Hydrothermal Synthesis,Crystal Structure and Property of a Threedimensional Compound [Zn(INAIP)(Bipy)]·3H2O

Chen Mansheng,Sheng Liangbing,Deng Yifang,Tang Haijun,Liu Yang,Lu Weihong,Sun Junbin,Fu Weiwei

(Key Laboratory of Functional Organometallic Materials of College of Hunan Province,Hengyang Hunan 421008,China;College of Chemistry and Materials Science,Hengyang Normal University,Hengyang Hunan 421008,China)

Abstract：A new zinc metal-organic coordination polymer [Zn(INAIP)(Bipy)]·3H2O (1) was obtained by hydrothermal assembly of Zn(OAc)·2H2O with 5-(isonicotinamido)isophthalic acid (H2INAIP) in the presence of N-donor ligands, namely 4,4’-bipyridine (Bipy). And the resulting complex were characterized by IR spectrum, EA and single-crystal X-ray diffraction. Coordination compound 1 belongs to monoclinic system, space group P21/c. Single-crystal X-ray diffraction analysis revealed that each INAIP2- ligand in turn uses its two carboxylate groups to connect three metal centers, then the two-dimensional (2D) layer is formed, lastly, the layer was pillared by Bipy to form 3D network. On the other hand, complex 1 exhibits a strong photoluminescence in the solid state at room temperature, which 1 displays selective adsorption behavior of CO2 over N2 at 273K.

Key words：Zn(II)complex;adsorption;topology

Recently, the rational design and synthesis of porous metal-organic frameworks (PMOFs) using metal salts and functional organic ligands are presently of considerable interest, which is not only for the intriguing topological structures encountered, but also for their potential applications as functional materials in catalysis,fluorescence,gas storage,chemical sensing and so on[1-6].However, it is still a great challenge to predict the exactly structures and compositions of MOFs because even very small tuning factors can dramatically effect on the framework structure.Among all the factors, the organic ligands act as a crucial role in the synthesis of functional MOFs,which can make some contributions to its structures and chemical properties[7-10]. Furthermore, mixed ligands with N-donor groups acting as ancillary connector has been proved to be effective building units to construct novel MOFs.According to the above considerations, in this paper, we selected the functional organic amide ligand 5- (isonicotinamido)isophthalic acid(H2INAIP), which is a enlarged ligand likely the pyridine-3,5-dicarboxylate, only some structures is reported now[11-12].In order to further study the influence of multicarboxylate ligands on the coordiantion framework and related chemical properties with 4,4’-bipyridine (Bipy)ligands, herein, we synthesize a new zinc complex,namely [Zn(INAIP)(Bipy)]·3H2O (1), which crystal structure and adsorption of CO2were explored in details.

1 Experimental

1.1 Materials and instruments

The regents were used as commercial sources without further purification. Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer.The ligand H2INAIP was prepared as reported previously[12].The IR spectra were recorded on Bruker Vector 22 FT-IR spectrophotometer using KBr discs. Thermogravimetric analyses were performed on a simultaneous SDT 2960 thermal analyzer under nitrogen with a heating rate of 10°C min-1.The luminescent spectra for the solid powdered samples were recorded at room temperature on an Aminco Bowman Series 2 spectrophotometer with xenon arc lamp as the light source. In the measurements of the emission and excitation spectra,the pass width was 5.0 nm.The Rigaku D/max-2500 Xray powder diffractometer with graphite monochromated Cu-Ka radiation (λ = 0.15418 nm) was applied to analyze the powder X-ray diffraction (PXRD) data at 296 K and using ω-2θ scan mode within 5-50° at 40 kV and 40 mA. Gas sorption isotherms were measured on a volumetric adsorption apparatus(Bel-max).

1.2 Synthesis of the compound 1

Complex 1 was synthesized by hydrothermal method as follows: a mixture of Zn(OAc)·2H2O (22.2 mg, 0.1 mmol), H2INAIP (28.8 mg, 0.1 mmol), Bipy(17.0 mg,0.1 mmol),8 mL H2O and heated at 140°C for 3 days in a 16 mL Teflon-lined autoclave container.After cooling to room temperature, colorless block crystals of 1 were collected washing by water for three times in 27 % yield.Anal. Calcd. for C24H22ZnN4O8: C 51.45; H 3.93; N 10.00; found: C 51.62; H 3.89; N 10.08%. IR(KBr pellet, cm-1): 3410 (m), 1675 (s), 1621 (m), 1574(s), 1558 (w), 1434 (s), 1418 (m), 1375 (s), 1269 (m),809 (m),785 (m),667 (w),628 (w),591 (w).

1.3 X-ray crystallography

The X- ray diffraction measurement for 1 was performed on the Bruker Apex-II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ =0.071073 nm) at room temperature. The data were integrated by using theSAINTprogram[13], which also did the intensity corrections for Lorentz and polarization effect.An empirical absorption correction was applied using theSADABSprogram[14]. The structures were solved by direct methods using the programSHELXS-97and all the non-hydrogen atoms were refined anisotropically on F2by the full-matrix least-squares technique using theSHELXL- 97crystallographic software package[15,16]. Crystal data and structure refinement parameters are listed in Table 1.The selected bond lengths and bond angles are given in Table 2.

