Titania−Silica Materials from the Molecular Precursor Ti[OSi(O<SUP>t</SUP><SUP></SUP>Bu)<INF>3</INF>]<INF>4</INF>:  Selective Epoxidation Catalysts

The tris(tert-butoxy)siloxy complex Ti[OSi(O^tBu)<INF>3</INF>]<INF>4</INF> (<BO>1</BO>) was pyrolytically converted at low temperatures to the theoretical yield of homogeneous TiO<INF>2</INF>·4SiO<INF>2</INF> materials. <BO>1</BO> also serves as a soluble model for isolated, tetrahedral titanium atoms in a silica matrix. The solid-state transformation of <BO>1</BO> to TiO<INF>2</INF>·4SiO<INF>2</INF> affords a low-surface-area material (<BO>2a</BO>, 22 m² g^-1 for samples heated to 500 °C) consisting of roughly spherical particles with an average diameter of ca. 25 nm (by transmission electron microscopy). The solution-phase thermolysis of <BO>1</BO> affords a xerogel (<BO>2b</BO>) consisting of smaller primary particles (≤ca. 5 nm) and possessing a much higher surface area (552 m² g^-1 for samples heated to 500 °C). Acid catalysis of the pyrolytic conversion in solution affords a xerogel (<BO>2c</BO>) with a lower surface area (399 m² g^-1 for samples heated to 500 °C). Supercritical drying of the “wet-gel” in CO<INF>2</INF> affords an aerogel (<BO>2d</BO>) with a morphology similar to that of the xerogel and a slightly higher surface area (677 m² g^-1 for samples heated to 500 °C). The TiO<INF>2</INF>·4SiO<INF>2</INF> materials are amorphous as initially formed, and subsequent crystallizations of anatase (<BO>2a</BO>, <BO>2b</BO>), rutile, and cristobalite (<BO>2a</BO>) occur at relatively high temperatures. <BO>1</BO> is a homogeneous catalyst for the selective epoxidation of cyclohexene to cyclohexene oxide (64% yield; 20 turnovers after 2 h) using cumene hydroperoxide (CHP) as the limiting reagent. <BO>2a</BO>−<BO>d</BO> are heterogeneous catalysts that produce 6% (<BO>2a</BO>)−49% (<BO>2d</BO>) yields of cyclohexene oxide after 2 h (with 0.05 g of catalyst). Supporting <BO>1</BO> on silica affords a highly active epoxidation catalyst that provides yields of 94% cyclohexene oxide after 2 h (with a standard amount of 0.05 g of catalyst). The molecular route to homogeneous titania−silica materials described here represents an alternative to the sol−gel method, affording materials with surface areas comparable to those of related aerogels and moderate-to-excellent activities for the epoxidation of cyclohexene.