1,2,4-triazole-based tunable aryl/alkyl ionic liquids

Article abstract of DOI:10.1021/jo101784v

Ionic liquids based on aryl-/alkyl-substituted imidazolium salts constitute a new generation of ionic liquids with a high degree of flexibility. The new concept could now also be extended to aryl-/alkyl-substituted 1,2,4-triazolium salts. The two differen

Full text of DOI:10.1021/jo101784v

pubs.acs.org/joc
SCHEME 1. Synthesis of 1-Phenyl-1H-1,2,4-triazole, 1, 4-Phenyl-
1,2,4-Triazole-Based Tunable Aryl/Alkyl
Ionic Liquids
4H-1,2,4-triazole, 2,21 and 1-Phenyl-1H-imidazole, 322,23
Dirk Meyer and Thomas Strassner*
€
Physikalische Organische Chemie, Technische Universitat
Dresden, Bergstrasse 66, 01069 Dresden, Germany
thomas.strassner@chemie.tu-dresden.de
Received September 21, 2010
most prominient, while only some examples of ILs based on
1,2,4-triazolium cations have been reported.9-18
Last year we introduced a new generation of ionic liquids
which differ from all previously known imidazolium ionic
liquids by the introduction of aryl substituents (TAAILs =
Tunable Aryl/Alkyl Ionic Liquids) at the imidazolium core.19
They allow the introduction of additional þM/-M effects to
tune their properties, a new concept with quite beneficial
effects.20 We decided to investigate whether the underlying
principle can also be transferred to other heterocycles and
present the results on the synthesis of phenyl-1,2,4-triazole-
based TAAILS with different chain lengths.
Synthesis of the different salts starts with the preparation
of the phenyl-substituted triazoles 1 and 2 (Scheme 1).
1-Phenyl-1H-1,2,4-triazole, 1, is accessible by the well-established
copper(I)-catalyzed coupling reaction of 1H-1,2,4-triazole and
iodobenzene, while 4-phenyl-4H-1,2,4-triazole, 2, was synthe-
sized according to a published procedure from 1,2-diformyl-
hydrazine and aniline (Scheme 1).21
Ionic liquids based on aryl-/alkyl-substituted imidazo-
lium salts constitute a new generation of ionic liquids with
a high degree of flexibility. The new concept could now
also be extended to aryl-/alkyl-substituted 1,2,4-triazo-
lium salts. The two different phenyl-substituted 1H- and
4H-1,2,4-triazoles have been synthesized by a coupling
reaction or a one-pot synthesis with diformylhydrazine,
respectively. Reaction with alkyl bromides provided the
1,2,4-triazolium bromides, and an anion-exchange reac-
tion led to the corresponding bis(trifluoromethylsulfonyl)-
imide salts with melting points well below 100 °C.
Ionic liquids (ILs) have recently received a lot of attention
as alternative solvents for organic synthesis,1-5 plating,6 the
dissolution of cellulose7 or as a component of dye sensitized
solar cells.8 Several classes of ionic liquids based on diffe-
rent organic cations have been investigated throughout
the last years with bisalkylimidazolium salts being the
Two types of aryl-/alkyl-substituted 1,2,4-triazolium salts
(1f 4a-g and 2f 5a-g) have been synthesized by reaction
of the corresponding triazoles with alkyl bromides of differ-
ent chain lengths (Table 1). For comparison, the analogous
(1) Giernoth, R. Top. Curr. Chem. 2007, 276, 1–23.
(2) Ionic Liquids in Synthesis, 2nd ed.; Welton, T., Wasserscheid, P., Eds.;
Wiley-VCH: Weinheim, 2008.
(3) Davis, J. H., Jr.; Wasserscheid, P. Ionic Liquids in Synthesis, 2nd. ed.;
Welton, T., Wasserscheid, P., Eds.; Wiley-VCH: Weinheim, 2008; Vol. 1,
pp 45-55.
