Polymorphism and Pseudopolymorphism in the Triaroylbenzene Derivative 1,3,5-Tris(4-cyanobenzoyl)benzene V. S. Senthil Kumar, F. Christopher Pigge,* and Nigam P. Rath
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1217-1222
Department of Chemistry & Biochemistry, University of Missouri-St. Louis, One University Boulevard, St. Louis, Missouri 63121-4499 Received June 28, 2004
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: The triaroylbenzene derivative 1,3,5-tris(4-cyanobenzoyl)benzene exhibits the phenomenon of concomitant polymorphism. Two topologically isomeric solid-state networks, hexagonal and ladder, were obtained upon crystallization from acetone/water solution. In contrast, crystallization of the title compound from EtOAc, 3-pentanone, MeNO2, DMSO, acetone, and methyl chloroacetate produced isostructural pseudopolymorphic inclusion complexes. In all these six cases, the triaroylbenzene host molecule self-assembles in a 2D lamellar pattern via C-H‚‚‚O and C-H‚‚‚N hydrogen bonding with guest solvent molecules residing in interlayer channels. A clathrate of different morphology was obtained upon crystallization from nitroethane. In all seven inclusion adducts, bifurcated C-H‚‚‚O hydrogen bonding between host donors and the guest acceptors was observed as a common feature. The network topologies in the polymorphic modifications of 1 are different from those observed in the structurally characterized inclusion complexes. Thus, C-H‚‚‚X (X ) N/O) hydrogen bonding appears to significantly influence supramolecular isomerism in this system. Introduction Polymorphism is a solid-state phenomenon defined by the existence of at least two different crystalline packing arrangements for the same chemical substance.1 Polymorphic modifications of a given compound are the consequence of a complex interplay of thermodynamic and kinetic factors operative during crystal nucleation and growth.2 In many instances, the free energies of polymorphs are comparable. Polymorphism has been an integral part of crystal engineering, and it has recently assumed greater importance in industry (e.g., pharmaceuticals, pigments) where product specifications (such as solubility and bioavailability) may be a function of solid-state structure.3 Polymorphism results when energetically comparable intermolecular interactions are recognized during crystal growth. This can result from self-assembly of different low-energy conformations of a single compound into different crystalline arrangements. The term “conformational polymorphism” has been used to describe such systems.4 Alternatively, polymorphism can also result from the packing of rigid molecules into different and topologically distinct crystalline arrays. Intuitively, one might imagine that conformationally flexible molecules may possess a greater propensity to exhibit polymorphism as energies required to rotate about single bonds are often comparable to lattice energy differences between polymorphs.5 In some instances, the same chemical substance will form a number of crystalline clathrates with various small molecule (usually solvent) species. In this regard, the formation of inclusion complexes is (at least superficially) related to polymorphism. The term “pseudopoly* To whom correspondence should be addressed. Phone: (314) 5165340. Fax: (314) 516-5342. E-mail:
[email protected].
