Assembly of polyoxometalates and Ni-bpy cationic units into the molecular core–shell structures as bifunctional electrocatalysts

Abstract

Four polyoxometalate-based supramolecular assemblies were designed and synthesized under hydrothermal conditions: [Ni(2,2-bpy)₃]₅[PW₁₁NiO₃₉(H₂O)]₂·1.08H₂O (1), [Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)BW₁₂O₄₀] (2), {[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)GeW₁₂O₄₀]}⁻ (3), and {[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)PW₁₂O₄₀]}²⁻ (4). These structures were characterized by IR, TG analysis, elemental analysis, X-ray powder diffraction and single-crystal X-ray diffraction. In compound 1, two mono-substituted [PW₁₁NiO₃₉(H₂O)]⁵⁻ anions were fused together via the H-bonding interactions forming a dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻, which was surrounded by Ni-bpy cationic units into the molecular core–shell structures. Compounds 2–4 were composed of saturated polyoxoanions [BW₁₂O₄₀]⁵⁻, [GeW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻, respectively. These saturated anions were surrounded by [Ni(2,2-bpy)₃]²⁺ and [Ni(2,2-bpy)₂(H₂O)]²⁺ cationic units forming isostructural core–shell organic–inorganic hybrid materials. These isostructural polyoxometalates with different heteroatoms and charges all formed the isostructural materials, indicating that their charges do not have a significant influence on the structure of supramolecular assemblies. A detail study showed that substituted polyoxometalate with the terminal H₂O molecule is essential for constructing the larger core–shell structure. An electrocatalytic study indicated that these four compounds are bifunctional electrocatalysts towards water oxidation and reduction of nitrite.

---

Introduction

Inorganic–organic hybrid materials have attracted considerable attention owing to their special and enhanced physical and chemical properties. They have a variety of structures due to multifarious interactions between the inorganic and organic components. Therefore, due to the diversity of their components and structures, they possess a wide range of applications, including catalysis, gas adsorption, and pharmaceuticals.¹ As is well known, inorganic anions are used widely to react with the organic and metal–organic units for constructing functional inorganic–organic assemblies. Many research groups have developed macrocycle, cage-like structures as receptors that are capable of hosting small guest anions (e.g. Cl⁻, NO₃⁻, ClO₄⁻).² In addition, a series of 4f clusters and the heterometallic 3d–4f clusters with graceful structures, [Gd₃₆Ni₁₂], [Gd₄₂Ni₁₀], [La₂₀Ni₃₀], [Gd₅₄Ni₅₄], [La₆₀Ni₇₆], [Er₄₈] and [Er₆₀], were designed and synthesized with the existence of small anions.^3,4 Although most of these small anions have a single structure and un-adjustable charges, numerous functional materials were designed and synthesized with their assistance.

