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Journal of Molecular Structure 837 (2007) 237–244

Synthesis and characterization of two novel high-dimensionalextended structures based on Keggin-type polyoxometalates

and potassium–glycine complex subunits

Jia Liu, Yangguang Li, Enbo Wang *, Dongrong Xiao, Linlin Fan, Zhiming Zhang, Yu Wang

Key Laboratory of Polyoxometalate Science of Ministry of Education, Institute of Polyoxometalate Chemistry, Department of Chemistry,

Northeast Normal University, Changchun, Jilin 130024, PR China

Received 25 September 2006; received in revised form 18 October 2006; accepted 21 October 2006Available online 26 December 2006

Abstract

Two novel compounds based on Keggin-type polyoxometalates and potassium–glycine coordination complexes, {K2(HGly)2[Si-W12O40]}Æ9H2O 1 and {K3HGly[SiW12O40]}Æ6H2O 2 (Gly = glycine), have been synthesized and characterized by elemental analyses,IR, TG analyses, cyclic voltammetry and single-crystal X-ray diffraction. Compound 1 is composed of the [a-SiW12O40]4� buildingblocks, which are linked through potassium–glycine acid fragments to form a novel two-dimensional (2D) network. Compound 2 con-tains an infinite 2D layer constructed from [SiW12O40]4� building blocks and potassium cations, and adjacent layers are further joined bypotassium–glycine complex subunits and potassium cations to yield an unusual three-dimensional (3D) framework. The crystal data fortwo compounds are the following: 1, monoclinic, space group P21/c, a = 9.829(4) A, b = 2.477(3) A, c = 9.777(4) A, b = 104.80(3)o,V = 4730.7(17) A3, Z = 4; 2, monoclinic, P2/c, a = 2.777(3) A, b = 7.529(4) A, c = 9.219(4) A, b = 99.60(3)�, V = 4244.2(15) A3, Z = 4.� 2006 Elsevier B.V. All rights reserved.

Keywords: High-dimensional structures; Keggin-type polyoxoanions; Potassium–organic complexes; Glycine

1. Introduction

The rational design and synthesis of the functionalizedpolyoxometalates (POMs) decorated by organic moleculeshave received extensive attention in recent years, not onlyowing to their application prospect in catalysis, sorption,medicine and electrical conductivity, but also to theircharming variety of architectures and topologies [1–4]. Acurrent focus in this field is the high-dimensional structuresbased on POMs building blocks and metal cations/metal–organic coordination complexes, because of sufficient char-ge density and various geometric topologies of the POMs[5–8]. So far, many multidimensional architectures of thePOMs modified by diversified metalate cations/metal–or-

0022-2860/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2006.10.037

* Corresponding author. Tel.: +86 431 5268787; fax: +86 431 5684009.E-mail addresses: wangenbo@public.cc.jl.cn, wangeb889@nenu.edu.cn

(E. Wang).

ganic coordination complexes have been detailed reported[9,10]. However, in contrast, the POMs decorated metal–aminophenol coordination complexes have not been soextensively studied [11,9a]. Therefore, the rational designand synthesis of extended structures containing POMsand metal–organic complexes in the presence of amino acidligands are one of great challenges in current syntheticchemistry.

Amino acid, as the basic unit of proteins, possesses high-ly biologic activity. It is the basic material of cells and tis-sues, and has important function in medical applications[12]. Moreover, there are one carboxyl group and one ami-no group in the amino acid molecule, so the amino acidmolecule can act as not only hydrogen-bonding acceptersand hydrogen-bonding donors, but also bidentate or tri-dentate ligands, which can coordinate to various metalsto give rise to inorganic–organic hybrid materials [13].The potassium atom, with highly biologic activity, is the