morphism” has been used to describe related inclusion complexes that differ only in the identity and/or stoichiometry of the solvate.6 In general, however, organic compounds show a tendency not to include solvent molecules within their crystalline lattices. Indeed, a relatively recent survey of the Cambridge Structural Database revealed that only ∼15% of all organic crystals contain solvent of crystallization.7 The occurrence of polymorphism and pseudopolymorphism in organic compounds and its potential impact upon bulk properties and solid-state topology provides the impetus for gaining a better understanding and control of these phenomena.8 The use of net topology to classify organic (and metalorganic) structures has greatly facilitated comparison of seemingly disparate solid-state assemblies, especially in the context of crystal engineering applications.9 Supramolecular isomerism is a term that describes the existence of multiple distinct crystalline networks composed from identical molecular components. Consequently, organic compounds that form more than one network architecture (i.e., polymorphs) exhibit a form of supramolecular isomerism (indeed, all polymorphs are supramolecular isomers).3d,10 For example, honeycomb, brick wall, ladder, and herringbone networks (Scheme 1) constructed from three-connected organic (or coordination polymer) building blocks represent topologically identical architectures that are related via supramolecular isomerism. Ladder, herringbone, and brick wall networks assembled from T-shaped precursors have been frequently observed in coordination polymers,9b,11 but these network structures seldom occur in organic crystals.12 On the other hand, honeycomb networks are rare in coordination polymers,9b,10,13 but are relatively common in organic crystal structures.14
10.1021/cg049792p CCC: $27.50 © 2004 American Chemical Society Published on Web 10/05/2004
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Table 1. Crystallographic Data for the Polymorphs and Pseudopolymorphs of 1 Form A formula crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z d µ T/K unique reflns reflns with I > 2σ(I) no. of parameters R1 [I > 2σ (I)] wR2
Form B
1‚EtOAc
1‚3-pentanone
1‚MeNO2
1‚DMSO
1‚acetone
1‚MCA
1‚EtNO2
C30H15N3O3 C30H15N3O3 (C30H15N3O3) (C30H15N3O3) (C4H8O2) (C5H10O) triclinic triclinic triclinic triclinic
(C30H15N3O3) (C30H15N3O3) (C30H15N3O3) (C30H15N3O3) (C30H15N3O3) (CH3NO2)1.5 (C2H6SO) (C3H6O) (C3H5O2Cl) (C2H5NO2) triclinic triclinic triclinic triclinic triclinic
P1 h
P1 h
P1 h
P1 h
P1 h
P1 h
P1 h
P1 h
P1h
5.9762(2) 13.9047(4) 14.2152(4) 71.790(2) 84.290(2) 87.967(2) 1116.51(6) 2 1.384 0.091 200 5486
7.7072(2) 11.3057(2) 13.7913(2) 73.284(1) 88.422(1) 83.420(1) 1143.33(4) 2 1.352 0.089 165 4900
8.7512(9) 11.2046(11) 14.7252(14) 89.930(5) 85.400(5) 71.494(5) 1364.3(2) 2 1.347 0.092 165 7093
8.3516(4) 11.5882(5) 14.8865(7) 88.831(3) 85.666(3) 76.197(3) 1395.11(11) 2 1.313 0.087 165 7323
8.782(1) 11.036(1) 14.664(2) 88.95(1) 84.30(1) 70.60(1) 1333.8(2) 2 1.387 0.099 165 6327
8.849(3) 11.270(4) 14.816(5) 90.586(19) 84.037(19) 70.934(18) 1387.4(8) 2 1.301 0.159 165 6673
8.7066(4) 11.2103(5) 14.6563(7) 89.465(2) 85.759(2) 72.136(2) 1357.68(11) 2 1.281 0.086 165 5902
8.9408(3) 11.3356(4) 14.3226(6) 89.952(2) 78.573(2) 71.502(2) 1346.30(9) 2 1.416 0.192 143 6108
11.3970(2) 15.4588(3) 15.8553(3) 73.8670(10) 82.2030(10) 88.4050(10) 2658.52(9) 4 1.350 0.093 143 10884
3708
2924
5129
4219
4818
4289
3879
4055
6933
325
325
388
379
397
389
361
380
758
0.0530 0.1361
0.0445 0.1084
0.0613 0.1726
0.0442 0.0945
0.0631 0.195
0.0878 0.2328
0.0592 0.1593
0.0609 0.1173
0.0595 0.1585
Scheme 1. Examples of Isomeric Three-Connected Networks Architecturesa
Scheme 2
Experimental Section
a
a - honeycomb; b - brick wall; c - ladder; d - herringbone.
