A novel inorganic-organic nanohybrid material H4SiW12O40/pyridino-MCM-41 as efficient catalyst for the preparation of 1-amidoalkyl-2-naphthols under solvent-free conditions

Article abstract of DOI:10.1039/c5dt00368g

A new inorganic-organic nanohybrid material H₄SiW₁₂O₄₀/pyridino-MCM-41 was prepared and performed as an efficient, eco-friendly, and highly recyclable catalyst for the one-pot multi-component synthesis of different substit

Full text of DOI:10.1039/c5dt00368g

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A novel inorganic–organic nanohybrid material
H4SiW12O40/pyridino-MCM-41 as efficient catalyst
for the preparation of 1-amidoalkyl-2-naphthols
under solvent-free conditions
Cite this: DOI: 10.1039/c5dt00368g
R. Tayebee,*a M. M. Amini,b M. Akbaria and A. Aliakbarib
A new inorganic–organic nanohybrid material H4SiW12O40/pyridino-MCM-41 was prepared and per-
formed as an efficient, eco-friendly, and highly recyclable catalyst for the one-pot multi-component syn-
thesis of different substituted 1-amidoalkyl-2-naphthols under solvent-free conditions. The nanohybrid
catalyst was prepared through electrostatic anchoring of Keggin heteropolyacid H4SiW12O40 on the
surface of MCM-41 nanoparticles modified by N-[3-(triethoxysilyl)propyl]isonicotinamide. The prepared
material was characterized by XRD, SEM, EDX, UV-Vis, DTA-TGA, DLS, and FT-IR spectroscopy. Findings
confirmed that the heteropolyacid is well dispersed on the surface of the solid support and its structure is
preserved after immobilization on the TPI modified MCM-41 nanoparticles. The recovered catalyst was
easily recycled for at least seven runs without considerable loss of catalytic activity.
Received 27th January 2015,
Accepted 8th April 2015
DOI: 10.1039/c5dt00368g
www.rsc.org/dalton
high dosages of a variety of drugs in its mesopores, called
drug delivery vehicles.8–10 Furthermore, mesoporous MCM-41
1. Introduction
Instability of homogeneous catalysts in reaction media and contains a large number of free silanol groups on its surface
difficulty in recovery are the main disadvantages which prevent which generate high specific surface areas and porosities;
their practical applications in both synthetic chemistry and therefore, the surface silanol groups can be modified by reac-
industrial processes. Therefore, preparing hybrid catalysts by tion with suitable organic modifiers and new inorganic–
linking homogeneous catalysts with organic linkers can organic hybrid mesoporous materials can thus be prepared.11,12
provide materials which are easier to handle, and may exhibit
1-Amidoalkyl-2-naphthols are raw materials for many
improved selectivity and activities.1 Inorganic–organic hybrid important biological purposes such as HIV protease inhibi-
materials can be classified into two classes: (i) compounds in tors,13,14 antibiotics,15 antitumors,16 antipsychotics,17 and
which weak interactions such as van der Waals forces, hydro- antihypertensives.18 Therefore, synthesis of these significant
gen bonds, or electrostatic interactions are created between compounds via catalytic green and efficient protocols is a pro-
organic and inorganic components, and (ii) inorganic–organic minent subject of interest. Although there are a number of
hybrid materials where both organic and inorganic com- reports on the three component condensation reaction of
ponents are bonded through strong covalent bonds.2
2-naphthol, aldehydes and amides in the presence of different
Since pure heteropolyacids generally show low catalytic catalysts for the synthesis of 1-amidoalkyl-2-naphthols, the
activity owing to their small surface area, the catalytic function majority of these methods suffer from several limitations such
of Keggin H4SiW12O40 (HPA) would be improved at the atomic as corrosive and expensive reagents, strongly acidic conditions,
and molecular level by anchoring to different porous materials and harsh reaction conditions.19,20
with high surface area, such as MCM-41, which shows high
In order to obtain an efficient heterogeneous catalyst, HPA
thermal stability, large surface area, and good adsorption was supported on TPI-MCM-41 to provide a highly dispersed
capacity for organic molecules.3–7 MCM-41 is able to carry hybrid material. The goal of this work was to characterize the
new nanohybrid material and evaluate the catalytic activity of
this heterogeneous catalyst in reference to the bulk hetero-
polyacid. Therefore, the prepared inorganic–organic nanohybrid
aDepartment of Chemistry, School of Sciences, Hakim Sabzevari University,
material H4SiW12O40/pyridino-MCM-41 would be a suitable
Sabzevar 96179-76487, Iran. E-mail: Rtayebee@hsu.ac.ir; Fax: +98-51-44410300;
candidate of choice for the efficient preparation of 1-amido-
Tel: +98-51-44410310
bDepartment of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113,
Iran
alkyl-2-naphthols under environmental benign conditions and
principles of safe chemistry (Scheme 1).
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Scheme 1 General formulation for the preparation of different 1-amidoalkyl-2-naphthols.
position SiO2 : 0.2 hexadecyltrimethylammonium bromide
(CTAB) : 120 H2O. For this purpose, 34.91 g sodium silicate
solution (7% Na2O, 27% SiO2) was diluted with 100 ml H2O
2. Experimental
2.1. Materials and methods
Reagents and solvents were purchased from Merck or Fluka and was added slowly to a rapidly stirred solution of 11.85 g
and used as received. The solid samples were identified by CTAB in 220 ml H2O. A precipitate formed immediately which
X-ray powder diffraction (XRD) using a STOE diffractometer was stirred at ambient temperature for 30 min. Dilute sulfuric
with Cu-Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA. acid (2 M) was then added dropwise to adjust the pH of the gel
The diffraction patterns were recorded in the Bragg angle (2θ) to 10. After being stirred for 3 h and readjusting the pH, the
range from 1° to 10° for small angle XRD and from 10° to 80° synthesized gel was transferred to a Teflon-lined autoclave and
for wide-angle XRD, with a position sensitive detector using a heated to 150 °C for 36 h. The solid particles produced were
step size of 0.06°. Infrared spectra were recorded (KBr pellets) collected by filtration, dried overnight at room temperature,
on a 8700 Shimadzu Fourier Transform spectrophotometer and calcined in air at 540 °C for 6 h to remove the template.
applying the KBr technique in the range of 400 to 4000 cm−1
.
