Cryst. Res. Technol. 41, No. 9, 874 – 879 (2006) / DOI 10.1002/crat.200510686

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Crystal structure and electrochemical behavior of a novel

polyoxometalate [Ni(bpy)3]2[W4V2O19] with Lindqvist-type

structure

Xin Wang, Baibin Zhou*, Chongmin Zhong, and Mingyan Ji

Laboratory of Inorganic Synthesis and Function Material, Department of Chemistry, Harbin Normal University, Heilongjiang 150025, People’s Republic of China

Received 28 September 2005, revised 12 April 2006, accepted 15 May 2006 Published online 15 August 2006

Key words hydrothermal synthesis, crystal structure, isopolyanions, redox property.

PACS 81.10.-h

A novel polyoxometalate [Ni(bpy)3]2[W4V2O19] (Ni2V2W4) has been synthesized by the hydrothermal method and the structure determined by X-ray crystallography. Ni2V2W4 crystallizes in a trigonal system with space group R-3c (a = 15.8984 (5) Å, b = 15.8984 (5) Å, c = 43.855 (3) Å). In the structure of the compound, terminal and bridging oxygen atoms are bond to the metal centers by σ or π bonds. The W6+ and V5+ ions in isopolyanion [W4V2O19]

4- statistically distribute over the six metal centers. Using cathodic adsorptive voltammetric method with a carbon paste electrode, the redox property and the electron transferring process were studied. The results show that electrochemical behavior about W(VI) and V(V) atoms give one-electron, three-electron and two-electron reduction waves. Three successive oxidation waves are observed too. The compound was also characterized by thermal gravimetric analysis and IR spectra.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Polyoxometalates (POMs) possess unusual electronic properties, and find potential applications in catalysis, functional materials and medicine.[1-4] Traditionally, isopolyanions are ascribed to a type of POMs. Many isoplyanions having the Lindqvist structure have been receiving extensive attention in recent years owing to their great people’s interests in studies and applications.[5-8] Hexametalate polyoxoanion-supported organometallic complexes, such as [{(η5-C5Me5)Rh}4V6O19]

2-, [9] [Ni(tpyrpyz)2]2[Mo4O12F2][Mo6O19]·2H2O,

[10] and [nBu4N]4[Ag2I4][W6O19] [11] have been studied extensively. However, Lindqvist structure with mixed metal atoms is relatively less studied, especially by the method of hydrothermal synthesis. Isopolyanions, which appeared as electron acceptor, may be constructed upon various metallic frameworks, to form a new kind of materials, which have potential applications to electrocatalysis. Design and synthesis of novel POMs with different structural characteristics are helpful to explore their properties. Following the former study [12], we reports the crystal structure of a new compound [Ni(bpy)3]2[W4V2O19], its redox property, and the electron transferring process.

2 Experimental

Materials and methods All chemicals were purchased in AR grade and used without further purification. Elemental analyses (C, N and H) were performed on Elemental Analyzer Perkin-Elmer Analyzer 2400 CHN. IR spectra were recorded in the range of 400-4000 cm-1 on a Perkin-Elmer 1730 FTIR spectrophotometer using ____________________

* Corresponding author: e-mail: zhou_bai_bin@sina.com, wangxin5569@163.com

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KBr pellets. Thermogravimetric analyses were performed on a Perkin-Elmer TGA-7 DTA-1700 Series. The TG-DTA experiments were carried out in N2 from 20°C to 900°C, at the rate of 10K·min-1.

Synthesis of [Ni(bpy)3]2[W4V2O19] The mixture of Na2WO4·2H2O (1.113g, 3.375mmol), Ni(CH3COO)2·4H2O (1.68g, 6.75 mmol), 2,2’-bipyridine (0.264g, 1.69 mmol), NH4VO3 (0.394g, 3.375 mmol) and water (40ml) in a molar ratio of 0.5:1:0.25:0.5:160 was stirred at room temperature for 80 minutes to form a solution, then sealed in a 30-ml Teflon-lined autoclave and heated at 180°C for 4 days. After cooling to room temperature, the orange needlelike crystals were isolated and given the yield of 60% (based on V). The isolated crystals were washed several times with distilled water and dried in air. Elemental analyses: Calculated (%) for [Ni(bpy)3]2[W4V2O19]: H, 2.18; N, 7.64; C, 32.75. Found (%): H, 2.19; N, 7.65; C, 32.79.

