NHC-Pd complex heterogenized on graphene oxide for cross-coupling reactions and supercapacitor applications

Article abstract of DOI:10.1002/aoc.5924

N-heterocyclic carbene (NHC)-palladium(II) complex (GO@NHC-Pd) was synthesized on graphene oxide (GO) support via a simple and cost-effective multistep approach. The spectroscopic, microscopic, thermal, and surface analyses of GO@NHC-Pd confirmed the successful formation of the catalyst. The investigation of catalytic activity showed that GO@NHC-Pd was very effective in Suzuki–Miyaura as well as Hiyama cross-coupling. Being heterogeneous in nature, GO@NHC-Pd was recovered after each reaction cycle easily and reused for up to nine and six cycles in Suzuki–Miyaura and Hiyama cross-coupling, respectively, without significant loss of activity. Further exploration of the supercapacitor performance of GO@NHC-Pd catalyst assembled in a two-electrode cell configuration shown a maximum attained capacitance of 105.26 F/g at a current density of 0.1 A/g with good cycling stability of 96.89% over 2,500 cycles.

Full text of DOI:10.1002/aoc.5924

Received: 30 April 2020
DOI: 10.1002/aoc.5924
Revised: 9 June 2020
Accepted: 17 June 2020
F U L L P A P E R
NHC-Pd complex heterogenized on graphene oxide for
cross-coupling reactions and supercapacitor applications
1
1
1
1
Vishal Kandathil | Aisha Siddiqa | Abhinandan Patra | Bhakti Kulkarni
|
1
2
3
Manjunatha Kempasiddaiah | B.S. Sasidhar | Shivaputra A. Patil
|
1
1
Chandra Sekhar Rout | Siddappa A. Patil
1
Centre for Nano and Material Sciences,
N-heterocyclic carbene (NHC)-palladium(II) complex (GO@NHC-Pd) was syn-
thesized on graphene oxide (GO) support via a simple and cost-effective multi-
step approach. The spectroscopic, microscopic, thermal, and surface analyses
of GO@NHC-Pd confirmed the successful formation of the catalyst. The inves-
tigation of catalytic activity showed that GO@NHC-Pd was very effective in
Suzuki–Miyaura as well as Hiyama cross-coupling. Being heterogeneous in
nature, GO@NHC-Pd was recovered after each reaction cycle easily and
reused for up to nine and six cycles in Suzuki–Miyaura and Hiyama cross-cou-
pling, respectively, without significant loss of activity. Further exploration of
the supercapacitor performance of GO@NHC-Pd catalyst assembled in a two-
electrode cell configuration shown a maximum attained capacitance of
Jain University, Jain Global Campus,
Kanakapura, Ramanagaram, Bangalore,
562112, India
2
Organic Chemistry Section, Chemical
Sciences & Technology Division, National
Institute for Interdisciplinary Science and
Technology, Thiruvananthapuram,
Kerala, -695019, India
3
Pharmaceutical Sciences Department,
College of Pharmacy, Rosalind Franklin
University of Medicine and Science, 3333
Green Bay Road,, North Chicago, IL,
60064, USA
Correspondence
1
05.26 F/g at a current density of 0.1 A/g with good cycling stability of 96.89%
Dr. Siddappa A. Patil, Centre for Nano
and Material Sciences, Jain University,
Jain Global Campus, Kanakapura,
Ramanagaram. Bangalore 562112, India.
Email: p.siddappa@jainuniversity.ac.in;
patilsiddappa@gmail.com
over 2,500 cycles.
K E Y W O R D S
graphene oxide, Hiyama cross-coupling, N-heterocyclic carbene-palladium(II), supercapacitor,
Suzuki–Miyaura cross-coupling
Funding information
The authors thank DST-SERB, India,
Grant/Award Number: (YSS/2015/
000010); DST-Nanomission, India, Grant/
Award Number: (SR/NM/NS-20/2014);
Jain University, India for financial support
1
| INTRODUCTION
heterocyclic carbenes (NHCs) are still of great impor-
[
3]
tance.
Their ease of synthesis, stability in air and
Organometallics is one of the most important research
moisture, relatively low toxicity, and ability to form
strong bonds with different transition metals are some
of the major advantages of NHCs over traditional ligand
[1]
areas in chemistry. The stability of the complex as
well as the ease of synthesis of the complex can be
judged during the ligand design itself. So, the designing
of the new ligand system for making the organometallic
[
3b,4]
systems such as phosphines and Schiff bases.
The
strong σ-donor property of the NHC ligand makes it a
[2]
[4a]
complex is very important. Among the ligand systems
perfect ligand system with high thermal stability.
employed in the organometallic chemistry, N-
Also, the steric and electronic properties of NHC
ligands can be fine-tuned by altering the substitutions
on the atom adjacent to carbene carbon of NHCs.
[4a]
Vishal Kandathil and Aisha Siddiqa contributed equally.
Appl Organomet Chem. 2020;e5924.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 19
https://doi.org/10.1002/aoc.5924

