Investigating the effect of immobilization of yttrium-Schiff base onto magnetite nanoparticles: Enhanced catalytic activity in the synthesis of dihydropyrimidinones under solvent-free conditions

Article abstract of DOI:10.1007/s11164-015-2137-8

A new complex of yttrium-salpr was synthesized through the reaction of Y(NO₃)₃·6(H₂O) and salpr Schiff base (salpr = N,N′-bis(3-salicylidenaminopropyl) amine). The prepared complex was characterized by Mid and Far-IR,

Full text of DOI:10.1007/s11164-015-2137-8

Res Chem Intermed
DOI 10.1007/s11164-015-2137-8
Investigating the effect of immobilization of yttrium-
Schiff base onto magnetite nanoparticles: enhanced
catalytic activity in the synthesis
of dihydropyrimidinones under solvent-free conditions
1
Narjes Tayyebi Sabet Khomami
1
Felora Heshmatpour
•
Received: 23 March 2015 / Accepted: 11 June 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A new complex of yttrium-salpr was synthesized through the reaction of
0
Y(NO ) Á6(H O) and salpr Schiff base (salpr = N,N -bis(3-salicylidenaminopropyl)
3
3
2
1
3
1
amine). The prepared complex was characterized by Mid and Far-IR, C-NMR, H-
NMR, UV–Vis, XRF, and CHN. The obtained data suggested a binuclear structure
for this compound. To our surprise, the catalytic activity of this complex was even
lower than its precursor, Y(NO ) Á6(H O), in one-pot synthesis of dihydropyrim-
3
3
2
idinones under solvent-free conditions. As one of the reasons of this reduced activity
should pertain to the binuclear structure of the complex that reduces the availability
of yttrium in the reaction, the complex was immobilized on modified Fe O4
3
nanoparticles, which not only increases the surface to the volume ratio of the
compound, but also hinders the formation of binuclear structure due to site (Schiff
base) isolation. These prepared nanoparticles depicted noticeably high catalytic
activity as well as good recyclability and very easy separation.
Keywords Yttrium Á Heterogeneous and homogeneous catalysis Á
Immobilization Á Schiff base Á Nanostructures
Introduction
Yttrium is a major constituent of the mineral-containing middle or heavy earth
group. It tends to occur in the same deposits as the lanthanides due to the similar
3
?
3?
3?
ionic radius of Y to Ho and Er [1]; however, yttrium is more abundant and
less toxic than almost any other lanthanide element. Different compounds of yttrium
have shown great catalytic activity; yttrium nitrate hexahydrate Y(NO ) Á6(H O)
3
3
2
&
Felora Heshmatpour
heshmatpour@kntu.ac.ir
1
Department of Chemistry, K.N. Toosi University of Technology,
P.O. Box 16315 - 1618, Tehran, Iran
123

