Synthesis and applications of novel imidazole and benzimidazole based sulfonic acid group functionalized Broensted acidic ionic liquid catalysts

Article abstract of DOI:10.1016/j.molcata.2011.06.003

In this study, a variety of imidazole/benzimidazole based sulfonic acid group functionalized Broensted acidic ionic liquids (BAILs) were synthesized. Catalytic activities of BAILs were assessed using multi-component coupling reactions. Catalytic activities of BAILs were high when compared with those of solid acid catalysts such as H-ZSM-5, H-BETA, and sulfonic acid functionalized SBA-15 catalysts. The Hammett acidity order determined from UV-visible spectroscopy of BAILs is consistent with their activity order observed in acid-catalyzed reactions. Theoretical studies demonstrate that the hydrogen bonding plays a key role in tuning the acidity of BAILs. Recycling experiments suggest that these novel BAILs can be reused without significant loss in catalytic activity. Novel BAILs offer several attractive features such as low cost, high catalytic activity, and recyclability.

Full text of DOI:10.1016/j.molcata.2011.06.003

Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis and applications of novel imidazole and benzimidazole based sulfonic
acid group functionalized Brönsted acidic ionic liquid catalysts
Rajkumar Kore, Rajendra Srivastava∗
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2011
Received in revised form 8 June 2011
Accepted 11 June 2011
Available online 22 June 2011
In this study, a variety of imidazole/benzimidazole based sulfonic acid group functionalized Brönsted
acidic ionic liquids (BAILs) were synthesized. Catalytic activities of BAILs were assessed using multi-
component coupling reactions. Catalytic activities of BAILs were high when compared with those of solid
acid catalysts such as H-ZSM-5, H-BETA, and sulfonic acid functionalized SBA-15 catalysts. The Hammett
acidity order determined from UV–visible spectroscopy of BAILs is consistent with their activity order
observed in acid-catalyzed reactions. Theoretical studies demonstrate that the hydrogen bonding plays
a key role in tuning the acidity of BAILs. Recycling experiments suggest that these novel BAILs can be
reused without significant loss in catalytic activity. Novel BAILs offer several attractive features such as
low cost, high catalytic activity, and recyclability.
Keywords:
Brönsted acidic ionic liquids
Sulfonic acid functionalized ionic liquids
Multi-component coupling reactions
Hammett acidity
© 2011 Elsevier B.V. All rights reserved.
Theoretical studies
1. Introduction
us more opportunity to manipulate their structures (with respect
to the organic cation, inorganic anion and the length of the side
The chemical industry routinely uses strong Brönsted acid cat-
alysts for fine chemical synthesis [1]. In this context, solid acid
catalysts are being most widely used as nonvolatile materials,
as they are deemed less noxious than traditional liquid acids
[2,3]. However, solid acids have several disadvantages as well.
Among the more troublesome of these are restricted accessibility
of the matrix-bound acidic sites, high molecular weight/active-
site ratios, and rapid deactivation from coking [3,4]. Bearing in
mind both the advantages and disadvantages of solid acids, the
identification of systems that are Brönsted acids with solid-like
non-volatility but that manifest the motility, greater effective sur-
face area, and potential activity of a liquid phase continues to be
useful.
Ionic liquids (ILs) have potential to substitute solid acid cata-
lysts in liquid phase catalytic reactions. ILs have been described
as one of the most promising new reaction mediums [5,6]. ILs
have the ability to dissolve many organic and inorganic sub-
strates. They can be readily recycled and are tunable to specific
chemical tasks [6–9]. A new class of reagents designed as task-
specific ILs (TSILs) has been developed: such derivatives combine
an IL-type part with an attached extra function designed for the
specific property [10]. Functionalization ability of the TSILs gives
chain attached to the organic cation) to achieve special proper-
ties according to the given reactions. The vast majority of TSIL
chemistry is based on nitrogen-containing heterocyclic compounds
[11].
Imidazoles and benzimidazoles have received significant atten-
tion because of their biological activities such as antihistaminic,
antiparasitic, antiulcer, antihypertensive, antiviral, antifungal and
anticancer activities [12,13]. They are used in diverse areas of chem-
istry and are very important intermediates in organic reactions. The
use of imidazole in the IL synthesis has been widely investigated [6].
However, only a few reports are available where benzimidazole has
been used as organic cations in ILs [14]. Recently, some significant
research findings have been reported for Lewis acidic ILs and Brön-
sted acidic ILs [15–18]. Compared to conventional homogeneous
and heterogeneous acid catalysts, acidic ILs demonstrate several
advantages that include the following: reactions can be carried out
under solvent-free condition, the product can be separated by easy
decantation, and ILs can be recycled. Brönsted acidic ILs can be pre-
pared by functionalizing SO3H (IL-SO3H) group on heterocyclic ring
or replacing Cl− with HSO4 (IL.HSO4−). Such functionalized ILs
−
have been reported as novel eco-benign catalysts for some acid
catalyzed reactions [15–18].
To the best of our knowledge, there is no report in litera-
ture which clearly state whether (IL-SO3H) or (IL.HSO4−) is highly
active. Enhancement in catalytic activity by increasing the num-
ber of –SO3H in functionalized ILs is not known very precisely.
∗
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.06.003

118
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
The objective of this study is to prepare a variety of imidazole
and benzimidazole based (one/two/three/four) sulfonic acid group
functionalized Brönsted acidic ionic liquids (BAILs) and investi-
gate their applications in suitable catalytic reactions. In this study,
catalytic activities of BAILs were assessed using multi-component
coupling reactions. The catalytic activities of BAILs were compared
with several solid acid catalysts such as H-ZSM-5, H-BETA, and
SO3H functionalized SBA-15 catalysts. Structure–activity relation-
ship was established using theoretical modeling and UV–visible
spectroscopic methods.
bromide (yield = 93%). 1-Benzyl-3-dodecyl-benzimidazolium bro-
mide was sulfonated using three equivalents of H2SO4 to obtain
BAIL-5 (yield = 86%).
