Fe3O4@SiO2-SO3H-DABCO: A novel magnetically retrievable bifunctional catalyst for ecofriendly synthesis of diheteroarylmethanes

Article abstract of DOI:10.1016/j.molstruc.2021.130960

Developing sustainable methodologies using green solvents, reusable catalysts and energy saving approaches is the need of time considering the growing stringent environmental regulations. The present study reports the novel three-step synthesis of magneti

Full text of DOI:10.1016/j.molstruc.2021.130960

Journal of Molecular Structure 1245 (2021) 130960
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Fe3O4@SiO2-SO3H-DABCO: A novel magnetically retrievable
bifunctional catalyst for ecofriendly synthesis of diheteroarylmethanes
Archana Yadav a, Pradeep Patila, Dattatray Chandamb, Sushilkumar Jadhavc, Anil Ghulea,
Shankar Hangirgekara, Sandeep Sankpala,∗
a Department of Chemistry, Shivaji University, Kolhapur 416004, India
b Department of Chemistry, Bhogawati Mahavidyalaya, Kurukali 416001, India
c School of Nanoscience and Technology, Shivaji University, Kolhapur 416004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Developing sustainable methodologies using green solvents, reusable catalysts and energy saving ap-
proaches is the need of time considering the growing stringent environmental regulations. The present
study reports the novel three-step synthesis of magnetically separable Fe3O4@SiO2-SO3H-DABCO core-
shell magnetic nanocomposite catalyst and evaluation of its catalytic efficiency involving the synthe-
sis of structurally diverse diheteroarylmethanes via one-pot reaction of indoles, aryl aldehydes and 4-
hydroxycoumarin in aqueous ethanol (50:50 v/v) at ambient conditions. The catalyst could be recycled for
six consecutive runs without significant change in catalytic activity or the product yields. The ecofriendly
synthetic approach offers several advantages such as the use of a green solvent system, ambient reaction
conditions, shorter reaction times, excellent yields and ease of separation of the catalyst using an external
magnet.
Received 3 April 2021
Revised 14 June 2021
Accepted 21 June 2021
Available online 28 June 2021
Keywords:
Ionic liquids
Organocatalyst
Fe3O4@SiO2-SO3H-DABCO
Magnetic nanocomposite
Multicomponent reactions
Diheteroarylmethanes
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
ization and reusability through magnetic separation [13], function-
alized Fe3O4 magnetic nanoparticles (MNPs) have been success-
Ionic liquids play an important role in the development of
acid as well as base catalyzed greener methodologies due to their
advantages of chemical and thermal stability, low toxicity and
non-flammability, etc. [1–3]. Among various organocatalysts, 1,4-
diazabiclo(2.2.2)octane (DABCO) has been well explored in the syn-
thesis of bifunctional ionic liquids which utilizes both acidic and
basic sites for catalyzing organic transformations [4–7]. However,
the reuse of such bifunctional ionic liquids for consecutive reac-
tions is difficult because of decreased catalytic activity and te-
dious recovery procedures [8]. In this context, immobilization of
bifunctional organocatalysts as well as ionic liquids through chem-
ical bonding on heterogeneous supports has attracted consider-
able attention in modern organic synthesis [9–10]. Among these,
nano-sized Fe3O4 is emerging magnetic support and has invoked
tremendous research interest in heterogeneous catalysis. It elim-
inates drawbacks associated with bulk and nano heterogeneous
supports, particularly inferior diffusion of reactants on less active
sites and difficulties in separation from the reaction mixture [11–
12]. Due to low cost, large surface area, ease of surface functional-
fully explored in the synthesis of a variety of heterocyclic frame-
works [12,14–26]. Indole and coumarin are important heterocy-
cles present in numerous biologically active natural products [27–
28]. Moreover, derivatives of indole and coumarin exhibit anti-
inflammatory [29], antimalarial [30], anti-HIV [31], antidiabetic
[32], antioxidant [33], anticoagulant [34], antiproliferative [35] and
anticancer [36] activities, etc. Additionally, heterocyclic hybrids of
indole and coumarin show promising anticancer [37], antileishma-
nial [38], antihyperlipidemic [39], antitumor and angiogenesis ac-
tivities [40] [Fig. 1].
