Octasilsesquioxanes

X-ray data are available only for the readily accessible octasilsesquioxanes and deca(methylsilsesquioxane) (Table 6). The crystal lattices of octa(alkylsilsesqui-... 

Very recently Muller, Kohne and Sliwinski (44) described the preparation of unsubstituted octasilsesquioxane (HSiOj.5)8. This interesting prototype octamer was obtained in about 1% yield by treating trichlorosilane with 80% sulfuric acid in hexamethyldisiloxane solution. Like the methyl-, ethyl-, and phenyl-substituted octamers, it is a high melting solid, readily sublimable under vacuum. 

Organosubstituted octasilsesquioxanes (Fig. lb) have also been prepared by cross-metathesis (CM) and silylative coupling of vinylsilsesquioxane with olefins in the presence of the ruthenium carbene complex Cl2(PCy3)2Ru(=CHPh) (Grubbs catalyst) and Ru-H (Ru-Si) complexes, for example, RuHCl(CO)(PCy3)2, respectively [57]. 

As for the tetramers and dendrimers previously described, the mesogenic arms of the octasilsesquioxane-based materials can arrange parallel to one another thus these supermolecules can pack together in layers to form a lamellar phase. Molecular simulations in the gas phase at absolute zero confirm this point of view and show that the minimized structure of the... 

Fig. 24 Octa(dimethylsiloxy)octasilsesquioxane cage-like dendrimers...

Neumann and coworkers [6] reported the first synthesis and characterization of a novel POSS, possessing eight isocyanate groups (XVIII) (Scheme 5) via the hydrosilylation of m-isopropenyl-Q ,Q -dimethylbenzyl isocyanate (m-TMI) (XVII) by octakis(dimethylsiloxy)octasilsesquioxane (Q8M8 ) (XVI). 

To sum up, octakis(3-hydroxypropyldimethylsiloxy)octasilsesquioxane (OHPS) has also been synthesized by direct hydrosilylation of allyl alcohol with octakis(dimethylsiloxy)octasilsesquioxane, (HSiMe20)8Si80i2... 

As expected, octasilacubane possesses high reactivity toward electrophiles. As shown in Scheme 3, mCPBA oxidation of silyl-substituted 1 resulted in a hi -yield formation of octasilsesquioxane (3) (Tg). Silsesquioxanes are well studied, but were previously only prepared by the hydrolysis-dehydration of halo- or alkoxysilanes. When the substituents are bulky, the reaction proceeds no further than the silanol stage, and silsesquioxanes are not obtained. For example, hydrolytic condensation of t-butyldimethylsilyltrichlorosilane gave only partly hydrolyzed silanols. Thus, 3 is the octasilsesquioxane with the bulkiest substituents, and the only silyl-substituted one ever reported. The structure was determined by X-ray ciystallography [7], and the bond lengths and angles are similar to those of known octasilsesquioxanes. 

Fig. 32 A nematogenic supermolecular octasilsesquioxane, 35, with laterally appended mesogenic groups...

Fig. 33 Local structure of the nematic phase of an octasilsesquioxane dendrimer, 35, possessing laterally appended mesogens...

The next family of materials, which are to be discussed all possess cyanobiphenyl moieties as the mesogenic units. The systematic studies of the liquid crystal properties are described in terms of spacer length and peripheral number density. The first material, 37, shown in Fig. 35, is an octamer based on the octasilsesquioxane core unit [86]. This material was prepared and purified in such a way that by HPLC and 29Si NMR spectroscopy it was shown to be a single compound without dispersity. The material was found to exhibit smectic polymorphism with SmC and SmA phases being formed. The incorporation of cyanobiphenyl mesogenic moieties means that the material has interesting... 

The effect of the length of the spacer chain on the liquid-crystalline properties of the octamers of octasilsesquioxane, of general structure 38, was... 

The point of bifurcation, so that 16 mesogenic units can be more easily located around the octasilsesquioxane core, can be achieved by moving the bifurcation point away from the central part of the scaffold as shown for... 

Fig. 43 Laterally appended octamer of octasilsesquioxane, 41, where the mesogens are chiral...

As with the terminally appended system 39, dendritic growth of the oc-tasilsesquioxane core allows twice the number of mesogenic units be bound to the central scaffold. Thus, the first generation octasilsesquioxane dendrimer 43, see Fig. 46, which is the size of a small protein (Mw = 14653) possessing 16 side-on attached mesogenic units, was reported [94]. After purification of this material it was found to be monodisperse by Maldi-fo/... 

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