English

• 1. INTRODUCTION

  Polyoxometalates (POMs) have a variety of applications in photochromism^{[1, 2]}, electrochemistry^{[3, 4]}, magnetism^[5-7], medicine^{[8, 9]}, and catalysis^{[10, 11]}. Especially, they have attracted much attention as environmentally friendly catalysts for the oxidative degradation of pollutants^[12]. However, the instability and easy aggregation of POMs in aqueous solution have hindered their further development in the field. The introduction of POM into coordination polymers not only improves the stability of the compound, but also gives the resulting material the advantages of both sides^[13]. Besides, the immobilization of POM on coordination polymers also increases the dispersion of POM compared to bulk POM, which is expected to facilitate the activity of POM catalysts. The strategy further broadens development prospects of POM in the field of catalysis. As reported, the copper compounds play an important role in oxidative catalysis because of their cheap, easy availability and activity. The combination of POM and Cu-based coordination polymer may further promote the activity of the resulting materials for oxidative degradation of pollutants. 1, 2, 4-triazole (trz) possesses diverse coordination modes including monodentate (μ₁), imidazole-like (μ_{1, 2}), pyrazole-like (μ_{1, 4}), and tridentate (μ_{1, 2, 4}) and has small steric hindrance^[14-20]. The use of trz as bridge ligand to construct coordination polymer increases the possibility of obtaining novel POM-based coordination polymers.

  Keeping these ideas in mind, we put our effort on the synthesis of POM-containing Cu-based coordination polymers. Herein, we report a novel compound, [Cl₂Cu₁₁^Ⅰ(trz)₈][H₃SiW₁₂O₄₀] (1), which was synthesized through one pot self-assembly reaction. Compound 1 exhibits a fascinating 3D framework in which [SiW₁₂O₄₀]⁴⁻ locates at the interlayer of adjacent 2D layers to further extend into a three-dimensional network through weak O(2)–Cu(6) bond. The layer contains two typically different Cu-trz rings, [Cu₄(trz)₄] and [Cu₈(trz)₈]. It is worth mentioning that the compound shows high stability in both acidic and alkaline solutions, as well as common organic solvents. Besides, compound 1 exhibits effective catalytic activity for light driven degradation of the dye pollutant.

  2. EXPERIMENTAL

  2.1 Materials and methods

  All chemical reagents were commercially available and used without further purification. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku desktop MiniFlex 600 diffractometer with CuKα radiation (λ = 1.54184 Å). Thermogravimetric analysis was performed with a TGA/DSC 1 STAR^e system under N₂ atmosphere from 30 to 900 ℃ at a heating rate of 10 ℃·min⁻¹. FT-IR spectrum in the range of 400~4000 cm⁻¹ was collected on a Bruker Vertex 70 spectrometer through KBr pellet. Elemental analyses for C, H, and N were carried out on an Elementar vario MICRO CHN analyzer.

  2.2 Synthesis of the title complex

  The mixture of H₄SiW₁₂O₄₀·nH₂O (0.5612 g, 0.195 mmol) and Cu(OAc)₂·H₂O (0.256 g, 1.28 mmol) was dissolved in 5 mL of distilled water at room temperature. After 15 min stirring, 1, 2, 4-triazole (0.091 g, 1.32 mmol) was added. The mixture was stirred for another 30 min and then the pH was adjusted to about 1.5 with 2.0 mol·L^-1 HCl. The suspension was put into a 15 mL Teflon-lined autoclave and heated under autogenous pressure at 160 ℃ for 4 days. After cooling down to room temperature slowly at a rate of 10 ℃·h⁻¹, 0.16 g dark-red block crystals was isolated and washed with distilled water and dried in air. Elemental analysis (%) calcd. for C₁₆H₁₉N₂₄O₄₀Cl₂Cu₁₁SiW₁₂ (4190.1): C, 4.58; H, 0.45; N, 8.02. Found (%): C, 4.67; H, 0.39; N, 8.15. IR (KBr, cm⁻¹): 3120 (w), 1514 (w), 1406(w), 925 (s), 783 (s), 534(w).

