H3PW12O40 supported on functionalized polyoxometalate organic-inorganic hybrid nanoparticles as efficient catalysts for three-component Mannich-type reactions in water

Article abstract of DOI:10.1039/c6ra27519b

Two new types of catalyst were synthesized by the immobilization of H₃PW₁₂O₄₀ (HPW₁₂) on the surface of organic-inorganic polyoxometalate nanoparticles of H₆Cu₂[PPDA]₆[SiW₉Cu₃O₃₇]·12H₂O (HybPOM) (PPDA = p-phenylenediamine) and Go/Fe₃O₄/HybPOM nanoparticles. These catalysts were characterized by thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and alternating gradient force magnetometry (AGFM). The IR spectroscopy as well as XRPD reveal that HPW₁₂ were immobilized on the support. The potentiometric titration with n-butylamine reveal that the catalysts can be classified as strong acids. The results show that the particles are mostly spherical in shape and have an average size in the range of 20-50 nm. The catalytic activities of the catalysts were probed through one-pot three-component Mannich-type reactions of aldehydes, amines and ketones in water at room temperature. The catalysts were re-used at least five times without any loss of their high catalytic activity.

Full text of DOI:10.1039/c6ra27519b

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H3PW12O40 supported on functionalized
polyoxometalate organic–inorganic hybrid
nanoparticles as efficient catalysts for three-
component Mannich-type reactions in water†
Cite this: RSC Adv., 2017, 7, 11510
*
Roushan Khoshnavazi, Leila Bahrami, Forugh Havasi and Elham Naseri
Two new types of catalyst were synthesized by the immobilization of H3PW12O40 (HPW12) on the surface of
organic–inorganic polyoxometalate nanoparticles of H6Cu2[PPDA]6[SiW9Cu3O37]$12H2O (HybPOM) (PPDA
¼ p-phenylenediamine) and Go/Fe3O4/HybPOM nanoparticles. These catalysts were characterized by
thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, scanning electron
microscopy (SEM), energy dispersive X-ray analysis (EDX) and alternating gradient force magnetometry
(AGFM). The IR spectroscopy as well as XRPD reveal that HPW12 were immobilized on the support. The
potentiometric titration with n-butylamine reveal that the catalysts can be classified as strong acids. The
results show that the particles are mostly spherical in shape and have an average size in the range of
20–50 nm. The catalytic activities of the catalysts were probed through one-pot three-component
Mannich-type reactions of aldehydes, amines and ketones in water at room temperature. The catalysts
were re-used at least five times without any loss of their high catalytic activity.
Received 29th November 2016
Accepted 8th February 2017
DOI: 10.1039/c6ra27519b
rsc.li/rsc-advances
made to study the support of HPAs on inert or weak acidic high
surface area materials such as, silica,18 zeolite,19 and alumina.20
1. Introduction
The application of supported HPAs increases the specic
surface area of the catalysts and modies their catalytic prop-
erties. Thus, immobilization of HPAs on supports show
a greater number of surface acid sites than their bulk compo-
nents. The use of nanoparticles (NPs) offers many advantages
for supporting homogeneous catalysts due to their unique size
and physical properties. NPs could have a higher catalyst
loading capacity and a higher dispersion than many conven-
tional support matrices. Recently, supported HPAs have been
developed for a wide range of organic reactions in fundamental
research.21,22
Easier recovery and recycling aer carrying out reaction
through simple separation processes are the advantages that
supported HPAs have compared to homogeneous examples.
In addition, heteropoly acids are more active catalysts for
various reactions in solution than conventional inorganic and
organic acids and they have many environmental benets.
