Preparation of ionic liquids bearing o-carborane anion via N,N′-dialkylimidazol-2-ylidene carbene

Article abstract of DOI:10.1016/j.jorganchem.2008.10.056

In the present study, a novel ionic liquid including o-carborane anion was prepared. After the carbene formation of 1-ethyl-3-methylimidazolium halide ([EMIm]⁺[X]^-) by reaction with n-BuLi, the subsequent reaction with o-carborane af

Full text of DOI:10.1016/j.jorganchem.2008.10.056

Journal of Organometallic Chemistry 694 (2009) 1612–1616
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Preparation of ionic liquids bearing o-carborane anion via
N,N0-dialkylimidazol-2-ylidene carbene
*
Noriyoshi Matsumi , Mari Miyamoto, Keigo Aoi
Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, a novel ionic liquid including o-carborane anion was prepared. After the carbene for-
mation of 1-ethyl-3-methylimidazolium halide ([EMIm]+[X]ꢀ) by reaction with n-BuLi, the subsequent
reaction with o-carborane afforded the desired ionic liquid in moderate yields. The structure of the ionic
liquid was supported by 1H NMR and 11B NMR spectra.
Received 16 September 2008
Received in revised form 30 October 2008
Accepted 31 October 2008
Available online 7 November 2008
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Carborane
Ionic liquid
Carbene
Ionic conductivity
1. Introduction
series of carborane anion based imidazolium salts via ion exchange
reaction of silver or cesium salts of carborane anions [11]. By a sim-
In recent years, numerous number of papers have been pub-
lished on ionic liquids (ILs) because of their intriguing characteris-
tics such as non volatility, non flammability and high ionic
conductivity. Their potential utilities range from reaction media
[1], solvents for biomaterials [2], electrolytes for electrochemical
devices [3] and so forth. A versatile types of ILs have been devel-
oped so far, however, boron containing ILs are relatively limited
except that BFꢀ4 is widely employed as anion. These days, a variety
of organoboron functional materials [4] has been developed espe-
cially in the field of light emitting materials, electron transporting
materials, and bio-sensors. Because of diversity of organoboron
chemistry, it is attractive approach to further expand the library
of ILs.
ilar method, alkylpyridinium caborane salts were also prepared by
Zhu et al. [12]. It was found that alkylpyridinium type carborane
salt showed a comparatively low melting point. An open-face car-
borane cluster anion was also recently incorporated into ILs [13].
These ionic carboranes are interesting as potential reaction media
for nucleophilic substitution reactions, because carborane anions
generally show low coordinating power. Such ILs are also consid-
ered important for further investigation of super acid chemistry.
Moreover, ionic carborane derivatives have potential utility as tu-
mor targeting reagent in boron–neutron-capture-therapy (BNCT)
[14] or liquid neutron moderators in nuclear processing [15]. Very
recently, ILs having trialkylammoniododecaborates were also re-
ported by Gabel et al. [16].
To improve the selectivity of target cation transport, imidazoli-
um type molten salts bearing organoboron anion receptor [5] were
prepared via hydroboration of 1-allylimidazolium type molten
salts. This strategy was extended to design of organoboron solid
polymer electrolytes [6] and organoborate type zwitterionic mol-
ten salt [7]. The ILs bearing a perfluoroalkyltrifluoroborates [8]
were also reported by Tatsumi et al. Watanabe et al. reported lith-
ium cation containing organoborate ILs bearing oligoethers [9]. ILs
including a boronium cation were also reported by Rogers and co-
workers [10].
In the present study, we have undertaken to develop an alterna-
tive method for the preparation of carborane anion based ILs. As
synthetic method for ILs, several different approaches such as (1)
neutralization of amine with Brønsted acid [17], (2) ion exchange
reaction of imidazolium salts [18] and (3) reaction of imidazolid-
ene carbene [19] are known so far (Scheme 1). We have examined
to utilize these methodologies for the preparation of carborane
anion based ILs. Because of relatively low brønsted acidity of
carboranes, neutralization of amines with o-carborane did not take
place at all. A reaction of imidazolium hydroxide with o-carborane
was also attempted under several reaction conditions, however,
the mixture of starting materials was recovered. On the other
hand, desired molten salts bearing carborane anion were success-
fully prepared via the reaction of imidazolylidene carbene with
o-carborane (Scheme 2).
On the other hand, more boron-rich type molten salts including
carborane were reported in 2000. Larsen and Reed et al. reported a
* Corresponding author.
E-mail address: matsumi@agr.nagoya-u.ac.jp (N. Matsumi).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.056

