Inorganica Chimica Acta 379 (2011) 151–157

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Inorganica Chimica Acta

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Four novel polytungstate compounds built up of paradodecatungstate clustersand transition-metal complexes/transition metal: Synthesis, structure,and electrochemical properties

Song Gao a, Jingxiang Zhao a, Baibin Zhou a,c,⇑, Kai Yu a,c, Zhanhua Su a, Lu Wang c, Yongkui Yin b,Zhifeng Zhao a, Yang Yu a, Ying Chen a

a Key Laboratory of Synthesis of Functional Materials and Green Catalysis, College of Heilongjiang Province, Harbin Normal University, Harbin 150080, PR Chinab School of Public Health, Mudanjiang Medical University, Mudanjiang 157011, PR Chinac Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e i n f o

Article history:Received 31 October 2010Received in revised form 13 August 2011Accepted 2 October 2011Available online 8 October 2011

Keywords:PolyoxometalateParadodecatungstateTransition metalTopologyElectrochemical property

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.10.011

⇑ Corresponding author at: Key Laboratory of Syntand Green Catalysis, Colleges of Heilongjiang ProvincHarbin 150080, PR China. Tel.: +86 451 88060770.

E-mail address: zhou_bai_bin@163.com (B. Zhou).

a b s t r a c t

A series of novel polytungstates, [{Cu(bim)(H2O)2}4(H4W12O42)]�14H2O 1, (Himi)6[{Mn(imi)(H2O)}(H4-

W12O42)]�2H2O 2, [{Cu0.5(H2O)}2{Cu0.5(H2O)2}2{Cu (bim)(H2O)2}2(H4W12O42)]�10H2O 3 and [{Na(H2O)4}2-{Cu0.5(H2O)}4{Cu0.5 (H2O)1.5}2(H4W12O42)]�3H2O 4 (imi = imidazole; bim = 2,20-biimidazole), have beenobtained by the routine synthetic reactions in aqueous solution and characterized by IR, X-ray powderdiffraction, UV, elemental analysis, TG and X-ray single-crystal diffraction. All the compounds containthe [H4W12O42]8� polyanions as building units. Compound 1 is isolated structure, which is modified withfour {Cu(bim)(H2O)2}2+ fragments. The compound 2 exhibits an infinite 1D ‘‘wave-like’’ chain based onthe interaction between the [H4W12O42]8� polyanion and [Mn(imi)(H2O)]2+ complex. The 2D layeredstructure of the compound 3 is formed by the interconnection 1D [{Cu0.5(H2O)}2(H4W12O42)]6� and[{Cu0.5(H2O)2}2(H4W12O42)]6� chains. While compound 4 can be abstracted of structural topology with(6,2)-connected 3D network. In addition, the electrochemical properties of these compounds areinvestigated.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Polyoxometalates (POMs) have attracted considerable attention[1–5] due to their diverse of properties and potential applicationsin catalysis, medicine, electro-conductivity, magnetism, and photo-chemistry. At present, the interesting research in this field isfocused on the exploration of frameworks based on a series ofPOMs building blocks and transition-metal complexes/transitionmetal linkers [6–8]. We note that many recent reports have con-centrated on the Keggin [9], Dawson [10], Anderson [11] andisopolymolybdates [12] POMs. However, few efforts have everbeen devoted to the isopolytungstates.

On the other hand, the isopolytungstate can be designed and syn-thesized different dimensions with novel architectures and charac-teristics due to the plenty room of their special structures [13]. Someisopolytungstates with novel structures have been characterized,such as [W10O32]4� [14], [W18O56]4� [15], [W22O74]16� [16], etc.Compared to the above compounds, it is more valuable to discuss

ll rights reserved.

