Silsesquioxane Nomenclature: T8 Cage and M/D/T/Q Notation
M, D, T, and Q Silicon Atom Classification in Siloxane Nomenclature
Because the systematic IUPAC names of polyhedral silsesquioxanes are often unwieldy, the literature commonly employs the nomenclature developed for siloxanes, which classifies silicon atoms into five connectivity types (see figure below). A type N silicon atom bears four organic substituents and no oxygen atom at all, as in a tetraorganosilane, and is therefore incapable of entering a siloxane network. A type M silicon atom is bonded to three organic substituents and one oxygen atom. Type D denotes a silicon atom bonded to two oxygen atoms, type T one bonded to three oxygen atoms, and type Q one bonded to four oxygen atoms. Si–O bonds in these frameworks may constitute either siloxane (Si–O–Si) or silanol (Si–OH) linkages. To differentiate these, a superscript is used to specify the number of Si–O–Si bonds formed by a given silicon atom; for example, T3 designates a silicon atom bearing one organic substituent and three siloxane bonds. All silicon atoms in polyhedral silsesquioxanes are of the T3 type.
Connection types for silicon atoms, including the non-siloxane N type (R = hydrogen, alkyl, or phenyl).
Where the Letter T Comes From and Why the Exponent Is 1.5
The four letters do not refer to shapes, ring sizes, or cage types. They abbreviate the functionality of the silicon atom, that is, the number of oxygen atoms through which it can be incorporated into a siloxane network. A monofunctional silicon atom of the type R3SiO1/2 is designated M, a difunctional silicon atom of the type R2SiO2/2 is designated D, a trifunctional silicon atom of the type RSiO3/2 is designated T, and a quadrifunctional silicon atom of the type SiO4/2 is designated Q. The letter T therefore stands for trifunctional, and it identifies the only building unit from which a silsesquioxane framework can be constructed, because a silsesquioxane is by definition a compound in which every silicon atom carries exactly one non-hydrolyzable substituent and three oxygen atoms. This classification originated in the industrial silicone literature of the 1940s and 1950s and was retained by the silsesquioxane community because it encodes connectivity far more compactly than any systematic name.
The fractional exponent in the empirical formula RSiO1.5 follows directly from this connectivity. Every oxygen atom in a fully condensed framework bridges two silicon atoms, so each silicon atom owns only one half of each oxygen atom bonded to it. A trifunctional silicon atom is bonded to three oxygen atoms, and three multiplied by one half gives 1.5. The formula RSiO3/2, which is chemically more transparent, and the formula RSiO1.5, which is more common in the materials literature, are therefore identical in meaning. The same arithmetic explains the other units, since M corresponds to R3SiO0.5, D corresponds to R2SiO1.0, which is why polydimethylsiloxane is written as (Me2SiO)n, and Q corresponds to SiO2.0, the empirical formula of silica. The ratio of 1.5 lies exactly halfway between the ratio of unity found in linear silicones and the ratio of two found in silica, which is the structural reason why silsesquioxanes are frequently described as molecular models of a silica surface decorated with organic groups.
An important consequence of writing the composition as an empirical formula is that RSiO1.5 carries no information about molecular size or architecture. Polyhedral cages, incompletely condensed cages, ladder polymers, and amorphous random networks all share the same empirical composition. For this reason the empirical formula is normally written as (RSiO1.5)n and is accompanied by an explicit structural designation whenever the architecture matters.
T8 Polyhedral Cage: R8T8 Shorthand Notation and IUPAC Systematic Naming
The number of silicon atoms in the cage is indicated by a subscript. The designation T8R8 or R8T8 refers to an octameric, cube-shaped silsesquioxane cage bearing eight organic substituents or hydrogen atoms. The formula Me8T8 denotes octamethyloctasilsesquioxane, whose systematic name is octamethylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, as illustrated in the figure below.
Structure of octamethyloctasilsesquioxane (Me8T8).
