WANG Ji-Jiang CAO Zhuang TANG Long WANG Xiao HOUXiang-Yang JU Ping REN Yi-Xia

?

Synthesis, Structure, Photoluminescence and Photocatalytic Properties of a New Cd(II) Metal-organic Framework Based on 1,4-Di(2,6-dimethyl-3,5-dicarboxypyridyl)benzene①

WANG Ji-Jiang②CAO Zhuang TANG Long WANG Xiao HOUXiang-Yang JU Ping REN Yi-Xia

(716000)

A new 3D coordination polymer, [Cd2(L)(bpb)2](1, H4L = 1,4-di(2,6-dimethyl- 3,5-dicarboxypyridyl) benzene, bpb =1,4-bis(4-pyridyl) benzene, has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, elemental analysis, TGA, IR spectroscopy and UV-Vis spectrum.Complex 1 crystallizes in orthorhombic, space group2221, with10.9393(12),= 20.900(3),= 20.027(2) ?,= 90°,= 4578.9(9) ?3,D= 41.668 Mg/cm3,M= 1149.74,(000) = 2312.0,= 0.996 mm–1,= 4, the final= 0.0316 and= 0.0786 for 4271 observed reflections with> 2(). Structural analysis shows that 1 possesses a 3D network based on the 2D layer bridged by L4-ligands, while the 2D layers are composed of the adjacent 1D chains joined by L4?ligand. The photoluminescent investigation indicates that one broad emission bandwith the maximum of 445 nm can be observed in 1. Moreover, compound 1 has high photocatalytic degradation effects on methylene blue under UV irradiation.

Cd(II), synthesis,structure, photoluminescence and photocatalytic properties;

1 INTRODUCTION

Metal-organic frameworks (MOFs) have attracted much attention owing to their diverse structures, intriguing topologies and potential applications in gas storage and separation[1-4], fluorescence[5-7], mag- netism[8-11]and catalysis[12-14]. It is generally known that the rational design and construction of MOFs are highly influenced by different factors such as metal ions[15], organic ligands[16], reaction temperature[17], ect. Among these factors, the organic ligands play a key role in theframework construction of metal complexs due to their different coordination atoms, flexible coordination sites, and rich coordination modes[18, 19]. To date, several cadmium complexes have been synthesized and utilized for their excellent photoluminescence and photocatalytic properties[5-7].

Noteworthy, some heterocyclic aromatic multicar- boxylic acids have been widely used as the ligand to construct various cadmium complexes because of the existences of both N and O atoms and their strong coordination abilities[20-22]. The ligand 1,4-bis(2,6- dimethyl-3,5-dicarbonyl pyridine) benzene, with both N and O atoms, has much richer coordination sites, and possibly can give various possibilities to form novel structures. Herein, we choose H4L as the organic ligand and Cd(II) as the metal center in the presence of bpb ligand to form a new coordination polymer, [Cd2(L)(bpb)2](1). Moreover, the lumine- scent and photocatalytic properties of the complex were also investigated.

2 EXPERIMENTAL

2. 1 Materials and methods

All chemicals and solvents employed for synthesis were commercially available and used without further purification. Elemental analyses for carbon, hydrogen, and nitrogen were performed with a Vario EL III elemental analyzer. The FT-IR spectra were recorded on a Bruker EQUINOX-55 spectrometer using KBr pellets in the range of 4000～400 cm-1. Thermal gravimetric analysis (TGA) was measured on a NETZSCH STA 449F3 thermal gravimetric analyzer in flowing nitrogen at a heating rate of 10 oC/min. Fluorescence spectra were collected on a Hitachi F-4500 fluorescence spectrophotometer at room temperature. The solid-state diffuse-reflectance UV-Vis spectra were performed at room temperature on a Shimadzu UV-2550 spectrophotometer equip- ped with an integrating sphere by using BaSO4as the reflectance standard.The concentrations of the me- thylene blue solutions were analyzed using a UV-Vis spectrophotometer (Shimadzu, UV-2550).

