Arrangements of enantiopure and racemic ionic liquids at the liquid/air interface: The role of chirality on self-assembly and layering

Article abstract of DOI:10.1039/c5ra23553g

Chiral ionic liquids (CILs) have been obtained in high yield using commercial propylene oxide or natural alcohols (citronellol and nopol) as building blocks. The self-assembly ability at the interface IL/air for some couples of enantiopure and racemic CILs was explored by angle resolved XPS.

Full text of DOI:10.1039/c5ra23553g

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Caporali, C. S. Pomelli, A. Iuliano, T. Ghilardi, G. Longhi and P. Margari, RSC Adv., 2016, DOI:
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Arrangements of enantiopure and racemic ionic liquids at the
liquid/air interface: the role of chirality on self-assembly and
layering
Received 00th January 20xx,
S. Caporali,a* C. Chiappe,b* T. Ghilardi,b A. Iuliano,c* G. Longhi,d P. Margarib and C. S. Pomellib
Chiral ionic liquids (CILs) have been obtained in high yield using commercial propylene oxide or natural alcohols (citronellol
and nopol) as building blocks. The self-assembly ability at the interface IL/air for some couples of enantiopure and racemic
CILs was explored by angle resolved XPS.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
the other hand, larger anions promote a disruption of the ionic layer
increasing the amount of free spaces.6
Introduction
In this regard, ILs possessing one or more chiral centers, namely chiral
ILs (CILs), can be considered a subclass of salts having special
Ionic liquids (ILs) are low-melting-point organic salts that have
attracted in the last years an increasing interest as solvents or
additives in numerous applications. ILs are indeed characterized by
unique properties, including negligible vapor pressure at ambient
conditions, good thermal stability, favorable solvation behavior and,
when used as solvents, they are able to guarantee high reactivity and
selectivity.1 An enormous number of different ILs can be prepared
with the possibility to fine-tune their physico-chemical properties by
variations in the molecular structure of cation and/or anion and by
mixing and matching different ion pairs.2
peculiarities, which have found application in analytical separation, in
chiral discrimination, in asymmetric synthesis, in chiral
chromatography, and as chiral solvents.7 A significant number of CILs
have been designed, synthesized and utilized recently.
Chirality is a fascinating property of molecular compounds and a
powerful tool for the generation of order, directionality and, as such,
of utility, in assembled nanoscale chemical devices. Chirality encoded
in specific building blocks, at molecular scale, can control the
nanoscale assembly of soft materials, such as gels, micelles,
polymers.8 Integration between chirality and functional aggregates is
therefore a field of great current interest.9
The coexistence of charged moieties and hydrophobic alkyl chains,
generally present on cation, determines the occurrence of both
electrostatic and van der Waals interactions. Moreover, depending on
cation and anion structure hydrogen bonds between these species
can be present. Consequently, ILs are often characterized by a high
degree of self-organization:3 in imidazolium based ILs with sufficiently
long side chains (butyl or longer) the formation of apolar domains
that coexist with an ionic networks characterizes both the liquid bulk4
and the interfaces (IL/solid, IL/air, IL/other liquids).3,5 In particular, at
the IL/air interface there is pronounced layering: the outermost
region of the interface is populated by the alkyl chains, whereas the
bulk is characterized by a dense and tightly packed layer formed by
oppositely charged ionic moieties.6 The increase in the alkyl chain
length on cation favors arrangements with the chains pointing to the
gas, thus increasing the coverage of the surface with alkyl groups. On
Despite the intense research activity in ILs and the fact that chiral
centers on cation and/or anion might determine the insurgence of a
supramolecular chirality in the resulting dynamic nanostructures
(both in the pure IL and in solution) investigations in this area are yet
limited.10 Actually, ILs consisting of a charged hydrophilic head group
and a long hydrophobic tail possess surface properties similar to
traditional surfactants and are able to give in water different kind of
aggregates, such as micelles, vesicles, bilayers, etc.11 Recently, long
chain triazolium-based CILs, showing good thermal stability and
surface-active behavior, have been prepared and characterized.12
However, practically no data have been reported about the
supramolecular chirality of the related self-assembled ionic systems,
as well as on the comparison with the corresponding racemic salts,
although these systems might be also very important for the
understanding of self-assembly processes, symmetry breaking
phenomena, and for the development of new materials.
a. Dipartimento di Chimica “Ugo Schiff”, Università di Firenze, Via della
Lastruccia, 3, 50019 Sesto Fiorentino, Italy. stefano.caporali@unifi.it
b. Dipartimento di Farmacia, Università di Pisa, via Bonanno 33, 56126 Pisa,
Italy. cinzia.chiappe@unipi.it
Herein, we report the synthesis and characterization of new chiral
long-chain ILs, using as building block the optically active propylene
oxide or commercially available natural alcohols and focusing our
attention in particular on aggregation behavior at the interface
IL/vacuum (air).
c. Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via
Moruzzi 13, 56124, Pisa, Italy. anna.iuliano@dcci.unipi.it.
d. Dipartimento di Medicina Molecolare e Traslazionale, Università di
Brescia, Viale Europa 11, 25123 Brescia, Italy.
