Ti silsesquioxane

Figure 14.5 DR-UV spectra of (a) Ti-silsesquioxanes complexes, with the assigned major species for each maximum, and (b) the Ti(OPr )4 grafted on MCM-41 and deconvolution. The fifth ligand in the pentacoordinated species is represented as an adjacent surface siloxane bridge, but water or surface silanols are also possible (see text). (From Reference [64].)...

Fig. 9.2 Catalytic activity in the epoxidation of 1-octene with TBHP of the Ti-silsesquioxanes in the screened parameter space.

Krijnen et al. reported on the noncovalent, nonionic immobilization of Ti-silsesquioxanes onto MCM-41, as an epoxidation catalyst.[132] An intriguing aspect... 

Two approaches deviate from this general scheme. First, catalysts were prepared by physisorption of the Ti silsesquioxane 1 [(c-C6Hii)7Si70i2]-TilV-CsHs) in the mesopores of the MCM-41 structure (27, 28). 

Figure 8.3. Ti-silicalite (right) and Ti-silsesquioxane (left) epoxidation systems.

FIGURE 1.3. Effect of environment on the catalytic active site. The active center of a Ti alkoxide is bonded to one of the three hydroxyl groups of an incompletely condensed silsesquioxane. If the other two hydroxyl groups condense with each other (bottom scheme), the steric hindrance to the active site is much less than if they are converted to alklylsilane groups (top scheme). The latter is significantly less active than the less hindered active site. 

The preparation of the heptanorbornyl silsesquioxane trisilanol 5 has been reported by Maschmeyer et al It is formed in admixture with the corresponding tetrasilanol. The norbornyl-substituted species still await further exploration as precursors in metallasilsesquioxane chemistry. The same applies for the isobutyl and isooctyl derivatives 6 and 7, respectively, which have been propagated in the patent literature by Lichtenhan and Abbenhuis. The application of high-speed experimentation techniques to optimize the preparation of silsesquioxanes as precursors for Ti catalysts has been reported by Maschmeyer et al ... 

The fulvene route was also successfully employed in the preparation of a compound, which can be regarded as one of the most advanced molecular models for a catalytically active titanium center on a silica surface. When Cp Ti(C5Me4CH2) was reacted with the monosilylated silsesquioxane precursor 12 in refluxing toluene a color change from deep purple to amber was observed. Crystallization afforded a bright-yellow material, which was subsequently shown to be the novel mo o(pentamethyleyclopentadienyl) titanium(IV) silsesquioxane complex 126 (69% yield). Its formation is illustrated schematically in Scheme 42. 

The surprising outcome of this reaction is the exclusive formation of a Cp Ti complex in which two silsesquioxanes are bonded in different ways to a single Ti... 

A major part of the work described in this section has been carried out with the aim of applying these silsesquioxane complexes of Ti, Zr and Hf in catalytic processes such as ethylene polymerization, olefin epoxidation and Oppenauer oxidation. These catalytic aspects have been highlighted in several recent review articles. 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

Titanium-silsesquioxanes tailored at modeling and understanding Ti-based heterogeneous catalysts form the largest family of M-POSS models. 

Table 14.1 Comparison between catalytic performances of Ti-based silsesquioxanes and MCM-supported Ti centers in the catalytic epoxidation of cyclohexene [61-63],... 

In the context of asymmetric catalysis, titanium silsesquioxanes containing the chiral ligand (lR,2S,5R)-(-)-menthoxo ligand (MentO) (figure 14.3) have been synthesized from the monosilanol la (R = cyclopentyl) and Ti(OPr )4 [70]. The molecular complexes were tested as asymmetric homogeneous catalysts for the epoxidation of cinammic alcohol with tert-butyl hydroperoxide and compared... 

Figure 14.6 A heterobimetallic Ti-Mg silsesquioxane, a homogeneous catalyst for ethylene polymerization after AlEtj activation, a possible model for the heterogeneous catalyst TiCl4/MgCl2/Si02 (one of the proposed surface structures) and a patented route to precatalyst, active after activation with MAO.

Scheme 14.4 Very electrophilic, formally eight-electron, Ti(IV) alkyl surface (a) and silsesquioxane (b) species.

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