BAIL-5: IR (KBr, ꢀ, cm−1) = 3392, 2892, 1699, 1635, 1309, 1145,
1025, 875, 751, 697. 1H NMR (D2O + DMSO-d6) ı = 9.97 (s, 1H),
8.26–7.67 (m, 7H), 5.98 (s, 2H), 4.16 (m, 2H), 1.37 (m, 20H), 1.06
(t, 3H). 13C NMR ı = 144.09, 142.80, 140.50, 134.17, 132.18, 129.54,
128.75, 127.39, 123.12, 114.46, 113.75, 50.88, 47.71, 32.62, 31.97,
31.28, 29.38, 29.16, 26.53, 22.18, 14.54. Elemental analysis for
C26H38N2S3O7: Theoretical (%): C 53.24, H 6.48, N 4.78; Experimen-
tal (%): C 53.21, H 6.51, N 4.83.
For the synthesis of BAIL-6, first, 1,3-dibenzyl-benzimidazolium
chloride was prepared using
a
similar procedure that
2. Experimental
was adopted for the synthesis of 1-benzyl-3-dodecyl-
benzimidazolium bromide. For this, 1-benzyl-benzimidazol
(10 mmol) and benzyl chloride (10 mmol) was reacted to
obtain 1,3-dibenzyl-benzimidazolium chloride (yield = 94%).
1,3-dibenzyl-benzimidazolium chloride was sulfonated using four
equivalents of H2SO4 to obtain BAIL-6 (yield = 87%).
2.1. Catalyst preparation
2.1.1. Synthesis of imidazole based BAILs
BAIL-1 was synthesized according to the reported procedure
[19]. BAIL-2 was synthesized according to the reported procedure
[18], except the sulfonation step, in which only two equivalents of
H2SO4 was taken.
For the synthesis of BAIL-3, first compound c (Scheme 1) was
synthesized using the reported procedure [20]. It was then sul-
fonated using three equivalents of H2SO4 (yield = 77.4%).
BAIL-3: IR (KBr, ꢀ, cm−1) = 3393, 3368, 3145, 3024, 2914, 1700,
1560, 1454, 1142, 1023, 875, 709, 611. 1H NMR (D2O) ı = 8.91
(s, 1H), 7.32–7.45 (m, 10H), 5.29 (s, 4H). 13C NMR ı = 136.61,
135.02, 129.76, 129.49, 126.97, 123.32, 53.18. Elemental analysis
for C17H18N2O10S3: Theoretical (%): C 40.32, H 3.56, N 5.53; Exper-
imental (%): C 39.93, H 3.84, N 5.81.
BAIL-6: IR (KBr, ꢀ, cm−1) = 3405, 3144, 3072, 2922, 2854, 2480,
1696, 1563, 1455, 1136, 1024, 877, 743, 703. 1H NMR (D2O + DMSO-
d6) ı = 9.76 (s, 1H), 7.82 (d, 4H), 7.64 (d, 4H), 7.48–7.42 (m, 3H),
5.78 (s, 4H). 13C NMR ı = 146.30, 142.93, 139.79, 135.20, 134.25,
130.48, 129.20, 128.26, 122.98, 116.83, 113.17, 50.50, 50.20. Ele-
mental analysis for C21H20N2S4O13: Theoretical (%): C 39.62, H 3.14,
N 4.40; Experimental (%): C 39.47, H 3.36, N 4.49.
For the synthesis BAIL-7, first, 3,3ꢀ-(hexane-1,6-diyl)bis(1-
benzyl-benzimidazolium)bromide was prepared using
a
similar procedure that was adopted for the synthesis of
1-benzyl-3-dodecyl-benzimidazolium bromide. For this, 1-benzyl-
benzimidazol (20 mmol) and 1,6-dibromohexane (10 mmol)
was reacted to obtain 3,3ꢀ-(hexane-1,6-diyl)bis(1-benzyl-
benzimidazolium)bromide (yield = 91%) [14]. 3,3ꢀ-(Hexane-
1,6-diyl)bis(1-benzyl-benzimidazolium)bromide was sulfonated
using six equivalents of H2SO4 to obtain BIL-7 (yield = 78.4%).
BAIL-7: IR (KBr, ꢀ, cm−1) = 3372, 3131, 3069, 2939, 2869, 2484,
1694, 1619, 1557, 1486, 1451, 1373, 1312, 1230, 1143, 1016, 861,
754, 703. 1H NMR (D2O + DMSO-d6) ı = 9.63 (s, 2H), 7.88–7.36 (m,
14H), 5.69 (s, 4H), 4.48 (t, 4H), 1.92 (m, 4H), 1.34 (m, 4H). 13C NMR
ı = 144.96, 143.02, 141.85, 135.34, 133.39, 129.34, 128.61, 125.51,
124.60, 116.37, 114.23, 51.10, 47.93, 27.61, 23.38. Elemental
analysis for C34H38N4S6O14: Theoretical (%): C 44.44, H 4.14, N 6.10;
Experimental (%): C 44.31, H 4.29, N 6.18.
For the synthesis of BAIL-4, first compound d (Scheme 1)
was synthesized from compound b (two equivalents) and 1,6-
dibromohexane (one equivalent) using the reported procedure
[20]. Compound d was then sulfonated using four equivalents of
H2SO4 (yield = 75.2%).
