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Electrocatalytic Oxidation of Formate by [Ni(PR2NR′2)2(CH3CN)]2+ Complexes

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posted on 17.08.2011, 00:00 by Brandon R. Galan, Julia Schöffel, John C. Linehan, Candace Seu, Aaron M. Appel, John A. S. Roberts, Monte L. Helm, Uriah J. Kilgore, Jenny Y. Yang, Daniel L. DuBois, Clifford P. Kubiak
[Ni(PR2NR′2)2(CH3CN)]2+ complexes with R = Ph, R′ = 4-MeOPh or R = Cy, R′ = Ph , and a mixed-ligand [Ni(PR2NR′2)(PR′′2NR′2)(CH3CN)]2+ with R = Cy, R′ = Ph, R′′ = Ph, have been synthesized and characterized by single-crystal X-ray crystallography. These and previously reported complexes are shown to be electrocatalysts for the oxidation of formate in solution to produce CO2, protons, and electrons, with rates that are first-order in catalyst and formate at formate concentrations below ∼0.04 M (34 equiv). At concentrations above ∼0.06 M formate (52 equiv), catalytic rates become nearly independent of formate concentration. For the catalysts studied, maximum observed turnover frequencies vary from <1.1 to 15.8 s–1 at room temperature, which are the highest rates yet reported for formate oxidation by homogeneous catalysts. These catalysts are the only base-metal electrocatalysts as well as the only homogeneous electrocatalysts reported to date for the oxidation of formate. An acetate complex demonstrating an η1-OC(O)CH3 binding mode to nickel has also been synthesized and characterized by single-crystal X-ray crystallography. Based on this structure and the electrochemical and spectroscopic data, a mechanistic scheme for electrocatalytic formate oxidation is proposed which involves formate binding followed by a rate-limiting proton and two-electron transfer step accompanied by CO2 liberation. The pendant amines have been demonstrated to be essential for electrocatalysis, as no activity toward formate oxidation was observed for the similar [Ni(depe)2]2+ (depe = 1,2-bis(diethylphosphino)ethane) complex.