View source for Chemistry:Silsesquioxane

{{Short description|Molecular compound with applications in ceramics}}
220x220px|thumbnail|right|A cubic silsesquioxane.
A '''silsesquioxane''' is an [[Chemistry:Organosilicon compound|organosilicon compound]] with the [[Chemistry:Chemical formula|chemical formula]] [RSiO<sub>3/2</sub>]<sub>n</sub>  (R = H, [[Chemistry:Alkyl|alkyl]], [[Chemistry:Aryl|aryl]], alkenyl or alkoxyl.).<ref name=Guizhi2001>{{cite journal
 |author1=David B. Cordes |author2=Paul D. Lickiss |author3=Franck Rataboul | year = 2010
 | title = Recent Developments in the Chemistry of Cubic Polyhedral Oligosilsesquioxanes
 | journal = Chem. Rev.
 | volume = 110
 | issue = 3
 | pages = 2081–2173
 | doi = 10.1021/cr900201r
 | pmid=20225901
}}</ref> Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices.  Silsesquioxanes are members of polyoctahedral silsesquioxanes ("POSS"), which have attracted attention as [[Chemistry:Preceramic polymer|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 [[Chemistry:Silica|silica]] core confers rigidity and thermal stability.

==Structure==
Silsesquioxanes are known in molecular form with 6, 8, 10, and 12 Si vertices, as well as polymers. The cages are sometimes labeled T<sub>6</sub> T<sub>8</sub>, T<sub>10</sub>, and T<sub>12</sub>, respectively (T = tetrahedral vertex). The T<sub>8</sub> cages, the most widely studied members, have the formula [RSiO<sub>3/2</sub>]<sub>8</sub>, or equivalently R<sub>8</sub>Si<sub>8</sub>O<sub>12</sub>. 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 ''O<sub>h</sub>''  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.<ref name="Janeta">{{Cite journal|last1=Janeta|first1=Mateusz|last2=John|first2=Łukasz|last3=Ejfler|first3=Jolanta|last4=Lis|first4=Tadeusz|last5=Szafert|first5=Sławomir|date=2016-08-02|title=Multifunctional imine-POSS as uncommon 3D nanobuilding blocks for supramolecular hybrid materials: synthesis, structural characterization, and properties|journal=Dalton Transactions|language=en|volume=45|issue=31|pages=12312–12321|doi=10.1039/C6DT02134D|pmid=27438046|issn=1477-9234|doi-access=free}}</ref><ref>{{cite journal |last=Larsson |first=Kare |title=Crystal structure of octa(methylsilsesquioxane), (CH<sub>3</sub>SiO<sub>1.5</sub>)<sub>8</sub> |journal=Arkiv för kemi |year=1960 |volume=16 |pages=203–8 |issn=0365-6128 }}</ref><ref>{{cite journal |last=Larsson |first=Kare |title=Crystal structure of (HSiO<sub>1.5</sub>)<sub>8</sub> |journal=Arkiv för kemi |year=1960 |volume=16 |pages=215–19 |issn=0365-6128 }}</ref><ref>{{cite journal|year=2014|title=The polyoctahedral silsesquioxane (POSS) 1,3,5,7,9,11,13,15-octa-phenyl-penta-cyclo-[9.5.1.13,9.15,15.17,13]octa-siloxane (octa-phenyl-POSS)|journal=Acta Crystallogr C|volume=70|issue=10|pages=971–974|doi=10.1107/S2053229614019834|pmid=25279598|author1=Chinnam P. R.|author2=Gau M. R.|author3=Schwab J.|author4=Zdilla M. J.|author5=Wunder S. L.}}</ref>
[[File:Janeta imine-POSS.png|thumb|222x222px|Molecular structure of imine-silsesquioxane<ref name="Janeta"/>]]

== Synthesis ==
Silsesquioxanes are usually synthesized by hydrolysis of organo[[Chemistry:Trichlorosilane|trichlorosilane]]s.<ref name="Jones">{{cite book|title=Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications|publisher=Kluwer Academic Publishers|year=2000|edition=1st|location=Dordrecht, The Netherlands|pages=157–183|author1=Jones, R. G.|author2=Ando, W.|author3=Chojnowski, J. |isbn=1-4020-0348-X }}</ref>  An idealized synthesis is:
:8 RSiCl<sub>3</sub>  +  12 H<sub>2</sub>O   →   [RSiO<sub>3/2</sub>]<sub>8</sub>  +  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.<ref>{{cite journal |last1=Hurkes |first1=N. |last2=Bruhn |first2=C. |last3=Belaj |first3=F. |last4=Pietschnig |first4=R. |title=Silanetriols as Powerful Starting Materials for Selective Condensation to Bulky POSS Cages |journal=Organometallics |year=2014 |volume=33 |issue=24 |pages=7299–7306 |doi=10.1021/om5010918|pmid=25550679 | pmc=4276717 |doi-access=free }}</ref>

