Electronic effects of para-substitution on the melting points of TAAILs

Article abstract of DOI:10.1002/asia.201000744

Owing to numerous new applications, the interest in "task- specific" ionic liquids increased significantly over the last decade. But, unfortunately, the imidazolium-based ionic liquids (by far the most frequently used cations) have serious limitations whe

Full text of DOI:10.1002/asia.201000744

DOI: 10.1002/asia.201000744
Electronic Effects of para-Substitution on the Melting Points of TAAILs
Tobias Schulz,[a] Sebastian Ahrens,[b] Dirk Meyer,[a] Christoph Allolio,[a]
Anke Peritz,[a] and Thomas Strassner*[a]
Abstract: Owing to numerous new ap-
plications, the interest in “task-specif-
ic” ionic liquids increased significantly
over the last decade. But, unfortunate-
ly, the imidazolium-based ionic liquids
(by far the most frequently used cat-
ions) have serious limitations when it
comes to modifications of their proper-
ties. The new generation of ionic liq-
uids, called tunable aryl–alkyl ionic liq-
uids (TAAILs), replaces one of the two
alkyl chains on the imidazolium ring
with an aryl ring which allows a large
degree of functionalization. Inductive,
mesomeric, and steric effects as well as
melting points of the corresponding
bromide and bis(trifluoromethanesulfo-
nyl)imide, (N(Tf)2 ), salts. In addition,
+
À
À
potentially also p p and p p interac-
tions provide a wide range of possibili-
ties to tune this new class of ILs. We
investigated the influence of electron-
withdrawing and -donating substituents
at the para-position of the aryl ring
(NO2, Cl, Br, EtO(CO), H, Me, OEt,
OMe) by studying the changes in the
À
we calculated (B3LYP/6-311++GACHTUNGTRENNUNG(d,p))
the different charge distributions of
substituted 1-aryl-3-propyl-imidazolium
cations to understand the experimen-
tally observed effects. The results indi-
cated that the presence of electron-do-
nating and -withdrawing groups leads
to strong polarization effects in the cat-
ions.
Keywords: computational chemis-
try · electronic effects · electrostatic
interactions · ionic liquids · taail
Introduction
Right now, there are many special applications that
demand new tunable ionic liquids, like the electrodeposition
of semiconducting materials and metals,[9–13] catalytic and
stoichiometric reactions,[14–21] or even wood processing and
pharmaceutical applications.[22–25]
Lately, research efforts have concentrated more on the
development of new anions and only very few new cations
have been published.[8,26–28] Recently, we introduced a new
class of imidazolium-based cat-
Since their discovery at the beginning of the 20th century,
ionic liquids have come a long way to become a class of
high performance materials with huge potential for various
applications.[1] In the middle of the 20th century, up to the
1970s, first-generation ionic liquids were investigated in alu-
minium-plating processes.[2–5]
The next steps towards a wide applicability of ionic liquids
were the development of water- and air-stable ionic liquids
ions and their salts, the aryl–
À
À
with, for example, BF4 and PF6 counterions and task-spe-
cific ionic liquids (TSILs), which were developed individual-
ly with specific problems in mind, for example, in organic
synthesis.[6–8]
alkyl
imidazolium
salts
(TAAILs, Scheme 1).[29] Owing
to the combination of sp2 and
sp3 carbon substituents in
TAAILs and the interaction of
the phenyl p-system with the
imidazolium core, the possibili-
Scheme 1. Tunable Aryl-Alkyl
Ionic Liquids (TAAILs)
[a] T. Schulz, D. Meyer, C. Allolio, A. Peritz, Prof. Dr. T. Strassner
Physikalische Organische Chemie
ties for variations and fine-tuning are far greater than in the
Technische Universitꢀt Dresden
101062 Dresden (Germany)
Fax : (+49)351-463-39679
E-mail: thomas.strassner@chemie.tu-dresden.de
common TSILs. In addition, they might allow the study of
+
À
À
p p and p p interactions as recently discussed for similar
systems,[30,31] or function as an anion receptor.[32,33]
[b] Dr. S. Ahrens
In our previous work concerning TAAILs, we kept the
aryl part constant (2,4,6-trimethylphenyl, mesityl) and stud-
ied the influence of different alkyl chains (C1–C8, C11, C14)
BASF SE, Ludwigshafen, M301
67056 Ludwigshafen (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000744.
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863

FULL PAPERS
À
À
À
and anions (BrÀ, BF4 , PF6 , N(Tf)2 ) on their melting
lated by alkyl bromides and the resulting bromide salts are
converted into the (N(Tf)2 ) salts by anion metathesis.
