CRYSTAL GROWTH & DESIGN
Varying the Interplay of Calix[5]arenes through Chloromethane Inclusion Thomas E. Clark, Mohamed Makha, Colin L. Raston,* and Alexandre N. Sobolev School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
2006 VOL. 6, NO. 12 2783-2787
ReceiVed August 4, 2006; ReVised Manuscript ReceiVed September 5, 2006
ABSTRACT: Three inclusion complexes of calix[5]arenes as dichloromethane (DCM) or chloroform solvates have been structurally authenticated, namely, the complexes of p-But-calix[5]arene, 1, or p-H-calix[5]arene, with DCM, 2, and a new polymorph of the complex of p-phenyl-calix[5]arene with chloroform, 3. Complex 1 has two adjacent tbutyl groups of one calixarene embracing the analogous groups from another calixarene. Complex 2 has a slipped head-to-head arrangement of calixarenes with a DCM molecule in the cavity of each calixarene. In complex 3, the calixarenes form dimers through interdigitation, and these dimers are assembled with back-to-back arrangement of calixarenes from adjacent dimers. All solvate complexes were crystallized in the presence of p-carborane, which has complementarity of size of the cavity of the calixarenes yet is not incorporated in the structures. Introduction Calixarenes are macrocyclic molecules formed by the condensation reaction of p-substituted phenols and formaldehyde in the presence of base or acid. Specific control over the reaction conditions leads to a variety of ring sizes, which are of interest in complexation of metal ions1 and as well-defined architectures for further synthetic modification.2 The most common ring sizes are the even numbered comprised of four, six, or eight phenolic units, while the odd numbered calixarenes made up of five or seven phenolic units are less well studied mainly due to lowyielding synthetic protocols.3 More recently, a new generation of calixarenes with extended cavity, the so-called deep-cavity calixarenes, have been prepared as potential hosts for binding large molecules.4 Such deep-cavity calixarenes are effective in binding fullerenes5 and mimicking enzyme catalysis,6 but nevertheless there are still difficulties in their synthesis and purification.7 The smaller ring sizes of calix[4,5]arenes adopt a cone- or bowl-shaped conformation due to the hydrogen-bonded network of the hydroxyls at the base of the calixarene, the so-called “lower rim”, and the resulting cavities can bind a wide range of guests including solvent molecules. The cavity of calix[4]arene is such that it is able to bind only small guests such as solvent molecules,8 whereas the significantly larger cavity of calix[5]arene is able to bind solvent molecules along with other guest molecules such as ferrocene9 and globular molecules such as fullerenes10 and carboranes.11 The guest molecule can also reside outside of the calixarene cavity filling the interstitial space between the macrocycles.12 The Cambridge Database has a number of calix[5]arene solvates, which includes four p-Butcalix[5]arenes,13 two p-H-calix[5]arenes,9,14 two p-Ph-calix[5]arenes,5a,15 one p-Bn-calix[5]arene (Bn ) benzyl),16 and one p-tetramethylbutylcalix[5]arene.17 In this paper, we report the crystal structures of three new complexes of calix[5]arenes. These include the complexes between p-But-calix[5]arene and p-H-calix[5]arene with dichloromethane (DCM) and a new polymorph of a chloroform complex with p-phenyl-calix[5]arene, which has a remarkably different arrangement of calixarenes relative to the earlier * To whom correspondence should be addressed. Fax: (internat.) +618 6488 1005. E-mail:
[email protected].
