Preparation and characterization of a novel Wells-Dawson heteropolyacid-based magnetic inorganic-organic nanohybrid catalyst H 6P2W18O62/pyridino-Fe 3O4 for the efficient synthesis of 1-ami

Article abstract of DOI:10.1039/c3dt51594j

A novel magnetic inorganic-organic nanohybrid material H₆P ₂W₁₈O₆₂/pyridino-Fe₃O₄ (HPA/TPI-Fe₃O₄) was fabricated and performed as an efficient, eco-friendly, and highl

Full text of DOI:10.1039/c3dt51594j

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Preparation and characterization of a novel
Wells–Dawson heteropolyacid-based magnetic
inorganic–organic nanohybrid catalyst
H6P2W18O62/pyridino-Fe3O4 for the efficient
synthesis of 1-amidoalkyl-2-naphthols under
solvent-free conditions
Cite this: Dalton Trans., 2014, 43,
1550
Reza Tayebee,*a Mostafa M. Amini,b Hooriyeh Rostamiana and Azam Aliakbarib
A novel magnetic inorganic–organic nanohybrid material H6P2W18O62/pyridino-Fe3O4 (HPA/TPI-Fe3O4)
was fabricated and performed as an efficient, eco-friendly, and highly recyclable catalyst for the solvent-
free, one-pot, and multi-component synthesis of various substituted 1-amidoalkyl-2-naphthols from the
reaction of β-naphthol, an aldehyde, and benzamide with good to excellent yields (47–94%) and in a
short span of time (25–60 min). The nanohybrid catalyst was prepared by the chemical anchoring
of Wells–Dawson heteropolyacid H6P2W18O62 onto the surface of modified Fe3O4 nanoparticles with
N-[3-(triethoxysilyl)propyl]isonicotinamide (TPI) linker. The magnetically recoverable catalyst was easily
recycled at least eight times without any loss of catalytic activity. XRD, TEM, UV-vis, and FTIR confirmed
that the heteropolyacid H6P2W18O62 is well dispersed on the surface of the solid support and its structure
is retained after immobilization on the pyridine modified Fe3O4 nanoparticles. This protocol is developed
as a safe and convenient alternate method for the synthesis of 1-amidoalkyl-2-naphthols utilizing an eco-
friendly, and a highly reusable catalyst.
Received 15th June 2013,
Accepted 7th October 2013
DOI: 10.1039/c3dt51594j
www.rsc.org/dalton
Preparation of 1-amidoalkyl-2-naphthols can be carried out
by condensation of aliphatic and/or aromatic aldehydes,
1 Introduction
1-Amidoalkyl-2-naphthols and their derivatives bearing 1,3- β-naphthol, and acetonitrile or amides in the presence of
amino oxygenated functional groups have attracted intense Lewis or Brönsted acid catalysts such as montmorillonite K10
attention due to their ubiquitous role as essential building clay,12 trichloroacetic acid and cobalt(II) chloride,13 P2O5,14
blocks for a variety of biologically important natural products, K5CoW12O40·3H2O,15 sulfamic acid/ultrasound,16 HClO4–
synthetic pharmaceuticals, and potent drugs, including a SiO2,17 cation-exchanged resins,18 polymer-supported sulfonic
number of nucleoside antibiotics and HIV protease inhibi- acid (NKC-9),19 silica-sulfuric acid,20 Nano-S (S8-NP),21
tors.1,2 These compounds can be converted into important Sr(OTf)2,22 ZrOCl2,23 oxalic acid,24 Mg(ClO4)2,25 dodecylphos-
‘drug-like’ materials by amide hydrolysis and formation of phonic acid (DPA),26 and wet cyanuric chloride.27 However,
1-aminoalkylnaphthols. The hypertensive and broad cardiac some of these catalysts suffer from the drawback from the
effects of these substrates and their potential biological perspective of green chemistry such as prolonged reaction
activities, including antibiotic,3 antitumor,4 analgesic,5 anti- times,28–33 using carcinogenic solvents,28–31 unsatisfactory
convulsant,6 antipsychotic,7 antimalarial,8 antianginal,9 anti- yields,32–35 toxicity and reusability of highly acidic and expen-
hypertensive,10 and antirheumatic11 properties have been sive catalysts.28,36,37 Therefore, proposing clean procedures
evaluated extensively.
and utilizing eco-friendly and efficient catalysts which can
be simply recycled at the end of the reaction have been under
permanent attention.
Super paramagnetic Fe3O4 nanoparticles, that can be mag-
netized in the presence of an external magnet, have attracted
worldwide attention and have been studied extensively due to
their biological and technological applications such as drug
aDepartment of Chemistry, School of Sciences, Hakim Sabzevari University, Sabzevar
96179-76487, Iran. E-mail: Rtayebee@yahoo.com; Fax: +98-571-4410300;
Tel: +98-571-4410310
bDepartment of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113,
Iran
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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.
Scheme 1 General formulation for the preparation of different 1-amido-
alkyl-2-naphthols.
2.2 Preparation of Wells–Dawson H6P2W18O62·24H2O acid
catalyst
delivery,38,39 magnetic resonance imaging (MRI),40 biosepara-
tion,41,42 biomolecular sensors43,44 and magneto-thermal
therapy.45,46 Recent reports show that magnetic nanoparticles
are efficient supports which can facilitate the isolation and
recycling of expensive catalysts from the reaction media.47,48
In this research, an advance in the context of synthetic
methodology towards the mentioned class of biologically
important materials is presented using a new magnetic in-
organic–organic nanohybrid material, H6P2W18O62/pyridino-
Fe3O4. This new environmentally benign, heterogeneous, and
highly reusable catalyst showed very good catalytic activity
towards condensation of β-naphthol with different aldehydes
and benzamide (Scheme 1).
The condensation of a mixture of benzaldehyde (1 mmol)
with β-naphthol (1 mmol) and benzamide (1.2 mmol) in the
presence of HPA/TPI-Fe3O4 (20 mg) was carried out at 100 °C
for 30 min under solvent-free conditions (Scheme 1). The
reaction proceeded smoothly and gave the corresponding
amidoalkylnaphthol as the sole product in 92% yield.
Wells–Dawson acid (H6P2W18O62·24H2O) was prepared from
an α/β-K6P2W18O62·10H2O isomer mixture as described
before.53–55 Concentrated H3PO4 in a 4 : 1 acid : salt ratio was
added to the boiling aqueous solution of Na2WO4·2H2O and
the mixture was kept boiling in a reflux system for 8 h. The salt
was precipitated by adding KCl, then purified by re-crystalliza-
tion and cooled overnight to 5 °C. The product consisted of a
mixture of the α- and β-isomers, was filtered, washed, and
then vacuum-dried for 8 h. The acid was obtained from an
aqueous solution of α/β K6P2W18O62·10H2O salt, which was
treated with ether and concentrated HCl (37%) solution. The
acid so released formed an addition compound with ether,
which allows it to be separated from the solution. After obtain-
ing the ether solution with the acid, the ether was eliminated
by flowing dry air and the remaining solution was placed in a
vacuum-desiccator until crystallization.
2.3 Synthesis of Fe3O4 and pyridine-functionalized Fe3O4
nanoparticles (TPI-Fe3O4)
Fe3O4 nanoparticles were synthesized according to a previously
reported procedure.56 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-
2 Experimental
2.1 Materials and methods
All starting materials and solvents were commercially available wise to the solution. After 15 min, the solid product was separ-
and were used as received. All products were identified by com- ated by a magnet and washed three times with NaCl (0.1 M)
parison of their spectral and physical data with those pre- solution. The identity of the prepared nanoparticles was
viously reported.49–51 Progress of the reactions was monitored confirmed by FTIR and X-ray powder diffraction. For the prepa-
by thin-layer chromatography (TLC). Melting points were ration of pyridine-functionalized Fe3O4, 1.0 g of Fe3O4 was sus-
recorded on a Bamstead electrothermal type 9200 melting pended in 50 ml toluene, and the mixture was stirred for 1 h.
point apparatus. Ultraviolet-visible spectra were recorded on a Then, 2 ml of TPI was added and the mixture was refluxed for
Shimadzu Model UV-2550 spectrophotometer. Infrared spectra 24 h. The black solid was removed by filtration and washed
were recorded (KBr pellets) on an 8700 Shimadzu Fourier with toluene and ethanol, then dried at room temperature.
Transform spectrophotometer. 1H and 13C NMR spectra were The synthesized Fe3O4 and pyridine-functionalized material
recorded on a Bruker AVANCE 300-MHz instrument using TMS (TPI-Fe3O4) were characterized by FTIR spectroscopy, X-ray
as internal reference. X-ray diffraction patterns were obtained diffraction, thermal analysis (TGA/DTA), and transmission
on a STOE diffractometer with Cu Kα radiation at 60 keV and electron microscopy (TEM).
15 mA with a scanning rate of 3° min−1 in the 2θ range from
5° to 80°. Transmission electron microscopy (TEM) was per-
2.4 Immobilization of Wells–Dawson
diphosphooctadecatungstic acid on TPI-Fe3O4
formed on a Philips CM120 with a magnification of 160 and
200 K. For observation of morphology by TEM, the sample was For the immobilization of H6P2W18O62·24H2O on TPI-Fe3O4, 50 ml
dispersed in ethanol using ultrasonication for several minutes. of methanol solution containing 0.3 g of H6P2W18O62·24H2O
A drop of the dispersed particles in ethanol was poured on a was added to freshly prepared TPI-Fe3O4 (0.6 g) and the
carbon-coated copper grid and then the grid was dried under mixture was refluxed for 3 h. Then, the heterogeneous catalyst
an infrared lamp, followed by inspection in the TEM. Thermal was filtered off and extracted in a Soxhlet extractor using
analysis (TGA-DTA) was carried out on a Bahr STA-503 instru- methanol as solvent for 12 h and thereafter was dried at
ment in air at a heating rate of 10 °C min−1. The hetero- 105 °C in a vacuum oven. The amount of H6P2W18O62·24H2O
polyacid catalyst H6P2W18O62, was prepared and characterized loading on TPI-Fe3O4 was found 0.02 mmol g−1, as determined
according to the literature procedure as described below.52 by ICP-OES after digestion in aqua regia, and this was
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(m, 4H), 7.66–7.82 (m, 4H), 8.02 (d, 1H), 8.66 (d, 1H), 10.16 (s,
1H). 13C NMR (DMSO-d6): δ = 32.46, 35.60, 46.70, 118.59,
119.62, 122.38, 125.66, 126.22, 126.96, 127.33, 128.181, 28.26,
128.35, 128.45, 128.52, 131.13, 132.02, 134.63, 141.68, 152.90,
165.55. Anal. Calcd: C, 81.86; H, 6.08; N, 3.67. Found: C, 81.86;
H, 5.98; N, 3.52.
