Paper

The co-crystallisation of pyridine with benzenepolycarboxylic acids:

The interplay of strong and weak hydrogen bonding motifs

Sophie H. Dale,a Mark R. J. Elsegood,*a Matthew Hemmingsb and

Alexandra L. Wilkinsonb

aChemistry Department, Loughborough University, Loughborough, Leicestershire, UK

LE11 3TUbFormerly of School of Natural Sciences (Chemistry), Bedson Building, University of

Newcastle-upon-Tyne, Newcastle-upon-Tyne, UK NE1 7RU

Received 26th March 2004, Accepted 28th May 2004

First published as an Advance Article on the web 9th June 2004

Co-crystallisation of pyridine with terephthalic, trimesic, phthalic, isophthalic and pyromellitic acids has been

investigated via single crystal X-ray diffraction and thermogravimetric analysis, concentrating on the nature of

the intermolecular interactions. The five new compounds are terephthalic acid bis(pyridine) solvate, trimesic

acid tris(pyridine) solvate, pyridinium hydrogen phthalate, isophthalic acid pyridine solvate and pyridinium

trihydrogen pyromellitate. A variety of supramolecular hydrogen bonded motifs involving interactions between

pyridine molecules and carboxylic acid groups are observed rather than just the R22(7) O–H…N/C–H…O motif;

while the common carboxylic acid head-to-tail ring motif is absent in all of the examples investigated. TGA

analysis of pyridine loss from two of the compounds has been shown to correlate with the strength of hydrogen

bonds in the crystal structures.

Introduction

The cyclic hydrogen bonded carboxylic acid ring motif R22(8) X

(Scheme 1),1 while being the second highest observed motifin the Cambridge Structural Database,2 has only a 33%probability of occurrence due to competing water, solventmolecules and other functionality in the structure.3 Variousstructures comprising benzenepolycarboxylic acids with sol-vents of crystallisation4 have been elucidated over the years.Benzenepolycarboxylic acids have been used extensively by thesupramolecular chemist for their numerous divergent carboxylgroups, useful in the creation of extensive arrays throughhydrogen bonding and metal-coordination bonds. Manysupramolecular structures have utilised trimesic acid due toits planar trigonal geometry,5 highly useful in the self-assemblyof two-dimensional structures.

Numerous authors have investigated the co-crystallisation ofcarboxylic acid-functionalised molecules with pyridine deriva-tives,6 with many notable examples utilising members of thebenzenepolycarboxylic acid family. These authors have clearlyhighlighted the carboxylic acid–pyridine supramolecular

synthon R22(7), Y, (Scheme 1) as a tool for the construction

of further supramolecular networks. Y exists through the com-plementarity of the strong O–H…N hydrogen bond7 andthe considerably weaker C–H…O interaction,8 and it wassuggested5,6a that Y, based on its predictable formation, ismore stable than the carboxylic acid head-to-tail dimer X. Arecent paper6c has added much to the study of the robustacid–pyridine synthon, with energy calculations showing theformation of Y to be more energetically favourable than thealternative acid–acid and pyridine–pyridine hydrogen bondedaggregates. Of course, whether the carboxylic acid–pyridinesynthon takes on the form Y or its ionic analogue Z (Scheme 1)depends greatly on the acidic nature of the carboxylic acidand the basicity of the pyridine derivative in question.6c,6f Ingeneral, the protophilic nature of pyridine leads to the forma-tion of a common species, the pyridinium ion, when acids areappreciably dissociated in the solvent.

Pyridine has a great influence on synthetic chemistry as anon-aqueous solvent,9 for a number of reasons:. Solubility of a wide range of compounds is aided by the

polarity of the pyridine molecule, even though it possesses arelatively low dielectric constant.. The stable aromatic nature of the pyridine molecule leaves it

the only truly aprotic organic solvent.. Pyridine’s heteroatom makes it ideal as a basic solvent

catalyst, particularly in nucleophilic substitution reactions, andgives solute–solvent interactions through its strong Lewis basecharacter.

This economic and widely available solvent provides an idealsimplistic building block which may model many of the moreelaborate co-crystals previously reported. Here we investigatethe existence of synthons Y and Z in the adducts of fivebenzenepolycarboxylic acids (Scheme 2) with pyridine, andanalyse the range of supramolecular structures resultingfrom the variation in the number and positioning of the poly-carboxylates’ divergent carboxyl functionality.

