A novel polyoxometalate templated microporous metal–organic framework with electrochemical properties

Abstract

A novel Wells–Dawson polyoxometalate-templated 3D microporous metal–organic framework (MOF), namely, H₆[Cu₃(H₂O)₆(P₂W₁₈O₆₂)₂(3-dpye)₆]·28H₂O (1) (3-dpye = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane), was hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR spectra, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). Each Cu(II) ion acts as a four-connected node linking four 3-dpye ligands to construct a 3D microporous host MOF containing left-handed helical chains, in which the P₂W₁₈O₆₂⁶⁻ anions reside. 1 represents the first example of 3D MOFs with an 8⁶ uniform net containing non-coordinating Wells–Dawson anions. The electrochemical properties have been investigated in detail.

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Introduction

Polyoxometalate (POM)-based metal–organic frameworks (MOFs) have attracted great attention, not only due to their novel and varied structures, but also stemming from their potential applications in the fields of catalysis, photocatalysis, electrocatalysis, adsorption, magnetism, etc.^1–3 Lots of POM-based MOFs constructed from various polyanions, appropriate organic ligands and metal ions have been obtained, in which polyanions mainly exert three roles:⁴ a linker,^4a a simple counter anion as a template,^4b or a combination role of linker and template.² So far, a series of POM-templated MOFs have been reported, in which the host frameworks usually show fundamental classes called uniform nets.^4c,d,5 However, the four-connected 8⁶ uniform net is very infrequent and only four examples have been obtained so far. Especially, a three-dimensional (3D) 8⁶ MOF induced by discrete POM anions has never been reported to our knowledge. Compared with the classical Keggin type anions,^5a–d the Wells–Dawson type anions acting as templates to construct MOFs are very limited,⁶ which may depend on the large size and high charge of the Wells–Dawson anions. Liu's group have reported two Wells–Dawson type POMs-templated 3D MOFs based on oxalate-bridged binuclear cobalt(II)/nickel(II) secondary building units and 4,4′-bipyridine linkers.⁷ Our group also reported a new bitrack sinusoid-like metal–organic chain templated by Wells–Dawson type polyoxometalate.⁸ However, it is still challenging and intriguing to design and construct Wells–Dawson POMs-templated MOFs, especially those containing micropores and acting as potential heterogeneous catalysts.

On the other hand, the selection of organic ligands play an important role in the assembly of POM-based MOFs.⁹ In recent years, the introduction of N-containing ligands has become popular in the POM-based hybrids, such as bis(pyridlyl)-,¹⁰ bis(imidazole)-,¹¹ bis(triazole)-,¹² and bis(tetrazole)-based¹³ derivatives ligands, which have been widely used to construct POM-based complexes. As a sort of N/O-donor ligands, flexible bis-pyridyl-bis-amide have attracted much attention based on the following structural characters: (i) this kind of ligands possess flexible –(CH₂)_n– spacers, which allow themselves to bend and rotate freely when coordinating to the metal centers; (ii) the pyridyl and amide group can provide more potential coordination sites; (iii) the amide groups with both the N–H hydrogen donor and C=O hydrogen acceptor promote the formation of supramolecular structures through hydrogen bonding interaction. Recently, our group introduced this kind of bis-pyridyl-bis-amide ligands into POM-based metal–organic complexes (MOCs), and obtained a series of Keggin-type and Anderson-type POM-based MOCs.¹⁴ However, the Wells–Dawson POM-based MOCs constructed from bis-pyridyl-bis-amide ligands are much less common.

As a continuous effort of our previous work, here we selected a flexible bis-pyridyl-bis-amide N,N′-bis(3-pyridinecarboxamide)-1,2-ethane (3-dpye) as the organic ligand, and a Wells–Dawson [P₂W₁₈O₆₂]⁶⁻ (abbreviated as P₂W₁₈) as the inorganic anion, to assemble with Cu²⁺ ions under hydrothermal conditions. The blue crystals of H₆[Cu₃(H₂O)₆(P₂W₁₈O₆₂)₂(3-dpye)₆]·28H₂O (1) was obtained, which exhibits an uncommon 3D MOF with 8⁶ uniform net containing the non-coordinating Wells–Dawson anion template. The electrochemical activities of the compound have been investigated in this work (Scheme 1).

Scheme 1 The ligand used in this paper.

Experimental

Materials and characterization

The 3-dpye ligand was prepared according to the reported procedure.¹⁵ All other reagents and solvents for syntheses were purchased from commercial sources and used as received without further purification. FT-IR spectra (KBr pellets) were taken on a Varian 640 FT-IR spectrometer by KBr pellets in the range 4000–500 cm⁻¹. Powder XRD investigations were carried out with an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα (λ = 1.5406 Å) radiation. Thermogravimetric analyses (TGA) were performed on a SDT 2960 Simultaneous DSC-TGA instrument under flowing N₂ atmosphere with a heating rate of 10 °C min⁻¹.

