Intramolecular electron transfer inner-sphere

Inner-sphere. Here, the two reactants first form a bridged complex (precursor)- intramolecular electron transfer then yields the successor which in turn dissociates to give the products. The first demonstration of this was provided by H. Taube. He examined the oxidation of ICrfHoOijj by lCoCl(NHr)< and postulated that it occurs as follows ... 

Ce4+ is a versatile one-electron oxidizing agent (E° = - 1.71 eV in HC10466 capable of oxidizing sulfoxides. Rao and coworkers66 have described the oxidation of dimethyl sulfoxide to dimethyl sulfone by Ce4+ cation in perchloric acid and proposed a SET mechanism. In the first step DMSO rapidly replaces a molecule of water in the coordination sphere of the metal (Ce v has a coordination number of 8). An intramolecular electron transfer leads to the production of a cation which is subsequently converted into sulfone by reaction with water. The formation of radicals was confirmed by polymerization of acrylonitrile added to the medium. We have written a plausible mechanism for the process (Scheme 8), but there is no compelling experimental data concerning the inner versus outer sphere character of the reaction between HzO and the radical cation of DMSO. 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

The discussed mechanisms represent a form of intramolecular catalysis of the oxidation of the FeII(CN)5 or Run(edta) centers by the Ruii(NH3)5 moiety. The first two moieties react sluggisly and, on the other hand, the electron in RuII(NH3)5 is readily accessible to the external oxidant and is given up. The rapid electronic isomerization processes aid in the consumption of the full oxidation process. This is not truly catalytic because the catalyst is the reactant itself, which, of course, is consumed in the reaction. A better description involves a net oxidation of the FeII(CN)5 or Run(edta) sites through activation by the facile intramolecular electron transfer between the metal centers. The mechanism is described in Fig. 23, bearing some resemblance to the classical chemical mechanism for inner sphere electron... 

More recently the same reactions have been monitored for Ru(III)-Os(II) analogs. These complexes are generated by the reduction of the Ru(III)—Os(III) dimers by e (aq) or C02 (aq) radicals in pulse radiolysis experiments 429). Because of the much lower inner-sphere reor-ganizational energy terms for the [Ru(NH3)5L]3+/2+ couples compared with Codll/II) analogs, the rates of intramolecular electron transfer in the Ru(III)—Os(II) dimers are much larger than those of Co(III)-Os(II) dimers 429). 

A variety of linkage isomer pairs have been produced from somewhat more complex ligands, such as substituted pyridines and benzoic acids, for example (5a) and (5b).77,78 These complexes have been employed in detailed studies of inner-sphere electron transfer reactions in order to assess the importance of the nature and orientation of the bridge between redox centres on intramolecular electron transfer rates.77-80... 

Two separate but somewhat interwoven themes have emerged from the study of inner-sphere reactions. The first is the use of product and rate studies to establish the existence of inner-sphere pathways and then the exploitation of appropriate systems to demonstrate such special features as remote attack . In the second theme the goal has been to assemble the reactants through a chemical bridge and then to study intramolecular electron transfer directly following oxidation or reduction of the resulting dimer (note equation 7). It is convenient to turn first to chemically prepared, intramolecular systems since many of the theoretical ideas and experimental results for outer-sphere reactions can be carried over directly as an initial basis for understanding the experimental observations. 

An inner-sphere intramolecular electron transfer has been observed also in the reaction of chromium(II) salts with [Co(A-acacCN)(NH3)5] + °. This reaction proceeds via the binuclear intermediate [(NH3)5Co(A-acacCN)Cr]" +, which evolves to [Co(NH3)5(H20)] and [Cr(0,0 -acacCN)(H20)4] + ° . The mechanism is supported by the observation that addition of non-reducing metal ions such as Zn(II), Ni(II) or Ba(II) to the reaction mixture causes a decrease of the rate constant. ... 

The importance of Marcus theoretical work on electron transfer reactions was recognized with a Nobel Prize in Chemistry in 1992, and its historical development is outlined in his Nobel Lecture.3 The aspects of his theoretical work most widely used by experimentalists concern outer-sphere electron transfer reactions. These are characterized by weak electronic interactions between electron donors and acceptors along the reaction coordinate and are distinct from inner-sphere electron transfer processes that proceed through the formation of chemical bonds between reacting species. Marcus theoretical work includes intermolecular (often bimolecular) reactions, intramolecular electron transfer, and heterogeneous (electrode) reactions. The background and models presented here are intended to serve as an introduction to bimolecular processes. 