Table 1Crystal data and structure refinement for 1 and 2

Complex 1 2

Chemical formula C4H30SiW12N2K2O53 C2H18K3SiW12NO48

Formula weight 3266.79 3175.76T (K) 293(2) 293(2)k (A) 0.71073 0.71073Crystal system Monoclinic MonoclinicSpace group P2(1)/c p2/ca (A) 19.829(4) 12.777(3)b (A) 12.477(3) 17.529(4)c (A) 19.777(4) 19.219(4)b (�) 104.80(3) 99.60(3)V (A3) 4730.7 (17) 4244.2(15)Z 4 4Dc (g/cm3) 4.587 4.970l (mm�1) 29.377 32.824Rint 0.1119 0.1035GOF on F2 1.031 1.012R1a 0.0456 0.0497wR2b 0.0868 0.1030R1 (all data) 0.0689 0.0717wR2 (all data) 0.0961 0.1123

a R1 = RiF0| � |Fci|/R|F0|.b wR2 = R [wðF 2

0 � F 2cÞ

2]/R [wðF 20Þ

2]1/2.

238 J. Liu et al. / Journal of Molecular Structure 837 (2007) 237–244

primary cation in the cell sap; it can adjust the penetratingpress of the cells and the acid–base balance of the body flu-id. Furthermore, the potassium atom can combine with thePOMs or organic ligands containing oxygen atoms ornitrogen atoms to form multidimensional structures,although their interaction sometimes is weak [14]. In viewof these, an incentive for design and synthesis of organic-inorganic hybrid compounds based on POMs and potassi-um–aminophenol coordination complexes has beenbrought forward. This may not only result in new hybridmaterials with high-dimensionality, but also shed light onthe biochemical activity of POMs.

In this paper, we choose the a-Keggin-type polyoxoa-nions as inorganic building blocks to synthesize two novelcompounds, {K2(HGly)2[SiW12O40]}Æ9H2O 1 and{K3HGly [SiW12O40]}Æ6H2O 2. Two compounds are bothconsists of [a-SiW12O40]4� anions, glycine and K+ cations.In compound 1, potassium–glycine coordination complex-es link [a-SiW12O40]4� anions together to form a 2D lay-ered structure. Compound 2 shows a 3D framework, inwhich K+ ions and potassium–glycine complex subunitsconnect [a-SiW12O40]4� anions together. To the best ofour knowledge, no compounds based on Keggin-typePOMs and potassium–glycine coordination complexeshave been reported. The synthesis and crystal structure oftwo new compounds are reported here.

2. Experimental section

2.1. Materials and methods

All chemicals were commercially purchased and usedwithout further purification. K4[a-SiW12O40]Æ17H2O wassynthesized according to the literature [15] and character-ized by IR spectrum and TG analysis. Elemental analyses(C) were performed on a Perkin-Elmer 2400 CHN elemen-tal analyzer; W and K were analyzed on a PLASMA-SPE(I) ICP atomic emission spectrometer. IR spectra wererecorded in the range 400–4000 cm�1 on an Alpha Cen-taurt FT/IR Spectrophotometer using KBr pellets. TGanalyses were performed on a Perkin-Elmer TGA7 instru-ment in flowing N2 with a heating rate of 10 �C min�1.

2.2. Syntheses

2.2.1. {K2(HGly)2[SiW12O40]}Æ9H2O 1K4[a-SiW12O40]Æ17H2O (0. 406 g, 0.15 mmol) was first

dissolved in 10 mL of distilled water with stirring. Theinsoluble material was removed by filtration. And then,Gly (0.045 g, 0.6 mmol) was added with vigorously stirring,the mixture was adjusted to pH 2 with diluted HCl andrefluxed for 3 h at 80 �C. Then cooled to room temperatureand filtered. The filtrate was kept for one week at roomtemperature, then colorless transparent block crystals ofcompound 1 were isolated in about 30% yield (based onW). Anal. Calcd for 1: W, 67.53; K, 2.39; C, 1.47 (%).Found: W, 67.19; K, 2.62; C, 1.76 (%).