After reflecting on the recent discussions led by Desiraju10

and Dunitz,11 we continue to adopt the term co-crystal in the

Scheme 1 Common acid–acid and acid–pyridine hydrogen-bondedsupramolecular synthons. X represents the homomeric carboxylic acid–carboxylic acid R2

2(8) synthon; Y represents the heteromeric carboxylicacid–pyridine R2

2(7) synthon; Z represents the ionic analogue of Y, withproton-transfer to the N atom of the pyridine group.

DOI: 10.1039/b404563g CrystEngComm, 2004, 6(36), 207–214 207

This journal is # The Royal Society of Chemistry 2004

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description of the product of co-crystallisation reactions, whileusing the term salt to describe the product of proton-transferreactions.12 The five carboxylic acid–pyridine co-crystals andsalts described herein are shown schematically in Scheme 3.

Experimental

All reagents were commercially available and used as received.

Syntheses

The relevant acids, except pyromellitic acid, were dissolved inneat pyridine with heating to give a saturated solution, andthen the filtered solutions were allowed to cool slowly andevaporate at room temperature, yielding colourless crystals ofX-ray quality, in essentially quantitative yield. Pyromelliticacid was crystallised with pyridine using an H-tube, where onevertical tube was filled with a methanolic solution of pyromel-litic acid and the other with pure pyridine, while the horizontalcross-tube was filled with pure methanol. The H-tube wassealed and stored at room temperature for five days, during

which time X-ray quality crystals grew in each of the verticaltubes.

Elemental analyses and FT-IR data

Terephthalic acid bis(pyridine) solvate 1. Colourless crystalsobserved to desolvate at 45 uC. Analysis calculated forC18H16N2O4: C, 66.66; H, 4.97: N, 8.64. Found: C, 66.18; H,5.02; N, 8.85%; IR nmax(Nujol)/cm21 3200–2500 (br, OH), 3086and 3056 (aromatic C–H), 1680 (s, CLO), 1574 and 1509(aromatic CLC), 1286, 1136, 1112 and 1020 (C–O), 934, 872and 723 (aromatic C–H).

Trimesic acid tris(pyridine) solvate 2. Colourless crystalsobserved to desolvate at 60 uC. Analysis calculated forC24H21N3O6: C, 64.43; H, 4.73; N, 9.39. Found: C, 64.64; H,4.79; N, 9.83%; IR nmax(Nujol)/cm21 3500–2500 (br, OH),3106, 3085 and 3065 (aromatic C–H), 1712 (s, CLO), 1681,1674, 1667 and 1659 (CLN), 1615, 1586 and 1557 (aromaticCLC), 1218, 1192 and 1177 (C–O), 756, 745, 707, 688, 678(aromatic C–H).

Pyridinium hydrogen phthalate 3. Colourless crystals, mp77–80 uC. Analysis calculated for C13H11NO4: C, 63.67; H,4.52; N, 5.71. Found: C, 63.81; H, 4.55; N, 5.94%; IRnmax(KBr)/cm21 3099 and 3064 (aromatic C–H), 2500 (OH),1713 (s, CLO), 1665 (w, CLN), 1551 (asymm. CO2

2), 1486,1374 (symm. CO2

2), 1250, 1195, 1131 and 1079 (C–O), 953,903, 853, 794, 806, 757 and 726 (aromatic C–H).

Isophthalic acid pyridine solvate 4. Colourless crystalsobserved to desolvate over the range 50–60 uC. Analysiscalculated for C13H11NO4: C, 63.67; H, 4.52; N, 5.71. Found:C, 63.30; H, 4.56; N, 5.98%; IR umax(Nujol)/cm21 3096 (w,pyridinium N–H, aromatic C–H), 1730–1680 region indistinct(most pronounced absorbances at 1732, 1698 and 1682 (CLO)),1608 (aromatic CLC), 1283, 1144, 1069 and 1057 (C–O), 759,725, 696 and 675 (aromatic C–H).

Pyridinium trihydrogen pyromellitate 5. Colourless crystalsobserved to desolvate at 30 uC. Analysis calculated forpyromellitic acid.1.40 pyridine, C17H13N1.4O8 C, 55.96; H,3.59; N, 5.37; Found (average of several samples): C, 55.99; H,3.65; N, 5.47%; IR nmax(KBr)/cm21 3234, 3179 (pyridiniumN–H), 3094 and 3068 (aromatic C–H), 2487 (br, OH), 1706(CLO), 1633 (CLN), 1579 (asymm. CO2

2), 1490, 1348 and 1327(symm. CO2

2), 1286, 1159, 1141, 1062 and 1019 (C–O), 980,951, 866, 805, 761, 755 and 732 (aromatic C–H).

