Magnesium oxide supported bimetallic Pd/Cu nanoparticles as an efficient catalyst for Sonogashira reaction

Article abstract of DOI:10.1016/j.jcat.2018.02.028

PdCu bimetallic nanoparticles with a diameter of about 3 nm are prepared and supported on a polymeric vinylimidazole ligand modified magnesium oxide. This new material is characterized using different analysis such as XRD, XPS, CHNS, TEM, SEM, and EDX-mapping. PdCu supported on MgO (MgO@PdCu) exhibits high catalytic activity in the Sonogashira coupling reaction of aryl iodides, bromides and chlorides with low Pd loading (0.05-0.2 mol%). This catalyst is recovered and recycled for 11 consecutive runs preserving its catalytic activity in the model reaction of iodobenzene with phenylacetylene for at least 8 cycles. Reused catalyst is characterized with TEM, XPS and EDX showing preservation of the catalyst structure. Using hot filtration and PVP poisoning tests, the catalyst shows a heterogeneous behavior for the model reaction.

Full text of DOI:10.1016/j.jcat.2018.02.028

Journal of Catalysis 363 (2018) 81–91
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Magnesium oxide supported bimetallic Pd/Cu nanoparticles as an
efficient catalyst for Sonogashira reaction
Mohammad Gholinejad a,b, , Maedeh Bahrami , Carmen Nájera , Biji Pullithadathil d
⇑
a
c,
⇑
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 5195-1159, Gavazang, Zanjan 45137-66731, Iran
Research Center for Basic Sciences & Modern Technologies (RBST), Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
Departamento de Química Orgánica and Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
Department of Chemistry, PSG College of Technology, Coimbatore 641004, India
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
PdCu bimetallic nanoparticles with a diameter of about 3 nm are prepared and supported on a polymeric
vinylimidazole ligand modified magnesium oxide. This new material is characterized using different
analysis such as XRD, XPS, CHNS, TEM, SEM, and EDX-mapping. PdCu supported on MgO (MgO@PdCu)
exhibits high catalytic activity in the Sonogashira coupling reaction of aryl iodides, bromides and chlo-
rides with low Pd loading (0.05–0.2 mol%). This catalyst is recovered and recycled for 11 consecutive runs
preserving its catalytic activity in the model reaction of iodobenzene with phenylacetylene for at least 8
cycles. Reused catalyst is characterized with TEM, XPS and EDX showing preservation of the catalyst
structure. Using hot filtration and PVP poisoning tests, the catalyst shows a heterogeneous behavior
for the model reaction.
Received 31 October 2017
Revised 20 February 2018
Accepted 26 February 2018
Keywords:
Sonogashira
Magnesium oxide
Palladium
Ó 2018 Elsevier Inc. All rights reserved.
Copper
Supported catalyst
1
. Introduction
The Sonogashira-Hagihara reaction is one of the most important
catalyzed reactions [20–22]. Among the different bimetallic cata-
lysts, Pd/Cu catalysts have attracted great attentions in different
organic transformations [23–43]. Along this line, some efforts have
been recently paid to use homogeneous or heterogeneous PdCu
bimetallic catalysts in various organic transformations particular
in Sonogashira reaction under efficient and enhanced catalytic
activity conditions [44–67]. Due to the expensive price and toxicity
of Pd, many efforts have been recently devoted to develop very effi-
cient heterogeneous and recyclable palladium catalysts in different
organic reactions [1,68–74], especially in the synthesis of pharma-
ceuticals in which allowable content of Pd is <5 ppm [75]. For this
purpose, different solid supports such as polymers [76], silica
[77,78], and naturally occurring polysaccharides [79] have been
used for stabilization of Pd nanoparticles in different coupling
reactions.
