Effects of alkyl chain length and anion size on thermal and structural properties for 1-alkyl-3-methylimidazolium hexafluorocomplex salts (C xMImAF6, x = 14, 16 and 18; A = P, As, Sb, Nb and Ta)

Article abstract of DOI:10.1039/c2dt11693f

A series of 1-alkyl-3-methylimidazolium hexafluorocomplex salts (C _xMImAF₆, x = 14, 16 and 18, A = P, As, Sb, Nb and Ta) have been characterized by thermal analysis, X-ray diffraction and polarized optical microscopy. A liquid crystalline mesophase is observed for all the C ₁₆MIm and C₁₈MIm salts. The C₁₄MIm⁺ cation gives a liquid crystalline mesophase only with PF₆ ^-. The temperature range of the liquid crystalline mesophase increases with an increase in alkyl chain length or with decrease in anion size. Single-crystal X-ray diffraction revealed that all the C₁₈MImAF ₆ salts (A = P, As, Sb, Nb and Ta) are isostructural with each other in the crystalline phase and have a layered structure. The interdigitated alkyl chain of the cation has a bent shape like a spoon near the imidazolium ring in the crystalline phase at -100 °C and is tilted with respect to the sheets of the imidazolium headgroups and anions. An increase of temperature increases the ratio of an all-trans conformation to the bent conformation in the crystalline phase. X-ray diffraction and polarized optical microscopy suggested that the liquid crystalline mesophase has a smectic A₂ structure. The interlayer distance increases with a decrease in the anion size since the smaller anion has a stronger coulombic interaction with the imidazolium headgroup, resulting in the decrease of the interdigitated part to give a larger layer spacing. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt11693f

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PAPER
Effects of alkyl chain length and anion size on thermal and structural
properties for 1-alkyl-3-methylimidazolium hexafluorocomplex salts
(
C MImAF , x = 14, 16 and 18; A = P, As, Sb, Nb and Ta)†
x
6
Fei Xu, Kazuhiko Matsumoto* and Rika Hagiwara
Received 7th September 2011, Accepted 21st December 2011
DOI: 10.1039/c2dt11693f
A series of 1-alkyl-3-methylimidazolium hexafluorocomplex salts (C MImAF , x = 14, 16 and 18, A = P,
x
6
As, Sb, Nb and Ta) have been characterized by thermal analysis, X-ray diffraction and polarized optical
microscopy. A liquid crystalline mesophase is observed for all the C MIm and C MIm salts. The
1
6
18
+
−
C MIm cation gives a liquid crystalline mesophase only with PF . The temperature range of the liquid
1
4
6
crystalline mesophase increases with an increase in alkyl chain length or with decrease in anion size.
Single-crystal X-ray diffraction revealed that all the C MImAF salts (A = P, As, Sb, Nb and Ta) are
1
8
6
isostructural with each other in the crystalline phase and have a layered structure. The interdigitated alkyl
chain of the cation has a bent shape like a spoon near the imidazolium ring in the crystalline phase at
−
100 °C and is tilted with respect to the sheets of the imidazolium headgroups and anions. An increase of
temperature increases the ratio of an all-trans conformation to the bent conformation in the crystalline
phase. X-ray diffraction and polarized optical microscopy suggested that the liquid crystalline mesophase
has a smectic A structure. The interlayer distance increases with a decrease in the anion size since the
2
smaller anion has a stronger coulombic interaction with the imidazolium headgroup, resulting in the
decrease of the interdigitated part to give a larger layer spacing.
1
0
Introduction
mesophases, ILCs based on an imidazolium cation with a
simple alkyl chain are common in previous reports which
examine the effects of the alkyl chain and the anion on the phys-
Ionic liquid crystals (ILCs) are composed of only ionic species
and have anisotropic structural organizations. This makes them
1
ical properties. A series of 1-alkyl-3-methylimidazolium cations
C MIm ) with long alkyl chains were combined with various
anions such as chloride (Cl ), bromide (Br ), tetrachlorome-
2
–4
5–7
+
attractive as one-dimensional or two-dimensional ion-con-
duction materials to move a specific ion, as well as anisotropic
reaction media. Ionic liquids are attracting more and more atten-
tion as functional materials because of their unique character-
istics such as wide liquid temperature range, negligible vapor
pressure and non-flammability. Because of the widespread use
of imidazolium-based ionic liquids with low melting points and
low viscosities, imidazolium-based ILCs are frequently studied.
Thermal behavior of ILCs based on imidazolium cations with
functional groups depends on the number of mesogenic units,
(
x
−
11
− 12
−
11
tallate (MCl4 , M = Co and Ni), trifluoromethylsulfonate
−
12
− 13
(
OSO CF ), tetrafluoroborate (BF4
)
and hexafluoropho-
2
3
−
14
sphate (PF ). The temperature range of the mesophase
6
8
observed for these salts increases with increasing alkyl chain
length, although the alkyl chain length where the liquid crystal-
line mesophase appears depends on the anionic species.
Although the C MImPF -type ILCs were studied by differen-
tial scanning calorimetry (DSC), polarized optical microscopy
POM) and X-ray diffraction (XRD), other C MImAF (AF
hexafluorocomplex anion) ILCs were not. Liquid crystalline
mesophase appears with x = 14 in the case of C MImPF and
9
x
6
1
0
14
−
mesogenic promoter and the counteranion. Although a variety
of functional groups, such as a cholesterol group, were coupled
with the imidazolium cation to form liquid crystalline
(
x 6 6
=
x
6
the temperature range of the mesophase rapidly increases with
increase in x. The cations of C MImPF and C MImPF in the
12
6
14
6
crystalline phase show a “spoon-like” shape with the alkyl
chains interdigitated to form nonpolar layers, separating the
polar layers made up of imidazolium headgroups and
Graduate School of Energy Science, Kyoto University, Yoshida, Sakyo-
ku, Kyoto606-8501, Japan. E-mail: k-matsumoto@energy.kyoto-u.ac.jp
†
Electronic supplementary information (ESI) available: Infra-red
14–16
spectra, Raman spectra, thermogravimetric curves, additional differential
scanning calorimetric curves, additional polarized optical microscopic
textures, additional X-ray diffraction patterns, additional layer spacings,
crystallographic data and additional geometrical parameters. CCDC
reference numbers 843137-843151. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt11693f
anions.
