Pd nanocatalyst adorned on magnetic chitosan?N-Heterocyclic carbene: Eco-compatible suzuki cross-coupling reaction

Article abstract of DOI:10.3390/molecules24173048

A novel magnetic-functionalized-multi-walled carbon nanotubes?chitosan N-heterocyclic carbene-palladium (M-f-MWCNTs?chitosan-NHC-Pd) nanocatalyst is developed in two steps. The first step entails the fabrication of a three-component cross-linking of chitosan utilizing the Debus-Radziszewski imidazole approach. The second step comprised the covalent grafting of prepared cross-linked chitosan to the outer walls of magnetically functionalized MWCNTs (M-f-MWCNTs) followed by introducing PdCl₂ to generate the m-f-MWCNTs?cross-linked chitosan with a novel NHC ligand. The repeated units of the amino group in the chitosan polymer chain provide the synthesis of several imidazole units which also increase the number of Pd linkers thus leading to higher catalyst effciency. The evaluation of catalytic activity was examined in the expeditious synthesis of biaryl compounds using the Suzuki cross-coupling reaction of various aryl halides and aryl boronic acids; ensuing results show the general applicability of nanocatalyst with superior conversion reaction yields, high turnover frequencies (TOFs) and turnover numbers (TON). Meanwhile, nanocatalyst showed admirable potential in reusability tests, being recycled for five runs without losing significant activities under optimum reaction conditions. The successfully synthesis of catalyst and its characterization was confirmed using the Fourier transform infrared spectrometer (FT-IR), spectrometer transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photo-electron spectroscopy (XPS) and thermogravimetric analysis (TGA).

Full text of DOI:10.3390/molecules24173048

molecules
Article
Pd Nanocatalyst Adorned on Magnetic
Chitosan@N-Heterocyclic Carbene: Eco-Compatible
Suzuki Cross-Coupling Reaction
1
,
1
1
1
Roya Sedghi *, Bahareh Heidari , Hatef Shahmohamadi , Pourya Zarshenas
and
2
,
Rajender S. Varma *
1
Department of Polymer& Materials Chemistry, Faculty of Chemistry & Petroleum Sciences, Shahid Beheshti
University, G.C, 1983969411 Tehran, Iran
Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of
Science, Palacky University, Šlechtitel u˚ 27, 783 71 Olomouc, Czech Republic
2
*
Correspondence: r_sedghi@sbu.ac.ir (R.S.); Varma.rajender@epa.gov (R.S.V.); Fax: +98-21-22431661 (R.S.)
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 20 June 2019; Accepted: 19 August 2019; Published: 22 August 2019
Abstract: A novel magnetic-functionalized-multi-walled carbon nanotubes@chitosan N-heterocyclic
carbene-palladium (M-f-MWCNTs@chitosan-NHC-Pd) nanocatalyst is developed in two steps.
The first step entails the fabrication of a three-component cross-linking of chitosan utilizing the
Debus–Radziszewski imidazole approach. The second step comprised the covalent grafting
of prepared cross-linked chitosan to the outer walls of magnetically functionalized MWCNTs
(
M-f-MWCNTs) followed by introducing PdCl to generate the m-f-MWCNTs@cross-linked chitosan
2
with a novel NHC ligand. The repeated units of the amino group in the chitosan polymer chain
provide the synthesis of several imidazole units which also increase the number of Pd linkers
thus leading to higher catalyst efficiency. The evaluation of catalytic activity was examined in the
expeditious synthesis of biaryl compounds using the Suzuki cross-coupling reaction of various aryl
halides and aryl boronic acids; ensuing results show the general applicability of nanocatalyst with
superior conversion reaction yields, high turnover frequencies (TOFs) and turnover numbers (TON).
Meanwhile, nanocatalyst showed admirable potential in reusability tests, being recycled for five runs
without losing significant activities under optimum reaction conditions. The successfully synthesis of
catalyst and its characterization was confirmed using the Fourier transform infrared spectrometer
(
FT-IR), spectrometer transmission electron microscopy (TEM), scanning electron microscopy (SEM),
X-ray photo-electron spectroscopy (XPS) and thermogravimetric analysis (TGA).
Keywords: cross-linked chitosan; N-heterocyclic carbene; MWCNTs; Suzuki cross-coupling reaction
1
. Introduction
The Suzuki–Miyaura cross-coupling reaction plays a significant industrial scale role for the
production of biaryl compounds which are widely used for a variety of industrial applications, such as
the synthesis of natural products, herbicides, pharmaceuticals, polymers and agrochemicals [ ].
1–4
Commonly, bulky phosphine ligands are employed to facilitate the Suzuki cross-coupling reaction;
however, critical setbacks for this ligand are toxicity, air sensitivity and typical susceptibility to metal-ligand
Molecules 2019, 24, 3048; doi:10.3390/molecules24173048
www.mdpi.com/journal/molecules

