Nonclassical Phosphomolybdates with Different Degrees of Reduction

Article pubs.acs.org/IC

Nonclassical Phosphomolybdates with Different Degrees of Reduction: Syntheses and Structural and Photo/Electrocatalytic Properties Zhao-yi Chen,†,‡ Jing-hua Lü,† Kai Yu,*,†,‡ He Zhang,†,‡ Lu Wang,§ Chun-mei Wang,†,‡ and Bai-bin Zhou*,†,‡ †

Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education and ‡Key Laboratory of Synthesis of Functional Materials and Green Catalysis, College of Heilongjiang Province, School of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, P. R. China § Department of Biochemical Engineering, Harbin Institute of Technology, Harbin 150090, China S Supporting Information *

ABSTRACT: Four nonclassical phosphomolybdates, formulated as (H2pytty)8[{Mn(H2pytty)(H2O)3}{Sr⊂P6Mo6V Mo12VIO73}]2· 16H2O (1), [{Mn(H3pytty)(H2O)3}2{Sr⊂P6Mo4VMo14VIO73}]· 18H2O (2), (H3pytp) (H2pytty)2[{Fe(H2O)4}{Sr⊂P 6 Mo 3 V Mo 15 VI O 73 }]·5H 2 O (3), and (H 2 pytty) 2 [{Cd(H2O)4}{Cd(H2O)3 (H3pytty)}{Sr⊂P6Mo5VMo13VIO73}]·9H2O (4) (pytty = 3-(pyrazin-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl; pytp = 4′-(4″-pyridyl)-2,4′:6′,4″-terpyridine) were hydrothermally synthesized and fully characterized. The penta- and hexa-reduced basket clusters represent the highest reduced level of basket-based polyoxometalate so far. In addition, transition metal complexes as bridge units were introduced to basket system for the first time to induce rare two-dimensional inorganic−organic hybrid layer. The results indicate that reduced degrees of the basket clusters of compounds 1−4 can be tuned by altering the molar ratio of organic ligand pytty and Na2MoO4. Compounds 1−4 exhibit bifunctional electrocatalytic behaviors for oxidation of dopamine and reduction of H2O2. Electrocatalytic mechanism, chronoamperometric experiments and electrocatalytic stability are studied in detail. In addition, the compounds show highly efficient catalytic ability for the degradation of typical dyes under UV irradiation.

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assemblies are based on classical POMs, such as the Keggin-type6 and the Wells−Dawson heteropoly acid,7 and their derivatives have widely been investigated. Compared with classical POMs, the unclassical POM compounds have been far unexplored for their lability in solution.8,9 In particular, the basketlike {P6Mo18O73} cages are still less common. Since basketlike [H2dmpip]5[K⊂P6Mo18O73] POM cluster was first synthesized by Zhang group in 2004,9a some organic−inorganic hybrids based on basketlike POMs and transition metal/transition metal complexes were reported by our group.9b,d,e However, only a handful of {P6Mo18O73}-based compounds have been reported so far, and most of them are zero-dimensional (0D) cluster.9 As a result, it is valuable for the rational synthesis of extented materials by using basket anions as the building blocks to extend basket cluster from 0D to one-dimensional (1D) and two-dimensional (2D) architectures and further investigate their structural diversities and properties.10

INTRODUCTION Polyoxometalate (POM)-based extended assemblies are ubiquitous materials whose diversity of composition and structure are manifested in intriguing physical and chemical properties bestowing their potential applications in catalysis,1 gas storage and adsorption,2 electromagnetic functional materials,3 biomedicine,4 and photochemistry.5 The synthesis of new POMbased extended assemblies, such as chains, nets, and open frameworks also play an important role in the design of new materials with novel electronic, magnetic, and topological properties. Generally, one effective approach for fabrication of novel POM-based extended assemblies is the incorporation of POM building blocks and various linking units such as transition metal complexes, organic ligand, and different metal ions. The introduction of organic ligands and transition metals into POMs can enrich their potential applications, such as catalysis, nonlinear optics, and electrical conductivity.6,7 Organic components can dramatically influence microstructure of hybrid materials. In addition, the synergic interactions between organic and inorganic components may exploit composite properties or even new characters. Until now, most of POM-based extended © XXXX American Chemical Society

Received: January 21, 2016

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DOI: 10.1021/acs.inorgchem.6b00158 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

