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Solvent-induced Mn(II)/Zn(II)/Co(II) organopolymolybdate compounds constructed by bis-pyridyl-bis-amide ligands through the Mo–N bond: synthesis, structures and properties† Na Xu, Ju-Wen Zhang,* Xiu-Li Wang,* Guo-Cheng Liu and Tian-Jiao Li Three transition metal organopolymolybdate hybrid compounds, namely, H2[Mn(H2O)4L3(γ-Mo8O26)]8H2O (1), H[M2(CH3O)(H2O)6L3(γ-Mo8O26)] [M = Zn (2) and Co (3)] [L = 1,4-bis(3-pyridinecarboxamido)benzene] have been synthesized under solvothermal conditions and characterized by IR spectroscopy, TG analysis, powder XRD and single-crystal X-ray diffraction. Compounds 1–3 were obtained by the onepot method, and the mixture of methanol and water with different ratios was used as the solvent. In compound 1, the γ-Mo8 anions were connected with pyridine groups of ligand L by the Mo–N bond, forming an uncommon 1D γ-Mo8–L chain. The adjacent chains were connected by [MnL2(H2O)4]2+ moieties through hydrogen bonding interaction to construct a 2D supramolecular network. Compounds 2 and 3 are isostructural, which show a 3D 2,4,6-connected {44·62}{44·66·84·10}{6} framework. The γ-Mo8 anions were connected by [M(H2O)2(CH3O)]+ [M = Zn (2) and Co (3)] subunits forming 1D M–Mo8 chains, which were connected by [ML2(H2O)4]2+ moieties to construct a 2D layer. In compounds 2 and 3, there also exist the same 1D γ-Mo8–L chains as in 1, which extended the 2D networks to 3D frameworks. The Mo–

Received 31st August 2015, Accepted 6th November 2015 DOI: 10.1039/c5dt03375f www.rsc.org/dalton

N bond with pyridyl groups was formed under the solvothermal conditions, which is scarcely reported to our knowledge. The effect of the solvent on the assembly of the title compounds and the formation of the Mo–N bond, as well as the role of metal–organic moieties on the construction of diverse organopolymolybdate compounds have been discussed in detail. Furthermore, the electrochemical and photocatalytic properties of 1–3 have been investigated.

Introduction Polyoxometalate (POM)-based inorganic–organic hybrid compounds have drawn extensive attention not only because of their abundant and intriguing structures but also due to their potential applications in catalysis, electrochemistry, magnetism, gas storage and others.1 There are many factors that can influence the assembly and final architectures of POM-based hybrid compounds, such as original reactants and their stoichiometry ratios, organic ligands, metal ions, solvents, pH value, temperature, etc.2 In particular, the choice of solvents is very important, because their structure and chemical properties can influence the rate of crystal growth and the final structures.3 Recently, an attractive branch in the POM

Department of Chemistry, Bohai University, Jinzhou, 121000, P. R. China. E-mail: [email protected]; Fax: +86-416-3400158; Tel: +86-416-3400158 † Electronic supplementary information (ESI) available: IR spectra, TG, and additional figures. CCDC 1411543–1411545. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt03375f

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field is the assembly of POM-based inorganic–organic hybrid compounds under solvothermal conditions.4 Ma and coworkers have reported a series of organic–inorganic hybrid materials constructed from Keggin-type POM clusters with methanol as the solvent.4a However, the examples based on solvent-induced synthesis of POM-based inorganic–organic hybrids are very rare, to our knowledge. Another remarkable branch in the POM field is the direct functionalization of isopolyanions with organic functional groups, which can improve the coordination ability of the isopolyanions with the metal atoms, and has attracted particular interest.5 So far, much more research studies are focused on the functionalization of Lindqvist-type POMs, especially the functionalization of a Mo6O192− anion with organoimido ligands under an anhydrous organic solvent.6 Peng, Wei and Xu’s groups have made contributions to this area.7 In these reports on organically derived Lindqvist-type POMs by Mo–N bonds, the N atoms are usually from the amino groups of organoimido ligands. Recently, several groups have also reported the direct functionalization of Mo8O264− anions through the

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Chart 1

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The semi-rigid L ligand used in this paper.

