Tunable Aryl Alkyl Ionic Liquids with Weakly Coordinating Tetrakis((1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)borate [B(hfip)4] Anions

Article abstract of DOI:10.1002/chem.201601063

Weakly coordinating borate or aluminate anions have recently been shown to yield interesting properties of the resulting ionic liquids (ILs). The same is true for large phenyl-substituted imidazolium cations, which can be tuned by the choice, position, or

Full text of DOI:10.1002/chem.201601063

DOI: 10.1002/chem.201601063
Full Paper
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Ionic Liquids
Tunable Aryl Alkyl Ionic Liquids with Weakly Coordinating
Tetrakis((1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)borate [B(hfip)4]
Anions
Maria Kaliner,[a] Alexander Rupp,[b] Ingo Krossing,*[b] and Thomas Strassner*[a]
Abstract: Weakly coordinating borate or aluminate anions
have recently been shown to yield interesting properties of
the resulting ionic liquids (ILs). The same is true for large
phenyl-substituted imidazolium cations, which can be tuned
by the choice, position, or number of substituents on the
aromatic ring. We were therefore interested to combine
these aryl alkyl imidazolium cations with the weakly coordi-
nating tetrakis((1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)borate
[B(hfip)4]ꢀ anions to study the physical properties and viscos-
ities of these ionic liquids. Despite the large size and high
molecular weight of these readily available ILs, they are
liquid at room temperature and show remarkably low glass
transition points and relatively high decomposition tempera-
tures.
Introduction
phenyl ring (and the length of the alkyl chain at the N-3 nitro-
gen atom of the imidazolium heterocycle), we can change the
electronic properties of the resulting ionic liquids through ad-
ditional mesomeric and steric effects, which can easily be seen
from the melting points.[9,10] By changing the core of the new
ionic liquids from imidazolium to triazolium, it was demon-
strated that not only the phenyl ring plays an important
role.[11]
Ionic liquids (ILs) are of interest for many different applications
as they combine various interesting physical properties. These
conductive salts, with melting points below 1008C (by defini-
tion), contain only organic cations together with organic or in-
organic anions. ILs with even lower melting points (below
258C) are called room temperature ionic liquids (RTILs).[1] The
individually sought after properties of ILs such as thermal sta-
bility, nonvolatility, melting point, acidity, or viscosity vary de-
pending on the combination of anions and cations used. ILs
have, for example, been used as solvents in organic synthesis
or catalysis,[1,2] for the extraction of metals,[3] as electrolytes in
batteries,[4] as additives in dye-sensitized cells,[5] to stabilize
nanoparticles[6] or even for the dissolution of cellulose.[7] Be-
cause of the large number of possible ILs, tools are under de-
velopment to predict some of the properties/physical data so
as to find ILs suitable for specific applications.[8]
It is widely known that the anions have a strong influence
on the properties of ionic liquids.[1,12] Exchanging the bromide
for the bis(trifluoromethylsulfonyl)imide ([NTf2]ꢀ) counterion
(cf.: m.p. [PhMesC3Im]Br: 2058C vs. [PhMesC3Im]NTf2: 278C) led to
a significant temperature drop of almost 1808C for this new
kind of ILs.[9a] The asymmetric [NTf2]ꢀ anion generally shows
a much weaker coordination to the cation in comparison to
the halides and has more available conformations and thus
leads to lower melting points.[8c,13] Therefore, we were interest-
ed in other weakly coordinating counterions and started to in-
vestigate their physical properties and to look for new suitable
applications for the TAAILs. Currently under investigation are
the extraction behavior of the 1-aryl 3-alkyl imidazolium NTf2
ILs as well as the synthesis of new nanoparticles near room
temperature.[14]
In the last few years a new generation of imidazolium-based
ionic liquids, called TAAILs (tunable alkyl aryl ionic liquids),[9]
has been developed, which can be distinguished from the
standard ionic liquids by the (substituted) phenyl ring at the
N-1 nitrogen atom of the imidazolium heterocycle. Depending
on the type, number, and position of substituent(s) at the
During the last decade, it was found through crystal-struc-
ture determinations that the previously so-called “noncoordi-
nating anions”, such as tetrafluoroborate [BF4]ꢀ, hexafluoro-
phosphate [PF6]ꢀ, or tetrahaloaluminate [Al(Hal)4]ꢀ do coordi-
nate and therefore larger and even more weakly coordinating
new anions were synthesized.[15] This led to the development
of various new anions, for example the tetrakis((1,1,1,3,3,3-hex-
afluoropropan-2-yl)oxy)aluminate [Al(hfip)4]ꢀ anion, which is
known to stabilize reactive cations.[16] Recent reports from the
Krossing group on their new anions[12,15a,17] raised the question
whether the combination of new cations with new anions will
[a] M. Kaliner, Prof. Dr. T. Strassner
Physikalische Organische Chemie, Technische Universitꢀt Dresden
01169 Dresden (Germany)
E-mail: thomas.strassner@chemie.tu-dresden.de
[b] Dr. A. Rupp, Prof. Dr. I. Krossing
Institut fꢁr Anorganische und Analytische Chemie und
Freiburger Materialforschungszentrum FMF
Albert-Ludwigs-Universitꢀt Freiburg, 79104 Freiburg im Breisgau (Germany)
E-mail: krossing@uni-freiburg.de
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201601063.
