Syntheses, Structures, and Electrochemical Properties of Three New

Jan 25, 2019 - Three new acetate-functionalized zirconium-substituted ... constructed from three [β2-GeW10O37]10– units and a new [Zr6O3(OH)3(OAc)(...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Syntheses, Structures, and Electrochemical Properties of Three New Acetate-Functionalized Zirconium-Substituted Germanotungstates: From Dimer to Tetramer Zhong Zhang, Hai-Lou Li, Yue-Lin Wang, and Guo-Yu Yang* MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China

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S Supporting Information *

ABSTRACT: Under hydrothermal conditions, three new acetate-functionalized zirconium-substituted polyoxometalates, H4Na2[Na6(H2O)22][Zr4(μ3O) 2 (OH) 2 (OAc) 2 (α-GeW 10 O 3 7 ) 2 ]·32H 2 O (1), H 8 K 3 Na 5 [Zr 6 (μ 3 O)3(OH)3(OAc)(H2O)(β-GeW10O37)3]·20H2O (2), H6K4Na12[{Zr5(μ3OH)4(OH)2}@{Zr2(OAc)2(α-GeW10O38)2}2]·22H2O (3), were synthesized and characterized. In 1, the sandwiched dimer [Zr4(μ3-O)2(OH)2(OAc)2(αGeW10O37)2]12− was linked to a hexameric [Na6(H2O)22]6+ cluster to form a 1D chain. In 2, the trimer was constructed from three [β2-GeW10O37]10− units and a new [Zr6O3(OH)3(OAc)(H2O)]14+ cluster. In 3, the tetramer was built by two novel sandwich-type dimers [Zr2(OAc)2(α2-GeW10O38)2]18− and one unique [Zr5(μ3-OH)4(OH)2]14+ core in an approximately orthogonal fashion, showing a staggered tetrahedral polyanion. Also, the electrochemistry and electrocatalytic properties of 3 were studied, exhibiting good catalytic activities toward the reduction of H2O2, BrO3−, and NO2−.

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INTRODUCTION The design and synthesis of transition-metal-substituted polyoxometalates (TMSPs) with high-nuclear or large clusters are a fascinating research work in polyoxometalate (POM) chemistry owing to their structural diversity and wide properties such as catalysis, electrochemistry, and magnetism.1 Up to now, the family of TMSPs has received considerable development, but there has been little progress in making zirconium-substituted POMs (ZrSPs) with various lacunary fragments under hydrothermal conditions. The class of ZrSPs was first obtained by Finke and co-workers in 1989.2 Subsequently, other types of ZrSPs were also reported,3 in which the number of Zr atoms is only up to 6: [Zr6O2(OH)4(H2O)3(β-SiW10O37)3]14−,3c [Zr6(O2)6(OH)6(γ-SiW10O36)3]18−,3d and [Zr6O4(OH)4(H2O)2(OAc)5AsW9O33)2]11−.3e Until 2014, Yang’s group reported a Zr24-cluster-substituted POM, [Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4 (GeW8O31)2]32−, in which a {Zr24O22(OH)10(H2O)2} core links with three different fragments of {B-α-GeW9O34}, {B-αGeW8O31}, and {W2O10}.3h Although some ZrSPs have been obtained, few ZrSPs with germanotungstate (GT) units have been reported. Besides Zr24-substituted POM, Xue et al. made a Zr 3 -substituted sandwiched dimer [Zr 3 O(OH) 2 (αGeW9O34)(β-GeW9O34)]12− based on α/β-GeW9O34 fragments.4 Recently, Yang et al. obtained two Tris-functionalized tetranuclear ZrSPs, [Zr4(μ3-O)2(Tris)2(α-GeW10O37)2]12−.5 To the best of our knowledge, the ZrSPs with GT units are much less than that functionalized by an organic ligand. © XXXX American Chemical Society

Because the GT precursors can easily transform into other derivative units, resulting in the design and synthesis of novel high-nuclear and multimeric ZrSPs functionalized by organic ligands, using GT fragments is still challenging under hydrothermal conditions. To further extend our ongoing research to prepare new ZrSPs, we report three new acetate-functionalized ZrSPs with GT fragments: H4Na2[Na6(H2O)22][Zr4(μ3O)2(OH)2(OAc)2(α-GeW10O37)2]·32H2O (1), H8K3Na5[Zr6(μ3-O)3(μ3-OH)3(OAc)(H2O)(β-GeW10O37)3]· 20H 2 O (2), and H 6 K 4 Na 12 [{Zr 5 (μ 3 -OH) 4 (μ-OH) 2 } @{Zr2(OAc)2(α-GeW10O38)2}2]·22H2O (3). One shows a new 1D chain made by a Zr4-substituted POM and a hexameric [Na6(H2O)22]6+ cluster. 2 is a novel trimer constructed from a [Zr6O3(OH)3(OAc)(H2O)]14+ core and three surrounding [β2-GeW10O37]10− units. In 3, the staggered tetramer was built by two sandwiched dimers [Zr2(OAc)2(α2GeW10O38)2]18− and one [Zr5(μ3-OH)4(OH)2]14+ core. The electrochemistry and electrocatalytic properties of 3 to the reduction of H2O2, BrO3−, and NO2− have been evaluated in a 0.5 mol L−1 (pH = 4.0) Na2SO4/H2SO4 buffer solution and display good electrocatalytic activities toward the H2O2, BrO3−, and NO2− reduction. Received: October 2, 2018

