Immobilized N-Heterocyclic Carbene-Palladium(II) Complex on Graphene Oxide as Efficient and Recyclable Catalyst for Suzuki–Miyaura Cross-Coupling and Reduction of Nitroarenes

Article abstract of DOI:10.1007/s10562-019-03083-0

Abstract: A new and efficient N-heterocyclic carbene (NHC)-palladium(II) complex immobilized on graphene oxide (NHC-Pd@GO) has been successfully designed and synthesized. The prepared NHC-Pd@GO heterogeneous catalyst was fully characterized using a combination of fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), energy-dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA) and Brunauer–Emmett–Teller surface area analysis (BET). This new air- and moisture-stable NHC-Pd@GO heterogeneous catalytic system was found to be a highly active catalyst in the Suzuki–Miyaura cross-coupling between phenylboronic acid and various aryl halides (bromides/chlorides/iodides) and in the reduction of nitroarenes. These organic transformations were best performed in an aqueous ethanol and aqueous methanol solvent system respectively with low catalyst loading under mild reaction conditions. Furthermore, NHC-Pd@GO heterogeneous catalyst could be recovered easily and reused at least eleven times in Suzuki–Miyaura cross-coupling and nine times in reduction of nitroarenes without any considerable loss of its catalytic activity. The stability and good selectivity of the NHC-Pd@GO heterogeneous catalyst in recycling experiments signify that it could be useful for practical application in various organic transformations. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-03083-0

Catalysis Letters
https://doi.org/10.1007/s10562-019-03083-0
Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex
on Graphene Oxide as Efcient and Recyclable Catalyst
for Suzuki–Miyaura Cross‑Coupling and Reduction of Nitroarenes
Vishal Kandathil1 · Bhakti Kulkarni1 · Aisha Siddiqa1 · Manjunatha Kempasiddaiah1 · B. S. Sasidhar2 ·
Shivaputra A. Patil3 · Siddappa A. Patil1
Received: 26 October 2019 / Accepted: 15 December 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A new and efcient N-heterocyclic carbene (NHC)-palladium(II) complex immobilized on graphene oxide (NHC-Pd@
GO) has been successfully designed and synthesized. The prepared NHC-Pd@GO heterogeneous catalyst was fully char-
acterized using a combination of fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optical emis-
sion spectroscopy (ICP-OES), energy-dispersive X-ray spectroscopy (EDS), feld-emission scanning electron microscopy
(FESEM), transmission electron microscopy (TEM), X-ray powder difraction (XRD), thermogravimetric analysis (TGA)
and Brunauer–Emmett–Teller surface area analysis (BET). This new air- and moisture-stable NHC-Pd@GO heterogeneous
catalytic system was found to be a highly active catalyst in the Suzuki–Miyaura cross-coupling between phenylboronic acid
and various aryl halides (bromides/chlorides/iodides) and in the reduction of nitroarenes. These organic transformations were
best performed in an aqueous ethanol and aqueous methanol solvent system respectively with low catalyst loading under
mild reaction conditions. Furthermore, NHC-Pd@GO heterogeneous catalyst could be recovered easily and reused at least
eleven times in Suzuki–Miyaura cross-coupling and nine times in reduction of nitroarenes without any considerable loss of
its catalytic activity. The stability and good selectivity of the NHC-Pd@GO heterogeneous catalyst in recycling experiments
signify that it could be useful for practical application in various organic transformations.
Vishal Kandathil and Bhakti Kulkarni have contributed equally.
* Siddappa A. Patil
p.siddappa@jainuniversity.ac.in; patilsiddappa@gmail.com
1
Centre for Nano and Material Sciences, Jain University, Jain
Global Campus, Kanakapura, Ramanagaram, Bangalore,
Karnataka 562112, India
2
Organic Chemistry Section, Chemical Sciences &
Technology Division, National Institute for Interdisciplinary
Science and Technology (CSIR), 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
Vol.:(0123456789)
1 3

V. Kandathil et al.
Graphic Abstract
Reduction of nitroarenes
Suzuki-Miyaura cross-coupling
NHC-Pd@GO
NH2
Aqueous medium
R1
R1
CN
Cl
N
Pd
N
2
9 Recycles
11 Recycles
Cl
Si
O O O
GO
1) Highly active and selective
2) Short reaction time
3) Eco-friendly
X
+
NO2
(HO)2B
R1
R1
4) Cost effective
5) Easy recovery and reusable
Keywords Graphene oxide · N-heterocyclic carbene-palladium(II) · Heterogeneous catalysis · Suzuki–Miyaura cross-
coupling · Nitroarene reduction
increased for numerous organic transformations. Neverthe-
less, most important drawback of homogeneous catalysts is
the difculty in their recovery and reuse from the reaction
medium. This problem is of economic and environmental
concern in bulk-scale syntheses.
1 Introduction
In recent years, Suzuki–Miyaura cross-coupling reaction
plays an important role towards the generation of new
carbon–carbon (C–C) bonds in both academic and indus-
trial sectors. Suzuki–Miyaura cross-coupling reaction
involves the transition metal palladium catalyzed cross-
coupling of organoboronic acids with organic halides [1–4].
Suzuki–Miyaura cross-coupling reaction has been applied to
the synthesis of numerous organic compounds like pharma-
ceutical drugs, liquid crystals, molecular wires, conducting
polymers, complex natural products and in supramolecu-
lar chemistry [4–11]. Consequently, huge eforts have been
done to the development of homogeneous ligand systems
for Suzuki–Miyaura cross-coupling reaction such as diazab-
utadienes, N-heterocyclic carbenes (NHCs), amines, imines,
oxime palladacycles and phosphines [12–17]. Conversely,
synthesis of aromatic amines through the reduction of
nitroarenes is an important concern in the preparation of dye
intermediates, agrochemicals, photographic chemicals, poly-
mers, rubber chemicals, pigments and pharmaceuticals [18].
The common methods to synthesize the amines are based
on; (i) amination of various functional groups (OH, H, F,
Cl, Br, I etc.) through the corresponding diazonium salts and
(ii) reduction of nitro compounds with homogeneous cata-
lysts [19–22]. Hence, the uses of transition metal complexes
in the form of homogeneous catalysts have tremendously
In recent years, NHC-metal complexes have been suc-
cessfully used as catalyst for numerous organic transfor-
mations [23, 24]. Stable NHC-metal complexes have been
efortlessly prepared from various transition metal ions with
diferent oxidation states. Of late, various types of NHC-
palladium(II) complexes have been developed as heteroge-
neous catalysts for various organic transformations through
immobilizing palladium(II) on various solid supports
[25–29]. Regrettably, these heterogeneous catalysts habitu-
ally sufer from many difculties such as inferior selectivity,
reduced activity and many more problems such as prepara-
tion complexity and metal leaching.
In this regard, graphene oxide (GO) has attracted wide
interest in heterogeneous catalysis because of its distinc-
tive properties such as huge surface area, two-dimensional
structure, easy recovery and recycling [30, 31]. In addition,
graphene oxide is largely oxygenated with hydroxyl, epox-
ide, carbonyl and carboxyl groups which provide plenty of
opportunities for functionality modifcations as active sites
for tethering metal ions and complexes for efcient chemi-
cal transformation. Hence, GO has been recognized as best
candidate for attaching many transition metal complexes
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
[32–34]. Fan et al. prepared a novel GO tethered palladium
complex catalyst through immobilizing palladium(II) on the
surface of GO for the Suzuki–Miyaura cross-coupling reac-
tion of aryl halides and phenylboronic acid. The catalyst
exhibited high activity and could be recycled and reused
numerous times without signifcant loss of activity [35].
