Development and Characterization of Polysiloxane Polymer Films for Use in Optical Sensor
Novel Nanoscale Hybrid Materials · Contents ix 5 Insights on Nanofiller Reinforced Polysiloxane...
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Transcript of Novel Nanoscale Hybrid Materials · Contents ix 5 Insights on Nanofiller Reinforced Polysiloxane...
Novel Nanoscale Hybrid Materials
Novel Nanoscale Hybrid Materials
Edited by
Bhanu P. S. ChauhanDepartment of Chemistry William Paterson University Wayne, NJ, USA
This edition first published 2018© 2018 John Wiley & Sons, Inc.
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Library of Congress Cataloging‐in‐Publication Data
Names: Chauhan, Bhanu P. S., editor.Title: Novel nanoscale hybrid materials / edited by Bhanu P. S. Chauhan.Description: Hoboken, NJ : John Wiley & Sons, 2018. | Includes bibliographical references and index. |Identifiers: LCCN 2017036464 (print) | LCCN 2017046280 (ebook) | ISBN 9781119156277 (pdf) | ISBN 9781119156260 (epub) | ISBN 9781119156246 (cloth)Subjects: LCSH: Nanostructured materials. | Composite materials.Classification: LCC TA418.9.N35 (ebook) | LCC TA418.9.N35 N675 2018 (print) | DDC 620.1/18–dc23LC record available at https://lccn.loc.gov/2017036464
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vii
List of Contributors xi
1 Silanols as Building Blocks for Nanomaterials 1Masafumi Unno and Hisayuki Endo
1.1 Introduction 11.2 Synthesis and Applications of Silanols 21.2.1 Silanetriols and Disiloxanetetraols 21.2.2 Cyclotetrasiloxanetetraol (Cyclic Silanols, All‐cis Isomer) 51.2.3 Cyclotetrasiloxanetetraol (Cyclic Silanols, Other Isomers) 141.2.4 Cyclotrisiloxanetriol 151.3 Structures and Properties of Nanomaterials Obtained
from Silanols 201.3.1 Structure of Laddersiloxanes 201.3.2 Thermal Property of Laddersiloxanes 231.3.3 Thermal Property of Other Silsesquioxanes 261.3.4 Refractive Indices of Silsesquioxanes 281.4 Summary and Outlook 29 References 29
2 Biomacromolecule‐Enabled Synthesis of Inorganic Materials 33Kristina L. Roth and Tijana Z. Grove
2.1 Introduction 332.2 DNA 342.3 Proteins and Peptides 362.3.1 Cage Proteins 372.3.2 Bovine Serum Albumin (BSA) 382.3.3 Engineered Peptides 402.3.4 Engineered Protein Scaffolds 422.4 Polysaccharides 442.5 Methods of Characterization 462.6 Conclusion 50 References 50
Contents
Contentsviii
3 Multilayer Assemblies of Biopolymers: Synthesis, Properties, and Applications 57Jun Chen, Veronika Kozlovskaya, Daniëlle Pretorius, and Eugenia Kharlampieva
3.1 Introduction 573.2 Assembly of Biopolymer Multilayers 583.2.1 Biopolymers and Their Properties 583.2.2 Growth and Thickness of Biopolymer Multilayers 593.2.3 Stability in Solutions and Enzymatic Degradation of
Biopolymer Multilayers 743.2.4 Hydration and Swelling of Biopolymer Multilayers 813.3 Properties of Biopolymer Multilayers 833.3.1 Surface Properties of Biopolymer Multilayers and Their
Interaction with Cells 833.3.2 Antibacterial Properties 843.3.3 Immunomodulatory Properties 853.3.4 Mechanical Properties of Biopolymer Multilayers 873.3.5 Other Properties 903.4 Applications 913.5 Conclusion and Outlook 95 Acknowledgment 96 References 96
4 Functionalization of P3HT‐Based Hybrid Materials for Photovoltaic Applications 107Michèle Chevrier, Riccardo Di Ciuccio, Olivier Coulembier, Philippe Dubois, Sébastien Richeter, Ahmad Mehdi, and Sébastien Clément
4.1 Introduction 1074.2 Design and Synthesis of Regioregular Poly(3‐Hexylthiophene) 1094.2.1 Metal‐Catalyzed Cross‐Coupling Reactions 1144.2.2 Functionalization of P3HT 1264.3 Morphology Control of P3HT/PCBM Blend by
Functionalization 1324.3.1 Introduction 1324.3.2 End‐Group Functionalization 1344.3.3 Side‐Chain Functionalization 1444.4 Polymer–Metal Oxide Hybrid Solar Cells 1544.4.1 Anchoring Method 1564.4.2 Surface Modification Using End‐ and
Side‐Chain‐Functionalized P3HT 1584.5 Conclusion 163 Acknowledgments 164 References 164
Contents ix
5 Insights on Nanofiller Reinforced Polysiloxane Hybrids 179Debarshi Dasgupta, Alok Sarkar, Dieter Wrobel , and Anubhav Saxena
5.1 Properties of Silicone (Polysiloxane) 1795.2 Nanofiller Composition and Chemistry 1835.2.1 Fumed Silica 1835.2.2 Aerogel Silica 1855.2.3 Carbon Black 1875.3 Polymer [Poly(dimethylsiloxane)]–Filler Interaction 1875.4 Polymer–Filler Versus Filler–Filler Interactions 1905.5 PDMS Nanocomposite with Anisotropic Fillers 1945.6 PDMS–Molecular Filler Nanocomposite 196 Acknowledgments 198 References 198
6 Nanophotonics with Hybrid Nanostructures: New Phenomena and New Possibilities 201Noor Eldabagh, Jessica Czarnecki, and Jonathan J. Foley IV
6.1 Introduction 2026.2 Theoretical Nanophotonics 2046.2.1 Mie Theory for Spherical Nanostructures 2056.2.2 Transfer Matrix Methods for Planar Structures 2086.2.3 The Finite‐Difference Time‐Domain Method 2146.2.4 The Discrete Dipole Approximation 2156.3 Hybrid Nanostructures 2166.3.1 Emergent Electrodynamics Phenomena: Inhomogeneous Surface
Plasmon Polaritons 2166.3.2 Advancing Imaging Beyond the Diffraction Limit with ISPPs 2206.3.3 Emergent Material‐Dependent Optical Response in Hybrid
