Structures of oligomeric silsesquioxanes (POSS)
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.
Because these compounds contain both an inorganic fragment and an organic group, they display hybrid properties. The inorganic Si–O–Si framework imparts chemical and thermal resistance, whereas the organic R group enhances solubility and provides the required reactivity. A wide range of polymeric structures with the general formula (RSiO₁.₅)ₙ can be synthesized, although those adopting polyhedral architectures are of greatest interest.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 and n = 6, 8, 10, or 12. A wide variety of POSS architectures is accessible, including ladder-like structures (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 to f). Polyhedral silsesquioxanes exhibit high chemical and thermal resistance,5–11 which makes them valuable precursors for 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
Appropriate modification of the POSS side groups makes it possible to tune their solubility, which enables the preparation of soluble silsesquioxane nanoparticles with diameters of up to 5 nm.54,55 In contrast to organosilicon materials obtained by the sol–gel method, which in most cases possess poorly defined structures (Figure 3, part a), polyhedral silsesquioxanes offer considerably better control over the structure and morphology of the resulting nanoparticles. Owing to their well-defined three-dimensional architecture, cage-type compounds constitute the most interesting group of silsesquioxanes (Figure 3, part c). Cages of various geometries and sizes can be obtained, including octamers, decamers, and dodecamers, denoted T8, T10, and T12, respectively. Their distinctive behavior arises from the three-dimensional core and the nanometric dimensions of these molecules, which give rise to properties that are not observed at the macroscale.4
Nomenclature and Designations
The English term silsesquioxane derives from Latin and may be parsed as silicium (silicon), sesqui (one and a half), and 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. Because systematic names for polyhedral silsesquioxanes are cumbersome, the nomenclature normally used for siloxanes is generally applied instead.57 This classification distinguishes five types of silicon atom (Figure 4). The M type denotes a silicon atom bonded to three organic groups and one oxygen atom, the D type a silicon atom bonded to two oxygen atoms, the T type a silicon atom bonded to three oxygen atoms, and the Q type a silicon atom bonded to four oxygen atoms.56
Siloxane and Silanol Linkages: Superscript Notation for Silicon Connectivity
Si–O bonds may form either siloxane groups (Si–O–Si) or silanol groups (Si–OH). To distinguish between the two, a superscript indicates the number of siloxane linkages formed by a given silicon atom. For example, T³ denotes a silicon atom bonded to one organic group and three siloxane linkages, and polyhedral silsesquioxanes contain T³-type silicon atoms. The number of silicon atoms is given as a subscript, so that the designation T₈R₈, or equivalently R₈T₈, refers to an octameric hexahedral silsesquioxane bearing eight organic side groups or hydrogen atoms. The formula Me₈T₈ therefore corresponds to octamethyl-octasilsesquioxane, a compound whose systematic name is octamethyl-pentacyclo[9.5.1.1³,⁹.1⁵,¹⁵.1⁷,¹³]octasiloxane and whose structure is shown in Figure 5.
The material presented in this dissertation has been published, or will be published, in English. For this reason the English number format, with a period as the decimal separator, has been adopted throughout, a convention that is particularly convenient in the interpretation of NMR spectra.
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