Green synthesis of benzimidazole derivatives under ultrasound irradiation using Cu-Schiff base complexes embedded over MCM-41 as efficient and reusable catalysts

Article abstract of DOI:10.1080/00958972.2020.1730335

We have synthesized two recoverable catalysts by covalently attaching complexes such as Cu-complex-phen and Cu-complex-bipy on MCM-41 through a greener synthetic route. FT-IR, EDX, SEM and TEM microscopy, XRD analysis, N₂ adsorption and desorpt

Full text of DOI:10.1080/00958972.2020.1730335

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Green synthesis of benzimidazole derivatives
under ultrasound irradiation using Cu-Schiff base
complexes embedded over MCM-41 as efficient
and reusable catalysts
M. Bharathi, S. Indira, G. Vinoth, T. Mahalakshmi, E. Induja & K. Shamuga
Bharathi
To cite this article: M. Bharathi, S. Indira, G. Vinoth, T. Mahalakshmi, E. Induja & K. Shamuga
Bharathi (2020): Green synthesis of benzimidazole derivatives under ultrasound irradiation using
Cu-Schiff base complexes embedded over MCM-41 as efficient and reusable catalysts, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2020.1730335
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1730335
Green synthesis of benzimidazole derivatives under
ultrasound irradiation using Cu-Schiff base complexes
embedded over MCM-41 as efficient and reusable catalysts
M. Bharathi, S. Indira, G. Vinoth, T. Mahalakshmi, E. Induja and
K. Shamuga Bharathi
Department of Chemistry, Periyar University, Salem, India
ABSTRACT
ARTICLE HISTORY
Received 28 June 2019
Accepted 3 February 2020
We have synthesized two recoverable catalysts by covalently
attaching complexes such as Cu-complex-phen and Cu-complex-
bipy on MCM-41 through a greener synthetic route. FT-IR, EDX,
SEM and TEM microscopy, XRD analysis, N2 adsorption and
desorption, ICP-OES and TGA were used to characterize the het-
erogeneous catalysts. The catalytic functioning of Cu-complex-
phen-MCM-41 (C1) and Cu-complex-bipy-MCM-41 (C2) were
shown yield up to 95% for the synthesis of benzimidazole deriva-
tives in ethanol/methanol solvent under ultrasonic irradiation
within 90 min. The catalyst was easily recovered and reused up to
three times without significant loss of its activity.
KEYWORDS
Schiff base; MCM-41;
copper catalyst;
benzimidazole; ultrasonic
irradiation
GRAPHICAL ABSTRACT
CONTACT K. Shamuga Bharathi
nksbharathi@Periyaruniversity.ac.in
Department of Chemistry, Periyar
University, Periyar Palkalai Nagar, Salem 636011, Tamil Nadu, India.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
M. BHARATHI ET AL.
1. Introduction
The goals of greener chemistry such as minimizing waste, reducing energy and time
can be achieved by using ultrasonic irradiation in chemical reactions [1]. This tech-
nique has advantages like increasing the reaction rate and yield of the product.
Sonication is used to promote a range of chemical reactions such as addition, substitu-
tion, coupling, condensation, oxidation, reduction, protection/deprotection, photo-
chemical and polymerization [2–5]. Furthermore, this method is efficiently applied for
homogeneous, heterogeneous and transition metal-catalyzed organic reactions [6].
Heterocyclic families are prominent in all fields of science viz agricultural, industrial,
pharmaceutical, medicinal and material chemistry. Benzimidazoles are prominent
among heterocyclic compounds, as building blocks for many bioactive compounds
and structural isosteres of naturally occurring nucleotides. Hence they can readily
interact with the biopolymers of living systems [7] and show enormous therapeutic
applications, including anti-HIV [8], anti-cancer [9], anti-inflammatories [10], anti-bacter-
ial [11], anti-fungal [12], anti-histaminic [13], anti-oxidant [14], anti-hypertensives [15]
and anti-coagulant agents [16]. A significant number of benzimidazole-based drugs
are commercially available. For example, thiabendazole, norastemizole, telmisartan and
omeprazole are well-known drugs of this class as anti-parasitic, anti-histaminic, antihy-
pertensive and anti-ulcer agents.
Benzimidazole derivatives are synthesized in two general ways. The first process is
coupling of o-phenylenediamines and carboxylic acids [17] or their derivatives (nitriles,
orthoesters or imidates) [18] under strongly acidic conditions and sometimes along
with very high temperatures. The second process involves two-step synthesis consist-
ing of formation of Schiff base by condensation of o-phenylenediamines with alde-
hyde followed by oxidative cyclodehydrogenation using oxidative and catalytic
reagents such as nitrobenzene [19], 1,4-benzoquinone [20], Ph(OAC)2 [21], Zn-proline
[22], heteropoly acids [23], thionyl chloride-treatment [24], 2,3-dichloro-5-6-dicyano-
benzoquinone (DDQ) [25], benzofuroxan [26], MnO2 [27], oxone [28], NaHSO3 [29],
H2O2/HCl [30], Na2S2O5 [31], air [32], copper complex [33], Hauland natural zeolite [34],
NaYzeolite [35], AlKIT-5 [36], etc. Some of these methods suffer from major drawbacks
such as low yield, expensive catalysts, prolonged reaction times, difficulty in separation
of the products from the reaction mixture, use of toxic solvents and requirement of
more reagent. Therefore, the focus on designing economical, stable, recoverable,
reusable and eco-friendly solid-supported heterogeneous catalyst for the synthesis of
benzimidazole derivatives is important.
Among mesoporous silica materials, MCM-41 has special properties like high ther-
mal stability, high pore volume, high surface area and well-ordered hexagonal
arrangement [37]. Hence MCM-41 is a suitable support for anchoring metal complexes.
Metal ions such as copper, nickel [38], zinc, manganese, cobalt, etc., in mesoporous sil-
ica framework, have made contributions in heterogeneous catalysis due to their low
toxicity and low cost. Anchoring of different ligands into the hexagonal channels can
be easily modified in the walls of the porous system of MCM-41 [39, 40] and allows
tuning catalytic activity.
Wanling Mo et al. [41] developed a heterogeneous catalyst, Cu/N,N-bidentate-lig-
and, that exhibits high catalytic activity, is easily recoverable and has excellent stability