Nanostructures 2226.3.4 Perspective on the Horizon of Health Applications of Hybrid
Nanostructures 2286.3.5 Photodynamic Therapy 2286.3.6 In Vivo Light Sources 2316.4 Concluding Remarks 233 References 233
7 Drug Delivery Vehicles from Stimuli‐Responsive Block Copolymers 239Prajakta Kulkarni and Sanku Mallik
7.1 Introduction 2397.2 Block Copolymers for Drug Delivery 2417.3 Polymeric Nanoparticles 2417.3.1 Micelles 2417.3.2 Hydrogels 2437.3.3 Polymersomes 244
Contentsx
7.4 Stimuli‐Responsive Drug Delivery 2457.4.1 Physical/External Stimuli‐Responsive Polymers 2467.4.2 Chemical/Internal Stimuli‐Responsive Polymers 2487.5 Challenges and Prospects 2527.6 Summary 252 References 253
8 Mechanical Properties of Rubber‐Toughened Epoxy Nanocomposites 263B. Zewde, I. J. Zvonkina, A. Bagasao, K. Cassimere, K. Holloway, A. Karim, and D. Raghavan
8.1 Introduction 2638.2 Epoxy Resins 2658.3 Rubber‐Toughened Epoxy Resin 2668.4 Nanoparticle Filled Epoxy Nanocomposites 2698.5 Carbon Nanotubes 2708.6 Rubber‐Toughened CNT Filled Epoxy Nanocomposites 2758.7 Nanoclay Filled Epoxy Nanocomposites 2778.8 Rubber‐Toughened Nanoclay Filled Epoxy Nanocomposites 2828.9 Silicon Dioxide Nanoparticles 2848.10 Rubber‐Toughened Nanosilica Filled Epoxy Nanocomposites 2868.11 Conclusions 289 Acknowledgments 290 References 290
9 Metal Complexes in Reverse Micelles 301Marc A. Walters
9.1 Introduction 3019.2 Location of Metal Complex Probes in the RM Core 3029.3 Metal Complexes in Confinement 3049.3.1 Substitution Reactions and Physical Methods 3049.3.2 Redox Reactions in Reverse Micelles 3099.3.3 Metal Ion Binding 3119.4 Conclusions 320 References 320
10 Heterogenized Catalysis on Metals Impregnated Mesoporous Silica 323Fatima Abi Ghaida, Sébastien Clément, and Ahmad Mehdi
10.1 Introduction 32310.2 Mesoporous Silica in Catalysis 32710.3 Modified Mesoporous Silica 32910.4 Recent Advances in SBA Applied to Catalysis 33210.5 Conclusion 341 References 342
Index 351
xi
A. BagasaoDepartment of ChemistryHoward UniversityWashington, DCUSA
K. CassimereDepartment of ChemistryHoward UniversityWashington, DCUSA
Jun ChenChemistry DepartmentUniversity of Alabama at BirminghamBirmingham, ALUSA
Michèle ChevrierInstitut Charles Gerhardt – UMR 5253Université de Montpellier CNRS ENSCM – CC1701MontpellierFrance
and
Laboratory of Polymeric and Composite MaterialsCenter of Innovation and Research in Materials and Polymers, University of Mons – UMONSMonsBelgium
Sébastien ClémentInstitut Charles Gerhardt – UMR 5253Université de Montpellier CNRS ENSCM – CC1701MontpellierFrance
Olivier CoulembierLaboratory of Polymeric and Composite MaterialsCenter of Innovation and Research in Materials and Polymers, University of Mons – UMONSMonsBelgium
Jessica CzarneckiDepartment of ChemistryWilliam Paterson UniversityWayne, NJUSA
List of Contributors
List of Contributorsxii
Debarshi DasguptaGlobal Research & Development Centre, India Technology CentreMomentive Performance Materials Pvt. LtdBangaloreIndia
Riccardo Di CiuccioLaboratory of Polymeric and Composite MaterialsCenter of Innovation and Research in Materials and Polymers, University of Mons – UMONSMonsBelgium
Philippe DuboisLaboratory of Polymeric and Composite MaterialsCenter of Innovation and Research in Materials and Polymers, University of Mons – UMONSMonsBelgium
Noor EldabaghDepartment of ChemistryWilliam Paterson UniversityWayne, NJUSA
Hisayuki EndoDepartment of Chemistry and Chemical Biology, Graduate School of Science and TechnologyGunma UniversityKiryuJapan
Jonathan J. Foley IVDepartment of ChemistryWilliam Paterson UniversityWayne, NJUSA
Fatima Abi GhaidaInstitut Charles Gerhardt – UMR 5253Université de Montpellier CNRS ENSCM – CC1701MontpellierFrance
and
Faculty of Sciences I, Inorganic and Organometallic Coordination Chemistry Laboratory (LCIO)Lebanese UniversityHadathLebanon
Tijana Z. GroveDepartment of ChemistryVirginia Polytechnic Institute and State UniversityBlacksburg, VAUSA
K. HollowayDepartment of ChemistryHoward UniversityWashington, DCUSA
A. KarimDepartment of Chemical and Biomolecular EngineeringUniversity of HoustonHouston, TXUSA
List of Contributors xiii
Eugenia KharlampievaChemistry DepartmentUniversity of Alabama at BirminghamBirmingham, ALUSA
Veronika KozlovskayaChemistry DepartmentUniversity of Alabama at BirminghamBirmingham, ALUSA
Prajakta KulkarniDepartment of Pharmaceutical SciencesNorth Dakota State UniversityFargo, NDUSA
Sanku MallikDepartment of Pharmaceutical SciencesNorth Dakota State UniversityFargo, NDUSA
Ahmad MehdiInstitut Charles Gerhardt – UMR 5253Université de Montpellier CNRS ENSCM – CC1701MontpellierFrance
Daniëlle PretoriusChemistry DepartmentUniversity of Alabama at BirminghamBirmingham, ALUSA
D. RaghavanDepartment of ChemistryHoward UniversityWashington, DCUSA
Sébastien RicheterInstitut Charles Gerhardt – UMR 5253Université de Montpellier CNRS ENSCM – CC1701MontpellierFrance
Kristina L. RothDepartment of ChemistryVirginia Polytechnic Institute and State UniversityBlacksburg, VAUSA