They were tested as catalysts for several reactions with the
nal objective of industrial use,23–25 as alcohol dehydration,26
alkylation27 or esterication28 reactions. Among heteropoly
acids, 12-tungstophosphoric acids are the most widely used
catalysts owing to their high acid strength and thermal
stability, and relatively low reducibility. Recently, the sup-
ported heteropoly acids as heterogeneous catalysts have
attracted much attention. H3PW12O40 have been immobilized
on Fe3O4 functionalized nanoparticles29–31 and successfully
Polyoxometalates (POMs), discrete anionic metal-oxo clusters,
can be linked together through cationic moieties to build
materials with incredible structural diversity which exhibit
a wide variety of compositions and structural versatility,1–3 as
well as important optical,4 catalytic,5–9 and magnetic10–13 prop-
erties. POM clusters occupy a vast parameter space between the
mononuclear metalate species and the bulk oxide.14 POMs can
also self-assemble into ordered patterns at the nanometer scale,
and thus have multilevel ordered structures.15,16 The great
diversity in structure and composition of POMs make them
attractive materials in many elds.17 One focus is to use them as
both homogeneous and heterogeneous catalysts in oxidation
and acid promoted reactions. Heteropoly acids (HPAs) such as
H3PW12O40 have been extensively studied as acid catalysts for
many reactions. Despite the above advantages, use of POMs is
restricted owing to the fact that they are non-porous solids with
low surface area (of less than 10 m2 gꢀ1). Because of their ionic
nature they dissolve well in polar solvents such as water, alco-
hols, ketones, acetonitrile and dimethylsulfoxide which makes
them difficult to separate from the reaction products and their
inevitable loss during recycling. Hence, efforts have also been
Department of Chemistry, University of Kurdistan, P.O. Box 66135-416, Sanandaj,
Iran. E-mail: r.khoshnavazi@uok.ac.ir; khoshnavazi@yahoo.com; Fax: +98 87
33624133; Tel: +98 87 33624133
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra27519b
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applied as active species in organic reaction catalysis.32,33 In
this study, organic–inorganic nanohybride of HybPOM was
used as a new kind of high surface area material for sup-
2. Experimental section
Materials and apparatus
porting homogeneous H3PW12O40
.
The functionalized
All reagents and solvents were commercially obtained and used
without further purication. The 12-tungstophosphoric acid
(H3PW12O40$xH2O) was prepared according to a reported
procedure.38 The GO–Fe3O4 was prepared according to the
literature.39
organic–inorganic polyoxometalates of HybPOM was synthe-
sized by a simple and high yield reaction.34 Ultimately, mixing
a suspension of HybPOM with an ethanolic solution of HPW12
leads to the formation of HybPOM/HPW12 NPs which involves
the reaction of H3PW12O40 with surface amine groups of
HybPOM. Protonation of amine groups gives positively
charged cations which bound electrostatically to the
heteropolyanions.
Despite the high surface areas of HybPOM/HPW12, ltra-
tion or centrifugation is needed and the separation process is
still a difficult and time consuming process. Magnetic nano-
particles are excellent supports for various catalysts35,36 and
can easily be separated and recycled from the products by an
external magnet. Surface functionalized Go/Fe3O4 nano-
particles are a kind of novel functional material which has
been widely used in biotechnology and catalysis. The Go/
Fe3O4/HybPOM magnetic nanoparticle was synthesized
simply by addition of HybPOM to the ethanol disperse solu-
tion of Go/Fe3O4 nanoparticle. The GO because of its excep-
tionally high surface area (2630 m2 gꢀ1) and abundant
oxygenate groups such as epoxy, hydroxyl, and carboxyl groups
Powder XRD was obtained by an X'PertPro Panalytical,
Holland diffractometer in 40 kV and 30 mA with a CuKa radiation
˚
(l ¼ 1.5418 A). Infrared spectra were recorded on a Bruker Vector
22 FT-IR using KBr plate. The morphology of nanocomposites
was revealed by a scanning electron microscope (FESEM-TESCAN
MIRA3). The elements in the nanocomposite samples were pro-
bed energy-dispersive X-ray (EDX) spectroscopy accessory to the
FESEM-TESCAN MIRA3 scanning electron microscopy. The
magnetic properties were investigated by a home-made alterna-
tive gradient force magnetometer (AGFM) in the magnetic eld
range of ꢀ5000 to 5000 Oe at room temperature. Thermo gravi-
metric analysis-differential thermal analysis (TGA-DTA) was
carried out using a STA PT-1000 LINSEIS.
Synthesis of nanohybrides
Preparation of H6Cu2[PDA]6[SiW9Cu3O37]$12H2O nano-
on their surface, is as an ideal support for immobilizing and hybrid (HybPOM). HybPOM was prepared by the following stated
improving POMs catalytic activity. The GO/Fe3O4 nano- method.34 A 2.00 g (0.725 mmol) of Na9H[b-SiW9O34]$18H2O was
particle, has key properties such as magnetic separation, the dissolved in water (100 mL) at room temperature. Cu(C2H3O2)2-
large surface area and ease of functionalizing with various $H2O (3.00 g, 15 mmol) was added to the solution while stirring,
chemical groups to increase their dependence toward target resulting in a blue solution. Then, 0.7 g (6.5 mmol) of PPDA was
compounds. Through a nucleophilic attack by available NH2 slowly added to this blue solution while stirring, producing
groups on the surface HybPOM hybrid with functional groups a blue-brown precipitate. Aer stirring for 4 h, the precipitate
such as epoxy, hydroxyl, and carboxyl groups on the surface of color changed from blue-brown to black. Obtained black
GO–Fe3O4 magnetic nanoparticle, the hybrid material is precipitate was centrifuged and washed several times with water
graed via C–N bond formation.34 Once again mixing and acetonitrile, then air-dried at room temperature.
a suspension of the Go/Fe3O4/HybPOM with an ethanolic
solution of HPW12 lead to the formation of Go/Fe3O4/ GO@Fe3O4@HybPOM was prepared as per the following
HybPOM/HPW12 NPs.
method.34 To disperse GO–Fe3O4 in 30 mL of absolute ethanol, an
Preparation
of
GO@Fe3O4@HybPOM
nanohybride.