Polyoxometalates (POMs), as a class of unique inorganic metal–oxygen clusters, possess structural diversity, and their structure and charge can be regulated easily.⁵ The assembly of the POMs and the metal–organic cationic units is a promising way to construct functional materials. In 2010, an innovative {Mo₁₅₀} wheel was designed and synthesized, in which a central {Mo₃₆} cluster appears to template the assembly of the surrounding {Mo₁₅₀} wheel.^6a Two host–guest complex [(SiMo₁₂O₄₀)Mo₂₄(Fe-edta)₁₂O₇₂]¹⁶⁻ and [(P₂W₁₈O₆₂)Mo₂₄(Fe-edta)₁₂O₇₂]¹⁸⁻ with the [SiMo₁₂] and [P₂W₁₈] anionic templates forming the {Mo₂₄Fe₁₂} macrocycle were reported in 2013.^6b In addition, various POMs were used widely as templates to isolate the silver(I)-cage-like clusters, such as the giant Ag₆₀ cluster [Ag₆₀(Mo₆O₂₂)₂(^tBuC≡C)₃₈](CF₃SO₃)₆, in the existence of polyoxoanion under the solvothermal conditions.^7a An Ag₇₀ cluster [Ag₇₀(PW₉O₃₄)₂(^tBuC≡C)₄₄(H₂O)₂][BF₄]₈·2[BMIm]BF₄·3H₂O (BMIm = 1-butyl-3-methylimidazolium) was reported using lacunary polyoxoanion [A-α-PW₉O₃₄]⁹⁻ as the template in ionic liquids.^7b Recently, a Mo₂O₇-based C₃-symmetric core–shell cluster [Ag₉(^tBuC₆H₄S)₇(dpph)₃(Mo₂O₇)_0.5]₂ was isolated under solvothermal conditions.^7c The core–shell structure has attracted considerable attention in the POM-based supramolecular assembly. Several typical assemblies: [(Cu₄Cl)(bpbb)₂(PMo₁₂O₄₀)], [Cu₄(bpbb)₂(SiMo₁₂O₄₀)], [Cu₃(C₂H₄N₄)₄][PW₁₂O₄₀], [Ag₈(C₂H₃N₄S)₄][SiW₁₂O₄₀], [Ag₆(C₂H₃N₄S)₄S₂][H₃PW₁₂O₄₀] and [Ni(dap)₂]₄[HV^IV₁₂V^V₆O₄₂(PO₄)] (dap = 1,2-diaminopropane) were designed and synthesized, which have been regarded as promising molecular structural models toward core–shell nanostructures.⁸

Herein, we report the synthesis and characterization of a series of supramolecular assemblies: [Ni(2,2-bpy)₃]₅[PW₁₁NiO₃₉(H₂O)]₂·1.08H₂O (1), [Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)BW₁₂O₄₀] (2), {[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)GeW₁₂O₄₀]}⁻ (3), and {[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)PW₁₂O₄₀]}²⁻ (4). Compound 1 represents the first core–shell structure composed of the dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻, where the substituted POM with the terminal H₂O molecule is essential for constructing a larger core–shell structure. An electrocatalytic study indicated that they possessed electrocatalytic activity of water oxidation and reduction of NO₂⁻.

Results and discussion

Synthesis

Compounds 1–4 were prepared from the mixture of Ni(Ac)₂, 2,2-bpy, Na₆K₄[Ni₄(PW₉O₃₄)₂], H₅BW₁₂O₄₀, K₈GeW₁₁O₃₉, and H₃PW₁₂O₄₀ under the hydrothermal conditions, respectively. Compound 1 was synthesized using the sandwich-type POM Na₆K₄[Ni₄(PW₉O₃₄)₂] as the precursor, and the resulting product contained a mono-nickel substituted polyoxoanion [PW₁₁NiO₃₉(H₂O)]⁵⁻. This showed that a transformation from a sandwich-type POM [Ni₄(PW₉O₃₄)₂]¹⁰⁻ to a mono-nickel substituted polyoxoanion [PW₁₁NiO₃₉(H₂O)] has occurred during the reaction process. The synthesis of 1 may supply a new method for constructing mono-nickel substituted-POM-based functional materials. Two mono-nickel substituted polyoxoanions [PW₁₁NiO₃₉(H₂O)]⁵⁻ were fused together by H-bonding interactions forming a dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻. The dimeric polyoxoanion was surrounded by Ni(bpy)₃ ligands into a core–shell organic–inorganic hybrid material. From the structure of 1, it can be concluded that the substituted POM with the terminal H₂O molecule is essential for constructing the larger core–shell structure. Furthermore, the saturated Keggin-type POM [BW₁₂O₄₀]⁵⁻ with identical negative charges to that of [PW₁₁NiO₃₉(H₂O)]⁵⁻ was used as the precursor to synthesize the core–shell materials. In addition, the [PW₁₂O₄₀]³⁻ and [GeW₁₂O₄₀]⁴⁻-based organic–inorganic hybrid materials isostructural to that of the [BW₁₂O₄₀]⁵⁻-based core–shell structure were also isolated under similar conditions. The double Keggin POM-based organic–inorganic hybrids materials in 1 could not be obtained with the saturated Keggin-type POMs. The synthesis of three isostructural materials using the POMs [PW₁₂O₄₀]³⁻, [GeW₁₂O₄₀]⁴⁻ and [BW₁₂O₄₀]⁵⁻ with different charges indicated that the charge of saturated POMs does not play key roles in synthesizing these supramolecular assemblies.

