Access to enhanced catalytic core-shell CuO-Pd nanoparticles for the organic transformations

Article abstract of DOI:10.1039/c6ra03883b

This paper describes the biosynthesis of core-shell CuO-Pd nanoparticles with the aid of a Cyperus rotundus rhizome extract. The crystallinity, surface chemistry, morphology and thermal properties of the synthesized nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and thermogravimetric analysis. The nanoparticles with Pd covered by a CuO enriched shell showed enhanced catalytic performance for organic transformations to synthesize 2,3-dihydroquinazolin-4(1H)-ones, spirooxindoles, and chromenylpyrimidin-2,4-diones in excellent yield. Moreover, the nanocatalyst can be recycled effectively for five consecutive reactions without significant loss of catalytic activity.

Full text of DOI:10.1039/c6ra03883b

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Access to enhanced catalytic core–shell CuO–Pd
nanoparticles for the organic transformations†
Cite this: RSC Adv., 2016, 6, 27974
*
Kanchan Mishra, Nagaraj Basavegowda and Yong Rok Lee
This paper describes the biosynthesis of core–shell CuO–Pd nanoparticles with the aid of a Cyperus
rotundus rhizome extract. The crystallinity, surface chemistry, morphology and thermal properties of the
synthesized nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy,
transmission electron microscopy, and thermogravimetric analysis. The nanoparticles with Pd covered by
a CuO enriched shell showed enhanced catalytic performance for organic transformations to synthesize
2,3-dihydroquinazolin-4(1H)-ones, spirooxindoles, and chromenylpyrimidin-2,4-diones in excellent yield.
Moreover, the nanocatalyst can be recycled effectively for five consecutive reactions without significant
loss of catalytic activity.
Received 12th February 2016
Accepted 9th March 2016
DOI: 10.1039/c6ra03883b
www.rsc.org/advances
the catalysts can be recovered by simple ltration and reused for
subsequent catalytic cycles without any loss of catalytic activity,
Introduction
even with the reduction of unwanted waste.
Transition metal bimetallic nanoparticles show higher catalytic
activity than their monometallic counterparts in many organic
reactions.1–3 Owing to the versatile catalytic assistance of palla-
dium, metals associated with Pd nanoparticles such as FePd,4
AuPd,5,6 PtPd,7,8 AgPd,9,10 and CuPd,11,12 have been used as
heterogeneous catalysts with excellent selectivity and activity.13–17
Many reports have emphasized the shape and size of these
nanoparticles to further tune the catalytic applications. Normally,
there are three different patterns of bimetallic nanoparticles;
heterodimer,18 core–shell19,20 and nano-alloys.21,22 The core–shell
pattern of nanoparticles, constitutes a single metal core covered
by another metal or metal oxide.23 This pattern of nanoparticles
results in superior catalytic activity to those of alloyed or
heterodimers.24–27
The surface chemistry of the palladium bimetallic catalyst is
fundamental for evaluating its catalytic activity. Despite the
continual improvement in the synthesis of bimetallic NPs,28–30
monodisperse bimetallic NPs with a controlled composition
and relevant sizes are still not available for catalytic studies.
Moreover, the recovery and recyclability of the catalyst puts
forward a crucial issue for the sustainable development of any
catalytic activity. The catalyst recyclability is a critical problem
that needs to be improved and searched for more efficient and
economically viable processes. This study focused on the
development of highly efficient and active heterogeneous cata-
lysts. Therefore, the development of heterogeneous catalysts
has shown promising results for sustainable purposes, in which
The biosynthesis of nanoparticles using plant extracts is
green, simple, cost effective, and eco-friendly. In the present
study, Cyperus rotundus L (Cyperaceae) was used, which is
a perennial plant distributed widely in tropical and subtropical
areas.31 The rhizome of C. rotundus has been used as a medicine
for the treatment of indigestion, diarrhea, fever, anorexia, CNS
disorders, and urinary retention. Phytochemical studies on C.
rotundus have revealed the presence of a variety of avonoids,
phenolic acids, cyperol, sitosterol, nootkatone and
valencene.32–35
To check the catalytic efficiency of the synthesized nano-
particles, we utilize a facile, and environmentally benign
protocol for the formation of 2,3-dihydroquinazolin-4(1H)-ones,
spirooxindoles, and chromenylpyrimidin-2,4-diones. 2,3-Dihy-
droquinazolinone derivatives are important fused heterocycles
that exhibit a wide range of pharmacological, biological and
medicinal properties.36–39
Spirooxindole derivatives are also an important class of
naturally occurring substances with highly pronounced biolog-
ical activities and properties.40–43 The unique structural array and
highly prominent pharmacological activities have subsequently
stimulated interest in the synthesis of spirooxindole derivatives.