N. Tayyebi Sabet Khomami, F. Heshmatpour
was found to be an efficient catalyst for the one pot synthesis of dihydropyrim-
idinones (DHPMs), named the Biginelli reaction [2]. Ever since this reaction was
discovered by the Italian chemist Pietro Biginelli [3], scientists have paid a great
deal of attention to it due to the unique biological activities of Biginelli compounds.
These are antihypertensive, antiviral, antitumor, antibacterial and antiflammatory
agents [4], which are also the basic skeleton of some pharmaceuticals that are used
to prevent or treat cancer and AIDS [5]. The first used catalyst in this reaction was
HCl [3]. To date, a broad spectrum of catalysts, including ammonium chloride [4],
Lewis acids (yttrium(III) acetate [6], Lanthanide Triflate [7]), heteropolyacids
(
doped Keggin [8], Wells–Dawson acid [9]), and organic–inorganic hybrid
materials [10], have been studied in this reaction to increase yield, ease reaction
conditions, lower temperature, shorten reaction time, or make the reaction
environmentally friendly.
Y(III) strongly interacts with hard bases, and makes strong complexes with
oxygen and nitrogen. Hence, Schiff bases can be considered very good ligands for
yttrium(III) [11], while their complexes have shown good catalytic activity in
different reactions such as ring opening polymerization [12] and transacylation [13].
However, many metal Schiff base complexes tend to form dimeric per-oxo or l-oxo
species that decrease or deactivate the catalytic activity [14]. Moreover, recycling
these catalysts is often tedious and time consuming. For overcoming the
aforementioned drawbacks, Schiff base complexes are immobilized onto different
solid supports. Silica-coated magnetic Fe O nanoparticles (SCMNPs) are one of
3
4
the newest discovered solid supports of the very recent decades. The magnetite
nanoparticles have superparamagnetic properties that enable them to be attracted to
an external magnetic field, but retain no residual magnetism after removing the
magnetic field [15]; moreover, formation of a silica layer on the surface of magnetic
nanoparticles could screen the magnetic dipolar attraction between magnetic
nanoparticles, which favors the dispersion of magnetic nanoparticles in liquid media
[
16].
In this study, we synthesized and characterized a new complex of Y(III) 1 by
condensation of yttrium nitrate hexahydrate and salpr Schiff base. The analytic data
revealed a binuclear structure where Y(III) centers were bridged by two
phenoxooxygens. Then, the catalytic activity of this complex was investigated in
one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones under solvent-free condi-
tions. The results were in contrast with our expectations when the catalytic activity
of the prepared complex was lower than with yttrium nitrate as the precursor. We
assumed that this decrease in catalytic activity was due to the binuclear structure
and steric hindrance around yttrium(III). Hence, we attempted to synthesize 2 and
immobilize this complex onto SCMNPs, not only to increase the catalytic activity
3
?
but also to ease the recyclability. Interestingly, the prepared Y nanoparticles
showed great activity in one-pot synthesis of Dihydropyrimidinones under solvent-
free conditions.
1
23

Investigating the effect of immobilization of yttrium…
Experimental
Materials and methods
Yttrium nitrate hexahydrate, diethylenetriamine, salicylaldehyde, triethylamine,
ferrous chloride tetrahydrate, ferric chloride hexahydrate, and tetraethyl orthosil-
icate were purchased from Merck Chemical Company and utilized without any
more purification. Ammonia (25 %) was obtained from Dr. Mojallali Chemical
Laboratories Co., Ltd, Iran. All water utilized in the experiments was doubly
distilled.
Infrared (IR) spectra were recorded on an ABB FTLA 2000 spectrometer using
KBr pellets. Elemental analyses (C, H and N) were performed using a Perkin-Elmer
CHN 2400 elemental analyzer. X-ray fluorescence (XRF) data were obtained on
PHILIPS model PW1480. A double-beam spectrophotometer (Shimadzu, UV-240)
1
13
was used for the UV–Vis absorption determination. H and C nuclear magnetic
resonance (NMR) spectra (300 MHz) were measured in DMSO solutions. Field
emission scanning electron microscopy (FE-SEM) images were obtained on a
Hitachi S-4160, Japan. Magnetic susceptibility measurements were carried out at
room temperature using a vibrating sample magnetometer (VSM), MDK6 Iran.
Atomic force microscopy (AFM) images were obtained on a DME 95-50E. The
transmission electron micrographs (TEM) of the nanoparticles were recorded using
a Zeiss EM 900 instrument with an accelerating voltage of 80 kV.
Synthesis of 1
Schiff base of Salpr was prepared based on the literature [17]. The synthesis of the
complex 1 was performed according to the procedure employed by Wu et al. [18].
Y(NO ) Á6(H O) (0.191 g, 0.5 mM) and triethylamine (0.3 mL) in EtOH (10 mL)
3
3
2
were added to a stirred solution of salpr (0.5 mM) in EtOH (10 mL). Yellow
sediment was generated rapidly. The precipitate was washed with ethanol and ether,
and then dried.
Synthesis of 2
SCMNPs/salpr was prepared according to the literature [15]. For preparation of
2
ethanol (50 ml). SCMNPs/salpr (1 g) was sonicated in ethanol (20 ml) for
, the excess amount of Y(NO ) Á6(H O) (574 mg, 1.5 mmol) was dissolved in
3
3
2
1
an external magnet, the product was washed with methanol to remove unreacted
0 min, added to Y(NO ) Á6(H O) , and refluxed for 12 h. After separation with
3
3
2
6
Y(NO ) Á6(H O).
3
3
2
General procedure for synthesis of 3,4-dihydropyrimidin-2(1H)-ones
A mixture of catalyst, benzaldehyde, ethyl acetoacetate and urea with mole ratio
(
0.05 or 0.02:1:1:1.1), was heated under stirring under solvent-free condition. The
123

