Novel features associated with the electrochemically driven bis(η 5-pentaphenylcyclopentadienyl)iron(II)–iron(III) redox transformation at an electrode–microcrystal–solvent (electrolyte) interface

Abstract

Electrochemical oxidation of microcrystals of the iron(II) compound, Fe(η 5-C 5Ph 5) 2 and reduction of the corresponding iron(III) [Fe(η 5-C 5Ph 5) 2]BF 4 salt, mechanically attached to graphite and gold electrodes placed in aqueous media and in a (70:30) water:acetonitrile solvent mixture containing electrolyte has been investigated by voltammetric, electrochemical quartz crystal microbalance, and micro-analytical techniques. When interconversion of Fe(η 5-C 5Ph 5) 2 to [Fe(η 5-C 5Ph 5) 2]X (X −=ClO 4 −, BF 4 −, Cl −, F −) and vice versa occurs at the microcrystal–electrode–aqueous electrolyte interface via redox cycling of the electrode potential, then the reaction can be summarised by the process [ Fe( η 5- C 5 Ph 5) 2] +[ X − ] ( solid) + e − ⇌Fe( η 5- C 5 Ph 5) 2( solid) + X − ( solution) However, when CH 3CN (in aqueous 0.1 M NaClO 4) is present at the interface, data obtained are consistent with co-insertion of the organic solvent into the structure to give formally the [Fe(η 5-C 5Ph 5) 2] 1+/0.5+/0 (solid) redox system containing interacting iron atoms in the solid structure. The formation of the new phase is voltammetrically associated with the conversion from the single chemically reversible one electron [Fe(η 5-C 5Ph 5) 2] +/0 process with a large separation in reduction and oxidation peak potentials ( E p red=385 mV, E p ox=980 mV) to two formally 0.5 electron processes with more closely spaced peak potentials (first 0.5 electron reduction: E p red=665 mV, E p ox=715 mV; second 0.5 electron reduction: E p red=545 mV, E p ox=610 mV). Mechanistic aspects of the substantial changes that are introduced by the incorporation of acetonitrile into the solid state structure are discussed.