In this study, a series of C MImAF salts (x = 14,
x 6
16 and 18, A = P, As, Sb, Nb and Ta) were synthesized and
characterized by DSC, POM and XRD to systematically investi-
gate effects of the alkyl chain length and anion size on their
thermal and structural properties.
3494 | Dalton Trans., 2012, 41, 3494–3502
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Table 1 Summary of DSC for C
As, Sb, Nb and Ta)
x
MImAF
6
(x = 14, 16 and 18, A = P,
temperature, ΔH and ΔS) for all the C MImAF salts. Fig. 1
x
6
shows the DSC curves of C MImAF . Two endothermic peaks
18
6
a
ΔH (kJ mol−1
ΔS (J mol−1 K−1
)
are observed in all the cases. The peaks at the lower and higher
temperatures correspond to the melting point (from crystal to
liquid crystal) and the clearing point (from liquid crystal to iso-
tropic liquid), respectively. The relatively large ΔH (and thus ΔS)
on melting indicates a large structural change, namely, the break-
up of a three-dimensionally ordered crystal lattice. The small ΔH
x
A
P
Transition
cryst-iso
T/°C
)
1
4
65.1
67.4
73.0
64.8
56.8
56.0
56.0
75.2
124.8
74.8
104.3
67.5
77.0
66.9
77.0
66.3
75.5
79.9
164.0
78.1
142.8
74.5
117.9
74.2
116.9
74.0
115.4
33.1
97.8
b
cryst-SmA
2
36.5 107.3
0.4 1.3
34.0 100.7
32.4
31.4
28.0
30.1
0.5
b
2
SmA -iso
As cryst-iso
Sb cryst-iso
Nb cryst-iso
Ta cryst-iso
98.2
95.6
85.2
86.4
1.3
(and thus ΔS) on clearing is mainly caused by the break-up of
van der Waals interactions between the alkyl chains, which is
reflected in the increase of ΔH (and thus ΔS) for clearing with
the increase in x for the same anion. Similar phenomena are also
observed for ILCs of C MImCl, C MImBr, C MImBF and
1
6
P
cryst-SmA
SmA -iso
As cryst-SmA
SmA -iso
Sb cryst-SmA
SmA -iso
Nb cryst-SmA
SmA -iso
Ta cryst-SmA
SmA -iso
cryst-SmA
SmA -iso
As cryst-SmA
SmA -iso
Sb cryst-SmA
SmA -iso
Nb cryst-SmA
SmA -iso
Ta cryst-SmA
SmA -iso
2
2
2
2
2
2
2
2
2
2
2
36.5 105.0
2
0.6
33.1
0.4
33.0
0.5
33.0
0.4
47.5 134.6
1.1 2.6
46.3 131.9
0.9 2.2
36.8 106.0
0.7
31.3
0.7
36.7 105.8
0.7 1.8
1.5
97.3
1.2
97.2
1.3
97.3
1.2
x
x
x
4
12–14,18
C MIm(FH) F.
Either a small peak (C MImPF ) or a
18 6
x
2
2
broad peak (C MImAF , A = As, Sb, Nb and Ta) is observed in
18 6
2
the crystalline phase below the melting point, which is probably
ascribed to a conformational change of the alkyl chains (see the
section of crystal structures below). An endothermic peak corre-
sponding to the clearing point is also observed in the thermo-
grams of C MImAF (Fig. S11, ESI†), proving all the
2
1
8
P
2
16
6
2
C MImAF salts give liquid crystalline mesophases, although
16
6
the temperature range of liquid crystal is quite narrow (∼10 °C)
2
1.7
90.1
1.9
for C MImSbF , C MImNbF and C MImTaF . The clearing
16
6
16
6
16
6
2
point was not observed for C MImAF (Fig. S10, ESI†) in the
14
6
−1
heating process at a scan rate of 5 °C min . It is noteworthy
that a small exothermic peak was observed for C MImPF at
2
14
6
a
A scan rate of 5 °C min−1 was used. The abbreviations, cryst, iso and
denote crystal, isotropic liquid and smectic A liquid crystal,
respectively. These transitions were determined from DSC recorded at
6
9.7 °C during the cooling process (Fig. 2). This transition temp-
SmA
2
2
b
erature is nearly the same as that for the melting in the heating
process and they seem to overlap. The lower scan rate of 0.5 °C
−1
0
.5 °C min .
−1
min confirmed the liquid crystalline mesophase (Fig. 2, inset)
although the transition temperature was slightly lower than those
14
reported in the previous work. The other four C MImAF
6
14
(
A = As, Sb, Nb and Ta) salts did not exhibit liquid crystalline
Results and discussion
mesophase even during the cooling process.
Fig. 3 shows the melting and clearing points of C MImAF as
x
6
Thermal properties
a function of the carbon number x and the radius of the central
atom of AF6−
.
19
Both the melting and clearing points increase
All the obtained salts are white hydrophobic solids at room
temperature and only C MImNbF is gradually hydrolyzed in air
x
6
with the increase in x or the decrease in the size of the anion.
The larger temperature dependence of the clearing point than
that of the melting point results in an increase in the temperature
−
even at room temperature. The AF6 anion is best identified by
IR and Raman spectroscopy (Experimental section and Fig. S1–
1
7
S6†). Table 1 summarizes the DSC data (phase transition
Fig. 2 Differential scanning calorimetric curves of heating and cooling
at 5 °C min− and heating process at 0.5 °C
1
Fig. 1 Differential scanning calorimetric curves (heating process) for
18MImAF (A = P, As, Sb, Nb and Ta).
processes for C14MImPF
min (inset).
