Modulating the catalytic properties of decavanadate hybrids using a mixed counterion strategy for selective oxidation of thiophene-based sulfides and detoxification of mustard gas simulant

Abstract

Selective oxidation of sulfides to sulfoxides, especially thiophene-based sulfides, is a challenging task. Herein, we report a mixed counterion strategy in polyoxometalate (POM) chemistry to tune the selectivity of sulfoxidation reaction catalyzed by decavanadate cluster-based hybrids using H₂O₂ as the oxidant under ambient conditions. By employing two different aryl sulfonium counterions (ASCIs) bearing different organic functional groups (phenol/aldehyde/salicylaldehyde/2,6-diformyl phenol) in a 1 : 1 synthetic feed ratio, we have generated a series of decavanadate-based hybrids HY1–HY6. Different functional groups on the periphery of hybrids HY1–HY6 helped control the efficiency and selectivity of the sulfoxidation reaction by fine-tuning the electronic and supramolecular effects of these hybrids as catalysts. Further, these hybrids were also applied as catalysts for detoxifying 2-chloroethyl ethyl sulfide (CEES), a mustard gas simulant. The hybrid HY5, with a structural formula (DFHPDS)₂(FPDS)₂[H₂V₁₀O₂₈](H₂O)₃ (DFHPDS = (3,5-diformyl-4-hydroxyphenyl)dimethylsulfonium, and FPDS = (4-formylphenyl)dimethylsulfonium) showed the best catalytic properties in the series, with up to 99% conversion and 85% and 99% selectivity towards sulfoxide in the case of dibenzothiophene (DBT) and CEES, respectively. This study's findings open new avenues for tuning the catalytic properties of POM-based hybrids toward selective organic transformation reactions by using a mixed counterion strategy.

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Introduction

Polyoxometalates (POMs) represent a large group of anionic metal-oxo nanoclusters, typically formed by early transition metals (Mo, W, V, Nb, and Ta) in their highest oxidation states.^1–3 The metal centers in POMs show reversible redox behavior, making them useful in catalysis,⁴ sensing,⁵ and biomedical applications.^6–8 The anionic charge of the POM clusters is balanced by positively charged counterions, commonly consisting of alkali and alkaline earth metal ions like Li⁺, Na⁺, K⁺, and Cs⁺. Besides these, several types of cationic organic moieties are also employed as counterions to charge-balance the POM clusters, leading to stable and robust Class I POM-hybrids. Choice and variation of the counterions can help fine-tune different interactions prevalent in POM-hybrids, including electrostatic, hydrogen bonding, van der Waals, and cation or anion-π interactions.⁹ Studies have shown that the counterions can modulate a POM-hybrid's solubility, structure, and redox behaviors.⁹ Therefore, in modern POM chemistry, the counterions are recognized as structural components that can control the overall properties of the cluster. Many reports have shown how the variation of counterions changes the catalytic,¹⁰ biological,¹¹ self-assembly,¹² photo coloration,¹³ and photocatalytic properties¹⁴ of POM-based systems. Streb et al. showed that replacing a placeholder cation (dimethyl ammonium) of a POM cluster with a reactive metal cation allows tuning its redox properties, making it useful for multi-electron transfer and storage applications.¹⁵ Further, Barteau et al. showed that increasing the countercations’ concentration can modify the POM cluster's redox behavior.¹⁶ These examples clearly show that the redox properties of a POM cluster may be tuned by altering the counterions, which in turn may affect its overall catalytic properties.

Polyoxovanadates (POVs) are an interesting sub-class of POMs consisting of vanadium metal centers. In recent years, researchers have shown increasing interest in the biological and catalytic activities of POVs due to their non-toxic nature and excellent redox properties.^17,18 Particularly, vanadium-containing POMs show superior activity as heterogeneous catalysts for sulfide oxidation reactions in the presence of H₂O₂.^19,20 Vanadium-based systems catalyze oxidation reactions in the presence of H₂O₂ by involving the V⁵⁺/V⁴⁺ redox pair²¹ and often their catalytic activity relies upon the efficient reversible conversion of this V⁵⁺/V⁴⁺ pair.^21,22 Recent studies have also shown that the noncovalent interactions (NCIs) such as hydrogen bonding, C–H⋯π, C–H⋯F, C–H⋯O, C–H⋯Cl, C–H⋯H–C, lone pair⋯π, and π⋯π interactions play significant roles in catalytic processes.²³ Theoretical studies have shown that the NCIs between the different entities of the catalysis reaction, such as the catalyst and substrate,²⁴ ligands in the catalyst,²⁵etc., can also play prominent roles in determining product selectivity.²⁶ In the POM-hybrids’ case, the organic moieties or counterions associated with a POM cluster are crucial in controlling its overall redox property and noncovalent interactions with the substrates. This, in turn, can affect the overall catalytic efficiency and selectivity of POM-hybrid catalyzed reactions.²⁶

Sulfide oxidation is an important organic transformation reaction critically connected with the oxidative desulfurization (ODS) of fuels, a successful strategy for removing sulfur components from fuel oils.²⁷ Thiophene-based sulfides in fossil fuels are harmful and must be removed for environmental reasons. However, in general, the thiophene-based sulfides are hard to oxidize. The oxidation of thiophene-based substrates often depends on the electron density of the sulfur; the higher the electron density of the sulfur, the more readily it can be removed by the oxidation process.²⁸ In general, the kinetics of sulfide oxidation reaction can be enhanced by using a catalyst at ambient temperature.²⁹ The oxidation of sulfide compounds in the presence of H₂O₂ and a catalyst can lead to the formation of different products, such as sulfoxides and sulfones. Most of the catalytic oxidation of sulfides reported in the literature results in complete substrate oxidation, generating sulfones. It is often difficult to control the selectivity of the reaction to yield the partially oxidized products, i.e., sulfoxides, predominantly. At the same time, the synthesis of sulfoxides from sulfides has attracted much attention recently due to their utility for synthesizing biologically relevant molecules³⁰ and chiral molecules.³¹ There have been several reports on polyoxovanadates (POVs)-based catalytic oxidation of sulfides,³² but most of those studies have focused on the conversion of sulfides but not on the selectivity.^33–38

The above discussions show that a POM hybrid's redox status and catalytic properties may be tuned by modulating the counterions. Further, the overall catalytic outcome of a reaction can be influenced by the noncovalent interactions between the catalyst and the substrate. In earlier work, we had shown that by employing organic counterions bearing salicylaldehyde-type functional groups, the catalytic activities of decavanadate-based hybrids could be controlled in the sulfide oxidation reaction.¹³ This discovery opened up huge potential for tuning POM-hybrid-based catalysts for organic transformations using counterions bearing functional groups of different acidities, H-bonding properties, steric effects, etc. However, there were some practical difficulties associated with this approach. One such difficulty was the synthetic challenge in introducing multiple functional groups onto a single counterion moiety. This issue can be addressed using a mixed counterion strategy in which two or more different counterions bearing different functional groups are used in a POM hybrid. This approach will allow easy introduction of different organic functional groups onto a hybrid's periphery. The synergistic effects of these functional groups in controlling the redox and supramolecular properties of the POM-hybrid are expected to control their overall catalytic properties. However, there are only a few reports on POM hybrids where a mixed counterion strategy has been used to fine-tune their properties.^12,39,40 Therefore, there is plenty of scope in exploring this strategy for developing novel POM-hybrid-based materials and catalysts. In this work, we used different aryl sulfonium counterions (ASCIs) bearing different organic functional groups, such as aldehyde, phenol, salicylaldehyde and 2,6-diformylphenol, to develop mixed counterion POM hybrids so that multiple functional groups are introduced comparatively easily onto a POV cluster, [H₂V₁₀O₂₈]⁴⁻ (decavanadate cluster). Accordingly, we synthesized mixed ASCIs based decavanadate hybrids having the formula (FHPDS)₂(HPDS)₂[H₂V₁₀O₂₈] (HY1), (FHPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY2), (HPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY3), (DFHPDS)₂(HPDS)₂[H₂V₁₀O₂₈] (HY4), (DFHPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY5), and (DFHPDS)₂(FHPDS)₂[H₂V₁₀O₂₈] (HY6) (where FHPDS = (3-formyl-4-hydroxyphenyl)dimethylsulfonium; DFHPDS = (3,5-diformyl-4-hydroxyphenyl)dimethylsulfonium; FPDS = (4-formylphenyl)dimethylsulfonium and HPDS = (4-hydroxyphenyl)dimethylsulfonium), as shown in Scheme 1. The use of the ASCIs also helped to tune the overall solubility properties of the decavanadate cluster. The hybrids HY1–HY6 were insoluble in ethanol, isopropyl alcohol and acetonitrile and were applied as heterogeneous catalysts for the sulfide oxidation reaction. These hybrids were also tested as catalysts for the oxidation of 2-chloroethyl ethyl sulfide (CEES), a simulant for the chemical warfare agent (CWA) bis-(2-chloroethyl)sulfide (mustard gas)⁴¹ that causes severe skin damage, lung infection, eye irritation and even death at high concentrations.^42,43 Only a few reports in the literature exist on using POVs, especially decavanadate-based systems, as sulfide oxidation and mustard gas detoxification catalysts.^19,44–48

