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

Abstract

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.

Ab Initio Calculation of UV–vis Absorption of Parent Mg, Fe, Co, Ni, Cu, and Zn Metalloporphyrins

“…Density matrix renormalization group (DMRG), − ,, and, more recently, heat-bath configuration interaction (HCI) , algorithm have emerged as an approximate but efficient solver for expensive porphyrin active spaces. Nevertheless, recovering dynamical correlation energy in metalloporphyrins using active space-based methods has often resorted to multireference perturbation theory (MRPT) − due to the high cost of other methods. , Recently, multiconfigurational pair-density functional theory (MC-pDFT) was used for a low-cost recovery of dynamic correlation within active space methods. , Still, the application of ab initio methods to metalloporphyrins has been largely limited to the ground state and low-lying ligand field states of different spin multiplicities originating from a partially filled metal 3 d shell, whereas the assignment of the characteristic UV–vis features has mostly relied on time-dependent (TD) density functional theory (DFT), which however suffers from the following major drawbacks: Its results depend on exchange-correlation functional choice, ,− poorly represent charge transfer and doubly or higher excited states, , and struggle with multiconfigurational ground states of transition metal porphyrins. In contrast, multireference active space-based methods adeptly handle multiconfigurational ground states.…”

“…Therefore, we deemed it essential to examine the UV–vis features of a series of parent metalloporphyrins within the relativistic (scalar relativistic (SR) and spin–orbit (SO)) MRPT/RASSCF framework. Ab initio UV–vis calculations were performed with the two widely employed second order MRPT methods, the most popular CAS perturbation theory (CASPT2), and the more recently developed N -electron valence state perturbation theory (NEVPT2). , These methods have been previously applied to certain porphyrin systems. ,− The two are generally expected to produce excitation energies of similar quality. , However, for parent metalloporpyrins, our NEVPT2 results with RAS wave functions (NEVPT2/RASSCF) are superior to CASPT2 results with RAS wave functions (CASPT2/RASSCF) and yield much better excitation energies. They allow an assignment of observed absorption features in terms of an intuitive orbital-based description of the ground state and valence excited states and can serve as benchmarks.…”