Design and preparation of ZnS-ZnFe2O4: a green and efficient hybrid nanocatalyst for the multicomponent synthesis of 2,4,5-triaryl-1H-imidazoles

Article abstract of DOI:10.1002/aoc.5008

In the present work, a new protocol was introduced for the preparation of an efficient hybrid nanocatalyst ZnS-ZnFe₂O₄ via the co-precipitation method as well as its application in the synthesis of 2,4,5-triaryl-1H-imidazoles derivatives starting from various aromatic aldehydes, benzil and ammonium acetate under ultrasonic irradiation in ethanol. ZnS-ZnFe₂O₄ was characterized by Fourier transform infrared (FT-IR) spectroscopy, energy-dispersive X-ray spectroscopy (EDS) analysis, scanning electron microscopy (SEM) image, X-ray diffraction (XRD) pattern and vibrating sample magnetometer (VSM) curve. This method has advantages such as high efficiency of the heterogeneous catalyst, the use of environmentally-friendly solvent, high yields, short reaction times and easy isolation of the products and chromatography-free purification. Our outcomes illustrated that the present nanocatalyst with nearly spherical and Cauliflower-like morphology and average particle size of 36?nm could be applied as an effective and magnetically recyclable catalyst without any significant decreasing of activity. Furthermore, the synergic effect of bimetallic Lewis acids was studied for the synthesis of imidazole derivatives.

Full text of DOI:10.1002/aoc.5008

Received: 17 February 2019
DOI: 10.1002/aoc.5008
Revised: 30 April 2019
Accepted: 1 May 2019
F U L L P A P E R
Design and preparation of ZnS‐ZnFe2O4: a green and
efficient hybrid nanocatalyst for the multicomponent
synthesis of 2,4,5‐triaryl‐1H‐imidazoles
Zahra Varzi | Ali Maleki
Catalysts and Organic Synthesis Research
In the present work, a new protocol was introduced for the preparation of an
efficient hybrid nanocatalyst ZnS‐ZnFe2O4 via the co‐precipitation method as
well as its application in the synthesis of 2,4,5‐triaryl‐1H‐imidazoles derivatives
starting from various aromatic aldehydes, benzil and ammonium acetate under
ultrasonic irradiation in ethanol. ZnS‐ZnFe2O4 was characterized by Fourier
transform infrared (FT‐IR) spectroscopy, energy‐dispersive X‐ray spectroscopy
(EDS) analysis, scanning electron microscopy (SEM) image, X‐ray diffraction
(XRD) pattern and vibrating sample magnetometer (VSM) curve. This method
has advantages such as high efficiency of the heterogeneous catalyst, the use of
environmentally‐friendly solvent, high yields, short reaction times and easy
isolation of the products and chromatography‐free purification. Our outcomes
illustrated that the present nanocatalyst with nearly spherical and Cauliflower‐
like morphology and average particle size of 36 nm could be applied as an
effective and magnetically recyclable catalyst without any significant decreas-
ing of activity. Furthermore, the synergic effect of bimetallic Lewis acids was
studied for the synthesis of imidazole derivatives.
Laboratory, Department of Chemistry,
Iran University of Science and
Technology, Tehran 16846‐13114, Iran
Correspondence
Ali Maleki, Catalysts and Organic
Synthesis Research Laboratory,
Department of Chemistry, Iran University
of Science and Technology, Tehran 16846‐
13114, Iran.
Email: maleki@iust.ac.ir
KEYWORDS
green chemistry, heterogeneous nanocatalyst, imidazole, recyclable, three‐component reaction
1 | INTRODUCTION
various organic reactions as effective photocatalysts.[6]
ZnFe2O4 is an important MTMO semiconductor with a
In recent years, mixed transition metal oxides (MTMOs),
compounded by two simple TMOs, have attracted much
attentions both in academic and industrial researches.
Categories of MTMOs include MMn2O4 (M = Zn, Ni and
Co), MCo2O4 (M = Mn, Ni, Zn and Fe) and MFe2O4
(M = Co, Ni, Mn and Zn).[1] Due to considerable magnetic
properties of spinel‐type complexes of MFe2O4 has widely
been used in various fields such as magnetic resonance
imaging (MRI), biomolecular sensors,[2] electronic stor-
age, frequency core, data storage and drug delivery.[3–5]
Spinel nanocrystalline zinc ferrites have been applied in
narrow band gap.[7] In cubic ferrites like MFe2O4, in which
M is a metal cation, the structural oxygen is located in an
FCC compact network and the M+2 and Fe+3 are located
in empty hollow tetrahedral and octahedral. Zinc ferrite
has widely been used in various fields including magnetic
fluids for the storage information,[8] photocatalysis[9] and
MRI.[10] ZnS nanoparticles are excellent semiconductors
with an energy level of 3.8 ev and have two crystalline
structures: hexagonal wurtzite and cubic zinc blend. In
the cubic structure of ZnS, there are three axes of x, y
and z in cubic crystal structure.[11] ZnS nanoparticles have
Appl Organometal Chem. 2019;e5008.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5008

