Preparation and characterization of two bronsted acid ionic liquids

Article abstract of DOI:10.1007/s10870-011-0038-2

Two Bronsted acid task specific ionic liquids based on the benizimidazolium cation (1-ethylbenzimidazolium tetrafluoroborate {[H-eBim]BF₄ (1)} and 1-propylbenzimidazolium tetrafluoroborate {[H-pBim]BF₄ (2)}) were synthesized and characterized. The crystal of 1 is in orthorhombic system with space group Pnma, and the unit cell parameters as follows: a = 9.9308(16), b = 6.8813(11), c = 15.345(3) A, V = 1048.6(3) A³, Z = 4, D_c = 1.482 g/cm³. Structure 2 crystallizes in the monoclinic space group P2₁/c, with unit cell a = 4.9688(14), b = 13.684(4), c = 17.295(5) A, β = 92.090(6)°, V = 1175.2(6) A³, Z = 4, D_c = 1.402 g/cm³. These molecules are stabilized in solid state by various intra and intermolecular hydrogen bonding interactions to give polymeric structures.

Full text of DOI:10.1007/s10870-011-0038-2

J Chem Crystallogr (2011) 41:1027–1031
DOI 10.1007/s10870-011-0038-2
ORIGINAL PAPER
Preparation and Characterization of Two Brønsted Acid Ionic
Liquids
•
Shuanhu Chen Mingtian Wang Fei Wang
•
•
•
Fenrong Yang Suqing Shi Nani Wang
•
Received: 7 October 2009 / Accepted: 3 February 2011 / Published online: 12 March 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Two Brønsted acid task specific ionic liquids
based on the benizimidazolium cation (1-ethylbenzimi-
dazolium tetrafluoroborate {[H-eBim]BF4 (1)} and 1-pro-
pylbenzimidazolium tetrafluoroborate {[H-pBim]BF4 (2)})
were synthesized and characterized. The crystal of 1 is in
orthorhombic system with space group Pnma, and the
catalysis [6]. These nonvolatile, nonflammable ILs are also
used as better heat-transfer fluids and lubricants [7]. In
parallel to these uses, a new class of reagents designed as
task specific ionic liquids (TSILs) have been developed [8].
TSILs are a subclass of ILs which possess unique func-
tional groups. Incorporation of functional groups allows
these ILs to interact with the dissolved substrates in spe-
cific ways [9]. The designability of the TSILs gives us
more chances to manipulate the structure_(with respect to
the organic cation, inorganic anion and the length of the
side chain attached to the organic cation) to achieve fitful
IL of special properties according to the given reactions.
The synthesis and application in esterification of compound
1 and our work 1-butylbenzimidazolium tetrafluoroborate
([H-bBim]BF4 have been reported [10, 11], but their
homologous crystal and molecular structures are not
known. The solid-state structures of IL compounds provide
the foundation in understanding the structural principles
that govern the organization of ions even in the liquid
state where direct determination of the structure is
rather difficult [9]. Recently, we have synthesized many
1-alkylbenzimidazolium tetrafluoroborate ([H-aBim]BF4)
unit cell parameters as follows: a = 9.9308(16), b =
3
˚
˚
6.8813(11), c = 15.345(3) A, V = 1048.6(3) A , Z = 4,
Dc = 1.482 g/cm3. Structure 2 crystallizes in the mono-
clinic space group P21/c, with unit cell a = 4.9688(14),
˚
b = 13.684(4), c = 17.295(5) A, b = 92.090(6)°, V =
3
3
˚
1175.2(6) A , Z = 4, Dc = 1.402 g/cm . These molecules
are stabilized in solid state by various intra and intermolecular
hydrogen bonding interactions to give polymeric structures.
Keywords Task specific ionic liquid Á Benzimidazole Á
DSC Á Crystal structure
Introduction
The past few years have witnessed a steady growth of
research in the field of ionic liquids (ILs) [1] because
of their particular properties, which opens up a vast range
of novel applications such as organic synthesis [2],
extraction [3, 4], electrochemistry [5], and biphasic
1
(Scheme 1), and characterized by HNMR, IR, elemental
analysis and thermogravimetric analysis. Meanwhile, the
crystals of compound 1 and 2 were prepared, and analysed
by single-crystal X-ray crystallography.
