Pd nanoparticles loaded on modified chitosan-Unye bentonite microcapsules: A reusable nanocatalyst for Sonogashira coupling reaction

Article abstract of DOI:10.1016/j.carbpol.2021.117920

This work investigates the preparation of a catalytic complex of palladium nanoparticles supported on novel Schiff base modified chitosan-Unye bentonite microcapsules (Pd NPs@CS-UN). The complex has been characterized by FT-IR, EDS, XRD, TEM, HRTEM, Raman

Full text of DOI:10.1016/j.carbpol.2021.117920

Carbohydrate Polymers 262 (2021) 117920
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Pd nanoparticles loaded on modified chitosan-Unye bentonite
microcapsules: A reusable nanocatalyst for Sonogashira coupling reaction
a
a
,
b
c
Nasrin Shafiei , Mahmoud Nasrollahzadeh *, Talat Baran , Nuray Yılmaz Baran ,
d
Mohammadreza Shokouhimehr
a
Department of Chemistry, Faculty of Science, University of Qom, PO Box 37185-359, Qom, Iran
b
Department of Chemistry, Faculty of Science and Letters, Aksaray University, 68100, Aksaray, Turkey
c
Aksaray University, Technical Vocational School, Department of Chemistry Technology, 68100, Aksaray, Turkey
d
Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul, 08826, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
This work investigates the preparation of a catalytic complex of palladium nanoparticles supported on novel
Schiff base modified chitosan-Unye bentonite microcapsules (Pd NPs@CS-UN). The complex has been charac-
terized by FT-IR, EDS, XRD, TEM, HRTEM, Raman, ICP-OES and elemental mapping analyses. Pd NPs@CS-UN
was used as a catalyst for Sonogashira coupling reactions between aryl halides and acetylenes, employing
Biopolymer-supported Pd complex
Catalysis
Chitosan
Unye bentonite
2 3
K CO as the base and EtOH as a green solvent under aerobic conditions in which it showed high efficacy. Pd
Sonogashira coupling reaction
NPs@CS-UN was regenerated by filtration after the completion of the reaction. This catalytic process has many
advantages including simple methodology, high yields, and easy work-up. The catalytic performance does not
notably change even after five consecutive runs.
1
. Introduction
combination with Cu(OAc)
of aryl iodides and bromides with terminal alkynes in 2006 (Deng, Xie,
Yin, & Li, 2006). Furthermore, Li and co-workers modified the Cu(OAc)
catalyst system by replacing the ligand with a coordinating solvent
(Et N) (Guo, Deng, & Li, 2007).
Typically, the disadvantages of Pd catalyzed SCR reactions including
2
and TBAF was used as a ligand for the SCR
The Sonogashira coupling reaction (SCR) is an extremely valuable
2
2
and useful reaction to form C(sp)-C(sp ) bonds for the coupling of vinyl
or aryl halides with terminal alkynes, which has been broadly used in
various fields including synthesis of different molecules with biological
features, natural products, heterocycles, pharmaceuticals, electron
conducting polymers, liquid polymer substances, etc. (Bunz, 2000;
Kamali, Habibi, & Nasrollahzadeh, 2009; Negishi & Anastasia, 2003;
Sonogashira, Tohda, & Hagihara, 1975). This reaction usually proceeds
using toxic phosphine ligands, a homogeneous Pd catalyst, a Cu salt as
the co-catalyst, and a stoichiometric quantity of a base in an organic
solvent, for example, benzene, THF, DMF, or an amine under environ-
mentally and economically disadvantageous and inert conditions
3
harsh reaction conditions, long reaction times, hard work-ups, applica-
tion of toxic polar solvents, inert atmosphere conditions, expensive li-
gands, contamination of the environment triggered by formation of
diyne as the side product and obtaining the main products with low
yields limit their utilization. Therefore, the development of suitable
approaches for SCR is a prominent objective (Nasrollahzadeh, Issaabadi,
Tohidi, & Sajadi, 2018; Shylesh et al., 2010; Sonogashira, 1991). In SCR,
copper salts assist in transmetallation by the in situ formation of copper
acetylide. Moreover, the homocoupling of the terminal acetylene may
occur as a side reaction (Sagadevan & Hwang, 2012; Shylesh et al.,
2010; Sonogashira, 1991; Yamane, Miwa, & Nishikata, 2017). In addi-
tion, the utilization of amines such as triethylamine and piperidine in
most SCR reactions causes bad smell and environmental hazard
(Albrecht & Stephenson, 1988; Kesson, Flor ´e n, & Skerfving, 1985).
Among different palladium catalysts, homogeneous ones have been
(
Fleckenstein & Plenio, 2007; Gholap et al., 2005; Huang, Liu, Jiang, &
Chen, 2008; Lu et al., 2013; Roy & Plenio, 2010; Shylesh, Schuenemann,
Thiel, 2010; Sonogashira, 1991). A protocol for SCR of aryl halides
&
with terminal alkynes was introduced by Fu et al. in the presence of CuBr
using 1,10-phenanthroline as the ligand and tetrabutylammonium bro-
mide (TBAB) as the phase transfer catalyst (Yang, Li, Yang, Fu, & Hu,
2
011).
In
addition,
1,4-diphenyl-1,4-diazabuta-1,3-diene
in
*
Corresponding author.
E-mail address: mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
https://doi.org/10.1016/j.carbpol.2021.117920
Received 25 August 2020; Received in revised form 20 January 2021; Accepted 5 March 2021
Available online 8 March 2021
0
144-8617/© 2021 Elsevier Ltd. All rights reserved.

