Elucidating the Electronic Structure of High-Spin [Mn^III(TPP)Cl] Using Magnetic Circular Dichroism Spectroscopy

Abstract

Manganese porphyrins are used as catalysts in the oxidation of olefins and nonactivated hydrocarbons. Key to these reactions are high-valent Mn−(di)oxo species, for which [Mn(Porph)(X)] serve as precursors. To elucidate their properties, it is crucial to understand the interaction of the Mn center with the porphyrin ligand. Our study focuses on simple high-spin [Mn III (TPP)X] (X = F, Cl, I, Br) complexes with emphasis on the spectroscopic properties of [Mn III (TPP)Cl], using variabletemperature variable-field magnetic circular dichroism spectroscopy and time-dependent density functional theory to help with band assignments. The optical properties of [Mn III (TPP)Cl] are complicated and unusual, with a Soret band showing a high-intensity feature at 21050 cm −1 and a broad band that spans 23200−31700 cm −1 . The 15000−18500 cm −1 region shows the Cl(p x/y ) → d π (CT (Cl,π) ), Q band, and overlap-forbidden Cl(p x/y )_d π → d x 2 −y 2 transitions that gain intensity from the strongly allowed π → π* (0) transition. The 20000−21000 cm −1 region displays the prominent pseudo A-type signal of the Soret band. The strongly absorbing features at 22500−28000 cm −1 exhibit A 1u ⟨79⟩/A 2u ⟨81⟩ → d π , CT (Cl,π/σ) , and symmetry-forbidden CT character, mixed with the π → π* (0) transition. The strong d x 2 −y 2 _B 1g ⟨80⟩ orbital interaction drives the ground-state MO mixing. Importantly, the splitting of the Soret band is explained by strong mixing of the porphyrin A 2u (π)⟨81⟩ and the Cl(p z )_d z 2 orbitals. Through this direct orbital pathway, the π → π* (0) transition acquires intrinsic metal-d → porphyrin CT character, where the π → π* (0) intensity is then transferred into the high-energy CT region of the optical spectrum. The heavier halide complexes support this conclusion and show enhanced orbital mixing and drastically increased Soret band splittings, where the 21050 cm −1 band shifts to lower energy and the high-energy features in the 23200−31700 cm −1 range increase further in intensity, compared to the chloro complex.

Reaction of a nitrosyl complex of Mn(ii)–porphyrinate with superoxide: NOD activity is favoured over SOD activity

“…13 Complex 1a , having low spin d 4 configuration, does not show any signal in X-band EPR in CH 2 Cl 2 solution even at 77 K. In UV-visible spectroscopy, the same solution displays a Soret band at 475 nm and Q bands at 530, 583 and 621 nm, which are typical of Mn( iii )–porphyrin complexes. 14 The crystal structure of 1a was reported earlier, which confirms that it is a five-coordinated square pyramidal complex with manganese at the center of the square plane formed by the porphyrin nitrogens and chloride as the axial ligand. 15 b…”

“…18 In a recent report from Galinato et al , it was reported that in the case of Mn( iii ) porphyrinates, the Soret band appeared around ∼475 nm. 14 Thus, the shift of the Soret band from 427 nm to 475 nm may be indicative of a one electron oxidation of complex 1 leading to the Mn( iii ) analogue.…”

A large negative magnetoresistance effect in semiconducting crystals composed of an octahedrally ligated phthalocyanine complex with high-spin manganese(iii)

“…With respect to the bond length, theoretical calculations predicted that the Mn–Cl distance would be notably larger for high-spin Mn III than for low-spin Mn III , and that the Mn–N distances would be insensitive to the Mn III spin state (Table 1). This suggests that the electronic configuration of Mn III in the high-spin state is (d yz , d zx ) 2 (d xy ) 1 (d z 2 ) 1 , as reported for porphyrin complexes, 27,28 and that in the low-spin state is (d xy ) 2 (d yz , d zx ) 2 . 19 The fact that the former possesses occupied d z 2 renders the Mn–Cl distance larger in the high-spin Mn III complex.…”

Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O2

“…We note that in the high-spin manganese(III) porphyrins, the normally intense Soret band splits into two less-intense bands; one of which is a prominent peak at lower energy (~470 nm), and the other one is a very broad band that occurs at higher energy (~380 nm). The ratios of bands well as the λ max values for these bands are particularly sensitive to the nature of the axial ligand(s) [ 32 , 33 ]. The X-ray structure of [(TPP)Mn III (MeTHF) 2 ]SbF 6 , as well as 1 H-NMR and IR data further support the identity of this product species as [(TPP)Mn III (MeTHF) 2 ] + (vide infra).…”