As part of continuing studies exploring the use of triaroylbenzene derivatives as supramolecular building blocks, we have examined the behavior of 1,3,5-tris(4cyanobenzoyl)benzene (1). This material has been isolated and structurally characterized as a mixture of concomitant polymorphs and, under different crystallization conditions, as a series of pseudopolymorphic inclusion complexes. The limited conformational flexibility inherent in the triaroylbenzene framework and the presence of numerous C-H hydrogen bond donor and acceptor groups appear to be important features that influence the crystalline motifs exhibited by 1 and its clathrates. Portions of this work have been previously communicated.15,16
Synthesis and Crystal Growth. Triaroylbenzene 1 was prepared from p-cyanobenzaldehyde as previously described (Scheme 2).17 Material used in crystallization experiments was obtained as an amorphous powder after purification by flash column chromatography (1:1 hexanes/EtOAc). X-ray quality crystals of concomitant polymorphs were obtained by slow evaporation of a solution of 1 in 1:1 acetone/water at ambient temperature. Solvated complexes of 1 were obtained by slow evaporation of neat solutions at ambient temperature. Solvents were used as received from the commercial supplier, and no effort was made to rigorously exclude moisture. X-ray Crystallography. Preliminary examination and data collection were performed using a Bruker SMART CCD area detector system single-crystal X-ray diffractometer. The SHELXTL-PLUS software package was used for structure solution and refinement.18 Hydrogens were fixed at idealized geometries and treated isotropically as riding groups. Selected crystallographic data for all the structures discussed below are given in Table 1.
Results and Discussion The preparative route utilized for accessing triaroylbenzene derivatives outlined in Scheme 2 illustrates the ease with which functionalized congeners can be obtained.19 Compound 1 was originally prepared for use as a ligand in new coordination polymers.17 Given the propensity of other substituted triaroylbenzenes to serve as inclusion hosts, however, an examination of the solidstate behavior of the purely organic material was undertaken.20 Initial recrystallization of 1 from 1:1 acetone/water solution yielded two concomitant conformational polymorphs (forms A and B).21 These two modifications were identified through random screening
Polymorphism in 1,3,5-Tris(4-cyanobenzoyl)benzene
Crystal Growth & Design, Vol. 4, No. 6, 2004 1219
Figure 1. Extended honeycomb network exhibited in 1, form A. C-H‚‚‚X (X ) N/O) hydrogen bonds are indicated in red. W A rotatable 3D image is available in PDB format suitable for viewing with the CHIME plug-in.
Figure 3. The 2D lamellar structure in 1‚EtOAc. Solid-state hydrogen bonds indicated in red. EtOAc solvate molecules have been omitted for clarity. W A rotatable 3D image is available in PDB format suitable for viewing with the CHIME plug-in.
Figure 2. Ladder network exhibited by 1, form B. Hydrogen bonds indicated in green. W A rotatable 3D image is available in PDB format suitable for viewing with the CHIME plug-in.
of single crystals as they are visually indistinguishable. Consequently, it was not possible to separate the two polymorphs. Structural characterization of form A revealed that three donor/acceptor sites participate in weak intermolecular bidendate C-H‚‚‚O/N hydrogen bonding, imparting a roughly Y-shape to individual molecules.22 Each molecule is connected to three of its inversion-related partners. Two approximately coplanar carbonyl oxygens and two phenyl hydrogens form a centrosymmetric four point recognition pattern via a pair of C-H‚‚‚O hydrogen bonds (d,θ: 2.43 Å, 169.8°; 2.61 Å, 138.8°). The other two nodes of the threeconnected molecules extend through C-H‚‚‚N centrosymmetric dimers involving phenyl hydrogens and nitrile acceptors from two cyanobenzoyl rings (2.77 Å, 123.7°; 2.78 Å, 125.5°). This hydrogen-bonding arrangment results in formation of a hexagonal honeycomblike