Concerning solid supports, metal oxides have received much
attention as resourceful materials for design of new heterogeneous
catalysts [80,81]. Along this line, MgO an organic ceramic material
with different properties such as high concentration of reactive
surface ions acting as Lewis acids, lattice bound and isolated
hydroxyl groups and anionic and cationic vacancies, is considered
as a promising support for the preparation of heterogeneous Pd
catalysts [82–103]. For instance, nanocrystalline MgO supported
palladium nanoparticles [NAP–Mg–Pd(0)] has been used as cata-
lyst for the selective reduction of nitro compounds [89], Heck reac-
tion of heteroaryl bromides [90], and oxidative coupling between
2
methodology for the bond formation between sp carbon and sp
carbon atoms forming substituted aryl-alkynes and conjugated
enynes through the coupling of aryl and vinyl halides or triflates
with terminal alkynes, respectively, by palladium catalysis and in
the presence of copper(I) as co-catalyst [1–11]. The obtained prod-
ucts from the Sonogashira reaction have wide utility in modern
organic chemistry and have been extensively applied in the syn-
thesis of natural products and pharmaceuticals, as well as in mate-
rial science [12–14]. In recent years, different copper-free
Sonogashira protocols (the original Cassar–Heck version of this
reaction) have been reported [15–19]. However, it has been proved
that in many cases, this cross-coupling reaction could be acceler-
ated in the presence of copper under oxygen free conditions to pre-
vent the formation of diynes as homo-coupling products [1–11].
Recently, an increasing attention has been paid to the applica-
tion of bimetallic catalysts in different organic transformations.
Due to new electronic and chemical properties of bimetallic cata-
lysts compared to the monometallic systems, high catalytic activity
and selectivity are expected for target products in these bimetallic
⇑
Corresponding authors.
E-mail addresses: gholinejad@iasbs.ac.ir (M. Gholinejad), cnajera@ua.es
(
C. Nájera).
https://doi.org/10.1016/j.jcat.2018.02.028
021-9517/Ó 2018 Elsevier Inc. All rights reserved.
0

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M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
N-aryl-2-aminopyridines and alkynes in the presence of copper(II)
chloride as additive [91]. In addition, Pd NPs supported on MgO
have been used for the regio selective hydrogenation of quinolines,
alkenes, and biodiesel under mild reaction conditions [92]. How-
ever, to the best of our knowledge there is no report on using
MgO as support for the stabilization of bimetallic PdCu nanoparti-
cles in any organic transformations. In this work, we introduce imi-
dazole as a simple nitrogen ligand modified MgO as an excellent
support for PdCu nanoparticles stabilization and the application
of this new material as a simple and efficient catalyst for Sono-
gashira coupling reactions of aryl halides with terminal alkynes
under efficient and phosphine-free conditions.
refluxed at 80 °C for 24 h. After centrifugation, the obtained solid
was washed with water (15 mL) and dichloromethane (3 Â 15 mL)
and dried at 60 °C.
2.3. Synthesis of bimetallic MgO@PdCu
CuSO O (18 mg, 0.07 mmol) was dissolved in a solution of
4
Á5H
2
ethylene glycol (5 mL) and polyvinylpyrrolidone (200 mg) and
the mixture was stirred for 2 h at 80 °C under an argon atmo-
sphere. In another flask, Pd(OAc)
2
(32 mg, 0.16 mmol) and
polyvinylpyrrolidone (400 mg) were dissolved in dioxane (5 mL)
and stirred for 2 h at room temperature under argon atmosphere.
