N-Heterocyclic Carbene–Palladium Complex onto Graphene Oxide and Poly (ethylene glycol) (PEG) Applied as Superior Catalyst for the Suzuki-Miyaura Cross-Coupling Reaction in Water

Article abstract of DOI:10.1002/aoc.5805

Using polymeric nanocomposites incorporated Pd to promote C-C coupling reactions has been found as one of the most successful strategies. In this paper we apply graphene oxide (GO) as an efficient surface immobilized by water-soluble poly (ethylene glycol)-imidazole followed by introduction of PdCl₂ salt to obtain the desired catalyst. Catalytic performance of this composite was investigated in the Suzuki–Miyaura cross-coupling reaction under mild reaction conditions and superior results were obtained. The hydrophilic nature of the catalyst and well distribution of Pd lead to superior catalytic activity in water media. Moreover, the Suzuki–Miyaura reaction proceed successfully with excellent yield and short reaction time without any loss of activity even after seven consecutive reaction cycles.

Full text of DOI:10.1002/aoc.5805

Received: 28 December 2019
DOI: 10.1002/aoc.5805
Revised: 10 April 2020
Accepted: 17 April 2020
F U L L P A P E R
N-Heterocyclic Carbene–Palladium Complex onto Graphene
Oxide and Poly (ethylene glycol) (PEG) Applied as Superior
Catalyst for the Suzuki-Miyaura Cross-Coupling Reaction in
Water
Majid M. Heravi
Elham Jaderi
| Shima Asadi | Seyede Mahdiye Hoseini Chopani |
Faculty of Chemistry and Physics,
Department of Chemistry, Alzahra
University, Vanak, Tehran, Iran
Using polymeric nanocomposites incorporated Pd to promote C-C coupling
reactions has been found as one of the most successful strategies. In this
paper we apply graphene oxide (GO) as an efficient surface immobilized by
Correspondence
Majid M. Heravi, Faculty of Chemistry
and Physics, Department of Chemistry,
Alzahra University, Vanak. Tehran, Iran.
Email: mmh1331@yahoo.com;
water-soluble poly (ethylene glycol)-imidazole followed by introduction of
PdCl salt to obtain the desired catalyst. Catalytic performance of this com-
2
posite was investigated in the Suzuki–Miyaura cross-coupling reaction under
mild reaction conditions and superior results were obtained. The hydrophilic
nature of the catalyst and well distribution of Pd lead to superior catalytic
activity in water media. Moreover, the Suzuki–Miyaura reaction proceed suc-
cessfully with excellent yield and short reaction time without any loss of
activity even after seven consecutive reaction cycles.
mmheravi@alzahra.ac.ir
Funding information
Iran National Science Foundation (INSF),
Grant/Award Number: 96006961
K E Y W O R D S
conducting polymer, graphene oxide, heterogeneous catalyst, N-heterocyclic carbene (NHC),
Suzuki-Miyaura reaction
1
| INTRODUCTION
scales owning to benign reaction condition, commercially
accessible of reagents, easy handling, good stability
Over the past two decades, Palladium-based catalysts
have been extensively employed in a cornerstone set
of cross-coupling reactions for the construction of
toward heat and moisture as well as non-toxicity of both
[4]
organoboron reagents and generated by-products. As
an added advantage, the biaryl derivatives are widely
found in the backbone of various natural products. They
are also considered as an important intermediate in the
production of pharmaceuticals, biochemical materials as
[1]
C–C bond. Additionally, it has considerably influenced
on synthetic organic chemistry ranging from industrial to
laboratory scales, notably those studying the total synthe-
sis of natural products with pharmaceutical and
[
5]
well as herbicides.
[2]
biochemical applications. After the successful introduc-
tion of Suzuki–Miyaura reaction by professor Akira
Suzuki in 1979, it has been regarded high admirable by
chemistry community which brought him the Noble
prize of chemistry in 2010 shared with Richard F. Heck,
Despite of high activity of Pd catalyzed Suzuki
reaction in homogenous system, it poses some severe
obstacles including high sensitivity toward moisture and
difficult isolation and recycling of metal catalysts from
the reaction mixture which frequently appear as contami-
[3]
[6]
Ei-ichi Negishi. Moreover, it has been broadly used in
different fields including both industrial and academia
nation in the final product. An alternative strategy to
tackle this issue is using heterogeneous system leading to
Appl Organomet Chem. 2020;e5805.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 13
https://doi.org/10.1002/aoc.5805

