PdCo bimetallic nanoparticles supported on three-dimensional graphene as a highly active catalyst for Sonogashira cross-coupling reaction

Article abstract of DOI:10.1002/aoc.3594

PdCo bimetallic nanoparticles (NPs) were decorated over three-dimensional graphene (3DG) in a facile manner by reducing palladium chloride and cobalt chloride in the presence of ethylene glycol as reducing, stabilizing and dispersing agent. The PdCo NPs–3

Full text of DOI:10.1002/aoc.3594

Received: 4 May 2016
DOI 10.1002/aoc.3594
Revised: 20 July 2016
Accepted: 31 July 2016
F U L L PA P E R
PdCo bimetallic nanoparticles supported on three‐dimensional
graphene as a highly active catalyst for Sonogashira cross‐coupling
reaction
*
Minoo Dabiri | Milad Pedarpour Vajargahy
Faculty of Chemistry, Shahid Beheshti University,
PdCo bimetallic nanoparticles (NPs) were decorated over three‐dimensional
Tehran 1983969411, Islamic Republic of Iran
graphene (3DG) in a facile manner by reducing palladium chloride and cobalt chlo-
ride in the presence of ethylene glycol as reducing, stabilizing and dispersing agent.
The PdCo NPs–3DG nanocomposite was characterized using Raman, X‐ray photo-
electron and energy‐dispersive X‐ray spectroscopies, X‐ray diffraction and transmis-
sion electron microscopy. The obtained catalyst can act as an efficient catalyst for
Sonogashira cross‐coupling reactions in aqueous media.
Correspondence
Minoo Dabiri, Faculty of Chemistry, Shahid
Beheshti University, Tehran 1983969411, Islamic
Republic of Iran.
Email: m‐dabiri@sbu.ac.ir
KEYWORDS
bimetallic, C─C coupling, graphene, nanoparticles, Sonogashira
1
| INTRODUCTION
compromises the superiority of the intrinsic high specific
surface area of graphene. Particularly, three‐dimensional
The Sonogashira reaction of terminal acetylenes with aryl or
vinyl halides provides a powerful tool for C─C bond forma-
tion, which has been widely applied to various fields such as
graphene materials have been attracting lots of attention,
since they not only maintain the intrinsic properties of two‐
dimensional graphene sheets, but also provide superior
advantages for the fast transfer of electrolyte ions, and thus
might possess great potential to overcome the above‐men-
[1]
natural product synthesis and material science. Typical pro-
cedures for Sonogashira coupling utilize catalytic palladium
[2]
[8]
with a metal co‐catalyst. The most widely employed co‐cat-
alysts are copper salts, which mediate homocoupling of termi-
nal alkynes when the copper acetylide is exposed to oxidative
tioned electron transfer and mass transfer issues.
Based on the above considerations and also a part of our
ongoing studies on the synthesis of graphene nanocomposite
[
3]
[9]
agents or air. Other co‐catalysts such as cobalt, zinc, tin,
catalysts for cross‐coupling reactions, herein we report a
boron, aluminium, Ag O and AgOTf have been developed
novel protocol for Sonogashira cross‐coupling reaction
2
[4]
to address this issue. Previews reports suggest that the
enhanced activity of PdCo alloys might be attributed to an
catalysed by
PdCo
nanoparticles–three‐dimensional
graphene (PdCo NPs–3DG) nanocomposite in aqueous solu-
tion under aerobic conditions. Additionally, the effects of
solvent polarity, base and temperature on yield, and the
recycling potential of the catalyst have been assessed.
The process for the preparation of PdCo NPs–3DG
