Iodosulfuron-Methyl-Based Herbicidal Ionic Liquids Comprising Alkyl Betainate Cation as Novel Active Ingredients with Reduced Environmental Impact and Excellent Efficacy.

A new family of bio-based herbicidal ionic liquids (HILs) has been synthesized starting from the renewable resource glycine betaine (a derivative of natural amino acids). After esterification, the obtained alkyl betainate bromides containing straight alkyl chains varying from ethyl to octadecyl were combined with a herbicidal anion from the sulfonylurea group (iodosulfuron-methyl). The melting points of the iodosulfuron-methyl-based salts were in a range from 51 to 99 °C, which allows their classification as ionic liquids (ILs). In addition, the new HILs exhibited good affinity for polar and semipolar organic solvents, such as DMSO, methanol, acetonitrile, acetone, and chloroform, while the presence of bulky organic cations reduced their solubility in water. The synthesized products turned out to be stable during storage at 25 °C for over 6 months; however, at 75 °C they underwent fast, progressive degradation and released volatile byproducts. The values of the logarithm of the octanol-water partition coefficient of ILs with alkyls longer than hexyl occurred in the "safe zone" (between 0 and 3); hence, the risk of their migration into groundwater after application or the possibility of their bioaccumulation in the environment is lower in comparison with the currently available commercial form (iodosulfuron-methyl sodium salt). Greenhouse studies confirmed a very high herbicidal efficacy for the obtained salts toward tested plants of oilseed rape, indicating that they may become an attractive replacement for the currently available sulfonylurea-based formulations.

Introduction

Iodosulfuron-methyl-sodium is known as a plant protection product that is widely applied to control monocotyledonous and dicotyledonous weeds in cereal cultivation. This compound belongs to the sulfonylurea compounds, which are used as postemergence herbicides.[1] The mechanism of action of these agrochemicals is based on the inhibition of the enzyme acetolactate synthase (ALS), which catalyzes the first branched-chain amino acid (BCAA) biosynthesis reaction—leucine, valine, and isoleucine.[2] According to recent reports, ALS-inhibiting herbicides are often characterized by high selectivity, high efficiency at low doses, and low toxicity to animals.[3] Interestingly, they were found to be efficient at doses corresponding to approximately 10–20 g/ha, whereas other herbicides, such as the phenoxyacids, require an application rate of the active ingredient many times greater. At present, all possible legal and formal measures are being taken to reduce the amount of plant protection products applied in agriculture.[4] This strategy is associated with concern for both food safety and the environmental impact of such compounds.[5] Simultaneously, much attention is being paid to the necessity of crop protection to enable the feeding of a growing global population of people.[6]

Many years of field studies confirm without doubt that the effectiveness of herbicides may be enhanced by the utilization of additives called adjuvants, even if the active ingredient is applied at a reduced dose.[7] Adjuvants allow the achievement of high activity and maintenance of satisfactory herbicidal efficacy, despite the occurrence of certain adverse conditions during or after treatment.[8] Lately, increasing attention is being paid to the safety of these substances. Since they are not considered as active ingredients, the limitations associated with their use are not as restrictive as in the case of the herbicides themselves.[9] The most famous example demonstrating the detrimental influence of adjuvants refers to the reported high toxicity of the adjuvant polyethoxylated tallow amine, which was added to the composition of plant protection products containing glyphosate.[10] This case justifies research focused on the development of new, efficient, and more eco-friendly forms of known herbicides, such as herbicidal ionic liquids (HILs). Generally, HILs are defined as salts composed of ions which occur in the liquid state below 100 °C and possess at least one ion exhibiting herbicidal activity.[11] They constitute a peculiarly interesting extension of the concept of biologically active ionic liquids (ILs), formerly focused on the pharmaceutical industry.[12] The application of ILs may become exceptionally profitable in the agrochemical industry, wherein the bioavailability and high absorption of the active ingredient is crucial. According to recent estimates, up to 99% of the currently applied pesticidal formulations might not reach the targeted pests directly and instead affect nontarget organisms, often causing irreversible changes in the natural biological balance of the agricultural landscape.[12−14] It has been suggested that efforts directly associated with fine-tuning of the physicochemical properties of HILs lead to the formation of structures with peculiar final properties. These include increased efficacy of new substances and formulations due to their improved permeability across biological membranes.[15] Moreover, the selection of appropriate cation–anion combinations allows a flexible design of HILs that exhibit the desired environmental advantages (e.g., low acute toxicity toward mammals,[11,16] good biodegradability,[17] or reduced mobility in soil[18−20]). Therefore, reports describing the synthesis and characterization of HILs derived from renewable sources such as carnitine,[17]d-glucose,[21] or acetylcholine[22] have appeared recently.

N,N,N-Trimethylglycine, commonly known as glycine betaine or betaine, constitutes an abundant raw material representing approximately 27% of sugar beet molasses. It is obtained after the extraction of sucrose and currently remains a poorly developed byproduct of the sugar industry. On a volumetric scale, its largest global application is for animal nutrition.[23] This naturally sourced compound is readily biodegradable and practically nontoxic (acute oral LD50 for rats ∼11000 mg per kg, fifth category according to GHS). It is nonmutagenic as well as nonallergenic and has been found to improve moisture retention of the skin. Thus, glycine betaine is used currently as an additive to skin creams and ointments, medicated cleansers, after-shave lotions, and deodorants.[24,25] Interestingly, betaine has been successfully applied in the pharmaceutical industry to prevent disturbances in liver methionine metabolism and in treatment of the metabolic disease “homocystinuria” caused by the inefficient recirculation of homocysteine to methionine.[26] Glycine betaine could also be utilized in crop production because it accumulates in many plant species under stress. Field experiments indicate that it may improve the yield of commercial fruits and vegetables by more than 20% under heat and salt stress when it is applied during the midflowering stage.[23,27] In addition to this dynamic development in science, the perception of betaine has changed dramatically—from what was previously called a “waste product and processing problem”—to a compound with great potential for application as well as a highly valuable reactant, useful for the conception of new eco-friendly chemicals.

