Abstract
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.