Chromene derivatives are important class of benzopyrans
found in bioactive natural products.44 They exhibit various
biological and pharmacological properties such as anticancer,
anticoagulant, antianaphylactic, and diuretic activities.45
Pyrimidine derivatives are found in a wide range of bioactive
materials46 and they show potent antibacterial, antiviral, and
antitumor activities.47 Chromenylpyrimidine derivatives, which
combine two bioactive heterocyclic cores, are expected to be
widely used as pharmacological agents. Therefore, the
School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic
of Korea. E-mail: yrlee@yu.ac.kr; Fax: +82-53-810-4631; Tel: +82-53-810-2529
† Electronic supplementary information (ESI) available: Characterization: XPS,
EDS, HRTEM, FTIR, UV, zeta potential, AFM, XRD, and NMR. See DOI:
10.1039/c6ra03883b
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development of new and simple synthetic methods for the anhydride, aldehydes, Isatins, 2-hydroxychalcone, 6-amino-1,3-
preparation of such derivatives bearing the biological activities dimethylpyrimidine-2,4(1H,3H)-dione, and amines were ob-
has become an interesting challenge.
tained from Aldrich chemicals. All chemicals were used as
Although a few green methods for the synthesis of Pd/CuO received. The air dried rhizome of Cyperus rotundus was ob-
nanoparticles have been described,48,49 the synthesis of core–shell tained at a local market in Yeongchon, South Korea. The
CuO–Pd nanoparticles using plant extracts has not been devel- Cyperus rotundus rhizome extract was prepared using the
oped. While several synthetic methods have been demonstrated previous method with slight modications.51 Powdered rhizome
for the preparation of 2,3-dihydroquinazolinones and spiroox- (15 g) was prewetted with a NH4OH solution and 100 mL of
indole derivatives by Lewis acid- and Brønsted acid-catalyzed deionized Milli-Q water was added, and followed by boiling for
reactions,50 to the best of our knowledge, there are no examples 10 min and ltration.
of the use of recyclable core–shell CuO–Pd nanocatalysts. To
further extend the applicability of the synthesized nanoparticles, Synthesis of CuO–Pd nanoparticles
we have performed novel reactions for the synthesis of biologically
interesting compounds. Importantly, there are no reports on the
A C. rotundus rhizome extract solution (50 mL) was added to
100 mL of a mixed aqueous solution of 1.0 mmol (181.6 mg)
synthesis of biologically interesting chromenylpyrimidin-
Cu(OAc)2 and 1.0 mmol (177.3 mg) PdCl2. The mixture solu-
2,4-diones. This paper reports a facile and green approach for
tion was then sonicated at 60 ꢀC for 2 h. Aer 2 h, the color of
the synthesis of core–shell CuO–Pd bimetallic nanoparticles using
the nal solution had changed to dark brown, which indicated
Cyperus rotundus rhizome extract as both a reductant and capping
the formation of CuO–Pd nanoparticles. The synthesized
agent. The synthesized nanoparticles were used as recyclable and
nanoparticles were isolated by centrifugation (10 000 rpm, 4 ꢀC
reusable catalysts for the synthesis of 2,3-dihydroquinazolin-4(1H)-
and 15 min), washed with water, and dried in an oven at 50 ꢀC
for 3 h. The yield of synthesized CuO–Pd NPs was 151.2 mg
(81%).
ones, spirooxindoles, and chromenylpyrimidin-2,4-diones without
signicant loss of catalytic activity, as shown in Scheme 1.