hesis of Functional Materialse, Harbin Normal University,

the [H4W12O42]8� polyanion, which could be easily modified to formdifferent dimensional covalent networks because of its highly activesurface oxygen and charge density. Furthermore, introducing thetransition metal bridging units into the polyanions not only dramat-ically enriches the inorganic backbone, but also enhances theirchemical properties [17,18]. As summarized in Table S1, (a list ofthe representative in this field), most structures are focused on 0D,1D to 2D, 1D to 3D, or 2D to 3D. However, few results were detectedon 0D to 3D. As a result, it seems that synthesizing 0D to 3D POMsbased on [H4W12O42]8� polyanions remain a lot of barriers in thisfield. In addition, some organic-ligands, such as en, bpy, and bbi,have been discovered as the part of transition-metal complexeswhich modified paradodecatungstate clusters [13,19,20]. However,the imi or bim ligands had not been reported. In the synthetic strat-egy, the two kinds of organic-ligands attract our attention becausethey are easy to form multi-core molecular aggregates or coordina-tion sites for their excellent acid–base conjugacy, proton acceptabil-ity, coordination selectivity and possessing multi coordination sites.Moreover, the imi-ring with multiple energy levels can act as elec-tron-rich systems with N to capture transition metals and to formthe complex of high drug activity. Therefore, imi and bim have greatapplications on drugs ligands and become good candidates to con-nect various transition metals [21].

152 S. Gao et al. / Inorganica Chimica Acta 379 (2011) 151–157

Herein, a series of compounds based on [H4W12O42]8�polyanionsbuilding units, [{Cu(bim)(H2O)2}4(H4W12O42)]�14H2O 1, (Himi)6

[{Mn(imi)(H2O)}(H4W12O42)]�2H2O 2, [{Cu0.5(H2O)}2{Cu0.5(H2O)2}2

{Cu(bim)(H2O)2}2(H4W12 O42)]�10H2O 3, and [{Na(H2O)4}2{Cu0.5(-H2O)}4{Cu0.5(H2O)1.5}2(H4W12O42)]�3H2O 4 (imi = imidazole; bim= 2,20-biimidazole) have been successfully synthesized in aqueoussolution synthesis methods. Moreover, IR, X-ray powder diffraction,UV, elemental analysis, TG, cyclic voltammetry for 1–4, and electro-catalytic activities of 3, 4 have been studied in detail.

2. Experimental

2.1. Materials and methods

All reagents and solvents for synthesis were used as receivedwithout further purification. Elemental analyses (C, H, and N) wereperformed on a Perkin–Elmer 2400 CHN Elemental Analyzer andW, Mn, and Cu were analyzed on a PLASMA-SPEC (I) ICP atomicemission spectrometer. The IR spectra was using the technique ofpressed KBr pellets on an Alpha Centauri Fourier transform FTIRspectrometer in the range of 400–4000 cm�1 region. The UV spec-trum was obtained on a Unican UV – 500 spectrometer in the rangeof 600–190 nm. The thermo gravimetric analyses (TGA) were car-ried out in N2 on a Perkin–Elmer DTA 1700 differential thermalanalyzer with a rate of 10 �C/min. Cyclic voltammetric measure-ments were carried on CHI650b electrochemical workstation. Elec-trochemical measurements were applied 0.4 M sodium acetatebuffer solution for 1.0 mM compounds 1–4 at room temperature.The surface of glass carbon electrode was washed with distilledwater before experiment. The working electrode was a glassy car-bon electrode, an Ag/AgCl electrode was made for a reference elec-trode and a platinum wire as the counter electrode. X-ray powderdiffractometry (XRPD) study of 1–4 were performed with aNETZSCHSTA 449C instrument, with a panalytical X-pert pro dif-fractometer and CuKa radiation.

