Ionic liquid immobilized palladium nanoparticle-Graphene hybrid as active catalyst for heck reaction

Article abstract of DOI:10.2174/1570178611666141201223344

We engineered, graphene based Pd-Nano composites (Pd/RGO) and tested them to catalyze the Heck reaction in [bmim] NTf2 solvent system. High yield, easy product isolation, recycling of the catalytic system and ligand free approach are the main outcomes of this proposed protocol. The proposed protocol was further exploited for the successful synthesis of 3-styryl coumarins in good yield.

Full text of DOI:10.2174/1570178611666141201223344

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Letters in Organic Chemistry, 2015, 12, 67-72
67
Ionic Liquid Immobilized Palladium Nanoparticle - Graphene Hybrid as
Active Catalyst for Heck Reaction
Vivek Srivastava*
Basic Sciences: Chemistry, NIIT University, NH-8 Delhi-Jaipur Highway, Neemrana, Rajasthan, India
Received May 20, 2014: Revised July 20, 2014: Accepted November 28, 2014
Abstract: We engineered, graphene based Pd -Nano composites (Pd/RGO) and tested them to catalyze the Heck reaction
in [bmim] NTf solvent system. High yield, easy product isolation, recycling of the catalytic system and ligand free ap-
2
proach are the main outcomes of this proposed protocol. The proposed protocol was further exploited for the successful
synthesis of 3-styryl coumarins in good yield.
Keywords: Heck Reaction, Ionic liquid, Graphene, Palladium nano particle, Nano composites.
1
. INTRODUCTION
Graphene is the two-dimensional sheet of sp hybridized
morillonite clay, silica, zeolite etc.) to anchor the palladium
catalysts as well as substrates in order to improve the yield,
selectivity along with the recycling of valuable palladium
catalyst etc [10-13]. In recent reports, palladium nanoparti-
cales immobilized reduced graphene oxide (Pd/rGO) was
found nearer to other commercially available palladium cata-
lysts (e.g. Pd on charcoal) in terms of yield and selectivity,
but such Pd/rGO also undergoes with various shortcomings
like high catalyst loading, catalyst leaching (via agglomera-
tion of Pd metals into the clusters), limited substrate scope,
requirement of polar solvents, etc.
2
carbon allotrop, having extraordinary combination of ther-
moelectrical and mechanical properties [1-6]. The graphene
(
graphene oxide or reduced graphene oxide) has attracted a
great interest of chemical researchers for the development of
a new kind of composite materials, particularly as a host to
carry metal nanoparticles such as palladium, gold, platinum,
titanium, tin and zinc. [1,2,7,8]. These nanoparticals sup-
ported graphenes were further exploited as catalyst for dif-
ferent organic reactions like oxidation, reduction, dehydro-
genation, hydration, condensation, esterification, hydrogena-
tion and coupling reactions [1,7,9-15] in a facile, recyclable
and eco-friendly manner [1,2,7]. It was found that immobili-
zation as well as stabilization of metal nanoparticals on a
carbon substance is favorable for improving their catalytic
activity. The presence of functional groups and the highly
specific surface area of chemically modified graphenes
A number of reports and reviews have been published on
exploring the application of ionic liquids as a reaction me-
dium for different organic transformations [16,17]. Recently
graphene-supported Pt nanoparticles were immobilized into
the [MTBD][bmsi] ionic liquid to catalyze electrocatalytic
reaction with high catalytic activity [18]. Gao et al. also
submitted report on the synthesis with complete characteri-
zation of Ag-Pd bimetallic nanoparticles on reduced gra-
phene oxide (rGO), which were also used to catalyze Suzuki-
Miyaura as well as Sonogashira carbon coupling reaction.
This method offers simple reaction protocol along with the
added advantages of catalyst recycling [19].
(
CMGs) enhances their capability of loading metal nanopar-
ticles (MNPs) with the aid of hydrophobic and electrostatic
interactions. These graphene based MNPs serve excellent
stability against air and moisture, hence they offer easy han-
dling during the reaction.
In catalysis, palladium metal is considered as an impor-
tant metal catalyst to catalyze various organic transforma-
tions [9-11], specially C-C bond forming coupling reactions.
Palladium metal offers the most effective combinations of
activity and selectivity throughout the reaction. Various ap-
proaches have been made to modify the palladium catalyzed
coupling reactions like incorporation of different or-
ganic/inorganic bases, ligands, solvent systems, different
organic-inorganic supports (polymers, ionic liquids, Mont-
In the search of ligand free, recyclable, selective and sta-
ble catalytic system, we engineered the Pd/rGO composite
and tested as a catalyst for the Heck reaction in [bmim]NTf
ionic liquid medium.
2
2. RESULTS AND DISCUSSION
Typically, Pd/rGO was prepared by reducing the mixture
of graphene suspension (2.5mg/mL) and palladium acetate
(
10 mg) in water. Sodium dodecyl sulfate (SDS, 0.1 mol/L)
was used as a surfactant as well as reducing agent [2] during
the process. Size distribution of Pd nanoparticales further
analyzed by transmission electron microscope (TEM) and
the mean size of Pd nanoparticales dispersed on the graphene
*
Address correspondence to this author at the Basic Sciences: Chemistry,
NIIT University, NH-8 Delhi-Jaipur Highway, Neemrana, Rajasthan, India;
Tel: +91-1494302423;
E-mail: vivek.shrivastava@niituniversity.com
1
875-6255/15 $58.00+.00
© 2015 Bentham Science Publishers

