Extended Water Tapes of Cyclic Hexamers Encapsulated in the Channels of a Metal Phosphonocarboxylate Network Xian-Ming Zhang,* Rui-Qin Fang, and Hai-Shun Wu School of Chemistry & Material Science, Shanxi Normal University Linfen, Shanxi 041004, China Received March 6, 2005;
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1335-1337
Revised Manuscript Received April 16, 2005
ABSTRACT: Water tapes consisting of fused cyclic water hexamers were formed in channels of a three-dimensional supramolecular framework. The geometrical parameters of water tapes are quite similar to those in ice Ih. The dehydration and rehydration of the title complex are partly reversible. Water is still not fully understood, although it is of fundamental importance in biological and chemical processes.1 Exploration of the possible structures of water clusters, one-dimensional water chains/tapes and twodimensional water sheets, is a step toward understanding the behavior of bulk water or ice.2 A variety of modern theoretical and experimental methods have been employed to characterize water clusters, including dimers, trimers, tetramers, and larger oligomers.3,4 Some of the exciting results obtained such as the single-crystal structural determination of octamers and decamers are yielding important insights into the cooperativity effects in hydrogen bonding, aqueous solvation, and hydrogen-bond network rearrangement dynamics, which promise to enhance our understanding of solid and liquid water behavior.5-7 However, how the discrete water clusters link to form chains, tapes, and layers remains relatively unexplored, although some polymeric water arrays have been reported in supramolecular hosts.8,9 Crystal engineering of microporous metal-organic frameworks has attracted much interest because of zeolite-like materials in molecular sieves, desiccants, ion exchangers, and catalysts. Our strategy toward microporous metalorganic frameworks in this work is based on supramolecular manipulation and control over weak π-π interactions. In this paper, we report a novel metal phosphonate complex [Cd(phen)(Hpppn)]‚4H2O 1 (phen ) 1,10-phenanthroline; pppn ) 3-phosphonopropionate) that has a three-dimensional supramolecular framework with one-dimensional channels that encapsulate water tapes consisting of fused cyclic water hexamers. The importance of this observation can be appreciated by noting that the cyclic hexamer, a higher energy isomer than a cage structure, is one of the prominent morphologies found in computer simulations of liquid water and is the structural motif for ice Ih. In addition, a one-dimensional water tape lies between water clusters and bulk water and shows physical properties closely associated with those of bulk water.8a A hydrothermal reaction of Cd(MeCO2)2‚2H2O, trimethyl 3-phosphonopropionate, and phen at 160 °C for 144 h produced colorless crystals of 1.10 X-ray crystallography reveals11 that 1 has a three-dimensional supramolecular framework formed by clay-like interdigitation of twodimensional layers with one-dimensional channels that encapsulate one-dimensional water tapes consisting of fused cyclic water hexamers. The asymmetric unit in 1 consists of one cadmium atom, one doubly deprotonated 3-phosphonopropionate, one phen, and four lattice water molecules (Figure 1). The cadmium atom in 1 has a * To whom correspondence should be addressed. Tel and Fax: 86-3572051402; e-mail:
[email protected].
Figure 1. Perspective view of the coordination environment of the cadmium atom with 35% thermal ellipsoids.
Figure 2. Perspective views of the two-dimensional layer of 1. For clarity, the carbon atoms of phen groups are omitted.
distorted octahedral geometry, coordinated by four oxygen atoms from three Hpppn groups and two nitrogen atoms from a chelate phen ligand. It should be noted that the O(2) atom is pendent and protonated, and the protonation of O(2) is confirmed by the finding of hydrogen from a Fourier map as well as a long P-O(2) distance of 1.562(3) Å. Each Hpppn group in a µ3 mode bridges three cadmium atoms. The coordination of phen and Hpppn groups to cadmium atoms results in a neutral two-dimensional layer with phen groups alternately decorated on both sides of the layer (Figures 2 and 3). There are aromatic π-π stacking interactions between phen groups of adjacent layers [offset face-to-face distance 3.4 Å], which induce adjacent two-dimensional layers interdigitated in a claylike fashion resulting in a three-dimensional supramolecular network featuring one-dimensional channels sized ca. 7.7
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Figure 6. View of water tape consisting of fused hexamers.
Figure 3. Side view of the three-dimensional supramolecular network showing the claylike interdigitation of layers.
Figure 7. Thermogravimetric analysis curve of 1. Figure 4. View of the 3-D supramolecular network showing 1-D channels.
Figure 5. View of the 3-D supramolecular network showing 1-D channels that ensapsulate water tapes.
