Formation of Calix [4] arene-Based Supramolecular Gels Triggered by

Aug 27, 2013 - Silverman, J. R.; John, G. Chem. Soc. Rev. 2013, 42, 924. (c) Steed, J. W. Chem. Commun. 2011, 47, 1379. (d) Foster, J. A.; Steed, J. W...
0 downloads 0 Views 3MB Size
Download PDF
Communication pubs.acs.org/crystal

Formation of Calix[4]arene-Based Supramolecular Gels Triggered by K+ and Rb+: Exemplification of a Structure−Property Relationship Doomi Hwang, Eunji Lee, Jong Hwa Jung, Shim Sung Lee,* and Ki-Min Park* Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, S. Korea S Supporting Information *

ABSTRACT: Supramolecular gels based on calix[4]arene tetraacetate were prepared in the presence of K+ and Rb+. The Rb gel shows higher mechanical stability than the K gel: crystals derived from the K gel have an H-bonded framework, while those obtained using the Rb gel have a framework stabilized by H bonds and coordination bonds. Chart 1. Structure of (Et3NH)2·H2CTA (1)

S

upramolecular gels1 constructed by the self-assembly of stimuli responsive low-molecular-mass gelators (LMMGs) have attracted particular interest because of their potential applications in many fields such as chiral recognition,2 tissue engineering,3 sensing devices,4 hydrogen storage,5 and drug delivery.6 Among them, container molecules such as resorcarenes,7 calix[4]pyrroles,8 cucurbiturils,9 and cyclodextrins10 have been used to develop supramolecular gels exhibiting a variety of host−guest interactions that include van der Waals interactions, π−π stacking, H bonds, and coordination bonds. Recently, we have reported the networking of calix[4]arene carboxylate derivatives by means of metalation11a,b and also found the alignment approach of olefinic dipyridyl species for the [2 + 2] photocyclization using a calix[4]arene-based template.11c The calixarene carboxylates due to their easy modification12 are also expected to provide a class of new reagents for the synthesis of supramolecular gels. Since the Shinkai group reported the first examples of calixarenes with long aliphatic chains as gelators,13 several other groups have reported LMMGs based on calixarene derivatives.14 However, despite the rapid growth in the syntheses of supramolecular gels,1−10 the investigation on their physical properties and their stabilities in terms of their tertiary molecular structures still remains lacking. In extending our prior studies on calix[4]arene carboxylate derivatives,10 we have prepared calix[4]arene tetraacetate (diprotonated form) as its triethylammonium salt, (Et3NH)2· H2CTA (1), in which the internal cavity and four arms are expected to show metal ion recognition but also provide sites for other multiple interactions, including H bonds and cation−π interactions, which might assist gel formation (Chart 1). In this work, we report the formation of supramolecular gels of 1 induced by K+ (K gel) and Rb+ (Rb gel). Notably, we were successful in growing corresponding single crystals suitable for X-ray structure analysis from both supramolecular gels with the use of simple procedures. Accordingly, correlation of the physical properties (and mechanical stability) of the gels with their solid structures at the molecular level is proved possible. © XXXX American Chemical Society

Recently, some physical properties of the gels have been explained in terms of their molecular structures.15 To the best of our knowledge, the outcome observed in the present work provides a rare example of two comparative model systems which correlate gel stability with corresponding structures in the solid state. The gelator 1, (Et3NH)2·H2CTA, was prepared from the reaction of calix[4]arene tetraacetic acid (H4CTA) and triethylamine in methanol (see the Experimental Section of the Supporting Information). The structure of 1 was also confirmed by single-crystal X-ray analysis.16 Single crystals of 1 were obtained by vapor diffusion of diethyl ether into the above methanol solution. In the crystalline state, the calix[4]arene moiety adopts a saddle-shaped 1,3-alternate conformation, with the four carboxyl groups oriented away from each other (Figure 1). Compound 1 is a nongelator in its free form but, as mentioned already, forms metallogels in the presence of K+ and Rb+ salts. No gelation was found for the addition of Li+, Na+, and Cs+ salts under the same conditions used for the above two ions (Figure 2). This suggests that the metal ion recognition based on the size effect (metal-to-cavity ratio concept) is one of the important factors for the gelation. For example, Li+ and Na+ are too small and Cs+ is too large to accommodate in the gelator 1. When we employed some selective transition metal salts (Cd2+, Cu2+, Ag+, and Cu+) under similar conditions, we Received: July 18, 2013 Revised: August 22, 2013