and
Department of Chemistry Radford University Radford, VAUSA
Alok SarkarGlobal Research & Development Centre, India Technology CentreMomentive Performance Materials Pvt. LtdBangaloreIndia
Anubhav SaxenaGlobal Research & Development Centre, India Technology CentreMomentive Performance Materials Pvt. LtdBangaloreIndia
List of Contributorsxiv
Masafumi UnnoDepartment of Chemistry and Chemical Biology, Graduate School of Science and TechnologyGunma UniversityKiryuJapan
Marc A. WaltersDepartment of ChemistryNew York UniversityNew York, NYUSA
Dieter WrobelResearch & Development CentreMomentive Performance Materials GmbHLeverkusenGermany
B. ZewdeDepartment of ChemistryHoward UniversityWashington, DCUSA
I. J. ZvonkinaDepartment of Chemical and Biomolecular Engineering,University of HoustonHouston, TXUSA
1
Novel Nanoscale Hybrid Materials, First Edition. Edited by Bhanu P. S. Chauhan. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.
1
1.1 Introduction
Silanols have been recognized as reactive intermediates and easily condense to form siloxanes. Therefore, most of the early research on the synthesis and isolation of organosilanols utilized an introduction of bulky substituents. The condensation of silanols is accelerated by an existence of acid or base; therefore preparation from chlorosilanes or alkoxysilanes often encountered further condensation reactions. However, when several important factors (acid, base, concentration, temperature) were carefully considered in the reac-tions, isolation of organosilanols even with smaller substituents was possible, and many isolated silanols have been shown to be stable in the air. Recently accumulating results have been recorded on the isolation of silanols [1, 2].
Silanols are silicon analogue of alcohols; however, in addition to the monosi-lanols (R3SiOH), silanediols and silanetriols (R2Si(OH)2 and RSi(OH)3) are also isolable. Because hydroxyl groups play an important role in interactions of molecules or biological species, silanols with multiple hydroxyl groups (often quoted as polysilanols) were expected to serve as useful components of hydrogen‐bonded supramolecular aggregates [3, 4] or host molecules [5]. In addition, polysilanols are also potential precursors of siloxanes or silsesqui-oxanes with well‐defined structures [6–10] because various bonding schemes are possible. Detailed chemistry of polysilanols can be found in reviews [11–13].
Most of silicone materials have been produced by controlled hydrolysis/ condensation from monosilanes like halosilanes or alkoxysilanes. However, this method affords only kinetically or thermodynamically most stable compounds,
Silanols as Building Blocks for NanomaterialsMasafumi Unno and Hisayuki Endo
Department of Chemistry and Chemical Biology, Graduate School of Science and Technology, Gunma University, Kiryu, Japan
Novel Nanoscale Hybrid Materials2
and usually well‐defined nanomaterials could not be obtained. In order to construct defined structure of siloxanes, coupling reactions from two different compounds (cross‐coupling reaction) are necessary. The reaction from chlorosilanes and silanols in the existence of organic base to form siloxane [8, 9] has been mostly utilized. More recently catalytic siloxane formation from hydrosilanes and silanols/alkoxysilanes was developed [14, 15] and has been widely employed for nanomaterial synthesis. As clearly seen in these examples, silanols are most potential precursors of well‐defined siloxanes.
In this chapter, our results in the last two decades regarding synthesis and structure identification of silanols and their application to the precursors of nanomaterials are summarized.
1.2 Synthesis and Applications of Silanols
1.2.1 Silanetriols and Disiloxanetetraols
Our first encounter with polysilanols was in the process of synthesis of cage silsesquioxanes [6]. Although cage octasilsesquioxanes have been prepared easily from trichlorosilanes or trialkoxysilanes, small cages, for example, hex-asilsesquioxanes, were hard to obtain [16]. In order to obtain smaller cages, we introduced bulky groups to stabilize strained cage framework. According to the typical synthetic method for cage silsesquioxane, we examined the hydro-lytic condensation from t‐BuSiCl3 or ThexSiCl3 (Thex = 1,1,2‐trimethylpropyl) to obtain cage hexasilsesquioxanes. Against expectation, obtained white solid was not the targeted hexasilsesquioxanes but silanetriols (t‐BuSi(OH)3 and ThexSi(OH)3). Interestingly, both compounds are air‐stable white solid and can be stored without problems. We then traced back the literatures and optimized the reaction condition (Scheme 1.1). Synthesis of t‐BuSi(OH)3 was effected by following the method by Roesky’s group [17]; thus t‐BuSiCl3
OH
Si
OH
OR Si
OH
OH
R
R = t-Bu, 1,1,2-trimethylpropyl (Thex)
aniline, H 2O
ether
KOH, SiO 2, H 2O
ethanol
RSiCl3 RSi(OH) 3
Si
OSi
O O
SiOSi
O
OO
Si
O
OSi
R R
R
R
RR
hexasilsesquioxaneRSiCl3
Scheme 1.1 Preparation of silanetriols and disiloxanetetraols.
