Synthesis of Zeolitic Macrocycles Using Site-Selective Condensation

Oct 30, 2018 - These compounds have more rigid ring structures than conventional cyclic organosiloxanes. Such an approach will lead to the design of a...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Synthesis of Zeolitic Macrocycles Using Site-Selective Condensation of Regioselectively Difunctionalized Cubic Siloxanes Shohei Saito,† Hiroaki Wada,† Atsushi Shimojima,*,† and Kazuyuki Kuroda*,†,‡ †

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/03/18. For personal use only.

Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan S Supporting Information *

ABSTRACT: The designed synthesis of inorganic cyclic compounds is a significant topic because of their many potential applications. In this study, we used a building block approach to synthesize siloxane-based macrocycles that resemble zeolite apertures. We synthesized a regioselectively functionalized cubic octasiloxane having two adjacent corners modified with Si−O− C bonds via the reaction of octa(hydridosilsesquioxane) (H8Si8O12) with 2,2′-(o-phenylenedioxy)diethanol. Hydrolysis and condensation of the Si−O−C bonds yield the cyclic compounds consisting of three, four, and five cage siloxane units. These compounds have more rigid ring structures than conventional cyclic organosiloxanes. Such an approach will lead to the design of a new class of host materials and molecular channels for transport and separation.

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INTRODUCTION Zeolites are crystalline microporous aluminosilicates that have various applications, including catalysis, adsorption, and separation.1,2 The properties of zeolites are strongly dependent on their framework structures.3−5 However, it remains difficult to design silicate frameworks using conventional hydrothermal synthesis. Researchers have devoted significant effort to constructing zeolite frameworks using well-defined building blocks.6−8 Cubic siloxanes with a double-four-ring structure, which is known as a secondary building unit (SBU) of some zeolites, have attracted particular attention because they have a rigid structure and corner functionalization capabilities.9−13 There have been many reports on the synthesis of nanoporous materials with high surface areas by covalent linking of cubic siloxanes through various reactions (e.g., hydrolysis and polycondensation of alkoxysilyl groups14 and homocoupling reactions of halogenated or alkynyl phenylsilsesquioxanes15,16). However, the frameworks for such materials have lower structural regularity compared with those for zeolites, mainly because it is difficult to kinetically control the connections between the cubic siloxanes with equivalent reaction sites at all corners. It is possible to construct crystalline assemblies using the intermolecular hydrogen bonding of silanol-modified cubic siloxanes;17,18 however, there are still some difficulties associated with covalently linking cubic siloxanes while retaining their ordered structures. One promising approach for achieving the controlled connection of cubic siloxanes is to restrict the number of available reaction sites via the partial functionalization of specific © XXXX American Chemical Society

corners. For example, monofunctionalized cubic siloxanes, which can be synthesized by corner-capping reactions of incompletely condensed polyhedral oligomeric silsesquioxanes (POSSs), have been used to form discrete oligomers with dumbbell-like structures10,11,19−21 and a radial structure.19 Difunctionalized cubic siloxanes are potentially more useful as building blocks because of their ability to form larger oligomers and polymers. However, the synthesis of difunctionalized cubic siloxanes is difficult because of the formation of regioisomers, which are hereafter denoted as the ortho, meta, and para types (Figure 1). Only a few reports exist on the synthesis of each isomer.22−27 Para-substituted divinyl-POSS can be synthesized using the selective corner-opening reaction of monovinyl heptaisobutyl-POSS and a subsequent corner-capping reaction with trichlorovinylsilane.22 This compound was used to

Figure 1. Three possible regioisomers of difunctional cubic siloxane. The red circles denote functional groups for connecting cubic siloxanes, and the blue circles denote other substituents. Received: August 24, 2018

