CRYSTAL GROWTH & DESIGN
Supramolecular Hydrogen Bond Isomerism in Organic Salts: A Transition from 0D to 1D
2006 VOL. 6, NO. 4 1022-1026
Darshak R. Trivedi and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar - 364 002, Gujarat, India ReceiVed December 26, 2005; ReVised Manuscript ReceiVed February 6, 2006
ABSTRACT: Based on nine single-crystal structures of a series of organic salts derived from dicyclohexylamine and n-alkyl monocarboxylic acids (CH3-(CH2)n-COOH, n ) 1-17), it is shown that salts having n ) 14 and 15 display 1D hydrogen-bonded networks, whereas majority of the salts having n ) 1-9 show 0D networks. Structural analyses indicate that both intra- and internetwork alkyl-alkyl interactions appear to be responsible for such a supramolecular transition. Introduction Building solid-sate structures with a predefined molecular organization constitutes the ultimate goal of crystal engineering,1 a subdiscipline of supramolecular chemistry.2 Crystal engineering is an interdisciplinary branch of structural science that seeks to understand the role of various noncovalent interactions that hold molecules together in the crystalline state, thereby building solid-state materials to order with desired material properties.3 One-dimensional (1D) arrays of molecules, ions, and functional groups are of utmost importance in a bottom-up approach in nanofabrication4 because of their potential application in developing nanowires and quantum dots. One-dimensional aggregates of molecules/ions are also found to be important5 in the gelation property of low molecular mass organic gelators (LMOGs).6 Our efforts in LMOG research7 have recently enabled us to establish a direct structure-property correlation between the crystal structure and gelation/nongelation property of a novel class of LMOGs based on organic salts.8 The design strategy exploited to generate a 1D hydrogen-bonded network (HBN) in this study8 depends on the preferential occurrence of 1D networks of secondary ammonium monocarboxylate out of the three plausible supramolecular networks (Scheme 1). In a recent paper, we have shown how to obtain 1D HBNs in secondary ammonium salts by introducing one more carboxylic acid moiety into the system.9 As depicted in Scheme 1, secondary ammonium monocarboxylate can display three different HBNs, and occurrence of 1D or 0D networks appears to be dependent upon the substituents of the acid moiety when that of the ammonium moiety is kept unchanged.8 For example, 1D HBNs were observed in various cinnamate salts of dicyclohexylamine, whereas a 0D network is observed in the corresponding various benzoate salts. Thus, we thought that it is worthwhile to study the effect of alkyl chain length of the acid moiety of salts of dicyclohexylamine on the resultant supramolecular networks in the crystalline state. Therefore, we have made a series of salts derived from alkyl monocarboxylic acids with various alkyl chain lengths and dicyclohexylamine (Scheme 2). In this paper, we report a supramolecular transition from a 0D to a 1D HBN as a result of long alkyl-alkyl chain interactions in some secondary ammonium monocarboxylate salts. * To whom correspondence should be addressed. E-mail: parthod123@ rediffmail.com. Fax: +91-278-567562.
Scheme 1
Scheme 2
Results and Discussion Efforts were made to crystallize all of the salts prepared. However, only salts 1-4, 6, 8, and 9 were crystallized from ethyl acetate and MeOH (few drops) and 14-15 were crystallized from n-heptane and MeOH (few drops). All these crystallized salts were subjected to single-crystal X-ray diffraction. The crystallographic parameters are listed in Table 1. Crystal Structures. Crystal Structure of Dicyclohexylammonium Propionate, 1. Salt 1 crystallizes in the monoclinic space group C2/c, and its asymmetric unit contains one ion pair and one water of crystallization. The oppositely charged ionic species are strongly hydrogen bonded with each other through N-H‚‚‚O interactions (N‚‚‚O ) 2.716(1)-2.755(1) Å; ∠NH‚‚‚O ) 159.6°-167.6°) resulting in a cyclic 0D network. The water molecule acts as a bridge between the 0D cyclic networks of the ion pairs in salt 1 by an O-H‚‚‚O hydrogen bond (O‚‚ ‚O ) 2.801(1) Å; ∠O-H‚‚‚O ) 160.2(2)°). Crystal Structure of Dicyclohexylammonium Butanoate, 2. Salt 2 crystallizes in the centrosymmetric space group (monoclinic, P21/c). Its asymmetric unit contains ion pairs, all located on general positions. Through N-H‚‚‚O interactions (N‚ ‚‚O ) 2.658(1)-2.754(1) Å; ∠N-H‚‚‚O ) 161.3°-166.7°), the ion pairs form a centrosymmetric 0D network.
