Silsesquioxane T8 Cube
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220x220px, thumbnail, A cubic silsesquioxane. A silsesquioxane is an
organosilicon compound Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, f ...
with the
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
SiO3/2sub>n (R = H,
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...
,
aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...
,
alkenyl In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as α-olefins. The International Union of Pu ...
or alkoxyl.). Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and
tetrahedral In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ("POSS"), which have attracted attention as preceramic polymer precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
core confers rigidity and thermal stability.


Structure

Silsesquioxanes are known in molecular form as well as polymers. The 6-, 8-, 10-, and 12-Si cages are sometimes labeled T6 T8, T10, and T12, respectively (T = tetrahedral vertex). The T8 cages, the most widely studied members, have the formula SiO3/2sub>8, or equivalently R8Si8O12. In all cases each Si center is bonded to three doubly bridging oxo groups, which in turn connect to other Si centers. The fourth group on Si is usually an alkyl, halide, hydride, alkoxide, etc. In the cubic clusters with ''Oh'' symmetry the Si-O-Si angles are in the range 145–152°, being bowed out, allowing the Si centers to better adopt tetrahedral geometry. The O-Si-O angle are in the range: 107–112°, Si-O bond: 1.55–1.65 Å.


Synthesis

Silsesquioxanes are usually synthesized by hydrolysis of organo
trichlorosilane Trichlorosilane (TCS) is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes ...
s. An idealized synthesis is: :8 RSiCl3 + 12 H2O → SiO3/2sub>8 + 24 HCl The formation of HCl negatively impacts the relative rates of hydrolysis and condensation of intermediate silanols. Consequently, silsesquioxanes can be obtained directly by condensation of the corresponding silanetriols which occurs at neutral pH and works even for sterically very bulky substituents. :8 RSi(OH)3SiO3/2sub>8 + 12 H2


Reactivity


Substituent modifications

Depending on the R substituent, the exterior of cage can be further modified. When R = H, the Si-H group can undergo
hydrosilylation Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds."Hydrosilylation A Comprehensive Review on Recent Advances" B. Marciniec (ed.), Advances in Silicon Science, Springer Science, 200 ...
or oxidation to the
silanol A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon c ...
. Bridged polysilsesquioxanes are most readily prepared from clusters that contain two or more trifunctional silyl groups attached to non-hydrolysable silicon-carbon bonds, with typical sol-gel processing. Vinyl-substituted silsesquioxanes can be linked by the
alkene metathesis In organic chemistry, Olefin Metathesis or Alkene Metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative sim ...
.


Cage-Rearrangement

Reorganization of the siloxane cage-like core (T8 → T10) can be induced by
trifluoromethanesulfonic acid Triflic acid, the short name for trifluoromethanesulfonic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest known acids. Triflic acid is mainly used in research as a catalyst for este ...
(CF3SO3H). In this case, reaction of hexahedral silsesquioxane and CF3SO3H in DMSO conducted in 1 : 12 molar ratio gives heptahedral silsesquioxane. In the first step CF3SO3H acid attacks siloxane Si-O-Si bonds and the formation of Si-O-SO2CF3 bond, opening one edge of the cage (Figure below). Such an inversion is observed at silicon atom during nucleophilic displacement reaction that is usually noticed when leaving groups are replaced by soft nucleophiles. Upon further acid attack, both T6(OH)4 C and siloxane dimer D are formed. Because this reaction takes place in an aqueous conditions, compound E of general formula T8(OH)4 as a consequence of hydrolysis reaction was obtained. E is prone to reaction with D and due to this, the abstraction of CF3SO anion occurs and the closure frame with the spontaneous cage-rearrangement to heptahedral T10 structure F is observed. Although, heptahedral F is less favorable energetically (MM2 data), in this case its creation is forces by the formation of a new Si4O4 moiety from much more less stable substrates D and E. thumbnail, 200px, Poly(phenylsilsesquioxane) does not adopt a cage structure, but is a polymer with a ladder-like repeating unit.


Polymeric silsesquioxanes

Polymeric silsesquioxanes have been prepared starting with the hydrolysis of phenyltrichlorosilane. Hierarchical organic-inorganic (hybrid) polysilsesquioxane (PSQ) materials using polyhedral oligomeric silsesquioxanes (POSS) cages as singular building blocks of the inorganic framework exhibit high specific surface area and hydrothermal stability and micro and/or mesoporosity. In addition, Marchesi et al. developed a series of amorphous POSS-based polysilsesquioxanes, in which the POSS cages (of partially-condensed T7-POSS or anionic completely-condensed T8-POSS molecules) act as structural units of the polymer-like inorganic network, both alone or in conjunction with metal ions of the lanthanide series (
europium Europium is a chemical element; it has symbol Eu and atomic number 63. It is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. Europium is the most chemically reactive, least dense, and soft ...
and
terbium Terbium is a chemical element; it has Symbol (chemistry), symbol Tb and atomic number 65. It is a silvery-white, rare earth element, rare earth metal that is malleable and ductile. The ninth member of the lanthanide series, terbium is a fairly ele ...
, especially).


