Two novel Anderson-type polyoxometalate-based metal–organic complexes with high-efficiency photocatalysis towards degradation of organic dyes under UV and visible light irradiation

Abstract

Two novel Anderson-type polyoxometalate (POM)-based metal–organic complexes (MOCs), namely, H{CuL_0.5¹ [CrMo₆(OH)₆O₁₈](H₂O)}·0.5L¹ (1) and {Cu₂(L²)₂[CrMo₆(OH)₅O₁₉](H₂O)₂}·2H₂O (2) (L¹ = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, L² = N,N′-bis(3-pyridinecarboxamide)-1.3-propane), were hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR spectra, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In complex 1, the bidentate [CrMo₆(OH)₆O₁₈]³⁻ (CrMo₆) polyoxoanions bridge the Cu^II ions to generate a 1D Cu–CrMo₆ inorganic chain, which is further connected by the μ₂-bridging L¹ ligands to form a 1D ladder-like chain. Complex 2 is a 3D POM-based metal–organic framework exhibiting a {4¹².6³} topology, which is constructed from the quadridentate CrMo₆ polyoxoanions and μ₂-bridging L² ligands. The flexible bis-pyridyl-bis-amide ligands with different spacer lengths have a significant effect on the final structures. In addition, pH shows great influence on the formation of the single-crystal phase. The photocatalytic activities of the title complexes on the degradation of methylene blue (MB) and rhodamine B (RhB) under UV and visible light have been investigated in detail.

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Introduction

Polyoxometalates (POMs), as one kind of well-defined metal oxide cluster with abundant structures and potential applications in catalysis, electrochemistry, photochemistry, magnetism, biology and medicine, are widely used as templates or inorganic building subunits to construct metal–organic complexes (MOCs) with various structures and desired properties.^1,2 As an important member of POMs family, Anderson-type polyoxoanions exhibit fascinating planar structures with abundant terminal oxygen atoms showing high-reactivity, which have attracted considerable attention and been employed as a synthetic source to construct MOCs with novel structures and appealing properties.³ For example, Ramanan's group have reported six new Anderson-type POM based complexes, (H₂4-pyc)₂[{Na-(4-pyc)₂}{CrMo₆(OH)₈O₁₆}]₃·4H₂O, (H₂4-pyc)₂[{Ni(4-pyc)2-(H₂O)₄}{CrMo₆(OH)₇O₁₇}]₃·10H₂O, (H₂3-pyc)₂[{Na(3-pyc)₂}-{CrMo₆(OH)₈O₁₆}]₃·2H₂O, and [{M₂(2-pzc)₂(H₂O)₄}{CrMo₆(OH)₇O₁₇}]₃·17H₂O (M = Co and Cu, 4-pyc = pyridine-4-carboxylate, 3-pyc = pyridine-3-carboxylate, and 2-pzc = pyrazine-2-carboxylate), in which Anderson-type polyoxoanions act as templates or building blocks.⁴

On the other side, the selection of organic ligands play an important role in the assembly of POM-based MOCs.⁵ Recently, the introduction of N-containing ligands has become popular in the POM-based hybrids, such as bis(pyridlyl)-,⁶ bis(imidazole)-,⁷ bis(triazole)-⁸ and bis(tetrazole)-based⁹ derivatives ligands, which have been widely used to construct POM-based complexes. As a sort of N/O-donor ligands, flexible bis-pyridyl-bis-amide have attracted much attention based on the following structural characters: (i) this kind of ligands possess flexible –(CH₂)_n– spacers, which allow themselves to bend and rotate freely when coordinating to the metal centers; (ii) the pyridyl and amide group can provide more potential coordination sites; (iii) the amide groups with both the N–H hydrogen donor and C=O hydrogen acceptor promote the formation of supramolecular structures by hydrogen bonding interaction. Recently, our group have reported several Keggin-type POM-based MOCs constructed from flexible bis-pyridyl-bis-amide ligands with different spacer lengths, in which the flexible bis-pyridyl-bis-amide ligands with different spacer length influence the formation and structures of the target complexes.¹⁰ More recently, by introducing the semi-rigid bis-pyridyl-bis-amide ligands into Anderson-type POMs systems, our group has obtained two Anderson-type POMs-based metal–organic frameworks.¹¹ However, no researches about the Anderson-type POM-based complexes constructed from flexible bis-pyridyl-bis-amide ligands have been reported.