N. Tayyebi Sabet Khomami, F. Heshmatpour
reaction was monitored by thin-layer chromatography (TLC) using ethyl acetate/
hexane (4:6) as eluent. After completion, the reaction mixture was washed with
water. Finally, the mixture was dissolved in hot EtOH. In the case of using 1 as
catalyst, the mixture was suction filtered. For 2, the catalyst was concentrated on the
side wall of the reaction vessel using an external magnet and washed with ethanol.
The crude product was recrystallized from EtOH. All products were known
compounds and were characterized by comparison with reported values.
Results and discussion
Characterization of 1
FT-IR
The Main FT-IR absorption bands of 1 and its corresponding Schiff base ligands are
shown in Fig. 1, and the numbers are given in Table 1. In the spectra of the Schiff
-
1
base ligand, a strong band occurring at 1633 (s) cm due to C=N stretching is
found to be shifted to lower frequency in 1, viz., 1630, indicating coordination
through azomethine nitrogens. The absorption bands of covalent nitrate with C2v
symmetry in the complex are observed at five different frequencies. The difference
-
1
between m1 and m4 is 202 cm , which is associated with bidentate nitrate
coordination [19].
-
Fig. 1 FT-IR spectra of a free salpr Schiff base, b 1 in the range of 400–4000 cm , and c 1 in the range
1
-1
of 300–400 cm
1
23

Investigating the effect of immobilization of yttrium…
Table 1 Main FT-IR absorption bands of 1 and salpr (cm
-1
)
Compound C=N
3
Phenolic Y–O Y–O–Y –NH– –OH NO (coordinated)
C–O
Free ligand 1633
1278
1282
–
–
–
3417 m1
m4
m2
m3
m5
(salpr)
–
–
–
–
–
1
1630
463
303
333
3271
3429 1545 1343 1036 860 791
1308
3
70
-
Two phenolic C–O stretching vibrations at 1282 and 1308 cm (Table 1) are
1
found for complex 1 that show they are not in a similar environment. The higher
frequency peak is related to phenolic C–O and yttrium interaction [20]. The lower
-
1
frequency peak at 1282 cm indicates that some of the oxygen atoms are affected
much more strongly by the Y(III), and are coordinated to two yttriums at the same
time [20, 21]. The Far-Ir spectrum exhibits vibrational modes at 303, 333, and
-
1
3
70 cm characteristic of Y–O–Y stretching [22].
-
1
A weak band observed at around 463 cm due to Y–O provides an additional
evidence for the coordination of oxygen to Y in this complex. Besides, the band
-
1
observed at 3271 cm is due to the presence of secondary amine (–NH–) in the
complex. This peak does not exist in the free ligand, probably due to the hydrogen
1
bonding. A broad band at 3429 cm in 1 is attributed to OH group of the crystal
-
water molecules. This indicates existence of water of hydration [23].
NMR
1
The H-NMR spectrum of the prepared complex 1 exhibits the following signals in
DMSO. d = 8.19 (s, 4H, CH=N), 6.44–7.21 ppm (m, 16 H, Ar–H), and
2
.4–3.8 ppm (complex multiplet, about 16H, CH ). Compared to free salpr ligand,
2
the complex depicts the disappearing of the phenolic signal and an upfield shift of
azomethine (from 8.50 ppm [14] to 8.19 ppm), which indicate the coordination of
phenolic oxygen and azomethine nitrogen to Y atom. Other signals, i.e., those due to
aromatic and CH protons, appear at nearly the same position as the free ligand.
2
Table 2 UV–Vis data of 1 and
salpr
kmax (nm)
Assignment
*
Salpr
264
12
271
54
p ? p
*
3
n ? p
*
1
p ? p
*
3
n ? p
123

