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(px/y) → dπ (CT(Cl,π)), Q band, and overlap-forbidden Cl(px/y)_dπ → dx2 -y2 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 dx2-y2 _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(pz)_dz2 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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry