Spin polarization assisted facile C-H activation by an S=1 iron(iv)-bisimido complex: a comprehensive spectroscopic and theoretical investigation

High valent iron terminal imido species (Fe= NR) have been shown to be key reactive intermediates in C-H functionalization. However, the detailed structure-reactivity relationship in FeNR species derived from studies of structurally well-characterized high-valent FeNR complexes are still scarce, and the impact of imido N-substituents (electron-donating vs. electron-withdrawing) on their electronic structures and reactivities has not been thoroughly explored. In this study, we report spectroscopic and computational studies on a rare S = 1 iron(iv)-bisimido complex featuring trifluoromethyl groups on the imido N-substituents, [(IPr)Fe(NC(CF3)(2)Ph)(2)] (2), and two closely related S = 0 congeners bearing alkyl and aryl substituents, [(IPr)Fe(NC(CMe3)(2)Ph)(2)] (3) and [(IPr)Fe(NDipp)(2)] (1), respectively. Compared with 1 and 3, 2 exhibits a decreased FeNR bond covalency due to the electron-withdrawing and the steric effect of the N-substituents, which further leads to a pseudo doubly degenerate ground electronic structure and spin polarization induced beta spin density on the imido nitrogens. This unique electronic structure, which differs from those of the well-studied Fe(iv)-oxido complexes and many previously reported Fe(iv)-imido complexes, provides both kinetic and thermodynamic advantages for facile C-H activation, compared to the S = 0 counterparts.