N-[4-Nitrophenyl-(2-hydroxynaphthalen-1-yl)-methyl]benz-
amide. FT-IR (neat) = 3420, 3062, 1627, 1572, 1534, 1489,
1
1347, 1271, 1026, 942, 822, 750 cm−1. H NMR (DMSO-d6): δ =
7.25 (m, 8H), 7.47 (t, J = 6.4 Hz, 3H), 7.53 (d, J = 6.8 Hz, 1H),
7.82 (m, 4H), 8.07 (d, J = 8 Hz, 1H), 9.03 (d, J = 8 Hz, 1H), 10.34
(br, s, 1H) ppm. 13C NMR (DMSO-d6): δ = 49.51, 117.86,
119.03, 123.11, 123.25, 123.86, 127.46, 127.91, 128.03, 128.90,
129.19, 130.45, 132.05, 132.75, 134.44, 146.61, 150.76, 153.94,
166.77.
N-[(2-Hydroxynaphthalen-1-yl)-phenylmethyl]benzamide.
White solid, m.p. 235.1–237.5 °C, 1H NMR (DMSO-d6) δ =
10.34 (br, s, 1H), 8.17–8.11 (m, 6H), 7.92–7.87 (m, 5H), 7.40
(br, s, 1H), 7.20–7.14 (m, 5H), 6.07 (s, 1H); 13C NMR (DMSO-d6)
δ = 169.5, 153.4, 148.2, 145.2, 134.8, 133.4, 132.8, 132.2,
130.8, 130.3, 129.7, 128.4, 128.1, 127.7, 127.1, 123.8, 122.6,
122.6, 121.0, 119.3, 118.1, 49.7; Anal. Calcd: C, 81.56; H, 5.42;
N, 3.96. Found: C, 81.73; H, 5.51; N, 3.90.
N-[4-Tolyl-(2-hydroxynaphthalen-1-yl)-methyl]benzamide.
Light yellow solid, m.p. 207.5–209.8 °C, 1H NMR (DMSO-d6,
TMS) δ = 9.91 (br, s, 1H), 8.04–7.98 (m, 6H), 7.63 (br, s, 1H),
7.23–7.14 (m, 9H), 6.07 (s, 1H), 1.91 (s, 3H); 13C NMR (DMSO-
d6) δ = 167.8, 153.1, 143.3, 139.5, 134.7, 132.9, 129.1, 128.8,
128.2, 126.3, 125.5, 123.4, 122.6, 119.4, 118.1, 48.7, 20.3; Anal.
Calcd: C, 81.72; H, 5.76; N, 3.81. Found: C, 81.53; H, 6.11; N,
3.87.
Scheme 2
nanocatalyst.
A
schematic for the preparation of HPA/TPI-Fe3O4
confirmed by UV-vis analysis with 5% uncertainty. The func-
tionalized TPI-Fe3O4 supported by H6P2W18O62·24H2O did not
leach significantly after prolonged contact with aqueous solu-
tion as confirmed by UV-vis. Therefore, N-[3-(triethoxysilyl)-
propyl]isonicotinamide (TPI) linker is a suitable agent for
the immobilization process and provided covalent anchoring
of the heteropolyacid onto the functionalized supporting
material. A schematic for the preparation of HPA/TPI-Fe3O4
nanocatalyst is shown in Scheme 2.
2.5 General procedure for the preparation of substituted
1-amidoalkyl-2-naphthols
N-[4-Chlorophenyl-(2-hydroxynaphthalen-1-yl)-methyl]benz-
amide. Off white solid, m.p. 179.0–181.6 °C, Rf = 0.24 (50%
1
In a typical reaction, a mixture of β-naphthol (1.0 mmol),
aldehyde (1 mmol), benzamide (1.2 mmol), and H6P2W18O62/
pyridino-Fe3O4 (20 mg) in a small test tube equipped with a
condenser, was heated to 100 °C in an oil bath for the required
time. After completion of the reaction (monitored by TLC), the
mixture was cooled to 25 °C, boiling ethanol was added and
the mixture was stirred for 5 min. The catalyst was recovered
by using an external magnet. Then, the solution was cooled to
room temperature and the resulting solid was filtered off and
re-crystallized from aqueous ethanol 15%.
AcOEt–hexane); H NMR (400 MHz, DMSO-d6, TMS) δ = 10.06
(br, s, 1H, OH), 8.06–7.97 (m, 6H, Ar–H), 7.43 (br, s, 1H, NH),
7.28–7.23 (m, 4H, Ar–H), 7.07–7.01 (m, 5H, Ar–H), 6.12 (s, 1H,
CH); 13C NMR (100 MHz, DMSO-d6) δ = 165.0, 153.3, 138.7,
134.1, 132.5, 132.1, 131.2, 130.6, 129.7, 128.4, 128.2, 128.0,
127.8, 127.3, 126.7, 126.2, 122.9, 122.4, 118.7, 116.6, 48.9; IR
(KBr pellets) νmax (cm−1) 3440 (N–H), 3325 (O–H), 3070 (C–H),
1644 (CvO); m/z (GC-MS) 387.10 [M+]. Anal. Calcd for
C24H18ClNO2: C, 74.32; H, 4.68; N, 3.61. Found: C, 74.26; H,
4.60; N, 3.86.
