CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 2 211-213
Communications Symmetry Mismatching as a Tool in the Synthesis of Complex Supramolecular Solids with Multiple Cavities Bao-Qing Ma and Philip Coppens* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York, 14260-3000 Received October 20, 2003;
Revised Manuscript Received January 8, 2004
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: Rotational symmetry of different parity of node and linker molecules has been used as a strategy in the synthesis of two architectural isomers with very different frameworks but identical cavities, both based on 1,3,5-tri(4pyridyl)-2,4,6-triazine and C-methylcalix[4]resorcinarene. One of the two cavities in each of the solids has an unusual pentagonal shape. Calixarenes, calixresorcinarenes, pyrogalloarenes, and related compounds play important roles in supramolecular host-guest chemistry, and have potential applications in materials science, gas storage, drug delivery, and chemical separation.1,2 The combination of C-methylcalix[4]resorcinarene (CMCR) with extended exo-bidentate pillar spacers, such as trans-1,2-bis(4-pyridyl)ethylene, bis(4pyridylmethylidyne)hydrazine and 4′-(4-octyloxyphenyl)4,2′:6′,4′′-terpyridine, can effectively expand cavities or channels, and generate frameworks capable of including single or multiple guest molecules as large as decamethylruthenocene, adamantanone, and [2.2] paracyclophane.3 In general, the size and shape of the cavity or channel determine the kind of guest that can be included in the host structure. For crystal engineering to become truly an engineering discipline, systematic procedures to incorporate guest molecules with specific properties must be devised. To achieve this goal, the nature of the host framework should be controlled. A pertinent strategy is to alter the nature of the spacer, which, because of the conformational flexibility of the CMCR molecule, may lead to dramatically different frameworks. We anticipated that the lack of symmetry matching between the node and spacer molecules could lead to complex supramolecular frameworks with more than one type of cavity in a single solid. We define symmetry mismatching to occur when the set of connecting atoms of one component has a 2m-fold principal axis, while in the second component the atoms forming the intermolecular links are related by a 2n+1-fold principal axis, m and n being integers g1. In pursuit of this concept, we have used the “threearmed” spacer 1,3,5-tri(4-pyridyl)-2,4,6-triazine (tpt, Scheme * To whom correspondence should be addressed. E-mail: coppens@ acsu.buffalo.edu.
Scheme 1
1), previously exploited in the synthesis of inorganicorganic hybrid coordination frameworks,4 to construct novel CMCR-based materials. Although numerous CMCR-based supramolecular complexes have now been reported,3 they have been exclusively based on bifunctional pillars. The bowl, chair, and boat conformations of CMCR in the solid state have an even symmetry with point groups C4v, C2v, and C2h, respectively. Thus, the tpt spacer with its odd (3-fold) symmetry provides a mismatch with the CMCR “node” molecules, no matter what kind of conformation the CMCR molecule adopts. One possible result of symmetry mismatching is that one of the components combines with a small species such as a water molecule or a halide ion to form a new molecular moiety with a symmetry matching that of the second component. A second possibility, illustrated here, is that the two components self-assemble to generate a solid with unusual architecture. The compounds [CMCR‚tpt]‚2nitrobenzene‚H2O (1) and [CMCR‚tpt]‚2toluene‚H2O (2) were prepared hydrothermally at 140 °C in the presence of nitrobenzene and toluene, respectively. Low-temperature X-ray diffraction analysis revealed both structures to consist of twodimensional sheets.5 Compound 1 crystallizes in the space group P1 h with six independent CMCR and tpt molecules per asymmetric unit and consequently has a very large unit cell (∼21 × 22 × 38 Å). Such a large asymmetric unit (498
10.1021/cg0341914 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/11/2004
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Figure 1. The two-dimensional noncentrosymmetric hydrogenbonded sheet formed by CMCR and tpt in 1. Only one position is shown for the disordered nitrobenzene guests in the pentagonal cavity A. W A 3D rotatable image in XYZ format is available.
