T12 POSS Cages: D6h and D2d Dodecasilsesquioxane Isomers

Dodecameric silsesquioxanes of the T₁₂ type, of general formula [RSiO_1.5]₁₂, differ from every smaller cage in one respect that governs the whole of their chemistry: the twelve silicon atoms can be arranged in two distinct ways. One isomer adopts idealized D_6h symmetry, the other idealized D_2d symmetry, and both are accessible under similar conditions. Neither the cubic T₈ cage nor the pentagonal prismatic T₁₀ cage offers that choice. Dodecamers are correspondingly the least studied members of the series, and isolated yields rarely exceed 30%.

Topics covered in this post: the two core geometries of T₁₂R₁₂ and their idealized symmetries; formation of dodecamers by hydrolytic condensation and by rearrangement of octameric cages; fluoride-, base- and amine-mediated routes; the difficulty of separating T₁₀ from T₁₂; the crystal structure of dodecakis(4-iodophenyl)dodecasilsesquioxane; photophysical behavior relative to T₈ and T₁₀; and the relationship between the D_6h cage and the double six-ring building unit of silicates.

Two Core Geometries: the D_6h and D_2d Isomers of T₁₂R₁₂

Silsesquioxanes of the T₁₂R₁₂ type, in contrast to T₁₀R₁₀ and T₈R₈, may possess two different core geometries. The first exhibits idealized D_6h symmetry and is built from two twelve-membered Si–O rings, each containing six silicon atoms alternating with six oxygen atoms, joined by six four-membered rings into a hexagonal prism. The second exhibits idealized D_2d symmetry and is composed of four ten-membered and four eight-membered Si–O rings, an arrangement that departs appreciably from prismatic regularity. The two frameworks are shown in Figure 1.

Figure 1. Isomers of T₁₂-type silsesquioxanes. The D_6h framework (left) is a hexagonal prism formed by two twelve-membered Si–O rings; the D_2d framework (right) consists of four ten-membered and four eight-membered Si–O rings.

The D_6h structures, although the less frequently encountered of the two, are of interest well beyond organosilicon chemistry, because the same hexagonal prismatic arrangement of twelve tetrahedra constitutes a secondary building unit of silicate frameworks. In the crystallographic nomenclature discussed in the entry on silsesquioxane nomenclature and the M/D/T/Q notation, that unit is designated a double six ring, abbreviated D6R, and it recurs throughout zeolite and silicate chemistry. A D_6h dodecasilsesquioxane is therefore a soluble molecular analogue of a structural motif otherwise encountered only in extended inorganic solids, which is precisely the argument that has long been made for T₈ cages as D4R analogues.

Formation of Dodecamers by Hydrolytic Condensation and Cage Rearrangement

The methods available for preparing T₁₂R₁₂ are substantially the same as those used for T₁₀R₁₀. They rest either on the hydrolytic condensation of trifunctional silanes RSiX₃, where X is chloride or ethoxide, or on the rearrangement of a preformed octameric cage. Isolated yields of dodecamers rarely exceed 30%, which is the single most important practical constraint on the field and the reason why so much of the T₁₂ literature consists of by-product characterization rather than targeted synthesis.

Dodecamers appeared in the earliest systematic work on the series. J. F. Brown obtained mixtures containing cages with eight, ten and twelve silicon atoms from the hydrolysis and condensation of trimethylethoxysilane catalyzed by potassium hydroxide in aqueous methanol, and recovered phenylsilsesquioxanes of the same three sizes from trichlorophenylsilane under comparable conditions. The solvent proved to be decisive rather than incidental, since equilibration in tetrahydrofuran directed the system toward phenyl-T₁₂ whereas benzene, pyridine or ethylene glycol dimethyl ether afforded phenyl-T₈, as set out in the entry on phenylsilsesquioxane equilibration from cage to ladder.

The formation of the thermodynamically less stable T₁₀ and T₁₂ frameworks occurs principally through reorganization of the T₈ core, and several groups have documented this transformation. V. Ervithayasuporn observed rearrangement of octakis(3-chloropropyl)octasilsesquioxane on treatment with sodium methacrylate, with sodium phenoxides and with potassium phthalimide, each of these reactions delivering a mixture of T₈, T₁₀ and T₁₂ species. The methacrylate- and acrylate-functionalized members of that series were subsequently isolated individually and their structure–property relationships compared directly.

Fluoride ions provide a second route. E. Rikowski and H. C. Marsmann found that a chloropropyl-substituted T₈ cage treated with Na₂SiF₆ and 18-crown-6 in acetonitrile converts to a mixture of 28% T₈, 61% T₁₀ and 11% T₁₂. The same authors demonstrated base-catalyzed reorganization of alkyl cages, in which T₈[C₂H₅]₈ heated with K₂CO₃ in acetone gave T₁₀[C₂H₅]₁₀ and T₁₂[C₂H₅]₁₂ in 55% and 4% yield respectively, with 41% of the starting material recovered unchanged. The asymmetry of those two figures is characteristic. Other alkyl cages of the type T₈[alkyl]₈ gave only the decamer, and in yields below 18%, so the decamer is reached far more readily than the dodecamer under conditions of this kind. The contrast with the superacid route is instructive, since treatment of an octameric amino cage with trifluoromethanesulfonic acid delivers the T₁₀ framework in 44% yield without producing the dodecamer at all.