measurement was performed on a CHI 660 electrochemical workstation at room temperature (25−30 °C). Synthesis of (H2pytty)8[{Mn(H2pytty)(H2O)3}{Sr⊂P6Mo6VMo12VIO73}]2·16H2O (1). A mixture of Na2MoO4·2H2O (1.210 g, 5.00 mmol), Mn(CH3COO)2·4H2O (0.599g, 3.00 mmol), pytty (0.543 g, 2.50 mmol), H3PO4 (1 mL, 15 mmol), SrCl2·6H2O (0.815 g, 3.06 mmol), and H2O (27 mL, 1.5 mol) was stirred at room temperature for 30 min; then the pH value was ajusted to ∼3.5 with 1 M NaOH, and it was sealed in a 50 mL Teflon-lined stainless steel reactor, which was heated at 160 °C for 4 d. The dark blue crystals were isolated and collected by filtration, washed thoroughly with distilled water, and dried at room temperature (Yield: 42% based on Mo). Anal. Calcd for C80H108Mn2Mo36N80O168P12Sr2 (8989.07): C, 10.69; H, 1.21; N, 12.47; P, 4.14; Mn, 1.22; Mo, 38.42; Sr, 1.95. Found: C, 10.67; H, 1.22; N, 12.46; P, 4.15; Mn, 1.21; Mo, 38.43; Sr, 1.96%. IR (KBr pellet, cm−1): 3402 (br), 3212 (sh), 1624 (s), 1489 (s), 1048 (s), 962 (s), 864 (s), 748 (s), 663 (s), 543(s). Synthesis of [{Mn(H3pytty)(H2O)3}2{Sr⊂P6Mo4VMo14VIO73}]·18H2O (2). A mixture of Na2MoO4·2H2O (1.210 g, 5.00 mmol), Mn(CH3COO)2·4H2O (0.599g, 3.00 mmol), pytty (0.326 g, 1.50 mmol), H3PO4 (1 mL, 15 mmol), SrCl2·6H2O (0.815 g, 3.06 mmol), and H2O (27 mL, 1.5 mol) was stirred at room temperature for 30 min; then the pH value was ajusted to ∼3.5 with 1 M NaOH, and it was sealed in a 50 mL Teflon-lined stainless steel reactor, which was heated at 160 °C for 4 d. The dark blue crystals were isolated and collected by filtration, washed thoroughly with distilled water, and dried at room temperature (Yield: 46% based on Mo). Anal. Calcd for C16H64Mn2Mo18N16O97P6Sr (4143.07): C, 4.64; H, 1.56; Mn, 2.65; Mo, 41.68; N, 5.41; P, 4.49; Sr, 2.11. Found: C, 4.67; H, 1.53; Mn, 2.70; Mo, 41.62; N, 5.45; P, 4.45; Sr, 2.10%. IR (KBr pellet, cm−1): 3413 (br), 3082 (w), 1632 (s), 1513 (s), 1042 (s), 942 (s), 858 (s), 756 (s), 638 (s), 541 (m). Synthesis of (H3pytz)(H2pytty)2[{Fe(H2O)4}{Sr⊂P6Mo3VMo15VIO73}]· 5H2O (3). A mixture of Na2MoO4·2H2O (1.210 g, 5.00 mmol), FeSO4· 7H2O (0.834 g, 3.00 mmol), pytp (0.110 g, 0.50 mmol), pytty (0.217 g, 1.00 mmol), H3PO4(1 mL, 15 mmol), and SrCl2·6H2O (0.802 g, 3.01 mmol), and H2O (27 mL, 1.5 mol) was stirred at room temperature for 30 min; then the pH value was ajusted to ∼3.5 with 1 M NaOH, and it was sealed in a 50 mL Teflon-lined stainless steel reactor, which was heated at 160 °C for 4 d. The dark blue crystals were isolated and collected by filtration, washed thoroughly with distilled water, and dried at room temperature (Yield: 48% based on Mo). Anal. Calcd for C36H46FeMo18N20O82P6Sr (Mr = 4127.14): C, 10.48; H, 1.12; N, 6.79; P, 4.50; Fe, 1.35; Mo, 41.84; Sr, 2.12. Found: C, 10.49; H, 1.11; N, 6.81; P, 4.49; Fe, 1.36; Mo, 41.83; Sr 2.11%. IR (KBr pellet, cm−1): 3456 (br), 3146 (br), 1637 (s), 1491 (s), 1041 (m), 964 (s), 842 (s), 751 (s), 622 (s), 511 (s). Synthesis of (H 2 pytty) 2 [{Cd(H 2 O) 4 }{Cd(H 2 O) 3 (H 2 pytty)}{Sr⊂P6Mo4VMo14VIO73}]·9H2O (4). A mixture of Na2MoO4·2H2O (1.210 g, 5.00 mmol), Cd(CH3COO)2·4H2O (0.747 g, 3.00 mmol), pytty (0.434 g, 2.0 mmol), H3PO4(1 mL, 15 mmol), SrCl2·6H2O (0.817 g, 3.06 mmol), and H2O (27 mL, 1.5 mol) was stirred at room temperature for 30 min; then the pH value was adjusted to ∼3.5 with 1 M NaOH, and it was sealed in a 50 mL Teflon-lined stainless steel reactor, which was heated at 160 °C for 4 d. The dark blue crystals were isolated and collected by filtration, washed thoroughly with distilled water, and dried at room temperature (Yield: 56% based on Mo). Anal. Calcd for C48H94Cd4Mo36N48O171P12Sr2 (8530.07): C, 6.76; H, 1.09; N, 7.88; P, 4.36; Cd, 5.27; Mo, 40.50; Sr, 2.05. Found: C, 6.75; H, 1.08; N, 7.89; P, 4.38; Cd, 5.26; Mo, 40.49; Sr, 2.06%. IR (KBr pellet, cm−1): 3412 (br), 3041 (s), 1621 (s), 1536 (m), 1074 (s), 968 (s), 853 (s), 762 (s), 631 (s), 520 (w). X-ray Crystallography. Single-crystal X-ray data of compounds 1− 4 were collected on a Bruker SMART CCD diffractometer equipped with graphite monochromatized Mo Kα radiation (λ = 0.710 73 Å). Semiempirical absorption corrections were applied using the SADABS program. The structure was solved by direct methods and refined by the full-matrix least-squares method on F2 using the SHELXTL-97 software package.11 All of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon atoms of organic ligands were included at calculated positions and refined with a riding model. The H atoms on

In fact, the basketlike {P6Mo18O73} unit is one of the ideal building blocks to construct novel POM-based extended arrays: (1) the compounds are mixed-valence molybdates, which are obtained by two or more electron reductions from the corresponding MoVI of polyoxoanions or molybdate. The mixed-valence Mo centers lead to the high negative charges of the whole clusters, which can induce more metal linkage units into the crystal frames and lead to plenty of structural topologies. (2) The calculations of geometry and studies of the density functional theory show that basket-shape heteropolyanion is a possible multidentate ligand to coordinate metal ions via external terminal phosphate−oxygen and handle−molybdate−oxygen sites.9c (3) The larger steric hindrance of basketlike POMs might be overcome if suitable ligands and metal cations are introduced into basket system under right reaction conditions. In addition, the basketlike POMs have significant absorption band in both UV and visible light region due to the introduction of transition metal and the reduction of molybdenum in framework, so they may be good photocatalyst. However, N-donor ligands play an important role in adjusting the structures of POM-based extended assemblies. They are structure-directing agents in the construction of {P6Mo18}-based assemblies: they can always act as reducing agents to reduce MoVI into MoV centers.10 Furthermore, they can induce the {P6Mo18}based inorganic fragments to form different dimensions or various packing arrangements in the final hybrid materials.10 Recently, 3-(pyrazin-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl (pytty) has attracted our attention, as it has more coordination sites than imidazole ligands. As mutidentate ligands, triazole and pyrazine rings can connect with the transition metal via various coordination modes, which not only can make POM frameworks robust but also have the potential to form π-electron system. Thus, pytty ligands are good candidates for the construction of {P6Mo18}-based extended hybrids. On the basis of previous work,9,10 we manage to utilize rigid ligand pytty to extend basket POMs with diverse degrees of reduction by altering the molar ratio of organic ligand and Na2MoO4, which leads to four basketlike compounds, namely, (H2pytty)8[{Mn(H2pytty)(H2O)3}{Sr⊂P6Mo6VMo12VI O73}]2· 16H2O(1),[{Mn(H3pytty)(H2O)3}2{Sr⊂P6Mo4VMo14VIO73}]· 18H2O (2), (H3pytz)(H2pytty)2 [{Fe(H2O)4}{Sr⊂P6Mo3VMo15VIO73}]·5H2O (3), and (H2pytty)2[{Cd(H 2 O) 4 }{Cd(H 2 O) 3 (H 2 pytty)}{Sr⊂P 6 Mo 4 V Mo 14 VIO 73 }]· 9H2O (4). Electrochemical and photocatalytic properties of 1−4 were investigated in detail.

