Decameric silsesquioxanes

There is little information available in the literature concerning pure, isolated larger cages like the aforementioned heptahedral T₁₀. A small amount of T₁₀ type POSS is usually formed as a by-product during the preparation of T₈ type silsesquioxanes. For example, during the preparation of T₈H₈ by the hydrolytic condensation of HSiCl₃, a small amount of T₁₀H₁₀.¹ is formed in addition to the main product.

The formation of more thermodynamically unstable POSS of the T₁₀ or even T₁₂ type occurs primarily as a result of the reorganization of the core of the T₈ cage. The transformation of the T₈ cage into a larger one has been published by several research groups.^2–4 For example, V. Ervithayasuporn observed the reorganization of octakis (3-chloropropyl) octasilsesquioxane by reaction with sodium methacrylate,⁵ sodium phenoxides,⁶ or potassium phthalimide.⁷ These reactions resulted in the formation of mixture of compounds of the type T₈, T₁₀ and T₁₂ as shown in the scheme below.

 Reorganization of the POSS core during synthesis its phthalimide derivative

In addition to the above methods, a method of obtaining T₁₀ cages by converting silsesquioxane with fluoride ions is known in the literature. E. Rikowski et al. Discovered that T₁₀ and T₁₂ type POSS can be formed by transforming a T₈ cage containing a 3-chloropropyl moiety. The reaction takes place in acetonitrile using Na₂SiF₆ and 18-crown-6 as catalyst. The yield of this conversion is: 28% (T₈), 61% (T₁₀) and 11% (T₁₂).⁸ The same authors⁸ also showed that T₈[alkyl]₈ analogs composed of cages containing 10 or 12 silicon atoms can be obtained. These compounds result from base catalyzed reorganization of the core. For example, T₈[C₂H₅]₈ when heated with K₂CO₃ in acetone forms a mixture of T₁₀[C₂H₅]₁₀ and T₁₂[C₂H₅]₁₂ with an efficiency of 55 and 4% respectively (41% remains unreacted). The remaining compounds of formula T₈[alkyl]₈ only form T₁₀[alkyl]₁₀ with low yield (less than 18%). Y. Kawakami et al. Described the formation of POSS T₈, T₁₀ and T₁₂ during the hydrolysis of 4-substituted phenyltriethoxysilane in the presence of tetrabutylammonium fluoride. hexane or ethanol/hexane mixtures.

Reorganization of the siloxane cage-like core (T₈ → T₁₀) can be easily performed, including isolation of intermediates, and cage rearrangement achieved by using superacid CF₃SO₃H (TfOH). Moreover, T₁₀-like SQs can be obtained in a one-step reaction by alkoxysilane condensation in trifluoromethanesulfonic acid conditions.⁹

Direct synthesis of T₁₀ POSS (3) from T₈ (1) using superacid CF₃SO₃H (RSC Advances, 2015, 5, 72340-72351)

General methods for obtaining T₁₀ and T₁₂ cages are shown in scheme belowe. It is usually difficult to obtain phase clean T₁₀ or T₁₂ cages. The separation of POSS is particularly demanding when the compounds, for example T₁₀ and T₁₂, have similar hysicochemical properties, such as retention factor or similar solubility. The known T₁₀-type silsesquioxanes were separated from the reaction mixture by means of fractional crystallization, sublimation, or using HPLC or SEC chromatography.

Methods of obtaining cage silsesquioxanes of T₁₀ type

R. Laine et al. described the crystal structure of decaphenyldecasilsesquioxane.¹⁰ These studies allowed for unequivocal determination of the structure of the silsesquioxane core containing 10 silicon atoms. As shown in figure below, it has an idealized D_5h symmetry.

Compounds of the T₁₀R₁₀ type typically are air-stable white powders. Their physical properties are similar to T₈R₈. The reactivity of T₁₀-type silsesquioxanes is less well understood than that of their T₈ analogs, but is usually similar. T. Goodson and R. Laine investigated the spectroscopic properties of T₈, T₁₀ and T₁₂ derivatives containing a stilbene moiety.¹¹ It turned out that with increasing the number of chromophore groups per silsesquioxane cage, the quantum luminescence yield decreased. It is related to self-absorption leading to non-radiative quenching. However, of the three frames (T₈, T₁₀ and T₁₂), POSS T₁₀ showed the highest two-photon absorption, indicating strong electron coupling and polarization.

X-Ray structure of decaphenylsilsesquioxane

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(10) Roll, M. F.; Kampf, J. W.; Kim, Y.; Yi, E.; Laine, R. M. Nano Building Blocks via Iodination of [PhSiO1.5]n, Forming [p-I-C₆H₄SiO_1.5]_n (n = 8, 10, 12), and a New Route to High-Surface-Area, Thermally Stable, Microporous Materials via Thermal Elimination of I2. J. Am. Chem. Soc. 2010, 132, 10171–10183.
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