Synthesis and Characterization of Double-Salt Herbicidal Ionic Liquids Comprising both 4-Chloro-2-methylphenoxyacetate and trans-Cinnamate Anions

Article abstract of DOI:10.1002/cplu.202000546

The synthesis and characteristics are presented of novel double-salt herbicidal ionic liquids (DSHILs) that contain 4-chloro-2-methylphenoxyacetate and trans-cinnamate anions. In the designed synthesis, an anion of natural origin and a herbicidal anion were combined with an amphiphilic bisammonium cation to obtain new DSHILs with high herbicidal activity while high biocompatibility is maintained. The NMR and HRMS spectral analysis confirmed that the target structures were formed. Furthermore, HPLC analyses indicated that, as assumed, both anions were present in equimolar amounts. Experiments regarding the herbicidal effectiveness confirmed that the synthesized DSHILs exhibited high biological activity. The solutions of DSHILs applied during greenhouse studies were characterized by a low contact angle (approx. 55–67°) and surface tension (approx. 32–35 mN m^?1), which facilitated the contact of the active substance with the plant surface and penetration of the herbicide into the plant tissues.

Full text of DOI:10.1002/cplu.202000546

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: Synthesis and Characterization of Double-Salt Herbicidal Ionic
Liquids Comprising both 4-Chloro-2-methylphenoxyacetate and
trans-Cinnamate Anions
Authors: Daria Szymaniak, Adam Maćkowiak, Kamil Ciarka, Tadeusz
Praczyk, Katarzyna Marcinkowska, and Juliusz Pernak
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content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.202000546
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ChemPlusChem
10.1002/cplu.202000546
FULL PAPER
Synthesis and Characterization of Double-Salt Herbicidal Ionic
Liquids Comprising both 4-Chloro-2-methylphenoxyacetate and
trans-Cinnamate Anions
Daria Szymaniak, Adam Maćkowiak, Kamil Ciarka, [b] Tadeusz Praczyk, [c] Katarzyna
[
a]
[a]
Marcinkowska, [c] and Juliusz Pernak*[a]
[
a]
MSc. D. Szymaniak, MSc. A. Maćkowiak, Prof. J. Pernak
Department of Chemical Technology
Poznan University of Technology
Berdychowo 4, Poznan 60-965, Poland
E-mail: juliusz.pernak@put.poznan.pl
MSc. K. Ciarka
[
[
b]
c]
Department of Chemistry
Adam Mickiewicz University in Poznan
Uniwersytetu Poznańskiego 8, Poznan 61-614, Poland
Prof. T. Praczyk, Dr. K. Marcinkowska
Institute of Plant Protection, National Research Institute
Węgorka 20, Poznan 60-318, Poland
Abstract: This study presents the synthesis and characteristics of
high risk of damaging these plants as a result of drift of spray
solutions from adjacent areas exposed to herbicides.[4]
Derivatives of phenoxy acids are biodegraded relatively quickly
(the half-life in soil varies from a few to several days), but they can
move in the soil profile and penetrate into groundwater, thus
contributing to environmental contamination.[5,6] One way to limit
the negative consequences of using herbicides belonging to the
synthetic auxin group is to transform them into ionic liquids (ILs),
which results in the formation of herbicidal ionic liquids (HILs).
HILs [cation-anion (herbicide)] are defined as salts with a melting
point that does not exceed 100 °C, which contain at least one ion
characterized by herbicidal activity in their structure.[7] They
belong to the third generation of ILs, exhibiting specific biological
activity while maintaining favorable physicochemical and
novel double salt herbicidal ionic liquids (DSHILs) containing 4-chloro-
2-methylphenoxyacetate and trans-cinnamate anions. In the
designed synthesis, anion of natural origin and herbicidal anion were
combined with an amphiphilic bisammonium cation, obtaining new
DSHILs with high herbicidal activity while maintaining high
biocompatibility. The NMR and HRMS spectral analysis confirmed
that the target structures were obtained. Furthermore, HPLC analyses
indicated that, as assumed, both anions were present in equimolar
amounts. Experiments regarding the herbicidal effectiveness
confirmed that the synthesized DSHILs exhibited high biological
activity. The solutions of DSHILs applied during greenhouse studies
were characterized by a low contact angle (approx. 55-67°) and
-
1
surface tension (approx. 32-35 mN·m ), which facilitated the contact
of the active substance with the plant surface and penetration of the
herbicide into plant tissues.