network, as shown in Figure 1. Cyanobenzoyl rings that are not incorporated into the honeycomb framework are oriented so as to fill hexagonal voids in the network. In the second polymorph (form B), individual molecules of 1 adopt a slightly different conformation relative to form A, producing an approximately T-shape geometry. Each molecule of 1 participates in a centrosymmetric C-H‚‚‚N hydrogen bond dimer involving a nitrile acceptor and an aromatic C-H donor (2.56 Å, 137.4°). The other two nodes of the three-connected building block extend via C-H‚‚‚N interactions (2.32 Å, 163.7°) involving translation related molecules to produce a ladder network as shown in Figure 2. Inversionrelated ladders are further connected via C-H‚‚‚O hydrogen bond dimers (not shown in Figure 2). These two isomeric networks have similar unit cell dimensions, while form A has a greater calculated density and
a more extensive hydrogen bonding network. Consequently, form A has been tentatively assigned the more stable modification. The inability to obtain either polymorph as a single species has precluded more extensive thermochemical characterization. Triaroylbenzene 1 was obtained as crystalline solvatefree material only when crystallized from acetone/H2O. Crystallization from other solvents resulted in formation of crystalline inclusion complexes or precipitation of an amorphous solid. For example, slow evaporation of an EtOAc solution deposited single crystals and X-ray diffractometry revealed an inclusion complex of stoichiometry 1‚EtOAc. The asymmetric unit has one host molecule (1) and one disordered guest molecule (EtOAc).23 In the crystal structure, a single cyanobenzoyl group of 1 self-assembles via pair of centrosymmetric C-H‚‚‚N and C-H‚‚‚O interactions to form an infinite 2D lamellar scaffold as shown in Figure 3. The central benzene ring and the other cyanobenzoyl rings act as pillars between inversion related 2D tapes, thus creating channels in the lattice that are filled by ethyl acetate guest molecules. Aromatic C-H hydrogen bond donors from host molecules 1 engage in relatively short bifurcated C-H‚‚‚O interactions with carbonyl acceptors originating from EtOAc guest molecules (2.47 Å, 177.0°; 2.29 Å, 165.3°). Compared to the nonsolvated structures of 1 discussed above, the triaroylbenzene network present in 1‚EtOAc represents a third supramolecular isomer of this material. A slight change in triaroylbenzene conformation acting in concert with the inclusion of solvent undoubtedly contributes to the observed change in network architecture. Interestingly, the intermolecular interactions operative in the inclusion complex between 1 and EtOAc appear to represent a general structural motif in that five additional clathrates involving 1 in combination with other solvates have been prepared and characterized. All of these inclusion complexes are isostructural with 1‚EtOAc. Solvents incorporated into this pseudopolymorphic family of clathrates include nitromethane (MeNO2), 3-pentanone, DMSO, acetone, and methyl chloroacetate (MCA).24 In each case, host molecules 1 adopt a lamellar sheetlike arrangement mediated by C-H‚‚‚O/N hydrogen bonding as shown in Figure 3.
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Figure 5. Extended packing of 1‚MCA with solvent molecules positioned within interlayer channels. W A rotatable 3D image is available in PDB format suitable for viewing with the CHIME plug-in. Table 3. Bifurcated Hydrogen Bonding Interactions between 1 and Various Solvatesa 1‚EtOAc 1‚pentanone 1‚MeNO2 1‚DMSO 1‚acetone 1‚MCA 1‚EtNO2 a
Figure 4. Bifurcated hydrogen bonding interactions between 1 and various solvates. Table 2. Parameters of Intermolecular C-H···O/N Hydrogen Bonding between Triaroylbenzenes in the Isostructural Inclusion Complexes of 1a d (H‚‚‚O) Å
θ (C-H‚‚‚O)°
d (H‚‚‚N) Å
θ (C-H‚‚‚N)°
2.52 2.48 2.52 2.62 2.52 2.85
131.1 136.0 130.2 129.2 132.0 121.0
2.57 2.83 2.71 2.86 2.68 2.68
166.0 152.1 162.0 168.1 162.2 156.2
1‚EtOAc 1‚3-pentanone 1‚MeNO2 1‚DMSO 1‚acetone 1‚MCA a
See Figure 3.
