Article pubs.acs.org/crystal
Host−Guest Complexes of a Tribenzotriquinacene-Based Tris(catechol) with Quaternary Ammonium Salts: Variation of H‑Bonding Pattern and Cationic Size on Supramolecular Architecture Chun-Fai Ng,†,‡ Hak-Fun Chow,*,†,‡ Dietmar Kuck,§ and Thomas C. W. Mak*,† †
Department of Chemistry and Center of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, HKSAR China State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong, Shatin, HKSAR China § Department of Chemistry and Center for Molecular Materials (CM2), Bielefeld University, 33615 Bielefeld, Germany ‡
S Supporting Information *
ABSTRACT: Hydrogen-bond-assisted host−guest complexation of a tris(catechol)-functionalized tribenzotriquinacene with various tetraalkylammonium halides yielded diversified supramolecular architectures ranging from octameric C3-symmetric cages and C2- and C4-symmetric channels to corrugated layers, depending on both the anionic and cationic components of the guest species. Single crystal X-ray analyses of this series of complexes revealed intricate anionic host networks formed by intermolecular O−H···O and O−H···X hydrogen-bonds involving catechol host molecules and halides, which accommodate the bulky quaternary ammonium guest ions in cavities, channels, or between corrugated sheets. With fine-tuning of the symmetry and size of the hydrophobic cationic guest and accompanying halide species, this simple “Mix and Match” of host and guest components provides an efficient route to the assembly of supramolecular architectures.
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INTRODUCTION Among countless organic host molecules, shape-persistent tribenzotriquinacene (TBTQ) derivatives1−4 constitute one of the popular categories having potential binding sites in three orthogonal directions for interaction with compatible guest species. This unique geometric feature has aroused considerable interest in their utilization for the construction of rectangular molecular structures. Sometime ago, one of us disclosed the formation of an octameric cube via enantioselective self-sorting from a racemic non-hydrogen-bonding tribromotrinitro-TBTQ derivative during crystallization.5 We later reported the metalmediated self-assembly of enantiopure metallosquares using diacetylene-functionalized TBTQ derivatives in the presence of Pt(II) ions,6 and a dumbbell-shaped nonchiral TBTQ-based metallocryptophane via a TBTQ-derived tricarboxylate with Cd(II) ions.7 However, Beuerle and Mastalerz independently demonstrated the use of reversible dynamic covalent bonding to furnish various TBTQ-based cubes8 and even cages.9,10 Mastalerz also reported the use of hydrogen-bonding to create a cubic-shaped octameric aggregate from a chiral TBTQ compound bearing three amide functionalities.11 Interestingly, these esthetically pleasing molecular architectures possess large host cavities to accommodate a wide variety of guest species. For example, the above-mentioned cubic-shaped hydrogenbonded aggregate was found to be an excellent host for various tetraalkylammonium salts as evidenced from extensive NMR and MS studies and semiempirical calculations. There are also reported X-ray structures showing the hydrogen-bond mediated formation of nanotubes with entrapped solvents from several racemic TBTQ alcohol derivatives.12 Despite these findings, detailed structural information on hydrogen-bond-mediated © XXXX American Chemical Society
self-assembly of TBTQ derivatives and their host−guest binding modes is still lacking, which may hinder progress in the future design of such large-size molecular capsules. In addition to the more conventional CO····H−X hydrogen bonding motif, anion-mediated hydrogen bonding (H····Y−····H) has also attracted much attention in the construction of extended supramolecular architectures.13,14 For example, MacLachlan reported the use of oligophenols in the presence of various anions to give sheet-like supramolecular structures15 and hexagonal nanotubes.16,17 The assembly of these crystalline anion-mediated complexes provided valuable structural information in the solid-state, which inspired us to explore the use of oligohydroxy-TBTQ derivatives in the preparation of large size molecular capsules. Herein we report our investigation of a wide variety of host−guest supramolecular assemblies obtained by mixing the known TBTQ tris(catechol) L118 with various quaternary ammonium halides. Depending on the size of the anion as well as that of the coexisting bulky hydrophobic organic cation, hydrogen-bonded octameric C3-symmetric cages, C2- and C4-symmetric channels, or corrugated layers could be prepared (Scheme 1). This study illustrates the rich structural diversity of supramolecular assemblies, in addition to the more studied cubic or rectangular structures, that can be generated from the tris(catechol)functionalized TBTQ skeleton. The detailed information reported here not only provides concrete structural data toward the future design of larger-size TBTQ hosts and Received: February 23, 2017 Revised: March 29, 2017 Published: April 7, 2017 A
