Inorganic Reaction Mechanisms Volume 3

A Review of the Literature Published Between December 1971 and June 1973

By J. Burgess

The Royal Society of Chemistry

Copyright © 1974 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-275-0

Contents

Part I Electron Transfer Processes,
Introduction, 3,
Chapter 1 Reactions Between Two Metal Complexes By A. McAuley, 5,
Chapter 2 Metal Ion–Ligand Redox Reactions By A. McAuley, 49,
Chapter 3 Reactions involving Oxygen and Hydrogen Peroxide By A. McAuley, 98,
Part II Substitution and Related Reactions,
Chapter 1 Non-metallic Elements By J. Burgess, 117,
Chapter 2 Inert Metal Complexes: Co-ordination Number Four By J. Burgess, 142,
Chapter 3 Inert Metal Complexes: Co-ordination Numbers Six and Higher By J. Burgess, 163,
Chapter 4 Labile Metal Complexes By D. N. Hague, 261,
Chapter 5 Reactions of Co-ordinated Ligands By J. Burgess, 297,
Chapter 6 Solvent Effects By J. Burgess, 312,
Part III Reactions of Biochemical Interest By D. N. Hague,
1 Introduction, 329,
2 Metal Ion Transport, 330,
3 Metal Complex Formation, 334,
4 Reactions involving Metals in Porphyrins and Related Ring Systems, 341,
5 Redox Reactions involving Metals in other Biological and Model Systems, 347,
Part IV Organometallic Compounds,
Chapter 1 Substitution By R. D. W. Kemmitt and M. A. R. Smith, 351,
Chapter 2 Metal–Alkyl, –Aryl, and –Allyl Bond Formation and Cleavage By R. D. W. Kemmitt and M. A. R. Smith, 372,
Chapter 3 Homogeneous Catalysis By R. D. W. Kemmitt and M. A. R. Smith, 384,
Chapter 4 Insertion Reactions By R. D. W. Kemmitt and M. A. R. Smith, 422,
Chapter 5 Reactions of Co-ordinated Ligands By R. D. W. Kemmitt and M. A. R. Smith, 432,
Chapter 6 Oxidative Addition and Reductive Elimination By R. D. W. Kemmitt and M. A. R. Smith, 451,
Chapter 7 Isomerization: Intramolecular Processes By R. D. W. Kemmitt and M. A. R. Smith, 460,
Errata, 485,
Author Index, 486,

CHAPTER 1

Part I

ELECTRON TRANSFER PROCESSES

By A. McAULEY

Introduction

BY A. McAULEY

The format of this Part follows closely that of the previous volumes in this series. Although some degree of selection has had to be imposed owing to the large number of papers involving electron-transfer processes, an attempt has been made to cover comprehensively all the areas in which studies are currently being undertaken. As in the previous volumes, compilations of data have been assembled to allow more direct comparison of rate constants and thermodynamic parameters of reactions of a similar type.

Differing aspects of the subject have been dealt with in several publications. A very readable account of the present ‘state of the art’ in inorganic mechanisms has been published by Tobel in which there are chapters dealing with redox systems involving both interactions between two metal-ion complexes and also reactions where ligands are oxidized or reduced. A series of review articles on this subject has appeared in a volume edited by Edwards, including an account of chromium(v1) oxidations of inorganic substrates, where both one-electron and two-electron systems are discussed, together with the role of CrV and the fate of CrIV in these reactions.

Electron-transfer processes between two metal ions continue to be investigated and in several papers the importance of medium effects has been noted. In a re-examination of the role of co-ordinated water as a bridging ligand in the CrII reduction of penta-amminecobalt(III) complexes, the use of lithium perchlorate yields data which are consistent with a single-term rate law instead of the two-term law noted previously in solutions where sodium perchlorate was used as the supporting electrolyte. Similar effects have also been observed in the corresponding reaction of the cobalt(III)–malonato-complexes. The metal-ion reduction of cobalt(III) complexes containing co-ordinated sulphur donor atoms has been studied with interesting differences in reactivity when compared with the corresponding N- or O-donor systems. Radical intermediates of sufficiently long life for spectrophotometric identification have been observed in the chromium(II) reduction of the corresponding carboxylatopenta-ammine complex ions. A review has also been written in which comparison has been made between vanadium(IV) and iron(II) as reductants in aqueous electron-transfer processes.