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VARZI AND MALEKI
been analyzed for their superior performance as a catalyst.
The enhancement of efficient techniques for the synthesis
of complex molecules is one of the essential assignment
for organic chemists. In this regard, the multicomponent
reactions (MCRs) are potentially important approaches.
Although, the synthesis of imidazole core as a major com-
ponent of many drugs is of prime importance. In addition,
they have various medicinal and biological activities like
antibacterial,[12] antifungal,[13] reducing inflammation,[14]
catalysts, harsh isolation and purification. Synthesis under
mild conditions using a suitable, non‐toxic, clean and
environmentally‐friendly and recoverable reagents and
catalysts is one of the important goals of green chemistry.
As a result, finding an efficient way to prepare these com-
pounds is important.[23] In this work, ZnS‐ZnFe2O4
MTMO as a magnetic recyclable nanocatalyst was pre-
pared and characterized. The results showed that ZnS‐
ZnFe2O4 MTMO has not only better catalytic performance
in comparison with both ZnS and ZnFe2O4, but also, could
be recovered simply by an external magnet and reused
several times in organic reactions. Therefore, the co‐
precipitation method was applied to control the morphol-
ogy of the ZnS‐ZnFe2O4 MTMO. The present investigation
has various superiority in the preparation of ZnS‐ZnFe2O4
MTMO nanocatalyst to exhibit enhanced nanocatalytic
activity and easy magnetic separation via the co‐
precipitation method[24] as well as environmentally‐
friendly conditions for the synthesis of diverse 2,4,5‐
triaryl‐1H‐imidazoles via MCRs under ultrasonic irradia-
tion in ethanol at 70 °C (Scheme 1).
anti‐cancer[15]
antiviral,[16]
anti‐diabetic,[17]
anti‐
allergic,[18] analgesic[19] and herbicidal.[20] Therefore, in
this work, 2,4,5‐triaryl‐1H‐imidazole derivatives were syn-
thesized via one‐pot and inexpensive method in the pres-
ence of ZnS‐ZnFe2O4 MTMO.[21] Disadvantages of
previous methods of ZnFe2O4 preparation include the
need for complex equipment, high temperature and
energy consumption, low efficiency, isolation problems,
as well as long reaction times to complete crystallization
of ferrite nanostructures.[22] On the other hand, many
of the methods for the synthesis of imidazoles had restric-
tions like low yields, long reaction times, expensive
SCHEME 1 Preparation of ZnS‐ZnFe2O4 and synthesis of 2,4,5‐triaryl‐1H‐imidazoles in the presence of ZnS‐ZnFe2O4 MTMO.

