Poster Presentation 21st International Conference on Biological Inorganic Chemistry 2025

Characterization of key intermediates in Fe-catalyzed cyanide reduction (#559)

Seungju Kang 1 , Jonas C Peters 1
  1. California Institute of Technology, Pasadena, CALIFORNIA, United States

Nitrogenase enzymes that catalyze nitrogen reduction to ammonia also exhibit reactivity towards multi-electron reduction of other small molecules including cyanide, acetylene, nitrous oxide, and carbon dioxide.1 Cyanide (CN-) is an interesting substrate due to its isoelectronic nature to N2 which may imply mechanistic parallels in their reduction processes, including the isolobalism between plausible intermediates. (M=CNH↔M=NNH, M≡CNH2↔M≡NNH2) Nitrogenases reduce CN- to produce NH3 and CH4 as major products along with methylamine and methylenimine through six, four, and two electron processes, respectively,2 which is reminiscent of differing selectivity in N2 reduction to form NH3 and N2H4 as 6e- and 4e- reduction products.3 Despite being a mechanistically rich target, cyanide reduction on well-defined molecular systems remains relatively underexplored. A recently reported catalytic cyanide reduction on a molecular iron site4 serves as a suitable platform for further mechanistic investigation in which [Fe]CHxNHy intermediates could be directly observed and studied. Herein we show that low-temperature reduction of a terminally bound [Fe]CNH+ species generates an S = 1/2 [Fe]CNH species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The 1H-hyperfine for [Fe]CNH which is diagnostic for the distally bound H atom (aiso = 7.8 MHz) suggests the isocyanide species as the earliest intermediate in cyanide reduction, reminding of its diazenido analogue in N2 reduction. ENDOR studies on [Fe]CN-/[Fe]CNH/[Fe]CNH2+ series show that the π electron density on carbon increases as a function of degree of protonation, denoting the increase in Fe-C multiple bonding character in the first half of the catalytic cycle. Studies towards later stage intermediates reveal that protonation of the aminocarbyne ([Fe]CNMe2) generates a cationic carbene species [Fe]CHNMe2+ instead of N protonation followed by release of NH3, which contrasts with the distal mechanism often invoked in N2 reduction. Further reduction of [Fe]CHNMe2+ results in the neutral carbene species [Fe]CHNMe2, which is characterized by CW and pulse EPR techniques. Characterization of novel intermediates presented herein provide insight into the mechanism of cyanide reduction and the electronic structure change throughout the transformation.

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