A Graphene Oxide Nanosheet Supported NHC-Palladium Complex as a Highly Efficient and Recyclable Suzuki Coupling Catalyst

Article abstract of DOI:10.1055/s-0037-1611726

A practical heterogeneous catalyst was prepared by anchoring a triazine-tethered N-heterocyclic carbene (NHC)-palladium complex on the surface of graphene oxide (GO) nanosheets. The immobilized complex was characterized by X-ray photoelectron spectroscopy, field-emission transmission electron microscopy, energy-dispersive X-ray spectroscopy, and surface area analysis. It proved to be a highly active and durable heterogeneous catalyst for Suzuki coupling reactions. At room temperature, the use of this catalyst enabled the preparation of various biaryls and heterobiaryls in short reaction times. The catalytic system could be recycled at least 10 times with almost consistent activity. The results reveal that the stable palladium complex is strongly anchored on the surface of GO nanosheets. Interestingly, an open planar network of the GO nanosheet support plays a role during the catalytic process in enhancing the catalytic activity.

Full text of DOI:10.1055/s-0037-1611726

SYNTHESIS
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Y. Qian et al.
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A Graphene Oxide Nanosheet Supported NHC–Palladium Complex
as a Highly Efficient and Recyclable Suzuki Coupling Catalyst
Yingjie Qian◊
Jaeil So◊
Sang-Yung Jung
Sosan Hwang
Myung-Jong Jin*
Sang Eun Shim*
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Department of Chemistry and Chemical Engineering, Inha
University, 100 Inha-ro, Nam-gu, Incheon 22212, South Korea
mjjin@inha.ac.kr
seshim@inha.ac.kr
◊
These authors contributed equally to this work.
Received: 30.10.2018
Accepted after revision: 17.01.2019
Published online: 20.03.2019
between the surface of a solid support and a ligand group in
the metal complex. A variety of highly active, heteroge-
neous palladium catalysts for Suzuki couplings have been
developed.6
DOI: 10.1055/s-0037-1611726; Art ID: ss-2018-h0732-op
Abstract A practical heterogeneous catalyst was prepared by anchor-
ing a triazine-tethered N-heterocyclic carbene (NHC)–palladium com-
plex on the surface of graphene oxide (GO) nanosheets. The immobi-
lized complex was characterized by X-ray photoelectron spectroscopy,
field-emission transmission electron microscopy, energy-dispersive X-
ray spectroscopy, and surface area analysis. It proved to be a highly ac-
tive and durable heterogeneous catalyst for Suzuki coupling reactions.
At room temperature, the use of this catalyst enabled the preparation
of various biaryls and heterobiaryls in short reaction times. The catalytic
system could be recycled at least 10 times with almost consistent activ-
ity. The results reveal that the stable palladium complex is strongly an-
chored on the surface of GO nanosheets. Interestingly, an open planar
network of the GO nanosheet support plays a role during the catalytic
process in enhancing the catalytic activity.
It should be noted that the support may play an import-
ant role, affecting the interactions between pendant cata-
lytic sites and substrates, together with the affinity be-
tween the support and substrates. The most typical sup-
ports include porous inorganic solids such as silica, zeolite,
7
mesoporous materials, and others. However, these materi-
8
als have high sensitivity towards acid and base conditions.