ꢀ
lead to ILs with specific, interesting properties. As the AlR4
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anions turned out to be very moisture sensitive,[17a] we focused
on the newer fluorinated tetrakis((1,1,1,3,3,3-hexafluoropropan-
2-yl)oxy)borate [B(hfip)4]ꢀ anions. In this paper we present the
results of the new ionic liquids with 1-aryl 3-alkyl imidazolium
cations and the weakly coordinating [B(hfip)4]ꢀ anion as shown
in Figure 1.
16–39 (Scheme 1). First, a nucleophilic substitution reaction
with different alkyl halides led to the imidazolium salts 4–15
by dissolving the imidazoles 1–3 in THF, addition of the alkyl
halides, and heating in a pressure tube from room temperature
to 808C for several days.[9a] A significant difference in the reac-
tion rates could be observed, with 1-aryl 3-methyl imidazolium
halide salts already forming at room temperature. Aryl imida-
zolium salts with longer alkyl chain lengths needed higher
temperatures to form. For the purification of the materials, it
was necessary to use different techniques. For some halide
salts, column chromatography was used, and in other cases,
the solid salts could be washed with immiscible solvents to
remove impurities, providing yields between 56–98%.
By anion metathesis, ionic liquids with [NTf2]ꢀ 16–27 and
[B(hfip)4]ꢀ 28–39 counterions (Scheme 1) were synthesized. No
influence of the halide salt used (bromide or iodide) in the
anion exchange reaction could be observed. For the ILs with
an [NTf2]ꢀ anion,[9a] the imidazolium halide salts 4–15 were dis-
solved in methanol and Li[NTf2] in water was added, immedi-
ately forming a two-phase system. After stirring the mixture at
room temperature for 12 h, the organic IL-containing phase
was extracted and washed with water. The solvent was re-
moved, and yields of over 90% could be obtained. ILs contain-
ing the [B(hfip)4]ꢀ counterion 28–39 were synthesized by dis-
solving the aryl imidazolium halide salts 4–15 in dichlorome-
thane, adding equimolar amounts of Na[B(hfip)4]*DME (DME.
1,2-dimethoxyethane), and stirring the solution at room tem-
perature for 24 h. After the reaction was complete, the precipi-
tate was filtered off and a clear filtrate could be obtained. The
solvent was removed and the ILs were isolated in yields be-
tween 81–97%.
Figure 1. General structure of the investigated borate ILs.
Results and Discussion
Synthesis
The imidazolium salts with borate [B(hfip)4]ꢀ anions were pre-
pared using the sodium borate Na[B(hfip)4],[17a] as reported by
Krossing et al. For comparison, the corresponding ILs with one
of the most commonly used anions, [NTf2]ꢀ, were also synthe-
sized.
