Decoration of multi-walled carbon nanotubes with NiO nanoparticles and investigation on their catalytic activity to synthesize pyrimidinone heterocycles

Article abstract of DOI:10.1007/s13738-014-0468-9

A supported carbon material is shown to be a highly efficient, eco-friendly and recyclable solid acid catalyst for the Biginelli reaction of aldehyde, β-ketoester and urea or thiourea under microwave irradiation in the absence of solvent. This method offe

Full text of DOI:10.1007/s13738-014-0468-9

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0468-9
ORIGINAL PAPER
Decoration of multi-walled carbon nanotubes with NiO
nanoparticles and investigation on their catalytic activity
to synthesize pyrimidinone heterocycles
•
Javad Safari Soheila Gandomi-Ravandi
Received: 8 December 2013 / Accepted: 8 May 2014
Ó Iranian Chemical Society 2014
Abstract A supported carbon material is shown to be a
highly efficient, eco-friendly and recyclable solid acid
catalyst for the Biginelli reaction of aldehyde, b-ketoester
and urea or thiourea under microwave irradiation in the
absence of solvent. This method offers significant advan-
tages such as efficiency, the excellent yield, avoidance of
the organic solvents, mild reaction conditions, easy sepa-
ration and simple operation. In addition, because of
employing microwave as heating source and reducing use
of organic solvents, this novel method emerges as a green-
approach leading to less harmful residues. Furthermore, a
mechanism was proposed to rationalize the reaction and the
role of NiO–MWCNTs was also investigated in these
transformations.
pharmacological activities such as calcium channel blocker
[2], neuropeptide Y (NPY) antagonists [3], selective a1a
receptor antagonists [4, 5] and antihypertensive [6, 7].
Their applications have been stimulated in the field of drug
research to develop a wide range of synthetic methods and
chemical transformations. Since the first report on the
synthesis of DHPMs by Pietro Biginelli in 1893 [8], the
most chemists have modified the original Biginelli reaction
by changing substrates and also reaction conditions [9–11].
Various catalysts have recently been suggested to improve
the reaction by different researchers. Some of these cata-
lysts included Zn(1–x)NixAl2O4 spinels [12], chloroindate
ionic liquid (BMI.InCl4) [13], Iron(III) tosylate [14], Cu/
SiO2 [15], nanocomposite Fe0.2Al0.8Zn1O4 [16], 1,3,5-tri-
azine-2,4,6-triyltrisulfamic acid [17], choline-based ionic
liquids [18], melamine sulfonic acid/ZnO nanotube [19]
and poly(4-vinylpyridine)-supported copper iodide nano-
particles [20]. The discovery of a new and efficient catalyst
has key importance to prepare 3,4-dihydropyrimidine-2-
(1H)-ones under mild conditions with high yield.
Keywords NiO–MWCNTs Á 3,4-Dihydropyrimidin-
2(1H)-one Á Microwave irradiation Á Heterogeneous
catalyst Á Multi-walled carbon nanotubes
Introduction
It is important to develop heterogeneous catalysts in
organic synthesis, because of the potential advantages of
these materials consist of ease of handling, lack of corro-
sion, recovery and regeneration [21]. Metal oxide nano-
particles have recently been used as heterogeneous catalyst
because of their interesting structure, high catalytic activ-
ities and improved selectivity [22–26]. More interest has
been focused on cheap transition metal oxides such as
nickel oxide (NiO), owing to its p-type semi-conductivity,
low toxicity and non-flammable properties which make it
safe to handle [27–29]. However, the specific surface area
of the pure NiO is not high enough for the practical
applications. Carbon nanotubes (CNTs) are cylindrical,
regular and symmetric nanostructures made of carbon
atoms rolled up with graphene [30]. Owing to inactivity of
Pyrimidinones or dihydropyrimidinones (DHPMs) are well
known for wide range of their bioactivities due to their
therapeutic and pharmaceutical properties. These cores are
regarded as one of the most important groups of drug-like
scaffolds [1]. They possess
a broad spectrum of
J. Safari (&) Á S. Gandomi-Ravandi
Laboratory of Organic Compound Research, Department of
Organic Chemistry, College of Chemistry, University of Kashan,
P.O. Box 87317-51167 Kashan, Islamic Republic of Iran
e-mail: safari@kashanu.ac.ir
S. Gandomi-Ravandi
e-mail: gandomi_so@yahoo.com
123

J IRAN CHEM SOC
the walls of CNTs, the functionalization processes must be
carried out before adsorption experiments to develop the
adsorption capacities of CNTs [31]. Also, the useful
applications of nanocomposites of metal oxides as catalyst
have been improved in organic synthesis owing to their
high catalytic activity and the large surface area. Combi-
nation of NiO and CNTs with excellent electron transfer
rate and large specific surface area is expected to provide a
chance for improvement of the durability and performance
of pure NiO particles [32]. Consequently, carbon nanotubes
decorated with NiO nanoparticles can influence their
physical, thermal, mechanical and electrical properties and
may give novel properties and promising applications with
the combination of features of nanoparticles and carbon
nanotubes [33]. However, there is no example of using
NiO–MWCNTs as a catalytic system to synthesize DHPMs
under microwave irradiation. Therefore, for the first time,
we would like to report a simple approach to prepare 3,4-
dihydropyrimidin-2(1H)-ones/thiones (Biginelli reaction)
via microwave-activated cyclocondensation of aromatic
aldehyde, b-dicarbonyl compound and urea or thiourea in
the presence of NiO–MWCNTs as catalyst (Scheme 1).