Scheme 1 Outline of the synthesis and applications of HY1–HY6.

Results and discussion

Synthesis and characterization

The aryl sulfonium counterion precursors, namely FHPDST ((3-formyl-4-hydroxyphenyl)-dimethylsulfonium triflate), HPDST ((4-hydroxyphenyl)-dimethylsulfonium triflate), FPDST ((4-formylphenyl)-dimethylsulfonium triflate) and DFHPDST ((3,5-diformyl-4-hydroxyphenyl)-dimethylsulfonium triflate) were synthesized following a reported procedure.⁴⁹ To synthesize the hybrids HY1–HY6, we followed a procedure reported for similar materials.¹¹ In a typical reaction, for example, the synthesis of HY1, sodium metavanadate [NaVO₃·2H₂O] (1 equivalent) was dissolved in water at elevated temperature (80 °C), then cooled down to room temperature and the pH was maintained at 4.5 with 4 M HCl to promote the formation of decavanadate anions in the solution. Parallely, a solution of 1 : 1 mixture of two different aryl sulfonium counterions precursors, i.e., FHPDST and HPDST, was prepared in acetonitrile, which was then added dropwise into the decavanadate solution prepared above, resulting in immediate precipitation. The obtained precipitate was stirred overnight, filtered, and washed at least three times with ethanol (EtOH) and diethyl ether (Et₂O) and dried at 40 °C. This process led to the formation of the hybrid (FHPDS)₂(HPDS)₂[H₂V₁₀O₂₈] (HY1). The other hybrids (FHPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY2), (HPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY3), (DFHPDS)₂(HPDS)₂[H₂V₁₀O₂₈] (HY4), (DFHPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY5), and (DFHPDS)₂(FHPDS)₂[H₂V₁₀O₂₈] (HY6) were also synthesized similarly by using a 1 : 1 mixture of the respective aryl sulfonium counterions precursors; see Scheme 1.

The formation of the mixed counterion hybrids HY1–HY6 was confirmed through several analytical and spectroscopic techniques, including elemental, NMR, FT-IR, XPS, and ESI-MS analyses. Elemental analyses of the hybrids HY1–HY6 indicated the presence of four organic counterions per decavanadate cluster in these hybrids. The details of the elemental analyses are given in the Experimental section. In the FT-IR spectra of HY1–HY6, characteristic stretching vibrations of the POM cluster, such as the stretching vibrations of the V–O bonds, ν_s(V–O_t) and ν_as(V–O_b–V), were observed in the range of 950–990 cm⁻¹ and 840–740 cm⁻¹, respectively. Additionally, the presence of the organic counterions was confirmed by aromatic ‘C=C’ stretch (1400–1600 cm⁻¹) and aromatic ‘C–H’ stretch (2910–3100 cm⁻¹) peaks. Notably, HY3 showed intense FT-IR peaks at around 1090 cm⁻¹ and 1692 cm⁻¹. These peaks can be assigned to the ν(O–H) bending of the ‘C–OH’ functionality of HPDS counterion and the hydrogen-bonded formyl group (ν(C=O)) of FPDS counterion, respectively; see Fig. 1. Therefore, it was clear that HY3 contains both the counterions, i.e., FPDS and HPDS. Likewise, HY2 showed a strong peak at 1702 cm⁻¹, indicating the presence of FPDS counterion with a formyl group, and another sharp peak at around 1680 cm⁻¹ corresponding to the hydrogen-bonded formyl group of the FHPDS counterion.⁵⁰ The FT-IR data of other hybrids also confirmed the presence of decavanadate clusters and mixed counterions, the details of which are given in ESI.†

Fig. 1 (a) FT-IR spectra of HY1–HY6; and (b) zoomed-in view of the FT-IR spectra of HY1–HY6 showing different stretching and bending modes of the counterions.

As the FT-IR analyses indicated the presence of mixed organic counterions with different functional groups in hybrids HY1–HY6, we performed NMR analyses to determine the ratio of different organic counterions present. These analyses showed that in hybrids HY1–HY6, the two different organic counterions are present in a more or less 1 : 1 ratio, which agrees with their synthetic feed ratio. The detailed NMR analyses of a representative hybrid, i.e., HY1, are given here, and similar analyses for the remaining hybrids are given in ESI (Fig. S2–S6†). Hybrid HY1 comprises the counterions FHPDS and HPDS along with the decavanadate cluster. The proton NMR spectrum of HY1 exhibited two singlets at δ 3.12 and 3.18 ppm in a 1 : 1 ratio, and each singlet integrated into six protons. These peaks are attributed to the –⁺S(CH₃)₂ protons of HPDS and FHPDS counterions. Further, it showed a singlet peak corresponding to one proton at δ 10.13 ppm, assignable to the aldehydic proton, and three singlet peaks corresponding to one proton each at δ 7.98, 7.85 and 6.85 ppm due to aromatic C–H protons.^11,13 The above peaks are assigned to the FHPDS counterion.¹³ Further, HY1 showed two doublets at around δ 7.88 and 7.02 ppm with two proton integrations each, assignable to the aromatic C–H protons of the HPDS counterion. As the integration ratio of –⁺S(CH₃)₂ protons observed at δ 3.12 and 3.18 ppm was approximately 1 : 1 (HPDS : FHPDS), it suggests that the original feed ratio (1 : 1) of the counterions is maintained in hybrid HY1; see Fig. 2. Similarly, the detailed NMR analyses of the hybrids HY2–HY6 also indicated the presence of two different organic counterions in a 1 : 1 ratio, matching the original synthetic feed ratio; see Fig. S2–S6, ESI.† Again, ⁵¹V NMR spectra of HY1–HY6 showed three peaks for three vanadium centers having different chemical environments. For example, the ⁵¹V NMR spectrum of HY1 exhibited three peaks at δ −418.67, −499.41, and −522.87 ppm, confirming the presence of a decavanadate cluster in the hybrid.⁵¹ The detailed ⁵¹V NMR analyses of the remaining hybrids are given in ESI (Fig. S7†).

Fig. 2 ¹H NMR spectra of HY1–HY3 in DMSO-D₆ solvent showing peak assignment due to different counterions.