Structure

Compound 1 crystallized in the monoclinic space group C2/c, and was composed of a dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻ and the [Ni(2,2-bpy)₃]²⁺ groups. As shown in Fig. 1a, the dimeric polyoxoanion was surrounded by twenty [Ni(2,2-bpy)₃]²⁺ groups forming a core–shell organic–inorganic hybrid material. The nickel center in the counter [Ni(2,2-bpy)₃]²⁺ units was coordinated by three 2,2-bpy ligands with a octahedral coordinated environment. In the anion [PW₁₁NiO₃₉(H₂O)], the Ni1 site was coordinated by five O atoms from the lacunary [PW₁₁O₃₉] unit and a water molecule. Two [PW₁₁NiO₃₉(H₂O)] anions were fused together via the H-bonding interactions forming a dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂ with a bond length of 3.0789 Å. Herein, the dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻ was first used to construct the core–shell molecular structure.

Fig. 1 (a) From the left to right, structure of [PW₁₁NiO₃₉(H₂O)]₂ in 1, the arrangement of the [Ni(2,2-bpy)₃]²⁺ groups in the shell, and core–shell structure in 1; (b) from left to right, the structure of the Keggin-type POM [BW₁₂O₄₀] in 2, the arrangement of the [Ni(2,2-bpy)₃]²⁺ and [Ni(2,2-bpy)₂(H₂O)] groups in the shell, and the core–shell structure in 2. Color code: P (yellow), B (pink), W (green), Ni (light blue), O (red), N (blue), C (gray).

Compounds 2–4, were composed using the saturated polyoxoanions [BW₁₂O₄₀]⁵⁻, [GeW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻, respectively. These saturated anions were surrounded by the [Ni(2,2-bpy)₃]²⁺ and [Ni(2,2-bpy)₂(H₂O)]²⁺ cationic units forming the core–shell organic–inorganic hybrid materials. These three compounds are isostructural, the detailed structure of compound 2 was selected as an example for a detailed description. Compound 2 crystallized in the monoclinic space group C2/c, which also exhibited a core–shell supramolecular structure consisting of [BW₁₂O₄₀]⁵⁻ and the [Ni(2,2-bpy)₃]²⁺ and [Ni(2,2-bpy)₂(H₂O)]²⁺ cationic units (Fig. 1b). The [Ni(2,2-bpy)₃]²⁺ unit has been observed in compound 1. The nickel center was coordinated by three 2,2-bpy ligands with six N atoms. For the [Ni(2,2-bpy)₂(H₂O)]²⁺ unit, the nickel center was coordinated by two 2,2-bpy ligands, one terminal water molecule and an oxygen atom from the saturated Keggin-type polyoxoanion. The Ni–N bond length is in the range, 2.037(12)–2.130(8) Å, and the Ni–H₂O and Ni–O bond lengths are 2.128(10) Å and 2.079(9) Å, respectively. The [Ni(2,2-bpy)₂(H₂O)]²⁺ unit was connected to the polyoxoanion [BW₁₂O₄₀]⁵⁻ via the Ni–O–W linking modes to decorate the POM (Fig. S6a†). The [BW₁₂O₄₀]⁵⁻ anion was surrounded by 9 [Ni(2,2-bpy)₃]²⁺ units and 4 [Ni(2,2-bpy)₂(H₂O)]²⁺ units forming the core–shell molecular structure (Fig. 1b and 2e). For compound 4, the size of the crystal is too small to be suitable for single crystal X-ray diffraction analysis. Powder X-ray diffraction (PXRD) studies showed that 4 was isostructural with 2, with the diffraction pattern matching that of 2, building the saturated [PW₁₂O₄₀]³⁻, replacing the [BW₁₂O₄₀]⁵⁻ in 2 (Fig. S5†).

