Supramolecular Synthon Transferability and Gelation by DiprimaryAmmonium Monocarboxylate SaltsUttam Kumar Das,† Vedavati G. Puranik,‡ and Parthasarathi Dastidar*,†

†Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A&2B Raja S. C. Mullick Road, Jadavpur,Kolkata 700032, West Bengal, India‡Centre for Materials Characterization, National Chemical Laboratories, Dr. Homi Bhaba Road, Pune 400008, India

*S Supporting Information

ABSTRACT: Earlier studies revealed that primary ammoniumdicarboxylate (PAD) salts possessed gelling ability, and manysuch salts displayed a 1D columnar hydrogen bonded network(observed in primary ammonium monocarboxylate (PAM)salts) on either side of the dicarboxylate end of the anion. Inthe present study, a new series of diprimary ammoniummonocarboxyate (DPAM) salts have been prepared with theaim of achieving supramolecular synthon transferability (thesame 1D columnar hydrogen PAM bonded network on eitherside of the diammonium cation) in these salts. Single crystal X-ray diffraction studies revealed that, in 47% of the DPAM salts, such supramolecular synthon transferability indeed took place.Some of the DPAM salts also showed gelation ability. The gels were characterized by DSC, rheology, electron microscopy, andatomic force microscopy. Structure property correlation using single crystal and powder X-ray diffraction data on a selected gelwas also attempted.

Low molecular weight gelators (LMWGs)1 are smallmolecules (molecular weight < 3000) that are capable of

solidifying (gelling) organic solvents (organogels), pure water,or aqueous solvents (hydrogels) and are in high demandbecause of their potential applications.2 Unfortunately, most ofthe gelators discovered are serendipitously found, andsubsequent modification of the parent gelator molecules ledto the development of a new generation of gelators.Designing gelators ab initio is a challenging task, as the

molecular level understanding of the gel formation mechanismis not fully deciphered. Various studies indicate that the LMWGmolecules form self-assembled fibrilar networks (SAFINs)3

driven by different noncovalent interactions, such as hydrogenbonding, π−π stacking, hydrophobic/hydrophilic, and van derWaals interactions; solvent molecules are then immobilizedwithin the SAFINs, resulting in gel formation. Key to anydesign strategy is to have a detailed understanding of thestructure of the material under study. Thus, determining thestructure of SAFIN is important in designing a gelatormolecule. Since SAFINs are too tiny to carry out single crystalX-ray diffraction (SXRD) and powder X-ray diffraction(PXRD) is not yet a routine methodology, an alternativeindirect approach is proposed by Weiss and co-workers4

wherein experimental and simulated PXRD patterns arecompared in order to determine the structure of SAFINs.Such a school of thought ultimately led to the workinghypothesis that indicates that 1D and 2D networks helppromote SAFIN formation whereas a 3D network is not asgood for such a task.5 The fact that such a hypothesis was based

on a logical foundation was explicitly demonstrated by ourgroup6 nearly a decade ago, and since then, we have beenengaged in exploiting the supramolecular synthon concept7−anessential tool in crystal engineering8−in designing LMWGs.9 Wehave thus far developed various supramolecular synthons, suchas secondary ammonium monocarboxylate (SAM),10 secondaryammonium dicarboxylate (SAD),11 primary ammonium mono-carboxyate (PAM),12 and primary ammonium dicarboxyate(PAD),13 in discovering a new series of gelators (Scheme 1).Among these synthons, the PAD synthon displayed

remarkable properties. Most often it is a 2D synthon by virtueof the bifunctionality of the anionic part (carboxyate) of the ionpair. We have shown that the 2D PAD synthon can be foldedinto a 1D nanotubular construct driven by alkyl−alkylhydrophobic interactions induced hydrogen bond isomerism.13

A close look at the detail of the hydrogen bonding interactionsin the 2D PAD synthon reveals the presence of a columnarPAM synthon (synthon W or X, Scheme 1) at each carboxylateend of the dianion.Supramolecular synthon transferability is an important issue

in crystal engineering, as it has farfetched implication indesigning solids with desired structures and properties.14 In anattempt to study the supramolecular synthon transferability ofthe 2D PAD synthon, we consider studying the reverse of thePAD salt wherein a α,ω-diamine (instead of α,ω-dicarboxylic