A

dx.doi.org/10.1021/cg401084w | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

Figure 1. Crystal structure of the anionic part of 1, (H2CTA)2−, showing the four carboxylate arms oriented away from the calix cavity: (a) side and (b) top view.

found that gel was not formed. In the formation of K gel and Rb gel, a 1:1 MeOH/water was used because the gelator 1 or the metal salts were not completely dissolved in common solvents except this mixed solvent (Table S4 of the Supporting Information). The K gel that formed was found to decompose after 3 h at room temperature, whereas the Rb gel remained stable (Figure S1 of the Supporting Information). Indeed, compound 1 was successfully gelated using a variety of rubidium salts (incorporating F−, Cl−, Br−, I−, NO3−, and ClO4− anions) in a 1:1 MeOH/water mixture (Figure S2 of the Supporting Information). In the case of potassium(I) salts, Cl−, Br−, I−, and ClO4− formed the gel, but F− and NO3− did not form the gel. So, we selected the chloride form in both metal salts to prepare the gels. In addition, the Rb+-induced gelation occurred over the pH range of 3−12 (Figure S3 of the Supporting Information). The SEM images of both xerogels obtained at pH 7 show fibrillar networks and the Rb xerogel showed a more tangled structure than the K xerogel (Figure 2, panels f and g). Because of the solubility problem, the pure water was not suitable for the gelation solvent. So, we tested the gelation ability in the mixtures of MeOH/H2O with different ratios. We

Figure 2. Gel formation test by mixing 1.0 wt % of 1 with 2 equiv of (a) LiCl, (b) NaCl, (c) KCl, (d) RbCl, and (e) CsCl in a 1:1 MeOH/ water mixture at pH 7. SEM images of (f) K xerogel and (g) Rb xerogel obtained from the above corresponding gels after drying.

Figure 3. Panel A: (a) growth process of single crystals of 2 through the thermal treatment of K gel, (b) crystal structure of 2, {[K2(H2CTA)H2O]· 2H2O}n, showing the intermolecular H bonds, and (c) pseudo 3D framework of 2 connected by H bonds. Panel B: (a) growth process of single crystals of 3 from Rb gel, (b) crystal structure of 3, {[Rb4(H2CTA)(H3CTA)2(MeOH)2]·MeOH·3H2O}n, showing the intermolecular H bonds and coordination bonds, and (c) pseudo 3D framework of 3 stabilized by intermolecular coordination bonds and H bonds. B

dx.doi.org/10.1021/cg401084w | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

As depicted in Scheme 1, the most striking difference between frameworks 2 and 3 lies in the connectivity patterns, which are strongly related to the gel stabilities. In both 2 and 3, it is found that the carboxylate arms in 1 are oriented away from the calix cavity but align with the axis of calix upon complexation. This aids the efficient formation of the intermolecular H bonds in 2 and 3. Cation−π interactions are also observed in both cases (Figure S8 and S9 of the Supporting Information). The simulated powder X-ray diffraction (PXRD) patterns for 2 and 3 are in agreement with those for the K and the Rb xerogels, respectively (Figure S10 of the Supporting Information), being consistent with the respective crystal structures that correspond with the structures of gels.17

