Journal of Molecular Structure 937 (2009) 75–80

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Journal of Molecular Structure

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Varieties in symmetry non-equivalent structural arrangements in solvatesof 2-(3-methylene-1,3,7-trioxo-6-(2-carboxy-phenyl)-3,5,6,7-tetrahydro-1H-pyrrolo[3,4-f]isoindol-2-yl)benzoic acid

Devendra Singh, Jubaraj B. Baruah *

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 July 2009Received in revised form 13 August 2009Accepted 19 August 2009Available online 23 August 2009

Keywords:Symmetry non-equivalenceSolvatesDicarboxylic derivativesPyromellitic diimide

0022-2860/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.molstruc.2009.08.019

* Corresponding author. Tel.: +91 361 2582301; faxE-mail address: juba@iitg.ernet.in (J.B. Baruah).

The crystal structures of few solvates of 2-(3-methylene-1,3,7-trioxo-6-(2-carboxy-phenyl)-3,5,6,7-tetra-hydro-1H-pyrrolo[3,4-f]isoindol-2-yl)benzoic acid (A) namely A�4H2O; A�2DMF; A�2Py; A�3Py are deter-mined (where Py = pyridine, DMF = dimethylformamide). The crystal structure of the hydrated form hasZ0 = 0.5, whereas other solvates with DMF and pyridine has Z0 = 1. The neighboring host molecules in thecrystal lattices of these solvates have different orientations; these orientations are related by translationor translation cum rotation of the orientation to the neighboring host molecules that has the most sym-metric structure.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The term Z0 in crystallography refers to the number of formulaunits present in an asymmetric unit [1–5]. The Z0 is decided bythe number of formula units in a unit cell divided by independentgeneral positions. Depending on Z0 value, numbers of moleculesthat requires independent set of coordinate per unit cell varies[1–5]. A good amount of discussions on the symmetry non-equiv-alent molecules [6–15] are available in literature. Solvent mole-cules during crystallization play a major role in formation oflattice with high Z0 value [16]. Systematic examples on solvatesof same host showing high Z0 value is difficult to obtain and it isa concern in understanding their mechanism of formation. Muchattention is needed to identify systems that would have differentZ0 since different orientations in lattices would enhance Z0. The pro-cesses like self assembling, solvation and host–guest interactionsmay cause symmetry non-equivalence among host moleculesand cause higher Z0. Symmetry non-equivalence may occur incocrystals [6–15] as well as in unsolvated molecules [17]. Sincethe above mentioned processes involve assembly formation, theoccurrence of high Z0 may be related to metastable [18,19] states.It is a well known fact that crystallization of same compound underdifferent conditions leads to polymorphs [20] and symmetry non-equivalent molecules [21]. Our interest has been to trap differentsolvated species to understand the role of solvents in making sym-

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metry non-equivalent hosts and vice versa. With such an approach,we present the structural features of few solvates of 2-(3-methy-lene-1,3,7-trioxo-6-(2-carboxy-phenyl)-3,5,6,7-tetrahydro-1H-pyr-rolo[3,4-f]isoindol-2-yl)benzoic acid (A).

2. Experimental

The X-ray single crystal diffraction data were collected at 296 Kwith MoKa radiation (k = 0.71073 Å) using a Bruker Nonius SMARTCCD diffractometer equipped with a graphite monochromator. TheSMART software was used for data collection and also for indexingthe reflections and determining the unit cell parameters; the col-lected data were integrated using SAINT software. The structureswere solved by direct methods and refined by full-matrix least-squares calculations using SHELXTL software. All the non-H atomswere refined in the anisotropic approximation against F2 of allreflections. The H-atoms, except those attached to nitrogen andoxygen atoms were placed at their calculated positions and refinedin the isotropic approximation; those attached to nitrogen andoxygen were located in the difference Fourier maps, and refinedwith isotropic displacement coefficients. Crystallographic data col-lection was done at room temperature and the data are tabulatedin Table 1.