DOI: 10.1021/acs.cgd.7b00278 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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6 H), 6.69 (s, ArH, 6 H), 3.97 (s, CH, 3 H), 1.48 (s, CH3, 3 H). 13C NMR (DMSO-d6): 144.8, 136.2, 110.6, 62.6, 62.1, 27.7. m/z (MALDI) 390 (M+, 25%), 374 (M−OH+, 100%). HRMS (MALDI) calcd for C23H18O6+, 390.1103; found, 390.1098. Synthesis of Inclusion Complexes of Tris(catechol) L1 with Quaternary Ammonium Halides. 4L1·3(n-Bu)4N+Cl− (1). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, a solution of tetrabutylammonium chloride (11 mg, 38 μmol) in methanol (0.5 mL) was added. Colorless block was obtained in ca. 70% yield after 3 d. Mp: 298−299 °C. Anal. Calcd for C280H360Cl6N6O48: C, 70.20; H, 7.57; N, 1.75; Cl, 4.44. Found: C, 70.90; H, 7.96; N, 1.93; Cl, 4.85. 4L1·3(n-Bu)4N+Br− (2). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, a solution of tetrabutylammonium bromide (12 mg, 38 μmol) in methanol (0.5 mL) was added. Colorless block was obtained in ca. 60% yield after 3 d. Mp: 246−248 °C. L1· (n-Bu)4N+F− (3). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, 1.0 M tetrabutylammonium fluoride solution in THF (40 μL) was added. Highly hygroscopic colorless square plate was obtained in ca. 40% yield after 1 d. 4L1· 3(C5H11)4N+Br− (4). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, a solution of tetrapentylammonium bromide (15 mg, 38 μmol) in methanol (0.5 mL) was added. Transparent brown block was obtained in ca. 60% yield after 2 d. Mp: 250−252 °C. L1·(n-Bu)3MeN+Cl− (5). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, aqueous tributylmethylammonium chloride solution (75% w/v, 0.5 mL) was added. Pale yellow block was obtained in ca. 50% yield after 2 d. Mp: 250−251 °C. L1·BnEt3N+Cl−·MeOH (6). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, a solution of benzyltriethylammonium chloride (9 mg, 38 μmol) in methanol (0.5 mL) was added. Pale brown block was obtained in ca. 70% yield after 1 d. Mp: 290−291 °C. L1·BnEt3N+Br−·MeOH (7). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, a solution of benzyltriethylammonium bromide (11 mg, 38 μmol) in methanol (0.5 mL) was added. Yellow block obtained in ca. 70% yield after 1 d. Mp: 258−259 °C. 2L1·2BnEt3N+Br−·EtOH (8). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in ethanol (0.5 mL) in a vial, a solution of benzyltriethylammonium bromide (11 mg, 38 μmol) in ethanol (0.5 mL) was added. Pale yellow plate was obtained in ca. 60% yield after 2 d. Mp: 258−260 °C. L1·BnEt3N+Br−·i-PrOH (9). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in isopropyl alcohol (0.5 mL) in a vial, a solution of tributylmethylammonium bromide (11 mg, 38 μmol) in isopropyl alcohol (0.5 mL) was added. Pale yellow plate was obtained in ca. 60% yield after 3 d. Mp: 258−259 °C. 2L1·2BnMe3N+F−·MeOH (10). To a solution of tris(catechol) L1 (15 mg, 38 μmol) in methanol (0.5 mL) in a vial, a solution of benzyltrimethylammonium fluoride (7 mg, 38 μmol) in methanol (0.5 mL) was added. Highly hygroscopic pale brown needle was obtained in ca. 40% yield after 2 d. Mp: 245 °C (dec.). X-ray Crystallography. Crystallographic data of complexes 1−10 were collected at either 173 or 296 K on a Bruker D8 VENTURE diffractometer with Mo Kα radiation (λ = 0.71073 Å) from a sealedtube generator. Data collection, reduction, and empirical absorption corrections were performed using APEX2 software.20 All the crystal structures were solved by direct methods with SHELXS21 program, and all non-hydrogen atoms were anisotropically refined against F2 with full-matrix least-squares techniques using SHELXL-97 program. All the hydrogen atoms were included in the structure factor calculation at idealized positions with fixed isotropic thermal displacement parameter relative to the attached atoms. Crystallographic data for all complexes 1−10 are given in the Supporting Information.
