A new 3D framework based on reduced Wells-Dawson arsenotungstates as eight-connected linkages

Abstract

A new hybrid compound based on reduced Wells-Dawson arsenotungstates as eight-connected linkages, [Cu₄(btb)₆(H₂O)₂][As₂W^V₂W^VI₁₆O₆₂]·10H₂O (1) (btb = 1,4-bis(1,2,4-triazol-1-y1)butane) has been synthesized under hydrothermal conditions. Structural analysis shows that 1 displays a 3D POMOF structure consisting of two crystal distinct motifs: a 3D MOF architecture and an eight-connected As₂W₁₈ cluster. It is worth mentioning that the 3D MOF architecture possesses two kinds of channels (A and B). The eight-connected As₂W₁₈ clusters as guests occupy the B channels and are incorporated within the MOF to achieve a 3D POMOF architecture with a novel (3²·4²·5²)(3³·4⁵·5³·6⁴)₂(3⁶·4⁹·5⁹·6⁴) topology. In addition, the electrochemical studies show that 1 has good electrocatalytic activities toward reduction of both hydrogen peroxide and nitrite molecules ascribed to W-centers. The photocatalytic properties of 1 indicate that the title compound presents a good degradation activity and may be a potential photocatalyst for oxidative decomposition of methylene blue dye.

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Introduction

Polyoxometalates (POMs), as a large family of metal–oxygen clusters, have been extensively studied due to their enormous structural variety and intriguing properties in chemistry, physics, catalysis, and materials science.^1–10 They cover an enormous range in size and structure and thereby provide access to a huge library of readily available and controllable secondary building units (SBUs). The POMs as SBUs can link variable transition-metal complexes (TMCs) for construction of various multifunctional hybrid materials with combination of key merits of both components.^11–14 Among the reported hybrid materials, those compounds with both high-connectivity and high-dimensionality structures have drawn more and more attention of chemists, because generally there exists a correlation between their complexity of structures and functionalities predicted early by Zubieta.¹⁵ To construct high-connected and high-dimensional POM based hybrid compounds, one effective method is to increase the surface charge density of POM polyanions. The higher charge density will lead to a higher affinity to interact with TMCs, which possibly results in forming the high-connected structures with ease. Generally, there are mainly two strategies widely adopted for increasing the surface charge density of POMs: (I) the transition metal substituted POMs (replacing high-oxidation state metals of POMs with other lower-valence metals) and (II) the heteropoly blue POMs (reducing the metal centers of POMs). To date, thanks to the work of POM chemists, some novel high-connected and high-dimensional hybrid compounds have been successfully synthesized with strategy I.^16–18 Nevertheless, the high-connected and high-dimensional hybrid compounds constructed by strategy II are rarely obtained,¹⁹ though such materials are of considerable significance in magnetochemistry and materials science.²⁰

In our previous work, a new 3D hybrid compound with dimeric monocopper-substituted Keggin polyoxoanions as ten-connected linkages, [Cu₅(en)₉][(PW₁₁CuO₃₉)₂]·18H₂O (en = ethylenediamine), has been synthesized via the strategy I.²¹ In our present, we try to obtain high-connected and high-dimensional hybrid compounds by strategy II. With this aim in mind, the reduced Wells-Dawson polyanion captures our attention since they possess more surface oxygen atoms (18 terminal O atoms and 36 μ₂-O atoms) than other well-known POMs, such as Keggin, Anderson, and Lindqvist. The surface oxygen atoms with high electron density in the reduced Wells-Dawson polyanion as the potential coordination sites can effectively connect with the TMCs. To our knowledge, the high-connected and high-dimensional hybrids based on Wells-Dawson arsenotungstates are rarely reported,²² and the highest number of the connecting sites of Wells-Dawson arsenotungstate anions is limited to seven. In addition, another important factor of the rational design of hybrids is judicious choice of organic ligands. Among many multidentate N-donor ligands, flexible 1,4-bis(1,2,4-triazol-1-y1)butane (abbr. btb) molecule is a good candidate for construction high-connected and high-dimensional hybrids. Because the 1,2,4-triazole group of btb ligand, with three N donors, is an integration of the coordination geometry of both imidazoles and pyrazoles to provide more potential coordination sites (Scheme S1†), and this kind of ligand exhibits a strong and typical coordination capacity to construct the metal–organic frameworks (MOFs) with high-dimension.²³

