Cation-Induced Supramolecular Isomerism in the Hydrogen-Bonded

Aug 2, 2006 - for gelation. The conformational flexibility of the dibenzyl cation and various intra- and internetwork C-H‚‚‚π and C-H‚‚‚O...
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CRYSTAL GROWTH & DESIGN

Cation-Induced Supramolecular Isomerism in the Hydrogen-Bonded Network of Secondary Ammonium Monocarboxylate Salts: A New Class of Organo Gelator and Their Structures

2006 VOL. 6, NO. 9 2114-2121

Darshak R. Trivedi and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar-364 002, Gujarat, India ReceiVed June 2, 2006; ReVised Manuscript ReceiVed July 3, 2006

ABSTRACT: A series of secondary ammonium monocarboxylate salts have been prepared by reacting variously substituted cinnamic acids and benzoic acids with dibenzylamine. Gelation tests reveal that 19 salts (9 cinnamates and 10 benzoates) are moderate to good gelators of various organic fluids, including commercial fuels such as gasoline and diesel fuel. Structure-property correlation studies based on single-crystal structures of 18 salts indicate that the one-dimensional hydrogen-bonded network is indeed important for gelation. The conformational flexibility of the dibenzyl cation and various intra- and internetwork C-H‚‚‚π and C-H‚‚‚O interactions appear to be responsible for the stabilization of the one-dimensional network in these salts. The gel fibrils in the xerogel state for 8 salts also adopt a 1D hydrogen-bonded network, as revealed by detailed X-ray powder diffraction studies, further supporting the importance of the one-dimensional network in the gelation process. Introduction Organic compounds (Mw < 1000) capable of arresting the flow of liquids (gel formation) are popularly known as low molecular mass organic gelators (LMOGs).1 LMOGs selfassemble into various types of aggregates such as fibers, strands, tapes, etc. in the gel state. Such aggregates are shown to crosslink among themselves through “junction zones”2 to form a 3D intertwined network of fibers within which the solvent molecules are immobilized, resulting in gels or viscous liquids. LMOGs have also been found to be used promisingly as structuredirecting agents (template) for making inorganic nanomaterials,3 in making microcellular materials,4 in a CO2-based coating process,4 in making dye-sensitized solar cells,5 in biomedical applications,6 etc. Therefore, studies on LMOGs have been an active research field in recent years in materials science and supramolecular chemistry. However, designing a gelator molecule is still a major challenge and most of the LMOGs reported thus far are either serendipitous or have been developed from a known gelator molecule. Moreover, making most of these gelators involves time-consuming nontrivial organic syntheses. Thus, the facile preparation of compounds as potential gelators is of utmost importance in order to find new and efficient gelling agents. To design a gelator molecule, it is important to understand the supramolecular architecture (crystal structure) of the metastable gel fiber in its native (gel) form. However, it is virtually impossible to determine the crystal structure of a gel fiber; only an indirect method using X-ray powder diffraction (XRPD) data may be applied.7 However, recording good-quality XRPD data of the gel fibers in its native form generally suffers from the scattering contribution of the solvent molecules and more poorly crystalline nature of the gel fibers and, therefore, in most of the cases attempts to record XRPD of gel fibers turn out to be a major disappointment. On the other hand, correlating the single-crystal structure of a molecule in its thermodynamically more stable crystalline state with its gelling/nongelling behavior seems to be more practical. We8 and others9 have shown, on the basis of a series of single* To whom correspondence should be addressed. Fax: +91-2782567562. E-mail: [email protected], [email protected].

Chart 1

crystal structures, that a 1D hydrogen-bonded network is important for gelation, whereas 0D (discrete cyclic), 2D, and 3D hydrogen-bonded networks are not as important. Therefore, we decided to work on compounds that might predictably aggregate in a 1D hydrogen-bonded network, as potential gelator molecules. Crystal engineering10sa powerful technique to gain control over many possible arrangements of molecules to produce solids (crystals) with desired structures and propertiess may be employed to generate new, efficient, easily prepared gelling agents. Out of many approaches to gain control over the arrangement of molecules in space, the incorporation of a small number of functional groups that can interact intermolecularly through noncovalent interactions (supramolecular synthon10) and, therefore, limit the possible arrangements of the molecules in the solid state with respect to one another, has been considered to be one of the most rational approaches. Thus, it is important to identify a suitable supramolecular synthon that might result in a 1D hydrogen-bonded network on self-assembly. We have recently shown that the supramolecular synthon approach is useful in designing new gelator molecules.11 In these studies,8b,c,11a we have shown that the single-crystal structures of various dicyclohexylammonium cinnamate salts show both of the plausible supramolecular networks, namely 0D and 1D (Chart 1); the corresponding benzoate salts display exclusively a 0D network, and the salts displaying a 1D hydrogen-bonded network show gelation properties with no exception whatsoever. It appeared to us that, in these salts, the cationic counterpart, namely the dicyclohexylammonium cation, is conformationally