Carbon materials as supports have attracted a fast-growing
interest in the recent past due to better corrosion resistance
9
in both alkaline and acidic environments. This is import-
ant from a durability point of view, since support corrosion
is one of the important factors for the aggregation of cata-
lytic metal particles, which causes considerable loss of cata-
lytic activity. Carbon nanotube supported palladium
nanoparticles have been prepared for various catalytic ap-
plications.10
Key words graphene oxide, palladium, immobilization, heteroge-
neous catalysis, Suzuki coupling
The palladium-catalyzed Suzuki coupling reaction of
aryl halides with arylboronic acids represents one of the
most important methods for the formation of a carbon–car-
Graphene oxide (GO) nanosheets, as a form of carbon
with a high specific surface area as well as chemical and
thermal stability, arise as an excellent candidate as a cata-
1
bon bond. Significant progress has been achieved in this
area with a variety of homogeneous palladium catalysts.2
Although homogeneous catalysts have attractive properties
such as high reactivity and selectivity, difficulties are en-
countered in the separation and recovery of these expen-
lyst support. Homogeneous metal complexes can be easily
11
immobilized through the formation of covalent bonds be-
tween the surface of GO nanosheets and silylated ligands,
as the nanosheets contain a range of reactive oxygen func-
3
12
sive catalysts from reaction mixtures for reuse. Moreover,
tional groups on their surface. Another important advan-
homogeneous catalysts tend to lose their activity owing to
palladium metal agglomeration and precipitation.4 Such
disadvantages lead to serious economic and environmental
concerns in large-scale synthesis. Therefore, the successful
development of homogeneous catalysts has been often fol-
lowed by attempts to immobilize the catalysts on insoluble
tage of GO nanosheet support is concerned with the hori-
zontal open structure. From the point of view of structure
openness, GO nanosheets seem to be a unique catalyst sup-
port which can lead to enhanced catalytic activity. Al-
though GO nanosheet supported palladium nanoparticles
have been developed in recent years,13 so far there are few
reports of the immobilization of homogeneous catalysts on
5
supports. A desirable method of supporting homogeneous
metal complexes involves the formation of a chemical bond
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Y. Qian et al.
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a GO nanosheet support. Wu and co-workers have reported
an efficient GO-supported palladium catalyst (aryldiimine
as ligand) for Suzuki coupling reactions.6a
Pd content of 0.19 mmol/g was prepared for this study; the
value was confirmed by inductively coupled plasma atomic
optical emission spectrometry (ICP-OES).
It has long been known that N-heterocyclic carbene–
palladium (NHC-Pd) complexes act as very effective cata-
To investigate the elemental composition and chemical
states of the compounds shown in Scheme 1, X-ray photo-
electron spectroscopy (XPS) measurements were per-
formed. As expected, there are only C 1s and O 1s peaks ob-
served in the XPS survey spectrum of GO nanosheets (Fig-
ures 1a and S1, see Supporting Information). The N 1s, Si
2p, and Si 2s peaks found in the XPS spectrum of 1 demon-
strate the effective silylation of GO nanosheets. Meanwhile,
Pd 3d peaks are present at the surface of complex 4, reveal-
ing the successful incorporation of Pd on the GO
nanosheets. To further investigate the chemical composi-
tion of the surface of the compounds, further study was
carried out on the N 1s and Pd 3d XPS spectra.
14
lysts in homogeneous couplings. The advantages of adopt-
ing NHCs as ligands in Pd-catalyzed couplings are as fol-
lows: (1) the strong σ-donating ability of NHCs into the Pd
center facilitates the oxidative addition process; (2) the ste-
ric bulkiness of NHC substituents renders reductive elimi-
nation; and (3) the strong NHC–Pd bond makes it more at-
tractive. Our interest in this area led us to explore NHC-Pd
complexes immobilized on commercial GO nanosheets
which may have advantages over the homogeneous coun-
terparts. Such heterogeneous catalysts with excellent cata-
lytic performance under mild conditions and long-term du-
rability would be highly desired. Herein, we present a tri-
azine-tethered NHC-Pd heterogeneous catalyst as a useful
catalyst for the room temperature Suzuki coupling of aryl
halides with arylboronic acids.
