A Three-Dimensional Cadmium(II) Coordination Network Based on 1,3-Di-(1,2,4-triazole-4-yl)benzene: Synthesis, Structure, and Luminescence Properties

: 1,2,4-Triazole and its derivatives have been investigated extensively in the construction of coordination polymers. Using a 1,2,4-triazole ligand 1,3-di-(1,2,4-triazole-4-yl)benzene (dtb), a new three-dimensional coordination polymer, {[Cd 2 (dtb) 2 (SO 4 )(H 2 O)] · (1,2-H 2 bdc) · SO 4 } n ( 1 ) (1,2-H 2 bdc = 1,2-benzenedicarboxylic acid), was synthesized under solvothermal conditions. Single-crystal X-ray di ﬀ raction analysis revealed that there are two crystallographically di ﬀ erent Cd(II) ions in 1 with distorted pentagonal bipyramidal [CdN 4 O 3 ] geometry and distorted octahedral [CdN 4 O 2 ] geometry, respectively. The Cd1 atoms are connected by dtb ligands to generate Cd 4 (dtb) 8 secondary building units (SUBs), and the SUBs are further linked by Cd2 atoms into a three-dimensional network with two di ﬀ erent one-dimensional channels of 14.63(2) × 14.63(2) and 7.54(2) × 7.54(2) Å 2 along the c axis. The topological analysis of the framework has also been discussed. In addition, compound 1 exhibits strong ﬂuorescence emission in the solid state at room temperature.


Introduction
The self-assembly of coordination polymers (CPs) has attracted considerable research interest due to their fascinating structural topologies and their potential applications in catalysis, gas storage, separation, magnetism, and luminescence [1][2][3][4][5][6]. In particular, luminescent CPs based on d 10 metal ions and conjugated organic ligands that potentially serve as chemical sensors in environmental monitoring applications have made considerable progress in recent years [7][8][9].
To synthesize targeted CPs, the choice of organic ligands is crucial since the final structures are mainly influenced by the conformation and the coordination ability of the ligands [10]. Aromatic polycarboxylate ligands have been widely accepted as the most effective building blocks in constructing CPs because of their structural rigidity and versatile coordination modes [11][12][13]. Meanwhile, ligands with nitrogen heterocycle rings are also frequently used as auxiliary linkers to finish the coordination geometries of metal centers in the crystallization process [14][15][16]. Combining these two kinds of ligands with metal ions tends to provide high-dimensional networks [17][18][19][20].

Reagents and Instruments
Dtb was prepared according to the literature method [25]. All other reagents and solvents were purchased from commercial sources and used without further purification. The IR spectrum was recorded on a Bruker EQUINOX55 spectrophotometer (Bruker, Karlsruhe, Germany) in the 4000-400 cm −1 region using KBr pellets. Powder X-ray diffraction (PXRD) measurement was carried out on a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.154 18 nm) at room temperature. The luminescent spectra in the solid states were performed on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at room temperature.

Single-Crystal X-ray Diffraction
The single crystal diffraction data for compound 1 were collected by using a Bruker SMART APEX II CCD area detector at 296 K. The structure was solved by direct methods and refined by fullmatrix least-squares methods with the SHELXL program and also refined by the Olex2 program [29,30]. Non-hydrogen atoms were refined with anisotropic thermal parameters, and hydrogen atoms were added to their calculation positions and refined using the riding model. The SQUEEZE routine in PLATON was used to identify a solvent-accessible volume of 2673 Å 3 . A summary of the crystallographic analysis is summarized in Table 1

Reagents and Instruments
Dtb was prepared according to the literature method [25]. All other reagents and solvents were purchased from commercial sources and used without further purification. The IR spectrum was recorded on a Bruker EQUINOX55 spectrophotometer (Bruker, Karlsruhe, Germany) in the 4000-400 cm −1 region using KBr pellets. Powder X-ray diffraction (PXRD) measurement was carried out on a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.154 18 nm) at room temperature. The luminescent spectra in the solid states were performed on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at room temperature.

Single-Crystal X-ray Diffraction
The single crystal diffraction data for compound 1 were collected by using a Bruker SMART APEX II CCD area detector at 296 K. The structure was solved by direct methods and refined by full-matrix least-squares methods with the SHELXL program and also refined by the Olex2 program [29,30]. Non-hydrogen atoms were refined with anisotropic thermal parameters, and hydrogen atoms were added to their calculation positions and refined using the riding model. The SQUEEZE routine in PLATON was used to identify a solvent-accessible volume of 2673 Å 3 . A summary of the crystallographic analysis is summarized in Table 1. Selected bond lengths and angles for 1 are listed in Table S1 (Supporting information). CCDC 1921813 contains the supplementary crystallographic data (supplementary materials) for 1. These 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: deposit@ccdc.cam.ac.uk.