10.1021/cg060325c CCC: $33.50 © 2006 American Chemical Society Published on Web 08/02/2006

Secondary Ammonium Monocarboxylate Salts

Crystal Growth & Design, Vol. 6, No. 9, 2006 2115

Chart 2

Figure 1. Plots of Tgel vs concentrations of the gelators. Numbers on the plots indicates the gelator numbers: 1, 4, and 7 represent mesitylene gel; 5, 8, 13, and 27 represent isooctane gel; 2 represents p-xylene gel.

rigid and does not offer nonbonded interactions other than hydrogen-bonding and dispersion forces. However, a conformationally more flexible aromatic analogue, namely the dibenzylammonium cation, which can also offer C-H‚‚‚π and π-π interactions in addition to hydrogen bonding, may be useful in imparting supramolecular isomerism in the hydrogen-bonded network with the hope of shifting the preference to a 1D hydrogen-bonded network exclusively over the 0D network in the corresponding cinnamate/benzoate salts. For this purpose, we have synthesized all of the dibenzylammonium analogues of dicyclohexylammonium cinnamate/benzoate salts studied previously by us.8b,c,11a This paper describes the preparation of these salts (Chart 2), their gelation properties, and structureproperty correlations based on single-crystal and powder X-ray diffraction data. It may be mentioned here that organic salt based LMOGs have become increasingly popular in recent years,12 since the preparation of such salts does not involve time-consuming nontrivial organic syntheses and because in a relatively short period of time many salts can be prepared and scanned for their gelation ability. Moreover, the supramolecular self-assembly in such salts is based on strong and directional hydrogen bonding as well as stronger but less directional electrostatic interactions between the cations and anions. Results and Discussion Gel Formation and Characterization. In a typical experiment, the gelator is dissolved in a suitable solvent with the aid of few drops of good solvent (MeOH for all the gelators except 11 (3-nitCIN), for which a mixture of MeOH/DMF was used) and heating. The solution is then cooled to room temperature under ambient conditions. The container (usually a test tube) is then inverted to examine the material’s deformity. If no deformation is observed, it is considered a gel. Table 1 gives the gelation data. Out of 14 cinnamate salts (including the hydrocinnamate 27) prepared, 9 salts are gelators, whereas 10 salts out of 13 benzoate

Figure 2. SEM micrographs of xerogels: (a) 27(HCIN) in isooctane, 2.0 wt % (bar 10 µm); (b) 7(4-MeCIN) in isooctane, 2.0 wt % (bar 10 µm); (c) 17(4-BrBEN) in cyclohexane, 10.0 wt % (bar 20 µm); (d) 24(3-NitBEN) in 1,2-dichlorobenzene, 10 wt % (bar 20 µm).

salts studied display gelation ability. These results are significant when compared with our previously reported results,8b,c,11a wherein dicyclohexylammonium hydrocinnamate and the corresponding benzoate salts did not show any gelation properties. On the other hand, most of the cinnamates, including hydrocinnamate and benzoate salts of dibenzylamine in the present study, display gelation ability. It is significant to note that quite a few dibenzylammonium cinnamate/benzoate salts (in the present study) also show the ability to harden commercial fuels such as gasoline and diesel fuel. The gel dissociation temperature Tgel as a function of gelator concentration of some of the cinnamate salts has been tested (Figure 1). A steady increase in Tgel with an increase in concentration of the gelator indicates that strong intermolecular interactions are responsible for the self-assembly in the gel state. SEM micrographs of the xerogels of most of the gelators display a typical intertwined network of fibers, within which the solvent molecules are understandably immobilized in the gelled state. Figure 2 displays representative micrographs of xerogels derived from some cinnamate and benzoate gelators. Intertwined fibers having lengths more than 100 µm and widths varying from sub-micrometer to a few micrometers are seen in

2116 Crystal Growth & Design, Vol. 6, No. 9, 2006

Trivedi and Dastidar Table 1. Gelation Dataa

1(4-ClCINN) sr no. 1 2 3 4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21

solvent CCl4 cyclohexane n-heptane isooctane gasolineb diesel fuelb benzene toluene chlorobenzene bromobenzene o-xylene m-xylene p-xylene mesitylene 1,2-dichlorobenzene DMF nitrobenzene methyl salicylate ethyl acetate DMSO

1 2 3 4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20

solvent CCl4 cyclohexane n-heptane isooctane gasolineb diesel fuelb benzene toluene chlorobenzene bromobenzene o-xylene m-xylene p-xylene mesitylene 1,2-dichlorobenzene DMF nitrobenzene methyl salicylate ethyl acetate