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Scheme 1. Schematic representation of the procedure for the organization of Pd NPs@CS-UN microcapsules.
widely used. However, heterogeneous catalysts are cheaper, easier to
separate from the reaction mixture and reusable for several runs (Fu
et al., 2017; Fuku et al., 2019). In addition, their thermal stability and
high catalytic activities make them suitable for application in various
organic syntheses (Bahammou et al., 2016). Therefore, polymer sup-
ported palladium catalysts have been developed as efficient catalysts
(
Nasrollahzadeh et al., 2020). Among different polymers, chitosan (CS)
was found to have a high efficacy as a support due to its biodegrad-
ability, biocompatibility, non-toxicity, positive charged, and availability
(
Nasrollahzadeh, Shafiei, Nezafat, Bidgoli, & Soleimani, 2020). Chitosan
is a natural polymer suitable for the organization of a heterogeneous
palladium catalyst (Motahharifar, Nasrollahzadeh, Taheri-Kafrani,
Varma, & Shokouhimehr, 2020; Nasrollahzadeh & Shafiei, 2020).
Clays can be used as ion exchangers, adsorbents or catalysts in
numerous industrial fields due to their low cost, environmental
compatibility, reusability, operation simplicity and easy accessibility
(
Ghassemzadeh & Sahimi, 2004; Harvey & Murray, 1997; Lagaly,
Ogawa, & D ´e k ´a ny, 2006; Murray, 2000; Zhou, Frost, He, & Xi, 2007).
Any clay of volcanic source containing montmorillonite is referred to as
bentonite. The crystalline structure of bentonite consists of an alumina
octahedral among two tetrahedral silica layers (Eren, 2009). In com-
parison with other kinds of clay, it has outstanding adsorption features
and contains adsorption sites in its interior layer space along with the
edges and outer surface (Abollino, Aceto, Malandrino, Sarzanini, &
Mentasti, 2003; Alvarez-Ayuso & Gar ´c ia-S a´ nchez, 2003; Tabak, Afsin,
Aygun, & Koksal, 2007). Unye bentonite comprises mainly of a dio-
ctahedral smectite together with quartz, cristobalite, tridymite, and
slight portions of anatase and feldspar (Caglar, Afsin, Koksal, Tabak, &
Eren, 2013). The cation exchange capacity and high specific surface area
of Unye bentonite and its noteworthy chemical and thermal stability
afford a variety of structural and surface properties. The combination of
this clay with chitosan improves the chemical, thermal, and mechanical
properties of the biopolymer. It also provides a good support for metal
ions in addition to its low cost and non-toxicity as it affords high
dispersion and good adsorption (Abdelkrim et al., 2020). The octahedral
aluminum hydroxide and tetrahedral silicon oxide sheets possessing
more hydroxyl functional groups on the surface of bentonite allow
chemical surface modifications, making it an appropriate stabilizer for
supporting nano-scaled Pd particles (Sajjadi et al., 2020).
Herein, we report amine, ligand, and copper free conditions for SCR
in ethanol as a green solvent using a Pd based nanocatalyst as a novel
efficient catalyst. Pd NPs supported on Schiff base modified chitosan-
Unye bentonite microcapsules (Pd NPs@CS-UN) have been designed
Fig. 1. FT-IR spectra of a) CS-UN, b) Pd NPs@CS-UN, and c) recycled Pd
NPs@CS-UN.
2

