Mechanistic insight into the reactions of hydride transfer versus hydrogen atom transfer by a transdioxoruthenium(VI) complex

Abstract

A mononuclear high-valent trans-dioxoruthenium(VI) complex, trans-[RuVI(TMC)(O)2]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was synthesized and characterized by various spectroscopic techniques and X-ray crystallography. Reactivity of the trans-[RuVI(TMC)(O)2]2+ complex was investigated in hydride transfer and hydrogen atom transfer reactions. The mechanism of hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues to trans-[RuVI(TMC)(O)2]2+, which proceeds via a protoncoupled electron transfer (PCET), followed by a rapid electron transfer (ET), has been proposed by the observations of a good linear correlation between log rate constants of trans- [RuVI(TMC)(O)2]2+ and p-chloranil (Cl4Q) and a large kinetic isotope effect (KIE) value of 13(1). In the case of the oxidation of alkyl hydrocarbons by the trans-[RuVI(TMC)(O)2]2+ complex, second-order rate constants were dependant on C-H bond dissociation energy (BDE) of substrates, and a large KIE value of 26(2) was obtained in the oxidation of xanthene and deuterated xanthene-d2 by the trans-[RuVI(TMC)(O)2]2+ complex, indicating that the C-H bond activation of alkyl hydrocarbons proceeds via an H-atom abstraction in the ratedetermining step.