The obtained solution was mixed with the CuSO
4
Á5H
2
O solution
2
. Experimental
and mechanically stirred under an argon atmosphere. During the
À1
stirring, a solution of NaOH (1 mol L ) was added upon reaching
2.1. Synthesis of MgO particles
pH to 9–10 and the obtained mixture was stirred for 2 h at
1
00 °C. Then, the reaction mixture was cooled to room tempera-
In a 250 mL flask, Mg(NO
water (100 mL) and an aqueous solution of Na
3
)
2
(3 g) was dissolved in distilled
CO (1.6 g in 100
ture and the resulting suspension was concentrated to 5 mL by
centrifugation (10000 rpm, 10 min). The obtained suspension was
diluted with acetone (5 mL) and the mixture was added to the pre-
pared imidazole functionalized MgO (1 g), which was previously
sonicated in acetone (10 mL). The mixture was stirred at room
temperature for 24 h under argon atmosphere and subjected to
centrifugation. The resulting solid was washed with water (3 Â 1
2
3
mL) was added slowly. The obtained mixture was then stirred vig-
orously for 12 h at room temperature. After centrifugation, the
resulting solid was isolated and dried at 100 °C. For achieving the
fine powder of MgO, the obtained solid was calcinated at 400 °C
for 2 h.
0
mL) and dichloromethane (3 Â 10 mL) and dried at 100 °C
affording MgO@PdCu.
2
.2. Synthesis of imidazole functionalized MgO
Acryloyl chloride (5 mmol, 0.4 mL) and Et
3
N (8 mmol, 1.1 mL)
2.4. General procedure for Sonogashira reaction
were added to the synthesized MgO (1g), which was sonicated in
THF (15 mL), at 0 °C under an argon atmosphere. Then, the mixture
was subjected to centrifugation and the obtained isolated solid was
washed with distilled water (2 Â 15 mL) and ethanol (2 Â 15 mL)
and then dried in an oven at 70 °C. For introducing the imidazole
group, MgO (1 g) was dissolved in EtOH (20 mL) and the mixture
was deoxygenated by bubbling argon for 5 min. Then, N-
vinylimidazole (5 mmol, 0.45 mL) and benzoyl peroxide (6 mg)
were added to the previously achieved mixture and the nit was
To a 5 mL flask, the catalyst MgO@PdCu (10 mg for ArI or 40 mg
for ArBr and ArCl), ArX (1 mmol), DABCO (168 mg, 1.5 mmol), and
TBAB (322 mg, 1 mmol), for aryl chloride and DMF (2 mL) were
added under argon atmosphere. The alkyne (1.5 mmol) was also
added and the resulting mixture was stirred at 60–120 °C for
appropriate reaction times (see Tables). Progress of reactions was
followed by GC. Then, the reaction mixture was cooled down to
room temperature and extracted with ethyl acetate (3 Â 5 mL),
Scheme 1. Synthetic steps for the preparation of MgO@PdCu.

M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
83
the organic phase was washed with H
orated. The resulting residue was purified by column or plate chro-
matography using hexane and ethyl acetate as eluents.
2
O (10 mL), dried and evap-
tion conditions. This process was repeated for 11 consecutive runs
(see, Fig. 9).
3
. Results and discussion
2
.5. Typical procedure for recycling of the catalyst for the reaction of
3.1. Materials and characterization
iodobenzene and phenyl acetylene
The preparation of MgO@PdCu is summarized in Scheme 1.
MgO was prepared by treatment of Mg(NO ) with Na CO in H O
3 2 2 3 2
After stirring the reaction in each run for 24 h and studying of
GC yield, the reaction mixture was centrifuged and the obtained
solid was washed with ethyl acetate and dried. The resulting solid
was used for another batch of reaction under the optimized reac-
and further calcination at 400 °C. Then, acryloyl chloride was
added in dry THF and the resulting solid was allowed to react with
vinylimidazole in the presence of benzoyl peroxide. To this MgO
bonded to the polymer containing imidazole, was added the
bimetallic PdCu nanoparticles in acetone.
In the first step, the formation of MgO was confirmed using
X-ray powder diffraction (XRD) showing related peaks to MgO in
2h = 37, 43.1, 62.4, 75.1, 78.7 [104] and small peaks in 2h = 30.3
and 38.2 correspond to Na
impurities (Fig. 1).