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DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for 1−3 C4H120O136Ge2Na8Zr4W20 (1)

C2H56O140Ge3K3Na5Zr6W30 (2)

C8H68O188Ge4K4Na12Zr9W40 (3)

6715.98 triclinic P1̅ 293(2) 0.71073 12.4911(4) 13.3472(5) 21.3247(8) 96.029(3) 103.217(3) 116.602(4) 3005.56(19) 2 3.710 20.033 3024 0.24 × 0.10 × 0.08 2.88−25.00 99.8 25875/10574 10574/402/722 1.057 0.0691/0.1872 0.0852/0.2097

8833.30 triclinic P1̅ 293(2) 0.71073 12.7512(2) 22.9304(7) 28.8268(8) 105.930(2) 100.854(2) 93.625(2) 7900.8(3) 2 3.713 22.866 7712 0.24 × 0.16 × 0.10 2.82−25.00 99.8 70902/27759 27759/48/1666 1.093 0.0449/0.1235 0.0689/0.1348

12070.24 orthorhombic I222 293(2) 0.71073 18.5428(6) 21.1272(6) 26.3537(8) 90 90 90 10324.3(5) 2 3.883 23.393 10552 0.18 × 0.14 × 0.10 3.09−25.01 99.7 17523/8979 8979/324/600 1.093 0.0357/0.0989 0.0410/0.1019

fw cryst syst space group T/K λ/Å a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalcd/g cm−3 μ(Mo Kα)/mm−1 F(000) cryst size/nm3 θ range/deg completeness/% reflns collected/unique data/restraints/param GOF on F2 R1/wR2 [I > 2σ(I)]a R1/wR2 (all data)

R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑w[(Fo)2 − (Fc)2]2/∑w[(Fo)2]2}1/2.

a

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W, 62.44; Zr, 6.20. Found: C, 0.35; H, 0.95; Ge, 2.35; K, 1.45; Na, 1.41; W, 62.81; Zr, 6.09. IR (KBr, cm−1): 3443(s), 2924(m), 2853(m), 1636(m), 1378(w), 1199(m), 1088(m), 1042(s), 952(s), 891(s), 795(s), 760(s), 574(s), 531(m), 464(w). Synthesis of H 6 K 4 Na 12 [{Zr 5 (μ 3 -OH) 4 (OH) 2 }@{Zr 2 (OAc) 2 (αGeW10O38)2}2]·22H2O (3). 3 was obtained by a procedure similar to that for 1, except that 0.5 mol L−1 sodium acetate buffer (pH = 4.8), Na2CO3 (1 mol L−1, 0.6 mL), and ZrOCl2·8H2O (0.210 g, 0.93 mmol) were replaced by 0.5 mol L−1 sodium acetate buffer (pH = 4.0), Na2CO3 (1 mol L−1, 0.75 mL), and ZrOCl2·8H2O (0.305 g, 0.93 mmol) (pHs = 4.21), and the solution was kept at 100 °C for 5 days (pHe = 4.05). Yield: 0.162 g (13%) based on ZrOCl2·8H2O. Anal. Calcd for 3: C, 0.80; H, 0.57; Ge, 2.41; K, 1.30; Na, 2.29; W, 60.92; Zr, 6.80. Found: C, 0.84; H, 0.73; Ge, 2.29; K, 1.43; Na, 2.13; W, 61.34; Zr, 6.89. IR (KBr, cm−1): 3432(s), 1625(m), 1380(w), 1124(m), 950(s), 892(s), 799(s), 753(m), 601(m), 452(w). Structural Determination. The intensity data of 1−3 were collected on a Gemini A Ultra CCD with graphite-monochromated Mo Kα (λ = 0.71073 Å) at 293(2) K. Then both structures were solved by direct methods and refined by a full-matrix least-squares fitting on F2 using the SHELXTL-97 program.9 The contribution of these disordered solvent molecules to the overall intensity data of all structures was treated using the SQUEEZE method in PLATON (5 lattice water molecules for 1, 2 lattice water molecules for 2, and 10 lattice water molecules for 3 are estimated from elemental analysis and TGA). To balance the charge of 1−3, four, eight, and six protons should be added, respectively. These phenomena are common in POM chemistry.10 The crystallographic data structure refinement information for 1−3 are shown in Table 1. CCDC 1525339 (for 1), 1838710 (for 2), 1852627 (for 3) contain the supplementary crystallographic data for this paper.