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1H), 1.20 (d, J=6Hz, 3H), 0.97 (t, J=1Hz, 9H); 13C NMR (75 MHz, CDCl3,
δ) 66.0, 53.1, 30.2, 26.1, 15.4.
Experimental
General methods
Unless otherwise specified the reagents were used without any
purification. (R)-Propylene oxide, (S)-(−)-β-citronellol were 99%
enantiopure, whereas (1R)-(-)-nopol was 98% .
1H NMR spectra were recorded in CDCl3 or D2O on a 200 or 300 MHz
NMR spectrometer. The following abbreviations are used: s=singlet,
d=doublet, dd=double doublet, t=triplet, m=multiplet, br=broad. 13C
NMR spectra were recorded at 75 MHz. The temperature was
controlled to ±0.1 oC. 1H and 13C NMR chemical shifts (ppm) are
referred to TMS as external standard.
Alkoxyacetic acids 2a-c: representative procedure
A solution of alcohols (50 mmol) in freshly distilled anhydrous 1,4-
dioxane (30 ml) was added dropwise under nitrogen atmosphere to a
suspension of NaH (60 mmol) in 1,4-dioxane (40 ml), and the mixture
was stirred at rt for 2 h. Then, sodium chloroacetate (100 mmol) was
added under nitrogen atmosphere and the reaction mixture was
refluxed for 93 h. After cooling to rt, water was added and the
mixture was concentrated under reduced pressure to remove the
organic solvent. The aqueous alkaline phase was extracted with Et2O
(3x30ml) discarding the organic phase, and the aqueous phase was
made acidic with HCl and extracted with Et2O (3x30ml). The final
collected organic phases were dried over anhydrous Na2SO4, then the
solvent was removed under vacuum, obtaining the pure product.
Circular Dichroic (CD) spectra have been recorded on a JASCO815SE in
quatz cuvettes of 1mm, 0.1 mm or demountable 0.01 mm. All spectra
have been recorded at 25°C. A test at different temperature (2°C-
90°C) on compound 6f shows only a wavelength shift of 3nm and no
substantial variation in intensity.
X-ray Photoelectron Spectroscopy (XPS) measurements were carried
out in an ultra-high vacuum (10-9 mbar) system equipped with a VSW
(R)-2-[(4-phenylbutan-2-yl)oxy]acetic acid (2a): 9.93 g (47.7 mmol,
95%) transparent liquid; (±)-2-[(4-phenylbutan-2-yl)oxy]acetic acid
(2b): 9.42 g (45.2 mmol, 91%) transparent liquid; 1H NMR (300 MHz,
CDCl3, δ) 7.24 (m, 5H), 6.79 (br s, 1H), 4.10 (d, J=7.8Hz, 2H), 3.55 (m,
1H), 2.69 (m, 2H), 1.92 (m, 1H), 1.76 (m, 1H), 1.21 (d, J=6Hz, 3H); 13C
NMR (75 MHz, CDCl3, δ)174.7, 141.9, 128.5, 128.5, 126.0, 76.4, 65.6,
38.0, 31.7, 19.4.
HAC 5000 hemispherical electron energy analyser and
a non-
monochromatized Mg-Kα X-ray source (1253.6 eV). The source power
was 100 W (10 kVx10 mA) and the spectra were acquired in the
constant-pass-energy mode at Epas = 44 eV. The recorded spectra
were fitted using CasaXPS vert. 2.3.15 software employing Gauss-
Lorentz curves after subtraction of a Shirley-type background. A drop
of the investigated CILs were loaded onto a gold foil (dimension 1cm x
1cm), previously maintained in HNO3 conc. The surface tension was
(R)-2-[(4,4-dimethylpentan-2-yl)oxy]acetic acid (2c): 5.16 g (29.65
1
mmol, 59%) transparent liquid; H NMR (300 MHz, CDCl3, δ) 6.33 (br
enough to form
a homogeneous and thick film of liquid as
s, 1H), 4.09 (d, J=13 Hz, 2H), 3.65 (m, 1H), 1.59 (dd, J1= 17.7Hz,
J2=9Hz, 1H), 1.28 (dd, J1=17.7Hz, J2=4.5Hz, 1H), 1.18 (d, J=7.2Hz, 3H),
0.92 (s, 9H); 13C NMR (75 MHz, CDCl3, δ) 173.6, 75.0, 65.2, 50.5, 30.1,
21.2.
demonstrated by the absence of signals attributable to the gold
substrate. The loaded sample holder was kept in the introduction
chamber for at least 12 hours before the measurements, allowing the
removal of volatile substances as confirmed by the pressure value
achieved (2x 10-8 mbar), just above the instrument base pressure.
2-Alkoxyethanols 3a-c: representative procedure
A solution of the alkoxyacetic acid (50 mmol) in anhydrous THF (15
ml) was added dropwise at 0°C to a suspension of LiAlH4 (50 mmol) in
anhydrous THF (60 ml) during 30 min. The reaction mixture was
stirred at rt overnight (20 h) and refluxed for 6 h, then was carefully
quenched with water under nitrogen atmosphere until collapse. After
addition of Et2O (30 ml), the solids were filtered off and the organic
phase was dried over anhydrous Na2SO4. The solvent was evaporated
under reduced pressure giving the pure product.