2 3 2
CO [105] and Mg(OH) [106]
Then, the acryloyl functionalized MgO was polymerized with
vinyl imidazole to introduce nitrogen ligands on MgO. Because imi-
dazole is considered as excellent ligand for coordination and stabi-
lization of different transition metals [107], vinylimidazole was
added for copolymerization with the acryloyl unit. After addition
of fresh prepared bimetallic PdCu nanoparticles, MgO@PdCu was
À1
obtained containing 0.05 and 0.01 mmol g Pd and Cu, respec-
tively, which has been determined by atomic absorption spec-
troscopy. Elemental analysis of MgO@PdCu showed a nitrogen
Fig. 1. XRD pattern of prepared MgO.
Fig. 2. (a–c) Representative TEM and HRTEM images, (insets in (b) shows the inverse FFT patterns of the selected area and (c) shows the corresponding SAED pattern) and (d)
corresponding EDS spectrum of unsupported PdCu nanoparticles.

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M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
content of 1.85%. According to this result the degree of introduced
materials, composed of electron donor and acceptor metals, can
exhibit desirable catalytic properties [108].
À1
imidazole in the MgO support was calculated to be 0.65 mmol g
.
Fig. 2(a)–(d) shows the TEM image analysis of PdCu nanoparti-
cles before anchoring onto MgO nanosheet supports. The HR-TEM
analysis (Fig. 2(b)–(c)) shows the presence of lattice fringes with a
d-spacing value of 0.225 nm and 0.21 nm corresponding to the (1
However, TEM images of PdCu nanoparticles before addition to
MgO nanosheets, showed slightly aggregated chain-like structure
confirming the effect of imidazole modified MgO in stabilization
of PdCu nanoparticles (Fig. 3). Fig. 3(a)–(d) reveals the effective
anchoring of PdCu particles on the surface of MgO nanosheets. It
is interesting to note that the PdCu nanoparticles were found to
be highly uniform, monodispersed with particle size in the range
of 3–4 nm after anchoring onto MgO nanosheets. Fig. 3f shows
the selected area diffraction pattern (SAED) analysis with promi-
nent ring patterns corresponding to the fcc CuPd structure (JCPDS
no. 48-1551) and cubic MgO (JCPDS no. 75-1585). The diffraction
pattern shows prominent rings with interplanar spacing of
1
1) plane of Pd and (1 1 1) plane of CuPd. Fig. 2b represents the
inverse FFT of the selected area which indicated two distinguish-
able periodic lattice fringes, corresponding to CuPd and Pd and
clearly validates the formation of PdCu bimetallic nanoparticles.
According to first principles studies, during the formation of PdCu
bimetallic nanoparticles, the electron donor atom, such as Cu and
an electron acceptor, such as Pd, the d-band of Pd is lowered and
the local electronic properties will be more similar to Pt and such
Fig. 3. (a–d) Representative TEM and HRTEM images, (e) corresponding EDS spectrum and (f) SAED pattern of MgO@PdCu.

M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
85
0
0
.275 nm, 0.235 nm and 0.146 nm corresponding to the (1 1 1), (2
0) and (3 1 1) of MgO and interplanar spacing of 0.211 nm, 0.133
plane of CuPd and (1 1 1) plane of Pd [JCPDS No: 46-1043] matched
well with the SAED pattern revealing the existence of PdCu
bimetallic nanoparticles on MgO. The preferential growth plane
nm and 0.100 nm corresponding to the (1 1 1) of CuPd. The (1 1 1)
Fig. 4. SEM images of MgO@PdCu in different magnifications.
Fig. 5. XPS spectra of MgO@PdCu in (a) Pd 3d, (b) Cu 2p, (c) Mg 1s and (d) N 1s regions.

8
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M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
of MgO was found to be (1 1 1) as confirmed from SAED. EDS spec-
tra of the unsupported (Fig. 2d) and MgO supported (Fig. 3e) PdCu
bimetallic nanoparticles clearly give evidence for the presence of
Cu, Pd and MgO in the samples.