The ESI-MS analyses of HY1–HY6 were performed in the negative ion mode. The sample solutions for these analyses were prepared by dissolving minimum amounts of the hybrids in DMSO, followed by diluting with acetonitrile to a final concentration of approximately 10⁻⁶ M. Negative-ion ESI-MS analyses of HY1, HY4, and HY6 revealed intense peaks at m/z 462.6262, 462.5788 and 462.5913 with 2⁻ charges, which are attributed to bare [V₁₀O₂₆]²⁻ units. Similarly, HY2, HY3, and HY5 exhibited intense peaks at m/z 462.6128, 462.7103, and 462.6941 with 1⁻ charge corresponding to the half cluster moiety [V₅O₁₃]¹⁻. Interestingly, all the hybrids showed envelopes corresponding to the mixed counterion cluster species, confirming the formation of the mixed counterion POM hybrids as expected. For example, the ESI-MS data of hybrids HY1–HY6 showed peaks at m/z 494.6133, 673.7867, 659.8673, 662.6035, 668.7777, and 676.7711 that are well matched to the mixed counterion species, [V₁₀O₂₈(FHPDS)(HPDS)₂(H₂O)₂]³⁻, [H₂V₁₀O₂₈(FHPDS)(FPDS)(H₂O)₂]²⁻, [H₂V₁₀O₂₈(FPDS)(HPDS)(H₂O)₂]²⁻, [H₂V₁₀O₂₈(DFHPDS)(HPDS)]²⁻, [H₂V₁₀O₂₈(DFHPDS)(FPDS)]²⁻, and [H₂V₁₀O₂₈(DFHPDS)(FHPDS)]²⁻, respectively. Therefore, the appearance of these peaks indicates the presence of mixed counterions in these hybrids. Detailed analyses of the negative-ion ESI-MS of the hybrids are given in Fig. S1–S6, ESI.† Further, the XPS analyses were conducted on HY1–HY6 to confirm the presence of organic counterions and the decavanadate cluster. Carbon C 1s was used as an internal reference to calibrate the XPS spectra. The XPS survey of HY1–HY6 showed peaks corresponding to vanadium, carbon, oxygen, and sulfur at their respective binding energies. Here, we have discussed the XPS data of only a representative hybrid, HY5, and the data of the rest of the hybrids are explained in Fig. S1–S6, ESI.†HY5 contains two counterions: DFHPDS bearing 2,6-diformyl phenol functional groups and FPDS bearing ‘–CHO’ functional group. To confirm the functional groups on the counterions in HY5, the carbon C 1s scan was deconvoluted. The C 1s peaks observed at 284.83, 285.49, and 286.25 were assigned to C=C/C–C, SP³C, and C–O, respectively. At the same time, the C 1s peaks observed at 287.29 and 289.08 eV for HY5 are due to the C=O groups on FPDS and DFHPDS.^13,52 The deconvoluted O 1s scan of HY5 showed peaks at 531.5 and 532.7 eV due to the presence of ‘C=O’ (of DFHPDS and FPDS) and ‘O–H’ (of DFHPDS) functionalities confirming the presence of both the counterions. Further, the deconvoluted O 1s scan of HY5 also showed peaks at 529.9 and 530.6 eV, corresponding to POM lattice oxygens.⁵² The deconvoluted V 2p scan of HY5 showed peaks at 517.29 eV and 524.70 eV, corresponding to V⁵⁺ 2p_3/2 and V⁵⁺ 2p_1/2, respectively. Similarly, the peaks observed at 516.06 eV and 523.17 eV are due to V⁴⁺ 2p_3/2 and V⁴⁺ 2p_1/2, respectively. Therefore, the XPS analyses revealed the presence of two different counterions along with the decavanadate cluster in the hybrids;⁵³ see Fig. 3.

Fig. 3 Representative XPS spectra of HY5 (a) C 1s; (b) O 1s; and (c) V 2p scans.

Single-crystal X-ray diffraction analyses

The formation of the mixed counterion hybrids HY1–HY6 was further confirmed by the single crystal X-ray analysis of a representative hybrid, HY3. The single crystals of HY3 were grown from the mother liquor by a slow evaporation method. The crystallographic data of HY3 are presented in Table S1, ESI.†HY3 crystallized in the monoclinic P2₁/c space group. The asymmetric unit consists of one-half of the decavanadate cluster and one unit each of the HPDS and FPDS counterions along with a water molecule, see Fig. 4. The aldehydic ‘C=O’ bond length of the FPDS counterion in HY3 is 1.157 Å. The V–O bond distances in HY3 agreed with the reported values for decavanadate-based hybrids.¹⁹ The S–C bond distances and C–S–C bond angles of HY3 are in the range of 1.773(5)–1.791(5) Å and 100.3(2)–105.30(19)°, respectively. As reported earlier, the C–S–C bond angles in HY3 (less than 109°) confirm that the sulfonium centers adopt a trigonal pyramidal geometry.⁵⁴

Fig. 4 Crystal structure of HY3. Color code: VO₆ – green octahedra; S – yellow; O – red; C – dark grey; H – black.

Our previous study has shown that the POM hybrids with mixed aryl sulfonium counterions bearing different functional groups can lead to intriguing self-assembled structures.¹² Therefore, we explored the morphological characteristics of HY1–HY6 using Field Emission Scanning Electron Microscopy (FE-SEM), taking ethanol as the dispersion medium. The hybrids were dispersed in ethanol and drop cast over a silicon wafer, followed by solvent evaporation at 60 °C. The hybrids HY1–HY6 showed different morphological features such as rods, spheres, sheets, etc., indicating different self-assembly properties as a function of composition. The differences in the self-assembled structures of HY1–HY6 indicate the existence of different interactions among the hybrids due to the presence of different functional groups on the counterions; see Fig. S10, ESI.† Further, we calculated the average particle sizes of HY1–HY6 using FE-SEM image analyses. The calculated average particle sizes were ∼373, 155, 448, 137, 197, and 130 nm, respectively, for HY1–HY6. These analyses show that the particle sizes of HY4–HY6 are comparatively smaller than those of HY1 and HY3, indicating larger surface areas; see Fig. S11, ESI.† Among HY4–HY6, HY5 has a uniform particle size distribution and is uniformly dispersed.

Catalytic oxidation of sulfides

To check the catalytic abilities of HY1–HY6 toward sulfide oxidation, we tested H₂O₂ mediated oxidation of dibenzothiophene (DBT) as a model reaction, as shown in Scheme 2. To optimize the reaction conditions, we first varied the experimental parameters, such as the catalyst dosage, reaction time, reaction temperature, and amount of oxidant. In each of these optimization reactions, HY5 was used as the catalyst, and the reaction products were identified and confirmed by NMR and gas chromatography (GC) analysis; see Table 1. Three different reaction temperatures were tested, i.e., 25 °C, 35 °C, and 45 °C, keeping the catalyst, substrate, and H₂O₂ amounts at 3 μmol, 0.246 mmol and 0.7 mmol, respectively. The results suggested that at 25 °C, the reaction proceeds only up to 71% in 2 hours under the above conditions. When the reaction temperature was increased to 35 °C and 45 °C (entries 3 and 4), the conversion reached 98% and 99% in 2 hours with 80% and 85% selectivity towards the sulfoxide derivative of DBT (i.e., DBTO), respectively. However, raising the reaction temperature further up to 55 °C resulted in no further increase in the conversion, probably due to the decomposition of H₂O₂ at the higher temperatures.⁵⁵ Accordingly, the reaction temperature was optimized as 45 °C. We also checked the influence of factors like the dosage of the catalyst (entries 7 and 8) and solvent types such as acetonitrile, methanol, isopropyl alcohol (IPA) and dichloromethane (entries 11, 12, 13 and 14) on this catalytic reaction. Changes in the catalyst dosage did not yield encouraging results. Out of the solvents tested, ethanol, IPA and acetonitrile (ACN) exhibited good to moderate conversions of 99%, 92% and 90%, respectively, for oxidizing DBT. In addition to the best conversion, ethanol as solvent also led to good product selectivity (85%) towards sulfoxide compared to IPA (60%) and ACN (75%). Therefore, the optimized reaction conditions for the oxidation of DBT were finalized as 120 minutes of reaction time and ethanol as a solvent^56–58 with the n_substrate : n_oxidant : n_catalyst mole ratio of 1 : 3 : 0.012. Under these optimized reaction conditions, the hybrids HY1–HY6 showed 10%, 79%, 75%, 94%, 99% and 98% conversion of DBT, respectively. Therefore, it can be noted that, among all the hybrids tested, hybrids HY4–HY6 showed maximum conversion of DBT in less than 120 min. We also tested the DBT oxidation using molecular oxygen as an oxidant instead of H₂O₂ in ethanol. The results showed that the reaction did not proceed well even after 24 hours of reaction time, probably because the oxidation using molecular oxygen requires high pressure and temperature, see Table 1 (entry 15).^36,59,60 To explore the kinetics of the DBT oxidation reactions using HY4–HY6 as catalysts, we calculated the reaction rate constants following the pseudo-first-order reaction kinetics; ln(C₀/C_t) = kt and ln k = −E_a/RT, where C₀ and C_t correspond to initial DBT concentration and DBT concentration at different reaction times (t); k and E_a represent the rate constant and the activation energy, respectively. The ln(C₀/C_t) vs. reaction time plots are shown in Fig. 5(a–c), and the corresponding data are presented in Table S3, ESI.† The activation energies (E_a) for the reactions using HY4–HY6 as the catalysts were calculated using the Arrhenius equation, which followed the order HY4 (19.76 kJ mol⁻¹) < HY5 (26.13 kJ mol⁻¹) < HY6 (30.02 kJ mol⁻¹), see Fig. 5d. Also, we calculated the turnover frequency (TOF) of these reactions²⁹ to identify the best catalyst, which followed the order HY5 (100 h⁻¹) > HY6 (75.8 h⁻¹) > HY4 (69.2 h⁻¹); Fig. S12, ESI.†

Scheme 2 Catalytic oxidation of DBT by using HY1/2/3/4/5/6 as the catalyst.

