One-pot electrochemical synthesis of palladium nanoparticles and their application in the Suzuki reaction

Article abstract of DOI:10.1039/c1nj20638a

A novel potential controllable, template free, electrodeposition/synthesis of palladium nanoparticles on an electrode surface as well as in the bulk electrolyte using ionic liquid as an electrolyte cum stabilizer is reported for the first time. The present method of preparation of palladium nanoparticles is advantageous as it eliminates the requirement of additional stabilizers or capping agents. The prepared palladium nanoparticles are characterized by different techniques such as TEM, SEM, EDAX and XPS analyses. These nanoparticles exhibited high catalytic activity for the Suzuki coupling reaction of unactivated halides in aqueous media with high TON.

Full text of DOI:10.1039/c1nj20638a

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LETTER
One-pot electrochemical synthesis of palladium nanoparticles and their
application in the Suzuki reactionw
Krishna M. Deshmukh,a Ziyauddin S. Qureshi,a Kushal D. Bhatte,a K. A. Venkatesan,b
T. G. Srinivasan,b P. R. Vasudeva Raob and Bhalchandra M. Bhanage*a
Received (in Montpellier, France) 22nd July 2011, Accepted 14th September 2011
DOI: 10.1039/c1nj20638a
A novel potential controllable, template free, electrodeposition/
synthesis of palladium nanoparticles on an electrode surface as well
as in the bulk electrolyte using ionic liquid as an electrolyte cum
stabilizer is reported for the first time. The present method of
preparation of palladium nanoparticles is advantageous as it
eliminates the requirement of additional stabilizers or capping agents.
The prepared palladium nanoparticles are characterized by different
techniques such as TEM, SEM, EDAX and XPS analyses. These
nanoparticles exhibited high catalytic activity for the Suzuki coupling
reaction of unactivated halides in aqueous media with high TON.
of various metal nanoparticles in ILs is possible without any
additives.6 Stabilization of nanoparticles in ILs may be due to the
weak anionic or cationic interactions with nanoparticle surfaces.7
Dupont et al. reported the stabilization of metal nanoparticles
in ionic liquids through a solvation force formed due to the ionic
liquid structure at the solid–liquid interface.8 Later Atkin et al.
studied that the IL structure in the vicinity of solid nanoparticles
produces solvation forces that are sufficient to prevent aggregation
and this was confirmed using AFM analysis.9
Electrochemical methods have several advantages in terms
of quickness, ease of operation, cost effectiveness and energy
efficiency. In spite of such attractive features little attention
has been paid towards the use of electrochemical methods for
PdNPs synthesis.10 Reetz and Helbig were the first to describe
an electrochemical procedure for the formation of palladium
and nickel nanoparticle colloids and in their method a metal
sheet is anodically dissolved. The intermediate metal salt
formed is reduced at the cathode, to provide metallic particles
which are stabilized by tetraalkyl ammonium salts along with
co-electrolyte acetonitrile/tetrahydrofuran.11 More recently,
Cha et al. reported the synthesis of PdNPs by means of anodic
dissolution of palladium metal foil using N-butyl-N-methyl-
morpholinium as an electrolyte and acetonitrile which acts as a
supporting electrolyte cum stabilizer.12 Endres et al. reported
nanocrystalline electrodeposition of Al, Fe, and an Al–Mn
alloy as well as nanocrystalline Pd from a Lewis basic ionic liquid
with or without nicotinic acid as an organic additive.13 These
reported methods utilize organic solvents, with requirement of
surfactants or stabilizers, and hence development of a clean and
neat process for PdNPs synthesis is desirable.
Palladium nanoparticles (PdNPs) synthesis has gained
considerable interest in the last few decades due to their potential
applications in catalysis, photonics, electronics, and biomedical
sensing.1 Because of their large number of applications in different
fields, the controlled synthesis of PdNPs with well-defined shape
and size has its special importance. Most of the previous
approaches required the use of organic or capping agents, such
as 15-membered triolefinic macrocycles,2 dendrimers,3 and surfac-
tants,4 which helps to prevent the agglomeration of metal nano-
particles. Even though it has potential for various applications
as described above, the conventional chemical reduction
process has a major limitation of contamination due to partial
incorporation of reducing agents or capping agents (e.g.
hydrogen or boron). The protecting agents used in most of
these processes have strong coordination affinity to surface atoms
of metal nanoparticles limiting their catalytic applications e.g.,
thiols that often inhibit catalytic activity.5 The alternative method
for palladium nanoparticles synthesis involving chemical
vapour deposition has a drawback of requirement of specialized
equipments and scale up issues.
