Activation Parameters Derived From a Temperature Dependent Large Amplitude ac Voltammetric Study of the Electrode Kinetics of the Cp2M0/+ Redox Couples (M = Fe, Co) at a Glassy Carbon Electrode

Abstract

Heterogeneous charge transfer rate constants (k°) for the oxidation of ferrocene, Fc (k° = 0.21 cm s(-1) at 20 °C), and the reduction of the cobaltocenium cation, Cc(+) (k° = 0.12 cm s(-1) at 18 °C), in CH(2)Cl(2) (0.5 M Bu(4)NPF(6)) at a glassy carbon (GC) electrode were determined as a function of temperature using the higher harmonic components available in Fourier transformed large amplitude ac voltammetry. The measured k° values lie at the upper end of the analytically useful range available for the acquisition of meaningful kinetic data for these very fast (i.e., close to reversible) processes. Measurements of the kinetics are facilitated by the ac technique because contributions resulting from uncompensated resistance and slow electrode kinetics, both of which also vary with temperature, can be distinguished via their different contributions to individual harmonics. Values of k° measured over the temperature range of -18 to +20 °C for the Fc(0/+) and Cc(+/0) processes gave linear Arrhenius plots yielding free energies of activation (ΔG(‡)) estimates of 17.0 and 18.0 kJ mol(-1), respectively, in excellent agreement with theoretical values derived from Marcus theory. An analogous temperature dependence is indicated in a study of the Fc(0/+) process in acetonitrile (0.1 M Bu(4)NPF(6)), but the greater k° value evident makes the temperature dependence of these parameters more difficult to quantify as departures from reversibility are minimal, even in the higher harmonics.