Synthesis of H₆[Cu₃(H₂O)₆(P₂W₁₈O₆₂)₂(3-dpye)₆]·28H₂O (1)

A mixture of CuSO₄·5H₂O (0.12 g, 0.48 mmol), 3-dpye (0.04 g, 0.15 mmol), K₆P₂W₁₈O₆₂·15H₂O (0.30 g, 0.06 mmol) and H₂O (10 mL) was stirred for 30 min at the room temperature. The pH value was then adjusted to about 3.5 using 1.0 M HCl. The suspension was transferred to a Teflon lined autoclave (25 mL) and kept at 120 °C for 4 days. After slow cooling to room temperature, blue block crystals of 1 were obtained. Yield 30% based on W. IR (KBr pellet, cm⁻¹): 3359 (m), 2343 (m), 1649 (s), 1606 (m), 1539 (s), 1475 (m), 1429 (m), 1342 (w), 1203 (w), 1091 (s), 1031 (m), 962 (s), 916 (s), 806 (s), 740 (m), 696 (m), 599 (m), 580 (w), 551 (w).

Preparation of compound 1 bulk-modified CPE

The compound 1 bulk-modified carbon paste electrode (1-CPE) was fabricated by mixing 0.11 g graphite powder and 0.01 g compound 1 in an agate mortar for approximately 30 min to achieve an uniform mixture; then 0.10 mL paraffin oil was added and stirred with a glass rod.¹⁶ The homogenized mixture was packed into a 3 mm inner diameter glass tube and the tube surface was wiped with weighing paper. The electrical contact was established with the copper wire through back of the electrode. The bare CPE was prepared by similar process without compound 1.

X-ray crystallographic study

All diffraction data were collected using a Bruker SMART APEX II with Mo Kα (λ = 0.71073 Å) by ω and θ scan mode. The structure was solved by direct methods SHELXS program of the SHELXTL package.¹⁷ All nonhydrogen atoms except some disordered water molecules were refined anisotropically, and the hydrogen atoms of the ligand were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. All hydrogen atoms attached to water molecules were not located, but were included in the structure factor calculations. The SQUEEZE routine of PLATON was applied to remove the contributions to the scattering from the solvent molecules. The reported refinements are of the guest-free structures using the *.hkp files produced by using the SQUEEZE routine. The amount of water came from the elemental analysis and TG experiment. The crystal data and structure refinement details for the title compound are given in Table 1. The Table S1 in the ESI† lists the data of selected bond distances and angles.†

Table 1 Crystal data and structure refinement for compound 1

Compound	1
a R1 = ∑‖Fo| − |Fc‖/∑|Fo|.b wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Empirical formula	C84H158Cu3N24O170P4W36
Formula weight	11 157.44
Crystal system	Cubic
Space group	I432
a (Å)	28.9058(4)
b (Å)	28.9058(4)
c (Å)	28.9058(4)
α (°)	90
β (°)	90
γ (°)	90
V (Å^3)	24 152.1(6)
Z	4
Dc (g cm^−3)	3.068
μ (mm^−1)	17.453
F (000)	20 004
GOF	1.029
R1^a [I > 2σ(I)]	0.0229
wR2^b (all data)	0.0594

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Results and discussion

Description of crystal structure of H₆[Cu₃(H₂O)₆(P₂W₁₈O₆₂)₂(3-dpye)₆]·28H₂O (1)

Single crystal X-ray diffraction study shows that 1 crystallizes in cubic space group I432. The asymmetric unit of 1 contains three Cu(II) ions, two noncoordinating [P₂W₁₈O₆₂]⁶⁻ (abbreviated to P₂W₁₈) anions, six 3-dpye ligands, six coordinated water molecules, and twenty-eight lattice water molecules. The P₂W₁₈ anion possesses the well-known Wells–Dawson structure, which consists of two [PW₉O₃₄]⁶⁻ units derived from an Keggin-type anion by removal of a set of three corner-shared WO₆ octahedra. Its W–O distances can be grouped into three sets: W–Ot (terminal oxygen) 1.707(5)–1.728(3) Å, W–Ob (bridge oxygen) 1.864(5)–1.967(3) Å, and W–Oc (central oxygen) 2.368(5)–2.389(7) Å. The central P atoms exhibit tetrahedral coordination mode with the P–O bond lengths in the range of 1.527(5)–1.560(8) Å. All the bond lengths and angles are within the normal ranges and in close agreement with those described in the literatures.^1a–c,5d Valence sum calculations¹⁸ show that all of the Cu ions are in +II oxidation state and all the W atoms are in +VI oxidation state in the title compound. To balance the charge, six protons are added, then 1 is formulated as H₆[Cu₃(H₂O)₆(P₂W₁₈O₆₂)₂(3-dpye)₆]·28H₂O.¹⁹

There is only one crystallographically unique Cu(II) ion with a distorted square–pyramidal geometry, which is five-coordinated by four nitrogen atoms of four different 3-dpye ligands and one oxygen atom from one coordinated water molecule, as shown in Fig. 1a.

