hydrogen bonded binary molecular adducts derived from exobidentate n-donor ligand with dicarboxylic...

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Hydrogen bonded binary molecular adducts derived from exobidentate

N-donor ligand with dicarboxylic acids: Acid imidazole hydrogen-bonding

interactions in neutral and ionic heterosynthons

Amal Cherian Kathalikkattil, Subin Damodaran, Kamal Kumar Bisht, Eringathodi Suresh

Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G.B. Marg, Bhavnagar 364 002, Gujarat, India

a r t i c l e i n f o

Article history:

Received 2 October 2010Accepted 10 November 2010Available online 16 November 2010

Keywords:

CocrystalOrganic saltSupramolecular assemblyHydrogen bondMolecular adductpKavalues

a b s t r a c t

Four new binary molecular compounds between a flexible exobidentate N-heterocycle and a series ofdicarboxylic acids have been synthesized. The N-donor 1,4-bis(imidazol-1-ylmethyl)benzene (bix) wasreacted with flexible and rigid dicarboxylic acids viz., cyclohexane-1,4-dicarboxylic acid (H2chdc), naph-thalene-1,4-dicarboxylic acid (H2npdc) and

1 H-pyrazole-3,5-dicarboxylic acid (H2pzdc), generating fourbinary molecular complexes. X-ray crystallographic investigation of the molecular adducts revealedthe primary intermolecular interactions carboxylic acid amine (via OH N) as well as carboxyl-ate protonated amine (via NH+ O) within the binary compounds, generating layered and two-dimensional sheet type H-bonded networks involving secondary weak interactions (CH O) includingthe solvent of crystallization. Depending on the differences in pKavalues of the selected base/acid (DpKa),diverse H-bonded supramolecular assemblies could be premeditated. This study demonstrates the H-bonding interactions between imidazole/imidazolium cation and carboxylic acid/carboxylate anion inproviding sufficient driving force for the directed assembly of binary molecular complexes. In the two-component solid form of hetero synthons involving bix and dicarboxylic acid, only H2chdc exist as cocrys-tal with bix, while all the other three compounds crystallized exclusively as salt, in agreement with the

DpKavalues predicted for the formation of salts/cocrystals from the base and acid used in the synthesis ofsupramolecular solids.

2010 Elsevier B.V. All rights reserved.

1. Introduction

Crystal engineering offers a rational approach to the design ofmaterials with new compositions, properties and crystal struc-tures. Much as an organic chemist employs the covalent bond inthe design of target molecules, non-covalent bonds can beexploited in the design of supramolecular assemblies [1,2]. Suchnon-covalent interactions include hydrogen bonding, van derWaals, pp stacking, and electrostatic interactions. These kind of

molecular interactions could be utilized in designing cocrystals,solvates, organic salts, polymorphs and pseudopolymorphs, etc.[37], out of which cocrystals are of widespread interest.

Cocrystal [810], also referred to as molecular complex, is ahomogeneous phase of two or more different components in a stoi-chiometric composition, and often relies on hydrogen bondedassemblies between neutral molecules. Strong and directionalhydrogen bonds have been used in rational strategies to designbinary/ternary cocrystals in crystal engineering, materials science,

hostguest inclusion compounds, and pharmaceutical solids[11].Understanding of highly specific and mutually complementarynon-covalent interactions between molecules is the basic aspectin molecular recognition, cocrystallization and supramolecularsynthetic chemistry[12].By the judicious choice of the supramo-lecular synthons with specific and directional non-covalentconnectivity, multidimensional solid state assembly can be gener-ated by the effective close packing of the discrete building blocks[1315]. Among all the non-bonded interactions, hydrogen bond-

ing has proved to be the most useful and reliable because of itsstrength and directional properties [16]. Many molecular solidswith novel properties have been prepared using hydrogen bondingas the main steering force[1719]. Molecular cocrystals have beenknown for a long time, still there is wide scope for further applica-tions and structural studies in this field. In recent years, many pre-meditated synthesis of binary and ternary cocrystals[2024]havebeen reported. Variety of cocrystals with multidimensional topol-ogy, based on hydrogen bonded networks can be achieved whena N-heterocycle moiety is allowed to interact with a suitablecarboxylic acid[14,25]. The ability of carboxylic acids to readilyaggregate through the dimer homosynthon have been observedtraditionally in structural chemistry.

0022-2860/$ - see front matter 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.molstruc.2010.11.022

Corresponding author. Tel.: +91 278 2567760x662/667 (office), +91 2782565006 (residence), moblie: +91 9426910756; fax: +91 278 2567562.

E-mail addresses:[email protected],[email protected](E. Suresh).

Journal of Molecular Structure 985 (2011) 361370

Contents lists available at ScienceDirect

Journal of Molecular Structure

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m o l s t r u c
http://dx.doi.org/10.1016/j.molstruc.2010.11.022mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.molstruc.2010.11.022http://www.sciencedirect.com/science/journal/00222860http://www.elsevier.com/locate/molstruchttp://www.elsevier.com/locate/molstruchttp://www.sciencedirect.com/science/journal/00222860http://dx.doi.org/10.1016/j.molstruc.2010.11.022mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.molstruc.2010.11.022