X-ray crystallography Suitable crystal for single-crystal X-ray diffraction was selected and had the size of 0.36×0.18×0.16 mm. Structure analyses was performed on a Bruker-Nonius diffractometer, using graphite– monochromated Mo-Kα radiation (λ = 0.71073 Å). The data were collected at 293 ± 2 K. Empirical absorption corrections were applied. The structures were solved by direct methods using the program SHELXS-97, and refined by the full-matrix least-squares methods based on F2 using the SHELXL-97 program package.

3 Results and discussion

Crystal structure description Vanadium has a very rich and complicated chemistry. From a structural point of view, polyvanadates may assume the structure of VO6 octahedra, VO5 square pyramids, or VO4 tetrahedrons. These polyhedra may occur either separately or two or three jointly [13]. The crystal structure of the title compound was determined by single crystal X-ray diffraction and its asymmetric unit consists of one [Ni(bpy)3]

2+ and one [W4V2O19]4-, as shown in figure 1a. One Ni atom is coordinated to three bpy molecules to

form a nearly regular octahedron. The complex anions and cations in the crystal are held together by electrostatic forces. The asymmetric unit of [Ni(bpy)3]2[W4V2O19] with only [W4V2O19]

4- is shown in figure 1b.

Fig. 1 a) The asymmetric unit of Ni2V2W4. b) View of [W4V2O19]

4-.

The well known symmetrical structure of the isopolyanions [W4V2O19]4- results from the fusion of six

octahedra sharing a common oxygen atom vertex, which is bound to six metal centers. In the [W4V2O19]4-

cluster the oxygen atoms can be classified as follows: (l) Oa: the hexacoordinated oxygen in the center of the (W-V)6 octahedron; (2) Ob: the triply-bridging oxygen; (3) Oc: the doubly-bridging oxygen on the surface of the [M6O19]

4- cluster; (4) Od: the terminal ones lying on the outer corners. According to the position and coordination environment in cluster [M6O19]

4-, the six metal atoms can be divided into three types: (l) M1: at the corner of the equatorial plane and bonded to one Oa, four Ocs and one Od; (2) M2: located above and below the equatorial plane and bonded to one Oa, two Obs, two Ocs and one Od; (3) M3: in the center of the equatorial plane and bonded to two Oas, two Obs and two Ocs. The distances of bridging-bond M-Ob compare very well with reported values [14] but the lengths of central M-Oc, especially terminal M-Od bonds are significantly longer than the average distances in literature [14].

Figure 2 shows a polyhedral view of the structure of the metallic oxide framework of [Ni(bpy)3]2[W4V2O19]. One [W4V2O19]

4- and two [Ni(bpy)3]2+ arrange alternatively along b-axis to form an extended molecular chain,

as shown in figure 2, which gives a view of the packing structure of Ni2V2W4 along a-axis. The crystallographic data for Ni2V2W4 are listed in table 1. It shows that Ni2V2W4 crystallizes in a trigonal

system with space group R-3c, unit cell parameters a = 15.8984 (5) Å, b = 15.8984 (5) Å, c = 43.855 (3) Å, V=9599.68(78) Å3, Z=6, R = 0.0180, wR = 0.0451. Interatomic distances and angles for Ni2V2W4 are given in

876 Xin Wang et al.: Crystal structure and electrochemical behavior of a novel polyoxometalate

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

table 2. An examination of the M-O bond distances (Table 2) shows that M-Ob, M-Oc, M-Od bonds are 190, 229, 176 Å, respectively, a little shorter than corresponding reported values 196, 237, 176 Å, presumablely due to the disordered metal centers [M = 1/3 V + 2 /3 W], which arised from similar ionic radius of W6+ and V5+.

Fig. 2 A view of the packing structure of Ni2V2W4 along the a axis. All H atoms are omitted for clearity.

Table 1 Crystal data and structure refinement for [Ni(bpy)3]2[W4V2O19].