JOURNAL OF COORDINATION CHEMISTRY
3
in consecutive catalytic runs for the oxidative carbonylation of methanol. Mohsen
Nikoorazm et al. [42] reported a heterogeneous catalyst, Ni-vanillin-Schiff base-MCM-
41, with high catalytic activity and low leaching of metal while reusing the catalyst for
the synthesis of polyhydroquinoline and 2,3-dihydroquinazolin-4(1H)-one derivatives.
Herein, we report the preparation and characterization of two heterogeneous cata-
lysts, Cu-complex-phen/bipy-MCM-41, for the synthesis of benzimidazoles by consider-
ing the significance of N,N-bidentate ligands, vanillin and Schiff base moiety.
Moreover, the catalytic reactions are carried out in a greener way i.e. using methanol/
ethanol as solvents and with ultrasonication method.
2. Experimental
2.1. Materials and instruments
3-Aminopropyltrimethoxysilane (3-APTMS), tetraethylorthosilicate (TEOS), cetyltrimethy-
lammoniumbromide (CTAB) and copper(II) acetate monohydrate were acquired from
Aldrich and SRL. IR spectra of prepared samples were examined on an Avatar 370
model FT-IR spectrometer with KBr pellets. Powder X-ray diffraction data were
recorded on a Bruker AXS D8 Advance diffractometer. Sonication was performed in an
ultrasonic cleaner, Sonica 2200 MH S3 (model no: 090.003.003). The SEM/EDX micro-
graphs were obtained on the JEOL Model JSM 6390LV microscope. HR-TEM analysis
was carried out on a JEOL JEM 2100 model. BET surface area was determined using
the NOVA 1200 Quanta chrome instrument at 77 K. ICP-OES was used to analyze the
Cu content in the samples. The TG/DTA micrographs were recorded on a Perkin Elmer
instrument from 0-900 ꢀC.
2.2. Preparation of MCM-41
An aqueous solution of CTAB (1 g) was added to 3.5 ml of (2 M) NaOH solution and
stirred at 80 ꢀC until the solution became uniform. Then, TEOS (5 ml) was progressively
added to the solution and reacted for 2 h at the same temperature. The collected
white product was washed with deionized water and dried at 70 ꢀC. The withered
powder was calcinated at 550 ꢀC for 5 h with the rate of 2 ꢀC/min to remove the
residual surfactant and finally got the pure MCM-41 [42].
2.3. Preparation of heterogeneous Cu-complex-phen/bipy-MCM-41
1 mmol (0.17 ml) of 3-APTMS in dichloromethane was added to 1 mmol (0.15 g) of van-
illin in methanolic solution, stirred and refluxed for 3 h. Then, a methanolic solution of
1 mmol (0.19 g) of Cu(OAc)2ꢁH2O and 1 mmol (0.19 g) of 1,10-phenanthroline was
added to the reaction mixture and continued to react for further 3 h at the same con-
ditions. After cessation of the reaction, the Cu-complex-phen was retrieved from the
reaction mixture using a rotary evaporator. 1 g of calcinated MCM-41 was added to
the synthesized complex in excess of toluene and stirred for 24 h under N2. Then, the
product Cu-complex-phen-MCM-41 (C1) was recovered by a rotary evaporator.