Mo–N bond, in which the organic ligands are imidazolyl derivatives, including bis(imidazole), bis(triazole) derivative ligands, etc.8 However, to our knowledge, there are scarce reports on the functionalization of isopolyanions through the Mo–N bond with the pyridyl groups of ligands. Considering that the isopolymolybdate anions may form organically derived POMs, as well as that solvents may show great effects on the assembly and structures of the POM-based complexes, in this work, we chose isopolymolybdates as building blocks and 1,4-bis(3-pyridinecarboxamido)benzene ligand L as linkers to assemble with transition metal ions under a methanol–water mixed solvent, aiming at investigating the feasibility of direct functionalization of isomolybdates with bis-pyridyl-bis-amide ligands under the solvothermal conditions, and exploring the influence of the mixed-solvent ratio on the assembly of the target compounds. Fortunately, three transition metal organopolymolybdate compounds have been successfully synthesized under the solvothermal conditions: H2[Mn(H2O)4L3(γ-Mo8O26)]·8H2O (1), H[Zn2(CH3O)(H2O)6L3(γ-Mo8O26)] (2) and H[Co2(CH3O)(H2O)6L3(γ-Mo8O26)] (3) [L = 1,4-bis(3-pyridinecarboxamido)benzene]. The electrochemical properties and the photocatalytic properties of 1–3 have been investigated. To our knowledge, the direct coordination of pyridyl groups from bis-pyridyl-bis-amide ligands to the Mo center has never been observed in the previous reports (Chart 1).

Experimental Materials and characterization All reagents and solvents for syntheses were purchased from commercial sources and were used as received. The ligand L was prepared according to the reported procedure.9 FT-IR spectra (KBr pellets) were recorded on a Varian 640 FT-IR spectrometer. Thermogravimetric analyses (TGA) were carried out under an N2 atmosphere on a Pyris Diamond TG instrument with a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) patterns were recorded on an Ultima IV with a D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα (λ = 1.5406 Å) radiation in the 2θ range of 5–50°. UV-Vis absorption spectra were obtained using a SP-1901 UV-Vis spectrophotometer. Electrochemical experiments were performed with a CHI 440 electrochemical workstation at room temperature. A conventional three-electrode system was used with a saturated calomel electrode (SCE) as the reference electrode, Pt wire as the counter electrode, and the title compound bulk-modified carbon paste electrodes (CPEs) as the working electrodes, respectively.