BAIL-4: IR (KBr, ꢀ, cm−1) = 3395, 3149, 3084, 2943, 2867, 2525,
1702, 1563, 1498, 1455, 1146, 1022, 875, 708. 1H NMR (D2O)
ı = 8.59 (s, 2H), 7.23–7.27 (m, 12H), 5.19 (s, 4H), 3.94 (t, 4H), 1.61
(quint, 4H), 1.05 (quint, 4H). 13C NMR ı = 134.64, 133.37, 131.52,
128.86, 128.09, 122.15, 121.96, 52.36, 48.96, 28.34, 24.1. Elemen-
tal analysis for C26H34N4O14S4: Theoretical (%): C 41.38, H 4.51, N
7.43; Experimental (%): C 41.58, H 4.24, N 7.55.
H-ZSM-5 [4], H-BETA [21], and SBA-15-pr-SO3H [22] were syn-
thesized following the reported procedures.
2.1.2. Synthesis of benzimidazole based BAILs
1-Benzyl-benzimidazole is the precursor of benzimidazole
based BAILs. Synthesis of 1-benzyl-benzimidazole is as follows: In
a two necked round-bottomed flask, K2CO3 (75 mmol), 40 ml ace-
tonitrile and benzimidazole (50 mmol) were taken and refluxed
for 30 min. Benzyl chloride (50 mmol) was subsequently added
drop wise over a period of 30 min and the mixture was refluxed
for 24 h. After the reaction, the reaction mixture was cooled to
room temperature, and the solvent was removed under reduced
pressure. Water was added in the reaction mixture. The aque-
ous layer was then separated and extracted three times with
dichloromethane. Organic phases were combined and the dried
over sodium sulfate and solvent was removed by Rota-evaporator
(yield = 92%).
For the synthesis of BAIL-5, first, 1-benzyl-3-dodecyl-
benzimidazolium bromide was prepared. In a typical synthesis,
1-benzyl-benzimidazole (10 mmol) was dissolved in toluene
(20 ml) and dodecylbromide (10 mmol) was added drop-wise and
the mixture was refluxed for 24 h. Upon completion of the reac-
tion, the solvent was evaporated under vacuum. The residue was
washed 3–4 times with ethyl acetate and then dried under vacuum
at 343 K for 4 h to afford 1-benzyl-3-dodecyl-benzimidazolium
2.2. Catalyst characterizations
FT-IR was recorded on Bruker Tensor-27 spectrometer in the
range of 400–4000 cm−1 (spectral resolution = 4 cm−1; number
of scans = 100). UV–visible spectra were recorded on Analytik-
jena Specord 250 PLUS spectrophotometer. 1H and 13C NMR was
recorded on Bruker AM-400 MHz NMR. X-ray diffraction (XRD) pat-
terns of solid samples were recorded in the 2ꢁ range of 5–50◦ with
a scan speed of 2◦/min on PANalytical X’PERT PRO diffractometer
using Cu K␣ radiation (ꢂ = 0.1542 nm, 40 kV, 20 mA) and a pro-
portional counter detector. Nitrogen adsorption measurement at
77 K of solid samples was performed by Autosorb IQ volumetric
adsorption analyzer of Quantachrome Instruments. Samples were
out-gassed at 150–300 ◦C for 4 h in the degas port of the adsorp-
tion apparatus. The specific surface area was determined by BET
method using the data points of P/P0 in the range of about 0.05–0.3.
The pore diameter was estimated using the Barret–Joyner–Halenda
(BJH) model. Si and Al contents in the solid catalysts were
estimated using a Rigaku 3070 E wavelength-dispersive X-ray

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
119
H
N
N
H2SO4
BAIL-1
N
N
HSO4
CH3
CH3
CH3
N
Ph
N
N
H2SO4
Toluene
N
BAIL-2
Cl
Ph
HSO4
+
(a)
N
N
Cl
CH3
CH3
SO3H
Ph
N
N
SO3H
PhCH2Cl
H2SO4
N
HSO4
N
Cl
BAIL-3
(c)
Ph
SO3H
N
N
KOH, CH3CN
+
Cl
Ph
N
H
N
Reflux
(b)
Ph
Br(CH2)6Br
HSO4
N
HSO4
N
Br
N
6
Br
N
N
N
H2SO4
6
N
N
Ph
Ph
(d )
SO H
HO3S
3
BAIL-4
Scheme 1. Synthesis of imidazole based BAILs.
fluorescence (XRF) spectrometer with Rh target energized at 50 kV
and 40 mA.
a 100 ml flask. Reaction was conducted at 313 K for 3 h. After the
reaction, ethyl acetate was added in the reaction mixture to dissolve
the product and the catalyst was removed by simple decanta-
tion/filtration. Products were isolated via column chromatography
(hexane/ethyl acetate, 3:1) and characterized by using FT-IR, NMR
and elemental analysis, which matches well with the reported
value in literature [23].
2.3. Catalytic reactions
2.3.1. Synthesis of dihydropyrimidinones
In a typical synthesis, benzaldehyde (5 mmol), urea (7.5 mmol),
ethylacetoacetate (5 mmol) and catalyst (0.1 mmol) were mixed in
Table 1
Textural properties of various solid acid catalysts investigated in this work.
a
Catalyst
SBET (m2/g)
Ext. surface area (m2/g)
Total pore volume (ml/g)
Concentration of active species
H-ZSM-5
H-BETA
SBA-15-pr-SO3H
350
595
690
67
212
618
0.2
0.44
0.95
Si/Al = 18b
Si/Al = 14b
Organic functional group (0.64 mmol/g)c
a
SBET is the surface area which was calculated using the Brunauer–Emmett–Teller equation.
Obtained from ICP.