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KANDATHIL ET AL.
Thus, NHCs can form a good ligand system for complex
formation with various transition metals that can be
utilized in the field of catalysis. The transition metal
complexes of NHCs can be employed as homogeneous
cyclic voltagram experiments at different scan rates. They
concluded that there was slow heterogeneous electron
transfer between compounds due to the short lifespan of
[
14]
the oxidized forms.
Akkoç et al. prepared chelated N-
[1]
catalysts as well as heterogeneous catalysts. However,
heterogeneous catalysts are considered to be significant
because of their excellent stability, accessibility to high
surface area for reaction, easy recovery of the catalyst,
heterocyclic carbene ruthenium(II) complex and obtained
a capacitance of 20.1 F/g, revealing the occurrence of a
[
15]
redox reaction at the ruthenium center.
As a continuation of our previous studies using a
green and sustainable approach in the field of
[5]
and facile recyclability.
[
16]
The allotropes of carbon such as graphene, carbon
nano tubes, and fullerene have attracted much attention
in almost all areas of science because of their applica-
tions in energy storage, catalysis etc. and experts in
every field of material science and catalysis are looking
catalysis,
in this study we have designed and devel-
oped a heterogeneous catalyst in which a N-heterocy-
clic carbene-palladium(II) complex was immobilized on
graphene oxide (GO@NHC-Pd) and studied the activity
of the prepared GO@NHC-Pd catalyst in Suzuki–
Miyaura and Hiyama cross-coupling reactions. The
newly synthesized GO@NHC-Pd catalyst was charac-
terized using various spectroscopic, thermal, surface,
and microscopic techniques, such as Fourier transform
infrared spectroscopy (FT-IR), thermogravimetric analy-
sis (TGA), Brunauer–Emmett–Teller (BET), transmis-
sion electron microscopy (TEM), field-emission
scanning electron microscopy (FE-SEM), energy-disper-
sive X-ray spectroscopy (EDS), inductively coupled
plasma optical emission spectrometry (ICP-OES), and
powder X-ray diffraction (XRD) analysis. The prepared
GO@NHC-Pd complex is a good catalyst in both the
cross-coupling reactions as it is simple, ecofriendly,
selective, easy to handle, and easy to recover by centri-
fugation. We also, for the first time, studied the elec-
trochemical performance of the GO@NHC-Pd complex.
[
6]
to realize their potential.
Exfoliated sheets of
2
graphene have a hexagonal honeycomb lattice with sp -
hybridized carbon atoms in two-dimensions. This
design is important as it ensures high electrical and
thermal conductivity, and a very large surface area,
[7]
which increases with exfoliation.
Oxidation of
graphene results in graphene oxide (GO) containing
hydroxyl, carbonyl, and carboxylic acid groups at the
edges of the sheets and a few epoxy bridging groups,
which can be efficiently utilized during their func-
[
8]
tionalization to form new heterogeneous catalysts.
The Suzuki–Miyaura cross-coupling reaction is a
key cross-coupling reaction in which palladium is used
[
9]
because it can form carbon–carbon (C–C) bonds. The
applications of the Suzuki–Miyaura cross-coupling
reaction are many, ranging from synthesis of drug mol-
ecules to natural products and from agrochemicals to
[9c,10]
polymer synthesis.
The main advantage of this
cross-coupling is the comparatively low sensitivity of
the reagents and large functional group tolerance.
2 | EXPERIMENTAL
2.1 | Materials
[
11]
Another important class of cross-coupling reactions for
C–C bond formation is the Hiyama cross-coupling
reaction. The Hiyama cross-coupling has the advantage
of utilization of cheaper starting materials along with
good functional group tolerance of the reactants during
All materials were purified as per the standard methods
prior to use. Unless otherwise stated, all the reactions were
carried out under aerobic conditions in oven-dried glass-
ware with magnetic stirring. Graphite, benzimidazole, 4-
methylbenzyl bromide, (3-chloropropyl)triethoxysilane,
trimethoxyphenylsilane, palladium(II) acetate, aryl
halides, phenylboronic acid, and nafion were purchased
from Sigma Aldrich, India and Avra, India and used with-
out further purification. Heating was accomplished by sili-
cone oil bath. Column chromatography was conducted on
silica gel of 60–120 mesh purchased from Merck and thin
layer chromatography (TLC) was carried out using
0.25 mm Merck TLC silica gel plates using UV light as a
visualizing agent. Concentration in the vacuo refers to the
removal of volatile solvent using a rotary solvent evapora-
tor connected to a dry diaphragm pump (10–15 mm Hg)
[10d,12]
the reaction.
In attempts to halt the ever-increasing energy demand,
scientists have tried to find new technology for storing
energy. Among these energy storage devices, super-
capacitors bridge the huge gap between conventional
capacitors and batteries, therefore we have investigated
the catalysis properties as well as the electrochemical
behavior of our material. In 2010 Buscemi et al. studied
the electronic properties of a series of N-heterocyclic
dicarbene palladium(II) complexes using cyclic
voltagrams and examined the electronic density of palla-
[13]
dium center. Peng et al. synthesized N-heterocyclic car-
bine complexes of rhodium and iridium, and performed

KANDATHIL ET AL.
3 of 19
followed by pumping to a constant weight with an oil
pump (<300 mTorr). All the organic products were known
and identified by comparison of their physical and spectral
data with those of authentic samples.
2.5 | Synthesis of 1-(4-methylbenzyl)-1H-
benzoimidazole (6)
Potassium hydroxide (0.34 g, 6.06 mmol) was dissolved
in 20 ml dimethyl sulfoxide (DMSO) in a 100-ml
ꢀ
round-bottomed flask and stirred at 100 C for 10 min.
2
.2 | Characterization
Benzimidazole (0.5 g, 4.23 mmol) was added to the
reaction mixture while stirring at the same temperature
for 45 min. The temperature was reduced to 40 C and
ꢀ
FT-IR was carried out with a Perkin-Elmer Spectrum
Two spectrometer, India. BET surface areas were
obtained by physisorption of nitrogen using a Microtrac
BELSORPMAX instrument, India. The elemental palla-
dium content of GO@NHC-Pd catalyst was determined
with a Perkin-Elmer Optima 5300 DV ICP-OES. TEM
images were obtained using a Jeol/JEM 2100 microscope.
FE-SEM along with EDS to observe morphology and
determine elemental distributions, respectively, were
conducted with a Jeol model JSM7100F. TGA was carried
out with a PerkinElmer Diamond TG/DTA with a
the 4-methylbenzyl bromide (0.78 g, 4.22 mmol) was
added slowly to the reaction mixture. The resultant
ꢀ
reaction mixture was stirred for 24 hr at 40 C. After the
reaction was complete, the reaction mass was poured
into 100 ml of ice-cold water and stirred for about 2 hr
to form a creamy white precipitate. The reaction mass
was then filtered and washed with water (3 × 25 ml)
ꢀ
and dried at 45 C until a constant weight of the prod-
1
uct was obtained. Yield 90%. H NMR (400 MHz,
CDCl ) δ: 7.94 (s, 1H), 7.86–7.81 (m, 1H), 7.34–7.25 (m,
3
ꢀ
heating rate of 10.0 C/min. Powder XRD patterns were
obtained using a Rigaku X-ray diffraction Ultima-IV. H
3H), 7.16 (d, J = 8.0 Hz, 2H), 7.10 (d, J = 8.0 Hz, 2H),
5.32 (s, 2H), 2.34 (s, 3H).
1
NMR spectra were recorded at 400 MHz and are reported
relative to deuterated chloroform (CDCl δ = 7.27 ppm).
3
1
H NMR coupling constants (J) are reported in hertz
2.6 | Synthesis of NHC grafted graphene
oxide (GO@NHC) (7)
(
Hz) and multiplicities are indicated as follows: s (sin-
glet), d (doublet), t (triplet), m (multiplet). The electrodes
were assembled in a two-electrode configuration using a
symmetric Swagelok cell and electrochemical measure-
ments were performed using this a Corrtest instrument
1-(4-methylbenzyl)-1H-benzimidazole (0.5 g, 2.25 mmol)
(6) in 10 ml of toluene was placed in a 50-ml round-
bottomed flask and 0.5 g of silane-functionalized GO
product (5) was added. The mixture was sonicated for
(CHI 660c, China).
ꢀ
about 30 min. The reaction mass was stirred at 110 C
2
.3 | Synthesis of GO (1)
for 48 hr and then cooled to room temperature. The
reaction mass was decanted and washed with toluene
(3 × 25 ml). The final black residue was collected by
centrifuging and the product was dried at 45 C until a
constant weight was obtained.
[17]
GO was synthesized using Hummer's method
slight modification (see the Supporting Information for
the detailed procedure).
with
ꢀ
2
.4 | Synthesis of silane functionalized GO
(
3)
2.7 | Synthesis of GO@NHC-Pd (8)
First, 0.5 g of GO was dispersed in 25 ml of ethanol in a
00-ml round-bottomed flask and sonicated intermittently
To a 50-ml round-bottomed flask containing a solu-
tion of 0.5 M sodium carbonate (0.53 g in 10 ml
water) in 10 ml dimethylformamide (DMF), was
added 0.11 g of palladium(II) acetate and the mixture
was sonicated for about 15 min followed by the addi-
tion of 0.5 g of NHC grafted GO (7). The resultant
mixture was sonicated for another 10 min. The reac-
1
for about 30 min. Subsequently, 7.5 ml of ammonia solu-
tion was added dropwise and the reaction mixture was
ꢀ
vigorously stirred at 40 C for about 3 hr. Subsequently,
2
1
ml of (3-chloropropyl)triethoxysilane dissolved in
2.5 ml of ethanol was added and the resultant reaction
ꢀ
mixture was stirred for about 8 hr at the same tempera-
ture. The obtained black precipitate was collected by cen-
trifugation and the residue was washed with ethanol
tion mixture was stirred at 50 C for 16 hr to obtain
the black product. The GO@NHC-Pd catalyst pre-
pared was then isolated by centrifugation followed by
ꢀ
(
4
2 × 20 ml) and water (1 × 20 ml) and kept in an oven at
5 C until a constant weight of the product was obtained.
washing with water (2 × 30 ml) and dried at 50 C for
ꢀ
about 24 hr.