6
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Letters in Organic Chemistry, 2015, Vol. 12, No. 1
Vivek Srivastava
O
O
I
Pd catalyst, Base
OEt
+
OEt
o
Solvent, 50-150 C, 30-120 min.
Yield =46-98%
Scheme 1. Reaction optimization for Heck Reaction.
sheets were found about 5 nm (average size) (Supporting
information Figure 1). The presence of a large number of Pd
nanoparticals on graphene was also confirmed by energy-
dispersive X-ray spectroscopy (EDX) (Supporting informa-
tion Figure 2). The oxygen and sulfur signal appear due the
presence of residual dodecanoate and sulfonate groups in
EDX data. The significant copper peaks also appeared in the
EDX data due to the presence of Copper grid.
from reaction mass along with the heavy loss of catalyst as
well as the reaction product.
Under conventional reaction conditions, homogeneous
Pd catalyst gets deteriorated and offers the deposition of Pd
black after the reaction. Recovery of the Pd catalyst is usu-
ally unrealistic, thus eliminating the possibility of recycling
the Pd catalyst. On the other hand, ionic liquid containing
Pd/rGO can be recycled as a catalyst itself, as shown in Fig-
ure 1. No significant loss in yield was found during catalyst
recycling experiments up to 10 runs but after extending the
The well characterized Pd/rGO was further tested for the
Heck reaction in [bmim]NTf ionic liquid medium. Here it
2
th
was expected that ionic liquids not only work as good sol-
vents for the Heck reaction (due to their physiochemical
properties), but their exceptional ionic environs may change
the progress of the reaction, activating and/or stabilizing
intermediates or transition states in the Heck reaction
mechanisms. As a result, while running the Heck reaction in
ionic liquids increases the reaction kinetics with respect to
conventional solvent systems. It is well documented that
onic liquids offer physiochemical stabilization of the metal
nanoparticales and also influences the regio- and stereoselec-
tivity of the Heck reaction products [20,21]. The Pd nanopar-
ticles before and after the coupling reaction were also studied
by TEM (Supporting information Figure 1). The presence of
Pd metal leaching in the reaction phase was confirmd with
the help of atomic absorption spectroscopy (AAS). The
results derived from above mentioned analytical studies,
indicate the proper dispersion of Pd/rGO species into the
recycling experiments up to 13 cycle, 5-7% drop in yield
was observed (Scheme 2). The drop in catalytic activity dur-
ing recycling experiments was mainly observed because of
agglomeration of rGO immobilized Pd nanoparticles, as the
particale size of Pd nanoparticales increased from 5 nm to 20
th
nm while performing the TEM analysis of Pd/rGO (13 run)
(Supporting information Figure 1b).
The optimized reaction conditions were further tested for
various combinations of aryl halides with different olifines
(
Table 2, Entry 1-15) [22]. The C-X bond strength affects the
activity towards the Heck reaction as the general cross cou-
pling reaction follows the C-I>C-Br reactivity order. The
yield of coupling reactions with different alkyl acrylates was
found to be more comparable to styrene. Boromobenzene
with electron withdrawing groups offered almost similar
activity with respect to iodobenzene towards Heck reaction
(
Table 2, Entry 5-12) while substituted bormobenzenes with
1
-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)
an electron donating groups gave the coupling product in
lower yield.
imide([bmim]NTf ) ionic liquid. Here, [bmim]NTf ionic
2
2
liquid works as a reservoir of catalytically active Pd/rGO
species. It was expected that Heck reaction probably pro-
ceeds via the oxidative addition of aryl halides on the surface
of Pd nanoparticles. After the reaction, the product was eas-
ily recovered by simple extraction with diethyl ether fol-
lowed by decantation of the upper organic layer. No aqueous
work-up and additives were required during the reaction.
Coumarins or benzopyranones, have attracted a great in-
terest of researchers due to their unique pharmacological
activities towards cancer, bacteria, etc [23-25]. We exploited
our proposed protocol for the synthesis of 3-styryl coumarins
using 3-bromo-6,7-dimethoxycoumarin with three different
derivatives of vinyl benzene (Table 2, Entry 13-15). The
overall yield for 3-styryl coumarin derivatives was isolated
in quantitative yield.
2
.2. Application of Pd/rGO Catalysts for Heck Reaction
Model test reaction was carried out with iodobenzene,
ethyl acrylate and potassium carbonate in different solvents
using Pd/rGO as catalyst (Scheme 1, Table 1, Entry 1-25).
3
. EXPERIMENTAL
All the chemicals were purchased from Sigma Aldrich,
Acros or Fluka. Nuclear Magnetic Resonance (NMR) spectra
were recorded on standard Bruker 300WB spectrometer with
an Avance console at 400 and 100 MHz for H and C NMR
respectively. The residue was purified by flash chromatogra-
The highest yield was obtained with Pd/rGO in [bmim]NTf
solvent system and because of that [bmim]NTf was used as
2
2
1
13
a reaction solvent to optimize rest of the reaction parameters
like time, temperature, catalyst loading, screening of bases
1
phy (FC) with hexane/ethyl acetate. The detailed H NMR
and ligands etc (Table 1, Entry 1-25). Pd/rGO/ [bmim]NTf
2
1
3
and C NMR of each eck reaction products were found
similar to the reported analytical data [25-27]. The purity of
reaction products (Table 2, Entry 1-15) was checked by gas
chromatographic analysis (Shimadzu GC-17A gas chro-
matograph with either a DB-Wax or a RTX-5 column).
system gave the best result in terms of yield, reaction time,
temperature and catalyst loading (Table 1, Entry 5). After
completion the reaction, the corresponding reaction product
was isolated by simple diethyl ether extraction method in
case of ionic liquid while in case of organic solvents, aque-
ous work-up was required to isolate the reaction product
1
-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)