× 3.3 Å (Figure 4). Calculation by PLATON12 shows that the channels occupy 24% of the crystal volume. The threedimensional supramolecular network can be viewed as array constructed from parallel ABAB stacking of twodimensional layers via induction of π-π stacking interactions. Interestingly, one-dimensional water tapes consisting of fused water hexamers were formed in the channels as shown in Figure 5 and Figure 6. According to Infantes’ classification,13 the water tapes in 1 have the symbol T6(2), which consists of edge-shared six-membered rings. The hydrogen-bonding association of the four crystallographic distinct water molecules and their equivalents generates two distinct six-membered rings with a chair conformation. However, the pucker degree in the two chair six-membered rings is different. In the R6-a and R6-b rings, the oxygen atoms not shared between rings deviate the plane defined by the four shared water 0.98 and 0.55 Å, respectively. The average O‚‚‚O distances in R6-a and R6-b
are 2.72 and 2.74 Å, which are quite close to the corresponding value of 2.759 Å in ice Ih. The average O‚‚‚O‚‚‚O angle in water tape is 114.9°. The one-dimensional water tapes are hydrogen-bonded to a supramolecular host framework via hydrogen bonds [O2‚‚‚O3W, O2W‚‚‚O4, O2W‚‚‚O5, O3w‚‚‚O3 distances are 2.615(4), 2.750(4), 2.743(4), and 2.779(4) Å, respectively.] The synthesis route of 1 is similar to a recently developed route by us in metal phosphonocarboxylates,14 namely, an in situ reaction of metal source and trialkyl phosphonocarboxylate. The preparation of metal phosphonocarboxylate from trialkyl phosphonocarboxylate not only removes the need to synthesize the corresponding phosphonocarboxylic acid but also ensures the growth of large single crystals suitable for X-ray crystallography. Thermogravimetric analysis of 1 in air shows the following clear and well-separated weight loss steps. As shown in Figure 7, a total weight loss of 14.2% occurred in the range of 80∼130 °C, corresponding to the removal of guest water molecules (13.9% calculated), followed by weight loss of 35.1% in the range of 280-425 °C consistent with the removal of coordinated phen ligands (34.9% calculated). No weight loss in the range 130-280 °C indicates a dehydrated phase formulated as [Cd(phen)(pppn)] is stable under 1 atm pressure below 280°. An X-ray powder diffraction pattern (Figure 8) of dehydrated 1 after calcinations at 130 °C for 6 h is similar to that of as-synthesized 1, indicating the supramolecular host framework can be retained after removal of one-dimensional water tapes. In addition, dehydrated 1 can rehydrate upon exposure to water vapor, and XRPD patterns of hydrated 1 have more similarities to those of the as-synthesized 1. All these observations indicate the dehydration and hydration of 1 are partly reversible. The π-π stacking interactions themselves are weak, but those in 1 induce calylike interdigitation of two-dimensional layers generating a microporous supramolecular framework that encapsulates water tapes of cyclic hexamers. The geometrical parameters of water tapes in 1 are quite similar to those in ice Ih. The formation of water tapes is derived from constraint of
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Figure 8. The XRPD patterns for simulated as-synthesized (A), measured as-synthesized (B), dehydrated (C), and rehydrated (D) samples of 1. Table 1. Geometrical Parameters of Hydrogen Bonds (Å, deg)a D-H‚‚‚A
D‚‚‚A
O1W-H‚‚‚O2W O1W-H‚‚‚O4Wa O3W-H‚‚‚O1W O4W-H‚‚‚O2Wd O4W-H‚‚‚O3W O2-H‚‚‚O3W O2W-H‚‚‚O4b O2W-H‚‚‚O5c O3W-H‚‚‚O3e
2.792(5) 2.703(7) 2.713(6) 2.722(6) 2.742(6) 2.615(4) 2.750(4) 2.743(4) 2.779(4)
∠ D-H-A 164(6) 164(6) 170(5) 172(7) 145(7) 175(4) 173(5) 169(5) 175(5)
a Symmetry codes: (a) 1 - x, 1 - y, 1 - z; (b) x - 1, 1/2 - y, 1/2 + z; (c) 1 - x, y, z; (d) 1 + x, y, z; (e) x, -y + 1/2, z + 1/2.
supramolecular host network and hydrogen-bonding interactions between neighboring water-water and waterOphosphonate. The work also provides a good paradigm of crystal engineering to manipulate microporous supramolecular structure through weak interactions. Acknowledgment. This work was financially supported by National Nature Science Foundation of China (20401011) and Shanxi Nature Foundation for Youth (20041009). Supporting Information Available: Crystallographic data in CIF format is available free of charge via the Internet at http://pubs.acs.org.
