Synthesis, structure, and spectroscopic characterization of a 2-D organic–inorganic hybrid Cu^II–Er^III heterometallic arsenotungstate

2.1 Materials and methods

The trivacant Keggin precursor Na₈[A-α-HAsW₉O₃₄]·11H₂O was synthesized according to the literature (Bi et al., 2001) and confirmed by IR spectra. Other chemicals were obtained from commercial resources and used without further purification. Elemental analyses (C, H, and N) were carried out on a Perkin−Elmer 2400-II CHNS/O analyzer. Inductively coupled plasma atomic emission spectrometry (ICP–AES) was performed on a Perkin–Elmer Optima 2000 ICP–AES spectrometer. IR spectra were recorded on solid samples palletized with KBr on a Nicolet 170 SXFT-IR spectrometer in the range 400–4000 cm⁻¹. The UV spectrum was obtained with a HITACHI U-4100 UV–Vis−NIR spectrometer in the range of 400–190 nm. XPS spectra were recorded on an Axis Ultra X-ray photoelectron spectrometer and XPS analyses were corrected with reference to C_1s (284.6 eV). Cyclic voltammetry was carried out on a CS electrochemical workstation (Wuhan Corrtest Instrument Co. LTD) at room temperature. A conventional three-electrode system was used. Platinum gauze was used as a counter electrode, and a Ag/AgCl electrode was referenced. The chemically bulk-modified carbon paste electrode (CPE) was used as the working electrode.

2.2 Synthesis of 1

A mixture of Na₈[A-α-HAsW₉O₃₄]·11H₂O (0.428 g, 0.161 mmol), CuCl₂·2H₂O (0.120 g, 0.704 mmol), ErCl₃ (0.143 g, 0.566 mmol), en (0.15 mL, 2.241 mmol) and H₂O (5 mL, 278 mmol) was stirred for 2.5 h (pH_i = 6.77), sealed in a 25 mL Teflon-lined steel autoclave, kept at 160 °C for 5 days and then cooled to room temperature (pH_f = 6.96). Purple prismatic crystals were separated by filtering, washed with distilled water and then dried in air at ambient temperature. Yield: ca. 32% based Na₈[A-α-HAsW₉O₃₄]·11H₂O. Elemental analysis (%) calcd for C₁₀H₅₅As₂Cu_2.5N₁₀Na₃O₈₄ErW₂₂ (1): C 1.92, H 0.89, N 2.24, Na 1.10, Cu 2.54, As 2.40, Er 2.68, W 64.72; found: C 1.83, H 1.02, N 2.09, Na 0.83, Cu 2.71, As 2.63, Er 2.49, W 64.55.

2.3 Preparation of 1-CPE

30 mg of graphite powder and 10 mg of 1 were mixed and ground together by an agate mortar and pestle to achieve a uniform mixture, and then 0.05 mL of Nujol was added with stirring. The homogenized mixture was packed into a glass tube with a 3.0 mm inner diameter, and the tube surface was wiped with paper. Electrical contact was established with a Cu rod through the back of the electrode.

2.4 X-ray crystallographic determination

Intensity data of 1 were performed at 296(2) K with a Bruker APEX-II CCD diffractometer (Mo Kα radiation, λ = 0.71073 Ǻ, graphite monochromator) by ω and θ scan modes. Routine Lorentz polarization and empirical absorption corrections were applied to intensity data. The structure was solved by direct methods and was refined by using Full-matrix least squares methods on F² with the SHELXTL 97 software (Sheldrick, 1997). No hydrogen atoms attached to isolated water molecules were located from the different Fourier map. The positions of hydrogen atoms attached to carbon atoms and nitrogen atoms were geometrically placed in ideal positions. All hydrogen atoms were refined isotropically as a riding model using the default SHELXTL parameters. Anisotropic thermal parameters were used to refine all non-hydrogen atoms except for some oxygen atoms, carbon atoms, nitrogen atoms and water molecules. A summary of crystallographic data and structure refinements for 1 is listed in Table 1.

