Tungsten oxides: green and sustainable heterogeneous nanocatalysts for the synthesis of bioactive heterocyclic compounds

Article abstract of DOI:10.1039/d0dt04238b

Tungsten oxide (WO₃) as an efficient heterogeneous catalyst was preparedviaa simple hydrothermal route for the synthesis of a wide range of bioactive heterocyclic compounds. The present investigation deals with the rapid and low-cost synthesis

Full text of DOI:10.1039/d0dt04238b

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Tungsten oxides: green and sustainable
heterogeneous nanocatalysts for the synthesis of
bioactive heterocyclic compounds†
Cite this: Dalton Trans., 2021, 50,
2032
T. A. J. Siddiqui,a Shoyebmohamad F. Shaikh, *b Balaji B. Totawar,c
b
Madhuri Dumpala,d Mohd Ubaidullah, b Badr M. Thamer,
b
Rajaram S. Mane *a and Abdullah M. Al-Enizi
Tungsten oxide (WO3) as an efficient heterogeneous catalyst was prepared via a simple hydrothermal
route for the synthesis of a wide range of bioactive heterocyclic compounds. The present investigation
deals with the rapid and low-cost synthesis of C-3-alkylated 4-hydroxycoumarin, chromene, and
xanthene derivatives. WO3 nanorods (NRs) are successfully envisaged to catalyze desired transformations,
demonstrating the wide range of their potential applications in catalysis. Synthetic transformation details,
smallest catalytic amounts, excellent product yields, and plausible reaction mechanisms for the formation
of these heterocyclic scaffolds are elicidated. As-prepared WO3 NRs are characterized to confirm their
structural, chemical, and morphological parameters by X-ray diffraction, X-ray photoelectron spec-
troscopy, and field emission scanning electron microscopy measurements, respectively. We discuss the
factors that govern the formation of products, and the active role of WO3 NRs, which are essential for the
activation of substrates in the present study of thermal conditions. Herein, detailed synthesis and spectro-
scopic information of the prepared compounds are reported.
Received 13th December 2020,
Accepted 2nd January 2021
DOI: 10.1039/d0dt04238b
rsc.li/dalton
chemistry compared with homogeneous catalysts in terms of
exceptional thermal and photochemical stability. Besides, the
1. Introduction
In this era, sustainable green chemistry plays a significant role in active sites of homogeneous catalysts are chemically better
decreasing the cost of energy for transforming molecules and defined, and facilitate the determination of reaction mechanisms
modifying the reaction pathways for the distribution of the final by theoretical computation towards an efficient catalyst develop-
product in a chemical reaction. The progression in impetus ment. Furthermore, homogeneous catalysts of high costs are
advancements in catalyst technologies has been indispensable for difficult to extract and recycle for long-term use in the chemical
a sustainable modern society and greener planet. In this manner, synthesis process. The limitations of homogeneous catalysts turn
the structural design of efficient and robust artificial catalysts by out to be the strengths of heterogeneous catalysts in terms of low
utilizing green approaches that can copy the synergist emulates cost, reusability, environmentally friendly nature, easy separation
catalytic features to receive incredible awareness in scientific from reaction mixtures, and high industrial throughput; these
research.1 Presently, catalysts are divided into two categories; advantages of heterogeneous catalysts over homogeneous catalysts
homogeneous and heterogeneous catalysts, depending on their created awareness in the scientific community across the world.2,3
relationship to the phase of the reaction in which they are In the different fields of heterogeneous catalysis, catalysis by
involved. Heterogeneous catalysts have played a crucial role in metal oxides is one of the most important, as it covers the greater
part of processes and catalyst families used industrially.4,5
Therefore, metal oxides have emerged as an important class of
multifunctional materials with a rich collection of properties, and
their ability to function in harsh environments make them very
suitable materials as photo and heterogeneous catalysts. Recently,
numerous efforts have been made to address the issues that arise
during the preparation of metal nanoparticles by various green
methods to attain unique properties that arise from their size,
shape, and composition.6–9 Several studies have demonstrated the
use of various metal oxides such as TiO2,10 ZnO,11 and ZrO2,12
aSchool of Physical Sciences, Swami Ramanand Teerth Marathwada University,
Nanded-431606, M.S., India. E-mail: rajarammane70@srtmun.ac.in
bDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451,
Saudi Arabia. E-mail: sshaikh1@ksu.edu.sa
cDepartment of Chemistry, SMT. G. G. Khadse College, Muktainagar, Jalgaon, India
dDepartment of Physics, Osmania University College for Women, Koti, Hyderabad-
50095, India
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
D0DT04238B
2032 | Dalton Trans., 2021, 50, 2032–2041
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with different morphologies for the synthesis of bioactive hetero- synthesis process, 1 g ammonium paratungstate was mixed in
cyclic compounds including nanowires, nanotubes, nanofibers, 95 mL distilled water (Milli-Q water; 18.2 MΩ cm), followed by
core–shell structures, and hollow nanostructures. Among these, the addition of 3 mL concentrated HCl. The resulting solution
WO3 is a popular n-type semiconductor with promising electro- was stirred for an appropriate time that was dependent on the
chemical properties, a high surface area, a small pore-size distri- amount of HCl added. The solution initially became transparent
bution, and several exciting chemical and physical properties, and light yellow, and then turned yellow with the formation of a
which have made it a suitable material for various applications.13 gelatinous precipitate. Following this, 2 mL H2O2 was added and
To date, myriads of chemical and physical preparation methods the mixture was stirred vigorously for 1 h to dissolve tungstic
such as thermal evaporation, chemical vapor deposition, sputter- acid. This resulted in the formation of a stable, transparent solu-
ing, and colloidal suspension have been applied for the synthesis tion. The as-prepared solution was transferred to a Teflon-lined
of WO3 nanostructures of different phases and one-dimensional autoclave, filling 80% of the vessel. Hydrothermal synthesis was
morphologies.14 Moreover, WO3 possesses an outstanding bio- conducted at 160 °C for approximately 14 h. After processing, the
photocatalytic properties in addition to its non-hazardous nature, autoclave was cooled naturally to room temperature (∼2 h). The
which has engrossed investigations on its possible applications in resulting tungsten hydroxide powder was collected, rinsed with
nano–bio-technology. All reported synthesis methods could be de-ionized water, and annealed at 500 °C for 1 h (Scheme 1).
either expensive, or time-consuming, or tedious, with huge hazar-
dous wastes such as toxic byproducts. Hydrothermal synthesis
2.2 Characterization details
has received great attention because of its simplicity, cost-effec-
The surface morphology of WO3 was confirmed with FE-SEM
tiveness, soft chemical approach, low-temperature processing,
(Nova-SEM 200-FEI) digital silhouettes. The HR-TEM image was
and uniqueness in producing controlled shapes/sizes of various
recorded using an FEI TECNAI G2 20 STWIN instrument. The
nanostructures.
