Highly efficient oxidation of various thioethers catalyzed by organic ligand-modified polyoxomolybdates

Abstract

The selective oxidation of thioethers is among the most straightforward and common methods to obtain the synthetic intermediates sulfoxides for application in chemical industry, medicinal chemistry and biology. In this study, we prepared four hybrid compounds, i.e. Cs₄[M(H₂O)₄][PMo₆O₂₁(PABA)₃]₂·nH₂O 1–4 (M = Co, Mn, Ni, Zn; PABA = p-aminobenzoic acid), based on carboxylic acid-modified polyoxomolybdates and metal cations with wonderful catalytic performance for the selective oxidation of thioethers. The compounds 1–4 comprise PABA ligand-modified polyoxomolybdates [PMo₆O₂₁(PABA)₃]³⁻ as building units and the Co²⁺/Mn²⁺/Ni²⁺/Zn²⁺ cations as linkers to generate novel dimeric architectures. Using compound 1 as a pre-catalyst, methyl phenyl sulfides were nearly entirely converted to sulfoxides with the selectivity of 98% at room temperature within 20 minutes. Satisfactory catalytic effects were also observed in the selective oxidation of various phenyl sulfides with substituent groups; in particular, another typical thioether, i.e. the chemical warfare agent simulant 2-chloroethyl ethyl sulfide (CEES), could be completely degraded to nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) with the selectivity of 99% within 12 minutes at room temperature. These pre-catalysts could be recycled at least 10 times by simple filtration with a negligible decrease in conversion and selectivity.

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1 Introduction

Oxidative transformations of organic compounds using green oxidants are of both industrial and academic interest.¹ In this area, the oxidation of sulfides to their corresponding sulfoxides and sulfones is a research field that is attracting extensive attention from researchers. On the one hand, this transformation can produce important intermediates for the construction of various medically and chemically active molecules. On the other hand, this oxidation process affords a sustainable way to a green environment via the oxidative decontamination of toxic chemicals, such as the sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES), and the desulfurization of oil.² To date, various materials, such as inorganic oxides, metal–organic frameworks (MOFs) and modified zeolites, have been synthesized and demonstrated for catalyzing the selective oxidation of sulfides. However, the current reported materials always display limited selectivity and take a long time to finish the catalytic reaction. Thus, the search for environmentally benign and more efficient materials that can highly selectively and rapidly oxidize sulfides is a challenging issue.

Polyoxometalates (POMs), as well-defined inorganic metal oxide clusters, possess significant applications in catalysis, optics, medicine, etc.^3–18 Among these applications, oxidation catalysis has been one of the most attractive aspects in the last few years owing to the resistance of POMs to oxidation. As an important branch, the selective oxidation of various sulfides catalyzed by POMs has been explored by Hill, Hu, Song, Niu and other groups.^19–35 In the early years, some homogeneous materials, such as [(n-C₄H₉)₄N]₄(α-Mo₈O₂₆), [{CH₂=CH(CH₂)₆Si}_xO_ySiW_wO_z]⁴⁻, and TBA₄H₂[BW₁₁Mn(H₂O)O₃₉]·H₂O, based on POMs have been developed for the selective oxidation of a wide range of thioethers.^21–23 Subsequently, to solve the problem of recyclability, supported heterogeneous materials based on POMs and some solid materials, such as zeolites, polymers, or silica, have been extensively studied and have gained ever-increasing attention. In 2016, Zhao et al. have reported that the POM anion {SiV₂W₁₀} can be combined with a linear positively charged polymer, poly(diallyldimethylammonium chloride) (PDDA), to obtain the composite material PDDA-SiV₂W₁₀ through electrostatic interaction, which can efficiently catalyze the oxidation of various thioethers.²⁴ The same year, Song and coworkers prepared a supported POM pre-catalyst by employing the facile condensation reaction between polytungstate {P₂W₁₅} and γ-Al₂O₃ spheres.²⁵ This supported material P₂W₁₅-γ-Al₂O₃ is active for the oxygenation of thioethers, and displays high stability, excellent reactivity and selectivity. Recently, Hu's group successively obtained several composite materials based on POMs, [C_nH_2n+1N(CH₃)₃]₇HNb₆O₁₉, Zn₂Cr-LDH-PW₁₁M and Mg₃Al-LDH-Nb₆, which exhibited good catalytic activities for the oxidative degradation of CEES.^26–28 However, most of these supported materials always suffer from a low loading efficiency, ill-defined structure and inhomogeneous pre-catalyst binding sites, which result in differences in both activity and selectivity. In addition, previous investigations mostly centered on the design and synthesis of heterogeneous POM materials, which can selectively catalyze various thioethers into their corresponding sulfones. Although sulfone compounds are of great importance, high selectivity to obtain sulfoxides in the oxidation reaction is also expected. Current catalytic systems based on POMs displayed low selectivity for the oxidation of sulfides to sulfoxides, such as PW₁₂@Al-MCF_en, {As₄W₄₀O₁₄₀(Ru₂(Ac)₂)} and V^V₁₇V^IV₁₂(C₆H₈O₄)₈.^19,29,30 Therefore, it is still highly desirable to exploit alternative catalytic materials based on POMs with high reactivity and selectivity to obtain sulfoxide compounds.