4
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KANDATHIL ET AL.
2
.8 | General procedure for Suzuki–
separated catalyst was washed with methanol (MeOH;
Miyaura cross-coupling reaction
2 × 15 ml) followed by water (2 × 15 ml) and dried at
ꢀ
4
5 C for about 12 hr. The dried GO@NHC-Pd catalyst
An oven-dried 50 ml round-bottomed flask was charged
with aryl halide (1 equiv.), phenylboronic acid (1.1 equiv.),
GO@NHC-Pd catalyst (0.03 mol % of Pd), and K CO (2.2
was utilized for the next cycle of organic transformations
without further purification.
2
3
equiv.) in 6 ml ethanol (EtOH). The reaction mixture was
ꢀ
sonicated for 10 min then stirred at 50 C for designated
2.11 | Fabrication of electrodes
time. Reaction progress was monitored by TLC. After com-
pletion of the reaction, the catalyst was separated by cen-
trifugation followed by the addition of dichloromethane
The electrodes were fabricated following some reported
methods. Working electrodes were prepared on a Ni-foam
substrate of 1 cm diameter (properly washed by 1 M HCl
followed by acetone, distilled water, and ethanol and
(DCM) (20 ml) and water (10 ml) and transferred to a sepa-
rating funnel. The biaryl products present in the organic
layer were separated out from the water and dried with
anhydrous sodium sulfate (Na SO ). Then the organic
ꢀ
dried at 60 C in the vacuum oven). Here 2 mg of
GO@NHC-Pd was taken as an active mass and a slurry
was made out of it by adding isopropyl alcohol (IPA) and
nafion (19:1). Afterwards the slurry was drop casted on
2
4
layer was concentrated under vacuum and purified via col-
umn chromatography using hexane and ethyl acetate as
eluting solvents to get the corresponding products in good
yields. All the cross-coupled products were known mole-
ꢀ
the Ni-foam and dried at 60 C in a vacuum oven for
[
18]
1 hr. The dried foam was pressed using a hydraulic pel-
1
[19]
cules and were further confirmed by comparing the H
let press with 5 tons of pressure to make the electrodes.
NMR spectral data with those of authentic samples (see
the Supporting Information).
3
| RESULTS AND DISCUSSION
2.9 | General procedure for Hiyama cross-
3.1 | Synthesis of GO@NHC-Pd catalyst
coupling reaction
In a continuation to our work on the development of
green and sustainable methods, we report here the prepa-
ration and characterization of GO anchored N-
heterocyclic carbene-palladium(II) complex and its use in
the Suzuki–Miyaura and Hiyama cross-couplings
between a range of aryl halides with phenylboronic acid
and trimethoxyphenylsilane. GO anchored N-heterocyclic
carbene-palladium(II) complex (8) was synthesized by a
multistep protocol as depicted in Scheme 1. First, GO (1)
was prepared from easily accessible graphite using the
An oven-dried 50 ml round-bottomed flask was charged
with aryl halide (1 equiv.), trimethoxyphenylsilane (1.5
equiv.), GO@NHC-Pd catalyst (0.01 mol % of Pd), and
Na CO (3 equiv.) in 5 ml of ethylene glycol. The reaction
2
3
ꢀ
mixture was sonicated for 5 min and stirred at 90 C for
the designated time. Reaction progress was monitored by
TLC. After completion of the reaction, the catalyst was
separated by centrifugation followed by the addition of
DCM (20 ml) and water (10 ml), and transferred to a sep-
arating funnel. The biaryl products present in the organic
layer were separated out from the water and dried with
anhydrous Na SO . The organic layer was concentrated
[
17]
modified Hummer's method.
Then, silyl chloride
functionalized GO (3) was prepared through the func-
tionalization of GO (1) with (3-chloropropyl)
2
4
ꢀ
under vacuum and purified via column chromatography
using hexane and ethyl acetate as eluting solvents to get
the corresponding products in good yields. All the cross-
coupled products were known molecules and were fur-
triethoxysilane (2) in EtOH at 40 C in the presence of
ammonia. At the same time, the intermediary
1-(4-methylbenzyl)-1H-benzoimidazole (6) was synthe-
sized from the reaction of benzimidazole (4) with
4-methylbenzyl bromide (5) in the presence of potassium
1
ther confirmed by comparing the H NMR spectral data
with those of authentic samples (see the Supporting
Information).
hydroxide
in
DMSO.
In
the
next
step,
1-(4-methylbenzyl)-1H-benzoimidazole (6) was treated
with silyl chloride functionalizedGO (3) in toluene at
ꢀ
1
10 C for 48 hr to obtain the corresponding GO tethered
2
.10 | Procedure for recovery of
N-heterocyclic carbene (7). Lastly, GO anchored N-
heterocyclic carbene-palladium(II) complex or
GO@NHC-Pd catalyst
GO@NHC-Pd (8) was prepared by the coordination of
palladium(II) acetate with GO anchored N-heterocyclic
carbene (7) in DMF solvent at 50 C for 16 hr.
After the completion of the reaction, the catalyst was sep-
arated from the reaction mixture by centrifugation. The
ꢀ