Fig. 2 Ball-and-stick representation of the (a) [Ni(2,2-bpy)₃] unit and the (b) [Ni(2,2-bpy)₂O(H₂O)] unit; (c) polyhedral and ball-and-stick representation of [PW₁₁NiO₃₉(H₂O)]₂ in 1, core–shell structure of (d) compound 1 and (e) compound 2. The 2,2-bpy were deleted for clarity. Color code: P (yellow), B (pink), W (green), Ni (light blue), O (red), N (blue), C (gray).

Electrochemistry and electrocatalytic reduction of NO₂⁻

POMs, can undergo a stepwise multi-electron reversible redox process without changing their original structure, and exhibit interesting electrochemical and electrocatalytic properties.⁹ As is well known, the carbon paste electrode (CPE) is inexpensive and easy to prepare and renew.¹⁰ In this study, the CPEs with compounds 1 and 2 and 4 were prepared carefully to study their electrochemical properties. The cyclic voltammogram (CV) curves of 1-CPE, 2-CPE were taken in 1 M H₂SO₄ aqueous solution with different scan rates (Fig. 3a and b). In the potential range of −0.5 V to 0.5 V, −0.6 V to 0.6 V, three pairs of redox peaks (I–I′, II–II′ and III–III′) were detected with E_1/2 = −0.515, −0.317 and −0.118 V for 1, E_1/2 = −0.597, −0.476 and −0.177 V for 2 (E_1/2 = (E_pa + E_pc)/2), respectively. These peaks corresponded to the redox process of the W centers in compounds 1 and 2, respectively.¹¹

Fig. 3 CV curves in 1 M H₂SO₄ aqueous solution at different scan rates (from inner to outer: 50, 100, 150, 200, 250, 300 mV s⁻¹) of (a) 1; (b) 2; electrocatalytic reduction of NO₂⁻ in 1 M H₂SO₄ aqueous solution by (c) 1; (d) 2. Scan rate: 100 mV s⁻¹.

POMs are used widely in the removal of pollutants from water by electrocatalytic processes, such as hydrogen peroxide, iodates, bromates, and nitrites.^9,12 Herein, the electrocatalytic properties of 1/2/4-CPE towards the reduction of nitrite in a 1 M H₂SO₄ aqueous solution were investigated. As shown in Fig. 3, on addition of different concentrations of NO₂⁻, the reduction peak current increased sharply, whereas the corresponding oxidation peak current decreased. In comparison, the blank electrode did not show catalytic activity at the maximum NO₂⁻ concentration. These results showed that compounds 1, 2 and 4 exhibited electrocatalytic activity toward the reduction of NO₂⁻ (Fig. 3c and d and S7†). No evident changes were observed in the CVs of these three electrodes after 200 cycles, which indicated the long-term stability of the 1/2/4-CPEs (Fig. S9†).