N. Tayyebi Sabet Khomami, F. Heshmatpour
1
1
3
The C-NMR spectrum of 1 is in agreement with H-NMR. It depicts the
following signals in DMSO. d = 166.6 ppm (HC=N), 112.7–166.0 ppm (Aromatic
Ar–H), 60.3 ppm (–CH –N–), and 49.0 ppm (–CH –C=N–).
2
2
UV–Vis
Table 2 provides electronic spectra of the prepared complex 1, along with its
assignments. The electronic absorption spectra of the free ligand and the complex
were measured in DMF at room temperature. The free ligand exhibited two bands
between 264 and 312 nm due to p ? p* and n ? p* ligand transitions. However,
in yttrium complex, these bands were found to be shifted toward higher wavelengths
(
red shift) that support the coordination of the ligand with the metal atom [24].
Elemental analyses
Efforts to make a single crystal of this complex were unsuccessful. However, Wu
et al. synthesized and characterized a complex of yttrium(III) and bis(N-
salicylidene)-3-oxapentane-1,5-diamine (a Schiff base similar to salpr) [18].
Single-crystal X-ray structure determination of this complex revealed that the
Y L (NO ) Á2H O formula of the complex had a centrosymmetric neutral
2
2
3 2
2
homobinuclear entity with two adjacent moieties coupled together via two
phenolato bridges belonging to the two ligands. Based upon the argument given
above and also taking into consideration that the IR spectrum proved the presence of
two kinds of phenolic C–O bond with yttrium and Y–O–Y bands, the proposed
structure for 1 is shown in Fig. 2.
For further investigation, the elemental analysis of the 1 was compared to the
proposed structure (Table 3). Interestingly, they were in good agreement, which
could prove the correctness of the proposed structure.
O
O
O
N
N
N
N
N
O
HN
O
Y
Y
NH
.
2H O
O
2
O
O
N
O
O
Fig. 2 The proposed structure for 1
1
23

Investigating the effect of immobilization of yttrium…
Table 3 Elemental analysis of 1
a
Y %
N %
C %
H %
Found
18.23
18.24
11.54
11.50
44.41
44.37
4.43
4.55
Calculated
a
Obtained by XRF
Characterization of 2
FT-IR
In order to confirm the modification of the magnetite surface, the FT-IR spectra of
the prepared SCMNPs/salpr and 2 were obtained (Fig. 3; Table 4). Two bands at
-
1
4
silica coating of magnetite nanoparticles was confirmed by observation of a band at
54–590 cm indicate the presence of Fe O in the prepared nanoparticles. The
3 4
-
1
079 cm assigned to Si–O–Si and Si–OH stretching vibrations [15].
1
-
In the FT-IR spectrum of SCMNPs/salpr a band at 1629 cm was assigned to
1
C=N stretching vibration. Upon reaction with Y(NO ) Á6H O, it shifted to lower
3
3
2
-
1
frequency (1620 cm ), indicating the complexation of C=N group of supported
ligand with yttrium. The presence of phenolic C–O in the supported free ligand is
-
confirmed by a peak at 1280 cm that shifted to 1295 cm after complextion. As
1
-1
-1
Fig. 3 FT-IR spectra of a free supported salpr Schiff base, b 2 in the range of 400–4000 cm , and c 2 in
the range of 300–400 cm
-
1
123