As a special subclass of hybrid POMs that can be rationally designed and synthesized, and increase the stability and activity of the precursors, organic ligand covalently modified POMs have recently attracted great attention in catalysis. Owing to their intrinsic nature and flexible tunability, a few hybrid POMs, such as a hybrid polymer based on polyoxovanadates and 1,3,5-benzenetricarboxamide and mixed carboxylic acid-modified polyoxomolydates, have been evidenced to be privileged pre-catalysts that enable the selective oxidative degradation of CEES.^31,32 Inspired by this idea, we predicted that the obtained carboxylic acid-modified POMs with the exposed metal cation linkers Cs₄[M(H₂O)₄][PMo₆O₂₁(PABA)₃]₂·nH₂O 1–4 (M = Co, Mn, Ni, Zn) would also be good candidates for catalyzing the selective oxidation of thioethers. As expected, these compounds displayed excellent ability to selectively oxidize various sulfide substrates. Within 20 minutes, various phenyl sulfides could be rapidly converted to the corresponding sulfoxide with high selectivity using these compounds as pre-catalysts. More importantly, the sulfur mustard simulant CEES can also be highly selectively and rapidly degraded to nontoxic CEESO within 12 minutes.

2 Experimental

Syntheses of Cs₄X[M(H₂O)₄][P^IIIMo₆O₂₁(PABA)₃]₂·nH₂O (M = Co 1, Mn 2, Ni 3, Zn 4; n = 25 1, 27 2; X = Na₂1)

In a typical synthesis procedure for 1–4, Na₂MoO₄ (0.145 g, 0.6 mmol), Na₂HPO₃·5H₂O (0.0216 g, 0.1 mmol), and PABA (0.0411 g, 0.3 mmol) were dissolved in 20 mL water. The pH value of the solution was adjusted from the original pH value of 4.5 to 4.0 with HCl solution under stirring. The solution was stirred for one hour at room temperature. Then, CoCl₂·6H₂O (0.0714 g, 0.3 mmol)/MnCl₂·2H₂O (0.0486 g, 0.3 mmol)/NiCl₂·6H₂O (0.0678 g, 0.3 mmol)/ZnSO₄·7H₂O (0.0861 g, 0.3 mmol) and CsCl (0.034 g, 0.2 mmol) were successively added into the reaction system under stirring. Finally, the solution was heated and stirred for one hour at 80 °C. The filtrate was kept undisturbed for two weeks under ambient conditions, and then crystals of 1–4 were isolated with about 45% yield (based on Mo). Elemental analyses: calcd for 1: Mo, 29.79; P, 1.60; Co, 1.52; Cs, 13.75; C, 13.05; N, 2.18; H, 2.58 (%). Found: Mo, 30.03; P, 1.82; Co, 1.66; Cs, 14.15; C, 13.53; N, 2.47; H, 2.54 (%). FTIR data (cm⁻¹) for 1: 3357 (s), 1605 (m), 1544 (s), 1413 (s), 1275 (w), 1179 (m), 928 (w), 894 (w), 780 (m), 672 (s), 567 (w), 539 (w), 444 (m). Calcd for 2: Mo, 29.89; P, 1.61; Mn, 1.43; Cs, 13.80; C, 13.10; N, 2.18; H, 2.90 (%). Found: Mo, 29.76; P, 1.28; Mn, 1.55; Cs, 13.43; C, 12.58; N, 2.34; H, 3.29 (%). FTIR data (cm⁻¹) for 2: 3447 (s), 1606 (m), 1545 (m), 1415 (s), 1274 (w), 1178 (m), 927 (w), 894 (m), 782 (w), 671 (s), 536 (w), 440 (m). FTIR data (cm⁻¹) for 3: FTIR data (cm⁻¹): 3444 (s), 1606 (w), 1547 (m), 1417 (s), 1272 (w), 1178 (m), 929 (w), 892 (w), 782 (m), 678 (s), 565 (w), 535 (w), 438 (m). FTIR data (cm⁻¹) for 4: FTIR data (cm⁻¹): 3434 (s), 1607 (w), 1549 (m), 1417 (s), 1271 (w), 1175 (m), 929 (w), 890 (w), 784 (m), 679 (s), 564 (w), 537 (w), 439 (m).

