English

• In recent years, the development of transition metal coordination polymers has been more oriented towards the development of practical applications, such as gas storage/separation, magnetism, catalysis and luminescent sensing^[1-7]. The suitable ligands are the key to the design and synthesis of transition metal coordination polymers with interesting structures and good application performances. In the O-donor ligands, the aliphatic carboxylates are widely used in making diverse transition metal coordination polymers due to good flexibility and diverse coordination modes^[8]. In our previous work, we obtained a number of transition metal coordination polymers with the fascinating structures and excellent properties based on a flexible itaconic acid^[9-10]. The citraconic acid is an isomer of itaconic acid; they are very cheap and excellent flexible ligands with diverse coordination modes. However, there are still few reports on the synthesis of transition metal complexes by citraconic acid as a ligand^[11-12].

  The N-donor ligands are often used to mediate the structures and properties of transition metal coordination polymers together with the carboxylate groups^[13-14]. In the N-donor ligands, 1, 2-bi(4-pyridyl)ethylene (bpe) is one of the most common auxiliary ligands to combine with carboxylates main ligands to obtain a series of transition metal complexes with different structures and properties^[15-18]. But as far as we know, there is no report about the 1-(1-imidazolyl)-4-(imidazol-1-ylmethyl)benzene(iimb) as an auxiliary N-donor ligand together with polycarboxylates ligands to synthesize the transition metal coordination polymers until now. In this work, we used flexible citraconic acid (H₂ca) as a dicarboxylate ligand and two N-donor ligands such as iimb and bpe as synergistic ligands to synthesize three coordination polymers by hydrothermal techniques, namely {[Zn(ca)(iimb)]·0.75H₂O}_n (1), {[Cd(ca)(iimb)(H₂O)]·0.75H₂O}_n (2) and {[Zn(ca)(bpe)_0.5(H₂O)]·H₂O}_n (3). The structures of three complexes are characterized by elemental analysis, infrared spectroscopy and single crystal X-ray crystallography. The fluorescence properties, thermal stabilities and powder X-ray diffraction for three complexes are given in this paper. The photocatalytic activity of 3 is evaluated by degradation of methylene blue under UV light.

  1.   Experimental

  1.1   Materials and methods

  Citraconic acid and bpe were purchased from J&K Scientific Ltd. Ligand iimb was purchased from Jinan Henghua Technology Co., Ltd. All other chemical reagents were of analytical grade and obtained from Tianjin Deen Chemical Reagent Co., Ltd. All chemical reagents were used without further purification. Elemental analyses for C, H and N were performed on a Flash 2000 organic elemental analyzer. Infrared spectra (4 000~600 cm^-1) were recorded on powdered samples using a Nicolet 6700 FT-IR spectrometer. Powder X-ray diffraction (PXRD) patterns were taken on a Bruker D8-ADVANCE X-ray diffractometer with Cu Kα radiation (λ=0.154 18 nm; generator voltage: 40 kV; generator current: 40 mA; scanning scope, 2θ: 5°~50°). Thermogravimetric analyses were carried out on a SⅡ EXStar 6000 TG/DTA6300 analyzer heated from 25 to 800 ℃ with a heating rate of 10 ℃·min^-1 in N₂ atmosphere. Solid fluorescence spectra were performed on an Aminco Bowman Series 2 luminescence spectrometer at room temperature. Solid-state ultraviolet/visible (UV/Vis) diffuse-reflectance spectrum was measured employing a UV-Vis spectrophotometer (Shimadzu, UV-3600 Plus) and a BaSO₄ plate was used as the reflectance standard. Solution UV spectra were tested on a Hitachi U-3010 UV-Vis spectro-photometer in the 10 mm quartz cuvette.