N. Matsumi et al. / Journal of Organometallic Chemistry 694 (2009) 1612–1616
1613
2a)
anion exchange
N
N
R'
R
MY
- MX
Y
2b)
R'X
(alkyl halide)
anion exchange
HY
N
N
N
N
N
N
N
N
R'
R'
R
R
R
R'
R
Y
X
OH
3)
1)
HX
(acid)
HY
base
N
N
N
N
R'
R'
R
R
N
NH
Y
R
carbene
X
Scheme 1. Synthetic method for ionic liquids. (1) Neutralization of amine with brønsted acid, (2) ion exchange reaction of imidazolium salts and (3) reaction of
imidazolylidene carbene.
[Step 1]
n
BuLi/ hexane
N
N
R'X
N
N
N
N
+
R
R'
R
R
0 oC ~ r.t.
R'
..
H
X
carbene
1
(R = Me)
(R = Me)
(R = Et)
(R' = Et, X = Br)
2
(R'= nBu, X = Cl)
1a (R = Me, R' = Et, X = Br)
(R' =
nBu, X = Cl)
2a (R = Me, R' = Et)
1b (R = Me, R'=
1c (R = Et, R' =
n
Bu, X = Cl)
2b (R = Me, R'=
2c (R = Et, R' =
n
Bu)
nBu, X = Cl)
nBu)
HC
HC
N
N
R'
R
[Step 2]
H
3
C
HC
THF
0 oC ~ r.t.
4
4a (R = Me, R' = Et)
4b (R = Me, R'=
4c (R = Et, R' =
n
Bu)
n
Bu)
Scheme 2. Synthesis of ionic liquids bearing o-carborane anion.
It is widely known that deprotonation of 1,3-dialkylimidazoli-
um cation leads to formation of a carbene which is stabilized by
adjacent nitrogen atoms [20]. It is reported that such imidazolylid-
ene carbene undergoes a reaction with methyl carboxylate whose
pKa is 25 to form the corresponding IL. Since pKa of o-carborane is
23.3 [21], o-carborane is expected to readily react with 1,3-dial-
kylimidazol-2-ylidene carbenes under a similar reaction condition.
In the present method, it is not necessary to prepare silver or ce-
sium salts of carborane anions prior to the synthesis of ILs. Such a
facile synthetic procedure should be industrially more accessible in
comparison with the reported methods.
n-hexane was added at 0 °C, and the reaction mixture was stirred
for 5–60 min. Then, THF solution of equimolar amount of o-carbo-
rane was added, and the resulting solution was further stirred for
12 h while the reaction mixture was gradually warmed up to room
temperature. After the evaporation of the solvent, the residue was
extracted with 2-propanol and then purified by reprecipitation
into diethylether. The products were obtained as yellowish soft so-
lid in moderate yields. The results of the reactions using various
molten salts are listed in Table 1. With increasing the reaction time
for carbene formation step, yields of the product were improved.
When 1-butyl-3-methylimidazolium chloride (1b) or 1-ethyl-3-
butylimidazolium chloride (1c) was employed as starting materi-
als, the yields of the products were relatively lower compared with
the case of 1-ethyl-3-methylimidazolium bromide (1a). This
should be due to higher viscosity of the reaction mixture including
1b or 1c. Moreover, some of 1b and 1c frozen during the reaction
with nBuLi at 0 °C. When the reaction mixture was extracted with
2. Results and discussion
The preparation of carborane anion based ILs was carried out as
depicted in Scheme 2. To a THF solution of N,N0-dialkylimidazolium
halide, an equimolar amount or slight excess amount of n-BuLi in

