How Axial Coordination Regulates the Electronic Structure and C-H Amination Reactivity of Fe-Porphyrin-Nitrene?

Detailed electronic structure and its correlation with the intramolecular C-H amination reactivity of Fe-porphyrin-nitrene intermediates bearing different "axial" coordination have been investigated using multiconfigurational complete active space self-consistent field (CASSCF), N-electron valence perturbation theory (NEVPT2), and hybrid density functional theory (DFT-B3LYP) calculations. Three types of "axial" coordination, -OMe/-O(H)Me ( 1-Sul / 2-Sul ), -SMe/-S(H)Me ( 3-Sul/4-Sul ), and -NMeIm (MeIm = 3-methyl-imidazole) ( 5-Sul ) mimicking serine, cysteine, and histidine, respectively, along with no axial coordination ( 6-Sul ) have been considered to decipher how the "axial" coordination of different strengths regulates the electronic integrity of the Fe-N core and nitrene-transfer reactivity of Fe-porphyrin-nitrene intermediates. CASSCF-based natural orbitals reveal two distinct classes of electronic structures: Fe-nitrenes ( 1-Sul and 3-Sul ) with relatively stronger axial coordination (-OMe and -SMe) display "imidyl" nature and those ( 2-Sul, 4-Sul, and 6-Sul ) with weaker axial coordination (-O(H)Me, -S(H)Me and no axial coordination) exhibit "imido-like" character. A borderline between the two classes is also observed with NMeIm axial coordination ( 5-Sul ). Axial coordination of different strengths not only regulates the electronic structure but also modulates the Fe-3d orbital energies, as revealed through the d - d transition energies obtained by CASSCF/NEVPT2 calculations. The relatively lower energy of Fe-3 d z 2 orbital allows easy access to low-lying high-spin quintet states in the cases of weaker "axial" coordination ( 2-Sul, 4-Sul, and 6-Sul ), and the associated hydrogen atom transfer (HAT) reactivity appears to involve two-state triplet-quintet reactivity through minimum energy crossing point ( 3,5 MECP) between the spin states. In stark contrast, Fe-nitrenes with relatively stronger "axial" coordination ( 1-Sul and 3-Sul ) undergo triplet-only HAT reactivity. Overall, this in-depth electronic structure investigation and HAT reactivity evaluation reveal that the weaker axial coordination in Fe-porphyrin-nitrene complexes ( 2-Sul, 4-Sul, and 6-Sul ) can promote more efficient C-H oxidation through the quintet spin state.