Synthesis , Crystal Structure , and Properties of a Zn ( II ) Coordination Polymer Based on a Difunctional Ligand Containing Triazolyl and Carboxyl Groups

A new compound, namely, [Zn(L)2]n (1) was obtained by the reaction of 2-methyl-4-(4H-1,2, 4-triazol-4-yl) benzoic acid (HL) with ZnSO4·7H2O, and the compound was characterized by single-crystal X-ray diffraction, infrared spectroscopy, elemental analysis, powder X-ray diffraction (PXRD), and thermogravimetric analysis. The linear HL ligands were deprotonated to be L− anions and act as two-connectors to link Zn2+ to form a two-dimensional (2D) lay structure with (4, 4) topology. The large vacancy of 2D framework allows another layer structure to interpenetrate, resulting in the formation of 2D + 2D→ 2D parallel interpenetration in 1. The weak interactions, such as hydrogen bonding and π–π stacking interactions, connect the adjacent 2D layers into a three-dimensional (3D) coordination polymer. The solid-state UV-visible spectroscopy and luminescent property have also been studied.


Introduction
In recent years, coordination polymers (CPs) built from metal ions and organic ligands have become a hot research topic because of their fascinating architectures and potent applications in the fields of luminescence [1][2][3][4][5], chemical sensors [6], gas adsorption or selective gas adsorption, gas separation [7,8], and heterogeneous catalysis [9].Generally speaking, the nature of metal ions and organic ligands can greatly affect the properties of CPs [10][11][12][13][14][15].Therefore, the elaborate design for organic ligands and the selection of metal ions are the most important factors to obtain desired architectures with favorable properties.Commonly, the N-or O-donor ligands with favorable coordination abilities are extensively employed to build diverse CPs.Among the N-heterocyclic ligands, triazole, imidazole, tetrazole, and pyrazole moieties can not only act as electrically neutral, but also negatively charged muti-dentate bridging ligands to construct CPs [16][17][18][19][20].For example, we have designed a series of polyazaheteroaromatic ligands including the derivatives of 1, 2, 4-triazole, for example, and 4-imidazoly-containing ligands, such as 1,3,5-tri(1H-imidazol-4-yl) benzene and 1,4-di(1H-imidazol-4-yl) benzene [21,22].Due to their favorable coordination abilities, they are used to build various porous metal Cu(II) or Co(II) imidzolate complexes, which have shown favorable gas adsorption properties in our previous work.Besides the polyazaheteroaromatic ligands, the carboxylic acids show diverse coordination modes and are extensively employed to assemble porous CPs via reticular synthesis by Yaghi's group [23,24].Significantly, the porous CPs possess a large surface area, as much as 10,000 m 2 /g, and greatly exceed those of traditional porous materials, such as zeolites and carbons [23].Moreover, the different electron configurations of N and O atoms cause polycarboxylates and N-donors ligands to exhibit different coordination preferences, which produces favorable compatibility for mixed polycarboxylates and N-donors ligands for building CPs [25,26].In this context, we have designed the difunctional organic molecule 4-(1H-imidazol-4-yl) benzoic acid including carboxyl and 4-imidazolyl groups, assembled into two series of Cu(II)/Cu(I) and Cd(II) complexes due to its diverse coordination modes [27].Due to their good adjustability for N/O donor ligands, we have designed the new ligand 2-methyl-4-(4H-1,2,4-triazol-4-yl) benzoic acid (HL) including the difunctional 1,2,4-triazol-4-yl and carboxyl groups (Scheme 1) to react with ZnSO

Materials and Instrumentation
All the reagents were purchased from Jinan Henhua reagent company (Jinan, China).IR spectra (Nicolet Instrument Inc., Madison, WI, USA) was measured on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets.Perkin-Elmer 240C Elemental Analyzer (Perkin-Elmer, Inc., Billerica, MA, USA) was used to carry out elemental analyses.Thermogravimetric analyses (TGA) (Perkin-Elmer, Inc., Billerica, MA, USA) was carried out on a simultaneous SDT 2960 thermal analyzer.Power X-ray diffraction pattern was recorded on a Shimadzu XRD-6000 X-ray diffractometer with CuKα (λ = 1.5418Å) radiation.A HORIBA FluoroMax-4 fluorescence spectrophotometer (Edinburgh Instruments, Edinburgh, UK) was used to measure photoluminescence spectra and the decay lifetimes.

