Electrocatalytic Ammonia Oxidation by a Low-Coordinate Copper Complex.

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 [PrNN]Cu-NH ([Cu]-NH ()) as a robust electrocatalyst for NH oxidation in acetonitrile under homogeneous conditions. Complex operates at a moderate overpotential (η = 700 mV) with a TOF = 940 h as determined from CV data in 1.3 M NH-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 [Cu]-NH forms {[Cu]-NH} (), which undergoes deprotonation by excess NH to form reactive copper(II)-amide ([Cu]-NH, ) unstable toward N-N bond formation to give the dinuclear hydrazine complex [Cu](μ-NH). Electrochemical studies reveal that the diammine complex [Cu](NH) () forms at high ammonia concentration as part of the {[Cu](NH)}/[Cu](NH) redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of the four-coordinate {[Cu](NH)} () by NH to give the copper(II) amide [Cu](NH)(NH) () (Δ = 31.7 kcal/mol) as compared to deprotonation of the three-coordinate {[Cu]-NH} by NH to provide the reactive three-coordinate parent amide [Cu]-NH (Δ = 18.1 kcal/mol) susceptible to N-N coupling to form [Cu](μ-NH) (Δ = -11.8 kcal/mol).