Anna Turgułaa,
Konrad Stęsika,
Katarzyna Maternaa,
Tomasz Klejdyszb,
Tadeusz Praczykb and
Juliusz Pernak*a
aFaculty of Chemical Technology, Poznan University of Technology, ul. Berdychowo 4, Poznan 60-965, Poland. E-mail: juliusz.pernak@put.poznan.pl
bInstitute of Plant Protection – National Research Institute, ul. Władysława Węgorka 20, Poznan 60-318, Poland
First published on 27th February 2020
Ionic liquids that belong to the third-generation designs due to their intended biological activity are compounds with high potential applications as plant-protection products. The present study describes the synthesis and characterization of novel ionic liquids with cations based on the alkyl derivatives of 1,4-diazabicyclo[2.2.2]octane (DABCO) and an anion derived from naturally occurring pelargonic acid. The developed synthesis method allowed obtaining products with a high yield (≥96%), and the liquids were characterized as high-viscosity liquids at room temperature. This allowed classifying the products as ionic liquids (ILs). The structures of the obtained ILs were confirmed on the basis of their NMR and IR spectra as well as by elemental analysis. All the products exhibited surface activity and were capable of partially wetting a hydrophobic surface. The tested ionic liquids exhibited higher herbicidal activity against winter oilseed rape (Brassica napus L.) and common lambsquarters (Chenopodium album L.) at a lower dose compared to a commercial preparation in greenhouse studies. The studied ionic liquids also exhibited different effects as antifeedants on various insect species. The best results were obtained against beetles belonging to the granary weevil species (Sitophilus granarius L.). The relation between the surface-tension-reduction efficiency pC20 and biological activity was investigated. The herbicidal activity was also correlated with the value of the contact angles for the studied pelargonates. All the obtained results indicate that the designed and synthesized ionic liquids possess double biological functions: herbicidal activity and deterrent activity.
The first herbicidal ionic liquids (HILs), which incorporated MCPA and 2,4-D, were reported in 2011 and their high herbicidal activity as well as the ability to regulate the toxicity of the herbicide by selecting an appropriate cation was highlighted.8 To date, HILs with the following herbicides in the anion structure have been described in the literature: 2,4-DP,12 MCPB,13 MCPP,14 dicamba,15 fomesafen,16 clopyralid,17 bentazone,18 glyphosate,19,20 metsulfuron methyl,21 nicosulfuron,9 nonanoic acid22 and picloram.23 The low volatility, which is characteristic for HILs, allows limiting the negative health effects during the application of the product.
Food deterrents, also known as antifeedants, are substances or mixtures that limit the feeding of pests by affecting the taste receptors of both the larval and adult forms of the pests. Obtaining natural deterrent agents is highly expensive, leading to the search for synthetic replacements. Sweet ILs that possess deterrent properties include anions such as saccharinate,10 cyclamate24 or acesulfame.25 Deterrent ILs with a pelargonate anion have also been reported.26
1,4-Diazabicyclo[2.2.2]octane (common name DABCO) is a bicyclic compound that belongs to the group of tertiary diamines. It is widely used as a catalyst for many reactions, both in pristine and modified forms.27–29 Long-chain alkyl DABCO derivatives, which are quaternary halides, qualify as surfactants due to their amphiphilic properties. The carbon chain of such compounds is the hydrophobic fragment, whereas the structure of the quaternized DABCO amine is the hydrophilic “head”.30 Quaternary DABCO derivatives exhibit bactericidal activity. Polycations containing DABCO moieties in their structure have exhibited activity against Gram-negative bacteria.31 Compounds containing 1,4-diazabicyclo[2.2.2]octane in their structure have proved useful in genetic engineering, especially during transfection of the gene, as they can inhibitor the mRNA sequence encoding the luciferase protein.32 Recently, HILs containing a monoalkyl derivative of DABCO as the cation and a herbicidal anion were shown to possess double biological activity. The anion derived from 4-chloro-2-methylphenoxyacid (MCPA) provided plant protection, while the cation introduced surfactant and bactericidal properties.33
Pelargonic acid is a naturally occurring compound in geranium flowers, where it is present in the form of essential oils. It exhibits non-selective herbicidal activity; however, the currently used commercial preparations are economically inefficient due to the necessity to apply high doses of around 12–16 L per hectare. Additionally, the deterrent activity of pelargonic acid has been confirmed against the common forest pest belonging to the large pine weevil species (Hylobius abietis)34 and the storage pest belonging to the hide beetle species (Dermestes maculatus).35
The third generation of ILs may incorporate a potentially wide range of numerous commercial and widely used pesticides, and have been well researched in terms of their properties, including toxicity in relation to non-target organisms. A thorough review of pesticides in the EU resulted in the withdrawal of many effective measures that had been used for a long period. It remains an open question as to whether the replacement of the active substance of a pesticide in the form of an ionic liquid may lead to a change in its properties and eliminate the reasons why the substance was withdrawn from the market.