Molecules 2019, 24, 3048
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degradation [5–7], posing significant limitations on their uses [8]. In contrast, nitrogen-based ligands
such as N-heterocyclic carbenes (NHCs) [9,10] have attracted attention as transition metal ligands; NHCs
feature a significantly stronger electron-donating capacity than phosphine’s ligands, which increases
the stability of resulting complexes towards the air, moisture and heat. On the other hand, the NHC
ligands have the same σ-donor and low π-acceptor properties while they can form stronger bonds to
a wide variety of metals such as Co [11], Pd [12], Cu [13] and Ni [14]. Pd complexes incorporating
NHC appear to be a superior catalyst to achieve excellent catalytic activities in cross-coupling reactions.
Also, the strong
σ-donating properties of the NHCs promote oxidative addition of reagent and can
assist in the reductive elimination of biaryl compounds [15,16]. On the other hand, recent advances
in ‘green’ Suzuki cross-coupling reactions have focused on designing easily recoverable catalysts, thus
simplifying product separation and adhering to sustainable green chemistry [17]. Although current
homogeneous metal catalysts have sufficient activity, heterogeneous catalysts offer superior and practical
aforementioned advantages. In recent years, the catalysts have been immobilized on a wide variety of
supports such as Fe O [18], SiO2 [19], TiO nanoparticles (NPs) [20] and mesoporous silica [21]; potential
3
4
2
supports should have high stability and large surface area. The MWCNTs are robust support materials
due to the high aspect ratio, thermal, chemical and mechanical stability, and conductivity and strong
interactions for grafting to the other material with suitable appending groups [22]. In the last few years,
techniques based on magnetic materials have attracted much attention to use in the wide variety of fields
such as immunoassay [21], targeted drug delivery [23], and environmental detection [24]. The main
advantage of magnetic material is to achieve easy separation strategy using an external magnetic field
without additional centrifugation or filtration [25
physical and chemical properties of the magnetic NPs, they have attracted much attention. In recent
years biopolymers have gained a great research interest to use as a support for catalysts [27 28]. Chitosan
,26]. Also, due to the large surface area, and unique
,
is a naturally occurring linear copolymer that can be produced from chitin—the naturally occurring
polysaccharide—via deacetylation [29]. Since the Chitosan is soluble in aqueous acidic media, it can be
easily processed, and a wide variety of shapes are known including membranes, fibers or spheres [30–32].
The most important advantage of using chitosan is because of the large number of available amino
groups in its structure which makes this polymer an ideal material for a wide variety of applications via
modification of side groups that assist in cross-linking or grafting with other polymers [33,34]. On the
other hand, imidazolium compounds are very versatile materials with a wide range of recent innovations
in the field of chemistry; the design and synthesis of imidazole derivatives and imidazolium-containing
polymers as a metal-ligand has received ample attention [35]. The Debus–Radziszewski imidazole
synthesis strategy has been used in designing the cross-linked chitosan derivatives via a rather convenient
one-pot approach involving the reaction of formaldehyde, a primary amine and a 1,2-dicarobonyl
compound [36]. Akin to NHC ligand design, herein, the in situ interaction of imidazolium salt and the
Pd source can be accomplished by deploying chitosan.
In this study, Debus–Radziszewski imidazole synthesis was used to fabricate cross-linked chitosan
using glyoxal and formaldehyde as dicarbonyl and carbonyl components, respectively. The prepared
cross-linked chitosan was anchored on the high surface area of m-f-MWCNTs via the esterification
reaction between the hydroxyl group of chitosan and the carboxylic acid group from the surface of
m-f-MWCNTs and then finally the prepared compound was applied to generate NHC@Pd (Scheme 1).
To the best of our knowledge, combining MWCNTs and cross-linked chitosan provides not only a high
surface area of MWCNTs but also enhanced the interaction of imidazolium cross-linked chitosan with
Pd NPs which leads to an excellent opportunity for the design and application of this ideal combination.

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Scheme 1. Schematicdepictionofthepreparationofmagnetic-functionalized-multi-walledcarbonnanotubes@chitosan
m-f-MWCNTs@chitosan) N-heterocyclic carbene-palladium (NHC-Pd) nanocomposite.
(
2
. Experimental
2
.1. Reagents and Materials
MWCNTs, FeCl .6H O, FeCl .4H O, H SO (98%), HNO (65%), NH OH (28%), Glyoxal,
3
2
2
2
2
4
3
4
formaldehyde, N,N’-Dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), palladium
chloride (PdCl ), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile (CH CN), ethyl
2
3
acetate, n-hexane and ethanol were purchased from Merck Company (Darmstadt, Germany). Chitosan,
boronic acids and differently substituted aryl halides were purchased from Sigma-Aldrich Company
(St. Louis, MO, USA). All employed solvents were dried using standard process and freshly distilled
prior to use.
2
.2. Instrumentation
The Fourier transform infrared (FT-IR) measurements were performed using a FT-IR spectrometer
(
BOMEM MB-Series, Quebec City, QC, Canada) in the form of KBr pellets over the wavenumbers
−1
of 400–4000 cm . The thermal properties of the nanocomposites were investigated using thermal
analyzer instrument (TGA/DTA BAHR: STA 503, Hüllhorst, Germany) from 25 to 1000 C at a heating
rate of 10 C/min under air. For determination of Pd loaded on nanocomposite an atomic absorption
◦
◦
spectrometry (Shimadzu model AA-680, Kyoto, Japan) was used with air-acetylene flame atomizer
and equipped with hollow cathode lamp. The SEM (Philips XL30, Leuven, Belgium) was used for
the investigation of surface morphological features and size of nanocatalyst and in order to provide
more quantitative data of surface, image of nanocatalyst was taken using TEM (Philips CM30, Milan,
Italy). The X-ray photoelectron analysis (XPS) was performed using a thermo fisher scientific k-alpha

Molecules 2019, 24, 3048
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instrument (Waltham, MA, USA). The Brunauer–Emmett–Teller (BET) surface area was determined
using a BET analyzer (Belsorp mini, Osaka, Japan). The progress of the Suzuki cross-coupling reaction
was monitored by gas chromatography (Shimadzu model GC instrument, Kyoto, Japan).
2
.3. Synthesis of Functionalized-MWCNTs (f-MWCNTs)
In order to improve the dispersion and reduce the aggregation tendency of MWCNTs and also
for improved interaction of cross-linked chitosan with surface of MWCNTs, surface functionalization
of MWCNTs (carboxylation) was carried out. According to the previously reported procedure [37],
◦
2
g MWCNTs were ultrasonically treated in a 100 mL of HNO /H SO (1:3) mixture at 60 C. After 2 h
3
2
4
stirring, the resulting product (f-MWCNTs) was collected and washed with distilled water and then
◦
dried at 60 C for 24 h.
2
.4. Synthesis of Magnetic f-MWCNTs (m-f-MWCNTs)
Firstly, 1 g of obtained f-MWCNTs nanocomposite was dispersed in 110 mL distilled water and
then 0.8 g FeCl .6H O was added to the reaction mixture under vigorous stirring and under nitrogen
3
2
atmosphere. The solution was stirred for 5 min and followed by addition of 1.2 g FeCl .4H O. Then
2
2
◦
0 mL diluted NH OH was added drop-wise to the reaction mixture and was kept at 60 C for 3 h.
4
5
The final products were isolated using a permanent magnetic field and washed several times with
◦
distilled water and ethanol. The obtained m-f-MWCNTs nanocomposite was dried at 25 C for 24 h.
2
.5. Synthesis of the Cross-Linked Chitosan Using Imidazole
Briefly, 1 g of medium molecular weight chitosan (6.1 mmol) was dissolved in 100 mL diluted
CH COOH (concentration 10%), then 0.35 mL (3.1 mmol) of glyoxal (40% in water) and 0.114 mL
3
(3.1 mmol) formaldehyde were added to chitosan solution while it was being stirred vigorously. After
attaining the homogeneous solution, a required amount of diluted NaOH (concentration 10%) was
added drop-wise till the pH reached 5. The ensuing solution kept under constant stirring over 48 h at
room temperature.
2
.6. Substitution Cross-Linked Chitosan with Activated m-f-MWCNTs (Esterification)
The cross-linked chitosan was covalently linked onto the m-f-MWCNTs via a chemical reaction
between the hydroxyl groups of the cross-linked chitosan and the active carboxyl groups of
m-f-MWCNTs. To accomplish the purpose, 0.1 g of m-f-MWCNTs and 0.16 mL of DCC and 0.8 g
DMAP were added to DMF (20 mL) while the mixture was stirred at room temperature for 30 min,
followed by the addition of 0.5 g of cross-linked chitosan. The reaction mixture was stirred for 48 h
and then the resulted product was rinsed with distilled water (3 times) and dried under vacuum for
◦
2
4 h at 25 C.
2
.7. Preparation of the m-f-MWCNTs@chitosan NHC-Pd
In order to increase the solubility of the PdCl in aqueous solution, CH CN was added to solubilize
2
3
the complex. M-f-MWCNTs@cross-linked chitosan (0.2 g) was added to a solution of PdCl (MeCN)
2
2
◦
(
5 mL) in DMF (10 mL) and the reaction mixture was stirred for 24 h at 60 C. The final precipitated
solid was collected using filtration under vacuum and washed with ethanol and distilled water to
remove any adsorbed Pd from the surface of the composite.
2
.8. General Procedure for the Suzuki Reactions
To establish the generality of this nanocatalyst, a wide variety of aryl halides and arylboronic acids
were used. In a typical experiment, to a 50 mL flask an appropriate aryl halide (1 mmol), arylboronic
acid (1.2 mmol), K CO (3 mmol), the above-mentioned catalyst (0.1–1 mol% based on aryl halide)
2
3
and 6 mL EtOH/water (1:1 V/V) were successively added. The mixture was stirred vigorously for the