For the cationic part, we chose phenyl alkyl imidazolium cat-
ions with various alkyl chain lengths (short, medium, long) and
with substituents in different positions on the aryl ring, leading
to a total of 24 new compounds (Scheme 1). The electron-do-
nating substituents like 2-methyl (2-Me), 4-methoxy (4-OMe)
and mesityl (2,4,6-Me, Mes) have a significant influence on the
electronic properties of the aryl ring. The inductive effect of
the methyl group in the 2-position is very different compared
to the mesomeric effect of the methoxy substituent, which in-
teracts via the p-system. The mesityl group allows us to addi-
tionally consider the steric influence, as the rings are orthogo-
nal to each other according to solid-state structures.[18]
The one-pot synthesis starting from the commercially avail-
able anilines led to the imidazoles 1–3[9a] and only two more
reaction steps were needed to synthesize the different TAAILs
Thermal behavior
After synthesis and basic characterization, differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA) curves
as well as temperature-dependent viscosities of the ILs were
measured. The results are shown in Table 1, together with the
data of known ILs for comparison.
Scheme 1. Synthesis of the reported ionic liquids: i) alkyl halide, THF, RT–808C, 2–5 d; ii) Li[NTf2], MeOH/H2O/DCM, RT, 12 h; iii) Na[B(hfip)4]*DME, DCM, RT,
24 h.
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Table 1. Overview of collected physicochemical data of the synthesized ILs. R1, R2 and anions are defined according to Figure 1. Vm+ =molecular volume
of the cation, Tm =melting temperature, Tgh =glass transition temperature upon heating, Tgc =glass transition temperature upon cooling, Td =decomposi-
tion temperature, h=viscosity. Volumes of [B(hfip)4]ꢀ and [NTf2]ꢀ are 0.559 and 0.233 nm3, respectively.[8a]
+
IL acronym
R2
R1
Anion
Vm [nm3]
Tm [8C]
Tgh [8C]
Tgc [8C]
Td [8C]
h (258C) [mPa -s]
28
32
36
16
20
24
29
33
37
17
21
25
30
34
38
18
22
26
31
35
39
19
23
27
Me (n=0)
Me
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
2-Me
4-MeO
Mes
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[NTf2]ꢀ
0.228
0.238
0.277
0.228
0.238
0.277
0.291
0.310
0.349
0.291
0.310
0.349
0.347
0.357
0.396
0.347
0.357
0.396
0.467
0.476
0.518
0.467
0.476
0.518
no thermal effects measured
no thermal effects measured
205
210
198/221
431
427
–
179
188/368
192
402
396
–
192
199/382
192
400
206/424
–
192/372
188/368
206/385
411
400
–
140[a]
401
445
–
213
308
456
242
328
–
204
276
403
143
411
–
Me
Me
Me
Me
ꢀ53
ꢀ54
ꢀ48
–
ꢀ60
ꢀ55
ꢀ53
ꢀ64
ꢀ53
–
–
ꢀ60
[NTf2]ꢀ
ꢀ50
[NTf2]ꢀ
71[9a]
–
–
nBu (n=3)
nBu
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[NTf2]ꢀ
ꢀ79[b]
–
–
–
ꢀ56
–
nBu
nBu
nBu
nBu
[NTf2]ꢀ
[NTf2]ꢀ
26[9a]
nHex (n=5)
nHex
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[NTf2]ꢀ
ꢀ62
ꢀ56
ꢀ56
–
ꢀ67
–
nHex
nHex
nHex
nHex
no thermal effects measured
[NTf2]ꢀ
[NTf2]ꢀ
ꢀ54
–
ꢀ60
40[9a]
–
ꢀ76
–
nUndec (n=10)
nUdec
nUndec
nUndec
nUndec
nUndec
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[B(hfip)4]ꢀ
[NTf2]ꢀ
ꢀ55
ꢀ62
ꢀ57
ꢀ55
ꢀ60
–
253
296
378
404
448
–
ꢀ59
ꢀ72
–
[NTf2]ꢀ
[NTf2]ꢀ
411
–
32[9a]
–
[a] Measured at 508C. [b] Crystallization temperature.
All ILs proved to be stable under the applied conditions in
TGA experiments (heating rate: 10 Kminꢀ1, N2 atmosphere) up
to temperatures between 179 and 4318C. In the case of six
salts, at least two degradation steps were observed, whereas
all other ILs showed a single degradation process, in which
a mass loss of typically 80–90% occurred. The presence of
both the methoxy residues and [B(hfip)4]ꢀ anions seems partic-
ularly inclined to induce two-step degradation events, with the
first decomposition step leading to a mass loss of approxi-
mately 48%, and the second event causing a mass loss be-
tween 10–35%. Hence, the first mass loss may be assigned to
the evolution of three equivalents of the volatile epoxide as
exemplarily shown for the first step in Scheme 2. This would
amount to a mass loss of 44–49%, depending on the total
molar mass of the respective IL.
