POM-based metal–organic compounds: Assembly,structures and properties

2.1.1 {[(Cu·L1’·H₂O)·(α-Mo₈O₂₆)_0.5]·H₂O}_n (1)

A mixed solution of L1 (0.0094 g, 0.0025 mmol), (NH₄)₆Mo₇O₂₄·4H₂O (0.0618 g, 0.05 mmol), CuCl₂ (0.017 g, 0.1 mmol) and H₂O (15 mL) were placed in 25 mL teflon reactor at room temperature and the pH value was adjusted to 3 by adding 4–6 drops of HCl (2 mol/L). The mixed solution was heated at 150°C for 72 h. After slow cooling to room temperature, the crystals of 1 were formed. Rectangular flaky crystals were filtered and washed with distilled water (yield 55%based on Mo). IR (KBr, cm^–1): 3476.13(m), 3117.65(m), 1656.25(m), 1572.56(m), 1453.26(w), 1405.26(s), 1284.12(m), 1143.58(s), 1051.25(s), 916.74(s), 801.74(s). Elemental Anal. Calc. for C₁₇H₂₂N₂O₁₉CuMo₄: C, 20.30; H, 2.20; N, 2.78; O, 30.23. Found: C, 20.41; H, 2.12; N, 2.85; O, 30.35.

2.1.2 {(Cu·L2’·H₂O)·(α-Mo₈O₂₆)_0.5}_n (2)

The synthesis of compound 2 was similar to that of compound 1, except that L1 was replaced by L2. Rectangular flaky crystals were filtered and washed with distilled water (yield 70%based on Mo). IR (KBr, cm^–1): 3389.63(m), 2929.45(m), 1659.68(m), 1574.33(s), 1411.00(w), 1406.00(s), 1145.97 (w), 923.78(s), 797.99(s). Elemental Anal. Calc. for C₁₉H₂₄N₂O₁₈CuMo₄: C, 22.46; H, 2.38; N, 2.75; O, 28.35. Found: C, 22.56; H, 2.24; N, 2.79; O, 28.46.

2.1.3 {[(Cu·L3’·H₂O)·(β-Mo₈O₂₆)_0.5]·5H₂O}_n (3)

The synthesis of compound 3 was similar to that of compound 2, except that L2 was replaced by L3. Light blue needle crystals were filtered and washed with distilled water (yield 72%based on Mo). IR (KBr, cm^–1): 3426.09(s), 2973.70(w), 2919.20(w), 1631.13(w), 1382.92(w), 1049.97(w), 1145.97 (w), 947.73(m). Elemental Anal. Calc. for C₂₀H₃₀N₂O₂₄CuMo₄: C, 21.26; H, 2.67; N, 2.47; O, 33.98. Found: C, 22.15; H, 2.71; N, 2.49; O, 34.12.

2.1.4 {[(Cu·L4’·H₂O)]·(β-Mo₈O₂₆)_0.5}_n (4)

The synthesis of compound 4 was similar to that of compound 3, except that L3 was replaced by L4. Petal-shaped pale blue crystals were filtered and washed with distilled water (yield 76%based on Mo). IR (KBr, cm^–1): 3417.21(s), 1636.66(m), 1568.62(s), 1458.54(w), 1371.10(s), 1135.97(w), 945.41(s), 715.07(s). Elemental Anal. Calc. for C₂₀H₂₀N₂O₁₉CuMo₄: C, 14.72; H, 1.23; N, 1.71; O, 31.38. Found: C, 14.75; H, 1.27; N, 1.69; O, 31.46.

