A new high-dimensional architecture constructed from paradodecatungstate and [Cu(2-Hpzc)]2+ complexes

Inorganic Chemistry Communications 12 (2009) 1242–1245

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A new high-dimensional architecture constructed from paradodecatungstate and [Cu(2-Hpzc)]2+ complexes Yuan Chen a, Jun Peng a,*, Hai-jun Pang a, Ai-xiang Tian a, Peng-peng Zhang a, Dan Chen a, Min Zhu a, Yonghui Wang a, Huiyuan Ma b a b

Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Harbin Normal University, Harbin 150025, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 18 July 2009 Accepted 28 September 2009 Available online 4 October 2009 Keywords: Paradodecatungstate 2-Pyrazinecarboxylic acid ligand Magnetic property

a b s t r a c t A new compound, H4{[Cu3(2-Hpzc)2(H2O)4](H2W12O42)}13H2O (1) (2-Hpzc = 2-pyrazinecarboxylic acid), was synthesized in conventional condition and characterized by elemental analyses, IR spectroscopy, thermal gravimetric analysis, X-ray powder diffraction (XRPD) and single X-ray diffraction. In the structure of compound 1, each paradodecatungstate anion acts as a quadridentate ligand coordinating to four Cu2+ cations through its terminal oxygen atoms to form a two-dimensional (2D) layer. The infinite [Cu(2Hpzc)]2+ chains connect adjacent layers forming a three-dimensional (3D) architecture. There exist two kinds of channels with sizes of ca. 5.737  4.628 Å2 and 9.104  8.640 Å2 along the b and c directions, respectively. The magnetic behavior of 1 exhibits antiferromagnetic interaction. Ó 2009 Elsevier B.V. All rights reserved.

Polyoxometalates (POMs), as a rich family of metal–oxygen clusters, have been employed as building blocks to construct inorganic–organic hybrid compounds owing to not only their intriguing structural and topological novelty but also potential applications in such as photochemistry, catalysis, electrical conductivity, material science and magnetism [1–8]. In this field, ongoing research is focusing on the design and construction of high-dimensional hybrids which possess novel structures and unusual properties [9–10]. Assembly of POM clusters through introducing secondary transition metals or transition metal complexes (TMCs) to extended structures is an effective strategy for designing such hybrids. With the introduction of secondary transition metals or TMCs, these polyanions acting as unusual inorganic ligands can multiply bind several transition metals or TMCs through terminal or bridging oxygen atoms [11–17]. Based on this point, many high-dimensional POM-based hybrids have been successfully synthesized. However, compared with other classical POMs, such as Keggin [14], Wells-Dawson [17], Anderson [18], Lindqvist [19] and Silverton [20] polyanions, such hybrids based on paradodecatungstate polyanions ([H2W12O42]10) are much less common. Note that the paradodecatungstate polyanion possesses 36 potentially coordinate oxygen atoms and relatively high negative charges, which indicate high coordination ability to various TMCs.

* Corresponding author. Address: Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, People’s Republic of China. Tel.: +86 43185099667; fax: +86 43185098768. E-mail address: [email protected] (J. Peng). 1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2009.09.032

Up to now, the typical examples include K6[Co(H2O)4]2[H2W12O42] 14H2O [21], Na5[{FeII(H2O)3}2{Fe(H2O)4}0.5(H2W12O42)]30H2O [22], [Cu(en)2]3[{Cu(en)2}2(H2W12O42)]12H2O [23], H2{[K(H2O)2]2[Ln(H2O)5]2(H2M12O42)}nH2O [24], Na2[Mn4(H2O)14(H2W12O42)] 16H2O [25], Na6[{Co(H2O)3}{Co(H2O)4}(H2W12O42)]29H2O [26] and KNa3[Cu(H2O)2{Cu(H2O)3}2(H2W12O42)]16H2O [27]. However, among these compounds, the paradodecatungstate are either connected by metal cations forming purely inorganic compounds or decorated by small organoamine molecules, such as ethylenediamine. The examples of high-dimensional hybrids based on paradodecatungstate POMs constructed from other organic ligands are seldom reported, so it will be necessary to further research this branch. A crucial factor for the construction of such architectures rests on the choice of appropriate organic ligands. Among the various ligands, multidentate N- and O-donor ligands, such as pyridinecarboxylic acid and pyridinedicarboxylic acid, have been used to obtain many interesting products [18,28–30]. 2-Pyrazinecarboxylic acid ligand (2-Hpzc) is also a versatile ligand due to its excellent coordination ability and various coordination modes, such as chelate, bridging or both of these [31–36], but it seems to be rarely exploited in POMs chemistry [37]. Thus, we believe that the coordination versatility of 2-Hpzc plus the nature of metal ions should be a favor of modifying paradodecatungstate. On the basis of above considerations, we employ 2-pyrazinecarboxylic acid ligand in reaction with Cu2+ cation to modify the paradodecatungstate. Here, we report the synthesis and structural characterization of a new paradodecatungstate (1) derivative H4{[Cu3(2-Hpzc)2(H2O)4](H2W12O42)}12H2O

