摘要:
Parthasarathi Dastidar* Silicates & Catalysis Division, Sophisticated Analytical Instrumentation Laboratory, Central Salt & Marine Chemicals Research Institute, G. B. Marg, Bhavnagar - 364 002, Gujarat, India. E-mail: salt@bhavnagar.com Received 18th February 2000, Accepted 15th March 2000, Published 24th March 2000 A lattice inclusion complex 1 of cholic acid (CA) with 4-aminopyridine (AP) has been prepared by a cocrystallization method, characterized by standard analytical techniques and structurally analyzed by single crystal X-ray diffraction in order to elucidate the characteristic supramolecular aggregation of host and guest molecules. X-Ray crystallographic studies of this inclusion complex revealed that a channel type inclusion complex has been formed with novel hydrogen bonded helical network of cholic acid instead of the usually observed hydrogen bonded multibilayer arrangement.Crystal structure of the inclusion complex of cholic acid with 4-aminopyridine: a novel supramolecular architecture of cholic acid molecules in their inclusion complexes.6 Here we report the crystal structure of the inclusion complex of cholic acid with 4-aminopyridine, which displays a novel supramolecular hydrogen bonded h–t helical network of host molecules instead of the usually observed hh–tt multibilayer arrangement. Scheme 1 Introduction Lattice inclusion complexes are the result of the supramolecular self-assembly of molecules in their crystal lattice with voids which are occupied by guest species.1 Deoxycholic acid (DCA) and cholic acid (CA) are naturally occurring bile acids which possess the ability of occluding various organic guests into their crystal lattice;2 the lattice inclusion ability of the latter host was discovered only a decade ago.3 Several crystal structures of inclusion complexes involving these two hosts revealed that host molecules are generally arranged in a hydrogen bonded multibilayer fashion and channel space is located between such bilayers.CA frequently forms a head-to-head–tail-totail (hh–tt) arrangement while DCA assembles in a head-totail (h–t) fashion4 in such bilayers (Scheme 1). CA also displays guest-responsive structural changes5 and in some cases, guest-participating reversion of molecular arrangement in the asymmetric multibilayer of host DOI: 10.1039/b001347l CrystEngComm, 2000, 8 Experimental Colorless rectangular shaped crystals of 1 were isolated from a methanol–water (1 : 1 v/v) solution containing CA and AP in a 1 : 1 molar ratio by slow evaporation at room temperature.Stoichiometry of the inclusion complex was established by 1H NMR, elemental analysis and thermogravimetric analysis (TGA). Single crystal X-ray structure determination established that it is a 1 : 1 host–guest inclusion complex with two fully occupied and some partially occupied disordered water molecules. The summary of crystallographic data for the complex is given in Table 1.Table 1 Summary of crystal data for 1a Data Properties C29H52.36O8.18N2 Formula FW Crystal system Crystal dimensions/mm–3 a/Å b/Å c/Å /º V/Å3 Dc/g cm–3 F(000) Total reflections 559.97 Monoclinic, P21 0.20 × 0.08 × 0.15 10.975(4) 7.620(3) 19.749(9) 97.53(3) 1637.4(12) 1.136 612 2326 1470 399 0.0699 0.2078 Observed reflections [I . I)] Parameter refined Final R1 on observed data Final wR2 on observed data a Click here for full crystallographic data (CCDC no.1350/14). 1H NMR. Spectra were recorded on a Bruker 200 MHz spectrometer in CD3OD. –CH3 protons of CA: 0.708 (3H, s), 0.913 (3H, s), 1.015 (3H, d); –CH2 and >CH protons of 1: 1.337–1.597 (11H, m), 1.829–2.031 (13H, m), 2.310–2.235 (3H, m); –OH protons of CA: 3.298 (1H, s, overlapped with solvent peak), 3.782 (1H, s), 3.955 (1H, s).Aromatic protons of AP: 6.75 (2H, d); 7.996 (2H, d). Considering the integration of the peaks of the guest at aromatic region and –CH3 attached to C20 (appears as doublet) of host, the stoichiometry is established as 1 : 1. X-Ray structure determination and refinement. The structure was solved by direct methods (SHELXL977). Subsequent difference Fourier syntheses located two fully occupied water molecules and three low electron density peaks which were assigned as partially occupied water molecules. The final occupancies for these partially occupied waters are 0.48, 0.40 and 0.29 amounting to 1.18 disordered water molecules. All of the non-hydrogen atoms except the partially occupied oxygen atoms of the disordered water molecules were refined anisotropically.Hydrogen atoms attached to carbon atoms were fixed