Magnetic inorganic-organic hybrid nanomaterial for the catalytic preparation of bis(indolyl)arylmethanes under solvent-free conditions: Preparation and characterization of H5PW10V 2O40/pyridino-Fe3O4 nanoparticles

Article abstract of DOI:10.1016/j.apcata.2013.07.065

Magnetically separable inorganic-organic hybrid material H ₅PW₁₀V₂O₄₀/pyridino-Fe ₃O₄ (HPA/TPI-Fe₃O₄) nanoparticles provide an efficient and economic route for the synthesis of substituted bis(indolyl)methanes by the electrophilic substitution of indole with various aldehydes and ketones under solvent-free conditions with excellent yields in <30 min. This catalyst was prepared by the chemically anchoring H ₅PW₁₀V₂O₄₀ onto Fe₃O ₄ nanoparticles via the use of N-[3-(triethoxysilyl)propyl] isonicotinamide (TPI) linker. High catalytic activity and ease of recovery using an external magnetic field are additional eco-friendly attributes of this new catalytic system. The catalyst was recycled at least eight times without significant loss of catalytic activity. Characterization techniques such as XRD, SEM, and FTIR for HPA/TPI-Fe₃O₄ verified that the Keggin structure of tungsto-vanadophosphoric acid on the surface of pyridine modified Fe₃O₄ nanoparticles is retained.

Full text of DOI:10.1016/j.apcata.2013.07.065

Applied Catalysis A: General 468 (2013) 75–87
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Magnetic inorganic–organic hybrid nanomaterial for the catalytic
preparation of bis(indolyl)arylmethanes under solvent-free
conditions: Preparation and characterization of
H5PW10V2O40/pyridino-Fe3O4 nanoparticles
Reza Tayebeea,∗, Mostafa M. Aminib, Nasrin Abdollahia, Azam Aliakbarib, Sima Rabieic,
Hassan Ramshinid
a Department of Chemistry, School of Sciences, Hakim Sabzevari University, Sabzevar, Iran
b Department of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
c Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
d Biology Department, Payam Noor University, 19395-4697 Tehran, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetically separable inorganic–organic hybrid material H5PW10V2O40/pyridino-Fe3O4 (HPA/TPI-
Fe3O4) nanoparticles provide an efficient and economic route for the synthesis of substituted
bis(indolyl)methanes by the electrophilic substitution of indole with various aldehydes and
ketones under solvent-free conditions with excellent yields in <30 min. This catalyst was pre-
pared by the chemically anchoring H5PW10V2O40 onto Fe3O4 nanoparticles via the use of
N-[3-(triethoxysilyl)propyl]isonicotinamide (TPI) linker. High catalytic activity and ease of recovery
using an external magnetic field are additional eco-friendly attributes of this new catalytic system.
The catalyst was recycled at least eight times without significant loss of catalytic activity. Characteri-
zation techniques such as XRD, SEM, and FTIR for HPA/TPI-Fe3O4 verified that the Keggin structure of
tungsto-vanadophosphoric acid on the surface of pyridine modified Fe3O4 nanoparticles is retained.
© 2013 Elsevier B.V. All rights reserved.
Received 1 June 2013
Received in revised form 18 July 2013
Accepted 21 July 2013
Available online xxx
Keywords:
Hybrid material
H5PW10V2O40
Fe3O4
Magnetic nanoparticles
Bis(indolyl)arylmethane
1. Introduction
stoichiometric amount of the Lewis acid, expensive and highly
toxic catalysts, and formation of hazardous by-products have
Indole derivatives are an important class of natural based
heterocyclic compound, which have demonstrated important bio-
logical and anticancer activities [1]. Among various substituted
indoles, bis(indolyl)arylmethanes have attracted much interest due
to their broad application in material sciences, in the pharma-
ceutical and in the agrochemical industries, and because of their
importance as potential antibacterial and antitumor reagents [2–5].
Due to the interesting biological activities displayed by various
kinds of indole derivatives, there is a great demand for novel syn-
thetic methods in this area and the development of new procedures
for the synthesis of bis(indolyl)arylmethanes is of current interest
[6]. Various routes using sodium p-toluene sulfonate [7], SBA-15
supported poly(4-styrenesulfonyl(perfluorobutylsulfonyl)imide)
[8], NaHSO4/SiO2 [9], zeolites [10], ionic liquids [11], and citric
acid modified H␤ zeolite [12] have been employed. Disadvantages
including low yield, prolonged reaction time, requirement of a
necessitated further development of environmentally benign and
economical alternatives.
Recently, metal oxide nanoparticles have emerged as poten-
tial heterogeneous catalysts because of their interesting structures
and high catalytic activities [13]. In particular, magnetic nanoparti-
cles have emerged as a useful class of materials for applications in
catalysis, biotechnology, and medicine [14]. Amongst these, Fe3O4
nanoparticles arguably have attracted most attention. The reason
for this is due to the ease of handling, ease of recovery with an exter-
nal magnetic field, as well as the high catalytic activity of Fe3O4
nanoparticles in various organic transformations [15].
Heterogenization of dodeca-tungstovanadophosphoric acid,
H5PW10V2O40, through immobilization onto the surface of a modi-
fied porous material and fabrication of an organic–inorganic hybrid
material has been the target of several studies in recent years
[16,17]. In this powerful strategy, the active homogeneous catalyst
chemically immobilizes/anchors onto an inert porous solid mate-
rial or is encapsulated in the pores and cavities of a mesoporous
material through the action of an organic linker to overcome the
potential negative aspects of catalyst leaching.
∗
Corresponding author. Tel.: +98 571 4410310; fax: +98 571 4410300.
E-mail address: Rtayebee@yahoo.com (R. Tayebee).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.07.065

