Construction of Four Zn(II) Coordination Polymers Used as Catalysts for the Photodegradation of Organic Dyes in Water

Hydrothermal reactions of Zn(OAc)2·2H2O with flexible bipyridyl benzene ligand and three dicarboxylic derivatives gave rise to four new coordination polymers, [Zn7(μ4-O)2(OAc)10(bpmb)]n (1), [Zn(5-OH-1,3-BDC)(bpmb)]n (2), [Zn(1,2-BDC)(bpmb)]n (3) and [Zn2(ADB)2(bpmb)]n (4) (bpmb = 1,4-bis(pyridine-3-ylmethoxy)benzene, 5-OH-1,3-H2BDC = 5-hydroxy-1,3-benzenedicarboxylic acid, 1,2-H2BDC = 1,2-benzenedicarboxylic acid, H2ADB = 2,2’-azodibenzoic acid). Their structures were characterized by single-crystal X-ray diffraction, elemental analyses, IR spectra, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). Compound 1 features a one-dimensional (1D) chain structure based on the rare heptanuclear [Zn7(μ4-O)(μ3-OAc)2(μ2-OAc)8] units. Compound 2 exhibits a novel 2D bilayer structure built from the two parallel 2D (4,4) layers. Compound 3 holds a 2D structure in which the 1,2-BDC ligands work as lockers interlocking 1D [Zn(bpmb)]n chain. Compound 4 comprises a 3D framework constructed by 2D wrinkled [Zn2(ADB)4]n networks and bpmb linkers with a six-connected pcu net. These results suggest that the motifs of the dicarboxylic ligands have significant effect on the final structures. These compounds exhibited relatively good photocatalytic activity towards the degradation of methylene blue (MB) in aqueous solution under a Xe lamp irradiation.

Photocatalysis is a green technology for the treatment of all kinds of contaminants that has many advantages over other treatment methods; for instance, the use of the environmentally friendly oxidant (O 2 or H 2 O 2 ), the ambient temperature reaction condition, and oxidation of the organic compounds, even at low concentrations [22,23].Recently, considering the novelty of this field in CPs, much effort has been devoted to developing new photocatalytic materials based on CPs in the degradation of many kinds of organic contaminants with up to 90% efficiency [24][25][26][27][28]. Compared to the traditional semiconductor metal oxide, the advantages of CPs as photocatalyst lie in the fact that their combination of inorganic and organic moieties results in different metal-ligand charge transfer, which can give rise to tunable photocatalysts [29].Lately, some metal CPs corresponding to the Cd(II), Cu(II) and Mn(II) ions, have been reported to be active in catalyzing the photodegradation of organic dyes [30][31][32][33][34].However, the exploration of Zn(II)-based coordination complexes as effective photocatalysts is relatively rare [35][36][37].

Chemicals and Characterization
The ligand bpmb was prepared according to the previously reported procedure with modification [38].All other chemicals and reagents were obtained from commercial sources and used as received.Infrared (IR) spectra were recorded with a Varian 800 Fourier transform infrared (FT-IR) spectrometer (Varian, Inc., Palo Alto, CA, USA) as KBr disks (4000-400 cm ´1).The elemental analysis for C, H, and N was performed on an EA1110 CHNS elemental analyzer (Carlo Erba, Inc., Milan, Italy).Powder X-ray diffraction (PXRD) was performed using a PANalytical X'Pert3 Powder instrument (PANalytical B.V., Almelo, The Netherlands) with Cu Kα radiation.Thermal gravimetric (TG) analysis was performed on a NETZSCH STA-449F3 instrument (Netzsch, Co., Selb, Germany) in flowing N 2 with a heating rate of 10 ˝C¨min ´1, coupled with a Bruker TENSOR27 Fourier Transform Infrared Spectrometer (Bruker Optics, Inc., Ettlingen, Germany).