:8 RSi(OH)<sub>3</sub>    →   [RSiO<sub>3/2</sub>]<sub>8</sub>  +  12 H<sub>2</sub>O

Depending on the R substituent, the exterior of cage can be further modified. When R = H, the Si-H group can undergo [[Physics:Hydrosilylation|hydrosilylation]] or oxidation to the silanol. 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.<ref name="Jones" /> Vinyl-substituted silsesquioxanes can be linked by the alkene metathesis.

== Reactivity ==

=== Cage-Rearrangement ===
Reorganization of the siloxane
cage-like core (T<sub>8</sub> → T<sub>10</sub>) can be performed, including
isolation of intermediates, and cage rearrangement achieved by using Bronsted
superacid, trifluoromethanesulfonic acid (CF<sub>3</sub>SO<sub>3</sub>H). In this case, reaction of hexahedral '''silsesquioxane''' and CF<sub>3</sub>SO<sub>3</sub>H in DMSO conducted in 1 : 12 molar ratio gives heptahedral silsesquioxane. In the first step CF<sub>3</sub>SO<sub>3</sub>H acid attacks siloxane Si-O-Si bonds and the formation of Si-O-SO<sub>2</sub>CF<sub>3</sub> 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 T<sub>6</sub>(OH)<sub>4</sub> '''C''' and siloxane dimer '''D''' are formed. Because this reaction takes place in an aqueous conditions, compound '''E''' of general formula T<sub>8</sub>(OH)<sub>4</sub> as a consequence of hydrolysis reaction was obtained. '''E''' is prone to reaction with '''D'''
and due to this, the abstraction of CF<sub>3</sub>SO{{su|b=3|p=−}} anion occurs and the closure frame with the spontaneous cage-rearrangement to heptahedral T<sub>10</sub> 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 Si<sub>4</sub>O<sub>4</sub> moiety from much more less stable substrates '''D''' and '''E'''.<ref>{{Cite journal|title = Novel organic–inorganic hybrids based on T8 and T10 silsesquioxanes: synthesis, cage-rearrangement and properties|journal = RSC Adv.|date = 2015|volume = 5|issue = 88|doi = 10.1039/c5ra10136k|first1 = Mateusz|last1 = Janeta|first2 = Łukasz|last2 = John|first3 = Jolanta|last3 = Ejfler|first4 = Sławomir|last4 = Szafert|pages=72340–72351| bibcode=2015RSCAd...572340J }}</ref>
[[File:POSS-Cage-Rearrangement.png|thumb|Reaction of OAS-POSS-Cl with CF<sub>3</sub>SO<sub>3</sub>H in DMSO. B-E constitute intermediates isolated during A → F cage-rearrangement.|alt= Reaction of OAS-POSS-Cl with CF3SO3H in DMSO. B-E constitute intermediates isolated during A → F cage-rearrangement.|centre|597x597px]]

thumbnail|200px|right|Poly(phenylsilsesquioxane) does not adopt a cage structure, but is a polymer with a ladder-like repeating unit.