the structure of the cation and the stability of the respective
IL. A comparison of [B(hfip)4]ꢀ and [NTf2]ꢀ ILs, however,
showed a generally higher thermal stability of the [NTf2]ꢀ ILs,
which typically lie 2008C above those of the [B(hfip)4]ꢀ ILs. Ad-
ditionally, the [NTf2]ꢀ ILs show similar thermal stabilities com-
pared to common ILs such as [C4MIm][NTf2] (Td =314–4228C)[20]
or [C4MIm][BF4] (Td =3618C).[20c] Interestingly, borate ILs with
nonfunctionalized imidazolium cations,[17a] or their correspond-
ing aluminate analogues,[13b,19] possess decomposition temper-
atures that are rather lower than the ILs reported here.
Altogether, the new ILs are sufficiently stable for most appli-
cations, although no long-term TGA measurements have yet
been performed. As is known from literature data,[21] results of
long-term measurements are of course more relevant for most
applications and can differ severely from short-term experi-
ments.
Analogous decomposition products have already been de-
tected in mass spectrometry experiments of [Al(ORF)4]ꢀ anions
and are also likely to form under thermal stress.[13b,19] Apart
from this effect, no clear correlation could be found between
Subsequent DSC measurements yielded typical results for
ionic liquids, that is, the tendency to form glasses instead of
true crystal lattices and broad hystereses of up to 178C. Ac-
cordingly, second-order transitions are the dominant pattern
seen in DSC thermograms. Furthermore, most of the ILs re-
mained liquid even at temperatures well below 08C, indepen
dent of the anion. Especially in the case of [B(hfip)4]ꢀ ILs, this
behavior is remarkable, as only one RTIL comprising of this
borate anion has been reported so far.[17a] Hence, the function-
alization of imidazolium cations indeed has a tremendously
beneficial effect on the overall properties.
Scheme 2. Epoxide-generating decomposition of the [B(hfip)4]ꢀ anion.
[Cat]=cation. Note that this reaction was only drawn in a unimolecular fash-
ion for the sake of clarity, but is likely to rather occur in a bi- or trimolecular
way.
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Viscosity measurements
Table 2. Parameters of the VFT fits according to Equation (1).
Viscosities were measured between 20 and 808C in the liquid
range of the ILs, and were fitted afterwards with a Vogel–
Fulcher–Tammann (VFT) approach [Equation (1)]. Graphic re
presentations of the data are given in Figures 2 and 3 and VFT
parameters are shown in Table 2. Data points of the measure-
ments are given in the Supporting Information. In the case of
32, no viscosity data could be collected due to its high melting
IL
h0 [mPa-s]
B [K]
960*
–
634
838
1171
832
1207
721
1556
1064
923
1073
1121
1861
1153
1077
1282
1063
1395
2167
T0 [K]
28
32
36
16
20
29
33
37
17
21
30
34
38
18
22
31
35
39
19
23
0.0419*
–
193[a]
–
0.1981
0.1119
0.0286
0.0830
0.0228
0.1672
0.0091
0.0496
0.0655
0.0448
0.0276
0.0015
0.0403
0.0448
0.0194
0.0428
0.0206
0.0013
226
196
177
192
171
207
145
177
183
175
181
135
173
173
165
181
157
129
[a] This sample could only be measured during cooling (cf. Supporting In-
formation).
Figure 2. Temperature-dependent viscosities of the investigated [B(hfip)4]ꢀ
ILs.
Room-temperature viscosities of both [B(hfip)4]ꢀ and [NTf2]ꢀ
ILs lie roughly between 100 and 500 mPa-s, and thus are quite
high compared to common, low-viscous ILs such as [C2MIm]
[NTf2] or [C2MIm][N(CN)2].[17c,22] However, their viscosities are
similar to those of other borate ILs with nonfunctionalized cati-
ons.[17a] The Vogel temperatures lie within ranges that are typi-
cal for most ILs, whereas the h0 values are generally approxi-
mately one order of magnitude lower than in common [NTf2]ꢀ
ILs.[23]
No clear general trends in viscosity with respect to the
choice of the cation can be drawn from the data. Molecular
volumes of the cations were calculated using the COSMO
model[8b] and are given in Table 1. The volumes cover a range
from 0.228 to 0.518 nm3, and hence may be compared to
common 1-n-alkyl-3-methyl imidazolium ions from [C5MIm]+
(0.220 nm3) to [C10MIm]+ (0.331 nm3) or to ammonium cations
+
+
from [N2225
] ]
(0.268 nm3) to [N1888 (0.604 nm3). In the case of
R1 =4-OMe or Mes, viscosities seem to decrease with increas-
ing chain length, but only when combined with the borate
anion. If R1 =2-Me, viscosities reach a minimum at R2 =Hex.
