Tunable aryl alkyl ionic liquids with weakly coordinating bulky borate anion

Article abstract of DOI:10.1016/j.tetlet.2016.06.082

A series of 1-aryl 3-alkyl imidazolium tetrakis[bis-3,5(trifluoromethyl)phenyl]borate [BArF_3,5] ionic liquids were synthesized and characterized. Increasing the alkyl chain length leads to lower melting points even resulting in room temperature

Full text of DOI:10.1016/j.tetlet.2016.06.082

Tetrahedron Letters 57 (2016) 3453–3456
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Tunable aryl alkyl ionic liquids with weakly coordinating bulky
borate anion
⇑
Maria Kaliner, Thomas Strassner
Physikalische Organische Chemie, Technische Universität Dresden, 01169 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 1-aryl 3-alkyl imidazolium tetrakis[bis-3,5(trifluoromethyl)phenyl]borate [BArF3,5] ionic
liquids were synthesized and characterized. Increasing the alkyl chain length leads to lower melting
points even resulting in room temperature ionic liquids (RTILs). In comparison to their corresponding
alkyl imidazolium [BArF3,5] counterparts these aryl imidazolium [BArF3,5] ionic liquids show significantly
lower melting points.
Received 24 March 2016
Revised 16 June 2016
Accepted 20 June 2016
Available online 21 June 2016
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquids
Imidazolium cations
Borate anions
TAAILs
ESP
Introduction
thermal stability, or acidity can be modified.12 By definition ionic
liquids are salts that contain organic cations and inorganic or
Tetraarylborate anions with many fluoro or fluoromethyl
groups on the phenyl part have attracted great interest as weakly
coordinating bulky counterions.1,2 The tetrakis[3,5-bis(trifluoro-
organic anions and have a melting point below 100 °C. ILs which
are even liquid below 25 °C are called room temperature ionic
liquids (RTILs).12 ILs attract interest in a lot of different applica-
tions like plating,13–18 as solvents in organic synthesis or
catalysis,12,19–24 as extracting agents,25–28 for the dissolution of
cellulose,29–34 or as electrolytes in dye sensitized solar cells.35–41
In 2006 the first ionic liquids with the [BArF3,5] anion were synthe-
sized based on ammonium, imidazolium, pyridinium, and isochi-
nolinium cations.42
These [BArF3,5] ILs were used in the investigation of ion transfer
across polarized water/IL interfaces,42,43 as ionic liquid salt
bridges44,45 that isolate the working from the reference electrode
in electrochemistry cells or to prepare poly(IL) functionalized par-
ticles46 where the polymerization is carried out via the functional
methyl)phenyl]borate [BArF3,5] anion was first introduced in
1983 by Kobayashi et al. and was originally used for anion
catalyzed phase transfer catalysis for diazo-coupling reactions.3–5 In
contrast Brookhart et al. prepared oxonium acid [H(OEt2)2][BArF3,5
]
from Na[BArF3,5] in ether and HCl and used the [BArF3,5] anion as
stabilization reagent for electrophilic cationic transition metal
complexes,6 e.g., to stabilize the palladium catalyst in the synthetic
copolymerization of olefins and carbon monoxide.7,8 Also in
organometallic chemistry the bulky [BArF] anions have frequently
been used.9–11 It was observed that the generally weakly coordi-
nating anions tetrafluoroborate ([BF4]) or hexafluorophosphate
([PF6]) reacted with highly reactive electrophilic complexes and
that abstraction of fluorine from the anion to the cationic complex
occurred. By using bulky fluorinated tetraarylborate anions the
fluoro transfer onto the metal center could be avoided.2,9–11
Weakly coordinating anions recently became more and more
important for ionic liquids (ILs) because they show a much lower
tendency to coordinate to the cation than the halide counterions.2
Due to the weaker interaction between the cation and the bulky
anion the properties of ILs like melting point, viscosity, high
⇑
Corresponding author. Tel.: +49 0351 46338571; fax: +49 351 46339679.
E-mail address: thomas.strassner@chemie.tu-dresden.de (T. Strassner).
