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DOI: 10.1039/C5RA09529H (Paper) RSC Adv., 2015, 5, 56687-56696

Application of flexible bis-pyrazine–bis-amide ligands to construct various polyoxometalate-based metal–organic complexes†

Xiu-Li Wang*, Dan-Na Liu, Hong-Yan Lin, Guo-Cheng Liu, Na Han, Jian Luan and Zhi-Han Chang
Department of Chemistry, Bohai University, Jinzhou, 121000, P. R. China. E-mail: wangxiuli@bhu.edu.cn; Fax: +86-416-3400158; Tel: +86-416-3400158

Received 21st May 2015 , Accepted 19th June 2015

First published on 22nd June 2015

Abstract

By introducing the flexible bis-pyrazine–bis-amide ligands into a polyoxometalates (POMs) system, five POM-based metal–organic complexes, {Cu3(L1)2[CrMo6(OH)6O18]2(H2O)2}·10H2O (1), [CuL1(Mo8O26)0.5]·H2O (2), [Cu2(L1)2(HPMoVI10MoV2O40) (H2O)2]·2H2O (3), [Cu2(L1)2(SiMo12O40) (H2O)2]·2H2O (4), [Cu2(L2)2(SiMo12O40)]·2H2O (5) [L1 = N,N′-bis(2-pyrazinecarboxamide)-1,3-propane, L2 = N,N′-bis(2-pyrizinecarboxamide)-1,6-hexane], were synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction analyses, elemental analyses, IR spectra, powder X-ray diffraction (PXRD) and thermal gravimetric analyses (TG). Single-crystal X-ray analysis reveals that compound 1 is a 1D infinite ribbon-like chain with Anderson type polyanions [CrMo6(OH)6O18]3− as building blocks. Compound 2 is a 2D layer constructed from 1D helical [Cu-L1]n2n+ chains and [Mo8O26]4− anions, in which the octamolybdates also act as inorganic building blocks. Compounds 3 and 4 are isostructural, and display 2D supramolecular networks based on the 1D [Cu-L1]n2n+ chains, in which the Keggin polyanions serve as noncoordinated templates. Compound 5 is a 3D supramolecular framework constructed from the 2D [Cu-L2]n2n+ layers and the Keggin [SiMo12O40]4− polyanion templates. In compounds 1–4, the L1 ligands display the same symmetric coordination mode with two chelating sites. When lengthening the spacer of the ligand, L2 exhibits an asymmetric coordination mode in compound 5. The structural diversities of 1–5 show that the different polyoxoanions and spacer lengths of the ligands play key roles in the construction of various architectures. The title compounds represent the first examples of introducing flexible bis-pyrazine–bis-amide ligands into the POMs system. In addition, the electrochemical properties of compounds 2–5 and the photocatalytic activities of the title compounds on the degradation of methylene blue (MB) under UV, visible light and sunlight irradiation have been investigated in detail.

Introduction

The design and synthesis of polyoxometalate (POM)-based metal–organic complexes (MOCS) through crystal engineering are currently attracting significant attention not only due to their potential applications in many areas but also as a result of their versatile structures.1 Different kinds of POMs, including Keggin, Anderson, Wells–Dawson, octamolybdates type and so on, have been extensively used to construct MOC materials, which may incorporate the merits of both components.2 In the process of synthesis, POMs can play different roles due to their features of oxygen-rich surface, high charge density and controllable size.3 One important synthesis strategy is to employ various POMs anions as inorganic building blocks to construct MOCs materials with charming configurations and desired properties.4 The other important synthesis strategy is the use of various POMs anions as noncoordinated templates to build MOCs. Several groups, including Su's, Meng's and Duan's group, have used various POMs as inorganic building blocks or templates to combine with diverse N-containing ligands constructing a lots of POMs-based MOCs.5 In our previous work, we also reported a series of POMs-based MOCs with various POMs as inorganic ligands or templates.6

As is known, the selection of organic ligands plays an important role in the self-assembly process of prospective POMs-based MOCs, because organic ligands may control and adjust the final structures of target compounds. Recently, some N-containing flexible ligands such as bis(pyridine), bis(imidazole), bis(triazole) and bis(tetrazole) derivatives have been used to construct the POM-based MOCs.7 However, to the best of our knowledge, no researches about the POMs-based MOCs with flexible bis-pyrazine–bis-amide ligands have been reported. The flexible bis-pyrazine–bis-amide ligands have some advantages as follows: (i) the flexible nature of the –(CH2)n– spacers allows the ligands to bend and rotate freely showing different configurations when coordinating to the metal centers; (ii) as neutral nitrogen/oxygen-donor ligands, the pyrazine and amide group of the ligands provide abundant potential coordination sites; (iii) the N of pyrazine and the O of amide group may chelate metal ions to generate diverse structures.

As an ongoing effort, in this paper, two flexible bis-pyrazine–bis-amide ligands (Scheme 1) with different spacer lengths were designed as the organic moieties to combine with different POMs and assemble with CuII ions, aiming at not only constructing novel POMs-based MOCs with the bis-pyrazine–bis-amide ligands, but also investigating the effect of different POMs cluster as inorganic building blocks or templates on the structures of target compounds. As a result, five POMs-based MOCs {Cu3(L1)2[CrMo6(OH)6O18]2(H2O)2}·10H2O (1), [CuL1(Mo8O26)0.5]·H2O (2), [Cu2(L1)2(HPMoVI10MoV2O40)(H2O)2]·2H2O (3), [Cu2(L1)2(SiMo12O40)(H2O)2]·2H2O (4), [Cu2(L2)2(SiMo12O40)]·2H2O (5) [L1 = N,N′-bis(2-pyrazinecarboxamide)-1,3-propane, L2 = N,N′-bis(2-pyrizinecarboxamide)-1,6-hexane] were obtained under hydrothermal conditions. The electrochemical properties and photocatalytic activities of the title compounds have been investigated in detail.


image file: c5ra09529h-s1.tif
Scheme 1 The bis-pyrazine–bis-amide ligands used in this paper.