2.2. Syntheses

2.2.1. Synthesis of compound 1The mixture of Na2WO4�2H2O (3.300 g, 10 mmol), CuCl2�2H2O

(0.422 g, 2.5 mmol) and bim (0.19 g, 1.4 mmol) were dissolved indistilled water to form a 40 mL solution which was heated at85 �C and stirred for 30 min, the pH value was adjusted to 5.0 with2 M H2SO4 until a precipitation appearing. After being cooled toroom temperature, the precipitate was removed by filtration. Afteraging for 3 weeks at room temperature, green oblong columnarcrystals suitable for X-ray diffraction were obtained (yield 58%,based on Cu). Anal. Calc. for C24H72N16Cu4O64W12 (4069.26): C,7.10; N, 5.51; H, 1.79; Cu, 6.25; W, 54.22. Found: C, 7.09; N,5.53; H, 1.78; Cu, 6.22; W, 54.30(%).

2.2.2. Synthesis of compound 2The preparation of 2 was similar to that of 1, except that

CuCl2�2H2O and bim were replaced by MnCl2�4H2O (0.36 g,1.8 mmol) and imi (0.29 g, 4.3 mmol), respectively. Additionally,the pH value was adjusted to 5.4 with CH3COOH. After aging for2 months at room temperature, yellow oblong columnar crystalssuitable for X-ray diffraction were obtained (yield 67%, based onW). Anal. Calc. for C21H44N14MnO45W12 (3475.72): C, 7.26; N,5.64; H, 1.28. Mn, 1.58; W, 63.47. Found: C, 7.23; N, 5.67; H,1.27; Mn, 1.60; W, 63.43%.

2.2.3. Synthesis of compound 3The preparation of 3 was similar to that of 1, except that bim

(0.13 g, 1.0 mmol) was added to the above solution, the pH value

was adjusted to 5.8. After aging for 3 weeks at room temperature,green oblong columnar crystals suitable for X-ray diffraction wereobtained (yield 62%, based on Cu). Anal. Calc. for C12H56N8Cu4O62

W12 (3764.93): C, 3.83; N, 2.98; H, 1.50; Cu, 6.75; W, 58.60. Found:C, 3.88; N, 3.01; H, 1.51; Cu, 6.72; W, 58.64%.

2.2.4. Synthesis of compound 4The preparation of 4 was similar to that of 1, except that bim

(0.054 g, 0.4 mmol) was added to the above solution, the pH valuewas adjusted to 6.5. After aging for 30 days at room temperature,green oblong columnar crystals suitable for X-ray diffraction wereobtained (yield 48%, based on Cu). Anal. Calc. for H40Cu3O60Na2W12

(3443.03): H, 1.17; Na, 1.34; Cu, 5.54; W, 64.08. Found: H, 1.19; Na,1.33; Cu, 5.51; W, 64.11%.

2.3. Crystal structure determination

Compounds 1–4 crystal data were collected on a Bruker SmartCCD diffractometer with Mo Ka radiation (k = 0.71073 Å) at273 K, which were collected for empirical absorption. The struc-tures of 1–4 were solved by direct methods and refined on F2 bythe full-matrix least squares using the SHELXTL-97 crystallographicsoftware [22,23]. Anisotropic thermal parameters refined to all ofthe non-hydrogen atoms. The hydrogen atoms were held in calcu-lated positions on carbon atoms and nitrogen atoms and that weredirectly included in the molecular formula on water molecules andprotonated oxygen atoms. The crystallographic data and structurerefinement were provided in Table 1 for compounds 1–4. Selectedbond lengths and angles were listed in Table S2, respectively.

3. Results and discussion

3.1. Synthesis

Compounds 1–4 were synthesized by the routine syntheticreactions in aqueous solution. Generally, there are many variableparameters (e.g., starting materials, pH, time, and temperature)can greatly affect the results in the routine synthesis technology.For compounds 1–4, the experiment shows that if starting materi-als, time and temperature are maintaining, while changing the pHvalue, the lower dimensional complexes are more liable to form incondition of lower pH value. There are more oxygen atoms on itssurface so it can get more coordination positions to form 0D com-plex, such as compound 1. If we reduce the starting materials ofbim and keep the other conditions constant, the coordinationbetween the ligand and copper will be reduced so as to form easilynon-ligand 3D complex, such as compound 4. The results indicatethat both the starting materials and pH value play an importantrole, as shown in Scheme 1.

