A molecular crown analogue templated by Keggin polyanions: synthesis, structure, and electrochemical and luminescent properties

2.1 Materials and methods

All reagents were purchased from Jinan Camolai Trading Company, China and used as received. Elemental analyses (C, H and N) were performed on a 2400 CHN Elemental Analyzer (Perkin-Elmer, USA), and that of Cu and W were carried out with a Leaman inductively coupled plasma (PLASMA-SPEC(I), USA) spectrometer. The IR spectrum was recorded from KBr pellets in the range of 4000–400 cm−1 with an AVATAR FT-IR360 spectrometer (Nicolet, USA). A CHI660 electrochemical workstation (CH Instruments, China) was used for control of the electrochemical measurements and data collection. A conventional three-electrode system was used, with a carbon paste electrode (CPE) as a working electrode, a commercial Ag/AgCl unit as reference electrode and a twisted platinum wire as counter electrode. The 1-CPE was fabricated according to the method reported in the literature [28]. A photoluminescence analysis was performed on an Edinburgh FLS920 fluorescence spectrometer (Edinburgh Instruments, UK). The powder X-ray diffraction (PXRD) data of the samples were obtained with a D/max 2500V PC diffractometer (Rigaku, Japan) with Cu-Kα radiation, and the scanning rate is 4°/s, 2θ ranging from 5–40°.

2.2 Synthesis of [Cu^I₄(bpmb)₄][PW^VI₁₁W^VO₄₀]

A mixture of H₃PW₁₂O₄₀ (0.06 mmol, 200 mg), CuCl₂·2H₂O (0.50 mmol, 67 mg) and bpmb (0.30 mmol, 63 mg) were dissolved in 12 mL of distilled water. The resulting suspension was stirred for 30 min, and the pH was adjusted to 3.0 with 1.0 m HCl. The suspension was then sealed in an 18 mL Teflon-lined reactor and heated at 160 °C for 4 days. After cooling slowly (10 °C h⁻¹) to room temperature, red block-shaped crystals were obtained. The crystals were stable in air at ambient temperature and insoluble in common organic solvents and water (37 % yield based on W). – Anal. for C₅₆H₅₆Cu₄N₁₆O₄₀PW₁₂ (4084.42) (1): Calcd. C 16.47, H 1.38, N 5.49, Cu 6.22, W 54.01; found: C 16.54, H 1.46, N 5.65, Cu 6.18, W 53.94 %.

2.3 X-ray crystallography

A single crystal of compound 1 was carefully selected for single-crystal X-ray diffraction analysis. Data collection for 1 was performed on a Bruker SMART Apex CCD diffractometer at 153 K. An absorption correction was applied by using the multiscan program Sadabs [29]. The structure was solved by Direct Methods, and non-hydrogen atoms were refined anisotropically by least-squares methods on F² using the program Shelxtl [30]. The hydrogen atoms of the organic ligands were generated geometrically. The hydrogen atoms of the water molecules could not be found from the residual peaks and were only included in the final molecular formula. During the refinement, weak reflections above 2θ = 50° were omitted. The restraint commands ‘ISOR’ were used to restrain the atoms with ADP problem. The restrained atoms are as follows: O10 O11 O16 O26 O9 C20 C28 O5 O13 O14 O18 O19 C1 C15 C19 C26 C28 O5 O7 O8 O14 O18 O19 O24 N8 O22 O24 and O22. Command ‘DELU’ was used to atoms C6 and C23 with Hirshfeld errors leading all together to 109 applied restraints. A summary of the crystal data, data collection and refinement parameters for 1 is given in Table 1. Selected bond lengths and angles are listed in Tables 2 and 3, respectively.

