Reusable nano-zirconia-catalyzed synthesis of benzimidazoles and their antibacterial and antifungal activities

Article abstract of DOI:10.3390/molecules26144219

In this article, a zirconia-based nano-catalyst (Nano-ZrO₂ ), with intermolecular C-N bond formation for the synthesis of various benzimidazole-fused heterocycles in a concise method is reported. The robustness of this reaction is demonstrated

Full text of DOI:10.3390/molecules26144219

molecules
Article
Reusable Nano-Zirconia-Catalyzed Synthesis of Benzimidazoles
and Their Antibacterial and Antifungal Activities
Tentu Nageswara Rao 1 , Suliman Yousef AlOmar 2,
*
, Faheem Ahmed 3 , Fadwa Albalawi 2, Naushad Ahmad 4,
Nalla Krishna Rao 5, M. V. Basaveswara Rao 5, Ravi Kumar Cheedarala 6,*, G. Rajasekhar Reddy 7
and Tentu Manohra Naidu 8
1
Department of Chemistry, Krishna University, Machilipatnam 521001, Andhra Pradesh, India;
tnraochemistry@gmail.com
Department of Zoology, College of Science, King Saud University, Riyadh 11451, Saudi Arabia;
2
fadwa_saad@hotmail.com
Department of Physics, College of Science, King Faisal University, Hofuf 31982, Al-Ahsa, Saudi Arabia;
3
fahmed@kfu.edu.sa
4
Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia;
anaushad@ksu.edu.sa
5
Department of Organic Chemistry, Krishna University, Machilipatnam 521001, Andhra Pradesh, India;
nallakrisharao@gmail.con (N.K.R.); professormandava@gmail.com (M.V.B.R.)
Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology,
6
UNIST-Gil, Ulsan 44919, Korea
Department of Chemical Engineering, A.C Tech Campus, Anna University, Chennai 600025, India;
7
gangireddy.df14@gmail.com
Department of Nuclear Physics, Andhra University, Visakhapatnam 530003, Andhra Pradesh, India;
t.manoharanaidu@gmail.com
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
8
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Rao, T.N.; AlOmar, S.Y.;
Ahmed, F.; Albalawi, F.; Ahmad, N.;
Rao, N.K.; Rao, M.V.B.; Cheedarala,
R.K.; Reddy, G.R.; Naidu, T.M.
Reusable Nano-Zirconia-Catalyzed
Synthesis of Benzimidazoles and
Their Antibacterial and Antifungal
Activities. Molecules 2021, 26, 4219.
https://doi.org/10.3390/
*
Correspondence: syalomar@ksu.edu.sa (S.Y.A.); rkchidrala@gmail.com (R.K.C.)
Abstract: In this article, a zirconia-based nano-catalyst (Nano-ZrO2), with intermolecular C-N
bond formation for the synthesis of various benzimidazole-fused heterocycles in a concise method is
reported. The robustness of this reaction is demonstrated by the synthesis of a series of benzimidazole
drugs in a one-pot method. All synthesized materials were characterized using 1HNMR, 13CNMR,
and LC-MS spectroscopy as well as microanalysis data. Furthermore, the synthesis of nano-ZrO2 was
processed using a standard hydrothermal technique in pure form. The crystal structure of nano-ZrO2
and phase purity were studied, and the crystallite size was calculated from XRD analysis using the
Debye–Scherrer equation. Furthermore, the antimicrobial activity of the synthesized benzimidazole
drugs was evaluated in terms of Gram-positive, Gram-negative, and antifungal activity, and the
results were satisfactory.
molecules26144219
Academic Editor: Mark McLaughlin
Received: 20 June 2021
Accepted: 7 July 2021
Published: 12 July 2021
Keywords: o-phenylenediamine; nano-ZrO2; aryl aldehydes; benzimidazole; XRD
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
1. Introduction
Heterocyclic compounds, particularly nitrogen-containing compounds, are extremely
important because their presence in many synthetic organic compounds promotes biologi-
cal activities [
significant and has been demonstrated to be challenging to medicinal chemists [
introduction of the amine functional group in a precise manner has been broadly examined
by chemists and was first reported by Ullmann and Goldberg [ ]. Given the predom-
inance and relevance of heteroatoms in molecules of interest, C-N bond formation via
direct formation of C-N bonds has attracted significant attention. In this context, numerous
1
]. The building of the C-N bonds of heterocyclic compounds is principally
2]. The
Copyright:
© 2021 by the authors.
3,4
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
transition metal complexes, especially of Rh [5], Ir [6], Co [7], Ru [8], and Pd [9], have
exhibited outstanding catalytic efficiency towards C-N bond formation reactions [10].
In recent decades, the application of engineered nanoparticles (NPs) has extended to
various fields, such as electronics, biomedical applications, and pharmaceuticals. Zirconia
Molecules 2021, 26, 4219. https://doi.org/10.3390/molecules26144219
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 4219
2 of 14
(ZrO2) NPs are one of the major nanomaterials used for manufacturing refractories, foundry
sands, and ceramics. Owing to the desirable mechanical strength, this material is also used
in the biomedical field, including biosensors, cancer therapy, implants, joint endoprostheses,
and dentistry [11]. However, the wide application of particles has raised concerns about
their potential risks to health and the atmosphere, of which ensuring occupational and
consumer safety is an essential concern. Thus far, toxicological studies on ZrO2 NPs are
limited, and the results were controversial. Zirconia (ZrO2) is widely used as a ceramic
material and has significant applications in catalysts as well [11,12]. Due to its unique
chemical properties (i.e., surface acidity and basicity), it is a valuable catalyst for elimination,
dehydration, hydrogenation, and oxidation in chemical transformations. Azole heterocyclic
aromatic compounds are divided into two categories, with the first type being older
than the second [13]. The conversion of lanosterol to ergosterol due to inhibiting fungal
cytochrome P450 3A-dependent C14-α-demethylase depletes ergosterol in the fungal cell
membrane [14]. The disturbance of hepatic enzymes may cause the interaction of triazoles
with cytochrome P450 [15]. Azole antifungal activity is different for each compound and
clinical efficiency may not coincide with in vitro activity [16].
Over the past two decades, the intensity of systemic fungal infections has increased
dramatically, mainly because of the Candida species. The increasing numbers of immune-
compromised patients due to HIV infection, cancer chemotherapy, and organ transplan-
tation are key factors contributing to this occurrence [17,18]. Recently, extended drug
research has been conducted because new antifungal agents are critical for treating these
life-threatening invasive fungal infections. The main causative fungi targeted by systemic
antifungal drugs are Aspergillus fumigates and Candida albicans, and mycosis drugs for the
treatment of patients with deep mycosis should have a wide antifungal spectrum, including
at least these microorganisms [19].
The benzimidazole nuclei are important synthetic intermediates in drug discovery.
Medicinal chemists have been encouraged by the high therapeutic activity of related drugs
to synthesize a large number of novel chemotherapeutic agents [20–22]. Nitrogen- and
sulfur-based heterocyclic compounds show a wide variety of biological activity as an-
titumor, antibacterial, antifungal, and antiviral agents [23]. The preparation of several
biologically active benzimidazole and benzotriazole derivatives has been described in
previous reports [24]. Recently, Salauddin et al. reported a review on benzimidazoles,
biologically active compounds, which were discussed in terms of antimicrobial, antiviral,
anticancer, antiprotozoal, anti-inflammatory, and analgesic activities [25]. Similarly, our
group has reported several novel benzimidazole ring systems at the second position of
the benzimidazole ring which contain replicas of various biologically active benzimida-
zoles [26–31]. However, it is still necessary to investigate the screening of various novel
benzimidazoles. In the current study, some new benzimidazole derivatives have been
synthesized as antifungal agents. However, because of the significant biological activity
of benzimidazole, it is still difficult to develop a convenient, high-yielding, and environ-
mentally benign procedure for the synthesis of this ring system, which is consistent with
our research work on the synthesis of this ring system. [32]. Here, we report a simple
synthetic protocol for the preparation of 2-arylbenzimidazoles from o-phenylenediamine
(o-PDA) and aromatic aldehydes in the presence of nano-ZrO2. Subsequently, we studied
antibacterial and antifungal activities using different benzimidazoles, which showed very
good results, as shown in Figure 1.

