Synthesis and spectroscopic properties of symmetrical ionic liquids based on (?)-menthol

Article abstract of DOI:10.1016/j.molliq.2016.08.112

Several symmetrical imidazolium salts were obtained from the natural chiral pool of (1R,2S,5R)-(?)-menthol. First 1,3-bis[(1R,2S,5R)-(?)-menthoxymethyl]imidazolium chloride, which is a prototype for ionic liquids, was prepared with the use of two different methods. Furthermore, metathesis of this symmetrical imidazolium chloride with various salts was carried out. The ion exchange reaction goes smoothly, with the satisfactory yield of 97.5 to 99.5%. Obtained 1,3-bis[(1R,2S,5R)-(?)-menthoxymethyl]imidazolium salts are stable in the air, in contact with water and in commonly used organic solvents. Moreover they are non-volatile and non-flammable. Discussed symmetrical salts belong to chiral ionic liquids (CILs) where the chirality resided in the cation and is associated with the presence of optically active (1R,2S,5R)-(?)-menthol. The diastereotopic protons in the ¹H NMR were thoroughly described. Moreover, the ¹H NMR and ¹³C NMR spectra indicated notable differences in the chemical shifts depending on the anion used. Comparing the differences between values of the chemical shifts, the anions were ordered according to their increasing shielding capacities. The changing of the electron density surrounding the imidazolium ring was discussed.

Full text of DOI:10.1016/j.molliq.2016.08.112

MOLLIQ-06273; No of Pages 8
Journal of Molecular Liquids xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and spectroscopic properties of symmetrical ionic liquids based
on (−)-menthol
Joanna Feder-Kubis
Wrocław University of Science and Technology, Faculty of Chemistry, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Several symmetrical imidazolium salts were obtained from the natural chiral pool of (1R,2S,5R)-(−)-menthol.
First 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride, which is a prototype for ionic liquids, was
prepared with the use of two different methods. Furthermore, metathesis of this symmetrical imidazolium chlo-
ride with various salts was carried out. The ion exchange reaction goes smoothly, with the satisfactory yield of
Received 1 March 2016
Received in revised form 22 August 2016
Accepted 30 August 2016
Available online xxxx
97.5 to 99.5%. Obtained 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium salts are stable in the air, in con-
tact with water and in commonly used organic solvents. Moreover they are non-volatile and non-flammable.
Discussed symmetrical salts belong to chiral ionic liquids (CILs) where the chirality resided in the cation and is
associated with the presence of optically active (1R,2S,5R)-(−)-menthol.
Keywords:
Symmetrical imidazolium salt
Chiral ionic liquid
Chemical shift
Nuclear magnetic resonance
Diastereotopic proton
The diastereotopic protons in the H NMR were thoroughly described. Moreover, the H NMR and 13C NMR spec-
tra indicated notable differences in the chemical shifts depending on the anion used. Comparing the differences
between values of the chemical shifts, the anions were ordered according to their increasing shielding capacities.
The changing of the electron density surrounding the imidazolium ring was discussed.
1
1
©
2016 Published by Elsevier B.V.
1
. Introduction
Moreover, other major benefits of testing them is higher flexibility in
the manipulation of their physicochemical properties [26–29].
Ionic liquids (ILs) attract much scientific interest due to their proper-
Previously, we described and tested ionic liquids based on
(1R,2S,5R)-(−)-menthol having asymmetrical structure [30–33]. The
presence of optical active monocyclic alcohol in the cationic part classi-
fied these compounds to the group of chiral ionic liquids (CILs). The
promising properties have already been shown, inter alia: in organic ca-
talysis [34], in stabilization and activation of laccase [35] and as
antielectrostatic materials [30,32,33]. Moreover, the microbiological ac-
tivity of some CILs with discussed terpene alcohol was definitely more
pronounced than that of commercially available pattern [36]. On the
other hand, the methods of synthesis of symmetrical imidazolium chlo-
ride based on (1R,2S,5R)-(−)-menthol have been also presented by our
group [37]. In this study the synthesis, physicochemical and spectro-
scopic properties of new symmetrical ionic liquids based on
(1R,2S,5R)-(−)-menthol are reported. It seems to be extremely inter-
esting to continue the research on the issue regarding CILs with natural
occurring substance: monocyclic terpene alcohol, mainly due to the
possible selection of the best of them for various applications. Moreover,
the impact of symmetry/asymmetry on physicochemical properties
and potential use for distinct purposes is also taken into account.
Furthermore, the selected anions for preparing symmetrical ionic liq-
uids presented herein [bis(pentafluoroethylsulfonyl)imide, 1,1,2,2-
tetrafluoroethanesulfonate, perfluorobutanesulfonate, dicyanamide
and thiocyanate] have been nowadays often investigated mainly due
to their very promising potential applications e.g. in catalysis [38], in
ties and the possibility of obtaining products for distinct purposes [1–4].
Therefore during the last decade a large number of publication appear-
ance both in the field of synthesis [5,6] and physicochemical properties
[
7,8], and in areas of their potential applications [9–11]. It should be
highlighted that one of their major features is the ‘designable’ structure
which can be achieved by selecting appropriate ions to obtain specific
utility for various applications (e.g. in separation technology [12,13],
biomass treatment [14], energy storage [15], lubrication [16,17], elec-
trochemistry [18], catalysis [19,20], and medical chemistry [21,22]).
The enormous combinations of different cations and anions for design-
ing ionic liquids have also impact on their tunable physicochemical
properties (e.g. thermal stability, density, viscosity, conductivity, polar-
ity, solubility, hydrogen bond ability).
The asymmetry/symmetry of the cation plays an important role in
changing the physicochemical properties of ionic liquids [23]. The sym-
metry in ILs is generally considered to the multiplicities of the cation
(
mainly dicationic and tricationic ILs) or to the presence of two (some-
times more) identical chains in the cation. The advantages of using
those salts seem to be justified mainly due to their higher thermal sta-
bility in comparison with their asymmetric counterparts [24,25].
