Formation of a molecularly and mesoscopically ordered structure from cage siloxanes with a long alkyl chain and dimethylsilanol groups

Structural control of siloxane-based materials at multiple length scales is important for various applications. In this study, we report the controlled assembly of cage oligosiloxane building blocks by both intermolecular hydrogen bonding and hydrophobic interactions. A cage siloxane molecule modified with seven dimethylsilanol groups and an octadecyldimethylsilyl group was synthesized by stepwise silylation of double-four-ring (D4R) cage octasilicate anions. This molecule self-assembled to form a molecularly and mesoscopically ordered structure by solvent evaporation. Furthermore, the silanol groups in the assembled solids were cross-linked by silylation with dichlorodimethylsilane. This approach will allow for the creation of various hierarchically ordered siloxane-based materials by molecular design. A cage siloxane molecule containing seven dimethylsilanol groups and an octadecyldimethylsilyl group was synthesized. The cage siloxane molecules formed an ordered solid with both molecular and mesoscale periodicities by self-assembly. The silanol groups in the ordered solid were cross-linked by dichlorodimethylsilane to form siloxane networks. A cage siloxane molecule containing seven dimethylsilanol groups and an octadecyldimethylsilyl group was synthesized. The cage siloxane molecules formed an ordered solid with both molecular and mesoscale periodicities by self-assembly. The silanol groups in the ordered solid were cross-linked by dichlorodimethylsilane to form siloxane networks.


Introduction
Ordered siloxane-based materials composed of regularly arranged SiO 4 tetrahedral units are useful for various applications, including catalysis and adsorption [1].Crystalline silicates such as zeolites [2] and layered silicates [3] are generally synthesized by hydrothermal treatments, but the complex crystallization mechanism makes it difficult to precisely control the structure and compositions.Controlled assembly of pre-designed molecular building blocks is a promising method to expand the structural and compositional diversity of molecularly ordered siloxane-based materials.Among various types of oligosiloxanes with well-defined structures [4][5][6][7][8][9][10], cage siloxanes have attracted particular attention as building blocks because of their rigid frameworks and various functional groups at the corners [7,8].Extensive studies have been made on cross-linking of cage siloxanes via Si-O-Si bonds and organic linkers [11,12]; however, most of the materials have random network structures, and controlled assembly of cage siloxanes has been an important issue.
The use of intermolecular interactions is crucial to realize the assembly of oligosiloxanes into molecularly ordered structures.Hydrogen bonds (H-bonds) of silanol groups are quite effective for the regular assembly of cage siloxanes.Several different arrangements of octa-silanol modified cage siloxanes, from one-dimensional (1D) to three-dimensional (3D) assemblies, have been reported [13][14][15][16][17][18][19].The silanol groups are also useful for intermolecular linking by the formation of Si-O-Si bonds either by dehydration condensation or by silylation.We have recently synthesized H-bonded molecular crystals of cage siloxanes modified with SiR 2 OH groups (R = Me and i Pr) [16][17][18][19].Furthermore, cross-linking of the H-bonded SiMe 2 OH groups of the cage siloxanes was achieved by silylation with trichlorosilane, resulting in the formation of a crystalline microporous material [17].Nevertheless, the crystalline structures assembled from octa-silanol modified cage siloxanes have been still limited, and the formation of mesoscale structures remains a challenge.
Introducing organic substituents to the corner Si atoms of cage siloxanes allows mesoscale assembly.Polyhedral oligomeric silsesquioxanes (POSS, R 8 Si 8 O 12 , R = H or organic groups) modified with various organic functional groups and/or polymers were designed to drive hierarchical assembly into complex architectures [20][21][22].However, the regularity of the cage siloxanes was not well-controlled in many cases.In addition, most of these POSS-based assemblies cannot be cross-linked by siloxane networks, except for those having SiH groups [20].On the other hand, we reported the formation of lamellar and two-dimensional (2D) hexagonal mesostructures by the sol-gel reactions of cage siloxanes modified with a long-chain alkyl group and seven ethoxy groups [23,24].The key step for the selfassembly was hydrolysis of the SiOEt groups into SiOH groups, forming amphiphilic molecules containing silanol groups and a long alkyl group.However, the cage siloxanes were not regularly arranged at the molecular level probably because condensation of the silanol groups partially proceeded prior to self-assembly.Synthesis and isolation of similar amphiphilic siloxane cages modified with relatively stable organosilanol groups and a long alkyl chain are essential.
In this paper, we report the synthesis of a novel cage siloxane compound modified with seven organosilanol groups and a long-chain alkyl group that can form a hierarchical siloxane-based material.Cubic octameric silicate was modified with SiMe 2 H groups and a SiMe 2 C 18 H 37 group by stepwise silylation (D4R-7SiH and D4R-7SiH-C18 in Scheme 1), followed by the conversion of the SiMe 2 H groups to SiMe 2 OH groups using a Pd/C catalyst (D4R-7SiOH-C18).Upon solvent evaporation, the molecule was assembled to form a crystalline solid with a regular arrangement of the cages induced by the intermolecular H-bonding of the silanol groups and a mesoscale periodicity induced by the long alkyl chains.Furthermore, cross-Scheme 1 Synthetic route to cage siloxane modified with a long alkyl chain and organosilanol groups (D4R-7SiOH-C18) starting from cubic octameric silicate linking of the silanol groups by siloxane bonds was performed by silylation with dichlorodimethylsilane.

