English

• 0.   Introduction

  In recent years, the inorganic-organic coordination polymers have become a unique class of functional materials because of their unique chemical applications and beautiful structural motifs^[1]. Polyoxometalates (POMs), defined as metal-oxide clusters based on polyanions, have received intensive attention because of their many important applications in electrical conductivity, catalysis, medicine, magnetism, materials science, and biological chemistry^[2]. As illustrated in the previous literature, some important applications are mainly based on the following reasons: (ⅰ) polyo-xometalates can be employed as electron and proton reservoirs; (ⅱ) their molecular properties are extremely variable such as shape, size, acidity and charge^[3]. Recently, many remarkable works about preparing the desired metal-organic frameworks (MOFs) are the utilization of the polyanions coordination ability to bind with versatile transition-metal organic units, which have been presented in coordination polymers^[4-6]. This synthetic strategy can bring the merits of each row-material together, such as intriguing structural motifs, good physical and chemical properties^[7].

  For the rational design and preparation of the POM-based coordination frameworks, judicious selective of organic linkers is quite important. The organic linkers can be divided into two parts: pre-synthesized and in-situ synthesized linkers^[8]. The former have been widely utilized in the POM-based reaction systems, including Nitrogen-donor linkers, poly-carboxylate linkers and so on^[9]. Among the Nitrogen-containing heterocyclic organic linkers, 1, 2, 4 -triazole and its derivatives are intriguing because the linkers combine the coordination geometries of both imidazoles and pyrazoles due to their three heteroa-toms arrangement. For instance, the organic linkers have been utilized to construct the open MOFs, which contain unsaturated metal clusters and possess even framework flexibility and high thermal stability^[10-11]. As depicted in Scheme 1, the multi-dentate mono- and bis-triazole derivatives, namely 4-pyridine-2-1, 2, 4-triazole (L₁), 3-(4H-1, 2, 4-triazol-4-yl)benzoic acid (HL₂) and trans-4, 4′-azo-1, 2, 4-triazole (L₃), which can afford strong coordination nitrogen donors and carboxylate donors to metal centers. It can be anticipated that L₁, HL₂ and L₃ can be utilized as multi-dentate coordina-tion donors to coordination metallic centers, which can further construct many novel inorganic-organic POM-based hybrid materials^[12].

  Scheme 1

  

  Scheme 1.  Three multi-dentate 1, 2, 4-triazole derivative ligands L₁, HL₂ and L₃

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  Previously we also began to explore the coordina-tion chemistry and functional application of Nitrogen-containing heterocyclic ligands and its derivatives^[13-16]. In this work under hydrothermal conditions, three novel polyoxometalate (POM)-based Cu(Ⅱ) and Cu(Ⅰ) hybrid materials with multi-dentate mono- and bis-triazole derivatives, namely {[Cu(L₁)₂(Mo₄O₁₃)]·2H₂O}_n (1), {[Cu_1.5(L₂)(HL₂)(H₂O)(Mo₄O₁₃)]·2H₂O}_n (2), {[Cu₂(L₃)_1.5(Mo₄O₁₃)]·H₂O}_n (3) have been designed and synthesized (Scheme 2). Their crystallographic structures have been determined by single crystal X-ray diffraction, FT-IR infrared spectra and PXRD analyses. Photo-catalytic activities for decomposition of different organic dyes, namely rhodamine B (RhB), methylene blue (MB) and methyl orange (MO), have been inves-tigated for 1~3.

  Scheme 2

  

  Scheme 2.  Hydrothermal assembly of a series of Cu(Ⅱ) and Cu(Ⅰ) hybrid polyoxometalate-based coordination frameworks

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  1.   Experimental

  1.1   General

  Ligands L₁ (4-pyridine-2-1, 2, 4-triazole), HL₂ (3-(4H-1, 2, 4-triazol-4-yl) benzoic acid) and L₃ (trans-4, 4′-azo-1, 2, 4-triazole) were prepared according to the literature methods^[17]. All the other reagents were commercially available and utilized without further purification. Perkin-Elmer 240 elemental analyzer was ulitilized to perform C, H and N microanalyses. Powder X-ray diffraction patterns were determined on a D/Max-2500 X-ray diffractometer using Cu Kα radiation (λ=0.154 1 nm, U=40 kV, I=40 mA) with a 2θ range of 5°~50°. A Lamdba 900 UV-Vis-NIR spectroscopy was utilized to measure the UV-Vis diffuse reflectance spectra at the ambident temperature.