X-ray crystallography

Experimental details of the X-ray analysis are provided inTable 1. Data for compound 1 were collected using a BrukerAXS SMART 1K CCD diffractometer using Mo Ka radiation(l ~ 0.71073 A). Data for compound 2 were collected using aBruker AXS SMART 1K CCD diffractometer using synchro-tron radiation (l ~ 0.6890 A) at Daresbury SRS Station 9.8.13

Data for compounds 3, 4 and 5 were collected using a BrukerAXS SMART 1000 CCD diffractometer using Mo Karadiation (l ~ 0.71073 A). All structures were solved bydirect methods and refined by full-matrix least-squaresmethods on F2. OH and NH hydrogens were initially placedin geometric positions using a riding model, then their coor-dinates were freely refined. All other hydrogens were placed ingeometrical positions using a riding model. U iso was con-strained to be 1.2 times (1.5 for OH) Ueq of the carrier atom. In4, the refined occupancies of the disordered H atom wereH(1):H(1X) ~ 0.58(6):0.42(6). Programs used were BrukerSMART,14a SAINT,14a SHELXTL14b and local programs.Scheme 3 The five co-crystals and salts described in this paper, 1–5.

Scheme 2 The five benzenepolycarboxylic acids used in this study.

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Results and discussion

Synthesis of pyridine co-crystals and salts

Synthesis of compounds 1–4 proceeded smoothly using themethod previously discussed, somewhat surprising in the caseof 1 considering the poor solubility of terephthalic acid inthe majority of organic solvents. Due to its low solubility inpyridine, the co-crystallisation of pyromellitic acid withpyridine was attempted using an H-tube apparatus to facilitatethe slow diffusion of the two reagents through a methanolicmedium. This method led to the isolation of two compoundsas X-ray quality single crystals, namely 5 at high acid con-centration and the known compound bis(pyridinium) dihydro-gen pyromellitate,15 [HNC5H5]2[C10H4O8], at high pyridineconcentration.

Description of the structures and packing

Terephthalic acid bis(pyridine) solvate 1. Terephthalic acidco-crystallises with two molecules of pyridine per acid molecule(Fig. 1) yielding 1. The symmetrical nature of terephthalic acidlends itself to its position on a centre of inversion, with theunique molecule of pyridine in the asymmetric unit hydrogen-bonded to the carboxylic acid group via synthon Y (Table 2);the same motif observed in the 4,4’-bipyridine:trimesic acidsystem.6a Terephthalic acid is known to pack in the crystallinestate in one-dimensional ribbons, comprising chains of rings,with synthon X linking the molecules.16 In contrast, in thepresence of pyridine, the formation of synthon X is prevented

and 1 packs as discrete, zero-dimensional, trimolecular entitieslinked by weak C–H…O interactions, producing a herringbonenetwork.

Thermogravimetric analysis of 1. A crystalline sample of 1was subjected to thermogravimetric analysis (TGA) over thetemperature range 30–500 uC, with a temperature increase of10 uC min21. Two mass loss steps were observed. The first massloss, commenced at 36 uC and peaked at 71 uC, representing theloss of approximately two equivalents of pyridine (46.8% of theinitial mass). Further heating led to the complete decomposi-tion of the sample, peaking at 339 uC. This represents the lossof 46.3% of the initial mass, corresponding to one equivalent ofterephthalic acid. Thus both pyridine molecules are lost at thesame time, which correlates with the observed centrosymmetricstructure. To our knowledge these TGA studies represent thefirst on pyridine/carboxylic acid compounds.

Trimesic acid tris(pyridine) solvate 2. Trimesic acid co-crystallises readily with three equivalents of pyridine, yieldingthe thought-provoking structure 2, determined using synchro-tron radiation on station 9.8 at the SRS, DaresburyLaboratory, due to small crystal dimensions. Although theunit cell dimensions have been reported previously byZaworotko et al.,6a full structural details are presented here.The inclusion of pyridine in the solid state form of trimesic acidprevents the association of synthon X, with pyridine beingretained in the crystal as a result of multipoint recognitionvia strong O–H…N and weak C–H…O hydrogen bonds,17 aproperty familiar to other commonly included organic solventssuch as DMF,4a,4b DMSO4d and dioxane.4e

Interestingly, each of the three independent pyridine mole-cules in the asymmetric unit (Fig. 2) has a unique hydrogen-bonding environment (Table 3), bonding to one, two or threedifferent acid molecules—giving mono-bound A, doubly-bridging B and triply-bridging C pyridines (Scheme 4). Theangles between the pyridine rings and the aromatic ring of theacid molecule are A 23.9u, B 88.7u and C 78.2u. The presence ofthree independent pyridine hydrogen-bonding modes gives riseto numerous n(CLN) absorptions in the range 1681–1650 cm21

in the infrared spectrum.Pyridine A is bound to just one carboxylate group on one

acid molecule via synthon Y, as expected from the work ofZaworotko et al.6a Pairs of pyridines B bind two stacked acidmolecules together via O–H…N and C–H…O bonds, thecombination resulting in a puckered R4

4(14) ring, with analternating pattern emerging along the stack of acid molecules.

