Cation-Induced Supramolecular Isomerism in the Hydrogen-BondedNetwork of Secondary Ammonium Monocarboxylate Salts: A NewClass of Organo Gelator and Their Structures

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 cinnamicacids and benzoic acids with dibenzylamine. Gelation tests reveal that 19 salts (9 cinnamates and 10 benzoates) are moderate togood gelators of various organic fluids, including commercial fuels such as gasoline and diesel fuel. Structure-property correlationstudies based on single-crystal structures of 18 salts indicate that the one-dimensional hydrogen-bonded network is indeed importantfor gelation. The conformational flexibility of the dibenzyl cation and various intra- and internetwork C-H‚‚‚π and C-H‚‚‚Ointeractions appear to be responsible for the stabilization of the one-dimensional network in these salts. The gel fibrils in the xerogelstate for 8 salts also adopt a 1D hydrogen-bonded network, as revealed by detailed X-ray powder diffraction studies, further supportingthe importance of the one-dimensional network in the gelation process.

Introduction

Organic compounds (Mw < 1000) capable of arresting theflow of liquids (gel formation) are popularly known as lowmolecular mass organic gelators (LMOGs).1 LMOGs self-assemble into various types of aggregates such as fibers, strands,tapes, etc. in the gel state. Such aggregates are shown to cross-link among themselves through “junction zones”2 to form a 3Dintertwined network of fibers within which the solvent moleculesare immobilized, resulting in gels or viscous liquids. LMOGshave also been found to be used promisingly as structure-directing agents (template) for making inorganic nanomaterials,3

in making microcellular materials,4 in a CO2-based coatingprocess,4 in making dye-sensitized solar cells,5 in biomedicalapplications,6 etc. Therefore, studies on LMOGs have been anactive research field in recent years in materials science andsupramolecular chemistry.

However, designing a gelator molecule is still a majorchallenge and most of the LMOGs reported thus far are eitherserendipitous or have been developed from a known gelatormolecule. Moreover, making most of these gelators involvestime-consuming nontrivial organic syntheses. Thus, the facilepreparation of compounds as potential gelators is of utmostimportance in order to find new and efficient gelling agents.

To design a gelator molecule, it is important to understandthe supramolecular architecture (crystal structure) of the meta-stable gel fiber in its native (gel) form. However, it is virtuallyimpossible to determine the crystal structure of a gel fiber; onlyan indirect method using X-ray powder diffraction (XRPD) datamay be applied.7 However, recording good-quality XRPD dataof the gel fibers in its native form generally suffers from thescattering contribution of the solvent molecules and more poorlycrystalline nature of the gel fibers and, therefore, in most ofthe cases attempts to record XRPD of gel fibers turn out to bea major disappointment.

On the other hand, correlating the single-crystal structure ofa molecule in its thermodynamically more stable crystalline statewith its gelling/nongelling behavior seems to be more practical.We8 and others9 have shown, on the basis of a series of single-

crystal structures, that a 1D hydrogen-bonded network isimportant for gelation, whereas 0D (discrete cyclic), 2D, and3D hydrogen-bonded networks are not as important. Therefore,we decided to work on compounds that might predictablyaggregate in a 1D hydrogen-bonded network, as potential gelatormolecules. Crystal engineering10sa powerful technique to gaincontrol over many possible arrangements of molecules toproduce solids (crystals) with desired structures and propertiessmay be employed to generate new, efficient, easily preparedgelling agents. Out of many approaches to gain control overthe arrangement of molecules in space, the incorporation of asmall number of functional groups that can interactintermo-lecularly through noncovalent interactions (supramolecularsynthon10) and, therefore, limit the possible arrangements of themolecules in the solid state with respect to one another, hasbeen considered to be one of the most rational approaches. Thus,it is important to identify a suitable supramolecular synthon thatmight result in a 1D hydrogen-bonded network on self-assembly.

We have recently shown that the supramolecular synthonapproach is useful in designing new gelator molecules.11 In thesestudies,8b,c,11awe have shown that the single-crystal structuresof various dicyclohexylammonium cinnamate salts show bothof the plausible supramolecular networks, namely 0D and 1D(Chart 1); the corresponding benzoate salts display exclusivelya 0D network, and the salts displaying a 1D hydrogen-bondednetwork show gelation properties with no exception whatsoever.

It appeared to us that, in these salts, the cationic counterpart,namely the dicyclohexylammonium cation, is conformationally

* To whom correspondence should be addressed. Fax:+91-278-2567562. E-mail: dastidar@csmcri.org, parthod123@rediffmail.com.