Because of promising biological activities, syntheses of het-
erodimers of coumarin and indole have aroused considerable at-
tention in medicinal chemistry research. The simplest approach
for the synthesis of heterodimers of coumarin and indole in-
volves a one-pot three-component reaction of 4-hydroxycoumarin,
aryl aldehyde and indole. Syntheses of heterodimers of 4-
hydroxycoumarin and indole have been studied in the presence
of InCl3 [41], L-proline [42], copper octoate [43] and Fe3O4@SiO2-
PEG/NH2 [44]. However, some of the methods are associated with
the formation of bis-indolyl methane as a side product along with
the formation of dihetroaryl methane. Other limitations such as
high-temperature conditions, use of toxic solvents, longer reac-
tion times, tedious purification procedures and lack of reusabil-
∗
Corresponding author.
E-mail address: sas_chem@unishivaji.ac.in (S. Sankpal).
https://doi.org/10.1016/j.molstruc.2021.130960
0022-2860/© 2021 Elsevier B.V. All rights reserved.

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Fig. 1. Biologically active indole-coumarin hybrid molecules.
Scheme 1. Fe3O4@SiO2-SO3H-DABCO catalyzed synthesis of diheteroarylmethanes of indole and 4-hydroxycoumarin.
ity of catalysts watered down the principles of green chemistry.
tra of synthesized compounds were recorded on a Waters Micro-
mass Q-TOF Micro instrument. Fourier transform-infrared (FT-IR)
spectra were recorded using a KBr pellet in the range of 400–
4000 cm−1 with a Bruker IR spectrophotometer. The X-ray pow-
der diffraction (XRD) pattern of the materials was recorded us-
ing a Bruker, D2 Phaser employing Cu target. The surface mor-
phology of the material was studied using field emission scanning
electron microscopy (FE-SEM, TESCAN, Mira3). The chemical com-
position of the Fe3O4@SiO2-SO3H-DABCO was determined using
energy-dispersive X-ray spectroscopy (EDX) coupled with FE-SEM.
The particle size of nanocomposite was investigated using a trans-
mission electron microscopy (TEM, Jeol JEM-2100, 200 KV). X-ray
photoelectron spectroscopy (XPS) analysis of the catalyst was per-
formed using PHI 5000 VersaProbe III. The thermal stability study
of the catalyst was performed using a thermogravimetric analyser
(TGA, Netzsch STA 409) under nitrogen atmosphere at the flow
rate of 20 mL min−1 with a heating rate of 10 °C min−1 from
30 °C to 800 °C. Magnetic properties of Fe3O4@SiO2-SO3H-DABCO
nanocomposite were measured using a vibrating sample magne-
tometer (VSM) of the Lakeshore 7410S model.
To circumvent these limitations, it is essential to design a cat-
alyst that provides weaker acidic sites that exclude the forma-
tion of bis-indolyl methanes and furnish excellent yields of di-
heteroarylmethanes. Considering greener perspectives of magnet-
ically separable nanocatalysts in organic synthesis and continua-
tion of ongoing research in green chemistry [45], herein, we re-
port an efficient and sustainable method for the synthesis of di-
heteroarylmethanes of indole and 4-hydroxy coumarin via one-
pot three-component reaction of 4-hydroxycoumarin, aryl aldehy-
des and indoles catalyzed by novel Fe3O4@SiO2-SO3H-DABCO bi-
functional magnetic nanocomposite in aqueous ethanol at ambient
conditions (Scheme 1).
2. Experimental
2.1. Materials and general procedure for the synthesis of novel
Fe3O4@SiO2-SO3H-DABCO catalyst
All required chemicals and solvents were procured from Merck,
Sigma-Aldrich and Spectrochem and used as received. Melting
points of synthesized compounds were measured in the open cap-
illary.
2.3. General procedure for the synthesis of
3-(1H-indole-3-yl)(phenyl)methyl)-4-hydroxy-2H-chromen-2-ones
The synthetic route of novel magnetically separable
Fe3O4@SiO2-SO3H-DABCO catalyst is depicted in Scheme 2.