Molecules 2019, 24, 3048
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◦
◦
varying particular length of time (2–8 h) and temperature at (40 C–60 C) depending on the substrate
◦
used. Upon completion of the reaction, the mixture was cooled down to ambient temperature (25 C)
and the solid catalyst was separated by using a magnet bar. The filtrate was extracted three times
with organic solvent (ethyl acetate). Then the organic phase was dried over anhydrous MgSO and
4
after removal of the solvent, corresponding cross-coupled Suzuki products were purified by short
chromatography on a silica gel (n-hexane/ethyl acetate) to afford the desired product and calculate
the yield.
3
. Result and Discussion
Homogenous Pd NPs have been extensively employed in various Pd catalyzed cross-coupling
reactions such as Suzuki [38], Sonogashira [39], Heck [40] and Negishi [41]. The inherent tendency
toward NPs agglomeration makes the use of supporting agents or stabilizers obligatory in most
reactions. The main purpose of this study has been to develop robust Pd NPs stabilized onto a
novel supported polymer which is abundant, renewable and biodegradable; such a decorated Pd
nanocatalyst on m-f-MWCNTs with larger surface area coupled with ease of its separation using just
an external magnet bodes well for its numerous Pd-catalyzed reactions.
3
.1. Characterization of Nanocatalyst
3
.1.1. FT-IR Spectroscopy
Figure 1 shows the FT-IR spectra for (a) MWCNTs, (b) f-MWCNTs, (c) m-f-MWCNTs, (d) chitosan,
(
e) cross-linked chitosan, and (f) m-f-MWCNTs@cross-linked chitosan. In Figure 1b, absorbance peaks
−1
−1
at 1718 cm and 3151 cm were attributed to stretching bands vibration of C=O and O–H, respectively
that confirmed successful carboxylation of MWCNTs (Figure 1a) [42 43]. For m-f-MWCNTs (Figure 1c),
the band at 583 cm is attributable to Fe–O stretching vibration. In the spectrum of cross-linked
chitosan (Figure 1e), the appearance of the new band at 1564 cm_1 and 1664 cm were accredited to
the aromatic C=C stretching and C=N ring stretching vibrations, respectively, due to the formation of
,
−1
_1
an imidazole ring [44
carboxyl groups emanating from m-f-MWCNTs was confirmed by observation of 1733 cm band
Figure 1f) [46].
,45]. The successful esterification reaction between OH groups of chitosan and
−1
(
Figure 1. FT-IR spectra of (
f-MWCNTs), (c) magnetic f-MWCNTs (m-f-MWCNTs), (d) chitosan, (e) cross-linked chitosan, and (f)
m-f-MWCNTs@cross-linked chitosan.
a) multi-walled carbon nanotubes (MWCNTs), (b) functionalized-MWCNTs
(

Molecules 2019, 24, 3048
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3
.1.2. XPS Analysis
XPS spectroscopy was applied to investigate the electronic state of Pd species that exist within the
nanocatalyst. As shown in Figure 2, two peaks were centered at 335.48 eV and 340.94 eV which are
0
ascribed to Pd 3d5 and 3d3/2, respectively, and two peaks were centered at 338.2 and 343.5 eV which
/2
accredited to Pd (II) 3d5/2 and 3d3/2, respectively [47]. These observations are reminiscent of previous
literature reports [48,49], wherein parts of Pd (II) species were likely reduced by chitosan backbone.
Figure 2. XPS of the m-f-MWCNTs@ chitosan NHC-Pd nanocomposite.
3
.1.3. TGA Analysis
Thermograms provide useful information about the catalyst contents in the nanocomposite
◦
powder (Figure 3). It is well known that the graphitic structure of MWCNTs is stable up to 600–700 C.
As can be seen, decomposition of f-MWCNTs with strong acids (carboxylic acid groups at the surface
of nanotube) occurs gradually in the whole range of temperature. Results showed that f-MWCNTs lost
◦ ◦
7 wt.% by 700 C while m-f-MWCNTs is thermally stable beyond this and lost 19 wt.% by 700 C.
2
The trend of weight loss for m-f-MWCNTs@chitosan is moderate and faster than of other samples up
◦
to 500 C due to the decomposition of polymeric portions of the nanocomposite.
Figure 3. TGA plots of (a) MWCNTs, (b) f-MWCNTs, (c) m-f-MWCNT, (d) m-f-MWCNTs@cross-linked.

