Selected POSS Compounds and Their Derivatives

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Polyhedral oligosilsesquioxanes constitute a class of nanometer-scale organosilicon compounds characterized by well-defined, highly symmetrical cage frameworks consisting of silicon and oxygen atoms surrounded by organic substituents in three-dimensional arrangements. With diameters between 1 and 3 nm, these materials represent the smallest possible particles of silica bearing surface organic groups, and their properties combine those of both organic and inorganic components, making them true nanocomposite materials.

Octahydridooctasilsesquioxane and Its Derivatives

Octahydridooctasilsesquioxane (14) contains a labile hydrogen atom bonded to a silicon atom, which enables the compound to undergo hydrosilylation reactions and renders it a valuable substrate for the preparation of a variety of silsesquioxane derivatives.

Hydrosilylation is a reaction in which an organosilicon compound bearing a Si–H bond adds across an unsaturated bond such as C=C, C=O, C=N, N=O, or N=N. Catalysts employed in this reaction include free radicals, organic bases, transition-metal complexes, and pure metals. In the case of silsesquioxanes, the most commonly used catalysts are H2PtCl6 and Karstedt's catalyst, owing to their high reactivity and selectivity.

A method for synthesizing silsesquioxanes bearing alkyl chains via hydrosilylation of octahydridooctasilsesquioxane, as developed by Y. Aziz, is presented in Scheme 11. Analogous reaction conditions were employed by W. Lin in reactions with styrene derivatives.

Reaction scheme for platinum-catalyzed hydrosilylation of octahydridooctasilsesquioxane forming alkyl cage derivatives

Scheme 11. Schematic representation of the synthesis of cage silsesquioxane derivatives via hydrosilylation.

A drawback of this approach is the possibility of forming two isomers, α and β (see Scheme 12). Hydrosilylation of cage silsesquioxanes typically yields a mixture of isomers, necessitating product purification. A further example is the synthesis reported by Y. Sheen’s group in 2008, in which octakis(dimethylsiloxy)octasilsesquioxane served as the substrate, yielding a range of both alkyl and aryl derivatives (Scheme 12).

Reaction scheme showing formation of alpha and beta isomers in POSS hydrosilylation

Scheme 12. Schematic representation of the synthesis of POSS derivatives via hydrosilylation.

Octa(3-aminopropyl)silsesquioxane

Octa(3-aminopropyl)silsesquioxane hydrochloride (OAS) was first described in a patent by Wacker-Chemie GmbH; however, that document neither details the preparative procedure nor provides the spectroscopic data required for full characterization. Subsequent investigators developed a modified synthesis employing commercially available (3-aminopropyl)triethoxysilane (APTES). Compounds 1–3 (see scheme below) can be obtained via a one-step hydrolytic condensation of APTES with an appropriate quantity of hydrochloric acid (3.6 equivalents). This approach improves the yield and minimizes the formation of deca- and other multiply substituted polyhedral silsesquioxanes, which are difficult to separate.

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