Selected POSS Compounds: Structures, Preparation and Reactivity

Polyhedral oligosilsesquioxanes constitute a class of nanometer-scale organosilicon compounds characterized by well-defined, highly symmetrical cage frameworks consisting of silicon and oxygen atoms surrounded by organic substituents in three-dimensional arrangements. With diameters between 1 and 3 nm, these materials represent the smallest possible particles of silica bearing surface organic groups, and their properties combine those of both organic and inorganic components, making them true nanocomposite materials. The compounds collected below are arranged according to the increasing complexity of the substituent carried by the T8 cage, beginning with the hydride and ending with functional groups that introduce optical activity or a coordinated metal center.

Contents

1. Octahydridooctasilsesquioxane and its siloxy analogue
2. Octamethylsilsesquioxane
3. Octavinyloctasilsesquioxane
4. Octaphenylsilsesquioxane
5. Octa(3-aminopropyl)silsesquioxane and its ammonium salts
6. Amido-functionalized silsesquioxanes
7. Imine-functionalized silsesquioxanes
8. Difluoroboron derivatives
9. Metal complexes supported on the cage
10. Derivatives with photoresponsive and emissive substituents
11. Monofunctionalized cages
12. Higher cages T10 and T12

Octahydridooctasilsesquioxane and Its Siloxy Analogue

Octahydridooctasilsesquioxane, H₈Si₈O₁₂, carries the simplest possible substituent and occupies the first position in any catalog of POSS building blocks. It contains a labile hydrogen atom bonded to a silicon atom, which enables the compound to undergo hydrosilylation reactions and renders it a valuable substrate for the preparation of a variety of silsesquioxane derivatives. Routes leading to the cage itself are collected in the survey of POSS synthesis, and the M, D, T and Q labels used throughout this page are explained in the article on silsesquioxane nomenclature.

Hydrosilylation is a reaction in which an organosilicon compound bearing a Si–H bond adds across an unsaturated bond such as C=C, C=O, C=N, N=O, or N=N. Catalysts employed in this reaction include free radicals, organic bases, transition-metal complexes, and pure metals. In the case of silsesquioxanes, the most commonly used catalysts are H₂PtCl₆ and Karstedt's catalyst, owing to their high reactivity and selectivity.

A method for synthesizing silsesquioxanes bearing alkyl chains via hydrosilylation of octahydridooctasilsesquioxane, as developed by Y. Aziz, is presented in Scheme 11. Analogous reaction conditions were employed by W. Lin in reactions with styrene derivatives.

Scheme 11. Schematic representation of the synthesis of cage silsesquioxane derivatives via hydrosilylation.

A drawback of this approach is the possibility of forming two isomers, α and β (see Scheme 12). Hydrosilylation of cage silsesquioxanes typically yields a mixture of isomers, necessitating product purification. A further example is the synthesis reported by Y. Sheen’s group in 2008, in which octakis(dimethylsiloxy)octasilsesquioxane served as the substrate, yielding a range of both alkyl and aryl derivatives (Scheme 12). The latter compound, in which each corner of the cage bears a dimethylsilyloxy group, extends the reactive Si–H function away from the rigid core and reduces the steric congestion that accompanies direct substitution at the cage vertices.

Scheme 12. Schematic representation of the synthesis of POSS derivatives via hydrosilylation.

Octamethylsilsesquioxane

Replacing the hydride by a methyl group gives (CH₃SiO_1.5)₈, the simplest cubic cage bearing a carbon substituent and the compound of highest symmetry in the whole family. The molecule belongs to the O_h point group, which reduces its ²⁹Si NMR spectrum to a single resonance and makes it the natural reference point for assigning spectra of substituted cages.

Its practical value follows from volatility rather than reactivity. The compound sublimes below 248 °C without decomposition, which permits purification by sublimation and deposition from the vapor phase. The methyl groups offer no site for further functionalization, and the compound therefore serves mainly as a structural model and as a thermally robust filler. A detailed account of its preparation, symmetry and thermal behavior is given in the article on octamethylsilsesquioxane, while the influence of the alkyl chain length on decomposition, which spans the range from 166 to 355 °C across the homologous series, is discussed in the article on thermal stability of POSS cages.

Octavinyloctasilsesquioxane

Octavinyloctasilsesquioxane, (CH₂=CHSiO_1.5)₈, is obtained by hydrolytic condensation of trichlorovinylsilane and combines a rigid core with eight terminal double bonds. It is the first compound in this list whose substituent is itself a reactive handle, and it therefore functions as a divergent platform for radial architectures.

