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Infrared and Raman Spectra of Inorganic and Coordination Compounds

Infrared and Raman Spectra of Inorganic and Coordination Compounds Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry Sixth Edition

Kazuo Nakamoto Wehr Professor Emeritus of Chemistry Marquette University

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright # 2009 by John Wiley & Sons, Inc. All rights reserved Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data is available. ISBN 978-0-471-74493-1

Printed in the United States of America 10 9

8 7 6 5

4 3 2 1

Contents

PREFACE TO THE SIXTH EDITION

ix

ABBREVIATIONS

xi

1. Applications in Coordination Chemistry 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9. 1.10. 1.11. 1.12. 1.13. 1.14.

1

Ammine, Amido, and Related Complexes / 1 Complexes of Ethylenediamine and Related Ligands / 14 Complexes of Pyridine and Related Ligands / 23 Complexes of Bipyridine and Related Ligands / 29 Metalloporphyrins / 37 Metallochlorins, Chlorophylls, and Metallophthalocyanines / 45 Nitro and Nitrito Complexes / 52 Lattice Water and Aquo and Hydroxo Complexes / 57 Complexes of Alkoxides, Alcohols, Ethers, Ketones, Aldehydes, Esters, and Carboxylic Acids / 62 Complexes of Amino Acids, EDTA, and Related Ligands / 67 Infrared Spectra of Aqueous Solutions / 74 Complexes of Oxalato and Related Ligands / 79 Complexes of Sulfate, Carbonate, and Related Ligands / 84 Complexes of b-Diketones / 96

v

vi

CONTENTS

1.15. 1.16. 1.17. 1.18. 1.19. 1.20. 1.21. 1.22. 1.23. 1.24. 1.25. 1.26. 1.27. 1.28.

Complexes of Urea, Sulfoxides, and Related Ligands / 105 Cyano and Nitrile Complexes / 110 Thiocyanato and Other Pseudohalogeno Complexes / 120 Complexes of Carbon Monoxide / 132 Complexes of Carbon Dioxide / 152 Nitrosyl Complexes / 155 Complexes of Dioxygen / 161 Metal Complexes Containing Oxo Groups / 175 Complexes of Dinitrogen and Related Ligands / 183 Complexes of Dihydrogen and Related Ligands / 189 Halogeno Complexes / 193 Complexes Containing Metal–Metal Bonds / 199 Complexes of Phosphorus and Arsenic Ligands / 206 Complexes of Sulfur and Selenium Ligands / 210 References / 222

2. Applications in Organometallic Chemistry 2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7. 2.8. 2.9.

Methylene, Methyl, and Ethyl Compounds / 275 Vinyl, Allyl, Acetylenic, and Phenyl Compounds / 281 Halogeno, Pseudohalogeno, and Acido Compounds / 283 Compounds Containing Other Functional Groups / 290 p-Bonded Complexes of Olefins, Acetylenes, and Related Ligands / 294 Cyclopentadienyl Compounds / 302 Cyclopentadienyl Compounds Containing Other Groups / 308 Complexes of Other Cyclic Unsaturated Ligands / 313 Miscellaneous Compounds / 318 References / 319

3. Applications in Bioinorganic Chemistry 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7.

275

Myoglobin and Hemoglobin / 335 Ligand Binding to Myoglobin and Hemoglobin / 340 Cytochromes and Other Heme Proteins / 350 Bacteriochlorophylls / 359 Hemerythrins / 363 Hemocyanins / 368 Blue Copper Proteins / 373

333

CONTENTS

vii

3.8. Iron–Sulfur Proteins / 378 3.9. Interactions of Metal Complexes with Nucleic Acids / 387 References / 393 Index

403

Preface to the Sixth Edition

Since the fifth edition was published in 1996, a number of new developments have been made in the field of infrared and Raman spectra of inorganic and coordination compounds. The sixth edition is intended to emphasize new important developments as well as to catch up with the ever-increasing new literature. Major changes are described below. Part A. Chapter 1 (‘‘Theory of Normal Vibrations”) includes two new sections. Section 1.24 explains the procedure for calculating vibrational frequencies on the basis of density functional theory (DFT). The DFT method is currently used almost routinely to determine molecular structures and to calculate vibrational parameters. Section 1.26 describes new developments in matrix cocondensation techniques. More recently, a large number of novel inorganic and coordination compounds have been prepared by using this technique, and their structures have been determined and vibrational assignments have been made on the basis of results of DFT calculations. Chapter 2 (‘‘Applications in Inorganic Chemistry”) has been updated extensively, resulting in a total number of references of over 1800. In particular, sections on triangular X3- and tetrahedral X4-type molecules have been added as Secs. 2.2 and 2.5, respectively. In Sec. 2.8, the rotational–vibrational spectrum of the octahedral UF6 molecule is shown to demonstrate how an extremely small metal isotope shift by 235U/238U substitution (only 0.6040 cm1) can be measured. Section 2.14 (‘‘Compounds of Carbon”) has been expanded to show significant applications of vibrational spectroscopy to the structural determination of fullerences, endohedral fullerenes, and carbon nanotubes. Vibrational data on a number of novel inorganic compounds prepared most recently have been added throughout Chapter 2. Part B. Chapter 1 (‘‘Applications in Coordination Chemistry”) contains two new Sections: Sec. 1.6 (‘‘Metallochlorins, Chlorophylls, and Metallophthalocyanines”) ix

x

PREFACE TO THE SIXTH EDITION

and Sec. 1.19 (‘‘Complexes of Carbon Dioxide”). The total number of references has approached 1700 because of substantial expansion of other sections such as Secs. 1.5, 1.18, 1.20, 1.22, and 1.28. Chapter 2 (‘‘Applications in Organometallic Chemistry”) includes new types of organometallic compounds obtained by matrix cocondensation techniques (Sec. 2.1). In Chapter 3 (‘‘Applications in Bioinorganic Chemistry”), a new section (Sec. 3.4) has been added, and several sections such as Secs. 3.3, 3.7, and 3.9 have been expanded to include many important new developments. I would like to express my sincere thanks to all who helped me in preparing this edition. Special thanks go to Prof. J. R. Kincaid (Marquette University), Prof. R. S. Czernuszewicz (University of Houston), and Dr. T. Kitagawa (Institute for Molecular Science, Okazaki, Japan) for their help in writing new sections of Chapter 3 of Part B. My thanks also go to all the authors and publishers who gave me permission to reproduce their figures in this and previous editions. Finally, I would like to thank the staff of John Raynor Science Library of Marquette University for their help in collecting new references. Milwaukee, Wisconsin March 2008

KAZUO NAKAMOTO

Abbreviations

Several different groups of acronyms and other abbreviations are used: 1. IR, infrared; R, Raman; RR, resonance Raman; p, polarized; dp, depolarized; ap, anomalous polarization; ia, inactive. 2. n, stretching; d, in-plane bending or deformation; rw, wagging; rr, rocking; rt, twisting; p, out-of-plane bending. Subscripts, a, s, and d denote antisymmetric, symmetric, and degenerate modes, respectively. Approximate normal modes of vibration corresponding to these vibrations are given in Figs. 1.25 and 1.26. 3. DFT, density functional theory; NCA, normal coordinate analysis; GVF, generalized valence force field; UBF, Urey–Bradley force field. 4. M, metal; L, ligand; X, halogen; R, alkyl group. 5. g, gas; l, liquid; s, solid; m or mat, matrix; sol’n or sl, solution; (gr) or (ex), ground or excited state. 6. Me, methyl; Et, ethyl; Pr, propyl; Bu, butyl; Ph, phenyl; Cp, cyclopentadienyl; OAc, acetate ion; py, pyridine; pic, pycoline; en, ethylenediamine. Abbreviations of other ligands are given when they appear in the text. In the tables of observed frequencies, values in parentheses are calculated or estimated values unless otherwise stated.

xi

Chapter

1

Applications in Coordination Chemistry 1.1. AMMINE, AMIDO, AND RELATED COMPLEXES 1.1.1. Ammine (NH3) Complexes Vibrational spectra of metal ammine complexes have been studied extensively, and uller [1]. Figure 1.1 shows the infrared spectra these are reviewed by Schmidt and M€ of typical hexammine complexes in the high-frequency region. To assign these NH3 group vibrations, it is convenient to use the six normal modes of vibration of a simple 1 : 1 (metal/ligand) complex model such as that shown in Fig. 1.2. Table 1.1 lists the infrared frequencies and band assignments of hexammine complexes. It is seen that the antisymmetric and symmetric NH3 stretching, NH3 degenerate deformation, NH3 symmetric deformation, and NH3 rocking vibrations appear in the regions of 3400–3000, 1650–1550, 1370–1000, and 950–590 cm1, respectively. These assignments have been confirmed by NH3/ND3 and NH3 =15 NH3 isotope shifts. The NH3 stretching frequencies of the complexes are lower than those of the free NH3 molecule for two reasons. One is the effect of coordination. On coordination, the NH bond is weakened and the NH3 stretching frequencies are lowered. The stronger the MN bond, the weaker is the NH bond and the lower are the NH3 stretching frequencies if other conditions are equal. Thus the NH3 stretching frequencies may be used as a rough measure of the MN bond strength. The other reason is the effect of the counterion. The NH3 stretching frequencies of the chloride Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, Sixth Edition, by Kazuo Nakamoto Copyright 2009 John Wiley & Sons, Inc.

1

2

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.1. Infrared spectra of hexammine complexes: [Co(NH3)6]Cl3 (solid line), [Cr(NH3)6]Cl3 (dotted–dashed line), and [Ni(NH3)6]Cl2 (dotted line).

are much lower than those of the perchlorate, for example. This is attributed to the weakening of the NH bond, due to the formation of the NH Cl-type hydrogen bond in the former. The effects of coordination and hydrogen bonding mentioned above shift the NH3 deformation and rocking modes to higher frequencies. Among them, the NH3 rocking mode is most sensitive, and the degenerate deformation is least sensitive, to these effects. Thus the NH3 rocking frequency is often used to compare the strength of the MN bond in a series of complexes of the same type and anion. As will be shown in Z

v2(A1)

v1(A1) X

v3(A1)

Y

Y

Y

vs(NH3)

v4(E)

va(NH3)

δs(NH3)

v5(E)

δd(NH3)

vs(M–N)

v6(E)

ρr(NH3)

Fig. 1.2. Normal modes of vibration of tetrahedral ZXY3 molecules. (The band assignment is given for an MNH3 group.)

TABLE 1.1. Infrared Frequencies of Octahedral Hexammine Complexes (cm1)a n(MN) Complex

na(NH3)

ns(NH3)

da(HNH)

ds(HNH)

rr (NH3)

IR

[Mg(NH3)6]Cl2

3353

3210

1603

1170

660

363

[Cr(NH3)6]Cl3

3257

3185 3130

1630

1307

748

[50 CrðNH3 Þ6 ](NO3)3

3310

1627

1290

770

[Mn(NH3)6]Cl2 [Fe(NH3)6]Cl2 [Ru(NH3)6]Cl2 [Ru(NH3)6]Cl3

3340 3335 3315

3250 3190 3160 3175 3210

495 473 456 471

1608 1596 1612 1618

592 633 763 788

302 315 409 463

818 654

452 325

[Os(NH3)6]OsBr6 [Co(NH3)6Cl2

3330

3250

1595 1602

1146 1156 1220 1368 1342 1339 1163

[Co(ND3)6]Cl3

3240

3160

1619

1329

831

[Co(ND3)6]Cl3

2440

2300

1165

1020

667

3077 3125

[Rh(NH3)6]Cl3

3200

1618

1352

845

498 477 449 462 442 415 472

[Ir(NH3)6]Cl3

3155

1587

857

475

685

335

645 613 950

300 298 530 516

[58 NiðNH3 Þ6 ]Cl2

3345

3190

1607

1350 1323 1176

[Zn(NH3)6]Cl2 [Cd(NH3)6]Cl2 [Pt(NH3)6]Cl4

3350 — 3150

3220 — 3050

1596 1585 1565

1145 1091 1370

3 a

All infrared frequencies are those of the F1u species.

Raman 335 243 465 412

(A1g) (Eg) (A1g) (Eg)

— 330 — — 500 475 — 357 255 500 445

(A1g)

(A1g) (Eg) (A1g) (Eg) (A1g) (Eg)

515 480 527 500 370 265 — 342 569 545

(A1g) (Eg) (A1g) (Eg) (A1g) (Eg) (A1g) (A1g) (Eg)

d(NMN)

Ref.

198

2

3

270

4

165 170 — 283 263 256 92

1,5 1,5 6 7 7 5

331

810

294

4

302

7,10

279 264 217

7,10

— — 318

1 5 12,13

8,11

4

APPLICATIONS IN COORDINATION CHEMISTRY

the next subsection, a simple 1 : 1 complex such as that shown in Fig. 1.2 has been prepared in inert gas matrices [30]. To assign the skeletal modes such as the MN stretching and NMN bending modes, it is necessary to consider the normal modes of the octahedral MN6 skeleton (Oh symmetry). The MN stretching mode in the low-frequency region is of particular interest since it provides direct information about the structure of the MN skeleton and the strength of the MN bond. The octahedral MN6 skeleton exhibits two n(MN) (A1g and Eg) in Raman and one n(MN) (F1u) in infrared spectra (see Sec. 2.8 of Part A). Most of these vibrations have been assigned on the basis of observed isotope shifts (including metal isotopes, NH3/ND3 and NH3 =15 NH3 ) and normal coordinate calculations. Although the assignment of the n(CoN) in the infrared spectrum of [Co (NH3)6]Cl3 had been controversial, Schmidt and M€ uller [4] confirmed the assignments that the three weak bands at 498, 477, and 449 cm1 are the split components of the triply degenerate F1u mode (Fig. 1.4). The intensity of the MN stretching mode in the infrared increases as the MN bond becomes more ionic and as the MN stretching frequency becomes lower. Relative to the Co(III)N bond of the [Co(NH3)6]3þ ion, the Co(II)N bond of the [Co(NH3)6]2þ ion is more ionic, and its stretching frequency is much lower (325 cm1). This may be responsible for the strong appearance of the Co(II)N stretching band in the infrared. As listed in Table 1.1, two Raman-active MN stretching modes (A1g and Eg) are observed for the octahedral hexammine salts. In general, n(A1g) is higher than n(Eg). However, the relative position of n(F1u) with respect to these two vibrations changes from one compound to another. Another obvious trend in n(MN) is n(M4þN) >n(M3þN) > n(M2þN). This holds for all symmetry species. Table 1.1 shows that the NH3 rocking frequency also follows the same trend as above. Normal coordinate analyses on metal ammine complexes have been carried out by many investigators. Among them, Nakagawa, Shimanouchi, and coworkers [13] have undertaken the most comprehensive study, using the UBF (Urey–Bradley Force) field. The MN stretching force constants of the hexammine complexes follow this order: PtðIVÞ CoðIIIÞ > CrðIIIÞ > NiðIIÞ CoðIIÞ 2:13 1:05 0:94 0:34 0:33 mdyn=A Acevedo and coworkers carried out normal coordinate calculations on the [Cr (NH3)6]3þ and [Ni(NH3)6]2þ ions [14,15]. On the other hand, Schmidt and M€uller [4,5] and other workers [8] calculated the GVF (generalized valence Force) constants of a number of ammine complexes by using the point mass model (where the NH3 ligand is regarded as a single atom having the mass of NH3), and refined their values with isotope shift data (H/D, 14 N=15 N, and metal isotopes). For the hexammine series, they obtained the following order: Pt4 þ > Ir3 þ > Os3 þ > Rh3 þ > Ru3 þ > Co3 þ > 2:75 2:28 2:13 2:10 2:01 1:86 Cr3 þ > Ni2 þ > Co2 þ > Fe2 þ Cd2 þ > Zn2 þ > Mn2 þ 1:66 0:85 0:80 0:73 0:69 0:67 mdyn=A

5

AMMINE, AMIDO, AND RELATED COMPLEXES

TABLE 1.2. Infrared Frequencies of Other Ammine Complexes (cm1) n(MN) na(NH3)

ns(NH3)

da(HNH)

ds(HNH)

rr(NH3)

IR

Raman

d(NMN)

Ref.

3340 3275 3233 3354

3260 3150

1610 1596

693 685

430 —

16 17,18

1617

670

370

405 (A1) 432 (A1) 412 (F2) —

195 156

3267

1240 1253 1239 1176

166 160

1,19

3270

3170

1630

1279

495

3236

3156

1563

1325

325 300 301

[Cu(NH3)4]So4H2O

3327 3253 3490 3220

3169

1669 1639 1571

1300 1283 1331

735

426 555

256 227 327 307 272

4,23

936 914

502 482 543 522 420 375 566 544

4,20,21

[Pt(NH3)4]Cl2

849 802 842

3320 3230 3265

3150

1642 1626 1605

1236 1222 1268

740 703 719

476 400 513

221 211 —

25,26 27

Complex Tetrahedral [Co(NH3)4](ReO4)2 [64 Zn(NH3)4]I2 [Cd(NH3)4](ReO4)2

Square-planar [104 Pd(NH3)4]Cl2H2O

[Au(NH3)4(NO3)3

Linear [Ag(NH3)2So4 [Hg(NH3)2Cl2

3105

3197

510

(A1g) (B1g) (A1g) (B1g) (A1g) (B1g)

372 (A1) 412

13,22,21

24

For a series of divalent metals, this order is parallel to the Irving–Williams series (Mn2þ < Fe2þ < Co2þ < Ni2þ < Cu2þ > Zn2þ). Schmidt and M€uller [1] discussed the relationship between the MN stretching force constant and the stability constant or the bond energy. Table 1.2 lists the observed infrared frequencies and band assignments of tetrahedral, square–planar, and linear metal ammine complexes. The Raman-active MN stretching frequencies are also included in Table 1.2. Normal coordinate analyses have been made by Nakagawa et al. [13] by using the UBF field; the following values were obtained for the MN stretching force constants: Hg2 þ > Pt2 þ > Pd2 þ > Cu2 þ 2:05 1:92 1:71 0:84 mdyn=A Normal coordinate calculations have also been made by Tellez [28] on the tetrahedral [Zn(NH3)4]2þ and [Cd(NH3)4]2þ ions. Using the GVF field and the point uller [5] obtained the following values: mass approximation, Schmidt and M€ Pt2 þ > Pt2 þ Co2 þ Zn2 þ Cu2 þ > Cd2 þ 2:54 2:15 1:44 1:43 1:42 1:24 mdyn=A

6

APPLICATIONS IN COORDINATION CHEMISTRY

As stated above, NH3 frequencies of ammine complexes are determined by the strength of the MN bond as well as the strength of the NH X hydrogen bond. The order of this synergetic effect has been studied for [M(NH3)6]X2-type complexes [29]. 1.1.2. Ammine Complexes in Inert Gas Matrices Infrared spectra of cocondensation products of alkali halide (MX) vapors with NH3 diluted in argon were measured by Ault [34]. ln the case of KCl, for example, the bands at 3365, 3177, and 1103 cm1 have been assigned to the na(NH3), ns(NH3), ds(HNH), respectively, of the 1 : 1 ion pair of type I shown below:

The type II structure was ruled out because of the following reasons: (1), the ds(HNH) frequency should be sensitive to the metal ion in (I) and to the anion in (II) [the fact that it shows relatively large shifts by changing the metal ion, but almost no shifts by changing the anion, supports (I)]; and (2) the na(NH3) and ns(NH3) in (II) are expected to be highly sensitive to the anion, owing to formation of the NH X hydrogen bonds; this is not the case in (I). The fact that they show only small shifts in going from CsCl to CsI supports (I). Further supports for structure (I) are given by the appearance of the rr(NH3) and n(MN) at 458 and 232 cm1 (KCl), respectively. These frequencies are much lower than those of transition metal complexes discussed earlier, because their MN bonds are much weaker (more ionic). uzer and Andrews [31] studied the IR spectra of cocondensation products of alkali S€ metal (M) vapors with NH3/Ar. They assigned the following bands:

ns(NH3) ds(NH3)

Li 3277 1133

Na

K

Cs

3294 1079

3292 1064

3287 1049

(all in cm1)

to the 1 : 1 adduct of C3v symmetry which is similar to that of the M(NH3)þ cation discussed earlier. The MNH3 bonding has been attributed to a small charge transfer from NH3 to M in the case of Li and Na, and to a reverse charge transfer in the case of K and Cs. At high concentrations of M and NH3, large aggregates of undefined stoichiometries were formed. Similar work including Fe and Cu was carried out by Szczepanski et al. [32] Loutellier et al. [33] have made the most extensive IR study on the Li(K)/NH3/Ar system. By varying the concentrations and relative ratios of M/NH3 in a wide range, they were able to observe bands characteristic of the 1 : 1, 1 : 2, . . ., 1 : n,

AMMINE, AMIDO, AND RELATED COMPLEXES

7

Fig. 1.3. IR spectra (1200–1050 cm1) of cocondensation products of Li atoms with NH3 molecules in Ar matrices. Left column: (a) Li/NH3/Ar ¼ 0.1/0.4/1000, (b) 0.1/1.2/1000, (c) 3/10/1000, and (d) 2.5/20/1000. Right column: (a) Li/NH3/Ar ¼ 0.1/0.4/1000, (e) 2/0.6/1000, (f) 2/2.5/1000, and (g) 8/2.5/1000 [33].

and 2 : 1, 3 : 1 m : 1 adducts. As an example, Fig. 1.3 shows the IR spectra of the Li/ NH3/Ar system in the ds(NH3) region. The molar ratios (Li/NH3) and the peaks characteristic of each species are indicated in the figure. In general, the 1 : 1 adduct is formed when the concentrations of Li and NH3 are close. If the concentration of NH3 is high relative to Li, the 1 : n (n ¼ 2,3,4, . . .) adducts are formed. On the other hand, the m : 1 (m ¼ 2,3,4. . .) adducts result when the concentration of Li is high relative to NH3. For the 1 : 1 adduct of Li, the bands at 381 and 320 cm1 have been assigned to the respectively. The LiN stretching force constant was found to rr(NH3) and n(LiN), be 0.3 mdyn/A. Photolysis of ammine complexes in inert gas matrices has been used to produce a number of new species. For example, Ault [34] obtained Cl2V(O)NH2 by the reaction in Ar

hn

OVCl3 þ NH3 ! OVCl3 NH3 ! Cl2 VðOÞNH2 þ HCl and its infrared spectrum was assigned by isotopic substitution (H/D and 14 N=15 N) and DFT calculations. Similar reactions have been utilized to prepare HSiNH2 [35] and HMNH2(M ¼ Al,Ga, In) [36]. 1.1.3. Halogenoammine Complexes If the NH 3 groups of a hexammine complex are partly replaced by other groups, the degenerate vibrations are split because of lowering of symmetry, and new

8

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.3. Skeletal Vibrations of Pentammine and trans-Tetrammine Co (III) Complex (cm1) [37,38] Complex Pentammine (C4v) [Co(NH3)5F]2þ A1 E [Co(NH3)5Cl]2þ A1 E [Co(NH3)5Br]2þ A1 E [Co(NH3)5I]2þ A1 E trans-Tetrammine (D4h) [Co(NH3)4Cl2]þ A2u Eu [Co(NH3)4Br2]þ A2u Eu

n(CoX)

Skeletal Bending

480, 438 498

343 —

308 345, 290, 219

476, 416 498

272 —

310 292, 287, 188

475, 410 497

215 —

287 290, 263, 146

473, 406 498

168 —

271 290, 259, 132

— 501

353 —

186 290, 167

— 497

317 —

227 280, 120

n(CoN)

bands belonging to other groups appear. Here we discuss only halogenoammine complexes. The infrared spectra of [Co(NH 3) 5 X] 2þ- and trans-[Co(NH 3) 4X 2] þtype complexes have been studied by Nakagawa and Shimanouchi [37,38]. Table 1.3 lists the observed frequencies and band assignments obtained by these workers. The infrared spectra of some of these complexes in the CoN stretching region are shown in Fig. 1.4. Normal coordinate analyses on these complexes [37] have yielded the following UBF stretching force constants (mdyn/A): K (CoN), 1.05; K(CoF), 0.99; K(CoCl), 0.91; K(CoBr), 1.03; and K(CoI), 0.62. Using the GVF force field, Chen et al. [39,40] also carried out normal coordinate analysis on [M(NH3)5X]-type complexes (M ¼ Co, Cr; X ¼ NH3, Cl, H2O, OH, etc.) to obtain the n(MN) and n(MX) force constants. Raman spectra of some chloroammine Co(III) complexes have been assigned [41]. In the series of the [Cr(NH3)5X]2þ ions, the n(CrN) are in the 475–400 cm1 region, and the n(CrX) are at 540, 302, 264, and 184 cm1, respectively, for X ¼ F, Cl, Br, and I [42]. For more information on halogenoammine complexes of Cr(III), see Ref. [43]. Detailed vibrational assignments are available for halogenoammine complexes of Os(III) [44] and of Ru(III), Rh(III), Os(III), and Ir(III) [45]. In regard to M(NH3)4X2- and M(NH3)3X3-type complexes, the main interest has been the distinction of stereoisomers by vibrational spectroscopy. As shown in

AMMINE, AMIDO, AND RELATED COMPLEXES

9

Fig. 1.4. Infrared spectra (600–300 cm1) of Co(III) halogenoammine complexes.

Appendix V of Part A, trans-MN4X2 (D4h) exhibits one MN stretching (Eu) and one MX stretching (A2u), while cis-MN4X2 (C2v) shows four MN stretching (two A1, B1, and B2) and two MX stretching (A1 and B1) vibrations in the infrared. For mer-MN3X3 (C2n), three MN stretching and three MX stretching vibrations are infrared-active, whereas only two MN stretching and two MX stretching vibrations are infrared-active for fac-MN3X3 (C3v). Nolan and James [12] have measured and assigned the Raman spectra of a series of [Pt(NH3)nCl6n](n2)þ-type complexes. Li et al. [46] carried out normal coordinate analysis on cis-Pt(NH3)2Cl4. Vibrational spectra of the planar M(NH3)2X2-type complexes [M ¼ Pt(II), Pd(II)] have been studied by many investigators. Table 1.4 summarizes the observed frequencies and band assignments of their skeletal vibrations, including those of “cis-platin”—the well-known anticancer drug. Figure 1.5 shows the infrared spectra of cis- and trans-[Pd(NH3)2Cl2] obtained by Layton et al. [51]. As expected, both the PdN and PdCl stretching bands split into two in the cis-isomer. Durig et al. [52] found that the PdN stretching frequencies range from 528 to 436 cm1, depending on the

10

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.4. Skeletal Frequencies of Square–Planar M(NH3)2X2-Type Complexes (cm1)a Complex

n(MN)

n(MX)

Bending

Ref.

trans-[Pd(NH3)2Cl2] IR 496 R 492

333 295

245, 222, 162, 137 224

47,21

cis-[Pd(NH3)2Cl2] IR

327, 306

245, 218, 160, 135

47

trans-[Pd(NH3)2Br2] IR 490 R 483

— 182

220, 220, 122, 101 172

47 21

cis-[Pd(NH3)2Br2] IR

258

225, 225, 120, 100

47

trans-[Pd(NH3)2I2] IR 480

191

263, 218, 109

47

trans-[Pt(NH3)2Cl2] IR 572 R 538

365 334

220, 195 —

48,49 21,48

cis-[Pt(NH3)2Cl2]b IR R

330, 323 253

250, 198, 155, 123 160

49 21

trans-[Pt(NH3)2Br2] IR 504 R 535

260 206

230 —

48,49 48

trans-[Pt(NH3)2I2] R

153

48

a b

495, 476

480, 460

510 507

532

For band assignments, see also Refs. 13 and 50. See Sec. 3.9.1.

nature of other ligands in the complex. In general, the PtN stretching band shifts to a lower frequency as a ligand of stronger trans influence is introduced in the position trans to the PtN bond. Using infrared spectroscopy, Durig and Mitchell [53] studied the isomerization of cis-[Pd(NH3)2X2] to its trans-isomer. Other studies on halogenoammine complexes include [Zn(NH3)2X2] (X ¼ Cl,Br, I) [54] and [Ir(NH3)Cl5]2 [55].

1.1.4. Linear Chain Ammine Complexes Mixed-valence compounds such as Pd11PtIV(NH3)4Cl6 and Pd11PdIV(NH3)4Cl6 take the form of a chain structure as shown below:

AMMINE, AMIDO, AND RELATED COMPLEXES

11

Both compounds exhibit an intense, extremely broad electronic absorption band in the visible region. The IR spectra of these mixed-valence compounds are approximately superpositions of those of each of the components. However, the RR spectra (Secs. 1.22 and 1.23 of Part A) obtained by using exciting lines in this

Fig. 1.5. Infrared spectra of trans- and cis-[Pd(NH3)2Cl2] [51].

12

APPLICATIONS IN COORDINATION CHEMISTRY

region are markedly different from the IR spectra. In the case of the PdPt complex, RR spectra involving the progressions of three totally symmetric metal– chlorine stretching vibrations were observed. Thus, the visible spectrum was attributed to a metal–metal mixed-valence transition. On the other hand, the PdPd complex exhibits a RR spectrum involving several stretching and bending fundamentals and their combinations and overtones that originate in the Pd (NH3)2Cl4 component only. Thus, Clark and Trumble [56] attributed the visible spectrum to the metal–ligand charge transfer transitions within this component. Later, this work was extended to the NiPt complex of ethylenediamine (Sec. 1.2.3). In the Magnus green salt, [Pt(NH3)4] [PtCl4], the Pt(II) atoms form a linear chain structure with relatively short PtPt distances (3.3 A). Originally, Hiraishi et al. [13] assigned the infrared band at 200 cm1 to a lattice mode that corresponds to the stretching mode of the PtPtPt chain. This high frequency was justified on the basis of the strong PtPt interaction in this salt. Adams and Hall [57], on the other hand, assigned this mode at 81 cm1, and the 201 cm1 band to a NH3 torsion. In fact, the latter is shifted to 158 cm1 by the deuteration of NH3 ligands [58]. Different from the mixed-valence complexes, the Raman spectrum of the Magnus green salt obtained by excitation in the visible absorption band does not display long overtone series [58]. This is expected since it has no axial bonds that would change the bond lengths on electronic excitation. Resonance Raman spectra of these and other linear chain complexes are reviewed by Clark [59].

1.1.5. Lattice Vibrations of Ammine Complexes Vibrational spectra of metal ammine complexes in the crystalline state exhibit lattice vibrations below 200 cm1. Assignments of lattice modes have been made for the hexammine complexes of Mg(II),2 Co(II) [60], Ni(II) [60,61], [Co(NH3)6]/[Co(CN)6] [62], and [Pt(NH3)4]Cl2 [63]. Lattice modes and low-frequency internal modes of hexammine complexes have also been studied by Janik et al. [64,65] using the inelastic neutron-scattering technique.

1.1.6. Amido (NH2) Complexes The vibrational spectra of amido complexes may be interpreted in terms of the normal vibrations of a pyramidal ZXY2-type molecule. Niwa et al. [66] carried out normal coordinate analysis on the ½HgðNH2 Þ2 ¥þ ion (infinite-chain polymer); the results are given in Table 1.5. Brodersen and Becher [67] studied the infrared spectra of a number of compounds containing HgN bonds and assigned the HgN stretching bands at 700–400 cm1. Alkylamido complexes of the type M(NR2)4,5 (M ¼ Ti,Zr,Hf,V,Nb,Ta) exhibit their MN stretching bands in the 700–530 cm1 region [68].

13

AMMINE, AMIDO, AND RELATED COMPLEXES

TABLE 1.5. Infrared Frequencies and Band Assignments of Amido Complexes (cm1) [66] Compound þ ðClÞ1 ½HgðNH2 Þ1 þ ½HgðNH2 Þ1 ðBrÞ1

n(NH2) 3200 3175 3220 3180

d(NH2)

rw (NH2)

rr (NH2)

n(HgN)

1540

1025

673

573

1525

1008

652

560

1.1.7. Amine(RNH2) Complexes Infrared spectra of methylamine complexes, [Pt(CH3NH2)2X2] (X: a halogen), have been studied by Watt et al. [69]. Far-infrared spectra of [M(R2NH)2X2][M ¼ Zn(II) or Cd(II); R ¼ ethyl or n-propyl; X ¼ Cl or Br] type complexes have also been reported [70]. The n(PtI) vibrations of Pt(RNH2)2I2-type complexes are in the 200–150 cm1 region [71]. Infrared and Raman spectra of metal complexes of aniline have been reviewed by Thornton [72]. 1.1.8. Complexes of Hydrazine and Hydroxylamine Hydrazine (H2NNH2) coordinates to a metal as a unidentate or a bridging bidentate ligand. No chelating (bidentate) hydrazines are known. For example, the hydrazine ligands in [M(N2H4)2]Cl2 [M(II) ¼ Mn,Fe,Co,Ni,Cu,Zn,Cd] are bridging bidentate (polymeric):

On the other hand, all hydrazine ligands in [Co(N2H4)6]Cl2 are coordinated to the Co atom as a unidentate ligand. According to Nicholls and Swindells [73], the complexes of the former type exhibit the n(NN) near 970 cm1, whereas those of the latter type show it near 930 cm1. The IR spectra of hydrazine complexes of M(II)(M ¼ Ni,Co, Zn,Cd) [74], Os(II) [75], and Ln(III) (Ln ¼ Pr,Nd,Sm) [76] have been reported. In these compounds, hydrazine acts as a unidentate or bridging bidentate ligand. The vibrational spectra of hydroxyalmine(NH2OH) have been reported by Kharitonov et al. [77]. Other related ligands include diazene (N2H2). Lehnert et al. [78] prepared a diazene-bridged Fe(II) dimer, [FeL(PPr3)2]2(m-N2H2), where L denotes 1,2-bis(2-mercaptophenylthio)ethane, and assigned the N2H2 vibrations on the basis of isotope shifths (H/D and 14 N=15 N) and normal coordinate analysis. Andersen and Jensen [79] assigned the IR spectra of M(I)2[M(IV)L2], where M(I) is an alkali metal ion; M(IV) is Ni, Fe, Mn, and V; and L is the C3H6N3O33 ion (hexahydro-1,3,5-triazine-1,3,5-triol). Assignments were based on isotope shift data

14

APPLICATIONS IN COORDINATION CHEMISTRY

(H/D, (14 N=15 N; 12 C=13 C; 58 Ni=62 Ni; and 54 Fe=57 Fe), and the IR spectrum of the free ligand, C3H6N3(OH)3.

1.2. COMPLEXES OF ETHYLENEDIAMINE AND RELATED LIGANDS 1.2.1. Chelating Ethylenediamine When ethylenediamine(en) coordinates to a metal as a chelating ligand, it may take a gauche (d and l) or a cis conformation, as shown in Fig. 1.6. Then, eight different conformations are probable for the [M(en)3]nþ ion if we consider all possible combinations of conformations of the three chelate rings (d or l) around the chiral metal center. They are designated as L(ddd), L(ddl), L(dll), L(lll), D(lll), D(lld), D(ldd), and D(ddd). According to X-ray analysis, all the en ligands in the [Co(en)3]3þ ion take the gauche conformation (d), and the configuration of the whole ion is L(ddd) [80,81]. Although it is rather difficult to obtain such information from vibrational spectra, Cramer and Huneke [82] have shown that some of these conformers can be distinguished by the number of IR-active CC stretching vibrations. For example, [Cr (en)3]Cl3 3.5H2O [L(ddd), D3 symmetry] exhibits only one band at 1003 cm1, whereas [Cr(en)3][Ni(CN)5] 1.5H2O [L(ddl, dll), C2 symmetry] exhibits three bands at 1008, 1002 (shoulder), and 995 cm1. Gouteron has shown [83] that racemic (dl) and optically active (d) forms of [Co(en)3]Cl3 can be distinguished in the crystalline state by comparing vibrational spectra below 200 cm1.

Fig. 1.6. Rotational isomers of 1,2-disubstituted ethane. X ¼ NH2 for en.

COMPLEXES OF ETHYLENEDIAMINE AND RELATED LIGANDS

15

Normal coordinate analyses on metal complexes of ethylenediamine have been made by several groups of workers. Fleming and Shepherd [84] carried out normal coordinate calculations on the 1:1(Cu/en) model of the [Cu(en)2]2þ ion. These workers considered a 9-atom system of C2v symmetry, assuming that the two hydrogen atoms bonded to the C and N atoms are single atoms having the double mass of hydrogen. The IR bands at 410 and 360 cm1 have been assigned to the n(CuN) that are coupled with other skeletal modes. The corresponding CuN stretching force constant (GVF) was 1.25 mdyn/A. Borch and coworkers [85–87] have carried out more complete calculations by considering all the 37 atoms of the [Rh(en)3]3þ ion ([L(ddd)] configuration of D3 symmetry), and the force constants (GVF) have been refined by using the vibrational frequencies obtained for the N–d12, C–d12, N,C–d24, and their 15 N analogs. In total, 38 force constants were employed, including the RhN stretch of 1.607 mdyn/ A. Three n(RhN) vibrations are at 545 (A1), 445 (A2), and 506 (E), although they are strongly coupled with other skeletal bending modes. Figure 1.7 shows the IR and Raman spectra of (N–d12) [Rh(en)3]Cl3D2O obtained by Borch et al. [87]. Later, their calculations (E modes) were improved by Williamson et al. [88], who assigned the polarized Raman spectra of tris(ethylenediamine) complexes of Co(III) and Rh(III) on the basis of similar calculations.

Fig. 1.7. Infrared (a) and Raman spectra (b) of N-deuterated (N-d12) [Rh(en)3]Cl3D2O [87].

16

APPLICATIONS IN COORDINATION CHEMISTRY

Empirical assignments of n(MN) have been reported for [M(en)3]3þ (M ¼ Cr, Co) [89] [M(en)3]2þ (M ¼ Zn, Cd, Fe, etc.) [90], and [M(en)2]2þ (M ¼ Cu,Pd,Pt) [91]. Bennett et al. [92,93] found that, in a series of the M(en)3SO4 complexes, the n(MN) frequencies follow the order M ¼ MnðIIÞ FeðIIÞ CoðIIÞ NiðIIÞ CuðIIÞ ZnðIIÞ n4 391 < 397 < 402 < 410 < 485 > 405 ðcm 1 Þ n5 303 < 321 319 < 334 < 404 > 291 As mentioned in Sec. 1.1.1, this is the order of stability constants known as the Irving– Williams series. These assignments have been confirmed by extensive isotope substitutions, including metal isotopes. Stein et al. [94] observed that the Raman intensities of the totally symmetric stretching and chelate deformation modes of the [Co(en)3]3þ ion at 526 and 280 cm1, respectively, display minima near 21.5 kK,* where the d–d transition shows its absorption maximum. Figure 1.8 shows the excitation profiles of these totally symmetric vibrations, as well as that of non-totally symmetric n(CoN) (Eg) at 444 cm1. Since this result is opposite to what one expects from resonance Raman spectroscopy (Sec. 1.22 of Part A), it is called “antiresonance.” These workers

Fig. 1.8. Excitation profiles for the [Co(en)3]3þ ion. The left-hand scale pertains to the excitation data and the right-hand scale, to the absorption spectrum. I M ClO 4 is the molar intensity relative to that of the n1 band of ClO4: (ICo /CCo)ðC ClO 4 =I ClO 4 Þ. The theoretical curves (– – –) are calculated with the A term frequency dependence given by A. C. Albrecht and M. C. Hutley [J. Chem. Phys. 55, 4438 (1971)]. 1 kK ¼ 1 kilokayser ¼ 103 cm1.

COMPLEXES OF ETHYLENEDIAMINE AND RELATED LIGANDS

17

attributed its origin to “interference” between the weak scattering from the ligand-field state and strong preresonance scattering from higher energy allowed electronic states. For more theoretical study on this phenomenon, see Ref. 95. Lever and Mantovani [96] assigned the MN stretching bands of M(NN)2X2[M ¼ Cu(II), Co(II), Ni(II); NN ¼ en, dimethyl-en, etc.; X ¼ Cl, Br, etc.]-type complexes by using the metal isotope technique. For these compounds, the CoN and NiN stretching bands have been assigned to 400–230 cm1 [96], and the CuN stretching vibrations have been located in the 420–360 cm1 [97]. A straight-line relationship between the square of the CuN stretching frequency and the energy of the main electronic d–d band was found [98], with some exceptions [97]. The infrared spectra of cis- and trans-[M(en)2X2]þ [M ¼ Co(III), Cr(III), Ir(III), Rh (III); X ¼ Cl, Br, etc.] have been studied extensively [99–102]. These isomers can be distinguished by comparing the spectra in the regions of 1700–1500 (NH2 bending), 950–850 (CH2 rocking), and 610–500 cm1 (MN stretching). 1.2.2. Bridging Ethylenediamine Ethylenediamine takes the trans form when it functions as a bridging group between two metal atoms. Powell and Sheppard [103] were the first to suggest that ethylenediamine in (C2H4)Cl2Pt(en)PtCl2(C2H4) is likely to be trans, since the infrared spectrum of this compound is simpler than that of other complexes in which ethylenediamine is gauche. However, an NMR study on this complex ruled out this possibility [104]. The trans configuration of ethylenediamine was found in (AgCl)2en [105], (AgSCN)2en [106], (AgCN)2en [107], Hg(en)Cl2 [108], and M(en)Cl2 (M ¼ Zn or Cd) [105]. The structure of these complexes may be depicted as follows:

A more complete study, including the infrared and Raman spectra, of M(en)X2-type complexes [M ¼ Zn(II),Cd(II),Hg(II); X ¼ Cl,Br,SCN] has been done by Iwamoto and Shriver [108]. Mutual exclusion of infrared and Raman spectra, along with other evidence, supports the C2h bridging structure of the en ligand in the Cd and Hg complexes (see Fig. 1.9).

1.2.3. Mixed-Valence Complexes In Sec. 1.1.3 we discussed the RR spectra of mixed-valence complexes such as PdPt (NH3)4Cl6 and Pd2(NH3)4Cl6. Analogous complexes can be prepared by changing the metal and the N-donor ligand. For example, Clark and Croud [109] measured the RR

18

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.9. Infrared (top) and Raman (bottom) spectra of [Cd(en)Br2 ] -.

spectra of single crystals of [Ni(en)2] [Pt(en)2X2] (ClO4)4 (X ¼ Cl, Br, I) in which a linear chain such as

is formed via halogen bridges. These complexes exhibit strong, broad bands due to the Ni(II)–Pt(IV) charge transfer transition in the visible region. Figure 1.10 shows the RR spectra of the chloro complex obtained by 488 nm excitation at 20 K. It is seen that the complex exhibits a series of overtones of the symmetric ns(ClPtCl) vibration up to 6n, which split into three peaks, due to mixing of 35 CI=37 CI isotopes. Similar overtone series were observed for X ¼ Br and I. Using these data, the frequencies corrected for anharmonicity and anharmonicity constants have been calculated (Sec. 1.23 of Part A). Polarized RR studies show that these vibrations are completely polarized along the Ni XPtX Ni axis. Similar work is reported for [Pt(en)2][Pt(en)2Cl2] (ClO4)4[110]. In the [Pt(en)2][Pt(en)2X2](ClO4)4 series, the IR-active chain phonon frequencies (antisymmetric stretching) are 359.1, 238.7, and 184.2 cm1, respectively, for X ¼ Cl, Br, and I [111]. The mixed-valence complex [Pt(en)2I2][Pt(CN)4] forms a quasi-one-dimensional chain, {Pt(II)IPt(IV)I}n, in the axial direction. The RR spectrum of its single crystal exhibits three totally symmetric vibrations at 138, 111, and 49 cm1 in the

COMPLEXES OF ETHYLENEDIAMINE AND RELATED LIGANDS

19

Fig. 1.10. Isotopomer band intensities for n1 6n1 of a single crystal of [Ni(en)2][Pt(en)2Cl2](ClO4)4. For ease of presentation, the 35 ClPt 35 Cl component (a) of each harmonic is lined up with the same abscissa value; (b) and (c) refer to the 35 ClPt 37 Cl and 37 ClPt 37 Cl components, respectively [109].

20

APPLICATIONS IN COORDINATION CHEMISTRY

low-frequency region. The band at 138 cm1 was assigned to the ns(IPtI) since the same mode was observed at 140 cm1 for the [Pt(IV)(en)I2]2þ ion [112]. Omura et al. [113] reported the far-IR spectra of Magnus-type salts [M(en)2][M0 Cl4] [M,M0 ¼ Pt(II) or Pd(II)]. Berg and Rasmussen [114] measured the IR and farIR spectra of the analogous complexes [M(en)2][M0 Br4] [M,M0 ¼ Pt(II) or Pd(II)] and [M(en)2][HgI4]. No bands assignable to the metal–metal stretching were observed in these complexes. 1.2.4. Complexes of Polyamines Polyamines such as these shown below coordinate to a metal as tridentate or tetradentate ligands: Diethylenetriamine(dien):

Triaminotriethylamine(tren):

Triethylenetetramine(trien):

The infrared spectra of diethylenetriamine (dien) complexes have been reported for [Pd(dien)X]X (X ¼ Cl,Br,I) [115] and [Co(dien)(en)Cl]2þ [116]. The latter exists in the four isomeric forms shown in Fig. 1.11. Their infrared spectra revealed that the oand k-isomers contain dien in the mer configuration; the p- and e-isomers contain dien in the fac configuration. The mer- and fac-isomers of [M(dien)X3] [M ¼ Cr(III), Co (III), and Rh(III); X: a halogen] can also be distinguished by infrared spectra [117]. The infrared spectra of b, b0 , b00 -triaminotriethylamine(tren) complexes with Co(III) [118] and lanthanides [119] have been reported. Buckingham and Jones [120] measured the infrared spectra of [M(trien)X2]þ, where trien is triethylene-tetramine, M is Co(III), Cr(III), or Rh(III), and X is a halogen or an acido anion. These compounds give three isomers (Fig. 1.12) that can be distinguished, for example, by the CH2 rocking vibrations in the 920–869 cm1 region. For [Co(trien)Cl2]ClO4, cis-a-isomer exhibits two strong bands at 905 and 871 cm1 and cis-b-isomer shows four bands at 918, 898, 868, and 862 cm1; trans-isomer gives only one band at 874 cm1 with a weak band at 912 cm1. Far-infrared spectra of some of these trien complexes have been reported [121].

COMPLEXES OF ETHYLENEDIAMINE AND RELATED LIGANDS

21

Fig. 1.11. Structures of the [Co(dien)(en)Cl] 2þ ion.

1.2.5. Complexes of 1,2-Disubstituted Ethanes As is shown in Fig. 1.6, 1,2-disubstituted ethane may exist in the cis, trans, or gauche form, depending on the angle of internal rotation. The cis form may not be stable in the free ligand because of steric repulsion between two X groups. The trans form belongs to point group C2h, in which only the u vibrations (antisymmetric with respect to the center of symmetry) are infrared-active. On the other hand, both gauche forms belong to point group C2, in which all the vibrations are infrared-active. Thus the gauche form exhibits more bands than the trans form. Mizushima and coworkers [122] have shown that 1,2-dithiocyanatoethane (NCSCH2CH2SCN) in the crystalline state

Fig. 1.12. Structures of the [M(trien)X2] þ ions.

22

APPLICATIONS IN COORDINATION CHEMISTRY

definitely exists in the trans form, because no infrared frequencies coincide with Raman frequencies (mutual exclusion rule). By comparing the spectrum of the crystal with that of a CHC13 solution, they concluded that several extra bands observed in solution can be attributed to the gauche form. Table 1.6 summarizes the infrared frequencies and band assignments obtained by Mizushima et al. It is seen that the CH2 rocking vibration provides the most clear-cut diagnosis of conformation: one band (Au) at 749 cm1 for the trans form, and two bands (A and B) at 918 and 845 cm1 for the gauche form. The compound 1,2-dithiocyanatoethane may take the cis or gauche form when it coordinates to a metal through the S atoms. The chelate ring formed will be completely planar in the cis, and puckered in the gauche, form. The cis and gauche forms can be distinguished by comparing the spectrum of a metal chelate with that of the ligand in CHCl3 solution (gauche þ trans). Table 1.6 compares the infrared spectrum of 1,2dithiocyanatoethanedichloroplatinum(II) with that of the free ligand in a CHCl3 solution. Only the bands characteristic of the gauche form are observed in the Pt(II) complex. This result definitely indicates that the chelate ring in the Pt(II) complex is gauche. The method described above has also been applied to the metal complexes of 1,2-dimethylmercaptoethane (CH3SCH2CH2SCH3) [123]. In this case, the free ligand exhibits one CH2 rocking at 735 cm1 in the crystalline state (trans), whereas the metal complex always exhibits two CH2 rockings at 920–890 and 855–825 cm1 (gauche). In the case of ethylenediamine complexes discussed earlier, the CH2 rocking mode does not provide a clear-cut diagnosis since it couples strongly with the NH2 rocking and CN stretching modes. TABLE 1.6. Infrared Spectra of 1,2-Dithiocyanatoethane and Its Pt(II) Complex (cm1) [122] Ligand Crystal trans — 2155(t) 1423(t) — 1291a(t) — 1220(t) 1145(t) — — 1037a — — 749(t) 680(t) 660(t) a b

CHCl3 Solution (gauche þ trans) 2170(g) 2170(t) 1423(t) 1419(g) — 1285(g) 1215(t) 1140(t) 1100(g) — (g)b — 918(g) 845(g) —b 677(t) 660(t)

Raman frequencies in the crystalline state. Hidden by CHCl3 absorption.

Pt Complex (gauche) 2165ðgÞ ------ 1410ðgÞ --- 1280ðgÞ

--- ---1110ðgÞ 1052ðgÞ ---) 929ðgÞ 847ðgÞ --- -------

Assignment n(C N) d(CH2)

rw(CH2) rt(H2) n(CC) rr(CH2)

n(CS)

COMPLEXES OF PYRIDINE AND RELATED LIGANDS

23

1.3. COMPLEXES OF PYRIDINE AND RELATED LIGANDS 1.3.1. Complexes of Pyridine On complex formation, the pyridine (py) vibrations in the high-frequency region are not shifted appreciably, whereas those at 604 (in-plane ring deformation) and 405 cm1 (out-of-plane ring deformation) are shifts to higher frequencies. Clark and Williams [124] carried out an extensive far-infrared study on metal pyridine complexes. Table 1.7 lists the observed frequencies of these metal-sensitive py vibrations and metal–py stretching vibrations. Clark and Williams showed that n(Mpy) and n(MX) (X ¼ a halogen) are very useful in elucidating the stereochemistry of these py complexes. For example, fac-[Rh(py)3Cl3] exhibits two n(Rh—py) (C3v symmetry), whereas mer-Rh(py)3Cl3 shows three n(Rh—py) (C2v symmetry) near 250 cm1. The infrared spectra of these two compounds are shown in Fig. 1.13. Vibrational spectra in the low-frequency region have been assigned for other halogeno pyridine complexes, including trans-[Tc(py)2X4] [128], cis/trans-[Os (py)2X4][129], cis/trans-[Pt(py)2Cl4] [130], mer-[Os(py)3X3] [131], trans-[Os (py)4X2] [132], and trans-[Pt(py)4F2]2þ [133]. The metal isotope technique has been used to assign the n(Mpy) and n(MX) vibrations of Zn(py)2X2 [134] and Ni(py)4X2 [135]. The former vibrations have been located in the 225–160 and 250–225 cm1 regions, respectively, for the Zn(II) and Ni (II) complexes. Figure 1.14 shows the infrared and Raman spectra of [64 ZnðpyÞ2 Cl2 ] and its 68 Zn analog. As expected from its C2v symmetry, two n(Znpy) and two n(ZnCl) are metal-isotope-sensitive. Thornton and coworkers [136,137] carried out an extensive study on a variety of pyridine complexes with emphasis on band assignments based on isotope shift data (py-d5, 15 N-py, and metal isotopes). As an example, Fig. 1.15 illustrates the IR spectra and band assignments of a series of the M(py)2Cl2-type complexes [137]. The shaded TABLE 1.7. Vibrational Frequencies of Pyridine Complexes (cm1) 124 Complex Co(py)2Cl2 Ni(py)2I2 Cr(py)2Cl2 Cu(py)2Cl2 Co(py)2Cl2 mer-[Rh(py)3Cl3] fac-[Rh(py)3Cl3] trans-[Ni(py)4Cl2] trans-Ir(py)4Cl2]Cl cis-[lr(py)4Cl2]Cl trans-[Pt(py)2Br2] cis-[Pt(py)2Br2] a b

Structure

pya

pya

n(Mpy)

Monomeric, tetrahedral Monomeric, tetrahedral Polymeric, octahedral Polymeric, octahedral Polymeric, octahedral Monomeric, octahedral Monomeric, octahedral Monomeric, octahedral Monomeric, octahedral Monomeric, octahedral Monomeric, square–planar Monomeric, square–planar

642 643 640 644 631 650 643 626 650 656 656 659 644

422 428 440 441 429 468 464 426 469 468 476 448

253b 240 219 268 243,235b 265,245,230 266,245 236 260,(255) 287,273 297 260,234

For band assignments of pyridine, see Refs. 125 and 126. Assignments made by Postmus et al. [127].

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APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.13. Far-IR spectra of (A) fac- and (B) mer-[Rh(py)3Cl3] [124].

Fig. 1.14. Infrared and Raman spectra of 64 Zn ðpy Þ2 Cl2 and its 68 Zn analog [134].

COMPLEXES OF PYRIDINE AND RELATED LIGANDS

25

Fig. 1.15. Infrared spectra (650–150 cm1) of the M(py)2Cl2 complexes: solid bands, n(MCl) and shaded bands, n(MN) [139].

bands (n3 and n4) are assigned to the n(M—N), while the solid bands (n1 and n2) are due to the n(MCl). The n5 is assigned to a bending mode. In a series of polymeric octahedral complexes shown in Fig. 1.15, both n(M—N) and n(M—Cl) follow the Irving–Williams order shown in Sec. 1.2.1.

Two n(M—Cl) and one n(M—N) are expected in IR spectra of polymeric octahedral complexes (C1 symmetry), whereas two n(MCl) and two n(M—N) are expected in IR spectra of tetrahedral complexes (C2v symmetry). The latter also holds for polymeric tetragonal Cu(II) complex. It should be noted that the violet Co(II) complex is polymeric octahedral while the blue Co(II) complex is monomeric tetrahedral. An Infrared and Raman spectra of metal pyridine complexes have been reviewed extensively by Thornton [137]. Far-infrared spectra of metal pyridine nitrate complexes, M(py)x(NO3)y, have been reported [138,139].

26

APPLICATIONS IN COORDINATION CHEMISTRY

1.3.2. Surface-Enhanced Raman Spectra of Pyridine In 1974, Fleischmann et al. [140] made the first observation of surface-enhanced Raman spectra (SERS) of pyridine adsorbed on a silver electrode. Figure 1.16 shows the Raman that spectra of pyridine in solution [(a) and (b)] and SERS of pyridine on an Ag electrode that has the potential 0 to 1.0 V [(c)–(h)] relative to the SCE. It is seen that the intensities of the bands at 1037, 1025, and 1008 cm1 (ring stretch) are changed markedly by changing the potential. The 1025 cm1 band was assigned to the uncharged species, that is, pyridine bonded directly on the electrode surface (Lewis acid site) since its intensity is maximized near zero potential. The remaining two bands were attributed to the pyridine that is hydrogen-bonded to water molecules on the electrode surface. These two environments of pyridine are illustrated in Fig. 1.17. As expected, the relative intensity of the 1025 cm1 band decreases and those of the 1037 and 1008 cm1 bands increase as the negative potential increases. While verifying these results in 1977, Jeanmaire and Van Duyne [141,142] noted that the Raman signals (3067, 1036, and 1008 cm1) from pyridine on Ag electrodes are enhanced by a huge factor (104–106) relative to normal Raman spectra in solution. In addition, they noted that the Raman intensity depends not only on the electrode potential but also on several other factors such as electrode surface preparation, concentration of pyridine in solution, and the nature and concentration of the supporting electrolyte anion. Almost simultaneously, Albrecht and Creighton [143] noted anomalous enhancements of Raman bands of pyridine adsorbed on a Ag electrode. These bands include those at 3076 (CH stretch), 1600 and 1050–1000 (ring stretch), 669 (ring deformation), and 239 cm1 [AgN(py) stretch] [144].

Fig. 1.16. Raman spectra of pyridine in solution and at the silver electrode: (a) liquid pyridine; (b) 0.05 M aqueous pyridine; (c) silver electrode, 0 V (SCE); (d) 0.2 V; (e) 0.4 V; (f) 0.6 V; (g) 0.8 V; (h) 1.0 V (514.5 nm excitation, 100 mW) [140].

COMPLEXES OF PYRIDINE AND RELATED LIGANDS

27

Fig. 1.17. A possible model of the structure of the double layer between silver and KCl solution containing pyridine: (a) At positive potentials, showing pyridine adsorbed to silver via nitrogen and in an ‘‘aqueous acidic environment’’; (b) at negative potentials, showing adsorbed pyridine in ‘‘aqueous environment’’ [140].

Yamada and Yamamoto [145,146] measured the SERS of pyridine adsorbed on metal oxides with UV excitation (363.8 nm). These workers were able to distinguish three types of adsorbed pyridine, as shown in Fig. 1.18: Type H Type L Type B

Pyridine hydrogen-bonded to the surface OH group (3075 and 999 cm1) Pyridine adsorbed on a Lewis acid site (3075 and 1025 cm1) Pyridine adsorbed on a Bronsted acid site (3090 and 1005 cm1)

N

N H

H+

N

O–

O M O

O H

M

M O

O L

O

O B

Fig. 1.18. Three types of adsorbed pyridines on metal oxide surfaces [146].

28

APPLICATIONS IN COORDINATION CHEMISTRY

The frequencies in the brackets are those observed for g-alumina. SERS of pyridine adsorbed on rhodium oxide have also been reported [147]. The SERS of the [Os(NH3)5py]nþ (n ¼ 2,3) and [Ru(NH3)5py]2þ ions adsorbed at the silver–aqueous interface exhibit internal modes of pyridine as well as metal–ligand modes, although the lone-pair electrons of pyridine nitrogen atoms in these ions are not available for bonding to the surface silver atoms. This result demonstrates the utility of SERS in obtaining vibrational data of coordination compounds that are sometimes difficult to obtain in bulk media [e.g., Os(II) complex] [148]. As stated in Sec. 1.21 of Part A, Raman intensity is proportional to P2, where P (induced dipole moment) is equal to aE (where a ¼ polarizability; E ¼ electric field strength). Thus, surface enhancement must be caused by the enhancement of either E or a or both. E increases significantly on the surface of fine metal particles or on rough metal surfaces (electromagnetic enhancement). In this case, the incident beam excites conduction electrons and generates “surface plasma resonance (plasmon resonance)” a increases through charge transfer or bond formation between the adsorbate and the metal surface (chemical enhancement). 1.3.3. Complexes of Pyridine Derivatives and Related Ligands The infrared spectra of metal complexes with alkyl pyridines have been studied extensively [149,150]. Using the metal isotope technique, Lever and Ramaswamy [151] assigned the Mpic stretching bands of M(pic)2X2 M ¼ Ni(II), Cu(II); pic ¼ picoline; X ¼ Cl,Br,I] in the 300–230 cm1 region. The infrared spectra of metal complexes with halogenopyridines have been reported [152,153]. Infrared spectra have been used to determine whether coordination occurs through the nitrile or the pyridine nitrogen in cyanopyridine complexes. It was found that 3- and 4-cyanopyridines coordinate to the metal via the pyridine nitrogen [154], whereas 2cyanopyridine coordinates to the metal via the nitrile nitrogen [155]. Vibrational spectra of M(3,5-lutidine)4X2 [156] (M ¼ Mn or Cu; X ¼ Cl or Br), and Zn{Hpic)(pic)Cl [157] and [Mn(Hpic)(pic)Cl]2 [158], where Hpic is 2-picolic acid, have been assigned. Infrared spectra of aromatic amine N-oxides and their metal complexes have been reviewed by Garvey et al. [159]. The N¼O stretching band of pyridine N-oxide (1265 cm1) is shifted by 70–30 cm1 to a lower frequency on complexation. The following references are given for three complexes: Fe(II) [160], Hg(II) [161], and Fe (III) [162]. Imidazole (Im) and its derivatives form complexes with a number of transition metal ions. Infrared spectra are reported for metal complexes of

COMPLEXES OF BIPYRIDINE AND RELATED LIGANDS

29

imidazole [163–165], 2-methylimidozole [166,167] 1 (or N)-methylimidazole [168], 4- and 5-bromoimidazole [169], and benzimidazole [170,171]. Among them, imidazole is biologically most important since imidazole nitrogens of histidyl residues coordinate to metal ions in many metalloproteins. Thus, the identification of MN (Im) vibrations in biological systems provides valuable information about the structure of the active site of a metalloprotein (Sec. 3.1). Using metal isotope techniques, Cornilsen and Nakamoto [165] assigned and MN stretching vibrations of 16 imidazole complexes of Ni(II), Cu(II), Zn(II), and Co(II) in the 325–210 cm1 region. Hodgson et al. [172] also assigned these vibrations in the same region. Salama and spiro [173] were first to assign the CoN stretching vibrations in resonance Raman spectra of Co(ImH)2Cl2 (274 and 232 cm1), [Co(ImH)4]2þ (301 cm1), and [Co(Im)4]2 (306 cm1). Caswell and Spiro [174] studied excitation profiles of imidazole, histidine, and related ligands including the [Cu(ImH)4]2þ ion in the UV region. These compounds exhibit maxima near 218 and 204 nm, where the p–p* transitions of the heterocyclic rings occur. Drozdzewski and coworkers prepared the {M(IA)2] [M ¼ Cu(II), Ni(II), Co(II), HIA ¼ 4-imidazoleacetic acid] and their IR spectra on the basis of isotopic shift data (H/D, 63 Cu=65 Cu) and DFT calculations [175], The far-IR spectrua of polymeric Zn (II) complexes, [ZnCl(IA)(HIA)] H2O (64 Zn=68 Zn), were also assigned [176]. Their work was extended to Pd(II) complexes of the type [Pd(hi)X2] [177] (X ¼ Cl,Br) and [Pd(hi)2]Br2 [178], where hi is 2-hydrazino-2-imidazoline.

1.4. COMPLEXES OF BIPYRIDINE AND RELATED LIGANDS 1.4.1. Complexes of 2,20 -Bipyridlne Infrared spectra of metal complexes of 2,20 -bipyridine (bipy) have been studied extensively. In general, the bands in the high-frequency region are not metal-sensitive since they originate in the heterocyclic or aromatic ring of the ligand. Thus, the main interest has been focused on the low-frequency region, where n(MN) and other metalsensitive vibrations appear. It has been difficult, however, to assign n(MN) empirically since several ligand vibrations also appear in the same frequency region. This difficulty was overcome by using the metal isotope technique. Hutchinson et al. [179] first applied this method to the tris-bipy complexes of Fe(II), Ni(II), and Zn(II), Later, this work was extended to other metals in various oxidation states [180]. Table 1.8 lists n(MN), magnetic moments, and the electronic configuration of these tris-bipy complexes. The results revealed several interesting relationships between n(MN) and the electronic structure: (1) In terms of simple MO theory, Cr(III), Cr(II), Cr(I), Cr(0), V(II), V(0), Ti(0), Ti (–I), Fe(III), Fe(II), and Co(III) have filled or partly filled t2g (bonding) and empty eg (antibonding) orbitals. The n(MN) of these metals (group A) are in the 300–390 cm1 region.

TABLE 1.8. MN Stretching Frequencies and Electronic Structures in [M(bipy)3]nþ-Type Compounds (cm1)a

30

–I d

I

II

3

III 374 335

V (3.67) (t2g)3 d4

374 339

Ti (0) d

Ti (1.74) (t2g)5-Is d

365 322

V (1.68) (t2g)5-Is

6

(t2g)4-Is 371 343

371 343

Cr (2.0) (t2g)5-Is

382 308

Cr (0) d

Mn (4.10) (t2g)5(eg)2 d8 Mn (3.71) (t2g)6(eg)2

258 227

235 184

244 194 (t2g)6(eg)2

Co (2.23) (t2g)6(eg)3

280 257

224 191

386 376

Co (4.85) (t2g)5(eg)2 Co (3.3)

d9

Mn (5.95) (t2g)3 (eg)-hs Fe (0) (t2g)6

(t2g)6 7

385 349

351 343

Cr (2.9)

(t2g)4-Is 5

Cr (3.78) (t2g)3

Ni (3.10) (t2g)6(eg)2

Fe (?)

Co (0) (t2g)6

384 367

378 370

266 228

282 258

291 268

Cu (?) (t2g)6(eg)3

d 10

230 184

Zn (0) (t2g)6(eg)4

The numbers at the upper right of each group indicate the MN stretching frequencies (cm1). The numbers in parentheses give the observed magnetic moments in Bohr magnetons. Is ¼ low spin; hs ¼ high spin.

a

COMPLEXES OF BIPYRIDINE AND RELATED LIGANDS

31

(2) On the other hand, Co(II), Co(I), Co(0), Mn(II), Mn(0), Mn(–I), Ni(II), Cu(II), and Zn(II) have filled or partly filled eg orbitals. The n(MN) of these metals (group B) are in the 180–290 cm1 region. (3) Thus no marked changes in frequencies are seen in the Cr(III)–Cr(0) and Co (II)-Co(0) series, although a dramatic decrease in frequency is observed in going from Co(III) to Co(II). (4) The fact that the n(MN) do not change appreciably in the former two series indicates that the MN bond strength remains approximately the same. These results also suggest that, as the oxidation state is lowered, increasing numbers of electrons of the metal reside in essentially ligand orbitals that do not affect the MN bond strength. Other work on bipy complexes includes a far-infrared study of tris-bipy complexes with low-oxidation-state metals [Cr(0), V(–I), Ti(0), etc.] [181], the assignments of infrared spectra of M(bipy)Cl2 (M ¼ Cu, Ni, etc.) [182], normal coordinate analysis on Pd(bipy)Cl2 and its bipy-d8 analog [183]. The [Fe(bipy)3]2þ ion and its analogs exhibit strong absorption near 520 nm, which is due to Fe(3d)–ligand(p) CT transition. When the laser wavelength is tuned in this region, a number of bipy vibrations (all totally symmetric) are strongly resonanceenhanced, as shown in Fig. 1.19 [184]. Excitation profile studies show that the intensities of all these bands are maximized at the main absorption maximum at 19,100 cm1 (524 nm) and that no maxima are present at the sideband near 20,500 cm1 (488 nm). Thus, Clark et al. [184] concluded that the latter band is due to a vibronic transition. The resonance Raman spectrum of the [Fe(bipy)3]2þ ion was also observed near the iron electrode surface in borate buffer solution containing bipy [185]. The electronic spectrum of the singly reduced [Fe(bipy)3]þ ion has also been assigned based on excitation profile studies [186]. 1.4.2. Time-Resolved Resonance Raman (TR3) Spectra The [Ru(bipy)3]2þ ion and related complexes have attracted much attention as potential compounds of solar energy conversion devices because of their excitedstate redox properties. When solutions of this ion are irradiated with 7-ns, highintensity pulses from the third harmonic (354.5 nm) of a Nd–YAG laser, the irradiated volume can be saturated with the longlived (600 ns) triplet ML CT state (A3) via efficient (f ffi 1) and rapid (t < 10 ps) intersystem crossing (A2 ! A3) as shown in Fig. 1.20. Since the A3–A4 (p–p*) transition is close to the 354.5-nm exciting line, conditions are favorable for efficient resonance Raman scattering from the A3 state; namely, it is possible to obtain the time-resolved resonance Raman (TR3) spectrum of the ion in the electronic excited state. The first observation of such spectra was made by Dallinger and Woodruff [187], who were followed by many investigators [188–191]. These workers found that the TR3 spectrum consists of two series of bipy vibrations; one series of bands are the same as those observed in the A1 state and the other correspond to those of Liþ(bipy). Figure 1.21 shows the spectra obtained by Mallick et al. [192]. It is seen that the TR3 spectrum (middle trace) is the addition of the RR

32

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.19. The RR spectra of the [Fe(bipy)3] 2þ ion. The asterisk indicates the 981 cm1 band of the SO24 ion (internal standard) [184].

Fig. 1.20. Energy-level diagram for the [Ru(bipy)3] 2þ ion.

COMPLEXES OF BIPYRIDINE AND RELATED LIGANDS

33

Fig. 1.21. The RR and TR3 spectra of the [Ru(bipy)3] 2þ ion. (A) RR spectrum with 350.7 nm excitation. (B) TR3 spectrum with 354.7 nm excitation. (C) RR spectrum of Li (bipy) with 350.7 nm excitation [192].

spectra of the [Ru(bipy)3]2þ ion (top trace) and Liþ(bipy) (bottom trace). Thus, the triplet ML CT state (A3) is formulated as [Ru(III)(bipy)2(bipy)]2þ, that is, the electron is localized on one bipy rather than delocalized over all three ligands (at the least vibrational time scale). The RR spectra of electron reduction products of several [Ru(bipy)3]2þ derivatives show similar electron localization [193]. Kincaid and coworkers [194,195] carried out normal coordinate analyses on the 1 : 1 (metal/ligand) model of the [Ru(bipy)3]2þ ion in the ground and the ML CT states. As expected, extensive vibrational couplings exist among those represented by local internal coordinates. The RuN stretching force constant was 2.192 mdyn/A in both states. In heteroleptic complexes, selective population of individual ligand-localized excited states is possible by using TR3 spectroscopy. For example, Danzer and Kincaid [196] have demonstrated that the triplet ML CT state of the [Ru (bipy)2(bpz)]2þ ion (bpz ¼ bipyrazine) should be formulated as [Ru(III) (bipy)2(bpz)]2þ, whereas that of the [Ru(bipy)(bpz)2]2þ ion should be formulated as [Ru(III)(bipy)(bpz)(bpz)]2þ.

34

APPLICATIONS IN COORDINATION CHEMISTRY

This work was extended to many other heteroleptic complexes such as [Ru (bipy)2(bpm)]2þ [197] and [Ru(bipy)2(dpp)]2þ [198,199], where bpm and dpp denote 4,40 -bipyrimidine and 2,3-bis(2-pyridyl)pyrazine, respectively. TR3 studies show selective population of the bpm-localized excited state in the former, and polarization of electron density toward the pyrazyl fragment in the latter. In the case of [Ru (pypz)3]2þ, where pypz is an inherently asymmetric ligand, 2-(2-pyridyl)pyrazine, the electronic charge is polarized toward the pyrazine fragment [200]. Other complexes studied include [Ru(bipy)2(bpdz)]2þ(bpdz ¼ 3,30 -bipyridazine) [201] and [Ru(bipy)2L]2þ (L; alkylated 2,20 -bipyridine) [202]. 1.4.3. Complexes of Phenanthroline and Related Ligands The metal isotope technique has been used to study the effect of magnetic crossover on the low-frequency spectrum of Fe(phen)2(NCS)2 (phen ¼ 1,10-phenanthroline). This compound exists as a high-spin complex at 298 K and as a low-spin complex at 100 K. Figure 1.22 shows the infrared spectra of 54 FeðphenÞ2 ðNCSÞ2 obtained by Takemoto and Hutchinson [203]. On the basis of observed isotopic shifts, along with other evidence, they made the following assignments (cm1):

High spin Low spin

n(FeNCS)

n[FeN(phen)]

252(4.0) 532.6(1.6) 528.5(1.7)

222(4.5) 379(5.0) 371(6.0)

The numbers in parentheses indicate the isotope shift, vð54 FeÞ vð57 FeÞ. Both vibrations show large shifts to higher frequencies in going from the high- to

Fig. 1.22. Infrared spectra of 54 Fe ðphenÞ2 ðNCSÞ2 . Numbers in parentheses indicate isotope shifts due to 54 Fe =57 Fe substitution.