A doublet peak in higher binding energy at 338 and 343 eV could
be assigned to the Pd(IV) species in PdO form (Fig. 5a) [111–
113]. These results indicated that 44% of Pd are in Pd(0) oxidation
state, 32% as PdO and 24% as PdO form. Formation of Pd(0) may
2
2
EDS mapping images of the material MgO@PdCu was also stud-
ied (Fig. 1, ESI). It can be seen that both Pd and Cu are mostly dis-
tributed evenly throughout each PdCu species, suggesting an alloy
structure [64,109,110]. Furthermore, energy-dispersive X-ray spec-
troscopy (EDS) analysis of MgO@PdCu confirmed the presence of
Pd, Cu, Mg and N species (Fig. 2, ESI).
In order to investigate the oxidation states of Pd and Cu in the
solid support, X-ray photoelectron spectroscopy (XPS) of the
MgO@PdCu was performed (Fig. 5). High resolution XPS analysis
of Pd region showed three doublets between 335 and 343 eV. An
intense doublets at 335 and 340 eV were related to Pd(0) and a
doublet peak at 336.5 and 341.6 eV related to Pd(II) in PdO form.
resulted from the addition of concentrate electron rich polyethy-
lene glycol and polyvinylpyrrolidone and oxidized palladium are
the results of the reaction with sodium hydroxide at high temper-
atures under the catalyst preparation steps.
XPS spectra of Cu 2p region showed two peaks centered at
932.8 and 952.9 eV which were assigned to Cu 2p3/2 and Cu
2p1/2 of CuO, respectively [113–115] (Fig. 5b). The small peak at
the binding energy of 935.1 was assigned to the presence of Cu
2
(OH) species [116]. Apart from the main peaks of CuO, three extra
shake-up satellite peaks were observed in higher binding energies
at 941.7, 943.8, 954.8 which can be attributed to Cu(II) states in
CuO and Cu(OH)
peaks at 931.5 and 951.5 which correspond to Cu
Since binding energies of Cu(0) and Cu O are very close and are dif-
2
[114–117]. The spectra also showed two smaller
2
O or Cu species.
2
ferent by only 0.1 eV, detection of true species by using this region
of binding energy is very difficult [114–117]. Therefore, we studied
it by LMM auger transition spectroscopy in the XPS spectra and it
was found out to be 568 eV for Cu(0) and 570 eV for Cu
118,119]. Fitting of these XPS data in this region showed appear-
ance of a peak at 570 eV confirming the peak in 931.5 eV is related
to Cu O species (Fig. 3, ESI). XPS studies of Mg [120] region con-
firmed the presence of Mg by showing related peaks to MgO at
304 eV (Fig. 5c). Furthermore, XPS analysis in N1s region confirm
2
O species
[
2
1
the presence of imidazole by two peaks at 398.9 and 401.02 eV
which can be attributed to the C@N and CAN bonds of the imida-
zole (Fig. 5d) [121,122].
XRD pattern of the PdCu suspensions before stabilization on
MgO were also studied (Fig. 6). The XRD diffractogram of
MgO@PdCu showed related peaks to MgO at 36.7, 42.9, 62.2, 74.5
and 78.5 [104] and a peak at 34.3 which is related to formation
7 4 4 00
of perovskite structure (Mg .0 C . O20.00) [123] (Fig. 6a). However,
this diffractogram did not show any significant peaks for Cu and Pd
due to low loading weight of them and the overlap of MgO peaks
Fig. 6. XRD spectra of: (a) MgO@PdCu, (b) PdCu suspension in 2h: 20–80, (c) PdCu
suspension in 2h: 20–37, (d) PdCu suspension in 2h: 37–50.
