Synthesis and characterization of MO[OSi(OtBu)3]4 and MO2[OSiOtBu)3]2 (M = Mo, W):: Models for isolated oxo-molybdenum and -tungsten sites on silica and precursors to molybdena- and tungsta-silica materials

The tri(alkoxy)siloxy complexes MO[OSi((OBu)-Bu-t)(3)](4) (1, M = Mo and 2, M = W) were prepared from MOCl4 and LiOSi((OBu)-Bu-t). Similarly, reactions of MO2Cl2(DME) with LiOSi((OBu)-Bu-t)(3) afforded the new siloxide complexes MO2[OSi((OBu)-Bu-t)(3)](2) (3, M = Mo and 4, M = W), which are themally unstable at ambient temperature. More stable compounds were obtained by the crystallizations of 3 and 4 in a coordinating solvent, to form the ether adducts MoO2[OSi((OBu)-Bu-t)(3)](2)(THF) (3a) and WO2[OSi((OBU)-B-t)(3)](2)(DME) (4a). These compounds serve as soluble models for isolated molybdenum or tungsten atoms on a silica surface and were characterized by H-1, C-13, Si-29, Mo-95, and W-183 NMR, FT-Raman, FT-IR, and UV-vis spectroscopies. Compounds 1, 2, 3a, and 4a were used to prepare metal-oxide silica composites via the thermolytic molecular precursor method. The xerogels obtained from the thermolyses of 1, 2, 3a, and 4a in toluene contained mesoporosity with surface areas of 10, 230, 106, and 270 m(2) g(-1), respectively. Despite the high surface areas for most samples, these xerogels contain MO3 domains. Complexes 1 and 2 were also used to introduce molybdenum and tungsten sites, respectively, onto mesoporous SBA-15 silica via displacement of the -OSi((OBu)-Bu-t)(3) ligand for a siloxyl group from the silica surface. All molybdenum- and tungsten-containing systems were tested as catalysts for the epoxidation of cyclohexene using tert-butyl hydroperoxide (TBHP) or aqueous H2O2 as the oxidant.