Secondary Ammonium Dicarboxylate (SAD) A Supramolecular

Article pubs.acs.org/crystal

Secondary Ammonium Dicarboxylate (SAD)A Supramolecular Synthon in Designing Low Molecular Weight Gelators Derived from Azo-Dicarboxylates Pathik Sahoo and Parthasarathi Dastidar* Department of Organic Chemistry, Indian Association for the Cultivation of Science (IACS), 2A and 2B Raja S C Mullick Road, Jadavpur Kolkata−700032, West Bengal, India S Supporting Information *

ABSTRACT: The supramolecular synthon namely secondary ammonium dicarboxylate (SAD) synthon has been exploited to design a new series of low molecular weight gelators (LMWGs) derived from azobenzene-4,4′-dicarboxylic acid and azobenzene-4,4′-diacrylic acid, and various secondary amines. Single crystal structures of six such salts exclusively established the presence of SAD synthon. Two such salts namely, dicyclohexylammonium azobenzene-4,4′diacrylate (2.DCHA) and dihexylammonium azobenzene-4,4′-diacrylate (2.DHA) displayed intriguing gelation properties. Powder X-ray diffraction in combination with single crystal X-ray data established existence of SAD synthon in the structure of the gel network of 2.DCHA. UV-irradiation of the salts as well as the gel did not show any trans−cis isomerization of the azo-moiety.

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INTRODUCTION Small molecules (molecular weight 2σ(I)] data/restraints/parameters goodness of fit on F2 final R indices [I > 2σ(I)]

2.CHMA

1.CHMA

crystal parameters

Table 1. Crystal Data

3750/0/447 1.024 R1 = 0.0429 wR2 = 0.1133 R1 = 0.0619 wR2 = 0.1256

819374 C38 H56 N4 O4 632.87 0.28 × 0.28 × 0.27 monoclinic P2/c 22.153(3) 9.7615(12) 17.488(2) 90.00 98.224(3) 90.00 3742.9(8) 4 1376 0.073 100(2) 0.0458 −21/21, −9/9, −17/7 0.93/20.49 22125/3750/2761

1.DCHA

1680/0/225 1.151 R1 = 0.1136 wR2 = 0.2861 R1 = 0.1261 wR2 = 0.2962

819372 C21 H30 N2 O2 342.47 0.40 × 0.18 × 0.17 monoclinic P21/c 7.721(3) 14.566(6) 17.327(7) 90.00 94.155(6) 90.00 1943.5(15) 4 744 0.075 100(2) 0.0494 −7/7, −13/13, −16/13 1.83/19.47 5416/1680/1470

2.DCHA

1693/2/162 1.578 R1 = 0.1186 wR2 = 0.3442 R1 = 0.1303 wR2 = 0.3589

819369 C30 H48 N4 O4 528.72 0.24 × 0.24 × 0.24 triclinic P-1 8.568(10) 9.238(10) 10.972(12) 98.503(14) 105.534(14) 103.485(14) 792.9(15) 1 288 0.074 100(2) 0.0210 −8/8, −9/9, −11/11 1.98/21.00 3767/1693/1383

1.DBuA

1831/3/180 1.208 R1 = 0.1536 wR2 = 0.3830 R1 = 0.1909 wR2 =0.4161

819371 C34 H52 N4 O4 580.80 0.27 × 0.22 × 08 triclinic P-1 8.1405(13) 11.1094(18) 11.6064(19) 108.919(3) 109.462(3) 101.428(4) 879.3(2) 1 316 0.072 293(2) 0.0291 −7/8, −11/11, −11/10 2.05/21.00 2612/1831/1283

2.DBuA

Crystal Growth & Design Article

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Figure 1. Illustration of the single crystal structure of nongelator salt 1.CHMA; (a) Hydrogen bonding interactions forming 1D SAD synthon; (b) parallel packing of the 1D networks.

Figure 2. Illustration of the single crystal structure of nongelator salt 2.CHMA; (a) Hydrogen bonding interactions forming 1D SAD synthon; (b) Parallel packing of the 1D networks.

Crystal Structure of Dibutylammonium Azobenzene-4,4′dicarboxylate (1.DBuA). The crystal of 1.DBuA crystallized in the centrosymmetric triclinic space group P1̅. The asymmetric unit is composed of half of the azobenzene-4,4′-dicarboxylate

moiety sitting on a center of symmetry and a dibutylammonium moiety located on a general position. Both the N atoms of the azo moiety of the anionic part were found to be disordered over two positions having site occupancy factors of 0.73171 and 5920

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of 1629 cm−1 and absence of 1685 cm−1 band in the FT-IR clearly supported the salt formation. The ionic species recognizes its opposite kind via N−H...O hydrogen bonding [N−H...O = 2.701(6)−2.706(6)Å; ∠N−H...O = 160.0−170.0°] resulting in the formation of 1D SAD synthon (Scheme 2). The 1D chains are packed in parallel fashion (Figure 5).