Molecules 2021, 26, 4219
3 of 14
Figure 1. Synthesis of nano-ZrO2 catalyst (Stage 1) and aryl benzimidazoles (
activities (Stage 3).
3, Stage 2) and antibacterial and antifungal
2. Results and Discussion
2.1. XRD, FT-IR, and TGA of Nano-ZrO2
Figure 2a shows the crystalline nature and phase purity of the resulting nano-ZrO2,
evaluated by measurements of room temperature XRD. The nano-ZrO2 sample diffraction
peaks were indexed according to JCPDS card number 80-0965. A tetragonal phase (space
group = P42/nmc) was confirmed by the XRD pattern of the investigated sample. In the
synthesized powder XRD pattern, no additional peaks were found. Scherrer’s formula was
used to calculate the average crystallite size for the most intense diffraction peaks.
Kλ
βCosθ
D =
(1)
where
factor,
β
(in radians) is the half maximum full width of XRD peaks, K = 0.94 is the shape
λ
= 1.54178 Åfor Cu-K X-rays, is the diffraction angle (in degrees) corresponding
α
θ

Molecules 2021, 26, 4219
4 of 14
to each plane. Crystallite size (D) was found to be 60
±
5 nm for chemically synthesized
nano-ZrO2. For (111) planes, the powder diffraction file for cubic and tetragonal ZrO2 has
the highest peak intensity and the second most intense is (220). By comparing the relative
intensities of (111) at 2θ
~30◦ and (220) at 50◦ in each sample with these data, it appears
that the (111) orientation is suppressed and the (110) growth is increased at 600 ◦C [32
,33].
Figure 2. (a) XRD, (b) FT-IR, and (c) TGA of nano-ZrO2.
2.2. FT-IR
FT-IR of nano-ZrO2 showed bending and stretching vibrations of the O-H functional
groups due to absorbed water molecules, attributed to the bands noted at 3425 cm−1
and 1638 cm−1, respectively [29]. The band at 1386 cm−1 is ascribed to the absorption of
non-bridging O-H groups. The sharp bands at 504 cm−1 and 734 cm−1 are attributed to the
vibration modes of the ZrO32−, and ZrO2 groups, which strongly confirm the formation of
nano-ZrO2, as shown in Figure 2b [34].
2.3. Thermal Stability Test (TGA)
Figure 2c shows the TGA analysis that was performed in an inert N2 gas atmosphere
to determine the thermal degradation of the sample due to the occurrence of air or oxygen
atmospheric oxidation in the sample and 92.6% char residue at 800 ◦C [22]. It is obvious
◦
that nano-ZrO2 exhibited continuous weight loss, and the weight loss below 300 C is
attributed to the release of physically adsorbed molecules, mainly water as moisture, and
the weight loss above 300 ◦C is due to the desorption of chemically bonded oxygenated
groups and the dehydration of surface hydroxyls. Based on the result, nano-ZrO2 is highly
thermally stable up to 800 ◦C [23]. Recently, Zhou et al. reported the dispersion behavior
of zirconia nanocrystals and their surface functionalization with vinyl group-containing
ligands. Our results are similar to their TGA data [35–37].
2.4. TEM, Size Distribution, SEM, and EDX Analysis of Nano-ZrO2
The morphology, size, and shape of the as-synthesized nano-ZrO2 were investigated
by using HR-TEM. The TEM image of nano-ZrO2 showed stretched spherical particles
with diameters of ~40–60 nm. Nano-ZrO2 has been well known for several years as
having a variety of sizes and shapes, however, stretched spherical nano-ZrO2, as shown in

