Synthesis, characterization, and crystal structure of several novel acidic ionic liquids based on the corresponding 1-alkylbenzimidazole with tetrafluoroboric acid

Article abstract of DOI:10.1016/j.crci.2010.06.009

A series of acidic task-specific ionic liquids 1-R₂-2-R ₁benzimidazolium tetrafluoroborate (R₁ = Me, R₂ = Me, Et, Pr-n, Bu-n, Pen-n) were prepared by simple acid-base neutralization of the corresponding 1-alkylbenzimidazole and tetrafluoroboric acid. The compounds were characterized by FTIR spectra, elemental analysis, ¹HNMR spectra and thermogravimetric analysis. These new ionic liquids are nonvolatile, and have potential use as alternatives to conventional organic solvents due to their solubility and thermal stability. These novel tetrafluoroborate salts show good catalytic activity to esterification of carboxylic acids with alcohols under mild reaction conditions, which could maintain good catalytic performance after recycling at least six times. Furthermore, a crystal of one compound, [H-bmBim]BF₄, was prepared, with the crystal structure determined by X-ray diffraction analysis. The molecular structure is formed by weak π-π interactions and intermolecular hydrogen bonds between the benzimidazole rings, yielding a three-dimensional net-like supramolecule.

Full text of DOI:10.1016/j.crci.2010.06.009

C. R. Chimie 13 (2010) 1391–1396
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´
Full paper/Memoire
Synthesis, characterization, and crystal structure of several novel acidic
ionic liquids based on the corresponding 1-alkylbenzimidazole with
tetrafluoroboric acid
*
Shuan-Hu Chen , Fen-Rong Yang, Ming-Tian Wang, Na-ni Wang
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), School of Chemistry & Materials Science, Northwest University,
Xi’an 710069, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of acidic task-specific ionic liquids 1-R2-2-R1benzimidazolium tetrafluoroborate
(R1 = Me, R2 = Me, Et, Pr-n, Bu-n, Pen-n) were prepared by simple acid-base neutralization
of the corresponding 1-alkylbenzimidazole and tetrafluoroboric acid. The compounds
were characterized by FTIR spectra, elemental analysis, 1HNMR spectra and thermo-
gravimetric analysis. These new ionic liquids are nonvolatile, and have potential use as
alternatives to conventional organic solvents due to their solubility and thermal stability.
These novel tetrafluoroborate salts show good catalytic activity to esterification of
carboxylic acids with alcohols under mild reaction conditions, which could maintain good
catalytic performance after recycling at least six times. Furthermore, a crystal of one
compound, [H-bmBim]BF4, was prepared, with the crystal structure determined by X-ray
Received 28 October 2009
Accepted after revision 4 June 2010
Available online 1 September 2010
Keywords:
Task-specific ionic liquids
Esterification
Crystal structure
diffraction analysis. The molecular structure is formed by weak
p-p interactions and
intermolecular hydrogen bonds between the benzimidazole rings, yielding a three-
dimensional net-like supramolecule.
ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
with unique properties have been developed and used as
catalysts in many types of reactions. However, as is true for
Owing to their unique properties, acidic ionic liquids
(ILs) have attracted increasing attention in many fields [1].
Although acidic ILs were initially introduced as alternative
green media because they are low-melting salts that are
nonvolatile, thermally stable, recyclable, and easy to
handle, there is currently a realization that there is a vast
range of novel applications of ILs, such as in electrochem-
istry [2], extraction [3], organic synthesis [4], biphasic
catalysis [5], physical chemistry [6], media for crystal
growth [7], and materials science. These ILs, which are
nonflammable and nonvolatile, can also be used as
lubricants [8] and heat transfer fluids. A number of ILs
conventional organic solvents, not all ILs are appropriate
for a particular reaction, and a single IL will not always be
the best for every reaction [9]. These ILs can be considered
as a unique subclass called task-specific ILs [10] which
otherwise are not present in ordinary ILs. Therefore, it is
worthwhile to design and synthesize task-specific ILs for
particular chemical processes [11].