===Polymeric silsesquioxanes===
Polymeric silsesquioxanes have been reported, first by Brown. High molecular weight tractable polymeric phenyl silsesquioxane featured a ladder-type structure.<ref name=Brown1960>{{cite journal|author1=Brown, J. F., Jr. |author2=Vogt, J. H., Jr. |author3=Katchman, A. |author4='Eustance, J.W. |author5=Kiser, K. M. |author6=Krantz, K. W. |title=Double Chain Polymers of Phenylsilsequioxane|journal=J. Am. Chem. Soc.|year=1960|volume=82|issue=23|pages=6194–6195|doi=10.1021/ja01508a054}}</ref> 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.<ref name=Suminoe>{{cite journal|author=Suminoe. T.: Matsumura. Y.: Tomomitsu. 0.|title=Methylpolysiloxane |journal=Chem. Abstr.|year=1978|volume=89|page=180824}}</ref> This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found applications in cosmetics,<ref name=Hase>{{cite journal|author1=Hase, N. |author2=Tokunaga, T. |journal=Chem. Abstr.|year=1993|volume=119|page=34107}}</ref> resins,<ref name=Dote>{{cite journal|author1=Dote, T. |author2=Ishiguro, K. |author3=Ohtaki, M. |author4=Shinbo, Y. |journal=Chem. Abstr.|year=1990|volume=113|page=213397}}</ref> and lithography.<ref name=Watanabe>{{cite journal|author1=Watanabe, H. |title=A novel silicon containing chemical amplification resist for electron beam lithography |author2=Todokoro, Y. |author3=Inoue, M. |journal=Microelectron Eng.|year=1991|volume=13|issue=1–4 |page=69|doi=10.1016/0167-9317(91)90050-N }}</ref> 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.<ref>{{Cite journal |last1=Marchesi |first1=Stefano |last2=Carniato |first2=Fabio |last3=Marchese |first3=Leonardo |last4=Boccaleri |first4=Enrico |date=June 2015 |title=Luminescent Mesoporous Silica Built through Self-Assembly of Polyhedral Oligomeric Silsesquioxane and Europium(III) Ions |url=https://onlinelibrary.wiley.com/doi/10.1002/cplu.201500143 |journal=ChemPlusChem |language=en |volume=80 |issue=6 |pages=915–918 |doi=10.1002/cplu.201500143|pmid=31973269 }}</ref><ref name="Chaikittisilp 4841–4843">{{Cite journal |last1=Chaikittisilp |first1=Watcharop |last2=Sugawara |first2=Ayae |last3=Shimojima |first3=Atsushi |last4=Okubo |first4=Tatsuya |date=2010-09-14 |title=Microporous Hybrid Polymer with a Certain Crystallinity Built from Functionalized Cubic Siloxane Cages as a Singular Building Unit |url=https://pubs.acs.org/doi/10.1021/cm1017882 |journal=Chemistry of Materials |language=en |volume=22 |issue=17 |pages=4841–4843 |doi=10.1021/cm1017882 |issn=0897-4756}}</ref><ref>{{Cite journal |last1=Sato |first1=Naoto |last2=Kuroda |first2=Yoshiyuki |last3=Wada |first3=Hiroaki |last4=Shimojima |first4=Atsushi |last5=Kuroda |first5=Kazuyuki |date=2018-11-16 |title=Preparation of Siloxane‐Based Microporous Crystals from Hydrogen‐Bonded Molecular Crystals of Cage Siloxanes |url=https://onlinelibrary.wiley.com/doi/10.1002/chem.201804441 |journal=Chemistry – A European Journal |language=en |volume=24 |issue=64 |pages=17033–17038 |doi=10.1002/chem.201804441 |pmid=30239060 |s2cid=52308066 |issn=0947-6539}}</ref><ref>{{Cite journal |last1=Gandhi |first1=Sakthivel |last2=Kumar |first2=Prem |last3=Thandavan |first3=Kavitha |last4=Jang |first4=Kiwan |last5=Shin |first5=Dong-Soo |last6=Vinu |first6=Ajayan |date=2014-06-18 |title=Synthesis of a novel hierarchical mesoporous organic–inorganic nanohybrid using polyhedral oligomericsilsesquioxane bricks |url=https://pubs.rsc.org/en/content/articlelanding/2014/nj/c4nj00292j |journal=New Journal of Chemistry |language=en |volume=38 |issue=7 |pages=2766–2769 |doi=10.1039/C4NJ00292J |issn=1369-9261}}</ref> In addition, Marchesi et al. developed a series of amorphous POSS-based polysilsesquioxanes, in which the POSS cages (of partially-condensed T<sub>7</sub>-POSS or anionic completely-condensed T<sub>8</sub>-POSS molecules) act as structural units of the polymer-like inorganic network, both alone<ref>{{Cite journal |last1=Marchesi |first1=Stefano |last2=Carniato |first2=Fabio |last3=Palin |first3=Luca |last4=Boccaleri |first4=Enrico |date=2015 |title=POSS as building-blocks for the preparation of polysilsesquioxanes through an innovative synthetic approach |url=http://xlink.rsc.org/?DOI=C4DT02887B |journal=Dalton Transactions |language=en |volume=44 |issue=5 |pages=2042–2046 |doi=10.1039/C4DT02887B |pmid=25515033 |issn=1477-9226}}</ref> or in conjunction with metal ions of the lanthanide series ([[Chemistry:Europium|europium]] and [[Chemistry:Terbium|terbium]], especially).<ref>{{Cite journal |last1=Marchesi |first1=Stefano |last2=Bisio |first2=Chiara |last3=Boccaleri |first3=Enrico |last4=Carniato |first4=Fabio |date=January 2020 |title=A Luminescent Polysilsesquioxane Obtained by Self‐Condensation of Anionic Polyhedral Oligomeric Silsequioxanes (POSS) and Europium(III) Ions |url=https://onlinelibrary.wiley.com/doi/10.1002/cplu.201900735 |journal=ChemPlusChem |language=en |volume=85 |issue=1 |pages=176–182 |doi=10.1002/cplu.201900735 |s2cid=213044695 |issn=2192-6506}}</ref><ref>{{Cite journal |last1=Marchesi |first1=Stefano |last2=Miletto |first2=Ivana |last3=Bisio |first3=Chiara |last4=Gianotti |first4=Enrica |last5=Marchese |first5=Leonardo |last6=Carniato |first6=Fabio |date=2022-11-12 |title=Eu3+ and Tb3+ @ PSQ: Dual Luminescent Polyhedral Oligomeric Polysilsesquioxanes |journal=Materials |language=en |volume=15 |issue=22 |pages=7996 |doi=10.3390/ma15227996 |pmid=36431482 |pmc=9694933 |bibcode=2022Mate...15.7996M |issn=1996-1944|doi-access=free }}</ref>