6
−1
C
6
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that the alkyl chain length where liquid crystalline mesophases
are observed depends on the anionic species in the cases of
11–14,18
C MIm salts.
x
Compared with symmetric N,N′-dialkylimi-
dazolium salts with long alkyl chains, C MIm salts show differ-
x
ent thermal behavior, probably due to the weaker van der Waals
interactions based on the different number of long alkyl
20–22
For PF6− salts, both the melting and clearing points
chains.
decrease by introducing another long alkyl chain (melting and
clearing points of C MImPF are 75.2 and 124.8 °C and
16
6
melting and clearing points of C C ImPF are 61.3 and
1
6
16
6
22
2−
1
03.3 °C). Another example, PdCl4 salts, gives a different
trend; melting and clearing points decrease and increase respect-
ively, by introducing another long alkyl chain (melting and clear-
ing points of (C MIm) (PdCl ) are 124 and 190 °C and melting
16
2
4
and clearing points of (C C Im) PdCl are 94.9 and
16
16
2
4
20,21
2
14.6 °C)
.
Fig. 3 Melting points (■) and clearing points (●) observed in the
DSC thermogram of C
x 6
MImAF (x = 14, 16 and 18, A = P, As, Sb, Nb
and Ta).
Structural properties
The five kinds of C MImAF salts are isostructural with each
18
6
range of the liquid crystalline mesophase with an increase in x or
a decrease in the size of anion. The increase of melting point
against x can be explained by an increase of the van der Waals
interactions between the alkyl chains, which leads to clearer sep-
aration of polar (imidazolium headgroup and anion) and non-
other since the cation with a long alkyl chain dominates crystal
packing, although a slight increase in cell volume is observed
with increasing anion size. This is in contrast with a series of
EMIm salts, where the PF , AsF and SbF salts belong to a
6
6
6
monoclinic cell (space group: P2 /c) and the NbF and TaF
6
1
6
polar (long alkyl chain) regions. When the size of AF6− is
changed with a fixed cationic structure, the difference in temp-
erature dependence of melting and clearing points reflects the
strength of the interaction between the imidazolium headgroup
and the anion. These trends suggest both the van der Waals inter-
action between the alkyl chains and the electrostatic interaction
between the cation and anion have an effect to stabilize the
liquid crystalline mesophase. This also explains the phenomenon
1
3
23
salts belong to an orthorhombic cell (space group: P2 2 2 ).
1
1 1
The crystal structure of C MImTaF at −100 °C, which is
18
6
shown in Fig. 4, is described as a typical case. The packing
mode of C MImAF is described as a layered structure of polar
18
6
and non-polar domains. The sheets consisting of the imidazo-
lium headgroups and TaF6− anions (polar domains) are separated
by interdigitated alkyl chains (non-polar domains) tilted relative
to the polar domain (Fig. 4(a)). The polar domain is made of a
−
Fig. 4 Crystal structure of C18MImTaF
region at −100 °C, (c) asymmetric unit with atom numbering scheme at −100 °C, (d) structures of C18MIm and TaF
6
. (a) Packing diagram at −100 °C, (b) configuration of the imidazolium headgroup and TaF
6
in the polar
+
−
6
at −100, 25 and 55 °C.
3496 | Dalton Trans., 2012, 41, 3494–3502
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Fig. 5 Schematic drawing of structural changes for the octadecyl chain of C18MImAF at −100, 25 and 55 °C.
6
double layer of a NaCl-type ion configuration, where one of the
two layers is not lying on the other in the ideal manner for a
NaCl-type structure (Fig. 4(b)). The alkyl chain keeps a trans
conformation from C8 to C23 and is bent near the imidazolium
ring like a spoon (Fig. 4(c) and (d)), as was reported for
always roughly starts from C9 regardless of the alkyl chain
length. The difference in the overlapped alkyl chain length
(12.947 Å, 15.497 Å and 20.617 Å for x = 12, 14 and 18,
Table S3, ESI†) directly corresponds to the number of the
methylene group added.
1
4–15
C MImPF and C MImPF .
Fig. 7 shows the cell parameters (a, b and c) of C MImAF6
1
2
6
14
6
18
Crystal structures were also determined at 25 and 55 °C for all
(A = P, As, Sb, Nb and Ta) at −100 °C. On increasing the anion
size, the a parameter shows a very small change and the b par-
ameter slightly increases, leading to an increase of the area in the
ionic sheets. The c parameter remains nearly constant for the
PF , AsF and SbF salts and becomes smaller for the NbF and
the five C MImAF salts (see Fig. 4(d) for the asymmetric
18
6
unit). Although the overall cation–anion configurations at −100,
5 and 55 °C are similar to each other, the carbon atoms in the
2
octadecyl chain are significantly disordered near the imidazolium
ring at 25 and 55 °C and it was hard to solve the structure com-
pletely. The atomic displacement parameters become large as the
temperature increases, especially for the C7, C8, C9 and C10
atoms. Although a slight sign of disordering was observed for
6
6
6
6
TaF salts, in spite of the larger sizes of NbF6 and TaF6−. The
−
6
value of c × sinβ, which is the distance between the layers,
decreases with increasing anion size (Table S4, ESI†), with the
¬
¬
the fluorine atoms in AF , the structure of AF was solved as a
6
6
non-disordered octahedron in this study. Although a similar
structural change was reported for C MImPF between −98 and
1
4
6
2
7 °C, the present diffraction studies at 25 and 55 °C propose a
more detailed model. The best model obtained is shown as a
schematic illustration in Fig. 5. The octadecyl chain which has a
bent conformation at −100 °C adopts an all-trans conformation
at 55 °C, whereas the electron density map obtained at 25 °C
suggests mixing of the all-trans and bent conformations. This
result suggests that the trans conformation is preferred near the
melting point and the ratio of the trans conformation to the bent
conformation becomes larger as the temperature increases. The
solid–solid phase transition observed in the crystalline phase
seems to result from such structural changes. It is notable that
the ratios of the two conformers at 25 °C are different in the five
cases and the solid–solid transition temperature is dependent on
the anion size. Packing of the ions at low temperatures is tighter
than those at or above room temperature, which is reflected in
the increase in tilt angle of the alkyl chains with regard to the
polar sheet. Since the imidazolium head groups and the anions
are connected by ionic interactions in the polar sheet and the
alkyl chains are connected with each other by van der Waals
interactions in the nonpolar sheet, such an increase of tilt angle
leads to a conformation change of C7, C8, C9 and C10 atoms.
