Recyclable and eco-friendly Pd-complexed graphene oxide/N-heterocyclic carbene catalyst for various coupling reactions in aqueous phase

Article abstract of DOI:10.1016/j.tetlet.2017.05.017

In this study, we report recyclable Pd-complexed graphene oxide (GO) catalysts with a bulky N-heterocyclic carbene (NHC) ligand (GO/NHC-Pd) for various cross-coupling reactions in the aqueous phase. To prepare GO/NHC-Pd, the NHC precursor and a trimethoxy-silane linker were combined on the GO surface via condensation, followed by the Pd chelation over GO/NHC. The GO-based catalysts were applied to three types of C–C cross-coupling reactions including Suzuki, Heck and Sonogashira reactions in the aqueous phase, to evaluate their catalytic performance. The reusability of the catalysts was tested by performing five consequent cross-coupling reactions.

Full text of DOI:10.1016/j.tetlet.2017.05.017

Accepted Manuscript
Recyclable and eco-friendly Pd-complexed graphene oxide/N-heterocyclic car-
bene catalyst for various coupling reactions in aqueous phase
Seongsoo Kim, Hong-Jun Cho, Dong-Sik Shin, Sang-Myung Lee
PII:
S0040-4039(17)30588-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.05.017
TETL 48910
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
16 February 2017
20 April 2017
8 May 2017
Please cite this article as: Kim, S., Cho, H-J., Shin, D-S., Lee, S-M., Recyclable and eco-friendly Pd-complexed
graphene oxide/N-heterocyclic carbene catalyst for various coupling reactions in aqueous phase, Tetrahedron
Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.05.017
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Tetrahedron Letters
Recyclable and eco-friendly Pd-complexed graphene oxide/N-heterocyclic carbene
catalyst for various coupling reactions in aqueous phase
a
b
c,
a,
Seongsoo Kim , Hong-Jun Cho , Dong-Sik Shin  and Sang-Myung Lee 
a
Department of Chemical Engineering, Kangwon National University, Chuncheon-si, Gangwon-do 24341, Republic of Korea
School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea
Department of Chemical and Biological Engineering, Sookmyung Women's University, Seoul 04310, Korea
b
c
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In this study, we report recyclable Pd-complexed graphene oxide (GO) catalysts with a bulky N-
heterocyclic carbene (NHC) ligand (GO/NHC-Pd) for various cross-coupling reactions in the
aqueous phase. To prepare GO/NHC-Pd, the NHC precursor and a trimethoxy-silane linker were
combined on the GO surface via condensation, followed by the Pd chelation over GO/NHC. The
GO-based catalysts were applied to three types of C–C cross-coupling reactions including
Suzuki, Heck and Sonogashira reactions in the aqueous phase, to evaluate their catalytic
performance. The reusability of the catalysts was tested by performing five consequent cross-
coupling reactions.
Available online
Keywords:
Heterogeneous catalyst
Graphene oxide
N-heterocyclic carbene
Palladium
2
017 Elsevier Ltd. All rights reserved.
C-C cross coupling
Graphene oxide (GO) is composed of complete honeycomb
As mentioned above, NHC-Pd complexes have been applied
as either homogeneous or heterogeneous catalysts in several C–C
cross-coupling reactions such as Suzuki, Heck, Sonogashira, and
Stille reactions. Owing to the disadvantages of using
homogeneous catalysts, solid or polymer-based NHC-Pd
catalysts with a number of advantages, e.g., easy separation,
recycling, efficient production cost, and utilization of eco-
friendly media, have been demonstrated by numerous research
3
2
lattices with sp and sp hybridized bonds. Apart from its large
surface area, GO has hydrophilic properties owing to the
presence of functional groups such as epoxy, hydroxyl, and
1
carboxyl groups derived from oxidative preparation methods.
Controlling the degree of oxidation of GO or its coordination
with metals can result in GO composites with a wide range of
applications, e.g., in electrodes, fuel cell catalysts, and catalysts
2
7
for chemical syntheses and so on.
groups. GO-supported NHC-Pd catalysts were reported for
Suzuki coupling reactions using aqueous-organic mixed solvent
system such as ethanol/water (1:1) and DMF/water (1:1) .
N-heterocyclic carbene (NHC) ligand has been widely studied
because of its prominent nucleophilicity compared with some
phosphines as well as thermal and chemical stabilities originating
8
9
Mesoporous polystyrene beads are one of the most famous
solid supports combined with NHC. This support is typically
used with NHC-Pd via stable immobilization of ionic liquids,
thereby affording excellent catalytic stability and large surface
3
from the nature of the carbine–metal bond. Over the last
decades, NHC-chelated transition metals have been used in
several types of homogeneous catalytic reactions, e.g., olefin
metathesis and polymerization, hydrosilylation and carbon–
carbon coupling reactions over NHC-Ru, NHC-Rh or NHC-Pt
1
0
area. In 2009, Fihri and coworkers developed a silica-based
1
1
NHC-Pd catalyst with high catalytic activity and good stability.
4
and NHC-Pd catalysts. Electron bulkiness and sterically stability
Additionally, graphite or GO-supported Pd complexes have been
shown to exhibit the excellent catalytic activity and stability
have been known to improve binding strength between NHC and
transition metals, thereby leading to a significant increase in the
1
2
during cross-coupling reactions. Moreover, NHCs derived from
bulky ionic liquids have been shown to exhibit good recyclability
5
catalytic efficiencies. Moreover, the development of novel NHC
9
ligands for homogeneous reactions and suitable solid supports for
heterogeneous processes has enabled aqueous reactions by virtue
in heterogeneous catalytic systems. GO has been covalently
bonded with NHC-Pd via silylation and used as a catalyst in
6
13
of the noticeable performance of the active species.
several cross-coupling reactions.
—
——