Figure 1. General structure of the investigated aryl alkyl imidazolium [BArF3,5] ILs.
http://dx.doi.org/10.1016/j.tetlet.2016.06.082
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

3454
M. Kaliner, T. Strassner / Tetrahedron Letters 57 (2016) 3453–3456
N
N
N
Na[BArF3,5],
DCM, rt, 24h
N
a: n = 0 (methyl)
b: n = 3 (butyl)
c: n = 5 (hexyl)
d: n = 10 (undecyl)
n
n
Hal
[BArF3,5
]
R
R
R = 2-CH3
n = 0,3,5,10 (4a-d)
1a d
( - )
R = 2-CH3
n = 0,3,5,10
5a d
4-OCH3 n = 0,3,5,10
(
- )
6a d
4-OCH3 n = 0,3,5,10
2,4,6-CH3 n = 0,3,5,10
(2a-d)
3a d
2,4,6-CH3 n = 0,3,5,10
(
- )
(
- )
Scheme 1. Synthesis of borate TAAILs by anion metathesis.
group at the cation. Even redox active [BArF3,5] ILs containing a
ferrocenyl alkyl ammonium cation were synthesized.47
influence of the anion on the melting points of the TAAILs. We used
different alkyl chain lengths (small, medium, and long) and three
different electron donating substituents (2-Me, 4-OMe, 2,4,6-Me
[Mes]) which have a different influence of the electron density of
the phenyl ring. The methyl and mesityl groups exhibit an +I effect
that donates electrons through the sigma bonds into the ring, while
mesityl also has an additional steric influence. The +M effect of the
We focus on imidazolium based ILs, so-called tunable aryl alkyl
ionic liquids (TAAILs) that feature a (substituted) phenyl ring
together with an alkyl chain on the other nitrogen atom of the
heterocycle.48–50 Through the aryl ring additional mesomeric and
steric effects change the properties of the ionic liquids. Variation
of the substituent at the phenyl ring leads to different melting
points using the same alkyl chain length and anion depending on
the mesomeric or inductive effects of the substituent. Generally,
electron donating substituents show lower melting points than
electron withdrawing substituents as previously reported.49 The
exchange of the anion also has an influence on the properties of
the TAAILs. Coordination of small anions (Br, I) leads to higher
melting points compared to the bigger anions ([BF4], [PF6]).48 Ionic
liquids with N,N-dialkyl imidazolium cations and [BArF3,5] anion
have been found to exhibit interesting properties. Therefore we
investigated the influence of the [BArF3,5] anion on different aryl
alkyl imidazolium cations by combining the large cations with
bulky weakly coordinating anions. In this Letter we report the syn-
methoxy group results in an electron donation through the p-sys-
tem. To visualize the electronic effects we calculated the electro-
static surface potential (ESP)54 of the substituted 1-aryl 3-butyl
imidazolium cations 4b, 5b, and 6b as shown in Figure 2. To com-
pare the ESP’s of the imidazolium cations with different sub-
stituents (2-Me, 4-OMe, Mes) we kept the chain length (C4H9)
constant. Cations carry a positive charge all over the molecule,
the ESP representations differentiate between a more positive
potential at the imidazolium core (blue) and a negative potential
at the end of the alkyl chain and/or at the functional groups
(red). Substituents in para position at the aryl ring, especially
OMe, show a more negative potential than the methyl group at
the ortho position.
theses and characterization of 1-aryl 3-alkyl imidazolium [BArF3,5
]
TAAILs with mesityl substituents 6a–d lead to higher melting
points due to the steric influence of the methyl groups at the phenyl
ring (Table 1). If there are two methyl groups in ortho position to
the imidazolium ring the planes of the phenyl ring and of the imi-
dazolium ring55 are orthogonal to each other as could be demon-
strated by quantum chemical calculations (Fig. 2). [BArF3,5] ILs
4a–5d with mono substituted 2-Me and 4-OMe show lower melt-
ing points than the sterically hindered mesityl [BArF3,5] ILs, cer-
tainly due to minor steric influences between the aryl ring and
the imidazolium core.
The melting points of the imidazolium halide salts with 2-Me
1a–d and 4-OMe 2a–d show a minimum for the medium sized
hexyl chain length whereas for the mesityl imidazolium halide
salts 3a–d the melting points decrease by increasing alkyl chain
length.
ILs. The acronym [PhRCnIm][BArF3,5] describes TAAILs 4a–6d where
PhR characterizes type and position of the substituent R at the phe-
nyl ring and Cn the length of the alkyl chain at the imidazolium
core (Im). The general structure is given in Figure 1.