Within the TAAILs with similar alkyl residues, the ordering of
the viscosities always follows 2-Me<4-OMe<2,4,6-Me
([B(hfip)4]-ILs) or 2-Me<4-OMe ([NTf2]-ILs). Although differen-
ces are small, this consistently observed trend may be attribut-
ed to a combination of steric and electronic effects: Thus, the
p-donating nature of the 4-OMe residue suggests that the ro-
tation around the bond of the aryl residue to the imidazolium
nitrogen atom is somewhat restricted, so that in addition to
the very slightly increased size, the conformational freedom is
also more restricted in the 4-OMe case. In contrast, the steric
demand of the 2-Me or 2,4,6-Me substitution now decouples
the p-systems of the arene and imidazolium moiety, allowing
for the adoption of at least two conformers by rotation around
Figure 3. Temperature-dependent viscosities of the investigated [NTf2]ꢀ ILs.
point. Furthermore, the measurement of 28 proved difficult, as
a homogeneous melt was only obtained after heating to 808C
for 10 min. Hence, in contrast to the other ILs, data for this IL
were recorded during cooling and should be considered with
caution due to possible hysteresis effects. For the VFT equation
for the description of temperature-dependent viscosities h, see
below:
ꢀ
ꢁ
B
h ¼ h0 ꢁ exp
ð1Þ
T ꢀ T0
in which h0 =viscosity at infinite temperature, B=constant,
T0 =Vogel temperature.
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the N-arene bond, which for the less-restricted 2-Me case ap-
pears to lead to more favorable properties than for the larger
and more restricted mesityl case.
Acknowledgements
This work was supported by the DFG in the normal procedure
as well as the SPP 1191 Ionic Liquids. We wish to thank. C.
Sturm for measuring several DSC and TGA traces. I. K. thanks
the ERC for the award of an Advanced Grant, which partly fi-
nanced the work (UniChem).
In an earlier study, by means of a modified Marcus theory,
we could show that increasing the anion diameter up to ap-
proximately 1 nm yields a minimal activation energy for the
movement of ions in an IL.[17c] Therefore, [B(hfip)4]ꢀ ILs should
also exhibit very good dynamic properties. Other studies, how-
ever, showed that the short BꢀO bonds lead to limited rota-
tional freedom, decreased melting entropy and worsening
transport properties.[8a,d,17d] Additionally, electrostatic interac-
tions are more pronounced, as in the case of the very similar
[Al(hfip)4]ꢀ anion. As a result, increasing the cation diameter
may have a different influence on ILs with [B(hfip)4]ꢀ anions
than with [Al(hfip)4]ꢀ ILs. With electrostatic forces not being
negligible, extending the alkyl chains of the cations leads to an
increased mean distance between the cations and anions and
decreases viscosities. In contrast to [Al(hfip)4]ꢀ salts, the in-
crease in dispersive interactions appears not to overbalance
this effect.
Keywords: aluminate anions · borate anions · imidazolium
salts · ionic liquids · TAAILs
[1] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis Vol. 1 (2nd ed.)
Wiley-VCH, Weinheim, 2008.
[2] a) P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis Vol. 2 (2nd ed.),
Wiley-VCH, Weinheim, 2008; b) P. Wasserscheid, W. Keim, Angew. Chem.
Int. Ed. 2000, 39, 3772–3789; Angew. Chem. 2000, 112, 3926–3945;
c) A. D. Sawant, D. G. Raut, N. B. Darvatkar, M. M. Salunkhe, Green Chem.
Lett. Rev. 2011, 4, 41–54; d) V. I. Pꢁrvulescu, C. Hardacre, Chem. Rev.
2007, 107, 2615–2665; e) C. M. Gordon, Appl. Catal. A 2001, 222, 101–
117; f) H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. A 2010,
373, 1–56.