Molecules 2019, 24, 3048
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3
.1.4. SEM and TEM Images and EDS Analysis
The morphological feature of the products before and after the assembly of imidazolium
cross-linked chitosan to m-f-MWCNTs and dimensions of the MWCNTs, m-f-MWCNTs and
m-f-MWCNTs@chitosan NHC-Pd were recorded using SEM (Figure 4). The surfaces of MWCNTs are
smooth and tidy (Figure 4A), whereas m-f-MWCNTs (Figure 4B) have a spherical shape and smooth
surface morphology. In contrast, the m-f-MWCNTs@chitosan NHC-Pd surfaces (Figure 4C) become
rough and they have many new branches. Compared to MWCNTs, the m-f-MWCNTs@ chitosan
NHC-Pd nanocomposite has a more aggregate morphology which possibly is due to the interactions
among the functional groups of cross-linked chitosan and the external wall of m-f-MWCNTs and thereby
affirming that cross-linked chitosan is successfully attached to m-f-MWCNTs surfaces. It is further
substantiated by FT-IR and TGA characterizations. Figure 4D shows a SEM image of the recycled
m-f-MWCNTs@ chitosan NHC-Pd nanocomposite. The morphology of m-f-MWCNTs@chitosan
NHC-Pd was examined by TEM, the result is shown in Figure 4E; apparently, the modification of
surface m-f-MWCNTs has occurred successfully, suggesting that the cross-linked chitosan was coated
onto the MWCNTs. The obtained data for EDX analysis affirmed the presence of Pd NPs in the catalyst
structure (Figure 4F).
Figure 4. SEM images for (
A
) f-MWCNTs, (
B
) m-f-MWCNT, (
C
) m-f-MWCNTs@chitosan NHC-Pd
) TEM image for
F) the EDX spectrum of m-f-MWCNTs@chitosan
nanocomposite, ( ) m-f-MWCNTs@chitosan NHC-Pd nanocomposite after five runs, (
D
E
m-f-MWCNTs@chitosan NHC-Pd nanocomposite and (
NHC-Pd nanocomposite.

Molecules 2019, 24, 3048
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3
.1.5. BET Analysis
The BET surface areas of the MWCNTs and the catalysts are provided. After modification
2
−1
of the MWCNTs, the BET surface area decreased dramatically from 430.06 m g in MWCNTs to
2
−1
283.85 m g in the final nanocatalyst; the obtained result indicated that most of the external surface
of the MWCNTs was impregnated with the Pd.
3
.2. Catalytic Performance of m-f-MWCNTs@chitosan NHC-Pd Nanocomposite
Heterogeneous catalysts stabilized by polymers have been extensively investigated for their
cross-coupling reactions [50
separable nanoparticles stabilized by hydrophilic polymers which catalyze the cross-coupling reactions
under greener conditions [25 52 56]. In this work, the catalytic activity of Pd NPs stabilized on
,51]. However, there are only a few reports on the application of magnetically
,
–
m-f-MWCNTs@cross-linked chitosan nanocomposite was investigated for the Suzuki reaction under
eco-friendly conditions. The flexibility in surface functionalities, large surface area to volume ratio and
superior mechanical performance of m-f-MWCNTs positively influenced the amount of catalyst loaded
and decreased the required reaction times. In this work, the reaction of bromobenzene with phenyl
boronic acid in the presence of the aforementioned catalyst was studied as a model reaction (Table 1).
According to the obtained data, the best result was found using a mixture of EtOH:H O as a solvent of
2
choice. The use of this mixture as the solvent provides excellent interaction between heterogeneous
stabilized Pd nanocomposite with hydrophilic compound (boronic acid) and hydrophobic compound
(
aryl halides) which consequently leads to an increase in the yield and rate enhancement of the Suzuki
reaction. According to the previous experiments—the base significantly affected the product yields
in this regard—K CO was used in a catalytic system. The amount of catalyst used and the time
2
3
of the reaction are two significant parameters investigated in this work. The best conversion in the
model reaction to provide biphenyl was identified by the deployment of 0.1 mol% of catalyst in
3
h for completion of the reaction (Table 1). As shown in Table 2, the coupling products obtained
from the reaction of boronic acid derivatives with various aryl halides gave corresponding biaryls
entries 1–18). As shown, the iodobenzene, 4-iodotoluene and 4-iodoanisole gave the corresponding
(
biaryls in the presence of 0.1 mol% catalyst in only 2 hrs (Table 2, entries 1–3). Both electron-donating
and -withdrawing groups on aryl bromides gave the corresponding biaryls in excellent yields in the
presence of 0.1 mol% catalyst in only 3 h (Table 2, entries 4–6). The prepared catalyst efficiently catalyzed
the Suzuki cross-coupling reaction of aryl iodides and aryl bromides. It has been well-established that
Suzuki reaction does not proceed smoothly when aryl chlorides are used; aryl chlorides are commonly
less reactive since C–Cl bond is stronger than those of C–Br and C–I, thus requiring longer reaction
times, and catalysts with higher activity are a prerequisite. Because of the ready availability of aryl
chlorides and being relatively less expensive than their bromo and iodo counterparts, using them in
cross-coupling reactions, including Suzuki reaction, is desirable. Thus, the most important advantage
of our novel catalyst system is that it can be successfully used in Suzuki reaction wherein a wide
variety of aryl chlorides can be coupled with aryl boronic acid to generate the corresponding products.
However, aryl chlorides due to strong C–Cl bond, relative to C–Br and C–I, are more difficult to react
than aryl bromides and aryl iodides and, therefore, increased amounts of catalyst and longer reaction
time are often required. By increasing the amount of catalyst loading to 1 mol% and extending the
◦
reaction time to 8 h in 60 C (entries 8–10 and 16–18), the Suzuki reaction of aryl chlorides could be
completed in good yields (40–62%). Furthermore, 4-tolylboronic acid was subjected to various aryl
halides and the obtained data are documented in entries 11–18.

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Table 1. Optimization of the reaction conditions for the Suzuki reaction of bromobenzene with phenyl
boronic acid catalyzed by m-f-MWCNTs@chitosan NHC-Pd nanocomposite.
Amount of
Catalyst (mol%)
◦
Entry
T ( C)
Solvent
H O
Time (h)
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
0.1
0.5
0.5
0.3
0.1
0.1
0.08
80
80
80
80
80
50
50
5
5
5
5
5
3
3
80
90
75
84
97
94
94
94
80
2
H O
2
H O-EtOH
2
100
98
H O-EtOH
2
H O-EtOH
2
97
H2O-EtOH
97
H O-EtOH
2
85
2 3
Reaction conditions: Phenyl boronic acid (1.2 mmol), bromobenzene (1 mmol), K CO (3 mmol), m-f-MWCNTs@chitosan
NHC-Pd nanocomposite as a catalyst, Solvent (H2O-EtOH: 1:1 (6 mL)), calculated by GC.
Table 2. Suzuki reaction of various aryl halides with aryl boronic acids catalyzed by m-f-MWCNTs@chitosan
NHC-Pd nanocomposite.
Amount of
Catalyst
Time Temperature Conversion Yield
Entry
R1
R2
X
TOF a
TON
◦
(h)
( C)
(%)
(%)
(
mol%)
0.1
0.1
0.1
0.1
0.1
0.1
1
1
2
3
4
5
6
7
8
9
H
4-Me
4-OMe
H
H
H
I
2
2
2
3
3
3
5
8
8
8
2
2
2
3
3
8
8
8
40
40
40
50
50
50
50
60
60
60
40
40
40
50
50
60
60
60
100
100
100
97
97
97
97
94
95
91
-
485
485
485
313
316
303
-
970
970
970
939
948
909
-
I
H
I
H
Br
Br
Br
Cl
Cl
Cl
Cl
I
4-NO2
4-Me
H
H
98
H
94
H
N.R.
60
H
H
1
53
50
62
95
94
93
89
83
50
47
40
6.6
52.8
50
4-Me
4-NO2
H
H
1
53
6.25
7.75
475
470
465
296
276
6.25
5.87
5
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
H
1
68
62
Me
Me
0.1
0.1
0.1
0.1
0.1
1
98
950
940
930
888
828
50
4-Me
I
97
4-OMe Me
I
96
H
Me
Me
Me
Me
Br
Br
Cl
Cl
Cl
92
4-Me
H
89
55
4-Me
1
50
46.96
40
4-OMe Me
1
45
Reaction conditions: Aryl boronic acid (1.2 mmol), Aryl halide (1 mmol), K
NHC-Pd nanocomposite as catalyst, Solvent (H O-EtOH: 1:1 (6 mL)), calculated by GC. N.R. means no reaction. TOF = Moles
of desired product formed/moles of catalyst/time = TON/time.
2 3
CO 2–3 (mmol), m-f-MWCNTs@chitosan
a
2

