Synthesis, Structure and Photocatalytic Activity of Two 4-(2,6-Di(pyrazin-2-yl)pyridin-4-yl)benzoate-Based Chain Complexes①

WANG Zhi-Hong HAO Rui ZHU Shuang ZHAO Xiao-Jun

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Synthesis, Structure and Photocatalytic Activity of Two 4-(2,6-Di(pyrazin-2-yl)pyridin-4-yl)benzoate-Based Chain Complexes①

WANG Zhi-Hong HAO Rui ZHU Shuang ZHAO Xiao-Jun②

(300387)

4-(2,6-di(pyrazin-2-yl)pyridin-4-yl)benzoate, photocatalytic activity,crystal structure,coordination polymer;

1 INTRODUCTION

Recently, rapid developments of printing, cosme- tics, plastics, rubber, and paper industry have dis- charged high level of hazardous colored dyestuffs. Over 10000 kinds of dyes with a total yearly production over 7×105tons worldwide are repor- ted to be commercially available and approximately 10% of dyestuffs are lost in the industrial effluents[1, 2]. The entering of these dye effluents into the receiving water body has already caused serious damage to aquatic organisms and humans by mutagenic and carcinogenic effects.Moreover, lots ofdyes are stable and have a large degree of aromaticity, which is difficult to degrade by conventional biological techniques.Thus, it is of great significance and importance to search effective and economical methods and/or techniques to efficiently degrade these organic dyes[3].

Acting as typically crystalline materials, coor- dination polymers (CPs) built from metal ions and/or metal-containing clusters and organic bri- dging connectors have exhibited excellent photoca- talyticproperties on the degradation of organic dyes (such as methyl orange, rhodamine B (RhB), me- thylene blue (MB) and so on) under the irradiations of UV, visible, and/or UV-vis lights due to their adjustable semiconductor nature and unsaturated metallic binding sites[4-6]. To date, lots of Cu(I/II)-, Co(II/III)-, Fe(II)/Fe(III)-, Zn(II)-, and Cd(II)-based CPs with diverse structures and intriguing topology have been prepared and used as photocatalysts to evaluate the degradation performance of organic pollutants in wastewater[7-12]. These interesting investigations have revealed that the-electron localization skeleton, binding group, andthe metal center of CPs can essentially dominate the strength and range of the illumination energy and the elec- tronic band gap of photocatalyst, which can domi- nate the photo-generated hole-electron separation process responsible for the enhanced catalytic activity. Herein, as continuous explorations on the effects of metal ion and coordination environment on the photocatalytic performance for degradation of organic pollutants, a bulky-conjugated organic ligand 4-(2,6-di(pyrazin-2-yl)pyridin-4-yl)benzoate (L-) was selected as a functional connector to self-assemble with transition metal ion and auxiliary 1,4-benzenedicarboxylate ligand (BDC). As a result, two approximately linear chains incorporated respectively with CuIIand ZnIIions were solvother- mally obtained. Their crystal structures, band gaps and photocatalytic performances towards the degradation of RhB and MB were reported.

2 EXPERIMENTAL

2. 1 Reagents and instruments

All initial chemicals were commercially purchased from either J&K Scientific or Tianjin Chemical Reagent Factory and used as received without further purification. Organic ligand 4-(2,6- di(pyrazin-2-yl)pyridin-4-yl)benzoic acid (HL) was prepared by a slightly modified method[13].Ele- mental analyses for C, H, and N were carried out with a CE-440 (Leeman-Labs) analyzer. Fourier transform (FT) IR spectra (KBr pellets) were taken on an Avatar-370 (Nicolet) spectrometer in the range of 4000～400 cm–1. Thermogravimetric analyses (TGA) were performed on a Shimadzu simultaneous DTG-60A compositional analysis instrument from room temperature to 800℃ under a N2atmosphere at a heating rate of 5℃×min–1. Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku D/max-2500 diffractometer at 60 kV and 300 mA for Curadiation (= 1.5406 ?), with a scan speed of 0.2o·min–1and a step size of 0.02oin 2. The simulated PXRD pattern was calculated using single-crystal X-ray diffraction data and processed by using the free Mercury v1.4 program provided by the Cambridge Crystallographic Data Center. UV/Vis diffuse reflectance spectra (DRS) were carried out on a U-4100 spectrophotometer (Shimadzu) equipped with an integrating sphere assembly. UV-vis absorp- tion spectra of the reaction mixture were recorded using a UV-2700 spectrophotometer (Shimadzu) in the range of 200～800 nm.Electrochemical impe- dance spectroscopy (EIS) was measured on an AMETEK Princeton Applied Research (Versa STAT 4) electrochemical workstation with 1/FTO or 2/FTO as the working electrode, a platinum foil as the counter electrode, and a saturated Ag/AgCl/KCl as the reference electrode. The working electrode was prepared by dropping 50L of suspension containing photocatalyst 1 or 2 (3.0 mg), ethanol (1.0 mL) and Nafion (20L) directly onto a FTO plate. The surface area of the working electrode exposed to the electrolyte was about 0.64 cm2. The EIS measurements were performed in 0.2 M Na2SO4aqueous solution (pH = 7) with a bias of 0 V under irradiation of 300W Xe lamp (≥ 350 nm).

