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

[Image: see text] Addition of anionic donors to the manganese(V)–oxo corrolazine complex Mn(V)(O)(TBP(8)Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [Mn(V)(O)(TBP(8)Cz)(X)](−) complexes (X = F(–), N(3)(–), OCN(–)) exhibit a ∼5 cm(–1) downshift of the Mn–O vibrational mode relative to the parent Mn(V)(O)(TBP(8)Cz) 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 Mn(V)(O)(TBP(8)Cz) complex. A Hammett study was performed with p-X-C(6)H(4)SCH(3) derivatives and [Mn(V)(O)(TBP(8)Cz)(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 [Mn(V)(O)(TBP(8)Cz)(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 Mn(V)(O)(TBP(8)Cz(•+)) 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 Mn(V)(O) porphyrinoid complexes.