Characterization of a Proposed Terminal Iron(III) Nitride Intermediate of Nitrogen Fixation Stabilized by a Trisphosphine‐Borane Ligand

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of Fe-mediated nitrogen fixation, and well-defined synthetic characterized in high oxidation states, including FeIV, FeV, FeVI. This study reports the generation low temperature characterization a terminally bound iron(III) nitride, P3BFe(N) (P3B=tris(o-diisopropylphosphinophenyl)borane), which is intermediate iron-mediated fixation by P3BFe-catalyst system. CW- pulse EPR spectroscopy (HYSCORE ENDOR), supported DFT calculations, help to define 2A ground state electronic structure this C3-symmetric nitride species, placing unpaired spin sigma orbital along B−Fe−N vector; distinct for an nitride. The unusual d5-configuration stabilized significant delocalization (≈50 %) electron onto axial boron ligands, with majority residing on boron.