2.3. Preparation of pyridine functionalizing agent
The pellets for FT-IR analysis were made of ∼3 mg of the nano-
hybrid catalyst combined with approximately 250 mg of KBr.
Thermogravimetry and differential thermal analysis (TGA-DTA)
were carried out on a Bahr STA-503 instrument in air at a
heating rate of 10 °C min−1. Ultraviolet–visible spectra were
recorded on a UV-Vis Array Spectrophotometer (Photonix Ar
2015, Iran) in the range of 200–800 nm. A Philips XL-30 ESEM
scanning electron microscope (acceleration voltage 26 kV)
equipped with an Oxford Energy Dispersive X-ray (EDX)
spectrometer was used to observe morphology and perform
microanalysis. The tungsten content in the nanohybrid catalyst
was determined using inductively coupled plasma optical
emission spectroscopy (ICP-OES) conducted on a Varian Vista-
Pro model ICP-OES spectrometer. To determine the tungsten
content, the wavelength of 239.708 nm was observed by
ICP-OES analysis. Melting points were recorded on a Bamstead
electrothermal type 9200 melting point apparatus. Hydro-
dynamic particle size analysis of HPA/TPI-MCM-41 in an ethanol
suspension was determined by measuring the dynamic light
scattering by a Zeta Sizer-HT (Brookhaven). Thin layer chromato-
graphy (TLC) was performed on Merck silica gel 60 F254
N-[3-(Triethoxysilyl)propyl]isonicotinamide (TPI) was syn-
thesized according to the previously reported procedure and
characterized by 1H NMR.25 Briefly, 4-pyridinecarboxylic acid
(2.0 g, 16 mmol) was suspended in dry CH2Cl2 (100 ml) under
nitrogen atmosphere. To this solution, oxalyl chloride (4.1 ml,
48 mmol) was slowly added and the mixture was stirred for
12 h. The CH2Cl2 was removed under reduced pressure and
the residue was suspended again in dry CH2Cl2 (100 ml). After
addition of triethylamine (6.7 ml, 48 mmol) to the reaction
mixture, 3-aminopropyltriethoxysilane (3.8 ml, 48 mmol) was
slowly added. The reaction mixture was stirred at room temp-
erature for 4 h and then the mixture was suspended in water to
remove impurities. The organic phase separated and the
solvent was removed under reduced pressure to obtain a
brownish viscous oil. The synthesis of TPI was confirmed by
1H NMR (CDCl3, ppm): 0.61 (t, 2H), 1.11 (t, 9H) 1.65 (m, 2H),
3.36 (m, 2H), 3.72 (q, 6H), 7.24 (d, 2H), 7.57 (s, 1H), 8.60 (d, 2H).
2.4. Synthesis of pyridine-functionalized MCM-41 (TPI-MCM-41)
The general sol–gel procedure for functionalization of nano-
porous silica was used to functionalize Si-MCM-41.26 In a
typical reaction, 1.0 g of MCM-41 was suspended in 80 ml
toluene and the mixture was stirred for 1 h; then, 2.5 ml of TPI
was added and the mixture was refluxed for 48 h. The white-
brown solid was removed from the solvent by filtration,
washed with toluene and acetone and then dried at room
temperature. FT-IR and TGA-DTA analyses showed TPI
anchored onto the mesoporous support.
plates using ethyl acetate and hexane (1 : 4) as eluting agents.
1
TLC plates were visualized by exposure to UV light. H and 13
C
NMR spectra were recorded on a Bruker AVANCE 100 MHz
instrument using TMS as internal reference. All products were
identified by comparison of their spectral and physical data
with those previously reported.21,22 Keggin heteropolyacid
H4SiW12O40 catalyst (HPA) was prepared according to the pre-
viously reported methods.23
2.2. Synthesis of MCM-41 mesoporous silica
2.5. Immobilization of silicotungstic acid H4SiW12O40 on
TPI-MCM-41 (HPA/TPI-MCM-41)
MCM-41 mesoporous silica was synthesized by the sol–gel
method according to the literature with slight modifications.24 For systematic access to the immobilized catalyst HPA/
This material was crystallized from a gel with the molar com- TPI-MCM-41, a modular system combining functionalized TPI
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Scheme 2 A schematic diagram for the pyridine functionalization of MCM-41 mesoporous silica and immobilization of HPA.
was developed to immobilize H4SiW12O40 on MCM-41 aqueous ethanol. Yields refer to the isolated yields in all cata-
(Scheme 2). For the immobilization of the heteropolyacid on lytic tests. This general procedure was used for all catalytic
the functionalized MCM-41, 50 ml of methanol containing tests, but with the mentioned amount of the used catalyst.
0.7
g of H4SiW12O40 was added to freshly prepared
TPI-MCM-41 (1.0 g) and the mixture was refluxed for 4 h.
2.7. Spectral data for some selected 1-amidoalkyl-
Then, the heterogeneous catalyst was filtered off and washed 2-naphthols
with methanol several times and thereafter was dried at room
2.7.1. N-[(2-Hydroxynaphthalen-1-yl)phenylmethyl]benz-
amide. White solid, H NMR (DMSO-d6) δ = 10.3 (br, s, 1H),
8.16–8.12 (m, 6H), 7.90–7.86 (m, 5H), 7.41 (br, s, 1H),
7.20–7.15 (m, 5H), 6.06 (s, 1H); 13C NMR (DMSO-d6) δ = 169,
153, 148, 145, 135, 133, 133, 132, 131, 130, 130, 128, 128, 128,
127, 124, 122, 121, 119, 118, 50.
temperature. XRD and FT-IR analysis of the prepared in-
organic–organic nanohybrid catalyst confirmed anchoring of
HPA onto the pyridine-functionalized silica. UV-Vis analysis
showed about 0.09 mmol g−1 of HPA is loaded on
TPI-MCM-41. The amount of 0.09 mmol g−1 was equivalent to
coordination of HPA to about 8% of the total amount of TPI
covalently attached to the MCM-41 surface, as calculated by
TGA-DTA analysis. A schematic representation for the prepa-
ration of the HPA/TPI-MCM-41 nanocatalyst is shown in
Scheme 2. The scheme shows grafting of TPI onto MCM-41 via
condensation of hydroxyl and ethoxy groups of the support
and linker, respectively; then, the HPA was anchored to
TPI-MCM-41 via electrostatic interaction in the second step.
