A new boat-like tungstoarsenate functionalized by carboxyethyltin and its catalytic properties

Abstract

A new polyoxometalate (POM) estertin derivative, {C(NH₂)₃}Na₁₀K₇[(W₄O₁₆){Sn(CH₂)₂COO}₄(B-α-AsW₉O₃₃)₂]·25H₂O (1), has been synthesized from K₁₄[As₂W₁₉O₆₇(H₂O)]·nH₂O ({As₂W₁₉}) and Cl₃SnCH₂CH₂COOCH₃, which exhibits an unusual boat-like structural feature: four [Sn(CH₂)₂COO]²⁺ groups surround a center W₄O₁₆ cluster and link two [AsW₉O₃₃]⁹⁻ subunits. 1 shows efficient oxidation catalytic activity and can be applied for mimicking enzyme catalysis.

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Organotin-decorated POMs, as a new class of organic–inorganic hybrid materials, have attracted great attention in recent years due to their tremendous structural variety and special properties which means they have extensive applications in catalysis, medicine, and multifunctional materials.^1–3 In contrast with mono-vacant POMs, multi-vacant POMs can provide more active sites for the incorporation of more organotin fragments into the inorganic clusters, which might realize multiple functionalization. However, many multi-vacant POMs are less stable in aqueous solution and can just exist in a narrow pH range, which limits the reactions between POMs and various organotin compounds. Especially, the crystalline POM-estertin derivatives are difficult to obtain. Pope's group reported the first example of three phenyltin-containing tungstophosphate [(PhSnOH)₃(PW₉O₃₄)₂]¹²⁻ in 1994.⁴ Recently, novel polyoxoanions containing organotin groups were found, which provided a platform for the development of inorganic–organic hybrids with special properties and potential functionalities.^3c–e In our previous work, some crystalline sandwich-type POMs modified by carboxyethyltin based on the trivacant Keggin and Dawson units were obtained.^1a,3c,3d,5 Considering that estertin and carboxyethyltin compounds have lower toxicities than those of alkyltin compounds, higher thermal stabilities,⁶ and combinative properties of both inorganic POM fragments and organic groups, and are more feasibly functionalized than the alkylating groups,⁷ we try to graft estertin/carboxyethyltin group into the trivacant Keggin anion B-α-[AsW₉O₃₃]⁹⁻ units to modulate the reactivity of POM building block, and to explore the catalytic activity of the POM-based organic–inorganic hybrid materials. Finally, a new boat-like tungstoarsenate functionalized by carboxyethyltin was isolated from aqueous solution. It displays good catalytic activities for the oxidation of cyclohexanol and photocatalytic degradation of rhodamine B (RhB). In addition, the title compound was firstly applied for mimic enzyme to form the o-phenylenediamine (OPD)/POM/H₂O₂ system, which provides a new approach to detect H₂O₂ with good sensitivity, wide linear range, low detection limit and fast response towards H₂O₂ instead of an enzyme biosensor.

Single crystal X-ray diffraction analysis reveals 1 consists of one large polyoxoanion [(W₄O₁₆){Sn(CH₂)₂COO}₄(B-α-AsW₉O₃₃)₂]¹⁸⁻, one [C(NH₂)₃]⁺, ten Na⁺, seven K⁺, and twenty-five water molecules (Fig. S1† and 1). Crystal data and structure refinement parameters of 1 were listed in Table S1.† Interestingly, the polyoxoanion of 1 displays boat-like structural feature, in which, two B-α-[AsW₉O₃₃]⁹⁻ subunits are connected by four [Sn(CH₂)₂COO]²⁺ moieties and a W₄O₁₆ cluster (Fig. 1a). As shown in Fig. 1b, four Sn groups and four WO₆ units are bridged by eight O atoms. Four Sn atoms have the same hexa-coordinated environments, each Sn atom was fulfilled by the coordination of four O atoms from WO₆ fragments, one C atom and one O atom from the carboxyethyl group. The Sn–O bond distances are 2.023(12)–2.207(15) Å, and the Sn–C bond distances are 2.098(17)–2.15(2) Å. The bond lengths of W–O are in the range of 1.677(18)–2.433(13) Å, respectively (see Table S2†). In 1, bond valence sum (BVS) calculations show that all W, Sn, O and As atoms are in the +6, +4, −2 and +3 oxidation states.⁸ Selected bond lengths and angles were given in Table S2.† The estertin precursor [Sn(CH₂)₂COOCH₃]³⁺ was found to have also hydrolyzed into the carboxyethyltin [Sn(CH₂)₂COO]²⁺ during the reaction process. As described by Boskovic, {As₂W₁₉} precursor can afford B-α-{AsW₉O₃₃} units and free tungstate {WO_x},⁹ thus in the synthesis of 1, these fragments and organotin groups were re-assembled into a new structure in a aqueous solution and form a 3D supramolecular framework (Fig. S2†).