À
points and properties.[34]
For the mesityl system, we observed a large anion effect,
which in some cases turned out to be as much as a melting
point difference of more than 1508C. It was remarkable
that, with increasing alkyl chain length, a nearly linear de-
crease in the melting points of the bromide salts was ob-
served. The decomposition temperatures of the mesityl
TAAILs were found to be dominated by anion effects and
to be nearly independent of the alkyl chain length. Those
with bromide anions decomposed at lower temperatures
The influence of the substituents can be clearly shown by
comparing the melting points and phase transitions of salts
with the same alkyl chain length, but different para-substitu-
tion. According to Figure 1 and Table 1, electron-withdraw-
ing groups lead to higher melting points, whilst electron-do-
nating groups lead to room-temperature ionic liquids (RT-
TAAILs), even for the bromide salts.
À
compared to the bis(trifluoromethylsulfonyl)imide (N(Tf)2 )
salts, which decompose at significantly higher temperatures
than the conventional ionic liquids.
Calculations of the charge distribution in TAAILs indicat-
ed that compared to conventional imidazolium-based cat-
ions like ethylmethylimidazolium (EMIM) or butylmethyl
imidazolium (BMIM), the new TAAILs exhibit a higher
positive charge on the imidazolium core. Therefore, we were
interested in finding out how much the electronic effects of
the substituents on the phenyl ring will influence the charge
distribution in the imidazolium ring as well as the general
properties of the resulting ionic liquids.
Figure 1. Melting points of the bromide salts.
Results and Discussion
Table 1. Phase transition temperatures of the (N(Tf)2À) salts.
R
R1 =C3H7
R1 =C6H13
R1 =C7H15
R1 =C14H29
Herein, we report the results for electron-withdrawing- and
electron-donating groups in the para-position of the phenyl
ring, but restricted the length of the linear alkyl chains to a
propyl, hexyl, heptyl, and tetradecyl chain, and the anions to
[a]
NO2
Cl
Br
EtO(CO)
H
Me
67
22
20
21
23
À45
21
26
35
36
36
[a]
30
[a]
22
À28
À
[a]
27
13
bromide and (N(Tf)2 ).
42
–
–
–
30
34
21
The synthesis is described in Scheme 2. To evaluate the
OEt
OMe
À51[b]
56
À53[b]; 27
1
substituent effect we looked at the melting point and the H
[a]
and 13C NMR shifts, and we calculated the electrostatic sur-
face potentials (ESPs) to assess the electronic structure.
As shown in Scheme 2, the synthesis starts from substitut-
ed anilines leading to the corresponding 1-phenyl-1H-imida-
zoles. Then the 1-phenyl-1H-imidazole derivatives are alky-
[a] liquid at room temperature. [b] glass transition.
There is little difference between the para-chloro- and
para-bromo-substituted imidazolium salts for both anions.
This is also true for the alkoxy-substituted salts, which show
nearly the same behavior with the only exception being the
À
(N(Tf)2 ) salt with a propyl chain. The methoxy-substituted
compound shows an unexpectedly high melting point where-
as the ethoxy-substituted salt exhibits only a glass-transition
point, no melting point could be found. With respect to the
bromide salts, the ester, nitro, and halo substituents lead to
ILs with higher melting points. The melting points for the
bromide salts with propyl and tetradecyl chains show similar
trends with a typical melting point difference of circa 1008C.
In contrast, the alkyl chains of intermediate chain lengths
show a different behavior; there, one can find almost the
same melting points (Br, EtO(CO), Me) or differences of
60–908C (NO2, OMe).
Scheme 2. Scheme of the synthetic approach, a) glyoxal, MeOH (60 mL),
RT; b) NH4Cl, CH2O, MeOH (400 mL), 1 h reflux; c) H3PO4, 4 h reflux;
d) THF, 80-1108C; e) H2O/CH2Cl2 RT. R =NO2 (1; 9; 17a,b; 25a,b;
33a,b; 41a,b), R=Cl (2; 10; 18a,b; 26a,b; 34a,b; 42a,b), R=Br (3; 11;
19a,b; 27a,b; 35a,b; 43a,b), R=EtO(CO) (4; 12; 20a,b; 28a,b; 36a,b;
44a,b), H (5; 13; 21a,b; 29a,b; 37a,b; 45a,b), R=Me (6; 14; 22a,b;
30a; 38a; 46a,b), R=OEt (7; 15; 23a,b; 31a,b; 39a; 47a,b), R=OMe
(8; 16; 24a,b; 32a,b; 40a; 48a,b). THF=tetrahydrofuran.