X-ray diffraction (XRD) pattern was used to investigate the crystal
phase (XRD-6000, X-ray diffractometer, Shimadzu) with a Cu-Kα
radiation tube. The X-ray photoelectron spectroscopy profile was
Al-Enizi et al. reported that anatase TiO2 catalysts playing a
vital role under solvent-free conditions exhibited remarkable
recyclability with up to five consecutive runs with negligible
acquired for determining the elemental composition using a PHI
reduction, which is superior to produce valuable C-3 alkylated
5000 Versa Probe (Ulvac-PHI). Moreover, the BET surface area and
4-hydroxycoumarins with 95% yield and high selectivity.15
pore-size distribution were measured using a Belsorp II, BET,
Bahador Karami reported the synthesis of xanthene derivatives
Japan Inc., instrument. UV-vis spectrum was recorded using a
by employing Fe3O4 nanoparticles as effective and magneti-
Shimadzu UV-2450, Varian Cary 5000 spectrophotometer in the
cally recoverable catalysts in water electrolysis.16 PbO nano-
wavelength range of 100–1000 nm. The chemical structure and
particles were also applied as heterogeneous catalysts under
phase were confirmed by the Raman microscopy (Renishaw, invia
solvent-free conditions for the synthesis of xanthenediones.17
Raman microscope, UK). The laser beam (λ = 532 nm) was
ZrO2 was also reported for the synthesis of xanthene deriva-
focused using a lens to produce a spot on the WO3 film surface.
tives as a recyclable catalyst.12
1H NMR and 13C NMR spectra of the product samples were
In the present investigation, considering the above-men-
recorded using a Varian 400 MHz instrument in CDCl3; tetra-
tioned advantages and scientific study of heterogeneous metal
methylsilane was used as an internal standard. The melting points
catalytic nanoparticles, green synthesis of WO3 NRs obtained by
of the samples were monitored by an open capillary method.
a hydrothermal method has been envisaged as heterogeneous
catalysts on probing the synthesis of a combinatorial library of
C-3 alkylated 4-hydroxycoumarin derivatives18,19 such as
warfarin,20–22 flocoumafen,23 and bromadialone24 (well-known
anticoagulants and rodenticides). Moreover, chromene and
xanthene derivatives have
a
wide range of biological
activities.25–28 Herein, the synthesis of these heterocyclic deriva-
tives using WO3 NRs as efficient green and sustainable hetero-
geneous catalysts to begin organic transformations is explored.
This novel process can be an example of exploring an environ-
mentally friendly, simple, and expedient synthetic route utilizing
WO3 NRs as heterogeneous catalysts under thermal conditions.
2. Experimental details
2.1 Synthesis of WO3 heterogeneous nanocatalysts
Analytical grade ammonium paratungstate ((NH4)10[H2W12O42]·
4H2O, 99%), hydrochloric acid (HCl, 37%), and hydrogen per-
oxide (H2O2, 30% w/w in water) were used as received. In a facile Scheme 1 Hydrothermal synthesis of WO3 NRs.
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Fig. 2a, the XRD pattern of the as-synthesized WO3 NRs reveals
all the specific diffraction peaks that are closely matched with a
= 0.7297 nm, b = 0.7539 nm, c = 0.7688 nm, and β = 90.91°,
3. Results and discussion
3.1 Surface morphology analysis
A delegate FE-SEM image demonstrates the consistently distribu- which is in good accordance with the standard (JCPDS card no.
ted surface morphology of WO3 nanorods. The side view (Fig. 1a) 43-1035) indexed to the monoclinic WO3 NRs crystal structure.