Experimental section

Materials and methods

All reagents and solvents for syntheses were purchased from commercial sources and used as received without further purification. The N-donor ligands L1 and L2 and Na3[CrMo6(OH)6O18]·8H2O were prepared according to the reported procedures.8 Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. FT-IR spectra (KBr pellets) were taken on a Magna FT-IR 560 Spectrometer. Powder X-ray diffraction (PXRD) patterns were measured on an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα (λ = 1.5406 Å) radiation. Thermal gravimetric analyses (TGA) were carried out in N2 atmosphere on a Pyris-Diamond thermal analyzer with a rate of 10 °C min−1. A CHI 440 Electrochemical workstation was used for the electrochemical experiments. A conventional three-electrode cell was used at room temperature. The title compounds bulk-modified carbon paste electrodes (CPEs) were used as the working electrodes. An SCE and a platinum wire were used as reference and auxiliary electrodes, respectively. UV-Vis absorption spectra were obtained using a SP-1901 UV-Vis spectrophotometer.

Preparation of compounds 1–5

Synthesis of {Cu3(L1)2[CrMo6(OH)6O18]2(H2O)2}·10H2O (1). Homogenized mixtures of CuNO3·3H2O (0.50 mmol), L1 (0.10 mmol), Na3[CrMo6(OH)6O18]·8H2O (0.24 mmol), and H2O (10 mL) was stirred for 30 min at the room temperature. The pH value was then adjusted to about 5.45 using 1.0 M NaOH. The suspension was transferred to a Teflon lined autoclave (25 mL) and kept at 120 °C for 4 days. After slowly cooled to room temperature, blue lath crystals of 1 were filtered off, washed with distilled water, and dried in a desiccator at room temperature with a final pH of 4.8 to give a yield of 15% based on Mo. Elemental analysis (%) calcd for C26H64Cr2Cu3Mo12N12O64 (3014.79): C 10.36, H 2.13, N 5.41. Found: C 10.45, H 2.06, N 5.52. IR (KBr pellet, cm−1): 3488w, 3378w, 2370s, 2321m, 1641s, 1538s, 1054m, 947m, 886s, 640s.
Synthesis of [CuL1(Mo8O26)0.5]·H2O (2). Compound 2 was prepared in the same manner as that for 1, except for using (NH4)6Mo7O24·4H2O (0.1 mmol) as the substitute of Na3[CrMo6(OH)6O18]·8H2O and the pH value was adjusted to about 4.38 using 1.0 M HCl. Blue block crystals of 2 were obtained with a final pH of 3.9. Yield 42% based on Mo. Elemental analysis (%) calcd for C13H16CuMo4N6O16 (959.62): C 16.27, H 1.68, N 8.76. Found: C 16.34, H 1.59, N 8.84. IR (KBr pellet, cm−1): 3499w, 3371s, 2335w, 1634s, 1538m, 1060s, 954s, 897s, 837s, 701s, 652m.
Synthesis of [Cu2(L1)2(HPMoVI10MoV2O40)(H2O)2]·2H2O (3). Compound 3 was prepared in the same process as that for 1 except that H3PMo12O40 (0.1 mmol) was used in place of Na3[CrMo6(OH)6O18]·8H2O and the pH value was adjusted to about 4.21 using 1.0 M HCl. Green block crystals of 3 were obtained with a final pH of 3.5. Yield 38% based on Mo. Elemental analysis (%) calcd for C26H37Cu2Mo12N12O48P (2595.01): C 12.03, H 1.44, N 6.48. Found: C 12.11, H 1.42, N 6.43. IR (KBr pellet, cm−1): 3448s, 2361m, 2333m, 1645s, 1551w, 1376s, 1055m, 911s, 879m, 810s, 641m.
Synthesis of [Cu2(L1)2(SiMo12O40)(H2O)2]·2H2O (4). Compound 4 was prepared in the same way as that for 1 except for Na2SiO3·9H2O (0.1 mmol) and Na2MoO4 (0.12 mmol) as the substitute of Na3[CrMo6(OH)6O18]·8H2O and the pH value was adjusted to 4.73 using 1.0 M HCl. Green block crystals of 4 were obtained with a final pH of 3.8. Yield 44% based on Mo. Elemental analysis (%) calcd for C26H36Cu2Mo12N12O48Si (2591.12): C 12.05, H 1.40, N 6.49. Found: C 12.16, H 1.35, N 6.41. IR (KBr pellet, cm−1): 3506m, 3275w, 3085w, 2370s, 2328m, 1634s, 1531m, 1129m, 1061s, 954s, 911s, 790s.
Synthesis of [Cu2(L2)2(SiMo12O40)]·2H2O (5). Compound 5 was prepared in the same method as that for 4 except that L2 was used instead of L1 and the pH value was adjusted to 5.07 using 1.0 M HCl. Green block crystals of 5 were obtained with a final pH of 4.2. Yield 44% based on Mo. Elemental analysis (%) calcd for C32H44Cu2Mo12N12O46Si (2639.24): C 14.56, H 1.68, N 6.37. Found: C 14.48, H 1.62, N 6.45. IR (KBr pellet, cm−1): 3435s, 2922m, 2847w, 2355w, 1639s, 1563s, 1420m, 1113s, 894m, 792m, 621s.

