Hydromagnesite sheets impregnated with cobalt-ferrite magnetic nanoparticles as heterogeneous catalytic system for the synthesis of imidazo[1,2-a]pyridine scaffolds

Article abstract of DOI:10.1039/d1ra02516c

This paper manifests an A³-coupling strategy assisted by novel hydromagnesite sheets impregnated with cobalt ferrite (CoFe₂O₄-HMS) magnetic nanoparticles (MNPs) as an environmentally benign nanocomposite to synthesize imid

Full text of DOI:10.1039/d1ra02516c

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Hydromagnesite sheets impregnated with cobalt–
ferrite magnetic nanoparticles as heterogeneous
catalytic system for the synthesis of imidazo[1,2-a]
pyridine scaffolds†
Cite this: RSC Adv., 2021, 11, 23207
a
a
Deepika Geedkar,a Ashok Kumar,
Kranti Kumarb and Pratibha Sharma
*
This paper manifests an A3-coupling strategy assisted by novel hydromagnesite sheets impregnated with
cobalt ferrite (CoFe2O4-HMS) magnetic nanoparticles (MNPs) as an environmentally benign
nanocomposite to synthesize imidazo[1,2-a]pyridine scaffolds under ultrasonication. The synthesis of
these biologically active derivatives was achieved through A3-coupling employing 2-aminopyridines
derivatives, pertinent aryl aldehydes, and phenylacetylene in the presence of polyethylene glycol 400
(PEG 400) as a green solvent under aerobic conditions. Based on its high product yield (up to 94%) in
a short reaction time, with a modest catalyst loading, excellent catalyst, and solvent recyclability without
substantial loss of operation (up to five synthetic cycles), as demonstrated by the high ecological
compatibility and sustainability factors, this strategy follows the principles of green chemistry. The
synthesized nanocomposite was characterized via several spectroanalytical techniques, including PXRD,
FE-SEM, HR-TEM, EDAX, ICP-AES, FT-IR, Raman spectroscopy, CO2-TPD, TGA-DTA-DTG analyses,
magnetic studies, and nitrogen porosimetry. Furthermore, the structures of synthesized compounds
Received 30th March 2021
Accepted 25th May 2021
DOI: 10.1039/d1ra02516c
1
rsc.li/rsc-advances
were confirmed based on FT-IR, H NMR, 13C NMR, mass spectroscopy, and elemental analysis data.
the framework has potential relevance in optoelectronics, dyes,
and sensing materials, which has resulted in the development
of many novel strategies for their preparation.8–10
Based on these observations, it is considered to be worth-
while to develop robust, economical, and environment-friendly
catalytic frameworks for the synthesis of pharmacologically
signicant imidazo-fused pyridine derivatives (VI). A systemic
perusal of the literature revealed that there are recent reports on
several transition-metal-catalyzed techniques for the synthesis
of imidazole-fused heterocyclic core nuclei. These catalytic
1. Introduction
Multicomponent coupling reaction strategies embracing green
chemistry protocols have blossomed as sustainable pathways
for achieving the synthesis of numerous heterocyclic scaffolds.
Particularly, nitrogen-containing fused frameworks are the
most prevalent structural motifs found in pharmaceutical
chemistry as biologically active synthetic equivalents and
natural products.1 Therefore, the development of efficient and
environmentally sustainable protocols for the synthesis of new
series of highly functionalized imidazo-fused pyridine scaffolds
has attracted considerable attention on account of their
synthetic and effective biological signicance. Their chemistry
has gained substantial interest in recent years owing to their
contribution in various medicinal applications as potent anti-
viral,2 anxiolytic,3 anticancer,4 antibacterial,5 and anti-
inammatory agents.6 Several biologically active representa-
tives such as zolpidem, alpidem, olprinone, zolimidine, and
miroprofen contain an imidazo[1,2-a]pyridine (I–V) nucleus
(Fig. 1).7 The presence of the imidazo[1,2-a]pyridine nucleus in
aSchool of Chemical Sciences, Devi Ahilya University, Indore-452001, MP, India.
E-mail: drpratibhasharma@yahoo.com
bUGC-DAE, Consortium for Scientic Research, Devi Ahilya University, Indore-452001,
MP, India
Fig. 1 Some representative examples of biologically active imidazo
fused pyridines (I–V) and our designed molecule (VI).
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra02516c
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strategies include the use of CuSO4$5H2O,11 MNP@BiimCu,12 given that organic solvents account for more than 80% of the
CuO/CuAl2O4,13 Fe3O4@SiO2 NPs,14 CuI/L-proline,15 Cu–NPs16 overall waste products of a process.24,25 Therefore, to circumvent
and nano-copper/nano-ZnAl2O4 under an N2 atmosphere,17 PW- all these bottlenecks, herein, we report a green and efficient
CIS500,18 and CoFe2O4@CNT-Cu19 towards the green synthesis protocol for the synthesis of amalgamated imidazo-pyridine
of amalgamated heterocyclic moieties. However, these reported derivatives in high to excellent yields considering recyclable
methods have some bottlenecks including the use of hazardous PEG 400 as the reaction medium under aerobic conditions.
and expensive catalysts, extensive moisture-sensitive metallic
Furthermore, despite the multifarious properties associated
reagents, harsh reaction conditions, prolonged reaction time, with heterogeneous nanocomposite catalysts and pharmaco-
tedious work-up processes, use of volatile organic solvents and logically signicant imidazo-fused pyridine scaffolds, and in
non-recyclable and large catalytic loading, leading to the continuation to our work towards nanocatalysis-oriented
generation of an enormous amount of waste. Hence, consid- synthesis,25,26 we demonstrate the unprecedented ultrasonic-
ering the diverse signicance of imidazo-fused pyridine scaf- assisted environmentally friendly synthesis of highly function-
folds, we endeavoured to develop hydromagnesite sheets alized imidazo[1,2-a]pyridines in the presence of hydro-
impregnated with cobalt–ferrite magnetic nanoparticles derived magnesite sheets impregnated with cobalt–ferrite magnetic
as an alternative strategy employing 2-aminopyridine deriva- nanoparticles (CoFe2O4-HMS) as a recyclable magnetic nano-
tives, pertinent aryl aldehydes, and phenyl acetylene as catalyst composite under mild and green reaction conditions
precursors. To the best of our knowledge, the synthesis of via A3-coupling followed by 5-exo-dig cyclization. To the best of
amalgamated imidazo-pyridine frameworks derived from our knowledge, this is the rst report to date on the use of
hydromagnesite sheets impregnated with cobalt–ferrite hydromagnesite sheets impregnated with cobalt–ferrite
magnetic nanoparticles has not been reported to date.
magnetic nanoparticles (CoFe2O4-HMS) as a heterogeneous
Furthermore, ultrasonic irradiation has attracted signicant nanocatalyst for the synthesis of imidazo[1,2-a]pyridines
attention from scientic and industrial researchers as an intu- scaffolds.
itive, green technique in many chemical processes.20,21
Currently, the use of ultrasonic irradiation as an alternative
source of energy for organic reactions results in a shorter
2. Results and discussion
reaction time with green and mild reaction conditions,
minimum chemical waste in high isolated yields, and
improvement in the eco-sustainability and selectivity of the
system. This acceleration of reactions under ultrasonic condi-
tions can be attributed to the acoustic cavitations arising from
the high pressures and energy produced instantly. Hydro-
magnesite sheets (HMS) have attracted signicant attention on
account of their specic applications in many areas such as the
sequestration of CO2 gas, corrosion prevention, re retardants,
RNA chromatography, photoluminescence, versatile magne-
sium oxide precursor, and endothermic coolant in yellow
smoke formulations for hand-held signals. However, despite
the diverse applications of HMS, its use as a catalyst has not
been substantially explored.22 Thus, based on the literature,22,23
the crystal structure models of HMS and CoFe2O4-HMS are
shown in Scheme 2.
2.1. Optimization of reaction conditions
To optimize the reaction conditions, a model condensation
reaction of 2-aminopyridine (1a), benzaldehyde (2a), and phenyl
acetylene (3) was carried out in the presence of various solvents
under ultrasonic irradiation. In the presence of organic solvents
such as ethanol, 1,4-dioxane, THF (tetrahydrofuran), DMF
(dimethylformamide), and DMSO (dimethylsulfoxide) the
reactions proceeded smoothly to afford the product (4a) in
quantitative yields (Table 1, entries 1–5). The same reaction was
attempted in aqueous medium and green solvents including
Table 1 Optimization of reaction conditions for the preparation of 3-
benzyl-2-phenyl imidazo[1,2-a]pyridine (4a)a
Entry Condition
Solvent
Timeb (min) Yieldc (%)
Interestingly, magnetic nanocomposites are considered
efficient and recoverable catalysts on account of their unique
properties and remarkable catalytic potential, simple prepara-
tion, non-corrosive nature, non-toxicity, physiochemical
stability, and environmental compatibility. Moreover, the
incorporation of support media has become important in
enhancing the surface area and overcoming the problems
associated with the aggregation of nanocomposites. Therefore,
considering these fascinating features, we synthesized hydro-
magnesite sheets impregnated with cobalt–ferrite magnetic
nanoparticles. The magnetic separation of a synthesized cata-
lyst is an intriguing alternative to ltration or centrifugation,
given that it reduces the loss of the catalyst and improves the
recovery and reusability of catalysts. Moreover, the eventual
substitution of conventional hazardous organic solvents by safe
and environmentally benign media is also substantially relevant
1
2
3
4
5
6
7
8
CoFe2O4-HMS (7 mol%) 1,4 dioxane 25
58
61
42
56
54
68
79
76
68
89
89
86
69
66
CoFe2O4-HMS (7 mol%) DMSO
CoFe2O4-HMS (7 mol%) THF
CoFe2O4-HMS (7 mol%) DMF
CoFe2O4-HMS (7 mol%) EtOH
CoFe2O4-HMS (7 mol%) H2O
CoFe2O4-HMS (7 mol%) Glycerine
CoFe2O4-HMS (7 mol%) EG
CoFe2O4-HMS (7 mol%) DEG
CoFe2O4-HMS (7 mol%) PEG 400
CoFe2O4-HMS (7 mol%) PEG 400
CoFe2O4-HMS (8 mol%) PEG 400
CoFe2O4 MNPs (7 mol%) PEG 400
25
25
25
25
25
25
25
60
25
60
25
30
30
9
10
11
12
13
14
CoCl2$6H2O (7 mol%)
PEG 400
a
Reaction conditions: 2-aminopyridine (1 mmol) (1a), benzaldehyde (1
b
mmol) (2a) and phenyl acetylene (1 mmol) (3). All reactions were
monitored by TLC. c Isolated yield.