  1.2   Preparation of coordination complexes 1~3

  {[Cu(L₁)₂(Mo₄O₁₃)]·2H₂O}_n (1). CuSO₄·5H₂O (75.00 mg, 0.3 mmol), L₁ (43.8 mg, 0.3 mmol) and MoO₃ (57.60 mg, 0.4 mmol) were added in 10 mL deionized H₂O. The mixture were transferred and placed into a Teflon vessel in a steel autoclave, heated at 165 ℃ for 72 h and then cooled to ambient temperature during 24 h. The blue strip-shaped single crystals of 1 were washed by H₂O, aether and air-dried. Elemental analysis Calcd. for C₁₄H₁₂CuMo₄N₈O_14.50(%): C, 17.31; H, 1.25; N, 11.53. Found(%): C, 17.55; H, 1.35; N, 11.82. FT-IR (cm^-1): 3 122(m), 2 615(w), 1 593(m), 1 528(m), 1 470(m), 1 441(m), 1 343(m), 1 253(m), 938(w), 912(m), 813(m), 568(m), 476(w).

  {[Cu_1.5(L₂)(HL₂)(H₂O)(Mo₄O₁₃)]·2H₂O}_n (2). CuSO₄·5H₂O (100.00 mg, 0.4 mmol), HL₂ (94.5 mg, 0.5 mmol) and MoO₃ (14.40 mg, 0.1 mmol) were added into 10 mL deionized H₂O. The mixture were transferred and putted into a Teflon vessel, heated at 165 ℃ for 72 h in a steel autoclave and then cooled to ambient temperature during 24 h. The green strip-shaped single crystals of 2 were washed by H₂O, aether and air-dried. Elemental analysis Calcd. for C₁₈H₁₇Cu_1.50Mo₄N₆O_19.50(%): C, 19.50; H, 1.55; N, 7.58. Found(%): C, 19.73; H, 1.63; N, 7.73. FT-IR (cm^-1): 3 180(m), 2 629(w), 1 704(m), 1 548(s), 1 459(m), 1 401(s), 1 332(w), 1 270(m), 948(w), 902(m), 820(m), 765(m), 699(m), 552(m), 479(w).

  {[Cu₂(L₃)_1.5(Mo₄O₁₃)]·H₂O}_n (3). CuSO₄·5H₂O (125.00 mg, 0.5 mmol), L₃ (49.2 mg, 0.3 mmol) and MoO₃ (28.80 mg, 0.2 mmol) were added in 10 mL deionized H₂O. The mixture were transferred and placed into a Teflon vessel in a steel autoclave, heated at 165 ℃ for 72 h and then cooled to ambient temperature during 24 h. The red block-shaped single crystals of 3 were washed by H₂O, aether and air-dried. Elemental analysis Calcd. for C₆H₈Cu₂Mo₄N₁₂O₁₄(%): C, 7.33. H, 0.82. N, 17.10. Found(%): C, 7.55; H, 0.98; N, 17.36. FT-IR (cm^-1): 3 120(bm), 1 513(s), 1 296(s), 1 176(s), 1 100(m), 1 048(m), 1 000(w), 922(m), 804(m), 661(m), 615(m).

  1.3   Photo-catalytic measurements of coordination complexes 1~3

  The photo-catalytic measurements of coordination polymers 1~3 were evaluated by the degradation of MB, RhB and MO under irradiation utilizing a 200 W high pressure mercury lamp. The photo-catalytic experi-ments have been measured in a classical method, 5 mg complex 1 or 2 or 3 were suspended in the water solutions containing three organic dyes (10 μmol·L^-1, 10 mL), respectively. The mixture was stirred in the dark for 30 min for the desorption-adsorption equili-brium, then the mixture was transferred and placed under the lighting of Hg lamp (200 W) with continuous stirring. Next, every 20 minutes intervals, aliquots of the mixture samples were extracted and centrifuged. Finally, the sample was analyzed by UV-Vis analytical technique. The photo-catalytic degradation efficiency (D) can be calculated as below:

  D=(A₀-A_t)/A₀×100%

  where A_t is the absorbance of dye at time t, A₀ represents the initial absorbance of dyes solution.

  1.4   X-ray crystallography

  Structure measurements of complexes 1~3 were performed on a computer controlled a Bruker SMART Apex Ⅱ CCD diffractometer equipped with graphite-monochromated Mo Kα radiation with radiation wavel-ength of 0.071 073 nm by using the ω-scan technique. The structures were solved by direct methods and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs^[18-19]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically; the hydrogen atoms of the water molecules were generated geometrically and refined with isotropic temperature factors. Crystal data collection and refinement details for complexes 1~3 are summarized in Table 1. Selected bond lengths and angles for complexes 1~3 are listed in Table 2. Hydrogen bonds analysis was carried out using the PLATON program, all the hydrogen bonds distances and angles are listed in Table 3.

