Catalytic activity and facile recovery of a cyclometalated N-heterocyclic carbene palladium(II) complex immobilized by non-covalent interactions on reduced graphene oxide

Article abstract of DOI:10.1002/aoc.4907

The growing concern about the potentially adverse effects of the production of chemical compounds on the sustainable development of the environment has led to a great deal of efforts to search for low-cost and environmentally friendly catalytic systems. A pyrene-tagged N-heterocyclic carbene palladacycle complex ([Pd{(C,N)C₆H₄CH₂NH(Et)}(Imd-P)Br]) was prepared by reacting imidazolium salt with dimer ([Pd₂{(C,N)C₆H₄CH₂NH(Et)}₂(μ-OAc)₂]). Then, it was immobilized onto the surface of reduced graphene oxide (rGO) via π–π stacking forces. The hybrid compound ((NHC)Pd-rGO) was made in a one-step process. Various techniques were employed to characterize the compound. In addition, computational studies were used to verify the interaction between the Pd complex and rGO. The catalytic activity of the molecular complex and hybrid material was evaluated in both Suzuki–Miyaura cross-coupling reactions and reduction of p-nitrophenol to p-aminophenol. The catalytic activity of the hybrid material was enhanced in comparison with the corresponding homogeneous analogue. Thus, rGO seems to play a significant role in catalytic activity. Hot filtration experiments show the heterogeneous nature of the catalyst resulting from the strong interaction between pyrene and graphene. The hybrid (NHC)Pd-rGO material could be recycled up to six times with no decrease in catalytic activity.

Full text of DOI:10.1002/aoc.4907

Received: 15 November 2018
DOI: 10.1002/aoc.4907
Revised: 20 February 2019
Accepted: 21 February 2019
F U L L P A P E R
Catalytic activity and facile recovery of a cyclometalated
N‐heterocyclic carbene palladium(II) complex immobilized
by non‐covalent interactions on reduced graphene oxide
1
1
2
3
Kazem Karami
| Azar Ramezanpour | Mostafa Zakariazadeh | Cristian Silvestru
1
Department of Chemistry, Isfahan
The growing concern about the potentially adverse effects of the production of
chemical compounds on the sustainable development of the environment has
led to a great deal of efforts to search for low‐cost and environmentally friendly
catalytic systems. A pyrene‐tagged N‐heterocyclic carbene palladacycle
University of Technology, Isfahan 84156/
83111, Iran
2
Research Institute for Fundamental
Sciences (RIFS), University of Tabriz,
Tabriz, Iran
3
complex ([Pd{(C,N)C H CH NH(Et)}(Imd‐P)Br]) was prepared by reacting
Department of Chemistry and Chemical
6
4
2
Engineering, Babes‐Bolyai University,
Cluj‐Napoca, Romania
imidazolium salt with dimer ([Pd {(C,N)C H CH NH(Et)} (μ‐OAc) ]). Then,
2
6
4
2
2
2
it was immobilized onto the surface of reduced graphene oxide (rGO) via
π–π stacking forces. The hybrid compound ((NHC)Pd‐rGO) was made in a
one‐step process. Various techniques were employed to characterize the com-
pound. In addition, computational studies were used to verify the interaction
between the Pd complex and rGO. The catalytic activity of the molecular com-
plex and hybrid material was evaluated in both Suzuki–Miyaura cross‐coupling
reactions and reduction of p‐nitrophenol to p‐aminophenol. The catalytic activ-
ity of the hybrid material was enhanced in comparison with the corresponding
homogeneous analogue. Thus, rGO seems to play a significant role in catalytic
activity. Hot filtration experiments show the heterogeneous nature of the cata-
lyst resulting from the strong interaction between pyrene and graphene. The
hybrid (NHC)Pd‐rGO material could be recycled up to six times with no
decrease in catalytic activity.
Correspondence
Kazem Karami, Department of Chemistry,
Isfahan University of Technology, Isfahan
84156/83111, Iran.
Email: karami@cc.iut.ac.ir
KEYWORDS
graphene, heterogeneous, p‐nitrophenol reduction, rGO docking, Suzuki–Miyaura
[
9]
1
| INTRODUCTION
(GO) and reduced graphene oxide (rGO) have been
used in the immobilization of metal complexes. Pd(II)
complexes of N‐heterocyclic carbenes (NHCs) have been
reported as immobilized homogeneous catalysts for
C&bond;C coupling reactions on various suitable sup-
Attempts to prepare immobilized transition metal cata-
lysts follow three major methods: (1) covalent bond for-
mation or ion pairing between support and complex,
[
1,2]
[
10–18]
(2) non‐bonding interactions including hydrogen bridg-
ports.
The formation of a covalent bond between
ing, halogen bridges, π‐stacking, sigma C–H– π‐stacking
an appropriate support and a complex is definitely the
most extensively applied approach for the immobilization
of a homogeneous catalyst. The non‐covalent method is
easier and more convenient for catalyst preparation with-
out chemical modification of the homogeneous catalyst in
or London forces (van der Waals interactions) with the
[3]
support and (3) encapsulation. Various supports includ-
[
4]
[5]
[6,7]
ing polymers, Fe O nanoparticles, silicates,
zeo-
3
4
[8]
lites and carbon materials such as graphene oxide
Appl Organometal Chem. 2019;e4907.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 15
https://doi.org/10.1002/aoc.4907