Khatri et al. prepared rhenium-oxo Schif-base complex on
GO nanosheets for the oxidation of numerous amines under
mild reaction conditions [36]. Mungse et al. reported oxo-
vanadium Schif-base complex attached on the surface of
GO as a heterogeneous catalyst for the oxidation of alcohols
which exhibited high catalytic activity [37].
out using 0.25 mm Merck TLC silica gel plates with UV
light as a visualizing agent. Yields refer to chromatography
pure material. Concentration in vacuo refers to the removal
of volatile solvent using a rotary evaporator attached to a
dry diaphragm (10–15 mm Hg) followed by pumping to a
constant weight with an oil pump (< 300 mTorr). All the
organic products were known and identifed by comparison
of their spectral data with those of authentic samples.
2.2 Characterization
Fourier transform infrared spectra (FTIR) were recorded
with a PerkinElmer Spectrum Two spectrometer.
Brunauer–Emmett–Teller (BET) surface areas were obtained
by physisorption of nitrogen using a Microtrac BELSORP-
MAX instrument. The elemental palladium content of
the nanomagnetic catalyst was determined with a Perkin
Elmer Optima 5300 DV inductively coupled plasma opti-
cal emission spectrometer (ICP‐OES). Transmission elec-
tron microscopy (TEM) images were obtained using a Jeol/
JEM 2100 microscope. Field‐emission scanning electron
microscopy (FESEM) along with energy‐dispersive X‐ray
spectroscopy (EDS) to observe morphology and determine
elemental distributions, respectively, were conducted with
a JEOL model JSM7100F. Thermogravimetric analysis
(TGA) was carried out with a PerkinElmer Diamond TG/
DTA with a heating rate of 10.0 °C min−1. Powder X‐ray dif-
fraction (XRD) patterns were obtained using a Bruker AXS
D8 Advance. 1H NMR spectra were recorded at 400 MHz,
and are reported relative to deuterated chloroform (CDCl3;
δ=7.27 ppm). 1H NMR coupling constants (J) are reported
in hertz (Hz) and multiplicities are indicated as follows: s
(singlet), d (doublet), t (triplet), m (multiplet).
Generally, NHCs are stabilized through bulky substitu-
ents on the nitrogen atoms. Hence, we have chosen 4′-bro-
momethyl-2-biphenylcarbonitrile for substitution on one
nitrogen and (3-chloropropyl)triethoxysilane attached GO
on the other nitrogen. In addition, the strong afnity of GO
towards silica makes silanes very useful for the function-
alization of GO. In a continuation of our very recent inves-
tigations of the applications of heterogeneous catalysts in
C–C cross-coupling and reduction of nitroarenes reactions
[38–42], herein we report the synthesis and characteriza-
tion of N-heterocyclic carbene-palladium(II) complex immo-
bilized on GO (NHC-Pd@GO). This new air- and mois-
ture-stable heterogeneous catalyst was found to be highly
active in Suzuki–Miyaura cross-coupling and reduction
of nitroarenes reactions in an aqueous ethanol and aque-
ous methanol as green solvents using an extremely small
amount of palladium under mild conditions (room tempera-
ture and short reaction time). Furthermore, NHC-Pd@GO
heterogeneous catalyst could be easily recovered from the
reaction media by simple fltration and reused at least 12
and ten cycles in the Suzuki–Miyaura cross-coupling and
reduction of nitroarenes respectively without signifcant loss
of its activity.
2.3 Synthesis of Graphene Oxide (GO) (1)
Graphene oxide was synthesized and purifed according to
modifed Hummers method from readily available graphite.
In a typical experiment, conc. H2SO4 (23 mL) was added
to the mixture of graphite (1.0 g) and NaNO3 (0.5 g) in a
round bottomed fask (250 mL) and the resulting reaction
mixture was cooled down to 0 °C in an ice bath under stir-
ring for 2 h. Then KMnO4 (3.0 g) was slowly added to the
above reaction mixture with vigorous stirring and the fask
was kept for oxidizing at 0 °C for another 2 h. Afterward, the
reaction mixture was transferred to a pre-heated water bath
at 35 °C and stirring was continued for further 2 h. Water
(H2O) (50 mL) was slowly added into the reaction system
and the temperature was increased to 95 °C for 15 min. Then
reaction mixture was treated with 30% H2O2 (10 mL) and
resulting solid was fltered, washed with 5% HCl and deion-
ized water until the pH reached neutral. The obtained solid
was dried at 70 °C till constant weight to get pure GO.
2 Experimental
2.1 Materials
All solvents were purifed according to standard methods
prior to use. Unless otherwise stated, all reactions were per-
formed under aerobic conditions in oven-dried glassware
with magnetic stirring. Graphite, NaNO3, H2SO4, H2O2,
KMnO4, benzimidazole, 4′-bromomethyl-2-biphenylcarbon-
itrile, palladium(II) acetate, (3-chloropropyl)triethoxysilane,
aryl halides, bases, and phenylboronic acid were purchased
from Avra and Sigma-Aldrich chemical company and were
used without further purification. Heating was accom-
plished by either a heating mantle or silicone oil bath. Col-
umn chromatography was conducted on silica gel 60–120
mesh (Merck) and thin layer chromatography was carried
1 3

V. Kandathil et al.
attached N-heterocyclic carbene was added to the above
reaction mixture and stirring was continued at 50 °C for
16 h. The reaction mixture was then cooled to room tempera-
ture. The resultant product was isolated by centrifugation,
washed with distilled water (2×40 mL) and dried at 40 °C
in vacuo for 12 h to yield graphene oxide tethered N-het-
erocyclic carbene-palladium(II) complex (NHC-Pd@GO).
2.4 Synthesis of Silyl Chloride Functionalized
Graphene Oxide (3)
The synthesised graphene oxide (2.0 g) was suspended
in ethanol (EtOH) (125 mL) through ultrasonicating for
30 min. Ammonia was added to the suspension and stirred at
40 °C for 3 h. Then (3-chloropropyl)triethoxysilane (8 mL)
was added to the above reaction solution and stirring was
continued for further 8 h at 40 °C. Then, reaction solution
was cooled to room temperature. The obtained black pre-
cipitate was collected by centrifugation and washed with
EtOH (3×25 mL) followed by H2O (1×25 mL) and dried
at 40 °C for 12 h.
2.8 General Procedure for Suzuki–Miyaura
Cross‑Coupling Reaction
A mixture of aryl halide (0.27 mmol), phenylboronic
acid (1 mmol), NHC-Pd@GO heterogeneous catalyst
(0.01 g) and potassium carbonate (1.1 mmol) was stirred in
EtOH:H2O (1:1) solvent system at room temperature for des-
ignated hours. The progress of the reaction was monitored
by TLC. After reaction completion, the mixture was cooled
to room temperature and NHC-Pd@GO heterogeneous cata-
lyst was separated through centrifugation. To the fltrate,
dichloromethane and water were added. Dichloromethane
layer was separated from the water layer using a separa-
tory funnel and dried with magnesium sulphate. The dried
dichloromethane was concentrated in vacuum and obtained
crude product was purifed from column chromatography
over silica gel using hexane and ethyl acetate as eluting sol-
vent to get the corresponding products in good to excellent
yields. All the coupled products were known molecules and
2.5 Synthesis of 4′‑((1H‑benzo[d]imidazol‑1‑yl)met
hyl)‑[1,1′‑biphenyl]‑2‑carbonitrile (6)
A mixture of benzimidazole (0.5 g, 4.23 mmol) and potas-
sium hydroxide (0.34 g, 6.06 mmol) in DMSO (20 mL) was
stirred at 100 °C for 2 h. The temperature of the reaction
mixture was reduced to room temperature and 4′-bromome-
thyl-2-biphenylcarbonitrile (1.15 g, 4.23 mmol) was added
and stirring was continued for further 12 h. Then reaction
mixture was poured into ice-cold water. The precipitated
solid product was washed with water and dried at 40 °C in
vacuo for 4–5 h to yield 4′-((1H-benzo[d]imidazol-1-yl)
methyl)-[1,1′-biphenyl]-2-carbonitrile as an of-white solid.
