A facile synthesis of PdCo bimetallic hollow nanospheres and their application to Sonogashira reaction in aqueous media

Article abstract of DOI:10.1039/b604581m

PdCo bimetallic hollow nanospheres with a diameter of about 80 nm were for the first time synthesized in polyethylene glycol solution. This new Pd-containing bimetallic hollow nanostructure was successfully applied to catalysis of the Sonogashira reaction

Full text of DOI:10.1039/b604581m

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LETTER
www.rsc.org/njc | New Journal of Chemistry
A facile synthesis of PdCo bimetallic hollow nanospheres and their
application to Sonogashira reaction in aqueous media
Yingguang Li, Ping Zhou, Zhihui Dai, Zhixin Hu, Peipei Sun* and Jianchun Bao*
Received (in Montpellier, France) 28th March 2006, Accepted 2nd May 2006
First published as an Advance Article on the web 15th May 2006
DOI: 10.1039/b604581m
PdCo bimetallic hollow nanospheres with a diameter of about
80 nm were for the first time synthesized in polyethylene glycol
solution. This new Pd-containing bimetallic hollow nanostruc-
ture was successfully applied to catalysis of the Sonogashira
reaction, which reveals obvious advantages such as environmen-
tally friendly reaction conditions (the reaction proceeded in
water), the recyclability of the catalyst, simple experimental
operation and high yields.
loss of catalytic activity.6 The high surface area of the hollow
spheres resulting from the nanoparticulate nature of the shell,
which is much larger than that of dense spheres of the same
diameter, is responsible for the high catalytic activity. Gen-
erally, this kind of ‘‘hard template’’ method can ensure
relatively good control over the morphology of the final
products and thus allows one to obtain a hollow nanostruc-
ture, but the addition and then removal of the templates may
complicate the synthetic procedures and limit the scale on
which a material can be synthesized. Therefore, direct solu-
tion-phase approaches to Pd-containing hollow spheres need
to be explored. On the other hand, in order to reduce the cost
of Pd catalysts and/or raise their utilization efficiency, bime-
tallic nanoparticles can be used. Hyeon and co-workers
synthesized Pd/Ni bimetallic nanoparticles with a cheap metal
core (Ni) and a noble metal shell (Pd), which can be used in
Sonogashira coupling reactions. The toxic organic solvent
diisopropylamine (DIA) or toluene, however, should be used
for this reaction.7
As an important route for the synthesis of aryl alkynes by
the coupling of terminal alkynes and aryl halides, the Sonoga-
shira reaction has always aroused the interest of chemists.1
This method has been applied for the synthesis of natural
products, bioactive compounds and materials.2 Numerous
palladium-complexes have been employed to catalyze this
transformation in recent years. It is thought that the basic
ligands in the complexes designed to provide the requisite
electron density on palladium in order to achieve oxidative
addition of Pd to aryl halides indeed facilitate successful
coupling reactions.3 The reaction generally proceeds in organ-
ic solvents such as amines, benzene, THF, DMF etc. in which
the catalyst can dissolve to form a homogeneous system. These
soluble palladium reagents tend to be expensive and some-
times difficult to manipulate and recover. Moreover, the
reactions are generally air-sensitive and the solvents also pose
recyclability (waste handling) problems of their own. In addi-
tion, amines such as piperidine, diethylamine and triethyla-
mine are required in most Sonogashira reactions and they add
to the environmental burden. Thus, it is essential to develop
new Pd-based catalysts and reaction media to overcome these
problems.
To date, only a few examples of Pd-containing bimetallic
hollow nanospheres exist in the literature.8 Furthermore, there
is no report of the use of such materials as catalysts in C–C
bond forming processes in aqueous solution. In this commu-
nication, PdCo bimetallic hollow spheres of about 80 nm
diameter were for the first time synthesized using a solution-
phase approach. These new bimetallic hollow nanospheres
were then successfully applied as catalysts in the Sonogashira
reaction. The reaction was carried out in water and good to
excellent yields were achieved (Scheme 1).
