Catalytic Reduction of Cyanide to Ammonia and Methane at a Mononuclear Fe Site

Nitrogenase enzymes catalyze nitrogen reduction (N₂R) to ammonia and also the reduction of non-native substrates, including the 7H⁺/6e^- reduction of cyanide to CH₄ and NH₃. CN^- and N₂ are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN^- and N₂ reduction. Here, we describe the catalytic reduction of CN^- to NH₃ and CH₄ by a highly selective (P₃^Si)Fe(CN) catalyst (P₃^Si represents a tris(phosphine)silyl ligand). Catalysis is driven in the presence of excess acid ([Ph₂NH₂]OTf) and reductant ((C₆H₆)₂Cr), with turnover as high as 73 demonstrated. This catalyst system is also modestly competent for N₂R and structurally related to other tris(phosphine)Fe-based N₂R catalysts. The choice of catalyst and reductant is important to observe high yields. Mechanistic studies elucidate several intermediates of CN^- reduction, including iron isocyanides (P₃^SiFeCNH^+/0) and terminal iron aminocarbynes (P₃^SiFeCNH₂^+/0). Aminocarbynes are isoelectronic to iron hydrazidos (Fe═N-NH₂^+/0), which have been invoked as selectivity-determining intermediates of N₂R (NH₃ versus N₂H₄ products). For the present CN^- reduction catalysis, reduction of aminocarbyne P₃^SiFeCNH₂⁺ is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P₃^SiFeCNMe₂), and associated computational studies, formation of a Fischer carbene (P₃^SiFeC(H)(NH₂)⁺) intermediate that is on path for either CH₄ and NH₃ (6 e^-) or CH₃NH₂ (4 e^-) products is proposed. From this carbene intermediate, pathways to the observed CH₄ and NH₃ products (distinct from CH₃NH₂ formation) are considered to compare and contrast the (likely) mechanism/s of CN^- and N₂ reduction.