VARZI AND MALEKI
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TABLE 1 Synthesis of 2,4,5‐triaryl‐1H‐imidazoles by ZnS‐ZnFe2O4 MTMO under ultrasonic irradiation conditionsa
M.p. (°C)
Time
(min)
Yieldb
(%)
Entry
Product
Found/Reported
1
60
50
25
68
62
85
197–199/196–198[25]
2
3
229–231/230–231[26]
229–231/230–231[20]
4
5
15
32
95
70
258–260/259–262[27]
275–277/273–275[28]
6
44
35
30
20
15
75
92
86
88
95
257–259/257–258[29]
264–266/265–266[26]
230–231/230–231[20]
199–201/200–201[29]
259–260/258–260[28]
7
8
9
10
11
45
51
298–299/296–299[30]
(Continues)

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VARZI AND MALEKI
TABLE 1 (Continued)
M.p. (°C)
Time
(min)
Yieldb
(%)
Entry
Product
Found/Reported
12
13
45
38
60
66
266/264–265[26]
210–211/210–212[31]
aReaction conditions: benzil (1 mmol), aldehyde (1 mmol), ammonium acetate (4 mmol), EtOH (1 ml) and ZnS‐ZnFe2O4 (20 mol%).
bIsolated yields.
analytical data with valid samples. Ultrasonic irradiation
were carried out in Elmasonic S60H.
2 | EXPERIMENTAL SECTION
2.1 | Materials and instruments
All commercially available solvents and chemicals were
provided from Aldrich and Merck. Melting points of
the samples were measured by an Electrothermal 9100
apparatus. The FT‐IR spectra were recorded using a
2.2 | Preparation of the ZnS‐ZnFe2O4
MTMO hybrid nanocatalyst
Zn (NO3)2·6H2O (0.3 g, 1 mmol), Zn (OAc)2·2H2O (0.2 g,
1 mmol) and thioacetamide (0.1 g, 1.6 mmol) were added
to 10 ml of deionized water solution including Fe
(NO3)3·9H2O (0.8 g, 2 mmol) during 30 min. Then, the
resulting mixture was sonicated. After that, 10 ml of an
aqueous solution of NH4OH was added dropwise into
the mixture. Eventually, the retrieved precipitate was
extracted by an external magnet and washed thoroughly
with deionized water and ethanol for two times. Finally,
the precipitate was dried at room temperature for 10 hr
to provide ZnS‐ZnFe2O4 MTMO.
1
Shimadzu IR‐470 spectrometer by using KBr pellet. H
NMR and 13C NMR spectra were recorded on a Bruker
DRX‐500 Avance spectrometer at 500 and 125 MHz.
VSM curves were measured by LBKFB. SEM images were
obtained by the MIRA3 TESCAN instrument. Elemental
analysis was performed by using EDS analysis that were
obtained by SAMx. The XRD pattern was taken with
the Panalytical X' Pert Pro diffractometer with Cu Kα
radiation (40 kV, 40 mA) from 20° to 80° (2θ). All prod-
ucts were identified by comparing of their spectral and
FIGURE 1 FT‐IR spectra of: (a) ZnFe2O4 and (b) ZnS‐ZnFe2O4 MTMO

VARZI AND MALEKI
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(ppm); 2.39 (s, 3H, CH3), 7.24–7.73 (m, 12H, Ar‐H),
7.77–7.79 (d, 2H, Ar‐H), 9.64 (s, 1H, N‐H); 13C NMR
(CDCl3): δC (ppm); 146.6, 139.2, 130.0, 129.0, 128.2,
127.5, 125.6, 21.8.
2.3 | General procedure for the
preparation of 2,4,5‐triaryl‐1H‐imidazole
derivatives
At first, 4‐chlorobenzaldehyde (0.14 g, 1 mmol), ammo-
nium acetate (0.3 g, 4 mmol), benzil (0.21 g, 1 mmol) and
ZnS‐ZnFe2O4 nanocatalyst (0.02 g) were stirred at 70 °C
in ethanol. The reaction was monitored by thin layer
chromatography. After completion of the reaction, the
precipitate was washed with deionized water and dried.
Pure precipitates were obtained by re‐crystallization by
EtOH. In all cases, the melting points of the products were
measured to be in accordance with reference samples
(Table 1).
2.4.2 | 2‐(4‐Chlorophenyl)‐4,5‐diphenyl‐
1H‐imidazole (4d)
Mp: 258–260 °C; Yield: 95%; FT‐IR (KBr, cm−1): 1448,
1
1485, 1602, 3060, 3272; H NMR (DMSO‐d6): δH (ppm);
7.21–7.56 (m, 12H, Ar‐H), 8.11 (d, 2H, Ar‐H), 12.79 (s,
1H, N‐H); 13C NMR (DMSO‐d6): δC (ppm); 144.3, 137.2,
134.9, 132.5, 130.81, 129.1, 128.7, 128.5, 128.4, 128.3,
128.1, 127.0, 126.5, 126.2.
2.4 | Chemical characterization data of
the selected products
3 | RESULTS AND DISCUSSION
The performance and synthesis of the ZnS‐ZnFe2O4
MTMO were accomplished for the first time by proce-
dures described in this literature. For preparation, in this
work, first of all, we presented a simple method for the
synthesis of the ZnS‐ZnFe2O4 MTMO as bimetallic Lewis
2.4.1 | 2‐(4‐Methylphenyl)‐4,5‐diphenyl‐
1H‐imidazole (4 h)
MP: 230–231 °C; Yield: 86%; FT‐IR (KBr, cm−1): 696, 767,
1
825, 1446, 1496, 1600, 3033, 3380; H NMR (CDCl3): δH
FIGURE 2 (a) SEM images and (b) particle size distribution diagram of the ZnS‐ZnFe2O4 MTMO