Synthesis of cubic octameric silicate
Tetramethylammonium silicate with a D4R structure (TMA 8 Si 8 O 20 •xH 2 O) was synthesized according to our previous report [25].TEOS (61.8 mL) was added to the mixture of TMA hydroxide•5H 2 O (50 g), ethanol (160.6 mL), and H 2 O (24.8 mL).The molar ratio was TEOS: TMA hydroxide: ethanol: H 2 O = 1: 1: 10: 10.After stirring for 3 d, the solution was concentrated to about half of the original volume by heating on a hot plate.The hydrated crystals precipitated upon cooling were filtrated and vacuum dried at room temperature to obtain a white solid (54.1 g). 29Si nuclear magnetic resonance (NMR) (99.37 MHz, ethanol-d 6 ): δ (ppm) −99.00 (Si(OSi) 3 O − ).Thermogravimetry (TG)-differential thermal analysis (DTA) revealed that the weight loss was 72% when the final white solid was heated up to 500 °C.Assuming that the residue was SiO 2 , the hydration number (x) was calculated to be 32.

Synthesis of D4R-7SiH
It is reported that incompletely trimethylsilylated and dimethylvinylsilylated cage siloxanes can be obtained by silylation of Si 8 O 20 8− using a THF solvent [26][27][28].On the basis of these reports, incomplete dimethylsilylation of cage siloxane was conducted.TMA 8 Si 8 O 20 •32H 2 O (10.6 g) dissolved in methanol (40 mL) was added dropwise to the mixture of THF (100 mL), hexane (100 mL), and DMSCl (12 mL).After stirring for 10 min, H 2 O (20 mL) and hexane (200 mL) were added to the reaction mixture.After further stirring for 10 min, the hexane layer was extracted, and the volatile compounds were removed under reduced pressure to obtain a white solid.The solid was washed with acetonitrile (20 mL) by stirring for 10 min to remove cage siloxane containing six or less silylated vertices.Then, a white solid was recovered by filtration (3.1 g, yield: 51% based on the fact that 7.5 corners were silylated on average, as calculated by the 29 Si NMR analysis).Although impurities such as Si 8 O 12 (OSiMe 2 H) 6 (OH) 2 and Si 8 O 12 (OSi-Me 2 H) 8 were still contained, the product was used for the subsequent reaction without separation of D4R-7SiH.