Taking these into account, we are trying to use two flexible bis-pyridyl-bis-amide ligands N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, L² = N,N′-bis(3-pyridinecarboxamide)-1.3-propane with different spacer lengths to construct Anderson-type POM-based MOCs, in order to explore effect of the flexible bis-pyridyl-bis-amide length on the architectures of target compounds. Fortunately, two novel POM-based MOCs, H{CuL_0.5¹[CrMo₆(OH)₆O₁₈](H₂O)}·0.5L¹ (1), {Cu₂(L²)₂[CrMo₆(OH)₅O₁₉](H₂O)₂}·2H₂O (2) are obtained under hydrothermal conditions with different pH values. The title complexes represent novel Anderson-type POM-based MOCs constructed from flexible bis-pyridyl-bis-amide ligands. The photocatalytic activities of the title complexes towards degradation of methylene blue (MB) and rhodamine B (RhB) under UV and visible light irradiation are investigated in this paper, respectively (Scheme 1).

Scheme 1 The ligands used in this paper.

Experimental

Materials and characterization

All reagents and solvents for syntheses were purchased from commercial sources and used as received without further purification. The N-donor ligands L¹, L² and Na₃[CrMo₆(OH)₆O₁₈]·8H₂O were prepared according to the reported procedure.¹² FT-IR spectra (KBr pellets) were taken on a Scimitar 2000 Near FT-IR Spectrometer. Thermogravimetric analyses (TGA) were performed on a Pyris Diamond TG instrument under a flowing N₂ atmosphere with a heating rate of 10 °C min⁻¹. 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. UV-vis absorption spectra were obtained using a SP-1901 UV-vis spectrophotometer.

Synthesis of H{CuL_0.5¹[CrMo₆(OH)₆O₁₈](H₂O)}·0.5L¹ (1)

A mixture of CuCl₂·2H₂O (0.085 g, 0.50 mmol), L¹ (0.027 g, 0.10 mmol) Na₃[CrMo₆(OH)₆O₁₈]·8H₂O (1.974 g, 0.24 mmol), and H₂O (10 mL) was stirred for 30 min at room temperature. The pH value was then adjusted to about 2.0 using 1.0 M HCl. The suspension was transferred to a Teflon lined autoclave (25 mL) and kept at 120 °C for 4 days, then the autoclave was slowly cooled to room temperature. Blue block crystals of complex 1 were filtered off, washed with distilled water, and dried in a desiccator at room temperature to give a yield of 32% based on Mo. IR (KBr pellet, cm⁻¹): 3421 (s), 1638 (s), 1554 (m), 1422 (m), 1118 (s), 941 (w), 911 (w), 873 (m), 620 (s), 559 (w).

Synthesis of {Cu₂(L²)₂[CrMo₆(OH)₅O₁₉](H₂O)₂}·2H₂O (2)

A mixture of CuCl₂·2H₂O (0.085 g, 0.50 mmol), L² (0.028 g, 0.10 mmol), Na₃[CrMo₆(OH)₆O₁₈]·8H₂O (1.941 g, 0.24 mmol), and H₂O (10 mL) was stirred for 30 min at room temperature. The pH value was then adjusted to about 3.9 using 1.0 M HCl. After transferred to a Teflon lined autoclave (25 mL) and kept at 120 °C for 4 days, the autoclave was slowly cooled to room temperature. Dark green green block crystals of complex 2 were filtered off, washed with distilled water, and dried in a desiccator at room temperature to give a yield of 22% based on Mo. IR (KBr pellet, cm⁻¹): 3305 (s), 1648 (s), 1548 (s), 1465 (m), 1202 (w), 954 (m), 874 (s), 698 (w), 646 (s), 558 (w).

