Two new POM-based compounds containing a linear tri-nuclear copper(II) cluster and an infinite copper(II) chain, respectively

2.1 Structure description of 1 and 2

2.1.1 [Cu₃(btz)₆(H₂O)₄(H₃P₂W₁₈O₆₂)₂]·6H₂O (1)

A single crystal X-ray diffraction study has revealed that compound 1 incorporates three Cu^II ions, six btz ligands, two [P₂W₁₈O₆₂]⁶⁻ anions (abbreviated to P₂W₁₈), and four coordinated and six crystal water molecules (Fig. 1). The valence sum calculations [17] show that all the W atoms are in the +VI oxidation states and all the Cu atoms are in the +II oxidation states. In order to balance the charge, six protons have been added to the formula.

Fig. 1:

Ball-and-stick view of the centrosymmetric basic molecular unit of 1. The hydrogen atoms and crystal water molecules are omitted for clarity.

In compound 1, there are two crystallographically independent six-coordinated Cu^II ions, both showing an octahedral coordination geometry. The Cu1 ion is coordinated by four N donors (two N4 and two N7) from four btz ligands and two O2W molecules, whereas the Cu2 ion is coordinated by three N donors (N1, N5, and N8) from three btz ligands, one O12 atom from one P₂W₁₈ anion and two water molecules (O1W and O2W). The Cu–N distances are in the range of 1.97(2)–2.007(16) Å, whereas the Cu–O bonds are from 1.93(8) to 2.472(19) Å.

In 1 there exists a linear tri-nuclear Cu^II cluster, in which three Cu^II ions are fused by six btz ligands. The btz molecule offers its two apical successive N donors to link two Cu^II ions. The water molecule O2W acts as a bridging factor to further strengthen this cluster. Two P₂W₁₈ anions provide two terminal O12 atoms to connect two apical Cu2 atoms of this tri-nuclear cluster, preventing a dimensional extension (Fig. 2a). Namely, two P₂W₁₈ anions cover the linear tri-nuclear Cu^II cluster, just like a POM “triple-decker” (Fig. 1). Between adjacent POM “triple-decker”, there are several hydrogen bonding interactions inducing a supramolecular 1D chain of 1, such as O34···O2W=2.73 Å (Fig. 3).

Fig. 2:

(a) The linear tri-nuclear cluster in compound 1. Two P₂W₁₈ anions linked Cu^II ions are symbolized as yellow spheres; (b) infinite Cu^II chain in the crystal structure of 2.

Fig. 3:

Supramolecular chain formation in the crystal structure of 1 formed by hydrogen bonding interactions (O34···O2W=2.79 Å). The molecular units depicted in Fig. 1 are shown here with different colors.

2.1.2 [Cu₂(btz)(tz)(H₂O)₂(SiMo₁₂O₄₀)]·5H₂O (2)

A single crystal X-ray diffraction analysis has shown that compound 2 consists of two Cu^II ions, one btz and one tz ligands, one [SiMo₁₂O₄₀]⁴⁻ anion (abbreviated to SiMo₁₂), two coordinated and five crystal water molecules (Fig. 4). The valence sum calculations [17] show that all the Mo atoms are in the +VI oxidation states and all the Cu atoms are in the +II oxidation states. In compound 2, the butyl “tail” –(CH₂)₃CH₃ of some btz molecules was lost to produce a tz ligand, which also has been observed in previous reports using similar ligands [18], [19]. Thus, in 2, the btz and tz molecules cooperate to modify the connectivity of the Keggin anions.

Fig. 4:

Ball-and-stick view of the basic molecular unit of 2. The hydrogen atoms and crystal water molecules are omitted for clarity.

In compound 2, there are three crystallographically independent Cu^II ions. The Cu1 ion is four-coordinated by two N5 atoms from two tz ligands and two O1W, showing a square planar coordination geometry. Considering the long-range coordinative Cu2–O28 bonds (2.868 Å), the Cu2 ion is five-coordinated by two N atoms (N2 and N4) from a btz and a tz ligand, respectively, two water molecules (O1W and O2W), and one O28 from one SiMo₁₂ anion, exhibiting a distorted tetragonal pyramidal geometry. The Cu3 ion exhibits an octahedral geometry, coordinated by two N1 atoms from two btz ligands, two O6 from two SiMo₁₂ anions, and two O2W. The Cu–O bonds are in the range 1.874(8)–2.86(8) Å and the Cu–N bonds are from 1.97(10) to 1.988(9) Å.

