title: Electrocatalytic oxidation of formate by [Ni(P(R)2N(R')2)2(CH3CN)]2+ complexes.
abstract: [Ni(P(R)(2)N(R')(2))(2)(CH(3)CN)](2+) complexes with R = Ph, R' = 4-MeOPh or R = Cy, R' = Ph , and a mixed-ligand [Ni(P(R)(2)N(R')(2))(P(R''(2))N(R'(2)))(CH(3)CN)](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 CO(2), 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 η(1)-OC(O)CH(3) 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 CO(2) 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.
authors.affiliation: Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
authors.forename: Brandon R
authors.initials: BR
authors.lastname: Galan
authors.forename: Julia
authors.initials: J
authors.lastname: Schöffel
authors.forename: John C
authors.initials: JC
authors.lastname: Linehan
authors.forename: Candace
authors.initials: C
authors.lastname: Seu
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Appel
authors.forename: John A S
authors.initials: JA
authors.lastname: Roberts
authors.forename: Monte L
authors.initials: ML
authors.lastname: Helm
authors.forename: Uriah J
authors.initials: UJ
authors.lastname: Kilgore
authors.forename: Jenny Y
authors.initials: JY
authors.lastname: Yang
authors.forename: Daniel L
authors.initials: DL
authors.lastname: DuBois
authors.forename: Clifford P
authors.initials: CP
authors.lastname: Kubiak