Graphene-Supported Palladium Complex: A Highly Efficient, Robust and Recyclable Catalyst for the Suzuki-Miyaura Cross-Coupling of Various Aryl Halides

Article abstract of DOI:10.1055/s-0035-1562607

We present a novel strategy based on the immobilization of a palladium complex on reduced graphene oxide for the development of a heterogeneous catalytic system with high efficiency and good recyclability. The prepared catalyst was tested for Suzuki-Miyaura cross-coupling reactions and shown to have a high catalytic efficiency, giving 83-98% conversion of the reactants under mild conditions. Moreover, it could be readily recycled and reused several times without significant loss of activity.

Full text of DOI:10.1055/s-0035-1562607

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–F
letter
A
A. Taher et al.
Letter
Synlett
Graphene-Supported Palladium Complex: A Highly Efficient,
Robust and Recyclable Catalyst for the Suzuki–Miyaura Cross-
Coupling of Various Aryl Halides
Abu Taher
Dong-Jin Lee
Ik-Mo Lee*
Department of Chemistry, Inha University, Incheon 402-751,
Republic of Korea
imlee@inha.ac.kr
Received: 02.06.2016
Accepted after revision: 30.06.2016
Published online: 28.07.2016
Graphene14 and reduced graphene oxide (rGO)15,16 sur-
faces have recently gained much attention as convenient
supports in heterogeneous catalysis due to their unique
two-dimensional structures, huge surface areas, and other
excellent properties.17,18 In fact, these materials have been
shown to be promising supports for many metallic nano-
catalysts, such as Pd,19–21 Pd–Ag nanorings,22 Au,23 Au–Pt
core–shell nanoparticles,24 or Fe3O4.25 Despite impressive
recent improvements in the scope of catalytic reactions, the
catalytic protocol still suffers from some disadvantages,
such as metal leaching and low reactivities for several im-
portant substrate classes, resulting in low yields. Therefore,
the development of a reusable catalyst for cross-coupling
reactions under mild conditions is highly desirable and is a
hot topic in both green chemistry and current organic syn-
thesis. Here, we report a highly efficient method for the im-
mobilization of Pd complexes on rGO as a reusable catalyst
for cross-coupling reactions in ethanol–water under mild
conditions (Scheme 1).
DOI: 10.1055/s-0035-1562607; Art ID: st-2016-u0378-l
Abstract We present a novel strategy based on the immobilization of
a palladium complex on reduced graphene oxide for the development
of a heterogeneous catalytic system with high efficiency and good re-
cyclability. The prepared catalyst was tested for Suzuki–Miyaura cross-
coupling reactions and shown to have a high catalytic efficiency, giving
83−98% conversion of the reactants under mild conditions. Moreover, it
could be readily recycled and reused several times without significant
loss of activity.
Key words graphene oxide, Suzuki coupling, heterogeneous catalysis,
palladium catalysis, catalyst support, aryl halides
The impact of metal-catalyzed coupling reactions in the
preparation of natural products, pharmaceuticals, and mo-
lecular organic compounds has been widespread.1,2 In this
context, palladium-catalyzed C–C bond-formation reac-
tions are among the most efficient techniques in organic
synthesis.3 In particular, the Suzuki–Miyaura cross-cou-
pling reaction has served as an innovative pathway for the
construction of biaryl units.4,5 Homogeneous metal cata-
lysts are frequently used in coupling reactions,6 and the im-
mobilization of homogeneous metal catalysts onto solid
supports is one of the major challenges in the field of catal-
ysis. In addition, recyclability of homogeneous metal cata-
lysts is considered a major objective in relation to green
chemistry.7 One approach to such an environmentally be-
nign process is based on the development of supported cat-
alysts that exhibit extremely high catalytic activities, even
in aqueous media.8 Thus, efforts have been made to immo-
bilize homogeneous catalysts on diverse support materials,
such as nanoparticles,9 inorganic solids,10 polymers,11 den-
drimers,12 or perfluorinated tags.13
Scheme 1 Preparation of graphene-supported palladium complex 3.
Reagents and conditions: (i) [3-(triethoxysilyl)propyl]amine, toluene,
100 °C, 10 h. (ii) Pd(OAc)2, acetone, 45 °C, 12 h.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F

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A. Taher et al.