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R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
R
R''
O
c
R''
Fe3O4/TPI-HPA
R'
+
2
C
solvent free, 100 oC
R
N
H
R'
R'
N
N
R= alkyl, halogen, hydroxyl, nitro, methoxy
H
H
R'= Me, H
R''= H, alkyl
HPA= H5PW10V2O40
Scheme 1. General route for the preparation of different bis(indolyl)arylmethanes.
Herein, we report the catalytic performance of a new magnetic
1 h. Then, 2 ml of TPI was added and the mixture was refluxed for
24 h. The black solid was removed by filtration and washed with
toluene and ethanol, then dried at room temperature. The synthe-
sized Fe3O4 and pyridine-functionalized material (TPI-Fe3O4) were
characterized by IR spectroscopy, X-ray diffraction, thermal analy-
sis (TGA/DTA), scanning electron microscopy (SEM), and elemental
analysis.
inorganic–organic hybrid material, HPA/TPI-Fe3O4 nanoparti-
cles, for the effective and economic synthesis of substituted
bis(indolyl)methanes through the electrophilic substitution of
indole with various aldehydes and ketones under solvent-free con-
ditions (Scheme 1).
2. Experimental
2.3. Immobilization of 10-tungsto-2-vanadophosphoricacid on
TPI-Fe3O4
2.1. Materials and methods
For the immobilization of H5PW10V2O40 on TPI-Fe3O4, 50 ml of
methanol solution containing 0.3 g of H5PW10V2O40·30H2O was
added to a freshly activated TPI-Fe3O4 (0.6 g) and the mixture
was refluxed for 3 h. Then, the heterogeneous catalyst was filtered
and extracted in a Soxhlet extractor using methanol as solvent for
12 h and thereafter was dried at 105 ◦C in a vacuum oven. After
one day’s washing and Soxhlet extraction, the solution phase was
subjected to UV–visible analysis and no characteristic bands for
10-tungsto-2-vanadophosphoricacid were found. This clearly indi-
cated retention of the heteropoly acid in the pores of TPI-Fe3O4.
The amount of H5PW10V2O40 loading on TPI-Fe3O4 was found
0.06 mmol/g as determined by UV–visible analysis.
All starting materials and solvents were commercially avail-
able, and their purities were monitored by gas chromatography.
All products were identified by comparison of their spectral and
physical data with those previously reported [18–20]. Progress of
the reactions was monitored by TLC. Melting points were recorded
on a Bamstead electrothermal type 9200 melting point apparatus.
Ultraviolet–visible spectra were recorded on a Shimadzu Model
UV-2550 spectrophotometer. Infrared spectra were recorded (KBr
pellets) on a 8700 Shimadzu Fourier Transform spectrophotome-
ter. 1H and 13C NMR spectra were recorded on a Bruker AVANCE
300-MHz instrument using TMS as internal reference. Data for 1
H
NMR are reported as follows: chemical shift (ı) and multiplicity (s:
singlet, d: doublet, t: triplet, q: quartet, m: multiplet, qt: quintet,
dq: doublet of quartets, br: broad). X-ray diffraction patterns were
obtained on a STOE diffractometer with Cu K␣ radiation at 60 KeV
and 15 mA with a scanning rate of 3◦/min in the 2ꢀ range from 5◦ to
80◦. Electron microscopy was performed on Phillips XL-30 scanning
electron microscope (SEM). For observation of morphology by SEM,
sample was poured directly on to a sample holder and then sputter
coated with a gold film. For identification of the elements present
in the catalyst, energy-dispersive spectroscopy (EDX) was used.
Thermal analysis (TGA-DTA) was carried out on a Bahr STA-503
instrument in air at a heating rate of 10 ◦C/min. The heteropoly acid
catalyst H5PW10V2O40 was prepared and characterized according
to the literature procedure [21].
The functionalized TPI-Fe3O4 supported H5PW10V2O40 did not
leach after prolonged contact with aqueous solution. Strong inter-
action between the functionalized material and H5PW10V2O40
made it
a suitable heterogeneous catalyst. Therefore, N-[3-
(triethoxysilyl)propyl]isonicotinamide (TPI) linker is a suitable
agent for immobilization process and provided covalent anchoring
of the heteropoly acid onto the functionalized support. A schematic
for the preparation of HPA/TPI-Fe3O4 nanocatalyst is shown in
Scheme 2.
2.4. General procedure for the preparation of
bis-(indolyl)phenylmethanes
A mixture of carbonyl compound (1 mmol), indole (0.234 g,
2 mmol) and the desired amount of the inorganic–organic hybrid
material HPA/TPI-Fe3O4 nanoparticles was stirred at elevated tem-
perature (100 ◦C) for an appropriate reaction time. After completion
of the reaction, as indicated by TLC, acetonitrile (3 ml) was added to
the reaction mixture and the catalyst was removed using an exter-
nal magnetic bar. Then, silica gel (∼1 g) was added to the filtrate,
and after evaporation of the solvent, a dark pinkish solid mixture
was obtained. Purification of the product was performed by short
column chromatography eluted with ethylacetate/petroleum ether
(1/9) to give a pinkish solid product in high yield. Products were
known compounds and were identified by means of IR and 1H NMR
spectroscopy and/or comparison of their melting points with those
reported in the literature.
2.2. Synthesis of Fe3O4and pyridine-functionalized Fe3O4
nanoparticles (TPI-Fe3O4)
Fe3O4 nanoparticles were synthesized according to a previously
reported procedure [22]. Briefly, 10.4 g of FeCl3·6H2O and 4.0 g of
FeCl2.4H2O were dissolved in 100 ml of deionized water, degassed
with nitrogen gas for 15 min and heated to 80 ◦C. Then, 15 ml of NH3
(32% aqueous solution) was added drop-wise to the solution. After
15 min, the solid product was separated by a magnet and washed
three times with NaCl (0.1 M) solution. The identity of the prepared
nanoparticles was confirmed by FTIR and X-ray powder diffraction.
For the preparation of pyridine-functionalized Fe3O4, 1.0 g of Fe3O4
was suspended in 50 ml toluene, and the mixture was stirred for