Firstly, Fe3O4 magnetic nanoparticles (MNPs) were prepared by
A mixture of 4-hydroxycoumarin (1 mmol), aryl aldehyde (1
mmol), indole/substituted indole (1 mmol) and Fe3O4@SiO2-SO3H-
DABCO (10 mg) in EtOH:H2O (5 mL, 50:50 v/v) was stirred at
room temperature. The progress of reactions was monitored by
thin-layer chromatography (TLC). After completion of the reaction,
Fe3O4@SiO2-SO3H-DABCO nanocomposite was separated using an
external magnet and washed with ethanol. The crude products
were further purified by recrystallization from ethanol affording
desired 3-(1H-indole-3-yl)(phenyl)methyl)-4-hydroxy-2H-chromen-
2-ones.
a
reported chemical co-precipitation method using Fe2+ and
Fe3+ salts [14,46]. To prevent oxidation and aggregation, Fe3O4
MNPs were further coated with a silica layer using tetraethyl
orthosilicate (TEOS) by using a sol-gel method [15,20,47]. After-
wards, the resultant Fe3O4@SiO2 was sonicated for half an hour
in n-hexane and further sulphonated with chlorosulfonic acid
which gave Fe3O4@SiO2-SO3H as a dark brown powder. Finally, the
reaction of DABCO with Fe3O4@SiO2-SO3H resulted desired novel
Fe3O4@SiO2-SO3H-DABCO magnetic nanocomposite (Scheme 2).
2.5. Spectral data of unknown compounds
2.2. Instruments
3-(1H-indol-3-yl)(4-methoxyphenyl)methyl-4-hydroxy-2H-
chromen-2-one [4f] (Table 2, entry 6):Pinkish white solid; M.p.
245-247 °C; IR (KBr): 3853, 3752, 2395, 1668, 1552, 766, 612
1H and 13C NMR spectra were recorded on a Bruker AC-300/400
spectrometer at 300/400 MHz and 75/100 MHz respectively in
DMSO-d6 and CDCl3 using TMS as an internal standard. Mass spec-
cm−1 1H NMR (300 MHz, DMSO-d6): δ 3.82 (3H, s, -OCH3), 6.07
.
2

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Scheme 2. Synthesis of magnetically separable Fe3O4@SiO2-SO3H-DABCO catalyst.
(1H, s, -CH), 6.86-6.89 (2H, d, Ar-H, J=8.1), 7.13-7.16 (2H, d, Ar-H,
J=7.8), 7.28-7.43 (4H, d, Ar-H, J=7.8), 7.62-7.64 (2H, d, Ar-H, J=6.9),
8.05 (2H, s, Ar-H), 11.30 (1H, s, -NH), 11.50 (1H, s, -OH). 13C NMR
(75 MHz, DMSO-d6): δ 35.55, 55.26, 104.30, 105.81, 114.06, 116.59,
124.36, 124.81, 126.99, 127.61, 132.75, 152.44, 158.48, 164.60,
165.59, 169.26. MS (ESI): 397.25 m/z.
4-hydroxy-3-(5-methyl-1H-indol-3-yl)(phenyl)methyl)-2H-
chromen-2-one [4g] (Table 2, entry 7): White solid; M.p. 173-175 °C;
IR (KBr): 3848, 3779, 1733, 1670, 1620, 1525, 1491, 762, 720, 580
.
cm−1 1H NMR (400 MHz, CDCl3): δ 2.50 (3H, s, -CH3), 5.98 (1H,
s, -CH), 6.85 (2H, d, Ar-H), 6.92 (8H, d, Ar-H), 7.44 (2H, s, Ar-H),
7.84 (1H, s, -NH), 10.42 (1H, s, -OH). 13C NMR (100 MHz, CDCl3):
δ 21.69, 37.76, 108.51, 111.47, 114.10, 116.30, 116.55, 118.37, 123.10,
123.58, 123.83, 124.72, 126.01, 127.49, 127.68, 127.97, 128.37, 131.74,
134.84, 142.77, 152.56, 160.94, 162.74. MS (ESI): 381.30 m/z.
4-hydroxy-3-(7-methyl-1H-indol-3-yl)(phenyl)methyl)-2H-
chromen-2-one [4h]: (Table 2, entry 8): Pinkish white solid;
.
M.p. 238-240 °C; IR (KBr): 3855, 1605, 1554, 1089, 753 cm−1 1H
NMR (400 MHz, CDCl3): δ 1.60 (3H, s, -CH3), 6.11 (1H, s, -CH),
7.25-7.41 (9H, t, Ar-H, J=8), 7.64 (2H, d, Ar-H), 8.02-8.07 (2H, d,
Ar-H, J=8.1), 11.32 (1H, s, -OH), 11.54 (1H, s, -NH). 13C NMR (100
MHz, CDCl3): δ 36.15, 103.90, 105.62, 116.43, 116.64, 116.92,124.40,
124.89, 126.47, 126.88, 128.64, 132.87, 135.16, 152.28, 152.54,
164.61, 165.82, 166.89, 169.34. MS (EI): 381.33 m/z.