2.1. Synthesis of compound A

A solution of pyromellitic dianhydride (1.090 g, 5 mmol) and2-aminobenzoic acid (1.31 g, 10 mmol) in acetic acid (25 ml) was

Table 1Crystallographic parameters of the solvates of A.

Compound name A.4H2O A.2DMF A.2Py A.3PyFormulae C24H20N2O12 C30H26N4O10 C34H22N4O8 C39H27N5O8

Moiety formula 0.5A.2H2O A.2DMF A.2Py A.3pyCCDC No. 728168 728171 728169 728170Mol. wt. 528.42 602.55 614.56 693.66Crystal system Monoclinic Triclinic Triclinic TriclinicSpace group C2/c P-1 P-1 P-1a/Å 25.5700(8) 10.2817(4) 10.9902(8) 10.1284(5)b/Å 7.8587(2) 13.0590(4) 12.4771(9) 13.6116(7)c/Å 13.3821(4) 13.1451(4) 13.2683(10) 14.2771(7)a/� 90.00 60.9530(10) 116.622(4) 92.706(3)b/� 118.488(3) 78.2450(10) 90.239(5) 110.091(3)c/� 90.00 72.6000(10) 107.989(5) 108.302(3)V/Å3 2363.49(12) 1468.75(9) 1525.12(19) 1728.26(15)Z 4 2 2 2Density/Mgm�3 1.485 1.362 1.338 1.333Abs. Coeff. /mm�1 0.121 0.104 0.097 0.095Abs. correction None None None NoneF(000) 1096 628 636 720Total No. of reflections 10293 16464 15651 21127Reflections, I > 2r(I) 1947 3655 2957 3680Max. h/� 26.00 28.31 25.00 28.23Ranges (h, k, l) �30 <= h <= 31 �13 <= h <= 13 �12 <= h <= 12 �13 <= h <= 13

�9 <= k <= 9 �16 <= k <= 17 �14 <= k <= 14 �17 <= k <= 17�16 <= l <= 16 �15<= l <= 17 �15 <= l <= 15 �18 <= l <= 18

Complete to 2h (%) 99.6 97.8 95.8 97.3Data/restraints/parameters 2361/0/189 7167/0/403 5142/0/417 8294/0/472Goof (F2) 1.056 1.056 1.016 1.068R indices [I > 2r(I)] R1 = 0.0406 R1 = 0.0751 R1 = 0.0556 R1 = 0.0790

wR2 = 0.1083 wR2 = 0.2223 wR2 = 0.1354 wR2 = 0.2401R indices (all data) R1 = 0.0506 R1 = 0.1375 R1 = 0.1043 R1 = 0.1635

wR2 = 0.1163 wR2 = 0.2665 wR2 = 0.1658 wR2 = 0.2882

76 D. Singh, J.B. Baruah / Journal of Molecular Structure 937 (2009) 75–80

refluxed for 3 h. The reaction mixture was cooled to room temper-ature; a brown colored precipitate of the product was obtained, fil-tered and washed several times with water to remove the aceticacid and dried in open air to give A in anhydrous form. Yield:92%. IR(KBr, cm�1): 3483 (bm), 2927 (w), 2620 (w), 1777 (s),1724 (s), 1492 (m), 1455 (m), 1380 (s), 1253 (s), 1191(m), 1118(s), 844 (w), 752 (m), 646 (w). 1H NMR (400 MHz, DMSO-d6):8.33 (s, 2H), 8.14 (d, 2H, J = 8.0 Hz), 7.64 (t, 2H, J = 7.6 Hz), 7.52(t, 2H, J = 7.6 Hz), 7.34 (d, 2H, J = 7.6 Hz). 13C NMR (100 MHz;DMSO-d6): 166.1, 165.6, 137.4, 133.4, 131.5, 131.2, 130.6, 129.4,129.0, 118.6. ESI-MS: 457.098 (M+H)+.

