Transition Metal Complexes Structures and Spectra PDF
Preview Transition Metal Complexes Structures and Spectra
55 erutcurtS dna Bonding Editors: M. J. Clarke, Chestnut Hill • J. B. Goodenough, Oxford J. A. Ibers, Evanston • C. K. J¢rgensen, Gen6ve J. B. Neilands, Berkeley • D. Reinen, Marburg R. Weiss, Strasbourg • R. J. P. Williams, Oxford Transition Metal Complexes Structures dna Spectra With Contributions by M. Bacci J. Fischer M.H. Gubelmann B. Korefi F. Mathey M. Meln~ J.H. Nelson P. )3viS F. Valach A.F. Williams With 39 Figures and 12 Tables galreV-regnirpS Berlin Heidelberg New York Tokyo 3891 Editorial Board Professor Michael .J Clarke Boston College, Department of Chemistry, Chestnut Hill, Massachusetts ,76120 U.S.A. Professor John B. Goodenough Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, Great Britain Professor James A. Ibers Department of Chemistry, Northwestern University Evanston, Illinois ,10206 U.S.A. Professor .C Klixbiill nesnegrCJ D~pt. de Chimie Minrrale de l'Universitr, 03 quai Ernest Ansermet, 1121-HC Gen~ve 4 Professor Joe B. Neilands Biochemistry Department, University of California, Berkeley, California ,02749 U.S.A. Professor Dirk Reinen Fachbereich Chemie der Philipps-Universit/it Marburg, Hans-Meerwein- StraBe, D-3550 Marburg Professor Raymond Weiss Institut L e Bell Laboratoire de Cristallochimie et de Chimie Structurale, 4, rue Blaise Pascal, F-67070 Strasbourg Cedex Professor Robert Joseph .P Williams Wadham College, Inorganic Chemistry Laboratory, Oxford 1XO 3QR, Great Britain tSBN 3-540-12883-6 Springer-Verlag Berlin Heidelberg New York Tokyo ISBN 0-387-12883-6 Springer Verlag New York Heidelberg Berlin Tokyo Library of Congress Catalog Card Number 67-11280 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54of the German Copyright Law here copies are made for other than for private use, a fee is payable to "Verwertungsgesellschaft ,"troW Munich. © Springer-Verlag Berlin Heidelberg 1983 Printed in Germany The use of general descriptive names, trade marks, etc. in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used fi'eely by anyone. Typesetting and printing: Schwetzinger Verlagsdruckerei. Bookbinding: J. Sch~'fer, Gr~stadt. 2152/3140-543210 Table of Contents The Structure and Reactivity of Dioxygen Complexes of the Transition Metals M. H. Gubelmann, A. F. Williams ................... The Role of Vibronic Coupling in the Interpretation of Spectroscopic and Structural Properties of Biomolecules M. Bacci ................................ 67 Crystal Structure Non-Rigidity of Central Atoms for Mn(II), Fe(II), Fe(III), Co(II), Co(Ill), Ni(II), Cu(II) and Zn(II) Complexes F. Valach, B. Korefi, P. ,)~viS M. Meln~ ................ 101 Complexing Modes of the Phosphole Moiety F. Mathey, J. Fischer, J. H. Nelson ................... 351 Author Index Volumes 1-55 ....................... 302 The Structure dna Reactivity of Dioxygen Complexes of the Transition Metals Michel H. Gubelmann and Alan F. Williams Ddpartement de Chimie Mindrate, Analytique et Appliqude, Universitd de Gen~ve, 30 quai Ernest Ansermet, CH-1211 Geneve 4, Switzerland This article gives a review of complexes in which a dioxygen ligand is bonded to a transition metal. Three aspects of these complexes are discussed in detail: the structure, the electronic structure, and the reactivity. The structural section summarises the recent X-ray crystal structure determinations, and the structural data obtained by other methods. The electronic structure is first considered in qualitative terms which allow the rationalisation of the different structures observed, and this qualitative model is compared with the results of calculations and with spectroscopic data. The reactivity of the complexes is discussed separately for each structural class in terms of the electronic structure. An attempt is made to compare the results obtained in historically different areas of research. Our objective is to give a clear summary of current knowledge of these compounds for workers interested in their application to catalysis and in their r61e in biochemical systems. A. Introduction ....................................... 