Fullerene Inclusion Based on Horning-Crown Macrocycles - Crystal

Dec 9, 2008 - University of Western Australia. , ‡. Monash University. , §. The Queen's University of Belfast. Cite this:Cryst. Growth Des. 2009, 9...
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CRYSTAL GROWTH & DESIGN

Fullerene Inclusion Based on Horning-Crown Macrocycles Mohamed Makha,† Janet L. Scott,‡ Christopher R. Strauss,§ Alexandre N. Sobolev,† and Colin L. Raston*,† Centre for Strategic Nano-Fabrication, School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Hwy, Crawley, Western Australia 6009, Australia, School of Chemistry, Monash UniVersity, Wellington Road, Clayton, Victoria 3800, Australia, QUILL Centre, The Queen’s UniVersity of Belfast, Northern Ireland BT9 5AG, U.K.

2009 VOL. 9, NO. 1 483–487

ReceiVed July 11, 2008; ReVised Manuscript ReceiVed September 29, 2008

ABSTRACT: Horning-crown macrocycles, 1(R ) (CH2)2) and 2(R ) (CH2)3) form inclusion complexes with C60 and C70 fullerenes which have been structurally authenticated using X-ray diffraction data. Macrocycle 1 affords a C60 complex as a toluene solvate with overall composition C60(1)2(tol)2, 3, whereas the larger macrocycle, 2, affords complexes with both C60 and C70 fullerenes, also as toluene solvates: C60(2)(tol)2, (4); C70(2)(tol)2, (5); and C70(2)2(tol)2, (6). The higher degree of flexibility of such macrocyles relative to classical calixarenes is noteworthy in forming fullerene complexes, with the smaller ring size of 1 favoring fullerene complexation toward the smaller C60 molecule. The supramolecular assemblies involve host-guest interactions as well as interaction of fullerenes in zigzag chains with interfullerene contacts close to the van der Waals limit. Introduction Calixarene-type macrocycles are renowned for their interplay with fullerenes and offer access to a diverse range of complexes.1,2 The selective retrieval of C60 using tBu-calix[8]arene3 was a significant finding in the supramolecular chemistry of fullerenes and relates to potential applications in separation and materials science.4 Complexation of fullerenes by calixarenes relies mostly upon complementarity of curvature between the host macrocycle and C60 or C70 fullerenes, at least for calix[4, 5, and 6]arenes with the prevalence of endo binding.5 Other inclusion complexes have the fullerene exo relative to the cavity of the calixarene with fullerene · · · fullerene contacts at the van der Waals limit.2,6 Although the classical calixarenes are flexible in binding and controlling the interplay of fullerenes, structural modifications of calixarene skeletons are likely to enhance the binding and stability of these complexes, and this is highlighted by the fullerene complexation prowess of bis-calix[n]arenes.7 Recently, other strategies have been deployed in the construction of calixarene-type macrocycles by substituting the methylene bridges, the phenolic units, or both, with a variety of moieties, such as in thiacalixarenes,8 calixpyrroles,9 and azacalixpyridines.10 In this regard, we have reported the synthesis of macrocycles possessing both polyether and phenolic functionalities (Horning-crown macrocycles) produced by sequential Claisen-Schmidt condensations.11 Initial supramolecular studies on this class of macrocycles revealed a tendency for selfdimerization and solvent dependency of association, both in solution and in the solid state.12,13 Herein, we report the supramolecular confinement of fullerenes C60 and C70 in the solid state by two Horning-crown macrocycles. Four C60 and C70 complexes based on the macrocycles 1 and 2; [1 · (C60)] · (C7H8)2, 3; [2 · (C60)] · (C7H8)2, 4; [2 · (C70)] · * Phone: +618 6488 3045. Fax: +618 6488 1005. E-mail: Colin.Raston@ uwa.edu.au. † University of Western Australia. ‡ Monash University. § The Queen’s University of Belfast.