clearly reveals that the surfaces of WO3 NRs were smooth with No other diffraction peaks or impurity phases were observed,
roughly 100 nm width and 0.5 μm length. The HR-TEM image of indicating the phase purity of the product material.31 The major
the as-prepared WO3 NRs displays the single-crystalline character Raman bands obtained at 810 and 719 cm−1 attributed to the
with fine and regular lattice fringe spacings over the high-resolu- O–W–O stretching vibration and at 273 cm−1 attributed to the
tion scanned area (Fig. 1b). The distance between the adjacent O–W–O bending vibration of bridging oxygen, as shown in
lattice spacing measured from the HR-TEM image (inset of Fig. 2b, are characteristic of monoclinic WO3.32 Finally, the
Fig. 1b) is 0.35 nm, which is well indexed with the d-spacing of surface composition and chemical state of the product were
the [200] crystallographic plane of monoclinic WO3 NRs and con- analyzed by XPS measurement. The W 4f spectrum presents the
sistent with the X-ray diffraction results (vide infra).29 The sharp 4f7/2–4f5/2 spin–orbit doublet that corresponds to the W6+ oxi-
diffractions observed in the selected area energy diffraction dation state in WO3 with well-resolved doublet peaks at 4f7/2
(SAED) pattern (inset of Fig. 1b) reveal the crystalline nature of the (35.7 eV) and 4f5/2 (37.6 eV) (Fig. 2c).33 The XPS profile, as
WO3 NRs. The HR-TEM-appended EDAX spectrum endows both depicted in Fig. 2d, also presents a high-resolution O 1s region
W and O components comparably distributed throughout the with a peak at 530.1 eV, which is related to oxygen in WO3. The
product, confirming the synthesis of WO3 NRs. The Cu peaks two O 1s peaks exhibit the binding energies at 530.4 and 531.5
additionally showed up because of the copper-grade substrate uti- eV, which has a place with WO3 NRs structure, separately. The
lized during scanning (Fig. 1c). The absorbance edge increases peak at 530.4 eV may be because of the oxygen lattice and O2−
near 460 nm, evidencing a visible-light transparency signature. ions and the peak at 531.5 eV could be identified with the O2−
,
The bandgap energy Eg can be estimated by extrapolating the O−, and OH− particles in the oxygen-deficient region.34 The
linear portion of the curve near the onset of the absorption edge results of the thermo-gravimetric analysis/differential thermal
to the energy axis. From the Tauc plot for the WO3 NRs, the Eg is analysis (TGA/DTA) of WO3 NRs are adduced in Fig. S2.†
2.84 eV, which closely matches to a previously reported number
(inset of Fig. 1d).30
3.3 Surface area and pore-size distribution studies
Nitrogen adsorption–desorption isotherms were obtained to
determine the specific surface area and pore-size distribution
3.2 Structural elucidation
The phase purity and crystal structure of the WO3 NRs were of WO3 NRs (Fig. 3). The nitrogen adsorption–desorption
determined with an X-ray diffraction pattern. As shown in traces demonstrate a type IV isotherm at relative pressures
Fig. 1 (a) FE-SEM image, (b) HR-TEM image (inset: high-resolution image and SAED pattern), (c) HR-TEM elemental analysis, and (d) UV-Vis absorp-
tion spectrum (inset: Tauc plot) of the WO3 NRs.
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Fig. 2 WO3 NR characterization results with (a) XRD pattern, (b) Raman spectrum, and (c, d) XPS core profiles of W 4f and O 1s.
4. Catalyst activity test
4.1 Catalyst activity test for the synthesis of C-3 alkylation of
4-hydroxycoumarin derivatives
At the outset, we optimized general reaction conditions by
using an equimolar mixture of 4-hydroxycoumarin (1) and sec-
ondary benzyl alcohol (2) as model substrates (Scheme 2). In
line with our hypothesis, under the influence of WO3 NR
heterogeneous catalysts under solvent-free conditions, the
reaction of (1) and (2) gave the C-3 alkylated 4-hydroxycou-
marin derivatives as final products (3).
4.2 Screening of reaction conditions
The reactivity of WO3 NRs is the benchmark C-3 alkylation of
4-hydroxycoumarin 1. The catalyst was used due to its non-
hazardous nature and stability compared to other metal
oxides; different solvents were screened. We found remarkable
solvent effects, as acetic acid proved to be the suitable solvent
for obtaining good yields under reflux conditions for 1 h
Fig. 3 Adsorption–desorption isotherm (inset: pore size distribution)
determination for the WO3 NRs.
(P/P0) between 0.4 and 1.0, suggesting the presence of a
mesoporous structure.35 The specific surface area, total pore
volume, and average pore diameter of the WO3 NRs were
obtained using the Brunauer–Emmett–Teller (BET) and
Barrett–Joyner–Halenda (BJH) methods. The specific
surface area was 38.74 m2 g−1. The large surface area
and average pore diameter (10.76 nm) provide attractive
abrupt charge intercalation–deintercalation properties,
which should favour efficient heterogeneous catalytic
performance.36
Scheme 2 General reaction condition for the synthesis of C-3 alkyl-
ation of 4-hydroxycoumarin derivatives.
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Table 1 Optimization of reaction conditions for the synthesis of C-3
compared with WO3 NRs is provided in Table S1.† Details for
the synthesis of 3a and comparative study of WO3 NRs with
reported catalysts are provided in Table S2.† With the opti-
mized reaction conditions in hand (10 mol%, WO3, Neat), we
explore the substrate scope for the alkylation of 4-hydroxycou-
marin 1 using a variety of structurally divergent reactants and
novel WO3 as a heterogeneous solid acid catalyst in the catalyst
field. The obtained results are summarized in Table 2.
alkylation of 4-hydroxycoumarina
Cat.
(mol%)
Time
(h)
Yieldb
(%)
Entry
Solvent
Temp.
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
5
MeOH
EtOH
CH3CN
IPA
Toluene
AcOH
AcOH
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
1
1
1
1
3
2
5
1
1
0
0
0
0
55
85
65
85
95
4.3 Plausible reaction mechanism
20
10
AcOH
Solvent-free
The plausible reaction mechanism for C-3 alkylation of
4-hydroxycoumarin involves the removal of the water molecule
as a by-product via formation of the stabilized carbocations
(stabilized due to conjugation with the benzene ring) I to act
as the alkylating species, derived from alcohol in the presence
of WO3 NRs using its Brønsted acidic site (C-3 Alkylation) for
the formation of final product II (Scheme 3).
Heat (100 °C)
a Reaction conditions: 4-Hydroxycoumarin (100 mg), secondary benzyl
alcohol (1 equivalent), WO3 (10 mol%), solvent free, with continuous
stirring under thermal conditions. b Isolation yield.
(Table 1 entry 6), and then tested solvents such as MeOH,
EtOH, CH3CN, and IPA, which did not give target product 1
(Table 1 entries 1–4). Hence, further optimization using
toluene gave moderate yields, although it took a longer reac-
tion time (Table 1 entry 5). A catalytic amount of 10 mol%
WO3 was sufficient to afford the desired compound in good
product yields using acetic acid (Table 1 entry 6). No signifi-
cant improvements in yields were observed if the amount of
catalyst increased (Table 1 entry 8). When the reaction was
catalyzed by 5 mol%, the reaction time was protracted and the
desired product was obtained in only 65% yield (Table 1 entry
6). Finally, C-3 alkylation of 4-hydroxycoumarin was optimal to
afford the final product 3 under solvent-free conditions
(Table 1 entry 9).
