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Construction of cadmium(II) and zinc(II) coordination frameworks based on 3,5-bis(benzoimidazo-1-ly)pyridine and carboxylate acids: Synthesis, crystal structures and photoluminescent properties
Inorganic Chemistry Communications 37 (2013) 49–53
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Construction of cadmium(II) and zinc(II) coordination frameworks based on 3,5-bis(benzoimidazo-1-ly)pyridine and carboxylate acids: Synthesis, crystal structures and photoluminescent properties Jiakun Xu a,b,⁎, Xiaochun Sun c, Xingchen Yan b,d, Shuaiyu Wang a, Mi Sun a,⁎ a
Key Laboratory of Sustainable Development of Marine Fisheries, Ministry of Agriculture, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China c National Oceanographic Center, Qingdao 266071, China d Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China b
a r t i c l e
i n f o
Article history: Received 2 June 2013 Accepted 16 September 2013 Available online 25 September 2013 Keywords: 3,5-Bis(benzoimidazo-1-ly)pyridine Hydrothermal synthesis Crystal structure Coordination polymer Luminescence
a b s t r a c t Two new metal coordination polymers [Cd2(ba)4(L)2]n (1) and [Zn(tda)(L)]n (2) [Hba = benzoic acid, H2tda = 2,5-thiophenedicarboxylic acid and L = 3,5-bis(benzoimidazo-1-ly)pyridine] have been successfully synthesized and structurally characterized by IR, elemental analysis, X-ray powder diffraction and X-ray single-crystal diffraction. For complex 1, two Cd(II) ions and four ba ligands form the dimetal clusters, which are further connected through the L ligands to give rise to a uninodal 3-connected hcb Shubnikov hexagonal plane net with the 63 topology. Complex 2 features the 3-connected topological net with 82 ∙ 10 topology (so-called “tongue-and groove” structure). The Zn(II) node together with the four surrounding coordinated ligands form a typical T-shaped molecular bilayer motif, which is rarely reported previously. Their X-ray powder diffraction patterns were then carried out to confirm the validity of the crystal structures. In addition, the luminescent properties of 1 and 2 are investigated in the solid state at room temperature, and the thermogravimetric analyses were carried out to study the thermal stability of the two complexes. © 2013 Elsevier B.V. All rights reserved.
In recent years, considerable attention has been devoted to metal– organic frameworks (MOFs) due to not only their appealing structures but also their diversified applications in gas storage [1], separation [2], luminescence [3], catalysis [4], magnetism [5], drug delivery [6] and sensor [7]. MOFs bearing diverse architectures as well as fascinating topological networks could be rationally designed and assembled by the judicious choice of bridging ligands, and the alterations in molecule size, symmetry, substituent group, and backbone flexibility of the bridging ligands could partly account for the structural and functional diversity [8–11]. Carboxylates and N-containing ligands are among the most important and widely used candidates to construct MOFs. Carboxylates can provide various coordination modes (monodentate, chelating and bismonodentate), while imidazole ligands can be used as a pillar or bridge to ligate metal nodes [12,13]. C.-C. Ji et. al. have synthesized a rarely reported typical T-shaped molecular bilayer unit with an 82 ∙ 10 topology by selecting an unsymmetrical tricarboxylate acid as the linker, with which a 3-D supermolecular framework with rectangle channels is constructed [14]. Differently, we herein adopted another strategy to construct T-shaped module, in which L ligand was utilized as the linker with carboxylate serving as the auxiliary ligand based on the following ⁎ Corresponding authors at: Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China. Tel.: +86 532 85833961. E-mail addresses: [email protected] (J. Xu), [email protected] (M. Sun). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.09.033
