Solvent and Electrolyte Effects on the Kinetics of Ferrocenium-Ferrocene Self-Exchange: A Reevaluation

Article abstract of DOI:10.1021/j100342a086

Rate constants, k_ex, for ferrocenium-ferrocene self-exchange have been evaulated by using the proton NMR line-broadening technique as a function of added electrolyte in acetonitrile, acetone, nitrobenzene, and methanol.Increasing the ionic strength over the range ca. 2 x 1E-4 - 0.25 M by adding tetraethylammonium hexafluorophosphate, tetrafluoroborate, or perchlorate salts yielded monotonic decreases in k_ex.These rate variations are relatively modest, typically 30 percent or less, and are substantially smaller than are anticipated on the basis of thermodynamic ion-pairing effects.Similar results were also obtained for cobaltocenium- cobaltocene and decamethyl(ferrocenium-ferrocene) self-exchange and for the addition of chloride and bromide anions in the latter case.However, the ferocenium-ferrocene k_ex values themselves deviate substantially (up to ca. 10-fold), particularly in nitrobenzene, methanol, and also dimethyl sulfoxide, from earlier published values obtained by using the same technique.These disparities are traced to systematic errors in the earlier measurements associated with uncertainties in the paramagnetic line widths.The new solvent-dependent k_ex data lead to a significant reevaluation of the factors controlling the electron-transfer dynamics for the ferrocenium-ferrocene system.They suggest that the barrier-crossing frequency is limited by donor-acceptor electronic coupling in addition to solvent polarization dynamics.This behavior is briefly compared and contrasted with that for other, more facile, metallocene redox couples.