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KARAMI ET AL.
comparison with conventional (covalent) immobilization.
Depending on the nature of support and complex, non‐
covalent immobilization has been realized via π–π stack-
ing, physical adsorption, electrostatic interaction, F–F
based interaction, non‐covalent interaction between
higher catalytic activities compared with the correspond-
ing homogeneous complex. The heterogenized catalyst
was recycled six times with no loss of activity. The
obtained hybrid material was also investigated in the
reduction of p‐nitrophenol. As a matter of fact, excellent
performance was observed for both catalytic reactions.
The benefits of supported molecular catalysis are shown
by this work and a practical methodology is proposed
with the capability to motivate future developments lead-
ing to efficient heterogeneous catalytic systems.
[19]
perfluorinated compounds
and self‐assembled mono-
layers. Most of these methods are rather simple and effi-
cient. This considerably simplifies the immobilization
process, hereby resulting in the easy availability of mate-
[20–22]
rials for industrial applications.
Graphene derivatives are interesting carbon com-
pounds considering their large specific surface area,
unique physical, chemical and mechanical properties,
chemical versatility and tunability as well as solution pro-
cessibility. Complete accessibility to all catalytic active
centres is provided by the two‐dimensional structure of
graphene. This is an advantage with regards to increasing
2 | EXPERIMENTAL
2.1 | Materials and instrumentation
All the experiments were performed in a nitrogen atmo-
sphere and high vacuum. All solvents were obtained from
commercial suppliers and degassed before use through
purging with dry nitrogen and stored over molecular
sieves. Aryl halides, phenylboronic acid (97%), cesium car-
bonate (Cs CO ), potassium phosphate (K PO ), potas-
[
23]
catalytic activity by avoiding diffusion problems.
In
addition to their inherent properties, graphene and its
derivatives provide a remarkable advantage for non‐
covalent modifications via π‐stacking interactions with
molecules which have polyaromatic parts, including
2
3
3
4
[
24]
promising molecules containing pyrene moiety.
The
sium carbonate (K CO ), sodium carbonate (Na2CO3), 1‐
2
3
solution pH does not affect polyaromatic hydrocarbon
bromoacetylpyrene and 1‐methylimidazole were supplied
by Sigma Aldrich Chemical Co. Graphite powder
[25]
immobilization onto rGO.
The non‐covalent interac-
tion between pyrene and rGO is based on van der Waals
interactions (π‐stacking) and control with thermody-
namic factors. Some of the valuable immobilization
methods developed recently are those involving π–π
stacking using graphene, carbon nanotubes, nanoparti-
(99.9995% pure), Pd(OAc) and N‐ethylbenzylamine were
provided by Merck Chemical Co. All the reagents were
used as received.
2
A Bruker spectrometer was utilized to record NMR
1
13
spectra at 500 MHz ( H) and 125 MHz ( C) by use of
standard pulse sequences at 298 K. A JASCO 680 Fou-
rier transform infrared (FT‐IR) spectrophotometer was
[26–49]
cles or polymers as supports.
Cyclopalladated complexes have been widely used in
organic synthesis, materials science, photochemistry and
−
1
used to record IR spectra in the range 400–4000 cm
[50–52]
the pharmaceutical industry.
The use of ligands such
using KBr pellets. A JASCO 7580 UV–visible double‐
beam spectrophotometer was used to record electronic
absorption spectra between 250 and 600 in a 1 cm path
length cell. Fluorescence spectra were recorded in
solution using a Cary Eclipse fluorescence spectropho-
tometer. Inductively coupled plasma atomic emission
spectroscopy (ICP‐AES) was used to determine Pd load-
ing. The samples were digested under reflux of a mix-
ture using concentrated nitric acid. Afterwards, the
recycling experiments were performed following several
as phosphines, amines, NHCs and dibenzylideneacetone
can tune the catalytic activity of Pd complexes. The best
choice for the preparation of an active catalyst is a combi-
nation of NHCs and metallacycle scaffolds. NHCs have
been of great interest as alternatives for phosphines due
to their higher dissociation energies, less toxicity, stability
in air and moisture, analogous σ‐donor and low π‐acceptor
abilities and stronger bonds with various transition
[53]
metals.
The preparation of pyrene‐containing catalysts non‐
covalently supported onto a graphene surface is a poten-
tially efficient form of catalyst heterogenization. Thus,
the synthesis of a ligand with a pyrene tag, which permits
the preparation of a Pd complex immobilized onto rGO
surface by π‐stacking interactions, is hereby reported.
The catalytic properties of the new hybrid compound in
the Pd‐mediated Suzuki–Miyaura cross‐coupling reaction
of a wide set of aryl halides and in p‐nitrophenol reduc-
tion were investigated. The supported catalyst showed
washes with Milli‐Q water (10 × 10 ml). The
Brunauer–Emmett–Teller (BET) method was applied to
determine the specific surface area of the samples at
77 K. A BELSORP‐II system was to carry out nitrogen
adsorption–desorption measurements. An STA 503A
(BÄHR) thermal analyser was used to perform thermo-
gravimetric analysis (TGA) in flowing argon atmosphere
−
1
(40 ml min ) from room temperature to 800°C at a
−
1
heating rate of 10°C min . Field emission scanning
electron microscopy (SEM) was used to investigate