Chart 1

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

rigid and does not offer nonbonded interactions other thanhydrogen-bonding and dispersion forces. However, a confor-mationally more flexible aromatic analogue, namely the diben-zylammonium cation, which can also offer C-H‚‚‚π andπ-πinteractions in addition to hydrogen bonding, may be useful inimparting supramolecular isomerism in the hydrogen-bondednetwork with the hope of shifting the preference to a 1Dhydrogen-bonded network exclusively over the 0D network inthe corresponding cinnamate/benzoate salts. For this purpose,we have synthesized all of the dibenzylammonium analoguesof dicyclohexylammonium cinnamate/benzoate salts studiedpreviously by us.8b,c,11aThis paper describes the preparation ofthese salts (Chart 2), their gelation properties, and structure-property correlations based on single-crystal and powder X-raydiffraction data.

It may be mentioned here that organic salt based LMOGshave become increasingly popular in recent years,12 since thepreparation of such salts does not involve time-consumingnontrivial organic syntheses and because in a relatively shortperiod of time many salts can be prepared and scanned for theirgelation ability. Moreover, the supramolecular self-assembly insuch salts is based on strong and directional hydrogen bondingas well as stronger but less directional electrostatic interactionsbetween the cations and anions.

Results and Discussion

Gel Formation and Characterization. In a typical experi-ment, the gelator is dissolved in a suitable solvent with the aidof few drops of good solvent (MeOH for all the gelators except11 (3-nitCIN ), for which a mixture of MeOH/DMF was used)and heating. The solution is then cooled to room temperatureunder ambient conditions. The container (usually a test tube) isthen inverted to examine the material’s deformity. If nodeformation is observed, it is considered a gel. Table 1 givesthe gelation data.

Out of 14 cinnamate salts (including the hydrocinnamate27)prepared, 9 salts are gelators, whereas 10 salts out of 13 benzoate

salts studied display gelation ability. These results are significantwhen compared with our previously reported results,8b,c,11a

wherein dicyclohexylammonium hydrocinnamate and the cor-responding benzoate salts did not show any gelation properties.On the other hand, most of the cinnamates, including hydro-cinnamate and benzoate salts of dibenzylamine in the presentstudy, display gelation ability. It is significant to note that quitea few dibenzylammonium cinnamate/benzoate salts (in thepresent study) also show the ability to harden commercial fuelssuch as gasoline and diesel fuel.

The gel dissociation temperatureTgel as a function of gelatorconcentration of some of the cinnamate salts has been tested(Figure 1). A steady increase inTgel with an increase inconcentration of the gelator indicates that strong intermolecularinteractions are responsible for the self-assembly in the gel state.

SEM micrographs of the xerogels of most of the gelatorsdisplay a typical intertwined network of fibers, within whichthe solvent molecules are understandably immobilized in thegelled state. Figure 2 displays representative micrographs ofxerogels derived from some cinnamate and benzoate gelators.Intertwined fibers having lengths more than 100µm and widthsvarying from sub-micrometer to a few micrometers are seen in

Chart 2

Figure 1. Plots ofTgel vs concentrations of the gelators. Numbers onthe plots indicates the gelator numbers:1, 4, and7 represent mesitylenegel; 5, 8, 13, and27 represent isooctane gel;2 representsp-xylene gel.

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

Secondary Ammonium Monocarboxylate Salts Crystal Growth & Design, Vol. 6, No. 9, 20062115

these micrographs which are typical of the morphologies ofmany of the xerogels derived from LMOGs.

Structure-Property Correlation. The present work isundertaken in order to attempt a structure-property correlationso that the question “Is a 1D hydrogen-bonded networkimportant for gelation?” can be addressed. For this purpose,we have tried to crystallize all the salts so that the supramo-lecular networks in the corresponding crystal structures can becorrelated with their properties (gelling/nongelling). Out of 14cinnamate salts, suitable single crystals of 6 salts for X-raydiffraction studies are obtained from suitable solvents (see theExperimental Section). On the other hand, out of 13 benzoatesalts that we have prepared, 12 salts could be crystallized forsingle-crystal X-ray diffraction studies. Table 2 gives thecrystallographic parameters of these salts. Space groups of thecinnamate salts are evenly distributed among three different

crystal systems, namely triclinic, monoclinic, and orthorhombic,whereas the majority of the benzoate salts belongs to monoclinicspace groups (Table 2).