Electrocatalytic water oxidation

To investigate the redox properties of compounds 1, 2, and 4, a combination of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed using glassy carbon (GC – 0.071 cm²) and 1/2/4-CPE (0.0314 cm²) as the working electrode, a platinum wire as the counter electrode and an Ag/AgCl as the reference electrode (∼0.197 V vs. normal hydrogen electrode, NHE). The CV curves of all electrodes were measured in a 0.2 M pH = 8 sodium borate buffer solution at 100 mV s⁻¹. At a positive potential, a large and irreversible oxidation wave with an onset potential at E_onset = 1.348, 1.389, and 1.353 V versus the normal hydrogen electrode (NHE) appeared for 1/2/4-CPE, respectively. The overpotentials of 1/2/4-CPE were ca. 591, 632, and 616 mV, respectively. The overpotential is defined as the difference between the onset potential minus the thermodynamic potential for water oxidation at this pH. As shown in Fig. 4, the oxidation current density of the 1/2/4-CPEs is 2.640, 1.564, and 1.857 mA cm⁻² at +1.697 V versus NHE, respectively, which is 16, 11 and 9 times enhanced compared to that of the blank GC electrode (0.165 mA cm⁻²), respectively. These results indicated that these core–shell organic–inorganic hybrids materials have efficient electrocatalytic activity for water oxidation.^13a The LSV curves of these materials were further measured (Fig. S8†); a large and irreversible oxidation wave with an onset potential at E_onset = 1.349, 1.387, 1.355 V vs. NHE was also observed for 1/2/4-CPE, respectively, which is consistent with the CV results. As shown in Fig. 4, the electrocatalytic activity of Na₅PW₁₁NiO₃₉, H₅BW₁₂O₄₀ and H₃PW₁₂O₄₀ was detected further using glassy carbon as the working electrode in the sodium borate buffer solution (0.2 M pH = 8) containing 0.2 mM Na₅PW₁₁NiO₃₉, H₅BW₁₂O₄₀ and H₃PW₁₂O₄₀. However, a very low current density ca. 0.216, 0.245, and 0.366 mA cm⁻² was observed, which was much lower than that of the core–shell organic–inorganic hybrid materials. After a 200-cycle canning, no evident changes were observed for the CVs of these three electrodes, which indicates the long-term stability of the 1/2/4-CPE in the electrocatalytic water oxidation process (Fig. S9†).

Fig. 4 CV curves of a blank GC electrode, Na₅PW₁₁NiO₃₉, H₅BW₁₂O₄₀, H₃PW₁₂O₄₀, 1/2/4-CPE in a 0.2 M pH = 8 sodium borate buffer solution at the scan rate of 100 mV s⁻¹.

Experimental section

Instrumentation

The IR spectrum was carried out in the range of 400–4000 cm⁻¹ on an Alpha Centaurt FT/IR spectrophotometer using KBr pellets. TG analyses were recorded using a Perkin-Elmer TGA7 instrument in flowing N₂ with a heating rate of 10 °C min⁻¹. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/max-2550 diffractometer with Cu Kα radiation. Elemental analyses for W and Ni were carried out on a Leaman inductively coupled plasma (ICP) spectrometer, and C and N were obtained with a PerkinElmer 2400 CHN Elemental analyzer.

Synthesis

[Ni(2,2-bpy)₃]₅[PW₁₁NiO₃₉(H₂O)]₂·1.08H₂O (1). A mixture of Ni(Ac)₂ (0.04 g, 0.226 mmol), Na₆K₄[Ni₄(PW₉O₃₄)₂] (0.1 g, 0.02 mmol), 2,2-bpy (0.05 g, 0.32 mmol), was added to 10.0 mL of distilled water. After stirring for 4 h, the pH of the mixture was adjusted to 4.6 with a 4 M CH₃COOH solution. The mixture was then transferred to a 25 mL Teflon-lined autoclave, kept at 180 °C for 3 days and then cooled to room temperature at a rate of 5 °C h⁻¹. Light pink crystals were isolated and washed with distilled water (46.8% yield based on W). Anal. calcd: C 22.09, N 5.15, Ni 5.04, W 49.58%, found: C 21.97, N 4.95, Ni 5.12, W 48.68%. IR (cm⁻¹): 3085 (m), 2091 (w), 1599 (s), 1473 (m), 1439 (s), 1314 (m), 1055 (s), 1022 (m), 947 (s), 878 (s), 803 (w), 759 (m), 652 (w).