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Amine-functionalized rGO 2 was prepared from com-
mercially available rGO (1) by treatment with [3-(triethox-
ysilyl)propyl]amine (APL) in toluene with mild sonication
for 10 minutes and then heating to 100 °C for ten hours.
Treatment of 2 with Pd(OAc)2 in acetone at 45 °C for 12
hours, followed by filtration and repeated washing with ex-
cess CH2Cl2 at room temperature gave the catalyst 3 con-
taining immobilized Pd complexes (Scheme 1). The palladi-
um complexes were encapsulated within the amine-func-
tionalized rGO layer through coordination,15a and the
nature of these complexes was examined by Fourier-trans-
form IR spectroscopy, field-emission transmission electron
microscopy (FE-TEM), high-resolution scanning electron
microscopy (SEM); X-ray photoelectron spectroscopy (XPS),
energy-dispersive X-ray spectroscopy (EDS), thermogravi-
metric analysis (TGA), inductively coupled plasma (ICP)
analysis, and Raman spectroscopy. The amount of immobi-
lized Pd in catalyst 3 was determined by ICP analysis to be
5.75%, which exactly matched the calculated value. The
morphology of 3 was studied by TEM and SEM. Figure 1 (a
and b) shows TEM images of fresh and recycled samples of
catalyst 3. SEM and the corresponding elemental mapping
by EDS analysis of 3 showed that palladium was homoge-
neously distributed in the hybrid material [Figure 1 (d)].
Figure 1 (e) shows typical FTIR spectra of 1, 2, and 3. rGO 1
showed typical absorption features that included coupled
C–O stretching vibrations at 1058 cm–1, O–H bending vibra-
tions at 1458 cm–1, C=O stretching vibrations at 1718 cm–1,
CH2 stretching vibrations at 2853 and 2921 cm–1, CH3
stretching vibrations at 2968 cm–1, and O–H stretching vi-
brations at 3434 cm–1.26–29 The vibration at 3434 cm–1 was
attributed to absorbed water.26 After silanization, the ap-
parent increases in intensity at 1060 and 2921 cm–1 are di-
rect evidence of Si–O stretching and the presence of methy-
lene groups from the silicon coupling agent APL, respective-
ly [Figure 1 (e); center]. Subsequently, on treatment with
Pd(OAc)2, the C–H peak increased in intensity and a new
peak appeared at 1718 cm–1, which was assigned to the car-
bonyl group [Figure 1 (e); bottom].28,29 This peak confirmed
the presence of Pd(OAc)2 on the surface of 2. However, an
intense broad band associated with hydroxy groups26 re-
mained, due to small solvent molecules adsorbed on the
surface of the graphene flakes or inserted between layers
during sonication.30
Figure 1 TEM images of catalyst 3 (a) before use and (b) after a fifth
catalytic run; (c) SEM image of 3; (d) corresponding quantitative EDS
mapping of Pd; (e) FTIR spectra and (f) Raman spectra of 1, 2, and 3.
The corresponding I(D)/I(G) values are indicated for each spectrum.
cm–1 across the whole supported area of the samples indi-
cates that the strain and doping are homogeneous.33 After
silanization of 1, the I(D)/I(G) ratio of the silanized
graphene increased rapidly from 1.30 to 1.41. This is due to
a decrease in the average size of the in-plane sp2 domains
upon silanization.26 For 3, the I(D)/I(G) ratio was drastically
decreased to 1.34, indicative of an increasing order of for-
mation of Pd(OAc)2 onto 2. The spectrum [Figure 1 (f); bot-
tom] shows an additional peak at around 432 cm–1 due to
the presence of palladium.34 Moreover, the EDX spectra
confirmed the presence of palladium on the rGO surface
(see Supporting Information, Figure S1).