General methods for catalyzing the selective oxidation of methyl phenyl sulfide and CEES

Methyl phenyl sulfide oxidation: Compound 1 (2.5 μmol), H₂O₂ (0.3 mmol), methyl phenyl sulfide (0.25 mmol), and ethanol (0.5 mL) were put in a glass bottle. The catalytic reaction was carried out at room temperature for 20 minutes. CEES oxidation: Compound 1 (2.5 μmol), H₂O₂ (0.3 mmol), CEES (0.25 mmol), and ethanol (0.5 mL) were put in a glass bottle. The catalytic reaction was carried out at room temperature for 12 minutes. After the catalytic reaction was finished, GC-FID, GC-MS and ¹H NMR were used to analyze the resulting mixture.

3 Results and discussion

Synthesis and characterization of these materials

In this paper, four hybrid compounds were assembled in a simple one-pot system of Na₂MoO₄, Na₂HPO₃·5H₂O, PABA and metal cation TM²⁺ at a ratio of 6 : 1 : 3 : 3 under reflux at 80 °C. These four hybrid POMs displayed similar dimeric structures. We did not obtain the single crystal data of compounds 3 and 4 due to the poor crystal quality, but IR and XRD results of the four compounds confirmed the isostructural architectures. Rare earth metal cations, such as La³⁺, Ce³⁺, and Nd³⁺, were also attempted as linkages to synthesize the similar compounds, but only the poor-quality crystal Cs₃[PMo₆O₂₁(PABA)₃]·nH₂O (compound 5) with Cs⁺ as the counter cation was produced. When we introduced Cu²⁺ into the reaction system, a coordination complex constructed from PABA and Cu²⁺ was obtained. More importantly, other counter cations, such as K⁺ and Rb⁺, were also introduced into the process in the presence of transition metal cations, but no crystal was obtained without Cs⁺. Thus, it can be seen that both metal cations and counter cations play an essential part in constructing these hybrid compounds. In addition, if PABA was replaced by other ligands, such as 3-aminobenzoic acid, 4-hydroxybenzoic acid, 3-hydroxybenzoic acid and salicylic acid, no similar dimeric structures were isolated. The parallel experiments described above demonstrated that the metal cations Co²⁺/Mn²⁺/Ni²⁺/Zn²⁺, counter cation Cs⁺ and the organic ligands play important roles for the construction of the dimeric structures with carboxylic acid ligand-modified POM units.

Single crystal X-ray diffraction analyses demonstrated that 1 and 2 were isostructural and crystallized in the same space group P1̄ constructed from the same polyoxoanion unit {PMo₆O₂₁}. The asymmetric unit of 1 contained one crystallographically independent [PMo₆O₂₁(PABA)₃]³⁻ anion, half of one Co²⁺ cation, one Na⁺ cation and two Cs⁺ cations (Fig. S1†). The unique Co(1) ion displayed a distorted octahedral geometry, defined by two terminal O atoms derived from two polyoxoanions [Co–O 2.136(4)–2.151(5) Å] and four water molecules [Co–OW 1.962(3)–2.144(4) Å]. First, the {PMo₆O₂₁} units were covalently coordinated to the carboxylic acid ligand PABA to produce the single-side modified POMs [PMo₆O₂₁(PABA)₃]³⁻ (Fig. 1a). Then, the two modified POMs were jointed together to form a dimer via the Co–O–Mo bond. Such dimers were next linked together to generate a 1D supramolecular chain through the hydrogen bonds (N1–H⋯O25 = 2.954 Å in 1) between the N atoms of PABA and the O atoms of the {PMo₆O₂₁} units (Fig. 1c). Finally, these 1D chains were further joined together to obtain a 2D supramolecular layer via the hydrogen bonds (N2–H⋯O20 = 2.744 Å, N2–H⋯O13 = 2.938 Å in 1) existing between the N atoms of PABA and the O atoms of the {PMo₆O₂₁} units (Fig. 1d).

