DOI: 10.1021/cg801054e
Influence of the Aliphatic Wrapping in the Crystal Structure of Benzene Tricarboxamide Supramolecular Polymers
2009, Vol. 9 4987–4989
Claudio A. Jimenez,*,† Julio B. Belmar,† Leandro Ortı´ z,† Paulina Hidalgo,† Oscar Fabelo,‡,§ Jorge Pas an,‡ and Catalina Ruiz-Perez*,‡ †
Departamento de Quı´mica Org anica, Facultad de Ciencias Quı´micas, Universidad de Concepci on, Casilla 160-C, Concepci on, Chile, and ‡Laboratorio de Rayos X y Materiales Moleculares, Departamento de Fı´sica Fundamental II, Facultad de Fı´sica, Universidad de La Laguna, Avenida Astrofı´sico Francisco S anchez s/n, E-38204 La Laguna (Tenerife), Spain. §Present address: Instituto de Ciencia de Materiales de Arag on, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain, and Institut Laue Langevin, B.P. 156, 6 Rue J. Horowitz, 38000, Grenoble, France Received September 19, 2008; Revised Manuscript Received September 29, 2009
ABSTRACT: The syntheses and crystal structures of two novel trisamides, the ethyl- (2) and propyl-substituted (3) N,N0 ,N00 trialkylbenzene-1,3,5-carboxamides, are reported. The compounds are prepared in good yields by aminolysis of the trimethyl-1,3,5benzenecarboxylate and the respective primary amine. Compound 2 crystallizes in the P212121 space group with a complete molecule in the asymmetric unit and four molecules per unit cell, whereas 3 does it in the R3c space group with one-third of the molecule in the asymmetric unit and six molecules per unit cell. Their solid-state structures show that the N;H 3 3 3 OdC hydrogen bond plays the most important role in the supramolecular framework of both cases, despite considerable differences in crystal packing. Supramolecular sheets are formed in 2, whereas molecules of 3 are packed in a H-bonded primitive cubic [46] three-dimensional network. The main difference in the molecular conformation is the tilting of the carboxamide group with respect to the aryl, which in the case of 3 occurs in the same direction, leading to the formation of “solid-state” chiral molecules. One of the aims of crystal engineering is to establish control over the preparation of crystalline solid materials so that their architecture and properties are, at least to some extent, predictable.1 From an esthetical and practical point of view, C3-symmetrical molecules are highly attractive building blocks for the formation of supramolecular architectures.2 Thus, a series of the benzene-1,3,5-tricarboxamide derivatives have been studied.3-8 The self-assembling into helical columnar aggregates9 and the quantitative analysis of the “majority rules” principle in aggregates of extended discotics were reported.10 In particular, trialkylsubstituted benzenetricarboxamides have attracted special attention, since they were reported to show discotic liquid crystalline phases, with the intermolecular hydrogen bonding being assumed to stabilize the columnar order.3,5,11-13 Matsunaga et al. have studied these compounds because of their thermotropic liquidcrystalline behavior,3,14 while Hanabusa et al. have observed that they formed organogels in selected solvents.5,15 The efficiency of gel formation was attributed to a combination of intermolecular hydrogen-bonding between N;H and CdO of amides and van der Waals interactions between the alkyl side chains. They concluded that longer and, hence, bulky alkyl groups substituted in the benzenetricarboxamide will prevent the molecular packing showed in the trimethyl-1,3,5-benzenetricarboxamide (1) stabilizing metastable gel-like states.5 To probe this assessment, we synthesized and crystallized the triethyl (2) and tripropylbenzene-1,3,5-tricarboxamides (3). The crystal structure of 216 (Figure 1) is very similar to that of the trimethyl-substituted molecule (1),5 and it consists of supramolecular sheets extended in the ab plane which are formed through CdO 3 3 3 H;N hydrogen bonds. From a topological point of view, the molecules in 1 and 2 act as 6-connected nodes forming a hexagonal [364653]-hxl 2D network.17 The ethyl groups of the triamide ligands in 2 are located between the sheets, with the ethyl groups of the adjacent layers occupying the space alternatively. A different situation is encountered in 1, where the methyl groups from adjacent layers are face-to-face. The *To whom correspondence should be addressed. E-mail: C.A.J., cjimenez@ udec.cl; C.R.-P.,
[email protected]. r 2009 American Chemical Society
torsion angles of the carboxamide with respect to the aryl group for 1 and 2 [-8.21(1), 16.16(1), and 46.09(1)° for 1 and -8.3(4), 14.1(4), and 44.9(4)° for 2] show little differences. The different packing of the sheets in 1 and 2 is a result of the noninnocent role played by the aliphatic chains in these substituted trisamides, in particular due to the increase in conformational flexibility when the methyl is substituted by an ethyl group. The longer the chain, the greater is its influence in the crystal packing. Thus, what are the changes in the crystal structure introduced by longer aliphatic wrappings? The crystal structure of N,N0 ,N00 -tris(2-methoxyethyl)benzene1,3,5-tricarboxamide (4) reported by Lightfoot et al.7 has been proposed as a model for the aggregation pattern of columnar mesophases.10 The molecules of 4 exhibit a columnar arrangement, with the π-stacking of the aromatic cores and the three intermolecular CdO 3 3 3 H;N hydrogen bonds being responsible for the linkage of the units along a 21 screw axis. The structure showed a triple-helical network of hydrogen bonds that resembled the R helix in proteins. The inter-ring separation was similar to those inferred for discotic liquid crystalline phases based on aromatic cores. Complex 4 crystallizes in the polar group P21, and spontaneous resolution of the supramolecular chirality arises. Chirality appears, since all the amide moieties are tilted in the same sense by about 40° with respect to the central benzene core [the torsion angles of the carboxamide with respect to the aryl are 38.1(7), 43.4(7), and 47.1(7)°]. This propeller shape of the wedges around the central core may therefore be the key for efficient transfer of chirality in all discotic compounds based on benzene-1,3,5-tricaboxamides. However, the crystal structure of the tripropylbenzene-1,3,5-tricarboxamide (3) shows that other conformations, where the alternation of chirality is not precluded, are possible. The crystal packing of 3 strongly differs from those of 1 and 2, and it builds up a primitive cubic [46]-pcu supramolecular 3D network through amide hydrogen bonds.18 Each molecule is, then, connected to another six (three above and three below the plane of the molecule) (Figure 2) through one crystallographically independent N;H 3 3 3 OdC hydrogen bond [the D 3 3 3 A and H 3 3 3 A distances are 2.804(4) A˚ and 2.062(2) A˚ whereas the D;H 3 3 3 A angle is 144.0(2)°]. The carboxamide group is Published on Web 10/22/2009
pubs.acs.org/crystal
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Jimenez et al.