Scheme 1. Reaction of Horning-Crown Macrocycles, 1 and 2 with Fullerene C60 and C70

(C7H8)2, 5; and [(2)2 · (C70)] · (C7H8)2, 6 have been prepared and structurally authenticated (Scheme 1). Results and Discussion Crystals of compound 3 were obtained by slow cooling of a toluene solution close to the boiling point of the solvent, and the structure was determined by single crystal X-ray crystallography (see the Experimental Section). The compound crystallizes in the triclinic space group P1 bar, Z ) 2, with the asymmetric unit consisting of two Et2H macrocycles, 1, one disordered C60 fullerene molecule (50:50), and two complete toluene molecules with significant positional disorder. The macrocycle 1 and C60 molecules were refined anisotropically, whereas all other non-hydrogen atoms were refined isotropically. The shape and dimensions of the C60 were fixed by distance and angle restraints (SHELX DFIX) during the structure refinement. Significantly disordered toluene molecules were refined with idealized geometries. Molecules of 1 adopt the saddle shape with distances between centroids of opposite rings of 7.0 and 6.9 Å for molecules A and B, respectively. The walls and the back of the saddle of 1 formed by benzene rings define a molecular tweezer, which is associated through the open cavity with another macrocycle, forming a dimeric unit (Figure 1). The hydrogen atoms of the OH groups form almost linear intramolecular H-bonds with O · · · O distances 2.765(6)-2.899(6) Å. In the continuous structure of 3, these dimeric assemblies form continuous corrugated layers parallel to the bc plane. The fullerene molecules are lined up along the c crystal axis, forming

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Figure 1. Perspective view of the association of two macrocycles of 1 in complex 3.

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Figure 3. Top perspective view of macrocycle 2 in compound 4 showing its shallow bowl conformation represented with ring 2,3,5,6 lined up and 1,4 down.

Figure 4. Top (left) and bottom (right) perspective view showing the confinement of macrocycle 2 in complex 4 (the disordered part of the toluene molecule (right) is omitted for clarity).

Figure 2. Packing diagram of compound 3 viewed down the a axis (the disordered parts are omitted for clarity).

zigzag arrays with short distances between fullerene centroids of 10.08 and 10.58 Å. These fullerene columnar arrays are positioned between layers of macrocycles with tunnels between the columns filled by toluene solvent molecules (Figure 2). The disordered distribution of the toluene molecules, with partial occupancy inside the channels between fullerene columns, presumably relates to the tendency for the crystals to lose solvent and associated potential material porosity. The shortest intermolecular C · · · C distances are 3.06 (fullerene-toluene), 3.08 (fullerene-macrocycle 1), and 3.42 Å (macrocycle 1-macrocycle 1). Compound 4 crystallizes in the triclinic space group P1 bar, Z ) 2, with the asymmetric unit consisting of one macrocycle, 2, one disordered C60 fullerene molecule, and two complete toluene molecules, albeit with significant positional disorder. One of the toluene molecules in the asymmetric unit is disordered (50:50 ratio) with a 50:50 ratio, with all nonhydrogen atoms except atoms of the disordered molecules refined anisotropically. Hydrogen atoms were calculated from geometrical considerations, being constrained during the refinement to the appropriate positional and thermal parameters of the bonded C and O atoms. The hydrogen atoms of OH groups are involved in intramolecular H-bonds with O · · · O distances of 2.774(3) and 2.798(3) Å. The conformation of macrocycle 2 is also saddle-shaped with four aromatic rings (2, 3, 5, 6) forming the walls of the open cavity and rings (1, 4) (Figure 3). A measure of the size of the cavity of the bowl is the distance