The prepared Go/Fe3O4/HybPOM/HPW12 hybrid composite appropriate amount of HybPOM was added and the mixture was
can be easily separated by magnetic separation from the sonicated for 10 min to form a homogeneous dispersion. Then,
medium aer reaction. The Go/Fe3O4/HybPOM/HPW12 hybrid the mixture was stirred under reux condition for 24 h. The ob-
composite show the same catalytic efficiency compared to the tained solid was then magnetically collected from the solution and
HybPOM/HPW12 and can be easily manipulated by external washed three times with water and ethanol, and dried at 50 ꢁC.
magnetic eld.
Preparation of HybPOM/HPW12 nanohybride. A 0.500 g of
In this paper, we synthesized HybPOM/HPW12 and Go/ HybPOM was dispersed in 25 mL of absolute ethanol by soni-
Fe3O4/HybPOM/HPW12 nanoparticles by immobilization of cation for 20 minutes. An appropriate amount of dissolved
Keggin type heteropoly acid H3PW12O40$xH2O on organic– H3PW12O40$xH2O in the absolute ethanol was added to the
inorganic nano hybrid of H6Cu2[PPDA]6[SiW9Cu3O37]$12H2O above solution while stirring. The resulting mixture was stirred
(HybPOM) and nanocomposite of Go/Fe3O4/HybPOM nano- under reux condition for 48 h. The crude product was sepa-
particles, respectively and assessed their catalytic activity in rated by ceꢁntrifuges, washed several times with ethanol and
Mannich-type reactions in water. Indeed, the venerable Man- dried at 50 C under reduced pressure.
nich reactions and its variants represent one of the more
Preparation of Go/Fe3O4/HybPOM/HPW12 nanohybride.
powerful constructs for alkaloid synthesis.37 To the best of our 0.500 g Go/Fe3O4/HybPOM was suspended in 25 mL of absolute
knowledge, this is the rst report containing immobilization of ethanol and then an appropriate amount of H3PW12O40$xH2O
a heteropolyacid on organic–inorganic nano hybrid. Synthesis, dissolved in absolute ethanol was added to the suspension
characterization and catalytic application of new nano catalysts while stirring. The mixture was stirred under reux condition
have been investigated.
for 24 hours. The obtained nanohybride were magnetically
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collected, washed three times with ethanol (3 ꢂ 5 mL), and and H2O (1 mL). The reaction mixture was vigorously stirred at
ꢁ
dried at 50 C under reduced pressure.
room temperature for an appropriate time. The progress of the
reaction was monitored by TLC. Aer completion of the reac-
tion, the catalyst was separated by external magnet and washed
with ethanol and water, vacuum dried and stored for subse-
quent runs. The reaction mixture was extracted with H2O and
EtOAc. The organic layer was dried over MgSO4 and then
evaporated under reduced pressure. The crude products were
puried either by crystallization from ethanol, giving of b-
amino ketones in good yield.
Titration of the catalysts acidity
The acidity of HybPOM/HPW12 and Go/Fe3O4/HybPOM/HPW12
was probed by potentiometric titration.40 HybPOM/HPW12 or
Go/Fe3O4/HybPOM/HPW12 (0.05 g) was suspended in acetoni-
trile, and stirred for 3 h. Then, the suspension was titrated with
0.05 M n-butylamine in acetonitrile at 0.05 mL minꢀ1. The
electrode potential variation was measured with a WTW digital
nanolab model using a double-junction electrode.
Characterization of organic products
2-(Phenyl(phenylamino)methyl)cyclohexanone (Table 2,
entry 1). White solid, mp 128–129 ꢁC;53 FT-IR (KBr, cmꢀ1) 1440–
1594 (C]C, aromatic), 1701 (C]O, carbonyl), 3349 (NH, second
General procedure for the synthesis of b-amino ketones
catalyzed by HybPOM/HPW12
Typically, the catalyst (0.005 g) was added to a mixture of
aldehyde (1 mmol), amine (1 mmol), cyclohexanone (2 mmol)
and H2O (1 mL). The reaction mixture was vigorously stirred at
room temperature for an appropriate time. The progress of the
reaction was monitored by TLC. Aer completion of the reac-
tion, the catalyst was separated by centrifuge, washed with
ethanol and water, vacuum dried and stored for subsequent
runs. The reaction mixture was extracted with H2O and EtOAc.