Ionic Liquid Immobilized Palladium Nanoparticle
Letters in Organic Chemistry, 2015, Vol. 12, No. 1 69
Table 1. Catalyst optimization for Heck reaction.
b
Catalyst
Solvent
(2mL)
Temperature
Reaction Time
(Minutes)
a
c
Entry
Base/Ligands
Yield (%)
o
(
0.25 mol%)
( C)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
.
.
.
.
.
.
.
.
.
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
Pd/rGO
DMSO
DMF
THF
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
100
100
100
100
100
100
100
100
100
100
50
60
60
60
60
60
60
60
60
60
60
60
60
60
60
30
90
120
60
60
60
60
60
60
60
60
57
53
53
57
98
65
61
63
97
90
73
97
98
97
47
98
97
77
72
80
89
95
96
98
71
NMP
[bmim]NTf
2
i
Water: prOH
Water:EtOH
Water:MeOH
[bmim]NTf
2
2
(3mL)
(1mL)
0.
1.
2.
3.
4.
5.
6.
7.
8.
9.
0.
1.
2.
3.
4.
5.
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
2
2
2
150
120
100
100
100
100
100
100
100
100
100
100
100
100
2
[bmim]NTf + DMSO
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
[bmim]NTf
2
2
2
2
2
2
2
2
2
2
2
Pyridine
ET
Na CO
CO :PPh
3
N
2
3
K
2
3
3
PdCl
2
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
Pd (OAc)
2
3
3
3
Pd/rGO (4 mol%)
Pd/rGO (1 mol%)
(
(
(
a.) The reaction was carried out with iodobenzene (0.50 mmol), ethyl acrylate (1 mmol), base (1.5 mmol), PPh
b.) The water : organic solvent ration 1:1.
c.) Isolated yields after column chromatography.
3
as ligand (0.05 mmol) solvent (2 mL).
O
O
I
Pd/rGO, K CO
OEt
2
3
+
OEt
o
[
bmim]NTf , 100 C, 60 min.
2
Yield = 90-98%
Scheme 2. Optimized reaction condition for Heck reaction.

