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 Pr2NNF6]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).

Electrochemical Ammonia Oxidation with Molecular Catalysts

“…42 Warren and co-workers recently described a low-coordinate Cu(I) complex 10 that oxidizes NH 3 in MeCN. 43 Unless otherwise specified, potentials measured in water are versus NHE; in organic solvents, potentials are versus Fc +/0 . Note that AO data obtained under different electrochemical conditions cannot be directly compared.…”

Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH−π Interactions

“…Molecular complexes that have shown activity as AO electrocatalysts are mainly based on Ru, Fe, and Cu metals and have been reported in the homogeneous phase. Table gathers their relevant catalytic parameters. − From a technological perspective it is important to use solid-state anodes for catalytic AO, since it greatly simplifies device engineering. Therefore, it is paramount to develop the chemistry for anchoring these molecular catalysts onto conductive surfaces, an endeavor which up to now has not been extensively pursued.…”

“…Cu complexes with β-diketiminato and 2-[(2,2′-bipyridin)-6-yl]­propan-2-ol ligands have also been reported to be active for AO. The former has been shown to oxidize NH 3 to N 2 in acetonitrile at an E app = 0.0 V vs Fc +/0 with a TON of 18.2 over 26.5 h (Table , entry 9) …”

“…The former has been shown to oxidize NH 3 to N 2 in acetonitrile at an E app = 0.0 V vs Fc +/0 with a TON of 18.2 over 26.5 h ( Table 1 , entry 9). 12 …”

“…The field has been appreciably enriched with the recent contributions reporting two Fe complexes based on the N4 polypyridyl type of ligands [Fe­(TPA)­(NH 3 ) 2 ] 2+ and [(bpyPy 2 Me)­Fe­(MeCN) 2 ] , (Table , entries 6–7) reaching TONs of ≈150, with relatively high E app , in the range of 0.85 to 1.10 V vs Fc +/0 , which implies an overpotential of ≈1.6 to 1.9 V for AO ( E o = −0.81 V vs Fc +/0 ). Cu complexes with β-diketiminato and 2-[(2,2′-bipyridin)-6-yl]­propan-2-ol ligands have also been reported to be active for AO. The former has been shown to oxidize NH 3 to N 2 in acetonitrile at an E app = 0.0 V vs Fc +/0 with a TON of 18.2 over 26.5 h (Table , entry 9) …”