Corresponding author. Tel.: +82-33-250-6335; fax: +82-33-250-5549; e-mail: sangmyung@kangwon.ac.kr
Corresponding author. Tel.: +82-2-2077-7236; fax: +82-2-2077-7450; e-mail: dshin@sm.ac.kr

2
Tetrahedro-1n16
In this study, we presented the water-compatible and
recyclable Pd-complexed GO catalysts with bulky NHC ligand
GO/NHC-Pd) for several kinds of cross-coupling reaction in
cm . Moreover, absorption bands from C2–H and (–C=)C–H
bendings in the Pd(II)-coordinated imidazole rings appeared at
-
1
(
1687 and 1123 cm , respectively. Inductively coupled plasma
atomic emission spectroscopy (ICP-OES) analysis revealed that
the palladium content of GO/NHC-Pd was calculated to be 0.55
mmol Pd /g catalyst.
water. Unlike the previous reports, our purpose is to perform
Suzuki coupling reaction as well as Heck and Sonogashira
coupling reactions in water as a sole solvent. To prepare
GO/NHC-Pd, NHC precursor with trimethoxy-silane linker was
combined on the GO surface by condensation reaction, and
followed by the Pd chelation with GO/NHC. The GO/NHC-Pd
catalysts were applied to Suzuki, Heck and Sonogashira carbon-
carbon cross-coupling reactions in water to evaluate the catalytic
performance. Finally, GO/NHC-Pd catalysts were applied in the
recycling test to evaluate their reusability efficiency.
GO was prepared from graphite by a modified Hummers
method and characterized by Raman spectroscopy. As shown in
Figure 1, a sharp and strong G band of graphite was clearly
-1
-
1
observed at 1581 cm . However, the D band at 1350 cm
Scheme 1. Preparation of GO/NHC-Pd catalyst.
associated with the edge distortion and destruction of the
structural symmetry was hardly detected in the graphite sample.
The D/G band intensity ratio (I /I ) of graphite was 0.23, while
D
G
GO showed a broad and strong D band with a higher ratio
1
4
(
0.89). Further modification of GO to GO/NHC and GO/NHC-
Pd displayed I /I intensity ratios of 0.94 and 0.95, respectively.
D
G
These results indicate that the chelation of Pd ions did not change
the band intensity ratio as much as the silanization step.
Graphite
GO
GO/NHC
GO/NHC-Pd
1.0
0.8
0.6
0.4
0.2
0.0
1000
1500
2000
2500
3000
3500
-
1
Raman Shift (cm )
Figure 2. FT–IR spectra of GO (red), GO/NHC (black line), and
Figure 1. Raman spectra of graphite, GO, GO/NHC and
GO/NHC-Pd.
the GO/NHC-Pd catalyst (blue line).
One may concern the coordination efficiency of Pd(II) ions
with imidazolium rings. If Pd(II) ions were reduced prior to a
stable chelation with NHC ligands, their nanoparticles can be
deposited on the GO/NHC surface. Thus, the generation of Pd
nanoparticles was critically evaluated by scanning electron
microscopy (SEM) analyses. For this purpose, the Pd precursors
were applied to GO without NHC ligands under the same
synthetic conditions as those used for GO/NHC-Pd. As seen from
the SEM images, a large number of Pd nanoparticles were
observed on the GO-Pd surface unlike GO/NHC-Pd (Figure S1
(a),(b), respectively). These findings indicate that Pd(II) ions
were thoroughly chelated over the NHC ligands on the GO
Scheme 1 describes the synthetic procedure to prepare the
GO/NHC-Pd catalysts. First, GO was coupled with the 1-(2,6-
diidopropylphenyl)-3-(propyltrimethoxysilyl)imidazol-2-ylidene
precursor in a THF solution by refluxing under a N atmosphere.
2
Thereafter,
Pd(OAc)2,
the
Pd-ion
precursor,
was
stoichiometrically chelated with NHC-silane imidazolium in the
deprotonated structure of carbene. Each step of the GO/NHC-Pd
synthesis was confirmed by Fourier transform infrared
spectroscopy (FT–IR) analyses (Figure 2). Initially, GO showed
strong absorbance bands originating from the oxidation of
graphite including C=O, C–O, and O–H stretching vibrations at
-
1
13, 17
1
692, 1082, and 3432 cm , respectively. Upon coupling the
surface, thereby avoiding the formation of Pd nanoparticles.
NHS-silane with GO, the absorbance bands corresponding to the
From the energy-dispersive X-ray spectroscopy (EDS) mapping
results, we could observe the reacting materials incorporated into
the GO complexes after each reaction step (Figure S2).
imidazolium ring vibration (C–H bending) appeared at 779 and
-
1
1
250 cm , thereby indicating that NHS-silane was successfully
15
combined with the GO template. The chelation of Pd(II) ion
As another characterization of GO/NHS-Pd catalysts, the
thermogravimetric analysis (TGA) was performed using GO,
GO-NHC, and GO/NHC-Pd (Figure S3). In the case of GO, 16%
weight loss at ca. 110 °C was initially observed due to the loss of
with GO/NHC was also characterized by FT–IR, as previously
1
3
reported. An absorbance band from C=C stretching in the
Pd(II)-coordinated imidazole group was clearly observed at 1562