Fig. 1 (a) The direct self-assembly protocol for the synthesis of carboxylic acid ligand-modified POMs; (b) ball-stick and polyhedral representation of the dimeric polyanion for 1; (c) schematic of the 1D supramolecular chain showing the hydrogen bonds; and (d) schematic of the 2D supramolecular sheet showing the hydrogen bonds (color code: P: light yellow, Mo: purple, O: red, Co: light blue, N: blue, and C: black).

Bond valence sum (BVS) calculations indicated that the oxidation states of Mo, and P were +6, and +3, respectively, and those of Co/Mn were +2.

Fig. S2† displays the IR spectra of compounds 1–4, indicating the isostructural characteristic of these compounds. The terminal Mo–O bonds and the bridging M–O–Mo units (M = P and Mo) were distributed at 850–950 cm⁻¹ and 580–690 cm⁻¹, respectively. The peaks from 1000 and 1610 cm⁻¹ are ascribed to the carboxylic acid ligands. The absorption maxima at 1605, 1541, and 1415 cm⁻¹ for 1 are assigned to the v_as(COO) and v_s(COO) stretching frequencies, which indicates the bidentate coordination manner of the carboxylate group of PABA. The diffuse reflectance spectra of 1–4 are shown in Fig. S5† to spot the absorption bands distributed in the ultraviolet and visible regions. Taking compound 1 as an example, the UV region (200–400 nm) exhibits two intense absorption peaks, 234 and 315 nm, which are attributed to the O → Mo charge transfer for the {PMo₆O₂₁} polyoxoanions. In addition, one absorption band located at 525 nm in the visible region is assigned to ⁴T_1g → ⁴T_2g and ⁴A_2g → ⁴T_1g for the low-spin Co²⁺ cations.

Fig. S6† displays the TGA of 1 and 2, which demonstrated multistep weight loss processes. The total weight loss of 34.9% for 1 and 35.2% for 2 are in agreement with the calculated weight losses of 35.4% and 35.6% from 50 to 850 °C. The PXRD patterns of these samples match the calculated peaks, which manifested the phase purities of 1 and 2. In addition, the similar diffraction patterns of 1–4 further indicate the isostructural characteristic (Fig. S7 and S8†).

Catalytic oxidation study for methyl phenyl sulfide

Given that sulfoxides are very important as intermediates for functional materials, obtaining sulfoxides is desirable through the selective oxidation of sulfides. Herein, the selective oxidation of various sulfides to sulfoxides using H₂O₂ as the oxidant and 1–4 as the pre-catalysts were tested in detail.

