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 [ i Pr 2 NN F6 ]Cu I -NH 3 ([Cu I ]-NH 3 ( 2 )) as a robust electrocatalyst for NH 3 oxidation in acetonitrile under homogeneous conditions. Complex 2 operates at a moderate overpotential (η = 700 mV) with a TOF max = 940 h -1 as determined from CV data in 1.3 M NH 3 -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 I ]-NH 3 forms {[Cu II ]-NH 3 } + ( 4 ), which undergoes deprotonation by excess NH 3 to form reactive copper(II)-amide ([Cu II ]-NH 2 , 6 ) unstable toward N-N bond formation to give the dinuclear hydrazine complex [Cu I ] 2 (μ-N 2 H 4 ). Electrochemical studies reveal that the diammine complex [Cu I ](NH 3 ) 2 ( 7 ) forms at high ammonia concentration as part of the {[Cu II ](NH 3 ) 2 } + /[Cu I ](NH 3 ) 2 redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of the four-coordinate {[Cu II ](NH 3 ) 2 } + ( 8 ) by NH 3 to give the copper(II) amide [Cu II ](NH 2 )(NH 3 ) ( 9 ) (Δ G = 31.7 kcal/mol) as compared to deprotonation of the three-coordinate {[Cu II ]-NH 3 } + by NH 3 to provide the reactive three-coordinate parent amide [Cu II ]-NH 2 (Δ G = 18.1 kcal/mol) susceptible to N-N coupling to form [Cu I ] 2 (μ-N 2 H 4 ) (Δ G = -11.8 kcal/mol).