ELECTROCHEMISTRY OF CHLORINATED FERROCENES - STABILITY OF CHLORINATED FERROCENIUM IONS

The cyclic voltammetries of 1,2,3,4,5-pentachloro- and decachloro-ferrocene have been studied in acetonitrile. The complexes undergo an irreversible two-electron oxidation consistent with an electrochemical-chemical-electrochemical mechanism at scan rates up to 10 V s-1. However, at the faster scan rates (up to 160 V s-1) available to electrodes of small radius and microelectrodes, chemically reversible one-electron oxidations to the chlorinated ferrocenium ions, [C10H10-xClxFe]+ are obtained under ambient conditions. The reversible [C10H10-xClxFe]+/0 couples when x = 10, 5, 2 and 1 are observed at +1.246, +0.774, +0.31 5 and +0.168 V vs. ferrocenium-ferrocene, respectively. A plot of E(f) vs. SIGMAsigma(p) (sigma(p) = Hammett para coefficient for the chloro substituent) shows that the neutral molecules are stabilised with respect to the corresponding ferrocenium cations by 0.16-0.12 V per Cl. The rate constants of decomposition of the [Fe(eta-C5Cl5)2] and [Fe(eta-C5H5)(eta-C5Cl5)]+ cations were calculated by both digital simulation and the method of Nicholson and Shain to be 40 +/- 20 and 200 +/- 50 s-1, respectively, at room temperature (ca. 20-degrees-C). The complexes [Fe(eta-C5H4Cl)2] and [Fe(eta-C5H5)(eta-C5H4Cl)] exhibit reversible oxidations at all scan rates down to 0.100 V s-1 under the same conditions. Both [Fe(eta-C5Cl5)2] and [Fe(eta-C5H5)(eta-C5Cl5)] undergo a series of irreversible two-electron reductions at potentials negative of -1.8 V, which lead to reductive dechlorination consistent with an electrochemical-chemical-electrochemical-chemical reaction.