Synthesis , Crystal Structures , and Photoluminescent Properties of Two Supramolecular Architectures Based on Difunctional Ligands Containing Imidazolyl and Carboxyl Groups

Two new supramolecular architectures, namely, [Cd(L1)2(H2O)]n (1) and [Ni(L2)2(H2O)]n (2), were synthesized by the reaction of corresponding metal salts of CdCl2·2.5H2O and NiCl2·6H2O with 2-(1H-imidazol-4-yl)benzoic acid (HL1) and 3-(1H-imidazol-4-yl)benzoic acid (HL2) respectively, and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and powder X-ray diffraction (PXRD). Both HL1 and HL2 ligands are deprotonated to be L1 and L2 anions that coordinate with Cd(II) and Ni(II) atoms to form two-dimensional (2D) layer structure. Topologically, complex 1 is a 2D network with (4, 4) sql topology, while 2 is a typical 63-hcb topology net. Complex 1 exhibits intense light blue emission in the solid state at room temperature.


Introduction
The rational design and construction of metal-organic frameworks (MOFs) has become an expanding research topic in the fields of supramolecular chemistry and crystal engineering not only because of their intriguing molecular architectures and intricate topologies but also their promising applications in various areas in fluorescence, gas adsorption, separation, ion exchange, magnetic properties, catalysis, and so on [1][2][3][4][5][6].Because the complexes incorporate metallic centers and bridging organic linkers, therefore, it is important to design suitable organic ligands and choose metallic centers to assemble desirable frameworks with specific structures and functions.
Due to their favorable coordination abilities for the N or O donor atoms, the organic linkers incorporating N-heterocyclic bridging or carboxyl groups ligands are extensively applied to construct coordination polymers and have made great progress in the field of crystal engineering [7,8].The N-heterocyclic bridging ligands include imidazole, triazole, tetrazole, and pyridine or its analogues such as pyridazine, tetrazine, triazine, and pyrazine moieties [9][10][11][12].In our previous studies, we have elaborately designed the series of novel 4-imidazole ligands with flexible coordination modes, and successfully employed the ligands to construct porous frameworks based on the 4-imidazolate-metal building units [13][14][15], and the porous frameworks exhibit favorable gas adsorption properties.Taking the various coordination modes of carboxyl group into consideration, we have further carried out studies to design another type of novel difunctional ligand containing 4-imidazolyl group and carboxyl group [16].Furthermore, the novel organic ligand was applied to construct two series Cu(II) and Cd(II) coordination polymers based on its flexible coordination modes.It should be noted that difunctional groups occupy the 1-and 4-position of benzene ring respectively, and as a result, the ligand often act as linear 2-connected node exhibited in resulting coordination polymers.As an extension of our previous work, we have designed two isomers of 4-(1H-imidazol-4-yl)benzoic acid [16] and synthesized two new organic ligands-2-(1H-imidazol-4-yl)benzoic acid (HL 1 ) and 3-(1H-imidazol-4-yl)benzoic acid (HL 2 ).Here, we report the synthesis and crystal structures of two new coordination polymers of [Cd(L 1 ) 2 (H 2 O) 2 ] n (1) and [Ni(L 2 ) 2 (H 2 O)] n (2) obtained by the reaction of HL 1 and HL 2 with corresponding metal salts under hydrothermal condition.

Results and Discussion
The X-ray single-crystal structural analysis reveals that complex 1 crystallizes in monoclinic C2/c space group, and the asymmetric unit has a half molecule of [Cd(L 1 ) 2 (H 2 O)], namely, a Cd(II) atom sitting on a special position with a half of occupancy, one L  1).
The organic molecule of HL 1 was deprotonated to L 1 − and act as a µ 2 -bridge to link two Cd(II) atoms using the imidazolyl and carboxyl coordination groups respectively.It should be noteworthy that these difunctional groups occupy the adjacent positions and result in serious steric effect, which promote these two group distortedly twist from the benzene plane.That is, the aromatic rings of phenyl and imidazolyl rings are nearly perpendicular, with the dihedral angle of 84.99º while the plane of carboxyl group twists from benzene ring with 52.367º respectively.In this sense, the HL 1 ligand is quite different from the ligand of 4-(1H-imidazol-4-yl)benzoic acid reported in our previous study, that the aromatic groups often are coplanar due to lack of steric hindrance.Each L 1 − ligand acts as 2-connector to link two Cd(II) atoms, while each Cd(II) atom in return connects four different L − ligands.In this connection, the Cd(II) atoms are linked to form a two-dimensional (2D) network along the ab plane (Figure 2).Topologically, the 2D layer is (4,4) 3).