COMPLEXES OF BIPYRIDINE AND RELATED LIGANDS

35

the low-spin complexes. This result suggests the marked strengthening of these coordinate bonds in goine from the high- to the low-spin complexes, as confirmed by X-ray analysis [204]. The work of Takemoto and Hutchinson has been extended to Fe(bipy)2(NCS)2 and Fe(phen)2(NCSe)2 [205]. Spin crossover by changing the temperature was also observed for [Fe(btr)2(NCS)2] H2O (btr ¼ 4,40 -bis-1,2,4-triazole). Thus, the na (NCS) of the low-spin (115 K) and high-spin (141 K) species were observed at 2099 and 2054 cm1, respectively. However, a large hysteresis effect was noted in this temperatural spin conversion [206]. It has been shown by infrared spectroscopy that a partial high- ! low-spin conversion occurs under high pressure [207]. Barnard et al. [208] studied the vibrational spectra of bis[tri-(2-pyridyl)amine]Co(II) perchlorate in the high-spin (293 K) and low-spin (100 K) states. In the infrared, the CoN stretching band is at 263 cm1 for the high-spin complex, whereas it splits and shifts to 312 and 301 cm1 in the low-spin complex. As stated above, Fe(phen)2(NCS)2 exhibits the room-temperature high-spin state (HS-1) and the low-temperature low-spin state (LS-1). Herber and Casson [209] found that, when the latter is irradiated by white light below 50 K, another high-spin state (HS2) is obtained (light-induced excited-state trapping), and annealing of this HS-2 state above 30 K produces another low-spin state (LS-2). These workers have shown that the n(CN) of the NCS ligands near the 2100-cm1 region are different among these four states. This work has been extended to Fe(bt)2(NCX)2 (bt ¼ 2,20 -bi-2-thiazoline and X ¼ S or Se) [210], and to Fe(5,6-dmp)2(NCS)2 (dmp ¼ dimethylphenanthroline) [211]. The TR3 spectrum of the [Cu(I)(DPP)2]þ ion (DPP ¼ 2,9-diphenyl-phenanthroline) shows that its ML CT state is formulated as [Cu(II)(DPP)(DPP)]þ [212]. Similar to the case of the [Ru(bipy)3]2þ ion, the TR3 spectrum of the [Ru(phen)3]2þ ion at the ML CT state was expected to show the bands due to the phen fragment. Since no such bands were observed, Turro et al. [213] concluded that charge localization seen for the corresponding bipy complex does not occur in this case. 1.4.4. Complexes of Other Ligands Simple a-diimines such as shown below form metal chelate compounds similar to bipy and phen, discussed earlier:

Figure 1.23 shows the IR spectra of Fe(II) and Ni(II) complexes of these ligands [214]. Normal coordinate calculations indicate extensive vibrational couplings among those represented by individual internal coordinates. Depending on the nature of the alkyl group (R), an alkyl-substituted a-diimine (RN¼CHCH¼NR) coordinates to the metal [Pt(II) or Pd(II)] as a unidentate or

36

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.23. Infrared spectra of a-diimine complexes of Fe(II) and Ni(II) [214].

bidentate (chelating) ligand. Van der Poel et al. [215] observed the C¼N stretching bands at 1615–1624 and 1590–1604 cm1 for the unidentate and bidentate coordinations, respectively. The metal isotope technique has been used to assign the MN vibrations of metal complexes with many other ligands. For example, Takemoto [216] assigned the NiN2 and NiN1 stretching vibrations of [Ni(DAPD)2]2 at 416–341 and 276 cm1, respectively:

In Ni(DAPD)2, where the Ni atom is in the þIV state, the NiN2 and NiN1 stretching bands are located at 509.8–472.0 and 394.8 cm1, respectively. These high-frequency shifts in going from Ni(II) (d8) to Ni(IV) (d6, diamagnetic) have also been observed for diarsine complexes (Sec. 1.25.2). Hutchinson and Sunderland [217] have noted that the MN stretching frequencies of the Ni(II) and Zn(II) complexes of 2,7-dimethyl-1,8naphthyridine (DMNAPY), shown above, are lower than those of the corresponding tris-bipy complexes by 16–24%. This was attributed to weakening of the MN bond due to the strain in the four-membered chelate rings of the DMNAPY complexes. Normal coordinate analysis has been carried out on the M(DMG)2 series (DMG ¼

METALLOPORPHYRINS

37

dimethylglyoximate ion) [218] and the MN stretching force constants (mdyn/A) have been found to be as follows: Pt(II) Pd(II) Cu(II) Ni(II) 3.77 >2.84 > 1.92 > 1.88 (GVF) This work was extended to bis(glyoximato) complexes of Pt(II), Pd(II), and Ni(II) [219]. The CoN(DMG) and CoN(py) stretching bands of Co(DMG)2(py)X (X ¼ a halogen) were assigned at 512 and 453 cm1, respectively, based on 15 N and py-d5 isotope shifts [220]. The metal isotope technique has been used to assign the MN stretching vibrations of metal complexes with 8-hydroxyquinoline [221] and 1,8naphthyridine [222]. Spectra–structure correlations [223] and detailed assignments [224] were made for metal complexes of 8-hyroxyquinoline. Assignments of the IR/Raman spectra of the [Ru (tpy)2]2þ ion (tpy ¼ 2,20 ,60 ,200 -terpyridine) were based on normal coordinate analysis [225]. 1.5. METALLOPORPHYRINS Vibrational spectra of metalloporphyrins have been studied exhaustively because of their biological importance as prosthetic groups of a variety of heme proteins (Chapter 3). Thus, many review articles have been published on this subject, and most of them discuss vibrational spectra of metalloporphyrins together with those of heme proteins. A review by Kitagawa and Ozaki [226], however, is focused on metalloporphyrins. 1.5.1. Normal Coordinate Analysis Because of relatively high symmetry and biological significance, normal coordinate analyses on metalloporphyrins have been carried out by many investigators [227– 234]. Figure 1.24 shows the planar D4h structure of a metalloporphyrin. The simplest

Fig. 1.24. Structures of metalloporphyrins. Porphin (Por), R1R8 ¼ H and R 0 ¼ H. Octaethylporphyrin (OEP), R1–R8 ¼ ethyl and R 0 ¼ H. Tetraphenylporphyrin (TPP), R1R8 ¼ H and R 0 ¼ phenyl. Tetramesitylporphyrin (TMP), R1–R8 ¼ H and R 0 ¼ mesityl. Protoporphyrin IX (PP), R1 ¼ R3 ¼ R5 ¼ R8 ¼ CH3, R2 ¼ R4 ¼ vinyl, R 0 ¼ H and R6 ¼ R7 ¼ (CH2)2COOH.

38

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.9. Classification of Normal Vibrations of a Metal Porphin Complex of D4h Symmetrya [227]. In-Plane Vibrations A1g (R) A2gb B1g (R) B2g (R) Eu(IR)

9 8 9 9 18

Out-of-Plane Vibrations A1u A2u (IR) B1u B2u Eg(R)c

3 6 5 4 8

a

R ¼ Raman-active; IR ¼ IR-active. The A2g vibrations become Raman-active under resonance conditions (see Sec. 1.23 of Part A). c The Eg vibrations are weak even under resonance condtions. b

porphyrin, porphin (Por), has 105 (3 376) normal vibrations, which are classified as shown in Table 1.9 [227]. As stated in Sec. 1.23 of Part A, metalloporphyrins are ideal for resonance Raman (RR) studies because they exhibit strong absorption bands in the visible and near-UV regions. It is well established that RR spectra obtained by excitation near the Q0 (or a) band are dominated by those of the B1g and B2g species (dp), while those obtained by excitation between the Q0 and Q1 (or b) band are dominated by those of the A2g species (ap). On the other hand, the RR spectra obtained by excitation near the B (or Soret) band are dominated by those of the A1g species (p). This information, combined with isotope shift data (H/D, 14 N=15 N, and metal isotopes), has been used extensively to refine the results of normal coordinate calculations. Spiro and coworkers [232–234] carried out the most extensive normal coordinate calculations on metalloporphyrins. As expected from their conjugated ring structures, strong vibrational couplings occur among vibrational modes represented by local internal coordinates. Thus, it is rather difficult to describe the normal modes by single internal coordinates. Table 1.10 lists the observed frequencies and major local coordinates responsible for the IR/Raman-active in-plane skeletal modes of Ni(Por), Ni(OEP), and Ni(TPP) (the RR spectra of Ni(OEP) are shown in Fig. 1.35 of Part A). Figure 1.25 shows the normal modes of the eight A1g vibrations obtained for Ni(OEP) and Ni(Por). Table 1.11 lists the normal modes to which the n(NiN) coordinates make significant contributions. Previously, empirical assignments have been made for IRactive MN stretching vibrations of a series of M(II)(TPP) complexes using metal isotopes such as 58 Ni=62 Ni; 63 Cu=65 Cu; and 64 Zn=68 Zn [235]. For normal coordinate analysis on the out-of-plane modes, see Ref. 234. It should be noted that the internal vibrations of the peripheral substituents such as the ethyl and phenyl groups also couple with the porphyrin core vibrations [232,233]. The vinyl group vibrations of Fe(PP) (PP ¼ protoporphyrin IX; see Fig. 1.24), which are commonly found in natural heme proteins, can be resonance-enhanced by excitation near 200 nm (near the p–p* transition of the vinyl group) [236,237]. Density Functional Theory (DFT) calculations (Sec. 1.24 of Part A) were carried out on large metalloporphyrin molecules to obtain structural information and to make band assignments. Spiro and coworkers [238,239] calculated both in-plane and

39

METALLOPORPHYRINS

TABLE 1.10. In-Plane Skeletal-Mode Frequencies (cm1) and Local-Mode Assignments for Ni(II) Complexes of OEP, Porphin, and TPP [232,233] Symmetry

ni

Descriptiona

NiOEP b

NiPor

NiTPP

A1g

n1 n2 n3 n4 n5 n6 n7 n8 n9

n(Cm-X) n(Cb-Cb) n(Ca-Cm)sym n(Pyr. half-ring)sym n(Cb-Y)sym n(Pyr. breathing) d(Pyr. def.)sym n(Ni-N) d(Cb-Y)sym

[3041] 1602 1520 1383 1138 804 674 360/343 263/274

[3042] 1579 1463 1380 [3097] 999 735 372 1070

1235 1572 1470 1374 [3097] 1004 889 402 1078

Big

n10 n11 n12 n13 n14 n15 n16 n17 n18

n(Ca-Cm)asym n(Cb-Cb) n(Pyr. half-ring)sym d(Cm-X) n(Cb-Y)sym n(Pyr. breathing) d(Pyr. def.)sym d(Cb-Y)sym n(Ni-N)

1655 1577 1331 1220 1131 751 746 305 168

1654 1509 1319 1189 [3097] 1007 734 1064 241

1594 1504 1302 238 [3097] 1004 [900] 1084 277

A2g

n19 n20 n21 n22 n23 n24 n25 n26

n(Ca-Cm)asym n(Pyr. quarter-ring) d(Cm-X) n(Pyr. half-ring)asym n(Cb-Y)asym d(Pyr. def.)asym d(Pyr. rot.) d(Cb-Y)asym

1603 1394 1307 1121 1058 597 551 [243]

1615 1358 1143 1009 [3087] 810 433 1321

1550 1341 [257] 1016 [3087] 828 560 1230

B2g

n27 n28 n29 n30 n31 n32 n33 n34 n35

n(Cm-X) n(Ca-Cm)asym n(Pyr. quarter-ring) n(Pyr. half-ring)asym n(Cb-Y)asym d(Pyr. def.)asym d(Pyr. rot.) n(Cb-Y)asym d(pyr. transl.)

[3040] 1483 1407 1160 1015 938 493 197 144

[3041] [1492] 1372 1007 [3088] 823 439 1197 201

1269 [1481] 1377 1004 [3087] 869 450 1191 109

Eu

n36 n37 n38 n39 n40 n41 n42 n43 n44 n45

n(Cm-X) n(Ca-Cm)asym n(Cb-Cb) n(Ca-Cm)sym n(Pyr. quarter-ring) n(Pyr. half-ring)sym d(Cm-X) n(Cb-Y)sym n(Pyr. half-ring)asym n(Cb-Y)asym

[3040] [1637] 1604 1501 1396 [1346] 1231 1153 1133 996

[3042] 1624 1547 1462 1385 1319 1150 [3097] 1033 [3087] (continued )

40

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.10. (Continued) Symmetry

a b

ni

Descriptiona

NiOEP

NiPor

n46 n47 n48 n49 n50 n51 n52 n53

d(Pyr.)asym n(Pyr. breathing) d(Pyr.)sym d(Pyr. rot.) n(Ni-N) d(Cb-Y)asym d(Cb-Y)sym d(Pyr. transl.)

927 766 [615] [534] [358] 328 263 [167]

806 995 745 366 420 1064 1250 282

NiTPP

See Ref. 232 for definitions of local coordinates. X,Y ¼ H,H for NiPor, H,C2H5 for NiOEP, and C6H5, H for NiTPP. [ ], calculated values.

out- of-plane vibrations of Ni(Por), Fe(Por), and Fe(Por)CO, and found that DFT force constants can reproduce the observed frequencies more accurately than NCA force constants and can predict IR/Raman intensities that are in good agreement with the observed spectra. The results of their calculations suggested slight ruffling distortion of the Ni(Por) core. DFT calculations on Ni(TPP) [240] indicated that the molecular symmetry is lowered to S4 as a result of the porphyrin core ruffling and the rotation of the phenyl groups. This distortion activates two out-of-plane vibrations, g12(330 cm1) and g13(547 cm1), of B1u symmetry that are forbidden under D4h, or D2d symmetry in the Soret-excited RR spectrum. An X-ray diffraction study on Ni (Por) by Jentzen et al. [241] showed that the porphin ring is planar with very small ruffling, and noted that its RR spectrum in the solid state is almost identical to that in solution. Thus, they concluded that any distortion of D4h symmetry, if it exists in solution, is rather small. In contrast, substituent-induced distortions are noted in Ni(II) complexes of mesotert-butylporphyrins [242] and tetracyclopentenyl–tetraphenyl porphyrin and its derivatives [243]. Since ethioporphyrins have one methyl and one ethyl substituent on each pyrrole ring, there are four possible isomers that differ only in the relative positions of the alkyl groups in the periphery. Etio I (R1 ¼ R3 ¼ R5 ¼ R7 ¼ CH3 and R2 ¼ R4 ¼ R6 ¼ R8 ¼ C2H5 in Fig. 1.24) is the most symmetric, whereas Etio-III (R1 ¼ R3 ¼ R5 ¼ R8 ¼ CH3 and R2 ¼ R4 ¼ R6 ¼ R7 ¼ C2H5) is the least symmetric. Rankin and Czernuszewicz [244] have shown that the Ni(II) and VO(II) complexes of ethioporphyrins I and III can be distinguished by RR spectra in the 1050–750 cm1 region. Li et al. [245] measured the surface-enhanced RR spectra (SERRS) of waterinsoluble Ni(Por) in aqueous silver sol (low concentration, 108 M), and observed extra enhancement of the A1g, B1g, and B2g modes but not of the A2g modes when the excitation was made at or near the Soret band. However, an antiresonance effect was noted for the A1g mode at 995 cm1 when the excitation wavelength was in the valley between the Soret and Q bands. Vibrational frequencies in the SERR spectra were in good agreement with those in hom*ogeneous solution. The effective symmetry of the adsorbed Ni(Por) must be lower than D4h, since many out-of-plane modes and IR-active in-plane modes were observed in the SERR spectra.

METALLOPORPHYRINS

41

Fig. 1.25. Atomic displacements and calculated skeletal frequencies (cm1) of A1g modes compared for Ni(OEP) and Ni(Por) [233 ].

TABLE 1.11. Normal Modes Containing NiN Stretching Vibrationsa,b Complex

n8(A1g)

n18(B1g)

n50(Eu)

n53(Eu)

Ni (Por) Ni (OEP) Ni (TPP)

369(27%) 360/343 (7%) 402 (24%)

237 (64%) 168 (37%) 277 (53%)

420 (50%) 358* (30%) 436 (21%)

282 (44%) 328 (28%)c 306 (32%)

a

The NiN stretching force constant of 1.68 mdyn/A was used for all three porphyrins. Numbers in front of brackets are observed frequencies (cm1) except for that with an asterisk, which is calculated. Numbers in brackets indicate % PED. c n51. b

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APPLICATIONS IN COORDINATION CHEMISTRY

1.5.2. Structure-Sensitive Vibrations Ozaki et al. [246] measured the resonance Raman spectra of a series of Fe(OEP) complexes and found linear relationships between the frequencies of their skeletal modes (n2, n3, n10, n11, n19) and the CtN distances (from the center of the porphyrin ring to pyrrole N atom) shown in Fig. 1.26. Similar relationships were found in a series of Fe(OEC) complexes (OEC ¼ octaethylchlorin) discussed in Sec. 1.6. The slope of each line increases in the order n19 > n10 > n3 > n2 > n11, which is parallel to the percent contribution to the n(CaCm) coordinate in each normal mode. This result indicates that when the core size increases, the CaCm bonds are weakened and the corresponding frequencies are lowered. In going from low to high spin state, the porphyrin core tends to expand or be domed, and this results in weakening of the CaCm bond. Thus, these vibrations serve as the spin state marker bands [247]. They are also metal-sensitive because the core size varies with the nature of the metal ion [248]. The n10 is sensitive to the number and the nature of axial ligands [249]. The n11 is sensitive to the nature of the peripheral substituents since it is due mainly to n(CbCb). Finally, n7 is a 16-membered porphyrin ring breathing motion. Thus, it is strong for planar complexes and weak for nonplanar (or domed) complexes [250]. The totally symmetric breathing mode, n4, is known to be the best marker for the oxidation state. It is near 1360 cm1 for Fe(II) complexes, and near 1375 cm1 for Fe (III) complexes with relatively small dependence on the spin state. The frequency increase in going from Fe(II) to Fe(III) is attributed to the decrease of p-backbonding

Fig. 1.26. Correlations between the n10, n2, n19, n11, and n3 frequencies and the porphyrin core size (CtN, A) for the Fe(OEP) complexes [246].

METALLOPORPHYRINS

43

from the metal dp orbital to the porphyrin p* orbital. In Fe(IV) and Fe(V) porphyrins, the n4 are at 1379 [251] and 1384 cm1 [252], respectively. Mylrajan et al. [253] included n4 and n28 and the crystallographically defined complexes, Fe(OEP), Fe (OEP)(NCS), [Fe(OEP)(N-MeIm)2]þ, [Fe(OEP)(DMSO)2]þ, Fe(OEC), and three Ni (OEP) complexes [triclinic (A and B) and tetragonal forms] in the correlations presented above. Structure-sensitive bands of other metalloporphyrins have also been studied; Fe (OEC) [254] and water-soluble porphyrins such as Fe(TMpy-P2) [TMpy-P2: tetrakis (2-N-methylpyridyl)porphyrin, R1–R8 ¼ H, R0 ¼ 2-N-methylpyridyl cation in Fig. 1.24] [255,256]. According to X-ray analysis, triclinic and tetragonal crystals of Ni(OEP) contain “flat” and “ruffled” porphyrin rings, respectively. All the frequencies above 1400 cm1 (in-plane skeletal modes) are lower in the “ruffled” form than in the “flat” form. The solution frequencies are intermediate between those of the triclinic and tetragonal forms, indicating that Ni(OEP) is definitely “ruffled” in solution [257,258]. In Ni(OETPP) (R1–R8 ¼ C2H5 and R0 ¼ C6H5 in Fig. 1.24), “saddle” distortion of the porphyrin ring occurs because of steric crowding of the peripheral substituents. Shelnutt and coworkers carried out RR as well as X-ray diffraction studies on this and related complexes [259–261]. Such distortion causes large-downshifts (70 cm1) relative to the planar porphyrin in a number of porphyrin skeletal modes, and activates three out-of-plane (g15, g16, and d4) vibrations. Among them, the g16 (tilting of the pyrrole rings) becomes one of the strongest bands in the Soret-excited RR spectrum [262]. 1.5.3. Axial Ligand Vibrations Table 1.12 lists the observed frequencies of FeL stretching vibrations of the iron porphyrins where L is an axial ligand. Polyatomic ligands such as N3 and pyridine (py) exhibit their own internal modes as well. These axial ligand vibrations can be assigned by using isotopic ligands (H/D, 14 N=15 N; 32 S=34 S, etc.) and metal isotopes (54 Fe=56 Fe, etc.). Kincaid and Nakamoto [270] observed the n(FeF) of 54 Fe(OEP)F at 595 cm1 with the 514.5 nm excitation. Kitagawa et al. [271] also observed the n(FeX) of Fe (OEP)X at 364 and 279 cm1 for X ¼ Cl and Br, respectively, and the ns(LFeL) of [Fe(OEP)L2]þ (L ¼ ImH) at 290 cm1 using the 488 nm excitation. These results show that the axial vibrations can be enhanced via resonance with in-plane p–p* transitions (a and b bands). According to the latter workers, vibrational coupling between these axial vibrations and totally symmetric in-plane porphyrin-core vibrations is responsible for their resonance enhancement. On the other hand, Spiro [272] prefers electronic coupling; namely, the p–p* transition induces the changes in the FeX (or L) distance, thus activating the axial vibration. Direct excitation is possible if the metal–axial ligand CT transition is in the visible region. Thus, Asher and Sauer [273] observed the n(MnX) of Mn(EP)X (X ¼ F,Cl,Br,I) with exciting lines in the

44

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.12. Observed Frequencies of Iron-Axial Ligand Stretching Vibrations Complexa Fe(OEP) ðImHÞ2þ Fe(PP) ðImHÞ2þ Fe(OEP) ðg-picÞ2þ

Fe(PP) (ImH)2 Fe(MP) (py)2 Fe(OEP)F Fe(OEP)Cl Fe(OEP)Br Fe(OEP)I Fe(OEP)NCS Fe(OEP)N3 Fe(OEP) (SC6H5) Fe(TPP) ðSC6 H5 Þ2 Fe(TPP)ðTHTÞ2þ Fe(TPP)F2 Fe(TPP)CCl2 Fe(TPP)CBr2

Mode na(LFeL) ns(LFeL) na(LFeL) ns(LFeL) ns(lFeL) n(FeL) n(FeL) n(FeL) n(FeL) n(FeL) n(FeL) n(FeL) na(LFeL) na(LFeL) ns(LFeL) n(FeL) n(FeL)

Obs. Freq. (cm1) b

385,319 200 373 200 179 605.5 357 270 246 315 420 341 345 328 453 1274c 1270c

Ref. 263 263 264 263 265 264 264 264 264 264 266 267 267 267 268 269 269

a

MP ¼ mesoporphyrin IX dimethyl ester; THT ¼ tetrahydrothiophene; pic ¼ picoline. These two bands are due to 50 : 50 mixing of two modes (antisymmetric FeL stretching and pyrrole tilting) [263]. c n(Fe¼C). b

460–490 nm region where the MnX CT bands appear. Here EP denotes etioporphyrin (R1 ¼ R3 ¼ R5 ¼ R7 ¼ CH3, R2 ¼ R4 ¼ R6 ¼ R8 ¼ C2H5 in Fig. 1.24). Similarly, Wright et al. [265] were able to observe totally symmetric pyridine (py) vibrations as well as ns[FeN(py)] of Fe(MP)(py)2 with exciting lines near 497 nm that are in resonance with the Fe(dp)–py(p*) CT transition. Here, MP denotes mesoporphyrin IX dimethylester (R1 ¼ R3 ¼ R5 ¼ R8 ¼ CH3, R2 ¼ R4 ¼ C2H5, and R6 ¼ R7 ¼ CH2CH2COOCH3 in Fig. 1.24). Ogoshi et al. [274] reported the IR spectra of M(OEC) (M ¼ Zn,Cu,Ni), Mg(OEC) (py)2, and Fe(OEC)X (X ¼ F,Cl,Br,I), and assigned n[MN(OEC)] and n[MgN(py)] using metal isotope techniques. Ozaki et al. [275] report RR spectra of these and other OEC complexes. Vibrations of axial ligands such as O2, NO, CO, and so on are discussed in later sections. Axial ligand vibrations provide valuable information about the structure and bonding of heme proteins containing these ligands. 1.5.4. Metal–Metal-Bonded Porphyrins As shown in Sec. 2.11.2 of Part A, RR spectra of metal–metal bonded complexes such as Re2 F28 exhibit the strong n(ReRe) at 320 cm1 and a series of its overtones and combination bands. The metal–metal bonded porphyrin dimer, ½RuðOEPÞn2 þ , exhibits the n(RuRu) at 285, 301, and 310 cm1 for n ¼ 0, 1, and 2, respectively [276]. The observed frequency increase indicates that the electron density of the RuRu bond is removed from its antibonding p* orbital as the complex ion is oxidized.

METALLOCHLORINS, CHLOROPHYLLS, AND METALLOPHTHALOCYANINES

45

However, this removal has almost no effect on the porphyrin core vibrations since the metal–metal bond is perpendicular to the porphyrin plane. Similar results are reported for the ½OsðOEPÞn2 þ series (233, 254, and 266 cm1 for n ¼ 0, 1, and 2, respetively) [277]. The RR spectra of asymmetric sandwich compounds such as Ce(OEP)(TPP) [278], triple-decker sandwich compounds such as Eu2(OEP)3, and their singly oxidized compounds are available [279]. 1.5.5. p–p Complex Formation and Dimerization The Cu(II) uroporphyrin I (R1–R8 ¼ CH2COO in Fig. 1.24) forms molecular adducts with a variety of aromatic heterocyclic compounds in aqueous alkaline solution. Using Raman difference spectroscopy, Shelnutt [280] observed small shifts (from þ 2.9 to 2.7 cm1) of the porphyrin skeletal vibrations resulting from the p–p charge-transfer (porphyrin to heterocycle), and showed that the planes of these two components are parallel to each other. This work has been extended to the study of dimerization of the Cu(II) and Ni(II) complexes of uroporphyrin I. In this case, the porphyrin skeletal modes were upshifted by 1–3 cm1 as a result of dimerization [281]. 1.5.6. Reduction of Metalloporphyrins Reduction of a metalloporphyrin results in lowering of the oxidation state of the central metal or the formation of a porphyrin anionic radical, depending on whether an electron enters in a metal or porphyrin orbital. One-electron reduction of Zn(TPP) yields Zn (TPP) containing an anionic radical of TPP, as confirmed by the observed pattern of isotopic shifts and polarizations [282]. In this case, an extra electron enters in the porphyrin eg* orbital causing Jahn–Teller effect. One-electron reduction of VO(OEP) also yields VO(OEP) [283]. Teraoka et al. [284] have shown by electronic absorption and RR and ESR spectroscopy that two- and three-electron reduction of Fe(III)(OEP) Cl in THF solution yield low-oxidation-state porphyrins that are formulated as [Fe(I) (OEP)]2 and [Fe(I)(OEP)]2, respectively. Anxolabehere et al. [285] carried out UV–visible and RR spectroelectrochemical studies of Fe(TPP) and its pentafluorophenyl derivative, and concluded that the two-electron reduced species should be formulated as Fe(0) complexes because their RR spectra did not provide any evidence for the formation of the porphyrin anion radical. On the other hand, De Silva et al. [286] obtained [Fe(TPP)]2 by three-electron reduction of Fe(III)(TPP)Cl, and observed the RR spectrum which is qualitatively similar to that of the two-electron reduction product, [Fe(I)(TPP)]. Thus, these workers formulated it as [Fe(I)(TPP)]2. 1.6. METALLOCHLORINS, CHLOROPHYLLS, AND METALLOPHTHALOCYANINES 1.6.1. Metallochlorins Metallochlorins differ from metalloporphyrins in that one of the four pyrrole rings of CbCb bonds is saturated, and serve as model compounds of chlorophylls that are

46

APPLICATIONS IN COORDINATION CHEMISTRY

(a)

(b)

R 3

α II

I N I

II N

III

IV H

III

IV

CH3

N

β N

N

M N

Mg

d

N

N

H H

γ

V

H

CO CO2Phy

H

M(OEC)

9 O CH3

Chlorophyll

Fig. 1.27. Structures of M(OEC) (a) and chlorophyll; in (b) R ¼ CH3 for Chl a, and CHO for Chl b. [Phy ¼ phytyl ¼ C20H39 ¼ CH2CH¼C(CH3){(CH2)3C(CH3)}3CH3].

involved in photosynthetic processes. Figure 1.27 shows the structures of metallooctaethylchlorin [M(OEC)] and chlorophyll a. The symmetry of a metallochlorin is regarded as C2v because the substituents on the reduced pyrrole ring normally take a trans conformation. Infrared and Raman selection rules of metallochlorins are markedly different from those of metalloporphyrins as a result of symmetry lowering from D4h, to C2v. According to the correlation table (Appendix IX of Part A), the symmetry species of the in-plane skeletal modes of a metalloporphyrin listed in Table 1.9 are changed as follows: A1g(A1), A2g(B1), B1g(A1), B2g(B1), and Eu (A1 þ B1). Here, the corresponding symmetry species under C2v symmetry are shown in parentheses. Since A1, B1, and B2 species are all IR-active, the IR spectrum of M (OEC) is expected to show more bands than the corresponding M(OEP). This is clearly demonstrated in Fig. 1.28 obtained by Ogoshi et al. [287]. These workers also located the metal isotope-sensitive bands of Zn(OEC), Cu(OEC), and Mg(OEC)(py)2 at 212.0 (1.0), 233.0 (1.6), and 176.5 (4.0) cm1, respectively, by 64 Zn=68 Zn; 63 Cu= 65 Cu; and 24 Mg=26 Mg substitutions. Here, the vibrational frequencies are listed for the first isotopic species of each metal, and the numbers in parentheses indicate the magnitudes of the isotope shifts. The corresponding mode of Ni(OEC) was assigned at 256 cm1. The number of vibrations observed in RR spectra also increases as a result of symmetry lowering. Normal coordinate analysis on M(OEC) was made by Bocian and coworkers [288,289]. Boldt et al. [290] carried out vibrational analysis of in-plane vibrations of Ni(OEC) and its derivatives using a semiempirical quantum mechanical force field (QCFF/PI) method [291]. On the basis of their theoretical calculations, these workers assigned 45 bands of the RR spectrum of Ni(OEC) observed in the 1650–340 cm1 region. The results show that few of the high-frequency modes of Ni(OEC) can be correlated with a single Ni(OEP) mode but the vibrations below 950 cm1 can be correlated with it. Their work was extended to Cu(OEC) and its isotopic species (H/D and 14 N=15 N) [292].

METALLOCHLORINS, CHLOROPHYLLS, AND METALLOPHTHALOCYANINES

47

ABSORPTION

Zn(OEP)

Zn(OEC)

Fe(OEC)CI

4000

3000

2000

CM-1

1500

1000

500

Fig. 1.28. IR spectra of Zn(OEP), Zn(OEC), and Fe(OEC)Cl [287].

Anderson et al. [293] compared the spectral properties of cis- and trans-isomers of planar Cu(OEC) and S4-ruffled Ni(OEC), and found that conformational differences of the peripheral substituents have marked effects on the spectral properties and that such localized changes strongly perturb the overall properties of chlorins. Ozaki et al. [294] measured the far-IR spectra (Fig. 1.29) of the Fe(OEC)X (X ¼ F,Cl,Br,I) and assigned the n(FeX) bands at 589 (605.5), 352 (357), 270 (270), and 240.5 (246) cm1 for X ¼ F, Cl, Br, and I, respectively. Here, the numbers in parentheses indicate the corresponding frequency of Fe(OEP)X. Ozaki et al. [295,296] also compared the RR spectra of Fe(OEC)L- and Fe(OEC)L2-type compounds with those of the corresponding Fe(OEP) complexes in the high- and low-frequency regions. RR spectra of meso-substituted chlorin complexes of Ni(II) and Cu(II) [297], Cu(II) [298], and Zn(II) [299] were measured and assigned by comparing them with the corresponding porphyrin complexes. 1.6.2. Chlorophylls Chlorophylls (Chl) and bacteriochlorophylls (BChl) are another “reduced” pigments that are involved in the process of photochemical energy transduction (Sec. 3.4). As shown in Fig. 1.27b, Chl is a Mg(II) macrocycle in which one pyrrole ring (ring IV) is reduced and ring V is fused to ring III. The peripheral R group on ring II is CH3 for Chl a and CHO for Chl b. Assignments of the vibrational spectra of Chls in the

48

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.29. Far-IR spectra of Fe(OEP)F and Fe(OEC)X (X ¼ F,Cl,Br,I) [294].

high-frequency region were based on the results of theoretical calculations obtained for in-plane vibrations of Ni(OEC) [290]. A similar approach was taken to assign lowfrequency vibrations (below 1000 cm1) of Mg(OEP), Mg(OEC), and Chl a [300]. Fujiwara and Tasumi [301,302] measured the Raman spectra (441.6 nm excitation) of Chl a and b in various solvents. Figure 1.30 compares their spectra obtained in diethylether and THF. The bands in the 1710–1690 cm1 region observed for both compounds are assigned to the n(C¼O) of the free C-9 carbonyl group of ring V, while those in the 1670–1669 cm1 of Chl b are assigned to the free C-3 formyl carbonyl group. The Raman spectrum of Chl a exhibits three bands at 1607 (weak), 1554 (strong), and 1529 (medium) cm1 in diethylether (group I solvent), and these bands are shifted to 1596, 1545, and 1521 cm1, respectively, in THF (group II solvent). Similar solvent behaviors were observed in going other group I solvents (n-hexane. CCl4, CS2, etc.) to group II solvents (dioxane, pyridine, and methanol). On the basis of these results together with other information, these workers concluded that Chl a in group I solvents is five-coordinate (with one axial ligand) whereas it becomes sixcoordinate (two axial ligands) in group II solvents. These two species coexist in I,IImixed solvents, although the six-coordinate species is dominant in pyridine solution. Similar results were obtained for Chl b. Heald and Cotton [303] observed that the RR spectrum (407.6 nm excitation) of the electrochemically generated cation radical of Chl a in CH2Cl2 exhibits the C-9 n(C¼O) vibration at 1717 cm1, which is 32 cm1 higher than that of Chl a

METALLOCHLORINS, CHLOROPHYLLS, AND METALLOPHTHALOCYANINES

49

Fig. 1.30. Raman spectra (1750–1450 cm1) of chlorophylls a and b: (a) Chl a in diethylether; (b) Chl a in THF; (c) Chl b in diethylether; (d) Chl b in THF [302].

(1685 cm1). This was attributed to a decrease in conjugation between the C-9 keto group and the macrocyclic ring caused by one-electron oxidation. It was suggested that the C-9 keto group plays a role in chlorophyll redox reaction in vivo. Vibrational spectra of chlorophylls and related compounds have been reviewed by several groups of investigators [302,304,305]. 1.6.3. Metallophthalocyanines Metallophthalocyanines [M(Pc)], shown in Fig. 1.31, are known for their exceptional thermal stability and extremely low solubility in any solvents. Under D4h, symmetry, their 165 (3 57 6) normal vibrations are classified into 14A1g þ 13A2g þ 14B1g 14B2g þ 13Eg þ 6A1u þ 8A2u þ 7B1u þ 7B2u þ 28Eu, of which the A1g, A2g, B1g, B2g, and Eu vibrations are in-plane modes, and the A1u, A2u, B1u, B2u, and Eg vibrations are

50

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.31. Structure of metallophthalocyanine.

out-of-plane modes. Only the A2u and Eu vibrations are IR-active, whereas the A1g, B1g, B2g, and Eg vibrations are Raman-active. Similar to metalloporphyrins, the A2g vibrations become Raman-active under resonance conditions. Melendres and Maroni [306] carried out normal coordinate analysis on Fe(Pc). As expected, extensive vibrational couplings occur among the local coordinates so that simple descriptions of normal modes by local coordinates cannot be justified. They estimated the FeN stretching force constant to be 1.00 mdyn/A . Using metal isotope techniques, Hutchinson et al. [307] assigned primary n(MN) modes of M(Pc) at 240.7 (64 Zn), 284.0 (63 Cu), 376.0 and 317.8 (58 Ni), and 308.4 cm1 (54 Fe). SERR and IR spectra of other M(Pc2) (M ¼ Mg,Cu,Zn,Pt,Pb) are reported together with band assignments (totally symmetric in-plane modes) obtained by normal coordinate analysis [308]. Homborg and coworkers carried out extensive IR and Raman studies of a variety of metallophthalocyanines. In a series of [Bi(III)(Pc2)2]n (n ¼ 1,0, þ 1), the Bi(III) atom is coordinated by eight N atoms of two slightly distorted Pc2 ligands in a square– antiprismatic conformation, and the na(BiN) and ns(BiN) were located at 116 (IR) and 150 cm1(RR), respectively [309]. The RR spectra (1064 nm excitation) of [M (Pc)2] [M ¼ a lanthanide(III) ion] obtained by anodic oxidation of [M(Pc2)] exhibit the ns(MN) in the range of 141 (La) to 168 cm1 (Lu), and show equal presence of the Pc p-radical(Pc)[310]. Thus, it may be formulated as [M(Pc2)(Pc)]. The IR spectra of the [M(Pc2)2] ion (M ¼ Y(III) or In(III)) show the na(MN) at 182 and 137 cm1, for M ¼ Y and In, respectively [311]. The n(ReRe) and na(ReN) vibrations of dimeric [Re(Pc2)2] are at 240 and 355 cm1, respectively [312]. The IR and Raman spectra of a monolayer film of Eu(Pc)2 were assigned by Berno et al. [313] on the basis of C4v symmetry.

METALLOCHLORINS, CHLOROPHYLLS, AND METALLOPHTHALOCYANINES

51

Figure 1.32 shows the IR and Raman spectra of a series of [Cr(III))(Pc2)X2] (X ¼ F,Cl,Br,I) below 600 cm1 obtained by Sievertsen et al. [314]. The shaded bands indicate the na(CrX2)(IR) and ns(CrX2)(Raman) vibrations. Axial n(MX) vibrations of other [M(Pc2)X2]-type complexes have been reported for M ¼ Ir(III) [315], Os(II) [316], Ru(III) [317], and Tl(III) [318]. The n(MX) vibrations of [Ru(III)(Pc2)(py) X]-type complexes are at 390, 360, 337, 260, and 204 cm1 for X ¼ CN, N3 , NCO, NCS, and NO2 respectively [319]. In the case of [M(O)(Pc2)X2]-type complexes [M ¼ Nb(V) and Ta(V) and X ¼ F,Cl,NCS and N3 ], three axial ligands (2X and O) are on one side of the Pc2 plane, and their n(M¼O), ns(MX)

Fig. 1.32. Vibrational spectra of [ n Bu 4 N][CrX2(Pc2)]: (a) 476.5 nm excitation, X ¼ F; (b) 496.5 nm excitation, X ¼ Cl, Br, I (all 496.5 nm excitation) (RRI ¼ relative Raman intensity) [314].

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APPLICATIONS IN COORDINATION CHEMISTRY

and na(MX) vibrations have been assigned [320]. In m-carbido diphthalocyanines of [(MPc2)2(m-C)] (M ¼ Fe,Ru), the MCM bond is linear, and the na(MCM) and ns(MCM) vibrations are at 997 and 477 cm1, respectively, for M ¼ Fe [321]. The na and ns vibrations of the MnNFe bridge in (TPP)MnNFe(Pc) are at 956/945 and 381 cm1, respectively [322]. Electronic and IR spectra of phthalocyanine molecular assemblies are reviewed by Cook [323]. 1.7. NITRO AND NITRITO COMPLEXES The NO2 ion coordinates to a metal in a variety of ways:

Vibrational spectroscopy is very useful in distinguishing these structures. 1.7.1. Nitro Complexes The normal vibrations of the unidentate N-bonded nitro complex may be approximated by those of a planar ZXY2 molecule, as shown in Fig. 1.33. In addition to these modes, the NO2 twisting and skeletal modes of the whole complex may appear in the low-frequency region. Table 1.13 summarizes the observed frequencies and band assignments for typical nitro complexes. It is seen that these complexes exhibit na(NO2) and ns(NO2) in the 1470–1370 and 1340–1320 cm1 regions, respectively. On the other hand, the free NO2 ion exhibits these modes at 1250 and 1335 cm1, respectively. Thus na(NO2) shifts markedly to a higher frequency, whereas ns(NO2) changes very little on coordination. Nakagawa and Shimanouchi [60] and Nakagawa et al. [324] carried out normal coordinate analyses to assign the infrared spectra of crystalline hexanitro cobaltic salts; both internal and lattice modes were assigned completely by factor group analysis. The results indicate that the complex ion takes the Th, symmetry in K, Rb, and

53

NITRO AND NITRITO COMPLEXES

Z v1(A1)

v2(A1)

v3(A1)

X Y

Y vs(NO2)

δs(ONO)

v(M—N)

+ v4(B2)

v5(B2)

v6(B1) +

ρr(NO2)

va(NO2)

+ ρW (NO2)

Fig. 1.33. Normal modes of vibration of planar ZXY2 molecules (the band assignment is given for an MNO2 group).

TABLE 1.13. Observed Infrared Frequencies and Band Assignments of Nitro Complexes (cm1) Complex

na(NO2)

ns(NO2)

d(ONO)

623

K3[Co(NO2)6]

1386

1332

Na3[Co(NO2)6]

1425

1333

K2Ba[Ni(NO2)6]

1343 1395 1375 1395 1374 1488 1458 1466 1397

1306

827 854 831 838

1330

K3[Ir(NO2)6] K3[Rh(NO2)6] K3[Ir(NO2)Cl5] [Pt(NO2)6]4 K2[Pt(15NO2)4] [Pd(NO2)4]2b

1408

K2[Pt(NO2)Cl3]

1401

a b

rw(NO2)

rr(NO2)a

Refs.

293 276 249 255

324

433

416 449 372 291

830

657

390

300

325

1340 1315

833 835

627 644

386 325

283 288

325 326

1328

834

621

368

294

327

1343 1364 1320 1325

) 847 839 833 834 824 844

This mode may couple with other low-frequency modes. Raman data in aqueous solution.

637

n(MN)

640 623

324

324

421

328,329

440

290

329

614

350

304

326

54

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.34. Infrared spectra of (a) K3[Co(NO2)6] and (b) Na3[Co(NO2)6] [330].

Cs salts but the S6 symmetry in the Na salt (see Fig. 1.10 of Part A). The IR spectra of K and Na salts are compared in Fig. 1.34. Kanamori et al. [330] obtained the RR spectra (632.8 nm excitation, 80 K) of these complexes, and showed that resonance enhancement occurs via the B term since nontotally as well as totally symmetric vibrations are observed. There are many nitro complexes containing other ligands such as NH3 and Cl. In these cases, the main interest has been the distinction of stereoisomers by the symmetry selection rules and the differences in frequency between isomers. It is possible to distinguish cis- and trans-[Co(NH3)4(NO2)2]þ [331] by the rule that the cis-isomer exhibits more bands than the trans-isomer, and to distinguish fac- and mer[Co(NH3)3(NO2)3] [332] by the observation that d(NO2) and rw(NO2) are higher for the fac-isomer (C3, 832 and 625 cm1, respectively) than for the mer-isomer (C2v, 825 and 610 cm1, respectively). Nakagawa and Shimanouchi [333] measured the infrared spectra of the [Co(NO2)n(NH3)6n](3n)þ series and carried out normal coordinate analysis on the mononitro and dinitro complexes. Nolan and James [334] studied the infrared and Raman spectra of [Pt(NO2)nCl6n]2-type salts in the crystalline state. 1.7.2. Nitrito Complexes If the NO2 group is bonded to a metal through one of its O atoms, it is called a nitrito complex. Table 1.14 lists the NO stretching frequencies of typical nitrito complexes. The two n(NO2) of nitrito complexes are well separated, n(N¼O) and n(NO) with at 1485–1400 and 1110–1050 cm1, respectively. Distinction between the nitro and nitrito coordination can be made on this basis. It is to be noted that nitrito complexes

55

NITRO AND NITRITO COMPLEXES

TABLE 1.14. Vibrational Frequencies of Nitrito Complexes (cm1) Complex

n(N¼O)

n(NO)

d(ONO)

Ref.

[Co(NH3)5(ONO)]Cl2 [Cr(NH3)5(ONO)]Cl2

1468 1460 1461 1445 1393 1485 1430 1405

1065 1048

825 839

335 335

1063

830

326

1114

825 835 825 824

336

[Rh(NH3)5(ONO)]Cl2 [Ni(py)4(ONO)2] trans-[Cr(en)2(ONO)2]ClO4 [Co(py)4(ONO)2](py)2

— 1109

337 338

lack the wagging modes near 620 cm1 that appear in all nitro complexes. The n(MO) of nitrito complexes were assigned in the 360–340 cm1 region for metals such as Cr (III), Rh(III), and Ir(III) [326]. Infrared spectra of nitro–nitrito isomeric pairs are reported for cis-[Ru(II)(bipy)2 (NO)X]2þ, where X is NO2 and ONO [339]. In many nitro complexes several types of nitro coordination are mixed. Goodgame, Hitchman, and their coworkers carried out an extensive study on vibrational spectra of nitro complexes containing various types of coordination. For example, all six nitro groups in K4[Ni(NO2)6]H2O are coordinated through the N atom. However, its anhydrous salt exhibits the bands characteristic of nitro as well as nitrito coordination. From UV spectral evidence, Goodgame and Hitchman [340] suggested the structure K4[Ni(NO2)4(ONO)2] for the anhydrous salt. Table 1.15 lists the observed frequencies of two Ni(II) complexes containing both nitro and nitrito groups. The red nitritopentammine complex, [Co(NH3)5(ONO)]Cl2, is unstable and is gradually converted to the stable yellow nitro complex. The kinetics of this conversion can be studied by observing the disappearance of the nitrito bands [342,343], and the rate of the photochemical isomerization in the solid state has been determined [344]. Burmeister [345] reviewed the vibrational spectra of these and other linkage isomers. 1.7.3. Chelating Nitrito Complexes If the nitrito group is chelating, the n(N¼O) and n(NO) of the nitrito group will be shifted to a lower and a higher frequency, respectively, relative to those of unidentate TABLE 1.15. Vibrational Frequencies of Ni(II) Complexes Containing Nitro and Nitrito Groups (cm1) Nitro Group

Nitrito Group

na(NO2)

ns(NO2)

rw(ONO)

n(N¼O)

n(NO)

Ref.

K4[Ni(NO2)6]H2O

1346

1319

340

K4[Ni(NO2)4(ONO)2]

1347

1325

1387

1206

340

Ni[2-(aminomethyl)-py]2-(NO2)(ONO)

1338

1318

427 423 414 —

1368

1251

341

Complex

56

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.16. Vibrational Frequencies of Chelating Nitrito Groups (cm1) Complex Co(Ph3PO)2(NO2)2

1266

Ni(a-pic)2(NO2)2

1272

ns(NO2) 1199 1176 1199

Re(CO)2(PPh3)2 (NO2) [Ni(N,N 0 -dimethyl-en)-(NO2)] ClO4 Cs2[Mn(NO2)4] Co(Me4-en)(NO2)2 Zn(py)2(NO2)2 Zn(isoquinoline)2(NO2)2 (o-cat) [Co(NO2)4]b

1241 1300 1302 1290 1351 1370 1390

1180 1230 1225 1207 1171 1160 1191

a b

na(NO2)

d(ONO)

Da

Ref.

856 866 862 887 — 841 850 850 — —

78

346

73

346

61 70 77 83 180 210 199

347 348 349 338 350 350 349

D ¼ na ns. o-cat ¼ [o-xylylenebis(triphenylphosphonium)]2þ ion.

nitrito complexes. As a result, the separation between these two modes (D) becomes much smaller than those of unidentate complexes. Table 1.16 lists the vibrational frequencies of chelating nitrito groups. It should be noted that the D value depends on the degree of asymmetry of the coordinated nitrito group; it is expected that the D value is the smallest when the two NO bonds are equivalent and increases as the degree of asymmetry increases. Relatively large D values observed for the last three compounds in Table 1.16 may be accounted for on this basis. According to X-ray analysis, the orange compound, K4[Ni(NO2)6]H2O, is an octahedral complex with six N-bonded nitro ligands and a water of crystallization. On dehydration at 100 C, it forms a mixture of red K3[Ni(NO2)4(chelated O2N)] and KNO2. The former exhibits the na, ns, and d vibrations of the chelated nitrite group at 1385, 1225, and 866 cm1 in IR spectra [351]. The corresponding vibrations are observed at 1293, 1223, and 826 cm1 in the IR spectrum of NaNO2 in Ar matrices, indicating the formation of a chelate ring with Na atom (C2v symmetry). On photoirradiation (248 nm), it is converted to the unidentate trans-Na–ONO complex with the three bands at 1446, 1159, and 787 cm1 [352]. 1.7.4. Bridging Nitro Complexes The nitro group is known to form a bridge between two metal atoms. Nakamoto et al. [335] suggested that among the three possible structures, IV, V, and VI, shown earlier, IV is most probable for

since its NO2 stretching frequencies (1516 and 1200 cm1) are markedly different from those of other types discussed thus far. Later, this structure was found by X-ray

LATTICE WATER AND AQUO AND HYDROXO COMPLEXES

57

analysis of [353]

This compound exhibits the NO2 stretching bands at 1492 and 1180 cm1. On 16 O!18 O substitution of the bridging oxygen, the latter is shifted by 10 cm1 while the former is almost unchanged. Thus, these bands are assigned to the n(N¼O) (outside the bridge) and n(NO) (bridge), respectively [354]. The [Co2{(NO2)(OH)2} (NO2)6]3 ion exhibits the NO2 bands at 1516, 1190, and 860 cm1, indicating the presence of a bridging nitro group [355]:

[Ni(b-pic)2(NO2)2]3C6H6 exhibits a number of bands due to coordinated nitro groups. Goodgame et al. [356] suggested the presence of two different types of bridging nitro groups (IV, V) and III, on the basis of the crystal structure and infrared data for this compound. Type IV absorbs at 1412 and 1236, type V at 1460 and 1019, and type III at 1299 and 1236 cm1. Goodgame et al. [357] also studied the infrared spectra of other bridging nitro complexes of Ni(II). For example, they found that Ni(en)(NO2)2 contains a type IV bridge (1429 and 1241 cm1), while Ni (py)2(NO2)2(13C6 H6 ) is similar to that of the analogous b-picoline complex. 1.8. LATTICE WATER AND AQUO AND HYDROXO COMPLEXES Water in inorganic salts may be classified as lattice or coordinated water. There is, however, no definite borderline between the two. The former term denotes water molecules trapped in the crystalline lattice, either by weak hydrogen bonds to the anion or by weak ionic bonds to the metal, or by both:

The latter term denotes water molecules bonded to the metal through partially covalent bonds. Although bond distances and angles obtained from X-ray and neutron

58

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.35. The three rotational modes of H2O in the solid state.

diffraction data provide direct information about the geometry of the water molecule in the crystal lattice, studies of vibrational spectra are also useful for this purpose. It should be noted, however, that the spectra of water molecules are highly sensitive to their surroundings. 1.8.1. Lattice Water In general, lattice water absorbs at 3550–3200 cm1 (antisymmetric and symmetric OH stretchings) and at 1630–1600 cm1 (HOH bending). If the spectrum is examined under high resolution, the fine structure of these bands is observed. For example, CaSO42H2O exhibits eight peaks in the 3500–3400 cm1 region [358], and its complete vibrational analysis can be made by factor group analysis (Sec. 1.28 of Part A). In the low-frequency region (600200 cm1) lattice water exhibits “librational modes” that are due to rotational oscillations of the water molecule, restricted by interactions with neighboring atoms. As shown in Fig. 1.35, they are classified into three types depending on the direction of the principal axis of rotation. It should be noted, however, that these librational modes couple not only among themselves but also with internal modes of water (HOH bending) and other ions (SO42, NO3, etc.) in the crystal. Tayal et al. [359] reviewed librational modes of water in hydrated solids. The presence of the hydronium (H3Oþ) ion in crystalline acid hydrates is well established, and their spectra were discussed in Sec. 2.3.1 of Part A. The existence of the H5 O2þ ion was first detected by X-ray analysis [360]. Pavia and Giguere [361] further confirmed its presence in HClO42H2O (namely, [H5O2]ClO4) by the absence of some characteristic bands of the H3Oþ and H2O species. Its structure is suggested to be centrosymmetric H2OHOH2 of approximately C2h symmetry. Both X-ray [362] and neutron diffraction [363] studies suggest the presence of the H5 O2þ ion in trans[Co(en)2Cl2]ClHCl2H2O. Thus it should be formulated as trans-[Co(en)2Cl2]Cl [H5O2]Cl. The existence of the H7 O3þ ion in crystalline HNO33H2O and HClO43H2O was confirmed by infrared studies [364]. The spectra are consistent with a structure in which two of the hydrogens of the H3Oþ ion are bonded to two H2O molecules through short, asymmetric hydrogen bonds. 1.8.2. Aquo (H2O) Complexes In addition to the three fundamental modes of the free water molecule, coordinated water exhibits other modes, such as those shown in Fig. 1.33. Nakagawa and

59

LATTICE WATER AND AQUO AND HYDROXO COMPLEXES

TABLE 1.17. Observed Frequencies, Band Assignments, and MO Stretching Force Constants of Aquo Complexes [365] Compound

rr(H2O)

[Cr(H2O)6]Cl3 [Ni(H2O)6]SiF6 [Ni(D2O)6]SiF6 [Mn(H2O)6]SiF6 [Fe(H2O)6]SiF6 [Cu(H2O)4]SO4H2O [Zn(H2O)6]SO4H2O [Zn(D2O)6]SO4D2O [Mg(H2O)6]SO4H2O [Mg(D2O)6]SO4D2O

800 (755)b — (655)c — 887, 855 — 467 — 474

a

rw(H2O) 541 645 450 560 575 535 541 392 460 391

n(MO)

K(MO)a

490 405 389 395 389 440 364 358 310 —

1.31 0.84 0.84 0.80 0.76 0.67 0.64 0.64 0.32 0.32

UBF field (mdyn/A). Ni(H2O)4Cl2. c Mn(H2O)4Cl2. b

Shimanouchi [365] carried out normal coordinate analyses on [M(H2O)6]- (Th, symmetry) and [M(H2O)4]-type ions (with D4h symmetry) to assign these lowfrequency modes. Table 1.17 lists the frequencies and band assignments, and Fig. 1.36 illustrates the far-infrared spectra of aquo complexes obtained by these authors. According to Stefov et al. [366], [Cr(H2O)6]Cl3 exhibits the rocking (rr), twisting (rt), and wagging (rw) modes of the coordinate water molecule at 825 (629), 575 (420),

Fig. 1.36. Infrared spectra of aquo complexes in the low-frequency region [365].

60

APPLICATIONS IN COORDINATION CHEMISTRY

and 500 (390) cm1, respectively, in IR spectrum (the number in the brackets indicates the frequency of the D2O complex). The MO6 skeletal vibrations of the octahedral [M(H2O)6]nþ ions have been assigned for M ¼ Mg(II) [367], Cd(II) [368], Al(III) [369], and Ga(III) [370]. In Cs2[InBr5(H2O)], the rr , rw , and n(InO) were assigned at 520, 380, and 260 cm1, respectively [371]. In solid K2[FeCl5(H2O)], however, the frequency order of these bands is different; 600(rw) > 460(rt) > 390 cm1 [n(FeO)] [372]. The n(CoO) of [Co(NH3)5(H2O)]Cl3 and [Co(NH3)5(OH)]Cl2 are assigned at 502 and 531 cm1( respectively [373]. Complete vibrational analysis has been made for single crystals of Ni(H2O)4Cl22H2O and related Co(II) complex [374]. a-Alums such as CsM (SO4)212H2O (M ¼ Co or Ir) contain the [M(H2O)6]3þ ions, and their single-crystal Raman spectra have been assigned by Best et al. [375]. An ab initio method has been employed to calculate vibrational frequencies of the [Na(H2O)n]þ ion (n ¼ l–4) [376]. Raman spectra of aqueous solutions of inorganic salts have been studied extensively. For example, Hester and Plane [377] observed polarized Raman bands in the 400–360 cm1 region for the nitrates, sulfates, and perchlorates of Zn(II), Hg(II), and Mg(II), and assigned them to the MO stretching modes of the hexa-coordinated aquo complex ions. Kameda et al. [378] measured the Raman spectra of NaX (X ¼ Cl,Br. ClO4,NO3) in concentrated (10 M%) aqueous solutions to determine the hydrated structure of the Naþ ion. The polarized bands at 183–187 cm1 were assigned to totally symmetric vibrations of the Naþ(H2O)n ion because they are downshifted by 10 cm1 in D2O solution. The value of n was close to 4. In the case of concentrated LiBr solution, the totally symmetric vibration of the LiðH2 OÞ4þ ion and the ion-pair ½LiðH2 OÞ4þ Br vibration were assigned at 190 and 340 cm1, respectively [379]. Vibrational spectroscopy is very useful in elucidating the structures of aquo complexes. For example, TiCl3 6H2O should be formulated as trans-[Ti (H2O)4Cl2]Cl2H2O since it exhibits one TiO stretching (500 cm1, Eu) and one TiCl stretching (336 cm1, A2u) mode [380]. Chang and Irish [381] showed from infrared and Raman studies that the structures of the tetrahydrates and dihydrates resulting from the dehydration of Mg(NO3)26H2O are as follows:

A number of hydrated inorganic salts have also been studied by the inelastic neutron scattering (INS) technique [382,383]. Since the proton scattering cross section is quite large, the INS spectrum reflects mainly the motion of the protons in the crystal. Furthermore, INS spectroscopy has no selection rules involving dipole moments or

LATTICE WATER AND AQUO AND HYDROXO COMPLEXES

61

polarizabilities. Thus, it serves as a complementary tool to vibrational spectroscopy in studying the hydrogen vibrations of hydrated salts. 1.8.3. Aquo Complexes in Inert Gas Matrices Cocondensation reactions of alkali halide vapors with H2O in Ar matrices (14 K) produce 1 : 1 adducts of a pyramidal structure (Cs symmetry):

These aquo complexes exhibit two n(OH) at 3300–3000 and two d(OH) at 700– 400 cm1 in agreement with Cs symmetry [384]. Cocondensation reaction of Li vapor with H2O in Kr matrices yields the 1 : 1 complex, Li(H2O), which can be characterized by three internal modes of water [385]. Similar studies with alkaline-earth metal vapors show that the d(H2O) is downshifted by 15 cm1 for Mg and 30 cm1 for Ca, Sr, and Ba on complex formation [386]. As stated in Sec. 1.26.2 of Part A, the reaction of a laser-ablated metal atom with a molecule such as CO in inert gas matrices produces a mixtures of a variety of novel species. In the case of metal atom–H2O reactions, assignments of IR frequencies and structures of reaction products were based on isotopic shifts (H/D and 16 O=18 O) and DFT calculations. For example, the reaction of laser-ablated Th atom with H2O in excess of Ar produced over 15 intriguing species [387]. Similar investigations were carried out with laser-ablated metal atoms such as Mn [388], Sc [389], Ti [390], Zr [390] and Hf [390]. 1.8.4. Hydroxo (OH) Complexes The spectra of hydroxo complexes are supposedly similar to those of the metal hydroxides discussed in Sec. 2.1 of Part A. The hydroxo group can be distinguished from the aquo group since the former lacks the HOH bending mode near 1600 cm1. Furthermore, the hydroxo complex exhibits the MOH bending mode below 1200 cm1. For example, this mode is at 1150 cm1 for the [Sn(OH)6]2 ion [391] and at 1065 cm1 for the [Pt (OH)6]2 ion [392]. The [P(OH)4]þ ion exhibits n(OH), da(POH), and ds(POH) vibrations at 3379/3262, 1112, and 1017 cm1, respectively [393]. The OH group also forms a bridge between two metals. For example

62

APPLICATIONS IN COORDINATION CHEMISTRY

exhibits the bridging OH bending mode at 955 cm1; this is shifted to 710 cm1 on deuteration [394]. For bridging hydroxo complexes of Pt(II), see Refs. [395] and [396]. The hydroperoxo ligand in the [Cu(II)(bppa)(OOH)]þ ion [bppa ¼ bis(6-pivalamide-2-pyridylmethyl)(2-pyridylmethyl)amine] exhibits n(OOH) vibration at 856 cm1, which is shifted to 810 cm1 by 16 O=18 O substitution [397].

1.9. COMPLEXES OF ALKOXIDES, ALCOHOLS, ETHERS, KETONES, ALDEHYDES, ESTERS, AND CARBOXYLIC ACIDS 1.9.1. Complexes of Alkoxides and Alcohols Metal alkoxides, M(OR)n (R: alkyl), exhibit n(CO) at 1000 cm1 and n(MO) at 600– 300 cm1 [398]. Infrared spectra have been reported for various alkoxides of Er(III) [399]. and isopropoxides of rare-earth metals [400]. Complete assignments of the IR and Raman spectra of M(OCH3)6 (M ¼ Mo,W) and the SbðOCH3 Þ6 ion have been based on normal coordinate analysis (C3i, S6 symmetry). The n(MO6) and d(MO6) vibrations are at 600–450 and 400–200 cm1, respectively [401]. The n(TiO) vibration of TiCl3(OCH3) in Ar matrices was assigned at 636 cm1 [402], and the n(FeO) frequencies of [Fe(III)(catecholato)]2 were 583 and 283 cm1 according to DFT calculations [403]. The infrared spectra of alcohol complexes, [M(EtOH)6]Y2, where M is a divalent metal and Y is ClO4 , BF4 , and NO3 , have been measured by van Leeuwen [404]. As expected, the anions have considerable influence on n(OH) and d(MOH). In ethylene glycol complexes with MX2 (X ¼ Cl,Br,I), n(OH) are shifted to lower frequencies and d(CCO) to higher frequencies relative to those of free ligand. It was shown that ethylene glycol serves as a bidentate chelating as well as a unidentate ligand, and that the gauche form prevails in the complexes [405]. Normal coordinate analyses have been carried out to assign the IR spectra of [M(ROH)6]X2 (M ¼ Mg,Ca; R ¼ CH3, C2H5; and X ¼ Cl,Br). The bands at 305 and 275 cm1 of Mg(CH3OH)6Br2 and its Ca analog are primarily the n(MO), and the corresponding force constants are 0.42 and 0.35 mdyn/A, respectively [406].

1.9.2. Complexes of Ethers The vibrational spectra of diethyl ether complexes with MgBr2 and MgI2 have been assigned completely [407]; n(MgO), at 390–300 cm1. The solid-state Raman spectra of 1 : 1 and 1 : 2 adducts of 1,4-dioxane with metal halides show that the ligand is bridging between metals in the chair conformation [408]. When the oxygen atoms of the crown ether (l8-crown-6) coordinate to Ba(II) [409] and Sb(III) [410], the n(COC) band near 1100 cm1 is shifted by 14 and 30 cm1, respectively, to a lower frequency. The n(GaO) vibrations of the [Ga(III)I2(18-crown-6)]þ ion are located at 396 and 352 cm1 [411]. In the case of Ln(NCS)3 (13-crown-4)2H2O (Ln ¼ La, Pr, etc.), the redshift of the n(COC) band is in the range of 76–64 cm1, and the n(LnO) band

COMPLEXES OF ALKOXIDES, ALCOHOLS, ETHERS, KETONES, ALDEHYDES

63

Fig. 1.37. Two conformers of the 18-crown-6 ring [437].

appears at 390–370 cm1 [412]. According to X-ray analysis, the 18-crown-6 ring takes the D3d structure in the Kþ complex and the Ci structure in the uncomplexed state (Fig. 1.37). Takeuchi et al. [413] assigned the IR/Raman spectra of these two compounds in the solid state via normal coordinate analysis, and elucidated the ring conformations of other metal complexes in methanol solution by comparing their Raman spectra with those of the known structures.

1.9.3. Complexes of Other Oxygen Donors There are many coordination compounds with weakly coordinating ligands containing oxygen donors. These include ketones, aldehydes, esters, and some nitro compounds. Driessen and Groeneveld [414–416] and Driessen et al. [417] prepared metal complexes of these ligands (L) through the reaction MCl2 þ 6L þ 2FeCl3 ! ½ML6 ðFeCl4 Þ2 CH3 NO2

in a moisture-free atmosphere; CH3NO2 was chosen as the solvent because it is the weakest ligand available. In acetone complexes, n(C¼O) are lower, and d(CO), p(CO), and d(CCC) are higher than those of free ligand [414]. Similar results have been obtained for complexes of acetophenone, chloracetone, and butanone [415]. In the [Li (acetone)4]þ ion, however, the n(C¼O), na(CC), and ns(CC) all shift to higher frequencies on coordination to the Li ion [418]. In metal complexes of acetaldehyde, n(C¼O) are lower and d(CCO) are higher than those of free ligand [416]. In ester complexes [417] n(C¼O) shifts to lower and n(CO) to higher frequencies by complex formation. When these shifts are dependent on the metal ions, the magnitudes of the shifts follow the well-known Irving–Williams order: Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II). Formamide(HCONH2) coordinates to metal ions via the O atom, and the n(MO) vibrations appear in the 304–230 cm1 range. In NiCl2(NMF)4 and NiCl2(DMF)4 (NMF ¼ N-methylformamide; DMF ¼ dimethylformamide), the Ni(II) ion is coordinated by the N as well as O atoms, and the n(NiN) and n(NiO) vibrations are observed at 500–480 and 420–380 cm1, respectively [419].

64

APPLICATIONS IN COORDINATION CHEMISTRY

1.9.4. Complexes of Carboxylic Acids Extensive infrared studies have been performed on metal complexes of carboxylic acids. Table 1.18 gives the infrared frequencies and band assignments for the formate and acetate ions obtained by Itoh and Bernstein [420]. The carboxylate ion may coordinate to a metal in one of the following modes:

Deacon and Phillips [421] made careful examinations of IR spectra of many acetates and trifluoroacetates having known X-ray crystal structures, and arrived at the following conclusions: (1) Unidentate complexes (structure I) exhibit D values ½na ðCO2 Þ--ns ðCO2 Þ that are much greater than the ionic complexes. (2) Chelating (bidentate) complexes (structure II) exhibit D values that are significantly less than the ionic values. (3) The D values for bridging complexes (structure III) are greater than those of chelating (bidentate) complexes, and close to the ionic values.

TABLE 1.18. Infrared Frequencies and Band Assignments for Formate and Acetate Ions (cm1) [420] [HCOO] Na Salt

Aqueous Solution

[CH3COO] Na Salt

2841 — 1366 — 772 — —

2803 — 1351 — 760 — —

2936 — 1414 924 646 — 2989

1567 — — 1377 —

1585 — — 1383 —

1578 1430 1009 460 2989

— — 1073

— — 1069

1443 1042 615

Aqueous Solution 2935 1344 1413 926 650 — 3010 or 2981 1556 1429 1020 471 2981 or 3010 1456 1052 621

C2v A1

Band Assignment

A2 B1

n(CH) d(CH3) ns(COO) n(CC) d(OCO) rt (CH3) n(CH)

B2

na(COO) d(CH3) rr(CH3) d(CH) or rr (COO) n(CH)

d(CH3) rr(CH3) p(CH) or p(COO)

COMPLEXES OF ALKOXIDES, ALCOHOLS, ETHERS, KETONES, ALDEHYDES

65

The bridging 2-thiopheneacetate (taa) ligand in [Cu(taa)2(DMF)]2 exhibits the na(COO) and ns(COO) at 1620 and 1284 cm1, respectively (D ¼ 336 cm1) [422]. Assignments of IR and Raman spectra of dimeric [Cu(OAc)2(H2O)]2 were based on 63 Cu=65 Cu isotopic shift data and DFT calculations. The n(CuCu) vibration was located at 178 cm1 in Raman spectra [423]. In [Mn2(dmb)4(bipy)2(H2O)2](bipy), where dmb is 2.6-dimethylbenzoate, terminal and bridging dmb ligands are mixed; the terminal unidentate dmb exhibits the na(COO) and ns(COO) at 1566 and 1404 cm1, respectively (D ¼ 162 cm1), whereas the bridging, chelating dmb exhibits these vibrations at 1604 and 1358 cm1, respectively (D ¼ 146 cm1) [424]. As seen in Table 1.19, these criteria hold except for asymmetric bidentates such as Ph2Sn(CH3COO)2 where the two SnO bond distances are markedly different:

In these cases, D values are comparable to those of unidentate complexes [427]. Table 1.19 also shows three carboxylate complexes in which two modes of coordination are mixed. Figure 1.38 shows the Raman spectra of Si(OAc)4 and Ge(OAc)4 that contain only unidentate acetato ligands [426]. According to Stoilova et al., [435], TABLE 1.19. Carboxyl Stretching Frequencies and Structures of Carboxylate Complexes (cm1) Compound

HCOO CH3COO (OAc) Rh(OAc)(CO)(PPh3)2 Ru(OAc)(CO)2(PPh3) Si(OAc)4 Ge(OAc)4 RuCl(OAc)(CO)(PPh3)2 RuH(OAc)(PPh3)2 Ph2Sn(CH3COO)2 Ph2Sn(CH2ClCOO)2 Ph2Te(CCl3COO)2 Rh2(OAc)2(CO)3(PPh3) [Ru(CO)2(C2H5COO)]n [Cr3O(OAc)6(H2O3)]þ [Mn2O2(OAc)]2þ [Pd(OAc)2(PPh3)]2 CrO2(OAc)2 Cp2Zr[Cr(CO)3(RCOO)]2c a

na(COO)a

ns(COO)a

D

Structure

Ref.

1567 1578 1604 1613 1745b 1710b 1507 1526 1610 1620 1705 1580 1548 1621 1548 1629 1580 1710 1610 1641 1542

1366 1414 1376 1315 1290b 1280b 1465 1449 1335 1240 1270 1440 1410 1432 1387 1314 1411 1240 1420 1329 1377

201 164 228 298 455 430 42 77 265 380 435 140 138 189 171 315 169 470 190 312 165

Ionic Ionic Unidentate Unidentate Unidentate Unidentate Bidentate Bidentate Asym. bidentate Asym. bidentate Asym. bidentate Bridging Bridging Bridging Bridging Unidentate Bridging Unidentate Bidentate Unidentate Bidentate

420 420 425 425 426 426 425 425 427 427 427 428 429 430 431 432 433 434

These correspond to the n(C¼O) (free) and n(CO) (coordinated) of the unidenlate carboxylates, respectively. IR frequency. c R ¼ C6H5. b

66

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.38. Raman spectra of Si(OAc)4 and Ge(OAc)4 in the solid state (514.5 nm excitation) [426].

unidentate acetates exhibit three bands (COO deformation) at 920–720 cm1 and a strong band [p(CO2)] at 540 cm1 that are absent in bridging complexes and reduced in number in bidentate complexes. Infrared spectra of formates have been reviewed by Busca and Lorenzelli. [436]. The linkage isomerism involving the acetate group has been reported by Baba and Kawaguchi [437]:

The O-isomer exhibits n(C¼O) at 1640 cm1, whereas the C-isomer shows n(C¼O) at 1670 and 1650 and n(OH) at 2700–2500 cm1. It is also possible to

COMPLEXES OF AMINO ACIDS, EDTA, AND RELATED LIGANDS

67

distinguish two acetato groups having different trans ligands by their frequencies:

Complex A exhibits the na(COO) and ns(COO) at 1665 and 1360 cm1, respectively, whereas complex B exhibits these vibrations at 1620 and 1300 cm1, respectively [438]. Assignments of the IR spectra of metal glycolato (CH2(OH)COO) complexes have been based on normal coordinate calculations [439]. Citric acid [C(OH)(COOH)(CH2COOH)2] contains one hydroxyl and three caroboxylate groups. X-Ray analysis by Matzapetakis et al. [440] shows that the Mn(II) ion in [Mn(II)(C6H5O7)2]4 ion is octahedrally coordinated by two citrate ligands in which three carboxyl groups are deprotonated and the COH group is not ionized. Although the structure of the [Mn(III)(C6H4O7)2]5 ion is similar, the COH group is also deprotonated. The IR spectrum of the former complex exhibits the na(COO) and ns(COO) at 1621–1588 and 1436–1386 cm1, respectively, while these vibrations are at 1636–1596 and 1441–1397 cm1, respectively, in the latter complex. 1.10. COMPLEXES OF AMINO ACIDS, EDTA, AND RELATED LIGANDS 1.10.1. Complexes of Amino Acids Amino acids exist as zwitterions in the crystalline state. Table 1.20 lists band assignments for the zwitterions of glycine [441] and a-alanine [442]. According to X-ray analysis, two glycino anions (gly) in [Ni(gly)2]2H2O, [443], for example, coordinate to the metal by forming a trans-planar structure, and the noncoordinating C¼O groups are hydrogen-bonded to the neighboring

molecule or water of crystallization, or weakly bonded to the metal of the neighboring complex. Thus n(CO2) of amino acid complexes are affected by coordination as well as by intermolecular interactions.

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APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.20. Infrared Frequencies and Band Assignments of Glycine and a-Alanine in the Crystalline State (cm1) [441,442] Glycine

a-Alanine

Band Assignment

1610 1585 1492 — 1445 1413 — 1333 — 1240 (R) 1131 1110 1003

1597 1623 1534 1455 — 1412 1355 — 1308 —

na(COO) dd (NH3þ ) ds(NH3þ ) dd (CH3) d(CH2) ns(COO) ds(CH3) rw(CH2) d(CH) rt(CH2)

1237 1113 1148 1026 1015 — 918 852 648 771 492 540

nr(NH3þ )a

— 910 893 694 607 516 504 a

na(CCN)a rr(CH3)a rr(CH2) ns(CCN)a rw(COO) d(COO) rt(NH3þ ) rr(COO)

These bands are coupled with other modes in a-alanine.

To examine the effects of coordination and hydrogen bonding, Nakamoto et al. [444] performed extensive IR measurements of the COO stretching frequencies of various metal complexes of amino acids in D2O solution, in the hydrated crystalline state, and in the anhydrous crystalline state. The results showed that, in any one physical state, the same frequency order is found for a series of metals, regardless of the nature of the ligand. The antisymmetric frequencies increase, the symmetric frequencies decrease, and the separation between the two frequencies increases in the following order of metals: NiðIIÞ < ZnðIIÞ < CuðIIÞ < CoðIIÞ < PdðIIÞ PtðIIÞ < CrðIIIÞ Although there are several exceptions to this order, these results generally indicate that the effect of coordination is still the major factor in determining the frequency order in a given physical state. The frequency order shown above indicates the increasing order of the metal–oxygen interaction since the COO group becomes more asymmetrical as the metal–oxygen interaction becomes stronger. To give theoretical band assignments on metal glycino complexes, Condrate and Nakamoto [445] carried out a normal coordinate analysis on the metal–glycino chelate ring. Figure 1.39 shows the infrared spectra of bis(glycino) complexes of Pt(II), Pd(II), Cu(II), and Ni(II). Table 1.21 lists the observed frequencies and theoretical band assignments. The CH2 group frequencies are not listed, since they

COMPLEXES OF AMINO ACIDS, EDTA, AND RELATED LIGANDS

69

Fig. 1.39. Infrared spectra of cis- or trans-bis(glycino) complexes of Pt(II), Pd(II), Cu(II), and Ni(II) [445].