Synthesis of Compound 1
[Zn 7 (µ 4 -O) 2 (OAc) 10 (bpmb)] n (1).A 10 mL Pyrex glass tube was loaded with Zn(OAc) 2 ¨2H 2 O (9 mg, 0.04 mmol), bpmb (6 mg, 0.02 mmol) and 4 mL of MeCN.The tube was then sealed and heated in an oven to 150 ˝C for four days, and then cooled to ambient temperature at a rate of 5 ˝C¨h ´1.The colorless blocks of 1 were formed four day later, which were collected and dried in air.Yield:

X-Ray Data Collection and Structure Determination
Single crystals of 1-4 were obtained directly from the above preparations.All measurements were made on a Bruker Smart Apex-II CCD area detector by using graphite monochromated Mo Kα (λ = 0.071073 nm).These crystals were mounted on glass fibers at 296 K for 1-4.Diffraction data were collected at f and ω modes with a detector distance of 35 mm to the crystals.Cell parameters were refined using the program Bruker SAINT.The collected data were reduced using the program Bruker SAINT A, and the absorption corrections (multi-scan) were applied.The reflection data were also corrected for Lorentz and polarization effects.The crystal structures of 1-4 were solved by direct method refined on F 2 by full-matrix least-squares techniques with the SHELXTL-97 program [42].A summary of the key crystallographic information for 1-4 is tabulated in Table 1.
, where n = number of reflections and p = total numbers of parameters refined.

Photocatalytic Activity Measurements
The photocatalytic activities of as-prepared samples were evaluated by the degradation of MB under irradiation of a 350 W Xe lamp with the whole spectrum.The as-synthesized single crystals obtained from the above preparations, which were further grinded into microcrystals with a size of about 70 µm (Figure S1).In a typical process, 20 mg grinded samples as photocatalysts were added into 50 mL of MB aqueous solution (4 ˆ10 ´5 mol¨L ´1).The MB aqueous solution was stirred for 30 min in the dark before irradiation to reach adsorption equilibrium between the catalyst and solution and then it was exposed to the Xe lamp irradiation.About 4 mL suspension was continually taken from the reaction cell and collected by centrifugation at each 30 min interval during irradiation.The resulting solution was analyzed on a Varian 50 UV-Vis spectrophotometer (Varian, Inc., Palo Alto, CA, USA).

Synthetic and Spectral Aspects
Treatment of bpmb with Zn(OAc) 2 ¨2H 2 O in MeCN followed by a hydrothermal condition at 150 ˝C for four days produced crystals of 1 (29% yield).Furthermore, similar reactions of Zn(OAc) 2 ¨2H 2 O with bpmb and dicarboxylic derivatives at 170 ˝C (2-3) and 150 ˝C (4) in water generated crystals of 2 (56% yield), 3 (48% yield) and 4 (36% yield), respectively.When the reaction temperatures were decreased to 120 ˝C, only precipitates were isolated and their PXRD patterns were inconsistent with those of 1-4.Compounds 1-4 were stable towards oxygen and moisture, and almost insoluble in common organic solvents.Their elemental analyses were consistent with the chemical formulas of 1-4.In order to check the phase purity of 1-4, the powder X-ray diffraction (PXRD) patterns were measured at room temperature (Figure 1).The identities of 1-4 were finally confirmed by single-crystal diffraction analysis.

Photocatalytic Activity Measurements
The photocatalytic activities of as-prepared samples were evaluated by the degradation of MB under irradiation of a 350 W Xe lamp with the whole spectrum.The as-synthesized single crystals obtained from the above preparations, which were further grinded into microcrystals with a size of about 70 μm (Figure S1).In a typical process, 20 mg grinded samples as photocatalysts were added into 50 mL of MB aqueous solution (4 × 10 −5 mol•L −1 ).The MB aqueous solution was stirred for 30 min in the dark before irradiation to reach adsorption equilibrium between the catalyst and solution and then it was exposed to the Xe lamp irradiation.About 4 mL suspension was continually taken from the reaction cell and collected by centrifugation at each 30 min interval during irradiation.The resulting solution was analyzed on a Varian 50 UV-Vis spectrophotometer (Varian, Inc., Palo Alto, CA, USA).