Fig. 1 A view of co-crystal 1. Hydrogen bonds are shown as dashedlines. Selected hydrogen bonding parameters for 1 are shown inTable 2. Click here to access a 3D interactive view.

Table 2 Selected hydrogen bonding parameters for 1

D–H…A D…A /A D–H/A H…A /A D–H…A /u

O(2)–H(2)…N(1) 2.6286(15) 1.00(2) 1.63(2) 175.8(18)C(5)–H(5)…O(1) 3.3086(18) 0.96 2.592 131.8

Table 1 Crystallographic data for compounds 1–5

1 2 3 4 5

Formula C18H16N2O4 C24H21N3O6 C13H11NO4 C13H11NO4 C15H11NO8

M/gmol21 324.33 447.44 245.23 245.23 333.25Crystal system Monoclinic Monoclinic Monoclinic Monoclinic OrthorhombicSpace group P21/n C2/c I2/a P21/c Pna21

T/K 160(2) 160(2) 150(2) 150(2) 150(2)a/A 9.9855(7) 24.427(3) 22.0544(19) 3.7969(3) 14.9369(10)b/A 7.3212(5) 13.9375(16) 3.7779(3) 17.2355(15) 12.9274(9)c/A 11.9778(9) 13.3061(16) 26.481(2) 17.2272(15) 14.2053(9)b/u 113.144(2) 100.554(3) 94.598(2) 90.374(2) 90V/A3 805.17(10) 4453.5(9) 2199.3(3) 1127.35(16) 2743.0(3)Z 2 8 8 4 8m/mm21 0.096 0.098 0.111 0.109 0.134Reflections measured 6560 17612 8987 9592 23185Unique reflections 1899 5594 2639 2673 6575Observed reflections {F2

w2s(F2)} 1727 3309 1701 2102 6176Rint 0.0115 0.0820 0.0357 0.0233 0.0150R1 {F2

w2s(F2)}a 0.0449 0.0543 0.0403 0.0340 0.0304wR2 {all data}b 0.1119 0.1331 0.1038 0.0936 0.0866Largest difference map features, eA23 0.253, 20.195 0.428, 20.306 0.292, 20.298 0.288, 20.186 0.401, 20.196a R ~ g||Fo| 2 |Fc||/g|Fo|. b wR2 ~ {S[w(F2

0 2 F2c )2/S [w(F2

0)2}½.

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Pyridine C binds three stacked acid molecules together via oneO–H…N hydrogen bond and two crystallographically inde-pendent C–H…O interactions, resulting in an infinite C1

2(6)chain pattern along the edge of this almost cylindrical tape(Figs. 3a and b). The p-stacked acid molecules exhibitalternating graphitic separations of 3.41 A between pairs ofmolecules held together by the doubly-bridging B pyridines,and 3.34 A between pairs of molecules held together by thetriply-bridging C pyridines.

Thermogravimetric analysis of 2. As observed with 1, 2showed stepwise mass loss when a crystalline sample wasanalysed by TGA. Three mass loss steps were observed. A massloss of 29.5% over the range 35–114 uC, peaking at 83 uC,corresponded to the loss of two equivalents of pyridine. Basedon the observation of the shattering and expansion of acrystalline sample of 2 at 66–70 uC, it may be assumed that thetwo equivalents of pyridine lost are pyridines B and C, theirloss resulting in the expansion of the acid layers. (It isimportant to note that B and C have C–H…O bond lengths

longer than those for A, hence they are less strongly bound andso less thermal energy would be required to break the inter-actions, allowing the more facile loss of B and C.) A furthermass loss of 19.9% was observed commencing close to theboiling point of pyridine (115 uC) peaking at 159 uC and tailing-off to 252 uC, corresponding to the loss of the final pyridinemolecule, A, over a temperature range in excess of thatrecorded for 1. The final mass loss of 50.3%, corresponding tocomplete decomposition of the acid, peaked at 366 uC.