found that a 1:1 MeOH/H2O is an optimized mixture solvent which contains a minimum amount of MeOH. The relative stabilities of the K and Rb gels were examined by strain sweep rheometry to probe the behavior of the gels against mechanical stress (Figure S4 of the Supporting Information). The plateau elastic (G′) and viscous (G″) moduli of the Rb gel are 102−103-fold higher than those of the K gel, suggesting that the Rb gel is more mechanically stable than the K gel. The stabilities of both gels were also confirmed from the transition temperatures (Tsol−gel) measured by differential scanning calorimetry (DSC). The Rb and K gels exhibited a sharp phase-transition at 68 and 51 °C, respectively, with both transition endothermic (Figure S5 of the Supporting Information). In order to rationalize how the gelator 1 can be gelated selectively in the presence of K+ and Rb+, and why the Rb gel is significantly more stable than K gel, we attempted to obtain the single crystals from the corresponding gels. Fortunately, we could isolate both products 2 and 3 from the K and Rb gels through the simple treatment, respectively, as X-ray quality single crystals. For example, upon heating the K gel in a vial to 60 °C, the gel phase was converted to a sol phase. Upon cooling the sol phase in the vial to room temperature under ambient conditions, the single-crystalline product 2 was obtained at the bottom of the vial after 5 min, together with the gel phase, as shown in Figure 3A, a. For 3, the Rb gel was dispersed in methanol and dissolved by heating to 60 °C. Allowing the solution to stand at room temperature for 1 month afforded colorless single crystals 3 (Figure 3B, a). Singlecrystal analysis of 2 and 3 confirmed that both feature pseudo 3D polymeric arrangements with different connectivity patterns. Complex 2 (derived from the K gel) crystallized in the monoclinic space group P21/n with Z = 4 and has a formula of {[K2(H2CTA)(H2O)]·2H2O}n.16 In 2, two crystallographically nonequivalent K atoms (K1 and K2) are present in the calix moiety and their coordination environments are similar, except that there is also one coordinated water molecule (O1W) to K1 atom (Figure 3A, b). Each K atom is coordinated by two monodentate carboxyl oxygens and two phenolic oxygens from different pendants in a cis arrangement. The association of the dipotassium(I) complex unit, [K2(H2CTA)(H2O)], by the cross-linking via intermolecular H bonds generates the pseudo 3D framework of 2 (Figure 3A, c). Notably, the coordination bonds of two potassium ions toward one calix moiety do not directly contribute to the network structure of 2. Complex 3 (derived from the Rb gel) crystallized in the orthorhombic space group Pbcn with Z = 4 and has a formula of {[Rb4(H2CTA)(H3CTA)2(MeOH)2]·MeOH·3H2O}n.16 Similar to 2, the pseudo 3D framework of complex 3 is formed by intermolecular H bonds. However, the connectivity pattern of 3 is also strongly associated with triply bridged two rubidium ions (Rb1 and Rb2) located in different calix moieties (Figure 3B, b). Importantly, two sets of the bridged Rb atoms generate the repeating unit, [Rb4(H2CTA)(H3CTA)2(MeOH)2], in which four Rb atoms occupy inner positions of the array and both of the terminal moieties in the repeating unit are terminated by one MeOH/water molecule (disordered by 50:50) via H bonds. As mentioned, these repeating units are further crosslinked by intermolecular H-bonds, resulting in the compact pseudo 3D framework (Figure 3B, c). Thus, unlike 2, the framework structure of 3 is reinforced by intermolecular coordination bonds as well as by H bonds.