Determined from the elemental analysis.
b
c

120
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
SO3H
Ph
N
N
N
N
H2SO4
Cl
HSO4
HO3S
Ph
BAIL-6
SO3H
PhCH2Cl
CH3
N
N
CH3(CH2)11Br
N
N
N
K2CO3
CH3CN
Reflux
Cl
Ph
Br
N
H2SO4
H
Ph
Ph
CH3
N
Br(CH2)6Br
HSO4
N
HO3S
BAIL-5
SO3H
SO3H
HO3S
H2SO4
N
N
N
N
N
N
6
6
N
N
Br
Br
HSO4 HSO4
BAIL-7
SO3H
SO3H
Scheme 2. Synthesis of benzimidazole based BAILs.
2.3.2. Synthesis of amidoalkylnaphthols
A mixture of 2-naphthol (1.0 mmol), benzaldehyde (1.0 mmol),
benzamide (1.1 mmol), catalyst (0.05 mmol) and 1,2-dicholoethane
(3 ml) was stirred at 298 K. The progress of the reaction was moni-
tored by TLC. After the reaction, crude product was washed with
water and purified by column chromatography using silica gel
(100–200) and hexane/ethyl acetate (95/5) as eluent to obtain the
desired compound in pure form. Product was characterized by
using FT-IR, NMR and elemental analysis, which matches well with
the reported value in the literature [24]. Water fraction having ILs
was evaporated under reduced pressure to remove the water and
BAILs were recycled.
2.0
1.5
1.0
0.5
0.0
Blank
BAIL-1
BAIL-2
BAIL-3
BAIL-4
Table 2
Calculation and comparison of H0 values of BAIL-1 to BAIL-4 in water at 298 K.
E. no.
ILs
Amax
[I] (%)
[IH+] (%)
H0
1
2
3
4
5
No BAIL
BAIL-1
BAIL-2
BAIL-3
BAIL-4
1.696
1.588
1.260
1.041
1.001
100
0
–
93.6
74.3
61.4
59.0
06.4
25.7
38.6
41.0
2.155
1.451
1.192
1.148
300
400
500
600
700
Wavenumber (nm)
Fig. 1. Absorbance spectra of 4-nitroaniline after addition of BAIL-1 to BAIL-4 in
water.
Indicator: 4-nitroanline.

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
121
Ph
Ph
O
EtOOC
H3C
O
O
EtOOC
+
COOEt
CH3
Catalyst
313 K
NH
+
+
PhCHO
H2N
NH2
H3C
OEt
N
H
O
H3C
N
H
[B]
[A]
Scheme 3. Three component coupling reaction of benzaldehyde, urea and ethylacetoacetate.
Table 3
Reaction of benzaldehyde, urea and ethylacetoacetate under different reaction condition.
E. no.
Catalyst
Reactant ratioa
Solvent
Yield (%)
A
TONc
B
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
BAIL-2
BAIL-2
BAIL-2
BAIL-2
1:1:1.5
1:1:1.5
1:1:1.5
1:1.25:1.5
1:1.25:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
1:1:1.5
Ethanol
Water
Ethanol/water (1:1)
Ethanol
None
None
None
None
None
None
None
None
None
None
None
None
None
48.6
16.6
21.1
43.7
45
50.2
48.4
29.4
68.1
72.2
47.1
81.5
78.7
78.4
0
0
10
8.9
6.3
5
0
0
0
0
0
0
0
0
0
0
0
0
24.3
13.3
15.0
25.0
25.0
25.1
24.2
14.7
34.0
36.1
23.5
40.8
39.4
39.2
–
BAIL-2
BAIL-2
BAIL-2b
BAIL-1
BAIL-3
BAIL-4
BAIL-5
BAIL-6
BAIL-6b
BAIL-7
HZSM-5
HBEA
SBA-15-pr-SO3H
3.4
7.3
1.7
3.6
Reaction conditions: reactant ratio as shown in the table (5 mmol = 1); solvent (5.0 g); catalyst (0.1 mmol); reaction time (3 h); reaction temperature (313 K).
a
Reactant ratio = benzaldehyde:ethylacetoacetate:urea.
Catalytic activity data of BAIL-2 and BAIL-6 in third cycle.
TON = moles of reactant converted per mole of catalyst.
b
c
2.3.3. Synthesis of 1-(phenyl (piperidin-1-yl) methyl)
naphthalene-2-ol (Betti base)
using FT-IR, NMR and elemental analysis. For comparative study,
solid acid catalysts such as H-ZSM-5, H-BETA, and SBA-15-pr-SO3H
were prepared. Phase purity of H-ZSM-5, H-BETA, and SBA-15-pr-
SO3H samples was confirmed using wide angle and low angle XRD
patterns. Solid acids were characterized by using N2-adsorption,
XRF and elemental analysis (Table 1).
Acidity of BAILs was measured using UV–visible spectropho-
tometer with a basic indicator by following the concept reported
in literature [26–28]. In this manuscript, acidity of imidazole based
BAILs (BAIL-1 to BAIL-4) was investigated using 4-nitroanline as
indicator. With the increase of acidity of the BAILs, the absorbance
of the unprotonated form of the basic indicator decreased, whereas
A mixture of benzaldehyde (1 mmol), 2-naphthol (1 mmol),
piperidine (1 mmol), catalyst (0.05 mmol) and water (3 ml) was
stirred at 298 K. After the reaction, the reaction mixture was
extracted with ethyl acetate. The aqueous-phase was extracted
with ethyl acetate. The combined organic layers were dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pres-
sure to get the crude product as a white solid, which was purified by
column chromatography with hexane/ethyl acetate (95/5). Prod-
uct was characterized by using FT-IR, NMR and elemental analysis,
which matches well with the reported value in the literature [25].
Water fraction having ILs was evaporated under reduced pressure
to remove the water and BAILs were recycled.
