posted on 2023-12-08, 18:06authored byMayank Mahajan, Bhaskar Mondal
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-3dz2 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 (3,5MECP) 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.