SCIENCE CHINA Chemistry • ARTICLES • · SPECIAL TOPIC · Coordination Polymer

September 2011

Vol.54 No.9: 1418–1422

doi: 10.1007/s11426-011-4340-9

Two new compounds with microporous constructed by Waugh-type polyoxoanion and transition metal ions TAN HuaQiao, CHEN WeiLin, LIU Ding, YAN AiXue & WANG EnBo* Key Laboratory of Polyoxometalate Science of Ministry of Education; Northeast Normal University, Changchun 130024, China Received April 13, 2011; accepted May 16, 2011

Two new compounds with microporous Co3[MnMo9O32]·15H2O (1) and Cu3[MnMo9O32]·15H2O (2) have been synthesized, and characterized by IR, element analysis, TG and single-crystal X-ray analysis. The structure analyses reveal that compounds 1 and 2 are isostructural. In crystal, the Waugh-type polyoxoanions [MnMo9O32]6– are connected by Co2+ or Cu2+ cations to a 3D open-framework, which possesses channels along the [1 2 2] direction of approximately 8.27 × 11.97 Å. The photocatalytic performances of compounds 1 and 2 for photodegradation of RhB with UV irradiation have been studied, which show a good photocatalytic activity for photodegradation of RhB. porous materials, Waugh-type polyoxoanion, single-crystal structure, photocatalysis

1 Introduction The design and synthesis of new porous materials have attracted intense interest owing to their extensive applications in gas storage, catalysis, separation and ion-exchange [1–6]. Polyoxometalates (POMs) [7, 8], a kind of soluble anionic metal oxide cluster with a wide range of magnetic [9], redox [10] and catalytic properties [11], are regarded as promising candidates for the design of new porous materials. To date, several novel POMs-based porous materials have been reported, for example: Liu and Su et al. synthesized a series of MOFs in which the catalytic active Keggin polyoxoanions were alternately arrayed as noncoordinating guests in the cuboctahedral cages of a Cu-benzentricaboxylate-based metal-organic framework [12, 13]; Ruiz-Salvador and Dolbecq et al. reported two zeolitic POMs-based MOFs [NBu4]3[PMoV8MoVI4O36(OH)4Zn4(BDC)2]·2H2O (BDC = benzenedicarboxylate) and [NBu4][PMoV8MoVI4O37(OH)3Zn4(im)(Him)] [14, 15]. In these compounds, the polyoxometalates act as guests or multiple metal nodes in the struc*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2011

ture of MOFs. It is well-known that in the construction of MOFs, an extensively employed strategy is the use of various rigid multidentate O- or N-donor organic ligands to coordinate and bridge metal ions into frameworks. As we have known, polyoxometalates are anions with O-enriched surface, thus it might be one of the most excellent inorganic multidentate O-donor ligands instead of organic ligands to construct new metal-inorganic porous materials, which might possess an important application in catalysis and separations. Based on the above-mentioned hypothesis, we have thus initiated a study on the reaction of polyoxoanions with simple transition-metal ions. Herein, we report two new pure inorganic microporous compounds: Co3[MnMo9O32]·15H2O (1) and Cu3[MnMo9O32]·15H2O (2), which represent a new kind of porous metal-inorganic framework (MIF).

2 Experimental 2.1

General

All reagents were purchased commercially and used without further purification. K3(NH4)3[MnMo9O32] was prepared according to ref. [16]. Co, Cu, Mn and Mo were chem.scichina.com

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determined by a Leaman inductively coupled plasma (ICP) spectrometer. The IR spectra were obtained on an Alpha Centaurt FTIR spectrometer in the 400–4000 cm–1 region with a KBr pellet. UV-vis absorption spectrum was obtained using a 752 PC UV-vis spectrophotometer. The TGA was performed on a Perkin-Elmer TGA7 instrument under flowing N2 with a heating rate of 10 °C min–1 at 20–600 °C. 2.2

Synthesis of Co3[MnMo9O32]·15H2O (1)

K3(NH4)3[MnMo9O32] (0.5234 g, 0.3 mmol) was dissolved in 50 mL of hot water. 6 mL of 1 M CoCl2 (6.0 mmol) aqueous solution was added to the mixture with stirring. The pH value of the mixture was carefully adjusted with a dilute H2SO4 solution (2 M) to approximately 2.00. Afterwards, the mixture was heated at 70 °C for 2 h, and then filtered. The filtrate was kept at room temperature, and slow evaporation for 5 d afforded the product as red crystals (yield: 55% based on K3(NH4)3[MnMo9O32]). Anal. found (%): Co, 9.28; Mn, 3.02; Mo, 46.10. calcd: Co, 9.42; Mn, 2.93; Mo, 45.99. IR (KBr pellet): 3430 (s), 1630 (m), 1470 (w), 1399 (w), 949(s), 907 (s), 647 (m), 588 (s), 540 (s), 493 (s) cm−1. 2.3