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Synthesis of H2[Mn(H2O)4L3(γ-Mo8O26)]·8H2O (1). MnCl2·4H2O (0. 20 g), L (0.015 g) and (NH4)6Mo7O24·4H2O (0.02 g) were dissolved in the mixed-solvent of 7 mL deionized water and 1 mL methanol (v : v = 7 : 1). The mixture was added in a 10 ml glass vial and heated at 85 °C for 4 days. After slowly cooling to room temperature, yellow block crystals of 1 were obtained and washed with distilled water, and dried in a desiccator at room temperature to give a yield of 32% based on Mn. IR (KBr pellet, cm−1): 3327 (m), 3172 (s), 2355 (m), 1674 (m), 1576 (m), 1539 (s), 1506 (s), 1397 (s), 1330 (s), 952 (m), 858 (m), 692 (w). Synthesis of H[Zn2(CH3O)(H2O)6L3(γ-Mo8O26)] (2). Compound 2 was prepared in the same way as 1 except that Zn(NO3)2·6H2O was used instead of MnCl2·4H2O and the solvent was a mixture of 4 mL deionized water and 4 mL methanol (v : v = 1 : 1). Yellow block crystals of 2 were obtained to give a yield of 25% based on Zn. IR (KBr pellet, cm−1): 3443 (s), 3187 (s), 2368 (m), 1654 (s), 1555 (m), 1512 (s), 1386 (m), 1312 (m), 941 (m), 884 (s), 642 (w). Synthesis of H[Co2(CH3O)(H2O)6L3(γ-Mo8O26)] (3). Compound 3 was prepared in the same way as 1 except that Co(NO3)2·6H2O was used instead of MnCl2·4H2O and the solvent was a mixture of 5 mL deionized water and 3 mL methanol (v : v = 5 : 3). Red block crystals of 3 were obtained to give a yield of 21% based on Co. IR (KBr pellet, cm−1): 3481 (s), 3284 (s), 2362 (w), 1651 (m), 1554 (m), 1514 (m), 1402 (m), 1312 (m), 938 (m), 884 (s), 690 (s). Preparation of 1-, 2- and 3-CPEs The compound 1 bulk-modified carbon paste electrode was fabricated by mixing compound 1 (0.01 g) and graphite powder (0.1 g) together in an agate mortar and grinding for about 30 min to achieve a uniform mixture, and then 0.05 mL paraffin oil was added with stirring. The homogenized mixture was packed into a glass tube with 3 mm inner diameter and the tube surface was wiped with weighing paper. The electrical contact was established with a copper wire through the back of the electrode. In a similar way, 2- and 3-CPEs were prepared with compounds 2 and 3, respectively. X-ray crystallography Crystallographic data for compounds 1–3 were collected on a Bruker SMART APEX II with Mo Kα (λ = 0.71073 Å) by ω and θ scan modes. The structures of the title compounds were solved by using direct methods and refined by full-matrix leastsquares using the SHELXL package.10 During refining, the crystal structures of all non-hydrogen atoms were refined anisotropically. The H atoms on the carbon atoms and nitrogen atoms were fixed in the calculated positions. Further details of crystallographic data and structures are listed in Table 1. The selected bond distances and bond angles of the three compounds are listed in Table S1 (ESI†). Crystallographic data for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC numbers 1411543–1411545.

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Table 1

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Crystal data and structure refinement for compounds 1–3

Compound

1

2

3

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dc (g cm−3) μ (mm−1) F (000) Reflection collected Unique reflections Rint GOF R1 a [I > 2σ(I)] wR2 b (all data)

C56H66MnMo8N12O44 2409.65 Triclinic ˉ P1 11.7431(10) 13.2703(10) 13.7193(11) 107.6610(10) 106.8970(10) 95.669(2) 1907.8(3) 1 2.097 1.538 1187 14 083 9498 0.0302 1.001 0.0447 0.1138

C56H57Mo8N12O39Zn2 2408.39 Triclinic ˉ P1 11.7091(11) 13.2967(11) 13.6131(11) 106.088(2) 106.707(2) 97.176(2) 1901.7 1 2.103 1.992 1179 10 975 6690 0.0354 1.030 0.0456 0.1293

C56H57Mo8N12O39Co2 2395.51 Triclinic ˉ P1 11.7021(8) 13.3591(9) 13.5639(9) 106.1960(10) 106.3980(10) 97.1570(10) 1905.3 1 2.088 1.795 1173 10 985 6701 0.0273 1.045 0.0416 0.1252

a

R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2.