E-mail address: joanna.feder-kubis@pwr.edu.pl.
http://dx.doi.org/10.1016/j.molliq.2016.08.112
0167-7322/© 2016 Published by Elsevier B.V.
Please cite this article as: J. Feder-Kubis, Synthesis and spectroscopic properties of symmetrical ionic liquids based on (−)-menthol, J. Mol. Liq.
2016), http://dx.doi.org/10.1016/j.molliq.2016.08.112
(

2
J. Feder-Kubis / Journal of Molecular Liquids xxx (2016) xxx–xxx
extraction [39–41], in dissolution of carbohydrates [42], in electrochem-
istry [43,44] and in processing of cellulose [45]. Surprisingly, they have
also appeared to effective stored product insect antifeedants [46].
by 1H NMR and 13C NMR and the results were fully comparable
with those presented earlier [37].
2.3.2. Synthesis of symmetrical chiral ionic liquids (2–6)
2
. Experimental
The solid salts (0.031 mol): potassium thiocyanate, lithium
bis(pentafluoroethylsulfonyl)imide, and potassium 1,1,2,2-
tetrafluoroethanesulfonate were dissolved in methanol at room
temperature. The potassium perfluorobutanesulfonate was also
dissolved in methanol but at the elevated temperature (around
40 °C), due to its limited solubility at room temperature. In case
of sodium dicyanamide the distilled water was used as a solvent
(because of very low solubility of sodium dicyanamide in metha-
nol). Then, the prepared solution was added to saturated methanol
containing 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
chloride (0.03 mol). The reaction was stirred at room temperature
for 1 day. Upon evaporation of the solvents, the product was dis-
solved in acetone and stirred for 30 min to precipitate the halide
salt (NaCl, KCl or LiCl), which was filtrated off (0.2 μm filter). The
filtrate was placed in the fridge (approx. −5 °C) for one night in
order to complete precipitation of the halide salt. In the case of pre-
cipitation of inorganic salt, another filtration was carried out and the
resulting filtrate was evaporated. Finally, the crude product was washed
with distilled water until chloride ions were no longer detected using
2
.1. Materials
The chemicals which have been used in the synthesis of the symmet-
rical chiral ionic liquids were of analytical grade. They were purchased
from different companies. (1R,2S,5R)-(−)-Menthol (≥99%), parafor-
maldehyde (power, 95%), imidazole (≥99%), triethylamine (anhydrous,
≥
99%), hydrochloric acid (35–38%), sulfuric acid (≥96%), sodium
dicyanamide (96%), potassium thiocyanate (≥99%) were purchased
from Sigma-Aldrich. Lithium bis(pentafluoroethylsulfonyl)imide
(
99%), potassium perfluorobutanesulfonate (N98%) and potassium
,1,2,2-tetrafluoroethanesulfonate (98%) were provided by Iolitec.
For NMR analysis, deuterated chloroform (CDCl ), deuterated di-
methyl sulfoxide [(CD SO], deuterated acetone (CD COCD ) and deu-
terated water (D O), purchased from Merck, were used. For all
1
3
)
3 2
3
3
2
measurements the concentration of the tested symmetrical
imidazolium salts in the deuterated solvent was constant with a value
3
3
of 20 mg (± 0.0001 g) in 0.60 cm (± 0.01 cm ) of each solvent.
All solvents used in synthesis and purification of chiral ionic liquids
were purchased from the commercial suppliers: Sigma-Aldrich and
Fluka and were dried before their use. All reagents were dried and puri-
fied before the use by usual procedures, reactions were performed
under anhydrous conditions and all substrates were freshly distilled be-
fore using.
3
AgNO . Obtained salts were further dried in vacuum (0.3 mm Hg) to
achieve analytically pure products. Prior to any measurements 1,3-
bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium salts (2–6) samples
were dried for 48 h under vacuum.
2.3.2.1.
bis(pentafluoroethylsulfonyl)imide (2). H NMR (600 MHz, CDCl
0.53 (d, J = 7.2 Hz, 6H, H9 or H10 and H9′ or H10′), 0.82–0.94 (m,
8H, Ha-4, H7, H9 or H10, Ha-6, Ha-3, Ha-4′, H7′, H9′ or H10′, Ha-6′
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
1
3
): δ
2
.2. Instrumentation
1
Melting points were determined by using an electrothermal digital-
melting-point apparatus model JA 9100 (temperature resolution
0.1 °C; accuracy ± 1%; choice of ramp rate of 1.0 °C/min). The type
and Ha-3′), 1.25–1.29 (m, 2H, H2 and H2′), 1.38–1.40 (m, 2H, H5 and
H5′), 1.61–1.67 (m, 4H, Hb-3, Hb-4, Hb-3′ and Hb-4′), 1.95–2.01 (m,
4H, H8, H8′Hb-6 and Hb-6′), 3.31 (td, J = 10.2 Hz, J = 4.2 Hz, 2H, H1
±
’
and the shape of the crystals were analysed via optical microscope
and H1′), 5.56 and 5.68 (d, J = 10.2 Hz, 4H, AB system, H11 and H11),
Axiolmager M1m (Zeiss) in reflection mode. Specific rotations at
7.475 (s, 1H, H12 or H13), 7.48 (s, 1H, H12 or H13), 9.28 (s, 1H, H14).
13
5
78 nm were measured using an Optical Activity Ltd. Model AA-5 auto-
matic polarimeter (resolution ± 0.01°, reproducibility ± 0.01°, accuracy
0.01°, temperature probe measurement accuracy ± 0.1 °C, the equip-
3
C NMR (300 MHz, CDCl ): δ 15.4 (C9 or C10 and C9′ or C10′), 20.8
(C7 and C7′), 21.9 (C9 or C10 and C9′ or C10′), 22.8 (C3 and C3′), 25.4
(C8 and C8′), 31.1 (C5 and C5′), 34.0 (C4 and C4′), 40.1 (C6 and C6′),
47.7 (C2 and C2′), 77.1 (C1 and C1′), 80.3 (C11 and C11′), 113.7,
±
ment provide four results for each measurement). The structure and pu-
rity of all of the synthesized salts were confirmed by spectral analysis.