TABLE 1.21. Observed Frequencies and Band Assignments of Bis(glycino) Complexes (cm1) [445] trans-[Pt(gly)2] 3230 3090 1643 1610 1374 1245 1023 792 745 620 549 415 2.10 2.10 a

UBF.

trans-[Pd(gly)2] 3230 3120 1642 1616 1374 1218 1025 771 727 610 550 420 2.00 2.00

trans-[Cu(gly)2] 3320 3260 1593 1608 1392 1151 1058 644 736 592 481 333 0.90 0.90

trans-[Ni(gly)2] 3340 3280 1589 1610 1411 1095 1038 630 737 596 439 290 0.70 0.70

Band Assignment n(NH2) n(C¼O) d(NH2) n(CO) rt(NH2) rw(NH2) rr(NH2) d(C¼O) p(C¼O) n(MN) n(MO)

K(MN)(mdyn/A)a K(MO)(mdyn/A)a

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APPLICATIONS IN COORDINATION CHEMISTRY

are not metal-sensitive. It is seen that the C¼O stretching, NH2 rocking, and MN and MO stretching bands are metal-sensitive and are shifted progressively to higher frequencies as the metal is changed in the order Ni(II) < Cu(II) < Pd(II) < Pt(II). Table 1.21 shows that both the MN and MO stretching force constants also increase in the same order of the metals. These results provide further support to the preceding discussion of the MO bonds of glycino complexes. To give definitive band assignments in the low-frequency region of bis(glycino) complexes of Ni(II), Cu(II), and Co(II), Kincaid and Nakamoto [446] carried out HD, 14 N 15 N; 58 Ni 62 Ni; and 63 Cu 65 Cu substitutions, and performed normal coordinate analyses on the skeletal modes of bis(glycino) complexes. Their results show that, in trans-[M(gly)2]2H2O, the infrared-active n(MN) and n(MO) are at 483 and 337 cm1, respectively, for the Cu(II) complex, and at 442 and 289 cm1, respectively, for the Ni(II) complex. Both modes are coupled strongly with other skeletal modes, however. Use of multiple isotope labeling techniques in assigning IR spectra of amino acid complexes has been extended to [Cd(gly)2]H2O [447], cis-[Ni(gly)2(ImH)2] [448], and [M(L–Ala)2] [M ¼ Ni(II),Cu(II)] [449]. Square–planar bis(glycino) complexes can assume the cis or the trans configuration. As expected from symmetry consideration, the cis-isomer exhibits more bands in infrared spectra than does the trans-isomer (see Fig. 1.39). In the low-frequency region, the cis-isomer exhibits two n(MN) and two n(MO), whereas the trans-isomer exhibits only one for each of these modes [445]. This criterion has been used by Herlinger et al. to assign the geometry of a series of bis(aminoacidato)Cu(II) complexes [450,451]. Octahedral tris(glycino) complexes may take the fac and mer configurations shown in Fig. 1.40. For example, [Co(gly)3] exists in two forms: purple crystals (dihydrate, a-form) and red crystals (mono-hydrate, b-form). The a-form is assigned to the mer configuration since it exhibits more infrared bands than does the b-form (fac configuration) [452]. Glycine also coordinates to the Pt(II) atom as a unidentate ligand:

The carboxyl group is not ionized in trans-[Pt(glyH)2X2] (X ¼ a halogen), whereas it is ionized in trans-[Pt(gly)2(NH3)2]. The former exhibits the un-ionized COO stretching band near 1710 cm1, while the latter shows the ionized COO stretching band near 1610 cm1 [453]. The distinction between unidentate and bidentate glycino complexes of Pt(II) can be made readily from their infrared spectra. Figure 1.41 illustrates the infrared spectra of trans-[Pt(glyH)2Cl2] and K[Pt(gly)Cl2] in the COO stretching and PtO stretching regions. The bidentate (chelated) glycino group absorbs at 1643 cm1, unlike either the ionized unidentate group (1610 cm1) or the un-ionized unidentate group (1708 cm1). Furthermore, the bidentate glycino group exhibits the PtO stretching band at 388 cm1, whereas the unidentate glycino group has no absorption between

COMPLEXES OF AMINO ACIDS, EDTA, AND RELATED LIGANDS

71

Fig. 1.40. Structures of fac- and mer-tris(glycino) complexes.

470 and 350 cm1. Figure 1.41 also shows the spectrum of [Pt(gly)(glyH)Cl], in which both the unidentate and bidentate glycino groups are present. It is seen that the spectrum of this compound can be interpreted as a superposition of the spectra of the former two compounds [453].

Fig. 1.41. IR spectra of K[Pt(gly)Cl2], [Pt(gly)(glyH)Cl], and trans-[Pt(glyH)2Cl2] [453].

72

APPLICATIONS IN COORDINATION CHEMISTRY

The Cu(III) complexes of tetraglycine and tetraglycineamide exhibit the N(amide)–Cu(III) CT absorption at 365 nm. Using the 363.8 nm excitation, Kincaid et al. [454] were able to resonance-enhance the n(CuN) vibrations at 420 and 417 cm1, respectively. Metal complexes with N-methylglycine (sarcosine) and N-phenylglycine, of the ML2nH2O type, take the chelate ring structures similar to those of the glycine complexes, and their IR spectra have been assigned by Inomata et al. [455] on the basis of normal coordinate calculations. These ligands also form metal complexes of the type CoCl2(HL)2H2O and MCl2(HL) (M ¼ Zn, Cd) in which the zwitterion of the amino acid is coordinated to the metal via the carboxyl oxygen atom [455]. Dro_zd_zewski et al. [456–459] assigned the metal–nitrogen stretching vibrations of histamine (hm) complexes on the basis of isotope shifts (H/D and metal isotopes) and DFT calculations. Histamine forms a chelate ring via its NH2 group and imidazole (Im) nitrogen:

Table 1.22 lists the n(MNH2) and n[MN(Im)] frequencies of the Ni(II), Cu(II), and Pd(II) complexes in IR spectra.

1.10.2. Complexes of EDTA and Related Ligands From the infrared spectra observed in the solid state, Busch and coworkers [460] determined the coordination numbers of the metals in metal chelate compounds of EDTA and its derivatives:

This method is based on the simple rule that the un-ionized and uncoordinated COO stretching band occurs at 1750–1700 cm1, whereas the ionized and coordinated

COMPLEXES OF AMINO ACIDS, EDTA, AND RELATED LIGANDS

73

TABLE 1.22. Metal-Nitrogen Stretching Frequencies (cm1) of Histamine Complexesa Compound

n(M-NH2)

n(M-N(Im))

Ref.

[Ni(hm)Cl2] 2H2O

58

62

Ni= Ni

423(2.5)

456

[Cu(hm)Cl2]

62

Cu=65 Cu

417(1.0)

[Pd(hm)2]Cl2

104

266(3.5) 249(2.0) 285(1.5) 270(3.5) 311(4.5)

a

Metal Isotopes

Pd 110 Pd

464(2.0)

457 458,459

Numbers in parentheses indicate the magnitude of metal isotope shift.

COO stretching band is at 1650–1590 cm1. The latter frequency depends on the nature of the metal: 1650–1620 cm1 for metals such as Cr(III) and Co(III), and 1610– 1590 cm1 for metals such as Cu(II) and Zn(II). Since the free ionized COO stretching band is at 1630–1575 cm1, it is also possible to distinguish the coordinated and free COO stretching bands if a metal such as Co(III) is chosen for complex formation. Table 1.23 summarizes the na(COO) of the un-ionized COOH, coordinated COO, and free COO groups of EDTA complexes. Faulques et al. [462] obtained the IR/ Raman spectra of [Co(H2O)(H2Y)]2H2O and [Co(H2O)6][Co(H2O)(HY)]22H2O by microspectroscopy.

TABLE 1.23. Antisymmetric COO Stretching Frequencies and Number of Functional Groups Used for Coordination in EDTA Complexes (cm1) [460,461] Compounda H4[Y] Na2[H2Y] Na4[Y] Ba[Co(Y)]24H2O Na2[Co(Y)Cl] Na2[Co(Y)NO2] Na[Co(HY)Cl]12 H2O Na[Co(HY)NO2]H2O Ba[Co(HY)Br]9H2O Na[Co(YOH)Cl]32 H2O Na[Co(YOH)Br]H2O Na[Co(YOH)NO2] [Pd(H2Y)]3H2O [Pt(H2Y)]3H2O [Pd(H4Y)Cl2]5H2O [Pt(H4Y)Cl2]5H2O

Un-ionized COOH 1698b 1668b — — — — 1750 1745 1723 — — — 1740 1730 1707, 1730 1715, 1530

Coordinated COO M

Free COO

— — — 1638 1648 1650 1650 1650 1628 1658 1654 1652 1625 1635 — —

— 1637b 1597b — 1600 1604 — — — — — — — — — —

Number of Coordinated Groups

6 5 5 5 5 5 5 5 5 4 4 2 2

a Y ¼ tetranegative ion; HY ¼ trinegative ion; H2 Y ¼ dinegative ion; H4Y ¼ neutral species of EDTA; YOH ¼ trinegative ion of HEDTA (hydroxyethylenediaminetriacetic add). b Reference 461.

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APPLICATIONS IN COORDINATION CHEMISTRY

Tomita and Ueno [463] studied the infrared spectra of metal complexes of NTA, using the method described above. They concluded that NTA

acts as a quadridentate ligand in complexes of Cu(II), Ni(II), Co(II), Zn(II), Cd(II), and Pb(II), and as a tridentate in complexes of Ca(II), Mg(II), Sr(II), and Ba(II). Krishnan and Plane [464] studied the Raman spectra of EDTA and its metal complexes in aqueous solution. They noted that n(MN) appears strongly in the 500– 400 cm1 region for Cu(II), Zn(II), Cd(II), Hg(II), and so on, and that its frequency decreases with an increasing radius of the metal ion, independently of the stability of the metal complex. McConnell and Nuttall [465] assigned the n(MN) and n(MO) of Na2[M(EDTA)]2H2O (M ¼ Sn, Pb) in their Raman and infrared spectra. 1.11. INFRARED SPECTRA OF AQUEOUS SOLUTIONS Since water is a weak Raman scatterer, Raman spectra of samples in aqueous solution can be measured without major interference from water vibrations. On the other hand, infrared spectroscopy of aqueous solution suffers from strong absorption of bulk water that interferes with IR absorption of the sample. Even so, it is sometimes necessary to measure aqueous IR spectra because some vibrations are inherently weak in Raman spectra. To measure IR spectra of aqueous solution, it is common to use very thin layers (0.01–0.05 mm thick) of solutions of relatively high concentrations (5–20%) which are sandwiched between two plates of water-insoluble crystals such as CaF2 and KRS-5 (TlBr/TlI). Figure 1.42 displays the IR spectra of H2O and D2O obtained by

Fig. 1.42. Infrared spectra of H2O versus air and D2O versus air [466].

INFRARED SPECTRA OF AQUEOUS SOLUTIONS

75

using a CaF2 cell (4000–1000 cm1, 0.03 mm thick) and a KRS-5 cell (1200– 250 cm1, 0.015 mm thick), showing that at least two regions, 2800–1800 and 1500–950 cm1 are relatively free from H2O absorption. These spectral “window” regions can be shifted to 2150–1250 and 1100–750 cm1, respectively, in D2O [466]. The combination of a more recently developed cylindrical internal reflection (CIR) cell [467] with a FTIR spectrometer may be best suited to IR studies of aqueous solution [468]. The following examples demonstrate the utility of aqueous IR spectroscopy in elucidating the structures of complex ions in solution equilibria. The C N stretching band (2200–2000 cm1) can be measured in aqueous solution since it is in the “window” region. Thus, the solution equilibria of cyano complexes have been studied extensively by using aqueous infrared spectroscopy (Sec. 1.16). Fronaeus and Larsson [469] extended similar studies to thiocyanato complexes that exhibit the C N stretching bands in the same region. They [470] also studied the solution equilibria of oxalato complexes in the 1500–1200 cm1 region, where the CO stretching bands of the coordinated oxalato group appear. Larsson [471] studied the infrared spectra of metal glycolato complexes in aqueous solution. In this case, the COH stretching band near 1060 cm1 was used to elucidate the structures of the complex ions in equilibria. The COO stretching bands of NTA, EDTA, and their metal complexes appear between 1750 and 1550 cm1 (Sec. 1.10). As stated above, this region is free from D2O absorption. Nakamoto et al. [472], therefore, studied the solution (D2O) equilibria of NTA, EDTA, and related ligands in this frequency region. By combining the results of potentiometric studies with the spectra obtained as a function of the pH (pD) of the solution, it was possible to establish the following COO stretching frequencies: Type A Un-ionized carboxyl (R2NCH2COOH), 1730–1700 cm1 Type B a-Ammonium carboxylate (R2NþHCH2COO), 1630–1620 cm1 Type C a-Aminocarboxylate (R2NCH2COO), 1585–1575 cm1 As stated in Sec. 1.10, the coordinated (ionized) COO group absorbs at 1650– 1620 cm1 for Cr(III) and Co(III), and at 1610–1590 cm1 for Cu(II) and Zn(II). Thus it is possible to distinguish the coordinated COO group from those of types B and C if a proper metal ion is selected. Tomita et al. [473] studied the complex formation of NTA with Mg(II) by aqueous infrared spectroscopy. Figure 1.43 shows the infrared spectra of equimolar mixtures of NTA and MgCl2 at concentrations of 5–10% by weight. The spectra of the mixture from pD 3.2 to 4.2 exhibit a single band at 1625 cm1, which is identical to that of the free H(NTA)2 ion in the same pD range [474]. This result indicates that no complex formation occurs in this pD range, and that the 1625 cm1 band is due to the H(NTA)2 ion (type B). If the pD is raised to 4.2, a new band appears at 1610 cm1, which is not observed for the free NTA solution over the entire pD range investigated. Figure 1.43 shows that this 1610 cm1 band becomes stronger, and the 1625 cm1 band becomes weaker, as the pD increases. It was concluded that this change is due mainly to a shift of the following equilibrium in the direction of complex formation:

76

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.43. Infrared spectra of Mg-NTA complex in D2O solutions: (* * * * ) pD 3.2; (– – – –) pD 4.2; (– – –) pD 5.5; (— ) pD 6.8; (– – –), pD 10.0; (– – –), pD 11.6 [473].

By plotting the intensity of these two bands as a function of pD, the stability constant of the complex ion was calculated to be 5.24. This value is in good agreement with that obtained from potentiometric titration (5.41). Martell and Kim [475–478] carried out an extensive study on solution equilibria involving the formation of Cu(II) complexes with various polypeptides. As an example, the glycylglycino–Cu(II) system is discussed below [476]. Figure 1.44 illustrates the infrared spectra of free glycylglycine in D2O solution as a function of

Fig. 1.44. Infrared spectra of glycylglycine in D2O solutions: (– – – –) pD 1.75; (– – –) pD 4.31; (—) pD 8.77; (– – – – –) pD 10.29 [476]. *

*

pD. The observed spectral changes were interpreted in terms of the solution equilibria shown below:

77

78

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.45. Infrared spectra of Cu(II)-glycylglycino complexes in D2O solutions: (— —) pD 3.58; (– – – – –) pD 4.24; (– – –) pD 5.18; (—) pD 10.65 [477]. *

*

Band assignments have been made by using the criteria given previously. In addition, Type D frequency (1680–1610 cm1) was introduced to denote the peptide carbonyl group. The exact frequency of this group depends on the nature of the neighboring groups. Figure 1.45 shows the infrared spectra of glycylglycine mixed with copper chloride at equimolar ratio in D2O solution [477]. At pD ¼ 3.58, the ligand exhibits three bands at 1720, 1675, and 1595 cm1 (Fig. 1.44). This result indicates that I and II are in equilibrium. At the same pD value, however, the mixture exhibits one extra band at 1625 cm1. This band was attributed to the metal complex (IV), which was formed by the following reaction:

At pD ¼ 5.18, the solution exhibits one broad band at 1610 cm1. This result was interpreted as an indication that the following equilibrium was shifted almost

COMPLEXES OF OXALATO AND RELATED LIGANDS

79

completely to the right side, and that the 1610-cm1 band is an overlap of two bands at 1610 and 1598 cm1:

The shift of the peptide carbonyl stretching band from 1625 (IV) to 1610 (V) cm1 may indicate the ionization of the peptide NH hydrogen, since such an ionization results in the resonance of the OCN system, as indicated by the dotted line in structure V. Kim and Martell [478] also studied the triglycine and tetraglycine Cu(II) systems. Later, Tasumi et al. [479] carried out similar studies in a wider frequency range (1800–1200 cm1). Kruck and Sarker [480] studied the equilibria of the Cu(II)L-histidine system in D2O.

1.12. COMPLEXES OF OXALATO AND RELATED LIGANDS 1.12.1. Oxalato Complexes The oxalato anion (ox2) coordinates to a metal as a unidentate (I) or bidentate (II) ligand:

The bidentate chelate structure (II) is most common. Fujita et al. [481] carried out normal coordinate analyses on the 1 : 1 (metal–ligand) model of the [M(ox)2]2 and [M(ox)3]3 series, and obtained the band assignments listed in Table 1.24. In the divalent metal series, n(C¼O) (average of n1 and n7) becomes higher, and n(CO) (n2 and n8) becomes lower, as n4(MO) becomes higher in the order Zn(II) < Cu(II) < Pd(II) < Pt(II) (see Fig. 1.46). This relation holds despite the fact that n2, n4, and n8 are all coupled with other vibrations. In the trivalent metal series, Hanco*ck and Thornton [482] found that n11 (MO stretching) follows the same trend as the crystal field stabilization energies (CFSE) of these metals, namely:

80 TABLE 1.24. Frequencies and Band Assignments of Chelating Oxalato Complexes (cm1) [481] K2[Zn(ox)2] 2H2O

K2[Cu(ox)2] 2H2O

K2[Pd(ox)2] 2H2O

K2[Pt(ox)2] 3H2O

K3[Fe(ox)3] 3H2O

K3[V(ox)3] 3H2O

K3[Cr(ox)3] 3H2O

K3[Co(ox)3] 3H3O

K3[Al(ox)3] 3H2O

[Cr(NH3)4 (ox)]Cl

1632 — 1433 1302 890 785 622 519 519 428, 419 377, 364 291

(1720) 1672 1645 1411 1277 886 795 593 541 481 420 382, 370 339

1698 1675, 1657 1394 1245 (1228) 893 818 610 556 469 417 368 350

1709 1674 1388 1236 900 825 — 575, 559 469 405 370 328

1712 1677, 1649 1390 1270, 1255 885 797, 785 580 528 498 366 340 —

1708 1675, 1642 1390 1261 893 807, 797 581 531 497 368 336 —

1708 1684, 1660 1387 1253 893 810, 798 595 543 485 415 358 313

1707 1670 1398 1254 900 822, 803 — 565 472 446 364 332

1722 1700, 1683 1405 1292, 1269 904 820, 803 — 587 436 485 364 —

1704 1668 1393 1258 914, 890 804 — 545 486,469 366 347 328

Band Assignment na(C¼O) na(C¼O) ns(CO) þ n(CC) ns(CO) þ d(OC¼O) ns(CO) þ d(OC¼O) d(OC¼O) þn(MO) Crystal water? n(MO) þn(CC) Ring. def. þ d(OC¼O) n(MO) þ ring def. d(OC¼O) þn(CC) p

n7 n1 n2 n8 n3 n9 n4 n10 n11 n5

COMPLEXES OF OXALATO AND RELATED LIGANDS

n(MO) (cm)1 CFSE (103 cm1)

Sc d0

V d2

Cr d3

Mn d4

Fe d5

Co d6

81

Ga d 10

340 < 367 < 416 > 372 > 354 < 446 > 368 0 < 10.2 < 21.2 > 10.2 > 0 < 27.0 > 0

Both quantities are maximized at the d3 and d6 configurations (d4 and d5 ions are in high-spin states). The IR spectra of [Ir(ox)Cl4]3 (C2v), [Ir(ox)2Cl2]3 (trans, D2h; cis, C2), and [Ir(ox)3]3 (D3) have been assigned by Gouteron [483]. The IR and Raman spectra of the [Co(ox)2]2 (D2h) [484] and [Os(ox)X4]2 (X ¼ Cl,Br,I) (C2v) ions have been assigned [485]. Vibrational spectra of bidentate chelating oxalato complexes were also assigned for [Os(ox)Cl4]2 [486], [Pt(ox)X2]2 [487] and trans-{Pt (ox)2X2]4 (X ¼ a halogen)) [488]. DFT calculations were made for [Fe(III)(ox)3]3 and [Fe(0)(ox)]2 [489]. The oxalato anion may act as a bridging group between metal atoms. According to Scott et al. [490], the oxalato anion can take the following four bridging structures:

Table 1.25 lists the n(CO) of each type. The spectrum of the tetradentate complex (VI) is the most simple. Because of its high symmetry [D2h (planar) or D2d (twisted)], it exhibits only two n(CO). The spectra of bidentate complexes (III and IV) show four n(CO), as expected from the C2v symmetry. The spectrum of the tridentate complex (V) should show four n(CO), although only three are observed. The tetradentate bridging structure (VI) is also found in [(MoFe3S4Cl4)2(ox)]4, which exhibits the n(CO) at 1630 cm1:

82

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.46. M–O stretching frequency versus C¼O and CO stretching frequencies in oxalato complexes of divalent metals [481].

This frequency is much lower than that of the chelating oxalato group in [MoFe3S4Cl4(ox)]3 (1670 cm1) [491]. The [Cr2(ox)5]4 ion contains four bidentate chelating ligands and one bridging oxalato ligand. The former exhibits the na(CO), ns(CO), and d(OCO) at 1678,1384, and 884 cm1, respectively, whereas the latter exhibits these vibrations at 1655,1357, and 804 cm1, respectively [492]. The Raman spectra of metal oxalato complexes have also been examined to investigate the solution equilibria and the nature of the MO bond [493]. TABLE 1.25. CO Stretching Vibrations of Co(III) Oxalato Complexes (cm1) Compound

Symmetry

n(CO)

I

Cs /C1

1761

II

C2v

III

C2v /C2

IV V VI

C2v /C2 Cs /C1 D2h /D2d

1696 1721 1701 1755 1650 —

1682 1665 1667 1629 1670 1626 1610 1628

1400

1260

1410 1439 1430 1318 1322 1345

1268 1276 1250 1284 — —

COMPLEXES OF OXALATO AND RELATED LIGANDS

83

1.12.2. Complexes of Related Ligands Vibrational assignments have been made on metal oxamido complexes of Vh symmetry [494].

and the cis- and trans-dimethyloxamido complexes of dimethylgallium [495]:

The IR and Raman spectra of the Ni(II) and Cu(II) complexes of oxamic hydrazine have been assigned using 58 Ni and 62 Ni isotopes [496]:

The n(NiNH) and n[NiN(NH2)] of the 58 Ni complex are at 439 and 428 cm1, respectively, in the IR spectrum. Biuret (NH2CONHCONH2) is known to form the following two types of chelate rings:

84

APPLICATIONS IN COORDINATION CHEMISTRY

Violet crystals of composition K2[Cu(biureto)2]4H2O are obtained when the Cu(II) ion is added to an alkaline solution of biuret, whereas pale blue-green crystals of composition [Cu(biuret)2]Cl2 result when the Cu(II) ion is mixed with biuret in neutral (alcoholic) solution. The former contains the N-bonded chelate ring structure (VIII), while the latter consists of the O-bonded chelate rings (VII). Kedzia et al. [497] carried out normal coordinate analyses of both compounds. The Co(II) complex forms the N-bonded chelate ring, whereas the Zn complex forms the O-bonded ring structure [497]. In [Cd(biuret)2]Cl2, the biuret molecules are bonded to the metal as follows [498]:

Saito et al. [499] carried out normal coordinate analysis on the ligand portion of the Cd complex. Thamann and Loehr [500] assigned the Raman spectra of N-bonded Cu(II) and Cu(III) complexes of biuret and oxamide based on normal coordinate calculations. The vibrations that are predominantly n(CuN) appear at 320–291 cm1 for the Cu(II) and at 344–320 cm1 for the Cu(III) complexes. The corresponding force constants were 1.04–0.96 mdyn/A for the former and 1.46–1.35 mdyn/A for the latter.

1.13. COMPLEXES OF SULFATE, CARBONATE, AND RELATED LIGANDS When a ligand of relatively high symmetry coordinates to a metal, its symmetry is lowered and marked changes in the spectrum are expected because of changes in the selection rules. This principle has been used extensively to determine whether acido anions such as SO24 and CO23 coordinate to metals as unidentate, chelating bidentate, or bridging bidentate ligands. Although symmetry lowering is also caused by the crystalline environment, this effect is generally much smaller than the effect of coordination. 1.13.1. Sulfato (SO4) Complexes The free sulfate ion belongs to the high-symmetry point group Td. Of the four fundamentals, only n3 and n4 are infrared-active. If the symmetry of the ion is lowered by complex formation, the degenerate vibrations split and Raman-active modes appear in the infrared spectrum. The lowering of symmetry caused by coordination

85

COMPLEXES OF SULFATE, CARBONATE, AND RELATED LIGANDS

is different for the unidentate and bidentate complexes, as shown below:

The change in the selection rules caused by the lowering of symmetry was shown in Table 2.6F of Part A. Table 1.26 and Fig. 1.47 give the frequencies and the spectra of typical Co(III) sulfato complexes obtained by Nakamoto et al. [501]. In [Co (NH3)6]2(SO4)35H2O, n3 and n4 do not split and n2 does not appear, although n1 is observed, it is very weak. They concluded, therefore, that the symmetry of the SO24 ion is approximately Td. In [Co(NH3)5SO4]Br, both n1 and n2 appear with medium intensity; moreover, n3 andn4 each splits into two bands. This resultcanbe explained byassuminga lowering of symmetry from Td to C3v (unidentate coordination). In

both n1 and n2 appear with medium intensity, and n3 and n4 each splits into three bands. These results suggest that the symmetry is further lowered and probably reduced to C2v, as indicated in Table 1.26. Thus, the SO24 group in this complex is concluded to be a bridging bidentate as depicted in the preceding diagram. The chelating bidentate SO24 group was discovered by Barraclough and Tobe [502], who observed three bands (1211, 1176, and 1075 cm1) in the n3 region of [Co(en)2SO4]Br. These frequencies are higher than those of the bridging bidentate complex listed in Table 1.26. Eskenazi et al. [503] also found the same trend in Pd(II) sulfato complexes. Thus the distinction between bridging and chelating sulfato complexes can be made on this basis. Table 1.27 lists the observed frequencies of the sulfato groups and the modes of coordination as determined from the spectra. TABLE 1.26. Vibrational Frequencies of Co(III) Sulfato Complexes (cm1) [501] Compound SO24

ion Free [Co(NH3)6]2 (SO4)35H2O [Co(NH3)6]5SO4]Br

a

Symmetry Td Td

n1

n2

n3

— — 973 (vw) —

C3v

970 (m)

C2v

995 (m)

vs ¼ very strong; s ¼ strong; m ¼ medium; vw ¼ very weak.

n4 a

1104 (vs) 613 (s) 1130–1140 (vs) 613 (s) 1032---1044 ðsÞ 645 ðsÞ 438 (m) 604 ðsÞ 1117---1143 ðsÞ 8 8 < 1050---1060 ðsÞ < 641 ðsÞ 610 ðsÞ 462 (m) 1170 ðsÞ : : 571 ðmÞ 1105 ðsÞ

86

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.47. Infrared spectra of [Co(NH3)6]2(SO4)35H2O (solid line); [Co(NH3)6]5SO4]Br (dot–dash line); and (No3)3 (dotted line) [501].

Unidentate coordination of the sulfato group in trans-[Ru(SO4)(NH3)4(nitotinamide)] Cl has also been confirmed [504]. The symmetries of the sulfate ions in metal sails at various stages of hydration have been studied using IR spectra [505]. Normal coordinate analyses have been carried out on Co(III) ammine complexes containing sulfato groups [506,507]. 1.13.2. Perchlorato (ClO4) Complexes In general, the perchlorate ðClO4 Þ ion coordinates to a metal when its complexes are prepared in nonaqueous solvents. The structure and bonding of metal complexes containing these weakly coordinating ligands have been reviewed briefly by Rosenthal [517]. Infrared and Raman spectroscopy has been used extensively to determine the mode of coordination of the ClO4 ligand.

COMPLEXES OF SULFATE, CARBONATE, AND RELATED LIGANDS

87

TABLE 1.27. Vibrational Frequencies and Modes of Coordination of Various Sulfato Complexes (cm1) Compound

Mode of Coordination

n1

n2

[Cr(H2O)5SO4]Cl H2 O

Unidentate

1002

[VO(SO4)2(H2O)3]2

Unidentate

483

[Cu(bipy)SO4]2H2O (polymeric)

Bridging bidentate

971

Ni(morpholine)2SO4 (polymeric)

Bridging bidentate

973

493

[Co2{(SO4)2OH}(NH3)6]Cl

Bridging bidentate

966

Pd(NH3)2SO4

Bridging bidentate

960

Pd(phen)SO4

Chelating bidentate

955

Pd(PPh3)2SO4

Chelating bidentate

920

Ir(PPh3)2(CO)I(SO4)

Chelating bidentate

856

549

K3[Fe(SO4)F]

Chelating bidentate

Tl[VO2SO4]

Chelating bidentate

1000

455 400

1 2

n3 1118 1068 1140 1046 1163 1096 1053–1035 1177 1094 1042 1180 1101 1048 1195 1110 1035 1240 1125 1040–1015 1265 1155 1110 1296 1172 880 1225 1130 1020 1255 1160 1125

n4

Ref.

508

640 619 —

509 510

628 612 593 645 598

511

512

503

503

513

662 610

514

515

720

516

570

The structures listed in Table 1.28 were determined on the basis of the same symmetry selection rules as used for sulfato complexes. The n3 and n4 frequencies of unidentate perchlorato complexes are 1194 and 1008 cm1, respectively, for [Au(ClO4)4](ClO2) [524], and 1204 and 1065 cm1, respectively, for [Pd(ClO4)4] (NO2)2 [525]. In Pd(ClO4)2, two ClO4 ligands form a square–planar complex around the Pd atom, and Cunin et al. [525] made complete band assignments of its IR/Raman spectra on the basis of DFT calculations. The terminal ClO2 group exhibits the na(ClO2) at 1288 and 1272 cm1 and ns(ClO2) at 1152 and 1130 cm1, whereas the coordinating ClO2 group exhibits these vibrations at 864 and 820 cm1, respectively, in IR spectra. Causse et al. [526] concluded from their IR and Raman study that [Al(ClO4)n](n3) contain two unidentate and two bidentate for n ¼ 4, four unidentate ligands and one bidentate for n ¼ 5, and six unidentate ligands for n ¼ 6. In polymeric M(ClO4)3 (M ¼ In, Tl), the ClO4 ion acts as a bridging bidentate ligand [527].

88

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.28. CIO Stretching Frequencies of Perchlorato Complexes (cm1) Complex

Structure

K[ClO4] Cu(ClO4)26H2O

Ionic Ionic

Cu(ClO4)22H2O

Unidentate

Cu(ClO4)2

Bidentate

Mn(ClO4)22H2O

Bidentate

Co(ClO4)22H2O

Bidentate

[Ni(en)2(ClO4)2]b

Bidentate

Ni(CH3CN)4(ClO4)2

Unidentate

Ni(CH3CN)2(ClO4)2

Bidentate

[Ni(4-Me-py)4](ClO4)2

Ionic

Ni(3-Br-py)4(ClO4)2

Unidentate

GeCl3 (ClO4)

Unidentate

a b

n3 1170–1050 1160–1085 1158 1030 8 < 1270---1245 1130 : 948---920 8 < 1210 1138 : 945 8 < 1208 1125 : 935 8 < 1130 1093 : 1058 1135 1012 8 < 1195 1106 : 1000

1040–1130 1165---1140 1025 1265, 1240

Ref.

n4 a

(935) (947)a

518

920

518

1030

518

1030

519

1025

519

962

520

912

521

920

521

(931)a

522

920

522

1030

523

Weak. Blue form.

According to Pascal et al., the ClO4 ligands in M(ClO4)2 (M ¼ Ni, Co) are bridging tridentate [528]:

In Ni(ClO4)2 (IR), the n(ClOt), na(ClOb), and ns(ClOb) are at 1300, 1030, and 960 cm1, respectively. This type of coordination has also been proposed for M(ClO4)3 (M ¼ Y, La, Nd, Sm, etc.) [529] and for Ce(ClO4)3 [530] and Mn(ClO4)2 [531]. 1.13.3. Complexes of Other Tetrahedral Ligands Many tetrahedral anions coordinate to a metal as unidentate and bidentate ligands, and their modes of coordination have been determined by the same method as is used for

COMPLEXES OF SULFATE, CARBONATE, AND RELATED LIGANDS

89

the SO24 and ClO4 ions. Thus, the PO34 ion is a unidentate in [Co(NH3)5PO4] and a bidentate in [Co(NH3)4PO4] [532]. Vibrational spectra have been reported for unidentate and bidentate complexes of the AsO34 [533], CrO24 , and MoO24 ions [534]. The SeO24 ion in [Co(NH3)5SeO4]Cl is a unidentate [535], whereas it is a bridging bidentate ligand and in [Co2{SeO4)2OH}-(NH3)6]Cl [512]. The latter structure is also reported for (NH4)2UO2(SeO4)24H2O [536]. The Raman spectrum of solid Ni(H2PO2)2 is interpreted as that of the two ions, [Ni(H2PO2)]þ and H2 PO2 ; the hypophosphite ion in the former is a chelating bidentate [537]. In polymeric UCl(H2PO2)32H2O, however, the H2 PO2 ion serves as a bridging bidentate with na(PO2) and ns(PO2) at 1234 and 1058 cm1, respectively [538]. Bridging bidentate phosphinates (R2 PO2 where R is a phenyl) of Ru(I) exhibit the na(PO2) and ns(PO2) at approximately 1145 and 1035 cm1, respectively [539]. The S2 O23 ion can coordinate to a metal in a variety of ways. According to Freedman and Straughan [540], na(SO3) near 1130 cm1 is most useful as a structural diagnosis: >1175 (S-bridging); 1175–1130 (S-coordination); 1130 (ionic S2 O23 ); < 1130 cm1 (O-coordination). On the basis of this criterion, they proposed polymeric structures linked by O-bridges for thiosulfates of UO22 þ and ZrO22 þ . In the [OsO2(S2O3)2]2 ion, the thiosulfate ion is a S-bonded unidentate with the n(SS) at 409 cm1, which is much lower than that of the free ligand (434 cm1) [541]. The fluorosulfate (SO3F) ion is a unidentate in [Sn(SO3F)6]2 [542], but is a unidentate as well as a bidentate in VO(SO3F)3 [543]. Similarly, only unidentate coordination is seen in [Ru(SO3F)6]2, whereas [Ru(SO3F)5] may contain both unidentate and bidentate ligands [544]. The na(SO2), ns(SO2), and n(SO)/n(SF) vibrations of Cs[Sb(SO3F)6] were assigned at 1451(IR), 1256(R) and 958(IR)/903 (R) cm1, respectiuvely. Band assignments are also reported for Cs2[M(SO3F)6] (M ¼ Sn, Pt) [545]. 1.13.4. Carbonato(CO3) Complexes The unidentate and bidentate (chelating) coordinations shown below are found in the majority of carbonato complexes:

The selection rule changes as shown in Table 1.18 of Part A. In C2v and Cs*, the n1 vibration, which is forbidden in the free ion, becomes infrared-active and each of the doubly degenerate vibrations, n3 and n4, splits into two bands. Although the number of infrared-active fundamentals is the same for C2v and Cs, the splitting of the degenerate The symmetry of the unidentate carbonato group is C2v if the metal atom is ignored.

90

APPLICATIONS IN COORDINATION CHEMISTRY

vibrations is larger in the bidentate than in the unidentate complex [501]. For example, [Co(NH3)5CO3]Br exhibits two CO stretchings at 1453 and 1373 cm1, whereas [Co(NH3)4CO3]Cl shows them at 1593 and 1265 cm1. In organic carbonates such as dimethyl carbonate, (CH3OI)2COII, this effect is more striking because the CH3OI bond is strongly covalent. Thus, the COII stretching is observed at 1870 cm1, whereas the COI stretching is at 1260 cm1. Gatehouse and co-workers [546] showed that the separation of the CO stretching bands increases along the following order: Basic salt < carbonato complex < acid < organic carbonate Fujita et al. [547] carried out normal coordinate analysis on unidentate and bidentate carbonato complexes of Co(III). According to their results, the CO stretching force constant, which is 5.46 for the free ion, becomes 6.0 for the COII bonds and 5.0 for the COI bond of the unidentate complex, whereas it becomes 8.5 for the COII bond and 4.1 for the COI bonds of the bidentate complex (all are UBF force constants in units of mdyn/A). The observed and calculated frequencies and theoretical band assignments are shown in Table 1.29. Normal coordinate analyses on carbonato complexes have also been carried out by other workers [548,549]. Vibrational spectra of bidentate carbonato complexes are reported for Na5[Sc(CO3)4]2H2O [550] and for GaCO3 radical formed in inert gas matrices [551]. As is shown in Table 1.29, normal coordinate analysis predicts that the highestfrequency CO stretching band belongs to the B2 species in the unidentate and the A1 species in the bidentate complex. Elliott and Hathaway [552] studied the polarized infrared spectra of single crystals of [Co(NH3)4CO3]Br and confirmed these symmetry properties. As will be shown later for nitrate complexes, Raman polarization studies are also useful for this purpose. According to X-ray analysis, the carbonate groups in [(NH3)3Co(m-OH)2(m-CO3) Co(NH3)3]SO45H2O [553] and [(teed)CuCl(CO3)CuCl(teed)] (teed: N,N,N0 ,N0 tetraethyl-ethylenediamine) [554] take the bridging bidentate and tridentate structures, respectively:

Vibrational spectra of bridging bidentate and tridentate complexes are also reported for [Ru(III)2(tacn)2(m-OH)2(m-CO3)]Br23.75H2O (tacn¼1.4.7-triazacyclononane) [555] and (m-CO3)[Ni(II)3(Medpt)3(NCS)4] [Medpt¼bis(3-aminopropyl)methylamine] [556]. No simple criteria have been established to distinguish these structures from common unidentate and bidentate (chelating) coordination on the basis of vibrational frequencies. However, Greenaway et al. [557] have demonstrated that the bridging and bidentate carbonate ligands can be distingished if the angular distortion (Da), the

TABLE 1.29. Calculated and Observed Frequencies of Unidentate and Bidentate Co(III) Carbonato Complexes (cm1) [547] Species (C2n)a Calculated Frequency Assignment

n1(A1) 1376 n(COII) þn(COI)

n2(A1) 1069 n(COI) þn(COII)

n3(A1) 772

n4(A1) 303

n5(B2) 1482

n6(B2) 676

n7(B2) 92

n8(B1) —

d(OIICOII)

n(CoOI)

n(CoOII)

rr(OIICOII)

d(CoOIC)

p

[Co(NH3)5CO3]Br [Co(ND3)5CO3]Br [Co(NH3)5CO3]I [Co(ND3)5CO3]I

1373 1369 1366 1360

1070 1072 1065 1063

756 751 776 742

362 351 360 341

1453 1471 1449 1467

678 687 679 687

— — — —

850 854 850 853

Species (C2n)a Calculated Frequency Assignment

n1(A1) 1595

n2(A1) 1038

n3(A1) 771

n4(A1) 370

n5(B2) 1282

n7(B2) 429

n8(B2) —

n(COII)

n(COI)

þ Ring Def. n(CoOI)

n(CoOI) þ Ring Def.

n(COI) þd(OICOII)

n6(B2) 669 d(OICOII) þn(COI) þn(CoOI)

n(CoOI)

p

1593 1635 1607 1602 1603

1030

760

395

1265

673

430

834

753

378

1268

672

418

832

762 765

392 374

1284 1292

672 676

428 415

836 835

[Co(NH3)4CO3]Cl [Co(ND3)4CO3]Cl [Co(NH3)4CO3]ClO4 [Co(ND3)4CO3]ClO4 a

(1031)

b

c

— —c

Symmetry assuming a linear CoOC bond (see Ref. 547). Overlapped with ds(ND3). c Hidden by [ClO4] absorption. b

91

92

APPLICATIONS IN COORDINATION CHEMISTRY

difference between the largest and smallest OCO angles, is known from X-ray analysis. These workers found that the frequency separation (Dn) between the two highest n(CO) bands increases linearly with Da. As an example, Na2[Cu(CO3)2] contains one bidentate ligand and one bridging carbonate ligand. Using their correlation, they were able to assign the 1610 and 1328 cm1 bands to the bidentate (Da ¼ 11.2 and Dn ¼ 282 cm1) and the 1525 and 1380 cm1 bands to the bridging carbonate ligands (Da ¼ 7.7 and Dn ¼ 145 cm1). The IR spectrum of K2CO3 in a N2 matrix indicates that the CO3 group coordinates in a bidentate fashion to one of the K atom and in a unidentate fashion to the other K atom [558]. Busca and Lorenzelli [559] reviewed the IR spectra and modes of coordination of carbonate, bicarbonate, and formate ions, and of CO2 in metal complexes. 1.13.5. Nitrato (NO3) Complexes The structures and vibrational spectra of a large number of nitrato complexes have been reviewed by Addison et al. [560] and Rosenthal [513]. X-Ray analyses show that the NO3 ion coordinates to a metal as a unidentate, symmetric, and asymmetric chelating bidentate, and bridging bidentate ligand of various structures. It is rather difficult to differentiate these structures by vibrational spectroscopy since the symmetry of the nitrate ion differs very little among them (C2v or Cs). Even so, vibrational spectroscopy is still useful in distinguishing unidentate and bidentate ligands. Originally, Gatehouse et al. [561] noted that the unidentate NO3 group exhibits three NO stretching bands, as expected for its C2v symmetry. For example, [Ni(en)2(NO3)2] (unidentate) exhibits three bands as follows: n5 (B2) n1 (A1) n2(A1)

1420 cm1 1305 cm1 (1008) cm1

na(NO2) ns(NO2) n(NO)

whereas [Ni(en)2NO3]ClO4 (chelating bidentate) exhibits three bands at the following: n1(A1) n5(B2) n2(A1)

1476 cm1 1290 cm1 (1025) cm1

n(N¼O) na(NO2) ns(NO2)

The separation of the two highest-frequency bands is 115 cm1 for the unidentate complex, whereas it is 186 cm1 for the bidentate complex. Thus Curtis and Curtis [562] concluded that [Ni(dien)(NO3)2] contains both types, since it exhibits bands due to unidentate (1440 and 1315 cm1) and bidentate (1480 and 1300 cm1) groups. Table 1.30 lists the three NO stretching frequencies mentioned above. The order of these frequencies is n5 > n1 > n2 for unidentate, and n1 > n5 > n2 for chelating bidentate complexes. In general, the separation of the first two bands of the latter is larger than that of the former if the complexes are similar. As seen in Table 1.30, however, this rule does not hold if the complexes are markedly different. More examples are found

COMPLEXES OF SULFATE, CARBONATE, AND RELATED LIGANDS

93

TABLE 1.30. NO Stretching Frequencies of Unidentate and Bidentate Nitrato Complexes (cm1) Compound

Mode of Coordination

n5

n1

n2

n5–n1

Ref.

Unidentate Unidentate Chelating bidentate Chelating bidentate Chelating bidentate Chelating bidentate Bridging bidentate Chelating bidentate Chelating bidentate Chelating bidentate Chelating bidentate

1497 1510 1630

1271 1275 1255

992 997 983

226 235 375

563 564 565

1555 1521 1619

1271

1025

566

1162

963

284 250 457

1490

1280

210

568

1519

1291

1041 1036 1008

228

569

1485

1300

185

570

1513

1270

1013

243

571

1530

1278

1023

252

572

1500

1295

1030

305

573

Re(CO)5NO3 cis-[Pt(NH3)2(NO3)2] Sn(NO3)4 K[UO2(NO3)3] Co(NO3)3 Na2[Mn(NO3)4] Cu(NO3)2MeNO2 Zn(bt)2(NO3)2a Ni(dmpy)2(NO3)2b Th(NO3)4 (tmu)2c Ln(NO3)3 (DMSO)n (Ln ¼ La,Ce,.)

567

a

bt ¼ benzothiazole. dmpy ¼ 2,6-dimethyl-4-pyrone. c tmu ¼ tetramethylurea. b

for C{Hg(NO3)}4H2O (unidentate, 223 cm1) [574], [V2O3Cl4(NO3)2]2 (chelatuig bidentate, 232 cm1) [575], and CrO2(NO3)2 (chelating bidentate. 280 cm1) [576]. Lever et al. [577] proposed the use of the combination band, n1 þ n4, of free NO3 that appears in the 1800–1700 cm1 region for structural diagnosis. On coordination, n4 (E0 , in-plane bending) near 700 cm1 splits into two bands, and the magnitude of this splitting is expected to be larger for bidentate than for unidentate ligands. This should be reflected on the separation of two (n1 þ n4) bands in the 1800–1700 cm1 region. According to Lever et al. [577], the NO3 ion is bidentate if the separation is 66– 20 cm1 and is unidentate if it is 26–5 cm1. As stated previously, the highest-frequency CO stretching band of the carbonato complexes belongs to the A1 species in the bidentate and to the B2 species in the unidentate complex. The same holds true for the nitrato complex. Ferraro et al. [578] showed that all the nitrato groups in Th(NO3)4(TBP)2 coordinate to the metal as bidentate ligands since the Raman band at 1550 cm1 is polarized (TBP ¼ tributylphosphate). This rule holds very well for other compounds [579]. According to Addison et al. [560], the intensity pattern of the three NO stretching bands in the Raman spectrum can also be used to distinguish unidentate and symmetric bidentate NO3 ligands. The middle band is very strong in the former, whereas it is rather weak in the latter. The use of far-infared spectra to distinguish unidentate and bidentate nitrato coordination has been controversial. Nuttall and Taylor [580] suggested that unidentate and bidentate complexes exhibit one and two MO stretching bands, respectively, in the

94

APPLICATIONS IN COORDINATION CHEMISTRY

350–250 cm1 region. Bullock and Parrett [581] showed, however, that such a simple rule is not applicable to many known nitrato complexes. Ferraro and Walker [582] assigned the MO stretching bands of anhydrous metal nitrates such as Cu(NO3)2 and Pr (NO3)3. Several workers studied the Raman spectra of metal nitrates in aqueous solution and molten states. For example, Irish and Walrafen [583] found that E0 -mode degeneracy is removed even in dilute solutions of Ca(NO3)2. This, combined with the appearance of 0 the A1 mode in the infrared, suggests C2v symmetry of the NO3 ion. Using FTIR and Raman spectroscopy, Castro and Jagodzinski [584] have shown that the Cu(NO3)þ ion of C2v symmetry is formed when copper nitrate hydrate is dissolved in H2O and acetone at a high solute concentration. The Raman band at 335 cm1 was assigned to the ns(CuO) of this chelating bidentate complex. Hester and Krishnan [585] studied the Raman spectra of Ca(NO3)2 dissolved in molten KNO3 and NaNO3. Their results suggest an asymmetric perturbation of the NO3 ion by the Ca2þ ion through ion-pair formation. Wick et al. [586] prepared the first peroxynitrite(OONO) complex, Na3[Co (CN)5(OONO)], which exhibits the n(N¼O), n(NO), and n(OO) vibrations at 1621,1399, and 915 cm1, respectively. 1.13.6. Sulfito (SO3), Selenito (SeO3), and Sulfinato (RSO2) Complexes The pyramidal sulfite ðSO23 Þ ion may coordinate to a metal as a unidentate, bidentate, or bridging ligand. The following two structures are probable for unidentate coordination:

If coordination occurs through sulfur, the C3v symmetry of the free ion will be preserved. If coordination occurs through oxygen, the symmetry may be lowered to Cs. In this case, the doubly degenerate vibrations of the free ion will split into two bands. It is anticipated [587] that coordination through sulfur will shift the SO stretching bands to higher frequencies, whereas coordination through oxygen will shift them to lower frequencies, than those of the free ion. On the basis of these criteria, Newman and Powell [588] showed that the sulfito groups in K6[Pt(SO3)4]2H2O and [Co(NH3)5(SO3)]Cl are S-bonded and those in Tl2[Cu(SO3)2] are O-bonded. Baldwin [589] suggested that the sulfito groups in cis- and trans-Na[Co(en)2(SO3)2] and [Co(en)2(SO3)X] (X ¼ Cl or OH) are S-bonded, since they show only two SO stretchings between 1120 and 930 cm1. According to Nyberg and Larsson [590], the appearance of a strong SO stretching band above 975 and below 960 cm1 is an indication of S- and O-coordination, respectively. Table 1.31 lists typical results obtained for unidentate complexes. The IR spectrum of fac-[Rh(SO3)3(NH3)3] Na32H2O in the crystalline state [592] shows that the symmetry of the S-bonded unidentate sulfite ligand is lowered from C3v to Cs. Comparison of the Raman spectra of cis- and trans-[Rh(SO3)2(NH3)4]þ ions

COMPLEXES OF SULFATE, CARBONATE, AND RELATED LIGANDS

95

TABLE 1.31. Infrared Spectra of Unidentate Sulfito Complexes (cm1) Compound

Structure

SO23

Free K6[Pt(SO3)4]2H2O [Co(NH3)5(SO3)]Cl trans-Na[Co(en)2(SO3)2] [Co(en)2(SO3)Cl]

— S-bonded S-bonded S-bonded S-bonded

Tl2[Cu(SO3)2]

O-bonded

(NH4)9[Fe(SO3)6]

O-bonded

n3(E) 933 1082–1057 1110 1068 1117–1075 902 862 943

n1(A1)

n2(A1)

967 964 985 939 984

620 660 633 630 625

989

673

815

638

n4(E) 469 540 519 — — 506 460 520

Ref. 588 588 589 589 588 591

reflects the difference between these geometries on the numbers of the observed SO3 vibrations [593]. The structures of complexes containing bidentate sulfito groups are rather difficult to deduce from their infrared spectra. Bidentate sulfito groups may be chelating or bridging through either oxygen or sulfur or both, all resulting in Cs symmetry. Baldwin [589] prepared a series of complexes of the type [Co(en)2(SO3)]X (X ¼ Cl, I, or SCN), which are monomeric in aqueous solution. They show four strong bands in the SO stretching region (one of them may be an overtone or a combination band). A chelating structure in which two oxygens of the sulfito group coordinate to the Co(III) atom was suggested. Newman and Powell [588] obtained the infrared spectra of K2[Pt(SO3)2] 2H2O, K3[Rh(SO3)3]2H2O, and other complexes for which bidentate coordination of the sulfito group is expected. It was not possible, however, to determine their structures from infrared spectra alone. Krieglstein and Breitinger [594] prepared [(en)Pt(SO3)2Pt (en)]3H2O and its Pd analog. According to X-ray analysis, the former contains two parallel m-S,O bridges whereas the latter contains two antiparallel (m-S,O bridges:

The former exhibits the ns(SO2), na(SO2), and n(SO) at 1116/1062, 1178/1153 and 923 cm1, respectively, while the corresponding frequencies of the latter are 1108, 1188, and 927 cm1, respectively. The mode of coordination of the selenite ion (SeO23 ) is similar to that of the sulfite ion. Two types of unidentate complexes are expected. The O-coordinated complex exhibits n3(E) and n1(A1) at 755 and 805 cm1, respectively, for [Co(NH3)5(SeO3)] BrH2O, [595] whereas the Se-coordinated complex, [Co(NH3)5(SeO3)]ClO4 [596], shows them at 823 and 860 cm1, respectively. Four types of coordination are probable for sulfinato (RSO2 , R ¼ CH3, CF3, Ph, etc.) groups:

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APPLICATIONS IN COORDINATION CHEMISTRY

The SO stretching bands at 1200–850 cm1 are useful in distinguishing these structures [597,598].

1.14. COMPLEXES OF b-DIKETONES 1.14.1. Complexes of Acetylacetonato Ion A number of b-diketones form metal chelate rings of type A:

Among them, acetylacetone (acacH) is most common (RI ¼ RIII ¼ CH3 and RII ¼ H). Infrared spectra of M(acac)2- and M(acac)3-type complexes have been studied extensively. Theoretical band assignments were first made by Nakamoto and Martell [599], who carried out normal coordinate analysis on the 1 : 1 model of Cu(acac)2. Mikami et al. [600] performed normal coordinate analyses on the 1 : 2 (square–planar) and 1 : 3 (octahedral) models of various acac complexes. Figure 1.48 shows the infrared spectra of six acac complexes, and Table 1.32 lists the observed frequencies and band assignments for the Cu(II), Pd(II), and Fe(III) complexes obtained by Mikami et al. In this table, the 1577- and 1529-cm1 bands of Cu(acac)2 are assigned to n(C%C) coupled with n(C%O) and n(C%O) coupled with n(C%C), respectively. Junge and Musso [601] have measured the 13 C and 18 O isotope shifts of these bands and concluded that the above assignments must be reversed. The n(MO) of acac complexes are most interesting since they provide direct information about the MO bond strength. Using the metal isotope technique, Nakamoto et al. [602] assigned the MO stretching bands of acetylacetonato complexes at the following frequencies (cm1): Cr(acac)3 463.4 358.4

Fe(acac)3

Pd(acac)2

Cu(acac)2

436.0 300.5

466.8 297.1 265.9

455.0 290.5

Ni(acac)2(py)2 438.0 270.8

Both normal coordinate calculations and isotope shift studies show that the bands near 450 cm1 are coupled with the CCH3 bending mode, whereas those in the lowfrequency region are relatively pure MO stretching vibrations. Figure 1.49 shows the actual tracings of the infrared spectra of 50 CrðacacÞ3 and its 53 Cr analog. It is seen that two bands at 463.4 and 358.4 cm1 of the former give negative shifts of 3.0 and 3.9 cm1, respectively, whereas other bands (ligand vibrations) produce negligible shifts by the 50 Cr 53 Cr substitution.

COMPLEXES OF b-DIKETONES

97

Fig. 1.48. Infrared spectra of bis- and tris-(acetylacetonato) complexes [600].

Sch€ onherr et al. [603] carried out normal coordinate analysis on Sn(acac)Cl4. The SnO stretching force constant (GVF) was 1.56 mdyn/A. Handa et al. [604] observed the following trends (cm1) in the RR spectra of the Fe(III)–acac system in CH3CN solution: FeðacacÞ2þ

Fe(acac)2þ n(CC)þn(CO) n(FeO)

1554 474

< >

1578 462

Fe(acac)3 < >

1603 451

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APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.32. Observed Frequenciesa and Band Assignments of Acetylacetonato Complexes (cm1) [600] Cu(acac)2

Pd(acac)2

Fe(acac)3

Predominant Mode

3072 ) 2987 2969 2920 1577 1552 1529 1461 1413

3070 ) 2990 2965 2920 1569 1549 1524 (1425) 1394

n(CH)

1353

1358

1274 1189 1019 936

1272 1199 1022 937

780

786 779

684

700

3062 2895) 2965 2920 1570 — 1525 1445 1425 1385o 1360 1274 1188 1022 930 ) 801 780 771 ) 670 664

653

678

656

612

661

559 548

451

463

433

n(CH3) nðC CÞ þ nðC CÞ combination nðC OÞ þ nðC CÞ dðCHÞ þ nðC CÞ dd ðCH3 Þ ds ðCH3 Þ

nðC--CH3 Þ þ nðC CÞ dðCHÞ þ nðC--CH3 Þ rr(CH3) nðC CÞ þ nðC OÞ p(CH) n(CCH3) þ ring deformation þ n(MO)

Ring deformation þ n(MO) n(MO) þ n(CCH3)

431

441

415 408

291

294

298

n(MO)

1.45

1.85

1.30

K(MO) (mdyn/A) (UBF)

a

Ring deformation

IR spectra in the solid state.

These orders suggest that the FeO bond becomes weaker as the number of the coordinated acac ligand increases because the Lewis acidity of the metal ion decreases in the same order. Complexes of the M(acac)2X2 type may take the cis or trans structure. Although steric and electrostatic considerations would favor the trans-isomer, the greater stability of the cis-isomer is expected in terms of metal–ligand p-bonding. This is the case for Ti(acac)2F2, which is “cis” with two n(TiF) at 633 and 618 cm1 [605]. In the case of Re(acac)2Cl2, however, both forms can be isolated; the trans-isomer exhibits n(ReO) and n(ReCl) at 464 and 309 cm1, respectively, while each of these bands splits into two in the cis-isomer [472 and 460 cm1 for n(ReO) and 346 and 333 cm1 for n(ReCl) in the infrared] [606]. For VO(acac)2L, where L is a substituted

COMPLEXES OF b-DIKETONES

99

Fig. 1.49. Infrared spectra of 50 Cr ðacacÞ3 and its 53 Cr analog [625].

pyridine, cis- and trans-isomers are expected. According to Caira et al. [607], these structures can be distinguished by their infrared spectra. The n(V¼O) and n(VO) of the cis-isomer are lower than those of the trans-isomer. For example, n(V¼O) of VO (acac)2 is 999 cm1, and this band shifts to 959 cm1 for 4-Et-py (cis) and to 973 cm1 for py (trans). Furthermore, the n(VO) of the cis-isomer splits into two bands:

Vibrational spectra of acac complexes have been studied by many other investigators. References are cited only for the following: Cs[Os(acac)X4] (X ¼ Cl,Br,I) [608], [Os (acac)3] [609], [M(acac)3] (M ¼ Ti,V,Cr,Mn,Fe,Co,Ni,Sc,Al) [610], [UO2(acac)2] [611], and [Ce(acac)3(H2O)]H2O [612]. Infrared spectra of metal complexes of b-diketones have been reviewed extensively by Thornton [613]. According to X-ray analysis [614], the hexafluoroacetylacetonato ion (hfa) in [Cu (hfa)2{Me2N–(CH2)2–NH2}2] coordinates to the metal as a unidentate via one of its O atoms. This compound exhibits n(C¼O) at 1675 and 1615 cm1, values slightly higher than those for Cu(hfa)2, in which the hfa ion is chelated to the metal (1644 and 1614 cm1). The n(C¼O) of a mixed-ligand complex, [Ru(II)(hfa)(acac)], is assigned at 1583 for the hfa ring and 1579 cm1 for the acac ring. These bands are shifted to 1550 and 1521 cm1, respectively, in the [Ru(I)(hfa)(acac)] ion [615].

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APPLICATIONS IN COORDINATION CHEMISTRY

The following dimeric bridging structure has been proposed for [CoBr (acac)]2:

n(CoOt) and n(CoOb) were assigned at 435 and 260 cm1, respectively [616]. In [Ni(acac)2]3 and [Co(acac)4]4, the O atoms of the acac ion serve as a bridge between two metal atoms [617]. However, no band assignments on these polymeric species are available. 1.14.2. Complexes of Neutral Acetylacetone In some compounds, the keto form of acetylacetone forms a chelate ring of type B:

This particular type of coordination was found by van Leeuwen [618] in [Ni(acacH)3] (ClO4)2 and its derivatives, and by Nakamura and Kawaguchi [619] in Co(acacH)Br2. These compounds were prepared in acidic or neutral media, and exhibit strong n(C¼O) bands near 1700 cm1. Similar ketonic coordination was proposed for Ni (acacH)2Br2 [620] and M(acacH)Cl2 (M ¼ Co,Zn) [621]. According to X-ray analysis [622], the acetylacetone molecule in Mn (acacH)2Br2 is in the enol form and is bonded to the metal as a unidentate via one of its O atoms:

The C O and C C stretching bands of the enol ring were assigned at 1627 and 1564 cm1, respectively.

COMPLEXES OF b-DIKETONES

101

1.14.3. C-Bonded Acetylacetonato Complexes Lewis and coworkers [623] reported the infrared and NMR spectra of a number of Pt(II) complexes in which the metal is bonded to the g-carbon atom of the acetylacetonato ion:

Behnke and Nakamoto carried out normal coordinate analysis on the [Pt(acac)Cl2] ion, in which the acac ion is chelated to the metal (type A) [624], and on the [Pt (acac)2Cl2]2 ion, in which the acac ion is C-bonded to the metal (type D) [625]. Table 1.33 lists the observed frequencies and band assignments for these two types, and Fig. 1.50 shows the infrared spectra of these two compounds. The results indicate that (1) two n(C¼O) of type D are higher than those of type A, (2) two n(CC) of type D are lower than those of type A, and (3) n(PtC) of type D is at 567 cm1, while n(PtO) of

TABLE 1.33. Observed Frequencies, Band Assignments, and Force Constants for K[Pt(acac)Cl2] and Na2 [Pt(acac)2C]2] 2H2O

K[Pt(acac)Cl2] (O-Bonded, Type A)

Na2[Pt(acac)2Cl2]2H2O (C-Bonded, Type D)

— 1563, 1380 1538, 1288 — 1212, 817 650, 478 —

1652, 1626 — — 1350, 1193 1193, 852 — 567

KðC OÞ ¼ 6:50 KðC CÞ ¼ 5:23 K(CCH3) ¼ 3.58 K(PtO) ¼ 2.46 K(CH) ¼ 4.68 r ¼ 0.43a

K(C¼O) ¼ 8.84 K(CC) ¼ 2.52 K(CCH3) ¼ 3.85 K(PtC) ¼ 2.50 K(CH) ¼ 4.48

a

Band Assignment

n(C¼O) nðC OÞ nðC CÞ n(CC) d(CH) or p(cH) n(PtO) n(PtC)

UBF constant (mdyn/A)

The stretching–stretching interaction constant (r) was used for type A because of the presence of resonance in the chelate ring.

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Fig. 1.50. Infrared spectra of Pt(II) acetylacetonato complexes: (1) K[Pt(acac)Cl2]; (2) K[Pt (acac)2Cl]; (3) Na2[Pt(acac)2Cl2]H2O, where A and D denote the bands characteristic of types A and D, respectively [624,625].

type A are at 650 and 478 cm1. Figure 1.50 also shows that the structure of K[Pt (acac)2Cl] is as follows

COMPLEXES OF b-DIKETONES

103

since its spectrum is roughly a superposition of those types A and D. Similarly, the infrared spectrum of K[Pt(acac)3] [623] is interpreted as a superposition of spectra of types A, D, and D0 , in which two CO bonds are transoid [626]:

The C-bonded acac ion was found in Hg2Cl2(acac) [627], Au(acac)(PPh3), [628], and Pd(acac)2(PPh3) [629]. In the last compound, one acac group is type A and the other, type D. In all these cases, the n(C¼O) of the type D acac groups are at 1700– 1630 cm1. As discussed above, K[Pt(acac)2Cl] contains one type A acac group and one type D acac group. If a solution of K[Pt(acac)2Cl] is acidified, its type D acac group is converted into type E:

This structure was first suggested by Allen et al. [630], based on NMR evidence. Behnke and Nakamoto [631] showed that the infrared spectrum of [Pt(acac)(acacH) Cl] thus obtained can be interpreted as a superposition of spectra of types A and E. That the two O atoms of the C-bonded acac group (type D) retain the ability to coordinate to a metal was first demonstrated by Lewis and Oldham [632], who prepared neutral complexes of the following type:

Using the metal isotope technique, Nakamura and Nakamoto [633] assigned the n(NiO) of Ni[Pt(acac)2Cl]2 at 279 and 266 cm1. These values are relatively close to

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APPLICATIONS IN COORDINATION CHEMISTRY

the n(NiO) of Ni(acacH)2Br2 (264 and 239 cm1), discussed previously. Thus, the newly formed Ni-acac ring retains its keto character and is close to type B. Other types prepared by Kawaguchi and coworkers include the following:

Kawaguchi [636] reviewed the modes of coordination of b-diketones in a variety of metal complexes. Gerisch et al. [637] determined the crystal structures of novel anionic tetranuclear platina-b-diketonates of platina-b-diketones, 12(BH)2[Cl2Pt(m-COMe)2Pt (COMe)2H)]2 (B ¼ n-BuNH2, NEt3, etc.), shown below:

The IR spectra exhibit the n(CO) of the bridging m-acyl group in the platina-bdiketonato unit at 1524–1534 cm1 and those of the platina-b-diketone unit, at 1552– 1556 cm1. 1.14.4. Complexes of Other b-Diketones In a series of metal tropolonato complexes, Hulett and Thornton [638] noted a parallel relationship between the n(MO) and the CFSE energy. These workers assigned the n(MO) of trivalent metal tropolonates in the 660–580 cm1 region, based on the 16 O 18 O isotope shifts observed for the Cu(II) complex [639]. Using the metal

COMPLEXES OF UREA, SULFOXIDES, AND RELATED LIGANDS

105

isotope technique, Hutchinson et al. [640] assigned the n(MO) at the following frequencies (cm1): V(III) 377 319

Cr(III)
>

338 268

Fe(III) > >

317 260

Co(III) <

444

Co(II) >

436

Zn(II) >

431

Fe(II) >

438 415

Mn(II) >

418

Griffiths and Thornton [685] made band assignments of these DMSO complexes based on d6 and 18 O substitution of DMSO. Ligands such as DPSO (diphenylsulfoxide) and TMSO (tetramethylenesulfoxide) do not exhibit the CH3 rocking bands near 950 cm1. Thus, the SO stretching bands of metal complexes containing these ligands can be assigned without difficulty. In a series of O-bonded DMSO and TMSO complexes, the S¼O stretching force constant decreases linearly as the MO stretching force constant increases [686]. Table 1.36 lists the SO stretching frequencies and the magnitude of band shifts in DPSO complexes [560]. Van Leeuwen and Groeneveld [687] noted that the shift becomes larger as the electronegativity of the metal increases. In Table 1.36, the metals are listed in the order of increasing electronegativity. In [M(DTHO2)3]2þ [M ¼ Co(II), Ni(II), Mn(II), etc.], the metals are O-bonded since the n(S¼O) of free ligand (1055–1015 cm1) are shifted to lower frequencies by 40–22 cm1:

On the other hand, the metals are S-bonded in M(DTHO2)Cl2 [M ¼ Pt(II), Pd(II)] since n(S¼O) are shifted to higher frequencies by 108–77 cm1 [688]. Dimethylselenoxide, (CH3)2Se¼O, forms complexes of the MCl2,(DMSeO)n type, where M is Hg(II), Cd TABLE 1.36. Shifts of SO Stretching Bands in DPSO and DMSO Complexes (cm1) [687] DMSO Complex

DPSO Complex Metal

n(SO)

Shift

Shift

Ca(II) Mg(II) Mn(II) Zn(II) Fe(II) Ni(II) Co(II) Cu(II) Al(III) Fe(III)

1012–1035 1012 983–991 987–988 987 979–982 978–980 1012, 948 942 931

0– (–23) –23 –45 –47 –48 –55 –56 –23, –87 –93 –104

— — –41 — — –45 –51 –58 — —

110

APPLICATIONS IN COORDINATION CHEMISTRY

(II), Cu(II), and so on, and n is 1, 112, or 2. The n(Se¼O) of the free ligand (800 cm1) is shifted to the 770–700 cm1 region, indicating the O-bonding in these complexes [689].

1.16. CYANO AND NITRILE COMPLEXES 1.16.1. Cyano Complexes The vibrational spectra of cyano complexes have been studied extensively and these investigations are reviewed by Sharp [690], Griffith [691], Rigo and Turco [692], and Jones and Swanson [693]. 1.16.1.1. CN Stretching Bands Cyano complexes can be identified easily since they exhibit sharp n(CN) at 2200–2000 cm1. The n(CN) of free CN is 2080 cm1 (aqueous solution). On coordination to a metal, the n(CN) shift to higher frequencies, as shown in Table 1.37. The CN ion acts as a s-donor by donating electrons to the metal and also as a p-acceptor by accepting electrons from the metal. s-Donation tends to raise the n(CN) since electrons are removed from the 5s orbital, which is weakly antibonding, while p-backbonding tends to decrease the n(CN) because the electrons enter into the antibonding 2pp* orbital. In general, CN is a better s-donor and a poorer p-acceptor than is CO. Thus, the n(CN) of the complexes are generally higher than the value for free CN, whereas the opposite prevails for the CO complexes (Sec. 1.18). According to El-Sayed and Sheline [702], the n(CN) of cyano complexes are governed by (1) the electronegativity, (2) the oxidation state, and (3) the TABLE 1.37. C N Stretching Frequencies of Cyano Complexes (cm1) Compound Tl[Au(CN)2] K[Ag(CN)2] K2[Ni(12 C 14 N)4] K2[Pd(12 C 14 N)4] K2[Pt(12 C 14 N)4] Na3[Ni(CN)5] Na3[Co(CN)5] K3[Mn(CN)6] K4[Mn(CN)6] K3[Fe(CN)6] K4[Fe(CN)6]3H2O K3[Co(CN)6] K4[Ru(CN)6]3H2O K3[Rh(CN)6] K2[Pd(CN)6] K4[Os(CN)6]3H2O K3[Ir(CN)6]

Symmetry D¥h D¥h D4h D4h D4h C4n C4n Oh Oh Oh Oh Oh Oh Oh Oh Oh Oh

n(CN) P Pþ 2164 ( g ), 2141 ( uþ ) Pþ P 2146 ( g ), 2140 ( uþ ) 2143.5 (A1g), 2134.5 (B1g), 2123.5 (Eu) 2160.5 (A1g), 2146.4 (B1g), 2135.8 (Eu) 2168.0 (A1g), 2148.8 (B1g), 2133.4 (Eu) 2130(A1), 2117 (B1), 2106 (E), 2090 (A1) 2115 (A1), 2110 (B1), 2096 (E), 2080 (A1) 2129 (A1), 2129 (Eg), 2112 (F1u) 2082 (A1g), 2066 (Eg), 2060 (F1u) 2135 (A1g), 2130 (Eg), 2118 (F1u) 2098 (A1g), 2062 (Eg), 2044 (F1u) 2150 (A1g), 2137 (Eg), 2129 (F1u) 2111 (A1g), 2071 (Eg), 2048 (F1u) 2166 (A1g), 2147 (Eg), 2133 (F1u) 2185 (F1u) 2109 (A1g), 2062 (Eg), 2036 (F1u) 2167 (A1g), 2143 (Eg), 2130 (F1u)

Ref. 694,695 696 697 697 697 698 698 699,700 699 699 699 699 699 699 701 699 699

CYANO AND NITRILE COMPLEXES

111

coordination number of the metal. The effect of electronegativity is seen in the following order: [Ni(CN)4]2 2128

2005 (cm1) This result indicates that the degree of the M ! CO p-backdonation increases in going from Co(II) to Fe(II) to Ru(II). The 1 : 1 adduct, Fe(TPP)(CO), exhibits the n(CO) at 1973 cm1, which is lower than that of Fe(TPP)(CO)2 because the net M ! CO p-backdonation decreases in the latter due to competition between the two CO ligands [1049]. Yu and coworkers carried out an extensive RR study on CO adducts of metalloporphyrins by using isotopic ligands, 13 C 16 O, 12 C 18 O, and 13 C 18 O. In the 700– 100 cm1 region, Fe(TpivPP)(CO)(1-MeIm) (TpivPP: “picket-fence” porphyrin shown in Fig. 1.64a) shows only one isotope-sensitive band at 489 cm1. This band has been assigned to the n (FeC) because it shifts from 485 to 481 to 477 cm1 in the order

Fig. 1.64. Structures of (a) ‘‘picket-fence’’ and (b) ‘‘strapped’’ porphyrins.