Fig. 1 (a) ORTEP diagram of the building unit of 1. (b) Ball-and-stick representations of the channel A in the 3D net viewed along the a/b/c-axis. (c) View of the 3D microporous framework formed by Cu(II) ions and 3-dpye ligands. (d) The helical tube constructed from double-helical chains (left) and the left-handed single-helical chain (right).

In 1, the 3-dpye acting as μ₂-bridging ligand links the adiacent Cu(II) ions via its pyridyl nitrogen atoms to produce a 3D microporous MOF (Fig. 1c). There exists a kind of left-handed helical chain with a pitch of 44.08 Å (Fig. 1d), which is the most striking feature for 1. Four left-handed helical chains enlace together to form a helical tube with small square channel A (Fig. 1b). The diagonal length of the square channel A is 11.24 Å. The other kind of larger channel B can also be observed along the a-axis with dimension of 25.03 Å × 19.49 Å (Fig. S1b and S2†), which is generated by six 3-dpye ligands in different directions. The channel B is large enough to accommodate the Wells–Dawson anion (ca. 12.24 Å × 10.37 Å, Fig. S1a†) to maintain stability of the whole framework (Fig. 2a). It is supposed that P₂W₁₈ anions may serve as templates during the process of self-assembly, so that the metal–organic cationic building blocks are aggregated around them, which leads to formation of the porous 3D cationic framework. Although the lattice water molecules and P₂W₁₈ anions are filled in the two different channels, respectively, it is worth noting that the large voids left (ca. 32.6%) can be observed by the calculation of PLATON¹⁷ software.

Fig. 2 (a) View of the 3D host framework with the channels (yellow balls) and the embedded P₂W₁₈O₆₂⁶⁻ polyanions (polyhedra). (b) The four-connected 8⁶ uniform mdf net with the long vertex symbol (8₆·8₆·8₇·8₇·8₇·8₇).

The topological analysis approach can be employed to understand the intricate structure better. Each Cu(II) ion connected to four near neighbours by four 3-dpye ligands can be regarded as a four-connected node, thus the overall 3D framework of 1 can be rationalized as a four-connected 8⁶ uniform mdf net with the long vertex symbol 8₆·8₆·8₇·8₇·8₇·8₇ (Fig. 2b). So far, only four 8⁶ nets have been reported (with long vertex symbols 8₃·8₃·8₃·8₃·8₂·8₂,^20a 8₂·8₂·8₅·8₅·8₅·8₅,^20b 8₆·8₆·8₇·8₇·8₇·8₇,^20c and 8₁·8₃·8₄·8₄·8₄·8₄ (ref. 20d)). In 1, P₂W₁₈ anions are embedded into the channels of the 3D MOF. To the best of our knowledge, this is the first example of 3D MOF constructed from flexible bis-pyridyl-bis-amide ligand with the non-coordinating Wells–Dawson POM as template. In addition, 1 also represents the first POM-templated 3D 8⁶ nets containing left-hand helical chains.

IR spectra

The IR spectrum of 1 is determined in the frequency range of 500–4000 cm⁻¹, as shown in Fig. S3.† The characteristic bands at 1091, 962, 916, 806 cm⁻¹ are attributed to the ν(P–O_a), ν(W–O_d) and ν(W–O_b–W) of P₂W₁₈ polyanion.²¹ The band around 1649 cm⁻¹ is characteristic of the ν_C=O group.^22a The presence of the characteristic bands at 1342 and 1031 cm⁻¹ suggests the ν_C–N stretching vibrations of the pyridyl ring from the 3-dpye.^22b The strong absorption peak observed at 3359 cm⁻¹ indicates the presence of –OH groups of water molecules.^22c

Thermal stability analysis and PXRD

Interestingly, the crystals of 1 may undergo the separation and absorption transformation of lattice water molecules when the crystals of 1 was heated to 100 °C and kept at this temperature for 90 min. The guest water-free form of 1 was marked as 1a. During the course of this process, the color of crystals changed from blue to purple (Fig. 3), while the crystals maintained its structural integrity, which is confirmed by the PXRD. After cooling to room temperature or taking the 1a powder into water, the crystals recovered to blue, whose PXRD patterns are almost identical with those of the as-prepared samples (Fig. S4†). The results indicated that the lattice water molecules may be removed from 1 during heating and reabsorbed into 1a after cooling to room temperature with the host framework keeping stable.

