English

• 1. INTRODUCTION

  Polyoxometalates (POMs) represent a class of metal-oxygen clusters of early transition metals^[1] and have stimulated interest in broad fields for their obviously physical and chemical properties^{[2, 3]}. In this field, one of the promising branches is to design and synthesize new POM-based coordination polymers (CPs), which not only maintain the advantages of POM and CPs but also generate new structures and properties that the precursors don't possess, leading to the formation of a variety of novel functional compounds^[4]. So far, numerous POM-based compounds have been obtained^{[5, 6]}, demonstrating strong potential not only for their charming topological structures but also for their potential applications in fluorescence, catalysis, magnetic devices and functional materials^{[6, 7]}.

  It is well known that numerous factors, such as initial reactants and their stoichiometry ratios, the properties of metal ions and organic ligands, temperature and pH value, can influence the ultimate architectures of POM-based hybrid materials. Of all these factors, the reasonable choice of organic ligands and POM units is an especially significant factor in the construction of novel POM-based frameworks.

  POMs are regarded as good candidates to construct abundant POM-based frameworks owing to irreplaceable advantages as follows. On one hand, compared with single metal ions, POM-based clusters with oxygen-rich surfaces make them possess high reactivity and conduce to coordinate with the transition metal ions^[8]. On the other hand, POMs are particularly attractive for their obviously physical and chemical properties. (ⅰ) Owing to remarkable catalytic property, they possess the ability to undergo fast, reversible, and stepwise multiple electron transfer reactions without changing their structures, which makes them ideal choice as catalysis^[9]. (ⅱ) Due to their tuned acidic and redox properties, in view of these advantages, POMs have been extensively utilized for building up POM-based framework materials with potential applications^[6-8].

  Organic ligands play an important role in the construction of novel POM-based framework materials. Among many organic ligands, nitrogen-containing heterocyclic ligands arouse our great interest owing to abundant coordination modes and predominant coordination ability^[10]. The past research has confirmed that pyridine, imidazole and their derivatives have been widely used in the syntheses of complexes, such as 1,4-bis(1-imidazoly)benzene, 1,4-bis(benzimidazol-1-yl)benzene and 1,3-bis(1-imidazoly)benzene^[11-13]. However, the ligand containing pyridine and double-diazole 2,6-bis(1-imidazoly)pyridine (L) has been paid less attention. In fact, 2,6-bis(1-imidazoly)pyridine as a rigid bridging ligand usually exhibits a bidentate coordination mode, but also pyridine and imidazole rings are capable of generating π···π stacking interactions, which may help to construct POM-based framework materials with novel topology types and potential applications.

  Based on the above, 2,6-bis(1-imidazoly) pyridine and NaWO₄ were adopted, self-assembled by Ni(NO₃)₂·6H₂O to obtain one new POM-based coordination polymer [Ni(L)(HL)(H₂O)(H₂W₁₂O₄₀)_0.5·H₂O]_n, of which one one-dimensional chain structure was constructed by coordination between [H₂W₁₂O₄₀]^6– polyoxoanions and Ni₂L₂ units, and was further extended into a 3D framework via different hydrogen bonding interactions. In addition, the electrochemistry, magnetism, photocatalysis and luminescence of the compound have also been investigated.

  2. EXPERIMENTAL

  2.1 Materials and general methods

  All the materials were commercially purchased and used as received without any further purification. The FT-IR spectrum was obtained on a Bruker VERTEX 70 using KBr pellets in the range of 450~4000 cm⁻¹. Elemental analysis was carried out on a Vario EL Ⅲ Etro Elemental Analyzer. X-ray powder diffraction (PXRD) data were collected on a X-PertPro diffractometer with CuKα radiation in the 2θ range of 5~50° at room temperature. Thermal gravimetric analysis (TGA) was measured by on a TGA/DSC³⁺ instrument in N₂ flow with the heating rate of 10 ℃·min⁻¹ from 25 to 1000 ℃. Magnetic measurements were accomplished on a Quantum Design MPMS-XL SQUID magnetometer. The data of electrochemistry were collected by RST5200F electrochemical workstation. UV-vis absorption spectrum was obtained with UV-6000PC spectrometer at room temperature. The fluorescent data were collected using an Edinburgh FLS980 Spectrometer.