  2.3 X-ray structure determination

  A single crystal of the title complex was mounted on an Oxford Diffraction/Agilent SuperNova (dual source) diffractometer. Data were collected at 100 K by using a graphite-monochromatic with CuKα radiation (λ = 1.54184 Å) in the ω-scan mode. The SHELXL-2018 program was used for structure solution by direct methods. Hydrogen atoms were located using the geometric method. Non-hydrogen atoms were refined with anisotropic thermal parameters. Crystal data for C₁₆H₁₉Cl₂Cu₁₁N₂₄O₄₀SiW₁₂ (M_r = 4190.1 g/mol): monoclinic system, space group I2/a, a = 16.7064(8), b = 20.5417(8), c = 19.6437(10) Å, β = 111.848(6)°, V = 6257.1(6) ų, Z = 4, T = 100 K, μ(CuKα) = 44.974 mm^‒1, D_c = 4.379 g/cm³, 12137 reflections measured (6.48°≤2θ≤146.12°), 6035 unique (R_int = 0.0491, R_sigma = 0.0474) which were used in all calculations. The final R = 0.0622 (I > 2σ(I)) and wR = 0.1685 (all data). The selected bond lengths and bond angles for 1 are listed in Table 1.

  Table 1

  

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

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  Bond	Dist.		Bond	Dist.
  O(19)–Cu(6)A	2.44(2)		N(14)–Cu(6)	1.92(2)
  O(9)–Cu(6)C	2.34(2)		Cu(5)–N(19)	1.891(17)
  Cu(6)–O(2)	2.518		Cu(5)–N(9)C	1.892(15)
  N(4)–Cu(6)D	2.12(3)
  Angle	(°)		Angle	(°)
  O(9)D –Cu(6)–O(19)A	118.3(8)		N(14)–Cu(6)–N(4)G	164.0(12)
  N(14)–Cu(6)–O(9)C	89.9(9)		N(4)G –Cu(6)–O(9)D	93.2(8)
  N(14)–Cu(6)–O(19)A	88.8(8)		N(4)G –Cu(6)–O(19)A	103.4(9)
  Symmetry codes: A: 0.5–x, y, 1–z; C: 0.5+x, –0.5+y, 0.5+z; D: –0.5+x, 0.5+y, –0.5+z; G: –0.5+x, –0.5+y, –0.5+z

  2.4 Photocatalytic experiment

  12 mg methylene blue was dissolved in 1000 mL distilled water. The solution was sealed and placed in dark condition. Then, 100 mL of the solution was taken out for each reaction. The photocatalytic experiments were carried out in six different experimental conditions: In the presence/absence of visible light, H₂O₂ was added solely; in the presence/absence of visible light, ground compound 1 was added solely; in the presence/absence of visible light, H₂O₂ and the ground compound 1 were added simultaneously. The specific steps are as follows: The solution prepared under different conditions was stirred for 15 min to reach surface adsorption equilibrium in the darkness. Under continuous stirring, 5 mL solution was taken out every 15 min. After centrifugation, the supernate was used for UV-vis absorption spectrum analysis.

  3. RESULTS AND DISCUSSION

  3.1 X-ray crystal structure

  The self-assembly of Cu(OAc)₂·H₂O, triazole and H₄SiW₁₂O₄₀ under hydrothermal conditions gives birth to a glamorous three-dimensional structure of compound 1. Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group I2/a. In the asymmetric unit, there are a half [SiW₁₂O₄₀]⁴⁻ anion, five and a half Cu ions, one Cl ion and four triazole ligands. Meanwhile, bond valence sums (BVS) calculations confirm that all Cu atoms are in +1 oxidation states (Table 2), which is consistent with the crystal color, charge neutrality and coordination environments^[21].

  Table 2

  

  Table 2.  Bond Valence for Compound 1

  DownLoad: CSV

  Bond	Bond distance	Bond valence	Sum of bond valence
  Cu(1)–N(1)	1.885	0.475569025	Cu(1) = 1.106
  Cu(1)–N(17)	1.886	0.474285439
  Cu(1)–Cl(1)	2.535	0.157024827
  Cu(2)–N(2)	1.876	0.487278759	Cu(2) = 1.138
  Cu(2)–N(6)	1.884	0.476856085
  Cu(2)–Cl(1)	2.497	0.174008931
  Cu(3)–N(7)	1.890	0.469185645	Cu(3) = 1.076
  Cu(3)–N(11)	1.902	0.454212926
  Cu(3)–Cl(1)	2.544	0.153251383
  Cu(4)–N(12)	1.904	0.451764345	Cu(4) = 1.052
  Cu(4)–N(16)	1.903	0.452986981
  Cu(4)–Cl(1)	2.557	0.14796037
  Cu(5)–N(9)	1.893	0.465396818	Cu(5) = 0.933
  Cu(5)–N(19)	1.891	0.467919288
  Cu(6)–N(4)	2.120	0.251986849	Cu(6) = 1.008
  Cu(6)–N(14)	1.915	0.438531197
  Cu(6)–O(2)	2.518	0.082084998
  Cu(6)–O(9)	2.337	0.133880079
  Cu(6)–O(19)	2.436	0.102450207