4(4-BrCINN)

5(3-BrCINN)

7(4-MeCINN)

8(3-MeCINN)

11(3-NitroCINN)

13(CIN)

27(HCIN)

MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C ppt 2.11 1.89 2.36 1.94 1.14 FC 3.47 2.62 FC ppt 3.20 2.59 3.08 2.08 FC VL 2.02

78 65 70 75

1.97 1.59 2.11 1.38 2.12 ppt 13.34 3.52 FC FC 3.35 3.84 3.24 VL L

64

L L L

L VL ppt

ppt L

3.13 L

65 81 90 84 86 68 72

ppt L 14(4-ClBEN)

sr no.

2(3-ClCINN)

62 70 65 82 63 62 57 50 76 75

15(3-ClBEN)

FC 1.16 0.93 1.00 1.49 2.93 3.00 3.14 ppt ppt 2.74 2.68 2.76 2.61 VL

56 69 71 70 70 66 72 72 79 86 68

73

16(2-ClBEN)

VL ppt 1.89 ppt ppt ppt ppt ppt ppt ppt ppt ppt VL ppt

74

ppt 1.12 1.33 0.94 2.87 2.75 ppt VL L L VL VL 2.33 2.18 L

68 73 82 71 90

57 72

C 1.41 1.42 1.49 VL 2.39 ppt FC L L ppt ppt L ppt L

53 61 71 73

FC FC FC FC FC FC ppt 2.28 FC FC VL VL VL FC 2.07

57

58

ppt VL ppt 1.35 FC FC ppt ppt ppt ppt L L L L L

64

ppt FC 1.32 1.39 ppt ppt ppt ppt L L L L L L L

66 60

ppt ppt ppt

L L L

L L FC

L L L

L L L

L L L

ppt ppt

PPT L

ppt L

FC L

ppt L

FC L

20(4-MeBEN)

22(2-MeBEN)

23(4-NitBEN)

17(4-BrBEN)

18(3-BrBEN)

19(2-BrBEN)

MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, MGC, Tgel, wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C wt % °C ppt 10.73 C C 10.02 FC FC FC FC FC L VL VL VL FC

83 77

ppt 9.61 FC FC 18.97 9.91 FC C FC FC C C C C FC

72 65 82

ppt 9.35 FC FC 12.17 FC FC 12.54 FC FC FC 9.31 FC 8.97 FC

83 69 68

68 75

ppt 6.83 FC FC 10.32 8.90 FC FC FC FC FC VL FC 8.69 FC

83 90 74

76

ppt 8.60 FC FC ppt 8.62 FC FC L L FC FC 8.60 FC L

66

74

72

ppt 7.39 FC FC 13.06 ppt C FC L L L L FC FC L

58 72

FC 7.75 FC FC FC ppt FC C FC FC FC FC FC FC L

72

ppt 8.43 FC FC FC 8.78 ppt FC L L L L L L L

78

86

FC ppt ppt ppt ppt VL FC 9.94 9.39 FC FC VL FC VL 8.86

L L VL

L L L

L L L

L FC L

L L L

L L L

L L L

L L L

L L L

C

FC

FC

FC

FC

FC

FC

FC

ppt

91 100

80

wt % ) g/100 mL of solvent. Abbreviations: MGC ) minimum gelator concentration at room temperature; Tgel ) gel-sol dissociation temperature; FC ) fibrous crystal; VL ) viscous liquid; L ) liquid; ppt ) precipitate. b In g/100 g of solvent. a

these micrographs which are typical of the morphologies of many of the xerogels derived from LMOGs. Structure-Property Correlation. The present work is undertaken in order to attempt a structure-property correlation so that the question “Is a 1D hydrogen-bonded network important for gelation?” can be addressed. For this purpose, we have tried to crystallize all the salts so that the supramolecular networks in the corresponding crystal structures can be correlated with their properties (gelling/nongelling). Out of 14 cinnamate salts, suitable single crystals of 6 salts for X-ray diffraction studies are obtained from suitable solvents (see the Experimental Section). On the other hand, out of 13 benzoate salts that we have prepared, 12 salts could be crystallized for single-crystal X-ray diffraction studies. Table 2 gives the crystallographic parameters of these salts. Space groups of the cinnamate salts are evenly distributed among three different

crystal systems, namely triclinic, monoclinic, and orthorhombic, whereas the majority of the benzoate salts belongs to monoclinic space groups (Table 2). The majority of the salts show one ion pair in the asymmetric unit, except for the salts 3(2-ClCIN), 16(2-ClBEN), 19(2BrBEN), and 24(3-NitBEN), wherein two ion pairs are seen in the asymmetric unit. It is interesting to note that the dibenzylammonium cation shows a syn-anti conformation in the majority of the structures, displaying corresponding C-CN-C torsion angles ranging from 54.2 to 88.3° and from 169.1 to 179.46° (Chart 3). In the salts 3(2-ClCIN), 16(2-ClBEN), 19(2-BrBEN), and 25(2-NitBEN), the conformation of the cation is anti-anti, displaying corresponding C-C-N-C torsion angles ranging from 175.9 to 179.2°. The most striking feature of the crystal structures of these salts is the presence of a 1D hydrogen-bonded network involving

