The influence of hydrogen-bond defects on the properties of ionic liquids

Article abstract of DOI:10.1002/anie.201100199

Counterintuitive: The preformation of ion pairs can explain the low melting points of imidazolium-based ionic liquids (ILs) and the resulting expanded range of working temperatures. This quasi-ion-pair formation is possible for ILs having cations with only one interaction site leading to local and directional hydrogen bonds with the corresponding anion (see structure; O red, N blue, F green, S yellow). Copyright

Full text of DOI:10.1002/anie.201100199

DOI: 10.1002/anie.201100199
Ionic Liquids
The Influence of Hydrogen-Bond Defects on the Properties of Ionic
Liquids**
Tim Peppel, Christian Roth, Koichi Fumino, Dietmar Paschek, Martin Kꢀckerling,* and
Ralf Ludwig*
Ionic liquids (ILs) are salts with uncommonly low melting
points that are formed by a combination of specific cations
and anions; they display distinctive properties and can be
This idea is also the basis of most of the structure–
property relations discussed in the literature including
quantitative structure–property relationships (QSPR) meth-
ods to correlate the melting points of ILs based on “molecular
descriptors” derived from quantum chemical calcula-
[1,2]
used in a variety of applications.
The working temperature
range of an ionic liquid is set by the melting point and the
boiling or decomposition point. In particular, the melting
[
17–21]
tions.
Such empirical correlations suffer from the fact
point (T ) varies substantially between different ILs for
that large experimental data sets are required and that the
statistical methods used are rather complex. In addition, no
interpretation of these fundamental physical properties at the
molecular level is provided. Krossing et al. have developed a
simple predictive framework to calculate the melting point of
a given ionic liquid based on lattice and solvation free
m
reasons presently not fully understood, but which we explore
[
3–9]
herein.
We show that the melting points of imidazolium
ionic liquids can be decreased by about 100 K if an extended
ionic and hydrogen-bond network is disrupted by localized
interactions, which can also be hydrogen bonds.
[
22]
Evidence for the presence of ion–ion interactions through
hydrogen bonds was reported by Dymek et al., Avent et al.,
energies. They showed that ILs are liquid under standard
ambient conditions because the liquid state is thermodynami-
cally favorable, owing to the large size and conformational
flexibility of the ions involved. This leads to small lattice
enthalpies and large entropy changes that favor the liquid
state. For such studies substituted imidazolium, pyrrolidi-
nium, pyridinium, and ammonium cations have been used
along with fluorometalate, triflate, and bis(trifluoromethyl-
sulfonyl)imide anions. Unfortunately, Krossingꢀs results do
not correlate with experimentally obtained melting points for
[
10–12]
and Elaiwi et al. some time ago.
It is reasonable to
assume that the interesting features of the melting points must
be related to the formation of structures in the solid and the
liquid phases of the ILs. Extended hydrogen-bond networks
in the liquid phase were reported with possible implications
[
13,14]
for both the structure and solvent properties of the ILs.
Dupont et al. described pure imidazolium ILs as hydrogen-
[
15]
bonded polymeric supramolecules.
Antonietti et al. sug-
[
23]
gested that these supramolecular solvent structures represent
an interesting molecular basis of molecular recognition and
protic ionic liquids (PILs) reported by Markusson et al. The
reason for the large deviations of the predicted from the
experimental melting points is probably related to the general
[16]
self-organization processes.
However, in all of these
examples it is suggested that hydrogen bonds strengthen the
structure of ILs leading to properties similar to those of
molecular liquids.
trend of increasing T with the increasing size of the anions.
m
We do not intend to present another framework for
predicting ionic liquid properties here. Instead we want to
demonstrate that in addition to the large size and conforma-
tional flexibility of the ions, local defects such as directional
hydrogen bonds can significantly decrease the melting points
of ionic liquids. For eight imidazolium-based ionic liquids we
show that these defects can increase their working temper-
ature range by up to 100 K and thus expand the spectrum of
potential applications. This was suggested previously by
Fumino et al. based on spectroscopic measurements and
DFT calculations on IL aggregates. They assumed that local
and directional types of interactions present defects in the
[*] Dipl.-Chem. C. Roth, Dr. K. Fumino, Dr. D. Paschek,
Prof. Dr. R. Ludwig
Universitꢀt Rostock, Institut fꢁr Chemie
Abteilung fꢁr Physikalische Chemie
Dr. Lorenz Weg 1, 18059 Rostock (Germany)
Fax: (+49)381-498-6517
E-mail: ralf.ludwig@uni-rostock.de
Prof. Dr. R. Ludwig
Leibniz-Institut fꢁr Katalyse, Universitꢀt Rostock e.V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Coulomb system which may lower the melting points,
[8,9]
viscosities, and enthalpies of vaporization.
In contrast,
Dr. T. Peppel, Prof. Dr. M. Kꢂckerling
Universitꢀt Rostock, Institut fꢁr Chemie
Abteilung fꢁr Anorganische Chemie
Albert Einstein Strasse 3a, 18059 Rostock (Germany)
Fax: (+49)381-498-6390
based on quantum chemical calculations, Hunt claimed that
an increase in the melting points and viscosities upon
[5,6]
methylation at C(2) stem from reduced entropy.
Noack
et al. showed very recently that neither the “defect hypoth-
esis” of Fumino et al. nor the “entropy hypothesis” of Hunt
alone can explain the changes in the physicochemical proper-
E-mail: martin.koeckerling@uni-rostock.de
[
**] This work was supported by the DFG priority programme 1191
[
7]
“
Ionic Liquids” and by the Sonderforschungsbereich SFB 652.
ties. However, in all these studies the data base was not
sufficiently large and other effects such as volume changes
could not be excluded for the ILs under investigation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100199.
Angew. Chem. Int. Ed. 2011, 50, 6661 –6665
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6661