2. 2 Synthesis of [Cd2(L)(bpb)2]n (1)

A mixture of CdO(0.10 mmol, 12.8 mg), bpb (0.05 mmol, 11.6 mg), H4L (0.10 mmol, 23.0 mg) and 15 mL H2O was stirred for 30 min.The mixture was then placed in a 25 mL Teflon-lined stainless- steel vessel and heated at 160 ℃ for 3 d. Colorless block crystals were obtained after the mixture was slowly cooled to room temperature.Yield: ca. 62% based on Cd.Calcd. for C56H40Cd2N6O8(%): C, 58.45; H, 3.48; N, 7.31. Found (%): C, 58.34; H, 3.52; N, 7.35. IR (KBr pellet, cm-1): 3422 s, 3051 w, 2924 w, 2367 s, 1605 s, 1541 s, 1433 m, 1400 m, 1360 m, 853 w, 806 w, 669 w, 442 w.

2. 3 Crystal structure determination

A colorless orthorhombic crystal of the complex (0.10mm × 0.05mm × 0.04mm) was selected for diffraction datacollection at 296(2) K on a Bruker Smart APEX II CCD diffractometer equipped with a graphite-monochromatic Mo-radiation (= 0.71073 ?). The absorption correction was applied using semi-empirical methods of SADABS pro- gram[23]. The structure was solved by direct methods using SHELXS-97 and refined by full-matrix least-squares on2using the SHELXL-97 programs, respectively[24, 25]. The positions of all non-hydrogen atoms were refined anisotropically. Hydrogen atoms were positioned in the geometrically calculated positions. In 1, a total of 11866 reflections were collected in the range of 2.03<25.50 (–13≤≤13, –24≤≤25, –9≤≤24) and 4271 were independent withint= 0.0366, of which 4271 with> 2() (refinement on2) were observed and used in the succeeding structure calculation. The final= 0.0316,= 0.0786 (= 1/[2(F2) +(0.0579)2+ 0.0000], where= (F2+ 2F2)/3), (Δ)max= 1.389 and (Δ)min= –0.805 e/?3. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) for Complex 1

Symmetry operations:A:, –+2,–+1; B: –,+1, –+3/2; C:,+1, z; D: –+1/2,+1/2, –+3/2; E:–1/2,+1/2,

2. 4 Catalysis experiments

The photocatalytic performance of complex 1 was measured for the degradation of methylene blue (MB) as a model complex of organic pollutant.During the photodegradation process, 15 mg catalysts and0.7 μL of 30% H2O2were added into 25 mL MB solution (10 mg/L), magnetically stirred in the dark environment for 30 min to establish the adsorption- desorption equilibrium. It was then conducted on an XPA-7 type photochemical reactor equipped with a 100 W mercury lampand the reaction temperature was maintained at about 25 ℃ by circulating cooling water. A certain volume of samples was collected and separated per 10 min by centrifugation to remove residual catalyst particles. Then thesolution was analyzed by using a Shimadzu UV-vis spectrometer. The concentration of MB was estimated by the absorbance at 664 nm, characteristic of MB.

The photocatalytic decomposition efficiency of MB is defined by the initial concentration of the dye solution C0(mg/L) and the concentration of the dye solution at reaction timeCt(mg/L):

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

The result of X-ray diffraction analyses indicates that complex 1 crystallizes in the orthorhombic2221space group and contains two Cd(II) ions, one L4-anion, and two bpb ligands. As shown in Fig. 1, the Cd(II) ions have two different coordination numbers (five and six). Cd(1) is coordinated by four O atoms from two different L4?ligands, and two nitrogen atoms from two bpb ligands, displaying a slightly distorted octahedron. Different from Cd(1), Cd(2) shows a slightly distorted trigonal-bipyramidal geometry coordinated by two O from two H2L2-ligands, two N from two bpb and one N from one L4-ligand. The Cd–O and Cd–N distances vary from 2.332(3) to 2.388(3) ? and from 2.312(3) to 2.337(3) ? (Table 1). All the bond lengths fall in the rational range[26].

Fig. 1. Coordination environment of Cd(II) in 1. Hydrogen atoms are omitted for clarity

As shown in Fig. 2a, the carboxylic acid groups of L4-adopt two different kinds of coordination modes. The four cadmium atoms are linked by six carboxyl oxygen atoms in each ligand (the 3- and 5-car- boxylates are linked to two cadmium atoms using monodentate coordination mode, and the 3?- and 5?-carboxylates are connected with two cadmium atoms using a bidentate coordination mode). The Cd(II) ions are connected through the bpb ligands to form a 1D chain, and then the cadmium atoms between adjacent chains are joined by the bidentate chelate carboxyl group carboxyl groups in the L4-ligand to form a 2D layer (Fig. 2b ). It is noteworthy that on the basis of connection mode, the 2D layers are further connected by L4-ligands (Fig. 3), forming a three-dimensional structure (Fig. 4).

