Recognition of Water-Acetonitrile-Water Cluster in a Tetraprotonated Picrate Salt of Octaaminocryptand I. Ravikumar, P. S. Lakshminarayanan, Eringathodi Suresh,* and Pradyut Ghosh* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute (CSIR Laboratory), BhaVnagar 364002, India
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2630-2633
ReceiVed June 16, 2006; ReVised Manuscript ReceiVed October 13, 2006
ABSTRACT: Octaamino cryptand L (1,4,11,14,17,24,29,36-octaazapentacyclo[12.12.12.2.6,92.19,22231,34]tetratetraconta-(43),7,9(44), 19(41),20,22(42),31(39),32,34(40)-nonaene, N(CH2CH2NHCH2-p-xylyl-H2NHCH2CH2)3N), in tetraprotonated form shows encapsulation of a water-acetonitrile-water cluster (1), whereas a molecule of water encapsulation inside the cryptand cavity is observed in the triprotonated state (2), with picrate as counteranions. These two compounds were examined crystallographically. Thermal analysis supports the crystallographic findings. Macropolycycles of appropriate binding sites and intramolecular cavities can be synthetically constructed to make them suitable hosts for selective guest inclusion.1 Cryptand as a host can recognize water2 molecules inside the cavity in addition to their popularity as a receptor for cations1 and anions.3 Encapsulation of several solvent molecules inside the cavity may facilitate the formation of a solvent cluster. Studies on such clusters are of considerable current interest, as they have been implicated in several contemporary problems.4 In recent years, a large number of water cluster systems have been characterized by X-ray crystallography;5 however, this is not so in the case of binary solvent systems such as acetonitrilewater. Acetonitrile is one of the aprotic solvents that are miscible in water in any ratio. Acetonitrile-water solvents are widely used in liquid chromatography, solvent extraction, organic synthesis, and electrochemistry. Several experimental and theoretical solution-state studies have been conducted on the physiochemical properties of acetonitrile-water mixtures.6 Studies by Bowman-James et al. and us have shown that azacryptand L, upon hexaprotonation, can become a good host for anions or anions with a molecule of water.7 It has also been observed that partial protonation of L can be suitable for multiple water molecules inside the cavity.7e Therefore, it is possible to tune the guest inclusion properties of L by altering the number of protonations as well as the counteranion. Herein, we report structural evidence of an encapsulated water-acetonitrile-water cluster in a tetraprotonated picrate salt of cryptand L (tritopic complex), whereas monotopic water encapsulation is observed in the triprotonated state.8 To the best of our knowledge, this is the first example of water-acetonitrile-water cluster encapsulation in a cryptand, though there is one such water-acetonitrile-water cluster structure available upon CSD search (refcode NUCXUE), where the two water molecules are coordinated to a metal center.9The macrobicyclic cryptand L was synthesized according to literature procedures.10 Complexes 1 and 2 were obtained by reacting L with picric acid in acetonitrile and toluene, respectively, and crystallizing in their respective moist solvents.11 Complex 1, [H4L(H2O)2CH3CN][(picrate)4]‚2.5CH3CN, crystallizes in tetraprotonated form, with the L containing 2 waters and 3.5 acetonitriles as the solvent of crystallization. Both of the water molecules (O29 and O30) are encapsulated at the two ends of tren units via O-H‚‚‚N hydrogenbonding interactions with protonated L, as depicted in Figure 1. These water molecules are further involved in intramolecular O-H‚ ‚‚N interactions with an acetonitrile positioned between the water molecules in a cascade fashion.7d Both oxygen atoms O29 and O30 are involved as donors in the above O-H‚‚‚N interactions, with O‚‚‚N distances ranging from 2.729(4) to 2.875(4) Å and the O-H‚ ‚‚N angles varying from 139(4) to 173(4)°. In the cavity, O29 and * Corresponding author. E-mail:
[email protected] (P.G); sureshe123@ rediffmail.com (E.S). Fax: 91-278-2426970.
Chart 1. Octaaminocryptand L; Tetraprotonated L Showing Water-Acetonitrile-Water Cluster Encapsulation, 1 (Tritopic Complex); Triprotonated L Encapsulating a Molecule of Water 2 (Monotopic Complex)
O30 further act as acceptors, and each of them is involved in two strong intramolecular N-H‚‚‚O interactions with the amino nitrogen of L with N‚‚‚O distances ranging from 2.774(4) to 2.832(4) Å and N-H‚‚‚O angles varying from 165(4) to 177(4)°. Details of all intramolecular hydrogen-bonding interactions between the encapsulated water-acetonitrile-water cluster and the tetraprotonated cryptand moiety in the cascade complex 1 are shown in Table 1. In the water-acetonitrile-water cluster, O29 and O30 are almost equidistant from their respective bridgehead amines, at distances of 3.02 and 3.01 Å, which are less than the sum of the van der Waals radii of nitrogen and oxygen (3.05 Å).12 The central N22 atom of the cluster is at distances of 5.67 and 5.54 Å from respective bridgehead nitrogen atoms, suggesting that the cluster is residing almost at the center of the receptor cavity. Interactions of the tetraprotonated L with the surrounding picrate, making various hydrogen bonds, is depicted in Figure 2. Each cryptand unit is in good hydrogen-bonding contact (via C-H‚‚‚O and N-H‚‚‚O) with eight picrate molecules. The phenolate oxygen is involved in N-H‚‚‚O hydrogen bonding with the amino nitrogen atoms. O1 is making contact with H5D, O8 with H3D, O15 with H8C, and O22 with H2D, respectively, and O8 makes an additional C-H‚‚‚O contact with H35B. The nitro oxygen atom O9 and O16 are making two C-H‚‚‚O contacts each with the methylene hydrogen atoms H32 and H34B and H3B and H34A, respectively,
10.1021/cg0603624 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/02/2006
Communications
Crystal Growth & Design, Vol. 6, No. 12, 2006 2631
Figure 2. H-bonding interaction of the picrate anions surrounding the tetraprotonated L via C-H‚‚‚O and N-H‚‚‚O contacts in complex 1.
Figure 1. PLATON diagram depicting the encapsulation of the wateracetonitrile-water cluster inside the cryptand cavity (a) showing various hydrogen-bonding interactions (via N-H‚‚‚O and O-H‚‚‚N) with the tetraprotonated L in complex 1; (b) view of complex 1 down the bridgehead nitrogen atoms. Table 1. Hydrogen-Bonding Interactions of Solvent Molecules of 1 interaction
H‚‚‚A (Å)
D‚‚‚A (Å)
2σ(I), and R1 ) 0.1177, wR2 ) 0.2079, GOF ) 0.976 for all 18987 reflections. (a) Spek, A. L. PLATON-97; University of Utrecht: Utrecht, The Netherlands, 1997. (b) Mercury 1.3, supplied with Cambridge Structural Database; CCDC: Cambridge, UK, 2003-2004. (c) Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, C34.
Crystal Growth & Design, Vol. 6, No. 12, 2006 2633 (9) Archibald, J. S.; Blake, J. A.; Parsons, S.; Schroder, M.; Winpenny, P. E. R. J. Chem. Soc., Dalton Trans. 1997, 173. (10) Chen, D.; Martell, A. E. Tetrahedron Lett. 1991, 47, 6895. (11) See the Supporting Information. (12) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. In Inorganic Chemistry: Principles of Structure and ReactiVity, 4th ed.; Harper Collins College Publishers: New York, 1993; p 292.
CG0603624