Molecules 2021, 26, 4219
5 of 14
Figure 3a, is rarely found. The corresponding size distribution histogram of nano-ZrO2
is shown in Figure 3b, and the calculated average particle size was about 60 nm, using
ImageJ software. Further, the surface morphology of nano-ZrO2 was confirmed using
SEM analysis, as shown in Figure 3c. The SEM image clearly showed that nano-ZrO2
appeared as aggregated spherical surface morphologies. The agglomeration of nano-ZrO2
is visible in the SEM images as well. Next, the elemental composition of nano-ZrO2 was
analyzed by EDX [30]. The host material of nano-ZrO2 exhibited three elemental peaks
which correspond to zirconium and oxygen at 0.1, 1.98, and 0.56 keV, respectively. From
the EDS data, the weight ratio of Zr:O was approximately 77:23 and the spectra suggested
that the nano-ZrO2 consisted of only Zr and O elements [38].
Figure 3. (a) TEM, (b) distribution histogram, (c) SEM, (d) EDX of nano-ZrO2.
2.5. Synthesis of 4-Methoxy Benzimidazole (3a)
2-(4-methoxy phenyl) benzimidazole (3a) was synthesized by a mixture of o-PDA (1)
◦
and 4-methoxy benzaldehyde (2a) in the presence of nano-ZrO2 at 60 C. The reaction was
performed with 2 mol% of nano-ZrO2 in diverse aprotic to protic solvent systems which
included dichloromethane (DCM), acetonitrile (ACN), methanol (MeOH), toluene, 1, 4-
dioxane, THF, DMF, DMSO, and dry ethanol (entries 1–9, Table 1). Among all the solvents,
the dry ethanol was suitable for condensation of 1 and 2a using nano-ZrO2 at 60 ◦C and
produced 2-(4-methoxy phenyl) benzimidazoles (3a) with 93% yield in 3 h (entry 1, Table 2).
A thorough examination of the reaction using the aforementioned solvents revealed lower
yields, longer reaction times, and recovered starting material as a result of an incomplete
condensation process. Mainly, DCM (45%), CH3CN (42%), MeOH (53%), toluene (55%),
1, 4-dioxane (35%), THF (32%), DMF (46%), DMSO (48%) showed poor condensation
between o-PDA (1) and 4-methoxy benzaldehyde (2a) due to poor product formation. To
overcome these practical issues, we chose dry ethanol as an alternative solvent to complete
the reaction with good to excellent yields. Less than 12% unreacted starting materials were
found in ethanol compared with the other solvents. Hence, the optimization was carried
out in ethanol by varying the amount of nano-ZrO2. The formation of 3a was observed
to have a high yield and the reaction with 2 mol% of nano-ZrO2 and 2.5 mol% of catalyst
showed a similar product yield of 92%. Therefore, the reaction was stabilized with 2 mol%
of nano-ZrO2 in dry ethanol [39–41].

Molecules 2021, 26, 4219
6 of 14
Table 1. Optimization study by solvent.
Nano-ZrO2
Mol %
Time
(h)
Yield
%
Entry
Solvent
1
2
3
4
5
6
7
8
9
10
11
12
13
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
1.5
2.0
2.5
DCM
CH3CN
MeOH
Toluene
1,4-dioxane
THF
DMF
DMSO
Dry ethanol
Dry ethanol
Dry ethanol
Dry ethanol
Dry ethanol
3
3
3
3
3
3
3
3
3
3
3
3
3
45
42
53
55
35
32
46
48
63
79
86
92
92
Table 2. Catalyst activity comparison [a]
.
Temp
[ C]
Conv
Product Yield [%] [c] [e]
Catalyst
Mol %
Time [h]
S. No
◦
[%] [b]
3a
Monoadduct
Starting Materials
1
2
3
4
5
6
7
[a]
2% nano-ZrO2
5% Pd/C [d]
60
60
60
60
60
60
60
3
3
3
3
3
3
3
>97
65
58
52
55
52
00
93
55
50
45
55
42
–
03
30
35
30
34
43
15
0
15
15
20
11
15
85
[d]
5% Pd/Al2O3
5% Pd/BaCO3
[d]
5% CuO NP [d]
5% Zn(NO3)2 XH2O [d]
No catalyst (blank) [d]
A solution of o-PDA (1mol) and 4-MeO-benzaldehyde (1.1 mol) in dry ethanol (20 mL) was heated at 60 ◦C under nitrogen for 3h in the
presence of various catalysts, [b] determined by gas chromatography, [c] isolated yield, [d] commercially available catalysts, [e] zirconium
was not detected in the filtrate with inductively coupled plasma (ICP) analysis.
Next, we investigated the preparation of 3a from
ious commercially available metal nano-catalysts, such as 5% Pd/c, 5% Pd/Al2O3, 5%
Pd/BaCO3, 5% CuO nanoparticles (CuO NPs), and 5% Zn(NO3)2 XH2O. Additionally, we
1 with 2a, performed using var-
·
conducted a blank experiment without nano-ZrO2 in dry ethanol and observed up to 15%
monoadduct was formed. In addition, the conversion of 3a was very poor even at higher
temperatures for 8 h, as shown in Table 2.
To check the reusability of nano-ZrO2, when the reaction of o-PDA with various
substituted aromatic aldehydes was over, the product formed was extracted with ethyl
acetate and the catalyst was purified. It was washed with ethyl acetate repeatedly, dried,
and reused for the reaction of o-PDA with various aryl aldehydes. The nano-ZrO2 was
found to be reusable for at least four cycles without any inactivity, and from the fifth cycle
the catalytic activity was decreased (Table 3) [41].
Table 3. Recycling of nano-ZrO2 for synthesis of 4-OMe benzimidazole (3a) [a]
.
Temp
[ C]
Time
(h)
Yield
%
Use
◦
[b]
1
2
3
4
5
60
60
60
60
60
3
3
3
3
3
93
92
90
85
83
[a]
A solution of o-PDA (1 mol) and 4-MeO-benzaldehyde (1.1 mol) in dry EtOH (20 mL) was heated at 60 ◦
C
under nitrogen for 3 h in the presence of 2 mol% of nano-ZrO2, [b] determined by gas chromatography.

Molecules 2021, 26, 4219
7 of 14
The main advantage of nano-ZrO2 is that besides its selectivity, it showed moderate
recyclability, and it can be recovered through simple filtration, or a decantation method
(
Table 3). To the best of our knowledge, nano-ZrO2 is a recyclable catalyst for the synthesis
of benzimidazoles by condensation of and 2a to produce 3a with a good yield [10,33].
1
When the catalyst was recovered and reused without any treatment, the reaction rate grad-
ually decreased (entry 1–5) due to moisture contamination. Therefore, for each successive
use, we gently dried the catalyst and reused it. Nano-ZrO2 showed moderate catalytic
activity after the fifth cycle, and produced 3a with an 82% yield.
With optimized reaction conditions, the scope of the different aryl aldehydes was exam-
ined for the synthesis of various aryl benzimidazoles. Initially, o-PDA was reacted with ben-
zaldehyde, in the presence of 2 mol % of nano-ZrO2 and produced 2-phenylbenzimidazole,
3a, with a 95% yield. Next, the reaction of o-PDA was carried out with various aryl
benzaldehydes with electron-donating groups using 2 mol % of nano-ZrO2 to produce
corresponding aryl benzimidazole in good to excellent yields, 3b
–3d (i.e., 93% (3b), 91%
(3c), and 90% (3d)), respectively. On the other hand, the reaction of o-PDA with aryl ben-
zaldehydes with electron-withdrawing groups at meta and ortho positions (e.g., 3-Cl and
3-Br) in the presence of 2 mol % of nano-ZrO2 produced corresponding aryl benzimidazoles
with 84% (3e) and 86% (3f) yields, respectively, as shown in Table 4.
Table 4. Nano ZrO2 catalysed synthysis of 2-aryl benzimidazoles a.
o-PDA
(1)
Aryl Aldehyde
(2)
Product
(3)
Yield
(%) b
M.P
( C)
Entry
3a
◦
226–228
93
95
(225–226) c
287–289
3b
3c
(290–292) c
91
246–248
262–264
3d
3e
3f
91
84
86
(268–270) c
233–234
(234–236) d
245–246
252–254
3g
3h
85
82
(256–257) d
201–292
(203–204) e