Pyridinium hydrogen phthalate 3. Phthalic acid reacts withpyridine in a 1:1 ratio, with full proton transfer yieldingthe pyridinium salt (Fig. 4 and Table 4). An intramolecularS(7) hydrogen bond results, a common occurrence in 1,2-disubstituted dicarboxylic acids in their neutral form1c andwhen deprotonated.15

Fig. 3 (a) and (b). Packing plots of 2, shown using different views ofthe stack. Hydrogen bonds are shown by dashed lines, while hydrogenatoms not involved in hydrogen bonding have been removed for clarity.Click here to access a 3D interactive view of Fig. 3a.

Fig. 2 A view of the asymmetric unit of co-crystal 2. Hydrogen bondsare shown as dashed lines. Selected hydrogen bonding parameters for 2are shown in Table 3. Labels A, B and C refer to the pyridine hydrogenbonding modes referred to in the text and in Scheme 4. Click here toaccess a 3D interactive view.

Table 3 Selected hydrogen bonding parameters for 2

D–H…A D…A/A D–H/A H…A /A D–H…A/u

O(2)–H(2)…N(1) 2.599(2) 0.96(3) 1.65(3) 175(2)O(4)–H(4)…N(2) 2.6406(18) 1.00(2) 1.64(2) 177(2)O(6)–H(6)…N(3) 2.6284(17) 1.03(2) 1.61(2) 168.4(19)C(15)–H(15)…O(3) 3.181(2) 1.00 2.583 118.5C(14)–H(14)…O(1)a 3.469(3) 0.98 2.520 162.2C(20)–H(20)…O(3)a 3.401(3) 0.97 2.468 160.7C(24)–H(24)…O(3)b 3.290(2) 0.93 2.513 141.2a Symmetry operations for equivalent atoms 12x, y, 1/22z. b 12x,2y, 1 2 z.

Scheme 4 Schematic of the three binding modes of pyridine moleculesin co-crystal 2. Acid molecules are shown as rectangular blocks forsimplicity, hydrogen bonds are shown by dashed lines.

Fig. 4 A view of the asymmetric unit of salt 3. Hydrogen bonds areshown as dashed lines. Selected hydrogen bonding parameters for 3 areshown in Table 4. Click here to access a 3D interactive view.

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It is clear that phthalic acid has enhanced acidity in pyridinecompared to terephthalic acid. Previous authors havesuggested9 that the hydrogen phthalate anion, like other 1,2-disubstituted acids, adopted a cyclic formation via intramole-cular hydrogen bonding, and that this was more preferred inpyridine than in water since pyridine ‘‘lacks hydrogen donorcharacter’’ associated with water. Loss of the second proton isthen at much greater pKa since disruption to this favourableintramolecular hydrogen bonding would need to occur. Incontrast, isophthalic and terephthalic acids, which have geo-metric structures that prevent the formation of intramolecularhydrogen bonds, have higher and relatively similar pK1 andpK2 values in pyridine.

The infrared spectrum of 3 confirms deprotonation hasoccurred, with strong asymmetric and symmetric carboxylateabsorptions at 1551 and 1374 cm21, respectively, in addition toan absorption at 1713 cm21 corresponding to the carbonyldouble bond of the protonated carboxylic acid group.

Instead of R22(7) synthon Y, 3 displays the alternative, ionic

R22(7) synthon Z due to deprotonation of the acid by the

pyridine, with N–H…O hydrogen bond interactions in place ofO–H…N interactions, and complementary C–H…O hydrogenbonds. No further strong hydrogen bonding exists outside theasymmetric unit, only weak hydrogen bonding maintains thepacking of the pyridinium salt co-molecules. Alternating pyri-dine and acid molecules are held in zigzag ribbons (Fig. 5) by Zand R1

2(5) motifs, interactions in the latter motif involvingweak hydrogen bonds between O(2A) and H(10) and H(11).This one-dimensional structure then extends into threedimensions when further C–H…O bonds between interlinkingribbons are considered. The zig–zag ribbons in 3 showsimilarities to recently reported co-crystals of phthalic acidand 4,4’-bipyridine derivatives, although proton transfer wasnot observed in these examples.18

Isophthalic acid pyridine solvate 4. Isophthalic acid co-crystallises with pyridine in a 1:1 ratio, with partial protontransfer yielding the pyridinium salt as a disordered minor{42(6)%} component (Fig. 6).