Crystal Structure Determination
The single-crystal structure for [Zn(L)2]n (1) was collected on a Bruker Smart APEX II charge couple device diffractometer using MoKα radiation (λ = 0.71073 Å) at 296(2) K.The structure was solved by the direct method and refined anisotropically on F 2 refined by the full-matrix least-squares procedure technique using the SHELX-97 program [28].The crystallographic data and structural refinement are shown in Table 1.

Materials and Instrumentation
All the reagents were purchased from Jinan Henhua reagent company (Jinan, China).IR spectra (Nicolet Instrument Inc., Madison, WI, USA) was measured on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets.Perkin-Elmer 240C Elemental Analyzer (Perkin-Elmer, Inc., Billerica, MA, USA) was used to carry out elemental analyses.Thermogravimetric analyses (TGA) (Perkin-Elmer, Inc., Billerica, MA, USA) was carried out on a simultaneous SDT 2960 thermal analyzer.Power X-ray diffraction pattern was recorded on a Shimadzu XRD-6000 X-ray diffractometer with CuKα (λ = 1.5418Å) radiation.A HORIBA FluoroMax-4 fluorescence spectrophotometer (Edinburgh Instruments, Edinburgh, UK) was used to measure photoluminescence spectra and the decay lifetimes.

Crystal Structure Determination
The single-crystal structure for [Zn(L) 2 ] n (1) was collected on a Bruker Smart APEX II charge couple device diffractometer using MoKα radiation (λ = 0.71073 Å) at 296(2) K.The structure was solved by the direct method and refined anisotropically on F 2 refined by the full-matrix least-squares procedure technique using the SHELX-97 program [28].The crystallographic data and structural refinement are shown in Table 1.

Preparation
In this study, we adopted the hydro/solvothermal reaction, because it is an effective method in construction of CPs, especially for the carboxylic acid ligands.Generally, the reaction of metal atoms with carboxylate ligands easily leads to the formation of precipitate, making it difficult to grow suitable single crystals for X-ray diffraction studies.However, a hydro/solvothermal reaction can facilitate crystal growth under the spontaneous reaction system.Coordination polymer 1 was readily prepared with 72% yield by hydrothermal reaction.The IR spectra exhibit the O-H/C-H stretching vibration absorption of ligand at the center of 3620-3245 cm −1 for 1 (Figure S1).Strong characteristic bands of carboxyl group are observed in the range of 1598-1528 cm −1 for asymmetric vibrations and 1361 cm −1 for symmetric vibrations, respectively.The typical vibrational band at 1701 cm −1 observed in the IR spectra of HL belongs to the characteristic peak of the carboxyl group, which disappeared in coordination with polymer 1, showing the carboxyl group was deprotonated and participated in coordination with Zn(II) metal atom.

Crystal Structure of [Zn(L) 2 ] n (1)
X-ray diffraction analysis showed that [Zn(L) 2 ] n (1) crystallizes in the monoclinic P2 1 /c space group.One crystallographically independent Zn(II) atom and two L − ligands are included in the asymmetric unit.As shown in Figure 1, each Zn(II) atom possesses a distorted tetrahedral geometry and is coordinated by two oxygen atoms (O1, O3) from two different L − ligands and two nitrogen atoms (N1) from another two distinct L − ligands.The Zn-O bond lengths are 1.931(2) and 1.938(2) Å, and the Zn-N bond lengths are 2.024(2) and 2.042(2) Å (Table S1).The bond angles around the center Zn(II) atom are in the range of 99.64(10)-124.08(10) • (Table S1).Both of the L − ligands in 1 have the same coordination mode and act as two-connectors to link two metal Zn(II) atoms.In 1, the L − ligands links Zn(II) ions to form a two-dimensional (2D) (4,4) layer structure (Figure 2 The study further demonstrates that the ligand has great effect on the resulting coordination framework.Moreover, the O and N atoms from imidazolyl or carboxyl groups can act as the acceptor for hydrogen bonding, thus easily benefiting the formation of supramolecular polymers [31].As a result, there exists rich hydrogen bonding interaction in 1, and the C- 4) 152 • hydrogen bonds are extensively distributed among the 2D layers (Table S2).Moreover, the two benzene rings of L − ligands from the neighboring 2D layers are parallel and separated by a centroid-centroid distance of 3.48 Å, exhibiting classic weak π-π stacking interactions [32].In this context, a three-dimensional (3D) coordination polymer is built through the connections originating from the weak interactions, including hydrogen bonding and π-π stacking interactions between the adjacent 2D layers (Figure 4).
X-ray diffraction analysis showed that [Zn(L)2]n (1) crystallizes in the monoclinic P21/c space group.One crystallographically independent Zn(II) atom and two L − ligands are included in the asymmetric unit.As shown in Figure 1, each Zn(II) atom possesses a distorted tetrahedral geometry and is coordinated by two oxygen atoms (O1, O3) from two different L − ligands and two nitrogen atoms (N1) from another two distinct L − ligands.The Zn-O bond lengths are 1.931(2) and 1.938(2) Å, and the Zn-N bond lengths are 2.024(2) and 2.042(2) Å (Table S1).The bond angles around the center Zn(II) atom are in the range of 99.64(10)-124.08(10)°(Table S1).Both of the L − ligands in 1 have the same coordination mode and act as two-connectors to link two metal Zn(II) atoms.In 1, the L − ligands links Zn(II) ions to form a two-dimensional (2D) (4,4) layer structure (Figure 2 S2).Moreover, the two benzene rings of L − ligands from the neighboring 2D layers are parallel and separated by a centroid−centroid distance of 3.48 Å, exhibiting classic weak π−π stacking interactions [32].In this context, a three-dimensional (3D) coordination polymer is built through the connections originating from the weak interactions, including hydrogen bonding and π−π stacking interactions between the adjacent 2D layers (Figure 4).