EXPERIMENTAL SECTION

Materials and Methods. The starting materials K8Na2[A-αGeW 9 O 34 ]·25H 2 O, 6 Na 10 [A-α-SiW 9 O 34 ]·18H 2 O, 7 and K 12 [αH2P2W12O48]·24H2O8 were prepared according to the literature. IR spectra were recorded on a Smart Omni-Transmission spectrometer over a range of 400−4000 cm−1. Elemental analyses were performed on a EuroEA3000 elemental analyzer. Power X-ray diffraction (PXRD) patterns were obtained using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/ DSC 1000 analyzer in an air atmosphere with a heating rate of 10 °C min−1. Energy-dispersive X-ray spectroscopy (EDS) measurement was performed on a JSM-6700F scanning electron microscope. Electrochemical measurements were performed with a CHI660E electrochemical workstation (Chenhua Instruments Co., Shanghai, China). Synthesis of H 4 Na 2 [Na 6 (H 2 O) 22 ][Zr 4 (μ 3 -O) 2 (OH) 2 (OAc) 2 (αGeW10O37)2]·32H2O (1). K8Na2[A-α-GeW9O34]·25H2O (1.003 g, 0.325 mmol), K12[α-P2W12O48]·24H2O (0.500 g, 0.126 mmol), and ZrOCl2·8H2O (0.210 g, 0.621 mmol) were successively added to 8 mL of a 0.5 mol L−1 sodium acetate buffer solution (pH = 4.8), and then Na2CO3 (1 mol L−1, 0.6 mL) was dropwise added under continuous stirring for 2 h (pHs = 5.27). The resulting mixture was sealed in a 25 mL Teflon-lined bomb, kept at 120 °C for 5 days, cooled to room temperature, and filtered (pHe = 5.12). Evaporation of the filtrate at room temperature resulted in colorless needlelike crystals of 1 after several days. Yield: 0.448 g (43%) based on ZrOCl2· 8H2O. Anal. Calcd for 1: C, 0.72; H, 1.80; Ge, 2.16; Na, 2.74; W, 54.75; Zr, 5.43. Found: C, 0.77; H, 2.15; Ge, 2.07; Na, 2.91; W, 54.93; Zr, 5.54. IR (KBr, cm−1): 3439(s), 2925(w), 2855(w), 1629(m), 1384(w), 1085(m), 1042(s), 956(s), 905(s), 879(s), 796(s), 566(s), 520(m), 458(w). Synthesis of H8K3Na5[Zr6(μ3-O)3(OH)3(OAc)(H2O)(β-GeW10O37)3]· 20H2O (2). 2 was obtained by a procedure similar to that for 1, except that K12[α-P2W12O48]·24H2O was replaced by Na10[A-α-SiW9O34]· 18H2O (0.508 g, 0.180 mmol) and extra N,N-dimethylformamide (0.1 mL) was added (pHs = 5.42), and the solution was kept at 180 °C for 5 days (pHe = 5.21). Yield: 0.320 g (35%) based on ZrOCl2· 8H2O. Anal. Calcd for 2: C, 0.27; H, 0.64; Ge, 2.47; K, 1.33; Na, 1.30;

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RESULTS AND DISCUSSION Synthesis and Structure. 1−3 were synthesized under hydrothermal conditions using two different kinds of POM precursors in the presence of Na2CO3; however, only GT fragments are present in 1−3 (Figures S1−S3). Also, they B

DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry could not be made by the conventional aqueous solution method at atmospheric pressure. When K12[α-H2P2W12O48]· 24H2O or Na10[A-α-SiW9O34]·18H2O was removed from the reactants, no crystals 1−3 and only amorphous precipitates were obtained, showing that K12[α-H2P2W12O48]·24H2O or Na10[A-α-SiW9O34]·18H2O plays a key role in the formation of 1−3. When the reaction temperature was raised or reduced more than 10 °C, only amorphous powders were received at the conditions of 1 and 2. When the reaction temperature was adjusted to 120 °C, only a Zr3-substituted POM,3i [Zr3(μ2OH)2(μ2-O)(A-α-GeW9-O34)(1,4,9-α-P2W15O56)]14−, was obtained with poor yield at the conditions of 3. If the reaction temperature of 3 was set at 80 or 140 °C, no crystalline products could be isolated. Under the same conditions, we expected to use Li2CO3, K2CO3, and Cs2CO3 in place of Na2CO3 to obtain analogous structures of 1−3, but we failed. The PXRD data display the good phase purities of 1−3 (Figure S10). Bond-valence-sum (BVS) calculations11 indicate that the oxidation states of W and Zr are 6+ and 4+, respectively (Table S1). Single-crystal X-ray structural analysis reveals that both 1 and 2 crystallize in the triclinic space group P1̅. The structure of 1 is constructed from two dilacunary [α1-GeW10O37]10− moieties (Figure 1b; abbreviated as {α1-GeW10}) and a central