X-ray crystallography

Crystallographic data for compounds 1–5 were collected on a Bruker Smart APEX II diffractometer with Mo Kα radiation (λ = 0.71069 Å) by ω and θ scan mode at 293 K. All the structures were solved by direct methods and refined on F2 by full-matrix least squares using the SHELXL package.9 The H atoms on the C and N atoms were fixed in calculated positions. However, the added H protons, the H atoms of the –OH groups and water molecules are not located in their crystal structure analysis, but were directly included in the final molecular formula. During the refinement, the command ‘isor’ was used to restrain the atoms C13, O1W, C14, C11 for compound 5. Otherwise, the command ‘DELU’ was used to restrain the atoms C2, C11, C13, C14, C50 for compound 5. The detailed crystal data and structures refinement for 1–5 are given in Table 1. Selected bond lengths and angles are listed in Table S1 (ESI†).
Table 1 Crystallographic data for compounds 1–5
Compound 1 2 3 4 5
a R1 = Σ‖Fo| − |Fc‖/Σ|Fo|.b wR2 = {Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]}1/2.
Empirical formula C26H64Cr2Cu3Mo12N12O64 C13H16CuMo4N6O16 C26H37Cu2Mo12N12O48P C26H36Cu2Mo12N12O48Si C32H44Cu2Mo12N12O46Si
Formula weight 3014.79 959.62 2595.01 2591.12 2639.24
Crystal system Triclinic Monoclinic Triclinic Triclinic Triclinic
Space group P[1 with combining macron] P2(1)/n P[1 with combining macron] P[1 with combining macron] P[1 with combining macron]
a (Å) 10.6780(6) 10.6557(4) 11.4981(11) 11.3935(6) 12.3112(7)
b (Å) 13.4480(8) 13.8735(6) 11.7035(11) 11.8235(6) 12.7440(8)
c (Å) 14.0150(8) 17.0837(7) 13.2243(13) 13.0713(6) 13.6911(9)
α (°) 105.5870(10) 90 64.013(2) 63.6790(10) 110.9730(10)
β (°) 102.5660(10) 99.7230(10) 89.207(2) 88.7650(10) 95.1360(10)
γ (°) 93.5890(11) 90 83.681(2) 82.9660(10) 117.4280(10)
V (Å3) 1876.32(19) 2489.24(18) 1588.7(3) 1565.35(14) 1694.33(18)
Z 1 4 1 1 1
Dc (g cm−3) 2.668 2.561 2.712 2.749 2.587
μ (mm−1) 3.163 2.889 3.077 3.117 2.880
F (000) 1455 1844 1238 1236 1264
Reflection collected 10[thin space (1/6-em)]737 12[thin space (1/6-em)]264 9069 7989 9708
Unique reflections 6592 4388 5589 5485 5959
Parameters 508 361 478 478 496
Rint 0.0197 0.0141 0.0190 0.0141 0.0157
GOF 1.045 1.041 1.073 1.062 1.051
R1a [I > 2σ(I)] 0.0412 0.0201 0.0503 0.0607 0.0641
wR2b (all data) 0.0813 0.0503 0.1501 0.1699 0.1980

Preparation of compounds 2–5 bulk-modified CPEs

The compound 2 bulk-modified CPE (2-CPE) was fabricated by mixing 0.10 g graphite powder and 0.030 g compound 2 in an agate mortar for approximately 30 min to achieve a uniform mixture; then 0.16 mL paraffin oil was added and stirred with a glass rod. The homogenized mixture was packed into a 3 mm inner diameter glass tube and the tube surface was wiped with weighing paper. The electrical contact was established with the copper wire through the back of electrode. In a similar manner, 3-CPE, 4-CPE and 5-CPE were made with compounds 3–5, respectively.

Results and discussion

Structural description

Crystal structure of compound 1. The asymmetric unit of 1 contains two crystallographically independent CuII ions, one L1 ligand, a [CrMo6(OH)6O18]3− (abbreviated to CrMo6) anion, one coordinated water molecules and five lattice water molecules (Fig. 1a). In compound 1, the CrMo6 anion possesses a B-type Anderson structure made up of seven edge-sharing octahedra. Six {MoO6} octahedra arranged hexagonally around the central {Cr(OH)6} octahedron. The Cu1 ion is five-coordinated by one O atom and one N atom from one L1 ligand, two terminal O atoms from one CrMo6 anion and one O atom from one coordinated water molecule. The bond distances and angles around Cu1 are 1.977(6) Å for Cu–N, 1.923(5)–2.404(6) Å for Cu–O, 87.60(3)–175.50(2)° for O–Cu–O and 82.7(2)–165.7(3)° for N–Cu–O. The Cu2 ion is six-coordinated by two pyrazine N atoms from two L1 ligands, two O atoms from two L1 ligands and two terminal O atoms from two CrMo6 anions showing a distorted octahedral geometry. The bond distances and angles around Cu2 are 1.961(6) for Cu–N, 1.970(5) Å for Cu–O, 179.998(1)° for N–Cu–N, 179.999(2)° for O–Cu–O and 82.60(2)–97.40(2)° for N–Cu–O. The L1 ligand adopts a “V-shape” conformation and the dihedral angle between the two pyrazine rings is 64.74°. Each L1 acts as a bridging ligand coordinating to Cu1 and Cu2 ions with its two N atoms and two O atoms in a chelating mode. While each CrMo6 anion also acts as an inorganic bridging ligand linking adjacent Cu1 and Cu2 ions with three O atoms. Thus, the adjacent crystallographically independent CuII ions are alternately connected by bidentate L1 ligands and CrMo6 anions, forming an infinite 1D wave-like chain. Two 1D wave-like chains crossed into a ribbon-like 1D chain with cycles formed by two L1 ligands, two CrMo6 anions and four CuII ions (Fig. 1b). The adjacent cycles are connected with each other by sharing Cu2 ions, as shown in Fig. 1c.
image file: c5ra09529h-f1.tif
Fig. 1 (a) The coordination environment of the CuII ion in 1. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: #1 − x − 1, −y + 2, −z + 2; (b) the ribbon-like 1D chain structure in 1. (c) Schematic diagram of the ribbon-like 1D chain structure.
Crystal structure of compound 2. The asymmetric unit of 2 contains one crystallographically independent CuII ion, one L1 ligand, a half of [Mo8O26]4− (abbreviated to Mo8) anion and one lattice water molecule (Fig. 2a). In compound 2, the Mo8 anion acts as an inorganic tetradentate building block. The Cu1 ion is six-coordinated by two N atoms and two O atoms from two L1 ligands and two terminal O atoms from two Mo8 anions, showing a distorted octahedral geometry. The bond distances and angles around Cu1 are 1.969(2) and 1.971(3) Å for Cu–N, 1.963(2), 1.962(2) and 2.322(2) Å for Cu–O, 175.21(10)° for N–Cu–N, 85.22(9)–175.64(9)° for O–Cu–O and 82.63(10)–101.66(10)° for N–Cu–O. The L1 ligand adopts “V-shape” conformation and the dihedral angle between the two pyrazine rings is 62.22°. As a bridging ligand, L1 connects the adjacent CuII ions to generate a 1D [Cu-L1]n2n+ helical chain (Fig. 2b). The Mo8 anion utilizes its four terminal O atoms as tetradentate linkage to coordinate with four CuII ions from adjacent [Cu-L1]n2n+ helical chains, thus a 2D network is generated (Fig. 2c). If the organic ligands are omitted, a 2D inorganic layer still exists (Fig. S1†). From a topological view, if the CuII ions and Mo8 anions are considered as four-connected nodes, the network of 2 can be described as a 2D 4-connected network (Fig. 2d).
image file: c5ra09529h-f2.tif
Fig. 2 (a) The coordination environment of the CuII ion in 2. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: #1 − x − 1/2, y + 1/2, −z + 1/2; (b) the [Cu-L1]n2n+ helical chain in 2. (c) The 2D layer structure in 2. (d) Schematic diagram of the 2D structure in 2.

