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Published in final edited form as: ACS Catal. 2024 Jun 5;14(12):9323–9327. doi: 10.1021/acscatal.4c01979

Selective and efficient detoxification of sulfur mustard gas analogues with H2O2 using bioinspired Mo and W dithiolene catalysts

Beria Tanriover 1, S M Supundrika Subasinghe 1, Neal P Mankad 1,*
PMCID: PMC12443344  NIHMSID: NIHMS2070749  PMID: 40969405

Abstract

Efficient and selective decomposition of chemical warfare agents (CWAs) is required to cope with threats from accidental or intentional releases from stockpiles. One of the most stockpiled CWAs is sulfur mustard (SM) gas. The most effective way to detoxify stockpiled SM is to oxidize the thioether functionality to its benign sulfoxide (SMO) state. However, overoxidation to the corresponding sulfone (SMO2), itself a potent toxin, should be avoided. Thus, catalysts for SM detoxification must be precisely tuned to promote the sluggish oxidation of SM while avoiding overoxidation of SMO to SMO2. In this study, Mo and W dithiolene catalysts, [MO2(dithiolene)2]2- (M = Mo or W), inspired by the active site structures of oxotransferase enzymes such as DMSO reductase were used as catalysts for oxidation of the SM research analogue, 2-chloroethyl ethyl sulfide (CEES), with aqueous H2O2 as an oxidant. Under optimized conditions, [WO2(mnt)2]2- and [MoO2(bdt)2]2- (mnt = maleonitriledithiolate, bdt = 1,2-benzenedithiolate) were found to promote selective CEES oxidation to sulfoxide CEESO without overoxidation to sulfone CEESO2 in as little as 5–15 min with catalyst loadings as low as 0.015 mol%. The W (pre-)catalyst was also found to be reusable without measurable loss of activity. Experimental and computational studies indicate the involvement of 𝜂2-peroxo species, [M(O)(𝜂2-O2)(dithiolene)2]2-, as the active oxidants formed in situ. Overall, the bioinspired catalysts in this study are shown to be promising candidates for developing convenient, inexpensive, efficient, and selective mustard gas detoxification technologies.

Keywords: sulfoxide, oxidation, molybdenum, tungsten, dithiolene, mustard gas, metal peroxo

Graphical Abstract

graphic file with name nihms-2070749-f0001.jpg

Since its first deployment as a weapon during World War I, sulfur mustard (SM, bis(2-chloroethyl) sulfide) has been recognized as a major chemical warfare agent (CWA), causing severe skin blistering, eye irritation, and respiratory damage.1 Although production, stockpiling, and use of CWAs including SM has long been banned by international treaty, SM still presents a significant threat because it is a simple molecule that can be synthesized readily by nefarious actors.2 In fact, deployment of SM by a terrorist organization was reported as recently as 2016.3 Therefore, it is critical to continue developing improved methods for detoxification of SM stockpiles.4 Although significant effort has been spent towards optimizing hydrolysis and dehydrochlorination processes, the most promising detoxification strategies for SM disposal involve oxidation of its sulfur center, as the corresponding bis(2-chloroethyl) sulfoxide (SMO) is benign and inert towards biological systems. Because SM undergoes oxidation significantly slower than other aliphatic thioethers,2 it is necessary to identify highly active sulfur oxidation catalysts for this application. However, overoxidation of SM produces bis(2-chloroethyl) sulfone (SMO2), which is also a potent toxin.5 Thus, SM detoxification requires a catalyst precisely tuned to promote the sluggish oxidation of SM to SMO without further oxidation to SMO2 (Figure 1a). Ideally, such a catalytic process would also employ a green oxidant (i.e., O2 or H2O2) that produces no chemical waste streams.6 Compounds including strong acids, molecular metal complexes, polyoxometalates (POMs), and metal oxide solids have been explored as catalysts for oxidation of the SM research analogue, 2-chloroethyl ethyl sulfide (CEES).2 Photoactive porous materials,6 especially metal-organic frameworks (MOFs),7 have received significant attention for their ability to catalyze selective CEES oxidation to 2-chloroethyl ethyl sulfoxide (CEESO) under UV or visible light-mediated conditions via 1O2 generation.8,9 Despite the successes of these photosensitization strategies, it is desirable to develop complementary thermal processes.

Figure 1.

Figure 1.