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 2. XRD patterns of a) Unye bentonite, b) Pd NPs@CS-UN, and c) recycled Pd NPs@CS-UN.
3

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 3. TEM analysis and histogram of particle size distribution of the Pd NPs@CS-UN.
4

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 4. HRTEM images of recycled Pd NPs@CS-UN.
following a simple surface modification process (Scheme 1). This
method is mild, highly efficient and naturally benign.
2.3. Preparation of Pd NPs on CS-UN microcapsules
1
g of CS-UN microcapsules and 0.2 g of PdCl
2
were stirred in 30 mL
◦
2
. Experimental
of EtOH at 70 C for 4 h. Upon the completion of the reaction, Pd NPs
loaded on the CS-UN microcapsules were filtered, washed with EtOH,
dried, and used as catalysts in organic reactions.
2
.1. Material and devices
All the solvents and reagents employed in this research were pro-
2
.4. General process for SCR
vided by Merck and/or Sigma Aldrich Chemical Co. (USA) and applied
as received. UN was obtained from Turkey. FT-IR (Fourier transform/
infrared) spectroscopy carried out using a Perkin Elmer 100 spectro-
photometer was applied to study the surface modification of the prod-
ucts obtained. The surface segments and EDS (energy-dispersive X-ray
spectroscopy) analyses of Pd NPs@CS-UN microcapsules were per-
formed using a FEI Quanta 450 FEG instrument. Raman spectra were
recorded using a via Raman microscope. The X-ray spectra of Pd
A terminal alkyne (1.1 mmol) and an aryl halide (1.0 mmol) were
vigorously stirred in a combination of Pd NPs@CS-UN (40.0 mg), K
2 3
CO
(
1.5 mmol), and ethanol (8.0 mL) in a glass flask under reflux conditions
◦
at 80 C for proper durations of time. Upon the completion of the re-
action (as followed by TLC), the mixture was cooled, the catalyst was
separated by filtration, and the solvent was removed. The precipitate
was washed with n-hexane and acetone to obtain the desired pure pre-
cipitate. All the products obtained were known and their analysis data
and melting points were similar to those reported in the literature.
NPs@CS-UN microcapsules were recorded using a Philips model PW
◦
1
373 diffractometer (Cu K
α
=1.5418 Å and 2θ = 10ꢀ 90 ). The size and
the morphology of the nanocatalysts were studied by TEM (transmission
electron microscopy, JEM-F200 JEOL). A Bruker DRX-400 AVANCE
spectrometer was used to record 1 C and H NMR spectra at 100.61 and
3
1
2.5. Characterization data of selected products
4
00.13 MHz, respectively.
1
1
,2-Diphenylethyne
HNMR (400 MHz, DMSO-d
m, 3 H); 13CNMR (100 MHz, DMSO-d
9.7.
-Methoxy-4-(phenylethynyl)benzene
6
) δ
H
= 7.58ꢀ 7.55 (m, 2 H) 7.45ꢀ 7.42
2
.2. Preparation of CS-UN microcapsules
(
6
) δ = 131.8, 129.3, 129.2, 122.7,
C
8
2
g of chitosan were dissolved in 100 mL of 2 % acetic acid solution
1
1
(
v:v), followed by the addition of 2 g of Unye bentonite to the reaction
HNMR (400 MHz, CDCl
CNMR (100 MHz, CDCl ) δ
23.6, 115.4, 113.9, 89.3, 88.1, 55.31.
-Methyl-4-(phenylethynyl)benzene
3
) δ
C
H
= 7.55ꢀ 6.89 (m, 9 H), 3.85 (s, 3 H);
mixture and stirring for 3 h at ambient temperature. Subsequently, the
reaction mixture was poured into an alkaline solution (water: methanol:
NaOH 40 mL: 60 mL: 12 g) to provide gelatinous microcapsules. After
the fabrication of gelatinous chitosan-Unye bentonite, the microcapsules
were extensively washed with water to neutrality. Finally, the micro-
capsules were dropped into the glyoxal solution (5 mL) in methanol (50
mL) and stirred under reflux conditions for one day for cross-linking.
After the cross-linking, CS-UN microcapsules were filtered, washed
with methanol and dried.
1
3
3
= 159.6, 133.1, 131,4, 128.3, 127.9,
1
1
1
HNMR (400 MHz, DMSO) δ
H
= 7.23–7.55 (m, 9 H), 2.34 (s, 3 H).
3
. Results and discussion
3
.1. Catalyst analysis
The structural, morphological, and physicochemical properties of Pd
5

