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 ( λ max = 462 nm, ε 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 ( λ max = 540 nm, ε 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 H 2 O 2 , a partial reduction product of dioxygen (O 2 ).