Ionic Liquids Synthesis
Chiral alcohols: representative procedure
Chiral alcohols 1 were synthesized according to reported procedures
with some variations that allow to avoid chromatographic
purifications.13 A solution of (R)-propylene oxide (5 g, 86.2 mmol)
containing cat. copper iodide (190.45 mg, 1 mmol) in anhydrous
diethyl ether was cooled to -30°C. To this solution the Grignard
reagent (1 mol L-1 , solution in ether, 100 ml, 100 mmol) was added
dropwise over a period of 10 min. After stirring for 2 h, the reaction
was quenched with saturated aqueous NH4Cl and extracted with Et2O
(3x20 ml). The organic phases were dried over Na2SO4 and after
removing the solvent the crude was distilled under reduced pressure
affording the pure product.
(R)-2-[(4-phenylbutan-2-yl)oxy]ethan-1-ol (3a): 9.3 g (47.9 mmol,
96%) yellowish liquid. (±)-2-[(4-phenylbutan-2-yl)oxy]ethan-1-ol (3b):
8.81 g (45.35 mmol, 91%) yellowish liquid. 1H NMR (300 MHz, CDCl3,
δ) 7.24 (m, 5H), 3.73 (m, 2H), 3.62 (m, 1H), 3.45 (m, 2H), 2.70 (m, 2H),
2.04 (br s, 1H), 1.86 (m, 1H), 1.73 (m, 1H), 1.18 (d, J=7.2Hz, 3H); 13C
NMR (75 MHz, CDCl3, δ) 142.3, 128.5, 125.9, 75.2, 69.4, 62.3, 38.3,
31.9, 19.7.
(R)-4-phenylbutan-2-ol (1a): 12.0
g
(80 mmol, 93%): (±)-4-
phenylbutan-2-ol (1b): 11.5 g (77 mmol, 90%) . 1H NMR (200 MHz,
CDCl3, δ) 7.20 (m, 5H), 3.80 (m, 1H), 2.68 (m, 2H), 1.77(m, 2H), 1.61 (s,
1H), 1.21(d, J=6Hz, 3H); 13C NMR (75 MHz, CDCl3, δ) 142.2, 128.5,
125.9, 67.6, 40.9, 32.2, 23.7.
(R)-2-[(4,4-dimethylpentan-2-yl)oxy]ethan-1-ol (3c): 6.56
g (40.96
mmol, 82%) transparent liquid. 1H NMR (300 MHz, CDCl3, δ) 3.69 (m,
2H), 3.54 (m, 2H), 3.40 (m, 1H), 1.97 (s, 1H), 1.54 (dd, J1=18Hz,
J2=9Hz, 1H), 1.23 (dd, J1=18Hz, J2=3Hz, 1H), 1.14 (d, J=6Hz, 3H), 0.92
(s, 9H) ; 13C NMR (75 MHz, CDCl3, δ) 73.8, 69.1, 62.3, 50.9, 30.2, 21.4.
(R)-4,4-dimethylpentan-2-ol (1c): 6.92 g (56.56 mmol, 62%) 1H NMR
(200 MHz, CDCl3, δ) 3.95 (m, 1H), 1.37 (d, J=6Hz, 1H), 1.34 (d, J=2Hz,
2 | J. Name., 2012, 00, 1-3
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2-alkoxyethylmethanesulphonates 4e-f: representative procedure
1H), 3.63 (m, 1H), 3.40 (m, 1H), 2.67 (s, 3H), 2.51 (m, 2H), 1.84-1,54
(m, 2H), 1.06 (d, J=7.5Hz, 3H); 13C NMR (75 MHz, CDCl3, δ) 141.9,
137.9, 128.5, 128.3, 125.9, 123.1, 122.9, 75.6, 66.4, 50.2, 39.5, 38.1,
36.4, 31.7, 19.4.
A solution of methanesulfonyl chloride (50 mmol) in CH2Cl2 (20 ml)
was added dropwise at 0°C to a solution of alcohol 3 (50 mmol) and
Et3N (51 mmol) in CH2Cl2 (25 ml) during 20 min, then the reaction
mixture was stirred at rt for 24h. The reaction was quenched with
saturated aqueous NaHCO3 solution and was extracted with CH2Cl2
(3x20ml). The collected organic phases were dried over anhydrous
Na2SO4, then the solvent was evaporated under reduced pressure
giving the pure product.