Scheme 1. Molecular Architectures of Supramolecular Host−Guest Complexes Generated through Hydrogen Bonding Interactions between the TBTQ-tris(catechol) Donor L1 and Anionic Acceptors Derived from Quaternary Ammonium Halides
capsules for various applications, but also suggests that the TBTQ skeleton can be used to prepare noncubic/rectangulartype hierarchical host−guest assemblies when combined with the suitable type of guest partners.
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EXPERIMENTAL SECTION
General. All reagents were purchased from commercial suppliers and used without further purification. CH2Cl2 was freshly distilled from CaH2. All reactions were carried out under N2 atmosphere unless otherwise stated. All reactions were monitored by thin-layer chromatography on precoated silica gel plates, which were visualized by UV irradiation at 254 or 365 nm and/or stained using 5% (w/v) dodecamolybdophosphoric acid in ethanol followed by heating. Flash column chromatography was performed on a glass column of silica gel (230−400 mesh), and solvent ratios were expressed in volume to volume. 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 MHz nuclear magnetic resonance spectrometer (1H, 400 MHz; 13 C, 100 MHz). Unless otherwise stated, all NMR measurements were conducted in CDCl3 at 25 °C. Chemical shifts were reported as parts per million in δ scale using solvent residual peak as internal standard. Coupling constants (J) were reported in hertz. All mass spectra were obtained on a Bruker Autoflex Speed MALDI-TOF mass spectrometer. The reported molecular mass (m/z) values were monoisotopic mass unless otherwise stated. Elemental analysis was carried out at MEDAC Ltd. Melting points were measured on a digital melting point apparatus and were uncorrected. Synthesis of 2,3,6,7,10,11-Hexamethoxy-12d-methyltribenzotriquinacene (L0). Compound L0 was synthesized by following the reported literature procedure.19 Mp: 234−235 °C. Rf: 0.7 (EtOAc). 1H NMR: 6.91 (s, ArH, 6 H), 4.30 (s, CH, 3 H), 3.88 (s, OCH3, 18 H), 1.68 (s, CH3, 3 H). 13C NMR: 149.2, 137.4, 107.4, 63.4, 63.3, 56.3, 27.5. m/z (MALDI) 474 (M+, 100%). RMS (MALDI) calcd for C29H30O6+, 474.2042; found, 474.2039. Synthesis of 2,3,6,7,10,11-Hexahydroxy-12d-methyltribenzotriquinacene (L1). A solution of the hexamethoxy-TBTQ derivative L0 (750 mg, 1.58 mmol) in CH2Cl2 (30 mL) was stirred at 0 °C for 5 min. Boron tribromide (0.9 mL, 9.48 mmol) was then added dropwise to the mixture over 10 min. After 6 h, the reaction was then quenched by addition of water. The solvent was evaporated in vacuo, and the residue was extracted with EtOAc (3 × 50 mL). The organic phase was washed with brine, dried with Na2SO4, and filtered, and the solvent evaporated to give the crude product. The solid residue was further purified by flash chromatography (EtOAc) to afford the target tris(catechol) L1 (610 mg, 1.56 mmol, 99%) as a pale pink solid. Mp: > 350 °C (dec.). Rf: 0.6 (EtOAc). 1H NMR (DMSO-d6): 8.64 (s, OH, B
DOI: 10.1021/acs.cgd.7b00278 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Treatment of tris(catechol) L1 with tetrabutylammonium chloride in either methanol or ethanol leads to the formation of host−guest complex 4L1·3(n-Bu4N+Cl−) (1). In the crystal structure, four of the eight equivalent tris(catechol) L1 molecules have their apical methyl groups pointing toward the center of the unit cell (henceforth designated as Type IN), and the other half have their apical methyl groups pointing in the opposite sense (Type OUT). Remarkably, intermolecular H···O−H hydrogen bonding involving two IN and two OUT L1 molecules generates three contingent tetrameric cycles, namely [A, N4 = R44(8)],22−24 [B, N4 = R44(44)], and [C, N4 = R44(44)], as shown in Figure 1a. The chloride ions further consolidate the L1 host network by forming acceptor Cl−···H− O hydrogen bonds with the remaining catecholic O−H groups.