Received: August 28, 2012Revised: October 11, 2012Published: October 12, 2012

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acid as in the case of PAD salt) is reacted with amonocarboxylic acid in 1:2 molar ratio. It is expected thatsuch an interchange should not drastically change the overallsupramolecular network of the 2D PAD synthon. Since it isderived from a primary diamine and a monocarboxylic acid, wehereafter designate this new synthon as diprimary ammoniummonocarboxyate (DPAM).In this article, we report the single crystal structures of 15

salts out of 26 DPAM salts that we prepared (Scheme 2); most

of the structures display the 2D network as expected; synthontransferability has been observed in 7 salts. Interestingly, someof the salts are capable of displaying organo- and hydrogelationproperties (Supporting Information). The gels are character-ized by DSC, rheology, SEM, AFM, etc. Structure−property(gelation) correlation based on SXRD and PXRD data has alsobeen reported.The DPAM salts were prepared by reacting the correspond-

ing diamines and the acids in MeOH at room temperature withnear quantitative yield. Crystals suitable for single crystal X-raydiffractions were grown following the slow evaporation methodfrom suitable solvent systems (Supporting Information). SXRDdata revealed that, out of 15 salts, 7 salts displayed the 2DDPAM synthon; none of the B3A(X) salt displayed the 2DDPAM synthon; 3 of the B4A(X) salts [B4A(3-NO2), B4Q,B4An] showed PAM synthon X at each ammonium endwhereas four salts [B4A(4-Cl), B4A(2-Br), B4A(4-Br), andB4A(4-NO2)] displayed PAM synthon W in the diammoniumsites.It may be noted that the diammonium cation B4 in the salts

displaying the 2D DPAM synthon adopted an extended all-staggered conformation which presumably helps the ion pairsto self-assemble in a 2D network. However, in salt B3A(4-Me),the diammonium cation B3 adopted a “U” shaped conformationthat resulted in 1D PAM synthon X (Figure 1).The rest of the salts [B3A(2-Cl), B3A(3-Cl), B3A(3-Br),

B3A(3-NO2), B3A(4-NO2), and B4A(2-NO2)] displayed a 2Dnetwork having various hydrogen bonding connectivities; thisappears to be due to the various conformations (all staggered,staggered-gauche, etc.) adopted by the diammonium cations inthese salts; in the case of B4A(4-Me), lattice occluded waterparticipated in hydrogen bonding, thereby displaying differenthydrogen bonding connectivity (Supporting Information).Thus, it is clear that interchanging of the dicarboxylic acid/primary amine to monocaroxylic acid/diprimaryamine did not

Scheme 1. Various Supramolecular Synthons Studied by Our Group

Scheme 2. List of 26 DPAM Salts Reported in This Paper

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influence the detail of the 2D hydrogen bonding network(HBN) in 7 DPAM salts, thereby confirming the synthontransferability (Scheme 3).

Our previous report on the gelation ability of the PAD saltsand the synthon transferability in the present case encouragedus to undertake detailed gelation studies of the DPAM saltsreported herein. Gelation tests were carried out in 15 selectedsolvents that include both the polar and the nonpolar solvents(Supporting Information). Interestingly, 5 salts displayedgelation ability. The minimum gelator concentration (MGC)and the gel−sol dissociation temperature (Tgel) are within therange 2.2−4.0 wt %, w/v, and 76−128 °C, respectivelyindicating that the gelators were quite efficient and the gelswere remarkably stable. Tgel vs [gelator] plots of 5 selected gelsdisplayed a steady increase in Tgel with the increase in [gelator](Figure 2). These results were attributed to the active

participation of the noncovalent interactions such as hydrogenbonding in forming the gel network.

Differential scanning calorimetry (DSC) was performed on aselected gel [a 6.0 wt % w/v methylsalicylate gel of B3A(4-Me)in order to study its thermoreversibility in a quantitativefashion. Clearly, two peaks at ∼100 °C and ∼58 °C inendothermic and exothermic cycles, respectively, could be seen;while the former represents the gel−sol dissociation temper-ature (Tgel), the later is the sol−gel transition temperature(Figure 3).

