Silsesquioxane models

Figure 14.4 Comparison between structural XAFS data measured for the active heterogeneous catalyst (a) before and (b) under the conditions of catalytic reactions (time averaged), along with respective stmctural X-ray determined bond distances (A) for the closest silsesquioxane models available [60, 62].

Scheme 14.5 Cationic Zr(iV) surface species obtained from Cp ZrMe3 with silica, and possible silsesquioxane models.

Zirconium Acetylacetonate Sihca supported Zr(lV) acetylacetonate surface complex [(=SiO)3Zr(acac)] (acac = MeCOCHCOMe) catalyzes transesterification reactions [114]. Satisfying silsesquioxane models have been reported [2, 114]. 

The use of small polyhedral silsesquioxanes as molecular models of silica surfaces has been studied increasingly in recent years, because catalysts, etc., attached to such species are relatively easy to handle and characterize when compared to bulk silica. The preparation and structures of silsesquioxanes used in this type of work are described later in Sections II,C, IV,E, and IV,F. Further recent examples may be found in Ref. 5. 

The incompletely condensed silsesquioxanes can be considered as intermediates in the synthesis of the whole family of silsesquioxanes, but at the same time they are the building blocks (e.g., initiators) for the new polymeric architectures.66 67 The importance of this class of materials in model studies of silica surfaces and silicate defects,68 and in creating various oligosilanols with novel frameworks,69 70 makes them the subject of extensive research.71 The use of the incompletely condensed silsesquioxanes in forming dimeric silsesquioxanes and metallasilsesquioxanes in attempts to design new routes to the well-defined Si-O assemblies has recently been reviewed in a well-illustrated research report.72... 

Controlled hydrolysis of RSiX3 compounds gives so-called silsesquioxanes or POSS compounds (Polyhedral Oligomeric SilSesquioxane), which can be used as models for silica surfaces or supports for catalysts [4] (Figure 18.2, schematic structure on the right). 

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

Scheme 5. Silica-supported species, which have been successfully modeled with the use of silsesquioxanes.

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 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 proposed IR assignments were supported by calculation of v(Zr-H) frequencies of computational models for the species involved in the reactions of Scheme 3.1 [16, 17] based on a polyhedral oligomeric silsesquioxane (POSS)... 

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]. 

Among several families of compounds used as molecular models for such surface silanolic grafting sites (mono or poly-silanols [10-12], heteropolyanions [13, 14], calixarenes [15, 16], tripodal amido complexes [17], etc.) some silsesquioxanes [18-23] can be considered among the most convincing molecular analogues of silica surface silanols. 

Figure 14.1 Silsesquioxanes used as models for Isolated, vicinal and geminal surface silanols.

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