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

The addition of an organic azide reagent to the square planar S = 1 complex (PNNP)FeII leads to sequential oxidation of the phosphine ligand substituents to phosphinimines. Control experiments suggest the intermediacy of an Fe-imido species, which is computationally predicted to adopt an FeIII-imidyl radical resonance form. [Display omitted] 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 Mössbauer spectroscopy and magnetic susceptibility measurements suggest a spin-state 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 ωB97X-D3/def2-SVP level of theory. Our computational analyses confirm that the driving force for this transformation is very favorable.