The organic layer was dried over MgSO4 and then evaporated
under reduced pressure. This solution was concentrated at
room temperature to yield the crude products. The crude
products were puried either by crystallization from ethanol,
giving of b-amino ketones in good yield. The reaction product
was characterized by IR and mass spectroscopy.
1
amine), 2925, 2860 (C–H, aliphatic), 3015 (C–H, aromatic). H
NMR (250 MHz, CDCl3) d (ppm): 7.80 (s, 1H), 7.45–7.26 (m, 7H),
6.68–6.64 (m, 2H), 4.80 (d, 0.38Hsyn, J ¼ 2.25 Hz), 4.63–4.60 (m,
0.36Hanti), 2.94–2.84 (m, 2H), 2.54–2.34 (m, 2H), 1.91–1.77 (m,
4H), 1.25 (s, 1H). (syn/anti: 50/50). EI-MS (70 eV): m/z (%) ¼ 279
(M+c, 11), 182 (M+c–C6H9O, 48).
2-((2-Chlorophenyl)(phenylamino)methyl)cyclohexanone
(Table 2, entry 2). White solid, mp 137–139 ꢁC;54 FT-IR (KBr,
cmꢀ1) 1434–1600 (C]C, aromatic), 1704 (C]O, carbonyl),
3380 (NH, second amine), 2933, 2857 (C–H, aliphatic), 3045
(C–H, aromatic); 1H NMR (250 MHz, CDCl3) d (ppm): 7.91 (s,
1H), 7.51–7.40 (m, 3H), 7.33–7.26 (m, 3H), 6.65–6.62 (m, 1H),
6.51 (d, 1H, J ¼ 8 Hz), 5.33 (d, 0.30Hsyn, J ¼ 3.25 Hz), 4.89 (d,
0.41Hanti, J ¼ 4.5 Hz), 2.77–2.54 (m, 7H), 1.96–1.92 (m, 2H).
(syn/anti: 42/58); EI-MS (70 eV): m/z (%) ¼ 315 [(M+c + 2), 3],
313 (M+c, 9), 216 (M+c–C6H9O, 100).
General procedure for the synthesis of b-amino ketones
catalyzed by Go/Fe3O4/HybPOM/HPW12
2-((4-Bromophenyl)(phenylamino)methyl)cyclohexanone
Typically, the catalyst (0.007 g) was added to a mixture of (Table 2, entry 3). White solid, mp 109–110 ꢁC;55 FT-IR (KBr,
aldehyde (1 mmol), amine (1 mmol), cyclohexanone (2 mmol) cmꢀ1) 1484–1610 (C]C, aromatic), 1709 (C]O, carbonyl),
Scheme 1 The schematic pathway for preparing Go/Fe3O4/HybPOM/HPW12 hybrid composite.
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3380 (NH, second amine), 2927, 2856 (C–H, aliphatic), 3029
1
(C–H, aromatic); H NMR (250 MHz, CDCl3) d (ppm): 7.49–
7.41 (m, 2H), 7.41–7.39 (m, 2H), 7.26–7.23 (m, 2H), 7.23–7.07
(m, 2H), 6.69–6.64 (m, 1H), 6.51 (d, 1H, J ¼ 6 Hz), 4.72 (d,
0.41Hsyn, J ¼ 4 Hz), 4.58 (d, 0.54Hanti, J ¼ 6.25 Hz), 2.78–2.76
(m, 1H), 2.53–2.38 (m, 1H), 2.36–2.31 (m, 1H), 1.94–1.71 (m,
6H). (syn/anti: 43/57); EI-MS (70 eV): m/z (%) ¼ 359 [(M+c + 2),
6], 357 (M+c, 7), 260 (M+c–C6H9O, 100).
2-((4-Chlorophenylamino)(phenyl)methyl)cyclohexanone
(Table 2, entry 5). White solid, mp 137–138 ꢁC;56 FT-IR (KBr,
cmꢀ1) 1490–1592 (C]C, aromatic), 1703 (C]O, carbonyl),
3369 (NH, second amine), 2940, 2850 (C–H, aliphatic), 3052
(C–H, aromatic); EI-MS (70 eV): m/z (%) ¼ 315 [(M+c + 2), 3],
313 (M+c, 9), 216 (M+c–C6H9O, 85).
Fig. 2 IR spectra of HPW12 (a) Go/Fe3O4/HybPOM/HPW12 (b) and Go/
Fe3O4/HybPOM (c).