Elemental analyses were carried out for all of the synthesized sub-
111.7, 109.75 (qxq, J1CF = 294 Hz, J2CF = 39 Hz, CF
118.9, 117.0, 115.1 (txt, J CF = 288 Hz, J CF = 33 Hz, CF
, anion), 120.6,
2
1
2
3
, anion), 122.1
1
13
stances using VARIO EL-III. The H NMR and C NMR spectra were re-
corded on a Bruker DRX with tetramethylsilane as the standard (at
(C13 and C12), 135.0 (C14).
Elemental analysis calc. (%) for C29H F N O S
45 10 3 6 2
(785.97): C 44.31, H
6
00 and 75 MHz, respectively). Total concentration of the metal in
5.78, N 5.35, S 8.17. Found: C 44.42, H 5.90, N 5.30, S 8.09.
each sample was measured in resulting solutions by high-resolution
continuum source flame atomic absorption spectrometry (HR-CS-
FAAS) with a contrAA 700 instrument (Analytik Jena AG).
2.3.2.2. 1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium 1,1,2,2-
tetrafluoroethanesulfonate (3). 1H NMR (600 MHz, CDCl
): δ 0.56 (d,
3
J = 6.6 Hz, 6H, H9 or H10 and H9′ or H10′), 0.84–0.98 (m, 18H, Ha-4,
H7, H9 or H10, Ha-6, Ha-3, Ha-4′, H7′, H9′ or H10′, Ha-6′ and Ha-3′),
2
2
.3. Synthesis of symmetrical chiral ionic liquids
1
.26–1.30 (m, 2H, H2 and H2′), 1.45–1.46 (m, 2H, H5 and H5′), 1.63–
.3.1. Synthesis of substrates
Chloromethyl (1R,2S,5R)-(−)-menthyl ether was obtained by pass-
1.69 (m, 4H, Hb-3, Hb-4, Hb-3′ and Hb-4′), 2.02 (sept d, J = 5.4 Hz,
J = 2.4 Hz, 2H, H8 and H8′), 2.07–2.08 (m, 2H, Hb-6 and Hb-6′), 3.37
(td, J = 10.8 Hz, J = 4.8 Hz, 2H, H1 and H1′), 5.62 and 5.76 (d, J =
10.8 Hz, 4H, AB system, H11 and H11′), 6.31, 6.22, 6.13 (txt, J =
53.0 Hz, J = 6.0 Hz, 1H, anion), 7.49 (s, 1H, H12 or H13), 7.495 (s, 1H,
ing HCl through a mixture of paraformaldehyde and (1R,2S,5R)-(−)-
menthol, following the published method [30]. 1-(1R,2S,5R)-(−)-
Menthoxymethylimidazole was prepared in a two-step reaction,
using: trimethylamine, chloromethyl (1R,2S,5R)-(−)-menthyl ether
and imidazole, following the published method [37]. 1,3-
Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride (1) was
obtained by two different methods and the details were described in
our previous publication [37].
H12 or H13),9.77 (s, 1H, H14). 13C NMR (300 MHz, CDCl
3
): δ 15.6 (C9
or C10 and C9′ or C10′), 20.9 (C7 and C7′), 22.0 (C9 or C10 and C9′ or
C10′), 22.8 (C3 and C3′), 25.4 (C8 and C8′), 31.1 (C5 and C5′), 34.0 (C4
and C4′), 40.2 (C6 and C6′), 47.8 (C2 and C2′), 77.3 (C1 and C1′), 80.1
1
2
(C11 and C11′), 111.70, 109.1, 107.4 (txt, J CF = 253 Hz, J CF = 29 Hz,
anion CHF
), 115.65, 113.8, 111.9 (txt, J1CF = 284 Hz, J2CF = 24 Hz,
anion CF ), 121.7 (C13 and C12), 136.4 (C14).
Elemental analysis calc. (%) for C27
7.92, N 4.77, S 5.47. Found: C 55.35, H 7.87, N 4.81, S 5.38.
All of the synthesized precursors of chiral ionic liquids
2
[
chloromethyl (1R,2S,5R)-(−)-menthyl ether, 1-(1R,2S,5R)-(−)-
menthoxymethylimidazole and 1,3-bis[(1R,2S,5R)-(−)-
menthoxymethyl]imidazolium chloride (1)] were characterized
2
46 4 2 5
H F N O S (586.85): C 55.26, H
Please cite this article as: J. Feder-Kubis, Synthesis and spectroscopic properties of symmetrical ionic liquids based on (−)-menthol, J. Mol. Liq.
2016), http://dx.doi.org/10.1016/j.molliq.2016.08.112
(

J. Feder-Kubis / Journal of Molecular Liquids xxx (2016) xxx–xxx
3
1
2
.3.2.3.