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 5. FFT (top) and STEM (bottom) images of the Pd NPs@CS-UN.
6

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
◦
◦
◦
◦
4
6.43 (200), 67.96 (220), 82.04 (311), and 86.51 (222) in the XRD
spectrum of the Pd NPs@CS-UN, corresponding to the face centered
cubic (FCC) structure of Pd (Khan et al., 2014), confirming the formation
of Pd NPs on the CS-UN microcapsule.
TEM analysis of the catalyst was applied to confirm the formation of
Pd NPs (Fig. 3). As observed in Fig. 3, the loading of the Pd NPs on the
CS-UN microcapsules prepared has been successfully carried out. Fig. 3
(
bottom) shows the particle size distribution histogram of Pd NPs@CS-
UN. Moreover, the average size of NPs was estimated to be about 13
nm. The HRTEM (Fig. 4) and fast Fourier transform (FFT) (Fig. 5, top)
images of the Pd NPs@CS-UN show that the nanoparticles are highly
crystalline. STEM image confirms a homogeneously assembled nano-
structured catalyst (Fig. 5, bottom).
The presence of the Pd NPs in the catalyst prepared was also
confirmed by Raman spectroscopy (Fig. 6). The Raman peak of Pd NPs
bond also verified the ambient Pd transition metal ions.
Fig. 6. Raman spectrum of the Pd NPs@CS-UN.
The element composition of the Pd NPs@CS-UN was determined by
EDS analysis. The EDS analysis of the catalyst was carried out using a Cu
grid to hold the sample. This spectrum showed the presence of Pd NPs on
the CS-UN superficial (Fig. 7). In the EDS spectrum, Pd, Si, O, Cl and C
peaks were observed, indicating the authenticity of Pd NPs@CS-UN.
The content of the Pd within Pd NPs@CS-UN, as determined by ICP-
OES (Inductively coupled plasma-optical emission spectrometry), was
found to be 14.16 wt%.
Elemental mapping of Pd NPs@CS-UN also confirmed the formation
of Pd NPs (Fig. 8). These analyses strongly confirmed the immobilization
of Pd NPs on the surface of the CS-UN.
The TGA analysis of Pd NPs@CS-UN (Fig. 9) showed that the catalyst
had high thermal stability and is thus suitable for different organic re-
actions, which require high reaction temperatures. As observed in Fig. 9,
the weight losses have occurred during three steps. The initial weight
Fig. 7. EDS spectrum of the Pd NPs@CS-UN.
◦
loss of up to 259.5 C is attributed to the organic solvents and the water
enclosed in the nanocatalyst structure (22.8 % weight loss) while the
NPs@CS-UN were determined by FT-IR, EDS, XRD, TEM, HRTEM,
Raman, ICP-OES and elemental mapping.
◦
◦
second and third steps at 424.6 C (11 % weight loss) and 800.1 C (3.1
weight loss) could be related to the thermal decomposition of the
glyoxal and chitosan groups, respectively.
%
2
+
The reduction potential of the Pd is very low, and it can be reduced
under mild conditions without the use of any harsh reductant. High