To obtain “green” ILs, the starting materials must be at least nontoxic, while for a perfect solution, they should be renewable. Moreover, the development of nonhazardous ILs still requires a relatively low cost synthetic route and easy preparation. Biorenewable natural compounds are ideal materials from both environmental and economic viewpoints.[24,28,29] Interest in research on and application of bio-based surfactants is progressively increasing due to their environmentally friendly nature and lower toxicity in comparison with fully synthetic surfactants.[30] Within this context, the aim of this study was focused on the development of a convenient method for the conversion of glycine betaine, known both as a cost-effective raw material of plant origin and as being nontoxic to humans and the surrounding environment, to multifunctional ionic agents for the effective control of weeds in cultivated plants. Because of the O-alkylation of glycine betaine, the novel salts contain glycine betaine esters as the cationic moiety, which allows their classification as “esterquats”. It should be mentioned that some of the previously synthesized glycine betaine esters were successfully used as cationic surfactants in the formulation of new emulsions with improved biodegradability.[24] Recently, the use of other forms of betaine (comprising an unesterified carboxylic group in the cation) in the synthesis of HILs was reported.[16,17] In this research, the subsequent combination of betaine ester cations with an herbicidal anion from the sulfonylurea group (iodosulfuron-methyl) led to the formation of new HILs. To our knowledge, the utilization of alkyl betainates for the synthesis of herbicidally active ILs has never been reported. This paper demonstrates the efficient synthesis and characterization of 13 new salts incorporating various alkyl chain lengths (from C2 to C18) in the cation. Furthermore, the influence of the alkyl chain length in the cation on the physicochemical properties (such as melting point, solubility, volatility, or logarithm of octanol–water partition coefficient) of the products as well as their biological activity toward oilseed rape was investigated to elucidate structure–property relationships and, if possible, to select the compound characterized by the most beneficial features.

Experimental Section

Materials

Bromoethane 98%,1-Bromopropane 99%, 1-bromobutane 99%, 1-bromopentane 98%, 1-bromohexane 98%, 1-bromoheptane 99%, 1-bromooctane 99%, 1-bromononane 98%, 1-bromodecane 98%, 1-bromododecane 97%, 1-bromotetradecane 97%, 1-bromohexadecane 97%, 1-bromooctadecane 97%, and betaine 99% were purchased from Sigma-Aldrich (Saint Louis, MO, USA). Iodosulfuron-methyl sodium salt (purity 96.6%) was obtained from PESTINOVA (Jaworzno, Poland). All solvents (methanol, DMSO, acetonitrile, acetone, isopropanol, ethyl acetate, chloroform, toluene, hexane) and potassium hydroxide were obtained from Avantor (Gliwice, Poland) and used without further purification. Deionized water with a conductivity of <0.1 μS cm–1, from Hydrolab HLP Smart 1000 demineralizer (Straszyn, Poland), was used.

Methods

General Considerations

1H NMR spectra were recorded on a Mercury Gemini 300 spectrometer operating at 300 MHz and a Varian VNMR-S 400 MHz spectrometer with TMS as the internal standard. 13C NMR spectra were obtained with the same instruments at 75 and 100 MHz, respectively. The IR spectra were collected by using a Mettler Toledo EasyMax 102 semiautomated system (Greifensee, Switzerland) connected to a ReactIR iC15 (Mettler Toledo) probe equipped with an MCT detector and a 9.5 mm AgX probe with a diamond tip. The data were sampled from 3000 to 650 cm–1 with 8 cm–1 resolution and processed by iCIR 4.3 software. Melting points of the obtained compounds were analyzed via a Mettler Toledo MP 90 melting point system.

Solubilities

The solubilities of the prepared ILs in 10 representative solvents were determined according to the protocols in ref (31). The solvents chosen for study were arranged in order of descending value of their Snyder polarity index: water, −9.0; methanol, −6.6; DMSO, −6.5; acetonitrile, −6.2; acetone, −5.1; ethyl acetate, −4.3; isopropanol, −4.3; chloroform, −4.1; toluene, −2.3; hexane, −0.0. A 0.1 g sample of each IL was added to a certain volume of solvent, and the samples were thermostated in a MEMMERT Model WNB 7 water bath at 25 °C. On the basis of the volume of solvent used, three types of behaviors were recorded: “soluble” applies to compounds which dissolved in 1 cm3 of solvent (>10%), “limited solubility” applies to compounds that dissolved in 3 cm3 of solvent (3.3–10%), and “not soluble” applies to the compounds which did not dissolve in 3 cm3 of solvent (<3.3%).