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
H NMR (600 MHz, CD
3
COCD
3
): δ 0.59 (d, J = 7.2 Hz, 6H, H9 or H10
1
’
’
perfluorobutanesulfonate (4). H NMR (600 MHz, CDCl
J = 7.2 Hz, 6H, H9 or H10 and H9′ or H10′), 0.83–0.97 (m, 18H, Ha-4,
H7, H9 or H10, Ha-6, Ha-3, Ha-4′, H7′, H9′ or H10′, Ha-6′ and Ha-3′),
1
1
3
): δ 0.56 (d,
and H9 or H10 ), 0.86–1.03 (m, 18H, Ha-4, H7, H9 or H10, Ha-6, Ha-3,
Ha-4′, H7′, H9′ or H10′, Ha-6′ and Ha-3′), 1.26–1.30 (m, 2H, H2 and
H2′), 1.48–1.50 (m, 2H, H5 and H5′), 1.63–1.69 (m, 4H, Hb-3, Hb-4,
Hb-3′ and Hb-4′), 2.07–2.10 (m, 2H, H8 and H8′), 2.22–2.24 (m, 2H,
Hb-6 and Hb-6′), 3.55 (td, J = 10.8 Hz, J = 4.2 Hz, 2H, H1 and H1′),
5.86 and 5.99 (d, J = 10.8 Hz, J = 10.2 Hz, 4H, AB system, H11 and
H11′), 8.045 (s, 1H, H12 or H13), 8.05 (s, 1H, H12 or H13), 9.88 (s, 1H,
.25–1.28 (m, 2H, H2 and H2′), 1.43–1.44 (m, 2H, H5 and H5′), 1.63–
.66 (m, 4H, Hb-3, Hb-4, Hb-3′ and Hb-4′), 2.00 (sept d, J = 7.2 Hz,
J = 2.4 Hz, 2H, H8 and H8′), 2.06–2.07 (m, 2H, Hb-6 and Hb-6′), 3.37
td, J = 10.2 Hz, J = 4.2 Hz, 2H, H1 and H1′), 5.61 and 5.75 (d, J =
0.8 Hz, J = 10.2 Hz, 4H, AB system, H11 and H11′), 7.49 (s, 1H, H12
(
1
H14). 13C NMR (300 MHz, CD
C10′), 20.4 (C7 and C7′), 21.6 (C9 or C10 and C9′ or C10′), 22.7 (C3
and C3′), 25.52 (C8 and C8′), 31.0 (C5 and C5′), 34.0 (C4 and C4′),
40.2 (C6 and C6 ), 47.9 (C2 and C2′), 77.0 (C1 and C1′), 78.9 (C11 and
C11′), 122.7 (C13 and C12), 131.0 (anion), 137.2 (C14).
3 3
COCD ): δ 15.7 (C9 or C10 and C9′ or
1
3
or H13), 7.495 (s, 1H, H12 or H13), 9.74 (s, 1H, H14). C NMR
(
300 MHz, CDCl ): δ 15.5 (C9 or C10 and C9′ or C10′), 20.8 (C7 and
3
’
C7′), 21.9 (C9 or C10 and C9′ or C10′), 22.75 (C3 and C3′), 25.3 (C8
and C8′), 31.0 (C5 and C5′), 34.0 (C4 and C4′), 40.1 (C6 and C6′), 47.7
(
C2 and C2′), 77.3 (C1 and C1′), 80.1 (C11 and C11′), 122.0 (C13 and
45 3 2
Elemental analysis calc. (%) for C26H N O S (463.84): C 67.32, H
C12), 136.0 (C14), 107.0–107.4, 108.3–109.5, 110.3–111.2, 111.7–
9.80, N 9.06, S 6.92. Found: C 67.27, H 9.89, N 9.13, S 6.88.
2.4. The purity of the prepared CILs – metal content in CIL
Samples (100 mg) of solid CILs were wet digested in ACS grade con-
1
1
12.9, 113.6–114.8, 115.6–116.7, 118.2–118.6, 120.1–120.6 (anion),
22.0 (C13 and C12), 136.0 (C14).
45 9 2 5
Elemental analysis calc. (%) for C29H F N O S (704.86): C 49.41, H
6
.45, N 3.97, S 4.55. Found: C 49.56, H 6.58, N 3.92, S 4.48.
3
centrated HNO (5 ml). A digestion block DigiPrep Jr. (SCP Science) was
used. The digestion was carried out at 120 °C for 2 h in semi-closed ves-
sels. Later on, the samples' solutions were evaporated nearly to dryness
and finally reconstituted in deionized water to 5.0 g. The total concen-
trations of the proper metal (Li in the case of [Men-Im-Men][PFSI] – 2;
K in the case of [Men-Im-Men][TFES] – 3, [Men-Im-Men][PFBS] – 4,
2
.3.2.4.
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
1
3
dicyanamide (5). H NMR (600 MHz, CDCl ): δ 0.60 (d, J = 6.6 Hz, 6H,
H9 or H10 and H9′ or H10′), 0.85–1.00 (m, 18H, Ha-4, H7, H9 or H10,
Ha-6, Ha-3, Ha-4′, H7′, H9′ or H10′, Ha-6′ and Ha-3′), 1.28–1.32 (m,
2
H, H2 and H2′), 1.45–1.47 (m, 2H, H5 and H5′), 1.63–1.70 (m, 4H,
[Men-Im-Men][SCN] – 6, and Na in the case of [Men-Im-Men][DCA] –
Hb-3, Hb-4, Hb-3′ and Hb-4′), 2.00–2.08 (m, 4H, H8, H8′, Hb-6 and
Hb-6′), 3.38 (td, J = 10.8 Hz, J = 4.2 Hz, 2H, H1 and H1′), 5.66 and
5) were measured in resulting solutions by high resolution continuum
source flame atomic absorption spectrometry (HR-CS-FAAS). Simple
standard solutions of K, Li or Na (0.05, 0.1, 0.2 and 0.5 μg/g) were used
for calibration. It was found that the samples' solutions of the investigat-
ed CILs contain proper metal below its limit of quantification (LOQ)
assessed for HR-CS-FAAS as 0.005 μg/g. Considering the sample masses
and the final volumes of the samples' solution it corresponded to the
metal content below 0.25 μg/g.
5
(
.75 (d, J = 10.8 Hz, J = 10.2 Hz, 4H, AB system, H11 and H11′), 7.57
s, 1H, H12 or H13), 7.575 (s, 1H, H12 or H13), 9.62 (s, 1H, H14). 13
C
NMR (300 MHz, CDCl
and C7′), 22.1 (C9 or C10 and C9′ or C10′), 22.9 (C3 and C3′), 25.5 (C8
and C8′), 31.3 (C5 and C5′), 34.0 (C4 and C4′), 40.3 (C6 and C6′), 47.8
3
): δ 15.8 (C9 or C10 and C9′ or C10′), 20.9 (C7
(
C2 and C2′), 77.5 (C1 and C1′), 80.4 (C11 and C11′), 119.9 (anion),
1
21.9 (C13 and C12), 135.9 (C14).