Synthetic and Spectral Aspects
Treatment of bpmb with Zn(OAc)2•2H2O in MeCN followed by a hydrothermal condition at 150 °C for four days produced crystals of 1 (29% yield).Furthermore, similar reactions of Zn(OAc)2•2H2O with bpmb and dicarboxylic derivatives at 170 °C (2-3) and 150 °C (4) in water generated crystals of 2 (56% yield), 3 (48% yield) and 4 (36% yield), respectively.When the reaction temperatures were decreased to 120 °C, only precipitates were isolated and their PXRD patterns were inconsistent with those of 1-4.Compounds 1-4 were stable towards oxygen and moisture, and almost insoluble in common organic solvents.Their elemental analyses were consistent with the chemical formulas of 1-4.In order to check the phase purity of 1-4, the powder X-ray diffraction (PXRD) patterns were measured at room temperature (Figure 1).The identities of 1-4 were finally confirmed by single-crystal diffraction analysis.

Crystal Structure of 1
Compound 1 crystallizes in the triclinic space group Pī, and its asymmetric unit contains three and a half crystallographically independent Zn atoms, one μ4-O group, five OAc − ligands and a half bpmb ligand.As shown in Figure 2a

Crystal Structure of 1
Compound 1 crystallizes in the triclinic space group Pī, and its asymmetric unit contains three and a half crystallographically independent Zn atoms, one µ 4 -O group, five OAc ´ligands and a half bpmb ligand.As shown in Figure 2a

Crystal Structure of 2
Compound 2 crystallizes in the monoclinic space group P2/c, its asymmetric unit contains one [Zn(5-OH-1,3-BDC)(bpmb)] unit.Each Zn atom is coordinated by two N atoms from two different bpmb ligands and two O atoms of bridging carboxylate groups from two 5-OH-1,3-BDC ligands to complete the tetrahedral geometry (Figure 3a).Each Zn II atom is interlinked by bis-monodentate 5-HO-1,3-BDC ligands to form a 1D [Zn(5-HO-1,3-BDC)]n chain extending along the a-axis (Figure 3b).Each chain is connected to adjacent chains via bpmb ligands to produce a 2D (4,4) layer (extending along the ac plane), with parallelogram-shaped meshes (9.635 Å × 16.363 Å, between Zn atoms at the corners) (Figure 3b).Interestingly, such 2D layer parallels to the equivalent one with a interleaving, resulting in a rare bilayer structure extending along the ac plane (Figure 3c,d).From the topological view [43], if the Zn centres are considered as nodes and the 5-HO-1,3-BDC and bpmb ligands are considered as linkers, the bilayer structure of 2 can be specified by a Schläfli symbol of 4 4 6 2 (Figure 3d).Further investigation of the crystal packing of compound 2 suggests that each bilayer structure is interconnected with adjacent ones through intermolecular H-bonding interactions among the uncoordinated O atoms of the carboxylate groups from 5-HO-1,3-BDC ligands and the H atoms of the hydroxyl groups [O7-H7•••O3, (-x, -y + 1, -z), 1.87(3) Å], which leads to the formation of a 3D supramolecular framework (Figure 3e).

Crystal Structure of 2
Compound 2 crystallizes in the monoclinic space group P2/c, its asymmetric unit contains one [Zn(5-OH-1,3-BDC)(bpmb)] unit.Each Zn atom is coordinated by two N atoms from two different bpmb ligands and two O atoms of bridging carboxylate groups from two 5-OH-1,3-BDC ligands to complete the tetrahedral geometry (Figure 3a).Each Zn II atom is interlinked by bis-monodentate 5-HO-1,3-BDC ligands to form a 1D [Zn(5-HO-1,3-BDC)] n chain extending along the a-axis (Figure 3b).Each chain is connected to adjacent chains via bpmb ligands to produce a 2D (4,4) layer (extending along the ac plane), with parallelogram-shaped meshes (9.635 Å ˆ16.363Å, between Zn atoms at the corners) (Figure 3b).Interestingly, such 2D layer parallels to the equivalent one with a interleaving, resulting in a rare bilayer structure extending along the ac plane (Figure 3c,d).From the topological view [43], if the Zn centres are considered as nodes and the 5-HO-1,3-BDC and bpmb ligands are considered as linkers, the bilayer structure of 2 can be specified by a Schläfli symbol of 4 4 6 2 (Figure 3d).Further investigation of the crystal packing of compound 2 suggests that each bilayer structure is interconnected with adjacent ones through intermolecular H-bonding interactions among the uncoordinated O atoms of the carboxylate groups from 5-HO-1,3-BDC ligands and the H atoms of the hydroxyl groups [O7-H7¨¨¨O3, (´x, ´y + 1, ´z), 1.87(3) Å], which leads to the formation of a 3D supramolecular framework (Figure 3e).

