Oxidative catalysis by Mn 4O 46+ cubane complexes

Abstract

The oxidation of a variety of substrates (thioethers, hydrocarbons, alkenes, benyzl alcohol and benzaldehyde) by t BuOOH catalyzed by Mn 4O 4(O 2PPh 2) 6 ( 1) and Mn 4O 4(O 2P( p-MePh) 2) 6 ( 2) is reported. These reactions illustrate the first examples of oxidative catalysis using a manganese-oxo complex with a Mn 4O 4 cubane core. These uncharged complexes contain Mn ions in a mixed valence oxidation state, formally Mn 4(2III, 2IV), and are bridged by bulky diphenylphosphinate chelates across each of the six faces of the cube. Using this system, methyl phenyl sulfide is selectively mono-oxygenated to methyl phenyl sulfoxide with high catalytic efficiency, and no evidence for further oxidation to the thermodynamically preferred sulfone. Toluene is oxidized to a mixture of benzyl alcohol, benzaldehyde, and benzoic acid with high catalytic efficiencies. Lower catalytic efficiencies are observed in the oxidation of styrene to a mixture of styrene oxide and benzaldehyde, of cyclohexene to a mixture of cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-1-one, and of cyclohexane to a mixture of cyclohexanol and cyclohexanone. The observed product distribution from the oxidation of hydrocarbons has the characteristics of a free radical-based oxidation mechanism. However, the sulfoxidation and epoxidation activity of the 1 / t BuOOH system, as well as the observed steric preferences for less congested substrates, suggest that a metal-oxo centered oxidation mechanism is active in the reactions studied here. An intermediate species, characterized by a UV–VIS band centered at 610 nm is observed in all reaction mixtures, and forms upon reaction of 1 or 2 with t BuOOH . Preliminary evidence suggests this reactive intermediate may correspond to a Mn(V)O species. Kinetic studies suggest two pathways for oxidation: one involving an oxygen atom transfer (two-electron branch), and the other involving a hydrogen atom abstraction (one-electron branch).