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VARZI AND MALEKI
acid that was characterized by VSM, FT‐IR, SEM and
EDS analysis. In fact, we examined the catalytic activity
of the ZnS‐ZnFe2O4 MTMO in 2,4,5‐triaryl‐1H‐imidaz-
oles. In this report, it was revealed that the catalytic
efficiency of the ZnS‐ZnFe2O4 for the MCRs of 2,4,5‐
triaryl‐1H‐imidazole using benzil 1, aromatic aldehyde 2
and ammonium acetate 3 under ultrasonic irradiation in
ethanol at 70 °C.
4 | CHARACTERIZATION OF THE
ZNS‐ZNFE2O4 HYBRID
NANOCATALYST
4.1 | FT‐IR spectra
FT‐IR spectral analysis was used to examine and iden-
tify the functional groups. The results obtained in the
FIGURE 3 (I) EDS diagram, and (II) elemental mapping images for (a) O, (b) S, (c) Fe and (d) Zn of the ZnS‐ZnFe2O4 MTMO

VARZI AND MALEKI
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FT‐IR spectrum was used to verify the synthesis of the
ZnS‐ZnFe2O4 MTMO and to confirm the ZnS and
ZnFe2O4 bond formations (Figure 1). According to the
interactions between cation and oxygen in the tetrahe-
dral and octahedral positions; in the spinel structure,
two vibrational frequencies of ZnFe2O4 were shown at
665 cm−1, 450 cm−1. The absorption band at 665 cm−1
related to vibration of the tetrahedral position (Zn–O
bond) and at 450 cm−1 is demonstrated with Fe3+ vibra-
tion frequencies at the octahedral positions (Fe–O
bond).[32] The major absorption band of ZnS was
observed at 966 cm−1. As a result, they confirm
stretching and bending absorption bands of the vibra-
tions of the Zn‐S. The intense absorption in the area
between 3300–3700 cm−1 is related to the O‐H
stretching vibrations of water molecules, with its bend-
FIGURE 4 VSM curve of the (a) ZnS‐ZnFe2O4 MTMO and (b)
ZnFe2O4
ing vibration in the area of 1627 cm−1 [33]
The absorp-
.
tion bands of 1350 and 1386 cm−1 relating to the
interactions between ZnFe2O4 and ZnS in ZnS‐ZnFe2O4.
4.2 | SEM analysis
SEM images of the synthesized nanocatalyst were
shown in Figure 2a. The structure of the nanocatalyst
was studied by this method and showed nearly spherical
or Cauliflower‐like morphology.[34] The approximately
uniform particle size distribution was below 50 nm
(Figure 2b).
4.3 | EDS analysis
Elemental composition of ZnS‐ZnFe2O4 was identified by
EDS analysis. As shown in Figure 3, iron, zinc, oxygen
and sulfur are present elements in the nanocatalyst. In
addition, the presence of all these elements in the
recycled nanocatalyst confirmed its reusability.
4.4 | VSM magnetization curve
Magnetic properties of the ZnS‐ZnFe2O4 MTMO were
recorded by using a vibrating sample magnetometer curve
(Figure 4). Due to the fact that the catalyst has a
supramagnetic property that allows it to be separated from
the solution by the use of external magnets. Magnetic sat-
urations of ZnS‐ZnFe2O4 and ZnFe2O4 were 52/9 and 66/9
electromagnetic units per gram (emu g−1), respectively. In
fact, the ZnS content in the ZnS‐ZnFe2O4 structure has
reduced its magnetic saturation value in comparison with
ZnFe2O4. However, this MTMO with an external mag-
netic force can be easily separated from the reaction
mixture.
FIGURE 5 XRD pattern of the ZnS‐ZnFe2O4 MTMO, ZnS and
ZnFe2O4