UV-visible analysis of total avonoids and phenolic acid
present in C. rotundus
Experimental section
Materials and methods
The total avonoids available in the C. rotundus rhizome extract
were determined using an aluminum chloride calorimetric
method with minor changes.52 A 2.0 mL aliquot of the rhizome
Copper(II) acetate (Cu(OAc)2, 98%) and palladium(II) chloride
(PdCl2, 99%) were obtained from Sigma-Aldrich. Isatoic
extract was added to 0.50 mL of a 10% aluminum chloride (w/v)
solution along with 0.10 mL of a 1 M potassium acetate solution
and 3.0 mL of distilled water. The resulting mixture was kept at
room temperature for 30 min. The absorbance of the solution
was observed at 415 nm, against a reagent blank, using
a UV-visible spectrophotometer. The result is reported as grams
of quercetin equivalent (QE)/(100 g dried weight of rhizome).
The total phenolic acid present in the C. rotundus rhizome
extract was determined by Folin–Ciocalteu's calorimetric
method.53 1.5 mL of diluted Folin–Ciocalteu's reagent (2/20) was
added to 1.0 mL of rhizome extract, and allowed to stand for
5 min at room temperature. Subsequently, 1.5 mL of 7% sodium
carbonate (w/v) was added to the mixture to neutralize it and
kept in the dark at room temperature for 90 min. The absor-
bance was taken at 765 nm against a reagent blank. The result
was assigned in grams of gallic acid equivalent (GAE)/(100 g
dried weight of rhizome).
Characterization of CuO–Pd nanoparticles
The surface chemistry of the nanoparticles and the chemical
state of each element were analyzed by X-ray photoelectron
spectroscopy (XPS) using a Thermo scientic, K-Alpha system
tted with an Al Ka X-ray light source. The ion source energy for
the survey was 100 V to 3 KeV. Powder X-ray diffraction (XRD,
PANalytical X'PertPRO MPD, operating at 40 kV and 30 mA) was
performed to determine the crystalline nature of the nano-
ꢀ
ꢀ
particles using Cu Ka (l ¼ 1.5406 A) over the 2q range, 30 to
Scheme 1 Synthesis and application of CuO–Pd NPs.
85ꢀ, and a scanning rate of 1.2ꢀ minꢁ1. The morphology, shape
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and size of the nanoparticles were analyzed by eld emission
transmission electron microscopy (FE-TEM, FEI Tecnai F20).
The samples were prepared on carbon-coated nickel grids by
placing a drop of the nanoparticle solutions on the grid and
allowing the solvent to evaporate in air at ambient tempera-
ture. The elemental make-up of the bimetallic nanoparticles
was analyzed by high-angle annular dark-eld scanning TEM
energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS) at
200 kV with a point resolution of 0.24 nm, a Cs and Cc of
1.2 mm each and a focal length of 1.7 mm, using Genesis
liquid nitrogen cooled EDS detector with an ultrathin
window. The Fourier transform infrared (FT-IR, JASCO FTIR)
spectra were recorded in transmittance mode over the range,
400–4000 cmꢁ1. The samples were prepared by adding ꢂ2 mg
of dry powder with 200 mg of KBr to produce pellet for both
the extract and nanoparticles. The formation of nano-
particles was monitored using an Optizen 3220 (Double
beam) UV-vis spectrophotometer with a quartz cuvette and
distilled water as the reference. The thermal properties of the
nanoparticles were analyzed by thermogravimetric analysis
coupled with differential scanning calorimetry (DSC-TGA,
SDT-Q600 V20.5 Build 15) and heated under a N2 atmo-
sphere to 700 ꢀC at a heating rate of 10 ꢀC minꢁ1. Atomic force
microscopy (AFM, NanoscopeIIIa, Digital Instruments, Inc.,
USA) was operated in tapping mode with standard silicon
nitride tips. Zeta-potential measurements were performed
using a dynamic light scattering apparatus (DLS, Malvern
Zetasizer, UK). The sample was equilibrated for 2 min at 25 ꢀC
and the measurements were repeated three times for
consistent results.
General procedure for the synthesis of chromenylpyrimidin-
2,4-diones (9a–9e)
CuO–Pd NPs (3.72 mg, 2 mol%) were added to a solution of
2-hydroxychalcones (1.0 mmol), and 6-amino-1,3-dimethylpyr-
imidine-2,4(1H,3H)-dione (1.0 mmol) in toluene (5 mL) at room
temperature. The reaction mixture was reuxed for 1 h. Aer the
reaction was completed, the solvent was removed under
reduced pressure evaporator. The residue was puried by
column chromatography on silica gel to afford nal product.