3.2. Structural description

The {W12O42} cluster is composed of four trimetallic units. Indetail, the four trimetallic units can be separated into two types,the triangular {W3O13} and the open {W3O14} subunits. All thesetrimetallic units are consisted of three edge-sharing {WO6} octahe-dra. Moreover, in the triangular {W3O13} units the three {WO6}octahedra are totalized together by sharing one oxygen atom. Bycontrast, in the open {W3O14} units the {WO6} octahedra are con-nected without sharing a common oxygen atom. All tungstenatoms are hexacoordinated with respect to oxygen with W-O dis-tances in the range 1.703(9)–2.296(8) Å, 1.715(9)–2.272(8) Å,1.720(7)–2.353(7) Å, and 1.708(11)–2.308(10) Å for compounds1–4, respectively. Bond valence sum calculations indicate that allthe W atoms have a +6 oxidation state in compounds 1–4.

Table 1Crystallographic data for compounds 1, 2, 3 and 4.

Compounds 1 2 3 4

Formula C24H72N16Cu4O64W12 C21H44N14MnO45W12 C12H56N8Cu4O62W12 H40Cu3O60Na2W12

Formula weight 4069.26 3475.72 3764.93 3443.03T (K) 273 273 273 273Crystal system monoclinic monoclinic triclinic triclinicSpace group C2/c P21/m P-1 P-1a (Å) 27.352(3) 11.7985(8) 11.5345(6) 10.7060(11)b (Å) 12.3708(14) 23.3231(17) 13.2238(7) 12.7124(14)c (Å) 24.821(3) 14.2971(10) 13.9066(8) 13.1664(14)a (�) 90.00 90.00 98.1800(10) 113.7600(10)b (�) 102.4340(10) 110.2220(10) 94.4010(10) 90.8230(10)C (�) 90.00 90.00 115.1530(10) 111.8290(10)V (Å3) 8201.4(16) 3691.7(4) 1878.08(18) 1493.8(3)Z 4 2 2 1l (MoKa) mm�1 17.881 18.865 19.501 24.162F (000) 7296 3062 1662 1483Reflections collected/unique (Rint) 32612/10228 32982/9295 18774/9321 15045/7443Goodness-of-fit (GOF) on F2 0.1079

1.0290.05701.055

0.04261.052

0.04481.031

Final R indices [I > 2r(I)] R1 = 0.0529 R1 = 0.0428 R1 = 0.0421 R1 = 0.0549wR2 = 0.1162 wR2 = 0.1055 wR2 = 0.1092 wR2 = 0.1414

R1 = R||Fo| � |Fc||/R|Fo|, wR2 = [R(w(F2o � F2

c )2)/R(wF2o )2]1/2.

Scheme 1. Schematic illustration of the synthesis routes of compounds 1–4. (Thepink nodes symbolize the [H4W12O42]8� clusters, the turquoise nodes symbolize Cu,the yellow nodes symbolize Mn, and the water molecules are omitted for clarity).

Fig. 1. Polyhedral and ball-and-stick representation of the 0D structure ofcompound 1. (WO6 octahedra, red; Cu, purple). Water molecules are omitted forclarity. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

S. Gao et al. / Inorganica Chimica Acta 379 (2011) 151–157 153

3.2.1. The structure of compound 1In monomer compound 1, the [{Cu(bim)(H2O)2}4(H4W12O42)]

metal fragment contains [H4W12O42]8� clusters and four [Cu(bim)(H2O)2]2+ cations (Fig. S1). There are two kinds of crystallographi-cally-independent Cu2+ cations, which exhibit the similar coordi-nation environments in compound 1 (as shown in Fig. 1). Cu(1)and Cu(2) atoms pentacoordintated by two nitrogen atoms fromone bim ligand and three oxygen atoms. These oxygen atoms canbe classified into two types according to their origins: (i) O(20)and O(21) come from WO6 octahedron group of one [H4W12

O42]8� polyanion with Cu(1)–O(20) distance of 1.923(11) Å andCu(2)–O(21) distance of 1.916(11) Å; (ii) O(22), O(23), O(24) andO(25) are from water molecules with Cu(1)–O(22), Cu(1)–O(23),Cu(2)–O(24) and Cu(2)–O(25) distances of 1.951(13) Å,2.251(14) Å, 2.283(12) Å and 2.002(13) Å, respectively.