Fig. 1 shows the XRD pattern of the sample prepared by the
redox reaction of CoSO4 and H2PdCl4 with NaBH4 in 0.25 g
mLÀ1 polyethylene glycol solution. The diffraction peaks in
the range of 10 p 2y p 701 can be indexed as cubic Pd (111),
(220), and hexagonal Co (002), (101), which are in good
accordance with ASTM standard 5-681 (Pd) and 5-727 (Co),
respectively, and mean that the sample is composed of metallic
Pd and Co instead of their alloy. The peaks are fairly broad,
even overlapping with neighbors, indicating the small particle
size. A rough estimation employing Scherrer’s equation in-
dicates the size of the particles forming the PdCo hollow
spheres is about 4.0 nm.9 Fig. 2 gives the energy-dispersive
spectrometry (EDS) analysis of the sample. The results
In recent years, there has been growing interest in the
fabrication of Pd nanoparticles and their applications in
organic reactions especially the formation of carbon–carbon
bonds.4 For example, ligand-free Pd nanoparticles of about
3.3 nm in size stabilized by tetraalkylammonium salts bearing
long alkyl chains were obtained and used as heterogeneous
catalyst for the Suzuki and Stille cross-coupling reactions of
aryl halides carried out in ionic liquids.5 Palladium hollow
spheres with diameters of about 300 nm were prepared by
using silica spheres as templates, and these showed good
catalytic activities in Suzuki coupling reactions and could be
reused several times by simple filtering and retrieving without
Department of Chemistry, Laboratory of Materials Science, Nanjing
Normal University, Nanjing, 210097, China. E-mail:
sunpeipei@njnu.edu.cn. E-mail: baojianchun@njnu.edu.cn
Scheme 1
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Fig. 3 XPS spectra of the PdCo bimetallic hollow nanospheres. The
insets a and b show the magnified analysis of the Co2p and Pd3d
peaks, respectively.
Fig. 1 XRD pattern of PdCo bimetallic hollow nanospheres.
demonstrate the coexistence of Pd and Co with an atom ratio
of 3.5 : 1. Also, carbon and oxygen were found in the EDS
analysis, meaning a small amount of polyethylene glycol
existed in the sample. Further evidence comes from the FTIR
spectra, which show the absorption peaks of the polyethylene
glycol (not shown here). Fig. 3 shows the XPS spectra of the
sample. It can be seen from the figure that the sample is
composed of Pd, Co, C and O elements. The binding energies
of Pd(3d5/2) and Pd(3d3/2) are 335.9 and 341.0 eV (Inset b of
Fig. 3), respectively, which are similar to those reported for Pd
metal in the literature.10 However there is a small chemical
shift toward higher binding energies for Co2p3/2 (783.5 eV)
and Co2p1/2 (799.4 eV) (Inset a of Fig. 2) as compared to
metallic Co,11 implying that the Co on the surface of the PdCo
particles is being oxidized. In addition, the C1s and O1s peaks
in Fig. 3 can be attributed to the polyethylene glycol, which is
in accordance with the EDS analysis. A typical TEM image of
the sample is shown in Fig. 4a. From the TEM image, we can
see pale regions in the central parts in contrast to the dark
edges, implying hollow spherical structures. The same contrast
difference between the center and edge in the TEM images of
one sphere is obtained when the sample grid is rotated by
different degrees and this further proves their hollow struc-
tures. The corresponding selected area electron diffraction
pattern of the hollow nanospheres shown in the inset of Fig.
4a shows regular diffraction rings, indicating that the hollow
nanospheres are composed of polycrystals. Those diffraction
rings can be indexed to cubic Pd (220), (420), and hexagonal
Co (103), (220), further confirming the formation of a bime-
tallic nanostructure. The average diameter of the hollow
spheres is about 80 nm with most of them in the range 60 to
90 nm. The high-resolution TEM (HRTEM) image of the
sample shown in Fig. 4b reveals that the shell wall is composed
of primary PdCo nanoparticles with an average diameter of
about 3.5 nm, and that the shell thickness is about 9 nm. To
the best of our knowledge, this is the first example of hollow
Pd-containing bimetallic nanospheres prepared by using the
direct solution-phase approach.
