Inorganic Chemistry Communications 13 (2010) 745–748

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Inorganic Chemistry Communications

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A new bitrack sinusoid-like chain templated by Wells–Dawson type polyoxometalate

Xiuli Wang ⁎, Hailiang Hu, Aixiang Tian, Hongyan Lin, Jin Li, Limin ShiFaculty of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121000, PR China

⁎ Corresponding author. Tel.: +86 416 3400158.E-mail address: wangxiuli@bhu.edu.cn (X. Wang).

1387-7003/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.inoche.2010.03.037

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 December 2009Accepted 20 March 2010Available online 27 March 2010

Keywords:Wells–Dawson PolyoxometalateInorganic-organic hybridsTemplate synthesisElectrochemical property

A new Wells–Dawson type polyoxometalate (POM)-templated compound, [Cu4(μ2-O)2(4,4′-bpo)4(INA)2(H2O)4][(P2W18O62)]·H2O (1) (4,4′-bpo=2,5-bis(4-pyridyl)-1,3,4-oxadiazole; INA=pyridine-4-carboxylicacid), has been hydrothermally synthesized. Compound 1 contains a novel bitrack sinusoid-like chainconstructed from alternating dicopper units and non-linear rigid 4,4′-bpo linkers. Induced by POMtemplates, the adjacent chains formed elliptical spaces to accommodate the Wells–Dawson type anions.

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Polyoxometalates (POMs), as unique metal–oxygen clusters,exhibit diverse structures and properties that endow them withapplications in a wide range of areas, such as catalysis, materialscience, and medicinal chemistry [1]. Up to now, the field of POMs hasbeen significantly enriched by introduction of transition metal (TM)complexes to this system. Many hybrids of this series have beenreported [2], in which the TM complexes usually act as modifier toconstruct supporting/pendanted or high-dimensional and high-connected frameworks; and polyoxoanions in this kind of compoundsusually act as linkers. However, in this attractive POMs field, theexamples of polyoxoanions acting as single inorganic templates arerelatively less [3]. Comparedwith other classical POMs, such as Keggintype [4], the Wells–Dawson type POMs acting as templates toconstruct hybrid compounds are much less common. The mainreason may rely on the large size and the high charge of the Wells–Dawson anions. Thus, it is quite appealing and challenging to designand construct Wells–Dawson type POM-templated hybrid com-pounds. As an extension of our ongoing effort in the design of POM-templated inorganic–organic hybrid compounds [5], we try to preparenew POM-templated architectures by utilizing uncommon[P2W18O62]6− anions. So far, only one notable example of a 3D Ni-4,4′-bipyridine framework templated by [P2W18O62]6− anion wasreported by Liu group [6]. They choose 4,4′-bipyridine ligand assynthon. Instead of the commonly used linear rigid ligand, we choosea non-linear rigid ligand, 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4,4′-bpo) (Chart S1) in this synthetic strategy. The 4,4′-bpowas selected asa suitable candidate mainly based on the following considerations: (i)the spacer length of the bis(pyridine) ligand was elongated byintroducing an oxadiazole ring, which may be in concert with the

large volume of the Wells–Dawson type anions; (ii) the rigidbackbones of this ligand exhibit a flexual angle, which may agreewith the ellipsoid-shape of the Wells–Dawson cluster. Such non-linear rigid ligand may be facilitated to form elliptical voids inprinciple [7]. Additionally, pyridine-4-carboxylic acid (INA) waschosen as the organic components with the expectation that it canact as effective auxiliary ligand to assist the construction of targetedcompound. Considering the above aspects, in this work, we introduce4,4′-bpo and a small bridging ligand INA into theWells–Dawson POMsystem and obtain a new POM-templated compound [Cu4(μ2-O)2(4,4′-bpo)4(INA)2][(P2W18O62)]·H2O (1), which was prepared underhydrothermal conditions [8]. To the best of our knowledge, this is thefirst example of non-coordinating Wells–Dawson type POM-tem-plated hybrid compound based on non-linear rigid ligand.