KANDATHIL ET AL.
5 of 19
SCHEME 1 Synthetic schemes for
(
(
a) silane functionalization of GO (3) and
b) NHC-palladium(II) grafted graphene
oxide (GO@NHC-Pd) (8)
1
−1 [21]
3
.2 | Characterization
1,135 cm− and for Si–OH stretching are at 1036 cm .
−
1
The peaks at 864 and 474 cm are for the stretching and
bending vibrations of the Si–O–Si network.
functional groups of GO@NHC
sent in the IR spectrum of GO@NHC (Figure 1c) and the
C=N stretching at 1,564 cm− along with the aromatic
[
22]
The FT-IR technique was used to determine the existence
of various functional groups in the synthesized samples
at each step. Each step of the newly synthesized
GO@NHC-Pd catalyst, where the NHC-Pd complex was
immobilized on the silane functionalized GO, was char-
acterized by FT-IR. The FT-IR spectra of (a) GO
All the
were found to be pre-
[
23]
1
−
1
C–H vibrations in the region of 756–615 cm confirm
the formation of GO@NHC.
[
23]
(
(
b) silane functionalized GO, (c) GO@NHC, and
d) GO@NHC-Pd catalyst are shown in Figure 1.
The presence of functional groups in the GO@NHC-
Pd catalyst can be confirmed from Figure 1d. The peak at
1,129 cm− corresponds to the exact planar asymmetric
1
The spectrum of GO (Figure 1a) showed the presence
of oxygen-containing functional groups, which confirms
stretching vibrations arising from the C–H bond of the
[20]
[24]
the successful oxidation of graphite.
The broad peak at
palladium-coordinated N-heterocyclic carbene.
peak at 1,716 cm is assigned to the presence of carbonyl
The
−
1
−1
3
,379 to 2,854 cm is due to the presence of carboxylic
[20]
OH groups and superimposed –OH groups.
absorption bands at 2,926 and 2,854 cm are due to
The
stretching and the most important peak to consider is
−
1
−1 [25]
that for C–Pd at 470 cm .
The C=N vibration mode
peak at 1,386 cm− and the C–H out of plane bending
1
asymmetric and symmetric CH stretching in the
2
−1
2
−1
GO. The peak at 1,578 cm gives the data of sp hybrid-
ized C=C stretching, the peak at 1,720 cm shows the
peak at 797–748 cm
confirm the formation of
−
1
GO@NHC-Pd catalyst.
existence of C=O stretching vibrations, and the peak at
1
Figure 1b shows the spectrum for silane-functionalized
GO where peaks for the Si–O–C group are found at
The thermal stability of both GO and GO@NHC-Pd
catalyst was investigated by TGA under a nitrogen atmo-
sphere from 35 to 800 C at a rate of 10 C per minute
(Figure 2). Figure 2a shows the weight loss in two
,128 cm− shows the C–O stretching of the epoxy group.
1
ꢀ
ꢀ

6
of 19
KANDATHIL ET AL.
FIGURE 1 FT-IR spectra of (a) GO, (b) silane-functionalized GO, (c) GO@NHC, (d) GO@NHC-Pd catalyst, (e) recycled GO@NHC-Pd
catalyst from Suzuki–Miyaura cross-coupling, and (f) recycled GO@NHC-Pd from Hiyama cross-coupling
consecutive steps. In the first step, there is a weight loss
GO@NHC-Pd catalyst, apart from the weight loss
observed in temperature range up to 150 C, the weight
ꢀ
of about 15–17% from GO in the temperature range of
ꢀ
3
5–150 C, which can be attributed to the removal of
loss was found to be less when compared with GO, which
can be attributed to the functionalization of the groups
present on the GO (Figure 2b). Also, the TGA results
clearly prove that the newly synthesized heterogeneous
physically adsorbed water molecules, which may be pre-
sent between the GO layers. The next step reveals a
weight loss of about 25% in the temperature range
ꢀ
ꢀ
1
50–230 C, which illustrates the decomposition of vari-
catalyst is very stable up to 200 C, which makes the
ous functional groups, such as –OH, –COOH, epoxy
bridging groups, etc., present in the GO. In the case of
GO@NHC-Pd catalyst suitable for reactions at higher
temperatures.

KANDATHIL ET AL.
7 of 19
2
GO was found to be 44.1 m /g and that of GO@NHC-Pd
2
catalyst was 8.4 m /g, which proves the formation of the
catalyst.
The size and surface morphology of the catalyst
were investigated by TEM. The TEM images of GO
and GO@NHC-Pd catalyst are shown in Figure 4.
The GO sheets have layered sheet like morphology
and are well separated after exfoliation, which is in
[
26]
the nanometer scale.
The immobilization of NHC-
Pd complex onto the layers of GO can be confirmed
by the presence of darker and thicker layers in the
TEM image (Figure 4b) when compared with that
of GO.
The FE-SEM analysis helped to determine the sur-
face morphology, size, shape, and topology of the
GO@NHC-Pd catalyst. The FE-SEM images of GO and
GO@NHC-Pd catalyst are shown in Figure 5a,b. The
FE-SEM images reveal that both GO and GO@NHC-Pd
catalyst have similar two-dimensional nanosheets with
wave-like structures.
The presence of each element in the newly synthe-
sized GO@NHC-Pd catalyst was confirmed by EDS. The
EDS spectra shown in Figure 6 for the GO@NHC-Pd cat-
alyst display different signals which reveal the existence
of elements C, N, Si, O, Cl, and Pd, confirming that the
FIGURE 2 TGA curves of (a) GO and (b) GO@NHC-Pd
catalyst
The surface areas of GO and GO@NHC-Pd catalyst
were measured by the BET technique. The nitrogen
adsorption–desorption curves for GO and GO@NHC-Pd
catalyst are shown in Figure 3a,b. The amount of nitro-
gen adsorbed on the surface of GO is high compared to
that for the GO@NHC-Pd catalyst. The surface area of
FIGURE 3 Nitrogen adsorption–desorption curves for (a) GO and (b) GO@NHC-Pd catalyst
FIGURE 4 TEM images of (a) GO
and (b) GO@NHC-Pd catalyst