  Cu ions show three kinds of coordination modes: (1) Cu(6) ion is five-coordinated by two nitrogen atoms from two triazole ligands and three oxygen atoms from three [SiW₁₂O₄₀]⁴⁻ anions in a triangular biconical coordination mode. The bond distances around Cu(6) are 1.92(2)~2012(3) Å for Cu(6)–N and 2.34(2)~2.518 Å for Cu(6)–O. (2) Cu(1), Cu(2), Cu(3) and Cu(4) are three-coordinated by two nitrogen atoms from two triazole ligands and one chlorine atom in a Y-type coordination environment. The bond lengths around Cu(1), Cu(2), Cu(3) and Cu(4) are 1.876(16)~1.903(15) Å (Cu−N) and 2.460(16)~2.577(8) Å (Cu–Cl). (3) Cu(5) is coordinated by two nitrogen atoms from two triazole ligands in a straight line with bond lengths of 1.891(17)~1.892(15) Å for Cu(5)−N. There are two substructures in the frameworks: Four Cu ions and four trz ligands compose [Cu₄(trz)₄] vis μ_{1, 2}-bridging modes. One chlorine locates at the center of [Cu₄(trz)₄] subunit and coordinates with four Cu ions (Fig. 1a). Meanwhile, eight Cu ions bridge eight trz ligands to generate a [Cu₈(trz)₈] subunit in a μ_{1, 4}-bridging mode (Fig. 1b). Each [Cu₄(trz)₄] is surrounded by four [Cu₈(trz)₈] sections, and each [Cu₈(trz)₈] section is next to four [Cu₄(trz)₄] and four [Cu₈(trz)₈], thus extending to generate a two-dimensional layer (Fig. 1c). SiW₁₂ polyoxoanions in the middle of [Cu₈(trz)₈] connect two adjacent identical layers by our terminal oxygen atoms (Fig. 1d). These double layers intersect each other to further create a 3D stable POMOF network through weak O–Cu bond (Cu(6)–O(2) 2.518 Å) (Fig. 1e, 1f).

  Figure 1

  

  Figure 1.  Crystal structure of compound 1: (a) [Cu₄(trz)₄] substructure, (b) [Cu₈(trz)₈] substructure, (c) two-dimensional layer omitting polyoxometalates, (d) coordination pattern of polyoxometalates and metals, (e) The resulting structure of three-dimensional frameworks along the a axis, (f) The resulting structure of three-dimensional frameworks along the b axis. Color code: W, dark blue; Si, orange; Cu, sky blue; O, red; N, blue; C, black; Cl, bright green. Symmetry codes: A: 0.5 – x, y, 1 – z; C: 0.5 + x, –0.5 + y, 0.5 + z; F: –x, –0.5 + y, 0.5 – z; G: –0.5 + x, –0.5 + y, –0.5 + z; H: 1 – x, –0.5 + y, 1.5 – z

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  3.2 PXRD patterns, TG and IR spectra

  The powder X-ray diffraction (PXRD) patterns of the as-synthesized compounds are in line with the simulated pattern from the single-crystal X-ray diffraction data, confirming their crystalline phase purity (Fig. 2a). IR spectra of compound 1 are shown in Fig. 2b. The characteristic bands at 925, 783 and 534 cm^-1 are attributed to the vibration of SiW₁₂O₄₀^4- anion. The vibrations at 1514 and 1406 cm^-1 are the absorption bands of the triazole-ring, and that at 3120 cm^-1 is attributed to ν(O–H) and ν(N–H). Furthermore, the TGA curve in Fig. 2c shows that the framework of 1 began to collapse at about 344 ℃, indicating its good thermal stability.

