Electrocatalytic alcohol oxidation by covalently immobilized ruthenium complex on carbon

Abstract

A dearth of discrete immobilized metal complexes exist that electrocatalytically oxidize methanol. Reported here is the covalent immobilization of a tris(2-pyridylmethyl)amine ruthenium complex [RuII(Cl)(DMSO)(ethynyl-TPA)]+ (ethynyl-TPA = (5-ethynyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine) to a glassy carbon (GC) electrode through a CuI catalyzed azide-alkyne cycloaddition (click) reaction between the ethynyl-TPA ligand and an azide derivatized carbon surface forming [RuII(Cl)(DMSO)(GC-click-TPA)]+. Following water substitution for DMSO and proton coupled electron transfer, [RuIV(O)(Cl)(GC-click-TPA)]+ electrooxidizes alcohols, including methanol, efficiently relative to other immobilized metal complexes. A primary kinetic isotope effect suggests rate-limiting Cα-H bond cleavage of benzyl alcohol. Approximately 40% of the [RuII(Cl)(DMSO)(GC-click-TPA)]+ undergoes the DMSO for water exchange to form an active oxidant, consistent with the 40% distribution of the more labile Cl-cis-amine isomer before immobilization. Using the benchmark of benzyl alcohol electrocatalytic oxidation, [RuIV(O)(Cl)(GC-click-TPA)]+ operates at ca. 250 mV lower overpotential, with a 15% increase in faradaic efficiency, and at least an order of magnitude increase in average turnover frequency (0.7 s−1 TOFavg) compared to the previously best immobilized discrete ruthenium complexes.