(4) Boesmann, A.; Schulz, P. S.; Wasserscheid, P. Monatsh. Chem. 2007,
138, 1159–1161.
(11) Mirzaei, Y. R.; Twamley, B.; Shreeve, J. M. J. Org. Chem. 2002, 67,
9340–9345.
(12) Xue, H.; Twamley, B.; Shreeve, J. M. J. Org. Chem. 2004, 69, 1397–
1400.
(13) Gao, Y.; Gao, H.; Piekarski, C.; Shreeve, J. M. Eur. J. Inorg. Chem.
2007, 4965–4972.
(14) Vestergaard, B.; Bjerrum, N. J.; Petrushina, I.; Hjuler, H. A.; Berg,
R. W.; Begtrup, M. J. Electrochem. Soc. 1993, 140, 3108–3113.
(15) Singh, R. P.; Verma, R. D.; Meshri, D. T.; Shreeve, J. M. Angew.
Chem., Int. Ed. 2006, 45, 3584–3601.
(5) Lombardo, M.; Pasi, F.; Trombini, C.; Seddon, K. R.; Pitner, W. R.
Green Chem. 2007, 9, 321–322.
(6) Koenig, U.; Sessler, B. Trans. Inst. Met. Finish. 2008, 86, 183–188.
(7) Pinkert, A.; Marsh, K. N.; Pang, S.; Staiger, M. P. Chem. Rev. 2009,
109, 6712–6728.
(16) Handy, S. T. Curr. Org. Chem. 2005, 959–988.
(17) Wang, R.; Jin, C.-M.; Twamley, B.; Shreeve, J. M. Inorg. Chem.
2006, 45, 6396–6403.
(8) Zakeeruddin, S. M.; Graetzel, M. Adv. Funct. Mater. 2009, 19, 2187–
2202.
(18) Trohalaki, S.; Pachter, R.; Drake, G. W.; Hawkins, T. Energy Fuels
2005, 19, 279–284.
(9) Mirzaei, Y. R.; Shreeve, J. M. Synthesis 2003, 24–26.
(10) Mirzaei, Y. R.; Xue, H.; Shreeve, J. M. Inorg. Chem. 2004, 43, 361–
367.
(19) Ahrens, S.; Peritz, A.; Strassner, T. Angew. Chem., Int. Ed. 2009, 48,
7908–7910.
(20) Giernoth, R. Angew. Chem., Int. Ed. 2010, 49, 5608–5609.
DOI: 10.1021/jo101784v
r
Published on Web 11/29/2010
J. Org. Chem. 2011, 76, 305–308 305
2010 American Chemical Society

Meyer and Strassner
JOCNote
TABLE 1. Triazolium Salts 4a-g and 5a-g, Imidazolium Salts 6a-g
n
mp [°C] yld %
mp [°C] yld %
mp [°C] yld %
2
3
5
6
7
4a
4b
4c
4d
4e
224
189
116
133
123
160
156
61
40
80
74
69
37
59
5a
5b
5c
5d
5e
5f
187
137
118
121
127
151
154
97
97
78
82
69
64
74
6a
6b
6c
6d
6e
6f
111
93
37
99
82
53
30
53
59
116
100
65
a
11 4f
14 4g
-
56
5g
6g
aLiquid at room temperature, no melting point observed.
FIGURE 1. Melting points of the triazolium and imidazolium salts.
TABLE 2. Triazolium Salts 7a-g and 8a-g, Imidazolium salts 9a-g
n
mp [°C] yld %
mp [°C] yld %
mp [°C] yld %
2
7a
7b
7c
7d
7e
51
99
46
39
41
40
38
98
88
90
62
91
84
87
8a
8b
8c
8d
8e
8f
55
73
45
36
44
51
68
80 9a
33
23
18
27
-
10
13
71
66
52
37
86
57
81
3
5
6
7
90 9b
90 9c
83 9d
100 9e
83 9f
81 9g
a
11 7f
14 7g
8g
aLiquid at room temperature, no melting point observed.