Elemental analysis calc. (%) for C27
45 5 2
H N O (471.74): C 67.88, H 9.51,
2.5. Solubility
N 5.86. Found: C 67.94, H 9.64, N 5.80.
The solubility of obtained CILs was determined according to Vogel's
2
.3.2.5. 1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium thiocyanate
Textbook of Practical Organic Chemistry [47]. The investigations were
carried out for the synthesized salts (2–6) at 20 °C (± 0.5 °C) and at
50 °C (± 0.5 °C) under ambient pressure using popular solvents. The
phrase complete solubility describe those salts which are soluble
(0.1 g ± 0.01 g of each) in 1 mL (± 0.01 mL) of solvent, while the limited
solubility specifies those CILs in which 0.1 g (± 0.01 g) of each salts is
soluble in 3 mL (± 0.1 mL) of solvent. The expression insoluble was
used to characterize insolubility of the compound (0.1 g ± 0.01 g of
the salt in 3 mL ± 0.1 mL of solvent).
1
3
(6). H NMR (600 MHz, CDCl ): δ 0.56 (d, J = 7.2 Hz, 6H, H9 or H10 and
H9′ or H10′), 0.80–0.96 (m, 18H, Ha-4, H7, H9 or H10, Ha-6, Ha-3, Ha-4′,
H7′, H9′ or H10′, Ha-6′ and Ha-3′), 1.24–1.28 (m, 2H, H2 and H2′), 1.43–
1
4
2
5
.455 (m, 2H, H5 and H5′), 1.59–1.65 (m, 4H, Hb-3, Hb-4, Hb-3′ and Hb-
′), 1.98 (sept d, J = 6.6 Hz, J = 2.4 Hz, 2H, H8 and H8′), 2.09–2.11 (m,
H, Hb-6 and Hb-6′), 3.42 (td, J = 10.8 Hz, J = 4.2 Hz, 2H, H1 and H1′),
.70 and 5.86 (d, J = 10.2 Hz, 4H, AB system, H11 and H11′), 7.50 (s, 1H,
1
3
H12 or H13), 7.55 (s, 1H, H12 or H13), 9.93 (s, 1H, H14). C NMR
(
300 MHz, CDCl ): δ 15.8 (C9 or C10 and C9′ or C10′), 21.0 (C7 and
3
C7′), 22.2 (C9 or C10 and C9′ or C10′), 22.9 (C3 and C3′), 25.5 (C8 and
C8′), 31.2 (C5 and C5′), 34.0 (C4 and C4′), 40.0 (C6 and C6′), 47.8 (C2
and C2′), 77.7 (C1 and C1′), 80.3 (C11 and C11′), 121.5 (C13 and C12),
3. Results and discussion
The studied new symmetrical chiral ionic liquids based on
(1R,2S,5R)-(−)-menthol (2–6) were synthesized as shown in Scheme
1. In the first step, chloromethyl (1R,2S,5R)-(−)-menthyl ether was ob-
tained by chloromethylation of (1R,2S,5R)-(−)-menthol [30]. Then 1-
(1R,2S,5R)-(−)-menthoxymethylimidazole was prepared following
the published method: trimethylamine reacted with terpene ether in
toluene, then imidazole was added in order to perform N-
alkoxymethylation [37]. Previously, we described two different methods
of obtaining 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chlo-
ride (1) [37]. In the first method chloromethyl (1R,2S,5R)-(−)-menthyl
ether reacted with stoichiometric amount of 1-(1R,2S,5R)-(−)-
menthoxymethylimidazole in hexane. In the second method the reaction
between two equivalents of chloromethyl (1R,2S,5R)-(−)-menthyl ether
and one equivalent of imidazole took place in DMF. In this study the sym-
metrical chloride with two monocyclic terpene moieties (1) has been
used as a precursor of new chiral ionic liquids via metathesis reaction.
1
32.0 (anion), 136.75 (C14).
1
3 2
H NMR (600 MHz, (CD ) SO): δ 0.47 (d, J = 6.6 Hz, 6H, H9 or H10
and H9′ or H10′), 0.81–0.92 (m, 18H, Ha-4, H7, H9 or H10, Ha-6, Ha-3,
Ha-4′, H7′, H9′ or H10′, Ha-6′ and Ha-3′), 1.15–1.20 (m, 2H, H2 and
H2′), 1.33–1.39 (m, 2H, H5 and H5′), 1.55–1.63 (m, 4H, Hb-3, Hb-4,
Hb-3′ and Hb-4′), 1.94 (sept d, J = 7.2 Hz, J = 2.4 Hz, 2H, H8 and
H8′), 2.04–2.505 (m, 2H, Hb-6 and Hb-6′), 3.28 (td, J = 10.8 Hz, J =
4
4
.2 Hz, 2H, H1 and H1′), 5.64 and 5.69 (d, J = 10.8 Hz, J = 10.2 Hz,
H, AB system, H11 and H11′), 7.99 (s, 1H, H12 or H13), 8.00 (s, 1H,
13
H12 or H13), 9.62 (s, 1H, H14). C NMR (300 MHz, (CD
3 2
) SO): δ
1
6.05 (C9 or C10 and C9′ or C10′), 21.3 (C7 and C7′), 22.5 (C9 or C10
and C9′ or C10′), 23.0 (C3 and C3′), 25.3 (C8 and C8′), 31.25 (C5 and
C5′), 34.1 (C4 and C4′), 39.6 (C6 and C6′), 47.7 (C2 and C2′), 77.6 (C1
and C1′), 78.4 (C11 and C11′), 123.2 (C13 and C12), 130.0 (anion),
1
37.7 (C14).
Please cite this article as: J. Feder-Kubis, Synthesis and spectroscopic properties of symmetrical ionic liquids based on (−)-menthol, J. Mol. Liq.
