Comparative Insight into Electronic Properties and Reactivities toward C–H Bond Activation by Iron(IV)–Nitrido, Iron(IV)–Oxo, and Iron(IV)–Sulfido Complexes: A Theoretical Investigation

A range of novel octahedral iron­(IV)–nitrido complexes with the TMC ligand (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) in the equatorial plane and one axial ligand trans to the nitrido have been designed theoretically, and a systematic comparative study of their geometries, electronic properties, and reactivities in hydrogen atom abstraction reactions regarding the iron­(IV)–oxo and −sulfido counterparts has been performed using density-functional theory methods. Further, the relative importance of the axial ligands on the reactivity of the iron­(IV)–nitrido systems is probed by sampling the reactions of CH₄ with [Fe^IVN­(TMC)­(L_ax)]^<i>n</i>+, (L_ax = none, CH₃CN, CF₃CO₂^–, N₃^–, Cl^–, NC^–, and SR^–). As we find, one hydrogen atom is abstracted from the methane by the iron­(IV)–nitrido species, leading to an Fe^III(N)–H moiety together with a carbon radical, similar to the cases by the iron­(IV)–oxo and −sulfido compounds. DFT calculations show that, unlike the well-known iron­(IV)–oxo species with the <i>S</i> = 1 ground state where two-state reactivity (TSR) was postulated to involve, the iron­(IV)–nitrido and −sulfido complexes stabilize in a high-spin (<i>S</i> = 2) quintet ground state, and they appear to proceed on the single-state reactivity <i>via</i> a dominantly and energetically favorable low-lying quintet spin surface in the H-abstraction reaction that enjoys the exchange-enhanced reactivity. It is further demonstrated that the iron­(IV)–nitrido complexes are capable of hydroxylating C–H bond of methane, and potential reactivities as good as the iron­(IV)–oxo and −sulfido species have been observed. Additionally, analysis of the axial ligand effect reveals that the reactivity of iron­(IV)–nitrido oxidants in the quintet state toward C–H bond activation enhances as the electron-donating ability of the axial ligand weakens