N-[(2-Hydroxynaphthalen-1-yl)-(4-hydroxyphenyl)methyl]-
benzamide. Light brown solid, m.p. 191.3–193.3 °C, H NMR
(DMSO-d6) δ = 11.24 (br, s, 1H), 8.94 (br, s, 1H), 8.11–8.05
2.6 Spectral data for some selected 1-amidoalkyl-
2-naphthols16,21,25,26
1
N-(1-(2-Hydroxynaphthalen-1-yl)-3-phenylallyl)benzamide. (m, 6H), 7.81–7.76 (m, 4H), 7.21–7.15 (m, 5H), 6.79 (s, 1H);
White solid, m.p. 234 °C, 1H NMR (DMSO-d6): δ = 7.08 (d, 1H), 13C NMR (DMSO-d6) δ = 169.6, 156.4, 152.4, 149.2, 144.7,
7.17 (d, 1H), 7.21 (m, 4H), 7.41 (m, 3H), 7.75 (m, 3H), 8.07 (d, 139.2, 134.5, 132.5, 132.1, 130.0, 129.6, 129.2, 128.8, 128.1,
1H), 9.01 (d, 1H), 10.43 (s, 1H). 13C NMR (DMSO-d6): δ = 48.15, 127.9, 127.1, 125.2, 124.4, 122.9, 122.2, 120.8, 119.3, 50.3; IR
123.51, 127.61, 128.30, 131.27, 132.47, 133.60, 135.81, 137.28, (KBr pellets) νmax (cm−1) 3488 (N–H), 3366 (O–H), 2955 (C–H),
141.84, 158.24, 165.54. Anal. Calcd: C, 82.30; H, 5.58; N, 3.69. 1655 (CvO); Anal. Calcd: C, 78.03; H, 5.18; N, 3.79. Found:
Found: C, 82.27; H, 5.55; N, 3.66.
C, 77.67; H, 5.22; N, 4.08.
N-(1-(2-Hydroxynaphthalen-1-yl)-3-phenylpropyl)benzamide.
N-[1-(2-Hydroxynaphthalen-1-yl)butyl]benzamide. White solid,
1
White solid, m.p. 158 °C, 1H NMR (DMSO-d6): δ = 2.04 m.p. 240.1–242.7 °C, H NMR (DMSO-d6) δ = 10.08 (br, s, 1H),
(m, 2H), 2.74 (t, 2H), 5.98 (d, 1H), 7.13–7.24 (m, 6H), 7.40–7.47 8.11–8.05 (m, 6H), 7.83–7.79 (m, 5H), 7.31 (br, s, 1H), 6.34
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(t, J = 6.5 Hz, 1H), 2.15 (s, 3H), 1.84–1.78 (m, 1H), 1.39–1.33
(m, 1H), 0.97 (t, J = 6.5 Hz, 3H); 13C NMR (DMSO-d6) δ = 162.7,
155.3, 148.1, 144.6, 139.7, 139.2, 135.1, 133.7, 132.4, 129.3,
128.8, 128.3, 127.7, 126.2, 125.3, 122.8, 119.6, 118.4, 48.7, 36.3,
19.4, 13.3; IR (KBr pellets) νmax (cm−1) 3469 (N–H), 3239 (O–H),
3010 (C–H), 1659 (CvO); Anal. Calcd: C, 78.97; H, 6.63; N,
4.39. Found: C, 79.11; H, 6.92; N, 4.45.
3 Results and discussion
The design and preparation of multifunctional materials
bearing different types of active sites have attracted more and
more attention from chemical engineers.57–62 Therefore,
various kinds of supports, including organic and inorganic
materials,63–66 have been employed in the field of catalytic
sciences to show unique properties in different catalytic
reactions.67–69 Generally, nanoparticles have been applied as
efficient supports for the preparation of functionalized
materials because of nano-effects. But, the separation pro-
cesses becomes a challenge, because nanoparticles are often
readily dispersed in the liquid media. This issue may be over-
come by using magnetic materials; they offer the advantage of
being magnetically separable by using an external magnetic
field. Amongst various metal oxide nanoparticles which have
emerged as potential heterogeneous catalysts, Fe3O4 nanopar-
ticles arguably have attracted the most attention. The reason
would be due to the ease of handling, ease of recovery, as well
as the high catalytic activity of Fe3O4 nanoparticles in various
organic transformations.48,70
Fig. 1 Stacked plot of UV-vis spectral changes after addition of
H6P2W18O62 to TPI-Fe3O4 under aerobic conditions with time.
In continuation of our recent activities on the utilization of
new organic–inorganic nanohybrid heterogeneous catalysts in
different areas of organic synthesis,71,72 a new environmentally
benign and highly reusable magnetic nanocatalyst is intro-
duced for the efficient synthesis of various substituted
1-amidoalkyl-2-naphthols under solvent-free conditions.
Fig. 2 FTIR spectra of (a) Fe3O4, (b) Fe3O4-TPI, and (c) H6P2W18O62
Fe3O4-TPI.
/
recorded for solutions after removing the solid catalyst by an
external magnetic field. As Fig. 1 shows, the main adsorption
has occurred during the first three hours and there was no sig-
nificant change in it even after one hour more. By comparison
with standard solutions, after 180 min refluxing, the hetero-
polyacid concentration had declined to 1400 ppm which
corresponded to 0.025 mmol g−1 loading of HPA on the
support.
3.1.2 FTIR spectroscopy. FTIR spectroscopy was applied to
characterize the composition of the Fe3O4-TPI nanoparticles.
N-[3-(Triethoxysilyl)propyl]isonicotinamide (TPI) was used to
modify the Fe3O4 nanoparticles in refluxing toluene. The TPI
was anchored onto the surface of Fe3O4 by condensation of the
surface hydroxyl groups of the mesoporous material with the
ethoxy groups of TPI as illustrated in Scheme 2. Incorporation
of the linker into the surface of Fe3O4 was confirmed by FTIR
(Fig. 2). The absorbance at about 590 cm−1 is ascribed to
Fe2+–O2−, which is consistent with the reported infrared
spectra for spinel Fe3O4.74 The detection of weak characteristic
stretching vibrations of pyridine at 1550–1650 cm−1 and the
3.1 Structural characterization of the magnetic inorganic–
organic nanohybrid material HPA/TPI-Fe3O4
3.1.1 UV-vis spectroscopy. The UV-vis spectrum of
the Wells–Dawson heteropolyacid shows two types of
ligand→metal charge-transfer bands originating from different
oxygen atoms. The highest energy absorption band at about
220 nm has been assigned to the Od→W charge-transfer band
due to the terminal oxygen atoms. Other weak bands at ∼260
and ∼310 nm are characteristic bands for diphosphooctadeca-
tungstic acid73 and have been attributed to the Ob→W or
Oc→W charge-transfer bands of the bridge-oxygen atoms.