two considerations: (1) both of the benzoimidazole rings in the L ligand can twist from the pyridine rings to point to different directions. Thus, the L ligand can serve as the arms of the T shape to connect the metal centers into this structure. (2) The benzoimidazole rings can only spin on the C\N single bonds to limit the flexibility of the linker, which would make the linkers point to the vertical directions rather than in the disordered state. We firstly selected Hba as the secondary ligand, but failed to construct the T shaped structure because the ba ligand is inclined to adopt the bidentate coordinating mode and contributed less to connect the framework. Finally, the T shaped structure has been successfully constructed by selecting H2tda as the secondary ligand. The two complexes were characterized by IR spectra, elemental analysis, X-ray powder diffraction and X-ray crystallography [15]. Furthermore, the luminescent properties of the L ligand and complexes 1 and 2 were studied. The crystallographic data and selected bond lengths and angles are given in the Supporting Information (Table S1 and Table S2). As shown in Fig. 1a, the asymmetric unit contains two Cd(II) ions, two L ligands and four ba ligands. Cd1 is seven coordinated by five O atoms from three carboxylate, one N atom from the pyridyl ring of an L ligand [Cd1–N3 = 2.411(9) Å], and one N atom from the imidazole ring of another L ligand [Cd1–N6 = 2.262(8) Å]. Cd2 is also coordinated by five carboxylate O atoms, one N atom from the pyridyl ring of an L ligand [Cd2–N2b = 2.261(9) Å], and one N atom from the imidazole ring of another L ligand [Cd2–N8 = 2.398(9) Å]. Interestingly, two of the ba
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Fig. 1. (a) The coordination environments for Cd atoms in complex 1 (all hydrogen atoms are omitted for clarity), symmetry codes: [a] -x+1, y+1/2, -z+1/2; [b]: x+1, y, z; [c] -x+1, y-1/2, -z+1/2; (b) the 2D structure for complex 1. (c) The schematic representation of the 2D framework for complex 1. (d) The schematic representation of the 3-connected 3D framework with 63 topology for complex 1.
ligands connect the Cd atom by their carboxylate groups with μ2–η2:η1 bridging mode [Cd1–O1 = 2.375(8) Å, Cd1–O2 = 2.510(9) Å, Cd1– O5a = 2.532(8) Å, Cd2–O5 = 2.380(8) Å, Cd2–O6 = 2.508(9) Å, Cd2–O1c = 2.563(8) Å], while two of the ba ligands connect the Cd atom by their carboxylate groups with μ2–η1:η1 bridging mode [Cd1–O3 = 2.318(9), Cd1–O4 = 2.396(9), Cd2–O7 = 2.418(9) and Cd2–O8 = 2.331(10)]. Two Cd(II) centers are bridged by two carboxylate ligands to form a bimetal cluster. Meanwhile, the L ligands can be viewed as the line linkers. Each L ligand connects two bimetal clusters with the pyridyl ring and the imidazole ring, respectively, forming a 1-D zigzag chain along the a axis. These 1D chains are further connected by the L ligands to give rise to a 2-D framework net (Fig. 1b). On the basis of the simplification, complex 1 possesses a uninodal 3-connected hcb Shubnikov hexagonal plane net (Fig. 1c). The topology of complex 1 calculated by TOPOS program [16] suggests the 3-connected net with the Point (Schläfli) symbol 63 (Fig. 1d). The coordination environment of Zn(II) center in complex 2 is illustrated in Fig. 2a. The Zn(II) ion situates in the center of the complex with two O atoms belonging to two molecules of tda [Zn1–O2 = 1.9041(15) Å, Zn1–O2a = 1.9041(15) Å] and two N atoms from L [Zn1–N1 = 2.0050(15) Å, Zn1–N1a = 2.0050(15) Å] occupying each vertex of the pyramid, forming a slightly distorted tetrahedron geometry. The tda ligands adopt the bis-monodentate coordinating modes, through which two adjacent Zn centers are connected to form a 16-membered ring