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DOI: 10.1021/acs.inorgchem.8b02402 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry synthesize linear polymers through hydrosilylation with 1,1,3,3,5,5-hexamethyltrisiloxane.22 Ortho and meta isomers have also been synthesized by postsynthetic modification of octafunctional POSSs.24−26 Three regioisomers of diadducts of octavinyl-POSS were successfully separated from their mixtures by column chromatography.24,25 The efficient synthesis of an ortho isomer was also achieved by the click reaction of octaazido-POSS with a parallel-oriented dialkyne.26 Unfortunately, the types of functional groups have so far been very limited, and these difunctionalized POSS-based compounds cannot form siloxane networks because all of the corners are terminated with stable Si−C bonds. Alkoxysilyl groups are particularly useful for siloxane bond formation, either by hydrolysis and condensation or by nonhydrolytic reactions with other functional groups (e.g., Si− H and Si−Cl).28 We recently reported the synthesis of dialkoxylated cubic siloxanes through the partial alkoxylation of octa(hydridosilsesquioxane) (H8Si8O12) with tert-butyl alcohol;27 however, we did not achieve regioselectivity control. It should be noted that we refer to the cubic compounds consisting of both T3 ((SiO)3SiH and (SiO)3SiC) and Q3 ((SiO)3SiOC and (SiO)3SiOH) silicons as cubic siloxanes. Among the three regioisomers of dialkoxylated cubic siloxanes, ortho- and meta-type isomers are attractive as building blocks for constructing zeolitic materials. These isomers are expected to form cyclic rather than linear structures by intermolecular siloxane bond formation because the angle between the two Si−O−C bonds in an ortho- or meta-type isomer is smaller than that for a para-type isomer (Figure S1). Cyclic siloxanes consisting of cubic siloxanes can be regarded as molecular zeolites because they have unique inner cavities that resemble micropores of zeolites. In this paper, we report the successful synthesis of an orthotype difunctionalized cubic siloxane, 1, in which two adjacent corners are selectively modified with Si−O−C bonds by using a dihydroxy compound with a specific molecular geometry, 2,2′(o-phenylenedioxy)diethanol (PDDE), as an intramolecular bridging agent for H8Si8O12 (Figure 2a). After the remaining six SiH groups were converted to more stable alkylsilyl groups to form 2, the disilanol compound 3 was obtained via hydrolysis of the Si−O−C bonds. Simple heating of 3 effectively generated the cyclic compounds cyclic-(R6Si8O13)n (n = 3, 4, 5) by intermolecular dehydrative condensation. As shown in Figure 2b and Movie S1, the cyclic structures formed by connecting three or four cubic siloxanes at the two adjacent corners can be regarded as partial structures of LTA-type zeolite. The apertures of LTA-type zeolite have been created in the molecules. Importantly, the cyclic structure formed by connecting five cubic siloxanes at the two adjacent corners has never been found in conventional zeolites listed in the International Zeolite Association (IZA) structure database.29 These results indicate the potential of the present method to design a new class of host materials and molecular channels for transport and separation.

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Figure 2. (a) Scheme for the regioselective synthesis of an ortho-type difunctional cubic siloxane with two Si−O−C bonds and the formation of cyclic compounds by hydrolysis and condensation of the difunctional cubic siloxane. (b) Structural models of cyclic-(R6Si8O13)n (n = 3, 4, 5) and the presence of the cyclic species in LTA zeolite. Organic substituents of cyclic-(R6Si8O13)n have been omitted for clarity. Pure Chemical Industries, Ltd., >99.5%), toluene (super anhydrous, Wako Pure Chemical Industries, Ltd., >99.5%), and trichlorosilane (Tokyo Chemical Industry Co., Ltd., >98.0%) were used as received. Dowex 50WX8 50−100 mesh (H) cation exchange resin (Wako Pure Chemical Industries, Ltd.) was dried under vacuum before use. Synthesis of [o-C6H4(OC2H4O)2]H6Si8O12 (1). 1 was synthesized by reacting H8Si8O12 with 2,2′-(o-phenylenedioxy)diethanol (PDDE) in the presence of Et2NOH as a catalyst. H8Si8O12 was synthesized from HSiCl3 by a previously reported procedure.30 H8Si8O12 (0.500 g, 1.18 mmol) and PDDE (0.233 g, 1.18 mmol) were dissolved in toluene (75 mL: 15 mL per 0.1 g of H8Si8O12) at 40 °C under a N2 atmosphere. Then a 0.5 wt % toluene solution of Et2NOH was added. The H8Si8O12/PDDE/Et2NOH molar ratio was 1:1:0.001. The mixture was stirred at 40 °C under a N2 atmosphere for 4 h, after which a strong-acid cation-exchange resin was added and the mixture was stirred overnight at room temperature to remove Et2NOH. The resin was removed by filtration, and the solvent was evaporated from the filtrate under reduced pressure to give a waxy solid containing a small amount of toluene. 1 was isolated as a white solid from the mixture by GPC separation (yield: 9% (64 mg)). The spectroscopic data (1H, 13C, and 29 Si NMR) of 1 are shown in the Supporting Information. Synthesis of [o-C6H4(OC2H4O)2]R6Si8O12 (R = Hexyl) (2). 2 was synthesized by reacting 1 with 1-hexene in the presence of a Pt catalyst (platinum(0)−1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex). 1 (95 mg, 0.153 mmol) and 1-hexene (574 μL, 4.59 mmol) were dissolved in toluene (9.5 mL: 10 mL per 0.1 g of 1) under a N2 atmosphere, and then the Pt catalyst was added. The 1/1-hexene/Pt catalyst molar ratio was 1:30:0.01. After the solution was stirred at 60 °C for 1 day, activated charcoal was added, and the mixture was stirred at room temperature for 3 h to remove the Pt catalyst. The activated neutral charcoal was removed by filtration, and the solvent was removed from the filtrate under reduced pressure. Finally, 2 was obtained as a

