Electrochemically Regenerable Hydrogen Atom Acceptors: Mediators in Electrocatalytic Alcohol Oxidation Reactions

A class of electrochemical mediators is described for electrocatalytic oxidation reactions that are catalyzed by metal hydrides. The octahedral ruthenium complex [Ru(acac)2(pyimN)] (RuIIIN 3) is shown to abstract a hydrogen atom from the ruthenium hydride [RuH(CNN)(dppb)] (RuH 2, CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) to generate [Ru(acac)2(pyimNH)] (RuIINH 4) and a reduced Ru(CNN)(dppb) complex. As RuIIIN 3 can be electrochemically regenerated from RuIINH 4 under appropriately basic and oxidizing conditions, we envisioned using RuIIIN 3 as a suitable electrochemically regenerable hydrogen atom acceptor in a tandem electrocatalytic cycle to reduce the overpotential for electrocatalytic alcohol oxidation by 450 mV. In the presence of a strong base, the previously reported alcohol oxidation electrocatalyst [RuX(CNN)(dppb)] (1, X = Cl, 2, X = H) catalyzes the oxidation of isopropanol to acetone with a turnover frequency (TOF) greater than 3 s–1 at −0.70 V versus ferrocenium/ferrocene (Fc+/0) in tetrahydrofuran. Upon adding RuIINH 4, RuCl 1 electrocatalytically oxidizes isopropanol to acetone at −1.20 V versus Fc+/0 in tetrahydrofuran with a TOF of ca. 1 s–1. Cyclic voltammetry and chemical hydrogen atom transfer studies suggest that the predominant electrocatalytic pathway involves hydrogen atom abstraction from RuH 2 by electrochemically generated RuIIIN 3.