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conditions, 2-aminopyridine derivatives, pertinent aromatic
aldehydes and, phenyl acetylene were employed to observe the
extent and prevalence of this reaction in the presence of PEG
400 (Scheme 1). The results are summarized in Table 1. The
correlation between catalytic activity and reaction efficiency for
the synthesis of the desired product (4a) is illustrated in Fig. S1
of the ESI.†
Scheme 1 Synthesis of imidazo[1,2-a]pyridine derivatives (4a–j).
Furthermore, the effects of various substituents were deter-
mined in a similar fashion, yielding the corresponding antici-
pated products (4a–j) in high to excellent yields (78–94%). The
nitro group at the ortho position of the benzene ring gave an
appreciable yield of 82% (Table 2, entry 4), whereas substitution
with chloro and hydroxyl groups at the para position resulted in
an improvement in the yield (Table 2, entries 2 and 3, respec-
tively). It was inferred that the electron-withdrawing and
electron-donating substituents at the para positions intensied
the rate of reaction, resulting in better yields in comparison to
Scheme 2 Synthesis of hydromagnesite sheets impregnated with the ortho position (Table 2, entries 2–5). Also, the presence of di-
cobalt–ferrite magnetic nanoparticles (CoFe2O4-HMS).
substituted aryl aldehydes resulted in the desired products in
satisfactory yields (Table 2, entries 5 and 10). In contrast, the
presence of strong electron-donating groups such as methyl at
water, glycerine, EG (ethylene glycol), DEG (diethylene glycol),
and PEG 400 (polyethylene glycol 400) (entries 6–14). It was
observed that the reaction did not proceed with a substantial
yield in water as the solvent (Table 1, entry 6). It can be inferred
from the data presented in Table 1 that the product yields with
EG and DEG were 76% and 68%, respectively, even aer
a reaction time of one hour (Table 1, entries 8 and 9, respec-
tively). Intriguingly, PEG 400 was considered to be the best
among the solvents used, affording the product yield of 89%
(Table 1, entry 10). The reaction was extended to one hour but
no further enhancement in the yield of the product was
observed (Table 1, entry 11). Furthermore, a high catalytic
loading (8 mol%) under similar conditions did not result in
a substantial improvement in the yield (Table 1, entry 12). Aer
the solvent optimization, the reaction was also performed in
CoCl2$6H2O as the catalyst and in bare CoFe2O4 MNPs, but the
yield of the product was not appreciable (Table 1, entries 13 and
14, respectively). Aer ascertaining the optimal reaction
the para position resulted in an enhancement in the yield from
87% to 94% (Table 2, entries 6–10).
To analyze the role of ultrasonic irradiation, the reactions
were performed in the presence of the same amount of
CoFe2O4-HMS in PEG 400 medium, under different conditions,
together with stirring and reuxing. The obtained results were
compared with the results from sonication and summarised in
Table 3. This revealed that the reactions under ultrasonic irri-
tation led to comparatively higher yields and shorter reaction
times compared to both stirring and reuxing conditions. It is
assumed that the efficiency with the use of ultrasound irritation
is due to the cavitation phenomenon. The chemical and phys-
ical effects of ultrasound are primarily due to acoustic cavita-
tions, which involve design, escalation and collapse of the
cavity. A substantial concentration of energy is instigated by an
alteration in the temperature of the bubble of kinetic energy of
the uid due to the collapse of the bubble in the uid. The high
temperatures and pressures arising from cavitations produce
Table 2 The sonochemical synthesis of imidazo[1,2-a]pyridine derivatives (4a–j) catalyzed by CoFe2O4-HMSa
R1 (amino
pyridine)
Entry
R2 (aryl aldehydes)
Productb
Time (min)
Yieldc (%)
1
2
3
4
5
6
7
8
9
10
H
H
H
H
H
4-Cl
4-OH
2-NO2
2-OMe, 4-OH
H
4-Cl
4-OH
4a
4b
4c
4d
4e
4f
4g
4h
4i
25
30
35
40
30
30
25
30
35
40
89
86
84
82
78
91
94
88
90
87
H
4-Me
4-Me
4-Me
4-Me
4-Me
2-NO2
2-OMe, 4-OH
4j
a
Reaction conditions: 2-aminopyridine derivatives (1 mmol) (1a–b), pertinent aryl aldehyde (1 mmol) (2a–e), phenyl acetylene (1 mmol) (3) and
a catalytic amount (7 mol%) of CoFe2O4-HMS in the presence of PEG 400. b All reactions were monitored by TLC. c Isolated yield.
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According to the literature and our results,11,14,22,25,31–35
a plausible reaction mechanism to account for the formation of
imidazo[1,2-a]pyridines is presented in Scheme 3. The domino
reaction begins with condensation between 2-aminopyridine
(1a–b) and the pertinent aryl aldehyde derivatives (2a–e), fol-
lowed by dehydration, resulting in the formation of a Schiff base
(A). In the A3-coupling strategy, where the basic sites of HMS
promote the formation of cobalt–ferrite acetylide complex (3)0
by the activation of a proton from phenyl acetylene (3), the
cobalt–ferrite acetylide (3)0 attacks the preformed iminium ion
(A) to form intermediate (B). Consequently, the latter undergoes
a 5-exo dig cyclization step initiated by an attack on the alkyne
unit by the pyridine nitrogen, which results in the formation of
imidazo-fused pyridine systems (4a–j).
Table
3 Synthesis of imidazo[1,2-a]pyridine using CoFe2O4-HMS
under stirring, refluxing and ultrasonic conditionsa
Time (min)
R1 (amino
Yieldb (%)
Entry pyridine)
R2 (aryl aldehydes) Sc Rd Ue Sc Rd Ue
1
2
3
4
5
6
7
8
9
H
H
H
H
H
4-Cl
4-OH
2-NO2
2-OMe, 4-OH
H
4-Cl
4-OH
45 40 25 85 76 89
55 45 30 74 69 86
65 50 35 80 73 84
75 55 40 79 67 82
55 40 30 71 64 78
50 45 30 85 79 91
55 40 25 88 81 94
60 45 30 72 63 88
55 40 35 83 75 90
75 60 40 76 66 87
H
4-Me
4-Me
4-Me
4-Me
4-Me
2-NO2
2-OMe, 4-OH
10
a
Reaction conditions: 2-aminopyridine derivatives (1 mmol) (1a–b),
pertinent aryl aldehyde (1 mmol) (2a–e), phenyl acetylene (1 mmol) (3) 2.2. Recycling and reusability of magnetic nanocomposite
and a catalytic amount (7 mol%) of CoFe2O4-HMS in the presence of
and solvent
b
c
PEG 400. Yields refer to isolated products. Stirring condition.
d
Reuxing condition. e Ultrasonic condition.
The reusability and recyclability of a catalytic framework are of
great relevance in industrial processes. Thus, to assess the level
of reusability and stability of the synthesized magnetic nano-
composite, we executed the reaction of 2-amino-4-methyl pyri-
dine (1b) with 4-chloro benzaldehyde (2b) and phenyl acetylene
(3) using PEG 400 as the reaction medium to acquire the desired
product (4g). Aer the completion of the initial reaction, the
magnetic nanoparticles were dragged to the inner sidewall of
the reaction ask with the use of an external magnet. Then, the
isolated catalyst was recovered according to the comprehensive
procedure for the recovery of catalyst, as mentioned in the
Experimental section.25,36 The magnetically recovered nano-
catalyst was further subjected to the model reaction for the
synthesis of (4g), (up to ve runs) under analogous conditions.
The outcomes of the recyclability of the nanocomposite are
presented in Fig. S2(i) in the ESI.† It can be observed in
Fig. S2(i)† that the recovered CoFe2O4-HMS catalyzed the reac-
tion to give the product in 94%, 93%, 90%, 89%, and 87% yield.
The signicant yields of the product conrmed that there was
no substantial deterioration in the catalytic activity. The
morphology and crystalline structure of the recycled catalyst
were characterized via FE-SEM and PXRD analyses (Fig. S3 and
S4, respectively, in the ESI†). The analysis of the results revealed
that there was no signicant changes in the morphology and
crystalline structure of the recovered catalyst.
a set of ultrasonic applications consisting of accelerating the
rate of the reaction, amending the reaction pathway, and
enhancing chemical reactivity, and thus have signicant use in
the synthesis of organic compounds.20,25 To assist the sustain-
ability of the protocol regarding the green chemistry approach,
the methodology was assessed based on the atom economy,27
carbon efficiency, reaction mass efficiency,28 eco-scale score,29
and E-factor values.30 The reaction of 2-amino-4methyl pyridine
(1b), 4-chlorobenzaldehyde (2b) and phenyl acetylene (3) were
considered for the estimation of these parameters. The atom
economy of 93.60% and 74.07% carbon efficiency with an eco-
scale factor of 75 demonstrated the feasibility of this method-
ology. The estimate of green chemistry metrics is summarized
in the ESI.† The E-factor, mass intensity and reaction mass
efficiency for the model reaction are 0.0639%, 1.0639% and
90.26%, respectively. These criteria support that this method is
a clean and green synthetic approach for the synthesis of
amalgamated imidazo-pyridine heterocyclic scaffolds.