The most interesting feature of 1 is that adjacent 1a is connected by hexameric [Na6(H2O)22]6+ clusters (abbreviated as {Na6}) to form a novel 1D chain (Figure 1e) and further connect together by hydrogen bonds [O20−H···O13W 2.818(2) Å; O22−H····O15W 2.765(2) Å], leading to a 2D supermolecular layer in the ac plane (Figure S4c). The hexameric {Na6} cluster chain is rarely observed in POM chemistry. As shown in Figure 1f, the Na2+ ion lies in an octahedral coordination atmosphere that is occupied by four bridging water molecules, one terminal water molecule, and one O atom from the {α1-GeW10} unit. The Na3+ and Na4+ ions are also located in the octahedral environments. The Na3+ ion is coordinated by five bridging water molecules and one terminal water molecule, while the Na4+ ion contains three bridging water molecules and three terminal water molecules. The Na−O distances are in the range of 2.310(30)−2.646(19) Å. The molecular unit of 2 consists of a trimeric [Zr6(μ3O)3(OH)3(OAc)(H2O)(β-GeW10O37)3]16− unit (2a; Figure 2d), 5 Na+ ions, 3 K+ ions, 8 protons, and 20 lattice water

Figure 2. (a and b) Polyhedral views of [A-α-GeW9O34]10− and [β2GeW 1 0 O 3 7 ] 1 0 − . (c) Ball-and-stick view of the [Zr 6 (μ 3 O)3(OH)3(OAc) (H2O)]14+ cluster in 2. (d) Polyhedral/ball-andstick view of 2a. Color code: WO6, red; GeO4, green. H atoms, lattice water molecules, and K+ and Na+ ions are omitted for clarity.

molecules. 2a is composed of three [β2-GeW10O37]10− moieties (Figure 2b; abbreviated as {β2-GeW10}) and a central hexanuclear [Zr6(μ3-O)3(OH)3(OAc)(H2O)]14+ cluster (Figure 2c; abbreviated as {Zr6}). The connection modes of the {Zr6} cluster exhibit three types of coordination geometries: three Zr4+ (Zr1, Zr3, and Zr4) centers with eight-coordinated trigonal dodecahedral geometry (Figure S5a), two Zr4+ (Zr2 and Zr6) centers showing seven-coordinated distorted capped octahedral geometry (Figure S5b), and one Zr4+ (Zr5) center displaying eight-coordinated square-antiprismatic geometry (Figure S5c). The BVS values for O3, O6, and O7 are approximately equal to 1 (Table S1), indicating that these three O atoms are actually monoprotonated. For O4, O5, and O8, the BVS values are nearly equal to 2 (Table S1) and strongly suggest deprotonation. In the {Zr6} cluster (Figure 2c), the Zr1 ion has one acetate chelating ligand, while the Zr6 ion has one coordination water molecule. The Zr3, Zr4, and Zr5 ions are centrally linked via two μ3-OH (O6 and O7) bridges. Both Zr4 and Zr5 ions join with Zr6 via a μ3-O8 bridge. Zr2 links Zr3/Zr4 and Zr1 via μ3-O4/O5 and μ-OH (O3) bridges, respectively. The Zr−O bond lengths are in the range of 2.020(11)−2.474(10) Å. The polyanions 1 and 2 have significant differences from [Zr4O2(OH)2(H2O)4(β-SiW10O37)2]10− (4) and [Zr6O2(OH)4(H2O)3(β-SiW10O37)3]14− (5) reported by Kortz et al. 3c These differences are (a) the [α 1 GeW10O37]10− and [β2-GeW10O37]10− fragments were derived

Figure 1. (a and b) Polyhedral views of [A-α-GeW9O34]10− and [α1GeW 1 0 O 3 7 ] 1 0 − . (c) Ball-and-stick view of the [Zr 4 (μ 3 O)2(OH)2(OAc)2]10+ unit in 1. (d) Polyhedral/ball-and-stick view of 1a. (e) 1D chain built by a dimer and the {Na6(H2O)22} cluster. (f) Ball-and-stick view of the {Na6(H2O)22} cluster in 1. Color code: WO6, red; GeO4, green; ZrO7/ZrO8, pink.

tetranuclear [Zr4O2(OH)2(OAc)2]8+ cluster (Figure 1c; abbreviated as {Zr4}), leading to a sandwiched structure [Zr4(μ3-O)2(OH)2(OAc)2(α-GeW10O37)2]12− (1a; Figure 1d). There are two kinds of Zr4+ cations in 1 (Figure S4). The Zr14+ ion exhibits a seven-coordinated distorted capped octahedral geometry (Figure S4a), while the Zr24+ center lies in a square-antiprismatic coordination environment (Figure S4b), in which one η2-OAc group is included. The Zr−O bond lengths are in the range of 2.038(12)−2.570(13) Å. The BVS confirms that O31 is actually monoprotonated and O32 is deprotonated (Table S1).11 C

DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry from [A-α-GeW9O34]10− precursors for 1 and 2, while the [β2SiW10O37]10− fragments were derived from [β-SiW10O36]8− precursors for 4 and 5; (b) the coordinating water molecules in the {Zr4O2(OH)2(H2O)4} (for 3) and {Zr6O2(OH)4(H2O)3} (for 4) clusters were replaced by acetate ligands, resulting in the {Zr4O2(OH)2(OAc)2} (for 1) and {Zr6O3(OH)3(OAc)(H2O)} (for 2) clusters, respectively. To the best of our knowledge, only one high-nuclear acetate-functionalized dimer ZrSP, [Zr6O4(OH)4(H2O)2(OAc)5(AsW9O33)2]11−, was reported.3e Accordingly, 2 is the first trimer ZrSP that not only included GT fragments but also was functionalized by the acetate ligand. Another interesting structural feature of 2 is that polyanions 2a are linked by Na+ and K+ ions to form a 2D structure. In the 2D structure, adjacent polyanions 2a are linked together by Na1+ ions, resulting in a 1D chain along the a axis (Figure S6a) and further linked by Na3+ ions to form a 1D double chain (Figure S6b). Simultaneously, neighboring polyanions 2a are linked to each other by K1+ and K3+ ions, producing another 1D chain (Figure S6c) and 1D double chain along the a axis (Figure S6d). These 1D double chains are cross-linked to form a 2D layer (Figure S6e). The Na−O and K−O distances are in the ranges of 2.625(17)−2.911(16) and 2.703(14)−3.349(16) Å, respectively. Different from 1 and 2, 3 crystallizes in the orthorhombic space group I222. The molecular structural unit of 3 contains 1 staggered tetrameric polyanion [{Zr 5 (μ 3 -OH) 4 (OH) 2 } @{Zr2(OAc)2(α-GeW10O38)2}2]24− (3a), 4 K+ ions, 12 Na+ ions, 6 protons, and 22 lattice water molecules. The polyanion 3a contains four divacant Keggin [α2-GeW10O38]12− units (Figure 3b; abbreviated as {α2-GeW10}), two {Zr2(OAc)2}

in an approximately orthogonal fashion to form a large staggered tetramer aggregate (Figure 3f). In addition, each outer Zr24+ ion of {Zr5} occupied the nonrotated triad vacancy of four [Zr(OAc)(α-GeW10O38)} subunits and linked Zr34+ by three Zr−O−Zr bridges to form a new {Zr5(μ3-OH)4(OH)2} core. So far, only a few staggered tetrameric TMSPs have been obtained, such as Co6-substituted silicotugstate [{Co3(B-βSiW9O33(OH))(B-β-SiW8O29(OH)2)}2]22−,12 Fe27-substituted tungstophosphate [H55P8W49Fe27O248]26−,13 and the manganese-substituted silicotugstate [W36Si4O136MnII10(OH)4(H2O)8]24−.14 Besides the abovementioned ZrSPs, 3 is not only the second high-nuclear ZrSP but also the first staggered tetrameric ZrSP functionalized by an acetate ligand. In polyanion 3a, two {Zr2} clusters and one {Zr5} cluster are linked by four GeO4 units to form a {Zr9Ge4} cluster (Figure 4a). The independent Zr4+ ions in the {Zr9Ge4} cluster possess

Figure 4. (a) Polyhedral representation of the {Zr9Ge4} cluster in 1a. (b) Capped octahedral geometry of the Zr14+ ion in 1a. (c) Squareantiprismatic geometry of the Zr24+ ion in 1a. (d) Trigonal dodecahedral geometry of the Zr34+ ion in 1a.

three types of coordination geometries: the Zr14+ ion exhibits a seven-coordinated distorted capped octahedral geometry (Figure 4b), the Zr24+ ion shows an eight-coordinated distorted square-antiprismatic geometry (Figure 4c), and the Zr34+ ion displays an eight-coordinated trigonal dodecahedral geometry (Figure 4d). The Zr−O bond lengths vary from 2.044(15) to 2.467(14) Å. The BVS confirms that O13 and O42 are actually monoprotonated (Table S2).6 In the {Zr9Ge4} cluster, four Zr14+ cations formed a parallelogram with side lengths of 11.933 × 3.486 Å (Figure S7). Four Ge4+ ions form a distorted quadrilateral with side lengths of 10.739 × 9.067 Å and a dihedral angle of 78.41(2)°. The other distorted parallelogram with side lengths of 6.170 × 3.444 Å and a dihedral angle of 79.53(6)° is derived from four Zr24+ atoms. The two distorted quadrilaterals indicate that Zr4+ ions play an important bridging role in the construction of staggered tetramer POMs. Furthermore, polyanions 3a are packed with the −AAA− mode at the bc plane (Figure S8). The structural transformations of {A-α-GeW9} → {α1GeW10}, {A-α-GeW9} → {β2-GeW10}, and {A-α-GeW9} → {α2-GeW10} should happen during the synthetic process of 1− 3, respectively. Our group has already reported the trans-

