New ionic liquid crystals based on imidazolium salts

Article abstract of DOI:10.1039/b600279j

The new ionic liquid crystal 1-(4-dodecyloxybenzyl)-3-methyl-1H-imidazol-3- ium bromide has been synthesized and its X-ray structure obtained; various salts containing BF₄^-, SCN^-, PF₆ ^-, CF₃

Full text of DOI:10.1039/b600279j

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LETTER
www.rsc.org/njc | New Journal of Chemistry
New ionic liquid crystals based on imidazolium saltsw
William Dobbs,a Laurent Douce,*a Lionel Allouche,b Alain Louati,c
Franc¸
ois Malboscd and Richard Weltere
Received (in Montpellier, France) 9th January 2006, Accepted 20th February 2006
First published as an Advance Article on the web 9th March 2006
DOI: 10.1039/b600279j
The new ionic liquid crystal 1-(4-dodecyloxybenzyl)-3-methyl-
imidazolium derivative which shows a smectic-A phase be-
tween 119 and 172 1C.9
1H-imidazol-3-ium bromide has been synthesized and its X-ray
ꢀ
structure obtained; various salts containing BF4ꢀ, SCNꢀ, PF6
,
Herein, we report the synthesis of an unsymmetrical imida-
zolium species (Scheme 1) designed to give a lower mesophase
transition temperature and to facilitate study of the influence
of the counteranion on the mesomorphism, thermal stability
and electrochemistry of imidazolium salts. Compound 1a,
1-(4-dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium bromide,
was obtained in a four-step procedure (Scheme 2). The last
step is a nucleophilic substitution on 1-(bromomethyl)-4-
(dodecyloxy)benzene by the methylimidazole which proceeds
in 88% yield. The bromide salt 1a is air stable and was purified
by column chromatography on silica. Distinctive signals of the
CF3SO3ꢀ, (CF3SO2)2Nꢀ have been prepared by anion metath-
esis in water from bromide species as starting materials, and
finally, the mesomorphism and electrochemical behavior have
been investigated.
Extensive studies of ionic liquids have revealed their many
useful properties such as extremely low volatility, high thermal
stability, non-flammability, high chemical stability, high ionic
conductivity and a wide electrochemical window.1,2 Ionic
liquids based on imidazolium salts are particularly well known
and have found application in dye-sensitized solar cells,
electrochemical devices, wet double-layer capacitors and ion
transport systems.3 Certain ionic materials are known also to
form amphitropic liquid crystals and such ‘‘ionic liquid crys-
tals’’ have been widely studied.4,5 They have especially great
potential as ordered reaction media that can impart selectivity
in reactions by organising reactants.6 Full conjunction of the
two fields of ionic liquid chemistry and liquid crystal chemistry
could lead to a vast new range of materials for research and
industry. Variation of the N,N0 substituents in imidazolium
systems is a facile means of creating this range, one of the
particularly interesting properties of these materials being
their ease of deprotonation to give, formally, N-heterocyclic
carbenes which are excellent ligands for transition-metal-based
catalysts.7
1
CH (1H-imidazolium) group appear in the H and 13C NMR
spectra at 10.89 ppm and at 136.54 ppm, respectively.
To explore the occurrence of aggregation in solution, the 1H
NMR spectra were recorded as a function of concentration.
Translational diffusion coefficients (D) were determined by
Diffusion Ordered NMR Spectroscopy (DOSY) experiments
in order to estimate the relative size of the molecular entities in
solution.10 The DOSY spectrum of 1a at low concentration
Despite growing interest in the properties of ionic liquids
based on imidazolium compounds, very little is known of
systems which form liquid crystals.8 Only a few reports of
thermotropic imidazolium compounds are available, one con-
cerning our recent characterisation of a symmetrical dialkyl
Scheme 1 1-(4-Dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium salts.