4
M. BHARATHI ET AL.
Scheme 1. Synthesis of Cu-complex-phen/bipy-MCM-41.
Cu-complex-bipy-MCM-41 (C2) was prepared by following the above procedure
using 1 mmol (0.15 g) of 2,20-bipyridine instead of 1,10-phenanthroline (Scheme 1).
2.4. General procedure for the synthesis of benzimidazole derivatives under
ultrasonic irradiation
30mg of catalyst C1 (Scheme 2) was added to an ethanolic solution of o-phenylenediamine
(1mmol) and aromatic aldehyde (1mmol) at 60 ꢀC. Then the mixture was subjected to son-
ication for 90min. After the reaction (monitored by TLC ethyl acetate:hexane ¼ 1:2), the
catalyst was separated by filtration and washed with ethanol to be reused for the next
cycle. The product was obtained from the filtrate and recrystallized to get the pure com-
pound. The compound was confirmed by 1H-NMR.

JOURNAL OF COORDINATION CHEMISTRY
5
Scheme 2. Synthesis of benzimidazole derivatives.
Another set of the catalytic mixture was carried out using the above procedure
with 30 mg of C2 (catalyst) in methanol (6 ml) instead of C1 in ethanol.
3. Results and discussion
The new heterogeneous catalysts were characterized by FT-IR, PXRD, SEM-EDX, HR-
TEM, ICP-OES, BET, TGA and analysis.
3.1. FT-IR spectroscopy
Figure 1 shows the FT-IR spectra of the synthesized ligand, Cu-complex-(phen/bipy),
C1 and C2. For the MCM-41 sample (Figure 1a), the peaks at 1100, 803 and 468 cmꢂ1
can be attributed to asymmetric stretching, symmetric stretching and bending vibra-
tions of Si-O-Si, respectively [43]. In addition, the band at 3453 cmꢂ1 is assigned to
vibration of the hydrogen-bonding silanol groups. For the ligand, the presence of an
absorption about 2900 cmꢂ1 is associated with the CH2 vibrations corresponding to
the C-H stretch. The appearance of peaks at 1647 cmꢂ1 and 1515 cmꢂ1 are due to for-
mation of Schiff base C¼N and aromatic C¼C bonds in 4-hydroxy-3-methoxybenzalde-
hyde (vanillin) (Figure 1b). The M-N and M-O bands, for Cu-complex-(phen/bipy), C1
and C2 are at 619 cmꢂ1 and 457 cmꢂ1, respectively, which confirmed that the metal
ion bonded to ꢂth1 e ligand. C-H stretching vibrations (from phen or bipy rings) appear
above 3000 cm and characteristic C-H out of plane bending vibrations are seen at
872 and 723 cmꢂ1, indicating the presence of diimine ligands (Figures 1c-1f) [44].
3.2. PXRD
Powder XRD patterns of pure MCM-41, C1, C2 and recovered catalysts are illustrated in
Figure 2. Three characteristic peaks assigned to 100, 110 and 200 reflections are
observed for pure MCM-41. The intensity of the diffraction for the 100 reflection is
decreased whereas 110 and 200 reflections almost disappear for both catalysts (C1, C2)
and recovered catalysts when compared to that of MCM-41, revealing that the copper
is highly dispersed on the inner channel pores of MCM-41 [45].
3.3. SEM, EDX and ICP-OES
SEM micrographs of MCM-41 and C1 and C2 show no significant difference in the sur-
face morphology occurred upon inclusion of the copper complex in MCM-41 (Figure 3).

6
M. BHARATHI ET AL.
Figure 1. FT-IR spectra of (a) MCM-41, (b) ligand, (c) Cu-complex-phen, (d) Cu-complex-phen-MCM-
41, (e) Cu-complex-bipy and (f) Cu-complex-bipy-MCM-41.
EDX elemental analysis indicates the presence of copper in the catalyst which is not pre-
sent in the MCM-41 (Figure 3). The exact loadings of copper on C1 and C2 are
2.7 ꢃ 10ꢂ3 mol/g and 3.0ꢃ 10ꢂ3 mol/g by ICP-OES technique.
3.4. Transmission electron microscopy
The TEM images of C1 and C2 are shown in Figure 4. C1 and C2 possess well-ordered
hexagonal arrangement even after immobilization of both copper complexes onto
MCM-41 [46].
3.5. N2 adsorption-desorption
The BET surface area, average pore diameter and pore volume were calculated using
the BJH method for pure MCM-41 [38], C1 and C2 (see Figure 5 and Table 1). They are
drastically decreased in both catalysts when compared to pure MCM-41, due to
incorporation of the copper complexes in the framework of MCM-41 [47].
3.6. Thermogravimetric analysis
Figure 6 shows the TGA curves for C1 and C2. Both have three-step weight losses. The
initial weight loss up to 100 ꢀC is due to the removal of physically adsorbed water and
solvent inside the pores. Then successive weight loss in the temperature range of
150-400 ꢀC is due to decomposition of organic groups. Upon further heating, final
weight loss to 700 ꢀC could correspond to decomposition of silanol groups [48].