(R)-3-{2-[(4,4-dimethylpentan-2-yl)oxy]ethyl}-1-methyl-1H-imidazol-3-
ium methanesulfonate (5c): 15.99 g (49.94 mmol, 99%) yellow solid;
1
mp: 76°C. H NMR (300 MHz, CDCl3, δ) 9.65 (br s, 1H), 7.41 (t, J=1.8
Hz, 1H), 7.37 (t, J=1.8Hz, 1H), 4.39 (dt, J1=7.8Hz, J2=4.2Hz, 2H), 3.95
(s, 3H), 3.76 (m, 1H), 3.59 (m, 1H), 3.43 (m, 1H), 2.69 (s, 3H), 1.39 (dd,
, J1=17.4Hz, J2=8.7Hz, 1H), 1.14 (dd, J1=17.4Hz, J2=4.2Hz, 1H), 1.00
(d, J=7.2Hz, 3H), 0.76 (s, 9H); 13C NMR (75 MHz, CDCl3, δ) 138.0,
123.1, 122.9, 74.0, 65.9, 50.6, 50.3, 39.7, 36.3, 30.0, 21.1.
(R)-2-[(4-phenylbutan-2-yl)oxy]ethyl methanesulfonate (4a): 13
g
(47.76 mmol, 96%) pale yellow liquid. (±)-2-[(4-phenylbutan-2-
yl)oxy]ethyl methanesulfonate (4b): 12.32 g (45.25 mmol, 91%)
amber-colored liquid. 1H NMR (300 MHz, CDCl3, δ) 7.24 (m, 5H), 4.36
(t, J=5.7Hz, 2H), 3.76 (m, 1H), 3.62 (m, 1H), 3.46 (m, 1H), 3.06 (s, 3H),
2.70 (m, 2H), 1.87 (m, 1H), 1.74 (m, 1H), 1.19 (d, J=7.2Hz, 3H); 13C
NMR (75 MHz, CDCl3, δ) 142.1, 128.5, 125.9, 75.5, 69.7, 66.2, 38.2,
37.7, 31.7, 19.5.
(S)-3-(3,7-dimethyloct-6-en-1-yl)-1-methyl-1H-imidazol-3-ium
methanesulfonate (5d): 15.65 g (49.47 mmol, 99%) yellowish liquid.
(±)-3-(3,7-dimethyloct-6-en-1-yl)-1-methyl-1H-imidazol-3-ium
methanesulfonate (5e): 15 g (49.47 mmol, 99%) yellowish liquid. 1H
NMR (300 MHz, CDCl3, δ) 9.31(s, 1H), 7.52 (t, J=2.4 Hz, 1H), 7.33 (t,
J=2.4 Hz, 1H), 5.01 (t, J=8.4Hz,1H), 4.23 (dt, J1=7.5Hz, J2=2.4Hz, 2H),
4.01 (s, 3H), 2.72 (s, 3H), 1.93 (m, 4H), 1.63 (s, 3H), 1.55 (s, 3H), 1.38
(m, 2H), 1.19 (m, 1H), 0.93 (d, J=7.8Hz, 3H); 13C NMR (75 MHz, CDCl3,
δ) 137.9, 131.7, 124.0, 123.7, 121.7, 48.1, 39.7, 37.3, 36.6, 36.3, 29.8,
25.7, 25.2, 18.9, 17.7.
(R)-2-[(4,4-dimethylpentan-2-yl)oxy]ethyl methanesulfonate (4c):
9.95g (41.78 mmol, 84%) pale yellow liquid. 1H NMR (300 MHz, CDCl3,
δ) 4.32 (m, 2H), 3.75 (m, 1H), 3.52 (m, 2H), 3.02 (s, 3H), 1.50 (dd,
J1=17.4Hz, J2=9Hz, 1H), 1.22 (dd, J1=17.4Hz, J2=4.2Hz, 1H), 1.13 (d,
J=7.2Hz, 3H), 0.91 (s, 9H); 13C NMR (75 MHz, CDCl3, δ) 74.20, 69.60,
65.82, 50.68, 37.73, 30.19, 21.23.
3-{[(1R,5S)-6,6-dimethylbicyclo[3.1.1]hept-2-en-2yl]
methyl}-1-
methyl-1H-imidazol-3-ium methanesulfonate (5f): 16.02 g (49.11
mmol, 98%) white solid; mp: 113-118°C. 1H NMR (300 MHz, D2O, δ)
7.44 (d, J=2.1Hz,1H), 7.38 (d, J=2.1Hz, 1H), 5.28 (br s, 1H), 4.22 (t,
J=8.1Hz, 2H), 3.85 (s, 3H), 2.77 (s, 3H), 2.53 (t, J=9Hz, 2H), 2.36 (m,
1H), 2.14 (br s, 2H), 2.04 (m, 2H), 1.23 (s, 3H), 0.915 (d, J=10.2Hz, 1H),
0.69 (s, 3H); 13C NMR (75 MHz, D2O, δ) 142.6, 122.9, 121.8, 120.1,
46.9, 44.0, 39.7, 38.0, 36.8, 35.7, 35.1, 30.7, 30.4, 25.0, 19.8.