Herein, we report the preparation and structure of a new 3D hybrid compound based on reduced Wells-Dawson arsenotungstates, [Cu₄(btb)₆(H₂O)₂][As₂W^V₂W^VI₁₆O₆₂]·10H₂O (1), which exhibits a 8-connected 3D framework with a novel (3²·4²·5²)(3³·4⁵·5³·6⁴)₂(3⁶·4⁹·5⁹·6⁴) topology. To the best of our knowledge, it represents the first hybrid compound based on the reduced-polyanion [As₂W^V₂W^VI₁₆O₆₂]⁸⁻ with a high connectivity and dimensionality structure to date. In addition, the electrocatalytic and photocatalytic properties of 1 were also investigated in details.

Experimental section

Materials and general methods

The α-K₆As₂W₁₈O₆₂·14H₂O was prepared according to the literature method.²⁴ All other chemicals were commercially purchased and used without purification. Elemental analyses (C, H and N) were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer, and that of As, Cu and W were carried out with a Leaman inductively coupled plasma (ICP) spectrometer. The FT-IR spectrum was recorded from KBr pellets in the range 4000–400 cm⁻¹ with a Nicolet AVATAR FT-IR360 spectrometer. A CHI660 electrochemical workstation was used for control of the electrochemical measurements and data collection. A conventional three-electrode system was used, with carbon paste electrode (CPE) as working electrode, commercial Ag/AgCl as reference electrode and twisted platinum wire as counter electrode. The powder X-ray diffraction (PXRD) data were collected on Rigaku RINT2000 diffractometer at room temperature. UV-vis absorption spectra were recorded on a 756 CRT UV-vis spectrophotometer.

Synthesis of [Cu₄(btb)₆(H₂O)₂][As₂W^V₂W^VI₁₆O₆₂]·10H₂O (1)

A mixture of α-K₆As₂W₁₈O₆₂·14H₂O (0.35 g, 0.073 mmol), Cu(NO₃)₂·3H₂O (0.15 g, 0.6 mmol), btb (0.085 g, 0.44 mmol) and Et₃N (0.2 mL) was dissolved in 20 mL of distilled water at room temperature. Then the pH value of the mixture was adjusted to about 3.8 with 1.0 mol L⁻¹ NaOH, and the obtained suspension was put into Teflon-lined autoclave and kept under autogenous pressure at 170 °C for 4 days. After slow cooling to room temperature, green rhombic block crystals of 1 were filtered, washed with distilled water and dried at room temperature. Yield: 51% (based on W). Anal. calcd for 1: H, 1.59; C, 9.49; N, 8.30; Cu, 4.18; As, 2.47; W, 54.48 (%). Found: H, 1.51; C, 9.59; N, 8.37; Cu, 4.04; As, 2.56; W, 54.61 (%).

X-ray crystallography

Single-crystal X-ray diffraction data collection of 1 was performed using a Bruker Smart Apex CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at 293 K. Absorption corrections were applied by using the multiscan program SADABS.²⁵ The structures were solved by direct methods, and non-hydrogen atoms were refined anisotropically by least-squares on F² using the SHELXTL program.²⁶ The hydrogen atoms of organic ligands were generated geometrically for 1, while the hydrogen atoms of water molecules can not be found from the residual peaks and were directly included in the final molecular formula. The reported refinement of 1 is of the guest-free structures using the *.hkp file produced by using the SQUEEZE routine. There are at least ten solvent water molecules in the crystal structure of 1. These water molecules were directly included in the final molecular formula based on the elemental analysis, TG and SQUEEZE analyses. A summary of the crystal data, data collection, and refinement parameters for 1 are listed in Table 1.†