The ligand 4,4′-bpo was prepared according to the literaturemethod [9]. Single crystal X-ray structural analysis [10] reveals thatthe asymmetric unit of 1 contains one Wells–Dawson type polyox-oanion, [P2W18O62]6− (abbreviated as P2W18), two [Cu2(μ2-O)(4,4′-bpo)2(INA)(H2O)2]3+ hybrid cations and one water molecule, asshown in Fig.1.

The template P2W18 polyoxoanion possesses the well-knownWells–Dawson type structure, and all of the bond lengths and anglesare in the normal ranges (see ESI, Table S1). Bond valence sumcalculations [11] show that all W atoms are in +VI oxidation state,and Cu atoms are in +II oxidation state.

The two crystallographically unique CuII ions exhibit similardistorted square–pyramidal geometry. Each CuII ion is defined bytwo nitrogen atoms from two 4,4′-bpo molecules, one oxygen atomfrom one INAmolecule, and two cis-/trans-related oxygen atoms fromone μ2-O and one water molecule. The average bond lengths aroundthe CuII ion are 2.034 Å for Cu–N and 2.025 Å for Cu–O. A pair ofsymmetry-related Cu1/Cu2 ions are fused together by one μ2-bridgingINA and one μ2-O to form two similar binuclear tetrahedron [Cu2O

Fig. 1. Stick/polyhedral view of the asymmetric unit of 1. All hydrogen atoms and freelattice water molecules are omitted for clarity.

746 X. Wang et al. / Inorganic Chemistry Communications 13 (2010) 745–748

(INA)(H2O)2] units (abbreviated as SBUs), with Cu…Cu separations of3.388 Å (SBUs-1) and 3.437 Å (SBUs-2), respectively (Fig. S1). More-over, the identical SBUs are bridged by 4,4′-bpo ligand in a trans-arrangement, forming a novel bitrack sinusoid-like chain (Fig. 2). Theaverage intrachain Cu…Cu separation bridged by 4,4′-bpo is 13.882 Å,which is obviously larger than that bridged by common linear rigidligand, such as 4,4′-bipyridine (∼11.0 Å) [12]. Interestingly, the 4,4′-bpoligand transfers its flexual angle to the whole chain. Thus, the zigzagangle (141.1°) agrees with that of 4,4′-bpo (142.9°).

The SBUs-bpo-SBUs unit adopts a boat-like conformation (Fig. S2),due to the bending of the non-linear rigid 4,4′-bpo ligand. Worth tomention that, to overcome the obstacle caused by the nanometric sizeof the Wells–Dawson anion, the boat-like metal–organic moieties areinterconnected with each other in an inversion-fashion to form a 1Dsinusoid-like chain (Fig. S3). Significantly, sinusoid-like chains cangenerate elliptical spaces in principle, which make it possible toenvelope P2W18 anions. As expected, the adjacent chains form

Fig. 2. The bitrack sinusoid-like chain exhibiting a zigzag angle t

elliptical voids via hydrogen bonds (Table S2). The 2D supramolecularmetal–organic layer is shown in Fig. 3a, in which the ellipticalmacrocycle is divided into two different cavities by fence-like INA. Theapproximate dimension of the large macrocyclic cavity (A-type) is14.6×17.5 Å, which is big enough to envelop the P2W18 anion (ca.10.4×12.2 Å) (Fig. 3b). The small square-like cavity (B-type) is about9.9×7.2 Å, which accommodates only one H2O guest molecule.

Additionally, two types of significant π–π stacking interaction witha near to perfect facial alignment have been observed within eachbitrack-chain architecture (Fig. S4). Type I occurs between twoneighboring pyrazinyl rings with the center-to-center separations of3.544 Å (Cg1) and 3.622 Å (Cg4), while Type II occurs between theoxadiazole planes with the corresponding values of 3.488 Å (Cg2) and3.742 Å (Cg3). Actually, such intermolecular aromatic contacts shouldbe considered as an effective force for the formation of double-chainmotif [13].