Three transformations dominate its chemistry. Thiol-ene addition proceeds under radical initiation and installs eight thioether arms in a single step. Hydrosilylation with hydrosilanes converts the vinyl groups into alkylsilyl arms and provides access to dendrimers built outward from the cage. Phosphination yields ligand-bearing cages of interest in coordination chemistry. The synthesis, the reported yields and the derivatives accessible along each route are collected in the article on octavinyloctasilsesquioxane.

Octavinylsilsesquioxane.

Octaphenylsilsesquioxane

The phenyl derivative introduces an aromatic shell around the inorganic core and, with it, two features absent from the aliphatic members of the series. The first concerns constitution. Alkaline equilibration of phenylsilsesquioxanes converges on a narrow set of products, namely discrete cages in the range from T8 to T12 together with ladder polymers, a result that resolves a long-standing ambiguity in the literature and is examined in the article on the equilibration of phenylsilsesquioxanes.

The second feature concerns reactivity of the aromatic ring. Bromination of octaphenyl POSS proceeds in the ortho position without any added catalyst, because the electron-withdrawing cage polarizes the halogen molecule and generates a dipole moment of 0.33 D sufficient to initiate electrophilic attack. The modeling study behind this conclusion is presented in the article on ortho self-bromination of octaphenyl POSS. The same aromatic derivative is also the most effective additive in polymer matrices, since it raises the peak degradation temperature of polypropylene from 330 to 379 °C, as described in the article on POSS polymer nanocomposites.

Octa(3-aminopropyl)silsesquioxane and Its Ammonium Salts

Octa(3-aminopropyl)silsesquioxane hydrochloride (OAS) was first described in a patent by Wacker-Chemie GmbH, however that document neither details the preparative procedure nor provides the spectroscopic data required for full characterization. Subsequent investigators developed a modified synthesis employing commercially available (3-aminopropyl)triethoxysilane (APTES). Compounds 1–3 (see scheme below) can be obtained via a one-step hydrolytic condensation of APTES with an appropriate quantity of hydrochloric acid (3.6 equivalents). This approach improves the yield and minimizes the formation of deca- and other multiply substituted polyhedral silsesquioxanes, which are difficult to separate.

The compound is isolated as a crystalline ammonium salt, and the identity of the counterion governs both solubility and the subsequent chemistry. Salts of this type are the entry point to the two largest families of functional cages described further below, namely the amides and the imines. Under strongly acidic conditions the cage itself rearranges, and treatment of the octameric ammonium salt with triflic acid affords the decameric T10 analogue in 44 percent yield, a transformation reported in the article on triflic acid rearrangement of T8 into T10 amino POSS salts. The preparation itself, together with the purification and the spectroscopic assignment, is described in the article on octa(3-aminopropyl)silsesquioxane.

Synthesis of [OAS-POSS-NH₃]X, after Chem. Eur. J. 2014, 20, 15966–15974.

Amido-Functionalized Silsesquioxanes

Acylation of the ammonium salts with acyl chlorides converts all eight arms into amide groups and proceeds in yields close to 95 percent, which places this route among the most efficient functionalizations of the cage reported to date. The products are homosubstituted, they retain the cage intact, and they self-assemble into nanoparticles of approximately 5 nm. The procedure, together with NMR, infrared and thermogravimetric characterization, is presented in the article on amide POSS obtained from OAS salts and acyl chlorides.

The choice of base determines whether the cage survives the reaction. A base of sufficient strength to deprotonate the ammonium group without attacking the siloxane framework preserves the polyhedron, whereas stronger nucleophilic bases promote cleavage of the Si–O–Si bonds and lead to uncontrolled condensation. This dependence, which governs the reproducibility of the whole route, is analyzed in the article on amide POSS and the role of base choice in cage integrity. Amide films of this type combine hydrophobic surfaces with high decomposition onsets and represent the most straightforward way of converting a water-soluble ammonium salt into a processable material.

Imine-Functionalized Silsesquioxanes

Condensation of the aminopropyl arms with aromatic aldehydes gives octa-imino cages, in which eight C=N bonds connect the inorganic core to eight aromatic rings. The reaction is reversible, which permits error correction during assembly and explains why these compounds crystallize readily despite their size. Five crystal structures of octa-imino T8 silsesquioxanes have been determined, revealing halogen-bonded three-dimensional networks and, in the case of hydroxy-substituted aldehydes, a keto-enamine tautomer in the solid state. The structural discussion is given in the article on imine-POSS crystal structures and supramolecular networks.

