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(O(t)Bu)(3)](4) (TiSi4), ((i)PrO)Ti[OSi(O(t)Bu)(3)](3) (TiSi3), and ((t)BuO)(3)TiOSi(O(t)Bu)(3) (TiSi) react with the hydroxyl groups of amorphous Aerosil, mesoporous MCM-41, and SBA-15 via loss of HO(t)Bu and/or HOSi(O(t)Bu)(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. In addition, the high surface area mesoporous silicas (MCM-41 and SBA-15) are more effective than amorphous silica as supports for these catalysts. Supporting TiSi3 on the SBA-15 affords highly active cyclohexene epoxidation catalysts (0.25-1.77 wt % Ti loading) that provide turnover frequencies (TOFs) of 500-1500 h(-1) after 1 h (TOFs are reduced by about half after calcination). These results demonstrate that oxygen-rich siloxide complexes of titanium are useful as precursors to supported epoxidation catalysts.

The crystal structure of the layered Sr 3 Ru 2 O 7 system has been analyzed by neutron powder diffraction methods. The structure is formed by stacking two blocks of distorted SrRuO 3 perovskite along the c axis, interleaved with SrO layers. The neighboring corner-sharing octahedra in each double perovskite block are rotated with respect to each other about the vertical axis so that the Ru-O-Ru angle in the RuO 2 planes is about 165°rather than 180°. These rotations are correlated within each double perovskite block, but they are not correlated along the c axis, resulting in an intrinsic disorder that emphasizes the layered nature of this material. The resulting structure has the symmetry of space group Pban and lattice parameters aϭbϭ5.5016(1) Å and cϭ20.7194(5) Å at 295 K. Data taken to 9 K showed no evidence of any structural phase transitions occurring below room temperature. We also searched for the development of long-range magnetic order to temperatures as low as 1.6 K, but no evidence of either ferromagnetic or antiferromagnetic long-range order was observed, with an upper limit of 0.05 B for any possible ordered moment. This result contrasts with a reported ferromagnetic ordering at 104 K with an ordered Ru moment of 1.3 B , which we believe was due to a phase other than Sr 3 Ru 2 O 7 . We also searched for an induced moment, for applied fields up to 7 T, but did not observe any induced ferromagnetic moment within the same experimental limit. ͓S0163-1829͑98͒00638-9͔

Two novel tris(tert-butoxy)siloxy complexes of Pt(II) and Pt(IV) were prepared in high yields, (cod)Pt[OSi(O t Bu)3]2 (1; 87%; cod = 1,5-cyclooctadiene) and Me3Pt(tmeda)[OSi(O t Bu)3] (2; 81%; tmeda = N,N,N′,N′-tetramethylethylenediamine). The structures of these compounds were determined by multinuclear NMR spectroscopy and by single-crystal X-ray analysis. The thermolytic chemistry of 1 and 2 in the solid state was studied by thermogravimetric analysis. The thermal decomposition of these complexes resulted in the formation of Pt metal, with the elimination of HOSi(O t Bu)3. Precursors 1 and 2 react with the surface Si−OH groups of mesoporous SBA15 silica to generate surface-supported Pt centers. The coordination environments of the supported Pt centers in these new materials, termed Pt(II)SBA15 and Pt(IV)SBA15, were investigated using Fourier-transform infrared spectroscopy, X-ray absorption near-edge spectroscopy, and extended X-ray absorption fine structure analysis. These materials were also characterized using N2 porosimetry, powder X-ray diffraction and transmission electron microscopy. Comparisons with the molecular precursors 1 and 2 revealed many similarities, and the results are indicative of isolated Pt(II) and Pt(IV) centers. In addition, isolated Pt centers proved to be robust in inert atmosphere to 150−200 °C, which is similar to the decomposition temperatures of 1 and 2.

The tri(alkoxy)siloxy complexes MO[OSi(O t Bu) 3 ] 4 (1, M ) Mo and 2, M ) W) were prepared from MOCl 4 and LiOSi(O t Bu) 3 . Similarly, reactions of MO 2 Cl 2 (DME) with LiOSi(O t Bu) 3 afforded the new siloxide complexes MO 2 [OSi(O t Bu) 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 MoO 2). These compounds serve as soluble models for isolated molybdenum or tungsten atoms on a silica surface and were characterized by 1 H, 13 C, 29 Si, 95 Mo, and 183 W 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 MO 3 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(O t Bu) 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 H 2 O 2 as the oxidant.