EXPERIMENTAL SECTION

Materials. Chloroform (Wako Pure Chemical Industries, Ltd., >99.0%), N,N-diethylhydroxylamine (Tokyo Chemical Industry Co., Ltd., >98.0%), granular activated charcoal (Wako Pure Chemical Industries, Ltd.), 1-hexene (Wako Pure Chemical Industries, Ltd., >95.0%), 1,3-bis(2-hydroxyethoxy)benzene (Tokyo Chemical Industry Co., Ltd., >98.0%), 1,8-octanediol (Tokyo Chemical Industry Co., Ltd., >97.0%), 2,2′-(o-phenylenedioxy)diethanol (Sigma-Aldrich, 97%), platinum(0)−1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in xylene (Pt content ∼ 2%) (Sigma-Aldrich), tetrahydrofuran (Wako B

DOI: 10.1021/acs.inorgchem.8b02402 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry clear oil (yield: 98% (169 mg)). The spectroscopic data (1H, 13C, and 29 Si NMR and high-resolution electrospray ionization mass spectrometry (ESI-MS)) of 2 are shown in the Supporting Information. Hydrolysis and Condensation of 2. Hydrolysis of 2 was performed by using Et2NOH as a catalyst. After 2 (169 mg, 0.151 mmol) was dissolved in a mixture of H2O (27.2 μL, 1.51 mmol) and THF-d8 (1272 μL), Et2NOH (4.65 μL, 4.53 × 10−2 mmol) was added. The 2/H2O/ Et2NOH/THF-d8 molar ratio was 1:10:0.3:100. After the solution was stirred at room temperature for 3 h, the solvent was removed under reduced pressure to give the disilanol compound R6(HO)2Si8O12 (3) as a viscous liquid. The spectroscopic data (1H, 13C, and 29Si NMR and ESI-MS) of 3 are shown in the Supporting Information. Compound 3 was heated at 100 °C for 1 day without any solvents to condense the Si−OH groups of 3. The compounds cyclic-(R6Si8O13)n (n = 3, 4, 5) were isolated as viscous liquids from the condensed mixture by gel-permeation chromatography (GPC) (yields: cyclic-(R6Si8O13)3, 14% (20 mg); cyclic-(R6Si8O13)4, 10% (14 mg); cyclic-(R6Si8O13)5, 8% (11 mg)). The low yields of these compounds are mainly due to the partial overlap of the GPC peaks of these fractions. The spectroscopic data (1H, 13C, and 29Si NMR and matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF-MS)) of cyclic-(R6Si8O13)n (n = 3, 4, 5) are shown in the Supporting Information. Characterization. The details of the measurement methods and conditions for solution-state 1H, 13C, and 29Si NMR, solid-state 29Si MAS NMR, high-resolution ESI-MS, low-resolution ESI-MS, MALDITOF-MS, and GPC and for molecular modeling are shown in the Supporting Information.