Results and discussion
The imidazolium halide salts 1a–3d were synthesized from the
commercially available anilines using previously described reac-
tions.48 The aryl imidazoles were accessible via a ring closing reac-
tion using
a one-pot procedure with glyoxal, formaldehyde,
ammonium chloride, and the respective anilines, followed by a
nucleophilic substitution with an alkyl halide. The aryl imida-
zolium [BArF3,5] salts 4a–6d were synthesized by anion metathesis
with Na[BArF3,5] from the aryl imidazolium halide salts 1a–3d as
By exchanging the halide anion with [BArF3,5] the behavior of
the methyl imidazolium ILs 4a–d changed, leading to generally
lower melting points with increasing chain length as can also be
shown in Scheme 1. After addition of an equimolar amount of Na
51
[BArF3,5
]
to the solution of the halide salts 1a–3d in
dichloromethane the reaction mixture was stirred at room
temperature for 24 h. After removal of the sodium halide salt and
solvent, TAAILs 4a–6d were obtained in excellent yields between
88% and 99%.52 The synthesized TAAILs show no transformation
when exposed to air and can be washed with water. They are air
and water stable and do not need to be handled or stored in a
glovebox. The sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]bo-
rate (Na[BArF3,5]) was synthesized through a modified route
according to Smith et al. using a Grignard reaction.53 Because of
the high and sometimes explosive reactivity of the Grignard reac-
tant the reaction should be handled with care. No problems with
the reaction could be observed following the instructions. The
azeotropic distillation was carried out with toluene instead of
benzene and provided the Na[BArF3,5] salt as a colorless solid.
We investigated the influence of the substituent at the aryl ring
and the alkyl chain length of the cations in comparison to the
Table 1
Melting points of 1-aryl 3-alkyl imidazolium [BArF3,5] ionic liquids 4a–6d and 1-aryl
3-alkyl-imidazolium halide salts 1a–3d
Anion
R
Me (C1)
Bu (C4)
Hex (C6)
Undec (C11)
[BArF3,5
]
2-Me
4-OMe
2,4,6-Me
71
83
102
70
44
86
41
68
70
À27d
À23d
À21d
47b
46b
60b
2-Me
4-OMe
2,4,6-Me
119a
127a
160a
61b
56a
90a
lb,c
Halide
la,c
68a
a
b
c
Imidazolium salts with iodide counterions.
Imidazolium salts with bromide counterions.
Ionic liquids are liquid (l) at room temperature.
d
Glass transition temperature upon cooling; all melting points are given in °C.

M. Kaliner, T. Strassner / Tetrahedron Letters 57 (2016) 3453–3456
3455
Figure 2. Electrostatic surface potential (ESP) of 1-aryl 3-butyl imidazolium cations 4b, 5b, and 6b.
observed for the mesityl imidazolium [BArF3,5] salts 6a–d. Only for
the methoxy imidazolium ILs 5a–d no dependence from the chain
length could be determined. The free rotation of the –OMe group
could favor different kinetic hindrance leading to alternating melt-
ing points with increasing alkyl chain length. At long chain lengths
the influence of the substituent at the aryl ring becomes less
important.
Apart from the 1-mesityl 3-methyl imidazolium [BArF3,5] IL 6a
all other synthesized borate salts fulfill the formal definition and
are ionic liquids, the TAAILs with n-undecyl chain are even RTILs
but appear to be highly viscous liquids. In DSC measurements
TAAILs 4d, 5d, and 6d show glass transition temperatures between
À27 and À21 °C. Around À70 to À50 °C a strong exothermic effect
could be observed due to recrystallization/post-crystallization
effects. Upon heating the formation of a melt around À24 to À17 °C
could be detected. The reason for the relatively high melting points
of the TAAILs can be explained by the interaction between the CF3
group and the phenyl ring of the borate anion which benefits an
easier crystallization. In comparison to the alkyl imidazolium
[BArF3,5] ILs42 derived from methylimidazole (MIM), they show
surprisingly lower melting points for the same chain lengths
given that the melting points decrease with increasing chain
length. This could be shown for halides, [BF4], [PF6]48 and now also
for the [BArF3,5] anion. The only exception so far are [PhMesCnIm]
ILs with [NTf2] counterion48 showing melting points near room
temperature independent from the alkyl chain length (Tables 1
and 2). This behavior was only observed for the mesityl substituted
[PhMesCnIm] ILs, other substituents show a different behavior
depending on the anion used as was presented above for the
methyl and methoxy substituents.
All TAAILs proved to be stable under the applied conditions in
TGA experiments (rt-850 °C, heating rate 5 K minÀ1, air) up to tem-
peratures between 308 and 373 °C. All salts show multiple degra-
dation processes with at least four to six degradation steps with
the exception of 5c which only showed three steps with a major
loss of 78 mass % in the first decomposition step. Details of the
TGA experiments are given in the Supporting information.
It could be shown that the anion has a considerable influence on
the properties of the ionic liquids but also the substituent or alkyl
chain length play an important role in varying the melting points
of the TAAILs.