Table 1 Crystal data and structure refinements for 1^a

Compound	1
a R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Formula	C48H96As2Cu4N36O74W18
Formula weight	6074.58
Crystal system	Monoclinic
Space group	C2/c
a/Å	16.207(5)
b/Å	25.341(5)
c/Å	29.606(5)
β/°	92.522(5)
V/Å^3	12 148(5)
Z	4
Dcalcd/g cm^−3	3.221
T/K	293(2)
μ/mm^−1	18.286
Refl. measured	30 559
Refl. unique	10 701
Rint	0.0730
GoF on F^2	0.901
R1/wR2 [I ≥ 2σ(I)]	0.0590/0.1639

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Results and discussion

Description of crystal structure

The single-crystal X-ray diffraction analysis study reveals that 1 crystallizes in the monoclinic space group C2/c (no. 15). The asymmetric unit of 1 includes one [As₂W^V₂W^VI₁₆O₆₂]⁸⁻ (abbreviated to As₂W₁₈) polyanion, four Cu cations, six btb ligands and two coordinated water molecules (Fig. 1a). The reduced-polyanion As₂W₁₈ shows a classical Wells-Dawson structure, and possesses higher charge density than common saturated Wells-Dawson polyanion, which makes them favorable for formation of complicated structures. As shown in Fig. 1b, each As₂W₁₈ cluster acts as octadentate inorganic ligand covalently bonding to eight Cu ions (Cu1, Cu2, Cu3, Cu1^#1, Cu1^#2, Cu1^#3, Cu2^#1 and Cu3^#1), which represents its highest connected number to date. There are three crystallographically independent Cu ions in 1, which shows similar distorted octahedral coordination geometry but different coordination environments (Fig. S1†). The coordination environments of Cu1 and Cu2 are identical, which are coordinated by two oxygen atoms from two As₂W₁₈ anions and four nitrogen atoms from four btb ligands. The Cu3 is coordinated by four oxygen atoms from two water molecules and two As₂W₁₈ anions as well as two nitrogen atoms from two btb ligands. The bond lengths and angles around the Cu ions are in the ranges of 1.96(3)–2.10(2) Å (Cu–N), 1.97(2)–2.39(2) Å (Cu–O), 81.0(10)–180.0(9)° (N–Cu–N) and 87.5(4)–92.5(4)° (N–Cu–O). All of these bond lengths and angles are within the normal ranges observed in other Cu(II)-containing complexes.²⁷

Fig. 1 View of (a) the basic crystallographic unit in 1 and (b) the polyhedral/ball representation of the coordination mode of As₂W₁₈ cluster (symmetry code: #1, x, −1 + y, z; #2, 1 − x, −y, 2 − z; #3, 2 − x, 1 + y, 1 − z).

One of the structural features of 1 is that 2D undulated layer with 48-membered macrocycle (Fig. S3†) was constructed by left-, right-handed helixes, and a meso-helix (Fig. 2 and 3a). There are four crystallography independent btb (btb1, btb2, btb3 and btb4) ligands in 1 and they show two types of conformation modes: “U”-type and “Z”-type. Meanwhile, there are two kinds of coordinated modes (μ₂ and μ₃) for btb ligands in 1: the 1,2,4-triazole group of imidazoles potential coordination N donors in btb ligands are utilized to coordinate with two or three Cu cations, respectively (Fig. S2†). Both the btb1 and btb2 ligands adopting the same μ₂ mode link Cu1 cations to form left- and right-handed helixes, and the btb4 ligands adopting μ₃ mode link Cu2 and Cu3 cations to form an extraordinary meso-helix. In fact, to the best of our knowledge, only two POM-based helical compounds possessing both common helix and meso-helix in one compound have been reported up to now.²⁸ Therefore, 1 represents a new example of such compound that is coexistence of common helix and meso-helix.