In our recent work, a 2D layered compound {[Cu(L1) (β-Mo8O26)0.5(H2O)2]·H2O}n (L1 = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane) with a bis-pyridine–bis-amide as ligand has been synthesized, which exhibits a 4-connected 32·62·72 topological network.10 Although the coordination modes of Mo8 anion are same as that in compound 2, the N-donor ligands and CuII exhibit distinct coordination modes. In compound 2, the CuII ions are chelated by two flexible bis-pyrazine–bis-amide ligands.

However, flexible bis-pyridine–bis-amide ligand L1 in above compounds only provides one N atom of pyridine ring to coordinate with CuII ion. In addition, the chelating mode of L1 and copper are rarely found in the POMs-based compounds built by the flexible bis-amides ligands.

Crystal structures of compounds 3 and 4. Crystal structure analyses reveal that compounds 3 and 4 are isostructural. Compound 3 is discussed as the representative example here. Single-crystal X-ray diffraction analysis reveals that compound 3 is a 2D supramolecular structure based on a [Cu-L1]n2n+ 1D chain and one [PMoVI10MoV2O40]5− (PMo12) anion. The bond valence sum shows that two of the twelve Mo atoms are in the +V oxidation state, and all the Cu atoms are in the +II oxidation state. To balance the charge of the compound, one proton is added, then 3 is formulated as [Cu2(L1)2(HPMoVI10MoV2O40)(H2O)2]·2H2O. There are two CuII ions, two L1 ligands, one PMo12 anion, two coordinated water molecules and two lattice water molecules in the asymmetric unit of compound 3 (Fig. 3a and S2† for compound 4). In compounds 3 and 4, the PMo12 and SiMo12 anions play the roles of noncoordinated templates. The Cu1 ion is six-coordinated by two N atoms and two O atoms from two L1 ligands and two O atoms from two coordinated water molecules, showing a distorted octahedral geometry. The bond distances and angles around Cu1 are 1.957(7) Å for Cu–N, 1.947(7), 1.948(7) and 2.405(11) Å for Cu–O, 180.000(1)° for N–Cu–N, 85.8(3)–180.0° for O–Cu–O and 81.1(3)–98.9(3)° for N–Cu–O. The Cu2 ion is four-coordinated by two N atoms and two O atoms from two L1 ligands. The bond distances and angles around Cu2 are 2.002(13) Å for Cu–N, 1.950(9) Å for Cu–O, 179.999(1)° for N–Cu–N, 179.999(1) for O–Cu–O and 83.2(4)–96.8(4)° for N–Cu–O. The L1 ligand adopts “V-shape” conformation and the dihedral angle between the two pyrazine rings is 63.95°. The adjacent CuII ions are connected by L1 ligands to form a 1D wave-like [Cu-L1]n2n+ chain. A detailed structural analysis reveals that the adjacent 1D chains are further connected by PMo12 anions via C–H⋯O hydrogen bonds C(7)–H(7A)⋯O(10), 3.3506 Å], forming a 2D supramolecular structure (Fig. 3b).
image file: c5ra09529h-f3.tif
Fig. 3 (a) The coordination environment of the CuII ion in 3. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: #2 − x − 1, −y + 1, −z − 1; #3 −x, −y, −z; (b) the 2D supramolecular structure formed via hydrogen bonds (C(7)–H(7A)⋯O(10), 3.3506 Å) in 3.
Crystal structure of compound 5. Single-crystal X-ray diffraction analysis reveals that compound 5 is a 3D supramolecular structure based on the 2D [Cu-L2]n2n+ layers and the [SiMo12O40]4− (SiMo12) anions. In compound 5, the Keggin anions SiMo12 are serving as templates. The asymmetric unit of 5 contains two CuII ions, two L2 ligands, one SiMo12 anion and two lattice water molecules (Fig. 4a). The Cu1 ion is six-coordinated by four N atoms from four L2 ligands and two O atoms from two L2 ligands showing a distorted octahedral geometry. The bond distances and angles around Cu1 are 1.963(8) Å for Cu–O, 1.990(10) Å and 2.430(9) Å for Cu–N, 179.999(1)° for O–Cu–O, 88.4(4)°, 91.6(4)° for N–Cu–N and 82.8(4)–97.2(4)° for N–Cu–O. The Cu2 ion is four-coordinated by two O atoms and two N atoms from two L2 ligands. The bond distances and angles around Cu2 is 1.928(7) Å for Cu–O, 1.946(9) Å for Cu–N, 179.999(1)° for O–Cu–O, 179.999(2)° for N–Cu–N and 83.4(3)–96.6(3)° for N–Cu–O, respectively. The L2 ligand adopts linear conformation and the dihedral angle between the two pyrazine rings is 86.14°. The adjacent CuII ions are connected by L2 ligands to form a 2D [Cu-L2]n2n+ layer (Fig. 4b and c). The 2D layers are further linked by SiMo12 anions via hydrogen bonds (N(5)–H(5B)⋯O(22), 3.0478 Å), forming a 3D supramolecular framework (Fig. 4d).
image file: c5ra09529h-f4.tif
Fig. 4 (a) The coordination environment of the CuII ion in 5. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: #2 − x − 1, −y, −z; #3 − x − 2, −y − 1, −z − 1; (b) and (c) the 2D layer structure viewed from different orientation in 5. (d) The 3D supramolecular structure formed via hydrogen bonds (N(5)–H(5B)⋯O(22), 3.0478 Å) in 5.