(Table 2). For example the [C10MIM] [BArF3,5] or [C12MIM] [BArF3,5
]
Conclusion
ILs have melting points around 70–85 °C whereas the [PhMesC11Im]
[BArF3,5] IL 6d is liquid at room temperature causing a temperature
difference of over 80 K. This can be traced back to the expansion of
In conclusion a series of 1-aryl 3-alkyl imidazolium [BarF3,5
]
ionic liquids were synthesized. These air and water stable TAAILs
show relatively low melting points in comparison to the large
anions and cations used. The melting points can be lowered by
increasing the alkyl chain length even leading to RTILs for the long
n-undecyl chain length. The aryl imidazolium [BarF3,5] ILs show
significantly lower melting points than their alkyl imidazolium
[BArF3,5] ionic liquid counterparts and have decomposition tem-
peratures between 308 and 373 °C. Electrostatic surface potentials
for the 1-aryl 3-butyl imidazolium cations were calculated to gain
insight into the electron density distribution.
the p-system as it was also observed for pyridinium versus isochi-
nolinium [BArF3,5] ILs.42 The additional phenyl ring in combination
with the [BArF3,5] anion leads to lower melting points. This could
also be confirmed for mesityl substituted [PhMesCnIm] [BArF3,5
]
ILs 6a–6d in comparison to [PhMesCnIm] [BF4] ILs. The sterically
more demanding [BArF3,5] anion leads to lower melting points
for mesityl ILs with C1–C6 alkyl chain lengths compared to the
smaller [BF4] anion, whereas for the long n-undecyl chain both
mesityl substituted [PhMesCnIm] ILs are liquid at room tempera-
ture. For different counterions it could be shown that the behavior
of ionic liquids with mesityl substituents is mostly predictable
Acknowledgements
We thank the Center for Information Services and High Perfor-
mance Computing (ZIH) in Dresden for the provided computation
time.
Table 2
42
Melting points (mp) of alkyl imidazolium [BArF3,5
]
and aryl imidazolium [NTf2]/
[BF4] ILs48
Alkyl imidazolium ILs
[EMIM] [BArF3,5
[BMIM] [BArF3,5
[C6MIM] [BArF3,5
Mp in °C
Aryl imidazolium ILs
Mp in °C
Supplementary data
]
]
134
104
82
85
72
[PhMesC1Im] [NTf2]
[PhMesC4Im] [NTf2]
[PhMesC6Im] [NTf2]
[PhMesC11Im] [NTf2]
[PhMesC1Im] [BF4]
[PhMesC4Im] [BF4]
[PhMesC6Im] [BF4]
[PhMesC11Im] [BF4]
71
26
40
0
137
92
Supplementary data (experimental and characterization data,
1H, 13C, and 19F NMR chemical shift values, elemental analyses,
DSC, and TGA data) for compounds 4a–6d are provided together
with the details of the quantum chemical calculations) associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.tetlet.2016.06.082.
]
[C10MIM] [BArF3,5
[C12MIM] [BArF3,5
]
]
[C12Py] [BArF3,5
[C12Iq] [BArF3,5
]
]
64
72
74
À49

3456
M. Kaliner, T. Strassner / Tetrahedron Letters 57 (2016) 3453–3456
41. Wu, J.; Lan, Z.; Lin, J.; Huang, M.; Huang, Y.; Fan, L.; Luo, G. Chem. Rev. 2015,
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51. 1 equiv imidazolium iodide salt was dissolved in 14 ml dichloromethane and
1 equiv Na[BArF3,5] was added. After stirring the reaction mixture at room
temperature for 24 h the precipitate was filtered off and the remaining solvent
was removed. The product was dried in vacuo.