Recyclability study
A recycling experiment was performed to evaluate the catalytic
activity and stability of the CuO–Pd NPs synthesized using the
green approach. The reaction was carried out as mentioned
above using the general procedure. Aer the reaction, the
catalyst was recovered by simple ltration, washed with hot
ꢀ
water and dried in a vacuum oven at 80 C for 3 h prior to the
next recycle.
Results and discussion
Characterization of CuO–Pd NPs
The corresponding powder XRD pattern of CuO–Pd NPs was
well resolved, as shown in Fig. 1a. The relative intensities and
positions of the XRD peaks were in accordance with those
indexed to the CuO (JCPDS le no. 48-1548) and to the Pd
(JCPDS le no. 46-1043). The XRD peaks at 39.92, 46.43, 67.77,
and 81.65ꢀ 2q were indexed to the (111), (200), (220), and (311)
planes of crystalline palladium, respectively.54 The reection
ꢁ
ꢁ
ꢁ
planes (002), (111), (202), (202), (113), and (311) were observed
at 35.47, 38.68, 48.76, 56.23, 61.60, and 66.31ꢀ 2q, respectively,
corresponded to copper oxide.55
Catalytic activity
The synthesized CuO–Pd nanoparticles were used as a heteroge-
neous catalyst for the synthesis of 2,3-dihydroquinazolin-4(1H)-
ones, spirooxindoles, and chromenylpyrimidin-2,4-diones.
The surface elemental chemistry and oxidation state of the
nanoparticles were investigated by XPS. The XPS survey scan of
CuO–Pd NPs (Fig. S1a, ESI†) revealed the presence of Cu, O, Pd,
and C without any other impurity in the nanoparticles. In the XP
General procedure for the synthesis of 2,3-dihydroquinazolin-
4(1H)-ones (4a–4e)
CuO–Pd NPs (3.72 mg, 2 mol%) were added to a solution of
isatoic anhydride (1.0 mmol), amines (1.0 mmol) and benzal-
dehydes (1.0 mmol) in water (10 mL) at room temperature. The
reaction mixture was reuxed for 30 min under a nitrogen
atmosphere. Aer completion of the reaction, the reaction
mixture was cooled to room temperature, ltered and recrys-
tallized in ethanol to produce the pure product.
General procedure for the synthesis of spirooxindole
derivatives (6a–6e)
CuO–Pd NPs (3.72 mg, 2 mol%) were added to a solution of
isatoic anhydride (1.0 mmol), amines (1.0 mmol) and isatins
(1.0 mmol) in water (10 mL) at room temperature. The reaction
mixture was reuxed for 30 min under a nitrogen atmosphere.
Aer completion of the reaction, the reaction mixture was
cooled to room temperature, ltered, and recrystallized in
ethanol to afford the pure product.
Fig. 1 (a) XRD patterns of the CuO–Pd NPs, (b) Cu 2p XPS spectra of
CuO–Pd NPs, (c) XP spectra of the O 1s electrons, and (d) Pd 3d
electrons of CuO–Pd NPs.
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spectra of the Cu 2p electrons (Fig. 1b), the binding energy a CuO shell, which might be because of more oxophilic nature
peaks at 931.26 and 950.97 eV were assigned to the Cu 2p3/2 and of copper and is attracted to the outside in presence of oxygen
Cu 2p1/2, respectively, with a spin–orbit splitting of 19.71 eV.56 atoms.60,61 The XRD peaks for palladium were not detected
The Cu2+ was attributed to CuO, hydrolyzed product of clearly because the size of the Pd core was too small, which may
Cu(OAc)2, which was also conrmed by XRD. In addition, the be due to the outer CuO shell in most of the NPs (Fig. S2, ESI†).
tted O 1s spectra (Fig. 1c) showed two bands at 525.21 and The enhanced catalytic activity of the CuO–Pd NPs may be due
528.90 eV, which was attributed to the absorption of metal to the structural characteristics of these NPs, i.e., the core shell
hydroxide and oxygen on the nanoparticles surface, and the one arrangement of catalytically active Pd in the CuO–Pd NPs.