To determine the optimum refluxing time that is required
for Fe3O4-TPI to adsorb HPA from the impregnating solution,
and also to determine the extent of HPA which can be loaded
on the support (as an ancillary method along with ICP ana-
lysis), 0.2 g Fe3O4-TPI was suspended in a 40 ml methanol
solution of HPA with an initial concentration 2500 ppm. The
suspension was refluxed and at different refluxing time
ranging from 1 to 240 minutes, adsorption studies were
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Fig. 3 The fingerprinting IR bands of H6P2W18O62 with the Wells–
Dawson structure.
appearance of shoulders at 2900–3000 cm−1 in the FTIR spec-
trum indicated grafting of TPI group on the surface of Fe3O4
nanoparticles. 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 region 3500–3700 cm−1 can be attributed to the
symmetrical stretching vibration modes of the adsorbed water
molecules.
The Wells–Dawson structure of H6P2W18O62 consists of two
half units composed of a central PO4 tetrahedron surrounded
by nine WO6 octahedra. Therefore, the H6P2W18O62 structure
involves four kinds of oxygen atoms. The first is P–Oa in which
the oxygen atom connects with the tungsten atom, the second
is W–Ob–W oxygen bridges (corner-sharing oxygen bridges
between different W3O13 groups), the third is W–Oc–W oxygen
bridges (edge-sharing oxygen bridge within W3O13 groups),
and the last is W–Od terminal oxygen atoms. As shown in
Fig. 3, the four characteristic bands of H6P2W18O62 appeared,
i.e. νas (W–Od): 965 cm−1; νas(W–Ob–W): 912 cm−1; νas(W–Oc–W):
Fig. 4 X-ray diffraction patterns of Fe3O4 (a), TPI-Fe3O4 (b), and HPA/
TPI-Fe3O4 (c).
(422), (511) and (440) reflections which is consistent with the
standard Fe3O4 crystal with the spinel structure.56a For the
XRD of the Fe3O4-TPI nanoparticles (Fig. 4b), compared to
Fig. 4a, the typical diffraction peaks were similar to those of
the unfunctionalized core–shell Fe3O4 microspheres though
the intensities of the main peaks decreased marginally, which
revealed that the crystal structure of the Fe3O4 cores was still
well-maintained after the introduction of organic functional
groups. The position and relative intensities of all diffraction
peaks match well with the standard magnetite peaks. There-
fore, the spinel structure of Fe3O4 is retained after functionali-
zation with TPI and immobilization of HPA onto TPI-Fe3O4. In
addition, the sample is crystalline and Fe3O4 with a spinel
structure is the only obtained phase together with some
776 cm−1 and νas(P–Oa): 1093 cm−1 52,75
The intense band
.
at 1090 cm−1 is assigned to the stretching mode of the P–O
species that is considered as the fingerprint of the Wells–
Dawson heteropolyanion. The vibration bands at 912 and
776 cm−1 were assigned to the ‘inter’ and ‘intra’ W–O–W
bridges, respectively. The IR spectrum of H6P2W18O62/
TPI-Fe3O4 nanoparticles possessed
a distinct band with
shoulders about 1030 cm−1, confirming a strong interaction
of the pyridine-modified Fe3O4 with the heteropolyacid. More-
over, observation of the fingerprinting IR bands of the Wells–
Dawson structure in the 800–1000 cm−1 region proved loading
of H6P2W18O62 onto the surface of TPI-Fe3O4.
additional peaks, indicated by asterisks, associated with the
56b
Well–Dawson structured H6P2W18O62
.
3.1.4 TEM studies. In this work, the size and morphology
of the H6P2W18O62/Fe3O4-TPI nanoparticles were investigated
by TEM and the result is shown in Fig. 5. The micrograph illus-
trates that the Fe3O4 core was coated with a TPI shell uni-
formly, and particles have semi-spherical morphology with
narrow size distributions and an average diameter of approxi-
mately 10 nm.
3.1.5 Thermal analysis. The thermal stability of HPA/
TPI-Fe3O4 nanoparticles was investigated by carrying out TGA–
DTA analysis (Fig. 6). The DTA profile exhibited a broad
endothermic peak from room temperature to 200 °C which is
attributed to the elimination of adsorbed water molecules.
3.1.3 X-ray diffraction analysis. The crystalline nature of
H6P2W18O62/Fe3O4-TPI nanoparticles was demonstrated by
XRD and along with X-ray diffraction patterns of Fe3O4 and
TPI-Fe3O4 as shown in Fig. 4. The positions and relative inten-
sities of all diffraction peaks matched well with those from the
JCPDS card (75-1610) for magnetite and the sharp, strong
peaks confirmed that the modified magnetic nanoparticles
were well crystallized, which suggested that the Fe3O4 core par-
ticles were well retained under modification with TPI. The
XRD pattern of the Fe3O4 exhibited the (220), (311), (400),
1554 | Dalton Trans., 2014, 43, 1550–1563
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Table 1 Effect of catalyst amount on the condensation of β-naphthol
with benzaldehyde and benzamide under solvent free conditionsa
HPA/TPI-Fe3O4
(mg)
H6P2W18O62
(mg)
Entry
Time (min)
Yield (%)
1
2
3
4
5
6
0
5
10
15
20
100
0
180
60
60
45
30
20
9
56
74
77
92
93
0.45
0.9
1.35
1.8
9
a A mixture of aldehyde (1 mmol), β-naphthol (1 mmol), benzamide
(1.2 mmol) and the catalyst was stirred at 100 °C for the appropriate
time. Progress of the reaction was monitored with TLC. Work-up was
carried out as described in the experimental section.