(Zn2C8O4S2) with a Zn⋯Zn distance of 7.927 Å (Fig. 2b). In addition, two L ligands link one Zn(II) ion in the bimetal unit in one direction, while two L ligands connect another Zn(II) ion in the bimetal unit in another direction. Each L ligand is coordinated to two Zn(II) ions with the imidazole N atoms. As both the two imidazole rings in the L ligand can twist away from the central pyridyl ring, the L ligands can point to different directions through the spin between them. In this particular case, the dihedral angles between the central pyridyl ring and the two imidazole rings are both 41.21°, making the L ligands which coordinate to the two Zn(II) ions in the bimetal cluster point to the two vertical directions. Thus, a typical T-shaped molecular bilayer motif, which is rarely reported, is successfully constructed directed by this consideration (Fig. 2c). The T-shaped structure of complex 2 can be described as the type in which the metal ions serve as the three-connecting nodes while two L ligands serve as the arms and the 16-membered rings formed by two tda serve as the legs, and the arms connect the legs in two vertical directions to form the 2D framework net (Fig. 2d) [14]. The 2D adjacent subunits are further polycatenated with each other to give rise to a uniform 3-connected net with Point (Schläfli) symbol of 82 ∙ 10, which represents a KIa type topology (Fig. 2e). Similar examples are often found if all nodes are 3-coordinated [17]. The simulated and experimental X-ray powder diffraction patterns of 1 and 2 are shown in Fig. 3. All the peaks presented in the measured curves approximately match the simulated curves generated from
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Fig. 2. (a) The coordination environment for Zn(II) atoms in complex 2 (all hydrogen atoms are omitted for clarity), symmetry codes: [a] −x + 1, −y + 1/2, z; (b) the structure of dinuclear Zn(II) unit linked by carboxylate ligands; (c) the 2D structure and the packing diagram for complex 2; (d) the 2D topological net for complex 2; (e) the 3D topological net for complex 2.
single-crystal diffraction data, which clearly confirms the phase purity of the as-synthesized products. The fluorescence emission spectra of 1, 2 and the L ligand were measured in the solid state at room temperature. Intense luminescence emission bands of 1 and 2 are observed at 349 nm (λex = 280 nm) and 335 nm (λex = 280 nm), respectively (Fig. 4), while the L ligand itself has an emission band at 352 nm (λex = 280 nm). Although the complexes share the same maximum excitation wavelength with L ligand, apparent changes of the emission spectra after coordination were observed, and coordination with the metal ions makes the emission spectrum of L ligand slightly shift toward lower wavelength,
which should be associated with the obvious structural difference. The luminous mechanism for complexes 1 and 2 might be intraligand fluorescent emission (π–π*) [18]. To investigate the thermal stability of the two complexes, TG analyses were carried out with a Netzsch STA409PC instrument at a heating rate of 10 °C min−1 from 20 °C to 900 °C under nitrogen atmosphere (flow rate = 60 mL min−1). As shown in Fig. 5, the TG curve of complex 1 indicates that complex 1 is stable up to 240 °C, where the decomposition of the framework starts. A rapid and significant weight loss of 81.05% (calcd. 80.72%) is observed in the temperature range of 240–900 °C, which could be assigned to the complete decomposition
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Fig. 5. The thermogravimetric analysis (TGA) curves for complexes 1 and 2.
of the L ligand. Similar to those reported in corresponding references [19–23], the carboxylate ligands coordinate to metal ions by monodentate, bidentate and bis-monodentate modes. Complex 1 possesses a uninodal 3-connected hcb Shubnikov hexagonal plane net with the 63 topology, while complex 2 features the 3-connected topological net with 82 ∙ 10 topology (so-called “tongue-and groove” structure). By strategically selecting the ligands, we have successfully constructed the rarely reported T shape module in complex 2. Their crystal structure validities have been confirmed by comparing the simulated X-ray powder diffraction patterns with the experimental ones. In addition, the complexes 1 and 2 also exhibit intense emission in the solid state at room temperature. Compared with the L ligand, both of their excitation and emission peaks have changed due to the structural differences. Acknowledgment
Fig. 3. The XRPD for (a): complex 1; (b) complex 2 in the solid state at room temperature.