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(a) Detoxification of sulfur mustard gas, (b) dimethyl sulfoxide reduction by Mo-dependent DMSO reductase with its active site’s resting state shown, (c) molybdopterin mimics used in this study.

In biological systems, selective oxygen atom transfer (OAT) reactions are catalyzed by oxotransferase enzymes10–12 whose active sites feature bis(dithiolene) ligation to Mo (and sometimes W)13,14 via the pyranopterin dithiolate cofactor. Particularly relevant to the SM problem is biological dimethyl sulfoxide reduction, i.e., the microscope reverse of sulfide oxidation, which is catalyzed by the DMSO reductase enzyme whose Mo(dithiolene)2 active site (Figure 1b) is proposed to shuttle between MoIV and MoVI=O states during OAT catalysis.15–17 Accordingly, synthetic oxomolybdenum(VI) compounds have been studied for catalytic OAT reactions including sulfur oxidations.18–22 A prototypical example, commercially-available MoO2(acac)2 (acac = acetylacetonate), efficiently catalyzes oxidations of sulfoxides to sulfones23 and is representative of the challenge of taming the oxidizing power of high-valent Mo and W catalysts to avoid overoxidation of SM to SMO2. Despite extensive synthetic modeling literature of the DMSO reductase active site and related Mo/W enzymes such as sulfite oxidases, aldehyde oxidoreductases, trimethyl N-oxide reductases,11,24–27 the use of close structural mimics of oxotransferases featuring bis(dithiolene) ligation to Mo or W in catalytic OAT reactions of sulfides has not yet been reported. Here, we report that [MO2(dithiolene)2]2- complexes (M = Mo or W, Figure 1c) catalyze oxidation of CEES with unusually high efficiency and with perfect selectivity for formation of CEESO rather than 2-chloroethyl ethyl sulfone (CEESO2). These catalysts operate at ambient conditions without requiring inert atmosphere, are robust enough for repeated use, and employ a green oxidant, H2O2. Thus, these bioinspired catalysts represent excellent candidates for SM detoxification technology development.

We began our investigation by attempting oxidation of a methanol solution of CEES with aqueous H2O2 (1.05 equiv.) at ambient temperature in the presence of [MoO2(mnt)2][NnBu4]2 (1, mnt = maleonitriledithiolate)28,29 as a catalyst at 1.5 mol% loading. Although only CEESO was observed at 5 min reaction time, some CEESO2 was observed after 1 h (Table 1, entries 1–2). A control experiment without the catalyst showed only trace CEES conversion (entry 3), and no conversion was observed using molecular oxygen in place of H2O2 (entry 4). Under the same conditions, changing the catalyst to either [WO2(mnt)2][NnBu4]2 (2)30 or [MoO2(bdt)2][NEt4]2 (3, bdt = 1,2-benzenedithiolate)25,31,32 resulted in quantitative conversion of CEES to CEESO without any overoxidation to CEESO2 (entries 5–6). Both catalysts 2 and 3 were found to be extremely active (Figure S41), showing complete consumption of CEES within 15 and 5 min, respectively (entries 7–8). These and all subsequent catalytic trials with 1 and 2 were performed on the benchtop without protection from room atmosphere, whereas 3 required an inert N2 atmosphere. Use of methanol solvent is critical,33,34 as experiments in other solvents (e.g., CH3CN or DMF) resulted in slightly lower sulfoxide selectivity. Moreover, running the reaction with minimal MeOH (0.2 mL) to simulate neat conditions led to significant sulfone formation (~80% sulfoxide selectivity). The oxygen-ligated catalyst, MoO2(acac)2 (acac = acetylacetonate), was found to be highly active but formed some sulfone initially, with sulfoxide selectivity further degrading as the reaction mixture was allowed to sit for longer than 1 h. Therefore, sulfur ligation to the catalytic metal site is critical to control selectivity.

Table 1.