X-ray crystallographic study

Crystallographic data for the title complexes were collected on a Bruker Smart APEX II diffractometer with Ka (λ = 0.71069 Å) by θ and ω scan mode at 296 K. All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL.¹³ The H atoms on the C and N atoms were fixed in calculated positions. However, the added protons and the H atoms from the water molecules and –OH groups are not located in their crystal structure analysis, but were directly included in the final molecular formula. The command “ISOR” was used to refine atoms C1, O4, O15 and O18 in 1. The command “DELU” was used to refine atoms Cr1 and O4 in 1. There are strong hydrogen bonds between O5 and O26 in 1, and the distances between them is reasonable for donor and acceptor atoms with strong hydrogen bonding interactions. Further details for crystallographic data and structures are listed in Table 1. The selected bond distances and bond angles are summarized in Table S1.†

Table 1 Crystal data and structure refinement for complexes 1 and 2

Complex	1	2
a R1 = Σ‖Fo| − |Fc‖/Σ|Fo|.b wR2 = Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]^1/2.
Empirical formula	C14H25CrCuMo6N4O27	C30H45CrCu2Mo6N8O32
Formula weight	1372.56	1784.46
Crystal system	Triclinic	Monoclinic
Space group	P1̄	C2/c
a (Å)	6.8870(17)	21.6870(13)
b (Å)	10.112(3)	22.2410(14)
c (Å)	22.292(6)	12.6300(8)
α (°)	92.269(5)	90
β (°)	94.582(4)	120.7550(10)
γ (°)	90.228(4)	90
V (Å^3)	1546.2(7)	5235.2(6)
Z	2	4
Dc (g cm^−3)	2.927	2.247
μ (mm^−1)	3.478	2.483
F (000)	1296	3432
Reflection collected	8080	13 224
Unique reflections	5809	6463
Parameters	478	359
Rint	0.0348	0.0206
GOF	1.063	1.039
R1^a [I > 2σ(I)]	0.0822	0.0224
wR2^b (all data)	0.2422	0.0576

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Results and discussion

Syntheses

In the process of hydrothermal synthesis, several factors can influence the formation of crystal phases, such as initial reactants, molar ratio, pH value, reaction time, temperature, etc. In this work, pH value of the reaction is crucial for the formation of the title complexes. Only at the pH value of 2.1–2.2 for 1 and in the pH value range from 3.8 to 4.3 for 2, the suitable crystals for single crystal X-ray diffraction can be obtained.

Description of crystal structures

The [CrMo₆(OH)₆O₁₈]³⁻ (CrMo₆) anions acting as inorganic building blocks adopt different coordination modes in complexes 1 and 2, which shows B-type Anderson structure made up of seven edge-sharing octahedra. Six of them are {MoO₆} octahedra arranged hexagonally around the central {Cr(OH)₆} octahedron. The Cr–O bond lengths are in the range of 1.951(6)–1.995(6) Å, while the O–Cr–O angles vary from 84.2(3) to 179.9(2)°. According to the coordination environments, there are four types of oxygen atoms existing in the POM unit: terminal oxygen Ot, terminal oxygen Ot′ linked to Cu ions, double-bridging oxygen Oa and central oxygen Ob. Thus, the Mo–O bond lengths fall into four types: Mo–Ot, 1.699(7)–1.704(6) Å; Mo–Ot′, 1.712(6)–1.719(6) Å; Mo–Oa, 1.895(6)–1.917(6) Å; and Mo–Ob, 2.288(6)–2.306(6) Å.