8
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KANDATHIL ET AL.
FIGURE 5 FE-SEM images of (a) GO, (b) GO@NHC-Pd catalyst, (c) recycled GO@NHC-Pd catalyst from Suzuki–Miyaura cross-
coupling, and (d) recycled GO@NHC-Pd from Hiyama cross-coupling
for GO and GO@NHC-Pd catalyst. In Figure 8a, the
ꢀ
strong and sharp peak at 2θ = 11.99 (200) represents the
[
27]
characteristic peak of the prepared GO
and the small
ꢀ
peak at 2θ = 42.35 (100) is the peak for carbon in the
graphite plane, which might occur because of the pres-
ence of a minute amount of graphite present in the
GO. On the other hand, the peak at 2θ = 11.99 disap-
pears and a broad diffraction peak at 2θ = 23.12 emerges
ꢀ
o
for the GO@NHC-Pd catalyst, confirming the successful
functionalization of the major oxygen-containing func-
tional groups of GO.
FIGURE 6 EDS spectrum of GO@NHC-Pd catalyst
3
.3 | Catalytic activity study of GO@NHC-
NHC-Pd complex is immobilized on the GO layers to
form the heterogeneous catalyst.
Pd catalyst in Suzuki–Miyaura cross-
coupling reactions
The elemental mapping of the GO@NHC-Pd catalyst
was carried out with the same technique to determine
the distribution of the corresponding elements on
GO@NHC-Pd catalyst (Figure 7). The analysis of the ele-
mental mapping shows an even distribution of all the ele-
ments on the heterogeneous catalyst. To determine the
exact Pd loading, ICP-OES analysis was performed. The
exact quantity of palladium loaded on the GO@NHC-Pd
catalyst was found to be 3.42%w/w.
After the characterization of the newly synthesized
GO@NHC-Pd catalyst using different spectroscopic, sur-
face, and microscopic techniques, the catalytic activity
and stability of the GO@NHC-Pd catalyst were studied in
Suzuki-Miyaura cross-coupling reactions. The reaction
conditions were optimized through a series of reactions
in which 1-bromo-4-nitrobenzene was chosen as the aryl
halide along with phenylboronic acid in the model reac-
tion, as shown in Scheme 2.
XRD is one of the most important techniques for
predicting and analyzing the crystalline and amorphous
state of a compound. Figure 8 shows the XRD patterns
Scheme 3 depicts a plausible mechanism for the cata-
lytic cycle of the Suzuki–Miyaura cross-coupling reaction.

KANDATHIL ET AL.
9 of 19
FIGURE 7 Elemental mapping of GO@NHC-Pd catalyst
SCHEME 3 Plausible mechanism for the catalytic cycle of
Suzuki–Miyaura cross-coupling catalyzed by GO@NHC-Pd catalyst
FIGURE 8 XRD pattern of (a) GO and (b) GO@NHC-Pd
catalyst
The catalyst ratio affects the rate of reaction in any
organic transformation. Here, the model Suzuki–Miyaura
cross-coupling reaction of 1-bromo-4-nitrobenzene with
phenylboronic acid was carried out with different
GO@NHC-Pd catalyst ratios, as shown in Table 1. The
optimum catalyst ratio was found to be 0.03 mol% of Pd
with respect to aryl halide (Table 1, entry 7).
The catalytic cycle starts with the preactivation of
Pd(II) species in GO@NHC-Pd catalyst to Pd(0) followed
by the oxidative addition of aryl halide to form the inter-
mediate (II), which then undergoes transmetallation to
form intermediate (III) followed by reductive elimination
to form the cross-coupled biaryl product.
SCHEME 2 Suzuki–Miyaura cross-coupling reaction
of 1-bromo-4-nitrobenzene with phenylboronic acid

10 of 19
KANDATHIL ET AL.
TABLE 1 Optimization of reaction conditions for the model Suzuki–Miyaura cross-coupling reaction in the presence of GO@NHC-Pd
[
a]
catalyst
ꢀ
[b]
Entry
Base
Solvent
EtOH
Temperature ( C)
Catalyst (mol% Pd)
0.015
0.015
0.015
0.015
0.03
Time (hr)
Yield (%)
80
1
2
3
4
5
6
7
8
9
K
K
K
K
K
K
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
RT
RT
RT
RT
RT
40
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
RT
50
60
50
50
50
50
50
8
EtOH:H
2
O (1:1)
6
50
MeOH
7
60
MeOH:H
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
2
O (1:1)
5
50
5
85
0.03
1.5
1.5
1.5
1.5
1.5
1.5
2
90
K
2
CO
3
0.03
92
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
0.0075
0.015
0.0225
0.035
0.03
49
3
76
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
3
83
3
92
Na
Cs
2
CO
3
85
2
CO
3
0.03
2
57
NaOH
KOH
0.03
2
60
0.03
2
72
Na
KF
TEA
3
PO
4
.12H
2
O
0.03
2
77
0.03
2
68
0.03
2
Trace
–
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
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
Acetonitrile
Toluene
DMF
0.03
6
0.03
5
42
0.03
6
60
Acetone
THF
0.03
14
7
42
0.03
45
DCM
0.03
15
14
1.5
0.25
0.5
0.75
1
Trace
83
IPA
0.03
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
0.03
92
0.03
50
0.03
55
0.03
65
0.03
78
0.03
2
92
a
Reaction conditions: 1-bromo-4-nitrobenzene (1.0 equiv.), phenylboronic acid (1.1 equiv.), base (2.2 equiv.), and solvent (6 ml) in air.
Isolated yield.
b
a
Note. DCM, dichloromethane; DMF, dimethylformamide; IPA, isopropyl alcohol; RT, room temperature; TEA, triethylamine; THF, tetrahydrofuran.
Bold represents the optimized reaction condition.
The activity of the GO@NHC-Pd catalyst was stud-
ied for different bases, including K CO , Na CO ,
triethylamine was used (Table 1, entry 18). The activity
of the GO@NHC-Pd catalyst was checked in different
2
3
2
3
Cs CO , NaOH, KOH, Na PO .12H O, KF, and
solvents, such as ethanol (EtOH), acetonitrile (CH CN),
2
3
3
4
2
3
triethylamine, as shown in Table 1 (entries 7, 12–18).
EtOH:H O (1:1), toluene, DMF, acetone, tetrahydrofuran
2
Among the bases employed, K CO showed a good con-
(THF), methanol (MeOH), DCM, and IPA. From the
results obtained, it can be concluded that the
GO@NHC-Pd catalyst showed very good conversion and
gave good yield with the polar solvents when compared
with less polar solvents. The highest yield for the cross-
2
3
version rate for the cross-coupling reaction (Table 1,
entry 7). Strong bases like NaOH and KOH did not
result in higher yields (Table 1, entries 14 and 15) and
there was only a trace amount of product formed when

