Solvent dynamical effects in electron transfer: evaluation of electronic matrix coupling elements for metallocene self-exchange reactions

Abstract

Abstract : The functional dependence of the rate constants for self exchange, k sub ex, for a series of metallocene redox couples to solvent-induced variations in the nuclear frequency factor, nu, engendered by alterations in the longitudinal solvent relaxation time, tau sub L, are utilized to deduce values of the electronic matrix coupling element, H12, for electron exchange. The analysis exploits the sensitivity of the k sub ex - 1/tau sub L dependence to the degree of reaction adiabaticity and hence H12 for a given electron-exchange reaction. Six metallocene couples are examined: Cp2Co+/o, Cp2Fe+/o (Cp = cyclopentadienyl) and the decamethyl derivatives Cp2Co+/o and Cp2Fe+/o scrutinized previously, additional solvent-dependent k sub ex values for carboxymethyl (cobaltocenium-cobaltocene) (Cp(e)Z Co+/o, e= 'ester') and hydroxymethyl (ferrocenium-ferrocene) (HMFc+/o.) Kinetic data are examined in 15 solvents, including 11 'debye' solvents for which it is anticipated that is proportioned to 1/tau sub L. Corrections to k sub ex for the solvent-dependent variations in the barrier height were obtained by corresponding measurements of the optical electron-transfer energies for the related binuclear complex biferrocenylacetylene, yielding 'barrier-corrected' rate constants, k sub ex. The relationship between H 12 superscript o and metallocene electronic structure is briefly discussed. The analysis also enables effective solvent relaxation times for adiabatic barrier crossing in non-Debye media, including primary alcohols, to be extracted.