Bond valence sum calculations¹⁴ show that all of the Cu atoms are in +II oxidation state and Mo atoms are in +VI oxidation state in the title complexes (Table S2†). To balance the charge, a proton is added in 1, then compound 1 is formulated as H{CuL_0.5¹[CrMo₆(OH)₆O₁₈](H₂O)}·0.5L.¹⁵ While for 2, to balance the charge, [CrMo₆(OH)₆O₁₈]³⁻ is deprotonated, so 2 is formulated as {Cu₂(L²)₂[CrMo₆(OH)₅O₁₉](H₂O)₂}·2H₂O.¹⁶

Crystal structure of H{CuL_0.5¹[CrMo₆(OH)₆O₁₈](H₂O)}·0.5L¹ (1)

Single-crystal X-ray diffraction analysis reveals that complex 1 is a two-dimensional (2D) supramolecular structure based on one-dimensional (1D) infinite ladder-like chains constructed from the bidentate L¹ ligands, Cu^II ions and CrMo₆ anions. The asymmetric unit of complex 1 consists of one Cu^II ion, half a coordinated L¹ ligand, one CrMo₆ anion, one coordinated water molecule and half one free L¹ ligand. Each Cu^II ion is four-coordinated by one N atom of the pyridine group from one L¹ ligand with Cu–N bond distance of 1.999(13) Å, two O atoms from two CrMo₆ anions and one O atom from one coordinated water molecule with Cu–O bond distances of 1.891(11)–1.952(11) Å, showing a seesaw-like tetrahedral geometry (Fig. 1a and b). The bond angles around the Cu ions are 90.2(5)–150.5(5)° for O–Cu–N and 90.6(5)–165.4(6)° for O–Cu–O, respectively.

Fig. 1 (a) The coordination environment of Cu^II ion in 1. The hydrogen atoms and lattice water molecules are omitted for clarity. (b) The seesaw-like coordination mode of the Cu^II ion in 1. (c) The 1D Cu–CrMo₆ inorganic chain in 1. (d) The 1D ladder-like chain in 1. (e) Schematic view of the ladder-like 1D chain in 1.

The CrMo₆ anions acting as bidentate inorganic ligands connect with the adjacent Cu^II ions to form an inorganic Cu–CrMo₆ chain (Fig. 1c). The bidentate L¹ ligands exhibit a liner configuration bridging two Cu^II ions from the adjacent Cu–CrMo₆ chains to construct an infinite ladder-like chain (Fig. 1d), which shows “Z”-like conformation along a axis (Fig. 1e). The L¹ ligands acting as the crossbeam of the ladder link two Cu^II ions with Cu⋯Cu distance of 2.838 Å, and its two pyridyl rings are parallel to each other. A detailed structural analysis reveals that the adjacent 1D ladder-like chains are further connected by the uncoordinated L¹ ligands via N–H⋯O hydrogen bonds [N(4)–H(4A)⋯O(13), 2.919 Å], to form a two-dimensional (2D) supramolecular structure (Fig. S1†).

Crystal structure of {Cu₂(L²)₂[CrMo₆(OH)₅O₁₉](H₂O)₂}·2H₂O (2)

Single crystal X-ray diffraction analysis reveals that complex 2 is a 3D metal–organic framework based on the CrMo₆ polyoxoanions. The asymmetric unit of 2 consists of two crystallographically independent Cu^II ions, two L² ligands, one CrMo₆ cluster, two coordinated water molecules and two lattice water molecules. In 2, the coordination sphere of each Cu1 ion is composed of two pyridyl N atoms from two L² ligands, two O atoms from two CrMo₆ anions, and two O atoms from two coordinated water molecules, showing a distorted octahedral geometry (Fig. 2a). The bond lengths and angles around Cu1 ion is 2.020(3)–2.021(3) Å for Cu–N bond, 2.153(2)–2.187(3) Å for Cu–O bond, 170.91(16)° for N–Cu–N, 85.88(11)–88.40(11)° for N–Cu–O and 87.35(10)–170.65(11)° for O–Cu–O, respectively. Similar to the Cu1 ion, Cu2 ion also adopts a six-coordination mode, except that two coordinated water molecules were replaced by two O atoms from two CrMo₆ anions, respectively. The bond distances around Cu2 ion are 1.990(3) Å for Cu–N, 1.935(2)–2.498(3)(11) Å for Cu–O, and the bond angles are 92.06(17)° for N–Cu–N, 87.50(11)–178.92(11)° for N–Cu–O and 82.19(13)–164.94(11)° for O–Cu–O, respectively.