A comparison of [NTf2]ꢀ ILs with their [B(hfip)4]ꢀ analogues
showed that in almost all cases, the [B(hfip)4]ꢀ salts are less vis-
cous. This is in accordance with earlier findings concerning
a comparison of [NTf2]ꢀ and [Al(hfip)4]ꢀ ILs,[17c] and can also
probably be attributed to low overall interaction potentials in
the large, highly fluorinated [B(hfip)4]ꢀ ILs. Additionally, it is
known that [B(hfip)4]ꢀ ILs are quite stable against (short-term)
exposure towards water and are completely stable against
oxygen at ambient temperatures.[17a,24] Therefore, even though
the use of such ILs in, for instance, electrochemistry, is unlikely,
the viscosity and thermal data of these functionalized ILs
should enable an application in fields such as lubrication re-
search or material processing.
[3] a) A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed, P. Shikotra, Inorg.
Chem. 2005, 44, 6497–6499; b) L. Fischer, T. Falta, G. Koellensperger, A.
Stojanovic, D. Kogelnig, M. Galanski, R. Krachler, B. K. Keppler, S. Hann,
Water Res. 2011, 45, 4601–4614; c) G. Cevasco, C. Chiappe, Green Chem.
2014, 16, 2375–2385; d) N. Hirayama, Solvent Extr. Res. Dev. Jpn. 2011,
18, 1–14.
[4] a) H. Shobukawa, H. Tokuda, M. A. B. H. Susan, M. Watanabe, Electro-
chim. Acta 2005, 50, 3872–3877; b) H. Shobukawa, H. Tokuda, S.-I.
Tabata, M. Watanabe, Electrochim. Acta 2004, 50, 305–309; c) S. Seki, Y.
Kobayashi, H. Miyashiro, Y. Ohno, A. Usami, Y. Mita, M. Watanabe, N.
Terada, Chem. Commun. 2006, 544–545; d) M. C. Buzzeo, R. G. Evans,
R. G. Compton, ChemPhysChem 2004, 5, 1106–1120; e) A. Lewandowski,
M. Galinski, J. Phys. Chem. Solids 2004, 65, 281–286; f) I. Osada, H. de V-
ries, B. Scrosati, S. Passerini, Angew. Chem. Int. Ed. 2016, 55, 500–513;
Angew. Chem. 2016, 128, 510–523.
[5] a) J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan, G. Luo, Chem. Rev.
2015, 115, 2136–2173; b) B. O’Regan, M. Graetzel, Nature 1991, 353,
737–740; c) P. Wang, S. M. Zakeeruddin, P. Comte, I. Exnar, M. Graetzel,
J. Am. Chem. Soc. 2003, 125, 1166–1167; d) W. Xu, C. A. Angell, Science
2003, 302, 422–425; e) P. Wang, S. M. Zakeeruddin, J.-E. Moser, M.
Graetzel, J. Phys. Chem. B 2003, 107, 13280–13285; f) P. Wang, S. M. Za-
keeruddin, R. Humphry-Baker, M. Graetzel, Chem. Mater. 2004, 16,
2694–2696; g) F. Mazille, Z. Fei, D. Kuang, D. Zhao, S. M. Zakeeruddin,
M. Graetzel, P. J. Dyson, Inorg. Chem. 2006, 45, 1585–1590.
[6] a) B. S. Lee, Y. S. Chi, J. K. Lee, I. S. Choi, C. E. Song, S. K. Namgoong, S.-G.
Lee, J. Am. Chem. Soc. 2004, 126, 480–481; b) H. Itoh, K. Naka, Y. Chujo,
J. Am. Chem. Soc. 2004, 126, 3026–3027; c) M. Antonietti, D. Kuang, B.
Smarsly, Y. Zhou, Angew. Chem. Int. Ed. 2004, 43, 4988–4992; Angew.
Chem. 2004, 116, 5096–5100; d) Z. He, P. Alexandridis, Phys. Chem.
Chem. Phys. 2015, 17, 18238–18261.
[7] a) R. P. Swatloski, S. K. Spear, J. D. Holbrey, R. D. Rogers, J. Am. Chem.
Soc. 2002, 124, 4974–4975; b) S. Zhu, Y. Wu, Q. Chen, Z. Yu, C. Wang, S.