4.4 Catalyst activity test for the synthesis of chromene
derivatives
Our synthesized WO3 NRs showed excellent catalytic behaviour
towards C-3 alkylation of 4-hydroxycoumarin derivatives,
which inspired us to discover their competency for the syn-
thesis of bioactive highly functionalized chromene derivatives.
We optimized the reaction conditions, i.e. equimolar dime-
done 4, aromatic aldehyde 5, and malononitrile 6 under
thermal conditions, which lead to the formation of functiona-
lized chromene derivative 7 (Scheme 4).
4.5 Screening of reaction conditions
Moreover, the detailed assessment of C-3 alkylation of
4-hydroxycoumarin with commercially available WO3 catalysts
The detailed screening of reaction conditions for the for-
mation of chromene derivatives is given in Table 3. To perform
a WO3 NR-mediated multicomponent reaction for the for-
mation of chromene derivatives, we surveyed the equimolar
(1 mmol scale) concentrations of dimedone, benzaldehyde,
and malononitrile under thermal conditions. The catalyst
Table 2 Expansion of substrate scope for C-3 alkylation of
4-hydroxycoumarina
Entry Alcohol (R1) Alcohol (R2) Product Time (h) Yieldb (%)
1
2
3
4
5
H
4-F
H
4-F
4-OMe
-CH3
-CH3
-C6H5
4-F-C6H5
-CH3
3a
3b
3c
3d
3e
1
2
1
2
95
80
93
85
92
2.5
a Reaction conditions: 4-Hydroxycoumarin (100 mg), secondary benzyl
alcohol (1 equivalent), WO3 NRs (10 mol%), with continuous stirring
under thermal conditions, solvent free. b Isolation yield.
Scheme 4 WO3 NR-catalyzed synthesis of chromene derivatives.
Scheme 3 Proposed mechanism of reaction for C-3 alkylation of 4-hydroxycoumarin.
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Table 3 Screening of reaction conditions for the synthesis of chro-
The detailed comparative study of known catalysts used for
the synthesis of chromene using WO3 is given in Table S3 of
ESI.† After optimizing the reaction conditions, we explored the
substrate scope of the thermally induced WO3 NRs catalysed in
a multicomponent reaction (MCR). In general, different alde-
hydes could be applied to provide the desired chromene
derivatives in excellent product yields (Table 4). A variety of
aldehydes containing a range of functionalities were tested.
Many useful functional groups were tolerated under reaction
conditions, providing simple opportunities for further derivati-
zation of the products. To summarize, this new protocol allows
the convenient, technically simple, and inexpensive synthesis
of various chromene derivatives and circumvents the need for
an expensive catalyst. A plausible mechanistic pathway for this
multicomponent reaction catalysed by WO3 NRs under
thermal conditions is proposed in Scheme S1.†
mene derivativesa
Catalyst
(mol%)
Temp.
(°C)
Time
(min)
Yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
0
0
5
Solvent-free
Solvent-free
Solvent-free
CH3OH
C2H5OH
CH3CN
H2O
IPA
CH2Cl2
Toluene
H2O/EtOH
Solvent-free
Solvent-free
Room temp.
70
70
180
180
30
40
30
90
120
30
30
30
0
Trace
50
70
85
Trace
40
Trace
Trace
65
95
97
10
10
10
10
10
10
10
10
10
20
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
70 °C
9
10
11
12
13
30
30
30
70 °C
97
a Reaction conditions: Dimedone (100 mg), benzaldehyde (1 equi-
valent), malononitrile (1 equivalent), WO3 NRs, different solvents with
continuous stirring at 70 °C. b Isolation yield.
4.6 Catalyst activity for the synthesis of 1,8-di-
oxooctahydroxanthenes
loadings were 0, 5, 10, and 20 mol% WO3 NRs, and solvent/
solvent-free conditions were applied. Initially, we optimized
the reactions at room temperature under thermal conditions
with different solvents as shown in Table 3. In the absence of
catalyst, solvent, heat energy no 7a product was observed
(Table 3 entries 1 and 2). However, upon addition of 5% WO3
NRs, 7a was obtained in 50% yield by heating at 70 °C under
solvent-free conditions (Table 3 entry 3). Initial results were
unsatisfactory, affording either low yields or numerous side
products that were produced with varying reaction parameters
(Table 3 entries 4–10). The best results in a solvent were
obtained with an ethanol/water mixture (Table 3 entry 11). In
terms of product selectivity, clean reactions were achieved in
the ethanol : water mixture (Table 3 entry 11) under solvent-
free conditions (Table 3 entry 12). Finally, to develop a method
in the context of green chemistry, we performed the reaction
under solvent-free conditions and obtained excellent yields of
7a (Table 3 entry 12). If the amount of catalyst is increased,
there is no difference observed in the percentage of product
yield (Table 3 entry 13).
The outstanding catalytic activity of the WO3 NRs for the syn-
thesis of chromene derivatives inspired us to discover their
competency for the synthesis of highly functionalized bioactive
1,8-di-oxo-octahydroxanthenes by the reaction of two equiva-
lents of 4 with aromatic aldehydes 5 (Scheme 5) to synthesize
xanthene derivative 8 under the reaction conditions similar to
those described for the chromene derivatives.
The list of different derivatives prepared by this method is
given in Table 5. The detailed comparative study of previously
Scheme 5 Synthesis of xanthene derivatives from WO3 NRs.
Table 4 Expansion of the substrate scope for the synthesis of chromenes using WO3 NRs as catalystsa
Entry
Aldehyde
Product
Time (min)
Yieldb (%)
MP (obs.) (°C)
MP (lit.) (°C)
1
2
3
4
5
6
7
8
9
C6H5-
4-OMe-C6H4
4-Br-C6H4
4-NO2-C6H4
3,4-Di-OMe-C6H3
3-OMe-C6H4
3,5-Di-Ome-C6H3
3-Br-C6H4
7a
7b
7c
7d
7e
7f
7g
7h
7i
30
45
25
30
60
90
120
30
90
130
97
92
95
95
90
92
85
90
80
86
233–234
202–204
209–210
181–182
229–230
197–198
233–234
230–231
208–209
235–237
232–23437
203–20537
208–21137
180–18237
129–23138
195–19738
—
229–23137
207–20937
—
4-F-C6H4
3,4,5-Di-OMe-C6H2
10
7j
a Reaction conditions: Dimedone (100 mg), benzaldehyde (1 equivalent), malononitrile (1 equivalent), WO3 NRs (10 mol%), H2O/EtOH solvent
system with continuous stirring at 70 °C. b Isolation yield.