Secondary Ammonium Monocarboxylate Salts

Crystal Growth & Design, Vol. 6, No. 9, 2006 2117

Table 2. Crystallographic Parameters of the Salts 3(2-ClCIN)

6(2-BrCIN)

7(4-MeCIN)

8(3-MeCIN)

9(2-MeCIN)

11(3-NitCIN)

empirical formula fw cryst size (mm) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) F(000) µ(Mo KR) (mm-1) temp (K) no. of obsd rflns (I > 2σ(I)) no. of params refined goodness of fit final R1 on obsd data final wR2 on obsd data

C23H23ClNO2.50 388.87 0.35 × 028 × 0.19 triclinic P1 8.552(3) 10.137(3) 12.372(4) 85.680(5) 78.101(5) 87.955(5) 1046.3(5) 2 1.234 410 0.202 293(2) 3302 520 1.067 0.0428 0.1229

C46H44Br2N2O4 848.65 0.49 × 0.26 × 0.21 orthorhombic Pbca 22.4126(17) 16.6990(13) 10.7237(8)

C24H25NO2 359.45 0.15 × 0.11 × 0.08 monoclinic P21/n 11.4535(17) 6.0276(9) 28.959(4)

C24H25NO2 359.45 0.25 × 0.12 × 0.06 monoclinic P21/c 5.959(2) 28.662(10) 11.742(4)

C24H25NO2 359.45 0.48 × 0.38 × 0.21 orthorhombic Pbca 10.7493(10) 22.797(2) 16.6738(16)

95.913(2)

90.291(8)

1988.6(5) 4 1.201 768 0.076 293(2) 2281 344 1.214 0.0639 0.1337

2005.5(12) 4 1.191 768 0.075 293(2) 758 245 0.757 0.0950 0.1146

C23H22N2O4 390.43 0.21 × 0.10 × 0.08 triclinic P1h 6.030(2) 11.418(4) 15.165(6) 102.599(7) 96.607(7) 90.049(9) 1011.8(6) 2 1.281 412 0.088 293(2) 1702 350 1.022 0.0565 0.1298

14(4-ClBEN)

15(3-ClBEN)

16(2-ClBEN)

17(4-BrBEN)

18(3-BrBEN)

19(2-BrBEN)

empirical formula fw cryst size (mm) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm-3) F(000) µ(Mo KR) (mm-1) temp (K) no. of obsd rflns (I > 2σ(I)) no. of params refined goodness of fit final R1 on obsd data final wR2 on obsd data

C21H20ClNO2 353.83 0.71 × 0.45 × 0.23 monoclinic P21/n 11.2663(6) 8.9435(5) 18.7571(10) 105.9260(10) 1817.42(17) 4 1.293 744 0.224 293(2) 2272 306 1.055 0.0315 0.0826

C21H20ClNO2 353.83 0.48 × 0.37 × 0.21 monoclinic P21/c 10.5042(8) 9.2210(7) 19.4703(15) 97.3840(10) 1870.2(2) 4 1.257 744 0.217 293(2) 2147 306 1.049 0.0518 0.1475

C42H40Cl2N2O4 707.66 0.47 × 0.31 × 0.26 monoclinic P21/n 8.5885(5) 20.7466(12) 21.5058(13) 100.1720(10) 3771.7(4) 4 1.246 1488 0.216 293(2) 3915 611 1.111 0.0566 0.1279

C21H20BrNO2 398.29 0.57 × 0.38 × 0.19 orthorhombic Pbca 8.8752(10) 17.898(2) 24.410(3)

C21H20BrNO2 398.29 0.58 × 0.39 × 0.23 monoclinic P21/c 10.4652(13) 9.2705(12) 19.788(3) 97.522(2) 1903.3(4) 4 1.390 816 2.173 293(2) 1983 306 1.057 0.0683 0.1785

C42H36Br2N2O4 792.55 0.48 × 0.23 × 0.40 monoclinic P21/n 8.5473(18) 21.118(4) 21.555(5) 99.894(4) 3832.8(14) 4 1.373 1616 2.158 293(2) 2272 409 0.926 0.0894 0.2523

20(4-MeBEN)