[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)BW₁₂O₄₀] (2). A mixture of Ni(Ac)₂ (0.04 g, 0.226 mmol), H₅BW₁₂O₄₀ (0.1 g, 0.035 mmol), 2,2-bpy (0.05 g, 0.32 mmol), was added to 10.0 mL of distilled water. After stirring for 4 h, the pH of the mixture was adjusted to 4.3 with a 4 M CH₃COOH solution. The mixture was then transferred to a 25 mL Teflon-lined autoclave, kept at 180 °C for 3 days and then cooled to room temperature at a rate of 5 °C h⁻¹. Pink block-shaped crystals were collected and washed with distilled water (44.8% yield based on W). Anal. calcd: C 19.35, N 4.51, Ni 3.64, W 54.68%, Found: C 19.05, N 4.25, Ni 3.47, W 54.03%. IR (cm⁻¹): 3089 (m), 2104 (w), 1598 (s), 1473 (m), 1441 (s), 1313 (m), 1063 (s), 1022 (m), 949 (s), 887 (s), 796 (w), 755 (m), 650 (w).

{[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)GeW₁₂O₄₀]}⁻ (3). The same synthetic procedure as that for 2 was used except that K₈GeW₁₁O₃₉ was used in place of H₅BW₁₂O₄₀ (4.8% yield based on W).

{[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)PW₁₂O₄₀]}²⁻ (4). The same synthetic procedure as that for 2 was used except that H₃PW₁₂O₄₀ was used in place of H₅BW₁₂O₄₀ (43.6% yield based on W). IR (cm⁻¹): 3072 (m), 2091 (w), 1598 (s), 1476 (m), 1439 (s), 1314 (m), 1065 (s), 1022 (m), 949 (s), 886 (s), 793 (w), 756 (m), 648 (w).

Preparation of 1-CPE

The 1, 2, 4-modified carbon paste electrodes (1-CPE, 2-CPE, 4-CPE) were fabricated as follows: graphite powder (90 mg) and 1/2/4 (1.2 μmol) were mixed and ground together by an agate mortar to achieve a uniform mixture. Subsequently, 80 μL of nujol was added to the mixture, which was stirred further for about 1 hour. The homogenized mixture was packed into a glass tube with a 2 mm inner diameter, and the tube surface was wiped with paper. Electrical contact was established with a copper rod through the back of the electrode.

X-ray crystallographic study

The single crystal X-ray diffraction data of compounds 1–3 were collected on a Bruker Smart Apex CCD diffractometer with Mo-Kα monochromated radiation (λ = 0.71073 Å) at 293(2) K. The structures were solved by direct methods and refined by the full-matrix least-squares method on F² using the SHELXTL crystallographic software package.¹⁴ In the refinement of 1, the nickel center is disordered over two sites with a ratio of about 87/13; it is not possible to locate the Ni position in the cluster. The hydrogen atoms attached to organic ligands were fixed in the calculated positions. The crystal data and structure refinements of compounds 1–3 are summarized in Table 1. Crystallographic details data can be obtained free of charge from the Cambridge Crystallography Data Centre (CCDC 1495993 for 1, 1495994 for 2, 1495995 for 3).†

Table 1 Crystal data and structure refinement for compounds 1–3^a

a R1 = ∑||F0| − |Fc||/∑|F0|; wR2 = ∑[w(F0^2 − Fc^2)^2]/∑[w(F0^2)^2]^1/2.
Compound	1	2	3
Formula	C150H120N30Ni7O81.08P2W22	C65H52N13Ni2.50O41BW12	C65H52N13Ni2.50O41GeW12
Fw	8157.64	4034.98	4096.76
Temp (K)	293(2)	293(2)	293(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	C2/c	C2/c	C2/c
a, Å	28.977(3)	46.643(3)	46.756(9)
b, Å	16.7308(2)	14.3057(8)	14.375(3)
c, Å	44.363(4)	25.9696(14)	26.151(5)
V, Å^3	21 328(4)	17 328.3(17)	17 576(6)
Z	4	4	4
μ/mm^−1	12.510	16.482	16.585
λ (Å)	0.71073	0.71073	0.71073
R1, wR2 [I > 2σ(I)]	0.0549, 0.1261	0.0442, 0.1008	0.0728, 0.1416
R1, wR2 (all data)	0.1051, 0.1471	0.0671, 0.1108	0.1475, 0.1692

