CRYSTAL GROWTH & DESIGN
Guest-Induced Molecular Capsule Assembly of p-Sulfonatothiacalix[4]arene
2006 VOL. 6, NO. 2 514-518
Daqiang Yuan, Mingyan Wu, Benlai Wu, Yanqing Xu, Feilong Jiang, and Maochun Hong* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Fujian, Fuzhou, 350002 China ReceiVed August 29, 2005; ReVised Manuscript ReceiVed NoVember 2, 2005
ABSTRACT: Two new compact molecular capsules of p-sulfonatothiacalix[4]arene with sodium ions have been synthesized by inducing suitable guest molecules, 1,2-bis(imidazol-1′-yl)ethane or 2,2′-bipyridine. Single-crystal X-ray diffraction analyses reveal that these supramolecular compounds possess open or close capsule conformations in a zigzag-like chain or a straight-line chain array. 1H NMR spectroscopic studies confirm that the above stable molecular capsules are present in solution. Their guest molecules in both compounds are well accommodated by p-sulfonatothiacalix[4]arene capsules via a variety of supramolecular interactions. Introduction In recent years, the design and investigation of self-assembled molecular capsules have attracted considerable interest in supramolecular chemistry.1 This is not only due to the large contribution to the development of supramolecular chemistry because of the ability of these compounds to bind molecular substrates by means of various intermolecular forces including electrostatic interactions, hydrogen bonding, π-π interactions, and van der Waals interactions but also due to their great importance in biological systems and for medical purposes, such as the mimicry of catalytic efficiency of enzymes, storage of molecules, and drug delivery.2 For biological and medical applications, it is prerequisite that these molecular capsules are water soluble. Among multifarious building blocks, watersoluble p-sulfonatocalix[4,5]arenas (CAS) have been exploited, and their 2- or 12-membered aggregates with ionic or molecular capsule structures have been reported.3 The cone conformation of p-sulfonatocalix[4,5]arenes presents a preorganized cavity that can act as a host for neutral or charged small substrate molecules.4 Capsules based on two p-sulfonatocalix[4,5]arenes shrouding other molecules including crown ethers, tetraprotonated cyclam, diprotonated [2,2,2] cryptand, amino acids, and H2SO4, have recently been established.5-9 Very recently, a new water-soluble p-sulfonatothiacalix[4]arene (TCAS) has been reported, in which sulfur atoms are used in place of the four methylene bridges (Scheme 1).10 Although the electron density of the calix cavity is reduced due to the utility of sulfur atoms as substitutes for methylene bridges, TCAS shows a higher inclusion ability than p-sulfonatocalix[4]arenas.11 Especially interesting, TCAS can recognize organohalogens and aromatics in water and has a trend to encapsulate them into the cavity.11,12 In the presence of suitable guest molecules that contain two aromatic moieties, it can be conjectured that the recognition ability will assemble TCAS into molecular capsules (Scheme 2). This idea prompts us to choose these kind of aromatics, such as 1,2-bis(imidazol-1′-yl)ethane (BIME) or 2,2′-bipyridine (BIPY), as guest molecules to induce molecular capsule formation. Herein, we describe a study on the syntheses and characterization of two new molecular capsule chains based on water-soluble p-sulfonatothiacalix[4]arenes. These molecular * To whom correspondence should be addressed. E-mail: hmc@ ms.fjirsm.ac.cn. Tel: 86-591-83792460. Fax: +86-591-83794946
Scheme 1
Scheme 2
