Efficient Isomeric Enrichment in Cocrystals of Cyclohexanediamines and Low Molecular Weight Diols Janet L. Scott,*,† Satoshi Hachiken,‡ and Koichi Tanaka*,‡ School of Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia, and Department of Chemistry and Material Engineering, Faculty of Chemistry, Materials and Bioengineering & High Technology Research Center, Kansai UniVersity, Suita, Osaka 564-8680, Japan
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2447–2452
ReceiVed January 10, 2008; ReVised Manuscript ReceiVed March 5, 2008
ABSTRACT: Cis and trans isomers of cyclohexanediamines are separated by cocrystallization with low molecular weight diols: 1,4-benzenedimethanol and cis- and trans-1,4-cyclohexanedimethanol. In six out of seven cocrystals formed, significant enrichment of one cyclohexanediamine enantiomer is achieved in a simple and efficient process. Crystal structures are examined to reveal the origin of this high degree of selectivity. Introduction As Dunitz and Gavezzotti so eloquently point out: the crystallization process involves the phenomenon of molecular recognition at an amazingly high degree of reliability, even where the crystal is composed of not one, but two (or even more) components, as occurs in a cocrystal.1 This reflects the “self-healing” nature of the crystallization process: attachment of new molecules to the growing crystal faces proceeds in a dynamic fashion, such that incorrectly oriented molecules (while transiently attached) provide regions of instability so that the molecule in the “wrong” orientation becomes detached (as it does not satisfy the requirement of greatest contribution to the lattice energy, presuming that the process is occurring at something approaching equilibrium conditions) and only those in the correct orientation will “stick”, leading to defectless crystal growth. This is, of course, why crystallization is a means of purification: exploitation of the tendency of molecules to pack in ordered arrays such that the greatest stabilization is achieved leads to ejection of “unfitting” molecules. Such processes of molecular recognition have long been applied in classical resolution of acids or bases by formation of salts with distinctly different solubilities, and form the basis for the so-called “Dutch resolution”2 in which families of closely related resolving agents are applied to racemic mixtures. The more subtle interactions occurring in cocrystals, without salt formation, have been exploited to effect the separation of isomers difficult to discriminate between by other physicochemical means. Thus ortho- and para-isomers of disubstituted aromatics, that frustrate attempts to achieve pure materials by cost-effective fractional distillation due to their very similar boiling points, may be greatly enriched by selective cocrystallization or enclathration. The two approaches may be effectively combined to achieve separation by distillation from slurries containing the molecules to be separated and a “host” compound, which complexes one of the isomers preferentially, thus retarding its volatilization to higher temperatures.3 Few reported cocrystals have arisen from completely random screening of possible combinations, and even large “combinatorial experimental arrays”,4 such as those becoming popular in the search for pharmaceutical cocrystals,5 are almost invariably based on sensible * Corresponding authors. E-mail:
[email protected]; ktanaka@ ipcku.kansai-u.ac.jp. J.L.S.: current address, Physical and Chemical Insights, Unilever HPC R&D, Port Sunlight, Bebington, CH63 3JW, U.K.; fax, +44 151 641 1852; e-mail,
[email protected]. K.T.: fax, +81 06 6368 0861. † Monash University. ‡ Kansai University.
choices of cocrystal formerssthe decision for which components to offer to the “gods of crystallization” being based on a knowledge of likely specific, predictable interactions, which yield supramolecular synthons.6 The predictability of these interactions forms the basis for “crystal engineering”.7 The exploitation of supramolecular synthons in organic cocrystals is not limited to interactions between the two different constituents, but, in analogous manner to the formation of metal coordination networks, may be exploited to form crystal frameworks of bulky molecules, which trap smaller molecules within interstitial spaces, as clathrate-type inclusion complexes.8 Such clathrate-type inclusion complexes have also been applied in the separation of substituted aromatics as exemplified by the use of hosts such as cephalosporin,9 diol10,11 or dicarboxy12 host compounds. Many further examples exist, but, in most cases, the cocrystal former is a large host molecule that thus (a) constitutes a significant mass of material in the experiment and (b) is of middling to great synthetic complexity (and some astonishingly beautiful, but complex, examples have been reported13), thus making the process unlikely to be competitive with simpler techniques, which do not require a large quantity of molecular “baggage” in the form of high MW hosts or resolving agents.