In compound 5, the L2 ligand displays an asymmetric coordination mode (Table 2), in which one pyrazine ring provides two coordinated sites, whereas the other provides only one. The asymmetric coordination mode has never been found in POMs-based compounds constructed from symmetric bisamide ligands.11 We have reported a compound [Cu2(L3)3(SiMo12O40) (H2O)6]·6H2O (L3 = N,N′-bis(3-pyridinecarboxamide)-1,6-hexane) based on the same central metal and SiMo12 anion.12 The L3 has same flexible spacer length and amide group as that of L2, which displays a symmetric coordination mode. The results indicate that the amount of coordination sites show great effect on the final structures of compounds.

Table 2 Coordination numbers and modes of CuII ions, POMs, organic ligands (L1 and L2) of compounds 1–5
Compound 1 2 3 4 5
Coordination modes of copper image file: c5ra09529h-u1.tif image file: c5ra09529h-u2.tif image file: c5ra09529h-u3.tif image file: c5ra09529h-u4.tif image file: c5ra09529h-u5.tif
Coordination sites of polyanions image file: c5ra09529h-u6.tif image file: c5ra09529h-u7.tif image file: c5ra09529h-u8.tif image file: c5ra09529h-u9.tif image file: c5ra09529h-u10.tif
Coordination modes and conformation of ligands image file: c5ra09529h-u11.tif image file: c5ra09529h-u12.tif image file: c5ra09529h-u13.tif image file: c5ra09529h-u14.tif image file: c5ra09529h-u15.tif
The dihedral angles 74.74 62.22 63.95 65.14 86.14

Influences of POMs on the structures of the title compounds

It is well known that POMs play an important role in the assembly process of the POMs-based MOCs and can induce structural diversities owing to their different charges and volume.13 In compounds 1–4, three distinct structures were synthesized under identical reaction conditions based on the Cu-L1 system, except for the alternation of polyoxoanions (Scheme 2). Anderson type CrMo6 anions were used in 1, which act as inorganic bidentate bridging ligands, and a 1D ribbon-like chain was formed. When Mo8 anions were employed in compound 2, a 2D network was obtained, in which the Mo8 anions act as tetradentate inorganic ligands. In compounds 3 and 4, Keggin-type SiMo12 and PMo12 anions were introduced as inorganic noncoordinated templates, the 2D supramolecular networks based on the Keggin anions and 1D [Cu-L1]n2n+ chains were constructed. As described above, the Keggin-templated compounds 3 and 4 are isostructural, which indicates that the Keggin anions have no obvious effect on the final networks due to their identical structures, negative charges and similar sizes. Due to the utilization of different types of POMs, the CuII ions also adopt distinct coordination modes in 1–5 (Table 2). The results indicate that the POMs anions with different volumes and charges play an important role in constructing various novel frameworks.
image file: c5ra09529h-s2.tif
Scheme 2 Schematic illustrations of different anions and ligands on the structures of 1–5.

Influences of the bis-pyrazine–bis-amide ligands with different spacer lengths on the structures

In this work, we selected two flexible bis-pyrazine–bis-amide ligands with different spacer lengths to investigate their influence on the assembly and structures of SiMo12-based MOCs. Firstly, we used L1 to construct SiMo12-based compound 4, and then a longer L2 ligand was employed in 5. As shown in Table 2, the SiMo12 anions act as templates in 4 and 5, and the CuII ions also exhibits same coordination modes, however, the ligands L1 and L2 display different coordination modes. The L1 exhibits a symmetric coordination mode utilizing two O atoms from two amide groups and two N atoms from two pyrazine rings to chelate two CuII ions. A 1D wave-like [Cu-L1]n2n+ chain was constructed in 4. The L2 displays an asymmetric coordination mode with two O atoms of two amide groups and three N atoms of two pyrazine rings coordinating with three CuII ions. Thus, a2D [Cu-L2]n2n+ network was obtained in 5. Finally, the compound 4 shows a 2D supramolecular structure based on 1D chains and SiMo12 anions, while compound 5 is a 3D supramolecular framework based on a 2D layer and SiMo12 anions. The results fully demonstrate that different spacer lengths of the flexible bis-pyrazine–bis-amide ligands have a significant effect on the architectures of the final compounds.

FT-IR spectra, powder X-ray diffraction and TG analyses of compounds 1–5

The IR spectra of complexes 1–5 are shown in Fig. S3.† The bands at 1054, 947, 886, 640 cm−1 for 1, 1060, 954, 897, 837, 701, 652 cm−1 for 2 can be attributed to the ν(Mo–O) and ν(Mo–O–Mo) of CrMo6 and Mo8 anions, respectively.14 The characteristic bands at 1055, 911, 879, 810, 641 cm−1 for 3 are attributed to ν(Mo–O) and ν(Mo–O–Mo) and ν(P–O) of PMo12 anion.15 The characteristic bands at 1061, 954, 911, 790 cm−1 for 4 and 1113, 894, 792, 621 cm−1 for 5 are due to ν(Mo–O) and ν(Mo–O–Mo) and ν(Si–O) of SiMo12 anions.16 The characteristic bands observed at 1641, 1538 cm−1 for 1, 1634, 1538 cm−1 for 2, 1645, 1551 cm−1 for 3, 1634, 1531 cm−1 for 4, 1639, 1563 cm−1 for 5, can be ascribed to the stretching and bending vibration of C[double bond, length as m-dash]O group.17 The bands around 3400 cm−1 can be attributed to the water molecules.