R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
77
OH
OH
O
OH
OH
OH
OH
OH
OH
OH
EtO
EtO
Si
N
H
+
N
EtO
(TPI)
Toluene
(Fe3O4)
Reflux
24 h
O
O
+
+
O
NH
Si
Si
NH2
HPA
O
O
(PAn-)
O
+
+
O
NH2
NH
O
(Fe3O4-pyridino/H5PW10V2O40)
Scheme 2. A schematic for the preparation of HPA/TPI-Fe3O4 nanocatalyst.
2.5. Spectral data for selected bis(indolyl)methanes
2.5.6. Bis(indolyl)-3,4-dimethoxyphenylmethane
1H NMR (CDCl3, 300 MHz), ı (ppm): ı 3.76 (s, 3H), 3.85 (s, 3H),
5.83 (s, 1H), 6.65 (d, 2H), 6.78 (d, 2H), 7.0 (t, 3H), 7.17 (t, 2H),
7.29–7.43 (m, 4H), 7.91 (br, NH, 2H). IR (KBr, cm−1) ꢁmax: 3480,
3020, 1604, 1512, 1456, 1418, 1336, 1216, 1091, 1033, 759.
2.5.1. Bis(indolyl)phenylmethane
1H NMR (CDCl3, 300 MHz), ı (ppm): 5.91 (s, 1H, CH), 6.68 (s,
2H), 7.10 (t, J = 7.3 Hz, 2H, arom), 7.22 (t, J = 7.3 Hz, 2H, arom), 7.23
(t, J = 7.2 Hz, 1H, arom), 7.30–7.40 (m, 2H, arom), 7.35–7.45 (m, 4H,
arom), 7.41 (d, J = 7.9 Hz, 2H, arom), 7.90 (s, br, 2H, NH). 13C NMR, ı
(ppm): 40.6, 111.4, 119.7, 120.2, 120.4, 122.4, 124.1, 126.6, 127.5,
128.6, 129.2, 137.1, 144.4. IR (KBr, cm−1) ꢁmax: 760, 820, 890, 960,
1080, 1200, 1410, 1640, 2380, 2880, 2900, 3340.
2.5.7. Bis(indolyl)-2-methoxylphenylmethane
1H NMR (CDCl3, 90 MHz), ı (ppm): 3.82 (s, 3H), 6.32 (s, 1H), 6.61
(s, 2H), 6.81–7.40 (m, ArH, 12H), 7.80 (br, NH, 2H). IR (KBr, cm−1
)
ꢁmax: 3408, 3056, 2932, 1597, 1486, 1450, 1335, 1102, 745.
2.5.8. Bis (indolyl)-2-chlorophenylmethane
2.5.2. Bis(indolyl)-4-nitrophenylmethane
1H NMR (CDCl3, 400 MHz), ı (ppm): 6.34 (s, 1H, Ar–CH), 6.66 (s,
2H), 7.02 (t, 2H, J = 8.0 Hz), 7.11–7.23 (m, 6H), 7.36–7.43 (m, 4H),
7.96 (br, s, 2H, NH). IR (KBr, cm−1) ꢁmax: 1010, 1037, 1093, 1337,
1417, 1455, 1616, 3052, 3412.
1H NMR (CDCl3, 300 MHz), ı (ppm): 5.92 (s, 1H, CH), 6.63 (s, 2H),
6.91 (t, J = 7.3 Hz, 2H, arom), 7.08 (t, J = 7.3 Hz, 2H, arom), 7.25 (d,
J = 7.9 Hz, 2H, arom), 7.32 (d, J = 8.1 Hz, 2H, arom), 7.44 (d, J = 8.6 Hz,
2H, arom), 8.04 (d, J = 8.6 Hz, 2H, arom), 9.31 (s, br, 2H, NH). 13C
NMR, ı (ppm): 40.5, 111.8, 117.8, 119.4, 119.7, 122.1, 123.8, 124.4,
127.0, 129.9, 137.3, 146.7, 152.8. IR (KBr, cm−1) ꢁmax: 736, 1086,
1343, 1416, 1457, 1506, 1636, 3447.
3. Results and discussion
Nanosized magnetic Fe3O4 has an inverse spinel structure,
which contains iron cations in mixed oxidation states. The oxy-
gen atoms form a face-centered cubic close-packed array and the
Fe2+ and half of the Fe3+ ions occupy octahedral sites and the
remaining half of the Fe3+ cations occupy tetrahedral holes. A major
advantage of the current study is the possibility of recovery of
the catalyst by magnetic separation. Apparently, magnetic sepa-
ration is typically more effective than filtration or centrifugation
as it prevents loss of the catalyst (Fig. 1). Furthermore, magnetic
separation of the nanoparticles is simple, economical, and promis-
ing for industrial application. The use of a more environmentally
benign catalyst coupled with the ease of catalyst separation and
reuse are important features of the present report. The versatility
of this approach is illustrated through the synthesis of a wide range
of bis(indoyl)methanes.
2.5.3. Bis(indolyl)-4-chlorophenylmethane
1H NMR (CDCl3, 300 MHz), ı (ppm): 5.82 (s, 1H), 6.54 (brs, 2H),
7.0 (t, 2H), 7.15 (t, 2H), 7.25–7.35 (m, 8H), 7.81 (br, 2H, NH). 13C
NMR, ı (ppm): 39.4, 111.2, 129.2, 129.4, 129.8, 122.0, 123.2, 126.4,
128.0, 130.0, 131.4, 136.2, 142.5. IR (KBr, cm−1) ꢁmax: 667, 761, 856,
1012, 1040, 1091, 1124, 1216, 1338, 1416, 1455, 1486, 1518, 1616,
1726, 2853, 2925, 3011, 3056, 3413.
2.5.4. Bis(indolyl)-(1-Methyl)methanediyl
1H NMR (CDCl3, 300 MHz), ı (ppm): ı 1.91 (d, 3H, J = 6.8 Hz), 4.50
(m, 1H), 6.85 (t, 2H, J = 6.8 Hz), 7.03 (m, 2H), 7.07 (t, 2H, J = 8 Hz), 7.25
(d, 2H, J = 8 Hz), 7.40 (d, 2H, J = 8 Hz), 7.84 (br, 2H). IR (KBr, cm−1
ꢁmax: 3399, 3389, 2958.
)
3.1. Structural characterization of the magnetic
inorganic–organic nanohybrid material HPA/TPI-Fe3O4
2.5.5. Bis(indolyl)-cyclohexanediyl
1H NMR (CDCl3, 300 MHz), ı (ppm): ı 1.61 (m, 6H), 2.54 (m, 4H),
3.25 (m, 1H), 4.60 (s, 1H), 6.89 (t, 2H, J = 7.2 Hz), 7.07 (m, 4H), 7.29
3.1.1. FTIR spectroscopy
(d, 2H, J = 8.1 Hz), 7.55 (d, 2H, J = 8.1 Hz), 7.92 (br, 2H). IR (KBr, cm−1
ꢁmax: 3450, 3030, 2929, 1658, 1550, 1461, 740.
)
Fe3O4
nanoparticles
were
modified
with
N-[3-
(triethoxysilyl)propyl]isonicotinamide (TPI) linker in refluxing

