Unveiling full mechanistic picture of Mo-catalysed nitrogen fixation driven by SmI₂-H₂O system

Ammonia is an essential chemical that underpins modern agriculture and industry, but the direct conversion of dinitrogen to ammonia under mild conditions remains highly challenging. To this end, molybdenum nitride complexes bearing 1,3-bis(di-tert-butylphosphinomethyl)benzimidazole-2-ylidene pincer ligands have been shown to catalyse ammonia formation from dinitrogen using samarium diiodide and water. However, the catalytic mechanism remains poorly understood owing to limited information on the key intermediates and the solution-state speciation of the samarium reductant. Here we show a full catalytic mechanism by combining mechanistic experiments and theoretical analyses. We isolate molybdenum methylimide and molybdenum methylamide complexes as stable analogues of catalytic intermediates, providing direct experimental evidence for the proposed catalytic pathway. Theoretical studies further suggest a plausible structure of the tetrahydrofuran-solvated samarium diiodide-water complex, and subsequent mechanistic analyses reveal that ammonia formation proceeds through proton-coupled electron transfer. Building on these experimental and theoretical findings, we identify the molybdenum-imide formation reaction as the most energy-demanding step. These findings provide a framework for understanding catalytic ammonia formation and inform future efforts to improve dinitrogen reduction catalysts.