Pd-aminoclay nanocomposite as an efficient recyclable catalyst for hydrogenation and suzuki cross coupling reactions

Article abstract of DOI:10.1166/jnn.2012.6132

A highly water dispersible Pd-aminoclay nanocomposite is found to be effective catalytic system for the hydrogenation of α,β-unsaturated carbonyl compounds and Suzuki coupling reactions in aqueous media. The catalytic hydrogenation of α,β-unsaturated carbonyl compounds proceeds at room temperature to afford the corresponding products in excellent yields with high chemoselectivity. The cross coupling of aryl bromides and iodides with aryl boronic acids proceeds efficiently under aqueous conditions at 90 °C to afford the corresponding biaryls in excellent yields with high selectivity. The Suzuki reaction proceeds smoothly even in the absence of external base due to the basic nature of the catalyst support. The catalyst could be easily recovered and recycled three times without a significant loss of activity in hydrogenation and Suzuki cross coupling reactions. Copyright

Full text of DOI:10.1166/jnn.2012.6132

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 2000–2007, 2012
Pd-Aminoclay Nanocomposite as an Efficient
Recyclable Catalyst for Hydrogenation and
Suzuki Cross Coupling Reactions
A. Sravanth Kumar1, K. K. R. Datta2, T. Srinivasa Rao1, K. V. Raghavan3,
M. Eswaramoorthy2, and B. V. Subba Reddy1ꢀ∗
1Natural Product Chemistry, Indian Institute of Chemical Technology, Hyderabad 500067, India
2Nanomaterials and Catalysis Lab, Chemistry and Physics of Materials Unit,
Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O, Bangalore 560064, India
3Reaction Engineering Laboratory, Indian Institute of Chemical Technology, Hyderabad 500067, India
A highly water dispersible Pd-aminoclay nanocomposite is found to be effective catalytic system
for the hydrogenation of ꢀ,ꢁ-unsaturated carbonyl compounds and Suzuki coupling reactions in
aqueous media. The catalytic hydrogenation of ꢀ,ꢁ-unsaturated carbonyl compounds proceeds at
room temperature to afford the corresponding products in excellent yields with high chemoselec-
tivity. The cross coupling of aryl ꢀbromides and iodides with aryl boronic acids proceeds efficiently
under aqueous conditions at 90 C to afford the corresponding biaryls in excellent yields with high
selectivity. The Suzuki reaction proceeds smoothly even in the absence of external base due to
the basic nature of the catalyst support. The catalyst could be easily recovered and recycled three
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times without a significant loss of activity in hydrogenation and Suzuki cross coupling reactions.
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Copyright: American Scientific Publishers
Keywords: Hydrogenation, Suzuki-Coupling, Aminoclay, Pd Nanoparticles, Nanocomposites.
1. INTRODUCTION
In the recent years, there is a growing interest in
developing eco-friendly supports for dispersing metal
nanoparticles.5ꢀ24 Specifically, amine functionalized Mg-
phyllo(organo)silicate25 is an emerging class of lay-
ered material which has shown its remarkable capping
action towards metal nanoparticles,26–27 fluorescent dye
molecules,28 as a filler to obtain highly ductile polymer
hybrid films29 and as a hard template for the preparation
of flexible porous layered carbon.30 One important fea-
ture of aminoclay is its exfoliation in water by the pro-
tonation of amine groups which can be restacked by the
addition of ethanol or less polar solvent.25–27 Also metal
nanoparticles can be synthesized by in-situ approach to
obtain metal nanoparticle-clay hybrids.26–27
In addition, performing organic transformations in water
contributes largely to the development of sustainable
chemistry because of its unique properties such as environ-
mental acceptability, abundance and cost effectiveness.31
It also exhibits unique reactivity and selectivity that can-
not be attained in conventional organic solvents.32 Herein,
we explore for the first time the catalytic activity of water
dispersible Pd-aminoclay nanocomposite for the hydro-
genation of ꢁ,ꢂ-unsaturated carbonyl compounds under
aerobic conditions. Furthermore, the catalyst can be recy-
cled for three times with minimal loss of activity.
The intrinsic chemical and physical properties that are
associated with multifunctional nanomaterials such as
metal/semiconducting nanoparticles and nanoporous mate-
rials make them potential candidates in the emerging
areas of catalysis, electronics, optics, sensors, storage and
biology.1–14 Increased surface to volume ratio and high
surface energies of the metal nanoparticles are impor-
tant assets that make them very useful in the field of
catalysis.15–16 The important prerequisite for noble-metal
nanoparticle catalysis is that they should be finely dis-
persed in preformed supports.15–16 Metal nanoparticles
often tend to agglomerate after every catalytic cycle which
dramatically reduces the reactivity, thereby reducing the
efficiency and its reusability.16 A major challenge in the
field of catalysis employing metal nanoparticles is to over-
come the agglomeration17 of the particles so as to improve
the recyclability of the catalyst.18–19 Recent methods that
are currently being used to minimize the agglomeration are
dispersing pre-synthesized nanoparticles onto high surface
area supports and encapsulating nanoparticles in hollow
interiors of inorganic shells.20–23
∗Author to whom correspondence should be addressed.
2000
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 3
1533-4880/2012/12/2000/008
doi:10.1166/jnn.2012.6132