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EXPERIMENTAL SECTION

Materials and Measurements. All chemicals were commercially purchased and used without further purification. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 CHN elemental analyzer; P, Mo, Fe, Mn, Cd, and Sr were analyzed on a PLASMA-SPEC (I) ICP atomic emission spectrometer. IR spectrum was recorded in the range of 400−4000 cm−1 on an Alpha Centaurt FT/IR Spectrophotometer using KBr pellets. X-ray photoelectron spectrum (XPS) analyses were performed on a VG ESCALAB MK II spectrometer with a Mg Kα (1253.6 eV) achromatic X-ray source. Thermogravimetric (TG) analyses were performed on a Perkin−Elmer TGA7 instrument in flowing O2 with a heating rate of 10 °C·min−1. X-ray diffraction patterns were collected on a Rigaku Dmax 2000 X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 0.154 nm) and 2θ ranging from 5 to 50°. UV−vis−NIR absorption spectroscopy was measured with a Cary 500 spectro-photometer. The electrochemical B


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1, 2, 3, and 4 compound formula Mr crystal size, mm3 crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V (Å3) Z Dcalcd, kg m−3 μ(Mo Kα), mm−1 F(000), e θ range, deg reflections collected/unique/ Rint data/restraints/parameters R1/wR2 [I ≥ 2σ(I)]a GOF (F2)a Δρfin (max/min), e Å−3 a

1 C80H108Mn2Mo36N80O168P12Sr2 8989.07 0.26 × 0.24× 0.20 triclinic P1̅ 16.645(7) 18.268(8) 23.747(10) 89.543(7) 74.209(7) 76.897(7) 6756(5) 1 2.209 2.270 4310 1.15−27.40 38 975/28 585/0.0328

2 C16H64Mn2Mo18N16O97P6Sr 4143.07 0.28 × 0.26× 0.22 monoclinic C2/c 27.6058(11) 26.5475(11) 17.5600(7) 90.00 90.3020(10) 90.00 12 868.9(9) 4 2.138 2.468 7928 1.06−28.34 53 033/16 044/0.0499

3 C36H46FeMo18N20O82P6Sr 4127.14 0.28 × 0.24× 0.22 monoclinic P2(1)/n 16.8296(9) 34.5086(19) 18.2360(10) 90.00 90.6960(10) 90.00 10 590.1(10) 4 2.589 2.895 7872 1.18−25.00 53 068/18 632/0.1497

4 C48H94Cd4Mo36N48O171P12Sr2 8530.07 0.28 × 0.24 × 0.20 monoclinic P2(1)/n 18.271(6) 14.132(5) 40.470(13) 90.00 95.797(7) 90.00 10 396(6) 2 2.725 3.216 8092 1.01−25.04 65 555/18 333/0.0389

28 585/57/1714 0.0796/0.1031 1.055 2.581/−2.397

16 044/46/705 0.0819/0.1249 1.038 2.340/−2.525

18 632/48/1477 0.0780/0.1105 1.106 2.574/−2.874

18 333/59/1415 0.0659/0.1265 1.083 2.286/−2.610

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = {Rw[(Fo)2 − (Fc)2]2/Rw[(Fo)2]2}1/2.

water molecules were not included and were just put into the final molecular formula. A summary of crystal data and structure refinement for compounds 1−4 was provided in Table 1. Selected bond lengths and angles of compounds 1−4 are listed in Tables S1−S4. Additional crystallographic information is available in the Supporting Information. Photodegradation Properties Measurements. Photocatalytic properties of the samples were examined in conventional processes. Typically, compounds 1−4 (50 mg) were placed with 200 mL of methylene blue (MB), methyl orange (MO), and Azon Phloxine (AP; 10 mg·L−1) solution in a tubular quartz reactor and stirred in the dark for 30 min to ensure the adsorption equilibrium before irradiation. Then, the solution was stirred while being irradiated by the surrounding UV lamp of 125 W. At certain intervals, 5 mL of samples were taken out of the reactor and separated by centrifuge to remove suspended particles. Photodegradation of the separated sample was investigated by measuring the absorption spectra of the solution using a UV−vis spectro-photometer.

Scheme 1. Schematic View of the Relationship between Reduction Degrees of Basket-Shaped POMs and the Molar Ratio of pytty and Na2MoO4

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RESULTS AND DISCUSSION Syntheses. The organic ligands play important roles in the formation of mixed valence {P6Mo18O73}-based POMs. They are essential reducing agents to reduce MoVI into MoV centers. The dosages of the ligand and the molybdenum source will directly affect the reduced degree of basket cluster. The basket-based POMs with diverse reduction degrees have different bonding activity and active position. Thus, the dosages of the ligand and the molybdenum source can further affect degree of modification, ligand number, different dimensions, and various packing arrangements of the final basket hybrid. To investigate the above relationship, the self-assembly hydrothermal reactions of Na2MoO4·2H2O, H3PO4, TM2+, SrCl2·6H2O, H2O, and rigid organic ligands pytty were performed under the same pH, reaction temperature, and time. Compounds 1−4 were synthesized by altering the molar ratio of organic ligand and Na2MoO4 of the reaction system (Scheme 1). At the beginning, when the molar ratio of pytty to Na2MoO4 is 4:5, compound 1 (a hexa-electron-reduced monosupported hybrid by {Mn-

