An Au(III)–amino alcohol complex for degradation of organophosphorus pesticides

Abstract

An Au(III)–amino alcohol complex has been used to cleave organophosphorous pesticides of the dithiophosphate family. P–S bond breaking was readily demonstrated by ¹H NMR, ³¹P NMR and MS. Thiol fragment release was also demonstrated using 2,4-dinitrobenzenesulfonyl fluorescein ethyl ester as a fluorescent sensor.

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Nowadays, the most widely used pesticides belong to the organophosphorous (OP) group, which have replaced environmentally persistent organochlorine reagents. Despite potential environmental problems, their use to protect plants from disease and insect damage not only in agriculture, but also in home gardens, has increased and is predicted to do so even more in the future as suitable substitutes are lacking.¹

Use of these toxic pesticides results in contamination of soils and groundwater and, consequently, traces of these compounds may be found in our food supply. OPs are not only highly toxic to insects, but they also can affect human beings. The toxicity of OP pesticides to insects, mammals and other organisms can be attributed mostly to the inhibition of acetylcholinesterase^2,3 (AChE), the enzyme responsible for the hydrolysis of neurotransmitter acetylcholine (ACh). This inhibition causes convulsion, paralysis, respiratory collapse, and finally death.⁴ In addition to this major biochemical interaction, there is growing concern about the varied toxicological effects that OPs induce in warm-blooded mammals as some are known to be mutagenic, teratogenic and neurotoxic.^5,6 The natural degradation of OP pesticides occurs by homogeneous and heterogeneous hydrolysis,^7,8 oxidation,⁹ photolysis¹⁰ and biodegradation,¹¹ whereas thermal, enzymatic or photochemical degradation, as well as chemical treatments through the use of strong oxidants or nucleophiles, are among the methods used for their destruction.^12–15 Some reports about the use of metal cations as catalysts in the hydrolysis of thionophosphates have been described^16,17 but, to the best of our knowledge, no examples related to dithiophosphates have been reported to date. On the other hand, many of the procedures used for the hydrolysis of dithiophosphates lead to transformation products (TPs), which are even more toxic than the parent compound. For this reason, interest in developing methods able to convert these dangerous compounds into innocuous products is ever-increasing.¹⁸ It is known that whereas the P–S cleavage in organothiophosphate pesticides yields non-toxic sub-products, the P–O cleavage produces the very toxic anionic phosphorothioite.^19,20 This fact creates a demand for reliable, fast and simple tools to be developed to break the P–S bond. A gold(III)–amino alcohol complex, [Au(L)Cl₂]Cl·2H₂O (1) (Scheme 1), was used herein to induce the P–S bond cleavage in OP pesticides. The design of the complex was based on our previous studies about the use of 2-(2-aminoethylamino)ethanol (L) for the hydrolysis of organophosphorous nerve agent simulant diethyl cyanophosphonate (DCNP).²¹ To ensure the cleavage of the P–S over the P–O bond, Au³⁺ was introduced into the system due to gold's high affinity for sulphur. Binding of Au(III) to sulphur should enhance either the electrophilicity of the P atom or the leaving group character of the thiolate.²²

Scheme 1 Chemical structures of selected OP pesticides and the metal-complex for cleavage.

Compound 1 was synthesised by simply mixing the corresponding metal salt (sodium tetrachloroaurate(III) dihydrate) with L in EtOH, and was further purified by recrystallisation from EtOH (see ESI† for details).

Degradation of the OPs in aqueous methanolic solution was followed by ³¹P-NMR spectroscopy, which provides valuable information about the nearest neighbours of the P atom in the molecule.^23–27 In addition, ¹H NMR and MS studies were carried out to acquire further information about the degradation process. In typical experiments, OP pesticides were combined with equimolecular amounts of 1 in a phial at room temperature in MeOD : D₂O 9 : 1, which was found to be a suitable solvent mixture to follow the hydrolysis extent by using NMR, and to also ensure complete solubility of 1. Addition of 1 to a methidathion solution induced the immediate appearance of an amorphous solid, which was separated by filtration. The solution was transferred to an NMR tube and its NMR spectra were recorded 2 min after the addition (see ESI† for details).

The ¹H NMR spectrum of methidathion showed a doublet at 5.19 ppm (J = 18 Hz), which corresponded to 2H_(a), a singlet at 4.04 ppm (3H_(b)) and a doublet at 3.77 ppm (6H_(c), J = 15 Hz). After treatment with 1, the methidathion concentration in the solution lowered and two new singlet peaks appeared at 5.16 and 5.09 ppm (Fig. 1).