Communications
To exclude effects from the different anions, we chose a
Table 1: Melting points T
m
of the ILs I–VIII.
set of eight ILs, all with the same bis(trifluoromethylsulfony-
l)imide anion. Only the imidazolium cations are different:
imidazolium (I), 1-monomethylimidazolium (II), 1,3-dime-
thylimidazolium (III), 1,2-dimethylimidazolium (IV), 2,4,5-
trimethylimidazolium (V), 1,2,3,-trimethylimidazolium (VI),
IL
Tm [8C]
Ref.
Tm [8C]
this work
I
II
73
9
[30]
[29]
[3,4,24–26]
69
52
–
III
IV
V
VI
VII
VIII
22, 23, 26
22
–
–
–
1
,2,4,5-tetramethylimidazolium (VII), and 1,2,3,4,5-pentam-
[29]
–
ethylimidazolium (VIII) (see Scheme 1).
57
106
40
118
118
[27,28]
Scheme 1. The imidazolium-based cations of the NTf salts I–VIII. The
different positions of the hydrogen bonds are indicated by bold dots
and the remaining interaction sites are marked with dotted lines.
2
À
+
Figure 1. Plot of Tm versus volume (A +C ) for the ionic liquids I–
VIII, which include various imidazolium cations but always the same
À
bis(trifluoromethylsulfonyl)imide anion (NTf2 ). As indicated by the
dotted lines, there is an increase in Tm with increasing volume for the
ILs that have no specific interaction site (*), with one interaction site
(
&), and with two interaction sites (^). Overall it is shown that the
presence of a single interaction site on the cation leads to a significant
To minimize conformational flexibility of the cation only
methyl groups and hydrogen atoms, but no extended alkyl
chains, were taken into consideration as groups at the
imidazolium ring. For a better understanding of our concept
possible interaction sites of the imidazolium cations through
CÀH groups are marked with bold dots and dotted lines in the
decrease in T , as found for ILs III, IV, and VII. The filled symbols
m
indicate further Tm values from the literature.
As shown in Figure 1 there is no linear trend for Tm as a
function of volume. The highest melting points are found for
the species I and VIII, in which neither cation, imidazolium
and 1,2,3,4,5-pentamethylimidazolium, have specific interac-
tion sites. In principle, IL I has five interaction sites through
CÀH or NÀH bonds as indicated by the black dotted lines in
structures in Scheme 1. The bold dots indicate local and
directional interactions through C(2)ÀH and/or NÀH groups
that are of importance for the structure and properties of ILs.
It had been shown earlier by experiments and theory that
interactions with C(4)ÀH and C(5)ÀH are significantly
weaker than interactions with C(2)ÀH. These negligible
Scheme 1, but no single interaction site is preferred over
others and thus the interactions are not specific. The melting
point increases with increasing volume as indicated by the
interaction sites are indicated as dotted lines.
The given T values for III, IV, and VIII are taken from
m
[
3,4,24–28]
the literature including our own work.
However, for
dashed line (Figure 1). It is observed that the T value of VI
m
the IL II a surprisingly low melting point of 98C had been
reported. Thus, we recrystallized this IL purchased from
IoLiTec GmbH and determined the melting point by DSC
also lies on that line, although the 1,2,3-trimethylimidazolium
cation can in principle interact with the anion through the
C(4)ÀH and C(5)ÀH groups. However, in earlier studies these
[
29]
(
differential scanning calorimetry) to be 528C. For ILVI, also
purchased from IoLiTec GmbH, the Tm was measured to be
068C. ILs I, V, and VII were synthesized and the T values
interactions were noted to be weak compared to those of the
[
31,32]
C(2)ÀH group, which is strongly acidic.
1
The largest decrease in T of about 100 K can be observed
m
m
[
30]
measured to be 698C (compared to 738C in the literature ),
78C, and 408C, respectively. We plotted the melting points
for ILs I–VIII (listed in Table 1) versus the ab initio computed
for the ILs III, IV, and VII. They are distinguished by single
interaction sites such as C(2)ÀH in III and NÀH in IVand VII.
5
The local and directional interaction through hydrogen
bonding leads to a preformation of ion pairs resulting in
lower “lattice energies” relative to the energy of isolated ion
pairs. In other words: the disruption of the hydrogen-bond
network increases the quasi-molecular character of the pure
À
+
volumes (A + C ) of the anions and cations (Figure 1). In
earlier work Krossing et al. and Markusson et al. showed that
the calculated volumes are in good agreement with those
[
22,23]
obtained from crystal structure or density measurements.
6
662 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 6661 –6665