Results and discussion Syntheses of the title compounds As is known, the solvent, as a significant factor in the assembly of target complexes, usually plays key roles in the reactions: (i) solvent as a ligand; (ii) solvent as a guest; (iii) solvent as both a ligand and a guest; and (iv) a structure-directing agent,11 which may not only affect the coordination environment of metal ions, but also change the connection mode of organic ligands.12 So the choice of the solvent is very important. In our original experiments, all the compounds were designed to be synthesized under hydrothermal conditions, unfortunately, we could only get an unidentified sediment rather than crystals. However, when we chose a mixture of water and methanol to replace the aqueous solution system, the title compounds 1–3 with different structures were obtained. In order to explore the role of methanol on the synthesis of the title compounds, we have chosen different ratios of CH3OH/H2O mixed solvents. However, when we used methanol as a single solvent at the process of the reaction, we could only get some unidentified sediments. Only when we used different ratios of CH3OH/H2O mixed solvents, we were able to obtain compounds 1–3. In addition, the yield of 1–3 increased with the increasing ratio of water and methanol (Scheme 1). At a ratio of 7 : 1 (v : v) for H2O/CH3OH, the yield of crystalline 1 reaches the highest value of 32%. Compound 2 can be obtained in the highest yield of 25% at the ratio of 1 : 1 (v : v) for H2O/CH3OH, and compound 3 shows the highest yield of 21% at a ratio of 5 : 3 (v : v) for H2O/CH3OH. Based on X-ray structure analysis of compound 1, there is not any methanol molecule in the final structure, so we speculated that methanol might act as a structure-directing

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Scheme 1 1–3.

Experimental conditions for the syntheses of compounds

agent in the formation of compound 1. In compounds 2 and 3, we could find clearly that methanol molecules as ligands coordinated with the center metal ions. So the solvents have a great influence on the synthesis of POM-based hybrid compounds, which may be the essential factor for the structural construction. Thus, the syntheses of 1–3 can be considered as solvent-controlled processes, in which the products can be synthesized and modified in different ratios of mixed solvents. In addition, we have used various transition metal salts with different cations (Cu2+, Ni2+, Co2+, Zn2+, Mn2+) and anions (Cl−, NO3−, SO42− and CH3COO−) to explore cations and anions on the formation and structures of 1–3 under the solvothermal conditions. Unfortunately, when the cations or anions were altered, we could not obtain any crystals but only the precipitate. The results indicate that both the metal cations and anions show obvious effects on the synthesis of compounds 1–3.

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Description of crystal structures Crystal structure of 1. Compound 1 crystallizes in the triˉ space group with the asymmetric unit consisting of clinic P1 one Mn(II), three L ligands, one γ-Mo8O264− anion (abbreviated as γ-Mo8), four coordinated water molecules, and eight lattice water molecules. The bond valence sum calculations suggest that13 all the Mo atoms of γ-Mo8 anions are in the +VI oxidation state, while the Mn atoms are in the +II oxidation state. To balance the charges of the compound, two protons are added.14 As shown in Fig. 1, the Mn ion shows a distorted octahedral coordination geometry completed by two N atoms of the pyridine groups from two L ligands with the Mn–N bond distance of 2.299(5) Å, and four O atoms from four coordinated water molecules with the Mn–O bond distances of 2.153(4)–2.202(4) Å. The bond angles around the Mn ions are 88.43(14)–180° for O–Mn–O, 86.07(17)–93.93(17)° for N–Mn–O, and 180° for N–Mn–N, respectively. Remarkably, one N donor (N2, Fig. 1) of the L ligand directly links the γ-Mo8 anion through the Mo3–N2 bond with a distance of 2.273(4) Å.8d The semi-rigid bis-pyridyl-bis-amide ligands have two N donors as potential coordination sites. In compound 1, the L ligand exhibits two kinds of coordination modes (Table 2):