of the ligands. In comparison with complex 1, complex 2 is stable up to 400 °C. The weight loss of 31.40% (calcd. 31.11%) during the temperature range of 400–470 °C could be attributed to the complete decomposition of the tda ligand, while the weight loss during the temperature range of 470–900 °C could be assigned to the decomposition of the L ligand. However, complex 2 does not decompose completely under the experimental temperature. In summary, we have successfully constructed two new coordination polymers [Cd2(ba)4(L)2]n (1) and [Zn(tda)(L)]n (2) through selfassemblies of rigid carboxylate with Cd(II) and Zn(II) in the presence
We thank the support of this work by NSFC (No. 31200642), Special Scientific Research Funds for Central Non-profit Institutes, Chinese Academy of Fishery Sciences (No. 2013A1002) and Qingdao Municipal Science and Technology Plan Project (No. 12-1-4-12-(2)-jch). Appendix A. Supplementary material Crystallographic data have been deposited at the Cambridge Crystallographic Data Center as supplementary publications (CCDC-918009 and 919391) containing the supplementary crystallographic data for 1–2. The data can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: [email protected]. Additional experimental methods, elemental analysis results, IR absorption peaks, crystallographic data (Table S1), and the selected bond lengths and angles (Table S2) are available as electronic supplementary information in the online version. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.09.033. References
Fig. 4. Comparison of the fluorescent emission spectra for complexes 1 and 2 with the L ligand in the solid state at room temperature.
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J. Xu et al. / Inorganic Chemistry Communications 37 (2013) 49–53 [6] R.C. Huxford, J.D. Rocca, W. Lin, Metal-organic frameworks as potential drug carriers, Curr. Opin. Chem. Biol. 14 (2010) 262–268. [7] G. Lu, J.T. Hupp, Metal-organic frameworks as sensors: a ZIF-8 based fabry-perot device as a selective sensor for chemical vapors and gases, J. Am. Chem. Soc. 132 (2010) 7832–7833. [8] C.H. Ke, G.R. Lin, B.C. Kuo, H. ManLee, Coordination polymers with bulky bis(imidazole) and aromatic carboxylate ligands: diversity in metal-containing nodes and threedimensional net topologies, Cryst. Growth Des. 12 (2012) 3758–3765. [9] X.P. Zhou, M. Li, J. Liu, D. Li, Gyroidal metal–organic frameworks, J. Am. Chem. Soc. 134 (2012) 67–70. [10] L. Schlechte, V. Bon, R. Grünker, N. Klein, I. Senkovska, S. Kaskel, Structural diversity of cobalt(II) coordination compounds involving bent imidazole ligand: a route from 0D dimer to 3D coordination polymer, Polyhedron 44 (2012) 179–186. [11] F. Guo, F. Wang, H. Yang, X.L. Zhang, J. Zhang, Tuning structural topologies of three photoluminescent metal–organic frameworks via isomeric biphenyldicarboxylates, Inorg. Chem. 51 (2012) 9677–9682. [12] J.K. Xu, X.C. Sun, Y.H. Fan, C.F. Bi, M. Sun, Hydrothermal synthesis of five new coordination polymers based on benzophenone-2, 4′-dicarboxylic acid and N-donor spacers, Z. Anorg. Allg. Chem. 638 (2012) 1512–1518. [13] S.S. Chen, Y. Zhao, J. Fan, T. Okamura, Z.S. Bai, Z.H. Chen, W.Y. Sun, Construction of coordination frameworks based on 4-imidazolyl tecton 1,4-di(1H-imidazol-4-yl) benzene and varied carboxylic acids, CrystEngComm 14 (2012) 3564–3576. [14] C.C. Ji, J. Li, Y.Z. Li, Z.J. Guo, H.G. Zheng, Solvothermal synthesis, structures and physical properties of four new complexes constructed from multi-variant tricarboxylate ligand and pyridylbased ligands, CrystEngComm 13 (2011) 459–466. [15] Crystal structure measurements for complexes 1–2: Diffraction intensity data of the single crystals of complexes 1 and 2 were collected on a Bruker SMART APEXII CCD diffractometer equipped with a graphite monochromated Mo–Kα radiation (λ = 0.71073 Å) by using a ω-scan mode. Empirical absorption correction was applied using the SADABS programs. All the structures were solved by direct methods
