Structures of oligomeric silsesquioxanes (POSS)

Polyhedral Silsesquioxanes

Research on compounds containing the Si–O bond has for years been dominated by silicon dioxide, minerals containing repeating SiO₂ fragments, and silicones composed of repeating R₂SiO units (R = alkyl or phenyl). Over the past 20–30 years, there has been a noticeable increase in studies on silsesquioxanes containing the RSiO₁.₅ unit. These compounds, due to the presence of both an inorganic fragment and an organic group, possess hybrid properties. The inorganic Si–O–Si fragment imparts chemical and thermal resistance to these compounds, while the organic R group increases their solubility and provides appropriate reactivity. A wide range of polymeric structures with the general formula (RSiO₁.₅)ₙ can be synthesized, but the most interesting are those with polyhedral architectures.⁴

Polyhedral oligomeric silsesquioxanes (POSS) are three-dimensional organosilicon compounds with the general empirical formula (RSiO₁.₅)ₙ (where R = H, alkyl, alkenyl, or aryl; n = 6, 8, 10, 12). A wide variety of POSS structures can be obtained, such as ladder-like (Figure 3, part b), cage-like structures differing in the number of silicon atoms (Figure 3, part c), and open cages (Figure 3, parts d–f). Polyhedral silsesquioxanes exhibit high chemical and thermal resistance,⁵–¹¹ making them useful precursors for the production of functional materials such as porous materials,¹²–¹⁸ catalysts,¹⁹–²¹ superhydrophobic materials,²²-³³ luminescent materials,³⁴-⁴² composites,⁴³–⁵² and others.⁵³

Through appropriate modification of POSS side groups, it is possible to "tune" their solubility, which allows for the preparation of soluble silsesquioxane nanoparticles with sizes up to 5 nm.⁵⁴,⁵⁵ In contrast to organosilicon materials obtained via the sol–gel method—which in most cases exhibit undefined structures (Figure 3, part a)—the use of polyhedral silsesquioxanes allows for better control over the structure and morphology of the resulting nanoparticles. Due to their well-defined three-dimensional architecture, the most interesting group of silsesquioxanes are the cage-type compounds (Figure 3, part c). It is possible to obtain cages of various geometries and sizes, such as octamers, decamers, or dodecamers (denoted as T₈, T₁₀, T₁₂). Their unique properties are related to their three-dimensional core and nanometric size, which allows for the emergence of properties not observed at the macroscale.⁴

 

Structures of silsesquioxanes (Chemistry - A EuropeanJournal 2014, 20, 15966-15974)

Nomenclature and Designations
The English term silsesquioxane derives from Latin and can be interpreted as follows: silicium (silicon), sesqui (one and a half), oxygenium (oxygen), indicating that the ratio of oxygen atoms to silicon atoms is 1.5.⁵⁶ In Polish-language literature, the term is commonly adapted as silseskwioksan. The use of systematic names for polyhedral silsesquioxanes can be cumbersome; therefore, the nomenclature typically used for siloxanes is often applied.⁵⁷ This classification distinguishes five types of silicon atoms (Figure 4). The "M" type denotes a silicon atom bonded to three organic groups and one oxygen atom. A "D"-type silicon atom is bonded to two oxygen atoms, a "T"-type to three oxygen atoms, and a "Q"-type to four oxygen atoms.⁵⁶

Figure 4. Types of silicon atom connections (R = hydrogen, alkyl, or phenyl).

Si–O bonds can form either siloxane groups (Si–O–Si) or silanol groups (Si–OH). To distinguish between them, a superscript is used to indicate the number of Si–O–Si connections formed by a silicon atom. For example, T³ denotes a silicon atom bonded to one organic group and three siloxane linkages. Polyhedral silsesquioxanes contain T³-type silicon atoms. The number of silicon atoms is given as a subscript. The designation T₈R₈ or R₈T₈ refers to an octameric hexahedral silsesquioxane containing eight organic side groups or hydrogen atoms. The formula Me₈T₈ corresponds to octamethyl-octasilsesquioxane, a compound with the systematic name octamethyl-pentacyclo[9.5.1.1³,⁹.1⁵,¹⁵.1⁷,¹³]octasiloxane, shown in Figure 5.

The material presented in this dissertation has been or will be published in English. Therefore, the consistent use of the English number format (with a period as the decimal separator) was adopted, which is especially convenient in the context of NMR spectra interpretation.

Figure 5. Structure of octamethyl-octasilsesquioxane (Me₈T₈).

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