It is necessary to point out that PdCo hollow nanospheres
can be easily obtained in high yields up to 70% by using
present method. The polyethylene glycol plays an important
role in the formation of the hollow spheres. When the experi-
ment was carried out in the absence of polyethylene glycol, the
Fig. 2 EDS of PdCo bimetallic hollow nanospheres.
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the [PdCl4]2À (the pH of the solution being 7–8) and Co21 ions
are mainly located on the surface of the aggregates to form a
Co- and Pd-containing coating which creates a nucleation
domain of Co and Pd for the subsequent redox reaction. As
the redox reaction proceeds, a metallic shell is formed on the
surface of the aggregates. The reason that pure Pd hollow
spheres failed to form might be that the polyethylene glycol
spherical aggregates can not form in the H2PdCl4–polyethy-
lene glycol solution because there is no complexation of
[PdCl4]2À ions with the oxygen atoms of the polyethylene
glycol. Similar spherical aggregates have also been observed
in a solution of Cd(OAc)2 and polystyrene-block-poly(2-vinyl-
pyridine) (PS-b-P2VP) in THF due to the complexation of
Cd21 ions with the 2-vinylpyridine units,12 and in an aqueous
solution of Cd(OAc)2 and the triblock copolymer poly(ethy-
lene oxide)poly(propylene oxide)poly(ethylene oxide) (EO20-
PO70EO20) via the complexation of Cd21 ions with the EO and
PO units.13 Further evidence comes from the FTIR (Fig. 6). It
reveals that although most of the IR spectrum of H2PdCl4-
(Fig. 6b) or CoSO4- (Fig. 6c) containing polyethylene glycol
aqueous solution is similar to that of pure polyethylene glycol
aqueous solution (Fig. 6a), but there is a change in the case of
CoSO4-containing solution in that the vibration of the C–O–C
Fig. 4 (a) TEM and (b) high-resolution TEM images of PdCo
is shifted to higher wavenumber from 1092 cmÀ1 to 1106 cmÀ1
.
bimetallic hollow nanospheres.
This results from the covalent properties increasing with the
partial donation of the oxygen atom lone pair electrons to the
vacant d-orbitals of the Co21 ions. This means that the
coordination of the oxygen atoms in the polyethylene glycol
with Co21 ions does indeed exist and plays a key role in the
formation of the polyethylene glycol spherical aggregates and
PdCo bimetallic hollow spheres. The details of the mechanism
of formation need further investigation. As to the polyethylene
glycol contained in the nanospheres, it can be removed from
the shells upon washing.13
products were solid PdCo particles of about 40 nm in size (Fig.
5). In addition, solid Pd particles, instead of hollow spheres, of
about 30 nm were obtained when the reaction was carried out
in the same conditions but without CoSO4. Contrarily, in the
absence of H2PdCl4 but keeping the other conditions constant,
Co hollow spheres with an average diameter of about 80 nm
were formed. These facts mean that cobalt ions also play an
important role in the formation of the PdCo bimetallic hollow
spheres. The polyethylene glycol chain contains many hydro-
philic and hydrophobic sites and has a high degree of flexibility
in that the C–O bond is easily rotated, and can form various
conformations such as extended chains or coils under different
conditions. We speculate that the formation of the PdCo
hollow spheres might result from polyethylene glycol spherical
aggregates formed mainly via the affinity of the oxygen atoms
of the polyethylene glycol for Co21 ions. In such aggregates,
the hydrophobic hydrocarbon chains of the polyethylene
glycol are oriented toward the interior of the aggregates, while
PdCo bimetallic hollow nanospheres were successfully used
to catalyze the Sonogashira reaction. The reaction conditions
were first studied. The coupling of phenylacetylene and
Fig. 6 (a) FTIR spectrum of the pure polyethylene glycol aqueous
solution; (b) and (c) FTIR spectra of H2PdCl4- and CoSO4-containing
polyethylene glycol aqueous solution, respectively.
Fig. 5 TEM images of PdCo bimetallic solid nanoparticles.
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p-iodotoluene was chosen as a model reaction for the investi-
gation. Solvents such as THF, acetonitrile, DMF, isopropa-
nol, isopropanol–H2O, H2O were employed as reaction media.