study.11,15 Overall, the ability to change the interplay of the calixarenes by inclusion of small chloromethane molecules is noteworthy and is important in preparing new material for gas storage, for example, as has been demonstrated for calix[4]arenes.18 Results and Discussion Although p-But-calix[5]arene has limitations in its synthetic procedure, having a 15% yield, it is an attractive host molecule given the larger size of its cavity relative to calix[4]arene. In a recent study on the complexation of calix[5]arenes with the three isomeric forms of carborane, o-, m-, and p-C2B10H12 we showed that p-But-calix[5]arene and p-H-calix[5]arene form complexes with o- and m-carborane but not with p-carborane in DCM/ hexane. In the presence of p-carborane a DCM solvate is isolated, [p-But-calix[5]arene∩0.75DCM‚0.5H2O].(DCM)0.75‚ (H2O)0.25, 1, for which the structure is reported herein, along with the structure of [p-H-calix[5]arene∩DCM]‚DCM, 2, which is similarly formed, Scheme 1. The same complexes also form in the absence of any carborane. The lack of complexation of p-carborane can be rationalized by o- and m-carborane forming two C-H‚‚‚π interactions with the aromatic rings of the cavity of calix[5]arenes, whereas p-carborane can only form one C-H‚‚‚‚π interaction, i.e., the energy gain associated with such interactions in binding p-carborane is half the energy gain for the o- and m-isomers, and is comparable to the energy gain for C-H‚‚‚π interactions associated with chloromethane solvents.11a The deep-cavity calix[5]arenes, in particular, p-phenyl-calix[5]arene, and p-benzyl-calix[5]arene, were also investigated, but no carborane complexes could be isolated from toluene. The same also applies on changing the solvent to chloroform/hexane. In the case of p-phenyl-calix[5]arene, a chloroform inclusion complex was isolated, as a new polymorph of [p-phenyl-calix[5]arene∩2CHCl3], 3, Scheme 1. This is formed regardless of which carborane or mixtures of carboranes are present. The same complex also forms in the absence of carborane. We note that there is only one example of a calix[4]arene binding a carborane, notably, the o-isomer with p-benzyl-calix[4]arene, which is in a symmetrically flattened cone conformation.19 Departure from this conformation, for example, a pinched cone conformation, would result in a cavity too small for the binding of carboranes. A related calix[4]resorcinarene which strictly has a symmetrical
10.1021/cg0605279 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/20/2006
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Scheme 1. General Synthetic Procedure for Preparing p-R-Calix[5]arene/Solvate Complexes, 1-3a
a (i) DCM/hexane (and in the presence of p-carborane), 1 and 2, and (ii) chloroform/hexane (and in the presence of o-, m-, p-carboranes), 3.
cone conformation, can also form host-guest complexes with o-carborane.20 We also note that in general there are only a few examples of carborane inclusion complexes,11,19,20 and examples where carborane facilitated crystallization of the calixarene but are not included in the structure.21 Structure of [p-But-calix[5]arene∩0.75DCM‚0.5H2O]‚ (DCM)0.75‚(H2O)0.25: 1. Complex 1 crystallizes in the monoclinic space group C2/m, Z ) 4, with the asymmetric unit comprised of one supermolecule [p-But-calix[5]arene∩0.75DCM‚ 0.5H2O] and 0.75 DCM and 0.25 water molecules located on a mirror plane. Two of the But groups are disordered over two positions, and the calixarene molecule adopts a distorted cone conformation as quantified by analysis of the angles of the phenyl rings relative to the plane of the five oxygen atoms varying from 116.5 to 153.25°. The calixarene forms the hydrogen-bonded array, which is expected for the cone conformation with the hydrogen atoms disordered in both directions around the rim, with O‚‚‚O distances in the range 2.87-2.91 Å. One DCM molecule resides in the cavity and forms a bifurcated C-H‚‚‚π interaction with the calixarene cavity at a distance of 2.74 Å. The calixarenes pack with back-to-back stacking with intermolecular phenolic O‚‚‚O separations of 2.82-3.15 Å. Apart from the included DCM, there are additional DCM solvent molecules occupying the interstitial space forming channels as shown in Figure 1. Most solvate complexes of p-But-calix[5]arene are based on the inclusion of one of the tbutyl groups in the cavity from another calixarene, which in turn engages in similar association with yet another calixarene forming a linear array, i.e., the calixarene are “hand-in-hand”, as seen in the hexane and ethyl acetate solvates.13 A subtle variation to this arrangement is found in the dimethylformamide solvate where the solvent molecule resides in the cavity, but a tbutyl group from a neighboring calixarene is directed into the cavity and embraces the included solvent. In the present structure, the interplay of the calixarenes has two adjacent tbutyl groups from one calixarene embracing the corresponding adjacent tbutyl groups from another calixarene. This is a new structural motif for the interplay of p-Butcalix[5]arenes, which is distinctly different to the aforementioned interplay of this calix[5]arene. Structure [p-H-calix[5]arene∩DCM]‚DCM: 2. Complex 2 crystallizes in the orthorhombic space group Pnma, Z ) 4, with the asymmetric unit comprised of one supermolecule [p-Hcalix[5]arene∩DCM] and one DCM molecule located on a mirror plane. The calixarene molecule adopts a slightly distorted cone conformation, which is evident by the variation in angles of the phenyl rings relative to the plane of the five oxygen atoms, ranging from 124.4 to 146.7°. Each calixarene forms the expected hydrogen-bonded array for a cone conformation with
Figure 1. Projection of 1: (a) side view of the packing of the calixarenes showing the back-to-back arrangement and channels of DCM molecules (along a axis), and (b) top view showing the interplay of the calixarenes and their columnar alignment with the included DCM solvents molecules (along c axis); space filling for the DCM molecules and disordered But groups, and water molecules have been omitted for clarity.