inequivalent Si atoms is evidence of an ortho-type structure (in contrast, meta- and para-type isomers have four and two inequivalent Si atoms, respectively, as shown in Figure 1). Unfortunately, MS analysis of this compound failed, probably because of the instability of the SiH groups during ionization. The next GPC peak for the shorter retention time exhibited several T3 signals ((SiO)3SiH: −83.01, −83.04, −83.76, and −83.77 ppm) and Q3 signals ((SiO)3SiOCH2: −104.14, −104.21, −104.77, and −104.79 ppm) in the 29Si NMR spectrum (Figure S5). The T3:Q3 intensity ratio was 1:1. The MALDI-TOF-MS spectrum (Figure S6) showed an isotope pattern corresponding to the Na + adduct of [oC 6 H 4 (OC 2 H 4 O) 2 ] 2 H 4 Si 8 O 1 2 (m/z 834.8; calcd for C20H28O20Si8Na+ [M + Na]+ 834.9). These results indicated that four corners of the cubic siloxane were modified with two PDDE molecules. Judging from the number of 29Si signals, it is likely that a mixture of three possible isomers of [oC6H4(OC2H4O)2]2H4Si8O12 was formed (see Figure S7 for details). It is noteworthy that meta- and para-type isomers of [oC6H4(OC2H4O)2]H6Si8O12 were not detected. If these two regioisomers of 1 had been generated, their eluted fractions should have appeared between those of H8Si8O12 and [oC6H4(OC2H4O)2]2H4Si8O12. Thus, these GPC and NMR results strongly indicate that the ortho-type isomer 1 was formed selectively among the three possible regioisomers. According to molecular models of PDDE and H8Si8O12, PDDE cannot bridge the two SiH groups at the meta or para positions of the cubic siloxane because the distances between the SiH groups are longer than that between the two hydroxyl groups of PDDE (Figure S8). In addition to the aforementioned intramolecular bridging reactions, intermolecular cross-linking of cubic siloxanes with PDDE occurred, as confirmed by the presence of highmolecular-weight fractions that were eluted after much shorter retention times than [o-C6H4(OC2H4O)2]2H4Si8O12 in the GPC profile (Figure S2). After removal of the solvent from the high-molecular-weight fraction, a white solid was obtained. The 29 Si magic-angle spinning (MAS) NMR spectrum (Figure S9) shows two broad signals corresponding to the T3 ((SiO)3SiH: −83 ppm) and Q3 ((SiO)3SiOCH2: −103 ppm) units. The Q3/ (T3 + Q3) intensity ratio was ca. 0.34. When one corner of the cubic siloxane is modified with PDDE, then the Q3/(T3 + Q3) intensity ratio becomes 0.125. These results indicated that an average of 2.7 corners of the cubic siloxanes were modified with PDDE to cause intermolecular cross-linking. No signal was observed in the Q4 region (−110 to −120 ppm), confirming that the occurrence of side reactions forming Si−O−Si bonds between the cage siloxanes was negligible. Further study is needed to suppress intermolecular bridging and increase intramolecular bridging. To investigate the critical role of PDDE as an intramolecular bridging agent, like the parallel-oriented dialkyne used to synthesize ortho-type difunctional POSSs by thiol−ene reactions,26 other dihydroxy compounds were reacted with H8Si8O12 under identical conditions (see the Supporting Information for details). When HO(CH2)8OH and mC6H4(OC2H4OH)2 were reacted with H8Si8O12, the GPC profile of each crude product showed several very weak peaks corresponding to intramolecular-bridged compounds (Figures S10 and S11, respectively). The intensities of these peaks are obviously weak compared with the case of using PDDE (Figure S12), implying that the formation ratios for (OC8H16O)H6Si8O12 and [m-C6H4(OC2H4O)2]H6Si8O12 are smaller than