Fig. 2 (a) The coordination environment of the Cu^II ions in 2. The hydrogen atoms and lattice water molecules are omitted for clarity. (b) View of the 1D left- and right- handed helical [Cu–L²]_n chains in 2. (c) The 2D network of 2. (d) The 1D Cu–CrMo₆ chain in 2. (e) Schematic representation of the 3D framework of 2.

The L² ligands act as bidentate linkers to bridge the Cu ions, with a Cu⋯Cu distance of 15.414 Å and the dihedral angle of 34.83° between the two pyridyl rings, forming the [Cu–L²]_n left- and right-handed helical chains, which have never been reported in the Anderson-type POM-based complexes (Fig. 2b). The adjacent left-handed helical chains and right-handed helical chains are linked by CrMo₆ anions to form a 1D crossed chain, in which CrMo₆ anions provide two O atoms to coordinate with Cu1 ions. The adjacent 1D crossed chains are further connected by other CrMo₅^VIMo^V anions to construct a 2D network (Fig. 2c and S2†), in which the CrMo₆ anions provide four O atoms to coordinate with Cu2 ions. Thus, it is obvious that the CrMo₆ anion shows two different coordination modes in the 2D network (Fig. S2†).

The adjacent 2D networks are further connected by CrMo₆ anions to construct a 3D framework (Fig. S3†). That is, the CrMo₅^VIMo^V anions serving as the quadridentate ligands connect the Cu1 and Cu2 ions in a row, forming a Cu–CrMo₆ inorganic ladder-like chain, in which the CrMo₆ anions act as crossbeams (Fig. 2d). From the topological point of view, the L² ligands can be regarded as linear linkers, while the [(Cu1)(Cu2)CrMo₆] units can be regarded as four 4-connected nodes. Thus, the whole framework of 2 can be classified as a 4-connected framework with the 4¹².6³ topology (Fig. 2e).

Effect of the coordination modes of POMs and configuration of bis-pyridyl-bis-amide with different spacer length on structures of the title complexes

Complexes 1 and 2 represent the rare examples of Anderson-type POM-based MOCs constructed from flexible bis-pyridyl-bis-amide ligands. Obviously, the Anderson-type anions, Cu^II ions and bis-pyridyl-bis-amide ligands with different spacer lengths show different coordination modes and configuration in complexes 1 and 2 (Table 2), having noticeable effect on the final structures. There are two types of coordination geometries for the Cu centers in 1 and 2, respectively: the Cu1 ion in 1 adopts the four-coordinated “seesaw” geometry. While in 2, the Cu1 and Cu2 ions show six-coordinated distorted octahedral geometry. In complex 1, the CrMo₆ anions provided two terminal oxygen atoms to coordinate with two Cu^II ions, constructing a 1D Cu–CrMo₆ chain. While in 2, the CrMo₆ anions provided six terminal oxygen atoms to coordinate with four Cu^II ions, forming a 1D ladder-like Cu–CrMo₆ inorganic chain, which is different from that in 1. The 1D Cu–CrMo₆ inorganic chains connect with the [Cu–L²]_n left- and right-handed helical chains by sharing Cu^II ions, finally resulting in the 3D framework. The different coordination modes of the Anderson-type anions may be attributed to their different charge intensity.

