Our currently limited understanding of the mechanism of biological dinitrogen fixation-that is, reduction of N2 to two equivalents of ammonia by addition of protons and electrons-motivates a sustained effort to prepare functional models of the nitrogenase MoFe cofactor. One plausible mechanism for N2 reduction is a so-called distal or Chatt-type cycle, in which successive addition of three hydrogen-atom equivalents to the distal N atom of a metal-bound dinitrogen molecule (M-N≡N) results in elimination of one equivalent of NH3 to yield an intermediate nitrido complex (M≡N). The latter is in turn converted to ammonia by reaction with three additional protons and electrons. Indeed, such a cycle has been suggested as operative for a mononuclear, molybdenum-based catalyst. The increasing evidence for substrate coordination at iron in the MoFe cofactor warrants the continued investigation of iron-based models in a related context.
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