Y. Kawakami and co-workers described the formation of T₈, T₁₀ and T₁₂ cages during the hydrolysis of 4-substituted phenyltriethoxysilanes in the presence of tetrabutylammonium fluoride, and a related amine-catalyzed condensation of silanols has likewise been shown to furnish higher polyhedral cages.

Separating T₁₂ from T₁₀ and T₈

Obtaining phase-pure T₁₀ or T₁₂ cages is generally difficult, and the separation is particularly demanding where the two higher homologues share closely similar physicochemical properties such as retention factor and solubility in common solvents. Kawakami and co-workers effected the separation by crystallization from acetonitrile and tetrahydrofuran mixtures, from pure hexane, or from ethanol and hexane mixtures. Fractional crystallization, sublimation, and HPLC or size-exclusion chromatography have all been applied to this problem. In size-exclusion chromatography the elution order follows hydrodynamic volume, so T₁₂ elutes ahead of T₁₀, which in turn elutes ahead of T₈, despite the modest differences in molar mass across the series.

Crystal Structure of Dodecakis(4-iodophenyl)dodecasilsesquioxane

The crystal structure of dodecakis(4-iodophenyl)dodecasilsesquioxane was described by R. M. Laine and co-workers, who obtained the compound by iodination of [PhSiO_1.5]_n to give [p-I-C₆H₄SiO_1.5]_n for n = 8, 10 and 12. The silsesquioxane core in the structure reported adopts D_2d symmetry rather than the hexagonal prismatic alternative, and the molecular structure in the solid state is shown in Figure 2.

Figure 2. Molecular structure of dodecakis(4-iodophenyl)dodecasilsesquioxane in the solid state. The twelve-silicon core adopts the D_2d geometry of Figure 1 rather than the hexagonal prismatic D_6h alternative. Rendered from the deposited crystallographic coordinates; structure originally reported by Roll, M. F.; Kampf, J. W.; Kim, Y.; Yi, E.; Laine, R. M. J. Am. Chem. Soc. 2010, 132, 10171.

The same study is notable for a second reason. Thermal elimination of I₂ from these iodinated cages was shown to provide a route to high-surface-area, thermally stable microporous materials, which makes the dodecamer not merely a structural curiosity but a precursor in its own right.

Photophysical Behavior Relative to T₈ and T₁₀

T. Goodson and R. M. Laine examined the spectroscopic properties of T₈, T₁₀ and T₁₂ derivatives bearing stilbene substituents. Increasing the number of chromophore groups per cage lowered the luminescence quantum yield, an effect attributable to self-absorption and the consequent non-radiative quenching. Among the three cage sizes, however, the decamer rather than the dodecamer displayed the largest two-photon absorption cross-section, which indicates strong electronic coupling and polarization among the peripheral chromophores of the D_5h framework. The result is worth stating plainly, because it runs against the intuition that a larger cage carrying more chromophores should perform better. Cage geometry, not chromophore count, determines the nonlinear optical response.

Outlook

Dodecameric silsesquioxanes remain the least developed members of the polyhedral series, and the reasons are straightforward. They form in low yield, they co-crystallize and co-elute with the decamer, and they exist as two isomers whose relative proportions are rarely controlled deliberately. What has not been systematically attempted is the selective synthesis of the D_6h isomer, which, as the molecular counterpart of the double six-ring unit of silicates, is the more interesting of the two from the standpoint of framework chemistry. Every reported structural characterization of a functionalized dodecamer to date has found the D_2d geometry instead. Establishing what directs a condensation toward one isomer rather than the other, whether solvent, template, counterion or substituent, would be the natural next step, and it is a question that the structural chemistry of the smaller cages does not answer, because the smaller cages do not have the choice.

References

(1) Brown, J. F.; Vogt, L. H.; Prescott, P. I. Preparation and Characterization of the Lower Equilibrated Phenylsilsesquioxanes. J. Am. Chem. Soc. 1964, 86, 1120–1125. https://doi.org/10.1021/ja01060a033

(2) Rikowski, E.; Marsmann, H. C. Cage-Rearrangement of Silsesquioxanes. Polyhedron 1997, 16, 3357–3361. https://doi.org/10.1016/S0277-5387(97)00092-2

(3) 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. Chem. Lett. 2007, 36, 792–793. https://doi.org/10.1246/cl.2007.792

(4) Ervithayasuporn, V.; Wang, X.; Kawakami, Y. Synthesis and Characterization of Highly Pure Azido-Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS). Chem. Commun. 2009, 5130–5132. https://doi.org/10.1039/B909802J

(5) 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. https://doi.org/10.1021/ja102453s

(6) 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. https://doi.org/10.1021/ic401994m

(7) 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. https://doi.org/10.1021/ja4043092

(8) Handke, M.; Jastrzębski, W. Vibrational Spectroscopy of the Double 4-, 6-Membered Rings in Silicates and Siloxanes. J. Mol. Struct. 2005, 744–747, 671–675. https://doi.org/10.1016/j.molstruc.2004.12.052

Full Citation 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." Journal of the American Chemical Society, 2010, 132, 10171–10183.
DOI: 10.1021/ja102453s
Full text: Journal of the American Chemical Society → ACS Publications

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Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2026). T12 Dodecameric Silsesquioxanes: D6h and D2d Cage Isomers. https://doi.org/