A mechanistic study of the electrochemical behavior of pendimethalin herbicide

Accepted Manuscript A mechanistic study of the electrochemical behavior of pendimethalin herbicide

Andressa Galli, Josiane Caetano, Paula Homem-de-Mello, Alberico Borges Ferreira da Silva, Antonio Gilberto Ferreira, Sthéfane Valle de Almeida, Sergio Antonio Spinola Machado PII: DOI: Reference:

S1572-6657(18)30575-7 doi:10.1016/j.jelechem.2018.08.038 JEAC 12577

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

11 May 2018 25 August 2018 26 August 2018

Please cite this article as: Andressa Galli, Josiane Caetano, Paula Homem-de-Mello, Alberico Borges Ferreira da Silva, Antonio Gilberto Ferreira, Sthéfane Valle de Almeida, Sergio Antonio Spinola Machado , A mechanistic study of the electrochemical behavior of pendimethalin herbicide. Jeac (2018), doi:10.1016/j.jelechem.2018.08.038

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ACCEPTED MANUSCRIPT A MECHANISTIC STUDY OF THE ELECTROCHEMICAL BEHAVIOR OF PENDIMETHALIN HERBICIDE Andressa Gallia, Josiane Caetanod, Paula Homem-de-Melloe, Alberico Borges Ferreira da Silvab, Antonio Gilberto Ferreirac, Sthéfane Valle de Almeidaa*, Sergio Antonio Spinola Machadob a

Departamento de Química, Universidade Estadual do Centro-Oeste do Paraná, C.P. 85040-080, Guarapuava – PR, Brazil. b

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Instituto de Química de São Carlos, Universidade de São Paulo, C.P. 780, 13560-970 São Carlos – SP, Brazil c

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Departamento de Química, Universidade Federal de São Carlos, C.P. 676, 13560-970 São Carlos – SP, Brazild Centro de Engenharias e Ciências Exatas, Universidade Estadual do Oeste do Paraná, C.P. 85903000 Toledo – PR, Brazil e

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Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, C.P. 09210580 - Santo André, SP - Brazil

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Abstract: The electrochemical reduction of pendimethalin was studied by controlled potential electrolysis, employing the mercury pool electrode, in Na2SO4 0.1

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mol L-1 pH = 8.0, with a constant potential of -1.0 V vs Ag/AgCl. The analysis of the products obtained was carried out by nuclear magnetic resonance (NMR), reaching a

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possible mechanism of reduction for this pesticide, proved by means of chemical

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quantum calculations. In such a mechanism, there was the reduction of the two nitro groups to hydroxylamine, involving 6 electrons, followed by the reduction of only one nitro group to amine, involving two more electrons. A structural rearrangement of the –

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NHCH(CH2CH3)2 cluster to a –CH2CH3 in the pesticide molecule was verified.

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Keywords: Pendimethalin reduction, mercury electrode, NMR. ___________________________________ Corresponding author at: Universidade Estadual do Centro-Oeste do Paraná, Departamento de Química, Campus CEDETEG, Rua Simeão Camargo Varela de Sá, 03 Vila Carli, CEP 85040-080, Guarapuava-PR, Brasil. *E-mail adress: [email protected] (S.V. Almeida)

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ACCEPTED MANUSCRIPT 1.

Introduction Pendimethalin is a selective herbicide used in the annual control of grasses and weeds

of wide leaves in crops of maize, potato, rice, cotton, soybean, tobacco, peanuts, sunflower, onions and pepper. It is used in pre-emergence, that is, before seed germination, as well as just after germination [1,2]. It has a mechanism of action by inhibition of cell division of the weeds, preventing the formation of microtubules during

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this period [3]. This herbicide is considered slightly toxic, since inhaling vapors or particulates may

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be only moderately irritating for the mucous membranes of the mouth, nose, throat and lungs. It is classified as carcinogenic (C class), even though the amount found as a

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residue in foods is too small to consider it a risk to the population. Studies carried out in small mammals also proved the cytotoxicity of the compound, as well as the capacity to

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cause possible damage to the DNA [4,5]. The molecule of pendimethalin is absorbed in insignificant amounts in the gastrointestinal tract, being excreted in feces without any

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changes. The remainder is quickly metabolized by the liver and kidneys, being excreted in the urine, in the form of metabolites [3].