To investigate the recyclability of the used PEG 400, the
solvent was collected aer completion of the initial reaction,
subjected to the normal workup and dried at 50 ꢀC under
vacuum. The recovered solvent was reused in the subsequent
synthetic runs (up to ve runs) for the model reaction. The
results of the solvent recyclability are shown in Fig. S2(ii) in the
ESI.† Furthermore, it can be seen from Fig. S2(ii)† that the
recovered PEG 400 gave the product in 87%, 85%, 85%, 83%,
and 82% yield.25
2.3. Heterogeneous nature of CoFe2O4-HMS magnetic
nanocatalyst
The heterogeneity of the CoFe2O4-HMS magnetic nanocatalyst
was studied by performing a hot ltration test experiment for
Scheme 3 Plausible mechanistic pathway for the formation of multi-
substituted imidazo[1,2-a]pyridine scaffolds.
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the model reaction of 2-amino-4-methyl pyridine (1b) with 4-
chlorobenzaldehyde (2b) and phenyl acetylene (3) in the pres-
ence of PEG 400 to afford the product (4g). The reaction was
stopped aer 15 min at around 50% conversion of the starting
materials and the catalyst was ltered from the reaction
mixture. There was no substantial improvement in the progress
of the reaction upon further continuing it with the ltrate for
the next 10 min aer the removal of the catalyst. Moreover, the
conversion remained constant with 57% yield even aer
a reaction time of 40 min, which revealed that the reaction
occurred only due to the presence of the CoFe2O4-HMS MNPs.
The ltrate was further analyzed by inductively coupled plasma
atomic emission spectroscopy (ICP-AES). The ICP-AES analysis
of the ltrate did not show a detectable amount of cobalt and
iron ions, which conrms the heterogeneous nature of the
catalyst with its negligible leaching into the reaction mixture.
The result of the hot ltration test and the leaching effect of the
CoFe2O4-HMS magnetic nanocatalyst is depicted in Fig. S5 of
the ESI.†
Fig. 2 PXRD diffraction pattern of CoFe2O4-HMS, CoFe2O4 MNPs and
HMS.
2.5. Characterization of nanocatalysts
2.4. Comparison of the results of the present protocol with
the reported methods
2.5.1. Powder X-ray diffraction (PXRD) analyses. The
structural properties of hydromagnesite sheets impregnated
with cobalt–ferrite magnetic nanoparticles (CoFe2O4-HMS) in
comparison with cobalt ferrite (CoFe2O4 MNPs) and hydro-
magnesite sheets (HMS) were examined by powder X-ray
diffraction (PXRD) pattern, as represented in Fig. 2. The
average crystallite size was calculated using the Debye–Scherrer
equation as follows:37
A comparative report of the literature protocols for the synthesis
of amalgamated imidazo-pyridine derivatives and our present
work is summarized in Table 4. Gratifyingly, in comparison to
the reported methodologies, our present approach offers
prominent advantages including environmental acceptability,
wide substrate scope, operational simplicity, practicability,
economic viability, excellent yield of the products in a shorter
reaction time, easy work-up, and reusability of both the
magnetic nanocatalyst and solvent. Consequently, performing
the reaction by utilizing a small amount of catalyst, reusability
within multiple runs, and ecofriendly nature of CoFe2O4-HMS
are among some of the value-added advantages of this newly
synthesized magnetic nano-composite, leading to the highly
efficient synthesis of multi-substituted imidazo-fused pyridine
derivatives.
D ¼ Kl/b cos q
where, D is the average crystallite size, K is the particle shape
factor whose value is 0.9, l is the X-ray wavelength of Cu-Ka
ꢀ
radiation (1.54056 A), b is the full width at half maximum
(FWHM) of a selected peak and q is the Bragg's angle.24,26
Accordingly, the average crystallite size for CoFe2O4-HMS,
CoFe2O4 MNPs, and HMS attained from the above-mentioned
equation were found to be 12.30, 16.97, and 14.78 nm, respec-
tively, which is in accordance with the results obtained by eld-
emission scanning electron microscopy (FE-SEM) (Fig. 3) and
nitrogen adsorption isotherm (Fig. 10).
Furthermore, a comparative account of the synthesized
catalyst with some reported catalytic systems for the formation
of the model compound (4a) via the A3 coupling strategy fol-
lowed by a 5-exo-dig cyclization approach is presented in Table
S1 of the ESI.†
Table 4 Comparison of the catalytic activity of CoFe2O4-HMS with the reported catalytic systems for the synthesis of imidazo[1,2-a]pyridine
derivatives
Entry
Condition
Solvent
Temperature (ꢀC)/time
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
10
CuSO4$5H2O (10 mol%)/sodium ascorbate (20 mol%)/SDS (10 mol%)
MNP@BiimCu (1.2 mol%)/CTAB (5 mg)
CuO/CuAl2O4 (15 mol%)/D-glucose (30 mol%)
Fe3O4@SiO2 NPs (5 mol%)/K2CO3
CuI (5 mol%)/L-proline
Cu-NPs (1 mol%)/N2
Nano-copper/nano-ZnAl2O4/N2
PW-CIS500 (0.9 mol%)
CoFe2O4@CNT–Cu (5 mol%)
H2O
H2O
Toluene
EtOH
DMSO
—
—
—
PEG 400
PEG 400
50/6–16 h
52–89
65–95
41–89
77–90
76–88
14–85
89–94
82–95
80–96
78–94
11
12
13
14
15
16
17
18
Reux/5–8.5 h
Reux/24 h
Reux/3–6 h
110/3 h
120/12–16 h
90/4–6 h
100/2.5–3.5 h
80/3–6 h
)))/25–40 min
19
Our work
CoFe2O4–HMS (7 mol%)
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spinel CoFe2O4 MNPs, respectively.25,39 These cores can be
indexed as inverse spinel and face-centred cubic (fcc) structure
(CoFe2O4 MNPs), according to the standard JCPDS card no. 10-
0325.40,41 The most intense diffraction peak is at 2q ¼ 35.678ꢀ
ꢀ
and has a plane spacing of d ¼ 2.5145 A, which corresponds to
the (311) plane and is characterized by cubic spinel ferrite. This
conrmed the formation of a single-phase face-centred cubic
spinel structure with no extra peaks corresponding to other
crystallographic phases.25,41 The presence of a peak at 30.100ꢀ
ꢀ
with d-spacing ¼ 2.9664 A (which corresponds to HMS) and
35.521ꢀ with d-spacing ¼ 2.5251 A (which corresponds to
ꢀ
CoFe2O4 MNPs) in the diffraction pattern of the CoFe2O4-HMS
composite, indicates the successful formation of the corre-
sponding hybrid material. The sharpness of the peaks indicates
the crystalline nature of the cobalt–ferrite nanoparticles and
their impregnation in the hydromagnesite sheets.
2.5.2. Field-emission scanning electron microscopy (FE-
SEM), EDAX and high-resolution transmission electronic
microscopy (HR-TEM). The morphology of the hydromagnesite
sheets hosted on cobalt–ferrite magnetic nanoparticles
(CoFe2O4-HMS) and the precursor hydromagnesite sheets
(HMS) and cobalt–ferrite (CoFe2O4 MNPs) was characterized via
eld-emission scanning electron microscopy (FE-SEM), as
shown in Fig. 3. The FE-SEM images of the hydromagnesite
sheets impregnated with cobalt–ferrite magnetic nanoparticles
(CoFe2O4-HMS) and hydromagnesite sheets (HMS) show
a ower-like morphology, while the magnetic ferrite composite
has a spherical shape.
The chemical purity of the as-synthesized nanocomposite
sample was veried via EDAX studies. The characteristic peaks
in the EDAX spectrum (Fig. 4) conrm the presence of cobalt
(Co), iron (Fe), oxygen (O), magnesium (Mg) and carbon (C) as
the only elements in the decorated magnetic nanoparticles. The
spectrum revealed that the amount of cobalt and iron stabilized
on the HMS was 12.33 at% and 25.57 at%, respectively. The
content of cobalt and iron in CoFe2O4-HMS was found to be
23.23 and 46.04 wt%, respectively, which are in accordance with
the amount of Co (23.14 wt%) and Fe (45.97 wt%) analyzed by
the ICP-AES technique.
Fig. 3 Field-emission scanning electron microscopy (FE-SEM) images
of (A–F) CoFe2O4-HMS and (G and H) CoFe2O4 MNPs and (I and J)
HMS at different magnifications.
The powder X-ray diffractograms of CoFe2O4-HMS, CoFe2O4
MNPs and HMS are shown in Fig. 2, showing broadened peaks
centred in the 2q range of 20ꢀ–70ꢀ. The diffraction pattern of
HMS could be indexed to monoclinic symmetry with rened
ꢀ
ꢀ
lattice parameters of a-axis of aboutꢀ10.12 A, b-axis of 8.93 A and
ꢀ
c-axis of 8.38 A with b ¼ 114.56 . The major peaks corre-
sponding to the (210), (120), (220), (130), (320), (230), (400) and
(240) planes at 2q positions of 21.33ꢀ, 23.04ꢀ, 27.60ꢀ, 31.66ꢀ,
34.24ꢀ, 35.77ꢀ, 38.57ꢀ and 45.44ꢀ, respectively, are the charac-
teristic peaks of hydromagnesite sheets according to the stan-
dard JCPDS card no. 25-513.38
The peaks centred at 30.33ꢀ, 35.71ꢀ, 37.20ꢀ, 43.40ꢀ, 53.80ꢀ,
57.44ꢀ and 63.08ꢀ are attributed to the presence of (220), (311),
(222), (400), (422), (511) and (440) crystal planes of inverse
Fig. 4 EDAX pattern of CoFe2O4-HMS.