As illustrated in Scheme 1, GO nanosheet supported
NHC-Pd complex 4 was easily prepared in four steps. Ami-
nopropyl-functionalized GO 1 was simply obtained by re-
fluxing a mixture of GO nanosheets and (3-aminopro-
pyl)triethoxysilane (APTES) in anhydrous toluene. Reaction
of 1 with cyanuric chloride in THF gave cyanuric chloride
functionalized GO 2. Subsequently, treatment of 2 with 1-
methylimidazole led to the formation of imidazolium-func-
tionalized GO 3. Complex 4 was obtained by treatment of 3
In the high-resolution N 1s XPS spectrum, 1 displays
one peak at 399.8 eV corresponding to primary amine
15
(RNH ) (Figure 1b). In the expanded XPS spectrum of 2,
2
the N 1s peaks at 399.46 and 401.1 eV can be assigned to
nitrogen in the form of triazine (C=N) and secondary amine
16
(R NH), respectively (Figure 1c). In Figure 1d, the N1s XPS
2
spectrum of 3 consists of three peaks at 399.46, 401.1, and
398.9 eV, corresponding to the binding energies of triazine
+
(C=N), R NH and N (imidazole), and R N (imidazole), re-
2
3
spectively.17
After the addition of Pd moieties, the triazine (C=N) and
imidazole (R N) bands shifted to higher binding energies of
3
399.66 and 399.09 eV, respectively (Figure 1e). This phe-
nomenon could be explained by donor–acceptor interac-
with Pd(OAc) in THF at 75 °C. Loading amounts of Pd on
2
18
the surface can be controlled in this step. Complex 4 with a
tions between the nitrogen in ligands and Pd moieties.
Moreover, the intensity of the peak at 401.1 eV decreased
and the peak at 399.09 eV increased owing to the conver-
+
sion of N into R N-type nitrogen after anchoring by Pd
3
moieties. Finally, we attempted to determine the oxidation
state of Pd in complex 4. The Pd 3d core level XPS spectrum
of complex 4 exhibits main peaks at 337.95 and 343.25 eV,
which are assigned to Pd(II) (Figure 1f).18
Elemental maps in energy-dispersive X-ray spectrosco-
py (EDX) of complex 4 show that N and Pd atoms were ho-
mogeneously dispersed on the surface of GO nanosheets.
The data also clarify that the Pd moieties are anchored on
the triazine-tethered NHC ligand (Figure 2).
In addition, structural elucidations of complex 4 and re-
used catalyst were performed using transmission electron
microscopy (TEM) analysis (Figure 3). For complex 4, it is
difficult to find agglomerated Pd nanoparticles, suggesting
that most of the Pd moieties are homogeneously dispersed
on the surface of the catalyst (Figure 3a,b). TEM images ob-
tained from recovered catalyst 4 show that some Pd
nanoparticles are formed at the surface of the catalyst (Fig-
ure 3c,d).
Scheme 1 Synthesis of complex 4. Reagents and conditions: a) APTES,
toluene, reflux, 10 h; b) cyanuric chloride, THF, r.t.; c) 1-methylimidaz-
The results for N adsorption–desorption containing the
Brunauer–Emmett–Teller (BET) surface area (SBET) and the
total pore volumes (Vtotal) of GO nanosheets and complex 4
2
ole, 75 °C, 2 h; d) Pd(OAc) , THF, 75 °C, 2 h.
2
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Y. Qian et al.
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Figure 1 N 1s XPS spectra of (a) GO, (b) 1, (c) 2, (d) 3, and (e) catalyst 4; (f) Pd 3d XPS spectrum of complex 4
are summarized in Table S1 (see Supporting Information).
The isotherms exhibited type IV hysteresis for GO
nanosheets and complex 4 (Figure S2, see Supporting Infor-
mation). The values for SBET and Vtotal of complex 4 were
slightly decreased compared to those of the starting materi-
al GO nanosheets.
With complex 4 as the heterogeneous catalyst in hand,
we carried out the Suzuki coupling of 4-bromoanisole with
phenylboronic acid as a model reaction (see Table S2, Sup-
porting Information). The reaction was initially performed
in the presence of 0.2 mol% 4 and 2.0 equivalents of K CO
2 3
in EtOH/H O, and an excellent yield was obtained within 1
2
hour at room temperature (entry 1). High catalytic activity
was observed with different bases in the presence of 0.1
mol% 4. K CO proved to be the most effective in terms of
2
3
yield (entries 1–4), whereas other inorganic bases gave
moderate yields (entries 5–7) and organic bases were less
favorable (entries 8–10). Next, we investigated solvents in
the presence of 0.1 mol% 4. A brief survey revealed that
Figure 3 (a) Low-resolution and (b) high-resolution TEM images of
fresh complex 4; (c) low-resolution and (d) high-resolution TEM images
of catalyst 4 after 12 runs
Figure 2 (a) TEM image; EDX mapping images of (b) carbon, (c) nitro-
gen, and (d) palladium for fresh complex 4
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Y. Qian et al.