Fig. 2 A 2D layer connected by the left-handed, right-handed helical and meso-helical chains.

Fig. 3 View of (a) the 2D undulated layer with 48-membered macrocycle, (b) the 3D MOF in 1, (c) the 3D POMOF structure and (d) schematic view of the (3²·4²·5²)(3³·4⁵·5³·6⁴)₂(3⁶·4⁹·5⁹·6⁴) topology in 1. Colour codes: blue, the Cu cations; gray, the As₂W₁₈ anions.

Another structural feature of 1 is its intricate 3D POMOF architecture that can be described in the as follows: the btb3 ligands by μ₂ mode link these parallel 2D undulated layers forming a 3D MOF architecture with two kinds of channels (A and B). Finally, the eight-connected As₂W₁₈ clusters as guests occupy the B channels and incorporate with the MOF to achieve 3D POMOF architecture (Fig. 3c). From the topological view, if Cu1 and Cu2 cations are considered as 6-connected nodes and the Cu3 cations as well as As₂W₁₈ clusters are considered as 4- and 8-connected nodes, respectively. The structure of 1 can be simplified as a novel 3D (4, 6, 6, 8)-connected framework with a (3²·4²·5²)(3³·4⁵·5³·6⁴)₂(3⁶·4⁹·5⁹·6⁴) topology (Fig. 3d).

Analyses of BVS, XPS, IR, XRPD and TG measurements

Compound 1 was synthesized under hydrothermal conditions. All copper atoms in 1 are in the +II oxidation state, confirmed by their octahedral coordination environments, blue crystal color and BVS calculations.²⁹ The XPS spectrum of 1 shows two overlapped peaks at 35.12 eV and 37.21 eV (Fig. S4†), which are attributed to W^V and W^VI, respectively.³⁰ All of these results consist with the structural analyses and charge balance. In the synthetic process of 1, the W(VI) was partly reduced to W(V). The triethylamine in this reaction may be acted as not only mineralization agent but also reducing agent. Such phenomena have often been observed in reactions of organic amine under hydrothermal conditions.^19e,31

As shown in Fig. S5,† the IR spectrum exhibits the characteristic peaks at 1069, 961, 869 and 784 cm⁻¹ in 1, which are attributed to ν(As–O), ν(W=Ot), ν_as(W–Ob–W) and ν_as(W–Oc–W), respectively. Compared to the typical Wells-Dawson type heteropolyanion As₂W₁₈, compound 1 has similar peaks in the range of 780–1080 cm⁻¹ except for slight shifts of peaks due to the interactions between the polyanions and the Cu²⁺ cations in the solid state, which indicates that the As₂W₁₈ cluster in title compound still retains the basic structure of Wells-Dawson As₂W₁₈ polyanion.³² Additionally, the characteristic peaks in the region 1621 to 1268 cm⁻¹ could be ascribed to the character peaks of btb ligands.^23a

The PXRD patterns for 1 are presented in the Fig. S6.† The diffraction peaks of both simulated and experimental patterns match well, thus indicating that the phase purities of the 1 are good. The difference in reflection intensities between the simulated and the experimental patterns is due to the different orientation of the crystals in the powder samples.

The TG curve of 1 (Fig. S7†) shows two-step weight loss in the temperature range of 30 to 600 °C. The first weight loss of 4.27% (calc. 4.15%) in the temperature range of 30 to 250 °C, corresponding to the loss of water molecules. The second weight loss of 22.58% (calc. 20.32%) in the temperature range of 250 to 600 °C is observed corresponding to the decomposition of organic btb molecules and collapse of As₂W₁₈ cluster. The whole weight loss of 26.85% is in agreement with the calculated value 24.47%.