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R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
Fig. 3. The fingerprint IR bands of H5PW10V2O40 with the Keggin structure.
Fig. 1. HPA/TPI-Fe3O4 nanoparticles dispersed in CH3CN (a) and the nanoparticles
adsorbed on a magnetic stirring bar (b).
[23,24]. The broad band at ∼1080 cm−1 belongs to the symmetric
stretching vibration of the PO4 tetrahedron. The band at ∼990 cm−1
is attributed to the stretching vibration of W Od, connecting with
the terminal oxygen atoms. The band at ∼885 cm−1 is assigned
to the stretching vibration of W Ob W, associated with corner
bridging oxygens, and the band at ∼780 cm−1 originates from the
vibration of W Oc W, associated with side bridging oxygens. The
IR spectrum of HPA/TPI-Fe3O4 nanoparticles possessed a broad
band in the range of 1020–1075 cm−1, proving strong interaction of
pyridino-Fe3O4 with the heteropoly acid. Moreover, consideration
of the fingerprint IR bands of the Keggin structure in 800–1100 cm−1
region revealed loading of H5PW10V2O40 on the surface of TPI-
Fe3O4. It should be noted that a slight shift in the frequencies after
immobilization could be as a result of the heteropoly acid inter-
action with the hydroxyl functional groups on the surface of the
support material. Furthermore, detection of absorption bands at
about 200 and 260 nm in the UV–visible spectrum of the cata-
lyst established retention of the Keggin structure. These bands are
caused by charge transfer of the terminal and the bridged oxygens
to the metal atoms, respectively [25].
toluene. The TPI was anchored onto the surface of Fe3O4 by
condensation of the hydroxyl groups of the mesoporous material
and the ethoxy groups of TPI as illustrated in Scheme 2. The
incorporation of the linker on the surface of Fe3O4 was confirmed
by FTIR investigation (Fig. 2). Detection of weak characteristic
stretching vibrations of pyridine at 1550–1650 cm−1 and appear-
ance of shoulders at 2900–3000 cm−1 in the FTIR spectra indicated
grafting of TPI group on the surface of the nanosized material.
Furthermore, the Si
O Fe, Fe O and FeO H stretching vibration
bands of Fe3O4 nanoparticles were detected at 1150, 950, and
>3200 cm−1, respectively. The broad band in the 3500–3700 cm−1
region can be attributed to the symmetrical stretching vibration
modes of the adsorbed water molecules.
The Keggin structure of H5PW10V2O40 consists of a central PO4
tetrahedron surrounded by four W3O9 groups formed by edge
sharing octahedra. These groups are connected to each other by
corner-sharing oxygen atoms. This structure resulted in different
types of oxygen atoms, being responsible for the four fingerprint
IR bands of the Keggin anion between 500 and 1100 cm−1 (Fig. 3)
3.1.2. X-ray diffraction study
The diffraction peaks of H5PW10V2O40 (Fig. 4) with the d values
of 8.76, 6.15, 5.01, 4.33, 3.87, 3.52, 3.26, 3.05, 2.87, 2.73, 2.60, 2.39,
´
˚
◦
2.22, 2.03, 1.97, 1.88, 1.72, and 1.54 A in the 2ꢀ range of 10–80
are in accordance with the JCPDS index for this compound. X-ray
diffraction (XRD) patterns of Fe3O4, TPI-Fe3O4 and HPA/TPI-Fe3O4
are shown in Fig. 5. The XRD pattern of the Fe3O4 exhibited the
(2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) reflections which
is consistent with the literature that standard Fe3O4 crystal with
the spinel structure has six diffraction peaks: (2 2 0), (3 1 1), (4 0 0),
Fig. 2. FTIR spectra of (A) Fe3O4, (B) Fe3O4-TPI, and (C) HPA/Fe3O4-TPI.
Fig. 4. X-ray diffraction pattern of H5PW10V2O40.