Jin, Y. Ding, G. Wu, Green Chem. 2006, 8, 325–327; c) H. Ohno, Y.
Fukaya, Chem. Lett. 2009, 38, 2–7; d) A. Pinkert, K. N. Marsh, S. Pang,
M. P. Staiger, Chem. Rev. 2009, 109, 6712–6728; e) M. Isik, H. Sardon, D.
Mecerreyes, Int. J. Mol. Sci. 2014, 15, 11922–11940; f) X. Yuan, G. Cheng,
Phys. Chem. Chem. Phys. 2015, 17, 31592–31607.
[8] a) U. Preiss, S. Bulut, I. Krossing, J. Phys. Chem. B 2010, 114, 11133–
11140; b) U. P. R. M. Preiss, J. M. Slattery, I. Krossing, Ind. Eng. Chem. Res.
2009, 48, 2290–2296; c) J. M. Slattery, C. Daguenet, P. J. Dyson, T. J. S.
Schubert, I. Krossing, Angew. Chem. Int. Ed. 2007, 46, 5384–5388;
Angew. Chem. 2007, 119, 5480–5484; d) U. P. Preiss, W. Beichel, A. M. T.
Erle, Y. U. Paulechka, I. Krossing, ChemPhysChem 2011, 12, 2959–2972;
e) S. Bulut, P. Eiden, W. Beichel, J. M. Slattery, T. F. Beyersdorff, T. J. S.
Schubert, I. Krossing, ChemPhysChem 2011, 12, 2296–2310; f) C.-W.
Cho, U. Preiss, C. Jungnickel, S. Stolte, J. Arning, J. Ranke, A. Klamt, I.
Conclusion
We synthesized the first TAAILs with a [B(hfip)4]ꢀ anion and in-
vestigated their thermal behavior and viscosities. The ILs with
[B(hfip)4]ꢀ and [NTf2]ꢀ anions both show remarkably low glass
transition temperatures around ꢀ608C and are stable up to
2008C ([B(hfip)4]ꢀ) or 4008C ([NTf2]ꢀ), which represents a large
window of more than 250/450 K. The viscosities of the borate
ILs are generally lower than those of the respective [NTf2]ꢀ ILs,
but slightly higher in comparison to standard alkyl imidazolium
[B(hfip)4]ꢀ ILs, which is likely induced by the additional phenyl
ring. Overall, the synthesized [B(hfip)4]ꢀ ILs are stable against
air and show good hydrolytic stability together with promising
physical properties. Furthermore, it is very interesting to note
that the melting points of these borate ILs are typically below
room temperature and are generally lower than those of the
traditional all-alkyl-imidazolium borate ILs. This indicates that
the aromatic residue appears to have an interesting and lique-
fying effect on these ILs.
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Krossing, J. Thoeming, J. Phys. Chem. B 2011, 115, 6040–6050; g) P.
Eiden, S. Bulut, T. Kochner, C. Friedrich, T. Schubert, I. Krossing, J. Phys.
Chem. B 2011, 115, 300–309.
[17] a) S. Bulut, P. Klose, I. Krossing, Dalton Trans. 2011, 40, 8114–8124; b) D.
Himmel, H. Scherer, D. Kratzert, I. Krossing, Z. Anorg. Allg. Chem. 2015,
641, 655–659; c) A. Rupp, N. Roznyatovskaya, H. Scherer, W. Beichel, P.
Klose, C. Sturm, A. Hoffmann, J. Tuebke, T. Koslowski, I. Krossing, Chem.
Eur. J. 2014, 20, 9794–9804; d) A. B. A. Rupp, S. Welle, P. Klose, H. Scher-
er, I. Krossing, ChemPhysChem 2015, 16, 1940–1947; e) M. Rohde, P.
Eiden, V. Leppert, M. Schmidt, A. Garsuch, G. Semrau, I. Krossing, Chem-
PhysChem 2015, 16, 666–675; f) N. Roznyatovskaya, A. B. A. Rupp, J.
Tuebke, I. Krossing, ChemElectroChem 2015, 2, 829–836.
[9] a) S. Ahrens, A. Peritz, T. Strassner, Angew. Chem. Int. Ed. 2009, 48,
7908–7910; Angew. Chem. 2009, 121, 8048–8051; b) R. Giernoth,
Angew. Chem. Int. Ed. 2010, 49, 2834–2839; Angew. Chem. 2010, 122,
2896–2901.