2.2.2. {K3HGly [SiW12O40]}Æ 6H2O 2K4[a- SiW12O40]Æ17H2O (0. 406 g, 0.15 mmol) was first

dissolved in 10 mL of distilled water with stirring. Theinsoluble material was removed by filtration. And then,Gly (0.023 g, 0.3 mmol) was added with vigorously stirring,the mixture was adjusted to pH 2 with diluted HCl andrefluxed for 3 h at 80 �C. Then cooled to room temperatureand filtered. The filtrate was kept for one week at roomtemperature, then colorless transparent block crystals ofcompound 2 were isolated in about 40% yield (based onW). Anal. Calcd for 2: W, 69.46; K, 3.69; C, 0.76 (%).Found: W, 69.82; K, 3.42; C, 1.04 (%).

2.3. X-ray crystallography

A colorless single crystal of 1 were carefully selectedunder a polarizing microscope and severally glued at theend of a glass capillary. The data of X-ray diffraction werecollected on a Rigaku R-AXIS RAPID IP diffractometerwith Mo Ka (k = 0.71073 A) operating at 293 K in therange of 3.07 < h < 25.00�. Empirical absorption correctionwas applied. A total of 35086 (8312 unique, Rint=0.1119)reflections were measured (�23 6 h 6 23, �13 6 k 6 14,�23 6 l 6 22). A colorless single crystal of 2 were carefullyselected under a polarizing microscope and severally gluedat the end of a glass capillary. The data of X-ray diffractionwere collected on a Rigaku R-AXIS RAPID IP diffractom-eter with Mo Ka (k = 0.71073 A) operating at 293 K in therange of 3.12 < h < 25.00�. Empirical absorption correc-tion was applied. A total of 30542 (7456 unique,Rint=0.1035) reflections were measured (�15 6 h 6 15,�20 6 k 6 20, �22 6 l 6 22).

Table 2Range of bond lengths (A) and angles (�) for 1 and 2

Compound 1 Compound 2

W–Oa 2.340–2.355 W–Oa 2.301–2.376W–Ob 1.878–1.937 W–Ob 1.860–1.964W–Oc 1.901–1.951 W–Oc 1.877–1.972W–Od 1.680–1.727 W–Od 1.678–1.719Si–O 1.622–1.633 Si–O 1.631–1.679K–O 2.670–3.364 K–O 2.664–3.412O–W–O 72.3–170.5 O–W–O 71.9–172.0O–Si–O 108.7–109.8 O–Si–O 108.5–110.3O–K–O 49.2–160.3 O–K–O 53.3–163.0

Symmetry transformations used to generate equivalent atoms: for 1: #1x,�y � 1/2,z � 1/2 #2 x,y + 1,z #3 x,�y + 1/2,z � 1/2 #4 x,y � 1,z #5x,�y1/2,z+1/2 #6 x,�y+1/2,z+1/2; for 2: #1 �x+2,�y+1,�z #2x,�y + 2,z + 1/2 #3 �x + 1,�y + 2,�z #4 �x + 1,y,�z1/2 #5�x + 1,�y+1,�z #6 x,�y+1,z + 1/2 #7 �x + 2,y,�z1/2 #8x,�y+1,z�1/2 #9 �x + 1,y,�z+1/2 #10 x,�y+2,z�1/2 #11�x + 2,y,�z + 1/2.