  Table 1

  

  Table 1.  Crystal data and structure refinement information for compounds 1~3

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  	1	2	3
  Empirical formula	C14H12CuMo4N8O14.50	C18H17Cu1.50Mo4N6O19.50	C6H8Cu2Mo4N12O14
  Formula weight	971.61	1 108.45	983.08
  Crystal system	Triclinic	Monoclinic	Monoclinic
  Space group	P1	C2/c	P21/c
  Temperature / K	296(2)	296(2)	296(2)
  a / nm	0.768 10(8)	2.598 7(4)	1.076 47(11)
  b / nm	0.819 54(9)	1.705 0(3)	1.078 40(11)
  c / nm	1.111 74(12)	1.569 0(2)	1.807 02(19)
  α / (°)	104.754(2)
  β / (°)	93.668(2)	115.779(2)	92.782(2)
  γ / (°)	100.439(2)
  V / nm3	0.661 07(12)	6.260 0(16)	2.095 2(4)
  Z	1	8	4
  F(000)	472	4 276	1 864
  Dc / (Mg·m-3)	2.468	2.352	3.117
  μ / mm-1	2.722	2.650	4.415
  Data, restraint, parameter	2 696, 0, 194	6 486, 42, 450	4 264, 12, 343
  GOF	1.024	1.077	1.098
  R1a [I≥2σ(I)]	0.025 1	0.044 5	0.044 4
  wR2a (all data)	0.088 0	0.130 3	0.087 0
  a R1=∑||Fo|-|Fc||/|Fo|, wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) for coordination polymers 1~3

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  1
  Cu(1)-O(1)	0.197 1(2)	Cu(1)-O(1)ⅰ	0.197 1(2)	Cu(1)-N(2)	0.197 2(3)
  Cu(1)-N(2)ⅰ	0.197 2(3)	Cu(1)-O(4)ⅰ	0.236 7(3)	Cu(1)-O(4)ⅱ	0.236 7(3)
  O(1)-Cu(1)-O(1)ⅰ	180.0	O(1)-Cu(1)-N(2)	89.12(11)	O(1)-Cu(1)-N(2)ⅰ	90.88(11)
  O(1)ⅰ-Cu(1)-N(2)ⅰ	90.88(11)	N(2)-Cu(1)-N(2)ⅰ	180.00(15)	O(1)-Cu(1)-O(4)ⅰ	95.94(10)
  O(1)-Cu(1)-O(4)ⅱ	84.06(10)	N(2)-Cu(1)-O(4)ⅰ	92.66(11)	N(2)-Cu(1)-O(4)ⅱ	87.34(11)
  O(4)i-Cu(1)-O(4)ⅱ	180.00(15)
  2
  Cu(1)-N(4)	0.198 8(5)	Cu(1)-O(15)ⅰ	0.199 3(4)	Cu(1)-N(1)	0.202 5(5)
  Cu(1)-O(1)	0.222 5(4)	Cu(2)-N(2)	0.201 6(5)	Cu(2)-N(2)ⅰ	0.201 6(5)
  Cu(2)-N(5)	0.205 6(5)	Cu(2)-N(5)ⅰ	0.205 7(5)	Cu(2)-O(18)	0.237 4(5)
  Cu(2)-O(18)ⅰ	0.237 4(5)	Cu(1)-O(12)ⅰ	0.193 8(4)
  O(12)ⅰ-Cu(1)-O(15)ⅰ	94.37(17)	N(4)-Cu(1)-O(15)ⅰ	86.90(19)	O(12)ⅰ-Cu(1)-N(1)	89.27(18)
  N(4)-Cu(1)-N(1)	89.5(2)	O(15)ⅰ-Cu(1)-N(1)	174.13(18)	O(12)ⅰ-Cu(1)-O(1)	87.15(16)
  N(4)-Cu(1)-O(1)	92.29(18)	O(15)ⅰ-Cu(1)-O(1)	98.36(18)	N(1)-Cu(1)-O(1)	86.41(17)
  N(2)-Cu(2)-N(2)ⅰ	180.0	N(2)-Cu(2)-N(5)	90.25(18)	N(2)ⅰ-Cu(2)-N(5)	89.75(18)
  N(2)-Cu(2)-O(18)	96.32(19)	N(5)-Cu(2)-O(18)ⅰ	94.58(19)	N(5)-Cu(2)-O(18)ⅰ	85.42(19)
  O(18)-Cu(2)-O(18)ⅰ	180.00(7)
  3
  Cu(1)-N(2)	0.193 1(7)	Cu(1)-N(7)ⅰ	0.200 1(7)	Cu(1)-O(4)	0.200 4(5)
  Cu(1)-O(4)	0.211 2(6)	Cu(2)-N(10)ⅱ	0.202 0(6)	Cu(2)-N(9)	0.207 7(7)
  Cu(2)-N(8)ⅰ	0.209 0(7)	Cu(2)-N(1)	0.220 8(7)
  N(2)-Cu(1)-N(7)ⅰ	116.2(3)	N(2)-Cu(1)-O(4)	128.7(3)	N(7)ⅰ-Cu(1)-O(4)	106.4(3)
  N(2)-Cu(1)-O(3)	104.1(3)	N(7)ⅰ-Cu(1)-O(3)	102.6(3)	O(4)-Cu(1)-O(3)	92.4(2)
  N(10)ⅱ-Cu(2)-N(9)	109.1(3)	N(10)ⅱ-Cu(2)-N(8)ⅰ	124.6(3)	N(9)-Cu(2)-N(8)ⅰ	105.7(3)
  N(10)ⅱ-Cu(2)-N(1)	114.4(3)	N(9)-Cu(2)-N(1)	94.2(3)	N(8)ⅰ-Cu(2)-N(1)	104.3(3)
  Symmetry codes: ⅰ x-1, y, z; ⅱ -x+1, -y+1, -z for 1; ⅰ -x+3/2, -y+3/2, -z+1 for 2; ⅰ x, y-1, z; ⅱ -x, -y+1, -z for 3.