Herein, we report a novel, one stroke potential controllable,
electrochemical method for the synthesis of PdNPs in the electrolyte
as well as on the working electrode. Here ionic liquid plays a role of
an electrolyte cum stabilizer. The catalytic activity of PdNPs was
studied for the Suzuki–Miyaura coupling reaction of unactivated
aryl halides using water as a benign reaction media.
Recently, Ionic Liquids (ILs) have been used as green
electrolytes for electrochemical synthesis and the stabilization
a Department of Chemistry, Institute of Chemical Technology,
Matunga, Mumbai-19, India.
Recent studies revealed that if there is any common anion
present between metal salt and ionic liquid, there is drastic
increase in the solubility of the metal salt in ionic liquid.14
Considering these observations we have conducted the electrode-
position of palladium metal from a 1-butyl-3-methylimidazolium
acetate IL ([BMim][OAc]), which has a common anion with a
E-mail: bm.bhanage@ictmumbai.edu.in; Fax: +91-22-24145614;
Tel: +91-22-33611111
b Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research,
Kalpakkam 603102, India
w Electronic supplementary information (ESI) available: Experimental
details, TEM, XRD, XPS, MS patterns and characterization data of
ionic liquid and products. See DOI: 10.1039/c1nj20638a
c
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Fig. 1 The cyclic voltammograms of [BMim][OAc] (A) and a 50 mM
solution of palladium acetate in [BMim][OAc] (B).
metal precursor Pd(OAc)2 responsible for the dissolution of
metal. The presence of a common acetate anion leads to
increase in solubility of the metal salt in IL. We observed that
at a certain potential, deposition of metallic PdNPs took place
at the working electrode as well as the formation PdNPs was
observed in the bulk electrolyte. We further investigated the
effect of various process parameters on the morphology of
formed nanoparticles.
A prominent reduction wave (Epc1) observed at ꢀ0.91 V
(vs. Pd) is due to the reduction of Pd(II) to Pd(0)15 and a couple
of oxidation waves (Epa1 and Epa2) at ꢀ0.38 V and 0.33 V may
be due to the oxidation of Pd (Fig. 1).16
Fig. 2 SEM and EDS images of the Pd deposit at a potential of ꢀ0.8 V,
electrolysis time 2500 s (A, B). Inset: Pd-deposit. The deposit at a
potential of ꢀ2.4 V, electrolysis time 2500 s (C, D).
Controlled potential electrolysis of
a
solution of
palladium(II) acetate (50 mM) in [BMim][OAc] was carried
out at three different potentials (ꢀ0.8 V, ꢀ1.6 V, ꢀ2.4 V vs.
Pd) using an aluminium plate (1 cm ꢁ 1 cm) as a cathode.
When we applied a low potential (ꢀ0.8 V), it was observed
that the palladium particles tend to get deposited on the electrode
surface itself. The deposition of palladium on the electrode
surface was confirmed by scanning electron microscopy coupled
with energy-dispersive X-ray spectroscopy (SEM-EDS). A
micrograph of the electrodeposit showed a pure metallic
Pd(0) deposit (Fig. 2A and B). This was further confirmed
by the X-ray Photoelectron Spectroscopy (XPS) analysis,
which gives a doublet with binding energies of 335.5 eV
(Pd 3d5/2) and 340.7 eV (Pd 3d3/2) (Fig. S1, ESIw). This shows
presence of Pd(0).17
Fig. 3 (A) The TEM image of PdNPs deposited from bulk electrolyte
at an applied potential of ꢀ1.6 V along with the diffraction pattern of
palladium (inset figure) and (B) XPS spectra of the nanoparticle from
bulk synthesized at an applied potential of ꢀ1.6 V, electrolysis time
1000 s.
peaks, one at a lower binding energy of 334.5 eV (Pd 3d5/2) and
another at 339.8 eV (Pd 3d3/2), which confirms the formation
of nanoparticles (Fig. 3B).
However, it was interesting to observe that the formation of
palladium nanoparticles in the bulk electrolyte took place
when the applied high potential was lowered to ꢀ2.4 V, which
was significantly lower as compared to that at the onset of
palladium deposition.
Studies at different potentials exhibited that palladium tends
to deposit at the working electrode at low potential (ꢀ0.8 V)
while at higher potential (ꢀ2.4 V) it tends to get stabilized in
the electrolyte. Whereas at an intermediate potential (ꢀ1.6 V)
one can achieve both the goals simultaneously (Fig. S3, ESIw).