  1.2   Synthesis of {[Zn(ca)(iimb)]·0.75H₂O}_n (1)

  A starting mixture containing Zn(OAc)₂·2H₂O (43.8 mg, 0.20 mmol), H₂ca (13.0 mg, 0.10 mmol), iimb (22.4 mg, 0.10 mmol) and H₂O (5 mL) was heated at 120 ℃ for 4 days under autogenous pressure in a 23 mL Teflon-lined autoclave, then cooled to room temp-erature. Colorless needle crystals of 1 were collected in 58% yield. Elemental analysis Calcd. for C₁₈H₁₇N₄ O_4.75Zn(%): C, 50.19; H, 3.98; N, 13.01. Found(%): C, 49.69; H, 4.02; N, 12.85. Selected IR(KBr, cm^-1): 3 129(w), 1 588(s), 1 524(s), 1 437(m), 1 401(s), 1 252(s), 1 102(s), 1 065(m), 1 031(m), 953(m), 864(m), 830(m), 754(s), 654(m).

  1.3   Synthesis of {[Cd(ca)(iimb)(H₂O)]·0.75H₂O}_n(2)

  A mixture of Cd(OAc)₂·2H₂O (54.0 mg, 0.2 mmol), H₂ca (0.013 g, 0.10 mmol), iimb (22.4 mg, 0.10 mmol), NaOH (4.0 mg, 0.10 mmol) and 5 mL H₂O was heated at 120 ℃ for 4 days under autogenous pressure in a 23 mL Teflon-lined autoclave, then colorless needle crystals were collected with ca. 72% yield. Elemental analysis Calcd. for C₁₈H_19.5N₄O_5.75Cd(%): C, 43.56; H, 3.96; N, 11.29. Found(%): C, 43.48; H, 4.01; N, 11.23. Selected IR (KBr, cm^-1): 3 134(w), 1 647(w), 1 563(w), 1 522(s), 1 493(w), 1 402(s), 1 360(m), 1 280(m), 1 230(m), 1 128(s), 1 108(s), 1 062(s), 1 028(m), 963(m), 937(s), 861(w), 822(m), 754(s), 689(m).

  1.4   Synthesis of {[Zn(ca)(bpe)_0.5(H₂O)]·H₂O}_n (3)

  A starting mixture containing Zn(OAc)₂·2H₂O (21.9 mg, 0.10 mmol), H₂ca (13.0 mg, 0.10 mmol), bpe (18.2 mg, 0.10 mmol), NaOH (4.0 mg, 0.10 mmol) and H₂O (5 mL) was heated at 120 ℃ for 4 days under autogenous pressure, then cooled to room temperature. Colorless sheet crystals of 3 were obtained in 82% yield. Elemental analysis Calcd. for C₁₁H₁₃NO₆Zn(%): C, 41.21; H, 4.09; N, 4.37. Found(%): C, 41.18; H, 4.13; N, 4.32. Selected IR (KBr, cm^-1): 3 072(w), 1 655(w), 1 612(s), 1 562(s), 1 509(s), 1 432(w), 1 401(s), 1 299(w), 1 262(s), 1 225(m), 1 069(m), 1 065(m), 1 026(s), 985(m), 956(s), 887(m), 872(s), 844(s), 794(m), 726(m).

  1.5   Photocatalytic experiments

  The photocatalytic activity of complex 3 in degradation of methylene blue (MB) was investigated at ambient temperature under UV light irradiation using a 500 W Xe lamp as light source. The degradation experiments were run as follows: 6 mg complex 3 was added to 6 mL MB (50 μmol·L^-1) aqueous solution with 10 μL H₂O₂ (30%) in a glass bottle. The aqueous suspensions were stirred for 30 min in the dark to ensure the adsorption-desorption equilibrium before the light irradiation. During the tracking photocatalytic reaction, the absorbance of 3 mL MB clear solution was measured at 20 min intervals with a UV-Vis spectrophotometer at a 665 nm absorption wavelength, then the solution was poured back into the original suspension. For comparison, the contrast experiment was completed under the same conditions without any catalyst.

