Redox reactions of a pyrazine-bridged Ru^III(edta) binuclear complex: spectrochemical, spectroelectrochemical and theoretical studies

The redox reactions of a pyrazine-bridged binuclear [(edta)Ru(III)pzRu(III)(edta)](2-) (edta(4-) = ethylenediaminetetraacetate; pz = pyrazine) have been investigated spectrochemically and spectroelectrochemically for the first time. The kinetics of the reduction of [(edta)Ru(III)pzRu(III)(edta)](2-) (Ru-III-Ru-III) with the ascorbic acid anion (HA(-)) was studied as a function of ascorbic concentration and temperature at a fixed pH 6.0. The overall reaction of Ru-III-Ru-III was found to consist of two-steps involving the initial formation of the mixed-valence [(edta)Ru(II)pzRu(III)(edta)](3-) (Ru-II-Ru-III) intermediate complex (lambda(max) = 462 nm, epsilon(max) = 10 000 M-1 cm(-1)), which undergoes further reduction by ascorbic acid to produce the [(edta)Ru(II)pzRu(II)(edta)](4-)(Ru-II-Ru-II) ultimate product complex (lambda(max) = 540 nm, epsilon(max) = 20 700 M-1 cm(-1)). Our studies further revealed that the Ru-II-Ru-III and Ru-II-Ru-II species are formed in the electrochemical reduction of the Ru-III-Ru-III complex at 0.0 and -0.4 V (vs. SHE), respectively. Formation of Ru-II-Ru-III and Ru-II-Ru-II was further corroborated by magnetic moment measurements and DFT calculations. Kinetic data and activation parameters are interpreted in terms of a mechanism involving rate-determining outer-sphere electron transfer between Ru(III) and the ascorbate monoanion (HA(-)) at pH 6.0. A detailed reaction mechanism in agreement with the spectral, spectro-electrochemical and kinetic data is presented. The results of the spectral and kinetic studies of the reaction of the Ru-II-Ru-II complex with molecular oxygen (O-2) reveal the ability of the Ru-II-Ru-II species to effect the oxygen reduction reaction (ORR) leading to the formation of H2O2, a partial reduction product of dioxygen (O-2).