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Conformationally flexible ligand 4-bis(imidazol-1-ylmethyl)-benzene (bix) has been proved as a suitable building block fordesigning the coordination polymers of various network topology.It has been employed to fabricate a large number of coordinationpolymers with a variety of structural diversity from low dimen-sional entities such as 1D helix, linear threads to high-dimensionalsupramolecular networks such as 3D interpenetrated networks,metal organic polyrotaxanes; brick-wall type networks, chiraland magnetic MetalOrganic Frameworks. These fascinating struc-tures hold promising applications in the area of non linear opticalproperties, magnetic properties, gas adsorption ability and cata-lytic properties[2632]. Herein, we report synthesis, characteriza-tion by various physicochemical methods and a systematicstructural examination of the cocrystal/salt resulted from cocrys-tallization between exobidentate N-heterocycle ligand 1,4-bis(imi-dazol-1-ylmethyl)benzene (bix) and a series of rigid and flexibledicarboxylic ligands such as cyclohexane-1,4-dicarboxylic acid(H2chdc), naphthalene-1,4-dicarboxylic acid (H2npdc) and 1H-pyr-azole-3,5-dicarboxylic acid (H2pzdc). The related primary intermo-lecular carboxylic acid amine (OH N)/carboxylate protonatedamine (NH+ O) interactions and secondary weak molecularinteractions including stacking have been investigated by crystallog-raphy on these binary cocrystals or organic salts in their supramo-lecular arrangement.

2. Experimental

2.1. Materials

All the chemicals were purchased from Sigma Aldrich, USA andused without any further purification. All solvents were freshlypurified by general distillation process and used as and whenrequired.

2.2. IR, NMR, CHNS and TGA measurements

IR spectra were recorded using KBr pellets on a PerkinElmerGX FTIR spectrometer. For each IR spectra 10 scans were recordedat 4 cm1 resolution. 1H NMR spectra for the compounds were re-corded on Bruker AX 500 spectrometer (500 MHz) at temperature25 C.1H NMR Spectra was calibrated with respect to TMS and TMSwas used as an internal reference for solvents such as d6-DMSO.Elemental analysis (CHNS estimation) was done by using theinstrument PerkinElmer 2400 CHNS/O analyzer. All the TGA spec-tra were recorded on METTLER TOLEDO STAR SW 7.01.

2.3. X-ray crystallography

The crystallographic data and details of data collection for all

the four compounds are given in supplementary information(Table S1). In each case, a crystal of suitable size was selected fromthe mother liquor and then mounted on the tip of a glass fiber andcemented using epoxy resin. Intensity data for all the six crystalswere collected using Mo Ka(k= 0.71073 ) radiation on a BrukerSMART APEX diffractometer equipped with CCD area detectoreither at 293 K or at 100 K. The data integration and reductionwere processed with SAINT[33]software. An empirical absorptioncorrection was applied to the collected reflections with SADABS[34]. The structures were solved by direct methods using SHELXTL[35]and were refined on F2 by the full-matrix least-squares tech-nique using the SHELXL-97 [36] package. In all compounds allnon-hydrogen atoms were refined anisotropically till convergenceis reached. Hydrogen atoms attached to the ligand moieties are

either located from the difference Fourier map or stereochemicallyfixed. The diagrams of the crystal structures are generated using

programs ORTEP[37], Mercury 1.4.1[38]or PLATON[39]. ORTEPdiagrams of all four compounds are shown inFig. 2, and structuraldrawings of the adducts for better understanding and clarity areincorporated inSupplementary material as Fig. S2. Relevant crys-tallographic data and selected bond lengths for the molecular ad-ducts are given in Supplementary material as Tables S1 and S2respectively. Hydrogen bonding interaction with symmetry codeand p

Ka value data for all four compounds are listed in Table 1

andTable 2respectively.

2.4. Synthesis of the N-donor ligand

1,4-Bis(imidazol-1-ylmethyl)benzene (bix) was synthesized byfollowing a procedure reported by Abrahams et al.[40]. Re-crystal-lization of the crude product by dissolving in hot water and keep-ing at constant temperature 23C yielded white crystallinematerial. The detailed synthetic procedure for the preparation of li-gand bix and its proton NMR spectra is given in Supplementarydata as S1 and Fig. S3 respectively.

2.4.1. 1 H NMR data for bix

d= 5.163(s, 4H; methylene), 6.885(s, 2H; imidazole), 7.715(s,2H; imidazole), 7.231(s, 4H; aryl ring), 7.731(s, 2H; imidazole)CHN data for bix: calculated (%): C = 61.29; H = 5.15; N = 20.45. Ob-served (%): C = 60.3; H = 6.21; N = 19.63. IR spectral data: (mmax(cm1)) 3438(b), 3108(m), 2553(w), 2949(m), 1677(m), 1602(w),1512(s), 1444(s), 1281(s), 1228(s), 1104(m), 1081(s), 1029(m),919(s), 828(m), 767(m), 708 (m), 665(m).

2.5. Synthesis of molecular complexes

2.5.1. Bix:H2chdc (CC1)

Bix (0.272 g, 1 mmol) and H2chdc (0.172 g, 1 mmol) was dis-solved separately in 5 ml methanol each and mixed gradually withconstant stirring at room temperature in 1 h duration. Five millili-

ters water was then added to this solution and continued stirringfor another three more hours. The clear colorless solution obtainedwas kept at room temperature for 1 week to get crystals suitable

Fig. 1. NMR spectra of CC1, CC2, CC3, CC4 in d6-DMSO solvent. The spectra of bixand used dicarboxylic acids are also presented for assessment of signals.

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for crystallographic studies. Melting point: 195197C. 1H NMRdata for CC1: d= 1.3161.885 (multiplet, 10H, cyclohexane ring),5.164 (s, 4H; methylene-bix), 6.888 (s, 2H; imidazole ring),7.159(s, 2H; imidazole ring), 7.231(s, 4H; aryl ring), 7.736(s, 2H;imidazole ring)CHN data: Calculated values (%): C = 64.4; H = 6.4;N = 13.66. Observed values (%): C = 64.44; H = 5.98; N = 13.19. IRspectral data: (mmax (cm

1)) 3135(s), 2952(s), 2859(s), 1695(m),1513(s), 1441(s), 1397(w), 1296(s), 1083(s), 819(w), 741(w),653(m), 615(s), 571(m), 491(s). CCDC number for the compound:CCDC 746435.