Synthesis of Cu3[MnMo9O32]·15H2O (2)

This compound was prepared similarly to 1, with 8 mL of 1 M CuCl2 aqueous solution (8 mmol) instead of CoCl2. Orange block crystals were obtained from the solution after one week (yield 66%, based on K3(NH4)3[MnMo9O32]). Anal. calcd for: Cu, 9.97; Mn, 2.81; Mo, 45.80. Found: Cu, 10.08; Mn, 2.90; Mo, 45.66. IR (KBr pellet): 3429 (m), 1637 (w), 1549 (w), 1460 (w), 944 (s), 908 (s), 653 (m), 594 (s), 534 (s), 487 (s) cm–1. 2.4

Photocatalysis

5.0 mg of compounds 1 or 2 as dissolved in 200 mL rhodamine-B (RhB) solutions (2.0 × 10–5 M) at pH 2.0 (the pH of the solutions was adjusted with dilute aqueous solutions of HClO4), then magnetically stirred in the dark for about 30 min. The solution was then exposed to UV irradiation from a 125 W Hg lamp at a distance of 4–5 cm between the liquid surface and the lamp. The solution was stirred during irradiation. At different time intervals, 3.0 mL of samples was taken out, and analyzed with UV-vis spectroscopy. 2.5

X-ray crystallography

Single-crystal X-ray crystallographic data were collected on a Rigaku R-AXIS RAPID IP diffractometer equipped with a normal focus 18 kW sealed tube X-ray source (Mo Kα radiation, λ = 0.71073 Å) operating at 50 kV and 200 mA at T =

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293 K. The structure was solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXL 97 software [17, 18]. All the H atoms on water molecules were directly included in the molecular formula. A summary of crystal data and structure refinements for compounds 1 and 2 is provided in Table 1. Further details of the crystal structures can be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (Emails: [email protected]) on quoting the deposited numbers CSD-422918 for 1 and CSD-422919 for 2.

3

Results and discussion

3.1

Structure description

Single crystal X-ray crystallographic analyses reveal that the heteropolymolybdate [MnMo9O32]6– in compounds 1 and 2 is a typical Waugh-type polyoxoanion [7]. This polyoxoanion was first prepared as its potassium and barium salts by Hall in 1907 [19], and was structurally determined as its ammonium salt by Waugh in 1954 [20]. The polyoxoanion is built around an octahedrally coordinated MnO6 motif. Three octahedral MoO6 motifs are arranged on the vertices of a triangle, which is coplanar with the central MnO6 octahedron. Meanwhile, two groups of edge-shared Mo3O13 triplets are positioned above and below the middle layer of the four octahedra, making the polyoxoanion with ideal D3 point symmetry. As revealed by X-ray crystallographic Table 1

Crystal data and structure refinement for 1 and 2

Compounds Empirical formula Mr Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dc (g cm−3) F(000) Reflns collected/ Unique/gt Rint GOF on F2 R1a) [I > 2σ(I)] wR2b) Largest residuals [e Å−3]

1 H30Co3MnMo9O47 1877.43 293 trigonal R32 14.832(2) 14.832(2) 13.953(3) 90.00 90.00 120.00 2658.2(8) 3 3.518 2670

2 H30Cu3MnMo9O47 1891.26 293 trigonal R32 15.544(2) 15.544(2) 12.513(3) 90.00 90.00 120.00 2618.4(7) 3 3.598 2688

7395/1110/1004

6832/1076/913

0.1285 0.986 0.0530 0.1390

0.1652 1.082 0.0817 0.2051

1.331/−1.168

1.830/−1.656

R1 = Σ||F0|-|Fc||/Σ|F0|; b) wR2 = Σ[w(F02-Fc2)2]/Σ[w(F02)2]1/2.