Structural Description
The title compound was obtained under a solvothermal condition of CdSO 4 ·8/3H 2 O, 1,2-benzenedicarboxylic acid and dtb in H 2 O/CH 3 CH 2 OH at 363 K. To understand the process, we did several parallel reactions under solvothermal conditions by changing metal salts, solvent, and temperature. For example, we used Cd(CH 3 COO) 2 ·2H 2 O or CdCl 2 , only distilled water or ethanol as solvent, 80 • C, or 120 • C. It was not possible to get compound 1 by changing any condition. To our surprise, although 1,2-H 2 bdc did not participate in coordination with Cd(II) ions in the structure, we could not obtain the framework in the absence of it. There is no doubt 1,2-H 2 bdc is present as a template in the construction of the framework.
Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in the tetragonal space group I-4m2. The asymmetric unit contains two Cd(II) ions, two dtb ligands, one 1,2-H 2 bdc molecule, one coordinated SO 4 2− anion, one uncoordinated SO 4 2− anion, and one coordinated water molecule. As represented in Figure 1, the Cd1 atom is seven-coordinated by four N atoms from four different dtb ligands, two oxygen atoms from two SO 4

Structural Description
The title compound was obtained under a solvothermal condition of CdSO4·8/3H2O, 1,2benzenedicarboxylic acid and dtb in H2O/CH3CH2OH at 363 K. To understand the process, we did several parallel reactions under solvothermal conditions by changing metal salts, solvent, and temperature. For example, we used Cd(CH3COO)2·2H2O or CdCl2, only distilled water or ethanol as solvent, 80 o C, or 120 o C. It was not possible to get compound 1 by changing any condition. To our surprise, although 1,2-H2bdc did not participate in coordination with Cd(II) ions in the structure, we could not obtain the framework in the absence of it. There is no doubt 1,2-H2bdc is present as a template in the construction of the framework.
Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in the tetragonal space group I-4m2. The asymmetric unit contains two Cd(II) ions, two dtb ligands, one 1,2-H2bdc molecule, one coordinated SO4 2-anion, one uncoordinated SO4 2-anion, and one coordinated water molecule. As represented in Figure 1, the Cd1 atom is seven-coordinated by four N atoms from four different dtb ligands, two oxygen atoms from two SO4 2-anions, and one oxygen atom from a coordinated water molecule, displaying a distorted pentagonal bipyramidal [CdN4O3] geometry.  In compound 1, the coordinated SO4 2-anions adopt μ3-bridging mode with three Cd(II) ions (Cd2Cd1Cd2), whereas both of the dtb ligands act as μ4-η 1 : η 1 : η 1 : η 1 mode connecting four metal ions (Cd1Cd2/Cd1Cd2). The dihedral angle between the triazole rings and the benzene ring of one dtb ligand is 30.56 (7)   by dtb ligands to give rise to Cd 4 (dtb) 8 secondary building units (SUBs), where the Cd···Cd distance is 10.347(2) Å. The Cd 4 (dtb) 8 SUBs show an interesting calixarene-like structure, as shown in Figure 2a. These SUBs are interconnected through the coordinate interactions of dtb and Cd2 to generate a three-dimensional framework with the Cd1···Cd2 distance of 3.948(1) Å (Figure 2b). The outstanding structural feature of the 3D framework is that there are two different one-dimensional channels of 14.63 (2) × 14.63 (2) and 7.54(2) × 7.54(2) Å 2 along the c axis (Figure 2c). The former is filled by uncoordinated SO 4 2− anions, and 1,2-H 2 bdc molecules reside in the latter (Figure 2d). Coordinated SO 4 2− anions and water molecules meet the coordination requirement of center Cd1 and Cd2 atoms, and by which the framework is consolidated. Uncoordinated SO 4 2− and 1,2-H 2 bdc did not participate in the formation of the framework directly, but as templates, they regulate the construction of the structure. Another porous metal-organic framework from dtb and Cd(II) ions has been reported previously [28]. The structure contains triangular nano-porous channels with edges of 13.622 Å, which is constructed by the Cd 3 (dtb) 12 trinuclear cluster. From a topological point of view, two Cd(II) atoms linked by one triazole ring can be regarded as 4-connected nodes, and the dtb ligands are reduced to linkers, so the whole framework can be simplified as a 4-connected net with {4 3 ·8 3 } topology, as depicted in Figure 2e. observed in other reported dtb-based coordination polymers [28]. Neighboring Cd1 atoms are bridged by dtb ligands to give rise to Cd4(dtb)8 secondary building units (SUBs), where the Cd···Cd distance is 10.347(2) Å. The Cd4(dtb)8 SUBs show an interesting calixarene-like structure, as shown in Figure 2a. These SUBs are interconnected through the coordinate interactions of dtb and Cd2 to generate a three-dimensional framework with the Cd1···Cd2 distance of 3.948(1) Å (Figure 2b). The outstanding structural feature of the 3D framework is that there are two different one-dimensional channels of 14.63 (2) ×14.63 (2) and 7.54(2) × 7.54(2) Å 2 along the c axis (Figure 2c). The former is filled by uncoordinated SO4 2-anions, and 1,2-H2bdc molecules reside in the latter (Figure 2d). Coordinated SO4 2-anions and water molecules meet the coordination requirement of center Cd1 and Cd2 atoms, and by which the framework is consolidated. Uncoordinated SO4 2-and 1,2-H2bdc did not participate in the formation of the framework directly, but as templates, they regulate the construction of the structure. Another porous metal-organic framework from dtb and Cd(II) ions has been reported previously [28]. The structure contains triangular nano-porous channels with edges of 13.622 Å, which is constructed by the Cd3(dtb)12 trinuclear cluster. From a topological point of view, two Cd(II) atoms linked by one triazole ring can be regarded as 4-connected nodes, and the dtb ligands are reduced to linkers, so the whole framework can be simplified as a 4-connected net with {4 3 ·8 3 } topology, as depicted in Figure 2e.