Fig. 2. FT-IR spectra of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2-SO3H (d)
Fe3O4@SiO2-SO3H-DABCO and (e) Fe3O4@SiO2-SO3H-DABCO after 6th run.
Fe3O4@SiO2 (Figure 2b) exhibits peaks at 1088 cm−1 and 800
cm−1 confirming asymmetric and symmetric stretching vibrations
of the Si-O-Si bond [51]. The FT-IR spectrum of Fe3O4@SiO2-SO3H
shows absorptions at 3365 cm−1 for O-H stretching, 1079 cm−1
for S=O stretching, 800 cm−1 for S-OH bending and 660 cm−1
for S-OH stretching indicate the presence of sulphonic acid group
(Figure 2c) [52]. The FT-IR spectrum of Fe3O4@SiO2-SO3H-DABCO
(Figure 2d) consists of bands at 2935 cm−1 and 1460 cm−1 as-
signed for stretching and deformation vibrations of C-H bond while
C-N stretching appeared at 1210 cm−1 corroborate successful graft-
ing of DABCO on Fe3O4@SiO2-SO3H [53].
3. Results and discussion
3.1. Characterization of Fe3O4@SiO2-SO3H-DABCO
The synthesized Fe3O4@SiO2-SO3H-DABCO nanocomposite was
characterized by various tools such as Fourier transform-infrared
(FT-IR) spectroscopy, X-ray diffraction spectroscopy (XRD), thermo-
gravimetric analysis (TGA), field emission scanning electron mi-
croscopy (FE-SEM), coupled with energy-dispersive X-ray spec-
troscopy (EDX), transmission electron microscopy (TEM), vibrat-
ing sample magnetometry (VSM) and X-ray photoelectron spec-
troscopy (XPS).
3.1.2. X-ray diffraction (XRD) patterns of Fe3O4, Fe3O4@SiO2,
Fe3O4@SiO2-SO3H and Fe3O4@SiO2-SO3H-DABCO
3.1.1. FT-IR spectra of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-SO3H and
Fe3O4@SiO2-SO3H-DABCO
The XRD patterns of synthesized Fe3O4, Fe3O4@SiO2 and
Fe3O4@SiO2-SO3H-DABCO are depicted in Figure 3. The reflections
observed at diffraction angles (2θ) 30.16°, 35.39°, 43.27°, 53.26°,
56.53°, 62.70° were successfully indexed with hkl values of (220),
(311), (400), (422), (511), (440) lattice indices of Fe3O4 [54]. The
XRD patterns of bare Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-SO3H-
DABCO are identical with the XRD pattern of reference Fe3O4
Figure 2(a-d) shows FT-IR spectra of Fe3O4, Fe3O4@SiO2,
Fe3O4@SiO2-SO3H and Fe3O4@SiO2-SO3H-DABCO catalyst. The peak
that appeared at 586 cm−1 (Figure 2a) confirms the Fe-O stretch-
ing while the absorption at 1640 cm−1 and 3365 cm−1 (Figure 2a)
corresponds to bending and stretching vibrations of surface O-
H groups of Fe3O4 [48–50]. Further, the FT-IR spectrum of the
3

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Fig. 3. Powder XRD patterns of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-SO3H-DABCO
Fig. 5. EDX of Fe3O4@SiO2-SO3H-DABCO MNPs (a) before reaction (b) after the 6th
run.
3.1.4. Energy-dispersive X-ray (EDX) spectroscopy analysis of
Fe3O4@SiO2-SO3H-DABCO
The purity of synthesized Fe3O4@SiO2-SO3H-DABCO catalyst is
confirmed using energy-dispersive X-ray spectroscopy (EDX) anal-
ysis. As illustrated in Figure 5a, the EDX result reveals the pres-
ence of carbon, nitrogen, oxygen, silicon, sulfur and iron confirm-
ing the successful synthesis of Fe3O4@SiO2-SO3H-DABCO. The EDX
analysis does not show extra peaks which imply high purity of
Fe3O4@SiO2-SO3H-DABCO catalyst. Furthermore, the amount of -S-
OH groups over catalyst surface was determined from EDX analysis
and found to be 0.109 mmol per gram of the catalyst. Subsequently,
the presence of nitrogen in its relevant energy position at 0.4 keV
confirms the loading of DABCO over the catalyst surface with an
amount of 0.116 mmol per gram of catalyst. The EDX spectrum of
the reused catalyst after the sixth run does not show any impurity
elucidating the structural integrity of the catalyst (Figure 5b).
