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) CH₃CN-H₂O media, the parent complex [Ru(bpy)₂(sqphen)]⁺ (I⁺) coexists with its conjugate acid [Ru(bpy)₂(sqphenH)]²⁺ (HI²⁺): HI²⁺⇌ H⁺ + I⁺; (deprotonation constant, K^ox_a= 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)]⁺ (HI⁺_R) (deprotonation constant,K^red_a = 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 Zn²⁺, increased E_1/2 but decreased the chemical reduction rates. Only an insignificant solvent isotope effect (k_H2O/k_D2O) was noted. An increased percentage of CH₃CN 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⁺.