10.1021/cg050674r CCC: $33.50 © 2006 American Chemical Society Published on Web 03/04/2006
Supramolecular Hydrogen Bond Isomerism
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Table 1. Crystallographic Parameters of the Salts 1-4, 6, 8, 9, 14, and 15 crystal data
1
2
3
4
empirical formula FW crystal size (mm3) crystal system space group a (Å) b (Å) c, (Å) β (deg) V (Å3) Z Dcalcd F(000) µ Mo KR (mm-1) temp (K) obsd reflns [I > 2σ(I)] params refined goodness of fit final R1 on obsd data final wR2 on obsd data
C15H30NO2.50 264.40 0.784 × 0.348 × 0.238 monoclinic C2/c 25.776(2) 8.4636(7) 18.9116(15) 131.1560(10) 3106.4(4) 8 1.131 1176 0.075 293(2) 1934 173 1.068 0.0336 0.0896
C16H31NO2 269.42 0.718 × 0.458 × 0.329 monoclinic P21/c 19.2547(15) 8.8429(7) 18.7850(14) 92.5820(10) 3195.2(4) 8 1.120 1200 0.072 293(2) 3806 345 1.036 0.0377 0.1024
C51H99N3O6 850.33 0.781 × 0.532 × 0.489 monoclinic P21/c 23.7670(16) 8.4456(6) 25.2049(17) 96.2880(10) 5028.9(6) 4 1.123 1896 0.072 293(2) 4574 544 1.001 0.0672 0.1717
C36H70N2O5 610.94 0.987 × 0.852 × 0.789 monoclinic P2/c 19.321(3) 8.6155(14) 23.943(4) 110.416(3) 3735.2(10) 4 1.086 1360 0.071 293(2) 4244 391 1.053 0.0525 0.1477
crystal data
6
8
9
14
15
empirical formula FW crystal size (mm3) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd F(000) µ Mo KR (mm-1) temp (K) obsd reflns [I > 2σ(I)] params refined goodness of fit final R1 on obsd data final wR2 on obsd data
C20H39NO2 325.52 0.539 × 0.390 × 0.299 orthorhombic Pbca 18.610 8.949 25.270
C44H86N2O4 707.15 0.281 × 0.083 × 0.053 monoclinic P21/n 19.004 8.358 27.425
C23H45NO2 367.60 0.658 × 0.432 × 0.316 monoclinic P21/c 14.754(8) 8.789(5) 19.241(11)
96.51
103.093(10)
4327.8 4 1.085 1584 0.067 100(2) 4761 453 1.029 0.0418 0.1051
2430(2) 4 1.005 824 0.062 293(2) 2012 236 1.112 0.1004 0.2496
C28H55NO2 437.73 0.048 × 0.043 × 0.035 triclinic P1h 5.594(6) 11.015(13) 23.69(3) 96.13(2) 90.55(2) 99.05(3) 1433(3) 2 1.014 492 0.062 293(2) 1116 281 0.816 0.1055 0.2411
C29H57NO2 451.76 0.065 × 0.062 × 0.058 triclinic P1h 5.642(2) 10.995(4) 24.187(9) 97.788(8) 90.533(8) 96.485(9) 1476.4(10) 2 1.016 508 0.061 293(2) 1913 290 1.019 0.0719 0.1505
4208.5 8 1.028 1456 0.064 293(2) 1566 209 1.054 0.1018 0.2644