KANDATHIL ET AL.
11 of 19
TABLE 2 Suzuki–Miyaura cross-coupling reactions between different aryl halides and phenylboronic acid with GO@NHC-Pd
[
a]
catalyst
[
b]
TOF (hr−1
)
Entry
Aryl halide
Product
Time (hr)
Yield (%)
TON
1
1
92
1823
1823
2
3
4
2
1
4
60
87
75
1,079
1,360
1,261
540
1,360
315
5
6
1.5
4
92
50
1,417
1,006
945
252
7
8
16
6
Trace
86
–
–
1,025
171
9
1
3
90
90
1,637
1,314
1,637
438
1
0
1
1
3
1
93
96
1,547
1,464
516
1
2
1,464
1
1
3
4
1
1
95
95
1,356
1,263
1,356
1,263
(
Continues)

12 of 19
KANDATHIL ET AL.
TABLE 2 (Continued)
[
b]
TOF (hr−1
)
Entry
Aryl halide
Product
Time (hr)
Yield (%)
TON
1
5
1
88
1,245
1,245
a
Reaction conditions: aryl halide (1.0 equiv.), phenylboronic acid (1.1 equiv.), base (2.2 equiv.), GO@NHC-Pd catalyst (0.03 mol % Pd loading
with respect to aryl halide), and solvent (6 ml) in air.
b
Isolated yield.
a
Note. TOF, turn over frequency; TON, turn over number.
coupling reaction was obtained with EtOH in less time
concluded that the reaction yield was not reliant on the
electronic properties of the substituent on the aryl
halides. On the other hand, aryl iodides gave excellent
yields as expected (Table 2 entries 12–15) whereas aryl
chloride gave a lower yield (Table 2, entry 6). These
results indicate that the newly synthesized GO@NHC-Pd
catalyst can catalyze the Suzuki–Miyaura cross-coupling
reaction with good to excellent yields.
(Table 1, entry 7) when compared with other solvent
system. The activity of GO@NHC-Pd catalyst was
checked at different temperatures to examine the effect
of temperature on the rate of the reaction. The
GO@NHC-Pd catalyst showed lower yields at lower
temperatures (Table 1, entries 5 and 6) and improve-
ment in the yield was observed with increasing temper-
ꢀ
ature. A temperature of 50 C was found to be the
optimum temperature (Table 1, entry 7) as there was
ꢀ
no improvement in the yield beyond 50 C (Table 1,
3.5 | Catalyst recyclability study in
Suzuki–Miyaura cross-coupling
entry 26). The catalytic activity was found to increase
with increasing reaction time with 0.03 mol% catalyst at
0 C (Table 1, entries 7, 27–31). The highest yield was
observed when the reaction was run for 1.5 hr
Table 1, entry 7) and a further increase in the reaction
time did not increase the yield (Table 1, entry 31).
Hence 1.5 hr was chosen as the optimum reaction time
for the model reaction of 1-bromo-4-nitrobenzene with
phenylboronic acid. Thus, the optimized conditions for
the model Suzuki–Miyaura cross-coupling reaction were
potassium carbonate (K CO ) as base, ethanol (EtOH)
ꢀ
5
Recovery of GO@NHC-Pd catalyst and its reuse in the
Suzuki-Miyaura cross-coupling reaction was studied for
the model reaction and the results are presented in
Figure 9. After each cycle of the Suzuki–Miyaura cross-
coupling reaction, the catalyst was successfully recovered
by centrifuging followed by washing with methanol and
drying, as explained in section 2.10. The catalyst recov-
ered was utilized in further cycles of the corresponding
(
2
3
as solvent, and 0.03 mol% of GO@NHC-Pd catalyst for
ꢀ
9
0 min at 50 C (Table 1, entry 7).
3
.4 | Study of substrate scope in Suzuki–
Miyaura cross-coupling reactions with
GO@NHC-Pd catalyst
After studying the effect of various parameters in
Suzuki–Miyaura cross-coupling reaction, the applicabil-
ity of GO@NHC-Pd catalyst in the Suzuki–Miyaura
cross-coupling of various aryl halides and phenylboronic
acid was studied out under the optimized reaction condi-
tions and the results are shown in Table 2. All the aryl
bromides with electron-donating and -withdrawing
groups reacted with arylboronic acid to afford the
corresponding products in high yields except
FIGURE 9 Recycling efficiency of GO@NHC-Pd catalyst in
the model Suzuki–Miyaura cross-coupling reaction
4
-bromoaniline (Table 2, entry 7). Therefore, it can be

KANDATHIL ET AL.
13 of 19
organic transformation without much significant loss in
catalytic activity, as shown in Figure 9. After three cycles
a slow and gradual decline in the yield was observed and
the yield reached 81% by the end of tenth recycle. The
FT-IR analysis of the GO@NHC-Pd catalyst after the
tenth recycle of the Suzuki–Miyaura cross-coupling reac-
tion shows that there is slight peak shift in the case of the
FT-IR spectrum (Figure 1e), and FE-SEM analysis
(Figure 5c) shows that there is no intense change in the
surface morphology of the GO@NHC-Pd catalyst, which
shows that the catalyst is highly stable in the given set of
reaction conditions.
3
.6 | Catalytic activity study of GO@NHC-
Pd catalyst in Hiyama cross-coupling
SCHEME 5 Plausible mechanism for the catalytic cycle of
Hiyama cross-coupling catalyzed by GO@NHC-Pd catalyst
The GO@NHC-Pd catalyst was further studied for its cat-
alytic activity in Hiyama cross-coupling reaction to
understand the wide range applicability. The cross-cou-
pling between 4-iodoanisole and trimethoxyphenylsilane
was selected as the model reaction to optimize the reac-
tion conditions (Scheme 4). Since organosilicon
compounds are weak nucleophiles, fluoride ions were
used to activate them. The nucleophilicity of the orga-
nosilicon compound can be increased by using an anion,
thereby forming a pentacoordinate aryl silicate that is
highly reactive. In our study, we used various bases for
SCHEME 4 Hiyama cross-coupling reaction of
4
-iodoanisole with trimethoxyphenylsilane
TABLE 3 Optimization of reaction conditions for the model Hiyama cross-coupling reaction in the presence of GO@NHC-Pd
[a.]
catalyst
ꢀ
[b]
Entry
Base
Solvent
Temperature ( C)
Catalyst (mol% Pd)
Time (hr)
Yield (%)
1
2
3
4
5
6
7
8
9
Na
Na
Na
Na
Na
Na
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
H
2
O
RT
RT
70
70
70
80
90
100
90
90
90
90
90
90
90
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.005
0.02
8
–
EtOH:H
EtOH:H
MeOH
2
2
O (1:1)
O (1:1)
6
–
6
60
50
85
90
95
95
70
90
91
86
92
83
95
6
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol: H
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
Ethylene glycol
1
1
Na
2
CO
3
0.5
0.5
1.5
0.5
0.5
1
Na
Na
2
CO
3
2
CO
3
2
O (1:1)
10
11
12
13
14
15
2 3
K CO
NaOH
KOH
Cs
2
CO
3
0.5
1
Na
Na
2
CO
3
3
2
CO
0.5
a.
b
Reaction conditions: 1-iodo-4-anisole (1.0 equiv.), trimethoxyphenylsilane (1.5 equiv.), base (3 equiv.), and solvent (5 ml) in air.
Isolated yield.
a
Note. RT, room temperature.
Bold represents the optimized reaction condition.

