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

Nitrogenase enzymes catalyze nitrogen reduction (N 2 R) to ammonia and also the reduction of non-native substrates, including the 7H + /6e - reduction of cyanide to CH 4 and NH 3 . CN - and N 2 are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN - and N 2 reduction. Here, we describe the catalytic reduction of CN - to NH 3 and CH 4 by a highly selective (P 3 Si )Fe(CN) catalyst (P 3 Si represents a tris(phosphine)silyl ligand). Catalysis is driven in the presence of excess acid ([Ph 2 NH 2 ]OTf) and reductant ((C 6 H 6 ) 2 Cr), with turnover as high as 73 demonstrated. This catalyst system is also modestly competent for N 2 R and structurally related to other tris(phosphine)Fe-based N 2 R catalysts. The choice of catalyst and reductant is important to observe high yields. Mechanistic studies elucidate several intermediates of CN - reduction, including iron isocyanides (P 3 Si FeCNH +/0 ) and terminal iron aminocarbynes (P 3 Si FeCNH 2 +/0 ). Aminocarbynes are isoelectronic to iron hydrazidos (Fe═N-NH 2 +/0 ), which have been invoked as selectivity-determining intermediates of N 2 R (NH 3 versus N 2 H 4 products). For the present CN - reduction catalysis, reduction of aminocarbyne P 3 Si FeCNH 2 + is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P 3 Si FeCNMe 2 ), and associated computational studies, formation of a Fischer carbene (P 3 Si FeC(H)(NH 2 ) + ) intermediate that is on path for either CH 4 and NH 3 (6 e - ) or CH 3 NH 2 (4 e - ) products is proposed. From this carbene intermediate, pathways to the observed CH 4 and NH 3 products (distinct from CH 3 NH 2 formation) are considered to compare and contrast the (likely) mechanism/s of CN - and N 2 reduction.