at 530.95 eV, was assigned to the lattice oxygen in crystalline Furthermore, the HRTEM image of a small region at a 2 nm
CuO.57 The Auger spectra were obtained to further clarify the scale revealed clear lattice fringes separated by 0.250 nm and
unknown species. Similarly, the XP spectra of the Pd 3d elec- 0.226 nm, corresponding to the face-centered cubic (fcc) of CuO
trons (Fig. 1d) appeared prominently at 336.51 and 341.73 eV (002) and Pd (111), respectively (Fig. 2c). The corresponding
binding energies corresponding to 3d5/2 and 3d3/2, respectively, selected area electron diffraction (SAED) pattern with multiple
due to Pd(0).58 On the other hand, the two Auger peaks observed diffraction rings from inside to outside (Fig. 2d) further
at 330.68 eV and 335.43 eV were also assigned to Pd2+, which conrms the crystalline structure of the CuO–Pd nano-
suggests that Pd(II) is present on the surface of Pd, which might particles.62 In addition, high angle annular dark-eld scanning
be due to a charge interaction between the metal oxide CuO and transmission electron microscopy (HAADF-STEM), along with
noble metal Pd.59 Similarly, the binding energies of Cu and Pd EDS elemental mapping conrmed the presence and spatial
shied slightly, indicating a change in the electron density on arrangement of Cu, O and Pd (Fig. 2e–h). The Pd was shown
CuO due to the interaction between CuO and Pd.59 Moreover, mainly in the core region and was barely observable in the shell
the XPS peak at 284.9 eV corresponds to the C 1s region region, whereas the signals for Cu and O showed a homoge-
(Fig. S1b, ESI†). These results also conrm the formation of neous distribution, which further conrmed the core–shell
CuO–Pd nanoparticles, which is in good accordance with the structure. This epitaxial arrangement governed the stability of
XRD patterns.
these interfaces.63,64
The size and morphology of the as synthesized nanoparticles
The elemental composition was assigned using EDS anal-
were observed by TEM. Fig. 2a presents TEM images of CuO–Pd ysis. TEM coupled with EDS revealed typical optical absorption
NPs at 5 nm. In the HRTEM image, the bright/dark contrast peaks for Cu, O and Pd (Fig. S3, ESI†), along with Ni, which was
clearly conrms the fabrication of the Pd@CuO core–shell due to the carbon coated nickel grid.
nanostructure. In Fig. 2b, the palladium core was covered with
The FT-IR spectrum analysis was obtained to investigate the
capping of the nanoparticles by the rhizome extract, i.e., the
surface property of the CuO–Pd NPs. Fig. S4, ESI† shows the
FTIR spectra of the rhizome extract and CuO–Pd NPs. In the IR
spectra of the nanoparticles, the absorption peaks at 3272 cmꢁ1
and 1628 cmꢁ1 were assigned to O–H and C]C stretching
vibrations, which was moved slightly from 3294 and 1621 cmꢁ1
,
respectively. The vibration at 1049 cmꢁ1 was due to a shi in the
C–O stretching vibration from 994 cmꢁ1. The peak at 529 cmꢁ1
was assigned to the Cu–O stretching vibration.65 Therefore, the
CuO nanoparticles are associated with Pd, which is in good
agreement with the XRD patterns. These results show that the
bioactive molecules present in the rhizome extract are respon-
sible for the reduction of metal ions and capping of the
nanoparticles.
The total avonoid and phenolic acid contents in the C.
rotundus rhizome extract were 0.16 ꢃ 0.008 g of QE/(100 g dried
weight of rhizome) and 0.4 ꢃ 0.08 g of GAE/(100 g dried weight
of rhizome), respectively.
The phytochemical constituents of the plant extracts include
catechin (avonoid), gallic acid (phenolic acid), cyperol, sitos-
terol, nootkatone and valencene (Fig. S5, ESI†),32–35 which are
likely responsible for the formation of the nanoparticles. The
role of polyols present in the extract for the formation of
core–shell nanoparticles is depicted in Scheme 2. For example,
the phenolic hydroxyl groups of catechin can chelate with metal
ions to form chelate rings, which forms the metal nanoparticles
by electron transfer.66–68 Thus, in the presence of free electrons
generated through bioreduction, Pd2+ ions rst reduced to Pd0
Fig. 2 TEM image of CuO–Pd NPs at (a) 5 nm, and (b) HRTEM image
(inset: FFT pattern of the corresponding HRTEM image), (c) lattice
fringes of CuO and Pd, (d) SAED pattern, (e) HAADF-STEM image of
CuO–Pd NPs (inset: STEM image at 20 nm), along with the EDS
elemental maps of (f) Cu Ka shell, (g) O Ka shell, and (h) Pd La shell.
