Synthesis , Structure , and Properties of Coordination Polymers Based on 1 , 4-Bis ( ( 2-methyl-1 H-imidazol-1-yl ) methyl ) benzene and Different Carboxylate Ligands

Three novel coordination polymers, formulated as {[Zn(1,4-bmimb)(PhAA)2]·H2O}n (1), [Cu(1,4-bmimb)0.5(2,6-PyDC)]n (2), and {[Cu(1,4-bmimb)0.5(2-PAC)(HCOO)]·2H2O}n (3) (1,4-bmimb = 1,4-bis((2-methyl-1H-imidazol-1-yl)methyl)benzene; PhAA = phenylacetic acid; 2,6-PyDC = pyridine-2,6-dicarboxylic acid; 2-PAC = 2-pyrazinecarboxylic acid), were synthesized by the self-assembly of mixed ligands with Zn(II) and Cu(II) under solvothermal conditions and characterized by means of single-crystal X-ray diffraction, X-ray powder diffraction, infrared spectra, thermogravimetric analysis, fluorescence spectra and UV-vis absorption spectra. 1 is shown as a Z-shaped chain, which is formed by Zn2+, PhAA ligands, and 1,4-bmimb ligands, and is assembled into a 3D structure by hydrogen bonding and π···π interaction. Similarly, 2 displays a single chain, which is built by Cu2+, 2,6-PyDC ligands, and 1,4-bmimb ligands, and is assembled into a 3D structure by hydrogen bonding and π···π interaction. 3 possesses a 1D ladder structure, which is formed by Cu2+, 2,6-PyDC ligands, and 1,4-bmimb ligands, and is assembled into a 3D structure by hydrogen bonding. The luminescence properties (for 1) and UV-vis spectrum (for 2 and 3) were also studied and discussed.


Introduction
Coordination polymers (CPs), a series of solid-state materials with an infinite framework structure, are generated by the self-assembly of metal ions and organic ligands [1][2][3][4].CPs are continuously gaining attention owing to their appealing structures and various potential applications in luminescence, catalysis, magnetism, drug delivery, gas adsorption and separation, and so on [5][6][7][8][9][10][11].It is apparent that certain structures give materials unique properties, which are crucial for the application of the material [12][13][14].However, the design and manufacture of novel functional CPs remain a serious challenge at present.
There are many factors (metal ions, ligands, solvents, pH, etc.) that can affect the synthesis of CPs.The most effective strategy for the synthesis of valuable CPs is selecting suitable organic ligands [15][16][17][18][19]. Crystal engineers select diverse ligands to purposefully synthesize CPs [20,21].Although it is a convenient strategy to construct a CP using a single ligand, to obtain unique structures, crystal engineers are more likely to use mixed ligands to manufacture CPs [22][23][24][25].

Materials and Physical Measurements
The reagents and drugs used in the experiment were obtained directly from commercial sources and used without further purification.Infrared (IR) spectra were recorded on a NEXUS 670 FTIR spectrometer (Thermo Nicolet Corporation, Madison, WI, USA) with KBr pellets in the range of 400-4000 cm −1 .Elemental analyses (C, H, and N) were performed using an EA 1110 elemental analyzer (Carlo-Erba Corporation, Sandwich, Italy).Powder X-ray diffraction (PXRD) measurements were executed using a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) at 40 kV and 40 mA with a Cu-target tube and a graphite monochromator.The recording speed was 6 • min −1 at room temperature.Thermogravimetric analysis (TGA) was performed using an SDT Q600 instrument (TA Instruments, New Castle, DE, USA) with a heating rate of 10 • C min −1 in a flowing N 2 atmosphere.The solid fluorescence test was performed using an Edinburgh Hitachi F-4500 fluorescence spectrophotometer (Hitachi Limited, Tokyo, Japan) with a power of 450 W at room temperature.During the test, the crystal powder was flatly adhered to a clean glass plate.The amount of sample used for each test was 20 mg.