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(4) Zubavicus, Y.; Grunze, M. Science 2004, 304, 974; Long, L.S.; Wu, Y.-R.; Huang, R.-B.; Zheng, L.-S. Inorg. Chem. 2004, 43, 3798; Tao, J.; Ma, Z.-J.; Huang, R.-B.; Zheng, L.-S. Inorg. Chem. 2004, 43, 6133. (5) Blanton, W. B.; Gordon-Wylie, S. W.; Clark, G. R.; Jordan, K. D.; Wood, J. T.; Geiser, U.; Collins, T. J. J. Am. Chem. Soc. 1999, 121, 3551; MacGillivray, L. R.; Atwood, J. L. J. Am. Chem. Soc. 1997, 119, 2592; Atwood, J. L.; Barbour, L. J.; Ness, T. J.; Raston, C. L.; Raston, P. L. J. Am. Chem. Soc. 2001, 123, 7192. (6) Liu, K.; Brown, M. G.; Carter, C.; Saykally, R. J.; Gregory, J. K.; Clary, D. C. Nature, 2002, 381, 501; Barbour, L. J.; Orr, G. W.; Atwood, J. L. Nature 1998, 393, 671. (7) Moorthy, J. N.; Natarajan, R.; Venugopalan, P. Angew. Chem., Int. Ed. 2002, 41, 3417. (8) (a) Ma, B.-Q.; Sun, H.-L.; Gao, S. Angew. Chem., Int. Ed. 2004, 43, 1374; (b) Neogi, S.; Bharadwaj, P. K. Inorg. Chem. 2005, 44, 816; (c) Pal, S.; Sankaran, N. B.; Samanta, A. Angew. Chem., Int. Ed. 2003, 42, 1741; (d) Janiak, C.; Scharman, T. G. J. Am. Chem. Soc. 2002, 124, 14010. (9) Liu, K.; Loeser, J. G.; Elrod, M. J.; Host, B. C.; Rzepiela, J. A.; Pugliano, N.; Saykally, R. J. J. Am. Chem. Soc. 1994, 116, 3507; Custelcean, R.; Afloroaei, C.; Vlassa, M.; Polverejan, M. Angew. Chem., Int. Ed. 2000, 39, 3094. (10) A mixture of Cd(MeCO2)2‚2H2O (0.133 g, 0.5 mmol), trimethyl 3-phosphonopropionate (0.098 g, 0.5 mmol), phen (0.108 g, 0.6 mmol), and water (7 mL, 390 mmol) in a mole ratio of 1:1:1.2:780 was stirred and adjusted with NaOH (2 M) solution to pH = 11, then transferred and sealed in a 15-mL Teflon-lined stainless container, which was heated to 160 °C for 144 h. After slow cooling of the sample to room temperature, colourless block crystals of 1 were recovered in 52% yield based on trimethyl 3-phosphonopropionate. Anal: Calc. for 1 C15H21Cd N2O9P: C, 34.87; H, 4.10; N, 5.42. Found: C, 34.67; H, 4.18; N, 5.36. IR, 3463s, 3012w, 2923w, 2422m, 1967w, 1638w, 1579s, 1521m, 1436s, 1291m, 1147s, 1108w, 1053s, 918m, 867s, 744m, 522m. (11) Data collection was performed at 293 K on a Bruker Apex diffractometer (Mo KR, λ ) 0.71073 Å). Multiscan absorption corrections were applied. The structures were solved with direct methods and refined with full-matrix least-squares technique (SHELX-97). Analytical expressions of neutralatom scattering factors were employed, and anomalous dispersion corrections were incorporated. In all cases, all non-hydrogen atoms were refined anistropically. Hydrogen atoms of organic ligands were geometrically placed; hydrogen atoms of water and phosphonate group were located from difference Fourier map and refined with isotropic temperature factors. Crystal data for 1: monoclinic, space group P21/c, Mr ) 516.7, a ) 8.1675(17), b ) 21.554(5), c ) 11.110(2) Å, β ) 93.822(3)°, V ) 1951.4(7) Å3, Z ) 4, Dc ) 1.759 g cm-3, µ ) 1.252 mm-1, F(000) ) 1040, Tmin ) 0.6901, Tmax ) 0.8060, 2θmax ) 54°, S ) 1.056. Final residuals (248 parameters) were R1 ) 0.0356 for 2886 I g 2σ(I) reflections, and wR2 ) 0.0703 for all 4011 unique reflections. (12) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Untrecht University: Utrecht, The Netherlands, 1999. (13) Infantes, L.; Motherwell, S. CrystEngComm 2002, 454. (14) Zhang, X.-M. Eur. J. Inorg. Chem. 2004, 544.
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