KARAMI ET AL.
3 of 15
morphology through a Tescan Mira 3‐LMU FEG instru-
ment equipped with energy‐dispersive X‐ray spectros-
copy (EDS). Transmission electron microscopy (TEM)
images of the samples were recorded with a Zeiss
EM10C 80 kV instrument. X‐ray photoelectron spectros-
copy (XPS) was conducted with a SPECS PHOIBOS 150
MCD system employing a monochromatic Al Kα source
(dd, 2H, CH , J = 4.4 Hz), 3.78 (dd, H , CH H,
2
a
a
2
3
J = 15.6 Hz, J = 4.4 Hz), 4.24 (dd, H , CH H,
J = 14.6 Hz, J = 5.6 Hz), 5.94 (br s, 1H, NH), 6.73–7.5
b
b
2
3
(m, 8H,C H , J = 7.6 Hz). Anal. calcd for C H N O Pd
6
4
22 30
2
4
2
(%): C: 44.0; H: 5.04; N: 4.67. Found (%): C: 43.9; H: 5.04;
[
55]
N: 4.65.
(
hν = 1486.6 eV), a hemispherical analyser, multichan-
2
.4 | Synthesis of [Pd{(C,N)
nel detector and charge neutralization device. An
Agilent Technologies 6890N chromatograph equipped
with a flame ionization detector and an HB 50+ column
C H CH NH(Et)}(Imd‐P)Br] ((NHC)Pd)
6
4
2
Amounts of 127 mg of imidazolium salt (0.334 mmol),
00 mg of [Pd {(C,N)C H CH NH(Et)} (μ‐OAc) ]
(
length = 30 m, inner diameter = 320 μm, film thick-
1
2
6
4
2
2
2
ness = 0.25 μm) was used to carry out GC analyses.
An AW‐XDM 300 instrument was used to obtain pow-
der X‐ray diffraction (XRD) patterns with continuous
scanning in the 2θ range 10–80° with a 2θ step size of
(
0.167 mmol) and 40 mg of KBr were refluxed in argon
atmosphere in 30 ml of MeCN for 24 h. The obtained
precipitates were then filtered, and the solvent was
evaporated in vacuo. The pure yellow solid product
was obtained on precipitation with CH Cl –hexane.
0
.01° and a scan step time of 1 s.
2
2
−
1
Yield: 67%. IR (KBr, cm ): υ(CO) = 1705, υ(CN) =
2.2 | Synthesis of imidazolium salt
1593, υ(CHaliphatic) = 2924, 2967, υ(CHaromatic) = 3124.
1
H
NMR (500 MHz, DMSO, δ, ppm): 8.82 (d,
A
solution of 1.13
g
of 1‐(bromoacetyl)pyrene
J = 9.4 Hz, 1H, CHAr‐pyrene), 8.64 (d, J = 8.1 Hz, 1H,
CHAr‐pyrene), 8.49 (d, J = 8.1 Hz, 1H, CHAr‐pyrene), 8.39
(m, 3H, CHAr‐pyrene), 8.33 (d, J = 2.9 Hz, 1H, CHAr‐
pyrene), 8.31 (d, J = 3.6 Hz, 1H, CHAr‐pyrene), 8.17 (t,
J = 7.5, 1H, CHAr‐pyrene), 7.47 (d, J = 7.5 Hz, 2H,
CHAr‐phenyl), 7.43 (s, 1H, CHimidazole), 7.38 (s, 1H,
CHimidazole), 6.95 (t, J = 7.3 Hz, 1H, CHAr‐phenyl), 6.82
(
(
3.50 mmol) and 0.289 ml of 1‐methylimidazole
3.50 mmol) in 30 ml of dry MeCN was refluxed in
nitrogen atmosphere for 24 h. The suspension obtained
was filtered to yield a yellow precipitate, which was
then washed with diethyl ether (2 × 10 ml). Yield:
−
1
1
8
5%. IR (KBr, cm ): υ(CO) = 1679, υ(CN) = 1592. H
NMR (500 MHz, DMSO, δ, ppm): 9.25 (s, 1H), 9.02–
.16 (m, 9H, CHAr‐pyrene), 7.87 (d, J = 11.1 Hz, 2H,
CH ), 6.34 (s, 2H, NCH &bond;), 4.02 (s, 3H,
(t, J = 7.2 Hz, 1H, CH
NimidazoleCH &bond;), 3.83 (dd, J = 10 Hz, J = 3.55,
2
), 6.02, 6.29 (s, 2H,
Ar‐phenyl
2
3
8
1H, N
CH H &bond;), 3.74 (s, 3H, NCH ), 3.49
imidazole
2
amine
a
b
3
3
13
2
NCH3). C NMR (126 MHz, DMSO, δ, ppm): 194.09
C&dbond;O), 138.77 (NCHN), [136.98, 134.35, 130.95,
(dd, J = 10 Hz, J = 3.5 Hz, 1H, N
CH H &bond;),
amine
a
b
(
2.83 (m, 2H, N
CH ), 1.27 (t, J = 7.0 Hz, 3H,
amine 2
13
1
1
30.52, 129.65, 128.15, 127.65, 127.19, 126.88, 126.49,
25.08, 124.84, 123.97, 123.78, 123.53, 123.16] (CHAr,
NamineCH3). C NMR (126 MHz, DMSO, δ, ppm):
196.02 (C&dbond;O), 175.52(CCarbene‐Pd), [151.91,
136.04, 133.85, 131.12, 130.77, 130.66, 130.38, 130.19,
129.71, 128.01, 127.69, 127.59, 127.55, 126.95, 126.59,
125.29, 124,85, 124.79, 124.68, 124.41, 123.71, 123.62,
CHimidazole), 57.10 (NCH &bond;), 36.23(NCH ). Anal.
calcd for C H N OBr (%): C: 65.20; H: 4.23; N: 6.91.
Found (%): C: 65.09; H: 4.16; N: 6.85.
2
3
22
17 2
[54]
1
5
3
21.19, 121.64] (CH , CHimidazole), 60.43 (NamineCH2),
Ar
9.53 (NimidazoleCH &bond;), 58.92 (N
CH &bond;),
2
2
amine
2
.3 | Synthesis of dimer [Pd{(C,N)
8.7 (NimidazoleCH ), 14.31 (N
CH ). Anal. calcd for
3
3
amine
C H CH NH(Et)}(μ‐OAc)]
6
4
2
2
C H BrN OPd (%): C: 57.65; H: 4.53; N: 6.51. Found
31
29
3
[
56]
(
%): C: 57.23; H: 4.31; N: 6.65.
An amount of 150 μl of N‐ethylbenzylamine
PhCH NH(Et); 1 mmol) was added to 0.224 g of
(
2
Pd(OAc) (1 mmol) in 20 ml of toluene. The obtained
2.5 | Preparation of GO
2
solution was then refluxed for 24 h. The solvent was then
removed from the resulting mixture and the dry residue
was recrystallized from CH Cl and hexane to yield the
A modified Hummers method was used to prepare GO
from flake graphite. In short, 1 g of graphite and
2
2
−
1
pure product. Yield: 72%. IR (KBr, cm ): υ(CO bridge)
0.75 g of NaNO were poured into a flask, which was
3
1
=
1566, 1414, υ(NH) = 3227. H NMR(400.13 MHz,
then placed in an ice–water bath. Afterwards, 75 ml of
DMSO, δ, ppm): 0.73 (t, 3H, CH , J = 7.2 Hz), 1.19
H SO was added with stirring, followed by the slow
3
2
4
(
t,3H, CH , J = 7.2 Hz),1.8 (s, 3H, MeCO ), 1.88 (s, 3H,
addition of 4.5 g of KMnO within an hour. This was
3
2
4
MeCO ), 2.77 (m, 4H, CH ), 3.33 (br s,1H, NH), 3.54
followed by stirring vigorously for 5 days at room
2
2