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APPLICATIONS IN COORDINATION CHEMISTRY

shown above for the isotopic CO ligands. It is also sensitive to the nature of the transligand (L); the weaker the ML bond is, the stronger the FeCO bond. However, no bands assignable to the d(FeCO) were resonance-enhanced [1051]. In CO adducts of simple metalloporphyrins such as Fe(TPP)(CO), the FeCO bond is linear and normal to the porphyrin plane. In the “strapped” porphyrins (Fig. 1.64b), the FeCO (linear) bond is tilted because of steric hindrance of the “strap.” According to Yu [1047], this tilting increases the electron donation from a pyrrole ring (p) to the CO (p*) orbital because of a better overlap between these orbitals. This would decrease the CO bond order and increase the FeC bond order. Thus, the following trends are observed as the “strap” is shortened [1052]: SP-15

SP-14

SP-13

n(CO)(cm1) n(FeC)(cm1)

1945 > 1939 > 1932 509 < 512 < 514 — 504 506 In the latter two complexes, the n(FeC) bands are split into two bands; the higherand lower-frequency components were attributed to the “tilted” and “upright” conformers, respectively. Similar trends in frequency were observed for a hybrid of the “picket-fence” and the “basket-handle” porphyrins [1053]. The RR spectra of these “strapped” porphyrins exhibit the d(FeCO), which shows the “zigzag” isotope shift pattern. For example, SP-14 exhibits this band at 578 cm1, which is shifted to 563 (13 C 16 O), 575 (12 C 18 O), and 561 (13 C 18 O) cm1 by the isotopic substitutions indicated in the parentheses. In contrast, the n(FeC) vibration near 510 cm1 shows a normal (monotonous) isotopic shift pattern. This difference has often been used to distinguish these two models. It should be noted, however, that the observation of a “zigzag” isotope shift pattern does not necessarily indicate the bending mode (Sec. 3.2.1). Yu et al. [1052] also observed that the degree of resonance enhancement of the d(FeCO) relative to the n(FeC) mode increases as the distortion of the FeC O linkage increases by shortening the “strap” length. As described above, Yu and coworkers originally assigned the d(FeCO) near 560 cm1, which are higher than the n(FeC) near 510 cm1, and their assignments have been followed by many other workers. However, an alternative assignment has been proposed for the CO adducts of heme proteins (Sec. 3.2.1). 1.18.5. Hydrocarbonyls Hydrocarbonyls exhibit bands characteristic of both MH and MCO groups. Kaesz and Saillant [1054] reviewed the vibrational spectra of metal carbonyls containing the hydrido group. Vibrational spectra of hydrido complexes containing other groups are discussed in Sec. 1.24. In general, the terminal MH group exhibits a relatively sharpand medium-intensity n(MH) band in the 2200–1800 cm1 region. The MH stretching band can be distinguished easily from the CO stretching band by the deuteration experiment. Edgell and coworkers [1055] assigned the infrared bands at 1934 and 704 cm1 of HCo(CO)4 to n(CoH) and d(CoH), respectively, and proposed structure I of Fig. 1.65,

COMPLEXES OF CARBON MONOXIDE

145

Fig. 1.65. Structures of hydrocarbonyls.

in which the H atom is on the C3 axis. Stammreich et al. [932] reported the Raman spectrum of HFeðCOÞ4 , which is expected to have a structure similar to that of HCo(CO)4. According to X-ray analysis [1056], the Mn(CO)5 skeleton of HMn(CO)5 takes the C4v structure shown in structure II. Kaesz and coworkers [1057,1058] assigned the infrared spectrum of HMn(CO)5 in the n(CO) region on the basis of this structure. The Raman spectra of HMn(CO)5 and HRe(CO)5 exhibit their n(MH) at 1780 and 1824 cm1, respectively [1059]. Edgell et al. [1061] have completed a complete vibrational assignment of gaseous HMn(CO)5. The infrared spectrum of H2Fe(CO)4 in hexane at 78 C exhibits three or more n(CO) above 2000 cm1 and a weak, broad n(FeH) at 1887 cm1. Thus, Farmery and Kilner [1061] suggested structure III. Table 1.51 lists the observed frequencies of other hydrocarbonyl compounds. It is rather difficult to locate the bridging n(MH) in polynuclear hydrocarbonyls. These vibrations appear in the region from 1600 to 800 cm1, and are rather broad at room temperature although they are sharpened at low temperatures. Higgins et al. [1066] were the first to suggest the presence of bridging hydrogens in Re3H3(CO)12 (structure IV) since no terminal n(ReH) bands were observed. Smith et al. [1067] observed a very weak and broad band at 1100 cm1 in the Raman spectrum of Re3H3(CO)12 and assigned it to the bridging n(ReH) since it shifted to 787 cm1 on deuteration. Although structure V was proposed for [M2H(CO)10] (M ¼ Cr, Mo and W) [1068], the WHWangle of the tungsten complex was found to be 137 [1069]. Shriver et al. [1070] located the antisymmetric and symmetric stretching vibrations of the WHW

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TABLE 1.51. Vibrational Frequencies of Metal Hydrocarbonyl Compounds (cm1)a Compound

n(CO)

n(MH)

d(MH)

Ref.

RhH(CO)(PPh3)3 IrH(CO)(PPh3)3 IrHCl2(CO)(PEt2Ph)2 IrHBr2(CO)(PEt2Ph)2 IrHCl2(CO)(PPh3)2 OsHCl(CO)(PPh3)2 OsH2(CO)(PPh3)2

1926 1930 2101 2035 2027 1912 2014 1990

2004 2068 2008 2232 2240 2097 1928 1873

784 822 — — — — —

1062 1062 1063 1063 1064 1064 1065

a

For the configurations of these molecules, see the original references.

bridge at 1683 and 900 cm1, respectively. At 80 K, the latter splits into four bands at 960, 869, 832, and 702 cm1. Although the origin of this splitting is not clear, the possibility of Fermi resonance with an overtone or a combination band involving the n(WC) or d(WCO) was ruled out on the basis of COC18 O isotopic shifts [1070]. Ginsberg and Hawkes [1071] suggested structure VI for [Re2H3(CO)6] since they could not observe any terminal n(ReH) vibrationals. The bridging n(FeH) band of FeHCo3(CO)12 in the infrared was finally located at 1114 cm1 by Mays and Simpson [1072], using a highly concentrated KBr pellet. This band shifts to 813 cm1 on deuteration. On the basis of mass spectroscopic and infrared evidence, they proposed structure VII, in which the H atom is located inside the metal atom cage. From the spectra in the n(CO) region, together with X-ray evidence, Kaesz et al. [1073] proposed the Td skeleton (structure VIII) for [Re4H6(CO)12]2. It showed no terminal n(ReH), but a broad bridging n(ReH) centered at 1165 cm1 was observed in its Raman spectrum. This band shifts to 832 cm1 with less broadening on deuteration. Bennett et al. [1074] found no terminal n(ReH) in the infrared spectrum of Re2H2(CO)8. However, its Raman spectrum exhibits bands at 1382 and 1272 cm1, which shift to 974 and 924 cm1, respectively, on deuteration. The D2h structure (IX) was proposed for this compound. Figure 1.66 shows the Raman spectra of Ru4H4(CO)12 and Ru4D4(CO)12 obtained by Knox et al. [1075]. Two n(RuH) bands at 1585 and 1290 cm1 of the former compound are shifted to 1153 and 909 cm1, respectively, on deuteration. Its infrared spectrum exhibits five n(CO) instead of the two expected for Td symmetry. Thus a structure of D2d symmetry, which lacks two H atoms from structure VIII, was proposed [1076]. The n(OsH) vibrations of the analogous Os complex have also been assigned [1077]. In the [Ru6H(CO)18] ion, the H atom is located at the center of an octahedron consisting of six Ru atoms [1078]. Oxton et al. [1079] located its n(RH) at 845 and 806 cm1 (95 K), which are probably split by Fermi resonance. As stated in Sec. 1.8 (on aquo complexes), the inelastic neutron scattering (INS) technique is very effective in locating hydrogen vibrations. White and Wright [1080] found two hydrogen vibrations at 608 and 312 cm1 in the INS spectrum of Mn3H3(CO)12. However, the nature of these vibrations is not clear.

COMPLEXES OF CARBON MONOXIDE

147

100

H4Ru4(CO)12 Solid sample

80

Relative intensity

60 40 20 0

D4Ru4(CO)12

80 60 40 20 0

200 2000 1900 1700 1600 1500 1400 1300 1200 1100 1000 900 ˆv (cm-1)

800

700

600

500

400

Fig. 1.66. Raman spectra of Ru4H4(CO)12 and its deuterated analog [1075].

1.18.6. Metal Carbonyls in Inert Gas Matrices General theory of matrix isolation spectroscopy and examples of matrix cocondensation reactions are given in Secs. 1.25 and 1.26 of Part A, respectively. Cocondensation reaction of metal atom vapors with CO yields a mixture of different compositions M þ nCO ! MðCOÞn

ðn ¼ 1 6Þ

and photolysis of coordinatively saturated metal carbonyls yields metal carbonyls of lower coordination numbers: hn

hn

MðCOÞn ! MðCOÞn 1 ! MðCOÞn 2 ! Andrews and co-workers [1081] prepared a variety of metal carbonyls and determined their structures using the methods described in Sec. 1.26. In most cases, their structures were elucidated on the basis of IR spectra in the high-frequency region, because Raman spectra are technically difficult to measure in inert gas matrices and because the spectra in the low-frequency region are difficult to measure even by IR spectroscopy. Table 1.52 lists the symmetry, structure, and the number of IR-active n(CO) vibrations of M(CO)n (n ¼ 1–6)-type molecules. As an example, Fig. 1.38 of Part A shows the IR spectra of Sc(CO)n obtained by cocondensation reaction in Ar matrices. Ni(CO)2 in Ar matrices is bent (C2v) and not linear as previously proposed. This was confirmed by complete assignments of all the nine modes, including 58=60 Ni and 12=13 C isotopomers [1082]. The linear Pd(CO) molecule in Ar matrices (site I) exhibits five palladium isotope peaks (104 Pd, 105 Pd, 106 Pd, 108 Pd, and 110 Pd) of the n(PdC) vibrations at 472.97, 472.48, 472.06, 471.18, and 470.32 cm1, respectively [1083]. As stated in Sec. 1.26, cationic MðCOÞnþ species such as Fe(CO)þ and FeðCOÞ2þ are produced by laser ablation techniques [1084], whereas anionic species such as

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TABLE 1.52. Number of Infrared-Active CO Stretching Vibrations for M(CO)x Molecule M(CO) M(CO)2 M(CO)2 M(CO)3 M(CO)3 M(CO)4 M(CO)4 M(CO)5 M(CO)5 M(CO)6

Symmetry and Structure C¥v D¥h C2v D3h C3v Td D4h C4v D3h Oh

Linear Linear Bent Trigonal–planar Trigonal–pyramidal Tetrahedral Square–planar Tetragonal–pyramidal Trigonal–bipyramidal Octahedral

IR-Active n(CO) Pþ Pþ u

A1 þ B2 E0 A1 þ E F2 Eu 2A1 þ E A002 þ E 0 F1u

NiðCOÞn (n ¼ 1–3) are produced by UVirradiation and electron bombardment [1085]. For all metal carbonyls, n(CO) frequencies follow the order, cations > neutrals > anions. For example, Ni(CO)þ (2206.3) > Ni(CO)(2006.6) > Ni(CO)(1860.6) (all in cm1) [1081]. Ultraviolet photolysis of Ni(CO)4 in O2-doped Ar matrices produces mixed-ligand species such as (Z2 –O2)Ni(CO)2, (Z2-O2)Ni(CO)3, and O¼Ni(CO)2. The n(16 O2 ) of the side-on O2 of the first species is at 978 cm1 [1086]. The n(CO) of linear M(CO)Cltype molecules are at 2218.7, 2156.8, and 2184.0 cm1 for M ¼ Ni, Cu, and Ag, respectively [1087]. Photon-induced isomerization reactions such as shown below are known to occur for hn MðCOÞ ! CMO ðM ¼ Nb;Th;UÞ hn OMCCO ðM ¼ Ti;Zr;Hf;Nb;Ta;Th;UÞ MðCOÞ2 !

For example, CNbO thus obtained exhibits two bands at 919.8 and 783.7 cm1 [1088]. The n(CO) of LiCO and LiOC were observed at 1806 and 1614 cm1, respectively, in Kr matrices [1089]. Such an “isocarbonyl” structure, O¼CAuO¼C, was also proposed for Au(CO)2 [1090]. The pair of OCBeO and COBeO was produced by the cocondensation reaction of pulsed-laser-evaporated Be atoms with CO2/Ar. The former exhibits the n(CO) and n(BeO) at 2139.4 and 1498.2 cm1, respectively, whereas the latter exhibits these bands at 2056.5 and 1533.9 cm1 [1091]. Poliakoff and Turner [1092] carried out UV photolysis of 13 CO-enriched FeðCOÞ5 hn in SF6 and Ar matrices [FeðCOÞ5 ! FeðCOÞ4 þ CO], and concluded that the structure of Fe(CO)4 is C2v since it exhibits four n(CO) (2A1 þ B1 þ B2) in the infrared spectrum. Graham et al. [1093] proposed the C4v structure for Cr(CO)5 produced by the photolysis of Cr(CO)6 in inert gas matrices. On the other hand, K€undig and Ozin [1094] proposed the D3h structure for Cr(CO)5 prepared by cocondensation of Cr atoms with CO in inert gas matrices. They derived a general rule that M(CO)5 species take the D3h structure when the number of valence-shell electrons is even [Cr (16), Fe (18)], and the C4v structure when it is odd [V (15), Mn (17)]. However, the D3h, structure of Cr(CO)5 has been questioned by Black and Braterman [1095] and Perutz and Turner [1096].

COMPLEXES OF CARBON MONOXIDE

149

The UV photolysis of Fe2(CO)9 in Ar matrices produces Fe2(CO)8 of C2v symmetry having two bridging CO groups [1097]. Photolysis of Mn2(CO)10 by plane-polarized light in Ar matrices produces Mn2(CO)9, which exhibits the n(CO) at 1764 cm1 [1098,1099]. Dunkin et al. [1098] proposed the semibridging structure on the basis of its polarization properties:

Ultraviolet photolysis of MnRe(CO)10 in Ar matrices yields MnRe(CO)9 with the n(CO) at 1759.8 cm1 [1100]. A semibridging structure similar to that proposed above may be expected. Carbonyl complexes of the type MX2CO are formed by reacting metal halide vapor directly with CO in inert gas matrices [1101,1102]. In this case, n(CO) shifts to higher frequencies by complexation, since the bonding is dominated by the donation of selectrons to the metal. On the other hand, n(MX) shifts to lower frequencies because the oxidation state of the metal is lowered by accepting s-electrons from CO. Figure 1.67 shows infrared spectra of the PbF2L system (L ¼ CO,NO,N2) in Ar matrices obtained by Tevault and Nakamoto [1102]. In this series, the magnitudes of the shifts of the PbF2 and L stretching bands (cm1) relative to the free state are as follows: PbF2CO PbF2NO PbF2N2 ns(PbF2) 10.8 8.8 5.8 10.9 8.5 5.0 na (PbF2) n(L) þ38.4 þ16.4 —

This result definitely indicates that CO is the best, NO is the next best, and N2 is the poorest s-donor. Other work involves the direct deposition of stable carbonyls in inert gas matrices, mainly to study the effect of matrix environments on the structure. Both Fe(CO)5 [1103] and M3(CO)12 (M ¼ Ru,Os) [1104] were found to be distorted from D3h symmetry in inert gas matrices. If a thick deposit is made on a cryogenic window while maintaining a relatively high sample/inert gas dilution ratio, it is possible to observe low-frequency modes such as n(MC) and d(MCO). It was found that these bands show splittings due to the mixing of metal isotopes. For example, the F1u n(CrC) of Cr(CO)6 in a N2 matrix exhibits four bands due to 50 Cr; 52 Cr; 53 Cr; and 54 Cr (see Fig. 1.36 of Part A). The magnitude of these isotope splittings may be used to estimate the degree of the n(MC)–d(MCO) mixing in the low-frequency vibrations [1105]. 1.18.7. Theoretical Calculations of Vibrational Frequencies Normal coordinate analyses on metal carbonyl compounds have been carried out by many investigators. Among them, Jones and coworkers have made the most

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Fig. 1.67. Infrared spectra of PbF2, PbF2CO, PbF2NO, and PbF2N2 in Ar matrices: (m) monomeric PbF2; (d) dimeric PbF2; (c) complex; (i) impurity (HF–CO) [1101].

extensive study in this field. For example, they performed rigorous calculations on the M(CO)6 (M ¼ Cr,Mo,W) series [936], Fe(CO)5 [935], and Mn(CO)5Br [1022], including their 13 C and 18 O analogs. For the last compound, 5 stretching, 16 stretching–stretching interaction, and 33 bending–bending interaction constants (GVF) were used to calculate its 30 normal vibrations. On the other hand, Cotton and Kraihanzel (C–K) [1106] developed an approximation (C–K) method for calculating the CO stretching and COCO stretching interaction constants, while neglecting all other low-frequency modes. For Mn (CO)5Br, they used only the five force constants [1107] shown in Fig. 1.68. Since only four CO stretching bands are observed for this type of compound, it was assumed that 12 kt ¼ kc ¼ kd holds. This was justified on the basis of the symmetry properties of the metal dp orbitals involved. This C–K method has since been applied to many other carbonyls in making band assignments, in interpreting intensity data, and in discussing the bonding schemes of metal carbonyls [924]. It is clear that the choice of a rigorous approach (Jones) or a simplified method (C–K) depends on the availability of observed data and the purpose of the

COMPLEXES OF CARBON MONOXIDE

151

Fig. 1.68. Definition of force constants for M(CO)5X.

investigation. Jones [1108] and Cotton [1109] discuss the merits of their respective approaches relative to the alternative. As mentioned earlier, n(CO) of metal carbonyls are determined by two factors: (1) donation of the 5s-electrons to the empty metal orbital tends to raise the n(CO) since the 5s orbital is slightly antibonding; and (2) backdonation of metal dp-electrons to the 2pp orbitals of CO tends to lower n(CO) since the 2pp orbitals are antibonding. Vibrational spectroscopy does not allow observation of these two effects separately since the observed n(CO) and the corresponding force constant reflect only the net result of the two counteracting components. It is possible, however, to correlate the CO stretching force constants (C–K) with the occupancies of the 5s and 2pp orbitals as calculated by MO theory. Table 1.53 lists the results obtained for d 6 carbonyl halides and dihalides by Hall and Fenske [1110]. It is interesting that the trans-CO in Fe(CO)4I2 and the cis-CO in Mn(CO)5Cl have almost the same force constants since the 5s occupancy of the former is smaller by 0.102 than that of the latter, while the 2pp occupancy of the former is larger by 0.108 than that of the latter. It is also noteworthy that the trans-CO in Fe(CO)4I2 and the cis-CO in Cr(CO)5Cl have identical 2pp occupancies (0.537) but substantially different force constants (17.43 and 15.58 mdyn/ A, respectively). In this case, the difference in force constants originates in the difference in the 5s occupancies (1.293 vs. 1.457). Hall and Fenske [1110] found a linear relationship between the C–K CO stretching force constants and the occupancies of the 5s and 2pp levels: k ¼ 11:73½2px þ 2py þ ð0:810Þ5s þ 35:81

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TABLE 1.53. Carbonyl Orbital Occupanciesa and Force Constants Compound Cr(CO)5Cl Cr(CO)5Br MnðCOÞ4 I2 Mn(CO)4IBr MnðCOÞ4 Br2 Cr(CO)5Br Cr(CO)5Cl Mn(CO)5Cl Mn(CO)5Br Mn(CO)5I MnðCOÞ4 I2 Mn(CO)4IBr MnðCOÞ4 Br2 Mn(CO)5I Mn(CO)5Br Fe(CO)4I2 Mn(CO)5Cl Fe(CO)4Br2 Fe(CO)5Brþ Fe(CO)5Clþ Fe(CO)4I2 Fe(CO)4Br2 Fe(CO)5Clþ Fe(CO)5Brþ a b

Structure

5s

2px

2py

k (mdyn/A)b

trans trans trans trans trans cis cis trans trans trans cis cis cis cis cis trans cis trans trans trans cis cis cis cis

1.407 1.405 1.354 1.355 1.357 1.456 1.457 1.352 1.350 1.349 1.402 1.404 1.406 1.394 1.394 1.293 1.395 1.295 1.287 1.289 1.337 1.338 1.325 1.325

0.355 0.353 0.302 0.302 0.302 0.261 0.261 0.286 0.286 0.286 0.251 0.241 0.242 0.213 0.212 0.252 0.211 0.250 0.233 0.233 0.221 0.205 0.171 0.171

0.355 0.353 0.330 0.327 0.325 0.282 0.276 0.286 0.286 0.286 0.251 0.252 0.242 0.240 0.228 0.285 0.218 0.272 0.233 0.233 0.221 0.205 0.177 0.193

14.07 14.10 15.48 15.48 15.50 15.56 15.58 16.28 16.32 16.37 16.75 16.77 16.91 17.29 17.39 17.43 17.46 17.53 17.93 17.95 17.95 18.26 18.99 19.00

The cis and trans designations of the CO groups are made with respect to the position of the halogen or halogens. Cotton–Kraihanzel force constants (see Ref. 1110).

A similar attempt has been made for a series of Mn carbonyls containing isocyanide groups [1111]. As described in Sec. 1.24 of Part A, density functional theory (DFT) has been used to determine the structures of a variety of compounds and to calculate their vibrational frequencies. In particular, this method was indispensable in elucidating the structures and in making vibrational assignments of novel metal carbonyls produced in inert gas matrices (Sec. 1.26).

1.19. COMPLEXES OF CARBON DIOXIDE Although CO2 is highly inert, a few complexes with metal atoms and metal ions in the low oxidation state are known. These complexes have been a subject of considerable interest because they have the potential to become catalysts in activating CO2, which is the most abundant source of C1 compounds. Thus far, vibrational studies on metal complexes of CO2 are limited to a small number of compounds, because stable complexes of CO2 are rare. The CO2 ligand may

COMPLEXES OF CARBON DIOXIDE

153

coordinate to a metal in any one of the following schemes:

Different from the linear CO2 molecule in the free state, the CO2 ligand in metal complexes is generally bent. Furthermore, the bond orders of the two CO bonds change markedly on coordination. Thus, the three vibrations observed for free CO2 show large downshifts in metal complexes. Table 1.54 lists the observed frequencies of typical complexes and their modes of coordination. The CO2 vibrations in metal complexes can easily be identified since they are sensitive to 13 C 12 O and 12 C 18 O substitutions. According to Jegat et al. [1115,1116], it is possible to distinguish the three modes of coordination on the basis of n3 n1 values and the magnitudes of isotopic shifts due to 13 C and 18 O substitutions. A variety of CO2 complexes were prepared by reacting CO2 with metal atom vapor produced by thermal heating. According to Mascetti and Tranquille [1120], matrix cocondensation reaction of Ti atom with pure CO2( 1/1000, 15 K) yields a side-on type complex, OTi(CO2):

TABLE 1.54. Observed CO2 Stretching Frequencies (cm1) and Modes of Coordination Compound

n3

n1

Mode of Coordination

Free CO2 [U(IV)(CO2)L]a cis-[Ru(bipy)2(CO)(CO2)] [{Ir4S2CH2CN}(CO2)]þ Cp2Ti(CO2)(PMe3) Mo(CO2)2 (PMe3)4

2349 2188 1443 1682 1671 1668

— End-onb End-on End-on C-bonded Side-on

330 (Chapter 2, Part A) 1112 1113 1114 1115 1116

Fe(CO2) (PMe3)4 Ni(CO2) (PCy3)2c

1623 1741

Side-on Side-on

1116 1117

Ni(CO2) (PEt3)2

1660 1635 1668 1630

1337 — — — 1187 1153 1102 1106 1150 1093 1203 1009 1165 1120

Side-on

1118

Side-on

1119

RhCl(CO2) (PBu3)2

Ref.

L ¼ (ad ArOH)3tacn ¼ l,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane. Almost linear end-on coordination. The UOC and OCO angles are 171 and 178 , respectively. c Cy ¼ cyclohexyl. a b

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1500

2000 10%

(cm-1)

1000 1132

1735

Ti 955 V

1176 1000 1745

939,932 1095

1692 1565 1625

1210

Fe Co

1190 1131

1818,1812 1215 1716

Cr 960

744 Ni 718 Cu

7% Fig. 1.69. IR spectra of the cocondensation reaction products of transition metal atoms with pure CO2 at 15 K (M/CO2 1/1000) (bands arising from H2O at 1615 cm1 and CO2 have been omitted for clarity) [1120].

It exhibits the n(C¼O), n(CO), n(Ti¼O), and n(TiO) at 1776/1735, 1132, 955, and 455 cm1, respectively. No simple Ti(CO2) was detected in this case because it is unstable and its decomposition product, TiO, reacts with the second CO2 to form OTi(CO2). The cocondensation product of Fe atom with CO2 exhibits IR bands at 1565 (n3) and 1210 (n1) cm1, which suggests a C-bonded structure. On the other hand, the cocondensation product of Cu atom with CO2 exhibits the n(C¼O), n(CO), and d(OCO) at 1722/1716, 1215, and 718 cm1, respectively. The end-on structure of cisconformation shown above was proposed. Figure 1.69 shows the IR spectra of these and other cocondensation products at 15 K [1120]. Solov’ev et al. [1121] measured the IR spectra of the cocondensation products of pure CO2 with Mg atom vapor produced by thermal heating. Three bands were observed at 1580, 1385, and 866 cm1. Theoretical calculations suggested the formation of a four-membered ring in which the Mg atom is chelated to the two oxygen atoms of CO2. Le Quere et al. [1122] carried out cocondensation reactions of thermally produced Al atom vapor with CO2/Ar, and found two geometric isomers of Al(CO2) that are reversibly interconvertable. The low-temperature form takes an endon structure of Cs symmetry and exhibits the n(C¼O), n(CO), d(OCO), and n(AlO) at 1780, 1146.5, 773, and 468.5 cm1, respectively, whereas the high-temperature form takes a four-membered ring structure of C2v symmetry similar to that of Mg(CO2) mentioned above. It exhibits the na(CO), ns(CO), d(OCO), ns(AlO), and na(AlO) at 1443.5, 1265,5, 796,5, 428, and 213,5 cm1, respectively. In the case

NITROSYL COMPLEXES

155

of Ag atom with CO2, the CO2 vibrations are essentially unperturbed, indicating only a weak interaction [1123]. Reactions of laser-ablated rhenium atoms with CO2 in Ar produce ORe(CO), ORe(CO)2, O2Re(CO), O2Re(CO)2, and anionic species, [ORe(CO)] and [ORe (CO)2], and elucidation of their structures has been based on the results of isotope shifts, warmup experiments, and DFT calculations [1124]. Reactions of laser-ablated CuX2 (X ¼ Cl,Br) with CO2/Ar yield XCuOCO-type complexes, which are linear and exhibit the n(CO) vibration at 2383.9 and 2381.4 cm1, respectively, for X ¼ Cl, and Br [1125].

1.20. NITROSYL COMPLEXES Like CO, NO acts as a s-donor and a p-acceptor. The NO contains one more electron than CO, and this electron is in the 2pp* orbital. The loss of this electron gives the nitrosonium ion, (NO)þ, which is much more stable than NO. Thus, the n(NO) of the nitrosonium ion (2273 cm1) is much higher than that of the latter (1880 cm1). On the other hand, the addition of one electron to this orbital produces the (NO) ion, which is less stable and gives a lower frequency (1366 cm1) than does NO. Such a charge effect has already been discussed in Sec. 2.1 of Part A. In nitrosyl complexes, n(NO) ranges from 1900 to 1500 cm1. X-Ray studies on nitrosyl complexes have revealed the presence of linear and bent MNO groups:

In the valence-bond theory, the hybridizations of the N atom in (I) and (II) are sp and sp2, respectively. If the pair of electrons forming the MN bond is counted as the ligand electrons, the nitrosyl groups in (I) and (II) are regarded as NOþ and NO, respectively. Thus, one is tempted to correlate n(NO) with the charge on NO and the MNO angle. It was not possible, however, to find simple relationships between them since n(NO) is governed by several other factors (electronic effects of other ligands, nature of the metal, structure, and charge of the whole complex etc.) [1126]. According to Haymore and Ibers [1127], the distinction between linear and bent geometry can be made by using properly corrected n(NO) values; the MNO group is linear or bent, respectively, if this value is above or below 1620–1610 cm1. Several review articles are available for vibrational spectra of nitrosyl complexes [1126,1128–1132]. 1.20.1. Inorganic Nitrosyl Complexes Table 1.55 lists the vibrational frequencies of typical nitrosyl complexes. Although the MNO group is expected to show n(NO), n(MN), and d(MNO), only n(NO) have been observed in most cases. The latter two modes are often coupled since their frequencies are

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TABLE 1.55. Vibrational Frequencies of Inorganic Nitrosyl Complexes (cm1) Compound

n(NO)

[Mo(NO)5]5þ

1721 1860 1795 1912, 1816, 1675

Cr(CO)3 (NO)2 Co(CO)3 (NO) Mn(CO)4 (NO) Mn(PF)3 (NO)3 cis[MoCl4(NO)2]2 NiCl2(NO)2 [RuCl5(NO)]2 [RuBr5(NO)]2 [Tc(NO) (CNCMe3)5]2þ,a

1705 1822 1781 1836, 1744 1720, 1600 1872, 1842 1904 1870 1865

Cr(NO)4 Co(NO)3

n(MN)

d(MNO)

Ref.

650 —

496 —

1133 1134

665, 633, 560, 516, 380 — 609 524 — — — 606 572 —

470, 320, 186

1135

— 566 657 — — — 588 300 —

1136 1137 1138 1139 1140 1141 1142 1143 1144

a CNCMe3: tert-butyl isocyanide. This complex is formulated as [Tc(I)(NO)þ(CNCMe3)5]2þ because of its high n(NO).

close to each other. Jones et al. [1137] carried out a complete analysis of the vibrational spectra of Co(CO)3(NO) and its 13 C, 18 O, and 15 N analogs. According to Quinby-Hunt and Feltham [1145], vibrational spectra of a wide variety of nitrosyl complexes can be accounted for on the basis of the simple three-body (MNO) model as long as the complex does not contain two or more NO groups attached to the metal. Vibrational spectra of nitroprusside salts have been studied extensively [1146]. Khanna et al. [1147,1148] assigned the IR and Raman spectra of the Na2[Fe (CN)5(NO)]2H2O crystal and its deuterated analog. On the basis of a comparison of n(CN), d(FeCN), and n(FeCN) between the Fe(II) and Fe(III) complexes of the [Fe(CN)5X]n-type ions, Brown [1149] suggested that the FeNO bonding of the [Fe (CN)5(NO)]2 ion be formulated as Fe(III)NO and not as Fe(II)NOþ. Tosi and Danon [1150] studied the IR spectra of [Fe(CN)5X]n ions (X ¼ H2O,NH3,NO2 , NO, SO23 ). The n(CN) of the nitroprusside (2170, 2160, and 2148 cm1) are unusually high in this series because the FeCN p-backbonding in this ion is much less than in other compounds owing to extensive FeNO p-backbonding. Vibrational spectra of nitroprusside salts of various forms have been assigned [1151–1153]. The IR spectra of K3[Mn(CN)5(NO)] and its 15 NO analog have been reported [1154,1155]. According to X-ray analysis, RuCl(NO)2(PPh3)2PF6 contains one linear MNO group and one bent M—NO group that exhibit the n(NO) at 1845 and 1687 cm1, respectively [1156]. CoCl2(NO)L2 [L ¼ P(CH3)Ph2] exists in two isomeric forms:

NITROSYL COMPLEXES

157

The n (NO) of the former is at 1750 cm1, whereas that of the latter is at 1650 cm1 [1157]. The n(NO) of the bridging nitrosyl group is much lower than that of the terminal nitrosyl group. For example, (C5H5)2Cr2(NO)3(NXY) (X ¼ OH,Y ¼ t-Bu) shown below exhibits the terminal n(NO) at 1683 and 1625 cm1 and the bridging n(NO) at 1499 cm1 [1158].

Similar frequencies are reported for an analogous compound [X ¼ Et, Y ¼ B(Et)2] [1159]. The structure of M3(CO)10(NO)2 (M ¼ Ru,Os) resembles that of Fe3(CO)12 (structure VIII in Fig. 1.59) with double nitrosyl bridges in place of the double carbonyl bridges in the latter. As expected, n(NO) of these nitrosyl groups are very low: 1517 and 1500 cm1 for the Ru compound, and 1503 and 1484 cm1 for the Os compound [1160]. The bridging n(NO) of [Ru2(m-NO)2(bipy)4] (ClO4)2 is much lower (1363 cm1) than that of [Ru2(m-NO)2(acac)4] (1575 cm1), which contains formally neutral NO bridges [1161]. In [Cp*3 Fe2Mn(m-CO)2(m2-NO)(m3-NO)] shown above, the n(m2-NO) and n(m3-NO) are observed at 1518 and 1320 cm1, respectively [1162]. The n(NO) is spin-state-sensitive in Fe(NO)(salphen) [salphen - N,N0 -o-phenylenebis(salicylideneimine)]: 1724 cm1 for the high-spin (room temperature) and 1643 cm1 for the low-spin (liquid N2 temperature) states [1163]. Photolysis of Cr(NO)4 in Ar matrices produces Cr(NO)3(NO*) where NO* denotes a bent NO group with an unusually low n(NO) (1450 cm1) [1164]. Similar observations were made for the photolysis products of Mn(CO)(NO)3 [1165] and Ni(C5H5)(NO) [1166]. 1.20.2. NO Adducts of Metalloporphyrins Table 1.56 lists the n(NO) of metalloporphyrins in which the NO groups take linear geometry. In Fe(TPP)(NO)2, however, the two n(NO) bands at 1870 and 1690 cm1 have been assigned to the linear Fe(II)(N0)þ and the bent Fe(II)(NO), respectively [1168]. The low-frequency modes, such as n(MNO) and d(MNO), have been observed by RR studies (Soret excitation). For example, a “strapped” porphyrin, Mn(SP-15)(NO) (N-MeIm) (Fig. 1.64) exhibits the n(NO), n(MNO), and d(MNO) at 1727, 631, and 578 cm1, respectively [1173]. A simple porphyrin such as Mn(PPDME)(NO)

158

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.56. NO Stretching Frequencies of Metalloporphyrins (cm1) Compound

IR/Raman

n(NO)

Ref.

Cr(TPP) (NO) Mn(TPP) (NO) Fe(TPP) (NO) Ru(TPP)(NO)Cl Ru(TPP)(NO)(ONO) Fe(TPP) (NO)2

IR IR IR IR IR IR

1167 1167 1168 1169 1170 1168

Fe(PPDME) (NO) Fe(PPDME) (NO)-(N-MeIm) Fe(TPP) (NO) Fe(TPP) (NO)

IR IR RR (in THF) RR (in THF)

1700 1760 1700 1845 1852 1870 1690 1660 1676 1681 1496

a

1171 1171 1172 1172

PPDME ¼ protoporphyrin IX dimethyl ester.

(N-MeIm) (PPDME ¼ protoporphyrin IX dimethylester) shows the n(NO) and n(MNO) at 1733 and 628 cm1, respectively. Thus, introduction of steric hindrance lowers the n(NO) and raises the n(MNO). Lipscomb et al. [1174] observed the RR spectra of NO adducts of iron porphyrins by Soret excitation. The n[Fe(II)NO] and n[Fe(III)NO] are at 527 and 600 cm1, respectively. The NO adducts of heme proteins exhibit the n(NO) at 554 cm1, which is much higher than that of simple porphyrins (527 cm1). Thus, the cage effect of proteins raises the n(NO) in this case. It was also noted that the n[Fe(II)NO] is insensitive to the nature of the trans ligand. This is markedly different from the n[Fe(II)CO], n[Fe(II)O2], and n[Fe(II)CN], which are sensitive to the trans ligand. The d(MNO) vibrations were not observed for iron porphyrins. 1.20.3. Metastable States of Nitroprussides When a sample of Na2[Fe(CN)5(NO)]2H2O is irradiated by the 488.0-nm line of an Ar–ion laser at 20 K, two electronically excited metastable states (MS1 and MS2) are produced. G€ uida et al. [1175,1176] have measured the IR spectra of these metastable states using an orientated single crystal. The upper traces of Fig. 1.70 show the polarized IR spectra of the ground-state complex (GS) with the electric vectors parallel to the a and c axes, respectively. The n(NO) near 1950 cm1 is strong and broad in the former but rather weak in the latter, because the linear FeNO axis [1177] is on the ab plane of the orthorhombic crystal. The d(FeNO) and n(FeNO) bands are seen at 667 and 658 cm1, respectively. On irradiation at 20 K, two sets of new bands appear as shown in the lower traces of Fig. 1.70. The bands at 1834, 583, and 565 cm1 (marked by 1) are assigned to the n(NO), d(FeNO), and n(FeNO) of MS1, and those at 1663, 597, and 547 cm1 (marked by 2) are assigned to the corresponding modes of MS2. Thus, all three bands are shifted markedly in going from GS to MS1 and MS1 to MS2. Similar redshifts are observed for the n(CN) near 2150 cm1, although the magnitudes of their shifts are much smaller than those observed for the FeNO group vibrations. This result indicates

NITROSYL COMPLEXES

159

Fig. 1.70. Low-temperature polarized IR spectra of ground-state (top traces) and metastable (bottom traces) anions in Na2[Fe(CN)5NO]2H2O single-crystal plate that was cut along the 010 plane; 1 and 2 denote the peaks resulting from the MS1 and MS2 states, respectively [1196].

that the electronic transitions mainly involve the FeNO bond. The MS1 is stable below 200 K, whereas MS2 is stable only below 150 K. According to the X-ray analysis at 50 K by Carducci et al. [1178], the NO ligand in the MS1 state is a linear O-bonded isonitrosyl (FeON), and the results of polarized Raman studies by Morioka et al. [1179] are consistent with such a structure. Extensive IR and DFT studies including 14 N=15 N, 16 O=18 O, and 54 Fe=56 Fe isotope shift data by Aymonino and coworkers [1180,1181] also support this structure. On the other hand, X-ray analysis [1178] shows that the NO ligand in the MS2 state is side-on with the FeN, FeO, and NO distances of 1.89(2), 2.07(2) and 1.14(2) A, respectively. This structure was also supported by the IR studies cited above [1181]. Similar photo-induced isomerization occurs for Ru(II) complxes. X-Ray analysis of trans-K2[Ru(NO2)4(OH)(NO)] indicates that the NO ligand in the MS1 state takes an almost linear RuON structure (RuON angle, 169 ) while it takes a side-on structure in the MS2 state [1182]. IR studies [1183] show that the n(NO) at 1914 cm1 in the ground state is shifted to 1790 cm1 in the MS1 state. Although trans-[Ru (NH3)4(NO)(nicotinamide)]3þ shows that the n(NO) at 1974 cm1 (with a shoulder at 1918) in the ground state is shifted to 1826 cm1 in the MS1 state, it was not observed in the MS2 state [1184]. IR and Raman studies were also carried out on K2[RuCl5(NO)] [1185]. The n(NO) was observed at 1765 in the MS1 state and 1554/1550 cm1 in the MS2 state. Kawano et al. [1186] carried out X-ray analysis on trans-[Ru(en)2(H2O) (NO)]Cl3, and confirmed the RuON bonding in the MS1 state. These workers

160

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.71. Effect of heating on the IR spectrum of Na2[Os(CN)5NO]2H2O in the MS1 state, which was produced by irradiation with an Ar–ion laser (457.9 nm) at 80 K; 1 and 2 denote the n(NO) bands of the MS1 and MS2 states, respectively [1211].

assigned the n(RuO) and d(RuON) at 492 and 484 cm1, respectively. The corresponding n(RuN) and d(RuNO) in the ground state are observed at 590 cm1. IR/ Raman studies on metastable states are reported for trans-[Ru(Hox)(en)2(NO)]Cl2 (Hox ¼ oxalic acid monoanion) [1187] and cis- and trans-[RuX(en)2(NO)]X2 (X ¼ Cl, Br) [1188]. For [Ru(OEP)(O-iC5H11)(NO)], the n(NO) are at 1791, 1645, and 1497 cm1 for the ground, MS1, and MS2 states, respectively [1189]. In the case of the analogous osmium complex, Na2[Os(CN)5(NO)]2H2O [1190], the GS, MS1, and MS2 states exhibit the n(NO) at 1897, 1790, and 1546 cm1, respectively. The MS1 and MS2 states can selectively be populated by irradiating the sample with an Ar–ion laser (457.9 nm) and a mercury lamp (280–340 nm), respectively, at 80 K. Figure 1.71 shows the effect of heating on the IR spectrum of MS1 thus produced. It is seen that the onset decay temperature (T2) of MS2 (220 K) is higher than that of MS1 (T1, 190 K). This is opposite to the case of the nitroprusside discussed above. Furthermore, the population of the MS2 state begins to increase at the expense of the MS1 state near the decay temperature T1. The photoexcited MS1 state was also detected for [Ni(Cp(NO)]. The n(NO) of the ground and MS1 states are observed at 1820–1786 and 1576/1566 cm1, respectively [1191]. Coppens and co-workers [1192] reviewed photoinduced linkage isomers of transition metal nitrosyl complexes and other linkage isomers of di- and triatomic ligands such as N2, NO2, and SO2. G€ utlich et al. [1193] reviewed photoswitchable coordination compounds including Na2[Fe(CN)5(NO)]2H2O.

COMPLEXES OF DIOXYGEN

161

Fig. 1.72. Infrared spectra in the 1900–1570 cm1 region for laser-ablated chromium atoms codeposited with 0.4% NO in excess argon on a 10 K CsI window: (a) after 1-h sample codeposition at 10 K; (b) after annealing to 25 K; (c) after broadband photolysis for 30 min; (d) after annealing to 30 K; and (e) after annealing to 35 K. Labels in the diagrams employ NO or (NO) to denote Z1- and [NO] to denote Z2-bonding to Cr [1195].

1.20.4. Matrix Cocondensation Reactions Similar to the reactions of CO complexes discussed in Sec. 1.18.6, matrix cocondensation reactions of laser-ablated metal atoms with NO have been studied by Andrews and coworkers [1194]. As an example, Fig. 1.72 shows the IR spectra of Cr atoms cocondensed with 0.4% NO in excess Ar [1195]. The band assignments shown were based on the results of warmup experiments, isotope shifts, and DFT calculations. The bands at 1726.0, 1696.8, 1663.5, 1623.3, and 1614.3 cm1 were assigned to Cr (NO)4, [NO]Cr(NO), Cr(NO)3, Cr(NO)2, and Cr(NO), respectively. Here, (NO) and [NO] denote the end-on and side-on coordinations, respectively. Other products such as NCrO (976.1 and 866.2 cm1) and Cr[NO] (1108.8, 528.2, and 478.0 cm1) were identified.

1.21. COMPLEXES OF DIOXYGEN Dioxygen (molecular oxygen) adducts of metal complexes have been studied extensively because of their importance as oxygen carriers in biological systems (Sec. 3.2)

162

APPLICATIONS IN COORDINATION CHEMISTRY

and as catalytic intermediates in oxidation reactions of organic compounds. A number of review articles are available on the chemistry of dioxygen adducts [1126,1196– 1205]. As discussed in Sec. 2.1 of Part A, the bond order of the OO linkage decreases as the number of electrons in the antibonding 2pp* orbital increases in the following order: ½O2þ AsF6 Bond order Bond distance (A) 1 n(O2)(cm )

2.5 1.123 1858

K½O2

O2 > < >

2.0 1.207 1555

> < >

1.5 1.28 1108

Na2 ½O22 > < >

1.0 1.49 760

The decrease in bond order causes an increase in OO distance and a decrease in n(O2). In fact, there is a good linear relationship between the OO bond order and the n(O2) of these simple dioxygen compounds. Dioxygen adducts of more complex molecules are generally classified into two groups; complexes that exhibit n(O2) in the 1200–1100 cm1 region are called “superoxo” because their frequencies are close to that of KO2, and complexes whose n(O2) are in the 920–750 cm1 region are called “peroxo” because their frequencies are close to that of Na2O2. As will be shown later, many compounds exhibit n(O2) outside these regions. Thus, this distinction of dioxygen adducts is not always clearcut. Structurally, the dioxygen adducts are classified into four types:

In structure I, two oxygens are not equivalent, whereas they are equivalent in other structures. Thus, the n(16 O 18 O) vibration splits into two bands in I, but not in II, III and IV. 1.21.1. Dioxygen Adducts of Metal Atoms As stated in Sec. 1.26 of Part A, a number of stable and unstable complexes of the MLn type have been synthesized via matrix cocondensation reactions of metal vapor (M) with gaseous ligands (L). Table 1.57 lists typical results obtained for the M(O2)-type compounds. It is seen that the n(O2) of these dioxygen adducts scatter over a wide range from 1120 to 920 cm1. Previously, we noted that the n(O2) decreases as the negative

COMPLEXES OF DIOXYGEN

163

TABLE 1.57. Vibrational Frequencies of M(O2)-Type Compounds (cm1) Compound 6

LiO2 LiO2 NaO2 KO2 RbO2 CsO2 AgO2 RhO2 InO2 GaO2 AuO2 TlO2 PdO2 NiO2 FeO2 PtO2

7

n(O2)

ns(MO)

na(MO)

Ref.

1097.4 1096.9 1094 1108 1111.3 1115.6 1082/1077 900 1084 1089 1092 1082 1024.0 966.2 946 926.6

743.8 698.8 390.7 307.5 255.0 236.5 — — 332 380 — 296 427 504 — —

507.3 492.4 332.8 — 282.5 268.6 — 422 277.7 285.5 — 250 — — — —

1206 1206 1206 1206 1206 1206 1207 1208 1209 1209 1210 1211 1212 1212 1213 1212

charge on the O2 increases. Thus, these results seem to suggest that the negative charge on the O2 can be varied continuously by changing the metal. In fact, Lever et al. [1214] noted that there is a linear relationship between the electron affinity of the M2þ ion and the MO2 CT transition energy in the M(O2)2 series and that the latter is linearly related to the n(O2). The dioxygen ligand may coordinate to a metal in the end-on or side-on fashion. These two structures can be distinguished by using the isotope scrambling technique. Andrews [1215] first applied this method to the structure determination of the ion-pair complex Li þ O2 ; a mixture of 16 O2 , 16 O 18 O, and 18 O2 was prepared by Tesla coil discharge of a 16 O2 18 O2 mixture, and reacted with Li vapor in an Ar matrix. Three n(O2) were observed in the Raman spectrum:

This result clearly indicates side-on coordination since four bands are expected for end-on coordination (see above). Using the same technique, Ozin and co-workers [1210–1212] showed that, in all cases they studied, O2 coordinates to a metal in the side-on fashion and that, in M(O2)2 (M ¼ Ni,Pd,Pt), the complexes take the spiro D2d structure. Some metal superoxides and peroxides are prepared by ordinary methods, and their n(O2) are reported by Evans [1216] and Eysel and Thym [1217]. Andrews and coworkers prepared a variety of novel complexes via cocondensation reactions of transition metal vapors produced by laser ablation with O2 diluted by inert

164

APPLICATIONS IN COORDINATION CHEMISTRY

Ni Triplet O

Singlet O–58NiO 954.9(v3)

58Ni–O

825.7

Singlet NiOO 1221.3

Triplet—C2v Singlet

Singlet—D2h

Ni(O2) 967.1

58Ni

2(O2) 650.2

ONi(O2) 1095.5

Singlet Triplet—almost D2h symmetry O–58NiOO 1393.7

(O2)58Ni(O2) 1063.9

Singlet—almost C2v symmetry (O2)NiOO 1135.8

Fig. 1.73. Optimized structures and IR frequencies (cm1) [1218].

gases. As an example, Fig. 1.73 shows the structures and vibrational frequencies of nine species obtained by the reaction of Ni atom vapor with O2/Ar [1218]. Here, (O2) denotes the side-on Z2-bonding. It is interesting to note that NiO2 takes three isomeric structures, while NiO3 and NiO4 take two isomeric forms. 1.21.2. Dioxygen Adducts of Transition Metal Complexes There are many transition metal complexes containing superoxo and peroxo ligands. Examples of superoxo complexes are [Cr(Z1-O2)(H2O)5]2þ [1240] and [Sm(Z2-O2) (TpMe2)2] [TpMe2 ¼ HB(3,5-Me2pz)3] [1241]. The former exhibits the n(O2) and n(CrO) at 1166 and 503 cm1, respectively, whereas the latter is a rare example of the side-on-type superoxo complex with the n(O2) at 1124 cm1. Peroxo complexes take the symmetric side-on structure and exhibit the n(O2) in the 900–800 cm1 region and the na(MO2) and ns(MO2) in the 650–430 cm1 region. In general, the na(MO2) is higher than the ns(MO2). Table 1.58 lists the observed frequencies of typical peroxo complexes. Some of these assignments have been

COMPLEXES OF DIOXYGEN

165

TABLE 1.58. Observed Frequencies of Peroxo Complexes (cm1) Complex

n(O2)

n(MO2)

Ref.

(NH4)3[Ti(O2)F5] K2[Ti(O2)(C2O4)] Zr(O2)(H2EDTA) (NH4)2 [ZrO(O2)F2] (NH4)3 [Zr(O2)F5] K3[V(O2)4] K3[Ta(O2)F4] A3[PO4{WO(O2)2}4]a [Fe(EDTA)(O2)]3 (aq.) Na2[UO2(O2)(CO3)] Pt(O2) (PPh3)2

905 895 840 850 837 854 866 843 815 980 821

600, 530 611, 536 650, 600 640, 585 550, 471 620, 567 592, 518 591, 526 —, — 615, 550 460, 437

1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231

a

A: [N(C6H13)4]þ ion.

confirmed by 16 O2 =18 O2 substitution. Nakamura et al. [1232] have made nonnal coordinate calculations on peroxo complexes. According to X-ray analysis [1233], [Cu(HB(3,5-R2pz)3)]2(O2) (R ¼ i-Pr,Ph; pz ¼ pyrazole) contains a rare symmetric side-on bridge:

The OO distance (1.412 A) is typical of peroxo complexes, and all the CuO distances are essentially the same (1.90–1.93 A). The RR spectrum exhibits the n(O2) at 741 cm1, which is much lower than those shown in Table 1.58. This compound serves as a model for dioxygen binding in hemocyanin (Sec. 3.6). 1.21.3. Dioxygen Adducts of Cobalt Ammine and Schiff-Base Complexes Extensive vibrational studies have been made on dioxygen adducts of cobalt ammine and Schiff base complexes. Table 1.59 lists the n(O2) and n(CoO) of representative compounds. The n(O2) of dinuclear cobalt complexes such as {[Co(NH3)5]2O2}nþ (n ¼ 4 or 5) are markedly different depending on whether the O2 group is of superoxo or peroxo type. The n(O2) of the {[Co(NH3)5]2O2}5þ ion appears strongly in Raman spectra (1122 cm1) but is forbidden in IR spectra because the OO bridge is centrosymmetric. However, the n(O2) of a dibridged complex ion, [(NH3)4Co(NH2)(O2)Co (NH3)4]4þ, is observed as 1068 cm1 in IR spectra [1236]. N,N0 -Ethylenebis(salicylideniminato)cobalt, Co(salen), binds dioxygen reversibility in the solid state [1243]. Figure 1.74 shows the resonance Raman spectra of [Co (salen)]2O2 at 100 K [1239]. The bands at 1011 and 533 cm1 are shifted to 943 and 514 cm1, respectively, by 16 O2 18 O2 substitution, and thus assigned to the n(O2) and n(CoO), respectively. The former frequency is unique in that it is between those of

166

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.59. Vibrational Frequencies and Structures of Dioxygen Adducts of Cobalt Ammine and Schiff Base Complexes (cm1) Structure

n(O2)

n(CoO)

Ref.

Co(salen)(py)O2 {[Co(NH3)5]2O2}Cl53H2O {[Co(NH3)5]2O2}(NO3)5 K5{[Co(CN)5]2O2}H2O [Co(salen)]2O2

Superoxo end-on Superoxo end-on Superoxo bridging Superoxo bridging Superoxo bridging Superoxo bridging

1146 1144 1122 1122 1104 1011

— 527 620, 441 — 493 533

[Co(salen)(pyO)]2O2b [Co(salen)(py)]2O2 [Co(J-en)(py)]2O2 [Co(DMG)(PPh3)]2O2c K6{[Co(CN)5]2O2}H2O {[Co(NH3)5]2O2}(NO3)4 {[Co(NH3)5]2O2}(NCS)4

Peroxo bridging Peroxo bridging Peroxo bridging Peroxo bridging Peroxo bridging Peroxo bridging Peroxo bridging

910 884 841 818 804 805 786

535 543 562 551 602 642, 547 —

1234 1235 1236 1237 1238 1239 1240 1241 1240 1234 1242 1238 1236 1237

Compound Co(J-en)(py)O2a

a

J-en ¼ N,N 0 -ethylenebis(2,20 -diacetylethylideneaminato) anion. pyO ¼ pyridine N-oxide. c DMG ¼ dimethylglyoximato anion. b

superoxo and peroxo complexes. However, this band is shifted to the normal peroxo range when the base ligands are coordinated trans to the dioxygen. Evidently, electron donation from the base to the bridging dioxygen is responsible for the shift of n(O2) to a lower frequency.

When a Co Schiff base (SB) complex in a nonaqueous solvent absorbs oxygen in the presence of a base (B), the following equilibria are established: ½CoðSBÞB þ O2 Ð ½CoðSBÞBO2 þ ½CoðSBÞB Ð

½CoðSBÞBO2 ½CoðSBÞB2 O2

The n(O2) of the 1 : 1 (Co/O2) adduct is near 1140 cm1, whereas that of the 1 : 2 adduct is between 920 and 820 cm1. Using these bands as the markers, it is possible to examine the effects of oxygen pressure, temperature, and solvent polarity on the equilibria shown above. Figure 1.75 shows the RR spectra of Co(J-en) in CH2Cl2 containing pyridine that were saturated with oxygen at various oxygen pressures and temperatures. [1234]. It is seen that the concentration of the 1 : 1 adduct (1143 cm1)

COMPLEXES OF DIOXYGEN

Fig. 1.74. The RR spectra of (a) [Co(salen)]2 (579 nm excitation, 100 K) [1239].

16

167

O 2 , (b) [Co(salen)]2 18 O 2 , and (c) Co(salen)

increases and that of the 1 : 2 adduct (836 cm1) decreases as the oxygen pressure increases (A ! B) and as the temperature decreases (D ! C ! B). It was also noted that 1 : 1 adduct is favored in a polar solvent containing a relatively strong base. 1.21.4. “Base-Free” Dioxygen Adducts of Metalloporphyrins and Related Compounds Table 1.60 lists the structures and observed frequencies of “base-free” dioxygen adducts of metalloporphyrins that were prepared mostly by matrix cocondensation reactions at low temperatures. The n(O2) varies continuously from the superoxo to the peroxo regions. The n(O2) is the highest in Co(TPP)O2 (superoxo) and the lowest in [MoO(TPP)O2] (peroxo), although some complexes exhibit the n(O2) in the intermediate region. The n(O2) of “base-free” Co(TPP)O2 is 133 cm1 higher than that of

168

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.75. The RR spectra of Co(J-en) in CH2Cl2 containing 3% pyridine that was saturated with O2 at various O2 pressures and temperatures (580 nm excitation): (A) 1 atm, 78 C, (B) 3 atm, 80 C; (C) 3 atm, 30 C; (D) 3 atm, þ20 C, where S and py denote the solvent and pyridine bands, respectively [1234]. For the structure of J-en, see footnote a in Table 1.59.

“base-bound” Co(TPP)(1-MeIm)O2 (1143 cm1) discussed in the following section. A shift of similar magnitude (127 cm1) is observed in going from [Co(salen)]2O2 to its pyridine adduct. Evidently, these shifts are caused by the base ligands, which increase the negative charge on the dioxygen (“base ligand effect”). Table 1.60 also indicates the “metal ion effect”; the n(O2) is lowered and the mode of coordination is shifted from the end-on to the side-on as the metal ion is changed in the following order: CoðTPPÞO2 nðO2 Þ ðcm 1 Þ 1278 ðend-onÞ

>

FeðTPPÞO2 1195 ðend-onÞ 1106 ðside-onÞ

MnðTPPÞO2 > 983 ðside-onÞ

The IR spectra of Fe(TPP)O2 shown in Fig. 1.76 are of particular interest since it exhibits two n(O2) at 1195 and 1106 cm1, corresponding to the end-on and side-on isomers, respectively [1248].

169

COMPLEXES OF DIOXYGEN

TABLE 1.60. Structures and Observed Frequencies (cm1) of ‘‘Base-Free’’ Dioxygen Adducts Complex

Structure

Co(TPP)O2 Co(OEP)O2 Co(TMP)O2 Co(J-en)O2 Co(salen)O2 [Co(salen)]2O2 Fe(TPP)O2

End-on End-on End-on End-on End-on Bridging End-on Side-on End-on Side-on End-on Side-on Bridging End-on Bridging Bridging Side-on Side-on Side-on

Fe(OEP)O2 Fe(Pc)O2 Fe(salen)O2 [Fe(salen)]2O2 Ru(TPP)O2 [Ru(TPP)]2O2 [Os(TPP)]2O2 Mn(TPP)O2 Mn(Pc)O2 [MoO(TPP)O2] a

nð16 O2 Þ

nð18 O2 Þ

Da

n(MO2)

1278 1275 1270 1260 1235 1011 1195 1106 1190 1104 1207 1106 1001 1167 1114 1090 983 992 876

1209 1202 1200 1192 1168 943 1127 1043 1124 1042 1144 1043 943 1101 1057 1030 933 935 —

69 73 70 68 67 68 68 63 66 62 63 63 58 66 57 70 50 57 —

345 404

509

Refs. 1244 1245 1246 1234 1247 1239 1248,1238 1248

488

433 521 490

1248,1250 1248 1251 1252 1252 1252 1253,1254 1255 1256

D ¼ nð16 O2 Þ nð18 O2 Þ.

Table 1.60 reveals another interesting trend; the n(O2) becomes lower as the inplane ligand is changed in the order

nðO2 Þðcm 1 Þ

FeðPcÞO2 1207

FeðTPPÞO2 > 1195; 1106

FeðsalenÞO2 1106

Here, Pc denotes the phthalocyanato ion. This result indicates that the larger the pconjugated system of the in-plane ligand is, the less the negative charge on the dioxygen, and the higher the n(O2) (“in-plane ligand effect”). 1.21.5. “Base-Bound’’ Dioxygen Adducts of Metalloporphyrins “Base-bound” dioxygen adducts of metalloporphyrins are highly important as models of respiratory heme proteins (Secs. 3.1, 3.2). Several reviews [1257,1126,226] are available on vibrational spectra of base-bound dioxygen adducts of metalloporphyrins. Table 1.61 lists the n(16 O2 ), n(M 16 O2 ), and 16 O2 =18 O2 isotope shifts observed for end-on, base-bound dioxygen adducts. As mentioned previously, the n(O2) of these base-bound adducts are lower than those of the corresponding “base-free” adducts because of the “base ligand effect.” In a series of base-bound adducts such as Co(TPP-d8)(B)O2 (B ¼ nitrogen donor base), the n(O2) decreases linearly as the pKa of the base increases [1264]. Thus, the n(O2) is the highest (1167 cm1) for the most acidic base (4-cyanopyridine, pKa ¼ 1.90) and

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TABLE 1.61. Observed Frequencies of ‘‘Base-Bound” Dioxygen Adducts of Metalloporphyrins (cm1, in Solution) Compounda Co(TPP)(py)O2 Co(TPP)(pip)O2 Fe(TPP)(pip)O2 Co(TpivPP)(1,2-Me2Im)O2 Co(TpivPP)(1-MeIm)O2 [Co(TpivPP)(SC6HF4)O2] Fe(TpivPP)(l,2-Me2Im)O2 Fe(TpivPP)(1-MeIm)O2 [Fe(TpivPP)(SC6HF4)O2] a b c

nð16 O2 Þ

Dn(O2)b

n(M16 O 2 )

Dn(MO2)c

Ref.

1144 1142 1157 1153 — 1126 1159 — 1140

60 64 64 65 — 66 66 — 60

519 509 575 — 517 — — 568 —

21 20 24 — 23 — — 23 —

1258 1259 1259 1259 1259 1260 1261 1262 1263

Tpiv PP ¼ picket-fence porphyrin (see Fig. 1.64a). Dn(O2) ¼ nð16 O2 Þ nð18 O2 Þ. Dn(MO2) ¼ n(M16 O 2 ) n(M18 O 2 ).

Fig. 1.76. Infrared spectra of (A) Fe (TPP) in Ar matrix, (B) Fe(TPP) cocondensed with 16 O 2 =Ar (1/10), and (C) Fe(TPP) cocondensed with 18 O 2 =Ar (1/10) at 15 K [1248].

COMPLEXES OF DIOXYGEN

171

the lowest (1151 cm1) for the most basic base (4-dimethylaminopyridine, pKa ¼ 9.70). If the pKa value is regarded as a rough measure of s-donation, this result suggests that the n(O2) is governed largely by the degree of s-donation of these bases. The n(O2) is shifted markedly to lower frequency when a thiolate ligand such as SC6 H5 coordinates to a metal. For example, the n(O2) of the [Co(TPP)(SC6H5)O2] ion (1122 cm1) is 21 cm1 lower than that of Co(TPP)(py)O2 (1144 cm1) in the same solvent (CH2Cl2) [1265]. As shown in Table 1.61, a similar downshift is noted for Fe(II) porphyrins. Since the pKa of SC6 H5 (6.5) does not differ appreciably from that of py (5.25), the observed shift must be attributed largely to an increase in p-donation. This p-donation is promoted by two factors: (1) relative to py, the thiolate ligand possesses an extra electron on the 2p orbital that overlaps on the dp orbital of the metal, and (2) the SC6 H5 ligand tends to take an orientation that maximizes the pp dp overlap and minimizes the steric repulsion from the mesophenyl groups. As discussed in Sec. 3.3.2, the active site of cytochomes P450cam is an iron porphyrin that has the mercaptide sulfur of a cystenyl residue as an axial ligand. When O2 coordinates to the axial position trans to the sulfur, it exhibits the n(O2) at 1140 cm1. Matsu-ura et al. [1266] synthesized a novel model compound of cytochrome P450, a twin-coronet porphyrin with a thioglycolate group. The hydrogenbonded O2 adduct of this model compound exhibits the n(O2) at 1137 cm1, which is shifted to 1073 cm1 by 16 O2 =18 O2 substitution. Table 1.61 also shows the ‘‘metal-ion effect”—the n(O2) of the Fe(II) adduct is higher than that of the corresponding Co(II) adduct. Although this is opposite to the case of base-free adducts discussed previously, the metal ion effect on the n(MO2) is the same in both cases; the n(FeO2) is always higher than the n(CoO2). These results can be accounted for in terms of the following bonding schemes [1251]:

When the O2 is bound to Co(II) (d7), the CoO2 bond is formed mainly by s-donation from Co(dz2) to the antibonding O2 (p*g ) orbital [i.e., Co(III)O2 ]. In the case of Fe(II) (d 6), however, the FeO2 bond is formed by s-donation from O2 (p*g ) to Fe(dz2), which is counteracted by a stronger p-donation in the opposite direction. This would strengthen the FeO2 bond and weaken the OO bond relative to those of the corresponding Co(II) adduct. In fact, “base-free” Fe(II) adducts exhibit lower n(O2) and higher n(MO2) than do the corresponding Co(II) adducts. In base-bound Co(II) complexes, s-donation from die base ligand causes a marked increase in the negative charge on O2, thus causing a large downshift in n(O2) relative to base-free complexes. However, the base ligand effect is much smaller in Fe(II) complexes because s-donation from the base is opposed by s-donation from the O2. As a result, the

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Fig. 1.77. Structure of O2 adducts of ‘‘picket-fence’’ Co(II) porphyrin and its derivative. See also Fig. 1.64a.

O2 in Fe(II) adducts is less negative than that of Co(II) complexes. Thus, the n(O2) of Fe(II) adducts become higher than those of Co(II) adducts. The n(FeO2) is always higher than the n(CoO2) because of the multiple bond character of the former. It is well established that the O2 bound to myoglobin and hemoglobin is stabilized by forming the NH O2 hydrogen bond with the distal imidazole of the peptide chain (Sec. 3.2). To mimic this “cavity effect,” “protected” porphyrins such as “picketfence” and “strapped” porphyrins (Fig. 1.64) have been prepared. Figure 1.77 compares the structures of the pickets in picket-fence porphyrin, Co(a4-TpivPP) and its derivative, Co(a4-TneoPP). It was found that the n(O2) decreases in the following order: nðO2 Þðcm 1 Þ

CoðTPP-d8 ÞðBÞO2 1167

>

CoðTpiv PPÞðBÞO2 1161

>

CoðTneo PPÞðBÞO2 1148

Here, B is 4-cyanopyridine and the spectra were measured in toluene [1264,1267]. Presumably, the NH O2 hydrogen bonding is weaker in Co(TpivPP) than in Co (TneoPP) because the repulsive force between the C(CH3)3 group and the bound dioxygen tends to push the pivalamide group outward. This repulsive force would be decreased when the pivalamide group is replaced by the neo-pentylcarboxamide group (Tneo). Odo et al. [1267] carried out an extensive RR study on n(O2) of Co(II) complexes of a variety of picket-fence porphyrins.

COMPLEXES OF DIOXYGEN

173

Fig. 1.78. Structure of ‘‘Pillard’’ dicobalt cofacial diporphyrin.

Pillard dicobalt cofacial diporphyrin, shown in Fig. 1.78, is another type of modified porphyrin which is of great interest in structural chemistry. Since a large base ligand such as 4-(dimethylamino)pyridine cannot enter the inter-porphyrin cavity, it forms a bridging O2 adduct that exhibits the n(O2) at 1098 cm1 (superoxo type) in RR spectra. This band is not observed if a small base such as g-picoline is first added to the CoCo complex solution and then the solution is oxygenated. This result indicates that a small base occupies the interporphyrin space so that formation of the CoO2Co bridge is blocked [1268]. However, the 1098-cm1 band is observed if the CoCo complex solution is first saturated by O2 and then a small base is added. Apparently, the CoO2Co bond once formed is too stable to be cleaved by the addition of a base ligand. The RR spectrum of Co(TPP-d8)(py)O2 in CH2Cl2 exhibits a single n(O2) band at 1143 cm1. However, this band becomes a doublet (1155 and 1139 cm1) when 1,2dimethylimidazole is used as the base. An extensive study involving a variety of base ligands [1258] has shown that vibrational coupling between n(O2) and a nearby base ligand vibration of the same symmetry is responsible for the observed doublet structure and resonance enhancement. In the case of Co(TPP-d8)(py)18 O2 in CH2Cl2, a weak py band appears at 1067 cm1 in addition to the n(18 O2 ) at 1084 cm1. The 1067 cm1 band of py is not observed in the case of the 16 O2 adduct. This is another example of vibrational coupling between the py mode and the n(18 O2 ) [1258]. Similar vibrational couplings were noted between the n(O2) and the axial ligand (3.5-dichloropyridine) for three picket-fence Co(II) porphyrins [1269]. The internal mode of a solvent molecule can also be resonance-enhanced via a similar mechanism [1264]. As shown in Fig. 1.79 the RR spectrum of Co(TPP-d8)(py) O2 in toluene exhibits two strong bands at 1160 and 1151 cm1 where the n(16 O2 ) band is expected (trace A). This doublet structure does not appear in toluene-d8 (trace C) and is not observed in the n(18 O2 ) region (trace B). Toluene exhibits three bands at 1210 (T1), 1178 (T2), and 1155 cm1 (T3) with an intensity ratio of approximately 6 : 1 : 1

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Fig. 1.79. The RR spectra of Co(TPP-d8) in toluene containing 3% pyridine at 85 C under 4 atm O2 pressure: (A) 16 O2 ; (B) 18 O2 ; (C) 16 O2 in toluene-d8 [1264].

(trace B). Thus, it is reasonable to attribute the observed splitting to a strong vibrational coupling between n(16 O2 ) and T3, which are very close in frequency. If n(16 O2 ) is shifted between T2 and T3 by using a weaker base (4-cyanopyridine), both internal modes of toluene are resonance-enhanced, as seen in Fig. 1.80. In this case, the magnitudes of frequency perturbation and resonance enhancement are less, relative to the previous case, since n(16 O2 ) is further from the solvent modes. As seen in Fig. 1.80, the multiple structure observed for Co(TPP-d8) disappears completely when picket-fence porphyrin, Co(TpivPP), is employed. This result indicates that the vibrational coupling observed for “unprotected porphyrin” cannot occur in picket-fence porphyrin because the four pivaloyl groups prevent the access of toluene to bound dioxygen. Thus, not only “frequency matching” but also “direct O2 solvent association” is necessary to cause such vibrational coupling. Vibrational couplings between n(O2) of bound dioxygen and internal modes of base ligands and/or solvents have been found in many other systems [1270,1271]. Thus, RR spectra of dioxygen adducts of metalloporphyrins must be interpreted with caution. Proniewicz and Kincaid [1272] carried out quantitative treatments of these vibrational couplings using a Fermi resonance scheme.

METAL COMPLEXES CONTAINING OXO GROUPS

175

Fig. 1.80. The RR spectra of (A) Co(TPP-d8) and (B) Co(TpivPP) in toluene containing 3% 4-cyanopyridine at 85 C under 4 atom O2 pressure [1264].

1.22. METAL COMPLEXES CONTAINING OXO GROUPS 1.22.1. Metal Complexes Containing Monooxo Groups There are many compounds containing monooxo groups (M¼O) in which relatively heavy metal atoms are bonded to oxygen via double bonds. In most cases, their n(M¼O) vibrations can be assigned without difficulty since they are relatively free from vibrational couplings and appear strongly in the 1100–900 cm1 region of IR spectra. Examples of M¼O group vibrations in inorganic compounds are found in the n(ZX) vibrations of ZXY3 (Table 2.6g), ZXY4 (Table 2.7b), and ZXY5 (Table 2.8c)type compounds discussed in Part A. Table 1.62 lists the n(M¼O) of metal complexes containing monooxo groups. A new type of isomerism involving monooxo groups was found by Wieghardt et al. [1277]. For example, the crystals of [PF6][W(O)LCI2] (L ¼ a tridentate ligand) can be obtained in the blue and green forms. X-Ray analyses show that the structures of these two forms are identical except for the W¼O and WN (trans to W¼O) distances; the W¼O bond length in the blue form (1.72 A) is shorter than that in the green form (1.89 A). Correspondingly, the n(W¼O) of the former (980 cm1) is higher than that of the latter (960 cm1). Similar isomerism has been reported for complexes containing the Nb¼O and Nb¼S groups [1278]. Thus, the yellow form of Nb(O)Cl3(PMe3)3 exhibits the

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APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.62. Vibrational Frequencies (cm1) of Monooxo Complexes Complex

Oxo Group a

(VO)(TBP)8(CzH) (MnO)(TBP)8(Cz)a H(TcO)(cys)2c [(ReO)(cys)2],c [(RhO)(bipy)2(py)]3þ

Ref.

n(M¼O) b

V(IV)¼O Mn(V)¼O Tc(V)¼O Re(V)¼O Rh(V)¼O

976(939) 979(938)b 940 969 845

1273 1274 1275 1275 1276

a

(TBP)8(Cz) ¼ octakis(p-tert-butylphenyl)corroiazine. nðM ¼ 18 OÞ: c Cys ¼ cysteine. b

n(Nb¼O) at 882 cm1 (Nb¼O distance, 1.78 A), whereas its green isomer shows it at 871 cm1 (Nb¼O distance, 1.93 A). In Nb(S)Cl3(PMe3)3, the orange form exhibits the n(Nb¼S) at 455 cm1 (Nb¼S distance, 2.196 A), while the green form shows it at 489 cm1, although its Nb¼S distance (2.296 A) is longer than that of the orange form. The origin of this anomaly is not clear. Moreover, the origin of this new type of isomerism (“bond-stretch” isomerism) is not understood. 1.22.2. Metal Complexes Containing Dioxo Groups* As stated in Sec. 2.2 of Part A, the dioxo groups (O¼M¼O) such as Mo(O)2, Ru(O)2, W(O)2, Re(O)2, Os(O)2, and U(O)2 exhibit strong- to medium-intensity IR bands in the 1100–850 cm1 region. Although the trans (linear) dioxo group exhibits only the na(O¼M¼O) vibration in IR spectra and only the ns(O¼M¼O) vibration in Raman spectra, the cis (bent) dioxo group is expected to show both vibrations in either spectra [1279]. Thus, trans-[Os(O)2(bipy)2] (bipy ¼ 2,20 -bipyridine) exhibits only one band at 872 cm1, whereas its cis-isomer shows two bands at 833 (ns) and 863 cm1 (na) in IR spectra [1280]. However, the trans-[Re(O)2(py)4]þ ion exhibits both symmetric and antisymmetric n(O¼Re¼O) at 907 and 822 cm1, respectively, in RR spectra (CH3CN solution) [1281]. The reason for this anomaly is not clear. The cis-V(O)2 groups show the ns and na(O¼Ru¼O) at 922–910 and 907–876 cm1, respectively [1282]. Similar results are reported for cis-Mo(O)2 [1283–1285] and cis-W(O)2 groups [1286,1287]. In the case of Ru(TPP)(O)2, the ns(O¼Ru¼O) vibration is observed at 808 cm1 in RR spectra in solution [1288]. The corresponding na(O¼Ru¼O) vibration appears at 821 cm1 in IR spectra [1289]. More recent references on dioxo complexes are summarized in Table 1.63. 1.22.3. Metal Complexes Containing Oxo Bridges If the oxo bridge (MOM) is linear, the na(MOM) is only IR-active and the ns(MOM) is only Raman-active. Although both become IR- and Raman-active in a bent geometry, the former is stronger in IR whereas the latter is stronger in Raman spectra. Table 1.64 lists the structures and observed frequencies of monooxo bridged complexes. To avoid confusion with dioxygen adducts (MO2), the dioxo groups are written as M(O)2.