The PXRD patterns of the title compounds (Fig. S4†) indicate that the synthesized compounds match well with the simulated ones except for some intensity differences, which can be due to the different orientation of the crystals in the powder samples, proving the good crystalline phase purity.18

Thermogravimetric (TG) analyses of compounds 1–5 were performed in flowing N2 atmosphere with a heating rate of 10 °C min−1 from the room temperature to 880 °C (Fig. S5†). Compounds 1–5 have two distinct steps of weight losses, respectively. The first weight losses of 7.19% for 1, 1.91% for 2, 2.71% for 3, 2.73% for 4, and 1.39% for 5 below the temperature of 138, 205, 220, 210 and 150 °C are consistent with the removal of lattice/coordinated water molecules (calc. 7.16%, 1.88%, 2.78%, 2.78% and 1.36%), respectively. The results show that the observed weight losses due to water match well with the calculated values. The TGA curves for compounds 1–5 suggested that their host networks were stable up to 322, 335, 323, 363 and 297 °C, respectively. Then, the second sharp weight losses are corresponding to the collapse of the frameworks and loss of the organic ligands. The residue can be attributed to a mixture of CuO, CrO3 and MoO3 for 1, CuO and MoO3 for 2 and 3, CuO, SiO2 and MoO3 for 4 and 5, respectively.

Electrochemical properties

It is meaningful to explore the electrochemical behaviors of such POMs-based complexes. The bulk-modified carbon paste electrodes (CPEs) with POMs-based compounds have been widely applied in electrochemistry due to their high stability, low solubility in water and common organic solvents and easy to handle. Their electrochemical behaviours in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution at different scan rates are presented. In the potential range from +500 to −300 mV, the 2-CPE exhibits three reversible redox peaks I–I′, II–II′ and III–III′ (Fig. 5a) corresponding to the redox processes of Mo centers, with the mean peak potentials E1/2 = (Epa + Epc)/2 of 183, 60, −159 mV (scan rate: 200 mV s−1), respectively.19 Three reversible redox peaks (I–I′,II–II′, III–III′) for 3-CPE appear in the potential range of +500 to −300 mV with the mean peak potentials of +242, +82, and −154 mV, respectively (Fig. S6a†),20 which corresponds to three consecutive two-electron processes of PMo12.21 The electrochemical behaviors of the 4-CPE and 5-CPE are similar. In the potential range from +500 to −300 mV, there are three reversible redox peaks I–I′, II–II′ and III–III′ with the mean peak potentials are +181, +62 and −131 mV (scan rate: 200 mV s−1) for 4-CPE (Fig. 5b), and +187, +69 and −132 mV (scan rate: 200 mV s−1) for 5-CPE (Fig. S6b†).22 As can be seen from Fig. 5 and S6,† all the peak potentials change gradually: the anodic peak potentials shifted toward the positive direction, while the corresponding cathodic peak potentials shifted toward the negative direction with increasing scan rates. In addition, all the peak currents are proportional to the scan rates up to 500 mV s−1, which indicates that the redox processes of 2-CPE, 3-CPE, 4-CPE and 5-CPE are surface-controlled (Fig. S7†).
image file: c5ra09529h-f5.tif
Fig. 5 Cyclic voltammograms of the 2-CPE (a) and 4-CPE (b) in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution at different scan rates (from inner to outer: 40, 80, 120, 160, 200, 250, 300, 350, 400, 450, 500 mV s−1 for 2-CPE and 40, 120, 160, 200, 250, 300, 350, 400, 450, 500 mV s−1 for 4-CPE). Cyclic voltammograms of 2-CPE (c) and 4-CPE (d) in 0.1 M H2SO4 + 0.5 M Na2SO4 solution containing 0.0–12.0 mM H2O2.

As is known, some POMs can be used as electrocatalysts to catalyze the reduction of hydrogen peroxide.23 In this work, we also investigate the electrocatalytic activity of 2-CPE, 3-CPE, 4-CPE and 5-CPE toward the reduction of hydrogen peroxide. As can be seen from Fig. 5c, d and S6c and d,† for 2-CPE, 3-CPE, 4-CPE and 5-CPE, with the addition of hydrogen peroxide, both the reduction peak currents increase gradually and the corresponding oxidation peak currents decrease, suggesting that the reduction of hydrogen peroxide is electrocatalyzed by the reduced species of POMs in compounds 2–5.

Photocatalytic property

It is well known that POMs often show photocatalytic activity in the degradation of some organic dyes under UV irradiation.24 Herein, we selected an organic dye methylene blue (MB) as a model pollutant in aqueous media to evaluate the photocatalytic effectiveness of compounds 1–5 under UV, visible light and sunlight irradiation. Firstly, we investigated the photocatalytic performance of the title compounds for the photodegradation of MB under UV light irradiation from a 125 W Hg lamp. As shown in Fig. 6a and S8a–S11a,† the absorption peak of MB decreased obviously with increasing reaction time for compounds 1–5 under UV irradiation. After 100 min, the degradation ratio of MB reaches 89.3% for 1, 74.0% for 2, 96.5% for 3, 88.4% for 4 and 95.9% for 5 (Fig. 7). Additionally, PXRD patterns of the title compounds after the photocatalytic reactions under UV irradiation have been recorded, which match with the simulated ones except for some intensity differences (Fig. S4†). The results suggest that the title compounds possess good stability as photocatalysts for the photodegradation of MB contaminant. In this work, we also carried out the same photocatalysis experiments under visible light irradiation from a 350 W Xe lamp equipped with a long-pass filter (400 nm cutoff). A few POMs-based hybrids can be used as one kind of cheap photocatalysts for the removal of organic pollutants under visible light irradiation.25 As shown in Fig. 6b and S8b–S11b,† the absorption peak of MB decreased obviously with increasing reaction time for compounds 1–5 under visible irradiation. After 150 min, the degradation ratio of MB reaches 83.4% for 1, 23.0% for 2, 95.6% for 3, 87.6% for 4 and 84.8% for 5 (Fig. 7).
image file: c5ra09529h-f6.tif
Fig. 6 Absorption spectra of the MB solution during the decomposition reaction under UV (a), visible (b) and sunlight (c) irradiation in the presence of the compound 3.

image file: c5ra09529h-f7.tif
Fig. 7 The degradation ratio of MB of compounds 1–5 under UV, visible light and sunlight irradiation.