In this context, the application of electroanalytical techniques for the determination of

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pendimethalin has been carried out discreetly, for both the development of a

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methodology, and the determination of the mechanism for the reduction of pendimethalin. A few studies on this theme are found in literature. Among them, the study by Sreedhar et al. [6], who employed the technique of differential pulse

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polarography for the determination of the pendimethalin mechanism based on the reduction of the nitro groups; they also developed an analytical methodology for this herbicide. Under the conditions studied, it was verified the presence of two well-defined

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peaks in an acid medium, and only one peak in a basic medium (pH = 8.0); as the electrochemical response was replicable in pH = 8.0, this was the medium used for the analytical applications. The detection limit found was 5.25 µg L-1. Kotoucek and Opravilová [7] employed direct current polarography, using the techniques of fast-scan differential pulse voltammetry and adsorptive voltammetry for the study of the mechanism of reduction of five nitropesticides (trifluralin, benfluralin, pendimethalin, bromofenoxim and fluoroglycofen-ethil), in an electrolyte composed of 47-49% (v/v) alcohol or of 10-20% (v/v) dimethylformamide. The authors proposed that

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ACCEPTED MANUSCRIPT the nitro groups were reduced to hydroxylamine or, even more drastically, to the respective amines. In turn, Wang et al. [8] proposed a mechanism of photodegradation for pendimethalin, using light from a xenon lamp and composites of Cu2O/SnO2/graphene and SnO2/graphene as photocatalysts. The transfer of electrons between SnO2 and pendimethalin is favorable from the thermodynamic point of view; also, the efficiency of SnO2 as a catalyst can be considered higher than those of other semi-conductors, due to

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the difference of potential between both. Thus, by combining Cu2O and SnO2, there is a vector flow of the electrons in this sense, so that a superoxide radical is produced, and

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this may lead to the formation of H2O2 and hydroxyl ions, which oxidize pendimethalin,

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degradating it. Photodegradation was verified by cyclic voltammetry and by differential pulse voltammetry using glassy-carbon electrode, and the analyses were carried out after

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different times of light exposure, being observed that the density of the peak current decayed according to the increase in exposure time.

At last, a mechanism of reaction of abiotic reduction for pendimethalin using a

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solution of Fe(II) was proposed by Wang and Arnold [9]. Juglone hydroquinone was used as an electron carrier, aiming at increasing the reaction speed. Thus, through the GC-MS

nitrobenzene-1,6-diamine.

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technique, only one product was observed, the N-(1-ethylpropyl)-3,4-dimethyl-2-

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Therefore, it was verified that the study on pendimethalin reduction was little explored, justifying a more thorough approach for a better clarification of the reaction mechanism of the molecule. Thus, in this work, pendimethalin was used as a model

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molecule of the dinitroaniline family with the aim of investigating the reduction mechanism, using controlled potential electrolysis and further identification of the

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products by nuclear magnetic resonance and theoretical calculations, as well as the intermediates formed during the process. 2.

Experiment The stock solution of the pendimethalin herbicide (BASF 98%) was prepared using

100% ethanol in the concentration of 1.90x10-2 mol L-1, and the other solutions were prepared through dilution from this stock. The supporting electrolyte used was Na2SO4 0.1 mol L-1, with pH adjusted to 8.0.

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ACCEPTED MANUSCRIPT The electrochemical measures were performed using Potentiostat/Galvanostat, Voltalab of the Radiometer Analytical model PGZ 402, attached to a microcomputer equipped with a Voltamaster 4.05 software, also Radiometer Analytical, with a mercury electrode (HDME) of 0.22 cm2 and a platinum wire counter electrode. The controlled potential electrolysis was carried out in a Pyrex® cell of two compartments – with capacity for 40.0 mL, separated by a Nafion membrane, having as working and the reference electrode used was Ag/AgCl/KCl 3.0 mol L-1.