between the centroids of opposite rings, 10.10 Å for rings 2 and 5, and 11.71 Å for rings 3 and 6, and the distance between the centroid of rings 1 and 4, 10.35 Å. Figure 4 shows the disposition of the complex components in the asymmetric unit as a divergent macrocycle with a C60 molecule located inside the major cavity and a toluene residing in the minor cleft/cavity of the macrocycle. The relative orientation of the host-guest components in this system can be described in different ways. For consistency, we calculated the angle between the perpendicular to the least-squares plane of the oxygen atoms and the line joining the centroids of two opposite 5-membered rings of C60 molecules, which is 41.0°. One of the toluene molecules is located above the fullerene; the second toluene (disordered) lies within the minor cleft of the macrocycle. The shortest C · · · C distance between C60 and macrocycle 2 is 3.19 Å, and the shortest fullerene-fullerene C · · · C distance is 3.46 Å. The arrangement of the complex components in the crystal packing is shown in Figure 5. The fullerene molecules are lined up along the crystallographic c axis forming zigzag arrays, which are lined with the macrocycles with distances between the fullerene centroids at 10.07 and 10.08 Å. The interstitial space is occupied by toluene molecules. Compound 5 is a 1:1 toluene solvated inclusion complex of the macrocycle 2 with C70 and is isostructural to its C60 derivative (compound 4), crystallizing in the triclinic space group P1 bar, Z ) 2. The fullerene and one of the two toluene molecules (above C70) are disordered between two positions. The general view of the asymmetric unit in the crystal is presented in Figure 6; the macrocycle has the same conformation (with the same conventional numbering system) as compound 4 (see Figure 3). The hydrogen atoms of OH groups are involved in H-bonds with O · · · O distances of 2.772(3) and 2.801(3) Å.

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Figure 5. Packing diagram of complex 4 viewed down the a axis (the disordered parts are omitted and the top layer of the macrocycles are presented in ball and stick form for clarity).

Figure 6. Perspective side view of complex 5 (disordered parts are omitted for clarity).

The distances between centroids of opposite aromatic rings are 10.04 Å (rings 2 and 5), 11.60 Å (rings 3 and 6), and 10.06 Å (rings 1 and 4). Within the macrocycle, the fullerene molecule takes two different positions that are related by a rotation of about 180 ° relative to each other around the principal 5-fold axis. The angles between the principal axes of C70 and the macrocycle are 53.4 and 46.5° for the two different orientations of the fullerene molecule. The uncertainty in the position of the C70 molecule is further compounded by the disorder of one of the solvent molecules. The shortest C · · · C distances between the C70 and 2 macrocycle are 3.19 and 3.11 Å for two different orientations of the fullerene. The lesser value for the second orientation indicates that, in this case, the fullerene is immersed slightly deeper into the cavity of the macrocycle and that the angle between the principal axes of C70 and the ligand is reduced. The crystal packing of this complex is similar to the packing described for compound 4 (Figure 7). Compound 6 is a 2:1 toluene solvated inclusion complex of the macrocycle 2 with C70 and crystallizes in the monoclinic space group C2/c, Z ) 4. The X-ray structural study of this compound was challenging, and the details can be found in the Supporting Information in the CIF file. The assembly of the components consists of the macrocycle, fullerene C70, and two toluene molecules. The 2-fold symmetry operation in the crystal (-x, y, 3/2-z) generates two macrocycles of 2, which encapsulate the fullerene molecule (Figure 8). The construction resembles a ball and socket inclusion, where the walls and base

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Figure 7. Packing diagram of the structure in complex 5 viewed down the a axis (disordered parts are omitted for clarity).

Figure 8. Perspective view of complex 6 viewed along the b axis with the fullerene molecule located on a 2-fold symmetry axis artificially fitted to an idealized form (the disordered parts of solvent molecules are omitted for clarity).

of the ball are formed by phenyl rings of the macrocycle. The conformation of the macrocycle differs from that found in the inclusion compounds 4 and 5 (see Figure 3). An aromatic ring, 6, in the present structure is trans relative to ring 3, and the macrocycle conformation can be regarded as a 1-2-3, 4-5-6 alternate chair, where rings 1-3 are up and rings 4-6 are down. The key distances between centroids of the opposite rings are 10.0 Å (rings 1 and 4), 10.5 Å (rings 2 and 5), and 13.2 Å (rings 2 and 6). According to our convention presented in Figure 3, ring 2 lies parallel to the central part of the fullerene carbon belts and the center of ring 5 points to the butt-end of C70, terminated by a 5-membered ring. The nearest carbon · · · carbon distances from these planes to the fullerene are 3.15, 3.18, and 3.00 Å. Corresponding distances between ring centroids are 3.34, 3.32, and 3.16 Å. Two symmetry related macrocycles above the fullerene complete the encapsulation of the fullerene. Aromatic ring 3 of each of these macrocycles is at the surface of the fullerene, forming π-stacking, with the shortest C · · · C distance at 3.18 Å. The plane of aromatic ring 4 is located along the surface of C70 with shortest C · · · C distances at 3.29 Å and 3.43 Å. These distances are in the range of typical π · · · π interactions, being 3.0-3.5 Å.14 Two disordered toluene molecules fill the interstitial space between the macrocycles. One is located at the bottom of the capsule or at the corner between rings 3 and 5 of the symmetrically related macrocycle