This study describes novel ILs with an alkyl derivative of 1,4-diazabicyclo[2.2.2]octane as the cation and an anion based on pelargonic acid. The obtained ILs combined two functions: herbicidal activity and deterrent activity.
The prepared ILs were dissolved in a mixture of water and methanol (the concentration of methanol was equal to 5%).
The prepared ILs were dissolved in a mixture of water and ethanol (1:1 v/v) at the amount corresponding to the doses of 5440 g and 8160 g of pelargonic acid per 1 ha. These doses accounted for ½ and ¾ of the recommended dose, respectively. The commercial product was a formulation containing 680 g of pelargonic acid in 1 L Beloukha 680 EC (Belchim Crop Protection, Londerzeel, Belgium). It was dissolved in water at doses of 5440 and 8160 g per ha of active substance. The applications were conducted at the 6 leaf growth stage using a moving sprayer (APORO, Poznan, Poland) with a TeeJet® VP 110/02 (TeeJet Technologies, Wheaton, IL, USA) flat-fan nozzle capable of delivering 200 L per ha of spray solution at 0.2 MPa operating pressure. The efficacy of the tested compounds was evaluated two weeks after treatment (2 WAT) using the fresh weight reduction method. Data are expressed herein as the per cent of fresh weight reduction with a standard error (SE), which was calculated using the following equation:
Choice and no-choice tests were carried out in accordance with the methodology developed by Prof. Jan Nawrot.38 Waffle discs (1 cm × 1 mm in diameter) were saturated by immersion in solvent, either methanol (control) or in a 1% solution of the tested compound. After evaporation of the solvent (after 30 min air drying), the discs were given to 3 beetles belonging to the S. granarius species, 20 beetles and 10 larvae belonging to the T. confusum species and 10 larvae belonging to the T. granarium species. Tests were carried out separately for each species. The number of insects used for the test was determined earlier, on the basis of the possibilities and pace of food intake by particular species and their developmental stages. The adult specimens used for the experiments were not divided by gender. Wafer discs were weighed prior to feeding and after a period of 5 days of feeding. Each test was performed in 5 replicates. Three coefficients (relative R, absolute A and total T) were calculated on the basis of the difference in weight of the discs before and after the feeding of the insects, using the formulas:
The measure of the deterrent activity of the tested compounds was the total coefficient of deterrence: T = A + R.
A simplified scheme of testing the deterrent activity of the obtained ILs towards the selected storage pests is shown in Fig. 1.
Fig. 1 Scheme for testing the deterrent activity of the obtained ILs towards the selected storage pests. |
The total coefficient T could reach values ranging from −200 to +200, and the following intervals were used to evaluate the biological activity of a given compound:
• Compounds with T values ranging from 151 to 200 were classified as very good deterrents,
• Compounds with T values between 101 and 150 were classified as good deterrents,
• Compounds with T in the range of 51–100 were classified as compounds with medium deterrent properties,
• Compounds with T values below 50 were classified as weak deterrents,
• Negative T values indicated that the compound acts as an attractant.
