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

摘要

Rate constants, K(ex), for ferrocenium-ferrocene self exchange have been evaluated 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. .0002 to 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% 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 ferrocenium-ferrocene K(ex) values themselves deviate substantially (up to ca. tenfold), particularly in nitrobenzene, methanol and also in dimethylsulfoxide, 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. Reprints. (aw)