METAL COMPLEXES CONTAINING OXO GROUPS

177

TABLE 1.63. IR Frequencies of Dioxo Complexes (cm1) Complex

cis/tran þ

[Re(O)2(P(CH2OH)3))] [Re(O)2(pyz)4]þ,a [Os(O)2(N2H2C2O2)2]2 [U(O)2F6]4 [V(O)2(HL)],b [Re(O)2(OTeF5)3] [Re(O)2F4] [Os(O)2F4] a b

na(OMO)

trans trans trans trans cis cis cis cis

ns(OMO)

880 810 852 913 882 978 973 930

Ref.

— — 900 (Raman) 951 (Raman) 918 1022 1011 940

1290 1291 1292 1293 1294 1295 1296 1297

pyz ¼ pyrazine. HL ¼ Schiff base ligand containing alkoxo group.

Figure 1.81 illustrates three types of monooxo bridges. Structures II and III contain one and two m-carboxylato groups in addition to single monooxo bridge. Structure III is important as a model compound of hemerythrin and other metalloproteins (Sec. 3.5). Sanders–Loehr et al. [1307] carried out a systematic study on electronic and RR spectra of oxo-bridged dinuclear Fe(III) complexes in proteins and their model compounds. As shown in Appendix VII of Part A, the FeOFe angle can be calculated using the observed values of na(FeOFe) and ns(FeOFe). These workers obtained excellent agreement between the observed (X-ray) and calculated FeOFe angles. They also noted that the molar Raman intensity of ns(FeOFe) is much larger in proteins than in model compounds, and suggested several possible reasons for this phenomenon. Bridging dioxo(m-O)2 and trioxo(m-O)3 complexes contain the structures shown below:

TABLE 1.64. Structures and Vibrational Frequencies (cm1) of Monooxo-Bridged Complexes Compound [Fe2(m-O)(TPP)2] [Fe2(m-O)(OEC)2] [Cr2(m-O)(TPP)2] {Cr(TPP)(m-O)Fe(TPP)} [Fe2(m-O)(H2O)10] 4þ [V2(m-O)(L-Hist)4] 2H2O [Fe)2(m-O)(mCH3CO2)2L2]2þ,a [Fe2(m-O)(m-CH3COO)2L0 2] b [V2(m-O)((m-CH3COO)2L0 2] b a

L ¼ tacn ¼ 1,4,7-triazacyclononane. L ¼ HB(pz)3 ¼ tris(1-pyrazolyl)borate ion.

b 0

Structure

na

ns

Ref.

Linear FeOFe Linear FeOFe Linear CrOCr Linear CrOFe Linear FeOFe Bent VOV Bent FeOFe Bent FeOFe Bent VOV

885 872 860 843 840 730 730 754 685

363 400 — — — 436 — 530 536

1298 1299 1300 1301 1302 1303 1304 1305 1306

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APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.81. Structures of three types of oxo bridges [1307].

For example, [AuIII2 (bipy)2(m-O)2]2þ [1308] and [(CuIII2(Sp)2(m-O)2]2þ [1309] exhibit the n(M2O2) at 682(662) and 619(591) cm1, respectively. Here the numbers in brackets indicate the frequencies of the corresponding 18 O vibrations, and Sp is () sparteine. Trioxo bridging vibrations are reported for [CrIII3(C2H5CO2)6F3(m-O)3]2 [1310]. The na(CrO3) and ns(CrO3) are at 663 and 165 cm1, respectively. 1.22.4. Oxoferrylporphyrins and Related Complexes Another type of monooxo comolex is oxoferrylporphyrin. As stated in Sec. 1.21.4, the O2 adducts of five-coordinate, base-free porphyrins such as Fe(TPP)O2 were prepared via matrix cocendensation reaction, and assignments of their IR spectra were based on 16 O2 =18 O2 isotopic shifts. During the measurements of the corresponding RR spectra in pure O2 at 15 K, Bajdor and Nakamoto [1311] observed the appearance of a new band at 853 cm1 on laser irradiation (406.7 nm, 1–2 mW), and noted that the intensity of this band peaks after 20 min. As shown in Fig. 1.82, this band is shifted to 818 cm1 by 16 O2 18 O2 substitution. Similar experiments with scrambled dioxygen (16 O2 =16 O 18 O 18 O2 ffi 1=2=1) produce only two bands at 852 and 818 cm1. These results clearly indicate that the bands at 852 and 818 cm1 are due to the n(Fe ¼16 O) and n(Fe18 O), respectively, of FeO(TPP), which were formed by the cleavage of the bound dioxygen in Fe(TPP)O2. 54 Fe56 Fe substitution experiments further confirmed these assignments. A simple diatomic approximation gives a FeO stretching force constant of 5.32 mdyn/A , which is much larger than that of the FeO bond in [Fe (TPP)]2O (3.8 mdyn/A) [1298]. A more detailed study by Proniewicz et al. [1312]

METAL COMPLEXES CONTAINING OXO GROUPS

179

Fig. 1.82. The RR spectra of Fe(TPP) cocondensed with O2 at 15 K (406.7 nm excitation): (A) Fe ðTPPÞ with 16 O 2 ; (B) NA Fe ðTPPÞ with 18 O 2 , and (C) NA Fe ðTPPÞ with isotopically scrambled O2. The broken lines in (A) and (B) denote the spectra of 54 Fe ðTPPÞ cocondensed with respective gases. All the spectra in (A), (B), and (C) (solid line) were obtained after 20-min laser irradiation. The dotted line in (C) indicates the spectrum obtained only after 3-min laser irradiation. NA Fe (Fe in natural abundance) contains 92% 56 Fe [1311]. NA

shows that the Fe atom in oxoferryl porphyrin is Fe(IV) and low-spin, and that the FeO 2 bond should be formulated as FeðIVÞ ----- O . Here, the arrowed line indicates a s-bond formed via the dz2–pz overlap and the broken lines represent two p-bonds formed via the dxz–px and dyz–py overlaps. It is conventionally written as Fe¼O. Similar experiments readily produced FeO(OEP) and FeO(salen) but not FeO(Pc). These results suggest that the OO bond strength decreases in the order Fe(Pc) O2 > Fe(TPP)O2 > Fe(salen)O2 as indicated in their n(O2) (Table 1.60).

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APPLICATIONS IN COORDINATION CHEMISTRY

n=3

n=2 dxz ,dyz

Π*

t2g Px,Py dxy 02– (a) 1200 cm-1

Π

1007

v(M4+ 02–)

1025 852 754

600

(b)

1200 cm-1

V 4+

Cr 4+

1017

1049

Mn4+

Fe4+ v(M5+ N3–)

876

600

(c) Cr 5+

Mn5+

Fe5+

Fig. 1.83. (a) Electron configuration of M4þO2 porphyrin; (b) variation of n(M4þO2) in M4þO2 porphyrins; (c) variation of n(M5þN3) in M5þN3 porphyrins [1314,1315].

Oxoferrylporphyrin, FeO(TMP) (TMP ¼ tetramesitylporphyrin), can also be produced by electrooxidation of Fe(TMP)(OH) in CH2Cl2 at 40 C. According to Czernuszewicz and Macor [1313], it exhibits the n(Fe¼O) at 841 cm1. Cooling is necessary because it is unstable and readily reacts with CH2Cl2 to form Fe(TMP)Cl at higher temperature. The n(M¼O) of stable oxo porphyrins are known for TPP(V¼O) (1007 cm1), TPP (Cr¼O) (1925 cm1), and (TPP)(Mn¼O) (754 cm1). Figure 1.83a is a simple molecular orbital (MO) diagram showing the relationship between the n(M¼O) and the d-electron configuration [1314]. As seen in Fig. 1.83b, the d electrons enter the nonbonding (dxy) orbital in V(IV) (d1) and Cr(IV) (d2) but antibonding orbitals (dxz and dyz) in Mn(IV) (d3) and Fe(IV) (d4) thus reducing the MO bond order from 3 to 2. As a result, the n(M¼O) drops abruptly in going from V(IV) and Cr(IV) to Mn(IV) and Fe (IV). In the Mn(IV) complex, the dxy orbital is raised near the dxz and dyz orbitals because of the special stability of the half-filled t2g subshell, resulting in the high-spin (dxy)1 (dxz)1 (dyz)1 configuration, and this is reflected in the n(M¼O) frequencies. The n(Mn¼O) is lower than the n(Fe¼O) primarily because of the higher effective nuclear charge on Fe(IV) relative to Mn(IV).

METAL COMPLEXES CONTAINING OXO GROUPS

181

1.22.5. Oxoferrylporphyrin p-Cation Radicals As shown in the preceding section, oxoferrylporphyrins such as (TPP)Fe¼O are formed by laser irradiation of (TPP)FeO2 in O2 matrices. A more detailed RR study by Proniewicz et al. [1316] revealed simultaneous formation of its p-cation radical. As shown in Fig. 1.84, (TPP)Fe¼16 O exhibits seven bands (shown shaded) that are sensitive to 16 O/18 O isotope substitution. The bands at 1195 (1129) and 1105 (1043) cm1 are due to the n(O2) of the end-on and side-on isomers of (TPP)FeO2, respectively (Sec. 1.21.4). Here, the numbers in brackets indicate the corresponding 18 O frequencies. Three bands at 508(487), 349(345) and 408(402) cm1 are also 16 O/ 18 O isotope-sensitive. The former two were assigned to the n(FeO2) and d(FeOO) of the end-on isomer, respectively, whereas the last band was assigned to the ns(FeO) of the side-on isomer. In the high-frequency region, two isotope-sensitive bands are

Fig. 1.84. RR spectra of Fe(TPP) cocondensed with dioxygen at 30 K (406.7 nm excitation): (A) 16O 2 ; (B) 18O 2 ; (C) scrambled O2 (16O 2 : 16O 18O : 18O 2 ¼ 1 : 2 : 1); (D) 54 Fe =16 O 2 ; (E) 54 Fe = 18 O 2 . Shaded bands are oxygen-isotope-sensitive.

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observed at 853(818) and 815(780) cm1. Although the former stronger band was previously assigned to the n(Fe¼O) of (TPP)Fe¼O, the nature of the latter, weaker band was not clear. It must be another n(Fe¼O) band since the magnitude of its isotope shift (35 cm1) is close to that of the former (37 cm1). To find out the difference between these two species, the RR intensities of (TPP-d8)Fe¼O were measured as a function of laser power as well as of laser irradiation time. It was found that the intensity of the former band at 853 cm1 increases rapidly as the laser power increases and reaches a maximum value after 30 min (laser power, 8.0 mW), whereas that of the latter band at 815 cm1 reaches a maximum at 3.5 mW, and then decreases exponentially with time. On the basis of these differences, the 815 cm1 band was attributed to the p-cation radical, (TPPþ-d8)Fe¼O, which was formed as an intermediate species during photocleavage of the O¼O bond. As will be shown in Sec. 3.3.3, such a p-cation radical serves as a model compound of HRP compound I. Hashimoto et al. [1317] first generated O¼Fe(TMP(þ) in CH2Cl2 solution mixed with CH3OH and observed the n(Fe¼O) at 828 cm1. On the other hand, Kincaid et al. [1318] observed it at 802 cm1 in CH2Cl2. Later, the former band was reassigned to the six-coordinated species with CH3OH as the axial ligand, whereas the latter band at 802 cm1 was reassigned to the six-coordinate species with Cl as the axial ligand [1319]. Since the n(Fe¼O) of the five-coordinated species, FeO(TPP,þ) in O2 matrices [1316] was observed at 815 cm1, the shift from 815 to 802 (13 cm1) was attributed to the trans-effect of the axial ligand on the Fe¼O moiety. This trans effect was confirmed in a series of FeO(TMP,þ)L-type complexes where the n(Fe¼O) band is shifted from 828 to 801 cm1 by changing L from CH3OH to m-CPBA (m-chloroperoxybenzoic acid). Czarnecki et al. [1319] conducted an extensive study using a variety of L, and observed the n (Fe¼O) near 835 cm1 for L ¼ ClO4 and CF3SO3, and near 800 cm1 for L ¼ F Cl, and m-CPBA. Their results clearly indicate that the electron-donating capabilities of the latter group are greater than that of the former group. Metalloporphyrin p-cation radicals can take either the 2A1u or 2A2u ground state because the two highest occupied orbitals are of a1u or a2u symmetry, which is nearly degenerate under D4h symmetry (Sec. 1.23 of Part A). Figure 1.85 shows the

Fig. 1.85. The atomic orbital (AO) structure of Mg (Por) in the two highest-occupied orbitals; the circle sizes are approximately proportional to the AO coefficients; the open circles represent negative signs of the upper lobe of the pp AOs [1341].

COMPLEXES OF DINITROGEN AND RELATED LIGANDS

183

coefficients of the atomic pz orbitals represented by the size of the circles [1320]. The open circle represents the negative sign of the upper lobes of the pz atomic orbital. It is seen that the CbCb bond is antibonding in the a1u orbial, whereas it is bonding in the a2u orbital. As shown in Table 1.10, the n2(A1g) is due to the n(CbCb). Therefore, this vibration should be downshifted in the 2A2u radical and upshifted in the 2A1u radical. It was concluded Ni(TPP), Cu(TPP), and ClFe(TPP) produce the A2u type, whereas Ni (OEP) and ClFe(OEP) form the A1u type [1320,1321]. VO(OEP) [1322,1323] also forms an A1u radical because its n2 is upshifted from 1580 to 1601 cm1. In general, TPP and OEP complexes form p-cation radicals of A2u and A1u types, respectively. The n(V¼O) near 1000 cm1 is insensitive to the radical type [1345]. Kincaid et al. [1324] concluded that six-coordinate FeO(TMPþ)L is of A2u type because both n2 and n4 are downshifted by 30 and 10 cm1, respectively, on radical formation [1318]. However, the A1u-type symmetry was found for FeO(TMTMPþ) [1324]. Here, TMTMP is tetramethyl–tetramesitylporphyrin. p-Cation radical types can also be differentiated by comparing the intensity of IRactive Eu modes. For example, the n(CaCm)a (n37) band of Cu(OEP) at 1551 cm1 markedly increases its intensity on radical formation. In contrast, the corresponding band at 1574 cm1 of Cu(TPP) shows almost no increase in intensity on radical formation. This result suggests that the changes in the du/dQ terms (Sec. 1.19 of Part A) are markedly different depending on the radical type [1325]. Resonance Raman studies on oxofenyl porphyrins and their p-cation radicals have been reviewed by Kitagawa and Mizutani [1326] and Nakamoto [1327].

1.23. COMPLEXES OF DINITROGEN AND RELATED LIGANDS 1.23.1. Dinitrogen Complexes of Transition Metals Since Allen and Senoff [1328] prepared the first stable dinitrogen (molecular nitrogen) compounds, [Ru(N2)(NH3)5]X2 (X ¼ Br, I, BF4 , etc.), a large number of dinitrogen compounds have been synthesized. The chemistry and spectroscopy of these compounds have been reviewed extensively [1126,1329–1332]. The structures of dinitrogen compounds are classified into three types:

The terminal end-on coordination is most common. The MN2 bonding is interpreted in terms of the s-donation and p-backbonding, which were discussed in Secs. 1.16 and 1.18. Since N2 is a weaker Lewis base than CO, p-backbonding may be more important in nitrogen complexes than in CO complexes [1333]. Free N2 exhibits n(N N) at 2331 cm1, and this band shifts to 2220–1850 cm1 on coordination to the metal. Table 1.65 lists the n(N N) of typical complexes. The n(N2) of the Fe(0) complex

184

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.65. Observed N N Stretching Frequencies (cm1) Complex Fe(N2)(DPE)2a

[FeCl(N2)(DPE)]þ,a [Ru(N2)(NH3)5]Br2 [Ru(N2)(NH3)5]I2 [Os(N2)(NH3)5]Cl2 [OsH(N2)(PPh2OCH3)4]þ Co(N2)(PPh3)3 Co(N2)H(PPh3)3 Ir(N2)Cl(PPh3)2 Ir(N2)Cl(H)(PPh3)2(BF4) trans-Mo(N2)2(DPE)2a cis-W(N2)2(PMe2Ph)4 Co(N2)(PR3)ðPR2 Þ2b 2 [Rh(I)(N2)2]þ,c

n(N N) 1955 2088 2105 2124 2022, 2010 2197 2093 2105 2105 2229 1970 (2020) 1998, 1931 1904 1864 2244. 2218

Ref. 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347

a

DPE ¼ Ph2P–(CH2)2–PPh2. R ¼ n-Bu or Ph. On zeolite surface.

b c

[1334] is much lower than that of the Fe(II) complex [1335], indicating stronger p-backdonation of the former to the N2 ligand. Very little information is available for n(M—N2) and d(MN N) in the lowfrequency region. Allen et al. [1333] assigned n(RuN2) of [Ru(N2)(NH3)5]2þ-type compounds in the 508–474 cm1 region, whereas other workers [1336,1337] attributed these bands to d(RuN N). Figure 1.86 shows the infrared spectrum of [Ru(NH3)5N2]Br2 obtained by Allen et al. According to Srivastava and Bigorgne [1348], Co(N2)H(PPh3)3 exists in two forms in the solid state; one form exhibits n(N N) at 2087 cm1, and the other shows two bands of equal intensity at 2101 and 2085 cm1. However, their structural differences are unknown. Darensbourg [1349] obtained a linear relationship between n(N N) and the absolute integrated intensity in a series of dinitrogen compounds.

Fig. 1.86. Infrared spectrum of [Ru(NH3)5N2]Br2 [1333].

COMPLEXES OF DINITROGEN AND RELATED LIGANDS

185

Armor and Taube [1350] postulated the occurrence of the side-on structure as a possible transition state in linkage isomerization: [(NH3)5Ru14 N 15 N]Br2 $ [(NH3)5Ru15 N 14 N]Br2. Kr€ uger and Tsay [1351] carried out X-ray analysis on [{(C6H5Li)3Ni}2(N2){(C2H5)2O}2]2 and confirmed the presence of the side-on coor dination in this compound; the NN distance was found to be extremely long (1.35 A). Formichev et al. [1352] prepared a photoinduced metastable state of [Os(NH3)5(N2)] (PF6)2 and confirmed its side-on coordination by X-ray diffraction. The n(N2) was observed at 1838 cm1(IR), which is 187 cm1 lower than that of the side-on ligand in the ground state. The linear bridging MN NM-type complex should not show n(N N) in the IR spectrum. However, it may show a strong n(N N) in the Raman spectrum. Thus [{Ru (NH3)5}2(N2)]4þ shows no infrared bands in the 2220–1920 cm1 region, whereas a strong n(N N) band appears at 2100 cm1 in the Raman region [1353]. In the case of (m-N2){Mo(III)[N(R) L]3}2, where R is C(CD3)2CH3 and L is 3.5C6H3Me2, the n(N2) of the linear MoN NMo bridge is at 1630 cm1, which is shifted to 1577 cm1 by 14 N=15 N substitution (Raman). The observed low frequency suggests the NN bond order close to 2 [1354]. If N2 forms a bridge between two different metals, n(N N) is observed in the infrared. For example, n(N N) is at 1875 cm1 in the infrared (spectrum of [(PMe2Ph)4ClReN2CrCl3(THF)2] [1355]. According to X-ray analysis [1356], an analogous compound, [(PMe2Ph)4 ClReN2MoCl4(OMe)], has a N N distance of 1.2 A, and its n(N N) is at 1660 cm1. As expected, the n(N¼N) of [(CO)5CrNH¼NHCr(CO)5] is very low (1415 cm1) [1357]. The side-on bridging structure (m–Z2–Z2–N2), similar to that of the O2 bridge (Sec. 1.21), was proposed for {[(Pri2PCH2SiMe2)2N]ZrCl}2(N2) by Cohen et al. [1358]. An intense Raman peak at 731 cm1 was assigned to one of the totally symmetric modes of the Zr2N2 moiety that is predominantly due to n(N2) in character. It is downshifted by 22 cm1 by 14 N=15 N substitution in THF solution. 1.23.2. Dinitrogen Adducts of Metal Atoms Similar to M(CO)n- and M(O2)n-type compounds discussed previously (Secs. 1.18 and 1.21), it is possible to prepare simple M(N2)n-type adducts by reacting metal atoms with N2 in inert gas matrices. Again the distinction between end-on and side-on geometries can be made by using the isotope scrambling techniques (14 N2 þ 14 N 15 N þ 15 N2 ). Figure 1.87 shows the IR spectra of Ni(N2)(end-on) [1359] and Co(N2)(side-on) [1360]. The observed frequencies (cm1) and assignments of the four bands of the former are as follows: Ni14 N 14 N 2089:9

Ni14 N 15 N 2057:4

Ni15 N 14 N 2053:6

Ni15 N 15 N 2020:6

Table 1.66 lists the n(N2) of M(N2)-type complexes. All these adducts take the endon structure except for Co(N2) and Th(N2) [1365]. The structure of M(N2)4, M(N2)3, and M(N2)2 are tetrahedral, trigonal–planar, and linear respectively, although slight

186

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.87. Matrix isolation IR spectra of Ni and Co atom vapors cocondensed with 14 N 2 / 14 N 15 N / N 2 /Ar at 10 K [1359,1360].

15

distortions from these ideal symmetries occur as a result of the matrix effect. In all these compounds, the N2 ligands take end-on geometry except Pt(N2)2, for which sideon coordination has been proposed [1361]. Most of the studies described above have been made in the n(N2) region since lowfrequency vibrations are generally weak and difficult to measure in inert gas matrics. However, the IR-active n(MN2) vibrations of Ni(N2)4, Rh(N2)4, and Pd(N2)2 have TABLE 1.66. Typical 1 : 1 (Metal/N2) Adducts Prepared by Matrix Cocondensation Techniques Adduct

n(N2)

Ref.

Ni(N2) Pd(N2) Pt(N2) Co(N2) V(N2) Nb(N2) Cr(N2) Th(N2)

2088 2211 2173/2168 2101 2216 1926/1931 2215 1829

1359,1359a 1359 1361 1360 1362 1363 1364 1365

COMPLEXES OF DINITROGEN AND RELATED LIGANDS

187

been observed at 283, 345, and 340 cm 1, respectively. The corresponding force constants are 0.81, 1.44, and 1.25 mdyn/A, respectively [1366]. It should be noted that the NiCO stretching force constant of Ni(CO) 4 (1.80 mdyn/A) is more than 2 times larger than that of Ni(N2)4 (0.81 mdyn/A). As stated in Chapter 2 of Part A, UN2 and PuN2 prepared by the spattering techniques take the linear NMN structures. Such insertion products are obtained by matrix cocondensation reactions of N2 with laser-ablated metal atom vapors. For example, the Fe atom reacts with N2/Ar at 10 K to form FeN (934.8), NFeN (903.6), cyclo-Fe2N (779,719) in addition to end-on Fe(N2)(2017.8), side-on Fe(N2)(l 826.8), and side-on Fe(N2)2 (1683.7) [1367]. Here, the numbers in parentheses indicate the characteristic frequency of each species. Similar results are reported for laser-ablated Be [1368] and Pt [1369] atoms. Maier et al. [1370] found that the reaction of thermally generated Si atom vapor with N2 produces NNSiNN, SiNNSi, SiNSiN, cyclo-SiN2, and cyclo-Si2N2, and confirmed their structures by DFT calculations. 1.23.3. Nitrido Complexes If the N3 ion coordinates to a metal, it is called a nitrido complex. Nitrido complexes of transition metals can be prepared by several methods, and their preparations, structures, and spectra have been reviewed by Griffith [1371]. The M N triple bonds are formed as a result of the strong p-donating property of the N3 ion. Cleare and Griffith [1372] carried out an extensive study on vibrational spectra of nitrido complexes. As shown in Table 1.67, the n(M N) of nonbridging nitrido complexes are generally found in the 1100–1000 cm1 region. However, an exception was found for Fe(N) (OEP), which exhibits the n(Fe N) at 876 cm1. This novel Fe(V) nitrido complex was prepared by laser photolysis of the corresponding azido complex at 30 K [1382]: hn

ðOEPÞFeN¼N N ! ðOEPÞFe N þ N2 TABLE 1.67. Vibrational Frequencies of Nonbridging Nitrido Complexes (cm1) Complex 3

[Nb(N)F5] [Ta(N)Cl5]3 Cr(N)(TTP)a [Cr(N)(CN)4]2 [Cr(N)(CN)5]3 Mo(N)(t-BuO)3 Mo(N)(TMP) W(N)(t-BuO)3 Mn(N)(TPP) [Tc(N)(py)4]2þ [Ru(N)Cl4] [Os(N)Cl5]2 [Os(N)(N3)5](PPh4)2 a

TTP ¼ tetra-p-tolylporphyrin.

n(M N)

Ref.

1050 1040 1017 1052 972 1020 1038 1010 1052 1072 1092 1081 1054

1373 1373 1374 1375 1375 1376 1377 1376 1378 1379 1380 1372 1381

188

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.88. The RR spectra of a thin film of Fe(N3)(OEP) at 30 K, 488.0 nm excitation with different excitation power: (a) 5 mW, eight scans added; (b) 10 mW, four scans added; (c) 20 mW, two scans added; (d) 40 mW; (e) 60 mW; (f) 100 mW; (g) 100 mW, after 10-min preirradiation with 488.0 nm (100 mW) [1382].

Figure 1.88 shows the RR spectra of a thin film of Fe(N3)(OEP) that was irradiated by 488.0 nm line of Ar ion laser. It is seen that, as the laser power increases, the bands characteristic of the azido group [266] become weaker and a set of new bands at 876, 752, and 673 cm1 become stronger. The 876 cm1 band can be assigned to the n(Fe N) on the basis of the results of isotope substitution experiments involving 15 NN2 ; 15N3 , and 54 Fe=56 Fe. The remaining two bands are attributed to porphyrin core vibrations of the nitrido complex. Similar results have been obtained for the TPP analogs. The large drop in the n(M N) in going from nitrido porphyrins of Cr(V) and Mn(V) to Fe(V) may be accounted for in terms of the MO schemes shown previously (Fig. 1.83c). Since the M(V) N3 system is one-electron-deficient relative to the M(IV)¼O2 system, the Fe(V) N3 bond is isoelectronic with the Mn(IV)¼O2 bond. Thus, the electronic configuration of the Fe(V) N system may be (dxy)l(dxz)l(dyz)1 (high spin) or (dxy)2(dxz)1 (low spin). The former is preferred because of the relatively small Fe N

COMPLEXES OF DIHYDROGEN AND RELATED LIGANDS

189

stretchingforceconstant(5.07mdyn/A).ThisisfurthersupportedbyMeyeretal.[1383], whostudiedtheESRandM€ ossbauerspectraofthetrans-[Fe(N)(N3)(cyclam)]þ ion.The frequencies of oxidation state marker bands of OEP and TPP porphyrins discussed in Sec. 1.5 indicate the Fe(V) state for their nitrido complexes. Dinuclear complexes containing linear and symmetric nitride bridges (MNM) exhibit the na(MNM) in IR and ns(NMN) in Raman spectra. These vibrations are observed at 985 and 228 cm1, respectively, for [Ta2(m-N)Br10]2 and at 904 and 203 cm1, respectively, for [Nb2(m-N)Brl0]2 [1384]. For [W2(m-N)Cl10], the na(WNW) was observed at 945 cm1 [1385]. The na(MNM) of [Mn2(m-N)(CN)10]6 is in the range of 976–935 cm1 depending on the cation [1386]. Similar frequencies are reported for [Nb2(m-N)Cl10]3 (951 cm1) [1387]. In [Fe(TPP)]2(m-N) containing low-spin Fe(III) centers, the ns(FeNFe) has been observed at 424 cm1 in RR spectra [1388]. The corresponding FeNFe stretching force constant (4.5 mdyn/A) is slightly larger than the FeOFe stretching force constant (3.8 mdyn/A) in (Fe(TPP))2O [1298]. : :: : :: Thus, the FeNFe bridge may be expressed as Fe NFe (bond order, 1.5). Clear and Griffith [1372] list the vibrational frquencies of other nitrido bridges containing Ru, Os, and Ir. In a trinuclear complex ion, [Ir3N(SO4)6(H2O)3]4, the nitrido atom is bonded to three Ir atoms trigonally and the na(Ir3N) is observed at 780 cm1 [1372]. 1.24. COMPLEXES OF DIHYDROGEN AND RELATED LIGANDS 1.24.1. Metal Complexes of Dihydrogen Dihydrogen is known to coordinate to a transition metal atom only in the side-on fashion. The metal H2 bonding is interpreted in terms of a delicate balance between s-donation to the metal and backdonation to s*, as illustrated below [1389]:

Photolysis of a mixture of Fe(CO)2(NO)2 with H2 in liquid Xe (104 C) produces Fe(CO)(NO)2(H2), which exhibits the n(H2), na(FeH2), and ns(FeH2) at 2973, 1374, and 870 cm1, respectively [1390]. These dihydrogen vibrations have been observed for M(CO)5(H2) (M ¼ Cr,Mo,W) [1391,1392], M(CO)3(Cp)(H2) (M ¼ V, Nb) [1393], and cis-W(CO)4(C2H4)(H2) [1394], which were prepared by similar methods. In V(CO)3(Cp)(H2), the n(H2) at 2642 cm1 is shifted to 2377and 1998 cm1, respectively, by HD and D2 substitution. The fact that the IR spectrum of the HD compound exhibits the n(HD) at a frequency intermediate between those of the H2 and D2 compounds and is not an overlap of the n(VH) and n(VD) bands provides definitive evidence that it is a dihydrogen compound and not a dihydride. More stable dihydrogen complexes of the type M(CO)3(PR3)2(H2), where M is Mo and W and R is cyclohexyl (Cy) or isopropyl (i-Pr), were first prepared by Kubas et al.

190

APPLICATIONS IN COORDINATION CHEMISTRY

Fig. 1.89. Infrared spectra of W(CO)3 (P(i-Pr)3)2(H2) and its D2 analog (Nujol mull) [1395,1396].

[1395,1396]. Figure 1.89 shows the IR spectra of W(CO)3[P(i-Pr)3]2(H2) and its D2 analog. The n(H2), na(WH2), ns(WH2), and d(WH2) are observed at 2695, 1567 (1140), 953 (704), and 465 (312) cm1, respectively. The corresponding frequencies of the D2 analog are given in parentheses. It should be noted that these complexes are still labile and must be kept in an H2-enriched atmosphere. Vibrational spectra and band assignment are also reported for trans-W(CO)3(Pcy3)2 (H2) (cy¼cyclohexyl) [1397] and [CpRu(Ph2PPPh2)(H2)]BF4 [1398], with n(H2) at 2690 and 2082 cm1, respectively. The latter frequency is very low, and the correanalysis shows that the HH distance in sponding n(D2) is at 1530 cm1. X-Ray Cr(CO)3(Pcy3)2(H2) is only 0.67(5) A, which is the shortest ligated HH bond. However, its n(H2) is hidden under other bands [1399]. 1.24.2. Hydrido Complexes Vibrational spectra of hydrocarbonyls have been discussed in Sec. 1.18.5. Metal complexes containing terminal hydrido groups (MH) exhibit the n(MH) and d(MH) in the 2250–1700 and 800–600 cm1 regions, respectively; Table 1.68 lists MH frequencies of typical complexes. The n(MH) is sensitive to other ligands, particularly those in the trans position in the square–planar Pt(II) complexes. Thus Chatt et al. [1410] found that the order of n(PtH) in trans-[Pt(H)X(PEt3)2] is as follows: < NO2 < SCN < CN X¼ NO3 < Cl < Br < I 1 n(PtH)(cm ) 2242 > 2183 > 2178 > 2156 > 2150 > 2112 > 2041

COMPLEXES OF DIHYDROGEN AND RELATED LIGANDS

191

TABLE 1.68. M–H Frequencies of Hydrido Complexes (cm1) Complex

[Al(H)(NPh2)3] H2Ga(m-Cl)2GaH2 trans,trans-[Cr(H)(CO)2(NO)(PEt3)2] [Mo(H)(CN)7]4 cis-[Fe(H)(CO)3P(OC6H5)3] trans-[Fe(H)Cl{C2H4(PEt2)2}2] [Co(H)(CN)5]3 [Ru(CO)(H)(NCO)(PPh2Me)3] mer-[Os(H)3(NO)(PPr3)2] [Rh(H)(CN)5]3 [Ir(H)(CN)5]3 Ir(H)(COD){As(C6H5)3}2a a

n(MH)

d(MH)

Ref.

1777 2042–1988 1661 1805 1895 1849 1840 1920 2032–1849 1980 2040 2030

— — — — — 656 774 — — 781 811 —

1400 1401 1402 1403 1404 1405 1406 1407 1408 1403 1403 1409

COD ¼ 1,5-cyclooctadiene.

This is the increasing order of trans influence. Church and Mays [1411] found that the NMR PtH coupling constant (J ) and n(PtH) decrease in the same order in the trans-[Pt(H)L(PEt3)2]þ series: L¼ py < CO < PPh3 < P(OPh)3 < P(OMe)3 < PEt3 J(PtH)(Hz) 1106 > 967 > 890 > 872 > 846 > 790 > 2067 < 2090 n(PtH)(cm1) 2216 > 2167 > 2100 > 2090 In this series, the s-donor strength of L increases as the J(PtH) value decreases and n(PtH) shifts to a lower frequency. Atkins et al. [1412] found linear relationships between the chemical shift of the hydride, the PtH coupling constant, n(PtH), and the pKa value of the parent carboxylic acid in a series of trans-[Pt(H)L(PEt3)2], where L is a carboxylate ligand. X-Ray analyses have shown that both terminal and bridging hydrido groups exist in each of the three complexes shown in Fig. 1.90. Compounds I and II exhibit the terminal and bridging n(MH) in the 2100–1800 and 1200–950 cm1 regions, respectively, while three n(TaH) vibrations (1810,1720, and 1650 cm1) are reported for compound III. Metal carbonyl ions such as [Co6(CO)15H] contain rare interstitial hydrogens. The neutron diffraction study on its [N(P(C6H5)3)2]þ salt indicates that the H atom is located at the center of the Co6 octahedron [1416]. The same conclusion has been reached by the inelastic neutron scattering (INS) study of the Csþ salt since it revealed the presence of a single n(CoH) (triply degenerate) at 1056 cm1 [1417]. Figure 1.91 shows the low-temperature IR spectra of its Kþ salt obtained by Stanghellini and Longoni [1418]. It is seen that two bands at 1086 and 949 cm1 are shifted to 772 and 677 cm1, respectively, by H/D substitution. Possible reasons for the observed splitting have been discussed by these workers. Corbett et al. [1419] observed two INS bands at 790 and 480 cm1 for Li6[Zr6Cl18H] at 15 K, and assigned them to the E and A1 vibrations, respectively, of the interstitial hydrogen at the trigonal (C3v) site within the octahedral Zr6 cluster.

Fig. 1.90. Structures of complexes containing both terminal and bridging hydrido groups.

Fig. 1.91. Infrared spectra of (a) K[Co6(CO)15H] and (b) K[Co6(CO)15D] in Nujol mull at 110 K [1418].

192

HALOGENO COMPLEXES

193

There are several metal hydrides of simple composition, and complete band assignments were based on their symmetries. These include Mg2[CoH5](C4v) [1420] and K2[PdH4](D4h) [1421]. 1.24.3. Metal–Hydrogen Complexes in Inert Gas Matrices Using methods similar to those described in Sec. 1.18.6, many metal–hydrogen complexes have been synthesized via matrix cocondensation reactions. Andrews and coworkers synthesized a number of novel metal–hydrogen complexes, and elucidated their structures and assigned their IR spectra on the basis of DFT calculations. Their results are summarized by Andrews [1422]. For example, the reactions of laser-ablated Mg atom vapor with H2/Ar at 10 K produced MgH (1422), linear MgH2 (1571.9 and 439.8), HMgMgH (1491.8), rhombic (MgH)2 (1022.8 and 605.4), and the bridged species, HMg(m-H)2MgH (1531.0, 1164.2, 1013.7 and 613.9) [1423], where the numbers in parenthses indicate the observed frequencies in cm1. 1.25. HALOGENO COMPLEXES Halogens (X) are the most common ligands in coordination chemistry. Several review articles [1424,1425] summarize the results of extensive infrared studies on halogeno complexes. Chapter 2 of Part A lists the vibrational frequencies of many halogeno compounds. Here the vibrational spectra of halogeno complexes containing other ligands are discussed. In most cases n(MX) can be readily assigned by halogen or metal (isotope) substitution. 1.25.1. Terminal Metal–Halogen Bond Terminal MX stretching bands appear in the regions of 750–500 cm1 for MF, 400– 200 cm1 for MCl, 300–200 cm1 for MBr, and 200–100 cm1 for MI. According to Clark and Williams [124], the n(MBr)/n(MCl) and n(MI)/n(MCL) ratios are 0.77–0.74 and 0.65, respectively. Several factors govern n(MX) [1426]. If other conditions are equal, n(MX) is higher as the oxidation state of the metal is higher. Examples have already been given for tetrahedral MX4- and octahedral MX6-type compounds, discussed in Chapter 2 of Part A. It is interesting to note, however, that in the [M(dias)2Cl2]nþ series [dias ¼ o-phenylenebis(dimethylarsine)]. n(MCl) changes rather drastically in going from Ni(III) to Ni(IV) (Fig. 1.89), while very little change is observed between Fe(III) and Fe(IV):

n(MCl)(cm1)

d4

d5

d6

d7

Fe(IV) 390

Fe(III) 384

Ni(IV) 421

Ni(III) 240

This was attributed to the presence of one electron in the antibonding eg orbital in the Ni(III) complex [1427]. The same trend was noted for the trans-planar-[NiL2X2]2þ

194

APPLICATIONS IN COORDINATION CHEMISTRY

TABLE 1.69. Structural Dependence of NiX Stretching Frequencies (cm1)a Stretching Frequency n(NiCl) n(NiBr)

Linear Triatomic

transPlanar

NiCl2b 521 NiBr2b 414

Ni(PEt3)2Cl2c 403 Ni(PEt3)2Br2c 338

0.80

0.84

n(NiI) nðNiBrÞ nðNiClÞ nðNiIÞ nðNiClÞ

cis-Planar

Tetrahedral

transOctahedral

Ni(DPE)Cl2d 341, 328 Ni(DPE)Br2d 290, 266 Ni(DPE)I2d 260, 212 0.83f

Ni(PPh3)2Cl2c 341, 305 Ni(PPh3)2Br2e 265, 232 Ni(PPh3)2I2e 215 0.77f

Ni(py)4Cl2 207 Ni(py)4Br2 140 Ni(py)4I2 105 0.68

0.70f

0.67f

0.51

a

DPE ¼ l,2-bis(diphenylphosphino)ethane. Reference 1429. Reference 1430. d Reference 1431. e Reference 1432. f This value was calculated by using average frequencies of two bands. b c

ion, where L is o-C6H4(PMe2)2 and X is Cl and Br [1428]. In the case of the bromide, the n(NiBr) changes from 180 to 306 cm1 in going from Ni(III) to Ni(IV). If other conditions are equal, n(MX) is higher as the coordination number of the metal is smaller. Table 1.69 indicates the structure dependence of n(NiX), obtained by Saito et al. [135]. According to Wharf and Shriver [1433], the SnX stretching force constants of halogenotin compounds are approximately proportional to the oxidation number of the metal divided by the coordination number of the complex. It is interesting to note that the n(SnCl) of free SnCl3 ion [289 (A1) and 252 (E) cm1] are shifted to higher frequencies on coordination to a metal. Thus n(SnCl) of [Rh2Cl2(SnCl3)4]2 are at 339 and 323 cm1. According to Shriver and Johnson [1434], the LX force constant of the LXn-type ligand will increase on coordination to a metal if X is significantly more electronegative than L. In the example above, chlorine is more electronegative than tin. Similar trend is reported for [Pt(SnCl3)5] (NBu4)3 [1435]. In metal amine complexes (see Sec. 1.1), n(NH) shifts to lower frequencies because nitrogen is more electronegative than hydrogen. As expected, the n(GeCl) of free GeCl3 ion [303 (A1) and 285 (E) cm1] are also shifted to higher frequencies in [Pd(PhNC)(PPh3)(GeCl3)Cl] (384 and 360 cm1) [1436]. The MX vibrations are very useful in determining the stereochemistry of the complex. Appendix V of Part A tabulates the number of infrared- and Raman-active vibrations of various MXnYm-type compounds. Using these tables, it is possible to determine the stereochemistry of a halogeno complex simply by counting the number of n(MX) fundamentals observed. Examples of this method will be given in the following sections. 1.25.1.1. Square–Planar Complexes Vibrational spectra of planar M(NH3)2X2 [M ¼ Pt(II) and Pd(II)] were discussed in Sec. 1.l. The trans-isomer (D2h) exhibits one n(MX) (B3u), whereas the cis-isomer (C2v) exhibits two n(MX)

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195

(A1 and B2) bands in the infrared. The infrared spectra of cis- and trans-[Pd(NH3)2Cl2] were shown in Fig. 1.5. Similar results have been obtained for a pair of cis- and trans[Pt(py)2Cl2] [1437], and PtL2X2, where L is one of a variety of neutral ligands [1438]. In planar Pt(II) and Pd(II) complexes, n(MX) is sensitive to the ligand trans to the MX bond. Thus the effect of “trans influence” [1439] has been studied extensively by using infrared spectroscopy. In the [PtCl3L] series [1440], n(PtCltrans) follows the order L¼ CO SMe2 C2H4 SEt2 AsEt3 PPh3 PMe3 AsMe3 PEt3 n(PtCl) (cm1) 322 > 310 309 307 > 280 279 275 272 271 Their order represents an increasing degree of trans influence, since n(PtCl) becomes lower as a ligand of stronger trans influence is introduced trans to the PtCl bond. It was found that n(PtClcis) is insensitive to the change in L. An order of trans influence such as Cl < Br < I CO < CH3 < PR3 AsR3 < H was noted from the order of n(MCltrans) in a series of octahedral Rh(III) and Os(III) complexes [1441]. Fujita et al. [1442] prepared two isomers of PtCl(C2H4)(L-ala), where L-ala is Lalanino anion:

Isomers I and II exhibit their n(PtCl) at 360 and 340 cm1, respectively. Since the trans influence of the N-donor is expected to be stronger than that of the O-donor, the structures of these two isomers have been assigned as shown above. Complexes of the type Ni(PPh2R)2Br2 (R ¼ alkyl) exist in two isomeric forms: tetrahedral (green) and trans-planar (brown). Distinction between these two can be made easily since the numbers and frequencies of infrared-active n(NiBr) and n(NiP) are different for each isomer. Wang et al. [1443] studied the infrared spectra of a series of compounds of this type, and confirmed that n(NiBr) and n(NiP) are at 330 and 260 cm1, respectively, for the planar form and at 270–230 and 200–160 cm1, respectively, for the tetrahedral form. The presence or absence of the 330-cm1 band is particularly useful in distinguishing these two isomers. According to X-ray analysis [1444], the green form of Ni(PPh2Bz)2Br2 (Bz ¼ benzyl) is a mixture of the planar and tetrahedral molecules in a 1 : 2 ratio. Ferraro et al. [1445] studied the effect of high pressure on the infrared spectra of this compound, and found that all the bands characteristic of the tetrahedral form disappear as the pressure is increased to 20,000 atm. This result indicates that the tetrahedral molecule can be converted to the planar form under high pressure if the energy difference between the two is relatively small. This

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Fig. 1.92. Effect of pressure on PtCl stretching bands of Pt(NBD)Cl2: (A) 1 atm; (B) 6000 atm; (C) 12,000 atm; (D) 18,000 atm; (E) 24,000 atm [1446].

conversion is completely reversible; the original green form is recovered as the pressure is reduced. High-pressure infrared spectroscopy has also been used to distinguish symmetric and antisymmetric MX stretching vibations. For example, Fig. 1.92 shows the effect of pressure on na(PtCl) and ns(PtCl) of Pt(NBD)Cl2 (NBD ¼ norbornadiene) [1446]. It is seen that by increasing pressure, the intensity of ns(PtCl) is suppressed to a greater degree than that of na(PtCl). For high-pressure vibratjonal spectroscopy, see a review by Ferraro [1447,1448]. 1.25.1.2. Octahedral Complexes cis-MX2L4 (C2v) should exhibit two n(MX), while trans-MX2L4 (D4h) should give only one n(MX) in the infrared. Thus cis[IrCl2(py)4]Cl shows two n(IrCI) at 333 and 327 cm1, while trans-[IrCl2(py)4]Cl exhibits only one n(IrCl) at 335 cm1 [124]. If MX3L3 is fac (C3v), two n(MX) are expected in the infrared. If it is mer (C2v), three n(MX) should be infrared-active. As is shown in Fig. 1.13, fac-[RhCl3(py)3] gives two bands at 341 and 325 cm1 and mer[RhCl3(py)3] shows three bands at 355, 322, and 295 cm1,124. In MX4L2-type compounds, the number of IR-active n(MX) is one for the transisomer (D4h) and four for the cis-isomer (C2v). For example, trans-[PtCl4(NH3)2] exhibits one n(PtCl) at 352 cm1 (with a shoulder at 346 cm1), whereas cis[ptCL4(NH3)2] exhibits four n(PtCl) at 353, 344, 330, and 206 cm1 [1449]. Using Sn isotopes, Ohkaku and Nakamoto [1450] confirmed that trans-[SnCl4L2] (L ¼ py,

HALOGENO COMPLEXES

197

THF, etc.) exhibits one n(SnCl) in the 342–370 cm1 region, while cis-[SnCl4(LL)] (LL ¼ bipy, phen, etc.) shows four n(SnCl) in the 340–280 cm1 region. For MX5L (C4v), one expects three n(MX) in the infrared. The n(InCl) of [InCl5(H2O)]2 were observed at 280, 271, and 256 cm1 [1451].

1.25.2. Bridging Metal–Halogen Bond Halogens tend to form bridges between two metal atoms. In general, bridging MX stretching frequencies [nb(MX)] are lower than terminal MX stretching frequencies [nt(MX)]. Vibrational spectra of simple M2X6-type ions having bridging halogens were discussed in Sec. 2.10 of Part A. Table 1.70 lists the nt(MX) and nb(MX) of bridging halogeno complexes containing other ligands. The trans-planar M2X4L2-type compounds (C2h) exhibit three infrared-active (Bu) n(MX) modes: one n(MXt), and two n(MXb). For the latter two

the higher-frequency band corresponds to n(MXb) trans to X, whereas the lowerfrequency mode is assigned to n(MXb) trans to L since it is sensitive to the nature of L [1452]. Strong coupling is expected, however, among these modes since they belong to the same symmetry species. In the {[Ru(NO)(Cl)(I)2](m-I)2}2 ion of C2h symmetry, the terminal n(RuI) are at 214 and 208 cm1 while the bridging v(RuI) are at 145 and 128 cm1 [1455].

TABLE 1.70. Terminal and Bridging Metal–Halogen Stretching Frequencies (cm1) nt (MX)

nb(MX)

nb/ ntb

Ref.

trans-Pd2Cl4L2

360–339 368–347

Pd2Br4L2

285–265

Pt2Br4L2

260–235

Pt2I4L2

200–170

Ni(py)2Cl2 Ni(py)2Br2 Co(py)2Cl2 Monomeric Polymeric

— —

0.86 0.75 0.91 0.78 0.74 0.66 0.89 0.74 0.92 0.77 — —

1452

trans-Pt2Cl4L2

308–294 283–241 331–317 301–257 220–185 200–165 230–210 190–175 190–150 150–135 193, 182 147

347, 306 —

— 186, 174

Compounda

a b

L ¼ PMe3, PEt3, PPh3, and so on. These values were calculated using average frequencies.

1452 1453 1453 1453 1454 1454 137 137

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The [Pt3Br12]2 ion takes a structure of nearly D2h symmetry in which two Pt(IV) Br6 octahedra share edges with one planar Pt(II)Br4 group:

Here, subscripts ax and eq denote the axial and equatorial atoms, respectively. Figure 1.93 shows the IR and Raman spectra of (TBA)2[Pt3Br12] obtained by Hillebrecht et al. [1456]. These spectra have been assigned completely via normal coordinate analysis. Four different PtBr stretching force constants were necessary to distinguish the Pt(IV)Brt,eq, Pt(IV)–Br t,ax, Pt(II)Brb, and Pt(IV)Brb bonds (1.75, 1.69, 1.10, and 1.05 mdyn/A, respectively). As discussed in Sec. 1.3.1, Co(py)2Cl2 exists in two forms: the monomeric tetrahedral (blue) and the polymeric octahedral (lilac). The n(CoClb) of the polymer is very low relative to that of the n(CoClt) because of an increase in coordination number and the effect of bridging [137]. Polymeric Ni(py)2X2 also exhibits n(NiXb) below 200 cm1 (Table 1.68) [1454].

Fig. 1.93. Infrared and Raman spectra of (TBA)2[Pt3Br12] (TBA ¼ tetra-n-butylammonium ion) [1456].

COMPLEXES CONTAINING METAL–METAL BONDS

199

The mixed-valence [Ru2(NH3)6X3]2þ ion (X ¼ Cl, Br) contains a triple halogeno bridge:

The totally symmetric n(RuX) and d(RuX3) vibrations (310 and 145 cm1 for X ¼ Cl, and 253 and 111 cm1 for X ¼ Br, respectively) are strongly enhanced in the RR spectrum by excitation in the visible region. Armstrong et al. [1457] were able to assign the electronic transition responsible for this resonance enhancement. The mixed-valence complex ion, [Pt2I8]2, assumes a chain structure: Pt(IV)IPt(II) IPt(IV). The n(Pt(IV)IPt(II)) frequency is reported to be 120 cm1 [1458]. Vibrational spectra of metal cluster ions such as [(M6X8)Y6]2 (M ¼ Mo,W; X ¼ a bridging halogen; Y ¼a terminal halogen) and [(M6X12)Y6]n (M ¼ Nb, Ta) are discussed in Sec. 2.12 of Part A. The low-frequency spectra of these compounds are difficult to assign empirically because of strong vibrational couplings among the n(MX), n(MY), n(MM), and bending modes. Finally, matrix cocondensation reactions such as Al þ X2 !AlXn þ Al2 X6 (where X ¼ F,Cl,Br, I; n ¼ 1–3) were carried out and their IR spectra assigned by Hassanzadeh et al. [1459].

1.26. COMPLEXES CONTAINING METAL–METAL BONDS A large number of complexes containing metal–metal (MM) bonds are known, and their vibrational spectra have been reviewed extensively [1460–1464]. In Part A, we reviewed the vibrational spectra of the X2Y6-, X2Y8-, and X2Y10-type compounds containing MM bonds (Secs. 2.10, 2.11) and metal clusters containing halogeno bridges (Sec. 2.12). In this section, we discuss other complexes containing MM bonds. In general, n(MM) appear in the low-frequency region (250–100 cm1) because the MM bonds are relatively weak and the masses of metals are relatively large. However, the n(MM) of some complexes are as high as 400 cm1 owing to the multiple-bond character of their MM bonds. If the dinuclear complex is centrosymmetric with respect to the MM bond, the n(MM) is forbidden in IR. However, the n(MM0 ) of a heteronuclear complex is allowed in IR spectra. In contrast, Raman spectroscopy has distinct advantages in that both n(MM) and n(MM0 ) appear strongly since large changes in polarizabilities are expected as a result of stretching covalent MM(M0 ) bonds. As shown in Sec. 2.11 of Part A, a long series of overtones of n(MM) can be observed under resonance conditions. Special caution must be taken, however, in measuring Raman spectra of metal–metal bonded compounds since they may

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TABLE 1.71. Metal–Metal Bond Stretching Frequencies (cm1) Complex 2þ,a

[Ag2(m-dcpm)] ½AuI2 ðdcpmÞ2 2 þ ; a ½AuI2 ðCS3 Þ2 2 [IrII(py)(Pc2)]2 [(NC)5Pt-Tl(CN)3]3 [(NC)5Pt-Tl(phen)2] [Pt(bipy)2][Pt(CN)4] HgI2 ðSCNÞ2 a

n(MM)

Ref.

80 88 116 135 159 172 54 179.158

1465 1466 1467 1468 1469 1470 1471 1472

dcpm ¼ bis(dicyclohexylphosphine)methane.

undergo thermal and/or photochemical decomposition on laser irradiation. Table 1.71 lists the n(MM) of metal complexes containing relatively weak metal–metal bonds. 1.26.1. Compounds Containing Metal–Metal Multiple Bonds A number of compounds containing unusually short MM bonds exhibit unusually high n(MM). For example, the MoMo distance of Mo2(OAc)4 is only 2.09 A and its n(MM) is at 406 cm1. According to Cotton [1473], this MoMo bond consists of one s-bond, two p-bonds, and a d-bond (bond order 4). Such a quadruple bond is also expected for [Re2Cl8]2, which exhibits the n(ReRe) at 272 cm1 with an ReRe distance of 2.22 A [1474,1475]. Table 1.72 lists n(MM) of typical compounds. It is seen that the n(MM) of dimolybdenum compounds of bond order 4 scatter over a wide range. TABLE 1.72. Bond Orders, Bond Distances, and Stretching Frequencies (cm1) of Metal–Metal Multiply Bonded Compounds Compound

Bond Order

Bond Distance

n(MM)

Ref.

Mo2(O2CCH3)4 Mo2(O2CCF3)4 Mo2(O2CCF3)4(py)2 K4[Mo2Cl8]2H2O K3[Mo2(SO4)4]3.5H2O

4 4 4 4 3.5

2.09 2.09 2.22 2.14 2.16

1476,1477 1478 1478 1478 1479

Re2(O2CCH3)4Cl2 [Bu4N]2[Re2Cl8] Re2Cl5(DTH)2a Re2OCl5(O2CCH2CH3)2(PPh3)2 Re2(CO)10 W2(O2Ct-Bu)4(PPh3)2 Os2(O2CCH3)4Cl2 Rh2(O2CCH3)4(PPh3)2 Rh2(O2CCH3)4(AsPh3)2 Rh2(O2CCH3)4(SbPh3)2 Rh2(OSCCH3)4(PPh3)2

4 4 3 2 1 4 3 4 4 4 4

2.24 2.22 2.29 2.52 3.02 2.22 2.31 2.45 2.43 2.42 —

406 397 367 345 386 373 289 272 267 216 122 287 229 289 298 306 226

a

DTH ¼ 2,5-dithiahexane.

1476 1474 1476 1476 1476 1480 1481 1482,1483 1484 1484 1485

COMPLEXES CONTAINING METAL–METAL BONDS

201

Fig. 1.94. n(ReRe) versus ReRe bond order.

In contrast, dirhenium compounds exhibit a nice n(MM)–bond order relationship as demonstrated by Fig. 1.94. Thus far, the highest n(MM) reported is 411 cm1 for [Mo2(CN)8]4 [1486,1487]. Littrell et al. [1488] noted that only the band at 369 cm1 of Mo2(CH2SiMe3)6 is shifted to 299 cm1 when the Mo atom is replaced by Watom (Raman). This provides definitive support for assigning them to the n(MM). Mixed-valence complexes such as RuII RuIII(O2C2H3)4Cl (bond order 2 or 2.5) exhibit the n(RuRu) in the range from 329 to 347 cm1 [1489]. Table 1.72 also shows that the n(RhRh) is sensitive to the nature of the axial ligand and is downshifted by 60 cm1 when the acetato group is replaced by the thioacetato group. The n(MoMo) of Mo2(O2CCH3)4 is upshifted by 9 cm1 when Mo in natural abundance (mainly 96 Mo) is replaced by the 92 Mo isotope. Such a metal–isotope shift provides definitive assignment for the metal–metal vibration [1490]. Normal coordinate analyses are reported for M2(O2CCH3)4 and M2 Xn5 (M ¼ Mo, Re; X ¼ Cl,Br). [1491,1492]. The n(MM) of porphyrin dimers are listed in Sec. 1.5.4. 1.26.2. Polynuclear Carbonyls The n(CO) of polynuclear carbonyls have been discussed in Sec. 1.17.2. Here, we discuss the n(MM) of polynuclear carbonyls in the low-frequency region. As an example, the Raman spectra of Mn2(CO)10, MnRe(CO)10, and Re2(CO)10 are shown in Fig. 1.95, where the n(MM) are indicated for each compound [1493]. Risen and coworkers [1494–1496] carried out normal coordinate analyses on many dinuclear and trinuclear metal carbonyls. Table 1.73 lists the observed n(MM) and the corresponding force constants obtained by these and other workers. It is noted that the MM stretching force constants obtained by rigorous calculations are surprisingly close to those obtained by approximate calculations considering only metal atoms. There is a

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Fig. 1.95. Low-frequency Raman spectra of polycrystalline Mn2(CO)10, MnRe(CO)10, and Re2(CO)10 (632.8 nm excitation) [1493].

general trend that as the MM stretching force constant increases, the n(MM) frequency decreases in going from lighter to heavier metals in the M2(CO)10 (M ¼ Mn,Tc,Re) [1493] and [M2(CO)10]2 (M ¼ Cr,Mo,W) series. In Sec. 1.18.3, we discussed the spectra of M2(CO)8X2-type compounds (M ¼ Mn, Tc, Re, Rh, etc.) in which the metals are bonded through halogen (X) bridges. Goggin and Goodfellow [1499] concluded, however, that the [Pt2(CO)2X4]2 ion (X ¼ Cl,Br) contains the direct PtPt bond:

They isolated two isomers of [N(nPr)4]2[Pt2(CO)2Cl4] that differ only in the angle of rotation about the PtPt bond. Both isomers exhibit n(PtPt) at 170 cm1. Fe2(CO)9 and Fe3(CO)12 exhibit very strong Raman bands at 225 and 219 cm1, respectively. San Filippo and Sniadoch [1500] assigned them to n(FeFe). Later studies [1501] showed, however, that these bands are due to decomposition products resulting from strong laser irradiation. Thus, the appearance of strong Raman bands in the low-frequency region does not necessarily mean that they are due to n(MM). It is also

COMPLEXES CONTAINING METAL–METAL BONDS

203

TABLE 1.73. Metal–Metal Stretching Frequencies (cm1) and Force Constants

Force Constant (mdyn/A)

Compound (CO)5MnMn(CO)5 (CO)5TcTc(CO)5 (CO)5ReRe(CO)5 (CO)5ReMn(CO)5 ðCOÞ5 Mn WðCOÞ5 ðCOÞ5 Mn MoðCOÞ5 ðCOÞ5 Mn CrðCOÞ5 Cl3SnCo(CO)4 Cl3GeCo(CO)4 Cl3SiCo(CO)4 Br3GeCo(CO)4 I3GeCo(CO)4 Br3SnCo(CO)4 I3SnCo(CO)4 H3GeRe(CO)5 H3GeMn(CO)5 H3GeCo(CO)4 (CO)4CoZnCo(CO)4 (CO)4CoCdCo(CO)4 (CO)4CoHgCo(CO)4

n(MM) 160 148 122 157 153 150 149 204 240 309 200 161 182 156 209 219 228 170, 284b 163, 218b 163, 195b

Rigorous Calculation

Approximate Calculationa

Ref.

0.59 0.72 0.82 0.81 0.71 0.60 0.50 1.23 1.05 1.32 0.96 0.52 1.05 0.64 — — — 1.30 1.28 1.26

0.41 0.63 0.82 0.62 0.55 0.47 0.37 0.97 1.11 1.07 — — — — 1.34 0.88 1.00 — — —

1493 1493 1493 1493 1494 1494 1494 1495 1495 1495 1496 1496 1496 1496 1497 1497 1497 995 995 995

a

Calculations considering only metal atom skeletons. Under D3d symmetry, these frequencies correspond to the A1g (symmetric) and A2u (antisymmetric) MCo stretching modes, respectively. b

noted that Re2(CO)8X2 (X ¼ Cl,Br), which does not contain ReRe bonds, shows strong Raman bands at 125 cm1 where n(ReRe) of Re2(CO)10 appears [1501]. Cooper et al. [1502] were able to locate the n(MM) of Fe2(CO)9 and Fe3(CO)12 at 260 and at 240 and 176 cm1, respectively. These assignments are based on the 54 Fe–56 Fe isotopic shifts observed in Raman spectra at 10 K. Onaka and Shriver [1503] observed three n(MM) bands at 235, 185, and 159 cm1 in acetone solution of Co2(CO)8 that correspond to the three isomers discussed in Sec. 1.18.2. They have shown that the n(MM) is higher than 200 cm1 for bridging carbonyls and between 190 and 140 cm1 for single-bonded nonbridged complexes. The RR band at 225 cm1 of [Cp(CO)Fe]2(m-CO)2 results from strong vibrational coupling between the n(FeFe) and Fe-(m-CO) breathing modes [1504]. Trinuclear complexes such as Ru3(CO)12 and Os3(CO)12 contain a triangular M3 skeleton for which two n(MM) are expected under D3h symmetry. Quicksall and Spiro [985] assigned the Raman bands at 185 and 149 cm1 of the Ru complex to n(RuRu) of the A10 and E0 species, respectively. The latter is coupled with other modes. The corresponding RuRu stretching force constant is 0.82 mdyn/A. Kettle and co-workers [1505,1506] have assigned the n(MM) of the [OsxRu3x(CO)12]- (x ¼ 0,1,2,3)-type 0 complexes. In Mn3H3(CO)12, Martin et al. [1507] assigned the n(MnMn) at 163 (A 1 ) and 146 cm1 (E0 ) and obtained the stretching force constant, K(MnMn) of 0.37

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mdyn/A with the interaction constant of 0.08 K. On the other hand, Jayasooriya and 0 Skinner [1508] assigned the RR bands at 198 (A 1 ) and 164 cm1 (E0 ) to the n(MnMn), and obtained K(MnMn) of 0.553 mdyn/A with the interaction constant of 0.055 0 mdyn/A. In Re3H3(CO)12, the n(ReRe) were observed at 126 (A 1 ) and 116/ 1 0 103 cm (E ) in the matrix-isolated IR spectrum [1509]. Kettle et al. [1510] carried out a detailed study on the IR and Raman spectra of the Fe3EE0 (CO)9 (E,E0 ¼ S,Se,Te)type complexes in the low-frequency region. These polynuclear complexes take pseudotrigonal–bipyramidal structures with two E(E0 ) atoms at axial positions, and they noted vibrational coupling between n(FeFe) and n(FeTe) modes. Quicksall and Spiro [1511] assigned the Raman spectrum of Ir4(CO)12, which consists of a tetrahedral Ir skeleton; three n(IrIr) bands were assigned at 207 (A1), 161 2 : 1.56 : 1.27, is far from (F2), and 131 (E) cm1. The ratio of these three pffiffifrequencies, ffi that predicted by a “simple cluster model” (2: 2:1) [1512], indicating the substantial coupling between the individual stretching modes. Their rigorous calculations gave K(IrIr) of 1.69 mdyn/A , together with interaction constants of 0.13 and þ0.13 mdyn/A for the adjacent and opposite IrIr bonds, respectively. The n(MM) of Rh4(CO)12 [1513] and Co4(CO)l2 [1514] have been assigned and the corresponding force constants calculated [1514]. As mentioned in Sec. 1.18.5, the M4 skeleton of M4H4(CO)12 (M ¼ Ru,Os) take D2d symmetry. Then, the six (3 4 6) normal vibrations are classified into 2A1(R) þ B1(R) þ B2(IR, R) þ E(IR, R). Kettle and Stanghellini [1515] have made complete assignments of these modes. 1.26.3. Metal Cluster Compounds Vibrational spectra of metal clusters including [(M6X8)Y6]2 (M ¼ Mo,W) and [(M6X12)Y6]n (M ¼ Nb,Ta), where X and Y are bridging and terminal halogens, respectively, have been discussed in Sec. 2.12 of Part A. The n(MM) are also reported for metal clusters of other types. In the mixed-valence ion, [Pt4(NH3)8(C5H4NO)4]5þ, shown in Fig. 1.96a, the intradimer (Pt1Pt2) and interdimer (Pt2Pt2) distances are 2.774 and 2.87 A, respectively. Correspondingly, the n(Pt1Pt2) and n(Pt2Pt2) are observed at 149 and 69 cm1, respectively, in RR spectra [1516]. Pt2(EtCS2)4I (EtCS2 ¼ dithiopropanoic acid anion) is a one-dimensional halogeno-bridged polymer consisting of a PtPtIPtPtI chain with an average valence state of Pt at þ2.5. It exhibits the n(PtPt) at 74 cm1 in Raman spectra [1517].

Fig. 1.96. Structures of metal–metal bonded complex ions: (a) [Pt4(NH3)8(C5H4NO)4] 5þ (the curves indicate a-pyridonate bridges); (b) [Au(CH2)2PPh2]2X2.

COMPLEXES CONTAINING METAL–METAL BONDS

205

The n(PtPt) of the SO4 bridged complex, [Pt2(SO4)4 (H2O)L]n (n ¼ 2 or 3) is sensitive to the nature of the axial ligand (L ¼ H2O, OH and Cl) [1518]. Sensitivity of the n(AuAu) to the nature of the trans ligand is seen in the series of [Au(CH2)2PPh2]2X2 shown in Fig. 1.96b; 162, 132, and 103 cm1, respectively, for X ¼ Cl, Br, and I [1519]. Two isomers of triangular phosphido-bridged Pt3(m-PPh2)3(Ph)(PPh3)2 have been isolated. Their differences are in the PtPt distances and Pt(m-PPh2)Pt angles. Both isomers exhibit the n(PtPt) in the range from 122 to 96 cm1 [1520]. 1.26.4. Metal–Metal Stretching Vibrations of Electronic Excited States Using TR3 spectroscopy (Sec. 1.4.2), it is possible to measure the n(MM) of electronic excited states. As mentioned in Sec. 2.11.2 of Part A, the n(ReRe) of die [Re2Cl8]2 ion at 275 cm1 in the ground state [(dd)2, 1 A g ] is shifted to 204 cm1 in the excited state [(dd) (dd*], 1 A 2u ] since an electron is promoted from a bonding (dd) to an antibonding (dd*) orbital [1521]. An opposite trend prevails when an electron is promoted from an antibonding to a bonding or less antibonding orbital. As an example, Figure 1.97 shows the RR spectra of the [Rh2b4]2þ ion (b ¼ 1,3-diisocyanopropane) in the electronic ground

Fig. 1.97. Lower trace—ground-state RR spectrum of the [Rh2b4]2þ ion obtained by CW excitation at 530.9 nm; upper trace—excited-state RR spectrum obtained by pulsed-laser excitation at 531.8 nm [1522].

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[(ds*)2, 1 A 1g ] andexcited [(ds*) (ps), 3 A 2u ] states obtained by Dallinger etal. [1522]. It is seenthat the n(RhRh) at 79 cm1 in theground state is upshiftedto 144 cm1 in theexcited state. Correspondingly, the RhRh stretching force constant in the ground state (0.19 mdyn/A) is increased 3 times (0.63 mdyn/A) by this electronic excitation. Similar results are reported for [Rh2(TMB)4]2þ(TMB ¼ 2,5-dimethyl-2,5-diisocycano hexane); n(RhRh) are at 50 and 151 cm1, respectively, in the ground and excited triplet states [1523]. In the case of the [Rh2(dimen)4]2þ ion (dimen ¼ 1,8diisocyanomenthane), the RhRh distance is very long (4.48 A) in the ground state, and its n(RhRh) frequency is very low (28 cm1). However, the lowest electronic excitation (ds* ! ps) produces an excited state with a much shorter RhRh distance (3.2 A) although the n(RhRh) band could not be observed because of its subnanosecond lifetime [1524]. The n(AuAu) vibrations of [Au2(dmpm)2]2þ [dmpm ¼ bis(dimethylphosphine)methane] are observed at 79 and 165 cm1, respectively, for the ground and the first electronic excited states [1525]. In the [Pt2(pop)4]4 ion [pop ¼ (P2O5H2)2 ion], the n(PtPt) at 118 cm1 in the ground state is upshifted to 156 cm1 when an electron is promoted from the ds* to the ps orbital [1526]. Similar trends are observed for M2(dppm) [M ¼ Pd, Pt; dppm ¼ bis(diphenylphosphino) methane] [1527]. For more details, see a review by Morris and Woodruff [1528]. 1.27. COMPLEXES OF PHOSPHORUS AND ARSENIC LIGANDS Ligands such as phosphines (PR3) and arsines (AsR3) (R ¼ alkyl, aryl, halogen, etc.) form complexes with a variety of metals in various oxidation states. Vibrational spectroscopy has been used extensively to determine the structures of these compounds and to discuss the nature of the metal–ligand bonding. 1.27.1. Complexes of Phosphorus Ligands Vibrational frequencies of pyramidal XY3-type ligands such as PH3, PF3, and their halogeno analogs are found in Sec. 2.3 of Part A. The most simple phosphine ligand is PH3. The vibrational spectra of Ni(PH3)4 [1529], Ni(PH3)(CO)3 [1530], and Ni(PH3) (PF3)3 [1531] have been reported by Bigorgne and coworkers. All these compounds exhibit n(PH), d(PH3), and n(NiP) at 2370–2300, 1120–1000, and 340–295 cm1, respectively. A series of the Ni(PH3)n (n ¼ 1–4)-type complexes have been prepared by matrix cocondensation reactions, and their n(NiP) have been assigned at 390–395 cm1 [1532]. Complete assignments based on normal coordinate calculations have been made on Ni(P(CH3)3)4 [1533]. The A1 and F2 n(NiP) vibrations of this compound have been assigned at 296 and 343 cm1, respectively. Trifluorophosphine (PF3) forms a variety of complexes with transition metals. According to Kruck [1534], the n(PF) of free PF3 (892, 860 cm1) are shifted slightly to higher frequencies (960–850 cm1) in M(PF3)n (n ¼ 4,5,6) and HM(PF3)4 and to lower frequencies (850–750 cm1) in [M(PF3)4] (M ¼ Co,Rh,Ir). These results have been explained by assuming that the PF bond possesses a partial double-bond character that is governed by the oxidation state of the metal. For individual compounds, only references are cited: M(PF3)4 (M ¼ Ni,Pd,Pt) [1535], M(PF3)5

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207

[1536], V(PF3)6 [1537], Au(PF3)Cl [1538], and cis-MH2(PF3)4 (M ¼ Fe,Ru,Os) [1539]. The d(PF3) and n(MP) of some of these compounds are assigned at 590– 280 and 250–180 cm1, respectively. Benazeth et al. [1540] showed that the skeletal symmetry of HCo(PF3)4 is C3v, while that of [Co(PF3)4] is Td. The n(CoP) of these compounds are at 250–210 cm1. Woodward and coworkers [1541] carried out complete vibrational analyses of the M(PF3)4 (M ¼ Ni,Pd,Pt) series. Their results show the following trends: Ni(PF3)4 n(MP) (cm1)

K(MP) (mdyn/A)

A1 F2

195 219 2.71

Pd(PF3)4

Pt(PF3)4

204 222 3.17

213 219 3.82

For the Ni(PX3)4 (X ¼ a halogen) series, Edwards et al. obtained the following: Ni(PF3)4 n(MP) (cm1)

K(MP) (mdyn/A)

A1 F2

195 219 2.71

Ni(PCl3)4 [1542] 135 208 2.35

Ni(PBr3)4 [1543]

Ni(PI3)4 [1544]

78 193 2.05

(55) 184 —

The n(NiP) of Ni(PMe3)4 arc observed at 182 (A1,) and 197 cm1 (F2) [1545]. The n(AuP) of Au(PMe3)X are observed at 220, 209, and 219 cm1, respectively, for X ¼ Cl,Br,I [1546]. In general, it is more difficult to assign the n(MP) of alkyl and phenyl phosphine complexes than those of halogeno phosphine complexes because the former ligands exhibit many internal modes in the region where the n(MP) are expected to appear. To overcome this difficulty, Shobatake and Nakamoto [1430] utilized the metal isotope technique (Sec. 1.17 of Part A). Figure 1.98 shows the infrared spectra of trans-½58;62 NiðPEt3 Þ2 X2 (X ¼ Cl,Br), and Table 1.74 lists the observed frequencies, metal isotope shifts, and band assignments. It is clear that the n(NiP) of these complexes must be assigned near 270 cm1, in contrast to previous investigations, which placed these vibrations near 450–410 cm1 [1453,1547–1549]. Triphenylphosphine (PPh3) is most common among phosphine ligands. It is not simple, however, to assign the n(MP) of PPh3 complexes since PPh3 exhibits a number of ligand vibrations in the low-frequency region [1550–1552]. Using the metal isotope technique, Nakamoto and a colleague [1430] showed that tetrahedral Ni(PPh3)2Cl2, for example, exhibits two n(NiP) at 189.6 and 164.0 cm1, in agreement with the result of previous workers [1553]. The n(MP) of M(PPh3)3Cl [M ¼ Cu(I), Co(I)] are located in the range from 233 to 219 cm1 [1554]. In Rh(PPh3)3Cl, the n(RhP) (550, 465, and 460 cm1) are higher than those of other n(MP). This has been attributed to the effect of the Rh(dp)–P(pp) bonding and the delocalization of the phenyl ring charge through the Rh and P atoms [1555]. As stated in Sec. 1.25.1, complexes of the type Ni(PPh2R)2Br2 (R ¼ alkyl) exist in two forms (tetrahedral and square–planar), which can be distinguished by the n(NiBr) and n(NiP) [1443]. For R ¼ Et, the n(NiP) of the planar complex is at 243 cm1,

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Fig. 1.98. Far-IR spectra of 58NiX 2 (PEt3)2 and its

62

Ni analogs (X ¼ Cl,Br) [1430].

whereas these vibrations are at 195 and 182 cm1 in the tetrahedral complex. Udovich et al. [1431] studied the infrared spectra of Ni(DPE)X2, where DPE is 1,2-bis (diphenylphosphino)ethane and X is Cl, Br, and I, by using the metal isotope technique. It was found that the n(NiX) are always lower and n(NiP) are always higher in the cis-Ni(DPE)X2 than in the corresponding trans-Ni(PEt3)2X2. This difference has been attributed to the strong trans influence of phosphine ligands. A number of investigators have discussed the nature of the MP bonding on the basis of electronic, vibrational, and NMR spectra [1556], and controversy has arisen about the degree of p-backbonding in the MP bond. For example, Park and Hendra TABLE 1.74. Infrared Frequencies, Isotopic Shifts, and Band Assignments of NiX2(PEt3)2 (X ¼ Cl and Br) (cm1) [1430] 58

PEt3 n 408 — 365 330 — 245

Dna

n 416.7 403.3 372.5 329.0 273.4

0.0 6.7 0.1 0.5 5.9

4.7

0.8 0.2 0.5

NiBr2 ðPEt3 Þ2

n

Dna

Assignmentb

413.6 337.8 374.0 327.8 265.0

1.2 10.5c 1.1 0.5c

d(CCP) n(NiX) d(CCP) d(CCP) n(NiP) d(CCP) d(CPC) d(NiX) d(NiP)

(hidden) 200.2 186.2 161.5

a

58

NiCl2 ðPEt3 Þ2

190.4 155.1

(hidden) 0.7 1.5 (hidden)

Dn indicates metal-isotope shift, n(58Ni)–n(62Ni). Ligand vibrations were assigned according to Ref. 1549. c Since these two bands are overlapped (Fig. 1.98), Dn values are only approximate. b

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209

[1557] suggest the presence of a considerable degree of p-bonding in square–planar Pd(II) and Pt(II) complexes of PMe3 and AsMe3. On the other hand, Venanzi [1558] claims from NMR evidence that the PtP p-bonding is much less than originally predicted [1559]. It is rather difficult, however, to discuss the degree of p-bonding from vibrational spectra alone since the MP stretching frequency and force constant are determined by the net effect, which involves both s- and p-bonding. 1.27.2. Complexes of Arsenic Ligands Complexes of the type M(CO)5L, where L is arsine (AsH3) and stibine (SbH3) and M is Cr, Mo, and W, have been prepared by Fischer et al. [1560]. n(AsH) and d(AsH3) are near 2200 and 900 cm1, respectively. Complexes of trimethylarsine (AsMe3) have been studied by several investigators. Goodfellow et al. [1561] and Park and Hendra [1557] measured the infrared spectra of M(AsMe3)2X2- (M ¼ Pt,Pd; X ¼ Cl,Br,I)type complexes and assigned n(MAs) in the 300–260 cm1 region. The latter workers assigned n(MSb) of analogous alkylstibine complexes at 200 cm1. Konya and Nakamoto [1427] assigned n(MAs) and n(MX) of [M(dias)2]2þ- and [M(dias)2X2]Yntype complexes by using the metal isotope technique. Figure 1.99 shows the infrared spectra of [58 Ni(dias)2X2]X and [58 Ni(dias)2X2](ClO4)2 (X ¼ Cl,Br) and their 62 Ni analogs. Their results show that the n(MAs) are very weak and appear at 325–295 cm1 for the Ni, Co, and Fe complexes and at 270–210 cm1 for the Pd and Pt complexes. For the n(MX) of these complexes, see Sec. 1.25.