4
of 15
KARAMI ET AL.
temperature. Then, 140 ml of aqueous 5% H SO was
the lowest free binding energy and reveal the possibility
of different non‐covalent interactions between these
molecules during molecular docking simulation. At first,
the chemical structure of the (NHC)Pd complex was
prepared by Gauss View 5.0 software. Molecular energy
optimization of the complex was done by Gaussian 09
software (Quantum Chemistry package). The density
functional theory method with Becke's three‐parameter
hybrid functional (B3LYP) using 3‐21G basis set was
applied in the minimization approach. Then, the chem-
ical structure of rGO was obtained from the PubChem
2
4
added to the mixture over a period of about 1 h with
stirring, the temperature being kept at 98°C. The tem-
perature of the reaction mixture was reduced to 60°C
and 3 ml of 30 wt% aqueous H O was added and the
2
2
stirring was continued for 2 h at room temperature.
The resulting GO was suspended in deionized water to
yield a brown dispersion, which was subjected to dialy-
sis for complete removal of residual salts and acids.
Pure GO powder was collected by centrifugation, dried
in air and dispersed in water to form a 0.05 wt% disper-
sion. Afterwards, the product was exfoliated by
ultrasonication for 1 h, during which the bulk GO pow-
[59]
compound database (PubChem CID = 124202900).
A 1.46 × 2.03 nm rGO nanosheet structure was modi-
fied where hydroxyl functional groups were added at
the edge of the rGO nanosheet. The rGO structural
geometry optimization was performed by employing
the UFF (Universal Force Field) in Avogadro 1.1.1 soft-
[57]
der was converted into GO nanoplatelets.
2.6 | Reduction of exfoliated GO with
hydrazine hydrate (rGO)
[60,61]
ware (Libavogadro Library, Pittsburgh, PA, USA).
The prepared compound molecular structures were used
for docking simulation by Auto Dock 4.2.6 and its
In a general experiment, 100 mg of GO was placed in a
round‐bottom flask and 100 ml of water was then added
to form a yellow‐brown dispersion. This dispersion was
subjected to sonication (100 W) until it became clear and
no particulate matter was observed. An amount of
graphical user interface, Auto Dock Tools 1.5.6 soft-
[62]
ware.
rGO nanosheet and complex structures were
introduced to the determined grid box with dimensions
at grid points in x, and directions of
0 × 80 × 60 Å with 0.375 Å grid spacing. Maximum
y
z
1
.00 ml of hydrazine hydrate (32.1 mmol) was then added,
followed by heating the solution in an oil bath at 100°C for
4 h, during which the rGO slowly precipitated as a black
3
8
of 200 runs were considered in docking process. So, all
of the parameter values in Auto Dock Tools software
were kept as a default in this docking study. The genetic
algorithm method was set to estimate the potential
interaction conformations of Pd‐containing complex.
2
solid. This product was isolated by centrifugation and
washed with water (5 × 20 ml) and EtOH (5 × 20 ml).
The obtained black solid was dried in vacuum at 308 K
[58]
for 2 h.
2
.7 | Preparation of (NHC)Pd‐rGO hybrid
2
.9 | Typical procedure for Suzuki–
Miyaura cross‐coupling reaction
Amounts of 200 mg of rGO and 40 ml of CH Cl were
2
2
poured into a round‐bottom flask. The suspension was
subjected to sonication for 30 min. For a loading of 2%
Pd, 25 mg of (NHC)Pd complex was added. The resulted
suspension was stirred at ambient temperature for 24 h
until the colour of solution disappeared. The black solid
was filtered and washed with CH Cl (3 × 10 ml) to afford
the (NHC)Pd‐rGO hybrid material as a black powder.
ICP‐AES shows 2.04% Pd in this material, representing
the solid‐supported catalyst.
(
(
(
NHC)Pd‐rGO nanocatalyst (0.2 mol% Pd), aryl halide
1.0 mmol), phenylboronic acid (1.2 mmol) and K CO
2
3
2 mmol) were placed in a round‐bottom flask. To the
mixture, EtOH (3 ml) was added and stirred at 75°C for
h. The progress of the reaction was monitored by GC.
2
2
2
After reaction completion, the mixture was cooled to
room temperature and the (NHC)Pd‐rGO nanocatalyst
was separated by centrifugation. EtOAc (15 ml) and
water (15 ml) were added to the filtrate. Organic layer
was separated using a separating funnel and dried with
2.8 | Molecular docking approach
MgSO . The final solution was concentrated in vacuo
4
and the purification of product was performed by col-
umn chromatography using n‐hexane and EtOAc as elu-
ents to afford the corresponding products in good to
A molecular docking study was conducted with the a
newly synthesized palladacycle containing organic chro-
mophore linked to rGO nanosheet in order to computa-
tionally analyse the interaction potential and favoured
binding mode of Pd complex–rGO system, calculate
excellent yields. All the coupling products were known
1
molecules and were confirmed by comparing the
NMR data and melting point with reference samples.
H

KARAMI ET AL.
5 of 15
2
.10 | General procedure for reduction of
p‐nitrophenol (p‐NP) to p‐aminophenol
p‐AP)
imidazolium salt with palladacycle dimer [Pd{(C,N)
C
6
H
CH
NH(Et)}(μ‐OAc)] in the presence of KBr in
2
4
2
(
60% yield (Scheme 1). Subsequently, KBr was added to
the reaction mixture to assist bromide complex forma-
tion. Complex (NHC)Pd is completely stable in air and
moisture. NMR spectroscopy, TGA and FT‐IR spectros-
copy were used to characterize the (NHC)Pd complex
and imidazolium salt.
An aqueous solution of 1 mM p‐nitrophenol (200 μl) and
fresh 0.1 M NaBH (200 μl) were loaded into a quartz cell
and 2.5 ml of deionized water was added. Then, 100 μl of
4
(NHC)Pd‐rGO aqueous suspension were added and the
progress of the reaction was monitored by UV–visible
spectroscopy. The rate constant of reaction was obtained
by the decreased intensity of the peak at 400 nm. The
decreased absorption at 400 nm was recorded at 0.4 min
intervals. GC‐MS analysis of the final mixture was used
to calculate the conversions.
3.2 | Structure characterization
Figure S1 depicts the FT‐IR spectra of GO, rGO, (NHC)
Pd‐rGO, (NHC)Pd complex and imidazolium salt. Peaks
−
1
at 3406, 1727, 1627, 1222 and 1051 cm , corresponding
to OH, C&dbond;O, C&dbond;C and C&bond;O
stretching vibrations, are observed in the GO spectrum.
The characteristic GO bands are weaker or completely
absent in the FT‐IR spectrum of rGO because of the
reduction by hydrazine hydrate. An additional peak is
observed in the FT‐IR spectrum of (NHC)Pd‐rGO, com-
pared with rGO, as a result of the carbonyl stretching
vibrations of the (NHC)Pd complex. The other peak is
3
3
| RESULTS AND DISCUSSION
.1 | Synthesis of materials
Imidazolium salt was synthesized via methylimidazole
alkylation with commercial 1‐(bromoacetyl)pyrene. The
(NHC)Pd complex was obtained by reacting the
SCHEME 1 Synthesis of (NHC)Pd‐rGO hybrid material

