Electrocatalytic Alcohol Oxidation with Ruthenium Transfer Hydrogenation Catalysts

Octahedral ruthenium complexes [RuX­(CNN)­(dppb)] (1, X = Cl; 2, X = H; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis­(diphenylphosphino)­butane) are highly active for the transfer hydrogenation of ketones with isopropanol under ambient conditions. Turnover frequencies of 0.88 and 0.89 s^–1 are achieved at 25 °C using 0.1 mol % of 1 or 2, respectively, in the presence of 20 equiv of potassium t-butoxide relative to catalyst. Electrochemical studies reveal that the Ru–hydride 2 is oxidized at low potential (−0.80 V versus ferrocene/ferrocenium, Fc^0/+) via a chemically irreversible process with concomitant formation of dihydrogen. Complexes 1 and 2 are active for the electrooxidation of isopropanol in the presence of strong base (potassium t-butoxide) with an onset potential near −1 V versus Fc^0/+. By cyclic voltammetry, fast turnover frequencies of 3.2 and 4.8 s^–1 for isopropanol oxidation are achieved with 1 and 2, respectively. Controlled potential electrolysis studies confirm that the product of isopropanol electrooxidation is acetone, generated with a Faradaic efficiency of 94 ± 5%.