Fig. 5 (a), (b) and (c) Pseudo-first-order rate constant for DBT conversion at different temperatures using HY4–HY6; and (d) Arrhenius plots of HY4–HY6.

Table 1 Conversion and selectivity of the oxidation of DBT to DBTO in the presence of HY5 with H₂O₂ as the oxidant in ethanol^a

Entry	Catalyst (μmol)	H2O2 (mmol)	Time (h)	Temp. (°C)	Conv.^b (%)	Selectivity^c (%)
a General reaction conditions (unless otherwise mentioned): ethanol as the solvent, 1.5 mL; substrate, 0.246 mmol; HY5, 3 μmol. b The conversion of DBT was confirmed by GC and NMR analysis. c Selectivity (towards sulfoxide) calculated using the formula, sulfoxide (mmol)/(sulfoxides (mmol) + sulfone (mmol)). d Methanol (1.5mL) as solvent. e Acetonitrile (1.5mL) as solvent. f 2-Propanol (1.5mL) as solvent. g Dichloromethane (1.5mL) as solvent. h Reaction condition: ethanol, 1.5 mL; substrate, 0.246 mmol; HY5, 3 μmol, O2 balloon.
1	3	0.7	2	rt	71	40
2	3	0.7	4	rt	85	61
3	3	0.7	2	35	98	80
4	3	0.7	2	45	99	85
5	3	0.5	2	45	78	51
6	3	0.8	2	45	87	60
7	1.5	0.7	2	45	80	36
8	6	0.7	2	45	81	42
9	3	None	6	45	<1	<1
10	None	0.7	6	45	20	<5
11^d	3	0.7	2	45	78	42
12^e	3	0.7	2	45	90	75
13^f	3	0.7	2	45	92	60
14^g	3	0.7	2	45	10	<1
15^h	3	None	24	45	<1	<1

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Several control experiments were also performed to fully understand the catalytic roles of HY1–HY6 in the sulfide oxidation reaction. Firstly, the control experiments were performed using the counterion precursors of the hybrids, such as FPDST, HPDST, FHPDST, and DFHPDST, as the catalysts. Similarly, the decavanadate POM cluster having sodium counterions (Na₆[V₁₀O₂₈]) and a 1 : 1 mixture of the counterion precursors used to synthesize HY5 (i.e., DFHPDST & FPDST) were also tested as catalysts in separate control experiments. All the above control experiments showed only poor to medium catalytic efficiency (<1–48% conversion); see Table 2. The results of the control experiments show that the Na₆[V₁₀O₂₈] cluster or the counterion precursors alone as catalysts are not adequate to give the high conversion, as observed in the case of HY1–HY6 catalyzed reactions under the given conditions. These results, therefore, highlight the importance of the hybrids HY1–HY6 in the observed catalysis.

Table 2 Conversion and selectivity of the oxidation of DBT to sulfoxide using different catalysts with H₂O₂ as the oxidant in ethanol^a

Entry	Catalyst	Conv.^b (%)	Selectivity^c (%)	System
a Reaction conditions: ethanol as the solvent, 1.5 mL; substrate, 0.246 mmol; catalyst, 3 μmol; temperature, 45 °C. b The conversion of DBT was confirmed by GC and NMR analysis. c Selectivity (towards sulfoxide, unless otherwise mentioned) calculated using the formula, sulfoxide (mmol)/(sulfoxides (mmol) + sulfone (mmol)). d Selectivity (towards sulfone) calculated using the formula, sulfone (mmol)/(sulfoxides (mmol) + sulfone (mmol)). 1a ((DFHPDS)4[H2V10O28])^13 and 1b ((FPDS)4[H2V10O28])^11 are previously reported decavanadate hybrids containing only a single type of aryl sulfonium counterions. These hybrids are used as control compounds to understand the roles of mixed counterions in the present study.
1	HY1	10	<5	Heterogenous
2	HY2	79	29	Heterogenous
3	HY3	75	20	Heterogenous
4	HY4	94	55	Heterogenous
5	HY5	99	85	Heterogenous
6	HY6	98	60	Heterogenous
7	1a	99	81^d	Heterogenous
8	1b	85	80	Heterogenous
9	Na6V10O28	48	10	Heterogenous
10	NaVO3	<15	<10	Heterogenous
11	DFHPDST	<1	<1	Homogeneous
12	FHPDST	<1	<1	Homogeneous
13	FPDST	<1	<1	Homogeneous
14	HPDST	<1	<1	Homogeneous
15	(DFHPDST : FPDST)	<1	<1	Homogeneous

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The catalytic activities of HY1–HY6 followed the order HY5 > HY6 > HY4 > HY2 > HY3 > HY1 in terms of percentage of conversion. Overall, HY4–HY6 gave comparatively better catalytic conversion of DBT (i.e., 94–99% conversion) than HY1–HY3 (10–75% conversion). To get an idea about the better catalytic efficiencies shown by HY4–HY6 compared to HY1–HY3, we used the Brunauer–Emmett–Teller (BET) method to calculate their surface areas using nitrogen adsorption–desorption isotherm (see Fig. S19, ESI†). The calculated surface areas varied for all the hybrids, where HY4–HY6 showed significantly higher values of 79.95, 19.31, and 12.80 m² g⁻¹, respectively, compared to HY1 (6.32 m² g⁻¹) and HY3 (6.88 m² g⁻¹). The only exception here was HY2, which had a calculated surface area of 15.43 m² g⁻¹, similar to HY5 and HY6, but exhibited comparatively poor catalytic activity. It is important to note here that while surface area can impact the activity of a catalyst, it is not always the case that a higher surface area leads to better catalytic efficiency. Several other structure–activity factors of the catalyst and substrate can also significantly determine the catalytic activity.⁶¹ Again, to get a deeper insight into the better catalytic efficiency shown by HY4–HY6, XPS analyses were performed using V 2p scans of HY1–HY6. XPS data revealed that the percentage of V⁴⁺ in these hybrids followed the order HY4 (24.1%) > HY5 (23.7%) ≈ HY6 (23.8%) > HY3 (10.8%) > HY2 (9.8%) > HY1 (7.1%), see Fig. 6. These data revealed that the hybrids HY4–HY6 possess a comparatively higher percentage of V⁴⁺ than HY1–HY3.

Fig. 6 Deconvoluted V 2p scans of (a) HY1; (b) HY2; (c) HY3; (d) HY4; (e) HY5; and (f) HY6.