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. PXRD patterns of (a) MCM-41, (b) Cu-complex-phen-MCM-41, (c) Cu-complex-bipy-MCM-
41, (d) recovered catalyst Cu-complex-phen-MCM-41 and (e) recovered catalyst Cu-complex-bipy-
MCM-41.
3.7. Catalytic activity
The catalytic activities of C1 and C2 have been carried out in the synthesis of benzimi-
dazole derivatives from o-phenylenediamine and with various substituted aldehydes
under ultrasonic irradiation.
In order to optimize the reaction conditions, benzaldehyde has been selected as a
model substrate and it was subjected to different reaction conditions such as the
amount of catalyst, time, solvent and temperature. To find the appropriate solvent, we

8
M. BHARATHI ET AL.
Figure 3. SEM images and EDX spectra of (a) (b) MCM-41, (c) (d) Cu-complex-phen-MCM-41 and
(e) (f) Cu-complex-bipy-MCM-41.

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 3. (Continued)
Figure 4. TEM images of (a) Cu-complex-phen-MCM-41 and (b) Cu-complex-bipy-MCM-41.
carried out the reaction in different solvents in the presence of C1/C2 (Table 2, entries
1-6). Ethanol and methanol were the best solvents (Table 2, entries 1 and 2) while
using C1 and C2, respectively. We carried out the reaction in neat ethanol/methanol
for 90 min. The reaction was very sluggish and gives only a small amount of product
(Table 2, entries 9 and 10). The reaction temperature was optimized as 60 ꢀC (Table 2,
entries 1 and 2).

10
M. BHARATHI ET AL.
Figure 5. BET images of (a) (b) Cu-complex-phen-MCM-41 and (c) (d) Cu-complex-bipy-MCM-41.
To investigate the effect of the catalysts, systematic studies were carried out in the
presence of different amounts of catalyst (20, 30, 40 and 50 mg) (Table 3). By increas-
ing the catalyst amount above 30 mg, there was a slight decrease in the yield of the
product (Table 3, entries 1 and 2). Thus, the best yield was found using 30 mg of cata-
lysts. In the next step, the influence of time in the presence of catalysts was examined.
When the reaction time was increased gradually from 30 min to 90 min, the rate of the
reaction increased (Table 4, entries 1 and 2). Optimization results reveal that 30 mg of
catalysts at 60 ꢀC for 90 min was most suitable for this reaction.
Comparative studies between pure MCM-41, pure complexes (Cu-complex-phen/
bipy) and catalysts (C1 and C2) were performed for the synthesis of benzimidazole
derivative; 2-nitro benzaldehyde in ethanol and vanillin in methanol were selected as
model substrates for C1 and C2, respectively (Figure 7). The yield of product is remark-
ably increased while using C1 and C2 when compared to pure MCM-41 and
pure complexes.

JOURNAL OF COORDINATION CHEMISTRY
11
Table 1. Surface properties of MCM-41, Cu-complex-phen-MCM-41 and Cu-complex-bipy-MCM-41.
Pore diameter
Entry
Materials
Surface area (m2/g)
Pore volume (cm3/g)
(nm)
1
2
3
MCM-41
Cu-complex-phen-MCM-41
Cu-complex-bipy-MCM-41
1091.165
3.719
10.883
0.805
0.013
0.016
2.7
1.6
1.1
Figure 6. TGA curves of (a) Cu-complex-phen-MCM-41 and (b) Cu-complex-bipy-MCM-41.
Table 2. Effect of solvent and temperature for the synthesis of benzimidazole deriva-
tives reaction.a
Yield %
Entry
Solvent
Ethanol
Methanol
Water
DCM
Toluene
Acetonitrile
Ethanol
Methanol
Ethanolb
Methanolb
Temperature (ꢀC)
Cu-complex-phen-MCM-41
Cu-complex-bipy-MCM-41
1
2
3
4
5
6
7
8
9
60
60
60
60
60
60
RT
RT
RT
RT
95
55
65
67
39
80
66
89
93
76
72
75
68
-
-
65
42
-
10
-
46
aReaction conditions: 1 mmol of o-phenylenediamine, 1 mmol of benzaldehyde, Cu-complex-phen/bipy-MCM-41
(30 mg) as catalyst in (6 ml) of solvent for 90 min under ultrasonic irradiation.
bReaction was carried out under neat condition (without catalyst).
Under optimized conditions, we have carried out the catalytic reactions with
o-phenylenediamine and a range of aldehydes bearing various electron donating
(Table 5, entries 2, 7 and 10)/electron withdrawing (Table 5, entries 4-6) and sterically
hindering groups. Good to excellent yields of products for all types of aldehydes were
obtained (Table 5). Our catalysts have also given good yields with the disubstituted
aldehydes (Table 5, entries 3 and 9) and the catalytic reaction of fused ring aldehyde