[8]
chemical properties. There are reports regarding the synthesis
of HILs containing synthetic auxins as an anion source (e.g. 2,4-
D, MCPA, mecoprop-P or dicamba).[
9-14]
Converting known
herbicides into the form of HILs increases their biological activity,
improves physicochemical parameters (thermal stability, volatility,
Introduction
water solubility, surface activity) and reduced negative effects on
the environment.[
11,14,15]
HILs, unlike commercial herbicides, are
characterized by low volatility, which eliminates the risk of
unintended contamination of other cultivated areas and minimizes
the likelihood of damaging neighboring dicotyledonous crops.
The advantage of HILs is associated with their higher herbicidal
activity than that of commercial herbicides, which allows for their
application at lower doses without the addition of adjuvants. The
Modern large-scale cultivation of plants is inherently associated
with the necessity to protect crops against weeds, pathogens and
animal pests. Among these factors, weeds are the greatest threat;
in the case of economically important plants, such as wheat, rice,
corn, potatoes, soybeans, and cotton, weeds can reduce crop
yields by up to 34%.[1] Therefore, herbicides are commonly used
in the cultivation of most plants.[2] It is currently the most effective
and economically viable method of reducing the harmful effects of
weeds. The derivatives of phenoxycarboxylic acids, particularly
[16]
use of HILs reduces the environmental pollution by reducing the
[13,17]
amount of chemicals that penetrate groundwater.
Double salt
ionic liquids (DSILs) are a special type of IL that contain more than
one cation or anion. Simultaneously, competitive interactions
between more than 2 ions with a different chemical structure
contribute to the unique physicochemical or biological properties
of DSILs. Therefore, DSILs should not be treated as mixtures, but
as unique ionic systems.[18,19] The structural design of DSILs
2
,4-dichlorophenoxyacetic
acid
(2,4-D),
4-chloro-2-
methylphenoxypropionic acid (MCPP) and acid 4-chloro-2-
methylphenoxyacetic acid (MCPA), have been used to selectively
control undesirable vegetation.[3] These compounds belong to the
group of synthetic auxins. MCPA is used in the form of salts or
esters to control dicotyledonous weeds, mainly in grain and
pasture cultivation. Most cultivated dicotyledonous plants (fruit
trees, vines, soybeans, cotton, oilseed rape, sugar beet) are
highly sensitive to phenoxy acid derivatives. Therefore there is a
allows the adjustment of their physicochemical parameters as
[20]
well as an increase in their herbicidal effectiveness. DSILs with
herbicidal properties are called "double salt herbicidal ionic
liquids" (DSHILs). DSHILs exhibit higher herbicidal effectiveness
1
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ChemPlusChem
10.1002/cplu.202000546
FULL PAPER
and a broader spectrum of weed control than single ILs.[19] Due to
the better designability of DSHILs, it is possible to reduce their
toxicity and increase their biocompatibility by introducing natural
ions into the structure, which are easily available and inexpensive
in terms of production costs.[21,22] trans-Cinnamic acid is a
compound of natural origin commonly used in the cosmetics,
pharmaceutical and food industries and is present in cinnamon,
shea butter and Peruvian balsam.[23] Cinnamic acid is
characterized by antioxidant, antibacterial and antiseptic activity.
It has been confirmed that both cinnamic acid and its derivatives
Table 1. Prepared bisammonium dibromides.
Measured
Literature
Yield
Dibromide
-(CH
)
2 n
-
R
T
m
T
m
[%]
[°C]
[°C]
202-203[
26]
1
2
3
4
5
6
7
8
9
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
2
2
2
2
2
2
2
2
2
2
)
6
6
6
6
6
8
8
8
8
8
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
C
8
H
H
H
H
H
17
97
99
98
99
97
98
97
96
99
97
98
97
96
97
95
203-205
215-217
225-227
220-222
218-220
116-118
114-117
195-198
125-127
136-140
113-115
181-184
135-137
108-110
110-112
[27]
29]
)
)
)
)
)
)
)
)
)
C
C
C
C
10
12
14
16
21
25
29
33
214-215
225-226[
[
28]
28]
223-224
220-221[
C
8
H
17
-
113-116[
27]
C
C
C
C
10
12
14
16
H
21
25
29
33
H
H
H
-
-
inhibit seed germination and exhibit herbicidal and fungicidal
activity, while being safe for the environment.[24,25] The aim of our
10
11
-
-
-(CH
-(CH
-(CH
-(CH
-(CH
2
)
12
12
12
12
12
C
8
H
17
180-182[
27]
1
1
2
3
2
2
2
2
)
)
)
)
C
C
C
C
10
12
14
16
H
21
25
29
33
study was to develop a new formula of DSHILs and investigate
the effect of presence of natural-based anion on both herbicidal
activity and physicochemical properties. The scientific novelty is
the combination of a cation ensuring good wettability of the leaf
surface with an anion of synthetic auxin with herbicidal properties
and an anion of natural origin with plant growth inhibitory activity,
which improves bio-applications of DSHILs.