2016), http://dx.doi.org/10.1016/j.molliq.2016.08.112
(

4
J. Feder-Kubis / Journal of Molecular Liquids xxx (2016) xxx–xxx
O
Cl
Method I
(
2 steps reaction)
1
eq. of ether,
hexane, rt
TEA,
O
N
N
o
toluene, 80 C
CH2O, HClg
toluene
o
N
N
H
5
-10 C, 5h
OH
O
Cl
O
N
N
O
Cl-
(1)
Method II
1 step reaction)
H2O or
MeOH/H2O
(
MX
2
eq. of ether,
o
DMF, 35 C
O
N
N
O
X-
(2-6)
2 5 2 2 2 4 3 4 9 3 2
Scheme 1. Preparation of symmetrical chiral ionic liquids (2–6). MX = (C F SO ) NLi, C F HSO K, C F SO K, NaN(CN) , KSCN.
The anion exchange processes proceeded smoothly, with the efficiency
exceeding 97% in each case (Table 1). Due to very low solubility of 1,3-
bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride in water the
metathesis reaction was conducted using saturated methanol solution
of salt 1. In the case of obtaining [Men-Im-Men][PFSI], [Men-Im-
Men][TFES] and [Men-Im-Men][SCN] the reaction was carried out in
methanol solution at room temperature, while [Men-Im-Men][PFBS]
was synthesized in the same solvent, but at elevated temperature, since
potassium perfluorobutanesulfonate shows limited solubility in methanol
at room temperature. The mixture of methanol/water was used for syn-
thesis of [Men-Im-Men][DCA] due to very low solubility of sodium
dicyanamide in methanol. Sodium, potassium or lithium chloride were
by-products of the exchange reactions and were removed using acetone,
in which those inorganic salts precipitate. The details about metathesis re-
action as well as structures and abbreviations of obtained salts are pre-
sented in the Table 1.
All of the synthesized 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]
imidazolium salts (2–6) are solid. The melting point, crystallization
conditions, crystal shape and specific rotation of each salt are given
in Table 2. The melting points of new symmetrical imidazolium
salts based on (1R,2S,5R)-(−)-menthol suggest that they should be
considered as ionic liquids, except 1,3-bis[(1R,2S,5R)-(−)-
menthoxymethyl]imidazolium thiocyanate (6) with higher melting
point: 134.1–134.5 °C. The general trend which can be observed for
discussed CILs is that there is a major influence of lowering the melt-
ing point with changing the type of the anion. In case of comparing
chloride (1) with perfluorobutanesulfonate (4) the difference is
over 70 °C: from 126 to 127.7 °C for salt 1 [37] to 51.7–52.3 °C for
salt 4. Surprisingly, the melting point of [Men-Im-Men][SCN] is
higher than its precursor.
(6) derivatives structure of plates were recognized, while crystals of
1,1,2,2-tetrafluoroethanesulfonate (3) and perfluorobutanesulfonate
(4) compounds are in the shape of needles (Table 2). Unfortunately, re-
peated efforts have not succeeded in obtaining the crystal structure for
[Men-Im-Men][DCA]. In spite of using different types of solvents for
crystallization the salt has exhibited the tendency to oil and the crystal
form hasn't been observed.
All of the synthesized 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]
imidazolium salts (2–6) are stable in air, in aqueous solutions and in
commonly used organic solvents. The solubility test has been performed
using popular polar and nonpolar solvents according to the method de-
scribed by Vogel et al. [47]. The results are shown in Table 2. All of the
obtained chlorides are soluble in various commonly used solvents,
e.g.: acetone, acetonitrile, chloroform, THF, DMSO, DMF and in low mo-
lecular weight alcohols. The solubility in toluene and ethyl acetate is
also excellent at room temperature in case of salts 2–5. The thiocyanate
derivative (6) might be completely soluble in toluene at elevated tem-
peratures, while in ethyl acetate it is totally insoluble even at elevated
temperatures (Table 3). All of the synthesized salts are not soluble in
diethyl ether and hexane independently from the set of the measure-
ment temperature. In case of water there is an interesting point to dis-
cuss: at room temperature majority of 1,3-bis[(1R,2S,5R)-(−)-
menthoxymethyl]imidazolium salts are insoluble, only [Men-Im-
Men][DCA] shows partial solubility. In 50 °C salts from 1 to 5 are
completely soluble in water, except [Men-Im-Men][SCN], which is total-
ly insoluble. Furthermore, it should be pointed that the change of the
anion influence the solubility of the obtained salts in water. This pattern
refers to CILs from 1 to 5, which are soluble in water at 50 °C, while its
chloride precursor is almost insoluble even at elevated temperatures.
The changes of the solubility of [Men-Im-Men][DCA] 5 were con-
1
The obtaining of the crystal structure was possible to achieve for ma-
jority of discussed salts. The solids: [Men-Im-Men][PFSI] and [Men-Im-
Men][PFBS] crystallize easily from mixture water/ethanol, while [Men-
Im-Men][TFES] from hexane/chloroform. In the case of performing the
crystal structure of [Men-Im-Men][SCN] the mixture of solvents of
ethyl acetate/acetone was used. The various form of the crystals were
observed: for bis(pentafluoroethylsulfonyl)imide (2) and thiocyanate
firmed by H NMR, using D
2
O as a solvent. The spectra were performed
for various amounts of the compound (m = 0.02 g, 0.01 g and 0.005 g;
3
V
D2O = 0.60 cm ; T = 20 °C) and various temperatures of the measure-
ment (T = 20 °C, 30 °C, 40 °C, 50 °C, 60 °C). With increasing the amount
of the dicyanamide salt and with increasing the temperature of the
measurement the observed picks are more pronounced and more
sharp (all spectra are provided in Supporting Information S22-S26).
Please cite this article as: J. Feder-Kubis, Synthesis and spectroscopic properties of symmetrical ionic liquids based on (−)-menthol, J. Mol. Liq.
