Characterization of an Fe≡N-NH₂ Intermediate Relevant to Catalytic N₂ Reduction to NH₃

The ability of certain transition metals to mediate the reduction of N₂ to NH₃ has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N₂ and mediate its protonation at one or more N atoms to furnish M(N_xH_y) species that can be characterized and, in turn, extrude NH₃. By contrast, the direct protonation of Fe-N₂ species to Fe(N_xH_y) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N₂)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N-NH₂]⁺, a doubly protonated hydrazido(2-) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N₂)][Na(12-crown-4)] can proceed along a distal or "Chatt-type" pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N-N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided. (Chemical Equation Presented).