Octanol–Water Partition Coefficients

The octanol–water partition coefficients (KOW) of the synthesized ionic liquids (1, 3, 5, 7, 9–13) as well as the sodium salt of iodosulfuron-methyl were estimated by the shake-flask method according to OECD guidelines (OPPTS 830.7550, partition coefficient (n-octanol/water), shake flask method). Measurements of KOW values were conducted using mutually saturated distilled water and n-octanol in a glass vial containing a magnetic stir bar. First, the synthesized product or iodosulfuron-methyl sodium salt was dissolved in 4 cm3 of distilled water in amounts corresponding to the dose applied in greenhouse experiments (98.77 μM), and then 4 cm3 of octanol was added. All vials were shaken at a constant temperature of 25 °C. After 15 min, all samples were centrifuged and the aqueous and octanolic phases were collected by a syringe. The concentrations of compounds in water (iodosulfuron-methyl sodium salt, 1, 3, 5) or octanol (7, 9–13) were determined spectrophotometrically using a UV/vis spectrophotometer (based on formerly made calibration curves with plots of absorbance (at λmax = 235 nm in water and at λmax = 236 nm in octanol) vs concentration for each substance). Three repetitions of each measurement were performed.

Thermal Stability

Before the experiment, the sodium salt of iodosulfuron-methyl and the synthesized compounds containing ethyl (P1, 1), hexyl (P5, 5), decyl (P9, 9), tetradecyl (P11, 11), and octadecyl (P13, 13) substituents were additionally dried in a Schlenk line at 40 °C for 6 h under reduced pressure (<1 mbar) in order to avoid the effects of residual water or other solvents. Then, 0.1000 g (±0.0005 g) of each compound was weighed using a Mettler Toledo MS204 electronic balance containing a vessel with 50 g of P4O10 and stored in an isothermal environment (MEMMERT UF55) at 75 °C under a constant flow of air. After 48 h (2 days) and 168 h (7 days), the mass of each compound was measured. The volatilization rate was calculated as the weight loss of the test compound divided by its original weight. The results are presented as the mean of triplicate experiments.

Surface Activity

Surface tension and contact angle measurements were performed with a DSA 100 analyzer (Krüss, Germany, accuracy ±0.01 mN m–1), at 25 °C. The shape drop method was utilized to determined surface tension. The principle of this method is based on the formation of an axisymmetric drop at the tip of a needle of a syringe, and the image of the drop (3 cm3) is taken with a CCD camera and digitized. The surface tension (γ in mN m–1) of the spray solutions used in the greenhouse experiments was calculated by analyzing the profile of the drop according to the Laplace equation. The temperature during the experiment was controlled using a Fisherbrand FBH604 thermostatic bath (Fisher, Germany, accuracy +0.1 °C).

Greenhouse Experiments

Oilseed rape (Brassica napus L., BRSNW) plants were used to test the biological activity of the examined compounds. Seeds of the selected plants were sown in plastic pots (1.0 dm3, 15 cm diameter) containing a peat-based substrate. Plants were grown in a greenhouse with a photoperiod of 16 h day/8 h night. The temperature was maintained at 25 ± 2 °C during the day and at 20 ± 2 °C during the night. The relative air humidity was approximately 60–80%. The soil moisture was maintained at 65–75% of soil water capacity. The seedlings were thinned 2 weeks after emergence to six uniform plants per pot. The greenhouse trial was designed as a randomized complete block with four replications. Herbicides were applied when the plants were at the four-leaf stage (BBCH 14) using a laboratory sprayer equipped with a spray chamber using Tee Jet 1102 (TeeJet Technologies GmbH, Schorndorf, Germany) nozzles delivering 200 dm3 ha–1 at 0.2 MPa. All tested products (1–13) were dissolved in water at amounts that correspond to a dose of 10 g of ai (active ingredient) per hectare. A commercial product containing iodosulfuron-methyl sodium (Autumn, Bayer CropScience) was used at the same dose. In the next stage of the experiment, the standard sodium salt of iodosulfuron-methyl and one of the most effective compounds (11) were applied at various doses equal to 0.25, 0.5, 0.75, 1, and 1.25 n (wherein n = 10 g of active ingredient per ha). Weed control was evaluated visually 21 days after treatment (DAT) using a scale of 0 (no effect) to 100% (completely destroyed plant). After the experiments, the weights of the plants were tested. Statistical analysis was conducted using Statistica software (Version 12, StatSoft Inc., Tulsa, OK, USA). Data were subjected to ANOVA followed by Tukey’s protected LSD test at the 0.05 probability level.

Preparation of ILs

(2-Alkyloxy-2-oxoethyl)trimethylammonium (alkyl betainate) bromides were synthesized according to a previously described protocol demonstrating the synthesis of alkylated analogues of l-carnitine.[32] Compounds were synthesized via the O-alkylation reaction of glycine betaine (in a zwitterionic form) and an appropriate linear 1-bromoalkane (from C2H5Br to C18H37Br). Betaine (0.02 mol) and 1-bromoalkane (0.025 mol) were mixed with 50 cm3 of acetonitrile, and the obtained mixture was heated at 40 °C (for P1), 70 °C (for P2), or 80 °C (for P3–P13) under reflux for 24–72 h. After evaporation of the solvent, the crude product was washed three times with 15 cm3 of ethyl acetate to eliminate unreacted 1-bromoalkane and eventually dried under vacuum (5–10 mbar) at 50 °C for 24 h.

All ion exchange reactions were performed using an Easy-Max reactor. The appropriate (2-alkyloxy-2-oxoethyl)trimethylammonium (alkyl betainate) bromide (0.01 mol) was dissolved in 15 cm3 of methanol in a 100 cm3 reaction vessel equipped with a mechanical stirrer. Next, a 2% molar excess (0.0102 mol) of the sodium salt of iodosulfuron-methyl in stoichiometric excess, dissolved in 15 cm3 of methanol, was added to perform the ion exchange reaction. The reaction mixture was stirred at 50 °C for 15 min and then cooled to 0 °C. Due to anion exchange, a sediment of sodium bromide precipitated from the postreaction mixture. Subsequently, the inorganic salt was filtered off and the solvent was evaporated from the filtrate. The obtained products were additionally purified through leaching with a small portion (10–15 cm3) of acetone (1–12) or a 1/1 v/v acetone/acetonitrile mixture (13) to remove the traces of inorganic impurities. In order to remove excess reagent, all of the products were dissolved in 15 cm3 of chloroform, and then the precipitate was filtered off and the solvent was evaporated from the filtrate. Finally, the obtained products were dried at 40 °C for 24 h under reduced pressure (1–2 mbar). All of the synthesized salts were stored in a vacuum desiccator over a drying agent (P4O10).

