R.M. Borade et al.
Catalysis Communications 159 (2021) 106349
for heteroaromatic compounds of biological interest [10]. Among the
benzo-fused heteroaromatics, benzimidazoles are the common struc-
tural scaffolds of biological interest [11]. Benzimidazoles are well
recognized for a wide range of pharmacological activities, including:
antibacterial, antifungal, antiviral, antihypertensive, anticancer, anti-
ulcer, and antihistaminic potential [12]. The scaffolds are the parts of
commercially marketed drugs such as esomeprazole that is an anti-
ulcerative drug [13]. Besides, imidazoles constitute the key in-
termediates in various organic reactions [14] and present in the skele-
tons of several functional materials [15]. Thus, the development of new
strategies for the synthesis of these heterocyclic compounds becomes
inevitable. Traditionally, benzimidazoles are synthesized by the
condensation of o-phenylenediamine with aldehyde / carboxylic acid or
its derivatives by using various catalysts and reagents such as clay
supported titanium catalyst [16], sulfonic acid functionalized graphene
oxide [17], lanthanum chloride [18], NaY zeolite [19], Fe3O4@-
SiO2@polyIonene/Br3-core-shell magnetic nanoparticles [20], magnetic
spectra were obtained on a Shimadzu-GCMS-QP2010 spectrometer
operating at 70 eV.
2.2. Sol-gel auto combustion method for the preparation of cobalt ferrite
magnetic nanoparticles
Cobalt ferrite magnetic nanoparticles were synthesized by the sol-gel
auto combustion method after using glycine as a green fuel. AR grade
chemicals such as cobalt nitrate Co(NO3)2.6H2O), ferric nitrate (Fe
(NO3)3.9H2O), and glycine (C2H5NO2) were used for the synthesis. The
metal nitrates to fuel ratio was taken as 1:4.4. Ammonia solution was
added to maintain the pH of solution at 7. The resulting powder was
sintered at 600 ◦C for 5 h, characterized, and then used for further
investigations.
2.3. Characterization of nano cobalt ferrite
ꢀ
core-shell nanoparticles containing I3 [21], p-toluene sulfonic acid
The synthesized nano cobalt ferrite was characterized for its surface,
morphology, optical, magnetic, and electrical properties. The material
was characterized by powder X-ray diffraction (XRD) (Regaku model).
The X-ray diffractograms of the sample were recorded at ambient tem-
[22], ceric mmonium nitrate / polyethylene glycol [23], silver carbon-
ate on celite [24], aluminum nitride / aluminum [25], 2,3-Dichloro-5,6-
dicyano-1,4-benzoquinone DDQ [26], and so on.
In continuation of our research on the development of magnetic
nanocatalysts for organic transformations [27], we herein report a green
synthesis of cobalt ferrite nanocatalyst by sol-gel auto combustion and
the development of an efficient mechanochemical protocol for the
synthesis of 2-aryl benzimidazoles by using cobalt ferrite NPs as a
magnetically separable heterogeneous catalyst (Scheme 1). The syn-
thesis of benzimidazoles was studied by grinding a mixture of o-phe-
nylenediamine and aldehydes in the presence of a catalytic amount of
cobalt ferrite nanocatalyst in an agate mortar-pestle to afford the
products in high yields. The protocol is solvent-free and environmentally
benign involving easy separation of the catalyst in a short reaction time.
perature in the 2θ range from 20◦ to 80◦ by using Cu-K
α radiation (λ =
1.54056 Å). Scanning Electron Microscope (SEM) images of the sample
were obtained on JEOL-JSM 840 SEM analyzer at an operating voltage
of 20 kV attached with EDX equipment.
The optical investigations of the nanomaterial were studied after
using PerkinElmer UV WinLab Lambda 900 UV/VIS/NIR, in the wave-
length range of 200–800 nm. Magnetic properties of the films were
studied by a vibrating sample magnetometer (Lakeshore VSM 7410) at
room temperature. Transmission electron microscopy (TEM, Philips
CM200) was used for microstructural analysis and surface morphology.
2.4. General procedure for the synthesis of 2-aryl benzimidazole
derivatives using CoFe2O4 nanocatalyst
2. Experimental
2.1. Materials and methods
A mixture of aldehyde (0.5 mmol), o-phenylenediamine (1 mmol),
and CoFe2O4 nanoparticles (5 mol%) was ground in an agate mortar and
pestle. The contents were turned into a pasty mass after 5 to 10 min of
grinding. After this, the remaining half quantity of the aldehyde (0.5
mmol) was added, and the grinding was continued for a further time as
specified in Table 1. The reaction progress was monitored on TLC by
using AcOEt: n-hexane (1:4) as the mobile phase. After completion of the
reaction as indicated by TLC, the reaction mixture was diluted with ethyl
acetate (10 mL) and the catalyst was separated by an external magnet.
The filtrate was evaporated under reduced pressure to obtain pure
All the required chemicals were Merck or SD Fine made and used
without further purification. The reaction progress was monitored by
Thin Layer Chromatography (TLC) on Aluchrosep Silica Gel 60 TLC
plates under UV- 254 nm light. Melting points of the products were
determined in capillaries open at one end on a TANCO® melting point
apparatus. The synthesized products were characterized by 1H and 13
C
NMR after using a Bruker Avance II 400 NMR spectrometer at 400 MHz
in CDCl3 solvent and using TMS as an internal standard. The mass
OH
H2N
Co(NO3)2.6H2O
O
600 oC
5 hr
CoFe2O4
NPs
CoFe2O4
Gel
H O;
Ammonia
2
2 Fe(NO3)3.9H2O
stirr 80 oC; 6 hr
O
NH2
+
NH2
H
R
R1
Mechanochemical synthesis
N
R
N
H
R1
R = H, -CH3
Scheme 1. Synthesis of CoFe2O4 NPs and their catalytic activity for the mechanochemical synthesis of benzimidazoles.
2