A. 2PyZ'=1

2.2. A�4H2O

Anhydrous form of A upon crystallization from ordinary ace-tone resulted in tetrahydrate product A�4H2O. IR(KBr, cm�1):3521 (m), 3429 (m), 2925 (w), 2629 (w), 1776 (s), 1716 (s), 1698(s), 1643 (s), 1492 (m), 1453 (m), 1384 (s), 1281 (m), 1264 (m),1121 (s), 1071 (w), 921 (w), 846 (w), 754 (m), 728 (w), 628 (w).1H NMR (400 MHz, DMSO-d6): 8.49 (s, 2H), 8.10 (d, 2H,J = 8.0 Hz), 7.82 (t, 2H, J = 7.6 Hz), 7.68 (t, 2H, J = 7.6 Hz), 7.60 (bs,2H), 2.08 (s, 8H).

NN

OOH

HOO

O

OO

O

Acetone/water

A

A. 4H2ODMF

A. 2DMF

Pyridine (Py)

Pyridine/Toluene

A. 3PyZ'= 1

Z'= 1 Z'= 0.5

Scheme 1. Different solvated species of A.

2.3. A�2DMF

The solvate A�2DMF was obtained as crystals from a solution ofA in N,N-dimethylformamide. It crystallized as colorless blocksafter 7 days. IR(KBr, cm�1): 3481 (m), 2925 (m), 2501 (w), 1776(s), 1721 (s), 1630 (s), 1600 (m), 1491 (m), 1455 (m), 1383 (s),1291 (m), 1256 (m), 1190 (w), 1121 (s), 845 (w), 757 (m), 728(m), 670 (w), 1H NMR (400 MHz, DMSO-d6): 8.50 (s, 2H), 8.11 (d,2H, J = 7.6 Hz), 7.95 (s, 2H), 7.83 (t, 2H, J = 7.6 Hz), 7.68 (t, 2H,J = 7.6 Hz), 7.59 (d, 2H, J = 7.2 Hz), 2.88 (s, 6H), 2.72 (s, 6H).

2.4. A�2Py

A solution of compound A in pyridine and toluene gave the sol-vate A�2Py as colourless blocks in quantitative yield. IR(KBr, cm�1):3446 (bm), 3077 (w), 2460 (w), 1778 (s), 1724 (s), 1601 (m), 1489(m), 1453 (m), 1437 (m), 1381 (m), 1284 (m), 1189 (m), 1119 (m),1063 (m), 1008 (w), 845 (w), 752 (m), 727 (w), 630 (w). 1H NMR(400 MHz, DMSO-d6): 8.57 (s, 4H), 8.50 (s, 2H), 8.11 (d, 2H,J = 7.6 Hz), 7.81 (m, 4H), 7.68 (t, 2H, J = 7.6 Hz), 7.60 (d, 2H,J = 7.6 Hz), 7.39 (t, 4H, J = 5.6 Hz).

2.5. A�3Py

The solvate A�3Py was obtained as colorless needles from thepyridine solution of compound A. IR(KBr, cm�1): 3502 (bm), 2924(w), 2623 (w), 1776 (s), 1720 (s), 1634 (s), 1492 (m), 1454 (m),1384 (s), 1281 (m), 1264 (m), 1190 (m), 1121 (s), 845 (w), 754(m), 728 (w). 1H NMR (400 MHz, DMSO-d6): 8.50 (s, 6H), 8.32

Table 2Hydrogen bond geometry (Å, �) for solvate A.4H2O.

D. Singh, J.B. Baruah / Journal of Molecular Structure 937 (2009) 75–80 77

(s, 2H), 8.11 (d, 2H, J = 8.0 Hz), 7.65 (m, 5H), 7.55 (t, 2H, J = 8.0 Hz),7.38 (d, 2H, J = 7.6 Hz), 7.24 (t, 6H, J = 6.0 Hz).

