Synthesis , Crystal Structure , and Properties of a New Coordination Polymer Built from N / O-Donor Mixed Ligands

The coordination polymer, namely, [Cd(H2L)(nobda)]n (1) was prepared by the reaction of Cd(NO3)2·4H2O with 4-amino-1,2-benzenedicarboxylic acid (H2nobda) and 1,4-di(1H-imidazol-4-yl)benzene (H2L), and characterized by single-crystal X-ray diffraction, elemental analysis, infrared (IR) spectroscopy, thermogravimetric analysis, and powder X-ray diffraction (PXRD). The carboxylic acid of H2nobda ligands was completely deprotonated to be nobda2− anions, which act as tridentate ligand to connect the Cd2+ to form two-dimensional (2D) network, while the neutral H2L ligands serve as a linear didentate bridge to connect two adjacent Cd2+ ions upper and down the 2D layer. The adjacent 2D layers were further linked into the three-dimensional (3D) supramolecular polymer by the weak interactions such as hydrogen bonds and π−π stacking interactions. The ultraviolet-visible (UV-vis) absorption spectra and luminescent properties in the solid state at room temperature have been investigated.


Introduction
In the past decades, the crystalline material of coordination polymers (CPs) has become an expanding research topic, not only due to their fascinating architectures and captivating topologies [1][2][3], but also for their potent applications in the fields of luminescence [4], gas adsorption/separation [5], chemical sensors [6], heterogeneous catalysis [7], and so on.Because the coordination polymers are composed of the metal ion and organic ligands, that is, the nature of metal ion and organic ligands are most important factors for constructing targeted CPs with desired properties [8][9][10].Especially, the design of organic ligands is the key factor to build CPs.O-donor carboxylic acids are extensively employed to build diverse CPs, due to their versatile coordination modes.For example, the Yaghi group has designed a series of carboxylic acid with expanded and variously functionalized organic linkers, and has made a prominent contribution for the construction of porous CPs by reticular synthesis based on metal paddle-wheel building or infinite metal-carboxylate secondary building units (SBUs) [11,12].Significantly, the surface area of CPs have great values ranging from 1000 to 10,000 m 2 /g, which have exceeded those of traditional porous materials, such as carbons and zeolites [11].Moreover, CPs with functional modifications exhibit favorable gas adsorption, which are typically selective gas adsorption properties for CO 2 or alkanes, and they can potentially alleviate the greenhouse effect or be employed as carriers to store energy gas [13,14].Meanwhile, polyazaheteroaromatic ligands, another series of N-donor ligands, including the imidazole, trizole and tetrazole, are also successfully used to construct coordination polymers [15][16][17].In our previous work, we have designed 4-imidazoly-containing ligands such as 1,3,5-tri(1H-imidazol-4-yl)benzene and,1,4-di(1H-imidazol-4-yl)benzene, and we use them to assemble porous metal-imidzolate complexes that show excellent gas adsorption or selective adsorption for CO 2 gas [18,19].Moreover, the polycarboxylates and N-donors ligands have different coordination preferences owing to the N and O atoms possessing different electron configurations.Due to their favorable compatibility for mixed polycarboxylates and N-donors ligands, we have synthesized a number of CPs with diverse structures by the reaction of mixed 4-imidazolyl and different carboxylate ligands, together with varied metal salts [20][21][22].Taking the favorable adjustability for the mixed 4-imidazolyl and carboxylate ligands, we have chosen 1,4-di(1H-imidazol-4-yl)benzene (H 2 L) and 4-amino-1,2-benzenedicarboxylic acid (H 2 nobda) as mixed ligand, to react with Cd(NO 3 )   (1) as an extension of our previous work.The UV-vis absorption spectra and luminescent properties in the solid state at room temperature have been investigated.