2
of 13
HERAVI ET AL.
the recovery of catalyst from the reaction medium and
consecutive recycling of the solid catalyst.
To obtain uniform and homogenous dispersion of
metal particles, different stabilizers, like polymers surfac-
tants, and dendrimers playing the role of capping agents
[7]
Synthesis of metal nanoparticles (MNPs) with
ultrafine size is in great demand because by decreasing
the size of noble metals, the accessible surface area and
abundance of the corner and edge atoms would
increase which lead to enhanced catalytic perfor-
[25]
are utilized in designing of new generation of catalyst.
Recently, the growing importance of conducting poly-
mers as stabilizer lies in the two main facts. First, coordi-
nation interaction between functional groups of the
polymer backbone and metal salt would efficiently
prevent the aggregation of metal particles. Second, the
conductivity of these types of polymers would facilitate
the charge transformations through catalytic pro-
[8]
mance. In spite of the key advantage of MNPs with
high specific surface area (SSA), they potentially tend to
aggregate during the catalytic cycle in order to reduce
[9]
total surface energy.
An innovative solution to
[
26]
optimize the catalytic activity and maximize SSA is
immobilizing of MNPs on a suitable support. In this
regard, various range of solid support such as metal
cesses.
Moreover, using soluble conducting polymers
in the template of insoluble graphene supported catalysts
serves the dual nature of hydrophilic and hydrophobic
which would perform powerfully in aqueous media.
Taking advantages of graphene and NHC properties and
also relying on our previous studies on heterogeneous
catalysis system, we have designed novel and efficient
GO-PEG-Imidazole-Pd (NHC) catalyst comprising of Pd
ions anchored on functionalized graphene oxide support
and evaluated its catalytic performance in Suzuki cross
coupling reaction. This synthesized catalytic system
performed successfully under eco-friendly and benign
reaction condition in extremely short reaction time
which could be recycled easily without remarkable
decrease of catalytic performance.
[10]
[11]
[12]
oxides,
polymers,
and carbon-based support
has been introduced in the area of heterogeneous catal-
ysis. Graphene as a kind of astonishing and ground-
breaking carbon-based material has profoundly impact
on research study, since its successful discovery and
[13]
isolation in 2004.
Due to high SSA, outstanding
physical and chemical properties including unique ther-
mal and electrical conductivity, special adsorption
capacity and high stability toward harsh reaction condi-
tion, graphene can be an attractive choice as catalyst
[14]
support.
Therefore, the experimental calculation dis-
closes that Pd particles can interact more strongly with
graphene substrate due to making more transmission
[
15]
channels and more interaction state between them.
Phosphine and nitrogen-based ligands are known as
2 | EXPERIMENTAL
2.1 | Characterization
[16]
the common ligands in Pd catalysis.
Though, phos-
phine based ligands are conventionally used in the fab-
rication of Pd catalysts, employing these ligands
encounters two major drawbacks. They are not only
toxic but also too sensitive and unstable toward air and
All chemical materials were purchased from international
commercial suppliers. The solvents were dried and purified
based on standard procedures. A Shimadzu model AA-680
flame atomic absorption spectrometer equipped with
deuterium background correction was used for the deter-
mination of Pd amount. Fourier transform infrared
(FT-IR) spectra were recorded on a BOMEM MB-series
FT-IR spectrometer. The crystalline phases of the Pd metal
particles were determined by X-ray powder diffraction
(XRD) measurements using Siemens D5000 diffractometer.
The Energy-dispersive X-ray spectroscopy (EDX)
(Philips XL-30) was used for elemental evaluation.
Thermogravimetric technique (TGA) was taken by a
thermal analyzer instrument (TGA/DTA BAHR: STA 503)
[17]
moisture which limit their usage.
On the other hand,
nitrogen-based ligands are mostly non-toxic, stable and
[18]
also abundant in nature.
In recent years, many
efforts have been made to develop some attractive
N-based
ligands
amines,
like
N-heterocyclic
carbenes
[19]
[20]
[21]
(NHCs),
oxime-based palladacycles,
[22]
and N4-Schiff-base ligand.
Introduction of first free
NHC has overwhelmingly impact on organometallic
chemistry due to their unique properties. The
NHCs perform as a strong electron σ-donor and poor
π-acceptor as well as phosphine which effectively
enhance the electron density at the center of Pd
and accelerate the oxidative addition step in Suzuki
ꢀ
ꢀ
from 25 to 1,000 C at a heating rate of 10 C/min under
air. Transmission electron microscope (TEM) image of
GO-PEG-Imidazole-Pd (NHC) composite was performed
using Philips CM-30 transmission electron microscope
with an acceleration voltage of 150 kV. Scanning
electron microscopy (SEM) was carried out using a
VEGA//TESCAN-LMU scanning electron microscope.
[23]
reaction.
More importantly, this phenomenon avoids
the decomposition of the active catalyst resulted in a
longer lifetime of the catalyst. Additionally, NHCs
generally benefit from higher dissociation energies in
contrast to other types of ligands which contribute to
[
24]
stability and insensitivity toward air and moisture.