The dynamic rheology for two selected gel samples [B4A(4-Me) and B4A(3-NO2)] has been carried out to study theviscoelastic behavior of the corresponding gels. Here, thestorage or elastic modulus, G′, and the viscous modulus, G″,were plotted as a function of angular frequency, ω, at theconstant strain 0.1%. It is observed (Figure 4) that the storagemodulus, G′, is invariable as a function of angular frequencyover a considerable time period and the magnitude of G′ (95.7and 56.1 kPa for B4A(4-Me) and B4A(3-NO2), respectively) ismuch larger than that of G″ (19 and 25 kPa for B4A(4-Me)and B4A(3-NO2), respectively), which supported strongly theviscoelastic behavior of these gels.To study the morphology of the gel network of the xerogels,

we have performed SEM, TEM, and AFM on the 1,2-

Figure 1. Synthons observed in the crystal structures of some of theDPAM salts: (A) 2D PAD synthon having PAM “X” observed inB4A(3-NO2), B4Q, B4An; (B) 2D PAD synthon having PAM “W”observed in B4A(4-Cl), B4A(2-Br), B4A(4-Br), and B4A(4-NO2); (C)1D PAM synthon “X” observed in B3A(4-Me).

Scheme 3. Transformation of 2D PAD Synthon into 2DDPAM Synthon

Figure 2. Tgel vs [gelator] plot (DCB-1,2-dichlorobenzene, MS-methylsalicylate, NB-nitrobenzene).

Figure 3. DSC trace of the ∼6 wt % methyl salicylate gel of B3A(4-Me).

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dichlorobenzene xerogel of some selected gelator salts. Thesamples for microscopic experiments were prepared by drop-casting a much diluted solution on the SEM stub/TEM grid inorder to get a clear view of the morphology (SupportingInformation). Unlike highly entangled 1D fibers usuallyobserved in the SEM of innumerous gels, aggregates ofspherical particles could be seen in most of the cases except inthe case of B3A(3-Cl), wherein a platelike morphology wasobserved. It is interesting to note that, in the case of B4A(4-Me), highly aligned aggregates of particles could be seen. Highresolution microscopic data such as TEM/AFM also confirmthe presence of a platelike and spherical morphology in B3A(3-Cl) and B3A(4-Me), respectively (Figure 5).

To determine the structure of the gel fiber in the xerogelstate, we have undertaken detailed X-ray diffraction experi-ments on a selected salt, namely B4A(3-NO2). Figure 6 depictsthe comparison plot of PXRD patterns15 of B4A(3-NO2) undervarious conditions; nearly superimposable PXRD patternsclearly establish that the structure of the gel network in thexerogel (4 wt % in water) is identical with that of the SXRDstructure of the salt.Thus, a library of 26 DPAM salts has been generated, and

single crystal structures of 15 such salts have been determined.SXRD data established that the 2D PAD synthon is indeedtransferable to the DPAM salt system in ∼47% of the saltsstudied herein. The conformational flexibility of the diammo-

Figure 4. Frequency sweep rheological traces of two selected gels.

Figure 5. SEM of (A) B3A(3-Cl); (B) B4A(4-Me); (C) B3A(3-Me); (D) B3A(3-Me); and (E) B3A(4-Me). TEM of (F) B3A(3-Cl). AFM of (G)and (H) B3A(3-Cl).

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nium alkane spacer plays a crucial role in shaping up the overallhydrogen bonding network in the resulting salts. Thus, 7DPAM salts failed to show synthon transferability, mainlybecause of the various conformations adopted by thediamonium cation. The effect of the conformational flexibilityof the diamonium backbone is remarkable in the salt B3A(4-Me); herein, the diammonium cation adopted a “U”conformation, which essentially prevented the formation of a2D HBN leading 1D PAM synthon. The fact that 5 salts outthe 26 DPAM salts reported herein displayed reasonablegelation ability goes well with the supramolecular synthonapplicability in designing LMWGs.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental data, single crystal structure description, molec-ular plot, hydrogen bonding parameter, crystallographicparameter, CIF data, and gelation data table. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ocpd@iacs.res.in; parthod123@rediffmail.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSP.D. and U.K.D. thank CSIR, New Delhi, for financial supportand a SRF fellowship, respectively. Single crystal X-raydiffraction data were collected at the DST-funded NationalSingle Crystal Diffractometer Facility at the Department ofInorganic Chemistry, IACS, and National Chemical Laboratory,Pune.

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Figure 6. PXRD plots of B4A(3-NO2) under various conditions.

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