Yield: 92%. 1H NMR (400 MHz, CDCl3) δ 8.03 (s, 1H),
7.88 (d, J=6.7 Hz, 1H), 7.78 (s, 1H), 7.66 (s, 1H), 7.56 (d,
J = 8.1 Hz, 2H), 7.49 (d, J = 7.8 Hz, 2H), 7.38 – 7.28 (m,
5H), 5.47 (s, 2H).
1
were confrmed by comparing the H NMR spectral data
with those of authentic samples.
1. [1,1′-Biphenyl]-4-carbonitrile (Table 7, entry 1): 1H
NMR (400 MHz, CDCl3) δ 7.74–7.71 (m, 2H), 7.70–
7.67 (m, 2H), 7.59 (d, J=5.3 Hz, 2H), 7.48 (d, J=8.1,
6.7 Hz, 2H), 7.43 (d, J=7.3 Hz, 1H).
2.6 Synthesis of Graphene Oxide Tethered
N‑Heterocyclic Carbene (NHC@GO) (7)
1
2. 1,1′-Biphenyl (Table 7, entries 2 and 11): H NMR
A mixture of 4′-((1H-benzo[d]imidazol-1-yl)methyl)-[1,1′-
biphenyl]-2-carbonitrile (2 g, 6.47 mmol) and silyl chloride
functionalized graphene oxide (4 g) in toluene (60 mL) was
sonicated for 30 min. The reaction mixture was kept for stir-
ring at 110 °C for about 48 h. Then, reaction mixture was
cooled to room temperature. The obtained black precipitate
was collected by centrifugation, washed with fresh toluene
(3 × 40 mL) and dried at 40 °C in vacuo for 12 h to yield
graphene oxide tethered N-heterocyclic carbene.
(400 MHz, CDCl3) δ 7.60 (d, J=7.7 Hz, 4H), 7.44 (t,
J=7.5 Hz, 4H), 7.35 (t, J=7.2 Hz, 2H).
3. [1,1′-Biphenyl]-4-ol (Table 7, entries 3, 15 and 16): 1H
NMR (400 MHz, CDCl3) δ 7.54 (d, J=8.3 Hz, 2H),
7.50– 7.44 (m, 2H), 7.41 (d, J=10.6 Hz, 2H), 7.31 (d,
J=7.4 Hz, 1H), 6.94– 6.88 (m, 2H), 5.02 (s, 1H).
4. 1-([1,1′-Biphenyl]-4-yl)ethanone (Table 7, entries
1
4 and 9): H NMR (400 MHz, CDCl3) δ 8.04 (d,
J=8.2 Hz, 2H), 7.69 (d, J=8.2 Hz, 2H), 7.62 (s, 2H),
7.47 (d, J=7.7 Hz, 2H), 7.41 (d, J=7.1 Hz, 1H), 2.64
(s, 3H).
2.7 Synthesis of Graphene Oxide Tethered
N‑Heterocyclic Carbene‑Palladium(II) Complex
(NHC‑Pd@GO) (8)
5. [1,1′-Biphenyl]-4-carbaldehyde (Table 7, entry 5): 1H
NMR (400 MHz, CDCl3) δ 10.06 (s, 1H), 7.99–7.94
(m, 2H), 7.79–7.74 (m, 2H), 7.65 (d, J=5.2 Hz, 2H),
7.52–7.45 (m, 2H), 7.43 (d, J=4.9 Hz, 1H).
To a solution of Na2CO3(0.5 M, 30 mL) in dimethylfor-
mamide (DMF) (30 mL), palladium(II) acetate (0.33 g,
1.47 mmol) was added and the reaction mixture was stirred
at room temperature for 10 min. Then graphene oxide
1
6. 4-Nitro-1,1′-biphenyl (Table 7, entry 6): H NMR
(400 MHz, CDCl3) δ 8.27 – 8.17 (m, 2H), 7.71–7.62
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
(m, 2H), 7.55 (d, J=5.3 Hz, 2H), 7.46–7.39 (m, 2H),
7.39–7.34 (m, 1H).
1H), 7.64 (d, J=8.7 Hz, 1H), 7.49–7.44 (m, 2H), 4.76
(s, 2H).
7. [1,1′-Biphenyl]-4-yl(phenyl)methanone (Table 7, entry
7): 1H NMR (400 MHz, CDCl3) δ 7.92–7.88 (m, 2H),
7.86–7.82 (m, 2H), 7.71 (d, J=8.3 Hz, 2H), 7.68–7.63
(m, 2H), 7.60 (t, J=7.4 Hz, 1H), 7.49 (d, J=9.8 Hz,
4H), 7.41 (t, J=7.4 Hz, 1H).
4. 2-Chloroaniline (Table 10, entry 5): 1H NMR (400 MHz,
CDCl3) δ 7.66 (d, J=5.7 Hz, 1H), 7.47 (d, J=5.7 Hz,
1H), 7.13 (d, J=7.6 Hz, 1H), 7.00 (d, J=8.2 Hz, 1H),
4.02 (s, 2H).
5. 4-Methoxyaniline (Table 10, entry 6): 1H NMR
(400 MHz, CDCl3) δ 6.76–6.71 (m, 2H), 6.66–6.62 (m,
2H), 3.73 (s, 3H), 3.34 (s, 2H).
8. 4-Methyl-1,1′-biphenyl (Table 7, entries 8 and 12): 1H
NMR (400 MHz, CDCl3) δ 7.57 (d, J=7.6 Hz, 2H),
7.49 (d, J=7.3 Hz, 2H), 7.41 (t, J=7.5 Hz, 2H), 7.31
(t, J = 7.3 Hz, 1H), 7.24 (d, J = 7.5 Hz, 2H), 2.39 (s,
3H).
1
6. m-Toluidine (Table 10, entry 7): H NMR (400 MHz,
CDCl3) δ 7.73 (d, J=5.7 Hz, 1H), 7.54 (d, J=5.7 Hz,
1H), 7.29 (d, J=2.4 Hz, 1H), 7.07 (d, J=8.1 Hz, 1H),
4.09 (d, J=6.7 Hz, 2H), 2.27 (s, 3H).
9. 4-Methoxy-1,1′-biphenyl (Table 7, entries 10 and 13):
1H NMR (400 MHz, CDCl3) δ 7.58–7.51 (m, 4H), 7.41
(t, J = 7.7 Hz, 2H), 7.30 (t, J = 7.4 Hz, 1H), 6.98 (d,
J=8.7 Hz, 2H), 3.85 (s, 3H).
2.10 Procedure for Recovery of Graphene Oxide
Tethered N‑Heterocyclic Carbene‑Palladium(II)
Complex (NHC‑Pd@GO)
10. [1,1′-Biphenyl]-2-carbaldehyde (Table 7, entry 14):
1H NMR (400 MHz, CDCl3) δ 9.99 (s, 1H), 8.03 (d,
J=7.8 Hz, 1H), 7.65 (d, J=1.3 Hz, 1H), 7.54–7.44 (m,
5H), 7.39 (d, J=7.7 Hz, 2H).
The stability and reusability of the catalyst are important
concern in organic transformations particularly for conven-
ient industrial applications. After the completion of reac-
tion, NHC-Pd@GO heterogeneous catalyst was isolated
from the reaction mixture by centrifugation. The separated
NHC-Pd@GO heterogeneous catalyst was then washed with
water (2×10 mL) followed by ethanol (2×10 mL) and dried
at 45 °C for 12 h. Then dried NHC-Pd@GO heterogene-
ous catalyst was used for next round of Suzuki–Miyaura
cross-coupling and reduction of nitroarenes without further
purifcation.