The chemical compositions of the as-prepared 3 com-
plexes were determined by means of TGA. Figure S2 (Sup-
porting Information) shows typical TGA curves for samples
of 1, 2, and 3. rGO materials are not thermally stable, and
mass loss started below 100 °C and was rapid at 150 °C.35
Mass loss between 350 °C and 750 °C was assigned to burn-
ing of graphitic carbon.36 The TGA curve for 3 showed a 12%
mass loss below 250 °C, which corresponded to loss of wa-
ter and trace amounts of oxygen.36 At temperatures up to
800 °C, total weight losses of 9, 19, and 33 wt% were ob-
served for 1, 2, and 3, respectively. We assume that the
weight loss that occurred during thermal decomposition of
Raman spectroscopy is a convenient and powerful tool
for probing the structure of graphene-based materials. Fig-
ure 1 (f) shows the featured regions of the Raman spectra
for 1, 2 and 3. In all the Raman spectra, two dominant peaks
for the D and G bands were observed at around 1308 and
1595 cm–1, respectively.29,31,32 The D band is due to the
breathing mode of the K point phonons of A1g symmetry,
and the G band corresponds to first-order scattering of the
E2g phonons. The constant position of the G band at 1595
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3 was due to palladium moieties. Furthermore, the results
of TGA also indicated that the graphene-supported catalyst
3 has excellent thermal stability.
to the production of fine chemicals. Initially, we examined
the Suzuki–Miyaura cross-coupling of 2-bromotoluene
with phenylboronic acid in the presence of various
amounts of catalyst 3 to determine the optimal loading. As
shown in Figure 3 (a), the yield of the reaction gradually in-
creased with increasing catalyst loading in the range 0.01–
0.2 mol%. A high catalytic activity was still observed at a
low catalyst loading. Whereas the reaction yield reached al-
most 100% conversion with 0.16 mol% of 3, 0.2 mol% of 3
gave a quantitative yield at 80 °C in ethanol–water. There-
fore, a 0.2 mol% loading of 3 was determined to be the opti-
mal loading under the reaction conditions.
To gain further insights into surface changes associated
with the chemical reaction shown in Scheme 1, we per-
formed XPS measurements. As expected, only C 1s and O 1s
peaks were seen in the XPS spectrum for 1 [Figure 2 (a); in-
sert]. The appearance of the Si 2s (152 eV), Si 2p (101 eV),
and N 1s (399.5 eV) peaks accompanying the C 1s and O 1s
peaks in the XPS spectrum of 2 [Figure 2 (a)] confirmed that
APS was covalently bonded to 1.26,37 The N 1s spectrum is
very useful for analyzing the nature of N functionalities.
The N 1s spectrum of 2 can be deconvoluted into three
components at 399.17, 400.04, and 400.89 eV, correspond-
ing to C–NH2, N–H, and NH2, respectively [Figure 2 (c)].38,39
A new peak for C–N (285.4 eV) in 2 confirmed the presence
of amino groups (Supporting Information, Figure S3a). Fur-
thermore, the O 1s spectrum could be reasonably fitted by
a single peak at 531.68 eV, indicating a high purity for prod-
uct 2 (Supporting Information; Figure S3b). The XPS survey
spectrum of 3 showed the adsorption of palladium onto 2
[Figure 2 (b)]. Furthermore, the presence of the Pd 3d3/2
(343.13 eV) and Pd 3d5/2 (337.60 eV) peaks indicated suc-
cessful coordination bonding of Pd(OAc)2 onto 2 [Figure 2
(d)].40,41
Figure 3 Suzuki–Miyaura cross-coupling of 2-bromotoluene with
phenylboronic acid in 1:1 ethanol–water catalyzed by 3. (a) Effect of
various catalyst loadings; (b) comparative catalytic study for 3, rGO +
APS + Pd(OAc)2, MWCNTs + APS + Pd(OAc)2, 2, and in the absence of
catalyst (top to bottom, respectively) with the same catalyst loading
(0.2 mol% Pd); (c) hot filtration test; and (d) recyclability test. In all cas-
es the reaction conditions were the same as those in Table 2, entry 2.
Next, various solvents were screened to improve the ef-
ficiency of the Suzuki–Miyaura cross-coupling reaction (Ta-
ble 1). Water alone was a poor solvent (Table 1, entry 1), re-
quiring a long reaction time. The results showed that 1:1
ethanol–water was the best solvent in terms of the catalytic
activity (entries 1–4). Next we screened various bases in
1:1 ethanol–water (entries 2 and 5–10) and we found that
relatively inexpensive K2CO3 gave the best result (entry 2),
whereas other inorganic bases showed moderate activities
(entries 5–7) and organic bases showed low activities (en-
tries 8–10).