Molecules 2021, 26, 4219
8 of 14
Table 4. Cont.
o-PDA
(1)
Aryl Aldehyde
(2)
Product
(3)
Yield
(%) b
M.P
( C)
Entry
3i
◦
87
85
254–255
278–280
3j
a
A solution of o-PDA (1 mol), and aryl benzaldehyde (1.1 mol), in dry. ethanol (20 mL) was heated at 60 ◦C in nitrogen gas for 3 h in the
presence of Nano ZrO2 [2 mol%], b determined by GC, c
1HNMR, & 13CNMR) analyses data.
[ [ [14]. All the products were showed satisfactory spectroscopic (IR,
12], d 13], e
2.6. Antibacterial Screening
Figure 4a shows that the cup–plate screening method is simple for measuring inhibi-
tion of microorganisms. Here, we have used this method for the antibacterial screening of
the tested compounds. Nutrient agar 2%, peptone 1%, beef extract 1%, sodium chloride
0.5%, and distilled water up to 100 mL were used in culture media. All the constituents
were weighed and added to the base medium water. The solution mixture was heated in
a water bath for about 1.5 h until a clear solution was obtained and the nutrient medium
was sterilized in an autoclave. Antibacterial activity was assessed against Gram-positive
bacteria (Streptococcus pneumonia (RCMB 010010), Bacillus subtilis (RCMB 010067)) and
Gram-negative bacteria (Pseudomonas aeruginosa (RCMB 010046), Escherichia coli (RCMB
010052)). Ampicillin was used as a standard in the assessment of antibacterial activity
against Gram-positive bacteria and gentamicin was used as a standard in the assessment
of the activity of the compounds tested against Gram-negative bacteria. The results were
expressed as the mean inhibition zone in mm
eter (6 mm) produced by microorganisms using 10 mg/mL of samples tested, as shown in
Table 5.
±
standard deviation beyond the well diam-
(a)35
(b)
Aspergillus niger
P.aeruginosa
E.coli
25
Aspergillus flavus
Candida albicans
S.pneumonia
B.substills
30
25
20
15
10
5
20
15
10
5
0
0
3a 3b 3c 3d 3e 3f 3g 3h 3i 3j Gtm ApnDmso
Antibactirial activities
3a 3b 3c 3d 3e 3f 3g 3h 3i 3j FczDmso
Antifungal activities
Figure 4. The antibacterial (a) and antifungal (b) activities of aryl benzimidazole derivatives.

Molecules 2021, 26, 4219
9 of 14
Table 5. Antibacterial activity of benzimidazole derivatives.
Zone of Inhibition (mm)
Bacteria
S. pneumonia
Compound Code
P. aeruginosa
E. coli
B. subtilis
3a
3b
3c
3d
3e
3f
3g
3h
3i
10
19
21
25
16
17
24
18
24
22
30
—
10
10
24
26
20
10
18
17
18
22
21
30
—
10
06
11
19
20
19
20
12
09
13
12
—
30
10
04
15
19
20
21
22
18
12
21
13
—
30
10
3j
gen-gentamicin
am-penicillin
DMSO
We investigated the microbial activity of the titled moiety, and the compounds 3b
3i, and 3j demonstrated the highest active potency against E. coli. The compounds 3c
,
,
3c,
3d
,
3g, and 3i showed maximum active potency against P. aeruginosa. The compounds 3d
and 3f showed moderate active potency against S. pneumonia. The compounds 3e and 3f
showed good active potency against B. subtilis. The rest of the compounds exhibited poor
to moderate active potency, as shown in Table 5.
We reported that tested compounds 3b, 3c, and 3i are more potent than other com-
pounds in terms of antifungal activity. The results also showed that alkyl benzimidazoles
are more active than alkyl benzotriazoles. All tested compounds in this study, except
3i, 3b, and 3c, exhibited desirable activity on Candida albicans. Some Aspergillus niger,
Aspergillus flavus, and Candida albicans, which are resistant to fluconazole, were affected by
the synthesized compounds 3i, 3b, and 3c, as shown in Table 6.
Table 6. Antifungal activity of benzimidazole derivatives.
Entry
Aspergillus niger
Aspergillus flavus
Candida albicans
3a
3b
3c
3d
3e
3f
3g
3h
3i
12
20
21
18
13
15
15
09
18
17
25
10
10
19
18
20
09
13
17
11
17
16
25
10
15
21
20
17
17
16
18
15
22
19
25
10
3j
fluconazole
DMSO
3. Methods and Materials
In this study, zirconium (IV) oxynitrate (ZrO(NO3)2
·
xH2O hydrate, sodium hydroxide
(NaOH) 5% Pd/c, 5% Pd/Al2O3, 5% Pd/BaCO3, and 5% CuO nanoparticles (CuO NPs)
were acquired from Sigma-Aldrich (St. Louis, MO, USA). 1HNMR and 13CNMR spectra
were recorded on a Bruker Avance (400 and 100 MHz) spectrometer (Billerica, MA, USA)
with tetramethylsilane (TMS) as an internal standard. Thin-layer chromatography using
silica gel F254 aluminum sheets was used to monitor the reaction progress. The crystal
structure and phase purity of nano-ZrO2 were investigated and the crystallite size was
calculated using the XRD spectrum Debye–Scherrer equation. SEM (TESCAN, CZ/MIRA