Table 4
Synthesis of 1-amidoalkyl 2-naphthol using different catalysts investigated in this
study.
3. Results and discussion
E. no.
Catalyst
Yield (%)
TONb
3.1. Synthesis and characterizations of BAILs
1
2
3
4
5
BAIL-1
41.6
68.8
64.5
81.2
83.3
57.2
93.8
91.2
90.7
0
8.3
13.8
12.9
16.2
16.7
11.4
18.8
18.2
18.1
–
BAIL-2
Except BAIL-1, all other BAILs were synthesized by mul-
tistep synthetic route. First 1-benzyl imidazole and 1-benzyl-
benzimidazole were synthesized and then they were used as
precursors to synthesize all kinds of imidazole and benzimidazole
based BAILs (Schemes 1 and 2). Samples were characterized by
BAIL-2a
BAIL-3
BAIL-4
BAIL-5
6
9
BAIL-6
10
11
12
13
14
BAIL-6a
BAIL-7
H-ZSM-5
H-BETA
SBA-15-pr-SO3H
NHCOPh
Ph
CHO
5.2
14.5
1.0
2.9
OH
OH
Catalyst
+
+
Reaction condition: 2-naphthol (1.0 mmol), benzaldehyde (1.0 mmol), benzamide
(1.1 mmol), catalyst (0.05 mmol), DCE (3 ml), temperature (298 K), reaction time
(4 h).
Ph-CONH2
DCE, 298K
a
Catalytic activity data of BAIL-2 and BAIL-6 in third cycle.
TON = moles of reactant converted per mole of catalyst.
Scheme 4. Three component coupling reaction of 2-naphthol, benzaldehyde and
benzamide.
b

122
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
CHO
N
H
N
OH
+
Catalyst
OH
+
H2O, 298K
Scheme 5. Three component coupling reaction of 2-naphthol, benzaldehyde and
piperidine.
the protonated form of the indicator could not be observed because
of its small molar absorptivity and its location, so the [I]/[IH+] (I
represents indicator) ratio can be determined from the differences
of measured absorbance after the addition of BAILs and Hammett
function, H0, can be calculated using Eq. (1). This value can be
regarded as the relative acidity of the BAILs:
Fig. 2. Optimized molecular structure of BAIL-1 using RHF/6-31G.
ꢀ
ꢁ
I
H0 = pK(I)aq + log
(1)
IH+
Under the same concentration of 4-nitroanline (5 mg/L,
pK(I)aq = pKa = 0.99) and BAILs (25 mmol/L) in water, H0 val-
ues of all BAILs were determined. The maximal absorbance of the
unprotonated form of the indicator was observed at 380 nm in
water. When the BAIL was added, the absorbance of the unproto-
nated form of the basic indicator decreased. As shown in Fig. 1, the
absorbance of the unprotonated form of the indicator on addition
of BAILs decreased as follows: BAIL-1 > BAIL-2 > BAIL-3 > BAIL-4.
Calculations suggest that the Hammett acidity (H0) of these ILs
follows the order: BAIL-4 > BAIL-3 > BAIL-2 > BAIL-1 (Table 2). It
may be noted that when similar UV–visible spectroscopy was used
to investigate the acidity of benzimidazole based BAILs (BAIL-5
to BAIL-7), the absorbance of basic indicator increased, rather
than decreased, upon addition of BAILs. This is due to the fact
that the absorbance of BAIL-5 to BAIL-7 was overlapping with the
4-nitroanline absorbance. Further study is underway to develop
a suitable probe molecule to evaluate the acidity of BAIL-5 to
BAIL-7 using UV–visible spectroscopic method. However, the
catalytic investigations and theoretical calculations described in
the following sections are able to assess and explain the difference
in activity observed in these BAILs.
Fig. 3. Optimized molecular structure of BAIL-2 using RHF/6-31G.
biological activity and are normally prepared using Biginelli reac-
tion (Scheme 3) [29]. A large number of reports are available
in literature for this protocol, including a few examples of Big-
inelli reaction in water [30,31]. The reaction is purely catalytic.
No product was obtained in the absence of catalyst. First the reac-
tion condition was optimized using BAIL-2. Using BAIL-2 catalyst,
approximately 50% yield of Biginelli product was obtained in the
ethanolic medium (Table 3, entry 1). When the reaction was con-
ducted in water or ethanol/water mixture, in addition to Biginelli
product (A), Hantzsch product (B) was also observed (entries 2 and
3) (Scheme 3). It was also observed that a mixture of products
was found in ethanolic medium when the ratio of ethylacetoac-
etate to benzaldehyde was greater than one (entries 4 and 5). It
is interesting to note that, when the reaction was carried out in
the absence of any solvent, only Biginelli product was observed
(Entry 6). However, this was possible only when the ethylacetoac-
etate to benzaldehyde ratio was one (compare entries 5 and 6).
A principle of green chemistry says that no solvent condition is
3.2. Catalytic activities
Catalytic activities of BAILs were assessed using multi-
component coupling reactions. Dihydropyrimidinones are an
important class of organic compounds, which show prominent
Table 5
Synthesis of 1-(phenyl (piperidin-1-yl) methyl) naphthalene-2-ol over different cat-
alysts investigated in this study.
E. no.
Catalyst
Yield (%)
TONb
1
2
3
4
5
6
7
8
9
BAIL-1
37.5
56.3
55.2
73.4
74.2
51.2
90.8
88.4
88.1
0
7.5
11.3
11.0
14.7
14.8
10.2
18.2
17.7
17.6
–
BAIL-2
BAIL-2a
BAIL-3
BAIL-4
BAIL-5
BAIL-6
BAIL-6a
BAIL-7
10
13
14
H-ZSM-5
H-BETA
SBA-15-pr-SO3H
0
<1
–
–
Reaction condition: 2-naphthol (1.0 mmol), benzaldehyde (1.0 mmol), piperidine
(1.0 mmol), catalyst (0.05 mmol), water (3 ml), temperature (298 K), reaction time
(4 h).
a
Catalytic activity data of BAIL-2 and BAIL-6 in third cycle.