Results and Discussion

Synthesis

Initially, we referred to the sustainable molecular design of an organic cation, which was obtained from the key reagent—naturally occurring glycine betaine. In the first step, a homologous series of (2-alkyloxy-2-oxoethyl)trimethylammonium (alkyl betainate) bromides was synthesized via the O-alkylation reaction of glycine betaine with an appropriate bromoalkane (Scheme ).

Scheme 1

Synthesis of Salts Comprising Alkyl Betainate as the Cation and Iodosulfuron-Methyl as the Anion

As shown in Table , the bromides (P1–P13), all containing a straight alkyl chain in the cation which varied from ethyl to octadecyl, were obtained in high yields (87–95%). The second synthesis step was based on a metathesis reaction in methanol, wherein the bromide anions in P1–P13 were replaced with the herbicidal iodosulfuron-methyl ion in alcoholic medium using an Easy-Max reactor. As a result of the ion exchange, a stoichiometric amount of the inorganic salt was separated from the postreaction mixtures. This means that the products obtained are not eutectic mixtures but new ionic pairs (details of the syntheses are provided in the Experimental Section). The halide level for each obtained salt, determined by the AgNO3 test, was below 1000 ppm. The utilized synthesis methodology has been proven to be substantially effective in comparison with ion exchange conducted in water. Thus, some issues such as the separation of phases during a two-phase extraction from an aqueous environment or formation of a foam during evaporation of water (caused by the presence of a highly surface active cation) can be completely eliminated. In effect, all iodosulfuron-based salts were obtained in high yields of above 90%.

Table 1

Synthesized Salts Comprising Alkyl Betainate as the Cation and Bromide (P1–P13) or Iodosulfuron-Methyl (1–13) as the Anion

salt	R	yield (%)	melting point (°C)	IL	yield (%)	melting point (°C)
P1	C2H5	87	154–156	1	92	91–93
P2	C3H7	92	106–108	2	98	51–52
P3	C4H9	95	99–101	3	95	66–68
P4	C5H11	92	129–131	4	97	72–74
P5	C6H13	89	110–112	5	98	54–56
P6	C7H15	93	103–105	6	96	52–54
P7	C8H17	94	93–95	7	97	71–73
P8	C9H19	90	92–94	8	98	76–78
P9	C10H21	92	103–105	9	98	74–76
P10	C12H25	95	99–101	10	94	80–81
P11	C14H29	90	110–112	11	99	81–83
P12	C16H33	91	102–104	12	96	83–84
P13	C18H37	88	115–117	13	97	98–99

According to the data in Table , alkyl betainate bromides are white solids with melting points ranging from approximately 93 °C (for a salt with the nonyl group (P8)) to approximately 155 °C (for a salt with the shortest ethyl group (P1)). Additionally, all of the synthesized iodosulfuron-methyl-based salts (1–13) turned out to be solids at room temperature. However, their melting points below 100 °C allow them to be classified as ILs. We also notice that the presence of the middle-length alkyl chains in the cation results in a substantial decrease in the melting points of the ILs. This trend has been repeatedly described in the literature for other tetraalkylammonium-, piperidinium-, and imidazolium-based ILs.[33] Such a phenomenon can be explained by the fact that the precisely chosen anion and cation are able to destabilize the solid phase of the crystal. Coulombic interactions are reduced due to the elongation of the cation alkyl chain, which leads to the disruption of lattice packing and reduction of the melting point of a compound.[34] Hence, less energy is required to break the lattice structure. This decreases the melting point of the compound. However, alkyls of too great a length (generally greater than C12) are responsible for an increase in the melting point despite the enhanced asymmetry. This phenomenon is ascribed to the increase in interactions between the lengths of nonpolar groups, as in the case of linear alkanes.[34]

The results in Figure reveal that the replacement of bromide anion by the structurally more complex and asymmetric iodosulfuron-methyl leads to a depression of the melting point by approximately 30–60 °C for compounds containing alkyls shorter than heptyl and approximately 20 °C for salts with longer chains. Moreover, bearing in mind that iodosulfuron-methyl sodium salt melts at approximately 154–157 °C, we may conclude that the combination of this herbicide with an organic cation may result in a reduction in the melting point of up to 100 °C. Interestingly, ILs comprising a structurally similar sulfonylurea, metsulfuron-methyl,[35] tend to be liquids or greases at room temperature, mostly independently of the structure of the utilized cation. The molecule of metsulfuron-methyl differs from iodosulfuron-methyl only in the presence of an iodine atom in the benzene ring, which means that this substituent is probably responsible for such alterations. One should take into consideration not only the increase in the molecular weight of the compound, which is known to increase the melting point, but also the fact that the polar C–I bond creates a molecular dipole, which may favor a better packing of the ions and eventually lead to an increase in the melting point. Recently, Rabideau et al. described the possibility of influencing the melting points of ILs through the adjustment of the dipole moment of the cation.[36] Therefore, the strength of the dipole moment, controlled through the incorporation of a halogen into the benzene ring, proves to be a useful tool for influencing this parameter that is often crucial in terms of the application of ILs.