  2.2 Synthesis of [Ni(L)(HL)(H₂O)(H₂W₁₂O₄₀)_0.5·H₂O]_n

  A mixture of Ni(NO₃)₂·6H₂O (0.087 g, 0.3 mmol), L (0.042 g, 0.2 mmol), NaWO₄ (0.198 g, 0.6 mmol) and H₂O (6 mL) was fully stirred in a 25 mL Teflon-lined stainless-steel vessel with the pH value adjusted to about 2.5 with 2.0 mol·L⁻¹ HCl. After heating at 160 ℃ for 4 days, blue block crystals of compound 1 were obtained. (Yield: 0.096 g, 27.6%, based on NaWO₄). Anal. Calcd.: NiW₆C₂₂H₂₄N₁₀O₂₂ (%): C, 13.51; H, 1.19; N, 7.22. Found (%): C, 13.59; H, 1.24; N, 7.21.

  2.3 X-ray crystallographic analysis

  A blue crystal of the title compound with approximate dimensions of 0.24mm × 0.22mm × 0.16mm was selected and mounted on a glass fiber. Crystallographic data were collected on Bruker Smart CCD X-ray single-crystal diffractometer with MoKα (λ = 0.71073 Å) at 296 K. A total of 36964 reflections were collected in the range of 2.52≤θ≤25.00º (–23≤h≤16, –27≤k≤28, –18≤l≤17), of which 6678 were independent (R_int = 0.0449) and 5903 were observed with I > 2σ(I). Data absorption correction and reduction were made with empirical methods. These structures were solved by directed methods using SHELXS-2014^[14] and refined by full-matrix least-squares techniques using SHELXL-2014^[15]. All hydrogen atoms were added in the riding model while the aqueous hydrogen atoms were located from difference Fourier maps. Anisotropic displacement parameters were refined for all non-hydrogen atoms. The final R = 0.0299, wR = 0.0551 (w = 1/[σ²(F_o²) + 103.6560P], where P = (F_o² + 2F_c²)/3), (Δρ)_max = 2.470 and (Δρ)_min = –1.266 e·Å^-3. The selected bond lengths and bond angles of the title compound are summarized in Table S1 (Supporting information), respectively.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of the title compound

  Single-crystal X-ray analysis reveals that the title compound crystallizes in orthorhombic system Pnma space group. The asymmetrical unit contains one Ni(Ⅱ) atom, one L ligand, one HL ligand, one coordinated water molecule, one free water molecule and one half [H₂W₁₂O₄₀]^6– polyoxoanion. As shown in Fig. 1, each Ni(Ⅱ) atom is six-coordinated by three oxygen and three nitrogen atoms in octahedral configuration, of which three oxygen atoms are derived from two individual [H₂W₁₂O₄₀]^6– polyoxoanions and one coordinated water molecule, and three nitrogen atoms from two L ligands and one HL ligand. The Ni–O and Ni–N distances fall in the ranges of 2.069(6)~2.113(6) and 2.052(7)~2.101(7) Å, respectively. Also, O–Ni–O, N–Ni–N and N–Ni–O angles are in the ranges of 84.9(2)~177.2(2)°, 95.6(3)~164.6(3)° and 87.2(3)~172.0(3)°, respectively. The ligands L adopt a bidentate coordination mode, while HL is in a monodentate mode with imidazole-N atoms protonated. Therefore, two Ni(Ⅱ) atoms are bridged by two ligands L to form one Ni₂L₂ unit with two HL symmetrically hanging in two sides, which is further connected to 1D chain by [H₂W₁₂O₄₀]^6– along the a axis.

  Figure 1

  

  Figure 1.  Coordination diagram of Ni(Ⅱ) atoms in the title compound with hydrogen atoms omitted for clarity. Symmetry codes: A: x, 1.5–y, z; B: –0.5+x, y, 0.5–z

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  In addition, it is obvious that there are three kinds of hydrogen bonding interactions within the one-dimensional chain (C(5)–H(5A)···O(16), O(2W)–H(2C)···O(1W) and O(2W)–H(2B)···O(14), Table S2): the C–H···O hydrogen bonding interaction exists between the oxygen atom of [H₂W₁₂O₄₀]^6– polyoxoanion and the carbon atom of pyridine rings, and two kinds of O–H···O hydrogen bonding interactions are from the oxygen atom of coordinated water molecule and [H₂W₁₂O₄₀]^6– polyoxoanion and free water molecule, respectively (Fig. 2), which make the whole chain structure more stable. At the same time, the neighboring 1D chains are reciprocally arranged into the 3D framework through different hydrogen bonding interactions from the oxygen atom of [H₂W₁₂O₄₀]^6– polyoxoanion and the carbon atom of ligand HL (C–H···O, Table S2, Fig. 3).