The majority of the salts show one ion pair in the asymmetricunit, except for the salts3(2-ClCIN), 16(2-ClBEN), 19(2-BrBEN) , and24(3-NitBEN), wherein two ion pairs are seenin the asymmetric unit. It is interesting to note that thedibenzylammonium cation shows a syn-anti conformation inthe majority of the structures, displaying corresponding C-C-N-C torsion angles ranging from 54.2 to 88.3° and from 169.1to 179.46° (Chart 3). In the salts3(2-ClCIN), 16(2-ClBEN),19(2-BrBEN), and 25(2-NitBEN), the conformation of thecation is anti-anti, displaying corresponding C-C-N-C torsionangles ranging from 175.9 to 179.2°.

The most striking feature of the crystal structures of thesesalts is the presence of a 1D hydrogen-bonded network involving

Table 1. Gelation Dataa

1(4-Cl-CINN)

2(3-Cl-CINN)

4(4-Br-CINN)

5(3-Br-CINN)

7(4-Me-CINN)

8(3-Me-CINN)

11(3-Nitro-CINN) 13(CIN) 27(HCIN)

srno. solvent

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

1 CCl4 ppt 1.97 62 FC ppt C FC ppt ppt2 cyclohexane 2.11 65 1.59 70 1.16 56 VL 1.12 68 1.41 53 FC VL FC3 n-heptane 1.89 81 2.11 65 0.93 69 ppt 1.33 73 1.42 61 FC ppt 1.32 664 isooctane 2.36 90 1.38 82 1.00 71 1.89 74 0.94 82 1.49 71 FC 1.35 64 1.39 605 gasolineb 1.94 84 2.12 63 1.49 70 ppt 2.87 71 VL FC FC ppt6 diesel fuelb 1.14 86 ppt 2.93 70 ppt 2.75 90 2.39 73 FC FC ppt8 benzene FC 13.34 62 3.00 66 ppt ppt ppt ppt ppt ppt9 toluene 3.47 68 3.52 57 3.14 72 ppt VL FC 2.28 57 ppt ppt10 chlorobenzene 2.62 72 FC ppt ppt L L FC ppt L11 bromobenzene FC FC ppt ppt L L FC ppt L12 o-xylene ppt 3.35 50 2.74 72 ppt VL ppt VL L L13 m-xylene 3.20 78 3.84 76 2.68 79 ppt VL ppt VL L L14 p-xylene 2.59 65 3.24 75 2.76 86 ppt 2.33 57 L VL L L15 mesitylene 3.08 70 VL 2.61 68 VL 2.18 72 ppt FC L L16 1,2-dichloro-

benzene2.08 75 L VL ppt L L 2.07 58 L L

17 DMF FC L L ppt L L L L L18 nitrobenzene VL L VL ppt L L L L L19 methyl

salicylate2.02 64 L ppt ppt L FC L L L

20 ethyl acetate ppt ppt 3.13 73 ppt PPT ppt FC ppt FC21 DMSO L L L ppt L L L L L

14(4-Cl-BEN)

15(3-Cl-BEN)

16(2-Cl-BEN)

17(4-Br-BEN)

18(3-Br-BEN)

19(2-Br-BEN)

20(4-Me-BEN)

22(2-Me-BEN)

23(4-Nit-BEN)

srno. solvent

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

MGC,wt %

Tgel,°C

1 CCl4 ppt ppt ppt ppt ppt ppt FC ppt FC2 cyclohexane 10.73 83 9.61 72 9.35 83 6.83 83 8.60 66 7.39 58 7.75 72 8.43 78 ppt3 n-heptane C FC FC FC FC FC FC FC ppt4 isooctane C FC FC FC FC FC FC FC ppt5 gasolineb 10.02 77 18.97 65 12.17 69 10.32 90 ppt 13.06 72 FC FC ppt6 diesel fuelb FC 9.91 82 FC 8.90 74 8.62 74 ppt ppt 8.78 86 VL8 benzene FC FC FC FC FC C FC ppt FC9 toluene FC C 12.54 68 FC FC FC C FC 9.94 9110 chlorobenzene FC FC FC FC L L FC L 9.39 10011 bromobenzene FC FC FC FC L L FC L FC12 o-xylene L C FC FC FC L FC L FC13 m-xylene VL C 9.31 68 VL FC L FC L VL14 p-xylene VL C FC FC 8.60 72 FC FC L FC15 mesitylene VL C 8.97 75 8.69 76 FC FC FC L VL16 1,2-dichloro-

benzeneFC FC FC FC L L L L 8.86 80

17 DMF L L L L L L L L L18 nitrobenzene L L L FC L L L L L19 methyl

salicylateVL L L L L L L L L

20 ethyl acetate C FC FC FC FC FC FC FC ppt

a 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.