2.5.2. H2bix:(Hnpdc)26H2O (CC2)

A solutioncontainingbix (0.272 g, 1 mmol)in 5 ml methanol wasadded slowly to H2npdc (0.2162 g, 1 mmol) in 25 ml methanol andstirred at room temperature for 2 h. The light orange solution ob-tainedwas filtered andupon slow evaporation at room temperatureyielded yellow micro-crystals. These micro-crystals were dissolved

inmethanolwatermixture(15 mland5 mlrespectively)withgentlewarming. The solution was filtered and upon slow evaporation atroom temperature yielded crystals suitable for Crystallographicstudies. Melting point: 183185 C; 1H NMR data for CC2: d = 5.182(s, 4H; methylene-bix), {7.223(s), 7.685(s), 8.121(s), 8.814(s),4.373(s); 22H} CHN data: Calculated (%): C = 58.63; H = 5.45;N = 7.19. Observed (%): C = 59.97; H = 5.39; N = 6.92. IRspectraldata:(mmax (cm

1)) 3410(br), 3111(s), 1692(s), 1512 (m), 1448(w),1395(w), 1227(m), 1082(m), 873(m), 824(m), 753(s), 658(m),612(m), 537 (m). CCDC number for the compound: CCDC 746434.

2.5.3. H2bix:(Hpzdc)22H2O and H2bix:pzdc2H2O (CC3 and CC4)

To a solution containing bix (0.272 g, 1 mmol) in 5 ml methanolwas added 3,5-pyrazole dicarboxylic acid (0.1742 g, 1 mmol)

dissolved in 5 ml ethanol with constant stirring. This was followedby slow addition of 5 ml water and the solution was stirred at room

temperature for 3 h to get clear solution. Slow evaporation of thisfiltered solution kept at room temperature gave microcrystallinematerial within a weeks time. Re-crystallization of this microcrys-talline materialby dissolving in hotwaterfollowedby slow evapora-tion resulted in twotypes of crystals with differentmorphology, viz.,diamond and plate shapes named as CC3 and CC4 respectively. Theoptical snap-shots of CC3 and CC4 are given inSupplementary dataFig. S5.

2.5.3.1. Melting point (CC3). 230232 C,CHN (CC3): Calculated val-ues (%): C = 49.14; H = 4.47; N = 19.11. Observed values (%):C = 50.02; H = 5.01; N = 18.98; 1H NMR data (CC3): d= 5.189 (s,4H; methylene-bix); 6.969 (s), 7.071 (s), 7.252 (s), 7.874(s), (12H,aromatic) IR spectral data (CC3): (mmax(cm

1))); 3424 (br),3189(s), 3135(s), 3036(m), 1658(m), 1597(s), 1539(s), 1443(m),1339(s), 1275(m), 1188(m), 858(m), 799(s), 765(m), 636(w),527(w) CCDC number for CC3: CCDC 746437.

2.5.3.2. Melting point (CC4). 202205 C,CHN (CC4): Calculated val-ues (%): C = 55.33; H = 4.89; N = 20.38. Observed values (%):C = 54.22; H = 5.32; N = 19.98.

2.5.3.3. NMR data (CC4). d= 5.180 (s, 4H; methylene-bix); 6.942 (s),7.054 (s), 7.244 (s), 7.823 (s), (11H, aromatic) IRspectral data (CC4):(mmax (cm

1)) 3421(br), 3188(s), 3136(s), 3103(s), 3037(m),1652(m), 1597(s), 1542(m), 1445(m), 1340(s), 1275(m), 1188(m),1102(s), 799(m), 763(s); 637(w), 522(w). CCDC number for CC4:CCDC 746436.

3. Results and discussion

Reliability of the COOH Im and COO

Him+

synthons in thepresence of potential weak intermolecular interactions such as

Fig. 2. ORTEP diagrams a, b, c, and d for the compounds, CC1, CC2, CC3 and CC4 respectively (the thermal ellipsoids are drawn with 50% probability factor).

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p p stacking and interactions with solvent molecules has beenexploited in the present investigation using ditopic N-donor ligandbix and dicarboxylic acids. The present system is chosen for a vari-ety of reasons. (a) Ability of dicarboxylic acids to involve in OH N hydrogen bonding interaction with the imidazole nitrogensof the bix moiety. (b) Possibility to fabricate organic salts by thetransfer of one or both protons from dicarboxylic acid to the imid-azole nitrogens of the bix moiety, depending on DpKavalues of theacid and bix used in the reaction via NH+ O interactions in thesupramolecular assembly. When carboxylic acids interact with

imidazole and closely related systems such as benzimidazole as aresult of proton transfer, the solid-state outcome is often an

organic salt [4143]. Attempted reactions between bix withH2chdc and H2npdc resulted in CC1 and CC2 which are cocrystaland monocrboxylate salt of the corresponding acids. Two moremolecular adducts (CC3 and CC4) were obtained from the samepot reaction between bix and H2pzdc with different acid/base stoi-chiometry. These adduct exhibit interesting combinations of acidto amine proton transfer. Characterization by various analytical

methods such as CHN, NMR, IR and TGA has been carried out toestablish the formation of the molecular adducts. Detailed struc-tural investigations of all these four compounds revealed themolecular interactions in establishing the versatile supramoleculararrangement within the binary complexes. In this contribution, wehave demonstrated the ability of carboxylic acid/carboxylate andamine/protonated amine functional groups exhibiting a variety ofhydrogen bonding motifs involving two supramolecular heterosynthons and correlated the formation of the cocrystal/salt basedon the DpKavalues of the respective acid and base used in the reac-tion. Upon the basis of information extracted from the CambridgeStructural Database (CSD-2010 release) [44], when a carboxylicacid is allowed to react with a symmetric ditopic base such asbix, there are strong bias toward cocrystal formation, in preference

to organic salts. Out of the five total crystal structures retrievedfrom the CSD containing bix and carboxylic acid, four are cocrystals

Table 1

Hydrogen bonding interactions in CC1, CC2, CC3 and CC4.