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(2-Hpzc = 2-pyrazinecarboxylic acid). To our knowledge, compound 1 represents the first high-dimensional hybrid based on paradodecatungstate and Cu-2-Hpzc complexes. Compound 1 was obtained by the conventional solution method (see the Supporting information). Single-crystal X-ray diffraction analysis [38] reveals that compound 1 consists of one [H2W12O42]10 polyanion, two 2-Hpzc ligands, three Cu2+ cations and thirteen water molecules (Fig. 1). The bond-valence sum (BVS) calculation [39] for compound 1 suggests that all W atoms are in +VI oxidation state. The copper atoms in 1 are all in +II oxidation state, confirmed by coordination environments, BVS calculations [39], and crystal colour. According to the BVS calculation and charge balance, four protons were added [13,40]. The paradodecatungstate is very similar to those previously reported [41,42]. The skeleton of the core [H2W12O42]10 anion is centrosymmetric and consists of four groups of two-type subunits: triangular cap-type W3O13 and open belt-type W3O14. Each captype W3O13 group consists of three edge-sharing WO6 octahedra with a common sharing oxygen atom, while in the belt-type W3O14 group, the three edge-sharing WO6 octahedra are linearly connected without common sharing oxygen atom to the three W atoms. Furthermore, each WO6 octahedron in W3O13 group has one terminal oxygen atom, while in W3O14 group each octahedron has two unshared oxygen atoms. The oxygen atoms in paradodecatungstate cluster can be grouped into three sets: (i) Ot, which bonding to one W atom with the W–O distance of 1.717– 1.802 Å; (ii) Ob, which connecting two W atoms with the W–O distance of 1.820–2.117 Å; (iii) Oc are combined with three W atoms with the W–O distance of 1.894–2.239 Å. It is well known that POMs tend to act as multidentate ligands to link secondary metals into high-dimensional structures through surface terminal or bridging oxygen atoms [18,21]. In the title compound, the [H2W12O42]10 cluster acts as a tetradentate ligand coordinating to four Cu2+ cations through terminal oxygen atoms of WO6 of the belt-type W3O14 groups. There are three crystallographically independent Cu centers (Cu1, Cu2 and Cu3) in the unit cell. Each Cu2+ cation exhibits distorted octahedral geometry. Cu1 is coordinated with four terminal oxygen atoms from two adjacent paradodecatungstate polyanion (Cu–O, 1.926–2.409 Å) and two nitrogen atoms from two ligands (Cu–N, 2.003 Å). Cu2 is coordinated by four oxygen atoms from two water molecules and two carboxyls from two ligands (Cu–O, 1.925–2.516 Å) and two nitrogen atoms from two ligands (Cu–N, 1.980 Å). The center of CuO6