1
2.7.2. N-(1-(2-Hydroxynaphthalen-1-yl)-3-phenylallyl)benz-
1
amide. White solid, H NMR (DMSO-d6): δ = 7.1 (d, 1H), 7.15
(d, 1H), 7.20 (m, 4H), 7.42 (m, 3H), 7.77 (m, 3H), 8.10 (d, 1H),
9.03 (d, 1H), 10.45 (s, 1H). 13C NMR (DMSO-d6): δ = 165, 158,
142, 137, 136, 133, 132, 131, 128, 127, 123, 48.
2.7.3. N-[4-Nitrophenyl-(2-hydroxynaphthalen-1-yl)methyl]-
benzamide. White solid, FT-IR (neat) = 3420, 3062, 1627,
1572, 1534, 1489, 1347, 1271, 1026, 942, 822, 750 cm−1
.
1H NMR (DMSO-d6): δ = 7.27 (m, 8H), 7.46 (t, 3H), 7.57 (d, 1H),
7.83 (m, 4H), 8.08 (d, 1H), 9.03 (d, 1H), 10.37 (br, s, 1H).
13C NMR (DMSO-d6): δ = 167, 154, 151, 146, 134, 133, 132, 130,
2.6. General procedure for the preparation of substituted
1-amidoalkyl-2-naphthols
In a typical reaction, a mixture of β-naphthol (1.0 mmol), alde- 129, 128, 127, 124, 123, 119, 118, 49.
hyde (1.0 mmol), benzamide (1.2 mmol), and HPA/ 2.7.4. N-[4-Tolyl-(2-hydroxynaphthalen-1-yl)methyl]benz-
1
TPI-MCM-41 (15 mg) in a small test tube equipped with a con- amide. Light yellow solid, H NMR (DMSO-d6, TMS) δ = 9.92
denser was conventionally heated to 100 °C for the required (br, s, 1H), 8.03–7.97 (m, 6H), 7.63 (br, s, 1H), 7.24–7.14
time. After completion of the reaction monitored by TLC, the (m, 9H), 6.08 (s, 1H), 1.90 (s, 3H); 13C NMR (DMSO-d6) δ = 168,
mixture was cooled to 25 °C, boiling ethanol was added and 153, 143, 140, 135, 133, 129, 129, 128, 126, 125, 123, 123, 120,
the mixture was stirred for 5 min. The catalyst was filtered off; 118, 48, 20.
then, the solution was cooled to room temperature and the
2.7.5. N-(1-(2-Hydroxynaphthalen-1-yl)-3-phenylpropyl)-
resulting solid was filtered and re-crystallized from 15% benzamide. White solid, 1H NMR (DMSOd6):
δ = 2.03
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(m, 2H), 2.75 (t, 2H), 5.98 (d, 1H), 7.12–7.24 (m, 6H), 7.41–7.47
(m, 4H), 7.68–7.83 (m, 4H), 8.02 (d, 1H), 8.68 (d, 1H), 10.17
(s, 1H). 13C NMR (DMSO-d6): δ = 165, 153, 142, 134, 132, 131,
128, 127, 126, 125, 122, 119, 118, 47, 35, 32.
2.7.6. N-[4-Chlorophenyl-(2-hydroxynaphthalen-1-yl)methyl]-
1
benzamide. White solid, H NMR (DMSO-d6) δ = 10.05 (br, s,
1H), 8.05–7.99 (m, 6H), 7.44 (br, s, 1H), 7.27–7.24 (m, 4H),
7.057–7.01 (m, 5H), 6.12 (s, 1H); 13C NMR (DMSO-d6): δ = 165,
153, 139, 134, 132, 131, 130, 128, 127, 126, 123, 122, 119, 116, 49.
2.7.7. N-[1-(2-Hydroxynaphthalen-1-yl)butyl]benzamide.
White solid, 1H NMR (DMSO-d6) δ = 10.08 (br, s, 1H),
8.12–8.05 (m, 6H), 7.84–7.78 (m, 5H), 7.31 (br, s, 1H), 6.35
(t, 1H), 2.15 (s, 3H), 1.85–1.79 (m, 1H), 1.39–1.33 (m, 1H), 0.98
(t, 3H); 13C NMR (DMSO-d6) δ = 163, 155, 148, 144, 139, 135,
134, 132, 129, 128, 126, 125, 123, 119, 118, 48, 36, 19, 13.
2.7.8. N-[(2-Hydroxynaphthalen-1-yl)(4-hydroxyphenyl)methyl]-
Fig. 2 Wide-angle XRD patterns of MCM-41, TPI-MCM-41 and HPA/
TPI-MCM-41.
benzamide. Light brown solid, 1H NMR (DMSO-d6) δ = 11.24 and pyridine-functionalized MCM-41, due to the incorporation
(br, s, 1H), 8.93 (br, s, 1H), 8.12–8.06 (m, 6H), 7.82–7.77 of the heteropolyacid. The broad band between 18° and 30° in
(m, 4H), 7.22–7.15 (m, 5H), 6.78 (s, 1H); 13C NMR (DMSO-d6) the wide-angle XRD patterns of all three compounds (Fig. 2)
δ = 170, 156, 152, 149, 144, 139, 134, 132, 130, 129, 128, 127, was characteristic of amorphous silica walls of the pristine
125, 124, 123, 122, 121, 120, 50.
material.28 Notably, no characteristic peak for H4SiW12O40
crystallites (20.5°, 25.4°, 29.4°, 34.6°, and 53.3°) was detected
in HPA/TPI-MCM-41.29 This clearly implied the fine dispersion
of the HPA on the support surface.30
3. Results and discussion
3.1.2. SEM and EDX of HPA/TPI-MCM-41. MCM-41 is a
short-range amorphous material containing a large number of
silanol groups available for grafting. The morphology of the
HPA/TPI-MCM-41 nanocatalyst was investigated by SEM as
shown in Fig. 3a. This micrograph illustrates that the nano-
particles have a semi-spherical morphology with a narrow size
distribution and an average diameter of approximately 80 nm,
and H4SiW12O40 is well dispersed on TPI-MCM-41. Concerning
the chemical composition of the HPA/TPI-MCM-41 nanocata-
lyst, energy dispersive X-ray (EDX) analysis was carried out
(Fig. 3b). The EDX analysis proved existence of tungsten in
addition to silicon in the nanocatalyst indicating silicotungstic
acid grafted successfully on MCM-41 mesoporous silica.