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

the ion pair, except in the salt3(2-ClCIN), wherein participationof solvate water molecules in hydrogen bonding prevents theformation of a 1D hydrogen-bonded network; the correspondingN‚‚‚O distances and∠N-H‚‚‚O angles are within the rangesof 2.609(2)-2.870(4) Å and 159.0(2)-180.0(3)°, respectively(Figure 3). The exclusive formation of a 1D hydrogen-bondednetwork in these salts is quite remarkable and is in contrast to

our previously studied structures of dicyclohexylammoniumcinnamate/benzoate salts,8b,c,11awherein all of the benzoate anda few cinnamate salts displayed a 0D network.

It may be seen in Figure 3 that the acid moieties in thecinnamate salt7(4-MeCIN) are oriented on the same side ofthe propagation axis of 1D network, whereas the reverse is truein the case of the benzoate salt20(4-MeBEN). The same trend

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 C23H23ClNO2.50 C46H44Br2N2O4 C24H25NO2 C24H25NO2 C24H25NO2 C23H22N2O4

fw 388.87 848.65 359.45 359.45 359.45 390.43cryst size (mm) 0.35× 028× 0.19 0.49× 0.26× 0.21 0.15× 0.11× 0.08 0.25× 0.12× 0.06 0.48× 0.38× 0.21 0.21× 0.10× 0.08cryst syst triclinic orthorhombic monoclinic monoclinic orthorhombic triclinicspace group P1 Pbca P21/n P21/c Pbca P1ha (Å) 8.552(3) 22.4126(17) 11.4535(17) 5.959(2) 10.7493(10) 6.030(2)b (Å) 10.137(3) 16.6990(13) 6.0276(9) 28.662(10) 22.797(2) 11.418(4)c (Å) 12.372(4) 10.7237(8) 28.959(4) 11.742(4) 16.6738(16) 15.165(6)R (deg) 85.680(5) 102.599(7)â (deg) 78.101(5) 95.913(2) 90.291(8) 96.607(7)γ (deg) 87.955(5) 90.049(9)V (Å3) 1046.3(5) 4013.5(5) 1988.6(5) 2005.5(12) 4085.9(7) 1011.8(6)Z 2 4 4 4 8 2Dcalcd(g cm-3) 1.234 1.404 1.201 1.191 1.169 1.281F(000) 410 1744 768 768 1536 412µ(Mo KR) (mm-1) 0.202 2.066 0.076 0.075 0.074 0.088temp (K) 293(2) 293(2) 293(2) 293(2) 293(2) 293(2)no. of obsd rflns (I > 2σ(I)) 3302 2426 2281 758 2174 1702no. of params refined 520 332 344 245 344 350goodness of fit 1.067 1.043 1.214 0.757 1.142 1.022final R1 on obsd data 0.0428 0.0264 0.0639 0.0950 0.0483 0.0565final wR2 on obsd data 0.1229 0.0655 0.1337 0.1146 0.1152 0.1298

14(4-ClBEN) 15(3-ClBEN) 16(2-ClBEN) 17(4-BrBEN) 18(3-BrBEN) 19(2-BrBEN)

empirical formula C21H20ClNO2 C21H20ClNO2 C42H40Cl2N2O4 C21H20BrNO2 C21H20BrNO2 C42H36Br2N2O4

fw 353.83 353.83 707.66 398.29 398.29 792.55cryst size (mm) 0.71× 0.45× 0.23 0.48× 0.37× 0.21 0.47× 0.31× 0.26 0.57× 0.38× 0.19 0.58× 0.39× 0.23 0.48× 0.23× 0.40cryst syst monoclinic monoclinic monoclinic orthorhombic monoclinic monoclinicspace group P21/n P21/c P21/n Pbca P21/c P21/na (Å) 11.2663(6) 10.5042(8) 8.5885(5) 8.8752(10) 10.4652(13) 8.5473(18)b (Å) 8.9435(5) 9.2210(7) 20.7466(12) 17.898(2) 9.2705(12) 21.118(4)c (Å) 18.7571(10) 19.4703(15) 21.5058(13) 24.410(3) 19.788(3) 21.555(5)â (deg) 105.9260(10) 97.3840(10) 100.1720(10) 97.522(2) 99.894(4)V (Å3) 1817.42(17) 1870.2(2) 3771.7(4) 3877.4(8) 1903.3(4) 3832.8(14)Z 4 4 4 8 4 4Dcalcd(g cm-3) 1.293 1.257 1.246 1.365 1.390 1.373F(000) 744 744 1488 1632 816 1616µ(Mo KR) (mm-1) 0.224 0.217 0.216 2.133 2.173 2.158temp (K) 293(2) 293(2) 293(2) 293(2) 293(2) 293(2)no. of obsd rflns (I > 2σ(I)) 2272 2147 3915 2152 1983 2272no. of params refined 306 306 611 306 306 409goodness of fit 1.055 1.049 1.111 1.065 1.057 0.926final R1 on obsd data 0.0315 0.0518 0.0566 0.0380 0.0683 0.0894final wR2 on obsd data 0.0826 0.1475 0.1279 0.0968 0.1785 0.2523