In the major {58(6)%} component, the 1:1 ratio leaves onecarboxylic acid group free to hydrogen bond to other suitablegroups. The pyridine molecule is twisted by 39.0u with respectto the aromatic ring and 28.6u with respect to the carboxylategroup positioned at C(1), preventing the formation of synthonY. Instead, O–H…N interactions occur within the asymmetricunit, while O(1) interacts with H(3)–O(3) of a symmetrygenerated acid molecule. O(4) is not involved in stronghydrogen bonding, instead it forms two short C–H…Ointeractions8 with H(12) and H(13) of a symmetry generatedpyridine molecule forming a R1

2(5) motif. These four hydrogenbonds combine to yield an infinite chain C3

3(11) motif, held as aspiral (Fig. 7) with pitch 3.797 A (equivalent to the shortest unitcell dimension) by several weaker C–H…O bonds, with everyproton on each pyridine ring being involved in C–H…Ointeractions. This contrasts with the zigzag chains seen in therecently reported co-crystallisation of isophthalic acid with4,4’-bipyridine.6b

The result of these extensive interactions is that the pyridinering is inclined at angles of 40.9u and 39.0u with respect to the

aromatic ring of its neighbouring acid molecules in the spiral.The aromatic rings of the two acid molecules linked throughO(3)–H(3)…O(1) are inclined at 41.7u with respect to each

Table 5 Selected hydrogen bonding parameters for 4

D–H…A D…A/A D–H/A H…A/A D–H…A/u

O(2)–H(1)…N(1) 2.5402(14) 0.97(6) 1.57(6) 171(3)N(1)–H(1X)…O(2) 2.5402(14) 0.89(7) 1.65(8) 175(5)O(3)–H(3)…O(1)a 2.6696(13) 0.895(19) 1.782(19) 171.5(17)C(12)–H(12)…O(4)b 3.0855(15) 0.95 2.473 122.2C(13)–H(13)…O(4)b 3.1031(15) 0.95 2.492 122.1a Symmetry operations for equivalent atoms x, 2y 1 1/2, z 1 1/2. b x 2 1,1/2 2 y, 21/2 1 z.

Table 4 Selected hydrogen bonding parameters for 3

D–H…A D…A/A D–H/A H…A/A D–H…A/u

O(2)–H(2)…O(3) 2.4244(18) 1.04(2) 1.38(2) 174(2)N(1)–H(1)…O(4) 2.553(2) 1.03(2) 1.53(2) 176.9(18)C(9)–H(9)…O(3) 3.221(2) 0.95 2.559 126.9C(10)–H(10)…O(2)a 3.317(2) 0.95 2.751 119.0C(11)–H(11)…O(2)a 3.237(2) 0.95 2.577 126.8a Symmetry operations for equivalent atoms 1/2 2 x, y 1 1, 2z.

Fig. 5 (a) and (b) A space filling diagram of hydrogen-bonded ribbonsof salt 3, (b) a close-up showing the combination of S(7), R2

2(7) andR1

2(5) motifs.

Fig. 6 A view of the asymmetric unit of co-crystal 4. Minor disordercomponent shown with open bond, while hydrogen bonds between thedisordered groups have been omitted for clarity. Selected hydrogenbonding parameters for 4 are shown in Table 5. Click here to access a3D interactive view.

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other. The infinite spirals, linked by numerous C–H…O inter-actions into a three-dimensional structure, stack when vieweddown the crystallographic a direction. Although microanalysisconfirmed the purity of the bulk material, infrared analysisshowed an indistinct region between 1730 and 1680 cm21, withX-ray data from two different crystals indicating the samepartial proton transfer.

Pyridinium trihydrogen pyromellitate 5. The known 1:2pyromellitate:pyridinium salt, found to be zero-dimensionalby Zaworotko et al,15 exhibits pyridinium proton transfer andintramolecular hydrogen bonding between the adjacent carboxy-late and carboxylic acid groups. Its 1:1 pseudopolymorph17 5,differing in the solvent:acid ratio, exhibits a three-dimensionalnetwork of hydrogen bonds due to the intramolecular S(7)hydrogen bonding motif of only one carboxylic acid group tothe ionised carboxylate in each of the two independent acidmolecules in the asymmetric unit (Fig. 8 and Table 6). The

limited intramolecular hydrogen bonding leads to six of theeight unique carboxylic acid groups lying within 13u of co-planarity with the aromatic rings to which they are attached,while two acid groups are twisted almost perpendicular to thearomatic rings (Table 7). As seen with the three pyridinemolecules in 2, the two pyridinium ions possess differentinteraction modes—one interacting with only one carbonyloxygen, O(7), while the other interacts with two carbonyloxygens, O(14) and O(15), forming an R2