  1.6   X-ray crystallography

  The single-crystal X-ray diffraction analyses were performed on a computer-controlled EosS2 diffracto-meter equipped with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) by using φ-ω scan technique at room temperature. Using Olex2^[19], the structure was solved with the SHELXS^[20] program and refined with the SHELXL^[21] refinement package. Hydrogen atoms were placed in their geometrically idealized positions and constrained to ride on their parent atoms. The U_iso of the H-atoms were constrained to 1.2 times U_eq of their bonding carbon atoms with d_C-H(aromatic)=0.093 nm and d_C-H(methylene)=0.097 nm, and 1.5 times U_eq of the hydrogen atoms at water and methyl groups with d_C-H(methyl)=0.096 nm. Crystallographic data for com-plexes 1~3 are listed in Table 1. Selected bond distances and angles and hydrogen bonds are listed in Table S1 and Table S2, respectively.

  Table 1

  

  Table 1.  Crystal and structure refinement data for complexes 1~3

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  Complex	1	2	3
  Empirical formula	C18H17N4O4.75Zn	C18H19.5N4O5.75Cd	C11H13NO6Zn
  Formula weight	430.72	496.27	320.59
  Crystal system	Orthorhombic	Monoclinic	Monoclinic
  Space group	Pbcn	C2/c	P21/n
  a/nm	2.164 01(6)	2.019 24(6)	0.795 82(3)
  b/nm	1.059 10(3)	1.439 64(5)	1.863 22(8)
  c/nm	1.610 72(3)	1.431 29(3)	0.864 82(4)
  β/(°)		101.396(3)	96.327(4)
  V/nm3	3.691 61(16)	4.078 7(2)	1.274 54(10)
  Z	8	8	4
  Dc/(g·cm-3)	1.550	1.616	1.671
  μ/mm-1	1.368	1.111	1.949
  F(000)	1 768	1 996	656
  θ range for data collection/(°)	3.31 to 25.50	3.39 to 25.50	3.15 to 25.50
  Reflection collected, unique	63 865, 3 423	26 249, 3 767	13 970, 2 367
  Rint	0.059 0	0.025 2	0.043 3
  Completeness/%	99.4	99.6	99.7
  Data, restraint, parameter	3 423, 103, 278	3 767, 124, 315	2 367, 0, 174
  Goodness-of-fit on F2	1.060	1.104	1.086
  R1, wR2 [I>2σ(I)]	0.056 6, 0.141 6	0.037 0, 0.095 4	0.032 4, 0.071 2
  R1, wR2 (all data)	0.082 3, 0.152 3	0.042 0, 0.098 2	0.038 6, 0.073 9
  Largest peak and hole /(e·nm-3)	890 and -530	990 and -1 000	310 and -310

  CCDC: 1913976, 1; 1913977, 2; 1913978, 3.

  2.   Results and discussion

  2.1   Structure description of {[Zn(ca)(iimb)]·0.75H₂O}_n (1)

  The complex 1 crystallizes in orthorhombic crystal system, space group Pbcn and features a 2D reticular structure. The asymmetric unit contains one crystallo-graphic Zn²⁺, one ca^2-, one iimb molecule, in addition to 0.75 free water molecules. The Zn²⁺ cation adopts a distorted tetrahedral [ZnN₂O₂] coordination environment with two carboxylic oxygen atoms from two symmetry-related ca^2- and two nitrogen atoms from two symmetry-related iimb co-ligands (Fig. 1a). The Zn-O bond lengths are 0.193 2(10) and 0.196 6(3) nm, and the Zn-N bond lengths are 0.199 9(3) and 0.201 1(3) nm, respectively.