The biological properties of the studied ILs were compared with the results of the studies of the best known and most practically used compound: azadirachtin.24
The methodology used and described above has been successfully used in previous studies regarding ionic liquids.39–41
The first step was the quaternization of bicyclic tertiary diamine (DABCO), which allowed preparing the 1-alkyl-1-azonia-4-azabicyclo[2.2.2]octane bromides according to the described methodology.42 The synthesized monoalkyl bromides, precursors of the ILs, were characterized in an earlier report, in which the physicochemistry of the compounds and their antimicrobial properties were presented.33 In the second step, the bromide anions were replaced with hydroxide anions in the exchange reaction. The reaction was carried out in anhydrous methanol at room temperature for 60 min, followed by filtration of the precipitated by-product (KBr). In the third step, the obtained solutions of 1-alkyl-1-azonia-4-azabicyclo[2.2.2]octane with hydroxides were neutralized with a stoichiometric amount of a methanolic solution of pelargonic acid. The resulting products were hot dissolved in a mixture of acetone–methanol in ratios from 3:1 to 3:2 (v/v), followed by filtration of the inorganic residues. After evaporation of the solvents, the products were dried in a vacuum oven at 60 °C for 24 h. High yields (≥96%) were obtained for 8 new pelargonates, which were liquids with high viscosity at room temperature, and could therefore be classified as ILs. The compounds contained from 4 to 18 carbon atoms in an alkyl substituent in the cation structure. The water content, which was lower than 400 ppm, was determined using the Karl Fischer method. The percentage amounts of precursors and ILs in the products of the individual reaction stages were also determined using the extraction titration method,36 in a two-phase system, commonly used for long-chain compounds. Titration was repeated three times for each compound. Both ILs and the base precursors were characterized by a high purity of ≥97%. The yields of the synthesis and the purity of the ILs are presented in Table 1.
IL | R | Short | Yield [%] | Purity [%] |
---|---|---|---|---|
1 | C4H9 | [D4][PEL] | 96 | — |
2 | C6H13 | [D6][PEL] | 96 | — |
3 | C8H17 | [D8][PEL] | 98 | 97 |
4 | C10H21 | [D10][PEL] | 99 | 99 |
5 | C12H25 | [D12][PEL] | 96 | 97 |
6 | C14H29 | [D14][PEL] | 98 | 97 |
7 | C16H33 | [D16][PEL] | 99 | 99 |
8 | C18H37 | [D18][PEL] | 99 | 97 |
The structures of the ILs were confirmed by 1H and 13C NMR spectroscopy, which allowed observing signals originating from the cation and the anion. Elemental CHN analysis was also carried out, which confirmed the purity of the ILs. Mass spectroscopy analysis (ESI-Q-TOF) of the obtained ionic liquids was also performed. To record the cations and anions in the obtained ILs, the positive ionization mode and negative ionization mode were applied, respectively. The NMR and ESI-MS spectra of the obtained ILs and a comparison of the IR spectra of the substrates and ionic liquid 6 and a description of the elemental analysis results are included in the ESI (Fig. A.1–A.16 and Table A.1).†
In the proton spectrum, signals originating from the hydrogen atoms in the bicyclic structure could be observed, which could be attributed to the protons in the CH2 groups at the tertiary nitrogen atom occurring at σ = 3.2 ppm (t, 6H) and at the quaternary nitrogen atom at approx. values of σ = 3.4 ppm (t, 6H). Between them, it was possible to observe the signal originating from the protons in the first CH2 group of the alkyl substituent bound to the quaternary nitrogen atom at approx. values of σ = 3.2–3.3 ppm (t, 6H). The carbon atom originating from the carboxylate anion in the ILs structure was visible at a shift value of approx. σ = 181.7–182.5 ppm.
Chemical shifts of the carbon atoms in the bicyclic structure of cation occurred equally at the tertiary and quaternary nitrogen atoms at σ = 46.1–46.3 ppm (3C) and σ = 53.4–53.6 ppm (3C), respectively. The carbon atom of the methylene group attached directly to the quaternary nitrogen atom was present at σ = 65.7–65.9 ppm.
In order to confirm the presence of appropriate functional groups, the transmittance of the infrared spectrum for the pelargonic acid precursor with the tetradecyl substituent in the cation structure and ionic liquid 6 with the same alkyl chain length in the cation were compared. In addition to the standard vibrations originating from the methyl and methylene groups, some differences were noted. The stretching vibrations originating from the CO in the pelargonic acid structure occurred at a value of approx. 1709 cm−1, while the stretching vibrations of the carboxylate anion occurred at lower values of approx. 1566 cm−1. Stretching vibrations in the bicyclic structure of the cation originating from O–N+–R bonds occurred in the spectrum at higher values compared to the vibrations attributed to the bonds of the nitrogen atom with a free electron pair.