Fig. 4. Thermogravimetric analysis of Fe3O4@SiO2-SO3H-DABCO.
(JCPDS#19-0629) which indicates that the crystalline spinel struc-
ture is restored even after functionalization.
3.1.5. Field emission scanning electron microscopy (FE-SEM) study of
Fe3O4@SiO2-SO3H-DABCO
The size and morphology study of the Fe3O4@SiO2-SO3H-
DABCO before and after the reaction was done using field emis-
sion scanning electron microscopy (FE-SEM) and the results are
shown in Figure 6. The FE-SEM images of Fe3O4@SiO2-SO3H-
DABCO nanocomposite shows a roughly spherical morphology with
an average grain size less than 85 nm (Figure 6a-b).
3.1.3. Thermogravimetric analysis (TGA) and differential thermal
analysis (DTA) of Fe3O4@SiO2-SO3H-DABCO catalyst
The thermal behaviour of Fe3O4@SiO2-SO3H-DABCO was in-
vestigated using thermogravimetric analysis (TGA) and differen-
tial thermal analysis (DTA) as shown in Figure 4. The TGA curve
showed a weight loss of 5% in the first step around 80 °C, which
is accountable for the loss of physico-adsorbed water. This is fur-
ther supported by the DTA thermogram showing an endothermic
peak at 70 °C. The TGA thermogram shows the second step of
weight loss (9%) in the range from 80-430 °C which is due to slug-
gish loss of adsorbed water and surface-bound -OH groups from
magnetite. The third step of weight loss in the range from 430-
790 °C is attributed to the decomposition of silica shell [18] and
the carbonization of organic matter. This is supported by the DTA
curve showing a broad endothermic peak at 500 °C. The TGA/DTA
study of Fe3O4@SiO2-SO3H-DABCO demonstrated that synthesized
nanocomposite is stable up to 430 °C.
3.1.6. Transmission electron microscopy (TEM) and SAED analysis of
Fe3O4@SiO2-SO3H-DABCO
To analyze the crystallinity and particle size of the synthesized
Fe3O4@SiO2-SO3H-DABCO nanocomposite, the HR-TEM and SAED
analysis was performed. Figure 7a-c shows the high-resolution
HR-TEM images of the Fe3O4@SiO2-SO3H-DABCO. As seen from
Figure 7a , the formation of core-shell was observed around the
small Fe3O4 nanoparticles, which is the main focus of the present
report for the preparation of the core-shell nanocatalyst. Figure 7b
and c show formation of nano spherical surface morphology with
4

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Fig. 8. Magnetization curve for Fe3O4 and Fe3O4@SiO2-SO3H-DABCO magnetic
nanocomposite at room temperature.
netization is reduced due to the coating of organic matter on the
surface of Fe3O4 MNPs.
Fig. 6. FE-SEM images of Fe3O4@SiO2-SO3H-DABCO (a, b) before reaction (c, d) after
the 6th run.
3.1.8. X-ray photoelectron spectroscopy (XPS) analysis of
Fe3O4@SiO2-SO3H-DABCO
The chemical valence states and surface chemical composition
of Fe3O4@SiO2-SO3H-DABCO magnetic nanocomposite were deter-
mined using X-ray photoelectron spectroscopy (XPS) (Fig. 9a-c).
The wide survey of the spectrum shows that the sample consists
of iron, oxygen, sulfur, silicon, nitrogen and carbon. The core-level
XPS spectrum of iron displays two peaks at binding energy 713 eV
and 728 eV assigned for Fe 2p1/2 and Fe 2p3/2 respectively. There
are two peaks observed in the core-level XPS spectrum of Si at
binding energy 104 eV and 154 eV for Si 2p and 2s state. The core-
level XPS spectrum of O 1s appeared at 532 eV while the peak for
sulfur 2p obtained at 168 eV validates anchoring of sulphonic acid
on silica. The binding energy for N 1s appeared at 400 eV while C
1s obtained at 284 eV revealed loading of DABCO on Fe3O4@SiO2-
SO3H.