D�H���A d(D�H)

d(H���A)

d (D���A) <D�H���A

O1–H1. . .O6 0.82 1.78 2.601(3) 179O5–H5A. . .O2 1.10 1.74 2.825(3) 170O5–H5B. . .O5 [�x, y, 1/2�z] 0.89 2.52 3.311(3) 148O6–H6A. . .O5 [x, 1�y, �1/

2 + z]0.84 2.54 2.870(3) 105

O6–H6B. . .O5 [x, 1�y, �1/2 + z]

0.89 2.01 2.870(3) 163

C3–H3. . .O1 0.93 2.38 2.714(2) 101C11–H11. . .O3 [�x, y, 1/2�z] 0.93 2.57 3.323(19) 139

3. Results and discussion

The solvates of 2-(3-methylene-1,3,7-trioxo-6-(2-carboxy-phenyl)-3,5,6,7-tetrahydro-1H-pyrrolo[3,4-f]isoindol-2-yl)benzoicacid (A) with water, dimethylformamide (DMF) and pyridine (Py)as illustrated in Scheme 1 are prepared and their structures aredetermined.

The structure of the hydrated form of A has four water of crys-tallization (A�4H2O); it has Z0 = 0.5. Each asymmetric unit of thissolvate contains one half of the parent molecule and two watermolecules. These two water molecules are in independent environ-ments in the lattice. A portion of the structure of the solvate show-ing hydrogen bond interactions in the lattice is shown in Fig. 1.There are two pairs of water molecules bearing O5 and O6 oxygenatoms, that are held in between the layers of host by H-bond inter-actions; these molecules are held by hydrogen bonds via two car-bonyl oxygens at the two ends of two independent A moleculesthrough O5–H���O2 and O5–H���O3 interactions (Fig. 1). These watermolecules are also held by the O–H���O interactions of the OHgroup of carboxylic acid of third host molecules. The H-bondparameters are shown in Table 2. Since we determined the

Fig. 1. A portion of lattice of A.4H2O showing the hydrogen bond interactions in it(drawn with 20% thermal ellipsoid).

Fig. 2. Structure of a portion of lattice of A.2DMF showing the hydro

structure at room temperature, some of the hydrogen bonds areless realistic; nevertheless they are being left as such, as theseare required to describe the position of host molecules.

The solvates of A with DMF and pyridine respectively has Z0 = 1.The asymmetric unit of A�2DMF comprises of two symmetry non-equivalent halves of A and two dimethylformamide molecules. Thedimethylformamide molecules are hydrogen bonded to carboxylicacid groups through C27–H���O5, C30–H���O2 and O6–H���O9, O6–H���O1 interactions (Fig. 2). There are other weak interactions suchas C23–H���O3, C10–H���O7, C24–H���O4 (Table 3) to hold thedimethylformamide molecules intact in the lattice. We have earliershown3 that in the glycinyldiimide of pyromellitic dianhydride, thesymmetry non-equivalent molecules in unit cells are generated bysolvents. The solvent provides symmetric H-bonding across a car-boxylic acid, such as dimethylsulphoxide leads to more symmetricstructure whereas less symmetric solvent such as dimethylform-amide makes less symmetric structure. The present case ofdimethylformamide solvate is also analogous to the earlier re-ported glycinyldiimide derivative [16].

Two forms of solvates of A with pyridine molecules are obtainedby varying crystallization conditions. Crystallization of A from tol-uene containing pyridine gave A�2Py (1:2 solvate). This solvatecrystallizes in P�1 space group. The asymmetric unit of this solvatecomprises of two halves of A at two independent symmetry posi-tions and two pyridines.

The carboxylic acid groups of A are in trans disposition acrossthe planar aromatic unit (Fig. 3a). The host molecules of the solvatepossess inversion center and half of the molecules are seen inasymmetric unit.

Crystallization of A from neat pyridine gave solvate A�3Py (1:3solvate). This solvate also crystallizes in P�1 space group (Fig. 3b).The asymmetric unit of A�3Py contains two halves of symmetry

gen bond interactions in it (drawn with 20% thermal ellipsoid).

Table 3Hydrogen bond geometry (Å, �) for solvate A.2DMF.