capsules were investigated by 1H NMR spectroscopy and X-ray diffraction analyses. Experimental Section BIPY and all the solvents were purchased and used as received. Na4TCAS and BIME were synthesized according to the literature methods.10,13,14 The IR spectra as KBr disk were recorded on a Nicolet Magna 750 FT-IR spectrophotometer in the range of 400-4000 cm-1. C, H, and N elemental analyses were determined on an Elementary Vario ELIII elemental analyzer. 1H NMR spectra were recorded on a Varian Unity-500 spectrometer (D2O as solvent), operating at 499.802 MHz. Synthesis of [H2BIME]⊂[Na2(H2TCAS)2(H2O)4]‚15H2O (1). BIME (8.1 mg, 0.05 mmol) and Na4TCAS (90 mg, 0.1 mmol) were dissolved in distilled water (10 mL). The solution was adjusted to pH ≈ 2 with nitric acid and was stirred at room temperature for 4 h. Then, the solution was allowed to evaporate slowly at room temperature. After several days, large crystals of 1 suitable for X-ray diffraction analysis were formed, yielding 25 mg (23%). Compound 1 was characterized to have a formula of C56H78N4Na2O51S16 (Mr ) 2182.24) by X-ray
10.1021/cg050446x CCC: $33.50 © 2006 American Chemical Society Published on Web 12/10/2005
Assembly of p-Sulfonatothiacalix[4]arene
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Table 1. Crystal Data and Structure Refinement for Complexes 1 and 2
CCDC deposit no. formula fw crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dc, g/cm3 F000 µ mm-1 λ MoK [Å] T, K 2θmax, deg reflections collected unique reflections Rint parameters final GoF R1 (I > 2σ(I))a wR (all data)b a
1
2
CCDC-279150 C56H78N4Na2O51S16 2182.16 monoclinic C2/c 28.1140(7) 18.7541(1) 18.1144(5) 90 92.6310(10) 90 9540.8(4) 2 1.519 4512 0.468 0.71073 293(2) 50.0 14147 8198 0.0499 639 1.063 0.0897 0.2159
CCDC-279151 C58H64N2Na2O53S16 2196.05 orthorhombic Pnnn 14.7050(1) 26.3196(7) 11.1021(3) 90 90 90 4296.84(17) 2 1.697 2256 0.522 0.71073 293(2) 50.2 9485 3610 0.0422 328 1.045 0.0889 0.2333
R ) ∑(||F0| - |Fc||)/∑|F0|. b wR ) [∑[w(F02 - Fc2)2]/∑w(F02)2]1/2.
crystallography. Anal. Calcd for C56H78N4Na2O51S16 (%): C, 30.82; H, 3.60; N, 2.57; found (%): C, 30.90; H, 3.76; N, 2.45. 1H NMR, δ (ppm): 8.022, (8H, s, Ar), 8.151 (2H, s, Ar), 6.352 (2H, s, Ar), 6.110 (2H, s, Ar), 3.834 (4H, s, -CH2), the H of -OH disappeared on exchange with D2O. IR (KBr) νmax. 3517, 3426, 3303, 3174, 3131, 3061, 1636, 1759, 1551, 1459, 1385, 1259, 1202, 1150, 1085, 1037, 773, and 623 cm-1. Synthesis of [H2BIPY]⊂[Na2(H2TCAS)2(H2O)4]‚17H2O (2). BIPY (7.8 mg, 0.05 mmol) and Na4TCAS (90 mg, 0.1 mmol) were dissolved in distilled water (10 mL). The solution was adjusted to pH ≈ 2 with nitric acid and was stirred at room temperature for 6 h. Then the solution was allowed to evaporate slowly at room temperature. After several days, large crystals of 2 suitable for X-ray diffraction analysis were formed, yielding 45 mg (41%). Compound 2 was characterized to have a formula of C58H64N2Na2O53S16 (Mr ) 2196.13) by X-ray crystallography. Anal. Calcd for C58H64N2Na2O53S16 (%): C, 31.72; H, 2.94; N, 1.28; found (%): C, 31.51; H, 3.06; N, 1.35. 1H NMR, δ (ppm): 8.185, (d, J ) 5 Hz, 2H, Ar), 8.011 (16H, s, Ar), 7.268 (d, J ) 8 Hz, 2H, Ar), 7.058 (t, J ) 16 Hz, 2H, Ar), 6.577 (t, J ) 11 Hz, 2H, Ar), the H of -OH disappeared on exchange with D2O. IR (KBr) νmax. 3415, 3077, 3034, 1603, 1587, 1534, 1516, 1548, 1441, 1280, 1234, 1176, 1140, 1113, 1087, 1047, 993, 939, 928, 765, 624, 610, and 542 cm-1. X-ray Crystallography. Intensity data for 1 and 2 were measured on a Siemens Smart CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at room temperature. All empirical absorption corrections were applied by using the SADABS program.15 The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXTL-97 program package.16,17 All C-H, N-H, and calixarene O-H hydrogens were fixed at geometrically calculated positions, assigned isotropic thermal parameters, and allowed to ride on their parent atoms before the final cycle of refinement. To satisfy charge balance, each TCAS must possess two protonated sulfonate groups, which are acceptable given the pH of the reaction solution. Not surprisingly, given the size of the crystal structure determination, protons were not located crystallographically. Crystal data and a structure determination summary for 1 and 2 are listed in Table 1.