temperature, use of EtOH as the solvent, and solid support with the
2
+
3
.2. Catalytic activity of Pd NPs@CS-UN in the SCR
amine groups work as a mild reduction condition to reduce the Pd
moieties. The prepared Schiff base also assists the Pd2 reduction (Choi,
Shokouhimehr, & Sung, 2013; Li, Wang, & Li, 2005; Shokouhimehr
et al., 2018). After the reaction was completed, it was observed that the
color of the solution turned dark grey.
+
The present work affords Schiff base supported Pd NPs as novel
heterogeneous catalysts for the ligand and copper-free SCR. This system
eliminated the need for expensive, difficult to prepare, and air and
moisture sensitive ligands.
The FT-IR spectra of CS-UN, Pd NPs@CS-UN, and recycled Pd
NPs@CS-UN are presented in Fig. 1. The FT-IR spectrum of CS-UN dis-
To optimize the reaction conditions, the effects of solvents, bases and
catalyst amount in the reaction of 1.1 mmol of phenylacetylene with 1.0
mmol of p-iodoanisole using Pd NPs@CS-UN catalyst, 1.5 mmol of base
and 8.0 mL of solvent as the model reaction were studied. As depicted in
Table 1, the SCR was first performed without any catalyst or in the
presence of Unye bentonite at 80 ℃ (Table 1, entry 1 and 2), which
resulted in no product formation. As observed in Table 1, the choice of
base or solvent affects the reaction. The efficacy of Pd NPs@CS-UN (40.0
ꢀ
1
played typical bands of chitosan and UN at 3340 cm (O
–
H stretch-
ꢀ 1
ꢀ 1
ꢀ 1
ing), 2926 cm
and 2871 cm
(C H stretching) and 1017 cm
–
(C O C stretching). More importantly, the peak observed at 1642
– –
1
ꢀ
cm is correlated to the imine bond vibration, confirming the formation
of CS-UN microcapsule. As for Pd NPs@CS-UN microcapsules, the imine
ꢀ 1
bond shifted to 1626 cm due to the strong interaction of Pd with CS-
UN microcapsule (Fig. 1).
mg) as a catalyst was evaluated in the presence of various bases (K
NEt , KOH, pyridine and Na CO ) and solvents (EtOH, toluene, H O and
O/EtOH). The best result was obtained using K CO and ethanol as
2 3
CO ,
The diffraction pattern of the samples was observed by XRD. Fig. 2
demonstrates the XRD pattern of the Unye bentonite, Pd NPs@CS-UN
and recycled Pd NPs@CS-UN. The specified peaks pointed by asterisks
in the XRD pattern of Pd NPs@CS-UN (Fig. 2b) are related to the typical
peaks of Unye bentonite (Fig. 2a) and the broad peak in the area of 2θ =
3
2
3
2
H
2
2
3
the base and solvent, respectively (Table 1, entry 11). Subsequently, the
amount of Pd NPs@CS-UN required for the SCR was optimized. The
amount of Pd NPs@CS-UN varied in the range of 20.0–50.0 mg in the
2
0 is related to the chitosan. In addition to the typical peaks of Unye
◦
◦
presence of EtOH at 80 C. Increasing the catalyst amount from 40.0 mg
bentonite and chitosan, new peaks were detected at 40.07 (111),
to 50.0 mg slightly increased the product yield. The reaction was
7