(H2pytty)(H2O)3} fragments) was prepared. The pentaelectron-reduced compound 4 exhibiting the 2D sheet structure was formed at molar ratio of pytty/Na2MoO4 = 3:5. The tetraelectron-reduced compound 2 exhibiting biarmed discrete cluster was obtained at molar ratio of pytty/Na2MoO4 = 2:5, and the trielectron-reduced 1D chain 3 was obtained at molar ratio of pytty/Na2MoO4 = 1:5. In addition, only the powder product was obtained when the molar ratio of pytty/Na2MoO4 was larger than 4:5, whereas the solid product was not obtained, as the molar ratio was lower than 1:5. Parallel experiments reveal C


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Inorganic Chemistry

Figure 1. (a) The basic building unit of compound 1. (b) The 1D chain of compound 1 based on {P6Mo18} units and {Mn(H2pytty)(H2O)3} fragments via supramolecular interactions. (c) The 2D supramolecular layer linked by protonated pytty in compound 1. (d) The 3D supramolecular network on the ac plane of compound 1.

Mo−O bond angles varying from 85.0(3) to 173.9(4)°. The six P centers display the tetrahedral coordination geometry. The P−O bond lengths are in the range of 1.48(3)−1.697(8) Å, while the O−P−O bond angles vary from 63.7(6) to 160.0(12)°. The bond lengths of Sr−O are in the range of 2.534(12)−2.894(12) Å (Tables S1−S4). Crystal Structures of 1. Compound 1 is hexa-electronreduced monosupported cluster, which represents the highest reduced degree of basket-based POM so far. The basic unit of 1 is built of one {Sr⊂P6Mo6VMo12VIO73}6− cluster, a {Mn(H2pytty)(H2O)3} fragment, four protonated pytty, and eight lattice water molecules (Figure 1a and Figure S2). There is one crystallographically independent Mn2+ cation that forms a distorted octahedral configuration with two terminal oxygen atoms from one external phosphates and one molybdates of the handle of the “basket”, a nitrogen donor from pytty ligands (Mn(1)−N(16) = 2.342(15) Å), and three oxygen atoms from coordinated water molecules. The Mn−O distances are in the range of 2.145(10)− 2.261(12) Å. The N−Mn−O and O−Mn−O angles are in the range of 87.8(4)−166.1(5)° and 84.5(5)−168.1(6)°, respectively. On the basis of such connection mode, each manganese complex {Mn(H2pytty)(H2O)3} is bonded to two terminal oxygen atoms of basketlike anion to form a monosupported structure. Two adjacent single-arm polyoxoanions are linked by water molecules via weak interaction to form dimeric cluster (Figure 1a). The adjacent dimeric units are further aggregated together to yield infinite 1D dimeric chains via supramolecular interaction (O55···O40 2.482 Å; Figure 1b). The adjacent dimeric chains are parallel with each other and bonded together by protonated pytty ligands to form 2D supramolecular layers along the ac plane via supramolecular interaction (N38···O72 2.969 Å and N38···O59 2.868 Å; Figure 1c). The threedimensional (3D) supramolecular framework (Figure 1d) was generated by hydrogen bond interactions (O5W···O57 3.40(2) Å and O5W···N15 2.86(3) Å shown in Table S5) between different 2D layers. Crystal Structures of 2. Compound 2 represents four-electron reduced bisupported structure when reducing molar ratio of pytty/Na2MoO4 = 2:5 under similar conditions. The basic unit of

that the molar ratio of pytty to Na2MoO4 varying in the range of 1:5−4:5 favors the formation of mixed-valence basket structure. Furthermore, in the range, the larger dosage of ligand is conducive to obtaining basket clusters with higher reduced level. Clearly, the reduction degree of basket-shaped POM can be regulated by adjusting molar ratio of the organic ligand and the molybdenum source. It is worth noting that the formation of compound 3 relies on the presence of the second ligand pytp, which plays the important induction role for the assembly of the 1D basket chains, and a series of similar reaction but without pytp ligands resulted in simple [P6Mo18]-based discrete cluster without any modification. In addition, it is unsuccessful for our attempts to use one transition metal (TM) to induce compounds 1−4, which shows that the kinds of TM also affect the formation of these compounds. Although four basket-shaped phosphomolybdate with diverse level of reduction are formed via altering the molar ratio of organic ligand and Na2MoO4 of the reaction system, the systematic exploration of the {TM/L/P6Mo18} system by using the same organic agent and TM centers under the same hydrothermal condition still remains a great challenge to us. Structure Descriptions. X-ray diffraction analysis reveals that the structures of compounds 1−4 comprise reduced basketlike {P6Mo18O73} polyoxoanion, which is composed of two parts: a tetravacant γ-Dawson-type {P2Mo14} unit and a handle-shaped {P4Mo4} segment (Figure S1). The {P2Mo14} block is derived from the γ-Dawson anion by removal of four adjacent {MoO6} octahedral of belt region to form the “body” of the basket. The {P4Mo4} unit consists of four {MoO6} octahedra and four {PO4} tetrahedra, which are corner-linked together to form the “handle” of the basket. The two parts are linked via the edge- and/or corner-sharing modes to form a cagelike structure with C2v symmetry, in which a Sr2+ cation is encapsulated in the central cavity. It is proved that the cavity size of the basketlike polyoxoanion inclined to capture larger alkali metals or alkaline earth cations.9a,b In fact, the incorporation of these large cations may, vice versa, stabilize the whole cryptand polyoxoanion. All Mo centers exhibit a hexa-coordination environment with Mo− O bond lengths in the range of 1.663(13)−2.463(8) Å and O− D


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Inorganic Chemistry

Figure 2. (a) The basic building unit of compound 2. (b) The 2D supramolecular layer in compound 2. (c) The 3D supramolecular network in ABAB mode on the ac plane of compound 2.