We have previously reported the selective inclusion of equatorial isomers of cyclohexane-polyols in phosphonium salt hosts,14 a process suffering from just such a mismatch in MW of resolving agent and target, but we now focus on the use of very simple small molecules to effect highly selective crystallization of 1:1 cocrystals of diols 1, 2 and 3 with cyclohexanediamines 4, 5 and 6. The focus here is not on separation of positional isomers of the cyclohexanediamines (a relatively simple process), but on differentiation by cocrystallization of cis and trans isomers. (These pure isomers may be used in turn as chiral building blocks,15b,16 in salts to effect topotactic control of reactions in crystals,17 or even as resolving agents in their own right.18) Results and Discussion 1,4-Benzenedimethanol (1) and trans-1,4-cyclohexanedimethanol (2) formed cocrystals with trans-1,2-diaminocyclohexane
10.1021/cg800032m CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
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Table 1. Cis/Trans Selectivity of Cyclohexanediamines 4-6 in 1:1 Cocrystals with Diols 1-3 amines (cis:trans)
1
2
4 (38:62) 5 (54:46) 6 (77:23)
11:89a 93:7c 5:95
8:92c 96:4 48:52
3 b
89:11 b
a Cis:trans ratios were determined by GC. b No complexation. c Crystal structures analyzed.
(4), cis-1,3-diaminocyclohexane (5) and trans-1,4-diaminocyclohexane (6), respectively, while cis-1,4-cyclohexanedimethanol (3) formed cocrystals with cis-1,3-diaminocyclohexane (5) only, Table 1. The degree of selection for one geometrical isomer over the other was significant in six of the seven cocrystals formed, with only 2 · 6 exhibiting poor selectivity. Most remarkably, in some cases the degree of selectivity was hugely in favor of the geometrical isomer present in lesser quantity in the initial mixture. Thus, for the combination of 1 with 6, an initial approximately 3:1 cis:trans mixture of 6 is converted via cocrystallization to a 1:19 cis:trans mixture. Trans-4 may be isolated by fractional crystallization of the sulfate salt19 (and R,R and S,S isomers of cis-4 separated by crystallization of diastereomeric salts with one or other antipode of optically active tartaric acid15). While trans-6 of high purity may be obtained by solventless, fractional crystallization of the liquid cis/trans mixture resulting from synthesis and separation of solid and liquid fractions,20 this does not provide a means for isolation of pure cis-6. Xu et al. report selectivity for cis-6 in the formation of germanate metal complexes by hydrothermal synthesis,21 but, while interesting, this is unlikely to provide a scalable commercial process (nor do the authors claim this, simply noting the unusual selectivity). 1,2-Diaminocyclohexane (4) and 1,3-diaminocyclohexane (5) prove even more intractable with regard to direct separation of cis and trans isomers. Brake, in 1970, reported a means of separation of cis and trans isomers of diaminocyclohexanes by the formation of “diaminocyclohexane polyol adducts”. These were assumed to be new molecular entities by virtue of their crystalline nature and melting points, which differed from those of either reactant.22 In this patent, examples of separation of cis-5 by crystallization with 2 are provided. Here we demonstrate that this principle of resolution of geometrical isomers by cocrystal formation is more widely applicable and that cis/trans selectivity may be switched by judicious choice of cocrystal former. To attempt to gain some insight into this highly effective purification of a single geometrical isomer by cocrystal formation, crystal structures were obtained of those complexes that could be isolated as good quality single crystals: 1 · trans-4 and 2 · cis-5. The complex 1 · trans-4 crystallizes in the triclinic space group P1j with one diamine molecule and two crystallographically independent ½ diol molecules in the asymmetric unit as shown in Figure 1. Five unique hydrogen bonds create an entirely cross-linked network structure. Thus one diol molecule forms tetrads of hydrogen bonds with the amine groups of 4 while the other forms part of an infinite chain of hydrogen bonds, Figure 2, Table 2. The fifth hydrogen bond, N1-H1N1 · · · O1A [-x + 1, -y + 2, -z + 1], serves to bond one tetrad to that above (and below) creating a 3-D hydrogen bonded network. While, at first glance, one N-H donor group (N2-H2N1) appears not to form part of the hydrogen bond network, leaving one hydrogen bond donor group “unfulfilled”, it is immediately obvious that the N-H group is oriented toward O1A and relevant geometric parameters are also listed in Table 2. The packing of the extensively hydrogen bonded structure is depicted in Figure 3.
