ASPECTS OF THE ELECTROCHEMISTRY, ELECTROPOLYMERIZATION AND ELECTROCATALYTIC ACTIVITY OF RHODIUM HYDRIDE AND RELATED COMPLEXES WITH TERTIARY PHOSPHINES AND NITROGEN LIGANDS

Electrochemical characterization of a series of [Rh-III(X)(Y)(P)(2)(L)](+) complexes (X, Y = H or Cl) containing a 2,2'-bipyridine derivative as the ligand L and a tertiary phosphine (PPh(3) or PPh(2)Et) as the ligand P is described. Dichloro (X = Y = Cl) and monohydrido (X = Cl, Y = H) complexes are characterized by a bielectronic reduction of the metal centre associated with a chemical reaction liberating chloride anions, followed by one or two reversible nitrogen-ligand-localized one-electron reductions. In contrast, the Rh(III) oxidation state is strongly stabilized in dihydrido complexes which present the only two successive reversible one-electron reductions of the ligand L in the negative potential region. In addition, hydride complexes undergo an irreversible anodic process with proton loss. Electrochemical and P-31 nuclear magnetic resonance data demonstrate the formation of dihydrido complexes upon reduction of the corresponding monohydrido and dichloro complexes in the presence of a proton source (water or formic acid). This behaviour was observed in homogeneous solution as well as in polymer films. In fact, polymer-coated electrodes were readily synthesized by oxidative electropolymerization of complexes containing pyrrole-substituted 2,2'-bipyridine ligands. Rhodium(III) complex films were used for the electrocatalytic hydrogenation of unsaturated organic substrates in aqueous electrolytes.