An acidic IL based on the benzimidazolium cation 1-
ethylbenzimidazolium tetrafluoroborate ([H-ebim]BF4)
was reported as an efficient medium for the preparation
of arylic esters [12]. In this article we present the synthesis
and characterization of five novel ILs based on the
benzimidazolium cation. Furthermore, we prepared a
crystal of 1-butyl-2-methylbenzimidazolium tetrafluoro-
borate and determined its crystal structure by single-
crystal X-ray diffraction. The preparation (Scheme 1) and
characterization are described. These new ILs show good
* Corresponding author.
E-mail addresses: csh318361@163.com, yangfenrong0603@163.com
(S.-H. Chen).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.06.009

[(Scheme_1)TD$FIG]
1392
S.-H. Chen et al. / C. R. Chimie 13 (2010) 1391–1396
sodium sulfate. The solvent was evaporated in a vacuum,
and the oil obtained was purified by distillation.
To a solution of 7.2 g (0.05 mol) of 1,2-bismethylbenzi-
midazole in 50 mL of ethanol was added, drop-wise and
with continuous stirring, 11.0 g (0.05 mol) of 40% w/w
aqueous tetrafluoroboric acid. The mixture was stirred for
3 h before most of the ethanol and water was removed
under vacuum in a rotary evaporator. The viscous residue
was dehydrated at 80 8C in a vacuum oven for 2 h. The
dehydrated residue solidified on cooling to a colorless
crystalline solid.
Scheme 1. Synthesis of 1-R2-2-R1benzimidazolium tetrafluoroborate,
R1 = Me, R2 = Me, Et, Pr-n, Bu-n, Pen-n.
Yield: 11.7 g (0.05 mol, 100%); IR,
2996, 2839, 1627, 1569, 1502, 1468, 1379, 1313,
1082(broad, BF4ꢀ), 766 cmꢀ1 1HNMR (CD3COCD3/TMS),
: 7.96–7.98 (m, 1H, Ar-H), 7.86–7.88 (m, 1H, Ar-H), 7.63–
n(KBr): 3308, 3027,
.
catalytic activity in the esterification of carboxylic acids
with alcohols, and they maintain good catalytic perform-
ance after recycling at least six times.
d
7.67 (m, 2H, Ar-H), 4.19 (s, 3H, N-CH3), 3.05 (s, 3H, =C–
CH3). Elemental analysis calculated (%) for C9H11N2BF4
(234.00): C, 46.19; H, 4.74; N, 11.97. Found: C, 46.25; H,
4.68; N, 12.03. UV, lmax (water): 275.5 nm.
2. Experimental
2.1. General remarks
2.2.2. 1-ethyl-2-methylbenzimidazolium tetrafluoroborate,
[H-emBim]BF4
All chemicals and solvents used were AR grade without
further purification and commercially available. Acidic ILs,
based on the benizimidazolium cation (Bim) and inorganic
anion BF4ꢀ, were prepared by acid-base neutralization at
0–5 8C. Elemental analyses of carbon, hydrogen, and
nitrogen were carried out with a Perkin-Elmer model
240C instrument. Infrared spectra on KBr pellets were
recorded on a Bruker EQUINOX-55 FTIR spectrophotome-
Yield: 12.4 g (0.05 mol, 100%); IR,
2978, 2889, 1625, 1565, 1501, 1465, 1448, 1383, 1320,
1082 (broad, BF4ꢀ), 763 cmꢀ1. 1HNMR (CD3COCD3/TMS),
8.02–8.04 (m, 1H, Ar-H), 7.86–7.88 (m, 1H, Ar-H), 7.63–
7.67 (m, 2H, Ar-H), 4.68 (q, J(H,H) = 7.4 Hz, 2H, N-CH2),
3.07 (s, 3H), 1.59 (t, J(H,H) = 7.4 Hz, 3H). Elemental analysis
calcd (%) for C10H13N2BF4 (248.03): C, 48.42; H, 5.28; N,
n(KBr): 3308, 3021,
d
:
11.29. Found C, 48.55; H, 5.36; N, 11.37. UV,
275.5 nm.
lmax (water):
ter in the range 4000–400 cmꢀ1 1HNMR spectra in
.
propanone-d6 solution were taken with a Varian INOVA-
400 spectrometer. The differential scanning calorimetry
experiment and the thermal stability of each IL was
performed using Q1000DSC + LNCS + FACS Q600SDT at a
heating rate of 5 8C minꢀ1 with nitrogen as the purge gas.