Figure 1

Influence of the alkyl chain length of the cation on the melting point of the obtained bromides (P1–P13) (top of the blue column) and ILs (1–13) (bottom of the blue column).

It is also noteworthy that the results of the following research provide a simple and effective solution that overcomes the limitations that prevent the formation of a successful combination of glycine betaine (and its zwitterionic analogues) in the hydrochloride form with some pesticidal anions, including sulfonylureas (such as iodosulfuron-methyl, metsulfuron-methyl, thifensulfuron-methyl, chlorosulfuron), pelargonate, or even glyphosate.

As was shown in Scheme , all previous attempts to obtain such ILs failed. This was attributed to proton transfer between betaine and the utilized pesticide during an ion exchange reaction. Our research group established that such a phenomenon was caused by a substantially higher binding strength between hydrogen and oxygen in the carboxyl group of the utilized pesticides, due to their higher values of pKa in comparison with betaine.[37] However, replacing the hydrogen with an alkyl group eliminated the risk of proton transfer and allowed us to obtain betaine-type ILs containing sulfonylureas such as iodosulfuron-methyl, which were previously far beyond our reach. Furthermore, it should be stressed that the presence of the alkyl chain brings other exceptional benefits, such as the possibility of adjusting physicochemical properties (e.g., melting point, solubility in water, viscosity) or surface activity and wettability, which are known to be essential in treatments relying on most sulfonylureas.

Scheme 2

Proton Transfer during the Reaction of Betaine Hydrochloride and the Sodium Salt of Iodosulfuron

Spectral Analysis of Synthesized ILs

The structures of the synthesized products were confirmed by UV, FT-IR, and 1H and 13C NMR spectroscopy. All spectral descriptions for salts P2, P6, P10, 1–13 are provided in Figures S1–S62 in the Supporting Information.

Generally, esters such as ethyl acetate in the UV region exhibit an n → π* transition (R band) and possess an absorption maximum (λmax) at approximately 210 nm. After dissolution in methanol, the synthesized betaine-type esterquats P2, P6, and P10 exhibited λmax at 201–202 nm with a molar absorptivity (ε202) varying from 4.24 × 103 to 4.50 × 103 M–1 cm–1. In contrast, the sodium salt of iodosulfuron-methyl possessed two distinct λmax values (λmax1 = 204 nm (E2 band); λmax2 = 239 nm (B band)) with almost 10-fold greater molar absorptivities (ε204 and ε239) equal to 2.87 × 104 and 2.95 × 104 M–1 cm–1, respectively. Analogously, ILs 1–13 containing the iodosulfuron-methyl anion possessed two maxima at 202–204 and 239–240 nm and values of molar absorptivity similar to those of the iodosulfuron-methyl sodium salt. However, their ε202–204 values were found to be strengthened by the n → π* transition from the cation. In effect, the λmax1/λmax2 ratio increased from 0.97 for the sodium salt to 1.06–1.11 for 1–13 (see Table S1 in the Supporting Information).

Comparison of the FT-IR spectra of either the precursors (Figure S63 in the Supporting Information) or the products (Figure S64 in the Supporting Information) revealed a successive growth of the intensity of the bands at 2800–3000 cm–1 as the length of the alkyl substituent in the cation was increased. Since signals in this region are attributed to alkyl C–H stretching vibrations, one can conclude that the designed homologous series was synthesized successfully. Additionally, all of the products possessed a characteristic signal at 930 cm–1 due to asymmetric stretching vibrations of the C–N group (ν(as) ∼ C–N) present in the utilized cations. Moreover, in the FT-IR spectra of ILs we can distinguish strong peaks originating from the iodosulfuron-methyl anion, such as the signal from the amide group (ν ∼ C=O) at 1670 cm–1 or the signal from the ester substituent (ν ∼ C=O) at 1740 cm–1, which overlaps with the peak from the stretching vibrations of the C=O present in the cation. We can also observe some other characteristic bands. These occur at 1520–1580 cm–1, originating from in-plane bending vibrations (δ ∼ N–H) as well as the conjugated stretching vibrations from aromatic ring (ν ∼ C=C), at 1520–1580 cm–1, which can be attributed to bending vibrations of methylene or the methyl group present in the alkyl chains (δ ∼ C–H), and at 1350–1380 cm–1, due to asymmetric vibrations of the sulfonamide group (ν (as) ∼ S=O). Moreover, all products exhibited very strong multiple signals in the range 1150–1250 cm–1, which may be assigned to stretching vibrations from one ether and two ester groups (ν ∼ C–O–C). A group of peaks between 1150 and 1050 cm–1 originates from the following vibrations: S=O symmetrical stretching (ν ∼ S=O), C–H aromatic in-plane bending (δ ∼ C–H,) and C–N stretching from the triazine ring (ν ∼ C–N), while bands appearing at 700–800 cm–1 are due to C–H out-of-plane bending vibrations from the aromatic rings.[38]

Additionally, NMR spectra confirmed the presence of both the herbicidal anion and the alkylated betaine cation in the obtained products. 1H NMR spectra of the reactants (iodosulfuron-methyl sodium salt and heptyl betainate bromide (P6)) as well as the final product (6) are depicted in Figure .

Figure 2

1H NMR spectra of iodosulfuron-methyl sodium salt ([Na][ISM]), heptyl betainate bromide (P6), and their final product (6).