The results showed that under basic conditions (K2CO3), in
the presence of a nanopalladium–cobalt catalyst, CuI and
PPh3, all of the above systems gave the coupling product with
moderate to good yields after 6 h at 70–80 1C. In water, the
yield (87%) was lower than that in isopropanol (95%), and
similar to that in isopropanol–H2O (90%) and THF (85%).
However in DMF and acetonitrile, the reaction gave only
moderate yields (80% and 70%, respectively). Considering
that water is an environmentally friendly solvent, it was chosen
as the solvent for this reaction. The experiment indicated that
5 mol% of catalyst (that is, about 3.9 mol% of Pd) was
sufficient to catalyze the reaction. As co-catalyst and ligand,
cuprous iodide and triphenylphosphine were essential to the
reaction. Without them, the reaction gave only traces of
product. A basic environment was also important for the
Sonogashira reaction: almost all reports and our experiment
showed this feature. As general bases, Na2CO3, K2CO3,
NaOH and KOH were tested and K2CO3 was found to be
the best. K2CO3 was therefore selected as the base for our
research. The reaction was carried out under a nitrogen atmo-
sphere to avoid the oxidative coupling of terminal alkynes
which generally takes place in the presence of oxygen.14
Another notable advantage with this reaction was the recycl-
ability of the PdCo catalyst. The catalyst could be reused after
being separated from the solution and washed sufficiently with
water and methanol. After being reused three times, the
catalytic activity did not decrease significantly. Elemental
analysis of the filtrate after the reaction demonstrated no
leaching of Pd, implying that the reaction proceeded through
a heterogeneous catalytic process.
differences. This means that the reaction is relatively insensi-
tive to the electronic characteristics of the substituents. It is
noteworthy that, compared to Hyeon’s method,7 the PdCo-
catalyzed Sonogashira reaction described here gives a similarly
high yield, but with marked differences: (i) the PdCo bimetallic
hollow nanospheres can be easily obtained using our simple
method and (ii) our reaction proceeds in water instead of the
toxic organic solvent DIA. In addition, to determine the role
of the cobalt in the PdCo hollow spheres in the catalytic
process, Co hollow spheres prepared under similar conditions
were used in the model reaction mentioned above. However
the reaction was sluggish. It is currently not clear whether the
catalytic activity of the PdCo bimetallic hollow spheres results
from true bimetallic catalysis or only from the Pd nanostruc-
ture with its special morphology. More detailed work is in
progress.
In summary, PdCo bimetallic hollow nanospheres of about
80 nm diameter and of about 9 nm thickness were synthesized
in polyethylene glycol solution. To the best of our knowledge,
this is the first example of Pd-containing bimetallic hollow
nanospheres using soft template synthesis. Furthermore, they
were successfully used to catalyze the Sonogashira reaction,
which reveals obvious advantages such as environmentally
friendly reaction conditions (the reaction proceeded in water),
the recyclability of the catalyst, simple experimental operation
and high yields. It is also a new application of palladium–
cobalt nanoparticles in organic synthesis. Similar fabrication
of Co, PdNi, Ag2S, and CdS hollow spheres via polyethylene
glycol-assisted route suggests that this simple, rapid and
effective approach can be extended to prepare a variety of
nanoscale hollow spheres. Further studies on organic synthesis
of these metallic hollow nanospheres are under way in our
laboratory.
Under optimized reaction conditions, the coupling of a
series of aryl halides with terminal alkynes was studied. The
results are shown in Table 1. Both aryl bromides and aryl
iodides could react with aromatic or aliphatic terminal alkynes
to give the corresponding coupling products with high yields.
Furthermore, the reactions of aryl halides with electron-with-
drawing or electron-donating groups did not show obvious
Experimental
Analytical-grade cobaltous sulfate (CoSO4 Á 7H2O), palladium
chloride (PdCl2), sodium borohydride (NaBH4), cuprous io-
dide (CuI), ammonium fluoride (NH4F), and triphenylpho-
sphine were purchased from Shanghai Chemical Reagents Co.