52. Characterization data of selected compounds: compound 4a: mp 71 °C, 1H
NMR (600 MHz, CDCl3): d = 8.07 (s, 1H, NCHN), 7.69 (s, 8H, B-Ar-H), 7.51 (s, 5H,
Ar-H + B-Ar-H), 7.38 (d, J = 8.0 Hz, 1H, Ar-H), 7.31 (t, J = 8.0 Hz, 1H, Ar-H), 7.25
(s, 1H, NCHCHN), 7.13 (s, 1H, NCHCHN), 7.09 (d, J = 7.5 Hz, 1H, Ar-H), 3.80 (s,
3H, NCH3), 2.07 (s, 3H, o-CH3) ppm, 13C NMR (150 MHz, CDCl3): d = 161.7 (q,
JB = 50 Hz, 4Ci-B), 134.7 (s, 8B-Ar-C), 134.3 (NCHN), 132.8 (Ci-o-CH3), 132.5 (Ar-
C), 132.4 (Ci-N), 132.4 (Ar-C), 129.0 (qq, JF = 31 Hz, JB = 2.4 Hz, 8Ci-CF3), 128.2
(Ar-C), 125.5 (Ar-C), 124.7 (NCHCHN), 124.5 (q, JF = 272 Hz, CF3), 123.7
(NCHCHN), 117.5 (m, 4B-Ar-C), 36.6 (NCH3), 16.8 (o-CH3) ppm, 19F NMR
(283 MHz, CDCl3): d = À63.0 (CF3) ppm, Elemental analysis for C43H25N2BF24
:
calcd C, 49.83; H, 2.43; N, 2.70, found C, 49.79; H, 2.25; N, 2.71. Compound 5b:
mp 44 °C, 1H NMR (300 MHz, CDCl3): d = 8.19 (s, 1H, NCHN), 7.69 (s, 8H, B-Ar-
H), 7.51 (s, 4H, B-Ar-H), 7.28 (t, J = 1.8 Hz, 1H, NCHCHN), 7.15 (d, J = 9.3 Hz, 2H,
Ar-H), 7.13 (t, J = 1.9 Hz, 1H, NCHCHN), 6.99 (d, J = 9.1 Hz, 2H, Ar-H), 4.05 (t,
J = 7.3 Hz, 2H, NCH2), 3.82 (s, 3H, p-OCH3), 1.82 (m, 2H, NCH2CH2), 1.32 (m, 2H,
CH2CH3), 0.91 (t, J = 7.2 Hz, 3H, CH2CH3) ppm, 13C NMR (75.5 MHz, CDCl3):
d = 162.0 (Ci-p-OCH3), 161.6 (q, JB = 50 Hz, 4Ci-B), 134.7 (s, 8B-Ar-C), 131.8
(NCHN), 129.0 (qq, JF = 31 Hz, JB = 2.8 Hz, 8Ci-CF3), 126.0 (Ci-N), 124.5 (q,
JF = 272 Hz, CF3), 123.7 (2Ar-C), 123.0 (NCHCHN), 122.7 (NCHCHN), 117.5 (m,
4B-Ar-C), 116.0 (2Ar-C), 55.8 (p-OCH3), 50.8 (NCH2) 31.6 (NCH2CH2), 19.3
(CH2CH3), 12.9 (CH2CH3) ppm, 19F NMR (283 MHz, CDCl3): d = -63.0 (CF3) ppm,
Elemental analysis for C46H31N2OBF24: calcd C, 50.48; H, 2.85; N, 2.56, found C,
50.14; H, 2.75; N, 2.82. Compound 6d: mp liquid at rt, 1H NMR (300 MHz,
DMSO-d6): d = 9.43 (s, 1H, NCHN), 8.09 (s, 1H, NCHCHN), 7.93 (s, 1H, NCHCHN),
7.62 (s, 12H, B-Ar-H), 7.10 (s, 2H, Ar-H), 4.26 (t, J = 7.0 Hz, 2H, NCH2), 2.28 (s,
3H, p-CH3), 1.98 (s, 6H, o-CH3), 1.87 (m, 2H, NCH2CH2), 1.15 (m, 16H, alkyl-
CH2), 0.75 (t, J = 6.4 Hz, 3H, CH2CH3) ppm, 13C NMR (75.5 MHz, DMSO-d6):
d = 161.0 (q, JB = 50 Hz, 4Ci-B), 140.2 (Ci-p-CH3), 137.2 (NCHN), 134.2 (2Ci-o-
CH3), 134.0 (s, 8B-Ar-C), 131.1 (Ci-N), 129.2 (2Ar-C), 128.4 (qq, JF = 32 Hz,
JB = 2.8 Hz, 8Ci-CF3), 124.0 (q, JF = 272 Hz, CF3), 124.0 (NCHCHN), 123.2
(NCHCHN), 117.5 (m, 4B-Ar-C), 49.3 (NCH2), 31.2 (NCH2CH2), 29.0, 28.8, 28.8,
28.7, 28.6, 28.2, 25.3 (alkyl-CH2), 21.9 (CH2CH3), 20.4 (p-CH3), 16.7 (2o-CH3),
13.6 (CH2CH3) ppm, 19F NMR (283 MHz, DMSO-d6): d = À62.6 (CF3) ppm,
Elemental analysis for C55H49N2BF24: calcd C, 54.83; H, 4.10; N, 2.33, found C,
54.85; H, 4.00; N, 2.52.
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