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 [^<i>i</i>Pr₂NN_F6]Cu^I-NH₃ ([Cu^I]-NH₃ (<b>2</b>)) as a robust electrocatalyst for NH₃ oxidation in acetonitrile under homogeneous conditions. Complex <b>2</b> operates at a moderate overpotential (η = 700 mV) with a TOF_max = 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 [Cu^I]-NH₃ forms {[Cu^II]-NH₃}⁺ (<b>4</b>), which undergoes deprotonation by excess NH₃ to form reactive copper(II)-amide ([Cu^II]-NH₂, <b>6</b>) unstable toward N-N bond formation to give the dinuclear hydrazine complex [Cu^I]₂(μ-N₂H₄). Electrochemical studies reveal that the diammine complex [Cu^I](NH₃)₂ (<b>7</b>) forms at high ammonia concentration as part of the {[Cu^II](NH₃)₂}⁺/[Cu^I](NH₃)₂ redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of the four-coordinate {[Cu^II](NH₃)₂}⁺ (<b>8</b>) by NH₃ to give the copper(II) amide [Cu^II](NH₂)(NH₃) (<b>9</b>) (Δ<i>G</i> = 31.7 kcal/mol) as compared to deprotonation of the three-coordinate {[Cu^II]-NH₃}⁺ by NH₃ to provide the reactive three-coordinate parent amide [Cu^II]-NH₂ (Δ<i>G</i> = 18.1 kcal/mol) susceptible to N-N coupling to form [Cu^I]₂(μ-N₂H₄) (Δ<i>G</i> = -11.8 kcal/mol).