3.1.3. FT-IR spectroscopy. Fig. 4 illustrates FT-IR spectra of
MCM-41, TPI-MCM-41, HPA/TPI-MCM-41 and H4SiW12O40. For
the parent MCM-41 material, asymmetric and symmetric Si–
O–Si characteristic stretching vibrations at 1076 cm−1 and
800 cm−1 along with Si–OH at 950 cm−1 and bending Si–O–Si
at 458 cm−1 are observed. The broad band around 3450 cm−1
is due to the surface silanols and adsorbed water molecules
and the band at 1635 cm−1 is assigned to the H–O–H bending
vibration of the free or absorbed water molecules.26,31 After
modification of MCM-41 with TPI, several new bands
appeared. The presence of organic groups was confirmed by
observation of the stretching vibrations for C–H aromatic, C–H
aliphatic, CvO and CvN at 3070, 2950–2850, 1650 and
1550 cm−1 regions, respectively. Decreasing intensities of the
O–H and Si–OH stretching vibrations for TPI-MCM-41 indi-
cated substitution of some of the surface silanols groups with
TPI reagent. Notably, the stretching vibration band of the
NH2 group is completely obscured by the broad stretching
vibration band of the O–H group.32,33 The Keggin structure of
3.1. Characterization of inorganic–organic nanohybrid
material HPA/TPI-MCM-41
3.1.1. X-ray diffraction. Fig. 1 and 2 show the small and
wide-angle, respectively, X-ray powder diffraction (XRD)
patterns of MCM-41, TPI-MCM-41, and HPA/TPI-MCM-41
samples. As Fig. 1 illustrates, reflections are assigned to (100),
(110), (200), and (210) planes in the highly ordered 2D hexa-
gonal arrangement of pores with d100 spacing of 4.63 nm for
pure MCM-41.27 For TPI-MCM-41 and HPA/TPI-MCM-41, three
intense reflections disappeared and only a sharp reflection
associated with (100) was observed. The intensity of this reflec-
tion after functionalizing with TPI was obviously lower than
that of the original MCM-41, which may be ascribed to a pore-
filling effect of TPI. After loading of HPA on TPI-MCM-41, the
intensity of XRD reflections on HPA/TPI-MCM-41 was weak-
ened further, in comparison with the unmodified MCM-41
Fig. 1 Sall-angle XRD patterns of MCM-41, TPI-MCM-41 and HPA/
TPI-MCM-41.
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Fig. 3 SEM micrograph (a) and EDX spectrum (b) of HPA/TPI-MCM-41
nanocatalyst.
Fig. 4 FT-IR spectra of MCM-41, TPI-MCM-41, HPA/TPI-MCM-41 and
H4SiW12O40
.
H4SiW12O40 consists of a central SiO4 tetrahedron surrounded
by four W3O9 groups formed by edge-sharing octahedrons.
These groups are connected to each other by corner-sharing
oxygen atoms. The FT-IR spectrum of the parent Keggin
H4SiW12O40·nH2O showed characteristic stretching and
bending vibration bands of H2O at 3410 and 1620 cm−1, and
Si–O and WvO stretching vibrations along with edge-sharing
and corner -sharing W–O–W stretching vibrations at 1019, 984,
926 and 787 cm−1, respectively (Fig. 4).34–39
Fig. 4 also shows the FT-IR spectrum of the immobilized
heteropolyacid on the pyridine-functionalized MCM-41.
Although characteristic bands associated with MCM-41-TPI are
clearly visible in the spectrum of the prepared nanocatalyst,
HPA bands were masked as a result of the strong background
of the silica support. Actually, from the characteristic bands in
the 700–1100 cm−1 region of HPA, only the edge-sharing W–O–W
stretching vibration band at 922 cm−1 is observed. A slight
shift of this band, in comparison with the bulk HPA, is prob-
ably due to interaction of the HPA with TPI-MCM-41.40
200–600 °C which is accompanied by exothermic peaks at
380 °C and 512 °C in DTA curves, respectively. According to
the weight loss in the TGA curve, the amount of pyridine agent
introduced into TPI-MCM-41 was 1.2 mmol g−1 32,41
On the
.
other hand, thermal analysis of H4SiW12O40·nH2O illustrates a
major weight loss between 25–300 °C and a minor loss at
350–600 °C. The first weight loss can be related to water loss
from H4SiW12O40·nH2O. These data are corroborated by differ-
ential thermal analysis: two main endothermic peaks observed
between 25 and 300 °C are attributable to the loss of water
molecules. The second minor weight loss probably is associ-
ated with the decomposition of the Keggin anion which is
accompanied by a sharp and relatively intense exothermic
transition in the region of 535 °C.42–44
The TGA-DTA curve of the synthesized catalyst is similar to
that of the pyridine functionalized material. There are two
weight losses below 300 °C due to water loss accompanied
with endothermic peaks in the same region and also another
weight loss between 300–700 °C which corresponds to the oxi-
dative decomposition of the organic part of TPI and degra-
dation of the heteropolyacid. This is accompanied by
exothermic peaks at 386 °C and 512 °C in the DTA curve,
respectively. Notably, according to thermal analysis data, the
prepared nanohybrid catalyst is stable up to 250 °C.
3.1.4. Thermal analysis. TGA-DTA curves of TPI-MCM-41,
HPA/TPI-MCM-41 and HPA are illustrated in Fig. 5.
TPI-MCM-41 has two weight losses due to the desorption of
physisorbed water on the external surface of the crystallites or
occluded in MCM-41 mesopores below 150 °C, and the oxi-
dative decomposition of the organic part of TPI between
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Fig. 5 TGA and DTA analyses for H4SiW12O40 (a), TPI-MCM-41 (b), and HPA/TPI-MCM-41 (c).