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 8. STEM image and elemental mapping of Pd NPs@CS-UN.
8

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 9. TGA analysis of Pd NPs@CS-UN.
2
018). Here, Pd NPs@CS-UN activates the phenylacetylene terminal
acetylene C H. Potassium acetylide intermediate is then efficiently
produced via the deprotonation of C H using K CO . Aryl palladium
Table 1
–
Optimization of reaction conditions for the SCR between p-iodoanisole with
–
◦
2
3
phenylacetylene using different solvents and bases at 80 C for 3 h.
halide complex (ArPdX) can be produced via the activation of ArX on the
active sites of Pd NPs@CS-UN. Consequently, aryl palladium acetylide is
generated as an intermediate. Finally, the expected coupling products
are obtained by reductive elimination.
Entry
Catalyst (mg)
Solvent
Base
Yield (%)
1
2
3
4
5
6
7
8
9
None
EtOH
EtOH
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
0
Bentonite (40.0)
0
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (40.0)
Pd NPs@CS-UN (20.0)
Pd NPs@CS-UN (50.0)
H
2
O
Trace
45
15
38
60
63
42
79
87
56
88
The comparison of the results of the present study with those of some
recent surveys regarding the catalytic SCR further confirms the efficacy
of our proposed system as the reaction is performed in a suitable time
and gives products in good to excellent yields. Table 3 briefly represents
the catalytic systems for SCR.
Toluene
O/EtOH
H
2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Pyridine
KOH
NEt
NaOH
Na CO
3
1
1
1
1
0
1
2
3
2
3
K
K
K
2
2
2
CO
CO
CO
3
3
3
3
.3. Catalyst recyclability
The recyclability of a catalyst is one of the vital properties, which
makes the catalyst advantageous for commercial applications. The
recyclability of Pd NPs@CS-UN catalyst was studied by model SCR for
phenylacetylene with p-iodoanisole. This catalyst was used in five
consecutive cycles under similar reaction conditions (Fig. 10). The
catalyst presented good yields of the products even on the fifth reaction
sequence for SCR. The microcapsule form and heterogeneous nature of
our catalyst allowed its easy separation from the reaction mixture. Upon
the completion of SCR, the catalyst is separated via filtration, washed
◦
completed within 3 h at 80 C and the product was isolated in 87 %
yield. No organic co-solvent, ligand, or surfactant was required. In
addition, no undesirable organization of a diyne as a homocoupling
product was observed, as no copper salts were employed.
Next, under the optimized conditions, the application of Pd NPs@CS-
UN was studied for the SCR between terminal acetylenes and different
aryl halides containing either electron-withdrawing or donating sites. As
specified in Table 2, our system is obviously general and practical for
various aryl halides. Products were obtained in high yields in all the
cases under the reaction conditions. To further investigate the efficacy of
Pd NPs@CS-UN, the chlorobenzene reaction was also checked with
phenylacetylene. Under these conditions, the desired product was ob-
tained in 70 % yield (Table 2, entry 8).
◦
with EtOAc, dried at 100 C for 4 h in a hot air oven and then used in the
next cycle. As shown in Fig. 10, the catalytic performance of Pd
NPs@CS-UN remained the same for the first cycle and then decreased
slightly in the next few cycles. To check the heterogeneity of this cata-
lyst, which is an important factor, the filtrate of each cycle was also
analyzed by ICP technique. Very low palladium contamination was
observed during these experiments. The content of the Pd within the
recycled Pd NPs@CS-UN was found to be 13.18 %, according to the ICP-
OES analysis. The XRD, TEM and FT-IR analyses exhibited no significant
changes in the size, shape, crystallinity and functional group of the
The role of the catalyst in SCR can be accounted for by the proposed
mechanism in Scheme 2, inspired from previous reports (Alinezhad,
Pakzad, & Nasrollahzadeh, 2020; Gazvoda, Virant, Pinter, & Ko ˇs mrlj,
9