Thermal Analysis and Powder X-Ray Diffraction Analysis
Thermogravimetric analysis (TGA) was performed to study the thermal stability of the coordination polymer in the N 2 atmosphere from 20-700 • C. Figure S2 shows no weight losses for the coordination polymer 1, and the collapse of the network happened at about 405 • C, which indicated the component of 1 has no lattice or coordinated water molecules or other organic components, completely fitting with crystal architecture of 1.The powder XRD experiment can be employed to measure the

Thermal Analysis and Powder X-Ray Diffraction Analysis
Thermogravimetric analysis (TGA) was performed to study the thermal stability of the coordination polymer in the N2 atmosphere from 20-700 °C. Figure S2 shows no weight losses for the coordination polymer 1, and the collapse of the network happened at about 405 °C, which indicated

Thermal Analysis and Powder X-Ray Diffraction Analysis
Thermogravimetric analysis (TGA) was performed to study the thermal stability of the coordination polymer in the N2 atmosphere from 20-700 °C. Figure S2 shows no weight losses for the coordination polymer 1, and the collapse of the network happened at about 405 °C, which indicated the component of 1 has no lattice or coordinated water molecules or other organic components,

Thermal Analysis and Powder X-Ray Diffraction Analysis
Thermogravimetric analysis (TGA) was performed to study the thermal stability of the coordination polymer in the N2 atmosphere from 20-700 °C. Figure S2 shows no weight losses for the coordination polymer 1, and the collapse of the network happened at about 405 °C, which indicated the component of 1 has no lattice or coordinated water molecules or other organic components, completely fitting with crystal architecture of 1.The powder XRD experiment can be employed to measure the phase purity of the as-synthesized compound.As shown in Figure S3, the simulated

Diffuse Reflectance Spectra
The UV-visible spectroscopy was investigated for the complex 1, as well as the free ligand HL.As shown in Figure 5, the compounds HL and 1 exhibited absorption peaks at 305 and 304 nm, Crystals 2018, 8, 424 6 of 11 respectively, which belong to π → π* and n → π* transitions, due to the conjugate character of the organic molecule [33].From the diffuse reflectance data, it can be easily transformed into a Kubelka-Munk function.And band gaps (Eg) can be obtained from the theory of optical absorption for a direct band gap semiconductor: (Ahν) 2 = B(hν-Eg), where B is a constant corresponding to the material itself [34].The Eg can be employed to evaluate the semiconductivity of the complexes.The Eg of 1 is estimated to be approximately 4.01 eV, which is obtained by extrapolation of the linear portion of the diffuse reflectance spectra (Figure 6), showing that 1 is an optical semiconductor [35].
sample from the crystal structure is quite consistent with the experimental pattern, indicating the phase purity of the sample.