Fig. 1 ¹H NMR spectra of (top) methidathion and (bottom) 1 + methidathion (MeOD : D₂O 9 : 1).

These two peaks can be assigned to the methylene group of compounds 2 and 3, respectively (Scheme 2), which are present in the solution at a 5 : 1 ratio. In addition, the ratios of compounds 2 + 3 versus non-hydrolysed methidathion in solution was higher than 80%. The ³¹P NMR spectrum showed two main signals (see ESI†). One appeared at 98.15 ppm and corresponded to methidathion, whereas the second appeared at 27.78 ppm can be attributed to the dimethyl thiophosphate SH tautomer 4.²⁸ A broad signal was observed at 44.32 ppm, which can be attributed to a small amount of dimethyl thiophosphate OH tautomer coordinated to gold. If we consider that the ³¹P NMR spectrum did not show any signal around 85 ppm that could be attributable to dimethyldithiophosphonate, the breaking of the C–S bond did not seem to be involved in the hydrolysis mechanism.²³ Consequently, compound 2 should originate from 3 through gold mediate substitution. Furthermore in the mass spectrum (ESI†) of the solution of mixture 1 + methidathion, peaks were observed at 304, 201, 184 and 165, which respectively corresponded to methidathion, [3 + Na]⁺, [2 + Na]⁺ and [4 + Na]⁺.

Scheme 2 Proposed products of methidathion hydrolysis.

Finally, the hydrolysis of the P–S bond was also demonstrated by the detection of the free thiol group through a reaction with 2,4-dinitrobenzenesulfonyl fluorescein ethyl ester (5), as described by Maeda and col.²⁹ (Scheme 3). Probe 5 was practically non-fluorescent in acetonitrile solution (Φ = 0.02), whereas a significant enhancement (Φ_{hyd_meth} = 0.33) of its emission at λ_em = 485 nm (λ_ex = 460 nm) took place upon the addition of the solutions that contained the organophosphorous pesticide previously combined with 1 (Fig. 2). Control experiments were performed by adding 5 to either 1 or the pesticide. No fluorescence response was observed in any case (see ESI†).³⁰

Scheme 3 Selective detection of the thiol group by the fluorescein derivative 5.

Fig. 2 The fluorescence emission spectra (λ_ex = 460 nm) of probe 5 (10⁻⁵ M in CH₃CN) before (grey trace) and after the addition of 1 + methidathion (MeOH : H₂O 9 : 1) (black trace).

Integration of signals into the ¹H NMR spectra demonstrated that the relationship between compounds (2 + 3)/4 was around 1.5. This result indicated that part of 4 had to be in the previously isolated amorphous solid. As the high insolubility of this solid precluded the recording of any NMR spectra, MS (ESI†) and EDXA were used to acquire information about its composition. Based on the obtained data (MS peak at 583 and presence of S, P and Au), one possible component of the solid could be compound 6 (Scheme 4). Similar structures have been described in the literature for complexes formed by dialkylthiophosphate and different metal cations.³¹

Scheme 4 Proposed compound present in the isolated solid.

In order to know the applicability of this reaction to other related OP pesticides, studies with azinphos-methyl were carried out under the same conditions. Similar results were obtained but, in this case, the hydrolysis in the presence of 1 was slower than with methidathion, as seen in Fig. 3. In addition, the solubility of the generated compounds was too low to be observed in the NMR spectrum of the solution, which were detected only in the solid fraction (see ESI†).

Fig. 3 Response of probe 5 with azinphos-methyl and methidathion 5 min after adding 1.

Finally, different sub-stoichiometric amounts of the gold complex (0.1, 0.25, 0.5 equiv.) were used to know how this factor changed the hydrolysis process. In all the experiments, the identified products were the same, but in different ratios. No complete hydrolysis took place, not even after long reaction times (up to 2 days).

Conclusions

Complex 1 was able to break the P–S bond in the studied OP pesticides, which gave rise to the corresponding thiol and dimethyl thiophosphate. During the degradation process, an amorphous and highly insoluble solid was generated. The preliminary results obtained in these experiments suggest that the system is useful for degradation of the OPs of the organothiophosphate family, but developing new complexes with other metal cations is necessary in order to obtain cheaper catalytic systems. These experiments are in progress.

Acknowledgements

The authors thank the DGICYT and European FEDER funds (MAT2012-38429-C04-01 and -02) and the Generalitat Valenciana (PROMETEOII/2014/047) for support. SCSIE (Universitat de Valencia) is gratefully acknowledged for all the equipment employed.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR and ME spectra, fluorescence studies. See DOI: 10.1039/c5ra20645f

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