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and refined by full-matrix least-squares methods on F2 using the program SHEXL 97. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located by geometrically calculations, and their positions and thermal parameters were fixed during the structure refinement. V.A. Blatov, TOPOS, a multipurpose crystallochemical analysis with the program package, Samara State University, Russia, 2009. T.G. Mitina, V.A. Blatov, Topology of 2-periodic coordination networks: toward expert systems in crystal design, Cryst. Growth Des. 13 (2013) 1655–1664. X.L. Sun, Z.J. Wang, S.Q. Zang, W.C. Song, C.X. Du, A series of Cd(II) and Zn(II) coordination polymers with helical subunits assembled from a versatile 3-(4hydroxypyridinium-1-yl) phthalic acid and N-donor ancillary coligands, Cryst. Growth Des. 12 (2012) 4431–4440. F.H. Zhao, Y.X. Che, J.M. Zheng, Three 2D complexes with helical chains constructed from pimelic acid and rigid bis(imidazole) ligand: syntheses, structures, thermal and photoluminescent properties, Inorg. Chem. Commun. 17 (2012) 99–103. Y. Wang, F.H. Zhao, Y.X. Che, J.M. Zheng, A 3D photoluminescent Cd(II) polymer based on mixed 3,5-bis-oxyacetate-benzoic acid and rigid bis(imidazole) ligands with an unusual (4,8)-connected topology, Inorg. Chem. Commun. 17 (2012) 180–183. H. Qu, L. Qiu, X.K. Leng, M.M. Wang, S.M. Lan, L.L. Wen, D.F. Li, Structures and photocatalytic activities of metal-organic frameworks derived from rigid aromatic dicarboxylate acids and flexible imidazole-based linkers, Inorg. Chem. Commun. 14 (2011) 1347–1351. R.H. Cui, Y.H. Xu, Z.H. Jiang, Syntheses, structures, and photoluminescence of two Cd(II) coordination polymers constructed from analogue (pyridyl)imidazole derivative and polycarboxylate acid, Inorg. Chem. Commun. 12 (2009) 933–936. J.D. Lin, C.C. Jia, Z.H. Li, S.W. Du, Syntheses, topological analyses and magnetic properties of two 3D supramolecular nickel–organic frameworks constructed from 1,3,5-benzenetricarboxylate and flexible imidazole-based ligands, Inorg. Chem. Commun. 12 (2009) 558–562.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Construction of cadmium(II) and zinc(II) coordination frameworks based on 3,5-bis(benzoimidazo-1-ly)pyridine and carboxylate acids: Synthesis, crystal structures and photoluminescent properties Jiakun Xu a,b,⁎, Xiaochun Sun c, Xingchen Yan b,d, Shuaiyu Wang a, Mi Sun a,⁎ a
Key Laboratory of Sustainable Development of Marine Fisheries, Ministry of Agriculture, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China c National Oceanographic Center, Qingdao 266071, China d Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China b
a r t i c l e
i n f o
Article history: Received 2 June 2013 Accepted 16 September 2013 Available online 25 September 2013 Keywords: 3,5-Bis(benzoimidazo-1-ly)pyridine Hydrothermal synthesis Crystal structure Coordination polymer Luminescence
a b s t r a c t Two new metal coordination polymers [Cd2(ba)4(L)2]n (1) and [Zn(tda)(L)]n (2) [Hba = benzoic acid, H2tda = 2,5-thiophenedicarboxylic acid and L = 3,5-bis(benzoimidazo-1-ly)pyridine] have been successfully synthesized and structurally characterized by IR, elemental analysis, X-ray powder diffraction and X-ray single-crystal diffraction. For complex 1, two Cd(II) ions and four ba ligands form the dimetal clusters, which are further connected through the L ligands to give rise to a uninodal 3-connected hcb Shubnikov hexagonal plane net with the 63 topology. Complex 2 features the 3-connected topological net with 82 ∙ 10 topology (so-called “tongue-and groove” structure). The Zn(II) node together with the four surrounding coordinated ligands form a typical T-shaped molecular bilayer motif, which is rarely reported previously. Their X-ray powder diffraction patterns were then carried out to confirm the validity of the crystal structures. In addition, the luminescent properties of 1 and 2 are investigated in the solid state at room temperature, and the thermogravimetric analyses were carried out to study the thermal stability of the two complexes. © 2013 Elsevier B.V. All rights reserved.
