Selected POSS
Polyhedral oligosilsesquioxanes comprise a group of nanometer-scaled organosilicon compounds with well-defined and highly symmetrical structures. These compounds em-body the hybrid organic-inorganic architectures, with inner inorganic frameworks con-sisting of silicon and oxygen atoms surrounded by organic substituents in three-dimensional arrangements. With diameters between 1 and 3 nm, polyhedral silsesquiox-anes can be considered the smallest possible particles of silica with surface organic groups. The properties of these hybrid compounds are combinations of those of the two components, making them really nanocomposite materials.
Octahydridooctasilsesquioxane and Its Derivatives
Octahydridooctasilsesquioxane (14) contains a labile hydrogen atom attached to a silicon atom, which allows the compound to undergo hydrosilylation reactions, making it a valuable substrate for obtaining a variety of new silsesquioxane derivatives.
Hydrosilylation is a reaction involving the addition of organosilicon compounds containing a Si–H bond to unsaturated bonds such as C=C, C=O, C=N, N=O, and N=N. Catalysts used in this reaction can include free radicals, organic bases, transition metal complexes, or pure metals. In the case of silsesquioxanes, the most commonly used catalysts are H₂PtCl₆ or Karstedt’s catalyst, due to their high reactivity and selectivity.
A method for synthesizing silsesquioxanes with alkyl chains via hydrosilylation of octahydridooctasilsesquioxane, as applied by Y. Aziz, is presented in Scheme 11. Similar reaction conditions were used by W. Lin in reactions with styrene derivatives.
Scheme 11. Schematic representation of the synthesis of cage silsesquioxane derivatives via hydrosilylation.
A drawback of this reaction is the possibility of forming two isomers: α or β (see Scheme 12). Typically, hydrosilylation of cage silsesquioxanes results in a mixture of isomers, making product purification necessary. Another example of a hydrosilylation application is the synthesis proposed by Y. Sheen's group in 2008, in which the substrate was octakis(dimethylsiloxy)octasilsesquioxane, yielding numerous derivatives—both alkyl and aryl (Scheme 12).
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, the document does not detail the method of its preparation or provide the spectroscopic data necessary for its full characterization. Later, researchers developed a modified method using commercially available (3-aminopropyl)triethoxysilane (APTES). Compounds 1–3 (see scheme below) can be synthesized via a one-step hydrolytic condensation of APTES with an appropriate amount of hydrochloric acid (HCl, 3.6 equivalents). This approach improves yield and minimizes the formation of deca- and other substituted polyhedral silsesquioxanes, which are difficult to separate.
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