English

• Polyoxometalates (POMs) are deemed as a crucial class of anion metal-oxo clusters constructed by early transition metals (M = Mo^VI, W^VI, V^V, Nb^V, and Ta^V), which possess superduper application prospects in medicine, catalysis, optics, magnetism and so on [1-10]. Up to now, the research of POMs has been prevailingly concentrated on lanthanide-substituted and transition-metal-substituted POMs (TMSPs) [11-19]. For lanthanide-substituted lacunary POMs (LSLPs), the reason is that the lanthanide ions (Ln³⁺) include unfilled 4f orbitals, which can be used for accepting lone pairs of electrons to form covalent bonds from diverse ligands. Hence, the lacunary POMs can be regarded as multi-dentate inorganic ligands to combine with lanthanide ions to obtain more LSLPs. Meanwhile, LSLPs have wide applications in fluorescence, photochromic, catalysis and so on [20,21]. For TMSPs, the introduction of transition metals into the skeleton of POMs can extend the adsorption range from UV to visible light, which is good for their applications in photocatalysis. The majority of the researches of TMSPs are focused on the noble-metal-substituted POMs such as Ru and Rh, and first transition series metal-substituted POMs such as Fe, Co, Ni, Cu, Zn, Ti, Mn [22-27]. Nevertheless, the metal-ions-substituted POMs with high valence like zirconium ion has been less developed. The development of zirconium-substituted POMs (ZrSPs) has been relatively slow probably because that it owns strong oxyphilic character, which makes it easy to bond with the oxygen atoms in the aqueous solution to form precipitate rather than crystallization. Also, the ZrSPs possess wonderful properties in oxygen sensing and catalysis [28]. Therefore, continuously exploring of ZrSPs is of great significance.

  Since the first sandwiched-type ZrSP has been reported by Finke et al. in 1989 [29], only a small number of ZrSPs have been achieved. In 2005, a sandwiched ZrSP was reported by Hill et al., which was acquired by mixing two chiral tartaric acid or malic acid and [P₂W₁₅O₅₆]^12– precursor in conventional aqueous solution [30]. From then on, ZrSPs have reported occasionally. For instance, a dimeric polyanion [Zr₄O₂(OH)₂(H₂O)₄(β-SiW₁₀O₃₇)₂]¹⁰⁻, and a trimer [Zr₆O₂(OH)₄(H₂O)₃(β-SiW₁₀O₃₇)₃]¹⁴⁻ were reported by Kortz et al. in 2006 [31]. Later, two dimeric polyanions of [{P₂W₁₅O₅₄(H₂O)₂}₂Zr]¹²⁻ and [{P₂W₁₅O₅₄(H₂O)₂}Zr{P₂W₁₇O₆₁}]¹⁴⁻ were gained by Hill's group in 2007 [32]. In 2009, Xue's group also reported a dimer Na₈K₄[Zr₄O₂(OH)₂(CH₃COO)₂(α-GeW₁₀O₃₇)₂]·33H₂O [33]. In addition, another sandwiched structure Cs₆Na₅[Zr₆O₄(OH)₄(H₂O)₂ (CH₃COO)₅(AsW₉O₃₃)₂]·80H₂O was obtained by Kortz et al. in 2010 [34]. In 2014, Yang's group got a high-nuclearity ZrSP Na₁₀K₂₂[Zr₂₄O₂₂(OH)₁₀(H₂O)₂(W₂O₁₀H)₂(GeW₉O₃₄)₄(GeW₈O₃₁)₂]·85H₂O under hydrothermal conditions [35]. It is the highest number of Zr⁴⁺ ions implanted into the skeleton of lacunary POMs up to now. Later, they also reported some other novel ZrSPs such as a tetrameric ZrSP K₂Na₆H₁₀(Hpy)₃[SbZr₇O₆(OH)₄(B-α-GeW₉O₃₄)₂(B-α-GeW₁₁O₃₉)₂]·28H₂O and a trimeric ZrSP [H₂N(CH₃)₂]₁₀H₁₄[(Zr₂P₂W₁₆O₆₁)₃]·7H₂O and so on [36-42].