the hydrogen atoms disordered in both directions around the rim, with O‚‚‚O distances in the range of 2.80-2.85 Å. Both DCM molecules are disordered over four positions with one located in the cavity and the other residing slightly above the cavity. The DCM molecule in the cavity is offset from the center and forms C-H‚‚‚π interactions with the walls of the cavity with C-H‚‚‚π (aromatic ring centroid) distances 2.37-2.81 Å, Figure 2a. The remaining DCM molecule fills up the interstitial space between molecules. It resides above one aromatic moiety of the calixarene (possibly associated with C-H‚‚‚π interactions having C-H‚‚‚centroid distances at 3.09-3.21 Å) and directly below the centroid of the five oxygen groups for another calixarene. The calixarenes pack into an alternating zigzag arrangement down the b axis with the row above being its mirror image and shifted by half of a calixarene unit, Figure 2b. The structure is comprised of slipped head-to-head arrangement of calixarenes with a DCM molecule in the cavity of each calixarene and form columnar arrays along the b axis. The arrays are slightly offset and twisted relative to each other, which maximizes the contacts between the arrays. The acetone and acetonitrile solvates have the same structural motif which is common for solvate complexes of this calixarene.9,14 Surprisingly, the ferrocene/toluene complex of calix[5]arene, (calix[5]arene)(ferrocene)0.45(toluene)0.55, has similar inclusion be-
Calix[5]arenes and Chloromethane Inclusion
Figure 2. Projection of 2: (a) side and top view of the host-guest supermolecule showing the hydrogen bonding from the methylene protons of DCM, and (b) side and top view of the packing diagram showing the interplay of the calixarenes and the included DCM molecules.
havior relative to inclusion complexes of small solvent molecules.9,14 Structure of [p-phenyl-calix[5]arene∩2CHCl3]: 3. Complex 3 crystallizes in the monoclinic space group P21/n, Z ) 4, with the asymmetric unit comprised of one supermolecule [p-phenyl-calix[5]arene∩2CHCl3]. The calixarene molecule adopts a distorted pinched cone conformation with the angle of the inner phenyl rings relative to the plane of the five oxygen atoms, varying from 93.9 to 152.6 °. One of the terminal phenyl rings is directed toward two adjacent terminal phenyl rings, which are in the 3,4 positions relative to the aforementioned phenyl ring. The other two phenyl rings in the 2,5 positions are splayed outward. All the terminal phenyl rings are twisted relative to the plane of their adjoining phenolic rings, at dihedral angles ranging from 19.7 to 39.7°. Despite the unsymmetrical cone conformation the calixarene retains the hydrogen-bonded array at the lower rim, with the hydrogen atoms disordered in both directions. The two chloroform molecules are highly disordered over three positions with one of the chloroform molecules sitting almost over the center of the cavity and involved in a C-H‚‚‚π
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interaction with a phenolic ring, C-H‚‚‚centroid 2.42 Å, and the other involved in such interactions with a terminal phenyl ring, C-H‚‚‚centroid 3.07 and 3.18 Å, Figure 3. The latter chloroform molecule is offset to one side, with the other side of the cavity occupied by a terminal phenyl ring of an adjacent symmetry equivalent calixarene, Figure 3a. The two terminal phenyl and phenolic rings associated with this interplay are coplanar with centroid‚‚‚centroid separations of 4.97 and 8.17 Å respectively. In addition, the interplay has one edge-to-face C-H‚‚‚π interaction involving two terminal phenyl groups, C-H‚‚‚centroid 2.83 Å. The calixarenes are arranged back-to-back relative to the cavity of the calixarenes with intermolecular phenolic O‚‚‚O separations of 2.86 to 3.22 Å, Figure 3b. The intramolecular phenolic O‚‚‚O separations are 2.85 to 3.07 Å, suggesting that the back-to-back arrangement is associated with intermolecular, as well as the expected intramolecular