In the 1H NMR spectra of iodosulfuron-methyl sodium salt we can distinguish three singlets at approximately 2.4 ppm (1) (protons from the methyl group), 3.9 ppm (2) (protons from the methyl in the ether and ester group), and two doublets and one multiplet (originating from three protons in the aromatic ring) at 7.3 ppm (3), 7.9 ppm (4), and 8.5 ppm (5), respectively. According to Figure , the signals from the O-alkylated betaine moiety appeared at approximately 3.4 ppm (9) (singlet from the three methyl groups) and 4.5 ppm (11) (singlet from the CH2 group), whereas peaks from the alkyl chain occurred at 0.9 ppm (6) (triplet from the methyl group), 1.3–1.7 ppm (7, 8) (multiplets from the CH2 group in the alkyl chain), and 4.2 ppm (10) (triplet from the CH2 attached to oxygen), accordingly. The absence of a signal from the proton of the amide group is caused by the exchange with the deuterated solvent used (CD3OD). As reported earlier, this signal is clearly visible in the case of using deuterated DMSO and chloroform.[35] Furthermore, when the obtained product with the herbicidal anion (6) is considered, data in Figure indicate that all characteristic signals from both ions can be seen and there are no significant shifts in the locations of peaks in comparison to both reagents.

Solubility

Information regarding the affinity of chemicals for solvents characterized by varying their polarity may be particularly useful in crop protection applications, which require simple and cost-effective solutions in the development of new and effective formulations.[39] Therefore, the solubilities in 10 representative solvents were determined for the sodium salt of iodosulfuron-methyl, the alkyl betainate bromides, and the prepared iodosulfuron-methyl-based ILs at room temperature, according to the procedure described by Vogel.[31] The results for the salts containing iodosulfuron-methyl (1–13) are presented in Table , whereas the solubilities of the bromides (P1–P13) are provided in Table S2 in the Supporting Information. As shown in Table , the sodium salt of iodosulfuron-methyl was miscible with polar organic solvents such as methanol and DMSO, while its solubility in water was moderate. The decreasing polarity of the solvent caused a notable deterioration of solubility; hence, the sodium salt was found to be insoluble in five of the less-polar solvents. The collected data revealed that incorporation of the organic betaine-based cation facilitates the affinity for acetonitrile, acetone, and chloroform. On the other hand, the presence of bulky organic cations instead of a sodium ion influences the solvation of ILs by water molecules and results in the reduction of hydrophilicity. Nonetheless, lower solubility in water can be a particularly desirable property for new agrochemicals to facilitate control of their soil and groundwater mobility.[20] Interestingly, the alkyl chain length in the cation of ILs proved to have an impact on their solubility in a few of the tested organic solvents. The most significant differences were noted for isopropanol, which effectively dissolved only ILs with alkyls comprising at least 12 carbon atoms (10–13). However, too great a length of the alkyls in the products was found to hinder their dissolution in acetonitrile (13) and acetone (11−), while a moderate length led to increased affinity for ethyl acetate (6–9).

Table 2

Solubility of the Iodosulfuron-Methyl Sodium Salt ([Na][ISM]) and the Prepared Iodosulfuron-Methyl-Based ILs (1–13) at 25 °C

Snyder polarity index: +, good solubility; ±, medium solubility; −, low solubility.

The differences in the solubilities in the least polar solvents support the thesis that ionic liquids are immiscible with liquids that possess low dielectric constants.[39] Hence, none of the obtained ILs dissolved in hexane (εr = 1.9) or toluene (εr = 2.4), whereas four were miscible with ethyl acetate (εr = 6.0) and almost all were soluble in isopropanol (εr = 19.9).

A thorough comparison of the solubilities of ILs (1–13) and alkyl betainate bromides (P1–P13) (Table S2) elucidated the effect of ion exchange on this parameter. Generally, the incorporation of the iodosulfuron-methyl anion led to a decrease in the compound’s affinity for water and isopropanol. In contrast, some bromides were characterized by a decreased miscibility with acetone and ethyl acetate. Interestingly, the structure of the anion did not influence the solubility in the more polar (methanol and DMSO) as well as the less polar (toluene and hexane) organic solvents.

Thermal Stability

Many currently applied herbicides exhibit a high potential for volatility, which may lead to their migration in the environment. The off-site movement of herbicides (occurring due to their volatilization after application) may cause significant damage to nontolerant crops as well as trees and other plants.[40] It should be noted that until recently ILs were broadly regarded as nonvolatile compounds. However, data published in the last several years have shed new light on this topic and these assumptions have had to be progressively reconsidered. The proof that ILs could be vaporized led to a wealth of intensive studies that were designed to determine the conditions for their volatilization as well as to discover their nature in the vapor phase.[41] The issue of the volatility of HILs, first raised and thoroughly investigated by Tang et al.[13] in the case of new ionic forms of bromoxynil herbicide, clearly demonstrated that such experiments are crucial for the development of plant protection products having a low potential for off-site drift. To address these problems, we attempted to determine the effect of the chemical structure of the obtained ILs (particularly the alkyl chain length) on the mass loss of the samples during storage at 75 °C, as was done in previous reports.[13,42,43]

