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 (P3Si)Fe(CN) catalyst (P3Si 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 (P3SiFeCNH+/0) and terminal iron aminocarbynes (P3SiFeCNH2+/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 P3SiFeCNH2+ is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P3SiFeCNMe2), and associated computational studies, formation of a Fischer carbene (P3SiFeC(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.

Catalytic transfer hydrogenation of N2 to NH3 via a photoredox catalysis strategy.

Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven transfer hydrogenations toward deep (>2 e-) reductions of small molecules are attractive compared to using harsh chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (N2RR), where a synthetic photocatalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH2) and related organic structures can behave as 2 e-/2 H+ photoreductants, we show here that, when partnered with a suitable catalyst (Mo) under blue light irradiation, HEH2 facilitates delivery of successive H2 equivalents for the 6 e-/6 H+ catalytic reduction of N2 to NH3; this catalysis is enhanced by addition of a photoredox catalyst (Ir). Reductions of additional substrates (nitrate and acetylene) are also described.