COMMUNICATION
Alkyl vs. alkoxy chains on ionic liquid cations{
Wesley A. Henderson,a Victor G. Young, Jr.,b Douglas M. Fox,a Hugh C. De Longc and Paul C. Trulove*a
Received (in Berkeley, CA, USA) 4th May 2006, Accepted 30th June 2006
First published as an Advance Article on the web 3rd August 2006
DOI: 10.1039/b606381k
The crystal structures and thermal behavior of the 1-(2-
methoxyethyl)-2,3-dimethylimidazolium chloride and hexa-
fluorophosphate salts are compared with the analogous
1-butyl-2,3-dimethylimidazolium salts to examine the influence
of the ether oxygen on salt thermal properties for a typical
constituent cation used in the preparation of ionic liquids.
Table 1 Crystallographic data of 1–4
16 2a
37
4a
Formula
Formula weight 188.69
T/K 213(1)
Wavelength/A 0.71073
C9H17ClN2 C8H15ClN2O C9H17F6N2P C8H15F6N2OP
190.67
173(2)
0.71073
298.22
213(1)
0.71073
Monoclinic Triclinic
P-1
300.19
173(2)
0.71073
˚
Crystal system Monoclinic Triclinic
Space group
P21/n
8.380(2)
P-1 Cc
7.3748(13) 15.389(3)
Ionic liquids (ILs)—salts or mixtures of salts which melt below
373 K (arbitrarily set at the boiling point of water)—are a topic of
rapidly expanding research since these materials often possess
unique combinations of physical properties not available in
molecular solvents.1,2 Frequently, ILs are referred to as ‘tuneable’,
‘tailored’, ‘task-specific’ or ‘designer’ solvents due to the ability to
vary the ions, thereby modifying and optimizing a salt’s physical
properties for specific applications.3–5 At present, this optimization
is greatly hindered by a lack of knowledge regarding how ion
structure affects salt properties. In particular, the addition of
functional groups (e.g., ether oxygens) to the ions may dramati-
cally change a salt’s thermal behavior.6–9 Here we compare the
a/s
8.3405(13)
8.7297(14)
10.4831(16)
67.755(2)
69.073(2)
62.821(2)
612.52(17)
2
1.628
0.291
6573
2665
b/s
11.710(2) 7.5274(13) 8.604(2)
10.760(2) 10.1293(18) 11.730(2)
90
90.74(3)
90
c/s
a/u
82.170(3)
76.176(3)
65.881(3)
90
122.46(5)
90
b/u
c/u
Volume/s3
1055.8(2) 497.88(15) 1310.4(5)
2
Z
r
4
1.187
4
1.512
calc/g cm23
1.272
0.342
5689
2177
0.0172
0.0296
0.0841
m/mm21
Reflns coll.
Reflns unique
Rint
0.0194
0.0373
0.1057
R1b
wR2b
1-butyl-2,3-dimethylimidazolium salts of Cl2 and PF6 with the
2
a
CCDC 606677 and 606678. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b606381k. I . 2s(I).
b
analogous salts of 1-(2-methoxyethyl)-2,3-dimethylimidazolium
cations:
DSC heating traces of the four salts are shown in Fig. 1. The
melting point (m.p.) of 1 is reported to be 370–377 K,10–13 in
agreement with the DSC data (Table 2). 2 exhibits a solid–solid
phase transition that may correspond to the onset of ion
disordering prior to the m.p. The m.p. of 3 is reported to be
313 K,11 also in agreement with the DSC data (Table 2). However,
several authors have previously reported the absence of a m.p. for
3, perhaps due to slow kinetics hindering crystallization (see
ESI{).13,14 It is interesting to note that the m.p.s of the salts
containing methoxyethyl groups are higher than those containing
butyl groups, and that the melting transition for 2 occurs slowly
over a wide temperature range (Fig. 1).
An ether oxygen, with its two electron lone-pairs, is comparable
in size (slightly larger) and shape to a methylene group, suggesting
that the two groups might have similar steric interactions with
neighboring atoms. However, unlike the methylene group, the
ether oxygen will exert repulsive interactions with neighboring
anions and ether oxygens. Thus, a study of salts 1–4{ represents an
excellent opportunity to examine how minor structural changes
affect ion crystal packing and how this, in turn, affects a salt’s
thermal properties. The crystal structures of 1 and 3 have been
previously reported.10,11 The crystal structures of 2 and 4 have
been determined and are reported here for comparison (Table 1).
aU. S. Naval Academy, Department of Chemistry, 572 M Holloway
Road, Annapolis, Maryland 21402, USA. E-mail: trulove@usna.edu;
Fax: (410) 293-2218; Tel: (410) 293-6641
bX-Ray Crystallographic Laboratory, Department of Chemistry,
University of Minnesota, S146C Kolthoff Hall, 207 Pleasant Street SE,
Minneapolis, Minnesota 55455, USA
cAir Force Office of Scientific Research, 875 N. Randolph St., Arlington,
Virginia 22203, USA
{ Electronic supplementary information (ESI) available: NMR spectra,
structure determination information, DSC traces and schematic of PF6
disorder in 4. See DOI: 10.1039/b606381k
2
Fig. 1 DSC heating traces (5 K min21) of (a) 1, (b) 2, (c) 3 and (d) 4.
This journal is ß The Royal Society of Chemistry 2006
3708 | Chem. Commun., 2006, 3708–3710