Synthesis and Catalytic Activity of Titanium Silsesquioxane Frameworks as Models of Titanium Active Surface Sites of Controlled Nuclearity

Abstract

The synthesis and catalytic activity of some new titanium(IV) complexes with incompletely condensed Cy 6 Si 6 O 7 (OH) 4 and endo-Cy 8 Si 8 O 11 (OH) 2 (Cy = c-C 6 H 11 ) silsesquioxane cages have been carried out with the aim of investigating new Ti silsesquioxane frameworks as molecular models of titanium active sites of catalysts prepared by grafting Ti(IV) precursors on the surface or in a matrix of mesoporous silica. The silsesquioxane Cy 6 Si 6 O 7 (OH) 4 , which bears four potentially reactive OH groups, when reacted with TiCpCl 3 (Cp = η 5 -C 5 H 5 ) affords the new species [Cy 6 Si 6 O 12 Ti 2 (Cp) 2 ], with two titanium atoms connected through one μ-oxygen bridge, as confirmed by a XANES and EXAFS spectroscopic investigation. By reaction of endo-Cy 8 Si 8 O 11 (OH) 2 with [Ti(o-C 6 H 4 O 2 )-(OCH 2 CH 3 ) 2 ] the new complex [Cy 8 Si 8 O 11 O 2 ] 2 Ti was obtained, which exhibits a tetrapodal coordination of Ti, as shown by a X-ray diffraction investigation. The catalytic activity of these two new Ti silsesquioxane complexes was investigated for the catalytic epoxidation of cyclohexene with tert-butyl hydroperoxide and compared with the catalytic activity of the known [Cy 7 Si 7 O 12 TiCp], widely used to model very active tripodal titanium single sites. It appears that a tripodal coordination of Ti sites facilitates the catalytic activity that is in contrast quite low for the two new complexes investigated in this work, probably due to the higher stability toward dissociation to generate a tripodal coordination of the Ti-O-Ti bond and to the steric hindrance of Ti tetrapodal coordination.

Cyclopentadienyl–Silsesquioxane Titanium Catalysts: Factors Affecting Their Formation and Activity in Olefin Epoxidation with Aqueous Hydrogen Peroxide

“…Consistently, 29 Si NMR spectra of these complexes show sets of 3 ( 5 ), 5 ( 6 ), and 7 ( 7 and 8 ) resonances, indicating a C 3 v , C s , and C 1 symmetry disposition, respectively. In addition, the absence of resonances above –60 ppm confirms the coordination of all three silenic oxygen atoms after deprotonation . In contrast to the NMR silicon data, the 1 H NMR and 13 C NMR spectroscopic data are much less informative because of the large number of similar organic groups present, which causes the overlap of many of their resonances, preventing us from obtaining relevant structural information about these species.…”

“…In addition, the absence of resonances above -60 ppm confirms the coordination of all three silenic oxygen atoms after deprotonation. [24][25][26] In contrast to the NMR silicon data, the 1 H NMR and 13 C NMR spectroscopic data are much less informative because of the large number of similar organic groups present, which causes the overlap of many of their resonances, preventing us from obtaining relevant structural information about these species. Nevertheless, the complete disappearance of the signals ascribed to the silanol protons (δ range ≈ 5.50-6.50 ppm) [26,27] corroborates the deprotonation of all of them.…”

Synthesis, reactivity and X-ray crystal structure of tris(pentafluorophenyl)silanol (C6F5)3SiOH

“…The investigation on the structure and reactivity of silica-anchored organometallic species has been improved in the last two decades also through the complementary investigation on the synthesis, structure and reactivity of molecular models bearing silanolate ligands which mimic the silanol groups of the silica surface [1][2][3][4][5][6][7]. For instance, some of us reported evidence for the peculiar stabilization of the rhenium pentacarbonyl species [Re(CO) 5 OSi≡] on the silica surface, but the synthesis of its molecular model [Re(CO) 5 OSiR 3 ] was hampered by the fact that nucleophiles, such as Me 3 SiO -, are exceptionally good cis-labilizing ligands that facilitate the instantaneous loss of carbonyl ligands and further nucleophilic attacks to give dimeric units bridged by silanolate groups such as [Re 2 (CO) 6 (-OSiMe 3 ) 3 ] - [8].…”

New Cu4Na4- and Cu5-Based Phenylsilsesquioxanes. Synthesis via Complexation with 1,10-Phenanthroline, Structures and High Catalytic Activity in Alkane Oxidations with Peroxides in Acetonitrile

“…The first type represented incompletely condensed cubane silsesquioxane of Si 6 composition ( Figure 2, center). Unlike widespread CLMSs, obtained using Si 7 -triols [2,4,6,[18][19][20], cage metallasilsesquioxanes including Si 6 -tetraol as a structural unit [60,61] are the rarest representatives of the CLMSs family. Noteworthy is that Si 6 -fragment in 2 was formed in situ during the self-assembly of complex 2, while Refs.…”

“…Noteworthy is that Si 6 -fragment in 2 was formed in situ during the self-assembly of complex 2, while Refs. [60,61] describe the use of corresponding Si 6 tetraol as a synthon. The second type of silsesquioxane skeleton in 2, to our opinion, was absolutely a unique one.…”