Solvent-controlled synthesis of tetranuclear cage-like copper() silsesquioxanes. Remarkable features of the cage structures and their high catalytic activity in oxidation with peroxides

Two principally different in their molecular architecture isomeric tetranuclear copper( ii ) silsesquioxanes, "Globule"-like compound [(PhSiO 1.5 ) 12 (CuO) 4 (NaO 0.5 ) 4 ] ( 1 ) and "Sandwich"-like derivative [(PhSiO 1.5 ) 6 (CuO) 4 (NaO 0.5 ) 4 (PhSiO 1.5 ) 6 ] ( 2 ), were synthesized by the partial cleavage of polymeric copper( ii ) silsesquioxane [(PhSiO 1.5 ) 2 (CuO)] n by tetraphenylcyclotetrasiloxanolate. The route leading to the formation of either 1 or 2 entirely depends on the nature and composition of the solvent used for this reaction. Thus, the process in an ethanol-1-butanol solution gives compound 1 . When a 1,4-dioxane-methanol mixture was used, compound 2 was prepared. The structures and unusual crystal packing of the cages were confirmed by the X-ray studies. It has been found that the reaction of benzene with H 2 O 2 in acetonitrile solution at 50 °C catalyzed by 1 requires addition of trifluoroacetic acid (TFA) in low concentration and gives phenol with a turnover number (TON) of 250 after 3 h. The initial reaction rate W 0 linearly depends on the concentration of catalyst 2 . The oxidation of 1-phenylethanol to acetophenone with hydrogen peroxide catalyzed by complex 1 in the presence of TFA is not efficient. In contrast, 1 exhibited excellent activity in the oxidation with tert -butyl hydroperoxide (TBHP) in the absence of any acid (the yield of acetophenone was close to the quantitative, TON attained 475 after 2 h). A kinetic study of this reaction led to the conclusion that the process occurs with the participation of radicals tert -BuO&z.rad; produced in the Cu-promoted decomposition of TBHP. The mode of dependence of W 0 on the initial concentration of TBHP indicates the formation of an intermediate adduct between the catalyst 1 and TBHP (characterized by the equilibrium constant K 1 2 M −1 for the conditions of conducted experiments) followed by subsequent decomposition of the adduct ( k 2 0.2 s −1 ) to generate an intermediate species tert -BuO&z.rad; which induces the alcohol oxidation. Compounds 1 and 2 catalyze oxidation with H 2 O 2 or TBHP.