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 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 similar to 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 approximate to thiocyanate approximate to nitrate < cyanate < azide < fluoride << cyanide. This trend correlates with DFT calculations of the binding of the axial donors to the parent Mn-V(O)(TBP(8)Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 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-electrode-oxidized cationic Mn-V(O)(TBP(8)Cz(center dot+)) complex yielded a linear Hammett relationship for all substrates (rho = -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.