Table 3Selected bond lengths (A) and angles (�) for 1 and 2

Compound 1 Compound 2

W(1)–O(2) 1.727(11) W(1)–O(35) 1.697(12)W(1)–O(25) 1.909(12) W(1)–O(31) 1.935(15)W(1)–O(14) 2.342(11) W(1)–O(37) 2.332(11)W(2)–O(23) 1.707(11) W(2)–O(29) 1.689(14)W(2)–O(42) 1.937(11) W(2)–O(5) 1.914(11)W(2)–O(14) 2.346(10) W(2)–O(37) 2.327(13)W(5)–O(44) 1.714(10) W(3)–O(15) 1.708(14)W(5)–O(34) 1.922(12) W(3)–O(27) 1.921(12)W(5)–O(13) 2.351(10) W(3)–O(37) 2.359(11)W(6)–O(4) 1.703(11) W(4)–O(12) 1.711(13)W(6)–O(39) 1.917(10) W(4)–O(6) 1.923(11)W(6)–O(26) 2.353(10) W(4)–O(25) 2.317(12)W(7)–O(3) 1.709(11) W(5)–O(13) 1.715(13)W(7)–O(16) 1.917(11) W(5)–O(30) 1.926(13)W(7)–O(12) 2.352(10) W(5)–O(23) 2.344(10)W(12)–O(41) 1.691(11) W(6)–O(28) 1.712(14)W(12)–O(32) 1.934(10) W(6)–O(18) 1.940(12)W(12)–O(26) 2.348(11) W(6)–O(24) 2.376(13)Si(1)–O(13) 1.622(12) Si(1)–O(24) 1.631(12)Si(1)–O(26) 1.633(10) K(1)–O(42) 2.82(2)K(1)–O(6) 2.670(12) O(24)–Si(1)–O(23) 108.5(7)K(2)–O(17) 2.845(16) O(31)–W(1)–O(37) 72.3 (5)O(14)–Si(1)–O(12) 109.8(6) O(4)–K(1)–O(42) 83.5(5)O(2)–W(1)–O(25) 100.3(5) OW5#9–K(2)–O(3)#9 95.4(5)O(1)–K(1)–O(4) 56.4(3) OW4–K(3)–O(13) 133.2(5)O(17)–K(2)–O(27) 69.3(4) O(31)–K(4)–OW6 67.9(4)

Symmetry transformations used to generate equivalent atoms: for 1: #1x,�y1/2,z � 1/2 #2 x,y + 1,z #3 x,�y + 1/2,z�1/2 #4 x,y � 1,z #5x,�y1/2,z + 1/2 #6 x,�y + 1/2,z + 1/2; for 2: #1 �x + 2,�y + 1,�z #2x,�y + 2,z + 1/2 #3 �x + 1,�y+2,�z #4 �x + 1,y,�z � 1/2 #5�x + 1,�y + 1,�z #6 x,�y + 1,z + 1/2 #7 �x + 2,y,�z � 1/2 #8x,�y+1,z � 1/2 #9 �x + 1,y,�z + 1/2 #10 x,�y + 2,z � 1/2 #11�x + 2,y,�z + 1/2.

Fig. 1. ORTEP drawing of compound 1 with thermal ellipsoids at 50%probability. Lattice water molecules and H atoms are omitted for clarity.

J. Liu et al. / Journal of Molecular Structure 837 (2007) 237–244 239

The structures of 1 and 2 were solved by the directmethod and refined by full-matrix least-squares on F2 usingthe SHELXL 97 software [16]. All the non-hydrogen atomsin 1 and 2 were refined anisotropically, in 1 and 2 positionsof the hydrogen atoms attached to nitrogen atoms andthose to carton atoms were fixed in ideal positions, and

other hydrogen atoms were not located. A summary ofthe crystallographic data and structural determination for1 and 2 is provided in Table 1. The range of bond lengthsand bond angles of 1 and 2 is listed in Table 2. Selectedbond lengths and bond angles of 1 and 2 are listed in Table3. Crystal data and structure refinement, atomic coordi-nates, bond lengths and angles, and anisotropic displace-ment parameters of 1 and 2 are available insupplementary crystallographic data.

CCDC reference number: (619051) for 1 and (619050)for 2.