14 of 19
KANDATHIL ET AL.
[
a.]
TABLE 4 Hiyama cross-coupling reactions between different aryl halides and trimethoxyphenylsilane with GO@NHC-Pd catalyst
[
b]
TOF (hr−1
)
Entry
1
Aryl halide
Product
Time (hr)
Yield (%)
TON
0.5
95
4,347
8,694
2
3
1
1
93
90
5,529
3,819
5,529
3,819
4
5
2
2
88
88
4,748
4,127
2,374
2064
6
7
0.5
1
95
90
3,561
4,159
7,123
4,159
8
9
0.5
1
92
90
3,939
4,912
7,878
4,912
1
0
1
88
3,855
3,855
1
1
1
1
2
3
0.5
1
95
92
93
3,789
4,592
4,770
7,579
4,592
4,770
1
1
1
4
5
0.5
0.5
90
92
3,387
3,869
6,775
7,737

KANDATHIL ET AL.
15 of 19
TABLE 4 (Continued)
[
b]
TOF (hr−1
)
Entry
Aryl halide
Product
Time (hr)
Yield (%)
TON
a.
Reaction conditions: 1-iodo-4-anisole (1.0 equiv.), trimethoxyphenylsilane (1.5 equiv.), base (3 equiv.), GO@NHC-Pd catalyst (0.01 mol %
Pd loading with respect to aryl halide), and solvent (5 ml) in air.
b
Isolated yield.
a
Note. TOF, turn over frequency; TON, turn over number.
the activation of the organosilicon reagent. Table 3 shows
the series of Hiyama cross-coupling reactions carried out
to determine the best reaction conditions.
3.7 | Study of substrate scope in Hiyama
cross-coupling reaction with GO@NHC-Pd
catalyst
A plausible mechanism for the catalytic cycle of
Hiyama cross-coupling is presented in Scheme 5. The base
acts as an activating agent to enhance the nucleophilicity
of the organosilicon compound in the absence of fluoride
ions. The cycle starts with the preactivation of the
Pd(II) species of GO@NHC-Pd catalyst to Pd(0) followed
by an oxidative addition step to form the intermediate (II).
The base activates the tetracoordinated silane to
pentacoordinate aryl silicate anion, which undergoes
transmetallation to form the intermediate (III) followed by
reductive elimination to form the biaryl product.
To explore the applicability of the optimized reaction
conditions with GO@NHC-Pd catalyst, we used the
Hiyama cross-coupling of various aryl halides with
trimethoxyphenylsilane. The substrate scope for the
Hiyama cross-coupling is given in Table 4. From the
results obtained it can be generalized that GO@NHC-Pd
catalyst is able to catalyze the Hiyama cross-coupling in
good to excellent yields with aryl halides containing
electron-withdrawing group as well as electron-donating
groups. In the case of aryl halides with electron-donating
groups, the reaction time was found to be higher when
compared with aryl halides with electron-withdrawing
groups. Aryl iodides were also faster to react with
trimethoxyphenylsilane to form the cross-coupled prod-
uct when compared with aryl bromides.
Initially, the model Hiyama cross-coupling was car-
ried out in water as solvent with Na CO as base at room
2
3
temperature and 0.01 mol% of Pd catalyst, which resulted
ꢀ
in zero conversion (Table 3, entry 1). At 70 C in EtOH:
H O (1:1) as solvent, product formation was observed
2
and a yield of 60% was obtained (Table 3, entry 3) while
at room temperature there was no product formation
(
Table 3, entry 2). When ethylene glycol was used and
ꢀ
the reaction temperature was raised to 90 C (Table 3,
entry 7), a maximum yield of 95% was observed, which
was less at 70 C (Table 3, entry 5) and 80 C (Table 3,
ꢀ
ꢀ
ꢀ
entry 6). Further increase in temperature up to 100 C
had no effect (Table 3, entry 8). Among the bases
employed, sodium carbonate was selected because of its
economic viability and ease of handling. The optimal cat-
alyst concentration was fixed at 0.01 mol% of Pd because
an increase in catalyst ratio did not increase the yield fur-
ther (Table 3, entry 15) and a decrease in the catalyst
ratio resulted in decreased yield (Table 3, entry 13). Thus,
the utilization of Na CO base and ethylene glycol sol-
2
3
ꢀ
vent at 90 C using 0.01 mol% of Pd GO@NHC-Pd catalyst
resulted in the highest yield in 30 min (Table 3, entry 7)
and was considered the optimized conditions for the
model reaction.
FIGURE 10 Recycling efficiency of GO@NHC-Pd catalyst in
the model Hiyama cross-coupling reaction

