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 sixcoordinate 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 oneelectron-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