In recent years, considerable attention has been devoted to metal– organic frameworks (MOFs) due to not only their appealing structures but also their diversified applications in gas storage [1], separation [2], luminescence [3], catalysis [4], magnetism [5], drug delivery [6] and sensor [7]. MOFs bearing diverse architectures as well as fascinating topological networks could be rationally designed and assembled by the judicious choice of bridging ligands, and the alterations in molecule size, symmetry, substituent group, and backbone flexibility of the bridging ligands could partly account for the structural and functional diversity [8–11]. Carboxylates and N-containing ligands are among the most important and widely used candidates to construct MOFs. Carboxylates can provide various coordination modes (monodentate, chelating and bismonodentate), while imidazole ligands can be used as a pillar or bridge to ligate metal nodes [12,13]. C.-C. Ji et. al. have synthesized a rarely reported typical T-shaped molecular bilayer unit with an 82 ∙ 10 topology by selecting an unsymmetrical tricarboxylate acid as the linker, with which a 3-D supermolecular framework with rectangle channels is constructed [14]. Differently, we herein adopted another strategy to construct T-shaped module, in which L ligand was utilized as the linker with carboxylate serving as the auxiliary ligand based on the following ⁎ Corresponding authors at: Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China. Tel.: +86 532 85833961. E-mail addresses: [email protected] (J. Xu), [email protected] (M. Sun). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.09.033
two considerations: (1) both of the benzoimidazole rings in the L ligand can twist from the pyridine rings to point to different directions. Thus, the L ligand can serve as the arms of the T shape to connect the metal centers into this structure. (2) The benzoimidazole rings can only spin on the C\N single bonds to limit the flexibility of the linker, which would make the linkers point to the vertical directions rather than in the disordered state. We firstly selected Hba as the secondary ligand, but failed to construct the T shaped structure because the ba ligand is inclined to adopt the bidentate coordinating mode and contributed less to connect the framework. Finally, the T shaped structure has been successfully constructed by selecting H2tda as the secondary ligand. The two complexes were characterized by IR spectra, elemental analysis, X-ray powder diffraction and X-ray crystallography [15]. Furthermore, the luminescent properties of the L ligand and complexes 1 and 2 were studied. The crystallographic data and selected bond lengths and angles are given in the Supporting Information (Table S1 and Table S2). As shown in Fig. 1a, the asymmetric unit contains two Cd(II) ions, two L ligands and four ba ligands. Cd1 is seven coordinated by five O atoms from three carboxylate, one N atom from the pyridyl ring of an L ligand [Cd1–N3 = 2.411(9) Å], and one N atom from the imidazole ring of another L ligand [Cd1–N6 = 2.262(8) Å]. Cd2 is also coordinated by five carboxylate O atoms, one N atom from the pyridyl ring of an L ligand [Cd2–N2b = 2.261(9) Å], and one N atom from the imidazole ring of another L ligand [Cd2–N8 = 2.398(9) Å]. Interestingly, two of the ba
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Fig. 1. (a) The coordination environments for Cd atoms in complex 1 (all hydrogen atoms are omitted for clarity), symmetry codes: [a] -x+1, y+1/2, -z+1/2; [b]: x+1, y, z; [c] -x+1, y-1/2, -z+1/2; (b) the 2D structure for complex 1. (c) The schematic representation of the 2D framework for complex 1. (d) The schematic representation of the 3-connected 3D framework with 63 topology for complex 1.