3. Results and discussion

3.1. Structure description

The single-crystal X-ray analysis reveals that compound1 is a 2D layered construction, which is composed of [a-SiW12O40]4� anions, potassium–glycine complex subunitsand the water molecules. The [a-SiW12O40]4� anion is thewell-known Keggin structure, which is made up of twelveWO6 octahedra, with the Si atom occupying tetrahedralcavity in the center of the anion. The twelve WO6 octahe-dra may be subdivided into four W3O13 groups, eachW3O13 is composed of three WO6 octahedra linked in a tri-angular arrangement by sharing edges, the four W3O13

groups are linked together by sharing corners and linkedto the SiO4 tetrahedron by the three-coordinated oxygenatoms (see Fig. 1). The W–O bond lengths can be catego-rized into five types in accordance with the oxygen coordi-nation: 1.680–1.727 A for the terminal oxygens (W–Od),1.901–1.947 A for the edge-shared oxygens (W–Oc),1.878–1.937 A for the corner-shared oxygens (W–Ob), and2.340–2.355 A for the oxygens of the SiO4 group (W–Oa).1.691–1.951 A for the oxygens commoning to W and K.The Si–O bond lengths in the SiO4 tetrahedron are in the

240 J. Liu et al. / Journal of Molecular Structure 837 (2007) 237–244

range 1.622–1.633 A. the W–O–W bond angles changebetween 91.13� and 153.16�.

There are two crystallization-independent potassiumions in 1 (see Fig. 1). K(1) is nine-coordinate, defined bythree terminal oxygen atoms from three polyoxoanions(average K(1)–O 3.104(7) A), two bridged oxygen atoms

Fig. 2. The ball-and-stick representation of a kalium–glycine complexsubunit for compound 1.

Fig. 3. (a) Polyhedral and ball-stick view of the 1D corrugated chain in 1. (b)(color code: WO6 octahedra, red; central SiO4 tetrahedra, yellow; K, green sphwater molecules have been omitted for clarity. (For interpretation of the referenthis paper.)

from two polyoxoanions (average K(1)–O 3.361(1) A),two oxygen atoms from two glycine molecules (K(1)–O1

2.925(1) A and K(1)–O6 2.670(1) A), and two water mole-cules (average K(1)–Ow 2.80(7) A). K(2) has a slightly dis-torted pentagonal bipyramidal coordination environment,formed by two terminal oxygen atoms from two {WO6}octahedra of different polyoxoanions (average K(2)–O2.994(1) A), one bridged oxygen atom from one polyoxo-anion (K(2)–O27 2.888(1) A), one oxygen atom from oneglycine molecule (K(2)–O17 2.845(2) A), and three watermolecules (average K(2)–Ow 2.757(4) A). There are alsotwo crystallographically independent glycine moleculesadopting two different coordination modes. One glycinemolecule, as a monodentate ligand, coordinates to K(1)by utilizing its one oxygen atom of carboxyl group. Theother glycine molecule, as a bidentate ligand, coordinatesto K(1) and K(2) by utilizing its two oxygen atoms of car-boxyl group. In this way, two glycine molecules and twopotassium ions form a potassium–glycine complex subunit(see Fig. 2).

A view along the a-axis illustrating the infinite 2D layered structure in 1

ere; O, red spheres, C, dark spheres; N, blue spheres). H atoms and latticeces to color in this figure legend, the reader is referred to the web version of

J. Liu et al. / Journal of Molecular Structure 837 (2007) 237–244 241

The [a-SiW12O40]4� anion link to two others to form a1D corrugated chain by two K(2) ions of two complex sub-units in a tridentate way. In this 1D corrugated chain, oneK(1) ion of one complex subunit is joined to two terminaloxygen atoms and two bridged oxygen atoms from twoadjacent [a-SiW12O40]4� anions of another 1D corrugatedchain to yield an unusual infinite 2D layered structure(see Fig. 3). To the best of our knowledge, no analogous2D layered architecture composed of Keggin-type POMsbuilding blocks and potassium–aminophenol complexeshas been reported in the literature. Valence bond calcula-tions [17] suggest that Si atom is in the +4 oxidation state,each K atom is in the +1 oxidation state and all W atomsare in the +6 oxidation state. These oxidation states areidentical with the charge balance considerations.