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Table 5 Expansion of the substrate scope for the synthesis of xanthene derivatives using WO3 NRs as catalystsa
Entry
Aldehyde
Product
Time (min)
Yieldb (%)
MP (obs.) (°C)
MP (lit.) (°C)
1
2
3
4
5
6
7
8
9
2-Cl C6H4-
2,4 Di-Cl-C6H3
4-Cl-C6H4
4-Br-C6H4
3-NO2-C6H4
4-Me-C6H4
2,3,4-Tri-Ome-C6H2
4-NO2-C6H4
3,4,5-Tri-Ome-C6H2
3,4-Di-Ome-C6H3
8a
8b
8c
8d
8e
8f
8g
8h
8i
30
120
35
90
120
35
120
30
120
125
95
90
95
90
80
95
80
90
85
83
226–227
249–251
231–232
240–241
166–167
214–215
189–200
221–222
200–201
177–178
227–22839
250–25140
231–23239
240–24139
166–16739
216–21740
—
219–22039
187–18940
179–18141
10
8j
a Reaction conditions: Benzaldehyde (1 equivalent), dimedone (2 equivalent), WO3 NRs (10 mol%), H2O/EtOH solvent system with continuous
stirring at 70 °C. b Isolation yield.
reported catalysts for xanthene synthesis using WO3 is given in the formation of biologically potent molecules in the
the ESI as Table S4.† The structural properties are supported near future. Exploration of the biological activities of these
by both spectral data and melting points. Table 5 shows that, series of organic scaffolds is now under study in our
in all cases, the reactions proceeded well to afford the corres- laboratory.
ponding products with good to excellent yields. A detailed
mechanism of the formation of xanthene derivatives is pro-
posed in Scheme S2.†
6. Spectral data of 1HNMR and
13CNMR
4.7 Reusability test
6.1 1H-NMR of 4-hydroxy-3-(1-phenylethyl)-2H-chromen-2-
Having delineated the scope of this reaction, we probed the
recyclability of WO3 NRs to demonstrate inexpensive, readily
available, heterogeneous, and robust catalyst features.
Experimentally, only simple filtration was required to separate
the catalyst before its reuse without further purification/treat-
ment. The reusability study was carried out using the for-
one (3a)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.67–1.69 (d, 3H, J = 8
Hz), 4.71–4.77 (q, 1H, J = 8 Hz), 6.51 (bs, 1H), 7.21–7.42 (m,
9H).
13CNMR of 4-hydroxy-3-(1-phenylethyl)-2H-chromen-2-one
(3a). 13CNMR (CDCl3, 100 MHz): δH (ppm) = 16.51, 29.69,
30.92, 34.51, 110.09, 116.10, 116.38, 122.87, 123.82,
127.31, 127.72, 129.62, 131.79, 141.75, 152.51, 159.68, 163.41.,
260.14.
mation of chromene derivatives as
a model reaction
(Scheme 4). Gratifyingly, the catalyst was efficiently recycled
through five consecutive catalytic cycles without any loss of
selectivity or reactivity, demonstrating its mechanical robust-
ness and underscoring the efficiency of the WO3 NR as a
catalyst.
6.2 1H-NMR of 3-(1-(4-fluorophenyl)ethyl)-4-hydroxy-2H-
chromen-2-one (3b)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.66–1.68 (d, 3H, J = 8
Hz), 4.68–4.73 (q, 1H, J = 8 Hz), 6.61 (bs, 1H), 7.06–7.45 (m,
8H).
5. Conclusions
We have developed a facile, rapid, and highly efficient green
synthetic protocol for a series of pharmaceutically interesting
heterocycles (3, 7, and 8) under thermal conditions using WO3
NRs (10 mol%) as eco-friendly catalysts under solvent-free con-
ditions. The notable advantages of this protocol include the
use of commercially available low-cost starting materials, eco-
friendly reusable heterogeneous catalysts, energy efficiency,
short reaction time, operational simplicity, clean reaction pro-
files, ease of product isolation/purification without the aid of
tedious column chromatography, large-scale synthetic applica-
13CNMR
of
3-(1-(4-fluorophenyl)ethyl)-4-hydroxy-2H-
chromen-2-one (3b). 13CNMR (CDCl3, 100 MHz): δH (ppm) =
16.81, 33.78, 109.86, 116.00, 116.22, 116.64, 122.85, 123.93,
128.93, 129.00, 131.92, 137.66, 137.69, 152.50, 159.81, 160.71,
163.16, 163.35, 200.47.
6.3 1H-NMR of 3-benzhydryl-4-hydroxy-2H-chromen-2-one
(3c)
bility, and good-to-excellent yields, thereby satisfying several 1H-NMR (CDCl3, 400 MHz): δH (ppm) = 5.98 (s, 1H), 6.34 (s,
principles of green and sustainable chemistry. Furthermore, 1H), 7.23–7.75 (m, 4H).
the use of volatile and hazardous solvents was avoided. All
13CNMR of 3-benzhydryl-4-hydroxy-2H-chromen-2-one (3c).
the relevant green criteria of the process were considered 13CNMR (CDCl3, 100 MHz): δH (ppm) = 47.36, 107.76, 115.97,
and satisfactorily met. We hope that WO3 NRs and other 116.49, 123.21, 123.97, 127.84, 128.80, 129.48, 132.17, 140.00,
morphologies will find more applications in MCRs, leading to 152.74, 160.74, 163.27.