2016), http://dx.doi.org/10.1016/j.molliq.2016.08.112
(

J. Feder-Kubis / Journal of Molecular Liquids xxx (2016) xxx–xxx
5
Table 1
Structure and metathesis conditions of symmetrical chiral ionic liquids (2–6).
Salt
number
Chiral ionic liquid
Abbreviation
Structure
Conditions of
metathesis
Yielda
(%)
2
3
4
5
6
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
bis(pentafluoroethylsulfonyl)imide
[Men-Im-Men][PFSI]
MeOH, 25 °C, 24
h
99.0
98.0
99.5
97.5
98.5
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
[Men-Im-Men][TFES]
[Men-Im-Men][PFBS]
[Men-Im-Men][DCA]
[Men-Im-Men][SCN]
MeOH, 25 °C, 24
h
1,1,2,2-tetrafluoroethanesulfonate
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
perfluorobutanesulfonate
MeOH, 40 °C, 24
h
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
dicyanamide
MeOH/H
°C, 24 h
2
O, 25
1,3-Bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium
thiocyanate
MeOH, 25 °C, 24
h
a
Accuracy ± 0.1%.
Synthesized new symmetrical chiral ionic liquids containing
Table 3
Solubility of prepared symmetrical chiral ionic liquids at 20 °C and 50 °C.
1
13
(
1R,2S,5R)-(−)-menthol moieties were characterized by H and
C
NMR (for spectra see Supporting Information), and by elemental
analysis.
Solvent
Temperature (°C)
CILs
2
3
4
5
6
Water
20 °C
−
+
+
+
+
+
+
−
−
−
+
+
+
+
+
+
+
+
−
+
+
+
+
+
+
−
−
−
+
+
+
+
+
+
+
+
−
+
+
+
+
+
+
−
−
−
+
+
+
+
+
+
+
+
±
−
−
+
+
+
+
+
−
−
−
+
−
−
+
±
Table 2
50 °C
+
+
+
+
+
+
−
−
−
+
+
+
+
+
+
+
+
Properties of symmetrical chiral ionic liquids (2–6).
Methanol
Ethanol
Propanol
Acetone
Acetonitrile
Diethyl ether
Hexane
20 °C
20 °C
20 °C
20 °C
20 °C
20 °C
20 °C
50 °C
20 °C
20 °C
50 °C
20 °C
20 °C
CILs M.p.a (°C)
Crystallization
solvents
Crystal shape
Specific
b,c,d
rotation
2
5
[
α]
D
2
3
81.2–81.4
94.2–94.6
Water/ethanol
Hexane/chloroform Long, regular
needles
Irregular plates
–77.02 (c 1.014)
–102.90 (c 1.076)
4
51.7–52.3
71.1–71.4
Water/ethanol
Long, regular
needles
–
Regular, squared
plates
–83.03 (c 1.034)
Chloroform
Ethyl acetate
5
6
–
–112.18 (c 1.054)
–123.38 (c 1.003)
134.1–134.5 Ethyl
acetate/acetone
THF
Toluene
50 °C
+
+
+
a
b
c
Accuracy ± 0.1 °C.
c in methylene chloride.
Standard uncertainty for specific rotation u is u(α) = ± 0.5°.
Standard uncertainty for concentration u is u(c) = ± 0.002 g/100 mL.
DMSO
DMF
20 °C
20 °C
d
Complete soluble: ‘+’. Limited solubility: ‘± ’. Insoluble: ‘−’.
Please cite this article as: J. Feder-Kubis, Synthesis and spectroscopic properties of symmetrical ionic liquids based on (−)-menthol, J. Mol. Liq.
2016), http://dx.doi.org/10.1016/j.molliq.2016.08.112
(

6
J. Feder-Kubis / Journal of Molecular Liquids xxx (2016) xxx–xxx
3
.1. Diastereotopic protons
quaternary nitrogen atom. The most significant chemical shifts were ob-
served in the case of the proton in imidazolium ring, numbered HC-14,
for all of the exchanged anions. A shift of 2.10 ppm between the salt
[Men-Im-Men][Cl] and the [Men-Im-Men][PFSI] in the case of the HC-
14 group, exemplifies the dramatic effect of the change in the electron
density resulting from changing the anion. Whereas the value of the
1
There are two characteristic doublets observed in the H NMR spec-
tra around 5.60 and 5.75 ppm, depending on the anion used. These sig-
nals described CH groups, which link menthol and amine derivatives,
2
appear in the spectra in the form of two doublets as an AB spin system.
This is a typical situation of diastereotopic protons presence, which are
chemical shift for protons in methylene groups, numbered H
2
C-11 and
generally seen in CH
2
group of chiral molecules. The diastereotopic pro-
2
H C-11′, amounts to 0.24 ppm, also for bis(pentafluoroethylsulfonyl)
tons are the ones that topologically are identical, but at the same time
they are non-alternate towards symmetry operations of the molecule.
For the discussed protons different chemical shifts are observed, be-
cause the molecule has C1 symmetry and due to it the methylene
groups are diastereotopic.
imide. Surprisingly, for other protons in the imidazolium ring, num-
bered HC-13 and HC-12, no particular effect of the change in the elec-
tron density was observed, since the value of the chemical shifts for
these protons where of a maximum of around 0.1 ppm.
The greatest effect of the chemical shifts for all the discussed pro-
tons, localized adjacent to the quaternary nitrogen atom, were observed
3
.2. Chemical shifts
for bis(pentafluoroethylsulfonyl)imide anion. The replacement of the
−
chloride anion with much bigger (C
2
F
5
SO
2
)
2
N
one was associated
NMR spectra are usually used to assess the purity and the stability of
with the chemical proton signal shifts in the direction of the higher
field. Such changes of the chemical shifts indicate lower electron accept-
ability of positively charged nitrogen atom, resulting from improved
solvation of the cation via the molecules of deuterated chloroform in
the case of anions with greater steric hindrance.