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VARZI AND MALEKI
of the catalyst was studied (Table 2). To optimize the reac-
tion, a mixture of aldehyde (1 mmol), benzil (1 mmol),
ammonium acetate (4 mmol), and 2 mg of MTMO catalyst
was sonicated at 70 °C for an appropriate time. To find the
optimal conditions for the reaction, in the absence of any
catalyst, the reaction yield was very low (entries 1–4). Max-
imum yield was gained (95%) when the reactions were
used with 2 mg of the catalyst (entry 14). The higher
amounts the catalyst loading did not lead to substantial
change in the yield and time of the reaction. The reaction
was investigated the effect of solvents such as EtOH, THF,
CH3CN and H2O. But, none of the solvents were found to
be more effective than ethanol. Thus, ethanol was used as
an appropriate solvent for the reaction at 70 °C under
ultrasonic irradiation (entries 13–16). A model reaction
was also carried out with ZnS, ZnFe2O4 and ZnS‐ZnFe2O4.
These results approved that ZnS‐ZnFe2O4 MTMO was
more suitable for this reaction.
4.5 | XRD pattern analysis
Figure 5 shows the XRD pattern of the ZnS‐ZnFe2O4.
In this figure, a comparison was made between the stan-
dard XRD patterns of ZnFe2O4, ZnS and the prepared
nanocatalyst. As shown in the patterns, ZnS nanoparti-
cles were confirmed with major peaks at 2θ = 29, 48°
and 57° by the standard diffraction card no. JCPDS,
05–0566. The XRD pattern of the ZnFe2O4 nanoparticles
defined according to the standard diffraction card no.
JCPDS, 22–1012 and major peaks at 2θ = 30°, 36°, 43°,
57° and 63°. ZnS and ZnFe2O4 have a cubic crystalline
structure based on Xpert software. The XRD pattern of
the ZnS‐ZnFe2O4 MTMO indicates major peaks at
2θ = 27°, 30°, 36°, 43°, 57° and 63° that confirms the pres-
ence of ZnS and ZnFe2O4 in the crystalline structure of
the nanocatalyst.
4.6 | Application of the ZnS‐ZnFe2O4
MTMO hybrid nanocatalyst for the
synthesis of 2,4,5‐triaryl‐1H‐imidazoles
4.7 | Comparison of the catalyst activity
ZnS‐ZnFe2O4 MTMO can be applied as an efficient
nanocatalyst in MCRs. As shown in Table 3, ZnS‐ZnFe2-
O4 showed superior activity in terms of time, temperature
and even the efficiency in comparison with other
reported catalysts such as Montmorilonite K10, Yb
(OPf)3 and Zr (acac)4.
ZnS‐ZnFe2O4 MTMO was predictable to be more effective
than ZnS and ZnFe2O4. In this regard and for a closer
research on the application of this nanocatalyst, in a model
reaction between benzil (1 mmol), 4‐chlorobenzaldehyde
(1 mmol) and ammonium acetate (4 mmol), the effect
TABLE 2 Optimization of the model reaction conditionsa
Entry
Catalyst
Catalyst loading (g)
Solvent
Temperature (°C)
Time (min)
Yieldb (%)
1
‐
‐
Solvent‐free
EtOH
EtOH
H2O
r.t.
290
270
220
250
155
140
95
‐
Trace
25
2
‐
‐
r.t.
3
‐
‐
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
Ultrasonic/30
Ultrasonic/50
Reflux/80
4
‐
‐
Trace
45
5
ZnS
0.02
0.02
0.02
0.02
0.02
0.03
0.03
0.03
0.03
0.02
0.01
0.005
EtOH
EtOH
EtOH
EtOH
EtOH
H2O
6
ZnFe2O4
40
7
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
ZnS‐ZnFe2O4
60
8
80
75
9
160
85
30
10
11
12
13
14
15
16
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
Ultrasonic/70
55
CH3CN
THF
120
145
50
46
40
EtOH
EtOH
EtOH
EtOH
68
15
95
70
65
78
60
aReaction conditions: 4‐chlorobenzaldehyde (1 mmol), benzil (1 mmol), ammonium acetate (4 mmol), EtOH (1 ml) and ZnS‐ZnFe2O4.
bIsolated yields.