When we change the quantity of glycine, compound 2

was obtained. The compound 2 is a three-dimensionalarchitecture, which is composed of [a-SiW12O40]4� anions,potassium–glycine complex subunits, K+ ions and thewater molecules. In Keggin-type [a-SiW12O40]4� cluster,there are five kinds of W–O distances according to the oxy-gen coordination: 1.689–1.719 A for the terminal oxygens(W–Od), 1.877–1.972 A for the edge-shared oxygens (W–Oc), 1.860–1.964 A for the corner-shared oxygens (W–Ob), and 2.301–2.376 A for the oxygens of the SiO4 group(W–Oa), 1.678–1.961 A for the oxygens commoning to Wand K. The Si–O bond lengths in the SiO4 tetrahedronare in the range 1.631–1.679 A. The W–O–W bond angleschange from 90.54� to 153.28�.

There are four crystallization-independent potassiumions in 2 (see Fig. 4). The eight-coordinated K(1) links tofour terminal oxygen atoms from four polyoxoanions(average K(1)–O 2.741(6) A), one bridged oxygen atomfrom one polyoxoanion (K(1)–O36 2.939(1) A), one oxygenatom from one glycine molecule (K(1)–O42 2.82(2) A), and

Fig. 4. ORTEP drawing of compound 2 with thermal ellipsoids at 50%probability. H atoms and non-coordinated water molecules have beenomitted for clarity.

two water molecules (average K(1)–Ow 2.75(7) A). K(2)completed its eight-coordination by four terminal oxygenatoms from four {WO6} octahedra of different polyoxoa-nions (average K(2)–O 2.890(6) A), two bridged oxygenatoms from two polyoxoanions (average K(2)–O3.264(1) A), and two water molecules (average K(2)–Ow2.758(2) A). K(3) is nine-coordinate, which is formed byfour terminal oxygen atoms from four polyoxoanions(average K(3)–O 2.966(6) A), two bridged oxygen atomsfrom two polyoxoanions (average K(3)–O 2.810(1) A),one oxygen atom from one glycine molecule (K(3)–O1

2.803(1) A), and two water molecules (average K(3)–Ow3.04(2) A). K(4) is in the center of a distorted octahedron,connected by two terminal oxygen atoms from two{WO6}octahedra of different polyoxoanions (average K(4)–O2.730(1) A), two bridged oxygen atoms from two poly-oxoanions (average K(4)–O 2.698(1) A), and two watermolecules (average K(4)–Ow 2.779(1) A).

The [a-SiW12O40]4� anion connect to two neighbors toproduce a 1D chain by one K(2) ion in a bidentate wayand two K(3) ions in a tridentate way. In this chain, oneK(3) ion and one K(4) ion are, respectively, linked to oneterminal oxygen atom from one [a-SiW12O40]4� anion ofanother 1D chain to generate an infinite 2D layer (seeFig. 5). Each potassium ion connects with these 2D layersto yield a 3D framework. (see Fig. 6). Valence bond calcu-lations [17] suggest that Si atom is in the +4 oxidationstate, each K atom is in the +1 oxidation state and all Watoms are in the +6 oxidation state. These oxidation statesare identical with the charge balance considerations.

3.2. FT-IR spectroscopy

In the IR spectrum of compound 1, the characteristicpeaks at 1018, 981, 971, 926, 880, 779, 533, and 476 cm�1

are attributed to m(Si–Oa), m(W–Od), m(W–Oa), m(W–Ob)and m(W–Oc) of the [a-SiW12O40]4� polyoxoanion andthe characteristic bands at 3553, 3481, 3198, 1751, 1727,1618, 1583, 1513, 1493, 1430, 1377, 1328, 1307, 1272,1248, 1118, and 1103 cm�1 can be regarded as features ofthe glycine molecule (see Fig. S1a). In the IR spectrum ofcompound 2 (Fig. S1b), the characteristic peaks at 1017,977, 922, 879, 784, 549, 529, and 419 cm�1 demonstratethat [a-SiW12O40]4� is a Keggin structure and characteristicbands at 3567, 3218, 3011, 1732, 1611, 1487, 1422, 1312,and 1252, 1135, and 1106 cm�1 can be regarded as featuresof the glycine molecule.

