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A.R. Hajipour and S.S. Malek
Molecular Catalysis 508 (2021) 111573
disadvantages of homogenous catalysts such as leaching of metal species
and recovery problems. Silica [39], polymer [40], and magnetic nano-
particles [16] (MNPs) have been used as support [41]. However, the
reported polymer or silica-supported catalysts have active sites in the
entire regions of the supports, which impede the diffusion of the re-
agents into the interior of the supports, resulting in a decrease in their
catalytic activity and reaction rate [23]. Among various supports,
magnetic nanoparticles (MNPs) [42,43] have been employed as a
promising alternative, because of their low toxicity, biodegradability,
and easy separation by an external magnetic field, with no other workup
processes [44]. On the other hand, shielding of MNPs has been carried
out to prevent oxidation and self-aggregation. Various protecting layer
have been reported in which chitosan are promising biopolymer due to
its conclusive properties such as biodegradability and cost effectiveness
[45–48].
temperature, the as-obtained precipitate was separated from the solu-
tion using an external magnetic bar, followed by washing several times
with deionized water (DI) and ethanol, and drying in an oven at 50 ◦C.
Subsequently, the surface modification of the magnetic nanoparticles
(MNPs) was performed with chitosan to address the agglomeration of
MNPs and also generate amine groups [34]. For this purpose, MNPs
(0.236 g) and 2 mL of acetic acid solution were mixed accompanied by a
mixture of DI (11 mL) and chitosan (0.472 g), and the resultant mixture
was then stirred for 15 min. Sodium sulfate solution (20 % W/V) was
dispersed in the as-obtained mixture and intensively stirred for 1 h at
room temperature. Finally, the magnetic chitosan (MC) was separated
from the solution using an external magnet, followed by washing with
ethanol several times and drying at 70 ◦C.
2.1.3. Preparation of MC supported Cobalt-NHC (Co-NHC@MC)
The MC (1.5 g) was added to a solution of [mim]Cl (5 mL) and tri-
methylamine (Et3N) (36 mmol, 5 mL) in 10 mL of tetrahydrofuran
(THF). The resulting mixture was allowed to reflux overnight at 60 ◦C.
The desired product (me-Im@MC) was collected using an external
magnetic bar, followed by washing with THF several times to remove
redundant, and drying in a vacuum oven at 50 ◦C. To attach the cobalt
ions to the (me-Im@MC), 1.2 g of this compound was added to 0.5 g of
Co(OAc)2⋅4H2O (2 mmol) together with 1 g of sodium carbonate (9
mmol) dissolved in 10 mL of ethanol. Afterward, the reaction mixture
was refluxed and stirred for 16 h at 75 ◦C. The resulting complex was
separated, washed several times with ethanol, and dried at room tem-
perature to afford Co-NHC@MC as a brown solid. At the end, the solid
was separated, washed with water, and dried at 50 ◦C. The detailed
preparation steps are illustrated in Scheme 1.
Although the reported catalytic systems were highly efficient, most
of them employ environmentally non-benign solvents, expensive metal,
high-temperature reaction conditions and limited substrate study. So we
were encouraged to design a heterogeneous catalyst in which Cobalt-
NHC immobilized on magnetic chitosan nanoparticles. The catalytic
activity of the prepared catalyst was evaluated in cross-coupling of
substituted phenylboronic acids and phenylacetylenes with aryl chlo-
rides under mild conditions in PEG as a green, thermally stable and
commercially available solvent. Indeed, conclusive properties such as
simple preparation, and easy recover ability of the magnetic catalyst
make this synthesized catalyst a promising candidate for examining it in
other cross-coupling reactions. Besides, the ability of the synthesized
catalyst to apply aryl chlorides in cited cross-coupling reactions is very
advantageous because they are cheaper and commercially available
than aryl iodides/bromides.