R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
79
Fig. 7. TGA and DTA curves of HPA/TPI-Fe3O4.
3.1.3. SEM studies
An SEM image of HPA/TPI-Fe3O4 is shown in Fig. 6a. This
micrograph clearly indicates homogeneity of the material and
also shows that the H5PW10V2O40 is well dispersed in TPI-
Fe3O4. Concerning the chemical composition of the HPA/TPI-Fe3O4
nanocatalyst, energy dispersive X-ray (EDX) analysis (Fig. 6b)
was undertaken. The analysis of EDX spectrum proved existence
of tungsten, vanadium, carbon, and silicon in the nanocata-
lyst.
Fig. 5. X-ray diffraction patterns of Fe3O4, TPI-Fe3O4, and HPA/TPI-Fe3O4. Asterisks
indicate the peaks associated with H5PW10V2O40
.
(4 2 2), (5 1 1) and (4 4 0) [22]. The position and relative intensity of
all diffraction peaks match well with the standard magnetite peaks.
Therefore, the spinel structure of Fe3O4 is retained after function-
alization with TPI and immobilization of HPA onto TPI-Fe3O4. In
addition, in HPA/Fe3O4-TPI pattern, some additional peaks, as indi-
cated by asterisks, can be associated with the Keggin structured
H5PW10V2O40. Obviously, the low intensity of these peaks is asso-
ciated with the amount of loaded HPA immobilized onto TPI-Fe3O4.
However, the refection at about 2ꢀ = 10◦ with medium intensity is
absent. The absence of this refection after immobilization on the
support probably can be explained by preferential orientation of
HPA [26]. Furthermore, XRD pattern of TPI.Fe3O4 shows broader
reflections in comparison with Fe3O4. This can be attributed to
its smaller crystalline size and more amorphous nature. It seems
TPI on the surface of nanoparticles acted as surfactant and pre-
vented from agglomeration of primary particles, and resulted in
wider reflections in the XRD pattern [27].
3.1.4. Thermal analysis
The thermal stability of HPA/TPI-Fe3O4 nanoparticles was stud-
ied by carrying out TGA-DTA analysis (Fig. 7). The DTA profile
exhibited a broad endothermic peak from room temperature to
200 ◦C which is attributed to the elimination of adsorbed water
molecules. A weight loss occurred between 200 ◦C and 500 ◦C which
is accompanied with a broad exothermic peak between 350 ◦C and
450 ◦C in DTA curve, corresponding to the oxidative decomposition
of the organic part N-[3-(triethoxysilyl)propyl]isonicotinamide
(TPI). Furthermore, according to thermal analysis, the prepared
nanocatalyst was stable up to 200 ◦C.
3.2. Effect of catalyst concentration on the condensation reaction
Catalytic reactions were carried out by the addition of the
heteropoly acid chemically adsorbed on the surface of pyridine
Fig. 6. The SEM image (a) and EDX spectrum (b) of HPA/TPI-Fe3O4.