Although the melting points are strikingly different, the
NMR spectra do not show remarkable differences. It is well
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Chem. Asian J. 2011, 6, 863 – 867

Electronic Effects of para-Substitution on the Melting Points of TAAILs
known that the hydrogen atom at the C2 carbon atom in
imidazolium ions is the most-acidic proton, and this fact is
widely exploited in metal–organic chemistry. Deprotonation
at the C2 carbon leads to the formation of N-heterocyclic
carbenes which are frequently used ligands in many com-
plexes.[35–37] It is also easily identifiable in the 1H- and
13C NMR spectra. Therefore, we chose these signals to
probe the change in the electronic structure of the imidazo-
lium core and observed changes of up to 0.3 ppm in the
1H NMR shifts, and, in most cases, less than Dd=1 ppm for
the 13C NMR shifts. In addition, there is virtually no influ-
ence of the alkyl chain on the observed NMR shifts, which
is in agreement with the results of the previous study on the
mesityl-substituted system, where we did not observe any
changes in the NMR spectrum in the presence of different
chain lengths and anions.
tentials, a wide range (0.05–0.235; Figure 2a, b) and a small-
er one (0.135–0.175; Figure 2c, d).
The ESP representation of the para-nitro-substituted
cation (Figure 2a) visualizes the negative polarization (red)
of the electron-withdrawing nitro group and the positive po-
larization (blue) of the imidazolium C2 position. The para-
methoxy-substituted compound (Figure 2b) exhibits visible
differences in the polarization of the phenyl ring, as can be
seen by the color coding of the ring (“green” vs. “yellow”).
In accordance with our expectations, the phenyl ring of the
para-nitro-substituted compound shows a larger positive po-
tential (Figure 2a, “green”) and the donating methoxy
group results in a smaller potential (Figure 2b, “yellow”) on
the phenyl ring.
Figures 2c and d demonstrate the different polarization of
the imidazolium cores and that the electronic effect of the
substituent is not restricted to the aryl ring, but also influen-
ces the conjugated p-system of the imidazolium core. From
the ESP representations, it becomes clear that the cation is
polarized along the aromatic rings. Although we were aware
of the potential problems involved, we decided to calculate
the dipole moments to check whether they also show a sub-
stituent effect (for these results, see Supporting Informa-
tion).
In conclusion, the melting points show a considerable sub-
1
stituent influence as well as the H and 13C NMR shifts (see
the Supporting Information) of the C2 imidazolium position.
Therefore, we started to investigate the nature of the sub-
stituent effect using density functional theory calculations.
As we did not observe a significant effect of the different
alkyl chains on the NMR spectra, we chose to restrict the
calculations to the shortest chain, the 1-phenyl-3-propylimi-
dazolium cations.
To visualize the electronic effects, we calculated the elec-
trostatic surface potentials (ESP) of all of the substituted 1-
aryl-3-propyl-imidazolium cations. In Figure 2, only the
ESPs for the para-nitro- and para-methoxy-substituted cat-
ions are compared, all other ESPs are given in the Support-
ing Information. The ESPs are plotted for two different po-
Conclusions
We synthesized and characterized substituted 1-phenyl-3-
alkyl-1H-imidazolium bromide and (N(Tf)2 ) salts and in-
À
vestigated their melting points. There is a significant sub-
stituent influence on the melting point, which can be best
explained by polarization effects. In addition, we were able
to calculate the electron distribution in the different substi-
tuted 1-phenyl-3-propyl-1H-imidazolium cations. We can
conclude that the considered macroscopic properties of the
new TAAILs are predominantly determined by the elec-
tronic character of the substituent on the phenyl ring.
Experimental Section
General
NMR spectra were recorded at 208C on a Bruker-AC-300P NMR spec-
trometer operating at 300 MHz for 1H and 75.5 MHz for 13C. [D6]DMSO
was used as solvent and special care was taken to measure at the same
concentration to be able to compare the chemical shifts. These shifts are
1
reported relative to Si
N
emental analysis was performed on
a
EUROVEKTOR Hekatech
EA3000, for DSC measurements a Setaram DSC121 was used (scan rate
5 KminÀ1). A more detailed description of the syntheses and the analyti-
cal characterization is presented in the Supporting Information. Owing to
the large number of synthesized compounds, we chose to report only one
example of each class here, all other syntheses are described in the Sup-
porting Information.