Synthesis of {[Zn
Zn(NO 3 ) 2 •6H 2 O (0.2974 g, 1 mmol), PhAA (0.0681 g, 0.5 mmol), and 1,4-bmimb (0.1282 g, 0.5 mmol) were dissolved in a mixture of DMF (2.5 mL) and deionized water (2.5 mL).The solution was then transferred to a 10 mL glass bottle.The sealed bottle was heated under autogenous pressure at 100 • C for 2 days and cooled in air to room temperature.After being collected by filtration, colorless crystals were washed several times with DMF/H 2 O (V DMF /V water = 1/1), and dried at 45 A mixture of Cu(NO 3 ) 2 •6H 2 O (0.2956 g, 1 mmol), 2,6-PyDC (0.0835 g, 0.5 mmol), and 1,4-bmimb (0.1282 g, 0.5 mmol) in ethanol/water (V ethanol /V water = 1/1, 5 mL) was placed in a 10 mL glass bottle.The sealed vessel was heated at 90 • C for 3 days and then cooled to room temperature, giving the blue block crystals of 2. In the same procedure as that described for 1, crystals were dried in air after filtration and washed with ethanol/H 2 O (V ethanol /V water = 1/1  (3) Compound 3 was synthesized using a procedure similar to 1 except that PhAA was replaced by 2-PAC (0.0620 g, 0.5 mmol).Blue block crystals suitable for single-crystal X-ray diffraction analysis

IR Spectra
The IR spectra of the obtained coordination polymers is in accordance with their single crystal structures.For instance, compounds show strong and broad absorption bands in the range 3400-3500 cm −1 , demonstrating the presence of the O-H stretching modes within the coordinated or lattice water molecules.The absence of any obvious absorption band in the region of 1700 cm −1 signifies full deprotonation of the original carboxylic ligands.The C-H vibration band for benzene exhibited characteristic absorptions in the range of 3000-3300 cm −1 , and the methyl C-H stretching vibration band on the 1,4-bmimb ligand was detected at 2900-3000 cm −1 .The C=N and C=C stretching vibration band of the ligands appeared in the range of 1500-1650 cm −1 .

Crystal Structure Determination
Three appropriately sized crystals (0.27 mm × 0.26 mm × 0.24 mm (1), 0.28 mm × 0.26 mm × 0.24 mm (2), and 0.27 mm × 0.24 mm × 0.22 mm (3)), were picked under a microscope and measured on a single crystal diffractometer named Siemens SMART (Siemens Limited, Berlin, Germany).This instrument is equipped with a graphite monochromator (Mo-Kα radiation, λ = 0.71073 Å).Absorption corrections were executed using the SADABS program (Version 2.03) [29].The structure was solved by direct methods using SHELXS-97 [30] and refined by full-matrix least-squares techniques using SHELXL-2014/7 (Sheldrick, 2014) [31].Hydrogen atoms were placed in calculated positions and included as riding atoms with isotropic displacement parameters 1.2-1.5 times the U eq of the attached atoms.The O-H hydrogen atoms of water were placed in calculated positions and refined using a SHELX DFIX restraint to fix their positions in the structure of 1, and using SHELX DFIX and DANG restraints in 3. The C-H hydrogen atoms on methyl were held in calculated positions by HFIX 137 in the structures of 1-3.Related information belonging to the crystallographic parameters and structure refinement is revealed in Table 1.Selected bond lengths and angles are listed in Tables S1-S3.Hydrogen bond parameters are listed in Tables S4-S6.

Results and Discussion
The asymmetric unit of 1 consists of half aZn(II) ion, half a solvate water molecule, a phenylacetic acid ligand, and half a 1,4-bmimb ligand.As depicted in Figure 1a S1.
As shown in Figure 1b, two 1,4-bmimb ligands bridge between neighboring Zn(II) ions in alternating layers, resulting in a Z-shaped chain structure that extends indefinitely in one dimension.These Z-shaped chains are assembled into a 2D supramolecular framework through O-H•••O hydrogen-bonding interactions, detailed in Figure 1c and Table S4.Furthermore, the neighboring 2D layers are connected by π•••π interactions (3.935 Å) from the imidazole rings to generate a 3D supramolecular framework, shown in Figure 1d.
As shown in Figure 1b, two 1,4-bmimb ligands bridge between neighboring Zn(II) ions in alternating layers, resulting in a Z-shaped chain structure that extends indefinitely in one dimension.These Z-shaped chains are assembled into a 2D supramolecular framework through O-H•••O hydrogen-bonding interactions, detailed in Figure 1c and Table S4.   Figure 2b shows that two 2,6-PDA ligands connect to two copper ions to form a secondary building units (SBU) and the 1,4-bmimb ligand forms a 1D Z-chain by linking adjacent SBUs.Different chains are assembled into 2D structures through C-H•••O hydrogen bonds, shown in Figure 2c and Table S5.In addition, the adjacent 2D layers are connected by π•••π interactions (3.935 Å) from the benzene rings to generate a 3D framework, as shown in Figure 2d.
highlighting the distorted octahedral geometry of the [CuN2O4] coordination mode, which consists of two nitrogen atoms originating from a 1,4-bmimb ligand and a 2-PAC ligand and four oxygen atoms originating from three different 2-PAC ligands.The Cu-O bond lengths are in the range of 1.9651(19) Å to 2.6320(19) Å, and the lengths of Cu-N bonds are 1.968(2) Å and 2.027(2) Å.The bond angles around the Cu1 centers range from 54.95(7)° to 174.30(8)°.The two oxygen atoms from the same carboxylic acid are alternately linked to copper ions, creating an infinitely-extended -Cu-O-Cu-chain, shown in Figure 3b.Additionally, the 1,4-bmimb ligand bridges two -Cu-O-Cu-chains to form an infinitely extended 1D ladder-shaped chain, as shown in Figure 3c.Different laddershaped structures build up a 3D framework that is relianton O-H•••O hydrogen bonds between ligands and solvate water molecules (see Figure 3d and Table S6).