3.1.5. UV-Vis spectroscopy. To determine the extent of
HPA loaded on TPI-MCM-41, UV-Vis spectra of solutions con-
taining HPA before and after addition of TPI-MCM-41 were
measured (Fig. 6). For this purpose, 1.0 g TPI functionalized
MCM-41 mesoporous silica was suspended in 50 ml methano-
lic solution of silicotungstic acid with an initial concentration
of 14 000 ppm. The suspension was refluxed for 4 h, and then
the catalyst was removed by filtration and the adsorption spec-
trum was recorded. Comparison of the absorbance band with
calibration curves for standard solutions of HPA in methanol
after 240 min refluxing showed that the heteropolyacid concen-
tration had declined to 8600 ppm. This indicated that
5400 ppm HPA grafted to 1.0 g of TPI-MCM-41, which corre-
sponds to 0.0933 mmol g−1 of HPA on the support. The band
observed in the UV-Vis spectrum at 268 nm is the character-
Fig. 6 UV-Vis spectra of H4SiW10O40 before and after immobilization
on TPI-MCM-41 after 4 h.
istic band of H4SiW12O40 and corresponds to the O→W charge with distilled water to 25 ml. Analysis of the solution by
transfer band.42–44 For confirmation of HPA loading on the ICP-OES showed the presence of 80 ppm tungsten in the
support, 0.01 g of heterogeneous catalyst was suspended in sample, which corresponds to 0.09 mmol g−1 HPA on the
100 µL HF to remove the siliceous skeleton. Then the mixture TPI-MCM-41 support. This finding is in good agreement with
was stirred for 4 h and filtered, and the solution was diluted the result of UV-Vis analysis. Comparison of TPI loading on
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Table 1 Effect of the heteropolyacid nature and structure on the
efficacy of the catalytic systema
Time Yield
(min) (%)
Catalyst
Structure
H3PMo12O40
H3PW12O40
H6P2W18O62
H6P2Mo18O62
H4SiW12O40
Keggin
Keggin
Wells-Dawson
Wells-Dawson
Si-substituted Keggin
35
25
15
20
7
78
79
75
79
95
75
H4SiW9Mo2O40 Si-substituted mixed metal lacunary
Keggin
15
H4SiW9Mo3O40 Si-substituted mixed metal Keggin
15
80
a Reactions were carried out as described in the experimental section.
0.5 mol% of catalyst was used in each case.
Fig. 7 Size distribution histogram of the synthesized HPA/TPI-MCM-41
from DLS analysis.
Table 2 Effects of the type of solid support material, organic linker,
and heteropolyacid on the efficacy of the catalytic reactiona
Mol%
(based
Time Yield
MCM-41 and HPA loading on TPI-MCM-41 showed that about
8% of pyridine groups on the surface of MCM-41 are co-
ordinated to the Keggin heteropolyacid.
3.1.6. Dynamic light scattering (DLS) analysis of HPA/
TPI-MCM-41. HPA/TPI-MCM-41 was suspended in ethanol
and the hydrodynamic particles size was determined by
measuring the dynamic light scattering (Fig. 7). The particle
size histogram of the nanohybrid catalyst showed that most of
the particles ranged in size from 70–120 nm and possessed an
average size of ∼75 nm.
Entry Catalyst
on HPA) (min) (%)
Ref.
1
2
3
4
5
H6P2W18O62/TPI-(Al)SBA-15 0.16
20
30
30
25
20
51
45
33
55
94
46
48
49
—
H5PW10V2O40/TPI-SBA-15
H5PW10V2O40/Pip-SBA-15
H6P2W18O62/TPI-CNT
0.10
0.10
—
H4SiW12O40/TPI-MCM-41
0.13
This work
a Reaction conditions selected as in the experimental section. Reactions
were performed with 15 mg of each catalyst. CNT refers to carbon
nanotubes. TPI and Pip refer to N-[3-(triethoxysilyl)propyl]
isonicotinamide and 1-[3-(triethoxysilyl)propyl]piperazine, respectively.
3.2. Catalytic tests
active sites from the supporting material. Therefore, we were
interested to study the effect of the type and nature of some
mesoporous supports modified with the two organic linkers
TPI and 1-[3-(triethoxysilyl)propyl]piperazine (Pip) which were
electrostatically anchored to H5PW10V2O40 and H6P2W18O62
heteropolyacids (Table 2). The results revealed that TPI
behaved better than the other linker in the case of SBA-15.
Moreover, among different supports such as MCM-41, SBA-15,
(Al)SBA-15, and CNT, the first was the best. Therefore, meso-
porous silica MCM-41, containing silanol functional groups
with high specific surface area which opened a wide field of
applications in chemical industries, was selected as the best
solid support in this study.
3.2.1. Effect of the heteropolyacid nature and structure on
the efficacy of the catalytic system. The catalytic activity of
H4SiW12O40 was compared with some familiar heteropolyacids
in the preparation of N-[(2-hydroxy-naphthalen-1-yl)phenyl-
methyl]benzamide (Table 1). All used Keggin and Wells–
Dawson heteropolyacids are strong acids and behave as good
catalysts in the target transformation.45 Although Wells–
Dawson heteropolyacids showed better catalytic activity than
Keggin H3PW12O40 and H3PMo12O40, the two Si-substituted
mixed metal Keggin heteropolyacids showed comparable
activity. Among the examined heteropolyacids, H4SiW12O40 as
Si-substituted Keggin-type heteropolyacid showed clearly the
best catalytic activity. The different reactivity patterns observed
in Table 1 can be explained considering many factors such as
total acidity of the heteropolyacid, negative charge density
smeared over oxygen atoms, structure, and modality of adsorp-
tion of organic reagents to the bulk of the heteropolyacid
during reaction.46,47
3.2.3. Studying the catalytic activity of the ingredients com-
posing HPA/TPI-MCM-41. Fig. 8 compares the catalytic activity
of HPA/TPI-MCM-41 with H4SiW12O40
, TPI-MCM-41, and
MCM-41 in the preparation of α-amidoalkyl-β-naphthol under
the standard reaction conditions. 4 mg of H4SiW12O40 pro-
vided 68% yield of the desired product after 20 min; whereas,
15 mg of the inorganic–organic hybrid material HPA/
TPI-MCM-41, which includes the same amount of HPA (4 mg),
achieved 94% yield after a short time of 20 min. The unmodi-
fied MCM-41 and the modified material TPI-MCM-41 were
also catalytically active; these materials led to 11% and 24% of
3.2.2. Studying the catalytic activity of some modified
mesoporous nanomaterials and heteropolyacids compared to
HPA/TPI-MCM-41. There are several reports on the electro-
static anchoring of different heteropolyacids on modified solid
supporting materials with a suitable organic linker. This stra-
tegy is a beneficial solution to overcome leaching of catalytic
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Fig. 8 Studying the catalytic activity of HPA/TPI-MCM-41 ingredients.