Cyclic voltammetry (CV) analyses of HY1–HY6 were performed to determine their electrochemical properties in a heterogeneous medium; see Fig. S21(a) and (b), ESI.† All the electrochemical experiments were conducted using a three-electrode electrochemical cell, which employed a glassy carbon (GC) working electrode, Ag/AgCl (3 M) reference electrode, and a Pt wire counter electrode. HY5 was used as a model compound to optimize the electrochemical conditions, i.e., the pH and the purged gas (N₂/O₂). Firstly, the CV analysis of HY5 was conducted by bubbling N₂ and O₂ gas separately in the electrolyte solution to investigate the influence of oxygen on the electrochemical properties of these materials, which are relevant in catalytic and oxidative environments. We found that in the presence of O₂, current density increases two times more than N₂, confirming that the presence of O₂ helps to regulate electron flow in the system; see Fig. 7(b). Interestingly, when we repeated the experiment with HY1, we observed no change in the current density, regardless of the presence or absence of O₂, see Fig. S20, ESI.† Based on the above observation, we can conclude that HY5, which has higher V⁴⁺ centers than HY1 (as confirmed by XPS analyses), can produce more current in the presence of O₂. This is because O₂, as an oxidizing agent, can oxidize the reduced species present in the system, thus helping to regulate electron flow more effectively. Again, we optimized the pH under the O₂ atmosphere by using 0.1 M Na₂SO₄ as an electrolyte at two different pH conditions: (i) catalytic medium (pH = 3.5) and (ii) neutral (pH = 7). However, we did not use the basic condition as decavanadate is unstable under these conditions.⁶² At pH 3.5, we observed more distinct peaks, corresponding to the V⁵⁺/V⁴⁺ couple for three vanadium centers in the decavanadate cluster, compared to pH = 7, see Fig. 7(a). Therefore, we analyzed the CV of HY1–HY6 in the scan range of +1.0 V to −0.8 V at pH 3.5 and in the presence of O₂. All the hybrids, HY1–HY6 showed three quasi-reversible oxidation potentials at E = +0.23, 0.41 and 0.63 V, respectively, that may be attributed to the oxidation–reduction steps of vanadium.⁶³

Fig. 7 Cyclic voltammograms (CVs) of (a) HY5 in pH 3.5 and 7; (b) HY5 in N₂ or O₂. Differential pulse voltammograms (DPVs) of HY1–HY6 (c) anodic scan in 0.1 M Na₂SO₄ at pH 3.5. All the CVs were recorded at the scan rate of 50 mV s⁻¹.

Again, differential pulse voltammetry (DPV) analyses of HY1–HY6 were performed in an acidic 0.1 M Na₂SO₄ solution (pH 3.5) bubbled with O₂ to establish a connection between their catalytic activity and the oxidation potentials, see Fig. 7(c). The oxidation potential of V⁵⁺/V⁴⁺ in HY1–HY3 shifts to a higher oxidation potential in HY4–HY6. This indicates that the oxidation of V⁴⁺ in HY4–HY6 is much easier than in HY1–HY3, increasing the likelihood of the formation of vanadium peroxo species and resulting in higher catalytic activity.

Therefore, the XPS analyses showed a higher percentage of V⁴⁺ in HY4–HY6 than in HY1–HY3, which is supported by the CV experiment. According to literature reports, often in vanadium-based catalytic oxidation reactions using H₂O₂, the vanadium (V⁴⁺) gets oxidized by H₂O₂, forming vanadium peroxo-species that act as active catalysts for oxidation reactions.^64,65 This peroxovanadium then leads to the oxidation of sulfide/DBT and the peroxovanadium species is restored by reforming the catalyst for the next cycle, as previously reported, see Fig. S17, ESI.†¹³ Therefore, in POV-based hybrids, forming the reversible redox pair V⁵⁺/V⁴⁺ is one of the key factors for the reaction to proceed. Therefore, a greater % of V⁴⁺ species in the hybrid can contribute to a greater number of catalytically active sites for sulfide oxidation reactions. Further, the EPR analyses of the isolated HY5 indicated the presence of some V⁴⁺, showing eight-line EPR spectra with a g value of 2.01; see Fig. S23, ESI.† Depending on the interactions with the POM cluster, the counterions can modulate the redox potential of the cluster in the hybrid, which can further affect the V⁵⁺/V⁴⁺ conversion and the catalysis rate.⁶⁶ Our earlier studies have shown that the DFHPDS counterions bearing a 2,6-diformyl phenol moiety can help in reducing the decavanadate cluster by providing acidic protons more readily.¹³ Therefore, DFHPDS counterions in HY4–HY6 could be a major factor behind their better catalytic properties compared to hybrids HY1–HY3, which do not contain any DFHPDS counterions. Among HY4–HY6, the percentage of V⁴⁺ in their composition was almost similar, and they showed almost similar conversion of DBT and the small variations in the catalytic activities among HY4–HY6 could be due to the influence of the second counterions bearing different functional groups.

Based on the above discussions, we believe that several factors, such as the redox behaviors of the hybrid under catalytic conditions as indicated by the percentage of V⁴⁺ in XPS analyses, the particle size and dispersibility of the catalyst, the surface area of the catalyst, etc. could be playing roles in determining the catalytic trend shown by the hybrids HY1–HY6 towards sulfide oxidation. However, more detailed theoretical and experimental analyses are required to fully understand the roles and reduction status of the catalyst in the catalytic oxidation reactions.

Selectivity of sulfide oxidation reaction

The oxidation of sulfides can lead to either sulfoxide or sulfone products, depending upon the reaction conditions. As mentioned earlier, sulfoxides are important precursors for synthesizing chiral³¹ and biological compounds.³⁰ POVs can act as sustainable redox catalysts in oxidative desulfurization reactions by oxidizing thiophene-based sulfides, including DBT, DMDBT and BT, found in fuel oils. Several studies have reported using vanadium-containing POM-based catalytic systems to oxidize DBT. However, many of those studies are concerned only about the removal or conversion of the DBT, not the selectivity of conversion.^33,67,68 A few reports have focussed on conversion and selectivity, but most report selectivity toward the sulfone derivative.¹³ On the contrary, the DBT oxidation data in Table 2 obtained using HY1–HY6 as catalysts show selectivity toward sulfoxide products in general. HY4–HY6 gave better selectivity towards sulfoxides (55–85%) than HY1–HY3, which gave a selectivity of (<5–29%) towards sulfoxides. Among all the hybrids, HY5 gave the best catalytic activity regarding conversion (99%) and selectivity (85%) toward sulfoxide. Since the POM cluster (decavanadate cluster) is the same in all the hybrids and the experimental conditions employed were also the same, a major reason behind the observed differences in the catalytic selectivity could be the differences in the organic counterions and their effects in HY1–HY6. To check if the nature of the ASCs present in the hybrid catalyst has any role in determining the product selectivity of the sulfide oxidation reaction, we performed some additional control experiments as follows. As the hybrid HY5 having the counterions DFHPDS and FPDS gave the best catalytic conversion and selectivity, we decided to perform control experiments using the hybrids having only one type of these counterions as catalysts. Accordingly, we synthesized two control compounds, (DFHPDS)₄[H₂V₁₀O₂₈] (1a)¹³ and (FPDS)₄[H₂V₁₀O₂₈] (1b),¹¹ and tested their catalytic activities toward the oxidation of DBT under the optimized reaction conditions. These control experiments were conducted to understand if using mixed counterions (i.e., a 1 : 1 combination of DFHPDS and FPDS in HY5) has any role in determining its catalytic properties. The results of these experiments suggested that when 1a (having only DFHPDS counterions) was used as the catalyst, the product selectivity was toward sulfone (up to 81%) rather than sulfoxide. However, when 1b (having only FPDS counterions) was used as the catalyst, the selectivity was toward sulfoxide (up to 80%). Therefore, under the experimental conditions, contradicting selectivities were obtained with hybrids with DFHPDS and FPDS counterions. Therefore, we can say that the nature of the aryl sulfonium counterions in HY1–HY6 play significant roles in controlling their catalytic properties, especially the conversion and selectivity; see Table 2.