  Figure 2

  

  Figure 2.  1D chain structure of compound. Most hydrogen atoms are omitted for clarity

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  Figure 3

  

  Figure 3.  3D network constructed by H-bonding interactions. Most hydrogen atoms and free water molecules are omitted for clarity

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  3.2 IR spectra, PXRD and TGA

  As shown in Fig. S1, the characteristic peaks at 649, 784, 879, 942 and 1067 cm⁻¹ can be attributed to v_as(W–O_c–W), v(W=O_t), v(W–O_b–W) and v(W–O_d)^[16]. The apparent peaks in the range of 1430~1660 cm⁻¹ can be assigned to the C=C and C=N stretching vibrations from the ligands^[6]. The above also confirm the existence of POMs and ligands in the title compound.

  The experimental powder X-ray diffraction patterns were carried out to check the phase purity of compound. As shown in Fig. S2, the main peaks of powder diffraction measured by the experiment are in good agreement with the theoretical peaks of single crystal data, demonstrating the good phase purity. The difference in strengths of the peaks may be due to the different orientations of the powder sample during the test^[17]. The thermogravimetric analysis (TGA) shows that the compound has high stability and exhibits similar two-step weight loss (Fig. S3). One consecutive weight loss of 1.65% occurs in the range of 150~340 ℃, which is corresponding to the loss of free water molecules and coordinated water molecules (calcd. 1.85%). The weight loss continues with the further increase of temperature for the collapse of skeleton at about 439 ℃.

  3.3 Cyclic voltammogram

  Fig. 4 shows the cyclic voltammogram of compound bulk-modified glassy carbon electrodes (GCE) in 0.5 mol·L⁻¹ Na₂SO₄ + 0.01 mol·L⁻¹ H₂SO₄ aqueous solution at different scan rates. It can be seen four obvious redox couples (Ⅰ–Ⅰ', Ⅱ–Ⅱ', Ⅲ–Ⅲ' and Ⅳ–Ⅳ') in the potential range from –700 to 600 mV. The mean peak potentials E1/2 = (Epa + Epc)/2 are –641.4, –486.7, –294.5 and –97.1 mV, which is attributed to two consecutive two-electron processes and two consecutive one-electron processes of W atoms^[18]. With the increase of scan rates from 80 to 200 mV, the anodic peak potentials shift toward the positive direction, while the corresponding cathodic peak potentials move to the negative direction. The peak currents are proportional to the square of scan rates, showing that the redox process of GCE is surface controlled^[19].

  Figure 4

  

  Figure 4.  Cyclic voltammetric behaviors at different scan rates in 0.5 mol·L⁻¹ Na₂SO₄ + 0.01 mol·L⁻¹ H₂SO₄ solution for GCE

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  3.4 Photocatalytic property

  The photocatalytic properties of POMs have potential prospects in the purification of water^[20] because of the high degradation of organic dyes under UV irradiation. Therefore, the photocatalytic activity of the title compound was studied in the Rhodamine (RhB) under UV light irradiation. Steps are as follows: 25 mg of compound was decentralized in a 40 mg/L RhB aqueous solution (50 mL) and magnetically stirred for about 2 h to ensure the equilibrium in the dark. Then, the mixed solution was exposed to a UV Hg lamp with continuous stirring. At every interval 30 min, 1.0 mL solution was taken out and diluted to 3 mL for analysis by using the UV-VIS spectrophotometer. It can be clearly observed that the absorbance peak of RhB is decreased obviously with increasing the reaction time (Fig. 5a), and the calculation reveals that the conversion of RhB is 43.8% for compound after 4.0 h (Fig. 5b). The results indicate that the compound has certain photocatalytic activity for the degradation of RhB.