1(7) motif.Although pyromellitic acid is highly oxygenated, the com-

monly observed acid–acid R22(8) motif is absent. The large size

of pyromellitic acid and the various twist angles of thecarboxylic groups result in slightly puckered acid layers(Fig. 9), with strong O–H…O hydrogen bonding abundantwithin the layers, forming cavities within which the pyridiniumions reside. As would be expected of such a highly oxygenatedcompound in the presence of the C–H proton donor rich,aromatic pyridine, C–H…O bonding combines with thestronger O–H…O and N–H…O bonding to create an extensiveweb of interactions. C…O lengths range from 3.012(2) A to3.900(2) A.

Tilt angles of 13.4u and 4.0u exist between the aromatic ringsof acid molecules and the aromatic rings of their respectivepyridinium ions, while the two acid molecules are tilted by14.0u with respect to each other, indicating that the three-dimensional nature of the structure is predominantly due to thecarboxylic acid group deviations from planarity, allowingextensive hydrogen bonding within and between layers. It isinteresting to note that the R2

2(7) motif Z involving thepyridinium ion, observed in 3 and by Zaworotko et al.15 in the1:2 adduct, is absent from 5.

5 desolvates rapidly under ambient conditions, so micro-analysis was carried out on a slightly wet sample, with a goodcorrelation to the formula pyromellitic acid?1.40 pyridine. FT-IR analysis of crystalline 5 correlates with the structure, withN–H absorptions at 3234 and 3179 cm21, absorptions at 1716and 1700 cm21 in the carbonyl region and carboxylateabsorptions at 1579 and 1348 cm21.

The co-crystallisation of pyridine and the benzenepolycar-boxylic acids used here in water and methanol (with theexception of terephthalic acid for solubility reasons) has alsobeen investigated, as it is well known that pKa values can varyconsiderably in different solvents.9 One, two or three equiva-lents of pyridine were used, depending on the acid in question,using heat to fully dissolve the acid where necessary. Only thecrystallisation of the pure acid was achieved in the case ofphthalic and isophthalic acids. The production of powderedmaterial was observed (as a precipitate on addition of pyridine)with both trimesic and pyromellitic acids. The H-tube method,successful in the crystallisation of 5 and its pseudopolymorphthrough a concentration gradient, was also employed for theco-crystallisation of trimesic acid with pyridine, but this again

Fig. 7 A view of the spiral structure of co-crystal 4. Hydrogen bondsare shown with dashed lines, while hydrogen atoms not involved inhydrogen bonding have been removed for clarity. Selected hydrogenbonding parameters for 4 are shown in Table 5. Click here to access a3D interactive view.

Fig. 8 A view of the asymmetric unit of salt 5. Hydrogen bonds areshown with dashed lines. Selected hydrogen bonding parameters for 5are shown in Table 6. Click here to access a 3D interactive view.

Table 7 Twist angles (u) of carboxylic acid groups in 5 with respect to the aromatic ring within the same molecule

C(7)O(1)O(2) C(8)O(3)O(4) C(9)O(5)O(6) C(10)O(7)O(8)

C(1)–C(6) aromatic ring 4.4 5.0 5.2 83.0C(17)O(9)O(10) C(18)O(11)O(12) C(19)O(13)O(14) C(20)O(15)O(16)

C(11)–C(16) aromatic ring 12.6 11.2 9.4 86.5

Table 6 Selected hydrogen bonding parameters for 5

D–H…A D…A/A D–H/A H…A/A D–H…A/u

N(1)–H(1)…O(7) 2.7980(19) 0.86(3) 1.96(3) 165(2)N(2)–H(2)…O(14) 2.8495(18) 0.94(2) 2.14(2) 131.0(18)N(2)–H(2)…O(15) 3.0387(18) 0.94(2) 2.29(2) 135.9(17)O(3)–H(3)…O(2) 2.3819(15) 1.05(2) 1.33(2) 179(2)O(11)–H(11)…O(10) 2.3801(15) 1.16(2) 1.23(2) 172(2)

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produced only powdered material. However, FT-IR analysisindicates that the powdered materials produced are identical tothe crystalline 2 and 5, suggesting no deprotonation occurredwith trimesic acid in MeOH, while mono-deprotonationoccurred with pyromellitic acid, as would be expected fromthe isolation of 5 from the methanolic H-tube at low pyridineconcentration.