2.9 General Procedure for Reduction of Nitroarenes
In a typical reduction reaction, nitroarene (0.81 mmol)
was mixed with the NHC-Pd@GO heterogeneous catalyst
(0.005 g) in MeOH:H2O (1:1) (10 mL) solvent system and
the reaction mixture was stirred for fve minutes at room
temperature. Consequently, NaBH4 (1.62 mmol) was added
to the reaction mixture and stirring was continued at room
temperature for designated hours. The progress of the reac-
tion was checked from TLC. After reaction completion,
NHC-Pd@GO heterogeneous catalyst was separated through
centrifugation and dichloromethane was added. The dichlo-
romethane layer was separated from the water layer using
a separating funnel and dried with anhydrous magnesium
sulfate. The dried dichloromethane layer was concentrated
in vacuum and obtained crude reduced product was purifed
through column chromatography over silica gel using hexane
and ethyl acetate as eluting solvents to aford the pure prod-
ucts in good to excellent yields. All the reduced products
were known molecules and were confrmed by comparing
the 1H NMR spectral data with those of authentic samples.
3 Results and Discussion
3.1 Synthesis of Graphene Oxide Tethered
N‑Heterocyclic Carbene‑Palladium(II) Complex
In continuation of our research work on the utilization of
immobilized N-heterocyclic carbene-palladium(II) complex
on diferent materials for various organic transformations
[38, 39], herein, we report a facile and efcient method for
Suzuki–Miyaura cross‐coupling between a range of aryl
halides with phenylboronic acid and also for the reduction
of nitroarenes in the presence of catalytic amounts of NHC-
Pd@GO heterogeneous catalyst.
1. Aniline (Table 10, entry 1): 1H NMR (400 MHz, CDCl3)
δ 7.14–7.07 (m, 2H), 6.72 (d, J=5.4 Hz, 1H), 6.66–6.53
(m, 2H), 3.47 (s, 2H).
The method for the synthesis of NHC-Pd@GO het-
erogeneous catalyst depicted in this work is outlined in
Scheme 1. Initially, graphene oxide (1) was prepared and
purifed according to a modifed Hummers method through
easily accessible graphite. Then, silyl chloride function-
alized graphene oxide (3) was synthesized by the func-
tionalization of graphene oxide(1) with (3-chloropropyl)
triethoxysilane (2) in EtOH at 40 °C in presence of ammo-
nia. On the other hand, the intermediate 4′-((1H-benzo[d]
1
2. Benzene-1,3-diamine (Table 10, entry 2): H NMR
(400 MHz, CDCl3) δ 7.66 (d, J=5.7 Hz, 1H), 7.59–7.53
(m, 1H), 7.47 (d, J = 5.7 Hz, 1H), 7.38 (d, J = 8.0 Hz,
1H), 4.02 (s, 2H), 4.01 (s, 2H).
1
3. 4-Aminobenzaldehyde (Table 10, entry 4): H NMR
(400 MHz, CDCl3) δ 8.19 (s, 1H), 8.08 (d, J=8.2 Hz,
1 3

V. Kandathil et al.
Cl
OH
OH
O
OH
Si
O
O
O
OH OHOH
GO
O
OH
EtO
EtO Si
EtO
Ethanol, NH3
(a)
GO
Cl
40 0C
Sonication
30 Minutes
O
1
2
3
O
OH
OH
Cl
CN
CN
Si
O
O
O
GO
H
N
N
N
N
KOH, DMSO
40 0C
3
Cl
(b)
Toluene, 110 0C
48 h
N
Br
N
CN
4
5
6
Si
O
O
O
CN
GO
7
DMF:H2O
50 0C
16 h
Pd(OAc)2
Cl
N
N
Pd
2
Cl
Si
O
O
O
GO
8
Scheme 1 Synthetic schemes for a silyl chloride functionalized graphene oxide (3) and b graphene oxide tethered N-heterocyclic carbene-
palladium(II) complex (NHC-Pd@GO) (8)
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
imidazol-1-yl)methyl)-[1,1′-biphenyl]-2-carbonitrile (6)
was prepared through the reaction of benzimidazole (4)
with 4′-bromomethyl-2-biphenylcarbonitrile (5) in the pres-
ence of potassium hydroxide in DMSO. In the next step,
4′-((1H-benzo[d]imidazol-1-yl)methyl)-[1,1′-biphenyl]-2-
carbonitrile (6) was treated with silyl chloride functional-
ized graphene oxide (3) in toluene at 110 °C for 48 h to get
the corresponding graphene oxide tethered N-heterocyclic
carbene (7). Finally, graphene oxide tethered N-heterocyclic
carbene-palladium(II) complex (8) was synthesized through
the coordination of palladium(II) acetate with graphene
oxide tethered N-heterocyclic carbene (7).
heterogeneous catalyst, 12 times recycled NHC-Pd@GO
heterogeneous catalyst from Suzuki–Miyaura cross-coupling
reaction and ten times recycled NHC-Pd@GO heterogene-
ous catalyst from the reduction of nitroarenes. Results dem-
onstrate that the chemical composition of recycled NHC-
Pd@GO heterogeneous catalyst from both the reactions is
not visible apart from some slight shift in the peak positions.
3.4 Thermogravimetric Analysis
The thermal stability of GO and NHC-Pd@GO heteroge-
neous catalyst and organic functionalization on GO was
inferred using TGA under nitrogen atmosphere from 35 to
800 °C at a rate of 10 °C per min. Figure 3a exhibited 15%
weight loss in the range of 50–230 °C which is due to the
loss of hydroxyl groups present on the surface of the GO
and~35% weight loss due to the pyrolysis of various oxygen
containing functional groups. In contrast, the NHC-Pd@GO
heterogeneous catalyst displayed higher thermal stability and
the total weight loss of~30% (Fig. 3b) shows the removal of
organic moiety functionalized on the GO support. Further-
more, on the basis of TGA results, it is clear that NHC-Pd@
GO heterogeneous catalyst is stable up to a temperature of
200 °C which in turn allows the usage of this NHC-Pd@GO
heterogeneous catalyst in organic transformations carried out
at higher temperatures.
3.2 Spectroscopic and Microscopic Characterization
The newly synthesized NHC-Pd@GO heterogeneous cata-
lyst was characterized from attenuated total refectance infra-
red spectroscopy, inductively coupled plasma-atomic emis-
sion spectroscopy, energy-dispersive X-ray spectroscopy,
feld-emission scanning electron microscopy, transmission
electron microscopy, X-ray powder difraction, thermogravi-
metric analysis and Brunauer–Emmett–Teller surface area
analysis.