Figure 3. (a) The basic building unit of compound 3. (b) The 1D wavelike chain of compound 3 based on {P6Mo18} units and {Fe(H2O)4} linkers via hydrogen bonds interactions. (c) The 2D supramolecular layer in compound 3. (d) The 3D supramolecular network on the ab plane of compound 3.

compound 2 consists of a {Sr⊂P6Mo4VMo14VIO73}10− polyoxoanion modified by two {Mn(H3pytty)(H2O)3} fragments and 18 lattice water molecules (Figure 2a and Figure S3). There is one crystallographically independent Mn2+ cation. Each Mn(1) cation possess six-coordinated octahedral configuration, which is defined by two nitrogen donors from pytty ligands, three oxygen donors from coordinated water molecules, and a terminal oxygen atom from one molybdate of basketlike polyoxoanion. The bond lengths of Mn−N and Mn−O are in the ranges of 2.09(2)−2.42(2) and 1.93(3)−2.42(2) Å, respectively. Two {Mn(H3pytty)(H2O)3} complexes are coordinated with a terminal molybdate−oxygen atom from two symmetrical “body” positions of basket to form bisupported structure. In the above connection mode, each bisupported basket cluster connects four same coplanar clusters by {Mn(H3pytty)(H2O)3} linkers via supramolecular interaction (O39···O30 3.015 Å) between water molecules of {Mn(H3pytty)(H2O)3} groups and oxygen atoms of polyoxoanion, forming 2D supramolecular layer (Figure 2b). The adjacent layers are bonded together to form infinite 3D supramolecular frameworks in ABAB mode via supramolecular interactions (C5···O34 3.074 Å and C3···O21

3.186 Å) between water molecules of {Mn(H3pytty)(H2O)3} groups and oxygen atoms of polyoxoanion (Figure 2c). Crystal Structures of 3. The basic units of compound 3 consist of a tri-electron-reduced polyoxoanion {Sr⊂P6Mo3VMo15VIO73} cluster, one {Fe(H2O)4} linker, two protonated H2pytty ligands, one H3pytz ligand, and five lattice water molecules (Figure S4). There is one crystallographically independent Fe2+ cation that forms a distorted octahedral configuration with two terminal oxygen atoms from external phosphates of two adjacent basketlike clusters and four oxygen donors from coordinated water molecules. The bonds distances of Fe−O are in the range of 1.925(13)−2.273(14) Å. The angles of O−Fe−O are in the range of 93.4(7)−174.4(7)°. Each basket cluster bonds to adjacent two basket units in ABAB mode via {Fe(H2O)4}2+ fragment to form infinite 1D wavelike chains (Figure 3). The adjacent wavelike chains are parallel with each other and are further aggregated together to yield a 2D supramolecular layer via protonated H2pytty ligands with hydrogen bond interactions (N9···O33 3.248(11) Å and N8··· O62 3.030(12) Å; Figure 3c). The 3D supramolecular framework (Figure 3d) was generated by hydrogen bonds and E


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Inorganic Chemistry

Figure 4. (a) The coordination pattern of {P6Mo18O73} cluster of compound 4. (b) The 1D zigzag-type chain of compound 4 based on {P6Mo18} units and {Cd(H2O)3(H2pytty)} linkers. (c) The 2D supramolecular layer of compound 4 linked by {Cd(H2O)4} linkers and {Cd(H2O)3(H2pytty)} complex segments. (d) The 3D supramolecular network on the ac plane of compound 4.

alternately bridged by two {Cd(H2O)4} linkers and two {Cd(H2O)3(H2pytty)} complexes to form a big aperture (Figure S6) with the dimensions of 8.30 × 15.23 Å (Cd(1)−Cd(2) distances 8.30 and 15.23 Å). Moreover, the 3D supramolecular framework (Figure 4d) was generated by supramolecular interaction (O45···O32 2.953 Å) between surface oxygens of adjacent 2D layers. All solvent water molecules and protonated ligands reside in the interspaces between two adjacent layers. Bond-valence sum (BVS) calculations12 average value of ca. 5.65 for Mo centers in compound 1, which is close to the result of 12 Mo6+ and six Mo5+ in the polyoxoanion cluster. BVS results are ∼5.80 and 5.84 for Mo centers of compounds 2 and 3, respectively, indicating that there are four and three Mo5+ in their basket cluster. BVS calculations display average value of ca. 5.74 for Mo centers in compound 4, which is about the result of 13 Mo6+ and 5 Mo5+ in each polyoxoanion (Tables S6−S9). In addition, BVS calculations confirm that all P and Sr centers are in the oxidation states of +5 and +2, respectively. BVS calculations also show that all transition metal centers in compounds 1−4 are in the oxidation states of +2. BVS indicates that water ligands linked to the TM centers (BVS values comprise in the range of 0.26−0.41). In addition, two or three extra protons should be added to the organic agents in compounds 1−4, for the charge balance. Spectroscopic and Thermal Analyses. Infrared Spectra Analyses. The IR spectra of compounds 1−4 recorded at 400− 4000 cm−1 with KBr pellet (see the Supporting Information, Figure S7a−d) are similar: the peaks at 1074−1041 cm−1 are attributed to v(P−Oa) vibrations; the strong peaks at 968−942 cm−1 are ascribed to v(Mo = Oterminal) vibrations; peaks at 864−748 cm−1 are assigned to v(Mo−Obridge) vibrations. The peaks located at 511−663 cm−1 can be attributed to ν(TM− O).13a The strong peaks at 1489−1536 cm−1 are indicative of v(C−N) vibrations of organic ligands. Furthermore, the broad