Compound DH A d(H A) () d(D A) () \DHA ()

CC1 O(1)H(1C) N(2)1 1.70 2.663(4) 172C(3)H(3) O(2)2 2.56(4) 3.287(5) 134(3)C(4)H(4B) O(2)2 2.49(4) 3.344(4) 141(3)

Symmetry code: (1) 1 x, 1 y, z; (2) x, 1 y, z

CC2 N(2)H(2A) O(2)1 1.71(5) 2.668(3) 173(4)O(3)H(3C) O(7)2 1.69(5) 2.644(3) 167(4)O(5)H(5C) O(6)3 1.89(5) 2.805(4) 177(6)O(5)H(5D) O(7)4 2.03(4) 2.933(3) 172(4)O(6)H(6C) O(5)4 1.97(3) 2.828(4) 175(3)O(6)H(6D) O(1)5 1.89(5) 2.772(3) 173(4)O(7)H(7C) O(2)6 1.89(4) 2.735(3) 154(3)O(7)H(7D) O(1)7 1.90(6) 2.741(4) 171(5)C(3)H(3) O(5)8 2.33 3.184(4) 153C(4)H(4B) O(4)9 2.46(3) 3.350(4) 152(3)C(13)H(13) O(4)10 2.30(3) 2.918(4) 120(3)C(14)H(14) O(6)11 2.58(3) 3.340(4) 136(2)C(16)H(16) O(2)10 2.46(3) 3.026(4) 119(2)

Symmetry code: (1) 1/2 +x, 3/2 y, 1/2 +z; (2) 1/2 x, 1/2 +y, 1/2 z; (3) 3/2 x, 1/2 +y, 1/2 z; (4) 1/2 + x, 1/2 y, 1/2 +z; (5) 1 x, 1 y, 1 z; (6) 1/2 x, 1/2 +y, 3/2 z; (7) 1 x, y, 1 z; (8) x, 1 y, 1 z; (9) 1/2 +x, 3/2 y, 1/2 +z; (10)x, y, z; (11) 1/2 +x, 1/2 y, 1/2 +z

CC3 N(2)H(2C) N(3)1 1.73(6) 2.816(5) 165(5)O(2)H(2D) O(5)2 1.80 2.596(4) 163N(4)H(4C) O(4)3 1.73(4) 2.688(5) 163(3)O(5)H(5C) O(3)4 1.65 2.697(4) 157O(5)H(5D) O(3)5 2.16 2.886(4) 161C(1)H(1) O(1)4 2.41 3.236(6) 147C(3)H(3) O(4)6 2.29 3.056(5) 139C(6)H(6) O(1)4 2.47 3.353(5) 159C(7)H(7) O(3)6 2.57 3.465(5) 161

Symmetry code: (1) 1 +x, y, 1 +z; (2) x, y, 1 +z; (3) 1 x, y, z; (4) 1/2 +x, 1/2 y, 1/2 +z; (5) 1/2 +x, 1/2 y, 1/2 +z; (6) 2 x, y, 1 z

CC4 N(2)H(2C) O(3)1 1.78(3) 2.707(3) 175(2)N(4)H(4C) O(2)2 1.44(4) 2.562(3) 176(3)N(5)H(5C) O(3)3 1.99(4) 2.796(3) 148(3)O(5)H(51) O(4)4 1.98 2.845(3) 173O(5)H(52) O(2)5 1.99 2.841(3) 163C(2)H(2) O(4)6 2.56(4) 3.317(4) 138(3)

C(3)H(3)

O(1)

7

2.39(2) 3.267(3) 151.4(19)C(3)H(3) N(6)7 2.34(2) 3.100(3) 135.6(18)C(6)H(6) O(1)7 2.38(3) 3.248(3) 150(2)C(9)H(9) O(2)7 2.38(3) 3.359(3) 171(2)

Symmetry code: (1) 1 +x, y, 1 +z; (2)x, y,z; (3) 2 x, 1 y, z; (4) 1 +x, y, z; (5)x, y,z; (6) 1 x, y, 1 z; (7) 1 x, 1 y, 1 z

Table 2

pKa values for the precursors of the salt/cocrystals.

Salt/cocrystal former pKa DpKa

1,4-Bis(imidazol-1-ylmethyl)benzene (base) 7.05trans-1,4-Cyclohexanedicarboxylic acid (CC1 precursor) 4.18 2.87

5.42 1.63

1,4-Naphthalenedicarboxylic acid (CC2 acid precursor) 2.97 4.08

1H-pyrazole-3,5-dicarboxylic acid (CC3 and CC4 precursor) 2.99 10.043.24 3.81



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involving dicarboxylic acid and the rest one is a 1:2 organic saltwhere the bix moiety is diprotonated due to the proton transferfrom the sulphonic acid functionality and the carboxylic acid re-mains intact[45,46].