Various experimental parameters, such as the solvent system, oxidant amount and oxidant type, can affect the selectivity of oxidative desulfurization reactions.⁵⁶ It has also been shown that soft interactions, such as hydrogen bonding interaction between the catalyst and the substrate, play a match-winning role in stabilizing the catalyst's transition state, affecting the catalysis reaction's selectivity.⁶⁹ Besides, it is well accepted that the reversible redox potential of POM hybrids can be manipulated based on the soft interaction, such as intramolecular charge transfer between POM and the counter-cationic part, leading to selectivity in oxidative catalysis reaction.²² The electronic and H-bonding effects of the functional groups on the aryl sulfonium counterions could play significant roles in determining the outcome of the reaction in the present case. HY4–HY6 contains DFHPDS as one of the counterions, but the 2^nd counterion varies in these hybrids. The organic functional groups on the 2^nd counterion vary from a simple phenol to aldehyde to salicylaldehyde moieties. It may be noted that the presence of such different functional groups on the catalyst could influence the interactions between the catalyst and the substrate and control the outcome of the reaction. Recently, Suzuki et al. reported that the presence of methyl ethyl ketone (MEK) solvent bearing a C=O group in the reaction medium influences the catalytic outcome of sulfoxidation reaction using a POV-based hybrid as the catalyst.⁷⁰ Therefore, the different functional groups on the aryl sulfonium counterions in HY1–HY6 could be playing different electronic and weak bonding interactions in the reaction media that affect their overall catalytic efficiency and selectivity. More detailed studies are, however, required to fully understand such an effect.

Considering the promising catalytic activity of HY5 toward selective sulfide oxidation, we wanted to check its wider applicability as a catalyst using various thiophene and thioether-based substrates; see Table 3. Substrates with bulky substituents, such as diphenyl sulfide, were converted up to 98% with 70% selectivity towards sulfoxide (entry 1). In the present study, HY5 was able to convert the thiophene-based sulfides, dibenzothiophene (up to 99% (entry 2)), benzothiophene (BT, up to 30% (entry 3)), and 4,6 dimethyl benzothiophene (up to 25% (entry 4)) to the corresponding sulfoxides. Here, BT has the least electron density on sulfur and showed a very low conversion rate. In the case of 4,6 dimethyl benzothiophene, the low conversion may be attributed to the steric effects of the two methyl groups besides the sulfur.⁷¹ Overall, it is concluded that HY5 can convert a variety of sulfides into their corresponding sulfoxides at 45 °C with impressive selectivity. Given this, the catalytic performance of HY5 is quite different than the reported catalysts such as C₈V/g-BN,⁴⁶ [C₈H₁₇N(CH₃)₃]H₃V₁₀O₂₈,⁷² which showed selectivity towards sulfone and [(C₂N₂H₈)₄(CH₃O)₄V^IV₄V^V₄O₁₆]·4CH₃OH (V₈-1),⁷³ which yielded sulfones and sulfoxides with less selectivity.

Table 3 Results of catalytic oxidation of different types of thiophene-based sulfides by HY5 as a catalyst with 30% of H₂O₂^a

Entry	Substrate	Time (min)	Conversion^b (%)	Selectivity^c (%)
a Reaction conditions: ethanol, 1.5 mL; substrate, 0.246 mmol; HY5, 3 μmol. b The conversion of DBT was confirmed by GC and NMR analysis. c Selectivity for sulfoxide = sulfoxide (mmol)/(sulfone (mmol) + sulfoxide (mmol)).
1		60	98	70^c
2		60	99	85^c
3		240	30	85^c
4		240	25	80^c

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Stability and recyclability of the catalyst

The stability and recyclability of HY1–HY6 as catalysts towards the oxidation of DBT were checked using a model catalyst, HY5, as it exhibited the highest catalytic activity among all the hybrids tested in this study. To confirm that the catalyst is stable under the reaction conditions and there is no leaching of vanadium, a thermal filtration experiment was performed in which the catalyst was filtered out from the reaction medium after 8 minutes, i.e., after 60% conversion of the reactants, and the reaction was allowed to continue further. It can be seen from Fig. 8(a) that there is only a negligible amount of substrate conversion after the removal of the catalyst. This confirms that there is no considerable vanadium leaching into the reaction medium during the oxidation process. After five consecutive catalytic cycles, the percentage of DBT conversion decreased by less than 2% and selectivity decreased by only 5%, again showing that vanadium leaching is negligible in the present case. In general, some amount of metal leaching has been noted for POM-based catalysts in the literature during catalytic oxidation reactions.^68,69

Fig. 8 (a) Catalytic dynamics (blue) and thermal filtration (red) of HY5; (b) ESI-MS analysis of the recovered catalyst; and (c) recyclability of catalyst (HY5) up to five cycles.

To check the recyclability of the catalyst HY5, it was recovered after five cycles of catalysis and subjected to ¹H NMR, FT-IR, ESI-MS, and XPS analyses. The ¹H NMR peaks of the recovered HY5 (see Fig. S24, ESI†) matched that of the original HY5, suggesting no compositional change after catalysis. The FT-IR data showed that the stretching frequencies of the recovered catalyst match those of the fresh catalyst (Fig. S14, ESI†). Similarly, the ESI-MS data of the recovered catalyst in negative ion mode also showed the intactness of HY5 after catalysis; see Fig. 8(b). Further, C 1s, O 1s and S 2p XPS scans of the recovered catalyst indicated there was no oxidation of the aldehyde functional group and the sulfonium center present in the counterion, discarding the possibility of any oxidation of the counterion moieties during catalysis. All these observations confirmed that the recovered catalyst remained stable; see Fig. S15, in ESI.† There was also no considerable decrease in the catalytic conversion after five cycles, which again proved the high recyclability of the catalyst.

Catalytic degradation of mustard gas simulant

Detoxification of hazardous chemical warfare agents (CWA) under mild conditions is extremely important in the current scenario of terrorist attacks and combats occurring in different parts of the world.^74,75 CWAs often cause damage to skin, lungs, eyes, and DNA. Considering the high toxicity and dangers of handling CWAs, experimentalists often use CWA simulants with similar structures and chemical behaviors for their studies. 2-Chloroethyl ethyl sulfide (CEES) is a promising and commonly used simulant for bis-(2-chloroethyl)sulfide (mustard gas or HD). Taking advantage of the better catalytic sulfide oxidation activity of HY4–HY6, we wanted to explore their potential to degrade CEES.^76–78 Most CWA detoxification studies using CEES simulant use O₂ (oxygen) as the oxidant, which requires a longer reaction time (∼1 hour).⁷⁹ A lesser reaction time is possible with H₂O₂ as an oxidant, but it may also generate highly toxic sulfone as a side product along with the poorly toxic sulfoxide. Therefore, catalysts that can control the selectivity of CEES oxidation reaction towards sulfoxide in the presence of H₂O₂ is highly desirable.⁸⁰HY4–HY6 were tested for the catalytic oxidation of CEES by using 3% H₂O₂ (diluted with water) as the oxidant; see (Scheme 3). Here, we used 0.246 mmol CEES as the substrate, 1.5 mL of ethanol as the solvent and 0.492 mmol of 3% H₂O₂ as the oxidant at 25 °C. The progress of the reaction was monitored by gas chromatography and ¹H NMR analyses. Within 30 min of the reaction, hybrids HY4, HY5 and HY6 catalyzed the conversion of CEES to its sulfoxide in 81%, 99.5%, and 87% yields with turnover numbers (TON) of 66.42, 81.59 and 71.34, respectively. Among the hybrids tested, HY5 gave the best catalytic activity. HY5 catalyzed the conversion of CEES to its sulfoxide in 85% and 99.5% yields in 5 and 10 min of the reaction, respectively; see Fig. 9. Further, the reaction kinetics of the catalytic detoxification of CEES was investigated using HY4–HY6 as catalysts under the optimized reaction conditions at 25 °C, and the conversion rates were calculated as a function of time. The ln(C₀/C_t) vs. time plot showed a linear correlation for all hybrids, indicating first-order kinetics. The first order rate constants k₁, k₂ and k₃ calculated for HY4, HY5 and HY6 are 0.021, 0.092 and 0.012 min⁻¹, respectively (Fig. S16, ESI†). Therefore, we can see that all these hybrid catalysts showed more or less similar catalytic behaviors, but HY5 showed comparatively better activity in terms of the conversion rate.

Fig. 9 Selected region of stacked ¹H NMR spectra of CEES, after 2, 5, 10 and 20 min of the reaction product by using CDCl₃ as a solvent.

Scheme 3 Catalytic oxidation of CEES to CEESO using HY4–6 as a catalyst.