21(3-MeBEN)

23(4-NitBEN)

24(3-NitBEN)

25(2-NitBEN)

26(BEN)

C88H92N4O8 1333.66 0.58 × 0.43 × 0.31 monoclinic P21/c 11.2369(9) 8.9082(7) 19.2612(17) 106.6240(10) 1847.5(3) 1 1.199 712 0.076 293(2) 2205 318 1.035 0.0290 0.0801

C22H23NO2 333.41 0.48 × 0.37 × 0.29 monoclinic P21/c 10.5691(13) 9.3135(11) 18.974(2) 99.028(2) 1844.6(4) 4 1.201 712 0.076 293(2) 2014 318 1.007 0.0353 0.0915

C21H20N2O4 364.39 0.40 × 0.34 × 0.28 monoclinic P21/c 11.6184(9) 8.7812(7) 19.5406(14) 107.604(4) 1900.2(3) 4 1.274 768 0.089 293(2) 2130 324 1.034 0.0374 0.0985

C42H40N4O8 728.78 0.36 × 0.28 × 0.18 orthorhombic Pca21 28.927(3) 5.9980(6) 21.499(2)

C21H20N2O4 364.39 0.57 × 0.34 × 0.29 monoclinic Cc 20.764(12) 9.877(6) 9.959(6) 106.380(9) 1960(2) 4 1.235 768 0.086 293(2) 2105 324 1.118 0.0340 0.0823

C21H21NO2 319.39 0.47 × 0.33 × 0.18 monoclinic P21/c 10.5943(8) 8.9347(7) 19.0320(15) 102.6070(10) 1758.1(2) 4 1.207 680 0.077 293(2) 2144 301 1.046 0.0297 0.0790

empirical formula fw cryst size (mm) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm-3) F(000) µ(Mo KR) (mm-1) temp (K) no. of obsd rflns (I > 2σ(I)) no. of params refined goodness of fit final R1 on obsd data final wR2 on obsd data

4013.5(5) 4 1.404 1744 2.066 293(2) 2426 332 1.043 0.0264 0.0655

the ion pair, except in the salt 3(2-ClCIN), wherein participation of solvate water molecules in hydrogen bonding prevents the formation of a 1D hydrogen-bonded network; the corresponding N‚‚‚O distances and ∠N-H‚‚‚O angles are within the ranges of 2.609(2)-2.870(4) Å and 159.0(2)-180.0(3)°, respectively (Figure 3). The exclusive formation of a 1D hydrogen-bonded network in these salts is quite remarkable and is in contrast to

3877.4(8) 8 1.365 1632 2.133 293(2) 2152 306 1.065 0.0380 0.0968

3730.2(6) 4 1.298 1536 0.091 293(2) 4210 647 1.092 0.0320 0.0797

4085.9(7) 8 1.169 1536 0.074 293(2) 2174 344 1.142 0.0483 0.1152

our previously studied structures of dicyclohexylammonium cinnamate/benzoate salts,8b,c,11a wherein all of the benzoate and a few cinnamate salts displayed a 0D network. It may be seen in Figure 3 that the acid moieties in the cinnamate salt 7(4-MeCIN) are oriented on the same side of the propagation axis of 1D network, whereas the reverse is true in the case of the benzoate salt 20(4-MeBEN). The same trend

2118 Crystal Growth & Design, Vol. 6, No. 9, 2006

Trivedi and Dastidar

Chart 3

is observed in all the cinnamate and benzoate salts reported herein, except in 24(3-NitBEN), which follows the trend of cinnamate salts. It is quite significant that, out of 27 dibenzylammonium salts presently studied, 19 of them turned out to be moderate to good gelators and all the gelator salts display a 1D hydrogen-bonded network in their crystal structures. These results clearly indicate that presence of a 1D hydrogen-bonded network in the crystal structure indeed plays a significant role in the gelation process. The cinnamate salts 6(2BrCIN) and 9(2MeCIN) and benzoate salts 21(3MeBEN), 25(2NitBEN), and 26(BEN), however, show no gelation ability with the solvents studied herein, despite having a 1D network in their crystal structures. Since solvents are immobilized in the intertwined network of fibers in the gelled state via surface tension or capillary force action, it is important to have the surface compatibility of the typical solvents used in the present study and the gel fibers. Thus, the most probable reason for the aforementioned cinnamate and benzoate salts (6, 9, 21, 25, and 26) being nongelators may be the lack of such compatibility. It is also significant to note that we have not come across any example in the present study or in our previous studies8b,c,11a that shows gelation ability despite having a 0D (cyclic) network in the crystal structure. What causes the dibenzylammonium cinnamate/benzoate salts to have a 1D network exclusively in their crystal structures? It may be noted in this context that the corresponding dicyclohexylammonium cinnamate/benzoate salts studied earlier by us8b,c,11a showed both 1D and 0D networks. This is an important question to be addressed in order to gain further insights into the structural aspect of designing new LMOGs. While the dicyclohexylammonium cation is conformationally rigid, having alicyclic substitutents, it is unable to offer secondary interactions such as C-H-π, π-π, etc. On the other hand, its dibenzylammonium counterpart is conformationally more flexible, allowing free N-C rotation, and is able to offer C-H-π and π-π interactions. There are only two ways a secondary ammonium monocarboxylate ion pair can self-assembles1D and 0D, as already discussed. Thus, preference toward one particular network (1D