20(4-MeBEN) 21(3-MeBEN) 23(4-NitBEN) 24(3-NitBEN) 25(2-NitBEN) 26(BEN)

empirical formula C88H92N4O8 C22H23NO2 C21H20N2O4 C42H40N4O8 C21H20N2O4 C21H21NO2

fw 1333.66 333.41 364.39 728.78 364.39 319.39cryst size (mm) 0.58× 0.43× 0.31 0.48× 0.37× 0.29 0.40× 0.34× 0.28 0.36× 0.28× 0.18 0.57× 0.34× 0.29 0.47× 0.33× 0.18cryst syst monoclinic monoclinic monoclinic orthorhombic monoclinic monoclinicspace group P21/c P21/c P21/c Pca21 Cc P21/ca (Å) 11.2369(9) 10.5691(13) 11.6184(9) 28.927(3) 20.764(12) 10.5943(8)b (Å) 8.9082(7) 9.3135(11) 8.7812(7) 5.9980(6) 9.877(6) 8.9347(7)c (Å) 19.2612(17) 18.974(2) 19.5406(14) 21.499(2) 9.959(6) 19.0320(15)â (deg) 106.6240(10) 99.028(2) 107.604(4) 106.380(9) 102.6070(10)V (Å3) 1847.5(3) 1844.6(4) 1900.2(3) 3730.2(6) 1960(2) 1758.1(2)Z 1 4 4 4 4 4Dcalcd(g cm-3) 1.199 1.201 1.274 1.298 1.235 1.207F(000) 712 712 768 1536 768 680µ(Mo KR) (mm-1) 0.076 0.076 0.089 0.091 0.086 0.077temp (K) 293(2) 293(2) 293(2) 293(2) 293(2) 293(2)no. of obsd rflns (I > 2σ(I)) 2205 2014 2130 4210 2105 2144no. of params refined 318 318 324 647 324 301goodness of fit 1.035 1.007 1.034 1.092 1.118 1.046final R1 on obsd data 0.0290 0.0353 0.0374 0.0320 0.0340 0.0297final wR2 on obsd data 0.0801 0.0915 0.0985 0.0797 0.0823 0.0790

Secondary Ammonium Monocarboxylate Salts Crystal Growth & Design, Vol. 6, No. 9, 20062117

is observed in all the cinnamate and benzoate salts reportedherein, except in24(3-NitBEN), which follows the trend ofcinnamate salts.

It is quite significant that, out of 27 dibenzylammonium saltspresently studied, 19 of them turned out to be moderate to goodgelators and all the gelator salts display a 1D hydrogen-bondednetwork in their crystal structures. These results clearly indicatethat presence of a 1D hydrogen-bonded network in the crystalstructure indeed plays a significant role in the gelation process.The cinnamate salts6(2BrCIN) and9(2MeCIN) and benzoatesalts 21(3MeBEN), 25(2NitBEN), and 26(BEN), however,show no gelation ability with the solvents studied herein, despitehaving a 1D network in their crystal structures. Since solventsare immobilized in the intertwined network of fibers in the gelledstate via surface tension or capillary force action, it is importantto have the surface compatibility of the typical solvents usedin the present study and the gel fibers. Thus, the most probablereason for the aforementioned cinnamate and benzoate salts (6,9, 21, 25, and26) being nongelators may be the lack of suchcompatibility. It is also significant to note that we have not comeacross any example in the present study or in our previousstudies8b,c,11athat shows gelation ability despite having a 0D(cyclic) network in the crystal structure.

What causes the dibenzylammonium cinnamate/benzoate saltsto have a 1D network exclusively in their crystal structures? Itmay be noted in this context that the corresponding dicyclo-hexylammonium cinnamate/benzoate salts studied earlier byus8b,c,11ashowed both 1D and 0D networks. This is an importantquestion to be addressed in order to gain further insights intothe structural aspect of designing new LMOGs.