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RESULTS AND DISCUSSION Synthesis of [o-C6H4(OC2H4O)2]H6Si8O12 (1). In the GPC profile of the crude product after the reaction of H8Si8O12 with PDDE (Figure S2), several components having different retention times were eluted. We found that the most delayed fraction was unreacted H8Si8O12 (Figure S3). Cubic siloxanes modified with PDDE were eluted earlier because of an increase in molecular size. The fraction eluted just before H8Si8O12was assigned as [o-C6H4(OC2H4O)2]H6Si8O12 (1), which was formed by the 1:1 reaction of H8Si8O12 and PDDE. The 1H NMR spectrum (Figure 3a) shows the signals originating from

Figure 3. (a) 1H NMR and (b) 29Si NMR spectra (in CDCl3) of 1 (the peak at the retention time of 30−31 min in Figure S2).

both H8Si8O12 and PDDE with an SiH/Si(OCH2CH2O)C6H4/ Si(OCH2CH2O)C6H4/Si(OCH2CH2O)C6H4 intensity ratio of 6:4:4:4. In the 13C NMR spectrum of 1 (Figure S4), the signal for −CH2OH (61.10 ppm) was slightly shifted downfield to 63.34 ppm, indicating that two of the hydroxyl groups in PDDE were reacted with the SiH groups to form −CH2−O−Si linkages.31 The 29Si NMR spectrum (Figure 3b) shows two T3 signals ((SiO)3SiH: −83.71 and −84.73 ppm) and a Q3 signal ((SiO)3SiOCH2: −105.00 ppm). The presence of three C

DOI: 10.1021/acs.inorgchem.8b02402 Inorg. Chem. XXXX, XXX, XXX−XXX

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SiOH group and the appearance of three new Q4 signals ((SiO)4Si: −110.00, −111.38, and −112.52 ppm), indicating the dehydrative condensation of the silanol groups. The GPC profile of the condensed products (Figure S16) showed three main components with apparently different sizes, and each component was isolated as a viscous liquid. The MALDI-TOFMS spectra of these components (Figure 5a,c,e) showed signals

that for 1. Therefore, we consider the molecular structure of PDDE (in which two −OC2H4OH groups are located at the ortho positions of the benzene ring) to significantly affect the synthesis of ortho-type difunctional cubic siloxanes in which two adjacent corners are modified with Si−O−C bonds. Synthesis of [o-C6H4(OC2H4O)2]R6Si8O12 (2) and Hydrolysis of the Si−O−C Bonds. Before the hydrolysis of the Si−O−C bonds in 1, the remaining six Si−H groups were converted into alkylsilyl groups in order to avoid side reactions, e.g., oxidation of the SiH groups. [o-C6H4(OC2H4O)2]R6Si8O12 (R = hexyl) (2) was easily obtained by Pt-catalyzed hydrosilylation with 1-hexene. In the 1H NMR spectrum (Figure S13), the signal due to Si−H groups completely disappeared after the reaction. The 29Si NMR spectrum (Figure 4a) showed the

Figure 4. 29Si NMR spectra (in THF-d8) of (a) 2 and (b) 3.

disappearance of the T3 signals for the SiH groups (−83.71 and −84.73 ppm) and the appearance of new T3 signals due to the formation of Si−C bonds (−65.68 and −66.62 ppm). In contrast, the Q3 signal due to Si−O−C bonds (−103.60 ppm) remained intact. Moreover, the HRMS spectrum showed an isotope pattern corresponding to a Na+ adduct of 2 (m/z 1145.4276). These results indicate the successful synthesis of 2. To connect the cubic siloxanes at adjacent corners, hydrolysis of 2 was performed to form disilanol compound 3 as an intermediate. When hydrolysis was performed at a 2/THF/ H2O/HCl molar ratio of 1:100:10:0.8, no significant reaction occurred (Figure S14). It is apparent that the bidentate structure of 2 provides a higher stabilization of Si−O−C bonds than pendant32 or intermolecular-bridged33 alkoxy groups on cage siloxanes. Among various catalysts tested, we found that diethylhydroxylamine (Et2NOH) can catalyze the hydrolysis of 2 without causing deterioration of the cubic structure, even though it is known that common base catalysts such as pyridine and tetraethylammonium hydroxide collapse the cubic structure.22,34 After hydrolysis of 2 at a 2/THF/H2O/ Et2NOH molar ratio of 1:100:10:0.3, the 29Si NMR spectrum (Figure 4b) showed the disappearance of the Q3 signal due to the Si−O−C bonds and the appearance of a new Q3 signal of Si−OH (−100.98 ppm).32 The two T3 signals due to Si−R remained intact, indicating the successful formation of R6(HO)2Si8O12 (3 in Figure 2a) without either cleavage of the Si−O−Si bonds or condensation of the SiOH groups. Formation of Zeolitic Macrocycles Consisting of Three, Four, or Five Cage Siloxane Units. The SiOH groups of 3 remained intact even after the removal of the solvent. They can be condensed by heating in the absence of solvent. After 3 was heated at 100 °C for 1 day, the 29Si NMR spectrum (Figure S15) showed the disappearance of the Q3 signal of the