supramolecular interactions between water molecular and oxygen atoms of polyoxoanion in different 2D layers (O3W··· O35 3.02(2) Å, O3W···O70 2.615(19) Å, and O1W···O70 2.866(17) Å, O1W···O48 3.064(18)). Crystal Structures of 4. Compound 4 exhibits 2D layer structure linked by the {Cd(H 2O) 4} linkers and {Cd(H2O)3(H2pytty)} complex segments. The asymmetric unit of compound 4 contains a penta-electron-reduced {Sr⊂P6Mo5VMo13VIO73} polyoxoanion, one {Cd(H2O)4} linker, one cadmium complex {Cd(H2O)3(H2pytty)} unit, two protonated H2pytty ligands, and nine water molecules (Figure S5). There are two kinds of crystallographically independent Cd atoms, which exhibit octahedron coordination geometry. Cd1 atom completes its hexa-coordinated environment by four terminal oxygen atoms of two external phosphates and two molybdates of the handle of the basket from two different basket clusters, two oxygen donors from coordinated water molecules. Cd2 cation possesses distorted octahedral configuration, which is defined by two nitrogen donors from one pytty ligand, two terminal oxygen atoms of a molybdate donor, and an external phosphate from two adjacent basketlike polyoxoanion, and two oxygen donors from coordinated water molecules (Figure S5 and Figure 4a). The bond lengths of Cd−N and Cd−O are in the range of 2.342(13)−2.399(11) and 2.184(11)−2.357(8) Å, respectively. Each basketlike anion connects with two adjacent basket-type clusters through two cadmium complex {Cd(H2O)3(H2pytty)} fragments to generate infinite 1D zigzagtype chains (Figure 4b). The adjacent chains are further bonded together by {Cd(H2O)4} linkers to form 2D layer (Figure 4c). The overall structure of compound 4 may be described that each basketlike anion links with four adjacent basket-type clusters through two {Cd(H 2 O) 4 } linkers and two {Cd(H2O)3(H2pytty)} complex segments to yield tetraconnected layer structures. In the structure, four basketlike clusters are F


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Inorganic Chemistry peaks between 3041 and 3456 cm−1 and the peaks at 1621−1637 cm−1 can be assigned to v(N−H) and/or v(O−H) of the protonated ligands and water molecules. X-ray Photoelectron Spectra Analyses. The oxidation states of Mo are further confirmed by XPS measurements, which were performed in the energy region of Mo 3d5/2 and Mo 3d3/2 (Figure S8a−d). The XPS spectrum of compounds 1−4 presents four overlapped peaks at 230.6−231.3, 234.0−234.2, 232.4− 232.6, and 235.0−235.3 eV in the Mo 3d region, which should be ascribed to the mixture of Mo5+ and Mo6+, respectively.13b The XPS results also support the BVS calculations for the Mo oxidation states, in which the deconvolutions of the spectra indicate that the ratio of MoVI/MoV is ∼2:1 for 1, 7:2 for 2, 5:1 for 3, and 13:5 for 4. Powder X-ray Diffraction Analysis. The simulated and experimental X-ray powder diffraction (XRPD) patterns of the compounds 1−4 are presented in Figure S9a−d. The diffraction peaks of both simulated and experimental patterns match in the key positions, indicating the phase purity of the compounds. The difference in intensity may be due to the preferred orientation of the powder samples. Thermogravimetric Analysis. The thermal stabilities of compounds 1−4 were investigated under an O2 atmosphere from 40 to 800 °C, and the TG curves are showed in Figure S10a−d. In TG curve of compound 1, the first weight loss of 4.25% (calcd 4.41%) at 150−300 °C corresponds to the loss of 16 lattice water and six coordinated water molecules. The second weight loss between 475 and 700 °C of 24.32% arises from the loss of protonated H2bytty ligands (calcd 23.94%, for 10 bth). For 2, the first weight loss of 9.92% at 180−320 °C is due to the loss of all lattice water molecules and coordinated water molecules (calcd 10.44% for 24 H2O). The second weight loss of 10.02% at 480−640 °C arises from the loss of all organic ligands. The value is close to the calculated value of 10.45% (for 2 H2bytty). For 3, the first weight loss of 3.99% at 150−280 °C corresponds to the loss of discrete water and coordinated water molecules (calcd 3.93%, for 9 H2O). The second weight loss between 380 and 630 °C of 17.82% relates from the evacuation of all pytp and bytty molecules (calcd 17.95%, for 1 H3pytp and 2 H2bytty). For 4, the first weight loss of 5.78% at 150−320 °C is due to the loss of all water molecules in the compound (calcd 6.74% for 16 H2O). The second weight loss between 490 and 720 °C of 15.02% arises from the loss of protonated H2bytty ligands (calcd 15.14%, for 3 H2bth). All the weight losses from the TG curves accord with the formulas of compounds 1−4. Ultraviolet Spectra Analyses. The UV−vis absorption spectra of compounds 1−4 were conducted in solid state at room temperature (shown in Figure S11a−d). The two strong bands in the ranges of 225−242 and 302−338 nm are attributed to pπ(Oterminal)→dπ*(Mo) electronic transitions in the MoO bonds and dπ−pπ−dπ electronic transitions between the energetic levels of the Mo−O−Mo bonds, respectively.13a The broad band at 622−655 nm is ascribed not only to the overlap of two bands of the intervalence transition from the MoV to MoVI via the Mo−O−Mo bond but also to the d−d transitions of MoV octahedral,13a which result in the dark blue coloration of these compounds. Electrochemical and Electrocatalytic Properties. Cyclic Voltammetric Behaviors of 1−4 Carbon Paste Electrodes. The electrochemical behaviors of compounds 1−4 were investigated with 1−4-modified carbon paste electrodes (1−4-CPE), for their insolubility in water or acidic aqueous solution.14a,b The cyclic voltammograms (CV) of 1−4-CPEs in 0.1 M H2SO4 aqueous

solution at different scan rates and 20 mV·s−1 were recorded (shown in Figure S12 and Figure 5). The CV of modified CPE

Figure 5. Cyclic voltammograms of 1−5-CPEs in 0.1 M H2SO4 solution. Scan rate: 20 mVs−1.

