4
92
Can. J. Chem. Vol. 82, 2004
Table 1. Bromide content of trialykylsulfonium bromides.
puriss) was purified by double vacuum sublimation, and
trimethylsulfonium bromide was recrystallized twice with
anhydrous ethanol (9) and vacuum dried at ~40 °C for
Theoretical
Br (%)
Experimental
Br (%)
Deviation
(%)
Salt
2
days. HCl gas (Mattheson) was electronic grade and
Me EtSBr
46.70
43.16
40.13
46.78
43.18
40.18
0.17
0.05
0.12
2
ethene-free. Organic solvents were freshly distilled before
use.
Et MeSBr
2
Ethyldimethylsulfonium bromide (EtMe SBr), diethyl-
Et3SBr
2
methylsulfonium bromide (Et MeSBr), and triethylsulfonium
2
bromide (Et SBr) were synthesized according to Saunders,
Jr. and co-workers’ method (10), with acetonitrile as the sol-
vent and an alkyl halide : dialkyl sulfide ratio of 2. For
3
Fig. 1. Ambient temperature density of trialkylsulfonium ionic
liquids at different compositions (25 °C): ᭜ AlCl –Me SBr
3
3
2
2
(
R = 0.9905); ꢀ AlCl –Me EtSBr (R = 1.000); AlCl –
3
2
3
Et SBr specifically, a yield of 25% was obtained after
3
2
2
Et MeSBr (R = 0.9935); ᭹ AlCl –Et SBr (R = 0.9959).
2
3
3
refluxing for 3 days; crystals were washed with ethanol and
vacuum dried at 40 °C. The method of Islam and Leffek
(
11), employing excess sulfide, was unsuccessful.
1
-Butyl-3-methyl-1H-imidazolium chloride (bmim Cl)
was prepared under atmospheric pressure, as described by
Rogers and co-workers (12). The yield was 90%.
Triethylsulfonium tetrafluoroborate and hexafluorophos-
phate were prepared by procedures akin to those for the
+
bmim salts, but since they had melting points close to 80 °C,
they were not studied further.
Chloroaluminate ionic liquids were prepared by adding
the required amount of AlCl slowly with stirring to the solid
3
organic salt so that a colorless or pale yellow liquid formed.
Cyclic voltammetry was carried out with a BAS 100W
system. A diamond-polished Pt working electrode, a Pt
counter-electrode, and an Al – acidic chloroaluminate refer-
ence electrode were employed.
To reduce the proton level in such a liquid from the esti-
–
1
–1
mated 20 mmol L level to <1 mmol L , either AlEtCl or
2
CaH was added in a roughly equivalent amount; any small
2
excess of AlEtCl does not affect the Lewis acidity.
2
Halohydrogenate(I) liquids were obtained by introducing
HCl gas slowly through a needle valve into a preweighed,
sealed vial containing solid organic salt. The added HCl was
determined gravimetrically or by titration.
Results and discussion
Salt purity
The experimental (12) and theoretical bromide contents of
the 3 trialkylsulfonium bromides synthesized are shown in
Table 1.
Physical measurements
Melting points were obtained with an Electrothermal
A9100 series Digital Melting Point Apparatus. Densities of
1
Density
liquids were measured at 25 °C in the glovebox with 2 mL
or 5 mL flasks calibrated with twice-distilled water. Viscos-
ities were measured with a Cannon-Fenske viscometer 150
Figure 1 shows the density of trialkylsulfonium chloro-
aluminate ionic liquids at 25 °C at different AlCl contents.
3
The range of composition for which the systems are liquid at
25 °C increases with the total number of carbon atoms,
reaching 23 mole % AlCl for AlCl –Et SBr. An approxi-
mate phase diagram for AlCl –Et SBr shows a minimum at
(46% AlCl , 10 °C) and a maximum at (50% AlCl , 40 °C)
such that while a basic liquid is obtainable at 25 °C, a neu-
tral one is a jelly. As the number of C atoms increases at
constant AlCl mole %, the density decreases linearly (R =
0.9974), a trend noted for other ionic liquid groups (13).
(
constant of 0.04591 at 40 °C, 0.04570 at 100 °C). It was
calibrated by the overlapping technique with aqueous glyc-
erol solutions. Measurements were made in a thermostated
paraffin oil bath in the glovebox, taking 10 min for equili-
bration and at least 120 s of flow time. Conductances were
measured with a YSI Model 31 Conductivity Bridge (Yellow
3
3
3
3
3
3
3
–
1
2
Spring Instrument Co.), using a commercial (1.0 cm cell
3
constant) or home-made cell (5.28 cm–1 cell constant) and
0
.1 daL aqueous KCl at 25 °C as calibrant.
When the densities of several 64% AlCl ionic liquids are
3
Fourier transform infrared (FTIR) spectra were obtained
compared, it is apparent that the trend is not a function of
cation size but rather a question of packing (15), as follows:
–
1
with a PerkinElmer instrument (range 400–2500 cm ) and
KBr salt plates. Provided the plates were dry and the ionic
liquid was placed between them in the glovebox, the film
was sufficiently air stable.
–
1
–1
bmim Cl (1.3229 g L ) (5) < emim Cl (1.3747 g L ) (1) <
–
1
–1
Et SBr (1.4607 g L ) < PyHCl (1.4759 g L ) < Me SBr
(1.5849 g L ) (9).
3
3
–
1
NMR spectra were obtained with a Bruker AC-200 MHz
spectrometer, using DMSO-d6, D O, CDCl3, and
Molarities
2
CD COCD as locking solvents; they were sealed into capil-
Although it has been customary to express the acidity of
3
3
laries and inserted into the 5 mm NMR tubes, which were
loaded with ionic liquid samples and capped in the glovebox
prior to measurements.
haloaluminate ionic liquids in terms of the AlX mole frac-
tion, the molarity of the various species present in the sys-
tem is determined by the density and average molecular
3
©
2004 NRC Canada