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3
.3. Study of Pd Leaching
The hot filtration test was performed to investigate whether the reaction proceeded in a
heterogeneous or homogeneous mode. In a typical experiment, phenyl boronic acid (1.2 mmol),
bromobenzene (1 mmol), K CO (3 mmol), m-f-MWCNTs@chitosan NHC-Pd nanocomposite as
2
3
catalyst (0.1 mol%) were taken in solvent (H O-EtOH: 1:1 (6 mL)) in a round-bottomed flask and stirred
2
◦
at 50 C for 30 min. After 30 min of reaction, the hot reaction solution was separated by a magnet and
the experiment was continued for another 3 h. According to the obtained data (monitored by GC),
no further progress in the Suzuki cross-coupling reaction was observed even after a 3 h reaction time.
Thus, barely any Pd leached into the reaction mixture, which confirmed that the reaction proceeded in
a heterogeneous fashion.
According to the literature review [57] and as evidenced by local leaching studies, the following
two essential factors have emerged: (1) the morphology of a heterogeneous catalyst is retained and
(
2) a very low level of metal leached in to the solution phase, thus implying that the catalyst system is
a ‘cocktail’-type. However, for our prepared composite structure, where Pd NPs were dispersed on
this composite structure, we cannot assign the Pd NPs shapes, but the hot filtration test confirmed the
absence of leached Pd in solution.
3
.4. The Reusability and Stability of Nanocatalyst
The reusability of the nanocatalyst was studied in the coupling reaction of bromobenzene and
phenyl boronic acid as a model reaction under the identified optimal conditions. In each cycle, 6 mL
aqueous solution containing reagents and the nanocatalyst was added into a 50 mL round-bottomed
flask at optimal temperature. Upon the completion of the reaction, nanocatalyst was separated from
the mixture using an external magnetic field, washed with water and reused for the next cycle in the
same reaction. The results showed that the nanocatalyst could be successfully reused for five runs,
without any considerable loss in its catalytic activity for the Suzuki cross-coupling reaction (Figure 5).
−1
−1
The remaining amount of Pd after the fifth cycle was decreased by 0.325 mmol g to 0.308 mmol g
as detected by AAS analysis (5% leaching).
Figure 5. Reusability studies for recycling of m-f-MWCNTs@chitosan NHC-Pd nanocomposite catalyst
in Suzuki reaction of bromobenzene with phenyl boronic acid.
3
.5. Comparison
To evaluate the efficiency of the m-f-MWCNTs@ chitosan NHC-Pd nanocomposite, and its
catalytic prowess, the activity was compared to similar earlier documented reactions from the literature.
According to the results summarized in Table 3, the reduced time reaction, high catalytic efficiency
under greener media at reduced reaction temperature, higher conversion and yield were obtained by
using our catalyst [58–64].

Molecules 2019, 24, 3048
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Table 3. Comparison of the cross-coupling reaction results from m-f-MWCNTs@chitosan NHC-Pd
nanocomposite with documented results from literature.
Yield or
Conversion
Time
Entry
Catalyst (Pd Loading (mol%))
Reaction Conditions
Reusability
Ref.
(h)
(%)
◦
1
2
poly(NIPAM-co-4-VP)-Pd (1)
GO-NH2-Pd(II) (1)
K2CO3, H2O, 60
C
5
4
95
71
5 times
[58]
[59]
K2CO3, EtOH:H2O,
10 times
◦
0 C
6
3
4
5
GO-NHC-Pd(II) (0.25)
K2CO3, EtOH:H2O,
8
20
1.5
3
94
71
89
6 times
6 times
5 times.
[60]
[61]
◦
0 C
rGO/Pd (0.5)
K2CO3, EtOH:H2O,
◦
0 C
6
m-f-MWCNTs@chitosan
NHC-Pd nanocomposite (0.1)
K2CO3, EtOH:H2O,
This work
◦
50 C
6
7
8
9
Poly(NIPAM-4-VP AC)-Pd
K2CO3, H2O, Ar
atmosphere, 90
1
4
6
4
3
92
73
92
99
94
6 times
10 times
6 times
-
[62]
[59]
◦
C
(
0.05), X = Br
GO-NH2-Pd(II) (1), X = Br
K2CO3, EtOH:H2O,
◦
0 C
6
◦
Fe3O4@C-Pd@mSiO2 (1.5),
X = I
K2CO3, iPrOH, 70
C
[63]
Pd@Fe3O4@C (1), X = Br
K2CO3, DMF:H2O,
[64]
◦
C
1
00
1
0
m-f-MWCNTs@chitosan
NHC-Pd (0.1), X = Br
K2CO3, EtOH:H2O,
5 times
This work
◦
50 C
4
. Conclusions
Our investigations demonstrate that the immobilization of imidazolium cross-linked chitosan to
m-f-MWCNTs support, followed by decorating with palladium, produces a well-defined and superior
catalyst for the Suzuki cross-coupling reactions. The repeated amino groups of chitosan led to imidazole
units’ in polymer chains that enhanced the number of Pd linkers and catalyst efficiency. Our best
prototype catalyst demonstrated an encouraging range of catalytic activity in the Suzuki cross-coupling
reactions of aryl chlorides in aqueous media. Further, this catalyst can be easily separated by using
a magnetic field and used for five runs without losing significant activities under optimum reaction
conditions; high TOF and TON, two important green chemistry metrics, bodes well for its utility in
other related Pd-catalyzed reactions.
Author Contributions: Conceptualization, R.S. and B.H.; methodology, H.S. and P.Z.; software, B.H.; validation,
R.S., B.H. and R.S.V.; formal analysis, R.S., B.H., H.S., P.Z. and R.S.V.; investigation, B.H., H.S. and P.Z.; resources,
R.S. and R.S.V.; data curation, B.H.; writing—original draft preparation, B.H.; writing—review and editing, R.S.
and R.S.V.; visualization, R.S., B.H., H.S., P.Z. and R.S.V.; supervision, R.S. and R.S.V.; project administration, B.H.;
funding acquisition, R.S.V.
Funding: R.S.V. was supported by the Operational Program Research, Development and Education-European
Regional Development Fund, project no. CZ.02.1.01/0.0/0.0/16_019/0000754 of the Ministry of Education, Youth
and Sports of the Czech Republic.
Conflicts of Interest: Authors declare no conflict of interest.
References
1
.
Magano, J.; Dunetz, J.R. Large-Scale Applications of Transition Metal-Catalyzed Couplings for the Synthesis
of Pharmaceuticals. Chem. Rev. 2011, 111, 2177–2250. [CrossRef] [PubMed]
2
.
Torborg, C.; Beller, M. Recent Applications of Palladium-Catalyzed Coupling Reactions in the Pharmaceutical,
Agrochemical, and Fine Chemical Industries. Adv. Synth. Catal. 2009, 351, 3027–3043. [CrossRef]