N-phenyl-substituted imidazolium bromides 6a-g have
been prepared from 1-phenyl-1H-imidazole 3.22,23 In general
(with the exception of 6c), the imidazolium salts show lower
melting points compared to those of both types of triazolium
salts. The position of the third, not substituted nitrogen atom
has no significant influence on the observed melting point.
The lowest melting points of the triazolium-based ILs are
observed for chains of medium lengths (C5-C7). The short
chains lead to significantly higher melting points while the
C11 and C14 chains seem to level out at around 150 °C.
Single crystals were grown from one of the triazolium salts
with a higher melting point (5a) by slow evaporation of a
methanol solution and subjected to X-ray diffraction. An
ORTEP style representation of the structure of 5a is given in
the Supporting Information.
Exchange of the bromide anions by anion metathesis with
lithium-bis(trifluoromethylsulfonyl)imide leads to the corre-
sponding bis(trifluoromethylsulfonyl)imide salts (4a-g f
7a-g and 5a-g f 8a-g) (Table 2). The imidazolium bis-
(trifluoromethylsulfonyl)imide salts 9a-g were prepared
from the bromides 6a-g using the same reaction conditions.
As expected, the melting points of the triazolium salts
decreased dramatically (differences of up to 173 °C were
observed) because of the counterion exchange (Figure 1).
Both types of triazolium salts investigated do not qualify as
room temperature ionic liquids (RTILs), as their melting
points are around 45 °C ((10 °C). Also in the case of the
FIGURE 2. Charge distribution in the imidazolium (center) and
triazolium cations.
bis(trifluoromethylsulfonyl)imide salts, the melting points of
the triazolium TAAILs are significantly higher compared to
those of the corresponding imidazolium TAAILs (Table 2).
It is obvious that for all bis(trifluoromethylsulfonyl)imide
salts with alkyl chains longer than C5 the influence of the
different alkyl chains is small. However, the exchange of
the counterions leads to a higher temperature stability, the
phenyl alkyl triazolium salts 7a-g and 8a-g start to decom-
pose at temperatures of 360-400 °C (see Supporting Infor-
mation for TGA spectra).
We also calculated the ESP surfaces (Figure 2) of both
types of phenyl-substituted triazolium cations by high-level
density functional theory calculations (B3LYP/6-311þþ
G(d,p))24 and compared the results to those of the corre-
sponding imidazolium salts. The ESP surface representa-
tions of the triazolium and imidazolium cations show a
similar charge distribution with the exception that the elec-
tron density of the additional nitrogen in the triazolium
backbone shows up as a center of negative charge.
(24) All calculations were performed with Gaussian03 using the density
functional/Hartree-Fock hybrid model Becke3LYP and the split valence
triple-ζ (TZ) basis set 6-311þþG(d,p). No symmetry or internal coordinate
constraints were applied during optimizations. The limits used for visualiza-
tion were 0.125 au (red) to 0.175 au (blue). Details and references are given in
the Supporting Information.
(21) Becker, H. G. O.; Hoffmann, G.; Kim, M. G.; Knuepfer, L. J. Prakt.
Chem. 1988, 330, 325–337.
(22) Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H. Synthesis 2003,
2661–2666.
(23) Strassner, T.; Ahrens, S.; Zeller, A. Patent WO/2006/058535, 2006.