The solubility of the synthesized ILs was investigated to determine their application potential. All tests were conducted in 10 solvents with a different index of polarity in the Snyder scale. All the tested ILs were completely soluble in the reaction medium, which was methanol. Some of the ILs were soluble in water, while the pelargonic acid was not soluble in this solvent. ILs 1 and 2 with a short alkyl substituent (with 4 and 6 carbons in the alkyl chain) were insoluble in isopropanol, but ILs with longer substituents were partially soluble in this solvent. Almost every IL was completely or partially soluble in acetone, except for 7 and 8. The synthesized ILs were insoluble in DMSO, acetonitrile, ethyl acetate, chloroform, toluene and hexane. The results of the solubility tests are presented in Table 2.
Compound | Water | Methanol | DMSO | Acetonitrile | Acetone | Chloroform | Isopropanol | Ethyl acetate | Toluene | Hexane |
---|---|---|---|---|---|---|---|---|---|---|
a + Complete solubility; ± limited solubility; − insoluble. Bold text – protic solvent. | ||||||||||
Snyder polarity index43 | 9.0 | 6.6 | 6.5 | 6.2 | 5.4 | 4.4 | 4.3 | 4.3 | 2.3 | 0.0 |
Pelargonic acid | − | + | + | + | + | + | + | + | + | + |
1 | + | + | − | − | ± | − | − | − | − | − |
2 | + | + | − | − | + | − | − | − | − | − |
3 | + | + | − | − | + | − | ± | − | − | − |
4 | ± | + | − | − | + | − | ± | − | − | − |
5 | − | + | − | − | ± | − | ± | − | − | − |
6 | − | + | − | − | ± | − | ± | − | − | − |
7 | − | + | − | − | − | − | ± | − | − | − |
8 | − | + | − | − | − | − | ± | − | − | − |
IL | Short | CMC (mmol L−1) | γCMC (mN m−1) | pC20 | Γmax × 106 (mol m−2) | Amin × 1019 (m2) | CA [°] |
---|---|---|---|---|---|---|---|
1 | [D4][PEL] | 25.1 | 26.5 | 2.10 | 2.67 | 2.89 | 30.7 |
2 | [D6][PEL] | 20.0 | 26.6 | 2.25 | 3.31 | 2.67 | 31.4 |
3 | [D8][PEL] | 6.31 | 26.5 | 2.75 | 4.45 | 2.52 | 33.6 |
4 | [D10][PEL] | 3.16 | 26.4 | 3.05 | 5.81 | 2.43 | 36.9 |
5 | [D12][PEL] | 2.24 | 26.1 | 3.45 | 6.59 | 2.36 | 37.2 |
6 | [D14][PEL] | 0.794 | 26.9 | 3.95 | 7.48 | 2.21 | 49.0 |
7 | [D16][PEL] | 0.251 | 27.0 | 4.30 | 8.49 | 1.91 | 56.1 |
8 | [D18][PEL] | 0.089 | 29.7 | 4.40 | 8.72 | 1.86 | 59.4 |
The surface tension of aqueous solutions (γCMC) of the studied ILs decreased from the value for the water–methanol mixture to a minimum located from 26.1 to 29.7 mN m−1. Generally, the γCMC values were very similar, as it was noticed for 8 only that this compound did not lower the surface tension as effectively as the others. At the same time, the CMC of this compound was the lowest. Similar to typical surfactants, the CMC value of the studied ILs was a function of the carbon atom numbers in the alkyl chain (Fig. 2). A significant decrease was observed, from 25.1 (1) to 0.089 mmol L−1 (8). Calculations of the surface excess concentration Γmax and the minimum surface area occupied by a molecule at the interface Amin showed that the values of these parameters differed with the elongation of the alkyl chain. For example, the values of Amin for the ILs decreased with the increasing number of carbon atoms in the alkyl chain of the IL molecule, i.e. from 2.89 × 10−19 to 1.86 × 10−19 m2. In the case of an excess surface concentration, higher values were observed for compounds containing longer alkyl chains, with the lowest values being obtained for 1, while the highest were obtained for 8. The analysis of the results in Table 3 allowed establishing the dependence of both parameters, which is a characteristic of surface-active compounds, whereby with the decrease in the maximum surface excess, the minimum surface per one molecule of surfactant adsorbed on the interface increased.