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to form the core part. In addition, the formation of CuO shell to the loss of oxygen in CuO lattice.56,72,73 The residual mass was
from Cu(OAc)2 can be attributed to the initial formation of found to be 56.72% (Fig. S7, ESI†), which conrmed the capping
copper hydroxide, which subsequently dehydrated and ther- of the nanoparticles by the rhizome extract leaving behind the
mally decomposed to form copper(II) oxide as conrmed by XRD residues of CuO and palladium nanoparticles.
pattern.69,70
The DSC thermogram (Fig. 3b) revealed heat variations
A preliminary facile approach to conrm the formation of associated with an exothermic and endothermic reaction. The
the CuO–Pd NPs was achieved by UV-visible spectroscopy. The two endothermic peaks observed were assigned to thermal
continuous absorption in the UV visible range and the disap- decomposition of the bioactive molecules present in the
pearance of the peak at 425 nm indicated the formation of CuO rhizome extract.
along with Pd NPs.71 The diminished plasmon band in the
The stability and the surface charge of the CuO–Pd NPs were
bimetallic nanoparticles might be due to the core shell structure measured using a zeta potentiometer. The zeta potential
of the CuO–Pd NPs (Fig. S6a, ESI†). The peak between 200 to distribution peak was found to be a single peak (Fig. S8, ESI†)
350 nm conrmed the presence of polyols present in the and the corresponding average zeta potential and zeta deviation
rhizome extract (Fig. S6b, ESI†).
of the CuO–Pd NPs were ꢁ9.94 ꢃ 0.22 mV and 5.34 ꢃ 0.86 mV,
The thermal stability of the CuO–Pd NPs was attempted by respectively (Table S1, ESI†), indicating the stability of the bio-
DSC-TGA. TGA revealed the weight loss % during heating upto synthesized nanoparticles in the colloidal solution. This nega-
700 ꢀC at a heating rate of 10 ꢀC minꢁ1 under a N2 atmosphere tive zeta potential was attributed to the capping of CuO–Pd NPs
(Fig. 3a). The thermal decomposition of the nanoparticles with negatively charged biomolecules present in the C. rotundus
proceeds in three steps. The initial weight loss (4.58%) below rhizome extract.74
ꢀ
150 C can be attributed to the evaporation of adsorbed mois-
AFM was used to characterize the surface roughness of the
ture and other volatile matters contained in the nanoparticles. nanoparticles. The nanoparticles were dispersed in water by
This feature was further reected by the endothermic DSC peak sonicating for 10 min. A drop of this colloidal solution was then
ꢀ
at approximately 50ꢀ C. The second stage weight loss (9.95%) placed onto a silica vapor glass plate and dried at room
ꢀ
from 150 C to 250 C, which correlates with the endothermic temperature. The AFM data was processed to 2D and 3-D color
peak on the DSC curve at approximately 221.12 ꢀC, is likely due map images (Fig. 4). A higher or lower quality image is a great
to the removal of the capping agent present in nanoparticles. challenge due to the position and nature of the tip and other
The last step from 250 ꢀC to 700 ꢀC (28.75%) with a prominent parameters, such as the applied force, scanning rate and
endothermic peak at approximately 289.62 ꢀC can be attributed oscillation amplitude.75 Roughness analysis at 5 mm2 showed
that the root mean square, mean roughness and maximum
height was 5.46 nm, 3.77 nm and 72.84 nm, respectively (Fig. S9,
ESI†). Interestingly, the roughness at higher magnication
(1.3 mm2) revealed the root mean square (1.55 nm), mean
roughness (1.08 nm), and maximum height (17.0 nm) (Fig. S10,
ESI†). The results showed that the nanoparticles have a rough
surface. Moreover, the particle distribution histogram showed
Scheme 2 Proposed mechanism for the formation of core–shell
CuO–Pd NPs.