Figure 5. Illustration of the single crystal structure of nongelator salt 1.DDBuA. (a) Hydrogen bonding interactions forming 1D SAD synthon; (b) Parallel packing of the 1D networks.

Figure 3. Illustration of the single crystal structure of nongelator salt 1.DCHA. (a) Hydrogen bonding interactions forming 1D SAD synthon; (b) packing of the 1D networks in criss-cross fashion.

Crystal Structure of Dibutylammonium Azobenzene-4,4′diacrylate (2.DBuA). The salt 2.DBuA crystallized in the centrosymmetric triclinic space group P1̅. The asymmetric unit contains half of the anionic moiety sitting on a center of

0.26829. Two terminal C atoms of one of the butyl chains were also found to be disordered and were isotropically refined. The C−O bond distances of 1.235(7)−1.265(7)Å and the presence

Figure 4. Illustration of the single crystal structure of gelator salt 2.DCHA. (a) Hydrogen bonding interactions forming 1D SAD synthon; (b) Parallel packing of the 1D networks. 5921

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Figure 6. Illustration of the single crystal structure of nongelator salt 2.DBuA. (a) Hydrogen bonding interactions forming 1D SAD synthon; (b) Parallel packing of the 1D networks.

gelator were measured by dropping ball method22 and the Tgel versus [gelator] plot was examined (Figure 7). Analysis of the

inversion and a cationic moiety located on a general position. The N atoms of the azo moiety were found to be disordered over two positions with site occupancy factors of 0.821940 and 0.178060. Two terminal C atoms of one of the butyl chains were also found to be disordered and were isotropically refined. The presence of 1641 cm−1 and absence of 1689 cm−1 in the FT-IR spectra and the C−O bond distances of 1.228(10)− 1.254(11)Å clearly supported the complete proton transfer (salt formation). In the crystal structure, the ionic species are involved in N−H...O hydrogen bonding [N−H...O = 2.684(10)− 2.743(11)Å; ∠N−H...O = 157.7−175.6°] resulting in the formation of 1D SAD synthon as depicted in Scheme 2; the 1D hydrogen bonded chains are further packed in parallel fashion (Figure 6). Gelation. As revealed from SXRD data that all the salts displayed typical 1D SAD synthon, it was considered worthwhile to evaluate their gelation behavior. Fifteen selected solvents, (from polar to nonpolar) were employed to testify the gelation ability of these salts. It was evidenced from the gelation Table S7 (Supporting Information) that none of the secondary ammonium azobenzene-4,4′-dicarboxylate salts exhibited the gelation property. When the anion was changed to azobenzene4,4′-diacrylate, it was quiet intriguing to note that, two salts, 2.DCHA and 2.DHA showed gelation behavior. It was found that the nonpolar solvents were unable to solubilize the organic salts, whereas, the aprotic solvents having high polarity (e.g., DMF and DMSO) were able to show gelation behavior with only two salts, 2.DCHA and 2.DHA. When the salts were heated in the polar-protic solvent, it was resulted into either clear solution or precipitate. It may be mentioned that the salt 2.DCHA exhibited the supergelation phenomena in the polar aprotic solvent like DMSO and DMF with the minimum gelator concentration (mgc) 0.88 and 0.80 wt %, respectively. Despite displaying 1D SAD synthon by all six of these SAD salts, only 2.DCHA and 2.DHA showed gelation behavior. Table top rheology was employed to assess the thermal stability of the gel derived from 2.DCHA in DMF. The gel−sol dissociation temperatures (Tgel) at various concentrations of the

Figure 7. Tgel vs [gelator] plots of 2.DCHA in DMF.

plot revealed that Tgel steadily increased with the increase in concentration of the gelators, indicating that the gel networks were mainly governed by strong supramolecular interactions such as hydrogen bonding. To study the morphological features of the gel fibers of 2.DCHA, several microscopic experiments namely, scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical microscopy (OM) were carried out. (Figure 8). Highly entangled microthin fiber in OM as well as in the corresponding SEM and AFM micrographs were observed. It was evidenced in SEM that several micrometer long fibbers form the 3D matrix wherein the solvent molecules were understandably immobilized to form gel. The diameter of these fibers was assigned to be 50 nm in AFM. It was curious to note that none of these gels as well as the single crystals of the SAD salts showed the photo responsiveness in the presence of ultraviolet light. The failure to undergo trans−cis isomerization of the azo moiety under UV irradiation led us to believe that there were not enough space in the solid state that allowed such a huge movement as the 1D SAD chains in the crystals remained closely packed. Similar 5922

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Figure 8. Morphology of the gel fibers of 2.DCHA as observed in various microscopy; (a) AFM of 0.09 wt % DMSO xerogel gel; (b) height profile in AFM; (c) SEM of 0.88 wt % DMSO xerogel gel; and (d) OM of 0.88 wt % DMSO gel.