HERAVI ET AL.
3 of 13
2
2
.2 | Preparation of the catalyst system
.2.1 | Synthesis of GO
(3.3 equiv.) and imidazole (2 equiv.) were added to resul-
tant mesylated product in DMF (10 ml) and stirred at
ambient temperature for 24 hr. When the reaction was
completed, the reaction mixture was filtered for the elim-
ination of excess solid material. To purify the product,
solvent was evaporated under vacuum followed by dis-
solving the residue in CH Cl (5 ml) and diethyl ether
(50 ml) which was stirred at 0 C for 30 min. The pre-
pared mixture was then filtered and washed several times
with diethyl ether (up to 100 ml) to obtain the white solid
of PEG-Imidazole as a soluble product.
Graphene oxide was initially synthesized from graphite
[27]
powder by adapting modified Hummers' method, based
on the previously reported method. In a typical procedure,
the graphite powder (5 g) was dispersed in deionized water
2
2
ꢀ
(100 ml) using an ultrasonic bath for 3 hr to obtain a sus-
pension. Then, concentrated H SO (25 ml), P O (5 g) and
K S O (5 g) were added to the suspension. This mixture
2
4
2 5
2
2 8
ꢀ
was subjected to magnetic stirring at 80 C for 80 hr. After
cooling the mixture (to room temperature), it was diluted
with distilled water. The filtrate was washed with distilled
water and then dried under room temperature. Pre-
oxidized graphite powder was added to 250 ml concen-
trated H SO , subsequently gradual addition of KMnO
2.2.4 | Synthesis of GO-PEG-imidazole
For the immobilization of PEG-Imidazole on the surface of
functionalized GO, first PEG-Imidazole (1.8 g) and NaI
(1.6 g) were added to the flask containing GO-CPTMS
(0.6 g) suspended in 50 ml of THF and the reaction mixture
was stirred under reflux reaction for 3 days. Finally, wash-
ing filtrate black precipitate and drying it under room tem-
perature afforded the desire GO-PEG-Imidazole support.
2
4
4
under magnetic stirring for 15 min. This solution was vig-
orously stirred for 4 hr at 35 C. The mixture was then
ꢀ
diluted with distilled water (500 ml) and stirred at room
temperature for 2 hr. H O 30% (40 mL) and distilled water
were added to the solution to obtain a golden color.
Finally, the prepared product was filtered and washed sev-
eral times with HCl (0.1 M) to remove residual metallic
ions and distilled water (250 ml) was added to reduce the
2
2
2.2.5 | Synthesis of GO-PEG-imidazole-Pd
(NHC)
ꢀ
acidity. The resulting brown suspension was dried at 70 C
in a vacuum oven.
For decoration of Pd metal particles, the mixture of
GO-PEG-Imidazole (1 g) and Pd (OAc) (0.056 g) was dis-
2
2.2.2 | Synthesis of GO-CPTMS
persed in DMSO (100 ml) using an ultrasonic bath. After
sonication for 30 min, the reaction was refluxed at 70 C
ꢀ
In order to functionalize the GO, it (250 mg) was firstly
dispersed in toluene (10 ml) using an ultrasonic bath for
for 20 hr. Ultimately, the resultant GO-PEG-Imidazole-
Pd (NHC) composite was obtained by filtration of the
reaction mixture and washing with distilled water,
ethanol and acetone to remove unanchored Pd metals
and finally dried at room temperature. The estimated
content of Pd ions was calculated 7.0 × 10− based on
atomic absorption spectroscopy (AAS).
1
hr. Then 12.5 mmol of CPTMS was introduced to the
suspension and the mixture was stirred at reflux condi-
tion under continuous purging of nitrogen atmosphere
for 24 hr. After 24 hr, the solution was cooled to room
temperature. The resultant GO-CPTMS solid was sepa-
rated by centrifuge and washed repeatedly with toluene,
ethanol and finally distilled water for removal of
5
ꢀ
unreacted CPTMS and then dried at 60 C for 2 hr.
2.3 | Investigations on the catalytic
activity
2
.2.3 | Synthesis of PEG-imidazole
2.3.1 | GO-PEG-imidazole-Pd (NHC)
catalyst in Suzuki–Miyaura reaction:
General procedure
A solution of PEG with molecular weight of 1,200
1 equiv.), methanesulfonyl chloride (1.2 equiv.) and
triethylamine (TEA, 5 ml) in CH Cl (15 ml) was stirred
(
In the typical experiment, the arylboronic acid (1.2 mmol),
aryl halide (1 mmol), K CO (2.92 mmol) and the prepared
2
2
ꢀ
at 40 C for 12 hr. When the mesylation reaction was
conducted, the solution was concentrated under reduced
pressure to remove excess TEA and solvent. To afford the
mesylated product, the concentrated mixture was dried
2
3
catalyst (2.8 × 10− mol% Pd based on aryl halide)
suspended in 5 ml water were added in the 10 ml flask.
The reaction mixture was magnetically stirred under reflux
4
ꢀ
ꢀ
in vacuum at 100 C for 2 hr. Subsequently, the Cs CO
condition at 80 C for suitable time. After the completion
2
3

4
of 13
HERAVI ET AL.
of the reaction, the reaction solution was cooled to the
room temperature. The reaction mixture was then diluted
with distilled water and the catalyst was isolated by filtra-
tion. Ethyl acetate was used for the extraction of organic
phase and dried with MgSO . The residue of MgSO was
ethanol and then reused in a model reaction of
bromobenzene and phenyl boronic acid under optimized
reaction condition.
4
4
separated from organic phase by filtration and then the
organic solvent was evaporated under reduced pressure.
The obtained residual was purified by column chromatog-
raphy on silica gel (n-hexane/ethyl acetate) to purify the
corresponding products. All products were confirmed and
3 | RESULTS AND DISCUSSION
It is found that multifunctional polymers can play two
roles of dispersing GO compound into aqueous media and
facilitating the interaction of its functional groups with
metal ions and metal nanoparticles in catalytic systems.
The synthetic route for the fabrication of GO-PEG-Imidaz-
ole-Pd (NHC) composite is illustrated in Scheme 1 which
began with the formation of GO from graphite powder via
1
identified by FT-IR and H NMR spectra.
2.3.2 | The reusability test
[
27]
modified Hummer's method.
In this respect, silanes
To examine the recyclability of synthesized catalyst, the
filtrated catalyst was washed with distilled water and
with various functionalities have been introduced for
tailoring and functionalizing on different surfaces.
SCHEME 1 The synthetic
route for fabrication of GO-PEG-
Imidazole-Pd (NHC) composite

