Incomplete condensation

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

Scheme 2 Incompletely condensed silsesquioxane (a), completely condensed silsesquioxane non-functional (b), completely condensed silsesquioxane functional (c), ladder-type silsesquioxane (d), and functional silicate (e).

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... 

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. 

Scheme 2. Incompletely condensed oligomeric silsesquioxanes (R = alkyl, cycloalkyl, aryl).

By far the most important and most thoroughly investigated representatives among the incompletely condensed polyhedral silsesquioxanes are the heptameric trisilanols R7Si70g(0H)3 (Scheme 4). 

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 3. Formation of completely and incompletely condensed silsesquioxanes upon hydrolysis of RSiXs pc = Cl, OR) followed by condensation steps.

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... 

Figure 5.7 The two most probable isomers [(14a) and (14b)j obtained from the reaction of [Mo(CHCMe2Ph)(NAr) (0S02CF3)2(DME)j with the incompletely condensed silsesquioxane [(c-C6Hii)Si70g(0SiMe3) (OTl)2j.

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]. 

Figure 7.5 (a) Schematic representation of an octosilsesquinoxane, (RSi01.5)8 (b) typical precursor of a SSQQ (c) incompletely condensed "T8 (0H)2" isomers. (Reprinted with permission from Fasce etal., 1999. Copyright 2001. American Chemical Society)... 

Fig. 9.1 Complexation of titanium to the incompletely condensed silsesquioxane R7Si7012H3 (R = cyclohexyl, cyclopentyl), yielding a catalyst active in the epoxidation of alkenes with TBHP.

Incompletely condensed silsesquioxanes different from a7 >3 may also act as precursors for titanium complexes that are catalytically active in the epoxidation of alkenes. 

From these considerations, the synthesis of silsesquioxanes was optimised, by means of HTE, as a function of the activity of the catalysts obtained after titanium coordination to the silsesquioxane structures. Therefore, this approach aimed at producing any incompletely condensed silsesquioxane that would result in active catalysts after titanium coordination rather than a specific structure (like silsesquioxane ulhS). The epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) as the oxidant was chosen as test reaction for the activity of the catalysts [26]. 

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