The plenary lectures at the 14th I.C.C.C. Meeting at Toronto have been published, as have the papers presented at the Bressanone Conference. l1 Several other useful reviews of reactions involving metal ions have been published. The oxidation–reduction of the cobalt centre in vitamin B12 has been discussed and recent developments in the bioinorganic chemistry of this complex have also been described. Electron-transfer catalysts involving metalloenzyme systems have also been reviewed by Williams. The role of transition metals in homogeneous catalysis has been described and homolytic oxidation and reduction reactions of organic compounds by metallic ions have been reviewed.

1

Reactions Between Two Metal Complexes

BY A. McAULEY

1 Reducing Agents

Chromium(II). — Quantitative data for this and for the other reducing species are tabulated in Tables 1 and 2 on pp. 17 and 24.

Many reactions have been studied using this reagent, especially those involving the inert ammine complexes of cobalt(III), and in the case of organic derivatives of these complexes a wide variety of reactivity patterns has emerged which has not yet been encountered with any other reducing centre. Electron transfer through organic structural units has been investigated in the case of the alkenecarboxylatopenta-amminecobalt(III) species, in which there is a double bond lying between the — CO2Co(NH3)5 unit and an aromatic ring. An interesting feature of these reactions is that some retardation of the reduction rate is observed at high hydrogen-ion concentrations which is not observed (over a sixty-fold range) in the case of the corresponding benzoato- and p-hydroxybenzoato-species. The reaction products for the systems involving the o-hydroxycinnamato- and acetylenecarboxylato-complexes are the corresponding carboxylatochromium(III) species, confirming the expected inner-sphere paths for these reactions. Specific rates for the reduction of αβ-unsaturated complexes of this type are significantly higher than those obtained for aromatic and straight-chain derivatives, and the decrease in reaction rate at high acidity appears to be a general effect for these species, the observed retardations being too large to be attributable to medium effects. The rate law may be described as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where the rate constants k1 and k2 are associated with the reactivity of the unprotonated and protonated forms of the cobalt(III) complex, KB being the protonation equilibrium constant. Comparison of the data for the protonated acetato-complex (in which the site of protonation is considered to be the carbonyl oxygen) with those of the unsaturated systems where the hydrogen ion may be associated at the double (or triple) bond suggests that the strong base-strengthening action of the — Co(NH3)5 group (when substituted for a hydrogen atom or alkyl group) is transmitted through the conjugated systems, giving rise to the acid dependences observed. In the reaction with the p-formylbenzoatopenta-amminecobalt(III) ion with CrII, however, the observed rate is strongly dependent on acidity, the rate law being expressed in the form

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The rate constant for the acid-independent term is much larger than that for the corresponding benzoato-complex, which may be due to attack by the reducing agent at the remote carbonyl of the group. Stopped-flow experiments confirm the existence of an intermediate complex of structure [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], in which the absorption maximum is ascribed to a π-π* transition, the band being shifted by interaction of the carbonyl group with the CrIII to 290 nm from the free-ligand position of 254 nm. The intermediate decays to yield [Cr(H2O)6]3+ and the free ligand in a unimolecular reaction independent of [H+] and of CrII at low concentrations. In the corresponding reaction with the m-formylbenzoato-complex, whereas 55% of the ligand in the cobalt complex is released, the remainder being bound to the chromium(III) as a carboxylato-complex, there is no evidence of rate promotion by hydrogen ions. The overall rate constant for this acid-independent route is about 170 times smaller than that for the p-formyl-benzoato-species but only a factor of two different from that of the unsubstituted benzoato-species. From values of product distribution as a function of temperature, the rate constants for both the reaction of the adjacent carboxyl leading to formation of a m-formylbenzoato intermediate and for the path leading to ligand release have been evaluated. In the reduction of m-formylbenzoic acid by chromium(II), however, the rates are a factor of 104 lower.