(i) all the terminal N donors are fully utilized acting as a bidentate linker to bridge two Mo atoms of the γ-Mo8 anions and (ii) it only uses one apical N donor to coordinate with one Mn ion, acting as a monodentate organic ligand. As shown in Fig. 2a, the adjacent γ-Mo8 anions were connected by the ligand L through the Mo–N bond to form a 1D γ-Mo8–L chain. The 1D γ-Mo8–L chains are connected by the [MnL2(H2O)4]2+ moieties through O(2W)–H(2WA)⋯O(11) hydrogen bonds (Table S2†) to construct a 2D supramolecular network (Fig. 2b). Crystal structures of 2 and 3. Single-crystal X-ray diffraction analyses reveal that compounds 2 and 3 are isostructural, and the unit cell dimensions, volumes, related bond distances and angles are only slightly different. So compound 2 has been taken as an example to describe the structure of the two compounds. Compound 2 also crystallizes in the triclinic space ˉ. As shown in Fig. 3, the asymmetric unit of comgroup P1 pound 2 consists of two ZnII ions, one γ-Mo8O264− anion, three L ligands, one coordinated methanol molecule, and six coordinated water molecules. The bond valence sum calculations suggest that8 all the Mo atoms of Mo8 anions are in the +VI oxidation state, while the Zn atoms are in the +II oxidation state. To balance the charge of the compound, one proton was added.9

Fig. 1 Stick/ball view of the asymmetric unit of 1. The hydrogen atoms are omitted for clarity. Symmetry code: #1 − x − 2, −y + 2, −z.

Table 2 Coordination numbers and modes of γ-Mo8 anions, metal ions and L in compounds 1–3

γ-Mo8 anions 1

Metal ions

L Fig. 2 (a) A view of the 1D Mo8–L chain in compound 1. (b) View of the 2D supramolecular network through hydrogen-bonding interactions in 1.

2

3

Fig. 3 Stick/ball view of the asymmetric unit of 2. The hydrogen atoms are omitted for clarity. Symmetry code: #2 − x − 1,−y, −z + 1.

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In compound 2, there are two crystallographically independent zinc ions (Zn1 and Zn2, Table 2). The Zn1 ion is sixcoordinated with one N atom from an L ligand, two O atoms (O3W and O4W) from two water molecules, one O atom (O11) from one methanol molecule and the other two terminal O atoms (O9 and O10) from one γ-Mo8 anion. The bond distances around the Zn1 ion are Zn(1)–O(11) = 2.064(14) Å, Zn(1)–O(3W) = 2.014(12) Å, Zn(1)–O(4W) = 2.113(13) Å, Zn(1)– O(10) = 2.557(8) Å, Zn(1)–O(9) = 2.167 (6) Å, and Zn(1)–N(6) = 2.225(13) Å and the angles around the Zn1 ion are 69.1(8)– 165.4(5)° for O–Zn–O, and 89.8(3)–99.7(4)° for N–Zn–O. The Zn2 ion is six-coordinated by two N atoms from two L ligands and four O atoms four water molecules. The Zn–O bond distances are 2.144(6) Å for Zn(1)–O(1W) and 2.101(5) Å for Zn(1)– O(2W), the Zn–N distance is 2.188(7) Å, and the angles around the Zn2 ion are 89.5(2)–180° for O–Zn–O, 87.8(3)–92.2(3)° for O–Zn–N, and 180° for N–Zn–N, respectively. In compound 2, all the L ligands acting as bidentate bridging ligands exhibit two types of coordination modes (Table 2): (i) two N donors of the L ligand link two Zn ions in a cis-configuration; and (ii) two N donors link two Mo ions from the γ-Mo8 anion through the Mo–N bond of 2.273(4) Å in a trans-configuration resulting in a 1D γ-Mo8–L chain.10 The linking roles of L ligands in compound 2 enhance the stability of the whole framework. Furthermore, there exists a [Zn(H2O)2(CH3O)]+ subunit. The γ-Mo8 anion provides two O atoms to coordinate with the Zn1 atoms of this subunit to construct a 1D Zn–Mo8 chain (Fig. 4a). The adjacent 1D chains are connected by the [ZnL2(H2O)4]2+ moieties forming 2D networks, which are further extended by the 1D γ-Mo8–L chains to a 3D metal–organic framework (Fig. S1†). To comprehend the structure better, if we consider the L ligand as the connector, and the γ-Mo8 anion, Zn1 and Zn2 ions are viewed as four-, six- and two-connected nodes respect-

Fig. 4 (a) View of the 1D Zn2–Mo8 chain in compound 2. (b) The 2D layer of compound 2. (c) Representation of the {44·62}{44·66·84·10}{6} framework of compound 2 (green ball: Mo8, yellow ball: Zn).