Crystal Structure of 4
Compound 4 crystallizes in the triclinic space group Pī, and its asymmetric unit contains half of [Zn2(ADB)2(bpmb)] unit.The Zn1 atom adopts a pyramidal coordination geometry and is fivecoordinated by four O atoms of four bridging carboxylate groups from four ADB ligands and one N atom of one bpmb ligand (Figure 5a).The Zn1 atom and its symmetry-related Zn1A atom are bridged by four carboxylate groups to generate a paddle-wheel [Zn2(μ2-CO2)4] unit (Figure 5b).The Zn•••Zn separation in this dinuclear unit is 2.9301 Å.Each paddle-wheel [Zn2(μ2-CO2)4] unit serves as a fourfold node, which links four equivalent ones via sharing of four ADB ligands to form a 2D wrinkled network extending along the bc plane (Figure 5b).Furthermore, the bpmb ligands are employed as linkers (pink) to bridge the 2D networks producing a 3D framework (Figure 5c).Topologically, the overall structure of 4 can be described as a pcu net with the six-connected 4 12 6 3 topology (Figure 5d).

Crystal Structure of 4
Compound 4 crystallizes in the triclinic space group Pī, and its asymmetric unit contains half of [Zn 2 (ADB) 2 (bpmb)] unit.The Zn1 atom adopts a pyramidal coordination geometry and is five-coordinated by four O atoms of four bridging carboxylate groups from four ADB ligands and one N atom of one bpmb ligand (Figure 5a).The Zn1 atom and its symmetry-related Zn1A atom are bridged by four carboxylate groups to generate a paddle-wheel [Zn 2 (µ 2 -CO 2 ) 4 ] unit (Figure 5b).The Zn¨¨¨Zn separation in this dinuclear unit is 2.9301 Å.Each paddle-wheel [Zn 2 (µ 2 -CO 2 ) 4 ] unit serves as a four-fold node, which links four equivalent ones via sharing of four ADB ligands to form a 2D wrinkled network extending along the bc plane (Figure 5b).Furthermore, the bpmb ligands are employed as linkers (pink) to bridge the 2D networks producing a 3D framework (Figure 5c).Topologically, the overall structure of 4 can be described as a pcu net with the six-connected 4 12 6 3 topology (Figure 5d).

Photocatalytic Activity
As is well-known, organic dyes such as MO, MB and RhB that were extensively used in the textile industry have been found to be toxic, stable and difficult to biodegrade.Thus, it is urgent to decompose such dye molecules into some simple molecules to reduce the environment pollution.Hence, in this work, the photocatalytic activities of the compounds 1-4 were evaluated by the degradation of MB under irradiation at room temperature as the test pollutant.To evaluate the band gaps, the UV-vis absorption spectrum of 1-4 is measured at room temperature (Figure S2).The results give Eg (band-gap energy) values of 3.46, 3.56, 3.62 and 3.64 eV for 1-4, respectively (Figure S3).As illustrated in Figure 7a-d, the absorption peaks of MB in water decreased patently following the reaction time in the presence of 1-4.The concentrations of organic dye were estimated by the absorbance at 665 nm (MB, absorption coefficient: 5.53 × 10 4 L•mol −1 •cm −1 ).The degradation efficiencies are defined as C/C0, where C and C0 represent the remnant and initial concentration of MB, respectively (Figure 7e).By contrast, the simple photolysis experiment was also performed under the same conditions without any catalyst.A comparison of the photocatalytic activities of 1-4 was presented in Figure 7e.The calculation results demonstrate that the photocatalytic activities increase from 23.0% (controlled experiment without any catalyst) to 95.3% for 1, 92.8% for 2, 95.5% for 3 and 95.2% for 4 after 120 min of irradiation, which are better than the commercial Degussa P25 TiO2 reference catalyst (84%), the pure ZnO (74%) and the ZnO@ZIF-8 materials [44].Even compound 1 holds the narrowest Eg among these compounds, however, it exhibits almost same photocatalytic activity compared with compounds 3 and 4, which may be due to the heptanuclear units in compound 1 that hinder the migration of excited electrons/holes and slow down the photocatalytic degradation process [34].It is clear that compound 2 possesses lower activity than other compounds, which may be ascribed to the bilayer structures in 2, leading to the MB molecules inaccessibility of the zinc centers.The catalyst was filtered and obtained a colorless solution, which was extracted by acetic ether, and the organic phase was analyzed by gas chromatography-mass spectrometer.No corresponding species of MB was observed, and thus we assumed that the dyes might be degradation of CO2 and H2O [22,45].
And the photocatalytic efficiencies of these compounds are comparable to those of other Zn-based CP materials.Such as, using the known CPs {[Zn2(Tipa)(4,4′-bpdc)1.