Figure 5. (a, c, e) MALDI-TOF MS spectra and (b, d, f) 29Si NMR spectra (in CDCl3) of cyclic-(R6Si8O13)n with (a, b) n = 3, (c, d) n = 4, and (e, f) n = 5. The broad signals ranging from −100 to −120 ppm in (b, d, f) originate from the quartz glass tube used for the measurements.

at m/z 2853.7, 3796.1, and 4739.9, which were assigned to the Na+ adducts of the cyclic compounds consisting of three, four, and five cage siloxane units (cyclic-(R6Si8O13)n, n = 3, 4, 5), respectively. The 29Si NMR spectrum of each component (Figure 5b,d,f) showed two T3 signals ((SiO)3SiC6H13) and a single Q4 signal ((SiO)3SiOSi(OSi)3) with an intensity ratio of approximately 2:1:1. This also indicates that cyclic compounds were formed. The preferential formation of cyclic oligomers rather than linear polymers may have been caused by the narrow angle between the two Si−OH bonds at the ortho position. The six-, eight-, and 10-membered rings of SiO4 tetrahedra in cyclic-(R6Si8O13)n (n = 3, 4, 5) are similar to the apertures of common zeolites. Figure 6 shows ball-and-stick models of the minimum-energy conformers of cyclic-(Me6Si8O13)n (n = 3, 4, 5). Here the hexyl groups were replaced with methyl groups because it is difficult to take into account the flexibility of the hexyl group. The cavity sizes of the six-, eight-, and 10membered rings were calculated as 2.24−2.40, 2.93−3.32, and 3.91−5.86 Å, respectively (details and a calculation method are described in the Supporting Information; see Figures S17−S19 and Table S1). cyclic-(Me6Si8O13)n (n = 3, 4, 5) should have more defined cavities than conventional cyclic siloxanes (typically (Me2SiO)n) D

DOI: 10.1021/acs.inorgchem.8b02402 Inorg. Chem. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02402. Experimental details, GPC profiles, spectral data (1H, 13C, and 29Si NMR and MALDI-TOF MS), and calculated molecular models (PDF) Movie S1 showing the relationship between the structures of (cyclic-(R6Si8O13)n (n = 3, 4) and the structure of LTAtype zeolite (AVI)

Figure 6. Ball-and-stick models of the minimum-energy conformers of cyclic-(Me6Si8O13)n: (a) n = 3, (b) n = 4, and (c) n = 5. These structure models were visualized by VESTA (version 3) after their minimum energies were calculated using the Gaussian 09 package.