16 of 19
KANDATHIL ET AL.
3
.8 | Catalyst recyclability study in the
and the results are given in Tables 5 and 6, respectively.
It can be seen that the GO@NHC-Pd catalyst showed
good to excellent activity when compared with other cat-
alytic systems under aerobic conditions and also could
catalyze the cross-coupling at relatively low temperatures
and shorter reaction times.
Hiyama cross-coupling reaction
A catalyst recyclability study was conducted for the
model Hiyama cross-coupling reaction and the results
are shown in Figure 10. After each cycle, the
GO@NHC-Pd catalyst was separated by centrifugation
and further processed, as explained in section 2.10.
The GO@NHC-Pd catalyst was found to be active up
to the end of five recycles without much reduction in
the yield and afterwards a notable decrease in the
yield was observed. The GO@NHC-Pd catalyst recov-
ered after the seventh recycle was subjected to FT-IR
and FE-SEM analyses. The FT-IR spectrum (Figure 1f)
shows that there is no major shift in peak position
and the FE-SEM image (Figure 5d) confirms that the
morphology remained unaltered which can be attrib-
uted to the higher stability of the GO@NHC-Pd
catalyst.
3.10 | Leaching study
To determine whether or not there is leaching out of pal-
ladium from the GO@NHC-Pd catalyst, a leaching study
was carried out. Initially, the GO@NHC-Pd catalyst and
ꢀ
the base were stirred for 1 hr at 50 C in ethanol. After
stirring for 1 hr, the GO@NHC-Pd catalyst was filtered
off and then aryl halide and phenylboronic acid were
added to the reaction mass and stirring continued for a
ꢀ
further hour at 50 C. The reaction was monitored by
TLC and it was found that there was no conversion of the
starting materials and the reactants remained unreacted.
Similarly, GO@NHC-Pd catalyst was stirred in ethylene
ꢀ
3.9 | Comparison with reported catalysts
glycol in the presence of base at 90 C for 1 hr followed by
the removal of the catalyst and the addition of aryl halide
and trimethoxyphenylsilane. TLC monitoring after con-
tinued stirring revealed that the starting materials
remained unreacted throughout. From this study it can
be confirmed that there is no palladium leaching out
To identify where the GO@NHC-Pd catalyst stands
among the reported catalysts with similar catalyst design
in the Suzuki–Miyaura as well as the Hiyama cross-
coupling reactions, a comparison study was carried out
TABLE 5 Comparison of yield in the Suzuki–Miyaura cross-coupling reaction between 1-bromo-4-nitrobenzene and phenylboronic
acid using GO@NHC-Pd catalyst with other reported catalysts
ꢀ
Entry
Catalyst
Pd/Fe
Solvent
Temperature ( C)
Time (hr)
Yield (%)
Ref.
[
[
[
[
28]
29]
30]
31]
1
2
3
4
5
6
7
3
O
4
/s-G
EtOH:H
2
O (1:1)
80
RT
80
60
80
60
50
0.75
1
86
96
99
98
95
80
92
Pd@AGu-MGO
2
H O
GO-CPTMS@Pd-TKHPP
NHC-Pd/GO-IL
EtOH:H
EtOH:H
EtOH:H
EtOH:H
EtOH
2
2
2
2
O (2:1)
O (1:1)
O (1:1)
O (2:1)
0.23
2.5
3
2
+
[24]
[32]
GO-NHC-Pd
2
+
GO-NH
2
-Pd
4
GO@NHC-Pd
1
Present work
TABLE 6 Comparison of yield in the Hiyama cross-coupling reaction between 4-bromoanisole and trimethoxyphenylsilane using
GO@NHC-Pd catalyst with other reported catalysts
ꢀ
Entry
Catalyst
Solvent
Temperature ( C)
Time (hr)
Yield (%)
Ref.
[
[
[
[
[
[
33]
34]
35]
36]
37]
38]
1
2
3
4
5
6
7
PdNP-GO/P123
Dioxane:H
2
O (2:1)
80
9
87
81
75
74
98
77
92
Pd (OAc)
2
H
2
2
O-PEG2000
O
60
6
Pd-LHMS-3
Coreshell Ni-Pd
Pd nanoparticle
Pd/C
H
100
65
14
24
3
THF
H
2
O
90
Toluene
Reflux
90
48
1
GO@NHC-Pd
Ethylene glycol
Present work

KANDATHIL ET AL.
17 of 19
from the catalyst to the reaction mass and this also
proves that the GO@NHC-Pd catalyst is heterogeneous
in nature.
symmetric Swagelok assembly by cyclic voltammetry
(CV), galvanostatic charge–discharge (GCD), and elec-
trochemical impedance spectroscopy (EIS). The aqueous
electrolyte was 0.5 M K SO and the material showed a
[39]
[40]
Mercury
or CS2
poisoning tests are usually car-
2
4
ried out to determine the nature of the active species pre-
sent in a catalyst and its leaching out. In this work, the Hg
poisoning test was performed to understand the kind of
active species and its leaching out from the GO@NHC-Pd
catalyst, if any. Here, 250 mol% of Hg was added to the
reaction mass of the model Suzuki–Miyaura cross-
coupling reaction under the optimized reaction condi-
tions. After maintaining the reaction for the required time,
the product was isolated and the yield obtained was 82%.
This test shows that Pd was not leached out from the cata-
lyst because of ligand coordination and at the same time
the presence of a small amount of Pd(0) species in the cata-
lyst can be confirmed, which during the course of the poi-
son test became amalgamated with Hg, resulting in the
slight decrease in biaryl product yield.
wide potential window of 0–1.75 V. Figure 11a shows
the CV curves at different scan rates of 5, 10, 20, 40,
60, and 80 mV/s and the capacitances obtained were
55.6, 45.05, 38.28, 32.42, 28.57, and 26.76 F/g, respec-
tively. From the CV curves it is evident that the mate-
rial has electric double layer capacitor type behavior.
Furthermore, the charge–discharge storage mechanism
was evaluated using GCD at different current densities.
Figure 11b shows the charge–discharge curves of
GO@NHC-Pd and the highest specific capacitance
obtained was 105.26 F/g at 0.1 A/g. With varying cur-
rent density, that is, 0.2, 0.3, 0.4, and 0.5 A/g, the spe-
cific capacitances obtained were 98.40, 89.35, 116.9,
81.8, and 61.6 F/g, respectively. Figure 11c shows the
orderly increment of specific capacitance with respect
to different current densities.
To understand more about its electrochemical behav-
ior, GO@NHC-Pd was examined by EIS. Figure 11d
shows the Nyquist plot for GO@NHC-Pd, which was per-
formed in the frequency range of 0.1–100 kHz at a poten-
tial of 5 mV. The intrinsic resistance of electrode material
3
.11 | Electrochemical performance
For the electrochemical measurements of GO@NHC-
Pd, previously prepared electrodes were tested using a
FIGURE 11 (a) CV curves at different scan rates, (b) GCD curves at different current densities, (c) specific capacitance vs. current
density, and (d) EIS and cycling stability

18 of 19
KANDATHIL ET AL.
(
Rs) was found to be 5.37 Ω and the interfacial transfer
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