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

Abstract

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