6
of 15
KARAMI ET AL.
overlapped by peaks corresponding to the other groups
present in the support. Intense bands at 1705 and
The superior thermal stability of (NHC)Pd‐rGO is shown
by TGA. The higher weight loss of the catalyst is presum-
ably due to the thermal decomposition of the (NHC)Pd
complex, which is anchored to the graphitic surface.
The TGA curves of imidazolium salt, complex, rGO and
final catalyst are presented in Figure S6 for comparative
purposes. Table 1 presents the TGA/DTG/DTA thermal
decomposition data for all compounds.
−
1
1
679 cm in the FT‐IR spectra of the complex and
imidazolium salt, respectively, are associated with car-
bonyl stretching vibrations. The C&dbond;N stretching
−
1
peak is observed at 1593 cm .
The removal of the acidic hydrogen signal of the
1
imidazolium salt at 9.25 ppm in the H NMR spectra of
the (NHC)Pd complex indicates a carbene‐type bonding
The UV–visible absorption spectra of (NHC)Pd‐rGO,
(NHC)Pd complex and rGO have been studied. UV–visible
spectroscopy was carried out in the range 250–600 nm. The
samples were suspended in dimethylformamide (DMF)
and subjected to sonication for 6 min prior to the measure-
of the imidazolylidene ligand. The characteristic
13
M&bond;Ccarbene signals at 175.52 ppm in the C NMR
spectrum of the (NHC)Pd complex further verify this.
Simultaneous TGA/DTG and DTA analyses of (NHC)
Pd complex and imidazolium salt were carried out at an
ments. The molecular complex was dissolved in DMF
−
1
−6
appropriately controlled heating rate of 10°C min in
argon atmosphere and the weight loss was measured
from room temperature to 800°C. Figures S2 and S3 show
TGA/DTG/DTA curves of imidazolium salt and (NHC)Pd
complex. Decomposition steps of imidazolium salt and
(10
M). The UV–visible spectrum of (NHC)Pd‐rGO
(Figure 1) shows the characteristic peaks of the (NHC)Pd
molecular complex. This clearly confirms the immobiliza-
tion of the complex on rGO, as also indicated by UV–visible
peaks of pyrene at 278 and 347 nm.
(
NHC)Pd complex are explained in detail in the
Typical bands are observed in the emission spectrum
of the (NHC)Pd complex at 388, 410 and 435 nm
(Figure 1). The fluorescence intensity of the complex
was quenched to almost 53% following the attachment
onto rGO. This indicates that the complex had been
immobilized on rGO by π–π stacking interactions. The
fluorescence emission of rGO appears to be increased by
the π–π interactions of rGO with (NHC)Pd, simulta-
neously quenching the fluorescence bands of (NHC)Pd.
Thus, the optical properties of rGO (e.g. for imaging)
are improved in the (NHC)Pd‐rGO hybrid via non‐
supporting information.
The (NHC)Pd complex was supported onto rGO
through mixing the molecular complex and rGO in CH Cl2
in an ultrasonic ice bath for 30 min, followed by stirring for
2
indication for the anchoring of the (NHC)Pd molecular
complex on the rGO surface. The yellow solution of
(
tion of the complex on rGO. The resulting hybrid, (NHC)
Pd‐rGO, was centrifuged and washed with CH Cl2
2
4 h. The disappearance of the solution colour is the first
NHC)Pd complex in CH Cl disappears upon the reten-
2 2
2
[
64]
(
3 × 10 ml). No trace of (NHC)Pd was found in the solution
covalent interactions.
according to the UV–visible spectrum of the filtrate,
confirming the quantitative deposition of (NHC)Pd onto
rGO. Thus, the disappearance of characteristic fluores-
cence band and UV absorption of supernatant of hybrid
following centrifugation show the successful attachment.
ICP‐AES analysis was used to determine the exact amount
of Pd in (NHC)Pd‐rGO. The obtained results show
The powder XRD patterns of GO, rGO and (NHC)Pd‐
rGO revealed a decrease in the layer‐to‐layer distance
for the hybrid material in line with the intercalation
[
65,66]
(Figure S7).
Further details of the analysis are given
in the supporting information.
A BELSORP‐II system was used to obtain nitrogen
adsorption–desorption isotherms at 77 K. The samples
were degassed for several hours at 100°C prior to analy-
sis. The adsorption data were used with the BET
method to calculate the specific surface area. Nitrogen
physisorption analysis at 77 K was used to analyse the
surface area of both (NHC)Pd‐rGO and rGO. The BET
2
.04 wt% of Pd in (NHC)Pd‐rGO.
TGA/DTG/DTA (Figures S4 and S5) was used to mea-
sure the thermal stability of the prepared (NHC)Pd‐rGO
catalyst and rGO. In the TGA curve of rGO, a small
weight loss is observed in the range 40.46–183°C with
TDTA at 105°C, attributed to the removal of water mole-
cules adsorbed between the rGO nanosheets. In addition,
the destruction of oxygenated functional groups left in
rGO accounts for the continuous weight loss in the 200–
2
−1
area of (NHC)Pd‐rGO was 77.61 m g , which is con-
siderably smaller than that of rGO raw material
2
−1
(130.7 m g ) (Figure 2).
The homogeneous distribution of Pd in (NHC)Pd‐rGO
was verified by elemental mapping via EDS in combina-
tion with SEM. SEM and EDS elemental mapping images
of (NHC)Pd‐rGO are shown in Figure 3. EDS of the
hybrid material clearly shows the elements Pd, O, N
and C that confirm the presence of the (NHC)Pd complex
on the surface of rGO (Figure S8). SEM and TEM images
4
7
00°C range. The residue remaining at 800°C is about
[63]
7.4% of the original sample weight.
A total weight
loss of about 29.12% is observed in the TGA curve of
(NHC)Pd‐rGO, which corresponds to the removal of
water molecules and O‐containing functional groups
and decomposition of immobilized (NHC)Pd complex.

KARAMI ET AL.
7 of 15
TABLE 1 TGA data and assignments of compounds
Temperature
Compound
Step
range (°C)
Weight loss (%) Assignment
Imidazolium salt
1st
50–152
268–298
330.2–398
398–800
800
4
2
1 H O (lattice water)
2nd
3rd
4th
3.08
44.56
18.2
30.07
Decomposition of ligand
Decomposition of ligand
Decomposition of ligand
Remaining
NHC)Pd complex 1st
nd
rd
(
92–175.21
176–215
215–276.21
280–348
348–800
800
2.76
4.25
6.57
14.87
21.22
49.64
2
1 H O(lattice water)
2
3
Decomposition of ligand
Decomposition of ligand
Decomposition of ligand
Decomposition of ligand
Pd and ligand
4
5
th
th
Remaining
rGO
1st
40.46–183
183–800
800
4.37
18.23
77.40
2
H O
2nd
O‐containing functional group
Remaining
(
NHC)Pd‐rGO
1st
Remaining
35–800
29.12
70.88
H
2
O, O‐containing functional group, decomposition of complex
800
FIGURE 1 UV–visible and fluorescence spectra of rGO, (NHC)Pd‐rGO and (NHC)Pd complex
FIGURE 2 Nitrogen physisorption of (NHC)Pd‐rGO (a) and rGO (b)
of the catalyst were obtained to study its morphology
and particle size. TEM images of (NHC)Pd‐rGO indicate
a sheet like morphology at different magnifications
(Figures 4 and S9). The folding and wrinkles characteriz-
ing the presence of two‐dimensional graphene sheets can
be observed.

