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

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 [MnIII(TPP)­X] (X = F, Cl, I, Br) complexes with emphasis on the spectroscopic properties of [MnIII(TPP)­Cl], using variable-temperature variable-field magnetic circular dichroism spectroscopy and time-dependent density functional theory to help with band assignments. The optical properties of [MnIII(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 ) → dπ (CT(Cl,π)), Q band, and overlap-forbidden Cl­(p x/y )_dπ → 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 A1u⟨79⟩/A2u⟨81⟩ → dπ, CT(Cl,π/σ), and symmetry-forbidden CT character, mixed with the π → π*(0) transition. The strong d x 2–y 2 _B1g⟨80⟩ orbital interaction drives the ground-state MO mixing. Importantly, the splitting of the Soret band is explained by strong mixing of the porphyrin A2u(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.