Materials and Instrumentation
All the reagents were of reagent grade in this experiment.IR spectra were carried out on a Bruker Vector 22 FT-IR spectrophotometer (Bruker, Billerica, MA, USA) using KBr pellets.Elemental analyses were analyzed on a Perkin-Elmer 240C Elemental Analyzer (Perkin-Elmer, Waltham, MA, USA).Power X-ray diffraction (PXRD) patterns were performed on a Shimadzu XRD-6000 X-ray diffractometer (Shimadzu, Kyoto, Japan) with CuKα (λ = 1.5418Å) radiation at room temperature.Thermogravimetric analyses (TGA) were carried on a simultaneous SDT 2960 thermal analyzer (Thermal Analysis Instrument Inc., New Castle, DE, USA).Photoluminescence spectra for the solid samples were recorded with a HORIBA FluoroMax-4 fluorescence spectrophotometer (Horiba, Kyoto, Japan) at room temperature.FLS920P fluorescence spectrometer (Edinburgh Instruments, Edinburgh, UK) was adopted to measure the decay lifetimes.

Crystal Structure Determination
The single crystal data of [Cd(H 2 L)(nobda)] n (1) was collected on a Bruker Smart APEX CCD diffractometer (Bruker, Billerica, MA, USA) The structure was solved by a direct method, and refined by full-matrix least squares on F 2 using the SHELX-97 program [23].The crystallographic data is listed in Table 1.

Results and Discussion
The result of X-ray diffraction analysis revealed that [Cd(H 2 L)(nobda)] n (1) crystallizes in monoclinic C2/c space group.The asymmetric unit of 1 consists of one crystallographically independent Cd(II) atom, one H 2 L ligand, and one completely deprotonated nobda 2− .As shown in Figure 1, the Cd1 had a distorted octahedral coordination geometry with N 3 O 3 binding set, in which the equatorial plane contains N4B and O3A from two distinct H 2 L and nobda 2− ligands respectively, and a pair of O1, O2 atoms from one chelating carboxylate group of nobda 2− ligand.The atoms N1 and N5C from two distinct H 2 L ligands occupy the axial positions with an N1-Cd1-N5C angle of 159.79(15) • (Table 2).The Cd-N distances are 2.264(4), 2.274(4), and 2.443(4) Å while the Cd-O distance is 2.340(3), 2.364(3), 2.424(4) Å, and the coordination angles around Cd(1) are in the range of 54.72(12) • ~159.79(15)• (Table 2).In this complex, two carboxyl groups from nobda 2− adopt µ 1 -η 1 :η 1 -chelating and µ 1 -η 1 :η 0 -monodentate coordination modes to coordinate with two Cd(II) atoms, while the amino from nobda 2− ligand also participate in coordination with another Cd(II) atom in this context; each nobda 2− ligand acts as a µ 3 -bridge to link the three Cd(II) atoms.Two such carboxylate groups from different nobda 2− ligands bridge two Cd(II) atoms to give a binuclear [Cd 2 (COO) 2 ] motif, with a Cd•••Cd distance of 6.14 Å.Each Cd 2 (COO) 2 binuclear unit acts as a 4-connected node to link other four identical motifs without considering the connection from H 2 L ligands.In this connection mode, the Cd(II) atoms are linked by nobda 2− ligands to form a two-dimensional (2D) Cd(II)(nobda) 2− layer structure with (4, 4) topology (Figure 2), where the [Cd 2 (COO) 2 ] motif is considered as a 4-connected node.The linear H 2 L ligands employ two-connector linkers to connect adjacent Cd(II) atoms of the same 2D layer, forming the Cd(II)(H 2 L)(nobda) 2− layer structure (Figure 3), which expands the 4-connected node of [Cd 2 (COO) 2 ] motif into a 6-connected node (Figure 4).According to the simplification principle, the resulting 2D layer structure of 1 can be considered as a uninodal 6-connected net with a Schläfli symbol (3 3 •4 10 •5•6) by taking the Cd 2 (COO) 2 binuclear motifs as network nodes and the H 2 L and nobda 2− ligands as 2-connected linkers (Figure 4) [24].Particularly, the carboxyl group can easily act as a hydrogen bonding acceptor, while the NH or N atom of the imidazolyl groups act as hydrogen bonding donors, and their interaction can easily benefit the construction of coordination polymers.As a result, the structure built from the mixed ligands exists rich hydrogen bonding interaction, and the C−H hydrogen bond exist between the 2D layers (Table 3).In addition to the hydrogen bond interaction, the π−π weak stacking interactions also exist between neighboring 2D structures.It could be found that the imidazole rings of H 2 L ligands from the neighboring 2D layers are parallel, showing the π−π stacking interactions with the centroid−centroid distance of 3.57 Å [25].In this context, the weak interactions including the π−π stacking and hydrogen bonding interactions extend 2D structure into a three-dimensional (3D) coordination polymer (Figure 5).3).In addition to the hydrogen bond interaction, the π−π weak stacking interactions also exist between neighboring 2D structures.It could be found that the imidazole rings of H2L ligands from the neighboring 2D layers are parallel, showing the π−π stacking interactions with the centroid−centroid distance of 3.57 Å [25].In this context, the weak interactions including the π−π stacking and hydrogen bonding interactions extend 2D structure into a three-dimensional (3D) coordination polymer (Figure 5).3).In addition to the hydrogen bond interaction, the π−π weak stacking interactions also exist between neighboring 2D structures.It could be found that the imidazole rings of H2L ligands from the neighboring 2D layers are parallel, showing the π−π stacking interactions with the centroid−centroid distance of 3.57 Å [25].In this context, the weak interactions including the π−π stacking and hydrogen bonding interactions extend 2D structure into a three-dimensional (3D) coordination polymer (Figure 5).