2 B. Properties ofM olecular Oxygen ............................. 3 C. Structural Classification of Dioxygen Complexes ..................... 5 D. The Electronic Structure of Dioxygen Complexes .................... 16 I. Qualitative Models of Dioxygen Binding ...................... 16 II. Calculations on Dioxygen Complexes ........................ 24 IlL Spectroscopic Studies ................................ 30 IV. Electronic Structure- Conclusions ......................... 34 E. Reactivity of Dioxygen Complexes ............................ 34 I. Reactions of 1/r Complexes .............................. 35 II. Reactions of 72 Complexes .............................. 36 III. Reactions of 1/r : rf Complexes ............................ 44 a) ~f : rf Complexes of Cobalt ........................... 44 b) ~/~ : ~/T Complexes Containing Metals Other Than Cobalt ............. 50 IV. Reactions of rf : 2/r and 2/r : 7~ 2 Complexes ...................... 51 F. Conclusions ......................................... 51 G. References ........................................ 53 Structure and Bonding 55 Berlin Heidelberg Springer-Verlag © 3891 2 .M H. Gubelmann dna A. F. smailliW A. In~oducfion The study of dioxygen complexes of the transition metals is generally accepted to have begun with the reporbty Frrmy in 1852 of the oxygenated ammoniacal salts of cobalt .)1 A satisfactory explanation of his results had however to await the development of a general theory of coordination compounds and the dioxygen bridged complexes of cobalt(III) figured among the many complexes studied byW erner at the turn of the century .)2 In the nineteen thirties the mechanism of the auto-oxidation of metal ions was studied and the first synthetoixcy gen carriers discovered. In 1936, Pauling and Coryell proposed the first of many theoretical models to explain the iron dioxygen interaction in haemoglobin .)3 The increasing availability of physical methods allowing the ready characterisation of dioxygen complexes and the determination of their molecular structures, coupled with a better understanding of the electronic structures, has given considerable encouragement to thes tudy of these compounds in recent years. The early work tended to concentrate on specific types of complex, and we may distinguish three basically different areas of research: (i) Complexes of cobalt with Schiff bases and nitrogen-containing ligands. (ii) Complexes of group VIII metals in low oxidation states. (iii) Biological systems where transition metals (especially iron and copper) are known to be intimately involved in reactions with molecular oxygen. This field includes innumerable simpler "model" complexes, and covers systems which act as oxygen carriers as well as those acting as redox systems. The distinctions between these topics have become somewhat blurred with the pas- sing of time, and the increasing availability of good crystal structure data has brought to light many similarities between apparently different complexes. In 1976 Vaska )4 pub- lished an important paper classifying dioxygen complexes according to their molecular geometry, and showing that the complexes for which data were available fell into four closely related categories, and that the well known peroxo complexes of the early transi- tion metals )5 were also structurally similar to many complexes of dioxygen. From a practical point of view the study of the chemistry of dioxygen complexes has considerable interest. Complexation of molecular oxygen by a transition metal has been widely adopted by biological systems as a means of reducing the considerable kinetic barrier to the reduction of 02. Quite apart from the inherent interest of the biological systems, the transition metal complexes offer the possibility of efficiceantta lysis of auto- oxidation reactions, and have recently attracted interest as possible catalysts for the reduction of 02 in fuel cells .)6 In this review we shall take Vaska's structural classification as the basis for the examination of the electronic structure and reactivity of dioxygen complexes. We wish to follow Vaska's unifying approacht o the chemistry of these systems