[29]
H
H
H
135-136
14
-
-
15
Tm – melting point
The products were purified by extraction in a two-phase
chloroform-water system. Reaction yields were in the range of
9
5–99%. The results of elemental analysis included in the
supporting Information (Table S1) confirmed the high purity of the
obtained salts. The water content of all products was determined
by the Karl-Fischer method and did not exceed 500 ppm. The
resulting DSILs with MCPA and trans-cinnamate (CINN) anions
Results and Discussion
Synthesis
1
a-15a and the IL with two MCPA 16a anions are presented in
Bisammonium dibromides were obtained using the Menchutkin
reaction between the alkyldimethylamine and the quaternizing
agent: 1,6-dibromohexane, 1,8-dibromooctane or 1,12-dibromo-
dodecane as presented in Scheme 1.
Table 2. The compounds were not previously described in the
literature. The structures of the synthesized DSILs were
1
13
confirmed by HRMS as well as H and C NMR analysis. The
results are included in the supporting Information (Figures S1-
S36). In the 1H NMR spectra of DSILs (1a-15a), resonance
signals originating from the cation and both anions were
observed. Characteristic chemical shifts associated with the
MCPA anion occurred at approx. σ [ppm] = 2.25 (attributed to the
3 2
CH at the aromatic ring) and 4.40 (attributed to the CH
-COO-).
Scheme 1. Synthesis of bisammonium dibromides.
The presence of the CINN anion was confirmed by chemical shifts
derived from protons in the olefinic moiety (approx. 6.5 and 7.4
13
ppm). After analysis of the C NMR spectrum, two signals
characteristic of carboxyl groups were observed at approx. 176
ppm for the MCPA anion and at approx. 175 ppm for the CINN
anion. The resonance signal at approx. 16 ppm confirmed the
presence of the methyl group attached to the aromatic ring in the
MCPA anion. In the case of IL 16a with two MCPA anions, all
resonance signals from the bisammonium cation and MCPA
anions were observed. The conducted HPLC analyses for DSILs
The resulting bisammonium dibromides were solids with melting
points ranging from 110 °C for dodecylmethylene-1,12-
bis(tetradecyldimethylammonium) dibromide (14) to 227 °C for
hexamethylene-1,6-bis(dodecyldimethylammonium) dibromide
(
3). The synthesized bisammonium dibromides along with the
measured and literature melting points are summarized in Table
.[26-29] During the analysis of the melting points of the obtained
1
dibromides, it was noted that elongation of the linker connecting
the two positively charged nitrogen atoms resulted in a decrease
of the melting point by 25-87 °C, which is confirmed in the
literature.[30] As a result of the ion exchange reaction between
potassium salts of trans-cinnamic acid, MCPA and synthesized
dibromides 1-15, novel DSHILs with unique physicochemical
properties were obtained. The exchange reaction was carried out
at room temperature in water as presented in Scheme 2
1
a-15a allowed calculation of the molar ratio of anions. The
results are summarized in Table 2. HPLC chromatograms are
provided in the supporting Information (Figures S37-S51). It was
confirmed that both anions were present at equimolar amounts.
The observed deviations were caused by a slight change in
conditions during the HPLC analysis, which affected the
[31]
measurement results. In addition, for DSILs 5a, 8a, 11a, and
5a, the HRMS spectra were collected, in which characteristic
1
signals for the bisammonium cation and MCPA and CINN anions
can be observed. NMR spectroscopy, HRMS and HPLC analysis
fully confirm the structures of the synthesized DSILs.
Thermal analysis
The results of thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) are presented in Table 2. The
analysis methodology is described in the supporting information.
Scheme 2. Synthesis of DSILs.
2
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ChemPlusChem
10.1002/cplu.202000546
FULL PAPER
Table 2. Synthesized DSILs and DSC and TG analysis.