Fig. 2. (a) Linking mode of the H4L ligand in 1; (b) Basic unit of the 2D layer formed by Cd(II) with bpb and L4?

Fig. 3. View of the 2D layer of complex 1

Fig. 4. View of the whole 3D framework. Hydrogen atoms are omitted for clarity

3. 2 IR spectra

In KBr pellets, the solid state IR spectra of complex 1exhibit the typical antisymmetric stre- tching bands of carboxyl groups (1558and 1605 cm-1) and symmetric stretching bands of the same groups (1360 and 1433 cm-1). The maximum and minimum values Δ(as(COO-) –s(COO-)) of 1 (125 and 245 cm-1) indicate the coordination of H4L ligands with Cd(II) not only in monodentate but also in bidentate- chelating modes[27].

3. 3 Thermal analysis

To study the thermal stability of the complex, thermal gravimetric analysis (TGA) was carried out from room temperature to 900 °C under nitrogen atmosphere (Fig. 5). Since complex 1 does not have solvent molecules, 1 does not obviously lose weight until 367 °C. The weight loss of 79.37% (calcd. 80.43%) from 367 to 750 °C implys the decompo- sition of bpb and L4-. TGA results of 1 indicate it possesses great thermal stability.

Fig. 5. Thermo gravimetric analysis for complex 1.

Fig. 6. Emission spectra of H4L and complex 1 in the solid state at room temperature

3. 4 Fluorescence properties

The photoluminescence spectra for complex 1 and the free ligand H4L were recorded on solid state at room temperature. As shown in Fig. 6, the free ligand H4L has a luminescent emission band maxima at 464 nm (ex= 387 nm).Usually, the emission for these organic ligands is attributable to*→and*→n transitions[28-30]. Complex 1 exhibits intense emission photoluminescence with the emission maxima at 445 nm upon excitation at 387 nm, indicating that the emissions of the complex should be ascribed to the combination of the intraligand transitions of H4L and bpb ligands rather than the charge transfer between Cd(II) and the ligands, considering that Cd(II) is difficult to oxidize or reduce owning to its10configuration[31, 32]. Compared with the free ligand H4L, the slight blue-shift of 1is due to thatthe non-coplanar aromatic rings for complex 1 have not formed an effective-conjugated system, which reduces virtually the electron delocalization range of the ligand[33].

3. 5 Photocatalytic properties

3. 5. 1 Absorption spectrum and optical gap

The solid state diffuse-reflectance UV-vis spectra for 1 were investigated at room temperatures. As shown in Fig. 7a, the absorption band is in the ultraviolet and visible spectrum region. The strong broad absorption band of 1 around 308 nm can be ascribed to-* transfer of the ligands. And the weakabsorption band of the visible spectrum region is not clear, which needs further study. Moreover, the band gap of 1 was calculated according to the intersection point between the energy axis and the line extrapo- lated from the linear portion of the adsorption edge in a plot of Kubelka-Munk function F versus energy E.[34]. Complex 1 behaves as a semiconductor with a band gap of 2.82 eV (Fig. 7b), suggesting that it may be used as as a photocatalyst using the UV irradiation.

Fig. 7. (a)UV-vis diffuse-reflectance spectra of complex 1 with BaSO4as background. (b) Kubelka Munk-tranformed diffuse reflectance spectra of 1

3. 5. 2 Photocatalytic degradation of the organic pollutant

To evaluate the photocatalytic activity of 1, herein MB was selected as a representative for investigating its degradation experiment of organic pollutants. A comparative experiment without any catalyst was also investigated under the same conditions. The photocatalytic activities were estimated by measuring the absorbance intensity at= 664 nm.

Fig. 8. Plots of concentration versus the irradiation time for MB without complex 1 (a) and in the presence of complex 1 (b) under irradiation

A comparison of Fig. 8a and Fig. 8b indicated that the maximum absorption peaks of MB decreased obviously with the reaction time in the presence of 1.In addition,the efficiencies of MB degradation forcomplex 1 are shown in Fig. 9. It can be seen that88.7% MB was successfully photodegraded with the presence of complex 1 after 60 min.In comparison, the degradation efficiency of the control experiment without any photocatalyst was no more than 15% after the same time.Additionally, it was found that the photodegradation of MB experiment follows pseudo-first-order kinetics, evidenced by the linear plot ofln(C/C0) versus the reaction time t.The rate constants (k) and the corresponding correlation coefficient (R2) for the photodegradation of MB with 1 were 0.036 min-1and 0.935, respectively. These results indicate that complex 1 is a good candidate for the photodegradation of MB in water, and may have potential applications in photocatalytic materials.