3.2. Catalytic studies of novel Fe3O4@SiO2-SO3H-DABCO
nanocomposite
After characterization of Fe3O4@SiO2-SO3H-DABCO nanocom-
posite, catalytic efficacy was investigated in the synthesis of
3-(1H-indole-3-yl)(phenyl)methyl)-4-hydroxy-2H-chromen-2-ones.
Initially, the model reaction of 4-hydroxycoumarin, benzaldehyde
and indole in an aqueous medium under catalyst-free condition
afforded inferior yield of the product 4a even after prolonged
reaction time (entry 1, Table 1). In the next step, the model
reaction was explored in the presence of the catalytic amount
of Fe3O4@SiO2-SO3H (5 mg) which resulted in poor yield of the
product 4a along with the formation of the bis-indolylmethane
as a major product (entry 2, Table 1). Further, the model reaction
catalyzed by Fe3O4@SiO2-SO3H-DABCO (5 mg) in an aqueous
medium resulted moderated yield of the 4a (entry 3, Table 1).
With this satisfactory result, the effect of solvent on the yield of
4a was investigated by selecting EtOH, iPrOH, CH2Cl2, CH3CN and
toluene (entries 4-8, Table 1). It was found that model reaction
in absolute ethanol shows massive improvement in the yield of
product 4a (entry 4, Table 1). In our endeavor to enhance the
yield of 4a, the model reaction was scrutinized using different
ratios of the EtOH:H2O solvent system (entry 9-11, Table 1). These
results demonstrated that the maximum yield of desired prod-
uct 4a was obtained with EtOH:H2O (50:50 v/v) within a short
reaction time (entry 11, Table 1). Gutmann’s parameter has been
Fig. 7. TEM images of Fe3O4@SiO2-SO3H-DABCO.
an average particle size of 14 nm. Furthermore, SAED patterns are
shown in Figure 7d. The SAED patterns show the formation of
ring patterns with bright spots which confirms the synthesized
Fe3O4@SiO2-SO3H-DABCO is in the polycrystalline form. The ob-
served HR-TEM and SAED analysis of Fe3O4@SiO2-SO3H-DABCO are
well accordances with FE-SEM and XRD data.
3.1.7. Vibrating sample magnetometer (VSM) analysis of
Fe3O4@SiO2-SO3H-DABCO catalyst
The magnetic properties of Fe3O4@SiO2-SO3H-DABCO nanocom-
posite were investigated by
a vibrating sample magnetometer
(VSM) at room temperature. The saturation magnetization of Fe3O4
and Fe3O4@SiO2-SO3H-DABCO are 93 emu/g and 40 emu/g at an
applied field of 1500 Oe respectively (Figure 8). VSM study of
Fe3O4@SiO2-SO3H-DABCO nanocomposite shows saturation mag-
5

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Fig. 9. (a) XPS survey spectrum of Fe3O4@SiO2-SO3H-DABCO, (b) Core level XPS spectrum of C, (c) Core level XPS spectrum of N.
Table 1
with the carbonyl group of aryl aldehyde which enhances the
reaction rate. The electrophilic character of ethanol is more than
isopropyl alcohol because the substitution of an acidic hydrogen
Optimization studyfor the synthesis of 3-(1H-indole-3-yl)(phenyl)methyl)-4-
hydroxy-2H-chromen-2-onea.
Entry
Solvent
Catalyst(mg)
Time (min)
Yieldb(%)
atom by
a bulkier alkyl group drastically decreases the elec-
trophilic properties [57]. The low acceptor number of acetonitrile
(18.9) and dichloromethane (20.4) affect solvating properties. The
order of increasing accepter strength of the tested solvents is
toluene < acetonitrile < CH2Cl2 < isopropyl alcohol < EtOH <
1
H2O
—
5c
5
245
55
89
68
75
83
97
126
35
30
40
15
15
27
35
65
75
71
46
41
39
79
81
85
93
95
2
H2O
3
H2O
4
EtOH
5
5
iPrOH
5
MeOH
< H2O, as obtained from spectroscopic measurements.