non-hydrogen atoms) is exceptional in the context of small molecule crystallography. The cell contains two crystallographically independent 2D hydrogen-bonded layers of identical connectivity, one of which includes an inversion center. All six CMCR molecules adopt the boat conformation. Two different bonding arrangements can be distinguished. Three of the CMCR molecules are involved in hydrogen-bonded molecular columns along the [100] direction. The other three molecules participate in dimers pointing along the [101] direction, the long axis of the dimer making an angle of ∼34° with the column direction (Figure 1). The column CMCR molecules are each linked to a pair of stacked, π-π interacting, tpt molecules through O-H‚‚‚N hydrogen bonding, and thus form a ribbon “decorated” with tpt dimers. The other two arms of the tpt dimer are linked to two different CMCR dimers along the axial and equatorial directions of CMCR, respectively. The decorated ribbons are thus connected into a two-dimensional framework (Figure 1). The framework encloses two types of cavities with approximately pentagonal (A) and rectangular (B) shapes, with an A/B ratio of 2:1. The A cavities are adjacent to the columns, while the B cavities are located halfway between two columns. Two free hydroxyl groups of each CMCR point into cavities of type A, leading to a hydrophilic region of the cavity wall, while the remainder of the cavity is lined by hydrophobic groups. Disordered hydrogen-bonded water molecules are located in the hydrophilic region of A, as are three nitrobenzene molecules, each disordered over several positions, but with all nitro groups hydrogen bonded to the water molecules and the CMCR hydroxyl groups. In contrast, the rectangular cavity B is lined with methyl groups and is formally hydrophobic. It contains two nitrobenzene molecules arranged in a back-to-back version (Figure 1), such as to allow C-H‚‚‚O interactions between the tpt molecules and the nitro groups. This orientation differs from that found in the carcerand-like capsule in 2CMCR‚4bipy‚2nitrobenzene in which two nitro groups are located in close proximity,6 demonstrating the defining influence of the host structure on the guest arrangement. The replacement of nitrobenzene with toluene under similar reaction conditions leads to 2, which crystallizes in the orthorhombic space group Pbcn. CMCR molecules
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Figure 2. The two-dimensional hydrogen-bonded sheet formed by CMCR and tpt in 2. Toluene and water molecules are omitted for clarity. The toluene dimer in the rectangular cavities is illustrated at the bottom of the picture. W A 3D rotatable image in XYZ format is available.
again adopt a boat conformation. Rather than forming molecular columns or dimers they crystallize as linear tetramers along the equatorial direction of CMCR, connected through intermolecular hydrogen bonds (O‚‚‚O separations 2.667-2.916 Å). The linear tetramers are arranged in a herringbone pattern with an intersecting angle of ∼39° (Figure 2). Four stacked tpt dimers are hydrogen bonded laterally to the tetramer in the axial CMCR directions, while two tpt dimers are linked to the end of the tetramer in equatorial CMCR directions (N‚‚‚O separations 2.651-2.761 Å). The tpt spacers stacked along the [001] direction show significant interactions, with center-to-center and interplanar distances of 3.84 and 3.59 Å, respectively. The remaining two ends of the laterally bonded tpt dimers are linked to other CMCR tetramers. Thus, each tpt dimer bonds to three CMCR tetramers, such as to produce a 2D pattern (Figure 2), which, notwithstanding the difference in CMCR structure contains the same A and B cavities as 1, with the same A/B ratio. The hydrophilic and hydrophobic regions of cavity A are occupied by water and toluene molecules, respectively, while the hydrophobic B cavities contain two inversion center related toluene molecules arranged in a tail-to-tail fashion. A striking feature of both compounds is their unusual frameworks with unique topological architectures, which to the best of our knowledge have not been realized before. While architectural or supramolecular isomers are common among CMCR-based frameworks, architectural isomers generally exhibit a different size and shape of their cavities.7 In contrast, the frameworks of 1 and 2 described here, while very different from each other, exhibit the same cavities, one of which has an unusual pentagonal shape, which has so far only been observed in some metal-organic frameworks.8 As the nitrobenzene and toluene guests have a similar size and shape, the difference in polarity and other electronic properties must play a role during the assembly of 1 and 2. In summary, two complex supramolecular solids have been synthesized through symmetry mismatching of the framework building blocks. Unlike 4,4′-bipyridine and trans-1,2-bis(4-pyridyl)ethylene, in which the nitrogen H-
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bond acceptor atoms are related by 2-fold symmetry, 1,3,5tri(4-pyridyl)-2,4,6-triazine is a 3-fold symmetric linker, which does not match any of the symmetry elements relating the donor atoms of the CMCR node molecules. Although it is not yet possible to reliably predict the structural motifs that will be formed from a specific reaction mixture under specific conditions, symmetry mismatching of framework components provides a pathway to complex topological architectures. Experimental Procedures
(4)
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[CMCR‚tpt]‚2nitrobenzene‚H2O (1). An aqueous solution (4 mL) of C-methylcalix[4]resorcinarene (0.05 mmol), tpt (0.1 mmol), and nitrobenzene (0.2 mL) was mixed and sealed in a heavy walled Pyrex glass tube (ca. 6 mL). The mixture was maintained at 140 °C for 1 day in an oven, and subsequently cooled to room temperature at a rate of 20°/day. Light yellow crystals were collected. [CMCR‚tpt]‚2toluene‚H2O (2). The complex was prepared under identical conditions as 1, but in the presence of toluene. An aqueous solution (4 mL) of C-methylcalix[4]resorcinarene (0.05 mmol), tpt (0.1 mmol), and toluene (0.2 mL) was mixed and sealed in a heavy-walled Pyrex glass tube (ca. 6 mL). The mixture was maintained at 140 °C for 1 day in an oven, and subsequently cooled to room temperature at a rate of 20°/day. Light yellow crystals were collected.