3.2.2. The structure of compound 2The building units of compound 2 is constructed by one

[{Mn(imi)(H2O)}{H4W12O42}]6� anion, six (Himi)+ cations and twolattice water atoms (Fig. S2). The compound 2 shows an infinite

1D ‘‘wave-like’’ chain built from the [H4W12O42]8� polyanion andone [Mn(imi)(H2O)]2+ cation. The manganese atoms, both in thecationic and anionic units, are hexadcoordinated according to the[5 + 1] mode (Fig. 2). The Mn(1) atom is coordinated by five oxygenatoms and one nitrogen atoms N(1) from one imi ligand withMn(1)–N(1) distance of 2.153(18) Å. O(15) and O(20) atoms whichis from WO6 octahedron group, join [Mn(imi)(H2O)]2+ unit to[H4W12O42]8� polyanion with Mn(1)–O(15) and Mn(1)–O(20) dis-tances of 2.181(9) Å and 2.219(9) Å, while O(22) atom is fromwater molecules with (Mn(1)–O(22) = 2.172(13) Å). The six pro-tonated imi molecules and two water molecules dissociated fromparadodecatungstate clusters.

Fig. 2. Polyhedral and ball-and-stick representation of the 1D ‘‘wave-like’’ chain structure of compound 2. (WO6 octahedra, orange; Mn, kelly). Dissociated imi molecules andwater molecules are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

154 S. Gao et al. / Inorganica Chimica Acta 379 (2011) 151–157

3.2.3. The structure of compound 3The structure of 3 differs from that of 1. Its particular structure

can be described that [H4W12O42]8� cluster is supported by threedistinct one [Cu0.5(H2O)]+, one [Cu0.5(H2O)2]+ and [Cu(bim)(H2O)2]2+ fragments (Fig. S3a). Interestingly, there are three crys-tallographically-independent Cu2+ cations exhibited three kindsof coordination environments in compound 3. The Cu(1) atom istetracoordinated by two bridging oxygen atoms originating fromtwo WO6 octahedron groups (Cu(1)–O(6) = 1.899(7) Å),Cu(1)–O(10) is weak interaction (Cu(1)–O(10) = 2.615 Å) and as-sorted with two terminal oxygen atoms deriving from water mol-ecules (Cu(1)–O(19) = 1.976(8) Å). So in the crystal of 3 the[H4W12O42]8� clusters are interconnected through [Cu0.5(H2O)]+

bridging groups to form an infinite 1D chain (Fig. S3b). WhileCu(2) is coordinated with four terminal oxygen atoms derivingfrom water molecules (Cu(2)–O(22) = 1.962(8) Å and Cu(2)–O(24) = 1.986(10) Å) and completed by two bridging oxygen atomsoriginating from two WO6 octahedron groups (Cu(2)–O(13) =2.287(7) Å). The clusters are interconnected through [Cu0.5(H2O)2]+

bridging groups to form an infinite 1D chain (Fig. S3c). The Cu(3)atom is pentacoordinated by two nitrogen atoms from one bim li-gand (Cu(3)–N(1) = 1.994(10) Å, Cu(3)–N(4) = 1.992(9) Å) andthree oxygen atoms, two of which are from water molecules(Cu(3)–O(23) = 2.288(10) Å, Cu(3)–O(25) = 1.984(9) Å), while theother one is from WO6 octahedron group (Cu(3)–O(20) =1.956(9) Å). Furthermore, 1D [{Cu0.5(H2O)}2 (H4W12O42)]6� chainlinked through [{Cu0.5(H2O)2}2(H4W12O42)]6� chains into a 2D layeralong the c axis (Fig. 3).