Sharper changes of some cell parameters, especially β, are
observed between 25 and 55 °C (Tables S1 and S2, ESI†) com-
pared with those between −100 and 25 °C, which are in good
accordance with the crystal–crystal phase transition temperatures
observed from the DSC results. Comparison of the crystal struc-
tures between C MImPF , C MImPF and C MImPF also
Fig. 6 Cell parameters (a, b and c) for C12MImPF
18MImPF . The overlapped alkyl chain length (lover) is also plotted
see Table S3† for calculation of lover). Data collection temperature:
6 6
, C14MImPF and
C
(
6
1
4
15
−
150 °C for C12MImPF
C MImPF (present study).
6
,
−98 °C for C14MImPF
6
and −100 °C for
1
8
6
12
6
14
6
18
6
gives interesting information on cell parameters (note: the data
1
4
collection temperatures are −150 °C for C MImPF , −98 °C
1
2
6
15
for C MImPF
and −100 °C for C MImPF ). The a and b
1
4
6
18
6
cell parameters, corresponding to cation–cation (or anion–anion)
distances within the NaCl-type layer, do not show a significant
change, whereas the c cell parameter increases as the alkyl chain
length increases (Fig. 6) and the alkyl–alkyl interdigitated part
6
Fig. 7 Cell parameters (a, b and c) for C18MImAF (A = P, As, Sb, Nb
and Ta) at −100 °C. The overlapped alkyl chain length (lover) is also
plotted (see Table S4† for calculation of lover).
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24–25
only exception of SbF . The overlapped alkyl chain length also
mesophase in these systems.
These POM results combined
6
increases with increasing anion size (Fig. 7 and Table S4, ESI†).
This observation means the alkyl chains interdigitate more
deeply as the anion becomes larger, which is probably due to the
weaker cation–anion interaction and the larger alkyl chain inter-
spacing caused by the larger anion.
with the XRD study later, suggest that the liquid crystalline
mesophase of C MImAF ILCs is assigned to a smectic A
x
,11,13–14,18
6
1
mesophase.
Such an enantiotropic smectic A liquid
crystalline mesophase was also observed for other 1-alkyl-3-
methylimidazolium ILCs in previous reports, since the large
11–14,18
Table 2 lists C–H⋯F interactions for C MImTaF . All F
cation dominates formation of the smectic layer.
1
8
6
atoms are involved in short contacts (below the sum of van der
Waals radii) with H atoms of the imidazolium ring (F1, F2, F3
and F5), or those of the alkyl side-chains (F1, F4, F6). Bifur-
cation of the C–H⋯F bonds is observed for H atoms (H2 and
H4) which interact with two F atoms (F1 and F5, F1 and F2,
respectively) of TaF6−. Similar phenomena were also observed
for the other C MImAF salts (A = P, As, Sb and Nb).
Fig. 9 and 10 show the XRD patterns (2° < 2θ < 30°) of the
crystalline phase and liquid crystalline mesophase for
C MImAF , respectively. In both the phases, sharp peaks are
18
6
observed in the low-angle region, indicating formation of layered
structures, and no peak was found in the region lower than 2.0°.
The similar crystal structures of C MImAF result in the similar
18
6
2θ-angle for the sharp peaks in the XRD pattern of the crystal-
line phase (see Figs. S15 and S16, ESI†, for C MImAF and
18
6
Fig. 8 shows a polarized optical microscopic (POM) texture of
C MImTaF which is typical of all the C MImAF ILCs (see
14
6
C MImAF ). Absence of additional peaks in the high-angle
1
8
6
x
6
16
6
Figures S12–S14, ESI† for the other salts) and characterized by
spontaneous formation of smooth fan-like or focal conic tex-
tures. The broken fan-like texture, which is indicative of the
smectic C mesophase, was not observed in the liquid crystalline
mesophase on cooling from the isotropic phase to the crystalline
mesophase. This suggests the absence of the smectic C
region in the cases of liquid crystals suggests the loss of pos-
itional ordering within the layer plane, which fits the character-
istics of a smectic A mesophase. Small peaks around 5.0° were
observed for C MImAF and C MImPF and were indexed as
the (002) diffraction of the smectic layers (Fig. 10 and S17†).
Measurements of the patterns of the liquid crystalline mesophase
18
6
16
6
Table 2 The C–H⋯F interaction geometries (Å, °) in C18MImTaF
6
D–H⋯A
D–H
H⋯A
D⋯A
∠D–H⋯A
a
C2–H2 ⋯F1
0.930
0.930
0.960
0.930
0.930
0.970
0.970
0.960
0.930
0.970
0.970
2.598
2.586
2.605
2.633
2.357
2.634
2.700
2.691
2.458
2.462
2.615
2.978
3.326
3.113
3.425
3.211
3.440
3.352
3.620
3.093
3.398
3.579
105.07
136.80
113.35
143.32
152.51
140.65
124.96
163.06
125.59
161.92
172.15
b
C4–H4 ⋯F1
d
C24–H24a ⋯F1
b
C4–H4 ⋯F2
e
C5–H5 ⋯F3
a
C6–H6b ⋯F4
d
C8–H8a ⋯F4
c
C23–H23b ⋯F4
d
C2–H2 ⋯F5
e
C6–H6a ⋯F6
C7–H7a⋯F6
a
b
c
d
Symmetry codes: −1 + x, y, z. −x, −y, −z. −1 − x, −y, 1 − z. −1/2
e
+
x, −1/2 − y, z. −1/2 + x, 1/2 − y, z.
6
Fig. 9 X-ray diffraction patterns for C18MImAF (A = P, As, Sb, Nb
and Ta) in the crystalline phase (40 °C).
Fig. 8 Polarized optical microscopic texture of
C
18MImTaF
6
at
Fig. 10 X-ray diffraction patterns for C18MImAF
6
(A = P, As, Sb, Nb
1
00 °C.
and Ta) in the liquid crystalline mesophase (100 °C).