---

Conclusions

In summary, four organic–inorganic hybrids materials constructed by POMs and [Ni(2,2-bpy)₃]²⁺/[Ni(2,2-bpy)₂(H₂O)]²⁺ units were synthesized by the hydrothermal method. In compound 1, the dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻ was first surrounded with Ni-bpy cationic units into the molecular core–shell structure. The substituted POM with the terminal H₂O molecule was fused together via H-bonds into a dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻, which is essential for constructing the larger core–shell structure. Compounds 2–4 also exhibited a core–shell structure constructed from the saturated Keggin-type polyoxoanions [BW₁₂O₄₀]⁵⁻, [GeW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻ with different charges. The isostructural core–shell organic–inorganic hybrids materials were obtained, indicating that the charge of the saturated POMs does not play a key role in the formation of the supramolecular assemblies. The electrocatalytic study indicated that these four compounds are bifunctional electrocatalysts towards water oxidation and reduction of nitrite.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21671032/21301020), the Science and Technology Development Project Foundation of Jilin Province (20150520001JH), the Science and Technology Research Foundation of the Thirteenth Five Years of Jilin Educational Committee ([2015]0056), Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), and Open Subject Foundation of Key Laboratory of Polyoxometalate Science of Ministry of Education.

Notes and references

1. (a) Q. Li, Y. G. Wei, J. Hao, Y. L. Zhu and L. S. Wang, J. Am. Chem. Soc., 2007, 129, 5810; (b) A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh and G. Izzet, Chem. Soc. Rev., 2012, 41, 7605; (c) T. Zhang and W. B. Lin, Chem. Soc. Rev., 2014, 43, 5982; (d) Z. M. Zhang, T. Zhang, C. Wang, Z. K. Lin, L. S. Long and W. B. Lin, J. Am. Chem. Soc., 2015, 137, 3197.
2. (a) G. T. Spence and P. D. Beer, Acc. Chem. Res., 2013, 46, 571; (b) M. S. Vickers and P. Beer, Chem. Soc. Rev., 2007, 36, 211; (c) F. J. Cui, S. G. Li, C. D. Jia, J. S. Mathieson, L. Cronin, X. J. Yang and B. Wu, Inorg. Chem., 2012, 51, 179.
3. (a) X. J. Kong, Y. L. Wu, L. S. Long, L. S. Zheng and Z. P. Zheng, J. Am. Chem. Soc., 2009, 131, 6918; (b) J. B. Peng, Q. C. Zhang, X. J. Kong, Y. P. Ren, L. S. Long, R. B. Huang, L. S. Zheng and Z. P. Zheng, Angew. Chem., Int. Ed., 2011, 50, 10649.
4. (a) X. J. Kong, Y. P. Ren, L. S. Long, Z. P. Zheng, R. B. Huang and L. S. Zheng, J. Am. Chem. Soc., 2007, 129, 7016; (b) M. Y. Wu, F. L. Jiang, D. Q. Yuan, J. D. Pang, J. J. Qian, S. A. AL-Thabaiti and M. C. Hong, Chem. Commun., 2014, 50, 1113.