  Figure 1

  

  Figure 1.  (a) View of the asymmetric unit showing the local coordination environments of Zn(Ⅱ) in 1; (b) Polyhedral view of a 2D layer extending in complex 1; (c) View of schematic representation of 2D layer

  Symmetry codes: ^ⅰ 1.5-x, -0.5+y, z; ^ⅱ 0.5+x, -0.5+y, 1.5-z

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  The adjacent Zn²⁺ ions are linked through ca^2- adopting the monodentate coordination mode to form a metal-carboxylate chain with the Zn…Zn distance of 0.5913 3(1) nm. The adjacent metal-carboxylate chains are further linked by iimb molecules adopting cross connection method along two different directions with the Zn…Zn distance of 1.255 41(3) nm to produce two-dimensional network structure (Fig. 1b). Each Zn(Ⅱ) ion is connected to four adjacent ones through two ca^2- and two iimb co-ligands. The 2D layer can be simplified as a commonly uninodal 4-connected net (Fig. 1c). There is no face-face stacking π…π interac-tion between the two pyridine and benzene rings of two iimb ligands. The 3D supramolecular structure is formed through weak van der Waals force.

  2.2   Structure description of {[Cd(ca)(iimb)(H₂O)]·0.75H₂O}_n (2)

  The complex 2 crystallizes in monoclinic crystal system, space group C2/c and features a one-dimensional double-stranded chain structure. The asymmetric unit contains one crystallographic Cd²⁺, one ca^2-, one iimb molecule, one coordinated water molecule, in addition to 0.75 free water molecule. The Cd(Ⅱ) ion is coor-dinated by two nitrogen atoms from two symmetry-related iimb co-ligands, four oxygen atoms from two symmetry-related ca^2- and one coordinated water molecule, thus creating a distorted octahedron coor-dination [Cd(O₄N₂)] geometry (Fig. 2a). The Cd-O bond lengths are in a range of 0.230 4(7)~0.266 2(5) nm, and the Cd-N bond lengths are 0.226 1(3) and 0.225 9(3) nm, respectively.

  Figure 2

  

  Figure 2.  (a) View of the asymmetric unit showing the local coordination environments of Cd(Ⅱ) in 2; (b) Polyhedral view of 1D quasi-trapezoidal shape chain with the dinuclear units

  Symmetry codes: ^ⅰ -0.5+x, -0.5+y, z; ^ⅱ 0.5-x, 0.5-y, 1-z

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  The two carboxylate groups of ca^2- adopt the chelating-bidentate and monodentate coordination mode, respectively. Two adjacent Cd(Ⅱ) ions are linked together to form a binuclear cluster with the Cd…Cd distance of 0.513 46(1) nm. The carboxylate-bridged dinuclears are extended along the [110] direction by mated iimb co-ligands to produce one 1D coordination polymer with quasi-trapezoidal shape (Fig. 2b).

  The adjacent chains are linked together by H-bond interactions to form its entire three-dimensional supramolecular structure (Fig.S1). There exist weak inter-chain offset π…π interactions between benzene ring and imidazole ring of iimb co-ligands, between two parallel benzene rings of iimb co-ligands. The centroids Cg1 (coordinate: -0.044 30, 0.925 04, -0.906 50), Cg3 (coordinate: 0.044 30, 0.925 04, -0.593 50) and Cg4 (coordinate: -0.044 30, 1.074 96, -0.406 50) are made up of atoms including C5, C6, C7, C8, C9 and C10 atoms, moreover, the centroid Cg2 (coordinate: -0.134 23, 1.033 17, 0.703 73) is made up of atoms including N1, C11, N2, C12 and C13 atoms. The centroid-centroid distance (Cg1-Cg2) and the dihedral angle between benzene ring and imidazole ring of iimb co-ligands are 0.402 8 nm and 23.651° respectively. The centroid-centroid distance (Cg3-Cg4) and the dihedral angle between two benzene rings are 0.411 3 nm and 0°, respectively (Fig.S2). It is obvious that the hydrogen bonds and π…π interactions among the coordination polymers play important roles in the self-assembly and enhanced stability of the resultant structure.