TON = moles of reactant converted per mole of catalyst.
b
Fig. 4. Optimized molecular structure of BAIL-3 using RHF/6-31G.

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
123
Fig. 5. Optimized molecular structure of BAIL-4 using RHF/6-31G.
the best medium. Hence, the catalytic activities of all other cat-
alysts were compared in neat condition. The activity of Brönsted
acidic ILs follows the order BAIL-6 > BAIL-7 > BAIL-4 > BAIL-3 > BAIL-
2 > BAIL-5 > BAIL-1. For comparative study, H-ZSM-5, H-BETA, and
SBA-15-pr-SO3H were investigated. H-ZSM-5 was inactive whereas
H-BETA and SBA-15-pr-SO3H were weakly active. Among all cata-
lysts investigated here, BAIL-6 was found to be the best (compare
TON) for the selective formation of 3,4-dihydropyrimidinones.
Recycling experiments confirm that the BAILs can be recycled
without significant loss in catalytic activity (Table 3, entries 7
and 13).
Three component coupling reaction of 2-naphthol, benzalde-
hyde, and benzamide was investigated to synthesize amidoalkyl-
naphthols (Scheme 4). The reaction proceeds through the formation
of o-quinone methide intermediates [32,33]. Further, nucleophilic
conjugate addition of amide on o-quinone methide intermedi-
ate, leads to the formation of amidoalkylnaphthol as a product.
BAIL-2 was chosen to optimize the synthetic protocol for this
Table 6
The geometry parameters of BAILs calculated at RHF/6-31G level.
Observed parameters
H–O bond distance of sulfonic acid (A)
BAIL-1
BAIL-2
BAIL-3
BAIL-4
28–O27 = 0.960
H75–O72 = 0.961
H12–O11 = 0.959
H82–O79 = 0.959
C2–H5 = 1.065
˚
H32–O31 = 0.960
H48–O47 = 0.961
H29–O28 = 0.960
H13–O12 = 0.959
H
−
˚
H–O bond distance of HSO4 (A)
H19–O18 = 0.959
C3–H8 = 1.070
H16–O15 = 0.959
C1–H4 = 1.066
(N)2C–H
C1–H4 = 1.065
C50–H52 = 1.065
H4· · ·O9 = 2.416
H4· · ·O10 = 2.115
H52· · ·O81 = 2.045
H52· · ·O80 = 2.535
H36· · ·O9 = 2.327
H29· · ·O10 = 2.283
H20· · ·O8 = 2.591
H46· · ·O81 = 2.508
H5· · ·O78 = 2.116
H59· · ·O81 = 2.416
H67· · ·O79 = 2.611
−3858.237388
18.8607
(N)2C–H· · ·O
H8· · ·O17 = 2.014
H4· · ·O14 = 2.099
H4· · ·O13 = 2.478
H4· · ·O11 = 2.131
H4· · ·O10 = 2.534
˚
Other hydrogen bonds in BAILs (A)
H5· · ·O16 = 1.665
H7· · ·O13 = 2.280
H33· · ·O14 = 2.264
H24· · ·O12 = 2.429
H50· · ·O10 = 2.245
H21· · ·O9 = 2.472
H40· · ·O10 = 2.402
Energy (a.u.)
Dipole moment (Debye)
−961.454827
−1851.652966
−2702.543759
13.4475
10.7362
8.0545
Observed parameters
BAIL-5
BAIL-6
33–O26 = 0.974
BAIL-7
˚
H–O bond distance of sulfonic acid (A)
H32–O31 = 0.961
H
H32–O31 = 0.961
H33–O26 = 0.983
H40–O39 = 0.960
H60–O59 = 0.974
H32–O31 = 0.975
H40–O39 = 0.974
H33–O26 = 0.993
H95–O87 = 0.979
H102–O99 = 0.987
H84–O83 = 0.960
H96–O91 = 0.959
C7–H11 = 1.075
C62–H64 = 1.066
H11· · ·O88 = 1.821
H64· · ·O90 = 2.482
H95· · ·O29 = 1.767
H33· · ·O90 = 1.660
H76· · ·O92 = 2.566
H79· · ·O93 = 2.474
−5406.887302
42.1129
(N)2C–H
C7–H11 = 1.074
C7–H11 = 1.069
(N)2C–H· · ·O
H11· · ·O42 = 1.936
H11· · ·O41 = 2.699
H45· · ·O42 = 2.523
H33· · ·O38 = 1.695
H11· · ·O41 = 2.030
H11· · ·O42 = 2.492
H44· · ·O42 = 2.166
H52· · ·O42 = 2.429
H18· · ·O38 = 2.546
H13· · ·O41 = 2.352
−3477.1618424
9.7614
˚
Other hydrogen bonds in BAILs (A)
Energy (a.u.)
Dipole moment (Debye)
−3055.1889438
16.1559

124
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
Table 7
−
Atomic charge analysis on the protons of –SO3H and HSO4 groups of BAILs.