3.2.4. The structure of compound 4Compound 4 exhibits a 3D open framework based on

[H4W12O42]8� clusters and CuII linkers (Fig. S4a). There are threecopper ions (Cu1, Cu2 and Cu3) in the unit cell. Cu(1) and Cu(2)are coordinated with two terminal oxygen atoms deriving fromwater molecules (Cu(1)–O(23) = 2.014(16) Å, Cu(2)–O(21) =2.003(14) Å) and completed by two bridging oxygen atomsoriginating from two WO6 octahedron groups (Cu(1)–O(12)= 2.403(12) Å, Cu(1)–O(5) = 1.940(12) Å and Cu(2)–O(19) =1.932(13) Å, respectively. Cu(3) is bonded with two bridging oxy-gen atoms originating from two WO6 octahedron groups(Cu(3)–O(6) = 1.970(13) Å) and assorted with two terminal oxygenatoms deriving from water molecules (Cu(3)–O(24) = 2.11(3) Å). Indetail, the [H4W12O42]8� anions are connected by Cu(1) and Cu(3)linkers into 2D layers parallel to ab plane, these layers packingalong the c-axis are further bound by Cu(2) linkers to form a 3Dstructure (seen the Fig. 4). The framework exhibits similarsquare-like 1D cavity along the crystallographic c axis with a sizeof ca. 13.23 � 12.71 Å (Fig. S4b) according to the Cu(1)� � �Cu(3) dis-tances. Similarly, the framework also generates square-like 1Dchannels along the crystallographic a axis with a size of ca.

13.17 � 13.23 Å (Fig. S4c) according to the Cu(1)� � �Cu(2) distances.Additionally, there is a Na ion connected [H4W12O42]8� cluster andfour terminal oxygen atoms with Na-O distances of 2.378(17)–2.44(2) Å.

Furthermore, the 3D structure is a (6,2)-connected networkwith (812123)(8)3 topology (shown in Scheme 2). In this simplifica-tion, [H4W12O42]8� clusters are taken as 6-connected nodes, Cu1,Cu2, Cu3 atoms are seemed as 2-connected nodes.

3.3. Properties

3.3.1. IR spectraIn deep insight into the structure, we further tested the IR of the

four compounds (as shown in Fig. S5). The IR of compounds 1–4 re-corded in the range of 400–4000 cm�1 with KBr pellet are similar:the characteristic peaks at 942, 878, 796, 685 and 493 (cm�1) for 1,940, 892, 844, 681 and 492 (cm�1) for 2, 935, 887, 796, 685 and 493(cm�1) for 3 and 942, 889, 796, 685 and 493 (cm�1) for 4 can be as-cribed to vas(W@Ot), vas(WAOb), vas(WAOa), vas(WAOc) andvas(MAO) (M@Cu, Mn) vibration modes [24]. The absorptions at1025, 1402, 1615 (cm�1) for 1, 1054, 1435, 1579 (cm�1) for 2and 1045, 1423, 1615 (cm�1) for 3 are corresponding to vas(CAN),d(CH2), and d(HOH) [25]. While the characteristic peaks at 2920–3371 (cm�1) are ascribed to vas(OAH). The results are consistentwith the feature of the structures.

3.3.2. XRPD patternsThe experimental and simulated XRPD patterns of compounds

1–4 are shown in Fig. S6. Their peak positions are in agreementwith each other, which indicate the good phase purity of com-pounds 1–4. The differences in intensity may be due to the pre-ferred orientation of the crystalline powder samples.

