Characterization of the Earliest Intermediate of Fe-N Protonation: CW and Pulse EPR Detection of an Fe-NNH Species and Its Evolution to Fe-NNH.

Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N-to-NH conversion (NRR) mediated by synthetic iron complexes and relatedly as intermediates of NRR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR), and also of further downstream intermediates (e.g., Fe(NNH)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N species, supported by a bulky trisphosphinoborane ligand (P), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The H-hyperfine for PFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom ( a = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH)] in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH)] intermediate of NRR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N-to-NH conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated NRR in a synthetic system.