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

The redox reactions of a pyrazine-bridged binuclear [(edta)RuIIIpzRuIII(edta)]2− (edta4− = ethylenediaminetetraacetate; pz = pyrazine) have been investigated spectrochemically and spectroelectrochemically for the first time. The kinetics of the reduction of [(edta)RuIIIpzRuIII(edta)]2− (RuIII–RuIII) 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 RuIII–RuIII was found to consist of two-steps involving the initial formation of the mixed-valence [(edta)RuIIpzRuIII(edta)]3− (RuII–RuIII) intermediate complex (λmax = 462 nm, ϵmax = 10 000 M−1 cm−1), which undergoes further reduction by ascorbic acid to produce the [(edta)RuIIpzRuII(edta)]4−(RuII–RuII) ultimate product complex (λmax = 540 nm, ϵmax = 20 700 M−1 cm−1). Our studies further revealed that the RuII–RuIII and RuII–RuII species are formed in the electrochemical reduction of the RuIII–RuIII complex at 0.0 and −0.4 V (vs. SHE), respectively. Formation of RuII–RuIII and RuII–RuII 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 RuII–RuII complex with molecular oxygen (O2) reveal the ability of the RuII–RuII species to effect the oxygen reduction reaction (ORR) leading to the formation of H2O2, a partial reduction product of dioxygen (O2).