Table 1 Crystal Data and Structure Parameters for Complex 1

Table 2 Selected Bond Lengths(nm)and Bond Angle (°)

2 Results and Discussion

2.1 Structure description

The results of X-ray crystallographic analysis revealed that[Zn(INAIP)(Bipy)]·3H2O exhibits a pillared three-dimensional (3D) structure.As shown in Fig. 1a,there is one Zn(II) atom, one INAIP2-ligand, one Bipy ligand and three non-coordinated water molecules in the asymmetric unit of 1. Each Zn(II) atom is five-coordinated by two nitrogen (N4A and N3) atom from two Bipy ligands, and other three oxygen (O1, O3B and O4C) atoms from three different INAIP2-ligands, respectively. From the table 2, it can be seen that the bond lengths and angles around the Zn(II) are in the range of 0.2029(3)- 0.2256(2) nm and 86.21(10)- 178.03(11) °,respectively. It is note that each INAIP2-ligand in complex 1 links with three Zn(II) ions making use of its two carboxylate groups,while the pyridyl group does not bond any metals.Therefore,the coordination interactions between the three-connecting INAIP2-ligand and Zn(II)atom as described above make 1 into a 2D layer along aaxis, as illustrated in Fig. 2. Finally, the 2D layers are further pillared by Bipy ligands to form a 3D framework.To further understand this 3D structure, from a topological perspective, the dimeric [Zn(OCO)]2units are considered as the nodes, the topology of complex 1,calculated by TOPOS[17], is a uniform 6-connected 3D pcu topology with the point symbols(412.63)(Fig.3).

Fig.1 Molecular structure of complex 1 with 30% thermal ellipsoids.All hydrogen atoms,free water molecules are omitted for clarity.

Fig.2 2D layer structure linked by INAIP ligands and Zn(II)centers without Bipy molecules.

Fig.3 View of the 3D pillared-layer structure of 1 (a)and pcu topology (b).

2.2 IR,TG,Photoluminescent Properties and PXRD Characterized

The infrared spectra of the title complex 1 has been measured and some important assignments are shown in the experimental section. No strong IR band from -COOH appeared at nearly 1700 cm-1, suggesting that the carboxylate of H2INAIP ligands are protonated, the strong absorption peaks at 3410 cm-1, which is correspond to the ν(O-H) stretching vibration of water molecules. These IR results are coincident with the crystallographic structural analyses. The thermal stability of the complex 1 was determined from 25 to 700 °C under a nitrogen atmosphere at a heating rate of 10 °C/min.The release of all lattice water molecules is observed in the temperature range of 30- 125 °C (calcd. 9.64%,found 9.72%) (Fig. 4a).After the loss of all the water molecules, the 3D framework is stable up to 280 °C,where the framework begins to decompose. The remaining is 15.7%, which corresponds to ZnO (calcd.14.5%). Powder X-ray diffraction (PXRD) patterns were measured for complex 1 and the outgassed 1 (Fig. 4b).The experimental PXRD patterns and are in good agreement with the corresponding simulated ones, indicating the purity of the synthesized material and the assimulated crystals. The photoluminescent properties for 1 and Na2INAIP were investigated in the solid state at room temperature.As shown in Fig. 4c, the maximum emission peak were observed at 465 nm (λex= 390 nm)for Na2INAIP ligand, 483 nm (λex= 390 nm) for 1, respectively. It could be concluded that a significant red shift compared with that of the constituent ligands.This result shows that the fluorescence of 1 may be attributed to the mixture effects of intraligand charge transmission and carboxylate groups to Zn(II)ions.On the other hand,these results further imply that complex 1 could be good candidate of potential hybrid inorganic-organic photoactive materials.

Fig.4 The TG curve of compound 1(a),the powder X-ray diffraction patterns calculated from the single-crystal data and that obtained from the experiments for 1 and activated 1 (b)and the solid photoluminescence of 1 (c).

2.3 Gas adsorption

As discussed above, the frameworks of 1 is stable even under the condition of more than 280 °C. In order to ensure whether the network can be maintained after the removal of the water molecules for 1,PXRD patterns were measured and the results are shown in Fig. 4b.The framework of the 1 still has good crystallinity without water molecules, therefore, the as-synthesized crystal samples were heated at an optimized temperature of 130°C for 20 hours.On the other hand,as a continuous work,we evaluated the capability of complex 1a for CO2storage and selectivity, the CO2over N2adsorption isotherm for the complex 1a (273K) are also measured.As shown in Fig.5, the results indicate that almost no N2adsorption was observed at 273K, while the significant amounts of CO2(273 K) were adsorbed and the isotherm exhibits classical type-I[18].The uptake of CO2increases rapidly with the increase of pressure, and reach to 31.78 cm3(STP)/g at 1 bar approximately 0.34 CO2molecules per formula unit. It is noteworthy that no N2were adsorbed, and the selective sorption of CO2rather than N2gas could facilitate dipole-quadrupole interactions between the acylamide groups in 1 and CO2,inducing CO2molecules may be provided with specific interactions and the host framework[19,20].

Fig.5 N2 and CO2 adsorption isotherm (273 K)of 1.

3 Conclusion

In summary, we have successfully synthesized a porous Zn(II) coordination frameworks with INAIP2-by simply adding the Bipy in the reaction system. In addition, the adsorption of complex 1 indicates that it has selective adsorptions of CO2over N2after the removal of the guest molecules within the pores.