geometrically whereas hydrogen atoms attached to oxygen and nitrogen atoms were located on the difference Fourier map. However, no parameters for these hydrogen atoms were refined in the final cycle of refinement. Elemental analysis: CA–AP–2H2O. Expected: C 64.6, H 9.35, N 5.20%. Found: C 64.26, H 9.03, N 5.55%. X-Ray structure shows that besides two fully occupied water molecules, there are partially occupied disordered water molecules amounting to additional 1.18 water molecules that could not be detected by elemental analysis. This could be due to the fact that these disordered water molecules are loosely bound (no short contact with any non-hydrogen atoms was observed in X-ray analyses) in the crystal lattice and therefore, desolvation of the partially occupied water molecules must have taken place before the elemental analyses were performed.Thermal analysis. In order to establish the stoichiometry of the complex 1, thermogravimetric analysis (TGA) and differential analysis (DTA) were carried out on a TG/DTA 32 instrument (Seiko, Japan). Freshly grown crystals of 1 were blotted dry and crushed. The sample weight was 2.637 mg. A constant flow of nitrogen (flow rate 100 ml min–1) was passed over the sample. The temperature range was 15–255 ºC at a heating rate of 10 ºC min–1. The peak temperatures of the two step guest release process were 87.0 and 179.9 ºC (see Fig.1). The first stage corresponds to an experimental weight loss of 10.99% which matches well with the expected 10.23% weight loss for 3.18 molecules of water (two fully occupied water and three partially occupied oxygen atoms of disordered water molecules as observed by X-ray analyses). The experimental weight loss found in the second stage was 16.96% which corresponds quite well with the expected weight loss of 16.80% for one molecule of AP. It may be pointed out here that release of a solid guest like 4',4'- bipyridine has been reported in the TGA analysis of some crystals of the coordination polymer.8 Fig. 1 TGA and DTA for 1 showing the two step guest release process. Results and discussion An ORTEP9 diagram of the complex with atom numbering scheme is depicted in Fig.2. All hydrogen bonding parameters are summarized in Table 2. The frequently occurring hydrogen bonded multibilayer arrangement of CA hosts is absent in this structure. Host molecules are self-assembled in a helical network via intermolecular hydrogen bonds involving O(3) and O(5) in a h–t fashion. These helical polymeric chains of CA run down the crystallographic b-axis. This supramolecular arrangement of CA exposes its other hydrogen bonding functionalities, Fig. 2 ORTEP view of the molecular structure of complex 1 at 50% probability level. Partially occupied oxygen atoms of disordered water molecules are not shown. Click image or here to access a 3D representation.Table 2 Hydrogen bonding parameters of 1a D–H···A Entry O(4)–H(4O)···N(2) N(1)–H(11N)···O(1)III O(1W)–H(11W)···N(1)IV O(1)–H(1)···O(2W)I 2.648 1 150.7 O(3)–H(3A)···O(5)II 2.854 2 113.8 3 2.631 123.5 2.954 4 168.7 2.884 5 148.6 6 O(1W)–H(12W)···O(2)V 2.814 157.2 O(2W)–(21W)···O(1W)V 2.732 7 162.9 O(2W)–H(22W)···O(1)VI 2.712 8 137.3 a D = donor, A = acceptor. Symmetry code: I = –x, 1/2 – y, –z; II = 1 – x, 1/2 + y, 1 – z; III = 1 – x, –1/2 + y, z; IV = 1 – x, 1/2 + y, –z; V = x, y, z; VI = x, –1 + y, z. Fig.3 Stacks of helical network of CA found in the crystal lattice of 1. Dotted lines represent hydrogen bonding. Green = carbon, red = oxygen. Click image or here to access a 3D representation. which also run down the b-axis (see Fig.3). In the crystal lattice, these polymeric helical chains are closely packed via the hydrophobic face of the host molecules and run along the a-axis. When such stacks of helical chains are arranged parallel to each other along the ab-plane, open channel spaces along the b-axis are created. Hydrogen bonding functionalities of CA, which are not involved in Fig. 4 Overall supramolecular arrangement in 1 showing the extensive hydrogen bonding network. Dotted lines represent hydrogen bonding. Blue = carbons of CA, yellow = carbons of AP, green = nitrogen, red = oxygen. Click image or here to access a 3D representation (disordered atoms included). D–H···A/º D···A/Å the helical network actually form the channel wall; thereby making hydrophilic channels that are occupied by the guest molecules, namely AP and water via extensive hydrogen bonding network (see Fig.4). It is interesting to note that in the crystal structure of inclusion complex of CA with