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EtOH/H O was the best solvent among those evaluated (en-
Table 1 (continued)
2
tries 11–17).
Entry (Hetero)aryl halide
Product
Time Yield
b
To further expand the scope of our catalytic system, we
next investigated the coupling of various aryl halides with
arylboronic acids in the presence of 0.1 mol% 4 at room
temperature (Table 1). Bromobenzene and activated aryl
bromides such as 1-bromo-2-nitrobenzene and 4-bromo-
benzonitrile were very well coupled in excellent yields
within 0.6 hour (entries 1–3). Deactivated aryl bromides in-
cluding 2-bromotoluene, 4-bromotoluene, 2-bromoanisole,
and 4-bromoanisole were found to furnish the correspond-
ing biaryl products in excellent yields (entries 4–7). More-
over, 4 was efficient for coupling 1-bromonaphthalene and
(h)
(%)
9
1
95
Br
10
2
93
Br
NH2
NH2
1
1
2
24
6
51
87
c
c
c
Cl
Cl
CN
CN
9
-bromoanthracene (entries 8, 9). Importantly, it was also
found to be active for unprotected 3-bromoaniline (entry
0). Electron-poor aryl chlorides are far more difficult to ac-
1
O2N
Me
O2N
Me
24
65
93
1
c
6
1
9
tivate than aryl bromides. Nevertheless, 4 showed satis-
factory efficiency toward 4-chlorobenzonitrile and 1-
chloro-2-nitrobenzene at 50 °C for 6 hours (entries 11, 12).
Complex 4 was also efficient for the coupling of deactivated
aryl halides with electron-rich arylboronic acids at room
temperature (entries 13, 14).
13
2
91
Br
Br
Me
1
4
5
1
96
Me
Me
Me
Table 1 Suzuki Coupling of Aryl and Heteroaryl Halides with Arylbo-
a
ronic Acids
1
Me
Me
4
1
83
90
c
c
4
(0.1 mol%)
(
Hetero)aryl halide (HO)2B
(Hetero)aryl
Br
Br
EtOH/H2O, K2CO3
r.t.
R
R
N
N
N
N
1
6
7
N
N
4
1
87
98
c
c
c
c
Entry (Hetero)aryl halide
Product
Time Yield
b
(
h)
(%)
1
4
1
91
99
1
O2N
O2N
0.5
99
Br
Br
Br
Br
1
8
9
S
4
1
89
99
S
c
c
c
c
2
3
4
5
0.6
0.5
0.8
1
99
99
99
99
1
S
S
S
S
4
1
87
98
Br
Br
Br
Br
CN
CN
20
CHO
CHO
2
1
87
99
c
c
OMe
OMe
a
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.5 mmol), 4
0.1 mol%), K CO (2.0 mmol), TBAB (0.5 mmol), H O (2 mL), EtOH (2 mL),
MeO
MeO
(
2
3
2
r.t.
b
Br
Br
GC yield, determined using n-dodecane as an internal standard.
The reaction was carried out at 50 °C.
c
6
7
1
1
99
99
Me
Me
The catalytic system was further extended to the cou-
pling of heteroaryl bromides with phenylboronic acid. The
Suzuki coupling of heteroaryl halides is of particular inter-
est to the pharmaceutical industry because many biologi-
cally active compounds are accessed through this method-
ology.20 Despite their importance, the coupling of het-
eroaryl bromides remains a challenge, especially at low
temperature. It is noteworthy that 3-bromopyridine, 2-bro-
Me
Me
Br
Br
8
1
97
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Y. Qian et al.