Data in Figure S65 and Table S3 in the Supporting Information revealed that the replacement of the sodium cation in [Na][ISM] by an alkylated betaine in ILs 1, 5, 9, 11, and 13 resulted in a significant increase in the loss of mass. The sodium salt of iodosulfuron-methyl was characterized by approximately 0.2% and 0.4% mass loss after 2 and 7 days of analysis, while the values noted for ILs were in the ranges 0.6–1.3% and 1.2–4.1% after 2 and 7 days, respectively. Interestingly, the obtained results regarding the volatility of HILs comprising other herbicides such as dicamba,[43] 2,4-D,[42] and bromoxynil[13] at 75 °C after 12 h revealed in some cases even greater mass losses that occurred in the ranges 0.2–1.0%, 1.9–8.5%, and 0.2–5.4%, respectively. However, none of these reports involved an analysis of the sample after the experiment. Interestingly, the 1H NMR analysis of the obtained ILs with the ethyl group (1) and decyl group (9) after the test revealed a substantial decomposition of both ions. The degradation of the betainate cation, assessed by a decrease in the intensity of the characteristic peak at 4.42 ppm, reached approximately 96% for 1 (see Figure ) and 90% for 9 (see Figure S66 in the Supporting Information), respectively. Furthermore, the decomposition of the anion (taking into consideration various functional groups) has been estimated at approximately 60% for 1 (see Figure ) and 85% for 9 (see Figure S66 in the Supporting Information). This means that the mass loss of samples during heating at 75 °C was caused by evaporation of the volatile decomposition products of ions. It is noteworthy that after 7 days the IL with an ethyl substituent (1) was found to exhibit lower mass loss than the product containing a decyl substituent (9). These results coincide with Figure and Figure S66, according to which IL 9 is characterized by significantly higher (approximately 30%) decomposition of the herbicidal anion that may lead to the formation of a greater amount of volatile compounds. The sodium salt of the tested herbicide exhibited substantially lower mass loss in comparison with the ILs. This phenomenon is due to the strong dependence of the thermal stability of ILs on the electrostatic cohesion of the ions as well as the charge distribution in the anion.[44] In addition, this experiment clearly demonstrates that all tested compounds, including ILs, are unstable at elevated temperature and exhibit substantial decomposition rather than volatility. It should also be emphasized that the obtained ILs, when they are stored at room temperature, exhibit no traces of decomposition in the 1H NMR spectra for over 6 months.

Figure 3

Comparison of 1H NMR spectra of IL 1 before (top) and after (bottom) 7 days of heating at 75 °C.

Further research is necessary to elaborate a new and more suitable methodology for assessing the risk to the environment and humans via vapor drift of such unstable ILs. Additionally, we recommend that future tests regarding the volatility of ILs containing herbicidally active ions should be supplemented with a spectral analysis of the compounds after the experiment.

Octanol–Water Partition Coefficient

The lipophilicity of compounds, including ILs, can be estimated by the logarithm of the octanol–water partition coefficient (log KOW). Hydrophilic compounds are characterized by values of log KOW lower than zero. Such compounds are known to permeate soil easily and pose a threat to watercourses, leading to pollution of the hydrosphere. In contrast, highly lipophilic substances, characterized by values of log KOW > 3, may persist in soil for months, increasing the plausibility of their bioaccumulation.[13,39] According to recent reports, the risk of environmental pollution dramatically increases when a herbicidally active compound is resistant to biodegradation by soil microorganisms.[45] In addition, the Cao research group recently revealed that in the case of HILs good lipophilicity could cause the active ingredients to easily penetrate the leaf surface, reaching the target tissues more quickly and exhibiting satisfactory herbicidal activity.[13−15,42,46] Therefore, in the process of designing novel environmentally friendly pesticides an assessment of their octanol–water partition coefficient is recommended, as a measure of not only their potential environmental impact but also their efficacy.

Because the KOW coefficients of ILs are known to depend on their concentrations, the synthesized ILs (1, 3, 5, 7, 9–13) were tested at a concentration recommended for application of iodosulfuron-methyl (equal to 10 g of the active ingredient per hectare, which corresponds to a concentration of 50 mg dm–3 of aqueous solution). The results, presented in Figure (and in Table S4 in the Supporting Information), confirmed the tendency that the log KOW value gradually increases with elongation of the alkyl group of ILs. This phenomenon is due to increasing hydrophobicity as the length of the nonpolar chain increases.[13,18,20,46] Thus, the highest value of KOW (log KOW = 0.42) was found for IL 13, containing the longest alkyl chain in the cation, while the lowest (log KOW = −0.27) was noted for the IL containing the shortest alkyl substituent chain (1) (Figure A). Interestingly, substitution of the alkyl betainate cation for the inorganic sodium cation leads to an increase in the compound’s affinity for the nonpolar phase. Therefore, as is shown in Figure B, the value of KOW for the sodium salt of iodosulfuron-methyl (log KOW = −1.26) was substantially lower than that of any of the analyzed ILs.

Figure 4

Influence of the alkyl chain length in the cation on the logarithm of the octanol–water partition coefficient of the obtained ILs (1, 3, 5, 7, 9–13) (A) in comparison to iodosulfuron-methyl sodium salt [Na][ISM] (B).

It is noteworthy that the transition from hydrophilic to hydrophobic character occurred in the alkyl elongation from hexyl (3) to octyl (4). This observation is consistent with other reports on ILs;[15,39] however, the hydrophobic character may also be demonstrated by ILs containing shorter substituents such as butyl.[18] Moreover, the increase in measured values of KOW due to chain elongation were not as drastic as in the case of HILs comprising other herbicides, such as bromoxynil,[13] fomesafen,[14] picloram,[15] and 2,4-DP.[18] In contrast, values of log KOW of HILs containing another sulfonylurea (nicosulfuron) were within a small range in spite of the presence of various short and long alkyl chains in the cation.[46] It is plausible that this phenomenon is caused by the extremely low amounts of sulfonylureas in spray solutions; hence, the transfer of these substances to the lipophilic phase is not as significant as in the case of herbicides that are usually applied at much greater doses (>100 g per hectare). Furthermore, it should be stressed that the values of log KOW for the majority of the synthesized ILs occurred between 0 and 3. Therefore, we assume that the risk of migration of ILs (7, 9–13) into groundwater after their application is significantly reduced in comparison to that of the currently available commercial form ([Na][ISM]).

