A Series of Coordination Polymers Constructed by 2-Phenylsuccinic Acid and Flexible Bis(imidazole) Ligands: Syntheses, Structures, and Photoluminescent Properties

Eight coordination polymers (CPs), namely, [Co(m-phsuc)(m-obix)] n (1), {[Co 2 (m-phsuc) 2 (m-pbix) 1.5 ]×2H 2 O} n (2), [Zn(m-phsuc)(m-obix)] n (3), {[Zn(m-phsuc)(m-mbix)]×H 2 O} n (4), {[Zn(m-phsuc)(m-pbix)]×H 2 O} n (5), {[Zn(m-phsuc)(m-pbix)]×1.5H 2 O×CH 3 OH} n (6), [Cd(m-phsuc)(m-obix)] n (7) and [Cd(m 3 - phsuc)(m-pbix) 0.5 ] n (8) (phsucH 2 = 2-phenylsuccinic acid, obix = 1,2-bis((imidazol-1-yl)methyl)benzene, mbix = 1,3-bis((imidazol-1-yl)methyl)benzene, pbix = 1,4-bis((imidazol-1-yl)methyl)benzene), were solvothermally synthesized and characterized by IR spectroscopy, elemental analysis, X-ray single-crystal diffraction, X-ray powder diffraction (PXRD), and thermal analyses. The structural analyses revealed that the compounds were 2D coordination polymers, in which metal ions were in tetrahedral coordination geometries in 1-6 and in octahedral coordination geometries in 2, 7 and 8. Compounds 1, 3, 4, 5, 7 and 8 displayed a 4-connected 2D wave sheets with the point symbol of {4 4 .6 2 }. Compound 2 was rare example exhibiting a 2D polyrotaxane structure. Compound 6 showed a 2D two-fold interpenetrating 3-connected hcb net with the point symbol of {6 3 }. In the compounds, the neighboring 2D layers were further assembled by p×××p stacking and weak C–H×××p interactions into the 3D supramolecular framework. The effects of the bis(imidazole) linkers and metals ions on the framework assemblies have been discussed. In addition, thermal and photoluminescent properties of the compounds were investigated.


Introduction
The design and construction of coordination polymers (CPs) have been drawn attention by researchers in the eld of crystal engineering owing to their intriguing architectures and application areas such as gas adsorption, sensor, magnetism, luminescence, dye removal, etc [1][2][3][4][5][6][7].In the construction of CPs with wellde ned architecture, the selection of appropriate ligands is a useful strategy.However, in addition to metal ions and ligands, reaction conditions, the solvent can also affect the nal structure of CPs [8,9].
As known, rigid or exible carboxylic acids have been widely utilized to provide charge balance of metal ions in the construction of CPs [10].Especially; exible ligands have been preferred to obtain an unpredicted structure with structural diversity [11,12].Recently, the combination of exible N-donor ligands and exible dicarboxylic acids (the mixed ligand strategy) has been widely employed to build unexpected CPs with intriguing architectures [13,14].In the literature, the coordination polymers synthesized with exible dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, etc, have been well documented and the ligands displayed several conformations to connect to metal ions resulting in versalite structures [15][16][17][18][19].In our previous studies, we used glutaric acid and substitute glutaric acid with semi-exible bis(imidazole) linkers in the synthesis of coordination polymers.They showed diverse coordination modes and conformations in the CPs with various dimensions [20][21][22].In this study, 2-phenylsuccinic acid (phsucH 2 ) containing phenyl side group, which was an asymmetric ligand, was used as an anionic linker.The phenyl side group on the succinate backbone can affect the coordination ability of the carboxylate group, orientation of the aliphatic skeleton, induce conformation ability, and reduce dimensionality in its compounds [23][24][25].In the literature, some coordination polymers with phsucH 2 were synthesized and characterized [25][26][27][28].However, there has been still need to investigate the effect of phenyl side group on succinate backbone on its compounds.Moreover, as an auxiliary ligand, we used semi-isomeric bis(imidazole) linker in the mixed-ligand assembly to investigate the effect of mixed ligand strategy on the dimensionality of phsuc 2− coordination polymers and also isomeric neutral ligands on the nal structure of CPs.The imidazole rings of bis(imidazole) ligand around -CH 2 -groups freely rotate to show various conformations, leading to interesting intriguing architecture [29].(7) and [Cd(µ 3 -phsuc)(µ-pbix) 0.5 ] n (8), with phsucH 2 and isomeric bis(imidazole) linkers in the presence of Co(II), Zn(II) and Cd(II) ions were hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, single crystal, and powder X-ray diffraction and thermal analysis.