HERAVI ET AL.
5 of 13
Moreover, silanes conveniently react with hydroxyl groups
that existed on the surface to construct a strong bound
connection. Among a wide variety of silane-based
materials, trimethoxysilane functionalized compounds are
widely used in the modification of the surface. As shown
in Scheme 1, first, the chloropropyltrimethoxysilane
(d) and GO-PEG-Imidazole-Pd (NHC) (e) compounds.
The broad band at 3421 cm− is attributed to the OH
stretching of GO (Figure 1a). Moreover, the absorption
spectra of C=O stretching vibration of carbonyl groups,
C=C stretching of aromatic rings and C-O stretching of
1
−
1
alkoxy are assigned at 1647, 1515 and 1,050 cm , respec-
tively. Also in the GO-CPTMS compound, the bands at
1924, 1086 and 778 cm− are due to the stretching vibra-
(
CPTMS) was condensed to the surface of GO which
enriched with hydroxyl groups to obtain GO-CPTMS (Step
) parallel with functionalization of terminal two-headed
1
1
tion of CH , Si-O-C and C-Cl respectively which approve
2
poly (ethylene)glycol (PEG) with imidazole (Step 2).
For the preparation of soluble PEG-Imidazole, first
the mesylation reaction was employed to convert termi-
nal hydroxyl groups of PEG to strong leaving groups
followed by reaction with commercially accessible imid-
azole to obtain desired polymeric network of PEG-Imid-
azole. In the following, the PEG-Imidazole was anchored
onto the GO surface. To be more specific, the proposed
mechanism for the reaction of PEG-Imidazole with
GO-CPTMS was shown in Scheme 2. This mechanism is
initiated by nucleophilic attack by N-3 of imidazole to
carbon attached Cl of GO-CPTMS which leads to the
formation of GO-Imidazole-PEG composite bearing two
N-heterocyclic carbene binding sites. After successful
anchoring of polymeric network on the GO surface, the
solid support was coordinated to Pd (ΙΙ) salt through both
carbenes to provide the desired catalyst.
the successful linkage of CPTMS to the surface of GO
(Figure 1b). In Im-PEG-Im compound, the spectra of C-H
sp and C-H sp were observed at 3139 and 2,887 cm
2
3
−1
(Figure 1c). Also, peaks at 1645, 1347, 1,115 and
961 cm− are ascribed to the stretching vibration C=N,
bending vibration C-H, stretching vibration of C-O of
ether and stretching vibration of C=C. The detected
1
3
bands around 3,420 (OH stretching), 2,921 (C-H sp ),
1,715 (C=O), 1,610 (C=N), 1,105 (C-O stretching) and
754 (C-H aromatic bending) indicate the successful
attachment of Im-PEG-Im to GO (Figure 1d). Decreasing
in relative intensity of 1,616 in contrast to GO-PEG-
Imidazole confirms the interaction of metal–ligand bond
(Figure 1e).
To investigate the surface morphology and particle
size of the catalyst, TEM analysis was carried out for as-
prepared catalyst. TEM images of GO-PEG-Imidazole-Pd
(NHC) are presented in Figure 2 and images from GO
sheets evident the flake-like shape of GO. Also, the high
transparency observed in Figure 2a, b displayed the
few-layer thickness of GO. As can be seen from
Figure 2c, d, the Pd particles were homogeneously
To confirm the as-synthesized GO-PEG-Imidazole-Pd
(NHC) structure, it was characterized by TEM, XRD,
TGA, SEM, EDX and infrared spectroscopy (FT-IR)
techniques. Figure 1 shows the FT-IR spectra of GO
(a), GO-CPTMS (b), Im-PEG-Im (c), GO-PEG-Imidazole
SCHEME 2 Proposed mechanism of reaction between PEG-Imidazole and GO-CPTMS

6
of 13
HERAVI ET AL.
total weight loss of nanocomposite was approximately
ꢀ
ꢀ
1
5% under 200 C and 30% under 350 C which high-
lights the acceptable thermal stability of the catalyst.
For investigation the morphology of the desired cata-
lyst, SEM technique was applied. SEM images of catalyst
exhibit the exfoliated structure of GO sheets. Moreover,
as shown in Figure 4, the uniform distribution of Pd ions
onto modified GO support are visible with the average
size of 10 nm. Furthermore, the presence of Pd content
and other elements including C, Si, N, and O was also
confirmed by EDX analysis (Figure 5). Additionally, any
impurities associated with solvents and other materials
used in catalyst fabrication were not detected by EDX
analysis.
The XRD technique was performed to determine the
crystalline structure of GO, GO-Imidazole-PEG and cata-
lyst. As can be observed in Figure 6a, b, the XRD patterns
of GO and GO-Imidazole-PEG show no specific crystal-
ꢀ
line structure. The broad and sharp peak at 2θ = 11.7
ꢀ
represents the interlayer distance of 7.6 A (002) of
ꢀ
ꢀ
GO. The diffraction peaks appeared at 2θ =42.39 , 48.44
ꢀ
and 63 are attributed to faced center cubic (fcc) of
crystalline Pd which correspond to (111), (200) and (220)
crystal planes of Pd (Figure 6c).
3
.1 | Catalytic performance of
GO-PEG-imidazole-Pd (NHC)
FIGURE 1 FT-IR spectra of GO (a), GO-CPTMS
b), Im-PEG-Im (c), GO-PEG-Imidazole (d) and GO-PEG-
(
In order to evaluate the catalytic performance of the
NHC-based catalyst, the Suzuki coupling reaction of
phenyl boronic acid and phenyl bromide was carried out
as a model reaction under green conditions. First, in
preliminary investigations, different amount of catalyst,
solvent, and base was employed to attain the optimized
reaction condition. As indicated in Table 1, in spite of
reducing the amount of catalyst, noticeably the GO-PEG-
Imidazole-Pd (NHC) composite still performed strongly,
and the coupling reaction of phenyl boronic acid and
phenyl bromide completed even in the presence of trace
amount of synthesized catalyst within very short reaction
time (5 min). As a result, the 2 mg of catalyst
(2.8 × 10− mol% Pd) was chosen as the optimum amount
of catalyst. After the examination of various solvents,
Imidazole-Pd (NHC) (e) compounds
dispersed on the surface of PEG-Imidazole modified GO
sheets with the diameter of 2–8 nm. Evidently, the Pd
particles are not appeared outside of the GO sheets. This
fact can suggest that the polymeric network helped to
control the distribution of Pd particles and strong interac-
tion between metal and polymeric network could prevent
aggregation of metals which approved the importance of
PEG-Imidazole in monodispersity of Pd.
The TGA analysis of GO-PEG-Imidazole-Pd (NHC)
catalyst was undertaken to determine the thermal stabil-
ity and portion of polymer deposited onto GO support.
As can be seen in Figure 3, the thermogram of the pre-
pared catalyst presented three main weight loss. The first
4
H O was selected as an environmentally benign solvent
2
ꢀ
decomposition was happened under 100 C indicating
with concerning for the green chemistry concept. More
precisely, the previous studies on the effect of water as
solvent suggest that it considerably accelerates the solu-
bility of the base which subsequently facilitates the tran-
smetalation step. However, water can easily solve boronic
acid compounds, the reactants with low polarity would
be unable to interact strongly which has detrimental
effects on the efficiency of the Suzuki-Miyaura reaction.
the corresponding loss of humidity and adsorbed water.
The second sharp weight loss observed between
ꢀ
1
50–300 C is assigned to the pyrolysis of oxygen-
containing functional groups of GO. Finally, the third
ꢀ
weight loss around 300–600 C is associated with the deg-
radation of carbon scaffold of GO as well as organic com-
pound anchored on the GO surface. Surprisingly, the