2. 2 Synthesis of {[Cu(L)(BDC)0.5]·3.5H2O}n (1)

A mixture containing CuSO4·5H2O (50.0 mg, 0.2 mmol), HL (17.7 mg, 0.05 mmol), 1,4-benzenedi- carboxylic acid (H2BDC, 28.2 mg, 0.17 mmol), doubly deionized water (5.0 mL), and DMF (5.0 mL) was sealed in a Teflon-lined stainless-steel vessel (23.0 mL) and heated at 120 ℃ for 72 h under autogenous pressure. After the mixture was cooled to room temperature at a rate of 2℃·h-1, green block-shaped crystals suitable for X-ray analysis were obtained directly, washed with cold water, and dried in air. Yield: 36% based on L-ligand. Calcd. for C48H42Cu2N10O15: C, 51.20; H, 3.76; N, 12.44%. Found: C, 51.23; H, 3.77; N, 12.48%. FT-IR (KBr pellet, cm-1): 3434 (br), 1611 (m), 1560 (s), 1458 (w), 1381 (s), 1176(w), 1150 (w), 1075 (w), 1039 (w), 1011 (w), 861(w), 823 (w), 789 (w), 745 (w), 591 (w), 516 (w), 466(w).

2. 3 Synthesis of {[Zn(L)(BDC)0.5]·H2O}n (2)

A mixture of ZnSO4·7H2O (57.5 mg, 0.2 mmol), HL (17.7 mg, 0.05 mmol), H2BDC (16.6 mg, 0.1 mmol), and H2O (10.0 mL) was stirred for 3.0 h in air. Then, the mixture was transferred into a 23.0 mL Teflon-lined stainless-steel vessel, and heated at 170℃ for 4 days. After the mixture was cooled slowly to room temperature, yellow block-shaped crystals of 2 were directly obtained. Yield: 46% based on L-ligand. Calcd. for C24H16N5O5Zn: C, 55.46; H, 3.10; N, 13.47%. Found: C, 55.35; H, 3.30; N, 13.57%. FT-IR (KBr pellet, cm-1): 3332 (br), 1642 (s), 1611 (m), 1582 (s), 1382(s), 1335 (s), 1179 (m), 1139 (w), 1034(m), 1010(w), 855 (m), 819 (m), 787 (m), 745 (m), 689 (w), 649(w), 590(w) 490(w), 462(w).

2. 4 Structure determination

Diffraction intensities of 1 (0.22mm′0.20mm′0.19mm) and 2 (0.22mm′0.21mm′0.18mm) were collected on a Bruker APEX-II CCD diffractometer equipped with graphite-monochro- mated Moradiation with radiation wavelength 0.71073 ? by using the-scan technique at 296 K, respectively. There was no evidence of crystal decay during data collection. Semi-empirical multi- scan absorption corrections were applied by SADABS[14]and the program SAINT was used for integration of the diffraction profiles. A total of 7622 reflections with 4781 unique ones (int= 0.0369) were measured in the range of 1.779≤≤26.499o, of which3704 were observed with> 2() for 1, and a total of 11673 reflections with 4278 unique ones (int= 0.0542) were measured in the range of 2.086≤≤26.494o, of which 2944 were observed with> 2() for 2.The structures were solved by direct methods and refined with full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 pro- grams[15, 16]. Anisotropic thermal parameters were assigned to all non-H atoms. The organic H atoms were generated geometrically. The final= 0.0575,= 0.1386 (= 1/[2(F2) + (0.0634)2+ 0.8562], where= (F2+ 2F2)/3),= 1.078, (D)max= 0.598, (D)min=-0.661 and (D/)max= 0.001 for 1, and the final= 0.0487,= 0.0907 (= 1/[2(F2) + (0.0386)2], where= (F2+ 2F2)/3),= 1.083, (D)max= 0.835, (D)min=-0.524and (D/)max= 0.001for 2. The selected bond lengths and bond angles for 1 and 2 are shown in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Symmetry codes for 1: a: 2-,-, 1-; 2: a:-,, 1/2-