nanocomposite is described in Scheme 1. The graphene oxide
(GO) starting material is synthesized by oxidation of graphite
[
5]
electronic stabilization of the added element in the alloy.
2
Graphene, a two‐dimensional monolayer of sp ‐hybrid-
ized carbon atoms patterned in a hexagonal lattice, with the
advantages of large specific surface area, outstanding electri-
cal conductivity, superior mechanical flexibility and easy
functionalization, has attracted widespread attention. It is
generally believed that graphene‐based catalysts not only
increase the nanosized catalyst surface area for electron trans-
port but also provide better mass transport of reactants to
[
6]
[
10]
with a modified Hummers method.
The 3DG with high
nitrogen content (17.44 wt% of N) was synthesized through
a one‐pot hydrothermal process using ethylenediamine as
the crosslinking agent in the presence of GO aqueous disper-
[
7]
catalysts. However, in graphene‐based composites, the
graphene sheets easily form irreversible agglomerates driven
by strong van der Waals interactions, which greatly
[
11]
sion. After freeze‐drying, a 3DG aerogel was produced.
Then, PdCo NPs were decorated over the 3DG in a facile
Appl. Organometal. Chem. 2016; 1–5
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
DABIRI AND VAJARGAHY
for PdCo NPs–3DG, possibly as a result of the growth of
[
15]
PdCo NPs at the defect regions on the 3DG surface.
A transmission electron microscopy (TEM) image shows
that the surfaces of 3DG are covered by distributed PdCo NPs
with an average size of 15–25 nm (Figure 2).
The electronic properties of the PdCo NPs–3DG
nanocomposite were probed using X‐ray photoelectron spec-
troscopy (XPS). As shown in Figure 3(a), the peaks corre-
sponding to C 1 s, N 1 s, O 1 s, Co 2p and Pd 3d are clearly
observed in the XPS survey spectrum. From the N 1 s XPS
scan shown in Figure 3(b), it is observed that three different
types of nitrogen are present in the PdCo NPs–3DG system,
namely pyridinic nitrogen (396.79 eV, 7.79%), pyrrolic nitro-
gen (399.88 eV, 82.95%) and graphitic nitrogen (403.30 eV,
[
16]
9
.26%).
This result indicates that the reaction occurs
between amine groups of ethylenediamine and hydroxyl or
carboxyl groups of GO. The Pd 3d XPS spectrum (Figure 3
SCHEME 1 Procedure for synthesis of PdCo NPs–3DG nanocomposite
manner by reducing palladium chloride and cobalt chloride in
the presence of ethylene glycol as reducing, stabilizing and
[
12]
dispersing agent.
2
| RESULTS AND DISCUSSION
Raman spectroscopy provides information on the structure,
quality and electronic properties of graphene including disor-
[
13]
der, defect structure, defect density and doping levels. The
Raman spectra of prepared GO, 3DG and PdCo NPs–3DG
composite are shown in Figure 1. As shown, after the hydro-
thermal synthesis of GO with ethylenediamine, the I /I ratio
D
G
is 1.93 for the resulting 3DG, while I /I for GO is 1.50. The
D
G
higher value of I /I observed for 3DG clearly demonstrates
D
G
FIGURE 2 TEM image of PdCo NPs–3DG nanocomposite
the heteroatomic doping of nitrogen into the 3DG frame and
[14]
the enhanced degree of disorder.
I /I decreases to 1.31
D G
FIGURE 3 (a) Full‐range XPS spectrum of PdCo NPs–3DG
nanocomposite. (b) N 1 s, (c) Pd 3d and (d) Co 2p core‐level XPS spectra of
PdCo NPs–3DG nanocomposite
FIGURE 1 Raman spectra of GO, 3DG and PdCo NPs–3DG composite