23212 | RSC Adv., 2021, 11, 23207–23220
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Fig. 6 FT-IR spectra of CoFe2O4-HMS, CoFe2O4 MNPs and HMS.
are located at 1644 and 1634 cmꢁ1 in the composite. The bands
in the range of 1476 to 880 cmꢁ1 are ascribed to the O–H
bending of the carboxylic groups, whereas the bands at 1348
and 1251 cmꢁ1 are due to the –CH bending. The band in the
spectrum at around 1155–1070 cmꢁ1 shows the presence of C–O
group vibrational modes. The metal–oxygen bands with peaks
range in the range of ꢂ723 to ꢂ534 cmꢁ1 are ascribed to the
deformation vibration of the Fe–OH groups. These two bands
are usually assigned as the vibration of ions in the crystal
lattices. This indicates the presence of continuously distributed
ferrite particles. The band at ꢂ588 cmꢁ1 corresponds to the
intrinsic stretching vibrations of the metal at the tetrahedral
site (Fe 4 O), whereas the band at ꢂ493 is assigned to the
octahedral-metal stretching (Co 4 O).25,39,41 Moreover, the FTIR
spectra of CoFe2O4 MNPs and HMS were also recorded and
compared with that of the hydromagnesite sheets impregnated
with cobalt ferrite, as displayed in Fig. 6. The functional groups
with their wavenumber values for the samples are summarized
in Table S2, in the ESI.†
Fig. 5 Selected area electron diffraction (SAED) pattern and high-
resolution transmission electron microscopy (HR-TEM) images of
hydromagnesite sheets (HMS) at different magnifications.
The surface and internal morphologies of the as-synthesized
hydromagnesite sheets (HMS) were further determined via
high-resolution transmission electronic microscopy (HR-TEM),
as shown in Fig. 5. The results revealed the existence of thin and
transparent porous akes with a 2D sheet morphology of HMS
at different magnications. The polycrystalline nature of the Mg
(CO3)4(OH)2$4H2O sheets (HMS) was authenticated using
selected area electron diffraction (SAED).
2.5.3. FTIR spectroscopy. The FT-IR spectrum of the
hydromagnesite sheets impregnated with cobalt ferrite
magnetic nanoparticles was recorded to determine the catalytic
sites associated with functional entities. This analysis also
revealed the structural constitution and exchange of cations
between the octahedral and tetrahedral sites, resulting in
vibrational frequencies.25,39
The broad band at around 3600–2500 cmꢁ1 can be ascribed
to the O–H stretching vibration of the carboxyl functional
groups produced on their surface. The bands at 2981 and
2891 cmꢁ1 are attributed to the stretching vibration of the C–H
group. The characteristic peaks of the C]O stretching vibration
Fig. 7 Raman spectra of CoFe2O4-HMS, CoFe2O4 MNPs and HMS.
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ꢀ
ꢀ
160 C and 400 C is most likely associated with the oxygen ꢀin
the Mg2+ and O2ꢁ pairs. Finally, the peaks higher than 400 C
can be attributed to the presence of strong basic sites, probably
corresponding to isolated O2ꢁ. The large-area peaks between
2.5.4. Raman spectroscopy. The Raman spectra of
CoFe2O4-HMS, CoFe2O4 MNPs, and HMS are shown in Fig. 7.
The bands at ꢂ200.995, ꢂ213.238 and ꢂ243.767 cmꢁ1 are
assigned to the A1g symmetric stretching (tetrahedral breath
mode) of the oxygen atoms associated with Fe and Co ions. The
T2g modes (asymmetric stretching) correspond to the bands at
ꢂ472.060 and ꢂ477.323 cmꢁ1, while the asymmetric bending
bands are located at ꢂ564.942 and ꢂ571.442 cmꢁ1. The T2g
mode at ꢂ686.863 and ꢂ702.847 cmꢁ1 belongs to the trans-
lation motion of the whole tetrahedron and the bands at
ꢂ613.556 and ꢂ618.078 cmꢁ1 are assigned to the Eg symmetric
bending of Fe(Co)–O.42–44 The calculated intensity ratios for
CoFe2O4-HMS and CoFe2O4 MNPs are 0.7965 and 0.7943,
respectively. These results indicate that the ratio increases aer
doping of HMS with CoFe2O4 MNPs as expected. This reveals
that the disorder density and defect of the hydromagnesite
sheets increase accordingly. The measured Raman shi, peak
position, and vibrational modes of the samples are summarized
in Table S3 in the ESI.†
ꢀ
ꢀ
ꢀ
160 C and 400 C centred at 160 C indicate the presence of
a large number of medium basic sites, which most likely
correspond to HMS. The ratios of the weak basic site/medium
basic sites and weak basic site/(weak + medium) basic sites
can be concluded from the amount of carbon dioxide adsorbed.
The ratios of the weak basic sites and medium basic sites on
HMS and CoFe2O4-HMS are 0.21 and 0.43, respectively. The TPD
results revealed that most of the surface of the catalyst is
covered by medium basic Mg2+ and O2ꢁ pairs. The number of
weak and medium basic sites on the surface of HMS and
CoFe2O4-HMS are summarized in Table S4 in the ESI.†
Aer impregnation of the cobalt ferrite magnetic nano-
particles on the hydromagnesite sheets, the positions of the
desorption peaks slightly changed. This implies that the
strength of the basic sites on the surface of CoFe2O4-HMS and
HMS remained the same, where the increase in the area of the
desorption peaks aer impregnation is attributed to the incre-
ment in surface area and coercivity with a decrease in the
crystalline size of CoFe2O4-HMS.
2.5.6. Magnetic studies. The magnetic properties of the
synthesized ferrites and their composite were evaluated using
a vibrating sample magnetometer. The magnetic hysteresis
curves were recorded at room temperature for the cobalt ferrite
and its composite impregnated in HMS (300 K), as demon-
strated in Fig. 9. The samples exhibited ferromagnetic behavior
at room temperature. According to the M–H hysteresis loops,
there was a decrease in the magnetic saturation of CoFe2O4-
HMS compared to that of the pure cobalt ferrites. The magnetic
saturation of CoFe2O4-HMS is lower than that of the corre-
sponding CoFe2O4 MNPs owing to the spin canting on the
surface of the ferrite magnetic nanoparticles.41 The magnetic
properties of materials with a particle size in the range of 1–
100 nm are suspected to be dependent on the shape of the
sample, direction of magnetization, crystalline nature, etc.
2.5.5. Surface basic property (temperature-programmed
desorption of CO2 (CO2-TPD)). The temperature-programmed
desorption of CO2 (CO2-TPD) was performed to measure the
strength of basic sites on the heterogeneous magnetic
nanoparticles/catalysts based on the amount of CO2 desorbed
from the catalyst. The strength of the basic sites on the indi-
vidual HMS and that supported with CoFe2O4 MNPs are shown
in Fig. 8. The peaks in the high- and low-temperature regions
can be ascribed to the desorption of CO2 from the strong and
weak basic sites, respectively. Based on the strength of the
various basic sites, these desorption peaks can be classied into
three groups exhibiting weak (CO2 desorption between 20 ꢀC
and 160 ꢀC), medium (CO2 desorption between 160 ꢀC and 400
ꢀC), and strong basicity (CO2 desorption higher than 400 C).45
ꢀ
The TPD prole indicates the presence of three desorption
peaks at <400 ꢀC (a at 80 ꢀC, b at 160 ꢀC, and g at 250 ꢀC). The
peaks between 20 ꢀC and 160 ꢀC can be attributed to the
interaction of CO2 with the weak basic sites, corresponding to
the hydroxyl groups on the surface. The second group between
Fig. 9 Hysteresis curves of CoFe2O4-HMS and CoFe2O4 MNPs at
Fig. 8 CO2-TPD profiles of HMS and CoFe2O4-HMS.
23214 | RSC Adv., 2021, 11, 23207–23220
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The decrease in the saturation magnetisation can be attrib-
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(DTG). The thermal decomposition prole of the chemical co-
precipitated as-synthesized CoFe2O4-HMS was achieved using
TGA-DTA-DTG, as exhibited in Fig. 11. The catalyst was entirely
uted to the non-collinear spin arrangement at or near the
surface of the particles. We observed that the saturation mag-
netisation for the smaller crystals has a signicantly lower value
for the ferrite impregnated in HMS than for the bare cobalt
ferrite. The values of Ms for the samples are attributed to the
greater fraction of surface spin glasslike state with a smaller net
moment. The hysteresis curve reveals an increment in the
coercivity value for CoFe2O4-HMS compared to that of the bare
CoFe2O4 magnetic nanoparticles. This increment may be due to
the decrease in the particle size with less magnetization. The
decrease in magnetization and increase in coercivity for smaller
particles is attributed to a decrease in domain wall motion and
different anisotropies of the surface and bulk material.39,41 The
trends for the various magnetic parameters are summarized in
Table 5.
2.5.7. Surface area and pore size distribution. The porous
structure of the synthesized CoFe2O4-HMS nanocomposite was
analyzed via N2 adsorption–desorption isotherms using the
multipoint BET method in the relative pressure (P/P0) range of
0.05–1.0 and DFT method to the analyze pore size distribution.