3.3 FTIR Spectroscopy
The structures of compounds from each step of the reac-
tion were confirmed through fourier transform infrared
spectroscopy. Figure 1a–e shows FTIR spectra of GO,
4′-((1H-benzo[d]imidazol-1-yl)methyl)-[1,1′-biphenyl]-
2-carbonitrile, silane functionalized GO, graphene oxide
tethered N-heterocyclic carbene and the NHC-Pd@GO
heterogeneous catalyst. The FTIR spectrum of GO shows
broad O–H bond stretching vibration at 3435.99 cm−1, vibra-
tion of carboxyl group at 1627.64 cm−1 and C–O stretch-
ing vibration at 1094.57 cm−1. The bands observed around
2221.63, 3024.82 and 1613.00 cm−1 (Fig. 1b) are ascribed to
the carbonitrile, C–H and C=C stretching vibrations of the
4′-((1H-benzo[d]imidazol-1-yl)methyl)-[1,1′-biphenyl]-2-
carbonitrile.The FTIR spectrum of silane functionalized GO
reveals typical bands at 698.34, 2926.8 and 1128.09 cm−1
attributed to C–Cl stretching, aliphatic C–H stretching and
Si–O–C stretching vibrations. The absence of C–Cl stretch-
ing peak around 700 cm−1 and the presence of carbonitrile,
aliphatic and aromatic C–H stretching vibrations peaks at
2221.0, 3118.7 and 2852.7 cm−1 confrms the formation
NHC@GO. The characteristic peaks at 2221.7, 2954.2 and
2921.11 cm−1 in the IR spectrum of the NHC-Pd@GO are
due to carbonitrile, aliphatic and aromatic C–H stretching
vibrations respectively, apart from the peak at 468.64 cm−1
which is due to C–Pd stretching which confrms the structure
of the NHC-Pd@GO heterogeneous catalyst. Figure 2a–c
shows the FTIR spectra of freshly prepared NHC-Pd@GO
3.5 BET Surface Area Analysis
The specifc surface area of GO and NHC-Pd@GO hetero-
geneous catalyst was measured by Brunauer–Emmet–Teller
(BET) analysis which is based on the adsorption and des-
orption of nitrogen gas molecules on the solid surface. The
nitrogen adsorption and desorption isotherms for (a) GO
and (b) NHC-Pd@GO heterogeneous catalyst are shown
in the Fig. 4. The amount of nitrogen adsorption on GO
is high compared to that on the NHC-Pd@GO heterogene-
ous catalyst. The surface areas of bare GO was found to be
44.12 m2g−1 which has got decreased to 6.23 m2g−1 upon
formation of the NHC-Pd@GO heterogeneous catalyst. The
decrease in surface area discloses the successful functionali-
zation of GO with the NHC-palladium(II) complex, which
is evident from the BET data.
3.6 TEM Analysis
The TEM images of GO and NHC-Pd@GO catalyst are
shown in Fig. 5a and b. The TEM image of the synthesized
GO possesses thin transparent sheet like surface morphology
with the presence of oxygen-containing functional groups
attached on both sides of the sheet. Whereas the TEM image
of newly synthesized NHC-Pd@GO heterogeneous catalyst
displays a crumpled and layered like structure revealing the
1 3

V. Kandathil et al.
Fig. 1 FTIR spectra of a GO, b 4′-((1H-benzo[d]imidazol-1-yl)methyl)-[1,1′-biphenyl]-2-carbonitrile, c silane functionalized GO, d NHC@GO
and e NHC-Pd@GO heterogeneous catalyst
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
catalyst (Fig. 6b) demonstrated that both contain similar form,
having two-dimensional nanosheets with wave-like structures.
After 12-times and ten-times recycle in Suzuki–Miyaura cross-
coupling and reduction of nitroarene, the surface morphology
of the NHC-Pd@GO heterogeneous catalysts remained almost
same as that of freshly prepared NHC-Pd@GO heterogeneous
catalyst as depicted in Fig. 6c and d.
3.8 EDS Analysis
To confrm the elemental composition present in the newly
synthesized NHC-Pd@GO heterogeneous catalyst, energy-
dispersive X-ray spectroscopy (EDS) was employed. EDS
spectra (Fig. 7) for NHC-Pd@GO heterogeneous catalyst
shows diferent characteristic signals corresponding to C,
N, O, Si, Cl, Pd atoms which defnes the formation of NHC-
Pd(II) complex on the surface of GO. Elemental mapping
of the NHC-Pd@GO heterogeneous catalyst was carried out
to understand the distribution of elements in the NHC-Pd@
GO heterogeneous catalyst as shown in the Fig. 8. From
elemental mapping it can be observed that all the elements
are distributed evenly.
Fig. 2 FTIR spectra of NHC-Pd@GO heterogeneous catalyst: a
freshly prepared, b recycled after Suzuki–Miyaura cross-coupling
reaction and c recycled after nitroarene reduction
3.9 ICP‐OES Analysis
Furthermore, the exact amount of palladium loading on
newly synthesized NHC-Pd@GO heterogeneous catalyst
was determined through ICP-OES analysis. The palladium
loading of the NHC-Pd@GO heterogeneous catalyst was
found to be 4.72% w/w by ICP-OES.
3.10 XRD Analysis
The crystalline nature of obtained GO and NHC-Pd@GO
heterogeneous catalyst was analyzed through the powder
XRD technique. The difraction patterns of GO and NHC-
Pd@GO are depicted in Fig. 9. The XRD pattern of GO
(Fig. 9a) demonstrates the characteristic difraction peak at
2θ = 11.99° corresponding to (001) plane which is due to
the introduction of oxygenic functional groups and trapped
water molecules between the layers of GO. After immo-
bilization of the NHC-palladium(II) complex, the peak (at
2θ=11.99°) was disappeared and one more broad difrac-
tion peak at 2θ=23.84° emerged, confrming that the major
oxygen-containing groups of GO have been successfully
functionalized (Fig. 9b).
Fig. 3 TGA curves of a GO and b NHC-Pd@GO heterogeneous cata-
lyst
successful synthesis of the NHC-Pd@GO heterogeneous
catalyst.
3.7 FESEM Analysis
FESEM analysis was carried out to get more information about
surface morphology of GO, NHC-Pd@GO heterogeneous cat-
alyst, 12-times recycled NHC-Pd@GO heterogeneous catalyst
in Suzuki–Miyaura reaction and ten-times recycled NHC-Pd@
GO heterogeneous catalyst in reduction reaction. FESEM
images are shown in Fig. 6. The FESEM images of the GO
(Fig. 6a) and freshly prepared NHC-Pd@GO heterogeneous
3.11 Catalytic Activity of NHC‑Pd@GO
Heterogeneous Catalyst in Suzuki–Miyaura
Cross‐Coupling Reactions
The obtained NHC-Pd@GO heterogeneous catalyst was
stable in air and moisture. After the characterization of
1 3

V. Kandathil et al.
Fig. 4 Nitrogen adsorption–desorption curves for a GO and b NHC-Pd@GO heterogeneous catalyst
Fig. 5 TEM images of a GO, b
NHC-Pd@GO heterogeneous
catalyst
newly synthesized NHC-Pd@GO heterogeneous cata-
lyst through spectroscopic and microscopic techniques,
the catalytic activity was investigated initially for the
Suzuki–Miyaura cross-coupling reaction. In the begin-
ning, reaction conditions were optimized on a model
Suzuki–Miyaura cross-coupling reaction between 4-bro-
mobenzonitrile and phenylboronic acid as shown in
Scheme 2. Much attention is given in solvent selection
for any chemical transformation nowadays because of the
environmental concern. Hence, during the optimization
of reaction conditions, environmentally benign solvents
like ethanol and water were given precedence. The reac-
tion conditions were optimized through a series of model
Suzuki–Miyaura cross-coupling reactions in the pres-
ence of NHC-Pd@GO heterogeneous catalyst as shown
in Table 1. The preliminary results shown that using
K2CO3 base, EtOH:H2O (1:1) solvent, NHC-Pd@GO het-
erogeneous catalyst (10 mg) at room temperature for 1 h
resulted in the highest yield (Table 1, entry 6). The cata-
lytic activity of the NHC-Pd@GO heterogeneous catalyst
with varying base, solvent, temperature, time, and catalyst
ratio were also investigated for model reaction as shown in
Tables 2, 3, 4, 5 and 6 respectively.
3.12 Efect of Base on Suzuki–Miyaura
Cross‑Coupling Reaction
The infuence of base on the catalytic activity of NHC-Pd@
GO heterogeneous catalyst was examined on model reac-
tion using diferent bases: NaOH, KOH, Na3PO4·12H2O,
Et3N, KF, K2CO3, Na2CO3 and Cs2CO3 (Table 2). Among
the bases employed, K2CO3 exhibited a good conversion
rate (Table 2, entry 6) and remaining base NaOH, KOH,
Na3PO4·12H2O, Et3N, KF, Na2CO3 and Cs2CO3 resulted in
lesser conversion (Table 2, entries 1–5 and entry 7–8).