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continuous zigzag chains with the fullerenes at the van der Waals limit for the larger macrocycle. This demonstrates an important finding in attempting to predict the nature of the complexes formed on complexation of C60 and C70 with flexible macrocycles. The structural diversity of fullerenes, which is further highlighted herein, is of interest in the design and construction of materials-based fullerenes for a variety of applications. Experimental Section

Figure 9. Packing diagram of the complex 6 viewed down the b-axis. Zig-zag columns of fullerene molecules are lined up along the c-axis and are shrouded by macrocycles and solvent molecules.

Figure 10. Fragment of compound 6 showing encapsulation of C70 pair by macrocycles in their dimeric form (the disordered parts are omitted for clarity).

unit, forming a wall of the capsule, whereas the other occupies the interstitial space. The shortest C · · · C distances between solvent molecules and other ligands are >3.3 Å. The symmetrytransformed C70 molecules in the crystal form zigzag chains along the c-axis, with the shortest C · · · C distance at 3.25 Å (Figure 9). These chains are shrouded by the macrocycles and solvent molecules. Molecules of 2 also form pairs of centrosymmetric dimers (Figure 10), where ring 4 is disposed approximately at the middle of a cleft between two propyl chains (Pr), with shortest C · · · C distances at 3.6 and 3.7 Å. The hydrogen atoms of the OH groups are involved in intramolecular H-bonds with O · · · O distances at 2.767(8) and 2.837(8) Å. Conclusions We have shown that flexible calixarene-type macrocycles can form supramolecular assemblies with fullerenes and that the complexation is macrocycle-ring-size-dependent. The macrocycles with the smallest ring size failed to form a complex with C70 fullerene, whereas the slightly larger homologue formed a “ball-and-socket” nanostructure with both C60 and C70. In addition to the steric considerations, each ring size directs the organization of the fullerenes into a specific mode of fullerene interplay, notably, dimeric association of the fullerenes in discontinuous zigzag chains with a smaller macrocycle versus