for similar basketlike molecular salt (H 2 pa) 3 (Hpa) 4 {Sr⊂P6Mo4VMo14VIO73}·11H2O (pa = α-pyridylamine) were also performed (abbreviated as 5-CPE) under identical conditions. It is necessary to make a comparison of CV measurements between 1−4-CPEs and 5-CPE (CPE of basket POM without modification of TM or TM-pytty) to identify redox processes of TM2+/TM or Mo6+/Mo5+. As shown in Figure 5, the five compounds exhibit similar cyclic voltammetric behaviors in certain potential range. There are three reversible redox peaks with close half-wave potentials E1/2 = (Epa + Epc)/2 at +0.31, +0.04, and −0.28 V for 1, +0.38, +0.08, and −0.21 V for 2, +0.41, +0.09, and −0.23 V for 3, +0.30, +0.02, and −0.31 V for 4, +0.32, +0.03, and −0.20 V for 5, respectively (based on the CV at 20 mV·s−1), which are ascribed to three consecutive two-electron processes of Mo centers in each basket-based {SrP6Mo18O73} polyoxoanion framework.9,10 Thus, no redox signal of transition metal cation TM2+ is detected. The cathodic peak potentials of 1−4 shift toward the negative direction, and the corresponding anodic peak potentials shift to the positive direction with increasing scan rates, as shown in Figure S13. The peak potentials change gradually following the scan rates from 20 to 160 mV·s−1. Furthermore, the peak-to-peak separations between the corresponding anodic and cathodic peaks increased, but the average peak potentials do not change on the whole. In plots of anodic peak current (II) versus scan rates (see inset plots in Figure S12), the peak currents were proportional to the scan rate, indicating that the redox processes of the 1−4-CPEs are surface-controlled.15,16 G


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Inorganic Chemistry

Figure 6. Cyclic voltammograms of (a) 1-, (b) 2-, (c) 3-, and (d) 4-CPE in 0.1 M H2SO4 solution in the presence of added concentrations of H2O2: black, 0; red, 0.5; green, 1.0; blue, 1.5; and light blue, 2.0 mM. (insets) The linear dependence of the anodic current of peak I for 1−4-CPE with H2O2 concentration. Potentials vs saturated calomel electrode. Scan rate: 60 mV·s−1 for 1−4-CPE.

Bifunctional Electrocatalytic Activities of 1−4 Carbon Paste Electrodes. The electrocatalytic reduction of H2O2 by 1−4-CPEs was also investigated. As shown in Figure 6, in the potential range from +1.0 to −1.0 V, with addition of H2O2, the I′ and II′ reduction peak currents of increase, while the corresponding I and II oxidation peak currents decrease for 1−4-CPEs, which suggests that reduction of H2O2 is mainly mediated by the I−I′ and II−II′ processes. As shown in inset plot, the nearly equal current steps for each addition of H2O2 demonstrate stable and efficient electrocatalytic activity of 1−4-CPE. The reaction mechanism of 1−4-CPE toward H2O2 can be described by the following equations: Electrochemical reactions:17

H4SrP6Mo VI(14 ‐ n)Mo V (n + 4)O73(6 + n) − + H 2O2 ⇌ H 2SrP6Mo VI(16 ‐ n)Mo V (n + 2)O73(6 + n) − + 2H 2O

However, dopamine (DA) is an important neurotransmitter, and the changes of its content can lead to some diseases, such as heart trouble, Parkinson’s disease, neuromuscular disorders, and a variety of diseases.18,19 Thus, it is significant to detect the DA by electrochemical method. 1−4-CPEs also exhibit the catalytic oxidation ability toward the DA. As shown in Figure 7, the oxidation and reduction peak currents of the three Mo-wave (III−III′) both increase accompanying the addition of increasing amounts of DA; however, the extent of enhancement of oxidation peak currents is far bigger than the reduction peak currents. This fact suggests that III−III′ couples of 1−4-CPEs are efficient toward the oxidation of DA. The nearly equal current steps for each addition of DA also demonstrate stable and efficient electrocatalytic activities of 1−4-CPE. All these results indicate that 1−4-CPEs have bifunctional electrocatalytic activities toward not only reduction of normal inorganic molecules H2O2 but also oxidation of biological molecules DA. The reaction mechanism of 1−4-CPE toward DA can be described by the following equations: Electrochemical reactions:20

SrP6Mo VI(18 ‐ n)Mo V nO73(6 + n) − + 2H+ + 2e ⇌ H 2SrP6Mo VI(16 ‐ n)Mo V (n + 2)O73(6 + n) − H 2SrP6Mo VI(16 ‐ n)Mo V (n + 2)O73(6 + n) − + 2H+ + 2e ⇌ H4SrP6Mo VI(14 ‐ n)Mo V (n + 4)O73(6 + n) − H4SrP6Mo VI(14 ‐ n)Mo V (n + 4)O73(6 + n) − + 2H+ + 2e ⇌ H6SrP6Mo VI(12 ‐ n)Mo V (n + 6)O73(6 + n) −

Catalytic chemical steps:

H 2SrP6Mo VI(16 ‐ n)Mo V (n + 2)O73(6 + n) −

H6SrP6Mo VI(12 ‐ n)Mo V (n + 6)O73(6 + n) − + H 2O2

⇌ SrP6Mo VI(18 ‐ n)Mo V nO73(6 + n) − + 2H+ + 2e

⇌ H4SrP6Mo VI(14 ‐ n)Mo V (n + 4)O73(6 + n) − + 2H 2O

Catalytic chemical steps: H


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Inorganic Chemistry

Figure 7. Cyclic voltammograms of (a) 1-, (b) 2-, (c) 3-, and (d) 4-CPE in 0.1 M H2SO4 solution in the presence of added concentrations of DA: black, 0; red, 0.5; green, 1.0; blue, 1.5; and light blue, 2.0 mM. (insets) The catalytic current of peak I vs DA concentration. Potentials vs saturated calomel electrode. Scan rate: 60 mV·s−1.

Figure 8. (a) Chart of the CAT vs concentration of H2O2 for 1−4-CPE. (b) Chart of the CAT vs concentration of DA for 1−4-CPE. (Ip values of cathodic peak I for H2O2 and anodic peak III for DA at a scan rate of 50 mV·s−1).

CAT = 100 × [I p(POM, substrate) − I p(POM)] /I p(POM)]

where Ip(POM, substrate) and Ip(POM) are the peak currents for the reduction of the POM with and without the presence of substrate (H2O2, DA), respectively. As shown in Figure 8, 1−4 CPEs show similar electrocatalytic efficiencies for H2 O 2 reduction, which indicate that basketlike POMs with different reduction degrees have universal excellent electrocatalytic activities for reduction of H2O2. The electrocatalytic efficiencies of 1−4-CPE are in the order of 1-CPE > 4-CPE > 2-CPE > 3CPE for DA oxidation. The results indicate that the catalytic

The electrocatalytic efficiency (CAT) of 1−4-CPEs can be calculated by using the CAT21 formula I


Article

Inorganic Chemistry

Figure 9. Plot of irradiation time vs concentration for MB in the presence of compounds 1−4 under UV irradiation. (insets) Absorption spectra of the MB solution during the decomposition reaction under UV irradiation in the presence of compounds (a) 1, (b) 2, (c) 3, and (d) 4.