a
´
Institut de Physique et Chimie des Materiaux de Strasbourg, Groupe
´
´
des Materiaux Organiques, UMR 7504, CNRS-Universite Louis
Pasteur, BP 43, 23 rue du Loess, F-67034 Strasbourg Cedex 2,
France. E-mail: laurent.douce@ipcms.u-strasbg.fr; Fax: þ33 388
107 246; Tel: þ33 388 107 107
b Service de R.M.N., Institut de Chimie de Strasbourg, CNRS-ULP,
1 rue Blaise Pascal, 67000 Strasbourg, France
c Laboratoire d’Electrochimie Analytique, Ecole Nationale Superieure
de Chimie de Mulhouse, 3 rue Alfred Werner, 68093 Mulhouse
Cedex, France
d SOLVIONIC SA, Parc Technologique Delta Sud, 09340 Verniolle,
France
Laboratoire DECOMET, UMR CNRS 7177-LC003, Universite
e
´
Louis Pasteur, 4 rue Blaise Pascal, 67000 Strasbourg, France
w Electronic Supplementary Information (ESI) available: Experimen-
tal data, DOSY spectra, DSC, TGA and XRD data and cyclic
voltammograms. See DOI: 10.1039/b600279j.
Scheme 2 Synthesis of the bromide 1a. Reaction conditions: (i) DMF,
K2CO3, BrC12H25, 60 1C, overnight; (ii) LiAlH4, THF, 4 h; (iii) SOBr2,
DCM; dry THF, methylimidazole under inert atmosphere.
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the anions, completely segregated from the flexible chains.
This is consistent with the amphiphilic character of the
molecules and indicates that they could show liquid crystal
behaviour at higher temperature.
In order to investigate the influence of the anion ꢀon the
mesomorphism, ꢀwe prepared compounds with BF4 (1b),
SCNꢀ (1c), PF6 (1d) and (CF3SO2)2Nꢀ (1f) anions in ex-
cellent yꢀield by anion metathesis in water as solvent, except for
CF3SO3 (1e) (see experimental details in ESI). All these
compounds were characterized by 1H NMR, C13 NMR
{1H}, FT-IR and elemental analysis. The IR spectra show
typical anion vibrations at 1036 cmꢀ1 (1b BF4ꢀ), 2063 cmꢀ1
(1c SCNꢀ), 822 cmꢀ1 (1d PF6ꢀ), 1260 and 1029 cmꢀ1 (1e
CF3SO3ꢀ), 1353 and 1180 cmꢀ1 (1f (CF3SO2)2Nꢀ). At low
concentration (1 mg in 0.3 ml CDCl3), the chemical shift for
the CH proton is very dependent upon the anion, with d 9.10
(1b), 9.63 (1c), 8.69 (1d), 9.40 (1e) and 8.93 ppm (1f).
The thermogravimetric analysis of compounds 1a–f showeꢀd
the general stability order to be SCNꢀ E Brꢀ o BF4ꢀ E PF6
o CF3SO3ꢀ E (CF3SO2)2Nꢀ (see Fig. S3 in ESI). For all the
compounds, the mesomorphic behavior and phase transition
temperatures were investigated by polarizing optical micro-
scopy (POM), differential scanning calorimetry (DSC, see Fig.
S5 in ESI), and X-ray powder diffractometry (XRD, see Fig.
S4 in ESI). To avoid possible effects of hydration of the
materials, all were dried under vacuum before X-ray and
DSC analyses. The phase transition temperatures and the
corresponding enthalpy changes derived for compounds 1a–f
are compiled in Table 1, while typical results are displayed in
Fig. 2. Compounds 1e and 1f, not unexpectedly,8d,e,f do not
show thermotropic behaviour, while the data for 1a–d give an
order of anion stabilisation of liquid crystal behaviour of
Fig. 1 Top: Partial crystal packing of the compound 1a projected on
to the (bc) plane. Bottom: ORTEP view of half asymmetric unit of
compound 1a with labelling scheme (left). Displacement ellipsoids are
shown at the 50% probability level. The unit cell content of 1a is
shown on the right. Selected non-classical H-bonds are presented
(dashed lines).