1614
N. Matsumi et al. / Journal of Organometallic Chemistry 694 (2009) 1612–1616
Table 1
Table 2
Solubility test for 4a.
Synthesis of 4 under various reaction conditionsa.
Entry
Ionic liquid, mg
(mmol)
nBuLib,
mmol
Timec,
min
3d, mg
(mmol)
Timee, Yield, mg
Solvent
Dipole moment [D]
Solubility
h
(%)
Hexane
Toluene
CHCl3
Et2O
iPrOH
EtOH
0.09
0.31
1.15
1.15
1.66
1.66
1.75
1.78
–
2.69
2.87
3.44
3.90
ꢂ
ꢂ
ꢂ
ꢂ
s
s
ꢂ
ꢂ
s
ꢂ
s
ꢂ
s
1
2
3
1a 204 (1.07)
1.20
2.00
1.65
60
10
5
145 (1.01) 12
231 (1.60) 12
232 (1.61) 12
249 (91)
317 (78)
242 (59)
306 (1.60)
307 (1.61)
4
5
1b 269 (1.70)
1.80
2.12
10
10
245 (1.70) 12
247 (1.71) 12
232 (49)
192 (37)
320 (1.83)
THF
H2O
6
1c 189 (1.00)
1.00
30
144 (1.00) 19
187 (69)
THF/H2Oa
Acetone
MeOH
CH3CN
DMSO
a
b
c
Under nitrogen atmosphere.
n-butyllithium, in n-hexane, 1.63 mol/l.
Reaction time for Step 1, 0 °C ꢃ r.t.
d
In THF (entry1; 0.42 ml, entry 2; 0.6 ml, entry3; 0.8 ml, entry 4; 0.8 ml, entry 5;
0.8 ml, entry 6; 0.4 ml).
e
a
Reaction time for Step 2; 0 °C ꢃ r.t.
THF/H2O (v/v) = 1/1.
2-propanol, some insoluble part whose structure was unclear re-
mained. On the other hand, from the TLC analysis (ethyl acetate:
methanol (v/v) = 10:1) of the 2-propanol soluble part using
PdCl3–HCl(aq.) [22], one main spot (Rf 0.36) was observed, while
no other spot due to by-product or unreacted carborane was de-
tected. The structures of the obtained organoboron ILs were char-
acterized by 1H (Fig. 1) and 11B NMR spectra. The 1H NMR spectrum
of [EMIm]+½closo-1; 2-C2B10H11ꢁꢀ (4a) is represented in Fig. 1. In the
1H NMR spectra measured in CD3OD, wavy peaks that are charac-
teristic for carborane structure were observed for each system. The
peak due to the proton at 2nd position of imidazolium ring did not
appear in the 1H NMR spectrum, similarly to the case of previously
reported carborane based molten salts. This should be due to rapid
proton–deuterium exchange in CD3OD as reported in Ref. [23].
Instead of o-carborane, m-carborane was also examined as a
starting material, however, the reaction did not take place at all
possibly due to significantly larger pKa of m-carborane (27.9) in
comparison with that of o-carborane (23.3) [21].
The obtained ILs were stable under air. After stored under air for
4 weeks, no significant spectroscopic change was observed in the
1H NMR spectra.
The solubility of 4a in various organic solvents was examined at
room temperature. The results are listed in Table 2. The IL 4a was
well soluble in 2-propanol, ethanol, methanol and DMSO, while it
was insoluble in n-hexane, toluene, chloroform, diethylether, THF,
H2O, acetone and acetonitrile. Although o-carborane was not solu-
ble in THF/H2O (v/v = 1/1), 4a was readily soluble in THF/H2O (v/
v = 1/1), indicating improved hydrophilicity of 4a.
The ionic conductivity of 4a was evaluated by ac impedance
method after the sample was dried throughly. The observed ionic
conductivity was 2.9 ꢂ 10ꢀ5 S cmꢀ1 at 51 °C, which was compara-
tively low among ILs. However, the observed bulk ionic conductiv-
ity was one order of magnitude higher than those for carborane
type ILs that were recently reported by Gabel et al. [16]. As repre-
sented in Fig. 2, 4a showed relatively large temperature depen-
dence of ionic conductivity possibly because of high viscosity.
c
b
e
f
N
d
N
a
H
C
HC
[EMIm]+[closo-1,2-C2B10H11]-
H2O
4a
CH3OH
[C2B10H11]-
(ppm)
Fig. 1. 1H NMR spectrum of 4a in CD3OD.