2. 5 Photocatalytic experiment

Typical procedure for photocatalytic reaction was as follows: a suspension containing photocatalyst 1 or 2 (3.0 mg), 30.0 mL aqueous RhB/MB (20.0/12.0 mg·L-1) solution and 30% H2O2(50L for RhB and 10L for MB) was stirred in the dark for about 30 min to ensure the absorption-desorption equili- brium. Then, the mixture was exposed to a visible light source (500 W xenon arc lamp) for irradiation. At different time intervals, 3.0 mL sample was withdrawn from the reaction mixture and the dispersed powder in the mixture was removed by centrifugation. The absorption of the as-resulted solutions was analyzed by UV-vis spectroscopy, in which the characteristic absorption bands around 553 nm for RhB and 664 nm for MB were employed to evaluate the degradation process. The catalyst after the first run was filtered, washed several times with water, dried at room temperature, and then dropped into the next reaction.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

As shown in Fig. 1b, the adjacent CuIIions of 1 are alternately bridged by a pair of L-and a cen- trosymmetric BDC2-connector, leading to an appro- ximately linear chain with the intrachain CuII···CuIIseparations of 11.4137(11) and 10.9307(1) ?, respectively. The angle of three neighboring CuIIions is 168.189(1)o. The neighboring 1chains of 1 are further packed into 2and 3supramolecular networks through interchain weak C-H···O and C-H···N hydrogen bonding interactions between aromatic ring and carboxylate O or pyrazinyl N acceptors (Table S1 and Fig. S1).

Fig. 1. (a) Local coordination environments of CuΙΙion in 1 (H atoms were omitted for clarity. Symmetry code: A = 2-,-, 1-). (b) 1D chain of 1 extended by the mixed ligands

3. 2 Crystal structure of 2

Similar to 1, 2 also features a 1chain with pentacoordinate ZnIIions periodically expanded by centrosymmetric BDC2-and pairs of L-connectors. Besides the replacement of CuIIion by the ZnIIsite, a quite distinction between 1 and 2 can be detected by careful checking their crystal structures. Much different from 1, 2 crystallizes in the monoclinic2/space group. The highly symmetric space group of 2 can result inthree aspects different from 1. Firstly, the coordination polyhedron of ZnIIion is more distorted (0.12, Fig. 2a). Secondly, the conjugated extent of L-ligand in 2 is much better than that in 1, and the dihedralangles between phenyl ring and polypyridyl plane are 4.8° and 45.8° for 1 and 2. Thirdly, chain structure of 2 is more bent and the angle of three adjacent ZnIIions is 16.558(2)o(Fig. 2b). Additionally, the weak interchain C-H···N hydrogen bonding interaction was absent during the crystal stacking process of 2 (Table S1 and Fig. S2).

Fig. 2. (a) Local coordination environments of ZnΙΙion in 2 (H atoms were omitted for clarity. Symmetry code: A =-,, 0.5-). (b) 1D chain of 2 extended by L-and BDC2-connectors

3. 3 PXRD, TGA, and FT-IR spectra

Powder X-ray diffraction (PXRD) patterns of the as-synthesized samples were in good agreement with the simulated ones (Fig. S3), suggestingthe phase purity and the structural consistency between the as-prepared sample and the single-crystal structures.

Both 1 and 2 display two separate weight-loss stages (Fig. S4). The first one that began at room temperature and ended at 155℃ for 1 should be ascribed to the removal of lattice water molecules (obsd. 11.8%, calcd. 11.2%). The breakof the chain of 1 began at 284℃, which was not completely finished till 800℃. The thermal stability of 2 is much higher than that of 1.The removal of lattice water molecule in 2 was between room temperature and 293℃ (obsd. 4.1%, calcd. 3.5%). The collapse of the chain structure of 2 occurred at 431℃. Similar to 1, the framework collapse of 2 was not completely finished at the highest temperature.

As compared with the free organic ligand, an absence of a strong band at 1679cm-1in the FT-IR spectra confirmed the complete deprotonation of H2BDC in both 1 and 2[18].Multiple strong bands corresponding to the asymmetric and symmetric stretching vibrations of carboxylate group appeared at 1611, 1560, and 1458, 1381 cm-1for 1 and at 1642, 1582, 1382, 133cm-1for 2.