3498 | Dalton Trans., 2012, 41, 3494–3502
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the imidazolium headgroup and the anion alternate with each
+
other. The octadecyl chain of C MIm in the C MImAF6
18
18
crystal structure at −100 °C has a bent structure, which changes
into a trans conformation as temperature increases. The layer
spacing (d) in the liquid crystalline mesophases satisfies the
relation of l < d < 2l (l is the fully extended length of the cation)
and monotonously decreases with increasing temperature, indica-
tive of a smectic A interdigitated bilayer structure. The layer
2
spacing in the mesophase decreases with increasing anion size,
due to the decrease of cation–anion interactions within the polar
region.
Experimental section
6
Fig. 11 Layer spacings of C18MImAF (A = P, As, Sb, Nb and Ta) in
the liquid crystalline mesophase.
Apparatus and materials
Volatile materials were handled in a vacuum line constructed of
SUS316 stainless steel and PFA (tetrafluoroethylene-perfluoro-
alkylvinylether copolymer). Nonvolatile materials were handled
under a dry Ar atmosphere in a glove box. Acetone (Wako
Chemicals, 99.5%) and dichloromethane (Aldrich, 99.8%) were
used as purchased. The 1-alkyl-3-methylimidazolium chlorides,
were difficult for C MImSbF , C MImNbF and C MImTaF
6
due to the relatively narrow temperature range of their
mesophases.
16
6
16
6
16
Fig. 11 shows the layer spacing (d) of C MImAF in the
1
8
6
liquid crystalline mesophase as a function of temperature. The
layer spacing d is found to satisfy l < d < 2l, where l is the fully
extended length of the cation, which indicates an interdigitated
bilayer structure is formed in the smectic A liquid crystalline
C MImCl (x = 14, 16 and 18), were prepared by reactions of 1-
x
methylimidazole (Aldrich, 99%) and equimolar quantities of the
corresponding chloroalkanes (1-chlorotetradecane (Aldrich,
1
2,18
mesophase (smectic A2).
an increase in temperature, which is also indicative of a smectic
A mesophase.
2
The layer spacing decreases with
9
8%), 1-chlorohexadecane (Wako Chemicals, 95%), 1-chloro-
octadecane (Wako Chemicals, 95%)) at temperatures between 70
and 100 °C for several days. Purification of the chlorides was
performed by dissolving the chloride salts in acetonitrile (dehy-
drated, Wako Chemicals, 99%) and then precipitating from the
solution by adding ethyl acetate (dehydrated, Wako Chemicals,
12,18
To our knowledge, all the 1-alkyl-3-methyl-
imidazolium ILCs known today show the same type of liquid
crystalline mesophase of smectic A , since the bulky cation is
2
much larger than the anion and dominates the type of mesophase
1
2–14,18
The layer spacing for PF and AsF6− is
−
for such ILCs.
6
−
9
9.5%). The hexafluorocomplex salts (KPF (Wako Chemicals),
6
−
−
larger than that for SbF6 , NbF6 and TaF6 , which can be
explained by changes in the cation–anion interactions within the
polar region. The bilayer structure interdigitates more deeply
with decreasing cation–anion interaction due to the increase of
thermal mobility of the cations, leading to a decrease of the layer
NaAsF (Aldrich), KSbF (Aldrich, 99%)) were used as pur-
6
6
chased. Potassium fluoride (Wako chemicals, 99%) was dried
under vacuum at 180 °C for a few days before use. Anhydrous
HF (Daikin Industries) was dried over K NiF6 prior to use.
2
Niobium pentafluoride (NbF , Aldrich, 99%) and tantalum
−
5
spacing. An increase of the AF6 anion size results in a longer
pentafluoride (TaF , Furuuchi Chemical Corp., 99.5%) were
5
cation–anion distance and weaker cation–anion interactions,
which facilitates the interdigitation of the alkyl chains and leads
to the smaller layer spacing. The same trend observed in the
crystal structure as shown above, also supports this explanation.
Similar phenomena were also observed for C MImPF and
used as purchased. The two potassium salts, KNbF and KTaF ,
6
6
were prepared by the reaction of KF and the corresponding metal
23,26
pentafluorides in HF according to the previous report.
white powder obtained was dried under dynamic vacuum at
0 °C.
The
1
6
6
7
C MImAsF (Fig. S18, ESI†).
1
6
6
Synthesis of C14MImPF
6
Conclusions
The two starting materials, C MImCl (3.150 g, 10.00 mmol)
14
Thermal and structural properties of 1-alkyl-3-methylimidazo-
and KPF6 (2.024 g, 11.00 mmol), were dissolved in water
lium hexafluorocomplex salts, C MImAF (x = 14, 16 and 18,
(30 mL) and the mixture was stirred vigorously overnight. A
x
6
−
A = P, As, Sb, Nb and Ta), were investigated. These salts exhibit
a liquid crystalline mesophase with a smectic A structure. The
temperature range of the mesophase increases with increasing
alkyl chain length or decreasing anion size, suggesting both the
van der Waals interactions between the alkyl chains and the elec-
trostatic interaction between the imidazolium headgroup and the
anion have an effect on stabilization of the liquid crystalline
mesophase. X-ray crystallography revealed that all five
C MImAF salts have the same layered structure where the non-
slight excess of KPF was used for the thorough removal of Cl .