Table 1
Optimization of reaction condition for the coupling of iodobenzene and phenylacetylene catalyzed by MgO@PdCu.a
Entry
Cat Pd (mol%)
Cat Cu (mol%)
Base
Solvent
T (°C)
Yield (%)b
1
2
3
4
5
6
7
8
9
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.025
0.05
–
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.005
–
DABCO
H
H
H
H
H
DMF
DMF
DMF
DMF
DMF
2
2
2
2
2
O
O
O
O
O
50
50
50
50
50
50
50
50
50
50
50
50
50
60
60
60
60
60
8
7
11
6
9
87
2
28
36
7
K
2
CO
t-BuOK
N(Et)
PO
DABCO
CO
t-BuOK
Et
3
3
K
3
4
Á3H
2
O
O
K
2
3
3
N
10
11
12
13
14
15
16
17
18
K
3
PO
4
Á3H
2
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
CH
3
CN
86
0
EtOH
Toluene
DMF
DMF
DMF
19
98
97
85
37
8
DMF
DMF
0.01
a
Reaction conditions: iodobenzene (0.5 mmol), phenylacetylene (0.75 mmol), base (0.75 mmol), solvent (1.5 mL), catalyst (see column).
GC yields.
b

M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
87
Table 2
Sonogashira-Hagihara reaction of terminal alkynes with aryl halides using MgO@PdCu.
a
Entry
1
Ar1
R2
Cat (mol%)
0.05
t (h)
24
T (°C)
Yield (%)b
97c
C
6
H
5
5
60
2
3
C
6
H
0.05
0.05
15
15
60
60
95
96
C
C
6
H
H
5
5
4
6
0.05
24
60
94
5
6
C
C
6
H
H
5
5
0.05
0.05
15
10
60
60
94
96c
6
7
8
9
C
C
C
6
H
6
H
6
H
5
5
5
0.05
0.05
0.05
24
24
24
60
60
60
97c
82
75
1
0
1
HOCH
HOCH
2
A
A
0.05
0.05
24
24
60
60
99c
98c
1
2
1
2
HOCH
2
A
0.05
24
60
90
13
14
15
16
4-MeC
H
6 4
0.05
0.05
0.02
0.2
24
24
24
6
60
60
80
60
94
88
83c
93
C
C
C
6
H
6
H
6
H
13
5
5
1
7
8
C
C
6
H
H
5
5
0.2
0.2
24
24
60
80
70c
85
1
6
1
9
0
C
C
6
H
H
5
5
0.2
0.2
24
24
60
60
88c
94
2
6
(
continued on next page)

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M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
Table 2 (continued)
Entry
Ar1
R2
Cat (mol%)
0.2
t (h)
24
T (°C)
Yield (%)b
95
2
1
C
6
H
5
60
2
2
2
3
C
C
6
H
H
5
5
0.2
0.2
24
24
80
80
82
90
6
2
4
5
C
C
6
H
H
5
5
0.2
0.2
24
24
60
80
55c,d
25c,d
2
6
2
2
6
7
C
C
6
H
H
5
5
–
24
48
60
10c,e
61c,f
6
0.2
100
2
8
C
6
H
5
0.2
24
120
94c,f
2
3
9
0
C
C
6
H
H
5
5
0.2
0.2
24
24
120
100
89c,f
95b,f
6
a
b
c
d
e
f
Reaction conditions: ArX (1 mmol), alkyne (1.5 mmol), DABCO (168 mg, 1.5 mmol), catalyst (10–40 mg), DMF (2 mL).
Isolated yields.
GC yields.
Reaction in the absence of Cu using Pd@MgO.
Reaction in the absence of Pd using Cu@MgO.
Reactions using 1 eq of TBAB.
with PdCu peaks. Therefore, we studied XRD of PdCu suspension
before stabilization on MgO support (Fig. 6b).
solvent and different bases such as DABCO, K
2 3 3
CO , t-BuOK, Et N,
and K PO in the presence of 0.1 mol% Pd loading, gave very low
3
4
Expanded XRD of PdCu suspension before stabilization on MgO
showed the presence of peaks at 2 theta value of 23.3°, which is
yields for this reaction (Table 1, entries 1–5). By changing the sol-
vent to DMF and using different bases (Table 1, entries 6–10), the
yields were improved. The best results were obtained using DABCO
as a base (Table1, entry 6). Using DABCO in other solvents such as
matched with (0 2 1) planes of Cu(OH)
tra also showed the presence of peaks at 24°, 34° and 39.9°, which
correspond to the lattice planes of (1 0 1), (1 1 0) and (1 1 0) of Cu
2 .