It is obvious that when electrolysis is carried out at high
negative potential, like ꢀ2.4 V, the bulk electrolyte undergoes
decomposition. Since palladium is only 50 mM as compared to
the electrolyte (B5 M), the electrode is covered only with ionic
liquid and no space seems to be available for electrodeposition
of palladium on the electrode. As a result, the palladium
deposit falls down rather than adhering to the working
electrode. This is not possible at ꢀ0.8 V since palladium only
is reduced and the IL is intact; but it also happens at ꢀ1.6 V
because the onset of decomposition of the electrolyte occurs
below ꢀ1.5 V (Fig. 1).
Formation of PdNPs in the bulk electrolyte is well
supported by the X-ray diffraction (XRD) analysis, which
revealed the characteristic diffraction patterns corresponding
to the (111), (200) and (220) reflections, respectively, for the
face centered cubic (fcc) phase of metallic palladium (JCPDS
card no. 5-681). The average crystallite size was 5.93 ꢂ 1 nm,
calculated by the Scherer equation, which is in good agreement
with results obtained from Transmission Electron Microscopy
(TEM) analysis (Fig. S2, ESIw). At the same time palladium
deposition was not observed at the working electrode, which
was also confirmed by SEM-EDS analysis (Fig. 2C and D).
When we applied an intermediate potential (ꢀ1.6 V) (Fig. 3A),
we found the deposition of metallic palladium on the electrode
as well as the formation of PdNPs in the bulk electrolyte. The
XPS spectrum of the palladium formed in the bulk shows two
To understand the formation of PdNPs in the electrolyte
bath, we have studied the electrolytic system comprised of
palladium(II) chloride in 1-butyl-3-methylimidazolium chloride
c
2748 New J. Chem., 2011, 35, 2747–2751
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[BMim][Cl]. In this system formation of PdNPs was not observed
as reported in the literature.15c,18 This indicates that the stabili-
zation and formation of particles is highly dependent on the
choice of the anion, here the acetate anion is acting as a stabilizer
or capping agent. Earlier, Endres et al. reported that the electro-
deposition of Al and of Ta is strongly influenced by ionic liquids.
In [Py1,4]TFSA, Al is obtained as a nanomaterial whereas in
[EMIm]TFSA, a microcrystalline material is obtained. Ta was
deposited from [Py1,4]TFSA, whereas from [EMIm]TFSA only
non-stoichiometric TaFx was obtained. These consequences were
explained on the basis of interfacial solvation layers.19 The other
factors which govern the size and shape of the palladium
nanoparticle such as effect of concentration and electrolysis time
were also studied (Fig. S4 and S5, ESIw).
Catalytic activity of isolated PdNPs was tested for the
Suzuki–Miyaura coupling reaction with a very low loading
of PdNPs in water as reaction media. Initially, the reaction of
iodobenzene with phenylboronic acid was studied as a model
reaction and the role of various bases like KOH (97%),
K2CO3 (87%), and triethylamine (48%), and solvents like
toluene (60%), 1,4-dioxane (72%), and water (97%) was
investigated. It was observed that the reaction was more
favourable using KOH as a base and environmentally benign
water as a solvent with a high TON number of up to 9.7 ꢁ 102.
The efficiency of the present system was further extended for the
coupling of various aryl halides with different steric and electronic
properties with phenylboronic acid (Table 1). Phenylboronic acid
was found to couple smoothly with iodobenzene providing an
excellent yield (97%) of the desired product. Encouraged by these
results we focused our attention on less reactive aryl bromides and
chlorides. Electron donating groups such as methoxy and with-
drawing groups such as nitro were well tolerated under the present
catalytic system (entries 2–5). Sterically hindered bromo-
naphthalene and benzyl bromide react efficiently providing
81% and 60% yield of the desired product respectively (entries
6 and 7). Heterocyclic aryl halides like 2-bromopyridine were
also found to react smoothly under the present conditions
(entry 8). Aryl chlorides fail to carry out the reaction under
present conditions; this is due to high C–Cl bond strength and
their unwillingness to undergo oxidative addition.
