Highly reactive catalysts for aerobic thioether oxidation: The Fe-substituted polyoxometalate/hydrogen dinitrate system

doi:10.1016/j.molcata.2005.10.006

Journal of Molecular Catalysis A: Chemical

Volume 246, Issues 1–2, 1 March 2006, Pages 11-17

https://doi.org/10.1016/j.molcata.2005.10.006 Get rights and content

Abstract

A new type of polyoxometalate (POM), one with a hydrogen dinitrate group associated with a d-electron-substituted polyoxometalate, Fe^III[H(ONO₂)₂]PW₁₁O₃₉⁵⁻ (1), has been developed that catalyzes highly effective aerobic sulfoxidation (RR′S + 0.5 O₂ → RR′SO) under ambient conditions (1 atm of air at room temperature). Comparison of the rates for aerobic sulfoxidation of the mustard simulant 2-chloroethyl ethyl sulfide (CEES) catalyzed by 1 and several other species reported to be catalysts for this class of reactions, including NO⁺, NO₂, (NH₄)₂Ce^IV(NO₃)₆ and Au^IIICl₂NO₃(CH₃CN) indicate that 1 is clearly the most effective catalyst. Conversions and selectivities for the desired sulfoxide decontamination product (CEESO in these studies) are both effectively quantitative. The low or nonexistent catalytic activity of Fe^II(NO)PW₁₁O₃₉⁶⁻, Fe(NO₃)₃ and HNO₃ argues strongly that nitrosyl and nitrate derivatives of the Fe polyoxometalate and nitric acid are not species important in catalytic turnover. The techniques of single crystal X-ray diffraction, solution NMR, FTIR, TGA, DSC, cyclic voltammetry, elemental and wet chemical analyses were applied to the characterization of 1 and these are collectively consistent with the α-Keggin structure with a strongly associated hydrogen dinitrate group and a more weakly associated nitrate of crystallization.

Graphical abstract

Introduction

The selective aerobic (O₂/air-based) oxidation of sulfides to sulfoxides is of considerable interest [1], [2], [3]. One important application is the aerobic sulfoxidation of mustard, bis(2-chloroethyl) sulfide (conventional abbreviation: “HD”) (Eq. (1)). If this could be achieved under very mild conditions, it would be of major interest in protection and decontamination technologies. The sulfoxide of mustard is far less toxic than the corresponding sulfone and some mustard hydrolysis products, thus high selectivity for Eq. (1) is highly desirable. The products of:total mustard mineralization, CO₂, H₂O, HCl and inorganic sulfide or sulfur, would be quite desirable, but this complex and challenging overall transformation cannot be achieved by any system in the dark. Furthermore, even photocatalytic mineralization systems would be questionably fast enough to be effective [4]. Eq. (1) is attractive for two additional reasons: it is atom efficient and it is green (no byproducts, etc.). Recently, we and other research groups have reported several catalytic systems for aerobic sulfoxidation in the absence of light [2], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], but some are based solely on generation of gaseous catalysts, such as NO, which can form electron donor–acceptor complexes with sulfides or NO₂ [5], [6]. Unfortunately such catalytic systems would very likely deactivate quickly under conditions and environments, where they would be needed by loss of the catalytically active gaseous specie(s). Liquids and solids are practically more attractive for stability as well as logistics reasons. However, sustained high rates of catalytic turnover for Eq. (1) under ambient conditions (1 atm of air at room temperature) with liquid or solid catalysts remains elusive.