Molecules 2021, 26, 4219
10 of 14
I LMH, Brno, Czech Republic) was used to investigate the surface morphology of nano-
ZrO2. The particle size was measured using TEM (FEI, TECNAI G2 TF20-ST, Brno, Czech
Republic). The FT-IR spectrum was recorded using a JASCO, FT/IR-6300 spectrometer
(Easton, MD, USA). The basic composition of nano-ZrO2 was determined by energy
dispersive X-ray (EDX) analysis. Thermogravimetric (TGA) analysis was performed on
a Perkin-Elmer (Waltham, MA, USA) instrument at a linear heating rate of 10 ◦C min−1
in a nitrogen environment. The temperature range was maintained at 30 to 800 ◦C. The
GC-MS system used consisted of an Agilent instrument (Carpinteria, CA, USA) Network
GC system 5975 C series MS Detector, 7683 B series, and an auto injector and was interfaced
with Chemstation Software with an Rxi-624Sil MS (30 m length
×
0.32 mm I.D.
×
1.8 µm
film thickness). Column oven temperature was maintained with a program, i.e., initial
temperature 40 ◦C held for 4 min, ramp 40 ◦C/min to 200 ◦C, held for 5 min. The injector
temperature was 200 ◦C, interface temperature was 220 ◦C, column flow (nitrogen) was
2.0 mL/min, and mass range was 50–300. Acquisition mode was electron ionization (EI)
mode and the injected sample volume was 1 µL with split mode (1:25).
3.1. Synthesis of Nano-ZrO2
The synthesis of nano-ZrO2 was processed using a standard hydrothermal technique.
Briefly, 0.1 M ZrO(NO3)2
of each solution ((ZrO(NO3)2
·
xH2O and 0.2 M NaOH were prepared in DI water. Fifty milliliters
·
xH2O:NaOH; 1:1) were mixed and poured into a hydrother-
mal flask, and heated at 150 ◦C. After 3 h, the separated nano-ZrO2 was centrifuged at
5000 rpm for 30 min using a cooling centrifuge machine, and washed 5 times with dry
◦
◦
ethanol. The obtained precipitate was dried at 100 C for 2 h, calcined at 600 C, and
crushed to obtain a fine powder of nano-ZrO2.
3.2. General Procedure for the Preparation of 2-Phenyl-1H-benzimidazole (3a–h)
Nano-ZrO2 (2 mol %) was added to the solution of o-PDA (1 mmol), and correspond-
◦
ing aryl aldehyde (1 mmol) in dry ethanol (20–25 mL), and stirred at 60 C. The progress of
the reaction was checked by TLC. Upon completion of the reaction, DI water (30–40 mL)
was added, and the separated solid mixture was purified using a gravity column to obtain
pure products.
2-Phenyl-1H-benzimidazole (3a)
1
IR (KBr):
CDCl3):
13CNMR (100 MHz, CDCl3)
ν
= 3356 (NH), 3051 (CH aromatic), 1459 (C=C) cm−1. HNMR (400 MHz,
δ
ppm = 7.72–7.24 (m 7H, Ar-H), 8.09–7.89 (m, 2H, Ar-H), 11.56 (s, 1H, NH);
ppm: 151.61, 142.06, 135.18, 131.29, 129.23, 128.24, 126.17,
δ
123.67, 122.55, 118.88, 112.58. Molecular formula: C13H10N2.
2-(4-methoxyphenyl)-1H-benzimidazole (3b)
1HNMR(400 MHz, CDCl3):
δ
3.70 (d, 3H, OCH3),6.12 (s, 1H, NH), 6.94 (d, 2H, aro-
matic), 6.98 (d, 2H, Ar-H, 7.20 (d, 2H, Ar-H), 7.58 (d, 2H, Ar-H); 13CNMR (100 MHz, CDCl3)
ppm: 162.54, 150.08, 129.81, 128.19, 123.46, 122.34, 114.15, 55.56. LCMS (m/z): 225 (M+ +
δ
H). Molecular formula: C14H12N2O.
2-(3,5-Dimethoxyphenylhenyl)-1H-benzimidazole (3c)
IR (KBr):
ν .
= 2884 (C-H, aliphatic), 3056 (C-H, Ar-H), 1521 (C=C), 1619 (C=N) cm−1
1HNMR (400 MHz, CDCl3):
δ
ppm = 3.74 (s, 3H, OCH3), 3.82 (s, 3H, OCH3), 7.18–7.87 (m,
7H, C-H, Ar-H), 12.15 (s, 1H, NH). Molecular formula: C15H14N2O2.
2-(4-Methylphenyl)-1H-benzimidazole (3d)
1
IR (KBr):
(400 MHz, DMSO):
13CNMR (100 MHz, CDCl3)
ν
= 3323 (NH), 3059 (C-H, Ar-H), 1448 (C=C), 1622 (C=N) cm−1. HNMR
δ
(ppm) = 12.83 (s, 1H, NH), 8.08–7.17 (m, 8H, Ar-H), 2.38 (s, 3H, CH3),
ppm: 152.94, 141.49, 130.56, 129.04, 128.25, 127.11, 125.83,
δ
123.56, 122.48, and 118.75, 111.90. Molecular formula: C14H12N2.
2-(3-Chlorophenyl)-1H-benzimidazole (3e)
1
IR (KBr):
ν = 3364 (NH), 3053 (CH, Ar-H), 1448 (C=C), 745 (C-Cl). HNMR (400 MHz,
CDCl3):
δ
ppm = 12.54 (s, 1H, NH), 8.10–7.36 (m, 8H, Ar-H); 13CNMR (100 MHz, CDCl3)