HERAVI ET AL.
7 of 13
FIGURE 2 TEM images of GO
(
a, b) and GO-PEG-Imidazole-Pd (NHC)
nanocomposite dispersed in water (c, d)
FIGURE 3 The representative
TGA diagram of (a) GO-PEG-Imidazole-
Pd (NHC)
The main disadvantage of water as a solvent is the poor
interaction of non-polar organic reactant with the active
site of catalyst. A practical way to cope with this obstacle
is designing the amphiphilic catalyst which in not only,
the polar part of water-soluble polymer interacts with
hydrophilic reactant including boronic acids and base,
but also the insoluble GO support interacts with hydro-
phobic aryl halides which enhance the Suzuki efficiency.
The best results in the reaction optimization were
achieved in the presence of water as a solvent, K CO as
2
3
ꢀ
base and 2.8 × 10− mol% of Pd as the catalyst at 80 C.
4
This reaction was also performed in the absence of the

8
of 13
HERAVI ET AL.
FIGURE 4 SEM images of GO-PEG-Imidazole-Pd (NHC) catalyst
FIGURE 5 EDX spectra of GO-PEG-
Imidazole-Pd (NHC) catalyst
resultant catalyst. As expected, no trace of the product
was formed showing the significance of metal-based cata-
lysts for the production of biaryls. Surprisingly, the reac-
tion proceeded in a clean condition and no side-products
related to homo-coupled and protodeboronation were
detected.
From an economic and industrial aspect, the condition
as mentioned above seems very attractive, that's why sol-
vent needless to pre-drying or purification. More impor-
tantly, the industrial scale of the Suzuki reaction
encounters some limitations due to the high cost of Pd
metal. In this regard, designing the corresponding catalyst
with low loading of Pd with high efficiency would be
appreciating to economize the expensive Pd metal for
Suzuki reaction which highlights the trace amount of Pd
content (2.8 × 10− mol% Pd) with excellent performance
applied in this protocol.
Encouraged by the excellent performance of our reac-
tion protocol, a wide variety of substituted aryl boronic
acids and aryl halides containing both electron-rich and
4

HERAVI ET AL.
9 of 13
FIGURE 6 XRD patterns of GO
(
a), GO-Imidazole-PEG (b) and
GO-PEG-Imidazole-Pd (NHC) (c)
nanocomposite
TABLE 1
Optimization of the reaction conditions for Suzuki reaction of bromobenzen with phenyl boronic acid catalyzed by
GO-PEG-Imidazole-Pd (NHC)
ꢀ
a
Entry
Amount of catalyst (mol%)
Base
T ( C)
Solvent
Time (hr)
Conversion (%)
1
2
3
4
5
6
7
8
9
-
Cs
Cs
Cs
Cs
Cs
Cs
Cs
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
80
80
80
80
80
80
80
80
80
80
80
80
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O:EtOH
O:EtOH
O:EtOH
O:EtOH
O:EtOH
O:EtOH
O
120
<5
<5
<5
<5
<5
5
-
4.2 × 10−
2
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
ꢁ99
2.24 × 10−
2.24 × 10−
5.6 × 10−
2.8 × 10−
2.8 × 10−
2.24 × 10−
2.24 × 10−
5.6 × 10−
2.8 × 10−
2.8 × 10−
2
3
4
4
4
2
3
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
O:EtOH
O:EtOH
O:EtOH
O:EtOH
O
<5
<5
5
4
1
1
1
0
1
2
4
5
4
5
Reaction conditions: phenyl boronic acid (1.2 mmol), bromobenzen (1 mmol), K
solvent (5 ml)
Calculated by TLC.
2 3
CO (2.92 mmol), GO-PEG-Imidazole-Pd (NHC) as catalyst,
a
electron-deficient was studied to investigate the scope
and limitation of synthesized catalyst. The results are
illustrated in Table 2. All products of corresponding aryl
bromides and iodides were obtained in excellent yield
reaction times (5–30 min), depending on the substrates
having either an electron donating or electron withdraw-
ing groups. Owing to the electronic effects of the substitu-
ent, the reactions of aryl halides substituted with electron
donating groups such as 4-iodoanisole and 4-iodotoluene
require longer time (Table 1, entries 4, 18) than electron
(76–100%) even when sterically hindered substrates were
tested and the reactions were completed in very short