Molar ratio
of anions
MCPA:CINN
Yield
%]
T
[°C]
g
T
[°C]
c
T
[°C]
m
T
[°C]
5%
T
50%
[°C]
DSILs
-(CH
)
2 n
-
R
[
1
2
3
4
5
6
7
8
9
a
a
a
a
a
a
a
a
a
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
2
2
2
2
2
2
2
2
2
2
)
6
6
6
6
6
8
8
8
8
8
-
-
-
-
-
-
-
-
-
-
C
8
H
17
97
97
96
99
97
99
95
98
98
98
96
96
96
95
95
98
0.97:1
0.95:1
0.98:1
1:1
-13
-10
-
-
-
194
195
190
196
198
197
193
192
193
192
192
191
191
190
193
206
226
242
223
268
303
275
250
260
277
286
250
267
276
299
307
330
)
)
)
)
)
)
)
)
)
C
C
C
C
10
H
21
25
29
33
-
-
12
14
16
H
H
H
39
35
24
7
35
29
11
50
64
-
-25
-
0.97:1
0.98:1
1:1
C
8
H
17
-11
-15
-27
-24
-
C
C
C
C
10
12
14
16
H
H
H
H
21
25
29
33
26
-
1:0.95
0.99:1
1:0.98
1:0.95
1:0.96
1:0.94
1:0.94
1:0.94
2:0
-
-
10a
11a
12a
13a
14a
15a
16a
24
26
44
24
64
11
-
93
60
-
-(CH
-(CH
-(CH
-(CH
-(CH
-(CH
2
)
12
12
12
12
12
12
-
-
-
-
-
-
C
8
H
17
-9
2
2
2
2
2
)
)
)
)
)
C
C
C
C
C
10
12
14
16
10
H
H
H
H
H
21
25
29
33
21
-13
-8
79
80
42
52
-29
-32
-19
T
m
– melting point; T
c g
– temperature of crystallization; T – glass transition temperature; T5% – decomposition temperature of 5% sample; T50% – decomposition
temperature of 50% sample
The melting points of 1a-16a did not exceed 100 °C, which allows
classification of the salts as DSILs. The synthesized DSILs
exhibited glass transition temperatures in the range of -32 °C for
presented in Table 3. Solubility of the obtained DSILs depended
on the polarity, protonity and ability to form hydrogen bonds
between the solvent and the examined compound.[33] All tested
DSILs were insoluble in low-polarity and nonpolar solvents such
as ethyl acetate, toluene and hexane. The exception is chloroform,
in which all salts were soluble despite its low polarity and aproticity,
which may be related to its donor ability to form hydrogen bonds
with the solute (α = 20). In the case of polar aprotic solvents, which
have poor donor ability (α < 20), it has been observed that the
solubility of DSILs varies and depends on the structure of the
bisammonium cation. All synthesized DSILs were soluble in protic
polar solvents, such as water or methanol. The solubility of
cinnamic acid and MCPA in water is limited and amounts to
1
5a to -8 °C for 13a. Low glass transition temperatures can be
caused by the interaction of long alkyl substituents in the cation
structure, which has been described in the literature.[28] A trend of
decreasing glass transition temperature with the elongation of the
alkyl substituent in the bisammonium cation was noted for DSILs
in the following order: 1a, 6a, 11a < 2a, 7a, 12a, 16a < 3a, 8a,
1
3a < 4a, 9a, 14a < 5a, 10a, 15a. In the case of DSILs with the
longest alkyl substituent (hexadecyl), no glass transition
temperature (5a, 10a) was observed. Crystallization temperatures
were recorded for eleven DSILs (3a-7a, 10a-15a) and ranged
from 7 °C for 6a to 64 °C for 14a. The TGA results indicated that
the studied ILs were thermally stable up to 190 °C. It was also
noted that the 5% mass decomposition of the sample was in a
narrow range from 190 °C for 14a to 204 °C for 16a, and the 50%
mass decomposition temperature increased with the extension of
the alkyl substituent in the bisammonium cation. Based on the
comparison of the results obtained for ILs with the dodecylene-
400 g·L-1 and 725 mg·L , respectively.
-1
[34,35]
The conversion of
cinnamic acid and MCPA into the ionic form improves their water
solubility.
Surface activity
Bisammonium dibromides 1-15 were water soluble. They can be
analysed using the two-phase titration according to EN ISO 2871-
2:2010 (results obtained in the range of 97-99%).[36] Surface
activity was determined for dibromide aqueous solutions in the
concentration range of 10-11 – 1 mol·L . The results are included
in the supporting information (Table S2 and Figures S52-S54).
The determined surface tension at the CMC point was between
1
,12-bis(decyldimethylammonium) cation (12a, 16a), it can be
concluded that the CINN anion reduced the thermal stability of the
tested salts. IL 16a with two MCPA anions exhibited higher
thermal stability than 15a, as confirmed by literature reports.[32]
It can therefore be concluded that anions, particularly the trans-
cinnamate anion, notably influence the thermal stability of the
obtained DSILs.