The thermal behavior of compound 1 was studied by TGA. Theresult is shown in Fig. S5. The TGA curve shows two weight loss steps:The first step corresponds to the loss of water molecules (calculated2.08%, found 2.10%) when the temperature is lower than 200 °C. Thesecond step in the range of 200–600 °C can be attributed to the loss oforganic components 19.39% (calculated 19.80%) for 1. The observedexperimental values are in consistence with the theoretical values.

The electrochemical behavior of compound 1 modified carbonpaste electrode (1-CPE) and its electrocatalytic reduction of nitritehave been investigated. The cyclic voltammograms for 1-CPE [14] in1 M H2SO4 aqueous solution at different scan rates are presented inFig. 4. In the potential range of +300 to −800 mV, there are threepairs of reversible redox peaks, I–I′, II–II′, and III–III′, with the meanpeak potentials E1/2=(Epc+Epa)/2 at −165, −394, and −623 mV(scan rate: 80 mV s−1) respectively, which should be ascribed tothree consecutive two-electron processes of P2W18 [15].With the scanrates increasing, the peak potentials changed gradually: the cathodicpeak potentials shifted to the negative direction and thecorresponding anodic peak potentials shifted towards the positivedirection. The peak-to-peak separation between the correspondingcathodic and anodic peaks increased, but themean peak potentials didnot change on the whole. The peak currents were proportional to thescan rates up to 500 mV s−1, suggesting that the redox process issurface-controlled (Fig. S6). In the experiment, 1-CPE shows highstability. The remarkable stability of the 1-CPE can be ascribed

hat ideally agrees with the flexual angle of 4,4′-bpo ligand.

Fig. 3. (a): Space-filling representations of the Wells–Dawson anion and the voids in the 2D supramolecular metal–organic layer; (b): Arrangement of [P2W18O62]6− anions in theelliptical voids.

Fig. 4. Cyclic voltammograms of the 1-CPE in 1 M H2SO4 solution at different scan rates(from inner to outer: 160, 180, 200, 250, 300, 350, 400, and 500 mV s−1).

747X. Wang et al. / Inorganic Chemistry Communications 13 (2010) 745–748

primarily to the insolubility of the hybridmaterial, avoiding the loss ofthe modifier during measurements.

As is well known, the electroreduction of nitrite requires a largeover potential [16], and therefore no obvious voltammetric responseis observed at the presence of nitrite in the potential range of +300 to

Fig. 5. Cyclic voltammograms of the 1-CPE in 1 M H2SO4 solution containing 0.0–8.0 mM KNO2 and a bare CPE in 6 mM KNO2 + 1 M H2SO4 solution. Potentials vs. SCE.Scan rate: 80 mV s−1.

−800 mV at a bare CPE. However, the 1-CPE displayed goodelectrocatalytic activity toward the reduction of nitrite (Fig.5). Atthe 1-CPE, with the addition of nitrite, all the reduction peak currentsincrease gradually while the corresponding oxidation peak currentsdecrease gradually, which indicates that the reduction of nitrite ismediated by the reduced species of P2W18 anions.

In summary, by introducing a non-linear rigid ligand 4,4′-bpo, anewWells–Dawson type POM-templated hybrid compound has beensuccessfully synthesized under hydrothermal conditions. The titlecompound exhibits a one-dimensional structure formed by analternating sequence of SBUsmoieties and non-linear 4,4′-bpo linkers.The [P2W18O62]6− ions are buried as template inside the ellipticalchannels. This work, to some extent, provides a good example ofreasonable design and controllable assembly of Wells–Dawson typePOM-templated compound.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (no. 20871022) and Talent-supporting ProgramFoundation of Liaoning Province (no. 2009R03).

Appendix A. Supplementary data

CCDC 754883 for 1 contains the supplementary crystallographicdata for this paper. These data can be obtained free of charge from theCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.inoche.2010.03.037.

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