===Hydridosilsesquioxanes===
A well known [[Physics:Hydrogen silsesquioxane|hydrogen silsesquioxane]] is [HSiO<sub>3/2</sub>]<sub>8</sub>.<ref name=Frye>{{cite journal|author1=Frye, C. L. |author2=Collins, W. T. |title=Oligomeric silsesquioxanes, (HSiO3/2)n |journal=J. Am. Chem. Soc.|year=1970|volume=92|issue=19|pages=5586–5588|doi=10.1021/ja00722a009}}</ref> Early syntheses involved treatment ([[Chemistry:Protonation|protonation]]) of [[Chemistry:Trichlorosilane|trichlorosilane]] with concentrated [[Chemistry:Sulfuric acid|sulfuric acid]], and fuming sulfuric acid, affording T<sub>10</sub>-T<sub>16</sub> oligomers. The T<sub>8</sub> cluster was also synthesized by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and [[Chemistry:Hydrochloric acid|hydrochloric acid]].  The Si-H groups are amenable to [[Physics:Hydrosilylation|hydrosilylation]].<ref name=Dijkstra>{{cite journal|author1=Dijkstra, T.W. |author2=Duchateau, R. |author3=Van Santen, R.A. |author4=Meetsma, A. |author5=Yap, G.P.A. |title= Silsesquioxane Models for Geminal Silica Surface Silanol Sites. A Spectroscopic Investigation of Different Types of Silanols|journal=J. Am. Chem. Soc.|year=2002|volume=124|issue= 33|pmid= 12175245|pages=9856–9864|doi= 10.1021/ja0122243}}</ref>

==Potential applications==

===Electronic materials===
Films of organosilsesquioxane, e.g., poly(methylsilsesquioxane), have been examined for semiconducting devices.<ref name=Hacker>{{cite patent |country=US |number= 6472076|status= |title= |pubdate= 2002|gdate= |fdate= |pridate= |inventor= N.P. Hacker|invent1= |assign1= |assign2= |class= |url=}}</ref><ref name=Voronkov>{{cite book|author1=Voronkov, M. G. |title=Polyhedral oligosilsesquioxanes and their homo derivatives |author2=Lavrent'yev, V. I. |journal= Top. Curr. Chem.|year=1982|volume=102|pages=199–236|doi=10.1007/3-540-11345-2_12 |series=Topics in Current Chemistry |isbn=978-3-540-11345-4 }}</ref> Poly(hydridosilsesquioxane), which has a linked-cage structure, was sold under the name Fox Flowable Oxide.<ref name=Hacker />