D�H���A d (D�H) d (H���A) d (D���A) <D�H���A

O1–H1. . .O10 [1�x, 1�y, 1�z] 0.82 2.13 2.652(5) 121O6–H6A. . .O9 [1�x, 1�y, 1�z] 0.82 1.79 2.596(3) 169C3–H3. . .O2 0.93 2.48 2.799(5) 101C10–H10. . .O7 0.93 2.54 3.396(4) 154C15–H15. . .O6 0.93 2.39 2.714(4) 100C23–H23. . .O3 [1�x, 1�y, 1�z] 0.93 2.52 3.076(3) 118C26–H26B. . .O4 [�x, 1�y, 1�z] 0.96 2.47 3.325(5) 149C26–H26C. . .O9 0.96 2.34 2.728(5) 104C27–H27. . .O5 0.93 2.63 3.294 128C29–H29A. . .O5 0.96 2.39 2.723(7) 100C30–H30. . .O2 0.93 2.71 3.277 120

78 D. Singh, J.B. Baruah / Journal of Molecular Structure 937 (2009) 75–80

independent molecules of A with three pyridine molecules. Theobservation of the two different pyridine solvates may be attrib-uted to the role of toluene in the crystallization process. The crys-tallization from neat pyridine provides more number of solventmolecules namely pyridine in the lattice as it is obvious that thepyridine being the solvent would try to make a structure via max-imization of its interactions with the host. Whereas, toluene doesnot have heteroatom to interact with a carboxylic acid; thus itwould weakly bind eventually, there is a competition to make tightpacked structures through strong O–H����N interactions by

Fig. 3. Structure of (a) A.2Py; (b) A.3Py showi

pyridine. Such process expels toluene molecules, leads to solvatewith two pyridine molecules. The weak interactions schemes inthe two solvates are different and these are shown in Fig. 4a andb. Each of them has R 2

2 ð7Þ type of hydrogen bond (i.e. seven mem-ber ring via two D–H and A����H in it) [22] among the carboxylicacid group with pyridine through C–H����O and O–H����N interac-tions (Fig. 4a and Table 4). In the case of A�2Py both the pyridine

are held in the lattice by R22 ð7Þ type of interactions supported by

another interactions of the C–H����O interactions between an orthohydrogen of pyridine with a carbonyl oxygen of pyromellitic partnamely C25–H����O7 interaction (Fig. 4a). In the solvate A�3Py, thepyridine molecules are involved in hydrogen bonding in multipleways. It has the conventional R 2

2 ð7Þ type interactions via O1–H����N5 and C23–H����O2 (Fig. 4b). Another set of pyridine in thelattice is positioned in slightly oblique to the carboxylic acid, sothey do not form cyclic hydrogen bond structure but has only theO5–H����N3 interactions. The third set of pyridine is weakly heldin the interstices through C5–H����N4 interactions. Some weak inter-actions to stabilize the lattices of the solvates are listed in Table 4.

The thermal properties of these two pyridine solvates are differ-ent. The differential scanning calorimetry (DSC) of the A�3Py showstwo endothermic processes at 89 �C and 173 �C. These correspondto the loss of one relatively weakly bound pyridine followed by twoother pyridine molecules respectively. Whereas, only one

ng different pyridine molecules in lattice.

Fig. 4. A portion of lattice (a) A.2Py and (b) A.3Py showing the hydrogen bond interactions in it (drawn with 20% thermal ellipsoid).

Table 4Hydrogen bond geometry (Å, �) for solvate A.2Py and A.3Py.