Fig. 5 TEM micrograph of nanohybrid catalyst H6P2W18O62/Fe3O4-TPI.
revealed that the heterogeneous catalyst exhibited high cataly-
tic activity in the desired transformation. Higher amounts of
the catalyst (>20 mg) neither increased, nor lowered the yield%.
Therefore, 20 mg of catalyst was found to be the optimal
quantity and sufficient to push the reaction forward.
3.3 Studying the catalytic activity of the ingredients
comprising the organic–inorganic hybrid nanomaterial
Table 2 compares the catalytic activity of HPA/TPI-Fe3O4 with
H6P2W18O62, TPI-Fe3O4, and Fe3O4 dispositions, separately, in
the preparation of α-amidoalkyl-β-naphthol under the standard
reaction conditions. 6.4 mg of H6P2W18O62 catalyst led to 42%
of the desired product after 40 min (entry 1); whereas, 20 mg
of the inorganic–organic hybrid material HPA/TPI-Fe3O4,
which includes only 6.4 mg of the heteropolyacid achieved
92% yield after a short time of 30 min (entry 4). The free
carrier material (Fe3O4) and the modified material TPI-Fe3O4
were also catalytically active; findings revealed that the first led
to 33% of product after 60 min and the latter produced 38% of
product after 30 min (entries 2 and 3, respectively). Although
the components of the composite material showed a little cata-
lytic activity, HPA/TPI-Fe3O4 gave the best result in the desired
organic transformation (entry 4). Clearly, immobilizing the
heteropolyacid onto the modified solid material TPI-Fe3O4
drastically increased its catalytic activity toward the conden-
sation reaction.
Fig. 6 TGA and DTA curves of HPA/TPI-Fe3O4.
A weight loss occurred between 200 °C and 500 °C which is
accompanied by a broad exothermic peak between 350 °C and
450 °C in the 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
3.4 Effect of organic solvent on the condensation reaction
The need to reduce the amount of toxic waste and byproducts
arising from chemical processes requires increasing emphasis
The catalytic efficiency of the HPA/TPI-Fe3O4 heterogeneous
catalytic system was studied for the preparation of α-amido-
alkyl-β-naphthol with various amounts of catalyst (Table 1).
A test reaction using β-naphthol, benzaldehyde, and benz-
amide at 100 °C without catalyst was performed in order to
establish the real effectiveness of the catalyst. It was found that
no conversion to product was obtained even after 3 h of
heating. In order to evaluate the appropriate catalyst loading, a
model reaction using β-naphthol, benzaldehyde, and benz-
amide was carried out using 5–100 mg of HPA/TPI-Fe3O4. It
was found that 5 mg of catalyst, incorporating 1.6 mg of
H6P2W18O62, afforded 56% of product after 1 h. This finding
Table 2 Studying the catalytic activity of H6P2W18O62, TPI-Fe3O4, and
Fe3O4 in the preparation of α-amidoalkyl-β-naphthola
Entry
Catalyst
Catalyst (mg)
Time (min)
Yield (%)
1
2
3
4
HPA
Fe3O4
TPI-Fe3O4
HPA/TPI-Fe3O4
6.4
20
20
40
60
30
30
42
33
38
92
20
a Reaction conditions are described below Table 1.
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the temperature from 25 to 120 °C. At temperatures lower than
100 °C, the reaction did not proceed efficiently and most of
the initial substances remained intact.
Table 3 Effect of different solvents on the synthesis of α-amidoalkyl-
β-naphthol in the presence of HPA/TPI-Fe3O4 at 60 °Ca
Entry
Solvent
Boiling point (°C)
Time (min)
Yield (%)
3.6 Synthesis of different α-amidoalkyl-β-naphthols in the
presence of HPA/TPI-Fe3O4 under solvent free conditions
1
2
3
4
5
6
7
H2O
100
81
65
78
61
83
—
120
120
120
120
120
120
30
17
83
41
57
62
86
92
CH3CN
CH3OH
C2H5OH
CHCl3
ClCH2CH2Cl
—
Multi-component reactions have drawn high efforts in recent
years owing to their exceptional synthetic efficiency, intrinsic
atom economy, procedural simplicity, and reducing the
number of steps, energy consumption, and waste production.
The multi-component, one-pot condensation of β-naphthol,
aromatic or aliphatic aldehydes, and benzamide constitutes a
valuable approach for the creation of structurally related drug-
like compounds.
The non-toxicity and chemical stability of the new syn-
thesized organic–inorganic hybrid material HPA/TPI-Fe3O4 is
amongst the most crucial characteristics of this super para-
magnetic iron oxide nanocatalyst. The generality of the proto-
col was confirmed by using structurally diverse aromatic and
aliphatic aldehydes carrying either electron-withdrawing, elec-
tron-donating, and halogen groups on their aromatic rings in
the condensation reaction and gave the desired products in
high yields in short reaction times (Table 4). It was shown that
aromatic aldehydes with electron withdrawing groups reacted
faster than their analogues with electron releasing groups. In
contrast to the previously reported methods,77–79 in which
aliphatic aldehydes were not transformed into the corres-
ponding 1-amidoalkyl-2-naphthols, they were converted into
the desired product in good yields in the current methodology.
Findings revealed that this protocol is effective for a wide
range of aldehydes. However, aliphatic aldehydes showed less
reactivity toward condensation reaction than aromatic
analogues.
a Reaction conditions are as described below Table 1 (except for
reaction temperature). 1.0 ml solvent was used.
on the use of less toxic and environmentally compatible
materials in the design of new synthetic methods.76 One of the
most promising approaches is focused on the use of solvent-
less conditions.