(S)-3,7-dimethyloct-6-en-1-yl methanesulfonate from (S)-(-)-β-
Citronellol (4d): 11.32 g (48.3 mmol, 97%) yellow liquid. (±)-3,7-
dimethyloct-6-en-1-yl methanesulfonate from (±)-β-Citronellol (4e):
11.0 g (48.0 mmol, 97%) yellow liquid. 1H NMR (300 MHz, CDCl3, δ)
5.06 (t, J=8.3 Hz, 1H), 4.25 (m, 2H), 2.98 (s, 3H), 1.97 (m, 2H), 1.67 (s,
3H), 1.68 (m, 3H), 1.59 (s, 3H). 1.33 (m, 1H), 1.19 (m, 1H), 0.92 (d,
J=7.8Hz, 3H); 13C NMR (75 MHz, CDCl3, δ) 131.7, 124.3, 68.7, 37.4,
36.9, 34.0, 29.0, 25.8, 25.4, 19.2, 17.7 .
Metathesis reactions. Representative procedure:
[(1R,5S)-6,6-dimethylbicyclo[3.1.1]hept-2-en-2-yl]methyl
An aqueous solution of LiTf2N (50 mmol) was added to an aqueous
solution of IL 5 (50 mmol) and the mixture was stirred at rt for 24 h.
The insoluble oily material was separated adding CH2Cl2 and the
organic layer was washed with water and brine and dried over
anhydrous Na2SO4. The solvent was evaporated under reduced
pressure to give the expected product.
methanesulfonate (4f) from (1R)-(-)-Nopol (3f): 11.58 g (47.39 mmol,
95%) pale yellow liquid. 1H NMR (300 MHz, CDCl3, δ) 5.33 (m, 1H),
4.19 (t, J=7.5Hz, 2H), 2.97 (s, 3H), 2.39 (m, 3H), 2.21 (m, 2H), 2.07 (m,
1H), 2.02 (m, 1H), 1.25 (s, 3H), 1.12 (d, J=10,5Hz, 1H), 0.81 (s, 3H); 13
C
NMR (75 MHz, CDCl3, δ) 142.6, 120.0, 68.0, 45.6, 40.6, 38.1, 37.5,
36.4, 31.6, 31.4, 26.2, 21.2.
(R)-1-methyl-3-{2-[(4-phenylbutan-2-yl)oxy]ethyl}-1H-imidazol-3-ium
bis[(trifluoromethyl)sulfonyl]imide (6a): 26.6 g (49.3 mmol, 99%). (±)-
1-methyl-3-{2-[(4-phenylbutan-2-yl)oxy]ethyl}-1H-imidazol-
iumbis[(trifluoromethyl)sulfonyl]imide (6b): 25.2 g (49 mmol, 99%)
pale yellow liquid. 1H NMR (300 MHz, CDCl3, δ) 8.62 (s, 1H), 7.34 (t,
J=2.1Hz, 1H), 7.28-7.07 (m, 6H), 4.27 (t, J=5.7Hz, 2H), 3.94 (s, 3H),
3.76 (m, 1H), 3.61 (m, 1H), 3.40 (m, 1H), 2.55 (m, 2H), 1.87-1.56 (m,
2H), 1.09 (d, J=7.5Hz, 3H); 13C NMR (75 MHz, CDCl3, δ) 142.0, 136.4,
128.5, 128.4, 126.8, 126.0, 123.4, 122.4, 117.3, 113.4, 75.8, 65.9,
50.5, 38.0, 36.3, 31.7, 19.2.
Synthesis
of
N-methylimidazolium
methanesulphonates:
representative procedure:
A mixture of the mesylate 4 (50 mmol) and N-methylimidazole (50
mmol) in CH3CN (20 ml) was refluxed for 72h. The solvent was
removed under vacuum obtaining the pure product.
(R)-1-methyl-3-{2-[(4-phenylbutan-2-yl)oxy]ethyl}-1H-imidazol-3-ium
methanesulfonate (5a): 17.61 g (49.70 mmol, 99%) yellowish liquid.
(±)-1-methyl-3-{2-[(4-phenylbutan-2-yl)oxy]ethyl}-1H-imidazol-3-ium
methanesulfonate (5b): 17.16 g (48.44 mmol, 97%) yellowish liquid.
1H NMR (300 MHz, CDCl3, δ) 9.40 (s, 1H), 7.37 (t, 1H, J=2.1Hz), 7.29 (t,
1H, J=2.1Hz), 7.13 (m, 5H), 4.39 (t, J=6Hz, 2H), 3.92 (s, 3H), 3.78 (m,
(S)-3-(3,7-dimethyloct-6-en-1-yl)-1-methyl-1H-imidazol-3-ium
bis[(trifluoromethyl)sulfonyl]imide (6d): 16 g (49.5 mmol, 99%) pale
yellowish
liquid.
(±)-3-(3,7-dimethyloct-6-en-1-yl)-1-methyl-1H-
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imidazol-3-ium bis[(trifluoromethyl)sulfonyl]imide (6e): 15 g (46.5
mmol, 93%) pale yellowish liquid. 1H NMR (300 MHz, CDCl3, δ) 8.71 (s,
1H), 7.32 (t, J=2.1Hz, 1H), 7.29 (t, J=2.1Hz, 1H), 5.04 (t, J=8.4Hz,1H),
4.16 (m, 2H), 3.91 (s, 3H), 2.07-1.11 (m, 7H), 1.65 (s, 3H), 1.57 (s, 3H),
0.93 (d, J=7.5Hz, 3H); 13C NMR (75 MHz, CDCl3, δ) 137.9, 131.7, 124.0,
123.7, 121.7, 48.1, 39.7, 37.3, 36.6, 36.3, 29.8, 25.7, 25.2, 18.9, 17.7.
