Simple Molecular\nSynthetic Approach to Dinuclear Titanium\nSites in Ti-Containing Silica-Based Catalysts

Ti-containing silica-based catalysts are widely used\nfor various\norganic oxidation reactions. Recently, increasing attention has been\npaid to the catalytic role of dinuclear (SiO)<sub>3</sub>Ti–O–Ti(OSi)<sub>3</sub> sites in titanosilicate zeolite TS-1. A promising experimental\napproach for elucidating the active sites is to use model compounds\nwith dinuclear Ti sites. In this study, we report the synthesis and\ncatalytic properties of a μ-oxo-bridged dinuclear titanosiloxane\ncompound with a (SiO)<sub>3</sub>Ti–O–Ti(OSi)<sub>3</sub> site. A Ti-containing cage siloxane compound (<sup><i>i</i></sup>Bu<sub>7</sub>Si<sub>7</sub>O<sub>12</sub>)TiO<sup><i>i</i></sup>Pr (mononuclear Ti-polyhedral oligomeric silsesquioxane\n(POSS)) is prepared by the reaction of incompletely condensed POSS\nwith titanium tetraisopropoxide. Then, it is converted to the dinuclear\nspecies (<sup><i>i</i></sup>Bu<sub>7</sub>Si<sub>7</sub>O<sub>12</sub>)<sub>2</sub>Ti<sub>2</sub>(μ-O) (dinuclear Ti-POSS)\nby the simple hydrolytic condensation of the TiO<sup><i>i</i></sup>Pr groups. The addition of H<sub>2</sub>O<sub>2</sub> to dinuclear\nTi-POSS leads to the formation of a bishydroperoxo complex, as revealed\nby Ti K-edge X-ray absorption fine structure analysis. Dinuclear Ti-POSS\npossesses high catalytic activity in the cyclohexene epoxidation reaction,\nindicating that the (SiO)<sub>3</sub>Ti–O–Ti(OSi)<sub>3</sub> sites can function as efficient catalytically active sites.\nThese findings contribute to the comprehension of the catalytic properties\nexhibited by dinuclear titanium sites within titanosilicate zeolite\ncatalysts.