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.<ref name=Abe>{{cite journal|doi= 10.1016/j.tet.2010.10.009|author1=Ervithayasuporn, V. |author2=Abe, J. |author3=Wang, X. |author4=Matsushima, T. |author5=Murata, H. |author6=Kawakami, Y. |title= Synthesis, characterization, and OLED application of oligo(p-phenylene ethynylene)s with polyhedral oligomeric silsesquioxanes (POSS) as pendant groups|journal= Tetrahedron|year=2010|volume=66|issue= 48|pages=9348–9355}}</ref><ref name=Renaud>{{cite journal|doi= 10.1007/s10854-008-9629-x|author1=Renaud, C. |author2=Josse, Y. |author3=Lee, C.-W. |author4=Nguyen, T.-P. |title= Investigation of defects in polyhedral oligomeric silsesquioxanes based organic light emitting diodes| journal= Journal of Materials Science: Materials in Electronics|year=2008|volume=19|issue= S1|pages=87–91|s2cid=95798868 }}</ref> 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 and [[Physics:Electron configuration|electronic]] properties.

Hydridosilsesquioxanes can be converted to silica coatings for potential application in integrated circuits.<ref name=Gentle>{{cite journal|last=Gentle|first= T.E.|journal=SPIE Proceedings|year=1991|volume=1595|page=146|title = Oxidation of hydrogen silsesquioxane, (HSiO3/2)n, by rapid thermal processing|doi = 10.1117/12.56672|s2cid= 136561326}}</ref><ref name=Chandra>{{cite journal|author=Chandra, G.|title=Low Temperature Ceramic Coatings for Environmental Protection of Integrated Circuits|journal=Mater. Res. Soc. Symp. Proc.|year=1991|volume=203|page=97|doi=10.1557/PROC-203-97}}</ref>

===LEDs===
For potential applications to light emitting diodes, cubic silsesquioxanes. have been functionalized.<ref name=Chan>{{cite journal|author1=Chan, K. L. |author2=Sonar, P. |author3=Sellinger, A. |journal= Journal of Materials Chemistry|year=2009|volume=19|issue=48 |page=9103|doi=10.1039/b909234j|title=Cubic silsesquioxanes for use in solution processable organic light emitting diodes (OLED)}}</ref>  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 techniques, [[Chemistry:Grignard reagents|Grignard reagents]], 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 [[Physics:Quantum yield|quantum yield]].

===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 <ref name=Chanmungkalakul>{{cite journal|author1=Chanmungkalakul, S. |author2=Ervithayasuporn, V. |journal= Chemical Communications|year=2017|volume=53|issue=89 |pages=12108–12111 |doi=10.1039/C7CC06647C|pmid=29072723 |title=Silsesquioxane Cages as Fluoride Sensors}}</ref> and other anions.<ref name=Kiatkamjornwong>{{cite journal|author1=Kiatkamjornwong, S. |author2=Chanmungkalakul, S.|journal= Chemical Science|year=2018|volume=9|issue=40|pages=7753–7765|doi=10.1039/C8SC02959H|pmid=30429984|title=Anion identification using silsesquioxane cages|pmc=6194494}}</ref>

===Antimicrobial silsesquioxanes===
Silsesquioxanes have been functionalized with biocidal quaternary ammonium (QASs) groups to produce [[Biology:Antimicrobial|antimicrobial]] coatings. QASs are disinfectants, antiseptics, and antifoulants that kill bacteria, fungi, and algae.<ref name=Russel>{{cite book|doi=10.1016/S0079-6468(08)70198-X|author=Russel, A.D.|title=The mechanism of action of some antibacterial agents.|journal=Prog. Med. Chem.|year=1969|volume=6|pages=135–199|pmid=4307054|series=Progress in Medicinal Chemistry|isbn=9780444533258}}</ref><ref name=Sauvet>{{cite journal|doi=10.1002/pola.10895|author1=Sauvet, G. |author2=Fortuniak, W. |author3=Kazmierski, K. |author4=Chojnowski, J. |title=Amphiphilic block and statistical siloxane copolymers with antimicrobial activity|journal=J. Polym. Sci. A Polym. Chem.|year=2003|volume=41|issue=19|pages=2939–2948|bibcode = 2003JPoSA..41.2939S }}</ref> The relatively small size of the silsesquioxane molecule, 2-5&nbsp;nm, allows a QAS functionalized molecule to have a charge density similar to dendrimers and thus the antimicrobial efficacy is prominent.  Dimethyl-n-octylamine was quaternized by octa(3-chloropropylsilsesquioxane), (T-ClPr)<sub>8</sub>.<ref name=Chojnowski>{{cite journal|author1=Chojnowski, J. |author2=Fortuniak, W. |author3=Rosciszewski, P. |author4=Werel, W. |author5=Łukasiak, J. |author6=Kamysz, W. |author7=Hałasa, R. |title=Polysilsesquioxanes and Oligosilsesquioxanes Substituted by Alkylammonium Salts as Antibacterial Biocides|journal=J. Inorgan. Organomet. Polym. Mater.|year=2006|volume=16|issue=3|pages=219–230|doi=10.1007/s10904-006-9048-5|s2cid=94388361 }}</ref>  The resulting material exhibited antimicrobial efficacy for the prevention of growth of both Gram-positive and [[Biology:Gram-negative bacteria|Gram-negative bacteria]].
 