10 of 13
HERAVI ET AL.
TABLE 2
Suzuki reaction of different aryl halides with aryl boronic acids catalyzed by GO-PEG- Imidazole-Pd (NHC)
a
−1 b
Entry
R
1
R
2
X
I
Time (min)
Yield (%)
100
95
TON/TOF (h )
1
2
3
4
5
6
7
8
9
H
H
15
15
30
30
45
5>
5
357/1428
339/1357
232/464
307/614
214/285
357/4284
350/4200
253/506
357/2142
296/1184
203/270
342/4104
317/1268
221/442
303/1212
271/465
167/83.5
325/650
346/1385
246/35
H
H
Br
Cl
I
H
H
65
4-Me
4-Me
H
H
86
H
Cl
I
60
4-Me
4-Me
4-Me
n-butyl
n-butyl
n-butyl
t-butyl
t-butyl
t-butyl
Ph-Ph
Ph-Ph
Ph-Ph
H
100
98
H
Br
Cl
I
H
30
10
15
45
5
71
H
100
83
10
11
12
13
14
15
16
17
18
19
20
21
22
H
Br
Cl
I
H
57
H
96
H
Br
Cl
I
15
30
15
35
120
30
15
420
360
120
89
H
62
H
85
H
Br
Cl
I
76
H
47
4-OMe
91
4-NO
4-NO
4-Me
4-NO
2
H
Br
Cl
I
97
2
H
69
4-Me
4-Me
35
125/20.83
239/119
2
I
67
Reaction conditions: aryl boronic acid (1.2 mmol), aryl halide (1 mmol), K
2
CO
3
(2.92 mmol), GO-PEG-Imidazole-Pd (NHC) as catalyst,
ꢀ
H
2
O (5 ml), 80 C
a
Isolated yield.
b
TOF was defined as mol product mol−1 Pd h−1
.
withdrawing group, conversely, aryl boronic acids con-
taining electron donating groups display higher reactivity
reaction times with high temperature, increasing catalyst
loading and using harmful solvents. Another noteworthy
feature of this synthesized catalytic system is high
efficiency in the treatment of aryl chloride with good
yields under the same reaction condition and in the
absence of hazardous solvents such as DMF or tetra butyl
ammonium bromide (TBAB).
(Table 1, entries 6–14).
More significantly, aryl chloride moieties are known
as appealing halides because of their availability and low
cost in a wide diversity. But their using in cross-coupling
reactions has some reactivity limitations due to their
stronger C–Cl bond and higher dissociation energy
(96 kcal/mol) in comparison with C–Br, and C–I. To be
more precise, aryl chlorides proceed slowly in the
3.2 | Leaching
oxidative addition step which is the rate determining step
[28]
in the reaction cycle.
Thus, the Suzuki reaction of
To determine the leaching of Pd ions and investigate if
Pd (II) was covalently bonded to the NHC modified GO
these inactivated compounds frequently needs longer

HERAVI ET AL.
11 of 13
surface, a hot filtration test and atomic absorption
spectroscopy (AAS) analysis was conducted. In this line,
catalytic performance in the Suzuki coupling reaction in
terms of utilizing green solvent, short reaction time, high
isolation yield and good recyclability with negligible Pd
leaching in contrast to previous reports.
a specific amount of catalyst dispersed in H O in the
2
ꢀ
presence of base was heated to 80 C and stirred for
1
5 min and then filter the hot mixture to obtain the
transparent solution. In the next step, the reactants of the
Suzuki reaction including phenyl boronic acid and bromo
benzene were introduced to the solution mentioned
3.4 | Recycles
above and the resulting mixture was heated again to
To investigate the reusability of catalyst, bromonitroben-
zene and phenyl boronic acid were utilized as model
substrates under optimized reaction condition because
ꢀ
8
0 C. The reaction development was monitored by TLC
and no product was detected after 2 hr, establishing the
heterogeneous nature of the prepared catalyst without
leaching of Pd into aqueous media. In addition, using
AAS of the same reaction solution after removing the cat-
alyst proved that no significant amounts of Pd are lost to
the reaction medium during the process.
3.3 | Comparison
Afterward, the catalytic performance of GO-PEG-Imidaz-
ole-Pd (NHC) was compared with some other Pd
supported catalysts reported in the literatures with
respect to yields, temperature, solvents and reaction time.
As shown in Table 3, the results disclosed that GO-PEG-
Imidazole-Pd (NHC) catalyst demonstrated unique
FIGURE 7 Reusability of GO-PEG-Imidazole-Pd (NHC)
nanocomposite catalyst in the Suzuki reaction of bromobenzene
with phenyl boronic acid
TABLE 3
Comparison the results obtained from coupling products using GO-PEG-Imidazole-Pd (NHC) catalyst with different type of
Pd-based catalysts selected by some previous literatures
Entry Catalyst (Pd loading mol%)
Reaction conditions
Yield (%) Time (hr) Refs.
ꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
Poly (NIPAM-co-4-VP)-Pd (1)
PNIPAM-HNT-Pd (1)
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
, H
2
2
O, 60 C
95
1
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[6]
ꢀ
, H
O, 70 C
97
1.5
ꢀ
G/MWCNTs/Pd (0.5)
, EtOH:H
, EtOH:H
, EtOH:H
, EtOH:H
2
2
2
2
O, 60 C
97
1
ꢀ
GO/Fe
3
O
4
/PAMPS/Pd (0.4)
O, 80 C
96
2
ꢀ
MWCNTs/Pd (0.5)
O, 60 C
99.2
94
3
ꢀ
Pd NPs-HNG
O, 60 C
2.5
ꢀ
GO-2 N-Pd (II) (0.5)
Poly (NIPAM-4-VP-AC)-Pd (0.05)
, EtOH, 80 C
82
4
ꢀ
, H
2
O, Ar atmosphere, 90 C 92
1
ꢀ
GO-NH
2
-Pd (II) (1)
, EtOH:H
, EtOH:H
2
2
O, 80 C
73
93
88
94
93
4
ꢀ
0
1
2
3
GO-NHC-Pd (II) (0.25)
GA-FSNP@Pd (0.28 mol%)
GO-NHC-Pd(1 mol%)
O, 60 C
3
ꢀ
Solvent-free, 90 C
0.5
[38]
[39]
This work
ꢀ
DMF:H
CO
2
O (1:1), 50 C
1
ꢀ
GO-PEG-Imidazole-Pd table 3(NHC)
K
2
3
, H
2
O, 80 C
5 min
4
(
2.8 × 10− mol%)

