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]Cu-NH3 ([Cu(I)]-NH3 (2)) as a robust electrocatalyst for NH3 oxidation in acetonitrile under homogeneous conditions. Complex 2 operates at a moderate overpotential (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 [Cu(I)]-NH3 forms {[Cu(II)]-NH3}+ (4) which undergoes deprotonation by excess NH3 to form reactive copper(II)-amide [Cu(II)]-NH2 (6) unstable towards N-N bond formation to give the dinuclear hydrazine complex [Cu(I)]2(mu-N2H4). Electrochemical studies reveal that the bisammine complex [Cu(I)](NH3)2 (7) 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 {[Cu(II)](NH3)2}+ (8) by NH3 to give the four-coordinate copper(II) amide [Cu(II)](NH2)(NH3) (9) (dG = 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 (dG = 18.1 kcal/mol) susceptible to N-N coupling to form [Cu(I)]2(mu-N2H4) (dG = -11.8 kcal/mol).

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

“…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 From the materials perspective, state-of-the-art electrocatalysts for AO are based on Pt metal at pH 14, typically operating at overpotentials in the range of 0.4 and 0.6 V. 27 In contrast, Ru metal has little to no activity for N 2 evolution from NH 3 oxidation, due to the formation of inactive N ads species on the surface. 28 However, a mix of RuO 2 and TiO 2 on a Ti electrode has been shown to promote AO in phosphate buffer with 12 mM NH 3 at pH 12.2, with an overpotentials of approximately 1.66 V. 29 In conclusion, we report the first example of molecular complex anchored on a conductive surface with proven electrocatalytic activity for AO to N 2 .…”

“…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+ 9 and [(bpyPy 2 Me)Fe(MeCN) 2 ] 9,10 (Table 1, 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 βdiketiminato 12 and 2-[(2,2′-bipyridin)-6-yl]propan-2-ol li-gands 26 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 1, entry 9).…”