DABIRI AND VAJARGAHY
3
c) shows two major peaks with binding energy at 335.67 and
3
41.49 eV, corresponding to Pd 3d5 and Pd 3d3/2. The peak
/2
0
at 335.67 eV corresponds to the Pd state, which is shifted by
0
.9 eV towards higher binding energy with respect to Pd/C,
[
17]
suggesting their alloy structure.
The peaks at 338.50 and
2+
3
43.69 eV for PdCo NPs–3DG can be attributed to Pd
FIGURE
4
(i) Sonogashira cross‐coupling reaction between terminal
states. The Co 2p XPS core‐level region is shown in
Figure 3(d), in which there are two major peaks with binding
energies at 781.53 and 798.16 eV, corresponding to Co 2p3/2
and Co 2p1/2, in good agreement with the literature value for
the PdCo alloy, which also matches well with the literature
alkynes and aryl halides and (ii) Suzuki cross‐coupling of benzeneboronic
acid with bromobenzene catalysed by PdCo NPs–3DG nanocomposite
a
TABLE 1 Screening of reaction conditions
reports for metallic cobalt. The peaks at 785.84 and
b
Entry
Solvent
Base
Yield (%)
2+
8
03.96 eV for PdCo NPs–3DG can be attributed to Co
1
2
3
4
5
6
7
CH
H
3
CN
O
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
51
94
86
90
78
85
64
55
NR
NR
78
61
states. Also, a shake‐up satellite at 792.16 eV is observed in
the XPS spectrum. This peak is typically associated with
cobalt in the bivalent and trivalent oxidation states, which is
2
Tetrahydrofuran
2+
3+
[18]
Dimethylformamide
assigned to Co and Co in Co O . The absence of chlo-
3
4
H
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
Na
i‐Pr
KOH
2
CO
3
rine in the XPS spectrum of the PdCo NPs–3DG
nanocomposite corroborates that this higher oxidation state
of Pd or Co is due to the partial oxidation surface of PdCo.
After careful investigation of the prepared PdCo NPs–
2
EtN
c
8
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
d
9
3
DG nanocomposite, it was used for catalysing two different
e
10
11
12
kinds of organic reactions: (i) Sonogashira cross‐coupling
reaction between terminal alkynes and aryl halides and (ii)
Suzuki cross‐coupling of benzeneboronic acid with
bromobenzene (Figure 4).
f
g
a
Ethynylbenzene (1.2 mmol), iodobenzene (1.0 mmol), solvent (3 ml), PdCo (1:1)
NPs–3DG nanocomposite (2 mol% Pd), PPh
base (1.5 mmol), 8 h, 80°C.
3
(0.02 mmol), CuI (0.003 mmol),
The catalytic activity of the PdCo NPs–3DG
nanocomposite was first tested in Sonogashira cross‐coupling
reactions between terminal alkynes and aryl halides. To opti-
mize the reaction conditions, ethynylbenzene and
iodobenzene were used as model substrates in the presence
of various bases and solvents (Table 1). The results indicate
that both base and solvent significantly influence the product
yield. Additionally, PdCo NPs–3DG exhibits high selectivity,
b
Isolated yield. NR, no reaction.
c
Pd NPs–3DG (2 mol% Pd).
d
Co NPs–3DG (2 mol% Co).
e
3
DG (0.05 g).
f
PdCo (2:1) NPs–3DG nanocomposite (2 mol% Pd).
g
PdCo (1:2) NPs–3DG nanocomposite (2 mol% Pd).
a
TABLE 2 Sonogashira cross‐coupling of aryl halides and terminal alkynes catalysed by PdCo NPs–3DG nanocomposite
1
2
b
Entry
R
X
R
Time (h)
Yield (%)
M.p. (°C)
[
19]
1
2
3
4
5
6
7
H
H
H
I
H
8
8
94
89
96
81
96
98
92
65
71
57
39
60–62 (61–63)
[
20]
19]
I
4‐CF
3
101–103 (102–104)
[19]
I
4‐Me
H
8
67–68 (67–70)
[
21]
2‐Me
I
8
Oil
96–98 (98–101)
[19]
4‐MeCO
I
H
8
[
NO
2
I
H
8
119–120 (116–118)
[19]
4‐Me
I
H
8
68–69 (68–70)
c
8
H
Br
Br
Br
Cl
H
20
20
20
100
c
9
NO
2
H
c
1
0
1
4‐Me
H
c
1
H
H
a
3 2 3
Aryl halide (1.0 mmol), terminal alkyne (1.2 mmol), PdCo (1:1) NPs–3DG (2 mol% Pd), PPh (0.02 mmol), CuI (0.003 mmol), K CO (1.5 mmol), 80°C.
b
Isolated yield.
c
PdCo (1:1) NPs–3DG (4 mol% Pd), 100°C.