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
Fig. 6 FESEM images of a GO, b NHC-Pd@GO heterogeneous catalyst, c 12 times‐recycled NHC-Pd@GO heterogeneous catalyst in Suzuki–
Miyaura cross-coupling reaction and d ten times recycled NHC-Pd@GO heterogeneous catalyst in reduction reaction
3.13 Efect of Solvent on Suzuki–Miyaura
Cross‑Coupling Reaction
The catalytic potential of NHC-Pd@GO heterogeneous cat-
alyst in diferent solvents were investigated with diferent
solvents such as acetone, DCM, toluene, THF, H2O, EtOH,
EtOH:H2O (1:1) and isopropyl alcohol. From the results
obtained as shown in Table 3, it is clear that reaction did
not proceed in acetone and THF (Table 3, entries 1 and 4).
When DMF, toluene, ethanol and isopropyl alcohol were
used cross-coupled product yield was medium (Table 3,
entries 2, 3, 6 and 8). Furthermore, the model reaction in
EtOH-H2O (1:1) mixture gave the highest yield (Table 3,
entry 7).
Fig. 7 EDS-spectrum of NHC-Pd@GO heterogeneous catalyst
1 3

V. Kandathil et al.
Fig. 8 Elemental mapping of NHC-Pd@GO heterogeneous catalyst
model reaction was carried out at diferent temperatures as
depicted in Table 4. Results demonstrate that the NHC-Pd@
GO heterogeneous catalyst is highly active at room tempera-
ture itself (Table 4, entry 1) and hence it is preferred over
higher temperatures even though there is no diference in the
yield (Table 4, entries 2–4).
3.15 Efect of Time on Suzuki–Miyaura
Cross‑Coupling Reaction
To investigate the time efect, the model Suzuki–Miyaura
cross-coupling reaction was conducted at diferent time
intervals in the presence of NHC-Pd@GO heterogeneous
catalyst (Table 5). It is understood from the results obtained
that the yield of reaction increased with increase in time
upto 1 h (Table 5, entries 1–4) and further yield improve-
ment was not observed (Table 5, entries 5–7). Hence, 1 h is
the optimum time required for the maximum yield (Table 5,
entry 4).
Fig. 9 XRD pattern of a GO and b NHC-Pd@GO heterogeneous
catalyst
3.16 Efect of Catalyst Ratio on Suzuki–Miyaura
Cross‑Coupling Reaction
3.14 Efect of Temperature on Suzuki–Miyaura
Cross‑Coupling Reaction
The optimal amount of the NHC-Pd@GO heterogeneous cat-
alyst required in the Suzuki–Miyaura cross-coupling reaction
has to be determined for the efective utilization of it in any
To fnd out the temperature efect in the catalytic activity
of newly prepared NHC-Pd@GO heterogeneous catalyst,
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
NHC-Pd@GO, base
N
Br
N
(HO) B
+
2
Solvent, temperature, time
Scheme 2 Suzuki–Miyaura cross‐coupling reaction of 4‐bromobenzonitrile with phenylboronic acid
used (Table 6, entry 2) and further increase in the catalyst
Table 1 Optimization of reaction conditions for model Suzuki–
Miyaura cross-coupling reaction in the presence of NHC-Pd@GO
heterogeneous catalyst
ratio did not increase the yield (Table 6, entries 3–5).
3.17 Suzuki–Miyaura Cross‑Coupling
Reactions of Substituted Aryl Halides
with Phenylboronic Acid
Entry Base
Solvent
Temp. (oC) Time (h) Yield (%)a
1
2
3
4
5
6
7
8
9
10
K2CO3
MeOH
H2O
EtOH
DMF
R.T
R.T
R.T
R.T
R.T
7
12
5
80
40
70
50
Trace
98
98
K2CO3
K2CO3
K2CO3
K2CO3
The Suzuki–Miyaura cross-coupling reaction is one of
the most imperative methods for C–C bond formation in
organic reaction. The Suzuki–Miyaura cross-coupling
reaction is used in the synthesis of broad range of C–C
coupled products and therefore it belongs to a vital set
of palladium catalyzed cross-coupling reactions. Hence,
catalytic potential of the NHC-Pd@GO heterogeneous
catalyst was tested in Suzuki–Miyaura cross-coupling
reactions of diferent aryl halides with phenylboronic acid
using the optimized conditions and the results are tabu-
lated in Table 7. The investigational procedure is simple
and convenient, and has the capability to tolerate a wide
range of functional groups. All the cross-coupled prod-
ucts were isolated and purifed by column chromatography.
The obtained results disclose that electronically diverse
substituted aryl bromides and aryl iodides are converted
into the corresponding products in good to excellent yields
(Table 7, entries 1–15). However, aryl bromides (Table 7,
entries 1–10) reacted slowly than the iodide derivatives
(Table 7, entries 11–15). Conversely, reaction of aryl chlo-
ride with phenylboronic acid gave cross-coupling products
in low yield (Table 7, entry 16). In contrast, selectivity
12
12
1
CH3CN
K2CO3 EtOH:H2O R.T
K2CO3
K2CO3
Na2CO3 EtOH:H2O R.T
Na2CO3 EtOH:H2O R.T
EtOH:H2O R.T
EtOH:H2O R.T
2
0.5
1
2
90
90
95
Reaction conditions: 4-bromobenzonitrile (0.27 mmol), phenylbo-
ronic acid (0.30 mmol), NHC-Pd@GO catalyst (0.02 mol % Pd with
respect to aryl halide), base (0.60 mmol) and solvent (5 mL) in air
The best reaction condition with the highest yield is given in bold
aIsolated yield
organic transformation. The study of efect of catalyst load-
ing in the model Suzuki–Miyaura cross-coupling reaction is
shown in Table 6. The quantity of NHC-Pd@GO heteroge-
neous catalyst was varied and studied for catalyst ratios with
0.01, 0.02, 0.03, 0.04, 0.05 mol% of Pd in the model reac-
tion (Table 6, entries 1–5). The yield of the biaryl product
increased with the catalyst loading (Table 6, entries 1 and 2).
A high yield of 98% was obtained when 0.02 mol % Pd was
Table 2 Suzuki–Miyaura cross-
coupling reaction of model
reaction in the presence of
NHC-Pd@GO heterogeneous
catalyst with diferent bases
Entry
Base
Solvent
Temp. (oC)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
NaOH
KOH
Na3PO4·12H2O
Et3N
KF
K2CO3
Cs2CO3
Na2CO3
EtOH:H2O
EtOH:H2O
EtOH:H2O
EtOH:H2O
EtOH:H2O
EtOH:H2O
EtOH:H2O
EtOH:H2O
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
4
8
4
3
4
1
1
1
20
95
95
60
50
98
90
90
Reaction conditions: 4-bromobenzonitrile (0.27 mmol), phenylboronic acid (0.30 mmol), NHC-Pd@GO
heterogeneous catalyst (0.02 mol % Pd with respect to aryl halide), base (0.60 mmol) and solvent (5 mL) in
air
The best reaction condition with the highest yield is given in bold
aIsolated yield
1 3

