The role of solvent dynamics in electron transfer: Electrochemical exchange kinetics of cobalt clathrochelates as a probe of reactant vibrational effects

Abstract

Rate constants for electrochemical exchange, k ex, have been measured for the cobalt clathrochelate couples Co(dmg) 3(BF) 2 +/0 and Co(dmg) 3(BC 4H 9) 2 +/0 (dmg = double deprotonated dimethylglyoxime) at gold and mercury electrodes in six solvents to ascertain whether the solvent relaxation, dynamics provide a significant contribution to the solvent-dependent reaction rates. Unlike most systems examined previously with regard to solvent dynamical effects, a substantial component of the activation barrier for the cobalt clathrochelate couples is associated with inner-shell (reactant vibrational) distortions in addition to outer-shell (solvent) reorganization. The solvents examined: acetonitrile, acetone, methylene chloride, benzonitrile, nitrobenzene and propylene carbonate, were chosen so as to provide large (up to ca. 30-fold) variations in the frequency of barrier passage from overdamped solvent relaxation, v os. The relative barrier-crossing frequencies, v n extracted from the k ex values by correcting for the solvent-dependent activation barriers, show a marked dependence upon v os, indicating that overdamped solvent relaxation contributes significantly to the barrier-crossing frequency for these systems. Qualitative comparisons are made with recent theoretical descriptions of the combined influence of reactant vibrational and solvent relaxation to v n.