Structural Description of [Ni(L2)2(H2O)]n (2)
Compound 2 crystallizes in the monoclinic space group P21/c, and the asymmetric unit contains one unique Ni(II) atom, two L2 − ligands, and one coordinated water molecule.As shown in

Structural Description of [Ni(L2)2(H2O)]n (2)
Compound 2 crystallizes in the monoclinic space group P21/c, and the asymmetric unit contains one unique Ni(II) atom, two L2 − ligands, and one coordinated water molecule.As shown in

Thermal Analysis and Powder X-ray Diffraction Analysis
The thermal stability of complexes 1 and 2 were examined by thermogravimetric analysis (TGA) to ascertain the stability of supramolecular architectures in the N 2 atmosphere from 25-700 ºC, and the result is shown in Figure 7.The complex 1 shows a weight loss of 3.50% in the temperature range of 100-125 ºC, corresponding to the release of coordinated water molecules (calc.3.57%), and the decomposition of the residue occurred at 330 ºC.The first weight loss of 3.83% around 85 ºC indicates the exclusion of coordinated water molecules (calc.3.99%) for 2, and the residue is stable up to 325 ºC.Powder XRD experiment was carried out to confirm the phase purity of bulk sample, and the experimental pattern of the as-synthesized sample can be considered comparable to the corresponding simulated one, indicating the phase purity of the sample (Figure 8).

Thermal Analysis and Powder X-ray Diffraction Analysis
The thermal stability of complexes 1 and 2 were examined by thermogravimetric analysis (TGA) to ascertain the stability of supramolecular architectures in the N2 atmosphere from 25-700 ºC, and the result is shown in Figure 7.The complex 1 shows a weight loss of 3.50% in the temperature range of 100-125 ºC, corresponding to the release of coordinated water molecules (calc.3.57%), and the decomposition of the residue occurred at 330 ºC.The first weight loss of 3.83% around 85 ºC indicates the exclusion of coordinated water molecules (calc.3.99%) for 2, and the residue is stable up to 325 ºC.Powder XRD experiment was carried out to confirm the phase purity of bulk sample, and the experimental pattern of the as-synthesized sample can be considered comparable to the corresponding simulated one, indicating the phase purity of the sample (Figure 8).

Photoluminescent Property
Considering that coordination polymers with d 10 closed-shell metal center and aromatic-containing system may exhibit excellent fluorescence properties and serve as good candidates for potential applications as photoactive materials, we investigate the solid-state fluorescence property of complex 1 as well as the HL1 ligand at room temperature [20][21][22].The free HL1 ligand shows emission band at 433 nm upon excitation at 407 nm, which may be attributed to π* → π transition of the intraligands [23,24], while the complex 1 exhibits light blue emission with maximum at 455 nm upon excitation at 399 nm as depicted in Figure 9.By contrast with the free ligand, the emission bands of complex 1 are 22 nm red-shifted.Such board emission bands may be tentatively assigned to ligand-to-metal charge transfer (LMCT) [25,26].

Photoluminescent Property
Considering that coordination polymers with d 10 closed-shell metal center and aromatic-containing system may exhibit excellent fluorescence properties and serve as good candidates for potential applications as photoactive materials, we investigate the solid-state fluorescence property of complex 1 as Crystals 2017, 7, 228 7 of 10 well as the HL 1 ligand at room temperature [20][21][22].The free HL 1 ligand shows emission band at 433 nm upon excitation at 407 nm, which may be attributed to π* → π transition of the intraligands [23,24], while the complex 1 exhibits light blue emission with maximum at 455 nm upon excitation at 399 nm as depicted in Figure 9.By contrast with the free ligand, the emission bands of complex 1 are 22 nm red-shifted.Such board emission bands may be tentatively assigned to ligand-to-metal charge transfer (LMCT) [25,26].