3.1.1 Crystal structures of compounds 1-2

Crystal structures of compounds 1-2 have linear cations as templates and have [α-Mo₈O₂₆]^4– anion clusters (Figs. 1-2). Take compound 2 as an example for detailed discussion. Single-crystal X-ray analysis reveals that compound 2 (Fig. 2) crystallizes in the triclinic with P-1 space group. The asymmetric unit of the compound 2 contains one Cu (II) cation, two half ligand L2’, one half [α-Mo₈O₂₆]^4– polyanion. Noticeably, the L2’ ligand was hydrolyzed and adopted a bidentate coordination mode, coordinating to two Cu5 ions form paddle-wheel-like structure, thus resulting in a 2D layer [25–27]. The Cu-O bond lengths are in the range of 1.966–1.984 Å, which are similar to those of other Cu (II) MOFs [28, 29]. The L2’ cation connects five coordination modes of Cu (II) in-situ generated a 2D structure with the dimension of 35.007×17.804 Ų. The metal organic loop of compound 2 is formed by the ligands L2’ connects adjacent Cu (II) cation with size of 17.439×21.613 Ų. Therefore, the ligand and Cu (II) cation of compounds 1-2 formed the metal organic framework which forms a rotaxane-like structure with [α-Mo₈O₂₆]^4–, if the size is larger. Powder X-ray diffraction (XRD) has been used to confirm the purity of the samples in the solid state. For compound 2 the as-synthesized PXRD patterns closely match the simulated patterns generated from the results of the single crystal diffraction data, indicative of pure products which were shown in Fig. 3.

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Fig. 1

(a) Structural unit of compound 1, (b) 2D layer structure of compound 1, (c) 3D supramolecular framework in compound 1.

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Fig. 2

(a) Structural unit of compound 2, (b) 2D layer structure of compound 2, (c) 3D supramolecular framework in compound 2.

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Fig. 3

Power X-ray Diffraction of compound 2.

The surface organic ligands react as template with a metal salt to form compounds 3-4. Single-crystal X-ray structural analysis compound 3 reveals that there are two kinds of Cu (II) cations, two kinds of L3’ ligands, and one kind of [β-Mo₈O₂₆]^4– anion in the asymmetrical unit. Then the O-H···O hydrogen bonds [O(12)-H(20A)···O(20) = 2.759 Å and O(4)-H(19B)···O(19) = 2.746 Å] between the [β-Mo₈O₂₆]^4– and the L3’ ligand (Fig. 4a). In compound 3, L3’ ligands link adjacent metal Cu(1) and Cu(2) ions to form a 1D metal-organic chain. Cu1 and Cu2 are hexagonal octahedral geometries in which Cu1 is surrounded by two oxygen atoms from two L3’ ligands and four oxygen atoms from two-molecular coordination water, and Cu2 is surrounded by four oxygen atoms from two L4’ ligands and two oxygen atoms from two-molecular coordination water (Fig. 4b). Furthermore, adjacent 1D metal-organic chains extended into 2D supramolecular framework by hydrogen C-H···O bonds and van der Waals forces (Fig. 4c).

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Fig. 4

(a) structural unit of compound 3; (b) 1D metal-organic chain in compound 3; (c) the 2D supramolecular layer of compound 3.

Compound 4 crystallizes in the triclinic space group P-1. Its basic structural units contain the one half [β-Mo₈O₂₆]^4–, one half L3’ ligand, one Cu (II) ion, and one H₂O molecule, as shown in Fig. 5(a). Cu1, six-coordinated mode, is coordinated by two oxygen atoms from two water molecules, two oxygen atoms from two L4’ ligands, and other two oxygen atoms from two [β-Mo₈O₂₆]^4– anion. Interestingly, the Cu-O_t bond lengths are obviously different, ranging from 1.944(6) to 2.583(2) Å and shows a distorted octahedral coordination geometry. As shown in Fig. 5(b), compound 4 is one independent copper center in which Cu1 is six-coordinated in elongated octahedral coordination geometry, which is respectively completed by two L4’ oxygen atoms, two oxygen atoms of [β-Mo₈O₂₆]^4– and two water oxygen atoms. It is notable that two [β-Mo₈O₂₆]^4– polyoxoanions, four Cu atoms and two L4’ form a 2D frame with the dimensions of 18.965×26.135 Ų. Compound 4 has hydrogen bonds between the 2D molecular layers, as shown in Fig. 5(c).

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Fig. 5

(a) asymmetric structural unit of compound 4; (b) 2D layer of compound 4; (c) the hydrogen bonds of compound 4.

To investigate the effects of organic ligands on the structures of polyoxoanions of compounds, four novel POM-based compounds were successfully synthesized under hydrothermal conditions by adjusting different organic ligands (Table 3). We found that the polyoxoanions in compounds 1-2 formed by linear cationic ligands were [α-Mo₈O₂₆]^4– anions, and the polyoxoanions formed by planar cationic ligands in compounds 3-4 were [β-Mo₈O₂₆]^4– anions. Therefore, it was found that the coordination mode of the polyoxoanions can be changed by increasing the conjugate surface in the ligand.