Co-ordination of a (NH3)5CoIII unit to glycine lowers the pKa by about 1.3 units and the reaction of substituted glycinato-complexes with CrII has been investigated to study the possibility of steric crowding as alkyl substitution on the ammonium nitrogen increases such that attack at the adjacent carboxy-group would be hindered. For the complex [(H3N)5CoO2C·CH2NH3]3+ there is no acid dependence on the rate constant within the range [H+] = 0.019 — 0.54M, the reaction proceeding via an inner-sphere mechanism. Increased substitution of H by methyl groups causes a decrease in the reaction rate; however, the factor of four decrease in rate for the trimethylglycinato-complex compared with the glycinato-species is reflected in only a small (3 cal deg-1 mol-1) change in ΔG≠. The reductions of malonatopenta-amminecobalt(III) and the corresponding dimethylmalonato-species by CrII have been reinvestigated, the acidity dependence observed previously for the malonato-complex in sodium perchlorate media being shown to be a medium effect. Whereas over the range [H+] = 1.0 — 3.95M (I = 4.0M) with NaC1O4 as supporting electrolyte the observed rate constant increases from 2.64 to 3.941 mol-1 s-1, no such variation in rate is seen in LiClO4 solutions, the value being 4.05 ± 0.141 mol-1 s-1. A similar effect is seen for the dialkylated species.

The outer-sphere reductions of penta-ammine pyridine and alkyl-substituted pyridine complexes have been investigated and have been compared with similar reactions involving Eu2+ as reductant. For the CrII reactions, the rate constants are in the range 10-3 — 10-3 l mol-1 s-1. In the reactions with the 4-ethoxycarbonylpyridine, 2-methylpyrazine, and 2,6-dimethylpyrazine (1) complexes, where there is a site for inner-sphere reduction and electron transfer through the conjugated system, the rates are appreciably greater, especially for the pyrazine complexes where the CrIII products are not the hexa-aquated ion but involve a metal ion-nitrogen bond. A probable pathway may be written as in Scheme 1. In the reduction of the pyrazinecarboxylatopenta-amminecobalt(III) ion (2) with CrII, however, physical evidence has been presented for a radical cation intermediate in the course of the reaction. Addition of excess Cr to oxidant (2) results in a marked increase in absorbance in the range 400 — 600 nm within 10 ms of mixing, followed by a slower decay to yield cobalt(n) and the chromium(III) product. Both reactions are first-order, the maximum concentration of absorbing species and its formation and decomposition rates being independent of the CrII added. The data are interpreted in terms of a reaction scheme (Scheme 2) where there is a precursor complex formed which undergoes an intramolecular electron transfer to yield the radical cation (ka = 263 s-1) with a subsequent slower transformation (kb = 2.4 s-1) to the successor complex, which then aquates rapidly in the acid media to yield the products. The corresponding reaction with the 4-cinnolinecarboxylato-derivative (3) again exhibits behaviour of this type, but in this case both formation and disappearance (kc) of the intermediate are first-order dependent on CrII, a mechanism of the type shown in Scheme 3 being invoked.

In the reduction of aromatic nitrile complexes of cobalt by chromium(II), second-order rate behaviour is observed. In the case of terephthalonitrile as ligand, no detectable ligand transfer occurs and in the case of complexes of 3- or 4-cyanophenol the reaction proceeds with only 10 — 15% of the bridging ligand attached to the chromium(III) product. These reactions are of interest from the point of view that the reducing agent cannot attack at the atom or group directly attached to the cobalt since there are no free electron pairs available for bonding to the reducing agent. Although at first sight it would appear that the data could be described by an outer-sphere mechanism, a consideration of the rate and activation parameters would require some modification of this view. For a simple outer-sphere route, little difference would be expected in the rates of reduction of the 3- and 4-cyano-substituted complexes whereas in fact they differ by ~ 30% and the terephthalonitrile species reacts about 30 times still faster. The mechanism postulated involves an outer-sphere reduction of the nitrile ligand where the electron is transferred to the ligand without co-ordination of a remote group to the chromium(II). A route of this type would be favoured by an easily reducible ligand containing a remote substituent which was not very basic towards CrII. In support of this view a correlation has been shown between the rate of reduction and Hammett σ constants, which themselves have been shown to be a measure of the polarographic half-wave reduction potentials of the uncomplexed aromatic nitriles.