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R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
Table 1
Table 3
Effect of catalyst amount on the condensation of benzaldehyde with indole under
solvent-free conditions.
The effect of different solvents on the synthesis of bis(indolyl)phenylmethane in the
presence of HPA/TPI-Fe3O4 at 60 ◦C.
Entry
HPA/TPI-Fe3O4 (g)
H5PW10V2O40 (mol%)
Time (min)
Yield (%)
Entry
Solvent
Boiling point (◦C)
Time (min)
Yield (%)
1
2
3
4
5
0
0
180
30
30
25
10
<5
66
74
96
99
1
2
3
4
5
6
7
H2O
100
81
65
78
61
83
–
120
90
50
120
60
50
15
52
35
46
50
34
96
0.002
0.005
0.01
0.15
0.01
0.025
0.055
0.8
CH3CN
CH3OH
C2H5OH
CHCl3
ClCH2CH2Cl
–
25
modified Fe3O4 nanoparticles, HPA/TPI-Fe3O4, to a mixture of
aldehyde and indole, with thorough stirring in the absence of sol-
vent under conventional heating to 100 ◦C. Bis(indolyl)methane
derivatives were formed in good to excellent yields. This two-
component condensation protocol was carried out in the presence
of a low amount of HPA/TPI-Fe3O4 (0.06 mol%), and selectively led
to excellent yields of bis(indolyl)arylmethanes under solvent-free
conditions in <25 min.
The catalytic efficacy of HPA/TPI-Fe3O4 was studied for the
preparation of bis(indolyl)phenylmethane with various amounts of
catalyst (Table 1). At first, a model reaction was run in the absence
of catalyst. The reaction of benzaldehyde with indole did not afford
an appreciable amount of the desired bis(indolyl)phenylmethane
in the absence of catalyst after an extended reaction time of 3 h;
Reaction conditions are as described below Table 1 (except for reaction tempera-
ture). 0.01 g of HPA/TPI-Fe3O4, including 0.0019 g of H5PW10V2O40 (0.06 mol%) was
used in all cases. 1.0 ml solvent was used. Total volume (∼1.3 ml) was controlled
using a reflux condenser.
after 25 min (entries 2 and 3, respectively). Although the compo-
nents of the composite material were active, HPA/TPI-Fe3O4 was
the best for the desired organic transformation (entry 4). Clearly,
immobilizing the heteropoly acid on the modified solid material
TPI-Fe3O4 increased its catalytic activity toward the condensation
reaction.
3.4. Effect of organic solvent on the condensation reaction
whereas, 0.002 g of catalyst containing 0.01 mol% of H5PW10V2O40
,
Different solvents such as water, ethanol, methanol, acetoni-
trile, 1–2 dichloroethane, and chloroform were investigated for
the model reaction of benzaldehyde and indole in the pres-
ence of 0.01 g of HPA/pyridino-Fe3O4 (Table 3). This study was
conducted at a fixed temperature, 60 ◦C, in all cases. The sol-
vent influenced the catalytic performance and the best yield
was achieved under solvent-free conditions. Almost, all reac-
tions were less efficient in the presence of solvent than in its
absence. However, acetonitrile and choloroform accomplished
resulted in better performance amongst all the examined sol-
vents.
afforded 66% yield after 0.5 h. This finding revealed that the het-
erogeneous catalyst exhibited high catalytic activity in the desired
transformation. Table 1 shows that enhancing the amount of cat-
alyst, initially, led to higher yields at short reaction time and
0.01 g of catalyst was sufficient. A higher amount of catalyst,
0.15 g, resulted in similar yield (96%) in shorter reaction time,
10 min.
A mixture of aldehyde (1 mmol), indole (2 mmol) and the cata-
lyst was stirred at 100 ◦C for the appropriate time. The catalyst mol%
was calculated based on the H5PW10V2O40 content with respect to
1 mmol indole. Aldehyde (liquid and/or solid) and indole (melting
point 52–53 ◦C) formed a homogeneous liquid mixture with a total
volume of ∼0.3 ml. Progress of the reaction was monitored with
TLC. Work-up was carried out as described in the experimental
section.
3.5. Effect of temperature on the condensation reaction
Effect
of
temperature
on
the
preparation
of
bis(indolyl)phenylmethane in the presence of HPA/TPI-Fe3O4
nanoparticles (0.01 g) was investigated under solvent-free condi-
tions (Table 4). The yield of products was enhanced by increasing
temperature from 25 to 120 ◦C. Even though the product yield at
120 ◦C was slightly higher than at 100 ◦C, considering the technical
and economical requirements, the lower temperature was selected
as the favored temperature for all reactions.
3.3. The studying the catalytic activity of the components from
which the organic–inorganic hybrid nanomaterial is comprised
Table 2 compares the catalytic activity of HPA/TPI-Fe3O4 with
the bare H5PW10V2O40, TPI-Fe3O4, and Fe3O4 components, sep-
arately, in the preparation of bis(indolyl)phenylmethane under
the standard reaction conditions. 1.9 mg of H5PW10V2O40 cata-
lyst led to the 37% yield after 40 min (entry 1); whereas, 0.01 g of
the inorganic–organic hybrid nanomaterial HPA/TPI-Fe3O4, which
includes only 1.9 mg of the heteropoly acid achieved 96% yield after
a shorter reaction time of 25 min (entry 4). It should be born in mind
that the free carrier material (Fe3O4) and the modified material
TPI-Fe3O4 are catalytically active; the first led to 45% of prod-
uct after 20 min and the later yielded 37% of the desired product
3.6. Synthesis of different bis(indolyl)methanes
Proper surface coating of nanosized Fe3O4 with N-[3-
(triethoxysilyl)propyl]isonicotinamide (TPI) linker and, then,
covalent attachment of H5PW10V2O40 on TPI-Fe3O4 generated
a unique nanocatalyst for the selective and efficient generation
of a wide range of bis(indolyl)methanes. The non-toxicity and
Table 4
Table 2
Effect of temperature on the synthesis of bis(indolyl)phenylmethane in the presence
of HPA/TPI-Fe3O4 under solvent-free conditions.
The catalytic activity of H5PW10V2O40, TPI-Fe3O4, and Fe3O4 in the preparation of
bis(indolyl)phenylmethane.
Entry
Time (min)
Yield (%)
Temperature (◦C)
Entry
Catalyst
Catalyst (mg)
Time (min)
Yield (%)
1
2
3
4
180
40
25
69
81
96
99
25
80
100
120
1
2
3
4
HPA
Fe3O4
TPI-Fe3O4
HPA/TPI-Fe3O4
1.9
10
10
40
20
25
25
37
45
37
96
20
10
Reaction conditions are as described below Table 1. 0.01 g of HPA/TPI-Fe3O4, includ-
ing 1.9 mg of H5PW10V2O40 (0.06 mol%) was used in all cases.
Reaction conditions are as described below Table 1.