Molecules 2019, 24, 3048
12 of 14
3.
4.
5.
6.
Corbet, J.-P.; Mignani, G.; Corbet, J. Selected Patented Cross-Coupling Reaction Technologies. Chem. Rev.
006, 37, 2651–2710. [CrossRef] [PubMed]
Hooshmand, S.E.; Heidari, B.; Sedghi, R.; Varma, R.S.; Hiedari, B. Recent advances in the Suzuki–Miyaura
cross-coupling reaction using efficient catalysts in eco-friendly media. Green Chem. 2019, 21, 381–405. [CrossRef]
Martin, R.; Buchwald, S.L. Palladium-Catalyzed Suzuki-Miyaura Cross-coupling Reactions Employing
Dialkylbiaryl Phosphine Ligands. Acc. Chem. Res. 2008, 41, 1461–1473. [CrossRef] [PubMed]
2
Barder, T.E.; Walker, S.D.; Martinelli, J.R.; Buchwald, S.L. Catalysts for Suzuki
Scope and Studies of the Effect of Ligand Structure. J. Am. Chem. Soc. 2005, 127, 4685–4696. [CrossRef]
PubMed]
−
Miyaura Coupling Processes:
[
7.
8.
9.
Miura, M. Rational Ligand Design in Constructing Efficient Catalyst Systems for Suzuki—Miyaura Coupling.
Angew. Chem. Int. Ed. 2004, 35, 2201–2203. [CrossRef] [PubMed]
Marziale, A.N.; Jantke, D.; Faul, S.H.; Reiner, T.; Herdtweck, E.; Eppinger, J. An efficient protocol for the
palladium-catalysed Suzuki–Miyaura cross-coupling. Green Chem. 2011, 13, 169–177. [CrossRef]
Kim, J.-H.; Jun, B.-H.; Byun, J.-W.; Lee, Y.-S. N-Heterocyclic carbene–palladium complex on polystyrene resin
surface as polymer-supported catalyst and its application in Suzuki cross-coupling reaction. Tetrahedron Lett.
2
004, 45, 5827–5831. [CrossRef]
1
0. Li, J.-H.; Liu, W.-J. Dabco as an Inexpensive and Highly Efficient Ligand for Palladium-Catalyzed
Suzuki−Miyaura Cross-Coupling Reaction. Org. Lett. 2004, 6, 2809–2811. [CrossRef]
1. Someya, H.; Ohmiya, H.; Yorimitsu, H.; Oshima, K. N-Heterocyclic Carbene Ligands in Cobalt-Catalyzed
Sequential Cyclization/Cross-Coupling Reactions of 6-Halo-1-hexene Derivatives with Grignard Reagents.
Org. Lett. 2007, 38, 1565–1567. [CrossRef] [PubMed]
1
1
1
1
2. Peris, E.; Crabtree, R.H. Recent homogeneous catalytic applications of chelate and pincer N-heterocyclic
carbenes. Coord. Chem. Rev. 2004, 248, 2239–2246. [CrossRef]
3. Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Group 11 Metal Complexes of N-Heterocyclic Carbene
Ligands: Nature of the MetalCarbene Bond. Organometallics 2004, 23, 755–764. [CrossRef]
4. Dorta, R.; Stevens, E.D.; Scott, N.M.; Costabile, C.; Cavallo, L.; Hoff, C.D.; Nolan, S.P. Steric and Electronic
Properties of N-Heterocyclic Carbenes (NHC): A Detailed Study on Their Interaction with Ni(CO)4. J. Am.
Chem. Soc. 2005, 127, 2485–2495. [CrossRef] [PubMed]
1
1
1
1
5. Fortman, G.C.; Nolan, S.P. N-Heterocyclic carbene (NHC) ligands and palladium in homogeneous
cross-coupling catalysis: A perfect union. Chem. Soc. Rev. 2011, 40, 5151–5169. [CrossRef] [PubMed]
6. Li, H.; Seechurn, C.C.C.J.; Colacot, T.J. Development of Preformed Pd Catalysts for Cross-Coupling Reactions,
Beyond the 2010 Nobel Prize. ACS Catal. 2012, 2, 1147–1164. [CrossRef]
7. Sydnes, M.O. The Use of Palladium on Magnetic Support as Catalyst for Suzuki–Miyaura Cross-Coupling
Reactions. Catalysts 2017, 7, 35. [CrossRef]
8. Fayazi, M.; Taher, M.A.; Afzali, D.; Mostafavi, A. Fe O and MnO assembled on halloysite nanotubes:
3
4
2
A highly efficient solid-phase extractant for electrochemical detection of mercury(II) ions. Sens. Actuators B
Chem. 2016, 228, 1–9. [CrossRef]
1
9. Guo, B.; Deng, F.; Zhao, Y.; Luo, X.; Luo, S.; Au, C.T. Magnetic ion-imprinted and –SH functionalized polymer
for selective removal of Pb(II) from aqueous samples. Appl. Surf. Sci. 2014, 292, 438–446. [CrossRef]
2
0. Deng, F.; Li, Y.; Luo, X.; Yang, L.; Tu, X. Preparation of conductive polypyrrole/TiO nanocomposite via
2
surface molecular imprinting technique and its photocatalytic activity under simulated solar light irradiation.
Colloids Surfaces A Physicochem. Eng. Asp. 2012, 395, 183–189. [CrossRef]
2
1. Liu, Y.; Liu, Z.; Gao, J.; Dai, J.; Han, J.; Wang, Y.; Xie, J.; Yan, Y. Selective adsorption behavior of Pb(II) by
mesoporous silica SBA-15-supported Pb(II)-imprinted polymer based on surface molecularly imprinting
technique. J. Hazard. Mater. 2011, 186, 197–205. [CrossRef] [PubMed]
2
2. Karaaslan, N.M.; Senkal, B.F.; Cengiz, E.; Yaman, M. Novel Polymeric Resin for Solid Phase Extraction and
Determination of Lead in Waters. CLEAN Soil Air Water 2010, 38, 1047–1054. [CrossRef]
2
3. Liu, Y.; Liu, Z.; Wang, Y.; Dai, J.; Gao, J.; Xie, J.; Yan, Y. A surface ion-imprinted mesoporous sorbent for
separation and determination of Pb (II) ion by flame atomic absorption spectrometry. Microchim. Acta 2011
72, 309–317. [CrossRef]
,
1
2
4. Murugan, E.; Gopi, V. Amphiphilic Multiwalled Carbon Nanotube Polymer Hybrid with Improved
Conductivity and Dispersibility Produced by Functionalization with Poly(vinylbenzyl)triethylammonium
Chloride. J. Phys. Chem. C 2011, 115, 19897–19909. [CrossRef]

