Notes Bull. Korean Chem. Soc. 2013, Vol. 34, No. 4 1243 http://dx.doi.org/10.5012/bkcs.2013.34.4.1243
An Unprecedented Trinodal (3,4,7)-Connected Metal-Organic Framework Containing Trizinc(II) Clusters
Wei-Guo Zhang,* Guang-hua Cui,†,* Shu-lin Xiao,† and Xu Du†
Department of Chemistry, Hebei Normal University of Science and Technology, Qinhuangda 066004, P.R. China
*E-mail: [email protected]
†College of Chemical Engineering, Hebei United University, Tangshan 063009, Hebei, P.R. China. *E-mail: [email protected] Received December 18, 2012, Accepted January 7, 2013
Key Words : Metal-organic framework, 1,2,4-Triazole, Trinuclear Zn(II) clusters
The construction of high-connected metal-organic frame- works (MOFs) with polynuclear metal clusters as secondary building units (SBUs) have attracted much attention due to their intriguing structural diversities and potential applications as functional materials.1,2 However, it is not easy to design and achieve a high-connected SBU, apart from trigonal prism trinuclear M3O-(COO)6 and tetranuclear zinc cluster Zn4O(COO)6 ones up to now.2f Further, the topological analysis of MOFs has been a topical research area not only for the importance of simplifying complicated frameworks of coordination polymers but also for the instructive role in the rational design of some predicted functional materials.
1,2,4-Triazole (Htrz) is an attractive ligand that can bridge metal ions to afford polynuclear compounds with diverse bridging fashions (μ1,2, μ2,4, and μ1,2,4) in the neutral and anionic ligand forms, respectively.3 As has been proved, the mixture ligands of Htrz and rigid carboxylate are capable of constructing 3D fascinating structures with polynuclear metal clusters.2c-e,4 In this work, 5-nitroisophthalic acid (H2nip) was selected as a rigid carboxylate ligand to react with zinc acetate and Htrz ligand under hydrothermal conditions. Successfully, a three-dimensional (3D) MOF {[Zn5(trz)4(nip)3·3H2O]·4H2O}n (1) has been obtained, which contains 7-connected trinuclear Zn(II) clusters and exhibits a unique trinodal (3,4,7)-connected (42·6)(42·64) (44·614·83) framework. To the best of our knowledge, this is the first example of a (3,4,7)-connected MOF containing 7- connected trinuclear Zn(II) cluster.5 Herein, we report its synthesis, crystal structure, thermal stability and fluorescent property.
Experimental Section
Materials and General Methods. All the solvents and reagents for synthesis were commercially available and used as received. Elemental analysis was performed on a Perkin- Elmer 240C analyzer. The IR spectrum was recorded in the 4000-400 cm−1 range using an FT-IR AVATAR 360 (Nicolet) spectrophotometer with KBr pellets. Powder X-ray diffrac- tion measurement was performed on a a Rigaku D/Max- 2500 diffractometer at 40 kV, 100 mA using Cu-Kα radiation (λ = 0.1542 nm) in the 2θ range of 5-50° with a step size of
0.02° and a scanning rate of 10° min−1. Fluorescent spectrum was recorded on a Cary Eclipse fluorescence spectrophoto- meter. Thermogravimetric analysis (TGA) was collected on a NETZSCH TG 209 thermal analyzer from room temper- ature to 800 °C with a heating rate of 10 °C min−1 under nitrogen.
Synthesis of {[Zn5(trz)4(nip)3·(H2O)3]·4H2O}n (1). A mix- ture of 5-nitroisophthalic acid (H2nip, 0.1 mmol, 21.1 mg), 1,2,4-triazole (Htrz, 0.1 mmol, 6.9 mg), Zn(OAc)2 (0.1 mmol, 18.3 mg), and 10 mL of water was sealed in a Teflon-lined autoclave and heated to 140 °C for 3 days under autogenous pressure. Colorless block crystals of 1 were obtained after the autoclave was cooled to room temperature at a rate of 5
°C h–1. Yield: 40% based on Zn. Anal. Calcd for C32H32N15- O25Zn5 (1353.6): C, 28.39; H, 2.38; N, 14.86%. Found: C, 28.67; H, 2.65; N, 14.59%. IR (KBr, cm−1): 3479s, 3110m, 1621s, 1530s, 1463s, 1350s, 1299m, 1180m, 1090s, 916w, 733s, 668m.