  Table 3

  

  Table 3.  Hydrogen bonds for coordination polymers 1~3

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  D-H…A	d(D-H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  1
  C(1)-H(1)…O(5)	0.093	0.239	0.326 29	156
  C(2)-H(2)…O(7)	0.093	0.230	0.318 86	159
  C(4)-H(4)…O(7)	0.093	0.245	0.336 99	172
  2
  O(18)-H(18A)…O(6)	0.096	0.216	0.310 65	167
  O(18)-H(18B)…O(19)	0.096	0.188	0.280 18	160
  C(10)-H(10)…O(6)	0.093	0.227	0.318 66	170
  C(13)-H(13)…O(4)	0.093	0.235	0.320 71	154
  C(15)-H(15)…O(2)	0.093	0.226	0.310 97	152
  3
  O(14)-H(14A)…O(2)	0.085	0.205	0.286 23	160
  O(14)-H(14B)…O(4)	0.085	0.206	0.290 84	177
  C(1)-H(1)…O(6)	0.093	0.226	0.306 73	145
  C(2)-H(2)…O(9)	0.093	0.241	0.325 61	151
  C(6)-H(6)…O(14)	0.093	0.239	0.326 94	157

  CCDC: 1474258, 1; 1477985, 2; 1544278, 3.

  2.   Results and discussion

  2.1   Structural description of coordination polymers 1~3

  Single-crystal X-ray diffraction analysis demon-strates that coordination compound 1 crystallizes both in P1 space groups and triclinic crystal system. The asymmetric unit of 1 contains one Cu(Ⅱ) ions, two L₁ ligands, one Mo₄O₁₃^2- anion and two lattice water molecules. As illustrated in Fig. 1, in coordination polymer 1, L₁ ligands adopt pyrazolate-like [N-N]-bidentate bridging coordination mode. The Cu(Ⅱ) center (Cu1) is six-coordinated by two nitrogen atoms (N2 and N2^ⅰ) from triazolyl groups and four oxygen atoms (O1, O1^ⅰ, O4^ⅰ and O4^ⅱ) from Mo₄O₁₃^2-. The oxygen atom O(1) coming from Mo₄O₁₃^2- anions further links Cu1 atoms. Six-coordinated Cu(Ⅱ) atoms (Cu1) and six-coordinated Mo centers are linked by bi-dentate L₁ ligands forming binuclear Cu(Ⅱ)-Mo structural units. As depicted in Fig. 2(a), the Mo₄O₁₃^2- anions are inter-linked via vertical oxygen atoms forming 1D chain structures, which are further joined by Cu(Ⅱ) centers and are arranged into the 2D hybrid metal-organic framework along the crystallographic c axis. From side view of the 2D hybrid metal-organic framework (Fig. 2(b)), it can be found that the pyridine groups of the bidentate L₁ ligands are not coordinated because of steric hindrance effect, therefore the pyridine groups of L₁ are decorated outside of the 2D coordination framework. Numerous non-classical C-H…O hydrogen bonding interactions (C(1)…O(5) 0.326 29 nm, C(2)…O(7) 0.318 86 nm, C(4)…O(7) 0.336 99 nm) also can be found in the coordination framework of 1, which also further extend 1 into a 3D supramolecular network.

  Figure 1

  

  Figure 1.  Structural unit of 1

  Thermal ellipsoids drawn at 30% probability level; Symmetry codes: ^ⅰ x-1, y, z; ^ⅱ-x+1, -y+1, -z

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  Figure 2

  

  Figure 2.  (a) Two dimensional layer coordination framework of 1 with Mo₄O₁₃^2- anions linked by Cu(L₁)₂ structural units; (b) Side view of 2D layer coordination framework of 1

  H atoms are omitted for clarity

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  Coordination polymer 2 crystallizes in C2/c space groups and monoclinic crystal system. The asymmetric unit of 2 contains 1.5 Cu(Ⅱ) ions (Cu2 and half of Cu1), one HL₂ ligand, one deprotonated L₂^- ligand, one Mo₄O₁₃^2- anion, one bridging H₂O molecule (O18) and two lattice H₂O molecules (O19 and O20). As depicted in Fig. 3, HL₂ link the neighboring Cu centers (Cu1 and Cu2) through its two nitrogen atoms (N1 and N2) of triazole forming bi-dentate bridging mode. While the deprotonated L₂^- ligands adopt tridentate bridging coordination mode linking Cu1, Cu2 and Cu1i through two triazole nitrogen atoms (N4 and N5) and its one oxygen atom (O15^ⅰ). The Cu(Ⅱ) center (Cu1) is six-coordinated by two nitrogen atoms (N1 and N4) from triazolyl groups, three oxygen atoms (O1, O12, O15) from Mo₄O₁₃^2- and one bridging H₂O molecule (O18). While the Cu(Ⅱ) center (Cu2) is also six-coordinated by four nitrogen atoms (N2, N2^ⅰ, N5 and N5^ⅰ) from four triazolyl groups and two bridging water molecule (O18, O18^ⅰ).