Molecules 2021, 26, 4219
11 of 14
δppm: 150.96, 141.80, 136.45, 129.21, 128.76, 127.43, 122.84, 119.08, 111.98. LCMS (m/z):
194.24. Molecular formula: C13H9ClN2.
2-(3-Bromophenyl)-1H-benzimidazole (3f)
IR (KBr):
ν .
= 3346 (NH), 3050 (C-H, Ar-H) 1434 (C=C) cm−1 1HNMR (400 MHz,
CDCl3):
CDCl3)
δ
ppm: 12.87 (s, 1H, NH), 8.02–7.27 (m, 8H, C-H, Ar-H); 13CNMR (100 MHz,
δ
ppm: 150.59, 141.48, 131.63, 129.57, 128.98, 128.15, 127.04. LCMS (m/z): 273.18
(M+ + 2); Molecular formula: C13H9BrN2.
2-(2-Hydroxy-5-bromophenyl)-1H-benzimidazole (3g)
1
IR (KBr):
CDCl3):
ppm = 11.94 (s, 1H, NH), 6.97–7.63 (m, 7H, Ar-H), 8.56 (s, 1H, -OH); 13CNMR
(100 MHz, CDCl3)
ν
= 3358 (NH), 3055 (C-H, Ar-H), 1442 (C=C) cm−1. HNMR (400 MHz,
δ
δ
ppm: 154.12, 148.38, 140.06, 132.79, 130.26, 126.44, 123.32, 119.76,
116.08, 115.57. LCMS (m/z): 289.26 (M+ + 2): Molecular formula: C13H9N2BrO.
2-(3-Nitrophenyl)-1H-benzimidazole (3h)
IR (KBr):
(400 MHz, CDCl3):
(100 MHz, CDCl3)
ν
= 3358 (NH), 3086 (CH aromatic), 1516 (N=O), 1431 (C=C) cm−1, 1HNMR
δ
(ppm) = 12.78 (s, 1H, NH), 8.14–7.28 (m, 8H, aromatic); 13NMR
δ
ppm: 153.08, 147.53, 136.74, 130.65, 128.42, 126.20, 122.87, 119.94,
119.07, 117.56, 112.08. LCMS (m/z): 240.12 (M+ + H). Molecular formula: C13H9N3O2.
2-(2-bromo-3,4-dimethoxyphenyl)-1H-benzimidazole (3i)
−1
,
IR (KBr):
ν
= 3358 (NH), 2923 (C-H, aliphatic), 3049 (C-H aromatic), 1440 (C=C) cm
ppm = 12.72 (s, 1H, NH), 7.55–7.12 (m, 6H, Ar-H), 3.72 (s,
1HNMR (400 MHz, CDCl3):
δ
6H, (OCH3)2); 13CNMR (100 MHz, CDCl3)
δ ppm: 152.98, 149.21, 147.73, 140.08, 137.47,
124.55, 122.73, 115.54, 111.64, 107.95, 61.09, 54.84. LCMS (m/z): 334.53 (M+ + H). Molecular
formula: C15H13BrN2O2.
2-(2-iodo-3, 5-dimethoxyphenyl)-1H-benzimidazole (3j)
IR (KBr):
1HNMR (400 MHz, CDCl3):
1H, Ar-H), 6.94 (s, 1H, Ar-H), 3.65 (s, 6H, (OCH3)2); 13CNMR (100 MHz, CDCl3)
ν ,
= 3372 (NH), 2879 (C-H, aliphatic), 3063 (CH aromatic), 1429 (C=C) cm−1
δ
ppm = 12.76 (s, 1H, NH), 7.46–7.24 (m, 4H, Ar-H), 7.08 (s,
ppm:
δ
158.21, 156.64, 149.55, 139.36, 136.47, 128.46, 126.54, 125.73, 122.75, 115.63, 55.09, 53.64.
LCMS (m/z): 380.22 (M+). Molecular formula: C15H13IN2O2. The NMR and Mass spectra
of Benzimidazoles are available in Supplementary Materials.
3.3. Antimicrobial and Antifungal Assays
Compounds were used to determine the antimicrobial activity using the agar well dif-
fusion test method. Agar nutrients (Oxoid Laboratories, UK) for bacteria were subcultured,
and Saboroud dextrose agar (Oxoid Laboratories, UK) for fungi was tested. As a positive
control for bacterial strains, ampicillin and gentamycin were used. As a positive control
for fungi, amphotericin was used. Triplicate plates were made. Bacterial cultures were
incubated at 37 ◦C for 24 h, while fungal cultures were incubated at 25–30 ◦C for 3–7 days.
The inhibition zone determined the antimicrobial activity. Antibacterial activity (Gram-
positive: Streptococcus pneumoniae (RCMB 010010), Bacillus subtilis (RCMB 010067) and
Gram-negative: Pseudomonas aeruginosa (RCMB 0100 46), Escherichia coli (RCMB 010052))
was assessed.
While ampicillin was the standard used to evaluate antibacterial activity against
Gram-positive bacteria, gentamicin was the standard used to evaluate the activity against
Gram-negative bacteria of the tested compounds. Results were expressed as the mean
inhibition zone in mm
by microorganisms using 10 mg/mL of the samples tested, as shown in Table 1.
±
standard deviation beyond the well diameter (6 mm) produced
3.4. Determination of MIC
In triplicate, the sample minimum inhibitory concentration (MIC) was estimated for
each of the tested organisms. The nutrient broth was added at varying sample concen-
trations (1000–0.007 µg/mL) and a loop of the test organism previously diluted to the 0.5
McFarland turbidity standard was introduced into the tubes. To serve as a control, a tube
containing broth media was only seeded with the test organisms. Tubes containing cultures

Molecules 2021, 26, 4219
12 of 14
of tested organisms were then incubated for 24 h at 37 ◦ C, while the fungal cultures were
◦
incubated at 25–30 C for 3–7 days. The tubes were then examined for growth using
turbidity observations [10].
4. Conclusions
In summary, this work demonstrated the design of nano-ZrO2 for the application
of nano-catalysis and use for the synthesis of benzimidazole ring systems. The main
advantages of the proposed nano-ZrO2 are that it is highly reproducible, easy to handle,
recyclable, has less mole %, and no leaching effect is found during the organic conversions.
The efficient condensation reaction was carried out between o-PDA (1) and various aryl
aldehydes (2) for the preparation of substituted-2-arylbenzimidazoles (3) in the presence
of the recyclable nano-ZrO2 catalyst. The nano-ZrO2 catalyst was prepared from readily
available reagents and thoroughly investigated using XRD, TEM, SEM, and FT-IR analyses.
2-Aryl benzimidazoles (3) were obtained in good to excellent yields, mainly from electron-
donating functional groups. More importantly, the nano-ZrO2 catalyst showed excellent
recyclability of up to five cycles in dry ethanol. In addition, dry ethanol works as an
eco-friendly solvent to protect the environment without producing any by-products. The
titled compounds have exhibited diverse antibacterial activity in terms of growth inhibition
of microorganisms. Derivatives of synthesized compounds have shown antibacterial
activities, manifested as growth inhibition of different Gram-positive bacteria (S. aureus,
B. subtilis) and two Gram-negative bacteria (P. aeruginosa, E. coli), while antifungal activity
against Aspergillus niger, Aspergillus flavus, and Candida albicans exhibited different active
potential. During the diffusion method, the newly synthesized compounds showed good
microbiological activity. The results confirmed their strong antibacterial and antifungal
activities against various microorganisms. Additionally, we are examining other one-pot
multicatalytic reactions based on the adaptable activity of the nano-ZrO2 catalyst.
Supplementary Materials: The following are available online. The NMR and Mass spectra of Benz-
imidazoles.
Author Contributions: Conceptualization, M.V.B.R. and N.K.R.; methodology, T.N.R.; validation,
S.Y.A.; formal analysis, T.N.R.; investigation, R.K.C.; data curation, F.A. (Fadwa Albalawi); writing—
original draft preparation, T.N.R.; writing—review and editing, S.Y.A.; visualization, G.R.R. and
T.M.N.; supervision, F.A. (Faheem Ahmed); project administration. F.A. (Faheem Ahmed) and N.A.
All authors have read and agreed to the published version of the manuscript.
Funding: The authors are grateful to the Deanship of Scientific Research at King Saud University for
its funding of this research through Research Group number RGP-1441-543.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The authors would like to express their gratitude to the Deanship of Scientific
Research at King Saud University for providing the reagents and chemicals used in this study.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Sample of compounds are not available from authors.
References
1.
Nakano, H.; Inoue, T.; Kawasaki, N.; Miyataka, H.; Matsumoto, H.; Taguchi, T.; Inagaki, N.; Nagai, H.; Satoh, T. Synthesis
and biological activities of novel antiallergic agents with 5-lipoxygenase inhibiting action. Bioorg. Med. Chem. 2000, 8, 373–380.
[CrossRef]
2.
Sherborne, G.J.; Adomeit, S.; Menzel, R.; Rabeah, J.; Brückner, A.; Fielding, M.R.; Willans, C.E.; Nguyen, B.N. Origins of high
catalyst loading in copper (I)-catalysed Ullmann–Goldberg C–N coupling reactions. Chem. Sci. 2017, 8, 7203–7210. [CrossRef]
[PubMed]