Fig. 9. Photocatalytic decomposition of MB solution under UV by 1 and the control experiment without any catalyst

4 CONCLUSION

In summary, a novel Cd(II) coordination polymer has been hydrothermally synthesized based on N- donor tetracarboxylic acid ligand (H4L) and N-donor ligand (bpb). The structural studies reveal that com- plex 1 features a 3D framework. The photolumi- nescence investigation shows that 1 exhibits ligand originated fluorescent emission. Additionally, the photocatalytic investigation indicates that complex 1 can promote the decomposition of MB.

(1) Li, B.; Wen, H. M.; Zhou, W. Porous metal-organic frameworks for gas storage and separation: what, how, and why?2014, 5, 3468–3479.

(2) Cui, Y.; Li, B.; He, H. Metal-organic frameworks as platforms for functional materials.. 2016, 49, 483–493.

(3) Yang, X.; Xu, Q. Bimetallic metal-organic frameworks for gas storage and separation.. 2017, 17, 1450–1455.

(4) Zhai, Q. G.; Bu, X.; Zhao, X. Pore space partition in metal-organic frameworks.. 2017, 50, 407–417.

(5) Wang, X. F.; Pan, Y.; Li, Z. Constructing three new metal-organic frameworks based on a mixed-donor ligand: topological analysis and fluorescence properties.2017, 122, 55–60.

(6) Wu, Y.; Wang, J.; Zou, L. K. Luminescence sensing of an unusual tetranodal 3-connected topology of Cu(I)-MOF.. 2016, 69, 13–15.

(7) Jin, J. C.; Wu, X. R.; Luo, Z. D. Luminescent sensing and photocatalytic degradation properties of an uncommon (4, 5, 5)-connected 3D MOF based on 3,5-di (3′,5′-dicarboxylphenyl) benzoic acid.. 2017,19, 4368–4377.

(8) Chen, Q.; Xue, W.; Lin, J. B. Windmill Co4{Co4(4-O)} with 16 divergent branches forming a family of metal-organic frameworks: organic metrics control topology, gas sorption, and magnetism.2016, 22, 12088–12094.

(9) Biswas, S.; Jena, H. S.; Adhikary, A. Two isostructural 3D lanthanide coordination networks (Ln = Gd3+, Dy3+) with squashed cuboid-type nanoscopic cages showing significant cryogenic magnetic refrigeration and slow magnetic relaxation.. 2014, 53, 3926–3928.

(10) Xu, Z.; Meng, W.; Li, H. Guest molecule release triggers changes in the catalytic and magnetic properties of a Fe(II)-based 3D metal-organic framework.2014, 53, 3260–3262.

(11) Cai, S. L.; Zheng, S. R.; Wen, Z. Z. Two types of new three-dimensionalheterometallic coordination polymers based on 2-(pyridin-3-yl)-1H-imidazole-4,5-dicarboxylate and oxalate ligands: syntheses, structures, luminescence, and magnetic properties.. 2012, 12, 4441–4449.

(12) Zeng, L.; Guo, X.; He, C. Metal-organic frameworks: versatile materials for heterogeneous photocatalysis.. 2016, 6, 7935–7947.

(13) Dhakshinamoorthy, A.; Asiri, A. M.; García, H. Metal-organic framework (MOF) complexs: photocatalysts for redox reactions and solar fuel production... 2016, 55, 5414–5445.

(14) Zhang, L.; Cui, P.; Yang, H. Metal-organic frameworks as promising photosensitizers for photoelectrochemical water splitting..2016, 3, 1500243(1–6).

(15) Wang, X. L.; Xiong, Y.; Liu, G. C. Effect of solvents and metal ions on the structural diversity of coordination polymers based on a dipyridylamide ligand: construction, fluorescent and photocatalytic properties.2016, 119, 590–596.

(16) Mu, Y.; Han, G.; Li, Z. Effect of organic polycarboxylate ligands on the structures of a series of zinc(II) coordination polymers based on a conformational bis-triazole ligand.. 2012, 12, 1193–1200.

