Stability and electrocatalytic activity of the oxo-bridged dimer [(bpy)2(H2O)RuORu(OH2)(bpy)2]4+ in basic solutions

Abstract Rotating disk electrode and cyclic voltammetry experiments with glassy carbon electrodes were applied to study the electrocatalytic properties of the oxo-bridged dimer, [(bpy) 2 (H 2 O)-Ru III ORu III (H 2 O)(bpy) 2 ] 4+ , I , (bpy = 2,2′-bipyridine) in basic solutions. When generated by electro-oxidation of I , the Ru(IV)Ru(V) form of the dimer electrocatalytically oxidizes alcohols, sugars and amino acids in reactions that the first order in catalyst and in substrate. The rate constants, k cat , for oxidation of glycine, L(+)arginine, D(+)glucose and benzyl alcohol by the Ru(IV)Ru(V) form of I were measured in 0.2, 0.1, 0.1 and 1.0 M NaOH, respectively, as equal to 12.0±1.3, 11.2±0.8, 63.2±4.0 and 6.2±0.5 × 10 3 M −1 s −1 , respectively. The electrocatalytic reaction involves prior proton dissociation from the complex [(bpy) 2 (HO)Ru III ORu IV (OH)(bpy) 2 ] 3+ (p K a3 =13.7) in the course of electrogenerating the Ru(IV)Ru(V) dimer. The pseudo-first order rate constant of this step was measured as 20±5 s −1 in 1.0 M NaOH. The proton dissociation reaction is too rapid to limit the electrocatalytic rate of oxidation of glycine, which is pH independent from pH 13 to 14. The rate constant for the more rapidly oxidizable benzyl alcohol is, on the other hand, strongly pH dependent and this reaction can be quantitatively shown to be proton dissociation rate-limited. In experiments supporting the main electrocatalytic objectives, the complex I in homogeneous 0.1 M NaOH solutions was found to oxidize D(+)glucose with a rate constant of 1.4±0.1×10 −2 M −1 s −1 . The Ru(III)-Ru(IV) dimer spontaneously self-reduces with a pseudo-first order rate constant of 8.6×10 −4 s −1 in 1.0 M NaOH, apparently by oxidation of a bpy ligand.