925411-67-2

Upstream product

• 925411-66-1 [(CH₃OC₆H₂(C(CH₃)3)(CH₂C₆H₂(O)(C(CH₃)3)CH₂)2C₆H₂(C(CH₃)3)O)Ti(OCH(CH₃)2)] 
• 791-31-1 triphenylhydroxysilane 

Relevant academic research and scientific papers

Structural assessment and catalytic consequences of the oxygen coordination environment in grafted Ti-calixarenes

Calixarene-Ti complexes were grafted onto SiO2 (0.18-0.24 Ti nm-2) to form isolated and accessible Ti centers persistently coordinated to multidentate calixarene ligands. Grafted Ti-tert-butylcalix[4] arenes gave Ti K-edge absorption spectra with pre-edge features at 4968.6-4968.9 eV, independently of Ti surface density and of their use in epoxidation catalysis. The structure and reactivity of grafted Ti-calix[4]arenes were weakly dependent on thermal treatment below 573 K, and the relative epoxidation rates of trans- and cis-alkenes showed that calixarene ligands did not restrict access to Ti centers more than corresponding calcined Ti-SiO2 materials. For all materials, 13C NMR and UV-visible spectroscopies confirmed the presence of Ti-O-Si connectivity and identical ligand-to-metal transitions. Grafted Ti-homooxacalix[3]arene complexes, however, gave weaker pre-edge features at higher energies (～4969.5 eV), consistent with greater Ti 3d occupancy and coordination numbers greater than four, and 20-fold lower cyclohexene epoxidation rate constants (per Ti) than on calix[4]arene-based materials. These different rates and near-edge spectra result from aldehyde formation caused by unimolecular cleavage of ether linkages in homooxacalix[3]arene ligands during grafting, leading to higher coordination and electron density at Ti centers. Materials based on tert-butylcalix[4]arene and homooxacalix[3]arenes led to similar epoxidation rates and near-edge spectra after calcination, consistent with the conversion of both materials to isolated Ti centers with identical structure. These materials provide a systematic approach for relating oxidation reactivity to Ti 3d occupancy, a descriptor of Lewis acid strength, and Ti coordination, because they provide Ti centers with varying electron density and coordination, but maintain accessible active centers with uniform structure and unrestricted access to reactants.

The role of outer-sphere surface acidity in alkene epoxidation catalyzed by calixarene-Ti(IV) complexes

Cooperativity between Bronsted acidic defect sites on oxide surfaces and Lewis acid catalyst sites consisting of grafted calixarene-Ti(IV) complexes is investigated for controlling epoxidation catalysis. Materials are synthesized that, regardless of the surface or calixarene substituent, demonstrate nearly identical UV-visible ligand-to-metal charge-transfer bands and Ti K-edge X-ray absorption near edge spectral features consistent with site-isolated, coordinatively unsaturated Ti(IV) atoms. Despite similar Ti frontier orbital energies demonstrated by these spectra, replacing a homogeneous triphenylsilanol ligand with a silanol on a SiO2 surface increases cyclohexene epoxidation rates with tert-butyl hydroperoxide 20-fold per Ti site. Supporting calixarene-Ti active sites on fully hydroxylated Al 2O3 or TiO2, which possess lower average surface hydroxyl pKa than that of SiO2, reduces catalytic rates 50-fold relative to SiO2. These effects are consistent with SiO2 surfaces balancing two competing factors that control epoxidation rates-equilibrated hydroperoxide binding at Ti, disfavored by stronger surface Bronsted acidity, and rate-limiting oxygen transfer from this intermediate to alkenes, favored by strongly H-bonding intermediates. These observations also imply that Ti-OSi rather than Ti-OCalix bonds are broken upon hydroperoxide binding to Ti in kinetically relevant steps, which is verified by the lack of a calixarene upper-rim substituent effect on epoxidation rate. The pronounced sensitivity of observed epoxidation rates to the support oxide, in the absence of changes to the Ti coordination environment, provides experimental evidence for the importance of outer-sphere H-bonding interactions for the exceptional epoxidation reactivity of titanium silicalite and related catalysts.