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6.4 1H-NMR of 3-(bis(4-fluorophenyl)methyl)-4-hydroxy-2H-
6.11 1H-NMR of 2-amino-4-(3-methoxyphenyl)-7,7-dimethyl-
chromen-2-one (3d)
5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (7f)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 5.91 (s, 1H), 6.48 (bs, 1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.05 (s, 3H), 1.10 (s,
1H), 7.04–7.77 (m, 12H).
3H), 2.22–2.23 (d, 2H, J = 4 Hz), 2.43–2.44 (d, 2H, J = 4 Hz),
13CNMR of 3-(bis(4-fluorophenyl)methyl)-4-hydroxy-2H- 4.37 (s, 1H), 4.56 (s, 2H), 3.78 (s, 3H), 6.72–6.77 (m, 2H),
chromen-2-one (3d). 13CNMR (CDCl3, 100 MHz): δH (ppm) = 6.81–6.83 (t, 1H, J = 8 Hz), 7.18–7.22 (t, 1H, J = 8 Hz).
45.80, 107.48, 115.74, 116.28, 116.49, 116.57, 123.19, 124.14,
6.12 1H-NMR of 2-amino-4-(3,5-dimethoxyphenyl)-7,7-
130.33, 130.41, 132.43, 135.56, 135.59, 152.75, 160.78, 160.96,
163.19, 163.42.
dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile
(7g)
6.5 1H-NMR of 4-hydroxy-3-(1-(4-methoxyphenyl)ethyl)-2H-
chromen-2-one (3e)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.07 (s, 3H), 1.11 (s,
3H), 2.24 (s, 2H), 2.44–2.45 (d, 2H, J = 4 Hz), 3.77 (s, 6H), 4.34
(s, 1H), 4.48 (s, 2H), 6.30–6.31 (t, 1H, J = 4 Hz), 6.38–6.38 (d,
2H, J = 2 Hz).
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.63–1.65 (d, 3H, J = 8
Hz), 3.83 (s, 3H), 4.63–4.69 (q, 1H, J = 8 Hz), 6.27 (s, 1H),
6.95–7.66 (m, 8H).
6.13 1H-NMR of 2-amino-4-(3-bromophenyl)-7,7-dimethyl-5-
13CNMR
of
4-hydroxy-3-(1-(4-methoxyphenyl)ethyl)-2H-
chromen-2-one (3e). 13CNMR (CDCl3, 100 MHz): δH (ppm) =
16.78, 33.83, 55.35, 110.07, 115.15, 116.13, 116.37, 116.72,
122.88, 123.85, 124.86, 128.49, 131.80, 132.65, 132.93, 152.49,
159.18, 159.80, 163.56.
oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (7h)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.05 (s, 3H), 1.11 (s,
3H), 2.23–2.23 (d, 2H, J = 2.4 Hz), 2.45–2.47 (d, 2H, J = 8 Hz),
4.37 (s, 1H), 4.58 (s, 2H), 7.14–7.26 (m, 2H), 7.31–7.34 (m, 2H).
6.14 1H-NMR of 2-amino-4-(4-fluorophenyl)-7,7-dimethyl-5-
6.6 1H-NMR of 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-
oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (7i)
tetrahydro-4H-chromene-3-carbonitrile (7a)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.02 (s, 3H), 1.11 (s,
3H), 2.21–2.23 (d, 2H, J = 8 Hz), 2.45 (s, 2H), 4.39 (s, 1H), 4.54
(s, 2H), 6.95–6.99 (t, 2H, J = 8 Hz), 7.19–7.26 (m, 2H).
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.04 (s, 3H), 1.11 (s,
3H), 2.21–2.23 (d, 2H, J = 8 Hz), 2.45 (s, 2H), 4.40 (s, 1H), 4.52
(s, 2H), 7.17–7.30 (m, 5H).
6.15 1H-NMR of 2-amino-7,7-dimethyl-5-oxo-4-(3,4,5-tri-
methoxyphenyl)-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile
6.7 1H-NMR of 2-amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-
oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (7b)
(7j)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.03 (s, 3H), 1.10 (s,
3H), 2.21–2.22 (d, 2H, J = 4 Hz), 2.44 (s, 2H), 3.76 (s, 3H), 4.36
(s, 1H), 4.48 (s, 2H), 6.80–6.83 (m, 2H), 7.13–7.17 (m, 2H).
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.03 (s, 3H), 1.11 (s,
3H), 2.23–2.24 (d, 2H, J = 4 Hz), 2.46–2.47 (d, 2H, J = 4 Hz),
3.77 (s, 3H), 3.81 (s, 6H), 4.71 (s, 1H), 6.51 (s, 2H).
6.8 1H-NMR of 2-amino-4-(4-bromophenyl)-7,7-dimethyl-5-
oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (7c)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.02 (s, 3H), 1.11 (s,
3H), 2.21–2.23 (d, 2H, J = 8 Hz), 2.45 (s, 2H), 4.37 (s, 1H), 4.57
(s, 2H), 7.11–7.13 (d, 2H J = 8 Hz), 7.40–7.42 (d, 2H, J = 8 Hz).
6.16 1H-NMR of 9-(2-chlorophenyl)-3,3,6,6-tetramethyl-
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8a)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.01 (s, 6H), 1.09 (s,
6H), 2.17–2.20 (d, 4H, J = 12 Hz), 2.44 (s, 4H), 4.99 (s, 1H),
7.03–7.07 (m, 1H), 7.13–7.15 (t, 1H, J = 4 Hz), 7.16–7.26 (m,
1H), 7.42–7.43 (d, 1H, J = 4 Hz).
6.9 1H-NMR of 2-amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-
6.17 1H-NMR of 9-(2,4-dichlorophenyl)-3,3,6,6-tetramethyl-
5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (7d)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.03 (s, 3H), 1.13 (s,
3H), 2.22–2.25 (d, 2H, J = 12 Hz), 2.485–2.487 (d, 2H, J = 8 Hz),
4.52 (s, 1H), 4.65 (s, 2H), 7.41–7.43 (dd, 2H, J = 2 and J = 6.8
Hz), 8.16–8.18 (dd, 2H, J = 1.6 and 6.8 Hz).
