Oxidation of phosphine donors to phosphinimines via a putative Fe-imido intermediate

The formation of FeII phosphinimine complexes by treatment of an S = 1 (PNNP)FeII complex with MesN3 (Mes = 2,4,6-trimethylphenyl) is reported. The resulting compounds have been characterized structurally and spectroscopically. Zero-field 57Fe Mo center dot ssbauer spectroscopy and magnetic susceptibility measurements suggest a spinstate change occurs once both phosphines have been converted to phosphinmines, commensurate with the weaker field nature of phosphinimine ligands compared to phosphines. We propose that phosphine oxidation proceeds through nitrene transfer to Fe to afford a formally FeIV-imido intermediate that is more accurately predicted to have FeIII-imidyl radical character. Although no such intermediate could be isolated due to its instability and rapid phosphine oxidation, we support this mechanistic proposal with stoichiometric experiments and in silico analyses at the omega B97X-D3/def2-SVP level of theory. Our computational analyses confirm that the driving force for this transformation is very favorable.