Table 1 The coupling of aryl halides with terminal alkynes using PdCo bimetallic hollow nanospheres as catalyst (see Scheme 1)
Entry
ArX
R
Time/h
Product
Yielda(%)
1
2
3
4
5
6
7
8
PhI
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
6
6
6
5
6
6
8
8.5
8
6
6
6
5.5
6
8
1a15
1b15
1c15
1d16
1e18
1f15
1a
92
87 (86, 87, 84)b
p-CH3C6H4I
m-CH3C6H4I
o-CH3C6H4I
p-ClC6H4I
p-O2NC6H4I
PhBr
p-CH3C6H4Br
p-ClC6H4Br
PhI
p-CH3C6H4I
m-CH3C6H4I
o-CH3C6H4I
p-ClC6H4I
PhBr
88
90
92
87
88
86
85
88
85
92
87
93
78
1b
1e
9
Ph
10
11
12
13
14
15
a
CH3(CH2)3
CH3(CH2)3
CH3(CH2)3
CH3(CH2)3
CH3(CH2)3
CH3(CH2)3
1
1g16
1h16
1i17
1j16
1k18
1g
b
Isolated yields. The products were characterized by H NMR and IR spectra. The catalyst was reused three times.
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Polyethylene glycol (Mw 20 000) was purchased from Beijing
Chemical Reagents Co. Phenylacetylene, p-iodotoluene, and
other terminal alkynes and aryl halides were purchased from
Aldrich. Organic solvents such as THF, acetonitrile, DMF,
and isopropanol were distilled before use. H2PdCl4 solution
was prepared by dissolving 500 mg PdCl2 into 25 mL of
hydrochloric acid (37%), then diluting with 50 mL of de-
ionized water.
1c: Light yellow oil. 1H NMR d (300 MHz, CDCl3)
7.58–7.61 (m, 2 H), 7.37–7.43 (m, 5 H), 7.27–7.29 (m, 1H),
7.19 (d, J = 7.8 Hz, 1H), 2.40 (s, 3 H). IR (neat) n 3015, 2228,
1645, 1556 cmÀ1
.
1d: Light yellow oil. 1H NMR d (300 MHz, CDCl3)
7.56–7.61 (m, 3 H), 7.38–7.42 (m, 3 H), 7.22–7.28 (m, 3 H),
2.57 (s, 3 H). IR (KBr) n 3025, 2218, 1625, 1530 cmÀ1
.
1e: Mp: 81 1C. 1H NMR d (300 MHz, CDCl3) 7.47–7.57 (m,
4 H), 7.32–7.40 (m, 5H). IR (KBr) n 3042, 2220, 1626, 1524
The PdCo bimetallic hollow nanospheres were fabricated as
follows. To a solution of 2.5 g polyethylene glycol (Mw 20 000)
in 10 mL of deionized water, 42 mg CoSO4 Á 7H2O, 25 mg
NH4F, 125 mg H3BO3 and 2.5 mL of H2PdCl4 solution were
added in turn. Then the pH was adjusted to 7–8 using con-
centrated ammonia to form solution A. To 6 mL of 0.25 g mLÀ1
polyethylene glycol aqueous solution, 10 mg NaBH4 were
added to form solution B. Solution A was added slowly into
solution B under sonication at 30–40 1C. The solution exhibited
a colour change from colourless to black. After the addition of
solution A, the resulting solution was further sonicated for 10
minutes. Finally, the black precipitate produced was separated
from the mixture by centrifugation. The deposit was collected,
washed with deionized water 4–5 times, and vacuum-dried.
Thus the PdCo hollow nanospheres were obtained.
cmÀ1
.
1f: Mp: 118 1C. 1H NMR d (300 MHz, CDCl3) 8.20 (d, J =
8.6 Hz, 2H), 7.66 (d, J = 8.6 Hz, 2H), 7.58–7.55 (m, 2H),
7.40–7.35 (m, 3H). IR (KBr) n 3038, 2218, 1623, 1538 cmÀ1
.