Table 2 The coordination modes of ligands, Anderson-type anions and Cu^II ions in the title complexes

	Ligand	POM	Cu ion
Complex 1
Complex 2

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The flexible bis-pyridyl-bis-amide ligands with different spacer lengths have a significant effect on the architectures of the final complexes. In 1, the L¹ ligand possesses a shorter –(CH₂)₂– spacer and less flexibility, which makes it act as a linear liker to connect adjacent Cu–CrMo₆ chains forming a 1D ladder-like chain. While in 2, the L² ligand with the longer spacer –(CH₂)₃–compared with L¹ ligand has more flexibility and conformational freedom, which makes it generate [Cu–L²]_n left- and right-handed helical chains, which have never been reported in the Anderson-type POM-based complexes. These results strongly suggest that the coordination modes of POM anions and Cu^II ions, as well as the flexible bis-pyridyl-bis-amide ligands with different spacers have important influence on the structures of target complexes.

IR spectra

The IR spectra of complexes 1 and 2 are shown in Fig. S4† The characteristic bands at 559, 620, 914 cm⁻¹ for 1 and 558, 646, 874 cm⁻¹ for 2 are attributed to the ν(Mo–Ot), ν(Mo–Ot′), ν(Mo–Oa) and ν(Mo–Ob), respectively.¹⁷ The bands observed in the region of 1118, 1422, 1638 cm⁻¹ for 1 and 1202, 1465, 1648 cm⁻¹ for 2 are due to the bis-pyridyl-bis-amide ligands.¹⁸ The bands around 3300 cm⁻¹ for the title complexes can be attributed to the water molecules.

Thermal stability analysis

Thermogravimetric analyses (TGA) of the title complexes were performed in flowing N₂ atmosphere with a heating rate of 10 °C min⁻¹ from the room temperature to 800 °C (Fig. S5†). The TG curve of complex 1 shows two distinct weight loss stages. The first weight loss starting from 61 °C to 89 °C corresponds to the loss of one coordinated water molecule 1.37% (calcd: 1.31%). The second weight loss in the range of 137–742 °C is ascribed to the loss of organic molecules and decomposition of the polyoxoanion 22.50% (calcd: 22.56%). The TG curve of complex 2 shows similar weight loss stages to that of 1 (Fig. S5†), with first weight loss of 4.29% (calcd: 4.03%) from 65 °C to 127 °C, which can be ascribed to the loss of two coordinated water molecules and two lattice water molecules. The second weight loss of 32.72% (calcd: 33.11%) at 239–632 °C can be attributed to the decomposition of L² ligands and polyoxoanion. The residue may be attributed to a mixture of CuO, CrO₃ and MoO₃.¹⁹

Photocatalytic activity

Previous investigations indicated that some POM-based complexes could serve as photocatalytic materials in the green decomposition of organic dye pollutants under UV and visible light irradiation.²⁰ Wang's group reported two Anderson-type POM-based complexes {Mn(salen)₂(H₂O)₂[AlMo₆(OH)₆O₁₈]}·16H₂O and{Mn(salen)₂(H₂O)₂[CrMo₆(OH)₆O₁₈]}·11H₂O (salen = N,N′-ethylene-bis(salicylideneiminate)), which showed photocatalytic activity toward the degradation of rhodamine B (RhB) under UV irradiation,²¹ and a Wells-Dawson-type POM-based ionic crystal [WO₄{Ni(en)₂(H₂O)}₃][Ni(en)₃]{P₂W₁₈O₆₂}·[Ni(en)₃]CO₃·H₂O (en = ethanediamine), which exhibited photocatalytic activity for the degradation of methylene blue (MB) under visible light irradiation.²² Our group also have reported two Anderson-type POM-based metal-organic frameworks (MOFs) with remarkable photocatalytic activities for the degradation of MB under UV, visible light and sunlight irradiation.¹¹ However, reports about the excellent photocatalytic activity of POM-based MOCs towards the degradation of RhB under visible light are very scarce. Accordingly, herein, we selected two organic dyes MB and RhB as model pollutants to evaluate the photocatalytic effectiveness of the title complexes under UV light irradiation from a 125 W Hg lamp and visible light irradiation from a 350 W Xe lamp equipped with a long-pass filter (400 nm cut off), respectively. In this case, 100 mg powder of the title complexes were added into a 90 mL MB aqueous solution (10 mg L⁻¹) and RhB aqueous solution (5 mg L⁻¹) magnetically stirred in the dark. 5 mL transparent sample solution was taken out every certain interval and analyzed by a UV-visible spectrometer. The control experiments were also accomplished under the uniform conditions without title complexes.