Figure 3. Crystal structure illustrations depicting the 1D hydrogenbonded network in dibenzylammonium cinnamate/benzoate salts: (a) the salt 7(4-MeCIN); (b) the salt 20(4-MeBEN). Identical 1D hydrogenbonding networks are observed in the rest of the salts, except for 3(2-ClCIN).

Figure 4. Scatter plot generated using C-H‚‚‚π distances and ∠CH‚‚‚π angles observed in the crystal structures of the dibenzylammonium salts reported in this study. The H‚‚‚π distance is the distance between the interacting hydrogen atom and the centroid of the interacting aromatic ring.

or 0D) may be dependent on other secondary interactions, such as C-H‚‚‚π, π-π, C-H‚‚‚O, halogen‚‚‚halogen, and C-H‚‚‚halogen, that a particular system might offer. An analysis of the presence and absence of these secondary interactions in dibenzylammonium (present study) and dicyclohexylammonium salts (previous study8b,c,11a) showed that both C-H‚‚‚π and C-H‚‚‚O interactions are the most significant secondary interactions in dibenzylammonium salts. It is observed that most of the dibenzylammonium salts display C-H‚‚‚π interactions except in the salts 11(3-NitCIN) and 18(3-BrBEN), whereas none of the corresponding dicyclohexylammonium salts8b,c,11a display any C-H‚‚‚π interactions. A scatter plot of C-H‚‚‚π distances and ∠C-H‚‚‚π angles observed in these salts show that the C-H-π distances vary from ∼2.7 to 3.5 Å and the ∠C-H‚‚‚π angles are within the range of ∼135-168°, which are well within the accepted range of C-H‚‚‚π interactions13 (Figure 4). While salts 6-9, 14, 16, 19-20, and 23 display internetwork C-H‚‚‚π interactions, salts 15, 17, 21, and 24-26 show intranetwork C-H‚‚‚π interactions (Figure 5). On the other hand, intranetwork C-H‚‚‚O interactions are present exclusively in all the dibenzylammonium salts, except for the salts 16(2-ClBEN) and 25(2-NitBEN), in which internetwork C-H‚‚‚O interactions are also present, in addition to intranetwork interactions. Although it cannot be concluded with certainty that C-H‚‚‚π interactions are mainly responsible for the exclusive 1D network in these salts, their contribution toward inducing such supramolecular hydrogen bond isomerism cannot be ruled out. The results presented here and the results we have recently reported8b,c,11a clearly indicate that a 1D hydrogen-bonded network in the thermodynamically more stable crystal form is an important factor, although it may not be a “necessary and sufficient” factor for a molecule to show gelation ability. It must be emphasized that the crystal structure (crystalline phase) of the gel fibril in the native (gel) state and that of the compound under study in its theromodynamically more stable crystalline state need not necessary be identical. To see whether the crystal phases of the gel fibril in the xerogel state and in the crystalline state are identical or not,

Secondary Ammonium Monocarboxylate Salts

Figure 5. Representative example of C-H‚‚‚π interactions in the salts studied here: (a) internetwork C-H‚‚‚π interactions in the salt 6(2BrCIN); (b) intranetwork C-H‚‚‚π interactions in the salt 15(3ClBEN).

Figure 6. XRPDs of the salt 16(2-ClBEN) under various conditions, The xerogel sample was prepared from a 10 wt % cyclohexane gel.

detailed X-ray powder diffraction (XRPD) studies have been undertaken. In this study, XRPDs of the gel fibrils in the xerogel state and bulk solid and XRPDs obtained by simulating singlecrystal X-ray data (wherever available) are compared. The results show that all three XRPD patterns in the salts 7(4-MeCIN), 8(3-MeCIN), 14(4-ClBEN), 15(3-ClBEN), 16(2-ClBEN), 17(4-BrBEN), 19(2-BrBEN), and 24(3-NitBEN) are virtually superimposable, meaning that the single-crystal structures of the salts represent their bulk solid’s crystalline phase and crystal structures of the gel fibrils in xerogel states are identical with those obtained from single-crystal data; one such comparison plot for salt 16(2-ClBEN) is shown in Figure 6. The rest of the plots are given in the Supporting Information (Figure S1).