Results and Discussion Compounds 1 and 2 were crystallized from acidic aqueous solution of Na4TCAS and BIME or BIPY over several weeks. As expected, the X-ray crystal structure revealed that the
Figure 1. The capsule structure of compound 1 showing partial atom labeling. Lattice water molecules and hydrogen atoms have been omitted for clarity (symmetry code: A -x, y, 3/2 - z).
molecular capsules were indeed formed. And the capsules were further associated with sodium ions to form one-dimensional (1D) infinite chains, so the two compounds can be defined as a coordination polymer based on the interlay of sodium ions with sulfonate groups from different capsules. Compound 1 crystallizes in the monoclinic space group C2/ c, with the capsule structure feature shown in Figure 1. In the asymmetric unit, there is one crystallographically distinct TCAS, one-half diprotonated BIME (H2BIME2+), two different onehalf sodium ions, two coordination water molecules, and sevenand-a-half lattice water molecules disordered at several positions. For a charge balance, two of four sulfonate groups in each TCAS should be protonated. Although the two crystallographically distinct sodium ions are both octahedral coordination geometry, they have a small difference: Na1 ligates to six oxygen atoms of four sulfonate groups belonging to four different TCAS (Na-O, 2.325(6)-2.571(6) Å); Na2 binds to four coordination water molecules and two oxygen atoms of two sulfonate groups (Na-O, 2.372(8)-2.471(11) Å). These coordination environments and associated bond lengths are typical for the structures containing Na-O coordination bonds.18 Three sodium ions are located at the upper rims of two calixarene molecules by coordination, creating a capsule with C2 symmetry. Every capsule sandwiches a H2BIME2+ ion as a charged guest through the electrostatic and “nonclassical” hydrogen-bonding interactions (C‚‚‚ aromatic ring centroid ) 3.574 Å).19 The H2BIME2+ ion has a substabilized gauche conformation with the N-C(chain)-C(chain)-N dihedral angle being 73.9°, where the conformation is stabilized with weak hydrogen bonding between the hydrogen atoms of imidazolyl groups in H2BIME2+ and the aromatic rings of TCAS. The capsule significantly keeps open, since one sulfonate group in each calixarene does not coordinate to a sodium center. Compared with the reported capsules compounds,5-8,20 the capsule in 1 is quite compact, in which the distance between the top S1A‚‚‚S2A‚‚‚S3A‚‚‚S4A plane and bottom S1‚‚‚S2‚‚‚S3‚‚‚S4 plane defined by bridging sulfur of TCAS is about 13.27 Å (Figure 1). Na1 hinges four different calixarenes of two capsules through sulfonate linkages, resulting in an infinite 1D array of capsules (Figure 2). The 1D array of capsules is not in a linear but in a zigzag-like way (Figure 2b), and the two adjacent capsules are antidirectional
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Figure 4. Packing view of compound 1 viewed along the c axis.
Figure 2. View of the 1D zigzag chain extended structure of compound 1. Lattice water molecules and hydrogen atoms have been omitted for clarity. (a) Side view; (b) top view.
Figure 5. The capsule structure of compound 2 showing partial atom labeling; lattice water molecules and hydrogen atoms have been omitted for clarity. (Symmetry codes: A 1/2 - x, 1/2 - y, z; B 1/2 - x, y, -z - 1/2; C x, 1/2 - y, -z - 1/2.)
Figure 3. The π-π staking interactions between the 1D chains of compound 1. Lattice water molecules and hydrogen atoms have been omitted for clarity.
(Figure 2a). Each chain is linked to four other chains through complicated hydrogen bonding and strong π-π packing interactions of aromatic rings between adjacent chains with approximate distances of 3.41 Å (Figure 3), and the chains are further packed into a three-dimensional (3D) supramolecule. Meanwhile, there are tunnels among four chains (Figure 4), in which the lattice water molecules and plentiful hydrogen bonding between the lattice water molecules and TCAS are found. Compound 2 crystallizes in the space group Pnnn and also has a capsule structure with D2 symmetry constructed by a pair of TCAS linked by sodium ions. The asymmetric unit of
Figure 6. View of the 1D chain extended structure of compound 2; lattice water molecules and hydrogen atoms have been omitted for clarity. (a) Top view; (b) side view.