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analysis, the Mo–O bond lengths range from 1.676(9) to 2.250 (7) Å. The central Mn–O distances are 1.884(1)– 1.889(7) Å, and the O–Mn–O angles are in the range of 86.2(4)–169.0(5)°. Compounds 1 and 2 are isostructural. Therefore, compound 1 is described as an example below. In compound 1, all Co atoms have the same coordination sphere and exhibit distorted octahedron structure. Each Co2+ is coordinated by four water molecules and two terminal oxygen atoms from two [MnMo9O32]6– polyoxoanions (Co–O, 2.162(1) Å; CoOw, 2.039(1)–2.059(1) Å). Each [MnMo9O32]6– acts as a hexadentate ligand and coordinates to six Co2+ cations through its terminal oxygen atoms (Figure 1). This connection mode generates a 3D framework with α-Po topology that possesses channels along the [122] direction of approximately 8.27 × 11.97 Å (Figure 2). The lattice water molecules occupy the channels (Figure 3). 3.2

PXRD

In order to investigate the thermal stability of compounds 1

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Figure 3 The polyhedral and ball-and-stick representation of the 3D framework in compound 1.

and 2, the variable temperature powder XRD of compound 2 has been studied as an example. As shown in Figure 4, the experimental PXRD pattern is consistent with the simulated one on the basis of the single-crystal structure, which proved the good phase purity of compound 2. In the temperature 20 to 75 °C, the PXRD patterns were changed slightly, indicating that the framework of 2 is intact after the partial departure of the lattice water molecules. As for the sample at 100 °C, the pattern has an obvious change. And at 150 °C, the low angle peaks of the pattern almost disappeared, implying the destroyed porous structure of compound 2. 3.3

Figure 1 The polyhedral and ball-and-stick representation of polyoxoanion [MnMo9O32]6– acting as a hexadentate ligand coordinating to six CoII cations in compound 1.

Figure 2 A polyhedral space-filling diagram of [MnMo9O32]6– connected by Co2+ into a 3D framework with channels along the [1 2 2] direction.

Photocatalysis property

It is well known that many POMs exhibit good photocatalytic activity in the degradation of organic dyestuff [21, 22]. Therefore, the photocatalytic performances of compounds 1 and 2 for photodegradation of RhB with UV irradiation have been studied. As shown in Figure 5, in the irradiation time 0–300 min, the A/A0 of RhB (λ = 550 nm) in solution decreased obviously from 1.2932 to 0.3492 for 1, and from 1.2052 to 0.3939 for 2, which indicated that compounds 1

Figure 4 PXRD patterns of compound 2: (a) simulated; (b) as-synthesized; (c) 50 °C; (d) 75 °C; (e) 100 °C; (f) 150 °C.

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4 Conclusions In summary, we have synthesized two new compounds with microporous by the reaction of the Waugh-type polyoxoanion [MnMo9O32]6– with Co2+ and Cu2+ respectively, which represent a new kind of porous metal-inorganic framework (MIFs). And the photocatalytic performances of these materials show a good photocatalytic activity for photodegradation of RhB under UV irradiation, which indicates its potential application in the treatment of organic dyes wastewater. This work was financially supported by the National Natural Science Foundation of China (20701005 & 20701006); the Fundamental Research Funds for the Central Universitues, Postdoctoral Station Foundation of Ministry of Education (20060200002), the Testing Foundation of Northeast Normal University (NENU), Science and Technology Creation Foundation of NENU (NENU-STC07009) and the Program for Changjiang Scholars and Innovative Research Team in University. 1 2 3 4

Figure 5 Changes in UV-vis absorption spectra of RhB solutions (2.0 × 10−5 M at a pH of 2.0) in the present of 5.0 mg compound 1 (a), compound 2 (b).

and 2 have a good photocatalytic activity for photodegradation of RhB under UV irradiation. And as a result of the changes of C/C0 with time, compounds 1 and 2 possess the similar kinetic process for photodegradation of RhB. As the irradiation time was increased, the C/C0 was declined from 1 to 0.27 for 1, and from 1 to 0.32 for 2. And with the increasing of irradiation time the rate of these declined were also decreased.

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3.4 Thermogravimetric analysis Compound 1 exhibits two weight loss steps (14.23 %) in the temperature range 30–384 °C, corresponding to the loss of the noncoordinated and coordinated water molecules, respectively. And at 384 °C, the polyoxoanion [MnMo9O32]6– would be decomposed. The TG curve of 2 exhibits three weight loss stages in the temperature ranges 30–600 °C. The first and second weight loss is 14.05% in the temperature range 30–350 °C, corresponding to the loss of noncoordinated and coordinated water molecules in 2 (14.27%). And at 350 °C, the polyoxoanion [MnMo9O32]6− would be decomposed.

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