VARZI AND MALEKI
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TABLE 3 Comparison of various catalysts with ZnS‐ZnFe2O4
MTMO in the synthesis of 2‐(4‐chlorophenyl)‐4,5‐diphenyl‐1H‐
imidazole (4d)
benzaldehyde. Then, ammonia as a nitrogen source
arisen by ammonium acetate attacks to the activated car-
bonyl to form imine by removing an H2O molecule. Next,
intermediate I was formed from the nucleophilic attack
of ammonia to imine. ZnS‐ZnFe2O4 activates carbonyls
of benzil to be reacted by I to result the formation of a
cyclic intermediate II. Finally, by removing of an H2O
molecule and in order to reach the aromatic heterocyclic
ring, the product of 2,4,5‐triaryl‐1H‐imidazoles was
provided.
Solvent/
conditions
Time Yield
(min) (%)
Entry Catalyst
[Lit.]
[35]
1
Montmorilonite EtOH/reflux
K10
95
75
[36]
2
Yb (OPf)3
Perflurodecalin/ 360
80 °C
83
[37]
3
4
Zr (acac)4
Ultrasonic/r.t.
35
15
91
95
ZnS‐ZnFe2O4
Ultrasonic/
70 °C
This
work
4.9 | Reusability of the ZnS‐ZnFe2O4
MTMO hybrid nanocatalyst
The recovered catalyst from the reaction mixture was
reused six times for subsequent reactions without signifi-
cant decrease of activity (Figure 6). As a result, a reusable,
eco‐friendly and efficient Lewis acid catalyst has been
successfully used for one‐pot synthesis of imidazole deriv-
atives. In addition, the simplicity of the preparation and
performance of the present nanocatalyst make this proto-
col as an effective and attractive approach for the future
works and large scale industrial production.[38–45]
4.8 | Proposed mechanism for the
synthesis of 2,4,5‐triaryl‐1H‐imidazoles in
the presence of ZnS‐ZnFe2O4 MTMO
The plausible mechanism for the synthesis of 2,4,5‐
triaryl‐1H‐imidazoles via MCR in the presence of ZnS‐
ZnFe2O4 MTMO is shown in Scheme 2. According to
the mechanism, the ZnS‐ZnFe2O4 MTMO as a Lewis acid,
interacts with the oxygen of the carbonyl group of
SCHEME 2 Proposed mechanism for the synthesis of 2,4,5‐triaryl‐1H‐imidazoles by using ZnS‐ZnFe2O4 MTMO.

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VARZI AND MALEKI
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Ali Maleki https://orcid.org/0000-0001-5490-3350

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How to cite this article: Varzi Z, Maleki A.
Design and preparation of ZnS‐ZnFe2O4: a green
and efficient hybrid nanocatalyst for the
multicomponent synthesis of 2,4,5‐triaryl‐1H‐
imidazoles. Appl Organometal Chem. 2019;e5008.
https://doi.org/10.1002/aoc.5008
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