Figure 1. Molecular diagram of the cocrystal components 1 · trans-4, from the single crystal structure. Atoms of the asymmetric unit (labeled) are depicted as ellipsoids at 50% probability, with symmetry generated atoms and hydrogen atoms depicted in stick mode.
Figure 2. Hydrogen bonded molecules form a network structure in 1 · trans-4. N and O atoms are depicted as spheres and H-bonds, 1, 2, 4 and 5, as dotted lines. The donor group of H-bond 3 is indicated by an arrow, but the bond is not shown to avoid overlaying tetrads.
The complex 2 · cis-5 crystallizes in the orthorhombic space group Pnma with half each of a diamine and diol molecule in the asymmetric unit, Figure 4. All potential hydrogen bond donors and acceptors participate in the extensive hydrogen bonded network, and geometric parameters describing these intermolecular interactions are listed in Table 3. As depicted in the packing diagram, Figure 5, the result is a crystal with packing stabilized by hydrogen bonds extending in 3 dimensions. Hydrogen bond interactions and, in particular, complete fulfillment of all hydrogen bonding potential appear to be implicit in the formation of the preferred (less soluble) cocrystals. Hirshfeld surface analysis23 reveals distinct differences in these intermolecular interactions in the respective cocrystals, but the sharp features that denote strong hydrogen bond interactions are clear, Figure 6. Hydrogen bond donor interactions are evidenced by sharp “horns” at low values of di vs de while corresponding acceptor close contacts yield similar horns at low values of de vs di. Thus, the shorter O-H · · · N D-H · · · A contact distance is reflected in the longer horn in diol donor and amine acceptor regions in the graphs.
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Table 2. Summary of Hydrogen Bond Geometry for 1 · trans-4
1 2 3a 4 5 6a,b
D-H · · · A
d(D · · · A)/Å
d(D-H)/Å
d(H · · · A)/Å
∠(D · · · A)/deg
N1-H1N2 · · · O1A O1A-H1OA · · · N1 [-x, -y + 2, -z + 1] N1-H1N1 · · · O1A [-x + 1, -y + 2, -z + 1] O1B-H1OB · · · N2 [x + 1, y, z] N2-H2N2 · · · O1B N2-H2N1 · · · O1A
3.073(1) 2.737(1)
0.92(2) 0.92(2)
2.17(2) 1.82(2)
168(1) 176(2)
3.167(1)
0.86(2)
2.32(2)
166(1)
2.787(1)
0.89(2)
1.91(2)
174(2)
3.114(1) 3.577(1)
0.91(2) 0.90(2)
2.21(2) 2.81(2)
176(2) 144(1)
a
Not depicted in Figure 2sbonds one layer, as shown in Figure 2, to that above. bond; see discussion and Figure 3.
b
d(D · · · A) is longer than generally considered to be a hydrogen
Figure 3. Packing of 1 · trans-4 viewed down [-1 0 0]. Hydrogen bonds in layers perpendicular to [-1 0 0] are depicted as dotted lines. Each hydrogen bonded sheet is also hydrogen bonded to that above and below and the remaining “unfulfilled” hydrogen bond interaction, N2-H2N1 · · · O1A (see inset), is, at 3.577(2) Å, a longer donor to acceptor distance than usually accepted as a hydrogen bond.
Figure 4. Molecular diagram of cocrystal components 2 · cis-5, from the single crystal structure. Atoms of the asymmetric unit (labeled) are depicted as ellipsoids at 50% probability with symmetry generated atoms and hydrogen atoms depicted in stick mode. The diamine is situated on a mirror plane (symmetry generated atoms: x, ½ - y, -z) and the diol on a centre of symmetry (symmetry generated atoms: 1 x, -y, 1 - z).