  2.3   Structure description of {[Zn(ca)(bpe)_0.5(H₂O)]·H₂O}_n (3)

  The complex 3 crystallizes in monoclinic crystal system, space group P2₁/n and features a two-dimensional wave layer. The asymmetric unit contains one crystallographic Zn²⁺, one ca^2-, half a bpe molecule, one coordinated water and one free water molecule. The Zn²⁺ cation adopts the distorted tetrahedral [ZnNO₃] coordination environment with three oxygen atoms from two symmetry-related ca^2- and one coordinated water, one nitrogen atom from one bpe co-ligand (Fig. 3a). The Zn-O bond lengths are 0.198 11(1), 0.198 79(1) and 0.202 47(1) nm, respectively, and the Zn-N bond length is 0.206 43(1) nm.

  Figure 3

  

  Figure 3.  (a) View of the asymmetric unit showing the local coordination environments of Zn(Ⅱ) in 3; (b) View of a section of the 1D chain by ca^2- and Zn²⁺; (c) Polyhedral view of a 2D wave layer extending in complex 3; (d) View of schematic representation of 2D layer

  Symmetry codes: ^ⅰ-0.5+x, 0.5-y, 0.5+z

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  The ca^2- indeed coordinates to two Zn(Ⅱ) ions from two monodentate sites. The adjacent Zn(Ⅱ) ions are connected by the ca^2- to form an interesting corrugated 1D metal-carboxylate chain with the Zn…Zn distance of 0.658 09(3) nm (Fig. 3b). The neighboring chains are connected by bpe molecules along b direction with the Zn…Zn distance of 1.349 50(4) nm to produce a 2D layer (Fig. 3c). Each Zn(Ⅱ) ion is connected to three adjacent atoms through two ca^2- and one bpe ligand. The 2D layer can be simplified as a commonly uninodal 3-connected grid (Fig. 3d).

  The adjacent layers are stacked in a -AAA- sequence along special direction to form a three-dimensional supramolecular polymer by interlayer H-bond interactions between the coordinatedwater O atom and the carboxylate O atom of ca^2- anions (O(1W)-H(1WA)…O(4^ⅰ): O(1W)…O(4^ⅰ) 0.271 3(3) nm, ∠O(1W)-H(1WA)…O(4^ⅰ)=133.2°), between the free water O atom and the carboxylate O atom of ca^2- (O(2W)-H(2WA)…O(2^ⅱ): O(2W)…O(2^ⅱ) 0.287 6(3) nm, ∠O(2W)-H(2WA)…O(2^ⅱ)= 171.2°, Symmetry codes: ^ⅰ x-1, y, z; ^ⅱ x-1/2, -y+1/2, z-1/2). There also exist abundant intralayer H-bond interactions between the coordinated water molecule and the guest water molecule (O(1W)-H(1WB)…O(2W): O(1W)…O(2W) 0.266 6(3) nm, ∠O(1W)-H(1WB)…O(2W)=151.4°), between the free water O atom and the carboxylate O atom of ca^2- anions (O(2W)-H(2WB)…O(3): O(2W)…O(3) 0.278 8(3) nm, ∠O(2W)-H(2WB)…O(3)=165.5°) (Fig.S3).

  2.4   Thermogravimetric analyses and powder X-ray diffraction

  The thermogravimetric analyses (TGA) of 1~3 were performed on the polycrystalline samples from 25 to 800 ℃ at a heating rate of 10 ℃·min^-1 under nitrogen atmosphere (Fig. 4). The TGA curve of 1 displayed the first weight loss of 3.78% from 60 to 100 ℃ corresponding approximately to release of 0.75 H₂O per formula unit (Calcd. 3.14%). The framework of 1 had a series of weight loss from 260 to 515 ℃ attributed to the decomposition of organic components. The final residue of 10.74% can′t be specifically identified because the theoretical remaining mass of 18.89% is calculated by assuming a final phase ZnO. The TGA curve of 2 displayed 8.59% weight loss in a region of 60~120 ℃, which is not agreement with the calculated value of 1.75 H₂O per formula unit (6.35%), so the 2.24% weight loss corresponds to the water because the sample is not completely dry. Over 250 ℃, the framework of 2 began to gradually decompose until 500 ℃. The final residue of 21.65% can be approximatively identified to final phase CdO because the calculated value of the mass remnant is 25.87%. For 3, there was no weight loss before 110 ℃, and the first weight loss of 11.16% between 110 and 143 ℃ corresponds to the loss of two water molecules from one coordinated water and one guest water (Calcd. 11.23%). The second weight loss began over 295 ℃ and ended until 454 ℃ corresponding to the decom-position of organic ligands. The observed final mass remnant of 26.63% is almost agreement with the calculated value of the final product ZnO (Calcd. 25.27%). All major peaks in experimental PXRD match quite well that of simulated from the respective single-crystal data, implying their good phase purity (Fig.S4~S6).

