Bottom-Up Crystal Engineering toward Nanoporosity Exemplified by a Zinc Carboxylate Coordination Polymer Adopting a Tenorite Analogue Network Topology Kuang-Lieh Lu,* Yen-Fu Chen, Yen-Hsiang Liu, Yi-Wei Cheng, Rong-Tang Liao, and Yuh-Sheng Wen
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 403-405
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan Received March 11, 2004;
Revised Manuscript Received December 8, 2004
ABSTRACT: A bottom-up approach to the design of a coordination polymer, [(K2)Zn(btec)‚8H2O]n, adopting a nanoscopic tenorite (CuO) analogue network topology with an increase in pore size from 0.343 to 1.134 nm is described (Kbtec ) potassium 1,2,4,5-benzenetetracarboxylate). One of the strategies in the crystal engineering design of 3D metal-ligand extended network solids with structurebased properties involves mimicking a naturally occurring mineral network topology.1 Frequently observed examples are diamondoid networks constructed from augmented tetrahedral (T) nodes and spacer ligands.2 The benefits derived from diamondoid network topology such as robustness, a porous architecture, and the favoring of acentric deposition can be exploited to generate porous as well as NLO active coordination networks.2 Some other examples of porous solids derived from decorated T nodes with zeolite-, feldspar-, and quartz-like topologies have also been demonstrated.3 In addition, R-polonium-like network, based on augmented Zn4(O)(O2C)6 octahedral cluster nodes, leads to a robust framework with a very high extraframework porosity.4 A nanoporous framework of NbO-like topology can also be assembled via square-planar Cu2(O2C)4 cluster units and [Ni(CN)4]-based building blocks.5 On the other hand, a combination of two types of decorated nodes such as octahedral and trigonal building units can lead to a rutile-like network.6 The exploition of both tetrahedral and square-planar building blocks can result in a cooperite (PtS)-like framework,7 and tenorite (CuO)-like network. To the best of our knowledge, the CuO-like network has not been reported probably because of challenges in generating distorted T nodes to fulfill the tenorite topology. Selfassembled molecular toolkits (e.g., metal coordination centers, multifunctional organic ligands, metal clusters, and organometallic ligands) not only provide versatile geometrical information and novel electronic properties, but also provide opportunities for the construction of chiral porous networks for enantiomeric separations that are achieved with difficulty by an inorganic zeolite.3d,8 In our continuing interests in the design and synthesis of functional coordination polymers,2k,9 we report herein the synthesis and characterization of a porous 3D polymeric network, [(K2)Zn(btec)‚8H2O]n (1) that adopts a tenorite topology (Kbtec ) potassium 1,2,4,5-benzenetetracarboxy-
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Figure 1. (a) The coordination environment of the Zn center in 1 has a distorted pseudotetrahedral geometry (angles: C(2)‚‚‚ Zn(1)‚‚‚C(2B), 87.1°; C(1C)‚‚‚Zn(1)‚‚‚C(2B), 120.6°; C(1C)‚‚‚ Zn(1)‚‚‚C(2), 124.0°; C(1B)‚‚‚Zn(1)‚‚‚C(1C), 85.0°); (b) unit cell packing diagram of CuO; (c) unit cell packing diagram of 1.
late). Our strategy lies in the recognition that rich metal carboxylate coordination chemistry may provide useful geometrical tools to meet the above challenge. Compound 1 was obtained by the diffusion of an aqueous solution of Kbtec into an ethanol solution containing Zn(ClO4)2‚6H2O at room temperature. Colorless needlelike crystals of 1 were formed during a 3-day period. Singlecrystal X-ray diffraction analysis reveals that the Zn centers reside on the crystallographic 2-fold symmetry site (S. O. F. ) 0.5), and the asymmetric unit contains onehalf of a Zn ion, one-half of a btec ligand, one K ion, and a total of four disordered guest water molecules.10 The btec ligand serves as an expanded square-planar node connecting to the four Zn centers, and each zinc(II) center is coordinated by four monodentate carboxylate groups. If the
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Figure 2. The ball-and-stick and space-filling drawings of 3D porous network of 1. Rhombic channels are observed along the c-axis (Key: pink, Zn; yellow, K; red, O; gray, C; cyan, guest water molecules).
the Zn(O2C)4 T node adopts an analogue site as an O atom in the CuO net, and the barycenter of the btec ligand adopts an analogue site as a Cu atom in the CuO net. Consequently, the combination of the elongated btec squareplanar nodes and distorted Zn(O2C)4 T nodes gives rise to a nanoscopic network for 1 that is topologically analogous to the condensed, angstrom-scale tenorite net. The 3D porous network of 1 is shown in Figure 2. There is about a 53% extraframework volume per unit cell.12 Rhombic channels with a Zn-to-Zn cross section of 1.134 × 1.538 nm (the unit cell dimensions) are observed along the crystallographic c-axis, which is occupied by potassium ions and guest water molecules. The porous nature of 1 was analyzed by heating sample of 1 at 180 °C for 90 min, and then naturally cooling it to room temperature by storing it in a chamber saturated with moist air for 48 h. The thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) study showed that compound 1 lost guest water molecules below 180 °C with a phase transition (Figure 3). However, its original network topology recovered after the readsorption of water molecules. In summary, rich metal carboxylate coordination chemistry provides a feasible way to fulfill the geometrical requirement in the bottom-up approach design of a nanoscale porous network adopting an angstrom-scale, condensed CuO analogue topology. Further extension of this approach to the synthesis of other functional polymeric networks is currently underway. Acknowledgment. We are grateful for funding support from Academia Sinica and National Science Council, Taiwan. Figure 3. PXRD diagram of (a) de-guest 1 measured at 180 °C, and (b) a heated sample of 1 after storage in a chamber saturated with moist air at room temperature for 48 h. (c) Simulated PXRD of 1; (d) TGA diagram of 1.
carboxylate groups are treated as one connecting point, the Zn(O2C)4 moiety represents a distorted, augmented pseudotetrahedral geometry (Figure 1a). The tetrahedral angles formed by the Zn(O2C)4 moiety range from 85 to 124°. In a careful comparison of the unit cell packing between CuO (Figure 1b)11 and compound 1 (Figure 1c), both CuO and 1 are crystallized in the form of a monoclinic crystal system with a space group of C2/c. In particular, the Zn center of
Supporting Information Available: X-ray crystallographic files in CIF format for 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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