Marcus theory of outer-sphere heterogeneous electron transfer reactions: High precision steady-state measurements of the standard electrochemical rate constant for ferrocene derivatives in alkyl cyanide solvents

The High Speed Channel Electrode is used to precisely measure the standard electrochemical rate constants under steady state conditions for the oxidation of substituted ferrocenes in alkyl cyanide solvents. First, the oxidation of ferrocene itself is kinetically analysed in the solvents: acetonitrile, propionitrile, butyronitrile and valeronitrile. Second, the oxidation kinetics of the ferrocene derivatives: ferrocene carboxylic acid, decamethyl ferrocene, bis(diphenylphosphino) ferrocene, ferrocene carboxaldehyde, α-hydroxyethyl ferrocene, t-butyl ferrocene, dimethyl aminomethyl ferrocene, dimethyl ferrocene and n-butyl ferrocene are measured in acetonitrile for comparison to ferrocene. The rate of electron transfer in the different alkyl cyanides is shown to depend on the longitudinal dielectric relaxation times of the solvents in accordance with solvent dynamic theory, whilst the electron transfer rates of the different ferrocene derivatives in acetonitrile are shown to depend on the hydrodynamic radii of the molecules. In both cases, a measured k = 1.0 ± 0.2 cm s for ferrocene oxidation in acetonitrile shows an excellent fit to the theoretical dependences.