6
The solid product was separated from the mixture by centrifu-
gation and the remaining water-soluble species were washed
with water several times. Testing for the presence of residual
chloride impurities in the water phase with 1 M AgNO aqueous
3
solution gave no precipitation of AgCl. The product was dried
under vacuum at room temperature and then at 80 °C (3.574 g,
8.420 mmol). Anal. Calcd. for C H N F P : C, 50.94; H,
18 35 2 6 1
8.25; N, 6.60; F, 26.89. Found: C, 50.77; H, 8.34; N, 6.72; F,
1
8
6
26.94. Raman (frequency/cm− (relative intensity)): 741(s)
1
polar layers of interdigitated alkyl chains and the polar layers of
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3494–3502 | 3499

View Article Online
−
−
−
(
PF , ν ), 566(w) (PF , ν ), 471(m) (PF , ν ). IR (frequency/
C MImCl (3.150 g, 10.00 mmol) and KSbF6 (2.886 g,
6
−
1
6
2
6
5
14
1
−
−
cm (relative intensity)): 847(w) (PF6 , ν ), 556(s) (PF6 , ν4)
10.50 mmol), were dissolved in 30 mL of acetone and the
mixture was stirred vigorously overnight. The solution was sep-
arated from the mixture by centrifugation and the acetone was
removed under vacuum at room temperature. Testing for the
presence of residual chloride impurities with a 1 M AgNO3
aqueous solution gave no precipitation of AgCl. The product
was dissolved in 20 mL of dichloromethane and the mixture was
stirred vigorously overnight. The dichloromethane solution was
separated from the precipitate (probably the potassium salt) by
centrifugation and dichloromethane was removed under vacuum
at room temperature initially and then at 80 °C for 3 days. Anal.
3
(
see S1–S6† for the vibrational spectra of C MImAF ). Here, s,
m and w for relative intensity denote strong, medium and weak,
respectively.
x 6
Synthesis of C MImPF
6
16
The same procedure as for C MImPF was used. Anal. Calcd.
1
4
6
for C H N F P : C, 53.10; H, 8.63; N, 6.19; F, 25.22. Found:
2
0 39 2 6 1
−1
C, 52.98; H, 8.55; N, 6.35; F, 25.44. Raman (frequency/cm
relative intensity)): 742(s) (PF , ν ), 565(w) (PF , ν ), 471
−
−
(
(
(
6 1 6 2
−1
−
Calcd. for C H N F Sb : C, 41.94; H, 6.80; N, 5.44; F, 22.14.
m) (PF6 , ν ). IR (frequency/cm (relative intensity)): 847(w)
18 35
2
6
1
5
−
−
Found: C, 41.84; H, 6.76; N, 5.44; F, 21.94. Raman (frequency/
PF6 , ν ), 554(s) (PF6 , ν4).
3
−
1
−
−
cm (relative intensity)): 645(s) (SbF , ν ), 280(m) (SbF ,
6
6
1
−1
−
ν5). IR (frequency/cm (relative intensity)): 654(m) (SbF6 , ν3).
Synthesis of C18MImPF
6
The same procedure as for C MImPF was used. Anal. Calcd.
1
4
6
Synthesis of C MImSbF
6
16
for C H N F P : C, 55.00; H, 8.96; N, 5.83; F, 23.75. Found:
2
2 43 2 6 1
−1
The same procedure as for C MImSbF was used. Anal. Calcd.
C, 54.76; H, 9.01; N, 5.94; F, 23.67. Raman (frequency/cm
relative intensity)): 741(s) (PF , ν ), 568(w) (PF , ν ), 471
14
6
−
−
for C H N F Sb : C, 44.20; H, 7.18; N, 5.16; F, 20.99. Found:
(
(
(
20 39
2
6
1
6
1
6
2
−1
−
−1
C, 44.20; H, 7.14; N, 5.25; F, 20.88. Raman (frequency/cm
m) (PF6 , ν ). IR (frequency/cm (relative intensity)): 845(w)
5
−
−
−
−
(
(
(
relative intensity)): 645(s) (SbF6 , ν ), 570(w) (SbF6 , ν ), 280
PF6 , ν ), 554(s) (PF6 , ν4).
1 2
3
m) (SbF6−, ν ). IR (frequency/cm (relative intensity)): 656(m)
−1
5
−
SbF6 , ν3).
Synthesis of C MImAsF
6
14
The same procedure as for C MImPF was used. Anal. Calcd.
1
4
6
18 6
Synthesis of C MImSbF
for C H N F As : C, 46.15; H, 7.48; N, 5.98; F, 24.36. Found:
1
8
35
2
6
1
−1
C, 46.06; H, 7.42; N, 5.96; F, 24.18. Raman (frequency/cm
relative intensity)): 678(s) (AsF , ν ), 575(w) (AsF , ν ), 368
The same procedure as for C14MImSbF
for C22 Sb : C, 46.23; H, 7.53; N, 4.90; F, 19.96. Found:
C, 46.07; H, 7.37; N, 4.86; F, 19.72. Raman (frequency/cm
6
was used. Anal. Calcd.
−
−
(
(
(
H
43
N
2
F
6
1
6
1
−1
6
2
−
−1
m) (AsF6 , ν ). IR (frequency/cm (relative intensity)): 693
5
m) (AsF6−, ν3).
−
−
(relative intensity)): 645(s) (SbF , ν ), 571(w) (SbF , ν ), 280
6 1 6 2
−
−1
(
(
m) (SbF6 , ν ). IR (frequency/cm (relative intensity)): 654(m)
5
−
SbF6 , ν3).
Synthesis of C16MImAsF
6
The same procedure as for C MImPF was used. Anal. Calcd.
1
4
6
Synthesis of C14MImNbF
6
for C H N F As : C, 48.39; H, 7.86; N, 5.65; F, 22.98. Found:
2
0
39
2
6
1
−1
C, 48.36; H, 7.65; N, 5.68; F, 23.21. Raman (frequency/cm
relative intensity)): 678(s) (AsF , ν ), 573(w) (AsF , ν ), 369
The same procedure as for C14MImSbF
for C18 Nb : C, 44.44; H, 7.20; N, 5.76; F, 23.46. Found:
C, 44.56; H, 7.37; N, 5.79; F, 23.26. Raman (frequency/cm
6
was used. Anal. Calcd.
−
−
(
(
(
H
35
N
F
2
6
1
6
1
−1
6
2
−
−1
m) (AsF6 , ν ). IR (frequency/cm (relative intensity)): 695
5
m) (AsF6−, ν3).
−
−
(relative intensity)): 681(s) (NbF
, ν
), 285(m) (NbF , ν ). IR
1 6 5
6
frequency/cm− (relative intensity)): 608(w) (NbF6 , ν3).
1
−
(
Synthesis of C18MImAsF
6
Synthesis of C MImNbF
6
16
The same procedure as for C MImPF was used. Anal. Calcd.