306 J. Org. Chem. Vol. 76, No. 1, 2011

Meyer and Strassner
JOCNote
We have synthesized a series of novel triazolium bromide
salts 4a-g and 5a-g containing one phenyl substituent and
an alkyl side chain. By changing the central heterocyclic
core from an imidazolium to a triazolium core we could
prove that the concept of an aryl(sp2)/alkyl(sp3) substitu-
tion can be extended to other heterocycles. The additional
nitrogen atom in the central heterocycle leads to an increase
of the melting point compared to the corresponding imi-
dazolium bromides 6a-g. After exchange of the counterion,
the triazolium salts 7a-g and 8a-g show melting points
around 45 °C together with a high temperature stability
(TGA spectra given in the Supporting Information). The
position of the third triazole nitrogen atom does not have a
significant influence on the melting point of the systems
studied here. But we know that substituents at phenyl rings
have an significant effect on the properties of the resulting
TAAILs and therefore plan to study the effect of substitu-
ents on the aryl ring also for this new class of triazolium
TAAILs.
110 °C. After cooling to room temperature, 2 mL of petro-
leum ether was added. The formed precipitate was separated
by filtration, washed with THF/petroleum ether (1:1), and
dried in vacuo. Yield: 0.23 g (97%); mp 187 °C. Found: C 46.9,
H 4.75, N 16.8. Calcd for C10H12N3Br: C 47.3, H 4.8, N 16.5.
δ
H (300 MHz, DMSO-d6, 25 °C) 1.56 (3 H, t, J 7.3), 4.49 (2 H,
q, J 7.3), 7.71 (3 H, m), 7.86 (2 H, d, J 7.5), 9.81 (1 H, bs), 10.87
(1 H, bs). δC (75.5 MHz, DMSO-d6, 25 °C) 13.4, 47.4, 122.4,
130.1, 130.3, 132.1, 141.2, 142.6.
Synthesis of 5b-5g follows an analogous procedure and is
described in detail in the Supporting Information.
Synthesis of 1-Phenyl-3-ethyl-1H-imidazolium Bromide, 6a.
An ACE pressure tube was charged with 0.20 g (0.0014 mol) of
1-phenyl-1H-imidazole, 3, and 0.11 mL (0.16 g, 0.015 mol) of
bromoethane in 5 mL of THF. The reaction mixture was stirred
for 72 h at 90 °C. After cooling to room temperature, a solid
separated from the reaction solvent. After decantation of the
solvent, the crude product was washed with THF or THF/
diethyl ether 1:2 and dried in vacuo. Yield: 0.13 g (37%); mp
111 °C. Found: C 49.9, H 5.15, N 10.6. Calcd for C11H13N2Br
3
0.6 H2O: C 50.1, H 5.4, N 10.6. δH (300 MHz, DMSO-d6, 25 °C)
1.52 (3 H, t, J 7.3), 4.30 (2 H, q, J 7.3), 7.67 (3 H, m), 7.81 (2 H, d,
J 7.4), 8.08 (1 H, s), 8.34 (1 H, s), 9.86 (1 H, s). δC (75.5 MHz,
DMSO-d6, 25 °C) 14.8, 44.7, 121.0, 121.8, 123.0, 129.7, 130.2,
134.8, 135.2.
Experimental Section
4-Phenyl-4H-1,2,4-triazole, 2,21 and 1-phenyl-1H-imidazole,
3,22 were prepared according to literature procedures.
Synthesis of 6b-6g follows an analogous procedure and is
described in detail in the Supporting Information.
Synthesis of 1-Phenyl-1H-1,2,4-triazole, 1. After standard
cycles of evacuation and refilling with argon, an oven-dried
Schlenk tube equipped with a magnetic stir bar was charged with
0.21 g of Cu2O (0.0015 mol), 0.52 g of 1,10-phenanthroline
(0.0029 mol), 1.00 g of 1H-1,2,4-triazole (0.0145 mol), and 6.01 g
of K2CO3 (0.044 mol). The tube was evacuated, filled with
argon, and capped with a rubber septum. Iodobenzene (2.42
mL, 4.43 g, 0.022 mol) was added via a syringe, followed by
anhydrous and degassed DMF (10 mL). The reaction mixture
was stirred for 48 h at 120 °C under a positive pressure of argon.
After cooling to room temperature, it was diluted with 20 mL of
dichloromethane and filtered through a plug of Celite, the filter
cake being further washed with dichloromethane (∼20 mL).