Preparation of the Phenyl Imidazoles
The aniline derivative (0.3 mol) is dissolved in 50–60 mL methanol,
slowly mixed with 34.2 mL aqueous glyoxal solution (40%), and stirred
at room temperature overnight. This reaction leads to a solid precipitate
Figure 2. ESP representations for 1-(4-nitrophenyl)-3-propyl-1H-imidazo-
lium (a,c) and 1-(4-methoxyphenyl)-3-propyl-1H-imidazolium (b,d)
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865

T. Strassner et al.
FULL PAPERS
or a highly viscous substance. After diluting the reaction mixture with
400 mL methanol, 32.1 g ammonium chloride and 48 mL formaldehyde
solution (37% in water) are added. The reaction mixture is heated under
reflux for 1 h and the resulting dark solution is allowed to cool down and
is mixed with 14 mL phosphoric acid. Finally, the reaction mixture is
heated for 4 h under reflux. 2/3 of the solvent is removed by evaporation
under reduced pressure, and the resulting crude solution is mixed with
200–300 mL ice water. Potassium hydroxide solution (40%) is added
until pH 9 is reached. The aqueous phase is extracted 4 times with di-
chloromethane and the combined organic phases are washed with water,
saturated sodium chloride solution, and dried over magnesium sulphate.
To isolate the crude product, the solvent is removed under reduced pres-
sure after filtration. The product is either purified by Kugelrohr distilla-
tion or recrystallization from ethyl acetate.
1-(4-(ethoxycarbonyl)phenyl)-3-heptyl-1H-imidazolium bromide 36a
Following the general synthetic procedure, 4.60 mmol (1.00 g) ethyl 4-
(1H-imidazol-1-yl)benzoate, and 5.50 mmol (0.99 g) 1-bromoheptane are
dissolved in THF and heated for 20 h at 958C. Yield: 1.14 g (62%); m.p.:
1128C; 1H NMR (300 MHz, [D6]DMSO, 258C): d=0.89 (m, 3H; CH3),
1.32 (m, 8H, 4xCH2), 1.37 (t, 3JH,H =7.4 Hz, 3H, CH3), 1.90 (m, 2H,
CH2), 4.26 (t, 3JH,H =7.4 Hz, 2H, NCH2), 4.38 (q, 2H, 3JH,H =7.4 Hz,
OCH2), 7.97 (d, 3JH,H =8.7 Hz, 2H, ArH), 8.10 (s, 1H, NCHCHN), 8.22
(d, 3JH,H =8.7 Hz, 2H, ArH), 8.44 (s, 1H, NCHCHN), 9.96 ppm (s, 1H,
NCHN); 13C NMR (125.8 MHz, [D6]DMSO, 258C): d=13.9, 14.1, 21.9,
25.5, 28.1, 29. 1, 31.0, 49.5, 61.3, 120.9, 121.9, 123.5, 130.7, 130.9, 135.8,
138.1, 164.6 ppm; elemental analysis: calcd. (%) for C19H27BrN2O2:
C 57.72, H 6.88, N 7.09; found: C 57.37, H 6.96, N 7.06.
1-(4-(ethoxycarbonyl)phenyl)-3-tetradecyl-1H-imidazolium bromide 44a
Ethyl 4-(1H-imidazol-1-yl)benzoate 12
Following the general synthetic procedure, 4.60 mmol (1.00 g) ethyl 4-
(1H-imidazol-1-yl)benzoate, and 5.50 mmol (1.50 g) 1-bromotetradecane
are dissolved in tetrahydrofuran and heated for 20 h at 958C. Yield:
1.40 g (61%); m.p.: 1268C; 1H NMR (300 MHz, [D6]DMSO, 258C): d=
Following the general synthetic procedure, 0.1 mol ethyl-4-aminoben-
zoate is used. The resulting product is recrystallized from 20 mL ethyl
acetate to give
a
solid product. Yield: 15.93 g (74%); 1H NMR
(300 MHz, [D6]DMSO, 258C): d=1.37 (t, 3JH,H =7.1 Hz, 3H, CH3), 4.36
(q, 3JH,H =7.1 Hz, 2H, CH2), 7.18 (s, 1H, NCHCHN), 7.31 (s, 1H,
NCHCHN), 7.42 (d, 3JH,H =8.7 Hz, 2H, ArH), 7.90 (s, 1H, NCHN),
8.11 ppm (d, 3JH,H =8.7 Hz, 2H, ArH); 13C NMR (125.8 MHz,
[D6]DMSO, 258C): d=14.2, 61.1, 117.6, 120.4, 129.2, 130.9, 131.2, 135.3,
140.5, 165.3 ppm; elemental analysis: calcd (%) for C12H12N2O2 (216.24):
C 66.65, H 5.59, N 12.96; found: C 66.42, H 5.54, N 12.57.