Synthesis of D4R-7SiH-C18
D4R-7SiH containing impurities (1.5 g) was dissolved in hexane (40 mL).The solution was added dropwise to the mixture of C 18 DMSCl (0.54 g), pyridine (190 μL), and hexane (5 mL) in a Schlenk flask under nitrogen atmosphere.After stirring at room temperature for 1 d, DMSCl (3 mL) and pyridine (3 mL) were added.After further stirring at room temperature for 3 h, H 2 O (5.0 mL) was added to the reaction mixture and the hexane layer was extracted.Volatile compounds were removed under reduced pressure to obtain a white solid.The solid was dissolved in chloroform and D4R-7SiH-C18 (a white solid, 0.16 g, yield: 8% based on D4R-7SiH) was isolated by gel permeation chromatography (GPC). 1

Characterization
Liquid-state 1 H, 13 C, and 29 Si NMR spectra were recorded on a JEOL JNM ECZ 500 spectrometer at resonance frequencies of 500.0MHz, 125.7 MHz, and 99.4 MHz, respectively, at ambient temperature using 5 mm glass tubes.Tetramethylsilane was used as an internal reference (set as 0 ppm).Chloroform-d, ethanol-d 6 , and THF-d 8 were used to obtain lock signals. 29Si NMR spectra were measured with a 45°pulse and a recycle delay of 30 s. Solidstate 13 C cross-polarization (CP)/magic-angle spinning (MAS) NMR and 29 Si MAS NMR spectra were recorded on a JEOL JNM ECA 400 spectrometer at resonance frequencies of 100.53 MHz and 79.43 MHz, respectively, at ambient temperature.The sample was put in a 5 mm zirconia rotor.Fourier transform infrared (FT-IR) spectra were recorded on a JASCO FT/IR-6100 spectrometer using the KBr method.X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima-III diffractometer with a parallel beam geometry equipped with a scintillation detector and parabolic multilayer mirror using Cu Kα radiation (λ = 1.5418Å).GPC was carried out using a Japan Analytical Industry LC-7080 system equipped with a refractive index detector RI-700LA.JAIGEL-HR series columns (1HR, 2HR, and 2HR plus) were used with chloroform as the eluent at a flow rate of 7.0 mL min -1 .Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass analysis was conducted using a Bruker autoflex maX instrument with dithranol as the matrix and sodium trifluoroacetate as the ionization aids.Scanning electron microscopy (SEM) images were obtained on a Hitachi S5500 electron microscope with an accelerating voltage of 1-5 kV.Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope with an accelerating voltage of 200 kV.Samples for TEM and SEM analyses were dispersed on a carbon-coated Cu grid (Okenshoji Co., Ltd.).Crosssectional sample was prepared using a focused ion beam (FIB) milling and imaging system (JIB-4700) equipped with an FIB and a scanning ion microscope.D4R-7SiOH-C18 powder deposited on the sample fixing table was cut out using Ga ion beams, and the flake was transferred to a carbon-coated Cu grid.Cross-sectional high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image was obtained on a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV.TG-DTA was conducted using a RIGAKU Thermo Plus EVO2 TG8121 under dry air at a heating rate of 10 K min -1 .