We report here the synthesis and characterization of a new iron-substituted Keggin heteropolytungstate with an associated hydrogen dinitrate group, Fe^III[H(ONO₂)₂]PW₁₁O₃₉⁵⁻ (1). The TBA₃H₂ salt (TBA: tetra-n-butylammonium) salt of 1, usually containing one additional nitric acid molecule of crystallization, is quite stable and when dissolved in aprotic solvents, such as acetonitrile, is the most active catalyst to date for the aerobic (air-only) sulfoxidation of 2-chloroethyl ethyl sulfide (CEES). In a thorough recent study, Khenkin and Neumann proposed the intermediacy of N^VO₂[H₄PV₂Mo₁₀O₄₀] in the oxidation of alkylarenes by nitrate salts in acetic acid [16]. CEES is the best and most frequently used mustard simulant and repeated studies by our laboratory in collaboration with others have shown that CEES and mustard are oxidized at quite similar rates. In dramatic contrast to 1, Fe^IIIPW₁₁O₃₉ⁿ⁻ (Fig. 1) and related Fe-substituted polytungstates in the presence of only nitrate are ineffective catalysts for selective aerobic sulfoxidation. These points are investigated in conjunction with the characterization of 1.

Section snippets

Materials and methods

NO₂BF₄, NOPF₆, (NH₄)₂Ce^IV(NO₃)₆, Fe(NO₃)₃·9H₂O, Ti(NO₃)₄, the mustard simulant 2-chloroethyl ethyl sulfide and the internal standard used in quantifying product distributions by gas chromatography (1,3-dichlorobenzene) were commercial samples, α-Na₇PW₁₁O₃₉·nH₂O [17], Au^IIICl₂NO₃(CH₃CN) [7] and TBA₆Fe^II(NO)PW₁₁O₃₉ [18], were prepared by the literature procedures. TBA₄Fe(H₂O)PW₁₁O₃₉·nH₂O was prepared by cation metathesis of an aqueous solution of Na₄Fe(H₂O)PW₁₁O₃₉·nH₂O made by the literature

Evaluation of catalysts for aerobic sulfoxidation

Table 2 compares the reactivities of a new ferric heteropolytungstate, TBA₃H₂Fe^III[H(ONO₂)₂]PW₁₁O₃₉, as a nitric acid solvate, TBA₃H₂1 and the many other previously reported catalysts for aerobic sulfoxidation under ambient conditions (1 atm of air at room temperature in acetonitrile solution) using the fairly unreactive mustard simulant, 2-chloroethyl ethyl sulfide, CEES, as the substrate [26]. TBA₃H₂1 is an unusually challenging complex to rigorously characterize structurally and a discussion

Conclusions

The most reactive catalyst yet for selective aerobic sulfoxidation, Fe^III[H(ONO₂)₂]PW₁₁O₃₉⁵⁻ (1), has been identified and investigated. Although NMR and X-ray crystallography, the two most definitive structural methods, can not be used to characterize 1 because of paramagnetism and positional disorder in the solid state, respectively, vibrational spectroscopy in conjunction with several wet chemical analyses, thermal studies, elemental analyses and the correlation of this data with catalytic

Acknowledgments

We wish to thank the Army Research Office (Grant numbers DAAD 19-01-1-0593 and currently W911NF-05-1-0200) and the Natick Soldier Center/Battelle for funding and Travis M. Anderson for help with manuscript preparation.

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Highly reactive catalysts for aerobic thioether oxidation: The Fe-substituted polyoxometalate/hydrogen dinitrate system

Abstract

Graphical abstract

Introduction

Section snippets

Materials and methods

Evaluation of catalysts for aerobic sulfoxidation

Conclusions

Acknowledgments

J. Photochem. Photobiol. A Chem.

J. Mol. Catal. A Chem.

Tetrahedron Lett.

J. Mol. Catal. A Chem.

Appl. Catal. A

J. Catal.

Spectrochim. Acta

Spectrochim. Acta

Polyhedron

Chem. Phys. Lett.

J. Am. Chem. Soc.

Curr. Opin. Drug Discov. Dev.

J. Org. Chem.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Inorg. Chem.

Chem. Mater.

Chem. Mater.

J. Am. Chem. Soc.

Can. J. Chem.

J. Am. Chem. Soc.

Inorg. Chem.

Zh. Neorg. Khim.