12
M. BHARATHI ET AL.
Table 3. Effect of catalyst amount.a
b
Entry
Catalyst
Solvent
Amount of catalyst (mg)
Yield (%)
1
2
Cu-complex-phen-MCM-41
Cu-complex-bipy-MCM-41
Ethanol
Methanol
20 / 30 / 40 / 50
20 / 30 / 40 / 50
78 / 95 / 87 / 90
87 / 93 / 90 / 92
aReaction conditions: 1 mmol of o-phenylenediamine, 1 mmol of benzaldehyde, catalyst in 6 ml of solvent at 60 ꢀC
for 90 min under ultrasonic irradiation.
bIsolated yield.
Table 4. Effect of time.a
Entry
Catalyst
Solvent
Time (min)
Yield (%)b
1
2
Cu-complex-phen-MCM-41
Cu-complex-bipy-MCM-41
Ethanol
Methanol
30 / 60 / 90
30 / 60 / 90
57 / 75 / 95
52 / 69 / 93
aReaction conditions: 1 mmol of o-phenylenediamine, 1 mmol of benzaldehyde, (30 mg) catalyst in 6 ml of solvent at
60 ꢀC under ultrasonic irradiation.
bIsolated yield.
Figure 7. Catalytic performance of the benzimidazole derivatives reaction.
(Table 5, entry 8) has also gone well. Steric hindrance of the catalyst and the substitu-
ent of the substrate affect the yield of the catalytic reaction. C1 gives higher yield
while using ortho-substituted aldehydes (Table 5, entries 6-8) whereas C2 gives higher
yield while using para-substituted aldehydes (Table 5, entries 2, 4, 5 and 10)
as substrates.
We have also compared the activity of our catalysts with some previously reported
homogeneous/heterogeneous catalysts (Table 6). The activity of our catalyst is much
better than entries 2, 3, 5, 6 and 8. Although the catalysts in entries 1, 4, 7 and 9 gave
similar or slightly higher yields when compared to our catalysts, they possess some
crucial disadvantages. Our catalyst overcomes those disadvantages and has given bet-
ter yields. In entry 1, TBAF (tetrabutylammonium fluoride) has been used as a

JOURNAL OF COORDINATION CHEMISTRY
13
Table 5. Synthesis of various aryl benzimidazole derivatives by Cu-complex-phen/bipy-MCM-
41 catalyst.a
Cu-complex-phen-MCM-41b Cu-complex-bipy-MCM-41c
TOF
TOF
f
Entry
1
Aldehyde
CHO
Product
Yield (%)d TONe (h-1)f Yield (%)d TONe (h-1)
95
64
115
77
77
51
93
69
100
74
67
49
H
N
N
CHO
H
N
2
3
OCH3
N
H
3CO
OH
OCH3
81
98
65
83
90
60
H
N
OH
N
OHC
Cl
OCH3
4
5
CHO
57
51
69
61
46
41
59
76
64
82
43
55
H
N
Cl
N
CHO
H
N
NO2
N
N
O2
N
O
CHO
NO2
6
7
8
95
43
62
115
52
77
35
50
73
40
60
79
44
65
51
29
43
H
N
2
N
CHO
OH
HO
H
N
N
N
CHO
OH
75
HO
H
N
N
CHO
OH
9
55
66
44
61
66
44
HO
H
N
Br
N
Br
(continued)

