Hybrid Silsesquioxane/Benzoate Cu 7 -Complexes: Synthesis, Unique Cage Structure, and Catalytic Activity.

A series of phenylsilsesquioxane-benzoate heptacopper complexes 1 - 3 were synthesized and characterized by X-ray crystallography. Two parallel routes of toluene spontaneous oxidation (into benzyl alcohol and benzoate) assisted the formation of the cagelike structure 1 . A unique multi-ligation of copper ions (from ( i ) silsesquioxane, ( ii ) benzoate, ( iii ) benzyl alcohol, ( iv ) pyridine, ( v ) dimethyl-formamide and ( vi ) water ligands) was found in 1 . Directed self-assembly using benzoic acid as a reactant afforded complexes 2 - 3 with the same main structural features as for 1 , namely heptanuclear core coordinated by ( i ) two distorted pentameric cyclic silsesquioxane and ( ii ) four benzoate ligands, but featuring other solvate surroundings. Complex 3 was evaluated as a catalyst for the oxidation of alkanes to alkyl hydroperoxides and alcohols to ketones with hydrogen peroxide and tert-butyl hydroperoxide, respectively, at 50 °C in acetonitrile. The maximum yield of cyclohexane oxidation products as high as 32% was attained. The oxidation reaction results in a mixture of cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone. Upon the addition of triphenylphosphine, the cyclohexyl hydroperoxide is completely converted to cyclohexanol. The specific regio- and chemoselectivity in the oxidation of n -heptane and methylcyclohexane, respectively, indicate the involvement of of hydroxyl radicals. Complex 3 exhibits a high activity in the oxidation of alcohols.