Hydridosilsesquioxanes

A well-known
hydrogen silsesquioxane 200px, thumbnail, Hydrogen silsesquioxane (R = H). Hydrogen silsesquioxane(s) (HSQ, H-SiOx, THn, H-resin) are inorganic compounds with the empirical formula SiO3/2sub>n. The cubic H8Si8O12 (TH8) is used as the visual representation for HSQ. TH8 ...
is SiO3/2sub>8. Early syntheses involved treatment (
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brø ...
) of
trichlorosilane Trichlorosilane (TCS) is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes ...
with concentrated
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
, and fuming sulfuric acid, affording T10-T16 oligomers. The T8 cluster was also synthesized by the reaction of trimethylsilane. The Si-H groups are amenable to
hydrosilylation Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds."Hydrosilylation A Comprehensive Review on Recent Advances" B. Marciniec (ed.), Advances in Silicon Science, Springer Science, 200 ...
.


Applications

Some high molecular weight soluble polymethylsilsesquioxanes have found applications in cosmetics. resins, and lithography.


Potential applications


Electronic materials

Films of organosilsesquioxane, e.g., poly(methylsilsesquioxane), have been examined for semiconducting devices. Poly(hydridosilsesquioxane), which has a linked-cage structure, was sold under the name Fox Flowable Oxide. Methylsilsesquioxanes have been examined for spin-on-glass (SOG) dielectrics. Bridged silsesquioxanes have been used for quantum confined nano-size semiconductors. Silsesquioxane resins have also been used for these applications because they have high dielectric strengths, low dielectric constants, high volume resistivities, and low dissipation factors, making them very suitable for electronics applications. These resins have heat and fire resistant properties, which can be used to make fiber-reinforced composites for electrical laminates. Polyhedral oligomeric silsesquioxanes have been examined as a means to give improved mechanical properties and stability, with an organic matrix for good optical and electrical properties. The mechanisms of degradation in these devices is not well understood, but it is believed that material defect understanding is important for understanding the
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
and
electronic Electronic may refer to: *Electronics, the science of how to control electric energy in semiconductors * ''Electronics'' (magazine), a defunct American trade journal *Electronic storage, the storage of data using an electronic device *Electronic c ...
properties. Hydridosilsesquioxanes can be converted to silica coatings for potential application in integrated circuits.


LEDs

For potential applications to
light emitting diodes A light-emitting diode (LED) is a semiconductor device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of pho ...
, cubic silsesquioxanes. have been functionalized. One of the first precursors used in light emitting application was octadimethylsiloxysilsesquioxane, which can be prepared in yields of >90% by treating tetraethoxysilane or rice hull ash with tetramethylammonium hydroxide followed by dimethylchlorosilane. The general method of hydrolyzing organotrichlorosilanes is still effective here. When brominated or aminated, these structures can be coupled with epoxies, aldehydes, and bromoaromatics, which enable attachment of these silsesquioxanes to π-conjugated polymers. These methods can use
copolymerization In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are som ...
techniques,
Grignard reagents Grignard reagents or Grignard compounds are chemical compounds with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...
, and different coupling strategies. There has also been research on the ability of conjugated dendrimer silsesquioxanes to behave as light emitting materials. Though, highly branched substituents tend to have π-π interactions, which hinder high luminescent
quantum yield In particle physics, the quantum yield (denoted ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. \Phi(\lambda)=\frac Applications Fluorescence spectroscopy The fluorescence ...
.


Sensors

For chemosensor applications, silsesquioxane cages conjugated with fluorescent molecules can be directly used to detect fluoride ions under a cage-encapsulation showing a change of color under naked eyes and other anions.


Antimicrobial silsesquioxanes

Silsesquioxanes have been functionalized with biocidal quaternary ammonium (QASs) groups to produce
antimicrobial An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...
coatings. QASs are
disinfectants A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than ...
,
antiseptics An antiseptic ( and ) is an antimicrobial substance or compound that is applied to living tissue to reduce the possibility of sepsis, infection, or putrefaction. Antiseptics are generally distinguished from ''antibiotics'' by the latter's abili ...
, and antifoulants. The relatively small size of the silsesquioxanes, 2-5 nm, allows a QAS functionalized molecule to have a charge density similar to that of some
dendrimers Dendrimers are highly ordered, branched polymeric molecules. Synonymous terms for dendrimer include arborols and cascade molecules. Typically, dendrimers are symmetric about the core, and often adopt a spherical three-dimensional morphology. The ...
and thus the antimicrobial efficacy is prominent. Dimethyl-n-octylamine was quaternized by octa(3-chloropropylsilsesquioxane), (T-ClPr)8. Array of QAS functionalized polyhedral oligomeric silsesquioxanes (Q-POSS) have been reported. These researchers varied the alkyl chain length from –C12H25 to –C18H37 and varied the counter ion between chloride, bromide, and iodine. The first reaction was the hydrosilylation between allydimethlamine and octasilane polyhedral oligomeric silsesquioxane via
Karstedt's catalyst Karstedt's catalyst is an organoplatinum compound derived from divinyl-containing disiloxane. This coordination complex is widely used in hydrosilylation catalysis. It is a colorless solid that is generally assumed to be a mixture of related Pt(0 ...
to make a tertiaryamino-functionalized silsesquioxane. The second step was the quaternization of the tertiaryamino groups with an alkyl halide. The alkyl halides used were 1-iodooctadecane, 1-bromohexadecane, and 1-chloroctadecane. The silsesquioxane core in these hybrid materials provides an increased
glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...
, improved mechanical properties, higher use temperature, and lower flammability. These desirable properties combined with the ability to readily functionalize a silsesquioxane with multiple antimicrobial groups allows for robust biocides with higher charge densities while maintaining a compact molecular structure. The organic functionalities provide high compatibility with polymers allowing for easy incorporation into many mediums. Of particular interest are silicone paints and coatings used in hospitals. Typical biocidal ammonium functionalized polymers are incompatible, but silsesquioxanes closely mimic the silicone structure. A silicone-based paint combined with QAS-functionalized silsesquioxanes could be used to paint medical and sanitary devices, biomedical devices, exam equipment, medical storage rooms, hospital rooms, clinics, doctor offices, etc. to prevent the formation and spread of bacteria. For example, the Q-POSS developed was combined with polydimethylsiloxane and catalysis to form a crosslinked network. The researchers found that coatings based on bromide and chloride had the best antimicrobial efficacy.