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supporting electrodes the mercury pool and the platinum wire, respectively. In both cases,

The products of the electrolysis were extracted using three aliquots of 50.0 mL of the

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organic solvent (chloroform), and later the sample was dried with anhydrous sodium

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sulfate (drying), filtered and then evaporated in a Büchi R-114. The extracts were analyzed by 1H NMR obtained in a 9.4 Tesla equipment (400 MHz for the hydrogen

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frequency), BRUKER brand, AVANCE model of 9.4 T (400 MHz for hydrogen frequency), at a temperature of 298K using DMSOd6 as a solvent. The spectra were referenced using tetramethylsilane (TMS) as an internal reference.

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The pendimethalin structures and the possible intermediates and products of reaction were pre-optimized by the molecular mechanics method (MM, with force field MM+ and

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convergence gradient of 0.01 Kcal mol-1) contained in the HyperChem 4.5 [10] program.

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To confirm the results, quantum calculations were used through the semi-empirical method PM3 (Parametric Model 3), contained in the computer program AMPAC 6.0 [11] of the working station “Ultra 1 of Sun” of the “Institute of Chemistry of São Carlos” (IQSC – “Instituto de Química de São Carlos”). It was used in the optimization of the

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pendimethalin molecular geometry, as well as in the calculation of the atomic charges

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derived from the electrostatic potential. 3.

Results and discussion

3.1.

Controlled potential electrolysis

At first, square wave voltammetry (SWV) measures were carried out to determine the working potential, as it can be seen in Figure 1.

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Figure 1. Square wave voltammetry for pendimethalin 1.0x10 -4 mol L-1 on the HMDE (Na2SO4 0.1

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mol L-1 pH = 8.0, a = 50 mV, f = 500 s-1, Es = 10 mV).

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Thus, the controlled potential electrolysis (Figure 2) was performed at -1.0 V, after the reduction of the two peaks, to guarantee the complete reduction of pendimethalin. The measures were carried out through sodium sulfate 0.1 mol L-1 in pH = 8.0, with the initial

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concentration of pendimethalin at 1.0x10-4 mol L-1.

Figure 2. Chronoamperogram for pendimethalin electrolysis (1.0x10 -4 mol L-1) at a potential of -1.0 V, in Na2SO4 0.1 mol L-1, pH = 8.0.

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ACCEPTED MANUSCRIPT The results obtained showed that, after approximately 2 hours electrolysis, there was consumption of pendimethalin and the formation of possible reaction products, through an exponential decay of the current due to the electrolysis time. The voltammetric analyses were performed before, during and after each electrolysis, in order to follow the decrease in the analytical sign during the process. The voltammograms obtained are illustrated in Figure 3. A decay of the pendimethalin signs

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was observed, without the appearance of new peaks.

Figure 3. Voltammograms obtained for the pendimethalin electrolysis (Na2SO4 0.1 mol L pH = 8.0, a = 50 mV, f = 500 s-1, Es = 10 mV) on the HMDE at the times: 0 (), 30 (), 60 -1

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() and 120 ()min.

It can be observed that, within 120 minutes of electrolysis, approximately 83% and

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77% of the initial concentration of pendimethalin was consumed, considering the first and the second peak, respectively. According to equation 1 and the data obtained by chronoamperometry, the number of electrons involved in the pendimethalin reduction was equal to 8.

Q = nFN

(1)

were, Q = charge, n = number of electrons, F = Faraday constant and N = number of mols consumed.

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ACCEPTED MANUSCRIPT 3.2.

Chemical quantum calculations

The molecular calculations were carried out to obtain more information on the geometrical aspects of the molecule and the probable intermediates and products of the reaction, as well as to observe their electrochemical behavior. The pendimethalin structure was optimized by using the DFT method (Density Functional Theory), with the B3LYP functional and the 6-31G(d) basis, contained in the

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Gaussian 03 [12] program. The charges on the atoms were also calculated, with the method of charges derived from the electrostatic potential [13], also implemented in the

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Gaussian 03 program. The results obtained for pendimethalin are shown in Figure 4 and

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in Table 1.