Scheme 1. Simplified Presentation Showing the Stability− Structure Relationship for the K gel Relative to the Rb gel

The structural information for the corresponding nongelation systems can be helpful to support the proposed struture−property relationship. Unfortunately, we failed to obtain the suitable crystals for Li+ and Na+ complexes. As we understand, such difficulty to prepare the crystalline complexes is probably due to the lower binding ability arising from size mismatching between the gelator 1 and these metal ions. It can be concluded that metalation by K+ or Rb+ of the calix tetraacetate groups primarily induces the observed H bond formations reflecting the orientation change of the carboxylate pendants and this, in turn, results in gelation. The observed intermolecular coordination bonds in the Rb gel, as already discussed, mainly reinforce the thermal stability of the gel. Overall, the combined approach of investigating gel properties along with corresponding solid state structural analysis has enabled in-depth information to be obtained concerning the property−structure relationship between the supramolecular gels presented in this study.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental details for synthesis of H4CTA, 1, 2, and 3, gelation test data, rheological measurements, DSC thermograms, SEM images, PXRD patterns for 2 and 3, additional figures, crystal data (PDF format), and X-ray crystallographic files (CIF format) for 1, 2, and 3 are included. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*K.M.P.: e-mail, [email protected]. S.S.L.: e-mail, sslee@gnu. ac.kr. C

dx.doi.org/10.1021/cg401084w | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

Notes

Soft Matter 2010, 6, 3541. (h) Tu, T.; Bao, X.; Assenmacher, W.; Peterlik, H.; Daniels, J.; Dötz, K. H. Chem.Eur. J. 2009, 15, 1853. (i) Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Commun. 2008, 2644. (j) Tu, T.; Assenmacher, W.; Peterlik, H.; Weisbarth, R.; Nieger, M.; Dötz, K. H. Angew. Chem., Int. Ed. 2007, 46, 6368. (k) Kumar, D. K.; Jose, D. A; Das, A.; Dastidar, P. Chem. Commun. 2005, 4059. (l) Gronwald, O.; Shinkai, S. Chem.Eur. J. 2001, 7, 4328. (m) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 122, 11679. (16) The X-ray data were collected on a Bruker SMART APEX II ULTRA diffractometer equipped with a graphite monochromated Mo Kα (λ = 0.71073 Å) radiation generated by a rotating anode and a CCD detector. The cell parameters for the compounds were obtained from a least-squares refinement of the spots (from 36 collected frames). Data collection, data reduction, and semiempirical absorption correction were carried out using the software package of APEX2.18 Semiempirical absorption corrections were applied to the data sets using the SADABS.19 The structures were solved by direct methods and refined by full matrix least-squares methods on F2 for all data using SHELXTL.20 In 1 and 3, the DFIX restraints in the structural model are applied during the refinement processes due to the large variation of some bond geometries. In 3, the EXYZ and EADP constraints have been applied in the refinement because the O20 atom in methanol and the O1W atom in water are sharing the same position with the site occupancy factors of 0.50. In all cases, all non-hydrogen atoms were refined anisotropically and all hydrogen atoms, except those of carboxyl groups and lattice water molecules of 1, 2, and 3 were placed in calculated positions and refined isotropically in a riding manner along with their respective parent atoms. In the case of carboxyl groups and the lattice water molecules, the initial positions of the hydrogen atoms were obtained from difference electron density maps and refined with riding constraints. In 3, however, the hydrogen atoms of the disordered lattice water molecules (O2W and O2W’) were not included in the model. The detailed crystallographic data and structure refinement parameters are summarized in Table S5 of the Supporting Information. Selected bond distances and angles and the hydrogen bond geometries are listed in Tables S6-S10 of the Supporting Information. Crystal data for 1: C50H70.5N2O14.25, Mr = 927.58, colorless, 0.05 × 0.05 × 0.30 mm3, orthorhombic, space group P21212, a = 13.849(4), b = 24.297(6), c = 7.644(2) Å, V = 2572.1(11) Å3, Z = 2, μ = 0.087 mm−1, θmax = 26.00°, 320 parameters, 5038 independent reflections, 5038 with I > 2.0σ(I), R = 0.0948, wR = 0.2304 (R = 0.1441, wR = 0.2695 for all data), and GOF = 1.040. Crystal data for 2: C36H36K2O15, Mr = 786.85, colorless, 0.10 × 0.15 × 0.23 mm3, monoclinic, space group P21/n, a = 12.3744(3), b = 17.4418(5), c = 16.2582(5) Å, β = 94.469(2)o, V = 3498.37(17) Å3, Z = 4, μ = 0.346 mm−1, θmax = 28.32°, 478 parameters, 8686 independent reflections, 8686 with I > 2.0σ(I), R = 0.0491, wR = 0.1278 (R = 0.0617, wR = 0.1351 for all data), and GOF = 1.029. Crystal data for 3: C111H110O42Rb4, Mr = 2457.87, colorless, 0.20 × 0.21 × 0.28 mm3, orthorhombic, space group Pbcn, a = 30.8147(5), b = 19.4575(3), c = 17.4680(3) Å, V = 10473.4(3) Å3, Z = 4, μ = 1.952 mm−1, θmax = 28.32°, 722 parameters, 12956 independent reflections, 12956 with I > 2.0σ(I), R = 0.0773, wR = 0.1597 (R = 0.1717, wR = 0.1877 for all data), and GOF = 1.078. CCDC 941532, 941533, and 941534 (1, 2, and 3). (17) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324. (18) Data Collection and Processing Software, Bruker APEX 2, version 2009, 1−0; Bruker AXS Inc.: Madison, WI, 2008. (19) Empirical Absorption and Correction Software, Bruker, SADAS, version 2.03; Bruker AXS Inc.: Madison, WI, 1999. (20) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by NRF (Grants 2010-0022675 and 2012R1A4A1027750) projects. REFERENCES