Hydrated electrons react with certain water-soluble metalloporphyrin complexes, reducing the porphyrin ligands to pi-radical species. When the metal centers are Zn(II), Pd(II), Ag(II), Cd(II), Cu(II), Sn(IV), and Pb(II), the radical complexes are produced at diffusion-controlled rates and decay with second-order kinetics.188 Fe(III) porphyrins, on the other hand, yield Fe(II) porphyrins.189 Rather different behavior is seen in the reaction of e (aq) with [Ru(bpy)3]3 + here, parallel paths generate the well-known luminescent excited-state [ Ru(bpy)3]2 + and another reduced intermediate, both of which decay to the ground-state [Ru(bpy)3]2+, 190 In a direct demonstration of the chemical mechanism of inner-sphere electron transfer, [Coni(NH3)5L]2+ complexes where L = nitrobenzoate and dinitrobenzoate react with e (aq) to form Co(III)-ligand radical intermediates, which then undergo intramolecular electron transfer to yield Co(II) and L.191... 

In principle, any of the steps in Scheme 2 can be rate limiting from the diffusion controlled formation of the association complex to the substitutional breakup of the successor complex. Because of the multiplicity of steps, a detailed interpretation of an experimentally observed rate constant can be extremely difficult. However, in some cases it has been possible to obtain direct or indirect information about the electron transfer step in an inner-sphere reaction using chemically prepared, ligand-bridged dimeric complexes. For example, reduction of the Co -Ru precursor to the product shown in equation (7) by Ru(NH3)6 + or Eu + occurs selectively at the Ru " site. The initial reduction to give Ru is followed by intramolecular electron transfer from Ru to Co which is irreversible since the Co site is rapidly lost by aquation. ... 

Reaction (4) implies a weak associative interaction, erroneously defined previously as of outer-sphere type (76). Encounter complexes have heen described and characterized in the nitrosylation/denitrosylation reactions of aromatic compounds and were proposed to he inner-sphere adducts containing weakly bound NO (77). They were considered as immediate precursors of transition states for intramolecular electron transfer, as in reaction (5). The equifihrium constants for reactions (6—7) should be high. The ligand interchange within the adduct-complex, reaction (6), is rate-controlled by the cleavage of the Fe —H2O bond, coupled to a fast NO -coordination. Therefore, for the kinetic analysis, processes (6—7) could be collected into a single kinetic constant fe h20-... 

The oxidation of [(NH3)5Ru (pyr)Ru (edta)] by S20g is biphasic. The first phase results in information of the mixed valence species, [(NH3)5Ru (pyr)Ru (edta)] , with a rate constant 2.5 x 10 Af s . From rate comparisons with the corresponding monomeric complexes this is thought to proceed with initial formation of [(NH3)5Ru (pyr)Ru (edta)] followed by rapid intramolecular electron transfer. The second phase consists of two pathways. One pathway is independent of [S20g ] and involves dissociation of the mixed-valence adduct, followed by rapid oxidation of [(NH3)5Ru (pyr)] . The second pathway involves oxidation of the adduct and has a rate constant 5.5x 10 M s at 25.0 °C and 0.10 M ionic strength. Kinetics of the oxidations of [Fe(phen)3] and [Fe(bpy)3] " by S20g have also been reported. Two pathways are involved. In one, ligand dissociation precedes the oxidation, while the other involves an outer-sphere adduct from which an inner-sphere intermediate is formed. 

The elementary electrochemical reactions differ by the degree of their complexity. The simplest class of reactions is represented by the outer-sphere electron transfer reactions. An example of this type is the electron transfer reactions of complex ions. The electron transfer here does not result in a change of the composition of the reactants. Even a change in the intramolecular structure (inner-sphere reorganization) may be neglected in many cases. The only result of the electron transfer is then the change in the outer-sphere solvation of the reactants. The microscopic mechanism of this type of reaction is very close to that for the outer-sphere electron transfer in the bulk solution. Therefore, the latter is worth considering first. 

The most striking application of electron transfer theory has been to the direct calculation of electron transfer rate constants for a series of metal complex couples.36 37 46 The results of several such calculations taken from ref. 37b are summarized in Table 2. The calculations were made based on intemuclear separations appropriate to the reactants in close contact except for the second entry for Fe(H20)j3+/2+, where at r = 5.25 A there is significant interpenetratidn of the inner coordination spheres. The Ke values are based on ab initio calculations of the extent of electronic coupling. k includes the total contributions to electron transfer from solvent and the trapping vibrations using the dielectric continuum result for A0. the quantum mechanical result for intramolecular vibrations, and known bond distance changes from measurements in the solid state or in solution. 

It is conventional to classify electrochemical reactions as outer-sphere and inner-sphere. The former involve the outer coordination sphere of a reacting ion. Thus, little if any change inside the ion solvate shell occurs they proceed without breaking-up intramolecular bonds. But in the latter, involving the inner coordination sphere, electron transfer is accompanied by breaking up or formation of such bonds. Often the inner-sphere reactions are complicated by adsorption of reactants and/or reaction products on the electrode surface. The electron transfer in the Fc(CN)62 /4 system is example of an outer-sphere reaction (with due reservation for some complications... 

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