Experimental
S. Chen (&) Á M. Wang Á F. Wang Á F. Yang Á S. Shi Á
N. Wang
Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry (Ministry of Education), College of Chemistry &
Materials Science, Northwest University, Xi’an 710069, China
e-mail: csh318361@163.com
General Remarks
All chemicals (AR grade) were commercially available and
without further purification unless otherwise stated.
Brønsted-acidic ILs, based on the benizimidazolium cation
M. Wang
e-mail: wmt1447@163.com
123

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J Chem Crystallogr (2011) 41:1027–1031
H
N
Crystal Structure Determination
N
N
-
BF4
1.NaOH
2.RI
HBF4
N
R
X-ray diffraction date were collected with graphite-mono-
chromated Mo Ka radiation (k = 0.071073 nm) using a
BRUKER SMART APEX CCD diffractometer for two
compounds at 296(2) K. The structures were solved by the
direct-methods using SHELXS-97 [12] and refined by full-
matrix least-squares methods on F2 with the SHELXTL-97
program [13]. All hydrogen atoms were added according to
theoretical models. Crystal data and structure refinement
details are summarized in Table 1. Selected bond lengths
and angles are given in Table 2.
N
R
N
H
Scheme 1 Synthesis of novel benzimidazolium salts R = Me, Et,
n-Pr, n-Bu, n-Hep
(Bim) and inorganic anion BF4-, were prepared by acid–
base neutralization at 0–5 °C. Infrared spectra on KBr
pellets were recorded on a Bruker EQUINOX-55 FT-IR
spectrophotometer in the range 4,000–400 cm-1. Elemen-
tal analyses were determined with a Perkin-Elmer model
240C instrument. 1HNMR spectra in propanone-d6 solution
were taken with a Varian INOVA-400 spectrometer; proton
chemical shifts are recorded relative to an internal TMS
standard. 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 °C min-1 with nitrogen as the purge gas. The esteri-
fication reactions were carried out in a glass reactor fitted
with a reflux condenser and oil-bath. The reaction was
monitored by GC–MS (AligentGC: 6890 N, MS:_5793 N).
Results and Discussion
Synthesis of Ionic Liquids
All the [H-aBim]BF4 ILs were prepared by simple acid–
base neutralization from the corresponding 1-alkylbenz-
imidazole with tetrafluoroboric acid. Ethanol was used as
Table 1 Selected crystallographic data for compounds 1 and 2
Compound
1
2
Syntheses of Brønsted-Acidic Ionic Liquids
Formula
C9H11BF4N2
234.01
C10H13BF4N2
248.03
The synthesis process was carried out according to the
previous literature [10]. 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.
Formula weight
T (K)
296(2)
296(2)
Crystal system
Space group
Crystal size (mm)
Orthorhombic
Pnma
Monoclinic
P21/c
1-ethylbenzimidazolium tetrafluoroborate, [H-eBim]BF4
Yield: 11.7 g [0.05 mol, nearly 100% (98%)]; mp: 108 °C
(we measured with a single-crystal, while it was reported
72 °C in literature [10]). Anal. Calc. (%) for C9H11N2BF4: C,
46.19; H, 4.74; N, 11.97. Found: C, 46.27; H, 4.65; N, 11.99.
IR mmax/cm-1: 3385 (m), 3123 (m), 3068 (m), 2948 (m), 2887
(w), 1618 (m), 1553 (m), 1505 (w), 1445 (s), 1394 (m), 1314
(s), 1080 (s), 759 (s). 1HNMR (400 Hz, CD3COCD3-d6), d:
9.57 (s, 1H, im-H), 8.09–8.12 (m, 1H, Ar–H), 7.91–8.02 (m,
1H, Ar–H), 7.69–7.75 (m, 2H, Ar–H), 4.74 (q, J(H,H) =
7.2 Hz, 2H, N–CH2), 1.72 (t, J(H,H) = 7.2 Hz, 3H).
0.31 9 0.26 9 0.14 0.30 9 0.25 9 0.13
˚
a (A)
9.9308(16)
6.8813(11)
15.345(3)
90
4.9688(14)
13.684(4)
17.295(5)
92.090(6)
1175.2(6)
4
˚
b (A)
˚
c (A)
B (°)
3
˚
V (A )
1048.6(3)
4
Z
Dc (g cm-3
Refl. No.