2.2. Catalytic activity
2. Experimental section
2.2.1. General procedure for the Suzuki reaction
2.1. Catalyst preparation
In a round-bottom flask containing a mixture of PEG (6 mL) as the
solvent, a mixture of aryl halide (1.0 mmol), phenylboronic acid (1.2
mmol), K2CO3 (2.0 mmol), and the synthesized catalyst (3 mol%) were
stirred at 70 ◦C. The reaction’s progress was assessed by TLC (hexane/
ethyl acetate, 80:20) and GC. After the completion of the reaction, the
organic layer was washed with water (3 × 10 mL) and ethyl acetate (3 ×
10 mL) and dried over anhydrous MgSO4. The related products were
purified using column chromatography (n-hexane/ethyl acetate, 5:1)
and characterized by 1H NMR and 13C NMR analyses and summarized in
the supporting information.
2.1.1. Synthesis of (1-(3-chloro-2-hydroxypropyl)-3-methyl imidazolium
chloride) ionic liquid ([mim]Cl)
A mixture of 0.1 mol hydrochloric acid (HCl, 37 %) and 50 mL
ethanol was poured into a flask equipped with a dropping funnel and a
reflux condenser. The flask was put in an oil bath with a magnetic
stirring bar. 8 mL of N-methyl imidazole (0.1 mol) was slowly added to
the flask over a period of 15 min. To avoid the formation of by-products,
the pH of the solution was adjusted to 6 using excess HCl. Afterward, 10
mL of epichlorohydrin (0.12 mol) was added dropwise into the resultant
aqueous solution at 5 ◦C under vigorous stirring. The temperature of the
mixture was then raised to 45 ◦C. After stirring at 45 ◦C for 4 h, 1-(3-
chloro-2-hydroxypropyl)-3-methyl imidazolium chloride, a viscous light
yellow ionic liquid (IL), was obtained. The remaining water, ethanol,
and epichlorohydrin were then removed by means of reduced pressure
distillation. For further purification, the as-prepared IL was passed
2.2.2. General procedure for the Sonogashira reaction
In a round-bottom flask containing a mixture of PEG (6 mL) as the
solvent, a mixture of aryl halide (1.0 mmol), phenylacetylene (1.2
mmol), K2CO3 (2.0 mmol) and the catalyst (6 mol%) was added and the
◦
obtained mixture was stirred at 100 C. The reaction was carried out
similarly to the Suzuki reaction. The corresponding products were pu-
rified using column chromatography (n-hexane/ethyl acetate, 5:1) and
characterized by 1H NMR and 13C NMR analyses.
◦
through a silica gel-G60 column and drying in vacuum at 60 C. The
1
product yield was about 90 % and, it was characterized by FT-IR, H
NMR, and 13C NMR analyses and summarized in the supporting infor-
mation [49].
3. Results and discussion
2.1.2. Synthesis of magnetic chitosan (MC)
3.1. Fabrication of Co-NHC@MC
In spite of various methods used for preparation of Fe3O4, co-
precipitation as the most facile and straightforward approach was
employed in this work [50]. In this regard, FeSO4⋅7H2O (5.2 mmol, 1.46
g) was first added to a round-bottom flask containing FeCl3⋅6H2O (7.4
mmol, 2 g) dissolved in 100 mL of distilled deionized water (DDI), fol-
lowed by ultrasonic for 30 min to achieve a homogenous solution. Then,
NH4OH solution (as an alkaline precipitation agent) was added dropwise
into the resultant solution until the pH reached 10 and a black precip-
itate formed, followed by stirring at 80 ◦C in a nitrogen atmosphere for
about 2 h to complete the crystallization. After being cooled to ambient
To prepare Co-immobilization and -stabilization, magnetic chitosan
(MC) were chosen as an appropriate support due to their magnetic
properties, high surface to volume ratio, and convenient separation of
the synthesized catalyst in combination with imidazolium chloride as
the stabilizer. In recent years, many studies on the catalytic activity of
imidazolium-based catalysts have been reported [51,52]. A schematic
illustration has been designed (Scheme 1) to summarize the formation of
the cobalt ion species on the me-Im@MC. The catalyst was prepared in 4
steps: in the first step, N-methyl imidazole reacted with epichlorohydrin
2