The possibility of bioaccumulation of the obtained ILs in the environment is also very low, considering that both the cation (alkylated betaine)[47] and the anion (sulfonylurea)[48] proved to be susceptible to biodegradation mainly within less than 30 days. Additionally, Carles et al. revealed that the coformulants (e.g., adjuvants) present in the commercially available sulfonylurea-based pesticides can have a strong negative effect on the biodegradation of the active ingredient.[48] This discovery reveals another advantage of the “HIL strategy”, wherein an appropriately selected cation may play the role of an adjuvant without the necessity of using any other chemicals.

Herbicidal Activity

The herbicidal efficacy of the obtained products (1–13) was investigated in greenhouse experiments. Oilseed rape (Brassica napus L.) was selected as a test plant because it is one of the most common weeds in cultivated crops. All ILs (1–13) as well as the reference herbicide containing the sodium salt of iodosulfuron-methyl were applied at a dose corresponding to 10 g of active ingredient per hectare. The results demonstrated in Figure (detailed results with statistical analysis are provided in Table S5 in the Supporting Information) allowed an assessment of the influence of the alkyl chain length on the herbicidal efficacy. No significant differences were observed in the reduction in the fresh weight values, allowing them to be described as equally effective members of a group of new herbicidal ionic liquids (HILs). However, according to visual assessment, the majority of the new forms of iodosulfuron-methyl were characterized by activity similar to that of the commercial preparation containing the sodium salt. Only HILs containing the shortest (1–3) and one of the longest (12) alkyl groups were found to be slightly less effective than the reference. It should be pointed out that the studies of other research groups indicated that compositions comprising surfactants containing C12–C13 alkyl chains showed high efficiency.[49] One may conclude that the tested HILs exhibited a similar optimum length of the alkyl chain. However, further research is required to validate this assumption in relation to other herbicides. Additionally, the efficacy of the synthesized HILs containing alkyl chains from pentyl (4) to octadecyl (13) (except 12) was comparable to that of the reference, which makes them promising replacements for the applied commercial preparation. Generally, the sulfonylurea-based formulations (including iodosulfuron-methyl) require the use of an adjuvant, such as crop oil concentrate, at doses multiple times greater than the active ingredient itself (approximately 1% v/v of spray solution). In order to elucidate the efficiency of the selected cations as adjuvants, the surface tension of spray solutions utilized in the greenhouse studies were determined. As is shown in Figure S67, the presence of short alkyls, such as ethyl (1), butyl (3), and hexyl (5), did not influence the values of surface tension, which along with [Na][ISM] were similar to the values recorded for pure water (approximately 73 mN m–1). However, a further increase in the length of alkyl chain leads to an increase in surface activity—HILs 10–13 were characterized by relatively low values of this parameter ranging from approximately 52 to 38 mN m–1. Therefore, one may conclude that, in the case of the obtained products, some of the betaine-type cations were found to play the role of an adjuvant, which eliminates the necessity of using additional compounds. This makes it possible to reduce the amount of chemicals that may persist in the environment and become a source of severe pollution.

Figure 5

Herbicidal efficacy of the synthesized HILs (1–13) compared with the reference herbicide containing the sodium salt of iodosulfuron-methyl (REF) toward oilseed rape (error bars represent LSD values).

Subsequently, the most effective HIL (11) was selected to elucidate its dose–response characteristics in comparison to the reference herbicide containing the sodium salt of iodosulfuron-methyl. The doses utilized in this test were equal to 0.25 n (2.5 g ha–1), 0.50 n (5 g ha–1), 0.75 n (7.5 g ha–1), 1.00 n (10.0 g ha–1) and 1.25 n (12.5 g ha–1), where n refers to the dose applied in the first experiment (10 g ha–1). The results, provided in Figure A (detailed values with a statistical analysis are provided in Table S6 in the Supporting Information), indicate that iodosulfuron-methyl sodium salt was more effective than 11 only at doses lower than 7.5 g ha–1. On the other hand, after application of 11 at the recommended dose or higher (10.0 and 12.5 g ha–1), the fresh weight reduction of the tested plants was superior to that of the preparation containing the sodium salt. A visual assessment (Figure B) confirmed the excellent efficacy of the HIL; however, the obtained results were almost equal to those using the commercial preparation.

Figure 6

Herbicidal activity of the obtained HIL (11) and iodosulfuron-methyl sodium salt toward oilseed rape after application at different doses of active ingredient: fresh weight reduction (A); visual assessment (B). Error bars represent LSD values.

As shown in Figure S68, the symptoms typical of the group of ALS enzyme inhibitors (such as chlorosis, necrosis, and stunting of plants) were observed in all combinations.[50] It should also be stressed that, after application of the new forms of iodosulfuron-methyl, thickening of oilseed rape stems was observed, especially in the case of HILs 1–3 and 13. This observation leads to the assumption that the betaine present in the cation might act as a growth stimulator.[51] This additional activity can be exceptionally favorable in the case of the cultivated crops. Further research is crucial to elucidate the potential benefits of the betaine-type cation in ILs on cultivated plants. Nevertheless, in this paper, we confirm that tuning the appropriate cation–anion combination during the molecular design of HILs is indeed a versatile tool. This enables a convenient study of their structure–property relationships and stimulates the creation of compounds with the desired properties.