ILs
Mullikan
NBO
CHELPG
−
−
−
HSO4
(N)2C–H
–SO3H
HSO4
(N)2C–H
–SO3H
HSO4
(N)2C–H
–SO3H
BAIL-1
BAIL-2
BAIL-3
BAIL-4
BAIL-5
BAIL-6
BAIL-7
0.407
0.405
0.389
0.393, 0.402
0.450
–
0.456
0.469
0.461
0.455, 0.465
0.467
0.317
0.312
0.312
0.305, 0.307
0.325
–
0.528
0.542
0.532
0.523, 0.534
0.538
0.285
0.225
0.190
0.204, 0.203
0.275
–
0.529
0.531
0.525
0.515, 0.548
0.531
0.477
0.538
0.579
0.470, 0.470
0.467, 0.471
0.476, 0.552
0.471, 0.478, 0.469
0.480, 0.555, 0.565,
0.464
0.537, 0.537
0.535, 0.537
0.570, 0.542
0.530, 0.543, 0.536
0.545, 0.577, 0.578,
0.533
0.573, 0.574
0.570, 0.570
0.568, 0.574
0.573, 0.582, 0.578
0.580, 0.615, 0.578,
0.538
0.409
0.474, 0.353
0.465
0.543, 0.457
0.322
0.331, 0.287
0.535
0.568, 0.528
0.195
0.353, 0.214
0.528
0.548, 0.659
reaction. First, the role of solvent was investigated. Activity of
BAIL-2 in protic polar solvents, such as water, methanol, and
ethanol, was found to be less than aprotic polar solvent such as
1,4-dioxane and 1,2-dicholoethane (DCE). The reaction can also
be performed without a solvent, but only if carried out at high
temperature (353 K). Activity of BAIL-2 was higher in DCE, hence
DCE was chosen as the reaction medium for this reaction for
further studies. Having found the optimized condition, all catalysts
were assessed for their catalytic acidity. The activity of Brönsted
acidic ILs follows the order BAIL-6 > BAIL-7 > BAIL-4 > BAIL-3 > BAIL-
2 > BAIL-5 > BAIL-1. For comparative study, H-ZSM-5, H-BETA,
and SBA-15-pr-SO3H were investigated. H-ZSM-5 was inac-
tive whereas H-BETA and SBA-15-pr-SO3H were weakly active.
Among all the catalysts investigated here, BAIL-6 was found
to be the best for the synthesis of amidoalkylnaphthols. Recy-
cling experiments confirmed that the BAILs can be recycled
without significant loss in catalytic activity (Table 4, entries 3
and 10).
6 > BAIL-7 > BAIL-4 > BAIL-3 > BAIL-2 > BAIL-5 > BAIL-1 (Table 5). All
solid acid catalysts investigated in this study were found to be
inactive for this reaction under the optimized reaction condition.
The activity of imidazole based Brönsted acidic ILs follows
the order BAIL-1 < BAIL-BAIL-2 < BAIL-3 ≈ BAIL-4, which is consis-
tent with the acidity measurement using UV–visible spectroscopy.
As shown in Tables 3–5, the activity of imidazole/benzimidazole
based multi-sulfonic acid group functionalized BAILs is compara-
tively higher than that of mono-sulfonic acid group functionalized
BAIL-2. However, it may be noted that each BAIL have differ-
ent numbers of acid sites (SO3H and HSO4−), hence all of them
have different TON per mol. If the number of acid sites (SO3H
and HSO4−) present in BAILs is taken into account for calcu-
lating TON, the activity of BAIL-2 is found to be higher than
other BAILs investigated in this study. This may be due to the
fact that the all –SO3H groups are not equivalent in all kinds of
BAILs. Hydrogen bonding may exist in these BAILs, which influ-
ences their catalytic activity. To understand this phenomenon,
structures of BAILs were optimized using theoretical modeling
(Section 3.3).
Further, multi-component Mannich type coupling reaction of
2-naphthol, benzaldehyde and piperidine was investigated for the
synthesis of 1-(phenyl (piperidin-1-yl) methyl) naphthalene-2-ol
(Betti base) in aqueous medium (Scheme 5) [25]. The reaction pro-
ceeds through the imine formation of the aldehyde and piperidine,
followed by the attack of 2-naphthol. Under the optimized reaction
condition the activity of Brönsted acidic ILs follows the order BAIL-
3.3. The molecular geometries of BAILs
The minimum-energy geometries of BAILs were determined
by performing ab initio geometry optimizations at the RHF/6-
31G level [34]. The fully optimized geometries of these BAILs are
presented in Figs. 2–8 and Table 6. In addition to this, atomic
−
charge analysis on the protons of –SO3H and HSO4 groups of
BAILs have also been performed and summarized in Table 7.
All calculations were carried out using the Gaussian 09 series
of programs. Theoretical study confirmed that a strong hydro-
gen bond network is presented in the BAILs. Molecular geometry
Fig. 6. Optimized molecular structure of BAIL-5 using RHF/6-31G.
Fig. 7. Optimized molecular structure of BAIL-6 using RHF/6-31G.

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
125
Fig. 8. Optimized molecular structure of BAIL-7 using RHF/6-31G.