(b)

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(a)

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HOMO

LUMO

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(c)

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Figure 4. Pendimethalin: (a) optimized structure and numbering adopted; (b) graphic representation of

Total Energy (u.a.)

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HOMO Energy (u.a.)

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the atomic charges calculated and (c) graphic representation of the HOMO and LUMO orbitals.

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LUMO Energy (u.a.)

-971.776 -0.240 -0.099 Charges on some atoms

1C

0.342

2C

-0.074

3C

-0.216

4C

0.119

5C

0.080

6C

-0.257

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ACCEPTED MANUSCRIPT -0.798

10 C

0.367

25 N

0.700

26 O

-0.398

27 O

-0.376

28 N

0.818

29 O

-0.409

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8N

-0.440

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30 O

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Table 1. Quantum properties calculated for Pendimethalin.

The most positive atomic charges are located on the 25N and 28N atoms, indicating that these atoms would be the most likely electron acceptors, as seen in Figure 4 (b).

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It is also important to perform the analysis of the HOMO and LUMO orbitals, since

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the former corresponds to the last molecular orbital occupied, and, so, it indicates the region where the most available electrons are found for a reaction of oxidation, while LUMO refers to the first molecular orbital unoccupied and, as a result, to the region that

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will be occupied by an electron received by the molecule. With regard to the energy of these orbitals, the more positive the HOMO energy is, the more favorable oxidation is, and, on the other hand, the more negative the LUMO energy is, the more likely the

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reaction of reduction is. In the case of pendimethalin, LUMO presents negative energy and the electron received in this process would be preferably located on atoms 28N, 29O, 30O, and on the ring. The difference in the structure of this intermediate (Figure 5) in relation to pendimethalin is that a NOO group (which presents higher contribution to LUMO, or else, the group linked to 6C) was reduced to NHOH. This molecule was optimized, and among the properties calculated, the charges and the graphic representation of LUMO are presented (Figure 6).

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Figure 5. Intermediate determined by the theoretical calculations.

Figure 6 illustrates that a new reduction process may occur mainly in 25N, since its

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charge is the molecule’s most positive one, and it is an atom that contributes most to the LUMO. In this way, the theoretical calculations showed that the reduction occurs in the

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two nitro groups of the pendimethalin molecule, being the first reduction in N28 close to

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the methyl group, and the second one in N25.

LUMO Atomic Charges (a)

(b)

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ACCEPTED MANUSCRIPT Figure 6. Graphic representations of the properties calculated for the intermediate proposed: (a) atomic charges derived from the electrostatic potential and (b) LUMO orbital.

3.3.

Characterization of the products

The NMR analyses were carried out for the characterization of the possible products from the electrolysis. In the NMR spectra of 1H, gCOSY, gHSQC and gHMBC, it was possible to

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unquestionably assign the chemical displacements and the constants of coupling for pendimethalin. Figure 7 shows the pendimethalin molecule with its respective

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numberings, being used as the stock material for electrolysis. Table 2 lists the values of

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chemical displacement  and coupling constant J (Hz) for the stock material.

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Figure 7. Pendimethalin structure before the electrolysis.

 (ppm) of 1H and J (Hz)

 of 13C

1

-----------

134.6

-----------

142.3

-----------

126.8

-----------

138.2

2

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3

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Number

4

gHMBC

18.6; 116.7; 5

8.10 s

128.0

126.8; 128.0; 135.7 and 138.2

6

-----------

135.7

7

7.22 d (J = 10.0)

-----------

27.3; 135.7 and 142.3 11

ACCEPTED MANUSCRIPT 3.03 dquint (J = 10.0 and

57.6

5.9) 1.52 ddq (J = 13.8; 7.4 and

9a

27.4

5.9) 1.42 ddq (J = 13.8; 7.4 and

9b

27.4

5.9)

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0.82 t (J = 7.4)

9.6

CH3 (3)

2.11 s

15.0

CH3 (4)

2.26 s

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126.8; 128.0 and 138.2 126.8; 138.2 and 142.3

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18.6

9.6; 27.4 and 57.6

Table 2. Data of chemical displacement  and coupling constant J (Hz) for pendimethalin.