Array of QAS functionalized polyhedral oligomeric silsesquioxanes (Q-POSS) have been reported.<ref name=Majumdar>{{cite journal|author1=Majumdar, P. |author2=He, J. |author3=Lee, E. |author4=Kallam, A. |author5=Gubbins, N. |author6=Stafslien, S.J. |author7=Daniels, J. |author8=Chishom, B.J. |title=Antimicrobial activity of polysiloxane coatings containing quaternary ammonium-functionalized polyhedral oligomeric silsesquioxane|journal=J. Coat. Technol. Res.|year=2010|volume=7|issue=4|pages=455–467|doi=10.1007/s11998-009-9197-x|s2cid=96227505 }}</ref>  These researchers varied the alkyl chain length from –C<sub>12</sub>H<sub>25</sub> to –C<sub>18</sub>H<sub>37</sub> and varied the counter ion between chloride, bromide, and iodine.  The first reaction was the hydrosilylation between allydimethlamine and octasilane polyhedral oligomeric silsesquioxane via [[Chemistry:Karstedt's catalyst|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 glass transition temperature, 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.<ref name=Majumdar /> The researchers found that coatings based on bromide and chloride had the best antimicrobial efficacy.

==Partially condensed silsesquioxanes: Si<sub>7</sub> species==
A well studied example of a partially condensed silsesquioxanes is the trisilanol Cy<sub>7</sub>Si<sub>7</sub>O<sub>9</sub>(OH)<sub>3</sub>, prepared by the slow (months) hydrolysis of trichlorocyclohexylsilane (C<sub>6</sub>H<sub>11</sub>SiCl<sub>3</sub>).<ref name=Brown1965>{{cite journal|author1=Brown, J.F. |author2=Vogt, L.H. |journal=J. Am. Chem. Soc.|year=1965|volume=87|issue= 19|pages=4313–4317|doi=10.1021/ja00947a016|title=The Polycondensation of Cyclohexylsilanetriol}}</ref> The same cage can be prepared by acid-mediated cleavage of fully condensed silsesquioxane.<ref name=Feher1998>{{cite journal|author1=Feher, F.J. |author2=Soulivong, D. |author3=Nguyen, F. |title= Practical methods for synthesizing four incompletely condensed silsesquioxanes from a single R8Si8O12 framework|journal=Chem. Commun.|year=1988|volume=12|issue= 12|pages=1279–1280|doi=10.1039/A802670J }}</ref>  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 [[Chemistry:Hydrogen bond|hydrogen bond]]ed networks.<ref name=Pietschnig2009>{{cite journal|author1=Spirk, S. |author2=Nieger, M.|author3=Belaj, F.|author4=Pietschnig, R. |journal=Dalton Trans.|year=2009|pages=163–167|doi=10.1039/B812974F|title=Formation and hydrogen bonding of a novel POSS-trisilanol|issue=1 |pmid=19081985 }}</ref> These dimers are retained in solution and a dynamic equilibrium has been elucidated using NMR.<ref name=Unno2008>{{cite journal|author1=Liu, H. |author2=Kondo, S.|author3=Tanaka, R.|author4=Oku, H.|author5=Unno, M. |journal=J. Organomet. Chem.|year=2008|volume=693|issue= 7|pages=1301–1308|title=A spectroscopic investigation of incompletely condensed polyhedral oligomeric silsesquioxanes (POSS-mono-ol, POSS-diol and POSS-triol): Hydrogen-bonded interaction and host–guest complex|doi=10.1016/j.jorganchem.2008.01.027}}</ref>

===Other partially condensed silsesquioxanes===
Other partially condensed species adopt ladder structures wherein in which two long chains composed of RSiO<sub>3/2</sub> units are connected at regular intervals by Si-O-Si bonds.   Amorphous structures include  RSiO<sub>3/2</sub> unit connections without any organized structure formation.<ref name=Chan />
[[File:Silsesquioxane Partial Cage.svg|200px|thumbnail|right|Partially condensed silsesquioxane.]]
 