ligands connect the Cd atom by their carboxylate groups with μ2–η2:η1 bridging mode [Cd1–O1 = 2.375(8) Å, Cd1–O2 = 2.510(9) Å, Cd1– O5a = 2.532(8) Å, Cd2–O5 = 2.380(8) Å, Cd2–O6 = 2.508(9) Å, Cd2–O1c = 2.563(8) Å], while two of the ba ligands connect the Cd atom by their carboxylate groups with μ2–η1:η1 bridging mode [Cd1–O3 = 2.318(9), Cd1–O4 = 2.396(9), Cd2–O7 = 2.418(9) and Cd2–O8 = 2.331(10)]. Two Cd(II) centers are bridged by two carboxylate ligands to form a bimetal cluster. Meanwhile, the L ligands can be viewed as the line linkers. Each L ligand connects two bimetal clusters with the pyridyl ring and the imidazole ring, respectively, forming a 1-D zigzag chain along the a axis. These 1D chains are further connected by the L ligands to give rise to a 2-D framework net (Fig. 1b). On the basis of the simplification, complex 1 possesses a uninodal 3-connected hcb Shubnikov hexagonal plane net (Fig. 1c). The topology of complex 1 calculated by TOPOS program [16] suggests the 3-connected net with the Point (Schläfli) symbol 63 (Fig. 1d). The coordination environment of Zn(II) center in complex 2 is illustrated in Fig. 2a. The Zn(II) ion situates in the center of the complex with two O atoms belonging to two molecules of tda [Zn1–O2 = 1.9041(15) Å, Zn1–O2a = 1.9041(15) Å] and two N atoms from L [Zn1–N1 = 2.0050(15) Å, Zn1–N1a = 2.0050(15) Å] occupying each vertex of the pyramid, forming a slightly distorted tetrahedron geometry. The tda ligands adopt the bis-monodentate coordinating modes, through which two adjacent Zn centers are connected to form a 16-membered ring
(Zn2C8O4S2) with a Zn⋯Zn distance of 7.927 Å (Fig. 2b). In addition, two L ligands link one Zn(II) ion in the bimetal unit in one direction, while two L ligands connect another Zn(II) ion in the bimetal unit in another direction. Each L ligand is coordinated to two Zn(II) ions with the imidazole N atoms. As both the two imidazole rings in the L ligand can twist away from the central pyridyl ring, the L ligands can point to different directions through the spin between them. In this particular case, the dihedral angles between the central pyridyl ring and the two imidazole rings are both 41.21°, making the L ligands which coordinate to the two Zn(II) ions in the bimetal cluster point to the two vertical directions. Thus, a typical T-shaped molecular bilayer motif, which is rarely reported, is successfully constructed directed by this consideration (Fig. 2c). The T-shaped structure of complex 2 can be described as the type in which the metal ions serve as the three-connecting nodes while two L ligands serve as the arms and the 16-membered rings formed by two tda serve as the legs, and the arms connect the legs in two vertical directions to form the 2D framework net (Fig. 2d) [14]. The 2D adjacent subunits are further polycatenated with each other to give rise to a uniform 3-connected net with Point (Schläfli) symbol of 82 ∙ 10, which represents a KIa type topology (Fig. 2e). Similar examples are often found if all nodes are 3-coordinated [17]. The simulated and experimental X-ray powder diffraction patterns of 1 and 2 are shown in Fig. 3. All the peaks presented in the measured curves approximately match the simulated curves generated from
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Fig. 2. (a) The coordination environment for Zn(II) atoms in complex 2 (all hydrogen atoms are omitted for clarity), symmetry codes: [a] −x + 1, −y + 1/2, z; (b) the structure of dinuclear Zn(II) unit linked by carboxylate ligands; (c) the 2D structure and the packing diagram for complex 2; (d) the 2D topological net for complex 2; (e) the 3D topological net for complex 2.