Packing differences between individual members of the series translate directly into thermal behavior, with decomposition onsets beginning near 305 °C for the cages designated POSS-6 through POSS-9, a comparison presented in the article on rationally designed octa-imine POSS. Extending the same condensation to polyfunctional aldehydes replaces the discrete molecule by an extended network and yields porous silsesquioxane-imine frameworks, which reach iodine vapor uptakes of 485 weight percent.

Difluoroboron Derivatives

Chelation of the imine arms with boron trifluoride converts each Schiff base arm into a rigid difluoroboron ring and produces a metal-free photocatalyst carrying eight equivalent active sites on one molecule. The rigidity imposed by chelation suppresses nonradiative decay and raises the efficiency of energy transfer to molecular oxygen. For the derivative abbreviated POSS-tert-BF₂ the singlet oxygen quantum yield reaches 48 percent, and photooxidation converts 99 percent of the sulfide substrate into the corresponding sulfoxide under mild conditions with complete conversion within 40 minutes. The scope of the reaction and the photophysical measurements behind these values are reported in the article on POSS difluoroboron photocatalysts for sulfide oxidation.

Metal Complexes Supported on the Cage

The imine arms also act as chelating ligands, and coordination of a metal ion converts the cage into a multinuclear catalyst of fixed geometry. In the zinc complex denoted Zn₄@POSS-1, four metal centers are held at defined separations by the rigid core, and the resulting quattro-site catalyst couples styrene oxide with carbon dioxide at a pressure of one atmosphere, conditions under which most homogeneous catalysts require elevated pressure. The catalytic study is presented in the article on the zinc imine-POSS quattro-site catalyst.

Beyond zinc, crystallographic evidence exists for complexes of copper, manganese, vanadium, tin and several lanthanides bound either to the functionalized arms or to the silanolate oxygen atoms of incompletely condensed cages. The corresponding structures, together with the coordination modes observed in each case, are surveyed in the article on crystallographically confirmed metal-POSS complexes.

Derivatives with Photoresponsive and Emissive Substituents

Attaching a chromophore to the cage exploits a property that none of the simpler derivatives possesses, namely the ability of the rigid core to hold several identical dyes at fixed distances from one another. Pyrene-functionalized cages emit from both the monomer and the excimer, and the ratio of the two bands responds to fluoride ions and to polycyclic aromatic hydrocarbons at micromolar concentration, which provides a ratiometric sensing mechanism described in the article on fluorescent silsesquioxane sensors.

Azobenzene arms introduce reversible photoisomerization instead of emission. The amphiphilic derivative known as Azo-POSS aggregates in water and removes cationic dyes with an efficiency exceeding 94 percent, and ultraviolet irradiation switches the geometry of the arms and thereby the aggregation state, as reported in the article on photoswitchable Azo-POSS.

Monofunctionalized Cages

All the compounds above are homosubstituted, which means that the eight vertices carry identical groups. Replacing a single vertex produces an amphiphilic molecule with one reactive site and seven inert ones, and this asymmetry governs self-assembly. Monofunctionalized cages bearing platinum or porphyrin units assemble into nanostructures whose morphology switches with the solvent, and the same compounds serve as supports in palladium-catalyzed transformations. The preparation of the single-substituted cage, which remains the principal synthetic obstacle, and the assemblies obtained from it are described in the article on monofunctionalized POSS self-assembly.

Higher Cages T10 and T12

The decameric and dodecameric cages accompany the octamer in almost every hydrolytic condensation and are separated from it only with difficulty. The T10 cage adopts a pentagonal prismatic framework of D_5h symmetry, which lowers the symmetry of the silicon environment relative to the cube and produces a characteristic signature in the ²⁹Si NMR spectrum. Their formation, isolation and properties are discussed in the article on T10 decameric silsesquioxanes, and the electronic consequences of cage size, including the insulating gap of 6 to 7 eV common to the family, are covered in the article on the electronic properties of octameric silsesquioxanes.

Further Reading

Confirming the identity of any compound listed above relies on the same small set of techniques, and annotated reference spectra of the salts, amides and imines are collected in the guide on how to interpret POSS spectra. The primary literature behind every section is gathered in the POSS literature references, and a complete list of articles published on this blog, grouped by topic, is available in the POSS Article Index.