The selective oxidation of methyl phenyl sulfide was chosen as a model catalytic reaction to evaluate the performance of compounds 1–4. To implement the catalytic oxidation reaction, methyl phenyl sulfide (0.25 mmol), the pre-catalyst (2.5 μmol), H₂O₂ (oxidant, 0.3 mmol), and naphthalene (internal standard, 0.25 mmol) were performed in ethanol (0.5 ml) at room temperature, and the results of this catalytic system were monitored by gas chromatography (GC). As expected, methyl phenyl sulfide was efficiently catalyzed to the corresponding sulfoxide within 20 min, and compound 1 displayed a relatively high catalytic effect (Fig. 2b). Indeed, compound 1 displayed more notable activity than most reported POM-based materials, and could oxidize 98.7% of methyl phenyl sulfide into sulfoxide with 98% selectivity within 20 min. The performances of these compounds were compared with the heterogeneous catalytic materials based on POMs in recent years, such as the organic–inorganic hybrid POM materials {Cu₃(ptz)₄(Co₂Mo₁₀)}, {[Cu(mIM)₄]V₂O₆}, V^V₁₇V^IV₁₂(C₆H₈O₄)₈, {As₄W₄₀O₁₄₀(Ru₂(Ac)₂)} and the supported materials PDDA-SiV₂W₁₀, P₂W₁₅-Al₂O₃, PW₁₂@Al-MCF_en, and SBA-15/K₆P₂W₁₈O₆₂. As summarized in Table S1,† the compound 1 displayed significantly higher activity than the first three hybrid POM compounds and the first composite material (obtained using TBHP or UHP as the oxidant or long reaction time); this catalytic system also showed a little higher activity than other pre-catalysts that were obtained using a little longer reaction time and displayed relatively low selectivity. Fig. 2c shows the recorded progress of the oxidation of methyl phenyl sulfide in the presence of 1 through GC signals, which illustrated that methyl phenyl sulfide was almost entirely oxidized to the corresponding sulfoxide. To further verify the accuracy of the catalytic process, ¹H NMR spectroscopy was employed to monitor the conversion and selectivity (Fig. 3a). The catalytic reaction at 4, 8, 16 and 20 minutes was investigated via¹H NMR spectroscopy in CDCl₃. In addition, the pure methyl phenyl sulfide, methyl phenyl sulfoxide, and methyl phenyl sulfone were also tested to further illuminate the reaction. The comparison of the ¹H NMR spectra of these actual processes and the pure substrate or products demonstrates that methyl phenyl sulfide can be almost converted to methyl phenyl sulfoxide within 20 minutes.

Fig. 2 (a) Catalytic oxidation pathway of methyl phenyl sulfide to methyl phenyl sulfoxide and methyl phenyl sulfone; (b) the conversion of methyl phenyl sulfide via oxidation using the compounds 1–4 and blank. Reaction conditions: 0.25 mmol sulfide, 2.5 μmol pre-catalyst, 0.25 mmol naphthalene (internal standard), 0.3 mmol H₂O₂ and 0.5 mL ethanol at room temperature for 20 min; (c) GC-FID signals indicating the progress of the oxidation of methyl phenyl sulfide (6.89 min) to methyl phenyl sulfoxide (10.38 min) and methyl phenyl sulfone (11.02 min), and the internal standard (8.495 min, naphthalene) in the presence of 1.

Fig. 3 (a) Selected regions of the ¹H NMR spectra of methyl phenyl sulfide, methyl phenyl sulfoxide, methyl phenyl sulfone, oxidation reaction (without internal standard) at 4 min, 8 min 16 min and 20 min in CDCl₃. The peaks used for calculation are labeled with chemical shift; (b) the oxidation of sulfide by different pre-catalysts; (c) the conversion and selectivity of methyl phenyl sulfide oxidation for different pre-catalysts.

Then, to understand the active center of compound 1 for the selective oxidation of methyl phenyl sulfide, we further investigated the catalytic performance of PABA, CoCl₂ salt, and compound 5 (Cs₃[PMo₆O₂₁(PABA)₃]·nH₂O). Compound 5 was confirmed by IR spectroscopy and EDS data because of the poor crystal quality (Fig. S3†). Fig. 3b reveals that both PABA and CoCl₂ have converted very little methyl phenyl sulfide, and the compound 5 originating from the PABA-modified POMs can catalyze 72% of methyl phenyl sulfide with a selectivity of 89%, which is far behind that in the case of 1. Thus, it can be seen that integrating the Co site and PABA-modified POMs together can improve the catalytic oxidation reaction with rapid and excellent selectivity for the oxidation of methyl phenyl sulfide. Furthermore, a comparison of the catalytic oxidation of compound 1 with those of {CoAsMo₆(PABA)₃} and {CoTeMo₆(PABA)₃}, which possessed different central atoms but similar architectures to the compound 1, was conducted to explore how the {ZMo₆} unit impacted the catalytic activity. {CoAsMo₆(PABA)₃} and {CoTeMo₆(PABA)₃} converted 96.8% and 95.5% of methyl phenyl sulfide with a selectivity of 96.1% and 96.3% under similar conditions, respectively, which indicates that the central heteroatoms played a certain role for the selective oxidation of methyl phenyl sulfide and the corresponding order is PMo > AsMo > TeMo (Fig. 3c). A similar phenomenon has been investigated by the Mizuno and Su's group.^36,37