  Based on the literature researches, we found the majority of the reported ZrSPs are sandwiched structures. The multimers in ZrSPs were sporadically reported. Herein, we attained a novel tetrameric compound K₈Na_19.5H_0.5[Zr₂(O₂)₂(β-As^VW₁₀O₃₈)]₄·68H₂O (1) utilizing K₁₄[As₂W₁₉O₆₇(H₂O)] as precursor via conventional aqueous solution [43]. Considering the introduction of peroxyl bond in the complex, the catalytic oxidation reactions of sulfides with H₂O₂ were implemented. The results demonstrate that 1 exhibits a good performance for the catalytic oxidation of sulfides to sulfones with high conversion (100%) and high selectivity (100%).

  Compound 1 is crystallized in the triclinic P-1 space group according to the analytic result of single crystal X-ray diffraction data (Table S1 in Supporting information). The result of the power X-ray diffraction pattern is in line with data obtained from the single crystal data, indicating that the high purity of the sample (Fig. S1 in Supporting information).

  The symmetric structure of 1 is comprised of a new tetrameric polyanion [Zr₂(O₂)₂(β-AsW₁₀O₃₈)]₄ ²⁸⁻ (1a), 8 K⁺, 19.5 Na⁺, 0.5H⁺, and 68 lattice water molecules. 1a is made up of four divacant [β-AsW₁₀O₃₈]¹¹⁻ {AsW₁₀} subunits, and four [Zr₂(O₂)₂]⁴⁺ {Zr₂} fragments. The [β-AsW₁₀O₃₈]¹¹⁻ subunit in 1a is rare compared with the common polyanion [α-AsW₁₀O₃₆]⁹⁻ (Fig. S3 in Supporting information). In the structure of 1a, the polyanion exhibits an interesting symmetry, whose whole configuration looks like a square (Fig. 1a). The structure of 1a can be considered as each {Zr₂} cation encapsuled by two {AsW₁₀} fragments, and the {Zr₂} group fills the role of the bridge to connect the four {AsW₁₀} subunits to form a fascinating square configuration (Fig. 1b). The distances of four As atoms are almost the same, exhibiting the length of 11.1 Å and 11.0 Å, respectively (Fig. 1c). The {Zr₂} group was constituted by two Zr atoms linked by two peroxyl ions, where the peroxyl bonds are in the vital position on enhancing stability of the structure. The dihedral angles of Zr1−O85−O86−Zr3 and Zr1−O87−O88−Zr3 are ca. 63.884° and 61.646°, respectively (Fig. 1d). The transformation of K₁₄[As₂W₁₉O₆₇(H₂O)] → {AsW₁₀} must have happened during the process of reaction (Figs. 2a and b). The structure of 1a can also be seen as each {AsW₁₀} fragment combined with two Zr atoms to form four [Zr₂(β-AsW₁₀O₃₈)]³⁻ building units (Fig. 2c), then every two such building units are bridged by two peroxyl bonds (Fig. 2d) via Zr−(O₂)₂−Zr (Zr1−(O₂)₂−Zr3 and Zr2−(O₂)₂−Zr4) to form the square-shaped structure [Zr₂(O₂)₂(β-AsW₁₀O₃₈)]₄ ²⁸⁻with the length about 2.16 nm (Fig. 2e). In 1a, all the Zr atoms exhibit the same coordination geometry which show an octa-coordinated, two capped triangular prism configurations (Fig. 2f and Fig. S4 in Supporting information). Specifically, each Zr atom is surrounded by four O atoms from the {AsW₁₀} building unit and another four O atoms from two peroxyl ions. Bond valence sum (BVS) calculation verified that the valence states of the W, Zr, As atoms were +6, +4, +5, respectively (Table S2 in Supporting information). It is obvious to find that the valence state of the As atoms in the obtained sample is different from the one that comes from the K₁₄[As₂W₁₉O₆₇(H₂O)] precursor, whose valence state of As atom is +3. The introduction of H₂O₂ in the synthetic process plays an important role to obtain the target product. On the one hand, it can act as an oxidant to oxidize As atom in the precursor from +3 to +5, which is verified by XPS spectra (Fig. S2 in Supporting information); on the other hand, it serves as a bridging ligand to link two Zr atoms. Up to date, there are only several hydrogen peroxide functionalized POMs in Zr-substituted POMs [44-46].

  Figure 1

  

  Figure 1.  (a) The ball-and-stick/polyhedral exhibition of 1. (b) A simplified pattern of 1. (c) The position and distance of four As atoms in the structure of 1. (d) The ball and stick exhibition of {Zr₂}.

