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

Terminal iron nitrides (Fe equivalent to 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 Fe-IV, Fe-V, and Fe-VI. This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, (P3Fe)-Fe-B(N) (P-3(B)=tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the (P3Fe)-Fe-B-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a (2)A ground state electronic structure of this C-3-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 d(5)-configuration is stabilized by significant delocalization (approximate to 50 %) of the unpaired electron onto the axial boron and nitrogen ligands, with a majority of the spin residing on boron.

Automated summary

This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, which is proposed as an intermediate in iron-mediated nitrogen fixation. Electron spin resonance spectroscopy and DFT calculations reveal the electronic structure of this nitride species, with the majority of the spin residing on boron.