| Abstract
| - MO[OSi(OtBu)3]4 and MO2[OSi(OtBu)3]2 (M = Mo, W) serve as soluble models for isolated molybdenum or tungsten atoms on a silica surface and as precursors to metal−oxide silica composites and metal-supported SBA-15 catalysts for the epoxidation of cyclohexene.
- The tri(alkoxy)siloxy complexes MO[OSi(OtBu)3]4 (1, M = Mo and 2, M = W) were prepared fromMOCl4 and LiOSi(OtBu)3. Similarly, reactions of MO2Cl2(DME) with LiOSi(OtBu)3 afforded the newsiloxide complexes MO2[OSi(OtBu)3]2 (3, M = Mo and 4, M = W), which are themally unstable atambient temperature. More stable compounds were obtained by the crystallizations of 3 and 4 in acoordinating solvent, to form the ether adducts MoO2[OSi(OtBu)3]2(THF) (3a) and WO2[OSi(OtBu)3]2(DME) (4a). These compounds serve as soluble models for isolated molybdenum or tungsten atoms ona silica surface and were characterized by 1H, 13C, 29Si, 95Mo, and 183W NMR, FT-Raman, FT-IR, andUV−vis spectroscopies. Compounds 1, 2, 3a, and 4a were used to prepare metal−oxide silica compositesvia 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 m2 g-1, respectively.Despite the high surface areas for most samples, these xerogels contain MO3 domains. Complexes 1 and2 were also used to introduce molybdenum and tungsten sites, respectively, onto mesoporous SBA-15silica via displacement of the −OSi(OtBu)3 ligand for a siloxyl group from the silica surface. Allmolybdenum- and tungsten-containing systems were tested as catalysts for the epoxidation of cyclohexeneusing tert-butyl hydroperoxide (TBHP) or aqueous H2O2 as the oxidant.
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