A type IV isotherm with a hysteresis curve was obtained from the
nitrogen adsorption–desorption isotherm, together with the
pore size distribution, BET surface area and t-plot analysis, as
depicted in Fig. 10A–D, respectively, which indicates the char-
acteristic feature of a mesoporous material. The isotherms
indicate that the samples possess cylindrical pores with an
opening at both ends.
The BET surface area of CoFe2O4-HMS was found to be 75 m2
gꢁ1. The samples exhibited mesoporosity with a pore volume of
0.4207 cm3 gꢁ1. The experimental point of the t-plot, as shown
in Fig. 10D is consistent with the Harkins and Jura isotherm
equation.46 It is evident from the plot that the experimental data
points fall in a straight line for t ¼ 0.17–0.30 nm (linear portion
of the curve). Thus, the hydromagnesite sheets impregnated
with cobalt–ferrite magnetic nanoparticles were found to be
porous in nature, given that the t-plot did not pass through the
origin. The tted linear line exhibits a positive intercept, which
further substantiates the presence of mesopores in nano-
composites.46–48 The values of the surface area, pore diameter,
and pore volume obtained from the above-mentioned studies
indicate that CoFe2O4-HMS possessed a wide mesopore size
distribution with the maximum centred at 10.50 nm.
2.5.8. Thermogravimetric analyses (TGA), differential
thermal analyses (DTA), and derivative thermogravimetric
Table 5 The magnetization values of the ferrites and their composites
with HMS
Saturation
Retentivity
magnetization
Mr
Coercivity
Hc (kOe)
S. no. Composition Ms (emu gꢁ1
)
(emu gꢁ1
)
1
2
CoFe2O4-HMS 20.16
9.32
13.57
0.27
0.21
CoFe2O4
MNPs
44.55
Fig. 10 (A) Nitrogen adsorption isotherm pattern of CoFe2O4-HMS,
(B) pore size distribution, (C) BET surface area and (D) t-plot analysis.
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PerkinElmer FTIR SP 10 STD. Thin-layer chromatography (TLC)
was performed using precoated aluminum sheets with Silica
Gel 60 F254. The Brunauer–Emmett–Teller (BET) surface area
analysis was carried out using a QuantachromeAutosorb iQ2
TPX gas sorption system equipped with highly accurate pres-
sure transducers and a thermostatic bath. The pore size distri-
bution was calculated using density functional theory (DFT).
Raman spectra were recorded on a Micro Raman microscope
(Jobin Yvon Horibra LABRAM-HR visible 800) equipped with
a 532 nm Nd:YAG solid state laser (10 mW). The PXRD
measurements were carried out on a Bruker D8 Advance X-ray
diffractometer using Cu Ka radiation with a wavelength of
0.154 nm. Field-emission scanning electron microscopy (FE-
SEM) images and the corresponding electron dispersive X-ray
analysis (EDAX) data were recorded using a Jeol microscope
(JSM7100F). The surface basic property was determined
through the temperature-programmed desorption of CO2 (CO2-
TPD) using a Chemisorb 2750 (Micromeritics) instrument.
Thermogravimetric analysis-differential thermal analysis (TGA-
DTA) was performed on a Hitachi (STA 7300) thermal analysis
instrument under an air ow in the temperature range of 25 ꢀC
to 1000 ꢀC at a heating rate of 10 ꢀC minꢁ1. Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis was
Fig. 11 TGA, DTA and DTG patterns for CoFe2O4-HMS.
decomposed in two stages of the weight loss process the
ꢀ
ꢀ
temperature range of 25 C to 1000 C.
The rst stage occurred in the temperature range of 100–
160 ꢀC, which is attributed to the dehydration of crystalline
adsorbed water on the catalyst surface with a weight loss of
17.80%, corresponding to the very feeble endothermic peak at
130–170 ꢀC in the DTA (green line) curve. Besides, there was
a further weight loss of about 36.70%, which is associated with
the evolution of CO2, in the temperature range of 170 ꢀC to
250 ꢀC. The second endothermic peak at around 250–340 ꢀC
corresponds to the decomposition of amorphous hydro-
carbonates, M2CO3(OH)2, which were generated at high
temperature before the production of metal oxide. Conse-
quently, the total weight loss over the CoFe2O4-HMs magnetic
nanocatalyst in both steps was determined to be 54.50%. The
DTA curve indicates that the decomposition took place faster
between 100 ꢀC to 340 ꢀC and slowly until the maximum
temperature of 1000 ꢀC. Therefore, given that the sample weight
remained constant aer 1000 ꢀC, this temperature is considered
a suitable calcination temperature for CoFe2O4-HMS in this
trans-esterication reaction.25,49
performed on
a PerkinElmer, Optima 2000. Magnetic
measurements were carried out on a vibrating sample magne-
tometer (VSM, JDM-13) at room temperature. A 2100F high-
resolution transmission electronic microscope (HR-TEM)
operating at 200 kV was used to examine the grain size and
morphology of the nanosheets.25,26
3.1. Preparation of hydromagnesite sheets (HMS)
The hydromagnesite sheets were synthesized according to the
procedure available in the literature.22 In a typical procedure,
120 mL of aqueous 1 M Na2CO3 was slowly added from a burette
to a 500 mL round-bottomed ask containing 200 mL of
aqueous 1 M MgCl2$6H2O solution under vigorous stirring at
room temperature to afford a white precipitate.
3. Experimental
3.2. Preparation of hydromagnesite sheets impregnated
with cobalt-ferrite magnetic nanoparticles (CoFe2O4-HMS)
All chemicals and solvents were purchased from Sigma Aldrich
and Merck India. The reactions were performed in an aerobic The synthesis of hydromagnesite sheets impregnated with
atmosphere without any specic precautions. Melting points cobalt–ferrite magnetic nanoparticles was executed via the
were determined in open capillary tubes on a Veego melting- chemical co-precipitation method according to a literature
point apparatus and are uncorrected. The ultrasound- protocol.19,25 The acquired HMS (0.16 g) were dispersed in 40 mL
promoted reactions were performed using a Cole Parmer of distilled water and sonicated at room temperature for 30 min.
ultrasonic processor Model CPX 130, with a maximum power of A homogenous dispersion of CoCl2$6H2O (1.19 g), FeCl3$6H2O
130 W, operating at an amplitude of 60% and frequency of 20 (2.70 g) and 10 mL of distilled water was added to the solution of
kHz. All products were characterized using spectroscopic data HMS. Aerward, 25 mL of 3 mol Lꢁ1 NaOH solution was slowly
1
1
(FTIR, H and 13C NMR). The H and 13C NMR spectra of the added to the above-mentioned solution under vigorous stirring.
synthesized compounds were recorded at 400 and 100 MHz The mixture was stirred continuously at 100 ꢀC for one hour.
respectively, using a Bruker Advance II 400 NMR spectrometer The obtained black solid of CoFe2O4-HMS was magnetically
in chloroform (CDCl3) solvent, and the chemical shis (d) are separated, washed with water and ethanol until the pH reached
ꢀ
expressed in parts per million. Spin multiplicities are described 7.0, and dried under vacuum at 60 C for 24 h. The structural,
as s (singlet), d (doublet), t (triplet), q (quartet), and m (multi- morphological and magnetic properties of HMS, CoFe2O4 MNPs
plet). Fourier transform infrared (FTIR) spectra were recorded and CoFe2O4-HMS were characterized via PXRD (Fig. 2), FE-SEM
as ATR spectra in the range of 4000–650 cmꢁ1 using a Frontier (Fig. 3), EDAX analysis (Fig. 4), HR-TEM (Fig. 5), FTIR analysis
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(Fig. 6), Raman analysis (Fig. 7), CO2-TPD (Fig. 8), magnetic
studies (Fig. 9), nitrogen BET adsorption–desorption pattern
(Fig. 10), and TGA-DTA-DTG pattern (Fig. 11).
3.3.3. 4-(3-Benzylimidazo[1,2-a]pyridin-2-yl)phenol
(4c).
Compound (4c) was prepared in 84% yield from 2-amino-
pyridine (1 mmol) (1a), 4-hydroylbenzaldehyde (1 mmol) (2c),
and phenyl acetylene (1 mmol) (3); white solid; M.P.: 137–
139 ꢀC; IR (ATR, n, cmꢁ1): 3449 (O–H), 2989 (C–Hasym, sp3), 2880
(C–Hsym, sp3), 1634 (C]C/C]N), 1613, 1482, 1453 (C/C ring
3.3. General procedure for the ultrasonic-assisted synthesis
of imidazo[1,2-a]pyridines derivatives (4a–j)
1
str.), 1087 (C–O); H NMR (400 MHz, CDCl3): dH ppm 4.52 (s,
A mixture of 2-aminopyridine derivatives (1 mmol) (1a–b),
pertinent aryl aldehyde (1 mmol) (2a–e), phenyl acetylene (1
mmol) (3) and a catalytic amount (7 mol% or 0.07 g) of CoFe2O4-
HMS in PEG 400 (5 mL) was irradiated employing an ultra-
sonicator at room temperature for the optimal time (25–40
min). Considering that the reaction time was very short, there
was no substantial increase in temperature due to ultrasonic
shock. The ultrasonic apparatus used showed the temperature
automatically, and thus the temperature was controlled and
xed at room temperature using a water circulator in the case of
any temperature increase.20 The progress of the reaction was
monitored through TLC (on aluminum sheets precoated with
silica) using n-hexane/ethyl acetate (4 : 1) as the eluting system.
