Electrocatalytic Ammonia Oxidation by a Low-Coordinate Copper Complex.

Abstract

Molecular catalysts for ammonia oxidation to dinitrogen represent enabling components to utilize ammonia as a fuel and/or source of hydrogen. Ammonia oxidation requires not only the breaking of multiple strong N-H bonds but also controlled N-N bond formation. We report a novel β-diketiminato copper complex [iPr2NNF6]CuI-NH3 ([CuI]-NH3 (2)) as a robust electrocatalyst for NH3 oxidation in acetonitrile under homogeneous conditions. Complex 2 operates at a moderate overpotential (η = 700 mV) with a TOFmax = 940 h-1 as determined from CV data in 1.3 M NH3-MeCN solvent. Prolonged (>5 h) controlled potential electrolysis (CPE) reveals the stability and robustness of the catalyst under electrocatalytic conditions. Detailed mechanistic investigations indicate that electrochemical oxidation of [CuI]-NH3 forms {[CuII]-NH3}+ (4), which undergoes deprotonation by excess NH3 to form reactive copper(II)-amide ([CuII]-NH2, 6) unstable toward N-N bond formation to give the dinuclear hydrazine complex [CuI]2(μ-N2H4). Electrochemical studies reveal that the diammine complex [CuI](NH3)2 (7) forms at high ammonia concentration as part of the {[CuII](NH3)2}+/[CuI](NH3)2 redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of the four-coordinate {[CuII](NH3)2}+ (8) by NH3 to give the copper(II) amide [CuII](NH2)(NH3) (9) (ΔG = 31.7 kcal/mol) as compared to deprotonation of the three-coordinate {[CuII]-NH3}+ by NH3 to provide the reactive three-coordinate parent amide [CuII]-NH2 (ΔG = 18.1 kcal/mol) susceptible to N-N coupling to form [CuI]2(μ-N2H4) (ΔG = -11.8 kcal/mol).