7
0
Letters in Organic Chemistry, 2015, Vol. 12, No. 1
Vivek Srivastava
2
Fig. (1). Recycling Results of Pd/rGO/[bmim]NTf for Heck reaction.
Table 2. Application of Pd/rGO catalysts for Heck reaction.
a
b
Entry
Aryl Halide
Olefin
Reaction Product
Yield (%)
O
I
O
1
O
98
92
O
O
I
I
I
O
2
3
4
O
O
73
90
O
O
O
O
O
O
Br
O
5
95
O
O
Br
O
O
O
6
7
92
65
O
O
O
O
O
Br
O
HO
HO

Ionic Liquid Immobilized Palladium Nanoparticle
Letters in Organic Chemistry, 2015, Vol. 12, No. 1 71
Table 2. contd….
a
b
Entry
Aryl Halide
Olefin
Reaction Product
Yield (%)
O
Br
O
8
O
58
O
O
Br
O
9
O
65
93
O
H N
2
H N
2
O
O
Br
O
1
0
O
O
O
N
N
N
N
O
1
1
1
2
78
71
O
Br
Br
S
O
O
S
O
O
Br
O
O
Br
O
1
3
O
63
O
O
O
O
O N
2
O
O
Br
O
NO2
1
4
O
O
67
O
O
O
O
O
O
Br
O
O
1
5
45
O
O
O
O
O
o
(
(
a.) The reaction was carried out with Pd/rGO (0.25mol%), haloaryl (0.50 mmol), olifin (1 mmol), K
b.) Isolated yields after column chromatography.
2 3 2
CO (1.5 mmol), [bmim] [NTf ] (2 mL) at 100 C for 60 minutes.
imide ([bmim]NTf
dure [28]. The Pd/rGO material was characterized by trans-
2
) was synthesized as per reported proce-
mission electron microscope (TEM) (Philips CM200) and
energy-dispersive X-ray spectroscopy (EDX) (PerkinElmer,

7
2
Letters in Organic Chemistry, 2015, Vol. 12, No. 1
Vivek Srivastava
PHI 1600 spectrometer). The palladium contents of the sam-
ples were determined quantitatively by atomic absorption
spectroscopy (AAS) on a HITACHI Z-2300 instrument.
ACKNOWLEDGEMENTS
Declared none.
SUPPLEMENTARY MATERIAL
3
.1. General Procedure for Heck Reaction
Supplementary material is available on the publishers
Web site along with the published article.
Add 50 mL round bottom flask with aryl halide (0.5
mmol), vinyl compound (1.5 mmol), catalyst in a solvent
system as per reported quantity in Table 1, Entry 1-25. Base
(
1.5 mmol) and phosphine ligand (0.05 mmol) were added as
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product was isolated with diethyl ether (5x2mL) washing.
Then the purification of reaction product was carried out
with flash chromatography (eluent: AcOEt: n-hexane=1: 3)
to achieve the pure Heck reaction product.
o
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CONCLUSION
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CONFLICT OF INTEREST
The authors confirm that this article content has no con-
flict of interest.