This apparent domination of O-H · · · N interactions is, interestingly, reflected in the conservation of tetrahedral geometry about the N centers as is illustrated in Figure 7. While crystal structures of cocrystals of the less preferred diol/amine combinations are not available, it appears that
hydrogen bonding interactions dominate the packing of the preferred cocrystals, and it appears reasonable to assume that optimization of H-bonds leads to great lattice energy stabilization and thus less soluble cocrystals, and so to enrichment in the “preferred” isomer. In conclusion, we present a means for separation of geometrical isomers of cyclohexanediamines by a facile cocrystallization with readily available diols, which, in some cases, provides a remarkable enrichment in the lesser isomer. Crystal structure analysis indicates the presence of close packed crystals hydrogen bonded in 3 dimensions. Analysis of hydrogen bonding indicates almost complete fulfillment of all hydrogen bond donor and acceptor capacity. Experimental Section General Remarks. 1,4-Benzenedimethanol, trans-1,4-cyclohexanedimethanol, cis-1,4-cyclohexanedimethanol, 1,2-diaminocyclohexane, 1,3-diaminocyclohexane, and 1,4-diaminocyclohexane used for the inclusion complexation study are commercially available and were purchased either from Wako chemicals or from Tokyo Kasei Kogyo. 1H NMR spectra were recorded in CDCl3 on a JEOL JNM-EX270 FT-NMR spectrometer.
Table 3. Summary of Hydrogen Bond Geometry for 2 · cis-5
1 2 3
D-H · · A
d(D · · · A)/Å
d(D-H)/Å
d(H · · · A)/Å
∠(D · · · A)/deg
O1-H1O · · · N1 N1-H1N · · · O1 [½ - x, -y, -z - ½] N1-H2N · · · O1 [x, y, z - 1]
2.745(1) 3.182(1)
0.83(2) 0.87(2)
1.93(2) 2.32(2)
171(2) 171(1)
3.069(1)
0.85(2)
2.23(2)
169(1)
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Figure 5. Packing diagram of 2 · cis-5 viewed down [0 0 1]. Hydrogen bonds are depicted as dotted lines and (in the inset) a close-up of the hydrogen bonds between layers is depicted, tilted a few degrees toward the viewer. The hydrogen bonding potential of all donors and acceptors is fulfilled.
Figure 6. Hirshfeld surface fingerprint plots of each molecular component of the cocrystals. Top row: 1 · trans-4. Bottom row: 2 · cis-5. Dashed diagonal lines serve to guide the eye to donor interactions with de > di (above the diagonal) and acceptor interactions with de < di (below the diagonal). There are clear differences in the intermolecular interactions of the two crystallographically independent diols in cocrystal 1 · trans-4, but both exhibit “wings” which are indicative of CH · · · π interactions, and closer inspection of the crystal structures indeed reveals such interactions, illustrated for diol A at bottom right (H · · · centroid for B ) 2.94 Å). IR spectra were recorded with a JASCO FT-IR 4100 spectrometer. Gas chromatographic analyses were performed on a Shimadzu GC-2014 instrument using a SUPELCO R-DEX 120 GC column. Selective Formation of 1:1 Cocrystals between 1,4-Benzenedimethanol (1) with trans-1,2-Diaminocyclohexane (4). A solution of 1 (0.6 g, 4.38 mmol) and a 38:62 mixture of cis-4 and trans-4 (1.0 g, 8.75 mmol) in toluene (10 mL) was kept at room temperature for
0.5 h to give a 1:1 inclusion complex of 1 and trans-4 (89% purity) as colorless needles (0.79 g, mp 70–75 °C). The purity was determined by GC analysis. Selective Formation of 1:1 Cocrystals between 1,4-Benzenedimethanol (1) with cis-1,3-Diaminocyclohexane (5). A solution of 1 (0.6 g, 4.38 mmol) and a 54:64 mixture of cis-5 and trans-5 (1.0 g, 8.75 mmol) in toluene (10 mL) was kept at room temperature for 0.5 h
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Figure 7. Geometry of contacts about hydrogen bond donor/acceptor groups in 1 · trans-4 (top four diagrams) and 2 · cis-5 (bottom diagram). Angles are measured as H · · · A-bonded atom (thick dotted lines) or D-H · · · A (thin dotted lines), and it is clear that N centers approach more closely ideal tetrahedral arrangements, while O centers may exhibit rather acute angles between hydrogen bonds from donor groups: 81.9° and 76.3° of for O1A of 1 · trans-4 and O1 of 2 · cis-5 respectively. to give a 1:1 inclusion complex of 1 and cis-5 (93% purity) as colorless needles (0.81 g, mp 90–93 °C). The purity was determined by GC analysis. Selective Formation of 1:1 Cocrystals between 1,4-Benzenedimethanol (1) with trans-1,4-Diaminocyclohexane (6). A solution of 1 (0.6 g, 4.38 mmol) and a 77:23 mixture of cis-6 and trans-6 (1.0 g, 8.75 mmol) in toluene (10 mL) was kept at room temperature for 0.5 h to give a 1:1 inclusion complex of 1 and trans-6 (95% purity) as colorless needles (0.85 g, mp 115–117 °C). The purity was determined by GC analysis.