Thermal Analysis and Powder X-ray Diffraction Analysis
The thermal stability of the framework was investigated by thermogravimetric analysis (TGA) in the N 2 atmosphere from 20-700 • C. As shown in Figure 6, no weight losses were observed for complex 1, until the framework collapse at about 380 • C, which was well consistent with the crystal structural composition of 1.The phase purity of the bulk sample of 1 can be confirmed by the powder XRD experiment.As shown from Figure 7, the phase purity of the sample could be proven because the experimental pattern of the as-synthesized sample was consistent with the simulated one.

Thermal Analysis and Powder X-ray Diffraction Analysis
The thermal stability of the framework was investigated by thermogravimetric analysis (TGA) in the N2 atmosphere from 20-700 °C.As shown in Figure 6, no weight losses were observed for complex 1, until the framework collapse at about 380 °C, which was well consistent with the crystal structural composition of 1.The phase purity of the bulk sample of 1 can be confirmed by the powder XRD experiment.As shown from Figure 7, the phase purity of the sample could be proven because the experimental pattern of the as-synthesized sample was consistent with the simulated one.

Diffuse Reflectance Spectra
The solid state optical diffuse reflection spectra at room temperature were investigated for 1 (Figure 8).The compound showed absorption peaks at 305 nm, which is attributable to the π → π * transition of the conjugated organic ligand [26].In order to study the semiconductivity of the complexes, the diffuse reflectance data were measured and transformed into a Kubelka-Munk function to obtain their band gaps (Eg).The band gap Eg of compound 1 can be determined based on the theory of optical absorption for the direct band gap semiconductor: (Ahν) 2 = B(hν − Eg), where B is a constant corresponding to the material itself [26].As shown in Figure 9, the optical band gap of complex 1 obtained by extrapolation of the linear portion of the diffuse reflectance spectra are estimated as approximately 3.12 eV, which exhibits the nature of semiconductivity, indicating that the compound is optical semiconductor [27].

Diffuse Reflectance Spectra
The solid state optical diffuse reflection spectra at room temperature were investigated for complex 1 (Figure 8).The compound showed absorption peaks at 305 nm, which is attributable to the π → π * transition of the conjugated organic ligand [26].In order to study the semiconductivity of the complexes, the diffuse reflectance data were measured and transformed into a Kubelka-Munk function to obtain their band gaps (Eg).The band gap Eg of compound 1 can be determined based on the theory of optical absorption for the direct band gap semiconductor: (Ahν) 2 = B(hν − Eg), where B is a constant corresponding to the material itself [26].As shown in Figure 9, the optical band gap of complex 1 obtained by extrapolation of the linear portion of the diffuse reflectance spectra are estimated as approximately 3.12 eV, which exhibits the nature of semiconductivity, indicating that the compound is optical semiconductor [27].