Electrochemical properties

The POMs possess the ability of undergoing reversible multi-electron redox processes, which makes them very attractive in chemically-modified electrode and the electrocatalytic study.³³ Due to the title compounds are insoluble in water and common organic solvents. Thus, the bulk-modified carbon paste electrode (CPE) becomes the optimal choice to study the electrochemical properties, which is inexpensive, easy to prepare and handle.³⁴

The cyclic voltammetric behavior for 1-CPE (carbon paste electrode) was studied in 1 M H₂SO₄ solution (Fig. 4). In the potential range of −0.7 to 0.4 V, there exist three redox peaks with half-wave potentials E_1/2 at −0.08 (II–II′), −0.28 (III–III′) and −0.54 V (IV–IV′), which can be ascribed to redox processes of tungstate in 1. In addition, there is one irreversible anodic peak (I) at +0.2 V, which is assigned to the oxidation of copper.³⁵ As shown in the insert of Fig. 4, when the scan rates are varied from 0.1 to 0.6 V 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 indicate that the redox processes are surface-controlled.³⁶

Fig. 4 Cyclic voltammograms for 1-CPE in 1 M H₂SO₄ solution at different scan rates (from inner to outer): 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 V s⁻¹. The inset shows plots of the anodic and the cathodic peak currents for III against scan rates.

The POMs have been exploited extensively in electrocatalytic reactions and further applications such as biosensors and fuel cells.³⁷ Herein, the reduction of hydrogen peroxide (H₂O₂) and nitrite (NO₂⁻) were chosen as test reaction to study the electrocatalytic activity of 1. As shown in Fig. 5, it displays good electrocatalytic activity toward the reduction of H₂O₂ and NO₂⁻ in 1 M H₂SO₄ solution. With the addition of H₂O₂ and NO₂⁻, the cathodic peak IV substantially increased, while the corresponding anodic peak currents decreased. The inset of Fig. 5a and b show the relationship between the fourth cathodic current and concentration of H₂O₂ and NO₂⁻. The electrocatalytic efficiency of 1-CPE (based on a rough calculation using CAT formula)³⁸ toward the reduction of H₂O₂ and NO₂⁻ were ca. 95% and 120% at 1 M H₂SO₄ containing 50 mM hydrogen peroxide and nitrite, which suggests that 1 has potential applications in detection of hydrogen peroxide and nitrite.

Fig. 5 Reduction of H₂O₂ (a) and NO₂⁻ (b) at 1-CPE in 1 M H₂SO₄ solution containing H₂O₂ and NO₂⁻ in various concentrations (from inner to outer): 0, 10, 20, 30, 40, 50 mM. The inset shows a linear dependence of the cathodic catalytic current of wave IV with H₂O₂ and NO₂⁻ concentration, respectively.

The performance of the parent POM clusters ((NBu₄)₆[As₂W₁₈O₆₂]) and compound 1 has been compared. As shown in Fig. S8,† it can be seen that the parent POM cluster (NBu₄)₆[As₂W₁₈O₆₂] almost have no electrocatalytic reduction activities towards hydrogen peroxide (H₂O₂) and nitrite (NO₂⁻). However, the electrochemical studies showed that compound 1 have good electrocatalytic activity toward the reduction of H₂O₂ and NO₂⁻, thanks to the complicated 3D structure of compound 1 that enhances the electrocatalytic reduction activities. Additionally, the stability experiment for 1-CPE has been investigated scanning for 40 cycles in 1 M H₂SO₄ solution. As shown in Fig. S9,† it can be seen that the electrode exhibits almost no loss in the current signal after 40 cycles, which suggests that catalyst of 1-CPE has high stability.

Photocatalytic properties

The use of POMs as photocatalysts to decompose waste organic molecules so as to purify the water resources has attracted great attention in recent years. The introduction of transition-metal complexes as functional groups into POMs can enrich their potential applications.³⁹ Herein, to investigate the photocatalytic activities of compound 1 as catalysts, the photodecomposition of methylene blue (MB) is evaluated under UV light irradiation through a typical process: the power of 50 mg was mixed together with 100 mL of a 2.0 × 10⁻⁵ mol L⁻¹ (C₀) solution of MB. Then, the mixture was continuously stirred under UV irradiation from a 250 W high-pressure Hg lamp. At 0, 30, 60, 90, 120 and 150 min, the sample (3 mL) was taken out from the beaker respectively, followed by several centrifugations to remove the new compound and a clear solution was obtained for UV-vis analysis.