3.1 Description of crystal structure

Single crystal X-ray diffraction indicates that 1 displays an organic–inorganic 2-D sheet structure. 1 crystallizes in the triclinic space group P-1 and its molecular structure unit contains an asymmetric subunit {[Cu(en)₂(H₂O)][Cu(en)₂]_1.5[H₃Er(α-AsW₁₁O₃₉)₂]}³⁻, three Na⁺ cations and four lattice water molecules. The asymmetric subunit {[Cu(en)₂(H₂O)][Cu(en)₂]_1.5[H₃Er(α-AsW₁₁O₃₉)₂]}³⁻ is constructed from a Er^III substituted sandwich-type fragment [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻, three bridging [Cu(en)₂]²⁺ cations (namely, [Cu1(en)₂]²⁺, [Cu2(en)₂]²⁺, [Cu3(en)₂]²⁺) and a pendant [Cu4(en)₂(H₂O)]²⁺ cation. The sandwich-type fragment [Er(α- AsW₁₁O₃₉)₂]¹¹⁻ consists of two monovacant Keggin [α-AsW₁₁O₃₉]⁷⁻ subunits in the staggered pattern linked together via a Er1 core (Scheme 2). The monovacant [α-AsW₁₁O₃₉]⁷⁻ subunit is derived from the well-known plenary Keggin [AsW₁₂O₄₀]³⁻ polyoxoanion by removal of a W⚌O group. Each [α-AsW₁₁O₃₉]⁷⁻ subunit is made up of three corner-shared W₃O₁₃ triads, a corner-shared W₂O₁₀ diad that is degraded from the W₃O₁₃ triad and a central AsO₄ group. In the [α-AsW₁₁O₃₉]⁷⁻ fragment, the As^V atom exhibits a tetrahedral coordination environment defined by three μ₄-oxygen atoms from three W₃O₁₃ triads and one μ₄-oxygen atom from W₂O₁₀ diad with the As—O distances of 1.623(16)–1.672(14) Ǻ; meanwhile, each W center in the [α-AsW₁₁O₃₉]⁷⁻ subunit shows a octahedral coordination geometry with the W—O distances of 1.684(16)–2.457(17) Ǻ. The bridging [Cu1(en)₂]²⁺, [Cu2(en)₂]²⁺ and [Cu3(en)₂]²⁺ cations are located on the special sites with atomic coordinates of (1/2, 1/2, 1/2), (1/2, 0, 1/2) and (1, 1/2, 1/2) leading to their occupanfcy of 50% for each. The [Cu1(en)₂]²⁺, [Cu2(en)₂]²⁺, [Cu3(en)₂]²⁺ and [Cu4(en)₂(H₂O)]²⁺ cations are all in the elongated octahedral geometries defined by four nitrogen atoms from two en ligands and two oxygen atoms. The Cu—N distances vary from 1.97(3) to 2.03(2) Å while the Cu—O band length is in the range of 2.53(2)–3.02(2) Ǻ, which proves the occurrence of Jahn–Teller distortion of the copper cations in ligand field (Fig. 1a). One remarkable structural characteristic of 1 is that four Cu-en complex cations are all connected with one [α-AsW₁₁O₃₉]⁷⁻ subunit. Of particular interest is that [Cu1(en)₂]²⁺ and [Cu3(en)₂]²⁺ cations are bound to the W₂O₁₀ diad, whereas [Cu1(en)₂]²⁺ and [Cu4(en)₂(H₂O)]²⁺ cations respectively link to other two W₃O₁₃ triads. The Er1 cation adopts a distorted square antiprismatic geometry formed by eight available oxygen atoms from two [α-AsW₁₁O₃₉]⁷⁻ moieties with Er—O distances of 2.326(15)–2.415(15) Å (Fig. 1b). In the coordination polyhedron around the Er1 cation, the O44, O47, O57 and O60 group and the O23, O27, O36 and O38 group constitute two bottom planes of the square antiprism and their average deviations from the ideal planes are 0.0037 and 0.0077 Å, respectively. The distances between the Er1 cation and two bottom planes are 1.273 and 1.248 Å, respectively and the dihedral angle between two bottom planes is 2.6°.

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Scheme 2 Combination of two [α-AsW₁₁O₃₉]⁷⁻ fragments, one Er³⁺ cation, three [Cu(en)₂]²⁺ cations and one [Cu(en)₂(H₂O)]²⁺ cation in the molecular unit of 1.

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Figure 1 (a) The molecular unit of 1 with the selected numbering scheme. (b) Square antiprismatic coordination geometry of the Er^III cation in 1 (symmetry codes: A: 1−x, 1−y, 1−z, B: 2−x, 1−y, 1−z, C: 1−x, −y, 1−z).