4. FTIR and proton NMR spectroscopic studies

The adduct formation between bix and various dicarboxylicacids was explored by FTIR and proton NMR spectroscopy. 1HNMR Spectra for the constituents (bix and dicarboxylic acids) andthe corresponding molecular adducts are shown inFig. 1. High res-olution proton NMR spectra of all adducts are also given inSupple-mentary data (Fig. S3). FTIR spectra of CC1 showed overtone bandsranging 18002500 cm1 indicating the presence of aromatic ringsystem supporting the presence of bix moiety in the adduct. Med-ium band at 1236 cm1 can be assigned to CN stretching of bix.Sharp peaks at 3135 and 1695 cm1 wavenumbers were assignedto OH and C@O stretching respectively from H2chdc. However,absence of the strong signal in the region around 3400 cm1 (forelectron deficient NH group) indicates the absence of protontransfer between acid and bix moieties in CC1. The supposition ac-quired from FTIR spectra are well supported by the 1 H NMR data.The proton NMR spectra of CC1 in d6-DMSO shows no prominentchange in chemical shifts of the constituents, i.e., the signals corre-sponding to bix and H2chdc moieties in CC1 appeared at the samepositions that of the pure bix and pure H2chdc. This observation in-fers that the interaction between bix and H2chdc is very feeble andthus rules out the possibility of proton transfer as confirmed withthe structural data. Moreover, integration of representative peakssuggests 1:1 stoichiometry of bix and H2chdc in the molecularadduct.

The FTIR spectra of CC2 encompass a medium band at3410 cm1 and a sharp bands at 1540 cm1 wavenumbers whichcan be attributed to NH stretch and bending vibrations of theelectron deficient heterocycles, apparently indicating bix is proton-

ated in CC2. Peaks at 3111 cm1 and 1692 cm1 were assigned toOH stretch and C@O stretch of carboxylic acid (COOH) grouprespectively. Moreover, peak at 1620 cm1 along with shouldersaround 1525 cm1 and 1570 cm1 can be attributed to the pres-ence of carboxylate (COO) functionality inferring part of H2ndpcis in deprotonated form. In the 1H NMR spectra imidazole protonsof bix were allocated at 7.71, 7.14 and 6.87 ppm; whereas in CC2corresponding signals appeared downfield at 8.82, 8.12 and7.67 ppm respectively. The considerable downfield shift of theimidazole protons clearly indicates the decrease in electron densityon imidazole rings that suggest the proton transfer in adduct. Inte-gration ratio of assigned peaks advocate 1:2 stoichiometry of bixand H2chdc in CC2.

FTIR spectra of CC3 and CC4 include a medium band around

3425 cm1

and a sharp band at 1655 cm1

. These two bands corre-spond to the NH stretch and bending in electron deficient hetero-cycles signifying protonated bix in these molecular adducts. Thesharp peaks at 3190 cm1 and 1658 cm1 observed for CC3 canbe assigned to OH and C@O stretch of carboxylic acid (COOH)group respectively. The presence of carboxylate (COO) functional-ity can be inferred for both CC3 and CC4 by assigning peaks at1600 cm1 (asym COO stretch) and 1540 cm1 (sym COO stretch)leading to the conclusion that CC3 and CC4 are molecular saltsformed by the proton transfer between bix and H2pzdc. The protonNMR spectra of CC3 and CC4 are highly perturbed indicating stronginteraction among the constituents. However, few protons particu-larly those are less likely to participate in hydrogen bonding areaccountable when spectra was recorded for CC3 and CC4 in d6-

DMSO solvent. Thus methylene protons of bix resonating at5.1 ppm (singlet) and the aromatic CH proton of H2pzdc that

resonates at 7.7 ppm (singlet) remains almost intact even after ad-duct formation. The ratio of integral values for methylene proton toaromatic CH proton evidently indicates the stoichiometrybix:H2pzdc in CC3 and CC4 as 1:2 and 1:1 respectively.

5. Thermal analysis

Thermogravimetric analysis for the compounds CC2, CC3, andCC4 which contains water molecules as solvent of crystallizationwas carried out on METTLER TOLEDO STAR SW 7.01 instrumentunder N2 flow in the temperature range 25400 C at a heatingrate of 10 C/min. The observed data finds good match with the cal-culated ones and reveal that the salts CC2, CC3 and CC4 lose theirlattice water nearby the boiling point of H2O and subsequently ob-tained anhydrous salts exhibit fairly thermal stability up to thetemperatures as high as 250 C. In the case of CC2 a mass loss of13.27% in the temperature range 30140 C (13.88% calculated va-lue) corresponds to the loss of all lattice water molecules. Thedehydrated compound remain intact up to 260 C temperatureafter that decomposition commences. For CC3 a weight loss of5.24% between 100 and 220 C indicates the loss of two water mol-

ecules located in the lattice (calculated weight loss 6.14%). Decom-position of the CC3 starts at 270 C. In the case of CC4 a weight lossof 7.04% between 100 and 160 C corresponds to the removal oftwo water molecules in the lattice (calculated weight loss 8.37%).The adduct starts to decompose at 250 C. The TGA data for, CC2,CC3 and CC4 is given inSupplementary information (Fig. S4)whichcorroborate well with the structural data.

6. Crystal and molecular structure of bix:H2chdc (CC1)

CC1 crystallizes in triclinic system with P-1 space group and theasymmetric unit of CC1 is composed of half a molecule each of bixand H2chdc as shown inFig. 2a. In the 1:1 cocrystal no protontransfer has been occurred between the acid and the base which

is reflected in the bond distance involving C8 carboxylic acid group(C8O1 = 1.318(3) and C8O2 = 1.198(3) ) form the crystalstructure (Table S2). Both bix and H2chdc molecules possesses cen-ter of symmetry at the mid-point of the phenyl and cyclohexanerings respectively. Expected primary OH N hydrogen bond iscoined between the N-donor ligand and the flexible dicarboxylicacid (from the OH group on the dicarboxylic acid with the imida-zol-1-yl nitrogen atom) of the symmetrically disposed moleculesfrom either side extending a one dimensional network oriented al-most diagonal to ab-plane. This is further associated via CH pinteraction between the imidazole ring of the symmetrically dis-posed bix and methyl hydrogen H4a (H4a Cg = 2.71 ) in the for-mation of two dimensional nets as depicted inFig. 3a.