Both the btz and tz molecules in 2 provide two successive N atoms to link Cu^II ions, inducing an infinite Cu^II chain with water molecules as bridges. The btz and tz ligands are attached to this chain up and down alternately (Fig. 2b). In compound 2, there are two types of SiMo₁₂ anions: Si1- and Si2-containing anions. Each Si1-containing anion provides two terminal O6 atoms to fuse Cu3 atoms of adjacent infinite Cu^II chains. Thus, a grid-like layer of 2 is formed (Fig. 5). Furthermore, considering the long-range coordinative Cu2–O28 bonds (2.868 Å), the Si2-containing anion can supply two terminal O28 atoms to connect adjacent layers. So a 3D framework is constructed (Fig. 6). In the crystal structure of 2, there exist large voids of about 324 ų, which may be important for potential applications of this material.

Fig. 5:

The grid-like layer (a) and its schematic view (b) of compound 2 with Si1-containing anions as pillars.

Fig. 6:

The 3D framework of compound 2 with Si2-containing anions as linkers.

2.2 FT-IR spectra

Figure S1 (Supplementary Information) shows the IR spectra of compounds 1 and 2. The bands at 1093, 954, 906, and 793 cm⁻¹ for 1 are ascribed to the modes of ν(P–O), ν(W–O_d), and ν(W–O_b/c–W), respectively. In the spectrum of 2, the characteristic bands at 1089, 968, 906 and 790 cm⁻¹ are attributed to ν(Si–O), ν(Mo–O_d), and ν(Mo–O_b/c–Mo), respectively. Furthermore, the bands in the regions of 1614–1214 cm⁻¹ for 1 and 1606–1227 cm⁻¹ for 2 are attributed to the btz and tz ligands.

2.3 Photocatalytic activity

Recently, the POM-based compounds have usually been used as photocatalysts to degrade organic dyes [20]. In this work, we also studied the photocatalytic activities of compounds 1 and 2 to degrade methylene blue (MB) and rhodamine B (RhB) solution under UV irradiation. The process of photocatalysis is as follows: 100 mg of compound 1 or 2 was suspended in 0.02 mmol L⁻¹ MB or RhB aqueous solution 250 mL. The suspension was magnetically stirred for about 10 min to ensure the equilibrium in the dark. In the continued stirring process, the solution was exposed to UV irradiation from an Hg lamp. Every 30 min, 5.0 mL samples were taken out for analysis by UV/Vis spectroscopy. It can be clearly observed from Fig. 7 that after 120 min the conversions of MB are 45% for 1 and 48% for 2. Figure 8 shows the photocatalytic degradation of RhB and the conversions are 33% for 1 and 35% for 2.

Fig. 7:

(a) Absorption spectra of the MB solution during the decomposition reaction under UV irradiation with compounds 1 and 2 as catalysts; (b) photocatalytic decomposition rates of the MB solution under UV irradiation with the use of 1 and 2.

Fig. 8:

(a) Absorption spectra of the RhB solution during the decomposition reaction under UV irradiation with compounds 1 and 2 as catalysts; (b) photocatalytic decomposition rates of the RhB solution under UV irradiation with the use of 1 and 2.

2.4 Voltammetric behavior and electrocatalytic activity

In this work, we studied the electrochemical properties of 1- and 2-CPEs (1/2 bulk-modified CPE; CPE=carbon paste electrode). The cyclic voltammograms for 1- and 2-CPEs are presented in Fig. 9 for 0.1 m H₂SO₄+0.5 m Na₂SO₄ aqueous solutions at different scan rates. In the potential range of +400 to −850 mV for 1-CPE, three reversible redox peaks II-II′, III-III′, and IV-IV′ are observed, respectively, with the half-wave potentials E_1/2=(E_pa+E_pc)/2 at –166 (I-I′), −478 (II-II′), −724 (III-III′) mV (scan rate: 80 mV·s⁻¹). These three redox peaks for 1-CPE should be ascribed to three consecutive two-electron processes of P₂W₁₈ [12], [21]. Moreover, there also exists one irreversible anodic peak I′ with the potential of +84 mV, which is assigned to the oxidation of the copper centers [12]. The 2-CPE shows three reversible redox peaks I-I′, II-II′, and III-III′ in the potential range of +600 to −200 mV, corresponding to three consecutive two-electron processes of SiMo₁₂ anions [22]. The half-wave potentials are at +229 (I-I′), +100 (II-II′), and −89 (III-III′) mV (scan rate: 80 mV·s⁻¹). With the scan rates increasing from 20 to 500 mV·s⁻¹, the peak potentials for 1- and 2-CPEs change gradually: the cathodic peak potentials are shifted toward the negative direction, whereas the corresponding anodic peak potentials are shifted to the positive direction.