Diffuse Reflectance Spectra
The UV-visible spectroscopy was investigated for the complex 1, as well as the free ligand HL.As shown in Figure 5, the compounds HL and 1 exhibited absorption peaks at 305 and 304 nm, respectively, which belong to π → π* and n → π* transitions, due to the conjugate character of the organic molecule [33].From the diffuse reflectance data, it can be easily transformed into a Kubelka-Munk function.And band gaps (Eg) can be obtained from the theory of optical absorption for a direct band gap semiconductor: (Ahν) 2 = B(hν-Eg), where B is a constant corresponding to the material itself [34].The Eg can be employed to evaluate the semiconductivity of the complexes.The Eg of 1 is estimated to be approximately 4.01 eV, which is obtained by extrapolation of the linear portion of the diffuse reflectance spectra (Figure 6), showing that 1 is an optical semiconductor [35].

Diffuse Reflectance Spectra
The UV-visible spectroscopy was investigated for the complex 1, as well as the free ligand HL.As shown in Figure 5, the compounds HL and 1 exhibited absorption peaks at 305 and 304 nm, respectively, which belong to π → π* and n → π* transitions, due to the conjugate character of the organic molecule [33].From the diffuse reflectance data, it can be easily transformed into a Kubelka-Munk function.And band gaps (Eg) can be obtained from the theory of optical absorption for a direct band gap semiconductor: (Ahν) 2 = B(hν-Eg), where B is a constant corresponding to the material itself [34].The Eg can be employed to evaluate the semiconductivity of the complexes.The Eg of 1 is estimated to be approximately 4.01 eV, which is obtained by extrapolation of the linear portion of the diffuse reflectance spectra (Figure 6), showing that 1 is an optical semiconductor [35].

Photoluminescent Property
Luminescent CPs, especially containing π-conjugated organic molecules and a d 10 metal center, can adjust the photoluminescent property through their interaction arising from metal and ligands [36,37].Therefore, we performed the study of the photoluminescent properties of complex 1 together with HL organic ligand in the solid-state.As shown in Figure 7, the broad emission band at 433 nm was recorded for the HL organic molecule upon excitation at 365 nm; this is attributable to π* → π transition because of the π-conjugated effect of the aromatic group from the HL ligand [38].Upon excitation at 365 nm, compound 1 shows similar broad photoluminescence emission at 442 nm.
For the complex 1, the emission band is 9 nm red-shifted in contrast to the free HL ligand (Figure 7), which may be intraligand fluorescence due to their similarity [39,40].
can adjust the photoluminescent property through their interaction arising from metal and ligands [36,37].Therefore, we performed the study of the photoluminescent properties of complex 1 together with HL organic ligand in the solid-state.As shown in Figure 7, the broad emission band at 433 nm was recorded for the HL organic molecule upon excitation at 365 nm; this is attributable to π* → π transition because of the π-conjugated effect of the aromatic group from the HL ligand [38].Upon excitation at 365 nm, compound 1 shows similar broad photoluminescence emission at 442 nm.For the complex 1, the emission band is 9 nm red-shifted in contrast to the free HL ligand (Figure 7), which may be intraligand fluorescence due to their similarity [39,40].Furthermore, we studied the corresponding decay lifetimes and quantum yield (QY) for the HL ligand and complex 1.As shown in Figure 8, the QY value of compound 1 is 0.74%.The QY value of coordination polymer 1 is 0.74%.The QY value of compound 1 is probably attributed to the immobilization of the HL ligand, as it is strongly coordinated to metal ions that effectively increase the rigidity of the ligands, and to the low-dimensional structure with a π-conjugated system that decreases the molecular band gap [41].Furthermore, the luminescence lifetime was also investigated for complex 1.The exponential function of I(t) = A exp(-t/τ) was employed to fit the luminescence decay curves, which shows the luminescence lifetime of 1 is 84 ns (Figure 9).Furthermore, we studied the corresponding decay lifetimes and quantum yield (QY) for the HL ligand and complex 1.As shown in Figure 8, the QY value of compound 1 is 0.74%.The QY value of coordination polymer 1 is 0.74%.The QY value of compound 1 is probably attributed to the immobilization of the HL ligand, as it is strongly coordinated to metal ions that effectively increase the rigidity of the ligands, and to the low-dimensional structure with a π-conjugated system that decreases the molecular band gap [41].Furthermore, the luminescence lifetime was also investigated for complex 1.The exponential function of I(t) = A exp(−t/τ) was employed to fit the luminescence decay curves, which shows the luminescence lifetime of 1 is 84 ns (Figure 9).
with HL organic ligand in the solid-state.As shown in Figure 7, the broad emission band at 433 nm was recorded for the HL organic molecule upon excitation at 365 nm; this is attributable to π* → π transition because of the π-conjugated effect of the aromatic group from the HL ligand [38].Upon excitation at 365 nm, compound 1 shows similar broad photoluminescence emission at 442 nm.For the complex 1, the emission band is 9 nm red-shifted in contrast to the free HL ligand (Figure 7), which may be intraligand fluorescence due to their similarity [39,40].Furthermore, we studied the corresponding decay lifetimes and quantum yield (QY) for the HL ligand and complex 1.As shown in Figure 8, the QY value of compound 1 is 0.74%.The QY value of coordination polymer 1 is 0.74%.The QY value of compound 1 is probably attributed to the immobilization of the HL ligand, as it is strongly coordinated to metal ions that effectively increase the rigidity of the ligands, and to the low-dimensional structure with a π-conjugated system that decreases the molecular band gap [41].Furthermore, the luminescence lifetime was also investigated for complex 1.The exponential function of I(t) = A exp(-t/τ) was employed to fit the luminescence decay curves, which shows the luminescence lifetime of 1 is 84 ns (Figure 9).