4 mg HPA and 15 mg of other components were used. Data were
obtained after 20 min.
Fig. 10 Effect of reaction temperature on the condensation reaction
catalyzed by HPA/TPI-MCM-41.
product, respectively, after the same time. Although the com-
ponents of the hybrid material showed slight catalytic activity,
HPA/TPI-MCM-41 performed the best in the desired organic
transformation. Obviously, immobilizing the heteropolyacid
onto the modified solid material TPI-MCM-41 increased its
catalytic activity towards the condensation reaction.
3.2.5. Effect of temperature on the condensation reaction.
Effect of reaction temperature on the preparation of α-ami-
doalkyl-β-naphthol was investigated in the presence of HPA/
TPI-MCM-41 nanoparticles (15 mg) under solvent-free con-
ditions (Fig. 10). Yield of the desired product was enhanced by
increasing the temperature from 25 to 130 °C. It was observed
that the reaction did not sufficiently proceed at room tempera-
ture and only 26% yield was achieved after 180 min; whereas,
at 60 °C the reaction poorly proceeded and maximum conver-
sion (52%) was attained after 75 min. The best result was
achieved at 100 °C. Further increase in temperature did not
significantly improve the conversion. Therefore, we kept the
reaction temperature at 100 °C.
3.2.6. Synthesis of different α-amidoalkyl-β-naphthols in
the presence of HPA/TPI-MCM-41 under solvent-free con-
ditions. The generality of the present protocol was evaluated
by examining structurally diverse aromatic and aliphatic alde-
hydes carrying either electron-withdrawing, electron-donating,
or halogen groups on the aromatic rings (Table 3). The reac-
tion of aldehydes bearing electron-withdrawing groups such as
4-nitrobenzaldehyde (entry 2) was successful and gave the
corresponding product in 70% yield after 35 min; but lower
yields were achieved and longer reaction times were necessary
for aldehydes involving strong electron-donating groups such
as 4-(N,N′-dimethylamino)benzaldehyde (entry 14). In addition
to the electronic factors, steric congestion also affected the
reaction in terms of time and yield. Comparison of entries
7 and 8 indicates that existence of bulky substituents on the
ortho-positions of the phenyl ring decreased the yield and
needed longer reaction time.
3.2.4. Effect of catalyst concentration. The catalytic
efficiency of the heterogeneous HPA/TPI-MCM-41 was investi-
gated for the preparation of α-amidoalkyl-β-naphthol with
various amounts of catalyst (Fig. 9). It was found that only
15% yield was attained after the long time of 3 h. In order to
evaluate the appropriate catalyst amount, a model reaction was
carried out by applying 5–40 mg HPA/TPI-MCM-41. It was
found that 5 mg of catalyst afforded 45% yield after 1 h. This
finding revealed that the heterogeneous catalyst exhibited high
catalytic activity in the desired transformation. As expected,
the percentage yield was increased with enhancing catalyst
concentration. However, higher amounts of catalyst (>15 mg)
neither increased nor lowered the percentage yield. Therefore,
15 mg of catalyst was chosen as the optimal quantity to push
the reaction forward.
3.3. Comparison of the catalytic activity of HPA/TPI-MCM-41
nanoparticles with some reported catalysts
The activity of the present heterogeneous catalyst H4SiW12O40/
TPI-MCM-41 was studied compared to some reported catalysts
(Table 4). The reaction of β-naphthol with benzaldehyde and
Fig. 9 Effect of catalyst concentration on the desired condensation
reaction.
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Table 3 Synthesis of different α-amidoalkyl-β-naphthol derivatives in the presence of HPA/TPI-MCM-41 under solvent-free conditionsa
Paper
Entry
1
Aldehyde
Time (min)
20
Yield (%)
94
m.p./lit. m.p. (°C)
Product
235–236/235–23750
2
35
70
237–238/238–24051
3
45
85
232–234/232–23452
4
45
83
175–176/176–17753
5
35
83
230–232
6
35
45
183–185/182–18454
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Table 3 (Contd.)
Entry
7
Aldehyde
Time (min)
40
Yield (%)
65
m.p./lit. m.p. (°C)
208/21022
Product
8
60
18
245–246
9
25
35
55
55
266–267/266–26755
10
169–171/168–17054
11
30
87
220–223
12
55
48
234–236
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Table 3 (Contd.)
Entry
13
Aldehyde
Time (min)
30
Yield (%)
89
m.p./lit. m.p. (°C)
236/23454
Product
14
45
50
221–222/220–22155
15
16
45
60
48
57
247–248
233–234/233–23553
a Reactions were carried out as described in the experimental section. Yields refer to the isolated pure products. The desired products were
characterized by comparison of their physical data with those of known compounds.
Table 4 Comparison of the catalytic efficiency of HPA/TPI-MCM-41 nanocatalyst with some reported catalysts for the reaction of β-naphthol with
benzaldehyde and benzamide
Entry
Catalyst
Catalyst (mol% or g); conditions
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
HPA/TPI-MCM-41
Montmorillonite K10
H3PMo12O40
NKC-9
H3PMo12O40/SiO2
Sulfamic acid
15 mg; solvent-free, 100 °C
0.1 g; solvent-free, 125 °C, neat
1.6 mol%; CH3COOEt, 65 °C
0.17 g; CHCl3, 65 °C
3.17 mol%; solvent-free, 120 °C
2.5 mol%; solvent-free, 28–30 °C
10 mol%; MW irradiation
0.4
1.5
4
4
1
4
0.1
94
78
92
89
82
74
92
This work
54
56
57
58
59
60
1-Hexanesulphonic acid sodium salt
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benzamide for the synthesis of the corresponding α-amidoalkyl-
β-naphthol was selected as a model reaction and the comparison
was in terms of mol% of the catalyst, temperature, reaction time,
and percentage yields. Obviously, the present new nanohybrid
material is better than most of the mentioned catalysts.
