Oxidative catalysis by Mn4O46+ cubane complexes

The oxidation of a variety of substrates (thioethers, hydrocarbons, alkenes, benyzl alcohol and benzaldehyde) by (BuOOH)-Bu-t catalyzed by Mn4O4(O2PPh2)(6) (1) and Mn4O4(O2P(p-MePh)(2))(6) (2) is reported. These reactions illustrate the first examples of oxidative catalysis using a manganese-oxo complex with a Mn4O4 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/(BuOOH)-Bu-t 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 6 10 nm is observed in all reaction mixtures, and forms upon reaction of 1 or 2 with (BuOOH)-Bu-t. 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). (C) 2002 Elsevier Science B.V. All rights reserved.