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8b)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.00 (s, 6H), 1.10 (s,
6H), 2.16–2.26 (d, 4H, J = 8 Hz), 2.47 (s, 4H), 4.69 (s, 1H),
7.17–7.20 (dd, 1H, J = 4 and 8 Hz), 7.26–7.27 (t, 1H, J = 4 Hz),
7.31–7.32 (d, 1H, J = 4 Hz).
6.10 1H-NMR of 2-amino-4-(3,4-dimethoxyphenyl)-7,7-
dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile
(7e)
6.18 1H-NMR of 9-(4-chlorophenyl)-3,3,6,6-tetramethyl-
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8c)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.05 (S, 3H), 1.11 (S, 1H-NMR (CDCl3, 400 MHz): δH (ppm) = 0.98 (s, 6H), 1.10 (s,
3H), 2.19–2.23 (d, 2H, J = 16 Hz), 2.44 (s, 2H), 3.83 (s, 3H), 3.86 6H), 2.18–2.21 (d, 4H, J = 12 Hz), 2.46 (s, 4H), 4.71 (s, 1H),
(s, 3H), 4.36 (s, 1H), 4.51 (s, 2H), 6.73–6.80 (m, 3H).
7.16–7.26 (m, 4H).
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6.19 1H-NMR of 9-(4-bromophenyl)-3,3,6,6-tetramethyl-
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8d)
Acknowledgements
The authors extend their appreciation to the Deanship of
Scientific Research at King Saud University for funding this
work through research group no. RG-1441-452.
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 0.98 (s, 6H), 1.10 (s,
6H), 2.18–2.22 (d, 4H, J = 12 Hz), 2.460–2.463 (d, 4H, J = 1.2
Hz), 4.69 (s, 1H), 7.16–7.18 (dd, 2H, J = 2 and 6.8 Hz),
7.32–7.34 (dd, 2H, J = 2 and 6.4 Hz).
References
6.20 1H-NMR of 3,3,6,6-tetramethyl-9-(3-nitrophenyl)-
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8e)
1 A. Dömling, Chem. Rev., 2006, 106, 17–89.
2 F. Liu, L. Wang, Q. Sun, L. Zhu, X. Meng and F. S. Xiao,
J. Am. Chem. Soc., 2012, 134, 16948–16950.
3 H. Liu, T. Jiang, B. Han, S. Liang and Y. Zhou, Science,
2009, 326, 1250–1252.
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.00 (s, 6H), 1.12 (s,
6H), 2.19–2.23 (d, 4H, J = 16 Hz), 2.51 (s, 4H), 4.84 (s, 1H),
7.39–7.43 (t, 1H, J = 8 Hz), 7.82–7.84 (d, J = 8 Hz), 7.98–8.01 (m,
2H).
4 T. A. J. Siddiqui, S. F. Shaikh, S. S. Sangale, S. D. Raut,
R. S. Mane, M. Ubaidullah, B. M. Thamer, A. M. Al-Enizi,
B. B. Totawar and M. S. Samdani, Catalysts, 2020, 10, 1369.
5 Y. Wang, H. Arandiyan, J. Scott, A. Bagheri, H. Dai and
R. Amal, J. Mater. Chem. A, 2017, 5, 8825–8846.
6 G. Manjari, S. Saran, S. Radhakrishanan, P. Rameshkumar,
A. Pandikumar and S. P. Devipriya, J. Environ. Manage.,
2020, 262, 110282.
6.21 1H-NMR of 3,3,6,6-tetramethyl-9-p-tolyl-3,4,5,6,7,9-hexa-
hydro-1H-xanthene-1,8(2H)-dione (8f)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 0.99 (s, 6H), 1.09 (s,
6H), 2.17–2.20 (d, 4H, J = 12 Hz), 2.23 (s, 3H), 2.45 (s, 4H), 4.70
(s, 1H), 7.00–7.02 (d, 2H, J = 8 Hz), 7.16–7.18 (d, 2H, J = 8 Hz).
6.22 1H-NMR of 3,3,6,6-tetramethyl-9-(2,3,4-trimethoxyphe-
nyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8g)
7 G. B. Dharma Rao, S. Nagakalyan and G. K. Prasad, RSC
Adv., 2017, 7, 3611–3616.
8 B. Karami, K. Eskandari, S. Khodabakhshi, S. J. Hoseini
and F. Hashemian, RSC Adv., 2013, 3, 23335–23342.
9 A. Saha, S. Payra and S. Banerjee, Green Chem., 2015, 17,
2859–2866.
10 S. Khodabakhshi, B. Karami, K. Eskandari and
S. J. Hoseini, C. R. Chim., 2014, 17, 35–40.
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.01 (s, 6H), 1.09 (s,
6H), 2.16–2.20 (d, 4H, J = 16 Hz), 2.43 (s, 4H), 3.75 (s, 3H), 3.78
(s, 3H), 3.94 (s, 3H), 4.81 (s, 1H), 6.53–6.55 (d, 1H, J = 8 Hz),
6.99–7.01 (d, 1H, J = 8 Hz).
6.23 1H-NMR of 3,3,6,6-tetramethyl-9-(4-nitrophenyl)-
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8h)
11 P. P. Ghosh and A. R. Das, J. Org. Chem., 2013, 78, 6170–
6181.
1H-NMR (CDCl3, 400 MHz): δH (ppm)
1.09 (s, 6H), 2.20–2.22 (d, 4H, J = 8 Hz), 2.45 (s, 4H),
4.66 (s, 1H), 6.56–6.58 (d, 2H, J = 8 Hz), 7.07–7.09 (d, 2H, J = 8
Hz).
= 0.99 (s, 6H),
12 K. Tanabe, Mater. Chem. Phys., 1985, 13, 347–364.
13 S. F. Shaikh, S. S. Kalanur, R. S. Mane and O. S. Joo, Dalton
Trans., 2013, 42, 10085–10088.