N. Matsumi et al. / Journal of Organometallic Chemistry 694 (2009) 1612–1616
1615
while the reaction mixture was gradually warmed up to room tem-
perature. Then a THF solution (0.4 ml) of o-carborane (145 mg,
1.01 mmol) was added to the reaction mixture, and the resulting
mixture was stirred for 12 h at room temperature. After the sol-
vents were removed under reduced pressure, the resulting solid
was extracted with 2-propanol. The insoluble part was removed
by filtration. Then, the solvent of the soluble part was removed,
—3.6
—3.8
—4.0
—4.2
—4.4
—4.6
—4.8
—5.0
and the obtained crude product was purified by reprecipitation
ꢀ
into diethylether. [EMIm]+[closo-1,2-C2B10H11
]
was obtained as a
yellowish soft solid (249 mg, 91% yield).
1H NMR (CD3OD, d, ppm) 0.8-3.0 (br, 14H, NCH2CH3 and
C2B10H11), 3.8 (3H, NCH3), 4.3 (2H, NCH2CH3), 7.5 (2H, NCHCHN).
11B NMR (CD3OD, d, ppm) ꢀ21.8 (d, 2B), ꢀ28.3 (d, 4B), ꢀ32.3 (d,
4B).
3.3. Synthesis of [BMIm]+[closo-1,2-C2B10H11]- (4b)
2.8
2.9
3.0
3.1
3.2
3.3
1000/T (K-1)
To
a
1-butyl-3-methylimidazolium bromide (269 mg,
1.70 mmol), 1.80 mmol of n-butyllithium (1.63 M in n-hexane)
was added at 0 °C, and the reaction mixture was stirred for
10 min. Then a THF solution (0.8 ml) of o-carborane (245 mg,
1.70 mmol) was added to the reaction mixture, and the resulting
mixture was stirred for 12 h at room temperature. After the sol-
vents were removed under reduced pressure, the resulting solid
was extracted with 2-propanol. The insoluble part was removed
by filtration. Then, the solvent of the soluble part was removed,
and the obtained crude product was purifiedꢀby reprecipitation
Fig. 2. Temperature dependence of ionic conductivity for 4a.
From the DSC (differential scanning calorimetry) measurement,
melting point of 4a was observed at 30.4 °C. Although the melting
point was slightly higher than that of [N-pentyl-C5H5N]+[closo-
CB11H12]
ꢀ (m.p. 19 °C) [12], incorporation of o-carborane anion re-
sulted in significantly lowered melting point of the obtained IL.
This would be useful for further design of carborane anion based
ILs showing lower melting point.
into diethylether. [BMIm]+[closo-1,2-C2B10H11
]
was obtained as
In conclusion, a series of novel organoboron ILs including o-car-
borane anion were facilely prepared via carbene formation of
N-,N0-dialkylimidazolium halides. Their structures were confirmed
by 1H and 11B NMR spectra. The obtained ILs were soluble in
H2O/THF (v/v = 1/1) at room temperature. [EMIm]+[closo-1,2-
a yellowish soft solid (232 mg, 49% yield).
1H NMR (CD3OD, d, ppm) 0.8–3.0 (br, 18H, NCH2CH2CH2CH3,
C2B10H11), 3.8 (3H, NCH3), 4.2 (2H, NCH2CH2CH2CH3), 7.6 (2H,
NCHCHN).
C2B10H11]
ꢀ (4a) showed its melting point at 30.4 °C, which was sig-
3.4. Synthesis of [BEIm]+[closo-1,2-C2B10H11
]
(4c)
ꢀ
nificantly lower than those for carborane anion based imidazolium
type molten salts reported in Ref. [11] ([EMIm]+[1-C3H7-CB11H11]ꢀ,
m.p. 45 °C; [EMIm]+[1-C4H9-CB11H11]ꢀ, m.p. 49 °C).
To a 1-butyl-3-ethylimidazolium bromide (189 mg, 1.00 mmol),
1.00 mmol of n-butyllithium (1.63 M in n-hexane) was added at
0 °C, and the reaction mixture was stirred for 30 min. Then a THF
solution (0.4 ml) of o-carborane (144 mg, 1.00 mmol) was added
to the reaction mixture, and the resulting mixture was stirred for
19 h at room temperature. After the solvents were removed under
reduced pressure, the resulting solid was extracted with 2-propa-
nol. The insoluble part was removed by filtration. Then, the solvent
of the soluble part was removed, and the obtained crude product
was purified ꢀby reprecipitation into diethylether. [BEIm]+[closo-
This indicated potential utility of these ILs as reaction media for
a variety of nucleophilic substitution rꢀeactions. The ionic conduc-
tivity of [EMIm]+[closo-1,2-C2B10H11
2.9 ꢂ 10ꢀ5 S cmꢀ1 at 51 °C.
]
(4a) was found to be
3. Experimental section
3.1. Instruments and materials
1,2-C2B10H11
]
was obtained as a yellowish soft solid (187 mg,
o-Carborane (Wako. Co. Ltd.), n-butyllithium in n-hexane
(1.63 M; Kanto Reagents. Co. Ltd.), 2-propanol (Kanto Reagents.
Co. Ltd.) were purchased and used without further purification.
Tetrahydrofuran was purchased from Kishida Chemicals Co. Ltd.
and used after distilled over sodium. N,N0-Dialkylimidazolium ha-
lides were prepared according to the reported method [24].
1H and 11B NMR spectra were recorded on Bruker ARX-400 or
JEOL-A400 Win Alipha FT-NMR system. DSC (differential scanning
calorimetry) measurements were made on DSC-6200 (Seiko Instru-
ments. Co. Ltd.). Ionic conductivity of ionic liquids was measured
by ac impedance method using Solartron 1260. Thin layer chroma-
tography (TLC) analysis for ILs was carried out using PdCl2/HCl aq.
solution that is capable of detecting boron cluster derivatives [22].
All the reactions were carried out under inert atmosphere.
69% yield).
1H NMR (CD3OD, d, ppm) 0.8–3.0 (br, 21H, NCH2CH3,
NCH2CH2CH2CH3 and C2B10H11), 4.3 (2H, NCH2CH2CH2CH3), 4.5–
4.6 (2H, –NCH2CH3), 7.4 (2H, NCHCHN).
Acknowledgements
The present study was supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan (#20031013).
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