There was an additional surface activity parameter, the adsorption efficiency, pC20, calculated based on the surface-tension measurements. This parameter is a function of the number of carbon atoms in the alkyl group, and increases as the number of carbon atoms increases. The higher pC20 values showed that the ILs were more efficiently adsorbed at the interface and more effectively reduced the surface tension by 20 mN m−1. The highest value of pC20 was obtained equal to 4.40 (Table 3).
The contact angle values obtained for the studied pelargonates (1–8) ranged from 30.7° to 59.4° (Table 3) and better wetting properties were exhibited by ionic liquids with a shorter alkyl chain. The obtained contact angle values were in the range of 0–90°, which means that the obtained ILs were liquids that could partially wet the paraffin surface.
IL | Short | Fresh weight reduction ( ± SE) [%] | |||
---|---|---|---|---|---|
Dose = ½ Nb | Dose = ¾ N | ||||
Common lambsquarters | Oilseed rape | Common lambsquarters | Oilseed rape | ||
a Beloukha 680 EC.b N recommended dose = 10880 g of pelargonic acid per 1 ha. | |||||
5 | [D12][PEL] | 93.48 ± 0.92 | 76.30 ± 9.52 | 93.28 ± 1.10 | 91.64 ± 3.20 |
6 | [D14][PEL] | 87.26 ± 3.46 | 82.82 ± 3.01 | 92.54 ± 1.57 | 95.93 ± 2.24 |
7 | [D16][PEL] | 77.81 ± 4.55 | 66.90 ± 5.50 | 77.95 ± 3.00 | 90.59 ± 3.22 |
8 | [D18][PEL] | 47.03 ± 2.93 | 20.70 ± 7.21 | 36.60 ± 3.10 | 44.16 ± 3.25 |
Referencea | −1.43 ± 4.03 | −10.81 ± 7.13 | 67.40 ± 8.44 | −2.76 ± 5.54 |
Fig. 3 Herbicidal activity of ILs 5–8 tested on common lambsquarters and oilseed rape in ½ and ¾ doses of compounds (C – Control, R – Reference). |
Compound | Adults granary weevil (Sitophilus granarius) | Adults confused flour beetle (Tribolium confusum) | ||||||
---|---|---|---|---|---|---|---|---|
A | R | T | Deterrent activity | A | R | T | Deterrent activity | |
1 | 58.5 | 98.7 | 157.2 | Very good | −18.8 | −0.5 | −19.3 | Attracting |
2 | 66.8 | 43.4 | 110.2 | Good | −0.7 | 12.5 | 11.8 | Weak |
3 | 56.8 | 98.6 | 155.4 | Very good | −18.6 | 39.2 | 20.6 | Weak |
4 | 84.0 | 100.0 | 184.0 | Very good | 9.7 | 41.2 | 50.9 | Weak |
5 | 71.2 | 97.2 | 168.4 | Very good | 5.1 | 44.1 | 49.2 | Weak |
6 | 79.0 | 100.0 | 179.0 | Very good | 29.4 | 32.6 | 62.0 | Medium |
7 | 90.9 | 100.0 | 190.9 | Very good | −3.0 | 65.0 | 62.0 | Medium |
8 | 79.7 | 100.0 | 179.7 | Very good | 37.4 | 61.0 | 98.4 | Medium |
Azadirachtin | 74.3 | 100.0 | 174.3 | Very good | 85.0 | 100.0 | 185.0 | Very good |
Compound | Larvae khapra beetle (Trogoderma granarium) | Larvae confused flour beetle (Tribolium confusum) | ||||||
---|---|---|---|---|---|---|---|---|
A | R | T | Deterrent activity | A | R | T | Deterrent activity | |
1 | 16.6 | 36.4 | 53.0 | Medium | −33.1 | 22.1 | −11.0 | Attracting |
2 | −0.1 | 44.6 | 44.5 | Weak | −27.1 | 48.8 | 21.7 | Weak |
3 | 43.5 | 44.0 | 87.5 | Medium | −1.4 | 20.2 | 18.8 | Weak |
4 | 68.7 | 80.9 | 149.6 | Good | −25.7 | 68.2 | 42.5 | Weak |
5 | 73.2 | 85.0 | 158.2 | Very good | −1.9 | 22.1 | 20.2 | Weak |
6 | 80.8 | 94.4 | 175.2 | Very good | −15.5 | 64.0 | 48.5 | Weak |
7 | 77.0 | 81.5 | 158.5 | Very good | −16.6 | 90.1 | 73.5 | Medium |
8 | 71.8 | 82.1 | 153.9 | Very good | 14.6 | 95.9 | 110.5 | Good |
Azadirachtin | 94.2 | 100.0 | 194.2 | Very good | 88.4 | 100.0 | 188.4 | Very good |
Total coefficient T | Deterrent activity |
---|---|
151–200 | Very good |
101–150 | Good |
51–100 | Medium |
50–0 | Weak |
<0 | Attracting |
Fig. 4 Deterrent activity depending on the number of carbons in the alkyl substituent of the cation in the ILs on the khapra beetle larvae and granary weevil. |
