Homogeneous redox catalysis of electrochemical reactions part II. Rate determining electron transfer. Evaluation of rate and equilibrium parameters

Abstract

The reduction of chlorobenzene in DMF with tetrabutylammonium as supporting cation is taken as an example illustrating the occurrence of homogeneous catalysis under the following conditions: (a) charge tranfer control of the uncatalyzed reduction of the substrate, (b) activation and diffusion control of the solution electron exchange between substrate and catalyst. Since chlorobenzene is reduced according to a two-electron process, the kinetic treatment previously given for a simple EC mechanism is extended to the two-electron case considering the occurrence of the second electron uptake either at the electrode or in solution. Eight catalyst couples involving aromatic hydrocarbons and heterocycles were employed to evaluate the catalytic efficiency as a function of the half-wave potential separation ΔE1/2. The variations of the catalytic efficiency with the excess factor for a given catalyst show that the rate determining step is in almost all cases the forward solution electron exchange. This allows a relatively simple determination of the rate constant of this reaction as a function of ΔE1/2. Two distinct regions in ΔE1/2 clearly appear corresponding to activation and diffusion control of the solution electron transfer, respectively. The behavior in the activation controlled region fits satisfactorily with a simplified version of the Marcus theory in which the quadratic term is neglected. Analysis of the results allows an indirect determination of the electrochemical standard rate constant, the standard potential and the isotopic homogeneous rate constant which could not have been obtained through standard electrochemical methods. The relationship between the homogeneous and heterogeneous reorganization energies is discussed in terms of Hush and Marcus theories.