bond motifs A and C (see Figure 1a) generates a large ellipsoidal cavity G surrounded by four IN or four OUT L1 molecules, as shown in Figure 1d. In the cubic unit cell, the G cavities are located at the face centers and the centers of the edges. Although cavity G looks smaller than cavity E in Figure 1d, it is prolate ellipsoidal in shape (Figure S1 in SI) and actually large enough to comfortably accommodate a 2-fold disordered (n-Bu)4N+ guest ion. The minimal and maximal methyl−methyl separations of the same type inside cavity G are ca. 4.8 and 15.9 Å, respectively, while the butyl chain maintains a minimum separation not less than ca. 3.8 Å from its α-carbon atom to the nearest surrounding chloride neighbor (Figure S2 in SI). Attempted crystallization of L1 with tetrabutylammonium salts containing larger monoanions, including iodide, acetate, nitrate, tetrafluoroborate, perchlorate, and hexafluorophosphate, as well as the oxalate dianion, in various solvents failed to yield new complexes. This suggests that the role of the anionic guest component is to facilitate the formation of a host network consolidated by a maximal number of hydrogen bonds, leaving empty voids that can comfortably accommodate quaternary ammonium guests of the appropriate size. To test the validity of this presumption, crystals of 4L1·3(nBu)4N+Br− (2) were prepared and then subjected to X-ray crystal structure analysis, which verified that 2 is indeed isostructural to 1 with a slight increase in unit-cell parameter a = 19.1059(6) Å. However, the failure to form a host−guest complex with the iodide anion indicated that the corresponding I−···H−O hydrogen bond may be too weak to sustain the network structure. As the fluoride ion is known to form stronger F−···H−O hydrogen bonds, simply mixing a 1.0 M THF solution of tetrabutylammonium fluoride with L1 in methanol gave L1·(nBu)4N+F− (3). In the crystal structure of 3, the tris(catechol) L1 molecule occupies a general position, two independent tetrabutylammonium ions N1 and N2 occupy different sites, each of symmetry 2, and two independent fluoride ions F1 and F2 occupy sites of symmetry −1 and 2, respectively. Tetrameric cycles involving intermolecular H···O−H hydrogen bonds of L1, namely, [A, N4 = R44(30)] and [B, N4 = R44(8)] found in 1, also occur in 3, as shown in Figure 2a. Stacking of A subunits along a crystallographic 2 axis at x = y = 1/4 affords the C2-symmetric channel shown in Figure 2b. Remarkably, adjacent A subunits related by noncrystallographic 41 local symmetry are bridged by two F1 fluorides in distorted square-planar fashion (H···F−···H = ca. 68.9°), as shown in Figure 2c. The cavities inside the channel are ellipsoidal in shape with minimal and maximal interior methyl−methyl separations of ca. 5.6 and 11.8 Å, respectively, such that each provides enough room for the accommodation of independent (n-Bu)4N+ cation N1. Furthermore, adjacent channels are interwoven along the a- and b-directions via tetrahedral F−···H−O hydrogen bonds involving the other independent fluoride ion F2 (see motif B in Figure 2a). It is noteworthy that the channel at x = y = 1/4 and the isometric counterpart at x = y = 3/4 are related by the diagonal n-glide such that lateral packing of four neighboring channels generates one additional cavity (at x = 3/4, y = 1/4), which has sufficient room to accommodate the other independent (n-Bu)4N+ cation N2 as shown in Figure 2d. To study the role of the cationic guest component in supramolecular assembly, we next explored variation of the alkyl chain length of the quaternary ammonium cation. Single
Figure 1. (a) Projection diagram along the b-axis showing the hydrogen bond patterns around the central empty cage and the periphery of the unit cell in 4L1·3(n-Bu4N+Cl−) 1. Tris(catechol) L1 molecules with their apical methyl groups pointing inward (Type IN) and outward (Type OUT) are highlighted by sky-blue and orange bonds, respectively, and the chloride ions are represented by green spheres. (b) Perspective diagram showing the discrete octameric C3symmetric empty cage E in the crystal structure of 1. (c) Projection diagram along the b-axis showing the central empty cavity E surrounded by four Type OUT methyl groups. (d) Projection diagram along the b-axis showing the large cavity G at (0, 0.5, 0) and four adjacent E cages at y = 0. All nonoxygen bonded hydrogen atoms and disordered cations are omitted for clarity. Detailed symmetry transformations for this and other figures are given in the Supporting Information.