14
M. BHARATHI ET AL.
Table 5. Continued.
Cu-complex-phen-MCM-41b Cu-complex-bipy-MCM-41c
TOF TOF
f
Entry
10
Aldehyde
CHO
Product
Yield (%)d TONe (h-1)f Yield (%)d TONe (h-1)
70 85 57 75 81 54
H
N
N
OH
HO
aReaction conditions: 1 mmol of o-phenylenediamine, 1 mmol of aldehyde (30 mg, 0.824%, 0.921% Cu) catalyst in
(6 ml) solvent under ultrasonic irradiation at 60 ꢀC for 90 min.
bIn ethanol (6 ml).
cIn methanol (6 ml).
dIsolated Yield.
eTurnover number (TON) ¼ (mmol of product)/(mmol of catalyst) after time t.
fTOF ¼ TON/time.
Table 6. Comparison of the different procedures with some reported catalysts, which has been
applied for the synthesis of benzimidazole derivative.a
Entry
1
Catalyst
Condition
Yield (%)
94
References
[51]
Tetrabutylammonium
fluoride (TBAF)
Graphene-oxide
Ammonium nickel sulphate
NiEuFe2O4
Sulfonated graphene oxide
HPFP-1(NP)
Water, RT, ultrasonic irradiation, 30 min
2
3
4
5
6
7
8
9
Methanol, 35 ꢀC, ultrasonic irradiation, 70 min
Water, RT, ultrasonic irradiation, 135 min
Water, RT, ultrasonic irradiation, 20 min
Solvent free, RT, 7 h
86
80
95
89
90
95
75
98
95
93
[52]
[53]
[54]
[55]
[56]
[36]
[38]
[57]
Ethanol, RT, 3 h
AlKIT-5
Acetonitrile, reflux, 4 h
Ni-MCM-41
Co/SBA-15
Cu-complex-phen-MCM-41
Cu-complex-bipy-MCM-41
Glycerol, 90 ꢀC, 4 h
Ethanol, 60 ꢀC, 4 h
10
11
Ethanol, 60 ꢀC, ultrasonic irradiation, 90 min
Methanol, 60 ꢀC, ultrasonic irradiation, 90 min
This work
This work
aBenzimidazole derivatives of o-phenylenediamine and benzaldehyde.
homogeneous catalyst. Hence, it is very difficult to recover and reuse. Although
NiEuFe2O4 has been used as a heterogeneous catalyst in entry 4, the cost of one of
the reactants (rare earth europium salt) is much higher than that of the simple copper
acetate, that we have used. Moreover, the calcination of their catalyst required much-
elevated temperature (800 ꢀC) over our MCM-41 (550 ꢀC). In entries 7 and 9, heteroge-
neous catalysts AlKIT in acetonitrile under reflux for 4 h and Co/SBA-15 in ethanol at
60 ꢀC for 4 h were used, respectively. From an environmental perspective, acetonitrile
is not a green solvent [49]. Our catalysts give the same/competitive yield by using
alcohols (ethanol/methanol) as solvents, in a shorter reaction time of 90 min under
ultrasonic irradiation.
Overall, our catalysts are more promising than others due to their following factors:
i) heterogeneous nature, reusability, ii) greener catalytic conditions (ultrasonication,
ethanol/methanol) and iii) low-cost catalyst. Above all, we have used a new class of
mixed ligand complexes as catalysts in order to enhance the activity by
“synergistic effect”.
Finally, the reusability of C1 and C2 was checked in 2-nitro benzaldehyde in ethanol
and vanillin in methanol, respectively, at 60 ꢀC for 90 min under ultrasonic irradiation.

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 8. Reusability of catalyst Cu-complex-phen/bipy-MCM-41 for benzimidazole deriva-
tives reaction.
Scheme 3. Plausible mechanism for the synthesis of benzimidazole derivatives.

16
M. BHARATHI ET AL.
The catalysts were separated by simple filtration after each cycle, washed with etha-
nol/methanol and then subjected to the next cycle. The catalysts were reused up to
three cycles without any significant loss of activity (Figure 8).
The proposed mechanistic path for the formation of benzimidazoles in the presence
of a catalyst is shown in Scheme 3 [50]. The catalyst coordinates with the carbonyl
group of aldehyde to assist the nucleophilic addition of o-phenylenediamine to the
activated aldehyde (I) and gives the intermediate (II). Then the intermediate (II) under-
goes intramolecular Michael-type addition to give intermediate III, which is oxidized in
air and gives the desired products.
3.8. Hot filtration test
To examine the leaching of metal from solid catalyst during the reaction, hot filtration
test was performed. In this test, a mixture of 2-nitro benzaldehyde and o-phenylenedi-
amine with C1 (30 mg) in ethanol was heated at 60 ꢀC for 90 min under ultrasonic
irradiation. Then, the catalyst was removed from the reaction mixture and the reaction
was continued with the filtrate for further 90 min at same conditions. It was observed
that the yield of the product was not increased after the removal of catalyst (95% and
94% yield for before and after the test). A similar result was obtained while using C2
with the substrate vanillin (84% and 83.5% yield before and after the test), confirming
that there is no leaching of metal during the catalytic reaction and the heterogeneous
nature of the catalyst.
4. Conclusion
We have synthesized and characterized two heterogeneous catalysts, Cu-complex-
phen/bipy-MCM-41. The catalysts were used for synthesis of benzimidazole derivatives
under a green pathway i.e., using methanol/ethanol as a solvent and ultrasonic irradi-
ation. We have also discussed the influence of the steric factor based on the position
of the substituent in the substrates on the yield of the catalytic product with good to
excellent yields and also easy recovery. Hot filtration test confirmed that there is no
leaching of the metal from the catalyst even after the third cycle which shows the sta-
bility and purity of the catalyst. Hence, the catalysts were successful for the catalytic
reaction up to three cycles without any significant loss in the yield of the product. In
particular, the new class of mixed ligand complexes have shown a new path in the
heterogeneous catalysis and enhance the activity by the synergistic effect.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
The authors are grateful for the financial support from the University Grants Commission (UGC-
BSR) F.30-319/2016 (BSR) New Delhi, India. The first author acknowledges to University Grants