Kumar et al.
Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
The catalytic hydrogenation is a useful method for
the reduction of various functional groups in particu-
lar carbon–carbon multiple bonds in both academia and
industry.33 In particular, the chemoselective hydrogenation
of ꢁ,ꢂ-unsaturated carbonyl compounds is of prime impor-
tance because of its innumerable applications in the syn-
thesis of drugs and fine chemicals.33 Although Pd/C is
known as the most universal catalyst for hydrogenation, it
shows poor selectivity in many cases.33 The most widely
used reducing agents are metal hydrides,34 which normally
reduces both carbonyl and olefinic bonds of conjugated
enones.35 The conjugate reduction of enones has been
achieved by means of transfer hydrogenation using irid-
ium complexes,36 rhodium,37 ruthenium,38 as catalysts and
phosphine, carbene, phebox (phenyl-bis(2-oxazolinyl)),
and bipyridine as ligands.36–38 However, the development
of simple, convenient and efficient method using envi-
ronmentally benign catalysts39 would expand the scope
of catalytic hydrogenation. We herein report the catalytic
activity of Pd-aminoclay nanocomposite for the hydro-
genation of ꢁ,ꢂ-unsaturated carbonyl compounds and for
carbon-carbon bond formation by Suzuki coupling under
aqueous medium. Though Pd nanoparticles prepared in
aminoclay are used for Suzuki reaction,40 the scope of
the basic support (aminoclay) has not been fully explored.
On a fancier note, we have accomplished the Suzuki cou-
pling reaction without using any external base exploiting
Scheme 1. Flowchart showing the in-situ synthesis of Pd-aminoclay
nanocomposite.
of MgCl2 · 6H2O was dissolved in 20 mL Millipore
water under constant stirring followed by the addition of
73 mg of PdCl2 under acidic condition. To the obtained
brown colored transparent solution, 2 mL of 3-amino-
propyltrimethoxysilane was added drop wise. The brown
slurry obtained was stiꢀrred overnight at room temperature
and then kept at 150 C for 24 h. It slowly turned dark
brown powder in colour due to the formation of palladium
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the basic nature of the Pd-aminoclay nanocomposite.
nanoparticles by thermal reduction without the addition of
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Copyright: American Scientific Publishers
external reducing agent. A flowchart depicting the synthe-
sis of Pd nanoparticles is given in Scheme 1.
2. EXPERIMENTAL DETAILS
2.1. Materials
2.3. Hydrogenation of Olefins
All of the reagents and solvents are commercially available
and are used as purchased, without any further purification.
Compounds were purified on axially compressed columns,
packed with Silica, and eluting with n-hexane/AcOEt mix-
tures. H1 NMR and C13 NMR were recorded on Brüker
Avance 200 or 300 MHz, in CDCl3 using TMS as the
internal standard, chemical shifts were reported in parts per
million (ppm) downfield from the tetramethylsilane. Mass
spectra were determined with a QP2010 Gas Chromato-
graph Mass spectrometer (EI ion source). UV-Vis absorp-
tion spectra were recorded between 200–900 nm with
Perkin Elmer Lambda 900 spectrophotometer. Transmis-
sion electron microscope (TEM) (Philips Tecnai G2 FEI
F12, operating at 80–100 kV) was used to investigate mor-
phology and size of the particles. TEM specimens were
prepared by drop casting one or two drops of aqueous solu-
tion onto carbon coated copper grids, which were allowed
to dry at room temperature overnight.
Pd-aminoclay nanocomposite and olefin (1 mmol) were
mixed in water (5 mL). The resulting mixture was stirred
at room temperature under hydrogen atmosphere. Upon
completion of the reaction based on TLC, the mixture
was extracted with ethyl acetate. Removal of the solvent
followed by purification on silica gel column chromatog-
raphy gave the pure saturated product.
2.4. Suzuki Coupling Reaction with Pd-Aminoclay
Nanocomposite
Aryl halide (1 mmol), boronic acid (1.1 mmol), and
K2CO3 (2 mmol), were added to 3 mL of water containing
Pd-aminoclay nanocomposite. The reaction mixture was
ꢀ
then stirred at 90 C for 7 to 12 h and then cooled to
room temperature (see Table I). After cooling the reac-
tion mixture to room temperature, the reaction mixture was
extracted twice with ethyl acetate (10.0 mL) and washed
with water (10 mL). Next the organic phase was dried
over Na2SO4, filtered, and concentrated in vacuum and the
residue was purified by chromatography to afford the cor-
responding pure biaryl derivative.
2.2. In-Situ Synthesis of Pd-Aminoclay Nanocomposite
In-situ synthesis of the clay stabilized Pd nanoparti-
cles was carried out by the following procedure. 1.68 g
J. Nanosci. Nanotechnol. 12, 2000–2007, 2012
2001

Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
Kumar et al.
Table I. Pd-aminoclay catalyzed hydrogenation of conjugated olefins.
Entry
Time (h)
2.0
Arylboronic acid
O
Product (2)a
Yield (%)b
O
Ph
Ph
a
b
c
97
96
O
O
O
O
Me
Me
Ph
2.0
Ph
Ph
2.0
94
Me
Me
O
O
O
Ph
d
e
f
2.0
2.0
97
95
MeO
MeO
O
O
Me
Ph
Me
Ph
MeO
Cl
MeO
Cl
O
O
2.0
2.0
96
93
O
g
h
i
OMe
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O
O
Copyright: American Scientific Publishers
3.0
91
95
O
O
3.0
MeO
OMe
MeO
OMe
O
O
j
4.0
92
H
H
O
O
k
l
4.0
3.0
90
95
aAll products were characterized by 1H NMR, IR and mass spectrometry. bYield refers to pure products after chromatography.
3. RESULTS AND DISCUSSION
colour. The absence of absorption band for the precursor
around 280 nm confirms the complete reduction of Pd ions
and the formation of Pd nanoparticles (Fig. 1).26
Accordingly, we have attempted the reduction of vari-
ous ꢁ,ꢂ-unsaturated carbonyl compounds in the presence
of molecular hydrogen employing Pd-aminoclay nanocom-
posite (10 mg of catalyst, 0.3 wt% of Pd for 1 mmol
The palladium aminoclay nanocomposite was prepared
by using our previously reported method.26 The detailed
synthetic procedure is given in Experimental Section
(Scheme 1). Thus prepared Pd-aminoclay nanocomposite
can be easily water dispersible and shows dark brown
2002
J. Nanosci. Nanotechnol. 12, 2000–2007, 2012