Fig. 1 Polyhedral and ball-and-stick representation of the boat-like polyoxoanion of 1 from the top (a) and side view (b). The connection mode of the four organotin groups and a {W₄O₁₆} cluster (c). Color code: WO₆ octahedra, green; Sn, yellow; As, violet; C, black; O, red.

The structure of 1 was further confirmed by IR spectrum (Fig. S3†) and NMR spectrum (Fig. 2). As shown in Fig. 2a, the chemical shift of ¹¹⁹Sn for the precursor Cl₃Sn(CH₂)₂COOCH₃ (CDCl₃/(CH₃)₄Sn) is at δ = −115.51 ppm, while the ¹¹⁹Sn NMR chemical shift of 1 (D₂O/(CH₃)₄Sn) shifts to high field (δ = −595.56 ppm) due to the increased electron density on Sn atom (Fig. 2b), indicating of organotin groups into the skeleton of {As₂W₁₉}. From Fig. 2b, we also found that compound 1 exhibits a broadening resonance. In order to explore the reason of broadening, the ¹¹⁹Sn NMR of Cl₃Sn(CH₂)₂COOCH₃ in D₂O/(CH₃)₄Sn (Fig. S4†) and compound DODA-1 in CDCl₃/(CH₃)₄Sn (Fig. 2c) were detected. From Fig. 2c, it is found that a sharp signal peak is gained at δ = −507.73 ppm of compound DODA-1 in CDCl₃/(CH₃)₄Sn. Compared with Fig. 2b and c, we argue that the existence of water molecule leads to the change of coordination environment of Sn atom, thereby, a broadening ¹¹⁹Sn NMR resonance was gained of compound 1 in D₂O. The purity of the as-synthesized compound was evaluated by XRPD patterns for experimental and simulated results of 1 (Fig. S5†).

Fig. 2 ¹¹⁹Sn NMR spectra in different solutions. (a) Cl₃SnCH₂CH₂COOCH₃ (CDCl₃/(CH₃)₄Sn). (b) 1 (D₂O/(CH₃)₄Sn). (c) Compound DODA-1 (CDCl₃/(CH₃)₄Sn).

This carboxyethyltin-POM hybrid shows good thermal stability. From the TG analysis (Fig. S6†) we can see that the introduced (CH₂)₂COO⁻ group can be stable before 333 °C, and within the temperature range of 800–1000 °C, the weight remains almost the same, inferring the stability of the polyoxoanion skeleton.

The optical property of 1 was investigated through the UV-Vis absorption spectra in an aqueous solution. As shown in Fig. S7,† the absorption behaviors for 1 and {As₂W₁₉} reveal the similar features, and the maximum absorbance at ca. 275 nm for 1 should be attributed to the characteristic absorption of POMs, i.e. the oxygen to metal (O_bridging → W) charge transfer. Besides, 1 also exhibits a broader band in the near-UV region. The cyclic voltammetry research shows 1 has redox property, and the redox process of 1 is irreversible (Fig. S8†). In order to investigate the oxidation catalytic activity of 1, according to the method reported in the literature (the specific conditions for catalytic oxidation experiments are in ESI†),¹⁰ i.e., using the catalytic oxidation of cyclohexanol to cyclohexanone with H₂O₂ as a model reaction (Scheme S1†), under the giving oxidation catalytic conditions, 89.7%, 65.2% and 7.6% of yields were obtained for 1, {As₂W₁₉}, and without catalyst respectively. This result indicates that the title boat-like hybrid exhibits enhanced oxidation catalytic activity compared with both {As₂W₁₉} and the reported sandwich-type carboxyethyltin functionalized POMs.¹⁰ In addition, catalyst 1 can be reused and its activity did not significantly change after four runs (see Fig. 3).