1
4
6
for C H N F As : C, 50.38; H, 8.21; N, 5.34; F, 21.76. Found:
The same procedure as for C14MImSbF
for C20 Nb : C, 46.69; H, 7.59; N, 5.45; F, 22.18. Found:
C, 46.90; H, 7.76; N, 5.50; F, 21.92. Raman (frequency/cm
6
was used. Anal. Calcd.
2
2
43
2
6
1
−1
C, 50.09; H, 8.20; N, 5.37; F, 21.74. Raman (frequency/cm
H
39
N
F
2
6
1
−
−
−1
(
(
(
relative intensity)): 678(s) (AsF , ν ), 573(w) (AsF , ν ), 368
6 1 6 2
−1
−
−
−
m) (AsF6 , ν ). IR (frequency/cm (relative intensity)): 698(s)
(relative intensity)): 681(s) (NbF
6
, ν
1
), 284(m) (NbF
6
, ν
).
). IR
5
5
−
−1
−
AsF6 , ν3).
(frequency/cm (relative intensity)): 604(w) (NbF
6
, ν
3
Synthesis of C MImSbF
Synthesis of C MImNbF6
18
14
6
The same procedure as for C MImPF was unsuccessful for the
The same procedure as for C MImSbF was used. Anal. Calcd.
14 6
1
4
6
synthesis of C MImSbF since addition of C MImCl and
for C H N F Nb : C, 48.71; H, 7.93; N, 5.17; F, 21.03. Found:
22 43 2 6 1
14
6
14
−1
KSbF into water gave a gel-like solution, making the separation
C, 49.00; H, 8.07; N, 5.26; F, 20.92. Raman (frequency/cm
6
−
−
of the product difficult. Instead of this method, the following
(relative intensity)): 681(s) (NbF , ν ), 285(m) (NbF , ν ). IR
6 1 6 5
(frequency/cm− (relative intensity)): 606(m) (NbF6 , ν3).
1
−
procedure was used for the SbF salts; the two starting materials,
6
3500 | Dalton Trans., 2012, 41, 3494–3502
This journal is © The Royal Society of Chemistry 2012

View Article Online
DSC-60, respectively, at a scan rate of 5 °C min− . The sample
was placed in a Ni open cell for TG analysis and in an alu-
minium sealed cell for DSC analysis. Polarized optical
microscopy was carried out using a VHX digital microscope
1
Synthesis of C14MImTaF
6
The same procedure as for C MImSbF was used. Anal. Calcd.
1
4
6
for C H N F Ta : C, 37.63; H, 6.10; N, 4.88; F, 19.86. Found:
18
35
2
6
1
−1
C, 37.66; H, 6.02; N, 4.88; F, 19.60. Raman (frequency/cm
(
Keyence, Co.) under cross-polarized light at ×100 magnification.
−
−
(
(
relative intensity)): 693(s) (TaF , ν ), 290(m) (TaF , ν ). IR
6 1 6 5
The sample was placed in a transparent cell made of sapphire
and covered with a piece of glass substrate. The temperature was
controlled by a TS1500 hot stage unit (Japan High Tech Co.,
Ltd.).
frequency/cm− (relative intensity)): 573(w) (TaF6 , ν3).
1
−
Synthesis of C16MImTaF
6
The same procedure as for C MImSbF was used. Anal. Calcd.
1
4
6
X-Ray diffraction analyses. a. Powder diffraction
for C H N F Ta : C, 39.87; H, 6.48; N, 4.65; F, 18.94. Found:
2
0
39
2
6
1
−1
C, 39.70; H, 6.25; N, 4.70; F, 19.09. Raman (frequency/cm
X-ray diffraction was performed using a Rigaku Ultima IV dif-
fractometer (Cu-Kα, λ = 1.542 Å). The output power was set as
−
−
(
(
relative intensity)): 693(s) (TaF , ν ), 287(m) (TaF , ν ). IR
6 1 6 5
frequency/cm− (relative intensity)): 575(w) (TaF6 , ν3).
1
−
4
(
0 kV–40 mA. The data were recorded in the 2θ ranges of 2–30°
scanning rate of 2° per minute) with a step of 0.02°. The sample
was sealed in a vacuum cell. The heating and cooling rate of
Synthesis of C MImTaF
18
6
−1
5
°C min was used.
The same procedure as for C MImSbF was used. Anal. Calcd.
1
4
6
for C H N F Ta : C, 41.90; H, 6.83; N, 4.44; F, 18.10. Found:
22
43
2
6
1
−1
b. Single-crystal diffraction
Crystals of C MImAF (A = P, As, Sb, Nb and Ta) were grown
C, 41.88; H, 6.82; N, 4.50; F, 17.87. Raman (frequency/cm
−
−
(
(
relative intensity)): 693(s) (TaF , ν ), 287(m) (TaF , ν ). IR
6 1 6 5
frequency/cm−1 (relative intensity)): 576(m) (TaF6 , ν3).
−
18
6
by slow evaporation of the solvent from an acetone (A = P, As,
Sb, Ta) or dichloromethane (A = Nb) solution. Suitable crystals
of C MImAF (A = P, As, Sb and Ta) were fixed on a glass rod
18
6
Spectroscopic and thermal analyses
with perfluoro polyether oil (fomblin YLVAC 06\6) and centered
Infra-red spectra were obtained by FTS-165 (Bio-Rad Labora-
tories). The samples were sandwiched between a pair of AgCl
windows fixed in a stainless steel airtight cell under dry Ar.
Raman spectra were obtained by FTS-175C (Bio-Rad Labora-
tories) using the 1064 nm line of a Nd:YAG laser as the exci-
tation line. The samples for Raman spectroscopy were loaded in
Pyrex glass tubes under dry Ar and sealed with a plastic cap.