CCDC 1040322 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Description of the structure

Single-crystal X-ray diffraction analysis reveals that 1consists of two basic subunits: a non-coordinated PW₁₂ cluster (a) and a crown-like [Cu₄(bpmb)₄]⁴⁺ macrocycle (b) (Fig. 1). The subunit a shows a well-known α-Keggin type structure [31], consisting of a central PO₄ tetrahedron with corner-sharing four triad {W₃O₁₃} clusters. The central four μ₄-O atoms are disordered over eight positions with each oxygen site half-occupied, which is a usual phenomenon for Keggin clusters [32–34]. The P–O bond lengths are in the range of 1.48(2)−1.57(3) Å, and the W–O bond lengths are in the range of 1.65(17)–2.54(2) Å. All of the bond lengths are within the normal ranges and in close agreement with those described in the literature [35]. In subunit b, there are three crystallographically independent Cu cations (Cu1, Cu2, Cu3, Cu1^#1) and four bpmb ligands. All Cu cations show an identical linear coordination geometry achieved by two N atoms from different bpmb ligands with Cu–N distances ranging from 1.87(2) to 1.90(2) Å, which are within the normal ranges observed in other Cu(I) complexes [36, 37]. The four crystallographically independent bpmb ligands link the Cu ions in a head-to-tail fashion to form a crown-like macrocycle (Fig. 2). Interestingly, there exists hydrogen bonding between neighboring PW₁₂ clusters and [Cu^I₄(bpmb)₄]⁴⁺ macrocycles, such as C18−H18A···O15 (2.573 Å) and C3−H3A···O20 (2.635 Å). As a result, the PW₁₂ clusters and the [Cu^I₄(bpmb)₄]⁴⁺ macrocycles are linked together in an ABAB fashion to form a chain (Fig. 3). Further, these chains are connected through hydrogen bondings of O14···H1A−C1 (2.645 Å) to form a 2D supramolecular framework (Fig. 4).

Fig. 1:

Ball-and-stick representation of the asymmetric unit of compound 1 (hydrogen atoms and coordinating water molecules omitted for clarity).

Fig. 2:

(a) View of a [Cu₄(bpmb)₄] subunit in compound 1; (b) view of a crown.

Fig. 3:

The supramolecular chain consisting of PW₁₂ clusters and [Cu^I₄(bpmb)₄]⁴⁺ macrocycles.

Fig. 4:

View of the hydrogen bonds between the adjacent chains of the 2D supramolecular framework.

3.2 Bond valence sum calculations, IR and powder X-ray diffraction spectra

In order to confirm the valences of the metal atoms in 1, bond valence sum (BVS) calculations [38] were done. In compound 1, all W atoms are in the oxidation state +VI as indicated by BVS calculations. The copper atoms are all in the oxidation state +I, in agreement with the coordination environments and confirmed by the BVS calculations. The Cu atoms were reduced from oxidation state +II to +I during the synthesis, which is often observed in hydrothermal reactions containing both N-donor organic ligands (bpmb in this work) and Cu^II ions [39].

The IR spectrum of compound 1 has characteristic bands at 977, 892, 807 and 1084 cm⁻¹, which are attributed to (W−Ot), (W−Ob−W), (W−Oc−W) and (P−O) modes [27], respectively. Bands at 1268–1636 cm⁻¹ are attributed to the bpmb ligand [36, 37].

To assess the phase purity of compound 1, powder X-ray diffraction (PXRD) experiments are carried on freshly prepared samples. As shown in Fig. 5, one can see that the diffraction peaks of both simulated and experimental patterns match well in key positions; however, some differences may be attributable to phase impurities.

Fig. 5:

Simulated (black) and experimental (red) PXRD patterns of compound 1.

3.3 Electrochemical properties

The cyclic voltammetric behavior of a CPE (1-CPE) was measured in 1 m H₂SO₄ solution. As shown in Fig. 6, the potential range of −650 to +1000 mV has three pairs of reversible redox peaks appearing with mean peak potentials E_1/2 = (Ep_a + kEp_c)/2 are 61 mV (II−II′), −410 mV (III−III′) and −620 mV (IIII−IIII′), respectively, at the scan rate 50 mV s⁻¹. These peaks should be ascribed to redox processes of W⁶⁺/W⁵⁺ for PW₁₂ in 1 [40]. In addition, there is one reversible anodic peak (I) at +484 mV, which is the one electron redox process of Cu⁺∕Cu [41]. As shown in the insert of Fig. 6, when the scan rates are varied from 40 to 120 mV s⁻¹, the peak potentials change gradually: the cathodic peak potentials shift toward the negative direction, and the corresponding anodic peak potentials to the positive direction with increasing scan rates. The peak currents are proportional to the scan rate, which indicates that the redox processes are surface-controlled, and the exchanging rate of electrons is fast [42–44].