physisorbed water. Subsequently, the thermal elimination of
labile oxygen containing hydroxyl or epoxy groups led to 30% of
4
I
6
91.6
1
8
weight loss at ca. 200-250°C. The further weight loss at 500°C
1
9
indicated a rapid decomposition of oxygen species. On the other
hand, a smaller elimination of water and oxygen containing
group was observed after the trimethoxysilane linker was
combined to GO by condensation reaction. The breakage of
siloxane groups led a degradation step between 300-400°C of
5
6
6
78.2
70.5
S
I
S
2
0
GO/NHC and GO/NHC-Pd.
H3C
I
12
H3C
Before applying the GO/NHS-Pd catalysts to various Suzuki
cross-coupling reactions, inorganic bases were examined as a co-
catalyst by coupling 4-iodoanisole and phenylboronic acid.
Inorganic bases are generally known to be more suitable for
cross-coupling reactions in the aqueous phase than organic
a
Conditions: aryl iodides and heterocyclic halide (0.114 mmol),
phenylboronic acid (0.137 mmol), the GO/NHC-Pd catalyst (1
mol%), and Na PO ·12H O (0.274 mmol) in water (2 mL) at 100
°C and reaction times (h).
3
4
2
2
1
bases. The coupling reactions were performed at 100 °C for 6 h.
As shown in Table S1, most of the carbonates (Cs CO , Na CO ,
b
2
3
2
3
GC yields.
and K CO ) and KOH showed GC yields between 70% and 80%,
2
3
whereas K PO showed the poorest coupling yield (62.3%).
3
4
However, Na PO showed the best Suzuki cross-coupling yield
3
4
In addition to Suzuki cross-coupling, Heck and Sonogashira
cross-coupling reactions using Pd catalysts were tested with aryl
halides and alkynes or alkenes to afford more diverse
with 4-methoxydiphenyl (83.3%). Thus, Na PO was chosen as a
3
4
base for the following Suzuki cross-coupling reactions in the
aqueous phase.
22
compounds. To evaluate the feasibility of the GO/NHC-Pd
catalyst for Heck cross-coupling reactions, a base screening was
performed in the aqueous phase, as demonstrated Suzuki
coupling in the previous section (120 °C, 12 h). As shown in
Table S2, three types of representative inorganic bases containing
different anions were used as an appropriate model reaction for
Based on the optimized base, various aryl iodides or 2-
iodothiophene were coupled with phenylboronic acid over the
GO/NHC-Pd catalyst (Table 1). In the aqueous phase, phenyl
iodide showed a high coupling yield of ca. 91.6% at 100 °C for 6
h (entry 4), while phenyl derivatives bearing an electron donating
group, e.g., 4-iodoanisole, showed poorer yields than phenyl
iodide under similar conditions (entries 1 and 2). However,
acetophenyl iodide was perfectly cross-coupled with
phenylboronic acid owing to the electronic deficiency effect in
the phenyl ring caused by the electron-withdrawing group
the coupling between 4-iodoanisole and styrene; K CO (a
2
3
carbonate) showed the highest yield (ca. 92%) in comparison to a
2
3
phosphate (entry 1) and hydroxide (entry 2). Thus, several Heck
cross-coupling reactions between styrene and aryl iodide
derivatives were demonstrated using the GO/NHC-Pd catalyst in
the presence of K CO as an optimum base in the aqueous phase.
(99.9%, entry 3). The coupling compounds of 2-iodothiophene
2
3
In Table 2, 4-Iodoacetophenone bearing an electron donating
group showed the maximum GC yield (>97%, entry 3).
Moreover, 1-nitro-4-iodobenzene and 4-iodoanisole afforded
and 4-iodotoluene were obtained in moderate GC yields of 78.2%
and 70.5%, respectively. As an example of bromobenzene, when
4
-bromoanisole was coupled with phenylboronic acid in the sole
water solvent at 100 °C for 12 h, the GC coupling yield was
1.2%. However, the yield increased to 88.0% when the ratio of
high efficiencies (92.8% and 91.6%, entries
2 and 4,
respectively), while iodobenzene showed a significantly lower
yield (55.0%).
2
the solvents was DMF/water (1:3).
Table 2. Heck cross-coupling reactions catalyzed by the
GO/NHC-Pd catalyst.
Table 1. Suzuki cross-coupling reactions catalyzed by the
GO/NHC-Pd catalyst.
a
a
Aryl or
heterocyclic
halides
OH
OH
Aryl Iodide
+
R
+
B
Styrene
R
Phenylboronic acid
b
Entry
1
Substrate
Product
Yield (%)
b
Entry
Substrate
Time
h)
Product
Yield
(
(%)
I
55.0
1
2
H3CO
HO
I
12
6
H3CO
HO
86.3
90.0
2
3
4
O2N
I
92.8
97.3
91.6
O2N
I
I
O
O
I
3
6
99.9
O
O
H3CO
I
H3CO