D�H���A d (D�H) d (H���A) d (D���A) <D�H���A

For A.2PyO5–H5A. . .N3 [1�x, �y, �z] 0.82 1.80 2.614(5) 172O1–H1. . .N4 0.82 1.86 2.681 173C3–H3. . .O1 0.93 2.46 2.768(5) 100C15–H15. . .O5 0.93 2.40 2.724(5) 101C22–H22. . .O4 [1�x, 1�y, 1�z] 0.93 2.53 3.285(4) 138C29–H29. . .O6 [1�x, �y, �z] 0.93 2.56 3.189(5) 125C30–H30. . .O2 0.93 2.61 3.173 118C33–H33. . .O8 [�1 + x, 1 + y, z] 0.93 2.49 3.219(8) 135

For A.3PyO1–H1. . .N5 0.82 1.86 2.680(5) 175O5–H5. . .N3 [1�x, 1�y, 1�z] 0.82 2.09 2.836(6) 150C3–H3. . .O1 0.93 2.39 2.718(5) 100C10–H10. . .O7 [1�x, 1�y, 1�z] 0.93 2.47 3.265(3) 143C15–H15. . .O5 0.93 2.40 2.726(5) 100C23–H23. . .O3 [1�x, 1�y, 1�z] 0.93 2.43 3.226(4) 144C39–H39. . .O2 0.93 2.63 3.245 123

D. Singh, J.B. Baruah / Journal of Molecular Structure 937 (2009) 75–80 79

endothermic process for the solvate A�2Py is observed at 164 �Cdue to simultaneous loss of the two pyridine molecules (for DSCplease refer to supplementary materials). These observations sup-port the differences among the weak bonds in the two pyridinesolvates.

The structural backbone of A consists of a planar aromatic ringon which aromatic rings bearing two carboxylic acid groups are at-tached. It is interesting to note that in all these crystal structures ofthe solvates symmetry of a host molecule A is present. Thus, in thecrystal packing, the central molecule A plays a crucial role. Thestructure of water solvate can be considered as reference(Z0 = 0.5) to compare the arrangements of such units in lattice ofthe other solvates. The spatial arrangements of two neighboringmolecules of A in the lattice, may be parallel, perpendicular, or ob-lique. The molecules may also be placed at different longitudinalpositions so as to minimize the intermolecular repulsions as wellas to accommodate guest molecules.

Some possible orientations of two neighboring molecules of Aare shown in Scheme 2. The three orientations shown in Scheme2 are found in the lattices of the solvates described in this manu-script. The parallel orientation shown at the center may be takenas the basis set to generate the other sets of orientations. TheA�4H2O has the most symmetric orientation and the other solvatesare less symmetric. From the most symmetric orientation, the twoless symmetric orientations can be generated by translation of onemolecule keeping a reference molecule static with or without acarbon–nitrogen bond rotation. A simple longitudinal translationof one molecule with respect to other in A�4H2O is good enoughto generate the orientations of two neighboring A in the lattice of

Scheme 2. The orientation of the neighboring pairs imides in crystal lattices of different solvates after omission of solvent molecules.

80 D. Singh, J.B. Baruah / Journal of Molecular Structure 937 (2009) 75–80

A�4DMF and A�2Py (left side of Scheme 2). However, translationwith a carbon–nitrogen bond rotation is necessary to obtain theobserved orientations in the case of A�3Py. Such observations sug-gest that weak interactions of the host molecules with solvent mol-ecules provide push and pull due to attractive and repulsiveinteractions. Such synergic effect makes molecules to organize inspecific orientations along with or without solvent molecules. Suchprocess generates symmetry non-equivalent molecules in the lat-tice. This supports the fact to consider the molecules with high Z0

as metastable species as pointed out earlier [18,19]. The compoundA crystallizes with pyridine to have two different structures andcompositions. The orientations and positions of the molecules ofA in the crystal lattice in the both cases differ (Fig. 1c and d;Scheme 2).

In conclusion, the symmetry non-equivalent solvates with sameZ0 value can have different orientations of the parent moleculesdepending on the solvent attached to it and it is the crystal packingrequirement that decides such orientations and such phenomenonarises from intermolecular or intramolecular push–pull interac-tions of various weak interactions.

Acknowledgements

The authors thank Department of Science and Technology, NewDelhi, India for financial support. The authors are grateful to one ofthe referee for very educative suggestions.

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