N. Tayyebi Sabet Khomami, F. Heshmatpour
Table 4 IR spectra of SCMNPs/salpr and its corresponding yttrium complex of 2
Compound
3
Fe O
4
SiO
2
C=N
Phenolic
C–O
Y–O
3
NO (coordinated)
SCMNPs/salpr
454
1079
1079
1629
1280
–
m1
m4
m2
m3
m5
5
3
90
50
–
–
–
–
–
a
a
a
2
454
1620
1295
1548
1350
800
5
3
90
50
a
3 4 2
Overlap with the broad peaks of Fe O @SiO
discussed, the C–O stretching vibration increases on yttrium complex formation [20,
1]. It is worth mentioning that the band indicating the interaction between the
2
oxygen of C–O and two yttriums does not exist here (around 1280 cm [20]).
-
1
-
1
Besides, the far-IR spectrum exhibits a broad vibrational mode at 350 cm
characteristic of Fe O ; however, Y–O–Y bands are not depicted [22]. Hence, it can
be inferred that the structure of 2 is different from its binuclear structure in the
3
4
-
1
homogeneous form. Appearance of three bands at 800, 1350, and 1548 cm proves
the presence of covalent nitrate with C v symmetry in the complex [19] (other bands
2
MeO
MeO Si
MeO
SiO2
SiO2
Cl
O
Fe O
Fe O
3 4
O
3
4
Si
Cl
O
Ethanol
OH N
HN
Ethanol
N
OH
SiO2
Fe O
OH
OH
N
N
O
O
O
Y(NO ) .6H O
3 3 2
SiO2
Fe O
O
O
N
O
O
3
4
Si
N
Y
Ethanol
3
4
Si
N
N
O
O
O
N
O
2
Fig. 4 The sequence of events in the preparation of 2
1
23

Investigating the effect of immobilization of yttrium…
of coordinated nitrate overlap with the bands of the support and cannot be seen
here). The proposed structure for 2 is depicted in Fig. 4.
FE-SEM, AFM, and TEM
In order to investigate the morphology and the size of the prepared nanoparticles
TEM, FE-SEM, and AFM analyses were conducted (Fig. 5). These images clearly
show that these nanoparticles have quasi-spherical morphologies, which is due to
the spherical shape of Fe O as the core of the nanoparticles with some
3
4
agglomeration; in fact, in spite of coating the particles with silica, the modified
silica coated magnetic nanoparticles still face coagulation as each particle is like a
magnet which attracts other particles. On the other hand, it has been proven that if
the specimen is magnetic, its magnetic field will deviate the electron beam of TEM
Fig. 5 a TEM, and its corresponding histogram, b FE-SEM, and c AFM images of 2
123

N. Tayyebi Sabet Khomami, F. Heshmatpour
instrument as it passes through and then the electrons that are used to form TEM
images will not be on the optic axis. All the images will be severely aberrated and
shift when the operator tries to focus on them [25].
However, to estimate the particle size, the histogram related to TEM was drawn
and compared to AFM. The results were in good agreement and showed that the
particle sizes are about 10–12 nm.
VSM
Magnetic properties of the prepared nanoparticles were investigated at room
temperature by using vibrating sample magnetometry (VSM). Magnetization curve
of SCMNPs and the prepared complex are shown in Fig. 6. It exhibited no hysteresis
loop and remanence effect, which proves its superparamagnetic properties. This
property is critical for its application, since it prevents aggregation and enables
particles to re-disperse rapidly when the magnetic field is removed. As it is shown in
Fig. 6, the saturation magnetization of SCMNPs is about 48 emu/g that is reduced to
3
6 emu/g in 2.
Elemental analysis
Yttrium content of the prepared nanomaterial was found to be 0.65 wt% on the basis
of XRF.
Catalytic activity
As Y(NO ) Á6H O has shown good catalytic activity in one pot synthesis of 3,4-
3
3
2
3
Dihydropyrimidin-2(1H)-ones, owing to the empty orbitals of Y that stabilize the
?
Fig. 6 Magnetization curves of a SCMNPs, b 2
1
23