Brꢀ > BF4 > SCNꢀ > PF6 (see Fig. 2).
ꢀ
ꢀ
As described in the literature4 and confirmed herein, it is
difficult to observe good defect textures for the mesophase of
an imidazolium salt by polarizing optical microscopy because
of the tendency of the salts to form spontaneously a single
homeotropic monodomain which gives a dark field between
crossed polarizers. To avoid the homeotropic alignment, we
observed the samples held between two microscope slides
separated by a 100 mm spacer. The optical textures then
observed upon slow cooling from the isotropic melt showed
the emergence of a smectic A phase with appearance of
batonnets, turning into focal conic structures co-existing with
homeotropic areas. The structure of the liquid crystal phase
characterises the molecule in solution (D = 700 mm2 sꢀ1); at
high concentrations, the DOSY spectrum shows a character-
istic polydisperse pattern (450 o D o 700 mm2 sꢀ1), signalling
the formation of aggregates in solution (see Fig. S1 and S2 in
electronic supplementary information, ESIw).
Single crystals of 1a suitable for X-ray diffraction were
obtained by slow diffusion of ether into a CH2Cl2 solution.
Both crystal and molecular structures are shown in Fig. 1. The
N-alkyl chains and the phenyl groups are approximately
orthogonal (82(1)1) to the imidazolium plane. The crystal
structure packing, as shown in Fig. 1a, can be described as a
pseudo-lamellar system perpendicular to the flexible (N-alkyl)
chains (B15 A). Many non-classical hydrogen bonds have
been detected in the crystal structure. This observation (par-
tially reported in Fig. 1) is in good accordance with the
segregation signaled above i.e. the Brꢀ anions are connected
to the rigid part of the molecule by these weak interactions.
Moreover, a CH–p interaction has been found between H10
(C10) and the C29–C34 phenyl group (distance of 2.92 A
between H10 and the centroid of the phenyl group). The layers
are separated by d E 25 A. Within the layer, the N-alkyl
groups form another pseudo-lamellar network, with a char-
acteristic distance of h E 9.8 A. The rigid phenyl and
imidazolium rings, forming a unit B7 A long, are, along with
Table 1 Phase transition temperaturesa and corresponding enthalpies
DH determined from the 1st heating
T/1C Cr -
I (DH/J gꢀ1
T/1C Cr -
T/1C LC -
Compound
)
LC (DH/J gꢀ1
)
I (DH/J gꢀ1
)
1ab
1b
1c
1d
1e
1f
81 (101.4)
65 (64.2)
54 (114.2)
77 (83.2)
249 (1.1)
203 (1.5)
188 (1.2)
148 (1.2)
67 (92.2)
49 (98.2)
a
Abbreviations: Cr = crystalline phase, LC = smectic A phase, I =
b
isotropic phase. Decomposition is observed after clearing point.
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Table 2 Main electrochemical peak potentials recorded in CH3CN
with NBu4PF6 (0.1 M)
Compound
Reduction processes/V
Oxidation processes/V
0.86 1.15
1a
1b
1c
1d
1e
1f
ꢀ1.74
ꢀ1.71
ꢀ1.67
ꢀ1.73
ꢀ1.76
ꢀ1.73
—
—
0.85
—
—
—
—
—
—
—
corresponds to the reduction of the cationic imidazolium
species. In the anodic scan, compound 1a showed two oxida-
tion steps, at 0.86 V and 1.15 V, while the compound 1c
exhibited one oxidation process at 0.85 V. We assume that the
two oxidation processes in 1a involve the formation of Br2 and
possibly then a higher-oxidation-state bromine species. For 1c,
the oxidation process can be attributed to the formation of
thiocyanogen, (NCS)2 (Fig. 4).11 Thus, these preliminary
results establish a cathodic limit of ca. ꢀ1.7 V for all com-
pounds, while the anodic limit is related to the oxidation of the
anionic species.