Crystal Growth & Design, Vol. 6, No. 9, 2006 2119

For the salt 11(3-NitCIN), however, both the bulk solid and xerogel appear to be less crystalline, making it difficult to determine the corresponding crystalline phases. The salt 18(3BrBEN) displays different XRPDs for the simulation, bulk solid, and xerogel (Figure S2, Supporting Information); this means that the single crystal that is analyzed does not represent the bulk solid’s crystal phase, which, in turn, is not identical with the crystal phase of the fibrils of the xerogel. For the salts 20(4-MeBEN) and 23(4-NitBEN), the XRPDs of the bulk solid and xerogel are almost superimposable (Figure S3, Supporting Information). However, the simulated patterns do not match those of the bulk solid and xerogel, indicating that the single crystals obtained for analyses do not represent the crystalline phases of the bulk solid as well as the xerogels. These results clearly indicate that the majority of the gelator salts for which single crystal structures are available display the same crystal structures for the xerogel fibers as for the single-crystal state. Thus, the crystal structures of the gel fibrils of these gelators in the xerogel state have been determined in an indirect manner and they indeed display 1D hydrogen-bonded networks. It may be mentioned here that a single crystal of a gelator is extremely difficult to grow and it is even more difficult to grow from its gelling solvent.14 Gelator crystals are, therefore, often crystallized from nongelling solvents. Consequently, crystal phase mismatches among single crystals, bulk solids and xerogels cannot be ruled out and the salts 18(3-BrBEN), 20(4-MeBEN), and 23(4-NitBEN) provide examples of such a situation. It may be pointed out here that crystalline phases of the gel fibers in the native gel state and xerogel state need not necessarily be identical because of the possibilities of having phase transitions triggered by a new nucleation event generated from some amount of dissolved gelator compound in the solvent during the solvent removal process of xerogel formation. There is no certainty that such a phase transition does occur during xerogel formation, but there is no assurance that it does not. Efforts to record XRPD patterns in the gel state have been proven unsuccessful, presumably due to strong scattering of the solvent molecules. Thus, it is not possible to comment on the crystal structure of the fibers in the gel state. On the other hand, the gelator salts 1(4-ClCIN), 22(2MeBEN), and 27(HCIN), for which no single-crystal structures are available, display reasonable matches of the XRPDs of their bulk solids with those of the xerogels, meaning that gel fibrils in the xerogel state retain the same crystal phase of the bulk solid (Figure S4, Supporting Information). However, the salts 2(3-ClCIN), 4(4-BrCIN), and 13(CIN), for which no crystal structures are available, display mismatches of the XRPDs of their bulk solids and xerogels, meaning that a crystal phase transition has taken place during xerogel formation (Figure S5, Supporting Information). The the xerogel of the gelator salt 5(3-BrCIN) is found be less crystalline, making it difficult to determine. Although the foregoing discussion on the crystalline phases of the material in various states seems to suffer from crystal phase transitions at times, local supramolecular architectures, e.g. the 1D hydrogen-bonded networks of the ion pairs in the present study, may remain identical while their packings in the crystal structures (crystal phase) might be different in different states. It is quite logical to think that the responsible driving forces for the growth of fibrils in the fibril axis direction and perpendicular to it must be different, since the growth in the former direction has to be faster than that of in the latter direction in order to have fiber morphology as observed in SEM micrographs of the xerogels. A 1D hydrogen-bonded network