  Figure 3

  

  Figure 3.  Structural unit of 2

  Thermal ellipsoids drawn at 30% probability level; Symmetry codes: ⅰ-x+3/2, -y+3/2, -z+Ⅰ

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  As depicted in Fig. 4, the Mo₄O₁₃^2- anions are inter-linked via the bridging oxygen atoms (O18), bi-dentate HL₂ and tri-dentate L₂^-, which are further joined by Cu(Ⅱ) centers and are arranged into the unique 3D micro-porous hybrid metal-organic framework along the crystallographic c axis. One dimensional micro-porous channels with 0.479 7(1) nm×0.641 6(1) nm also can be found, in which lattice H₂O molecules are located in the 1D channel. Numerous O-H…O and O-H…O non-classical hydrogen-bonding interactions (O(18)…O(6) 0.310 65 nm, O(18)…O(19) 0.280 18 nm, C(10)…O(6) 0.318 66 nm, C(13)…O(4) 0.320 71 nm, C(15)…O(2) 0.310 97 nm) also can be found in the coordination framework of 2, which also further stabilize the 3D coordination network of 2.

  Figure 4

  

  Figure 4.  Mo₄O₁₃^2- anions linked by bridging L₂^- forming 3D micro-porous framework of 2

  1D micro-porous channels with 0.479 7(1) nm×0.641 6(1) nm can be observed, in which lattice water molecules are located

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  Coordination polymer 3 crystallizes in P2₁/c space group and monoclinic crystal system. The asymmetric unit of 3 contains two Cu(Ⅰ) ions (Cu1 and Cu2), 1.5 L₃ ligands, one Mo₄O₁₃^2- and one lattice H₂O (O14). As depicted in Fig. 5, Cu(1) are four-coordinated by two triazole nitrogen atoms (N2 and N7^ⅰ) and two oxygen atoms (O3 and O4) forming the tetrahedral coordination mode while Cu2 are four-coordination by four triazole nitrogen atoms (N1, N8^ⅰ, N9 and N10) forming the tetrahedral coordination mode. The L₃ ligands are linked via four neighboring Cu(Ⅱ) ions adopting the tetra-dentate bridging mode.

  Figure 5

  

  Figure 5.  Structural unit of 3

  Thermal ellipsoids drawn at 30% probability level; Symmetry codes: ^ⅰ x, y-1, z; ^ⅱ-x, -y+1, -z

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  As depicted in Fig. 6(a), the L₃ ligands bridge neighboring Cu(Ⅰ) centers forming 1D double [Cu₂(L₃)_1.5] chains. Further the Mo₄O₁₃^2- anions are utilized as pillars to link the neighboring 1D double [Cu₂(L₃)_1.5] chains forming the 3D POM-based Cu(Ⅰ) hybrid micro-porous framework. It is noted that, as depicted in Fig. 6(b), two POM-based 3D micro-porous frame-works self-interpenetrate with each other, which ultimately form two-fold interpenetrating 3D coordination framework of 3 viewed along the a-axis direction. The C-H…O, N-H…O and O-H…N hydrogen bonding interactions (O(14)…O(2) 0.286 23 nm, O(14)…O(4) 0.290 84 nm, C(1)…O(6) 0.306 73 nm, C(2)…O(9) 0.325 61 nm, C(6)…O(14) 0.326 94 nm) can be found, which also further stabilize the 3D hybrid framework of 3 (Table 3)^[20].

  Figure 6

  

  Figure 6.  (a) Mo₄O₁₃^2- anions used as pillars to link the neighboring 1D double [Cu₂(L₃)_1.5] chains forming 3D micro-porous framework; (b) Two-fold interpenetrating 3D coordination framework of 3 viewed along the a-axis direction

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  2.2   Powder X-ray diffraction (PXRD) and FT-IR characterizations

  PXRD patterns of coordination compounds 1~3 were also determined at ambient temperature to verify the phase purity. As depicted in Fig. 7, the experimental patterns positions were well consistent with those of the theoretical patterns, demonstrating the as-synthesized samples 1~3 are pure phases. The slight differences in reflection intensities between experi-mental and theoretical PXRD patterns can be ascribed to the crystal orientation variation of the powder samples.

