Helical or Polar Guest-Dependent Z′ ) 1.5 or Z′ ) 2 Forms of a Sterically Hindered Bis(urea) Clathrate Adam M. Todd, Kirsty M. Anderson, Peter Byrne, Andre´s E. Goeta, and Jonathan W. Steed* Department of Chemistry, UniVersity of Durham, South Road, Durham, U.K. DH1 3LE
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1750-1752
ReceiVed May 30, 2006
ABSTRACT: A sterically hindered bis(pyridylurea) species forms clathrates with water, methanol, and ethanol, all exhibiting unusual polar structures with Z′ > 1. This crystal packing is due to the hindered geometry about the urea oxygen acceptor atoms. Urea is well-known to form hexagonal clathrates, with guests running in channels walled by urea helices. The urea molecules are held together by NH‚‚‚O hydrogen bonding in which the urea oxygen atoms act as acceptors for four hydrogen bonds.1 In contrast, N,N′-disubstituted ureas often form a urea tape motif (Figure 1a). A recent thorough study by Nangia2 has shown that in aryl urea derivatives this kind of urea tape motif involving a bifurcated oxygen acceptor is retained, except in cases where the aryl groups incorporate an electron-withdrawing substituent. Thus, ureidopyridines tend to crystallize with NH‚‚‚Npyridyl hydrogen-bonding interactions (Figure 1b), despite the lower hydrogen bond basicity of the pyridyl nitrogen atom.3 This behavior is because the electrondeficient nature of the pyridyl ring results in relatively acidic H-bonded pyridyl CH groups, and hence, the urea oxygen is rendered unavailable for intermolecular hydrogen bonding interactions as a result of strong intramolecular CH‚‚‚O hydrogen bonding. These CH‚‚‚O interactions tend to result in planar molecular conformations. We have observed competition between NH‚‚‚O and NH‚‚‚‚anion interactions in coordination complexes of ureidopyridines,4 and our work on the free ligands reinforces the conclusions offered by Nangia.5 The aggregation of bis- and tris(ureas) is of considerable current interest because of their behavior as low-molecular-weight gelators (LMWG)6-8 as well as the opportunity they afford to study more robust hydrogen-bonded frameworks arising from multiple interactions linking molecules together. In this communication we report the solid-state packing and clathrate chemistry of the bipyridyl bis(urea) 1. Compound 1
was designed as an LMWG (i.e. to be poorly crystalline and exhibit awkward crystal packing). The molecule has one electronwithdrawing pyridyl unit and a sterically hindering -CMe2- group as urea substituents. The urea oxygen atom is thus expected to be relatively unavailable as a hydrogen bond acceptor and is situated in an unsymmetrical, hindered environment. While the compound has not been found to act as an LMWG to date, it does exhibit awkward and interesting crystal packing. Compound 1 was prepared by reaction of 1,3-bis(1-isocyanato1-methyl-ethyl)benzene with 3-aminopyridine and was fully characterized. The host crystallizes very readily from methanol solution as the 3:1 solvate 13‚MeOH in the unusual chiral space group C2 with 1.5 independent molecules in the asymmetric unit (Z′ ) 1.59) (Figure 2).23 Structures with Z′ > 1 have attracted a great deal of recent attention because of the general insights they offer into crystal nucleation and packing.10-14 The key feature of the polar, chiral packing arrangement is an irregular (noncrystallographic) 3-fold * To whom correspondence should be addressed. Fax: +44 (0)191 384 4737. Tel: +44 (0)191 334 2085. E-mail:
[email protected].