Herein, we also present a sonochemical approach for the
fabrication and uniform dispersion of NiO nanoparticles on
the surface of multi-walled carbon nanotubes. Ultrasonic
irradiation prevents the agglomeration of the nanoparticles
and improves the dispersion ability of the coatings’
formulation.
Perkin Elmer FT-IR 550 spectrophotometer using KBr
pellets. NMR spectra were recorded on a Bruker DRX-400
spectrometer and chemical shifts are expressed in d ppm
downfield from TMS as an internal standard. To charac-
terize NiO–MWCNTs nanocomposites, fourier transform
infrared spectroscopy (FT-IR) was employed to analyze the
functional groups on the purified MWCNTs. X-ray dif-
fraction (XRD) pattern of the as-synthesized material was
obtained using a Holland Philips X-ray diffraction (XRD)
diffractometer model X’PertPro (CuK, radiation, k =
0.154056 nm), at a scanning speed of 28/min from 108 to
1008 (2h). The nanocatalysts were determined also using a
TESCAN model VEGA II scanning electron microscope
(SEM) operated at a 15-kV accelerating voltage. Recrys-
tallization was carried out in suitable solvent. The Raman
spectra were recorded on a Bruker SENTERRA spectrom-
eter with Laser wavenumber 785 nm and spectral range
200–3,500 cm-1. All products were identified through their
spectroscopic data and the melting points were confirmed
by comparison with those reported in the literature.
Preparation of NiO-supported on the acid-treated
MWCNTs
Oxidized carbon nanotubes were prepared by purification
in a ratio of 3:1 70 % nitric acid and 98 % sulfuric acid
using sonication for 6 h. The purified samples were washed
and neutralized by distilled water and were dried at 100 °C
for 2 h. Composite material of NiO–MWCNTs was pre-
pared by thermal decomposition of Ni(OH)2/MWCNTs
[34]. In short, 0.06 M NiCl2Á6H2O was mixed with 0.1 g
acid-treated MWCNTs. Then, ammonium hydroxide was
dropped into this mixed suspension at pH 8 and was son-
icated for 20 min. After that, the suspension was soaked in
the solution for 5 h and was separated by centrifugation to
achieve intermediate product (Ni(OH)2/MWCNTs).
Indeed, Ni2? ions were enucleated by chelation with car-
boxyl functional groups on the surface of MWCNTs due to
the electrostatic attraction. Then, OH- reacts in situ with
the Ni2? and Ni(OH)2 precipitates were formed on the
surface of MWCNTs through electrovalent bonding. Next,
the composite material was calcined at 400 °C for 2 h and
NiO–MWCNT nanocomposites are achieved after these
heat treatments (Fig. 1). Finally, the resulting products
were washed with deionized water several times and col-
lected by centrifugation. Now the material is ready to be
used as a supported catalyst.
Experimental
All chemicals were of research grade obtained from Sigma-
Aldrich and were used without any further purification.