3. Results and discussion
Characterization of nanomaterials
The XRD diffraction patterns of Go/Fe3O4/HybPOM (a)
HybPOM/HPW12 (b) and Go/Fe3O4/HybPOM/HPW12 (c) are
shown in Fig. 3. X-ray diffractions pattern of HybPOM, as
a typical amorphous state material is observed in the X-ray
spectra of HybPOM/HPW12 and Go/Fe3O4/HybPOM/HPW12. In
Fig. 3, peaks corresponding to the Fe3O4 are observed at 2q ¼
30.52ꢁ, 35.85ꢁ, 43.58ꢁ, 53.96ꢁ, 57.43ꢁ, 63.16ꢁ and 74.65ꢁ which
match well with the standard Fe3O4 sample (JCPDS le no. 19-
0629).40
The results of IR spectrum showed that all amine groups on the
surface HybPOM were not linked to the GO–Fe3O4 magnetic
nanoparticle. Therefore, H3PW12O40 might interact with the
reminded free amine groups on the surface GO–Fe3O4 for the
formation of Go/Fe3O4/HybPOM/HPW12 nanoparticles. The
catalytic ability of the prepared HybPOM/HPW12 and Go/Fe3O4/
HybPOM/HPW12 hybrid composite was evaluated in synthesis
of various b-amino ketones via Mannich reactions in aqueous
media. The schematic pathway for preparation Go/Fe3O4/
HybPOM/HPW12 hybrid composite is presented in Scheme 1.
IR spectra of HybPOM, HybPOM/HPW12 and HPW12 samples
are shown in Fig. 1. The PW12O403ꢀ Keggin ion structure is well
known and shows typical bands for absorptions at 1081 (P–O),
The peaks corresponding to HPW12 at 2q ¼ 30.52ꢁ, 35.85ꢁ,
43.58ꢁ, 53.96ꢁ, are observed in X-ray powder diffractions of
HybPOM/HPW12 and Go/Fe3O4/HybPOM/HPW12 which match
well with the X-ray powder diffraction pattern of H3PW12O40
-
$6H2O.30 The X-ray powder diffractions thus conrm that the
Keggin structure of the HPW12 compound was preserved with
its immobilization on the supports.
985 (W]O), 897 and 803 (W–O–W) cmꢀ1 41 As a result, the new
.
peaks are observed at 1079 and 962 cmꢀ1 in IR spectra of
HybPOM/HPW12 indicating that HPW12 was anchored onto
HybPOM successfully. IR spectra of Go/Fe3O4/HybPOM, Go/
Fe3O4/HybPOM/HPW12 and HPW12 samples are shown in Fig. 2.
In Fig. 2, peaks at 573–630 cmꢀ1 in the Go/Fe3O4/HybPOM/
HPW12 composite are attributed to Fe–O stretching vibration
and the peak at 1080 cmꢀ1 is attributed to the asymmetric
stretching vibration P–O.
Fig.
3 The XRD diffraction patterns of Go/Fe3O4/HybPOM (a)
Fig. 1 IR spectra of HPW12 (a), HybPOM/HPW12 (b) and HybPOM (c).
HybPOM/HPW12 (b) and Go/Fe3O4/HybPOM/HPW12 (c).
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Fig. 4 The potentiometric titration with n-butylamine curves of (A) HybPOM (a) HybPOM/HPW12 (b) HybPOM/HPW12 after five consecutive runs
(c); and (B) Go/Fe3O4/HybPOM (a) Go/Fe3O4/HybPOM/HPW12 (b) and Go/Fe3O4/HybPOM/HPW12 after five consecutive runs (c).
Numerous methods have been employed to describe the the plateau is reached (meq. n-butylamine/total amount of meq.
acidity of POMs in the solid state both qualitatively and quan- H+) indicates the total number of acid sites that are present in
titatively. Titration with Hammett indicators, temperature pro- the titrated solid. On the other hand, the acid strength of these
grammed desorption of adsorbed molecules such as ammonia sites may be classied according to the following scale: Ei >
or pyridine, adsorption microcalorimetry, NMR spectroscopy 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), ꢀ100 <
and catalytic probe reactions42 are the most common methods Ei < 0 (weak sites) and Ei < ꢀ100 mV (very weak sites).43 The
for characterizing solid acids. The nature of the support HybPOM (Fig. 4A(a)) and Go/Fe3O4/HybPOM (Fig. 4B(a)) showed
apparently affecting the acidity of the supported HPAs by means very weak and weak acid sites (Ei ¼ ꢀ165.9 and ꢀ73.8 mV),
of potentiometric titration with n-butylamine allows the esti- respectively. Difference in the acid strength of HybPOM and Go/
mation of the number of acid sites and their distribution.22
Fe3O4/HybPOM is attributed to the total number of free NH2
The titration curves obtained for HybPOM and HybPOM/ groups on them. For Go/Fe3O4/HybPOM, some of the NH2
HPW12 are illustrated in Fig. 4A and those of Go/Fe3O4/HybPOM groups are involved in the bonding with Go/Fe3O4
and Go/Fe3O4/HybPOM/HPW12 are shown in Fig. 4B. It is nanoparticles.
believed that the initial electrode potential (Ei) indicates the
As expected, the strength of the acid sites of the HPW12
maximum strength of the acid sites and the value from which decreases with its addition to the HybPOM and Go/Fe3O4/
Fig. 5 The SEM images of HybPOM (a), HybPOM/HPW12 (b), Go/Fe3O4/HybPOM (c) and Go/Fe3O4/HybPOM/HPW12 (d).