Aer the reaction was complete, the mixture was diluted with
ethyl acetate (2 mL). The organic layer was washed with water
and dried over MgSO4 and concentrated under vacuum. The
catalyst was separated via a magnetic bar to obtain pure prod-
ucts (4a–j) (Table 2).25
2H), 6.28 (s, 1H), 6.92–6.95 (m, 1H), 7.07–7.12 (m, 2H), 7.17–7.21
(m, 2H), 7.23–7.26 (m, 1H), 7.27–7.30 (m, 3H), 7.31–7.38 (m,
2H), 7.69–7.73 (m, 2H); 13C NMR (100 MHz, CDCl3): d 29.9 (C-
16), 112.5 (C-9), 116.5 (C-12, 14), 117.4 (C-7), 117.8 (C-6), 123.6
(C-8), 125.5 (C-10), 127.1 (C-20), 127.2 (C-5), 129.1 (C-19, 21),
129.3 (C-11, 15), 130.0 (C-18, 22), 136.8 (C-17), 144.1 (C-4), 144.8
(C-2), 158.6 (C-13); HRMS (ESI) m/z: 300.1256 [M + H]+; Anal.
Cald. for C20H16N2O: C, 79.98; H, 5.37; N, 9.33%. Found C,
80.02; H, 5.41; N, 9.38%.
3.3.4. 3-Benzyl-2-(2-nitrophenyl)imidazo[1,2-a]pyridine
(4d). Compound (4d) was prepared in 82% yield from 2-ami-
nopyridine (1 mmol) (1a), 2-nitrobenzaldehyde (1 mmol) (2d),
and ꢀphenyl acetylene (1 mmol) (3); white solid; M.P.: 166–
167 C; IR (ATR, n, cmꢁ1): 2948 (C–Hasym, sp3), 2916 (C–Hsym
,
sp3), 1634 (C]C/C]N), 1543 (N–O), 1611, 1481, 1453 (C/C
ring str.); 1H NMR (400 MHz, CDCl3): dH ppm 4.51 (s, 2H, CH2),
6.91–6.93 (m, 1H), 7.16–7.19 (m, 2H), 7.21–7.24 (m, 1H), 7.27–
7.31 (m, 3H), 7.67–7.71 (m, 2H), 7.78–7.81 (m, 1H), 7.93–7.96
(m, 1H), 8.08–8.11 (m, 2H); 13C NMR (100 MHz, CDCl3): d 29.9
(C-16), 112.6 (C-9), 117.4 (C-7), 117.8 (C-6), 123.6 (C-8), 124.5 (C-
12), 124.9 (C-5), 125.3 (C-10), 127.1 (C-20), 128.5 (C-19, 21), 128.9
(C-18, 22), 129.7 (C-13), 132.7 (C-15), 135.4 (C-14), 136.8 (C-17),
144.2 (C-4), 144.9 (C-2), 148.9 (C-11); HRMS (ESI) m/z:
329.1156 [M + H]+; Anal. Cald. for C20H15N3O2: C, 72.94; H,
4.59; N, 12.76%. Found: C, 72.97; H, 4.62; N, 12.79%.
The analytical and spectroscopic data for the synthesized
imidazo[1,2-a]pyridines derivatives (4a–j) are given below:
3.3.1. 3-Benzyl-2-phenylimidazo[1,2-a]pyridine
(4a).
Compound (4a) was prepared in 89% yield from 2-amino-
pydrine (1 mmol) (1a), benzaldehyde (1 mmol) (2a), phenyl-
ꢀ
acetylene (1 mmol) (3); white solid; M.P.: 121–122 C; IR (ATR,
n, cmꢁ1): 2942 (C–Hasym, sp3), 2872 (C–Hsym, sp3), 1637 (C]C/
C]N), 1611, 1480, 1434 (C/C ring str.); 1H NMR (400 MHz,
CDCl3): dH ppm 4.53 (s, 2H, CH2), 6.92–6.96 (m, 1H), 7.17–7.22
(m, 2H), 7.23–7.26 (m, 1H), 7.29–7.40 (m, 4H), 7.43–7.48 (m,
2H), 7.70–7.73 (m, 2H), 7.80–7.83 (m, 2H); 13C NMR (100 MHz,
CDCl3): d 29.9 (C-16), 112.5 (C-9), 117.5 (C-7), 117.8 (C-6), 123.5
(C-8), 124.6 (C-5), 127.0 (C-13, 20), 127.8 (C-11, 15), 128.4 (C-19,
21), 128.8 (C-18, 22), 129.2 (C-12, 14), 134.3 (C-10), 136.8 (C-17),
144.0 (C-4), 145.0 (C-2); HRMS (ESI) m/z: 284.1306 [M + H]+;
Anal. Cald. for C20H16N2: C, 84.48; H, 5.67; N, 9.85%. Found: C,
84.49; H, 5.69; N, 9.89%.
3.3.5. 4-(3-Benzylimidazo[1,2-a]pyridin-2-yl)-3-
methoxyphenol (4e). Compound (4e) was prepared in 78% yield
from 2-amino pyridine (1 mmol) (1a), 4-hydroxy-2-
methoxybenzaldehyde (1 mmol) (2e), and phenyl acetylene (1
mmol) (3); white solid; M.P.: 164–165 ꢀC; IR (ATR, n, cmꢁ1): 3470
(O–H), 2942 (C–Hasym, sp3), 2912 (C–Hsym, sp3), 1636 (C]C/C]
N), 1613, 1483, 1439 (C/C ring str.), 1287, 1082 (C–O); 1H NMR
(400 MHz, CDCl3): dH ppm 3.94 (s, 3H, OCH3), 4.53 (s, 2H, CH2),
6.30–6.74 (s, 2H), 6.96–6.99 (m, 1H), 7.07–7.11 (m, 1H), 7.17–
7.20 (m, 2H), 7.21–7.23 (m, 1H), 7.25–7.34 (m, 3H), 7.38–7.41
(m, 2H), 7.71–7.74 (m, 1H); 13C NMR (100 MHz, CDCl3): d 29.9
(C-16), 56.1 (C-110), 106.2 (C-12), 108.8 (C-14), 112.6 (C-9), 115.6
(C-10), 117.4 (C-7), 117.9 (C-6), 124.3 (C-8), 126.0 (C-20), 127.8
(C-5), 129.1 (C-19, 21), 130.1 (C-18, 22), 132.0 (C-15), 137.0 (C-
17), 144.1 (C-4), 144.8 (C-2), 154.8 (C-13), 158.7 (C-11); HRMS
(ESI) m/z: 330.1354 [M + H]+; Anal. Cald. for C21H18N2O2: C,
76.34; H, 5.49; N, 8.48%. Found C, 76.37; H, 5.51; N, 8.52%.
3.3.6. 3-Benzyl-7-methyl-2-phenylimidazo[1,2-a]pyridine
(4f). Compound (4f) was prepared in 91% yield from 2-amino-4-
methyl pyridine (1 mmol) (1b), benzaldehyde (1 mmol) (2a), and
phenyl acetylene (1 mmol) (3); white solid; M.P.: 160–162 ꢀC; IR
(ATR, n, cmꢁ1): 2956 (C–Hasym, sp3), 2928 (C–Hsym, sp3), 1634
3.3.2. 3-Benzyl-2-(4-chlorophenyl)imidazo[1,2-a]pyridine
(4b). Compound (4b) was prepared in 86% yield from 2-ami-
nopyridine(1 mmol) (1a), 4-chlorobenzaldehyde (1 mmol) (2b),
phenylacetylene (1 mmol) (3); white solid; M.P.: 144–145 ꢀC; IR
(ATR, n, cmꢁ1): 2953 (C–Hasym, sp3), 2928 (C–Hsym, sp3), 1634
1
(C]C/C]N), 1613, 1481, 1436 (C/C ring str.), 698 (C–Cl); H
NMR (400 MHz, CDCl3): dH ppm 4.58 (s, 2H, CH2), 6.91–6.94 (m,
1H), 7.16–7.21 (m, 2H), 7.22–7.26 (m, 1H), 7.27–7.29 (m, 3H),
7.30–7.38 (m, 2H), 7.69–7.73 (m, 4H); 13C NMR (100 MHz,
CDCl3): d 29.9 (C-16), 112.5 (C-9), 117.4 (C-7), 117.6 (C-6), 123.5
(C-8), 127.2 (C-20), 127.2 (C-5), 128.6 (C-19, 21), 129.1 (C-11, 15),
129.2 (C-18, 22), 129.5 (C-12, 14), 129.8 (C-10), 134.4 (C-13),
136.6 (C-17), 144.0 (C-4), 144.8 (C-2); HRMS (ESI) m/z:
318.0915 [M + H]+; Anal. Cald. for C20H15ClN2: C, 75.35; H,
4.74; N, 8.79%. Found: C, 75.38; H, 4.77; N, 8.82%.
1
(C]C/C]N), 1612, 1487, 1449 (C/C ring str.); H NMR (400
MHz, CDCl3): dH ppm 2.47 (s, 3H, CH3), 4.57 (s, 2H, CH2), 7.16–
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7.19 (m, 1H), 7.29–7.32 (m, 1H), 7.34–7.40 (m, 3H), 7.43–7.48 for C21H17N3O2: C, 73.45; H, 4.99; N, 12.24%. Found: C, 73.48;
(m, 2H), 7.52 (s, 1H), 7.70–7.73 (m, 3H), 7.81–7.84 (m, 2H); 13
NMR (100 MHz, CDCl3): d 21.4 (C-80), 29.9 (C-16), 116.9 (C-9),
C
H, 5.02; N, 12.29%.