We examined the target transformation under similar con-
ditions in different solvents. Solvents such as water, ethanol,
methanol, acetonitrile, 1-2 dichloroethane, and chloroform
were screened in a model reaction of β-naphthol with benz-
aldehyde and benzamide in the presence of 0.02 g of HPA/pyri-
dino-Fe3O4 nanoparticles (Table 3). This study was conducted
at a fixed temperature, 60 °C, in all cases. The solvent influ-
enced the catalytic performance; however, the best yield was
picked up under the solvent free condition (Table 3, entry 7).
Almost all the reactions were less efficient in the presence of
solvent than in its absence, considering the reaction time and
yield%. Acetonitrile and 1-2 dichloroethane resulted in better
performance amongst all the examined solvents. The target
multi-component reaction under solvent-free conditions is
fascinating, since it involves the best reaction medium with
“no medium”.
3.7 Hot filtration test
3.5 Effect of temperature on the multi-component
condensation reaction
In order to confirm that the catalytic activity was generated
from the conjugated Fe3O4 nanoparticles on HPA-TPI and not
from leached components in the reaction mixture, a hot fil-
tration test was carried out. In this technique, the conden-
sation reaction was performed at 100 °C for 20 min in the
presence of the Fe3O4-TPI/HPA magnetic nanocatalyst. At this
stage, the yield of the product was 63%. The catalyst was then
filtered off under hot conditions, and with the filtrate, which
was obtained after separation of the catalyst after 20 min, the
reaction was continued for another 20 min at the same reac-
tion temperature. But, no corresponding increase in product
yield beyond 63% was observed. This result confirmed the
heterogeneous nature of the magnetic nanocatalyst in this con-
densation reaction and that no leaching of the heteropolyacid
occurred during the course of the reaction.
The effect of temperature on the preparation of α-amidoalkyl-
β-naphthol was investigated in the presence of HPA/TPI-Fe3O4
nanoparticles (0.02 g) under solvent-free conditions (Fig. 7).
The yield of the desired product was enhanced by increasing
3.8 Reusability and reproducibility of HPA/TPI-Fe3O4
nanoparticles
To investigate the reusability of the HPA/TPI-Fe3O4 nanoparti-
cles, it was easily separated from the reaction mixture by an
external magnet and washed thoroughly with chloroform.
Fig. 7 Effect of reaction temperature on the condensation of
β-naphthol, benzaldehyde, and benzamide catalyzed by HPA/TPI-Fe3O4
under solvent free conditions.
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Table 4 Synthesis of various α-amidoalkyl-β-naphthol derivatives in the presence of HPA/TPI-Fe3O4 under solvent free conditionsa
Entry
1
Aldehyde
Time (min)
30
Yield (%)
92
M.P. (lit. m.p.)
Product
235 (235–237)23
2
25
94
237 (239–241)24
3
4
30
30
88
91
239 (240–242)27
177 (176–177)27
5
6
35
35
75
83
286 (284–285)14
230–232
7
35
87
243–244
8
9
40
45
85
88
215 (215–216)27
202 (204–205)27
10
25
61
267 (266–267)14
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Table 4 (Contd.)
Entry
11
Aldehyde
Time (min)
35
Yield (%)
76
M.P. (lit. m.p.)
232
Product
12
13
30
35
87
86
220–223
265–267
14
15
25
30
74
93
234–236
194 (193–194)16
16
45
60
221 (220–221)14
17
18
30
30
57
50
175–178
168
19
30
75
159 (158)26
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Table 4 (Contd.)
Entry
20
Aldehyde
Time (min)
45
Yield (%)
48
M.P. (lit. m.p.)
Product
247–248
21
22
60
30
57
52
235 (233–235)27
237–239
23
24
30
60
47
71
241 (240–242)20
183
25
25
83
236 (234)26
a Reaction conditions are described below Table 1.
Then, the catalyst was slowly dried in air and then was acti-
vated in a vacuum oven at 80 °C for 2 h. Finally, the recycled
catalyst was reused for another condensation reaction. Find-
ings revealed the same catalytic activity as the fresh catalyst,
without any loss of its activity (Fig. 8). Moreover, to ensure
reproducibility of the transformation, repeated typical exper-
iments were carried out under identical reaction conditions.
The obtained yields were found to be reproducible within 2%
variation.
3.9 Comparison of the catalytic activity of HPA/TPI-Fe3O4
nanoparticles with some reported catalysts
The merit of the present heterogeneous catalyst H6P2W18O62/
pyridino-Fe3O4 was studied over some reported catalysts
(Table 5). The reaction of β-naphthol with benzaldehyde and
benzamide for the synthesis of 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 conventional catalysts
Fig. 8 Yield% as a function of reusability of H6P2W18O62/pyridino-
Fe3O4 nanoparticles.
mentioned. The present methodology utilized a very low
amount of the supported heteropolyacid under solvent-free
conditions. Although some introduced additives catalyzed the
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Table 5 Comparison of the catalytic efficiency of HPA/TPI-Fe3O4 nanocatalyst with some reported catalysts for the reaction of β-naphthol with
benzaldehyde and benzamidea
Entry
Catalyst
Catalyst (mol% or g); conditions
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
HPA/Py-Fe3O4
H3PMo12O40
NKC-9
Mg(OOCCF3)2
H3PMo12O40/SiO2
DPA
Sulfamic acid
1-Hexanesulfonic acid sodium salt
0.02 g; solvent-free, 100 °C
1.6 mol%; CH3COOEt, 65 °C
0.17 g; CHCl3, 65 °C
0.12 mmol; solvent-free, 100 °C
3.17 mol%; solvent-free, 120 °C
10 mol%; solvent-free, 90 °C
2.5 mmol; solvent-free, 28–30 °C
10 mol%; microwave-irradiation
0.5
4
4
0.4
1
0.3
4
92
92
89
87
82
90
74
92
This work
84
19
85
86
26
16
87
0.1
a Mole ratio of aldehyde–2-naphthol = 1 : 1.
reaction, even though at lower temperature, they required toxic atoms), affects the acid strength of the heteropolyacid as well
and expensive solvents, higher mol% of catalyst, and longer as its catalytic activity and solubility in a wide range.80
reaction times.
