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.