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mopyridine, sterically hindered 2-bromo-3-methylpyri-
dine, 3-bromothiophene, 2-bromothiophene, and 5-bro-
mo-2-thiophenecarboxaldehyde could be coupled in good
yield at room temperature, although longer time was re-
quired (Table 1, entries 15–20). In addition, the heteroaryl
halides underwent the coupling at 50 °C to afford the corre-
sponding desired products in high yields in 1 hour. The ex-
cellent catalytic activity of 4 could be explained by the fact
that the triazine-tethered NHC could increase the electron
density of the center Pd, consequently to greatly enhance
A leaching test was carried out in the coupling of 3-bro-
moaniline with phenylboronic acid. No further reaction
took place after catalyst 4 was removed by filtration (Figure
S4, see Supporting Information). Also, ICP-OES analysis of
the solution indicated that little Pd metal (0.11 ppm)
leached into the solution during the reaction. These results
demonstrated that the high sustainability of the catalyst
could be explained by a strong binding of the triazine-teth-
ered NHC to Pd.
We have compared the results obtained in this work
with various reported heterogeneous catalysts (see Table
S3, Supporting Information), which verified that 4 is highly
efficient in room-temperature Suzuki coupling reactions.
Along with the inherent nature of the free Pd complex, the
excellent reusability and reactivity of 4 could be explained
by the strong binding of triazine-tethered electron-rich
NHCs to the metal center and the structural openness of GO
nanosheets.
In summary, we have developed a highly active, recycla-
ble, GO nanosheet supported, triazine-bridged NHC catalyst
for Suzuki coupling reactions. A wide range of aryl and het-
eroaryl bromides were very well coupled at room tempera-
ture. The catalytic system is also effective for less reactive
aryl chlorides. In particular, the catalytic activity remained
high after several reuses. Along with the inherent nature of
the free Pd complex, the high efficiency appears to be due
to the good stability of the catalytically active Pd species on
GO nanosheets. The simple preparation of 4, in combina-
tion with its high catalytic activity and recyclability, may
hopefully stimulate future research in the field of heteroge-
neous catalysis. Further studies of other coupling reactions
are currently in progress.
14a
the performance of 4.
To demonstrate the support effect on catalytic activity,
we compared Pd supported on the most common support
SiO (Pd-NHC@SiO ) with 4 (Figure S3, see Supporting In-
2
2
formation). Pd-NHC@SiO2 was prepared using the same
method as for catalyst 4, except for the support. Firstly, we
coupled activated 4-bromobenzonitrile and phenylboronic
acid. After 30 minutes, the reaction showed complete con-
version with 4, in comparison to 95% with Pd-NHC@SiO2.
Secondly, we selected deactivated 4-bromoanisole as aryl
halide. Within 50 minutes, 4 and Pd-NHC@SiO yielded the
2
coupling product in 99% and 84% yield, respectively. In the
case of 4-bromoanisole, the difference in activity between
the catalysts was much more conspicuous. Finally, we se-
lected 4-chlorobenzonitrile as aryl halide and the reaction
was carried out at 50 °C. After 6 hours, the yields were 87%
and 58% for 4 and Pd-NHC@SiO , respectively. On the basis
2
of these findings, we could carefully assert that the struc-
tural openness of GO facilitates the catalysis. It is also note-
worthy that homogeneous triazine-based NHC-Pd catalysts
require somewhat higher temperatures and longer reaction
21
times to achieve effective couplings and would suffer
from catalyst reuse and recycling problems.
Catalyst reuse is one of the most attractive advantages
of the heterogeneous reaction. The reusability of 4 was in-
vestigated in the reaction of deactivated 4-bromoanisole
with phenylboronic acid. As shown in Figure 4, after 10
times reuse, the excellent catalytic efficiency was still re-
tained without loss of catalytic activity. The catalytic activi-
All chemicals are commercially available and were used as purchased
without any further purification. Graphene oxide (GO) was obtained
from Anstrong Materials. All other chemicals were purchased from
Sigma Aldrich, TCI, and Alfa Aesar. X-ray photoelectron spectroscopy
(
XPS) data were collected using an ESCA 2000 photoelectron spec-
trometer (Thermo Scientific) with monochromated Al Kα radiation
hν = 1486.6 eV). NMR spectra were recorded on a Bruker DIGITAL
th
ty gradually decreased from the 11 run. TEM studies have
(
confirmed that Pd nanoparticles are formed at the surface
of the GO nanosheets after 12 runs (Figure 3c,d).