compound 2 consists of one-half TCAS and sodium ion, onequarter diprotonated BIPY (H2BIPY2+), a coordination water molecule, and four-and-a-quarter disordered solvent water molecules occupying six positions. Figure 5 shows the capsule
Assembly of p-Sulfonatothiacalix[4]arene
Crystal Growth & Design, Vol. 6, No. 2, 2006 517
Table 2. Chemical Shifts for Hydrogen Atoms of Guests in Compounds 1 and 2
chemical shift of free guest δ (ppm) chemical shift of compounds δ (ppm) ∆δ (ppm) chemical shift with an excess of guest δ (ppm)
Ha 4.428 3.834 -0.594 3.963
Hb 7.410 8.151 0.741 7.608
unit resulting from the combination of two TCAS with cone conformations through four sodium cations. Different from the open capsule of 1 where one out of four sulfonate groups of TCAS is uncoordinated, the capsule of 2 here is a closed one bound completely by all sulfonate groups. Every sodium ion in 2 adopts an octahedral coordination geometry, which is coordinated by four sulfonate groups from four TCAS in two capsules (Na-O, 2.372(7)-2.62(2) Å) and by two terminal water molecules (Na-O, 2.454(9) Å). Thus, a 1D chain arrangement of capsules forms. Because of the double NaO6 hinges held together from two sides (Figure 6a), the capsule array in 2 is a straight-line chain (Figure 6), which is comparable with the zigzag-like chain found in 1. A H2BIP2+ cation inhabits the cavity of the capsule with a 2-fold axis passing through it. The two aromatic rings of H2BIP are not coplanar with a dihedral angle of 53.21(1)°. The meta and para hydrogen atoms of aromatic rings interact with the calixarene core through CH‚ ‚‚π interactions, and H‚‚‚centroid distances range from 2.52 to 3.19 Å. Additionally, those chainlike structures further assemble into a 3D supramolecule through complicated interchain hydrogen bonding and π-π interaction of aromatic rings separated by 3.453 Å (Figure 7). From another side view, the packing structures for both compounds 1 and 2 have conventional bilayer arrangements of
Hc 7.016 6.110 -0.906 6.536
Hd 7.016 6.352 -0.664 6.753
He 8.848 8.185 -0.663 8.457
Hf 8.418 7.058 -1.360 7.907
Hg 8.382 6.577 -1.805 7.151
Hh 7.876 7.268 -0.608 7.716
up-down alternating calix[4]arene separated by a hydrophilic layer containing guest cations, sodium ions, and water molecules (Figures 4 and 7). And such a bilayer arrangement has been widely seen for CAS or TCAS containing complexes.21 Furthermore, sodium ions in 1 and 2 have played an important role in the formation of open or close capsules, where a part of or all of sulfonate groups from TCAS coordinate to sodium centers. To study the solution properties of these molecular capsules for comparison with their solid-state structures, we examined spectroscopically the solution obtained from crystalline 1 and 2. 1H NMR spectra of free TCAS, BIME, BIPY, compounds 1 and 2 were recorded in D2O (pD ≈ 2.5). Interestingly, significant complexation-induced upfield shifts for the signals from the guest molecules (BIME or BIPY) were observed (Table 2), as expected from the shielding effects of the aromatic rings of the host cavity. Comparison with the resonance of free BIME, the signals corresponding to Ha, Hc, and Hd of BIME in 1 were obviously shifted upfield by 0.594, 0.906, and 0.664 ppm, respectively, while the signal of Hb was unexpectedly shifted downfield. Furthermore, in compound 1 the signals corresponding to Hc and Hd are clearly distinguished, although the chemical shifts of Hc and Hd for free BIME in D2O appear in the same positions. Similar to compound 1, the signals corresponding to He, Hf, Hg, and Hh of BIPY for compound 2 were also shifted upfield by 0.663, 1.841, 1.324, and 0.608 ppm, respectively, in which the signals of Hf and Hg were clearly distinguished. In 1H NMR experiments with an addition of an excess of the guest molecules (host: guest ) 1:1) for 1 and 2, the signals of guests were shifted downfield (Table 2) and appeared in the positions between the original signals of free guest and included guest, indicating their rapid exchange on the NMR time scale. The 1H NMR results are therefore consistent with the interpretation that the guest-induced compact molecular capsules 1 and 2 are significantly stable in water solution and maintain their solid-state structures.12,22 Conclusion
Figure 7. Packing view of compound 2 illustrating the bilayer arrangement of TCAS.
TCAS with sodium ions linkage in water solution can be aggregated into new compact molecular capsules by inducing suitable guest molecules containing conjugate groups, such as imidazol rings in BIME and pyridine rings in BIPY. Singlecrystal X-ray diffraction analyses reveal that the open capsules of compound 1 are in a zigzag-like chain array, while the close capsules of 2 in a straight-line chain array. 1H NMR characterizations indicate that both water-soluble 1 and 2 maintain their molecular capsules in solution. BIME and BIPY as the guest molecules inhabiting inside the cavities in 1 and 2 are well accommodated by TCAS-Na capsules via a variety of supramolecular interactions.
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Acknowledgment. We thank National Natural Foundation of China (No. 20231020) and the Natural Science Foundation of Fujian Province for funding this research. Supporting Information Available: X-ray crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG050446X