Materials and measurements
All chemicals were received from commercial sources except that bis(imidazole) ligands were synthesized according to literature [29].IR spectra in the range 4000-400 cm −1 were taken with a Bruker Tensor FT-IR spectrometer with the use of KBr pellet.Elemental analyses of the compounds for C, H, N were performed with a Perkin Elmer 2400 Series II device.A Perkin Elmer Diamond TG/DTA Thermal Analyzer were utilized to thermal analysis curves under a dry air atmosphere in the range 30-1000 o C.
Photoluminescence spectra of the compounds were gained with a Perkin-Elmer LS-55 spectrophotometer.
A Panalytical Emperian X-ray diffractometer with Cu-Kα radiation was used to record powder X-ray diffraction patterns.A Bruker Smart Apex II CCD diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å) was used for single crystal X-ray diffraction data collection.SHELXT-2015 program in conjunction with the OLEX2 was used to solve the structures with direct methods and the structures were re ned by full-matrix least-squares on all F 2 data using SHELXL [30][31][32].In the structures, all hydrogen atoms were calculated and re ned using riding and free modes.The crystal structures were drawn using Mercury program [33].ToposPro software was used for topological analyses of the compounds [34].
Table 1 shows the crystal data parameters and Tables S1-S4 indicate the selected bond distances, angles, and hydrogen bond geometries.[Zn(µ-phsuc)(µ-obix)] n (3) The complex was prepared in a similar manner as 1 except for the use of Zn(NO [Cd(µ-phsuc)(µ-obix)] n (7) The compound was obtained in a similar method to that of 1 except for the use of 3CdSO 3. Results And Discussion

Syntheses and Characterizations
The eight coordination polymers were hydrothermally synthesized with phsucH 2 and isomeric bis(imidazole) linkers in the presence of diverse metal ions and characterized with several techniques.Compound 1 was not reproduced repeatedly.Hence, only single crystal analysis result was given to determine the effect of isomeric bis(imidazole) linkers on the structure.Elemental analysis results are agreed with the given formulations.IR spectra of compounds 2, 4-6 indicate that the broad bands observed at about 3429 cm −1 are attributed to the ν(OH) stretchings of crystal water molecules.The aromatic and aliphatic ν(CH) stretchings appeared at between 3134 and 2922 cm −1 in all compounds.In the compounds, asymmetric (ν as COO − ) and symmetric (ν s COO − ) stretching vibrations of carboxylate groups of phsuc 2− are observed in the range of 1616-1560 cm −1 and 1419-1359 cm −1 , respectively.The asymmetric stretching vibration of phsucH 2 shifts to a lower frequency after connecting to metal ions (Figs.S1-S8).Moreover, ν(C=N) stretching vibrations of imidazole rings are observed at 1520 cm −1 in the compounds.