single-crystal diffraction data, which clearly confirms the phase purity of the as-synthesized products. The fluorescence emission spectra of 1, 2 and the L ligand were measured in the solid state at room temperature. Intense luminescence emission bands of 1 and 2 are observed at 349 nm (λex = 280 nm) and 335 nm (λex = 280 nm), respectively (Fig. 4), while the L ligand itself has an emission band at 352 nm (λex = 280 nm). Although the complexes share the same maximum excitation wavelength with L ligand, apparent changes of the emission spectra after coordination were observed, and coordination with the metal ions makes the emission spectrum of L ligand slightly shift toward lower wavelength,
which should be associated with the obvious structural difference. The luminous mechanism for complexes 1 and 2 might be intraligand fluorescent emission (π–π*) [18]. To investigate the thermal stability of the two complexes, TG analyses were carried out with a Netzsch STA409PC instrument at a heating rate of 10 °C min−1 from 20 °C to 900 °C under nitrogen atmosphere (flow rate = 60 mL min−1). As shown in Fig. 5, the TG curve of complex 1 indicates that complex 1 is stable up to 240 °C, where the decomposition of the framework starts. A rapid and significant weight loss of 81.05% (calcd. 80.72%) is observed in the temperature range of 240–900 °C, which could be assigned to the complete decomposition
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Fig. 5. The thermogravimetric analysis (TGA) curves for complexes 1 and 2.
of the L ligand. Similar to those reported in corresponding references [19–23], the carboxylate ligands coordinate to metal ions by monodentate, bidentate and bis-monodentate modes. Complex 1 possesses a uninodal 3-connected hcb Shubnikov hexagonal plane net with the 63 topology, while complex 2 features the 3-connected topological net with 82 ∙ 10 topology (so-called “tongue-and groove” structure). By strategically selecting the ligands, we have successfully constructed the rarely reported T shape module in complex 2. Their crystal structure validities have been confirmed by comparing the simulated X-ray powder diffraction patterns with the experimental ones. In addition, the complexes 1 and 2 also exhibit intense emission in the solid state at room temperature. Compared with the L ligand, both of their excitation and emission peaks have changed due to the structural differences. Acknowledgment
Fig. 3. The XRPD for (a): complex 1; (b) complex 2 in the solid state at room temperature.
of the ligands. In comparison with complex 1, complex 2 is stable up to 400 °C. The weight loss of 31.40% (calcd. 31.11%) during the temperature range of 400–470 °C could be attributed to the complete decomposition of the tda ligand, while the weight loss during the temperature range of 470–900 °C could be assigned to the decomposition of the L ligand. However, complex 2 does not decompose completely under the experimental temperature. In summary, we have successfully constructed two new coordination polymers [Cd2(ba)4(L)2]n (1) and [Zn(tda)(L)]n (2) through selfassemblies of rigid carboxylate with Cd(II) and Zn(II) in the presence
We thank the support of this work by NSFC (No. 31200642), Special Scientific Research Funds for Central Non-profit Institutes, Chinese Academy of Fishery Sciences (No. 2013A1002) and Qingdao Municipal Science and Technology Plan Project (No. 12-1-4-12-(2)-jch). Appendix A. Supplementary material Crystallographic data have been deposited at the Cambridge Crystallographic Data Center as supplementary publications (CCDC-918009 and 919391) containing the supplementary crystallographic data for 1–2. The data can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: [email protected]. Additional experimental methods, elemental analysis results, IR absorption peaks, crystallographic data (Table S1), and the selected bond lengths and angles (Table S2) are available as electronic supplementary information in the online version. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.09.033. References
Fig. 4. Comparison of the fluorescent emission spectra for complexes 1 and 2 with the L ligand in the solid state at room temperature.
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