[124] (Fig. 6c) These spec-
3
-
3
CH CN, EtOH, or toluene gave lower yields (Table 1, entries 11–13).
Pd. (JCPDS no. 07-0138) (Fig. 6c, d). These results indicated the
presence of peaks at 29.1° and 36.2°, which are related to the plane
By increasing the reaction temperature to 60 °C and using DABCO
and DMF, yield of this reaction was increased to 98% (Table 1, entry
14). However, in order to insure about Pd loading, this model reac-
tion was performed in the presence of 0.05 and 0.025 mol% of Pd
and results indicate the formation of the desired products in 97%
and 63% yield, respectively (Table 1, entries 15 and 16). Therefore,
we selected 0.05 mol% of Pd, DABCO as a base and DMF as solvent
at 60 °C to be most efficient and optimized reaction conditions
(Table 1, entry 15). It is worth mentioning that for showing the
important effects of the both Cu and Pd in the reaction efficiency,
the model reaction in the presence of similar catalyst without Cu
and also without Pd were studied. The corresponding results indi-
cated that only 37 and 8% GC yield were obtained, respectively
(Table 1, entries 17 and 18).
(
3
(
1 1 0) and (1 1 1) of Cu
2.2, 35.5 and 38.5 were ascribed to the (1 1 0), (0 0 2), and
2 0 0) reflections of CuO [126] (Fig. 8c,d). In addition, the appear-
ance of a peak at 2theta value of 41.7° is related to the lattice plane
1 1 0) of CuPd (JCPDS no. 48-1551) (Fig. 8d). Furthermore, peaks
2
O [125] (Fig. 8c). The peaks at 2 theta =
(
related to Pd were observed at 2theta value of 40.5° and 46.3°,
which correspond to (2 2 0) plane of Pd [127] (Fig. 6c,d).
3.2. Catalytic performance
Catalytic activity of prepared PdCu catalyst was studied in
Sonogashira alkynylation reaction of aryl halides with terminal
alkynes. Initially, for finding optimized reaction conditions, the
reaction of iodobenzene with phenylacetylene was selected as a
model reaction and effect of different factors such as solvent, reac-
tion temperature and catalyst loading were studied. Using water as
Using the optimized reaction conditions, the Sonogashira reac-
tion of structurally different aryl halides with alkynes were stud-
ied. Under the optimized reaction conditions, alkynylations of
aryl iodides containing electron-donating groups such as methyl,

M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
89
isopropyl and methoxy and electron-withdrawing groups such as
NO , Cl, and F as well as the heteroaromatic2-iodothiophene, pro-
of Cu using Pd@MgO (Table 2, entries 24–25) and also in the
absence of Pd using Cu@MgO (Table 2, entry 26) gave very low
yields for desired products. Reactions of 4-chlorobenzaldehyde
under the optimized reaction conditions was slow, therefore the
reaction temperature was increased to 100 °C and TBAB (1 eq)
was added to the reaction mixture. Under these reaction condi-
tions, the desired product was obtained in 61% yield (Table 2, entry
27). However, by increasing the temperature to 120° C, the yield
was improved to 94% (Table 2, entry 28). Under this new reaction
conditions, other aryl chlorides such as 4-chloronitrobenzene and
4-chlorobenzonitrile proceed well and gave 89–95% yield for the
corresponding products, respectively (Table 2, entries 29–30).