T8, T10, and T12 Cage Sizes and the D4R, D5R, and D6R Descriptors
Because the subscript counts trifunctional silicon atoms, the symbols T8, T10, and T12 denote fully condensed polyhedra, consistent with the history of T8, T10, and T12 cage synthesis, containing eight, ten, and twelve silicon atoms and, correspondingly, twelve, fifteen, and eighteen siloxane bridges. The three species are the members of the homologous series that are isolated most often from hydrolytic condensation, and they are almost always obtained together, which is why their separation and their relative ratio are recurring practical concerns. The T8 cage has the shape of a slightly distorted cube built from six four-membered Si4O4 rings and possesses idealized Oh symmetry. The T10 cage is a pentagonal prism composed of two five-membered rings joined by five four-membered rings, with idealized D5h symmetry. The T12 framework is not unique, since a hexagonal prism of idealized D6h symmetry and a lower-symmetry isomer of D2d symmetry are both known, which is one reason why the number of resonances in the 29Si NMR spectrum, rather than the molecular formula alone, is used to identify the species present.
The descriptor D4R belongs to a different naming tradition and is frequently misread. It is not related to the difunctional D unit at all. The abbreviation is borrowed from silicate and zeolite crystal chemistry, where it stands for double four ring, meaning a secondary building unit formed by two four-membered rings of tetrahedra stacked one above the other and connected by four additional bridges. The T8 cage is exactly such a unit, so a T8 silsesquioxane is described in the zeolite and silicate literature as a D4R species, and the octaanion Si8O208− is commonly called the D4R silicate. By the same logic T10 corresponds to a double five ring, abbreviated D5R, and the prismatic isomer of T12 corresponds to a double six ring, abbreviated D6R. The practical rule is that a symbol of the form Tn counts silicon atoms and implies an organic substituent on each of them, whereas a symbol of the form DnR describes the topology of the polyhedron and is normally applied to purely inorganic silicate anions or to the framework of a zeolite.
A related distinction concerns cages that are not fully condensed. When one silicon vertex is missing from the T8 framework, the resulting open cage retains seven silicon atoms and three silanol groups and is written R7T7(OH)3 or, more informally, as a trisilanol POSS. In such compounds the superscript notation introduced above becomes essential, because the three silicon atoms bearing hydroxyl groups are of the T2 type, while the remaining four are of the T3 type. This pattern is directly observable in the 29Si NMR spectrum and is the standard criterion for confirming that a cage has been opened.
Substituent Notation: T8-Me, Me8T8, and Acronyms Such as OAS-POSS
Three conventions coexist for indicating which organic group occupies the vertices of a cage. The first places the substituent before the cage symbol, as in Me8T8, Ph8T8, or Vi8T8, and is preferred in the organosilicon literature because it mirrors the molecular formula. The second reverses the order and gives T8Me8 or T8R8. The third, written with a hyphen as T8-Me, T8-Ph, or T8-Vi, is common in polymer and materials science, where the cage is treated as a nanoscale filler and the suffix simply names the corona of substituents. All three refer to the same compound, and the hyphenated form should be read as a cage of eight trifunctional silicon atoms carrying eight methyl groups rather than as a cage bearing a single methyl group. When a cage carries mixed substituents, the composition is written explicitly, for example R7R'T8 for a monofunctionalized derivative, which is the standard designation for the mono-substituted cages used in the construction of hybrid architectures.
Alongside these systematic shorthands, a large family of acronyms has entered the literature. They are generally formed by combining a prefix denoting eight substituents with an abbreviation of the substituent name and the term silsesquioxane, giving designations such as OVS for octavinylsilsesquioxane, OPS for octaphenylsilsesquioxane, OHS for octahydridosilsesquioxane, and OAPS for octa(aminophenyl)silsesquioxane. The composite forms encountered in the applied literature, such as OAS-POSS, follow the same pattern and simply append the general POSS label to the acronym in order to signal that the compound is a polyhedral cage rather than a resin or a network. These acronyms are convenient but they are not standardized, the same string is occasionally used by different authors for different amino-functionalized or anilino-functionalized derivatives, and the isomeric purity of the cage is rarely encoded in them. For this reason an acronym of this type should always be defined at first occurrence together with the explicit formula of the cage, and the Tn notation should be preferred whenever the number of silicon atoms or the degree of condensation is part of the argument being made.
Further Reading
DOI: 10.1021/cr00036a004
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