Figure 1. View of the hexagonal [364653]-hxl 2D hydrogen-bonded network of 2 (left) and its topological representation (right).
Scheme 1
Figure 2. View of the packing of adjacent molecules in 3 as a schematic representation (top) and as a space-filling model (bottom).
partially tilted with respect to the aryl ring [the torsion angle of the carboxamide with respect to the aryl is 27.4(4)°] due to the counterbalance between the demand of conjugation and that of hydrogen bonding. Since all the amide groups are tilted in the same direction, the tricarboxamide molecules become chiral units in the solid state, with the propeller shape of the wedges observed in 4.10 However, in 3 the packing is in the R3c space group; then,
due to the presence of the glide plane, the two molecular enantiomers appear in the crystal structure. Each one of the enantiomers is connected through hydrogen bonds to six of the other kind (Figure 2). The molecules in 3 are located in nonH-bonded sheets within the ab plane and are stacked along the c axis following the regular sequence ABAB with alternating chirality. The chemical differences in the molecular structure of 3 and 4 are reduced to the longer chain (four atoms) of 4 including an ether group which is not H-bonded and does not participate in the formation of the helical network. The columnar packing of 4 can only be achieved by stacking propeller-like molecules of the same chirality; however, the “cubic” packing of 3 needs the presence of the two enantiomers. Since the free molecules are not chiral, the formation of both crystal structures (3 and 4) during the crystal growth is not precluded for chirality reasons. Thus, what causes lead the molecules to pack following the structure of 3 or 4? There are two main sources for conformational differences in 3 and 4, the torsion of the carboxamide with respect to the aryl group and the torsion of the alkyl chain with respect to the carboxamide. Concerning the former one, although the torsion angles of 3 and 4 are different, the three carboxamide groups are tilted in the same sense with respect to the aryl, leading to a propeller-like shape for the molecule: a situation not encountered either in 1 or 2. Thus, the structural feature leading to different packings in 3 and 4 is the torsion of the alkyl chain with respect to the carboxamide. In 3, all the propyl groups point toward the same direction (i.e., “up-up-up”) with torsion angles of the alkyl chain with respect to the carboxamide of 90.6(4)°, which is that of the nitrogen atom of the tilted carboxamide. However, in 4, the 2methoxyethyl substituents are located “up-up-down”, with torsion angles of -89.8(6), 82.3(7), and 145.7(5)°. Since these conformations can be interchanged in solution and the trialkylbenzene-1,3,5-tricarboxamides with substituents longer than butyl behave as columnar liquid crystals,3,14 the constraints for
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Crystal Growth & Design, Vol. 9, No. 12, 2009
“cubic” packing of the slightly longer alkyl-wrapped molecules of 4 seem to be the reason for the columnar arrangement. The drastic change of the hydrogen bonding pattern from 2 to 3 could be the reference mark for the occurrence of these processes. On the increasing of the alkyl-chain wrapping, the “aside” hydrogen bonding is less favored and the tilting of the carboxamide groups tends to align in the same direction, leading to the chiral conformations observed in 3 and 4. The crystal structures of the triethyl- (2) and tripropylbenzene-1,3,5-tricarboxamides (3) together with those of already reported methyl- (1) and methoxyethyl- (4) substituted triamides can offer better clues to the nature of the mesophases3 and lyotropism5 observed. Attempts to crystallize the butyl-substituted triamide continue. Acknowledgment. Funding for this work was supported by the Ministerio Espa~ nol de Ciencia e Innovaci on through Projects MAT2007-60660 and “Factorı´ a de Crystalizaci on” (ConsoliderIngenio2010, CSD2006-00015) and by the AECI through Project A/012620/07. A predoctoral (O. F.) fellowship from Ministerio Espa~ nol de Ciencia e Innovaci on is acknowledged. J.P. also thanks the CSD2006-00015 for a postdoctoral contract. Support for this work is also provided by FONDECYT 11080179. Supporting Information Available: Details of the synthesis and Xray crystallography of 2 and 3, their structures, and bond lengths and angles, as well as their CIF data. This material is available free of charge via the Internet at http://pubs.acs.org.
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