6
CH3CN
5
The acceptor number of toluene is zero and the donor number
is also very low due to its non-polar nature. From the study of
acceptor and donor number of solvents, we selected ethanol:water
solvent system in the present study due to its high acceptor
number (AN). Ethanol: water solvent system directly influences
reaction rate and product yield owing to the higher solubility of
reactants in aqueous ethanol than water [58] (Table 1, entries
9-13). To obtain excellent results, the effect of the catalytic load
of Fe3O4@SiO2-SO3H-DABCO (10 and 15 mg) on the product yield
of the model reaction was examined in EtOH:H2O (50:50 v/v)
(entries 12 and 13, Table 1). As can be evident from Table 1, the
model reaction using 10 mg of Fe3O4@SiO2-SO3H-DABCO catalyst
in EtOH:H2O (50:50 v/v) furnished an excellent yield of the desired
product 4a (entry 12, Table 1). With optimized reaction conditions
in hand, the catalytic efficiency of Fe3O4@SiO2-SO3H-DABCO was
generalized by exploring structurally and electronically diverse aryl
aldehydes and indoles (entries 4a-n, Table 2). It was noticed that
the reaction of aryl aldehydes, indoles and 4-hydroxycoumarin
smoothly furnished the desired 3-(1H-indole-3-yl)(phenyl)methyl)-
7
CH2Cl2
5
8
Toluene
5
9
EtOH:H2O (80:20 v/v)
EtOH:H2O (70:30 v/v)
EtOH:H2O (50:50 v/v)
EtOH:H2O (50:50 v/v)
EtOH:H2O (50:50 v/v)
5
10
11
12
13
5
5
10
15
a
Reaction conditions: 4-hydroxycoumarin (1 mmol), benzaldehyde (1 mmol),
indole (1 mmol) and Fe3O4@SiO2-SO3H-DABCO in solvent (5 mL) at room tem-
perature (entries 3-13).
b
Isolated yield. cModel reaction catalyzed by Fe3O4@SiO2-SO3H.
used to study the effect of solvents on reaction rate depending
upon the acceptor number and donor number. These numbers
for various solvents are calculated from Gutmann’s parameter
by using triethylphosphine oxide as a reference standard [55,56].
The acceptor number and donor number of water is 54.8 and 33
respectively, indicate more electrophilic and nucleophilic character
toward reactant species. Water forms strong hydrogen bonding
6

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Table 2
Fe3O4@SiO2-SO3H-DABCO catalyzed synthesis of 3-(1H-indole-3-yl)(phenyl)methyl)-4-hydroxy-2H-chromen-
2-ones.a
Sr. No.
R1
R2
R3
Product
Time (min)
Yieldb (%)
M.p. (˚C) [Literature]
1
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4a
4b
4c
4d
4e
4f
15
30
25
30
35
50
50
25
28
45
30
27
93
80
82
78
77
89
74
75
79
76
83
85
198-200[198-199][40]
169-171[169-170][40]
187-189[190][43]
191-193[195-197] [42]
134-136 [138-140] [42]
245-247c
2
4-CH3
4-Cl
H
3
H
4
4- F
H
5
H
5-Br
H
6
4-OCH3
H
7
5-CH3
7-CH3
H
4g
4h
4i
173-175c
238-240c
8
H
9
4-NO2
2-OCH3
3-Br
Piperonal
204-206 [207][43]
185-186[186-186] [43]
203 [206] [43]
10
11
12
H
4j
H
4k
4l
H
241-243[242-244] [42]
a
Reaction conditions:4-hydroxycoumarin (1 mmol), aryl aldehyde (1 mmol), indole/substituted indole (1
mmol) and Fe3O4@SiO2-SO3H-DABCO (10 mg) in (5mL, 50:50 v/v) EtOH:H2O at room temperature.
b
Isolated yield. cNovel compound.
Fig. 10. Reusability study of Fe3O4@SiO2-SO3H-DABCO.
cilitates Michael addition of C-3 indole on the [B]. This reaction
generates an intermediate [C] which finally undergoes deprotona-
tion to afford desired product 4a.
3.4. Comparative study of catalytic activity of
Fe3O4@SiO2-SO3H-DABCO with reported catalysts
To explore the merits of the present methodology, the cat-
alytic efficiency of novel Fe3O4@SiO2-SO3H-DABCO nanocomposite
for the synthesis of 3-(1H-indole-3-yl)(phenyl)methyl)-4-hydroxy-
2H-chromen-2-ones (4a) has been compared with reported meth-
ods. From the comparison as depicted in Table 3, it is observed
that the present methodology is superior to reported procedures
in terms of reaction time and percentage yield.