Selective Formation of 1:1 Cocrystals between trans-1,4-Cyclohexanedimethanol (2) with trans-1,2-Diaminocyclohexane (4). A solution of 2 (0.28 g, 1.94 mmol) and a 38:62 mixture of cis-4 and trans-4 (0.44 g, 3.88 mmol) in toluene (5 mL) was kept at room temperature for 0.5 h to give a 1:1 inclusion complex of 2 and trans-4 (92% purity) as colorless needles (0.24 g, mp 76–77 °C). The purity was determined by GC analysis. Selective Formation of 1:1 Cocrystals between trans-1,4-Cyclohexanedimethanol (2) with cis-1,3-Diaminocyclohexane (5). A solution of 2 (0.28 g, 1.94 mmol) and a 54:64 mixture of cis-5 and trans-5
2452 Crystal Growth & Design, Vol. 8, No. 7, 2008 (0.44 g, 3.88 mmol) in toluene (5 mL) was kept at room temperature for 0.5 h to give a 1:1 inclusion complex of 1 and cis-5 (96% purity) as colorless needles (0.29 g, mp 126–127 °C). The purity was determined by GC analysis. Formation of 1:1 Cocrystals between trans-1,4-Cyclohexanedimethanol (2) with 1,4-Diaminocyclohexane (6). A solution of 2 (0.28 g, 1.94 mmol) and a 77:23 mixture of cis-6 and trans-6 (0.44 g, 3.88 mmol) in toluene (5 mL) was kept at room temperature for 0.5 h to give a 1:1 inclusion complex of 1 and a 48:52 mixture of cis-6 and trans-6 as colorless needles (0.44 g, mp 138–139 °C). The purity was determined by GC analysis. Selective Formation of 1:1 Cocrystals between cis-1,4-Cyclohexanedimethanol (3) with cis-1,3-Diaminocyclohexane (5). A solution of 3 (0.28 g, 1.94 mmol) and a 54:64 mixture of cis-5 and trans-5 (0.44 g, 3.88 mmol) in toluene (5 mL) was kept at room temperature for 0.5 h to give a 1:1 inclusion complex of 3 and cis-5 (89% purity) as colorless needles (0.16 g, mp 126–127 °C). The purity was determined by GC analysis. X-ray Diffraction Studies. Crystals suitable for single crystal diffraction experiments were obtained as described above. Data were collected on either an Enraf-Nonius Kappa CCD diffractometer or a Brüker Apex II KAPPA CCD diffractometer at 123 K using graphite monochromated Mo KR radiation (λ ) 0.71073 Å). Structures were solved by direct methods using the program SHELXS-9724 and refined by full-matrix least-squares refinement on F2 using the programs SHELXL-9725 and X-Seed.26 Non-hydrogen atoms of the hosts were refined anisotropically and methylene and methine group hydrogen atoms inserted in geometrically determined positions with temperature factors fixed at 1.2 times that of the parent atom. Hydrogen atoms of the hydroxyl or amino groups (i.e., potentially involved in hydrogenbonding) were located from electron density difference maps and refined without restraints.
Acknowledgment. This work was supported by “High-Tech Research Center” Project for Private Universities: matching fund subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2005–2009. We thank Ms. Ann Almesaker for collection of crystallographic data. Supporting Information Available: Data pertaining to crystal structure analysis and refinement. This material is available free of charge via the Internet at http://pubs.acs.org.