Molecules 2021, 26, 4219
13 of 14
3.
4.
Bariwal, J.; Van der Eycken, E. C–N bond forming cross-coupling reactions: An overview. Chem. Soc. Rev. 2013, 42, 9283–9303.
[CrossRef]
Takemoto, S.; Shibata, E.; Nakajima, M.; Yumoto, Y.; Shimamoto, M.; Matsuzaka, H. Ruthenium-Sulfonamide-Catalyzed Direct
Dehydrative Condensation of Benzylic C–H Bonds with Aromatic Aldehydes. J. Am. Chem. Soc. 2016, 138, 14836–14839.
[CrossRef] [PubMed]
5.
6.
7.
8.
9.
Shu, W.; Nevado, C. Visible-Light-Mediated Remote Aliphatic C–H Functionalizations through a 1,5-Hydrogen Transfer Cascade.
Angew. Chem. Int. Ed. 2017, 56, 1881–1884. [CrossRef]
Wang, S.; Chen, S.Y.; Yu, X.Q. C–H functionalization by high-valent Cp* Co (III) catalysis. Chem. Commun. 2017, 53, 3165–3180.
[CrossRef]
Ruan, Z.; Zhang, S.K.; Zhu, C.; Ruth, P.N.; Stalke, A.L. Ruthenium(II)-Catalyzed meta C H Mono- and Difluoromethylations by
−
Phosphine/Carboxylate Cooperation. Angew. Chem. Int. Ed. 2017, 56, 2045–2049. [CrossRef]
Hartwig, J.F. Evolution of a Fourth Generation Catalyst for the Amination and Thioetherification of Aryl Halides. Acc. Chem. Res.
2008, 41, 1534–1544. [CrossRef] [PubMed]
Kwon, M.S.; Seong, N.K.; Seo, H.; Park, I.S.; Cheedrala, R.K.; Park, J. Recyclable Palladium Catalyst for Highly Selective
α
Alkylation of Ketones with Alcohols. Angew. Chem. Int. Ed. 2005, 44, 6913–6915. [CrossRef]
10. Salam, M.S.A.; Betiha, M.A.; Shaban, S.A.; Elsabagh, A.M.; El-Aal, R.M.A.; Elkady, F.Y. Synthesis and characterization of
MCM-41-supported nano zirconia catalysts. Egypt. J. Pet. 2015, 24, 49–57. [CrossRef]
11. Gondaliya, M.B.; Maheta, T.; Mamtora, M.J.; Shah, M.K. Reusable Nanocatalyst: Zirconia and Sulfated Zirconia. Int. Lett. Chem.
Phys. Astron. 2014, 36, 295–302. [CrossRef]
12. Dunham, S.J.B.; Ellis, J.F.; Li, B.; Sweedler, J.V. Mass Spectrometry Imaging of Complex Microbial Communities. Acc. Chem. Res.
2017, 50, 96–104. [CrossRef]
13. Pour, M.; Špulák, M.; Balšánek, V.; Kuneš, J.; Kubanová, P.; Buchta, V. Synthesis and structure–antifungal activity relationships of
3-aryl-5-alkyl-2, 5-dihydrofuran-2-ones and their carbanalogues: Further refinement of tentative pharmacophore group. Bioorg.
Med. Chem. 2003, 11, 2843–2866. [CrossRef]
14. Sharma, D.; Narasimhan, B.; Kumar, P.; Judge, V.; Narang, R.; Clercq, E.D.; Balzarini, J. Regioselective synthesis of isoxazole–
mercaptobenzimidazole hybrids and their in vivo analgesic and anti-inflammatory activity studies. J. Enzym. Inhib. Med. Chem.
2009, 24, 1161–1168. [CrossRef]
15. Charvalos, E.; Tsatsakis, A.; Tzatzarakis, M.; Dolapsakis, G.; Stiakakis, J. New nystatin polymeric complexes and their in vitro
antifungalevaluation in a model study with Fusarium oxysporum. Mycopathologia 2002, 153, 15–19. [CrossRef] [PubMed]
16. Mobinikhaledi, A.; Forughifar, N.; Zendehdel, M.; Jabbarpour, M. Conversion of aldehydes to benzimidazoles using NaY zeolite.
Synth. React. Inorg. Met. Org. Chem. 2008, 38, 390–393. [CrossRef]
17. Kumar, A.; Maurya, R.A.; Saxena, D. Diversity-oriented synthesis of benzimidazole, benzoxazole, benzothiazole and quinazolin-4
(3H)-one libraries via potassium persulfate–CuSO 4-mediated oxidative coupling reactions of aldehydes in aqueous micelles.
Mol. Divers. 2010, 14, 331–341. [CrossRef]
18. Kattimani, P.P.; Kamble, R.R.; Meti, G.Y. Expedient synthesis of benzimidazoles using amides. RSC Adv. 2015, 5, 29447–29455.
[CrossRef]
19. Cheedrala, R.K.; Sachwani, R.; Krishna, P.R. Lipase mediated kinetic resolution of benzimidazolyl ethanols. Tet. Asym. 2008
19, 901–905. [CrossRef]
,
20. Cheedarala, R.K.; Sunkara, V.; Park, J.W. Facile synthesis of second-generation dendrons with an orthogonal functional group at
the focal point. Synth. Commun. 2009, 39, 1966–1980. [CrossRef]
21. Elnima, E.I.; Zubair, M.U.; Al-Badr, A.A. Antibacterial and antifungal activities of benzimidazole and benzoxazole derivatives.
Antimicrob. Agents Chemother. 1981, 19, 29–32. [CrossRef] [PubMed]
22. Cheedarala, R.K.; Park, E.J.; Park, Y.B.; Park, H.W. Highly wettable CuO: Graphene oxide core–shell porous nanocomposites for
enhanced critical heat flux. Phys. Status Solidi 2015, 212, 1756–1766. [CrossRef]
23. Arena, A.; Prete, F.; Rambaldi, E.; Bignozzi, M.C.; Monaco, C.; di Fiore, A.; Chevalier, J. Nanostructured zirconia-based ceramics
and composites in dentistry: A state-of-the-art review. Nanomaterials 2019, 9, 1393. [CrossRef]
24. Montaña, M.; Aparicio, M.S.L.; Ocsachoque, M.A.; Navas, M.B.; Barros, I.D.C.L.; Castellón, E.R.; Casella, M.L.; Lick, I.D. Zirconia-
Supported Silver Nanoparticles for the Catalytic Combustion of Pollutants Originating from Mobile Sources. Catalysts 2019
9, 297. [CrossRef]
,
25. Shaharyar, M.; Mazumder, A. Benzimidazoles: A biologically active compounds. Arab. J. Chem. 2017, 10, S157–S173.
26. Ramaiah, K.; Dubey, P.K.; Kumar, R.; Cheedarala, R.K.; Grossert, J.S.; Hooper, D.L. Condensation of O-Phenylenediamine with
Cinnamic Acids. Synth. Comm. 2001, 31, 3439–3446.
27. Dubey, P.K.; Cheedarala, R.K.; Ramanatham, J.; Kumar, R. Studies on Syntheses of 1-alkyl/aralkyl-2-Cinnamoylbenzimidazoles.
Indian J. Het. Chem. 2000, 9, 259–262.
28. Dubey, P.K.; Cheedarala, R.K.; Balaji, B. A convenient one-pot synthesis of 1-alkyl-benzimidazole-2-substituted aminothiazoles.
Indian J. Heterocylic Chem. 2002, 12, 95–98.
29. Dubey, P.K.; Cheedarala, R.K.; Naidu, A. Studies on syntheses of 1-alkyl/aralkyl-2-substituted thiazolylbenzimidazoles. Indian J.
Chem. 2003, 42B, 931–934.