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  Figure 2

  

  Figure 2.  (a-d) The ball-and-stick/polyhedral exhibition of [As₂W₁₉O₆₇(H₂O)]¹⁴⁻, [β-AsW₁₀O₃₈]¹¹⁻, [Zr₂(β-AsW₁₀O₃₈)]³⁻ and [Zr₂(β-AsW₁₀O₃₈)]₂ ⁶⁻, respectively. (e) The size of 1. (f) The coordination environment of Zr atom in 1.

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  This square-shaped ZrSP reminds us of another three square-shaped ZrSPs reported by Yang's group [39], which are obviously different from our square-shaped ZrSP. It is obvious to see that the building block of the compound 1 obtained by us is [β-As^VW₁₀O₃₈]¹¹⁻, while the buliding block of another three compounds obtained by Yang's group is [β-SiW₁₀O₃₈]¹²⁻. Moreover, the square-shaped compound 1 is linked by peroxyl bonds, while another three compounds obtained by Yang's group are linked by O-bridges or [BO(OH)₂]⁻ groups.

  Organic synthesis plays an important role in the synthesis of high value-added chemicals. The high value-added chemicals such as the oxygenation product (sulfones and sulfoxides) of sulfides possess wide applications in the domain of function materials, synthetic chemistry, biomedicine and so on [47-49]. Therefore, it has attracted more and more researchers to explore a better catalyst to catalyze the oxidation reaction of sulfides under mild conditions. POMs owning their peculiar properties such as high stability, redox properties, are widely accepted as the candidates for catalysis. The catalytic oxidation of sulfides via different methods offers a firsthand synthesis strategy for preparing the sulfone. In this research, 1 was chosen as a heterogeneous catalyst to catalyze the sulfides into sulfones in 3 mL acetonitrile solution. The hydrogen peroxide (H₂O₂) was employed as the oxidant for the reaction, because it is regarded as a green oxidant in organic reaction. The methyl phenyl sulfide (MPS) (0.5 mmol) was employed as the model material and carried out plenty of disparate experiments to investigate the details of the experiments (Scheme 1). The results were decided by gas chromatography-mass spectrometry (GC–MS) and utilized naphthalene as an internal standard to evaluate the conversion of the substrates.

  Scheme 1

  

  Scheme 1.  The catalytic oxidation of sulfides with H₂O₂ catalyzed by 1.

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  Compound 1 was selected as catalyst to verify the optimum conditions. First of all, the influence of the amount and type of oxidant on the experiment were investigated. When the amount of H₂O₂ is 0.5 mmol, the conversion of MPS is 73% and the selection of RR'SO₂ is 26%. The conversion of MPS and selectivity of RR'SO₂ increased significantly when the amount of H₂O₂ was added to 1.5 mmol, which changed from 73% and 26% to 100% and 100%. Tert-butyl hydroperoxide (TBHP) was also used as oxidant to the oxidation of sulfides. The results exhibit the conversion of MPS and the selectivity of RR'SO₂ is only 40% and 4%, respectively, which is much lower effect than H₂O₂ (Fig. 3a). Additionally, the reaction time also plays an important role in sulfides oxidation reaction. As exhibited in Table S3 (Supporting information), a good conversion of MPS can reach 100%, but a relatively lower yield of RR'SO₂ is only 90% during 20 min. when the time was delayed to 50 min, the substrate can totally transform into the target product. The amount of compound 1 was also explored, the results show the best amount of 1 is 0.45 µmol (Fig. 3b). Moreover, the temperature of reaction was also a crucial factor in the process of sulfides oxidation. The substrate can be totally converted but the yield of RR'SO₂ is less than 80% in 40 ℃. The yield was reached 100% when the temperature of the reaction was increased to 60 ℃ (Fig. 3c).

  Figure 3

  

  Figure 3.  (a) Impact of the oxidant amount and type of for the catalytic capacity of 1. (b) Impact of different amount of catalyst on the catalytic capacity of 1. (c) Impact of the temperature on the catalytic capacity of 1. (d) Impact of different solvents on the catalytic capacity of 1. Conditions: MPS (0.5 mmol), catalyst (0.39 μmol), naphthalene (0.39 mmol), solvent (3 mL), 60 ℃, and 50 min.