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Table 2
a
Substrate scope for Pd NPs@CS-UN catalyzed SCR of terminal alkynes with various aryl halides .
b
ꢀ 1
)
Entry
1
R
Aryl halide
Product
Time (h)
3
Yield (%)
TON
1800
TOF (h
600
Ph
90
2
3
4
5
6
7
8
9
Ph
3
87
88
90
77
73
80
70
82
78
83
71
1740
1760
1800
1540
1460
1600
1400
1640
1560
1660
1420
580
Ph
3
586.6
720
Ph
2.5
4.5
4.5
4.5
5.5
8
Ph
342.2
324.4
355.5
254.5
205
Ph
Ph
Ph
Me-(CH
Me-(CH
Me-(CH
Me-(CH
2
)
2
)
2
)
2
)
5
-
5
-
5
-
5
-
1
1
1
0
1
2
8
195
8
207.5
177.5
8
a
b
◦
2 3
CO (1.5 mmol), catalyst (40.0 mg), EtOH (8 mL), 80 C.
Reaction conditions: Aryl halide (1.0 mmol), terminal acetylene (1.1 mmol), K
Isolated yield.
Scheme 2. Proposed pathway for the SCR using Pd NPs@CS-UN.
reused Pd NPs@CS-UN after the 5th recycle (Figs. 1c, 2c and 11). We
have also checked the particle size distribution of the reused catalyst
sufficient to indicate that the Pd NPs@CS-UN catalyst exhibits high
chemical and thermal stability.
(
Fig. 11). The average size of Pd NPs calculated from the TEM images is
found to be about 14 nm. In addition, the FT-IR spectrum of the reused
Pd NPs@CS-UN also shows peaks which assigned to the catalyst
4. Conclusion
(
Fig. 1c). The XRD results also confirmed that the structure of the reused
In summary, SCR has been carried out in ethanol as a green solvent
using Pd NPs@CS-UN catalyst. Several characterization methods
Pd NPs@CS-UN is similar to the fresh catalyst (Fig. 2c). Thus, it is
1
0