Each chloride ion is tetrahedrally bound to two IN and two OUT L1 molecules such that the extensive hydrogen-bonded network furnishes a discrete octameric C3-symmetric cage at the center of the unit cell, as shown in Figure 1b. Notably, this confined cavity surrounded by the methyl groups of four IN L1 molecules is too small (henceforth designated as cavity E) to accommodate any molecular guest species, as the closest methyl−methyl separation is ca. 4.8 Å (see Figure 1c). Fusion of empty E cavities in the a and c axial directions via hydrogenC
DOI: 10.1021/acs.cgd.7b00278 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. (a) Projection diagram along the c-axis showing the hydrogen-bond patterns around a channel within the unit cell in L1·(nBu)4N+F− 3. (b) Perspective diagram showing a unidirectional C2symmetric channel at x = y = 1/4 formed by the stacking of A subunits. (c) Perspective diagram showing the linkage of two tetrameric A subunits related by a 90° rotation in the c direction (noncrystallographic 41 operation), which are highlighted by orange and sky-blue bonds, through hydrogen-bonding by two fluoride ions. (d) Projection diagram along the c-axis showing the two independent (n-Bu)4N+ ions bearing disordered alkyl chains (represented by transparent blue spheres) filling cavities inside the channels. All nonoxygen bonded hydrogen atoms are omitted for clarity.
Figure 3. (a) Projection diagram along the a-axis showing the small A and large B cycles in the crystal structure of L1·(n-Bu)3MeN+Cl− 5. (b) Perspective diagram showing a corrugated layer within the unit cell. (c) Perspective diagram showing stacking of two corrugated layers. (d) Projection diagram along the b-axis showing the (n-Bu)3MeN+ ions, represented by transparent blue spheres, located between two corrugated layers. All nonoxygen-bonded hydrogen atoms were omitted for clarity.
crystals of 4L1·3(C5H11)4N+Br− (4) were obtained from crystallization of tris(catechol) L1 and tetrapentylammonium bromide in methanol. It was found that 4 is isostructural with 1 and 2 with lattice parameter a = 19.2382(6) Å. However, crystallization of L1 with quaternary ammonium salts bearing longer alkyl substituents such as tetraheptylammonium bromide only led to the formation of an amorphous solid. In the other extreme, crystallization of L1 with tetramethylammonium salts also failed to yield single crystals. This suggests that a cation of appropriate size is crucial to the formation of a compatible host network by filling the empty voids. We next explored the use of quaternary ammonium ions bearing mixed alkyl legs of suitable lengths, and indeed, tributylmethylammonium chloride and L1 combine to form inclusion complex L1·(n-Bu)3MeN+Cl− (5), which has a corrugated layer structure constructed by extensive intermolecular H···O−H hydrogen bonding between tris(catechol) L1 molecules. Unlike the hydrogen-bonded cyclic tetramers observed in 1, 2, 3, and 4, small and large trimeric cycles, namely, [A, N3 = R33(6)] and [B, N3 = R33(33)], respectively, are formed such that A is surrounded by three Bs, and vice versa, throughout the corrugated layer, as shown in Figure 3a. The corrugated layer is further consolidated by intermolecular Cl−···H−O acceptor hydrogen bonds in nearly trigonalpyramidal manner in which each chloride ion bridges three adjacent L1 molecules in larger cycle B. Throughout each corrugated layer, the apical methyl groups of consecutive tris(catechol) L1 molecules point to the same direction along
the b-axis but flip alternately along the c-axis, as shown in Figure 3b. Notably, two adjacent layers are slightly offset (Figure 3c), while the interlayer spacing is ca. 8.3 Å, so that the guest (nBu)3MeN+ cations are accommodated in the interlayer region, as shown in Figure 3d. The crystalline inclusion complexes of trialkyl benzylammonium salts with L1 were also investigated. Single crystals of triethylbenzylammonium chloride and bromide with L1 can be readily grown from different alcoholic solvents, including methanol, ethanol, and isopropyl alcohol. Remarkably, a series of complexes, L1·BnEt3N+Cl−·MeOH (6), L1·BnEt3N+Br−· MeOH (7), 2L 1 ·2BnEt 3 N + Br − ·EtOH (8), and L 1 · BnEt3N+Br−·i-PrOH (9), are isostructural with 5 except for inclusion of their corresponding solvate molecules. Crystallization of benzyltrimethylammonium fluoride with L1 in methanol yielded solvated inclusion complex 2L 1 · 2BnMe3N+F−·MeOH (10). Figure 4a shows that a tetrameric cycle [A, N4 = R88(20)], in which four tris(catechol)s L1 are connected by intermolecular H···O−H hydrogen bonds, exists in the crystal structure. Such A subunits are further stacked through the support of hydrogen bonds along the c-axis to give the C4-symmetric unidirectional channel shown in Figure 4b. This channel (henceforth designated as channel E) has a nearly square cross-section with a diagonal of ca. 6.8 Å (from O1 to O1b), which is too narrow for the accommodation of guest D
DOI: 10.1021/acs.cgd.7b00278 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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should be considered in the designed assembly of supramolecular aggregates.