R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
81
Table 5
Synthesis of various bis(indolyl)methane derivatives in the presence of HPA/TPI-Fe3O4 under solvent-free conditions.
R
R''
O
c
R''
Fe3O4/TPI-HPA
R'
2
+
C
solvent free, 100 oC
N
H
R
R'
R'
N
N
R= alkyl, halogen, hydroxyl, nitro, methoxy
H
H
R'= Me, H
R''= H, alkyl
HPA= H5PW10V2O40
Entry
Aldehyde
Substrate
Time (min)
Yield (%)
MP (m.p./lit.m.p. (◦C))
147–148/145
Product
CHO
H
C
1
25
96
N
H
N
H
N
H
NO2
CHO
H
2
15
92
219–222/222
C
N
H
NO2
CHO
N
N
H
H
Cl
C
H
3
40
65
78–79/79
N
H
Cl
N
H
N
H
CHO
Cl
H
Cl
C
4
5
6
25
10
40
98
85
45
107–109/107
189–191/190
95–97/97
N
H
N
N
H
H
Br
CHO
H
C
N
H
Br
N
H
N
H
CHO
Me
H
Me
C
N
H
N
H
N
H

82
R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
Table 5 (Continued)
Entry
Aldehyde
Substrate
Time (min)
Yield (%)
MP (m.p./lit.m.p. (◦C))
Product
CHO
MeO
H
OMe
C
7
15
75
140–141/142
N
H
N
H
N
H
OMe
MeO
CHO
H
8
10
85
195
N
H
C
OMe
OMe
N
N
H
H
NO
2
CHO
H
C
9
20
95
220–222/220
N
NO2
H
N
N
H
H
CHO
CHO
NO2
H
NO
2
C
10
10
90
137–139/137
N
H
N
H
N
H
OH
H
11
10
65
90
C
N
H
OH
N
H
N
H
HO
CHO
H
C
12
10
98
98
N
H
OH
N
H
N
H
H
H
O
C
C
13
14
40
99
35
150
H
N
H
H
N
N
H
H
H
CH CH CH
2
3
2
C
O
CH CH CH C
140
118–120/118
3
2
2
N
H
N
H
H
N
H

R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
83
Table 5 (Continued)
Entry
Aldehyde
Substrate
Time (min)
Yield (%)
40
MP (m.p./lit.m.p. (◦C))
194–196/194
Product
CH
O
3
CH CHCH C
15
160
3
2
N
H
H
H
N
CHO
H
16
140
35
189–190/190
C
N
H
N
H
N
H
N
H
O
CH
H
17
35
20
80
C
N
H
N
H
N
H
CHO
H
CH3
C
18
20
65
92–93/92
N
H
Me
Me
N
H
N
H
H
H
O
C
19
15
56
119
C
H
N
H
H
N
N
H
H
H
CH3CH2CH2
C
O
CH CH CH C
20
21
35
30
50
61
118–119/119
3
2
2
N
H
H
N
H
N
H
CH
O
3
CH CHCH C
121
3
2
N
H
H
H
N
CHO
H
22
20
25
139
C
N
H
N
H
N
N
H
H
O
CH
H
C
23
40
86
98–100
N
H
N
H
N
H

84
R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
Table 5 (Continued)
Entry
Aldehyde
Substrate
Time (min)
Yield (%)
MP (m.p./lit.m.p. (◦C))
Product
CHO
CHO
H
CH3
C
24
15
92
105–110
N
H
Me
Me
N
H
N
H
F
CH3
H
25
10
95
248
N
C
H
Me
F
Me
N
N
H
H
H
CHO
Me
CH3
CH3
CH3
Me
C
26
27
28
20
10
15
65
82
97
255
170
201
N
H
Me
Me
N
N
H
H
CHO
MeO
H
OMe
C
N
H
Me
Me
N
N
H
H
H
CHO
Cl
Cl
C
N
H
Me
Me
N
N
H
H
Reaction conditions are as described below Table 1. 0.01 g of HPA/TPI-Fe3O4, including 1.9 mg of H5PW10V2O40 (0.06 mol%) was used in all cases.
chemical stability of this new organic–inorganic hybrid material
is amongst the most important characteristics of this catalyst. The
general nature of the method was demonstrated by using struc-
turally diverse aromatic and aliphatic aldehydes bearing electron
withdrawing and electron donating substituents in the reaction
with indole under the standard reaction conditions (Table 5). The
results revealed that this methodology is effective for a wide
range of aldehydes. However, aliphatic aldehydes showed less
reactivity toward the condensation reaction than their aromatic
analogs.
2-Methyl-indole was also used in the target condensation
reaction with some substituted aldehydes bearing electron with-
drawing and electron donating substituents (entries 24–28).
Comparison of the results obtained for indole and 2-methyl indole
clearly proved that the substitutedindole was more reactive toward
the condensation reaction than indole. Benzaldehyde produced 96%
yield with indole after 25 min (entry 1); whereas, 2-Me-indole led
to 96% yield in <15 min (entry 24). It seems that the electronic
influence of the methyl substituent on 2-methyl-indole positively
affected the electrophilic attack of indole group to the aldehyde
molecule and facilitated the generation of indolyl carbinol inter-
mediate (see Section 3.10).
3.7. Hot filtration test
A hot filtration test was carried out in order to confirm that
the catalytic activity was generated from the conjugated Fe3O4
nanoparticles on HPA-TPI and not from the leached components
in the reaction mixture. In this investigation, the condensation
reaction of benzaldehyde, and indole was performed at 100 ◦C for
15 min in the presence of the Fe3O4-TPI/HPA magnetic nanocata-
lyst. At this stage, the yield of the product was 75%. The catalyst
was then filtered off under hot conditions and with the filtrate,
which was obtained after separation of the catalyst after 15 min,
the reaction was continued for another 15 min at the same reac-
tion temperature. But, no corresponding increase in product yield
beyond 75% was observed. This result demonstrated the hetero-
geneous nature of the magnetic nanocatalyst in this condensation