2120 Crystal Growth & Design, Vol. 6, No. 9, 2006

thus might play a significant role in the elongated growth of the fibril in one direction, while interactions with the solvent may prevent or reduce the growth perpendicular to the fibril axis. Conclusions A series of dibenzylammonium cinnamate/benzoate salts have been prepared on the basis of the supramolecular synthon approach. Out of 27 salts prepared, 9 cinnamate and 10 benzoate salts have been found to be moderate to good organo gelators; a few of them are even capable of hardening commercial fuels such as gasoline and diesel fuel. All of the salts (6 cinnamate and 12 benzoate) for which single-crystal structures could be determined display a 1D hydrogen-bonded network; 12 of them (3 cinnamate and 9 benzoate) are gelators, emphasizing the importance of a 1D hydrogen-bonded network for gel formation. Crystal structure analyses of all these salts indicate that the conformationally flexible geometry of the cation and both intraand internetwork C-H‚‚‚π and C-H‚‚‚O secondary interactions may contribute toward inducing such supramolecular hydrogen bond isomerism in these salts. Crystal structures of the gel fibers in the xerogel state for 8 gelator salts, which were determined by comparing the XRPDs of the xerogel and simulated XRPDs obtained from the corresponding single-crystal data, revealed the presence of a 1D hydrogen-bonded network. It may be noted that the corresponding cinnamate/benzoate salts of dicyclohexylamine previously studied by us8b,c,11a provided only a few gelators; none of the benzoate salts showed any gelation properties. Moreover, all the benzoate salts displayed a 0D network. In the present study, however, the introduction of a conformationally flexible aromatic analogue of dicyclohexylamine results in the formation of 19 gelators out of 27 salts that we have studied and all the crystal structures displayed a 1D hydrogen-bonded network. Thus, subtle changes in the cationic species produce a profound effect on the resultant supramolecular structures and properties. We believe these results are important in the context of designing new gelling agents on the basis of a crystal engineering approach. Experimental Section Materials and Physical Measurements. All reagents (Aldrich) and the solvents used for gelation (AR grade, S. D. Fine Chemicals, India) were used without further purification. All of the oils were procured from local sources. Microanalyses were performed on a Perkin-Elmer 2400 elemental analyzer, Series II. FT-IR and 1H NMR spectra were recorded using Perkin-Elmer Spectrum GX and 200 MHz Bruker Avance DPX200 spectrometers, respectively. X-ray powder diffraction patterns were recorded on an XPERT Philips (Cu KR radiation) diffractometer. Scanning electron microscopy (SEM) was performed with a LEO 1430VP instrument. Syntheses. (a) Salts 1 and 4. A solution of the corresponding acid (1.0 mmol) in hot nitrobenzene was prepared with the aid of few drops of MeOH. To this solution was slowly added dibenzylamine (1.0 mmol), and the reaction mixture was kept at room temperature. After a few hours, the salts 1 and 4 as white precipitates were isolated by filtration (near-quantitative yield) and used for gelation and other studies. (b) Salts 10-12. A solution of the corresponding acid (1.0 mmol) in hot DMF was prepared with the aid of a few drops of MeOH. To this solution was slowly added dibenzylamine (1.0 mmol), and the reaction mixture was kept at room temperature. After a few hours, the resulting salts as precipitates (near-quantitative yield) were used for gelation and other studies. (c) Salts 2, 3, 5-9, and 13-27. The corresponding acid (1 mmol) was dissolved in MeOH by sonication. Dibenzylamine (1.0 mmol) was added slowly to the methanolic solution of the acid at room temperature. The reaction mixture was then evaporated to dryness at room temper-

Trivedi and Dastidar ature. The resulting salts as precipitates (near-quantitative yield) were used for gelation and other studies. Single-Crystal X-ray Diffraction. X-ray-quality single crystals were grown under slow evaporative conditions at room temperature. The corresponding salts were dissolved in the crystallizing solvent with the aid of a few drops of MeOH in most of the cases. For salt 11 DMF was used. Crystals of 16, 18, 19, and 23 were grown from benzene. Salts 3, 6, 14, 15, 17, 20, 21, and 26 were crystallized from toluene. 8 was crystallized from carbon tetrachloride. 7 was crystallized from acetonitrile. 24 was crystallized from p-xylene. Salt 25 was crystallized from 1,2-dichlorobenzene. Salts 9 and 11 were crystallized from n-heptane. Diffraction data were collected using Mo KR (λ ) 0.7107 Å) radiation on a Bruker AXS SMART APEX CCD diffractometer. All calculations were performed by using the software package of the SMART APEX instrument. All structures were solved by direct methods and refined in a routine manner. In all cases except 19, non-hydrogen atoms were treated anisotropically. In 19, the aromatic ring of one of the acid moieties in the asymmetric unit was found to be disordered and was refined isotropically with the constraint of a regular hexagon. The hydrogen atoms attached to nitrogen were located in most of the cases, except for salts 8 and 19, on a difference Fourier map and refined. Whenever possible, the other hydrogen atoms were located on a difference Fourier map and refined. In the rest of the cases, the hydrogen atoms were geometrically fixed. Gel to Sol Dissociation Temperature (Tgel) Measurement. Tgel was measured by using the following method. A 1.0 mL portion of the gel was prepared in a test tube. A locally made glass ball weighing 0.19 g was placed on the gel surface. The test tube was then heated in an oil bath. The temperature (Tgel) was noted when the ball fell to the bottom of the test tube.

Acknowledgment. The Ministry of Environment and Forests, New Delhi, India, is gratefully acknowledged for financial support. D.R.T. thanks the CSIR for an SRF fellowship. Supporting Information Available: Text, tables, and figures giving melting point, analytical, FT-IR, and 1H NMR data, hydrogen-bonding parameters, and XRPD patterns for the salts and CIF files giving data for the single-crystal structure determinations. This material is available free of charge via the Internet at http://pubs.acs.org.

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