helix of pitch 12.50 Å (the crystallographic c axis). The helix comprises an AABAAB type repeat of the two independent molecules linked together by a twisted, chiral variation on the urea tape motif (cf. Figure 1a) in which one of the three crystallographically independent urea-urea interactions comprises a nonbifurcated acceptor NH‚‚‚O interaction (N‚‚‚O ) 2.822 and 3.347 Å) (Figure 3). This departure from the more common six-membered-ring motif is due to steric interactions with the CMe2 group, which would otherwise be in close proximity with the central aryl ring on the acceptor molecules. It is this anomalous donor that is the independent half-molecule in the Z′ ) 1.5 structure. Thus, the unusual Z′ value arises from steric congestion of the dominant urea tape motif. Unlike diarylureas containing pyridyl substituents,2 the urea oxygen is not blocked by CH‚‚‚O interactions on both sides, and the NH donors approach on the opposite side of the oxygen to the pyridyl ring in each case. The axes of the irregular 3-fold helices run in a distorted-hexagonal arrangement approximately perpendicular to an interwoven (but not interpenetrating) layer arrangement of the host molecules. This layer is linked by long CH‚‚‚Npyridyl interactions. The OH group does not apparently interact with the host. The overall topology is complex and is represented in Figure 4. Figure 4 also shows the positions of the disordered methanol guest, which is somewhat too small to fill the distorted 66 Å3 hexagonal cavity. The methanol is held in place by long hydrogen bonds from methanol CH to pyridyl N atoms. Overall, this chiral structure arises from the steric constraints imposed on the urea oxygen atom by the combination of intramolecular CH‚‚‚O interactions and steric congestion directly resulting in the Z′ ) 1.5 arrangement. The trapped methanol appears to be tightly held without obvious egress. Analysis of the compound by TGA confirmed that methanol loss is extremely difficult and the guest is not released until 197 °C in a distinct weight loss step shortly before decomposition. This represents a stabilization of methanol some 132 °C above its normal boiling point.15 Given the fact that methanol is too small for the cavity (and is hence regularly disordered over two sites), we sought to include larger guests such as ethanol and 1,2-diaminoethane, simply by recrystallizing the host from these solvents. The 1,2-diaminoethane sample yielded crystals of 16‚H2O, isomorphous with 13‚MeOH. Each cavity is occupied by two partially occupied water sites some 2.7 Å apart. TGA analysis indicates onset of water loss at 214 °C, consistent with the methanol analogue. The water arises from residual water in the ethylenediamine, which is itself too large to occupy the cavity. However, a completely different packing arrangement is observed for the ethanol clathrate, which is a 1:1 complex with Z′ ) 2. Ethanol inclusion in the same framework as the methanol and water clathrates of 1 should result in 77% occupancy of the 66 Å3 cavity, which is somewhat higher than the 55-70% range for stable clathrates.16 The new phase 1‚EtOH crystallizes in a polar but achiral space group, Pn. Once again, the interesting polar packing arises from the steric frustration of the urea tape motif. The host molecules crystallize in a U-shaped
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Figure 1. Hydrogen-bonding arrangements in (a) the planar urea tape motif and (b) pyridylureas and (c) environment around the urea carbonyl in 1.
Figure 2. Asymmetric unit in 13‚MeOH, showing the atom-numbering scheme. The methanol is disordered. Most hydrogen atoms are not included for clarity. Figure 4. Host network topology in 13‚MeOH. Vertical red lines are 3-fold helices obtained by linking urea O atoms. Nodes on the horizontal blue network are pyridyl N atoms, with long edges representing the molecular axis and short edges being intermolecular CH‚‚‚N interactions. The positions of the included methanol are respesented by spheres.
Figure 3. Threefold urea helix observed in 13‚MeOH: (a) overall packing (crystallographically independent half-molecule shown in blue); (b) detail showing the urea arrangement.
conformation very different from the S-conformation in 13‚MeOH, which allows the formation of one repeat of the urea tape six-membered ring adjacent to a very short aryl CH‚‚‚O interaction, both to the second independent molecule (Figure 5). Steric interactions to the CMe2 group prevent the double urea tape interaction seen in other bis- and tris(ureas), particularly LMWGs,6,7,17 and instead the second urea NH functionalities on each molecule are orientated outward, away from the CMe2 group, and form a six-membered ring hydrogen bond to the included ethanol. It must be stressed that the environment and principal interactions of the two independent molecules are similar to one another but the frustrated hydrogen-bonding interaction between them does not conform to crystallographic symmetry and hence necessitates the Z′ ) 2 structure. The urea NH donors interact with the ethanol solvent, forming a six-membered hydrogen-bonded ring with the ethanol oxygen acting as a bifurcated acceptor. The two independent
Figure 5. Urea NH‚‚‚O and aryl CH‚‚‚O interactions between the two independent molecules in 1‚EtOH (most hydrogen atoms are not included for clarity).