The esterification reactions were carried out in a glass
reactor fitted with a reflux condenser and oil bath. The
reaction was monitored by GC-MS (AligentGC: 6890N, MS:
5793N). The melting points were determined by Melting
Point Apparatus XT-4.
2.2.3. 1-propyl-2-methylbenzimidazolium tetrafluoroborate,
[H-pmBim]BF4
Yield: 13.1 g (0.05 mol, 100%); IR,
2930, 2883, 1647, 1567, 1516, 1465, 1390, 1311, 1087
(broad, BF4ꢀ), 751 cmꢀ1 1HNMR (CD3COCD3/TMS),
d:
n(KBr): 3444, 3018,
.
7.89–8.02 (m, 1H, Ar-H), 7.81–7.83 (m, 1H, Ar-H), 7.44–
7.57 (m, 2H, Ar-H), 4.50 (t, J(H,H) = 7.2 Hz, 2H, N-CH2), 2.94
(s, 3H), 1.95–2.03 (m, 2H), 1.04 (t, J(H,H) = 7.2 Hz, 3H).
Elemental analysis calcd (%) for C11H15N2BF4 (262.05): C,
50.42; H, 5.77; N, 10.69. Found: C, 50.54; H, 5.74; N, 10.74.
UV, lmax (water): 275.5 nm.
2.2. Synthesis of ionic liquids
The synthesis process was carried out according to the
literature [13]. A crystal suitable for single-crystal X-ray
analysis was grown by diffusion of Et2O in a mixture of
ethanol and chloroform at room temperature.
2.2.4. 1-butyl-2-methylbenzimidazolium tetrafluoroborate,
[H-bmBim]BF4
Yield: 13.8 g (0.05 mol, 100%); IR,
2962, 2894, 1648, 1561, 1501, 1466, 1381, 1318, 1078
(broad, BF4ꢀ), 761 cmꢀ1. 1HNMR (CDCl3/TMS),
: 7.76–7.78
n(KBr): 3274, 3027,
d
2.2.1. Typical procedure of preparation for the ionic liquids
2-methylbenzimidazole (13.2 g, 0.1 mol), 50 mL of 50%
sodium hydroxide (aqueous solution), and methyl iodide
(8.7 mL, 0.11 mol) were placed in a three-necked 250 mL
flask equipped with a mechanical stirrer, reflux condenser
and thermometer. The mixture was stirred for 10 min;
when it solidified, the temperature of the water bath was
raised to 30–40 8C and the contents became liquid. After
5 min, an exothermic effect was observed and the
temperature of the mixture was maintained at 30–40 8C
for 10 min. The organic layer was extracted with CHCl3
(3 ꢁ 30 mL), washed with water, and dried with anhydrous
(m, 1H, Ar-H), 7.49–7.58 (m, 3H, Ar-H), 4.35 (t,
J(H,H) = 8 Hz, 2H, N-CH2), 2.91 (s, 3H), 1.84–1.92 (m,
2H), 1.42–1.47 (m, 2H), 1.00 (t, J(H,H) = 7.2 Hz, 3H).
Elemental analysis calcd (%) for C12H17N2BF4 (276.08): C,
52.21; H, 6.21; N, 10.15. Found: C, 52.34; H, 6.26; N, 10.26.
UV, lmax (water): 275.5 nm.
