Kinetics of oxidation of quinol and ascorbic acid with the phen substituted semiquinone ligand, (5,6-dioxolene-1,10-phenanthroline-O,O), bound to the Ru II(bipy) 2 moiety

Abstract

In 10% (v/v) CH 3CN–H 2O media, the parent complex [Ru(bpy) 2(sqphen)] + ( I +) coexists with its conjugate acid [Ru(bpy) 2(sqphenH)] 2+ (H I 2+): H I 2 + ⇌ H + + I +; (deprotonation constant, K a ox = 0.01; (bpy = 2,2 ′-bipyridine; sqphen = 5,6-dioxolene-1,10-phenanthroline-O,O). Electrochemical reduction, and also chemical reductions with quinol and ascorbic acid, produced the corresponding catechol complex [Ru(bpy) 2(catphen)] ( I R ), which also in solution coexists with its conjugate acid, [Ru(bpy) 2(catphenH)] + ( H I R + ) (deprotonation constant, K a red = 0.002). Progressive increase in [ascorbic acid] led to rate saturation, indicating adduct formation (formation constant, Q = 990 ± 80 M −1). Added H +, and also redox-innocent Lewis acid Zn 2+, increased E 1/2 but decreased the chemical reduction rates. Only an insignificant solvent isotope effect ( k H 2 O / k D 2 O ) was noted. An increased percentage of CH 3CN in the solvent also retards the rate. [Ru(bpy) 2(sqphen)] + ( I +) reacts at a much slower rate than [Ru(bpy) 2sq] + (sq = the unsubstituted semiquinone). Phen in the substituted ligand sqphen is known to become a much weaker base (sqphen < phen); O,O-coordination to the Ru II(bpy) center further lowers the basicity of sqphen. Catphen (O,O) in I R, produced on reduction of I +, is a stronger base than the O,O-coordinated sqphen in I +.