Empirical formula C60H48N12Ni2O19V2W4

Formula weight 2195.80

Temperature (K) 293 (2)

Wavelength (Å) 0.71073

Crystal system Trigonal

space group R-3c

a (Å) 15.8984 (5)

b (Å) 15.8984 (5)

c (Å) 43.855(3)

Volume (Å3) 9599.6 (7)

Z 6

Calculated density (g⋅cm-3) 2.279

Absorption coefficient (mm) 8.095

F (000) 6252

Crystal size (mm) 0.36 × 0.18 × 0.16

Limiting indices -17 ≤ h ≤ 19, -17 ≤ k ≤ 19, -49 ≤ l ≤ 54

Reflections collected / unique 16971/ 6957 [ R (int) = 0.0421]

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 6957 / 0 /151

Goodness-of-fit on F2 1.044

Final R indices [I > 2sigma (I) ] R1 = 0.0180, wR2 = 0.0451

R indices (all data) R1 = 0.0236, wR2= 0.0485

aR1 = ||Fo|–|Fc|| / |Fo|. bwR2 = [w (|Fo| –|Fc|)2] / [(|Fo|)2 ]1/2.

IR spectra and thermogravimatric analysis In the IR spectrum of the compound (Fig. 3), the strong bands at 945 and 926 cm-1 arise from stretching vebration of terminal V–O, and bonds at 967 cm-1 from that of W–Od. The bonds at 779, 735, and 631 cm-1 are attributed to νasV–O–V and νsV–O–V stretching, while bonds at 800-760 cm-1 to W–Oc–W stretching. The 1174, 1153 cm-1 bands stem from νC–C. The peak at 1021 cm-1

can be assigned to νsW–Oa. The peak assigned to νW–Ob–W splitted into two peaks at 1099 and 1117 cm-1,

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comparing with those from νV–Ob–V at 1247, 1224 cm-1 . The 1488, 1470 cm-1 bands are due to νasC–H

stretching.

Table 2 Bond lengths (Å) and angles (deg).

Ni-N(1) 2.072 (2) N(1)#1-Ni-N(2) 172.76(9) O(5)-V-O(3) 154.48(8) Ni-N(2) 2.101 (2) N(1)-Ni-N(2) 78.77(9) V#6-O(1)-W 179.172(7) W-O(2) 1.684 (2) N(2)#2-Ni-N(2) 95.42(9) W-O(1)-W#3 89.533(6) W-O(5) 1.9092(13) O(2)-W-O(5) 102.35(9) V-O(1)-W#3 89.533(6) W-O(3) 1.9168(17) O(3)#3-W-O(5) 87.84(6) V-O(1)-V#3 89.533(6) W-O(4) 1.9368(13) O(5)-W-O(3) 154.48(8) V#7-O(1)-V#3 179.174(7) W-O(1) 2.2919(15) O(2)-W-O(1) 179.21(8) V#3-O(1)-V#5 91.058(8) W-W#3 3.2281 (3) O(2)-W-W#3 135.30(7) W#4-O(3)-W 115.33(9) W-W#5 3.2380 (3) O(5)-W-W#3 81.67(4) V#7-O(4)-W 115.22(12) V-O(2) 1.6840 (2) O(4)-W-W#3 122.10(6) V#5-O(5)-V 115.99(12) V-O(5) 1.9092 (13) O(1)-W-W#3 45.24(3) C(1)-N(1)-C(5) 118.2(2) V-O(3) 1.9168 (17) W#3-W-W#5 60.78(6) C(1)-N(1)-Ni 126.24(19) V-O(4) 1.9368 (13) W#4-W-W#5 90.29(2) C(5)-N(1)-Ni 115.18(19) V-O(1) 2.29190(15) O(2)-V-O(5) 102.35(9) C(10)-N(2)-C(6) 118.0(3) O(3)#3-V-O(5) 87.84(6) C(10)-N(2)-Ni 127.3(2)

Fig. 3 IR spectrum of Ni2V2W4.

The temperature for TG experiment ranged from 20°C to 900°C. In the TG curves (Fig. 4), there are two

weight loss steps. The first step from 444°C to 493°C, which is exothermic and has a weight loss of 20.6 %,

corresponds to the anion decomposition. The second step occurs at 565°C with a loss of 14 %, indicating

releasing of ligand bpy. Therefore, during 565-601°C, the compound cluster decomposes, losing the remnant

about 11.0%.