Molecules 2019, 24, 3048
13 of 14
2
2
2
5. Baig, R.B.N.; Varma, R.S. Magnetically retrievable catalysts for organic synthesis. Chem. Commun. 2013, 49,
7
52–770. [CrossRef] [PubMed]
6. Baig, R.B.N.; Varma, R.S. Organic synthesis via magnetic attraction: Benign and sustainable protocols using
magnetic nanoferrites. Green Chem. 2013, 15, 398–417. [CrossRef]
7. Huang, K.; Xue, L.; Hu, Y.-C.; Huang, M.-Y.; Jiang, Y.-Y. Catalytic behaviors of silica-supported
starch–polysulfosiloxane–Pt complexes in asymmetric hydrogenation of 4-methyl-2-pentanone. React.
Funct. Polym. 2002, 50, 199–203. [CrossRef]
2
8. Zhang, J.; Xia, C.-G. Natural biopolymer-supported bimetallic catalyst system for the carbonylation to esters
of Naproxen. J. Mol. Catal. A Chem. 2003, 206, 59–65. [CrossRef]
2
9. Leonhardt, S.E.S.; Ondruschka, B.; Ondruschka, J. Comment on Aspects of Chitosan Preparation. Chem. Eng.
Technol. 2008, 31, 917–921. [CrossRef]
3
3
0. Agboh, O.; Qin, Y. Chitin and chitosan fibers. Polym. Adv. Technol. 1997, 8, 355–365. [CrossRef]
1. Knaul, J.Z.; Hudson, S.M.; Creber, K.A.M. Improved mechanical properties of chitosan fibers. J. Appl. Polym.
Sci. 1999, 72, 1721–1732. [CrossRef]
3
2. Silvestri, D.; Wacławek, S.; Sobel, B.; Torres-Mendieta, R.; Novotn
ý, V.; Nguyen, N.H.A.; Ševc u˚ , A.;
Padil, V.V.T.; Müllerov , J.; Stuchlik, M.; et al. A poly(3-hydroxybutyrate)–chitosan polymer conjugate for
á
the synthesis of safer gold nanoparticles and their applications. Green Chem. 2018, 20, 4975–4982. [CrossRef]
3. Baba, Y.; Hirakawa, H.; Yoshizuka, K.; Inuoe, K.; Kawano, Y. Adsorption Equilibria of Silver(I) and Copper(II)
Ions on N-(2-Hydroxylbenzyl)chitosan Derivative. Anal. Sci. 1994, 10, 601–605. [CrossRef]
3
3
3
4. Guibal, E. Interactions of metal ions with chitosan-based sorbents: A review. Sep. Purif. Technol. 2004, 38,
4
3–74. [CrossRef]
5. Heeres, J.; Backx, L.J.J.; Mostmans, J.H.; Van Cutsem, J. Antimycotic imidazoles. Part 4. Synthesis and
antifungal activity of ketoconazole, a new potent orally active broad-spectrum antifungal agent. J. Med.
Chem. 1979, 22, 1003–1005. [CrossRef] [PubMed]
3
6. Debus, H. Ueber die einwirkung des ammoniaks auf glyoxal. Eur. J. Org. Chem. 1858, 107, 199–208.
[CrossRef]
3
7. Huong, L.T.T.; Nam, N.H.; Doan, D.H.; Nhung, H.T.M.; Quang, B.T.; Nam, P.H.; Thong, P.Q.; Phuc, N.X.;
Thu, H.P. Folate attached, curcumin loaded Fe3O4 nanoparticles: A novel multifunctional drug delivery
system for cancer treatment. Mater. Chem. Phys. 2016, 172, 98–104. [CrossRef]
3
8. Miyaura, N.; Yamada, K.; Suzuki, A. A new stereospecific cross-coupling by the palladium-catalyzed reaction
of 1-alkenylboranes with 1-alkenyl or 1-alkynyl halides. Tetrahedron Lett. 1979, 20, 3437–3440. [CrossRef]
9. Sonogashira, K. Development of Pd–Cu catalyzed cross-coupling of terminal acetylenes with sp2-carbon
halides. J. Organomet. Chem. 2002, 653, 46–49. [CrossRef]
3
4
4
0. Heck, R.F. Palladium-Catalyzed Vinylation of Organic Halides. Org. React. 1982, 345–390.
1. Negishi, E.-I.; Hu, Q.; Huang, Z.; Qian, M.; Wang, G.; Brown, H. Palladium-catalyzed alkenylation by the
Negishi coupling. Aldrichim. Acta 2005, 38, 71–87. [CrossRef]
4
4
2. Chang, X.; Jiang, N.; Zheng, H.; He, Q.; Hu, Z.; Zhai, Y.; Cui, Y. Solid-phase extraction of iron(III) with an
ion-imprinted functionalized silica gel sorbent prepared by a surface imprinting technique. Talanta 2007, 71,
3
8–43. [CrossRef] [PubMed]
3. Rofouei, M.K.; Payehghadr, M.; Shamsipur, M.; Ahmadalinezhad, A. Solid phase extraction of ultra traces
silver(I) using octadecyl silica membrane disks modified by 1,3-bis(2-cyanobenzene) triazene (CBT) ligand
prior to determination by flame atomic absorption. J. Hazard. Mater. 2009, 168, 1184–1187. [CrossRef]
[PubMed]
4
4
4. Sarker, R.B.; Bhuiyan, A. Electrical conduction mechanism in plasma polymerized 1-Benzyl-2-
methylimidazole thin films under static electric field. Thin Solid Films 2011, 519, 5912–5916. [CrossRef]
5. Silva, S.M.; Braga, C.R.; Fook, M.V.; Raposo, C.M.; Carvalho, L.H.; Canedo, E.L. Application of Infrared
Spectroscopy to Analysis of Chitosan/Clay Nanocomposites. In Infrared Spectroscopy—Materials Science,
Engineering and Technology; InTech Open: Rijeka, Croatia, 2012.
4
6. Chang, X.; Chen, D.; Jiao, X. Chitosan-Based Aerogels with High Adsorption Performance. J. Phys. Chem. B
2
008, 112, 7721–7725. [CrossRef] [PubMed]
4
7. Xu, Y.; Zhang, L.; Cui, Y. Catalytic performance of cellulose supported palladium complex for Heck reaction
in water. J. Appl. Polym. Sci. 2008, 110, 2996–3000. [CrossRef]