In an attempt to improve reaction conversion of aryl
chlorides, a phase transfer agent like tetrabutylammonium
bromide (TBAB) was added, which enhanced the yield significantly
(entry 9) with prolonged reaction time. Presumably, here TBAB
plays a role of PTC and stabilizer for the nanocatalyst.20
Moreover, the catalyst efficiency of PdNPs was compared
with the present heterogeneous catalyst Pd/C under the
present reaction conditions. It was found that Pd/C gives
78% yield of the desired product as compared to the PdNPs
that give 97% yield. The enhancement of the yields may be due
to the excellent dispersion of PdNPs in water.21 To study a
greener and economical aspect of the present protocol,
catalytic activity was investigated for five consecutive recycles
for a model substrate iodobenzene and phenylboronic acid under
optimized reaction conditions. The PdNPs were effectively
recycled up to five cycles. The TEM image of the recycled PdNPs
shows that the catalyst was stable enough (Fig. S6, ESIw) and
could be used for the further consecutive recycles.
In summary, for the first time we have successfully demon-
strated a potential, controllable, one stroke electrochemical
method for synthesis of PdNPs using ionic liquid as an electrolyte
cum stabilizer. Furthermore, PdNPs were successfully applied as
catalysts in the Suzuki coupling reaction of several unactivated
aryl/heteroaryl halides in environmentally benign water as a
solvent with effective recyclability.
Experimental section
Materials and methods
All commercial reagents and solvents were used directly as
purchased without further purification. Palladium acetate was
purchased from Parekh Platinum Ltd (PPL) Mumbai, India,
in its highest purity available and was used as it is. Optimized
yields were based on GC (Perkin-Elmer, Clarus 400) analysis by
the external standard method. All the products are known, and
1
representative products were characterized by H NMR (Varian,
300 MHz) and 13C NMR (Varian, 75 MHz) spectroscopy. The
ionic liquid 1-butyl-3-methylimidazolium acetate [Bmim][OAc] was
prepared using the reported method available in the literature.22
Purity of the ionic liquids prepared was checked using 1H NMR,
13C NMR and IR analyses.
Table 1 Suzuki reaction of aryl/heteroaryl halide catalyzed by IL-
PdNPs in aqueous mediaa
Scanning electron microscopy (SEM) analysis (FEI, Quanta
200) was carried out to study the surface morphology. The
morphology of prepared nanoparticles (Phillips Model
CM 200, operating voltage 200 kV) was studied using a
transmission electron microscope (TEM). X-Ray diffraction
(XRD) measurements were carried out on a Rigaku X-ray
diffractometer with a Cu Ka (Ka = 1.5405 A) radiation
source operating at 40 kV. X-Ray photoelectron spectroscopic
(XPS) measurements were carried out on a VG Micro Tech
ESCA 3000 instrument at a pressure of 41 ꢁ 10ꢀ9 Torr (pass
energy of 50 eV, electron take off angle 601 and the overall
resolution was B0.1 eV).
Entry Aryl/heteroaryl halide (1a) Product (3a–i) Time/h Yieldb (%)
1
2
3
4
5
6
7
8
9
Iodobenzene
4-Iodoanisole
1-Iodo-4-nitrobenzene
4-Bromoanisole
3-Bromoacetophenone
1-Bromomethylbenzene
1-Bromonaphthalene
2-Bromopyridine
Chlorobenzene
3a
3b
3c
3d
3e
3f
3g
3h
3i
1
2
1
7
5
15
12
15
18
97, 78c
91
98
87
90
60
81
52
64d
Procedure for electrodeposition/synthesis of palladium
nanoparticles
a
Reaction conditions: 1a (1 mmol), 2a (1.2 mmol), KOH (2 mmol),
b
IL-PdNPs (0.001 mmol), water (2 mL), temp., 100 1C. Isolated
yields. Pd/C. Tetrabutyl ammonium bromide (TBAB) (1 mmol).
In a typical process, in a flat bottom beaker 50 mM solution of
palladium acetate in 1-butyl-3-methyl imidazolium acetate
c
d
c
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[Bmim][OAc] was prepared by dissolving the required quantity
of palladium acetate in [Bmim][OAc], which was then placed
in an electrolytic cell maintained at a temperature of 80 1C
under a nitrogen atmosphere. The electrolysis study was
conducted on an aluminium (1 cm ꢁ 1 cm) plate, with a
platinum rod acting as a counter electrode and a palladium
wire acting as a quasi-reference electrode. After electrolysis
electrodeposits on the aluminium plate were washed extensively
with acetone and de-ionized water before subjecting them to
surface morphology analysis. The mother electrolyte which
contains nanoparticles was first extensively washed with
de-ionized water and subjected to centrifugation to separate
out metal particles. It was observed that at higher negative
potential (ꢀ2.4 V) decomposition of ionic liquids leads to the
color change to dark brown.
dC (CDCl3, 75 MHz): 198.10, 141.73, 140.19, 137.65, 131.65,
129.07,128.95, 127.21, 26.79 ppm.