5. (a) U. Kortz, A. Müller, J. Slageren, J. Schnack, N. S. Dalal and M. Dressel, Coord. Chem. Rev., 2009, 253, 2315; (b) J. W. Zhao, Y. Z. Li, L. J. Chen and G. Y. Yang, Chem. Commun., 2016, 52, 4418; (c) H. L. Wu, Z. M. Zhang, Y. G. Li, X. L. Wang and E. B. Wang, CrystEngComm, 2015, 17, 6261; (d) Z. J. Liu, X. L. Wang, C. Qin, Z. M. Zhang, Y. G. Li, W. L. Chen and E. B. Wang, Coord. Chem. Rev., 2016, 313, 94; (e) X. B. Han, Z. M. Zhang, T. Zhang, Y.-G. Li, W. Lin, W. You, Z. M. Su and E.-B. Wang, J. Am. Chem. Soc., 2014, 136, 5359.
6. (a) H. N. Miras, G. J. T. Cooper, D. L. Long, H. Bögge, A. Müller, C. Streb and L. Cronin, Science, 2010, 327, 72; (b) X. K. Fang, L. Hansen, F. Haso, P. C. Yin, A. Pandey, L. Engelhardt, I. Slowing, T. Li, T. B. Liu, M. Luban and D. C. Johnston, Angew. Chem., Int. Ed., 2013, 52, 10500; (c) H. Liu, C. Y. Song, R. W. Huang, Y. Zhang, H. Xu, M. J. Li, S. Q. Zang and G. G. Gao, Angew. Chem., Int. Ed., 2016, 55, 3699; (d) W. M. Xuan, A. J. Surman, Q. Zheng, D. L. Long and L. Cronin, Angew. Chem., Int. Ed., 2016, 55, 12703.
7. (a) J. Qiao, K. Shi and Q. M. Wang, Angew. Chem., Int. Ed., 2010, 49, 1765; (b) Z. G. Jiang, K. Shi, Y. M. Lin and Q. M. Wang, Chem. Commun., 2014, 50, 2353; (c) X. Y. Li, H. F. Su, R. Q. Zhou, S. Feng, Y. Z. Tan, X. P. Wang, J. Jia, M. Kurmoo, D. Sun and L. S. Zheng, Chem.–Eur. J., 2016, 22, 3019.
8. (a) J. J. Gong, W. S. Zhang, Y. Liu, H. C. Zhang, H. L. Hu and Z. H. Kang, Dalton Trans., 2012, 41, 5468; (b) Y. A. Yang, J. J. Gong, W. S. Zhang, H. C. Zhang, L. L. Zhang, Y. Liu, H. Huang, H. L. Hu and Z. H. Kang, RSC Adv., 2012, 2, 6414; (c) Y. Y. Zhou, Y. J. Kong, Q. Q. Jia, S. Yao and J. H. Yan, RSC Adv., 2016, 6, 33946.
9. (a) G. F. Hou, L. H. Bi, B. Li and L. X. Wu, Inorg. Chem., 2010, 49, 6474; (b) X. L. Wang, Y. F. Bi, B. K. Chen, H. Y. Lin and G. C. Liu, Inorg. Chem., 2008, 47, 2442; (c) S. B. Li, W. Zhu, H. Y. Ma, H. J. Pang, H. Liu and T. T. Yu, RSC Adv., 2013, 3, 9770.
10. (a) R. N. Adams, Anal. Chem., 1958, 30, 1576; (b) L. Gorton, Electroanalysis, 1995, 7, 23.
11. R. A. Prados and M. T. Pope, Inorg. Chem., 1976, 15, 2547.
12. (a) X. L. Wang, Z. H. Kang, E. B. Wang and C. W. Hu, J. Electroanal. Chem., 2002, 523, 142; (b) B. X. Dong, L. Chen, S. Y. Zhang, J. Ge, L. Song, H. Tian, Y. L. Teng and W. L. Liu, Dalton Trans., 2015, 44, 1435; (c) L. Cheng, X. M. Zhang, X. D. Xi and S. J. Dong, J. Electroanal. Chem., 1996, 407, 97.
13. (a) T. T. Li and Y. Q. Zheng, Dalton Trans., 2016, 45, 12685; (b) Y. N. Gong, T. Ouyang, C. T. He and T. B. Lu, Chem. Sci., 2016, 7, 1070.
14. (a) G. M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997; (b) SHELXTL, 2014/7 ed., Bruker AXS, Madison, WI, 2014; (c) SHELXL, ed. G. M. Sheldrick, vol. 2012 – 4, 2012 – 1, 2012.

---

Footnote

† Electronic supplementary information (ESI) available: Structural figures, TGA, IR and CIF file. CCDC 1495993 for 1, 1495994 for 2 and 1495995 for 3. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra19257b

---