Cocrystals incorporating bix with a series of both aliphatic and

aromatic dicarboxylic acids is reported by Aakery et al. to under-stand the driving force for the directed assembly in a wide range ofbinary cocrystals shows similar pattern with OH N distancewithin the range reported in the present investigation[14].

In an attempt to understand the orientation and arrangement ofthe constituents within this cocrystal, secondary hydrogen-bond-ing interactions and packing mode have been analyzed in detail.Packing and hydrogen-bonding interactions of the cocrystalviewed downc-axis is given inFig. 3b showing OH N hydrogenbonded layers of the cocrystal are oriented diagonal to ab-plane. Inaddition to the primary OH N interaction, secondary hydrogenbonding interaction involving H3 from the Imidazole ring andmethylene hydrogen H4B from bix via bifurcated CH O contactwith O2 of the carboxylic acid is also observed. Hence, the carbonyl

oxygen O2 acting as an acceptor in a bifurcated CH O H-bondingto generate two-dimensional hydrogen bonded sheet like network

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(Fig. 3b) linking the zigzag chain of heteromeric synthon betweenacid and bix. Details of all these pertinent hydrogen-bonding inter-actions with symmetry code is given inTable 1. In order to makeeffective OH N, CH O interactions with carboxylic acid thebix moiety has adopted the anti conformation in CC1. The mean

plane angle between the symmetrically disposed imidazole moie-ties with the central phenyl ring is 80.05. Additional packing dia-gram with hydrogen bonding interaction viewed down b-axis forCC1 is given asFig. S6 in Supplementary material.

7. Crystal and molecular structure of H2bix:(Hnpdc)26H2O (CC2)

The asymmetric unit ofCC2 consists of half portion of proton-ated bix moiety possessing center of symmetry at the mid-pointof the aryl ring, one monoanionic 5-carboxynaphthalene-1-carbox-ylate (Hnpdc) along with three molecules of water as solvent ofcrystallization (Fig. 2b). N+H O interaction (N2 O2 =2.668(3)) between the carboxylate oxygen O2 from Hnpdc andH2A of the protonated imidazole nitrogen from either side of the

symmetrically disposed ditopic ligand create a trimeric supramo-lecular arrangement as shown inFig. 4a.

The protonation of the imidazole nitrogen is reflected in thelonger N2C3 bond (N2C3 = 1.331(4) A

0

) compared to the free li-gand[22]. The bond length of the carboxylate group indicate thatcarboxyl group involving C18 is deprotonated (O(1)C(18) = 1.249(3) A

0

, O(2)C(18) = 1.273(3) A0

) and C19 carboxyl

group is protonated (O(4)C(19) = 1.218(3) A

0

, O(3)C(19) =1.315(3) A0

)(Table S2) signifying the carboxylate ligand is in mono-anionic from. The trimeric supramolecular units are oriented al-most diagonal to ab-plane. The offset arrangement of the trimericH-bonded moiety oriented along b-axis make effective stackinginteraction between the protonated imidazole ring with the arylring of the Hnpdc with the stacking distance (3.678 and 3.436 ),between the centroids of the rings as depicted inFig. 4b.

It is interesting to note that all the three water molecules pres-ent in the asymmetric unit are involved in strong OH O hydro-gen bonding interaction generating a zigzag one-dimensionalwater cluster involving O5 and O6 and a dangling water moleculeO7 attached to O5 alternatively along b-axis as depicted inFig. 4c.Thus, H6C from O6 is making contact with O5 and H5C from O5 ismaking contact with O6 with O O distances 2.828(4) and2.805(4) and the corresponding OH O angles 175(3) and

Fig. 3. (a) Mercury diagram depicting the interaction between the bix and H2chdc in the formation of extended 2D supramolecular network in CC1; (b) packing diagramviewed downc-axis showing the diagonal orientation of the H-bonded cocrystal (bix-H2chdc) layer and the H-bonding interaction between these layers in the formation oftwo dimensional networks.

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177(6)in building the zigzag water chain. The second hydrogenatom H5D from O5 further acts as a donor to attach the danglingwater molecule O7 alternatively at a distance O5 O7 =2.933(3) and \O5H5D O7 = 172(4) respectively. In an at-tempt to understand the interaction of the water cluster with thetrimeric molecular adduct, we have analyzed the packing modeand hydrogen bonding interactions in detail. Packing diagram ofthe compound with various hydrogen-bonding interactions is de-picted inFig. S7.

It is observed that the trimeric moiety is involved in various

hydrogen-bonding interactions with the water cluster. Thus, thehydrogen atom H3C of the monoanionic carboxylic acid O3 of thesymmetrically disposed trimeric supermolecule acts as a donorand is involved in OH O interaction with the dangling oxygenof the water cluster O7 (O3 O7 = 2.644(3) A

0

). H7C and H7D ofthe dangling water molecule O7 is involved in hydrogen bondingwith the carboxylate oxygens of the acid O1 and O2 of the Hnpdcrespectively. H6D which is not involved in the cluster formationis drawn in OH O contact with O1. Imidazole hydrogen H3and the methylene hydrogen H4B is making intramolecular CH O contact with water oxygen O5 from the cluster and the car-bonyl oxygen O4 of the acid moiety respectively. Hydrogen atomsH13, H14 and H16 of the naphthalene rings are involved in weakCH O interaction with O4, O6 and O2 respectively. The C

H O contact between methylene hydrogen H4B of the bix moietywith the O4 of the acid described above is effective in bringing the

trimeric species closer towards the stacking of the protonatedimidazole ring with the phenyl moiety of the acid down b-axis.The binary salt aggregate via N+H O interaction regulate assupramolecular trimeric species is further involved in various interand intramolecular OH O and CH O interactions with theone-dimensional water cluster in the formation of stable two-dimensional hydrogen bonded network. Details of these varioushydrogen-bonding interactions with symmetry code are given inTable 1.