2.2.5. 1-pentyl-2-methylbenzimidazolium tetrafluoroborate,
[H-pemBim]BF4
Yield: 14.5 g (0.05 mol, 100%); IR,
2958, 2889, 1647, 1564, 1516, 1467, 1383, 1311, 1089
(broad, BF4ꢀ), 754 cmꢀ1 1HNMR (CD3COCD3/TMS),
d:
n(KBr): 3443, 3034,
.

S.-H. Chen et al. / C. R. Chimie 13 (2010) 1391–1396
1393
7.88–8.02 (m, 1H, Ar-H), 7.78–7.82 (m, 1H, Ar–H), 7.46–
7.53 (m, 2H, Ar-H), 4.52 (t, J(H,H) = 5.6 Hz, 2H, N-CH2), 2.92
(s, 3H), 1.92–2.00 (m, 2H), 1.38–1.48 (m, 4H), 0.90 (t,
J(H,H) = 3.6 Hz, 3H). Elemental analysis calcd (%) for
The results in Table 1 show that the esterification of
aliphatic acids with primary alcohols was very satisfac-
tory, followed by secondary alcohols and tertiary
alcohols due to an increase in steric interaction. The
length of alkyl chains of the primary alcohols did not
affect the conversion. In the reaction of oxalic acid, no
monoesterification products were detected when the
molar ratio of acid to alcohol was 1:2. Esterification of
aromatic acids gave lower yields compared with those of
aliphatic acids; it is possible that the positive charge of
the carbon cation formed during reaction was delocalized
to benzene.
After the reaction, the ILs could be easily recycled. The
mixture was decanted and the ILs separated and regen-
erated by distillation in a vacuum at 80 8C for 2 h. The IL [H-
bmBim]BF4 was recycled six times in the esterification of
acetic acid with ethanol, and its catalytic activity was
unchanged (Table 3).
Compared with the inorganic acid catalyzed esterifi-
cation, these new ILs showed good catalytic activity
under mild reaction conditions, no corrosion and no
waste acid. At the same time, several noteworthy
features of this approach as follows: (1) they could
maintain good catalytic performance after recycling
at least six times; (2) the esters produced can be
separated by easy decantation with high yields. These
advantages make this methodology became a green
alternative to the conventional mineral acid catalyzed
esterification.
C13H19N2BF4 (290.11): C, 53.82; H, 6.60; N, 9.66. Found:
C, 53.84; H, 6.57; N, 9.71. UV, lmax (water): 275.5 nm.
2.3. Typical preparation for esterification in novel ionic
liquids (demonstrated by the example of 1-butyl-2-
methylbenzimidazolium tetrafluoroborate [H-bmBim]BF4)
Acetic acid (0.01 mol), equivalent ethanol (0.01 mol)
and ILs [H-bmBim]BF4 (0.005 mol) were added to a flask
with a reflux condenser in an oil bath. The mixture was
stirred for 2 h with the oil bath at 80 8C and progress of the
reaction was monitored by GC-MS. Then the mixture was
decanted and the ester isolated, and the IL [H-bmBim]BF4
was reused after removal of water under vacuum
(0.01 Torr) at 80 8C for 2 h.
2.4. Crystal structure determination
X-ray diffraction data were collected with graphite-
monochromated Mo Ka radiation (l = 0.071073 nm) using
a
BRUKER SMART APEX CCD diffractometer for the
compound at 296(2)K. The structure of [H-bmBim]BF4
was determined by direct methods and refined by full-
matrix least-squares methods on F2 with the SHELXTL-97
program [14]. All hydrogen atoms were added according to
theoretical models. Further details on crystal data,
structure refinement and selected bond lengths and angles
are given in Section 3.5.