(1) (a) Meazza, L.; Foster, J. A.; Fucke, K.; Metrangolo, P.; Resnati, G.; Steed, J. W. Nat. Chem. 2013, 5, 42. (b) Jung, J. H.; Lee, J. H.; Silverman, J. R.; John, G. Chem. Soc. Rev. 2013, 42, 924. (c) Steed, J. W. Chem. Commun. 2011, 47, 1379. (d) Foster, J. A.; Steed, J. W. Angew. Chem., Int. Ed. 2010, 49, 6718. (e) Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Rev. 2010, 110, 1960. (2) (a) Lloyd, G. O.; Piepenbrock, M.-O. M.; Foster, J. A.; Clake, N.; Steed, J. W. Soft Matter 2012, 8, 204. (b) Chen, X.; Huang, Z.; Chen, S.-Y.; Li, K.; Yu, X.-Q.; Pu, L. J. Am. Chem. Soc. 2010, 132, 7297. (c) Zheng, Y.-S.; Ran, S.-Y.; Hu, Y.-J.; Liu, X.-X. Chem. Commun. 2009, 1121. (d) Zhou, J.-L.; Chen, X.-J.; Zhou, J.-L. Chem. Commun. 2007, 5200. (3) (a) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699. (b) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47, 8002. (4) (a) Lee, H.; Kang, S.; Lee, J. Y.; Jung, J. H. Soft Matter 2012, 8, 2950. (b) Lee, H.; Jung, S. H.; Han, W. S.; Moon, J. H.; Kang, S.; Lee, J. Y.; Jung, J. H.; Shinkai, S. Chem.Eur. J. 2011, 17, 2823. (5) Murray, J.; Dinca, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294. (6) (a) Ghost, I.; Nau, W. M. Adv. Drug Delivery Rev. 2012, 64, 764. (b) Fleige, E.; Quadir, M. A.; Haag, R. Adv. Drug Delivery Rev. 2012, 64, 866. (c) John, G.; Shankar, B. V.; Jadhav, S. R.; Vemula, P. K. Langmuir 2010, 26, 17843. (d) McKinlay, A. C.; Morris, R. E.; Horcajada, P.; Férey, G.; Gref, R.; Couvreur, P.; Serre, C. Angew. Chem., Int. Ed. 2010, 49, 6260. (e) Yang, Z.; Ho, P. L.; Liang, G.; Chow, K. H.; Wang, Q.; Cao, Y.; Guo, Z.; Xu, B. J. Am. Chem. Soc. 2007, 129, 266. (7) Haines, S. R.; Harrison, R. G. Chem. Commun. 2002, 2846. (8) Verdejo, B.; Rodriguez-Llansola, F.; Escuder, B.; Miravet, J. F.; Ballester, P. Chem. Commun. 