Scheme 3. Plausible mechanism for the synthesis of 3-(1H-indole-3-
yl)(phenyl)methyl)-4-hydroxy-2H-chromen-2-one (4a).
4-hydroxy-2H-chromen-2-ones in high to excellent yields (entries
4a-l, Table 2). Particularly, the electronic nature of substituents on
aryl aldehydes and indoles had no critical effect on product yield.
3.5. Recyclability of Fe3O4@SiO2-SO3H-DABCO
The reusability of catalysts is an important criterion in the
viewpoint of green chemistry applications in academic and indus-
trial research. The reusability study of Fe3O4@SiO2-SO3H-DABCO
nanocomposite was investigated using model reaction under op-
timized reaction conditions. After completion of the model reac-
tion, Fe3O4@SiO2-SO3H-DABCO was recovered by an external mag-
net. The catalyst was washed thoroughly with diethyl ether and
kept for drying at 60 °C for 2 h in vacuum oven. The quantity of
regenerated catalyst was found to be almost same as used in the
fresh run. The catalytic efficiency of the regenerated catalyst was
tested in the first cycle and afforded 4a in 89% yield. The procedure
of isolation, washing and drying of the catalyst was repeated after
every cycle. Reusability study shows that Fe3O4@SiO2-SO3H-DABCO
could be reused for six runs without significant loss of catalytic
activity (Fig. 10). The structural rigidity of recycled Fe3O4@SiO2-
SO3H-DABCO nanocomposite was confirmed by FT-IR (Figure 2e),
3.3. Plausible mechanism
A plausible mechanism of a one-pot three-component reaction
of 4-hydroxycoumarin, benzaldehyde and indole for the synthe-
sis of 3-(1H-indole-3-yl)(phenyl)methyl)-4-hydroxy-2H-chromen-
2-one (4a) is proposed in Scheme 3. As illustrated in Scheme 3,
both cationic and basic nitrogen of DABCO of the immobilized cat-
alyst participates in the formation of hydrogen bond and abstrac-
tion of acidic proton of the reactant molecules [59]. In the first
step, abstraction of the hydrogen atom of the hydroxyl group of 4-
hydroxycoumarin (1) by the basic site of catalyst and protonation
of carbonyl carbon of benzaldehyde (2a) by acidic site [5] gener-
ates an intermediate [A] which upon dehydration converted into
intermediate [B]. In the second step, the acidic site of the catalyst
protonates carbonyl carbon of an intermediate [B] [60] which fa-
7

A. Yadav, P. Patil, D. Chandam et al.
Journal of Molecular Structure 1245 (2021) 130960
Table 3
Comparison of catalytic efficiency of Fe3O4@SiO2-SO3H-DABCO with reported catalysts in the synthesis of di-
heteroarylmethanes of indole and 4-hydroxycoumarin.
Entry
Catalyst
Conditions
Time (min)
Yield(%)
Reference
1
2
3
4
5
InCl3
Toluene, reflux
180
60
75
75
88
90
93
[41]
L-proline
Solvent-free, grinding
n-Hexane, reflux
EtOH, 80oC
[42]
Copper octoate
Fe3O4 @SiO2–PEG/NH2
Fe3O4@SiO2-SO3H- DABCO
60
[43]
15
[44]
EtOH: H2O (50:50 v/v), r.t.
15
Present work
and FE-SEM (Fig. 6c-d) coupled with EDX (Fig. 5b) indicating no
apparent change in the functional group, composition, morphology
and size of the recycled catalyst.
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In conclusion, a robust magnetically separable Fe3O4@SiO2-
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Credit author statement
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nanoparticles (LPMNP):
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Archana Yadav, Pradeep Patil, Sushilkumar Jadhav: Synthesis of
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Declaration of Competing Interest
The authors affirm no conflicts of interest.
Acknowledgment
[16] A. Khazaei, N. Sarmasti, J.Y. Seyf, Waste to wealth: conversion of nano-
magnetic eggshell (Fe3O4@Eggshell) to Fe3O4@Ca(HSO4)2: cheap, green and
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Sandeep Sankpal thankful to Shivaji University, Kolha-
pur for financial assistance through Research Strengthening
Grant (SU/C&U.D. Section/90/1387). Archana Yadav grateful
to BARTI for the award of a fellowship is also acknowledged
(BARTI/Fellowship/BANRF-2018/19-20/3036).
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a novel dual-function
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