Acknowledgment. Support of this work by the National Science Foundation (CHE9981864 and CHE0236317) and the Petroleum Research Fund of the American Chemical Society (PRF32638AC3) is gratefully acknowledged. Supporting Information Available: X-ray crystallographic information files (CIF) for 1 and 2 are available. This material is available free of charge via the Internet at http://pubs.acs.org.
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B. Q.; Coppens, P. Chem. Commun. 2003, 412. (f) Cave, W. V.; Hardie, M. J.; Roberts, B. A.; Raston, C. L. Eur. J. Org. Chem. 2001, 3227. (g) MacGillivray, L. R.; Atwood, J. L. J. Am. Chem. Soc. 1997, 119, 6931. (a) Fujita, M.; Fujita, N.; Ogura, K.; Yamaguchi, K. Nature 1999, 400, 52. (b) Biradha, K.; Fujita, M. Angew. Chem., Int. Ed. 2002, 41, 3392. (c) Abrahams, B. F.; Batten, S. R.; Hamit, H.; Hoskins, B. F.; Robson, R. Chem. Commun. 1996, 1313. (d) Abrahams, B. F.; Batten, S. R.; Grannas, M. J.; Hamit, H.; Hoskins, B. F.; Robson, R. Angew. Chem., Int. Ed. 1999, 38, 1475. Crystal data: Compound 1: formula C372H336N48O78, Mr ) 6726.89, triclinic, space group P1 h , a ) 20.969(5), b ) 22.052(5), c ) 37.511(10) Å, R ) 81.838(6), β ) 76.680(6), γ ) 87.209(7)°, U ) 16705(7) Å3, Z ) 2, Fcalcd ) 1.337 Mg/m3, µ ) 0.095 mm-1, F(000) ) 7056, crystal size 0.28 × 0.20 × 0.12 mm, GoF ) 1.028. Among 79 993 reflections, 34 570 are unique (Rint ) 0.0929). The final R1 and wR2 are 0.1210 and 0.3147, respectively, for 3769 parameters and 23 303 reflections [I > 2σ(I)]. Compound 2: formula C128H124N12O18, Mr ) 2118.39, orthorombic, space group Pbcn, a ) 39.852(4), b ) 38.196(4), c ) 15.095(2) Å, V ) 22977(4) Å3, Z ) 8, Fcalcd ) 1.225 Mg/m3, µ ) 0.083 mm-1, F(000) ) 8960, crystal size 0.20 × 0.18 × 0.12 mm, GoF ) 1.171. Among 184 967 reflections, 19 539 are unique. The final R1 and wR2 are 0.1465 and 0.3878, respectively, for 1177 parameters and 5702 reflections [I > 2σ(I)]. The relatively high values of the merging and final R factors are related to the complexity of the structures and small sample size, resulting in weakness of the reflections in the data set. Non-hydrogen atoms of tpt and CMCR molecules in both compounds were refined anisotropically, whereas those of nitrobenzene and water molecules were refined isotropically. Hydrogen atoms of host were fixed in calculated positions and others were not added. Due to huge cell content and poor quality data obtained, some of the guest molecules in cavity A were poorly modeled. However, the guests in cavity B were modeled well. The data were collected on a Bruker SMART1000 CCD with MoKR radiation (λ ) 0.71073 Å) at 90(1) K. Reflections were reduced by the SAINT program. The structures were solved by direct methods and refined by a full matrix least squares technique based on F2 using SHELXL 97 program. MacGillivray, L. R.; Diamente, P. R.; Reid, J. L.; Ripmeester, J. Chem. Commun. 2000, 359. (a) Moulton, B.; Zaworotko, M. Chem. Rev. 2001, 101, 1629. (b) Holman, K. T.; Martin, S. M.; Parker, D. P.; Ward, M. D. J. Am. Chem. Soc. 2001, 123, 4421. (a) Keller, S. W.; Lopez, S. J. Am. Chem. Soc. 1999, 121, 6306. (b) Moulton, B.; Lu, J.; Zaworotko, M. J. Am. Chem. Soc. 2001, 123, 9224. (c) Liu, S.; Meyers, E. A.; Shore, S. G. Angew. Chem., Int. Ed. 2002, 41, 3609.
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