2011, 47, 2017. (9) Hwang, I.; Jeon, W. S.; Kim, H. J.; Kim, D.; Kim, H.; Selvapalam, N.; Fujita, N.; Shinkai, S.; Kim, K. Angew. Chem., Int. Ed. 2007, 46, 210. (10) (a) Deng, W.; Yamaguchi, H.; Takashima, Y.; Harada, A. Angew. Chem., Int. Ed. 2007, 46, 5144 and references therein.. (11) (a) Park, K.-M.; Lee, E.; Park, C. S.; Lee, S. S. Inorg. Chem. 2011, 50, 12085. (b) Kim, K.; Park, S.; Park, K.-M.; Lee, S. S. Cryst. Growth Des. 2011, 11, 4059. (c) Lee, E.; Ju, H.; Lee, S. S.; Park, K.-M. Cryst. Growth Des. 2013, 13, 992. (12) (a) Böhmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (13) (a) Aoki, M.; Murata, K.; Shinkai, S. Chem. Lett. 1991, 1715. (b) Aoki, M.; Nakashima, K.; Kawabata, H.; Tsutsui, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1993, 347. (14) (a) Xing, B.; Choi, M.-F.; Xu, B. Chem. Commun. 2002, 362. (b) Xing, B.; Choi, M.-F.; Zhou, Z.; Xu, B. Langmuir 2002, 18, 9654. (c) Zhou, J.-L.; Chen, X.-J.; Zheng, Y.-S. Chem. Commun. 2007, 5200. (d) Zheng, Y.-S.; Ran, S.-Y.; Hua, Y.-J.; Liu, X.-X. Chem. Commun. 2009, 1121. (e) Becker, T.; Goh, C. Y.; Jones, F.; McIldowie, M. J.; Mocerino, M.; Ogden, M. I. Chem. Commun. 2008, 3900. (f) Goh, C. Y.; Becker, T.; Brwon, D. H.; Skelton, B. W.; Jones, F.; Mocerino, M.; Ogden, M. I. Chem. Commun. 2011, 47, 6057. (g) Zhang, J.; Guo, D.S.; Wang, L.-H.; Wang, Z.; Liu, Y. Soft Matter 2011, 7, 1756. (h) Xing, B.; Choi, M.-F.; Xu, B. Chem.Eur. J. 2002, 8, 5028. (15) (a) Samai, S.; Ghosh, P.; Biradha, K. Chem. Commun. 2013, 49, 4181. (b) Ahn, J.; Park, S.; Lee, J. H.; Jung, S. H.; Moon, S.-J.; Jung, J. H. Chem. Commun. 2013, 49, 2109. (c) Zhang, Y.; Liang, C.; Shang, H.; Ma, Y.; Jiang, S. J. Mater. Chem. C 2013, 1, 4472. (d) Xu, Y.; Kang, C.; Chen, Y.; Bian, Z.; Qiu, X.; Gao, L.; Meng, Q. Chem.Eur. J. 2012, 18, 16955. (e) Jung, S. H.; Lee, H.; Park, S.; Jung, J. H. New J. Chem. 2012, 36, 1957. (f) He, Y.; Bian, Z.; Kang, C.; Gao, L. Chem. Commun. 2011, 47, 1589. (g) Piepenbrock, M.-O. M.; Clarke, N.; Steed, J. W. D

dx.doi.org/10.1021/cg401084w | Cryst. Growth Des. XXXX, XXX, XXX−XXX