hydrogen bonding. This type of back-to-back arrangement has been noted for other calix[5]arene structures, for example, in C60 complex of p-benzylcalix[5]arene.22 Within the overall packing, there is an intermolecular edge to face C-H‚‚‚π interactions between non-interdigitating calixarenes with C-H‚‚‚centroid distances at 2.74 Å, and a plethora of longer interactions with corresponding distances ranging from 3.30 to 3.50 Å, Figure 3a,b. The interdigitated dimer of the calixarene is the central building block in building up the array, which is common in all solvent inclusion complexes of p-phenyl-calix[5]arene regardless of the type of solvent used.5a,15 However, the interplay of these dimers in the extended structure is significantly different to that established elsewhere and seems to be solvent and solute dependent, at least for toluene, chloroform, and chloroform/ hexane. In the present structure, the dimers are packed with back-to-back arrangement of dimers, and with a head-to-head type arrangement, essentially resulting in skewed capsules. In addition, a terminal phenyl group from one dimer penetrates the upper cavity of a calixarene of another dimer, and vice versa, which is possibly due to the distortion of the cone conformation of the calixarenes, Figure 3a-c. The interplay of the dimers of the calixarenes is distinctly different to that in the structure of the polymorph of the same complex reported by No et al., where the dimers are arranged orthogonally to each other, Figure 3d.15 Conclusion We have structurally authenticated three new solvates of selected calix[5]arenes and their reproducible syntheses with and without the presence of the globular carboranes. The work highlights the ability to access new structural motifs based on the assembly of bowl shaped calix[5]arenes. This has implications in building up material for application in gas storage as well as gas separation, a trajectory we are currently pursuing. Experimental Section p-But-calix[5]arene,3a
p-H-calix[5]arene,23 p-Ph-calix[5]arene,24 and p-Bn-calix[5]arene4c were prepared as described previously in the literature. All starting materials and solvents were purchased commercially and used as supplied. The DCM solvates of complexes 1 and 2 were prepared by slow evaporation of DCM/hexane solutions containing 3 mol equiv of p-carborane, with crystals suitable for singlecrystal diffraction studies forming over 3 days. The chloroform solvate of complex 3 was prepared by slow evaporation of a pure chloroform/ hexane solution containing 3 mol equiv of p-carborane, with crystals suitable for single-crystal diffraction studies forming over one week. Control experiments in the absence of p-carborane gave the same complexes 1-3. The uniformity of each sample was checked by determining cell dimensions on crystals from the same preparation,
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Figure 3. Projection of 3: (a) top and side view of the interdigitated calixarenes and associated included chloroform molecules, (b) back-to-back interaction showing the inclusion of chloroform molecules, (c) the overall packing of calixarenes within the extended structure, and (d) the irregular and orthogonal arrangement of the inter-digitated calixarenes in the structure of No et al.;15 hydrogen atoms have been omitted for clarity. and from different preparations. In the case of complex 3 samples contained crystals of the previously reported chloroform inclusion complex15 as well as the new polymorph. Crystallography. The X-ray diffracted intensities were measured from single crystals of the title compounds on a Bruker ASX CCD diffractometer at about 150 K, using monochromatized Mo-KR (λ ) 0.71073 Å.) Data were corrected for Lorentz and polarization effects and absorption correction applied using multiple symmetry equivalent reflections. The structures were solved by direct method and refined on F2 using Bruker SHELXTL crystallographic package. A full matrix least-squares