8
of 15
KARAMI ET AL.
FIGURE 3 SEM image of (NHC)Pd‐rGO and elemental mapping images showing the homogeneous distribution of Pd
Pd 3d3/2 doublet peaks, which are located near the
expected positions for Pd(II) at around 337.73 and
343.13 eV, respectively. These results confirmed immobi-
lization of (NHC)Pd complex in molecular form. The C 1s
spectrum of the catalyst showed a strong peak at
2
2
84.58 eV and lower intensity peaks at 286.6 and
88.34 eV corresponding to sp carbon, epoxide and car-
2
boxyl functional groups, respectively (Figure 5b). The
ratio of C/O for (NHC)Pd‐rGO is 5.5. Furthermore, the
O 1s spectrum was reasonably fitted into a couple of
peaks at 532.46 eV for C&bond;O and 535.17 eV for
O&bond;C&dbond;O, indicating the presence of the car-
bonyl group in (NHC)Pd‐rGO (Figure 5d). The N 1s XPS
spectrum of (NHC)Pd‐rGO showed the N&bond;Pd and
N&bond;C binding energies at 400.1 and 399.43 eV,
respectively (Figure 5c). According to the Pd 3d spectrum
of the catalyst after the first catalytic cycle in the presence
of NaBH as reducing agent for the reduction of p‐NP, the
4
presence of characteristic Pd 3d5/2 and 3d3/2 peaks related
to Pd(0) located at 335.76 and 341.3 eV confirms the con-
version of Pd(II) to Pd(0) under experimental conditions
(Figure 5e). According to the C 1s spectrum, the peaks
located at 286.6 and 288.34 eV approximately disappeared
(Figure 5f). The N 1s spectrum after the first recycle
showed two components at around 400.0 and 399.53 eV
with lower intensity compared to fresh catalyst
(Figure 5g). The decreased intensity of peaks related to
FIGURE 4 TEM image of (NHC)Pd‐rGO
To prove the reduction of Pd(II) in nitro reduction
reaction and determine the composition of the (NHC)
Pd‐rGO catalyst, XPS was utilized. Figure 5 illustrates
XPS spectra of (NHC)Pd‐rGO catalyst before and after a
first catalytic cycle. The Pd 3d spectrum in Figure 5a of
the fresh catalyst shows the characteristic Pd 3d5/2 and

KARAMI ET AL.
9 of 15
FIGURE 5 XPS spectra of fresh (NHC)Pd‐rGO catalyst: (a) Pd 3d, (b) C 1s, (c) N 1s, (d) O 1s; and after first catalytic cycle: (e) Pd 3d, (f) C
s (g) N 1s, (h) O 1s
1
−
1
N 1s and C 1s and C/O ratio to 4.4 could indicate the
removal of ligand from (NHC)Pd complex after reduction
of Pd(II).
−12.79 kcal mol . This result indicates the high
potency of the (NHC)Pd complex to interact with rGO
nanosheet. The lowest binding free energy conformation
of complex and rGO is indicated in Figure 6a. Having
analysed all of the binding conformations, it is evident
that the complex tends to interact with rGO in the cen-
tre of the nanosheet. The presence of pyrene polycyclic
aromatic hydrocarbon in the complex structure facili-
tates the formation of face‐to‐face π–π stacking interac-
tions with rGO nanosheet. Figure 6b indicates π–π
stacking interactions formed between polycyclic
[67,68]
3.3 | Molecular docking result
According to the result obtained from the output file
of docking simulation, the calculated free binding
energy between the newly synthesized palladacycle con-
taining organic chromophore and rGO nanosheet is
FIGURE 6 Lowest binding free energy conformation of complex and rGO: (a) yellow solid long lines represent potential π–π stacking
interactions; (b) solid spheres indicate the close contacts, and (c) the small violet spheres represent the hydrogen bond; (d) the
hydrophobic interactions are indicated by wireframe spheres

10 of 15
KARAMI ET AL.
aromatic moiety of complex and rGO nanosheet. Due to
the nature of graphene structure (π‐electron system)
and the presence of pyrene in the complex chemical
structure, the π–π stacking interactions are the predom-
inant non‐covalent interactions in this system. Also, the
ability to form hydrogen bonds via amine group with
carboxyl and hydroxyl functional groups at the edge of
GO nanosheet seems to noticeably help the adsorption
and stabilization of the complex with rGO. In Figure 6
c, a hydrogen bond formed between carboxyl group of
rGO and amino group of the complex is represented
by violet spheres. Further analysis of molecular docking
indicated that hydrophobic interactions occurred
between rGO nanosheet and complex (Figure 6d). The
distance of the best docked conformer of (NHC)Pd com-
plex with rGO nanosheet was measured from the
docking results. Measured distance of pyrene and Pd
centre with rGO were 3.5 and 5 Å, respectively. The
obtained distance of hydrogen binding of amine group
and edge of rGO is 2.08 Å. It is evident that the π–π
stacking interactions have a pivotal role in non‐covalent
interactions between the complex and bulky π‐electron
system of rGO nanosheet.
Miyaura cross‐coupling reaction between bromobenzene
and phenylboronic acid (Scheme 2).
The investigation of effect of solvent on the activity of
(NHC)Pd‐rGO demonstrates that the reaction proceeds
suitably in EtOH solvent (Table 2). This can be related
to the good solubility of the reactants in EtOH. This sol-
vent is preferred to toluene due to its green nature and
higher stability of the catalyst in it. In the more polar
EtOH medium, non‐polar (NHC)Pd complex may prefer
to bond to the rGO surface by stronger π–π stacking since
the hydrophobic interaction between complex, rGO and
the medium may be enhanced. Average and low coupled
product yields are obtained in DMF and MeCN solvents.
One possible explanation for this is that MeCN and
DMF are basic solvents, which could compete with the
surrounding ligands coordinating with the Pd centre to
[
71]
give less active complexes.
In addition, DMF is less
basic than MeCN because of conjugation. Thus it is less
accessible for coordination and the reaction progresses
to 59% product in DMF while a yield of only 9.62% is
obtained in MeCN over the same period of time. All in
all, very low coupled product yields were obtained in tet-
rahydrofuran (THF).
The effect of bases on the efficiency of the (NHC)Pd‐
rGO catalyst was investigated. K CO was shown to be
2
3
the most efficient base (Table 2, entry 11).
3
.4 | Catalytic properties of (NHC)Pd‐rGO
The impact of catalyst dosage was studied at three dif-
ferent concentrations using K CO and EtOH as the base
2 3
and solvent, respectively. To optimize the catalyst
amount, the reaction was carried out in EtOH in the pres-
nanohybrid catalyst
3
.4.1 | Suzuki–Miyaura cross‐coupling
reaction
ence of K
CO at 75°C with catalyst loadings of 0.05, 0.1,
2 3
0
.2, 0.3 and 0.4 mol% based on the metal amount (the
Following structural characterization by spectroscopic,
microscopic and computational methods, the catalytic
activity of the synthesized (NHC)Pd‐rGO catalyst, as
an efficient and stable nanocatalyst in Suzuki–Miyaura
cross‐coupling reactions of a number of aryl halides
with phenylboronic acid, was investigated. One of the
most effective techniques for the synthesis of biaryl
and alkene derivatives, which are structural components
of natural products, agrochemicals, pharmaceuticals,
polymers and advanced materials, is Pd‐catalysed
solid amount was calculated on the basis of the Pd
amount determined by ICP‐AES). A loading of 0.2 mol%
Pd of (NHC)Pd‐rGO gave the highest conversion or
maximum yield (Table 2, entry 15). In addition, having
more catalyst does not markedly enhance the conversion
(Table 2, entry 17).
Studying the effect of temperature on the reaction
shows a high conversion at 75°C (Table 2, entry 21).
Lower yields are obtained at lower temperatures (40, 50
and 60°C; Table 2, entries 18–20). The yield decreases at
high temperatures (85°C; Table 2, entry 22). Therefore,
all the reactions were performed at 75°C. Optimum time
for reaction was 2 h (Figure 7 and Table 2).
[69]
Suzuki–Miyaura coupling reaction.
This reaction
allows the coupling of such organic electrophiles as aryl
and alkenyl halides and organoboron compounds. The
advantages of this reaction include high efficiency,
non‐toxic boronic acid by‐product and high functional
(NHC)Pd‐rGO was used in the reaction of
phenylboronic acid with different types of aryl halides
[70]
group tolerance.
A number of parameters, including
solvent, base type, temperature and catalyst amount,
determine the performance of a Pd‐catalyzed Suzuki–
Miyaura cross‐coupling reaction. Therefore, the impacts
of various solvents, bases, temperatures and catalyst
ratios have been investigated in a model Suzuki–
SCHEME 2 Suzuki–Miyaura cross‐coupling reaction between
bromobenzene and phenylboronic acid in the presence of (NHC)
Pd‐rGO catalyst

