Silsesquioxane

A silsesquioxane is an organosilicon compound with the chemical formula n (R = H, alkyl, aryl or alkoxyl). Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ('POSS'), which have attracted attention as precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The silica core confers rigidity and thermal stability. A silsesquioxane is an organosilicon compound with the chemical formula n (R = H, alkyl, aryl or alkoxyl). Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ('POSS'), which have attracted attention as precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The silica core confers rigidity and thermal stability. Silsesquioxanes are known in molecular form with 6, 8, 10, and 12 Si vertices, as well as polymers. The cages are sometimes labeled T6 T8, T10, and T12, respectiively (T = tetrahedral vertex). The T8 cages, the most widely studied members, have the formula 8, or equivalently R8Si8O12. In all cases each Si center is bonded to three oxo groups, which in turn connect to other Si centers. The fourth group on Si is usually an alkyl, halide, hydride, alkoxide, etc. In the cubic clusters with Oh symmetry the Si-O-Si angles are in the range 145–152°, being bowed out, allowing the Si centers to better adopt tetrahedral geometry. The O-Si-O angle are in the range: 107–112°, Si-O bond: 1.55–1.65 Å. Silsesquioxanes are usually synthesized by hydrolysis of organotrichlorosilanes. An idealized synthesis is: The formation of HCl negatively impacts the relative rates of hydrolysis and condensation of intermediate silanols. Consequently, silsesquioxanes can be obtained directly by condensation of the corresponding silanetriols which occurs at neutral pH and works even for sterically very bulky substituents. Depending on the R substituent, the exterior of cage can be further modified. When R = H, the Si-H group can undergo hydrosilylation or oxidation to the silanol. Bridged polysilsesquioxanes are most readily prepared from clusters that contain two or more trifunctional silyl groups attached to non-hydrolysable silicon-carbon bonds, with typical sol-gel processing. Vinyl-substituted silsesquioxanes can be linked by the alkene metathesis. Reorganization of the siloxanecage-like core (T8 → T10) can be performed, includingisolation of intermediates, and cage rearrangement achieved by using Bronstedsuperacid, trifluoromethanesulfonic acid (CF3SO3H). In this case, reaction of hexahedral silsesquioxane and CF3SO3H in DMSO conducted in 1 : 12 molar ratio gives heptahedral silsesquioxane. In the first step CF3SO3H acid attacks siloxane Si-O-Si bonds and the formation of Si-O-SO2CF3 bond parallel with cage opening process is observed and compound B is obtained (Figure below). Such an inversion is observed at silicon atom during nucleophilic displacement reaction that is usually noticed when leaving groups are replaced by soft nucleophiles. Uponfurther acid attack, both T6(OH)4 C and siloxane dimer D are formed. Because this reaction takes place in an aqueous conditions, compound E of general formula T8(OH)4 as a consequence of hydrolysis reaction was obtained. E is prone to reaction with Dand due to this, the abstraction of CF3SO−3 anion occurs and the closure frame with the spontaneous cage-rearrangement to heptahedral T10 structure F is observed. Although, heptahedral F is less favorable energetically (MM2 data), in this case its creation is forces by the formation of a new Si4O4 moiety from much more less stable substrates D and E. Polymeric silsesquioxanes have been reported, first by Brown. High molecular weight tractable polymeric phenyl silsesquioxane featured a ladder-type structure. Brown's findings were the basis for many future investigations. Brown's synthesis proceeded in three-steps:(1) the hydrolysis of phenyltrichlorosilane, (2) equilibration of this hydrolyzate with potassium hydroxide at a low concentration and temperature to give the prepolymer, and (3) equilibration of the prepolymer at a high concentration and temperature to give the final polymer. Other notable silsesquioxane polymers include the soluble polymethylsilsesquioxane with high molecular weights described by Japan Synthetic Rubber. This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found applications in cosmetics, resins, and lithography. A well known hydrogen silsesquioxane is 8. Early syntheses involved treatment of trichlorosilane with concentrated sulfuric acid, and fuming sulfuric acid, affording T10-T16 oligomers. The T8 cluster was also synthesized by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and hydrochloric acid. The Si-H groups are amenable to hydrosilylation. Films of organosilsesquioxane, e.g., poly(methylsilsesquioxane), have been examined for semiconducting devices. Poly(hydridosilsesquioxane), which has a linked-cage structure, was sold under the name Fox Flowable Oxide.