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ively, a 3D {44·62}{44·66·84·10}{6} topology of 2 is formed, as shown in Fig. 4c. The role of the solvent and the metal–organic moiety in the construction of organopolymolybdate compounds It is well-known that octamolybdate (Mo8) possesses eight isomers (α, β, γ, δ, ε, ζ, η and θ) due to the different numbers of [MoO4], [MoO5] and [MoO6] polyhedra.15 It is noted that the γ-Mo8 anion has two [MoO5] and six [MoO6] polyhedra. The [MoO5] polyhedron shows a tetragonal pyramid geometry, in which the Mo atom is five-coordinated. It is easy for this Mo atom to coordinate with another atom in order to form the more stable octahedron coordination mode (Fig. S2†). So the γ-Mo8 anion possesses the unique merits to construct organopolymolybdate compounds through the Mo–N bond. Su’s group and Li’s group have constructed a series of organopolymolybdate compounds through the Mo–N bond under the hydrothermal conditions, in which the N atoms of the Mo–N bonds are mainly from imidazolyl of the organic ligands, such as 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine and 4-amino-3,5bis(3/4-pyridyl)-1,2,4-triazole.8a–c However, the pyridyl groups have never been used for the formation of the Mo–N bond. In this work, we obtained three transition metal organopolymolybdate compounds with bis-pyridyl-bis-amide ligands, in which the γ-Mo8 anions connect with the pyridyl groups of the semi-rigid bis-pyridyl-bis-amide L by the Mo–N bond forming the 1D γ-Mo8–L chains. In our previous reports, we have employed bis-pyridyl-bis-amide ligands to assemble with octamolybdate and transition metal ions under the hydrothermal conditions, and obtained two compounds by utilizing the octamolybdate cluster as an inorganic building block and choosing two flexible bis-pyridyl-bis-amide ligands with different spacer lengths as the organic moiety, in which no Mo–N bond was formed.16 In this work, the mixture of methanol and water was used as the solvent, the Mo–N bond was formed in compounds 1–3. The results indicate that the solvents play an important role in the formation of the Mo–N bond and construction of novel architectures. It should be noted that the metal–organic moiety in compounds 1–3 plays different linkage roles. In compound 1, the adjacent 1D γ-Mo8–L chains are connected by [MnL2(H2O)4]2+ moieties through the hydrogen bonding interaction to form a 2D supramolecular layer. In compounds 2 and 3, there exist [Zn(H2O)2(CH3O)]+ and [Co(H2O)2(CH3O)]+ subunits, which connect γ-Mo8 anions to form 1D Zn–Mo8 and Co–Mo8 chains, respectively. The adjacent 1D Zn–Mo8 and Co–Mo8 chains are linked by [ZnL2(H2O)4]2+ and [CoL2(H2O)4]2+ moieties to form 2D layers, which are further extended by the γ-Mo8–L chains to 3D frameworks. Thus, the different linkage roles of the metal– organic moieties resulted in different coordination modes of the γ-Mo8 anions (Table 2) and various structures of the title compounds. FT-IR spectra The IR spectra of compounds 1–3 are shown in Fig. S5.† In the spectra, the characteristic bands at 692, 858, 952 cm−1 for 1,

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642, 884, 941 cm−1 for 2 and 690, 884, 938 cm−1 for 3 are attributed to ν(MovOt) and ν(Mo–O–Mo) of the Mo8O264− anion,17 respectively. The bands in the region of 1330–1674 cm−1 for 1, 1312–1654 cm−1 for 2, and 1312–1651 cm−1 for 3 can be assigned to the L ligand.18 The bands around 3300 cm−1 are due to the water molecules.