Horning-crown macrocycles 1 and 2 were synthesized according to methods described previously,11 and fullerenes C60 and C70 were purchased from Bucky USA and used as received. A toluene solution of Horning-crown macrocycles 1 or 2 (100 mg, 0.15 mmol) was heated and allowed to slowly evaporate under ambient conditions, affording crystals of compounds 4-6 which were suitable for X-ray diffraction studies. X-ray Crystallography. The X-ray diffracted intensities were measured from single crystals on a Bruker and Enraf-Nonius CCD instrument using monochromatized Mo KR (λ ) 0.71073 Å, T ) 153 K), an Oxford Diffraction Gemini-R Ultra CCD diffractometer using monochromatized Cu KR (λ ) 1.54178 Å, T ) 100 K), and synchrotron (λ ) 0.67750 Å, T ) 100 K) X-ray sources. Data were corrected for Lorentz and polarization effects, and absorption corrections were applied using multiple symmetry equivalent reflections. The structures were solved by direct method and refined on F2 using a Bruker SHELXTL crystallographic package.15 A full matrix least-squares refinement procedure was used, minimizing w(Fo2 - Fc2), with w ) [s2(Fo2) + (AP)2 + BP]-1, where P ) (Fo2 + 2Fc2)/3. Agreement factors (R ) Σ|Fo| - |Fc|/Σ|Fo|, wR2 ) {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2 and GOF ) {Σ[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. Crystal/Refinement Details for 3. Moiety formula C60, 2(C44H40O6), 2(C7H8): C162H96O12, M ) 2234.39, F(000) ) 2348 e, triclinic, P1 bar, Z ) 2, T ) 150 K, a ) 18.262(2), b ) 18.663(2), c ) 19.717(2) Å, R ) 84.807(2), β ) 74.192(2), γ ) 69.421(2)°, V ) 6053.2(11) Å3; Dc ) 1.236 g cm-3; λ ) 0.67750 Å, sin Θ/λmax ) 0.5268; N (unique) ) 14700 (merged from 35958, Rint ) 0.0388, Rsig ) 0.0484), No (I > 2σ(I)) ) 10293; R ) 0.1253, wR2 ) 0.3425 (A, B ) 0.25, 14.0), GOF ) 1.049; |∆Fmax| ) 1.1(1) e Å-3. CCDC 694509. Crystal/Refinement Details for 4. Moiety formula C60, C46H44O6, 2(C7H8): C120H60O6, M ) 1597.68, F(000) ) 1656 e, triclinic, P1 bar, Z ) 2, T ) 150 K, a ) 13.834(7), b ) 17.166(4), c ) 18.809(4) Å, R ) 116.079(2), β ) 98.419(3), γ ) 103.934(4)°, V ) 3727(2) Å3; Dc ) 1.424 g cm-3; λ ) 0.67750 Å, sin Θ/λmax ) 0.6261; N (unique) ) 14913 (merged from 32084, Rint ) 0.0459, Rsig ) 0.0682), No (I > 2σ(I)) ) 9539; R ) 0.0722, wR2 ) 0.1951 (A, B ) 0.125, 1.56), GOF ) 1.006; |∆Fmax| ) 0.80(5) e Å-3. CCDC 694510. Crystal/Refinement Details for 5. Moiety formula C70, C46H44O6, 2(C7H8): C130H60O6, M ) 1717.78, F(000) ) 1776 e, triclinic, P1 bar, Z ) 2, T ) 100 K, a ) 13.8439(3), b ) 18.2149(4), c ) 19.2699(6) Å, R ) 118.887(3), β ) 96.793(3), γ ) 105.722(2)°, V ) 3912.32(17) Å3; Dc ) 1.458 g cm-3; λ ) 1.54178 Å, sin Θ/λmax ) 0.5980; N (unique) ) 13695 (merged from 87350, Rint ) 0.0402, Rsig ) 0.0410), No (I > 2σ(I)) ) 8175; R ) 0.0664, wR2 ) 0.1851 (A, B ) 0.14, 0), GOF ) 1.007; |∆Fmax| ) 1.02(6) e Å-3. CCDC 694511. Crystal/Refinement Details for 6. Moiety formula C70, 2(C46H44O6), 2(C7H8): C176H104O12, M ) 2410.59, F(000) ) 5024 e, monoclinic, C2/c, Z ) 4, T ) 153 K, a ) 29.983(11), b ) 23.503(9), c ) 18.700(7) Å, β ) 91.832(5)°, V ) 13171(9) Å3; Dc ) 1.216 g cm-3; λ ) 0.71073 Å, sin Θ/λmax ) 0.61; N (unique) ) 11111 (merged from 38768, Rint ) 0.0521, Rsig ) 0.0659), No (I > 2σ(I)) ) 5523; R ) 0.1554, wR2 ) 0.3372 (A, B ) 0.035, 284.0), GOF ) 1.06; |∆Fmax| ) 1.0(1) e Å-3. CCDC 694512. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC 694509-694512). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: [email protected]).

Acknowledgment. We thank the Australian Research Council and the University of Western Australia for support of this work.

