One-dimensional helical chain in the co-crystal of CHCl₃ and trans-2,3,5,6,11,11b-hexahydro-3-oxo-1H-indolizino[8,7-b]indole-5-carboxylic acid methyl ester: a channel-type inclusion complex

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Abstract

X-Ray analysis of the title co-crystal shows a channel-type inclusion structure. The helical network of the indolizino–indole moiety contains a helical arrangement of CHCl₃.

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Introduction

The rational construction of extended structures incorporating large cavities or channels capable of hosting small molecules is an area of considerable current interest due to their potential application in, for example, catalysis or separation procedures.¹ There are now many examples based upon silica and alumina frameworks, coordination compounds and ordered networks constructed from organic compounds.^2–9 A potentially attractive feature of organic compounds is that by careful selection of constituent molecules one can potentially modify the inclusion cavities. However, solvents are rarely included in crystals when organic compounds are crystallized. A recent survey of the Cambridge Structural Database (CSD) shows that 85% of all organic crystals do not contain solvent of crystallization.¹⁰ In a previous paper, we reported the syntheses of L-tryptophan derivatives, (5S)-2,3,5,6,11,11b-hexahydro-3-oxo-1H-indolizino[8,7-b]indole-5-carboxylic acid methyl esters [trans (1) and cis (2) forms].¹¹

Crystallization of 1 from CHCl₃ results in the formation of crystalline inclusion compounds. However, the closely related compound 2 displays no inclusion properties whatsoever. This suggests that the steric difference of constituent molecules may bring about marked changes in the crystal structure. Here, in order to investigate the molecular recognition and organization via weak intermolecular interactions,^12,13 compound 1 is characterized by X-ray single-crystal diffraction and thermal analysis.

Experimental

Compound 1 was prepared from L-tryptophan methyl ester and 2-oxoglutaric acid according to the method reported previously,¹¹ and recrystallized from CHCl₃ to give single crystals of 1·CHCl₃ (1∶1). Thermogravimetric analysis (TGA) was carried out with a Seiko Instruments SSC5200 thermo-analyzer in a nitrogen atmosphere (heating rate 10 K min⁻¹).

Crystallographic data collection and refinement of the structure

Data collection for compound 1·CHCl₃ was carried out on a MAC Science MXC3 with graphite-monochromated Mo-Kα radiation. The structure was solved by direct methods (Rigaku teXsan crystallographic software package¹⁴) and refined with full-matrix, least squares on F² (SHELXL97).¹⁵ All non-H atoms were refined anisotropically. All of the hydrogen atoms were located on the difference Fourier map and not refined. A summary of crystallographic data is given in Table 1.

Table 1 Summary of crystal data for 1·CHCl₃^a

Parameter	1·CHCl3
a Click b104821.txt for full crystallographic data (CCDC 164739).
Empirical formula	C17H17Cl3N2O3
Crystal dimensions/mm	0.20⊕×⊕0.20⊕×⊕0.05
M	251.06
Crystal system	Orthorhombic
Space group	P212121
a/Å	13.619(5)
b/Å	13.668(5)
c/Å	10.242(4)
V/Å^3	1906(1)
Z	4
T/K	273.2
D c/g cm^−3	1.406
μ(Mo-Kα)/cm^−1	4.98
F(000)	832
2θmax/°	55
Total reflections	2805
Observed reflections	1754
Parameters refined	227
R [I⊕>⊕2σ(I)]	0.057
wR 2 (all data)	0.173
R int	0.029
GOF	1.09

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Results and discussion

The ORTEP drawing of 1·CHCl₃ with atom numbering scheme is shown in Fig. 1.¹⁶ The indole ring and lactam ring within 1 are twisted with respect to each other. The torsion angle N(2)–C(8)–C(1)–C(2) is 54.9°.

Fig. 1 ORTEP view of the molecular structure of compound 1·CHCl₃ at the 50% probability level. Click image or 1.htm to access a 3D representation.

As shown in Fig. 2(a), compound 1 is linked to a neighboring molecule by the N–H⋯O interaction between the indole NH and the lactam carbonyl oxygen [N(2)–O(1′)⊕=⊕2.88 Å], forming one-dimensional helical chains running parallel to the a-axis. Compound 1 is synthesized as an optically resolved material and the crystal contains a single enantiomer at the molecular level. This local chirality translates throughout the crystal into the formation of only left-handed helices at the supramolecular level. There are intermolecular aromatic stacking interactions between the helices [C(14)–C(15′)⊕=⊕3.57 Å]. Consequently, there exist large chiral channels parallel to the crystallographic c-axis. These channels contain an ordered arrangement of CHCl₃ molecules generated around crystallographic 2₁ screw axes. The primary interaction between the CHCl₃ and the walls of the channels results from the C–H⋯O hydrogen bonding between CHCl₃ and the ester carbonyl oxygen in 1 [C(17)–O(3)⊕=⊕3.24 Å]. Furthermore, relatively close intermolecular Cl⋯O contact is present: Cl(3)–O(1′) contact of 3.52 Å.¹⁷ A portion of one of the CHCl₃ arrangements is illustrated in Fig. 2(b). The chirality of the channel appears to induce chirality into the helical arrangements that are formed by CHCl₃.^18–20

Fig. 2 (a) Molecular packing and hydrogen bonding of 1·CHCl₃. Dashed lines represent hydrogen bonds. (b) Representation of the one-dimensional helical arrangement of CHCl₃ molecules.

Thermogravimetric analysis of 1 indicated that CHCl₃ molecules are liberated at 109 °C (Fig. 3). The high cohesion of 1 is demonstrated by its thermal stability, when compared with the boiling point of CHCl₃ (bp 61 °C). The closely related compound 2 displays no inclusion properties. Early study suggests that 2 is more planar than 1.¹¹ It is believed from this study that the shape of the building block is a key factor to control the structural features in the sophisticated process of molecular self-recognition and self-assembly crystal engineering.

Fig. 3 Thermogravimetric analysis data for 1·CHCl₃.

Conclusions

Compound 1 represents prototypes of a new class of compounds, i.e. hydrogen bond-supported helical arrangements with chiral cavities that are suitable for the incorporation of CHCl₃. Chiral porous materials clearly have implications for the developing fields of stereospecific synthesis and enantioselective separations. The compound 1 therefore effectively combines the presence of polarity and porosity in the solid state, both of which are of significant relevance to applications in chemistry and materials science. Further experimentation, aimed at investigating the applications of 1 in enantioselective chemistry, is in progress.

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Footnote

† Present address: Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.

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