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As is shown in Fig. 2a, the [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments are linked by [Cu2(en)₂]²⁺ and [Cu3(en)₂]²⁺cations to form 1-D organic–inorganic chains, and then each chain is connected with two adjacent 1-D chains through [Cu1(en)₂]²⁺ to yield a 2-D sheet structure (Fig. 2b). As shown in Fig. 2b, [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments exhibit two types of spatial orientation. If we look on [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments with one spatial orientation as A type and [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments with the other spatial orientation as B type, a simplified connection mode of the 2-D sheet comes into being (Fig. 2c). From the viewpoint of topology, the [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments are defined as 3-connected nodes and the [Cu(en)₂]²⁺ cations serve as linkers; therefore, the resulting 2-D sheet can be simplified into a 2-D (6,3)-network topology and adjacent sheets are aligned in the mode of —AAA— (Fig. 2c). From the viewpoint of supramolecular chemistry, intermolecular hydrogen bonds are also observed between the neighboring clusters in the 3D supramolecular architecture (Fig. 3a). For each [Cu(en)₂]²⁺ cation, the N atoms of en ligands as hydrogen-bonding donors interact with the O acceptors from the close [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments with the N—H⋯O distances of 2.97(3)–3.35(3) Å (Table 2). Several types of hydrogen-bond interactions based on different [Cu(en)₂]²⁺ cations are shown in Fig. 3b–d.

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Figure 2 (a) Polyhedral and ball-stick view of 1-D organic–inorganic hybrid chains in 1. (b) 2-D sheet structure in 1. (c) Simplified connection mode of the 2-D sheet. (d) The topological (6,3)-network with the packing mode of —AAA—. The balls represent the [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragments.

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Figure 3 (a) The 3D supramolecular architecture of 1 formed by the hydrogen-bonding interactions. (b) The hydrogen-bonding patterns of [Cu1(en)₂]²⁺ cation. (c) The hydrogen-bonding patterns of [Cu2(en)₂]²⁺ cation. (d) The hydrogen-bonding patterns of [Cu2(en)₂]²⁺ cation. Symmetry codes: D: −1 + x, y, z, E: −x, 1−y, 1−z, F: −x, −y, 1−z.

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3.2 IR spectra

The IR spectrum of 1 was recorded as KBr pellets in the range of 4000–400 cm⁻¹ and displays four characteristic ν_as(As—O_a), terminal ν_as(W—O_t), corner-sharing ν_as(W—O_b) and edge-sharing ν_as(W—O_c) asymmetric stretching vibration bands derived from the Keggin-type AT framework, which are observed at 891, 955, 828, and 768, 721 cm⁻¹, respectively. Compared with Na₈[A-α-HAsW₉O₃₄]·11H₂O (2) [837, 945, 799 and 745 cm⁻¹ for ν_as(As—O_a), ν_as(W—O_t), ν_as(W—O_b) and ν_as(W—O_c)] (Fig. 4), the ν_as(W—O_t) vibration band in 1 is almost not shifted, suggesting the weak effect of the Cu-en cations on the terminal oxygen atoms on the [H₃Er(α-AsW₁₁O₃₉)₂]⁸⁻ fragment, which is in good agreement with the long Cu—O_t distances in 1. The signals at 3353–3143 cm⁻¹ and 2990–2890 cm⁻¹ are ascribed to the ν(NH₂) and ν(CH₂) stretching vibration, while the resonances at 1639–1560 cm⁻¹ and 1476–1450 cm⁻¹ are assigned to the δ(NH₂) and δ(CH₂) bending vibration, respectively, which confirms the presence of en in 1. Furthermore, the broad vibration band at 3447–3437 cm⁻¹ suggests the presence of lattice water molecules or coordination water molecules. In conclusion, the occurrences of these characteristic signals of 1 are in good agreement with its single-crystal structural analysis.

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Figure 4 The IR spectra of 1 and 2.

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3.3 UV spectra

The solid-state UV absorption spectrum of 1 in the range of 190–400 nm has been recorded at room temperature and it displays two obvious absorption bands centered at 219–221 nm and 299–302 nm, respectively (Fig. 5). The higher energy absorption band can be assigned to the pπ–dπ charge-transfer transitions of the O_t → W bonds, whereas the lower one can be attributed to the pπ–dπ charge-transfer transitions of the O_b(c) → W bonds (Feng et al., 2005; Chang et al., 2009; Chen et al., 2010).

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Figure 5 The UV spectrum of 1.