ethanol OH groups act as hydrogen bond donors to one of the two pyridyl nitrogen atoms on each independent molecule (Figure 6). Thus, the ethanol guests exhibit saturated hydrogen bonding.18 The remaining pyridyl N atoms engage only in long CH‚‚‚N interactions, as seen for 13‚MeOH. Interestingly, the overall crystal polarity does not arise from the fact that all of the urea groups line up in the same direction (the directional sense of adjacent chains is antiparallel). It in fact arises from the urea‚‚‚ethanol‚‚‚‚pyridyl interactions that are all in the same direction in the crystal. Thus, the chiral Z′ ) 2 asymmetric unit exhibits two essentially perpendicular polar axes. While the n glide packing allows stacks of these units to interdigitate effectively, the glide (reflection) operation can only
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References
Figure 6. Saturated hydrogen bonding to the enclathrated ethanol in 1‚ EtOH.
Figure 7. Antiparallel stacking of urea motifs parallel to b (blue) and parallel stacking of interactions to ethanol parallel to c (green) leading to bulk polarity (ethanol shown as vdW spheres).
invert the polarity of the stacking perpendicular to it, not parallel (Figure 7). This specific observation gives a general clue in the design of polar crystalline materials for NLO applications.19,20 Consistent with the more open structure of 1‚EtOH, TGA analysis indicates a distinct 7.4% weight loss step at 116 °C corresponding to loss of 75% of the ethanol guest. This loss of guest at a much lower temperature than in 13‚MeOH is consistent with the more open structure of 1‚EtOH. In this paper we have shown that deliberate incorporation of features within a molecule that frustrate or place constraints on well-recognized supramolecular synthons, such as the urea tape motif, can lead to interesting polar or chiral packing arrangements that, unlike 91.2% of the compounds in the CSD,21,22 are forced to exhibit Z′ > 1. Despite a gross change of structure on moving to a guest molecule that is too large for the C2 phase, the frustration leading to unusual packing is retained. We are thus able to achieve some measure of control over the supramolecular synthon. This kind of control is surely a key step toward a “genuine crystal engineering” and increases our understanding of molecular features leading to polar packing for applications in NLO materials19 or the kind of “arrested crystallization” found in supramolecular gels, for example. Acknowledgment. We are grateful to Prof. Ashwini Nangia for helpful discussions and the EPSRC for a postdoctoral grant (K.M.A.). Supporting Information Available: CIF files for all structures presented herein. This material is available free of charge via the Internet at http:// pubs.acs.org.
(1) Hollingsworth, M. D.; Harris, K. D. M., Urea, Thiourea and Selenourea. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon: Oxford, U.K., 1996; Vol. 6, pp 177-237. (2) Reddy, L. S.; Basavoju, S.; Vangala, V. R.; Nangia, A. Cryst. Growth Des. 2006, 6, 161-173. (3) Laurence, C.; Berthelot, M. Perspect. Drug Discuss. Des. 2000, 18, 39-60. (4) Turner, D. R.; Smith, B.; Goeta, A. E.; Evans, I. R.; Tocher, D. A.; Howard, J. A. K.; Steed, J. W. CrystEngComm 2004, 6, 633-641. (5) Turner, D. R. Ph.D. Thesis, King’s College London, 2004. (6) de Loos, M.; Ligtenbarg, A. G. J.; van Esch, J.; Kooijman, H.; Spek, A. L.; Hage, R.; Kellogg, R. M.; Feringa, B. L. Eur. J. Org. Chem. 2000, 3675-3678. (7) Brinksma, J.; Feringa, B. L.; Kellogg, R. M.; Vreeker, R.; van Esch, J. Langmuir 2000, 16, 9249-9255. (8) Applegarth, L.; Clark, N.; Richardson, A. C.; Parker, A. D. M.; Radosavljevic-Evans, I.; Goeta, A. E.; Howard, J. A. K.; Steed, J. W. Chem. Commun 2005, 5423-5425. (9) Steed, J. W. CrystEngComm 2003, 5, 169-179. (10) Das, D.; Banerjee, R.; Mondal, R.; Howard, J. A. K.; Boese, R.; Desiraju, G. R., Chem. Commun. 