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

Electrochemical oxidation of microcrystals of the iron(II) compound, Fe(eta (5)-C5Ph5), and reduction of the corresponding iron(III) [Fe(eta(5)-C5Ph5)( 2)]BF4 salt, mechanically attached to graphite and gold electrodes placed i n aqueous media and in a (70:30) water:acetonitrile solvent mixture contain ing electrolyte has been investigated by voltammetric, electrochemical quar tz crystal microbalance, and micro-analytical techniques. When interconvers ion of Fe(eta(5)-C5Ph5)(2) to [Fe(eta(5)-C5Ph5)(2)]X (X- = ClO4-, BF4-, Cl- , F-) and vice versa occurs at the microcrystal-eleotrode-aqueous electroly te interface via redox cycling of the electrode potential, then the reactio n can be summarised by the process [Fe(eta(5)-C5Ph5)(2)] + [X-]((solid)) e(-) reversible arrow Fe(eta(5)-C5Ph5)(2(solid)) + X (-)((solulion)) Howeve r, when CH,CN tin aqueous 0.1 M NaClO4 is present at the interface, data ob tained are consistent with co-insertion of the organic solvent into the str ucture to give formally the [Fe(eta(5)-C5Ph5)(2)]((solid))(1 + /0.5) stem c ontaining interacting iron atoms in the solid structure. The formation of t he new phase is voltammetrically associated with the conversion from the si ngle chemically reversible one electron [Fe(eta(5)-C5Ph5)(2)](+ /O) process with a large separation in reduction and oxidation peak potentials (E-p(re d) = 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 struct ure are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.