While the dicyclohexylammonium cation is conformationallyrigid, having alicyclic substitutents, it is unable to offersecondary interactions such as C-H-π, π-π, etc. On the otherhand, its dibenzylammonium counterpart is conformationallymore flexible, allowing free N-C rotation, and is able to offerC-H-π andπ-π interactions.

There are only two ways a secondary ammonium monocar-boxylate ion pair can self-assembles1D and 0D, as alreadydiscussed. Thus, preference toward one particular network (1D

or 0D) may be dependent on other secondary interactions, suchas C-H‚‚‚π, π-π, C-H‚‚‚O, halogen‚‚‚halogen, andC-H‚‚‚halogen, that a particular system might offer.

An analysis of the presence and absence of these secondaryinteractions in dibenzylammonium (present study) and dicy-clohexylammonium salts (previous study8b,c,11a) showed that bothC-H‚‚‚π and C-H‚‚‚O interactions are the most significantsecondary interactions in dibenzylammonium salts. It is observedthat most of the dibenzylammonium salts display C-H‚‚‚πinteractions except in the salts11(3-NitCIN) and18(3-BrBEN),whereas none of the corresponding dicyclohexylammoniumsalts8b,c,11adisplay any C-H‚‚‚π interactions. A scatter plot ofC-H‚‚‚π distances and∠C-H‚‚‚π angles observed in thesesalts show that the C-H-π distances vary from∼2.7 to 3.5 Åand the∠C-H‚‚‚π angles are within the range of∼135-168°,which are well within the accepted range of C-H‚‚‚π interac-tions13 (Figure 4).

While salts6-9, 14, 16, 19-20, and23display internetworkC-H‚‚‚π interactions, salts15, 17, 21, and 24-26 showintranetwork C-H‚‚‚π interactions (Figure 5).

On the other hand, intranetwork C-H‚‚‚O interactions arepresent exclusively in all the dibenzylammonium salts, exceptfor the salts16(2-ClBEN) and25(2-NitBEN), in which inter-network C-H‚‚‚O interactions are also present, in addition tointranetwork interactions.

Although it cannot be concluded with certainty thatC-H‚‚‚π interactions are mainly responsible for the exclusive1D network in these salts, their contribution toward inducingsuch supramolecular hydrogen bond isomerism cannot be ruledout.

The results presented here and the results we have recentlyreported8b,c,11a clearly indicate that a 1D hydrogen-bondednetwork in the thermodynamically more stable crystal form isan important factor, although it may not be a “necessary andsufficient” factor for a molecule to show gelation ability. It mustbe emphasized that the crystal structure (crystalline phase) ofthe gel fibril in the native (gel) state and that of the compoundunder study in its theromodynamically more stable crystallinestate need not necessary be identical.

To see whether the crystal phases of the gel fibril in thexerogel state and in the crystalline state are identical or not,

Chart 3

Figure 3. Crystal structure illustrations depicting the 1D hydrogen-bonded network in dibenzylammonium cinnamate/benzoate salts: (a)the salt7(4-MeCIN); (b) the salt20(4-MeBEN). Identical 1D hydrogen-bonding networks are observed in the rest of the salts, except for3-(2-ClCIN) .

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

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

detailed X-ray powder diffraction (XRPD) studies have beenundertaken. In this study, XRPDs of the gel fibrils in the xerogelstate and bulk solid and XRPDs obtained by simulating single-crystal X-ray data (wherever available) are compared.

The results show that all three XRPD patterns in the salts7(4-MeCIN), 8(3-MeCIN), 14(4-ClBEN), 15(3-ClBEN), 16-(2-ClBEN), 17(4-BrBEN), 19(2-BrBEN), and24(3-NitBEN)are virtually superimposable, meaning that the single-crystalstructures of the salts represent their bulk solid’s crystallinephase and crystal structures of the gel fibrils in xerogel statesare identical with those obtained from single-crystal data; onesuch comparison plot for salt16(2-ClBEN) is shown in Figure6. The rest of the plots are given in the Supporting Information(Figure S1).