  Figure 7

  

  Figure 7.  Powder X-ray patterns for coordination framework (a) 1, (b) 2 and (c) 3 confirming purities of the bulky samples

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  In coordination polymers 1~3, the FT-IR bands at 912, 813 cm^-1 for 1, 902, 820 cm^-1 for 2 and 922, 804 cm^-1 for 3 can be assigned to ν(Mo-O) and ν(Mo-O-Μo). FT-IR spectra also demonstrated classical absorption bands for triazole moieties of L. The bands around ca. 3 100 cm^-1 and bands located at 1 100~ 1 300 cm^-1 should be correlated with ν(C-H) and ν(C-N) or ν(N-N) vibrations of triazole moieties. The triazole out of plane ring absorption is also found at around 600 cm^-1 (568 cm^-1 for 1, 552 cm^-1 for 2 and 615 cm^-1 for 3)^[21].

  2.2   Photo-catalytic capability for organic dyes by coordination polymers 1~3

  As it known to us, large numbers of commercial organic dyes are devoted to our personal life, however, they are also carcinogenic and toxic to us. Therefore how to rational utilize and purify the waste water can be the burning question to solve. Some reported about coordination polymers can show excellent photo-catalytic capability in the organic dyes degradation through the UV irradiation method^[22]. To determine the catalytic capability of coordination polymers 1~3 for dye degradation, three commercial available common organic dyes RhB, MB and MO have been utilized for the photo-catalytic experiments.

  The photo-catalytic activities of coordination polymers 1~3 in three different dye solutions are depicted in Fig. 8~10, respectively. The variation trend of the concentration ratios of dyes (C_t/C₀) against time (min) for coordination polymers 1~3 were plotted, where C₀ is the initial concentration of dyes after stirred in the dark environment for 20 minutes. As depicted in Fig. 8, coordination polymer 1 showed remarkable photo-catalytic capacity for the two organic dyes MO and RhB, and could be almost fully degraded (95.8% for RhB, and 97.5% for MO) in 70 min. As depicted in Fig. 9, coordination polymer 2 also showed photo-catalytic capacity for the three organic dyes, and could be almost totally degraded (91.1% for MB, 98.6% for RhB and 99.7% for MO) in nearly 70 min. As depicted in Fig. 10, coordination polymer 3 showed remarkable photo-catalytic capacity for two organic dyes MO and RhB, and could be nearly degraded completely (97.9% for RhB and 84.1% for MO) in 70 min. The experiment results demonstrate that all the coordination polymers 1~3 also could be excellent candidates in the photo-catalytic degradation of some organic dyes.

  Figure 8

  

  Figure 8.  Plot of concentration ratios (C_t / C₀) against irradiation time of MB, RhB and MO degraded by coordination polymer 1

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  Figure 9

  

  Figure 9.  Plot of concentration ratios (C_t / C₀) against irradiation time of MB, RhB and MO degraded by coordination polymer 2

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  Figure 10

  

  Figure 10.  Plot of concentration ratios (C_t / C₀) against irradiation time of MB, RhB and MO degraded by coordination polymer 3

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  With a view to the coordination environment, the types of the central metal ions and the coordinated mode of the ligand have significant influence on the photocatalytic activities. As described in the previous literature^[21], during the photocatalytic process of the POM-based hybrid coordination polymers, UV-Vis light can induce POM/organic ligands to produce oxygen and/or nitrogen-metal charge transfer by promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO strongly demands one electron to return to its stable state. Thus, one electron is captured from H₂O molecules, which are oxygenated into the ·OH active species, which should further decompose certain organic dyes and effectively complete the photo-catalytic experiment^[23-27].

  3.   Conclusions

  In summary, under hydrothermal conditions, three novel polyoxometalate (POM)-based Cu(Ⅱ) and Cu(Ⅰ) hybrid materials with multi-dentate mono- and bis-triazole derivatives, namely {[Cu(L₁)₂(Mo₄O₁₃)]·2H₂O}_n (1), {[Cu_1.5(L₂)(HL₂)(H₂O)(Mo₄O₁₃)]·2H₂O}_n (2), {[Cu₂(L₃)_1.5(Mo₄O₁₃)]·H₂O}_n (3) have been designed and synthesized. In 1, the Mo₄O₁₃^2- anions and Cu(Ⅱ)centers are inter-linked by bidentate L₁, which are arranged into the 2D hybrid metal-organic framework. In 2, the Mo₄O₁₃^2- anions and Cu(Ⅱ) centers are inter-linked via bridging aqua atoms (O18), bi-dentate HL₂ and tri-dentate L₂^-, which are arranged into the 3D micro-porous hybrid metal-organic framework. In 3, the L₃ ligands bridge neighboring Cu(Ⅰ) centers and Mo₄O₁₃^2- anions, which ultimately form the two-fold interpenet-rating 3D coordination framework of 3. Photocatalytic activities for decomposition of different organic dyes RhB, MB and MO have been investigated, and the results indicate that 1~3 are good candidates for photocatalytic degradation of the organic dyes. The experiment result also demonstrates that great potential in the construction of the novel POM-based hybrid metal organic frameworks employing different multi-dentate mono- and bis-triazole derivatives and versatile polyoxometalate (POM)-based building blocks. On the basis of this work, further preparation, structural analyses and functional properties investiga-tions of the POM-based hybrid coordination polymers utilizing the versatile building blocks are also under way in our laboratory.