The resulting organic layer was washed with water and brine
and then dried over Na2SO4. The solvent was removed in vacuo.
The crude product was purified by flash column chromatogra-
phy on silica gel (gradient ethyl acetate/petroleum ether 2:8 to
ethyl acetate) to provide 1 (1.36 g, 65%) as a pale-yellow solid.
Found: C 66.0, H 5.0, N 28.9. Calcd for C8H7N3: C 66.0, H 4.9,
N 29.0. δH (300 MHz, DMSO-d6, 25 °C) 7.41 (1 H, t, J 7.4), 7.58
(2 H, t, J 7.4), 7.87 (2 H, d, J 7.5), 8.25 (1 H, s), 9.31 (1 H, s). δC
(75.5 MHz, DMSO-d6, 25 °C) 119.4, 127.8, 129.8, 136.7, 142.3,
152.4.
Synthesis of 1-Phenyl-4-ethyl-1H-1,2,4-triazolium Bromide,
4a. A sealed ACE pressure tube was charged with 0.22 g
(0.0015 mol) of 1-phenyl-1H-1,2,4-triazole, 1, and 1.00 mL
(0.013 mol) of bromoethane. The reaction mixture was stirred
for 24 h at 110 °C. After cooling to room temperature 2 mL of
petroleum ether was added. The formed precipitate was sepa-
rated by filtration, washed with THF/petroleum ether (1:1), and
dried in vacuum. Yield: 0.23 g (61%); mp 224 °C. Found: C
47.25, H 4.9, N 16.5. Calcd for C10H12N3Br: C 47.3, H 4.8, N
16.5. δH (300 MHz, DMSO-d6, 25 °C) 1.57 (3 H, t, J 7.3), 4.37
(2 H, q, J 7.3), 7.69 (3 H, m), 7.94 (2 H, d, J 7.5), 9.52 (1 H, s),
11.01 (s, 1 H). δC (75.5 MHz, DMSO-d6, 25 °C) 14.2, 43.4, 120.5,
130.1, 130.3, 135.0, 141.4, 144.8.
Synthesis of 1-Phenyl-4-ethyl-1H-1,2,4-triazolium Bis(trifluoro-
methylsulfonyl)imide, 7a. 4a [0.10 g (0.0004 mol)] was dissolved in
5 mL of water and reacted with 0.12 g (0.0004 mol) of lithium
bis(trifluoromethylsulfonyl)imide. The product separated from the
aqueous phase. After addition of 10 mL of dichloromethane, the
phases were separated. The aqueous phase was extracted twice
with 5 mL of dichloromethane. The combined organic phases were
washed with 10 mL of water and dried over Na2SO4. The solvent
was removed in vacuo, yielding the product as a white solid. Yield:
0.17 g (98%); mp 51 °C. Found: C 31.9, H 2.6, N 12.5, S 13.7. Calcd
for C12H12F6N4O4S2: C 31.7, H 2.7, N 12.3, S 14.1. δH (300 MHz,
DMSO-d6, 25 °C) 1.56 (3 H, t, J 7.3), 4.35 (2 H, q, J 7.3), 7.71 (3 H,
m), 7.92 (2 H, m), 9.46 (1 H, s), 10.87 (1 H, s). δC (75.5 MHz,
DMSO-d6, 25 °C) 14.1, 43.4, 117.3, 120.5, 130.1, 130.4, 135.0,
141.3, 144.8. δF (282 MHz, DMSO-d6, 25 °C) -78.71.
Synthesis of 7b-7g follows an analogous procedure and is
described in detail in the Supporting Information.