Descriptions of crystal structures
[Co(µ-phsuc)(µ-obix)] n (1).X-ray crystallographic analysis indicates that compound 1 crystallizes in the orthorhombic Pbca space group, and the asymmetric unit of 1 contains one Co(II) ion, one phsuc and one obix ligand (Fig. 1a).Co1 ion is in a slightly distorted tetrahedral coordination geometry with two nitrogen atoms from two different obix ligands and two oxygen atoms from two different phsuc ligands.Each phsuc ligand adopts bis(monodentate) coordination mode linking Co(II) ions leading to a 1D linear chain with the intrachain Co⋅⋅⋅Co distance 7.546 Å (Fig. 1b).Furthermore, the adjacent 1D chains are connected by obix ligands into a 2D wave network (Fig. 1c.).This 2D network can be clari ed as a 4-connected sql/Shubnikov tetragonal topology with the point symbol of {4 4 .6 2 } (Fig. 1d).
The asymmetric unit of 2 contains two independent Co(II) ions, two phsuc, one and a half pbix ligands and two crystal water molecules.As shown in Fig. 2a, there are two cobalt centres (Co1 and Co2) in the compound with different coordination environments.Co1 is coordinated to two pbix ligands via two nitrogen atoms of the imidazole groups and two oxygen atoms of two carboxylate groups from two different phsuc ligands, showing a distorted tetrahedral geometry.Whereas, Co2 is six-coordinated with four carboxylate oxygen atoms from four different phsuc ligands to form a distorted octahedral geometry with N 2 O 4 donor set.The basal plane is constructed by the coordination of four carboxylate oxygen atoms, whereas, the axial site of Co2 ion is occupied by two nitrogen atoms from two symmetry-related pbix ligands.The carboxylate groups of phsuc ligands are completely deprotonated and adopt the bis(monodentate) coordination modes.Co1 and Co2 are bridged by phsuc anions into a 1D double chain with four metallic 28-member rings (Fig. 2b).Then, the adjacent two 1D double chains are further connected by two nitrogen atoms from pbix ligands with bimetallic 26-member rings, thus generating a 2D tubular structure along the b-axis (Fig. 2c).As shown in Figs.2d and 2e, pbix ligand adopts two different conformations, namely gauche and trans, to generate a 2D polyrotaxane sheet.The values of the Co⋯Co separation through these bridges are 7.221, 7.943 and 11.001 Å.The 2D framework can be clari ed as a 6-nodal net and the point symbol for net is {4.8 3 .12 2 } 2 {4} 2 {8 4 .12 9.16 2 }{8} 5 (Fig. 2f).
[Zn(µ-phsuc)(µ-obix)] n (3).X-ray diffraction result indicates that compound 3 crystallizes in the monoclinic space group P2 1 /n and features a two-dimensional (2D) network.The asymmetric unit consists of one Zn(II) ion, one phsuc and one obix ligand.As shown in Fig. 3a, the Zn1 ion is four-coordinated by two carboxylate oxygen atoms from two phsuc ligands and two nitrogen atoms from obix ligands, showing a distorted tetrahedral structure.The Zn-O and Zn-N bond lengths of previously reported Zn(II) complexes based on succinate ligands are similar to those of compound 3 [35].The phsuc ligand is fully deprotonated and acts as bridging ligands with bis(monodentate) coordination modes to connect the Zn(II) ions to form a 1D linear structure (Fig. 3b.).Two adjacent 1D units are connected by obix ligands to give a 2D wave network (Fig. 3c).The distances between the closest Zn(II) ions are 7.746 and 13.563 Å.
{[Zn(µ-phsuc)(µ-pbix)]⋅1.5H 2 O⋅CH 3 OH} n (6).It is very surprising that when solvent mixture (H 2 O (30 mL) and C 2 H 5 OH (3.5 mL)) for the synthesis of 5 is replaced with solvent mixture (H 2 O (5 mL), DMF (5 mL) and CH 3 OH (5 mL)), two-fold interpenetrated network of 6 is observed.Compound 6 shows a 2D network and it crystallizes in monoclinic system C2/c.The asymmetric unit consists of one Zn(II) ion, one phsuc, one pbix, one methanol and one and a half water molecules (Fig. 6a).The Zn1 atom is four-coordinated and forms a disordered tetrahedral geometry, which is completed by two carboxylate oxygen atoms from two distinct phsuc moieties and two nitrogen donors from two pbix ligands.The lengths of the Zn-O (1.951(6)-1.959(6)Å) and Zn-N (2.002(7) and 2.000(7) Å) bonds are comparable to those observed in the related literature [35].The adjacent Zn(II) ions are connected by bridging carboxylate groups of phsuc ligands into a 1D double chain with the Zn⋅⋅⋅Zn distance of 9.291 Å.Further, every Zn(II) ion are coordinated with two pbix ligands showing gauche form, and a two-fold interpenetrating 2D structure with bimetallic 26-member rings is observed (Fig. 6b.)Two adjacent 2D networks interpenetrate each other to form a 2D + 2D → 2D interpenetration network (Fig. 6c).The nal framework of 6 can be considered as a uninodal 3-connected hcb Shubnikov hexagonal plane net with the point symbol of {6 3 } (Fig. 6d).
[Cd(µ-phsuc)(µ-obix)] n (7).X-ray diffraction shows that 7 crystallizes in monoclinic system with P2 1 /n space group.The asymmetric unit contains one Cd(II) ion, one phsuc and obix ligands.As shown in Fig. 7a, the Cd1 ion is coordinated by four oxygen atoms from two different phsuc ligands and two nitrogen atoms from two different obix ligands.The Cd-O/N bond lengths are in the range of 2.245(4) to 2.470(4) Å.The phsuc ligands in 7 adopt a bis(bidentate) bridging coordination mode linking the neighbouring Cd(II) ions to generate 1D linear chain along the a-axis with Cd(II)⋅⋅⋅Cd(II) distances of 7.570 Å (Fig. 7b).

Powder X-ray diffraction and thermal properties
The phase purities the compounds were determined with powder X-ray diffraction (PXRD) measurements (Figs.S9-S15.).All the major peaks of the recorded PXRD patterns are agreed with those of the simulated patterns, demonstrating the phase purities of the compounds.
Thermal analysis curves of the compounds were recorded in the temperature range 30-1000 °C under a dry air atmosphere to determine the thermal behaviors of the compounds (Fig. S16

Photoluminescence properties
Solid-state photoluminescence spectra of compounds 3-8 the d 10 metal center were recorded under the same conditions at room temperature.As seen in Fig. 9, phsucH 2 displayed very weak emission at 360 and 425 nm upon excitation at 300 nm.In our previous study, isomeric bis(imidazole) linkers showed emissions in the range of 409-428 nm with the excitation at 300 nm.These emissions were assigned to π*→π or π*→n transitions of the ligands [40].Compounds 3-8 showed emission maxima at 440, 384, 368, 422, 440 and 382 nm, respectively upon excitation at 300 nm.The shifted emissions of the compounds, when compared to free ligands, could be assigned to formation of coordination bonds.

Conclusion
Eight coordination polymers were prepared with phsucH 2 and isomeric semi-exible bis(imidazole) linkers in the presence of diverse M(II) ions and characterized by several techniques.X-ray results showed that the phsuc 2− connected the metal ions to form one dimensional chains which were extended to 2D structures by coordination of bis(imidazole) linkers.Moreover, all compounds were extended to 3D supramolecular structure via π⋅⋅⋅π stacking and weak C-H⋅⋅⋅π interactions.The phsuc 2− ligand displayed diverse coordination modes in the compounds and pbix ligand showed transand gaucheconformations due to its exible nature.The emissions of the compounds were assigned to the intra-ligand transition of bis(imidazole) linkers because it was di cult to oxidize or reduce the d 10 metal ions  (vi) -x, -y+2, -z)