Conclusions
A new coordination polymer [Zn(L) 2 ] n was synthesized by the reaction of Zn(II) salt with an HL organic ligand including difunctional triazolyl and carboxyl groups.The deprotonated L − ligands act as linear two-connectors to link Zn 2+ atoms to form a 2D layer structure.The large vacancy of the 2D framework allows another layer structure to interpenetrate, resulting in the system of 2D +2D → 2D parallel interpenetration in 1.The coordination polymer 1 exhibits an emission at 442 nm upon excitation at 365 nm.The luminescence lifetime and quantum yield of compound 1 are 84 ns and 0.74%, respectively.The study has further demonstrated that the difunctional organic linkers containing triazolyl and carboxyl groups are effective moieties to generate desired architectures.The coordination polymer 1 shows good photoluminescence properties, and this study has further confirmed that CPs with the combination of d 10 metal atoms and conjugated organic ligands could be potentially used as luminescent material.

Figure 1 .
Figure 1.The coordination environment of Zn(II) ion in 1, with the ellipsoids drawn at the 30% probability level.All of the hydrogen atoms are omitted for clarity.Symmetry code: A 2.5 − x, 0.5 + y, 1.5 − z, B 0.5 − x, −0.5 + y, 1.5 − z.

Figure 1 .
Figure 1.The coordination environment of Zn(II) ion in 1, with the ellipsoids drawn at the 30% probability level.All of the hydrogen atoms are omitted for clarity.Symmetry code: A 2.5 − x, 0.5 + y, 1.5 − z, B 0.5 − x, −0.5 + y, 1.5 − z.

Figure 4 .
Figure 4. 3D coordination polymer of 1 packed by intramolecular and intermolecular hydrogen bonds and π−π interactions with a centroid−centroid distance of 3.48 Å.

Figure 4 .
Figure 4. 3D coordination polymer of 1 packed by intramolecular and intermolecular hydrogen bonds and π−π interactions with a centroid−centroid distance of 3.48 Å.

Figure 4 .
Figure 4. 3D coordination polymer of 1 packed by intramolecular and intermolecular hydrogen bonds and π−π interactions with a centroid−centroid distance of 3.48 Å.

Figure 4 .
Figure 4. 3D coordination polymer of 1 packed by intramolecular and intermolecular hydrogen bonds and π-π interactions with a centroid-centroid distance of 3.48 Å.

Figure 5 .
Figure 5.The UV-Vis spectra in the solid-state of HL and 1.

Figure 6 .
Figure 6.The band gap with 4.01 ev by the treatment with the Kubelka-Munk function.

Figure 5 .
Figure 5.The UV-Vis spectra in the solid-state of HL and 1.

Figure 5 .
Figure 5.The UV-Vis spectra in the solid-state of HL and 1.

Figure 6 .
Figure 6.The band gap with 4.01 ev by the treatment with the Kubelka-Munk function.

Figure 6 .
Figure 6.The band gap with 4.01 ev by the treatment with the Kubelka-Munk function.

Figure 7 .
Figure 7. Solid-state photoluminescent spectra of HL ligand and complex 1.

Figure 7 .
Figure 7. Solid-state photoluminescent spectra of HL ligand and complex 1.

Figure 7 .
Figure 7. Solid-state photoluminescent spectra of HL ligand and complex 1.

Table 1 .
Crystallographic data and structure refinement for 1.