3. 4 Optical properties

Complex 1 exhibits three strong absorption bands centered at 286, 368 and 699 nm (Fig. 3), respec- tively, which can be ascribed to intraligand→* transition, ligand-to-metal charge transfer (LMCT), andspin-allowed transition of CuIIion. By contrast, only two absorptions are observed for 2 at 275 and 358 nm, which correspond to the intra- ligand→* transition and LMCT of 2.Energy band gaps (Eg) for 1 and 2 are 2.04 and 2.81 eV obtained from the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a plot of Kubelka- Munk function (Fig. 3 inset), indicating that the CPs have semiconductor nature and can be potentially used as photocatalysts.

Fig. 3. UV-Vis absorption spectra of HL, H2BDC, 1, and 2 (Inset: Diffuse reflectance UV-vis spectra of K-M function. energy of 1 and 2)

3. 5 Photocatalytic properties

Catalytic performance of the as-synthesized sample as a photocatalyst was evaluated by photo- degradation of RhB and MB in aqueous solution. As shown in Fig. 4, in the presence of 1, the charac- teristic absorptions of RhB (664 nm) and MB (553 nm) decreased remarkably with the extension of the irradiation time, suggesting a detectable degradation of RhB and MB with the aid of 1. The photoca- talytic efficiency of RhB and MB by 190% and 95% after 150 min reaction. By contrast, the degradation efficiency of the organic dyes by 2 was 53% and 60% for RhB and MB, respectively. Under the same experimental conditions, the degradation efficiency of RhB and MB without a photocatalyst was only 28% and 32%. Thus, both 1 and 2 can exhibit good photocatalytic activities upon the photodegradation of organic dyes, in which 1 has a much higher catalytic performance than that of 2. Herein, the lower photodegradation efficiency of 2 than 1 is significantly due to the large band gap and the weak response of 2 to visible irradiation. Furthermore, to compare the difference on the charge separation and transfer process between 1 and 2, the EIS measurements were carried out. As shown in Fig. 5, the smaller semicircle diameter of 1 than 2 indicates the resistance of 2/PTO electrode is bigger than that of 1/FTO, that is to say, the faster interfacial charge transfer and lower charge recombination occur in 1. On the other hand, as compared with the previously reported CuII-based photocatalysts exhibiting high degradation per- formance of RhB and MB, such as centrosymmetric dinuclear [Cu2(2,2?-bipy)2(pfbz)4] (pfbz = pen- tafluorobenzoate)[19], pyridine-2,6-dicarbohydrazide based imine linked ligands extended one-dimen- sional CuIIchains with distinct coordination en- vironments[20]and three-dimensional [Cu(mip)(bpy)0.5](mip = 5-methylisophthalate and bpy = 4,4?-bipyridine) with jsm topology[21], the dimensionality, unsaturated coordination site of CuIIcenter and the chemical structure of the organic ligand play important roles for the enhancement of the photocatalytic activity.

3. 6 Stability and reusability of the photocatalysts

The stability of photocatalyst was confirmed by comparison of the PXRD patterns before and after the photocatalytic reactions. After this reaction, the PXRD patterns of 1 and 2 were almost the same as those of the as-prepared samples (Fig. S3), sug- gesting the robust stability of the two photocatalysts during the process of photocatalytic reaction. Additionally, the reusability of 1 with better catalytic performance was examined by performing three consecutive runs. The degradation efficiency of 1 only varied from 90% to 64% after three runs (Fig. 6), implying that 1can be re-used without a significant loss of the catalytic activity.

Fig. 4. UV-vis absorption spectra and degradation efficiency for degradation of RhB and MB by using 1 and 2 as photocatalysts under visible irradiation at different time intervals (insert: photographs of RhB and MB solution before and after photocatalytic reaction by 1 and 2)

Fig. 5. EIS Nyquist plots of 1 and 2 as electrode materials under the irradiation of visble light

Fig. 6. Cycle performance of 1 on the photodegradation of aqueous RhB solution

4 CONCLUSION

Two bulky conjugated 4-(2,6-di(pyrazin-2-yl)py- ridin-4-yl)benzoate-derived one-dimensional chains were solvothermally obtained by varying the transition metal ions and were used as photocata- lysts to degrade organic dyes. Due to the narrower band gap and broader response to visible irradiation, the CuII-based chain exhibits better photocatalytic activity than those of ZnII-chain.

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27 September 2017;

20 December 2017 (CCDC 1573596 for 1 and 1573597 for 2)

① This work was supported by NNSFC (No. 21671149)

. Zhao Xiao-Jun, born in 1955, professor, majoring in coordination chemistry. E-mail: xiaojun_zhao15@163.com

10.14102/j.cnki.0254-5861.2011-1834