refinement procedure was used, minimizing w(Fo2 - Fc2), with w ) [σ2(Fo2) + (AP)2 + BP]-1, where P ) (Fo2 + 2Fc2)/3. Agreement factors (R ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo2 - Fc2)2]/ ∑[w(Fo2)2]}1/2 and GOF ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2 are cited, where n is the number of reflections and p is the total number of parameters refined). CCDC 615792-615794. Crystal/refinement details: Complex 1: [p-But-calix[5]arene∩0.75DCM‚0.5H2O]‚(DCM)0.75‚(H2O)0.25, C56.5H74.5Cl3O5.75, M ) 952.01, F(000) ) 2042 e, monoclinic, C2/m, Z ) 4, T ) 153(2) K, a ) 22.347(4), b ) 15.636(3), c ) 17.756(3) Å, β ) 98.506(3)°, V ) 6136.0(19) Å3; Dc ) 1.031 mg m-3; sin θ/λmax ) 0.5953; N (unique) ) 5392 (merged from 18758, Rint ) 0.0398, Rsig ) 0.0437), No [I >
2σ(I)] ) 3698; R ) 0.1108, wR2 ) 0.2615 (A,B ) 0.03, 35.7), GOF ) 1.087; |∆Fmax| ) 1.1(1) e Å-3. Complex 2: [p-H-calix[5]arene∩DCM]‚DCM, C37H34Cl4O5, M ) 700.44, F(000) ) 1456 e, orthorhombic, Pnma, Z ) 4, T ) 153(2) K, a ) 23.140(3), b ) 14.940(3), c ) 9.609(1) Å, V ) 3321.9(9) Å3; Dc ) 1.401 mg m-3; sin θ/λmax ) 0.5953; N (unique) ) 3014 (merged from 19827, Rint ) 0.00259, Rsig ) 0.0166), No [I > 2σ(I)] ) 2413; R ) 0.0929, wR2 ) 0.2544 (A,B ) 0.16, 6.98), GOF ) 1.068; |∆Fmax| ) 1.1(1) e Å-3. Complex 3: [p-Ph-calix[5]arene∩2CHCl3], C67H52Cl6O5, M ) 1149.79, F(000) ) 2384 e, monoclinic, P21/n, Z ) 4, T ) 150(2) K, a ) 18.639(4), b ) 11.066(2), c ) 28.419(6) Å, β ) 108.043(3)°, V ) 5573(2) Å3; Dc ) 1.370 mg m-3; sin θ/λmax ) 0.5946; N (unique) ) 9464 (merged from 33482, Rint ) 0.064, Rsig ) 0.073), No [I > 2σ(I)] ) 5771; R ) 0.0977, wR2 ) 0.2421 (A,B ) 0.114, 29.99), GOF ) 1.046; |∆Fmax| ) 1.1(1) e Å-3.
Acknowledgment. We thank the Australian Research Council and the University of Western Australia for a SIRF award to T.C.
Calix[5]arenes and Chloromethane Inclusion Supporting Information Available: Crystallographic information file (cif). This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713-1734. (2) For comprehensive reviews see (a) Gutsche, C. D. Calixarenes; Royal Society of Chemistry: Cambridge, 1989. (b) Vicens, J.; Bo¨hmer, V., Eds.; Calixarenes; Kluwer Academic Press: Dordrecht, 1991. (c) Gutsche, C. D. Calixarenes ReVisited; Royal Society of Chemistry: Cambridge, 1998. (3) (a) Stewart, D. R.; Gutsche, C. D. Org. Prep. Proc. Int. 1993, 25, 137. (b) Nakamoto, Y.; Ishida, S. I. Makromol. Chem. Rapid Commun. 1982, 3, 705. (c) Vocanson, F.; Lamartine, R.; Lanteri, P.; Longery, R.; Gauvrit, J. New J. Chem. 1995, 18, 825. (4) (a) Gutsche, C. D.; No, K. H. J. Org. Chem. 1982, 47, 2708-2712. (b) Arduini, A.; Pochini, A.; Rizzi, A.; Sicuri, A. R.; Ungaro, R. Tett. Lett. 1990, 31, 4653-4656. (c) Souley, B.; Asfari, Z.; Vicens, J. Pol. J. Chem. 1992, 66, 959-961. (5) (a) Makha, M.; Hardie, M. J.; Raston, C. L. Chem. Commun. 2002, 1446-1447. (b) Isaacs, N. S.; Nichols, P. J.; Raston, C. L.; Sandova, C. A.; Young, D. J. Chem. Commun. 1997, 1839-1840. (6) Juneja, R. K.; Robinson, K. D.; Johnson, C. P.; Atwood, J. L. J. Am. Chem. Soc. 1993, 115, 3818-3819. (7) (a) Makha, M.; Raston, C. L.; Skelton, B. W.; White, A. H. Green Chem. 2004, 6, 158-160. (b) Atwood, J. L.; Hardie, M. J.; Raston, C. L.; Sandoval, C. A. Org. Lett. 1999, 1, 1523-1526. (8) (a) Andreetti, G. D.; Ungaro, R.; Pochini, A. J. Chem. Soc. Chem. Commun. 1979, 1005-1007. (b) Ungaro, R.; Pochini, A.; Andreetti, G. D.; Domiano, D. J. Chem. Soc., Perkin Trans. 2 1985, 197201. (9) Hardie, M. J. Supramol. Chem. 2002, 14, 7-10. (10) (a) Atwood, J. L.; Barbour, L. J.; Raston, C. L. Cryst. Growth Des. 2002, 2, 3-6. (b) Haino, T.; Yanase, M.; Fukazawa, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 259-260. (11) (a) Clark, T. E.; Makha, M.; Raston, C. L.; Sobolev, A. N. Dalton, in press, http://dx.doi.org/10.1039/b611034g. (b) Hardie, M. J.; Raston, C. L. Eur. J. Inorg. Chem. 1999, 195-200. (12) Ungaro, R.; Pochini, A.; Andreetti, G. D.; Sangermano, V. J. Chem. Soc., Perkin Trans. 2 1984, 1979-1985.