1,10-Methanediyldibenzene (Table 1, entry 6). dH (300 MHz,
CDCl3): 7.16–7.30 (10H, m), 3.97 (2H, s) ppm; dC (CDCl3, 75
MHz): 141.16, 128.99, 128.51, 126.12, 41.99 ppm.
1-Phenylnaphthalene (Table 1, entry 7). dH (300 MHz,
CDCl3): 7.80–7.90 (3H, m), 7.36–7.51 (9H, m) ppm; dC
(CDCl3, 75 MHz): 140.82, 140.32, 133.86, 131.68, 130.14,
128.81, 128.31, 127.70, 127.29, 126.99, 126.76, 126.08,
125.83, 125.44 ppm.
2-Phenylpyridine (Table 1, entry 8). dH (300 MHz, CDCl3):
8.66 (1H, d, J = 4.8 Hz), 7.95–7.99 (2H, m), 7.67 (2H, dd,
J = 0.9 Hz, 3.3 Hz), 7.37–7.48 (3H, m), 7.16 (1H, qr) ppm; dC
(CDCl3, 75 MHz): 157.06, 149.41, 139.07, 136.56, 128.76,
128.53, 126.70, 121.95, 120.29 ppm.
Experimental procedure for the Suzuki coupling reaction
A 10 mL tube was filled with aryl halide (1.0 mmol), phenylboronic
acid (1.2 mmol), KOH (2.0 mol), IL-PdNPs (0.001 mmol) in
water (3 mL) and was properly sealed. The reaction mixture
was heated at 100 1C for a desired time and was cooled to
room temperature on completion of the reaction. Aryl halide
conversion as well as product formation was monitored by gas
chromatography. The product was extracted in ethyl acetate
(3 ꢁ 5 mL) and evaporated on a rota vac. The residue
obtained was purified by column chromatography (silica gel,
60–120 mesh; PE–EtOAc, 95 : 05) to afford the desired
product. The structures of the obtained products were confirmed
by 1H NMR and 13C NMR analyses.
Acknowledgements
The author KMD would like to thank B. R. Sathe from Dr
B.A.M.U. Aurangabad, India, and D. G. Rathod from I.T.T.
Dublin, Ireland, for their help in XPS analysis and interpretation.
This work was supported by the Indira Gandhi Center For
Atomic Research (IGCAR), Kalpakkam, India.
Notes and references
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(300 MHz, CDCl3): 10.05 (1H, s), 7.45 (1H, s), 7.36 (1H, t,
J = 4.5 Hz), 4.29 (2H, t, J = 7.8 Hz), 4.05 (3H, s), 2.00
(3H, s), 1.86 (2H, m), 1.37 (2H, m), 0.95 (3H, t, J = 7.5 Hz)
ppm; dC (75 MHz, CDCl3) d: 175.29, 136.89, 123.48, 121.75,
49.29, 36.10, 31.75, 22.06, 19.07, 13.10 ppm; IR (neat): 3142,
2961, 2874, 1711, 1571, 1465, 1383 1248, 1172, 1008, 878 cmꢀ1
.
Biphenyl (Table 1, entry 1). dH (300 MHz, CDCl3): 7.58
(4H, t, J = 7.8 Hz), 7.43 (4H, t, J = 7.2 Hz), 7.35 (2H, d, J =
7.2 Hz) ppm; dC (CDCl3 75 MHz): 141.28, 128.82, 127.30,
127.22 ppm.
4-Methoxylbiphenyl (Table 1, entry 2). dH (300 MHz,
CDCl3): 7.51–7.56 (4H, m), 7.41 (2H, t, J = 7.2 Hz),
7.3 (1H, t, J = 7.2 Hz), 6.98 (2H, d, J = 8.7 Hz), 3.85
(3H, s) ppm; dC (CDCl3 75 MHz): 159.21, 140.89, 133.83,
128.79, 128.21, 126.79, 126.72, 114.30, 55.39 ppm.
4-Nitrobiphenyl (Table 1, entry 3). dH (300 MHz, CDCl3):
8.30 (2H, d, J = 9.0 Hz), 7.73 (2H, d, J = 8.4 Hz), 7.64 (2H, d,
J = 8.4 Hz), 7.42–7.53 (3H, m) ppm; dC (CDCl3 75 MHz):
147.56, 147.04, 138.69, 129.12, 128.89, 127.74, 127.34,
124.04 ppm.
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