-1
-1
34 and 40 mN·m . The lowest CMC occurs for dibromides 3, 4, 8,
9, 13 and 14 with dodecyl and tetradecyl substituents. However,
the highest value was determined for dibromides 1, 6, and 11 with
an octyl substituent. The effect of anions on surface activity was
examined based on DSILs 12a with the MCPA and CINN anions,
16a with two MCPA anions and dibromide 12. The following
parameters were determined: critical micellization concentration
Solubility studies
The solubility test was carried out using popular organic solvents
ordered in ascending order of their polarity index according to
Snyder's scale. Detailed information regarding the test is given in
the supporting information. The results of the research are
(CMC), surface tension at CMC (γCMC) and adsorption
3
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FULL PAPER
Table 3. Solubility of synthesized DSILs at 25 °C.
ethyl
acetate
water
.0[b]
methanol
DMSO
acetonitrile
acetone
isopropanol
chloroform
toluene
hexane
DSILs
9
6.6
6.5
6.2
5.4
4.3
4.3
4.1
2.3
0.0
1
.17[c]
0.98
0.00
0.19
0.08
0.76
0.00
0.20
0.00
0.00
1
2
3
4
5
6
7
8
9
a
a
a
a
a
a
a
a
a
+[a]
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+/-
+/-
-
+/-
+/-
+/-
+
-
-
-
+
+
-
-
-
+/-
+/-
-
-
+/-
+/-
+/-
+/-
+/-
+/-
+/-
+/-
+/-
+/-
-
+
-
+/-
+/-
+/-
-
-
+
+
-
10a
11a
12a
13a
14a
15a
16a
-
-
+/-
+/-
+/-
+/-
+
+
+
+/-
-
+/-
+
+/-
+/-
+/-
+
[
a] + high solubility,+/- moderate solubility, - low solubility, [b] – Snyder polarity index, [c] – Kamlet and Taft solvatochromic parameter α
efficiency (pC20), defined as the negative logarithm of the surface-
active compound concentration in the bulk phase required to
reduce the surface tension of the water by 20 mN·m . The results
(Table S3, Figure S55). The determined surface tension values
ranged from 32.20 to 36.84 mN·m . Analogous surface tension
values were achieved in the case of bisammonium salts with a
-1
-1
are presented in Table 4 and Figure 1.
more complex cation with hydroxyl moieties and halide
anions.[
37,38]
The introduction of the MCPA and CINN anion into
the structure of bisammonium DSILs enabled the reduction of the
surface tension of spray solutions without complex modification of
the cation, which is responsible for surface activity. The exchange
of bromide anions in 1-15 with MCPA and CINN resulted in a
Table 4. Surface activity of DSILs 12a and 16a and dibromide 12.
CMC
γ
CMC
Compound
pC20
-
1
[mN·m-1]
[
mmol·L ]
-1
decrease in surface tension by up to 12 mN·m . Contrary to
common beliefs that anions in cationic active agents provide
adequate solubility and aggregation only in water, it has been
confirmed that the surface activity of bisammonium salts can be
modified by selecting appropriate counterions.[34] These findings
may become crucial from the point of view of designing new
surface active ILs.
1
2
1.09
38.38
36.81
35.77
3.82
3.97
3.97
1
2a
6a
0.76
1
0.72
Exchange of the bromide anion in 12 with MCPA and CINN (12a)
anions resulted in decrease in the CMC value by
.33 mmol·L . For 16a, the introduction of two MCPA anions into
a
-1
0
-1
the structure reduced the CMC by 0.37 mmol·L . In the case of
the pC20 parameter, the difference between dibromide 12 and
DSILs 12a and 16a was equal to 0.15. Exchange of the bromide
anion with organic acid anions reduces the surface tension, and
the structure of the surface inactive ion has a significant impact
on the ability of DSIL to reduce the surface tension of an aqueous
solution. The surface tension of aqueous solutions of the
synthesized DSILs applied in greenhouse studies (concentration
equal to 10-3 mol·L ) was beyond the CMC point; i.e., it was
determined. Only the surface tension and contact angles (CA)
were determined during herbicidal studies of DSIL aqueous
solutions. The results were compared with the measurements
carried out for solutions of dibromides 1-15 at a concentration of
-
1
Figure 1. Surface tension as a function of concentration (log C mmol·L ) of 12,
2a, 16a at 25 °C in water.