Specific hydrogen bond distances and angles for all six inclusion complexes are given in Table 2. In addition to essentially identical triaroylbenzene networks, the interactions between host and solvate molecules are also similar across the clathrate series. As shown in Figure 4, each solvent guest is imbued with a hydrogen bond acceptor that participates in a bifurcated interaction with a host molecule. Individual hydrogen bond distances and angles for these interactions are provided in Table 3. The identities of the solvating species and their ability to function as hydrogen bond acceptors are presumably crucial characteristics necessary for maintaining isostructurality throughout the series. Indeed, attempted crystallization of 1 from solvents lacking
d (H‚‚‚O) Å
θ (C-H‚‚‚O)°
2.47 2.29 2.49 2.44 2.70 2.51 2.49 2.33 2.45 2.28 2.49 2.48 2.42 2.16
177.0 165.3 136.6 142.2 172.0 159.8 126.5 135.9 178.6 161.2 177.6 164.6 172.7 178.8
See Figure 4.
hydrogen bond acceptors (CHCl3, CH2Cl2, benzene, toluene) was unsuccessful. Despite their similarities, there are differences between the individual solvates in terms of overall size (volume). Nonetheless, the triaroylbenzene lattice is able to accommodate individual guest demands. Such behavior is reminiscent of so-called “soft” materials that are capable of expanding and contracting to facilitate guest inclusion.25 Unfortunately, the inclusion complexes of 1 were not stable after removal of solvate and crystallinity was lost after a short time once crystals were taken from the mother liquor. Consequently, guest exchange experiments were not pursued. The presence of a hydrogen bond acceptor alone is not sufficient to ensure inclusion complex formation, and this observation may provide an indication as to the scope of the clathrating ability of 1. Attempted crystallization of 1 from potential solvates such as cyclohexanone, cyclopentanone, γ-butyrolactone, acetylacetone, methyl acetoacetate, and nitrobenzene failed to produce crystalline material. One other noteworthy observation concerning the structure of these isomorphous inclusion complexes concerns their resemblance to neutral organic clay mimics.26 The guest channels within the lattice are the result of a pillared lamellar network as shown in Figure 5. Such an architecture is the hallmark of naturally occurring inorganic and synthetic organic clays. The instability of the host framework in the absence of included solvent, however, renders this comparison superficial at best.
Polymorphism in 1,3,5-Tris(4-cyanobenzoyl)benzene
Crystal Growth & Design, Vol. 4, No. 6, 2004 1221 Table 4. Comparison of the Solid State Conformations Exhibited by 1 in Various Polymorphic and Pseudopolymorphic Structures
Figure 6. View of the extended packing of 1‚EtNO2. W A rotatable 3D image is available in PDB format suitable for viewing with the CHIME plug-in.