Figure 2 XPS spectra of 1 [a (insert)], 2 (a), and 3 (b); High-resolution
XPS spectra of N1s for 2 (c) and Pd 3d for 3 (d).
The Suzuki–Miyaura cross-coupling reaction is one of
the most significant reactions for the construction of biaryl
units in organic synthesis,42 and various Pd catalysts have
been used for this reaction.43 Aryl iodides have been em-
ployed as suitable coupling substrates in most coupling re-
actions with arylboronic acids. However, the use of inex-
pensive aryl bromides or chlorides would be highly desir-
able for large-scale applications. Furthermore, recycling
and recovery of catalysts have been problematic in relation
After successfully screening the cross-coupling reaction,
we carried out Suzuki–Miyaura reactions of several nonac-
tivated aryl bromides and chlorides. Catalyst 3 maintained
a high catalytic reactivity for bromobenzene and nonacti-
vated 2-bromotoluene, 4-bromotoluene, 2-bromoanisole,
4-bromoanisole and 4-bromophenol (Table 2, entries 1–6).
The activated aryl bromides 1-bromo-3-nitrobenzene, 1-
bromo-4-nitrobenzene, 4-bromoacetophenone, and 1-bro-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F

D
A. Taher et al.
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Table 1 Optimization of Solvent and Base for Suzuki–Miyaura Cross-
Coupling Reaction of 2-Bromotoluene with Phenylboronic Acid Cata-
lyzed by 3a
Table 2 Suzuki–Miyaura Cross-Coupling of Aryl Halides with PhB(OH)2
in Ethanol–Watera
3 (0.2 mol%), K2CO3
X
(HO)2B
+
3 (0.2 mol%)
EtOH–H2O (1:1)
HBr
(HO)2B
+
R
R
Solvent, Base
80 °C
X = Br, Cl
Me
Me
Entry
R
X
Time (h)
Yieldb (%)
Entry Solvent
Base
K2CO3
Time (h)
Yieldb (%)
1
2
H
Br
0.6
1
96
98
95
97
98
95
97
98
98
96
98
96
88
96
92
83
1
2
H2O
6
94 (91)
100 (98)
81
2-Me
4-Me
2-MeO
4-MeO
4-HO
3-O2N
4-O2N
4-Ac
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
1:1 EtOH–H2O
1:1 DMF–H2O
1:1 1,4-dioxane–H2O
1:1 EtOH–H2O
1:1 EtOH–H2O
1:1 EtOH–H2O
1:1 EtOH–H2O
1:1 EtOH–H2O
1:1 EtOH–H2O
K2CO3
K2CO3
K2CO3
K3PO4
Na2CO3
KOAc
1
3
1
3
3.5
3
4
0.6
0.6
0.6
0.5
0.5
0.5
0.6
4
4
86
5
5
1
86 (85)
81 (78)
58
6
6
1
7
7
1
8
8
Et3N
1
51
9
9
piperazine
Et2NH
1
35
10
11c
12c
13d
14d
15d
16d
4-NC
4-Ac
10
1
trace
a Reaction conditions: 2-bromotoluene (1.0 mmol), PhB(OH)2 (1.2 mmol),
base (2.0 mmol), solvent (3.5 mL), 80 °C.
3-O2N
2,6-Me2
H
4
b GC yields; isolated yields are given in parentheses.
3
7
2-Me
4-HO
7
mo-4-isocyanobenzene rapidly gave the corresponding
coupling products in quantitative yields (entries 7–10). The
cross-coupling reactions of two bromides with electron-
withdrawing aryl groups, 4-bromoacetophenone and 1-
bromo-3-nitrobenzene, were efficiently promoted, even at
room temperature, giving the corresponding products in al-
most quantitative yields (entries 11 and 12). As expected,
aryl bromides with electron-withdrawing groups showed
somewhat higher reactivities than those with electron-do-
nating groups. Sterically hindered 2,5-dimethylbromoben-
zene also efficiently coupled with phenylboronic acid to
give the coupling product in 88% yield (entry 13). It is nota-
ble that catalyst 3 also smoothly promoted the Suzuki–Mi-
yaura reaction of aryl chlorides with phenylboronic acid to
provide the corresponding coupling products in up to 96%
yield (entries 14–16). These results demonstrate that cata-
lyst 3 successfully promotes the coupling reactions of vari-
ous aryl bromides or chlorides with either electron-with-
drawing or electron-donating substituents under mild con-
ditions.