3.3.10. 4-(3-Benzyl-7-methylimidazo[1,2-a]pyridin-2-yl)-3-
117.5 (C-7), 122.1 (C-6), 124.9 (C-5), 125.7 (C-20), 127.7 (C-11, methoxyphenol (4j). Compound (4j) was prepared in 87% yield
15), 128.7 (C-19, 21), 128.8 (C-13), 128.9 (C-18, 22), 129.3 (C- from 2-amino-4-methyl pyridine (1 mmol) (1b), 4-hydroxy-2-
12, 14), 134.1 (C-10), 136.9 (C-17), 139.8 (C-8), 144.1 (C-4), methoxybenzaldehyde (1 mmol) (2e), and phenyl acetylene (1
144.8 (C-2); HRMS (ESI) m/z: 298.1462 [M + H]+; Anal. Cald. mmol) (3); white solid; M.P.: 167–168 ꢀC; IR (ATR, n, cmꢁ1): 3436
for C21H18N2: C, 84.53; H, 6.08; N, 9.39%. Found: C, 84.56; H, (O–H), 2945 (C–Hasym, sp3), 2928 (C–Hsym, sp3), 1635 (C]C/C]
6.11; N, 9.44%.
N), 1613, 1488, 1437 (C/C ring str.), 1126, 1085 (C–O); 1H NMR
(400 MHz, CDCl3): dH ppm 2.47 (s, 3H, CH3), 3.95 (s, 3H, OCH3),
3.3.7. 3-Benzyl-2-(4-chlorophenyl)-7-methylimidazo[1,2-a]
pyridine (4g). Compound (4g) was prepared in 94% yield from 2- 4.52 (s, 2H, CH2), 6.30–6.73 (s, 2H), 7.08–7.13 (m, 1H), 7.17–7.19
amino-4-methyl pyridine (1 mmol) (1b), 4-chlorobenzaldehyde (m, 1H), 7.24–7.29 (m, 1H), 7.34–7.40 (m, 3H), 7.43–7.48 (m,
(1 mmol) (2b), and phenyl acetylene (1 mmol) (3); white 2H), 7.56 (s, 1H), 7.73–7.78 (m, 1H); 13C NMR (100 MHz, CDCl3):
solid; M.P.: 148–149 ꢀC; IR (ATR, n, cmꢁ1): 2941 (C–Hasym, sp3), d 21.5 (C-80), 29.9 (C-16), 56.2 (C-110), 106.0 (C-12), 108.9 (C-14),
2917 (C–Hsym, sp3), 1633 (C]C/C]N), 1612, 1482, 1438 (C/C 112.6 (C-9), 115.6 (C-10), 117.5 (C-7), 122.2 (C-6), 126.1 (C-20),
ring str.), 686 (C–Cl); 1H NMR (400 MHz, CDCl3): dH ppm 2.48 (s, 128.0 (C-5), 128.8 (C-19, 21), 130.2 (C-18, 22), 132.9 (C-15),
3H, CH3), 4.55 (s, 2H, CH2), 7.17–7.19 (m, 1H), 7.28–7.31 (m, 137.0 (C-17), 139.9 (C-8), 144.2 (C-4), 144.8 (C-2), 154.9 (C-13),
1H), 7.33–7.41 (m, 3H), 7.43–7.49 (m, 2H), 7.53 (s, 1H), 7.71–7.76 158.8 (C-11); HRMS (ESI) m/z: 344.1519 [M + H]+; Anal. Cald.
13
(m, 4H); C NMR (100 MHz, CDCl3): d 21.4 (C-80), 29.9 (C-16), for C22H20N2O24: C, 76.72; H, 5.85; N, 8.13%. Found: C, 76.75; H,
116.9 (C-9), 117.5 (C-7), 122.2 (C-6), 124.9 (C-5), 125.8 (C-20), 5.89; N, 8.17%.
128.7 (C-19, 21), 129.1 (C-11, 15), 129.2 (C-18, 22), 129.4 (C-12,
14), 131.2 (C-10), 134.4 (C-13), 137.0 (C-17), 139.8 (C-8), 144.2
3.4. General procedure for the ultrasonic-assisted synthesis
of 2-benzyl-3-(4-chlorophenyl)-7-methylimidazo[1,2-a]
pyridine (4g) (with maximum yield) on a gram-scale level
(C-4), 144.9 (C-2); HRMS (ESI) m/z: 332.1068 [M + H]+; Anal.
Cald. for C21H17ClN2: C, 75.78; H, 5.15; N, 8.42%. Found: C,
75.81; H, 5.18; N, 8.46%.
3.3.8. 4-(3-Benzyl-7-methylimidazo[1,2-a]pyridin-2-yl)
A mixture of 2-amino-4-methyl pyridine (10 mmol) (1b), 4-
phenol (4h). Compound (4h) was prepared in 88% yield from 2- chlorobenzaldehyde (10 mmol) (2b), phenyl acetylene (10
amino-4-methyl pyridine (1 mmol) (1b), 4-hydroxybenzaldehyde mmol) (3) and a catalytic amount (7 mol% or 0.07 g) of CoFe2O4-
(1 mmol) (2c), and phenyl acetylene (1 mmol) (3); white HMS in PEG 400 (50 mL) was irradiated using an ultrasonicator
solid; M.P.: 172–173 ꢀC; IR (ATR, n, cmꢁ1): 3442 (O–H), 2987 (C– at room temperature for the optimum time (25 min). The
H
asym, sp3), 2884 (C–Hsym, sp3), 1635 (C]C/C]N), 1613, 1483, progress of the reaction was monitored through TLC (on
1
1451 (C/C ring str.), 1089 (C–O); H NMR (400 MHz, CDCl3): aluminum sheets precoated with silica) using n-hexane/ethyl
dH ppm 2.47 (s, 3H, CH3), 4.53 (s, 2H, CH2), 6.29 (s, 1H), 7.06– acetate (4 : 1) as the eluting system. Aer the completion of
7.11 (m, 2H), 7.16–7.19 (m, 1H), 7.21–7.25 (m, 1H), 7.29–7.34 the reaction, the mixture was diluted with ethyl acetate (20 mL).
(m, 3H), 7.43–7.48 (m, 2H), 7.51 (s, 1H), 7.71–7.74 (m, 2H); 13
C
The organic layer was washed with water and dried over MgSO4
NMR (100 MHz, CDCl3): d 21.4 (C-80), 29.9 (C-16), 116.6 (C-12, and concentrated under vacuum. The catalyst was separated
14), 116.9 (C-9), 117.5 (C-7), 122.3 (C-6), 125.8 (C-10), 127.2 (C- using a magnetic bar to give the anticipated product (4g) in 90%
20), 127.2 (C-5), 129.2 (C-19, 21), 129.2 (C-11, 15), 130.1 (C-18, yield.25
22), 136.9 (C-17), 139.8 (C-8), 144.2 (C-4), 144.9 (C-2), 158.6 (C-
13); HRMS (ESI) m/z: 314.1406 [M + H]+; Anal. Cald. for
3.5. Recovery of CoFe2O4-HMS magnetic nanocatalyst
C
21H18N2O: C, 80.23; H, 5.77; N, 8.91%. Found: C, 80.26; H,
5.79; N, 8.94%.
To evaluate the reusability and recovery of the magnetic nano-
catalyst, the reaction of 2-amino-4-methyl pyridine (1 mmol)
3.3.9. 3-Benzyl-7-methyl-2-(2-nitrophenyl)imidazo[1,2-a]
pyridine (4i). Compound (4i) was prepared in 90% yield from 2- (1b), 4-chlorobenzaldehyde (1 mmol) (2b) and phenyl acetylene
amino-4-methyl pyridine (1 mmol) (1b), 2-nitrobenzaldehyde (1 (1 mmol) (3) with a catalytic amount (7 mol% or 0.07 g) of
mmol) (2d), and phenyl acetylene (1 mmol) (3); white CoFe2O4-HMS in PEG 400 (5 mL) was selected as the model
solid; M.P.: 191–193 ꢀC; IR (ATR, n, cmꢁ1): 2943 (C–Hasym, sp3), reaction to acquire the desired product (4g). The reaction
2926 (C–Hsym, sp3), 1638 (C]C/C]N), 1537 (N–O), 1611, 1487, mixture was ultrasonically irradiated at room temperature for
1
1448 (C/C ring str.); H NMR (400 MHz, CDCl3): dH ppm 2.46 25 min. The progress of the reaction was monitored through
(s, 3H, CH3), 4.56 (s, 2H, CH2), 7.17–7.19 (m, 1H), 7.21–7.24 (m, TLC (on aluminum sheets precoated with silica) using n-
1H), 7.28–7.33 (m, 3H), 7.59 (s, 1H), 7.68–7.71 (m, 2H), 7.76–7.80 hexane/ethyl acetate (4 : 1) as the eluting system. According to
(m, 1H), 7.92–7.95 (m, 1H), 8.08–8.10 (m, 2H); 13C NMR (100 the knowledge from the literature,25,36 aer the completion of
MHz, CDCl3): d 21.4 (C-80), 29.9 (C-16), 117.5 (C-7), 117.9 (C-9), the reaction, the catalyst was separated from the reaction
122.2 (C-6), 124.6 (C-12), 125.0 (C-5), 125.3 (C-10), 127.1 (C-20), mixture by using an external magnet. The isolated catalyst was
128.7 (C-19, 21), 128.9 (C-18, 22), 129.8 (C-13), 132.8 (C-15), washed with dichloromethane (DCM) and dried for 3 h at 60 ꢀC.
135.5 (C-14), 136.8 (C-17), 139.7 (C-8), 144.2 (C-4), 145.0 (C-2), The magnetically recovered nanocatalyst was further subjected
149.0 (C-11); HRMS (ESI) m/z: 343.1314 [M + H]+; Anal. Cald. to the model reaction and this process was repeated for up to
23218 | RSC Adv., 2021, 11, 23207–23220
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ve runs under similar conditions to afford the product (4g) in
94%, 93%, 90%, 89%, and 87% yield.