  Figure 5

  

  Figure 5.  (a) Absorption spectra of RhB (10 mg·L^–1) solution during the decomposition reaction under UV light irradiation with the use of compound. (b) Comparison of the title compound and no crystal decomposition rate of RhB in the same conditions

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  3.5 Luminescent property

  The solid-state photoluminescence of the L ligand and compound was carried out at room temperature. As shown in Fig. 6a, upon excitation at 253 nm, the compound exhibits photoluminescence with maximum emission peaks at about 372 and 389 nm. The free L ligand displays two intense emissions at 445 and 462 nm upon excitation at 232 nm. Compared with the emission spectrum of L ligand, there is an obvious blue shift of about 73 nm for the title compound. It can be inferred that those peaks originate from the π*-π transition of L ligand^[21]. In order to further investigate the photoluminescent property, luminescent decay curve was carried out by checking the emission at 389 nm for the title compound, which can be well fitted into a double-exponential decay function as I = A₁exp(–t/τ) + A₂exp(–t/τ), affording the lifetime: τ₁ = 0.833 µs (73.52%) and τ₂ = 0.489 µs (26.48%), the pre-exponential factor: A₁ = 1106.54, A₂ = 67.84 (Fig. 6b). Therefore, the average lifetime is 1.908 µs for the title compound based on the formula τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂)^[22].

  Figure 6

  

  Figure 6.  (a) Solid-state photoluminescence spectra of compound at room temperature. (b) Luminescence decay curve was taken by monitoring the emission at 389 nm for the compound

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  3.6 Magnetic property

  The temperature-dependent magnetic susceptibility data of the compound were investigated at 1000 Oe from 2 to 300 K. As shown in Fig. 7a, the χ_mT value is 1.402 cm³·mol⁻¹·K at 300 K, which is higher than the expected (1.199 cm³·mol⁻¹·K) for Ni(Ⅱ) ion (S = 1.0, g = 2.19)^[23], and the χ_mT value reduces to 0.556 cm³·mol⁻¹·K with the temperature decrease, demonstrating the existence of antiferromagnetic interaction. What's more, the plot of χ_m⁻¹ for compound well obeys the Curie-Weiss law χ = C/(T − θ) from 300–2 K (Fig. 7b), with a Curie constant C of 1.4081 and the Weiss constant θ to be −3.535, further demonstrating the compound displays antiferromagnetic behaviors.

  Figure 7

  

  Figure 7.  (a) χ_mT vs. T and χ_m vs. T plots of compound; (b) χ_m⁻¹ vs. T plots of compound

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  4. CONCLUSION

  In summary, one new POM-based coordination polymer has been successfully synthesized based on 2,6-bis(1-imidazoly) pyridine through hydrothermal methods, of which one one-dimensional chain structure was constructed by coordination between Ni₂L₂ unit and [H₂W₁₂O₄₀]^6– polyoxoanions, and was further extended into a 3D framework via different hydrogen bonding interactions. The title compound exhibits antiferromagnetic behaviors, luminescent behaviors at solid state and photocatalytic activity to RhB. Further researches for the POM-based coordination polymers are ongoing in our work.

  
  
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  Figure 1  Coordination diagram of Ni(Ⅱ) atoms in the title compound with hydrogen atoms omitted for clarity. Symmetry codes: A: x, 1.5–y, z; B: –0.5+x, y, 0.5–z

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  Figure 2  1D chain structure of compound. Most hydrogen atoms are omitted for clarity

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  Figure 3  3D network constructed by H-bonding interactions. Most hydrogen atoms and free water molecules are omitted for clarity

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  Figure 4  Cyclic voltammetric behaviors at different scan rates in 0.5 mol·L⁻¹ Na₂SO₄ + 0.01 mol·L⁻¹ H₂SO₄ solution for GCE

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  Figure 5  (a) Absorption spectra of RhB (10 mg·L^–1) solution during the decomposition reaction under UV light irradiation with the use of compound. (b) Comparison of the title compound and no crystal decomposition rate of RhB in the same conditions

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  Figure 6  (a) Solid-state photoluminescence spectra of compound at room temperature. (b) Luminescence decay curve was taken by monitoring the emission at 389 nm for the compound

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  Figure 7  (a) χ_mT vs. T and χ_m vs. T plots of compound; (b) χ_m⁻¹ vs. T plots of compound

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