CEES oxidation with bio-inspired Mo and W dithiolene catalysts.a

graphic file with name nihms-2070749-t0002.jpg
Entry Catalyst Time Conversion (%)b Sulfoxide selectivity (%)c
1 [MoO2(mnt)2]2- (1) 5 min 100 100
2 [MoO2(mnt)2]2- (1) 1 h 100 94
3 None 1 h 3 100
4d [WO2(mnt)2]2- (2) 1 h 0 n/a
5 [WO2(mnt)2]2- (2) 1 h 100 100
6 [MoO2(bdt)2]2- (3) 1 h 100 100
7 [WO2(mnt)2]2- (2) 15 min 100 100
8 [MoO2(bdt)2]2- (3) 5 min 100 100
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a

Reaction conditions: CEES (2.0 mmol), H2O2 (2.1 mmol), Catalyst (1.5 mol%), MeOH (6 mL), room temperature.

b

Determined by GC using mesitylene as an internal standard.

c

Determined by GC. Sulfoxide selectivity = [%sulfoxide / (%sulfoxide + %sulfone)]*100.

d

O2(g) in place of H2O2.

Investigations with other thioether substrates were performed with catalysts 1 and 2. Whereas 2 consistently gave perfect sulfoxide selectivity (see Supporting Information), sulfone products were observed in some cases for 1 (Table 2). Like CEES (entry 1), diethyl sulfide was oxidized completely within 1 h and showed slightly better sulfoxide selectivity (entry 2). Interestingly, a control experiment with Et2S in the absence of catalyst resulted in formation of 2-(ethylthio)ethan-1-ol, a product that was completely suppressed in the catalytic trials (Figures S8–S10). For thioanisole, quantitative conversion was observed with 94% selectivity for methyl phenyl sulfoxide (entry 3). Diphenyl sulfide was found to undergo oxidation sluggishly under these conditions, with 13% sulfide remaining after 1 h (entry 4) and complete conversion requiring 4 h.

Table 2.

Oxidation of other thioethers.a

graphic file with name nihms-2070749-t0003.jpg
Entry R1 R2 Conversion (%)b Sulfoxide selectivity (%)c
1 CH2CH2Cl Et 100 94
2 Et Et 100 100
3 Ph Me 100 94
4 Ph Ph 86 98
5 Ph CH2CH=CH2 95 98
6 CH3CH2 CH2CH=CH2 100 100d
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a

Reaction conditions: Thioether (2.0 mmol), H2O2 (2.1 mmol), 1 (1.5 mol%), MeOH (6 mL), room temperature, 1 h.

b

Determined by GC using mesitylene as an internal standard.

c

Determined by GC. Sulfoxide selectivity = [%sulfoxide / (%sulfoxide + %sulfone)]*100.

d

Thiane 1-oxide was the major product, see Scheme 1.

Because allylic sulfoxides are valuable building blocks that participate in the Mislow-Evans rearrangement,35 we probed the compatibility of the bioinspired catalysts with allylic sulfides. Phenyl allyl sulfide was efficiently oxidized under the catalytic conditions to phenyl allyl sulfoxide (Table 2, entry 5). Surprisingly, allyl ethyl sulfide converted to thiane 1-oxide under these conditions (entry 6). Unlike the background reactivity observed for Et2S, here only trace reactivity was observed in the absence of catalyst (Figures S19–S21). Probing the mechanism and generality of this unusual cyclization reaction (Scheme 1) will be subjects of future studies in our laboratory.

Scheme 1.

Scheme 1.

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Catalytic oxidation and cyclization of allyl ethyl sulfide.

Next, we experimented with oxidation of CEES using different catalyst loadings of 2 (Table 3). For catalyst loadings ranging from 3.0 mol% to 0.15 mol%, quantitative oxidation of CEES with perfect CEESO selectivity was achieved within 15 min (entries 1–4). Good catalytic activity was maintained even with 0.015 mol% catalyst loading, but completion of the reaction required 90 min in this case (entry 5). Overall, these experiments indicate that catalyst 2 can achieve turnover numbers of >6.6 x 103 and turnover frequencies of >4.4 x 103 h−1, making this system competitive with leading POM catalysts36–38 in terms of CEES oxidation efficiency while still avoiding overoxidation.

Table 3.

CEES oxidation with different catalyst loadings.a

graphic file with name nihms-2070749-t0004.jpg
Entry Catalyst loading (mol%) Time (min)b
1 3.0 15
2 1.5 15
3 0.75 15
4 0.15 15
5 0.015 90
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a

Reaction conditions: CEES (2.0 mmol), H2O2 (2.1 mmol), 2 (x mol%), MeOH (6 mL), room temperature, 15 min.

b

Time to reach quantitative conversion of CEES as determined by GC using mesitylene as an internal standard. No CEESO2 sulfone was observed in any of the trials.