Figure 3. (a−c) Polyhedral views of [A-α-GeW9O34]10−, [α2GeW10O38]12−, and {Zr2} cluster in 3a. (d and e) Polyhedral views of sandwich-type polyoxoanion 3b and {Zr5} cluster in 3a. (f and g) Ball-and-stick and polyhedral view of polyoxoanion 3a. Color code: GeO4, green; WO6, red; ZrO7/ZrO8, pink. H atoms, lattice water molecules, and K+ and Na+ ions are omitted for clarity.

clusters (Figure 3c; abbreviated as {Zr2}), and one {Zr5(μ3OH)4(OH)2} cluster (Figure 3e; abbreviated as {Zr5}). The belt vacancy of each divacant {α2-GeW10} unit was filled by Zr14+ ions coordinated with an acetate ligand. The substituted Keggin species {Zr(OAc)(α2-GeW10O38)} are linked by two bridging oxo ligands from {α2-GeW10} fragments to form a sandwich-type unit [Zr2(OAc)2(α2-GeW10O38)2]18− (3b; Figure 3d). Two 3b moieties are connected by a {Zr5} cluster D

DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry formations of {A-α-GeW9O34} → {α1-GeW10O37} in [Zr4(μ3O)2(Tris)2(α-GeW10O37)2]12− and {A-α-GeW9O34} → {α2GeW10O38} in [KFe3Sm3(H2O)6(α-GeW10O38)3]17−.15 As the case stands, structural transformation from {A-α-GeW9} to {αGeW6},16 {α-GeW8},3h {B-α-GeW9},3h or {α-GeW11}17 fragments has also been observed (Figure 5). However, there is no

performed cyclic voltammetry (CV) and UV−vis spectrometry (UV) studies in the conditions of a 0.5 mol L−1 (pH = 4.0) Na2SO4/H2SO4 buffer solution at room temperature. The CV and UV of this solution were checked seven times in 3 days, and the CV and UV characteristics of this solution show no obvious changes (Figures S12 and S13), indicating that structure 3 is stable in a 0.5 mol L−1 (pH = 4.0) Na2SO4/ H2SO4 buffer solution. The electrochemical properties of 3 (concentration: 5 × 10−4 mol L−1) were investigated in a solution of 0.5 mol L−1 (pH = 4.0) Na2SO4/H2SO4 at a scan rate of 100 mV s−1. As shown in Figure S14a, CV curve of 3 exhibits two groups of redox peaks and a single oxidation peak from −1.0 to +0.2 V. The midpoint potentials (E1/2) of the two redox peaks are −0.828 V (I/I′) and −0.619 V (II/II′). In addition, the single oxidation peak is 0.083 V (III). Clearly, I/I′, II/II′, and III are ascribed to the oxidation−reduction procedure of the WVI centers in the {α2-GeW10} fragments, which is similar to those of the reported POMs.18 The CV curves of the different scan speed rates, which varies from 20 to 200 mV s−1, are surfacecontrolled, as revealed by the peak current (Ipc) and scan speed rate showing a linear relationship. This linear function is Ipc = −20.20451 to −0.14374 with a consistency factor of 0.995 (Figure S14b), and numerous POMs have been investigated for this phenomenon.19 For 3, the electrocatalytic activities to reduce hydrogen peroxide (H2O2), bromate (BrO3−), and nitrite (NO2−) were examined in a 0.5 mol L−1 (pH = 4.0) Na2SO4/H2SO4 aqueous solution using a scan rate of 100 mV s−1. Following the addition of H2O2, NaBrO3, or NaNO2, the cathodic reduction peak (I′ and II′) currents of WVI-based waves show increases, while the corresponding oxidation peak (I−III) currents decrease dramatically, indicating that the reduction of H2O2, BrO3−, and NO2− is primarily controlled by the Wbased waves in 3.18b,20 (Figure S15a−c). On the basis of the reduction peak current at −0.893 V (I′), the catalytic efficiencies (CATs) of 3 at H2O2 concentrations in the range of 0, 3 × 10−4, 5 × 10−4, 7 × 10−4, 9 × 10−4, 1 × 10−3, and 3 × 10−3 mol L−1 are calculated as 0, 140.70%, 267.92%, 307.27%, 346.09%, 385.44%, and 509.16%, the CAT values for BrO3− are 0, 61.73%, 123.46%, 149.10%, 183.52%, 258.10%, and 353.91%, and the CAT values for NO2− are 0, 51.34%, 75.27%, 138.90%, 232.53%, 287.63%, and 308.87% under the same measurement conditions (Figure 6). Upon reduction of H2O2, BrO3−, and NO2−, the electrocatalytic capability of 3 obeys the order of CATNO2− < CATBrO3− < CATH2O2.