1g: Light yellow oil. 1H NMR d (300 MHz, CDCl3)
7.40–7.42 (m, 2H), 7.28–7.32 (m, 3H), 2.41 (t, J = 6.9 Hz,
2H), 1.45–1.62 (m, 4H), 0.95 (t, J = 6.9 Hz, 3H). IR (neat) n
3025, 2988, 2225, 1620, 1526 cmÀ1
.
1h: Light yellow oil. 1H NMR d (300 MHz, CDCl3) 7.26 (d,
J = 8.4 Hz, 2H), 7.08 (d, J = 8.4 Hz, 2H), 2.39 (t, J = 7.2 Hz,
2H), 2.33 (s, 3H), 1.48–1.60 (m, 4H), 0.94 (t, J = 7.2 Hz, 3H).
IR (neat) n 3022, 2945, 2210, 1625, 1528 cmÀ1
.
1i: Light yellow oil. 1H NMR d (300 MHz, CDCl3)
7.08–7.26 (m, 4H), 2.42 (t, J = 6.8 Hz, 2H), 2.33 (s, 3H),
1.48–1.63 (m, 4H), 0.95 (t, J = 6.8 Hz, 3H). IR (neat) n 3038,
The synthesis of alkynes catalyzed by PdCo hollow nano-
spheres is as follows. To 3 mL of water were added 1 mmol of
aryl halide and 1 mmol of terminal alkyne, then 0.05 mmol of
palladium–cobalt nanoparticles, 0.2 mmol of PPh3, 0.03 mmol
of CuI and 1.5 mmol of K2CO3 were added in turn. The
mixture was stirred at 80 1C under a nitrogen atmosphere for
the appropriate time (see Table 1, monitored by TLC) till
reaction was complete, then centrifuged. The solution was
separated and the precipitate was washed with ether (5 mL Â
3). The solutions were combined and extracted with ether and
purified by column chromatography on silica gel with hex-
ane–ethyl acetate (20 : 1) as eluent to yield the product
(1a–1k, see Table 1). The precipitate was further washed
sufficiently with water and methanol then dried, and the
palladium–cobalt nanoparticles were recovered. After being
reused three times, the yield of the product did not obviously
decrease (a new batch of CuI and PPh3 should be added when
the catalyst was reused).
2958, 2216, 1620, 1534 cmÀ1
.
1j: Light yellow oil. 1H NMR d (300 MHz, CDCl3) 7.39 (d,
J = 7.1 Hz, 1H), 7.12–7.20 (m, 3H), 2.48 (t, J = 7.0 Hz, 2H),
2.45 (s, 3H), 1.50–1.66 (m, 4H), 0.98 (t, J = 7.0 Hz, 3H). IR
(neat) n 3042, 2962, 2225, 1631, 1543 cmÀ1
.
1k: Light yellow oil. 1H NMR d (300 MHz, CDCl3) 7.34 (d,
J = 8.5 Hz, 2H), 7.26 (d, J = 8.5 Hz, 2H), 2.42 (t, J = 7.2 Hz,
2H), 1.45–1.63 (m, 4H), 0.97 (t, J = 7.2 Hz, 3H). IR (neat) n
3045, 2968, 2234, 1630, 1554 cmÀ1
.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China for the project (No. 20473038,
20505010), the Natural Science Foundation of Jiangsu pro-
vince (No. BK2005139), and the Natural Science Foundation
of the Education Committee of Jiangsu province (No.
04KJB150066). We thank Professor J. Yao of Center for
Materials Analysis for her help in the measurement and
discussion of FTIR spectra.
The characterization of the PdCo hollow nanospheres was
performed by X-ray diffraction (XRD) using a D/Max-RA
diffractometer with Cu Ka radiation, X-ray photoelectron
spectroscopy (XPS) with Mg Ka ray source, energy dispersive
spectroscopy (EDS), transmission electron microscopy (JEM-
200CX TEM) and high-resolution transmission electron mi-
croscopy (JEOL 2010 HRTEM). For the structural determi-
nation of the Sonogashira reaction products, 1H NMR spectra
were determined on a Bruker spectrometer (300 MHz) with
TMS as the internal standard. FTIR spectra were obtained
with a Nexus 670 spectrometer. Melting points are uncor-
rected.
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