{{Image frame|align=center|width=440|caption=Scheme for the synthesis of silsesquioxanes.
|content=<math chem>
\begin{array}{ll}
\ce{{RSiZ3} + 3H2O -> {RSi(OH)3} + 3HZ} & \text{hydrolysis} \\
\ce{3RSi(OH)3 ->[\text{cat.}] 3RSiO_{3/2}} + 4.5\ce{H2O} & \text{condensation} \\
\ce{{RSi(OH)3} + RSiZ3 ->[\text{cat.}] {2RSiO_{3/2}} + 3HZ} & \text{condensation} \\
\mathrm{Z = OR', Cl, OAc} & \text{cat. = catalyst}

\end{array}
</math>}}

===Metal complexes of partially condensed silsesquioxanes and metal-containing POSS===
The incompletely condensed silsesquioxanes bind numerous metals, including Na<sup>+</sup>, Li<sup>+</sup>, and Be<sup>2+</sup> as well as transition metals.<ref name=LorenzCoordChem>{{cite journal|author1=Lorenz, V. |author2=Fischer, A. |author3=Edelmann, F.T. |title= Disiloxanediolates and polyhedral metallasilsesquioxanes of the early transition metals and f-elements|journal=Coord. Chem. Rev.|year=2000|volume=206|issue= 1|pages=321–368|doi=10.1016/S0010-8545(00)00299-X}}</ref><ref name=Feher>{{cite journal|author1=Feher, F.J. |author2=Budzichowski, T.A. |title= Silasesquioxanes as ligands in inorganic and organometallic chemistry|journal=Polyhedron|year=1995|volume=14|issue= 22|pages=3239–3253|doi=10.1016/0277-5387(95)85009-0}}</ref><ref name=Duchateau>{{cite journal|author1=Duchateau, R. |author2=Gerritsen, G. |author3=Van Santen, R.A. |author4=Yap, G.P. |title= Boron, Aluminum, and Gallium Silsesquioxane Compounds, Homogeneous Models for Group 13 Element-Containing Silicates and Zeolites|journal=Organometallics|year=2003|volume=22|issue= 1|pages=100–110|doi=10.1021/om0200858}}</ref>  Cubic metal-silsesquioxane derivatives of the core stoichiometry MSi<sub>7</sub>O<sub>12</sub> can be prepared by treating the incomplete cage with a metal halide in the presence of a base such as triethylamine.<ref name=Murugavel>{{cite journal|author1=Murugavel, R. |author2=Voigt, A. |author3=Walawalkar, M.G |author4=Roesky, H.W. |title= Hetero- and Metallasiloxanes Derived from Silanediols, Disilanols, Silanetriols, and Trisilanols|journal=Chem. Rev.|year=1996|volume=96|issue= 6|pmid= 11848826|pages=2205–2236|doi=10.1021/cr9500747}}</ref>  Another route of synthesis involves first deprotonating the trisilanol group using [[Chemistry:Lithium bis(trimethylsilyl)amide|LiN(SiMe<sub>3</sub>)<sub>2</sub>]].<ref name=Feher1995>{{cite journal|author1=Feher, F.J. |author2=Rahimian, K. |author3=Budzichowski, T.A. |author4=Ziller, J.W. |title= Thallium-Stabilized Silsesquioxides: Versatile Reagents for the Synthesis of Metallasilsesquioxanes, Including High-Valent Molybdenum-Containing Silsesquioxanes|journal=Organometallics|year=1995|volume=14|issue= 8|pages=3920–3926|doi=10.1021/om00008a043}}</ref>  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.<ref name=LorenzCoordChem/> Marchesi et. al synthesised a luminescent fully-condensed [[Chemistry:Rare-earth element|rare-earth]]-doped POSS bearing in the structure an [[Chemistry:Europium|europium]] ion by reaction of the open-corner heptaisobutyl trisilanol T<sub>7</sub>-POSS ((C<small><sub>4</sub></small>H<small><sub>9</sub></small>)<small><sub>7</sub></small>Si<small><sub>7</sub></small>O<small><sub>9</sub></small>(OH)<small><sub>3</sub></small>) with anhydrous [[Chemistry:Europium(III) chloride|EuCl<sub>3</sub>]] under basic conditions.<ref>{{Cite journal |last1=Marchesi |first1=Stefano |last2=Carniato |first2=Fabio |last3=Boccaleri |first3=Enrico |date=2014 |title=Synthesis and characterisation of a novel europium( iii )-containing heptaisobutyl-POSS |url=http://xlink.rsc.org/?DOI=C4NJ00157E |journal=New J. Chem. |language=en |volume=38 |issue=6 |pages=2480–2485 |doi=10.1039/C4NJ00157E |issn=1144-0546}}</ref> Furthermore, a combination of partially-condensed tetrasilanolphenyl POSS with [[Chemistry:Terbium compounds|terbium acetate]] and/or [[Chemistry:Europium(III) acetate|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 [[Chemistry:Lanthanide|lanthanide]] ion(s) act as both structural and functional agents in the final compound.<ref>{{Cite journal |last1=Marchesi |first1=Stefano |last2=Bisio |first2=Chiara |last3=Carniato |first3=Fabio |date=April 2022 |title=Synthesis of Novel Luminescent Double-Decker Silsesquioxanes Based on Partially Condensed TetraSilanolPhenyl POSS and Tb3+/Eu3+ Lanthanide Ions |journal=Processes |language=en |volume=10 |issue=4 |pages=758 |doi=10.3390/pr10040758 |issn=2227-9717|doi-access=free }}</ref>