Partially condensed silsesquioxanes: Si7 species

A well studied example of a partially condensed silsesquioxanes is the tri
silanol A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon c ...
Cy7Si7O9(OH)3, prepared by the slow (months) hydrolysis of trichlorocyclohexylsilane (C6H11SiCl3). The same cage can be prepared by acid-mediated cleavage of fully condensed silsesquioxane. This process results in silanediols that can further be used to create new metallasilsesquioxanes. These partially condensed silsesquioxanes are intermediates en route to the fully condensed cages. In general, such silsesquioxane trisilanols form discrete dimers in the solid held together by cooperatively enhanced cyclic
hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
ed networks. These dimers are retained in solution and a dynamic equilibrium has been elucidated using
NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...
.


Other partially condensed silsesquioxanes

Other partially condensed species adopt ladder structures wherein in which two long chains composed of RSiO3/2 units are connected at regular intervals by Si-O-Si bonds. Amorphous structures include RSiO3/2 unit connections without any organized structure formation.


Metal complexes of partially condensed silsesquioxanes and metal-containing POSS

The incompletely condensed silsesquioxanes bind numerous metals, including Na+, Li+, and Be2+ as well as transition metals. Cubic metal-silsesquioxane derivatives of the core stoichiometry MSi7O12 can be prepared by treating the incomplete cage with a metal halide in the presence of a base such as triethylamine. Another route of synthesis involves first deprotonating the trisilanol group using strong bases such as LiN(SiMe3)2.
rare-earth The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set of ...
-doped POSS were prepared from heptaisobutyl trisilanol T7-POSS ((C4H9)7Si7O9(OH)3) with EuCl3. Furthermore, a combination of partially-condensed tetrasilanolphenyl POSS with terbium acetate and/or europium acetate lead to novel double-decker silsesquioxane (DDSQ) materials consisting of lanthanide-doped POSS units with intrinsic luminescent properties, in which the
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium (el ...
ion(s) act as both structural and functional agents in the final compound.


Catalytic properties

Although lacking commercial applications, metallasilsesquioxanes have been investigated as catalysts and models for heterogenerous catalysts. The coordination environment provided by Cy7Si7O9(OH)3 has been proposed to approximate that of silica-based
catalyst support In chemistry, a catalyst support or carrier is a material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the ma ...
s. Some of these complexes catalyze
alkene metathesis In organic chemistry, Olefin Metathesis or Alkene Metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative sim ...
,
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
,
epoxidation In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...
, and Diels-Alder reactions. They serve as Lewis acid-catalysts for Oppenauer oxidation and Meerwein-Pondorf-Verley reductions. A number of metallasilsesquioxanes have been reported that can polymerize
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...
, akin to the
Phillips catalyst The Phillips catalyst, or the Phillips supported chromium catalyst, is the catalyst used to produce approximately half of the world's polyethylene. A heterogeneous catalyst, it consists of a chromium oxide supported on silica gel. Polyethylene, ...
. The catalyst is activated with trimethylaluminum and typically proceeds with high turnover number. As ligands, the silsesquioxane frameworks are approximately as electron-withdrawing as a trifluoromethyl group. One of the few crystallographically characterized metalated silsesquioxane-based catalytic systems involves the use of silsesquioxanes as soluble models for tethered osmium(IV) and rhodium(II) complexes. A mononuclear organopalladium-functionalized T8-Pd POSS functions as an active
homogeneous catalyst In chemistry, homogeneous catalysis is catalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in di ...
for the Suzuki–Miyaura cross-coupling reaction. A Zn4-POSS catalyzes the cycloaddition of carbon dioxide (CO₂) with terminal epoxides, using tetrabutylammonium iodide (TBAI) as a co-catalyst.


References

{{oxygen compounds Organosilicon compounds Oxygen compounds