In contrast to the inclusion complexes described above, crystallization of 1 from nitroethane (EtNO2) afforded solvated single crystals of different morphology. There are two host molecules and two disordered guest molecules in the asymmetric unit in 1‚EtNO2. One of the crystallographically independent molecules self-assembles via centrosymmetric C-H‚‚‚O dimers involving carbonyl oxygen acceptors and aromatic C-H donors. The other independent molecule forms a centrosymmetric dimer mediated by C-H‚‚‚N hydrogen bonding. The independent host molecules are further connected through additional C-H‚‚‚N hydrogen bonding, the net result being the generation of a pair of interconnected 2D tapes. Each pair of 2D tapes is connecting to adjacent pairs via solid-state hydrogen bonding, producing a network with two distinct channels in which disordered EtNO2 guest molecules reside (Figure 6). Despite the dissimilarity between this structure and the previously described inclusion complexes, a bifurcated interaction between 1 and EtNO2 is also observed (see Figure 4 and Table 3). While it is unclear why the inclusion complex between 1 and EtNO2 constitutes a departure from the previously observed isomorphism, the significant role of weak hydrogen bonding in the transformation of host network architectures has been reported in a recent study.27 Thermochemical characterization of several inclusion complexes was briefly explored. The thermal properties of clathrates between 1 and EtOAc, MeNO2, acetone, and MCA were examined using differential scanning calorimetry (DSC).28 In each case, DSC analysis proved unremarkable with each complex exhibiting two endothermic transitions that corresponded to loss of solvate (at a temperature near the boiling point of the included species) followed by melting of 1 (T ) 212-218 °C). The crystallographically independent triaroylbenzene molecules observed in the various polymorphic and pseudopolymorphic structures differ in the rotation about the C-C single bonds between the cyanobenzoyl groups and the central 1,3,5-substituted benzene ring. The conformation of 1 may be defined according to the three torsion angles τ1, τ2, and τ3 (Table 4) and comparison of these values seemingly provides a convenient means for categorizing these structures. For example, the isomorphous clathrates of 1 adopt nearly identical conformations as revealed by the magnitudes of the torsion angles listed in Table 4. In contrast, the dimorphic modifications of 1 possess significantly different torsion angles, hence, these concomitant polymorphs
polymorph Form A Form B 1‚EtOAc 1‚3-pentanone 1‚MeNO2 1‚DMSO 1‚acetone 1‚MCA 1‚EtNO2
τ1 (C1-C2C7-O1) [deg]
τ2 (C3-C4C8-O2) [deg]
τ3 (C5-C6C9-O3) [deg]
39.2(2) 27.7(3) 27.7(2) 25.4(2) 27.8(3) 27.3(4) 27.8(3) 28.1(2) 30.8(3) 20.0(3)
36.8(3) 17.3(2) 34.2(3) 30.8(2) 33.2(3) 34.3(4) 33.3(3) 29.2(2) 36.5(3) 32.7(3)
146.1(2) 164.9(2) 150.93(18) 152.08(15) 150.93(19) 151.2(3) 150.8(2) 153.16(17) 153.3(8) 148.4(2)
also may be described as conformational polymorphs. In addition to conformational differences, each polymorph of 1 exhibits a distinct network structure mediated by weak solid-state hydrogen-bonding interactions. Moreover, two additional distinct network structures are exhibited by 1 in the two types of inclusion complexes that have been characterized. Thus, a total of four supramolecular isomers have been identified for 1 in its various polymorphic and pseudopolymorphic manifestations. Conclusions The polymorphism and pseudopolymorphism exhibited by 1 provides salient examples of the influence conformational flexibility and weak hydrogen bonding can exert over crystallization processes. While the dimorphic modifications were found to possess divergent crystal packing, six of the seven pseudopolymorphs of 1 are isostructural. It appears, therefore, that crystallization of 1 from certain solvents capable of participating in hydrogen bonding interactions directs crystallization toward formation of a recurring 2D lamellar pattern. Solvates are then accommodated within channels and engage in bifurcated interactions with triaroylbenzene host molecules (Figure 5). This view is somewhat oversimplified and many other subtle physical and chemical factors are certainly important as well. Such factors undoubtedly contribute to the unique structure found for 1‚EtNO2. Nonetheless, the results of this work indicate the feasibility of controlling supramolecular isomerism through guest inclusion. Simple and readily accessible triaroylbenzene frameworks have previously been shown to serve as inclusion hosts and as components of coordination polymers.17,20 More recently, a methyl-substituted analogue has been found to display an unusual type of organic network interpenetration.29 The present study further illustrates the potential utility of triaroylbenzenes in supramolecular chemistry in general, and in the exploration of polymorphism in particular.30 Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered
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by the American Chemical Society, for support of this research (ACS PRF# 37468-AC4). We thank Prof. J. S. Chickos (UM-St.Louis) for assistance in obtaining calorimetric data. Supporting Information Available: X-ray data with details of refinement procedure (cif files) are available free of charge via the Internet at http://pubs.acs.org.