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Supporting Information Available: Crystallographic information file (cif). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) Makha, M.; Purich, A.; Raston, C. L.; Sobolev, A. N. Eur. J. Inorg. Chem. 2006, 50, 7–517. (b) Raston, C. L. Complexation of Fullerenes. In Encyclopedia of Supramolecular Chemistry; Atwood, J. L., Steed, J., Eds.; Marcel Dekker: New York, 2004; p 302. (2) Makha, M.; Raston, C. L.; Sobolev, A. N.; Turner, P. Cryst. Growth Des. 2005, 6 (1), 224–228. (3) Atwood, J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994, 368, 229. (4) (a) Diederich, F.; Gomez-Lopez, M. Chem. Soc. ReV. 1999, 28, 263. (b) Zhong, Z.-L.; Ikeda, A.; Shinkai, S.; Complexation of Fullerenes in Calixarenes 2001; Asfari, Z., Bohmer, V., Vicens, J., Saadioui, M., Eds.; Kluwer Academic Publishers: The Netherlands, 2001; pp 476-495. (5) (a) Atwood, J. L.; Barbour, L. J.; Nichols, P. J.; Raston, C. L.; Sandoval, C. A. Chem.sEur. J. 1999, 5, 990. (b) Atwood, J. L.; Barbour, L. J.; Heaven, M. W.; Raston, C. L. Angew. Chem., Int. Ed. 2003, 42, 3257. (c) Makha, M.; Hardie, M. J.; Raston, C. L. Chem. Commun. 2002, 1446–1447. (d) Makha, M.; McKinnon, J. J.; Sobolev, A. N.; Spackman, M. A.; Raston, C. L. Chem.sEur. J. 2007, 13, 3907– 3912. (6) (a) Mizyed, S.; Tremaine, P. R.; Georghiou, P. E. J. Chem. Soc., Perkin Trans. 2001, 2, 3–6. (b) Makha, M.; Raston, C. L.; Sobolev, A. N.; Barbour, L. J.; Turner, P. CrystEngComm 2006, 8, 306–308. (7) (a) Wang, L.; Gutsche, C. D. J. Am. Chem. Soc. 1998, 120, 12226.

(8)

(9) (10)

(11) (12) (13) (14)

(15)

(b) Wang, J.; Bodige, S. G.; Watson, W. H.; Gutsche, C. D. J. Org. Chem. 2000, 65, 8260. (c) Haino, T.; Yanase, M.; Fukazawa, Y. Angew. Chem. 1998, 110, 1044. (d) Haino, T.; Araki, H.; Fujiwara, Y.; Tanimoto, Y.; Fukazawa, Y. Chem. Commun. 2002, 2148. (a) Iki, H.; Kabuto, C.; Fukushima, T.; Kumagai, H.; Takeya, H.; Miyanari, S.; Miyashi, T.; Miyano, S. Tetrahedron 2000, 56, 1437. (b) Akdas, H.; Bringel, L.; Graf, E.; Hosseini, M. W.; Mislin, G.; Pansanel, J.; De Cian, A.; Fischer, J. Tetrahedron Lett. 1998, 39, 2311. (a) For example, Gale, P. A.; Anzenbacher, P.; Sessler, J. L. Coord. Chem. ReV. 2001, 222, 57. (b) Sliwa, W. Heterocycles 2002, 57, 169. (a) Wang, M.-X.; Zhang, X.-H.; Zheng, Q.-Y. Angew. Chem., Int. Ed. 2004, 43, 838. (b) Zhang, E.-X.; Wang, D.-X.; Zheng, Q.-Y.; Wang, M.-X. Org. Lett. 2008, 10, 2565–2568. Higham, L. T.; Kreher, U. P.; Raston, C. L.; Scott, J. L.; Strauss, C. R. Org. Lett. 2004, 6, 3257–3259. Higham, L. T.; Kreher, U. P.; Mulder, R. J.; Strauss, C. R.; Scott, J. L. Chem. Commun. 2004, 2264. Higham, L. T.; Kreher, U. P.; Scott, J. L.; Strauss, C. R. CrystEngComm 2004, 6 (79), 484. Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.; Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc. 1999, 121, 10487–10485. (a) Sheldrick, G. M. SHELX-97: Structure solution and refinement programs; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (b) Bruker SMART, SAINT, SADABS and SHELXTL, V5.1; Bruker AXS Inc.: Madison, Wisconsin, 1997. (c) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189–191. (d) Atwood, J. L.; Barbour, L. J. Cryst. Growth Des. 2003, 3, 3–8.

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