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Powder X-ray diffraction The PXRD patterns indicate that the experimental peaks of the synthesized compounds 1–3 match well with the simulated ones except for some intensity differences, which can be owed to the different orientation of the crystals in the powder samples (Fig. S6†), proving the crystalline phase purity. Thermal stability analysis Thermogravimetric analyses (TGA) of compounds 1–3 were performed under a flowing N2 atmosphere with a heating rate of 10 °C min−1 from room temperature to 800 °C (Fig. S7†). The TG curve of compound 1 shows two weight loss steps. The first weight loss step from room temperature to 220 °C corresponds to the loss of water molecules and methanol molecules by 9.18% (calcd: 8.98%). The second weight loss at 320–730 °C is ascribed to the loss of organic molecules by 40.42% (calcd: 39.59%). For compound 2, six water molecules and one methanol molecules are released successively from the room temperature to 200 °C with the weight loss of 8.96% (calcd: 5.98%). The next weight loss of 39.78% from 310 °C to 700 °C is ascribed to the decomposition of three L molecules (calcd 39.61%). The TG curve of compound 3 shows two distinct weight loss steps. The first weight loss step below 190 °C corresponds to the loss of six water molecules and two methanol molecules, 6.36% (calcd 6.01%). The second weight loss step from 300 °C to 670 °C ascribes to the loss of three L molecules, 42.05% (calcd 39.82%).

Fig. 5 (a) Cyclic voltammograms of 1-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution at different scan rates (from the inner to the outer: 40, 60, 80, 100, 120, 140, 160, 180, 200, 250, 300, 350, 400, 450, 500 mV s−1). The inset shows the relationship of the redox peak currents II–II’ versus the scan rates; (b) 1-CPE in a 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution containing 0.0–6.0 mM H2O2 (scan rate: 40 mV s−1).

Electrochemical properties of compounds 1–3 Compounds 1–3 are insoluble in water and common organic solvents. Thus, the 1–3 bulk-modified CPEs become the optimal choice to study the electrochemical properties of these compounds. The electrochemical behaviors of compounds 1–3 are similar (Fig. 5 and S8†), so the 1-CPE cases have been taken as an example to investigate the electrochemical properties. Fig. 5a shows the cyclic voltammogram behaviors at a potential range from 600 to −450 mV for 1-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution at different scan rates. It can be clearly seen that 1-CPE shows three reversible redox peaks I–I′, II–II′ and III–III′, which can be attributed to the redox of Mo8 anions.19 The mean peak potentials E1/2 = (Epa + Epc)/2 are + 178 (I–I′), + 25 (II–II′) and −174 mV (III–III′) (40 mV s−1) for 1-CPE. The peak potentials change gradually following the scan rates from 40 to 500 mV s−1. With the scan rates increasing, the cathodic peak potentials shifted to the negative direction and the corresponding anodic peak potentials toward the positive direction. The peak (II–II′) currents are proportional to the scan rates (inset of Fig. 5a), which indicates that the redox process of 1-CPE is surface-controlled.20

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Fig. 5b exhibits the cyclic voltammograms for the electrocatalytic reduction of H2O2 at 1-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution. With the addition of H2O2, the second and the third reduction peak currents gradually increase, while the corresponding oxidation peak currents decrease, which suggests that H2O2 can be electrocatalytically reduced by the 4- and 6-electron reduced Mo8 species. In addition, we also investigated the electrocatalytic behaviors of Mo8-CPE and L-CPE as the working electrodes under the same conditions, as shown in Fig. S9a and S10a.† Compared with 1-CPE, Mo8-CPE exhibited similar electrochemical behaviors and electrocatalytic activities to that of 1-CPE except for the different mean peak potentials of three reversible redox peaks (I–I′, II–II′, and III–III′), which can be attributed to the combination of metal–organic ligand cations.7d As shown in Fig. S10b,† the reduction peak currents and the corresponding oxidation peak currents are almost unchanged at L-CPE with the addition of H2O2. This indicates that the ligand L has almost little effect on the electrocatalytic activity for the

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reduction of H2O2. The above results further indicated that the Mo8 anions maintained their electrochemical and electrocatalytic activities in the title compounds.