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3.4 XPS spectra

Bond valence sum (BVS) calculations (Brown and Altermatt, 1985; Trzesowska et al., 2004; Brese and Keeffe, 1991) of 1 indicate that the oxidation states of all W, Cu and Er atoms are +6,+2 and +3, respectively, which are further confirmed by X-ray photoelectron spectroscopy (XPS) (Fig. 6). The spin–orbit components (2p_3/2 and 2p_1/2) of the Cu2p peak are well deconvoluted by two curves at 933.2 and 953.0 eV, respectively, which indicate the presence of the Cu^II cations in 1. These values are in good agreement with the previous results (Yu et al., 2007). The Er^III 4d_3/2 peak is found at 178.5 eV (Akitsu and Einaga, 2006). The W4f_7/2 and W4f_5/2 binding energies of 33.8 and 35.9 eV are coincident with the reported values (Martinez et al., 2004; Szilágyi et al., 2006; Khyzhuna et al., 2005), which confirm that all the W centers in 1 are +6. The XPS profile exhibits a peak at 36.5 eV, which is ascribed to the As^III 3d_3/2 in 1. These results are consistent with BVS calculations from the X-ray structural analysis.

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Figure 6 (a) The XPS spectrum for Cu2p_3/2 and Cu2p_1/2 in 1. (b) The XPS spectrum for Er 4d_3/2 in 1. (c) The XPS spectrum for W4f_7/2 and W4f_5/2 in 1. (d) The XPS spectrum for As3d_3/2 in 1.

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3.5 Electrochemistry and electrocatalytic properties

Cyclic voltammetry (CV) measurements were performed to study the solid-state electrochemistry behavior and electrocatalytic properties for 1 in the 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ aqueous solution (a medium suitable for testing electrocatalytic processes) by entrapping 1 in carbon paste electrode (CPE) because of the inorganic–organic hybrid 1 synthesized under hydrothermal conditions being insoluble in water and having poor solubility in common organic solvents. In the sulfate pH = 3.32 medium, the cyclic voltammogram of 1 at a scan rate of 20 mV s⁻¹ at room temperature shows four pairs of redox peaks in the potential range of −1.4 V to 1.0 V (Fig. 7a). The mean potentials E_1/2 = (E_pa + E_pc)/2 are +0.130 V (I/I′), −0.328 V (II/II′), −0.655 V (III/III′) and −1.014 V (IV/IV′), respectively. The first oxidation peak I (+0.182 V) and its reduction counterpart I′ (+0.078 V) are attributed to the redox process of the Cu^II centers (Zhao et al., 2013; Zhang et al., 2013). The second, third and fourth oxidation peaks II, III and IV (−0.14 V, −0.502 V and −1.014 V, respectively) and their reduction counterparts II′, III′ and IV′ (−0.511 V, −0.790 and −1.132 V) are ascribed to the W^VI reductions (Zhang et al., 2013; Chen et al., 2014). The peak potentials change gradually with the scan rate from 20 to 200 mV s⁻¹: anodic peak potentials move to the positive direction and the corresponding cathodic peak potentials shift to the negative direction (Fig. 7b). As shown in Fig. 7c, the cathodic peak current intensity (I_pc) is proportional to the scan rate (ν) with the linear equation of I_pc = −0.0000001.1171ν − 0.00001 with the correlation coefficient of 0.996, which suggests the redox process of the 1-CPE is diffusion-controlled. In order to evaluate the electrocatalytic activity, 1-CPE is employed to investigate the electrocatalytic reduction of nitrite, bromate, and hydrogen peroxide in 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ aqueous solution (pH = 3.32), respectively. With the addition of nitrite, bromate or hydrogen peroxide, peak currents are almost unaffected by either the Cu-based reduction wave or the W-based reduction peaks (Fig. 7d–f). These results manifest that 1-CPE does not show obvious electrocatalytic activities toward nitrite, bromate or hydrogen peroxide.

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Figure 7 (a) Cyclic voltammogram of 1-CPE in pH = 3.32, 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ aqueous solution. (b) Cyclic voltammograms of 1-CPE at different scan rates. (c) The linear relationship between cathodic peak currents (III′) and the scan rates for 1-CPE. (d) Cyclic voltammograms of 1-CEP in pH = 3.32, 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ aqueous solution containing various concentrations (a 3 × 10⁻³, b 5 × 10⁻³, c 7 × 10⁻³, d 9 × 10⁻³) of NaNO₂. (e) Cyclic voltammograms of 1-CEP in pH = 3.32, 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ aqueous solution containing various concentrations (a 1 × 10⁻⁵, b 3 × 10⁻⁵, c 5 × 10⁻⁵, d 7 × 10⁻⁵) of NaBrO₃. (f) Cyclic voltammograms of 1-CEP in pH = 3.32, 0.5 mol L⁻¹ Na₂SO₄ + H₂SO₄ aqueous solution containing various concentrations (a 5 × 10⁻⁴, b 1 × 10⁻³, c 1.5 × 10⁻³, d 2 × 10⁻³) of H₂O₂.

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