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  In order to verify the universal catalytic effect of 1, different solvents were selected as reaction medium. As it turned out, the 1 exhibits great catalytic property for oxidation of sulfides in the solvents of acetonitrile (MeCN), methyl alcohol (MeOH), N, N‑diethyl acetamide (DMA), acetone (AC), and ethyl alcohol (EtOH), which demonstrates that the MPS can totally be transformed into RR'SO₂ (Fig. 3d). As a contrast, the blank experiment was also carried out to better verify the catalytic capacity of 1 under same conditions. The results demonstrate that the conversion of the substrate and the yield of the target product was only 35% and 2%. The raw material such as ZrOCl₂·8H₂O, K₁₄[As₂W₁₉O₆₇(H₂O)] were also employed as catalyst to oxidize MPS (Table S4 in Supporting information). The results demonstrate 1 was a terrific catalyst for the oxidation of MPS. Additionally, the different substrates with electron-withdrawing group or electron-donating groups catalyzed by 1 were investigated. It is easy to find that the conversion of the substrates with disparate groups and yield of the conduct are all quite high (Table 1, entries 1-5). The derivatives of sulfides with groups in different position were also explored. The results verify that the substrate with the group in the "meta-", "para-", are easy to be oxidized than that in the "ortho-" position (Table 1, entries 6-8). 1 can be used as catalyst to catalytic most of the substrates included some derivatives with high stereo hindrance (Table 1, entries 9 and 10). Compared with those catalysts summarized in Table S5 (Supporting information), the target compound 1 is superior to the most catalysts with high conversion of sulfides to sulfones (Table S5). The recycling experiments of 1 were carried out. The results demonstrate the excellent yield of RR'SO₂ are not decreased obviously after 3 runs (Fig. S6 in Supporting information). Meanwhile, the thermal filtration experiment of 1 was conducted to further verify the heterogeneous nature of this catalyst. 1 was rapidly removed from the reaction mixture by filtration after 5 min and the filtrate continued for another 45 min. The result demonstrated the conversion rate is not significantly improved, indicating the heterogeneity of 1 (Fig. S7 in Supporting information). Moreover, the IR spectrum and PXRD pattern after catalysis was measured and matched well with the fresh one, demonstrating that the catalyst keeps its structure integrity after catalysis (Figs. S8 and S9 in Supporting information).

  Table 1

  

  Table 1.  The catalytical performance of 1 for MPS oxidation.

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  As shown in Fig. S10 (Supporting information), the reaction mechanism was speculated by us. First, the catalyst reacts with the H₂O₂ to form the electrophilic peroxide intermediates; then the substrate of the sulfides got the O atoms from the electrophilic peroxide intermediates; finally, the electrophilic peroxide intermediates are reduced into the initial conditions for the next recycling. Some similar mechanisms were proposed in the literature formerly [41,50,51].

  In conclusion, a novel tetrameric Zr-substituted polyoxotungstate 1, was successfully obtained by a one-pot reaction. Compound 1 is a tetramer linked by peroxyl bonds. Moreover, the catalytic oxidation of sulfides for 1 has been explored. Notably, compound 1 exhibits prominent performance for the catalytic oxidation of sulfides with high conversion (100%) and high selectivity (100%).

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgment

  This work was financially supported by the National Natural Science Foundation of China (No. 22071043).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109717.

  
  
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• 

  Figure 1  (a) The ball-and-stick/polyhedral exhibition of 1. (b) A simplified pattern of 1. (c) The position and distance of four As atoms in the structure of 1. (d) The ball and stick exhibition of {Zr₂}.

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  Figure 2  (a-d) The ball-and-stick/polyhedral exhibition of [As₂W₁₉O₆₇(H₂O)]¹⁴⁻, [β-AsW₁₀O₃₈]¹¹⁻, [Zr₂(β-AsW₁₀O₃₈)]³⁻ and [Zr₂(β-AsW₁₀O₃₈)]₂ ⁶⁻, respectively. (e) The size of 1. (f) The coordination environment of Zr atom in 1.

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  Scheme 1  The catalytic oxidation of sulfides with H₂O₂ catalyzed by 1.

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  Figure 3  (a) Impact of the oxidant amount and type of for the catalytic capacity of 1. (b) Impact of different amount of catalyst on the catalytic capacity of 1. (c) Impact of the temperature on the catalytic capacity of 1. (d) Impact of different solvents on the catalytic capacity of 1. Conditions: MPS (0.5 mmol), catalyst (0.39 μmol), naphthalene (0.39 mmol), solvent (3 mL), 60 ℃, and 50 min.

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  Table 1.  The catalytical performance of 1 for MPS oxidation.

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