  Figure 2

  

  Figure 2.  (a) PXRD patterns of compound 1, (b) IR spectra of compounds 1, (c) TGA curve of compound 1

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  3.3 Stability of compounds

  Compound 1 displays high stability in aqueous acid, alkali as well as organic solvents. We have soaked 40 mg samples in 10 mL aqueous solutions with pH = 1, 2, 11, 12, 13 and 14 (achieved by HCl or NaOH) and in the common organic solvents such as ethanol, methanol, dichloromethane, DMF, acetonitrile and acetone for 24 h, respectively. Then the samples were filtered, washed with distilled water and dried under ambient condition. The PXRD analyses reveal that 1 maintained crystalline integrity with the pH range of 1~13 and in the common organic solvents, whereas its structure began to collapse at pH = 14 (Fig. 3). Such high chemical stability of 1 is rare due to the restrain from decomposition, recombination and aggregation of polyoxometalate in water^[22]. These results suggest the good potential of 1 as a catalyst.

  Figure 3

  

  Figure 3.  (a) PXRD patterns for compound 1 and the sample after being soaked in aqueous solutions with different pH values; (b) PXRD patterns for compound 1 and the sample after being soaked in different organic solvents

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  3.4 Photocatalysis

  Methylene blue (MB) is an organic dye. Its effusion significantly pollutes the water. In view of excellent chemical and thermal stability of compound 1, it may be an excellent candidate for degrading dye contaminant. Thus, we examined the activity of compound 1 to degrade MB. The specific steps are as follows: Firstly, 100 mL MB solution (12 mg/L) with mashed compound 1 was stirred for 15 min to reach surface adsorption equilibrium in the darkness. Then, the solution was irradiated by visible light (λ ≥ 420 nm) with continuous stirring. During the period, 5 mL solution was taken out every 15 min and centrifuged for UV-vis absorption spectrum analysis. As is presented in Fig. 4, dyes hardly degrade without H₂O₂ or compound 1 in the absence of light. When 1 mL H₂O₂ and 30 mg mashed 1 were synchronously added into the solution, only 14% MB was degraded in 1 h. Under the radiation of Xe lamp with 420 nm filter, if H₂O₂ or compound 1 was added solely, the degradation rate of MB reached up to 20% in 1 h, implying light was indispensable for the reaction. However, the degradation rate of MB was as high as 94% when H₂O₂ and compound 1 were added at the same time under the radiation of Xe lamp. The results confirm that compound 1 is an excellent photocatalyst for the degradation of MB. The excellent activity of 1 is attributed to the combination of polyoxometalate and metal-organic frameworks, which solves the disadvantage of easy aggregation of polyoxo-metalate, and enhances the photocatalytic degradation ability. The PXRD analysis of used 1 filtered out after reaction was carried out. The result matched to the pattern of pristine sample well, indicating that the structure of 1 remained unchanged during photocatalytic experiments (Fig. 2a).

  Figure 4

  

  Figure 4.  Photodegradation of MB under visible light irradiation

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  It is proposed that the photocatalytic reaction proceeded via a radical mechanism that has been previously reported^[23-29]. Initially, the catalyst is excited by visible light radiation to produce excited 1*, which is interchangeable with 1(e^- + h⁺) (equation (1)). Then 1(e^- + h⁺) can directly oxidize MB into the final product MB_ox (equation (2)), or react with water to produce ·OH radicals (equation (3)). When H₂O₂ is present in the solution, H₂O₂ can easily trap an electron to produce ·OH groups (equation (4)). Finally, ·OH causes the degradation of MB (equation (5)).

  $ {\bf{1}} + {\rm{hv}} \to {{\bf{1}}^ * } \leftrightarrow {\bf{1}}\left( {{{\rm{e}}^ - } + {{\rm{h}}^ + }} \right) $

  (1)

  $ {\bf{1}}\left( {{{\rm{e}}^ - } + {{\rm{h}}^ + }} \right) + {\rm{MB}} \to {\rm{M}}{{\rm{B}}_{{\rm{ox}}}} + {\bf{1}}\left( {{{\rm{e}}^ - }} \right) $

  (2)

  $ {\bf{1}}\left( {{{\rm{e}}^ - } + {{\rm{h}}^ + }} \right) + {{\rm{H}}_2}{\rm{O}} \to \cdot {\rm{OH}} + {{\rm{H}}^ + } + {\bf{1}}\left( {{{\rm{e}}^ - }} \right) $

  (3)

  $ {{\rm{H}}_2}{{\rm{O}}_2} + {\bf{1}}\left( {{{\rm{e}}^ - }} \right) \to \cdot {\rm{OH}} + {\rm{O}}{{\rm{H}}^ - } $