Scheme 2
3-{[(1R,5S)-6,6-dimethylbicyclo[3.1.1]hept-2-en-2yl]
methyl}-1-
methyl-1H-imidazol-3-ium bis[(trifluoromethyl) sulfonyl]imide (6f):
24.73 g (48.4 mmol, 97%) pale yellow liquid. 1H NMR (300 MHz,
CDCl3, δ) 8.71 (br s, 1H), 7.30 (t, J=2.1Hz 1H), 7.28 (t, J=2.1Hz, 1H),
5.31 (m, 1H), 4.19 (t, J=9Hz, 2H), 3.92 (s, 3H), 2.52 (t, J=9Hz, 2H), 2.40
(m, 1H), 2.19 (br s, 2H), 2.06 (m, 2H), 1.27 (s, 3H), 1.00 (d, J=10.5Hz, 1
H), 0.76 (s, 3H); 13C NMR (75 MHz, CDCl3, δ) 142.3, 136.2, 127,6,
123.7, 122.4, 122.4, 121.1, 112.3, 48.2, 45.1, 40.6, 38.1, 37.1, 36.4,
31.7, 31.4, 26.1, 21.0.
From alcohols 3, CILs were prepared in three steps (Scheme 3). The
reaction of the alcohols with methanesulphonyl chloride afforded the
corresponding mesylates, in very high yields, which could be
quantitatively transformed into the corresponding salts 5 by reaction
with
role of anion on the assembly properties of CILs, ILs 5a-b and 5d-f
were transformed into the corresponding
N-methylimidazole. To obtain information about the eventual
bis[(trifluoromethyl)sulfonyl]imide based ILs 6, by anion exchange.
O
O
S
N
N
Tf2
N
O
LiTf2N
R
O
O
R
MsCl, CH2Cl2
Et3
OH
R
N
N
CH3
Results and discussion
CH3
R
S
N
N
O
CH3CN,
N
3
4
reflux 72h
5
6
To obtain ILs possessing a bulky non-polar chiral architecture capable
to give rise to stereochemically dependent hydrophobic interactions
or, in the case of CILs containing aromatic groups, π−π interactions,
chiral secondary alcohols were synthesized starting from propylene
oxides. In particular, the enantiopure commercial (R)-propylene oxide
(or the corresponding racemic epoxide) was reacted with a Grignard
reagent in the presence of a catalytic amount of CuI, affording the
desired secondary alcohol in 60-90% yield (Scheme 1).
;
O
;
O
;
R =
O
c
b
a
;
;
e
d
f
Scheme 3
It is noteworthy that with the exception of 5c and 5f all the other salts
were uncolored or pale yellow liquids at room temperature. The
presence of an ethereal group on the side alkyl chain appears able to
give moderately or low viscous liquids also in the presence of a
phenyl ring.
Scheme 1
Furthermore, to favor the self-organization of the chiral hydrophobic
chains of the final planned imidazolium salts, we decided to lengthen
the synthesized secondary alcohols introducing a proper spacer (a C2
tether). The tether was introduced as described in Scheme 2. The
chiral alcohols 1 were treated with sodium hydride and the resulting
sodium alkoxides were reacted with sodium chloroacetate, affording
the alkoxyacetic derivatives 2 in good-excellent yields. The LiAlH4
reduction of the carboxylic group gave alcohols 3 in excellent yields.
Longer spacers were avoided to prevent an excessive conformational
freedom. The applied synthetic strategy was supported by the
possibility to easily convert the primary hydroxyl groups into the
corresponding mesylates (4), which, in turn, can be displaced by the
nucleophilic nitrogen of the imidazole ring.
Chiroptical Properties
A preliminary CD investigation was conducted initially on the less
viscous bis[(trifluoromethyl)sulfonyl]imide based CILs 6a, 6d and 6f
and, to obtain information on the role anion, on CIL 5d. The neat
samples have been measured in quartz cuvettes at 25°C. Results are
reported in Figure 1 and the asymmetry factor g values at CD band
maximum are given in Table 1.
Finally, two chiral natural alcohols ((R)-nopol and (S)-citronellol) and
racemic citronellol were used to prepare imidazolium based CILs: in
this case, however, being the chiral centre sufficiently distant from
the hydroxyl group, alcohols were directly converted to the
corresponding mesylates.
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Table 1. Asymmetry factor g values for CILs.
were always carried out by changing the sample orientation with
respect to the electron analyzer in order to change the thickness of
the probe depth. The spectra were recorded at different angles from
normal (0° or 90° take off) to grazing (75°, 15° take off) emission
angle.