Photoluminescent Property
Luminescent CPs, especially consisting of the d 10 closed-shell metal center and π-conjugated organic ligand system, have been proved to have the ability to adjust the emission because of their interaction between metal and ligands [28,29].Therefore, we carry out the solid-state photoluminescent property of complex 1, and the organic ligands are depicted in Figure 10.The complex 1 shows strong broad photoluminescence emission at 435 nm upon excitation at 359 nm.The intense emission band at 455 nm was observed for a free H2L ligand upon excitation at 342 nm, which may be ascribed to π* → π transition because the conjugative effect of H2L ligand [30].

Photoluminescent Property
Luminescent CPs, especially consisting of the d 10 closed-shell metal center and π-conjugated organic ligand system, have been proved to have the ability to adjust the emission because of their interaction between metal and ligands [28,29].Therefore, we carry out the solid-state photoluminescent property of complex 1, and the organic ligands are depicted in Figure 10.The complex 1 shows strong broad photoluminescence emission at 435 nm upon excitation at 359 nm.The intense emission band at 455 nm was observed for a free H 2 L ligand upon excitation at 342 nm, which may be ascribed to π* → π transition because the conjugative effect of H 2 L ligand [30].However, the H 2 nobda ligand shows weak emission maxima at 420 nm upon excitation at 338 nm, much lower than that of the π-conjugated H 2 L ligand, because the fluorescent emission of the π* → n transition resulting from benzene-dicarboxylate ligands could nearly be neglected in comparison with that arising from the π* → π transition of the π-conjugated H 2 L ligand.Therefore, benzene-carboxylate ligands made almost no contribution to the fluorescent emission of supermolecular polymer [31,32].In this compound, the emission bands of complex 1 is 20 nm blue-shifted, and it shows intensive emission in comparison to the free H 2 L ligand (Figure 11), which may be the intraligand fluorescence, since the free ligand exhibited a similar emission under the same condition [33,34].However, the H2nobda ligand shows weak emission maxima at 420 nm upon excitation at 338 nm, much lower than that of the π-conjugated H2L ligand, because the fluorescent emission of the π* → n transition resulting from benzene-dicarboxylate ligands could nearly be neglected in comparison with that arising from the π* → π transition of the π-conjugated H2L ligand.Therefore, benzene-carboxylate ligands made almost no contribution to the fluorescent emission of supermolecular polymer [31,32].In this compound, the emission bands of complex 1 is 20 nm blue-shifted, and it shows intensive emission in comparison to the free H2L ligand (Figure 11), which may be the intraligand fluorescence, since the free ligand exhibited a similar emission under the same condition [33,34].Furthermore, we carried out the study of the corresponding quantum yield (QY) and decay lifetimes for complex 1.The QY value of compound 1 is 0.86% (Figure 12).In addition, the luminescence lifetime of complex 1 is 81.25 ns (Figure 13), that luminescence decay curves was fitted by exponential function as I(t) = A exp(−t/τ).Therefore, the emissions of 1 should arise from a singlet state, because the luminescence lifetime is much shorter than the ones resulting from a triplet state (>10 −3 s) [35].Furthermore, we carried out the study of the corresponding quantum yield (QY) and decay lifetimes for complex 1.The QY value of compound 1 is 0.86% (Figure 12).In addition, the luminescence lifetime of complex 1 is 81.25 ns (Figure 13), that luminescence decay curves was fitted by exponential function as I(t) = A exp(−t/τ).Therefore, the emissions of 1 should arise from a singlet state, because the luminescence lifetime is much shorter than the ones resulting from a triplet state (>10 −3 s) [35].

Figure 8 .
Figure 8.The solid-state diffuse reflectance UV-Vis spectra for the complex.

Figure 8 .Figure 8 .
Figure 8.The solid-state diffuse reflectance UV-Vis spectra for the complex.

Figure 9 .
Figure 9.The band gap with 3.12 ev by the treatment with the Kubelka-Munk function.

Figure 9 .
Figure 9.The band gap with 3.12 ev by the treatment with the Kubelka-Munk function.

Figure 10 .
Figure 10.Solid-state photoluminescent spectra of 1 at room temperature.

Figure 11 .
Figure 11.The photographic picture exposed at 365 nm under an ultraviolet lamp (H2L ligand,

Table 1 .
Crystallographic data and structure refinement for