Photocatalytic Activity
As is well-known, organic dyes such as MO, MB and RhB that were extensively used in the textile industry have been found to be toxic, stable and difficult to biodegrade.Thus, it is urgent to decompose such dye molecules into some simple molecules to reduce the environment pollution.Hence, in this work, the photocatalytic activities of the compounds 1-4 were evaluated by the degradation of MB under irradiation at room temperature as the test pollutant.To evaluate the band gaps, the UV-vis absorption spectrum of 1-4 is measured at room temperature (Figure S2).The results give E g (band-gap energy) values of 3.46, 3.56, 3.62 and 3.64 eV for 1-4, respectively (Figure S3).As illustrated in Figure 7a-d, the absorption peaks of MB in water decreased patently following the reaction time in the presence of 1-4.The concentrations of organic dye were estimated by the absorbance at 665 nm (MB, absorption coefficient: 5.53 ˆ10 4 L¨mol ´1¨cm ´1).The degradation efficiencies are defined as C/C 0 , where C and C 0 represent the remnant and initial concentration of MB, respectively (Figure 7e).By contrast, the simple photolysis experiment was also performed under the same conditions without any catalyst.A comparison of the photocatalytic activities of 1-4 was presented in Figure 7e.The calculation results demonstrate that the photocatalytic activities increase from 23.0% (controlled experiment without any catalyst) to 95.3% for 1, 92.8% for 2, 95.5% for 3 and 95.2% for 4 after 120 min of irradiation, which are better than the commercial Degussa P25 TiO 2 reference catalyst (84%), the pure ZnO (74%) and the ZnO@ZIF-8 materials [44].Even compound 1 holds the narrowest E g among these compounds, however, it exhibits almost same photocatalytic activity compared with compounds 3 and 4, which may be due to the heptanuclear units in compound 1 that hinder the migration of excited electrons/holes and slow down the photocatalytic degradation process [34].It is clear that compound 2 possesses lower activity than other compounds, which may be ascribed to the bilayer structures in 2, leading to the MB molecules inaccessibility of the zinc centers.The catalyst was filtered and obtained a colorless solution, which was extracted by acetic ether, and the organic phase was analyzed by gas chromatography-mass spectrometer.No corresponding species of MB was observed, and thus we assumed that the dyes might be degradation of CO 2 and H 2 O [22,45].
And the photocatalytic efficiencies of these compounds are comparable to those of other Zn-based CP materials.Such as, using the known CPs {[Zn  [26] as catalysts, ca.82.0%, 92.7% and 88.7% of MB was degraded in 120, 140 and 180 min, respectively.Combined with the UV-Vis adsorption spectra of 1-4 in the solid state (Figure S2), we inferred that the photocatalytic activities of these compounds could be attributed to the ZnO units.The valence and the conduction bands of ZnO are mainly due to O(2p) and Zn(4s) orbitals, respectively, this electronic transition can basically be described as an O 2´Z n 2+ Ñ O ´Zn + LMCT.The organic linker acts as a photon antenna that could efficiently transfer the energy to the ZnO units [46].In addition, the PXRD patterns of each powder for 1-4 were basically identical to those of the parent compounds, indicating that these compounds are stable during photocatalysis.
Polymers 2016, 8, 3 10 of 13 {[Cd3(bcb)2(H2O)5]•H2O}n (H3bcb = 3,4-bi(4-carboxyphenyl)-benzoic acid) [26] as catalysts, ca.82.0%, 92.7% and 88.7% of MB was degraded in 120, 140 and 180 min, respectively.Combined with the UV-Vis adsorption spectra of 1−4 in the solid state (Figure S2), we inferred that the photocatalytic activities of these compounds could be attributed to the ZnO units.The valence and the conduction bands of ZnO are mainly due to O(2p) and Zn(4s) orbitals, respectively, this electronic transition can basically be described as an O 2− Zn 2+ → O − Zn + LMCT.The organic linker acts as a photon antenna that could efficiently transfer the energy to the ZnO units [46].In addition, the PXRD patterns of each powder for 1-4 were basically identical to those of the parent compounds, indicating that these compounds are stable during photocatalysis.