KARAMI ET AL.
11 of 15
a
TABLE 2 Optimization of reaction conditions for Suzuki–Miyaura reaction of bromobenzene with phenylboronic acid
Entry
Solvent
Base
CO
Temperature (°C)
Time (h)
Catalyst (mol%)
Yield (%)
1
2
3
4
5
6
7
8
9
EtOH
EtOH–H
MeOH
MeCN
THF
K
2
3
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
40
50
60
75
85
75
75
75
75
75
75
2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.05
0.1
0.2
0.3
0.4
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
98
81
73
10
Trace
60
82
21
34
18
98
24
70
75
98
90
86
3
2
O
K
K
K
K
K
K
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
2
3
2
3
3
3
2
2
DMF–H
Toluene
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
2
O
2
3
2
Na
2
CO
3
2
K
3
PO
NaOH
CO
CO
4
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
2
K
2
3
2
Cs
2
3
2
K
K
2
2
CO
3
3
2
CO
2
K
2
CO
3
2
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
2
3
2
3
3
3
2
2
36
75
98
86
42
61
76
98
97
98
2
K
2
CO
3
2
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
2
3
0.5
1
3
3
1.5
2
2 3
K CO
K
K
2
2
CO
3
2.5
3
CO
3
a
2
Reaction conditions: bromobenzene (1.0 mmol), PhB(OH) (1.2 mmol), base (2.0 mmol) and EtOH (3 ml).
under the optimal reaction conditions. The (NHC)Pd‐
rGO nanocatalyst is highly efficient for a wide range of
substrates (Table 3). Generally, substrates with electron‐
withdrawing groups yield better results. The (NHC)Pd
complex on rGO is comparably more effective than other
palladacycle solid supports (Table 4). It is noteworthy that
for all of the catalysts mentioned, the palladacycle is con-
nected to the support by a covalent bond; while in this
work, the palladacycle is immobilized on the support in
a non‐covalent manner.
carried out with the (NHC)Pd complex as a homogeneous
catalyst under identical reaction conditions (Table 5,
entry 2). The reaction yield is considerably lower than
those using the (NHC)Pd‐rGO hybrid, according to the
results of GC analysis. Thus, rGO immobilization
enhances the catalytic activity of the (NHC)Pd complex.
In this regard, the results indicate that rGO has an impor-
tant catalytic effect due to its surface area which may
accelerate the interaction between the substrates and
complex.
GO and rGO have been investigated in various pro-
Having obtained acceptable yields with the (NHC)Pd‐
rGO nanocatalyst in cross‐coupling reactions, it was
decided to investigate the catalyst stability with respect
to its recycling capability. Recycling experiments of
the Pd‐containing catalyst were performed under the
[76]
cesses due to their inherent catalytic properties.
Con-
trol experiments using rGO under identical reaction
conditions show that the Pd‐free material is completely
inefficient (Table 5, entry 1). Coupling reaction was also

12 of 15
KARAMI ET AL.
conditions previously described using bromobenzene as a
model substrate, and GC analysis was used to monitor the
reaction. After each run (2 h), the reaction mixture was
allowed to cool to ambient temperature and the reaction
mixture was then centrifuged to separate the solid cata-
lyst, washed with CH Cl , dried and reused in the next
2
2
run. The base was only required in the first cycle, which
may be due to the complex transformation into the active
catalyst during the first run or adsorption of residual
[
77]
K CO to the rGO surface.
High catalytic activity is
2
3
shown by the (NHC)Pd‐rGO catalyst (100 to 98% conver-
sion) and the catalyst can be reused up to six times with
no appreciable loss of catalytic activity (Figure 8).
In this study, ICP‐AES was used to analyse the solid
resulting from the recycling experiments. The Pd content
of (NHC)Pd‐rGO was 1.98 wt%, according to ICP‐AES
analysis of the solids. Thus, there was a total loss of
0.06 wt% of Pd after all six reaction cycles. This small loss
of Pd content may have resulted from the catalyst collec-
tion during the subsequent runs because of the adsorption
of substances from the catalyst reactions and not desorp-
tion or leaching of the metal complex. The markedly high
stability and reusability of the synthesized catalyst may
be associated with the strong non‐covalent interaction
between the pyrene moiety and the graphitic surface. After
the sixth run, TEM images of (NHC)Pd‐rGO indicate the
retention of rGO morphology (Figure S10).
FIGURE 7 GC profile for the catalytic reaction of bromobenzene
with phenylboronic acid using (NHC)Pd‐rGO catalyst for various
times
TABLE 3 Suzuki–Miyaura cross‐coupling reactions of various
aryl halides and phenylboronic acids using (NHC)Pd‐rGO as
a
catalyst
Entry
Aryl halide
Product
Yield (%)
1
100
Hot filtration experiments were performed to investi-
gate the possibility of release‐and‐return of the molecular
complex from the graphene surface. This kind of experi-
ment is used to differentiate between homogeneous and
heterogeneous catalytic processes, particularly in the case
of supported molecular catalysts. The experiment was
performed under the general conditions previously
described using bromobenzene and phenylboronic acid
as substrates. The catalyst was filtered off at 75°C, 1 h
through the reaction (GC yield of 61%). The filtrate was
allowed to stir for 2 h more under identical conditions,
after which no further coupling reaction took place based
on the results of GC analysis (GC conversion remained at
61%). This result verifies the absence of catalytic active
species in the reaction medium resulting from Pd
leaching or desorption of the molecular catalyst by the
boomerang effect. Therefore, the (NHC)Pd‐rGO catalyst
is heterogeneous in nature.
2
3
98
57
4
5
99
95
6
46
7
8
98
98
9
82
89
1
1
0
1
3.4.2 | Reduction of p‐NP
The catalytic activity of the (NHC)Pd‐rGO catalyst was
also evaluated in the reduction of p‐NP to p‐AP using
NaBH4 in aqueous medium at ambient temperature.
UV–visible absorption spectroscopy was performed to
80
a
Yield determined by GC.