Figure 10. UV−vis absorption spectra of the (a) MO and (b) AP solutions during the decomposition reaction under UV irradiation in the presence of compound 4. Plot of irradiation time vs concentration for (c) MO and (d) AP in the presence of compound 4 under UV irradiation.

constant potential of 0.6 V (Figure S13). With the addition of DA (2.0 mM), significant current decreases are produced in the experimental time of 0.25 s, and electrocatalytic current differences were 50, 22, 11, and 33 mA for 1−4-CPE, respectively. The order of the current differences is in accord

activities were enhanced with the increasing of reduction extent of the anion. Compared with 2−4-CPEs, compound 1 with the highest reduction degree shows the highest CAT value (453.48%) toward oxidation of DA at 2.0 mM DA (Figure 8). Chrono-amperometric experiments with and without presence of DA in 0.1 M H2SO4 solutions for 1−4-CPEs were studied at J


Article

Inorganic Chemistry

based POM so far. In addition, transition metal complexes as bridge units were introduced to basket system for the first time to induce rare 2D inorganic−organic hybrid layer. The compounds indicate good bifunctional electrocatalytic behavior for oxidation of DA and reduction of H2O2. It is promising to use them as a new style of electrochemical sensor. In addition, compounds 1− 4 as photocatalysts exhibit universal highly efficient degradation ability for typical dyes such as MB, MO, AP, etc. The successful synthesis of compounds 1−4 to some extent provides useful prototype for further research on 3D high-connected basketbased hybrid compounds with diverse reduction degrees.

with the performance order of electrocatalytic DA oxidations by 1−4-CPE (1-CPE > 4-CPE > 2-CPE > 3-CPE).22 Electrocatalytic stability toward the oxidation of DA for 1−4CPE were investigated by scanning 40 cycles of CV measurement in 0.1 M H2SO4 solutions with concentration of 2.0 mM DA at the scan rate of 80 mV·s−1. As shown in Figure S14, the CVs remained almost unchanged after 40 cycles. The results indicate that 1−4-CPEs exhibit better electrocatalysis stability, and the electrocatalytic activities of 1−4-CPEs for oxidation of DA were maintained during the repeating catalytic experiments. It is promising to use them as a new style of electrochemical sensor. Photocatalysis Properties of Compounds 1−4. To study the photodegradation performance of the compounds, the MB as a typical organic pollutant in printing and dyeing wastewater was used for target molecules to evaluate the degradation ability of compounds 1−4 under UV irradiation. It can be clearly observed that the absorption peaks of MB decreased obviously with increasing reaction time (Figure 9). As shown in Figure 9, the photocatalytic decomposition rate, defined as 1 − C/C0, increased from 5% (without catalysts) to 96.22% for 1, 95.93% for 2, 96.60% for 3, and 98.48% for 4 after 28 min of irradiation. Obviously, all compounds show excellent photocatalytic activities for the degradation of MB in short time. Compared with reported other hybrid POMs, the basketlike POMs exhibit better degradation ability for MB dyes in shorter time under similar reaction condition.23 For example, only 63%, 94%, and 96.52% of MB were decomposed by the Keggin-, Dawson-, and {P4Mo6}-based hybrid after 180 min in refs 23a and e. In addition, the degradation efficiency of 4 under UV light is higher than that of 1−3, which may be caused by two reasons: first, cadmium complexes as bridge units promote the transfer of electrons among adjacent basket-shaped POMs clusters, which facilitate photogenerated electrons transfer to the surface of basket POMs rapidly. Second, the large pores in compound 4 increase the contact area between catalysts and substrate and promote more active centers to involve in the photocatalytic reactions. As a result, the catalytic properties of compound 4 are improved. Compound 4 was chosen as a representative catalyst to investigate the photocatalytic activities on different substrates by the degradation of organic contaminants MO and AP under similar conditions. The dynamic degradation curve and decomposition rate of MO and AP were recorded in Figure 10. The degradation rate of 89.15% for MO and 80.68% for AP molecules are observed after 40 and 45 min, respectively. Comparatively, photodegradation of MO for Keggin and Dawson hybrids are not ideal in ref 23a, and only 82.5% of MO was decomposed by the Keggin phosphomolybdates after 90 min with the aid of H2O2 in ref 23d, which demonstrates that compound 4 exhibits unique highly efficient degradation ability for typical dyes MO under UV light. In addition, the degradation rate of compound 4 for resistant dye AP still reaches 80.68% just 45 min later. The results show that the basket-based POMs exhibit the universal advantage to decompose many kinds of dyes that arise from selectively using one of the activated species to decompose many kinds of dyes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00158. Tables of selected bond lengths (Å) and bond angles (deg) for compounds 1−4; IR, UV, TG, and CV behaviors of compounds 1−4, ORTEP plots and part structural figures of compounds 1−4 (PDF) Crystallographic data for the structures have been deposited in the Cambridge Crystallographic Data Centre with CCDC 1418984 for 1, 1418985 for 2, 1418986 for 3, and 1418987 for 4 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported the National Natural Science Foundation of China (Grant Nos. 21371042, 21271056, and 21571044), the Ministry of Education and Specialised Research Fund for the Doctoral Program of Higher Education (20122329110001), the Natural Science Foundation of Heilongjiang Province (ZD2015001 and JJ2016JQ0037), the Scientific Innovation Project for Graduate of Harbin Normal University (No. HSDSSCX2015-05), and the Program for Scientic and Technological Innovation Team Construction in Universities of Heilongjiang Province (No. 2011TD010).

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REFERENCES

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CONCLUSIONS In this paper, four basket-shaped phosphomolybdates with diverse degrees of reduction were synthesized by altering the molar ratio of organic ligand and Na2MoO4 of the reaction system. The penta- and hexa-reduced basket clusters were obtained, which represents the highest reduced level of basketK


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Nonclassical Phosphomolybdates with Different Degrees of Reduction

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