Silsesquioxane Precursors

The hydrolysis/condensation process takes a somewhat different course when cyclopentyl and cycloheptyl substituents are employed instead of cyclohexyl. In 1991 Feher et al reported the preparation of 2 and 4 via hydrolytic condensation reactions of c-CsHgSiCls (9) and c-C7Hi3SiCl3 (10), respectively. It was reported that these straightforward syntheses produce multigram quantities of incompletely condensed silsesquioxanes within a few days. In these two cases no completely condensed silsesquioxane derivatives were formed. While 2 was obtained as the sole 

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  

Group 1 Metal Derivatives (Li, Na, K) Useful Starting Materials 

Fully metalated silsesquioxane derivatives of the type Cy7Si709(0M)3 (M = Li, Na, K) would constitute highly desirable precursors for the construction of realistic catalyst model compounds, including novel heterobimetallic species. However, such alkali metal derivatives of 2-7 were unknown until recently, and structural information on such materials was lacking. There have also been contrasting reports in the literature concerning the metalation of 3 by alkali metal 

The lithiated species 15 and 16 represent highly valuable intermediates in the synthesis of novel metallasilsesquioxanes. For this purpose it is not essential to isolate these lithium derivatives as crystalline solids. It has been found a convenient synthetic route to prepare 15 in situ by metalation of 3 with three equivalents of LiN(SiMe3)2 

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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. 

While previously reported Cr silsesquioxane complexes contained chromium in the hexavalent oxidation state, the recently synthesized lithium silsesquioxane precursors such as 15-17 enabled the synthesis of an unprecedented heterobimetallic chromium species.Treatment of 16 with anhydrous CrCh in THF resulted in formation of green crystalline 154, which was structurally characterized by X-ray diffraction (Scheme 52). 

Exploration of the Synthesis of Silsesquioxane Precursors for Epoxidation Titanium Catalysts I 213... 

The epoxidation activity of the titanium catalysts, as a function of the different solvents and R groups varied in the synthesis of the silsesquioxanes precursors, is reported in Fig. 9.2. Values are normalised to the activity of the complex obtained by reacting TifOPr1 with the pure cyclopentyl silsesquioxane 7fc3 in THF. The results show some general trends ... 

The Ti-catalyst derived from the silsesquioxanes synthesised by the hydrolytic condensation of cyclopentyltrichlorosilane in acetonitrile presents the highest catalytic activity (Fig. 9.2). This activity is 87% of that of the Ti catalyst obtained using pure silsesquioxane alhS as precursor. The relevance of this result lies in the fact that the synthesis of these silsesquioxane precursors does not require any purification process and is much less time-consuming than the synthesis of silsesquioxane a7bi. 

A new parameter space for the synthesis of silsesquioxane precursors was defined by six different trichlorosilanes (R=cyclohexyl, cyclopentyl, phenyl, methyl, ethyl and tert-butyl) and three highly polar solvents [dimethyl sulfoxide (DMSO), water and formamide]. This parameter space was screened as a function of the activity in the epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) [26] displayed by the catalysts obtained after coordination of Ti(OBu)4 to the silsesquioxane structures. Fig. 9.4 shows the relative activities of the titanium silsesquioxanes together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile. The values are normalised to the activity of the complex obtained by reacting Ti(OBu)4 with the pure cyclopentyl silsesquioxane o7b3 [(c-C5H9)7Si7012Ti0C4H9]. 

Fig. 9.4 Screening of the epoxidation activity of the titanium silsesquioxanes as a function of the solvent and of the trichlorosilanes used in the synthesis of the silsesquioxane precursors.

One classification criterion could be the question of whether only one single source or two (or more) different precursors are utilized. Then, for instance, materials made of two different silsesquioxane precursors have to be classified into the same class as a material that is made of TEOS and another terminal organotrialkoxysilane, because in both cases a co-condensation approach is followed is this truly justified ... 

Another criterion could be whether the organic functions are only incorporated into the framework or only the pore wall interface is functionalized. However, how then should a co-condensation reaction between a silsesquioxane precursor and a terminal organotrialkoxysilane be classified ... 

Figure 18.2 A sample of pendant and bridged silsesquioxane precursors suitable for creating hybrid nanostructured porous materials.

Of special significance in the development of PMO materials was the synthesis of porous materials with crystalline order of the bridged silsesquioxane inside the framework walls, initially achieved through the use of tt-tt interactions between 1,4-phenylene-bridged silsesquioxane precursors.33 Molecular scale periodicity in... 

A second method that has been described as a means to deal with the limited accessibility issue involves a copolymerization route. In this approach, the porous matrix is copolymerized with the template. Burleigh and coworkers, for example, described the preparation of imprinted polysilsesquioxanes for the recognition of metal ions.79 Polysilsesquioxanes are hybrid porous materials synthesized from bridged alkoxide precursors such as shown in Figure 20.2.80 In this example, the bridged silsesquioxane precursors were copolymerized with the metal ion complex in the presence of surfactant. Once the surfactant and metal ion were removed, a porous network was formed that showed a high affinity for the metal ion.79... 

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