Mechanistic studies of the selective reduction of ruthenium(III) containing trinuclear oxo complexes by l-ascorbic acid in aqueous solution

The reduction of the Ru(III) oxo-centred trinuclear acetate cations, [Ru3(μ3-O)(μ2-CH3CO2)6(H2O)3]+ and [Ru2Cr(μ3-O)(μ2-CH3CO2)6(H2O)3]+, by the biological reductant l-ascorbic acid was studied spectrophotometrically under pseudo first-order conditions over the ranges 3.05 ≤ pH ≤ 4.83 (acetate buffer), 15 °C ≤ θ ≤ 30 °C and at I = 0.5 mol dm−3 (NaClO4). The first electron transfer in the redox process resulted in mixed-valence species [Ru2M(μ3-O)(μ2-CH3CO2)6(H2O)3]0, where M = Ru or Cr, followed by the slow consecutive reduction of other Ru(III) ions. The kinetics of the formation of the mixed-valence species was studied in detail, and a mechanism in support of these data is proposed. The intricate mechanistic details of the subsequent reactions are unclear as the spectral characteristics of the species involved could not be resolved from those of the first intermediate. The final products, however, were found to be Ru(II) (and Cr(III) for the mixed-metal species) in acetate buffer. The electron-transfer mechanism has been proposed to be inner-sphere, as deduced from Marcus cross-relationship. In an aqueous acetate buffer at I = 1.0 mol dm−3 (NaClO4), the cyclic voltammograms of the complexes were found to be quasi-reversible and pH dependent and have values of 0.18 and 0.19 V (relative to SHE) at pH = 3.41 for the [Ru3(μ3-O)(μ2-CH3CO2)6(H2O)3]+ and [Ru2Cr(μ3-O)(μ2-CH3CO2)6(H2O)3]+ cations, respectively.