We also conducted experiments to establish catalyst reusability. Using 1.5 mol% (pre-)catalyst loading of 2, results were monitored over 4 consecutive injections of CEES into the same reaction mixture. For all 4 injections, full conversion of CEES to CEESO was observed (Figure 2). Given that no loss of catalytic activity was evident, it can be concluded that 2 (or the active species that arises) is sufficiently robust for repeated uses.

Figure 2.

Figure 2.

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Reusability of catalyst 2 (1.5 mol%) over 4 consecutive CEES injections into the same reaction mixture (GC traces with mesitylene internal standard, IS = internal standard).

Based on the accepted catalytic mechanisms for the DMSO reductase enzyme family,10–12,15 one might expect the bioinspired Mo and W dithiolene catalysts to employ the dioxo intermediate, [MO2(dithiolene)2]2-, as the active oxidant to convert CEES to CEESO, producing [MO(dithiolene)2]2- as the immediate byproduct that would undergo reoxidation by H2O2. However, literature precedents with synthetic model complexes indicate either that [MO2(dithiolene)2]2- intermediates should react sluggishly with thioethers25 or that the microscopic reverse, sulfoxide reduction by [MO(dithiolene)2]2-, should be thermodynamically favored.26 In accord with the previous literature, no OAT reactivity was observed in an attempted stoichiometric reaction between CEES and 2. Additionally, our computational modeling using DFT calculations indicates that OAT from [MO2(mnt)2]2- to Me2S to produce [MO(mnt)2]2- and DMSO is thermodynamically unfavorable with reaction free energies of ΔG = +6 and +26 kcal mol−1 for M = Mo (1) and W (2), respectively (Figure S44).

An alternative mechanism that has been proposed for Mo-promoted OAT in some cases involves further oxygenation of the dioxo intermediate to an oxo/𝜂2-peroxo species that serves as the active oxidant generated in situ.22,39,40 In agreement with this peroxo mechanism, OAT from [M(O)(𝜂2-O2)(mnt)2]2- to Me2S to produce [MO2(mnt)2]2- and DMSO was calculated to be thermodynamically favorable with reaction free energies of ΔG = -30 kcal mol−1 for both M = Mo (1) and W (2). Reoxygenations of the dioxo intermediates with H2O2 to regenerate the oxo/𝜂2-peroxo intermediates were also calculated to be thermodynamically favorable (ΔG = -8 kcal mol−1 for both cases), thus providing substantial support for this proposed catalytic mechanism (Figure 3). The calculated O-O bond distances for the 𝜂2-peroxo ligands (M = Mo: 1.407 Å; M = W: 1.428 Å) are consistent with the [O2]2- formulation.

Figure 3.

Figure 3.

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Proposed mechanism for Me2S oxidation by [MO2(mnt)2]2- involving an oxo/𝜂2-peroxo intermediate as the active oxidant. Optimized structures and reaction free energies are shown for M = W; nearly identical structures and energetics were calculated for M = Mo. Calculations were conducted at the M062X-D3//def2TZVPP/6–31+G** level of DFT with implicit methanol solvation (SMD model).

In conclusion, Mo and W catalysts with bioinspired bis(dithiolene) ligation were found to be unusually efficient and selective catalysts for oxidation of the sulfur mustard analogue, CEES, to its corresponding sulfoxide using a green oxidant, aqueous H2O2. A mechanistic investigation indicated that catalysis likely proceeds through an oxo/𝜂2-peroxo intermediate as the active oxidant rather than the dioxo form proposed for biological oxygen atom transfer processes. Because sulfur mustard undergoes oxidation ~5 times more slowly than CEES,2 it is critical to identify detoxification catalysts in research settings that exhibit extremely high activity along with good selectivity. The results presented in this study indicate that bioinspired Mo and W catalysts represent promising candidates with which to develop sulfur mustard detoxification technologies.

Supplementary Material

SI

ACKNOWLEDGMENT

Funding was provided by NIH/NIGMS under grant R35 GM140850. Computational resources and services were provided by the Advanced Cyberinfrastructure for Education and Research (ACER) group at UIC.

Footnotes

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Experimental methods & supporting data (PDF)

Computational output (XLSX)

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Supplementary Materials

SI
NIHMS2070749-supplement-SI.pdf (1.1MB, pdf)

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