MWCNTs were purchased from Nanotech Port Co. (Tai-
wan). These MWCNTs were produced via the chemical
vapor deposition (CVD, or sometimes called catalytic
pyrolysis) method. The outer diameter of MWCNTs was
between 10 and 20 nm. Melting points were determined on
an Electrothermal MK3 apparatus using open capillaries
and are uncorrected. FT-IR spectra were recorded on a
R
R
O
O
NiO-CNTs
Microwave
CHO
NH2
OMe
HN
OMe
2
X
NH2
O
X
N
H
Preparation of the Biginelli compounds
3
1
4
X = O, S
A mixture of (thio) urea (1 mmol), aldehyde (1 mmol) and
methyl acetoacetate (1.5 mmol) was irradiated in the pre-
sence of NiO–MWCNTs (0.04 g) into microwave oven at
Scheme 1 Microwave-mediated multi-component synthesis of dihy-
dropyrimidinones (thiones)
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J IRAN CHEM SOC
Fig. 1 Schematic illustration of
decoration of the MWCNTs
with nickel oxide nanoparticles
milder reaction conditions, easier workup and green
chemistry. The reaction conditions were optimized with the
model substrates of benzaldehyde, methyl acetoacetate and
urea in the presence of NiO–MWCNTs nanocomposites as
heterogenous catalyst under microwave irradiation. At first,
we examined the reaction parameters such as amount and
type of catalyst, reactant ratios and also reusability of
catalyst. Initially, the various catalysts were studied for
their activities in the model reaction. As shown in Table 1,
NiO–MWCNTs were found to be highly active in term of
reaction yield under MW irradiation.
Table 1 Comparison of the results of the synthesis of 4a using dif-
ferent catalysts under microwave irradiation
Entry
Catalyst
Time (min)
Yield (%)
1
2
3
4
5
6
MWCNTs
MWCNTs–COOH
NiO NPs
10
10
10
10
10
10
20
35
70
60
49
98
NiO
NiCl2
NiO–MWCNTs
Next, we performed the model reaction with different
amounts of NiO–MWCNTs nanocomposite to evaluate the
effect of the catalyst. It was found that the highest yield
was obtained using 0.04 g catalyst (Table 2). There is no
significant improvement in the yield by increasing in the
amount of catalyst (Table 2, Entry 4).
Table 2 Optimization of reaction conditions of 4a using various
concentrations of catalyst
Entry
Catalyst (g)
Time (min)
Yield (%)
1
2
3
4
5
0.01
0.02
0.03
0.04
0.05
35
30
20
10
15
80
85
90
98
98
Moreover, the recyclability of the supported catalyst is
also important. The resulting mixture was washed using hot
ethanol and the catalyst was recovered by filtrating of the
reaction mixture and was reused after dilution with ethyl
acetate for subsequent experiments under similar reaction
conditions. It was found from Fig. 2 that there was no
significant decrease in activity after four runs using the
NiO–MWCNTs.
300 W for the appropriate time. After the reaction was
completed, hot ethanol was added to mixture and the cat-
alyst was removed by filtration and the filtrate was poured
onto crushed ice. The resulting solid was collected by fil-
tration and was recrystallized from ethanol to afford the
pure product in high yield.
To investigate the efficiency of microwave irradiation,
the model reaction was also examined in conventional
conditions. As shown in Table 3, it was apparent that the
reaction proceeded better under microwave irradiation.
The same reaction was investigated with different molar
ratios of reactants to determine the appropriate molar ratio
and the highest yield of the desired product 4a was
obtained with 1:1:1.5 ratios of benzaldehyde, methyl ace-
toacetate and urea in the presence of 0.04 g NiO–
MWCNTs under microwave irradiation. Afterward, a wide
range of aromatic aldehydes carrying either electron-
Results and discussion
Nickel oxide supported on multi-walled carbon nanotubes
afforded a valuable catalytic system to promote the syn-
thesis of DHPMs. In addition, the microwave technique
represented a better procedure owing to the higher yield,
123

J IRAN CHEM SOC
donating or -withdrawing substituents was investigated
under the optimized reaction conditions to study the gen-
erality of this process. Most importantly, aromatic alde-
hydes carrying either substituents reacted very well to give
the corresponding dihydropyrimidinones in excellent yields
with high purity. The results are summarized in Table 4.
Also, thiourea has been used with similar approach to
produce the corresponding thio-derivatives of DHPMs.
Figure 3 presents the typical Scanning Electron
microscopy (SEM) images of NiO–MWCNTs nanocom-
posites. These micrographs show the presence of metal
oxide nanoparticles. It can be clearly observed that the NiO
nanoparticles were dispersed with the homogeneous dis-
tribution on the outside wall of the MWCNTs.