0.85 (t, 3JH,H =6.9 Hz, 3H, CH2CH2CH3), 1.27 (m, 22H, alkyl CH2), 1.40
À
(t, 3JH,H =7.1 Hz, 3H, OCH2CH3), 1.93 (m, 2H, NCH2CH2), 4.25 (t,
3JH,H =7.3 Hz, 2H, NCH2), 4.39 (q, 3JH,H =7.1 Hz, 2H; OCH2), 7.97 (d,
3JH,H =8.8 Hz, 2H, ArH), 8.10 (s, 1H, NCHCHN), 8.23 (d, 3JH,H =8.8 Hz,
2H, ArH), 8.45 (s, 1H, NCHCHN), 9.96 ppm (s, 1H, NCHN); 13C NMR
(125.8 MHz, [D6]DMSO, 258C): d=13.9, 14.1, 22.1, 25.5, 28.4, 28.7, 28.8,
28.9, 29.0, 29.0, 31.3, 49.5, 61.3, 120.9, 121.9, 123.5, 130.7, 131.0, 135.8,
138.1, 164.6 ppm; elemental analysis: calcd. (%) for C26H41BrN2O2:
C 63.28, H 8.37, N 5.68; found: C 63.28, H 8.42, N 5.76.
The synthetic procedures and analytical characterizations of compounds
9–16 are given in the Supporting Information.
Preparation of the Imidazolium Bromides
A detailed description of the preparation and analytical characterization
for all bromide salts 17a–48a is given in the Supporting Information.
Alkylation of the 1-N-substituted imidazoles is carried out in an ACE
pressure tube, within a temperature range of 80–1108C. 1 Equivalent of
the substituted imidazole derivative is dissolved in 10 mL tetrahydrofuran
and a slight excess (1.1 equiv) of the bromoalkane is added. The reaction
mixture is heated for 8 to 10 hours. By washing with tetrahydrofuran the
resulting crude product is purified and dried under vacuum.
Preparation of the Corresponding Bis(trifluoromethanesulfonyl)imides
After completely dissolving 1 equivalent of imidazolium bromide in
water, or, for the longer alkyl chains,
a water/methanol mixture,
1.1 equivalents of lithium bis(trifluoromethansulfonyl)imide is added.
This leads to a biphasic system where dichloromethane is added for
better phase separation. The organic phase is separated from the aqueous
phase by a separatory funnel and the aqueous phase is washed twice with
dichloromethane. The combined organic phases are dried over MgSO4,
filtered, and evaporated in a rotary evaporator under vacuum. The result-
ing ionic liquid is dried under vacuum.
1-(4-(Ethoxycarbonyl)phenyl)-3-propyl-1H-imidazolium bromide 20a
Following the general synthetic procedure, 4.6 mmol (1.00 g) ethyl 4-(1H-
imidazol-1-yl)benzoate, and 5.5 mmol (0.68 g) 1-bromopropane are dis-
solved in THF and heated for 20 h at 958C. Yield: 0.838 g (52%); m.p.:
1
3
1798C; H NMR (300 MHz, [D6]DMSO, 258C): d=0.97 (t, JH,H =7.3 Hz,
3H, CH2CH2CH3), 1.42 (t, 3JH,H =7.3 Hz, 3H, OCH2CH3), 1.99 (hept,
1-(4-(Ethoxycarbonyl)phenyl)-3-propyl-1H-imidazolium
bis(trifluoromethanesulfonyl) imide 20b
3JH,H =7.3 Hz, 2H, NCH2CH2), 4.25 (t, JH,H =7.3 Hz, 2H, NCH2), 4.72 (q,
3
3JH,H =7.1 Hz, 2H, OCH2), 8.00 (d, 3JH,H =8.8 Hz, 2H, ArH), 8.12 (s, 1H,
NCHCHN), 8.25 (d, 3JH,H =8.8 Hz, 2H, ArH), 8.48 (s, 1H, NCHCHN),
10.02 ppm (s, 1H, NCHN); 13C NMR (125.8 MHz, [D6]DMSO, 258C):
d=10.5, 14.1, 22.6, 51.0, 61.3, 121.0, 121.9, 123.5, 130.7, 131.0, 135.8,
Following the general synthetic procedure, 1.47 mmol (0.50 g) 1-(4-ethox-
ycarbonyl-phenyl)-3-propyl-1H-imidazolium bromide and 1.64 mmol
(0.47 g) lithium bis(trifluoromethane-sulfonyl)imide are dissolved in a bi-
phasic system consisting of 50 mL water/20 mL dichloromethane and
stirred at room temperature. Yield: 0.73 g (92%); m.p.: 678C; 1H NMR
138.1,
164.6 ppm;
elemental
analysis:
calcd
(%)
for
C15H19BrN2O2·0.35H2O: C 52.14, H 5.75, N 8.11; found: C 52.19, H 5.79,
N 8.11.