4
DABIRI AND VAJARGAHY
TABLE 3 Reusability of PdCo NPs–3DG nanocomposite in Sonogashira
cross‐coupling of ethynylbenzene and iodobenzene
or electron‐donating groups show obvious differences in reac-
tivity (Table 2), indicating that the reaction is sensitive to the
electronic characteristics of the substituents under the present
reaction conditions. Additionally, a hindered substrate such as
a
Reaction cycle
1
2
3
4
5
6
7
b
Yield (%)
94
94
91
89
87
85
80
2
‐iodotoluene is converted to the corresponding product with
a
Ethynylbenzene (4.8 mmol), iodobenzene (4 mmol), H
2
O (12 ml), PdCo NPs–
a moderate yield (Table 2, entry 4). In the case of aryl bro-
mides and chlorides, low to good coupled yields are obtained
after prolongation of reaction and an increase in the amount of
Pd to 4 mol% (Table 2, entries 8–11).
3
8
b
DG (2 mol% Pd), PPh
h, 80°C.
3
(0.08 mmol), CuI (0.012 mmol), K CO (6 mmol),
2
3
Isolated yield.
Reusability is a key property for a heterogeneous catalyst
that should be studied. In this regard, we performed a reus-
ability test of the PdCo NPs–3DG catalyst in Sonogashira
cross‐coupling of ethynylbenzene and iodobenzene under
the optimized conditions. The PdCo NPs–3DG catalyst is sta-
ble during the catalytic reaction and is easily separated by
centrifugation from the reaction solution at the end of the
reaction. A new coupling reaction was started by using the
separated PdCo NPs–3DG catalyst and this process was con-
tinued up to the seven cycles. As evident from Table 3, the
PdCo NPs–3DG catalyst is still active and provides 80%
yield in 8 h after seven runs. Moreover, no important change
is observed in the morphology of the PdCo NPs–3DG cata-
lyst after the seven runs, which is evident from the TEM
image shown in Figure 5. To elucidate whether PdCo NPs–
3
DG is the real catalyst or just serves as a source for Pd(II)
FIGURE 5 TEM image of PdCo NPs–3DG nanocomposite after seven
cycles in Sonogashira cross‐coupling of ethynylbenzene and iodobenzene
or Co(II) ions that leach out into the solution to form the
active catalyst species, a hot filtration test was performed.
The Sonogashira coupling of ethynylbenzene and
iodobenzene was studied as the test reaction for the filtration
test. Once 50% of the reaction was completed (followed using
GC), the PdCo NPs–3DG catalyst was separated by centrifu-
gation and the reaction was then continued without catalyst in
the reaction flask for an additional 12 h at 80°C. There is no
noticeable coupled product formation that reveals the amount
of Pd leaching out is negligible and the catalyst is indeed het-
erogeneous in nature.
Inspired by the high activity and stability of the PdCo
NPs–3DG nanocomposite, the Suzuki–Miyaura cross‐cou-
pling reaction was employed as another model reaction to
investigate further the performance of the PdCo NPs–3DG
nanocomposite. We chose the Suzuki–Miyaura coupling
reaction of benzeneboronic acid with bromobenzene in water
as solvent at 80°C in the presence of K CO as base and with
FIGURE
6
Suzuki cross‐coupling of benzeneboronic acid with
bromobenzene catalysed by PdCo NPs–3DG nanocomposite
which may be related to the presence of Co species. The
enhanced reactivity is attributed to the promotional effect of
Co dopants, which provide more Pd active sites for the reac-
[
19]
tants.
Notably, the reaction using pristine 3DG without
any PdCo NPs gives no conversion in the present system
under the same reaction conditions (Table 1, entry 10). From
Table 1, it is clear that PdCo (1:1) NPs–3DG is more active
than PdCo NPs–3DG nanocomposites with other ratios of
Pd to Co (Table 1, entries 11 and 12).
2
3
Table 2 demonstrates that PdCo (1:1) NPs–3DG is an
active catalyst for Sonogashira cross‐coupling reactions of
various aryl halides and terminal alkynes. The reactions of
aryl iodides and terminal alkynes with electron‐withdrawing
a catalyst loading of 4 mol% of Pd for 2 h. Under these con-
ditions, we find that the cross‐coupling reaction proceeds
well, affording excellent yield (98%) of the corresponding
biphenyl (Figure 6).
TABLE 4 Comparison of efficiency of various PdCo NP catalysts in Sonogashira cross‐coupling reaction of ethynylbenzene and iodobenzene
Catalyst
Conditions
Yield (%)
Time (h)
Ref.
PdCo NPs–3DG
Pd (2 mol%), PPh
Pd (5 mol%), PPh
Pd (2 mol%), PPh
3
3
3
(2 mol%), CuI (0.3 mol%), K
(2 mol%), CuI (0.3 mol%), K
(2 mol%), CuI (0.3 mol%), K
(2 mol%), CuI (0.3 mol%), Et
2
2
2
3
CO
CO
CO
3
3
3
(150 mol%), H
(150 mol%), H
(150 mol%), H
2
2
2
O
O
O
94
92
99
98
8
This work
[
[
[
22]
23]
16]
PdCo hollow nanospheres
Hollow Pd–Co nanospheres
PdCo/G
6
5
Pd (2 mol%), PPh
3
N (120 mol%), tetrahydrofuran–H
2
O (1:1)
12

DABIRI AND VAJARGAHY
5
A few previous studies have involved the application of
free or supported PdCo NPs as catalysts in Sonogashira
cross‐coupling reactions. Table 4 compares the efficiency of
the PdCo NPs–3DG nanocomposite with that of other
reported PdCo NP‐based catalysts in Sonogashira cross‐cou-
pling reaction of ethynylbenzene and iodobenzene.
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work. The prepared PdCo NPs–3DG nanocomposite exhibits
a high catalytic activity for Sonogashira cross‐coupling reac-
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