-1
1
It was confirmed that in the case of bisammonium DSILs the
contact angle depends on the structure of the cation.[20,40,41] The
length of the linker and alkyl substituent in the bisammonium
-3
-1
1
0
mol·L , which are included in the supporting information
4
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cation determines the value of the contact angle. The calculated
values of wetting angle on the paraffin surface, which is similar to
the actual surface of leaves, are summarized in Table S3. The
elongation of the alkyl substituent resulted in a decrease in the
contact angle from 67° to 59° for 1a-4a, from 63° to 56° for 6a-9a
and from 56° to 55° for 11a-14a. The salts with a hexadecyl
substituent (5a, 10a, 15a) with a CA exceeding 82° were an
exception, exhibiting the worst wettability of the paraffin surface.
DSILs with a 1,12-dodecylene 11a-15a linker exhibited the lowest
contact angles. In the case of the linker, it was observed that the
wettability of DSILs improved with elongation.
anion previously described by our research team (Table
5).[19,20,32,44,45] Based on the analysis of the DSHIL and HIL activity
presented in Table 5, it was noted that the combination of a
natural anion with plant growth inhibitory activity (CINN) with a
synthetic auxin anion (MCPA) and an amphiphilic cation allowed
to increase herbicidal activity against common lambsquarters.
The synthesized DHILs showed a higher herbicidal activity than
the quoted HILs by 15-64%. This was also confirmed by the fact
that the IL containing two MCPA anions (16a) in its structure
exhibited lower herbicidal activity than salts with the same cation
structure, but with different anions (12a) used at the same dose
of the active substance. In all cases, the dose of DSILs resulted
in up to three times higher reduction of fresh rapeseed mass than
that of the reference substance. In the case of common
lambsquarters, the effectiveness of all DSILs was higher by
approx. 40% than the herbicide Chwastox Extra 300 SL. The
effectiveness of all DSILs in the case of winter oilseed rape was
above 95% and was not influenced by changes in the chemical
structure of the tested DSILs. The obtained results allowed us to
classify the DSILs (1a-16a) as new double salt herbicidal ionic
liquids (DSHILs).
Herbicidal studies
The herbicidal activity of the obtained DSILs was determined on
the basis of biological tests performed in a greenhouse, using
common lambsquarters (Chenopodium album L.) and winter
oilseed rape (Brassica napus L.) as test plants. The methodology
of the herbicidal activity test is described in the supporting
information. The test results for 1a-15a and 16a are presented in
Figure 2 for common lambsquarters (A) and winter oilseed rape
(
B), respectively.
Figure 3. Effect DSHILs 6a-10a (A) and 16a (B) on the condition of common
Figure 2. Herbicidal activity against common lambsquarters (A) and oilseed
rape (B) of DSHILs 1a-16a.
lambsquarters.
Herbicidal activity is influenced to substantially by the surface
activity and surface wettability of the applied compounds.
The observed symptoms of damage to plants sprayed with the
tested DSILs were typical for herbicides from the group of
phenoxy acid derivatives, i.e., leaf and shoot deformations,
chlorosis and necrosis in the final stage, and are presented in
Figure 3. The greenhouse test indicated that the synthesized
DSILs were characterized by higher herbicidal effectiveness in
comparison to a commercial herbicide containing MCPA in the
form of potassium and sodium salts. The use of two different
anions in the synthesis of DSILs was a factor that contributed to
high biological activity since both the MCPA anion and the CINN
anion inhibit plant cell growth.[25,42,43] The positive effect of the
trans-cinnamate anion on herbicidal activity can be observed by
comparing DSILs 1a-16a with DSHILs and HILs with the MCPA
[46,47]
The effect of the contact angle of DSHIL aqueous solutions on
herbicidal activity against common lambsquarters was
investigated (Figure 4). In the case of the synthesized DSHILs, it
was observed that the contact angle decreases upon elongation
of the linker in the bisammonium cation, which has a positive
effect on the herbicidal effectiveness. An increase in herbicidal
activity with a decrease in contact angle can be observed for
DSHILs containing octyl, decyl and tetradecyl substituents. In the
case of the dodecyl and hexadecyl substituents, the contact angle
also decreased with increasing linker length, but no simple
relationship in terms of herbicidal activity was noted. The obtained
DSHILs exhibited good wettability of the hydrophobic surface and.
5
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Table 5. Comparison of DSHILs and HILs with the MCPA anion.