Conclusions

A mixed counterion strategy used two different aryl sulfonium counterions (ASCIs) bearing different organic functional groups in a 1 : 1 synthetic feed ratio to generate a series of decavanadate cluster-based hybrids for the first time. The use of different ASCIs allowed the facile introduction of different organic functional groups, such as phenol/aldehyde/salicylaldehyde/2,6-diformyl phenol, onto a POM hybrid. The use of mixed counterions bearing different functional groups has proven to be a powerful tool for controlling the supramolecular interactions and redox properties of the decavanadate hybrids, which in turn controlled their catalytic properties towards the selective oxidation of thiophene-based-sulfides to sulfoxides. In general, hybrids HY4–HY6 having DFHPDS (ASCI with 2,6-diformyl phenol functional group) as one counterion showed better catalytic conversions than other hybrids. Hybrid HY5 yielded 99% conversion and 85% selectivity towards sulfoxides in dibenzothiophene (DBT) oxidation. The ability of the counterions to control the V⁵⁺/V⁴⁺ redox status and the supramolecular interactions of the cluster with the substrates played significant roles in controlling the catalytic conversion and selectivity in this study. Further, the organic counterions also helped to modulate the particle size and dispersibility of the catalysts, changing their surface area and affecting catalytic properties. In short, we have developed a series of mixed counterion incorporated decavanadate-based organic–inorganic hybrid materials exhibiting efficient catalytic properties towards the oxidation of thiophene-based sulfides and detoxification of mustard gas simulant. This strategy may be explored with different POM clusters and similar anionic clusters to generate a new series of selective catalysts for organic transformations. However, more detailed studies, including computational studies, are required to fully understand the selectivity switching observed here due to changes in the organic counterions of the hybrid catalysts.

Experimental section

Materials and method

Hexamethylenetetramine (HMTA) and trifluoro acetic acid (TFA) were purchased from Loba chemicals and Avra chemicals, respectively. 4-(Hydroxy) mercapto phenol, 4-(thiomethyl) benzaldehyde, dibenzothiophene 5,5′ dioxide, benzothiophene, and diphenyl sulfone were purchased from TCI chemicals. Dibenzothiophene, 4,6 dimethyl benzothiophene and silver trifluoromethane sulfonate were purchased from Sigma-Aldrich, and NaVO₃·2H₂O (sodium metavanadate) was purchased from Loba Chemicals. All the solvents used were of AR grade, and acetonitrile was distilled over CaH₂ before use.

Physical measurements

¹H, ¹³C and ¹⁹F NMR spectra were recorded on Jeol-JNM 500 MHz NMR spectrometer using CDCl₃/D₂O/DMSO-d₆ solvents and TMS as internal standard. FT-IR spectra were recorded on Agilent Technology Cary 600 series instrument. ESI-MS spectra were recorded on Bruker HD compact instrument. The X-Ray Photoelectron Spectroscopy (XPS) of the hybrids were recorded on a Thermo Scientific NEXSA photo-emission spectrometer using Al-Kα (1486.6 eV) X-ray radiation. The raw data obtained from the instrument were processed using Avantage software. CHNS(O) elemental analysis was carried out in a Thermo Finnigan FLSH EA 1112 elemental analyzer. Inductively coupled plasma–mass spectrometry (ICP–MS) measurements were carried out using an Agilent 7850 LC-ICP-MS instrument after digesting the samples in concentrated nitric acid. Thermogravimetric analyses (TGA) were performed by using PerkinElmer Pyris 1 instrument. The sample (2 mg) was heated under a nitrogen atmosphere from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ with a flow rate of 20 mL min⁻¹ in all of the TGA experiments. The X-ray powder diffraction data were recorded on a Rigaku SmartLab 9 kW rotating anode X-ray diffractometer in Bragg–Brentano configuration using a Cu-sealed tube (Cu-Kα X-rays of 0.1541 nm) operating at 45 kV and 100 mA. The measurements were taken in the 2θ range of 3–50° with a scan rate of 2° min⁻¹ and a step size of 0.02°. The samples’ morphological characterizations were performed using field emission scanning electron microscopy (FESEM, FEI Nova Nano SEM-450). Electron paramagnetic resonance (EPR) spectra were recorded at a temperature of 6 K using a Bruker ELEXSYS 580 EPR spectrometer.

Electrochemical measurements

Electrochemical experiments were conducted using a standard three-electrode cell. A catalyst-coated glassy carbon electrode (GCE) was used as the working electrode, Ag/AgCl (3 M) as the reference electrode, and Pt-wire as the counter electrode. The electrolytes used were 0.1 M H₂SO₄ aqueous solutions under an oxygen (O₂) atmosphere. To prepare the sample-modified electrodes, 4 mg of the compound, 1 mg of acetylene black carbon, and 20 μL of Nafion (aq) binder were mixed in 1 mL of isopropyl alcohol (IPA)/water mixture (4 : 1; v/v) and sonicated until a homogeneous suspension was formed. 20 μL of this mixture was coated onto the GCE using the drop-casting method, and the GCE was then dried under an IR lamp (temperature ∼70 °C). All electrochemical experiments were conducted at ambient temperature.

X-ray crystallography

Single crystal X-ray diffraction data of HY3 were collected on an Agilent SuperNova diffractometer equipped with multilayer optics, monochromatic dual source (Cu and Mo), and Eos CCD detector, using Mo Kα (0.71073 Å) radiation at 293 K. Data acquisition, reduction, and analytical face-index-based absorption correction were performed by using CrysAlisPRO program.⁸¹ The structures were solved with ShelXS and refined on F² by full-matrix least-squares techniques using the ShelXL program provided in Olex² (v.1.2) program package.^81,82 Anisotropic displacement parameters were applied for all the atoms, except hydrogen atoms and some less intensely scattered carbon atoms. CCDC No: 2346413† contains supplementary crystallographic data.

Gas chromatography (GC) analyses

Gas chromatography analyses were performed on an Agilent 7890A series gas chromatograph equipped with an HP-5 column. A flame ionization detector (FID) and N₂ carrier gas were used for the sample analyses. Agilent open lab control panel software A.01.05 (1.3.19.115) was used to analyze the results and operate the GC system.

General procedure for the synthesis of HY1–HY6

The synthesis of a model hybrid, HY1, having the counterions FHPDS and HPDS, is described in detail here.

NaVO₃·2H₂O (0.5 g, 4.10 mmol) was dissolved in 20 mL of distilled water and heated to 80 °C for a clear, colourless solution. 4 M HCl was added dropwise to the reaction mixture to adjust the pH to 4.5. The color of the solution was changed to orange during this. Parallely, the counterion precursors FHPDST  and HPDST were mixed in 1 : 1 molar ratio in acetonitrile to prepare a mixed counterion solution. This mixed counterion solution was then added dropwise into the decavanadate solution, leading to the formation of an instant yellow precipitate, which was stirred at room temperature for 12 h. After completion of the reaction, the reaction mixture was filtered and the precipitate obtained was washed with water (H₂O), ethanol (EtOH) and diethyl ether (Et₂O) several times and dried under vacuum at 40 °C to get a yellow-colored powder as the product.

Other hybrids HY2–HY6 were also synthesized in a similar manner, using different counterion precursor mixtures FHPDST : FPDST/FPDST : HPDST/DFHPDST : HPDST/DFHPDST : FPDST/DFHPDST : FHPDST, respectively. The control catalysts, 1a and 1b, were synthesized using the methods reported earlier.^11,13

(FHPDS)₂(HPDS)₂[H₂V₁₀O₂₈] (HY1). Yield: 30% based on V. Anal. calc. for HY1·H₂O: C, 24.68%; H, 2.92%; S, 7.75%; V, 30.79%; found: C, 24.31%; H, 2.7%; S, 7.32%; V, 31.19%. ¹H NMR (500 MHz, DMSO-d₆): FHPDS⁺ cations, δ ppm 10.13 (s, 1H, Ar–CHO), 7.98 (s, 1H, Ar–H), 7.84 (s, 1H, Ar–H), 6.85 (s, 1H, Ar–H), 3.18 (s, 6H, –S(CH₃)₂); HPDS⁺ cations, δ ppm 7.87 (d, 2H, J = 10 Hz, Ar–H), 7.01 (d, 2H, J = 10 Hz, Ar–H), 3.12 (s, 6H, –S(CH₃)₂). FT-IR (cm⁻¹): 3019 (w), 2923 (w), 1681 (sh), 1649 (sh), 1597(w), 1577(w), 1500(w), 1432 (w), 1228 (m), 1185 (m), 1087 (m), 1043 (m), ν (V=O, V–O–V), 963 (sh), 836 (sh), 750 (sh), 540 (m), 442 (w). ⁵¹V NMR (105.2 MHz, DMSO-D₆): δ ppm −418.67, −499.41, −522.87.

(FHPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY2). Yield: 20% based on V. Anal. calc. for HY2·H₂O: C, 25.76%; H, 2.88%; S, 7.64%; V, 30.35%; found: C, 25.42%; H, 2.521%; S, 7.22%; V, 30.62%. ¹H NMR (500 MHz, DMSO-d₆): FHPDS⁺ cations, δ ppm 10.17 (s, 1H, Ar–CHO), 8.03 (s, 1H, Ar–H), 7.92 (s, 1H, Ar–H), 6.94 (s, 1H, Ar–H), 3.15 (s, 6H, –S(CH₃)₂); FPDS⁺ cations, δ ppm 10.09 (s, 1H, Ar–CHO), 8.27 (d, 2H, J = 10 Hz, Ar–H), 8.17 (d, 1H, J = 10 Hz, Ar–H), 3.34 (s, 6H, –S(CH₃)₂). FT-IR (cm⁻¹): 3012 (w), 2917 (w), 1702 (sh), 1680 (sh), 1654 (sh), 1595(w), 1572(w), 1474 (w), 1229 (m), 1182 (m), 1087 (m), 1043 (m), ν (V=O, V–O–V), 970 (sh), 833 (sh), 765 (sh), 535 (m), 450 (w). ⁵¹V NMR (105.2 MHz, DMSO-D₆): δ ppm −412.43, −499.88, −524.23.

(HPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY3). Yield: 28% based on V. Anal. calc for HY3·H₂O: C, 25.17%; H, 2.98%; S, 7.91%; V, 31.40%; found: C, 24.85%; H, 2.79%; S, 7.51%; V, 31.68%. ¹H NMR (500 MHz, DMSO-d₆): HPDS⁺ cations, δ ppm 10.63(s, 1H, Ar–OH), 7.88–7.86 (d, 2H, J = 10 Hz, Ar–H), 7.02–7.00 (d, 2H, J = 10 Hz), 3.18 (s, 6H, –S(CH₃)₂); FPDS⁺ cations, δ ppm 10.09 (s) and 9.88 (s) (1H, Ar–CHO), 8.29–8.18 (dd, J = 10 Hz), 7.79–7.77 (d, J = 10 Hz), 7.41–7.40 (d, J = 5 Hz) (4H, Ar–H), 3.34 (s, 6H, –S(CH₃)₂). FT-IR (cm⁻¹): 3357(w), 3083 (w), 3000 (w), 2917 (w), 1692 (sh), 1670 (w), 1632 (w), 1594 (w), 1573 (w), 1431 (w), 1228 (m), 1175 (m), 1090 (m), 1043 (m), ν (V=O, V–O–V), 970 (sh), 833 (sh), 765 (sh), 557 (m), 445 (w). ⁵¹V NMR (105.2 MHz, DMSO-D₆): δ ppm −421.46, −503.65, −521.40.

(DFHPDS)₂(HPDS)₂[H₂V₁₀O₂₈] (HY4). Yield: 22% based on V. Anal. calc. for HY4·3H₂O: C, 24.76%; H, 3.0%; S, 7.34%; V, 29.17%; found: C, 24.23%; H, 3.15%; S, 6.89%; V, 29.63%. ¹H NMR (500 MHz, DMSO-d₆): HPDS⁺ cations, δ ppm 10.71 (s, 1H, Ar–OH), 7.87 (d, 2H, J = 10 Hz, Ar–H), 7.01 (d, 2H, J = 10 Hz, Ar–H), 3.18 (s, 6H, –S(CH₃)₂); DFHPDS⁺ cations, δ ppm 10.17 (s, 2H, Ar–CHO), 8.17 (s, 2H, Ar–H) 3.13 (s, 6H, –S(CH₃)₂). FT-IR (cm⁻¹): 3017 (w), 2921 (w), 1691 (w), 1639 (m), 1588 (w), 1531 (m), 1448 (w), 1231 (m), 1106 (w), 1090 (w), 1048 (w), ν (V=O, V–O–V), 963 (sh), 838 (sh), 750 (sh), 556 (m), 453 (w). ⁵¹V NMR (105.2 MHz, DMSO-D₆): δ ppm −420.13, −502.31, −524.21.

(DFHPDS)₂(FPDS)₂[H₂V₁₀O₂₈] (HY5). Yield: 28% based on V. Anal. calc. for HY5·3H₂O: C, 25.78%; H, 2.96%; S, 7.24%; V, 28.77%; found: C, 26.11%; H, 2.94%; S, 7.49%; V, 28.45%. ¹H NMR (500 MHz, DMSO-d₆): FPDS⁺ cations, δ ppm 10.09 (s, 1H, Ar–CHO), 8.27 (d, 2H, J = 10 Hz, Ar–H), 8.17 (d, 2H, J = 10 Hz, Ar–H), 3.34 (s, 6H, –S(CH₃)₂); DFHPDS⁺ cations, δ ppm 10.15 (s, 2H, Ar–CHO), 8.11 (s, 2H, Ar–H) 3.11 (s, 6H, –S(CH₃)₂). FT-IR (cm⁻¹): 3012 (w), 2926 (w), 1695 (w), 1643 (m), 1588 (w), 1531 (m), 1447 (w), 1234 (m), 1103 (w), 1046 (w), ν (V=O, V–O–V), 964 (sh), 838 (sh), 746 (sh), 575 (m), 459 (w). ⁵¹V NMR (105.2 MHz, DMSO-D₆): δ ppm −420.24, −501, −522.84.

(DFHPDS)₂(FHPDS)₂[H₂V₁₀O₂₈] (HY6). Yield: 31% based on V. Anal. calc. for HY6·3H₂O: C, 25.32%; H, 2.91%; S, 7.12%; V, 28.26%; found: C, 24.91%; H, 2.78%; S, 6.74%; V, 28.61%. ¹H NMR (500 MHz, DMSO-d₆): DFHPDS⁺ cations, δ ppm 10.16 (s, 2H, Ar–CHO), 8.02 (s, 2H, Ar–H), 3.09 (s, 6H, –S(CH₃)₂); FHPDS⁺ cations, δ ppm 10.16 (s, 1H, Ar–CHO), 8.23 (s, 1H, Ar–H), 8.13 (s, 1H, Ar–H), 7.22 (s, 1H, Ar–H), 3.21 (s, 6H, –S(CH₃)₂). FT-IR (cm⁻¹): 3007 (w), 2926 (w), 1687 (w), 1644 (m), 1590 (w), 1533 (m), 1448 (w), 1234 (m), 1102 (w), 1048 (w), ν (V=O, V–O–V), 963 (sh), 831 (sh), 746 (sh), 557 (m), 457 (w). ⁵¹V NMR (105.2 MHz, DMSO-D₆): δ ppm −421.46, −500.57, −521.91.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

C. P. P. thanks SERB, DST, Govt. of India for financial support (project no. CRG/2023/003151) and AMRC, IIT Mandi for infrastructural facilities. K. R. thanks the Council of Scientific and Industrial Research [CSIR SRF: 09/1058(0019)/2019-EMR-I] for a fellowship. K. R. thanks Suman Dolai for the assistance he provided in EPR experimental analysis. Rajesha Kumar Swain is sincerely thanked for his assistance in this study.

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Footnote

† Electronic supplementary information (ESI) available: Characterization data of all hybrids, details of catalytic oxidation of thiophene-based sulfide (PDF). CCDC 2346413. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt00102a

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