Fig. 3 The photographs of 1 and 1 after heating to 100 °C (1a).

Thermal gravimetric (TG) curves of compound 1 and 1a were performed under N₂ atmosphere with a heating rate of 10 °C min⁻¹ in the temperature range of 20–800 °C. As shown in Fig. S5,† the TG curve of 1 gives the first weight loss of 5.42% from 40 to 280 °C corresponding to the loss of the lattice and coordinated water molecules, which agree with the calculated value of 5.49%. The second weight loss from 280 to 750 °C can be ascribed to the loss of organic molecules 14.45% (calcd: 14.52%). The first weight loss for 1a starting from 20 °C to 226 °C corresponds to the loss of the coordinated water molecules 0.73% (calcd: 1.01%). The second weight loss of 16.13% (calcd: 15.74%) at 264–800 °C can be attributed to the decomposition of organic molecules.

Electrochemical properties

In order to investigate the electrochemical properties of the title compound, complex 1 bulk-modified CPE is the optimal choice, which is due to its insolubility in water and common organic solvents, and even if in acidic and weak basic aqueous solutions. The cyclic voltammetry (CV) of 1-CPE was measured in the potential range from +600 to −810 mV in 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution. As shown in Fig. 4a, three pairs of reversible redox peaks I–I′, II–II′ and III–III′ appeared with the mean peak potentials E_1/2 = (E_pa + E_pc)/2 of −175 (I–I′), −420 (II–II′) and −650 (III–III′) mV (scan rate: 120 mV s⁻¹), respectively. The three redox peaks I–I′, II–II′ and III–III′ are deemed to be identical to the native P₂W₁₈ anion, which can be ascribed to the redox reaction of the W centers.^23,24 When the scan rate is increased, the peak potentials change gradually: the cathodic peak potentials shift toward the negative direction and the corresponding anodic peak potentials to the positive direction. The peak currents are proportional to the scan rates up to 450 mV s⁻¹, indicating that the redox processes of the 1-CPE is surface-controlled (Fig. S6†).

Fig. 4 (a) Cyclic voltammogram of the 1-CPE in 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution at different scan rates (from inner to outer: 40, 80, 120, 160, 200, 250, 300, 350, 400, 450 mV s⁻¹ for 1-CPE). (b) Cyclic voltammogram of a bare CPE in the 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution containing 5.0 mM KNO₂ and 1-CPE in 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution containing NO₂⁻. (c) Cyclic voltammogram of a bare CPE in the 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution containing 5.0 mM H₂O₂ and 1-CPE in 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution containing H₂O₂.

Many studies have revealed that POMs can act as electrocatalysts for multi-electron reduction.^25,26 In this work, the electrocatalytic reduction of nitrite and H₂O₂ at the 1-CPE in 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution was investigated. It is well known that the electroreduction of nitrite and H₂O₂ requires a large overpotential at most electrode surface, and no obvious response was observed in the range of +600 to −810 mV on a bare CPE in 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution containing 5.0 mM KNO₂ or H₂O₂. However, as shown in Fig. 4b and c, with the addition of KNO₂ or H₂O₂, all the reduction peak currents gradually increase and the corresponding oxidation peak currents gradually decrease. The results indicate that all of the three reduced species of P₂W₁₈ in the 1-CPE show good electrocatalytic activity for the reduction of nitrite or H₂O₂.

Conclusion

A 3D microporous bis-pyridyl-bis-amide-based MOF with a rare 8⁶ net containing the non-coordinating P₂W₁₈ anion is successfully constructed. It represents the first example of Wells–Dawson POM-templated MOF with 8⁶ net, and it is also the first example of non-coordinated Wells–Dawson POM-templated MOF based on flexible bis-pyridyl-bis-amide ligand with left-hand helical chains, showing electrochemical properties. Further work for preparing novel POM-based MOFs constructed from other flexible bis-pyridyl-bis-amide ligands and other POMs is in progress.

Acknowledgements

Financial supports of this research by the National Natural Science Foundation of China (no. 21171025, 21471021), New Century Excellent Talents in University (NCET-09-0853), and Program of Innovative Research Team in University of Liaoning Province (LT2012020) are gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR spectra, TG, and additional figures. CCDC 938554. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra06204g

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