As shown in Fig. 6, after irradiation compound 1 for 150 min, the photocatalytic decomposition rate, defined as 1 − C/C₀, is 67.8% for 1. In contrast, the photocatalytic decomposition rate using insoluble (NBu₄)₆[As₂W₁₈O₆₂] as catalyst is 31.7% and the blank MB solution without any catalyst is 15.9%, which illustrates that the formation of organic–inorganic hybrid compound based on POMs could improve the photocatalytic performance of the (NBu₄)₆[As₂W₁₈O₆₂]. The enhanced photocatalytic property may be arisen from the following: one is the Cu–btb MOFs of 1, which may act as photosensitizer under UV light, namely, promoting transition of electrons onto POMs; Another is the reduced-polyanion [As₂W^V₂W^VI₁₆O₆₂]⁸⁻, which possesses the higher charge density and exert considerable influence on the pseudo-liquid phase behaviour of POMs. The photocatalytic mechanisms may be deduced as follows: during the photocatalytic reaction, [*As₂W^V₂W^VI₁₆O₆₂]⁸⁻ (*POM) abstract electrons from water molecules and hold the electrons (eqn (1) and (2)). The reduced POM (POM⁻) is quite stable, but is rapidly re-oxidized in the presence of O₂ through eqn (3). The main function of O₂ in the POM reactions seems to be the re-oxidation (regeneration) of the catalyst. The re-oxidation step accompanies the generation of superoxides. These cycles occur continuously whilst the system is exposed to UV light. Furthermore, the MB dye is also excited by UV light to generate *MB molecules, as shown in eqn (4). Finally, after several photo-oxidation cycles, degradation of the MB dye by hydroxyl radicals and superoxides occurs (eqn (5)).⁴⁰

[*As₂W^V₂W^VI₁₆O₆₂]⁸⁻ + hv → [*As₂W^V₂W^VI₁₆O₆₂]⁸⁻ (1)

[*As₂W^V₂W^VI₁₆O₆₂]⁸⁻ + H₂O → [As₂W^V₂W^VI₁₆O₆₂]⁹⁻ + H⁺ + OH˙ (2)

[As₂W^V₂W^VI₁₆O₆₂]⁹⁻ + O₂ → [*As₂W^V₂W^VI₁₆O₆₂]⁸⁻ + O₂⁻ (3)

MB + hv → *MB (4)

*MB + OH˙ + O₂⁻ → intermediate products → degradation (5)

Fig. 6 Absorption spectra of the MB aqueous solution during the photodegradation under 250 W Hg-lamp irradiation with (a) no catalyst, (b) (NBu₄)₆[As₂W₁₈O₆₂] power, (c) compound 1, and (d) conversion rate of MB with the reaction time.

Conclusion

In summary, a novel compound with 3D framework have been synthesized under hydrothermal conditions, in which reduced Wells-Dawson arsenotungstates As₂W₁₈ clusters are connected to eight copper ions. The successful isolation 1 not only provides intriguing example of high dimensional and highly connected POM-based hybrid compound, but also shows that the reduced Wells-Dawson polyoxoanions, thanks to their high negative charge density, present a very useful ability to act as multi-connectors towards metal cations, opening the door to the synthesis of many more charming high-connected and high-dimensional hybrid compounds with a simple synthetic strategy. A further work in this field is underway in our laboratory.

Acknowledgements

This work was financially supported by the NSF of China (no. 21371041 and 21101045), the NSF of Heilongjiang Province (no. B201103) and innovative research team of green chemical technology in university of Heilongjiang Province, China.

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Footnote

† Electronic supplementary information (ESI) available: CIF file of 1; additional figures. CCDC 989568. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02239d

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