JOURNAL OF COORDINATION CHEMISTRY
17
Commission-Rajiv Gandhi National Fellowship (UGC-RGNF) RGNF-2015-17-SC-TAM-21024
research grant for their financial support.
References
[1] M.A. Schiel, A.B. Chopa, G.F. Silbestri, M.B. Alvarez, A.G. Lista, C.E. Domini. Green Synthetic
Approaches for Biologically Relevant Heterocycles, p. 571. Elsevier, Amsterdam (2015).
[2] R. Patil, P. Bhoir, P. Deshpande, T. Wattamwar, M. Shirude, P. Chaskar. Ultrason.
Sonochem., 20, 1327 (2013).
[3] S. Puri, B. Kaur, A. Parmar, H. Kumar. Curr. Org. Chem., 17, 1790 (2013).
[4] G. Cravotto, S. Tagliapietra, M. Caporaso, D. Garella, E. Borretto, A.D. Stilo. Chem.
Heterocycl. Comp., 49, 811 (2013).
[5] R. Cella, H.A. Stefani. Tetrahedron, 65, 2619 (2009).
[6] D. Nagargoje, P. Mandhane, S. Shingote, P. Badadhe, C. Gill. Ultrason. Sonochem., 19, 94
(2012).
[7] J.A. Horig, P. Renz. Eur. J. Biochem., 105, 587 (1980).
[8] T. Pan, X. He, B. Chen, H. Chen, G. Geng, H. Luo, H. Zhang, C. Bai. Eur. J. Med. Chem, 95,
500 (2015).
[9] (a) B. Chu, F. Liu, L. Li, C. Ding, K. Chen, Q. Sun, Z. Shen, Y. Tan, C. Tan, Y. Jiang. Cell
Death Dis., 6, e1686 (2015). (b) J. Zhao, Y. Guo, J. Hu, H. Yu, S. Zhi, J. Zhang. Polyhedron,
102, 163 (2015).
[10] K.C.S. Achar, K.M. Hosamani, H.R. Seetharamareddy. Eur. J. Med. Chem., 45, 2048 (2010).
[11] (a) B. Narasimhan, D. Sharma, P. Kumar. Med. Chem. Res., 21, 269 (2012). (b) K. Mahmood,
W. Hashmi, H. Ismail, B. Mirza, B. Twamley, Z. Akhter, I. Rozas, R.J. Baker. Polyhedron, 157,
326 (2019).
[12] E.I. Elnima, M.U. Zubair, A. Badr. Antimicrob. Agents Chemother, 19, 29 (1981).
[13] X.J. Wang, M.Y. Xi, J.H. Fu, F.R. Zhang, G.F. Cheng, D.L. Yin, Q.D. You. Chin. Chem. Lett., 23,
70 (2012).
[14] Z. Ates-Alagoz, B. Can-Eke, T. Coban, M. Iscan, E. Buyukbingol. Arch. Pharm. Pharm. Med.
Chem., 337, 188 (2004).
[15] Y. Zhang, J. Xu, Y. Li, H. Yao, X. Wu. Chem. Biol. Drug Des., 85, 54 (2015).
[16] Y. Bansal, O. Silakari. Bioorg. Med. Chem., 20, 6208 (2012).
[17] R.W. Middleton, D.G. Wibberley. J. Heterocycl. Chem., 17, 1757 (1980).
[18] A. Czarny, W.D. Wilson, D.W. Boykin. J. Heterocycl. Chem., 33, 1393 (1996).
[19] B. Yadagiri, J.W. Lown. Synth. Commun., 20, 955 (1990).
[20] E. Verner, B.A. Katz, J.R. Spencer, D. Allen, J. Hataye, W. Hruzewicz, H.C. Hui, A. Kolesnikov,
Y. Li, C. Luong, A. Martelli, K. Radika, R. Rai, M. She, W. Shrader, P.A. Sprengeler, S. Trapp,
J. Wang, W.B. Young, R.L. Mackman. J. Med. Chem., 44, 2753 (2001).
[21] L.H. Du, Y.G. Wang. Synthesis, 5, 675 (2007).
[22] V. Ravi, E. Ramu, K. Vijay, A. Srinivas Rao. Chem. Pharm. Bull., 55, 1254 (2007).
[23] M.M. Heravi, S. Sadjadi, H.A. Oskooie, R.H. Shoar, F.F. Bamoharram. Catal. Commun., 9, 504
(2008).
[24] A.B. Alloum, K. Bougrin, M. Soufiaoui. Tetrahedron Lett., 44, 5935 (2003).
[25] J.J. Vanden Eynde, F. Delfosse, P. Lor, Y. Van Haverbeke. Tetrahedron, 51, 5813 (1995).
[26] F. Patzold, F. Zeuner, T.H. Heyer, H.J. Niclas. Synth. Commun., 22, 281 (1992).
[27] I. Bhatnagar, M.V. George. Tetrahedron, 24, 1293 (1968).
[28] P.L. Beaulieu, B. Hache, E. Von Moos. Synthesis, 11, 1683 (2003).
[29] M.A. Weidner-Wells, K.A. Ohemeng, V.N. Nguyen, S. Fraga-Spano, M.J. Macielag, H.M.
Werblood, B.D. Foleno, G.C. Webb, J.F. Barrett, D.J. Hlasta. J. Bioorg. Med. Chem. Lett., 11,
1545 (2001).
[30] K. Bahrami, M.M. Khodaei, I. Kavianinia. Synthesis, 4, 547 (2007).