Fig. 4 Tapping mode AFM images of the as synthesized CuO–Pd NPs;
flatten view at (a) 5 mm2, & (b) 1.3 mm2, and 3-D view at (c) 5 mm2, & (d)
Fig. 3 (a) TGA trace, and (b) DSC curve of CuO–Pd NPs.
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Table 1 Comparative results of the different catalysts for the synthesis
of 4aa
that the CuO–Pd nanoparticles have a particle height or
substrate depth of 10.56 nm (Fig. S11, ESI†). The size of the
nanoparticles was conrmed further by section analysis of the
AFM image at higher magnication. The vertical distance
observed from section prole analysis was 3.94 nm and 4.24 nm
at two different lines (Fig. S12, ESI†).
Entry
Catalyst
Time (min)
Yields
Ref.
1
2
3
4
CuO–Pd NPs
CuO NPs
Pd NPs
Cyperus rotundus
rhizome extract
EDDA
Cu–MWCNTs
Cu(C6H5SO3)2$
6H2O
30
30
30
30
97%
71%
50%
NR
Present work
Catalytic activity
5
6
7
300
25
42
94%
78%
92%
50
76
77
To examine the versatility and catalytic performance of CuO–Pd
NPs, a reaction of isatoic anhydride (1), aniline (2a) and benz-
aldehyde (3a) was performed under reux for 30 min in water in
the presence of 2 mol% of the CuO–Pd NPs (3.72 mg). The ex-
pected product 4a was obtained in high yield (97%), which was
determined by an analysis of its spectral data and by a direct
comparison with the reported data (data presented in ESI†).50
To synthesize a variety of 2,3-dihydroquinazolin-4(1H)-one
derivatives, the three-component reactions with a range of aryl
amines bearing either electron-donating or electron-
withdrawing groups and benzaldehyde carrying electron-
donating group on the benzene ring afforded the desired
products 4b–4e in 92–96% yield (Scheme 3).
To prove the catalytic efficiency of bio-synthesized core–shell
CuO–Pd nanoparticles, we have compared with other catalysts
including nanocatalysts. The results are depicted in Table 1. To
our delight, the biosynthesized CuO–Pd NPs provided a rapid
synthetic route to 2,3-dihydroquinazolin-4(1H)-one (4a) with
excellent yield compared to the other reported catalysts.76–81
Moreover, the reaction using its monometallic nanoparticles
like CuO NPs or Pd NPs afforded the desired product 4a in 71
and 50% yield, respectively, but no product was formed with
only C. rotundus extract (entries 2–4, Table 1). The results
further conrmed that the core–shell pattern of CuO–Pd NPs
increased the catalytic efficiency by increasing the yield.
8
9
10
11
Cu(MS)2$4H2O
Nano-In2O3
Nano CuO
ZnO–NPs
60
85%
87%
85%
92%
78
79
80
81
240
180
180
a
NR-no reaction.
To broaden the application of core–shell CuO–Pd NPs,
various spirooxindole derivatives bearing the dihy-
droquinazolinone moiety were synthesized. CuO–Pd NPs cata-
lyzed three-component reactions of isatoic anhydride (1),
aniline (2a), and isatin (5) were performed. The multi compo-
nent reactions of 1 with 2a and 5 in the presence 2 mol% CuO–
Pd NPs (3.72 mg) reuxed in water for 30 min gave compound
6a in 98% yield. Treatment of 1 with anilines bearing both
electron-donating and electron-withdrawing groups on the
benzene ring or benzylamine and 5 afforded products 6b–6e in
93–98% yield (Scheme 4).
To further demonstrate the applicability of the synthesized
CuO–Pd NPs, we have performed novel reactions for the
Scheme 3 CuO–Pd NPs catalyzed synthesis of a variety of 2,3-dihy-
a
droquinazolin-4(1H)-ones 4a–4ea,b
Reaction conditions: isatoic Scheme 4 CuO–Pd NPs catalyzed synthesis of a variety of spiroox-
anhydride (1.0 mmol), amine (1.0 mmol), and aldehydes (1.0 mmol), indole derivatives 6a–6ea,b Reaction conditions: isatoic anhydride
a
CuO–PdNPs catalyst (2 mol%), H2O (10.0 mL), reflux for 30 min. (1.0 mmol), amine (1.0 mmol), and isatin (1.0 mmol), CuO–PdNPs
b
Isolated yields.
catalyst (2 mol%), H2O (10.0 mL), reflux for 30 min. b Isolated yields.