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• 

  Scheme 1  Three multi-dentate 1, 2, 4-triazole derivative ligands L₁, HL₂ and L₃

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  Scheme 2  Hydrothermal assembly of a series of Cu(Ⅱ) and Cu(Ⅰ) hybrid polyoxometalate-based coordination frameworks

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  Figure 1  Structural unit of 1

  Thermal ellipsoids drawn at 30% probability level; Symmetry codes: ^ⅰ x-1, y, z; ^ⅱ-x+1, -y+1, -z

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  Figure 2  (a) Two dimensional layer coordination framework of 1 with Mo₄O₁₃^2- anions linked by Cu(L₁)₂ structural units; (b) Side view of 2D layer coordination framework of 1

  H atoms are omitted for clarity

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  Figure 3  Structural unit of 2

  Thermal ellipsoids drawn at 30% probability level; Symmetry codes: ⅰ-x+3/2, -y+3/2, -z+Ⅰ

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  Figure 4  Mo₄O₁₃^2- anions linked by bridging L₂^- forming 3D micro-porous framework of 2

  1D micro-porous channels with 0.479 7(1) nm×0.641 6(1) nm can be observed, in which lattice water molecules are located

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  Figure 5  Structural unit of 3

  Thermal ellipsoids drawn at 30% probability level; Symmetry codes: ^ⅰ x, y-1, z; ^ⅱ-x, -y+1, -z

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  Figure 6  (a) Mo₄O₁₃^2- anions used as pillars to link the neighboring 1D double [Cu₂(L₃)_1.5] chains forming 3D micro-porous framework; (b) Two-fold interpenetrating 3D coordination framework of 3 viewed along the a-axis direction

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  Figure 7  Powder X-ray patterns for coordination framework (a) 1, (b) 2 and (c) 3 confirming purities of the bulky samples

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  Figure 8  Plot of concentration ratios (C_t / C₀) against irradiation time of MB, RhB and MO degraded by coordination polymer 1

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  Figure 9  Plot of concentration ratios (C_t / C₀) against irradiation time of MB, RhB and MO degraded by coordination polymer 2

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  Figure 10  Plot of concentration ratios (C_t / C₀) against irradiation time of MB, RhB and MO degraded by coordination polymer 3

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  Table 1.  Crystal data and structure refinement information for compounds 1~3

  	1	2	3
  Empirical formula	C14H12CuMo4N8O14.50	C18H17Cu1.50Mo4N6O19.50	C6H8Cu2Mo4N12O14
  Formula weight	971.61	1 108.45	983.08
  Crystal system	Triclinic	Monoclinic	Monoclinic
  Space group	P1	C2/c	P21/c
  Temperature / K	296(2)	296(2)	296(2)
  a / nm	0.768 10(8)	2.598 7(4)	1.076 47(11)
  b / nm	0.819 54(9)	1.705 0(3)	1.078 40(11)
  c / nm	1.111 74(12)	1.569 0(2)	1.807 02(19)
  α / (°)	104.754(2)
  β / (°)	93.668(2)	115.779(2)	92.782(2)
  γ / (°)	100.439(2)
  V / nm3	0.661 07(12)	6.260 0(16)	2.095 2(4)
  Z	1	8	4
  F(000)	472	4 276	1 864
  Dc / (Mg·m-3)	2.468	2.352	3.117
  μ / mm-1	2.722	2.650	4.415
  Data, restraint, parameter	2 696, 0, 194	6 486, 42, 450	4 264, 12, 343
  GOF	1.024	1.077	1.098
  R1a [I≥2σ(I)]	0.025 1	0.044 5	0.044 4
  wR2a (all data)	0.088 0	0.130 3	0.087 0
  a R1=∑||Fo|-|Fc||/|Fo|, wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

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  Table 2.  Selected bond lengths (nm) and angles (°) for coordination polymers 1~3