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CONCLUSION In summary, we have demonstrated the facile preparation of TBTQ-based host−guest complexes by simply mixing the organic host L1 with various quaternary ammonium halides. Remarkably, by fine-tuning the size of both anionic and cationic guest components, a large structural diversity in their supramolecular assemblies has been observed. It was found that the use of moderate-size halide ions, including fluoride, chloride, and bromide, as hydrogen-bonding acceptors led to linkage between tris(catechol) molecules, whereas larger and weakly coordinating anions like iodide, tetrafluoroborate, and perchlorate fail to furnish an anionic host network. Notably, host−guest complexation between TBTQ catechol and tetraalkylammonium salts does not involve simple encapsulation of the bulky hydrophobic guest by the host network created solely by intermolecular hydrogen bonding between TBTQ catechol molecules. The halide anion plays an essential role as a component in creating the hydrogen-bonded host network with tris(catechol) molecules, while the bulky, hydrophobic quaternary ammonium cation further stabilizes the structure by filling the empty cavities. This host−guest complexation approach provides a simple yet effective strategy to generate various supramolecular architectures, and the principle could also be applied to higher TBTQ congeners with poly(catechol) moieties, such as the extended wizard-hatshaped TBTQ derivatives,33 to create even larger-size supramolecular species.
Figure 4. (a) Projection diagram along the c-axis showing the intermolecular hydrogen-bonded tetrameric subunit in 2L 1 · 2BnMe3N+F−·MeOH 10. (b) Perspective diagram showing a C4symmetric host channel. (c) Projection diagram along the c-axis showing a cross-section of the large host channel G (at 0.5, 0.5, z) and four adjacent narrow E channels. Four disordered BnMe3N+ cations, represented by transparent blue spheres, occupy chambers in channel G. All nonoxygen bonded hydrogen atoms and disordered solvent molecules are omitted for clarity.
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ASSOCIATED CONTENT
S Supporting Information *
cations. Cross-linkage of E channels in the a- and b-axial directions by strong F−···H−O hydrogen bonds, with the fluoride ions occupying sites of symmetry m, generates a large G channel, which is surrounded by four E channels, and vice versa, as shown in Figure 4c. Remarkably, the C4-symmetric channel G with irregular propeller-like cross sections contains large chambers each accommodating four 2-fold disordered BnMe3N+ guest ions. Based on the crystal structures of fluoride-containing complexes 3 and 10, we speculate that the fluoride ion could facilitate the construction of a closely packed supramolecular channel structure via formation of four adaptive strong F−···H− O hydrogen bonds in either tetrahedral or distorted squareplanar configuration. Undoubtedly, a guest cation of smaller size, such as BnMe3N+ in 10 compared to (n-Bu)4N+ in 3, can be accommodated more readily to attain higher symmetry of the crystal structure. However, isotypicity found in 1 and 2, as well as in 6 and 7, reveals that the use of weaker hydrogen-bond acceptors, like chloride and bromide ions, makes no significant difference in their corresponding crystal structures. It is worthwhile pointing out that the construction of molecular cage architectures involving TBTQ derivatives presents a formidable challenge owing to the difficulty in maintaining centripetal concave surface orientation during crystallization. Reported single-crystal X-ray structures of TBTQ derivatives12,25−28 mostly involve simpler packing modes with weak π−π interaction, and crystallization in high-symmetry space groups5,29−32 rarely occur. Therefore, stronger noncovalent interaction like hydrogen bonding involving matching functional groups, together with host−guest size compatibility,
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00278. 1 H and 13C NMR spectra and supporting figures (PDF) Accession Codes
CCDC 1526955, 1526957, 1526959−1526963, 1526965, 1527027, and 1527057 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Chun-Fai Ng: 0000-0002-9445-6396 Hak-Fun Chow: 0000-0002-7621-0851 Dietmar Kuck: 0000-0001-7400-1696 Thomas C. W. Mak: 0000-0002-4316-2937 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Research Grants Council of Hong Kong (14303816) for the financial support. E
DOI: 10.1021/acs.cgd.7b00278 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.cgd.7b00278 Cryst. Growth Des. XXXX, XXX, XXX−XXX