λ (nm)
g factor
cell
pathlength
(mm)
Figure 2 displays the O s1 XPS spectra collected at normal and grazing
5 × 10-3
1 × 10-4
1 × 10-4
3.6 × 10-5
0.2
1
14
6f
6d
5d
6a
338
287
293
263
angle for 5a and 5b. In accordance with literature data the spectra
were fitted by means of two components attributable to the oxygen
atoms of methansulfonate anion (BE 532.2 eV) and of the ethereal
group on imidazolium cation, R-O-R (BE 530.7 eV). At normal emission
angle, racemic mixture (5b) is characterized by a nearly stoichiometric
composition (3 : 1, ratio between the two components) (Figure 2A).
Vice versa, in the spectrum of the enantiopure IL 5a the stoichiometry
is not satisfied: an enrichment in the cation component can be
envisaged (atomic ratio is close to 1:1) (Figure 2C). In both samples,
the decrease in the probing depth leads to an enhancement of the
peak attributable to the methansulfonate anion (Figure 2B and 2D).
This effect is more pronounced in the enantiopure liquid 5a with
respect to the racemic mixture, as shown by Figure 2.
0.1
̴≈0.01
The comparison of the CD spectra of ILs 5d and 6d seems to
suggest a negligible effect of the anion on the Cotton effect, despite
of the fact that the interactions of the two anions (mesylate and
bis[(trifluoromethyl)sulfonyl]imide) with the imidazolium cation
should be significantly different.
Finally, it is noteworthy that only the presence of a bulky chiral group,
dimethylbicyclo[3.1.1]hept-2-en-2yl, in 6f originates a quite intense
CD band. Moreover, for this CIL the CD signal (considering the g
factor, which can be evaluated without explicit knowledge of
concentration) is the same for the neat sample and acetonitrile
solutions.
6a
6d
Fig 2. O 1s Xps spectra collected at normal (90° take off angle) and
grazing angle (15°) on 5b (A, B) and 5a (C, D) sample.
5d
6f
Fig 1. CD spectra on neat ILs, for g values and pathlengths see Table 1
Surface Properties
In order to verify if the outermost regions of the IL/air interface,
which are often primarily populated in imidazolium salts by the cation
alkyl chains, are affected by the presence of a chiral center, the
structure of CILs 5a and 5d at the liquid/vacuum interface was
investigated by means of angle resolved-XPS (AR-XPS). Furthermore,
since enantiopure compounds and racemic mixtures can be
characterized by different interaction ability and consequently by a
distinct self-assembly behavior, measurements were carried out also
on the corresponding racemic mixtures, ILs 5b and 5e. Experiments
Fig 3. Normalized C 1s spectra as function of take-off angle for 5a
sample.
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Fitting the C 1s peak is much more complicated with respect to the O
1s due to the larger number of overlapping components contributing
the XPS spectra of carbon. Therefore, the unambiguous assignment of
all the different components was not reliable. Nevertheless, the
peaks can be regarded as the sum of at least two different groups of
peaks. One, at lower binding energy values (284.8 eV), attributable to
the carbons of the aliphatic chain (and, probably, of the phenyl ring)
and the another one, located at higher energy (286.3 eV), attributable
to the carbon of the imidazolium ring and methansulfonate anion. In
accordance with previous studies,15 the contribution at lower B.E.
value is dominant in the spectra collected at grazing angle (thinner
probing depth) (see Figure 3). This observation undoubtedly shows
that the contribution of aliphatic carbons increases in the uppermost
portion of the liquid; i.e. these groups are pointing away from the
bulk liquid. This trend characterizes both the enantiopure CIL 5a and
the corresponding racemic mixture 5b. However, side chain carbons
seem to contribute more extensively to the surface composition in
the case of the racemic mixture, as evidenced by the diagram
reported in Figure 4.
Fig 5. Cartoon-like representation of the preferred orientations of 5a
and 5b at the air-liquid interface
It is noteworthy that substitution of the aromatic ring on the side
alkyl chain of cation with a double bond, ILs 5d and 5e, induces some
interesting phenomena at the liquid/vacuum interface can be
evidenced. As observed for 5a and 5b, the outermost portion of these
liquids results enriched in the aliphatic components, i.e. cation alkyl
side chains. However, both for 5d and 5e the surface enrichment is
much less marked (see Figure 6). Furthermore, there are no evident
differences between the racemic and enantiopure IL. These data
suggest less “structured” surfaces in the case of 5d/5e with respect to
5a/5b.
Fig 4. Contributes to the C 1s peak by aliphatic and imidazolium
carbons as function of take-off angle for 5b (full symbols) and 5a
(empty symbols) sample.
A different preferential orientation of the side chains with respect to
the liquid surface, attributable to the chiral center and consequently
to the different packing ability of the racemic mixture and
enantiopure CIL, could determine the observed differences in the XPS
spectra of 5a and 5b. A cartoon-like representation of the preferred
orientations of these two ILs at the air-liquid interface, accentuating
the principal differences between racemic and enantiopure IL, is
reported in Figure 5.
Fig 6. Contribute to the 1s peak of aliphatic and imidazolic carbon as
function of take off angle for 5e (full symbols) and 5d (empty
symbols) sample.