Structure Discussion
As described above, three co-ligands with different coordination sites have marked effects on the structure.For 1, the structural extension is terminated due to a lack of coordination sites in the PhAA ligands, which makes 1 possess a 1D structure only.Although 2 and 3 have one of the same coordination sites (the functional groups circled by a dotted line in Figure 4), the type and position of another functional group results in different chain structures.As for 2, three coordination sites chelate copper ions like "crab claws", and as there is no coordination site present on the opposite side of the 2,6-PyDC ligand, the extension into a 2D structure is impossible.In the case of 3, the coordination sites circled by a dotted line chelate the copper ion and leave no suitable coordination conditions for the third functional group, resulting in 3 possessing a 1D ladder-shaped chain.In summary, a co-ligand with diverse coordination sites enriches structures with the same dimension.

Structure Discussion
As described above, three co-ligands with different coordination sites have marked effects on the structure.For 1, the structural extension is terminated due to a lack of coordination sites in the PhAA ligands, which makes 1 possess a 1D structure only.Although 2 and 3 have one of the same coordination sites (the functional groups circled by a dotted line in Figure 4), the type and position of another functional group results in different chain structures.As for 2, three coordination sites chelate copper ions like "crab claws", and as there is no coordination site present on the opposite side of the 2,6-PyDC ligand, the extension into a 2D structure is impossible.In the case of 3, the coordination sites circled by a dotted line chelate the copper ion and leave no suitable coordination conditions for the third functional group, resulting in 3 possessing a 1D ladder-shaped chain.In summary, a co-ligand with diverse coordination sites enriches structures with the same dimension.

Structure Discussion
As described above, three co-ligands with different coordination sites have marked effects on the structure.For 1, the structural extension is terminated due to a lack of coordination sites in the PhAA ligands, which makes 1 possess a 1D structure only.Although 2 and 3 have one of the same coordination sites (the functional groups circled by a dotted line in Figure 4), the type and position of another functional group results in different chain structures.As for 2, three coordination sites chelate copper ions lik

XRD Analysis and Thermal Analysis
The XRD patterns of the three compounds were measured at room temperature using a scanning range of 5 • to 40 • .Figures S1-S3 show the PXRD patterns of the compounds 1-3 (red line) and the XRD pattern simulated from the single crystal structure (black line).The two lines are essentially in agreement, demonstrating that the synthesized compound is phase pure, i.e., does not contain an impurity phase.The difference in the reflection intensity between the simulated and experimental patterns is due to a certain degree of preferred orientation of the powder samples during data collection.
To examine the thermal stability of the three compounds, TGA was implemented in the temperature range of 25-800 • C under a steady N 2 flow, the outcome of which is given in Figure 5.For 1, the decomposition starts at 158 • C. All uncoordinated water molecules (calculated for the asymmetric unit) were lost in the temperature range of 158 • C to 200 • C (obsd 2.6%, calcd 2.9%).The compound then decomposed further at 420 • C with a weight loss of 49% (calcd 48.12%), which was most probably due to the removal of PhAA ligands from the metal.At a temperature of 450 • C, ZnO (obsd 13.12%, calcd 13.14%) was the final residue.The TGA curve of compound 2 shows the loss of 2,6-PyDC ligands in the temperature range of 200 • C to 240 • C (obsd 32%, calcd 45.64%).The observed weight loss is much less than the theoretical value, which is most likely due to the fact that the weight-loss process was not complete before the next step started.Finally, a residue of CuO (obsd 22.21%, calcd 21.98%) was left at 600 • C. For 3, the first weight loss (obsd 6.9%, calcd 9%) was observed in the temperature range of 90 • C to 200 • C, due to the loss of two uncoordinated water molecules (calculated for the asymmetric unit).The observed weight loss of 20% in the temperature range of 200 • C to 240 • C (calcd 30.68%) most likely corresponds with the loss of 2-PAC ligands.The reason this weight loss is much less than the theoretical value is similar to the reason given for 2. At 670 • C, the metal center was freed from all ligands, leaving behind the residue of CuO (obsd 19.54%, calcd 19.86%).