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From the analysis of the NMR spectrum of 1H (Figure 8) for the products of the reaction, it can be stated that:

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(a) a great amount of the stock material (indicated in spectrum 1 as P) was left,

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observed by the chemical displacements of: aromatic hydrogen  8.11 ppm, aromatic methyles at  2.26 and 2.14 ppm, aliphatic methyles at  0.79 ppm, hydrogen of the NH  7.10 ppm (d J=10.0Hz) and the hydrogens of the methylenes at  1.25-1.60 ppm (m).

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(b) the presence of aromatic methyles and aromatic hydrogen of two major compounds was observed; they are highlighted in spectrum 1, compound 1 (C1) and

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compound 2 (C2).

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Figure 8. NMR spectrum of 1H of the pendimethalin product at a potential of –1.0 v.

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For compound C2 (Figure 9), the presence of an ethyl radical in position 1 is proposed, since only with the inclusion of this cluster it was possible to better accommodate the values of chemical displacement experimentally and the simulated

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ones. The simulated signs were obtained using the program ACD version 1.1. Another

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change proposed was the reduction of the groups linked to carbons 2 and 6, changing from -NO2 to -NHOH. Again, these groups accommodate the experimental values better

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than the simulated ones.

Figure 9. Structure regarding product C2.

In the same way, it was also possible to propose the structure for compound C1 (Figure 10), where one of the groups -NO2 is reduced to –NHOH, and the other from NO2 to NH2; it is also proposed the presence of the ethyl radical in position 1. The data of the chemical displacements observed are presented in Table 3.

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Figure 10. Structure regarding product C1.

Compound C1

1

3

4

-----

---

5.84 s

130.5

106.2

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--

n.o.

1.85 s

12.6

6

CH3 (3)

CH3(4)

---

2,00 s

20.1

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-----

-----

-----

20.1; 108.7 and 117.4

-----

141.6; 130.5 and 108.7 130.5; 108.7 and 106.2

H

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141.6

108.7

----

-----

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---

117.4

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5

--

1

gHMBC

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2

C

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1

13

H

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Number

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Compound C2

----

-

---------6.6 2s ---1.9 2s 2.1 1s

13

C

gHMBC

123.1

-----

148.7

-----

114.8

-----

131.2

-----

117.2

n.o.

13.2

19.5

19.5; 114.8 and 132.1

-----

148.7; 131.2 and 114.8 131.2; 117.8 and 114.8

*CH2CH3

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ACCEPTED MANUSCRIPT n.o. sign not observed *sign hidden by other signs Table 3. Chemical displacement  (ppm) of 1H and 13C for compounds 1 and 2.

In addition, it was observed, in the spectrum of hydrogen (Figure 8), chemical displacements which indicate the presence of the group NHCH(CH2CH3)2 (Figure 11),

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derived from the loss of its rupture with the aromatic ring. The values of displacement for

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this compound can be seen in Table 4.

Figure 11. Structure regarding the propylimine.

H

13

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1

Number

C via HSQC

2.59-2.66 m

2

1.20-1.35 m

26.2

3

0.88 t J=7.45

10.2

58.0

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Table 4. Chemical displacement  (ppm) of 1H and 13C of the propylimine.

The results presented in this study, for both, the NMR technique and the quantum

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calculations, allow to establish a new mechanistic proposal for this reaction. Thus, a

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possible mechanism of reduction for the pendimethalin molecule is shown in Figure 12.

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Figure 12. Mechanism proposed for the reduction of pendimethalin.

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It can be observed, in product C1, that the reduction involved 10 electrons, and it differs from the number of electrons found by coulometry (8 electrons). This probably occurred due to the formation of several products during the electrolysis, with different

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number of electrons transferred, from 4 for a reduction to NHOH, to 12 for two

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reductions for NH2. In this way, it is impossible to decide how much, from each species determined by NMR, was produced, and, therefore, perform a coherent balance of the number of electrons transferred. Conclusion

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4.