Photoluminescent Property
Considering that coordination polymers with d 10 closed-shell metal center and aromatic-containing system may exhibit excellent fluorescence properties and serve as good candidates for potential applications as photoactive materials, we investigate the solid-state fluorescence property of complex 1 as well as the HL1 ligand at room temperature [20][21][22].The free HL1 ligand shows emission band at 433 nm upon excitation at 407 nm, which may be attributed to π* → π transition of the intraligands [23,24], while the complex 1 exhibits light blue emission with maximum at 455 nm upon excitation at 399 nm as depicted in Figure 9.By contrast with the free ligand, the emission bands of complex 1 are 22 nm red-shifted.Such board emission bands may be tentatively assigned to ligand-to-metal charge transfer (LMCT) [25,26].

Materials and Instrumentation
All the commercially available chemicals and solvents were of reagent grade and used as received without further purification.Elemental analyses were performed on a Perkin-Elmer 240C Elemental Analyzer (PerkinElmer, Waltham, MA, USA).IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer (Instrument Inc., Karlsruhe, Germany) using KBr pellets.Thermogravimetric

Materials and Instrumentation
All the commercially available chemicals and solvents were of reagent grade and used as received without further purification.Elemental analyses were performed on a Perkin-Elmer 240C Elemental Analyzer (PerkinElmer, Waltham, MA, USA).IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer (Instrument Inc., Karlsruhe, Germany) using KBr pellets.Thermogravimetric analyses (TGA) were performed on a simultaneous SDT 2960 thermal analyzer (Thermal Analysis Instrument Inc., New Castle, DE, USA) under nitrogen with a heating rate of 10 ºC min −1 .Power X-ray diffraction (PXRD) patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) with CuKα (λ = 1.5418Å) radiation at room temperature.The fluorescent spectra were measured using a Perkin Elmer LS-55B fluorescence spectrometer (PerkinElmer, Billerica, MA, USA).

Synthesis of [Cd
Reaction mixture of HL 1 (0.021 g, 0.1 mmol), CdCl 2 •2.5H 2 O (0.0228 g, 0.1 mmol) and 10 mL H 2 O was adjusted to pH = 7 with 0.5 mol L −1 NaOH solution.The mixture was then sealed in a 20 mL Teflon-lined stainless steel container and heated at 120 ºC for 48 h.After cooling to the room temperature, colorless block crystals of 1 were collected with a yield of 82% by filtration and washed with ethanol and water for several times.Anal

Crystal Structure Determination
The single crystal data of [Cd(L 1 ) 2 (H 2 O)] n (1) and [Ni(L 2 ) 2 (H 2 O)] n (2) were collected on a Bruker Smart APEX CCD diffractometer (Bruker, Karlsruhe, Germany) with graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293(2) K.The structure was solved by direct method and refined by full-matrix least squares on F 2 using the SHELX-97 program [27].The hydrogen atoms were generated geometrically except that H5B of water molecule in 1 was found at reasonable positions in the differential Fourier map and located there.The crystallographic data and structural refinement are listed in Table 3. Crystallographic data for the structure reported in this paper has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication Nos.CCDC 1554214 for 1 and 1554215 for 2. Copy of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk).

2. 2 .
Structural Description of [Ni(L 2 ) 2 (H 2 O)] n (2) Compound 2 crystallizes in the monoclinic space group P2 1 /c, and the asymmetric unit contains one unique Ni(II) atom, two L 2 − ligands, and one coordinated water molecule.As shown in Figure 4, the Ni1 has a distorted octahedral environment, in which the equatorial plane contains O3, O4 atoms of one chelating carboxylate groups from one L 2 − ligand, and N3A, O2B from other two distinct L 2 − ligands, and the atoms N1 and O5 from L − ligand and coordinated water molecule occupy the axial positions with an N1−Ni1−O5 angle of 175.19(5)º (