8. Crystal and molecular structure of H2bix:(Hpzdc)22H2O (CC3)

Re-crystallization of the microcrystalline material obtained byreaction between bix and H2pzdc dissolving in hot water followedby slow evaporation resulted in two types of crystals with differentmorphology (diamond shaped CC3 and plate shaped CC4 crystals,Fig. S5). Structure determination by X-ray diffraction methods re-vealed that the compound CC3 crystallized in monoclinic systemwith P21/n space group. The asymmetric unit of CC3 is composedof half a molecule of fully protonated bix moiety with the centerof symmetry, one molecule of 5-carboxypyrazole-3-carboxylatein monoanionic form (Hpzdc) and water as solvent of crystalliza-tion with 1:2 ratio of H2bix and Hpzdc in the crystal lattice.

As mentioned earlier the acidic component is monoanionic inCC3, in which one terminus remains as carboxylic acid (O(1)C(11) = 1.192(4) , O(2)C(11) = 1.326(4) while the other one ex-ists as carboxylate (O(3)C(12) = 1.258(4) and O(4)C(12) =1.238(4) ), which is obvious form the bond distances calculatedfrom the X-ray diffraction studies. In the 1:2 salt imidazole nitro-gen N2 of bix ligand is protonated which shows longer bond dis-tance (N2C2 = 1.363(5) ) with the neighboring carbon. It isinteresting to note that, the pyrazole monocarboxylates (Hpzdc)are associated as a centro-symmetric dimer by NH O interac-tions between the pyrazole hydrogen H4C and free carboxylateoxygen O4 (N4 O4 = 2.688(5) , \N4H4C O4 = 163(3)). Four

pairs of such centro-symmetric dimers positioned at the cornersof the rectangle are bridged via OH O interaction with fourwater molecules located at the center of the dimeric units generat-ing rectangular grid (Fig. 5a) with dimensions 18.267 10.118across the hydrogen bonded water molecules. Thus, water mole-cule O5 acts as both donor and acceptor in the OH O interactionin which H5C is making contact with the carboxylate oxygen O3(O5 O3 = 2.697(4) , \O5H5C O3 = 157) and H2D of the car-boxylic acid is involved in OH O interaction with O5(O2 O5 = 2.596(4) , \O2H2D O5 = 163) from either side inthe formation of the two dimensional rectangular grids which istranslated alongbc-plane. These two dimensional grids are furtherbridged via OH O hydrogen bonds from the water hydrogenH5D with carboxylate oxygen O3 of the adjacent 2-D grid layer

down a-axis, generating a three dimensional hydrogen bondednetwork with through channels. It is remarkable that the proton-ated bix moiety is encapsulated within this channel by varioushydrogen-bonding interactions as depicted in packing diagramviewed down a-axis Fig. 5b. Hence, the symmetrically disposedprotonated imidazole nitrogen of the bix moiety is making N+H N interaction (N(2) N(3) = 2.816(5) ; \N(2)H2C N(3) =165(5)) with the pyrazole nitrogen of the carboxylate dimer atthe corners of the hydrogen bonded rectangular grid by orientingalmost diagonal to the rectangular channel. In addition to thisN+H N interaction, CH O contacts do exist between imidaz-ole hydrogens H1 and H3 from either ends of the symmetricallyoriented H2bix as well as phenyl hydrogen H6 and H7 with theoxygen atoms of the carboxylate ligand O1, O4 and O1, O3 respec-

tively in encapsulating the bix moiety strongly inside the rectangu-lar cavity (Fig. S8). Details of all these pertinent hydrogen-bonding

Fig. 4. (a) Building blocks in CC2, H2bix and Hnpdc are linked via NH Ointeraction into a trimeric supermolecule; (b) stacking interaction between the

protonated imidazole rings of the H2bix moiety and the aryl ring of the monocarb-oxylate Hnpdc anion in CC2; (c) hydrogen bonding interactions of the watermolecules present in the asymmetric unit generating one-dimensional watercluster in CC2.

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interactions with symmetry code is given in theTable 1. The driv-ing force in the cavity containing supramolecular assembly is thedimeric association of the 5-carboxypyrazole-3-carboxylate inthe formation of the rectangular grids with lattice water moleculesand the association of the H2bix in the grids by N +H N and CH O interactions rather than the expected N+H O contactsbetween the heterosynthons.

9. Crystal and molecular structure of H2bix:pzdc2H2O (CC4)

Re-crystallization of the micro-crystals obtained by the reactionof bix with and H2pzdc from hot water gave single crystals of CC4

with plate shaped morphology suitable for X-ray diffraction stud-ies along with diamond shaped CC3. CC4 crystallized in triclinic

system with P-1 space group and the crystal structure of the com-pound revealed that complete proton transfer has been occurredfrom the acid to the bix moiety resulting an 1:1 organic salt. Theasymmetric unit of the salt is constituted by two half bix moietiesboth possessing a center of symmetry at the mid-point of the phe-nyl ring, one fully deprotonated dianionic pzdc moiety along withwater as solvent of crystallization (Fig. 1d). The stoichiometry be-tween the protonated bix to completely deprotonated acid is thus1:1 in the unit cell. The deprotonation of the dicarboxylic acid is re-flected in the bond distances involving the carbon atoms C18 andC19 of the carboxylate group (O(1)C(18) = 1.230(3) , O(2)C(18) = 1.284(3) and O(3)C(19) = 1.259(3) , O(4)C(19) = 1.237(3) ) observed from the crystal structure. Slight dif-ference in bond distance around protonated nitrogens N(2) andN(4) with the adjacent carbon atoms of different imidazole moietyin bix indicates the protonation of the imidazole nitrogen(Table S2).