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Fig. 6 EDX analysis of HybPOM/HPW12 (a) and Go/Fe3O4/HybPOM/HPW12 nanoparticles (b).
HybPOM surfaces. This effect depends on the extent of the respectively. A decrease in saturation magnetization observed
interaction of HPW12 with the support. According to potentio- for Go/Fe3O4/HybPOM and Go/Fe3O4/HybPOM/HPW12 could be
metric titration curves, HPW12 presented very strong acidic sites attributed to the increased mass and the loading of the
(Ei ¼ 572.2 mV).44 Potentiometric titration results of HybPOM/ HybPOM and HPW12. Even with this reduction in the saturation
HPW12 (Fig. 4A(b)) and Go/Fe3O4/HybPOM/HPW12 (Fig. 4B(b)) magnetization, complete magnetic separation of Go/Fe3O4/
samples showed the lower Ei value of 74.1 and 80 mV, respec- HybPOM and Go/Fe3O4/HybPOM/HPW12 was achieved in <10 s
tively compared to the HPW12 which might be a result of by placing a magnet near the vessels containing the aqueous
a strong interaction with the HybPOM and Go/Fe3O4/HybPOM dispersion of the nanoparticles.
support. Here, differences in the acid strength of HybPOM/
Fig. 6 shows the SEM images of HybPOM (a), HybPOM/HPW12
HPW12 and Go/Fe3O4/HybPOM/HPW12 samples are attributed (b), Go/Fe3O4/HybPOM (c) and Go/Fe3O4/HybPOM/HPW12 (d). The
to the total number of free NH2 groups on HybPOM and Go/ images indicate the nano compounds aer immobilization of
Fe3O4/HybPOM support.
HPW12 preserved the relatively uniform spherical nanometer
The magnetization of GO–Fe3O4, Go/Fe3O4/HybPOM and Go/ particles of diameter in the range 20–50 nm. EDX analysis (Fig. 7)
Fe3O4/HybPOM/HPW12 samples was measured at room of HybPOM/HPW12 (a) and Go/Fe3O4/HybPOM/HPW12 (b) nano-
temperature as shown in Fig. 5. The specic saturation particles show peaks of W, O, Si, C, N, Cu, P and C, O, Fe, W, Si, N,
magnetization of GO–Fe3O4, is 29.2 emu gꢀ1 value is smaller Cu, P respectively, the obtained element percentages of which are
than the reported value of bulk Fe3O4 of 92 emu gꢀ1
.
The fairly consistent with the TGA results.
45
ꢁ
specic saturation magnetization, of Go/Fe3O4/HybPOM and
Go/Fe3O4/HybPOM/HPW12 are 22.47 and 16.64 emu gꢀ1
TGA was conducted in the range of 30–800 C and the TGA
,
plot of Go/Fe3O4/HybPOM (a) and Go/Fe3O4/HybPOM/HPW12
Fig. 8 The yield (%) of five consecutive cycles for the preparation of 2-
Fig. 7 TGA plot of Go/Fe3O4/HybPOM (a) and Go/Fe3O4/HybPOM/ (phenyl(phenylamino)methyl)cyclohexanone of HybPOM/HPW12 (a)
HPW12 (b).
and of Go/Fe3O4/HybPOM/HPW12 (b).
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Fig. 9 The FT-IR spectra of (A) HybPOM/HPW12 (a) and HybPOM/HPW12 after five consecutive run (b); (B) Go/Fe3O4/HybPOM/HPW12 (a) and
Go/Fe3O4/HybPOM/HPW12 after five consecutive run (b).