The effects of the two organic linkers, TPI and (EtO)3Si-
(CH2)3NH2 (named RNH2), were investigated on the catalytic
efficiency of the heterogenized vanadium substituted Keggin
type heteropolyacid H5PW10V2O40. Obviously, the type of solid
supporting material and identity of the organic modifier
strongly affected the efficiency of the fabricated catalyst.
Amongst H5PW10V2O40/TPI-Fe3O4 and H5PW10V2O40/RNH2-
Fe3O4, the first behaved more efficiently (entries 7 and 8). As is
expected, the type of supporting material played an important
role; since, TPI-modified magnetite (entry 8) behaved better
than zinc oxide modified with TPI (entry 6). Among various
simple oxides (entries 9–14), zinc oxide behaved better and
produced 23% of the desired product in a short time of
20 min. No further improvement in yield% was observed for
these simple oxides after prolonging reaction times.
3.10 Effect of different catalysts on the condensation of
β-naphthol with benzaldehyde and benzamide
The catalytic activity of H6P2W18O62 was compared with other
well-known heteropolyoxomatalates and metal (non-metal)
oxides in the target transformation (Table 6). The heteropoly-
acids introduced in Table 6 structurally divided into two
important sub-classes, Keggin and Wells–Dawson. Clearly,
Wells–Dawson H6P2W18O62 showed higher catalytic activity
than the Keggin analogue in this catalytic transformation.
Amongst the examined Keggin-type heteropolyacids,
H4SiW12O40 showed the best reactivity; whereas, H3PMo12O40
showed the lowest. Substitution of two tungsten atoms with
vanadium in H3PW12O40 decreased the catalytic activity, there-
fore, H5PW10V2O40 revealed distinctly less reactivity than
H3PW12O40. It is difficult to offer an exact explanation for the
different catalytic reactivity pattern observed for these two
structural types of heteropolyacids; since there is a complex
relationship between the activity, structure, and solubility of
the polyanion in the reaction medium. Changing the constitu-
ent elements of the polyanion (both hetero and addenda
3.11 Proposed reaction pathway for the catalytic system
A plausible mechanism explaining the sequence of the events
is shown in Scheme 3.81–83 Presumably, the reaction proceeded
via the ortho-quinonemethide (I) formed by the nucleophilic
addition of β-naphthol to the carbonyl group of the aldehyde
which was activated by the heteropolyacid. Finally, (I) reacts
with the amide via Michael addition to afford the desired
1-amidoalkyl-2-naphthol.
The results displayed in Table 5, confirmed the mechanism.
As indicated, electron-releasing substituents slightly decreased
the yields and enhanced the reaction times (entries 9–11 and 16);
whereas, aldehydes bearing electron-withdrawing substituents
behaved slightly better considering yield% and reaction time.
In fact, electron-releasing substituents deactivate the aldehyde
to accept nucleophilic attack of β-naphthol; however, electron-
withdrawing groups activate the carbonyl group of the
aldehyde. These findings are in accord with the proposed
mechanism.
Table 6 Three-component condensation of β-naphthol, benzaldehyde,
and benzamide in the presence of some simple and complex metal
oxides
Entry
Catalyst
Time (min)
Yield (%)
1
2
H6P2W18O62/TPI-Fe3O4
H4SiW12O40
60
15
60
25
45
60
60
45
35
120
120
120
120
20
74
50
48
26
24
10
40
48
<6
<9
<9
12
21
23
3
4
5
6
7
8
9
H3PW12O40
H5PW10V2O40
H3PMo12O40
H5PW10V2O40/TPI-ZnO
H5PW10V2O40/RNH2-Fe3O4
H5PW10V2O40/TPI-Fe3O4
ZrO2
a
10
11
12
13
14
MCM-41
MCM-48
SBA-15
KH2PO4
4 Conclusion
ZnO
Heterogenization of Wells–Dawson diphosphooctadecatungstic
acid (H6P2W18O62·24H2O) through immobilization on
a RNH2: (EtO)3Si(CH2)3NH2. 10 mg of catalyst was used in each case.
1560 | Dalton Trans., 2014, 43, 1550–1563
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Paper
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modified TPI-Fe3O4 and fabrication of
a new organic–
inorganic nanohybrid material has been the target of this
research. In this powerful strategy, the active homogeneous
catalyst is chemically immobilized onto the surface of the
inert solid material through the action of the organic linker
N-[3-(triethoxysilyl)propyl]isonicotinamide to overcome the
potential negative aspects of catalyst leaching. The magnetic
nanohybrid material H6P2W18O62/pyridino-Fe3O4 was used in
an environmentally friendly route for the one-pot preparation
of different 1-amidoalkyl-2-naphthols. The present approach
offers several promising advantages such as higher yield, easy
handling, reduced reaction time, easy and green work-up,
and versatility. These advantages, in general, highlight this
protocol as a useful and attractive methodology, among the
methods reported in the literature, for the rapid synthesis of
biologically active 1-amidoalkyl-2-naphthols.
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