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AUTHOR INFORMATION

Corresponding Authors

because the rotation of the Si−O−Si bonds in cubic siloxanes is restricted. To confirm the existence of defined cavities in cyclic(Me6Si8O13)n (n = 3, 4, 5), the minimum-energy conformer for cyclic-(Me6Si8O13)3 was compared with that for dodecamethylcyclohexasiloxane [(Me2SiO)6] (Figure S20). A structural model of (Me2SiO)6 shows that the cavity is partially blocked by the methyl groups because of the flexible Si−O−Si bonds that allow the rotation of the Me2Si units relative to the ring plane. In contrast, the cavity in cyclic-(Me6Si8O13)3 is not blocked by methyl groups. These results suggest that cyclic-(R6Si8O13)n (n = 3, 4, 5) have more defined ring structures than conventional cyclic siloxanes. Such nanoscale cavities will be useful for the adsorption or separation of small ions or molecules. It is noteworthy that cyclic-(R6Si8O13)n (n = 3, 4, 5) are obtained as liquids and are soluble in organic solvents. These properties are in marked contrast to those of conventional zeolitic materials, which are insoluble solids. Molecularly dispersed cyclic-(R6Si8O13)n in solutions should provide highly accessible cavities that allow for the efficient inclusion of guest species. We expect that variations in the properties and functions of cyclic-(R6Si8O13)n based on the types of organic substituents (R) introduced to the cubic siloxanes will lead to diverse applications. This paper has disclosed for the first time that cyclic(R6Si8O13)n (n = 3, 4) fragments exceeding secondary building units of LTA-type zeolite can be synthesized, which may trigger a new stream of zeolite syntheses. cyclic-(R6Si8O13)n (n = 5) seems to be rare as a building unit of zeolites, but the presence of such a new unit might also be meaningful for developing the synthesis of hypothetical zeolites.35−37 In this sense, these rigid cyclic compounds are promising as building blocks to construct novel porous architectures.

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Atsushi Shimojima: 0000-0003-2863-1587 Kazuyuki Kuroda: 0000-0002-1602-0335 Author Contributions

S.S. carried out all of the experiments and data curations and wrote the manuscript. H.W. discussed the results and contributed to the final manuscript. A.S. is a project administrator of JSPS KAKENHI (Grant-in-Aid for Scientific Research (B), 15H03879) and conceived of the presented idea. K.K. is a project administrator of the Grant-in-Aid for Strategic International Collaborative Research Program (SICORP) “France−Japan Joint Call on Molecular Technology” from the Japan Science and Technology Agency (JST) and discussed the results and contributed to the final manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. N. Sugimura (Materials Characterization Central Laboratory, Waseda University) for his help with molecular simulations. We also thank Dr. M. Yoshikawa, Mr. S. Sakamoto, and Mr. N. Sato (Waseda University) for fruitful discussions. This work was supported in part by JSPS KAKENHI (Grant-inAid for Scientific Research (B), 15H03879) and a Grant-in-Aid for Strategic International Collaborative Research Program (SICORP) “France−Japan Joint Call on Molecular Technology” from the Japan Science and Technology Agency (JST). We are grateful to the Materials Characterization Central Laboratory38 at Waseda University for supporting NMR and MS measurements.

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CONCLUSIONS We have successfully synthesized a novel macrocyclic siloxane through the stepwise and site-selective reactions of cubic siloxanes. We found it effective to use a dihydroxy compound with a specific geometry to synthesize difunctional cubic siloxanes in which two adjacent corners were modified with Si−O−C bonds. This type of site-selective modification of cubic siloxanes is of great importance because it is generally very difficult to separate regioisomers. The Si−O−C bonds were hydrolyzed and subsequently condensed to form cyclic compounds (cyclic-(R6Si8O13)n; n = 3, 4, 5) possessing zeolitelike structures with six-, eight-, and 10-membered rings, respectively. This synthesis method, which is based on the site-selective reactions of cubic siloxanes, will allow for the precise design of zeolite-like porous materials.

REFERENCES

(1) Davis, M. E. Ordered porous materials for emerging applications. Nature 2002, 417, 813−821. (2) Corma, A. From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis. Chem. Rev. 1997, 97, 2373−2419. (3) Li, Y.; Corma, A.; Yu, J. H. Synthesis of new zeolite structures. Chem. Soc. Rev. 2015, 44, 7112−7127. (4) Van Speybroeck, V.; Hemelsoet, K.; Joos, L.; Waroquier, M.; Bell, R. G.; Catlow, C. R. A. Advances in theory and their application within the field of zeolite chemistry. Chem. Soc. Rev. 2015, 44, 7044−7111. (5) Wakihara, T.; Okubo, T. Hydrothermal Synthesis and Characterization of Zeolites. Chem. Lett. 2005, 34, 276−281. (6) Marler, B.; Gies, H. Hydrous layer silicates as precursors for zeolites obtained through topotactic condensation: a review. Eur. J. Mineral. 2012, 24, 405−428.

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DOI: 10.1021/acs.inorgchem.8b02402 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b02402 Inorg. Chem. XXXX, XXX, XXX−XXX