  (4)

  $ \cdot \;\;\;\;{\rm{OH}} + {\rm{MB}} \to {\rm{M}}{{\rm{B}}_{{\rm{ox}}}} $

  (5)

  4. CONCLUSION

  In summary, a new 3D POM-based coordination polymer has been successfully synthesized by hydrothermal reaction. The framework of compound 1 not only demonstrates ultrahigh chemical stability, but also shows outstanding catalytic activity to degrade organic dye pollutants. This work demonstrates an example for constructing excellent POM-based catalysts for light-driven elimination of organic contamination. It is meaningful for clean environment.

  
  
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  Figure 1  Crystal structure of compound 1: (a) [Cu₄(trz)₄] substructure, (b) [Cu₈(trz)₈] substructure, (c) two-dimensional layer omitting polyoxometalates, (d) coordination pattern of polyoxometalates and metals, (e) The resulting structure of three-dimensional frameworks along the a axis, (f) The resulting structure of three-dimensional frameworks along the b axis. Color code: W, dark blue; Si, orange; Cu, sky blue; O, red; N, blue; C, black; Cl, bright green. Symmetry codes: A: 0.5 – x, y, 1 – z; C: 0.5 + x, –0.5 + y, 0.5 + z; F: –x, –0.5 + y, 0.5 – z; G: –0.5 + x, –0.5 + y, –0.5 + z; H: 1 – x, –0.5 + y, 1.5 – z

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  Figure 2  (a) PXRD patterns of compound 1, (b) IR spectra of compounds 1, (c) TGA curve of compound 1

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  Figure 3  (a) PXRD patterns for compound 1 and the sample after being soaked in aqueous solutions with different pH values; (b) PXRD patterns for compound 1 and the sample after being soaked in different organic solvents

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  Figure 4  Photodegradation of MB under visible light irradiation

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

  Bond	Dist.		Bond	Dist.
  O(19)–Cu(6)A	2.44(2)		N(14)–Cu(6)	1.92(2)
  O(9)–Cu(6)C	2.34(2)		Cu(5)–N(19)	1.891(17)
  Cu(6)–O(2)	2.518		Cu(5)–N(9)C	1.892(15)
  N(4)–Cu(6)D	2.12(3)
  Angle	(°)		Angle	(°)
  O(9)D –Cu(6)–O(19)A	118.3(8)		N(14)–Cu(6)–N(4)G	164.0(12)
  N(14)–Cu(6)–O(9)C	89.9(9)		N(4)G –Cu(6)–O(9)D	93.2(8)
  N(14)–Cu(6)–O(19)A	88.8(8)		N(4)G –Cu(6)–O(19)A	103.4(9)
  Symmetry codes: A: 0.5–x, y, 1–z; C: 0.5+x, –0.5+y, 0.5+z; D: –0.5+x, 0.5+y, –0.5+z; G: –0.5+x, –0.5+y, –0.5+z

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  Table 2.  Bond Valence for Compound 1

  Bond	Bond distance	Bond valence	Sum of bond valence
  Cu(1)–N(1)	1.885	0.475569025	Cu(1) = 1.106
  Cu(1)–N(17)	1.886	0.474285439
  Cu(1)–Cl(1)	2.535	0.157024827
  Cu(2)–N(2)	1.876	0.487278759	Cu(2) = 1.138
  Cu(2)–N(6)	1.884	0.476856085
  Cu(2)–Cl(1)	2.497	0.174008931
  Cu(3)–N(7)	1.890	0.469185645	Cu(3) = 1.076
  Cu(3)–N(11)	1.902	0.454212926
  Cu(3)–Cl(1)	2.544	0.153251383
  Cu(4)–N(12)	1.904	0.451764345	Cu(4) = 1.052
  Cu(4)–N(16)	1.903	0.452986981
  Cu(4)–Cl(1)	2.557	0.14796037
  Cu(5)–N(9)	1.893	0.465396818	Cu(5) = 0.933
  Cu(5)–N(19)	1.891	0.467919288
  Cu(6)–N(4)	2.120	0.251986849	Cu(6) = 1.008
  Cu(6)–N(14)	1.915	0.438531197
  Cu(6)–O(2)	2.518	0.082084998
  Cu(6)–O(9)	2.337	0.133880079
  Cu(6)–O(19)	2.436	0.102450207

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