Powder X-ray Diffraction (PXRD) and Thermal Analysis
The powder X-ray diffraction (PXRD) experiment was carried out to confirm the purity of compound 1. As shown in Figure 3, the major peak positions of the experimental PXRD patterns are almost in agreement with the simulated ones from the single-crystal data, indicating the high phase purity of the bulk sample.

Powder X-ray Diffraction (PXRD) and Thermal Analysis
The powder X-ray diffraction (PXRD) experiment was carried out to confirm the purity of compound 1. As shown in Figure 3, the major peak positions of the experimental PXRD patterns are almost in agreement with the simulated ones from the single-crystal data, indicating the high phase purity of the bulk sample.
The thermal stability of complex 1 was examined by TG analysis in the range of 25-900 o C. The result is given in Figure S1. Compound 1 lost its coordinated water molecule and uncoordinated 1,2-H2bdc molecule at 290 o C (Obsd. 17.94, Calcd. 17.75%). Then the further losses resulted from the decomposition of 1.

Luminescent Properties
The solid-state luminescence spectra of 1 and free dtb were recorded at room temperature ( Figure 4). Upon excitation at 318 nm, dtb displays an emission maximum at 380 nm, which can be attributed to the π-π* transitions. The maximum emission peak of 1 is located at 409 nm (λex = 326 nm), the emission may originate from intraligand π-π* transitions since a similar peak shape with dtb is found; similar luminescence was also found in other dtb CPs [31]. Compared with the emission spectra of free dtb, the red shift (29 nm) is probably attributed to the coordination of dtb to Cd(II) atoms.  The thermal stability of complex 1 was examined by TG analysis in the range of 25-900 • C. The result is given in Figure S1. Compound 1 lost its coordinated water molecule and uncoordinated 1,2-H 2 bdc molecule at 290 • C (Obsd. 17.94, Calcd. 17.75%). Then the further losses resulted from the decomposition of 1.

Luminescent Properties
The solid-state luminescence spectra of 1 and free dtb were recorded at room temperature ( Figure 4). Upon excitation at 318 nm, dtb displays an emission maximum at 380 nm, which can be attributed to the π-π* transitions. The maximum emission peak of 1 is located at 409 nm (λ ex = 326 nm), the emission may originate from intraligand π-π* transitions since a similar peak shape with dtb is found; similar luminescence was also found in other dtb CPs [31]. Compared with the emission spectra of free dtb, the red shift (29 nm) is probably attributed to the coordination of dtb to Cd(II) atoms.

Powder X-ray Diffraction (PXRD) and Thermal Analysis
The powder X-ray diffraction (PXRD) experiment was carried out to confirm the purity of compound 1. As shown in Figure 3, the major peak positions of the experimental PXRD patterns are almost in agreement with the simulated ones from the single-crystal data, indicating the high phase purity of the bulk sample.
The thermal stability of complex 1 was examined by TG analysis in the range of 25-900 o C. The result is given in Figure S1. Compound 1 lost its coordinated water molecule and uncoordinated 1,2-H2bdc molecule at 290 o C (Obsd. 17.94, Calcd. 17.75%). Then the further losses resulted from the decomposition of 1.

Luminescent Properties
The solid-state luminescence spectra of 1 and free dtb were recorded at room temperature ( Figure 4). Upon excitation at 318 nm, dtb displays an emission maximum at 380 nm, which can be attributed to the π-π* transitions. The maximum emission peak of 1 is located at 409 nm (λex = 326 nm), the emission may originate from intraligand π-π* transitions since a similar peak shape with dtb is found; similar luminescence was also found in other dtb CPs [31]. Compared with the emission spectra of free dtb, the red shift (29 nm) is probably attributed to the coordination of dtb to Cd(II) atoms.

Conclusions
In summary, a three-dimensional network {[Cd 2 (dtb) 2 (SO 4 )(H 2 O)]·(1,2-H 2 bdc)·SO 4 } n has been synthesized. The structure was characterized by IR, PXRD, and single-crystal X-ray crystallography. An interesting calixarene-like Cd 4 (dtb) 8 SUB was found in the structure, and the SUBs were joined to give a three-dimensional framework with two different one-dimensional channels. The channels are occupied by SO 4 2− anions and 1,2-H 2 bdc molecules, and both act as templates to decide the structure.
The CPs exhibits a ligand-based emission, and further research is in progress.
Author Contributions: Z.L. and C.X. synthesized the title compound. D.D. and C.X. performed the X-ray structure determination and analyzed the results. S.M. and B.J. wrote the paper.