  Figure 4

  

  Figure 4.  TGA curves for complexes 1~3

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  2.5   Luminescent properties

  The solid-state luminescent properties of three complexes 1~3 and free ligands were investigated in the solid state at room temperature, as illustrated in Fig. 5. The free H₂ca ligand had no emission peak in a very wide excitation wavelength because of the absence of its conjugation effect. The free bpe and iimb ligands showed fluorescent emission bands at 367 nm (λ_ex=334 nm) and 595 nm (λ_ex=376 nm). The emission maximums for 1 and 2 occurred at 595 nm upon excitation at a wavelength of 376 nm^[9]. The emission spectra of complexes 1 and 2 are completely consistent with that of the powdered free iimb molecule, which should be assigned as the π-π^* and/or n-π^* transition of the iimb linker. The emission maximum for complex 3 located at 375 nm upon excitation at 280 nm, which displays similar emission spectrum feature to that of the powdered free bpe ligand. The slight red-shift may be attributed to the increase of the ligand conformational rigidity because of the unique coordination of bpe to the Zn(Ⅱ) ion.

  Figure 5

  

  Figure 5.  Solid-state emission spectra of complexes 1~3 along with the free iimb and bpe ligands

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  2.6   Photocatalysis property

  The organic dyes are one of the largest sources of industrial pollutants. Furthermore, it is difficult to remove organic dyes from industrial wastewater due to their high photo and chemical stability. In recent years, transition metal coordination polymers have been used as a new class of photocatalyst for the decomposition of organic dyes^[22-24]. The UV-Vis diffuse reflectance spectrum of complex 3 showed absorption bands at 328 nm, which can be attributed to π-π^* transitions of the ligand^[25]. The band gap (E_g) derived from Kubelka-Munk function plot is 3.24 eV (Fig.S7). Methylene blue (MB) is one of the largest sources of industrial pollutants. So we chose MB as a represen-tative to investigate the degradation process in detail. There is almost no reduction in concentration of MB with time, and the degradation is almost negligible without H₂O₂ or light irradiation. The photocatalytic behavior of the complex 3 in MB solution is displayed in Fig. 6. The organic dye concentrations were estimated by the absorbance at 665 nm (MB). According to Lambert-Beer′s law, the absorbance of a solution is proportional to the concentration of the substance. As shown in Fig. 6, approximately 70% of MB was decom-posed after 40 min. After 120 min, the photocatalytic efficiency of MB was found to be 97% for 3. In comparison, the degradation efficiency of MB without any photocatalyst was only about 20% after the same time under the same experimental conditions^[26-27]. The apparent rate constant of MB was 0.033 6 min^-1, which can be confirmed by the pseudo-first-order reaction model: ln(C_t/C₀)=-kt^[28] (Fig.S8), where C_t and C₀ are the concentration of MB at time t and at t=0, and k is the apparent rate constant. To evaluate the stability of photocatalyst 3, after the photocatalytic process, the complex 3 was recycled by the centrifugation method and monitored by powder X-ray diffraction (PXRD) (Fig.S6). The results demonstrated that the complex 3 still maintained a good crystal state after the photocatalytic experiments.