Fig. 6:

Cyclic voltammograms for 1-CPE in 1 m H₂SO₄ solution at different scan rates (from inner to outer): 40, 60, 80, 100 and 120 mV s⁻¹.

3.4 Electrocatalytic activity

The electrocatalytic properties of 1-CPE have also been investigated. The results show that 1-CPE has bifunctional electrocatalytic activities toward not only reduction of NO₂⁻ and IO₃⁻ ascribed to W centers but also oxidation of biologically relevant molecules like ascorbic acid (AA) ascribed to the Cu centers, which is seldom reported in the literature [45]. As shown in Figs. 7 and 8, 1-CPE displays good electrocatalytic activity toward the reduction of NO₂⁻ and IO₃⁻ in 1 m H₂SO₄ solution. With addition of NO₂⁻ and IO₃⁻, the cathodic peak currents for I, II and III, especially peak III for NO₂⁻ and peak I for IO₃⁻, are increased, while the corresponding anodic peak currents are markedly decreased. The nearly equal current steps for each addition of NO₂⁻ and IO₃⁻ demonstrate stable and efficient electrocatalytic activity of 1-CPE (see the inserts of Figs. 7 and 8. As shown in Fig. 9, with addition of AA, the anodic peak currents of copper-centered redox waves are increased, while the corresponding cathodic peak currents are decreased, suggesting that 1 can catalyze the oxidation of AA.

Fig. 7:

Reduction of NO₂⁻ at 1-CPE in 1 m H₂SO₄ solution containing NO₂⁻ in various concentrations (from outer to inner): 0.0, 0.2, 0.4, 0.6 and 0.8 mm. Scan rate: 0.05 V s⁻¹. The insets show a linear dependence of the current peaks III′ of 1-CPE with NO₂⁻ concentration, respectively.

Fig. 8:

Reduction of IO₃ at 1-CPE in 1 m H₂SO₄ solution containing IO₃⁻ in various concentrations (from outer to inner): 0.0, 0.2, 0.4, 0.6 and 0.8 mm. Scan rate: 0.05 V s⁻¹. The insets show a linear dependence of the current peaks II′ of 1-CPE with IO₃⁻ concentration, respectively.

Fig. 9:

Reduction of oxidation of AA at 1-CPE in 1 m H₂SO₄ solution containing AA in various concentrations (from outer to inner): 0.0, 0.2, 0.4, 0.6 and 0.8 mm. Scan rate: 0.05 V s⁻¹. The insets show a linear dependence of the current peaks I of 1-CPE with AA concentration, respectively.

3.5 Luminescence properties

The solid-state luminescence of compound 1 was investigated at room temperature. In order to understand the nature of the luminescence, the emission spectra of the free ligand (bpmb) were also investigated under identical experimental conditions (Fig. 10). The free bpmb ligand exhibits one fluorescent emission band at λ_max = 400 nm upon excitation at 310 nm, probably attributable to the π – π* transitions [46–48]. Compound 1 displays one strong broad emission band at λ_max = 420 nm (λ_ex = 310 nm). Compared with the free bimb ligand, we can observe that the emission spectrum of compound 1 is red-shifted, which may be due to the effect of the coordination between ligands and metal ions [49]. The origin of the emission of compound 1can be tentatively attributed to ligand-to-metal charge transfer [50–52].

Fig. 10:

Solid-state emission spectra of bpmb and 1 (λ_ex = 310 nm) at room temperature.