ILs. This is similar to Dupontꢀs suggestion for the addition of
other molecules and macromolecules to ILs. He stated that
the disruption of the hydrogen-bond network can generate
[13,15]
nanostructures with polar and nonpolar regimes.
Our hypothesis of preformed ion pairs is supported by ab
initio calculations of the energies of IL aggregates. When we
subtracted four times the energy of the isolated ion pairs from
the tetramer energies for each IL, we obtained larger
interaction contributions for the ILs with high Tm values
and significantly smaller ones for the ILs with low T values;
m
this can be explained by preformation of ion pairs. The simple
result is that more strongly bound ion pairs give smaller
“
lattice energies” resulting in low T values. Again, all the ILs
m
Figure 2. a) Tm values plotted versus calculated interaction energies
per ion pair for tetramers of the ionic liquids I–VIII. b) The same
interaction energies are given for the number of interaction sites in the
corresponding imidazolium cations. Both plots show that Tm depends
on the interaction energies between preformed ion pairs in the lattice.
If most of the overall interaction energy is included in the cation–
anion interaction of the preformed ion pair, which is best obtained
with one interaction site, the melting point is low. The filled symbols
indicate further Tm values from the literature.
having one interaction site give Tm values that follow a
straight line with increasing volume in the order III, IV, and
VII. This line has about the same slope as the plot of the Tm
values of I, VI, and VIII (see Figure 1). Here, it is interesting
to note that the similar molecular volumes of III and IV result
in similar melting points. Both cations also allow interactions
through C(4)ÀH and C(5)ÀH, which apparently have no
effect on the T , in agreement with the finding for VI.
m
Following the argument that single, local and directional
interactions can lead to low melting points, the ILs II and V
have Tm values between those of the other two groups (^).
given imidazolium cation. It can be seen clearly that the
À1
These compounds have two interaction sites, C(2)ÀH and NÀ interaction energies range from + 2 to À6 kJmol for cations
À1
H for II and NÀH and NÀH for V. Local and directional
with one interaction site, from À10 to À19 kJmol for
species having two interaction sites, and from À18 to
interactions are possible, leading to T values lower than
m
À1
those of ILs I, VI, and VIII. However, more than one
interaction site hinders the preformation of ion pairs and
increases the “lattice energy” towards extended hydrogen-
À34 kJmol for cations with no interaction sites. When the
preformation of ion pairs is possible as in ILs III, IV, and VII,
the lowest melting points are found (Figure 3).
[
13–16]
bond networks.
As the interaction energy between
Further evidence for this scenario can be derived from X-
À
“
unsaturated” ion pairs increases, higher Tm values result
ray single-crystal structures. For the ILVIII the anion NTf2 is
than those for III, IV, and VII. This finding is also supported
by ab initio calculations (see Table 2, Figure 2).
found in conformations in which the two SCF groups are
3
[
28]
trans to one another. For the IL III the cis conformation is
[
3]
In Figure 2a the melting points are plotted versus
interaction energies per ion pair defined as E = (E À4E )/
dominant. Holbrey et al. conjectured that this behavior is
caused by structural constraints of extensive CÀH···O and NÀ
ip
4
1
4
. Four times the calculated energies of isolated ion pairs are
H···O cation–anion hydrogen bonding between neighboring
[3]
subtracted from the corresponding tetramer energies. It is
observed that the interaction energies increase almost linearly
with increasing melting points. The larger scatter for the high-
melting-point ILs (*) may result from not considering the
different volumes of the species. A better representation of
our concept is shown in Figure 2b, where these interaction
energies are referred to the number of interaction sites at the
molecules. The cis configuration, which is energetically
Table 2: Ab initio calculated total energies (in Hartrees) for tetramers
(
E ) and monomers (E ) of ion pairs of the ILs I–VIII. In addition, the
4
1
À1
energies per ion and per ion pair are given in kJmol
.
IL
Tetramer
E4 [Hartree]
Monomer
E1 [Hartree]
Energy
per ion
Energy per
ion pair
À1
À1
[kJmol
]
[kJmol
]
I
II
À8141.850660
À8297.077549
À8452.315770
À8452.395913
À8607.768535
À8607.612327
À8763.014030
À8918.206619
À2035.449504
À2074.265686
À2113.080019
À2113.097402
À2151.934921
À2151.896047
À2190.751088
À2229.540757
À213.43
À192.21
À175.97
À185.08
À191.37
À164.84
À180.69
À155.73
À34.55
À9.72
III
IV
V
VI
VII
VIII
+2.83
À4.14
À18.94
À18.47
À6.35
Figure 3. Calculated tetramer of the ionic liquid (1,2,4,5-MIm)NTf2
VII). The hydrogen bonds are indicated by the dotted lines. The single
(
interaction site NH of each cation leads to a preformation of ion pairs
in this IL.
À58.61
Angew. Chem. Int. Ed. 2011, 50, 6661 –6665
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6663

