Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis.

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon-neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth-abundant transition metal complex, trans-[LFe^II(MeCN)₂][PF₆]₂, where L is a macrocyclic ligand bearing four N-heterocyclic carbene (NHC) donors. Using triarylaminium radical cations in MeCN, up to 182 turnovers of N₂ per Fe were obtained from chemical catalysis with an extremely low loading of the Fe catalyst (0.043 mM, 0.004 mol % catalyst). This chemical catalysis was successfully transitioned to mediated electrocatalysis for the oxidation of ammonia. Molecular electrocatalysis by the Fe catalyst and the mediator (p-MeOC₆H₄)₃N exhibited a catalytic half-wave potential (E_cat/2) of 0.18 V vs [Cp₂Fe]^+/0 in MeCN, and achieved 9.3 turnovers of N₂ at an applied potential of 0.20 V vs [Cp₂Fe]^+/0 at -20 °C in controlled-potential electrolysis, with a Faradaic efficiency of 75 %. Based on computational results, the catalyst undergoes sequential oxidation and deprotonation steps to form [LFe^IV(NH₂)₂]²⁺, and thereafter bimetallic coupling to form an N-N bond.