study reveals that anion (HSO4−) interacts with the –SO3H and
imidazole/benzimidazole ring through C–H· · ·O or O–H· · ·O type
hydrogen bonding. In case of BAIL-1, there are two hydrogen bonds
(H8· · ·O17 = 2.014 and H5· · ·O16 = 1.665) which are shorter than the
activity data, t−he acidity of BAIL-6 (having three –SO3H groups
and one HSO4 group) is more active than BAIL-7 (having four
–SO3H and two HSO4 groups). This can also be explained using
−
the optimized structure obtained through theoretical modeling. In
−
˚
Van der Waals distance of 2.67 A (Fig. 2 and Table 6) [35]. The
case of BAIL-6, all three –SO3H groups have no influence of HSO4
,
−
acidity in BAIL-1 is due to HSO4 and the acidic proton in imi-
whereas two –SO3H groups in BAIL-7 are under strong influence of
HSO4− (H33· · ·O90 = 1.660, H95· · ·O29 = 1.767, and H11· · ·O88 = 1.821)
(Figs. 7 and 8). Optimized structure of BAIL-7 clearly shows that one
dazole ring. The activity of BAIL-2 should be high compared to
BAIL-1 due to the presence of additional –SO3H group. Catalytic
activity experiments reveal that the activity of BAIL-2 is approx-
imately 1.5 times higher than BAIL-1. Optimized structure and
hydrogen bonding revealed that imidazolium acid proton ((N)2C–H
bond distance of BAIL-2 < BAIL-1) and HSO4− of BAIL-2 is less acidic
than BAIL-1(Fig. 3 and Table 6). In BAIL-2, benzene and imidazole
ring hydrogen form weak hydrogen bonding with oxygen (O12,
−
of the HSO4 (S85) and –SO3H (S28) are so closely associated that
they completely lose their activity, due to strong hydrogen bond-
ing (Table 6). In addition, one of the –SO3H (S98) group in BAIL-7
−
is under the influence of second HSO4 (S86). Due to these interac-
tions a close network structure is formed in which some of the acid
protons are masked (except two terminal –SO3H groups acid pro-
tons) and inaccessible to the reactant molecules. Hence, the overall
activity of BAIL-7 is less than the activity of BAIL-6. Theoretical
modeling was able to correlate well with the activity–acidity order
obtained using catalytic investigations.
−
O13 and O14 of S O) of HSO4 (H4· · ·O14 = 2.099, H4· · ·O13 = 2.478,
H7· · ·O13 = 2.280, H33· · ·O14 = 2.264, H24· · ·O12 = 2.429). Whereas in
BAIL-1, imidazolium ring hydrogens (H5 and H8) form compar-
atively strong−hydrogen bonding with oxygen (O16 and O17 of
S
O) of HSO4 (H5· · ·O16 = 1.665, H8· · ·O17 = 2.014) which makes
−
proton of HSO4 −in BAIL-1 more labile and acidic compared to the
proton of HSO4 in BAIL-2. This is the reason why the activity
of BAIL-2 is only 1.5 times higher than BAIL-1. Catalytic activ-
ity of BAIL-4 (having two imidazolium acid protons, two –SO3H,
4. Conclusions
In this study, the synthesis of several novel BAILs that contain
one or multi-sulfonic acid group on imidazolium/benzimidazolium
cation by simple and straight-forward procedures from cheap
starting materials in good yields has been reported. BAILs
served as efficient and reusable catalysts and realized the
green synthesis of dihydropyrimidinones, amidoalkylnaphthols,
and 1-(phenyl (piperidin-1-yl) methyl) naphthalene-2-ol. The
activity of Brönsted acidic ILs follows the order BAIL-6 > BAIL-
7 > BAIL-4 > BAIL-3 > BAIL-2 > BAIL-5 > BAIL-1, which is consistent
with the acidity measurement using UV–visible spectroscopy and
minimum-energy geometries determined by ab initio calculations.
Based on the results obtained, one can also conclude that more
−
and two HSO4 groups) was found to be only marginally more
active than BAIL-3 (having one imidazolium acid proton, two –SO3H
−
groups and one HSO4 group). Optimized structure and hydrogen
bonding clearly show that in both cases, –SO3H groups and imida-
zolium acid protons are equally acidic ((N)2C–H−bond distance is
same in both the cases, Table 6). Whereas HSO4 proton of BAIL-
4 form comparatively strong hydrogen bonding (H52· · ·O81 = 2.045,
H4· · ·O10 = 2.115, H5· · ·O78 = 2.116) than BAIL-3 (H4· · ·O11 = 2.131)
−
(Table 6) that makes HSO4 −protons of BAIL-4 less acidic as com-
pared to the proton of HSO4 in BAIL-3. Hence, the overall activity
of BAIL-4 is only marginally higher than BAIL-3 (Figs. 4 and 5 and
Table 6). Based on the above observations, Tables 2–5 and Figs. 2–8,
it can be concluded that, imidazolium acid proton, –SO3H and
HSO4− groups contribute to the acidity of ionic liquids. It looks that
both the catalytic activity and acidity increase with the number of
−
numbers of –SO3H and HSO4 functionalization does not really
improve the catalytic activity, when the catalytic activity is cal-
culated per acid sites. Catalyst systems have several noteworthy
features: (1) reactions can be carried out at relatively lower tem-
perature with shorter reaction time; (2) products can be separated
easily with high yields and purity; (3) BAILs can be reused several
times (4) BAILs are cheap, environmentally benign and safe.
−
–SO3H and HSO4 in BAIL-1 to BAIL-4.
As indicated by the catalytic activity data, the activity of benzim-
idazole based BAIL-5 is less than BAIL-3. This can be explained using
the optimized structure obtained through theoretical modeling. In
−
case of BAIL-3, none of the –SO3H group has any influence of HSO4
,
Acknowledgements
whereas one –SO3H group in BAIL-5 is under the strong influence
−
of HSO4 (H33· · ·O38 = 1.695) (Fig. 6). That makes proton of one of
Authors thank Department of Science and Technology, New
Delhi and Director, IIT Ropar for financial assistance. Authors
thank Dr. T.J. Dhilip Kumar, Chemistry Department, IIT Ropar
the –SO3H group in BAIL-5 less acidic (labile) than BAIL-3 and thus
its activity is lower than the BAIL-3. As indicated by the catalytic

126
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 345 (2011) 117–126
for providing software and valuable suggestions. They also thank
Prof. B.K. Dhindaw, Dean, Academic and Research, and Prof. P.K.
Raina, Head, Department of Chemistry, IIT Ropar, for their constant
encouragement.
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