Elucidating the Electronic Structure of High-Spin [Mn^III(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 [Mn^III(TPP)X] (X = F, Cl, I, Br) complexes with emphasis on the spectroscopic properties of [Mn^III(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 [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) → d_π (CT^(Cl,π)), Q band, and overlap-forbidden Cl(p)_d_π → d_- 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_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)_d 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.