Silanols as Building Blocks for Nanomaterials 3
was treated with aniline/water in ether, and target compound was obtained. For ThexSi(OH)3, we applied Takiguchi’s method [18] and silanetriols were obtained in 64% yield from ThexSiCl3.
Similarly, disiloxanetetraols were also obtained from trichlorosilanes. By fol-lowing the Sommer and Tyler’s method [19], reaction of ThexSiCl3 with KOH, silica gel, and water in ethanol afforded 1,3‐bis(1,1,2‐trimethylpropyl)-1,1,3, 3‐tetrahydroxydisiloxane in 47% yield. Disiloxanetetraol with tert‐butyl groups was obtained from t‐BuSiCl3 by the reported method by Lickiss et al. [20].
With four silanols in hand, we then tried the synthesis of silsesquioxanes from silanols. At that time, many silanols were isolated and structures were determined; however, reactions starting from silanols were not known. In order to stabilize silanols against dehydration, most of isolated silanols possess bulky substituents, and their reactivity was low. We determined to use dicy-clohexylcarbodiimide (DCC) as dehydrating reagent, and after several exami-nations of reaction conditions, we found that reactions at higher temperature in polar solvent afforded the target compounds. Thus treating silanetriols or disiloxanetetraols with DCC in dimethyl sulfoxide (DMSO) or DMF at 120°C, cage hexasilsesquioxanes were obtained in 25–41% yields (Scheme 1.2). We carefully examined the reaction mixture, but no other cage silsesquioxanes (T8 or T10) were not obtained. The structure of hexasilsesquioxane was estab-lished unequivocally by X‐ray crystallographic analysis. Prior to our work, synthesis of cyclohexyl‐substituted hexasilsesquioxane was reported by the reaction from cyclohexyltrichlorosilane in acetone/water for 4 months [21, 22]. Our method offered more facile access to hexasilsesquioxanes.
Later we applied this reaction to other silanols and obtained several cage silsesquioxanes (Scheme 1.3). Cage hexasilsesquioxanes with bulky mesityl
RSi(OH)3
DCC
DMSO or DMF
SiO
Si
O O
SiOSi
O
OO
Si
O
OSi
R R
R
R
RR
R = t-Bu : 26% Thex : 41%
DCC
DMSO or DMF
SiO
Si
O O
SiOSi
O
OO
Si
O
OSi
R R
R
R
RR
R = t-Bu : 25% Thex : 38%
OH
Si
OH
OR Si
OH
OH
R
Scheme 1.2 Synthesis of hexasilsesquioxanes from silanols.
Novel Nanoscale Hybrid Materials4
(Unno, M., Imai, Y., Matsumoto, H., unpublished result) or Tip (triisopropyl-phenyl) groups [10] were obtained from respecting silanols by the reactions with DCC. In the case of mesityl‐substituted silanols, both hexasilsesquioxane (6%) and octasilsesquioxane (20%) were obtained (Unno, M., Imai, Y., Matsumoto, H., unpublished result). Similar reaction starting from dicy-clohexyldisiloxanetetraol afforded octasilsesquioxane in 13% yield [7].
It should be noted that hexasilsesquioxane with the most bulky Tip group showed interesting structure [10]. With bulky triisopropylphenyl groups, six Tip groups can only rotate with gear‐like motion. In the single crystal, a pair of enantiomer exists and Tip groups are twisted like three blades of a propeller (Figure 1.1). In solution, gear‐like rotation could be observed by 1H NMR; two different singlet peaks are observed for aryl protons at low temperature (exo‐proton: outer of the cage; endo‐proton: inner of the cage (see Figure 1.1)), and Tip groups start rotating by elevating temperature. The coalescence tempera-ture was 38°C, and above this temperature, Tip groups freely rotate to show only single acrylic proton. The activation free energy (ΔG‡) was calculated to be 14.8 kcal mol−1.
OH
Si
OH
OMes Si
OH
OH
Mes
Mes = 2,4,6-trimethylphenyl
SiO
Si
O O
SiOSi
O
OO
Si
O
OSi
Mes Mes
Mes
Mes
MesMes
TipSi(OH)3
Si
OSi
O O
SiOSi
O
O
O
Si
O
OSi
Tip Tip
Tip
Tip
TipTip
Tip = 2,4,6-triisopropylphenyl
SiSi
MesSi
Mes
Si
Mes
SiMes
Si
Mes
Si
Mes
Si
Mes
O
O
OO
O
O
O
O
O
O
O
OMes
6% 20%
DCC in DMSO
DCC in DMSO
DCC in DMSO
180 °C, 3 d
180 °C, 3 d
20%
Si
OSi
O O
SiOSi
O
O
O
Si
O
OSi
Mes Mes
Mes
Mes
MesMes
140 °C, 3 d
13%
OH
Si
OH
OMes Si
OH
OH
Mes
Scheme 1.3 Synthesis of hexasilsesquioxanes from silanols.