Fig. 2 Phase transition temperatures of compounds 1a–f.
was established using XRD. The smectic A structure was
characterized by patterns containing one to three sharp,
equidistant reflections in the small angle region, related to
the smectic layering, and a broad band in the wide angle
region, related both to the disordered alkyl chains and the
liquid-like lateral correlations of the aromatic cores within the
layers. The d-spacing values obtained for the compounds at 80
1C were d1 = 37.7 (1a Brꢀ), 37.2 (1b BF4ꢀ), 36.5 (1c SCNꢀ)
and 35.8 A (1d PF6ꢀ). Note that the d-spacings of the smectic
layers of the compounds 1a–d were less than two times the
length of the molecules. This can be interpreted as indicative of
the formation of an ionic double layer with interdigitated
polar groups (Fig. 3). In our case, the d-spacing is not
correlated with the anion size. The small differences in thick-
ness (around 2 A) originates from the fact that, owing to van
der Waals and dipole interactions, the cationic centres adapt
to allow the anions to penetrate to different extents within the
aromatic layers.
In conclusion, we report the full characterisation of new
mesomorphic compounds based on an imidazolium unit with
different anions. NMR experiments have shown that these
materials can form aggregates in solution. We are now in-
vestigating the ability of the conjugate base of the cation to
form thermotropic carbene complexes with, for example,
palladium(II). We are also attempting to introduce different
length tails in order to obtain room temperature ionic liquid
crystals.
Experimental
General
All commercially available chemicals were used without
further purification. Dichloromethane (DCM) and tetrahy-
drofuran (THF) were carefully dried and distilled prior to use.
Evaporation and concentration were done at water pressure
and drying in vacuo at 10ꢀ2 torr. Column chromatography:
Cyclic voltammetry was used to determine the electroche-
mical windows for the compounds 1a–f, the voltammograms
being recorded in CH3CN solutions containing 0.1 M
NBu4PF6 as supporting electrolyte at a platinum working
electrode and with a SCE reference electrode (see Fig. S6 in
ESI). The results are gathered in Table 2.
For all compounds, only an irreversible reduction process
was observed at ca. ꢀ1.7 V. It is evident that this process
Fig. 4 Cyclic voltammogram of 1c in CH3CN (0.1 M NBu4PF6).
Scan rate 100 mV sꢀ1
Fig. 3 Schematic drawing of the possible arrangement of the imida-
.
zolium species and anions in an ionic double layer.
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silica gel 60 (230–400 mesh, 0.040–0.063 nm) was purchased
from E. Merck.
phenyl); 121.57; 123.38 (CH imidazolium); 124.17 (C phenyl);
130.60 (CH phenyl); 136.56 (N–CH–N); 160.13 (C phenyl).