  1
  Cu(1)-O(1)	0.197 1(2)	Cu(1)-O(1)ⅰ	0.197 1(2)	Cu(1)-N(2)	0.197 2(3)
  Cu(1)-N(2)ⅰ	0.197 2(3)	Cu(1)-O(4)ⅰ	0.236 7(3)	Cu(1)-O(4)ⅱ	0.236 7(3)
  O(1)-Cu(1)-O(1)ⅰ	180.0	O(1)-Cu(1)-N(2)	89.12(11)	O(1)-Cu(1)-N(2)ⅰ	90.88(11)
  O(1)ⅰ-Cu(1)-N(2)ⅰ	90.88(11)	N(2)-Cu(1)-N(2)ⅰ	180.00(15)	O(1)-Cu(1)-O(4)ⅰ	95.94(10)
  O(1)-Cu(1)-O(4)ⅱ	84.06(10)	N(2)-Cu(1)-O(4)ⅰ	92.66(11)	N(2)-Cu(1)-O(4)ⅱ	87.34(11)
  O(4)i-Cu(1)-O(4)ⅱ	180.00(15)
  2
  Cu(1)-N(4)	0.198 8(5)	Cu(1)-O(15)ⅰ	0.199 3(4)	Cu(1)-N(1)	0.202 5(5)
  Cu(1)-O(1)	0.222 5(4)	Cu(2)-N(2)	0.201 6(5)	Cu(2)-N(2)ⅰ	0.201 6(5)
  Cu(2)-N(5)	0.205 6(5)	Cu(2)-N(5)ⅰ	0.205 7(5)	Cu(2)-O(18)	0.237 4(5)
  Cu(2)-O(18)ⅰ	0.237 4(5)	Cu(1)-O(12)ⅰ	0.193 8(4)
  O(12)ⅰ-Cu(1)-O(15)ⅰ	94.37(17)	N(4)-Cu(1)-O(15)ⅰ	86.90(19)	O(12)ⅰ-Cu(1)-N(1)	89.27(18)
  N(4)-Cu(1)-N(1)	89.5(2)	O(15)ⅰ-Cu(1)-N(1)	174.13(18)	O(12)ⅰ-Cu(1)-O(1)	87.15(16)
  N(4)-Cu(1)-O(1)	92.29(18)	O(15)ⅰ-Cu(1)-O(1)	98.36(18)	N(1)-Cu(1)-O(1)	86.41(17)
  N(2)-Cu(2)-N(2)ⅰ	180.0	N(2)-Cu(2)-N(5)	90.25(18)	N(2)ⅰ-Cu(2)-N(5)	89.75(18)
  N(2)-Cu(2)-O(18)	96.32(19)	N(5)-Cu(2)-O(18)ⅰ	94.58(19)	N(5)-Cu(2)-O(18)ⅰ	85.42(19)
  O(18)-Cu(2)-O(18)ⅰ	180.00(7)
  3
  Cu(1)-N(2)	0.193 1(7)	Cu(1)-N(7)ⅰ	0.200 1(7)	Cu(1)-O(4)	0.200 4(5)
  Cu(1)-O(4)	0.211 2(6)	Cu(2)-N(10)ⅱ	0.202 0(6)	Cu(2)-N(9)	0.207 7(7)
  Cu(2)-N(8)ⅰ	0.209 0(7)	Cu(2)-N(1)	0.220 8(7)
  N(2)-Cu(1)-N(7)ⅰ	116.2(3)	N(2)-Cu(1)-O(4)	128.7(3)	N(7)ⅰ-Cu(1)-O(4)	106.4(3)
  N(2)-Cu(1)-O(3)	104.1(3)	N(7)ⅰ-Cu(1)-O(3)	102.6(3)	O(4)-Cu(1)-O(3)	92.4(2)
  N(10)ⅱ-Cu(2)-N(9)	109.1(3)	N(10)ⅱ-Cu(2)-N(8)ⅰ	124.6(3)	N(9)-Cu(2)-N(8)ⅰ	105.7(3)
  N(10)ⅱ-Cu(2)-N(1)	114.4(3)	N(9)-Cu(2)-N(1)	94.2(3)	N(8)ⅰ-Cu(2)-N(1)	104.3(3)
  Symmetry codes: ⅰ x-1, y, z; ⅱ -x+1, -y+1, -z for 1; ⅰ -x+3/2, -y+3/2, -z+1 for 2; ⅰ x, y-1, z; ⅱ -x, -y+1, -z for 3.

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  Table 3.  Hydrogen bonds for coordination polymers 1~3

  D-H…A	d(D-H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  1
  C(1)-H(1)…O(5)	0.093	0.239	0.326 29	156
  C(2)-H(2)…O(7)	0.093	0.230	0.318 86	159
  C(4)-H(4)…O(7)	0.093	0.245	0.336 99	172
  2
  O(18)-H(18A)…O(6)	0.096	0.216	0.310 65	167
  O(18)-H(18B)…O(19)	0.096	0.188	0.280 18	160
  C(10)-H(10)…O(6)	0.093	0.227	0.318 66	170
  C(13)-H(13)…O(4)	0.093	0.235	0.320 71	154
  C(15)-H(15)…O(2)	0.093	0.226	0.310 97	152
  3
  O(14)-H(14A)…O(2)	0.085	0.205	0.286 23	160
  O(14)-H(14B)…O(4)	0.085	0.206	0.290 84	177
  C(1)-H(1)…O(6)	0.093	0.226	0.306 73	145
  C(2)-H(2)…O(9)	0.093	0.241	0.325 61	151
  C(6)-H(6)…O(14)	0.093	0.239	0.326 94	157

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•