Crystallography. X-ray single-crystal diffraction data of 1 was collected on a Bruker Smart APEX-CCD diffracto- meter with graphite-monochromated Cu-Kα radiation (λ = 1.54178 Å) and ω-2θ scan mode at 293 K. The structure of 1 was solved by direct methods and refined on F2 by full- matrix least-squares technique using Bruker’s SHELXTL program package.6 All non-hydrogen atoms were located in difference Fourier maps and refined anisotropically. The hydrogen atoms of organic ligands were generated theoreti- cally onto the specific atoms and refined isotropically. In the structure of 1, four lattice water molecules were disordered and were refined by the SQUEEZE routine of the PLATON program.7 The unligated water molecules occupy a solvent- accessible incipient space comprising 17.8% of the unit cell volume. The crystallographic data for 1 is summarized in Table S1, and the selected bond lengths and angles are listed in Table S2.
Results and Discussion
Single crystal X-ray diffraction reveals that structure of 1 can be described as a dense three-dimensional framework that is built from alternating Zn polyhedra and trz molecules.
Complex 1 crystallizes in the monoclinic system with space
1244 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 4 Notes
group P21. The asymmetrical unit contains five crystallo- graphically independent Zn(II) ions, four μ3-trz– ligands, three coordinated water molecules, one μ2-η1:η1-nip2− anion, and two μ3-η2:η1-nip2− anion (Scheme 1). As shown in Figure 1, Zn1, Zn5 and Zn3 display similar four-coordinated slightly distorted tetrahedral geometries, Zn2 has an octa- hedral geometry and Zn4 adopts a distorted tetragonal pyramid geometry. Zn1 and Zn3 are both coordinated by two nitrogen atoms from two different trz– ligands and two oxygen atoms from one one μ3-nip2− and one μ2-nip2− anion;
Zn2 is bonded with four nitrogen atoms from four trz– ligands and two oxygen atoms from two distinct μ3-nip2−
anions; Zn4 is completed by two nitrogen atoms from two trz– ligands and three oxygen atoms from one μ3-nip2− anion and two coordinated water molecules; Zn5 is coordinated by two nitrogen atoms from two trz– ligands and two oxygen atoms from one μ3-nip2 anion and one coordinated water molecule. The trz– ligand adopts typical μ1,2,4-coordination mode, connecting three Zn(II) ions, and the nip2− anions employ μ3-η2:η1 and μ2-η1:η1 coordination modes (Scheme
1), liking three Zn(II) ions and two Zn(II) ions, respectively.
The Zn-N distances of 2.001(10)-2.180(9) Å and the Zn-O distances of 1.937(9)-2.268(12) Å are in good agreement with literature values (Table S2).2c-e The average bond lengths around Zn2 (2.159 Å) are somewhat longer than those of other zinc atoms (2.000 Å), and this is consistent with the fact that bond lengths in an octahedral geometry are generally longer than those in other geometries. It is can be observed that the Zn metal sites in 1 exhibit a variety of coordination polyhedra: tetrahedron, tetragonal pyramid, and octahedron.
In 1, μ3-trz– ligands and μ3-, μ2-nip2− anions connect Zn(II) atoms to form 1D [Zn3(trz)4(H2O)3]n2n+ cation chain and 1D anionic [Zn2(nip)3]n2n− chains (Figure S1a-b), re- spectively. That two infinite chains are interconnected in different direction to fabricate a 3D complicated framework (Figure 2). To well understand the architecture of 1, the intricate framework was simplified and analyzed by TOPOS4.0.8 Three Zn(II) atoms (Zn2, Zn3, Zn4) are connected by four μ2-trz and three carboxylate forming a [Zn3(COO)3N8] SBUs. Each Zn1 and Zn5 ions connecting four and three trinuclear Zn(II) clusters, can be regarded as 4-connected node and 3-connected node, respectively, and each trinuclear Zn(II) cluster connecting four Zn1 ions and three Zn5 ions, should be considered as a 7-connected node. The 3-con- nected nodes, 4-connected nodes, and 7-connected nodes are in the ratio of 1:1:1. Therefore, each 7-connected node links four 4-connected nodes and three 3-connected nodes, each 4-connected node and 3-connected node links four and three 7-connected nodes, respectively, generating a novel trinodal (3,4,7)-connected (42·6)(42·64)(44·614·83) topology (Figure 3), and the extended point symbol is (4·4·4·4·6·62·62·62· 62·62·62·63·63·64·64·64·65·65·87·89·814). The resulting (3,4,7)- connected topology is a rare trinodal-connected nets, and has been deposited in the TOPOS database (http://www.topos.
samsu.ru) as cgh3. This framework represents the first (3,4,7)-connected network involving 7-connected trinuclear Zn(II) clusters as building blocks. It is notable that Zhang et al. reported another compound using the same mixed ligands, Scheme 1. View of the coordination mode of nip2 ligand in
complex 1.
