Geometric and electronic structure of a peroxomanganese(iii) complex supported by a scorpionate ligand

A monomeric Mn^II complex has been prepared with the facially-coordinating Tp^Ph2 ligand, (Tp^Ph2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate). The X-ray crystal structure shows three coordinating solvent molecules resulting in a six-coordinate complex with Mn-ligand bond lengths that are consistent with a high-spin Mn^II ion. Treatment of this Mn^II complex with excess KO₂ at room temperature resulted in the formation of a Mn^III-O₂ complex that is stable for several days at ambient conditions, allowing for the determination of the X-ray crystal structure of this intermediate. The electronic structure of this peroxomanganese(iii) adduct was examined by using electronic absorption, electron paramagnetic resonance (EPR), lowerature magnetic circular dichroism (MCD), and variableerature variable-field (VTVH) MCD spectroscopies. Density functional theory (DFT), time-dependent (TD)-DFT, and multireference ab initio CASSCF/NEVPT2 calculations were used to assign the electronic transitions and further investigate the electronic structure of the peroxomanganese(iii) species. The lowest ligand-field transition in the electronic absorption spectrum of the Mn^III-O₂ complex exhibits a blue shift in energy compared to other previously characterized peroxomanganese(iii) complexes that results from a large axial bond elongation, reducing the metal-ligand covalency and stabilizing the σ-antibonding Mn d_z² MO that is the donor MO for this transition.