3.3.3. UV spectraTo further research the structure of compounds 1–4, the UV

spectra of them were performed in the aqueous solution(Fig. S7). The four compounds all exhibit two absorption bands,which seemed that their peak positions have no relations to thecoordinate transition metals in the UV spectra. Therefore, thebands at 268 nm for 1, 276 nm for 2, 273 nm for 3 and 271 nmfor 4 are attributed to pp(Od) ? dp⁄(W) transitions in the W@Obands and dp–pp–dp transitions between the energetic levels ofthe WAOAW tricentric bonds, respectively. The bands at 206,213, 211 and 209 nm may assigned to Ot ? W charge transterwhere electrons are promoted from the low energy electronicstates, which is mainly comprised of oxygen 2p orbital, to thehigh-energy states mainly composed of metal d orbital [26].Description of polyanion is the characteristic like Keggin-typestructure, which is in conformity with single crystal structureanalysis.

Fig. 3. Polyhedral and ball-and-stick representation of the 2D layer in 3 constructed by [H4W12O42]8� building blocks and Cu(1), Cu(2) linkers viewed along c axis. (WO6

octahedra, yellow; Cu, blue). Water molecules are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the webversion of this article.)

Fig. 4. Polyhedral and ball-and-stick representation of the 3D structure of compound 4. (WO6 octahedra, green; Cu(1) and Cu(2), pink; Cu(3), dark red). Water molecules areomitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

S. Gao et al. / Inorganica Chimica Acta 379 (2011) 151–157 155

3.3.4. TG analysesTo study the thermal stabilities of compounds 1–4, TG analyses

are carried out for the four compounds (Fig. S8). The TG curve of 1shows a two-step weight loss process. The first weight loss of9.73% in the temperature range of 43–210 �C corresponds to alllattice water molecules (calc. 9.74%). The second weight loss of13.21% in the temperature range of 220–680 �C is attributed tothe release of all bim ligands (calc. 13.18%). For 2, there are four

obvious weight loss steps. The first weight loss of 1.22% in the tem-perature range of 60–140 �C corresponds to all of isolated latticewater molecules (calc. 1.21%). The loss of 0.52% occurred to thesecond step is attributed to coordinated lattice water moleculesin the range of 150–180 �C (calc. 0.52%). The third weight loss of11.76% at 200–385 �C is considered as the six protonated imi mol-ecules (calc. 11.75%). In the range of 400–678 �C, the last weightloss of 1.99% corresponded to the release of imi molecule (calc.

Scheme 2. Schematic view of the 3D topology of compound 4. (The green nodes symbolize the [H4W12O42]8� clusters, the pink nodes symbolize {Cu(1)} and {Cu(2)}, the darkred nodes symbolize {Cu(3)}, and the water molecules are omitted for clarity).

Fig. 5. Cyclic voltammograms of the 4-CPE in the 0.4 M sodium acetate buffersolution at different scan rates. (from inner to outer: 0.050, 0.080, 0.110, 0.140,0.170, 0.200 and 0.230 V s�1).

Fig. 6. Cyclic voltammograms of 4-CPE in the 0.4 M sodium acetate buffer solutioncontaining different concentrations of NaNO2: (a) 0, (b) 3.0, (c) 6.0 and (d) 10.0 mMat a scan rate of 0.020 V s�1.

156 S. Gao et al. / Inorganica Chimica Acta 379 (2011) 151–157

1.96%). The TG of compound 3 displays two-step weight loss pro-cess, the first weight loss of 9.57% in the temperature range of40–400 �C, corresponding to the loss of all the isolated and coordi-nated lattice water molecules (calc. 9.56%). The second step occursto the loss of two bim molecules of 7.16% in the range of420–787 �C (calc. 7.12%). Then the TG of 4 shows two-step weightloss process, the first weight loss of 1.59% in the temperature rangeof 35–53 �C, and the following weight loss of 7.88% during60–456 �C, which corresponds to the set free of all the isolated lat-tice water molecules (calc. 1.57%) and the coordinated lattice watermolecules (calc. 7.85%), respectively. All the weight loss from theTG curves accord with the formulas of compounds 1–4 [27].