and to give as general a coverage as possible, and it is therefore impossible to discuss all the published work in a review of this length. Many reviews on the chemistry of these complexes have been published, dealing with general properties 4'7'8), complexes of cobalt with Schiff bases and nitrogen ligands '9 ,)0~ complexes of group VIII metals ,)21,11 catalysis by dioxygen complexes )61-a1 and biological subjects '71 )81 including oxygen carriers )12-91 and redox sys- tems .)62-22 These reviews may be consulted for more detailed discussion of particular topics. Complexes Dioxygen of Metals Transition the B. Properties of Molecular Oxygen For the purposes of this review we shall reserve the term molecular oxygen for free gaseous 02; in cases where the 02 entity is bound to other atoms we shall use the term dioxygen. This is a structural definition which requires only the continued existence ofO - O bonding in the complex molecule, and gives no information on the bonding of the 02 species. The electronic structure of molecular oxygen in its ground state gg13 is well known (Fig. 1). The two lowest excited states of molecular oxygen, the lag and lye- are obtained by pairing the spins of the two electrons in the z~ orbital, and lie respectively at 94.2 and 156.9 kJ/mol above the ground state 3~g .)72 *%3 -12ev __¢____+_ 71 eV go3 - Fig. .1 orbitals The molecular of oxygen molecular The triplet ground state of molecular oxygen provides a considerable kinetic barrier to the auto-oxidation of normally diamagnetic organic molecules where reactions involving change of spin are generally very slow, and where products formed in triplet states are unstable. This barrier may be circumvented in three ways: (i) the formation of the lowest singlet state (by photochemical activation) where the spin conservation barrier is removedEa); (ii) reaction with radical species or the free electron in two distinct steps to give diamagnetic products; (iii) reaction with a heavy element such as a transition metal where greater spin-orbit coupling considerably reduces the kinetic barrier to change of spin, and where the formation of a metal dioxygen complex may itself provide sufficient energy to pair the spins. The ions arising from simple one electron reductions or oxidations of molecular oxygen are well characterised and their properties summarised in Table .1 The possible importance of partially reduced dioxygen species in biochemical reac- tions has led to a reinvestigation of their equilibria in aqueous solutions )74-s4 and the values obtained are summarised in Table 2. Several interesting points arise from these data. The potential for the first one elec- tron reduction of molecular oxygen is unfavourable and gives the unstable superoxide anion. This and the other possible intermediates (H202 and HO) are notably more reactive than free molecular oxygen and most biological systems appear to have taken steps to eliminate them. "~O is eliminated by superoxide dismutase )94-74 which catalyses the highly favourable reaction: 4 M. H. Gubelmanna nd A. F. Williams Table 1. Properties of some dioxygen species Species Bond Compound O-O distance o.oV Bond energy order /~ Ref. cm 1- Ref. kJ/mol Ref. "~O 2.5 6FsA2O 321.1 03 8581 13 526 23 );.Y3(~O 2 02 702.1 03 5551 32 094 91 O:(tA~) 2 02 612.1 92 4841 33 693 23 0 2 5.1 2OaN 33.1 43 2OK 1.32-1.35 53 6411 73 O~(g) 43.1 63 -2~C 1 )g(zO2H 574.1 83 764.1 93 )1(202H 088 73 )s(202H 354.1 93 202aN 05.1 04 497 24 402 91 94.1 14 837 a 202aN • 8H20 248 24 BaO 2 94.1 34 2OdC 94.1 44 There are two different peroxide ions in the unit cell Table 2. Equilibria in aqueous solution Reaction Electrode potential (volts) pH = 0 pH = 7 02 + e- ~ 02 -0.33 -0.33 -~O + 2 + H + e- ~-- 2021-1 +1.69 +0.87 H2Oz+H ++e- ~ H20+HO +0.793 +0.38 HO + + + H e- ~-- 021-1 +2.76 +2.33 02 + 4H + + 4e- --. 