R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
85
Table 6
Comparison of the catalytic efficiency of HPA/TPI-Fe3O4 nanocatalyst with some reported catalysts for the reaction of indole with benzaldehyde.
Entry
Catalyst
Catalyst (mol%)
Time (min)
Yield (%)
Conditions
Ref.
1
2
3
4
5
6
7
8
9
HPA/TPI-Fe3O4
HPA/TPI-SBA-15
H6P2W18O62
LiClO4
ZrOCl2.8H2O
0.06a
25
25
15
5 h
40
12 h
30
16
15
12 h
96
84
>80
90
84
64
93
80
92
95
Solvent-free/100 ◦
Solvent-free/100 ◦
Solvent-free/120 ◦
CH3CN/r.t
C
C
C
This work
[16]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
0.1b
2
10
5
10
150
5
10
Solvent-free/50 ◦
CH2Cl2/r.t
C
TiO2(nano)
Aminosulfonic acid
(PhCH2PPh3)+Br3−/SiO2
Trichloro-1,3,5-triazine
Lanthanide triflate
EtOH/H2O(N2 atm)
Slovent-free, 90 ◦
CH3CN
C
10
0.1 M solution
EtOH/H2O
[35]
a
1.0 g of the heterogeneous catalyst contains ∼0.15 g of the heteropoly acid and the catalyst mol% is calculated as describe below Table 1.
b
1.0 g of the heterogeneous catalyst contains 0.19 g of the heteropoly acid and the catalyst mol% is calculated based on H5PW10V2O40 content with respect to 1 mmol
indole.
n+
n-
{Fe O /TPI-H} PW
3
4
H
R
H
O
O
C
+
O
N
H
+
O
NH
Si
Si
NH
2
O
O
O
O
O
N
H
+
+
NH
2
NH
O
C
n+
n-
{Fe O /TPI-H} PW
3
4
H
R
H
H
O
H
C
H
R
R
C
N
H
N
H
N
H
R
C
OH
H
H
H
H
(PW)
R
C
N
H
N
H
N
H
R
(PW)
H
R
H
C
C
N
H
N
H
N
H O
2
(PW)
(PW)
H
N
H
Scheme 3. Proposed reaction pathway for the condensation of two molecules of indole with one molecule of aldehyde by the mediation of HPA/TPI-Fe3O4.

86
R. Tayebee et al. / Applied Catalysis A: General 468 (2013) 75–87
of indole to generate indolyl carbinol which further converts to
azafulvenium intermediate (I). The produced azafulvenium salt, as
an electrophile, can undergo nucleophilic addition of the second
indole molecule yielding the corresponding bis(indolyl)methane
derivative [36,37]. Our preliminary studies on the role of het-
eropoly acid showed that structure and acidity of the catalyst
contribute to its catalytic activity [28]. Although, all common
strongly acidic heteropoly acids achieved high catalytic activity,
H5PW10V2O40 with the Keggin structure was selected for this
study.
4. Conclusion
In
H5PW10V2O40/pyridino-Fe3O4 nano-particles are found to be
highly efficient and economically viable catalyst for the
fast and efficient synthesis of wide range of substituted
summary
the
magnetically
recoverable
a
Fig. 8. Yield% as a function of reusability of HPA/TPI-Fe3O4 nanoparticles.
a
bis(indolyl)methanes under solvent-free conditions. The salient
features are short reaction times, high yields, solvent-free con-
ditions, and the application of an easily magnetically recyclable
catalyst without any loss of catalytic activity. Applying surface
modified iron oxide nanoparticles makes this methodology sim-
ple, convenient, and practical for the large-scale synthesis of
bis(indolyl)methanes under mild conditions.
reaction and no leaching of active components occurred during the
course of the reaction.
3.8. Reusability and reproducibility of HPA/TPI-Fe3O4
nanoparticles
To investigate reusability of HPA/TPI-Fe3O4 nanoparticles, it
was magnetically separated easily from the reaction mixture and
washed with acetonitrile. The catalyst was slowly dried in air and
then was activated in a vacuum oven at 80 ◦C for 2 h. Finally, the
recycled catalyst was reused for a further condensation reaction.
The results revealed that the catalyst could be reused for at least
eight cycles without a significant loss of catalytic activity (Fig. 8).
Moreover, to ensure reproducibility of the transformation, repeated
typical experiments were carried out under identical reaction con-
ditions. The obtained yields were found to be reproducible within
2.5% variation.
Acknowledgement
Partial financial support from the Research Councils of Hakim
Sabzevari and Shahid Beheshti universities is greatly appreci-
ated.
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Comparison of the present heterogeneous catalyst was made
with some reported alternative methodologies (Table 6). The
reaction of benzaldehyde with indole for the synthesis of bis(3-
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a model reaction and the
comparison was made in terms of mol% of the catalysts, tem-
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3.10. Proposed reaction pathway for the heterogeneous catalytic
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