Fresh weight reduction of common
lambsquarters [%]
Compounds
Structure
1a-16a
70-97%
A[32]
60%, 32%
4
3
1-61%
3-54%
B[20]
C[19]
48-70%
51-82%
D[44]
E[45]
21%, 34%
Gly – N-(phosphonomethyl)glycine anion, Dicamba - 3,6-dichloro-2-methoxybenzoate anion
high herbicidal effectiveness using a lower dose than commercial Conclusion
herbicides without the addition of surface tension lowering agents.
Modification of the structure of DSHILs allows herbicides with the
desired physicochemical parameters to be obtained.
A series of novel compounds was designed, obtained and
examined. They consisted of a natural-based trans-cinnamate
anion and herbicidal MCPA anion, which were linked with cation
responsible for surface activity. This is cutting–edge approach in
development of DSHILs. Based on the NMR, HRMS and HPLC
analyses, it was confirmed that both anions were present at
equimolar amounts. The research showed that the basic
physicochemical properties – solubility, phase transitions and
thermal stability – were notably influenced by the structure of the
bisammonium cation. The solubility of DSHILs in DMSO, acetone,
acetonitrile and isopropanol depended on both the length of the
alkyl substituent and the linker in the cation. The obtained DSHILs
were insoluble in low-polarity, aprotic and low donor properties
solvents such as ethyl acetate, hexane and toluene. In polar protic
solvents (water, methanol), the solubility of DSHILs was good.
Figure 4. Impact of contact angles of aqueous DSHILs 1a-15a on herbicidal
activity against common lambsquarters.
6
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The advantage of the obtained DSHILs is associated with their
water solubility, which eliminates the need to introduce toxic
solvents during the preparation of formulations. TG and DSC
analysis confirmed that the obtained salts were characterized by
melting points lower than 100 °C; therefore, they can be classified
as ionic liquids. DSHILs exhibited high thermal stability, and their
decomposition temperature for 5% of the sample mass exceeded
mode was applied to record anions in ILs. HPLC analyses were
conducted using a Dionex UltiMate 3000 unit equipped with a UV-
DAD detector operating at 280 nm wavelength. Thermo Scientific
Hypersil Gold C18 150 mm/4,6 mm with stationary particle size of
5 μm was used as a column. The mobile phase was a gradient
mixture of 0.1 M ammonium formate and acetonitrile at a ratio of
100:0 at 0 min, 40:60 at 11.6 min, 90:10 at 20 min and 100:0 at
22 min at a flow rate of 1 mL·min . The time of analysis was 20
minutes, and the injection volume was equal to 20 μL.
Concentrations were determined on the basis of a calibration
curve in the range of 10 – 500 ppm. The water content was
determined by using an Aquastar volumetric Karl-Fischer titration
with Composite 5 solution as the titrant and anhydrous methanol
as the solvent.
-1
1
90 °C. The resulting bisammonium dibromides and DSHILs were
characterized by surface activity at concentrations of 10 mol·L-1.
The contact angles of the synthesized DSIL in the case of the
paraffin surface, which is similar to the actual leaf surface, were
in the range of 55-72°, except for salts with a hexadecyl
substituent in a cation in the case of which the angles were in the
range of 82-105°. It was observed that the contact angle of the
hydrophobic surface decreased upon extension of the alkyl linker
in the bisammonium cation. The introduction of MCPA and trans-
cinnamate anion reduced the CMC by almost 30% compared to
the original bisammonium dibromides. Greenhouse studies
confirmed that all the synthesized DSHILs were characterized by
efficacy several times higher than that of the commercial herbicide
-3
Preparation of DSHILs
Bisammonium salts with the trans-cinnamate and MCPA anions
were synthesized using a two-step approach. In the first step, 0.02
mol of the appropriate dibromoalkane was introduced into a
round-bottom flask. Next,
a
0.04 mol solution of
(
Chwastox Extra 300 SL). The effectiveness of reducing the fresh
alkyldimethylamine in 100 mL of acetonitrile with a 10% excess
was added. The synthesis was conducted at the boiling point
temperature of the solvent for 48 hours. Then, the solvent was
removed by a vacuum evaporator, and 100 mL of ethyl acetate
was added. The product, which precipitated in the form of a white
solid, was isolated by filtration, washed with small portions of ethyl
acetate (3·10 mL) to remove the unreacted amine, and dried
under reduced pressure at 60 °C for 48 hours. In the next step,
the appropriate bisammonium dibromide (0.05 mol) was
dissolved in 50 mL of deionized water, followed by the addition of
weight of common lambsquarters was higher than that of the
commercial herbicide by over 30%. In the case of winter oilseed
rape, the effectiveness of DSHILs was close to 100% despite the
application of an active substance dose much lower than the
recommended value. Low surface tension values of aqueous
solutions and low contact angle values determine the high
herbicidal effectiveness of synthesized DSHILs.