In view of previous research studies and the abovementioned results, we speculate that the peroxo-metal species originating from 1 and H₂O₂ play a crucial part in the catalytic oxidation process, which have been widely investigated based on POMs.^26–28,38 In addition, to exclude the free radical mechanism, radical scavengers (such as diphenylamine and 1,4-benzoquinone) were added into the catalytic reactions, and no change of the conversion was observed. Hence, we proposed a possible mechanism of methyl phenyl sulfide selective oxidation in Fig. 4a. To verify the interaction between 1 and H₂O₂, UV-Vis spectra of the mixed solution were measured before and after the addition of H₂O₂ into solution 1. Indeed, two red shifts in the ultraviolet region (276 → 284 nm) and in the visual region (540 → 544 nm), and a new band at 760 nm confirm the interaction between compound 1 and H₂O₂ (Fig. 4b and c). As shown in Fig. 4a, the active peroxomolybdenum and peroxocobalt species derived from compound 1 and H₂O₂ worked on the S atom of the sulfides to yield the corresponding products.

Fig. 4 (a) Proposed mechanism of the catalytic oxidation of sulfides; (b) UV-Vis spectra of the compound 1 in dimethyl sulfoxide with one-drop portions of H₂O₂ in the UV region (200–400 nm); (c) UV-Vis spectra of the compound 1 in dimethyl sulfoxide with one-drop portions of H₂O₂ in the visible region (400–800 nm).

Subsequently, we also investigated the recyclability and stability of this pre-catalyst. Just as in Fig. 5a, the conversion of methyl phenyl sulfide remained almost steady in the following 10 cycles, and only a negligible decrease occurred using the recovered pre-catalyst by simple filtration. No catalytic activity was observed when the pre-catalyst was filtered. Furthermore, no distinct change could be detected from the IR spectra and PXRD patterns before and after the catalytic reaction (Fig. 5b and c). From the abovementioned results, we can conclude that these compounds are excellent heterogeneous oxidation pre-catalysts.

Fig. 5 (a) Recycling of the catalytic system for the oxidation of methyl phenyl sulfide using compound 1; (b) IR spectra for the (a) as-synthesized compound 1 and (b) recovered pre-catalyst after catalytic reaction; (c) powder X-ray diffraction (PXRD) patterns of 1: calculated pattern from crystal data, experimental pattern before reaction and experimental pattern of the recovered pre-catalyst after the oxidation of methyl phenyl sulfide.

Given that compounds 1–4 exhibited excellent catalytic activities in the oxidation of methyl phenyl sulfide, and compound 1 outperformed other compounds with respect to the catalytic activity, various organic sulfides with different electronic and steric effects were continuously investigated using compound 1 (Table 1). The conversion of these substrates did not obviously decrease despite the introduction of the electron donor or acceptor units with less steric hindrance into monophenyl sulfides. We were also delighted to find that even the sulfides with large steric hindrance, such as dibenzyl sulfide and diphenyl sulfide, did not greatly influence the catalytic activities. The above results demonstrate that these compounds possessed superior catalytic capability in the oxidation of sulfides. In addition, to further clarify the function of the central heteroatoms of the {ZMo₆} units, the selective oxidation of these organic sulfides were also performed in the presence of {CoAsMo₆(PABA)₃} and {CoTeMo₆(PABA)₃} (Tables S2–S7†), which indicated that the heteroatoms indeed made a difference in the catalytic activity. We also investigated the catalytic oxidation reaction kinetics of methyl phenyl sulfide. The time profiles for the selective oxidation reaction process of methyl phenyl sulfide are shown in Fig. 2. A plot of the reaction time against ln(C_t/C₀) displays a linear correlation, which shows that the oxidation of methyl phenyl sulfide is in accordance with the first order reaction. The first-order kinetics constants k₁–k₄ for 1–4 are 0.23842 min⁻¹, 0.1624 min⁻¹, 0.20015 min⁻¹, and 0.17483 min⁻¹, respectively (Fig. S9†).