Molecules 2019, 24, 3048
14 of 14
4
4
8. Wu, X.; Lu, C.; Zhang, W.; Yuan, G.; Xiong, R.; Zhang, X. A novel reagentless approach for synthesizing
cellulose nanocrystal-supported palladium nanoparticles with enhanced catalytic performance. J. Mater.
Chem. A 2013, 1, 8645–8652. [CrossRef]
9. Xiong, R.; Lu, C.; Wang, Y.; Zhou, Z.; Zhang, X. Nanofibrillated cellulose as the support and reductant for the
facile synthesis of Fe3O4/Ag nanocomposites with catalytic and antibacterial activity. J. Mater. Chem. A 2013
, 14910–14918. [CrossRef]
,
1
5
5
5
0. Paul, S.; Islam, M.M. Suzuki–Miyaura reaction by heterogeneously supported Pd in water: Recent studies.
RSC Adv. 2015, 5, 42193–42221. [CrossRef]
1. Yin, L.; Liebscher, J. Carbon—Carbon Coupling Reactions Catalyzed by Heterogeneous Palladium Catalysts.
Chem. Rev. 2007, 38, 133–173. [CrossRef]
2. Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U.-I.
Design and Fabrication of a High-Strength Hydrogel with Ideally Homogeneous Network Structure from
Tetrahedron-like Macromonomers. Macromolecules 2008, 41, 5379–5384. [CrossRef]
5
3. Hong, M.C.; Choi, M.C.; Chang, Y.W.; Lee, Y.; Kim, J.; Rhee, H. ChemInform Abstract: Palladium Nanoparticles
on Thermoresponsive Hydrogels and Their Application as Recyclable Suzuki-Miyaura Coupling Reaction
Catalysts in Water. Adv. Synth. Catal. 2012, 43, 1257–1263. [CrossRef]
5
5
5
4. Sheldon, R.A.; Wallau, M.; Arends, I.W.C.E.; Schuchardt, U. Heterogeneous Catalysts for Liquid-Phase
Oxidations: Philosophers’ Stones or Trojan Horses? Accounts Chem. Res. 1998, 31, 485–493. [CrossRef]
5. Baig, R.B.N.; Leazer, J.; Varma, R.S. Magnetically separable Fe3O4@DOPA–Pd: A heterogeneous catalyst for
aqueous Heck reaction. Clean Technol. Environ. Policy 2015, 17, 2073–2077. [CrossRef]
6.
Sá, S.; Gawande, M.B.; Velhinho, A.; Veiga, J.P.; Bundaleski, N.; Trigueiro, J.; Tolstoguzov, A.;
Teodoro, O.M.N.D.; Zbo rˇ il, R.; Varma, R.S.; et al. Magnetically recyclable magnetite–palladium (Nanocat-
Fe–Pd) nanocatalyst for the Buchwald–Hartwig reaction. Green Chem. 2014, 16, 3494–3500. [CrossRef]
5
5
7. Eremin, D.B.; Ananikov, V.P. Understanding active species in catalytic transformations: From molecular
catalysis to nanoparticles, leaching, “Cocktails” of catalysts and dynamic systems. Coord. Chem. Rev. 2017
46, 2–19. [CrossRef]
8. Lee, Y.; Hong, M.C.; Ahn, H.; Yu, J.; Rhee, H. Pd nanoparticles immobilized on poly(NIPAM-co-4-VP)
hydrogel: Highly active and reusable catalyst for carbon–carbon coupling reactions in water. J. Organomet.
Chem. 2014, 769, 80–93. [CrossRef]
,
3
5
9. Shang, N.; Feng, C.; Zhang, H.; Gao, S.; Tang, R.; Wang, C.; Wang, Z. Suzuki–Miyaura reaction catalyzed by
graphene oxide supported palladium nanoparticles. Catal. Commun. 2013, 40, 111–115. [CrossRef]
0. Shang, N.; Gao, S.; Feng, C.; Zhang, H.; Wang, C.; Wang, Z. Graphene oxide supported N-heterocyclic
carbene-palladium as a novel catalyst for the Suzuki–Miyaura reaction. RSC Adv. 2013, 3, 21863–21868.
6
[CrossRef]
6
6
6
6
1. Song, H.-Q.; Zhu, Q.; Zheng, X.-J.; Chen, X.-G. One-step synthesis of three-dimensional graphene/multiwalled
carbon nanotubes/Pd composite hydrogels: An efficient recyclable catalyst for Suzuki coupling reactions.
J. Mater. Chem. A 2015, 3, 10368–10377. [CrossRef]
2. Zhang, Y.; Yang, J.; Zhang, X.; Bian, F.; Yu, W. A novel thermo and pH-double sensitive hydrogel immobilized
Pd catalyst for Heck and Suzuki reactions in aqueous media. React. Funct. Polym. 2012, 72, 233–241.
[CrossRef]
3. Sun, Z.; Yang, J.; Wang, J.; Li, W.; Kaliaguine, S.; Hou, X.; Deng, Y.; Zhao, D. A versatile designed synthesis of
magnetically separable nano-catalysts with well-defined core–shell nanostructures. J. Mater. Chem. A 2014, 2,
6
071–6074. [CrossRef]
4. Woo, H.; Lee, K.; Park, J.C.; Park, K.H. Facile synthesis of Pd/Fe O /charcoal bifunctional catalysts with
3
4
high metal loading for high product yields in Suzuki–Miyaura coupling reactions. New J. Chem. 2014, 38,
626–5632. [CrossRef]
5
Sample Availability: Samples of the compounds are not available from the authors.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).