h (°)
)
1.482
1.402
5241
5829
2.44–25.78
0.138
1.90–25.10
0.128
l (mm-1
F (000)
)
1-propylbenzimidazolium tetrafluoroborate, [H-pBim]
BF4 Yield: 12.4 g [0.05 mol, nearly 100% (98%)]; mp:
117 °C (measured using a single-crystal). Anal. Calc. (%)
for C10H13N2BF4: C, 48.42; H, 5.28; N, 11.29. Found C,
48.49; H, 5.22; N, 11.33. IR mmax/cm-1: 3383 (m), 3149
(m), 3032 (w), 2969 (m), 2880(w), 1617 (m), 1551 (m),
1503 (w), 1473 (w), 1449 (s), 1383 (m), 1318 (s), 1034 (s),
480
512
Limiting indices
-11 B h B 12
-8 B k B 8
-11 B l B 18
1.094
-5 B h B 5
-16 B k B 16
-16 B l B 20
1.165
Goodness-of-fit on F2
Ra1, wRb2 [I [ 2r(I)]
R1, wR2 (all data)
0.0489, 0.1490
0.0597, 0.1690
0.0611, 0.1311
0.1612, 0.1473
1
753 (s). HNMR (400 Hz, CD3COCD3-d6), d: 9.57 (s, 1H,
Largest diff peak and hole 0.251 and -0.182 0.307 and -0.171
-3
im-H), 8.11–8.13 (m, 1H, Ar–H), 7.99–8.02 (m, 1H, Ar–
H), 7.71–7.74 (m, 2H, Ar–H), 4.69 (t, J(H,H) = 7.4 Hz,
2H, N–CH2), 2.09 -2.15 (m, 2H), 1.04 (t, J(H,H) = 7.2 Hz,
3H).
˚
(e A
)
P
P
a
R = ||F0|-|Fc||/ |F0|
P
P
b
wR2 = { [w(F20-F2c)2]/ [(F20)2]}1/2
123

J Chem Crystallogr (2011) 41:1027–1031
1029
˚
the solvent, which is easy recycled after reaction. The
reactions were carried out in an ice bath and required little
time or effort. The molecular structures were characterized
Table 2 Selected bond lengths (A) and angles (°) of 1 and 2
Compound 1
Compound 2
1
N1–C6
1.371(3)
1.311(3)
0.8600
N1–C1
1.384(5)
1.318(5)
1.466(4)
1.317(5)
1.376(5)
0.8600
by HNMR and FT-IR spectroscopy and elemental analy-
N1–C7
N1–C7
sis. Single-crystal X-ray crystallography of [H-bmBim]BF4
was obtained from a prepared crystal. All characterization
data accord with expected compositions and structures.
N1–H1
N1–C8
N2–C7
1.322(3)
1.383(3)
1.475(3)
1.385(3)
1.389(3)
1.407(3)
1.357(4)
108.3(3)
109.1(2)
107.71(18)
126.6(2)
106.74(18)
131.7(2)
110.4(2)
112.8(2)
106.0(2)
N2–C7
N2–C1
N2–C6
Solubility
N2–C8
N2–H2
C1–C6
C1–C6
1.380(5)
1.374(5)
1.502(6)
1.361(8)
107.6(10)
107.8(5)
125.8(6)
108.6(5)
110.8(7)
105.8(5)
107.7(6)
110.1(6)
111.4(6)
It has been reported that ILs show good solubility to natural
polymers and biopolymers, such as cellulose [14], chitin
[15], various polysaccharides [16], Bombyx Mori silk [17],
and wool keratin [18], most of which are insoluble in water
or in conventional organic solvents. These IL compounds
have a very low vapor pressure, so that they are candidates
to replace volatile organic solvents in organic reactions
[19]. The solubility properties of these ILs in organic sol-
vent approximately conform to the law of similar mutual
solubility. [H-ebim]BF4 and [H-pbim]BF4 were completely
soluble in water, ethanol, methanol, acetonitrile and ace-
tone, but insoluble in benzene, toluene, cyclohexane, ethyl
acetate and diethyl ether. But in chloroform, the IL with the
longer 1-substitution alkyl chain had a better solubility than
the shorter one.