Although some introduced additives catalyzed the reaction,
even though at lower temperature, they required toxic and/or
expensive solvents, higher mol% of catalyst, and longer reaction
times.
3.4. Reusability and reproducibility of HPA/TPI-MCM-41
nanocatalyst
A major drawback of homogeneous free HPA is the separation
and recovery of the catalyst when the reaction is completed.
Heterogenization of HPA via anchoring to TPI-MCM-41 is an
attractive strategy allowing easy separation. Moreover, reusabi-
lity and regeneration of the catalyst reduce wastage and pro-
duction of wasteful materials. To investigate the reusability of
HPA/TPI-MCM-41, it was easily separated from the reaction
mixture and washed thoroughly with ethanol. Then, the cata-
lyst 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 another condensation reaction. Findings revealed nearly
the same catalytic activity as for the fresh catalyst, albeit with
slightly decreasing activity (Fig. 11 and 12). 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 3% variation. It could thus be concluded that HPA/
TPI-MCM-41 could be a satisfactory catalyst for this reaction
with high activity and good reusability.
Fig. 12 FT-IR spectra of fresh and recovered catalysts.
TPI-MCM-41 (15 mg). At this stage, the yield of the product
was 76%. Then, the solid catalyst was filtered off under hot
conditions and with the filtrate, which was obtained after sepa-
ration of the catalyst, the reaction was continued for another
25 min at the same reaction temperature. However, no corres-
ponding increase in yield beyond 76% was observed. This
result clearly confirmed the heterogeneous nature of the meso-
porous nanohybrid catalyst in this condensation reaction and
no leaching of the heteropolyacid occurred during the course
of the reaction.
3.6. Proposed reaction pathway for the catalytic system
Scheme 3 shows a plausible reaction pathway for the three-
component condensation reaction.61,62 Presumably, the reac-
tion proceeded via ortho-quinonemethide (I) formed by the
nucleophilic addition of β-naphthol to the carbonyl group of
the aldehyde which was activated by the nanocatalyst. Finally,
(I) reacts with the amide via Michael addition to afford the
desired 1-amidoalkyl-2-naphthol. According to this mechan-
ism, electron-releasing substituents deactivate the aldehyde to
accept the nucleophilic attack of β-naphthol; however, elec-
tron-withdrawing groups activate the carbonyl group of the
aldehyde.
3.5. Hot filtration test
To establish that the catalytic activity was generated from the
heterogeneous nanohybrid catalyst, and not from the leached
heteropolyacid in the reaction mixture, a hot filtration test was
planned. In this technique, the condensation reaction of
β-naphthol (1.0 mmol), aldehyde (1 mmol), and benzamide
(1.2 mmol) in a small test tube equipped with a condenser per-
formed at 100 °C for 25 min in the presence of HPA/
4. Conclusions
HPA/TPI-MCM-41 catalyst was prepared from commercially
available starting materials, and efficiently catalyzed the syn-
thesis of 1-amidoalkyl-2-naphthols through the condensation
of β-naphthol and benzaldehyde with benzamide under
solvent-free, one-pot, and multi-component conditions. The
advantages of this method include short reaction time, high
yields, and easy purification as well as the reusability and
economic viability of the catalyst. This protocol is developed as
a safe and convenient alternate method by applying an eco-
friendly and highly reusable catalyst.
Fig. 11 Reusability of HPA/TPI-MCM-41 nanocatalyst.
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Scheme 3 Proposed reaction pathway for the preparation of 1-amidoalkyl-2-naphthol catalyzed by HPA/TPI-MCM-41.
11 X. S. Zhao, G. Q. Lu, A. J. Whittaker, G. J. Millar and
Acknowledgements
H. Y. Zhu, J. Phys. Chem. B, 1997, 101, 6525.
Partial financial support from the Research Councils of 12 S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and
Hakim Sabzevari and Shahid Beheshti Universities is greatly
appreciated.
O. Terasaki, J. Am. Chem. Soc., 1999, 121, 9611.
13 T. Dingermann, D. Steinhilber and G. Folkers, Mol. Biol.
Med. Chem., 2004, 21, 381.
14 S. Knapp, Chem. Rev., 1995, 95, 1859.
15 Y. Kusakabe, J. Nagatsu, M. Shibuya, O. Kawaguchi,
C. Hirose and S. Shirato, J. Antibiot., 1972, 2, 44.
16 S. Remillard, L. I. Rebhun, G. A. Howie and S. M. Kupchan,
Science, 1975, 189, 1002.
17 J. L. Peglion, J. Vian, B. Gourment, N. Despaux, V. Audinot
and M. Millan, Bioorg. Med. Chem. Lett., 1997, 7, 881.
18 R. D. Clark, J. M. Caroon, A. F. Kluge, D. B. Repke,
A. P. Roszkowski, A. M. Strosberg, S. Baker, S. M. Bitter and
M. D. Okada, J. Med. Chem., 1983, 26, 657.
19 S. Samantaray, G. Hota and B. G. Mishra, Catal. Commun.,
2011, 12, 1255.
Notes and references
1 A. P. Wight and E. Davis, Chem. Rev., 2002, 102, 3589.
2 C. Sanchez and F. Ribot, New J. Chem., 1994, 18, 1007.
3 S. Udayakumar, S. Ajaikumar and A. Pandurangan, Appl.
Catal., A, 2006, 302, 86.
4 B. E. Ali, J. Tijani, M. Fettouhi, M. E. Faer and A. A. Arfaj,
Appl. Catal., A, 2005, 283, 185.
5 R. R. Jermy and A. Pandurangan, Appl. Catal., A, 2005, 295,
185.
6 R. Maheswari, K. Shanthi, T. Sivakumar and S. Narayanan,
Appl. Catal., A, 2003, 248, 291.
20 D. Kundu, A. Majee and A. Hajra, Catal. Commun., 2010,
11, 1157.
21 M. A. Amrollahi, F. Mirjalili and H. Emtiazi, J. Chem. Sci.,
2013, 125, 561.
7 R. Tayebee and B. Maleki, J. Chem. Sci., 2013, 125, 335.
8 M. Vallet-Regi, A. Ramila, R. P. Del Real and J. Perez-
Pariente, Chem. Mater., 2001, 13, 308.