18
M. BHARATHI ET AL.
[31] G. Navarrete-Vazquez, H. Moreno-Diaz, F. Aguirre-Crespo, I. Leon-Rivera, R. Villalobos-
Molina, O. Munoz-Muniz, S. Estrada-Soto. Bioorg. Med. Chem. Lett., 16, 4169 (2006).
[32] S. Lin, L. Yang. Tetrahedron Lett., 46, 4315 (2005).
[33] S. Hashem, H.S. Mona, M. Fatemeh. Can. J. Chem., 86, 1044 (2008).
[34] M.M. Heravi, M. Tajbakhsh, A.N. Ahmadi, B. Mohajerani. Monatsh. Chem., 137, 175 (2006).
[35] A. Mobinikhaledi, N. Forughifar, M. Zendehdel, M. Jabbarpour. Synth. React. Inorg. Met.-
Org. Nano-Met. Chem., 38, 390 (2008).
[36] M.A. Chari, D. Shobha, E.-R. Kenawy, S.S. Al-Deyab, B.V.S. Reddy, A. Vinu. Tetrahedron Lett.,
51, 5195 (2010).
[37] (a) C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck. Nature, 359, 710 (1992).
(b) J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu,
D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker. J. Am. Chem. Soc.,
114, 10834 (1992).
[38] M. Bharathi, S. Indira, G. Vinoth, K. Shanmuga Bharathi. J. Porous Mater., 26, 1377 (2019).
[39] X. Yuan, G.F. Shan, L.X. Li, J. Wu, H.A. Luo. Catal. Lett., 145, 868 (2015).
[40] C. Venkatesan, A.P. Singh. Catal. Lett., 88, 193 (2003).
[41] W. Mo, H. Liu, H. Xiong, M. Li, G. Li. Appl. Catal. A, 333, 172 (2007).
[42] M. Nikoorazm, A. Ghorbani-Choghamarani, M. Khanmoradi. RSC Adv., 6, 56549 (2016).
[43] M. Khanmoradi, M. Nikoorazm, A. Ghorbani-Choghamarani. Catal. Lett., 147, 1114 (2017).
[44] M. Mureseanu, V. Parvulescu, R. Ene, N. Cioatera, T.D. Pasatoiu, M. Andruh. J. Mater. Sci.,
44, 6795 (2009).
[45] J. Zhou, R. Zhou, L. Mo, S. Zhao, X. Zheng. J. Mol. Catal. A: Chem., 178, 289 (2002).
[46] Y. Gang, C. Xing, W. Xiaoli, X. Weihong, X. Nanping. Chin. J. Catal., 34, 1326 (2013).
[47] N. Noori, M. Nikoorazm, A. Ghorbani-Choghamarani. Microporous Mesoporous Mater., 234,
166 (2016).
[48] M. Nikoorazm, A. Ghorbani-Choghamarani, M. Khanmoradi. Appl. Organometal. Chem., 30,
236 (2016).
€
[49] C. Capello, U. Fischer, K. Hungerbuhler. Green Chem., 9, 927 (2007).
[50] S.A. Mirfarjood, M. Mamaghani, M. Sheykhan. J. Nanostruct. Chem., 7, 359 (2017).
[51] R.S. Joshi, P.G. Mandhane, S.K. Dabhade, C.H. Gill. J. Chin. Chem. Soc., 57, 1227 (2010).
[52] K.B. Dhopte, S.R. Zambare, V.A. Patwardhan, R.P. Nemade. RSC Adv., 6, 8164 (2016).
[53] S.D. Pardeshi, J.P. Sonar, S.S. Pawar, D. Dekhane, S. Gupta, A.M. Zine, S.N. Thore. J. Chil.
Chem. Soc., 59, 2335 (2014).
[54] A. Ziarati, A.S. Nasab, M.R. Nasrabadi, M.R. Ganjali, A. Badiei. J. Rare Earths, 35, 374 (2017).
[55] M.B. Swami, A.H. Jadhav, S.R. Mathpati, H.G. Ghuge, S.G. Patil. Res. Chem. Intermed., 43,
2033 (2017).
[56] A. Dutta, J. Mondal, A.K. Patra, A. Bhaumik. Chem. Eur. J., 18, 13372 (2012).
[57] F. Rajabi, S. De, R. Luque. Catal. Lett., 145, 1566 (2015).