Interesting information have been obtained also substituting the
methansulfonate
anion
of
5d
and
5e
with
bis[(trifluoromethyl)sulfonyl]imide. The XPS spectra of 6d and 6e
show dramatic differences in the region characteristic of C 1s. Along
the large and complex shaped peak due to the superimposing of
several contributes of carbons in different chemical environments, at
very high energy a lonely peak attributable to the -CF3 group of
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bis[(trifluoromethyl)sulfonyl]imide anion appears in the spectrum Therefore, it is possible to state that regardless the enantiomeric
(Figure 7).
purity of the liquid at the very top layer there is a slightly depletion in
bis[(trifluoromethyl)sulfonyl]imide anion.
90
60
30
0
-CF3
C imidazolic
C aliphatic
-CF3
C imidazolic
C aliphatic
80
60
40
20
Take off angle (°)
296
294
292
290
288
286
284
282
Binding Energy (eV)
Fig 7. XPS (C1s) spectra for 6d at normal emission (black curve) and
grazing angle (red curve). The inset displays the relative contributes
for 6d (full symbols) and 6e (empty symbols).
Fig 8. N 1s Xps spectra collected at normal (90° take off angle) and
grazing angle (15°) recorded on 6d (A, B) and 6e (C, D) sample.
Also in this case, by reducing the take-off angle (i.e. decreasing the
thickness of the investigated samples) the contribution of aliphatic Conclusions
carbons increases. The intensity ratio between the two components
Chiral ionic liquids have been obtained in high yield using as building
(aliphatic/imidazolium carbons) constantly increases from 1.3 at 90°
(normal incidence) to about 1.7 at 15° (grazing angle). Very little
differences, within the experimental error, were observed between
the enantiopure IL (6d) and the racemic mixture (6e, inset of Figure
7). At the same time, the relative area of the peak attributable to –CF3
carbon decreases in both the samples on going from 90 to 15°.
block commercial propylene oxide and natural alcohols. The self-
assembly at the interface IL/vacuum, investigated for some couples of
enantiopure and racemic CILs, show that the presence of a chiral
centre on the cation side chain and the enantiomeric purity of the
related imidazolium based CILs can affect the orientation and layering
of the liquid components in the outmost regions of the interface. This
ability depends primarily by the structural features of the side chain.
Contrarily to the case of 5a and 5b, where we observed an increase of
the component related to the anion as the probing depth decreased,
in this case the component related to the anion (-CF3 of
bis[(trifluoromethyl)sulfonyl]imide) appears depleted in the
outermost portion of the surface. Actually, two main processes can
account for this experimental behavior: (a) shadowing effects played
by fluorine atoms that are positioned pointing towards vacuum, (b) a
real depletion of bis[(trifluoromethyl)sulfonyl]imide anion at the
surface. In order to shed some light about these possibilities we
analyzed the N 1s peaks. This region of the spectrum presents two
well separated peaks (see Figure 8). In accordance with the literature
data these peaks can be easily assigned to the nitrogen atoms in the
The presence of
a phenyl ring, probably able to give π−π
interactions, appears fundamental to have significantly different
IL/vacuum (air) surfaces going from racemic and enantiopure ILs. This
feature, associated with the optical purity of some of the investigated
ILs, might be useful when the surface of ILs is used for synthetic
processes, for example for polymerization reactions. Air/ionic liquid
interface properties of achiral ILs containing metal salts have been
used17 for the one-pot fabrication of large-area polymeric film
through oxypolymerization processes: the structural surface features
of racemic or enantiopure ILs might affect polymer uniformity and
thickness. On the other hand, the different assembly of racemic and
enantiopure ILs surely affects also the dynamic nanostructures
characterizing the pure ionic liquid bulk, as well as the different kind
of aggregates (micelles, vesicles and so on) formed by ILs in an
immiscible solvent. These features might exert an important role in
metal nanoparticles and/or other (nano)material synthesis.
imidazolium
ring
(B.E.
401.7
eV)
and
in
bis[(trifluoromethyl)sulfonyl]imide anion (399.1 eV).16 Since the
sensitivity factor is the same, the atomic ratio between these
chemical species can be straightforwardly obtained by the direct
comparison of the peaks area values. At normal emission, the area
ratios resulted nearly identical for the two liquids (2.03:1 for 6d and
2.00:1 for 6e, respectively) and very close to the expected
stoichiometric value, 2:1. Reducing the sampling depth, this ratio
slightly changed toward higher values (2.06:1 for 6d and 2.08:1 for
6e, respectively).
Acknowledgements
The authors thank MIUR for the financial support (PRIN 2010-2011-
PROxy).
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Notes and references
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8 | J. Name., 2012, 00, 1-3
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Graphical Abstract
Arrangements of enantiopure and racemic
ionic liquids at the liquid/air interface: the
role of chirality on self-assembly and
layering
S. Caporali, C. Chiappe, T. Ghilardi, A. Iuliano, G.
Longhi, P. Margari and C. S. Pomelli
This paper reports on the self-assembly ability
at the interface IL/air for some couples of
enantiopure and racemic chiral ILs