Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry

Dodecameric silsesquioxanes of the T 12 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 T 8 cage nor the pentagonal prismatic T 10 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 T 12 R 12 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 T 10 from T 12 ; the crystal structure of dodecakis(4-iodophenyl)dodecasilsesquioxane; photophysical behavior relative to T 8 and T 10 ; and the relationship ...

Among the large and structurally diverse family of polyhedral oligomeric silsesquioxanes (POSS) , octamethylsilsesquioxane — formally designated Si 8 O 12 (CH 3 ) 8 and commonly abbreviated as T 8 -Me or OMS — occupies a unique position as the structurally simplest and historically most studied member of the cubic T 8 series. Its eight methyl groups, one attached to each silicon vertex of the Si 8 O 12 cage, make it the prototype for understanding the geometry, dynamics, spectroscopy, and reactivity of the entire POSS family. Despite its apparent simplicity, T 8 -Me continues to attract sustained scientific interest across disciplines as diverse as materials science, polymer chemistry, computational chemistry, and ceramic engineering.

Phenylsilsesquioxanes have been known since the 1870s , when chemists first found that treatment of the condensation products of phenylsilanetriol with alkali yields soluble compounds of the empirical formula (C 6 H 5 SiO 1.5 ) x . These materials, described variously as phenylsilsesquioxanes, phenyl-T resins, or silicobenzoic anhydride, were first prepared in the search for silicon analogs of the carboxylic acids. Despite more than half a century of subsequent study, their molecular constitution remained obscure until a systematic examination of their equilibrium chemistry finally rendered the problem tractable. Adapted from the work of J. F. Brown, L. H. Vogt, and P. I. Prescott.

The publication titled “Novel organic–inorganic hybrids based on T 8 and T 10 silsesquioxanes: synthesis, cage-rearrangement and properties,” published in RSC Advances in 2015, presents a controlled route to amino- and amido-functionalized cage silsesquioxanes together with a detailed account of T 10 silsesquioxane cage rearrangement promoted by trifluoromethanesulfonic acid. Treatment of the octameric hydrochloride salt with twelve equivalents of this Brønsted superacid in dimethyl sulfoxide delivered the decameric triflate in 44% yield, and the species formed along the reaction path were isolated and identified. The two long-chain amido derivatives obtained from these salts decompose at 471 and 451 °C and form optically transparent films whose water contact angles reach 104° and 110°. For a general introduction to decameric cages, their D 5h geometry, and isolation methods, see T10 Decameric Silsesquioxanes: Structure, Symmetry, and Isolation .

Decameric silsesquioxanes of the T 10 cage type (general formula [RSiO 3/2 ] 10 ) represent a structurally distinct class of polyhedral oligomeric silsesquioxanes (POSS) . Unlike the more common cubic T 8 cages possessing O h symmetry, T 10 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, T 10 silsesquioxanes are considerably less studied than their T 8 counterparts, primarily due to the challenges associated with their isolation in pure form. Topics covered in this post:  formation of T 10 POSS as a by-product of T 8 synthesis; cage reorganization pathways (nucleophilic substitution, fluoride-catalyzed, base-catalyzed, and superacid-mediated routes); separation and purification methods; crys...

Hydrolytic condensation of organotrichlorosilanes remains the workhorse preparation of octameric polyhedral oligomeric silsesquioxanes (POSS). This entry treats that single route in detail, covering the hydrolysis and condensation sequence, the steric control that favors the T 8 cage, and the practical influence of solvent, acidity and reaction time. A comparison of this route with the two remaining synthetic strategies is given on the dedicated overview page linked below. This entry is a focused companion to the main reference page Synthetic Routes to Octameric POSS , which surveys all three principal categories of cage synthesis, including corner capping of incompletely condensed precursors and post synthetic modification of preformed cages. Figure 1. Hydrolysis of an organotrifunctional silane and subsequent polycondensation of the resulting silanols to the octameric T 8 framework.

Polyhedral Silsesquioxanes Research on compounds containing the Si–O bond has long been dominated by silicon dioxide, by minerals built from repeating SiO₂ units, and by silicones composed of repeating R₂SiO units (R = alkyl or phenyl). Over the past 20 to 30 years, however, interest in silsesquioxanes based on the RSiO₁.₅ unit has increased markedly.

Silsesquioxane nomenclature governs how chemists name and communicate the structures of polyhedral oligomeric silsesquioxane (POSS) compounds and related organosilicon frameworks. The term silsesquioxane derives from the Latin silicium (silicon), sesqui (one and a half), and oxygenium (oxygen), reflecting the characteristic oxygen-to-silicon atomic ratio of 1.5. Because systematic IUPAC designations for polyhedral silsesquioxanes are unwieldy — reaching 40 or more characters for even the simplest T 8 cage — the field relies on the concise M, D, T, and Q silicon atom classification and on the compact R n T n shorthand notation.