Figure 5. Transformations between [A-α-GeW9O34]10− and different GT units.

report for the transformation of {A-α-GeW9O34} → {β2GeW10O37}, which is first observed in the structure of 2. From that mentioned above, although some structural transformations of {A-α-GeW9} have been reported, investigations on the transformation between {A-α-GeW9} and different vacant GTs are rare and still challenging work. Thermal Properties. The TGA curves indicate three weight loss steps for 1 and 2 and two weight loss steps for 3 between 25 and 1000 °C (Figure S11). The first weight loss of 13.23% (calcd 13.14%) from 25 to 398 °C corresponds to the release of 49 lattice water molecules for 1; 4.69% (calcd 4.27%) from 25 to 449 °C are involved in the release of 20 lattice water molecules and 1 coordination water molecule for 2; 4.18% (calcd 4.00%) from 25 to 166 °C corresponds to the release of 22 lattice water molecules for 3. The second weight loss of 3.39% (calcd 3.10%) from 398 to 812 °C is assigned to the loss of five lattice molecules and the decomposition of two OAc ligands for 1; 1.22% (calcd 1.10%) from 449 to 578 °C is attributed to the dehydration of three hydroxy groups and the decomposition of one OAc ligand for 2; 2.93% (calcd 2.56%) from 166 to 521 °C is assigned to the decomposition of four OAc ligands and the dehydration of six hydroxy groups and six protons for 3. The third weight loss of 1.18% (calcd 0.80%) approximately corresponds to the dehydration of two hydroxy groups and the dehydration of four protons for 1; 0.83% (calcd 0.81%) is assigned to the dehydration of eight protons for 2. Electrochemistry and Electrocatalytic Properties. In order to investigate the aqueous solution stability of 3, we

Figure 6. Graph of CAT versus concentrations of H2O2, NaBrO3, and NaNO2 for 3. E

DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ORCID

For the {A-α-GeW9} precursor, its CV curve has three redox peaks located at −0.708, −0.459, and −0.257 from −1.0 to +0.2 V (Figure S16a). Compared with compound 3, the CV curve of the {A-α-GeW9} precursor exhibits a potential shift, which may be because of the different GT fragments, extra Zr clusters, and different negative charges. Following the addition of H2O2, NaBrO3, or NaNO2, the cathodic reduction peak currents have a moderate increase, while the corresponding oxidation peak currents decrease dramatically, indicating that the reduction reactions are primarily controlled by the Wbased waves in {A-α-GeW9} (Figure S16b−d). Therefore, the electrocatalytic capability of 3 is well beyond that of {A-αGeW9} to reduce H2O2, BrO3−, and NO2− under the same conditions, showing that 3 has good electrocatalytic activity toward the reduction of H2O2, BrO3−, and NO2−, respectively. In addition, the electrocatalytic activity of ZrOCl2·8H2O toward the reduction of H2O2 was also studied under the same conditions, whereas the redox peaks of the Zr4+ cations were not observed in our case, showing that ZrOCl2·8H2O has no effect on the reduction of H2O2. To date, the efficient electrocatalytic reduction properties of some POMs have been exploited, in which only several ZrSPs are included, such as [Zr4(OH)6(OAc)2(α-PW10O37)2]10− with efficient activity to the reduction of nitrite.3g Our group reports two examples of electrocatalytic reduction for H2O2 and BrO3− of {[Zr3(OAc)W7(H2O)O25][B-α-SbW9O33]2}15− and {Zr2[SbP2W4(OH)2O21][α2-PW10O38]}220−.21

Guo-Yu Yang: 0000-0002-0911-2805 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grants 21571016, 21831001 and 91122028) and NSFC for Distinguished Young Scholars (Grant 20725101).