===Catalytic properties===
Although lacking commercial applications, metallasilsesquioxanes have been investigated as catalysts.  The coordination environment provided by Cy<sub>7</sub>Si<sub>7</sub>O<sub>9</sub>(OH)<sub>3</sub> has been proposed to approximate β-[[Chemistry:Tridymite|tridymite]] and β-[[Chemistry:Cristobalite|cristobalite]].  Some of these complexes are active as catalysts for alkene metathesis, [[Chemistry:Polymerization|polymerization]], epoxidation and Diels-Alder reactions of enones, as well as other Lewis acid-catalyzed reactions like Oppenauer oxidation and Meerwein-Pondorf-Verley reductions.
.<ref name=Feher1994 /> A number of metallasilsesquioxanes have been reported that can polymerize ethene, akin to the [[Chemistry:Phillips catalyst|Phillips catalyst]].<ref name=Abbenhuis>{{cite journal|author= Abbenhuis, H.C.|title= Advances in Homogeneous and Heterogeneous Catalysis with Metal-Containing Silsesquioxanes|journal=Chem. Eur. J.|year=2000|volume=6|issue= 1|pages=25–32|doi=10.1002/(SICI)1521-3765(20000103)6:1<25::AID-CHEM25>3.0.CO;2-Y|pmid= 10747385}}</ref> The catalyst can be easily activated with trimethylaluminum and typically proceeds with high turnover number.<ref name=Abbenhuis /> [[Chemistry:Vanadium|Vanadium]] complexes as well as Ziegler–Natta type catalysts also catalyze the polymerization of ethylene.<ref name=Karol>{{cite journal|author=Karol, F.J.|last2=Karapinka|first2=George L.|last3=Wu|first3=Chisung|last4=Dow|first4=Alan W.|last5=Johnson|first5=Robert N.|last6=Carrick|first6=Wayne L.|title=Chromocene catalysts for ethylene polymerization: Scope of the polymerization|journal=J. Polym. Sci. A|year=1972|volume=10|issue=9|pages=2621–2637|doi=10.1002/pol.1972.150100910|bibcode = 1972JPoSA..10.2621K }}</ref> The coordination of metals  to the silsesquioxane framework gives electrophilic centers that are approximately as electron-withdrawing as a trifluoromethyl group, leading to increased catalytic activity.<ref name=Feher1994>{{cite journal|author1=Feher, F. J. |author2=Tajima, T. L. |title= Synthesis of a Molybdenum-Containing Silsesquioxane Which Rapidly Catalyzes the Metathesis of Olefins|journal=J. Am. Chem. Soc.|year=1994|volume=116|issue= 5|pages=2145–2146|doi=10.1021/ja00084a065}}</ref>

==References==
{{reflist}}
{{silicon compounds}}
{{oxygen compounds}}

[[Category:Organosilicon compounds]]
[[Category:Oxygen compounds]]

{{Sourceattribution|Silsesquioxane}}