V. Kandathil et al.
Table 3 Suzuki–Miyaura cross-coupling reaction of model reaction
in the presence of NHC-Pd@GO heterogeneous catalyst with various
solvents
Table 5 Suzuki–Miyaura cross-coupling reaction of model reaction
in the presence of NHC-Pd@GO heterogeneous catalyst at various
time intervals
Entry Base
Solvent
Temp. (oC) Time (h) Yield (%)a
Entry Base
Solvent
Temp. (oC) Time (h) Yield (%)b
1
2
3
4
5
6
7
8
K2CO3 Acetone
K2CO3 DMF
K2CO3 Toluene
K2CO3 THF
K2CO3 H2O
K2CO3 EtOH
K2CO3 EtOH:H2O R.T
K2CO3 IPA R.T
R.T
R.T
R.T
R.T
R.T
R.T
12
12
3
7
12
5
Trace
30
30
Trace
40
70
01
02
03
04
05
06
07
K2CO3 EtOH:H2O
K2CO3 EtOH:H2O
K2CO3 EtOH:H2O
R.T
R.T
R.T
0.25
0.50
0.75
1.00
1.50
1.75
2.00
80
90
95
98
98
98
98
K2CO3 EtOH:H2O R.T
K2CO3 EtOH:H2O
K2CO3 EtOH:H2O
K2CO3 EtOH:H2O
R.T
R.T
R.T
1
7
98
70
Reaction conditions: 4-bromobenzonitrile (0.27 mmol), phenylbo-
ronic acid (0.30 mmol), NHC-Pd@GO heterogeneous catalyst
(0.02 mol % with respect to aryl halide), base (0.60 mmol) and sol-
vent (5 mL) in air
Reaction conditions: 4-bromobenzonitrile (0.27 mmol), phenylbo-
ronic acid (0.30 mmol), NHC-Pd@GO heterogeneous catalyst
(0.02 mol % Pd with respect to aryl halide), base (0.60 mmol) and
solvent (5 mL) in air
The best reaction condition with the highest yield is given in bold
aIsolated yield
The best reaction condition with the highest yield is given in bold
aIsolated yield
Table 6 Suzuki–Miyaura cross-coupling reaction of model reaction
in the presence of NHC-Pd@GO heterogeneous catalyst with difer-
ent catalyst ratios
Table 4 Suzuki–Miyaura cross-coupling reaction of model reaction
in the presence of NHC-Pd@GO heterogeneous catalyst at various
temperatures
Entry Base
Pd (mol %) Temp. (oC) Time (h) Yield (%)a
Entry Base
Solvent
Temp. (oC) Time (h) Yield (%)a
1
2
3
4
5
K2CO3 0.01
R.T
R.T
R.T
R.T
R.T
1
1
1
1
1
85
98
98
96
93
1
2
3
4
K2CO3 EtOH:H2O R.T
1
1
1
1
98
98
98
97
K2CO3 0.02
K2CO3 0.03
K2CO3 0.04
K2CO3 0.05
K2CO3 EtOH:H2O
K2CO3 EtOH:H2O
K2CO3 EtOH:H2O
40
50
60
Reaction conditions: 4-bromobenzonitrile (0.27 mmol), phenylbo-
ronic acid (0.30 mmol), NHC-Pd@GO heterogeneous catalyst
(0.02 mol % with respect to aryl halide), base (0.60 mmol) and sol-
vent (5 mL) in air
Reaction conditions: 4-bromobenzonitrile (0.27 mmol), phenylbo-
ronic acid (0.30 mmol), base (0.60 mmol) and solvent (5 mL) in air
The best reaction condition with the highest yield is given in bold
aIsolated yield
The best reaction condition with the highest yield is given in bold
aIsolated yield
which gives the C–C cross-coupling products via possible
of the NHC-Pd@GO heterogeneous catalyst was estab-
lished through the existence of a negligible amount of the
homocoupled product obtained. For more explanation on
selectivity, Suzuki–Miyaura cross-coupling reactions were
carried out without aryl halides under optimized condi-
tions. The biphenyl homocoupled product obtained was
in trace amount, which concluded that the NHC-Pd@GO
heterogeneous catalyst is highly selective. Moreover, a
plausible mechanism for our newly synthesized NHC-Pd@
GO heterogeneous catalyst is demonstrated in Scheme 3
mechanistic intermediates (I–III).
3.18 Catalyst Recyclability Study in Suzuki–Miyaura
Cross‑Coupling Reactions
The recyclability study of NHC-Pd@GO was carried out
on the model Suzuki–Miyaura cross-coupling reaction.
The prepared catalyst shown good activity upto 11 recycles
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
Table 7 Suzuki–Miyaura cross-coupling reactions of diferent aryl halides with phenylboronic acid catalyzed by NHC-Pd@GO heterogeneous
catalyst
Reaction conditions: aryl halide (0.27 mmol), phenylboronic acid (0.30 mmol), NHC-Pd@GO heterogeneous catalyst (0.02 mol% Pd), base
(0.60 mmol) and EtOH-H2O (5 mL) in air
aIsolated yield after separation by column chromatography; average of two runs
1 3

V. Kandathil et al.
NHC-Pd(II)@GO
Preactivation
it can be concluded that NHC-Pd@GO heterogeneous cata-
lyst showed good to excellent activity when compared with
other catalytic systems.
Ar X
NHC-Pd(0)@GO
I
Ar Ph
3.20 Activity of NHC‑Palladium(II) Grafted
Graphene Oxide Catalyst in Reduction
of Nitroarenes
Oxidative addition
reductive
ellimination
Ar
NHC-Pd(II)@GO
Subsequent to the catalytic activity study of NHC-Pd@
GO in Suzuki–Miyaura cross-coupling, the prepared catalyst
was studied for its activity in reduction of nitroarenes. The
reduction of nitrobenzene to aniline was chosen as the model
reaction to optimize the reaction conditions (Scheme 4). The
reaction conditions were optimized through a series of reac-
tion by varying conditions like solvent, reducing agent ratio,
and temperature and so on as shown in Table 9. The sol-
vent choice was predetermined to be aqueous and green and
hence aqueous ethanol and methanol were only studied for
solvent system. The reaction yield was less when only water
was used as solvent which may be due to the low solubility
of the starting material in water. The optimal ratio of sodium
borohydride was found to be two equivalents. An increase
in the ratio of sodium borohydride resulted in no change
where as decrease in the ratio resulted in lower yields of the
product. Also, an increase or decrease in catalyst ratio from
0.003 mol% of Pd resulted in lower yields. The optimized
condition obtained for reduction was by using 2 equivalent
reducing agent (NaBH4) and 1:1 ratio of MeOH:H2O solvent
(5 mL) in presence of 0.003 Pd mol % catalyst at room tem-
perature in ten minutes.
X
II
Ar
NHC-Pd(II)@GO
(HO)2HB Ph
Ph
III
K2CO3
transmetallation
(HO)2B Ph
B(OH)3
Scheme 3 Proposed mechanism of Suzuki–Miyaura cross-coupling
reaction
3.21 Reduction of Various Nitroarenes
With the optimized conditions available, we then pro-
ceeded to study the reduction of different nitroarenes as
shown in the Table 10. The range of substrates studied
contained different electron withdrawing as well as elec-
tron donating groups. The NHC-Pd@GO could catalyze
the reduction of nitroarenes in good to excellent conver-
sion rates and yield in less time duration under optimized
conditions. The use of eco-friendly solvent at room tem-
perature in less time is a few advantages of the protocol
presented here. Furthermore, a plausible mechanism for
the reduction of nitroarene using NaBH4 catalysed by
NHC-Pd@GO heterogeneous catalyst through probable
mechanistic intermediates (I–V) is given in Scheme 5.
Fig. 10 Recycling efciency of NHC-Pd@GO heterogeneous catalyst
in Suzuki–Miyaura cross-coupling reaction
as shown in Fig. 10. Also, it is observed that there is a
slow and steady decrease in the activity from sixth recycle
onwards which may be due to the blocking of active sites
on the catalyst by reactant left over even though the wash-
ings of the recovered catalyst were performed.