Fig. 1.99. Far-IR spectra of octahedral nickel dias complexes; vertical lines marked by A and B indicate NiAs and NiX stretching bands, respectively [1427].

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Tertiary phosphine oxides and arsine oxides coordinate to a metal through their O atoms. The n(P¼O) of triphenylphosphine oxide (TPPO) at 1193 cm1 is shifted by 35 cm1 to a lower frequency when it coordinates to Zn(II) [1562]. The shift is much larger in MX4(TPPO)2 (160–120 cm1), where MX4 is a tetrahalide of Pa, Np, and Pu [1563]. A similar observation has been made for n(As¼O) of arsine oxide and its complexes. Exceptions to this rule are found in MnX2(Ph3AsO)2 (X ¼ Cl,Br); their n(As¼O) are higher by 30–20 cm1 than the frequency of the free ligand (880 cm1) [1564]. Rodley et al. [1565] have assigned the n(MO) of tertiary arsine oxide complexes at 440–370 cm1.

1.28. COMPLEXES OF SULFUR AND SELENIUM LIGANDS A large number of metal complexes of ligands containing sulfur and selenium are known. Here the vibrational spectra of typical compounds are reviewed briefly. For SO3 and thiourea complexes that form metal–sulfur bonds, see Secs. 1.13 and 1.15, respectively.

1.28.1. Complexes of Sn and Sen (n ¼ 2–6)

uller et al. [1566], most complexes containing the S22 ligand take the According to M€ following structures:

The n(S2) of free S2 is 623 cm1 (Table 2.1b of Part A). On coordination, the n(S2) is shifted to 560–510 cm1 in structures I (Z2-S2) and III {(m-S)(m-S)}, and to 510– 480 cm1 in structure II(m-S2). It is rather difficult, however, to distinguish these structures by vibrational spectroscopy. An extensive compilation of structural and vibrational data of disulfur complexes is found in a review by M€uller et al. [1566]. Here, some references for various types of coordination and n(S2) are given: MoO (S2)2(bipy) (I, 540 cm1) [1567], Ti(TPP)(S2) (I, 551 cm1) [1568], and Co2(CO)6(S2) (III, 615 cm1). The n(S2) of the last compound is unusually high because the SS bond is unusually short (1.98 A) [1569]. The Mo3 S213 ion contains three type I and three type II S2 ligands and one bridging S atom, and their n(S2) have been assigned using 92 Mo=100 Mo and 32 S=34 S isotope shift data [1570]. The n(Se2) of the free Se22 ion is at 349 cm1, and this band is shifted to 310 cm1 in [Ir(Se2)(DME)2]Cl [DME ¼ 1,2-bis(dimethylphosphino)ethane] [1571]. In general, the n(MS) and

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211

Fig. 1.100. Raman spectra of solid Cp2MSe5 (M ¼ Hf,Zr) in the 600–50 cm1 region (514.5 nm excitation) [1576].

n(MSe) are more difficult to assign empirically; the former is in the 380–300 cm1 region and the latter is lower. Although the S2n and Se2n ions (n ¼ 3–5) take open-chain configurations in the free state (Sec. 2.18 of Part A), they tend to form chelate rings in metal complexes. The n(SS) vibrations of the [Cu(Z2-S4)2]3 ion were assigned at 474 and 401 cm1 [1572]. The [M2O2(S2)(S4)]2 (M ¼ Mo,W) exhibits the n(S2) at 521–500 and 490– 420 cm1 for the S2 and S4 rings, respectively [1573]. The n(Se2) of the [Zn(Se4)2]2 ion are observed at 276 and 250 cm1 [1574], while those of the [Ni(Se4)2]2 ion are reported to be 364 and 348 cm1 [1575]. Figure 1.100 shows the Raman spectra of Cp2M(Se5) (M ¼ Hf and Zr) obtained by Butler et al. [1576]. The bands at 266(261), 247(245), and 200(214) cm1 have been assigned to ns(Se2), na(Se2), and n(MS), respectively. Raman frequencies are reported for the [M(S6)2]2 ion, where M is Zn, Cd, and Hg [1577]. In polymeric PdCl2(Z1-Se6), the Se6 rings are connected to two PdCl2 units by forming PdSe bonds, and the n(SeSe) ring vibrations are assigned at 275, 256, and 237 cm1 [1578]. 1.28.2. Complexes of S- and Se-Containing Ligands The n(NS) of N-bonded thionitrosyl (NS) complexes such as Cr(NS)(Cp)(CO)2 [1579] and Re(NS)(PMe2Ph)Cl2 [1580] are observed at 1180 cm1, which is 40 cm1 lower than that of free NS molecule. The n(NSe) of [Os(Tp)(NSe) Cl2]

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[Tp ¼ hydro-tris(1-pyrazolyl)borate] is 1156 cm1 [1581]. The n(SO) of Ru(SO)(Cl) (NO)(PPh3)2 at 1061 cm1 is 63 cm1 lower than that of free SO (1124 cm1) [1582]. Thiocarbonyl (CS) may form metal complexes of the following types:

The n(CS) of free CS is at 1285 cm1. The n(CS) of the C-bonded terminal CS group (structure I) is higher than that of free CS (1360–1290 cm1) [1583–1585]. The C-bonded bridge structure such as shown above (structure II) was found in [Co2(m-CS) (m-S2C2R2)(CO)3(m-dppm)] [R ¼ COOMe,COOEt); dppm ¼ 1,2-bis(diphenylphosphino)methane]. It exhibits the n(CS) at 1147 cm1 [1586]. The bridging C- and S-bonded structure (III) was suggested for (DPE)2(CO)W(m-CS)W(CO)5 [DPE ¼ l,2(diphenylphosphino)ethane], but its n(CS) was hidden by the DPE band near 1095 cm1 [1587]. An extremely low n(CS) (910 cm1) is reported for Co3Fe(Cp)(CO)9(CS) in which the CS group acts as a six-electron donor to the three Co atoms [1588]. Normal coordinate analyses have been made on M(CO)5(CX) (M ¼ Cr,W; X ¼ S,Se) [1589] and (C6H6)Cr(CO)2(CX) (X ¼ O,S,Se) [1590]. On coordination, N2S2 may form a chelating ring (Z2-N2S2) or a bridge between two metals (m-N2S2, structure II):

The N2S2 ligand in [Pt(Z2-N2S2)(PPh3)2] forms a five-membered chelate ring (structure I), and exhibits the n(NS) at 1045 cm1 [1591]. On the other hand, the N2S2 ligand in the [(VCl5)2(m-N2S2)]2 ion forms a bridge between two V atoms (structure II), and exhibits the n(N2S2) vibration at 858 cm1 [1592]. 1.28.3. Complexes of SO2, CS2, and Related Ligands Sulfur dioxide (SO2) may take one of the following structures when it coordinates to a metal:

COMPLEXES OF SULFUR AND SELENIUM LIGANDS

213

Free SO2 exhibits the na(SO2) and ns(SO2) at 1351 and 1147 cm1, respectively (Sec. 2.2 of Part A). Table 1.75 lists the modes of coordination and stretching frequencies of SO2 complexes. It is clearly not possible to determine the coordination geometry by vibrational data. In fact, most of the structures shown in the table were determined by X-ray analysis. According to Kubas [1608], the coordination geometry of the SO2 ligand can be deduced by combining spectroscopic and chemical properties. The SO2 stretching frequencies are useful, however, in distinguishing the O- and S-bonded complexes; a complex is O-bonded if (na ns) is larger than 190 cm1, and S-bonded if it is smaller than 190 cm1 [1600]. Johnson and Dew [1609] first oberved a linkage isomerization of the SO2 ligand in [Ru(NH3)4(SO2)Cl]Cl that changes from structures II to V by solid-state photolysis (365 nm, 25–195 K). The former stable isomer exhibits the na and nb at 1255 and 1110 cm1, respectively, whereas the latter unstable isomer exhibits them at 1165 and 940 cm1, respectively. Later, Kovalevsky et al. [1610] confirmed their results by photocrystallographic and IR studies on trans-[Ru(NH3)4(H2O)(SO2)](C6H5SO3)2. In the ground state, the SO2 ligand coordinates to the Ru as a unidentate (structure II) with TABLE 1.75. Structures and Observed Frequencies (cm1) of SO2 Complexes Complex IrCl(CO)(PPh3)2(SO2) PtBr(C6H3(CH2NMe2)2-o,o0 )(SO2) [Ru(NH3)4(SO2)Cl]Cl [Ru6C(CO)15(m-SO2)]2 Fe(CO)2{P(OMe3)}2(SO2) RuCl(C5Me5){P(i-Pr)3}(SO2) Ni{P(C6H5)3}3(SO2) Ni{P(C6H5)Me2}3(SO2) SbF5(SO2) {(C5H5)Fe(CO)2}2(m-SO2) Fe2(CO)8(m-SO2) RuCl(NO)(PPh3)2(SO2) OsCl(NO)(PPh3)2(SO2) [Fe6(CO)15C(m-SO2)]2 [Pt3Au(m-CO)2(m-SO2){P(C6H11)3}4]þ [Pd3(m-SO2)2(m-N3)(PPh3)3]

Structure I I II VI II II II I III IV IV V V VI VI VI

na 1198–1185 1231 1301–1278 1186 1225 1249 1195 1170 1327 1135 1203 — 1133 1180 1233 1202

ns 1048 1074 1100 1052 1095 1095 1052 1030 1102 993 1048 895 846 1045 1075 1062, 1051

Ref. 1593 1594 1595 1596 1597 1598 1599 1599 1600 1601 1602 1603 1604 1605 1606 1607

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the n(SO2) at 1279 and 1126 cm1. On photolysis (488 nm at 90 K), the metastable MS-2 state (structure V) similar to that of NO complexes (Sec. 1.20.3) is formed. As a result, the na and nb are downshifted to 1181 and 946 cm1, respectively. The increased separation of the na and nb reflects the difference in SO2 bond length between the two structures. Sulfur dioxide reacts with [MFe3(CO)14]2 (M ¼ Cr,Mo,W) to form [Fe3(CO)9(m3, 2 Z -SO2)]2 and [Fe3(CO)8(m-SO2)(m3-S)]2, shown below [1611] [all as the bistriphenylphosphonium imidium (PPN) salts]:

The n(SO) of the former are at 1162 and 902 cm1, while the latter exhibits it at 1066 cm1. Vibrational spectra of transition metal complexes of CS2 and CS have been reviewed by Butler and Fenster [1612]. According to Wilkinson et al. [1584,1613], CS2 coordinates to the metal in four ways:

The linear CS2 molecule in the free state exhibits the na(CS2)(IR), ns(CS2) (R), and d(CS2)(IR) at 1533, 658, and 397 cm1, respectively (Sec. 2.2 of Part A). In metal complexes, the SCS bond is bent, except for structure II shown above. Most vibrational studies on CS2 complexes report only the n(CS2) in the high-frequency region. The n(CS2) of structure I (Z2-bonded) and structure II (Z1-bonded) are at 1100–1150 and 1510 cm1, respectively. The n(CS2) of the bridging CS2 groups (structures III–IV) are in the 1155–1120 cm1 region [1613–1615]. From infrared and other evidence, [Ir(CS2)(CO)(PPh3)3]BPh4 was originally thought to be a six-coordinate complex with a Z2-bonded CS2 [1616]. However, X-ray analysis [1617] revealed an unexpected structure; a five-coordinate complex of the type [Ir(CO)(PPh3)2(S2CPPh3)]BPh4. Matrix cocondensation reactions of

COMPLEXES OF SULFUR AND SELENIUM LIGANDS

215

Ni atoms with CS2/Ar produce a mixture of Ni(CS2)n, where n ¼ 1,2,3. It was not possible, however, to determine the mode of coordination from their IR spectra [1618]. A mixed-carbon dichalcogenide such as SCSe can form a pair of geometric isomers

where L4 represents (CO)(CNR)(PPh3)2 (R ¼ p-tolyl). The n(CSe) of the former and the n(CS) of the latter have been observed at 1015 and 1066 cm1, respectively [1619]. The na(NSN) and ns(NSN) of the sulfur–diimine ligand (bent NSN) in Ag(NSN)Ag are at 1115 and 1012 cm1, respectively (IR) [1620]. Krieglstein and Breitinger [1621] have noted that the two types of sulfito (m-S,O) bridgings shown below can be distinguished by vibrational spectroscopy:

Complexes of monoalkylsulfides such as Hg(SR)2 (R ¼ CnH2nþ1, n ¼ 1–10,12) exhibit n(HgS) in the 415–220 cm1 region [1622], and the n(AgS) of a dialkylsulfide complex, Ag[S(CH3)2]NO3, is at 255/240 cm1 in IR spectra [1623]. Allkins and Hendra [1624] carried out an extensive vibrational study on cis-and trans-[MX2Y2] and their halogen-bridged dimers, where M is Pd(II) and Pt(II); X is Cl, Br, and I; and Y is (CH3)2S, (CH3)2Se, and (CH3)2Te. The n(ms), n(MSe), and n(MTe) were assigned in the ranges 350–300, 240–170, and 230–165 cm1, respectively. The vibrational spectra of PtX2L2 [1625], PdX2L2, [1626] AuX3L, and AuXL, [1627], where X is a halogen and L is a dialkylsulfide, have been assigned. Aires et al. [1628] reported the infrared spectra of MX3L3-type compounds, where M is Ru(III), Os(III), Rh(III), and Ir(III); X is Cl or Br; and L is Et2S and Et2Se. The n(MS) and n(MSe) of these compounds were assigned at 325–290 and 225–200 cm1, respectively, based on the fac structure. On the other hand, Allen and Wilkinson [1629] proposed the mer structure for these compounds, based on far-infrared and other evidence. According to X-ray analysis, Pd2Br4(Me2S)2 is bridged via Br atoms, whereas Pt2Br4(Et2S)2 is bridged via S atoms [1630]:

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Adams and Chandler [1631] have shown that halogen-bridged Pd2Cl4(SEt2)2 exhibits n(PdClt), n(PdClb), and n(PdS) at 366, 266, and 358 cm1, respectively, whereas sulfur-bridged Pt2Cl4(SEt2)2 exhibits n(PtClt) and n(PtS) at 365–325 and 422– 401 cm1, respectively. 1.28.4. Complexes of Thiometalates Vibrational frequencies of thiometalate ions such as MSn4 , MS3 On , and MS2 On3 are found in Sec. 2.2 of Part A. These ions form trinuclear bridging complexes such as shown below:

uller and coworkers have carried out an extensive study on structures and spectra of M€ transition metal complexes of thiometalates [1632–1634]. For example, they assigned the IR spectra of the [Ni(MoS4)2]2 ion using on normal coordinate analysis involving 58 Ni=62 Ni and 92 Mo=100 Mo isotopes [1635]. The IR-active metal-sulfur stretching frequencies follow the order

~nðcm 1 Þ

nðMoSt Þ 494ðB3u Þ

nðMoSb Þ 456ðB2u Þ 443ðB3u Þ

nðNiSÞ 332ðB3u Þ 324ðB2u Þ

Here, the asterisks indicate vibrational coupling between them. Figure 1.101 shows the electronic and Raman/RR spectra of the [Fe(WS4)2]3 ion obtained by M€uller and Hellmann [1636]. It is seen that the excitation at 647.1 nm gives a normal Raman spectrum, whereas those at 514.5 and 488.0 nm yield RR spectra. In the latter case, a series of overtones of n2 [totally symmetric n(WSt)], as well as a combination with n(FeS), are observed because the absorption band near 500 nm originates in an electronic transition within the WS4 ligand. For [ReOCl(WS4)2]2, the n(WSt) and n(WSb) were observed at 495 and 460 cm1, respectively [1637]. Vibrational spectra are also reported for metal-sulfur bridging complexes formed by the ligands such as PS34 [1638] and PS2 R22 (R ¼ Me, Ph, etc.) [1639]. The infrared spectra of thiocarbonato complexes of the type [M(Z2-CS3)2]2 [M ¼ Ni(II), Pd(II), and Pt(II)] have been studied by Burke and Fackler [1640] and Cormier el al. [1641]. The latter workers carried out normal coordinate analyses on the [58 Ni(Z2–CS3)2]2 ion and its 62 Ni analog. The n(NiS) were assigned at 385 and 366 cm1, with a corresponding force constant of 1.41 mdyn/A (UBF). The (n(C¼S) and n(CS) are at 1010 and 858/507 cm1, respectively. Band assignments are also reported for the [AuCl2(Z2-CS3)] [1642] and [Mo(Z2-CS3)4]3 [1643]. Vibrational assignments are available for metal complexes of other sulfur

COMPLEXES OF SULFUR AND SELENIUM LIGANDS

217

Fig. 1.101. Electronic absorption (top trace) and Raman/RR spectra (bottom traces) of [(Ph)3PNP (Ph)3]2 [N(Et)4][Fe(WS4)2 ]2CH3CN; the terminal n(WSt) (n2 and n3) and bridging n(WSb) (n1) are strongly coupled in this case, and the n(FeS) is at 270 cm1 [1636].

ligands such as K3[Pu(Z2-PS4)3] [1644] and Cp2Mo(Z2–S2SO2), which contains a dithiosulfate ion [1645]. Bis(dithioacetato)palladium, Pd(CH3CS2)2, exists in three different crystalline forms. Piovesana et al. [1646] characterized each of these forms by IR spectroscopy.

218

APPLICATIONS IN COORDINATION CHEMISTRY

1.28.5. Complexes of Relatively Large Ligands 2,5-Dithiahexane(dth) forms metal complexes such as [ReCl3(dth)]n and Re3Cl9(dth)1.5. Cotton et al. [1647] showed from infrared spectra that dth of the former forms a chelate ring in the gauche conformation, whereas that of the latter forms a bridge between two metals by taking the trans conformation (see Sec. 1.2.5). Infrared spectra have been used to show that ethanedithiol forms a chelate ring of the gauche conformation in Bi(S2C2H4)X, where X is Cl and Br [1648]. Schl€apfer et al. [1649] assigned the n(NiS) and n(NiN) of dth, ete [2-(ethylthio)ethylamine], and mea [mercaptoethylamine] complexes using the metal isotope technique:

The infrared spectra of N,N-dialkyldithiocarbamato complexes have been studied extensively. All these compounds exhibit strong n(C¼N) bands in the 1600– 1450 cm1 region. These compounds are roughly classified into two types:

The former exhibits n(CS) near 1000 cm1 as a single band, whereas the latter shows a doublet in the same region [1650]. Also, the n(C¼N) of the former, (above 1485 cm1) is higher than that of the latter (below 1485 cm1) [1651]. The n(MS) of the bidentate complexes are observed at 400–300 cm1[1652]. Dithiocarbamato complexes of Fe(III) undergo the high-spin (6 A1 )–low-spin (2 T 2 ) crossover. This change can be induced by applying high pressure or by lowering temperature [1653]. Sorai [1654] assigned the n(FeS) of Fe(S2CN(Et)2)3 of the high- and lowspin states to the IR bands at 355 and 552 cm1, respectively. Hutchinson et al. [1655] have shown by 54 Fe 57 Fe isotope shifts and variable-temperature studies that the n(FeS) of Fe(III) dialkyldithiocarbamates appear at 250–205 and 350– 305 cm1, respectively, for high- and low-spin states, and that intermediate-spin complexes show n(FeS) in both regions. Similar n(FeS) frequencies are reported for high-spin and low-spin Fe(III) complexes of [Fe(Z2-S2CNRR0 )3], where R ¼ CH2CH2OH, and R0 ¼ Me and Et [1656]. Nakamoto et al. [1657] performed normal coordinate analysis on the 1 : 1 (metal/ ligand) model of dithiocarbamato Pt(II) complex [Pt(S2CNH2)2] and its ND2 analog. The infrared spectra of diselenocarbamato complexes have been reported [1658] and assigned on the basis of normal coordinate analysis [1659]. In the Ni(II) complex,

COMPLEXES OF SULFUR AND SELENIUM LIGANDS

219

n(NiSe) is assigned at 298 cm1, which is lower by 85 cm1 than the n(NiS) of the corresponding dithiocarbamato complex. The infrared spectra of xanthato complexes, namely

have been studied by Watt and McCormick [1660]. The n(CO), n(cs), and n(MS) were assigned at 1325–1250, 760–540, and 380–340 cm1, respectively. Savant et al. [1661] roughly classified monothiobenzoato complexes into three categories:

In the Hg(II), Cu(I), and Ag(I) complexes, coordination occurs mainly via sulfur. In the Cr(III) complex, however, the CrO bond is stronger than the CrS bond. The Ni(II) complex is between these two cases and is close to symmetrical coordination. This is reflected in the frequency trends shown above. In the [In(PhCOS)4] ion, the ligand is bonded to In(III) through sulfur as a unidentate and exhibits the n(CO) and n(CS) vibrations at 1615 and 922 cm1, respectively [1662]. Similar frequencies (1630 and 910 cm1) are reported for the S-bonded As(PhCOS)3. In Bi(PhCOS)3, however, the ligand is chelated to the Bi atom through the S and O atoms (1589 and 923 cm1) [1663]. In thiocarboxylato complexes of the type Ni(RCOS)212 (EtOH) (R ¼ CH3, C2H5, Ph), the ligand serves as a bridge between two metals as shown

and their n(CO) are reported to be at 1580–1520 cm1 [1664]. The infrared spectra of metal complexes of thiosemicarbazides

and thiosemicarbazones

220

APPLICATIONS IN COORDINATION CHEMISTRY

have been reviewed by Campbell [1665]. For metal complexes of dithiocarbazic acid, infrared spectra support the N,S-chelated structure (shown below) rather than the S,Schelated structure normally found for dithiocarbamato complexes [1666]:

In 1 : 1 complexes of N,N0 -monosubstituted dithiooxamides, Desseyn et al. [1667] concluded from infrared spectra that metals such as Ni(II) and Cu(II) are primarily bonded to the N atom, whereas metals such as Hg(II), Pb(II), and Pd(II) are bonded to the S atom:

Infrared spectra are reported for [Au(HL)X2] (HL ¼ dialkylated dithiooxamide; X ¼ Cl or Br) in which the Au(III) atom is chelated to HL via its two S atoms. The n(AuS) is observed at 390–380 cm1 [1668]. Monothiooxamide coordinates to Cu(I) as a unidentate via S but as a bidentate to Cu(II) via S and O atoms [1669]. 2-Thiooxamic acid coordinates to Mn(II) as a bidentate via S and O atoms [1670]:

The vibrational spectra and band assignments of these metal complexes are reported. Dithiooxalato (DTO) complexes [M(DTO)2](PPh4)2 [M(II) ¼ Ni,Pd,Pt] are all chelated to the metal via two S atoms, and exhibit two n(MS) at 420/370,395/ 360, and 430/400 cm1, respectively, in the order of metals shown above [1671]. Two n(PtS) vibrations of [Pt(mnt)2]2 are observed at 332/318 and 357/330 cm1 for the Pt(II) and Pt(III) complexes, respectively [1672]. Here, mnt (maleodinitriledithiolato ion) is an analog of DTO in which its two oxygen atoms are replaced by CN groups. Coucouvanis et al. [1673] synthesized novel tin halide adducts of Ni(II) and Pd(II) dithiooxalto complexes:

COMPLEXES OF SULFUR AND SELENIUM LIGANDS

221

The n(NiS) of the [Ni(DTO)2]2 ion is at 349 cm1. In SnX4 (X ¼ Cl,Br,I) adducts, this band shifts to 385–375 cm1, indicating a strengthening of the NiS bond by complexation. It was found that Cr(DTO)3[Cu(PPh3)2]3 exists in two isomeric forms [1674], one in which the Cr atom is bonded to sulfur, and another in which it is bonded to oxygen of the DTO ion. As expected, n(C¼O), n(cs), and n[CrO(S)] are markedly different between these two isomers. Metal complexes of 1,2-dithiolates (or dithienes) have been of great interest to inorganic chemists because of their redox properties [1675]. Schl€apfer and Nakamoto [1676] prepared a series of complexes of the type [Ni(S2C2R2)2]n where R is H, Ph, CF3, and CN and n is 0, 1, or 2, and carried out normal coordinate analysis to obtain rough estimates of the charge distribution on the basis of the calculated force constants. Infrared spectra of metal complexes with thio-b-diketones have been reviewed briefly [1677,1678]. Siimann and Fresco [1679] and Martin et al. [1680] carried out normal coordinate analysis on metal complexes of dithioacetylacotone, monothioacetylacetone, and related ligands. For dithioacetylacetonato complexes, two n(MS) have been assigned at 390–340 and 300–260 cm1. Dithiomalonamide(H2A) is thiob-acetylacetone in which its two CH3 groups are replaced by NH2 groups. The n(NiS) of the [Ni(H2A)2]2þ ion are at 335 and 233 cm1 in IR spectra [1681]. L-Cysteine has three potential coordination sites (S,N,O), and infrared spectra have been used to determine the structures of its metal complexes. For example, the Zn(II) complex shows no n(SH), and its carboxylate frequency indicates the presence of a free COO group. Thus, the following structure was proposed [1682]:

On the other hand, the (S,O) chelation has been suggested for the Pt(II) and Pd(II) complexes [1683]. IR spectra show that the Fe atom in Fe(cyst)(H2O)1.5 is bonded to the S, N, and O atoms whereas that in Na2[Fe(cyst)2]H2O is bonded only to the S and N atoms [1684]. The n(SS) of L-cystine complexes of Cu(II), Ni(II), and Zn(II), and so on are observed near 500 cm1 [1685]. McAuliffe et al. [1686] studied the IR spectra of metal complexes of methionine [CH3SCH2CH2CH(NH2)COOH]. They found that most of the metals they studied [except Ag(I)] coordinate through the NH2 and COO groups, and that the CH3S groups of these complexes are available for further coordination to other metals. McAuliffe [1687] suggested that complexes of the type M(methionine)Cl2 [M ¼ Pd(II), Pt(II)] take the polymeric structure:

222

APPLICATIONS IN COORDINATION CHEMISTRY

Infrared spectra of metal complexes of sulfur-containing ligands are reported for 2-methylthioaniline, [1688] 8-mercaptoquinoline [1689], cyclic thioethers, [1690], N-alkylthiopicolinaimdes [1691], dithizone [1692], and N,N0 -dimethylthioformate [1693].

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Chapter

2

Applications in Organometallic Chemistry Vibrational spectra of organometallic compounds have been reported extensively [1], and comprehensive reviews are found in several monographs [2,3]. More limited reviews are available on specific metal elements or functional groups, and will be quoted in respective sections. Reference books on vibrational spectra of organic compounds [4–6] are useful in making band assignments since vibrational spectra of organometallic compounds in the high-frequency region are largely due to organic ligands or moieties. Spectral charts [7,8] and an index [9] of vibrational spectra of organic and organometallic compounds are also useful for this purpose. In the following, we review vibrational spectra of organometallic compounds with emphasis on metal-carbon streching vibrations in the low-frequency region since they provide structural and bonding information on the metal-carbon skeleton. 2.1. METHYLENE, METHYL, AND ETHYL COMPOUNDS The smallest organometallic compound may be a metal carbene in which the carbon atom of the methylene group is s-bonded to a metal (MCH2). Such a compound can be prepared via cocondensation reaction of metal atom vapor with diazomethane in inert gas matrices: M þ CH2 N2 ! MCH2 þ N2

Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, Sixth Edition, by Kazuo Nakamoto Copyright 2009 John Wiley & Sons, Inc.

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Margrave and co-workers obtained the IR spectra of CuCH2 [10], FeCH2 [11], and CrCH2 [12] in Ar matrices. These molecules are planar and exhibit the six normal vibrations shown in Fig. 1.33. In the case of CrCH2, the na(CH2), ns(CH2), d(CH2), rr(CH2) and n(CrC) are observed at 2967, 2907, 688, 450, and 567 cm1, respectively. The n(CuC) in CuCH2 is at 614 cm1, and the n(FeC) in FeCH2 is at 624 cm1. Bare et al. [13] also obtained MgCH2 and its carbenoid radicals XMgCH2 by the reaction of laser-ablated Mg atoms with CH3X (X ¼ H,F,Cl,Br) diluted in Ar. The n(MgC) of MgCH2 was at 502.2 cm1. There are many compounds in which the carbon atom of the methyl group is s-bonded to a metal (MCH3). Vibrational spectra of these methyl compounds can be interpreted in terms of the normal modes of a 1:1 (metal/methyl) model as shown in Fig. 1.2. The na(CH3), ns(CH3), dd(CH3), ds(CH3), and rr(CH3), and n(MC) are at 3000– 2800, 3000–2700, 1400–1350, 1300–1100, 950–700, and 700–400 cm1, respectively. The simplest is the 1:1 (metal/methyl) complex such as NaCH3 and KCH3, which can be prepared in Ar matrices at 20 K [14]. Table 2.1 lists the Raman frequencies of typical M (CH3)4-type compounds. Figure 2.1 shows the Raman spectra of Si(CH3)4 and Si(CD3)4 obtained by Fischer et al. [17]. In Fig. 2.2, the observed frequencies are plotted as a function of the atomic mass of the group IVB elements [18]. It is seen that the ds(CH3), rr(CH3), n(MC), and d(CMC) are shifted progressively to lower frequencies as the atomic mass increases. The rr(CH3) and n(MC) are particularly sensitive to the nature of these metals. Under Td symmetry, two n(MC) (A1 and F2) are expected for the M (CH3)4-type molecule. These vibrations are reported to be 508 and 530 cm1, respectively, for Sn(CH3)4, [19] and 598 and 696 cm1, respectively, for Si(CH3)4 [20]. Tables 2.2 and 2.3 list the MC stretching and CMC bending frequencies of various M(CH3)n-type molecules and ions observed in IR and/or Raman spectra. The number of IR- and Raman-active skeletal vibrations expected for each structure are found in Appendix Vof Part A. As already seen in the M(CH3)4 series, both n(MC) bands are downshifted progressively as the mass of the central metal increases in the same family of the periodic table. Thus, the orders of n(MC) are AlðCH3 Þ3 > GaðCH3 Þ3 > InðCH3 Þ3 TABLE 2.1. Raman Frequenciesa of M(CH3)4-Type Molecules (cm1) [15,16] Compound

na(CH3)

ns(CH3)

dd (CH3)

ds(CH3)

rr(CH3)

n(MC)

d(CMC)

C(CH3)4

2959 2963 (2959) 2964 (2910) (2981) 2982 (2984) 2988 2996 2924

2922

(1475) 1457 (1430) 1421

926 (926) 870 (870)

733 1260 593 698

418 332 239 190

— (828) — (768) 767 700

561 599 509 527 478 459

196 188 137 133 145 130

Si(CH3)4

Ge(CH3)4 Sn(CH3)4 Pb(CH3)4 a

( ) ¼ IR frequency.

2913

2920 2920 2924

(1430) 1420 (1447) — 1450 1400

1271

1259 1211 1170 1154

METHYLENE, METHYL, AND ETHYL COMPOUNDS

Fig. 2.1. Polarized and depolarized Raman spectra of Si(CH3)4 and Si(CD3)4 [17].

Fig. 2.2. Vibrational frequencies of M(CH3)4-type compounds [18].

277

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

TABLE 2.2. Metal–Carbon Skeletal Frequencies of M(CH3)n-Type Compounds (cm1) Compound

Structure

Be(CH3)2 Zn(CH3)2 Cd(CH3)2 Hg(CH3)2 Se(CH3)2a Te(CH3)2a B(CH3)3 Al(CH3)3 Ga(CH3)3 In(CH3)3 P(CH3)3 As(CH3)3 Sb(CH3)3 Bi(CH3)3 Si(CH3)4 Ge(CH3)4 Sn(CH3)4 Pb(CH3)4 Ti(CH3)4 Sb(CH3)5

Linear Linear Linear Linear Bent Bent Planar Planar Planar Planar Pyramidal Pyramidal Pyramidal Pyramidal Tetrahedral Tetrahedral Tetrahedral Tetrahedral Tetrahedral Trigonal–bipyramidal

W(CH3)6

Octahedral

na(MC)

ns(MC)

d(CMC)

Ref.

1081 604 525 538 604 528 1177 760 577 500 703 583 513 460 696 595 529 476 577 514b 456c

— 503 460 515 589 528 680 530 521.5 467 653 568 513 460 598 558 508 459 489 493b 414c

21 22–26 22,24–27 22,24–26 28,29,31 28–31 32–34 35–37 36–40 39,41,42 43–45 43,44,46 47 47 48,49 16,49 18,49 49,50 51 52

482

— 157 140 160 233 198 341,321 170 162.5 132 305,263 238,223 188 171 239,202 195,175 157 120 180 213b 199b 104c —

53

a

New assignments have been proposed on the basis of D3d model containing a linear CMC skeleton (see Ref. 31). b Equatorial. c Axial.

TABLE 2.3. Metal–Carbon Skeletal Frequencies of [M(CH3)n]mþ-Type Compounds (cm1) na(MC)

ns(MC)

d(CMC)

Refs.

— Linear Linear Linear Planar Planar Nonplanar Nonplanar Tetrahedral

— 566 559 582 557 582 602 534 783

557 502 498 529 521 536 580 — 649

Tetrahedral Tetrahedral

652 574

590 535

— — 114 180 152 166 272 — 285 170 217 178

54 55 56,57 58 59 60 61 62 63 64 47 47

Compound

Structure

[Zn(CH3)]þ [In(CH3)2]þ [Tl(CH3)2]þ [Sn(CH3)2]2þ [Sn(CH3)3]þ [Sb(CH3)3]2þ [Se(CH3)3]þ [Te(CH3)3]þ [P(CH3)4]þ [As(CH3)4]þ [Sb(CH3)4]þ

METHYLENE, METHYL, AND ETHYL COMPOUNDS

279

and PðCH3 Þ3 > AsðCH3 Þ3 > SbðCH3 Þ3 > BiðCH3 Þ3 An exception is found in the linear M(CH3)2 series: Zn(CH3)2 ns(MC) (cm1) na(MC) (cm1)

503 604

Cd(CH3)2 > >

460 525

Hg(CH3)2 <
Ga > In Si > Ge > Sn > Pb P > As > Sb > Bi Among the six modes, t and u are particularly metal-sensitive because they correspond to the n(MC) and d(CMC), respectively. The number of these bands reflects the local symmetry of the MCn skeleton. For example, in the M(C6H5)3 (M ¼ P, As,Sb,Bi) series, two n(MC) bands (A1 and E) are expected in IR as well as in Raman

Fig. 2.4. Metal-sensitive modes of the M C6H5 moiety.

283

HALOGENO, PSEUDOHALOGENO, AND ACIDO COMPOUNDS

TABLE 2.6. Vibrational Frequencies of Metal-Sensitive Modes of Metal Phenyls (cm1) Compound

q

r

y

t

x

u

Ref.

Hg(C6H5)2

1067

661

456

207

139,140

10

893

600

245

408

141,142

Al(C6H5)3

1285 1248 1085

258 252 248 650

460

420

207

332

142,143

Ga(C6H5)3

1085

670 643 665

315

180

1070

673

270

180

Si(C6H5)4

1108

709

Ge(C6H5)4

1091

435 239 332

Sn(C6H5)4

1075

616

1061

645

185 171 187 168 193 152 147

145,147,148

Pb(C6H5)4

181

145

P(C6H5)3

1089

— 667

Sb(C6H5)3

1082 1074 1065

651

457

216

Bi(C6H5)3

1055

448

270 257 237 220

209 190 192 183 166

147,149

As(C6H5)3

519 511 481 465 459 448 450 440 501 509 474

245 225 248 195 261 223 232 214 225

142

In(C6H5)3

453 445 465

207

157

147,149,150

BðC6 H5 Þ3

268 212 223 201 428 398 313

248 268 237

142,144 145 145,146

147,149 147,149

spectra since, the symmetry of the MC3 skeleton is C3v. This is clearly demonstrated in Fig. 2.5, where the IR and Raman spectra of these compounds in benzene solution are shown [149]. The spectra obtained in the solid state [149] show further band splittings due to lowering of symmetry and intermolecular interaction.

2.3. HALOGENO, PSEUDOHALOGENO, AND ACIDO COMPOUNDS There are many organometallic compounds containing functional groups other than those discussed in the preceding sections. Vibrational spectra of these compounds can be interpreted approximately as the overlap of bands discussed previously and those described below. 2.3.1. Halogeno Compounds Table 2.7 lists metal-carbon and metal-halogen stretching frequencies of typical compounds. The n(MCH3) and n(MC6H5) are observed in the, 750–450 and

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

Fig. 2.5. (a) Infrared and (b) Raman spectra of M(C6H5)3 (M ¼ P, As, Sb, Bi) in benzene solution [149].

300–190 cm1 regions, respectively, as previously found in methyl and phenyl compounds. The n(MX) (terminal) vibrations (Sec. 1.23) appear in the following regions: nðMFÞ 800--400

nðMClÞ 550--200

nðMBrÞ 450--140

nðMIÞ 260--100 ðcm1 Þ

These ranges are rather wide because the n(MX) varies markedly depending on the structure. This is clearly demonstrated in Table 2.7. As shown previously for n(MC), the n(MX) also becomes lower as the mass of M increases in the same family of the periodic table:

nðM ClÞðcm1 Þ

SiðCH3 Þ3 Cl 472

>

GeðCH3 Þ3 Cl 378

>

SnðCH3 Þ3 Cl 318

GeðC6 H5 ÞCl3 SiðC6 H5 ÞCl3 SnðC6 H5 ÞCl3 nðM ClÞðcm Þ 383;363 596;518 > 427;407 > 1

The same trends are observed for the n(MCH3) and n(MC6H5) of these compounds. Normal coordinate analyses have been carried out on Si(CH3)3Cl [165], Si(CH3)2 Cl2 [166], Si(CH3)Cl3 [167], Si(CH3)Cl3 [168], Si(tBu)X3(X ¼ F, Cl, Br, etc.) [169], Sb(CH3)3X2 [159], Ti(CH3)Cl3 [152], cis-[Au(CH3)2X2] [160], Si(C2H5)3F[170], and Tl(C6H5)X2 [162]. Detailed band assignments are reported for the Sn(CH3)nCl4n [19] and M(C6H5)Cl3 (M ¼ Si,Ge,Sn) series [171].

HALOGENO, PSEUDOHALOGENO, AND ACIDO COMPOUNDS

285

TABLE 2.7. Metal–Carbon and Metal–Halogen Stretching Frequencies of Organometal Halogeno Compounds (cm1) Compound

n(MC)

Cd(CH3)Cl Cd(CH3)Br Cd(CH3)I Ti(CH3)Cl3

476 475 482 536

Al(CH3)Cl2

653

Al(CH3)2Cl

603

Si(CH3)3F

704 635 704 635 623 576 612 569 548 518 591 546

Si(CH3)3Cl Ge(CH3)3F Ge(CH3)3Cl Sn(CH3)3Cl Sb(CH3)3F2a

Ref.

Compound

247 206 167 551 390 564 425 691 453 898

151 151 151 152

Sb(CH3)3Cl2a

472

155

623

156

378

157

318

19

484 465

158 159

n(MX)

n(MC)

n(MX)

Ref.

Tl(C6H5)Cl2

273

Tl(C6H5)Br2

206

Bi(C6H5)3F2a Bi(C6H5)3Cl2a

202 213 195 215 195 276 232

282 272 215 168 144 122 272 268 197 179 525 500 382 332 270 258 445 253

158 159 158 159 158

[Au(CH3)2Cl2]– (cis) [Au(CH3)2Br2]– (cis) Tl(C6H5)F2

577 538 569 526 559 508 572 563 558 552 283

153

163

339

164

Sb(CH3)3Br2a Sb(CH3)3I2a

153 153 154

Bi(C6H5)3Br2a Sn(C6H5)3Cl

a

160 160 161 162 162 163 163

The halogens occupy the axial positions of trigonal bipyramias.

In condensed phases, halogeno compounds tend to polymerize by forming halogeno bridges between two metal atoms. As discussed in Sec. 1.25, the bridging frequencies are much lower than the terminal frequencies. For example, in [Al(CH3)Cl2]2

the n(AlClt) are at 495 (Ag) and 485 (Bu), whereas the n(AlClb) are at 345 (Ag), 380 (Au), and 322 cm1 (Bu) [153]. As shown in Table 2.7, the n(AlCl) of monomeric Al(CH3)Cl2 are at 425 (ns) and 564 cm1 (na). Thus, it is possible to distinguish monomeric and polymeric structures by vibrational spectroscopy. It was found that B(CH3)2X (X ¼ F,Cl,Br) is monomeric [172], whereas Al(CH3)2F, Ga(CH3)2F, and In(CH3)2Cl are tetrameric [173], trimeric [174], and dimeric [175], respectively, in benzene solution. Alkyl silicon and germanium halides tend to be monomeric, whereas alkyl tin and lead halides tend to be polymeric in the

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

liquid and solid phases. For example, Sn(CH3)2F2 and Sn(CH3)3F are polymerized through the fluorine bridges [176]:

The n(SnF) for terminal bonds are at 650–625 cm1, whereas those for bridging bonds are at 425–335 cm1. In the solid state, the coordination number of tin is five or six. Dialkyl compounds prefer six-coordinate structures, while trialkyl compounds tend to form five-coordinate structures. In both cases, the favored positions of the alkyl groups are those shown in the diagrams above. Normal coordinate calculations have been made on the trans-[Sn(CH3)2X4]2 (X ¼ F,Cl,Br) series [177]. Pb(CH3)3X (X ¼ F,Cl,Br,I) are monomeric in benzene but polymeric in the solid state; n(PbCl) of the monomer and polymer are observed at 281 and 191 cm1, respectively [178]. In the [Au(CH3)2X]2 series, the Au atom takes a square–planar arrangement with two bridging halogens. The n(AuXb) of these compounds are at 273 and 256 for X ¼ Cl, 181 for X ¼ Br, and 144 and 131 cm1 for X ¼ I [179]. Figure 2.6 shows the tetrameric structure of [Pt(CH3)3X]4-type compounds. Vibrational spectra have been reported for [Pt(CH)3X]4 (X ¼ Cl,Br,I) [180,181] and [Pt(CH3)2X2]n (X ¼ Cl,Br,I; n is probably 4) [182].

Fig. 2.6. Structure of [Pt(CH3)3X]4; R denotes CH3.

HALOGENO, PSEUDOHALOGENO, AND ACIDO COMPOUNDS

287

2.3.2. Pseudohalogeno Compounds Vibrational spectra of coordination compounds containing pseudohalogeno groups such as CN, SCN, CNO, and N3 ions were discussed in Sec. 1.17. The IR spectra of organometallic compounds involving these ligands have been reviewed by Thayer and West [183]. Although these compounds may have corresponding linkage isomers, only one of the isomers is generally stable for a given metal. The linear triatomic pseudohalogeno groups exhibit three vibrations in the regions 2300–2000 (na), 1500–850 (ns), and 700–450 cm1 (d). The na is most useful as a diagnostic test of these groups because of its strong appearance in the, IR region, which is free from interference by other bands. As an example, Fig. 2.7 shows the IR spectrum of Si(CH3)3N3 obtained by Thayer and Strommen [184]. Vibrational spectra of azido compounds are reported for Zn(CH3)N3 [185], Hg(CH3)N3 [186], A1(CH3)2N3 [187], Ga(CH3)2N3 [188], Ge(CF3)3N3 [189], Si(CH3)3N3 [190], and Sn(CH3)3N3 [191]. The n(MN) of these compounds are in the 600–400 cm1 region. The NCO group is always bonded to the metal via the N atom (isocyanato compound). Vibrational spectra of monomeric CH3HgNCO [192], (CH3)3SiNCO [190,193], (CH3)3GeNCO [194], (CH3)3Sb(NCO)2 [192], and (C6H5)3Bi(NCO)2 [163] have been reported. The NCS group may be bonded to a metal through the N or S atom or may form a bridge between two metals by using the N, the S, or both atoms. It is not easy to distinguish all these possible structures by vibrational spectra. Table 2.8 lists the modes of coordination and the vibrational frequencies of the MNCS group. The NCSe group is N-bonded in (CH3)3SiNCSe [200] and (CH3)3GeNCSe [201] but is Se-bonded in (CH3)3PbSeCN [202]. Only a very few compounds containing the MCNO (fulminato) group are known. The spectrum of (CH3)2Tl(CNO) is similar to that of K[CNO], and the TlCNO bond may be ionic [203]. No isofulminato complexes are reported. The CN group is usually bonded to the metal through the C atom. In the case of (CH3)3M(CN) (M ¼ Si,Ge), however, the cyano and isocyano complexes are in equilibrium in the liquid phase, although the mole fraction of the latter is rather small. The CN stretching frequencies (cm1) of these isomers are as follows:

Fig. 2.7. Infrared spectrum of Si(CH3)3N3 (liquid film) [184].

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

TABLE 2.8. Vibrational Frequencies of Thiocyanato and Isothiocyanato Compounds (cm1) Mode of Coordination

Compound

n(CN) 2190 2140

[CH3Zn(SCN)]¥ [CH3Hg(SCN)3]2

[(CH3)3A1(SCN)]

d(NCS)

195

276

196

Al—SCN

2097

845

485

627

501 438

779

474 467

2050

781

485

2163

775

444 430

(CH3)3Sn(NCS), solid (polymer)

Sn—NCS—Sn

2098 2079 2046

(CH3)3Sn(NCS), CS2 solution (monomer)

Sn—NCS

[(CH3)2Au(NCS)]2

)

Ref.

445

2119

2075

n(MS) or n(MN)

685

Hg—SCN

[(CH3)2A1(SCN)]3

a

n(CS)

a

335

197

198

199

478

199

179

These bands may be assigned to n(AuN).

M ¼ Si M ¼ Ge

(CH3)3MCN

(CH3)3MNC

2198 2182

2095 2090

(gas phase) [204] (CHC13 solution) [205]

Complete band assignments for (CH3)3GeCN and its deuterated analog have been made based on normal coordinate analysis [206].

2.3.3. Acido Compounds The free acetate ion (CH3COO) exhibits the na(COO) and ns(COO) at 1578 and 1414 cm1, respectively (Sec. 1.9.4). Ifit is covalently bonded to a metal as a unidentate ligand, the na and ns are shifted to higher and lower frequencies, respectively:

HALOGENO, PSEUDOHALOGENO, AND ACIDO COMPOUNDS

289

Figure 2.8 shows the IR spectra of the Si(CH3)n(CH3COO)4n (n ¼ 0–3) obtained by Okawara et al. [207]. The n(C¼O) are observed at 1765, 1748, 1732, and 1725 cm1, respectively, for n ¼ 0,1,2,3. In contrast, analogous Sn compounds such as Sn(CH3)3(CH3COO) and Sn(CH3)2Cl(CH3COO) exhibit two n(COO) near 1600

Fig. 2.8. Infrared spectra of (a) Si(CH3)3(OCOCH3), (b) Si(CH3)2(OCOCH3)2, (c) Si(CH3) (OCOCH3)3, (d) Si(OCOCH3)4 [207].

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

and 1420 cm1, indicating the ionic character of the tin-acetate interaction [207,208]. In Sb(CH3)3(CH3COO)2, however, the two acetate groups occupy the axial positions of the trigonal–bipyramidal structure, and are covalently bonded to the metal as unidentate ligands [209]. As stated earlier, the trigonal-planar Sn(CH3)3 group in Sn(CH3)3F is polymerized by forming the SnFSn bridges in the solid state. Similar polymerization occurs in Sn(CH3)3L where L is CH3COO [210], NO3 [211], ClO4 [212], and other acido groups:

As expected for symmetric bidentate coordination, the separation of the na (1600 cm1) and ns (1363 cm1) of the first compound (237 cm1) is much smaller than those observed for unidentate coordination (460 cm 1) [207]. In agreement with the local symmetry of the NO3 group in the structure shown above (C2v or lower), Sn(CH3)3(NO3) exhibits three n(NO) (1452, 1300, and 1021 cm 1) in IR spectra [211]. Symmetric bidentate coordination of the ClO4 group in Sn(CH3)3(ClO4) is also confirmed by the observation of four n(ClO) at 1212, 1112, 998, and 908 cm 1 in IR spectral [212]. If Sn(CH3)3L is tetrahedral, two n(Sn C) vibrations should be IR-active. Thus, the tetrahedral Sn(CH3)3Cl molecule exhibits two n(Sn C) bands at 545 and 513 cm 1 [207]. On the other hand, only one n(Sn C) vibration is expected in IR spectra if it contains a trigonal-planar Sn(CH3)3 group. Examples of the latter are found in the polymeric Sn(CH3)3 L-type compounds discussed above. These compounds exhibit only one n(Sn C) band near 550 cm 1. Tetrahedral Sn(CH3)2X2 (X ¼ Cl,Br,I) molecules exhibit two n(Sn C) vibrations at 560 and 515 cm 1 in IR spectra because their C Sn C groups are bent [207]. The C Sn C groups in polymeric [Sn(CH3)2]2O(CO3) and [Sn(CH3)2(NCS)]2O may be bent since they show two n(Sn C) bands in IR spectra [213]. Only one n(Sn C) vibration is expected for a linear C Sn C group.

2.4. COMPOUNDS CONTAINING OTHER FUNCTIONAL GROUPS 2.4.1. Nitrogen Donors The n(MN) of ammine and related ligands have been discussed in Sec. 1.l. The n(MN) of [Hg(CH3)(NH3)]þ [214], Ga(CH3)3(NH3) [215], and [Sn(CH3)3(NH3)]þ [216] are observed at 585, 350, and 503 cm 1, respectively. The IR spectrum of Al(CH3)3(NH3) in Ar matrices shows significant nonplanarity of the AlC3 skeleton [217]. In

COMPOUNDS CONTAINING OTHER FUNCTIONAL GROUPS

291

Sn(CH3)2(bipy)Cl2, the Sn(CH3)2 group is linear because its IR spectrum shows only one n(SnC) near 575 cm1 [218]. The IR bands at 427 and 346 cm1 were tentatively assigned to the n(SnN). The IR and Raman spectra of [Pt(CH3)3(NH3)3]þ are interpreted on the basis of the fac structure (C3n): the n(PtN) are at 390 (A1, Raman) and 410 and 377 cm1 (E, IR) [219]. 2.4.2. Oxygen and Sulfur Donors Compounds containing the hydroxo (OH) group exhibit n(OH), d(MOH), and n(MO) at 3760–3000, 1200–700, and 900–300 cm1, respectively. As expected, these frequencies depend heavily on the metal and the strength of the hydrogen bond involved. The n(SiO) of Si(CH3)3(OH) is at 915 cm1 [220], whereas the n(SnO) of Sn(CH3)3(OH) is at 370 cm1 [221,222]. Polymeric structures involving the trigonalplanar Sn(CH3)3 group and SnOHSn bridges were proposed for the latter compound because it exhibits only one n(SnC) at 540 cm1. Figure 2.9 shows the IR spectra of Sn(CH3)3(OH) obtained by Okawara and Yasuda [221]. References on other hydroxo compounds are as follows: Si(C2H5)2(OH)2 [223], Sb(CH3)4(OH) [224], M(CH3)2(OH)2 (M ¼ Ge,Sn,Pb) [225], [Ga(CH3)2(OH)]4 [226], and [Pt(CH3)3(OH)]4 [227]. Si(CH3)3(SH) exhibits the n(SH) and n(SiS) at 2580 and 454 cm1, respectively, in the liquid state [228]. The IR spectra of Sn(CH3)2(OR)2 (R ¼ Me, Et, etc.) exhibit the n(CO) and n(SnO) in the 1070–1000 and 650–550 cm1, respectively [229]. Other pertinent references are Al(CH3)2(OCH3) [230] and Si(CH3)2(OCH3)2 [231]. The n(MS) of Si (CH3)3(SC2H5) [232], Si(CH3)3(SC6H5) [233], Ge(CH3)3(SCH3) [234], and Sn (CH3)2 (SCH3)2 [235] are assigned at 486, 459, 394, and 347 cm1, respectively. The vibrational spectra of aquo complexes are characterized by the bands discussed in Sec. 1.82. References are cited for [Sn(CH3)3(OH2)2þ [236], [Hg(CH3)(OH2)]þ [237], and [Pt(CH3)3(OH2)3]þ [238]. Vibrational assignments are also reported for the IR spectrum of Ge(CH3)2O, which exhibits the n(Ge¼O), na(GeC2) and ns(GeC2) at 943, 584, and 546 cm1, respectively [239]. Methyltrioxorhenium(VII), Re(CH3)O3, exhibits the ns(ReO), na(ReO), and n(ReC) at 998, 947, and 575 cm1, respectively, in IR spectra [240].

Fig. 2.9. IR spectra of Sn(CH3)3(OH). — and - - - - - - indicate the spectra obtained in Nujol (or hexachlorobutadiene) mull and CCl4 solution, respectively [221].

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

Vibrational spectra of O-bonded (chelated) acetylacetonato (acac) complexes were discussed in Sec. 1.14. Some references for acac complexes of metal alkyls are Ga(CH3)2(acac) [226], Sn(CH3)2(acac)2 [241], Pb(CH3)2(acac)2 [242], Sb(CH3)2 (acac)Cl2 [243], and Au(CH3)2(acac) [244]. The structure of Sn(CH3)2(acac)2 was originally suggested to be trans in solution and in the solid state on the basis of NMR and vibrational spectra [245]. Later, the cis structure was proposed because of the large dipole moment (2.95 D) in benzene solution [246]. X-Ray analysis showed, however, that the compound is trans in the solid state [247]. Ramos and Tobias [241] suggest that the structure remains trans in solution and that the large dipole moment may originate in the nonplanarity of the SnO4 plane with the remainder of the acac ring.

2.4.3. Hydrido Compounds Metal hydrido (H) complexes exhibit the n(MH) and d(MH) in the 2250–1700 and 800–600 cm1 regions, respectively (Sec. 1.24.2). The n(MH) and n(MC) of typical compounds are listed in Table 2.9. It is seen that the n(MH) decreases as the mass of the metal increases in the same family of the periodic table and as more methyl groups are bonded to the metal in the M(CH3)nH4n (n ¼ 1–3) series. Vibrational spectra of B2H6-type molecules were discussed in Sec. 2.10 of Part A. Methyldiboranes, (CH3)nB2H6n (n ¼ 1–4), exhibit terminal and bridging n(BH) at 2600– 2500 and 2150–1525 cm1, respectively [258]. The dimeric [(CH3)2AlH]2 species is predominant in the gaseous dimethylaluminum hydride; the na(AlH) and ns(AlH) of the AlHAl bridges are observed at 1353 and 1215 cm 1, respectively [259].

TABLE 2.9. Metal–Hydrogen and Metal–Carbon Stretching Frequencies of Typical Hydrido Compounds (cm 1) Compound CH3SiH3 (CH3)2SiH2 (CH3)3SiH (CH3)2GaH CH3GeH3 (CH3)2GeH2 (CH3)3GeH CH3SnH3 (CH3)2SnH2 (CH3)3SnH (CH3)3PbH (CH3)2PH (CH3)2AsH

n(MH) 2166,2169 2145, 2142 2123 1869.5 2086,2085 2080,2062 2049 1875 1869 1837 1709 2288 2080

n(MC)

Ref.

701 728,659 711,624 589.5 604 604,590 601,573 527 536,514 521,516 — 703,660 580, 565

248 249 249 250 251 252 252 253 254 254 255 256 257

COMPOUNDS CONTAINING OTHER FUNCTIONAL GROUPS

293

TABLE 2.10. Metal–Metal Stretching Frequencies of Metal Alkyl Compounds (cm1) n(MM0 )

Compound (CH3)3SiMn(CO)5 (CH3)3GeCr(CO)3(p-Cp) (CH3)3GeMn(CO)5 (CH3)3GeCo(CO)4 (CH3)3SnMo(CO)3(p-Cp) (CH3)3SnMn(CO)5 (CH3)3SnRe(CO)5 (CH3)3SnCo(CO)4

297 (R) 119 (R) 191 (R) 192 (R) 172 (IR) 182 (IR, R) 147 (R) 176 (IR, R)

Ref. 260 261 262 263 264 264 262 263

2.4.4. Metal–Metal Bonded Compounds As discussed in Sec. 1.26, the n(MM0 ) of metal–metal bonded compounds are generally strong in the Raman and weak in the infrared. Table 2.10 lists the n(MM0 ) of some metal–metal bonded compounds. Figure 2.10 shows the n(SnMn) bands observed in far-IR spectra of (CH3)3nClnSnMn(CO)5-type compounds [264]. The vibrational spectra of (CH3)3MM(CH3)3 (M ¼ Si,Ge,Sn,Pb) [265] and M[Si (CH3)3]4 (M ¼ Si,Ge,Sn) [266] are reported.

Fig. 2.10. Far-IR spectra of (CH3)3nClnSnMn(CO)5 where n ¼ 0, 1, 2, or 3; the arrow indicates the n(SnMn) band [264].

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

2.5. p-BONDED COMPLEXES OF OLEFINS, ACETYLENES, AND RELATED LIGANDS Vibrational spectra of p-bonded complexes of ethylene, acetylene, and related ligands have been reviewed by Davidson [267]. In contrast to s-bonded complexes (Sec. 2.2), the n(C¼C) and n(C:C) bands of p-bonded complexes show marked shifts to lower frequencies relative to those of free ligands. 2.5.1. Complexes of Monoolefins Ethylene and other olefins form p-complexes with transition metals. Free ethylene Exhibits 12 (3 6 6) normal modes that are classified into 3Ag(R) þ Au (i.a.) þ 2B1g(R) þ B1u(IR) þ B2g(R) þ 2B2u(IR) þ 2B3u(IR) under D2h(Vh) symmetry. Figure 2.11 shows the approximate normal modes and observed frequencies of these vibrations [6]. The simplest and best-studied complex is Zeise’s salt, K[Pt(C2H4)Cl3]H2O, in which the ethylene molecule replaces one of the Cl atoms of the square–planar PtCl42 ion with its C¼C axis perpendicular to the PtCl4 plane. According to Chatt et al. [268], the Pt(II)–ethylene interaction is described in terms of two bonding schemes: (A) a stype bond is formed by electron donation from the filled 2pp bonding orbital of the olefin to the vacant dsp2 bonding orbital of the metal, and (B) a p-type bond is formed

Fig. 2.11. Approximate normal modes of vibration of ethylene; symmetry, vibrational assignments, and observed frequencies (cm 1) are given for each vibration; the n(C¼C) and ds(CH2) are vibrationally coupled in the Ag species.

p-BONDED COMPLEXES OF OLEFINS, ACETYLENES, AND RELATED LIGANDS

295

by backdonation of electron from a filled dp hybrid orbital of the metal to the 2pp* antibonding orbital of the olefin. This is illustrated below:

The real bonding is somewhere between A and B, and the latter becomes more predominant as the oxidation state of the metal becomes lower. The results of X-ray analysis [269] as well as MO calculations [270] seem to indicate that bonding scheme A is predominant in the case of Zeise’s salt. The vibrational spectra of Zeise’s salt have been studied by several investigators [271–274]. The effects of coordination on the vibrational frequencies of free ethylene are (l) The n(C¼C) coupled with the ds(CH2) is shifted markedly from 1623 to 1526 [271] or 1243 cm1 [273], (2) the rr(CH2) are shifted to lower frequencies (1030 to 840/720 cm 1), and (3) the rw(CH2) and rt (CH2) are shifted to higher frequencies (945 to 1010/975 and 1023 to 1180 cm 1, respectively) [273]. The directions of these shifts are anticipated since (1) and (2) are in-plane, whereas (3) are out-of-plane vibrations. Approximate band assignments of IR spectra of Zeise’s salt and its deuterated analog were first made by assuming that Zeise’s anion is a composite of a perturbed C2H4 and the PtCl3 moiety [271]. The band at 407 cm 1 was assigned to the n(Pt C2H4) (scheme A) since it did not belong to either components. More elaborate treatments employ a triangular model involving two Pt C bonds (scheme B). Hiraishi [273] carried out normal coordinate analysis on such a model and assigned the Raman bands at 493 (dp) and 405 cm 1 (p) to the na(Pt C2H4) and ns(Pt C2H4), respectively. The corresponding force constants for these two modes were calculated to be 1.92 and 1.45 mdyn/A, respectively. The na(Pt C2H4) may be called the “tilt” mode since it involves a tilting motion of the ethylene against the rest of the anion. Crayston and Davidson [274] carried out similar calculations on several ethylene complexes. These workers assigned the bands at 455 and 380 cm 1 to the na(Pt C2H4) and ns(Pt C2H4) of Pt(0)(C2H4)(PPh3)2. Both bands are downshifted on going from the Pt(II) to Pt(O) complexes because a “metallocyclopropane” form (scheme B) becomes more predominant in the latter. Mink and coworkers [275,276] have assigned the IR and Raman spectra of M(C2H4)3 (M ¼ Pt,Pd,Ni) using normal coordinate analysis. These molecules take a D3h structure, as shown in Fig. 2.12, and their 51 (3 19 – 6) normal vibrations are grouped into: 0

00

00

00

5A 1 ðRÞ þ 4A 2 ði:a:Þ þ 10E ðIR; RÞ þ 4A1 ði:a:Þ þ 4A2 ðIRÞ þ 7E ðRÞ

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APPLICATIONS IN ORGANOMETALLIC CHEMISTRY

Fig. 2.12. Skeletal modes of tris(ethylene)metal complexes; symmetry and vibrational assignments are indicated for each mode (L denotes ethylene) [276].

Figure 2.12 illustrates their skeletal modes, and Table 2.11 compares their band assignments and force constants for Pt(C2H4)3 and Zeise’s anion. Figure 2.13 shows the IR and Raman spectra of Pt(C2H4)3 obtained by Csaszar et al. [276]. As shown in Table 2.11, the calculated force constants for the tilt and symmetric stretching modes are relatively close in Zeise’s anion, whereas the former is larger in Pt(C2H4)3. This result may suggest that the p-bonding (scheme B) is more predominant in the latter compound [275]. Vibrational spectra are reported for many other complexes of ethylene. Some references are Li(C2H4)(in Ar matrix) [277], Ni(C2H4)(in Ar matrix) [278], [Pd(C2H4)Cl3] [279], trans-M(C2H4)2 (CO)4 (M ¼ Mo,W) [280], Fe(C2H4)(CO)4 [281], [Rh(C2H4)2Cl]2 [282], Ir(C2H4)4Cl [283], and [Pt(C2H4)Cl2]2 [271]. TABLE 2.11. Observed Frequencies, Band Assignments, and Force Constants of Zeise’s Anion and Pt(C2H4)3 [275,276] [Pt(C2H4)Cl3] (C2v)

Pt(C2H4)3 (D3h)

Band Assignment

Observed Frequencies (cm1) 1517 (A1) 501 (B1) 408 (A1)

n(C¼C) þ d(CH2)

1617 (A 1 ) 1501 (E 0 ) 448(E 0 ) 0 398 (A 1 ) 332 (E 0 )

Tilt n(PtC2H4)

Force Constants (mdyn/A) 2.84 2.54

2.04 1.66

Tilt Pt C2H4 stretch

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297

Fig. 2.13. Far-IR and low-frequency Raman spectra of Pt(C2H4)3 in petroleum ether [276].

Tetracyanoethylene (TCNE) also forms p-complexes with transition metals, and their vibrational spectra are reported for M(TCNE)(CO)5 (M ¼ Cr, W) [284], Fe(TCNE) (CO)4 [285], and Pt(TCNE)(PPh3)2 [286]. Mrozek and Weaver [287] measured surface-enhanced Raman spectra (Sec. 1.3.2) of chemisorbed ethylene on metal electrode surfaces of Au, Pd, Rh, Pt, and Ir. The results indicate the extensive formation of p-bonded ethylene on each metal surface. The n(C¼C) coupled with d(CH2) vibrations (cm1) were observed at Au 1540 1275

Pd

Rh

Pt

Ir

1514 1244

1506 1232

1495 1210

1495 1188

These values were obtained at the electrode potential of 0.2 V versus SCE. The decreasing order of these frequencies indicates the increasing order of p-backbonding from the metal to the 2pp* orbital of ethylene. It was noted that the formation of ethylidyne (:CCH3) from chemisobed ethylene increases as the electrode potential is lowered. This was confirmed by the appearance of the bands characteristic of the CH3 group vibrations.

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2.5.2. Allyl Complexes According to X-ray analysis [288], the two Pd atoms in [Pd(p-allyl)Cl]2 are bridged by two Cl atoms to form a square–planar (PdCl)2 group, and the ally (C3H5) groups are bonded to the Pd atoms with their planes tilted by 112 with respect to the (PdCl)2 plane so that the overall symmetry is C2h:

Vibrational spectra of [Pd(p-allyl)Cl]2 and related compounds in the low-frequency region were assigned on the basis of isotope shifts due to 104 Pd=110 Pd substitution. The bands at 402 and 379 cm 1 were assigned to the na(Pd-allyl) and ns(Pd-allyl), respectively, since both bands are downshifted by 3 cm 1 by such substitution [289]. References on other allyl complexes are M(p-C3H5)2 (M ¼ Ni,Pd), M(p-C3H5)3 (M ¼ Rh,Ir) [290], [Pd(p-C3H5)X]2 (X ¼ CL,Br) [291], Fe(p-C3H5)(CO)3X (X ¼ Br,NO3) [292], and Mn(p-C3H5)(CO)4 [293]. Chenskaya et al. [294] assigned the metal-olefin and metal-halogen vibrations of p-allyl complexes of transition metals.

2.5.3. Complexes of Diolefins and Oligoolefins Nonconjugated diolefins such as norbornadiene (NBD, C7Hg) and 1,5-hexadiene (C6H10) form metal complexes via their C¼C double bonds (Figs. 2.14a, 2.14b). Complete vibrational assignments have been made for M(NBD)(CO)4 (M ¼ Cr,Mo, W) [295], Cr(NBD)(CO)4 [296], and Pd(NBD)X2 (X ¼ Cl,Br) [296]. The metal– olefin vibrations are assigned in the region from 305 to 200 cm 1. The spectrum of K2[(PtCl3)2(C6H10)] is similar to that of the free ligand in the trans conformation [297]. Thus, its structure may be shown as in Fig. 2.14b. However, the spectrum of Pt (C6H10)Cl2 is more complicated than that of the free ligand and suggests a chelate structure such as that shown in Fig. 2.14a. Free butadiene (C4H6) is trans-planar. However, it takes a cis-planar structure in Fe(C4H6)(CO)3 [298] and Fe(C4H6)2CO [299]. For K2[C4H6(PtCl3)2], the infrared spectrum indicates the trans-planar structure of the olefin [300]. In [Rh(COT)Cl]2, cyclooctatetraene (COT) takes a tub conformation and coordinates to a metal via the 1,5 C¼C double bonds, the 3,7 C¼C double bonds being free (Fig. 2.14c). The C¼C stretching bands of free COTare at 1630 and 1605 cm 1, whereas those of the complex are at 1630 (free) and 1410 (bonded) cm 1 [301]. According to X-ray analysis [302], only two of the four C¼C double bonds of COT are bonded to the metal in Fe(COT) (CO)3 (Fig. 2.14d). In this case, the C¼C stretching band for free C¼C double bonds is at 1562 cm 1, whereas that for coordinated C¼C double bonds is at 1460 cm 1 [303]. In [Rh(COD)Cl]2 (COD, C8H12, 1,5-cyclooctadiene), the Rh atom is bonded to COD via the 1,5 C¼C bonds in a manner similar to its COT analog (Fig. 2.14c).

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299

Fig. 2.14. Structures of p-complexes.

The Rh–olefin stretching vibrations were assigned in the range from 490 to 385 cm1 [304]. Bailey et al. [305] measured the resonance Raman excitation profiles of Co(Cp) (COD), and noted interference effects between multiple excited states. The n(CoCOD) and n(CoCp) were assigned at 470 and 354 cm1, respectively. Figure 2.15 shows the far-IR spectra of 104 PdðCOTÞCl2 , 104 PdðCODÞCl2 , and their 110 Pd analogs [306]. The COT complex exhibits four isotope-sensitive bands at 344, 319, 238, and 219 cm1. The first two are assigned to the na(PdCl) and ns(PdCl), respectively, whereas the last two are attributed to the na(Pd-olefin) and ns(Pd-olefin), respectively. Similar assignments can be made for the COD complex. These n(Pd-olefin) frequencies are much lower than the n(PdC2H4) (427 cm1) because of weaker metal-olefin bonds and larger olefin masses.

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Fig. 2.15. Far-IR spectra of Pd(COD)Cl2 and Pd(COT)Cl2; the solid and dashed lines indicate the spectra of the complexes containing 104Pd and 110Pd isotopes, respectively; vertical lines show metal–isotope-sensitive bands [306].