Molecules 2021, 26, 4219
14 of 14
30. Dubey, P.K.; Cheedarala, R.K.; Naidu, A.; Reddy, P.V.V.P. Studies on syntheses of 1-alkyl/aralkyl-benzimidazolyl-2-substituted-
0
0
[1 -5 ]-Benzothiazepins. Indian J. Chem. 2003, 42B, 1701–1705.
31. Ramaiah, K.; Dubey, P.K.; Ramanatham, J.; Cheedarala, R.K.; Grossert, J.S. Benzimidazolium dichromate’s: Efficient Reagents for
selective oxidation of Alcohols to Carbonyl compounds. Indian J. Chem. 2003, 42B, 1765–1767.
32. Kumar, P.; With, P.; Srivastava, V.C.; Gläser, R.; Mishra, I.M. Efficient ceria-zirconium oxide catalyst for carbon dioxide conversions:
Characterization, catalytic activity and thermodynamic study. J. Alloys Compd. 2017, 696, 718–726. [CrossRef]
33. Ardakani, H.A.; Rakati, T.H. Zirconium Oxide Nanoparticles as an Efficient Catalyst for Three-Component Synthesis of
Pyrazolo[1,2-A][1,2,4]Triazole-1,3-Diones Derivatives. Orient. J. Chem. 2016, 32, 1625–1629. [CrossRef]
34. Hazmi, M.H.A.; Choi, Y.; Apblett, A.W. Preparation of Zirconium Oxide Powder Using Zirconium Carboxylate Precursors. Adv.
Phys. Chem. 2014, 2014, 429751.
35. Sigwadi, R.; Dhlamini, M.; Mokrani, T.; Nemavhola, F. Preparation of a high surface area zirconium oxide for fuel cell application.
Int. J. Mech. Mater. Eng. 2019, 14, 5. [CrossRef]
36. Pashavan, C.T.; Gondaliya, M.B.; Shaw, M.K. Synthesis of Benzimidazole Using Reusable Nanocatalyst Zirconia and Sulfated
Zirconia. World Sci. News 2019, 117, 235–242.
37. Jeon, J.H.; Cheedarala, R.K.; Kee, C.D.; Oh, I.K. Dry-type artificial muscles based on pendent sulfonated chitosan and functional-
ized graphene oxide for greatly enhanced ionic interactions and mechanical stiffness. Adv. Funct. Mater. 2013, 23, 6007–6018.
[CrossRef]
38. Cheedarala, R.K.; Jeon, J.H.; Kee, C.D.; Oh, I.K. Bio-inspired all-organic soft actuator based on a π–π stacked 3D ionic network
membrane and ultra-fast solution processing. Adv. Funct. Mater. 2014, 24, 6005–6015. [CrossRef]
39. Zhou, S.; Garnweitner, G.; Niederberger, M.; Antonietti, M. Dispersion behavior of zirconia nanocrystals and their surface
functionalization with vinyl group-containing ligands. Langmuir 2007, 23, 9178–9187. [CrossRef]
40. Rao, N.K.; Rao, T.N.; Parvatamma, B.; Prashanthi, Y.; Cheedarala, R.K. Novel Synthesis of 4-Benzylidene-2-((1-phenyl-3,4-
dihydroisoquinoline-2(1H)-yl)methyl) oxazol-5(4H)-one Derivatives Using 1,2,3,Tetrahydroisoquinoline and their Antimicrobial
Activity. Curr. Org. Synth. 2020, 17, 396–403. [CrossRef]
41. Kwon, M.S.; Kim, S.J.; Park, S.; Bosco, W.; Cheedrala, R.K.; Park, J. One-Pot Synthesis of Imines and Secondary Amines by
Pd-Catalyzed Coupling of Benzyl Alcohols and Primary Amines. J. Org. Chem. 2009, 74, 2877–2879. [CrossRef] [PubMed]