Electrochemistry of chlorinated ferrocenes: stability of chlorinated ferrocenium ions

Abstract

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, [C₁₀H_10^–xCl_xFe]⁺ are obtained under ambient conditions. The reversible [C₁₀H_10^–xCl_xFe]^+/0 couples when x= 10,5,2 and 1 are observed at + 1.246, +0.774, +0.315 and +0.168 V vs. ferrocenium–ferrocene, respectively. A plot of E_fvs.Σσ_p(σ_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(η-C₅Cl₅)₂] and [Fe(η-C₅H₅)(η-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 °C). The complexes [Fe(η-C₅H₄Cl)₂] and [Fe(η-C₅H₅)(η-C₅H₄Cl)] exhibit reversible oxidations at all scan rates down to 0.100 V s^–1 under the same conditions. Both [Fe(η-C₅Cl₅)₂] and [Fe(η-C₅H₅)(η-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.