DOI: 10.1021/cg100226u
An Unusual Three-Dimensional Water Cluster in Metal-Organic Frameworks Based on ZnX2 (X = ClO4, BF4) and an Azo-Functional Ligand
2010, Vol. 10 2054–2056
Chuan-Ming Jin,*,† Zhu Zhu,† Zhan-Fen Chen,† Yan-Jun Hu,† and Xiang-Gao Meng*,‡ †
Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, College of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi, Hubei 435002, P. R. China, and ‡ Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, Central China Normal University, Wuhan 430079, P. R. China Received February 15, 2010; Revised Manuscript Received March 11, 2010
ABSTRACT: A highly symmetrical three-dimensional (3D) water network containing cage-shaped tetradecameric water cluster units (H2O)14 is observed in the crystalline 3D metal-organic framework materials 1 and 2. This is a rare demonstration of a 3D water clathrate network trapped in a cavity of a 3D metal-organic framework, and there are no classical hydrogen bonding interactions within the skeleton of the metal-organic framework. Recently, considerable attention has been focused on the structural investigations of water molecules, including both theoretical and experimental studies, not only because of the diversity of water patterns in nature but also because of the fundamental importance of water for human life and in biological and chemical processes.1-3 A variety of discrete water clusters, including hexamers, octamers, decamers, etc., have been identified, and significant progress has been made in terms of characterizing one-dimensional (1D) water chains and tapes.4-6 Some two-dimensional (2D) water/ice layers containing large water rings have also been obtained.7,8 However, three-dimensional (3D) water clathrate networks have rarely been reported. In fact, the structure of 3D water clathrates should elucidate novel structural aspects of water and help us to understand the relationship between cluster and bulk water.9 In this paper, we describe a symmetrical 3D water clathrate network containing cage-shaped tetradecameric water cluster units (H2O)14 that is observed in the crystalline 3D metal-organic framework (MOF) materials [Zn3(AP-BIM)6(BF4)6] 3 34H2O (1) and [Zn3(AP-BIM)6(ClO4)6] 3 34H2O (2), wherein ligand AP-BIM is 1,1-bis[(2-phenylazo)imidazol-1-yl]methane (Scheme 1). All compounds were prepared at room temperature from a 2:1 stoichiometric mixture of ZnX2 (X = ClO4, BF4) and ligand APBIM in a mixture of CH3OH and CH2Cl2 by using the solvent diffusion method.10 The products were obtained as an orange-red hexagonal plate crystalline solid in a yield of 74% and 67%. They were then characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single-crystal X-ray crystallography. These analyses showed that 1 and 2 are isomorphous structures which have a 3D MOF and an unusual 3D water clathrate consisting of cage-shaped tetradecameric (H2O)14 units and six independent bridged water molecules. Single-crystal X-ray analysis revealed that they crystallize in the high symmetry cubic space group Ia3d. Their asymmetric unit contains one crystallographically independent Zn2þ ion, two unique AP-BIM ligands, and two uncoordinated ClO4- or BF4- anions. Each Zn(II) center is fourcoordinated by four nitrogen atoms from four different AP-BIM ligands to form a slightly distorted tetrahedron geometry (Figure 1). The N-Zn-N angles vary from 102.04(7)° to 125.64(16)° for 1 and from 102.57(6)° to 124.38(15)° for 2; the Zn-N distances of 2.028(3) A˚ for 1 and 2.029(3) A˚ for 2 are within the range expected for such species.11 Each AP-BIM ligand is coordinated to two ZnII cations. To serve as a bridge ligand, each group of six *To whom correspondence should be addressed. pubs.acs.org/crystal
Published on Web 04/05/2010
Figure 1. Thermal ellipsoids drawn at the 30% probability level shows the tetrahedron coordinated geometry of the Zn(II) center. Symmetry codes: (i) y - 3/4, -x þ 3/4, -z þ 1/4; (ii) y - 3/4, x þ 3/4, -z þ 1/4; (iii) -x, -y þ 3/2, z.
Scheme 1
AP-BIM ligands was linked alternatively to each other by six metal centers, resulting in a 48-membered macrometallacycle (Figure 2a). Each metal center exists in four different 48-membered macrometallacycles to propagate the coordination polymer throughout the 3D MOFs. The tetra-coordinated zinc metalorganic network can be simplified as an lcs topological array (Figure 2b) with long and short Schl€ afli symbols of 6.6.6(2).6(2).6(2).6(2) and 66, respectively. Consequently, the benzene ring of each azophenyl group in bridging AP-BIM ligands pointing into the center of macrometallacycle is alternatively in the up or down mode to form a cavity of about 10.0 8.0 5.8 A˚3 inside each macrometallacycle (Figure 2a). r 2010 American Chemical Society
Communication More interestingly, an unusual 3D water clathrate network containing cage-shaped structural tetradecameric water cluster (H2O)14 units was encapsulated in the 3D MOF (Figure 3a). There is no classical hydrogen bonding between the 3D water clathrate and the organic ligands in the MOF. Figure 3b shows the simplified array between the host metal-organic framework
Figure 2. (a) View of a cavity inside a 48-membered macrometallacycle; (b) the topological array of a metal-organic network.