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construction of biologically interesting chromenylpyrimidin-
2,4-diones (Scheme 5). The reaction of 2-hydroxychalcone (7a)
with 6-amino-1,3-dimethylpyrimidine-2,4(1H,3H)-dione (8) in
the presence of 2 mol% of CuO–Pd NPs (3.72 mg) in reuxing
toluene for 1 h provided product 9a in 87% yield. The structure
of 9a was determined by analysis of its spectral data (presented
in ESI†). Additional reactions of 2-hydroxychalcones 7b–7e,
bearing substituents on the aromatic ring and vinyl group such
Fig. 5 (a) Recovery and reuse of the synthesized CuO–Pd NPs for the
synthesis of compound 4a, and (b) TEM image of CuO–Pd NPs after
recycled for five times.
as –CH3 and –Br, with 8 provided desired products 9b–9e in the
range of 81–85% yield.
Scheme 6 presents the proposed reaction mechanism for the
formation of 4a, 6a and 9a. The nucleophilic addition of aniline
(2a) to isatoic anhydride (1), in the presence of the CuO–Pd NPs,
followed by decarboxylation, produced 2-aminobenzamide 10.
The condensation of 10 with benzaldehyde (3a), in the presence
of CuO–Pd NPs, gave imine 11, which underwent intra-
molecular cyclization to produce the nal product 4a. Similarly,
the condensation of 10 with isatin (5), in the presence of
CuO–Pd NPs, gave imine 12, which on intramolecular cycliza-
tion gave product 6a. Moreover, the 1,4 addition of 8 on
2-hydroxychalcone (7a), in the presence of CuO–Pd NPs,
provided intermediate 13, which on intramolecular cyclization
gave the nal product 9a.
To carry out the recycling experiment, the CuO–Pd nano-
catalyst was recovered by simple ltration aer the organic
reaction. The ltered catalyst was washed with hot water and
dried in a vacuum oven at 80 ꢀC for 3 h before the next recycle.
The recovered catalyst was then reused for the subsequent
reaction under identical conditions for ve times without any
signicant loss of activity, as shown in Fig. 5a. The TEM image
(Fig. 5b) and XRD patterns (Fig. S13, ESI†) of CuO–Pd NPs were
unchanged, even aer being recycled ve times, which
conrmed the retention of the catalytic activity without leaching
the palladium nanoparticles.
Scheme 5 CuO–Pd NPs catalyzed synthesis of a variety of chrome-
a
nylpyrimidin-2,4-diones 9a–9ea,b
Reaction conditions: hydrox-
ychalcone (1.0 mmol), and dimethylpyrimidinone (1.0 mmol), CuO–
b
PdNPs catalyst (2 mol%), toluene (5.0 mL), reflux for 1 h. Isolated
yields.
Conclusions
Facile and stable core–shell CuO–Pd NPs were synthesized and
their chemistry was analyzed. This eco-friendly synthetic protocol
was totally free of additives or by-products. The synthesized
nanoparticles were used as a more convenient and green
heterogeneous catalyst, exhibited enhanced catalytic activity with
excellent yield for the synthesis of 2,3-dihydroquinazolin-4(1H)-
ones, spirooxindoles, and chromenylpyrimidin-2,4-diones.
Furthermore, the catalyst was separated easily from the reac-
tion mixture by a simple ltration due to the heterogeneous
nature of the nanoparticles. The recovered unchanged catalyst
was then reused in ve subsequent reactions under identical
conditions without any signicant change in the catalytic activity.
Therefore, such core–shell CuO–Pd catalysts are expected to be
Scheme 6 Plausible mechanism for the formation of 4a, 6a, and 9a.
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Acknowledgements
This work was supported by the National Research Foundation 26 J. K. Edwards and G. J. Hutchings, Angew. Chem., Int. Ed.,
of Korea (NRF) grant funded by the Korea government (MSIP) 2008, 47, 9192–9198.
(NRF-2014R1A2A1A11052391), the Nano Material Technology 27 B. Nakhjavan, M. N. Tahir, F. Natalio, M. Panthofer, H. Gao,
¨
Development Program (2012M3A7B4049675), and the Ministry
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