Analysis of hydrogen bonding parameters from structuresheld in the Cambridge Structural Database2 (hereafter theCSD; Version 5.25; Januray 2004 update) indicates thatpyridine hydrogen bonds to the carboxylic acid OH group in231 structures (constraints applied to search: O…N contactdistances in the range 2–3 A, O–H…N angles in the range120–180u; hits returned were analysed for statistical outliers,however redeterminations were not removed from the statis-tical analysis), while proton transfer leads to N–H…Ohydrogen bonds between pyridinium cations and carboxylateanions in 203 structures (search constraints as before exceptthat the angle constraint involved N–H…O angles).

The addition of constraints for C–H…O hydrogen bonding,thus narrowing the search to those structures containing theR2

2(7) carboxylic acid–pyridine synthon in either its neutral orionic forms, (constraints applied to search: C…O contactdistances in the range 2.5–4 A, C–H…O angles in the range 90–180u) reduces the number of hits returned to 173 in the case ofO–H…N hydrogen bonding (neutral form of R2

2(7)) and 86 inthe case of N–H…O hydrogen bonding (ionic form of R2

2(7)).When C–H…O hydrogen bonding is also present, the meanN…O contact distances are shortened only very slightly, withperhaps the most obvious difference being that the mean C…Ocontact distance in the ionic R2

2(7) synthon are almost 0.1 Ashorter than in the neutral analogue. The hydrogen bondingparameters found in co-crystals 1, 2 and 4, and in salts 3 and 5

are clearly comparable to the mean values determined from theCSD (Table 8), particularly so in co-crystals 1 and 2, and salt 3which contain the R2

2(7) hydrogen bonded synthon.CCDC reference numbers 192135–192139.See http://www.rsc.org/suppdata/ce/b4/b404563g/ for crys-

tallographic data in CIF format.

Conclusion

Low temperature single crystal X-ray diffraction has enabledthe elucidation of five examples of pyridine–benzenepoly-carboxylic acid co-crystals, novel contributions to the extensiveresearch into the occurrence of carboxylic acid–pyridine motifsin co-crystals. TGA analysis usefully relates pyridine lossfor two of the examples, to hydrogen bond length/strengthobserved in the structures. Extensive hydrogen bonding existswithin the structures, with the common carboxylic acid R2

2(8)head-to-tail ring motif absent in all cases. Interestingly, whilethe carboxylic acid–pyridine R2

2(7) synthon (in both its neutraland ionic forms), considered by many authors to be robust andpredictable6a,6c,6g is observed in 1 and 3, it is absent from 4 and5 and only exists using one of the three carboxylic acid groupsin 2. This perhaps indicates a weakness in the predictability ofthe R2

2(7) synthon when using simple pyridine derivatives incombination with highly oxygenated carboxylic acids. In theirplace are a series of motifs in which strong hydrogen bonds(O–H…N and N1–H…O2) combine with weaker interactions(C–H…O). This variety, coupled with the varying geometriesand extent of carboxylic acid substitution of the benzene-polycarboxylic acids employed, has led to the creation of arange of supramolecular arrays, from discrete units to ribbonsand helices.

Table 8 Statistical results from a search of the Cambridge Structural Database (see text for constraints applied to search)

No. of hits D…A/A (mean) D…A/A (range) D–H…A /u (mean) D–H…A/u (range)

O–H…N (neutral) 231 2.641(3) 2.347–2.897 168.3 123.5–179.9N–H…O (ionic) 203 2.697(6) 2.478–2.992 162.8 121.8–179.3O–H…N (neutral) 173 2.631(3) 2.347–2.897 169.6 129.7–179.9C–H…O 3.416(13) 3.049–3.998 122.6 90.6–141.4N–H…O (ionic) 86 2.672(9) 2.478–2.883 168.7 133.5–179.3C–H…O 3.32(2) 2.954–3.972 122.4 92.5–148.3

Fig. 9 Packing plot of salt 5, viewed along the crystallographic b-axis. Hydrogen atoms not involved in hydrogen-bonding have been removed forclarity. Hydrogen bonds are shown by dashed lines.

CrystEngComm, 2004, 6(36), 207–214 213

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Acknowledgements

We are grateful for funding from the EPSRC for a studentship(SHD) and acknowledge EPSRC for the provision of beam timeat the CLRC Daresbury Laboratory, UK. Microanalyses werecarried out by the Departmental Services of LoughboroughUniversity and the University of Newcastle-upon-Tyne. Wethank Prof. M. J. Zaworotko for valuable discussions andpermission to publish the full structural results for 2.

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