C1–C2
C1–C2
C5–C6
C8–C9
B1–F1
B1–F1
F1–B1–F3
C7–N1–C6
C7–N2–C1
C7–N2–C8
N2–C1–C6
N2–C1–C2
N1–C7–N2
N2–C8–C9
N1–C6–C1
F1–B1–F3
C7–N1–C1
C7–N1–C8
C7–N2–C6
C8–C9–C10
C6–C1–N1
C1–C6–N2
N2–C7–N1
N1–C8–C9
anion. The N1 atom adds a proton to form the cation. The
anion in general position balances the charge. The bond
Thermal Stability and DSC Measurements
˚
lengths of N1–C7 and N2–C7 (1.315 and 1.326 A,
The thermal stabilities of the novel ILs were determined by
the thermogravimetric analysis experiments. Although the
study [20] indicated that these ILs began to decompose at
slightly lower temperatures than the imidazole series, they
also display high thermal stability. The decomposition
temperature (Td) added along with the increase of alkyl-
chain while the melting point (m.p.) appear to have no law
to comply. It may be caused by the combined action of
alkyl-chain length and the steric hindrance. In this study,
the compound 1 and 2 were stable below 210 °C and some
weight loss for 220–270 °C. When the temperature reached
290 °C, the amounts of residue were both beneath 0.5%.
Therefore, due to their solubility and thermal stability,
the salts have potential usage as alternative to conventional
organic solvents.
respectively) are shorter than normal C–N bond lengths,
indicating that they have double bond characteristics. But
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) ben-
˚
˚
zene, and 1.310(6) A for N1–C27 and 1.365(5) A for N2–
C27 in l,3,5-tri(benzimidazol-1-ylmethyl) -2,4,6-trimethyl-
benzene) [21, 22]. The other two C–N bond lengths in the
˚
imidazole ring are 1.377 and 1.389 A, respectively, which
˚
is shorter than the single-bond length of 1.48 A, but longer
˚
than the typical C=N length of 1.28 A, indicating appre-
ciable charge delocalization in the ring. The benzimidazole
moiety with the conjunction carbon atom C8 and atom B1
in the anion is planar.
The molecular structure of 1 is defined by weak p–p
interactions and intermolecular hydrogen bonds between
the benzimidazole rings, yielding a three-dimensional
(3-D) net-like supramolecule. The 1-ethylbenzimidazole
cation is connected with the tetrafluoroborate anion by
hydrogen bonds as C–HÁÁÁF and N–HÁÁÁF, forming 1-D
polymeric chains running along the crystallographic a axis,
as shown in Fig. 2. All possible hydrogen bonding in the
compound 1 crystal is listed in Table 3. There are six kinds
of intermolecular and two intramolecular hydrogen bonds
Description of Structures
The crystal data and structural details of compounds 1 and
2 as determined by single-crystal X-ray diffraction are
presented in Table 1. Selected bond lengths and bond
angles are listed in Table 2. The structure determined from
these data are shown in Fig. 1.
[H-ebim]BF4(1) The title compound consists of the
1-ethylbenzimidazole cation and the tetrafluoroborate
-
in the packing structure. The distance between two BF4
123

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J Chem Crystallogr (2011) 41:1027–1031
Fig. 1 (Left) Crystal structure
of compound 1. The atom B1 is
co-planar with the
benzimidazole moiety. (Right)
Crystal structure of compound
2. The tetrafluoroborate anion
and the stretched n-propyl group
are in the same side of
benzimidazole plane
Fig. 2 Chain of molecules connected by strong hydrogen bonds in
compound 1 extending along the crystallographic a axis
Fig. 3 Packing in the crystal of 1, viewed down the crystallographic
a axis
Table 3 Hydrogen bonding parameters for [H-ebim]BF4 (1) crystal
system
[H-pbim]BF4(2) The title compound consists of the
1-propylbenzimidazole cation and the tetrafluoroborate
anion. The benzimidazole moiety with the conjunction
carbon atom C8 is approximately co-planar, with the
D–HÁÁÁAA
HÁÁÁA/A
DÁÁÁA/A
\D–HÁÁÁA/°
˚
D–H/A
˚
˚
N1–H1ÁÁÁF2a
N1–H1ÁÁÁF2Aa
C4–H4ÁÁÁF3Ab
C8–H8AÁÁÁF2c
C8–H8AÁÁÁF2Ad
C7–H7ÁÁÁF3Ae
N1–H1ÁÁÁF1Ae
C7–H7ÁÁÁF1Ae
A
0.8600
0.8600
0.9300
0.9700
0.9700
0.9300
0.8600
0.9300
2.293
2.293
2.552
2.635
2.635
2.153
2.399
2.508
2.996
2.996
3.374
3.528
3.528
3.069
2.925
2.993
138.94
138.94
147.67
153.19
153.19
167.50
119.98
112.73
˚
maximum departure of 0.081 A. The tetrafluoroborate
anion and the stretched n-propyl group are on the same side
of the benzimidazole plane. The distance between the atom
˚
B and the planar is 1.595 A.