XRD Analysis and Thermal Analysis
The XRD patterns of the three compounds were measured at room temperature using a scanning range of 5° to 40°.Figures S1-S3 show the PXRD patterns of the compounds 1-3 (red line) and the XRD pattern simulated from the single crystal structure (black line).The two lines are essentially in agreement, demonstrating that the synthesized compound is phase pure, i.e., does not contain an impurity phase.The difference in the reflection intensity between the simulated and experimental patterns is due to a certain degree of preferred orientation of the powder samples during data collection.
To examine the thermal stability of the three compounds, TGA was implemented in the temperature range of 25-800 °C under a steady N2 flow, the outcome of which is given in Figure 5.For 1, the decomposition starts at 158 °C.All uncoordinated water molecules (calculated for the asymmetric unit) were lost in the temperature range of 158 °C to 200 °C (obsd 2.6%, calcd 2.9%).The compound then decomposed further at 420 °C with a weight loss of 49% (calcd 48.12%), which was most probably due to the removal of PhAA ligands from the metal.At a temperature of 450 °C, ZnO (obsd 13.12%, calcd 13.14%) was the final residue.The TGA curve of compound 2 shows the loss of 2,6-PyDC ligands in the temperature range of 200 °C to 240 °C (obsd 32%, calcd 45.64%).The observed weight loss is much less than the theoretical value, which is most likely due to the fact that the weightloss process was not complete before the next step started.Finally, a residue of CuO (obsd 22.21%, calcd 21.98%) was left at 600 °C.For 3, the first weight loss (obsd 6.9%, calcd 9%) was observed in the temperature range of 90 °C to 200 °C, due to the loss of two uncoordinated water molecules (calculated for the asymmetric unit).The observed weight loss of 20% in the temperature range of 200 °C to 240 °C (calcd 30.68%) most likely corresponds with the loss of 2-PAC ligands.The reason this weight loss is much less than the theoretical value is similar to the reason given for 2. At 670 °C, the metal center was freed from all ligands, leaving behind the residue of CuO (obsd 19.54%, calcd 19.86%).

Photoluminescent Properties and UV-Vis Spectra
The luminescent properties of CPs with d 10 metal centers have attracted intense interest because of their potential applications in chemical sensors, photochemistry, and electroluminescent displays [32][33][34].Therefore, synthesizing a novel Zn(II)-CP is a method for obtaining novel luminescent

Photoluminescent Properties and UV-Vis Spectra
The luminescent properties of CPs with d 10 metal centers have attracted intense interest because of their potential applications in chemical sensors, photochemistry, and electroluminescent displays [32][33][34].Therefore, synthesizing a novel Zn(II)-CP is a method for obtaining novel luminescent materials.The fluorescence properties of 1 and 1,4-bmimb ligand are shown in Figure 6 (λ ex = 350 nm).When excited at 350 nm, compound 1 and the 1,4-bmimb ligand showed strong fluorescence emission peaks at 442 nm and 453 nm, respectively.The difference in peak position may be owing to the interaction of metal and ligand, or the interaction between molecules, such as π•••π stacking.materials.The fluorescence properties of 1 and 1,4-bmimb ligand are shown in Figure 6 (λex = 350 nm).When excited at 350 nm, compound 1 and the 1,4-bmimb ligand showed strong fluorescence emission peaks at 442 nm and 453 nm, respectively.The difference in peak position may be owing to the interaction of metal and ligand, or the interaction between molecules, such as π•••π stacking.The UV-vis absorption spectra of 2 and 3 were given under the conditions of solid state and room temperature (Figure 7).In the UV region of the spectrum (200-350 nm), it is intraligand transitions that result in the absorption maxima from 200 nm to 270 nm. 2 and 3 also exhibit broad bands in the visible region with absorption maxima at 562 nm for 1 and 583 nm for 2, due to d-d transitions (2Eg to 2T2g transitions) of copper(II) compounds [35][36][37].