3.2. Synthesis of ionic liquids
The tetrafluoroborate salts were prepared by simple
acid-base neutralization from the corresponding 1-alkyl-
benzimidazole with tetrafluoroboric acid. The acid-base
neutralization is an atom-economical reaction – both
atoms in the tetrafluoroboric acid and all atoms in the
corresponding 1-alkylbenzimidazole molecules are in-
volved in the final products. The atom economy for these
reactions was 100%. Ethanol was used as the solvent,
though not stoichiometrically due to its high solubility and
easy reuse. The reactions were carried out in an ice bath
and required little time or effort. The molecular structures
were characterized by 1HNMR and FTIR spectroscopy and
elemental analysis. Single-crystal X-ray crystallography of
[H-bmBim]BF4 was obtained from a prepared crystal. All
characterization data accord with expected compositions
and structures.
3. Results and discussion
3.1. Esterification in the novel ionic liquids
The novel ILs were used as catalysts in esterification
resulting in high yields. The esterification of a variety of
acids with common alcohols was carried out using the
synthesized ILs as catalysts (Table 1). Good yields were
obtained in all cases. Because the tetrafluoroborate salts
reported here are miscible with water but immiscible with
ethers, esterification proceeded smoothly to completion
without removal of the water produced, even though
esterification is a reversible reaction. Table 2 shows the
results of esterification of acetic acid and butanol
isomeride in [H-bmBim]BF4 at 120 8C.
Table 1
Results of esterification for different acids and alcohols in ionic liquids (ILs).
Entry
Acid
Alcohol
ILs
Time (h)
T (8 C)
Yield (%)
1
2
3
4
5
6
7
8
9
Acetic
Ethanol
[H-bmBim]BF4
[H-bmBim]BF4
[H-bmBim]BF4
[H-bmBim]BF4
[H-bmBim]BF4
[H-bmBim]BF4
[H-emBim]BF4
[H-emBim]BF4
[H-emBim]BF4
2.0
2.0
4.0
4.0
8.0
8.0
2.0
8.0
8.0
80
120
80
95
95
90
90
85
88
96
89
84
Acetic
1-butanol
Ethanol
Oxalic acid
Oxalic acid
Benzoic acid
Benzoic acid
Acetic
1-butanol
Ethanol
120
80
1-butanol
Ethanol
120
80
Oxalic acid
Benzoic acid
1-butanol
Ethanol
120
80

1394
S.-H. Chen et al. / C. R. Chimie 13 (2010) 1391–1396
Table 2
Table 5
Results of esterification for acetic acids and butanol isomeride in [H-
Selected crystallographic data for the compound.
bmBim]BF4 at 120 8C.
Compound
[H-bmBim]BF4
Entry
Butanol isomer
Time (h)
Yield (%)
Formula
C12H17BF4N2
276.09
1
2
3
4
1-butanol
2.0
2.0
2.0
2.0
95
89
70
53
Formula weight
T/K
iso-butyl alcohol
sec-butyl alcohol
tert-butyl alcohol
296(2)
Crystal system
Space group
Crystal size/mm
Orthorhombic
Pna2(1)
0.31 ꢁ 0.24 ꢁ 0.15
13.566(4)
15.593(4)
6.8193(17)
90
˚
a/A
Table 3
˚
b/A
Reuse of [H-bmBim]BF4 in the synthesis of acetic acetate.
˚
c/A
b
V/A
/(8)
˚
Run
Time (h)
T (8 C)
Yield (%)
3
1442.5(6)
4
1
2
3
4
5
6
2.0
2.0
2.0
2.0
2.0
2.0
80
80
80
80
80
80
95
95
95
95
95
95
Z
Dc/(g cmꢀ3
)
1.271
Refl. No.
7034
u
/(8)
/mmꢀ1
F(000)
1.99–25.09
0.111
m
576
Limiting indices
ꢀ16 ꢃ h ꢃ 13
ꢀ18 ꢃ k ꢃ 18
ꢀ7 ꢃ l ꢃ 8
0.967
3.3. Solubility
Goodness-of-fit on F2
b
R1a, wR2 [I > 2
s
(I)]
0.0684, 0.1690
0.1812, 0.2298
0.198 and ꢀ0.165
Due to their polarity and/or hydrogen bonding, polar
solvents such as methanol show good solubility in ILs,
while ILs are insoluble in nonpolar solvents. The novel ILs
were completely soluble in ethanol, water, acetonitrile,
methanol and acetone, but insoluble in toluene, benzene,
ethyl acetate, cyclohexane and diethyl ether. Chloroform
appeared to be borderline, as it is partly soluble in the
novel ILs. The solubility in chloroform can be used to purify
the products. We studied 1HNMR spectra in a propanone-
d6 solution rather than in chloroform, as some novel ILs are
immiscible with chloroform.