Synthesis of 4-Phenyl-1-ethyl-1,2,4-triazolium Bis(trifluoromethyl-
sulfonyl)imide, 8a. 5a [0.10 g (0.0004 mol)] was dissolved in 5 mL of
water and reacted with 0.12 g (0.0004 mol) of lithium bis(trifluoro-
methylsulfonyl)imide. The product separated from the aqueous
phase. After addition of 10 mL of dichloromethane, the phases
were separated. The aqueous phase was extracted twice with 5 mL of
dichloromethane. The combined organic phases were washed with
10 mL of water and dried over Na2SO4. The solvent was removed in
vacuo, yielding the product as a white solid. Yield: 0.14 g (80%); mp
55 °C. Found: C 31.8, H 2.4, N 12.2, S 14.3. Calcd for C12H12-
F6N4O4S2: C 31.7, H 2.7, N 12.3, S 14.1. δH (300 MHz, DMSO-d6,
25 °C) 1.56 (3 H, t, J 7.3), 4.46 (2 H, q, J 7.3), 7.71 (3 H, m), 7.80
(2 H, m), 9.75 (1 H, s), 10.71 (1 H, s). δC (75.5 MHz, DMSO-d6,
25 °C) 13.4, 47.4, 117.3, 122.4, 130.2, 130.4, 132.1, 141.2, 142.8. δF
(282 MHz, DMSO-d6, 25 °C) -78.71.
Synthesis of 8b-8g follows an analogous procedure and is
described in detail in the Supporting Information.
Synthesis 1-Phenyl-3-ethyl-1H-imidazolium bis(trifluoromethyl-
sulfonyl)imide, 9a. 6a (0.06 g [0.0002 mol]) was dissolved in 5 mL
of water and reacted with 0.08 g (0.0003 mol) of lithium bis-
(trifluoromethylsulfonyl) imide. Due to the anion exchange, the
product separated from the aqueous phase. After addition of 10 mL
of dichloromethane, the phases were separated. The aqueous phase
was extracted twice with 5 mL of dichloromethane. The combined
Synthesis of 4b-4g follows an analogous procedure and is
described in detail in the Supporting Information.
Synthesis of 4-Phenyl-1-ethyl-4H-1,2,4-triazolium Bromide,
5a. An ACE pressure tube was charged with 0.15 g (0.001 mol)
of 4-phenyl-4H-1,2,4-triazole, 2, and 0.22 mL (0.003 mol)
of bromoethane. The reaction mixture was stirred for 24 h at
J. Org. Chem. Vol. 76, No. 1, 2011 307

Meyer and Strassner
JOCNote
organic phases were washed with 10 mL of water and dried over
Na2SO4. The solvent was removed in vacuo, yielding the product as
a colorless oil. Yield: 0.07 g (71%). mp 33 °C. Found: C 34.3, H 2.8,
N 9.4, S 13.9. Calcd for C13H13F6N3O4S2: C 34.4, H 2.9, N 9.3, S
14.1. Yield: 0.07 g (71%). δH (500 MHz, DMSO-d6, 25 °C) 1.51
(3 H, t, J 7.3), 4.28 (2 H, q, J 7.3), 7.59 (1 H, t, J 7.5), 7.68 (2 H, t,
J 7.4), 7.78 (2 H, d, J 8.3), 8.06 (1 H, t, J 1.7), 8.32 (1 H, t, J 1.7), 9.78
(1 H, s). δC (125.8 MHz, DMSO-d6, 25 °C) 14.7, 44.7, 118.1, 121.1,
121.8, 129.7, 130.1, 130.1, 134.8, 135.1.
Acknowledgment. D.M. thanks the “Evonik-Stiftung” for
financial support. We are grateful to the Center for High-
Performance Computing (ZIH) of the TU Dresden for the
generous allocation of computer time.
Supporting Information Available: Cartesian coordinates,
details of the quantum chemical calculations, references
(Gaussian 03, ref 4 in SI), experimental details, and 1H and 13
C
Synthesis of 9b-9g follows an analogous procedure and is
described in detail in the Supporting Information.
spectra for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
308 J. Org. Chem. Vol. 76, No. 1, 2011