2
ceed very efficiently and produced the desired products in excel-
lent yields (Table 2, entries 1–9). Also, reactions of aryl iodides
with other alkynes such as propargyl alcohol and 4-
ethynyltoluene, and 1-octyneproceed efficiently affording the cor-
responding alkynes in high to excellent yields (Table 2, entries 10–
1
4). Furthermore, reactions of different aryl bromides as well as
heterocyclic 5-bromopyrimidine and 2-bromopyridine proceed
efficiently at 60–80 °C by increasing the loading to 0.2 mol% Pd
affording the desired products in high to excellent yields (Table 2,
entries 15–23). It should be noted that reactions of
4
-nitrobromobenzene and 4-bromobenzaldehyde in the absence
3.3. Recycling of the catalyst
Recovering and reusing of heterogeneous catalysts are very
essential factors from economical and sustainable chemistry points
of view. Along this line, we have studied recyclability of the cata-
lyst for the reaction of iodobenzene and phenylacetylene under
the optimized reaction conditions. For this purpose, after 24 h pro-
gress of the reaction, the catalyst was separated by centrifugation
(
6000 rpm), washed with ethyl acetate and after drying reused in
another batch of reaction. By using this method, the catalyst was
recovered and reused for 8 consecutive runs with very small
decreasing in activity (97–90%). However, the yield of the reaction
was decreased to 84% in run 9 and to 63% in run 11 (Fig. 7).
Fig. 7. Recovering and reusing of the catalyst for the reaction of iodobenzene with
phenyl acetylene.
Fig. 8. TEM images of the reused catalyst after the 9 run.

9
0
M. Gholinejad et al. / Journal of Catalysis 363 (2018) 81–91
Fig. 10. MgO@PdCu catalyzed reaction of iodobenzene with phenylacetylene in the
presence of PVP and hot filtration conditions.
The presence of Cu was demonstrated to be crucial to obtain good
yields. The catalyst was recovered and recycled using simple cen-
trifugation and reused for 8 consecutive runs with very small
decrease in activity. Heterogeneous properties of the catalyst was
confirmed using addition of PVP and hot filtration test.
Acknowledgements
Fig. 9. XPS spectra of reused catalyst after 9 run: (a) Pd 3d and (b) Cu 2p regions.
The authors are grateful to Institute for Advanced Studies in Basic
Sciences (IASBS) Research Council and Iran National Science Foun-
dation (INSF-Grant number of 95844587) for support of this work.
C. Nájera is also thankful to the Spanish Ministerio de Economía y
Competitividad (MINECO) (projects CTQ2013-43446-P and
CTQ2014-51912-REDC), the Spanish Ministerio de Economía,
Industria y Competitividad, Agencia Estatal de Investigación (AEI)
and Fondo Europeo de Desarrollo Regional (FEDER, EU) (projects
CTQ2016-76782-P and CTQ2016-81797-REDC), the Generalitat
Valenciana (PROMETEOII/2014/017) and the University of Alicante
for financial support.
TEM images of the reused catalyst after 9 runs showed preser-
vation of the catalyst structure and also the presence of nanoparti-
cles with slight aggregation (Fig. 8). Also, XPS analysis of reused
catalyst after 9 runs showed in Pd 3d and Cu 2p region showed
very similar pattern to fresh catalyst (Fig. 9). In the case of palla-
dium, Pd(0), Pd(II) and Pd(IV) percentage determined to be 38,
2
6 and 36%, respectively. Furthermore, elemental EDS analyses
showed the presence of Pd, Cu, Mg, and N in the structure of the
reused catalyst (Fig. 4, ESI).
Finally, in order to find information about heterogeneous or
homogeneous nature of the catalyst, two important tests were per-
formed for the reaction of iodobenzene and phenylacetylene under
the optimized reaction conditions. Initially, after 2 h stirring of the
catalyst in DMF (2 mL) at 60 °C, the catalyst was removed under
hot filtration condition and iodobenzene, phenylacetylene and
base were added to the obtained solution. The resulting mixture
was stirred at 60 °C and GC analysis of the reaction after 24 h
showed only 31% conversion of iodobenzene to diphenylacetylene
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.02.028.
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