Conclusions
In summary, we demonstrated that treatment of Zn(OAc) 2 ¨2H 2 O with bpmb and dicarboxylic ligands with different motifs formed four different CPs 1-4.These compounds exhibit various structural features.The photocatalytic activities of 1-4 were evaluated by the decomposition of organic dyes in aqueous solutions under the Xe lamp irradiation.These compounds showed good catalytic performance for the degradation of MB, which were excellent candidates for decomposing other organic dyes.
, Zn1 atom adopts an octahedral coordination geometry and is six-coordinated by two O atoms of two μ4-O groups, four O atoms of four bridging carboxylate groups from four OAc − anions.While each Zn2 atom adopts a trigonal bipyramidal coordination geometry, coordinated by one O atom of μ4-O group, four O atoms of four bridging carboxylate

Figure 3 .
Figure 3. (a) View of the coordination environments of Zn center in 2 with labeling schemes.Symmetry codes: (A) -1 + x, 1 + y, -1 + z; (B) 1 + x, y, z; and (C) -1 + x, 1 − y, − 1/2 + z; (b) View of the 2D (4,4) layer in 2; (c) View of the 2D bilayer structure in 2 looking down b-axis; (d) View of the 2D bilayer model in 2. Each single net represents a topology with a Schläfli symbol of 4 4 6 2 ; (e) View of a 3D supramolecular framework in 2. Green dashed lines represent the hydrogen-bonded interactions.Atom color codes: Zn, cyan polyhedrons; O, red; N, blue; C, dark green and pink.All hydrogen atoms except those related to H-bonding interactions are omitted for clarity.

Figure 3 .
Figure 3. (a) View of the coordination environments of Zn center in 2 with labeling schemes.Symmetry codes: (A) ´1 + x, 1 + y, ´1 + z; (B) 1 + x, y, z; and (C) ´1 + x, 1 ´y, ´1/2 + z; (b) View of the 2D (4,4) layer in 2; (c) View of the 2D bilayer structure in 2 looking down b-axis; (d) View of the 2D bilayer model in 2. Each single net represents a topology with a Schläfli symbol of 4 4 6 2 ; (e) View of a 3D supramolecular framework in 2. Green dashed lines represent the hydrogen-bonded interactions.Atom color codes: Zn, cyan polyhedrons; O, red; N, blue; C, dark green and pink.All hydrogen atoms except those related to H-bonding interactions are omitted for clarity.

Figure 4 .
Figure 4. (a) View of the coordination environments of Zn center in 3 with labeling schemes.Symmetry codes: (A) 1 ´x, 1/2 + y, 1 ´z; (B) 1 + x, 1 + y, z; (b) View of the 1D [Zn(bpmb)] n chain in 3; (c) View of the 2D structure in 3 looking down b-axis; (d) Schematic view of the six-connected 4 12 6 3 net of 3. Atom color codes: Zn, cyan; O, red; N, blue; C, dark green and pink.All H atoms are omitted for clarity.

Figure 7 .
Figure 7. (a-d) Absorption spectra of the MB solution (4 × 10 −5 mol•L −1 , 50 mL) during the decomposition reaction under the Xe lamp irradiation with the presence of compounds 1-4 (20 mg); (e) Concentration changes of MB at different time intervals under Xe lamp irradiation with 1-4 as catalysts and without catalyst.

Figure 7 .
Figure 7. (a-d) Absorption spectra of the MB solution (4 ˆ10 ´5 mol¨L ´1, 50 mL) during the decomposition reaction under the Xe lamp irradiation with the presence of compounds 1-4 (20 mg); (e) Concentration changes of MB at different time intervals under Xe lamp irradiation with 1-4 as catalysts and without catalyst.