Crystal Growth & Design, Vol. 6, No. 12, 2006 2787 (13) (a) Dumazet, I.; Ehlinger, N.; Vocanson, F.; Lecocq, S.; Lamartine, R.; Perrin, M. J. Inclusion Phenom. Mol. Recognit. Chem. 1997, 29, 175-185. (b) Atwood, J. L.; Juneja, R. K.; Junk, P. C.; Robinson, K. D. J. Chem. Crystallogr. 1994, 24, 573-576. (c) Gallagher, J. F.; Ferguson, G. Acta Crystallogr. 1994, C50, 73-77. (d) Ripmeester, J. A.; Ratcliffe, C. I. J. Inclusion Phenom. Mol. Recognit. Chem. 1992, 13, 93-96. (14) Coruzzi, M.; Andreetti, G. D.; Bocchi, V.; Pochini, A.; Ungaro, R. J. Chem. Soc., Perkin Trans. 2 1981, 1133-1138. (15) No, K.; Kwon, K. M.; Kim, B. H. Bull. Korean Chem. Soc. 1997, 18, 1034-1036. (16) Thue´ry, P.; Nierlich, M.; Souley, B.; Asfari, Z.; Vicens, J. J. Inclusion Phenom. Mol. Recognit. Chem. 1998, 31, 357-365. (17) Perrin, M.; Lecocq, S. J. Inclusion. Phenom. Mol. Recognit. Chem. 1991, 11, 171-183. (18) (a) Atwood, J. L.: Barbour, L. J.; Jerga, A. Science 2002, 296, 23672369. (b) Atwood, J. L.: Barbour, L. J.; Jerga, A.; Schottel, B. L. Science 2002, 298, 1000-1002. (c) Atwood, J. L.; Barbour, L. J.; Jerga, A. Chem. Commun. 2002, 2952-2953. (d) Brouwer, E. B.; Enright, G. D.; Udachin, K. A.; Lang, S.; Ooms, K. J.; Halchuk, P. A.; Ripmeester, J. A. Chem. Commun. 2003, 1416-1417. (e) Atwood, J. L.; Barbour, L. J.; Lloyd, G. O.; Thallapally, P. K. Chem. Commun. 2004, 922-923. (f) Atwood, J. L.: Barbour, L. J.; Jerga, A. Angew. Chem. Int. Ed. 2004, 43, 2948-2950. (g) Thallapally, P. K.; Wirsig, T. B.; Barbour, L. J.; Atwood, J. L. Chem. Commun. 2005, 35, 44204422. (i) Thallapally, P. K.; Lloyd, G. O.; Wirsig, T. B.; Bredenkamp, M. W.; Atwood, J. L.; Barbour, L. J. Chem. Commun. 2005, 42, 5272-5274. (j) Thallapally, P. K.; Lloyd, G. O.; Atwood, J. L.; Barbour, L. J. Angew. Chem., Int. Ed. 2005, 44, 3848-3851. (19) Makha, M.; Raston, C. L.; Sobolev, A. N.; Barbour, L. J.; Turner, P. CrystEngComm 2006, 8, 306-308. (20) Raston, C. L.; Cave, G. W. V. Chem.sEur. J. 2004, 10, 279-282. (21) Makha, M.; Raston, C. L.; Sobolev, A. N. Aus. J. Chem. 2006, 59, 260-262. (22) Atwood, J. L.; Barbour, L. J.; Nichols, P. J.; Raston, C. L.; Sandoval, C. Chem.sEur. J. 1999, 5, 990-996. (23) Bo¨hmer, D.; Rathay, H.; Ka¨mmerer, H. Org. Prep. Proc. Int. 1978, 10, 113. (24) Makha, M.; Raston, C. L. Tetrahedron Lett. 2001, 42, 6215-6217.
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