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observation has also been reported by Hanabusa and coworkers.23 X-ray Powder Diffraction. To study as to what extent the 1D SAD synthon is responsible for gelation, we have made an attempt to carry out structure−property correlation using SXRD and PXRD data data. Near superimposable PXRD patterns under various conditions (simulated, bulk and gel) of 2.DCHAclearly established that the 1D SAD is not only present in the bulk sample but also is the main supramolecular entity in the gel network as well (Figure 9). The bulk PXRD

EXPERIMENTAL SECTION

Methods. All of the chemicals (Aldrich) and solvents were (A.R. grade, commercially available, India) used without any further purification. Petrol used in the gelation experiments had been purchased from the local market. Microanalyses were performed on a Perkin-Elmer elemental analyzer 2400, Series II. FT-IR spectra were recorded using Perkin-Elmer Spectrum GX. Powder X-ray patterns were recorded on XPERT Philips (CuKα radiation, λ = 1.5418 Å) Diffractometer. Scanning Electron Microscopy (FT-SEM) was performed on a JEOL; JSM-6700F. Optical microscopy was done in Leica MZ 16. AFM was performed by diCP-II, model FE-0100. Single Crystal X-ray was done by BRUKER axs, SMART APEX II. Preparation of Salts. Salts were prepared by reacting azobenzene4,4′-dicarboxylic acid24 and azobenzene-4,4′-diacrylic acid with various secondary amines, namely dicyclohexylamine, dibenzylamine, cyclohexylmethylamine, and dihexylamine in 1:2 (acid:amine) molar ratio in methanolic medium. The resultant mixture was subjected to sonication for a few minutes to ensure the homogeneous mixing of the two components. An orange precipitate was obtained after complete removal of MeOH by rotavapor, which were subjected to various physicochemical analyses and gelation test. Single Crystal Preparation. Single crystals were grown from methanol/methylsalicylate mixtures (∼25 mg of salt in ∼5 mL solvents in 10 mL beaker) by slow evaporation at room temperature. Typically X-ray quality crystals were appeared after a few weeks. Crystal Structures. Data were collected using MoKα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/ refinement were carried out using the software package of SMART APEX. All structures were solved by direct method and refined in a routine manner. Nonhydrogen atoms were treated anisotropically, except the atoms where the disorder was observed. All of the hydrogen atoms were geometrically fixed. CCDC 819369 (1.DBuA), 819370 (1.CHMA), 819371 (2.DBuA), 819372 (2.DCHA), 819373 (2.CHMA), and 819374 (1.DCHA), contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223−336−033; or [email protected]. uk). Photoirradiation. 1.0 mL gel was made in a pyrex glass vial and subjected to UV irradiation at 350 nm in a Rayonet photoreactor for 3 h.

Figure 9. PXRD comparison plots of 2.DCHA at various conditions.

patterns of other salts also matched quite well with that of the corresponding simulated pattern establishing the fact that 1D SAD synthon is indeed present in the bulk sample in each salt.

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CONCLUSIONS In summary, we have prepared a series of secondary ammonium dicarboxylate salts derived from azobenzene-4,4′dicarboxylic acid and azobenzene-4,4′-diacrylic acid with various secondary amines in 1:2 (acid:amine) molar ratio. Single crystal structures of six such salts revealed the presence of the 1D SAD synthon. Two such salts (2.DCHA and 2.DHA) displayed intriguing gelation ability. The structure− property correlation established the existence of 1D SAD synthon in the gel network of DMF gel of 2.DCHA. The fact that only two such salts out of eight salts synthesized displayed gelation ability indicates that much more efforts are needed to understand the crucial role played by gel-network/solvent interactions in the process of gelation. 5923

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Article

Nakano, K.; Iida, K.; Miyata, M.; Tohnai, N.; Shibayama., M. Chem. Mater. 2005, 17, 741. (18) Trivedi, D. R.; Dastidar, P. Cryst. Growth Des. 2006, 6, 2114. (19) Trivedi, D. R.; Dastidar, P. Cryst. Growth Des. 2006, 6, 1022. (20) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Crys. Growth Des. 2005, 5, 1545. (21) Sahoo, P.; Krishna Kumar, D.; Trivedi, D. R.; Dastidar, P. Tetrahedron Lett. 2008, 49, 3052. (22) Raghavan, S. R.; Cipriano, B. H. In Molecular Gels. Materials with Self-Assembled Fibrillar Networks; Weiss, G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2005; Chapter 8, p 241. (23) (a) Inoue, D.; Suzuki, M.; Shirai, H.; Hanabusa, K. Bull. Chem. Soc. Jpn. 2005, 78, 721. (b) Sahoo, P.; Chakraborty, I.; Dastidar, P. Soft Matter 2012, 8, 2595. (24) Jilani, J. European Pat. EP 1 688 413 A1Hikama Pharmaceuticals Co. Ltd, August, 09, 2006.

ASSOCIATED CONTENT

S Supporting Information *

Physico-chemical data for the salts, single-crystal X-ray data, molecular plots and hydrogen bonding parameters for the compounds, and gelation data (Table S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S. thanks IACS for a research fellowship and P.D. thanks CSIR, New Delhi for financial support. REFERENCES

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