(b) are depicted in Fig. 7. Although, the TGA prole of Go/Fe3O4/
Recycling experiments for catalysts were performed by the
HybPOM/HPW12 shows continuous weight loss in the range of Mannich condensation of benzaldehyde, aniline and cyclohexa-
30–800 ꢁC, only three weight loss steps cab be observed. The rst none in water. The model reaction was carried out by using 0.01 g
weight loss (ca. 2.5%) that occurred below 150 ꢁC is related to the of the catalysts. Fig. 8 shows the yield of ve consecutive cycles for
loss of physisorbed water at the hybrid material surface. A main the preparation of 2-(phenyl(phenylamino)methyl)cyclohexanone
weight loss (14.00%) in the range of 150–425 ꢁC is assigned to the in the presence of HybPOM/HPW12 (a) and Go/Fe3O4/HybPOM/
decomposition of the Go/Fe3O4/HybPOM nanocomposite HPW12 (b). When the reaction was completed, catalysts are
support.34 The third weight loss (ca. 8.28%) in the range of 425– separated and water was removed from the mixture to leave
600 ꢁC is due to partial decomposition of H3PW12O40 (ref. 29) and a residue. Then, the product was dissolved in ethyl acetate and
residual Go/Fe3O4/HybPOM nanocomposite. The TGA analysis the catalyst easily separated from the product by centrifuge or
indicates that the prepared Go/Fe3O4/HybPOM/HPW12 is ther- with the aid of an external magnet, followed by decantation of the
mally stable at temperatures lower than 150 ꢁC. The total weight product solution. The remaining catalysts were washed with
loss of Go/Fe3O4/HybPOM/HPW12 (27.7 wt%) was less than that ethanol and water to remove the residual product, dried under
of Go/Fe3O4/HybPOM (32.4 wt%) because of additional residual vacuum and re-used in a subsequent reaction. The catalysts have
from partial decomposition of H3PW12O40.30
been observed to be re-usable for at least ve times without
Table 1 Optimization of different parameters for the synthesis of 2-(phenyl(phenylamino)methyl)cyclohexanone catalyzed by HybPOM/HPW12
(1) and Go/Fe3O4/HybPOM/HPW12 (2)a
Entry
Cat. (g)
Solvent
Time (min) Cat. 1(2)
Yieldb (%) Cat. 1(2)
1
2
3
4
5
6
7
8
0
H2O
H2O
H2O
H2O
EtOH
PEG
EtOAc
CH2Cl2
250(250)
90(110)
60(90)
60(55)
120(115)
70(80)
Trace(trace)
80(75)
96(88)
97(98)
89(90)
70(65)
60(88)
75(85)
0.003
0.005
0.007
0.007
0.007
0.007
0.007
160(180)
210(200)
a
b
Reaction conditions: aldehyde (1 mmol), amine (1 mmol), ketone (2 mmol). Isolated yield.
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Table 2 One-pot Mannich reactions for the synthesis of b-amino carbonyl compounds catalyzed by HybPOM/HPW12 (1) and Go/Fe3O4/
HybPOM/HPW12 (2)
Entry
1
Product
Time (min) Cat. 1(2)
Yielda,b (%) Cat. 1(2)
Mp (ꢁC) [ref.]
60(55)
96(98)
128–129 (ref. 53)
2
3
80(75)
40(50)
92(95)
90(88)
137–139 (ref. 54)
109–110 (ref. 55)
4
65(70)
94(90)
131 (ref. 56)
5
6
7
60(55)
90(80)
80(70)
95(96)
92(95)
90(90)
137 (ref. 56)
136–138 (ref. 53)
132–133 (ref. 57)
8
30(40)
97(92)
134–136 (ref. 58)
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Table 2 (Contd.)
Entry
9
Product
Time (min) Cat. 1(2)
Yielda,b (%) Cat. 1(2)
Mp (ꢁC) [ref.]
70(80)
93(95)
136–137 (ref. 57)
10
75(60)
95(98)
122 (ref. 59)
11
12
80(65)
45(50)
90(85)
93(78)
167–169 (ref. 55)
262–263 (ref. 60)
13
50(50)
95(90)
211–213 (ref. 60)
a
All the products were identied and characterized by comparison of their physical and spectral data with those of authentic samples. b Isolated
yield.
a detectable catalytic leaching or an appreciable change in most widely utilized chemical transformations for constructing
activity. High catalytic activity and signicantly low leaching of b-amino ketones and other b-amino carbonyl compounds.46,47
catalysts (during catalytic reactions) were observed by titration of Recently, direct Mannich reactions of aldehydes, ketones and
acid content. The potentiometric titration curve of the re-used aryl amines have been realized via Lewis acids, lanthanides,
catalysts showed that all of their acidic sites were preserved transition metal salt catalysis and organocatalytic
during ve cycles and HPW12 leaching was negligible (Fig. 4A(c) approaches.47–50 Most of these methods suffer from severe
and B(c)). The FT-IR spectra showed no signicant structural drawbacks including the use of a large amount of catalysts,
changes for catalysts aer ve consecutive runs (Fig. 9).