4a
Tetrahedron
Conditions: aryl iodide (0.114 mmol), styrene (0.137 mmol),
the GO/NHC-Pd catalyst (1 mol%), and K CO (0.274 mmol) in
piperidine (0.274 mmol) and CuI (0.001 mmol) in water (2 mL) at 80
°C and 12 h.
2
3
water (2 mL) at 120 °C for 12 h.
b
GC yields.
b
GC yields.
Heterogeneous Pd catalysts have been actively developed
mainly because they exhibit outstanding recyclability without
causing a significant loss in efficiency. Thus, the GO/NHC-Pd
catalysts were reused four times for the Suzuki coupling between
4-iodoacetophenone and phenylboronic acid and Sonogashira
coupling between 2-idodothiophene and phenyl acetylene after
the first test. As shown in Figure 3, the initial Suzuki coupling
yield was 99.9%, which was effectively maintained (>90%) after
the fourth recycling test. However, this yield decreased to 79%
after the fifth test, which might be associated with the
deactivation or dissociation of NHC-Pd complex form the GO
surface. Similarly, the efficiency of the Sonogashira reaction
remained nearly constant at the initial yield value (69.9%) during
the first four recycling tests, followed by a sharp drop to 50% for
subsequent cycles. As seen from the transmission electron
microscopy (TEM) images before and after 5 cycles of cross-
coupling reaction, we could conclude that there was no
significant geometric difference even though some salts were
observed (Figure S4). From X-ray photoelectron spectroscopy
The GO/NHC-Pd catalyst was used for performing the
Sonogashira cross-coupling reactions in the aqueous phase.
Piperidine,
a
well-known effective organic amine for
Sonogashira coupling, was used without additional base
screening, while CuI was used as a co-catalyst to react with
terminal alkynes to form Cu(I) acetylide species, which then
2
4
undergoes oxidative addition with aryl halide. As shown in
Table 3, various aryl or heterocyclic halides were cross-coupled
with acetylene in the aqueous phase. Unexpectedly, most of the
substrates provided moderate GC yields (55.3%–69.9%) despite
the functionalities of the aryl halides. Thus, the GO/NHC-Pd
catalyst was less effective in Sonogashira coupling compared
with the other two cross-coupling reactions. These results suggest
that homocoupling reactions between aryl halides are rapid in
water that the functional groups (related to the electron density of
the aryl halide) hardly influence the efficiency of the GO/NHS-
Pd catalyst.
(XPS) results, two single Pd peaks corresponding to NHC-Pd(II)
Table 3. Sonogashira cross-coupling reactions catalyzed by
GO/NHC-Pd.
were observed from the fresh GO/NHC-Pd (Figure S5). Also, we
could observe Pd metal peaks after 3 cycles of coupling
reaction. Although the recyclability test of GO/NHC-Pd is
proceeded until three times, the substantial area of Pd(II) peaks
a
2
5
Aryl or
heterocyclic
halides
(
Pd 3d5/2 and Pd 3d ) of NHC-Pd(II) was maintained. Our
3/2
+
R
finding indicates that our heterogeneous GO/NHC-Pd catalyst is
suitable for performing 4 or 5 cycles of cross-coupling reactions.
Phenyl acetylene
b
Entry
1
Substrate
Product
Yield (%)
58.1
63.1
61.2
55.3
69.9
55.4
62.6
H3CO
I
H3CO
2
3
4
5
6
7
HO
I
HO
I
O
O
Figure 3. Recycling efficiencies of the GO/NHC-Pd catalysts
for the Suzuki and Sonogashira cross-coupling reactions.
I
In summary, we prepared a recyclable GO/NHC-Pd catalyst
suitable for the Suzuki, Heck, and Sonogashira cross-coupling
reactions in the aqueous phase. Every synthetic step was
confirmed by FT–IR and SEM analyses, including the
identification of Pd(II)-ion chelation on GO/NHC without the
formation of Pd nanoparticles. The optimized inorganic bases for
the Suzuki and Heck cross-coupling reactions were Na PO and
S
I
S
H3C
O2N
I
H3C
O2N
3
3
K CO at each specific reaction condition, respectively. Various
2
3
aryl iodide substrates were cross-coupled in the aqueous phase,
with phenylboronic acid for the Suzuki reactions and styrene for
the Heck reactions resulting in GC yields of 86%~99.9% and
I
5
5%~97%, respectively. Moreover, the GO/NHC-Pd catalyst
a
Conditions: aryl iodine and heterocyclic halide (0.114 mmol),
phenylacetylene (0.171 mmol), the GO/NHC-Pd catalyst (1 mol%),
showed relatively moderate yields of 55.4%~69.9% for coupling
reactions between several phenyl iodides and phenyl acetylene