Thermogravimetric and DSC analyses were performed under a
dry Ar gas flow using Shimadzu DTG-60H and Shimadzu
on an X-ray diffractometer (R-axis Rapid II, Rigaku controlled
27
by the program RAPID AUTO 2.40) equipped with an
imaging plate area detector and graphite-monochromated Mo-
Kα radiation (0.71073 Å). The crystal of C MImNbF was
18
6
transferred into a quartz capillary (0.5 mm o.d., dried under a
vacuum at 500 °C) under a dry Ar atmosphere. The capillary
was tentatively plugged with vacuum grease and sealed using an
oxygen burner. Data collection was performed at −100, 25 and
55 °C. The measurements consisted of 12 ω scans (130–190°,
Table 3 Summary of crystal data and refinement results for C18MImAF
6
(A = P, As, Sb, Nb and Ta) at −100 °C
P
As
Sb
Nb
Ta
Formula
Fw
Crystal size/mm
T/°C
Crystal system
Space group
a/Å
C
22
H
43
N
2
F
6
P
C
22
H
43
N
2
F
6
As
C
22
H
43
N
2
F
6
Sb
C
22
H
43
N
2
F
6
Nb
22 43 2 6
C H N F Ta
630.53
0.43 × 0.13 × 0.04
−100
Monoclinic
480.55
524.50
571.34
542.49
0.80 × 0.30 × 0.05
−100
Monoclinic
0.70 × 0.20 × 0.05
−100
Monoclinic
0.75 × 0.30 × 0.03
−100
Monoclinic
0.70 × 0.30 × 0.05
−100
Monoclinic
P2
1
/a
P2
1
/a
P2
1
/a
P2
1
/a
1
P2 /a
9.1715(7)
9.8807(8)
28.520(2)
95.707(2)
2571.7(3)
4
9.1894(2)
9.9779(3)
28.5321(7)
96.0148(7)
2601.74(12)
4
9.2091(3)
10.1539(5)
28.5250(10)
96.2477(10)
2651.47(18)
4
9.1865(5)
10.1457(7)
28.4689(15)
96.1335(16)
2638.2(3)
4
9.2100(3)
10.1744(3)
28.4495(11)
96.1688(11)
2650.46(15)
4
b/Å
c/Å
β (°)
3
V/Å
Z
/g cm−
3
1.241
1.339
1.431
1.366
1.576
ρ
c
F(000)
μ/mm
1032
0.161
1104
1.360
1176
1.093
1136
0.509
1264
4.188
−
1
λ/Å
0.71073
22 634/5864
0.0465
0.71073
24 782/5884
0.0418
0.71073
25 221/6035
0.0262
0.71073
15 005/5954
0.0470
0.71073
11 776/3944
0.0349
Reflns collected/unique
a
R
1
(F
o
)
2
b
wR (F
2
o
)
0.1059
0.0981
0.0656
0.1125
0.1100
R
int
0.0556
0.0452
0.0294
0.0536
0.0402
a
b
2
o
2 2
2 2 1/2
R
1
= Σ∥F
o
| − |F
c
∥/Σ|F
o
| for I > 2σ(I). wR
2
= {Σ[w(F
c o
− F ) ]/Σ[w(F ) ]} for I > 2σ(I).
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3494–3502 | 3501

View Article Online
5
(
°/frame) at the fixed φ (30°) and χ (45°) angles and 32 ω scans
0–160°, 5°/frame) at the fixed φ (180°) and χ (45°) angles for
C MImPF , C MImAsF and C MImSbF and 20 ω scans
10 K. Goossens, P. Nockemann, K. Driesen, B. Goderis, C. Görller-Walrand,
K. Van Hecke, L. Van Meervelt, E. Pouzet, K. Binnemans and
T. Cardinaels, Chem. Mater., 2008, 20, 157–168.
11 C. J. Bowlas, D. W. Bruce and K. R. Seddon, Chem. Commun., 1996,
1625–1626.
1
8
6
18
6
18
6
(
130–190°, 3°/frame) at the fixed φ (30°) and χ (45°) angles and
4 ω scans (0–162°, 3°/frame) at the fixed φ (180°) and χ (45°)
angles for C MImNbF and C MImTaF . The exposure time
12 A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston, S. E. J. McMath
and M. Nieuwenhuyzen, Chem. Mater., 2002, 14, 629–635.
5
18
−1
6
18
6
13 J. D. Holbrey and K. R. Seddon, J. Chem. Soc., Dalton Trans., 1999,
was 300 s deg . Integration, scaling and absorption corrections
were performed using RAPID AUTO 2.40. The structure was
solved using SIR-92 and refined by SHELXL-97 linked to
Win-GX. Anisotropic displacement factors were introduced for
all atoms except for hydrogen. The crystallographic data and
refinement results of C MImAF are summarized in Table 3.
2133–2139.
14 C. M. Gordon, J. D. Holbrey, A. R. Kennedy and K. R. Seddon, J. Mater.
Chem., 1998, 8, 2627–2636.
28
29
30
15 J. De Roche, C. M. Gordon, C. T. Imrie, M. D. Ingram, A. R. Kennedy,
F. Lo Celso and A. Triolo, Chem. Mater., 2003, 15, 3089–3097.
16 W. M. Reichert, J. D. Holbrey, R. P. Swatloski, K. E. Gutowski,
A. E. Visser, M. Nieuwenhuyzen, K. R. Seddon and R. D. Rogers, Cryst.
Growth Des., 2007, 7, 1106–1114.
1
8
6
1
7 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordi-
nation Compounds Part A: Theory and Applications in Inorganic Chem-
istry, 5th Edn, Wiley Interscience, New York, 1997.
Acknowledgements
1
8 F. Xu, K. Matsumoto and R. Hagiwara, Chem.–Eur. J., 2010, 16, 12970–
12976.
9 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor.
Gen. Crystallogr., 1976, 32, 751–767.
0 C. K. Lee, H. H. Peng and I. J. B. Lin, Chem. Mater., 2004, 16, 530–536.
1 C. Hardacre, J. D. Holbrey, P. B. McCormac, S. E. J. McMath,
M. Nieuwenhuyzen and K. R. Seddon, J. Mater. Chem., 2001, 11, 346–
This work was financially supported by the Grant-in-Aid for
Scientific Research of Japan Society for the Promotion of
Science, # 20246140.
1
2
2
3
50.
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