6 L. Almirante, L. Polo, A. Mugnaini, E. Provinciali, P. Rugarli,
A. Biancotti, A. Gamba and W. Murmann, J. Med. Chem.,
1965, 8, 305–312.
7 A. K. Bagdi, S. Santra, K. Monir and A. Hajra, Chem.
Commun., 2015, 51, 1555–1575.
4. Conclusions
8 N. Nagarajan, G. Velmurugan, A. Prakash, N. Shakti,
M. Katiyar, P. Venuvanalingam and R. Renganathan,
Chem.–Asian J., 2014, 9, 294–304.
9 J. Y. Lee, J. Y. Shim, H. K. Kim, D. Ko, M. H. Baik and
E. J. Yoo, J. Org. Chem., 2017, 82, 4352–4361.
10 Y. Prostota, O. D. Kachkovsky, L. V. Reis and P. F. Santos,
Dyes Pigm., 2013, 96, 554–562.
11 Z. T. Bhutia, D. Das, A. Chatterjee and M. Banerjee, ACS
Omega, 2019, 4, 4481–4490.
12 M. Tajbakhsh, M. Farhang, R. Hosseinzadeh and Y. Sarra,
RSC Adv., 2014, 4, 23116–23124.
An efficient ultrasonic-assisted and environmentally sustain-
able methodology for the synthesis of pharmacologically
signicant imidazo[1,2-a]pyridine scaffolds through a domino
A3-coupling strategy catalyzed by hydromagnesite sheets
impregnated with cobalt-ferrite magnetic nanoparticles was
developed using green media. The role of CoFe2O4-HMS as
a heterogeneous catalyst was envisaged to intensify the viability
and yield of the products. This novel approach affords several
advantages for achieving sustainable chemistry and opens up
a new scope to advance the catalytic potential of hydro-
magnesite sheets impregnated with cobalt-ferrite magnetic
nanoparticles (CoFe2O4-HMS) for the synthesis of biologically
active synthetic equivalents. The high environmental compati-
bility and sustainability factors including a high eco-scale score,
small E-factor, and considerable atom economy and carbon
efficiency place this protocol under the umbrella of green
chemistry principles. The reaction protocol is also feasible for
multi-gram scale, which presents an economically affordable
methodology on a large scale.
13 U. Balijapalli and S. K. Iyer, Dyes Pigm., 2015, 121, 88–98.
14 A. Maleki, Helv. Chim. Acta, 2014, 97, 587–593.
15 E.
Allahabadi,
S.
Ebrahimi,
M.
Soheilizad,
M. Khoshneviszadeh and M. Mahdavi, Tetrahedron Lett.,
2017, 58, 121–124.
16 C. Zong, R. Zeng and J. Zou, Chem. Res. Chin. Univ., 2014, 30,
632–638.
17 T. R. Mandlimatha and K. I. Sathiyanarayanan, RSC Adv.,
2016, 6, 3117–3125.
18 G. Purohit, A. Kharkwal and D. S. Rawat, ACS Sustainable
Chem. Eng., 2020, 8, 5544–5557.
19 M. Zhang, J. Lu, J. N. Zhang and Z. H. Zhang, Catal.
Commun., 2016, 78, 26–32.
Conflicts of interest
The authors declare no conicts of interest.
20 S. Sadjadi and M. Eskandari, Ultrason. Sonochem., 2013, 20,
640–643.
21 A. Maleki and M. Aghaie, Ultrason. Sonochem., 2017, 39, 534–
539.
22 U. C. Rajesh, U. Gulati and D. S. Rawat, ACS Sustainable
Chem. Eng., 2016, 4, 3409–3419.
23 M. Akao, F. Marumo and S. Iwai, Acta Crystallogr., 1974, 30,
2670–2672.
24 S. Verma, R. B. N. Baig, C. Han, M. N. Nadagouda and
R. S. Varma, Chem. Commun., 2015, 51, 15554–15557.
25 D. Geedkar, A. Kumar and P. Sharma, J. Heterocycl. Chem.,
2020, 57, 4331–4347.
26 D. Geedkar, A. Kumar, G. K. Reen and P. Sharma, J.
Heterocycl. Chem., 2020, 57, 1963–1973.
Acknowledgements
The authors thank MPCST-JRF (File no. A/R&D/RP-2/Phy&Engg./
2017-18/269 Dated 31/03/18) for the fellowship to D. Geedkar.
The authors wish to thank SAP (no. F.540/13/DRS-I/2016 (SAP-I)
dated 7 Nov., 2016) UGC, New Delhi research grant to the
department for developing SIC facilities. We would like to thank
UGC-DAE CSR, Indore for their great support in providing HR-
TEM, VSM and Raman analyses facilities.
References
1 C. E. Gueiffier and A. Gueiffier, Mini-Rev. Med. Chem., 2007,
7, 888–899.
27 R. A. Sheldon, Pure Appl. Chem., 2000, 72, 1233–1246.
2 A. Kamal, J. S. Reddy, M. J. Ramaiah, D. Dastagiri, 28 D. J. C. Constable, A. D. Curzons and V. L. Cunningham,
E. V. Bharathi, M. V. PremSagar, S. N. C. V. L. Pushpavalli, Green Chem., 2002, 4, 521–527.
P. Ray and M. P. Bhadra, Med. Chem. Commun., 2010, 1, 29 K. V. Aken, L. Strekowski and L. Patiny, Beilstein J. Org.
355–360. Chem., 2006, 2, 1–7.
3 S. C. Goodacre, L. J. Street, D. J. Hallett, J. M. Crawforth, 30 R. A. Sheldon, Chem. Commun., 2008, 39, 3352–3365.
S. Kelly, A. P. Owens, W. P. Blackaby, R. T. Lewis, 31 O. Ebenezer, P. Awolade, N. Koorbanally and P. Singh, Chem.
J. Stanley, A. J. Smith, P. Ferris, B. Sohal, S. M. Cook,
Biol. Drug Des., 2020, 95, 162–173.
A. Pike, N. Brown, K. A. Wafford, G. Marshall, J. L. Castro 32 J. B. Bharate, S. K. Guru, S. K. Jain, S. Meena, P. P. Singh,
and J. R. Atack, J. Med. Chem., 2006, 49, 35–38.
4 S. S. Kataev, B. Y. Syropyatov, N. E. Gavrilova and
V. V. Zalesov, Pharm. Chem. J., 2004, 38, 181–182.
5 Y. Rival, G. Grassy, A. Taudou and R. Ecalle, Eur. J. Med.
Chem., 1991, 26, 13–18.
S.
Bhushan,
B.
Singh,
S.
B.
Bharate
and
R. A. Vishwakarma, RSC Adv., 2013, 3, 20869–20876.
33 N. Chernyak and V. Gevorgyan, Angew. Chem., Int. Ed., 2010,
49, 2743–2746.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23207–23220 | 23219

View Article Online
RSC Advances
Paper
34 B. V. S. Reddy, P. S. Reddy, Y. J. Reddy and J. S. Yadav, 42 Z. Wang, R. T. Downs, V. Pischedda, R. Shetty, S. K. Saxena,
Tetrahedron Lett., 2011, 52, 5789–5793.
35 Y. Volkova and V. Gevorgyan, Chem. Heterocycl. Compd.,
2017, 53, 409–412.
36 N. Basavegowda, K. Mishra and Y. R. Lee, Mater. Technol.,
2019, 34, 558–569.
C. S. Zha, Y. S. Zhao, D. Schiferl and A. Waskowska, Phys.
Rev. B, 2003, 68, 941011–941016.
43 P. Chandramohan, M. P. Srinivasan, S. Velmurugan and
S. V. Narasimhan, J. Solid State Chem., 2011, 184, 89–96.
44 V. Bartunek, D. Sedmidubsky, S. Huber, M. Svecova,
P. Ulbrich and O. Jankovsky, Materials, 2018, 11, 1241–1252.
37 B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction
Boston, Prentice-Hall, Addison-Wesley, MA, USA, 3rd edn, 45 J. Hu, K. Zhu, L. Chen, C. Kubel and R. Richards, J. Phys.
2001. Chem. C, 2007, 32, 12038–12044.
38 T. Selvamani, T. Yagyu, S. Kawasaki and I. Mukhopadhyay, 46 W. D. Harkins and G. Jura, J. Am. Chem. Soc., 1944, 66, 1366–
Catal. Commun., 2010, 11, 537–541. 1372.
39 S. Iikhara, M. F. Warsia, S. Haiderb, S. Musaddiqc, 47 C. Santhosh, P. Kollu, S. Felix, S. K. Jeong and A. N. Grace,
I. Shakird and M. Shahid, Ceram. Int., 2019, 45, 21150–
21161.
40 S. Qu, J. Wang, J. L. Kong, P. Y. Yang and G. Chen, Talanta,
2007, 71, 1096–1102.
RSC Adv., 2015, 5, 28965–28972.
48 S. Chella, P. Kollu, E. V. P. R. Komarala, S. Doshi, M. Saranya,
S. Felix, R. Ramachandrana, P. Saravanand, V. L. Konerue,
V. Venugopal, S. K. Jeongf and A. N. Grace, Appl. Surf. Sci.,
2015, 327, 27–36.
41 P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy and
C. Muthamizhchelvan, Appl. Surf. Sci., 2012, 258, 6648–6652. 49 N. F. Sulaiman, W. A. W. A. Bakar, S. Toemen, N. M. Kamal
and R. Nadarajan, Renewable Energy, 2019, 135, 408–416.
23220 | RSC Adv., 2021, 11, 23207–23220
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