Reduction outer-sphere

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

X = H2O, OH , CP, NCS , and Nj in all these cases the rates of reduction are rapid compared with the rates of substitution of X" in Fe(H20)6. No evidence is evinced for formation of VX complexes. It is noteworthy that OH exerts little effect on the rate of this reaction although it has a pronounced influence on the rate of other reactions of Fe(IlI) . This result, together with the observation that the other anions have similar effects on the rate, would seem to indicate that the FeX " reactions proceed via an outer sphere path. The... 

Reductions of various Co(ni) complexes by Fe(II) have been studied under high pressures . The motivation for performing such experiments resides in the possibility that the volume of activation (AF ), like the entropy of activation, might be a criterion for distinguishing between inner- and outer-sphere reactions. For reactions of the type... 

It is of incidental interest that a little work has been done on dicobalt systems. Doyle and Sykes have made a study of the reduction of decammine-//-amidodi-cobalt(Ill), (NH3)5Co NH2-Co(NH3)5 , by V(II). Since the rate is independent of hydrogen-ion concentration the mechanism cannot involve an amide bridge and must be outer-sphere, as it is in the case of the reduction of Co(NH3)6 by V(1I) . Both the binuclear complex and Co(NH3)6 are inert to substitution but the former is capable of functioning as a two-equivalent oxidant. Thus the two likely mechanisms are... 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

These present an interesting dichotomy in their reductions by tm(l,10-phen-anthroline)iron(ri) (ferroin) °. That of CIO2 to CIOJ is rapid, is first-order in each component ki = 1.86 0.13 l.mole sec at 35 °C) and is independent of acidity. Ferriin is the immediate product and an outer sphere electron-transfer is proposed. The reduction of CIO2 is much slower, proceeding at the same rate as dissociation of ferroin at high chlorite concentrations and a major product is feriin dimer, possibly [(phen)2Fe-0-Fe(phen)2] . Clearly the reaction depends on ligand-displacement followed by an inner-sphere electron transfer. 

The appearance of FeCl in the Fe (aq) reduction suggests an inner-sphere path. By contrast, the reduction by tris-1 lO-phenanthrolineiron(II) or ferroin is outer-sphere , for ferriin is formed in high yield. The kinetics are simple second-order with (25 °C, n = I M) = 2.2+0.2 (independent of acidity). 

The reduction of 804 in the rapid second step is, on the other hand, of the outer-sphere type (for otherwise further CrS04 would be found), viz. 

Electrochemical reactions only involving a change of charge of simple or complex ions but not any change in inner geometry are commonly called outer-sphere electron transfer reactions. For some time, the reduction and oxidation of simple and... 

In typical outer sphere electron transfer on metal electrodes, A is in the weakly adiabatic region and thus sufficiently large to ensure adiabaticity, but too small to lead to a noticeable reduction of the activation energy. In this case, the rate is determined by solvent reorganization, and is independent of the nature of the metal [Iwasita et al., 1985 Santos et al., 1986]. 

Anderson AB, Alhu TV. 1999. Ah initio determination of reversible potentials and activation energies for outer-sphere oxygen reduction to water and the reverse oxidation reaction. JAm Chem Soc 121 11855-11863. 

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). 

Release of superoxide during ORR catalysis indicates that the ferric-superoxo intermediate (Fig. 18.20) has a substantial residence time at 0.2 V (the potential of the maximum production of superoxide), suggesting that the potential of the ferric-superoxo/ferric-peroxo couple, (Fig. 18.20), is more reducing than 0.2 V. The fraction of superoxide detected at potentials >0.2 V probably reflects the fact that 02, which is a strong outer-sphere reductant [Huie and Neta, 1999], was oxidized by the mostly ferric catalytic film before it could escape the film. There are two plausible explanations for the decrease in the fraction of superoxide byproduct released at... 

Experimental tests of the theoretical predictions have involved the electrochemical reduction of alkyl and benzyl halides as well as their reduction by homogeneous electron donors.22,29-31 In the first case, AG° = E - rx r.+x=f where E is the electrode potential and rx r.+x=f is the standard potential of the RX/R + XT couple. In the homogeneous case, AG° = E q — rx r-+xt> where E Q is the standard potential of the outer-sphere electron donor or acceptor couple P/Q, and + stands for a reduction and — for an oxidation. 

---