Crystal Structure of Dicyclohexylammonium Pentanoate, 3. Salt 3 crystallizes in the centrosymmetric space group (monoclinic, P21/c). The asymmetric unit contains three ion pairs, all located on general positions. The ionic species are held together by hydrogen bonding between the nitrogen donor of the cation and the oxygen acceptor of the carboxylate anion (N‚‚‚O ) 2.675(2)-2.740(2) Å; ∠N-H‚‚‚O ) 159.1°-167.6°) resulting in a 0D cyclic network. Crystal Structure of Dicyclohexylammonium Hexanoate, 4. Two ion pairs and two water molecules are located in the asymmetric unit of salt 4 (monoclinic, P2/c). The oppositely charged ionic species are strongly held together by hydrogen bonding through N-H‚‚‚O interactions (N‚‚‚O ) 2.731(2)2.768(2) Å; ∠N-H‚‚‚O ) 159.6°-168.2°) resulting in a cyclic 0D network. Water molecules act as a bridge between the 0D cyclic networks of the ion pairs by O-H‚‚‚O hydrogen bonds (O‚‚‚O ) 2.808-2.822 Å) resulting in a 1D chain. Crystal Structure of Dicyclohexylammonium Octanoate, 6. Salt 6 crystallizes in a centrosymmetric space group (orthorhombic, Pbca), and its asymmetric unit contains one anionic moiety and one cationic moiety, located on general positions. The ionic species are held together by hydrogen bonding between the nitrogen donor of the cation and the oxygen
acceptor of the carboxylate anion (N‚‚‚O ) 2.718(4)-2.762(5) Å; ∠N-H‚‚‚O ) 166.0°-166.1°) resulting in a 0D cyclic network. Crystal Structure of Dicyclohexylammonium Decanoate, 8. Salt 8 crystallizes in the monoclinic P21/n space group. Two anionic and two cationic moieties are located in the asymmetric unit. Through N-H‚‚‚O interactions (N‚‚‚O ) 2.682(2)-2.754(2) Å; ∠N-H‚‚‚O ) 160.3°-169.1°), the ion pairs display a 0D cyclic network. Crystal Structure of Dicyclohexylammonium Undecanoate, 9. Salt 9 crystallizes in the centrosymmetric space group (monoclinic, P21/c). Its asymmetric unit contains one dicarboxylate ion and one ammonium ion, located on general positions. Through N-H‚‚‚O interactions (N‚‚‚O ) 2.716(5) Å; ∠N-H‚‚‚O ) 162.8°-167.4°), the ion pairs form a centrosymmetric 0D hydrogen-bonded network. Crystal Structure of Dicyclohexylammonium Hexadecanoate, 14. Salt 14 crystallizes in the centrosymmetric space group P1h. Both the anion and the cation are located on general positions in the asymmetric unit. The oppositely charged ionic species are strongly hydrogen-bonded with each other through N-H‚‚‚O (N‚‚‚O ) 2.700(7)-2.708(8) Å; ∠N-H‚‚‚O ) 165.7°-175.3°) interactions resulting in a 1D hydrogen-bonded chain.
1024 Crystal Growth & Design, Vol. 6, No. 4, 2006
Trivedi and Dastidar
Figure 2. 1D HBN observed in salt 15. Salt 14 displayed an identical network. Alkyl chains are shown in space filling model to emphasize the alkyl-alkyl interactions.
Figure 1. (a) 0D HBN observed in salt 9 (identical HBNs are observed in salts 1-4, 6, and 8 and (b, c) water-mediated bridging of 0D HBNs in salts 1 and 4 respectively.
