Trimanganese complexes bearing bidentate nitrogen ligands as a highly efficient catalyst precursor in the epoxidation of alkenes

Abstract

A series of trinuclear manganese complexes coordinated with neutral bidentate nitrogen ligands, [Mn3L2(OAc)6], were prepared from manganese acetate and the corresponding ligands. Using peracetic acid as the oxidant, the air- and moisture-stable manganese clusters exhibited excellent catalytic activity and selectivity in the epoxidation of olefins under mild conditions. The highest activity was observed with a trinuclear complex containing a 2-pyridylimino ligand, [Mn3(ppei)2(OAc)6] (ppei = 2-pyridinal-1-phenylethylimine). With this system, the substrate scope was extremely wide to include terminal and electron-deficient double bonds of both aliphatic and aromatic alkenes. The high activity was undiminished under the reaction conditions even directly using a mixture of the pyridylimino ligands and manganese acetates, making this process more convenient. It was also observed that analogous trinuclear complexes, such as [Mn3(bipy)2(OAc)6] and [Mn3(phen)2(OAc)6], displayed excellent activities. While radical intermediacy was inferred from the product distribution, kinetic data revealed that the epoxidation is roughly first-order in manganese cluster precursor and oxidant, respectively, and zero-order in olefin. These results led us to propose that the trinuclear complexes [Mn3L2(OAc)6] serve as catalyst precursors that dissociate into monomeric species with the formulation of [MnL2(OAc)2] under the reaction conditions.