Figure 1. Coordination environment of the Zn(II) ions in 1 with 30% thermal ellipsoids. (Symmetry codes: #1 = x−1, y, z;#2 = x+1, y, z).
Figure 2. The 3D network of complex 1. The black, blue, red and bright purple spheres represent the C, N, O, Zn atoms, respectively.
Notes Bull. Korean Chem. Soc. 2013, Vol. 34, No. 4 1245
namely, [Zn4(trz)4(Hnip)2(nip)]n, which was a 3D open frame- work consisting of {[Zn4(trz)4]4+}n layers with (4·82) topo- logy arranged in the –ABAB– stacking sequence and nip pillars.2e That structure is quite different with complex 1 characterized here.
IR Spectrum and XRPD Pattern. The IR spectrum displays characteristic absorption band for water molecule, carboxyl groups of nip2–, and trz– ligand. The strong and broad absorption at 3479 cm−1 corresponds to the O-H group of water molecules. The bands at 3110 cm−1 and 1299, 1269, 1180 cm−1 are related to ν(C-H) and ν(C-N) or ν(N-N) of the trz– ligand, respectively. The strong bands at 1621, 1463 and 1350 cm1 may be assigned to νas(COO) and νsym(COO) of the carboxyl groups of nip2–. The value, Δν[νas(COO)- νsym(COO)], is 158, 271 cm−1 and indicates the presence of the two coordination mode of carboxyl groups of nip.2-9 The IR spectrum also shows characteristic bands at 1530 and 1299 cm−1 for the nitro group. The crystalline phase purity of 1 was evidenced by the similarity of the simulated and experimental XRPD patterns, as shown on Figure 4. The positions of the diffraction peaks in both patterns correspond well, indicating that the phase purity of the as-synthesized
samples.
Thermal Analysis. TGA was performed to gauge the thermal stability of complex 1 (Figure 5). There are two separate weight loss steps in the TGA curve. The first weight loss of 6.04% (calcd 5.33%) occurs from 62.5 °C to 138.9
°C, which is attributed to the loss of free water molecules.
There is a large region of plateau between 138.9 and 371.3
°C, then the framework starts to collapse as the organic ligands decompose from 371.3 °C to 689.6 °C (61.78%
weight loss observed, 64.61% calcd). The residue holds a weight of 32.18% of the total sample (the calculated ZnO weight is 30.06%), indicating that the final product is ZnO.
Fluorescent property. The fluorescent property of 1 was studied in the solid state at room temperature. As shown in Figure 6, the maximum emissions occur at 472 nm (λex = 390 nm) for 1 and 366 nm (λex = 300 nm) for free Htrz ligand. As previously reported, the solid H2nip ligand is nearly nonfluorescent in the range 400-600 nm at room temperature,10 The emission of complex 1 can be assigned to the a red shift of the Htrz ligand’s peak on the MOF formation since the Zn2+ ion is difficult to oxidize or to reduce due to its d10 configuration.2e,11,12
In summary, we have successfully synthesized a novel Figure 3. Schematic view of the trinodal (3,4,7)-connected
(42·6)(42·64)(44·614·83) topology of 1. The trinuclear Zn(II) unit is represented by red balls, the 4-connected Zn1 is represented by green balls, and the 3-connected Zn5 is represented by blue balls.
Figure 4. X-ray powder diffraction patterns of complex 1.
Figure 5. The TG curves of complex 1.
Figure 6. The emission spectra of 1 and the free Htrz ligand.
1246 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 4 Notes
zinc(II) coordination polymer of {[Zn5(trz)4(nip)3(H2O)3]·4H2O}n (1) based on 1,2,4-triazole and 5-nitrosophthalic acid under hydrothermal conditions, which is characterized by IR, elemental analysis, thermal analysis and single crystal X-ray diffraction. Compound 1 represents the first trinodal (3,4,7)- connected network containing 7-connected trinuclear Zn(II) cluster as building block. In addition, 1 exhibits fluorescence at 472 nm in the solid state at room temperature.
Supplementary Material. CCDC number: 912887 for complex 1. The data can be obtained free of charge via http:/
/www.ccdc.cam.ac.uk/deposit (or from the Cambridge Cryst allographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (44)1223-336-033(44); or deposit@ccdc .cam.ac.uk).
Acknowledgments. The publication of this paper was supported by the Korean Chemical Society.
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