Electrochemical behaviour It is well known that heteropolyanions can accept a limited numbers of

electrons without decomposition. Cyclic voltammmetry curve of Ni2V2W4 shows one one-electron, one three-

electron and one two-electron reduction waves at 95.50, 68.34 and 25.28 mV (vs. SCE), respectively. Three

successive oxidation waves are located at 74.45, 43.90 and 11.23 mV (vs. SCE). (Fig. 5(a) and (b)) According

to the formula E1/2 = |Epa + Epc| / 2, E1/2 = 1.52 (2.303 RT / nF) (mV) [15] (n denotes the electrons that

transferred), figure 6a shows the cyclic voltammetric behavior of the compound in solution at different scan

rates, using cathodic adsorptive voltammetric method of a carbon paste electrode. With the scan rate varying

from 100 to 250 mV·s-1, the peak potentials changed gradually: the cathodic peak potentials shifted toward the

negative direction and the corresponding anodic peak potentials to the positive direction with increasing scan

rates. The peak-to-peak separation between the corresponding cathodic and anodic peaks increases with the

scan rate increasing.

878 Xin Wang et al.: Crystal structure and electrochemical behavior of a novel polyoxometalate

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Fig. 4 TG-DTA curves of Ni2V2W4.

Figure 5 shows that the pH of the supporting electrolyte effects on the electrochemical behavior of the

compound in aqueous solutions. The cathodic potentials gradually shift to the negative direction, and the

corresponding anodic peak potentials shift to the positive direction along with reducing pH.

Fig. 5 a) Cyclic Voltammmetry curves of Ni2V2W4 with different scan rates (A: 100 mV·s-1, B: 150 mV·s-1,

C: 200 mV·s-1 , D: 250 mV·s-1). b) Cyclic Voltammmetry curves of Ni2V2W4 with different pH. (A: 0.25 mol·L-1 H2SO4, B: 0.5 mol·L-1 H2SO4, C: 1.0 mol·L-1 H2SO4). Conditions: Ag-AgCl working electrode, platinum SCE referenece electrode, making Ni2V2W4 as carbon paste electrode (CPE), using N2 as protected atmosphere. Using 0.25, 0.5 and 1.0 mol·L-1 H2SO4 aqueous solutions change the pH.

4 Conclusion

In conclusion, the synthesized compound shows some interesting properties. The hydrothermal synthesis is a

promising method for the preparation of isopolyanions in the presense of organic ligands and it provides a new

mild method for the design of solid materials by linking POMs and organic ligands. Furthermore, this new

material shows some interesting redox properties and has potential applications to electrocatalysis. It is

expected that many novel compounds can be synthesized based on this method.

Acknowledgments This work was supported by NSFC (No. 20371014).

References

[1] Hai-Yan An, En-Bo Wang, and Lin Xu, Angew. Chem. 118, 918–922 (2006). [2] Laurent Lisnard, Anne Dolbecq, and Pierre Mialane, Dalton Trans. 24, 3913–3920 (2005). [3] Lei Qian and Xiurong Yang, Electrochem. Commun. 5, 547–551 (2005). [4] Bastian Ewald, Yigit Öztan, and Rüdiger Kniep, Z. Anorg. Allg. Chem. 631, 1615–1621 (2005).

Cryst. Res. Technol. 41, No. 9 (2006) 879

www.crt-journal.org © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[5] Qiang Li, Pingfan Wu, and Hongyou Guo, J. Org. Chem. 691, 1223–1228 (2006). [6] R. John Errington, Sagar S. Petkar, and Benjamin R. Horrocks, Angew. Chem. 117, 1280–1283 (2005). [7] Elmer C. Alyea, Don Craig, and Ian Dance, Cryst. Eng. Comm. 81, 491–503 (2005). [8] R. F. Klewdowa, E. N. Jurtchenko, L. A. Glinskaya, J. Strukt. Chem. 84, 88 (1988) (in russian). [9] Jin-Yu Xie and Jiang-Gao Mao, J. Mol. Struct. 750, 186–189 (2005).

[10] Eric Burkholder and Jon Zubieta, Inorg. Chim. Acta 357, 279–284 (2004). [11] H.-W. Hou and J. Huang, Acta. Cryst. C 51, 2013–2015 (1995). [12] Zhi-Feng Zhao, Bai-Bin Zhou, Zhan-Hua Su, Xu Zhang, and Guang-Hua Li, Cryst. Growth Des. 6, 632–635

(2006). [13] Y. J. Wei, K. W. Nam, and K. B. Kim, Solid State Ionics 9, 2243–2249 (2005). [14] M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer Verlag, Berlin, 125–138 (1983). [15] Xiao-Xia Gao, Introduction to Electroanalysis Chemistry, Beijing: Scientific Press, 359 (1991).