Simultaneous electrochemical and quartz crystal microbalance studies of non-conducting microcrystalline particles of trans-Cr(CO)2(dpe)2 and trans-[Cr(CO)2(dpe)2]+ (dpe = Ph2PCH2CH2PPh2) attached to gold electrodes

Abstract

Simultaneous cyclic voltammetric double potential step and electrochemical quartz crystal microbalance (EQCM) experiments on water insoluble trans -Cr(CO) 2 (dpe) 2 and trans -[Cr(CO) 2 (dpe) 2 ]X (dpe = Ph 2 PCH 2 CH 2 PPh 2 , X = Cl − , Br − and I − ), attached as an array of microcrystals, have been employed to probe mechanistic aspects of the redox chemistry of the [ trans -Cr(CO) 2 (dpe) 2 ] +/0 process at the electrode |solid| solvent (electrolyte) interface in a variety of aqueous electrolytes. EQCM experiments show that the oxidation of solid trans -Cr(CO) 2 (dpe) 2 involves the slow incorporation of non-solvated anions from the electrolyte solution into the solid. Interestingly, on the reverse scan of cyclic voltammetric experiments, EQCM data reveal that some but not all the anions are rapidly expelled from the crystal lattice. Double potential step experiments with the neutral chromium compound confirm that the oxidation reaction is a relatively slow process. The conclusion reached from all experiments is that the reduction process predominantly expels the anions that are relatively close to the solid|solution interface. EQCM investigations of trans -[Cr(CO) 2 (dpe) 2 ]X compounds in electrolytes containing a different anion to that in the compound show that the anion originally in the salt is rapidly replaced by the anion in the aqueous electrolyte at open circuit potential, presumably via a rapid ion exchange process. The anion from the electrolyte is then expelled and incorporated into the solid during the reduction and oxidation steps respectively.