Data were analyzed using a one-way ANOVA statistical approach. In cases where the ANOVA test values reached a 5% significance level, the Tukey test was performed and the LSD obtained values are presented in the ESI (Table A.2).†
Fig. 7 Relationship between the plant's fresh weight reduction and contact angle for ¾ N dose of ILs 5–8. |
However, the deterrent activity increased with the higher values of pC20 (Fig. 8). The most effective homologues start with a pC20 value of approx. 3. To the best of our knowledge, this is the first report that presents a dependence between the deterrent activity and surface activity.
All the ILs were completely soluble in methanol. Some of the ILs were soluble in water (1–4), isopropanol (3–8) and acetone (1–6). The synthesized ILs were insoluble in DMSO, acetonitrile, ethyl acetate, chloroform, toluene and hexane.
The tested ILs were surface-active compounds capable of lowering the surface tension of the solvent at the same level as classical surfactants. The obtained values of contact angles were in the range of 0–90°, which means that they are liquids capable of partially wetting the hydrophobic surface (paraffin). All the ILs were completely soluble in methanol. Some of the ILs were soluble in water (1–4), isopropanol (3–8) and acetone (1–6). The synthesized ILs were insoluble in DMSO, acetonitrile, ethyl acetate, chloroform, toluene and hexane.
The highest herbicidal activity was observed in the cases of ILs 5 and 6, which possessed 12 and 14 carbon atoms in the alkyl chain, respectively, to the following target weed species: common lambsquarters and oilseed rape. Regardless of the dose of the active substance, ILs 5 and 6 were more effective than the commercial formulation used.
Some of the obtained ILs were characterized by very good deterrent properties and effectively limited the feeding of such globally important storage pests as the granary weevil (1, 3–8) and khapra beetle (5–8). The mentioned insects belong to two taxonomically distant super-families: Curculionoidea (weevils) and Bostrichoidea (khapra beetle). The fact that these species are not closely related and react similarly to the synthesized ILs may be the basis for concluding that, in the case of other species, not only storage but also field pests, these substances may find use as an alternative to the currently used, often very toxic and environmentally harmful insecticides.
The obtained correlations allowed observing a reverse dependence, whereby the herbicidal activity decreased with the increasing value of pC20. At the pC20 value of 4.40, the herbicidal activity of the compounds was lost. At the same time, it was noticed that the value of the contact angle increased with the increase in the number of carbon atoms in the alkyl substituent, which resulted in a lower wetting of the leaf surface and, consequently, a lower herbicidal efficacy. During the deterrence tests, the most effective homologues started with a pC20 value of approx. 3.
The obtained results regarding the biological activity allow us to conclude that the synthesized ILs are effective as herbicides and deterrents. Based on this conclusion, these compounds can be referred to as bifunctional ILs.
Footnote |
† Electronic supplementary information (ESI) available: Identification of compounds (1H, 13C NMR, ESI-MS and IR spectra, results of elemental analyses), feeding-deterrent activity and the relationship between the surface-tension-reduction efficiency pC20 and biological activity. See DOI: 10.1039/d0ra00766h |
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