Table 1 Results of selective oxidation of various sulfides catalyzed by 1 using H₂O₂ as oxidant^a

Entry	Substrates	Products	Con. (%)	Sel.^b (%)
a Reaction conditions: Sulfide (0.25 mmol, 1 equiv.), pre-catalyst (1 mol%), H2O2 (1.2 equiv.), ethanol (0.5 mL), for 20 min, at room temperature. b Selectivity to sulfoxides, the byproduct was sulfone.
1			99.5	98
2			98.4	97.5
3			98	96.5
4			97.8	95.7
5			98.6	96.5
6			98.1	94.3
7			95.1	92.9

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Catalytic oxidation study for CEES

In light of the excellent catalytic activity and selectivity of compounds 1–4 in the oxidation of methyl phenyl sulfide, it is of great interest to explore their catalytic performance in the selective oxidation of sulfur mustard (one of the most toxic chemical warfare agents). Owing to the high toxicity of sulfur mustard, the simulant molecule 2-chloroethyl ethyl sulphide (CEES) with a similar architecture is always employed. The catalytic reaction condition was similar to methyl phenyl sulfide: CEES (0.25 mmol), pre-catalyst (2.5 μmol), H₂O₂ (oxidant 0.3 mmol), naphthalene (internal standard 0.25 mmol) and the solvent ethanol (0.5 ml) at room temperature. As expected, these compounds exhibited superb catalytic activity for the selective oxidation of CEES. The results demonstrate that CEES can be almost entirely degraded within 12 min (Fig. 6b). As the best pre-catalyst among these compounds, compound 1 can oxidize 98.8% of CEES into the nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) with the selectivity of 99.0%. In addition, the process of the selective oxidation of CEES in the presence of 1 was also recorded by GC-FID signals, and it indicated that CEES was almost completely catalyzed within 12 min under mild conditions (Fig. 6c). Similarly, ¹H NMR spectroscopy monitored the oxidation of CEES (Fig. 6d). Also, for assessing the recyclability and stability of this hybrid compound, the pre-catalyst (recovered by simple filtration) can be reused in selective oxidation reactions at least ten times with no significant loss of activity (Fig. 6e).

Fig. 6 (a) Catalytic oxidation pathway of CEES to CEESO and CEESO₂; (b) the conversion of CEES via oxidation using the compounds 1–4 and blank. Reaction conditions: 0.25 mmol CEES, 2.5 μmol pre-catalyst, 0.25 mmol naphthalene (internal standard), 0.3 mmol H₂O₂ and 0.5 mL ethanol at room temperature for 12 min; (c) GC-FID signals indicating the progress of the oxidation of CEES (4.56 min) to CEESO (8.92 min), CEESO₂ (9.23) and internal standard naphthalene (8.495 min) in the presence of 1; (d) selected regions of the ¹H NMR spectra of CEES, CEESO, CEESO₂ oxidation reaction (without internal standard) at 6 min and oxidation reaction (without internal standard) at 12 min with two compounds in CDCl₃. The peaks used for calculation are labeled with the chemical shifts; (e) recycling of the catalytic system for the oxidation of CEES using the compound 1.

4. Conclusions

In conclusion, four hybrid compounds were prepared through a simple one-pot reaction and used as the oxidative pre-catalyst for selectively catalyzing various phenyl sulfides and the sulfur mustard simulant CEES. As a superb pre-catalyst, these compounds can rapidly and selectively convert methyl phenyl sulfides to their corresponding sulfoxide, as well as catalyze the degradation of CEES to the nontoxic CEESO. Such remarkable catalytic activities are possibly ascribed to the synergistic effect between the carboxylic acid modified POMs and metal cations. In addition, the catalytic performance of heteropolymolybdates can be also influenced by altering the heteroatom of {ZMo₆} in the order: {P^IIIMo} > {As^IIIMo} > {Te^IVMo}. More importantly, the pre-catalyst can be recycled at least ten times with no obvious loss of activity. These results encourage us to design and prepare the highly active and novel heterogeneous hybrid materials and investigate the catalytic oxidation performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (21371027, 20901013), Natural Science Foundation of Liaoning Province (2015020232) and Fundamental Research Funds for the Central Universities (DUT15LN18, DUT19LK01).

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Footnote

† Electronic supplementary information (ESI) available: ORTEP drawing of 1; IR spectra, UV–Vis diffuse reflective spectra, TG plots, PXRD figures for 1–4; kinetic analysis of methyl phenyl sulfide; comparison of methyl phenyl sulfide oxidation in recent years; oxidation of various sulfides to the corresponding sulfoxides and sulfones; crystal data and structure refinement for 1 and 2; selected distances and angles for 1 and 2. CCDC 1949665 and 1949666. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qi01098j

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