Silanols as Building Blocks for Nanomaterials 5
As shown in these results, hexasilsesquioxanes with relatively smaller sub-stituents were hard to be obtained because octasilsesquioxanes generate preferably. We applied a reaction from cyclic silanols (details are described in the following section), and reaction with tetrachlorodisiloxane afforded isopropyl‐substituted hexasilsesquioxane [8]. This is the hexasilsesquioxane ever known with the smallest substituents (Scheme 1.4). Cage octasilsesquioxane was also obtained from cyclic silanols [8].
Octasilsesquioxanes could also be prepared from disiloxanetetraol. From cyclohexyl disiloxanetetraol, reaction with DCC in DMSO at 120°C afforded octasilsesquioxane in 13% yield (Scheme 1.5) [7]. When cyclohexyltrichlorosi-lane was hydrolyzed and condensed in acetone/water, hexasilsesquioxane was obtained as reported [21]. More recently Bassindale and Taylor’s group reported the synthesis of cage silsesquioxanes by nonaqueous hydrolysis of trichlorosi-lanes with DMSO [23]. In our case starting from silanol, high reaction temperature favored the generation of more thermodynamically stable T8 cage.
1.2.2 Cyclotetrasiloxanetetraol (Cyclic Silanols, All‐cis Isomer)
At the same time we investigated cage silsesquioxanes, we were also interested in ladder silsesquioxanes. Although the structure was proposed in 1960 [24]
Endo-proton
Exo-protonO
O
OO
OSi
Si
Figure 1.1 Side view of the crystal structure and two inequivalent acrylic protons of Tip‐T6.
Novel Nanoscale Hybrid Materials6
and various superior properties were expected, obtaining well‐defined ladder silsesquioxanes is still challenging. Our first trial to obtain ladder silsesquiox-anes was targeting bicyclic ones, and we treated tetrachlorodisiloxane with disiloxanetetraol. The reaction is simple dehydrochlorination reaction, and we succeed to obtain a mixture of stereoisomers with expected molecular weight. We isolated all isomers by recycle‐type HPLC, and the structures were deter-mined by X‐ray crystallographic analysis. To our surprise, obtained compound was not bicyclic ladder silsesquioxanes but a stereoisomer containing two six‐membered rings (Scheme 1.6) (Unno, M.; Suto, A.; Matsumoto, H., unpub-lished result).
With this result, we realized that fused eight‐membered ring ladder silses-quioxanes could only be obtained from the precursors containing cyclotetrasi-loxane rings. Then we set the target compound to be the cyclotetrasiloxanes
DCC in DMSO
120 °C, 3 d
OH
Si
OH
OCy Si
OH
OH
Cy
Cy = cyclohexyl
SiSi
CySi
Cy
Si
Cy
Si
CySi
Cy
Si
Cy
SiCy
O
O
OO
O
O
O
O
O
O
O
O
Cy
13%
Scheme 1.5 Synthesis of octasilsesquioxanes from disiloxanetetraol.
Cl
Si
Cl
Oi-Pr Si
Cl
Cl
i-Pr
SiO
Si
O O
SiOSi
O
OO
Si
O
OSi
i-Pr i-Pr
i-Pr
i-Pr
i-Pri-Pr
pyridine
0 °C, 1 h
25%
DCC in DMSO
180 °C, 3 dSi
Sii-Pr
Si
i-Pr
Si
i-Pr
Sii-Pr
Si
i-Pr
Sii-Pr
Sii-Pr
O
O
OO
O
O
O
O
O
O
O
O
i-Pr
45%
SiO
O SiOSiOSi
OH
i-Pr
i-Pr
HO
i-Pr
HOOH
i-Pr
SiO
O SiOSiOSi
OH
i-Pr
i-Pr
HO
i-Pr
HOOH
i-Pr
Scheme 1.4 Synthesis of cage silsesquioxanes from cyclic silanols.
Silanols as Building Blocks for Nanomaterials 7
with reactive substituents. As we already knew that silanols were stabilized with even relatively smaller substituents, we chose isopropyl groups as sub-stituents. As shown in Scheme 1.7, we first applied the stepwise synthesis [4]. After obtaining the target compound, we also succeeded to prepare the com-pound directly from isopropyltrichlorosilane [8].
The key step of laddersiloxane synthesis is substitution reaction of phenyl groups into chlorine atoms. Aluminum chloride has been known to cleave Si–O bonds [25], so this dephenylchlorination was not applied to siloxanes.
Si
O
Si
O Si
O
SiO O Si
O Si
O
Ph
Ph
Ph
Ph
Si
O
Si
Cl
Cl
Ph
Ph
i-Pr
+
HO Si
O
SiHO OH
i-Pri-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pri-Pri-Pr
OH
2pyridine
OSi O
SiOSi
OSiO
SiO Si
O
Ph
Ph
Ph
Ph
No
5 stereoisomers
Scheme 1.6 Attempted synthesis of bicyclic laddersiloxane.
Si O Si
Ph
Ph
i-Pr
Ph
i-Pr
Ph
Si O Si
Cl
Cl
i-Pr
Cl
i-Pr
Cl96%
acetone
41%
HCl, AlCl3
THF, PhH
H2O, NaOH
66%
i -PrMgBr
87%
H2O
PhH
Ph2SiCl2THF
i-PrSiCl322%
H2O / acetone
i-PrPh2SiCl
SiO
O SiOSiOSi
OH
i-Pr
i-Pr
i-Pri-Pr
i-Pr
i-Pr i-Pri-Pr
HOHO
OH
SiO
O SiOSiOSi
OHHO
HO
OH
Scheme 1.7 Synthesis of cyclic silanols.
Novel Nanoscale Hybrid Materials8
However, if substituent on silicon atom is larger than methyl, replacement reaction occurs in quantitative yield. This reaction is also an important step to obtain well‐defined laddersiloxanes [9].