Raman spectroscopy is a powerful and sensitive tech-
nique to investigate the structural properties of carbona-
ceous materials and the extent of disorder in the
functionalized CNTs and CNTs based composite materials
[40, 41]. Figure 4 compares the Raman spectra of acid-
treated MWCNTs and NiO–MWCNTs between 200 and
4,000 cm-1. Two broad bands were located at 1,596 (G
bands) and 1,307 (D bands) cm-1 region corresponding to
the acid-treated MWCNTs, which show the C–C bond
vibrations of carbon atoms with sp2 electronic configura-
tion in graphene sheet structure and the disordered and
imperfect structures of carbon materials, respectively [42].
Attaching NiO nanoparticles led to the red shift of D-band
(1,274 cm-1) and G-band (1,555 cm-1) peak position rel-
ative to oxidized multi-walled carbon nanotubes (Fig. 4b).
In addition, it appears a peak at 515 cm-1 confirmed to the
stretching modes of NiO [43]. The stretching modes of NiO
have also been detected at 200–800 cm-1 region [44, 45].
The intensity ratio of the ID/IG corresponding to integrated
intensity ratios was found to be 1.86 for the oxidized
MWCNTs samples; however, it was about 1.47 for the
NiO–MWCNTs samples.
Fig. 2 Reusability of catalyst in the synthesis of 4a under microwave
irradiation
Table 3 Synthesis of compound 4a under heating and microwave
conditions
Entry
Conditions
Time (min)
Yield (%)
1
2
3
4
5
6
7
r.t stirring
110
90
70
50
35
30
10
5
20
30
45
60
65
98
Thermal 40 °C
Thermal 60 °C
Thermal 80 °C
Thermal 100 °C
Thermal 120 °C
Microwave
The XRD patterns of MWCNTs–COOH, nanocrystal-
line NiO and NiO–MWCNTs composites are presented in
Fig. 5. The diffraction peaks at 37.428, 43.468, 63.018,
75.448 and 79.408 correspond to (111), (200), (220), (311)
and (222) reflections, respectively, which they are
Catalyst (0.04 g)
Table 4 Catalytic synthesis of
dihydropyrimidinones (thiones)
catalyzed by NiO–MWCNTs
under microwave conditions
Entry
R
X
Product
Mp (°C)
Time (min)
Yield (%)
Obs.
Lit.
1
H
O
O
O
O
O
O
4a
4b
4c
4d
4e
4f
208–210
204–205
235–237
254–255
206–208
240–241
225–226
233–235
245–247
178–180
152–154
151–153
209–211 [35]
204–206 [36]
236–238 [35]
252–253 [36]
207–208 [37]
240–242 [38]
225 [39]
10
15
15
20
15
10
25
15
20
20
15
20
98
92
95
91
90
88
85
93
87
92
93
89
2
4-Cl
3
4-NO2
2,4-Cl2
3-OMe
2-Br
4
5
6
7
3-OMe-4-OH
H
O
S
S
S
S
S
4g
4h
4i
8
232–234 [38]
245–246 [38]
179–180 [38]
153–154 [38]
152–155 [36]
9
4-OH
10
11
12
4-OMe
4-Br
4j
4k
4l
4-NMe2
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J IRAN CHEM SOC
Fig. 3 SEM images of NiO–
MWCNTs
Fig. 5 XRD patterns of oxidized MWCNTs, nanocrystalline NiO and
NiO–MWCNTs
Fig. 4 Raman spectra of a acid-treated MWCNTs and b NiO–
MWCNTs composite
correlated with NiO in cubic phase. This confirms the
crystalline nature and phase purity of NiO nanoparticles.
Also, a peak is clearly observed near 26.328 [(002) plane],
which is typical for multi-walled CNTs [46]. The XRD
result clearly indicates that NiO nanoparticles were deco-
rated on the surface of MWCNTs. The average crystal size
of the NiO–MWCNTs was calculated by the Debye–
Scherrer formula to be 18.93 nm.
frequencies. The C=O stretching frequency is owing to the
presence of carbonyl and carboxylic groups produced during
the oxidation of pristine MWCNTs. Also, C=C stretching
frequency is owing to the inherent structure of MWCNTs.