2.5.4. Complexes of Acetylene and Related Ligands Free HC:C(C6H5) exhibits the C:C stretching band at 2111 cm1. In the case of s-bonded complexes (Sec. 2.2), this band shifts slightly to a lower frequency (2062– 2016 cm1) [307]. In M[C:C(C6H5)]2 [M ¼ Cu(I) and Ag(I)], it shifts to 1926 cm1. This relatively large shift was attributed to the formation of both s- and p-type bonding, shown in Fig. 2.14e [308]. Ti[C:C(C6H5)]2(p-Cp)2 reacts with Ni(CO)4 to form the complex shown in Fig. 2.14f. The C:C stretching band of the parent compound at 2070 cm1 is shifted to 1850 cm1 by complex formation [309]. According to Chart and coworkers [310], the C:C stretching bands of disubstituted alkynes (2260–2190 cm1) are lowered to 2000 cm 1 in Na[Pt(RC:CR0 )Cl3] and [Pt(RC:CR0 )Cl2]2, and to 1700 cm 1 in Pt(RC:CR0 )(PPh3)2. Here R and R0 denote various alkyl groups. The former represents a relatively weak p-bonding similar to that found for Zeise’s salt, whereas the latter indicates strong p-bonding in which the C:C triple bond is almost reduced to the double bond (Fig. 2.14g). Similar results were found for (RC:CR0 )Co2(CO)6, which exhibits the C:C stretching bands near 1600 cm 1 [311]. In the case of (HC:CH)Co2(CO)6, the C:C stretching band was observed at 1402 cm 1, which is 570 cm 1 lower than the value for free acetylene (1974 cm 1). The spectrum of the coordinated acetylene in this complex is similar to that of free acetylene in its first excited state, at which the molecule takes a trans-bent structure. Considering possible steric repulsion between the hydrogens and the Co(CO)3 moiety, a structure such as that shown in Fig. 2.14h was proposed [312]. There are many other p-bonded acetylenic complexes for which vibrational data are available. For example, the chloro-bridged dimer, [WC14(IC:Cl)]2, exhibits the n(C:C) at 1619 and 1590 and the n(WC) at 928 865 and 848 cm 1. The former are 510 cm 1 lower than that of free IC:CI (2118 cm 1) [313]. In the case

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301

of (t-BuC:Ct-Bu)2Fe2(CO)4, the C:C bonds are p-bonded to the FeFe bridge as shown below:

Its IR spectrum shows a very weak n(C:C) at 1670 cm1. The weak band at 531 cm1 and a strong band at 284 cm1 were assigned to the n(Fe-acetylene) coupled with d(FeCO) and n(FeFe), respectively. According to normal coordinate analysis [314], the FeFe stretching force constant (3.0 mdyn/A) is about twice that of the Fe Fe single bond (1.3 mdyn/A) of Fe2S2(CO)6 [315]. Thus, the Fe Fe bond of the compound shown above must be close to a double bond.

2.5.5. Complexes of Nitriles The C:N stretching frequency of CF3 C:N is 2271 cm 1. This band is shifted to 1734 cm 1 in Pt(CF3CN)(PPh3)2 because of the formation of a Pt–nitrile p-bond [316]. A similar p-bonding has been proposed for Mn(CO)3IL, where L is ocyanophenyldiphenylphosphine:

In the latter case, the C:N stretching band of the free ligand at 2225 cm to 1973 cm 1 in the complex [317].

1

is shifted

2.5.6. Metal–Olefin Complexes in Inert Gas Matrices A number of olefin complexes of the type M(olefin)n have been prepared via cocondensation reactions of metal vapors with olefins in inert gas matrices: M þ nðolefinÞ ! MðolefinÞn Vibrational studies show that the olefins in these cocondensation products are all p-bonded to metal atoms. Moskovitz and Ozin [318] report the IR spectra of M(C2H4)n

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where M is Co, Ni, Cu, Pd, Ag, and Au. Andrews and coworkers measured and assigned the IR spectra of Li(C2H4)n (n ¼ l,2,3) [319] and In(C2H4) [320]. In the latter, the n(C¼C) coupled with the d(CH2) and n(InC) were observed at 1201 and 238 cm1, respectively. These frequencies are also reported for Ni(C2H4)n (n ¼ 1,2,3) [321]. The first RR spectrum of such a cocondensation product was obtained for Cu (C2H4)3, which exhibits the n(C¼C) and n(CuC) at 1530 and 302 cm1, respectively [322]. Similar studies have been extended to Li(C2H2) [323], Ni(C2H2) [324], Ni(C4H6) [325], (HgCl2)(olefin) [326], and Fe(TPP)(C2H4) [327]. Matrix cocondensation reaction of Mo(CO)6 with acethylene in Ar matrices yield: Mo(CO)6(C2H2), which exhibits a very low n(C:C) at 1820 cm1 [328].

2.5.7. Metal Methylidenes and Metylidynes Metal methylidene, H2C¼Re(O)2(OH), was obtained by photoexcitation (254 nm) of CH3ReO3 in Ar matrices. The n(C¼Re) vibration was located at 780 cm [329]. Metal methylidyne, trans-[HC:W(PMe3)4Cl], exhibits the n(C:W) band at 911 cm 1 that is shifted to 871 cm 1 by deuteration of the CH hydrogen [330]. Similar frequencies are reported for HC:W(CO)2(Tp) where Tp is hydridotris(3,5-diethylpyrazolyl) borate [331].

2.6. CYCLOPENTADIENYL COMPOUNDS The infrared spectra of cyclopentadienyl (C5H5 or Cp) complexes have been reviewed extensively by Fritz [332], who roughly classified them into four groups, each of which exhibits its own characteristic spectrum.

2.6.1. Ionic Complexes These are complexes such as MCp (M ¼ Kþ, Rbþ, Csþ) and MCp2 (M ¼ Ca2þ, Sr2þ, Ba2þ, Mn2þ) [333,334], in which Mnþ and Cp are ionically bonded. The spectra of these compounds are essentially the same as that of the C5H5 ion, which takes a planar pentagonal structure of D5h, symmetry. The 24 (3 10 6) normal vibrations of this ion are classifed into 0

00

00

00

2A 1 ðRÞ þ A 2 ði:a:Þ þ A2 ðIRÞ þ 3E 1 ðIRÞ þ E1 ðRÞ þ 4E 2 ðRÞ þ 2E2 ðiaÞ Figure 2.16 illustrates the approximate normal modes and observed frequencies (KþCp ) of these vibrations. Four IR-active and seven Raman-active vibrations are expected for the Cp ion. In fact, the ionic complexes mentioned above exhibit four IR bands: n(CH), 3100–3000 cm 1, n(CC), 1500–1400 cm 1, d(CH), 1010–1000 cm 1, and p(CH), 750–650 cm 1.

CYCLOPENTADIENYL COMPOUNDS

303

Fig. 2.16. Normal modes of vibration of the cyclopentadienyl group. These figures are approximate, and only the displacements of the H or C atoms are shown. Symmetry, band assignments, and observed frequencies (cm1) are given for each mode.

2.6.2. Ion-Paired and Centrally s-Bonded Complexes These are complexes such as MCp (M ¼ Li,Na), in which the metal ion is bonded to the center of the ring by forming a tight ion pair, or MCp2 (M ¼ Be,Mg,Ca) [333–336], in which the metal is covalently bonded to the center of the ring. In this case, the local symmetry of the Cp ring is regarded as C5v, and its 24 normal vibrations are classified into 3A1 ðIR; RÞ þ A2 ði:a:Þ þ 4E1 ðIR; RÞ þ 6E2 ðRÞ Thus, seven bands are expected to appear in IR spectra. These vibrations are observed in the following regions: n(CH), 3100–3000 cm1; n(CH), 2950– 2900 cm1; n(CC), 1450–1400 cm1; n(CC), 1150–1100 cm1; d(CH), 1010– 990 cm1; two p(CH), 890–700 cm1. In addition, these complexes are expected

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to show one n(MCp) (A1) and one ring-tilt (E1) vibration in the low-frequency region. The former is observed at 426 cm1 for LiþCp and at 196 cm1 for NaþCp in IR spectra [337] and the latter, the 150–130 cm1 region in Raman spectra [338]. According to electron diffraction studies [339], the two rings of SnCp2 and PbCp2 form angles of 45 and 55 , respectively, in the vapor state. On the assumption of angular structure in the solid state, two bands at 240 and 170 cm 1 of SnCp2 have been assigned to the antisymmetric and symmetric M Cp stretching modes, respectively [340]. The IR spectrum of BeCp2 in solution [341] exhibits the bands characteristic of the centrally s-bonded ring (similar to CpBeCl) and those of the diene-type (s-bonded) ring (HgCp2) discussed in the later subsection. Thus, a structure such as (A) shown below, has been proposed. X-Ray analysis [342] as well as IR studies [343] show that BeCp2 in the gaseous and solid phases take a “slip-sandwich” structure as shown in B. A highly symmetric ferrocene-like structure (Z5-ring, D5d) and a centrally s-bonded structure (Z1-ring, C5v) can be ruled out because seven IR bands are observed in the n(CH) region:

On the basis of vibrational analysis of metal–ligand and out-of-plane r(CH) vibrations, a similar “slip-sandwich” structure was proposed for ZnCp2 in THF solution. The ns(Zn Cp2), na(Zn Cp2), and d(CpZnCp) were assigned at 315,344, and 171 cm 1 respectively. The Z5-ring exhibits the tilt vibrations at 494 and 349 cm 1, while the d(ZnCC) of the Z1-ring is observed at 260 cm 1 [344]. 2.6.3. Centrally p-Bonded Complexes* These are Z5-complexes such as FeCp2 and RuCp2, in which the transition metals are bonded to the center of the ring via the d–p-bond. It is interesting to note that two rings in solid ferrocene take the staggered configuration (D5d), while those in ruthenocene take the eclipsed configuration (D5h):

*

Or pentahapto (Z5) complexes.

CYCLOPENTADIENYL COMPOUNDS

305

Since the number of infrared- or Raman-active fundamentals is the same for both conformations, they cannot be distinguished on the basis of the number of fundamentals observed. Under D5d symmetry, the 57 (3 21 6) normal vibrations of ferrocene are classified into 4A1g ðRÞ þ 2A1u ði:a:Þ þ A2g ði:a:Þ þ 4A2u ðIRÞ þ 5E1g ðRÞ þ 6E1u ðIRÞ þ 6E2g ðRÞ þ 6E2u ðiaÞ Thus, 10 vibrations are expected to be IR-active. These include the seven Cp bands discussed previously and three skeletal modes (n3, n5, and n6) illustrated in Fig. 2.17. Table 2.12 lists the observed IR frequencies of the centrally p-bonded MCp2-type complexes, and Fig. 2.18 shows the IR spectra of NiCp2 and FeCp2. As shown in Table 2.12, the IR bands at 492 and 478 cm 1 of FeCp2 have been assigned to the ringtilt (n5) and n(M Cp) (n3), respectively. Both bands show marked isotope shifts (7–8 cm 1) by 54 Fe=57 Fe substitution [350]. The Raman spectra of Fe(Cp2)þ [351] and Fe(Cp*2)þ [352] exhibit the ns(M Cp or M Cp*) of these complexes at 311 and 173/160 cm 1, respectively. Here Cp* denotes the pentamethyl derivative of Cp. Lippincott and Nelson [346] carried out normal coordinate analysis on the C5H5 ring of ferrocene assuming D5h symmetry. Fritz [332] calculated the approximate M Cp stretching force constants using the equation available for the antisymmetric stretching vibration of a linear YXY-type molecule: 2my k ð5:89 10 2 Þ~n23 ¼ 1 þ mx my

Fig. 2.17. Skeletal modes of dicyclopentadienyl metal complexes (D5h symmetry); R denotes the C5H5 ring in band assignments.

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TABLE 2.12. Observed Infrared Frequencies and Band Assignments of Centrally p-Bonded MCp2-Type Compounds (cm 1) Compound FeCp2 RuCp2 OsCp2 CoCp2 NiCp2 FeCpþ 2 CoCpþ 2 IrCpþ 2 a

n(CH) — — 3061 3041 3075 3108 3100 3094 3077

n(CC)

3077 3076 3061 3041 3075 3108 3100 3094 3077

1110 1095 1098 1101 1110 1116 1110 1113 1106

d(CH)

1410 1410 1400 1412 1430 1421 1412 1419 1409

1005 1005 998 995 1000 1017 1001 1010 1009

Ring Tilt n(MR)a d(RMR)a Ref.

p(CH) 820 808 823 778 773 805 779 860 818

855 834 831 828 839 860 841 895 862

492 450 428 464 355 501 490 495 —

478 380 353 355 355 423 405 455 —

179 170 160 — — —

344 344 332,345 332,345 346 347

172 —

348 348

R denotes the Cp ring. For the Raman spectra of MCp2 (M ¼ Mn, Cr, V, Ru, Os), see Ref. 349.

Here, ~n3 is the observed na(MCp) in cm1, mx and my are the point masses (atomic weight unit) of the metal and the Cp ring, respectively, and k is the MCp stretching force constant in mdyn/A. The results are Os

Fe

Ru

Cr

Co

V

Ni

Zn

k(M Cp)(mdyn/A) 2.8 > 2.7 > 2.4 1.6 1.5 1.5 1.5 1.5 na(M Cp)(cm 1) 353 478 379 408 355 379 355 345

Fig. 2.18. Infrared spectra of Ni(C5H5)2 (solid line) and Fe(C5H5)2 (dashed line).

CYCLOPENTADIENYL COMPOUNDS

307

This may indicate the order of the MCp bond strength. Table 2.12 shows that the MCp stretching band of FeCp2 at 478 cm1 is shifted to a lower frequency when it is ionized to FeCp2þ. Apparently, the deviation from the inert gas electronic configuration due to the ionization weakens the MCp bond. More accurate calculations on MCp stretching force constants of ferrocene and its derivatives have been made by Phillips et al. [353], who employed two observed frequencies [478 (n 3) and 306 (n1) for ferrocene]. The FeCp stretching force constant was 3.11 mdyn/A with the stretch stretch interaction constant of 0.48 mdyn/A. In another approach, Hyam [354] considered five Fe C bonds between the Fe atom and the Cp ring, and obtained a “pseudoring”-Fe stretching force constant of 1.4 mdyn/A. Sch€afer et al. [355,356] carried out the most complete normal coordinate analysis by assuming 10 Fe C bonds between the Fe atom and the two Cp rings (D5h symmetry). According to Yokoyama et al. [357], the observed skeletal frequencies of FeCp2 and NiCp2 are as follows (cm 1): FeCp2 NiCp2

n1 (R)

n3(IR)

n4(R)

n5(IR)

306 245

476 355

390 198

400 355

These workers have explained the marked difference in n4 on the basis of their electronic structures. More references are available on vibrational spectra of FeCp2 [353–359] and RuCp [360]. Diana et al. [361] carried out approximate normal coordinate calculations of the M-(Z5-Cp) units of a variety of metal–Cp complexes, and found a correlation between the metal–ring stretching force constant and the metal–carbon distance. 2.6.4. Diene-Type (s-Bonded) Complexes* These are complexes such as HgCp2 and CH3HgCp [362,363] in which the metal is s-bonded to one of the C atoms of the Cp ring:

The spectra of these compounds are similar to that of C5H6 (cyclopentadiene), and are markedly different from those of the other groups discussed previously. Figure 2.19 shows the infrared spectrum of HgCp2 [364]. Band assignments of these compounds can be made on the basis of those obtained for C5H6 [365]. Infrared and NMR evidence suggests the presence of diene-type bonding for (Cp)M(CH3)3 (M ¼ Si,Ge,Sn) [366]. There are many other complexes in which the p-bonded (Z5) and the s-bonded 1 (Z ) cyclopentadienyl groups are mixed. As expected, these compounds exhibit *

Or monohapto (Z1) complexes

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Fig. 2.19. Infrared spectrum of Hg(C5H5)2 in CS2 (2–6 mm and 7.1–15.5 mm). CHCl3 (6–6.6 mm), and CCl4 (6.6–7.1 m) [364].

bands characteristic of both groups. Typical examples are as follows: VCp3 (two p and one s) [367], NbCp4 (two p and two s) [368], ZrCp4 (three p and one s) [369–371] and MoCp4 (three p and one s) [372]. Infrared [368], X-ray [370], and NMR [373] evidence indicates the presence of two p- and two s-bonded Cp rings in TiCp4. In addition to the Z5- and Z1-bondings discussed above, X-ray analysis [374] reveals the presence of Z3-type bonding in allylic [Ni(Cp)(C3H4)]2.

2.7. CYCLOPENTADIENYL COMPOUNDS CONTAINING OTHER GROUPS 2.7.1. Carbonyl Compounds The vibrational spectra of carbonyl compounds were discussed in Sec. 1.18. Here we discuss only those containing cyclopentadienyl rings. It has been well established that the number of CO stretching bands observed in the infrared depends on the local symmetry of the M(CO)n group in M(Cp)m(CO)n-type compounds [332]. For example, only two CO stretching bands have been observed for the following compounds, in accordance with the prediction from local symmetry:

CYCLOPENTADIENYL COMPOUNDS CONTAINING OTHER GROUPS

309

In the case of MCp(CO)3 (M ¼ Mn, Re), breakdown of the C5n selection rule for the Cp vibrations was noted in solution IR spectra [375]. The FT Raman spectra of MnCp (CO)3 as well as Cr(arene)(CO)3 are reported [376]. Other references for carbonyl compounds are [M(Cp)(CO)3] (M ¼ Cr,Mo,W) [377], Mn(Cp)(CO)3 [378], Re(Cp) (CO)3 [379], and V(Cp)(CO)4 [380]. In M(Cp)(CO)3-type compounds [381], the CO stretching frequencies increase in the order V1 < Cr0 < Mnþ1 < Fe2þ. This indicates that the higher the oxidation state of the metal, the less the MC p-backbonding and the higher the CO stretching frequency. Originally, Fe(Cp)2(CO)2 was thought to contain two p-bonded Cp rings [382]. However, an infrared and NMR study [383] showed that one ring is p-bonded and the other s-bonded to the metal. Later, X-ray analysis confirmed this structure [384]. The structure of Fe2(Cp)2(CO)4 has been studied extensively. In the solid state, it takes a trans-bridged structure (Fig. 2.20a) [385], or a cis-bridged structure (Fig. 2.20b) if crystallized in polar solvents at lower temperatures [386]. The cis-isomer exhibits two terminal (1975 and 1933 cm1) and two bridging (1801 and 1766 cm1) bands. Although the trans-isomer also exhibits two terminal (1956 and 1935 cm1) and two bridging (1769 and 1755 cm1) bands, these splittings are probably due to the crystal field effect.

Fig. 2.20. Structures of cyclopentadienyl carbonyl and nitrosyl complexes. The bridging CO groups are not shown in (i).

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The structure of Fe2(Cp)2(CO)4 in solution has been controversial. Early infrared studies [387] suggested the presence of the cis-bridged structure (Fig. 2.20b) mixed with a trace of noncentrosymmetric, nonbridging isomer (Fig. 2.20c). Manning [388] proposed, however, an equilibrium involving the three isomers, a, b, and c, of Fig. 2.20. This was confirmed by Bullet et al. [389] who gave the following assignments for the spectrum in a CS2C6D5CD3 solution: trans-isomer (a), 1954 and 1781 cm1; cisisomer (b), 1998, 1954, 1810, and 1777 cm1. The frequencies of nonbridged species could not be determined because of their very low concentration. In the case of Ru2(Cp)2(CO)4, Bullitt et al. [389] proposed an equilibrium containing four isomers: a, b, d, and e of Fig. 2.20. It is interesting to note that the bridging CO groups of Fe2(Cp)2(CO)4 form an adduct with trialkylaluminum [390] (see Sec. 1.18):

This indicates that the basicity of the bridging CO group is greater than that of the terminal CO group. The CO stretching bands of the parent compound (R: isobutyl) are at 2005 and 1962 (terminal) and 1794 (bridging) cm1 in heptane solution. These bands are shifted to 2041 and 2003 (terminal) and 1680 (bridging) cm1 by adduct formation. X-Ray analysis has been carried out on [Fe2(Cp)2(CO)4][Al(C2H5)3]2 [391]. Formation of adducts such as [Fe2(Cp)2(CO)4]BX3 (X ¼ Cl,Br) and [Fe(Cp) (CO)]4BX3 (X ¼ F,Cl,Br) has also been confirmed [392]. These compounds exhibit bands at 1470–1290 cm1 for bridging CO groups, which are bonded to a Lewis acid via the O atom. Ni2(Cp)2(CO)2 exhibits two bridging CO stretching bands at 1854 and 1896 cm1 in heptane solution. The structure shown in Fig. 2.20f with a puckered Ni(CO)2Ni bridge was proposed for this compound [393]. In heptane solution FeNi(Cp)2(CO)3 shows a strong terminal CO stretching at 2004 cm1 and two bridging CO stretching bands at 1855 and 1825 cm1. Since the 1855 cm1 band (symmetric type) is very weak, the Ni(CO)2Fe bridge in this compound was thought to be virtually planar, as shown in Fig. 2.20g [393]. According to X-ray analysis [394], the structure of Mo2(Cp)2(CO)6 is transcentrosymmetric, as shown in Fig. 2.20h. The infrared spectrum in the CO sketching region is consistent with this structure, both in the solid state and in solution [395]. In solvents of high dielectric constants, however, the trans-rotamer is rearranged into the gauche-rotamer [396]. For the infrared spectra of analogous tungsten compounds, see Refs. [377] and [397]. According to X-ray analysis [398], Fe4(Cp)4(CO)4 takes a regular tetrahedral structure such as that shown in Fig. 2.20i. It exhibits a bridging CO

CYCLOPENTADIENYL COMPOUNDS CONTAINING OTHER GROUPS

311

stretching band at 1649 cm [390] in the infrared and a FeFe stretching band at 214 cm1 [399] in the Raman. 2.7.2. Halogeno Compounds Cyclopentadienyl complexes containing metal-halogen bonds exhibit metal–halogen vibrations (Sec. 1.25), together with those of the cyclopentadienyl rings. The lowfrequency spectra of these compounds are complicated [400,401] because metal– ring skeletal modes couple with metal–halogen modes. The infrared spectra of M(Cp)2X2-type compounds (M ¼ Ti,Zr,Hf; X ¼ Cl,Br,I) have been studied by several investigators [400–403]. Also, infrared spectra have been reported for Mo(Cp)(CO)3X [375] and Mo(Cp)(CO)2X3 (X ¼ Cl,Br,I) [404]. 2.7.3. Nitrosyl Compounds Vibrational spectra of nitrosyl compounds were discussed in Sec. 1.20. The vibrational spectra of Ni(Cp)(NO) and its deuterated and 15 N species have been assigned completely [405]; the NO stretching, NiN stretching, NiCp stretching, and NiCp tilt vibrations are at 1809, 649, 322, and 290 cm1, respectively. If this compound in an Ar matrix is irradiated by UV light, the bands near 1830 cm1 disappear and a new band emerges at 1390 cm1 [406]. This has been interpreted as indicating the following photoionization: hn

ðCpÞNiNO ! ðCpÞNiþ NO Mn2(Cp)3(NO)3 exhibits two NO stretching bands at 1732 and 1510 cm1 [407]. With the former attributed to the terminal and the latter to the bridging NO, the structure that is shown in Fig. 2.20j was proposed. The infrared spectrum of Mo(Cp) (CO)2(NO) has been reported [408,375]. Figure 2.20k shows the structure of Mn3(Cp)3(NO)4, containing doubly and triply bridging NO groups. The bands at 1530 and 1480 cm1 were assigned to the doubly bridged NO groups, whereas the 1320-cm1 band was attributed to the triply bridged NO group [409]. 2.7.4. Hydrido Complexes Vibrational spectra of hydrido complexes were reviewed in Sec. 1.24. The metalhydrogen stretching bands for Mo(Cp)2H2 [410], Re(Cp)2H2, and W(Cp)2H2 [411,412] have been observed in the 2100–1800 cm1 region. X-Ray analysis on Mo(Cp)2H2 [413] suggests that the coordination around the Mo atom is approximately tetrahedral. In polymeric [Zr(Cp)2H2]n [414], the bridging ZnH stretching vibration is observed as a strong, broad band a 1540 cm1 [415]. A similar bridging TiH vibration is found at 1450 cm1 for [Ti(Cp)2H]2 [416]. In {[Rh(Cp*)]2HCl3}, where Cp* denotes the pentamethyl-Cp group, the bridging RhH vibration was assigned at 1151 cm1 [417]:

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An extremely low CoH stretching frequency (950 cm1), together with an unusually high-field proton chemical shift observed for [Co(Cp)H]4, was attributed to the triply bridged structure shown above (only one face of the tetrahedron is shown) [418].

2.7.5. Complexes Containing Other Groups As discussed in Sec. 1.17 the mode of coordination of the pseudohalide ion can be determined by vibrational spectroscopy. Burmeister et al. [419] found that all NCS and NCSe groups are N-bonded in M(Cp)2X2-type compounds, where M is Ti, Zr, Hf, or V, and X is NCS or NCSe. In the case of analogous NCO complexes, Ti, Zr, and Hf form O-bonded complexes, whereas V forms an N-bonded complex. Later, Jensen et al. [420] suggested the N-bonded structure for the titanium complex. The n(CH) (3311 cm1) and n(CN) (2097 cm1) of free HCN are shifted to 3188 and 2155 cm1, respectively, when it coordinates to the Ti atom in the [Ti (Cp)2(HCN)2]2þ ion. This result has been attributed to the Ti HCN s-donation [421]. The cis- and trans-isomers of the isonitrile complex, MoCp(CO)2 (t-BuNC)I, exhibit the n(NC) at 2153 and 2138 cm1, respectively:

The n(NC) of the trans isomer is lower because its I ! CN p-donation may be larger relative to the cis isomer [422]. The n(NC) of MoCp(CO)2(EtNC)I at 2168 cm1 is shifted to 1869 cm1 in Na [MoCp(CO)2(EtNC)]. The marked downshift of n(NC) may reflecta loweroxidationstate (O)of theMo atom in the latter sincetheMo ! CN-Et p-backdonation increases as the oxidation state becomes lower [423]. A strong N:N stretching band is observed at 1910 cm1 in the Raman spectrum of L2(Cp)MoN:NMo(Cp)L2 (L: PPh3) [424]. Thiocarbonyl complexes of the (Cp) Mn(CO)3n(CS)n-(n ¼ l,2,3) type exhibit the C:S stretching bands at 1340– 1235 cm1 [425]. In (Cp)Nb(S2)X-type compounds (X ¼ Cl,Br,I,SCN), the S2 is

COMPLEXES OF OTHER CYCLIC UNSATURATED LIGANDS

313

probably coordinated to the metal in a side-on fashion, and its SS stretching band may be assigned at 540 cm1 [426]. 2.8. COMPLEXES OF OTHER CYCLIC UNSATURATED LIGANDS In addition to those discussed in the preceding sections, there are many other cyclic unsaturated ligands that form p-complexes with transition metals. Some of these complexes are discussed below. 2.8.1. Complexes of Cyclobutadiene (C4H4) The local symmetry of the C4H4 ring in Fe(C4H4)(CO)3 is regarded as C4v, and its 18 (3 8 6) vibrations are classified into 3A1 ðIR; RÞ þ A2 ðiaÞ þ 4B1 ðiaÞ þ 2B2 ðiaÞ þ 4EðIR; RÞ Thus, sevenvibrations are IR- as well as Raman-active. Two skeletal modes, ring tilt (E) and ns (Fe C4H4) (A1) should be added if the Fe atom is included. These vibrations were observed near 475 and 406 cm 1, respectively [427,428]. Normal coordinate analysis on Fe(C4H4)(CO)3 was carried out by Andrews and Davidson [429]. The IR spectra of Ni(C4(CH3)4)Cl2, M(C4(C6Hs)4)X2 (M ¼ Ni,Pd; X ¼ Cl,Br,I) and Pt(C4R4)Cl2 (R ¼ alkyl) [430] are reported [431]. 2.8.2. Cyclopentadiene (C5H6) and Cyclohexadienyl (C6H7) Complexes Cyclopentadiene forms p-complexes such as MCp(C5H6) (M ¼ Co,Rh) in which the two H atoms of the CH2 group exhibit two separate bands; n(CHendo) and n(CHexo) at 2750 and 2945 cm 1, respectively [432]:

A later study [433] shows, however, that the lower-frequency band at 2750 cm 1 must be assigned to n(CHexo), since replacement of the exo hydrogen by the phenyl or perfluorophenyl group results in the disappearance of this band. In a cyclohexadienyl complex such as Mn(C6H7)(CO)3, the bands at 2970 and 2830 cm 1 were assigned to n(CHendo) and n(CHexo), respectively [434].

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2.8.3. Complexes of Benzene (C6H6) Under D6h symmetry, the 30 (3 12 6) normal vibrations of benzene are classified into 2A1g ðRÞ þ A2g ðiaÞ þ A2u ðIRÞ þ 2B1u ðiaÞ þ 2B2g ðiaÞ þ 2B2u ðiaÞ þ E1g ðRÞ þ 3E1u ðIRÞ þ 4E2g ðRÞ þ 2E2u ðiaÞ Thus only four vibrations are IR-active and only sevenvibrations are Raman-active. Figure 2.21 shows the normal modes and observed frequencies of these vibrations. Dibenzene chromium, Cr(C6H6)2, takes a ferrocene-like sandwich structure of D6h symmetry (eclipsed form). Then, its 69 (3 25 – 6) normal vibrations are grouped into 4Alg þ 2A1u þ A2g þ 4A2u þ 2B1g þ 4B1u þ 4B2g þ 2B2u þ 5E1g þ 6E1u þ 6E2g þ6E2u of which only A2u and E1u vibrations are IR-active and A1g, E1g, and E2g vibrations are Raman-active. Thus 10- (4 A2u and 6 E1u) bands should be observed in IR spectra. Table 2.13 lists these 10 frequencies, including the ring tilt, ns(Cr C6H6), and d(C6H6 Cr C6H6). Figure 2.22 shows the IR spectrum of Cr(C6H6)2 [436]. Using the same approximation as used previously for ferrocene (Sec. 2.6.3), the Cr C6H6 stretching force constant of Cr(C6H6)2 is calculated to be 2.43 mdyn/A, which is smaller than that of ferrocene (2.7 mdyn/A). Cyvin et al. [437] carried out normal coordinate analysis on Cr(C6H6)2. Infrared spectra have been reported for V(C6H6)2 [438] and Fe(C6H6)2 [439], which were prepared by matrix cocondensation techniques (Sec. 2.5.6). Resonance Raman spectra of V(C6H6)2 thus prepared exhibit the progression of the n(V C6H6) (257 cm 1) up to the ninth overtone [440]. The IR spectra of V(C6H6)þ and V(C6H6)2þ in the gaseous state have been assigned [441]. The CH out-of-plane vibrations of aromatic rings tend to shift to higher frequencies by forming sandwich complexes. Saito et al. [442] noted, however, that some vibrations are upshifted, while others are downshifted, when benzene forms the sandwich complex [Cr(C6H6)2]I. The IR and Raman spectra of Cr(C6H6)(C6F6) (C6v symmetry) have been assigned empirically [443]. Normal coordinate analyses of Cr(C6H6)(CO)3 (C3v symmetry) have been made by two groups of investigators [444,445]. The UV photolysis of Cr (C6H6)(CO)3 in the gas phase produces Cr(C6H6)(CO)1,2, which is characterized by n(CO); Cr(C6H6)(CO)2 is predominant on 355 nm photolysis [446]. 2.8.4. Tropylium Cation (C7H7þ) and p-C7H7 Metal Complexes Under D7h, symmetry, the 36 (3 14 cation are classified into 0

00

6) normal vibrations of the planar tropylium 00

00

2A 1 þ A 2 þ A 2 þ 3E 1 þ E 1 þ 4E 2 þ 2E 2 þ 4E 3 þ 2E 00

00

00

3

of which A 2 and E1 vibrations are IR-active, whereas A1 ; E 1 , and E2 vibrations are Raman-active. Thus, four IR and seven Raman bands are expected for the C7H7þ cation. The four IR bands of (C7H7)Br are observed at 3020 [n(CH)], 1477 [n(CC)], 992 [d(CH)], and 633 cm 1 [p(CH)] [447].

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315

Fig. 2.21. Approximate normal modes of vibration of benzene. Symmetry, band, assignments, and observed frequencies (cm1) of representative modes are given.

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TABLE 2.13. Infrared Frequencies of Dibenzene-Metal Complexes (cm 1) [332,435] Complex Cr(C6H6)2 CrðC6 H6 Þþ 2 Mo(C6H6)2 W(C6H6)2 V(C6H6)2 a

n(CH) 3037 3040 3030 3012 3058

— — 2916 2898 —

n(CC)

d(CH)

n(CC)

p(CH)

Ring Tilt

1426 1430 1425 1412 1416

999 1000 995 985 985

971 972 966 963 959

833 857 811 882 818

794 795 773 798 739

490 466 424 386 470

n(MR)a 459 415 362 331 424

d(RMR)a (140) (144) — — —

R denotes the C6H6 ring.

The IR frequencies of several metal p-complexes such as M(C7H7)(CO)3 (M ¼ Cr, Mo) are summarized by Fritz [332]. In these complexes, the symmetry of the M(C7H7) moiety is regarded as C7v, and its 39 (3 15 – 6) vibrations are grouped into 4A1 ðIR; RÞ þ A2 ðiaÞ þ 5E1 ðIR; RÞ þ 6E2 ðRÞ þ 6E3 ðiaÞ Thus, nine vibrations are IR-active while 15 vibrations are Raman-active. These vibrations include the ring tilt (E1) and ns(M C7H7) (A1), which are IR- as well as Raman-active. The Raman spectrum of [Mo(C7H7)(CO)3]BF4 in the solid state exhibits the ring tilt at 331 and 324 cm 1, and the ns(Mo C7H7) at 309 cm 1, with two shoulder bands at 302 and 295 cm 1. The observed splitting of the former is due to lowering of symmetry in the crystalline state (site symmetry, C1) [448]. 2.8.5. Complexes of Cyclooctadienyl Anion (C8H82 ) The C8H82 ion takes an octagonal planar structure of D8h symmetry, and its 42 (3 16 6) vibrations are grouped into 2A1g þ A2g þ A2u þ 2B1u þ 2B2g þ2B2u þ Elg þ 3E1u þ 4E2g þ 2E2u þ 4E3g þ 2E3u of which A2u and E1u vibrations are IR-active, whereas A1g, E1g, and E2g vibrations are Raman-active. Thus, four vibrations are IR-active and seven vibrations are Raman-active. The former bands of K2(C8H8) are observed at 2994 [n(CH)], 1431

Fig. 2.22. Infrared spectra of crystalline Cr(C6H6)2 in (A) KBr pellet and (B) Hostaflon oil suspension (2–7.5 mm) and Nujol mull suspension (7.5–29 mm) [436].

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317

[n(CC)], 880 [d(CH)], and 684 cm1 [p(CH)] [332]. Metal complexes such as M(C8Hg)2 (M ¼ Th,U) take sandwich structures similar to that of ferrocene, but the two rings are eclipsed so that the overall symmetry becomes D8h. In this case, the 93 (3 33 6) normal vibrations are classified as 4A1g þ 2A1u þ A2g þ 4A2u þ 2B1g þ 4B1u þ 4B2g þ 2B2u þ 5E1g þ 6E1u þ 6E2g þ 6E2u þ 6E3g þ 6E3u Then, 10 vibrations (4 A2u þ 6 E1u) are IR-active and 15 vibrations (4 A1g þ 5E1g þ 6 E2g) are Raman-active. The IR spectra of biscyclooctadienyl complexes mentioned above have been assigned by Hocks et al. [449]. The ring tilt (E1u) and na(M C8H8) (A2u) of these complexes are observed at 695(698) and 250(240) cm 1, respectively (the numbers in parentheses are for the uranium complex). In Ti(C8H8)2, however, one ring is symmetrically bonded (local symmetry, C8h), while the other is asymmetrically bonded to the metal (local symmetry, Cs). Under C8h, symmetry, the 45 (3 17 6) vibrations of the M(C8H8) moiety are classified into 4A1 þ A2 þ 2B1 þ 4B2 þ 5E1 þ 6E2 þ 6E3 Then, nine (4A1 þ 5E1) vibrations are IR-active, whereas 15 (4A1 þ 5E1 þ 6E2) vibrations are Raman-active. The IR spectra of M(C8H8)2 (M ¼ Ti,V) have been assigned partly on this basis [449]. Similar assignments can be made for the Ti(C8H8) moiety of Ti(C8H8)(C5H5) [450]. The IR spectra of the K[Ln(C8H8)2] (Ln ¼ Ce,Pr,Nd, Sm) can be assigned on the basis of the sandwich structure (D8h) [451]. Cyclooclatetrane (COT) takes a tub conformation in the free state. As discussed in Sec. 2.5.3, it takes a tub conformation in [Rh(COT)Cl]2 and a chair conformation in Fe (COT)(CO)3. 2.8.6. Indenyl Complexes The indenyl group may coordinate to the metal through a s- or a p-bond:

An example of the former is seen in Hg(C9H7)Cl, which exhibits an aromatic CH stretching at 3060–3050 cm 1 and an aliphatic CH stretching band at 2920– 2850 cm 1. The latter band should be absent in the p-bonded complex [452]. The IR spectra of p-bonded sandwich complexes such as Ru(C9H7)2 (fully eclipsed) and Fe(C9H7)2 (staggered) have been assigned. No appreciable differences were noted between these two complexes in the low-frequency region [452].

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Fig. 2.23. Structures of some metal sandwich compounds.

2.8.7. Complexes of Larger Ligands Infrared spectra are reported for a mixed-valence-state complex, biferrocene (Fe2þ, Fe3þ) picrate [453] and bis(pentalenyl)Ni [454], whose structures are shown in Fig. 2.23a and 2.23b, respectively. The two ferrocene moieties in the former are not independent since only one set of the skeletal modes is observed. The spectrum of a triple-decker compound, [Ni2 (Cp0 )3]BF4(Cp0 : CH3Cp) (Fig. 2.23c), is similar to that of Ni(Cp0 )2 [455]. In the case of the [Ni2Cp3]þ ion, the skeletal frequencies of the NiCp (terminal) moiety was found to be 25–10 cm1 higher than those of the NiCp (bridging) moiety [456].

2.9. MISCELLANEOUS COMPOUNDS There are many other organometallic compounds that have not been covered in the preceding sections. For these, the reader should consult general references cited previously. Other review articles on specific groups of compounds are listed below: Alkyldiboranes: W. J. Lehmann and I. Shapiro, Spectrochim. Acta 17, 396 (1961). Organoaluminum compounds: E. G. Hoffman, Z. Elektrochem. 64, 616 (1960). Organosilicon compounds: A. L. Smith, Spectrochim. Acta 16, 87 (1960). Organogermanes: R. J. Cross and R. Glockling, J. Organomet. Chem. 3, 146 (1965). Organotin compounds: R. Okawara and W. Wada, Adv. Organomet. Chem. 5, 137 (1967). Organophosphorus compounds: D. E. C. Corbridge, The Structural Chemistry of Phosphorus, Elsevier, Amsterdam, 1974; L. C. Thomas, Interpretation of the Infrared Spectra of Organophosphorus Compounds, Heyden, London, 1974. Organometallic compounds of P, As, Sb, and Bi: E. Maslowsky, Jr., J. Organomet. Chem. 70, 153 (1974).

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REFERENCES 1. Spectroscopic Properties of Inorganic and Organometallic Compounds, Vol. 1 to present, The Chemical Society, London. 2. K. Nakaraoto, “Characterization of Organometallic Compounds by Infrared Spectroscopy,” in M. Tsutsui,ed., Characterization of Organometallic Compounds, Part I, Wiley, New York, (1969). 3. E. Maslowsky, Jr., Vibrational Spectra of Organometallic Compounds, Wiley, New York, (1977). 4. L. J. Bellamy, The Infrared Spectra of Complex Molecules, 3rd ed., Chapman and Hall, London, Vol. 1, 1975; Vol. 2, 1980. 5. D. Lin-Vien, N. B. Colthup, W. G. Fateley, and J. G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, San Diego, CA, (1991). 6. T. Shimanpuchi, Tables of MolecularVibrational Frequencies, Consolidated Volume, Natl. Stand. Ref. Data Ser., US Natl. Bur. Stand. Vol. 39, June (1972). 7. B. Schrader, Raman/IR Atlas of Organic Compomias, VCH, New York, (1989). 8. The Sadtler Standard Spectra, Sadtler Research Laboratories, Division of Bio-Rad Laboratories, Inc., Philadelphia, PA. 9. N. N. Greenwood, E. J. F. Ross, and B. P. Straughan, Index of Vibrational Spectra of Inorganic and Organometallic Compounds, Vols. 1–3, Butterworth, London, 1972–1977. 10. S. C. Chang, Z. H. Kafafi, R. H. Hauge, W. E. Billups, and J. L. Margrave, J. Am. Chem. Soc. 109, 4508 (1987). 11. S. C. Chang, R. H. Hauge, Z. H. Kafafi, J. L. Margrave, and W. E. Billups, J. Am. Chem. Soc. 110, 7975 (1988). 12. W.E. Billups, S. C. Chang, R. H. Hauge, and J. L. Margrave, Inorg. Chem. 32, 1529 (1993). 13. W. D. Bare, A. Citra, C. Trindle, and L. Andrews, Inorg. Chem. 39, 1204 (2000). 14. K. Burczyk and A. J. Downs, J. Chem. Soc. Dalton Trans. 2351 (1990). 15. E. Maslowsky, Jr., Chem. Soc. Rev. 9, 25 (1980). 16. A. M. Pyndyk, M. R. Aliev, and V. T. Aleksanyan, Opt. Spectrosc. (Engl. transl.) 36, 393 (1974). 17. D. Fischer, K. Klostermann, and K. -L. Oehme, J. Raman Spectrosc. 22, 19 (1990). 18. W. F. Edgell and C. H. Ward, J. Am. Chem. Soc. 77, 6486 (1955). 19. W. F. Edgell and C. H. Ward, J. Mol. Spectrosc. 8, 343 (1962). 20. K. Shimizu and H. Murata, J. Mol. Spectrosc. 5, 44 (1960). 21. R. A. Kovar and G. L. Morgan, Inorg. Chem. 8, 1099 (1969). 22. I. S. Butler and M. L. Newbury, Spectrochim. Acta. 33A, 669 (1977). 23. B. Nagel and W. Br€user, Z. Anorg. Allg. Chem. 468, 148 (1980). 24. J. R. Dung, and S. C. Brown, J. Mol. Spectrosc. 45, 338 (1973). 25. C. Kippels, W. Thiel, D. C. McKean, and A. M. Coats, Spectrochim. Acta 48A, 1067 (1992). 26. M. A. Bochmann, M. A. Chesters, A. R. Coleman, R. Grinter, and D. R. Linder, Spectrochim. Acta 48A, 1173 (1992).

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Chapter

3

Applications in Bioinorganic Chemistry Metal ions in biological systems are divided into two classes. The first class consists of ions such as Kþ, Naþ, Mg2þ, and Ca2þ, which are found in relatively high concentrations. These ions are important in maintaining the structure of proteins by neutralizing negative charges of peptide chains and in controlling the function of cell membranes that selectively pass certain molecules. In the second class, ionic forms of Mn, Fe, Co, Cu, Zn, Mo, and so on exist in small to trace quantities, and are often incorporated into proteins (metalloproteins). The latter class is divided into two categories: (A) transport and storage proteins and (B) enzymes. Type A includes oxygen transport proteins such as hemoglobin (Fe), myoglobin (Fe), hemerythrin (Fe), and hemocyanin (Cu), electron transfer proteins such as cytochromes (Fe), iron–sulfur proteins (Fe), blue-copper proteins (Cu), and metal storage proteins such as ferritin (Fe) and ceruloplasmin Cu). Type B includes hydrolases such as carboxypeptidase (Zn) and aminopeptidase (Zn,Mg), oxidoreductases such as oxidase (Fe,Cu,Mo) and nitrogenase Mo,Fe), and isomerases such as vitamin B12 coenzyme (Co). To understand the roles of these metal ions in metalloproteins, it is first necessary to know the coordination chemistry (structure and bonding) of metal ions in their active sites. Such information is difficult to obtain since these active sites are buried in a large and complex protein backbone. Although X-ray crystallography would be ideal for this purpose, its application is hampered by the difficulties in growing single crystals of large protein molecules and in analyzing diffraction data with high resolution. As will

Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, Sixth Edition, by Kazuo Nakamoto Copyright 2009 John Wiley & Sons, Inc.

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be discussed later, these difficulties have been overcome in some cases, and knowledge of precise geometries has made great contribution to our understandings of their biological functions in terms of molecular structure. In other cases where X-ray structural information is not available or definitive, a variety of physicochemical techniques have been employed to gain structural and bonding information about the metal and its environment. These include electronic, infrared, resonance Raman, ESR, NMR, ORD, CD, M€ ossbauer spectroscopy, EXAFS, and electrochemical, thermodynamic, and kinetic measurements. Resonance Raman (RR) spectroscopy (Sec. 1.22 of Part A) has been used extensively for the study of active sites of metalloproteins. The reason for this is twofold: (1) Most metalloproteins have strong electronic absorptions in the UV–visible region that originate in a chromophore containing a metal center. By tuning the laser wavelength into these bands, it is possible to selectively enhance the vibrations localized in this chromophore without interference from the rest of the protein. (2) Owing to strong resonance enhancement of these vibrations, only a dilute solution is needed to observe their RR spectra. This enables one to obtain spectra from a small volume of dilute aqueous solution under biological conditions. This is particularly significant in assigning metal–ligand vibrations by using metal–isotope techniques, because isotopes such as 54 Fe and 68 Zn are expensive. In some cases, however, the vibrations of interest may not be enhanced with sufficient intensity. A typical example is the n(O2) of oxyhemoglobin. Then, one must resort to IR spectroscopy, which exhibits all vibrations allowed by IR selection rules. It should be noted, however, that IR measurements in aqueous media are generally limited to the regions where water does not absorb strongly (Sec. 1.11). Furthermore, it is often necessary to use difference techniques to cancel out interfering bands due to the solvent and some solute bands. In the following, we will review typical results to demonstrate the utility of vibrational spectroscopy in deducing structural and bonding information about large and complex biological molecules. Marked progress has been made in biomimetic chemistry where the active site is modeled by relatively simple coordination compounds. For example, a number of iron porphyrins have been prepared to mimic heme proteins, and the vibrational spectra of some of these compounds have been discussed in Sec. 1.5 and other sections. Thus, we compare vibrational spectra of biological molecules and their model systems whenever appropriate or necessary. Since biospectroscopy is one of the most exciting areas of modern research, the volume of literature on biological compounds is increasing explosively. It is clearly not possible to cover all important topics in a limited space. Several excellent monographs [1–6] and review articles cited in each section should be consulted for further information.

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3.1. MYOGLOBIN AND HEMOGLOBIN Myoglobin (Mb, MW 16,000) is an oxygen storage protein found in animal muscles. Figure 3.1 shows the structure of sperm-whale myoglobin as determined by X-ray analysis. It is a monomer consisting of 153 amino acids, and its active site is an iron protoporphyrin (see Fig. 1.24) that is linked axially to the proximal histidine (F8). In the deoxy state, the iron is divalent and high-spin, and the Fe atom is out of the porphyrin core plane by 0.6 A as shown in Fig. 3.2. On oxygenation, the dioxygen molecule coordinates to the vacant axial position, and the heme core becomes planar. The Fe atom in oxy-Mb is low spin, and its oxidation state is close to Fe(III) (see discussion below). Hemoglobin (Hb, MW 64,000) is an oxygen transport protein found in animal blood. It consists of four subunits (a1, a2, b1, and b2), each of which takes a structure similar to that of Mb. However, these four subunits are not completely independent of each other. Oxygen uptake studies show that the oxygen affinity of each subunit depends on the number of other subunits that are already oxygenated (cooperativity).

Fig. 3.1. Structure of sperm whale myoglobin.

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Fig. 3.2. Structures of deoxy- and oxymyoglobins.

This phenomenon has been explained in terms of two quaternary structures called the T and R (tense and relaxed) states. Deoxy-Hb is in the T0 (most tense) state. As it gradually absorbs dioxygen, the R state becomes more stable than the corresponding T state. Finally, oxy-Hb assumes the R4 (most relaxed) state [7]. Several review articles are available on RR [8–11] and IR [12] spectra of heme proteins. 3.1.1. Selective Excitation of RR Spectra As stated previously, one of the great advantages of RR spectroscopy is its ability to selectively enhance chromophor vibrations by tuning the excitation wavelength to the electronic transition of a particular chromophor in a large and complex molecule such as myoglobin and hemoglobin. This is clearly demonstarated in Fig. 3.3, obtained by Asher [13]. The absorption spectrum of the fluoride complex of sperm whale myoglobin is shown in the bottom (b). The absorption near 500 nm is due to the Fe F (axial ligand) CT and/or the p–p* transition of the porphyrin core. Thus, the RR spectrum obtained by excitation near 500 nm (inset f) shows strong enhancement of the porphyrin core as well as the n(Fe F) vibrations. Excitation at the Soret band near 400 nm (p–p* transition of the porphyrin core) produces the RR spectrum (inset e) in which totally symmetric porphyrin vibrations are strongly enhanced (Sec. 1.23 of Part A). In contrast, excitation below 300 nm produces RR spectra that exhibit peptide chain vibrations with no major interference from porphyrin core vibrations. The absorption band in the 270–220 nm region originates in the p–p* transitions of aromatic amino acids such as tyrosine and tryptophan (structures shown). Thus, their phenyl and indole ring vibrations are enhanced by excitation in this region (inset c). Strong enhancement of peptide chain vibrations can occur by excitation below 220 nm (inset d), since the electronic absorption in this region is due to the p–p* transition of the peptide backboned [14].

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Fig. 3.3. (a) Structure of iron protoporphyrin; (b) absorption spectrum of Mb fluoride; (c) UV excitation at 225 nm enhances tyrosine and tryptophan bands; (d) excitation further in the UV region enhances amide vibrations; (e) excitation near 400 nm enhances totally symmetric vibrations of the heme core; (f) the absorption bands in the 600–500 nm region are due to the porphyrin core as well as Fe–axial ligand CT transitions, and excitation in this region enhances nontotally symmetric porphyrin core vibrations as well as Fe–axial ligand vibrations [13].

In Mb(III)F, Hb(III)F and their model compounds, the vinyl group vibrations of protoporphyrin (PP) such as the n(C¼C) near 1620 cm 1 are overlapped by strong porphyrin core vibrations when RR spectra are measured by Soret excitation [15]. In the case of the [Fe(PP)(CN)2] ion, however, the vinyl stretching as well as the vinyl– heme stretching (1125 cm 1) vibrations are observed without interference by porphyrin core vibrations if excitation lines in the UV region (225 nm) are used [16]. UVRR techniques* have been utilized extensively to elucidate the structures of biological macromolecules [17]. UV laser lines down to 175 nm can be obtained by combining the third (355 nm) and fourth (266 nm) harmonics of the Nd–YAG laser with dye lasers or hydrogen Raman shifters.

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Deoxy-Hb and deoxy-Mb exhibit a weak charge transfer absorption near 764 nm (not shown in Fig. 3.3). RR spectra obtained by excitation in this region are markedly different from those obtained by B- and Q-state excitations; such an excitation exhibits a number of relatively intense low-frequency modes including those at 168 and 115 cm 1, which may be due to out-of-plane deformation of the porphyrin ring [19]. 3.1.2. Porphyrin Core Vibrations “Structure-sensitive bands” of porphyrin core vibrations in heme proteins were first discovered by Spiro and Strekas [19] in 1974. Table 3.1 lists four structure-sensitive bands reported by these workers. Bands I and IV are an oxidation-state-marker and a spin-state marker, respectively, while bands II and Vare sensitive to both oxidation and spin states. On the basis of these results, they proposed that the Fe O2 bond in oxy-Hb should be formulated as Fe(III) O2 . In Sec. 1.5, we discussed structure-sensitive bands of model compounds such as Ni(OEP) and Ni(TPP) on the basis of the results of normal coordinate analyses. The normal modes obtained for these model systems are not directly transferable to heme proteins since the effects of peripheral substituents, axial ligands, and peptide chains on porphyrin core vibrations must be considered. Approximate correlations may be made, however, between these two systems. Thus, bands I, II, IV, and V listed in Table 3.1 are often referred to as the n4, n3, n19, and n10 of the model compound, respectively (see Table 1.10). The oxidation-state-sensitive bands (I, II, IV, and V) contain n(CaCm) or n(CaN) as the major contributors in their potential energy distribution. By lowering the oxidation state, backdonation of d-electrons to the porphyrin p* orbitals increases. Thus, the porphyrin p-bonds are weakened, and their stretching frequencies are lowered. As seen in Table 3.1, this is most clearly demonstrated by band I, which is a relatively pure oxidation-state marker. In general, axial coordination of p-acceptor ligands (CO, O2, etc.) raises its frequency, while that of p-donor ligands (RS , etc.) lowers it. In fact, cytochrome P450 [Fe(II), high spin] exhibits band I at 1346 cm 1, which is much lower than that of deoxy-Hb (1358 cm 1) because its axial ligand is a mercaptide sulfur of a cysteinyl residue [20]. TABLE 3.1. Structure-Sensitive Bands of Heme Proteins (cm 1)a Protein Ferricytochrome c CN-Met-Hb F-Met-Hb deoxy-Hb Ferrocytochrome c oxy-Hb

Oxidation State

Spin State

Band I (p)

Band II (p)

Band IV (ap)

Band V (dp)

Fe(III) Fe(III) Fe(III) Fe(II) Fe(II) Fe(II)

Low spin Low spin High spin High spin Low spin Low spin

1374 1374 1374 1358 1362 1377

1502 1508 1482 1473 1493 1506

1582 1588 1555 1552 1584 1586

1636 1642 1608 1607 1620 1640

a The bands are numbered following the convention given by T. G. Spiro and J. M. Burke [J. Am. Chem. Soc. 98, 5482 (1976)]. Bands I, II, IV, and V (where p ¼ polarized, ap ¼ anonymous polarization, dp ¼ depolarized) correspond approximately to n4, n3, n19, and n10, respectively, of metalloporphyrins (see Sec. 1.5.2). For a more complete listing, see Ref. 8.

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As discussed in Sec. 1.5, the sensitivity of RR bands to spin state is attributed to expansion or out-of-plane deformation of the porphyrin core. In high-spin iron, electrons populate the antibonding dx2 y2 orbital, and the lengthened Fe N bonds are accommodated by expansion of the porphyrin core or displacement of the Fe atom from the porphyrin core plane. This results in weakening of the methine bridge bonds in high-spin complexes. Thus, the frequencies of spin-state-sensitive bands (n3, n19, and n10) are lower in high-spin than in low-spin complexes since all these vibrations contain n(CaCm) as the major contributor in their normal modes (see Table 1.10). The spin-state-sensitive bands are also metal sensitive since electron occupation in the antibonding dx2 y2 orbital is varied in a series of transition metals [21]. 3.1.3. Fe–Histidine Vibrations [22] As shown in Fig. 3.2, the iron protoporphyrin is linked to the nitrogen (N) atom of the proximal histidine (F8) in Mb, Hb, and many other heme proteins. Thus, the n[Fe N(His)] vibration is highly important in understanding the nature of the T and R states mentioned previously. Nagai et al. [23] have shown definitively that the n[Fe N(His)] is near 220 cm 1, which is much lower than those proposed by others. Their assignments were confirmed by the observed 54 Fe= 58 Fe isotope shifts (2 cm 1) of these bands. As seen in Fig. 3.4, this band is at 215 cm 1 for the T state of deoxy Hb,

Fig. 3.4. The RR spectra (441.6 nm excitation) of deoxy-Hb at the T and R states; Hb A and NES des-Arg (141 a)Hb were used to represent these states, respectively; the 215 cm 1 band is asymmetric and broad because contributions from the a and b chains are not equivalent [23].

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and at 221 cm 1 for the R state of deoxy NES des–Arg141a Hb [23]. It is the only band that can differentiate between the T and R states of deoxy Hb. The observed upshift in going from the T to R state indicates that the Fe N bond is stretched in the T state because of a strain exerted by the globin [24]. Stein et al. [25] proposed an alternative explanation that partial donation of the NdH proton to an acceptor such as the COO group of the peptide backbone would strengthen the Fe N bond, and that the degree of such partial donation might be less in the T than in the R state. The n[Fe N(His)] of oxyMb [Fe(III), low spin] is much higher (271 cm 1) [9] than that of deoxyMb [Fe(II), high spin] (218 cm 1). Extensive isotope labeling studies have shown that the latter should be regarded as a vibration of the whole imidazole moiety against the Fe center, and not as a simple Fe N diatomic vibrator [26]. The RR spectrum of horse myoglobin exhibits the n[Fe N(His)] at 220 cm 1 in solution at atmospheric pressure (0.1 MPa). This band is upshifted by 2.8 cm 1 when high pressure (175 MPa) is applied to the sample in a hydrostatic cell. The observed upshift may be due to the sliding of the F helix (Fig. 3.1), which alters the tilt angle of the proximal histidine (F8) relative to the heme plane [27]. The n(Fe N(His)] of Nireconstututed Hb was located at 236 cm 1. This band is shifted to 229 cm 1 by 58 Ni=64 Ni substitution [28]. 3.1.4. Low-Frequency Vibrations Resonance Raman spectra of Hb in the low-frequency region provide structural information on the subtle differences between two types of subunits (a and b). Deconvolution studies show that the intense band at 300 cm 1 of Hb consists of two bands at 299 and 304 cm 1, which are attributed to the a- and b-subunits, respectively. These bands are due largely to the out-of-plane bending vibrations of the methane carbons, which are useful for detecting distorsion resulting from interactions between subunits [29].

3.2. LIGAND BINDING TO MYOGLOBIN AND HEMOGLOBIN When a diatomic (XY) ligand such as CO, NO, and O2 binds to myoglobin and hemoglobin, the n(XY) as well as the n(Fe XY) and d(FeXY) vibrations are expected in IR and Raman spectra. In RR spectra, the origin of resonance enhancement of these axial vibrations is attributed to “direct coupling” between the n(Fe XY) vibration and the porphyrin p–p* electronic transition [9]. These axial vibrations provide valuable information about the steric and electronic effects of the heme cavity on the Fe X Y moiety. 3.2.1. CO Adducts [5,30] In general, the n(CO) is strong in IR but weak in RR spectra. The opposite trend holds for the n(Fe CO) and d(FeCO) in the low-frequency region. Tsubaki et al. [31] first

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341

assigned the n(CO), d(FeCO), and n(Fe CO) of HbCO at 1951, 578, and 507 cm 1, respectively. Isotopic substitution experiments show 12

d(FeCO) (cm 1) n(Fe CO) (cm 1)

C 16 O

578 507

13

> >

C 16 O

563 503

12

< >

C 18 O

13

> >

576 498

C 18 O

560 494

These assignments were supported by approximate normal coordinate analysis on a linear (tilted) Fe CO model. The d(FeCO) exhibits a “zigzag” pattern (Sec. 1.18.7), whereas the n(Fe CO) changes monotonously as the mass of CO increases. Although the trend d(FeCO) > n(Fe CO) is somewhat unusual, it has been reported for some metal carbonyls (Table 1.48). Hirota et al. [32] assigned the d(FeCO) at 365 cm 1 and attributed the band at 578 cm 1 to a combination of the d(FeCO) with a porphyrin or a Fe C deformation mode (displacement of the C atom parallel to the porphyrin plane). Their assignments are based on the Raman difference spectra of HbCO obtained by using four isotopomers of CO. On the other hand, Hu et al. [33] observed two strong bands at 574 and 495 cm 1 in the IR spectrum of Fe(OEP)(py)(CO) that were downshifted by 17 and 5 cm 1, respectively, by 12 CO= 13 CO substitution. However, they could not observe any isotope-sensitive bands near 360 cm 1. This observation led them to support the original assignment by Tsubaki et al. Further support was provided by RR studies of selectively deuterated hemes [34] and DFT calculations [35,36]. As discussed in Sec. 1.18, s-donation from the CO to the metal tends to raise the n(CO) while p-backdonation from the metal to the CO tends to lower the n(CO). It is expected, therefore, that the Fe C bond order would increase, and the CO bond order would decrease, as p-backbonding increases. In fact, a negative linear relationship was found between the n(CO) and n(Fe CO) in a series of heme proteins containing imidazole as the axial ligand [37]. Deviation from the straight-line relationship occurs when (1) electron-donating substituents are introduced in the porphyrin ring, (2) the donor strength of the trans axial ligand is increased, and (3) the coordinated CO interacts with the distal histidine. In a protein-free environment, the Fe CO bond is perpendicular to the porphyrin plane. In the heme cavity, however, it may be bent and/or tilted owing, to steric hindrance and/or electronic interaction with distal histidine (E7). Three probable geometries are illustrated in Fig. 3.5. According to X-ray analysis [38], the Fe CO bond in MbCO is linear but tilted by 13 from the normal to the porphyrin plane. Such distortion is expected to raise the n(CO) since it decreases the Fe(dp) ! CO(p*) O

O

C

C

C

O

Fe

Fe

Fe

His

His

His

UPRIGHT

BENT

TILTED

Fig. 3.5. Three geometries of Fe CO bond in heme proteins.

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Fig. 3.6. Orientation of distal histidine at neutral and acidic pH.

backbonding. However, this effect is overcome by an increase in the pyrrole (p) CO (p*) overlap resulting from such tilting. As a result, tilting lowers the n(CO) and raises the n(Fe CO) as discussed for “strapped” porphyrins in Sec. 1.18.4. In heme proteins, the upright Ð tilted conformational change is observed by changing the pH of the solution. Thus, the n(Fe CO) of the sperm whale MbCO at 507 cm 1 in neutral pH solution is shifted to 488 cm 1 in acidic solution [39]. This result corresponds to the previous IR observation that the n(CO) of soybean legHbCO at 1947.5 cm 1 in neutral pH is upshifted to 1957 cm 1 in acidic solution [40]. As illustrated in Fig. 3.6, the Ne atom of the distal histidine is protonated in acidic solution. This may induce the displacement of the histidine to accommodate the upright geometry of the Fe CO bond. Similar upshifts of the n(CO) are observed when the b-chain distal histidine of Hb is replaced by nonpolar residues such as glycine and valine [41], and when the degree of hydration of hydrated films of Hb and Mb is changed [42]. The IR spectra of sperm whale MbCO exhibit three n(CO) near 1967, 1944, and 1933 cm 1 in solution as well as in the crystalline state. Three different environments of the Fe CO moiety were proposed to account for this observation [43]. In human HbCO, two n(CO) bands of a and b subunits overlap to give a single band at 1951 cm 1. In contrast, rabbit HbCO exhibits two n(CO) at 1951 (b) and 1928 (a) cm 1; the latter frequently is unusually low, and its intensity is about half that of the former. It has been suggested that the distal histidine acts as a nucleophilic donor to the CO in the a subunit [44]. All the observations of n(CO) mentioned above were made by using aqueous IR techniques (Sec. 1.11). 3.2.2. O2 Adducts [5,30] As discussed in Sec. 1.21, dioxygen coordinated to metalloporphyrins can take endon, side-on, and bridging structures. However, the bridging structure is too bulky to occur in a heme cavity. Although the side-on coordination is stereochemically

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343

possible, it may be too unstable under biological conditions. Thus, the end-on coordination, such as shown in Fig. 3.2, is most probable. In fact, this structure was found by X-ray [45] and neutron diffraction [46] on MbO2 and X-ray diffraction on HbO2 [47]. These studies also revealed the presence of hydrogen bonding between the bound O2 and the Ne atom of the distal imidazole (Fig. 3.2). The N HO2 distance in MbO2 is 2.97 A, whereas in HbO2 it is 3.7 and 3.2–3.4 A, respectively, for the a- and b-subunits. The coordinated dioxygen of the end-on type exhibits the n(O2), n(Fe O2), and d(FeOO). Thus far, the n(O2) of heme proteins have been observed only in IR spectra. Attempts to resonance-enhance this mode have been unsuccessful because the “direct coupling” mechanism invoked for CO adducts does not work or because the oscillator strength of the Fe ! O2 CT transition is too small [9]. Exceptions are found in fivecoordinate Fe(TPP)O2 (Sec. 1.21) and O2 adducts of cytochrome P450 (Sec. 3.3). In contrast, the n(Fe O2) and d(FeOO) in the low-frequency region have been observed exclusively by RR spectroscopy. Thus, the n(Fe O2) vibration of HbO2 was first observed at 567 cm 1 by Brunner [48], and the end-on geometry was confirmed by 16 18 O O experiments that showed two n(Fe O2) vibrations due to mixing of the Fe–16 O-18 O and Fe–18 O–16 O bonds [49]. Hirota et al. [50] were able to locate the d(FeOO) of HbO2 at 425 cm 1 using Raman difference techniques. As shown in Fig. 3.7, difference features are observed at 568 and 425 cm 1. The same results were obtained independently by Jeyarajah et al. [51].

Fig. 3.7. The RR spectra (427.0 nm excitation) of HbO2 and their difference spectrum as specified [50].

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The n(O2) of HbO2 was first located at 1107 cm 1 by IR spectroscopy [52]. This was followed by similar work on MbO2 that gave almost the same frequency [53]. Later IR studies revealed, however, that HbO2 exhibits two n(O2) bands at 1156 and 1107 cm 1, although a single n(18 O2 ) was observed at 1065 cm 1. Therefore, the observed splitting was attributed to Fermi resonance between n(16 O2 ) near 1130 cm 1 and the first overtone of the n(Fe O2) at 567 cm 1 [11]. The interpretation presented above was challenged by Tsubaki and Yu [54], who observed the n(O2) of cobalt(II)-reconstituted HbO2 (CoHbO2) using Soret excitation, which is in resonance with the Co O2 CT transition. These workers observed three oxygen–isotope-sensitive bands at 1152 (weak), 1137 (strong), and 1107 cm 1 (very weak). The origin of this multiple-band structure was attributed to the presence of two conformers; conformer I is responsible for the bands at 1137 and 1107 cm 1, which result from Fermi resonance between the unperturbed n(O2) (1122 cm) and the porphyrin mode at 1121 cm 1, whereas conformer II is responsible for the 1152 cm 1 band. This interpretation is based on X-ray analysis of MbO2 in which the Fe O O plane can take two orientations relative to the porphyrin plane [45]. Thus, in conformer I, the Co O O plane is in the direction that permits the formation of the N HO2 bond mentioned earlier. In conformer II, this plane is rotated by about 40 from that of conformer I so that the O2 is free from hydrogen bonding. As a result, the n(O2) of the latter (1152 cm 1) is much higher than that of the former (1122 cm 1). The observed upshift of the 1137 cm 1 band (2 cm 1) by D2O/H2O (solvent) exchange was regarded as evidence to support their interpretation [55]. More recent IR studies by Potter et al. [56] confirmed the presence of the three bands mentioned above. These workers noted, however, that the observed difference in n(O2) (30 cm 1) between the two conformers is too large to attribute it to the effect of hydrogen bonding alone, and proposed a structure of conformer I in which the Fe O O and imidazole planes are eclipsed on the N Fe N axis of the porphyrin ring since p-electron donation from the imidazole to the O2 mediated through the metal would cause a marked reduction in the n(O2). Quite contrary to these investigations, Bruha and Kincaid [57] interpret the RR spectra of CoMbO2 and CoHbO2 in terms of a single conformer. Figure 3.8 shows the RR spectra of CoHbO2 obtained by these authors. The complicated features arise because of two reasons. First, several porphyrin vibrations appear in this region. They are easily identified at 1228, 1174, 1136, and 1123 cm 1 because they show no oxygen–isotope shifts and appear in all the compounds studied. Second, the remaining oxygen–isotope-sensitive bands are analyzed by considering the possibilities of vibrational couplings between the n(O2) fundamental and internal modes of imidazole (proximal or distal). In this case, vibrational coupling occurs between the n(16 O2 ) near 1136 cm 1 and the imidazole mode near 1160 cm 1. Similar coupling occurs between the n(18 O2 ) near 1063 cm 1 and the second imidazole mode near 1100 cm 1 (these imidazole bands are seen in the spectrum of histidine shown by the dotted line in trace A). Thus, the n(16 O2 ) and n(18 O2 ) of HbO2 are assigned near 1136 and 1063 cm 1, respectively. As stated in Sec. 1.21.5, these vibrational couplings have been analyzed quantitatively by using the Fermi resonance scheme.

LIGAND BINDING TO MYOGLOBIN AND HEMOGLOBIN

Fig. 3.8. The RR spectra (406.7 nm excitation) of CoHbO2 and its

345

18

O2a and scrambled O2 adducts in H2O and D2O. The dashed line in (A) indicates the Raman spectrum of l-histidine [57].

Vibrational spectra of O2 adducts of heme protein model compounds such as “picket-fence” and “strapped” porphyrins have also been discussed in Sec. 1.21.5. 3.2.3. NO Adducts [12,30] Similar to the case of O2 adducts, the NO groups bonded to ferrous Mb and Hb take a bent end-on geometry. The bent Fe N O group is expected to show the n(NO) at 1700–1600 cm 1, and the n(Fe NO) and d(FeNO) in the 600–400 cm 1 region. The NO has been used to probe conformational changes of the heme moiety when the quaternary structure of Hb is switched from the R to the T state. Human HbNO in the R state has four six-coordinate hemes, whereas the T state is a hybrid of five- and sixcoordinate NO moieties. Using IR spectroscopy, Maxwell and Caughey [58] observed the n(NO) of six-coordinate heme (R state) at 1618 cm 1 and that of five-coordinate heme (T state induced by adding IHP (inositol hexaphosphate) at 1668 cm 1. The lack of discernible pH effects on these frequencies suggested that a polar (donor–acceptor) interaction is more likely than hydrogen bonding between the NO and the distal imidazole. Spiro and coworkers [59,60] observed the n(Fe NO) of these six- and five-coordinate hemes near 550 and 590 cm 1, respectively (413.1 and 454.5 nm

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APPLICATIONS IN BIOINORGANIC CHEMISTRY

excitation). However, Yu and coworkers [61,62] could not detect the latter by 406.7 nm excitation. According to Benko and Yu [63], the band near 554 cm 1 in ferrous MbNO is the d(FeNO) and not n(Fe NO). Their assignment is based on the zigzag isotopic shift pattern in the order of NOð554 cm 1 Þ > 15 NOð545 cm 1 Þ < N18 Oð554 cm 1 Þ. Figure 3.9 shows the RR spectra of NO adducts of ferrous Mb obtained by Hu and Kincaid [64]. These workers assigned the bands at 554 and 449 cm 1 to the n(Fe NO) and d(FeNO), respectively, although substantial mode mixing was noted. The former shows a zigzag isotope shift pattern, whereas the frequency of the latter decreases monotonously as the total mass of the NO ligand increases. Thus, the observation of a zigzag isotope shift pattern does not necessarily indicate a bending mode. Normal coordinate analysis on a bent FeNO model shows that local internal coordinates are mixed substantially and the degree of contribution of each coordinate to these vibrations depends on the FeNO bending angle. The NO can also bind to ferric heme proteins, although ferric nitrosyl complexes have a tendency for spontaneous autoreduction. Since the Fe(III) NO is isoelectronic with the Fe(II) CO, it may take a linear geometry which would be distorted in a heme cavity. The n[Fe(III) NO] and d[Fe(III)NO] of MbNO are observed at 595 and 573 cm 1, respectively [63]. In this case, the latter shows a zigzag isotope shift pattern.

Fig. 3.9. The RR spectra (406.7 nm excitation) of MbNO with four NO isotopomers [64].

LIGAND BINDING TO MYOGLOBIN AND HEMOGLOBIN

347

The NO group vibrations are also reported for the NO adducts of Co(II)-[65,66] and Mn(II)-reconstituted [67] Mb and Hb. The following trend is found in the Mb series: v½MnðIIÞ NOŠ > v½FeðIIIÞ NOŠ > v½CoðIIÞ NOŠ > v½FeðIIÞ NOŠ v~ðcm Þ 627 595 576 554 1

The RR spectra of NO adducts of Mn(II) complexes of “unprotected” and “strapped” porphyrins (Fig. 1.64b) have been compared to study the steric effects of the “strap” [68]. Vibrational spectra of NO adducts of heme proteins have been reviewed by Wang et al. [69]. 3.2.4. Adducts of Other Axial Ligands [11,30] The CN ion binds strongly to ferric heme proteins. The n(CN) of ferric MbCN and HbCN are observed at 2125 cm 1 in IR spectra [70]. This frequency is higher than that of free CN ion (2083 cm 1) for the reason discussed in Sec. 1.16. Since the Fe(III) CN bond is linear, it may be distorted in a heme cavity. Although an X-ray diffraction study on ferric (or met) HbCN confirmed such a distortion, the exact geometry has not yet been known because of poor resolution (2.8 A) [71]. Yu et al. [72] suggest that the linear Fe CN bond is tilted only because both the nðFe–13 CNÞ and nðFe–C15 NÞ bands appear at 450 cm 1. The CN ion can also bind to ferrous heme proteins, but its affinity is much lower and the corresponding complexes are readily photodissociated. The N3 ion binds to ferric heme proteins to form a mixture of high-spin (hs) and low-spin (ls) complexes at room temperature. Thus, the IR spectrum of metMbN3 exhibits two na(N3) bands at 2045 and 2023 cm 1 that were assigned to the hs and ls complexes, respectively. Similar bands were observed at 2047 (hs) and 2025 (ls) cm 1 for metHbN3 [70]. In RR spectra, Tsubaki et al. [73] observed two sets of porphyrin vibrations corresponding to the low- and high-spin states of metMbN3. They also assigned the RR band at 411 cm 1 to the n(Fe N3) of the low-spin complex, although the band at 413 cm 1 was previously assigned to the high-spin complex [74]. The n(Fe OH) of metHbOH is observed at 495 cm 1 [75]. In the abnormal subunit of Hb M Boston, the heme iron is bonded to the phenolate oxygen of tyrosine (E7) [76] instead of the proximal histidine (F8). Nagai et al. [77] assigned the band at 603 cm 1 to the n[Fe O(tyrosine)]. In metMbF, the n(Fe F) vibrations were observed at 461 and 421 cm 1 which were attributed to the nonhydrogen-bonded and hydrogen-bonded (to water) Fe F moieties, respectively [78]. 3.2.5. Photochemistry of HbCO and HbO2 Time-resolved resonance Raman (TR3) spectroscopy (Sec. 1.4.2) has been utilized extensively to study the structures and dynamics of extremely short-lived transient species [in the order of nanosecond (10 9) and picosecond (10 12)] that are created by photolysis of HbCO, HbO2, and other proteins [79–81].