Comparing the differences between the values of the chemical shifts,
the anions are ordered according to their increasing shielding capacities
as follows:
ILs. A more thorough analysis of the chemical shifts value provides in-
formation about the acidity of the hydrogen atoms and of the electron
density distribution within the cation and anion. The signal of H-2
atom in imidazolium ring (numbered as HC-14 in figure in Table 4) is
registered in the lowest field, which means the lowest electron density
surrounds this atom.
1H and 13C NMR spectra indicated notable differences in the chemi-
cal shifts depending on the anion used. The chemical shifts of the 1,3-
bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium salts, but also num-
bered protons and carbons, are presented in Table 4. These chemical
shifts are evident for the protons and the carbons localized adjacent to
the quaternary nitrogen atom. It is caused mainly by the steric effect
of electron clouds and changing of the electrostatic interactions be-
tween ionic charges.
Cl bSCN bCHF2CF2SO3−bC4 F9SO3−bðCNÞ N bC2 F5SO2Þ2N
−
−
−
−
2
As mentioned earlier, the 13C NMR spectra also indicated notable dif-
ferences in the chemical shifts of the carbons, depending on the anion
used. The shifts were most evident for carbon surrounded of two nitro-
gen atoms in the imidazolium ring (numbered HC-14) and were of the
1H NMR spectra of the 1,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]
imidazolium chloride and the additional 1,3-bis[(1R,2S,5R)-(−)-
menthoxymethyl]imidazolium salts indicate different chemical shifts
for the imidazolium ring protons and methylene protons adjacent to
the oxygen atoms. A strong anionic effect was evident. The substitution
ranged of 2.7 ppm for (C
F
SO
)
N , around 2 ppm for (CN)
−
−
and
2
5
2
2
2
N
−
−
−
C F SO
4 9 3
anions, 1.3 ppm for CHF
2
CF
2
SO
3
and nearly 1 ppm for SCN .
The ordering of the anions according to their increasing shielding capac-
ities is identical as the one presented for protons.
Some examples about the chemical shifts depending on the anion
used have already been shown in the literature [48–50]. In those manu-
scripts the differences in the chemical shifts have been noticed only in
−
−
−
−
−
of the Cl anion with (C
2
F
5
SO
2
)
2
N , CHF
2
CF
2
SO
3
, C
4
F
9
SO
3
, (CN)
2
N
−
and SCN resulted in changes in the electron density near the
Table 4
The chemical shifts in 1H NMR.
1
,3-bis[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium salts with numbered atoms
Salt number
Abbreviation
Anion X−
Characteristic protons
1
2
3
4
5
6
[Men-Im-Men][Cl]
Cl−
(C
CHF
11.38
9.28
9.77
9.74
9.62
9.93
7.48
7.48
7.495
7.495
7.575
7.55
5.68 and 5.92
5.56 and 5.68
5.62 and 5.76
5.61 and 5.75
5.66 and 5.75
5.70 and 5.86
3.39
3.31
3.37
3.37
3.38
3.42
−
[Men-Im-Men][PFSI]
[Men-Im-Men][TFES]
[Men-Im-Men][PFBS]
[Men-Im-Men][DCA]
[Men-Im-Men][SCN]
2
F
5
SO
2
)
SO
2
N
7.475
7.49
7.49
7.57
7.50
−
2
CF
2
3
−
C
4
F
9
SO
3
(CN)
SCN
2
N−
−
3
Spectra in CDCl ; shifts in ppm.
Please cite this article as: J. Feder-Kubis, Synthesis and spectroscopic properties of symmetrical ionic liquids based on (−)-menthol, J. Mol. Liq.
2016), http://dx.doi.org/10.1016/j.molliq.2016.08.112
(

J. Feder-Kubis / Journal of Molecular Liquids xxx (2016) xxx–xxx
7
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NMR spectra indicated no significant variation in the carbon signal
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+
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[
)
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of the synthesized salts are solid, are stable in the air, in contact with
water and in commonly used organic solvents. The salts 2–5 fall within
the category of ionic liquids (i.e. m.p. b100 °C). The only exception is
[
[
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[
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to the group of chiral ionic liquids due to the presence in cationic part
two molecules of natural, monocyclic terpene alcohol with three chiral
centers.
1
In H NMR, characteristic diastereotopic protons of the CH
2
group
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1H and 13C NMR spectra indicated notable differences in the chemi-
cal shifts depending on the anion used. The most pronounced shielding
abilities were noted for the bis(pentafluoroethylsulfonyl)imide and
dicyanamide salts with the value of chemical proton shifts 2.1 and
[
1
.76 ppm respectively. On the other hand, in the 13C NMR spectra the
[
[
[
shift of 2.7 ppm was observed again for bis(pentafluoroethylsulfonyl)
imide salt. The most significant chemical shifts were noted for the pro-
ton and the carbon surrounded of two nitrogen atoms in the
imidazolium ring.
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The selection of the anions for preparing symmetrical ionic liquids:
bis(pentafluoroethylsulfonyl)imide, 1,1,2,2-tetrafluoroethanesulfonate,
perfluorobutanesulfonate, dicyanamide and thiocyanate were intended,
since currently these anions are strongly investigated in various applica-
tions, e.g. separation science, electrochemistry, catalysis. Further re-
search to evaluate the potential used of the salts described herein is
currently ongoing in our laboratories. We believe we will find applica-
tions for ionic liquids based on (1R,2S,5R)-(−)-menthol with untested
earlier anions, especially since successful examples with different an-
ions we already have.
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[
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The work was financed by the National Science Centre of Poland
grant no. 2013/09/D/ST5/03904.
Author is deeply grateful to Prof. Paweł Pohl for the atomic absorp-
tion spectrometry measurements and to Mr. Paweł Dąbrowski for re-
cording the NMR spectra.
[
[
(
1R,2S,5R)-(−)-menthyl group, Tetrahedron-Asymmetry 17 (2006) 1728–1737.
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