In order to make the photocatalytic process more conducive to practical application, we performed photocatalytic tests of the title compounds towards the degradation of MB under sunlight irradiation. To the best of our knowledge, reports on the photocatalytic activity of POMs-based compounds under sunlight irradiation are very limited up to now.26 After being in sunlight for 5 h, the degradation efficiency is 81.2% for 1, 50.0% for 2, 91.3% for 3, 81.2% for 4 and 84.7% for 5 (Fig. 6c and S8c–S11c†). In comparison, we also measured the absorption spectra of the MB solution during the decomposition reaction in the presence of the compounds 1–5 under the dark environment. After 5 h, the conversions of MB are only 25.8% for 1, 14.5% for 2, 24.7% for 3, 22.9% for 4 and 35.5% for 5, respectively (Fig. S12†). The results further indicate that UV, visible light or sunlight irradiation is very important for the degradation of MB.

The different photocatalytic activity of 1–5 on the decomposition of MB may be due to different POMs in the title compounds and their different structures. The results suggest that the title compounds, especially the Keggin-type and Anderson-type POMs-based compounds, have excellent photocatalytic activity and potential application in the degradation of some organic dyes in the polluted water.

Conclusions

Utilizing two flexible bis-pyrazine–bis-amide ligands (L1 and L2), five POMs-based MOCs with different structures have been hydrothermally synthesized. Different types of POMs act as diverse roles in building compounds. Anderson CrMo6 anions in compound 1 and [Mo8O26]4− anions in 2 act as 2-connected and four-connected linkers, respectively. In 3–5, the Keggin anions act as noncoordinated templates. The structural diversities show that different polyoxoanions and spacer lengths of the ligands play key roles in the construction of various architectures. The title compounds represent the first examples of introducing flexible bis-pyrazine–bis-amide ligands into the POMs system. Compounds 2–5 exhibit good electrocatalytic behaviors toward the reduction of H2O2 and compounds 1–5 exhibit remarkable photocatalytic activities for the degradation of MB under UV, visible light and sunlight irradiation. The title compounds may be potential candidates for electrocatalytic or photocatalytic materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21171025 and 21471021) and Program of Innovative Research Team in University of Liaoning Province (LT2012020).