Fig. 9:

The cyclic voltammograms of the 1- and 2-CPEs in 0.1 m H₂SO₄+0.5 m Na₂SO₄ aqueous solution at different scan rates (from inner to outer: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 250, 300, 350, 400, 450, and 500 mV·s⁻¹, respectively).

The electrocatalytic reduction of nitrite by 1- and 2-CPEs in 0.1 m H₂SO₄+0.5 m Na₂SO₄ aqueous solutions is shown in Fig. 10a. With the addition of nitrite gradually, all the reduced species of P₂W₁₈ in 1-CPE and SiMo₁₂ in 2-CPE present electrocatalytic activity for the reduction of nitrite. We also studied the electrocatalytic reduction of bromate by these CPEs (Fig. 10b). With the addition of bromate, the first redox peak remains almost unchanged, showing that only four- and six-electron-reduced species of P₂W₁₈ in 1-CPE and SiMo₁₂ in 2-CPE present electrocatalytic activity for the reduction of bromate. It was also noted that the six-electron-reduced species presented the largest catalytic activity.

Fig. 10:

Cyclic voltammograms of the 1- and 2-CPEs in 0.1 m H₂SO₄+0.5 m Na₂SO₄ aqueous solution containing KNO₂ (a) and KBrO₃ (b). Scan rate: 100 mV·s⁻¹.

4.1 Materials and methods

All reagents and solvents for the syntheses were purchased from commercial sources and used as received without further purification. Elemental analyses (C, H, and N) were carried out with a Perkin-Elmer 2400 CHN elemental analyzer. The FT-IR spectra were taken on Alpha Centaurt FT-IR spectrometer in the 400–4000 cm⁻¹ region (KBr pellets). Electrochemical measurements were performed with a CHI 440 electrochemical workstation. A conventional three-electrode system was used, with modified CPEs as the working electrodes. A saturated calomel electrode was used as a reference electrode, and a Pt wire as a counter electrode. UV/Vis absorption spectra were obtained using an SP-1901 UV/Vis spectrophotometer.

4.2 Synthesis

4.3 X-ray crystallographic study

X-ray intensity data for compounds 1 and 2 were collected on a Bruker Smart Apex CCD diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å) at 293 K. The structures were solved by Direct Methods and refined on F² by full-matrix least-squares methods using the Shelxtl package [23], [24]. All the hydrogen atoms attached to carbon atoms were generated geometrically. In compound 2, the five water molecules were highly disordered and could not be modeled properly. As a consequence, the SQUEEZE routine of Platon [25], [26], [27] was applied to remove the contributions of the solvent molecules to the scattering power. The reported refinements are of the guest-free structures using the structure factors produced by SQUEEZE [27]. Table 1 shows a summary of the crystallographic data and structure determinations. Selected bond lengths and angles are listed in Table S1 (Supplementary information).

CCDC 1474765 (1) and 1474766 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

4.4 Preparation of compounds 1 and 2 bulk-modified CPEs

Compound 1 bulk-modified CPE (1-CPE) was used as the working electrode, which was prepared as the following process: 90 mg of graphite powder and 8 mg of 1 were mixed and ground together by an agate mortar and pestle to achieve a uniform mixture. Then 0.1 mL of Nujol was added with stirring. The homogenized mixture was packed into a glass tube with a 1.5 mm inner diameter. The tube surface was wiped with weighing paper. Electrical contact was established with a copper rod through the back of the electrode. A 2-CPE was made with compound 2 in a similar manner.