As depicted inFig. 6a, both symmetrically protonated bix pres-ent in the asymmetric unit are placed alternatively between thecompletely deprotonated dianionic pzdc involving NH+ O

(N2 O3 = 2.707(3) ; N4 O2 = 2.562(3) ) interaction with thecarbonyl oxygen of the carboxylate group from either side generat-ing a one dimensional hydrogen bonded inter-twined strands.Within the inter-twined strands effective p pstacking betweenthe central phenyl rings of the bix moiety is observed(Cg Cg = 3.74 ). The carboxylate oxygen O3 is involved in abifurcated NH O with the pyrazole hydrogen H5C(N(5) O(3) = 2.796(3) ) stabilising the inter-twined strands inthe crystal lattice. Eventhough, both symmetrically disposed bixmolecule present in the unit cell isanticonformation, it is observedthat the mean plane involving the central phenyl ring with respectto the imidazole ring in the respective bix ligand shows slightvariation (Mean plane between the phenyl ring C5 to C7 withimidazole ring N1C1C3N5 = 70.80 and C11 to C14 with N3C10C12N4 = 74.80). This may be due to the self orientation ofthe flexible bix moiety for making effective hydrogen bonding

and weak p p stacking interaction between the phenyl ringswithin the strands while packing. In an attempt to understandother secondary hydrogen bonding interaction of these strandswith the lattice water molecules and neighboring strands we haveanalyzed the packing mode and hydrogen bonding interactions indetails. The crisscross p p stacked strands are oriented almostdiagonal to bc-plane. The lattice water hydrogen acts as donorsin strong OH O interaction with the terminal carboxylateoxygens O4 and O2 of the deprotonated dianionic 3,5-pyrazoledicarboxylate (pzdc). Intermolecular CH O and CH N inter-actions do exist between the carboxylate oxygen of the dicarboxyl-ate ligand and the imidazole hydrogens of the bix moiety withinthe twined strands. Thus, imidazole hydrogens H2 and H3 are mak-ing contacts with carboxylate oxygens O4 and O1 respectively and

H3 is making an additional contact with the pyrazole nitrogen N6.In addition to the above, phenyl hydrogen H6 is in contact with thecarboxylate oxygen O6 in stabilization of the hydrogen bondedcrisscross strands within the crystal lattice. The adjacent layeredinter-twined double strands are bridged along c-axis via imidazolehydrogen H9 of the symmetrically disposed bix from either side togenerate a two-dimensional hydrogen bonded network as depictedinFig. 6b. Details of all these hydrogen-bonding interactions withsymmetry codes are given inTable 1.

10. Prediction of salt vs cocrystal formation based on pKavalues

The pKa values for the constituent acids and base used in the

present investigation are obtained from different references andare listed inTable 2[47]. Formation of salt/cocrystal from the reac-

Fig. 5. (a) Ball and stick representations of the rectangular hydrogen bonded gridsin which the H2bix moiety is encapsulated; (b) packing diagram of CC3 with varioushydrogen bonding interaction viewed down a-axis showing the hydrogen bondedrectangular grids with encapsulation of the H2bix in the two dimensional network.

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structural and molecular recognition features. The salient featuresof recognition patterns in terms of the pKavalues of the constitu-ents clearly indicates that the proposed rule for the cocrystal/saltformation is in well agreement considering the DpKa value. Thisinformation may be quite useful in further target-oriented supra-molecular synthesis and also in many computational studies con-cerning intermolecular interactions including polymorph andcrystal structure predictions. Studies pertinent to multi-compo-nent cocrystals/salts offer structural diversity, composition, andproperties which will continue to attract attention from solid-statechemists and pharmaceutical scientists.

Acknowledgments

Authors gratefully acknowledge the Department of Science andTechnology (DST), New Delhi, India (Grant No. SR/S1/IC-37/2006)and CSIR, India (Grant No. NWP-0010) for financial support. ACKand KKB acknowledges DST (India) and CSIR for a SRF and JRFrespectively. SD is thankful to Dr. Parimal Paul, CSMCRI, Bhavnagarfor his M.Sc. dissertation work in analytical discipline. The authorsare grateful to Mr. Hitesh Bhatt for recording NMR spectra, Mrs.Sheetal N. Patel for TGA data, Mr. Viral Vakani for microanalysis,Mr. Vinod Kumar Agrawal for IR data, and Dr. P. Paul for all-roundanalytical support.

Appendix A. Supplementary material

CCDC 746434 to 746437 contains the supplementary crystallo-graphic data for compounds 14. These data can be obtained freeof charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: [email protected]. Synthesis of bix; proton NMR spectra of allreported compounds; TGA plots for CC2, CC3 and CC4; opticalimages of CC3 and CC4, additional crystallographic diagrams; sum-mary of crystallographic data and selected bond distances and an-

gles for all compounds are given in supplementary material.Supplementary data associated with this article can be found, inthe online version, atdoi:10.1016/j.molstruc.2010.11.022.

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http://www.ccdc.cam.ac.uk/conts/retrieving.htmlhttp://dx.doi.org/10.1016/j.molstruc.2010.11.022http://research.chem.psu.edu/brpgroup/pKa_compilation.pdfhttp://research.chem.psu.edu/brpgroup/pKa_compilation.pdfhttp://research.chem.psu.edu/brpgroup/pKa_compilation.pdfhttp://research.chem.psu.edu/brpgroup/pKa_compilation.pdfhttp://dx.doi.org/10.1016/j.molstruc.2010.11.022http://www.ccdc.cam.ac.uk/conts/retrieving.html

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