Synthesis of cage siloxanes modified with seven
SiMe 2 OH and one SiMe 2 C 18 H 37 groups (D4R-7SiOH-C18) The first step for the synthesis of D4R-7SiOH-C18 was dimethylsilylation of Si 8 O 20 8− to form cage siloxane modified with SiMe 2 H groups at 7 corners (D4R-7SiH).The progress of dimethylsilylation was confirmed by the 1 H NMR spectrum of the silylated product (Fig. S1) showing the signals of the OSiMe 2 H groups (0.25 and 4.73 ppm) [16].The 29 Si NMR spectrum (Fig. 1A(a)) showed multiple signals at the M 1 (SiOSiMe 2 H: −0.9 to −1.5 ppm), Q 3 (Si(OSi) 3 OH: −100.4 to −100.6 ppm), and Q 4 (Si(OSi) 4 : −108.4 to −108.9 ppm) regions.The integral ratio of the M 1 , Q 3 , and Q 4 signals was 7.5: 0.5: 7.5, indicating that the average number of dimethylsilylated corners was 7.5 per Si 8 O 20 and the products were mixtures of cage siloxanes with different numbers of silylated corners.Note that mono-/hepta-substituted cage siloxanes consist of four chemically inequivalent Si atoms [29,30].In the case of D4R-7SiH, the Q 3 signal of the unsilylated SiOH group and the Q 4 signals of the silylated corner Si atoms first, second, and third closest to the SiOH group should be observed with the 1: 3: 3: 1 ratio.The largest Q 4  and M 1 signals were probably derived from the symmetrical cage siloxane modified with eight SiMe 2 H groups, and the smaller signals were assumed to be derived from the cage siloxanes modified with six and seven SiMe 2 H groups.The MALDI-TOF mass spectrum (Fig. 1B(a))  Although detailed signal assignments are difficult, the four Q 4 signals can be attributed to the cage corner Si atoms first, second, third, and fourth closest to the SiMe 2 C 18 H 37 group (1: 3: 3: 1 ratio).Similarly, the three D 1 signals can be attributed to the Si(CH 3 ) 2 OH groups first, second, and third closest to the SiMe 2 C 18 H 37 group (3: 3: 1 ratio).The MALDI-TOF mass spectrum (Fig. 1B(c)) shows the peak corresponding to [Si 8 O 12 (OSiMe 2 OH) 7 (OSiMe 2 C 18 H 37 ) + Na] + .The isotope pattern was consistent with that obtained by simulation.These results indicated that the cage siloxane was successfully modified with seven SiMe 2 OH groups and one SiMe 2 C 18 H 37 group.

Self-assembly of D4R-7SiOH-C18
Self-assembly of D4R-7SiOH-C18 easily occurred upon solvent evaporation.The 29 Si MAS NMR spectrum of the product (Fig. 2(a)) showed the M 1 , D 1 , and Q 4 signals corresponding to D4R-7SiOH-C18.No signal was observed at the D 2 region (around −20 ppm), indicating that the silanol groups remained intact without condensation to form siloxane bonds.This solid product was therefore resoluble in THF.The 13 C CP/MAS NMR spectrum (Fig. 2(b)) shows the signals for SiCH 3 (0.5 and 1.0 ppm) and SiC 18 H 37 (15.2, 20.1, 24.8, 30.7, 33.7, and 36.9 ppm) [31,32].The interior methylene carbons of the C 18 H 37 groups are observed at 30.4 ppm and 33.8 ppm, which correspond to mixed trans-gauche and all-trans conformations, respectively [33][34][35].Because the latter signal (33.8 ppm) is mainly observed, it is likely that the alkyl chains are in an ordered all-trans state with some gauche defects.The FT-IR spectrum (Fig. 2(c)) showed bands due to alkyl chains, SiMe groups, SiOH groups, and SiOSi networks.The stretching vibration of the SiO-H groups was observed as a broad band centered at 3295 cm −1 , which was attributed to the H-bonded silanol groups [16][17][18][19]36].
The powder XRD pattern of self-assembled D4R-7SiOH-C18 (Fig. 3a) exhibited the most intense peak at d = 3.1 nm along with second-and third-order peaks.These peaks were assignable to a lamellar structure and differed from other mesophases such as 2D hexagonal and cubic phases [37].In fact, when calcined in air at 250 °C for 1 d to remove alkyl chains, most of the diffraction peaks disappeared (Fig. S5), as in the previously reported lamellar alkylsiloxanes [9].The d value of the first-order peak (3.1 nm) was slightly smaller than the molecular length of D4R-7SiOH-C18 when the alkyl group is assumed to be all-trans (~3.6 nm).Taking into account the relatively large occupying volume of the cage siloxane units, it is reasonable to consider that an interdigitated monolayer-type structure rather than a bilayer-type structure was formed.The other peaks observed at d = 1.1 nm and higher angles can be attributed to the arrangements of H-bonded cage siloxanes and The SEM observation that most of the particles have plate-like morphology (Fig. 3b).A stripe pattern characteristic of a lamellar structure was observed near the edge of the particles (Fig. 3c, d).The HAADF-STEM observation of the cross-section of the particle cut vertically by FIB processing also showed a stripe pattern oriented parallel to the particle surface (Fig. S6).The SAED pattern of another view (Fig. 3e, f) showed spots with a d value of about 0.95 nm.These spots are likely associated with the H-bonding arrangement of the cage molecules because similar square arrays of spots were reported for the H-bonded molecular crystals of D4R siloxanes modified with SiMe 2 OH groups at all corners [16,17].All the above results suggested that the cage siloxanes were arranged to form a molecularly and mesoscopically ordered structure.