Figure 3. (a) View of an unusual 3D water clathrate network encapsulated in 3D MOF; (b) the topological array of the host MOF and the 3D water clathrate. The blue and red networks represent the simplified lcs MOF and the 3D bcs water clathrate, respectively.
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and the 3D water clathrate. To the best of our knowledge, this is a rare demonstration of a 3D water clathrate network trapped in a cavity of the MOF and there are no typical hydrogen bonding interactions within the skeleton of the MOF. Most of the previous reported water clusters usually exist within hydrophilic inner walls in MOFs, and there is obvious hydrogen bonding between the water cluster and the -OH, -NH2 groups of organic ligand in the MOF.12-15 A view of a single (H2O)14 cluster unit in compound 1 is shown in Figure 4a. The cage-shaped (H2O)14 unit consists of six fused cyclic hexamers in a book conformation and six cyclic pentamers. This novel configuration of a (H2O)14 cluster has not been found experimentally. The O-O distance ranges from 2.682 to 2.848 A˚, resulting in an average O-O distance of 2.756 A˚, which is similar to that reported for supramolecular (H2O)12 morphology and very close to the corresponding value of 2.759 A˚ in ice Ih.16,17 The Ow-Ow-Ow bond angles vary from 99.99° to 119.76°, which are slightly different from the tetrahedral angle found in ice Ih and Ic.1 It also should be noted that there are only a few structural reports on discrete (H2O)14 clusters.18 The fascinating feature is that a single (H2O)14 cluster links to six other (H2O)14 clusters via hydrogen bonding between the O3 and O1 water-bridged molecules. The O3-O1 distance of 2.894 A˚ is close to the distance of 2.85 A˚ found in liquid water (Figure 4b). Extending to a 3D water clathrate network (Figure 4c), if the (H2O)14 cluster is regarded as a node and O1 as a bridge, then the 3D water clathrate forms a bcs network (Figure 4d), in which the long and short Schl€ afli symbols are 4.4.4.4.4.4.6(4).6(4).6(4).6(9).6(9).6(9).6(9).6(9).6(9) and (46)(69), respectively. The structure of 3D water clathrates in 2 is similar to that of compound 1 (see Supporting Information). The FTIR spectra of 1 and 2 show broad bands centered at 3424 cm-1 and 3410 cm-1 (see Supporting Information). The O-H stretching frequency of the water clathrates in 1 and 2 are comparable to liquid water whose corresponding stretching vibration is 3490 cm-1. In comparison, the O-H stretching frequency of ice appears at 3220 cm-1.17 X-ray powder diffraction (XRPD) peaks measured for 1 and 2 closely match those in the simulated patterns generated from
Figure 4. (a) View of a single (H2O)14 cluster unit; (b) view of one six-connected 3D water clathrate; (c) the 3D water clathrate is sustained mainly by the Ow-Ow hydrogen bonds; (d) the topological array of the 3D water clathrate.
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single-crystal diffraction data, indicating pure phases are formed. Thermal gravimetric analyses show that gradual weight loss begins from room temperature to 95 and 78 °C for a total loss of 18.4% and 16.7%, respectively. This corresponds to the loss of all water molecules (the calculated weight loss values are 17.6% and 17.3%, respectively) in 1 and 2. The complete decomposition of compounds 1 and 2 is achieved at approximately 272 °C (see Supporting Information). These results indicate that the 3D water clathrate is easily removed from the 3D MOF, which is stable up to 272 °C.12b They also confirm that there are not strongly hydrogen bond interactions between the 3D water clathrate and the cavity of host molecule, and that the water clathrate exhibits liquid water properties.19 We also found those single-crystals unchanged over six months at room temperature. To examine whether the crystal lattice of 1 resists the removal of 3D water cluster molecules without collapsing, we heated part of the crystalline material of 1 for 2 days at 110 °C under a vacuum. Subsequent X-ray analysis showed that the unit cell remain unchanged (see Supporting Information), maybe demonstrating that such thermal treatment does not destroy the crystalline nature of the material, or suggesting that the dehydration and reabsorption of the 3D water clathrate in 1 is reversible and fast. The reversible adsorption of water clusters in porous MOFs has been observed by other groups.12,18c In summary, we have characterized a novel and fascinating 3D water clathrate in a cavity within a MOF. This new mode of the association of water molecules (H2O)14 has rarely been predicted theoretically and previously found experimentally. Our results also possibly suggest an example of the formation of water clusters in nature and biological systems. Further studies will focus on the synthesis of other 3D MOFs containing larger cavities and their properties on trapping other larger 3D water clathrates with different sizes and shapes in our lab. Acknowledgment. This work was supported by NSFC (No. 20803019), the Natural Science Foundation of Hubei Province (2009CDB349, 2006ABB038), the Distinguished Young Scholars’ Programs, HBDE (Q200722003, Z201022001, CXY2009B028), and the Science and Technology Foundation for Creative Research Group of HBNU (2009). Supporting Information Available: Experimental section; FTIR spectra of 1 and 2; thermogravimetric analysis and X-ray powder diffraction measurements for 1 and 2; table of crystallographic data and structure refinement for 1, 1 (heated), and 2; view of a single (H2O)14 cluster in compound 2; crystallographic information files. This material is available free of charge via the Internet at http:// pubs.acs.org.
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