Notes and references

  1. (a) K. Yu, B. B. Zhou, Y. Yu, Z. H. Su and G. Y. Yang, Inorg. Chem., 2011, 50, 1862 CrossRef CAS PubMed; (b) F. J. Ma, S. X. Liu, C. Y. Sun, D. D. Liang, G. J. Ren, F. Wei, Y. G. Chen and Z. M. Su, J. Am. Chem. Soc., 2011, 133, 4178 CrossRef CAS PubMed; (c) K. Nomiya, T. Yoshida, Y. Sakai, A. Nanba and S. Tsuruta, Inorg. Chem., 2010, 49, 8247 CrossRef CAS PubMed; (d) J. Sha, J. Peng, Y. Zhang, H. Pang, A. Tian, P. Zhang and H. Liu, Cryst. Growth Des., 2009, 9, 1708 CrossRef CAS.
  2. (a) W. W. He, S. L. Li, H. Y. Zang, G. S. Yang, S. R. Zhang, Z. M. Su and Y. Q. Lan, Coord. Chem. Rev., 2014, 279, 141 CrossRef CAS PubMed; (b) J. W. Zhao, J. L. Zhang, Y. Z. Li, J. Cao and L. J. Chen, Cryst. Growth Des., 2014, 14, 1467 CrossRef CAS; (c) J. Y. Guo, D. Zhang, L. Chen, Y. Song, D. R. Zhu and Y. Xu, Dalton Trans., 2013, 42, 8454 RSC.
  3. (a) M. Mirzaei, H. Eshtiagh-Hosseini, M. Alipour and A. Frontera, Coord. Chem. Rev., 2014, 275, 1 CrossRef CAS PubMed; (b) Q. G. Zhai, X. Y. Wu, S. M. Chen, Z. G. Zhao and C. Z. Lu, Inorg. Chem., 2007, 46, 5046 CrossRef CAS PubMed; (c) P. P. Zhang, J. Peng, H. J. Pang, J. Q. Sha, M. Zhu, D. D. Wang and M. G. Liu, CrystEngComm, 2011, 13, 3832 RSC.
  4. (a) M. X. Liang, C. Z. Ruan, D. Sun, X. J. Kong, Y. P. Ren, L. S. Long, R. B. Huang and L. S. Zheng, Inorg. Chem., 2014, 53, 897 CrossRef CAS PubMed; (b) A. Iturrospe, B. Artetxe, S. Reinoso, L. S. Felices, P. Vitoria, L. Lezama and J. M. Gutié rrez-Zorrilla, Inorg. Chem., 2013, 52, 3084 CrossRef CAS PubMed; (c) L. Luo, H. Lin, L. Li, T. I. Smirnova and P. A. Maggard, Inorg. Chem., 2014, 53, 3464 CrossRef CAS PubMed.
  5. (a) M. L. Wei, C. He, W. J. Hua, C. Y. Duan, S. H. Li and Q. J. Meng, J. Am. Chem. Soc., 2006, 128, 13318 CrossRef CAS PubMed; (b) Y. G. Li, L. M. Dai, Y. H. Wang, X. L. Wang, E. B. Wang, Z. M. Su and L. Xu, Chem. Commun., 2007, 2593 RSC; (c) C. Y. Sun, S. X. Liu, D. D. Liang, K. Z. Shao, Y. H. Ren and Z. M. Su, J. Am. Chem. Soc., 2009, 131, 1883 CrossRef CAS PubMed.
  6. (a) X. L. Wang, Q. Gao, A. X. Tian and G. C. Liu, Cryst. Growth Des., 2012, 12, 2346 CrossRef CAS; (b) X. L. Wang, N. Li, A. X. Tian, J. Ying, T. J. Li, X. L. Lin, J. Luan and Y. Yang, Inorg. Chem., 2014, 53, 7118 CrossRef CAS PubMed; (c) A. X. Tian, Y. Yang, N. Sun, J. C. Li, J. Ying, J. W. Zhang and X. L. Wang, J. Coord. Chem., 2014, 67, 1550 CrossRef CAS PubMed.
  7. (a) J. X. Meng, Y. Lu, Y. G. Li, H. Fu and E. B. Wang, Cryst. Growth Des., 2009, 9, 4116 CrossRef CAS; (b) P. P. Zhang, J. Peng, H. J. Pang, J. Q. Sha, M. Zhu, D. D. Wang, M. G. Liu and Z. M. Su, Cryst. Growth Des., 2011, 11, 2736 CrossRef CAS; (c) X. L. Wang, H. L. Hu and A. X. Tian, Cryst. Growth Des., 2010, 10, 4786 CrossRef CAS; (d) A. X. Tian, J. Ying, J. Peng, J. Q. Sha, Z. G. Han, J. F. Ma, Z. M. Su, N. H. Hu and H. Q. Jia, Inorg. Chem., 2008, 47, 3274 CrossRef CAS PubMed.
  8. (a) M. Sarkar and K. Biradha, Cryst. Growth Des., 2006, 6, 202 CrossRef CAS; (b) V. Lee and Y. Sasaki, Chem. Lett., 1984, 1297 CrossRef.
  9. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112 CrossRef CAS PubMed.
  10. X. L. Wang, C. Xu, H. Y. Lin, G. C. Liu, J. Luan and Z. H. Chang, RSC Adv., 2013, 3, 3592 RSC.
  11. (a) H. Chen, H. Y. An, X. Liu, H. L. Wang, Z. F. Chen, H. Zhang and Y. Hu, Inorg. Chem. Commun., 2012, 21, 65 CrossRef CAS PubMed; (b) X. L. Wang, C. Xu, H. Y. Lin, G. C. Liu, J. Luan, Z. H. Chang and A. X. Tian, J. Coord. Chem., 2013, 66, 1451 CrossRef CAS PubMed; (c) X. L. Wang, C. Xu, H. Y. Lin, G. C Liu, J. Luan, A. X. Tian and J. W. Zhang, Inorg. Chem. Commun., 2012, 24, 43 CrossRef CAS PubMed.
  12. X. L. Wang, C. Xu, H. Y. Lin, G. C. Liu, S. Yang, Q. Gao and A. X. Tian, CrystEngComm, 2012, 14, 5836 RSC.
  13. (a) X. Wang, J. Peng, M. G. Liu, D. D. Wang, C. L. Meng, Y. Li and Z. Y. Shi, CrystEngComm, 2012, 14, 5836 RSC; (b) B. K. Tripuramallu and S. K. Das, Cryst. Growth Des., 2013, 13, 2426 CrossRef CAS; (c) X. L. Wang, N. Li, A. X. Tian, J. Ying, G. C. Liu, H. Y. Lin, J. W. Zhang and Y. Yang, Dalton Trans., 2013, 42, 14856 RSC.
  14. R. Dessapt, M. Gabard, M. Bujoli-Doeuff, P. Deniard and S. Jobic, Inorg. Chem., 2011, 50, 8790 CrossRef CAS PubMed.
  15. H. Y. Liu, H. Wu, J. Yang, Y. Y. Liu, J. F. Ma and H. Y. Bai, Cryst. Growth Des., 2011, 11, 1786 CAS.
  16. C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck and R. Thouvenot, Inorg. Chem., 1983, 22, 207 CrossRef CAS.
  17. K. Uehara, T. Oishi, T. Hirose and N. Mizuno, Inorg. Chem., 2013, 52, 11200 CrossRef CAS PubMed.
  18. A. X. Tian, J. Ying, J. Peng, J. Q. Sha, H. J. Pang, P. P. Zhang, Y. Chen, M. Zhu and Z. M. Su, Cryst. Growth Des., 2008, 8, 3717 CAS.
  19. (a) X. Xu, W. W. Ju, W. T. Hou, D. R. Zhu and Y. Xu, CrystEngComm, 2014, 16, 82 RSC; (b) X. L. Wang, N. Han, H. Y. Lin, A. X. Tian, G. C. Liu and J. W. Zhang, Dalton Trans., 2014, 43, 2052 RSC.
  20. H. L. Hu, W. S. Zhang, J. J. Gong, H. Dong, F. F. Zhao, H. Huang, Y. Liu, G. J. Zhang and Z. H. Kang, CrystEngComm, 2014, 16, 5642 RSC.
  21. A. X. Tian, Y. L. Ning, J. Ying, X. Hou, T. J. Li and X. L. Wang, Dalton Trans., 2015, 44, 386 RSC.
  22. A. X. Tian, Y. L. Ning, J. Ying, X. Hou, T. J. Li and X. L. Wang, Dalton Trans., 2015, 44, 386 RSC.
  23. M. L. Qi, K. Yu, Z. H. Su, C. X. Wang, C. M. Wang, B. B. Zhou and C. C. Zhu, Dalton Trans., 2013, 42, 7586 RSC.
  24. (a) J. Tucher, Y. L. Wu, L. C. Nye, I. Ivanovic-Burmazovic, M. M. Khusniyarov and C. Sterb, Dalton Trans., 2012, 41, 9938 RSC; (b) A. Dolbecq, P. Mialane, B. Keita and L. Nadjo, J. Mater. Chem., 2012, 22, 24509 RSC; (c) P. P. Zhang, J. Peng, H. J. Pang, J. Q. Sha, M. Zhu, D. D. Wang, M. G. Liu and Z. M. Su, Cryst. Growth Des., 2011, 11, 2736 CrossRef CAS.
  25. Z. J. Liu, S. Yao, Z. M. Zhang and E. B. Wang, RSC Adv., 2013, 3, 20829 RSC.
  26. X. L. Wang, Z. H. Chang, H. Y. Lin, A. X. Tian, G. C. Liu and J. W. Zhang, Dalton Trans., 2014, 43, 12272 RSC.

Footnote

† Electronic supplementary information (ESI) available: IR Spectra, PXRD, TG and additional figures. CCDC 1045303–1045307. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra09529h
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