A control experiment carried out under exactly the
same conditions showed that 2 was ineffective as a catalyst,
as it gave similar results to those obtained in the absence of
a catalyst [Figure 3 (b)]. When a mixture of rGO, APS, and
Pd(OAc) or a mixture of multiwall carbon nanotubes
(MWCNTs), APS, and Pd(OAc)2 was used, a relatively poor
yield was obtained. Reports in the literature suggest that
this might be due to the absence of coordinating ligands.15a
On the other hand, the supported catalyst 3 gave a quanti-
7
a Reaction conditions: aryl halide (1.0 mmol), PhB(OH)2 (1.2 mmol), K2CO3
(2.0 mmol), 1:1 EtOH–H2O (3.5 mL), 80 °C.
b Isolated yield.
c At r.t.
d At 90 °C.
tative yield under the same reaction conditions. These re-
sult demonstrate that the presence of connecting ligands
between the rGO and Pd moieties promotes the catalytic
reaction in the Suzuki–Miyaura cross-coupling.
Next, we turned our attention to testing the reusability
of the catalyst under mild conditions. A critical issue with a
heterogeneous catalyst is the possibility that a proportion
of the active molecules can migrate from the solid to the
liquid phase by leaching. The leached molecules might then
be responsible for a significant proportion of the catalytic
activity. To test this possibility, two identical reactions of
2-bromotoluene with phenylboronic acid were carried out
with continuous monitoring of the conversion after 20 min-
utes. One reaction was carried out in the presence of the
catalyst until the conversion reached 39% and then the sol-
ids were removed from the reaction mixture by filtration.
The liquid phase was then transferred to a new reaction vial
and allowed to react further, but no significant change in
conversion was observed [Figure 3 (c)]; after one hour, the
conversion was 40%, indicating that no active species was
present in the reaction vial.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F

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The reusability of the catalyst was also examined. A ma-
jor advantage of the catalyst is that it can be easily recov-
ered by simple filtration. The recycling of the catalyst was
examined in the coupling of 2-bromotoluene with phenyl-
boronic acid in 1:1 ethanol–water [Figure 3 (d)]. Similar
yields (100% to 98%) were obtained over the six runs, with-
out any significant loss of activity. The possible leaching of
Pd after completion of the reaction was monitored by ICP-
MS, which showed a total loss of 0.7 wt%. A comparative
TEM study of the catalyst 3 before and after the experi-
ments [Figure 1 (a and b)] showed that Pd nanoparticles
were dispersed on the rGO after several reaction runs. The
results of this study agreed with reports in the literature
that the formation of Pd nanoparticles on the rGO surface
as an active species developed during the reaction.45,46 The
extremely high stability and reusability of the prepared cat-
alyst might be due to the strong interactions between rGO
and the Pd moieties, which promote the catalytic reaction
under mild conditions.
In conclusion, we have designed and developed a highly
active heterogeneous palladium catalyst supported on re-
duced graphene oxide for coupling reactions in ethanol–
water. We showed that the catalyst could be used to couple
a wide range of less reactive aryl halides with phenylboron-
ic acid under mild conditions. Repeated reactions per-
formed by using a recycled catalyst showed similar yields
without a significant loss in activity. Advantages of the
present method include simple workups, high yields, and
good recyclability of the catalyst.
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(44) Suzuki–Miyaura Reaction; General Procedure
The appropriate aryl halide (1.0 mmol), PhB(OH)2 (1.2 mmol),
K2CO3 (2.0 mmol), and catalyst 3 (0.2 mol%) were heated in 1:1
EtOH–H2O (3.5 mL). When the reaction was complete, the
mixture was diluted with H2O and CH2Cl2. The organic layer was
separated, dried (MgSO4), and concentrated under reduced
pressure. The crude product was purified by column chroma-
tography on silica gel. As all of the products are known, charac-
terization was by comparison of their 1H and 13C NMR spectra
with literature data.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F