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Fig. 3.10. Soret band excited RR spectrum of deoxyHb and Hb* with 416-nm, 7-ns pulses [82].

Dasgupta and Spiro [82] measured the RR spectra of deoxy-Hb and the photoproduct of HbCO (abbreviated as Hb*) using 7 ns Nd–YAG laser pulses at wavelengths of 416 and 533 nm. As stated in Sec. 1.23 of Part A, Soret excitation (416 nm) enhances the totally symmetric modes (n2, n3, and n4), whereas Q excitation (532 nm) enhances nontotally symmetric modes (n10, n11, and n19). Figure 3.10 compares the RR spectra (Soret excitation) of these two compounds. It is seen that the bands at 1562 (n2), 1468 (n3), and 1357 cm 1 (n4; see Fig. 3.10 inset) of deoxy-Hb are downshifted by 2–3 cm 1 in Hb*. Similar downshifts are observed for nontotally symmetric vibrations of Hb*. These results suggest that the photoproduct, Hb*, has a slightly expanded porphyrin core because of the out-of-plane displacement of the Fe atom by 0.1 A relative to the deoxy-Hb structure. To gain more detailed information, Kincaid et al. [83] prepared two hybrid Hb such as (ab*)2 and (a*b)2 where a* and b* denote protoheme-d4 subunits, and measured the RR spectra of their native states and the photoproducts of HbCO with 10 ns Nd–YAG laser line at 532.1 nm. The n19 frequency (subunit-specific structural marker band) shows no difference between the two photoproducts of the above two hybrid Hb in spite of significant differences observed for their equilibrium deoxy forms. Thus, subunit heterogeneity does not exist in the photoproducts. Similar work on HbO2 [84] shows that the photoproduct obtained by 30-ps pulses (532 nm) exhibits the n10 and n11 at frequencies lower by 10 and 5 cm 1, respectively, than the HbCO photoproduct. These large shifts were attributed tentatively to the formation of an electronically excited deoxy-Hb. Kaminaka et al. [85] studied the dynamics of quaternary structural changes of HbCO after the photolysis by using UV TR3 spectroscopy (218 nm). Finally, ultrafast

LIGAND BINDING TO MYOGLOBIN AND HEMOGLOBIN

349

femtosecond (10 15) IR spectroscopy was used to characterize 13 CO bonded to the a- and b-subunits of Hb M Boston [86]. Mizutani and Kitagawa [87] carried out an extensive TR3 study on ultrafast dynamics of photodecomposition of MbCO and its derivatives. Since Mb has no quaternary structures, their interest was focused on the structure and the timeframe of the photodissociated product (Mb*). Figure 3.11 compares temporal changes of the Raman intensities of the n(Fe N) of the proximal histidine at 220 cm 1 and the porphyrin core (out-of-plane) vibrations, g7, at 301 cm 1. These vibrations serve as monitors in detecting structural changes in the protein matrix and the porphyrin core, respectively. As seen in the figure, the temporal intensity change of g7 shows that structural changes in the porphyrin core by photodissociation are completed within the instrumental response time of the apparatus used (2 ps) and the equilibrium structure is reached within a few picoseconds. On the other hand, the temporal intensity change

Fig. 3.11. Temporal changes of Raman intensities of the Stokes n(Fe His) (circles) and g7 (triangles) of photo-dissociated MbCO. The solid lines indicate the calculated fits. The lower panel shows a close-up of the curve in the early time region [87].

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of the n(Fe N) shows a gradual increase up to 20 ps after instantaneous rise on photodissociation. Thus, changes in the tertiary structure of the protein are much slower than that of the heme core caused by the Fe CO bond cleavage.

3.3. CYTOCHROMES AND OTHER HEME PROTEINS 3.3.1. Cytochrome c [88] Cytochromes (a, b, and c) are electron carriers in the mitochondrial respiratory chain. Among them, cytochromes c are relatively small (MW 13,000) and relatively easily crystallized. The structures of cytochromes c from various sources have been determined by X-ray crystallography [89]. These studies show that the prosthetic group of cytochrome c is a heme in which the vinyl sidechains of iron protoporphyrin are replaced by cysteinyl thioether bonds and to which the imidazole (His 18) nitrogen and the methionine (Met 80) sulfur (thioether) atoms are coordinated axially. One of the structural features of cytochrome c is the presence of an “opening” at the edge of the heme cavity through which the electron transfer may occur. In most cytochrome c, the iron atoms are in the low-spin state, and the basic structure of the heme is unchanged by changing the oxidation state of the iron [90]. As shown in Table 3.1, bands I (n4), II (n3), and V (n10) are shifted markedly to higher frequencies in going from the ferrous to ferric states [19]. Cytochrome c takes five different structures depending on the pH with

pK values shown above [91]. As stated above, the heme iron is axially bonded to the imidazole nitrogen (His 18) and the methionine sulfur (Met 80) at neutral pH (III). However, these axial ligands are replaced by water at acidic pH (I and II). At alkaline pH, the Fe S (Met 80) bond is cleaved and may be replaced by another ligand (Lys 79), although the Fe N (His 18) bond is intact (IV). At extremely alkaline pH, both of these axial ligands may be replaced by other ligands. Thus, vibrational studies of cytochrome c as a function of pH are of particular interest. The RR spectra of ferricytochrome c as a function of pH were first studied by Kitagawa et al. [92]. These workers noted that the bands at 1375 (n4), 1504 (n3), 1563 (n11), and 1637 cm (n10) are shifted by 2–3 cm 1 to higher frequencies when the pH is increased from 7 to 10.8. This result is expected since a weak p-backdonation from the Met 80 to the porphyrin (p*) via the Fe(dp) orbital is disrupted at alkaline pH. As mentioned above, both axial ligands are replaced by water at pH ¼ 2.5. Lanir et al. [93] observed that the bands at 1563 (n11), 1585 (n19), and 1637 cm (n10) are downshifted to 1556, 1569, and 1623 cm 1, respectively, by decreasing the pH from 7.0 to 2.0. Thus, these workers concluded that structure II mentioned above is high spin.

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The RR spectra of ferrocytochrome c at pH ¼ 7–11.2 exhibit many bands below 500 cm 1. At pH ¼ 13.6, however, this feature is replaced by a much simpler spectrum in this region. Valance and Strekas [94] interpret this result as follows. In neutral to alkaline solution, the heme is rigidly held by the peptide chain, and the resulting asymmetric heme activates many Raman bands. At pH over 13, however, the protein structure is relaxed (unfolded) and the symmetry of the heme becomes effectively higher, resulting in fewer Raman bands. Thus far, not much information is available on axial vibrations. The n[Fe N (His 18)] is observed at 182 cm 1, and the n[Fe S (Met 80)] is estimated to be near 344 cm 1 [95]. Hu et al. [96] measured the RR spectra (Soret and Q excitation) of ferrocytochrome c and its isotopomers (meso-d4 and pyrrole-15 N, etc.) at neutral pH, and assigned most of the in-plane and out-of-plane skeletal modes according to the assignments obtained for Ni(OEP) (Sec. 1.5). This is justifiable because cytochrome c does not have the conjugating vinyl groups of protoporphyrins that complicate the vibrational assignments. Their results manifest the out-of-plane distorsions of the porphyrin core found by high-resolution X-ray diffraction studies [97]. For example, two anomalously polarized bands (n19 and n21) gain substantial intensity by Soret excitation, and the depolarized band (n15) becomes extraordinarily strong. These and other observations suggest that the D4h symmetry of the porphyrin core is lowered by “suddle-shaped distortion” [96]. X-Ray analysis has been reported on several model compounds of cytochrome c such as [Fe(TPP)(THT)2]ClO4 and [Fe(TPP)(PMS)2]ClO4, where THT and PMS denote tetrahydrothiophene and pentamethylene sulfide, respectively [98,99]. Oshio et al. [100] assigned the na(S Fe S) of these compounds at 328 and 323.5 cm 1, respectively, based on 54 Fe= 56 Fe isotope shifts observed in IR spectra. 3.3.2. Cytochrome P450 [101] Cytochromes P450 (MW 50,000) are monooxygenase enzymes that catalyze hydroxylation reactions of substrates such as drugs, steroids, pesticides, and carcinogens:

One of the microbial species in which cytochrome P450 is found is Pseudomonas putida. When this bacterium is grown in air with camphor as the substrate, cytochrome P450cam can be isolated in a crystalline form. Thus far, most spectroscopic studies have been made on this compound. The term P450 was used because its CO adduct exhibits the Soret band at 450 nm. The active site of cytochrome P450 is an iron protoporphyrin with the iron center axially bound to the mercaptide sulfur of a cysteinyl residue. The axial Fe S linkage is retained throughout its reaction cycle shown in Fig. 1.12 [101]. This was confirmed by X-ray analysis [102] of cytochrome P450cam (b state). The resting state (a) is a six-coordinate ferric low-spin porphyrin with H2O as the axial ligand trans to the Fe S linkage. Binding of a substrate (SH) disrupts the Fe OH2 bond and converts it to

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[LS, 6C] H2O

SOH

SH Fe3+ [HS, 5C]

(a) Fe3+

Fe3+

SOH

SH

H2O

e

(b) 2e-+2H+

.

O -2 H2O2 OH 4+

Fe

Fe2+

S.

(c)

.. . ..

(g) O Fe5+

Fe3+ SH

.. (2H+) . ..

SH O2

SH (d)

e-,2H+ (f) H2 O

Fe3+

SH

(e)

Fig. 3.12. Catalytic cycle of cytochrome P450 [101]. The axial Fe S bond is not shown.

b state, which is a ferric five-coordinate (5C) high-spin (HS) state. In general, this conversion produces a mixture of high- and low-spin complexes, and the fraction of the high-spin species depends on the substrate; it is 95% for a large substrate such as camphor but only 43% for a small substrate such as norcamphor [103]. b state is converted to c state (five-coordinate, ferrous, high-spin complex) by accepting electrons from iron–sulfur proteins (Sec. 3.8) and other reducing agents. Oxygenation of c state yields an O2 adduct (d state) that is the last detectable intermediate in the reaction cycle. Thus, the structures of e–g states shown in Fig. 3.12 were proposed without definitive evidence. As stated in Sec. 3.1, Ozaki et al. [20] observed the oxidation-state marker band of cytochrome P450cam in the c state at 1346 cm 1, which is much lower than those of other Fe(II) porphyrins. Similar observations have been made for cytochromes P450 from other sources [104]. This anomaly was attributed to the extra negative charge transmitted to the porphyrin p*(eg) orbital from the mercaptide sulfur (RS ), which has two lone-pair electrons. Champion et al. [105] first observed the n(Fe S) of cytochrome P450cam (b state) at 351 cm 1 in the RR spectrum (364 nm excitation). This band is shifted by 54 Fe= 56 Fe and 32 S= 34 S substitutions by 2.5 § 0.2 and 4.9 § 0.3 cm 1, respectively. Later, excitation profile studies on the n(Fe S) and other modes were carried out by Bangcharoenpaurpong et al. [106]. The n(Fe S) of model compounds such as Fe(TPP)(SC6H5) are observed at 345–335 cm 1 in IR spectra [100].

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According to IR studies by O’Keefe et al. [107], cytochrome P450cam CO exhibits the n(CO) at 1940 cm 1 (nonlinear FeCO bond), whereas the camphor-free compound exhibits two n(CO) at 1963 (linear) and 1942 cm 1 (nonlinear). The n(CO) of model compounds such as Fe(TpivPP)(CH3S)(CO) is observed at 1945 cm 1 [108]. Uno et al. [109] assigned the bands at 1940, 558, and 481 cm 1 to the n(CO), d(FeCO), and n(Fe CO), respectively. This n(Fe CO) is markedly lower than that of HbCO (507 cm 1) because of the trans effect of the mercaptide sulfur discussed earlier. The n(O2) of cytochrome P450cam O2 (d state) was first observed at 1140 cm 1 in RR spectra (420 nm excitation) by Bangcharoenpaurpong et al. [110]. This frequency is very close to the n(O2) of a model compound, [Fe(TpivPP)(SC6HF4)(O2)] (1139 cm 1), observed in IR [111] as well as in RR spectra [112]. In a Co(II)substituted model compound, [Co(TPP)(SC6H5)(O2)] , the n(O2) is at 1122 cm 1, which is 22 cm 1 lower than that of Co(TPP)(py-d5)(O2) [113]. The n(Fe O2) of cytochrome P450cam O2 was first located at 541 cm 1 in RR spectra by Hu et al. [114]. As seen in Fig. 3.13, this band is rather weak, but its presence is confirmed by the difference spectrum (trace C). These workers also noted that two n(O2) are observed at 1139 and 1147 cm 1 when camphor is replaced by adamantanone. This may indicate the existence of two conformers that have different types of interactions between the bound O2 and the substrate. The

Fig. 3.13. The RR spectra (441.6 nm excitation) of O2 adducts of cytochrome P450cam: (A) 16O2, (B)

18

O2 and their difference spectrum (C) [114].

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TABLE 3.2. Effect of Substrates on FeNO Group Vibrations of Cytochrome P450–NO (cm 1) [117] Fe(III)–NO (Linear) Substrate Substrate-free Norcamphor Camphor Adamantanone Mb–NO

n(Fe–NO) 528 524 522 520 594a

d(FeNO)

Fe(II)–NO (Bent) n(Fe–NO)

— — 546 542 573a

547 545 553 554 554b,c

d(FeNO) 444 441 445 446 450b

a

Reference 63 Reference 64 c This band was previously (Ref. 63) assigned to the d(FeNO). b

n(Fe O2) of a model compound of cytochrome P450cam, “twin-coronet” iron porphyrin with an axial thiolate ligand, was observed at 578 cm 1 (413.1 nm excitation, 80 C). This band is shifted to 552 cm 1 by 16 O2 =18 O2 substitution [115]. Macdonald et al. [116] located the d(Fe O O) mode of oxycytochrome P450cam at 401 cm 1. A simple three-body calculation gives the Fe O O angle of 125–130 . This frequency is higher than those of the corresponding modes of MbO2 and HbO2, suggesting a strained Fe O O moiety relative to those of the latter. Hu and Kincaid [117] studied the effect of changing the substrate structure on the Fe(III) NO bond of cytochrome P450. Table 3.2 summarizes their results together with those obtained for the Fe(II) NO series. The Fe(III) NO bond is expected to be linear since it is isoelectronic with the Fe(II) CO bond. It is seen that the n(Fe NO) of ferricytochrome P450 NO is by 70 cm 1 lower than that of ferriMb NO because of the thiolate ligand in the former. Furthermore, this band is shifted sensitively by changing the substrate. These results have been explained by considering electronic and kinematic effects for a slightly bent Fe(III) NO bond in the substrate-bound form. Their work has been extended to the cyanide adducts of cytochrome P450cam [118]. According to Fig. 3.12, hydroxylation of the substrate molecule is accomplished by the activated oxygen released from the oxoferrylporphyrin (f state). Although such a state has not yet been characterized spectroscopically, oxoferryl stretching [Fe(IV)¼ O] vibrations have been observed for model compounds such as FeO(TPP) (852 cm 1) (Sec. 1.22.3) and for horseradish peroxidase compound II (HRP-II) at 780 cm 1. 3.3.3. Horseradish Peroxidase [119,120] Peroxidases are enzymes that catalyze the oxidation of a substrate, AH2, by H2O2: AH2 þ H2 O2 ! A þ 2H2 O Among them, reaction mechanisms of horseradish peroxidase (HRP) (MW 40,000) have been studied most extensively. The active site of HRP is the same as that of Mb, namely, iron protoporphyrin, which is axially bonded to the proximal histidine. However, there are marked differences between the two; HRP binds O2 irreversibly,

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355

whereas Mb does so reversibly. Also, HRP is active biologically in the ferric state, whereas Mb is active in the ferrous state. This may be due to the difference in the heme environment; the proximal histidine in HRP is strongly hydrogen-bonded to nearby amino acid residues, and this hydrogen bonding increases s-basicity of the proximal histidine. As a result, the n[Fe N(His)] of HRP is at 244 cm 1, which is much higher than that of Mb (220 cm 1) [121]. The reaction cycle of HRP involves two intermediates, HRP-I and HRP-II: HRPðferricÞ þ H2 O2 ! HRP-I þ H2 O HRP-I þ AH2 ! HRP-II þ AH HRP-II þ AH ! HRPðferricÞ þ A þ H2 O Thus, HRP-I (green) and HRP-H (red) have oxidation states higher than the native Fe (III) state by two and one oxidizing equivalents, respectively. It has been found that both intermediates are oxoferryl [Fe(IV)¼O] porphyrins and that HRP-II is low-spin Fe(IV), whereas HRP-I is its p-cation radical, which is one electron deficient in the porphyrin p-orbital of HRP-II. As expected from its high oxidation state, HRP-II exhibits the n4 at 1381 cm 1, which is the highest among heme proteins [122]. The n(Fe¼O) of HRP-II was reported by Hashimoto et al. [123] and Terner et al. [124] almost simultaneously. Figure 3.14 shows the RR spectra of HRP-II obtained by the former workers. On reacting HRP with H2O2 at alkaline pH, a new band appears at 787 cm 1 (trace C) that is shifted to 790 cm 1 by 56 Fe=54 Fe substitution (trace B), and to 753 cm 1 by H2 16 O2 =H2 18 O2 substitution (trace D). Thus, this band was assigned to the n(Fe¼O) of HRP-II. In neutral solution, the n(Fe¼O) band was observed at 774 cm 1, which was shifted to 740 cm 1 by H2 16 O2 =H2 18 O2 substitution. The observed downshift (from 787 to 774 cm 1) in going from alkaline solution to neutral solution has been attributed to the formation of a hydrogen bond between the oxoferryl oxygen and the NH group of the distal histidine, which disappears in alkaline pH. As discussed in Sec. 1.22.3, the n(Fe¼O) of model compounds such as FeO(TPP) were first observed near 852 cm 1 in O2 matrices. These frequencies are much higher than that of HRP-II because the former is a five-coordinate complex. In fact, the n(Fe¼O) of six-coordinate model compounds such as FeO(TPP)(1-MeIm) (820 cm 1) are lower than that of five-coordinate complexes. HRP-I is a p-cation radical (Sec. 1.22.5) produced by one-electron oxidation of HRP-II, and is much more unstable and photolabile than HRP-II. Thus several groups of workers reported different RR spectra. Ogura and Kitagawa [125] measured the RR (406.7 nm excitation) and electronic absorption spectra of HRP-I. However, they could not determine the radical type (2 A1u or 2 A2u ) because no prominent band shifts were observed between HRP-I and HRP-II in the 1700–1200 cm 1 region. They suggested that a clear cation radical state may not exist for HRP-I because of delocalization of electrons through the metal atom and the axial ligand. Oertling and Babco*ck [126] measured the RR spectrum (390 nm excitation, 10-ns pulses) using the rapid mixing and flow sample techniques, and also noted similarity between them; the n(Fe¼O) of HRP-I was 791 cm 1, which was almost identical to that of HRP-II

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Fig. 3.14. The RR spectra (406.7 nm excitation) of HRP-II (pH ¼ 11.2) containing isotopomers as indicated [123].

(787 cm 1). These workers suggested two possibilities: (1) the electron delocalization proposed by Ogura and Kitagawa [125] and (2) conversion of HRP-I to HRP-I* via rapid and efficient photoinduced electron transfer. The latter symbol indicates a nonradical porphyrin similar to HRP-II produced by one electron transfer from a nearby amino acid residue to the porphyrin cation radical. Paeng and Kincaid [127] used a microdroplet sample device to reduce the sample residence time in the laser beam (406.7 nm) to less than 5 ms, and assigned the n(Fe¼O) of HRP-I at 737 cm 1. Their spectrum in the high-frequency region suggested 2 A2u formulation. Using a similar device, Chuang and Van Wart [128] observed it at 721 cm 1. Palaniappan and Terner [129] obtained a spectrum (350 nm excitation) that is definitely different from that of HRP-II. Their spectrum in the high-frequency region suggested the 2 A2u formulation of HRP-I. Finally, Kincaid and co-workers [130] measured the RR spectra of HRP-I with 356.4 nm excitation as a function of laser power. They found that, with low laser power (1 mW), the conversion of HPR-I to HRP-I* and/or HRP (resting state) [129] is minimized, and concluded that the spectrum obtained by Palaniappan and Turner [129] is due to HRP-I. It exhibits the n(Fe¼O) at 790 cm 1 (pH ¼ 7.5), which is similar to that reported by Oertling and

CYTOCHROMES AND OTHER HEME PROTEINS

357

Babco*ck [126], but markedly different from that of Paeng and Kincaid [127]. Their RR spectra obtained by high laser power (5 25 mW) [130] suggest the conversion of HRP-I to a HRP-II-like photoproduct. Nakamoto [131] reviewed the RR spectra and biological significance of high-valence iron (IV,V) porphyrins including the n(Fe¼O) of other heme proteins. Resonance Raman studies of cyanide-coordinated HRP in the 5.5–12.5 pH range indicate the presence of two conformers [132]. In conformer I, the Fe C N linkage is essentially linear with the axial n(Fe CN) and d(Fe C N) at 453 and 405 cm 1, respectively (pH ¼ 5.5). In conformer II, the Fe C N linkage is bent and the corresponding frequencies are 360 and 422 cm 1, respectively, at the same pH. The n(Fe CN) of these conformers are pH-dependent, and the origins of their pHdependent shifts have been discussed. 3.3.4. Cytochrome c Oxidase Cytochrome c oxidases (CcO) are the terminal enzymes in the respiratory chains of mitochondria and aerobic bacteria, and catalyze the O2-reducing and proton-pumping reaction: 4½cyt: c; FeðIIފ þ O2

4Hþ ; 4e

! 4½cyt: c; FeðIIIފ þ 2H2 O

CcO

The number of subunits depends on the source of CcO. Bovine cytochrome c oxidase (MW, 2.1 105) in the mitochondrial inner membrane consists of 13 subunits. However, only subunits I and II are involved in the enzymatic reaction above. Figure 3.15 is a schematic representation of the structures of these subunits in the fully oxidized form of bovine heart CcO based on X-ray analysis by Tsukihira et al. [133,134]. Subunit I contains one CuB atom and one five-coordinate high-spin heme a3 (a)

(b) Heme a3 Met 207

Tyr 244

His 161

CuA Cys 200 His 376

CuB

Cys 196

His 240

O C

His 291 His 290

His 204

Glu 198

Fig. 3.15. Structures of subunit I (a) and subunit II (b). In (b), heme a is not shown [133].

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HO

O

C16H27

N N

Fe

HO N N

O

C16H27 Nhis N N

N N

Nhis

Nhis COO–

Fe

COO–

a3

COO–

COO–

a

Fig. 3.16. Structures of the hemes (heme a3 and heme a) and Cu sites in subunits I and II of CcO. C16H27 ¼ CH2-[CH2CH¼C(CH3)CH2]2 CH2 CH¼C(CH3)2.

(Fig. 3.16) with the N atom of His 376 occupying one axial position, and the other axial position is utilized for O2 coordination. The distance between the Fe(a3) and CuB atoms is 4.9 A, but no bridging ligands are detected between them despite their strong antiferomagnetic coupling. Subunit II contains one six-coordinate low-spin heme a (Fig. 3.16) and two CuA atoms that are bridged by two sulfur atoms of Cys196 and Cys200 to form a planar Cu2S2 ring, as shown in Fig. 3.15b. Electrons from cytochrome c are transferred to heme a via CuI $ CuII shuttling, and eventually transferred to heme a3 of subunit I. Resonance Raman spectra (840 nm excitation) of bovine CcO exhibit the CuA N (His) and CuA S(Cys) stretching vibrations of subunit II at 356 and 330 cm 1, respectively [135]. The CcO fragment obtained from bacterium Paracoccus denitrificans contains a similar bridging structure, and Andrew et al. [136] made complete assignments of its dinuclear CuA center by combining isotopic shift data with normal coordinate analysus (C2h symmetry). Figure 3.17 shows the RR spectra (488 nm excitation) of six isotopic species (32 S=34 , 63 Cu=65 Cu, and 14 N=15 N) in the 375–200 cm 1 region. Most of these bands are due to coupled vibrations of n(Cu S), n(Cu N), and n(Cu Cu), and the vibrations at 339 (Ag), 260 (Ag), and 216 cm 1(Bu) have major contributions from the n(Cu S), as they are shifted by 5.1, 4.1 and 1.5 cm 1, respectively, to lower frequencies on 32 S=S34 substitution. Since the enzymatic reaction occurs in subunit I, it is important to identify the structure of the intermediate species involved in the passageway. Ogura and Kitagawa [137,138] carried out TR3 studies coupled with electronic absorption spectroscopy on CcO obtained from bovine heart muscle. On the basis of isotope shift data coupled with temperature dependence studies, they were able to characterize the intermediate species in the catalytic cycle. The vacant axial position of heme a3 can also be coordinated by other ligands such as CO, CN , and NO. Hosler et al. [139] obtained the IR and Raman spectra of the CO adduct of wild-type Rb. sphaeroides CcO, and assigned the n(CO) at CuB, n(CO) at Fe(a3), n(Fe CO), and n(Fe N) at 2060, 1964, 516, and 214 cm 1, respectively. Other investigations include UV resonance studies of model compounds of the CuB site [140,141]. Kim et al. [142] reviewed synthetic models of CcO.

BACTERIOCHLOROPHYLLS

359

Fig. 3.17. Effect of isotope substitution on RR spectra (488 nm excitation) of P. denitrificans CuA: (a) CuA fragment from cells grown on 32S- or -3 4S-sulfate; (b) CuA apoprotein fragment reconstituted with 63Cu or 65Cu; (c) CuA fragment from cells grown on.14N- or 15N-ammonium chloride. Peak frequencies are listed for the lighter isotope spectra (—) with frequency shifts for the heavier isotope spectra () indicated above [136].

3.3.5. Other Heme Proteins Infrared and Raman studies have also been made on many other heme proteins. These include cytochrome c peroxidase (CCP), myeloperoxidase (MPO), lactoperoxidase (LPO), and catalase (CAT). Several review articles mentioned previously should be consulted for vibrational studies of these and other heme proteins.

3.4. BACTERIOCHLOROPHYLLS In purple photosynthetic bacterium such as Rb. sphaeroides, the reaction center (RC) contains four bacteriochlorophylls (BChl), two bacteriopheophytins (BPh), two quinones, a carotinoid, and a nonheme iron or manganese atom. Figure 3.18 compares the structure of BChl a with that of chlorophyll a (Sec. 1.6). The structure of BPh is the

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APPLICATIONS IN BIOINORGANIC CHEMISTRY

Fig. 3.18. Structures of (a) chlorophyll a (R ¼ CH3) and (b) bacteriochlorophyll a.

same as that of BChl with the Mg2þ ion replaced by two protons. Similar to chlorophylls, BChl is a Mg2þ macrocycle in which the two pyrrole rings (II and IV) are reduced and the fifth isocyclic ring (V) is fused to ring III. Figure 3.19 illustrates the arrangement of the BChl and BPh cofactors in the RC that was adapted by Czarnecki

Fig. 3.19. Arrangement of the BChl and BPh cofactors in RCs from Rb. sphaeroides; for clarity, the protein matrix, the other cofactors, and the phytyl substituents of the BChls and BPhs have been removed [143,144].

BACTERIOCHLOROPHYLLS

361

et al. [143] from the results of X-ray analysis of Rb. sphaeroides [144]. Here, P denotes a special BChl pair where the primary charge separation for electron transfer occurs. The subscripts “L” and “M” denote the L and M peptide subunits, respectively. Robert and Lutz [145] measured the RR spectra of the RC of this bacterium at 20–80 K with 363.8 nm excitation (Soret resonance) in the high-frequency region, and proposed the structure and bonding of the PMPL pair in the ground state on the basis of observed n(C¼O) frequencies. Raman difference techniques were used because the Soret bands of P and BChl are almost perfectly overlapped. The band at 1684 cm 1 was assigned to the keto carbonyl group of P, which is moderately interacting with the peptide chain. Two bands at 1660 and 1637 cm 1 were assigned to the two acetyl group of P; the former to the n(C¼O) group free from intermolecular interaction, and the latter to the acetyl group interacting with the peptide chain. No binding interactions were proposed between the two BChl’s. Czarnecki et al. [143] studied the RR spectra of the RC of Rb. sphaeroides in the low-frequency region. Figure 3.20 shows the electronic absorption spectrum of the RC in the near-IR region at 10 K. These workers were able to resonance-enhance the vibrations due to P, BChl, and BPh separately with excitation lines near 900, 800, and 760 nm, respectively, and obtained the RR spectra (894 nm excitation, 25 K) of P for the naturally abundant, 26 Mg-labeled and 15 N-labeled RCs. The SERDS (shifted excitation Raman difference spectroscopy) technique [146] was used to remove the large fluorescence background. In this method, RR spectra are measured with two exciting lines of slightly different wavenumbers (typically 10 cm 1), and

Fig. 3.20. Low-temperature (10-K) near-infrared absorption spectrum of RCs from Rb. sphaeroides. The cofactors contributing to the various absorptions are indicated. The arrows mark the different excitation wavelengths used to acquire RR spectra [143].

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Fig. 3.21. Two views of P in RCs from Rb. sphaeroides. The bottom view is along the approximate C2 symmetry axis of the dimmer; the top view in approximately orthogonal to this axis [143].

then the subtraction between them yields a spectrum that is free from background interference. The RR spectrum of P thus obtained exhibits a band near 137 cm 1 that is known to be a marker band of the primary electron donor. On the basis of observed isotope shifts and normal coordinate calculations, it was assigned to a strongly coupled vibration involving the in-plane deformation of the C2 acetyl group, a doming motion of the Mg2þ ion, and a core deformation that includes all four pyrrole rings. Such vibrational coupling may be intrinsic of the structure of P shown in Fig. 3.21 [144] since the overlap between PL and PM primarily involves ring I of BChl and the positions of the two C2-acetyl groups are close to the core of the neighboring macrocycle. As stated earlier, the RC of Rb. sphaeroides contains two quinone molecules (QA and QB) that play different roles; QA is a one-electron carrier while QB leaves the RC as dihydroquinone after accepting two electrons and two protons. To account for this difference, Zhao et al. [147] measured the RR spectra of QA and QB and their 13 C analogs to assign the n(C¼O) and n(C¼C) vibrations, and concluded that environmental differences are responsible for their different roles at the RC. Figure 3.22 illustrates the X-ray crystal structure of one subunit of the BChl a–protein complex from Prosthecochloris aestuari [148]. It contains seven BChl a molecules that are noncovalently bonded to protein. Lutz et al. [149] measured the RR spectra of this complex in the 1710–1630 cm 1 region and observed many acetyland ketocarbonyl bands. A review article, “Chlorophylls and the Photosynthetic Membrane” by Lutz and Robert [150], is available. *

*

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363

Fig. 3.22. One subunit of the bacteriochlorophyll protein showing the seven bacteriochlorophylls enclosed within an envelope of protein. For clarity, the phytyl tails of each bacteriochlorophyll have been omitted. In this figure, the threefold symmetry axis extends from left to right across the front of the molecule [148].

3.5. HEMERYTHRINS [151–155] Hemerythrins (Hr) are molecular oxygen carriers found in invertebrate plyla. Different from Hb and Mb, Hr have no heme groups. Thus far, spectroscopic investigations have been concentrated on hemerythrin isolated from Golfingia gouldii, a sipunculan worm (MW 108,000) consisting of eight identical subunits. Each unit contains 113 amino acids and two Fe atoms, and each pair of Fe atoms binds one molecule of dioxygen. However, its oxygen affinity is slightly lower than hemoglobin, and no cooperativity is found in its oxygenation reaction. Deoxy-Hr (colorless) turns to pink on oxygenation (“pink blood”). Figure 3.23 shows the primary structure of Hr obtained from G. gouldii, while Fig. 3.24 shows the tertiary structure of monomeric myohemerythrin obtained by low-resolution X-ray analysis [156]; it consists of four nearly parallel helical segments, 30–40 A long, connected by sharp nonhelical turns. Figure 3.25 shows the electronic spectra of deoxy-, oxy-, and Met-Hr obtained by Dunn et al. [157]. The oxy form exhibits a band at 500 nm that does not exist in the deoxy form. When the laser wavelength falls under this electronic absorption, two bands are resonance-enhanced at 844 and 500 cm 1 that are shifted to 798 and 478 cm 1, respectively, by 16 O2 18 O2 substitution (Fig. 3.26). These two bands are assigned to the n(O2) and n(Fe O2) of the oxy form, respectively. Apparently, the

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Asp

40 Asn

Ala

Ala

35 Leu

Asp

His

Lys

Trp

Asn

Phe

Gly

Asn

Leu

Asp 85

Gly

Ile

Asp 90 Vol

Leu

Glu

30 Asn

Lys

Ala

Leu

Phe

Trp

His

Arg

Leu

Ala

Ile

Arg

Gly

Thr Lys 25

Ile 5 Pro

Phe

Lys 95 Ser Trp

Tyr

Asn

Trp

Fe

Ile

Glu

Lys

Asp

Arg Thr 15

Asn Gln Glu

Tyr 70

Thr Phe

Phe

His

His

Tyr Ser

Lye

Leu

Fe

Ser Pro

Gly

His

Phe 55

Ile

Asp

Glu

Lys

His 20

Thr

Lys 75

100

Ile

Vol

Cys 50

Gly

Glu Vol

Asp

45

Phe 80

Leu

Asp

Gly

Pro

10

His Glu

Pro

Asp

Asp

Ile

105

Vol 60

Phe

Ile

Leu

Gln Asp Tyr Phe

Mol Gln

Lys

65 Ser

Tyr

Gln Ala

Lys 110

Gly Lly Ile

Fig. 3.23. Primary structure of hemerythrin from erythrocytes of G. gouldii [156].

electronic transition at 500 nm is due to Fe ! O2 charge transfer. Also, the observed frequency of n(O2) (844 cm 1) suggests that the dioxygen is not of “superoxo” but of “peroxo” type (Sec. 1.21). In order to gain more information about the geometry of O2 binding, Kurtz et al. [158] measured the RR spectra of the oxy-Hr with isotopically scrambled oxygen (16 O2 =16 O 18 O=18 O 1=2=1). Figure 3.27 shows that the central band due to the 16 18 O O adduct clearly splits into two peaks, indicating the nonequivalence of the two oxygen atoms. This conclusion is also supported by the RR spectrum in the n(Fe O2)

HEMERYTHRINS

Fig. 3.24. Tertiary structure of monomeric myohemerythrin [156].

Fig. 3.25. Electronic spectra of hemerythrin in the deoxy, oxy, and Met forms [157].

365

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Fig. 3.26. The RR spectra (488.0 nm excitation) of oxyhemerythrin (16O2 and denotes laser plasma lines [157].

18

O2), where P

Fig. 3.27. The RR spectra (514.5 nm excitation) of oxyhemerythrin (16O2/16O18O/18 O/18 O2 ¼ 1/2/ 1 [158]. (A) The v(O2) region. The smooth curves represent deconvolution of the 822 cm 1 feature into two components. The difference between the observed and fitted curves is shown below the spectrum near 822 cm 1. Lines a–d show the calculated peak positions for models I and II of Fe-16O2 (845 cm 1), Fe 16O18O (825 cm 1), Fe 18O16O (818 cm), and Fe 18O2 (797 cm 1), respectively. (B) The n(Fe O2) region. Lines a–d represent calculated positions for the isotopic species defined in (A). Lines e–g show, for models III and IV, the calculated peak positions and estimated relative intensitives for 16O2 (502 cm 1), 16O18O (495 cm 1), and 18O2 (489 cm 1), respectively.

HEMERYTHRINS

367

region. As is seen in Fig. 3.27B, the spectrum consists of two composite bands, one near 502 cm 1 and the other near 483 cm 1. Simple normal coordinate calculations on models I and II indicate

that the n(Fe O2) of the Fe–16 O 16 O (a) and Fe–16 O 18 O (b) adducts nearly overlap, as do those of the Fe–18 O 16 O (c) and Fe–18 O 18 O (d) adducts (a–d refer to the vertical lines in Fig. 3.27B). If the two oxygen atoms were equivalent as shown below

a three-peak spectrum with 1 : 2 : 1 intensity ratio would have appeared in the positions indicated by e–g in Fig. 3.27B. Later, X-ray analyses were carried out on met-azidohemerythrin [159] and oxyhemerythrin [160]. Figure 3.28 shows the structure of the active site of the latter; the two Fe atoms are separated by 3.25 A, and bridged by an oxo atom and two carboxylate groups of the peptide chain. The structure of the former is similar except that the protonated peroxide ion is replaced by the azide ion. Shiemke et al. [161] observed the na(FeOFe) and ns(FeOFe) of the oxo bridge at 753 and 486 cm 1, respectively, in the RR spectrum (363.8 nm excitation). They also noted that both n(O2) (844 cm 1) and n(Fe O2) (503 cm 1) of oxy-Hr in H2O are shifted by þ4 and 3 cm 1, respectively, in D2O solution. These shifts are consistent with the protonated peroxide structure shown in Fig. 3.28. Their subsequent study [162] revealed the

Fig. 3.28. Structures of the active site of hemerythrin in the oxy form [160].

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O Fe

Fe

Monobridged

O

O

Fe O

Fe O

Dibridged

Fe

Fe

O O

O O

Tribridged

Fig. 3.29. Structures of three types of oxo bridges [164].

presence of two ns(FeOFe) vibrations at 492 and 506 cm 1, which correspond to the cis-isomer involving the intramolecular hydrogen bond shown in Fig. 3.28 and its trans-isomer having a free OH group, respectively. Kaminaka et al. [163] found via RR studies that, in a cooperative hemerythrin (Lingula unguis), hydrogen bonding between bound O2 and a nearby amino acid residue is responsible for cooperativity in oxygen affinity. As seen in Fig. 3.28, oxyhemerythrin takes a tribridged structure with one oxo bridge. Figure 3.29 illustrates three types of oxo bridges [164]. Vibrational spectra of oxo-bridged complexes containing a variety of metals have been discussed in Sec. 1.22.3. RR spectra of dibridged compounds containing one oxo group and one carboxylate group are reported for ribonucleotide reductase [165] and stearoyl–ACP desaturase [166].

3.6. HEMOCYANINS [151,167] Hemocyanins (Hc) are oxygen transport nonheme proteins (MW 105–107) found in the blood of some insects, crustaceans, and other invertebrates. One of the smallest Hc (MW 450,000) extracted from spiny lobster Panulirus interruptus consists of six subunits each containing two Cu atoms. On oxygenation, the deoxy form [Cu(I), colorless] turns blue [Cu(II), “blue blood”] by binding one O2 molecule per two Cu atoms. Oxy-Hc extracted from Cancer magister (Pacific crab) and Busycon canaliculatum (channeled whelk) exhibit absorption bands near 570 and 490 nm. Freedman et al. [168] measured the RR spectra of these compounds with 530.9 and 457.9 nm

HEMOCYANINS

Fig. 3.30. The RR spectra of oxyhemocyanins (16O2 and

369

18

O2) with 530.9 and 457.9 nm

excitation [168].

excitations. The results shown in Fig. 3.30 clearly indicate that the bands near 747 cm 1 are sensitive to 16 O2 18 O2 substitution and must be assigned to the n(O2) characteristic of the peroxo(O22 ) type. Excitation profiles of the n(O2) consist of two components and indicate that the absorption bands near 570 and 490 nm are due to O22 ! Cu(II) charge transfer. These workers proposed a nonplanar (C2) structure to account for the appearance of the two CT bands:

The equivalence of the two oxygen atoms in this structure was confirmed by the RR spectrum of oxy-Hc, which exhibits a single n(O2) band at 728 cm 1 for the 16 O 18 O adduct [169]. This is a marked contrast to oxy-Hr discussed in the preceding section.

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APPLICATIONS IN BIOINORGANIC CHEMISTRY

Fig. 3.31. Structures of the active site of oxyhemocyanin.

The bands in the 290–260 cm 1 region in Fig. 3.30 are not sensitive to 16 O2 =18 O2 substitution, and are assigned to the n[Cu N(His)] vibrations. Larrabee and Spiro [170] observed n(Cu N(Im)) below 300 cm 1 in the RR spectra of oxy-Hc with 363.8 nm excitation. Their assignments were confirmed by 63 Cu–65 Cu and H2O D2O frequency shifts. In 1980, Brown et al. [171] carried out an EXAFS study on oxy- and deoxy-Hc of B. canaliculatum, and proposed the structure shown in Fig. 3.31a for the oxy form; the two Cu atoms are bound to the protein via three histidine ligands each, and bridged by the O22 and an X atom from a protein, possibly tyrosine. Later, Gaykema et al. [172] carried out X-ray analysis (3.2 A resolution) on colorless single crystals of Hc extracted from Panulirus interruptus. This molecule consists of six subunits (MW 75,000), each folded into three domains. The structure of the second domain in which two Cu atoms are located is shown in Fig. 3.32. The Cu Cu distance is 3.8 A, and each Cu atom is coordinated by three histidyl residues as suggested by Brown et al. [171] for the deoxy form. No evidence for a bridging protein ligand was found, although it was not possible to rule out such a possibility from low-resolution X-ray analysis. X-Ray analysis by Magnus et al. [173] revealed that the two Cu(II) atoms in oxy-Hc (from Limulus polyphemus) are bridged by a side-on peroxide as shown in Fig. 3.31b. Here, each Cu(II) atom takes a square–pramidal structure with four equatorial bonds (two Cu N and two Cu O bonds) and one axial Cu N bond so as to obtain the overall C2h symmetry. Ling et al. [174] have measured the RR spectra of oxy-Hc from several sources, and made complete band assignments via normal coordinate analysis using isotopic shift data (16 O=18 O, 63 Cu=65 Cu, and H/D). The na (Cu2O2) and its first overtone are located at 542 and 1085 cm 1, respectively, for oxy-Hc from Octopus dofleini. The n[Cu N(His)] of oxy-Hc (L. polyphemus) appear in the 370–190 cm 1 region, although some of these are coupled with the n(Cu O) modes. fa*ger and Alben [175] studied the FTIR spectra of HcCO using 13 C 16 O and 12 18 C O, and proposed a structure in which the CO is coordinated to one Cu via the O atom in a trigonal–planar fashion while the second Cu is free from such interaction. Pate et al. [176] proposed the m-1,3 bridging structure for met-HcN3 based on RR spectra obtained by using the isotopic 14 N2 15 N ligand:

HEMOCYANINS

371

Fig. 3.32. Structures of the second domain of Hc extracted from P. interruptus. The Cu atoms are indicated by diamonds. The cylinders (2.1–2.7) indicate the a-helical structure, while the strips (2A–2E) represent the b structure of the peptide chain [172].

For the 14 N-- 14 N--15 N ion, two n(N3) bands were observed at 2035 and 2024 cm 1. This observation suggests nonequivalence in the two Cu N interactions that originates in differences between the two Cu environments in the protein.

Fig. 3.33. Structures of model compounds of oxyhemocyanin: (a) Py denotes the 2-pyridyl group; (b) R denotes the Me, i-Pr, or Ph group.

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APPLICATIONS IN BIOINORGANIC CHEMISTRY

Fig. 3.34. TheRRspectraofmodelcompoundA(Fig.3.33)with 16O2, 18O2,and 16O18O(amixtureof

16 O2, 16O18O, and 18O2 in 1/2/1 ratio): (a) the n(O2) region (488.0 nm excitation). The asterisk indicates the peak due to CH2Cl2; (b) the n(Cu O) region (647.1 nm) excitation [178].

Karlin et al. [177] first prepared a model compound of Hc, which is shown in Fig. 3.33a. This complex performs reversible oxygenation at 70 C. As seen in Fig. 3.34, the 16 O 18 O adduct exhibits a broad n(O2) centered at 780 cm 1 (peroxidetype) and two n(Cu O) bands at 486 and 465 cm 1 in RR spectra. Through normal coordinate analyses and computer simulations of the observed band shapes. Pate et al. [178] have shown that the peroxide is asymmetrically bonded to the Cu atoms, although the nature of asymmetry is not clear. Kitajima et al. [179] prepared another type of model compounds that mimic HcO2. As shown in Fig. 3.33b, their compounds contain two Cu atoms that are bonded via the peroxo bridge without a phenoxo bridge. Figure 3.35 shows the RR spectra of one of their complexes, [Cu(HB(3,5-R2pz)3)]2(O2) (R ¼ i-Pr), which were obtained with 16 O2 (A) and isotopically scrambled dioxygen (16 O2 =16 O 18 O=18 O2 ¼ 1=2=1) (B) at 40 C. The latter spectrum shows that the intensity ratio of the three n(O2) is close to 1/2/1 and their band widths are nearly identical. These results confirm that the n(16 O2 ) is at 741 cm 1, and that the peroxide is symmetrically coordinated as that shown in Fig. 3.31b (m-Z2 Z2-type). Electronic and vibrational spectra of the model compound shown in Fig. 3.33a were also studied by Baldwin et al. [180]. Karlin [181] reviewed the reaction of O2 with copper complexes. As shown above, the peroxo bridging complex of oxy-Hc exhibits the n(O2) at 750 cm 1, [174]. A similar m-Z2 Z2 peroxo structure was also proposed for the dioxygen adduct of the dinuclear Co(II) complex [Co(HB(3,5-R2pz)3)]2(O2) (R ¼ i-Pr) [182]. It exhibits an electronic absorption band at 350 nm ( ( 8900 M 1 cm 1) with an intensity less than half that of the corresponding Cu(II) complex (340 nm, ( 21,000 M 1 cm 1). The RR spectrum of the Co(II) complex (514.5 nm excitation) in acetone at 80 C exhibits the peroxo n(O2) at 651 cm 1, which is shifted to 617 cm 1 by 16 O2 =18 O2 substitution. This may be the lowest n(O2) thus far

BLUE COPPER PROTEINS

373

Fig. 3.35. The RR spectra (514.5 nm excitation) of model compound b (Fig. 3.33) in acetone at

40 C; (A) 16O2; (B) a mixture of 16O2,

16

O18O, and 18O2 in 1/2/1 ratio [179].

observed. High electron density on both of the antibonding peroxo orbitals might weaken the O O bond more in the Co(II), complex than in the Cu(II) complex [179].

3.7. BLUE COPPER PROTEINS [183,167] Blue copper proteins are found widely in nature. For example, oxidized plastocyanin (electron transport protein) and azurin (copper oxidase) contain one Cu(II) atom per protein, and exhibit an intense absorption band near 600 nm that is due to the S (Cys) ! Cu charge transfer near 600 nm. In addition, these copper proteins have unusual properties such as extremely small hyperfine splitting constants (0.003 0.009 cm 1) in ESR spectra and rather high redox potential (þ0.2 0.8 V) compared to the Cu(II)/Cu(I) couple in aqueous solution. In 1978 the crystal structure of poplar plastocyanin was first determined by X-ray diffraction with 2.7 A resolution [184], and later refined to 1.6 A resolution [185]. Figures 3.36a and 3.36b show the location of the Cu atom in the peptide chain and the environment around the Cu atom, respectively. It was found that Cu atom is coordinated by two histidyl nitrogens (His 37 and 87), one cysteinyl sulfur (Cys 84), and one methionyl sulfur (Met 92) in a distorted tetrahedral environment. The two Cu N(His) distances are 2.10 and 2.04 A, and the Cu S(Cys) distance is 2.13 A, while the Cu S(Met) is 2.90 A. This distorted tetrahedral structure approaches to the distorted trigonal–planar structure as the Cu S(Met) bond lengthens. These

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Fig. 3.36. Crystal structure of plastocyanin: (a) location of the Cu atom in the peptide chain; (b) environment around the Cu atom [184].

structures (type 1 site) are responsible for the unusual properties of blue copper proteins mentioned above. Since then, several groups of investigators measured the RR spectra of type 1 blue copper proteins with excitation lines near the S(Cys) ! Cu charge transfer band, and assigned the n[Cu N(His)], n[Cu S(Cys)], and n[Cu S(Met)] vibrations on the basis of isotopic shifts (63 Cu=65 Cu, 32 S=34 S and 14 N=15 N). Although these low-frequency vibrations are strongly coupled with each other, the vibration containing the larger contribution from the n[Cu S(Cys)] coordinate is expected to be more strongly resonance-enhanced and to show the larger isotope shift by 32 S=34 S substitution. Figure 3.37 shows the RR spectra (647.1 nm excitation) of naturally abundant (NA) and isotopically labeled poplar plastocyanins obtained by Qui et al. [186]. Two intense bands at 429.1 and 419.9 cm 1 of the NA show the largest isotope shifts (2.2 cm 1) due to major contribution from the n[Cu S(Cys)] coordinate. As expected, the n[Cu N(His)] band at 267.3 cm 1 (not shown) gives no isotope shift by 32 S=34 S substitution. Dong and Spiro [187] measured the RR spectra (647.1 nm excitation) of isotopically labeled plastocyanins (H/D, and 14 N= 15 N) in the 1500–150 cm 1 region at low-temperature. Their results revealed the presence of vibrational coupling between the n[Cu S(Cys)] and ligand internal vibrations that helped in assigning internal modes of cysteine and histidine residues. Wu et al. [188] measured UVRR spectra (229 nm excitation) of poplar plastocyanine in H2O/D2O solution, and compared the NH/ND exchange rate of His 37 and His 87 (Fig. 3.36b). Intensity variations of their imidazole ring modes at 1389 and 1344 cm 1 caused by H/D exchange were slower than those of the pair at 1398

BLUE COPPER PROTEINS

375

Fig. 3.37. 647.1-nm excited resonance Raman spectra of isotopically labeled and natural abundance plastocyanin from poplar [186 ].

and 1354 cm 1. Thus, the former pair was assigned to His 37, which is hydrogenbonded to the backbone carbonyl group, whereas the latter pair was assigned to His 87, which is exposed to the solvent. The frequencies of the former pair are 10 cm 1 lower than those of the latter due to hydrogen bonding. The X-ray crystal structure of azurin (Alcaligenes denitriflavans) shows that the Cu site takes a distorted trigonal–planar or a trigonal–bipyramodal structure rather than a distorted tetrahedral structure [189]. Dave et al. [190] measured the RR spectra (568.2 nm excitation) of azurin from Pseudomonas aeruginosa in natural abundance (WT), and its two mutants, M121G (Met 121 is replaced by Gly) and H46D (His 46 is replaced by Asp) to study the relationship between the n[Cu S(Cys)] frequency and metal site geometry. Figure 3.38 shows the RR spectra of these three azurins and their 34 S(Cys)-labeled analogs. Each spectrum consists of four bands in the 430–360 cm 1 region, which is typical of type 1 structure. The two strong bands near 400 cm 1 are of primary interest because the remaining two bands near 428 and 373 cm 1 show only small or no 32 S=34 S isotope shifts. Wild-type azurin exhibits the most intense band at 408.6 cm 1 with a shoulder at 400 cm 1. The former is down-shifted by 3.8 cm 1, while the latter shows almost no shift. In mutant M121G and H46D, these bands are separated more distinctly into two bands of almost equal intensities. In WT, the higher

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APPLICATIONS IN BIOINORGANIC CHEMISTRY

Fig. 3.38. Low-temperature (77-K) RR spectra of P. aeruginosa azurins (thick-line traces) and their 34S-Cys-labeled proteins (thin-line traces) in the region between 360 and 435 cm 1: top, wild type (WT) excited at 647.1 nm; middle, M121G mutant, and bottom, H46D mutant, both excited at 568.2 nm [190].

frequency band at 408.6 cm 1 can be definitively assigned to the n[Cu S(Cys)] vibration. In the mutant azurins, however, the lower-frequency bands near 400 cm 1 show much larger isotope shifts than do the higher frequency bands. Thus, the lowerfrequency bands are due mainly to the n[Cu-S(Cys)] vibration. These results suggest that the Cu S(Cys) bonds of the mutant azurins are lengthened relative to WT azurin probably because the Cu atoms are displaced from the trigonal N2S plane because of strong interaction with an axial ligand [191]. Van Gastel et al. [192] measured the RR spectra (635.5 nm excitation) of azurin from P. aeruginosa and observed that the bands at 409.0 and 401.9 cm 1 are downshifted by 4.6 and 5.9 cm 1, respectively, in D2O solution probably because these vibrations contain d (Cu S H) character. Ni(II)-substituted P. aeruginosa azurin exhibits the S(Cys) ! Ni(II) charge transfer absorption at 440 nm (pale yellow). Czernuszewicz et al. [193] measured the RR spectra of Ni(II)(58 Ni=62 Ni)-reconstituted azurins with 413.1 nm excitation at 77 K. The strong bands at 360 and 346 cm 1 were assigned to the n[Ni–S(Cys)] coupled with d(SCC) and d(CCN) vibrations, respectively. These frequencies are much lower than those of the corresponding n[Cu S(Cys)] vibrations (408 and 401 cm 1). Their excitation profile

BLUE COPPER PROTEINS

377

studies show that the intensities of these n[Ni S(Cys)] bands are maximized with exciting lines close to the S(Cys) ! Ni(II) charge transfer band. As described above, type 1 (blue) copper proteins are characterized by the electronic absorption near 600 nm and the n[Cu S(Cys)] near 400 cm 1. On the other hand, type 2 (yellow) copper proteins such as amine oxidase and superoxide dismutase are characterized by the electronic absorption near 400 nm and the n[Cu S (Cys)] near 320–290 cm 1, indicating that the Cu S(Cys) bond in type 2 is longer and weaker than that of type 1. The Cu center of P. aeruginosa azurin mutant, H117G (His 117 is replaced by Gly), is accessible to exogenous ligands through the opening on the surface created by the removal of the endogenous imidazole ligand. den Blaauwen et al.[194] measured the RR spectra of the solution of H117G mutant by adding unidentate ligands such as Cl , Br and N3 and bidentate ligands such as histidine and histamine. Excitation lines at 647.1 and 413.1 nm were used for the solutions containing unidentate and bidentate ligands, respectively. Similar to those mentioned above, the former solutions exhibit strong n[Cu S(Cys)] bands in the 410–390 cm 1 region, which are typical of type 1 copper site, whereas the latter solutions show them in the 320–290 cm 1 region, which are characteristic of type 2 copper site. For example, the histidine mutant exhibits strong bands at 319 and 298 cm 1 as shown in Fig. 3.39a. A similar spectrum was obtained for the aqueous solution of H117G

Fig. 3.39. Resonance Raman spectra of type 2 sites in H117G azurins obtained with 413.1 nm excited: (a) H117G plus histidine; (b) H117G plus histamine; (c) H117G in H2O (no ligand added) [194].

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APPLICATIONS IN BIOINORGANIC CHEMISTRY

Cu

N

N

Cu L

L

S

S

(b)

(a)

L

N Cu

L

S

(c) Fig. 3.40. Proposed structures for types 1 and 2 sites in H117G azurin: (a) trigonal type 1 site, with Cys 112 (S), His 46 (N), and an exogenous ligand (L) in a trigonal–planar array (in wild type, L ¼ His 117 with met 121 as a weak axial ligand); (b) tetrahedral type 1 site, where Cu is displaced from the trigonal ligand plane owing to stronger coordination of the axial ligand; (c) tetragonal type 2 site, which has four strong ligands [e.g., Cys 112 (S), His 46 (N), and a bidentate exogenous ligand (2L)] in a square–planar array with one or more ligands at a longer distance [194].

(no ligand added) This may imply that two H2O molecules occupy two coordination sites like one bidentate ligand. Figure 3.40 illustrates the structures of types 1 and 2 copper sites of H117G azurin proposed by these workers [194]. Here, an exogenous ligand coordinates as a unidentate (L) or a bidentate (L–L). The type 1 site has two possible structures: trigonal (a) and tetrahedral (b). For the type 2 site, the tetragonal geometry (c) was proposed on the basis of on electronic absorption, RR and ESR studies. Andrew and Sanders-Loehr [195] reviewed the relationship between RR spectra and coordination chemistry of copper–sulfur sites.

3.8. IRON–SULFUR PROTEINS [196,197] Iron–sulfur proteins are found in a variety of organisms, bacteria, plants, and animals, and serve as electron transfer agents via a one-electron oxidation–reduction step [redox potential (Em), 0.43 V in chloroplasts to þ0.35 V in photosynthetic bacteria]. For example, ferredoxin in green plants (chloroplasts) is involved in the electron transfer system of photosynthesis. The molecular weights of iron–sulfur proteins range from 5600 (rubredoxin from Clostridium pasteurianum, Cp) to 83,000 (beef heart aconitase). All these compounds show strong absorptions in the visible and near-UV regions that are due to Fe S CT transitions. Thus, laser excitation in these regions is expected to resonance-enhance n(Fe S) vibrations of iron–sulfur proteins.

IRON–SULFUR PROTEINS

379

Fig. 3.41. ORTEP drawing of [Fe(S2-o-xyl)2] viewed down the C2 axis [198].

The most simple iron–sulfur protein is rubredoxin (Rd), which contains one Fe atom per protein. The Fe atom is coordinated by four sulfur atoms of cysteinyl residues in a tetrahedral environment. Figure 3.41 shows the crystal structure of a model compound, [Fe(S2-o-xyl)2] (S2-o-xyl ¼ o-xylene-a,a0 -dithiolate) [198]. Long et al. [199] first obtained the RR spectrum of oxidized rubredoxin, and assigned two bands at 368(n3) and 314(n1) cm 1 to the n(Fe S) and those at 150(n4) and 126(n2) cm 1 to the d(FeS4) of the FeS4 tetrahedron. Later, Yachandra et al. [200] attributed three bands observed near 371, 359, and 325 cm 1 of oxidized rabredoxins to the splitting components of the n3(F2) vibration. Figure 3.42 shows the RR spectra of oxidized rubredoxin from Desulfovibrio gigas (Dg) at 77 K obtained by Czernuszewicz et al. [201]. The split components are clearly seen at 376, 363, and 348 cm 1 with the n(A1) at 314 cm 1. These workers were able to assign all the fundamentals as well as overtones and combination bands as indicated in Fig. 3.42. Czernuszewicz et al. [202]

Fig. 3.42. The RR spectra of oxidized D. gigas rubredoxin obtained in a liquid N2 Dewar using the excitation lines indicated; the asterisk indicates a band due to ice [201].

380

APPLICATIONS IN BIOINORGANIC CHEMISTRY

Fig. 3.43. ORTEP drawing of [Fe2S2(S2-o-xyl)2]2 in its Et4Nþ salt [206].

studied the origins of the F2 mode splitting by using model compounds, [FeL4] (L¼SMe ; and SEt ; (L)2¼S2-o-xyl). Saito et al. [203] carried out normal coordinate analysis on rubredoxins, considering over 1000 internal coordinates around the FeS4 site. Their results show the presence of extensive vibrational couplings between the n(Fe S) and bending modes of the peptide skeleton. The RR spectra of rubrerythrin (from Desulfovibrio vulgaris) demonstrates the presence of a rubredoxin-type FeS4 site as well as a (m-oxo) diiron(III) cluster [204]. Two-iron proteins are found in ferredoxin from chloroplast (MW 10,000) and in adrenodoxin from adrena cortex of mammals (MW 13,000), and so on. These proteins contain the Fe2S2(cysteinyl)4 cluster in which two Fe atoms are bridged by two “labile” (inorganic) sulfur atoms and each Fe atom is tetrahedrally coordinated by two bridging and two cysteinyl sulfur atoms (2Fe 2S cluster). This structure was confirmed by X-ray analysis of the ferredoxin from Spirulina platensis (Sp Fd) [205]. 0 The Fe2S2S 4 core (D2h symmetry) is modeled by the [Fe2S2(S2-o-xyl)2]2 ion whose structure is shown in Fig. 3.43 [206]. Yachandra et al. [207] measured the RR spectra of oxidized spinach ferredoxin and its 34 S-enriched analog containing such a 2Fe 2S cluster. Later, Han et al. [208] remeasured the RR spectra of bovine adrenodoxin (Ado) and ferredoxin (Fd) from Porphyra umbilicalis with 34S substituted at the bridge positions. Figure 3.44 shows the RR spectra of the native and 34 Sb -reconstituted Fd measured at 77 K, and Table 3.3 lists the band assignments for Fd and its model compound, which were confirmed by normal coordinate calculations. These workers 0 also carried out normal coordinate analysis on model compounds for the [Fe2S2]S 4 type proteins to study vibrational couplings between n(Fe S) and bending modes [209]. Kuila et al. [210] measured the RR spectra of Thermus(thermophilus) Rieske protein (TRP) and phthalate dioxygenase (PDO) from Pseudomonas cepacia and 0 discussed possible structures for the [Fe2S2]S 2 N2-type core. Vidakovic and coworkers [211] prepared the Ala45Ser mutant of the 2Fe 2S ferredoxin from vegetative cells of the cyanobacterium Anabaena sp. 7120, and studied its biochemical and biophysical properties. The RR spectrum was distinctly different from that of the wild-type (WT) protein and showed exceptional similarity to those of higher plant ferredoxins such as spinach ferredoxin. However, the terminal Fe S stretching vibration of the mutant showed a considerably larger deuterium isotope shift than that of the WT protein. This was attributed to the formation of more hydrogen bonding between the protein matrix and the cysteinyl sulfur atoms in the

381

IRON–SULFUR PROTEINS

Fig. 3.44. The RR spectra (77 K) of native and 34Sb-reconstituted P. umbilicalis Fd [208].

mutant than in the WT protein. For example, the sidechain hydroxyl group of Ser 45 in the mutant is hydrogen-bonded to the sulfur atom of Cys 41. One of the most common Fe S clusters in iron–sulfur proteins is the 4Fe 4S cube containing interpenetrating Fe4 and S4 tetrahedra, the Fe corners of which are bound to cysteinyl sulfur atoms. Figure 3.45 shows the X-ray crystal structure of a bacterial ferrodoxin from Peptococcus aewgenes (MW 6000) containing two such clusters TABLE 3.3. RR frequencies and Vibrational Assignments of the Fe2S2(SR)4 Core (cm 1) [208] Vibrational Modea n(Fe–Sb) n(Fe–Sb) n(Fe–Sb) n(Fe–St) n(Fe–St) n(Fe–St) n(Fe–Sb) n(Fe–St) a

Symmetryb B2u Ag B 3u ) B 1u B 2g Ag B1g B3u

Fdc

[Fe2S2(S2-o-xyl)2]2

426(7.2) 395(5.6) 367 (2.0)

415(6.0) 391(5.9) 342(3.2)

357(0.8)

339(1.2) 329(4.1) 282(3.5)

323(2.0) 313(3.2) 276(3.2)

Sb and St denote the bridging and terminal sulfur atoms, respectively. D2h symmetry. c Numbers in parentheses indicate the 32 S 34 S isotopic shift. b

,c

382

APPLICATIONS IN BIOINORGANIC CHEMISTRY

Fig. 3.45. Structure of bacterial ferredoxin: , *, , and * indicate Fe, S (inorganic), S (cysteinyl), and C atoms, respectively [212].

[212]. The geometry of this 4Fe 4S cluster is in good agreement with that of the synthetic analog, [Fe4S4(SR)4]2 (R¼CH2C6H5, C6H5, etc.) prepared by Berg and Holm [213]. In both cases, the 4Fe 4S cube is slightly squashed with four short and eight long Fe S bonds (approximately D2d symmetry). The RR spectra of 4Fe 4S proteins have been reported by several investfgators [214–217]. Figure 3.46 shows the RR spectra of the high-potential iron protein (HiPIP) from Chromatium vinosum (Cv), ferredoxin from Clostridium pasteurianum (Cp), and their model compounds, (Et4N)2 [Fe4S4(SCH2Ph)4], in the solid state and in solution obtained by Czernuszewicz et al. [216]. Table 3.4 lists the observed frequencies and band assignments for these compounds. From normal coordinate calculations using 32 S=34 S isotopic shift data, these workers confirmed that the symmetry of the model compound above is Td in solution but D2d in the solid state. If the symmetry of the 0 (Fe4S4)S 4 core shown in Fig. 3.47 is Td, it should give five n(Fe-Sb) (A1 þ E þ F1 þ 2F2) and two n(Fe St) (A1 þ F2) modes. Here, Sb and St denote the bridging and terminal S atoms, respectively. If it is D2d, all the degenerate vibrations under Td symmetry should split into two bands. As a result, a total of 12 vibrations are expected. Thus, the results shown in Table 3.4 and Fig. 3.46 support their conclusions. Furthermore, the RR spectra of HiPIP and Cp Fd are similar to that of the model compound in solution and in the solid state, respectively. Thus, the symmetries of the Fe4S4 cores of these proteins must be Td and D2d, respectively.

IRON–SULFUR PROTEINS

383

Fig. 3.46. The RR spectra (77 K) of Cn HiPIPred (457.9 nm excitation), Cp Fdox (488.0 nm excitation), and (Et4N)2[Fe4S4(SCH2Ph)4] in solution (room temperature, 457.9 nm excitation) and solid state (KCl pellet, 488.0 nm excitation) [216].

Maes et al. [218] measured the RR spectra of a series of 4Fe 4S proteins of the type (n-Bu4N)2[Fe4S4(SR )4], where SR is thiophenol (tp) and its dimethyl derivatives at positions 3.5, 2.4, and 2.6. Assignments of their Fe S vibrations were based on 32 S=34 S isotope shifts and normal coordinate analysis. It was found that the Fe4S4 cores of these proteins take a significantly distorted D2d structure, and that the general rule n(Fe St) > n(Fe Sb) is violated when their frequencies are compared for the thiphenolate and 2,6-dimethylthiophenolate clusters as shown in Fig. 3.48.

384

APPLICATIONS IN BIOINORGANIC CHEMISTRY

TABLE 3.4. RR Frequencies (cm 1) and Band Assignments for the [Fe4S4(SCH2Ph)4]2 Ion, Oxidized Cp Fd and Reduced Cv HiPIPa D2d

[Fe4S4(SCH2Ph)4]2 (Solid)

Cp Fd

Fe4S4(SCH2Ph)4]2 (Solution)

Td

384(1) 358(1)

A1 F2

395

384(1)

F2

337

333(7) 268(3)

A1 E

268(3)

F1

241(6)

F2

Cv HiPIP

Mainly Terminal n (Fe–S) A1 B2(F2) E(F2)

391(1) 367(1) 359(2)

395(3.9) 351(0.7) 363(2.0)

395 362

Mainly Bridging n(Fe–S) B2(F2) E(F2) A1 A1(E) B1(E) E(F1) A2(F1) B2(F2) E(F2) a

385(6) — 335(8) 298(5) 283(4) 283(4) 270(3) 249(6) 243(5)

380(5.6) — 338(7.0) 298(4.9) 276(4.5) 276(4.5) 266(4.0) 251(6.2) —

273 273 249

Numbers in parentheses indicate downshifts due to S substitution for the bridging S atoms [216].

In the active form of aconitase that catalyzes the isomerization of citrate to isocitrate, one of the Fe atoms in the Fe4S4 cluster is coordinated by an OHx (x ¼ 1 or 2) group instead of a cysteinyl residue. Kilpatrick et al. [219] assigned the n(Fe S) of aconitase and its model compounds via normal coordinate analysis. A number of Fe S proteins contain 3Fe centers. ln some cases, the 3Fe center can be converted to the 4Fe center, and vice versa. The structures of 3Fe clusters were controversial. In 1980, Stout et al. [220] determined the crystal structure of ferredoxin I extracted from Azotobacter vinelandii (Av Fd I; MW, 14,000) which contains a 3Fe 3S cluster in addition to a 4Fe 4S cluster. For the former, they proposed a novel Fe3S3 planar ring structure. However, Beinert et al. showed that four labile sulfides, not three, are associated with the 3Fe center [221]. Thus, these workers as well as Johnson et al. [214] proposed cubane-like structures in which one of the corner Fe atoms is lost (Fig. 3.49).

Fig. 3.47. Structure of the (Fe4S4)S0 4 cluster.

IRON–SULFUR PROTEINS

400

[Fe4S4(S

380

A 1 390 (1.0) b T 2 379 (9.5)

t

)4]2–

[Fe4S4(S

385

)4]2– 388 (6.5) 382 (6.5)

b

T2

t

T 2 379 (3.0) 368 (3.5)

Fe–S stretching frequency (cm–1)

360

b T2

t

T2

359 (5.0) 351 (2.0)

b

A 1 345 (8.0) 340

336 (7.0)

b

A1

320 307 (3.5) E

300 E

b

295 (4.5) 291 (4.0)

280

279 (3.5) b T1

b

T1

271 (2.0) 264 (2.5)

260

258 (4.0)

b

T2

b T2

240

b

240 (4.5)

244 (4.5)

Fig. 3.48. Correlation diagram of RR frequencies for [Fe4S4(tp)4]2 and [Fe4S4(2,6-dmtp)4]2 (b refers to bridging Fe S modes; t refers to terminal Fe S (aryl) modes) [218].

Fig. 3.49. Structures proposed for 3Fe–4S clusters [221].

386

APPLICATIONS IN BIOINORGANIC CHEMISTRY

Johnson et al. [214] first measured the RR spectra of Av Fd I and Tt Fd (from Thermus thermophilus), both of which contain 3Fe as well as 4Fe clusters. As seen in Fig. 3.50, their RR spectra are dominated by the 3Fe spectra, which exhibit bands at 390, 368, 347,

Fig. 3.50. Low-temperature RR spectra (488.0 nm excitation) of oxidized Av Fd I, oxidized Tt Fd, oxidized Dg Fd II, and ferricyanide-tetrated Cp Fd [222].

INTERACTIONS OF METAL COMPLEXES WITH NUCLEIC ACIDS

387

285, and 266 cm 1. The weak band at 334 cm 1 is attributed to the 4Fe 4S cluster. Oxidized Dg Fd II [222] and ferricyanide-treated Cp Fd, [223], which are known to contain only 3Fe clusters, show no such bands. The 34S sulfide substitution in Tt Fd and ferricyanide-treated Cp Fd produced downshifts of the bands near 266, 285, and 347 cm 1. Therefore, these bands must be assigned to the bridging n(Fe S). The strong band at 347 cm 1 is due to the totally symmetric breathing-cluster mode, while the remaining bands near 390 and 368 cm 1 are assigned to the terminal n(Fe S). Normal coordinate calculations by Johnson et al. [214] have shown that the RR spectra ofAv FdI crystalsand3Fe-bacterialferredoxins(CpFd and Tt Fd)are compatible with cubane-like 3Fe 4S structures shown in Fig. 3.49, but not with the 3Fe 3S structure reported by Stout et al. The RR spectra of aconitase (inactive form) and Desulfovibrio desulfuricans [224] are also very similar to those mentioned above, indicating the possibility of the cubane-like 3Fe 4S structures in these proteins. Later, the planar Fe3S3 structure originally proposed by Stout et al. [220] was found to be in error [225]. 3.9. INTERACTIONS OF METAL COMPLEXES WITH NUCLEIC ACIDS Nucleic acids interact with a variety of ligands such as metal ions, metal complexes, anticancer drugs, and carcinogens. Their modes of interactions with DNA can be classified into intercalation, groove binding, covalent bonding, and strand-breaking, and these are often reinforced by hydrogen bonding and/or Coulombic interactions. Figure 3.51 shows the structure of one strand of DNA backbone consisting of deoxyriboses connected via phosphate groups Bases such as guanine (G), cytosine (C), adenine (A), and thymine (T) are attached to deoxyriboses, and the chain direction is defined as 50 O to 30 O (50 ! 30 ). The well-known double-helix structure is formed by hydrogen bondings between G–C and A–T bases on the complementary strands (Fig. 3.52). Interaction of DNA with a ligand results in changes in the double-helix structure, thereby inhibiting DNA replication and transcription, which are necessary preconditions for cell division. In the following, vibrational studies on interactions of metal complexes with nucleic acids are discussed using two examples. More detailed and extensive discussions are found in several review articles [226] and monographs [227,228]. 3.9.1. Cisplatin [229] A square–planar platinum complex, cis-diamminedichloroplatinum(II), cis-[Pt(NH3)2 Cl2] (abbreviated as cisplatin) and its derivative, carboplatin, shown in Fig. 3.53a are well-known anticancer drugs that are currently in clinical use. When reacted with DNA, cisplatin forms covalent Pt N bonds by replacing its two Cl ligands with the N7 atoms of guanine (G) or adenine (A) bases. Although four types of platinum–DNA complexes shown in Fig. 3.53b (structures a, b, c, and d) are formed, the major products are the 1,2-intrastrand crosslinking complexes at the 50 -GG-30 sequence (a, 60%) and 50 -AG-30 sequence (b, 25%). Minor products are 1,3-intrastrand crosslinking (c) and 1,2-interstrand crosslinking (d) complexes between two G bases that are formed less than 10%.

388

APPLICATIONS IN BIOINORGANIC CHEMISTRY

5′ HOCH2 O

O

Base

H H H H O H O P O

N1 2

6 5 3 4

N

H2N

N

N3

7 98

N

2

4

5 1 6

N

O

irection Chain d

R Guanine (G)

Base

OCH2 O

NH2

H

R Cytosine (C)

Nakamoto B IR Raman Spectra of Coordination Compounds - PDFCOFFEE.COM (2024)
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