Kinetics of oxidation of ascorbic acid and 1,4-dihydroxybenzene by semiquinone radical bound to ruthenium(II)

Abstract

In 40% (v/v) MeOH-H₂O media, containing [H⁺] (0.001-0.038 mol dm^-3), the semiquinone (sq) radical, bound to Ru(II) in [Ru(bpy)₂(sq)]⁺1, oxidises ascorbic acid (H₂A) to dehydroascorbic acid (A), and 1,4-dihydroxybenzene (H₂Q) to p-benzoquinone (Q); 1 is itself reduced to [Ru(bpy)₂(Hcat)]⁺2H. The reactions are centred at sq not Ru(II). The sq/cat couple in 1 is reversible and its E_1/2 increases with increasing [H⁺]; rate of chemical reduction of 1 to 2H increases in parallel. Rate increases also with increasing mole percent of D₂O in the solvent suggesting a preliminary protonation equilibrium producing 1H, in which a H⁺ binds to the π-electron cloud of Ru(II)-bound sq. Under the experimental conditions, the kinetically significant species are 1H, H₂Q, H₂A and HA^-. The kinetic activity of HA^- ion is only ≈200 times more than that of H₂A. This testifies against a purely outer-sphere mechanism and suggests significant electronic interaction between the redox partners. Increased percentage of MeOH in the solvent decreases λ_max for the LMCT band; reaction rate for ascorbic acid decreases in parallel.