The role of co-ordinated water as a bridging ligand in redox systems has been discussed, the data for the reaction in lithium perchlorate media being consistent with a single-term rate law. The hydrogen-ion-independent pathway is considered too insignificant to be measured directly and it is postulated that for all reactions of CrII with complexes containing water as the only bridging ligand any hydrogen-ion-independent pathway proceeds via an outer-sphere mechanism.

The reduction of thioether and carboxylate chelates of cobalt(III) by CrII has been investigated. The complexes [(en)2Co(NH2CH2CO2)]2+ and [(en)2Co(O2CCH2SMe)]2+ react via an inner-sphere mechanism with carboxylate bridging, whereas the species [(en)2Co(NH2CH2CH2SMe)]3+ interacts in an outer-sphere manner. The rate for [(en)2Co(O2CCH2SMe)]2+ is very high and may be due to the cis non-bridging influence of the thioether function. Symmetry rules have been applied to electron-transfer reactions of CrII (and VII) with complexes of the penta-amminecobalt(III) type including a bridging ligand, the reactivity of the co-ordination compounds increasing with decreasing symmetry.

The reductions of di- and tetra-nuclear cobalt(III) complexes have been studied by Sykes and his co-workers. In the reaction with µ-amido-µ-formatobis[tetra-amminecobalt(III)] the only observable stage in the reaction may be represented by the rate law when the reductant is present in large excess. No dependence on hydrogen ion was detected in the range [H+] = 0.04 — 0.98M. The observed rate law has been interpreted by assuming an equilibrium between the µ-formato-species and an aquoformato-complex, both of which react with the chromium(II), e.g. as shown in Scheme 4. The reaction was also found to be strongly catalysed by chloride ions. In the corresponding reaction of the µ-acetato-complex, however, there is no evidence for the second term in equation (1), the observed rate constants being a factor of four lower than for the former system. The reaction of the µ-amido-µ-oxalato-complex (4) shows two reduction stages with the formation of an intermediate. In this case, the reaction is much faster than those previously described and probably proceeds via an innersphere mechanism. The first two stages of the reduction of the superoxo-complex [(en)2Co·µ-(NH2,O2)·Co(en)2]4+

The second redox reaction involves one cobalt(III) centre, but the second cobalt(III) site reacts only after the peroxo-bridge has been reduced. The Cr2+ reduction of (5) is too rapid for detection by flow techniques, but the [Cr(H2O)6]3+ product is indicative of an outer-sphere route. The reduction of (6) is also fast and here the reaction product is not a mononuclear chromium(III) species but the first example of a µ-peroxo-complex involving different metal atoms,

[FORMULA NOT REPRODUCIBLE IN ASCII]

confirming an inner-sphere pathway for this reaction.

In the case of the trinuclear µ-oxalato-complexes, parallel inner- and outer-sphere routes have been observed. It is interesting to note, however, that whereas the reaction of the species [Co3(NH3)11(NH2)(OH)(C2O4)]5+ follows second-order kinetics, reaction of the corresponding complex [Co3(NH3)11(OH)2(C2O4)]5+ is more complicated and at least two stages are observed in the chromium(II) reduction. The reactions of three tetranuclear µ-oxalato-cobalt(III) complexes have been reported where in a 40 — 400-fold excess of reductant all four Co centres undergo reduction, the first stage of the reaction in each case being rate determining. The rate of reduction of µ4-oxalato-[di-µ-hydroxo-bis{triamminecobalt(III)}]-[µ-amido-bis{tetraamminecobalt(III)}] (7) is found to be independent of hydrogen-ion concentration over the range 0.1 — 0.9M, the rate law being

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

A similar rate law was observed for the 6+ ion µ4-oxalato-bis[di-µ-hydroxobis{triamminecobalt(III)}] (8) and for the 7+ species µ4-oxalato-[µ-amido-µ-hydroxo-bis{triamminecobalt(III)}]-[µ-amido-bis {tetra-amminecobalt(III)}](9).

In these systems no rate-determining hydroxo-bridge cleavage was encountered and the mechanism of reduction in each case is outer-sphere. It is considered that the secondary and tertiary redox stages involving tri- and di-nuclear µ-oxalato-complexes are fast, since in these species there are possible sites for attack via an inner-sphere route.

(Continues…)Excerpted from Inorganic Reaction Mechanisms Volume 3 by J. Burgess. Copyright © 1974 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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