The significant peaks at 428, 567 and 1,400 cm-1 are
assigned to the Ni–O bond stretching vibration mode in FT-
IR spectrum of NiO–MWCNTs (Fig. 5b). Also, Fig. 5c
shows clearly the recovered nanocomposites.
FT-IR spectra of acid-treated MWCNTs, NiO–MWCNTs
nanocomposites and recovered NiO–MWCNTs are pre-
sented in Fig. 6. In case of oxidized MWCNTs, the pre-
sentative peaks observed around 3,438, 1,726 and
1,627 cm-1 are assigned to O–H, C=O and C=C stretching
EDX spectrum is also recorded to determine the chemical
composition of NiO–MWCNTs (Fig. 7). The results indicate
the presence of carbon, nickel and oxygen elements in the
nanocomposites. Results of element content in EDS patterns
reveal that the content of Ni is 24.63 W %.
123

J IRAN CHEM SOC
Fig. 8 The ATR FT-IR spectra of oxidized MWCNTs and NiO–
MWCNTs
Fig. 6 FT-IR spectra of a acid-treated MWCNTs, b NiO–MWCNTs
catalyst and c recovered NiO–MWCNTs composites
A reasonable mechanistic pathway of Biginelli con-
densation via N-acylimine intermediate I is shown in
Scheme 2. This intermediate is formed from the reaction
of the aldehyde and urea and then is stabilized by cata-
lyst through coordinate bonds owing to metal empty
orbital. Meanwhile, the metal support acts as a Lewis
acid and increases the electrophilic character of the
carbonyl groups. Then, methyl acetoacetate enolate is
stabilized by complexing the two oxygen atoms with
metal ion. The iminium ion intermediate I interacts with
methyl acetoacetate enolate to produce an open chain
intermediate ureide II. At last, dihydropyrimidinones are
afforded during the cyclization and dehydration of
intermediate II.
10000
9000
α
NiL
8000
7000
6000
5000
4000
3000
2000
1000
0
α
C K
AuM
β
O K
α
AuM
α
NiK
α
ClK
β
α
ClK
AuLl
NiK
β
AuL
α
keV
0
5
10
Fig. 7 The EDX spectrum of NiO–MWCNTs
Conclusions
In summary, this investigation deals with a novel and
practical approach to synthesize 3,4-dihydropyrimidin-
2(1H)-ones, which was achieved by applying the Biginelli
cyclocondensation reaction in the presence of nickel
oxide supported on the multi-walled carbon nanotubes as
a heterogenous catalyst. The catalyst is highly efficient,
selective, easy to handle and it is easily removed from the
reaction mixture. This recyclable heterogeneous catalyst
provides a simple strategy to prepare a variety of het-
erocycles under green conditions. Our interest in envi-
ronmentally friendly organic synthesis prompted us to
investigate the utility of NiO–MWCNTs as a catalyst for
the Biginelli reaction under microwave irradiation. A
collection of DHPMs has been generated by a parallel
The ATR FT-IR spectra of the MWCNT–COOH and
NiO–MWCNTs are shown in Fig. 8. A sharp peak was
observed for Ni–O stretching vibration mode at 667 cm-1
.
Also, it is shown that characteristic frequencies correspond to
the signature of bands of graphitic carbon materials and the
functional groups containing oxygen in carbon frameworks.
Therefore, we have introduced NiO–MWCNTs nano-
composites as a novel and highly effective heterogenous
catalyst for the condensation of aldehyde, urea (thiourea)
and 1,3-dicarbonyl compound under microwave condi-
tions in excellent yields. All the products were charac-
1
terized by FT-IR and H NMR analysis and their melting
points are like the known compounds reported in the
literature.
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J IRAN CHEM SOC
NiO-CNT
NH2
Scheme 2 Plausible
X
X
mechanism for the Biginelli
reaction in the presence of NiO–
MWCNTs nanocomposites
NiO-CNT
TNC-ONi
X
O
HN
NH2
N
+
R1
H
-H2O
R1
O
R1
H
O
H2N
NH2
I
NiO-CNT
O-CNT
Ni
O
OMe
O
R1
O
R1
O
R1
MeO
NH
MeO
NH
MeO
NH
-H2O
HO
N
H
X
X
O
N
H
X
H2N
NiO-CNT
TNC-ONi
II
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Acknowledgments We gratefully acknowledge financial support
for this work by University of Kashan (Grant No. 256722/33).
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