Silsesquioxane
A silsesquioxane is an
Structure
Silsesquioxanes are known in molecular form with 6, 8, 10, and 12 Si vertices, as well as polymers. The cages are sometimes labeled T6 T8, T10, and T12, respectively (T = tetrahedral vertex). The T8 cages, the most widely studied members, have the formula [RSiO3/2]8, or equivalently R8Si8O12. In all cases each Si center is bonded to three 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.[2][3][4][5]
Synthesis
Silsesquioxanes are usually synthesized by hydrolysis of organotrichlorosilanes.[6] An idealized synthesis is:
- 8 RSiCl3 + 12 H2O → [RSiO3/2]8 + 24 HCl
The formation of HCl negatively impacts the relative rates of hydrolysis and condensation of intermediate
- 8 RSi(OH)3 → [RSiO3/2]8 + 12 H2O
Depending on the R substituent, the exterior of cage can be further modified. When R = H, the Si-H group can undergo
Reactivity
Cage-Rearrangement
Reorganization of the siloxane
cage-like core (T8 → T10) can be performed, including
isolation of intermediates, and cage rearrangement achieved by using Bronsted
superacid, trifluoromethanesulfonic acid (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 parallel with cage opening process is observed and compound B is obtained (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. Uponfurther 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−
3 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.[8]
Polymeric silsesquioxanes
Polymeric silsesquioxanes have been reported, first by Brown. High molecular weight tractable polymeric phenyl silsesquioxane featured a ladder-type structure.[9] Brown's findings were the basis for many future investigations. Brown's synthesis proceeded in three-steps: (1) the hydrolysis of phenyltrichlorosilane, (2) equilibration of this hydrolyzate with potassium hydroxide at a low concentration and temperature to give the prepolymer, and (3) equilibration of the prepolymer at a high concentration and temperature to give the final polymer. Other notable silsesquioxane polymers include the soluble polymethylsilsesquioxane with high molecular weights described by Japan Synthetic Rubber.[10] This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found applications in cosmetics,[11] resins,[12] and lithography.[13] Hierarchical organic-inorganic (hybrid) polysilsesquioxane (PSQ) materials using polyhedral oligomeric silsesquioxanes (POSS) cages as singular building blocks of the inorganic framework were synthesized by different research groups, exhibiting high specific surface area and hydrothermal stability and micro and/or mesoporosity.[14][15][16][17] 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[18] or in conjunction with metal ions of the lanthanide series (europium and terbium, especially).[19][20]
Hydridosilsesquioxanes
A well known hydrogen silsesquioxane is [HSiO3/2]8.[21] Early syntheses involved treatment (protonation) of trichlorosilane with concentrated sulfuric acid, and fuming sulfuric acid, affording T10-T16 oligomers. The T8 cluster was also synthesized by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and hydrochloric acid. The Si-H groups are amenable to hydrosilylation.[22]
Potential applications
Electronic materials
Films of organosilsesquioxane, e.g., poly(methylsilsesquioxane), have been examined for semiconducting devices.[23][24] Poly(hydridosilsesquioxane), which has a linked-cage structure, was sold under the name Fox Flowable Oxide.[23]
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.
Hydridosilsesquioxanes can be converted to silica coatings for potential application in integrated circuits.[27][28]
LEDs
For potential applications to
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 [30] and other anions.[31]
Antimicrobial silsesquioxanes
Silsesquioxanes have been functionalized with biocidal quaternary ammonium (QASs) groups to produce
Array of QAS functionalized polyhedral oligomeric silsesquioxanes (Q-POSS) have been reported.[35] 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 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
Partially condensed silsesquioxanes: Si7 species
A well studied example of a partially condensed silsesquioxanes is the trisilanol Cy7Si7O9(OH)3, prepared by the slow (months) hydrolysis of trichlorocyclohexylsilane (C6H11SiCl3).[36] The same cage can be prepared by acid-mediated cleavage of fully condensed silsesquioxane.[37] 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
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.[29]
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.[40][41][42] 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.[43] Another route of synthesis involves first deprotonating the trisilanol group using LiN(SiMe3)2.[44] Aspinall et al. later succeeded in doing the same using three equivalents of n-BuLi in hexanes and further results indicate that alkali metal derivatives of deprotonated silsesquioxanes could also be prepared using alkali metalbis(trimethylsilyl) amides.[40] Marchesi et. al synthesised a luminescent fully-condensed rare-earth-doped POSS bearing in the structure an europium ion by reaction of the open-corner heptaisobutyl trisilanol T7-POSS ((C4H9)7Si7O9(OH)3) with anhydrous EuCl3 under basic conditions.[45] Furthermore, a combination of partially-condensed tetrasilanolphenyl POSS with terbium acetate and/or europium acetate (at different molar ratio with both metals) led to novel double-decker silsesquioxane (DDSQ) materials consisting of lanthanide-doped POSS units with intrinsic luminescent properties, in which the lanthanide ion(s) act as both structural and functional agents in the final compound.[46]
Catalytic properties
Although lacking commercial applications, metallasilsesquioxanes have been investigated as catalysts. The coordination environment provided by Cy7Si7O9(OH)3 has been proposed to approximate β-
References
- PMID 20225901.
- ^ PMID 27438046.
- ISSN 0365-6128.
- ISSN 0365-6128.
- PMID 25279598.
- ^ ISBN 1-4020-0348-X.
- PMID 25550679.
- .
- .
- ^ Suminoe. T.: Matsumura. Y.: Tomomitsu. 0. (1978). "Methylpolysiloxane". Chem. Abstr. 89: 180824.
{{cite journal}}
: CS1 maint: numeric names: authors list (link) - ^ Hase, N.; Tokunaga, T. (1993). Chem. Abstr. 119: 34107.
{{cite journal}}
: Missing or empty|title=
(help) - ^ Dote, T.; Ishiguro, K.; Ohtaki, M.; Shinbo, Y. (1990). Chem. Abstr. 113: 213397.
{{cite journal}}
: Missing or empty|title=
(help) - .
- PMID 31973269.
- ISSN 0897-4756.
- S2CID 52308066.
- ISSN 1369-9261.
- PMID 25515033.
- S2CID 213044695.
- PMID 36431482.
- .
- PMID 12175245.
- ^ a b US 6472076, N.P. Hacker, published 2002
- )
- .
- S2CID 95798868.
- S2CID 136561326.
- .
- ^ doi:10.1039/b909234j.
- PMID 29072723.
- PMID 30429984.
- )
- .
- S2CID 94388361.
- ^ S2CID 96227505.
- .
- doi:10.1039/A802670J.
- PMID 19081985.
- .
- ^ .
- .
- .
- PMID 11848826.
- .
- ISSN 1144-0546.
- ISSN 2227-9717.
- ^ .
- ^ PMID 10747385.
- .