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

Area of Science:

• Inorganic Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Manganese porphyrins are vital catalysts for oxidizing olefins and hydrocarbons.
• High-valent Mn-(di)oxo species are key intermediates, with [Mn(Porph)(X)] complexes as precursors.
• Understanding Mn-ligand interactions is crucial for elucidating catalyst properties.

Purpose of the Study:

• To investigate the spectroscopic properties of high-spin [Mn^III(TPP)X] complexes, focusing on [Mn^III(TPP)Cl].
• To elucidate the electronic interactions between the manganese center and the porphyrin ligand.
• To explain the unusual optical properties and Soret band splitting in these complexes.

Main Methods:

• Variable-temperature, variable-field magnetic circular dichroism (MCD) spectroscopy.
• Time-dependent density functional theory (TD-DFT) calculations for spectral band assignment.
• Synthesis and characterization of [Mn^III(TPP)X] complexes (X = F, Cl, I, Br).

Main Results:

• The optical spectrum of [Mn^III(TPP)Cl] exhibits complex features, including a prominent Soret band and broad absorption bands.
• Charge transfer (CT) transitions and porphyrin π → π* transitions were identified and assigned.
• Strong mixing between porphyrin A_2u(π) and Cl(p)_d orbitals explains the Soret band splitting.
• Heavier halide complexes show enhanced orbital mixing and larger Soret band splittings.

Conclusions:

• The splitting of the Soret band is attributed to strong orbital mixing between the porphyrin ligand and the halide ligand.
• This orbital interaction facilitates charge transfer character into the π → π* transitions, influencing the optical spectrum.
• The findings provide fundamental insights into the electronic structure and catalytic mechanisms of manganese porphyrin complexes.