R1, wR2 (all data)
Largest diff. peak and hole (e A
ꢀ3
˚
)
P
P
a
R = ||F0| ꢀ |Fc||/ |F0|.
h
i h
i
1=2
P
P
2
2
b
wR2
¼
½wðF02 ꢀ Fc2Þ ꢄ =
½ðF02Þ ꢄ
.
tion, are presented in Table 5. Selected bond lengths and
bond angles are listed in Table 6. The structure determined
from these data is shown in Fig. 1 which shows that the
atom B1 with the benzimidazole moiety is co-planar and
the tetrafluoroborate anion and the stretched n-butyl
group are on the same side of the benzimidazole plane.
The compound [H-bmBim]BF4 is composed of the 1-
butyl-2-methylbenzimidazole cation and the tetrafluor-
oborate anion. The N1 atom adds a proton to form the
cation, and the anion balances the charge. The bond
3.4. Thermal stability and differential scanning calorimietry
measurements
Thermogravimetric data of the new ILs are shown in
Table 4. Although research indicates that the new salts
begin to decompose at slightly lower temperatures than
the imidazole series [15], they also display high thermal
stability. In each case, the samples showed no weight loss
below 250 8C and some weight loss for 250–280 8C.
Thorough degradation was observed until the temperature
reached ꢂ300 8C. Therefore, the salts have potential usage
as alternatives to conventional organic solvents due to
their solubility and thermal stability.
˚
˚
lengths of C-N, N1-C7 and N2-C7 (1.321 A and 1.332 A,
respectively) are greater than normal C-N bond lengths,
indicating that they have double bond characteristics.
However, they are different from those of other neutral
˚
substituted benzimidazole compounds (1.320(3) A for N1-
˚
C7 and 1.359(2) A for N2-C7 in 1,4-bis(2-benzimidazolyl)
Table 6
˚
Selected bond lengths (A) and angles (8) of the compound.
3.5. Description of the structure of [H-bmBim]BF4
Bond
Dist.
Bond
Dist.
The crystal data and structural details of this new IL, [H-
bmBim]BF4, as determined by single-crystal X-ray diffrac-
N(1)-C(1)
N(1)-C(7)
N(2)-H(2)
N(1)-C(9)
N(2)-C(6)
N(2)-C(7)
1.400(6)
1.321(6)
0.8600
C(1)-C(2)
C(5)-C(6)
C(7)-C(8)
B(1)-F(2)
C(1)-C(6)
1.380(7)
1.381(7)
1.496(7)
1.389(9)
1.388(6)
Table 4
1.465(6)
1.405(6)
1.332(6)
Thermal analysis data of novel benzimidazolium salts.
Entry
ILs
MP (8)
Td (8)
Angle
(8)
Angle
(8)
1
2
3
4
5
[HmmBim]BF4
[HemBim]BF4
[HpmBim]BF4
[HbmBim]BF4
[HpemBim]BF4
125.5
127.9
120.0
110.9
106.5
274
260
266
260
257
F(1)-B(1)-F(4)
C(7)-N(1)-C(1)
C(7)-N(2)-C(6)
C(7)-N(1)-C(9)
N(1)-C(1)-C(6)
110.7(9)
109.1(4)
109.0(4)
127.0(5)
106.6(5)
N(1)-C(1)-C(2)
N(1)-C(7)-N(2)
N(1)-C(9)-C(10)
N(1)-C(7)-C(8)
N(2)-C(6)-C(1)
132.5(5)
109.6(5)
113.7(6)
126.6(5)
105.7(4)
MP: melting point; Td: decomposition temperature.