1-(4-Dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium
tri-
Syntheses
fluoromethanesulfonate, 1e. 1-(4-Dodecyloxybenzyl)-3-methyl-
1H-imidazol-3-ium trifluoromethanesulfonate was obtained
by mixing a solution of 1-(4-dodecyloxybenzyl)-3-methyl-1H-
imidazol-3-ium bromide (250 mg, 0.57 mmol) in 10 ml of
DCM and a solution of silver trifluoromethanesulfonate (147
mg, 0.57 mmol) in 5 ml of DCM. Silver bromide was removed
by filtration over alumina and the solvent removed. The crude
product was finally crystallized in good yield by slow diffusion
of ether into a THF solution. Yield: crystalline white solid 260
mg, 90% (Found C, 56.8; H, 7.4; N, 5.6. C25H37F3N2O4S
requires C, 56.9; H, 7.4; N, 5.5%). nmax/cmꢀ1 3122 (C–H
aromatic), 2918 and 2850 (C–H aliphatic), 1516 (CQC aro-
matic), 1260 and 1029 (CF3SO3ꢀ). dH(300 MHz; CDCl3): 0.89
(3 H, t, J 6.8, CH3 aliphatic chain); 1.27 (16 H, br s, CH2
aliphatic chain); 1.35–1.48 (2 H, m, CH2 aliphatic chain); 1.78
(2 H, q, J 6.6, O–CH2–CH2); 3.95 (2 H, t, J 6.6, OCH2); 3.99 (3
H, s, CH3–N); 6.92 (2 H, d, J 8.6, CH phenyl); 7.10 (1 H, m,
CH imidazolium); 7.16 (1 H, m, CH imidazolium); 7.32 (2H,
d, J 8.6, CH phenyl); 9.40 (1 H, s, N–CH–N). dC (75 MHz;
CDCl3): 14.02 (CH3 aliphatic chain); 22.60; 25.94; 29.10;
29.26; 29.26; 29.32; 29.50; 29.53; 29.58; 31.83 (CH2 aliphatic
chain); 36.26 (N–CH3); 52.96 (N–CH2-Ph); 68.10 (O–CH2);
115.23 (CH phenyl); 120.6 (quadruplet, J = 320 Hz, CF3);
121.76; 123.61 (CH imidazolium); 124.31 (C phenyl); 130.46
(CH phenyl); 136.34 (N–CH–N); 160.02 (C phenyl).
1-(4-Dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium
bro-
mide, 1a. This reaction was carried out under argon atmo-
sphere. 4-(Dodecyloxyphenyl)methanol (8.2 g, 28 mmol) and
thionyl bromide (2.25 ml, 28 mmol) were dissolved in dry
DCM. The progress of the reaction was monitored by TLC
until no more alcohol was observed. After three hours, the
solvent was removed under vacuum and the crude 1-bromo-
methyl-4-dodecyloxybenzene was used directly without
further purification. This bromo derivative and 1-methylimi-
dazole (2.3 ml, 28 mmol) were stirred in dry THF at room
temperature overnight. After evaporation to dryness, the
residue was purified by flash chromatography (silica gel col-
umn, elution DCM–MeOH, MeOH 2% to 8%). Yield: crys-
talline white solid, 10.8 g, 88% (Found: C, 62.7; H, 8.55; N,
6.45. C23H37BrN2O requires: C, 63.15; H, 8.5; N, 6.4%). nmax
/
cmꢀ1 3133 (C–H aromatic), 2919 and 2850 (C–H aliphatic),
1513 (CQC aromatic), 1248 (aromatic ether). dH (300 MHz;
CDCl3): 0.88 (3 H, t, J 6.9, CH3 aliphatic chain); 1.27 (16 H, br
s, CH2 aliphatic chain); 1.39–1.47 (2 H, m, CH2 aliphatic
chain); 1.78 (2 H, q, J 6.9, O–CH2–CH2); 3.95 (2 H, t, J 6.5,
OCH2); 4.08 (3 H, s, CH3–N); 5.47 (2 H, s, N–CH2-Ph); 6.91 (2
H, d, J 8.6, CH phenyl); 7.09 (1 H, m, CH imidazolium); 7.13
(1 H, m, CH imidazolium); 7.40 (2 H, d, J 8.6, CH phenyl);
10.89 (1 H, s, N–CH–N). dC (75 MHz; CDCl3): 13.86 (CH3
aliphatic chain); 22.42; 25.76; 28.92; 29.08; 29.15; 29.31; 29.34;
29.36; 29.40; 31.64 (CH2 aliphatic chain); 36.52 (N–CH3);
52.56 (N–CH2–Ph); 67.90 (O–CH2); 114.94 (CH phenyl);
121.58; 123.47 (CH imidazolium); 124.61 (C phenyl); 130.39
(CH phenyl); 136.54 (N–CH–N); 159.71 (C phenyl).
X-Ray crystallography
A suitable crystal of 1a was mounted on a Nonius Kappa-
CCD area detector diffractometer (MoKa, l = 0.71073 A).