Cyclic silanols with the similar structure were previously reported, and substituents were phenyl [26] and cyclohexyl [21] groups. However, there was no evidence for the all‐cis conformation. Out of four possible stereoiso-mers (Scheme 1.8), cis,cis,trans‐isomer can be identified with NMR spectra because three independent silicon atoms and substituents exist. On the other hand, identification of other three isomers is impossible from spectro-scopic analysis. In order to construct ladder structures, stereostructure of the starting compound is very important; therefore we tried to obtain a single crystal to determine the structure. Unfortunately, crystallinity of obtained isopropyl cyclic silanol was not good, and no suitable crystals were obtained after several months of trials. Finally a single crystal generated from the solvent containing water, and the result clearly indicated the struc-ture was all‐cis [4]. Almost at the same time, Feher’s group succeeded to determine the structure of phenyl‐substituted cyclic silanol [27]. We also reported the crystallographic analysis of supramolecular aggregates of this cyclic silanol with i‐Pr2Si(OH)2 [3].
As expected, obtained cyclic silanol served as a quite potential starting material for well‐defined siloxanes. For example, perfect ladder silsesquioxanes (we termed “laddersiloxane” for those whose structure was unequivocally determined) up to nonacyclic could be obtained starting from cyclic silanols. As an example, synthesis of pentacyclic laddersiloxane is shown in Scheme 1.9 [9].
This synthesis was based on the coupling reaction of silanols with chlorosi-lanes. We investigated several combinations of base and solvent and found out that using pyridine as solvent afforded highest yield. As mentioned in the previous section, exchange reaction of phenyl groups into chlorine atoms is also a key step of this synthesis. Obtained laddersiloxanes were mixture of isomers because of the orientation of terminal isopropyl and phenyl groups; however, the stereostructure of starting cyclic silanol (all‐cis) was perfectly maintained throughout the synthesis, and no stereoisomerization of siloxane framework was observed. The structure of pentacyclic laddersiloxane was determined by X‐ray crystallography [9].
all-trans all-cis
SiO O SiOSiOSi
OH
HOHO
OHSiO
O SiOSiOSi
OH
OH
HO
HO
SiOO Si
OSiOSi
OH
i-Pri-Pr
i-Pri-Pri-Pr
i-Pr
i-Pri-Pr
i-Pri-Pr
i-Pr
i-Pri-Pr
i-Pr
i-Pr
i-Pr HOHO
OH
SiOO Si
OSiOSi
OH
HO
HOOH
cis-trans-cis cis-cis-trans
Scheme 1.8 Four stereoisomers of cyclotetrasiloxanetetraols.
Silanols as Building Blocks for Nanomaterials 9
Syntheses of well‐defined siloxanes starting from all‐cis cyclic silanol are summarized in Scheme 1.10. Not only laddersiloxanes, cage silsesquioxanes, or metallasiloxanes [28] (obtained from cyclic silanols and (CpTiCl2)2O) could also be accessed from this single starting compound, showing its versatility. In the synthesis of longer laddersiloxane, we developed a stereocontrolled approach by use of single diastereomer [29]. Chemistry of laddersiloxanes is summarized in a recent review [30].
Even though the stepwise synthesis afforded the highly ordered laddersi-loxanes, preparation of ladder polysilsesquioxanes is still a challenging topic. Condensation of cyclic silanols seems to offer an effective pathway to ladder polysilsesquioxanes; however, cyclic silanols are basically stable and no condensation is observed in neutral condition. Kawakami’s group reported that phenyl cyclic silanols showed stereoisomerization by an action of acid [31]. In the existence of base, siloxane bond cleavage may occur and random structure is possibly obtained. We developed the neutral dehydration reac-tion using DCC, but only cage silsesquioxanes were obtained from all‐cis cyclic silanols (Scheme 1.4). Therefore, currently possible pathway to perfect ladder polysilsesquioxane can be (a) cross‐coupling reactions of cyclic silanols with other substituted cyclotetrasiloxanes in a condition to avoid stereoisomerization or (b) self‐condensation of cyclic silanols other than
Si
O
Si
Ph
Ph
Cl
Cl
SiO
Si O SiO
Si
O
Si O Si O Si O Si
OO O
Ph
Ph
Ph
Ph
pyridine
85%
SiO
O Si
OSiOSi
OHHO
HOOH
SiO
Si O SiO
Si
O
Si O Si O Si O Si
OO O
Cl
Cl
Cl
Cl
isomeric mixture
isomeric mixture
AlCl3 , HCl
Si O Si O Si O Si
O
Si O Si O Si O Si
OO O
OH
OH
HO
HO
H2O, pyridine
98%
isomeric mixture
pyridine
Si O Si O Si O Si
O
Si O Si O Si O Si
OO O
OO Si
O Si
O
Ph
Ph
Si
OSi
O
Ph
Ph
PhH Et2O
isomeric mixture
37%
98%
Si O Si i-Pri-Pri-Pri-Pri-Pr i-Pr i-Pr
i-Pri-Pri-Pri-Pr
i-Pr
i-Pr
i-Pri-Pr
i-Pri-Pr
i-Pr
i-Pri-Pri-Pr
i-Pri-Pri-Pri-Pr
i-Pri-Pri-Pri-Pr
i-Pr i-Pr i-Pr i-Pr i-Pr
i-Pr i-Pr i-Pr i-Pr i-Pr i-Pri-Pri-Pri-Pr i-Pr
Ph Ph
Cl Cl
Scheme 1.9 Synthesis of pentacyclic laddersiloxane.
