Influence of Ligand Architecture on Oxidation Reactions by High-Valent Nonheme Manganese Oxo Complexes Using Water as a Source of Oxygen

Mononuclear nonheme Mn-IV=O complexes with two isomers of a bispidine ligand have been synthesized and characterized by various spectroscopies and density functional theory (DFT). The Mn-IV=O complexes show reactivity in oxidation reactions (hydrogen-atom abstraction and sulfoxidation). Interestingly, one of the isomers (L-1) is significantly more reactive than the other (L-2), while in the corresponding Fe-IV=O based oxidation reactions the L-2-based system was previously found to be more reactive than the L-1-based catalyst. This inversion of reactivities is discussed on the basis of DFT and molecular mechanics (MM) model calculations, which indicate that the order of reactivities are primarily due to a switch of reaction channels (sigma versus pi) and concomitant steric effects.