[10] T. Schulz, S. Ahrens, D. Meyer, C. Allolio, A. Peritz, T. Strassner, Chem.
Asian J. 2011, 6, 863–867.
[11] D. Meyer, T. Strassner, J. Org. Chem. 2011, 76, 305–308.
[12] A. B. A. Rupp, I. Krossing, Acc. Chem. Res. 2015, 48, 2537–2546.
[13] a) I. Krossing, J. M. Slattery, C. Daguenet, P. J. Dyson, A. Oleinikova, H.
Weingaertner, J. Am. Chem. Soc. 2006, 128, 13427–13434; b) S. Bulut, P.
Klose, M.-M. Huang, H. Weingaertner, P. J. Dyson, G. Laurenczy, C. Frie-
drich, J. Menz, K. Kuemmerer, I. Krossing, Chem. Eur. J. 2010, 16, 13139–
13154.
[18] D. J. Nielsen, C. Pettinari, B. W. Skelton, A. H. White, Acta Crystallogr.
Sect. C 2004, 60, o542–o544.
[19] I. Raabe, K. Wagner, K. Guttsche, M. Wang, M. Gratzel, G. Santiso-Qui-
nones, I. Krossing, Chem. Eur. J. 2009, 15, 1966–1976.
[20] a) K. Noack, P. S. Schulz, N. Paape, J. Kiefer, P. Wasserscheid, A. Leipertz,
Phys. Chem. Chem. Phys. 2010, 12, 14153–14161; b) F. Heym, B. J. M.
Etzold, C. Kern, A. Jess, Green Chem. 2011, 13, 1453–1466; c) C. P. Fred-
lake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, J. F. Brennecke, J. Chem.
Eng. Data 2004, 49, 954–964.
[21] C. Maton, N. De Vos, C. V. Stevens, Chem. Soc. Rev. 2013, 42, 5963–5977.
[22] D. R. MacFarlane, J. Golding, S. Forsyth, M. Forsyth, G. B. Deacon, Chem.
Commun. 2001, 1430–1431.
[14] a) http://gepris.dfg.de/gepris/projekt/237028221,
2014;
b) http://
www.isc.fraunhofer.de/presse-medien/presseinformationen/pressede-
tails/archiv/2015/09/30/meldung/minsem-forschungsprojekt-zur-schlies-
sung-von-wertstoffkreislaeufen-im-bereich-mineralische-aufbereitungs-
und-produktionsrueckstaende/, 2015.
[15] a) I. Krossing, I. Raabe, Angew. Chem. Int. Ed. 2004, 43, 2066–2090;
Angew. Chem. 2004, 116, 2116–2142; b) S. H. Strauss, Chem. Rev. 1993,
93, 927–942; c) A. B. A. Rupp, P. Klose, H. Scherer, I. Krossing, ChemPhys-
Chem 2014, 15, 3729–3731.
[23] X. Wang, Y. Chi, T. Mu, J. Mol. Liq. 2014, 193, 262–266.
[24] S. Bulut, PhD thesis, Albert-Ludwigs-Universitꢂt (Freiburg) 2010.
[16] T. A. Engesser, M. R. Lichtenthaler, M. Schleep, I. Krossing, Chem. Soc.
Rev. 2016, 45, 789–899.
Received: May 3, 2016
Published online on && &&, 0000
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Full Paper
FULL PAPER
Bulky TAAILs: Combining weakly coor-
dinating bulky tetrakis((1,1,1,3,3,3-hexa-
fluoropropan-2-yl)oxy)borate [B(hfip)4]ꢀ
anions with aryl alkyl imidazolium cat-
ions results in tunable alkyl aryl ionic
liquids (TAAILs; see figure) that show re-
markably low melting points despite of
their large size and high molecular
weight. These TAAILs are liquid at room
temperature.
&
Ionic Liquids
M. Kaliner, A. Rupp, I. Krossing,*
T. Strassner*
&& – &&
Tunable Aryl Alkyl Ionic Liquids with
Weakly Coordinating
Tetrakis((1,1,1,3,3,3-hexafluoropropan-
2-yl)oxy)borate [B(hfip)4] Anions
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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These are not the final page numbers! ÞÞ