Communications
disfavored in the gas-phase calculations, indicates local and
melting points were determined by DSC. Suitable single crystals for
X-ray crystallography of II were obtained by drying liquid II in high
directional interactions between cation and anion. The
finding that both SO2 groups point towards the cation
supports our hypothesis of preformed ion pairs, which is
also supported by the order of energies per ion pair in
À5
[33, 34]
vaccum (10 mbar) at ambient temperature.
The energies of monomeric and tetrameric ion pairs for all ionic
liquids I–VIII were calculated at the Hartree–Fock level, using the
[35]
internal stored 3-21G basis set of the Gaussian03 program. The
[
33]
Figure 2. The structure of II (this work ) is made up of two
independent anions and cations, with both anions having the
trans conformation. Nevertheless, the anions show strong
binding energies per ion and per ion pair were corrected for the basis
[
36]
set superposition error (BSSE). The geometries of the monomers
and tetramers can be obtained from the authors upon request.
(
short) hydrogen bonds with two such contacts to only one
Received: January 10, 2011
Revised: March 18, 2011
Published online: May 31, 2011
cation (see Figure 4). This indicates the preformation of ion
pairs, which lowers the melting point.
[4–9]
Keywords: ab initio calculations · crystal structures ·
.
hydrogen bonding · ionic liquids · melting points
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