aniline,10 the host molecules are self-assembled in a hh–tt multibilayer as normally observed in many other CA inclusion complexes.2 Whereas, structurally similar AP induces a completely different supramolecular host architecture. AP is structurally similar but chemically different from aniline. Moreover, two fully occupied water molecules are also present in the crystal lattice. These facts might have played a crucial role in constructing the unusual supramolecular architecture of the resulting lattice inclusion complex. In the crystal structure of the cholic acid–aniline complex,10 the nitrogen atom of aniline, being the only basic group, forms hydrogen bonds with both of the oxygen atoms of –COOH of CA.Whereas in AP, the ring nitrogen N(2) is more basic than N(1) i.e. amino nitrogen and therefore, N(2) forms a strong hydrogen bonding interaction with the –COOH group of CA at a distance N(2)···O(4) = 2.631 Å. On the other hand, N(1) forms two hydrogen bonding interactions: one between one of its hydrogens and O(1) and the other with a proton on one of the two fully occupied water molecules at distances N(1)···O(1) = 2.954 Å and O(1W)···N(1) = 2.884 Å, respectively. This network actually bridges between symmetry related helical chains of CA. Two fully occupied water molecules also display interesting hydrogen bonding network.Beside hydrogen bonding one of its protons with N(1) (see above), the water molecule H2O(1W) also associates its other proton with a symmetry related CA molecule and interacts with a hydrogen from the other water molecule, namely H2O(2W), at distances O(1W)···O(2) = 2.814 Å and O(2W)···O(1W) = 2.732 Å, respectively. H2O(2W) also links with two other symmetry related CA molecules by intermolecular hydrogen bonding at distances O(1)···O(2W) = 2.648 Å and O(2W)···O(1) = 2.712 Å. This extensive network of hydrogen bonding also acts as bridge between symmetry related helical chains of CA. Conclusions To the best of our knowledge, this type of helical network of CA in its inclusion complexes has not been observed previously even though other types of helical network are reported for CA, other bile acids and their derivatives.2 The size, shape and chemical nature of guests play significant roles in shaping the supramolecular architecture of the resulting inclusion solid.In the present study, AP with multifunctional hydrogen bonding groups (i.e., the ring nitrogen as a hydrogen bonding acceptor and the amino group as both donor and acceptor) and two water molecules (acting as hydrogen bonding donors as well as acceptors) are actually serving as linkers among the parallel stacks of helical chains of CA via an extensive hydrogen bonding network; thereby occupying the channel space and stabilizing the crystal lattice.We are currently investigating the effect of different pyridine derivatives as guest molecules on the resulting supramolecular arrangement of CA in its inclusion complex. Acknowledgements We sincerely thank Dr P. K. Ghosh and Dr R. V. Jasra of this institute for their keen interest and encouragement in this work. We also thank both of the referees for their comments and suggestions. References 1 P. Dastidar and I. Goldberg, in Comprehensive Supramolecular Chemistry, ed. D. D. MacNicol, F. Toda and R. Bishop, Pergamon Press, Oxford, 1996, vol. 6, ch. 10, p. 305. 2 M. Miyata and K. Sada, in Comprehensive Supramolecular Chemistry, ed. D. D. MacNicol, F. Toda and R. Bishop, Pergamon Press, Oxford, 1996, vol. 6, ch. 6, p. 147. 3 M. Miyata, M. Shibakami, W. Goonewardena and K. Takemoto, Chem. Lett., 1987, 605. 4 K. Miki, A. Masui, N. Kasai, M. Miyata, M. Shibakami and K. Takemoto, J. Am. Chem. Soc., 1988, 110, 6594; M. Shibakami, M. Tamura and A. Sekiya, J. Am. Chem. Soc., 1995, 117, 4499. 5 M. Miyata, M. Shibakami, S. Chirachanchai, K. K. Takemoto, N. Kasai and K. Miki, Nature, 1990, 343, 446. 6 K. Nakano, K. Sada and M. Miyata, J. Chem. Soc., Chem. Commun., 1995, 953. 7 G. M. Sheldrick, SHELXL97, a program for the structure solution and refinement of crystal structure, 1997, University of Gottingen, Germany. 8 J. Lu, T. Paliwala, S. C. Lim, C. Yu, T. Niu and A. J. Jacobson, Inorg. Chem., 1997, 36, 923. 9 C. K. Johnson, ORTEP, 1965, report ORNL-3794, Oak Ridge National Laboratory, TN. 10 M. R. Caira, L. R. Nassimbeni and J. L. Scott, J. Chem. Soc., Chem. Commun., 1993, 612; M. Shibakami and A. Sekiya, J. Chem. Soc., Chem. Commun., 1994, 429. CrystEngComm © The Royal Society of Chemistry 2000
ISSN:1466-8033
DOI:10.1039/b001347l
出版商:RSC
年代:2000
数据来源: RSC