3.3. Thermal analyses

The TG curve of compound 1 gives a total weight loss of9.01% in the range of 38–774 �C (see Fig. S2a), whichagrees with the calculated value of 9.62%. The weight lossof 1.65% at 38–195 �C corresponds to the release of thenon-coordinated water molecules (calc. 2.21). The weightloss of 7.36% at 275–774 �C corresponds to the loss ofthe coordinated water molecules and the organic molecules

Fig. 5. (a) Polyhedral and ball-stick view of the 1D chain of 2. (b) The infinite 2D layer of 2 viewing along the b-axis (color code: WO6 octahedra, purple;central SiO4 tetrahedra, yellow; K, green sphere; O, red spheres, C, dark spheres; N, blue spheres). H atoms and non-coordinated water molecules havebeen omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

Fig. 6. A view along the a-axis illustrating the 3D framework in 2 (color code: WO6 octahedra, purple; central SiO4 tetrahedra, yellow; K, green sphere; O,red spheres, C, dark spheres; N, blue spheres). H atoms and non-coordinated water molecules have been omitted for clarity. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this paper.)

242 J. Liu et al. / Journal of Molecular Structure 837 (2007) 237–244

(calc. 7.41%). The TG curve of compound 2 is shown inFig. S2b. The TG curve exhibits a total weight loss of5.43% in the range of 37–443 �C, which agrees with the cal-culated value of 5.80%. The weight loss of 3.82% at 37–

124 �C corresponds to the release of all non-coordinatedand coordinated water molecules (calc. 3.4%). The weightloss of 1.61% at 195–443 �C corresponds to the loss ofthe organic molecules (calc. 2.4%).

Fig. 7. The cyclic voltammograms of compound 1 (black line) and 2 (redline) at the scan rate of 10 mV/s�1. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of thispaper.)

J. Liu et al. / Journal of Molecular Structure 837 (2007) 237–244 243

3.4. Electrochemical behavior

The cyclic voltammogram of compound 1 in a pH 4.7buffer solution (1 M CH3COONa + CH3COOH) at the scanrate of 10 mV/s�1 shows three reduction peaks of the WVI

centers [18] (see Fig. 7). Thereinto two one-electron reduc-tion peaks located at �216 and �473 mV, respectively, onetwo-electron reduction peak at �933 mV. The typical cyclicvoltammetric behavior for compound 2 in a pH 4.7 buffersolution (1 M CH3COONa + CH3COOH) at the scan rateof 10 mV/s�1 shows two one-electron reduction peaks locat-ed at�218 and�465 mV, respectively, and one two-electronreduction peak at�935 mV. The three reduction peaks cor-respond to the reduction of the WVI centers [18] (see Fig. 7).Therefore, the peaks can be described by the followingequations.

a-SiW12O404� þ e� ¼ a-SiW12O40

5�

a-SiW12O405� þ e� ¼ a� SiW12O40

6�

a-SiW12O406� þ 2e� þ 2Hþ ¼ a�H2SiW12O40

6�

4. Conclusions

In summary, we report here for the first time two high-di-mensional extended complexes constructed from dod-ecatungstosilicate polyanions building blocks andpotassium–glycine coordination complexes, which may pos-sess new and unique properties in contrast to discrete poly-oxometalates and Amino acid. The successful syntheses oftwo compounds are expected to be effective for the furtherconstruction of novel polyoxometalate-based extendedstructure.

Acknowledgement

This work was financially supported by the NationalNatural Science Foundation of China (20371011).

Appendix A. Supplementary data

The IR spectrums and the TG curves of compound 1

and 2 and the additional tables are available. Supplementa-ry data associated with this article can be found, in theonline version, at doi:10.1016/j.molstruc.2006.10.037.

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