  Figure 6

  

  Figure 6.  (a) Absorption spectra of the MB solutions during the photodegradation reaction under UV light in the presence of 3; (b) Concentration changes of MB as a function of irradiation time in the present of 3 and the control experiment without any catalyst

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  3.   Conclusions

  In summary, three Zn/Cd(Ⅱ) coordination polymers have been successfully synthesized under hydrothermal conditions. Both complexes 1 and 3 exhibit two dimensional layers containing the metal-carboxylate chains. The complex 2 features a 1D chain structure containing carboxylate-bridged dinuclears. All three complexes show good phase purity and high thermal stability. The emission spectra of three complexes are similar to those of the corresponding N-donor ligands. The photocatalytic experiment result indicates that complex 3 has good photocatalytic activity in decom-posing MB in the presence of H₂O₂. The result shows that complex 3 may be a good candidate as photo-catalyst with a potential application in wastewater treatment.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  (a) View of the asymmetric unit showing the local coordination environments of Zn(Ⅱ) in 1; (b) Polyhedral view of a 2D layer extending in complex 1; (c) View of schematic representation of 2D layer

  Symmetry codes: ^ⅰ 1.5-x, -0.5+y, z; ^ⅱ 0.5+x, -0.5+y, 1.5-z

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  Figure 2  (a) View of the asymmetric unit showing the local coordination environments of Cd(Ⅱ) in 2; (b) Polyhedral view of 1D quasi-trapezoidal shape chain with the dinuclear units

  Symmetry codes: ^ⅰ -0.5+x, -0.5+y, z; ^ⅱ 0.5-x, 0.5-y, 1-z

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  Figure 3  (a) View of the asymmetric unit showing the local coordination environments of Zn(Ⅱ) in 3; (b) View of a section of the 1D chain by ca^2- and Zn²⁺; (c) Polyhedral view of a 2D wave layer extending in complex 3; (d) View of schematic representation of 2D layer

  Symmetry codes: ^ⅰ-0.5+x, 0.5-y, 0.5+z

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  Figure 4  TGA curves for complexes 1~3

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  Figure 5  Solid-state emission spectra of complexes 1~3 along with the free iimb and bpe ligands

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  Figure 6  (a) Absorption spectra of the MB solutions during the photodegradation reaction under UV light in the presence of 3; (b) Concentration changes of MB as a function of irradiation time in the present of 3 and the control experiment without any catalyst

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  Table 1.  Crystal and structure refinement data for complexes 1~3

  Complex	1	2	3
  Empirical formula	C18H17N4O4.75Zn	C18H19.5N4O5.75Cd	C11H13NO6Zn
  Formula weight	430.72	496.27	320.59
  Crystal system	Orthorhombic	Monoclinic	Monoclinic
  Space group	Pbcn	C2/c	P21/n
  a/nm	2.164 01(6)	2.019 24(6)	0.795 82(3)
  b/nm	1.059 10(3)	1.439 64(5)	1.863 22(8)
  c/nm	1.610 72(3)	1.431 29(3)	0.864 82(4)
  β/(°)		101.396(3)	96.327(4)
  V/nm3	3.691 61(16)	4.078 7(2)	1.274 54(10)
  Z	8	8	4
  Dc/(g·cm-3)	1.550	1.616	1.671
  μ/mm-1	1.368	1.111	1.949
  F(000)	1 768	1 996	656
  θ range for data collection/(°)	3.31 to 25.50	3.39 to 25.50	3.15 to 25.50
  Reflection collected, unique	63 865, 3 423	26 249, 3 767	13 970, 2 367
  Rint	0.059 0	0.025 2	0.043 3
  Completeness/%	99.4	99.6	99.7
  Data, restraint, parameter	3 423, 103, 278	3 767, 124, 315	2 367, 0, 174
  Goodness-of-fit on F2	1.060	1.104	1.086
  R1, wR2 [I>2σ(I)]	0.056 6, 0.141 6	0.037 0, 0.095 4	0.032 4, 0.071 2
  R1, wR2 (all data)	0.082 3, 0.152 3	0.042 0, 0.098 2	0.038 6, 0.073 9
  Largest peak and hole /(e·nm-3)	890 and -530	990 and -1 000	310 and -310

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