L 2 − 10 Figure 4 ,
Figure4, the Ni1 has a distorted octahedral environment, in which the equatorial plane contains O3, O4 atoms of one chelating carboxylate groups from one L2 − ligand, and N3A, O2B from other two distinct L2 − ligands, and the atoms N1 and O5 from L − ligand and coordinated water molecule occupy the axial positions with an N1−Ni1−O5 angle of 175.19(5)º (Table1).Two carboxyl groups from different L2 − ligand adopt μ1-η 1 :η 0 -monodentate and μ1-η 1 :η1 -chelating coordination modes to coordinate with Ni(II) atoms while the imidazolyl groups both employ the external N atoms to link Ni(II) atoms.As a result, two different L2 − both act as μ2-bridge to link two Ni(II) atoms, and in return, each Ni(II) atom link other three Ni(II) atoms by the L2 − ligand.Topologically, L2 − ligands bridge the Ni(II) atoms to form a 2D layer with 6 3 -hcb topology where the Ni(II) atom and L − ligand act as 3-and 2-connected nodes respectively (Figures 5).Compared with HL1 ligand, the dihedral angle between two different imidazolyl groups and benzene rings from HL2 are 21.126 and 16.304º, not twisting distortedly as 1, because the imidazolyl and carboxyl groups occupy the meta-position, avoiding the steric hindrance.There exist weak interactions including rich hydrogen bond and π−π stacking interactions.Noticeably, the NH or N atom of imidazolyl groups and carboxyl group can act as hydrogen bonding donor or acceptor, which benefits the construction of supramolecular structures.In 2, it can be seen clearly that the N−H•••O, O−H•••O hydrogen bonds (N(4)•••O(1) 2.833(18) Å, N(4)-H(4)•••O(1) 144º; O(5)•••O(4) 2.781(16) Å, O(5)-H(5B)•••O(4) 176º) connect the adjacent 2D layers to produce 3D supramolecular polymer (Figure 6, Table 2).Particularly, two benzene rings of the L2 − ligands between the adjacent 2D layers are nearly parallel with a dihedral angle of 3.16º and are separated by a centroid−centroid distance of 3.56 Å, indicating the presence of strong π−π stacking interactions (Figure 6).Therefore, the overall framework of 2 is a 3D supramolecular polymer by rich N−H•••O and O−H•••O hydrogen bonds and strong π-π stacking interactions.The total void value of the channel is estimated to be 76.5 Å 3 , 4.0 % of the total crystal volume of 1910.9Å 3

Figure 5 .
The 2D network structure (left) and the representation of 6 3 -hcb framework of 2 (right).

Figure 6 .
Figure 6.3D supramolecular network structure of 2 linked by hydrogen bonds and π-π stacking interactions highlighted by pink and blue dotted line, respectively.

Figure 6 .
Figure 6.3D supramolecular network structure of 2 linked by hydrogen bonds and π-π stacking interactions highlighted by pink and blue dotted line, respectively.

Figure 7 .
Figure 7. Thermal analysis curve of the complexes 1 and 2.Figure 7. Thermal analysis curve of the complexes 1 and 2.

Figure 8 .
Figure 8.The X-ray powder diffraction of the complexes 1 and 2.

Figure 8 .
Figure 8.The X-ray powder diffraction of the complexes 1 and 2.

Figure 8 .
Figure 8.The X-ray powder diffraction of the complexes 1 and 2.

Figure 9 .
Figure 9. Solid-state photoluminescent spectra of 1 at room temperature.

Figure 9 .
Figure 9. Solid-state photoluminescent spectra of 1 at room temperature.
Complex 2 was obtained by the same procedure used for preparation of 1 except that the CdCl 2 •2.5H 2 O and HL 1 were replaced by NiCl 2 •6H 2 O (0.024 g, 0.1 mmol) and HL 2 respectively.Green block crystals of 2 were collected in 78% yield.Anal.Calcd.(%) for C 20 H 16 N 4 O 5 Ni: C, 53.26; H,

Table 3 .
Crystallographic data and structure refinement for 1 and 2.