The study of the mechanism of the reaction of reduction through analyses of the

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products of electrolysis showed complex and surprising results. According to the theoretical calculations, there was a reduction of the two nitro groups. These support the data obtained by nuclear magnetic resonance (NMR) for hydrogen and for carbon, where a possible mechanism of partial reduction of the nitro groups for hydroxylamines was identified, followed by a reaction of replacement of the group –NHCH(CH2CH3)2 by a group –CH2CH3. Such conclusions presuppose the transfer of 4 electrons for each one of the two –NO2 of the molecule. In addition, this reduction originates a further chemical reaction, generating the product detected by NMR.

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ACCEPTED MANUSCRIPT Acknowledgements The autors would like to thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação Araucária and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support. Also, we thank the IQSC – USP. References

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[1] A. Saha, D. Bhaduri, A. Pipariya, N.K. Jain, B.B. Basak, Behaviour of pendimethalin and oxyfluorfen in peanut field soil: effects on soil biological and biochemical activities, Chem. Ecol. 31 (2015) 550-556. [2] V.B. Megadi, P.N. Tallur, R.S. Hoskeri, S.I. Mulla, H.Z. Ninnekar, Biodegradation of pendimethalin by

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Bacillus circulans, Indian J. Biotechnol. 9 (2010) 173-177.

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[3] P.J. Christoffolleti, R.F.L. Ovejero, J.C. Carvalho, Aspectos de Resistência de Plantas Daninhas a Herbicidas, fourth ed., Associação Brasileira de Ação e Resistência de Plantas aos Herbicidas, Campinas, 2016.

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[4] US Environmental Protection Agency, Reregistration Eligibility Decision (RED): Pendimethalin. www.epa.gov/oppsrrd1/REDs/ 0187red.pdf, 1997 (accessed in 18.03.17). [5] S. Patel, M. Bajpayee, A.K. Pandey, D. Parmar, A. Dhawan, In vitro induction of cytotoxicity and DNA

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strand breaks in CHO cells exposed to cypermethrin, pendimethalin and dichlorvos, Toxicol. in vitro 21 (2007) 1409-1418.

[6] M. Sreedhar, J. Damodar, N.V.V. Jyothi, S.R.J. Reddy, Polarographic behavior and determination of

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pendimethalin in formulations and environmental samples. Bull. Chem. Soc. Jpn. 73 (2000) 2477-2480. [7] M. Kotouček, M. Opravilová. Voltammetric behaviour of some nitropesticides at the mercury drop

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electrode, Anal. Chim. Acta 329 (1996) 73-81.

[8] Z. Wang, Y. Du, F. Zhang, Z. Zheng, X. Zhang, Q. Feng, C. Wang, Photocatalytic degradation of pendimethalin over Cu2O/SnO2/graphene and SnO2/ graphene nanocomposite photocatalysts under visible

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light irradiation. Mater. Chem. Phys. 140 (2013) 373-381. [9] S. Wang, W. A. Arnold, Abiotic reduction of dinitroaniline herbicides. Water Research 37 (2006) 41914201.

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[10] Hypercube, HyperChem: Molecular Visualization and Simulation, Ontario, 1992. [11] Semichem, AMPAC 6.0: User’s manual, Shawnee, KS, 1994. [12] Gaussian. GAUSSIAN 03, Pittsburgh, 2003. [13] U.C. Singh, P.A. Kollman, An approach to computing electrostatic charges for molecules, J. Comput. Chem., Jpn. 5 (1984) 129-145.

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HIGHLIGHTS  A possible mechanism for pendimenthalin reduction was proposed;  There was reduction of the two nitro groups to hydroxylamine of pendimethalin;  Also, one nitro group of the molecule reduced to amine;  It was observed a rearrangement of –NHCH(CH2CH3)2 cluster to a –CH2CH3;  Intermediates and products was proposed, using chemical quantum calculations.

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