The complete conditions of data collection (Denzo software)
and structure refinements are given below. The cell parameters
were determined from reflections taken from one set of ten
frames (1.01 steps in f angle), each at 20 s exposure. The
structures were solved using direct methods (SHELXS97) and
refined against F2 using the SHELXL97 software. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were generated according to stereochemistry and refined using
a riding model in SHELXL97.z Colorless crystal; crystal
dimension: 0.10 ꢃ 0.07 ꢃ 0.05 mm3; C23H37N2O1,Brꢀ,
1-(4-Dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium tetra-
fluoroborate, 1b. The compound 1b was obtained by anion
metathesis in water as solvent (5 ml aqueous solution of
sodium tetrafluoroborate (64 mg, 0.57 mmol) was added to
1-(4-dodecyloxybenzyl)-3-methyl-1H-imidazol-3-ium bromide
(250 mg, 0.57 mmol) dissolved in 15 ml of water). A precipitate
of 3-(4-dodecyloxybenzyl)-1-methyl-1H-imidazolium tetra-
fluoroborate occurred immediately. 1b was filtered and washed
with water and finally crystallized by slow diffusion of ether
into a THF solution. Yield: crystalline white solid, 250 mg,
98% (Found C, 60.6; H, 8.25; N, 6.1. C23H37BF4N2O ꢂ 1/2H2O
requires C, 60.9; H, 8.4; N, 6.2%). nmax/cmꢀ1 3154 (C–H
aromatic), 2919 and 2848 (C–H aliphatic), 1515 (CQC aro-
matic), 1249 (aromatic ether), 1036 (BF4ꢀ). dH(300 MHz;
CDCl3): 0.89 (3 H, t, J 6.8, CH3 aliphatic chain); 1.27 (16 H,
br s, CH2 aliphatic chain); 1.39–1.45 (2 H, m, CH2 aliphatic
chain); 1.76 (2 H, q, J 6.8, O–CH2–CH2); 3.94 (2 H, t, J 6.5,
OCH2); 3.98 (3 H, s, CH3–N); 5.29 (2 H, s, N–CH2–Ph); 6.91
(2 H, d, J 8.6, CH phenyl); 7.09 (1 H, m, CH imidazolium);
7.16 (1 H, m, CH imidazolium); 7.33 (2 H, d, J 8.6, CH
phenyl); 9.1 (1 H, s, N–CH–N). dC (75 MHz; CDCl3): 14.10
(CH3 aliphatic chain); 22.67; 26.00; 29.16; 29.33; 29.39; 29.57;
29.59; 29.62; 29.65; 31.90 (CH2 aliphatic chain); 36.37
(N–CH3); 53.15 (N–CH2–Ph); 67.18 (O–CH2); 115.35 (CH
M = 437.46 g molꢀ1; monoclinic; space group P1 (no. 2);
ꢀ
a = 8.7610(2) A; b = 9.7620(3) A; c = 27.2570(5) A; a =
93.675(1)1; b = 96.278(1)1; g = 95.237(4)1; V = 2301.03(1)
A3, Z = 4; Dc = 1.263 g cmꢀ3; m(MoKa) = 1.801 mmꢀ1; a
total of 16426 reflections; 1.511 o y o 27.521, 10512 indepen-
dent reflections with 7277 having I > 2s(I); 487 parameters;
Final results: R1 = 0.0438; Rw2 = 0.1147, Gof = 1.073,
maximum residual electronic density = 0.541 eꢀ Aꢀ3
.
Acknowledgements
We are especially grateful to Dr D. Guillon and J. Harrowfield
for the critical evaluation of the manuscript; we also thank
z CCDC reference numbers 293491. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b600279j
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Dr A. De Cian for the X-ray diffraction studies. This work was
supported by the Institute for Physics and Chemistry of
Materials Strasbourg (I.P.C.M.S.), Louis Pasteur University
and NANORGYSOL A.N.R.
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