with piperidine as a co-catalyst. Finally, the GO/NHC-Pd
catalysts were recycled during the Suzuki and Heck reactions,
and their performance was mostly maintained for four
consecutive cycles. We believe that this GO/NHC-Pd catalyst
can be used in further eco-friendly applications to produce noble
pharmaceuticals.
24. Alami M, Crousse B, Ferri F. J Organomet Chem. 2001; 624:
114-123.
2
1
5. Yang H, Han X, Li G, Wang Y. Green Chem. 2009; 11: 1184-
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Supplementary Material
Acknowledgments
Supplementary material is available.
This work was supported by the Ministry of Trade, Industry &
Energy (MOTIE) and Korea Institute for Advancement of
Technology (KIAT) through Encouragement Program for The
Industries of Economic Cooperation Region (R0004027), the
Ministry of Science, ICT & Future Planning and National
Research Foundation of Korea through Basic Science Research
Program (NRF-2014R1A1A1006711) and 2014 Research Grant
from Kangwon National University (C1010942-01-01).
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6
Tetrahedron
Graphical Abstract
Recyclable and eco-friendly Pd-complexed
graphene oxide/N-heterocyclic carbene
catalyst for various coupling reactions in
aqueous phase
Leave this area blank for abstract info.
Seongsoo Kim, Hong-Jun Cho, Dong-Sik Shin and Sang-Myung Lee

Highlight



Recyclable Pd-complexed graphene oxide (GO) catalysts were prepared.
Cross-coupling reactions were performed in the aqueous phase.
The GO/NHC-Pd catalysts were recycled during the Suzuki and Heck reactions.