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

Abstract

High valent iron terminal imido species (FeNR) 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)₂Ph)₂] (2), and two closely related S = 0 congeners bearing alkyl and aryl substituents, [(IPr)Fe(NC(CMe₃)₂Ph)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 β 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.