Electrocatalytic Oxidation of Formate by [Ni(P^R₂N^R′₂)₂(CH₃CN)]²⁺ Complexes

[Ni(P^R₂N^R′₂)₂(CH₃CN)]²⁺ complexes with R = Ph, R′ = 4-MeOPh or R = Cy, R′ = Ph , and a mixed-ligand [Ni(P^R₂N^R′₂)(P^R′′₂N^R′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.