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Table 3
Comparison of recent reports on SCR.
Entry
Catalyst
Reaction conditions
Time (h)
Yield
%)
Ref.
(
◦
1
2
Pd NPs@CS-UN (40.0 mg)
K
2
CO
3
, EtOH, 80
C
2.5ꢀ 8
70ꢀ 90
Current study
Br a¨ se, Dahmen, Lauterwasser, Leadbeater, &
Sharp, 2002
Polymer supported Pd triazene complex (3.0 mol% Cat.)
CuI, dioxane, r.t.
16
71
a
◦
3
4
5
PS -Pd(II) -anthra complex (0.5 mol% Pd)
Et
Et
Et
3
3
3
N, DMF, 90
C
12ꢀ 24
8ꢀ 12
3
14ꢀ 96
68ꢀ 96
23ꢀ 99
Islam, Mondal, Tuhina et al., 2010
Islam, Mondal, Roy, Mondal, Mobarak et al., 2010
Bakherad, Keivanloo, Bahramian, & Jajarmi,
Polystyrene anchored Pd(II) azo complex (0.5 mol% Pd)
N, H
2
O, TBAB, 70ꢀ 80 ◦
C
a
b
PS -PPDOT -Pd(II) (1.0 mol% Cat.)
N or pyridine, r.t.
2
010
a
c
6
PS -PEG resin bound triarylphosphine ligand (5.0 mol %
Et
3
N or CsOH, H
2
O, N
2
,
6ꢀ 24
13ꢀ 99
Suzuka, Okada, Ooshiro, & Uozumi, 2010
◦
Pd)
40ꢀ 100
CuI, Et
CuI, K
C
◦
7
8
9
Pd(II) Schiff base complex (3.72 mol% Pd)
3
N, H
2
O, 70
C
8ꢀ 20
32ꢀ 99
10ꢀ 94
35ꢀ 99
Islam, Mondal, Roy, Mondal, & Hossain, 2010
Tao et al., 2011
d
e
◦
FDU -NHC /Pd(II) complex (1.0 mol% Cat.)
Polymer-supported Schiff base palladium complex (0.5
mol% Pd)
2
CO
3
, DMA, 100
C
1ꢀ 24
◦
Piperidine, H
2
O, 100
C
0.5ꢀ 12
He et al., 2011
◦
10
11
12
Fe
3
O
4
@SiO
2
-polymer-imid-Pd (0.03 g)
Et
Et
3
N, DMF, 80
C
0.5ꢀ 10
1.5ꢀ 7
87ꢀ 96
75ꢀ 95
23ꢀ 95
Esmaeilpour, Javidi, Mokhtari, & Nowroozi, 2014
Jadhav, Kumbhar, Mali, Hong, & Salunkhe, 2015
Mohammadi et al., 2018
f
◦
Pd-NHC@SP-PS or Pd-NHC@PS (1.0 mol% Cat.)
3
N, 90
C
g
h
◦
Ps[PdCl-(SeCSe)] (0.3 mol% Pd)
NMP , TBAF, 70 C
0.25ꢀ 24
a
b
c
d
e
f
Polystyrene-supported.
-Phenyl-1,2-propanedione-2-oxime thiosemi-carbazone.
Poly-(ethylene glycol).
Fudan University)-15 mesopolymer.
N-Heterocyclic carbene.
1
(
Polymer supported air-stable palladium NHC complex with a spacer (Pd-NHC@SP-PS) and without a spacer (Pd-NHC@PS).
Palladium complex of polystyrene-supported SeCSe pincer ligand.
N-Methylpyrolidone.
g
h
Fig. 10. Reusability of Pd NPs@CS-UN in SCR.
including FTIR, EDS, XRD, Raman, ICP-OES, elemental mapping and
TEM were used to characterize this Schiff base modified heterogeneous
catalyst. It was shown that the catalyst comprised Pd NPs with a particle
size of about 14 nm. The catalyst appeared to be extremely efficient,
stable and recyclable for five runs in SCR. The present catalyst was
prepared keeping in mind the low cost and the benefits of chitosan and
Unye bentonite based materials and the usefulness of solid acid cata-
lysts. A range of different substituted aryl halides and terminal acety-
lenes were employed to obtain products in high yields. The easy
separation, eminent reusability and low cost of this catalyst make it even
more attractive for catalytic applications in various chemical
transformations.
1
1

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Fig. 11. TEM images and histogram of particle size distribution of recycled Pd NPs@CS-UN.
1
2

N. Shafiei et al.
Carbohydrate Polymers 262 (2021) 117920
Declaration of Competing Interest
Huang, H., Liu, H., Jiang, H., & Chen, K. (2008). Rapid and efficient Pd-catalyzed
Sonogashira coupling of aryl chlorides. The Journal of Organic Chemistry, 73(15),
6
037–6040.
Authors declare no conflict of interest.
Islam, M., Mondal, P., Tuhina, K., Roy, A. S., Mondal, S., & Hossain, D. (2010). Highly
efficient recyclable heterogeneous palladium catalyst for C–C coupling, amination
and cyanation reactions. Journal of Organometallic Chemistry, 695(21), 2284–2295.
Islam, S. M., Mondal, P., Roy, A. S., Mondal, S., & Hossain, D. (2010). Heterogeneous
Suzuki and copper-free Sonogashira cross-coupling reactions catalyzed by a reusable
palladium(II) complex in water medium. Tetrahedron Letters, 51(15), 2067–2070.
Islam, S. M., Mondal, S., Roy, A. S., Mondal, P., Mobarak, M., Hossain, D., … Pandit, P.
Acknowledgments
The supports from Iranian Nano Council and the University of Qom
are appreciated.
(
2010). Synthesis and characterization of a polymer-anchored palladium(II) Schiff
base complex and its catalytic efficiency in phosphine-free Sonogashira coupling
reactions. Transition Metal Chemistry, 35(3), 305–313.
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