12 of 13
HERAVI ET AL.
FIGURE 8 TEM micrographs of the recycled catalyst after 7th run
the recycling of heterogeneous catalysts is rewarding in
practical applications. Then, the reaction mixture was fil-
tered off and the black solid of the catalyst was isolated
and washed with ethyl acetate. As can be observed in
Figure 7, the catalyst could be recycled for 8 runs without
a significant loss of activity. The minimal decline in cata-
lytic deficiency would be owing to the gradual physical
loss of the GO-PEG-Imidazole-Pd (NHC) composition
through filtration and washings between each cycle or on
account of the passivation of the catalyst surface. Accord-
ingly, the prominent efficiency, stability and reusability
of this catalyst reveal the presence of strong interaction
between Pd ions and NHC groups of functionalized GO
which increase the charge density over Pd and also
inhibits Pd leaching from the GO surface. The TEM
images (Figure 8) of GO-PEG-Imidazole-Pd (NHC)
recycled after 7th run shows the surfaces of GO are
covered with good dispersion of Pd particles with an
average size of ꢁ5 nm.
project no. 96006961 for granted Post-Doc position under
supervision of Prof. M.M. Heravi. We are also grateful to
Alzahra University for financial support.
ORCID
Majid M. Heravi https://orcid.org/0000-0003-2978-1157
REFERENCES
[1] a) T. J. Colacot, Platinum Met. Rev. 2011, 55, 84. b) M. Heravi,
E. Hashemi, N. Ghobadi, Curr. Org. Chem. 2013, 17, 2192.
[
2] a) J. Magano, J. R. Dunetz, Chem. Rev. 2011, 111, 2177.
b) J.-P. Corbet, G. Mignani, Chem. Rev. 2006, 106, 2651.
c) S. E. Hooshmand, B. Heidari, R. Sedghi, R. S. Varma, Green
Chem. 2019, 21, 381.
[3] a) N. Miyaura, K. Yamada, A. Suzuki, Tetrahedron Lett. 1979,
20, 3437. b) A. Suzuki, Angew. Chem. Int. Ed. 2011, 50, 6722.
[
4] a) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457. b) List of
contributors, in Advances in Metal-Organic Chemistry,
(Ed: L. S. Liebeskind) Vol. 6, JAI 1998 vii. c) A. Suzuki,
J. Organomet. Chem. 1999, 576, 147.
[
5] a) G. Bringmann, S. Rüdenauer, T. Bruhn, L. Benson,
R. Brun, Tetrahedron 2008, 64, 5563. b) J. Hassan,
M. Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev.
4
| CONCLUSION
2002, 102, 1359.
[6] N. Shang, S. Gao, C. Feng, H. Zhang, C. Wang, Z. Wang,
In conclusion, we have developed a highly active and
reusable polymeric Pd-NHC catalyst on the surface of GO
sheets. This heterogeneous composite showed superior
catalytic performance through Suzuki-Miyaura coupling
reaction of aryl halides with boronic acid in aqueous
media. Combination of hydrophilic character of PEG
with hydrophobic feature of GO led to powerful catalytic
activity in water. This work could open new gate on poly-
meric Pd-based catalysis.
RSC Adv. 2013, 3, 21863.
[7] B. Çelik, Y. Yıldız, H. Sert, E. Erken, Y. Ko s¸ kun, F. S¸ en,
RSC Adv. 2016, 6, 24097.
8] a) C. Bock, C. Paquet, M. Couillard, G. A. Botton, B. R.
MacDougall, J. Am. Chem. Soc. 2004, 126, 8028. b) H. Lee,
S. E. Habas, G. A. Somorjai, P. Yang, J. Am. Chem. Soc. 2008,
[
130, 5406.
[
9] a) R. J. White, R. Luque, V. L. Budarin, J. H. Clark,
D. J. Macquarrie, Chem. Soc. Rev. 2009, 38, 481. b) D. Astruc,
F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed. Engl. 2005, 44,
7852.
[
[
10] K. An, G. A. Somorjai, Catal. Lett. 2015, 145, 233.
11] R. Rajesh, E. Sujanthi, S. Senthil Kumar, R. Venkatesan, Phys.
Chem. Chem. Phys. 2015, 17, 11329.
ACKNOWLEDGEMENT
The financial support provided by the Iran National
Science Foundation (INSF) is gratefully acknowledged;