(17) Dikhtiarenko, A.; Serra-Crespo, P.; Castellanos, S. Temperature-dependent supramolecular isomerism of lutetiumaminoterephthalate metal-organic frameworks: synthesis, crystallography, and physical properties.. 2016, 16, 5636–5645.

(18) Liang, M.; Zou, D. H. Synthesis, crystal structure and catalytic activity of dioxidomolybdenum(Ⅵ) complex with tridentate ONO aroylhydrazone ligand.2016, 63, 180?185.

(19) Liu, C.; Yu, Q. Q.; Liu, M.; Zhang, L. A 3D nickel(Ⅱ) complex constructed by bis(2-dimethylaminoethyl) ether: solvothermal synthesis, crystal structure and catalytic properties..2017, 36, 286–293.

(20) Wang, H. H.; Yang, H. Y.; Shu, C. H. Five new Cd(II) complexes induced by reaction conditions and coordination modes of 5-(1H-tetrazol-5-yl) isophthalic acid ligand: structures and luminescence..2016, 16, 5394–5402.

(21) Gu, J.; Cui, Y.; Liang, X. Structurally distinct metal-organic and H-bonded networks derived from 5-(6-carboxypyridin-3-yl) isophthalic acid: coordination and template effect of 4,4′-bipyridine..2016, 16, 4658–4670.

(22) Farhadi, S.; Amini, M. M.; Dusek, M. A new nanohybrid material constructed from Keggin-type polyoxometalate and Cd(II) semicarbazone Schiff base complex with excellent adsorption properties for the removal of cationic dye pollutants.. 2017, 1130, 592–602.

(23) Sheldrick, G. M.,. University of G?ttingen, Germany 1997.

(24) Sheldrick, G. M.,. University of G?ttingen, Germany 1997.

(25) Sheldrick, G. M.,. University of G?ttingen, Germany 1997.

(26) Shi, X.; Wang, X.; Li, L. Secondary ligand-directed assembly of Cd(II) coordination architectures: from 0D to 3D complexes based on ferrocenyl carboxylate.. 2010, 10, 2490–2500.

(27) Bellamy, L. J. F. C.. Wiley, New York 1958.

(28) Dong, Y.; Zhang, L.; Zhang, Y. A new 3D rtl-type Cd(II) framework constructed from 1,3,5-benzenetricarboxylic acid: urothermal synthesis, crystal structure, and photoluminescent property.. 2017, 47, 73–77.

(29) Liang, L. L.; Gao, Y. Y.; Yue, Q. Photoluminescence, magnetic and photocatalytic properties of three overlapping layered homochiral coordination polymers based on semi-rigid alanine ligands.2017, 461, 102–110.

(30) Song, W. C.; Cui, X. Z.; Wang, X. G. Structural diversity of Zn(II)/Cd(II) coordination polymers constructed from mixed ligand systems of conformationally flexible azo functionalized bis-imidazolate and dicarboxylates.2017, 127, 266–277.

(31) Cepeda, J.; Rodríguez-Diéguez, A. Tuning the luminescence performance of metal-organic frameworks based on10metal ions: from an inherent versatile behaviour to their response to external stimuli.. 2016, 18, 8556–8573.

(32) Wang, L. N.; Fu, L.; Zhu, J. W. Mn(II), Zn(II) and Cd(II) complexes based on oxadiazole backbone containing carboxyl ligand: synthesis, crystal structure, and photoluminescent study.2017, 64, 202–207.

(33) Li, X. J.; Yu, Z. J.; Guan, T.; Li, X. X.; Ma, G. C. Substituent effects of isophthalate derivatives on the construction of zinc(II) coordination polymers incorporating flexible bis(imidazolyl) ligands.. 2015, 15, 278–290.

(34) Qiao, Y.; Zhou Y. F.; Guan, W. S. Syntheses, structures, and photocatalytic properties of two new one-dimensional chain transition metal complexes with mixed N,O-donor ligands.2017, 466, 291–297.

17 December 2017;

21 May 2018 (CCDC 1543968)

① Supported by the National Natural Science Foundation of China (No. 21373178 and 21503183), the Scientific Research Foundation of Shaanxi Provincial Education Department (No. 16JK1857), and the Natural Scientific Research Foundation of Yan’an City Technology Division of China (No. 2016kg-01)

10.14102/j.cnki.0254-5861.2011-1926