octahedron, Cu3, receives contributions from four terminal oxygen atoms from two adjacent paradodecatungstate polyanion and two oxygen donors from two water molecules (Cu–O, 1.85–2.42 Å). The axial Cu–O distance is considerably longer than equatorial Cu–O distance due to the Jahn–Teller effect. Based on the above connection modes, the [H2W12O42]10 clusters are connected through [Cu(H2O)2]2+ bridging groups into one-dimensional chains along the a axis. The chains are further linked through Cu1 into a twodimensional layer (shown in Fig. 2). As shown in Fig. 1, the Cu2+ cations are coordinated by two nitrogen atoms and one carboxylate oxygen atom of the 2-Hpzc ligand. The 2-Hpzc, with N- and O-donors, is an integration of the coordination geometry of both pyrazine and carboxylate to provide more potential coordination sites. Based on this coordination mode, an infinite chain of Cu1, Cu2 and 2-Hpzc was formed. The interesting feature of the title compound is that adjacent layers are linked together by the infinite chain ([Cu(2-Hpzc)]2+) to generate a complicated three-dimensional framework. Another fascinating structural feature is that the three-dimensional framework contains two kinds of channels with sizes of ca. 5.737  4.628 Å2 and ca. 9.104  8.640 Å2 along the b and c directions, respectively, water molecules fill in the channels (shown in Fig. S1). The topological analysis of the structure has been performed by considering [H2W12O42]10 cluster and Cu1 atom as 4-connected nodes, which can be symbolized as a network with a (6581)2 topology (the first symbol for [H2W12O42]10 cluster; the second for Cu1 atom), shown in Fig. 3. To the best of our knowledge, this kind of network has never been described so far in POM chemistry. The solid UV–Vis spectrum in the region of 200–1150 nm shows the absorptions of compound 1 (shown in Fig. S2). The electronic spectrum of compound 1 exhibits two absorption bands at 235 and 690 nm. The main peak in higher energy can be attributed to O ? W charge transition [43]. The other absorption band at 690 nm may be assigned to d–d transition of distorted octahedral Cu2+ [44]. Thermal gravimetric (TG) measurements also support the chemical composition. As shown in Fig. S4, the TG curve for compound 1 exhibits two weight loss steps in 45–600 °C. The first weight loss of 8.12% (calc. 8.0%) from 45 to 250 °C corresponds to the loss of water molecules. The second weight loss of 6.67% (calc. 6.83%) from 250 to 520 °C can be ascribed as the decomposition of the 2-Hpzc ligands and [H2W12O42]10 polyanions. Compound 1 is based on the paradodecatungstate–Cu system which possesses electrochemically activity. To check whether the redox ability of [H2W12O42]10 anion can be maintained in the hybrid solids, cyclic voltammetry measurement is carried out in 1 M H2SO4 aqueous solution. As the insufficient solubility of compound

Fig. 1. Stick/ball/polyhedral view of the asymmetric unit of compound 1. The hydrogen atoms and crystal water molecules are omitted for clarity.

Fig. 2. Polyhedral representation of the 2D layer in compound 1. The hydrogen atoms and crystal water molecules are omitted for clarity.

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Fig. 3. The topological view of compound 1. The pink balls represent [H2W12O42]10 cluster.

1 in water and common organic solvents, the bulk-modified carbon paste electrode (CPE) becomes the optimal choice to study its electrochemical property [26]. The CV curve of 1-CPE shows three pairs quasi-reversible waves (shown in Fig. S5). The mean peak potentials, E1/2 = (Epa + Epc)/2, are +0.030 (I–I0 ), 0.31 (II–II0 ), 0.50 V (II–II0 ) in the range of 600 to +500 mV at the scan rate of 100 mV s1. Redox peaks II–II’ and III–III’ correspond to two consecutive two-electron processes of W centers [45]. The first redox wave can be attributed to the redox of the Cu2+ ions to Cu+, which is similar to the results of Cu-substituted POMs [46]. Compared to the same building block in other compounds, the mean peak potentials of 1 are slight different, which may be explainable due to their different chemical environment. The magnetic behavior of compound 1 has been investigated. The dc magnetic susceptibility (vm) data were measured in the temperature range of 2–300 K in 0.1 T magnetic field and are plotted as vmT and v1 m versus T, as displayed in Fig. 4. The effective magnetic moment (leff) determined from equation leff = 2.828 (vmT)1/2 is 3.23 lB, lower than expected spin-only value (3.87 lB) for three isolated system with S = 1/2 and g = 2, and such phenomenon is also observed in previous reported POMs with Cu(II) complexes [47–49].When the sample was cooled, the vmT value slowly decreased from 1.30 cm3 K mol1 at 300 K to 0.39 cm3 K mol1 at 2 K. The v1 m versus T plot is almost linear in the temperature range of 134–300 K, and is well fitted by the Curie–Weiss law with

C = 1.52 cm3 K mol1 and h = 42.1 K, indicating the existence of antiferromagnetic interactions between copper(II) complexes. In summary, a new framework based on paradodecatungstate, 2-pyrazinecarboxylic acid and Cu2+ cations has been successfully synthesized under conventional conditions. In the structure point of view, compound 1 represents the first example of 3D framework of paradodecatungstate connected by [Cu(2-Hpzc)]2+ Complexes. Magnetic measurement reveals that compound 1 shows antiferromagnetic interactions. Considering that two kinds of channels exist in compound 1, the study on adsorption property is underway. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (20671016), and the Analysis and Testing Foundation of Northeast Normal University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2009.09.032. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

Fig. 4. The temperature dependence of

v

1 m

(inset) and

vm T for compound 1.

[20]

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