For the salt11(3-NitCIN), however, both the bulk solid andxerogel appear to be less crystalline, making it difficult todetermine the corresponding crystalline phases. The salt18(3-BrBEN) displays different XRPDs for the simulation, bulk solid,and xerogel (Figure S2, Supporting Information); this meansthat the single crystal that is analyzed does not represent thebulk solid’s crystal phase, which, in turn, is not identical withthe crystal phase of the fibrils of the xerogel. For the salts20-(4-MeBEN) and23(4-NitBEN), the XRPDs of the bulk solidand xerogel are almost superimposable (Figure S3, SupportingInformation). However, the simulated patterns do not matchthose of the bulk solid and xerogel, indicating that the singlecrystals obtained for analyses do not represent the crystallinephases of the bulk solid as well as the xerogels. These resultsclearly indicate that the majority of the gelator salts for whichsingle crystal structures are available display the same crystalstructures for the xerogel fibers as for the single-crystal state.Thus, the crystal structures of the gel fibrils of these gelators inthe xerogel state have been determined in an indirect mannerand they indeed display 1D hydrogen-bonded networks. It maybe mentioned here that a single crystal of a gelator is extremelydifficult to grow and it is even more difficult to grow from itsgelling solvent.14 Gelator crystals are, therefore, often crystal-lized from nongelling solvents. Consequently, crystal phasemismatches among single crystals, bulk solids and xerogelscannot be ruled out and the salts18(3-BrBEN), 20(4-MeBEN),and23(4-NitBEN) provide examples of such a situation.

It may be pointed out here that crystalline phases of the gelfibers in the native gel state and xerogel state need notnecessarily be identical because of the possibilities of havingphase transitions triggered by a new nucleation event generatedfrom some amount of dissolved gelator compound in the solventduring the solvent removal process of xerogel formation. Thereis no certainty that such a phase transition does occur duringxerogel formation, but there is no assurance that it does not.Efforts to record XRPD patterns in the gel state have beenproven unsuccessful, presumably due to strong scattering of thesolvent molecules. Thus, it is not possible to comment on thecrystal structure of the fibers in the gel state.

On the other hand, the gelator salts1(4-ClCIN), 22(2-MeBEN), and27(HCIN), for which no single-crystal structuresare available, display reasonable matches of the XRPDs of theirbulk solids with those of the xerogels, meaning that gel fibrilsin the xerogel state retain the same crystal phase of the bulksolid (Figure S4, Supporting Information). However, the salts2(3-ClCIN), 4(4-BrCIN) , and13(CIN), for which no crystalstructures are available, display mismatches of the XRPDs oftheir bulk solids and xerogels, meaning that a crystal phasetransition has taken place during xerogel formation (Figure S5,Supporting Information). The the xerogel of the gelator salt5-(3-BrCIN) is found be less crystalline, making it difficult todetermine.

Although the foregoing discussion on the crystalline phasesof the material in various states seems to suffer from crystalphase transitions at times, local supramolecular architectures,e.g. the 1D hydrogen-bonded networks of the ion pairs in thepresent study, may remain identical while their packings in thecrystal structures (crystal phase) might be different in differentstates. It is quite logical to think that the responsible drivingforces for the growth of fibrils in the fibril axis direction andperpendicular to it must be different, since the growth in theformer direction has to be faster than that of in the latter directionin order to have fiber morphology as observed in SEMmicrographs of the xerogels. A 1D hydrogen-bonded network

Figure 5. Representative example of C-H‚‚‚π interactions in the saltsstudied here: (a) internetwork C-H‚‚‚π interactions in the salt6(2-BrCIN) ; (b) intranetwork C-H‚‚‚π interactions in the salt15(3-ClBEN).

Figure 6. XRPDs of the salt16(2-ClBEN) under various conditions,The xerogel sample was prepared from a 10 wt % cyclohexane gel.

Secondary Ammonium Monocarboxylate Salts Crystal Growth & Design, Vol. 6, No. 9, 20062119

thus might play a significant role in the elongated growth ofthe fibril in one direction, while interactions with the solventmay prevent or reduce the growth perpendicular to the fibrilaxis.

