Oxygen-Atom Transfer Reactivity of Axially Ligated Mn(V)–Oxo Complexes: Evidence for Enhanced Electrophilic and Nucleophilic Pathways

Addition of anionic donors to the manganese­(V)–oxo corrolazine complex MnV(O)­(TBP8Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [MnV(O)­(TBP8Cz)­(X)]− complexes (X = F–, N3 –, OCN–) exhibit a ∼5 cm–1 downshift of the Mn–O vibrational mode relative to the parent MnV(O)­(TBP8Cz) complex as seen by resonance Raman spectroscopy. Product analysis shows that the oxidation of thioether substrates gives sulfoxide product, consistent with single OAT. A wide range of OAT reactivity is seen for the different axial ligands, with the following trend determined from a comparison of their second-order rate constants for sulfoxidation: five-coordinate ≈ thiocyanate ≈ nitrate < cyanate < azide < fluoride ≪ cyanide. This trend correlates with DFT calculations on the binding of the axial donors to the parent MnV(O)­(TBP8Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 derivatives and [MnV(O)­(TBP8Cz)­(X)]− (X = CN– or F–) as the oxidant, and unusual “V-shaped” Hammett plots were obtained. These results are rationalized based upon a change in mechanism that hinges on the ability of the [MnV(O)­(TBP8Cz)­(X)]− complexes to function as either an electrophilic or weak nucleophilic oxidant depending upon the nature of the para-X substituents. For comparison, the one-electron-oxidized cationic MnV(O)­(TBP8Cz•+) complex yielded a linear Hammett relationship for all substrates (ρ = −1.40), consistent with a straightforward electrophilic mechanism. This study provides new, fundamental insights regarding the influence of axial donors on high-valent MnV(O) porphyrinoid complexes.