SiOO Si
OSiOSi
OH
i-Pr
i-Pr
HO
i-Pr
HOOH
i-Pr
SiO O SiOSiOSi
O
i-Pr
O
i-Pri-Pr
i-Pr
OO
Si OOSi
O Si O Si
O
i-Pr
O
i-Pri-Pr
i-Pr
OO
TiCpClClCpTi
TiCpClClCpTi
SiSi
RSi
i-Pr
Si
i-Pr
Sii-Pr
Sii-Pr
Sii-Pr
Sii-Pr
O
O
OO
O
O
O
O
O
O
O
OR
SiO
Si
O O
SiOSi
O
OO
OSi
i-Pr i-Pr
i-Pr
i-Pr
i-Pr
i-PrSi
O
Si O Si O Si O SiOSi O Si O Si O Si
OO O
Ph
Phi-Pri-Pri-Pr
Ph
Phi-Pr
i-Pri-Pr i-Pri-Pr
Si O Si O Si O Si O Si O Si
O
Si O Si O Si O Si O Si O Si
OO O O O
hP
Ph
Ph
i-Pr i-Pr i-Pr i-Pr i-Pr i-PrPh
i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr
Si O Si
O
Si O Si
O
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr
Ph
Ph
i-Pri-Pri-Pri-Pri-Pri-Pri-Pri-Pr
Ph
Ph
Si O Si
O
Si O Si
O
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
O Si
O
SiO
O
O
i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr
i-Pri-Pri-Pri-Pri-Pri-Pri-Pr
Si
O
Si
O Si
O
SiO
i-Pr i-Pr
Ph
Ph
i-Pri-Pr
Si O
O
Si O
i-Pr
i-Pr
Ph
Ph
hexasilsesquioxane
octasilsesquioxane
titanium complex
tricyclic laddersiloxane
pentacyclic laddersiloxane
heptacyclic laddersiloxane
nonacyclic laddersiloxane
Scheme 1.10 Well‐defined siloxanes prepared from cyclic silanols.
Silanols as Building Blocks for Nanomaterials 11
all‐cis structure (e.g., cis,trans,cis‐isomer). In Scheme 1.11, our approach to ladder polysilsesquioxanes using cis,trans,cis‐tetrabromocyclotetrasiloxane is depicted. Starting from readily available methylphenyldichlorosilane, dehydration/condensation in THF/water afforded a mixture of 6‐, 8‐, and 10‐membered cyclic siloxanes with various stereostructures. Surprisingly, targeted cis,trans,cis‐cyclotetrasiloxane could be obtained as white solid by direct recrystallization of the mixture. Treatment of cyclotetrasiloxane with bromine afforded tetrabromide, and the stereostructure was confirmed by transfer to tricyclic laddersiloxane. This tetrabromide was moisture sensi-tive, and when the compound was in the air, spontaneous hydrolysis and following condensation occurred to give white solid. Although X‐ray struc-ture analysis was not possible, the product indicated the highly ordered structure from NMR and IR spectra and from high thermal stability [32]. Unfortunately obtained white solid was insoluble to organic solvents. Then we are now investigating the synthesis of soluble ladder polysilsesquioxanes with different substituents.
Our synthesis of cyclic silanols was accomplished by treatment of chlorosi-lanes with water in an appropriate solvent. Then various products were possible and the yields were not satisfactory. Following our report, Shchegolikhina’s group developed a high‐yield synthesis of cyclic silanols by isolating sodium or potassium silanolate [33–35]. Metal salts of organosilanols or organosilanolates were isolated and identified by Feher et al. [36], Roesky et al. [37], or Corriu’s group [38], and the Russian group showed the first result of cyclotetrasiloxane silanolates. As shown in Scheme 1.12, phenyltrialkoxysilane was treated with
MeOH / Et2O
recrystallization
SiO
O SiO
SiOSi
Me
Ph
Me
Ph
Me
PhMe
Ph
11% from dichlorosilane
(MePhSiO)nn = 3, 4, 5
MePhSiCl2THF, H2O
Br2 sublimation
–30°C
SiO
OSi
OSiOSi
Me
Br
Me
Br
Me
BrMe
Br
SiO
O SiOSiOSi
Me
OH
Me
HO
Me
HOMe
OHormoisture
Scheme 1.11 Approach to ladder polysilsesquioxane.
Novel Nanoscale Hybrid Materials12
aqueous NaOH in butanol. After the reaction, generated white needles were collected and then treated with hydrochloric acid. The target tetraphenylcyclo-tetrasiloxanetetraol was obtained in 80% yield. The structure (all‐cis) was determined by X‐ray crystallography.
By this method, cyclic silanols with various substituents were obtained in relatively good yield with simple procedure. We applied this synthesis to vari-ous alkoxysilanes and obtained silanolates in good yields. For example, isobu-tyl‐substituted cyclic silanol was obtained in 74% yield from commercially available alkoxysilane. By optimizing the condition, we improved the yield from the reported one (34%) (Scheme 1.13).
Both sodium silanolate and silanol possessed all‐cis configuration, indicating no stereoisomerization occurred. It should be noted that stereoisomerization of cyclic silanols easily occurred by an action of HCl. As shown in Scheme 1.14, all‐cis phenylcyclotetrasiloxanetetraol afforded all four isomers after treatment of 1 M HCl in acetone for 10 min at room temperature. In order to avoid stereoisomerization, reaction with aqueous HCl must be maintained at low temperature and keep the reaction time short.
i-BuSi(OMe)3
NaOH, H2O
hexane, r.t.
HClaq.SiO
O SiOSiOSi
ONa
i-Bu
i-Bu
NaO
i-Bu
NaO
ONa
i-BuSi
OO Si
OSiOSi
OH
i-Bu
i-Bu
HO
i-Bu
HO
OH
i-Bu
ether, 0 °C
74% from alkoxysilane
Scheme 1.13 Facile synthesis of isobutyl‐substituted cyclotetrasiloxanetetraol.