HERAVI ET AL.
13 of 13
[
12] S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia,
Coord. Chem. Rev. 2016, 312, 99.
13] Y. Gogotsi, Nat. Mater. 2015, 14, 1079.
[28] A. F. Littke, G. C. Fu, Angew. Chem. Int. Ed. 2002, 41, 4176.
[29] Y. Lee, M. C. Hong, H. Ahn, J. Yu, H. Rhee, J. Organomet.
Chem. 2014, 769, 80.
[30] M. C. Hong, H. Ahn, M. C. Choi, Y. Lee, J. Kim, H. Rhee,
Appl. Organomet. Chem. 2014, 28, 156.
[
[
14] J. Zhang, T. Yao, C. Guan, N. Zhang, X. Huang, T. Cui, J. Wu,
X. Zhang, J. Colloid Interface Sci. 2017, 491, 72.
[
15] I. Cabria, M. J. López, J. A. Alonso, J. A. Phys. Rev. B 2010, 81,
[31] H.-Q. Song, Q. Zhu, X.-J. Zheng, X.-G. Chen, J. Mater. Chem.
A 2015, 3, 10368.
[32] S. Asadi, R. Sedghi, M. M. Heravi, Catal. Lett. 2017, 147, 2045.
[33] R. Nie, J. Shi, W. Du, Z. Hou, Appl. Catal. A. Gen. 2014, 473, 1.
[34] S. K. Movahed, M. Dabiri, A. Bazgir, Appl. Catal. A 2014,
488, 265.
0
35403.
16] a) X. Jin, K. Zhang, J. Sun, J. Wang, Z. Dong, R. Li, Cat. Com.
012, 26, 199. b) P. D. Stevens, J. Fan, H. M. R. Gardimalla,
[
2
M. Yen, Y. Gao, Org. Lett. 2005, 7, 2085. c) B. Baruwati,
D. Guin, S. V. Manorama, Org. Lett. 2007, 9, 5377.
[
17] P. Das, U. Bora, A. Tairai, C. Sharma, Tetrahedron Lett. 2010,
[35] C. Bai, Q. Zhao, Y. Li, G. Zhang, F. Zhang, X. Fan, Catal. Lett.
2014, 144, 1617.
[36] Y. Zhang, J. Yang, X. Zhang, F. Bian, W. Yu, React. Funct.
Polym. 2012, 72, 233.
5
1, 1479.
18] J.-H. Kim, B.-H. Jun, J.-W. Byun, Y.-S. Lee, Tetrahedron Lett.
004, 45, 5827.
[
2
[
[
19] J.-H. Li, W.-J. Liu, Org. Lett. 2004, 6, 2809.
20] P. Das, C. Sarmah, A. Tairai, U. Bora, Appl. Organomet. Chem.
[37] N. Shang, C. Feng, H. Zhang, S. Gao, R. Tang, C. Wang,
Z. Wang, Cat. Com. 2013, 40, 111.
2
011, 25, 283.
[38] M. Tanhaei, A. Mahjoub, R. Nejat, Catal. Lett. 2018, 148, 1549.
[39] S. K. Movahed, R. Esmatpoursalmani, A. Bazgir, RSC Adv.
2014, 4, 14586.
[
[
[
[
[
21] W. Susanto, C.-Y. Chu, W. J. Ang, T.-C. Chou, L.-C. Lo,
Y. Lam, Green Chem. 2012, 14, 77.
22] B. Banik, A. Tairai, N. Shahnaz, P. Das, Tetrahedron Lett.
2
012, 53, 5627.
23] B. Zhang, J. Song, H. Liu, J. Shi, J. Ma, H. Fan, W. Wang,
P. Zhang, B. Han, Green Chem. 2014, 16, 1198.
24] L.-A. Schaper, S. J. Hock, W. A. Herrmann, F. E. Kühn,
Angew. Chem. Int. Ed. 2013, 52, 270.
How to cite this article: Heravi MM, Asadi S,
Hoseini Chopani SM, Jaderi E. N-Heterocyclic
Carbene–Palladium Complex onto Graphene
Oxide and Poly (ethylene glycol) (PEG) Applied as
Superior Catalyst for the Suzuki-Miyaura Cross-
Coupling Reaction in Water. Appl Organomet
Chem. 2020;e5805. https://doi.org/10.1002/aoc.
25] a) R. Narayanan, M. A. El-Sayed, J. Am. Chem. Soc. 2004, 126,
7
2
194. b) R. Narayanan, M. A. El-Sayed, Langmuir 2005, 21,
027.
[
[
26] a) M. Ma, Q. Zhang, D. Yin, J. Dou, H. Zhang, H. Xu, Cat.
Com. 2012, 17, 168. b) K. Liang, X. Li, S.-Z. Kang, L. Qin,
G. Li, J. Mu, Carbon 2014, 80, 716.
5805
27] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80,
1
339.