(300 MHz, [D6]DMSO, 258C): d=0.98 (t, 3JH,H =7.3 Hz, 3H, CH3), 1.98
3
(hept, 2H, CH2), 4.23 (t, 3JH,H =7.3 Hz, 2H, NCH2), 4.48 (q, JH,H
=
1-(4-(Ethoxycarbonyl)phenyl)-3-hexyl-1H-imidazolium bromide 28a
7.1 Hz, 2H, CH2), 8.00 (d, 3JH,H =8.8 Hz, 2H, ArH), 8.11 (s, 1H,
NCHCHN), 8.27 (d, 3JH,H =8.8 Hz, 2H; ArH), 8.48 (s, 1H, NCHCHN),
9.99 ppm (s, 1H, NCHCHN), 13C NMR (125.8 MHz, [D6]DMSO, 258C):
d=10.5, 14.1, 22.6, 51.0, 61.3, 119.5, 121.0, 121.6, 123.5, 130.7, 131.0,
135.8, 164.6 ppm; elemental analysis: calcd (%) for C17H19F6N3O6S2:
C 37.85, H 3.55, N 7.79, S 11.89; found C 38.07, H 3.20, N 7.92, S 11.74.
Following the general synthetic procedure, 1.62 mmol (0.35 g) ethyl 4-
(1H-imidazol-1-yl)benzoate, and 1.78 mmol (0.294 g) 1-bromohexane are
dissolved in THF and heated for 20 h at 958C. Yield: 0.417 g (68%);
m.p.: 1138C; 1H NMR (300 MHz, [D6]DMSO, 258C): d=0.89 (m, 3H,
CH3), 1.32 (m, 6H, 3xCH2), 1.37 (t, 3JH,H =7.4 Hz, 3H, CH3), 1.90 (m,
3
3
2H, CH2), 4.26 (t, JH,H =7.4 Hz, 2H, N-CH2), 4.38 (q, JH,H =7.4 Hz, 2H,
OCH2), 7.97 (d, 3JH,H =8.7 Hz, 2H, ArH), 8.10 (s, 1H, NCHCHN), 8.22
(d, 3JH,H =8.7 Hz, 2H, ArH), 8.44 (s, 1H, NCHCHN), 9.97 ppm (s, 1H,
NCHCHN); 13C NMR (125.8 MHz, [D6]DMSO, 258C): d=13.9, 14.1,
21.9, 25.2, 29.0, 30.6, 49.5, 61.3, 120.9, 121.9, 123.5, 130.7, 130.9, 135.8,
138.1, 164.6 ppm; elemental analysis: calcd. (%) for C18H25BrN2O2:
C 56.70, H 6.61, N 7.35; found: C 56.48, H 6.67, N 7.25.
A detailed description of the preparation and analytical characterization
for all bis(trifluoromethanesulfonyl)imide salts 17b–48b is given in the
Supporting Information.
Computational Details
Density functional (DFT) calculations were carried out on the 1-aryl-3-
propyl-imidazolium cations of the prepared imidazolium salts. Geometry
optimizations of the structures were performed using the B3LYP hybrid
density functional combined with the split valence triple-z (TZ) basis set
6–311++GACTHNUTRGNEUNG
(d,p).[38–42] No symmetry or internal coordinate constraints
866
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Chem. Asian J. 2011, 6, 863 – 867

Electronic Effects of para-Substitution on the Melting Points of TAAILs
were applied during optimizations. Vibrational frequency calculations
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Tobias Schulz thanks the Fonds der Chemischen Industrie for financial
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Published online: January 19, 2011
Chem. Asian J. 2011, 6, 863 – 867
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