2006, 555-557. (11) Hao, X.; Siegler, M. A.; Parkin, S.; Brock, C. P. Cryst. Growth Des. 2005, 5, 2225-2232. (12) Hao, X.; Chen, J.; Cammers, A.; Parkin, S.; Brock, C. P. Acta Crystallogr., Sect. B 2005, 61, 218-226. (13) Nichol, G. S.; Clegg, W. Cryst. Growth. Des. 2006, 6, 451-460. (14) Pidcock, E. Acta Crystallogr., Sect. B 2006, 62, 268-279. (15) Atwood, J. L.; Barbour, L. J.; Jerga, A. Science 2002, 296, 23672369. (16) Nakano, K.; Sada, K.; Kurozumi, Y.; Miyata, M. Chem. Eur. J. 2001, 7, 209-220. (17) Stanley, C. E.; Clarke, N.; Anderson, K. M.; Lenthall, J. P.; Steed, J. W. Chem. Commun., in press (DOI: 10.1039/b606373j). (18) Loehlin, J. H.; Franz, K. J.; Gist, L.; Moore, R. H. Acta Crystallogr., Sect. B 1998, 54, 695-704. (19) Sarma, J. A. R. P.; Allen, F. H.; Hoy, V. J.; Howard, J. A. K.; Thaimattam, R.; Biradha, K.; Desiraju, G. R., Chem. Commun. 1997, 101-102. (20) Marder, S. R., Metal Containing Materials for Nonlinear Optics. In Inorganic Materials, 2nd ed.; Wiley: Chichester, U.K., 1996; pp 121-169. (21) Anderson, K. M.; Goeta, A. E.; Hancock, K. S. B.; Steed, J. W. Chem. Commun. 2006, 2138-2140. (22) Steiner, T. Acta Crystallogr., Sect. B 2000, 56, 673-676. (23) Crystal data for 13‚MeOH: C73H88N18O7, Mr ) 1329.61, colorless block, 0.20 × 0.20 × 0.20 mm3, monoclinic, space group C2 (No. 5), a ) 24.905(8) Å, b ) 11.351(4) Å, c ) 12.511(4) Å, β ) 90.066(12)°, V ) 3536.7(19) Å3, Z ) 2, Dc ) 1.249 g/cm3, F000 ) 1416, SMART 1k, Mo KR radiation, λ ) 0.710 73 Å, T ) 120(2) K, 2θmax ) 46.5°, 13 050 reflections collected, 5068 unique (Rint ) 0.0510), final GOF ) 1.055, R1 ) 0.0440, wR2 ) 0.0923, R indices based on 4334 reflections with I >2σ(I) (refinement on F2), 476 parameters, 1 restraint, Lp and absorption corrections applied, µ ) 0.083 mm-1. Crystal data for 16‚H2O: C24H28.60N6O2.31, Mr ) 438.04, colorless block, 0.20 × 0.20 × 0.20 mm3, monoclinic, space group C2 (No. 5), a ) 25.306(3) Å, b ) 11.1907(11) Å, c ) 12.3470(13) Å, β ) 90.410(2)°, V ) 3496.5(6) Å3, Z ) 6, Dc ) 1.248 g/cm3, F000 ) 1398, Smart 1k, Mo KR radiation, λ ) 0.710 73 Å, T ) 120(2) K, 2θmax ) 46.7°, 15 733 reflections collected, 5100 unique (Rint ) 0.0448), final GOF ) 1.057, R1 ) 0.0337, wR2 ) 0.0703, R indices based on 4535 reflections with I >2σ(I) (refinement on F2), 471 parameters, 1 restraint, Lp and absorption corrections applied, µ ) 0.083 mm-1. Crystal data for 1‚EtOH: C26H34N6O3, Mr ) 478.59, colorless block, 0.30 × 0.20 × 0.10 mm3, monoclinic, space group Pn (No. 7), a ) 11.6615(7) Å, b ) 12.1566(7) Å, c ) 18.5303(11) Å, β ) 99.5820(10)°, V ) 2590.3(3) Å3, Z ) 4, Dc ) 1.227 g/cm3, F000 ) 1024, SMART 1k, Mo KR radiation, λ ) 0.710 73 Å, T ) 120(2) K, 2θmax ) 46.6°, 22 291 reflections collected, 7447 unique (Rint ) 0.0920), final GOF ) 0.990, R1 ) 0.0559, wR2 ) 0.0963, R indices based on 5432 reflections with I >2σ(I) (refinement on F2), 681 parameters, 4 restraints, Lp and absorption corrections applied, µ ) 0.083 mm-1.
CG060318O