Conclusions

A series of dibenzylammonium cinnamate/benzoate salts havebeen prepared on the basis of the supramolecular synthonapproach. Out of 27 salts prepared, 9 cinnamate and 10 benzoatesalts have been found to be moderate to good organo gelators;a few of them are even capable of hardening commercial fuelssuch as gasoline and diesel fuel. All of the salts (6 cinnamateand 12 benzoate) for which single-crystal structures could bedetermined display a 1D hydrogen-bonded network; 12 of them(3 cinnamate and 9 benzoate) are gelators, emphasizing theimportance of a 1D hydrogen-bonded network for gel formation.Crystal structure analyses of all these salts indicate that theconformationally flexible geometry of the cation and both intra-and internetwork C-H‚‚‚π and C-H‚‚‚O secondary interactionsmay contribute toward inducing such supramolecular hydrogenbond isomerism in these salts. Crystal structures of the gel fibersin the xerogel state for 8 gelator salts, which were determinedby comparing the XRPDs of the xerogel and simulated XRPDsobtained from the corresponding single-crystal data, revealedthe presence of a 1D hydrogen-bonded network. It may be notedthat the corresponding cinnamate/benzoate salts of dicyclohexyl-amine previously studied by us8b,c,11a provided only a fewgelators; none of the benzoate salts showed any gelationproperties. Moreover, all the benzoate salts displayed a 0Dnetwork. In the present study, however, the introduction of aconformationally flexible aromatic analogue of dicyclohexy-lamine results in the formation of 19 gelators out of 27 saltsthat we have studied and all the crystal structures displayed a1D hydrogen-bonded network. Thus, subtle changes in thecationic species produce a profound effect on the resultantsupramolecular structures and properties. We believe theseresults are important in the context of designing new gellingagents on the basis of a crystal engineering approach.

Experimental Section

Materials and Physical Measurements.All reagents (Aldrich) andthe solvents used for gelation (AR grade, S. D. Fine Chemicals, India)were used without further purification. All of the oils were procuredfrom local sources. Microanalyses were performed on a Perkin-Elmer2400 elemental analyzer, Series II. FT-IR and1H NMR spectra wererecorded using Perkin-Elmer Spectrum GX and 200 MHz BrukerAvance DPX200 spectrometers, respectively. X-ray powder diffractionpatterns were recorded on an XPERT Philips (Cu KR radiation)diffractometer. Scanning electron microscopy (SEM) was performedwith 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 dropsof MeOH. To this solution was slowly added dibenzylamine (1.0 mmol),and the reaction mixture was kept at room temperature. After a fewhours, the salts1 and4 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. Tothis solution was slowly added dibenzylamine (1.0 mmol), and thereaction mixture was kept at room temperature. After a few hours, theresulting salts as precipitates (near-quantitative yield) were used forgelation 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) wasadded slowly to the methanolic solution of the acid at room temperature.The reaction mixture was then evaporated to dryness at room temper-

ature. The resulting salts as precipitates (near-quantitative yield) wereused for gelation and other studies.

Single-Crystal X-ray Diffraction. X-ray-quality single crystals weregrown under slow evaporative conditions at room temperature. Thecorresponding salts were dissolved in the crystallizing solvent with theaid of a few drops of MeOH in most of the cases. For salt11 DMFwas used. Crystals of16, 18, 19, and23 were grown from benzene.Salts3, 6, 14, 15, 17, 20, 21, and26 were crystallized from toluene.8was crystallized from carbon tetrachloride.7 was crystallized fromacetonitrile.24was crystallized fromp-xylene. Salt25was crystallizedfrom 1,2-dichlorobenzene. Salts9 and 11 were crystallized fromn-heptane. Diffraction data were collected using Mo KR (λ ) 0.7107Å) radiation on a Bruker AXS SMART APEX CCD diffractometer.All calculations were performed by using the software package of theSMART APEX instrument.

All structures were solved by direct methods and refined in a routinemanner. In all cases except19, non-hydrogen atoms were treatedanisotropically. In19, the aromatic ring of one of the acid moieties inthe asymmetric unit was found to be disordered and was refinedisotropically with the constraint of a regular hexagon. The hydrogenatoms attached to nitrogen were located in most of the cases, exceptfor salts8 and19, on a difference Fourier map and refined. Wheneverpossible, the other hydrogen atoms were located on a difference Fouriermap and refined. In the rest of the cases, the hydrogen atoms weregeometrically fixed.

Gel to Sol Dissociation Temperature (Tgel) Measurement.Tgel wasmeasured by using the following method. A 1.0 mL portion of the gelwas prepared in a test tube. A locally made glass ball weighing 0.19 gwas placed on the gel surface. The test tube was then heated in an oilbath. The temperature (Tgel) was noted when the ball fell to the bottomof the test tube.

Acknowledgment. The Ministry of Environment and For-ests, New Delhi, India, is gratefully acknowledged for financialsupport. D.R.T. thanks the CSIR for an SRF fellowship.

Supporting Information Available: Text, tables, and figures givingmelting point, analytical, FT-IR, and1H NMR data, hydrogen-bondingparameters, and XRPD patterns for the salts and CIF files giving datafor the single-crystal structure determinations. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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