KARAMI ET AL.
13 of 15
TABLE 4 Comparison of reported results for Suzuki–Miyaura cross‐coupling reactions using supported palladacycle catalysts
Time (h)/
Entry
Catalyst
temperature (°C)
Medium
Yield (%)
Ref.
[
[
[
[
72]
73]
74]
75]
1
2
3
4
5
Palladacycle‐Fe
3
O
4
24/100
17/130
2/50
Toluene
Toluene
99
72
73
99
98
Palladacycle‐silica
Oxime palladacycle‐polymer
Palladacycle‐polystyrene
(NHC)Pd‐rGO
2
DMF–H O
5/130
2/75
o‐Xylene
EtOH
This work
TABLE 5 Comparision catalyst activity of (NHC)Pd complex, (NHC)Pd‐rGO and rGO
Entry
Catalyst
Solvent
Time (h)
Temperature (°C)
Base
Yield (%)
1
2
3
rGO
EtOH
EtOH
EtOH
2
2
2
75
75
75
K
2
K
2
K
2
CO
CO
CO
3
3
3
0
52
98
(NHC)Pd complex
(NHC)Pd‐rGO
FIGURE 8 Recycling experiment of (NHC)Pd‐rGO catalyst in
Suzuki–Miyaura cross‐coupling reaction
FIGURE 9 UV–visible absorption spectrum of p‐NP reduction
with NaBH catalysed by (NHC)Pd‐rGO at ambient temperature
4
follow the reduction of p‐NP using the evaluation of the
4
00 nm absorption of the p‐nitrophenolate ions that are
induction period at the start of the reaction that is related
to the rearrangement of the surface atoms before the prog-
ress of the reaction. Since hydrolysis of borohydride
depends on pH (the lower the pH, the faster the hydrolysis
of borohydride), the induction time partially related to the
original pH of the solution, and a greater induction time
−
formed in the presence of the strong BH4 base. In the
absence of the catalyst, no reduction was observed.
Adding the catalyst leads to a decrease in the 400 nm sig-
nal and, at 300 nm, the absorption band typical for p‐AP
appears (Figure 9).
[
78]
Assuming pseudo‐first‐order kinetics, we calculated a
occurs in alkaline solution.
−
3 −1
rate constant Kapp of 8.6 × 10 s , which is relatively high
in view of the low amount of Pd in the (NHC)Pd‐rGO
catalyst. The pseudo‐first‐order rate constant (kobs) was
measured from the linear relation of ln(C /C ) versus time
with a good linear correlation (Figure S11). Also, p‐AP for-
mation was verified by GC. The results provided valid evi-
dence for the efficient p‐NP reduction to p‐AP. There is an
The precursor compound is reduced by NaBH to form
4
catalytically active metallic Pd. The classical Langmuir–
Hinshelwood model describes the possible reaction
mechanism. Electrons are transferred to the surface of Pd
t
0
−
nanoparticles by the borohydride ions (BH ), adsorbed
4
onto the Pd surface to form Pd hydride, and p‐NP mole-
cules are adsorbed on the Pd surface. In addition, p‐NP

14 of 15
KARAMI ET AL.
reduction to p‐AP occurs on the surface of the Pd nanopar-
[4] S. M. Islam, K. Ghosh, R. A. Molla, A. S. Roy, N. Salam, M. A.
[79]
Iqubal, J. Organometal. Chem. 2014, 774, 61.
ticles. When this reaction was investigated with rGO in
the absence of the (NHC)Pd complex, no reduction
occurred, indicating that Pd plays a main role in the
reaction.
[5] H. Keypour, M. Balali, M. M. Haghdoost, M. Bagherzadeh, RSC
Adv. 2015, 5, 53349.
[6] D. Do Van, T. Hosokawa, M. Saito, Y. Horiuchi, M. Matsuoka,
Appl. Catal. A 2015, 503, 203.
[
[
[
7] L. L. Lou, S. Jiang, K. Yu, Z. Gu, R. Ji, Y. Dong, S. Liu,
Micropor. Mesopor. Mater. 2011, 142, 214.
4
| CONCLUSIONS
8] C. Martinez‐Macias, P. Serna, B. C. Gates, ACS Catal. 2015, 5,
A palladacycle molecular complex containing polyaro-
matic NHC ligand was synthesized. It was also
immobilized on rGO surface through non‐covalent inter-
actions under mild reaction conditions to yield the
5
647.
9] S. K. Movahed, R. Esmatpoursalmani, A. Bazgir, RSC Adv.
014, 4, 14586.
2
[
[
[
[
[
[
10] K. Vishal, B. D. Fahlman, B. S. Sasidhar, S. A. Patil, S. A. Patil,
(NHC)Pd‐rGO hybrid compound. The incorporation of
Catal. Lett. 2017, 147, 900.
well‐defined molecular complex onto chemically modified
graphene is allowed by this modular methodology, in
which the complex properties are not changed by the
immobilization effect. Computational results confirmed
π–π stacking interactions are dominant for immobilization
of (NHC)Pd complex on rGO and the (NHC)Pd‐rGO
hybrid is stable. The catalytic behaviour of the (NHC)Pd
complex and the (NHC)Pd‐rGO hybrid material has been
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Ramezanpour A, Zakariazadeh M, Silvestru C.
Catalytic activity and facile recovery of a
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