Crystal Structure of Dicyclohexylammonium Heptadecanoate, 15. The asymmetric unit of salt 15 contains one anion moiety and one ammonium cation. The ion pair is located on the general position in triclinic space group P1h. The ionic species are held together by hydrogen bonding between the nitrogen donor of the cation and the oxygen acceptor of the carboxylate anion (N‚‚‚O ) 2.677(4)-2.720(4) Å; ∠N-H‚‚‚ O ) 162.0°-171.0°) resulting in a 1D hydrogen-bonded network. The majority of the crystal structures belong to monoclinic crystal system, except salts 6 (orthorhombic) and 14 and 15 (triclinic). While salts 1 and 4 are crystallized with water solvate, the rest of the structures are solvent-free. Analyses of the structures reveal that salts 1-4, 6, 8, and 9 display 0D networks (synthon B, Scheme 1) via N-H‚‚‚O hydrogen bonding (average N‚‚‚O ) 2.719 Å; average ∠N-H‚‚‚O ) 164.5°). Despite having solvate water molecules in the crystal lattice, salts 1 and 4 both display synthon B; water molecules act as a hydrogen bonding bridge between such 0D HBNs involving the ion pairs (Figure 1a-c). On the other hand, both the salts 14 and 15, having reasonably longer alkyl chains, crystallize in the same space group (P1h) and display identical 1D HBNs (synthon A, Scheme 1 and Figure 2). Thus it is clear that a supramolecular transition of 0D to 1D HBNs does take place while going from relatively shorter alkyl chains to longer alkyl chains in this series of salts. To address the key structural reasoning for such a transition, we examined both intra- and internetwork interactions. Because only the alkyl chain length is varied in the series of salts studied herein, we mainly concentrated our focus on the behavior of the alkyl groups in the crystal structures. It can be recognized that alkyl groups are oppositely oriented in the 0D network; thereby they unable to interact with each other within the network (intranetwork) (Figure 1a). However, that is not the case in 1D network; the alkyl groups are oriented on the same side of the 1D chain and interact significantly via hydrophobic interactions (Figure 2). It is interesting to note that the alkyl chains recognize each other via hydrophobic interactions in the 0D salts (Figure 3). In salts 3-9, the alkyl-alkyl interactions can be observed. Interdigitation of the internetwork alkyl chains appears to be more effective in the 1D salts (Figure 4). Thus,
Figure 3. Illustration of crystal structures depicting internetwork alkylalkyl interactions in 0D salts: (a) salt 1; (b) salt 2; (c) salt 3; (d) salt 4; (e) salt 6; (f) salt 8; (g) salt 9. Double-headed red arrows indicate alkyl-alkyl interactions.
both intra- and internetwork alkyl-alkyl interactions observed in salts 14 and 15 play a major role for a transition from 0D to 1D HBNs. Although we do not have the crystal structures of salts 1013, 16 and 17, remarkable resemblance of the XRPD patterns of these salts with those of 14 and 15 indicate that they might also have adopted 1D HBNs in their crystal structures (Figure 5). Remarkable differences of the XRPD patterns of 0D salts with those of 1D salts can also be recognized (Figure 5). It is also interesting to note that COO- FT-IR bands for the 0D salts 1-9 appear in the range of 1621-1625 cm-1, whereas those for 1D salts 10-17 fall in the range of 1639-1643 cm-1
Supramolecular Hydrogen Bond Isomerism
Figure 4. Illustration of the crystal structure of salt 14 depicting internetwork interdigitation of long alkyl chains. Interacting L-shaped ion pairs are shown in purple and orange (space filling model). Salt 15 showed an identical assembly.
Crystal Growth & Design, Vol. 6, No. 4, 2006 1025
Figure 6. SEM picture of the xerogel of salt 18 derived from (a) nitrobenzene solvent (bar 100 µm) and (b) diesel (bar 3 µm).