AVANCE III (400 MHz) spectrometer. Field-emission transmission
electron microscopy (FE-TEM) images were acquired by employing a
JEOL JEM-2100F microscope. The specific surface area and pore size
distribution of the catalyst were investigated by measuring the nitro-
gen adsorption and desorption isotherms at 77 K on a BELSORP-MAX
system (BEL, Japan). GC/GC-MS analyses were performed on an Agi-
lent 6890N GC coupled to an Agilent 5975 Network Mass Selective
Detector.
Aminopropyl-Functionalized GO 1
GO (200 mg) was dispersed in anhydrous toluene (100 mL) under son-
ication for 1 h. (3-Aminopropyl)triethoxysilane (APTES) (0.05 mL,
0.21 mmol) was dropwise added into the GO suspension. The mixture
was stirred under reflux for 10 h. After allowing the mixture to cool to
r.t., the mixture was filtered, washed with methylene chloride, and
dried at 75 °C under vacuum to give 1.
Figure 4 Reuse of catalyst 4 in the Suzuki coupling of 4-bromoanisole
with phenylboronic acid
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Y. Qian et al.
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Cyanuric Chloride Functionalized GO 2
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for 10 min at 0 °C, the solution was warmed to r.t. over 20 min. The
resulting mixture was filtered, washed with methylene chloride, and
dried at 75 °C under vacuum to give 2.
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2
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J. Mater. Chem. A 2015, 3, 10368.
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Suzuki Coupling Reaction; General Procedure
Aryl halide (1.0 mmol), arylboronic acid (1.5 mmol), TBAB (0.5
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mmol), K CO (2.0 mmol), and catalyst 4 (0.1 mol%) were mixed in
2
3
H O/EtOH (2 mL/2 mL). The mixture was stirred at r.t. in an air atmo-
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(14) (a) Valente, C.; Calimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.;
Organ, M. G. Angew. Chem. Int. Ed. 2012, 51, 3314.
(b) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F.
Nature 2014, 510, 485.
2
sphere. The reaction progress was monitored by GC. After reaction
completion, the mixture was diluted with H O and Et O. The organic
2
2
layer was separated from mixture, dried over anhydrous MgSO , and
4
concentrated under reduced pressure. The crude reaction product
was purified using column chromatography on silica gel to afford the
corresponding product in good yield. In the recycling experiment, af-
ter the completion of the reaction, catalyst was separated from the re-
action mixture by filtration. The separated catalyst was washed with
H O, EtOH, and Et O. It was directly used in the next run under the
2
2
(15) Xiao, L. H.; Zheng, X.; Zhao, T. Y.; Sun, L. Y.; Liu, F. Q.; Gao, G.;
Dong, A. Colloid Polym. Sci. 2013, 291, 2359.
same reaction conditions.
(
16) Fu, Y. S.; Zhu, J. W.; Hu, C.; Wu, X. D.; Wang, X. Nanoscale 2014,
, 12555.
6
(
17) Liu, W. S.; Koh, K. L.; Lu, J. L.; Yang, L. P.; Phua, S.; Kong, J. H.;
Funding Information
Chen, Z.; Lu, X. H. J. Mater. Chem. 2012, 22, 18395.
This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF), funded by
(18) Zhao, Q. S.; Zhu, Y. Z.; Sun, Z.; Li, Y.; Zhang, G. L.; Zhang, F. B.;
Fan, X. B. J. Mater. Chem. A 2015, 3, 2609.
the Ministry of Education (grant number: NRF-2015R1A4A1042434).
N
atiRonesaFeloaurc
h
n-
(19) Alonso, F.; Beletskaya, I. P.; Yus, M. Tetrahedron 2008, 64, 3047.
(20) Tyrrell, E.; Brookes, P. Synthesis 2003, 469.
d
ati
o
n
of
K
ore
a
(N
R
F-2
0
1
5
R
1
A
4
A
1
0
4
2
4
3
4)
(21) Gao, C.; Zhou, H. J.; Wei, S. P.; Zhao, Y. S.; You, J. S.; Gao, G. Chem.
Commun. 2013, 49, 1127.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611726.
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©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–F