[(Fig._1)TD$FIG]
S.-H. Chen et al. / C. R. Chimie 13 (2010) 1391–1396
1395
Table 7
Hydrogen bonding parameters for [H-bmBim]BF4 crystal system.
a
D-H. . .A
D-H
H. . .A
D. . .A
nD-H. . .A
C(8)-H(8A). . .F(3)a
C(9)-H(9B). . .F(3)a
C(8)-H(8C). . .F(1)b
C(10)-H(10A). . .F(1)b
N(2)-H(2). . .F(2)c
N(2)-H(2). . .F(4)d
0.960
0.970
0.960
0.960
0.860
0.860
2.457
2.447
2.381
2.641
1.977
2.459
3.394
3.346
3.278
3.508
2.813
3.166
165.58
154.15
155.44
149.02
163.45
139.81
˚
and of C9 0.047 A. The tetrafluoroborate anion and the
stretched n-butyl group are on the same side of the
benzimidazole plane. The distance between the atom B and
Fig. 1. Crystal structure of [H-bmBim]BF4. The atom B1 with the
benzimidazole moiety is co-planar. The tetrafluoroborate anion and
the stretched n-butyl group are in the same side of benzimidazole plane.
˚
the plane is 3.673 A. The 1-butyl-2-methylbenzimidazole
cation is connected with the tetrafluoroborate anion by
hydrogen bonds as C–HꢅꢅꢅF and N–HꢅꢅꢅF, yielding a two-
dimensional (2-D) sheet. All possible hydrogen bonding in
ꢀ
benzene [16]). The other two C-N bond lengths in the
the compound crystal between a cation and adjacent BF4
˚
imidazole ring are in the range 1.401–1.405 A, which is
anions and symmetry codes is listed in Table 7. There are
six kinds of intermolecular and no intramolecular hydro-
gen bonds in the packing structure. Furthermore, the
neighboring 2-D sheets are extended through interlayer
˚
shorter than the single-bond length of 1.48 A, but longer
than the typical C N length of 1.28 A, indicating appreci-
˚
able charge delocalization in the ring.
The molecular structure is formed by weak
p
-p
hydrogen bonds and p-p interactions between the
interactions and intermolecular hydrogen bonds between
the benzimidazole rings, yielding a three-dimensional (3-
D) net-like supramolecule. The benzimidazole moiety with
the conjunction carbon atoms C8 and C9 are almost in the
benzimidazole rings [the centroid of the imidazolium ring
(C1, N1, C7, C6, N2) to the centroid of the phenyl ring (C6,
˚
C5, C4, C3, C2, C1) distance is 3.672 A (symmetry code 1-x,
1-y, 1/2 + z)], leading to a 3-D supramolecular structure
˚
same plane, with the maximum departure of C8 0.023 A
[(Fig._2)TD$FIG]
shown in Fig. 2.
Fig. 2. Packing in the crystal of [H-bmBim]BF4, with the view being down the crystallographic c axis.

1396
S.-H. Chen et al. / C. R. Chimie 13 (2010) 1391–1396
4. Conclusions
Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336
033; e-mail: deposit@ccdc.cam.ac.uk).
In conclusion, several task-specific BrØnsted-acidic ILs
based on the 1,2-bisalkylbenizimidazolium cation and
inorganic anion BF4ꢀ have been designed, synthesized and
fully characterized. These new ILs show good catalytic
activity to esterification of carboxylic acids with alcohols,
which could maintain good catalytic performance after
recycling at least six times. Finally, we prepared a crystal of
[H-bmBim]BF4, and the crystal structure has been deter-
mined by X-ray diffraction analysis.
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This project was supported by the Natural Science
Foundation of Shanxi Province (No. 03B19), China.
Supporting information: crystallographic data for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Center as supple-
mentary publication No. CCDC-743508. Copies of the data
can be obtained free of charge on application to CCDC, 12