expensive reagents or catalysts, sometimes long reaction times
and low yield. The use of environmentally benign solvents in
organic reactions have attracted much attention and another
key research area of green chemistry, with great advances being
seen in aqueous catalysis. Use of water in addition to its being
Catalytic studies
The Mannich-type reactions are very important carbon–carbon
bond-forming reactions in organic synthesis and one of the
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Scheme 2 Mannich reactions of aromatic aldehydes, anilines, and cyclohexanone.
an environmentally benign solvent show unique selectivity and coupling constant (J) for the anti-isomer is higher than that of
reactivity. In this study, the catalytic activity of HybPOM/HPW12 the syn. The anti/syn ratio was determined by 1H NMR judged by
and Go/Fe3O4/HybPOM/HPW12 nanoparticles were investigated the intensity of the H1 (Scheme 2).51
through one-pot three-component Mannich-type reactions of
aldehydes, amines and cyclohexanone in water as a novel, effi-
cient and heterogeneous catalyst under mild conditions. First,
in order to optimize the Mannich-type reaction conditions, the
reaction was performed with benzaldehyde, aniline and cyclo-
hexanone as a model reaction (Table 1). As shown in Table 1,
0.005 and 0.007 g of catalysts were found to be ideal for the
Mannich-type reactions and the best results were obtained
using 1 mL water as solvent. The control experiments carried
out using homogenous H3PW12O40$xH2O and Go/Fe3O4/
HybPOM showed that the mixed Mannich product and imine
were formed. When using H3PW12O40$xH2O as a homogenous
catalyst, approximately 80% and 15% of the Mannich product
and imine were formed, respectively. However, when Go/Fe3O4/
HybPOM was used as a catalyst, approximately 65% of the
Mannich product was formed. The reaction was extended to
a series of aldehydes, cyclohexanone and amines to explore the
generality of this catalytic system (Table 2).
The formed anti and syn isomers in the Mannich-type reac-
tions were identied by the coupling constants (J) of the vicinal
Scheme 4 Proposed syn- and anti-intermediates for the Mannich-
protons adjacent to C]O and NH in their 1H NMR spectra. The
type reaction in the presence of Go/Fe3O4/HybPOM/HPW12
.
Scheme 3 Proposed mechanism of the Mannich-type reaction in the presence of HybPOM/HPW12 and Go/Fe3O4/HybPOM/HPW12
.
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A possible mechanism of the Mannich-type reaction in the 12 J. M. Clemente-Juan and E. Coronado, Coord. Chem. Rev.,
presence of HybPOM/HPW12 and Go/Fe3O4/HybPOM/HPW12 as 1999, 193–195, 361.
Bronsted acid catalysts is proposed (Scheme 3). The reaction 13 D. L. Long, P. Kogerler, L. J. Farrugia and L. Cronin, Dalton
proceeded typically through the imine formation of the alde- Trans., 2005, 1372.
hyde and amine, protonation of the imine, and the attack of the 14 D. L. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int.
enol derived from the ketone to the protonated imine, leading
to the formation of Mannich products.
Ed., 2010, 49, 1736.
15 D. Volkmer, A. Du Chesne, D. G. Kurth, H. Schnablegger,
¨
As shown in Scheme 4, an intermediate are formed via
P. Lehmann, M. J. Koop and A. Muller, J. Am. Chem. Soc.,
hydrogen bonds formation among Go/Fe3O4/HybPOM/HPW12
,
2000, 122, 1995.
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mediate the aryl groups of aldimine and the methylene groups Chem., Int. Ed., 2007, 46, 3342.
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be less steric repulsion. Therefore, the most stable transition 18 X. M. Yan, J. H. Lei, D. Liu, Y. C. Wu and W. Liu, Mater. Res.
state would produce the anti isomer.52
Bull., 2007, 42, 1905.
19 N. Dubey, S. S. Rayalu, N. K. Labhsetwar and S. Devotta, Int. J.
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20 M. A. Alibeik, Z. Zaghaghi and I. M. Baltork, J. Chin. Chem.
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4. Conclusion
The new nanoparticle catalysts were synthesized by the immo-
bilization of H3PW12O40 on the surface of H6Cu2[PPDA]6[SiW9- 21 (a) T. Okuhara, N. Mizuno and M. Misono, Appl. Catal., A,
Cu3O37]$12H2O (HybPOM) (PPDA
organic–inorganic hybrid polyoxometalates and Go/Fe3O4/
¼
p-phenylenediamine)
2001, 222, 63; (b) E. Raee, S. Eavani, S. Rashidzadeh and
M. Joshaghani, Inorg. Chim. Acta, 2009, 362, 3555.
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We gratefully acknowledge the Research Council of the 30 J. B. Mioc, R. Z. Dimitrijevi, M. Davidovic, Z. P. Nedic,
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