The photocatalytic performance of the degradation of MB–RhB solutions under different conditions were investigated. The UV-vis spectra of the samples solution are related to the concentration of MB–RhB aqueous solution. As shown in Fig. 3, the absorption peaks of MB–RhB decreased obviously with increasing reaction time at the presence of complexes 1 and 2, respectively. In addition, the changes in the C/C₀ plot of the solutions versus irradiation time are shown in Fig. 4 (wherein C₀ is the initial concentration of the MB–RhB solutions and C is the concentration of the MB–RhB solutions after catalysis). The degradation ratios of MB and RhB under different conditions are listed in Table 3. Recently, Li's group synthetized a kind of N–TiO₂/g–C₃N₄ composite with enhanced visible light photocatalytic activities for the degradation of organic dyes, and the degradation rates of MB and RhB is about 99.00% and 99.42% within 90 min, respectively.²³ More recently, Wu's group prepared a hierarchical SnO₂ nanostructure, which was used as the efficient photocatalyst for the degradation of MB and RhB under UV irradiation, and the degradation ratios of MB and RhB is about 99.12% within 20 min and 97.62% within 60 min, respectively.²⁴ Compared with the reported compounds showing photocatalytic activity towards the degradation of dyes under UV or visible irradiation, our results indicate that the novel POM-based MOCs show good photocatalytic activities for the degradation of MB–RhB under both UV and visible irradiations.

Fig. 3 Absorption spectra of the MB–RhB solution during the decomposition reaction under UV and visible irradiation at the presence of complexes 1 and 2.

Fig. 4 Photocatalytic decomposition rates of MB solution under UV (a) and visible (c) irradiation, and RhB solution under UV (b) and visible (d) irradiation.

Table 3 Degradation ratios of MB and RhB under different conditions

Light irradiation		UV		Visible
Organic dye		MB	RhB	MB	RhB
Degradation ratios	Without catalyst	1.11%	1.31%	1.12%	1.15%
	Complex 1	89.41%	98.35%	82.63%	68.31%
	Complex 2	90.63%	99.93%	85.37%	92.21%
	N–TiO2/g–C3N4 composites^23			99.00%	99.42%
	Hierarchical SnO2 nanostructures^24	99.12%	97.62%

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Additionally, the PXRD patterns of the title complexes after the photocatalytic reactions have been carried out, which match with the simulated ones except for some intensity difference²⁵ (Fig. S6†). The results suggest that the title complexes possess good stability as photocatalysts for the photodegradation of MB and RhB contaminants during the heterogeneous catalytic reaction.

Conclusion

Two novel Anderson-type POM-based MOCs constructed from two flexible bis-pyridyl-bis-amide ligands with different spacer lengths have been hydrothermally synthesized. The coordination modes of Anderson-type polyoxoanions and the conformation of the bis-pyridyl-bis-amide ligands conduce to the diverse structures. Complexes 1 and 2 exhibit remarkable photocatalytic activities for the degradation of MB and RhB under UV and visible light irradiation, respectively, which not only further confirm the excellent photocatalytic activity of Anderson-type POM-based MOCs, but also can be used as prospective photocatalyst materials. Further work for preparing novel Anderson-type POM-based MOCs constructed from other flexible bis-pyridyl-bis-amide ligands is in progress.

Acknowledgements

Financial supports of the National Natural Science Foundation of China (21171025, 21471021), New Century Excellent Talents in University (NCET-09-0853), and Program of Innovative Research Team in University of Liaoning Province (LT2012020) are gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR Spectra, TG, and additional figures. CCDC 1026253 and 1026254. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra15608k

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