Formation, Characterization, and O–O Bond Activation of a Peroxomanganese(III) Complex Supported by a Cross-Clamped Cyclam Ligand

Although there have been reports describing the nucleophilic reactivity of peroxomanganese­(III) intermediates, as well as their conversion to high-valent oxo-bridged dimers, it remains a challenge to activate peroxomanganese­(III) species for conversion to high-valent, mononuclear manganese complexes. Herein, we report the generation, characterization, and activation of a peroxomanganese­(III) adduct supported by the cross-clamped, macrocyclic Me₂EBC ligand (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]­hexadecane). This ligand is known to support high-valent, mononuclear Mn^IV species with well-defined spectroscopic properties, which provides an opportunity to identify mononuclear Mn^IV products from O–O bond activation of the corresponding Mn^III–peroxo adduct. The peroxomanganese­(III) intermediate, [Mn^III(O₂)­(Me₂EBC)]⁺, was prepared at low-temperature by the addition of KO₂ to [Mn^II(Cl)₂(Me₂EBC)] in CH₂Cl₂, and this complex was characterized by electronic absorption, electron paramagnetic resonance (EPR), and Mn K-edge X-ray absorption (XAS) spectroscopies. The electronic structure of the [Mn^III(O₂)­(Me₂EBC)]⁺ intermediate was examined by density functional theory (DFT) and time-dependent (TD) DFT calculations. Detailed spectroscopic investigations of the decay products of [Mn^III(O₂)­(Me₂EBC)]⁺ revealed the presence of mononuclear Mn^III–hydroxo species or a mixture of mononuclear Mn^IV and Mn^III–hydroxo species. The nature of the observed decay products depended on the amount of KO₂ used to generate [Mn^III(O₂)­(Me₂EBC)]⁺. The Mn^III–hydroxo product was characterized by Mn K-edge XAS, and shifts in the pre-edge transition energies and intensities relative to [Mn^III(O₂)­(Me₂EBC)]⁺ provide a marker for differences in covalency between peroxo and nonperoxo ligands. To the best of our knowledge, this work represents the first observation of a mononuclear Mn^IV center upon decay of a nonporphyrinoid Mn^III–peroxo center