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1034. (d) Vishweshwar, P.; Nangia, A.; Lynch, V. M. J. Org. Chem. 2002, 67, 556. (e) Aitipamula, S.; Thallapally, P. K.; Thaimattam, R.; Jasko´lski, M.; Desiraju, G. R. Org. Lett. 2002, 4, 921. (f) Kumar, V. S. S.; Nangia, A.; Kirchner, M. T.; Boese, R. New. J. Chem. 2003, 27, 224. (g) Bhogala, B. R.; Nangia, A. Cryst. Growth Des. 2003, 3, 547. (a) Choi, H. J.; Suh, M. P. J. Am. Chem. Soc. 1998, 120, 10622. (b) Brooks, N. R.; Blake, A. J.; Champness, N. R.; Cunningham, J. W.; Hubberstey, P.; Teat, S. J.; Wilson, C.; Schro¨der, M. J. Chem. Soc., Dalton Trans. 2001, 2530. (a) Comprehensive Supramolecular Chemistry; MacNicol, D. D.; Toda, F.; Bishop, R. Eds.; Pergamon: Oxford, 1996; Vol. 6. (b) Nangia, A. Curr. Opin. Solid State Mater. Sci. 2001, 5, 115. Kumar, V. S. S.; Pigge, F. C.; Rath, N. P. New J. Chem. 2003, 27, 1554. Kumar, V. S. S.; Pigge, F. C.; Rath, N. P. Cryst. Growth Des. 2004, 4, 651. Pigge, F. C.; Burgard, M. D.; Rath, N. P.; Cryst. Growth Des. 2003, 3, 331. Sheldrick, G. M. SHELXTL-PLUS, Bruker Analytical X-ray Division, Madison, WI, 1999. For discussions concerning this preparative route, see (a) Balasubramanian, K. K.; Selvaraj, S.; Venkataramani, P. S. Synthesis 1980, 29. (b) Elghamry, I. Synthesis 2003, 15, 2301. (a) Pigge, F. C.; Ghasedi, F.; Rath, N. P. Tetrahedron Lett. 1999, 40, 8045. (b) Pigge, F. C.; Zheng, Z.; Rath, N. P. New J. Chem. 2000, 24, 183. (c) Pigge, F. C.; Ghasedi, F.; Zheng, Z.; Rath, N. P.; Nichols, G.; Chickos, J. S. J. Chem. Soc., Perkin Trans. 2 2000, 2458. Bernstein, J.; Davey, R. J.; Henck, J.-O. Angew. Chem., Int. Ed. 1999, 38, 3440. Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999. The designation of “host” and “guest” is somewhat arbitrary for the inclusion complexes under discussion. The triaroylbenzene derivative 1 has been assigned the role of host, while the enclathrated solvent molecules assume the role of guest. The host: guest stoichiometry was 1:1 for all inclusion complexes except MeNO2 (1:1.5). Soldatov, D. V.; Ripmeester, J. A. Chem. Eur. J. 2001, 7, 2979. Brouwer, E. B.; Udachin, K. A.; Enright, G. D.; Ripmeester, J. A.; Ooms, K. J.; Halchuk, P. A. Chem. Commun. 2001, 565. (b) Kumar, V. S. S.; Nangia, A. Chem. Commun. 2001, 2392. Thaimattam, R.; Xue, F.; Sarma, J. A. R. P.; Mak, T. C. W.; Desiraju, G. R. J. Am. Chem. Soc. 2001, 123, 4432. Measurements were performed on a Perkin-Elmer DSC 7 at a scan rate of 5 °C min-1. Kumar, V. S. S.; Pigge, F. C.; Rath, N. P. New J. Chem., in press. For another example of a polymorphic triaroylbenzene derivative, see: Kumar, V. S. S.; Pigge, F. C.; Rath, N. P. CrystEngComm 2004, 6, 102.
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