3.3.5. Electrochemistry propertiesOwing to the solubility of compounds 1–4 in water, the CPEs

become optimal choices to study their electrochemical properties.We explore the electrochemical behaviors of the 1-, 2-, 3-, and4-CPE in 0.4 M sodium acetate buffer solution at different scanrates in the potential range from 1.000 to �0.300 V for 1, from1.400 to �0.300 V for 2, from 1.000 to �0.300 V for 3 and from0.200 to �1.400 V for 4. Cyclic voltammograms of 1–3 CPE and

4-CPE are shown in Fig. S9 and Fig. 5. One pair of reversible redoxpeaks (I–I0) appeared, and the mean peak potentials E1/2 = (Epc +Epa)/2 were �0.127, 0.742, �0.111 and �0.502 V, respectively, atscan rate of 0.080 V s�1. The one pair of redox waves correspondingto the electrochemical process of Cu(II)/Cu(I) for 1, 3, 4-CPE andMn(III)/Mn(II) for 2-CPE. The peak potentials changed gradually:the cathodic peak potentials shifted to the negative direction andthe corresponding anodic peak potentials shifted to the positivedirection with increasing scan rate [28–30]. The peak currentswere proportional to the scan rates, which reveals that the redoxprocesses of 1–4 are surface controlled.

The pH value in the 0.4 M sodium acetate buffer solution thereis a slight change in the compounds 1–4 electrochemical behaviors.As seen in Fig. S10, in the pH of value of 3, 3.5, 4, 4.5 and 5, thereduction peak potentials of the four compounds are all relatedto the pH value. For the reversible electrode reaction is accompa-nied by the protons, the slope of E � pH line is �0.059 m/n(m and n represent the number of protons and electrons). Theirreversible of their cumulative reduction strengthened in casethe E � pH slope of the compounds 1–4 deviate from the reversiblecase of �0.059 V.

S. Gao et al. / Inorganica Chimica Acta 379 (2011) 151–157 157

The electrocatalytic reductions of NaNO2 in 0.4 M sodium ace-tate buffer solution were investigated at 3 and 4-CPE (Fig. S11and Fig. 6). The studies indicate that 3 and 4-CPE have goodelectrocatalytic activities toward the reduction of NaNO2. It canbe seen that an evident anodic peak appears and the currents grad-ually rise accompanied by the peak potentials shifting toward thenegative direction when the concentration of NaNO2 increases. Atthe same time, the cathodic currents increase to a small extent.

4. Conclusions

In conclusion, a series of novel polytungstate compounds 1–4,from 0D to 3D, have been successfully obtained using the routinesynthetic reactions in aqueous solution. Compounds 1–4 representthe paradodecatungstate [H4W12O42]8� anions decorated by transi-tion-metal complexes or transition metal. The successful synthesesof these four compounds enrich POMs materials, which havepotential applications such as catalysis.

Acknowledgments

This work is supported by the National Natural Science Founda-tion of China (20671026, 20971032), The Study Technological Inno-vation Project Special Foundation of Harbin (2009RFXXG202), theScience and Technology Project of Education Office of HeilongjiangProvince (11551122), Technological innovation team building pro-gram of college of Heilongjiang Province (2009td04) and InnovationTeam Research Program of Harbin Normal University (KJTD200902).

Appendix A. Supplementary data

CCDC 787783, 772481, and 772480 contain the supplementarycrystallographic data 1, 2, and 3, respectively. These data can beobtained free of charge from The Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif. Crystallographicdata for the structure of 4 reported in this paper has been appliedCSD number 421586. Copy of the data can be obtained free of chargefrom the FIZ Karlsruhe, Eggenstein-Leopoldshafen, Germany(Fax: +49 7247 808 666; E-mail: crysdata@fiz-karlsruhe.de).

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2011.10.011.

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