SiO
O SiOSiOSi
OH
Ph
Ph
HO
Ph
HO
OH
Ph1 M HClaq.
acetone, r,t,10 min
Si O Si OH
OSi O Si OH
O
Ph
Ph
Ph
HO
HO
Ph
4 isomers
Scheme 1.14 Stereoisomerization of cyclotetrasiloxanetetraol.
PhSi(OBu)3
NaOH, H2O
BuOH
HClaq.SiO
O SiOSiOSi
ONa
Ph
Ph
NaO
Ph
NaOONa
PhSi
OO Si
OSiOSi
OH
Ph
Ph
HO
Ph
HOOH
Ph
Scheme 1.12 Facile synthesis of cyclic silanols.
Silanols as Building Blocks for Nanomaterials 13
We then investigated the preparation of silanolates in detail in order to obtain the optimized condition. We treated commercially available phenyltri-methoxysilane with aqueous NaOH in 2‐propanol. By adjusting the concentra-tion to 2 mol l−1, highest yield 99% was recorded after 17 h of reaction at room temperature. Obtained solid indicated single peak in 29Si NMR in D2O (−54.8 ppm), and all‐cis structure was determined by treatment with dimethyl-chlorosilane to give all‐cis siloxycyclotetrasiloxane, whose structure was established by X‐ray crystallography (Scheme 1.15) [39].
Similar transformation could be applied to isobutyl‐ and vinyl‐substituted silanolates. As shown in Scheme 1.16, substitute dimethylsiloxycyclotetrasi-loxanes were obtained by facile single‐step reaction from trimethoxysilanes. In these reactions, no purification of sodium or potassium silanolates was nec-essary; silylation after filtration and drying of the crude product afforded good results [39]. In the case of vinyl‐substituted compound, potassium hydroxide gave better yield. In addition, treatment of dimethylchlorosilane resulted in the
PhSi(OMe)3
NaOH, H2O
i-PrOH
SiO
O SiOSiOSi
ONa
Ph
Ph
NaO
Ph
NaO
ONa
Ph SiO
O SiOSiOSi
O
Ph
Ph
O
Ph
O
O
Ph
99% SiMe2H
SiMe2H
SiMe2H
SiMe2H
Me2SiHCl
hexane
68%
Scheme 1.15 Synthesis of siloxycyclotetrasiloxane.
i-BuSi(OMe)3NaOH, H2O
i-PrOH
SiO
O SiOSiOSi
ONa
i-Bu
i-Bu
NaO
i-Bu
NaO
ONa
i-Bu SiO
O SiOSiOSi
O
i-Bu
i-Bu
O
i-Bu
O
O
i-Bu
SiMe2H
SiMe2H
SiMe2H
SiMe2H
Me2SiHCl
hexane
69% oveall yield
ViSi(OMe)3KOH, H2O
ether-hexane
SiO
O SiOSiOSi
OK
Vi
Vi
KO
Vi
KO
OK
Vi SiO
O SiOSiOSi
O
Vi
Vi
O
Vi
O
O
Vi
SiMe2H
SiMe2H
SiMe2H
SiMe2H
Me2SiHCl / NEt3
hexane
Vi = vinyl
12% overall
Scheme 1.16 Synthesis of various siloxycyclotetrasiloxane.
Novel Nanoscale Hybrid Materials14
stereoisomerization in the case of vinyl compounds. Therefore, triethylamine was added to the reaction mixture to avoid isomerization. In each reaction, selection of the solvents is important because solubility of these salts is dif-ferent by substituents.
It is noteworthy that these extended cyclic siloxanes contain reactive vinyl or hydrosilyl groups. Therefore these compounds are potential precursors for highly ordered siloxanes or siloxane‐containing organic polymers.
In addition, oxidation of these hydrosilanes with m‐chloroperbenzoic acid (mCPBA) afforded silanols (Scheme 1.17). These silanols are also interesting in view of precursors of siloxanes as well as host molecule for anions with extended silanol arms [39]. The structures of both compounds were estab-lished by X‐ray crystallographic analysis. In the crystal, the molecules form hydrogen‐bonded dimer.
1.2.3 Cyclotetrasiloxanetetraol (Cyclic Silanols, Other Isomers)
As shown in Scheme 1.10 and following schemes, all‐cis cyclic silanol showed its capacity as the precursors of various well‐defined siloxanes. This fact also indicates the potential of other three stereoisomers. For example, as shown in scheme 1.11, cis,trans,cis‐isomer is the most promising starting compound for ladder polysilsesquioxanes, because only this isomer is capable of generating straight‐chain polymers with unlimited molecular weight. With this point of view, we investigated the synthesis of other three isomers. When we started this project, there was a report of the isomerization of all‐cis‐[PhSiO(OH)]4 with catalytic amount of organochlorosilane (Me3SiCl, Me2SiCl2), HCl, or
SiO
O SiOSiOSi
O
Ph
Ph
O
Ph
OO
Ph
SiMe2HSiMe2H
SiMe2H
SiMe2H
SiO
O SiOSiOSi
O
i-Bu
i-Bu
O
i-Bu
OO
i-Bu
SiMe2HSiMe2H
SiMe2H
SiMe2H
mCPBA
benzene
68%
mCPBA
benzene
67%
SiO
O SiOSiOSi
O
Ph
Ph
O
Ph
OO
Ph
SiMe2OHSiMe2OH
SiMe2OH
SiMe2OH
SiO
O SiOSiOSi
O
i-Bu
i-Bu
O
i-Bu
OO
i-Bu
SiMe2OHSiMe2OH
SiMe2OH
SiMe2OH
Scheme 1.17 Synthesis of cyclotetrasiloxanes with extended silanol arms.