0
.05 mol of potassium salt of cinnamic acid and 0.05 mol of
Experimental Section
potassium salt of MCPA dissolved in 25 mL of water. The reaction
was conducted for 30 minutes at 25 °C. Then, 50 mL of chloroform
Materials
1
1
9
,6-Dibromohexane (purity 96%), 1,8-dibromooctane (purity 98%), was added to the solution. The mixture was shaken in a
,12-dibromododecane (purity 98%), trans-cinnamic acid (purity
7%), dimethyloctylamine (purity 97%), decyldimethylamine
separatory funnel. The collected organic phase was evaporated
by means of a vacuum evaporator. The obtained product was
dried at 50 °C for 48 hours under vacuum.
(
purity
dimethyltetradecylamine (purity 95%), hexadecyldimethylamine
purity 95%) were purchased in Sigma-Aldrich (Poznan, Poland).
-Chloro-2-methylphenoxyacetic acid [MCPA] (purity 97%) was
90%),
dimethyldodecylamine
(purity
97%),
(
4
Surface activity
The surface tension and contact angle values of the synthesized
active compounds were determined using a DSA 100 analyser
obtained from CIECH Sarzyna (Nowa Sarzyna, Poland).
Potassium hydroxide (purity 85%) and all solvents (methanol,
acetonitrile, acetone, hexane, toluene, chloroform, isopropanol,
DMSO and ethyl acetate) were from Avantor (Gliwice, Poland).
-1
(KRÜSS GmbH, Germany, accuracy ± 0.01 mN·m ) at 25 °C. The
surface tension was carried out using the shape drop method.
Basically, the principle of this method is to form an axisymmetric
drop at the tip of a needle of a syringe. An image of the drop (3
mL) was obtained by a CCD camera and digitized. The surface
−
1
Deionized water with
a conductivity <0.1 μS⋅cm , from
demineralizer HLP Smart 1000 (Hydrolab, Poland) was used for
solubility and surface activity measurement. All reagents and
solvents were used without further purification. As a reference, a
commercial herbicide was used Chwastox Extra 300 SL (300 g
MCPA per 1 L, CIECH Sarzyna, Poland).
-1
tension (γ in mN·m ) was calculated by analysing the profile of
the drop according to the Laplace equation. The temperature was
controlled using a Fisherbrand FBH604 thermostatic bath (Fisher,
Germany, accuracy ± 0.1 °C). The values of the critical micelle
concentration (CMC) and the surface tension (γCMC) at CMC were
determined based on the intersection of the two straight lines
drawn in the low- and high-concentration regions of surface
tension curves (γ vs log C curves) using a linear regression
analysis method. The surface active parameter, such as the
adsorption pC20 (pC20 – molality of the compound that leads to a
General
1
H and 13C NMR studies were performed using a Varian XL300
spectrophotometer. Deuterated methanol was used as the
solvent and tetramethylsilane was the reference. The elemental
analyses (CHN) were performed by using an Elementar Analyser
Vario EL III at Adam Mickiewicz University, Poznan (Poland).
High-resolution mass spectrometry (HRMS) was performed using
a Bruker Impact HD ESI-Q-TOF (electrospray ionization -
quadrupole - time of flight) mass spectrometer. Positive ionization
mode was used to record cations, whereas negative ionization
-1
reduction of the surface tension of the solvent by 20 mN·m ), is
defined by using the following equation:
pC20 = -logC20
7
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The contact angle was determined using the sessile drop method,
where drops of liquid above CMC were deposited on a solid
paraffin surface (capable of imitating the hydrophobic surface of
the leaf). After the actual drop shape and contact line were
defined, the drop shape was adapted to fit a mathematical model
used to calculate the contact angle. The most precise method to
calculate this value is Young-Laplace fitting (sessile drop fitting),
in which the entire drop contour is evaluated. After successful
fitting of the Young-Laplace equation, the CA is determined as the
slope of the contour line at the three-phase contact point (solid-
liquid and liquid-air).
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Entry for the Table of Contents
There is great need for new green herbicides due to the threat, which weeds pose to modern large-scale cultivation of plants. In our
study we developed series of novel double salt ionic liquids, which exhibit excellent herbicidal activity. Anion of natural origin combined
with herbicidal anion showed synergistic effect in bioactivity, while amphiphilic bisammonium cation guaranteed desired
physicochemical properties.
9
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