14 S. F. Shaikh, B. G. Ghule, P. V. Shinde, S. D. Raut,
S. K. Gore, M. Ubaidullah, R. S. Mane and A. M. Al-Enizi,
Mater. Res. Express, 2020, 7, 15076.
15 A. M. Al-Enizi, T. A. J. Siddiqui, S. F. Shaikh,
M. Ubaidullah, A. Yousef, R. S. Mane and A. ul H. S. Rana,
RSC Adv., 2020, 10, 14826–14836.
16 B. Karami, S. J. Hoseini, K. Eskandari, A. Ghasemi and
H. Nasrabadi, Catal. Sci. Technol., 2012, 2, 331–338.
17 T. Lambat, R. Chaudhary, A. Abdala, R. Mishra,
S. H. Mahmood and S. Benerjee, RSC Adv., 2019, 9, 31683–
31690.
18 N. Venugopalrao, V. Ravi and P. Zubaidha, Int. J. Org.
Chem., 2012, 2, 287–294.
19 P. Theerthagiri and A. Lalitha, Tetrahedron Lett., 2010, 51,
5454–5458.
20 P. W. Hansen, T. S. G. Sehested, E. L. Fosbøl, C. Torp-
Pedersen, L. Køber, C. Andersson and G. H. Gislason, PLoS
One, 2018, 13, e0194295–e0194295.
6.24 1H-NMR of 3,3,6,6-tetramethyl-9-(3,4,5-trimethoxyphe-
nyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8i)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.03 (s, 6H), 1.11 (s,
6H), 2.23–2.24 (d, 4H, J = 20 Hz), 2.46–2.47 (d, 4H, J = 4 Hz),
3.77 (s, 3H), 3.81 (s, 6H), 4.71 (s, 1H), 6.51 (s, 2H).
6.25 1H-NMR of 9-(3,4-dimethoxyphenyl)-3,3,6,6-tetramethyl-
3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (8j)
1H-NMR (CDCl3, 400 MHz): δH (ppm) = 1.00 (s, 6H), 1.10 (s,
6H), 2.20–2.22 (d, 4H, J = 8 Hz), 2.46 (s, 4H), 3.79 (s, 3H), 3.85
(s, 3H), 4.70 (s, 1H), 6.70–6.77 (m, 2H), 6.90–6.90 (d, 1H, J = 1.6
Hz).
21 J. Tan, S. Bae, J. B. Segal, J. Zhu, G. C. Alexander,
D. L. Segev and M. McAdams-DeMarco, Nephrology, 2019,
24, 234–244.
Conflicts of interest
There are no conflicts to declare.
2040 | Dalton Trans., 2021, 50, 2032–2041
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View Article Online
Dalton Transactions
Paper
22 H. Xing, S. D. Houston, X. Chen, D.-Y. Jin, G. P. Savage, 31 V. Ganbavle, G. Agawane, A. Moholkar, J. Kim and
J.-K. Tie and C. M. Williams, Bioorg. Med. Chem. Lett., 2019,
29, 1954–1956.
23 J. E. Gill, Int. Biodeterior. Biodegrad., 1992, 30, 65–76.
K. Rajpure, J. Mater. Eng. Perform., 2014, 23, 1204–1213.
32 S. S. Kalanur, I.-H. Yoo, I.-S. Cho and H. Seo, Electrochim.
Acta, 2019, 296, 517–527.
24 F. P. Rowe, C. J. Plant and A. Bradfield, J. Hyg., 1981, 87, 33 A. Kanwat and J. Jang, J. Mater. Chem. C, 2014, 2, 901–
171–177. 907.
25 Mayank, B. Kaur Billing, P. K. Agnihotri, N. Kaur, N. Singh 34 T. Wang, W. Quan, D. Jiang, C. Linlin, D. Li, S. Meng and
and D. O. Jang, ACS Sustainable Chem. Eng., 2018, 6, 3714–
3722.
26 Subodh, N. K. Mogha, K. Chaudhary, G. Kumar and
D. T. Masram, ACS Omega, 2018, 3, 16377–16385.
27 T. Paik, M. Cargnello, T. R. Gordon, S. Zhang, H. Yun,
M. Chen, Chem. Eng. J., 2016, 280–290.
35 J. Li, X. Liu, Q. Han, X. Yao and X. Wang, J. Mater. Chem. A,
2013, 1, 1246–1253.
36 S. Tian, X. Ding, D. Zeng, S. Zhang and C. Xie, Sens.
Actuators, B, 2013, 186, 640–647.
J. D. Lee, H. Y. Woo, S. J. Oh, C. R. Kagan, P. Fornasiero 37 M. A. Wanzhen, A. G. Ebadi, M. S. Sabil, R. Javahershenas
and C. B. Murray, ACS Energy Lett., 2018, 3, 1904–1910. and G. Jimenez, RSC Adv., 2019, 9, 12801–12812.
28 J. Balou, M. A. Khalilzadeh and D. Zareyee, Sci. Rep., 2019, 38 M. Esmaeilpour, J. Javidi, F. Dehghania and F. N. Dodeji,
9, 3605. RSC Adv., 2015, 5, 26625–26630.
29 J. Zhang, Z. Liu and Z. Liu, ACS Appl. Mater. Interfaces, 39 O. G. Jolodar and F. Shirini, J. Iran. Chem. Soc., 2016, 13,
2016, 8, 9684–9691. 1077–1092.
30 P. P. González-Borrero, F. Sato, A. N. Medina, M. L. Baesso, 40 K. Venkatesan, S. S. Pujari, R. J. Lahoti and
A. C. Bento, G. Baldissera, C. Persson, G. A. Niklasson, K. V. Srinivasan, Ultrason. Sonochem., 2008, 15, 548–553.
C. G. Granqvist and A. da Silva, Appl. Phys. Lett., 2010, 96, 41 A. Ilangovan, S. Malayappasamy, S. Muralidharan and
61909.
S. Maruthamuthu, Chem. Cent. J., 2011, 5, 81.
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