Kinetics of the reduction of a tetra-µ-acetato-dirhodium cation by substituted 1,2- and 1,4-dihydroxybenzene compounds in aqueous perchlorate media

The kinetics of the reduction of the tetra-µ-acetato-dirhodium cation, [Rh2(O2CCH3)4(OH2)2]+([Rh2]+), by substituted 1,2- and 1,4-dihydroxybenzene compounds (H2Q) have been investigated in aqueous perchlorate media. The observed rate equation (below) is interpreted in terms of a rate-–D[Rh2+]/dt= 2(k1+k2K1/[H+])[Rh2+][H2Q] determining one-electron oxidation of H2Q or HQ– to a semiquinone radical intermediate. The rate constants for the cross-reactions are correlated with the semiquinone reduction potentials by means of the Marcus relationship. Self-exchange rate constants for the dihydroxybenzene/semiquinone and semiquinone/quinone couples are discussed in terms of the inner-sphere and solvent reorganization energies for electron exchange.