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

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