22 M. Zandi and A. R. Sardarian, C. R. Chim., 2012, 15, 365.
23 E. North, Inorg. Synth., 1939, 1, 129.
24 C. Nunes, A. Valente, M. Pillinger, A. Fernandes, C. Romão,
J. Rocha and I. Gonçalves, J. Mater. Chem., 2002, 12,
1735.
9 Y. F. Zhu, J. L. Shi, W. H. Shen, X. P. Dong, J. W. Feng,
M. L. Ruan and Y. S. Li, Angew. Chem., Int. Ed., 2005, 44,
5083.
10 S. P. Rigby, M. Fairhead and C. F. van der Walle, Curr.
Pharm. Des., 2008, 14, 1821.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.

View Article Online
Paper
Dalton Transactions
25 J. Hoogbomm, P. M. L. Garcia and M. B. Otten, J. Am. 45 R. Tayebee, F. Nehzat, E. Rezaei-Seresht, F. Z. Mohammadi
Chem. Soc., 2005, 127, 11047. and E. Rafiee, J. Mol. Catal. A: Chem., 2011, 351, 154.
26 T. P. B. Nguyen, J.-W. Lee, W. G. Shim and H. Moon, Micro- 46 R. Tayebee, M. M. Amini, S. Pouyamanesh and A. Aliakbari,
porous Mesoporous Mater., 2008, 110, 560. Dalton Trans., 2015, 44, 5888.
27 Z. Qu, D. Chen, Y. Sun and Y. Wang, Appl. Catal., A, 2014, 47 R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155.
487, 100.
48 R. Tayebee, M. M. Amini, F. Nehzat, O. Sadeghi and
M. Armaghan, J. Mol. Catal. A: Chem., 2013, 366, 140.
49 R. Tayebee, M. M. Amini, M. Ghadamgahi and
M. Armaghan, J. Mol. Catal. A: Chem., 2013, 366, 266.
50 R. R. Nagawade and D. B. Shinde, Acta Chim. Slov., 2007,
54, 642.
28 H. Zhao, J. C. Zhou, H. Luo, C. Y. Zeng, D. H. Li and
Y. J. Liu, Catal. Lett., 2006, 108, 49.
29 L. Pesaresi, D. R. Brown, A. F. Lee, J. M. Montero,
H. Williams and K. Wilson, Appl. Catal., A, 2009, 360, 50.
30 Z. Shanhui, Z. Yulei and H. Shunli, Catal. Lett., 2012, 142, 267.
31 E. Vunain, N. Opembe, K. Jalama, A. Mishra and 51 S. A. M. K. Ansari, N. S. Jiaprakash, D. K. S. Nagnnath,
R. Meijboom, J. Therm. Anal. Calorim., 2014, 115, 1487.
32 M. Behbahani, A. Ali Akbari, M. M. Amini and A. Bagheri,
Anal. Methods, 2014, 6, 8785.
33 Y. Shuijin, Y. Xieqing and S. Shaomin, Front. Chem. China,
2006, 1, 82.
W. Pravin and D. B. Shinde, Indian J. Chem. Technol., 2010,
17, 71.
52 M. Wang, Y. Liang, T. T. Zhang and J. J. Gao, Chin. Chem.
Lett., 2012, 23, 65.
53 G. H. Mahdavinia and M. A. Bigdeli, Chin. Chem. Lett.,
2009, 20, 383.
34 M. Ammam and J. Fransaer, J. Electrochem. Soc., 2011, 158, A14.
35 F. J. Berry, G. R. Derrick and M. Mortimer, Polyhedron, 54 S. Kantevari, S. V. N. Vuppalapati and L. Nagarapu, Catal.
2014, 68, 17. Commun., 2007, 8, 1857.
36 G. Yang, Y. Pan, F. Gao, J. Gong, X. Cui, C. Shao, Y. Guo 55 G. C. Nandi, S. Samai, R. Kumar and M. S. Singh, Tetra-
and L. Qu, Mater. Lett., 2005, 59, 450. hedron Lett., 2009, 50, 7220.
37 L. B. Yang, Y. H. Shen, A. J. Xie, J. J. Liang and B. C. Zhang, 56 J. Wen-Qing, A. Li-Taoa and Z. Jian-Ping, Chin. Chem. Lett.,
Mater. Res. Bull., 2008, 43, 572. 2008, 26, 1697.
38 Q. Li, Z. Jin, Z. Peng, Y. Li, S. Li and G. Lu, J. Phys. Chem. C, 57 A. Li-Tao, L. Xiao-Huab, D. Fei-Qingb, J. Wen-Qingb and
2007, 111, 8237. Z. Jian-Ping, Chin. J. Chem., 2008, 26, 2117.
39 Q. Zhang, Y. Tan, C. Yang and Y. Han, J. Mol. Catal. A: 58 A. Zare, A. R. Hasaninejad, E. Rostami, A. R. Moosavi-zare,
Chem., 2007, 263, 149.
40 W. Chu, X. Yang, Y. Shan, X. Ye and Y. Wu, Catal. Lett.,
1996, 42, 201.
41 R. Tayebee, M. M. Amini, H. Rostamian and A. Aliakbari,
Dalton Trans., 2014, 43, 1550.
42 Y. Q. Zhang, L. H. Gao, K. Z. Wang, H. J. Gao and
Y. L. Wang, J. Nanosci. Nanotechnol., 2008, 8, 1248.
43 K. Z. Wang and L. H. Gao, Mater. Res. Bull., 2002, 37, 2447.
N. Pishahang, M. Roshankar, F. Khedri and M. Khedri,
Electron. J. Chem., 2010, 7(4), 1162.
59 S. B. Patil, P. R. Singh, M. P. Surpur and S. D. Samant,
Ultrason. Sonochem., 2007, 14, 515.
60 K. S. Niralwad, B. B. Shingate and M. S. Shingare, Chin.
Chem. Lett., 2011, 22, 551.
61 M. M. Khodaei, A. R. Khosropour and H. Moghanian,
Synlett, 2006, 916.
44 J. Gong, X. J. Cui, Z. W. Xie, S. G. Wang and L. Y. Qu, Synth. 62 S. B. Patil, P. R. Singh, M. P. Surpur and S. D. Samant,
Met., 2002, 129, 187. Synth. Commun., 2007, 37, 1659.
Dalton Trans.
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