Kumar et al.
Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
OH
OH
OH
OH
Honokiol
Magnolol
OH
OH
OH
OH
OH
OH
O
OH
OH
OH
OH
Isoperrottin A
OH
Isoplagiochin D
Riccardin
200
300
400
500
600
700
800
Fig. 2. Examples of biaryl containing natural products.
Wavelength (nm)
biaryls can be prepared by Ullmann reaction.42 The clas-
sical Ullmann reaction normally requires high tempera-
ture and stoichiometric amounts of Cu(0) or Cu(I) salts.43
Subsequently, the Suzuki coupling reaction has become
popular method for the synthesis of both symmetric and
unsymmetric biaryls which works under extremely mild
conditions.44 The Suzuki coupling typically requires palla-
dium phosphine complexes such as Pd(PPh3ꢃ4, Pd2(dba)2,
Pd(tBu3P)4 and its analogues.45–46 Most of these palla-
dium complexes are toxic, air sensitive and are homo-
Fig. 1. UV-Vis absorption spectrum of Pd-aminoclay nanocomposite.
of olefin). Initially, phenylstyryl ketone was subjected to
catalytic hydrogenation to give the corresponding 1,3-
diphenylpropan-1-one (Scheme 2, Entry a, Table I).
The hydrogenation of conjugated olefin proceeds well
in water at room temperature without affecting other func-
tionalities such as aldehydes and ketones or halides in
the presence of catalyst to afford the saturated ketones in
excellent yields and the results are presented in Table I.
No dehalogenation was observed in the case of reduction
geneous in nature. Moreover, these phosphine complexes
cannot be recycled for further reactions. Subsequently,
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of halogen containing substrate (Entry f , Table I). In case
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Copyright: American Scientific Publishers
of bisbenzylidene ketones, both double bonds underwent
non-phosphine palladium complexes such as heterocyclic
carbenes, oxime palladacycles, imine and amine pallada-
cycles, diazabutadienes and 2-aryl-2-oxazolines have been
developed as catalysts for the Suzuki coupling reaction.44
However, these Pd complexes are expensive which limit
their usage in large scale production of biaryls.
smooth hydrogenation under identical conditions (Entries
h and i, Table I). Notably, acid sensitive cinnamaldehyde
also underwent smooth reduction of double bond without
affecting aldehyde functionality (Entry j, Table I). This
method works not only with activated olefins but also with
styrene derivative (Entry l, Table I).
Recyclability of the catalyst was tested in cat-
alytic hydrogenation of phenylstyryl ketone to pro-
duce 1,3-diphenylpropan-1-one. The recovered catalyst
could efficiently be recycled three times without any
considerable loss of activity. For instance, hydrogena-
tion of phenylstyryl ketone in the presence of Pd
aminoclay-nanocomposite gave the corresponding 1,3-
diphenyl ketone in 97, 95 and 92% yields over three
cycles. Encouraged by the results obtained in hydrogena-
tion of olefins, we further evaluated the catalytic perfor-
mance of Pd nanoparticles in Suzuki coupling reaction.
Biaryls are frequently found in many biologically
important molecules (Fig. 2).41 Generally, symmetrical
Initially, we evaluated the catalytic properties of water
dispersible Pd-aminoclay nanocomposite in the cross cou-
pling of 4-bromoanisole with phenyl boronic acid (1 mmol
ꢀ
of each). The reaction was performed in water at 55 C
for 10 h using 10 mg of the catalyst in the presence
of K2CO3. The corresponding 4-methoxybiphenyl 5a was
formed only with 50 % yield. By increasing the reaction
temperature up to 90 ꢀC with same loading of catalyst, the
desired product 5a was obtained in 95% yield (Scheme 3).
The reactions are clean, high yielding and no side
products were obtained. Moreover, the catalyst is air
stable. Encouraged by the above result, we examined the
scope of this methodology to various aryl halides and aryl
OMe
O
O
Br Pd-aminoclay, K2CO3
H2O, 90 ºC
B(OH)2
Pd-aminoclay
+
MeO
Ph
Ph
H2O, H2, 25 ºC
3
4
5a
2a
1
Scheme 3. Coupling reaction of phenyl boronic acid with
Scheme 2. Catalytic hydrogenation of phenylstyryl ketone.
J. Nanosci. Nanotechnol. 12, 2000–2007, 2012
4-bromoanisole.
2003

Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
Kumar et al.
boronic acids. Initially, we attempted the cross coupling of
aryl boronic acids with aryl iodides such as iodobenzene,
1,3-diiodobenzene, 4-iodophenol, 1-iodonaphthalene,
4-iodonitrobenzene, 2-amino-5-iodotoluene, 4-iodoanisole
and 4-iodotoluene. These iodo derivatives participated
well in the cross coupling reaction. In the case of
1,3-diiodobenzene, 1,3-bis-arylated product was obtained
(Table II, entry d). It is noteworthy to mention that
Table II. Pd-aminoclay catalyzed Suzuki coupling for the synthesis of biaryls.
a
Yield (%)b
Time (h)
Entry
Arylboronic acid
Iodoarene
5
Product ( )
OMe
OMe
B(OH)2
a
b
8.0
8.0
95
92
I
B(OH)2
B(OH)2
I
Me
Me
c
d
e
89
9.0
Br
B(OH)2
Ph
11.0
10.0
90
88
I
I
NH2
NH2
B(OH)2
B(OH)2
Br
CHO
CHO
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10.0
85
90
f
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Br
Copyright: American Scientific Publishers
OH
B(OH)2
OH
10.0
8.0
g
I
Ph
I
B(OH)2
h
91
NO2
NO2
i
j
8.0
8.0
95
92
B(OH)2
B(OH)2
I
NH2
Me
NH2
Me
I
I
OMe
Me
OMe
B(OH)2
B(OH)2
8.5
90
k
Me
l
8.0
8.5
92
85
Br
NO2
m
NO2
B(OH)2
Br
aAll products were characterized by 1H NMR, IR and mass spectrometry. bYield refers to pure products after chromatography.
O2N
O2N
2004
J. Nanosci. Nanotechnol. 12, 2000–2007, 2012

Kumar et al.
Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
even amino or hydroxyl substituted aryl iodides gave the
desired biaryls in excellent yields without the formation of
diaryl amine or biaryl ether respectively. It is known that
aryl amines or phenols undergo smooth cross coupling
with aryl boronic acids in the presence of Cu(OAc)2 to
furnish diaryl amines or biaryl ethers respectively which
are not observed under present reaction conditions.
of 4-nitrophenylboronic acid with 4-bromonitrobenzene
gave the respective product 2m in 85% yield under identi-
cal conditions. The high catalytic activity of the catalyst is
attributed to the uniform size and high particle dispersity
of the palladium nanoparticles which are finely encapsu-
lated in the clay matrix.
Eventually, the recycling of the Pd-aminoclay nanocom-
posite was evaluated in the coupling of 4-bromo-
benzaldehyde with phenylboronic acid (Table II, entry f )
in water under optimized conditions. The product was sep-
arated by simple extraction with ethyl acetate. The residual
Pd nanocomposite in aqueous solution was reused for three
cycles. The desired product 2f was isolated in 94, 87, 80,
73, 63% yields over five cycles.
The decrease in the yield was attributed to the leach-
ing of palladium nanoparticles, which is further supported
by transmission electron microscope (TEM) studies. TEM
image (Fig. 3(a)) of the Pd-aminoclay nanocomposite
clearly shows that the Pd nanoparticles (black spherical
dots) are uniformly embedded over the clay surface. The
histogram shows a large number of Pd nanoparticles are in
the size range between 3.5 and 5.3 nm (Fig. 3(b)). TEM
image of the Pd nanoparticles-aminoclay after carrying
out two-cycles of reaction did not show any aggregation
of nanoparticles usually observed over the spent catalyst.
However, there is a reduction in the density of the Pd
47
Next, we investigated the reactivity of aryl bro-
mides such as 4-bromotoluene, 4-bromobenzaldehyde,
4-bromoaniline and 4-bromonitrobenzene. Interestingly,
these aryl bromides reacted well under similar condi-
tions to afford the corresponding biaryls in excellent
yields. However, no significant electronic or steric effects
either on reaction rates or yields were observed. Both
electron-rich as well as electron-deficient aryl halides
gave the products in comparable yields. Notably, steri-
cally hindered either 1-iodonaphthalene (Table II, entry h)
or 2-naphthylboronic acid also gave the products in
excellent yields. In general, electron rich substrates like
4-iodoanisole (Table II, entry k), 4-bromoaniline (Table II,
entry e), took long reaction time (9 h) to achieve
good conversions. Next, we tested functional group tol-
erance under the reaction conditions. Interestingly, the
reaction conditions are compatible with various func-
tionalities such as amine, hydroxyl, aldehyde and nitro
groups. Also, we tested the catalytic performance of
palladium nanocomposite with nitro substituted phenyl
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boronic acid (Table II, entry m). For instance, the coupling
nanoparticles observed on the clay surface probably, due to
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Copyright: American Scientific Publishers
160
(b)
(a)
120
80
40
0
50 nm
2
4
6
8
10
Particle size (nm)
(c)
(d)
20
15
10
5
0
50 nm
2
4
6
8
10
Particle size (nm)
Fig. 3. TEM images of (a) as prepared Pd-aminoclay nanocomposite, (b) corresponding histogram showing Pd nanoparticle size distribution, (c) Pd-
aminoclay nanocomposite after two catalytic cycles, (d) corresponding histogram showing Pd nanoparticle size distribution.
J. Nanosci. Nanotechnol. 12, 2000–2007, 2012
2005

Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
Kumar et al.
Table III. Pd-aminoclay catalyzed Suzuki coupling for the synthesis of biaryls in the absence of base.
Product (5)a
Yield (%)b
Entry
Iodoarene
Time (h)
Arylboronic acid
OMe
OMe
B(OH)2
8.0
8.0
a
b
90
92
Br
I
B(OH)2
B(OH)2
Me
Me
c
87
82
9.0
10.0
10.0
Br
CHO
CHO
B(OH)2
B(OH)2
d
Br
OH
OH
85
90
e
f
I
NO2
NO2
8.0
B(OH)2
I
aAll products were characterized by 1H NMR, IR and mass spectrometry. bYield refers to pure products after chromatography.
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leaching of Pd nanoparticles under the reaction conditions
(Fig. 3(c and d)).
thank Dr. B. Sreedhar for his support. The authors thank
Professor C. N. R. Rao, FRS for his kind support and
encouragement.
Copyright: American Scientific Publishers
Finally, we attempted the Suzuki cross coupling in the
absence of K2CO3 with a slight increase in the amount of
the catalyst (up to 15 mg). To our surprise, the reaction
went well even without base in excellent yields and the
results are presented in Table III. In this case aminoclay
provides the basic nature to the reaction mixture.
References and Notes
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4. CONCLUSIONS
In conclusion, we have successfully demonstrated the use
of Pd nanoparticles-aminoclay nanocomposite for conju-
gate reduction of olefins in the presence of other reducible
functionalities, such as aromatic carbonyls and halides.
Also, the catalyst exhibits high catalytic activity for both
Suzuki reaction and hydrogenation in aqueous media. This
method does not require any co-solvent along with water.
In addition, the catalyst functions not only as a source
of palladium but also acts as a base. The catalyst can be
recycled for more than three times with minimal loss of
activity in Suzuki cross coupling. No precautions needed
to be taken to exclude air and the reactions proceeds well
under aerobic conditions. Further studies of the scope of
application of aminoclay composite with other noble met-
als are under investigation in our laboratory.
Acknowledgments: ASK and TSR thank CSIR, New
Delhi for the award of fellowships. ASK and BVSR
2006
J. Nanosci. Nanotechnol. 12, 2000–2007, 2012

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Pd-Aminoclay Nanocomposite for Hydrogenation and Suzuki Coupling Reactions
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Received: 20 September 2011. Accepted: 14 October 2011.
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