Investigating the effect of immobilization of yttrium…
acylimine intermediate [2], we decided to evaluate 1 and 2 in this reaction. The
reaction procedure is illustrated in Fig. 7.
The obtained data are listed in Table 5. Entries 4 and 5 show the catalytic
performance of 1. These data depict the lower catalytic activity of 1 compared to
Y(NO ) Á6H O (entries 2 and 3). This reduction can be discussed from two
3
3
2
3
?
perspectives. Firstly, according to the proposed structure for 1, Y groups are
hindered by Schiff bases and are not easily available to react in the catalytic
reaction. Secondly, reducing the electron-withdrawing nitrate groups in 1 compared
to Y(NO ) Á6H O could lessen the Lewis acidic property, and lead to decreasing its
3
3
2
catalytic activity. The reaction was also conducted at 90 °C (entries 6 and 7).
Although the catalytic activity increased at higher temperature, the yields remained
relatively low.
Interestingly, the same reaction with sample 2 depicted high yields of product
(
entries 9 and 10) compared to both 1 and Y(NO ) Á6H O. This phenomenon is
3
3
2
attributed to immobilization of the yttrium complex onto silica coated magnetite
nanoparticles. The newly prepared nanoparticles have higher surface area; hence,
there is increased catalytic activity as more catalytic reactions can occur at the same
time. Moreover, it can be suggested that site (Schiff base) isolation, due to primary
immobilization of Schiff base on to SCMNPs, hinders the formation of binuclear
complexes after adding the yttrium salt, which leads to increasing the catalytic
activity of 2 in comparison with 1. Another advantage of 2 is very easy recovery and
reusability. After each run, the used catalyst was separated from the reaction
mixture by means of a permanent magnet, washed and reused in the same catalytic
reaction. The recycled catalyst after each run was characterized by IR. It showed
characteristic peaks similar to the original unused catalyst. Entry 10 depicts the
recyclability of 2 for three cycles.
In order to study the generalizability of the catalytic procedure for 2, a variety of
benzaldehyde derivatives with different electronic properties were used. The results
showed that both electron-donating and electron-withdrawing groups afforded high
yields of the desired products (Fig. 8).
X
X
O
O
Cat. (1 and 2)
O
H
EtO C
2
NH
EtO
+ 2 H O
2
+
Solvent-free
H N
2
O
N
H
O
H N
2
Fig. 7 Synthesis of 3,4-Dihydropyrimidin-2(1H)-ones using 1 and 2
123

N. Tayyebi Sabet Khomami, F. Heshmatpour
Table 5 Effect of catalysts under different reaction conditions for condensation of benzaldehyde, ethyl
acetoacetate and urea
b
Isolated yield (%)
Entry
Catalyst
Mol%
Temp. (°C)
a
1
2
3
4
5
6
7
8
9
No catalyst
–
5
2
5
2
5
2
5
5
2
70
70
70
70
70
90
90
70
90
90
28 [2]
97 [2]
77 [2]
56
Y(NO
3
)
)
3
3
Á6H
Á6H
2
O
O
Y(NO
3
2
1
1
1
1
2
2
2
44
63
48
43
95
c
1
0
88, 88, 83
Reaction condition: benzaldehyde (5 mmol), ethylacetoacetate (5 mmol), urea (5.5 mmol), in 20 min
a
60 min
b
c
All products were identified by their melting point in comparison with known samples [26]
Catalyst was recycled for three times
Fig. 8 Catalyzed synthesis of
dihydropyrimidinones (DHPMs)
under solvent free condition.
100
80
60
40
20
0
(Reaction condition:
benzaldehyde (5 mmol),
ethylacetoacetate (5 mmol),
urea (5.5 mmol), and 2 mol%
catalyst, in 20 min, 90 °C)
Benzaldehyde derivatives
Conclusions
In this study, a novel binuclear complex of yttrium(III) was synthesized. In order to
enhance the catalytic activity of this complex in one-pot synthesis of dihydropy-
rimidinones under solvent-free conditions, it was immobilized onto SCMNPs. The
newly prepared nanoparticles not only showed high activity, but also were
recyclable and could be separated fast and easily from the reaction mixture.
Acknowledgments The authors acknowledge K. N. Toosi University of Technology for supporting this
study.
1
23

Investigating the effect of immobilization of yttrium…
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