Conclusions
In conclusion, three novel CPs with diverse chain structures (Z-shaped chain, single chain and ladder-shaped chain) were successfully synthesized by the assembly of a 1,4-bmimb ligand, three other co-ligands, and zinc or copper ions.Each structure is assembled in a 3D supramolecular structure through intermolecular forces (hydrogen bonds and π•••π interactions for 1 and 2, hydrogen bonds for 3).In addition, the thermal stability, XRD patterns, fluorescence spectra, and UV-vis absorption spectra of CPs were discussed in detail.The UV-vis absorption spectra of 2 and 3 were given under the conditions of solid state and room temperature (Figure 7).In the UV region of the spectrum (200-350 nm), it is intraligand transitions that result in the absorption maxima from 200 nm to 270 nm. 2 and 3 also exhibit broad bands in the visible region with absorption maxima at 562 nm for 1 and 583 nm for 2, due to d-d transitions (2Eg to 2T2g transitions) of copper(II) compounds [35][36][37].
materials.The fluorescence properties of 1 and 1,4-bmimb ligand are shown in Figure 6 (λex = 350 nm).When excited at 350 nm, compound 1 and the 1,4-bmimb ligand showed strong fluorescence emission peaks at 442 nm and 453 nm, respectively.The difference in peak position may be owing to the interaction of metal and ligand, or the interaction between molecules, such as π•••π stacking.The UV-vis absorption spectra of 2 and 3 were given under the conditions of solid state and room temperature (Figure 7).In the UV region of the spectrum (200-350 nm), it is intraligand transitions that result in the absorption maxima from 200 nm to 270 nm. 2 and 3 also exhibit broad bands in the visible region with absorption maxima at 562 nm for 1 and 583 nm for 2, due to d-d transitions (2Eg to 2T2g transitions) of copper(II) compounds [35][36][37].

Conclusions
In conclusion, three novel CPs with diverse chain structures (Z-shaped chain, single chain and ladder-shaped chain) were successfully synthesized by the assembly of a 1,4-bmimb ligand, three other co-ligands, and zinc or copper ions.Each structure is assembled in a 3D supramolecular structure through intermolecular forces (hydrogen bonds and π•••π interactions for 1 and 2, hydrogen bonds for 3).In addition, the thermal stability, XRD patterns, fluorescence spectra, and UV-vis absorption spectra of CPs were discussed in detail.

Conclusions
In conclusion, three novel CPs with diverse chain structures (Z-shaped chain, single chain and ladder-shaped chain) were successfully synthesized by the assembly of a 1,4-bmimb ligand, three other co-ligands, and zinc or copper ions.Each structure is assembled in a 3D supramolecular structure through intermolecular forces (hydrogen bonds and π•••π interactions for 1 and 2, hydrogen bonds for 3).In addition, the thermal stability, XRD patterns, fluorescence spectra, and UV-vis absorption spectra of CPs were discussed in detail.

3. 3 .
Crystal Structures of {[Cu(1,4-bmimb) 0.5 (2-PAC)(HCOO)]•2H 2 O} n (3)Acrystallographically independent Cu(II) ion, a 2-PAC ligand, a formate ligand, half a 1,4-bmimb ligand, and two solvate water molecules make up the asymmetric structural unit of compound 3. Figure3agives a description of the coordination environment of each Cu(II) ion, highlighting the Crystals 2018, 8, 288 6 of 12 distorted octahedral geometry of the [CuN 2 O 4 ] coordination mode, which consists of two nitrogen atoms originating from a 1,4-bmimb ligand and a 2-PAC ligand and four oxygen atoms originating from three different 2-PAC ligands.The Cu-O bond lengths are in the range of 1.9651(19) Å to 2.6320(19) Å, and the lengths of Cu-N bonds are 1.968(2) Å and 2.027(2) Å.The bond angles around the Cu1 centers range from 54.95(7) • to 174.30(8) • .The two oxygen atoms from the same carboxylic acid are alternately linked to copper ions, creating an infinitely-extended -Cu-O-Cu-chain, shown in Figure 3b.Additionally, the 1,4-bmimb ligand bridges two -Cu-O-Cu-chains to form an infinitely extended 1D ladder-shaped chain, as shown in Figure 3c.Different ladder-shaped structures build up a 3D framework that is relianton O-H•••O hydrogen bonds between ligands and solvate water molecules (see Figure 3d and Table