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Photocatalytic activity Photocatalytic degradation of organic dyes is one of the effective methods to eliminate pollutants. In order to investigate the photocatalytic activities of compounds 1–3 as catalysts, photodecomposition of methylene blue (MB) as a model organic pollutant was performed under UV irradiation using a 125 W Hg lamp. In the process of the photocatalytic reactions, 150 mg of the sample was dispersed in 90 mL aqueous solu-

Fig. 6 Absorption spectra of the MB solution during the decomposition reaction under UV irradiation in the presence of compounds 1 (a); 2 (b) and 3 (c).

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tion of MB (10 mg L−1) under magnetic stirring for about 30 min in the dark in order to rule out the effect of its absorption on the particle surfaces. Then, every 30 min, 5.0 mL samples were taken out for analysis by UV measurement. In addition, we further studied the photocatalytic properties of the title compounds in 90 mL aqueous solution of MB (10 mg L−1) under visible light irradiation using a 350 W Xe lamp, and the samples of 5 mL were taken out every certain time for UV measurement. As shown in Fig. 6, the absorption peak of MB decreased gradually with increasing reaction time for compounds 1–3 under UV irradiation. The changes in C/C0 of MB solutions versus the reaction time of compounds 1–3 are plotted in Fig. 7. After 180 min, the calculation results show that the degradation ratios reach approximately 52.92% for 1, 67.17% for 2 and 63.47% for 3 under UV irradiation, respectively (Fig. 7). As shown in Fig. S11,† the absorption peak of MB also decreased with increasing reaction time for compounds 1–3 under visible light irradiation. The changes in C/C0 of MB solutions versus the reaction time for the title compounds are plotted in Fig. S12.† After 180 min, the degradation efficiency is 25.59% for 1, 31.04% for 2 and 28.26% for 3 under visible light irradiation, respectively (Fig. S12†). The results indicate compounds 1–3 show a good photocatalytic efficiency for the degradation of MB under UV irradiation. PXRD patterns of compounds 1–3 after the photocatalytic reactions have been recorded, which match well with the simulated ones in the peak position (Fig. S6†). The results suggest that compounds 1–3 also possess good stability as photocatalysts for the photodegradation of the MB contaminant. In addition, photodegradation of MB catalyzed by the parent POMs was performed under UV irradiation in order to compare with the title compounds (Fig. S13a†). After 180 min, the degradation ratio of MB is 30.13% for Mo8 (Fig. S13b†), which is lower than those of the title compounds as catalysts. So compounds 1–3 can improve the photocatalytic activity of the parent POMs toward the degradation of MB.

Fig. 7 Photocatalytic decomposition rate of the MB solution under UV irradiation with the use of compounds 1–3.

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Conclusions Three metal organopolymolybdate hybrid compounds have been solvothermally synthesized. The mixed-solvent system plays an important role in the formation of the Mo–N bond with pyridyl groups, which represents the first example of organopolymolybdate hybrid compounds through the Mo–N bond from bis-pyridyl-bis-amide ligands. In addition, different ratios of water and methanol show great effects on the formation and yield of the final compounds. Compounds 1–3 show good electrocatalytic activity toward the reduction of H2O2 and exhibit photocatalytic activity for the degradation of MB under UV irradiation. This work may provide a new strategy for the construction of novel organopolymolybdate hybrid compounds with various pyridyl-based derivatives.

Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21171025, 21471021 and 21201021) and the Program of Innovative Research Team in the University (LT2012020).

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