T10 Decameric Silsesquioxanes: Synthesis and Cage Reorganization

Decameric silsesquioxanes of the T₁₀ cage type (general formula [RSiO_3/2]₁₀) represent a structurally distinct class of polyhedral oligomeric silsesquioxanes (POSS). Unlike the more common cubic T₈ cages possessing O_h symmetry, T₁₀ silsesquioxanes adopt a prismatic cage geometry with idealized D_5h symmetry, in which ten silicon atoms are arranged in two parallel pentagonal rings connected by five bridging Si–O–Si linkages, consistent with the T10 cage nomenclature conventions used throughout this series. Despite their interesting geometry and properties, T₁₀ silsesquioxanes are considerably less studied than their T₈ counterparts, primarily due to the challenges associated with their isolation in pure form.

Topics covered in this post: formation of T₁₀ POSS as a by-product of T₈ synthesis; cage reorganization pathways (nucleophilic substitution, fluoride-catalyzed, base-catalyzed, and superacid-mediated routes); separation and purification methods; crystal structure and D_5h symmetry; and spectroscopic and optical properties of T₁₀ derivatives.

Formation and Cage Reorganization

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 T₁₀H₁₀¹ 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 a mixture of compounds of the type T₈, T₁₀ and T₁₂ as shown in the scheme below. The cage expansion proceeds through partial hydrolysis of the Si–O–Si framework followed by recondensation into larger polycyclic structures, a process that is thermodynamically driven by relief of steric strain at the cage vertices.

 Reorganization of the POSS core during synthesis of 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 in 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). This approach offers a particularly efficient route, as the strongly acidic medium promotes rapid equilibration of the siloxane framework, allowing selective formation of the T₁₀ product under appropriate reaction conditions. Moreover, T₁₀-type silsesquioxanes 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)

Separation and Purification

General methods for obtaining T₁₀ and T₁₂ cages are shown in the scheme below. It is usually difficult to obtain phase-pure T₁₀ or T₁₂ cages. The separation of POSS cage types is particularly demanding when the compounds, for example T₁₀ and T₁₂, have similar physicochemical properties, such as similar retention factors or comparable solubility in common solvents. The known T₁₀-type silsesquioxanes were separated from the reaction mixture by means of fractional crystallization, sublimation, or using HPLC or SEC chromatography. In size-exclusion chromatography, the larger T₁₂ cage elutes before T₁₀, which in turn elutes before T₈, reflecting the systematic increase in hydrodynamic volume with cage size, despite the relatively modest differences in molecular weight among the three cage types.

Methods of obtaining cage silsesquioxanes of T₁₀ type

Crystal Structure and Symmetry

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 the figure below, it has an idealized D_5h symmetry. The T₁₀ cage may be envisioned as a pentagonal prismatic framework in which ten silicon atoms are arranged in two parallel pentagonal rings, connected by Si–O–Si linkages. This geometry contrasts sharply with the cuboid O_h symmetry of T₈ cages and gives rise to distinct spectroscopic signatures: T₁₀ silsesquioxanes typically display a single ²⁹Si NMR resonance when all ten substituents are equivalent, consistent with the equivalence of all silicon environments in the D_5h-symmetric cage. In the infrared spectrum, T₁₀ silsesquioxanes display characteristic Si–O–Si stretching bands in the 1050–1200 cm^–1 region, which are distinguishable from those of the T₈ cage by their pattern and relative intensities.

X-Ray structure of decaphenylsilsesquioxane

Physical and Optical Properties

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 due to self-absorption and consequent non-radiative quenching. However, of the three cage types (T₈, T₁₀ and T₁₂), POSS T₁₀ showed the highest two-photon absorption cross-section, indicating strong electron coupling and polarization among the peripheral chromophore groups. The enhanced two-photon absorption of T₁₀ stilbene derivatives compared to T₈ and T₁₂ analogs has been attributed to the unique geometric arrangement of the ten chromophore units around the D_5h-symmetric cage, which facilitates cooperative electronic interactions and maximizes the hyperpolarizability of the assembly. This finding underscores the significance of cage geometry and symmetry in determining the nonlinear optical response of POSS-based chromophoric systems.

(1) Frye, C. L.; Collins, W. T. Oligomeric Silsesquioxanes, (HSiO_3/2)_n. J. Am. Chem. Soc. 1970, 92, 5586–5588.
(2) Kawakami, Y.; Yamaguchi, K.; Yokozawa, T.; Serizawa, T.; Hasegawa, M.; Kabe, Y. Higher Polyhedral Silsesquioxane (POSS) Cage by Amine-Catalyzed Condensation of Silanols and Related Siloxanes. Chemistry Letters 2007, 36, 792–793.
(3) Bassindale, A. R.; Liu, Z.; MacKinnon, I. A.; Taylor, P. G.; Yang, Y.; Light, M. E.; Horton, P. N.; Hursthouse, M. B. A Higher Yielding Route for T 8 Silsesquioxane Cages and X-Ray Crystal Structures of Some Novel Spherosilicates. Dalton transactions 2003, No. 14, 2945–2949.
(4) Ervithayasuporn, V.; Wang, X.; Kawakami, Y. Synthesis and Characterization of Highly Pure Azido-Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS). Chem. Commun. 2009, No. 34, 5130–5132.
(5) Ervithayasuporn, V.; Chimjarn, S. Synthesis and Isolation of Methacrylate- and Acrylate-Functionalized Polyhedral Oligomeric Silsesquioxanes (T₈, T₁₀, and T₁₂) and Characterization of the Relationship between Their Chemical Structures and Physical Properties. Inorg. Chem. 2013, 52, 13108–13112.
(6) Chimjarn, S.; Kunthom, R.; Chancharone, P.; Sodkhomkhum, R.; Sangtrirutnugul, P.; Ervithayasuporn, V. Synthesis of Aromatic Functionalized Cage-Rearranged Silsesquioxanes (T₈, T₁₀, and T₁₂) via Nucleophilic Substitution Reactions. Dalton Trans. 2015, 44, 916–919.
(7) Jaroentomeechai, T.; Yingsukkamol, P.; Phurat, C.; Somsook, E.; Osotchan, T.; Ervithayasuporn, V. Synthesis and Reactivity of Nitrogen Nucleophiles-Induced Cage-Rearrangement Silsesquioxanes. Inorg. Chem. 2012, 51, 12266–12272.
(8) Rikowski, E.; Marsmann, H. C. Cage-Rearrangement of Silsesquioxanes. Polyhedron 1997, 16, 3357–3361.
(9) Janeta, M.; John, Ł.; Ejfler, J.; Szafert, S. Novel Organic-Inorganic Hybrids Based on T8 and T10 Silsesquioxanes: Synthesis, Cage-Rearrangement and Properties. RSC Advances 2015, 5, 72340–72351.
(10) Roll, M. F.; Kampf, J. W.; Kim, Y.; Yi, E.; Laine, R. M. Nano Building Blocks via Iodination of [PhSiO_1.5]_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.
(11) Furgal, J. C.; Jung, J. H.; Goodson, T.; Laine, R. M. Analyzing Structure–Photophysical Property Relationships for Isolated T₈, T₁₀, and T₁₂ Stilbenevinylsilsesquioxanes. J. Am. Chem. Soc. 2013, 135, 12259–12269. 

Full CitationFrye, C. L.; Collins, W. T. "Oligomeric Silsesquioxanes, (HSiO3/2)n." Journal of the American Chemical Society, 1970, 92, 5586-5588.
DOI: 10.1021/ja00722a009
Full text: Journal of the American Chemical Society → ACS Publications