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(1) (a) Zheng, S. T.; Yang, G. Y. Recent Advances in ParamagneticTM-Substituted Polyoxometalates (TM = Mn, Fe, Co, Ni, Cu). Chem. Soc. Rev. 2012, 41, 7623−7646. (b) Wang, S. S.; Yang, G. Y. Recent Advances in Polyoxometalate-Catalyzed Reactions. Chem. Rev. 2015, 115, 4893−4962. (c) Lv, H. J.; Geletii, Y. V.; Zhao, C. C.; Vickers, J. W.; Zhu, G. B.; Luo, Z.; Song, J.; Lian, T. Q.; Musaev, D. G.; Hill, C. L. Polyoxometalate Water Oxidation Catalysts and The Production of Green Fuel. Chem. Soc. Rev. 2012, 41, 7572−7589. (d) Oms, O.; Dolbecq, A.; Mialane, P. Diversity in Structures and Properties of 3dIncorporating Polyoxotungstates. Chem. Soc. Rev. 2012, 41, 7497− 7536. (e) Ma, X.; Li, H. L.; Chen, L. J.; Zhao, J. W. The Main Progress over The Past Decade and Future Outlook on High-nuclear Transition-Metal Substituted Polyoxotungstates: From Synthetic Strategies, Structural Features to Functional Properties. Dalton Trans. 2016, 45, 4935−4960. (f) Gu, Y. N.; Chen, Y.; Wu, Y. L.; Zheng, S. T.; Li, X. X. A Series of Banana-Shaped 3d-4f Heterometallic Cluster Substituted Polyoxometalates: Syntheses, Crystal Structures, and Magnetic Properties. Inorg. Chem. 2018, 57, 2472−2479. (g) Zhao, J. W.; Li, Y. Z.; Chen, L. J.; Yang, G. Y. Research Progress on Polyoxometalate-Based Transition-Metal-RareEarth Heterometallic Derived Materials: Synthetic Strategies, Structural Overview and Functional Applications. Chem. Commun. 2016, 52, 4418−4445. (2) Finke, R. G.; Rapko, B.; Weakley, T. J. R. Polyoxoanions Derived From A-β-SiW9O3410‑: Synthesis and Crystallographic and 183W NMR Characterization of Si2W18Zr3O71H311‑, Including Its Organic Solvent Soluble Bu4N+ Salt. Inorg. Chem. 1989, 28, 1573−1579. (3) (a) Fang, X. K.; Anderson, T. M.; Hill, C. L. Enantiomerically Pure Polytungstates: Chirality Transfer Through Zirconium Coordination Centers to Nanosized Inorganic Clusters. Angew. Chem., Int. Ed. 2005, 44, 3540−3544. (b) Fang, X. K.; Anderson, T. M.; Hou, Y.; Hill, C. L. Stereoisomerism in Polyoxometalates: Structural and Spectroscopic Studies of Bis(malate)-Functionalized Cluster Systems. Chem. Commun. 2005, 44, 5044−5046. (c) Bassil, B. S.; Dickman, M. H.; Kortz, U. Synthesis and Structure of Asymmetric ZirconiumSubstituted Silicotungstates, [Zr6O2(OH)4(H2O)3(β-SiW10O37)3]14‑ and [Zr4O2(OH)2(H2O)4(β-SiW10O37)2]10‑. Inorg. Chem. 2006, 45, 2394−2396. (d) Bassil, B. S.; Mal, S. S.; Dickman, M. H.; Kortz, U.; Oelrich, H.; Walder, L. 6-Peroxo-6-Zirconium Crown and Its Hafnium Analogue Embedded in A Triangular Polyanion: [M6(O2)6(OH)6(γ-SiW10O36)3]18‑ (M = Zr, Hf). J. Am. Chem. Soc. 2008, 130, 6696−6697. (e) Al-Kadamany, G.; Mal, S. S.; Milev, B.; Donoeva, B. G.; Maksimovskaya, R. I.; Kholdeeva, O. A.; Kortz, U. Hexazirconium- and Hexahafnium-Containing Tungstoarsenates(III) and Their Oxidation Catalysis Properties. Chem. - Eur. J. 2010, 16, 11797−11800. (f) Kato, C. N.; Shinohara, A.; Hayashi, K.; Nomiya, K. Syntheses and X-ray Crystal Structures of Zirconium(IV) and Hafnium (IV) Complexes Containing Monovacant Wells-Dawson and Keggin Polyoxotungstates. Inorg. Chem. 2006, 45, 8108−8119. (g) Zhang, W.; Liu, S. X.; Zhang, C. D.; Tan, R. K.; Ma, F. J.; Li, S. J.; Zhang, Y. Y. An Acetate-Functionalized Tetranuclear Zirconium Sandwiching Polyoxometalate Complex. Eur. J. Inorg. Chem. 2010, 2010, 3473−3477. (h) Huang, L.; Wang, S. S.; Zhao, J. W.; Cheng, L.; Yang, G. Y. Synergistic Combination of Multi-ZrIV Cations and Lacunary Keggin Germanotungstates Leading to A Gigantic Zr24Cluster-Substituted Polyoxometalate. J. Am. Chem. Soc. 2014, 136,

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CONCLUSION In conclusion, three new ZrSPs with different dilacunary GT fragments derived from the [A-α-GeW9O34]10− precursor have been made by the hydrothermal method. The dimeric polyanion 1a consists of two {α1-GeW10} units sandwiching a {Zr4O2(OH)2(OAc)2} cluster and further extended to a 1D chain by the hexameric {Na6} cluster linkers. The polyanion 2a is the first ZrSP trimer with GT fragments, in which three {β2GeW10} units are linked by a {Zr6O3(OH)3(H2O)(OAc)} cluster. The staggered tetramer 3a was first made and built by two sandwiched subunits and a {Zr5(μ3-OH)4(OH)2} cluster core. In addition, 3 displays good electrocatalytic activity toward the reduction of H2O2, BrO3−, and NO2−.

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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.8b02805. BVS calculations, PXRD, IR spectra, TGA, EDS, UV−vis spectra, electrochemistry and electrocatalytic CVs, and additional structures (PDF) Accession Codes

CCDC 1525339, 1838710, and 1852627 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.-Y.Y.). F

DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02805 Inorg. Chem. XXXX, XXX, XXX−XXX