3.19 Comparison of Catalysts
In order to understand the advantages and disadvan-
tages and to scrutinize how important is NHC-Pd@GO
heterogeneous catalyst among the reported catalysts in
Suzuki–Miyaura cross-coupling reaction, we have com-
pared the yields obtained from our protocol with other
related protocols which is given in Table 8. From this table
3.22 Catalyst Recyclability in Nitroarene Reduction
Recyclability study of NHC-Pd@GO heterogeneous
catalyst was also carried out for the model reaction in
nitroarene reduction under optimized conditions. The
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
Table 8 Comparison of
Entry
Catalyst
Solvent
Temp. (°C)
Time (h)
Yield (%)
Refs.
results in Suzuki–Miyaura
cross-coupling reaction
between 4-bromotoluene and
phenylboronic acid using
NHC-Pd@GO heterogeneous
catalyst with other reported
catalysts
1
2
3
4
5
6
7
GO-2N-Pd(II)
GO-CPTMS-Pd
NHC-Pd/GO-IL
GO-NHC-Pd2+
Catalyst
EtOH
80
80
60
80
R.T
60
R.T
4
3
2.5
20
1
4
1
77
95
85
94
99
70
92
[35]
[43]
[44]
[45]
[46]
[47]
Present work
EtOH:H2O (2:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (2:1)
EtOH:H2O (1:1)
GO-NHC-Pd2+
NHC-Pd@GO
The best reaction condition with the highest yield is given in bold
Table 11. The comparison data reveals that NHC-Pd@GO
heterogeneous catalyst performed with comparable product
yields in the reduction reaction with the additional merits
of usage of environmentally benign solvent, mild reaction
conditions and less time period for reaction completion.
NHC-Pd@GO
NO2
NH2
MeOH:H2O (1:1), RT
NaBH4
Scheme 4 Catalytic activity of NHC-Pd@GO heterogeneous catalyst
in reduction of nitrobenzene
3.24 Heterogeneity Study
In order to confirm the heterogeneity of NHC-Pd@
GO, leaching study was carried out for the model
Suzuki–Miyaura cross-coupling reaction. Here, the reaction
mass containing NHC-Pd@GO heterogeneous catalyst and
the base was stirred for one hour and fltered before the addi-
tion of aryl halide and phenylboronic acid. After the removal
of NHC-Pd@GO heterogeneous catalyst, aryl halide and
phenylboronic acid were added to the fltrate and allowed to
stir for another one hour and the reaction was monitored by
TLC and found that there is no product formation and the
reactants remained unreacted. This study confrms that there
is no palladium in the soluble form which is leaching out
from the catalyst to reaction mass. Thus, it proves that NHC-
Pd@GO heterogeneous catalyst is heterogeneous in nature.
NHC-Pd@GO heterogeneous catalyst was recovered by
the centrifugation of reaction medium after reaction com-
pletion. Separated catalyst was then washed with metha-
nol (2 × 10 mL) followed by water (10 mL) and dried at
40 °C in hot oven. Further, the recovered catalyst was
reused for reduction of nitroarenes and it was observed
that there was no change in catalytic activity up to ten
recycles as shown in Fig. 11.
3.23 Comparison of Catalysts
A comparison data of the catalytic activity of NHC-Pd@
GO heterogeneous catalyst with other reported catalyst in
nitroarene reduction was also carried out which is shown in
Table 9 Optimization of
reaction conditions for the
reduction of nitrobenzene in
presence of NHC-Pd@GO
heterogeneous catalyst
Entry
Solvent
NaBH4 ratio
Pd (mol %)
Time (h)
Yield (%)a
01
02
03
04
05
06
07
08
09
10
H2O
3
3
3
3
3
1
2
2
2
2
0.006
0.006
0.003
0.009
0.002
0.003
0.003
0.003
0.003
0.003
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.16
60
95
95
93
75
70
95
Trace
Trace
60
MeOH:H2O
MeOH:H2O
MeOH:H2O
MeOH:H2O
MeOH:H2O
MeOH:H2O
MeOH
EtOH
EtOH:H2O
Reaction conditions: nitrobenzene (0.81 mmol) and solvent 10 mL
The best reaction condition with the highest yield is given in bold
aIsolated yield
1 3

V. Kandathil et al.
Table 10 Nitroarenes reduction catalyzed by NHC-Pd@GO heterogeneous catalyst
Reaction conditions: nitrobenzene (0.81 mmol), sodium borohydride (1.6 mmol) and catalyst (0.003 Pd mol% with respect to nitroarenes) and
solvent 10 mL
aIsolated yield after separation by column chromatography; average of two runs
1 3

Immobilized N‑Heterocyclic Carbene‑Palladium(II) Complex on Graphene Oxide as Efcient…
NO2
NH2
OH
NHC-Pd@GO
NaBH
4 , RT
O
N
O
OH
H
H
N
N
N
H
-H2O
O
O
OH
H
III
I
II
OH
NH
H
OH2
NH
H
N
H
-H2O
H
IV
V
Scheme 5 Proposed mechanism for reduction of nitroarene using NaBH4 catalysed by NHC-Pd@GO nanomagnetic catalyst
4 Conclusions
In summary, NHC-Pd@GO heterogeneous catalyst was suc-
cessfully prepared and characterized by FTIR spectroscopy,
TGA, BET, TEM, FESEM, EDS, ICP-OES and XRD analy-
sis. The new NHC-Pd@GO heterogeneous catalyst showed
good catalytic activity in both Suzuki–Miyaura cross-cou-
pling reaction and reduction of nitroarenes. The main advan-
tages of this catalyst are its simple design, eco-friendliness
and recyclability. The recovered catalyst can be reused up to
eleven recycles without signifcant loss of catalytic activity
in Suzuki–Miyaura cross-coupling reactions and up to nine
recycles in case of nitroarenes reduction. The novel NHC-
based palladium complexes can be prepared and immobi-
lized on graphene oxide following this protocol which can be
used in various organic transformations like Mizoroki–Heck
reaction, Sonogashira-Hagihara coupling reaction, oxidation
reactions apart from Suzuki–Miyaura cross coupling and
nitroarenes reduction.
Fig. 11 Recycling efciency of NHC-Pd@GO heterogeneous catalyst
in nitroarene reduction
Table 11 Comparison of results
Entry
Catalyst
Solvent
Temp. (°C)
Time (h)
Yield (%)
Refs.
in nitrobenzene reduction using
NHC-Pd@GO heterogeneous
catalyst with other reported
catalysts
1
2
3
4
5
6
7
Pd/C (5%)
Catalyst 1
Pd-gCN
THF:H2O
MeOH
EtOH
H2O:MeOH (2:1)
H2O:MeOH (3:7)
H2O:MeOH (3:7)
H2O:MeOH (1:1)
60
50
70
R. T
R. T
R. T
R. T
3
12
4
0.16
0.16
0.08
0.16
85
95
99
99
99
99
95
[48]
[49]
[50]
[51]
[52]
[53]
Present work
PdNPs
rGO-Fe48Pd52
rGO@Ag/Pd
NHC-Pd@GO
The best reaction condition with the highest yield is given in bold
1 3

V. Kandathil et al.
28. Wang W, Cui L, Sun P, Shi L, Yue C, Li F (2018) Chem Rev
118:9843–9929
Acknowledgements The authors thank DST-SERB, India
(YSS/2015/000010), DST-Nanomission, India (SR/NM/NS-20/2014),
and Jain University, India for fnancial support.
29. Yang J, Wu Y, Wu X, Liu W, Wang Y, Wang J (2019) Green Chem
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Compliance with Ethical Standards
31. Tan R, Li C, Luo J, Kong Y, Zheng W, Yin D (2013) J Catal
298:138–147
Conflict of interest All authors declares that they have no conficts to
declare.
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1 3