Tris(tert-butoxy)siloxy metal complexes as single-source molecular precursors to multicomponent oxides and surface-bound metal catalysts

Chapter 1. A molecular precursor approach involving simple grafting procedures was used to produce site-isolated titanium-supported epoxidation catalysts of high activity and selectivity. The tris( tert-butoxy)siloxy titanium complexes Ti[OSi(OtBu) 3]4 (TiSi4), (iPrO)Ti[OSi(O tBu)3]3 (TiSi3), and ( tBuO)3TiOSi(OtBu) 3 (TiSi) react with the hydroxyl groups of amorphous Aerosil, mesoporous MCM-41 and SBA-15 via loss of HOtBu and/or HOSi(OtBu)3 and introduction of titanium species onto the silica surface. Powder X-ray diffraction, nitrogen adsorption/desorption, infrared and diffuse reflectance ultraviolet spectroscopies were used to investigate the structures and chemical natures of the surface-bound titanium species. The titanium species exist mainly in isolated, tetrahedral coordination environments. Increasing the number of siloxide ligands in the molecular precursor decreases the amount of titanium that can be introduced this way, but also enhances the catalytic activity and selectivity for the epoxidation of cyclohexene with cumene hydroperoxide as oxidant. Chapter 2. The tri(alkoxy)siloxy complexes MO[OSi(O tBu)3]4 (1, M = Mo and 2, M = W) were prepared from MOCl4 and LiOSi(O tBu)3. Similarly, reactions of MO2Cl 2(DME) with LiOSi(OtBu)3 afforded the new siloxide complexes MO2[OSi(OtBu) 3]2 (3, M = Mo and 4, M = W), which are thermally 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(O tBu)3]2(THF) (3a) and WO2[OSi(OtBu)3] 2(DME) (4a). These compounds serve as soluble models for isolated molybdenum or tungsten atoms on a silica surface, and were characterized by 1H, 13C, 29Si, 95Mo, and 183W 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 m2g-1, respectively. Despite the high surface areas for most samples, these xerogels contain MO3 domains. Chapter 3. Reaction of [(diene)RhCl]2 with 2 equiv of KOSi(OtBu)3 afforded {(diene)Rh[mu-OSi(OtBu)3]} 2, where diene = cod (1) and nbd (2). Multinuclear NMR studies reveal that 1 and 2 have a dimeric structure with bridging tris(tert-butoxy)siloxy ligands. These dimers are folded along the O &cdots; O axis. Complexes 1 and 2 reacted with PR 3 (R = Me, Ph) to give monomeric products, the formula of which depends on the amount of PR3 added, ((diene)Rh[OSi(OtBu) 3](PR3) and Rh[OSi(OtBu) 3](PMe3)3). Chapter 4. The ( tert-butoxy)siloxy complex (cod)Pt[OSi(OtBu) 3]2 (1, cod = 1,5-cyclooctadiene) was prepared by reaction of (cod)PtCl2 with 2 equiv of KOSi(O tBu)3 and examined as a single-source precursor to Pt/SiO2 materials. Thermogravimetric analysis (TGA) of 1 under both nitrogen and oxygen reveals rather sharp, low-temperature conversions to the final ceramic compositions (onset temperatures: ca. 140°C under nitrogen; ca. 120°C under oxygen). (Abstract shortened by UMI.).