Electron transfer kinetics for the cobaltocene (+1/0) couple at platinum disk electrode in acetonitrile/dichloromethane binary solvent system

The oxidation of Cobaltocene (CoCp2) to cobaltocenium cation (CoCp2) (where Cp=cyclopentadienyl) was investigated by means of cyclic voltammetry in acetonitrile (ACN), dichloromethane (DCM), and acetonitrile (ACN)/dichloromethane (DCM) binary mixtures with n-tetrabutylammonium hexafluorophosphate (TBAPF(6)) as background electrolyte at the temperature of 298.15 K. The half-wave potentials (E-1/2), the diffusion coefficients (D), and the heterogeneous electron transfer rate constants (k(s)) were determined. The activation free energies for the electron transfer process (Delta G(exp)(not equal)) were also extracted and compared with the calculated theoretical values (Delta G(cal)(not equal)). The one-electron transfer process was found to be reversible and diffusion controlled in all the investigated solvent media. The variation of E-1/2 with the solvent composition was found to be linear. The rate of the electron transfer was found to vary inversely with the viscosity (eta) and the specific resistance (rho) of the solvent medium. The changes on structural reorganization of the electroactive species upon the electron transfer were found to be insignificant in ACN-rich solutions while that was not true in DCM-rich solutions. The results demonstrated the effect of the solvent dynamic and dielectric on the electrochemical properties of the CoCp2+/0 couple. (C) 2007 Elsevier B.V. All rights reserved.