The molecular of the compound is connected together
by the hydrogen bonds (C–HÁÁÁF and N–HÁÁÁF), yielding a
two-dimensional (2-D) wave-like sheet, as is shown in
Fig. 4. All possible hydrogen bonding in the compound 2
Symmetry transformations used to generate equivalent atoms:
(a) x, y, z; (b) -1/2 ? x, 3/2 - y, ‘ - z; (c) 1 - x, -1/2 ? y, -z;
(d) 1 - x, 1/2 ? y, -z; (e) 1/2 ? x, 3/2 - y, 1/2 - z
-
between a cation and adjacent BF4 anions and symmetry
codes is listed in Table 4. There are six kinds of intermo-
lecular and two intramolecular hydrogen bonds in the
packing structure, which is the same with compound 1.
Figure 4 also shows the channels made by cation pillars in
anions in the chain is too large to have any specific inter-
action. The tetrafluoroborate anions acted as a bridge
between the chains, creating a two-dimensional sheet as
shown in Fig. 3. Furthermore, the neighboring 2-D sheets
are extended through interlayer hydrogen bonds and p–p
interactions between the benzimidazole rings [the centroid
of the imidazolium ring (C1, C6, N1, C7, N2) to the cen-
troid of the phenyl ring (C1, C2, C3, C4, C5, C6) distance
-
which BF4 anions are accommodated as a chain. Fur-
thermore, the neighboring 2-D wave-like sheets are
extended through interlayer hydrogen bonds and p–p
interactions between the benzimidazole rings [the centroid
of the imidazolium ring (C1, C6, N1, C7, N2) to the cen-
troid of the phenyl ring (C1, C2, C3, C4, C5, C6) distance
˚
is 3.565 A (symmetry code: 1-x, -y, 1-z)], leading to a
3-D supramolecular structure as shown in Fig. 3.
˚
is 3.551(2) A (symmetry code: -1 ? x, y, z)], leading to a
3-D supramolecular structure shown in Fig. 5.
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J Chem Crystallogr (2011) 41:1027–1031
1031
CCDC No. 727524 and No. 727523. Copy of these infor-
mation may be obtained free of charge from the Cambridge
Crystallographic Data Center, 12, Union Road, Cambridge,
CB2 1EZ, UK (e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgments This project was supported by the Natural Sci-
ence Foundation of Shaanxi Province (No.03B19), China.
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Fig. 4 The 2-D wave-like sheet structure of compound 2 connected
together by hydrogen bonds, viewed down the crystallographic a axis
Table 4 Hydrogen bonding parameters for [H-pbim]BF4 (2) crystal
system
D–HÁÁÁAA
HÁÁÁA/A
DÁÁÁA/A
\D–HÁÁÁA/°
˚
D–H/A
˚
˚
C8–H8BÁÁÁF1a
C8–H8AÁÁÁF2b
C7–H7ÁÁÁF2b
C7–H7ÁÁÁF3c
N2–H2ÁÁÁF3c
C7–H7ÁÁÁF3d
N2–H2ÁÁÁF4e
C5–H5ÁÁÁF4e
A
0.9700
0.9700
0.9300
0.9300
0.8600
0.9300
0.8600
0.9300
2.392
2.415
2.358
2.569
2.297
2.867
2.277
2.594
3.346
3.274
3.149
2.957
2.810
3.097
2.974
3.257
167.61
147.34
142.69
105.50
118.51
95.46
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Symmetry transformations used to generate equivalent atoms:
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Fig. 5 Packing in the crystal of 2, viewed down the crystallographic
c axis
Supplementary Material
The X-ray crystallographic files, in CIF format, have been
deposited with the Cambridge Crystallographic Data Center,
123