2H20 +1.23 +0.82 Values calculated for aqueous solutionsa t 52 °C with Pth = 1 atmosphere, [02] = [H202] = [HO] = i M 20~ + + H 2 -o 02 + H202 and ensures that the molecular oxygen liberated is exclusively in the less reactive ~X3 state. H202 is eliminated by peroxidases. The OH radical, generally thought to be formed in acidic aqueous solutions of Fenton's reagent (hydrogen peroxide and a ferrous salt) is extremely reactive towards organic substrates '°5 )15 and probably reacts in a non- specific way before it may be eliminated. The very reactivity of these intermediates justifies the use by biological systems of metal ion mediated pathways of molecular oxygen reduction. Much of the experimental work on oxygen chemistry has been carried out in non- aqueous solvents and the thermodynamic data of Table 2 may not be applied to such systems. Molecular oxygen is appreciably more soluble in organic solvents than in water (the solubilities differ by a factor of ten between water and diethyl ether) .)25 Groves )15 Dioxygen Complexes of the Transition Metals 5 has shown that Fenton's reagent in non-aqueous solutions does not produce OH radicals but rather a reactive Fe(IV) (ferryl) species. A change of solvent can thus have a consid- erable effect on the mechanism of oxygen reduction. C. Structural Classification of Dioxygen Complexes In his review ,)4 Vaska showed that every dioxygen complex whose structure was then known fell into one or other of 4 structural types. This structural classification has proved extremely useful in discussing the properties of dioxygen, and in this section we discuss the classification and review structural data published since 1976. Vaska's four structural types are the first four entries in Table 3; he grouped the four structures into the superoxo compounds (types I a and I b) where the O-O distance is roughly constant (-1.3/~) and close to the value reported for the superoxide anion (Table 1), and the peroxo compounds (types II a and II b) where the O-O distance is close to the values reported for H202 and 022- (~ 1.48 A). The a or b classification distinguishes complexes where the dioxygen is bound to one metal atom (type a) or bridges two metal atoms (type b). In this review we shall use a "hapto" nomenclature in which the structures are classified by the number of atoms of dioxygen bound to the metal ion; although this does not distinguish explicitly between types Ib and IIb, it does avoid assigning a possibly misleading oxidation state to the dioxygen (see Sect. D) and may readily be applied to the structural types discovered since Vaska's review. elbaT 3. Structural classification of dioxygen complexes Structure type Structural designation Vaska classification Example o/O r/ldioxygen Type a I (superoxo) 3- [Co(CN)502] I M O.~M ~ negyxoid2/t Type II a (peroxo) (PhaP)2PtO2 M--O,.. O-- M 1~ r/ldioxygen : Type b I (superoxo) ]s)3HN(oC2OoCs)N3H([ +5 M--O 1/r t/tdioxygen : Type IIb (peroxo) ]5)3HN(oC2OoCs)N3H([ +4 xO--M .(cid:127)O/M 2/t t/2dioxygen : - ]202)31C2OU([ 4- M~o~'q M 1/t r/2di°xygen : - 202]hR1C2)P3hP([ 6 M.H. Gubelmann and A. F. Williams The 2/r : 2/r structure with a "sideways" bound dioxygen bridging two metal atoms has been suggested for the complex [{Rh(diene)}202] )35 and a crystal structure showing this geometry has been reported for the uranium complex [(UO2C13)202] 4- )45 and a complex of La +3 .)55 The 1/r ]t 2 Structure (Fig. 2) is known only for [RhCI(O2)(PPh3)2]2 .)65 : rr Fig. 2. The structure of 2])2O(IChR2)P3hP([ (Ref. )65 The only completely unambiguous method of structure determination has proved to be X-ray diffraction. Vaska noted however that the stretching frequencies attributed to the O-O vibration were closely related to the structural type .)4 Type I complexes show O-O stretching vibrations around 1125 cm -1 and type II around 860 cm -1. This sharp difference enables the O-O stretching frequency as measured by infra-red or Raman spectroscopy to be used to assign the structure type, provided (as is usually the case) the formation of a dinuclear species can be confirmed or excluded by other means. If X-ray diffraction gives an unambiguous description of the structure, it should nevertheless be noted that the accurate determination ofb ond lengths and angles for the coordinated dioxygen is not always easy. When the dioxygen is bonded closely to a very ! S I )tc( (b) Fig. 3 a, b. Disorder in dioxygen complexes
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