Cross-linking of D4R-7SiOH-C18 by silylation
The self-assembled solids of D4R-7SiOH-C18 were treated with dichlorodimethylsilane to bridge the adjacent silanol groups by siloxane bonds.The solid-state 29 Si MAS NMR spectrum of the product (Fig. 4a) showed the M 1 (OSi-Me 2 C 18 H 37 : 13.2 ppm), D 1 (OSiMe 2 OH: −10.3 ppm), D 2 (SiMe 2 (OSi) 2 : -17.5 ppm), and Q 4 (Si(OSi) 4 : −108.9 ppm) signals.The sharp Q 4 signal accompanying no Q 3 signal suggested that the silylated cage siloxane units were almost retained without cleavage of the Si-O-Si bonds.The integral ratio of M 1 , D 1 , D 2 , and Q 4 signals was calculated to be 1.0: 1.7: 7.7: 8.0.Assuming that one dichlrodimethylsilane reacts with two silanol groups, the theoretical ratio of M 1 , D 2 , and Q 4 units is calculated to be 1.0: 10.5: 8.0.The difference between the theoretical and measured ratios indicated that ~75% of the silanol groups underwent crosslinking.FT-IR analysis also confirmed that the band assignable to Si-O-H deformation (887 cm −1 ) significantly decreased (Fig. 4b).After silylation, the XRD peaks observed for D4R-7SiOH-C18 became weak and/or broad, and no new peaks appeared (Fig. S7).Sheet-like morphology was still observed by SEM (Fig. 4c) and stripe patterns were observed by TEM (Fig. 4d).These results suggested that the lamellar structure was at least partially retained after the cross-linking.We expect that the structural regularity can be improved by optimizing the reaction conditions and/ or by the choice of the silylating agent used.
The synthetic method of crystalline 2D silica-based materials presented in this study will be useful for the preparation of functional coatings and membranes.The structure of the siloxane frameworks is the factor for these applications.Until now, the top-down synthesis of crystalline silica nanosheets by exfoliation of 2D zeolites [38] and silicates [39], as well as their applications barrier coatings and gas separation membranes, have been reported.However, the types of structures that can be obtained by exfoliation of those hydrothermally synthesized layered compounds are limited.The surfactant-directed selfassembly process allows for the preparation of lamellar silica-and organosiloxane-based materials [40][41][42], and a recent report has shown that colloidal silica nanosheets can be obtained by exfoliation [43].However, these materials have amorphous siloxane networks unless post-synthetic hydrothermal treatment is employed [44].The bottom-up self-assembly method using well-defined cage siloxanes has the potential to overcome these limitations, although further research is needed to retain the crystal structure upon crosslinking.The direct formation of single-layer nanosheets may also be possible by the Langmuir-Blodgett technique.

Conclusions
We have demonstrated the hierarchical assembly of cage siloxane building blocks by hydrophobic interactions and hydrogen bondings.A long-chain alkyl group and organosilanol groups were successfully introduced to cage siloxane with a D4R structure by stepwise silylation using chlorodimethylsilane and chloro(octadecyl)dimethylsilane followed by conversion of the SiH groups into SiOH groups.Self-assembly of the resulting compound, D4R-7SiOH-C18, occurred upon solvent evaporation.Furthermore, cross-linking of D4R-7SiOH-C18 using dichlorodimethylsilane proceeded while retaining the sheet-like morphology.This approach will allow for the creation of various hierarchically ordered siloxane-based materials by the molecular design of the building blocks.