Fig. 3 The catalytic activity of compound 1, {As₂W₁₉} and non-catalyst for the oxidation of cyclohexanol to cyclohexanone with H₂O₂, and the reusability of 1 for four runs (cyclohexanol to H₂O₂ molar ratio, 1 : 2.2; catalyst (based on W) to cyclohexanol molar ratio, 1 : 250; reaction time, 3.5 h; reaction temperature, 80 °C; acetonitrile, 10 mL).

To further explore the peroxidase-like property of POMs, the mimic enzyme system of OPD/POM/H₂O₂ was formed by using 1 to replace horseradish peroxidase (Fig. S9†), in which the catalytic oxidation of the peroxidase substrate OPD in the presence of H₂O₂ was measured. A typical experiment was carried out as follows: 3.0 mg of 1 was added to 1.0 mL buffer solution (pH = 4.0) with 50 μL (50 mmol L⁻¹) OPD as substrate and the concentration of H₂O₂ was changed. The absorption spectrum of the reaction solution was determined after 2 min using UV-Vis spectroscopy. The catalytic activity of 1 depends on the H₂O₂ concentration was detected. Fig. S10† shows that a H₂O₂ concentration as low as 2.3 × 10⁻⁴ mol L⁻¹ was detected, a linear range is from 2.3 × 10⁻⁴ to 9 × 10⁻³ mol L⁻¹. For further analysis of the catalytic mechanism, the Michaelis–Menten kinetic model was used to investigate the peroxidase-like activity of 1.^11,12 The apparent steady-state kinetic analysis of compound 1 with H₂O₂ as substrate was shown as Fig. S11a† and the kinetic parameters were calculated using a Lineweaver–Burk plot (Fig. S11b†).¹¹ The kinetics behavior followed the Michaelis–Menten kinetics mechanism, and the Menten constant (K_m) of 1 with H₂O₂ was 19.68 mmol L⁻¹ at pH = 4. This result confirms that compound 1 behaves as a peroxidase mimetic.

As is known, some POM-based compounds can be used as excellent photocatalysts to degrade organic dyes. In this work, the photocatalytic performance of the title compound was investigated through the degradation of RhB solution under visible light (the special photocatalytic experiments are detailed in ESI†).¹³ Interesting, when 1 dissolved and remained in the solution after once degradation, after five runs of photocatalytic tests, the photoactivity of 1 did not display significant loss when RhB was re-added in the system.¹⁴ In addition, when the dosage of the catalyst was reduced, 1 still have good catalytic performance. As shown in Table S3,† with the reduction of the amount of catalyst, the degradation rate of the dye is not obviously weakened, and the catalyst can be recycled. The catalytic efficiency could reach above 95% when the molar ratio of catalyst to RhB is 1 : 8 (Fig. 4). Besides, the degradation rate of 1 was far higher than that of without 1 (see Fig. S12†), and also higher than that of the parent {As₂W₁₉} (Fig. S13†), indicating the functionalization of carboxyethyltin group. Meanwhile, the maximum peak shifted gradually towards shorter wavelengths, which was probably caused by the existence of a number of intermediates.¹⁵

Fig. 4 UV-visible absorption spectra of RhB solution during the decomposition reaction under visible light in the presence of 1 (the molar ratio of catalyst to RhB is 1 : 8). The inset is the corresponding degradation versus reaction time.

Conclusions

In summary, the tungstoarsenate {As₂W₁₉} and estertin precursors were re-assembled into a new boat-like structure through conventional synthetic method. The carboxyethyltin fragment-functionalized POM presents a green, efficient and eco-friendly catalyst for the oxidation of cyclohexanol. Besides, it was firstly applied for the mimic enzyme catalysis, which provides a new approach to detect H₂O₂ instead of an enzyme biosensor. Meantime, its visible light photocatalytic degradation of organic dye RhB was explored. More works will focus on the exploration of new reaction systems of multi-vacant POM species with various organotin groups in order to obtain a series of new functional POM-based organotin derivatives.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21503103), the open project fund Northeast Normal University of the Key Laboratory of polyoxometalate science of the Ministry education (no. 130026512) and Youth scientific research project of Liaoning Normal University (no. LS2015L008 and LS2015L006).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ORTEP views of compound 1; IR, NMR, XRPD, TG-DTA, absorption spectra, cyclic voltammograms and catalytic results. CCDC 1436349. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra27781g

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