New room-temperature ionic liquids with C -symmetrical imidazolium
2
cations†
Sergei V. Dzyuba and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, USA.
E-mail: richard.bartsch@ttu.edu; Fax: +1(806) 742 1289; Tel: +1(806) 742 3069
Received (in Corvallis, OR, USA) 16th May 2001, Accepted 18th June 2001
First published as an Advance Article on the web 23rd July 2001
New 1,3-dialkylimidazolium hexafluorophosphates with
two butyl, pentyl, octyl, nonyl or decyl groups are room
temperature ionic liquids.
Table 1 Structure and phase transition temperatures of 1,3-dialkyimidazo-
lium ionic liquids
Phase transition temperature
Room-temperature ionic liquids (RTILs),1 such as 1-alkyl-
IL
Alkyl group
T/°C (DC
/J g2 K21
1
)
m
T )
/°C (DH/kJ mol21
p
3-methylimidazolium hexafluorophosphates, are finding ever-
expanding applications as alternative reaction media for organic
1
2
3
4
5
6
7
8
9
CH
C H
3
—
—
—
89 (6.8)
70 (6.6)
43 (6.3)
—
synthesis2 and separation science.
,3
4–6
Although our under-
2
5
standing of why these salts have such low melting points is
incomplete, the unsymmetrical nature of the cation is believed
C
C
3
H
H
7
4
9
269 (0.3)
272 (0.2)
—
to play a major role.1 Thus, the 100 °C lower melting points of
,7
C H
—
5
11
13
15
17
19
C
C
C
C
C
6
7
8
9
H
H
H
H
73 (8.8)
47 (5.2)
19 (5.2)
11 (5.4)
1
-ethyl-3-methylimidazolium salts than their N-butylpyridin-
—
2
ium analogues have been attributed to the C symmetry of the
280 (0.2)
270 (0.2)
—
latter.1
By extension of such reasoning, 1,3-dialkylimidazolium salts
symmetry would seem to be poor candidates
1
0
10
H
21
227 (0.4)
that also have C
2
1
6 (6.7)
as RTILs. In agreement, 1,3-dialkylimidazolium hexafluoro-
phosphates are reported to be solids at room temperature when
8
the two alkyl groups are hexadecyl. The salts with elongated
(DSC)§ are presented in Table 1. As the alkyl groups are
changed from methyl to ethyl to propyl, the melting point drops
from 89 to 70 to 43 °C. This suggests poorer packing into the
crystal lattice as the alkyl group is elongated. With dibutyl and
dipentyl substituents, the salts are RTILs with glass transitions,
but no melting points. In sharp contrast when the alkyl groups
are elongated to hexyl, a melting point of 73 °C, but no glass
transition, is observed. There is a remarkable, greater than
140 °C difference between the phase transition temperatures for
alkyl groups are of interest because they exhibit liquid
crystalline behavior above their melting points.8
,9
We now report the synthesis of ten 1,3-dialkylimidazolium
hexafluorophosphates [(C ) -Im]PF with dialkyl moieties
n 2 6
ranging from dimethyl to didecyl and their phase transition
behavior. Surprisingly, 1,3-dialkylimidazolium hexafluoro-
phosphates with dibutyl, dipentyl, dioctyl, dinonyl, and didecyl
substituents are found to be RTILs.
The [(C
1
)
2
-Im]PF
6
was prepared by reaction of commercially
5 2 6 6 2 6
[(C ) -Im]PF and [(C ) -Im]PF . Apparently the hexyl groups
available 1-methylimidazole with iodomethane to form [(C
1
)
2
-
pack into the crystal lattice almost as well as methyl and ethyl
groups. As the alkyl groups are lengthened from hexyl to heptyl,
the 73 °C melting point for the former diminishes to 47 °C. For
octyl and nonyl groups, both glass transitions and melting points
are observed. When the alkyl groups are decyl, there are two
melting points. Since their melting points are below room
Im]I10 followed by its metathesis with potassium hexa-
fluorophosphate in water.‡ Since higher homologues of 1-me-
thylimidazole are not available commercially, a method was
sought to convert imidazole directly into a 1,3-dialkylimidazo-
lium halide. (Reported methods for the preparation of 1,3-dia-
lkylimidazolium halides require multiple steps, long reactions
temperature, [(C
Im]PF are also RTILs. Studies are currently underway to
compare the other physical properties of these readily accessible
8 2 6 9 2 6 2
) -Im]PF , [(C ) -Im]PF , and [(C10) -
times, and/or special precautions.8
,9,11,12
) We have found that
6
reaction of imidazole with one equivalent of sodium hydride in
THF followed by addition of two equivalents of a 1-bromoalk-
ane and refluxing (Scheme 1) gives high yields (75–89%) of
n 2 6
[(C ) -Im]PF RTILs with the more common 1-alkyl-3-methyl-
imidazolium hexafluorophosphates.
We thank the Texas Advanced Research Program for support
of this research under Grant No. 003644-0395-1999, Professor
Dominick J. Casadonte, Jr. for sharing the DSC equipment, and
Professor Sindee L. Simon for helpful discussions.
[
(C
bromide salts with potassium hexafluorophosphate in water
gave high yields (89–95%) of [(C -Im]PF with n
–10.†‡
The phase transition temperatures (mid-points of glass
transitions and/or melting points) for [(C -Im]PF with n =
–10 as determined by differential scanning calorimetry
n
)
2
-Im]Br with n = 2–10. Metathesis of the imidazolium
n
)
2
6
=
2
n
)
2
6
Notes and references
‡
1
Representative procedures for the preparation of [(C
n 2 n 2
) -Im]Br and [(C ) -
Im]PF . Synthesis of 1,3-dinonylimidazolium bromide. A flask containing a
6
magnetically stirred mixture of 95% sodium hydride powder (0.75 g, 29.4
mmol) in THF (20 ml) was cooled in an ice bath and a solution of imidazole
(2.00 g, 29.4 mmol) in THF (20 ml) was added dropwise. The ice bath was
removed and the mixture was stirred for 2 h at room temperature. Following
dropwise addition of 1-bromononane (11.23 ml, 58.8 mmol) at room
temperature, the mixture was refluxed for 7 h and filtered. The precipitate
was thoroughly rinsed with THF. The filtrate was evaporated in vacuo and
the residue was dissolved in dichloromethane. The mixture was filtered and
the filtrate was evaporated in vacuo. The residue was rinsed with diethyl
ether (20 ml) and dried in vacuo to give 10.40 g (82% yield) of the product
as a waxy-like solid. Synthesis of 1,3-dinonylimidazolium hexafluor-
Scheme 1 Synthesis of 1,3-dialkylimidazolium bromides and hexa-
fluorophosphates.
1
†
Electronic supplementary information (ESI) available: H NMR and IR
spectra and elemental analysis results consistent with the indicated
structures for all of the [(C -Im]Br and [(C -Im]PF compounds. See
n
)
2
n
)
2
6
1466
Chem. Commun., 2001, 1466–1467
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104512c
Dzyuba