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Scott et al. (4) Remenar, J. F.; MacPhee, J. M.; Larson, B. K.; Tyagi, V. A.; Ho, J. H.; McIlroy, D. A.; Hickey, M. B.; Shaw, P. B.; Almarsson, O. Org. Process Res. DeV. 2003, 7, 990–996. (5) (a) Almarsson, O.; Zaworotko, M. Chem. Commun. 2004, 1889–1896. (b) Almarsson, O.; Hickey, M. B.; Peterson, M. L.; Morissette, S. L.; Soukasene, S.; McNulty, C.; Tawa, M.; MacPhee, J. M.; Remenar, J. F. Cryst. Growth Des. 2003, 3, 927–933. (6) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311–2327. (7) Desiraju, G. R. Angew. Chem., Int. Ed. 2007, 46, 8342–8356. (8) Nassimbeni, L. R. Acc. Chem. Res. 2003, 36, 631–637. (9) Kemperman, G. J.; de Gelder, R.; Dommerholt, F. J.; Klunder, A. J. H.; Zwanenburg, B. Eur. J. Org. Chem. 2002, 345, 250. (10) Caira, M. R.; Nassimbeni, L. R.; Toda, F.; Vujovic, D. J. Chem. Soc., Perkin Trans. 2 1999, 2681–2684. (11) Caira, M. R.; Nassimbeni, L. R.; Vujovic, D.; Weber, E.; Wierig, A. Struct. Chem. 1999, 10, 205–211. (12) Beketov, K.; Weber, E.; Seidel, J.; Köhnke, K.; Makhkamov, K.; Ibragimov, B. Chem. Commun. 1999, 91, 92. (13) Some particularly synthetically challenging examples include:(a) Kerckhoffs, J. M. C. A.; ten Cate, M. G. J.; Mateos-Timoneda, M. A.; van Leeuwen, F. W. B.; Snellink-Ruel, B.; Spek, A. L.; Kooijman, H.; Crego-Calama, M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2005, 127, 12697–12708. (b) Kusukawa, T.; Fujita, M. J. Am. Chem. Soc. 1999, 121, 1397–1398. (14) Tanaka, K.; Nakashima, A.; Shimada, Y.; Scott, J. L. Eur. J. Org. Chem. 2006, 2423, 2428. (15) (a) Galsbøl, F.; Steenbøl, P.; Sørensen, B. S. Acta Chem. Scand. 1972, 26, 3605–3611. (b) Larrow, J. F.; Jacobsen, E. N.; Gao, Y.; Hong, Y.; Nie, X.; Zepp, C. M. J. Org. Chem. 1994, 59, 1939–1942. (16) Too many excellent examples exist to list here, but from our own work:(a) Correa, W. H.; Scott, J. L. Molecules 2004, 9, 513–519. (b) Tanaka, K.; Hiratsuka, T.; Urbanczyk-Lipkowska, Z. Eur. J. Org. Chem. 2003, 3043, 3046. (17) Natarajan, A.; Mague, J. T.; Venkatesan, K.; Ramamurthy, V. Org. Lett. 2005, 7, 1895–1898. (18) Ratajczak-Sitarz, M.; Katrusiak, A.; Gawroñska, K.; Gawroñski, J. Tetrahedron: Asymmetry 2007, 18, 765–773. (19) Smith, A. I. US Patent 3,187,045, 1965. (20) For this reason commercial producers of 1,4-diaminocyclohexane, such as DuPont, market pure crystalline trans-6 and a liquid mixture of cis- and trans-6. (21) Xu, Y.; Fan, W.; Elangovan, S. P.; Ogura, M.; Akubo, T. Eur. J. Inorg. Chem. 2004, 4547, 4549. (22) Brake, L. D. US Patent 3,491,149, 1970. (23) (a) McKinnon, J. J.; Mitchell, A. S.; Spackman, M. A. Chem. Eur. J. 1998, 4, 2136–2141. (b) Spackman, M. A.; Byrom, P. G. Chem. Phys. Lett. 1997, 267, 215–220. (c) Spackman, M. A.; McKinnon, J. J. CrystEngComm 2002, 4, 378–392. (d) McKinnon, J. J.; Spackman, M. A.; Mitchell, A. S. Acta Crystallogr. B 2004, B60, 627–668. (24) Sheldrick, G. M. SHELXS-97; University of Gottingen: Gottingen, 1997. (25) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen, 1997. (26) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189–191.
CG800032M