In summary, we have successfully demonstrated with the help of various crystal structures of a series of dicyclohexylammonium monocarboxylate salts having varying alkyl chain length that significant alkyl-alkyl interactions (both intra- and internetwork) lead to the formation of 1D HBNs (synthon A, Scheme 1) in salts 14 and 15. XRPD patterns also indicate that the transition of 0D to 1D HBNs takes place from salt 10 onward. The results reported herein should contribute significantly toward the development of supramolecular synthetic methods of producing 1D arrays of molecules, ions, etc., which is important in various materials applications including gels. Experimental Section
Figure 5. XRPDs of salts 1-17. Remarkable resemblance of the XRPDs of salts 10-13, 16, and 17 with those of salts 14 and 15 can be recognized. XRPDs of salts 1-9 are distinctly different from those of salts having n > 9; the XRPD of salt 6 could not be recorded due to its semisolid nature.
meaning that hydrogen-bonding interactions in the 0D network are comparatively stronger than those in the 1D salts. However, none of these salts show any gelation properties, which may be attributed to the noncompatibility of the 1D network and the typical organic solvents8 used for gelation experiments. At this stage, we thought it might be interesting to see the effect of an aromatic backbone instead of an alycylic backbone of the ammonium cation on the resultant material property (gelation). Thus, we prepared dibenzylammonium heptadecanoate 18, which is an analogous salt of 15 having aromatic cationic backbone. To our delight, salt 18 gave gels with diesel and nitrobenzene. The SEM micrographs of xerogels derived from nitrobenzene and diesel showed typical fibrous morphology within which the solvent molecules are understandably immobilized to form gel (Figure 6). Despite our best efforts, we are unable to get X-ray quality single crystals of salt 18. Thus, it is difficult at this stage to comment on the HBN in this salt.
Materials. All chemicals (Aldrich) and the solvents (A. R. grade, S. D. Fine Chemicals, India) for syntheses and gelation are used without further purification. Diesel used for gelation is procured from the local market. Microanalyses are performed on a Perkin-Elmer elemental analyzer 2400, series II. FT-IR data are recorded using Perkin-Elmer Spectrum GX. Scanning electron microscopy (SEM) is performed on a LEO 1430VP. Syntheses. The salts 1-17 were prepared by dissolving the acid component (1 mmol) in MeOH followed by addition of the corresponding amine (1 mmol). The mixture was then sonicated and kept at room temperature for evaporation. The white solid isolated after evaporation is used for further characterization and analyses. Salt 18 was prepared following an identical procedure using ethyl acetate as solvent.
Acknowledgment. P.D. thanks Ministry of Environment and Forests, New Delhi, India, for financial support. D.R.T. thanks CSIR, New Delhi, India, for a SRF. Supporting Information Available: Melting points and analytical and FT-IR data for salt 1-18, crystallographic information files (CIF), hydrogen-bonding parameters, and XRPDs. This material is available free of charge via the Internet at http://pubs.acs.org
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Trivedi and Dastidar Chem., Int. Ed. 2000, 39, 2263. (e) Sangeetha, N. M.; Maitra, U. Chem. Soc. ReV. 2005, 34, 821. (7) (a) Dastidar, P.; Okabe, S.; Nakano, K.; Iida, K.; Miyata.; Tohnai, N.; Shibayama, M. Chem. Mater. 2005, 17, 741. (b) Trivedi, D. R. Ballabh, A.; Dastidar, P. Chem. Mater. 2003, 15, 3971. (c) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Chem. Mater. 2003, 15, 2136. (d) Krishna Kumar, D.; Jose, D. A.; Das, A.; Dastidar, P. Chem. Commun. 2005, 4059. (e) Krishna Kumar, D.; Jose, D. A.; Dastidar, P.; Das, A. Langmuir 2004, 20, 10413. (f) Krishna Kumar, D.; Jose, D. A.; Dastidar, P.; Das, A. Chem. Mater. 2004, 16, 2332. (8) (a) Trivedi, D. R.; Ballabh, A.; Dastidar, P. J. Mater. Chem. 2005, 15, 2606. (b) Trivedi, D. R.; Ballabh, A.; Dastidar, P.; Ganguly, B. Chem.sEur. J. 2004, 10, 5311. (9) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Cryst. Growth Des. 2005, 5, 1545.
CG050674R
