A. Maleki et al.
Journal of Physics and Chemistry of Solids 136 (2020) 109200
active catalytic site).
images were taken by a Zeiss Sigma microscope with a camera attached.
Magnetic measurements of the solid samples were performed with
Lakeshore 7407 and Meghnatis Kavir Kashan Co. (Iran) vibrating sample
magnetometers. Elemental analysis of the nanocatalyst was performed
by energy-dispersive X-ray (EDX) analysis with a Numerix DXP-X10P
instrument. X-ray diffraction (XRD) measurements were performed
Because of the great importance of biphenyl derivatives as in-
termediates in materials science, pharmaceuticals, polymers, and agro-
chemicals, several methods have been reported for the synthesis of
biphenyl compounds [14–16]. Among the different strategies, the
Suzuki–Miyaura coupling reaction is a powerful synthetic method for
preparation of biphenyls from aryl halides and phenylboronic acids
because of the mild reaction conditions and functional group tolerance
[17–19]. Usually, coupling reactions are conducted with a soluble
Pd/organic complex as the homogeneous catalyst, but tedious workup
procedures, low catalytic activity (due to wasting), and contamination
by residual metals in the final product are the major drawbacks of this
method. However, it seems that use of heterogeneous Pd-containing
catalysts is the best solution to address the aforementioned de-
ficiencies [20–28]. Recently, several methods have been reported in
which organic and inorganic solid supports (such as silica, carbon,
zeolite, polymer, and ionic liquids) were used for preparation of het-
erogeneous Pd catalysts [29,30]. In this regard, we present a novel
Pd-supported magnetic nanocatalyst (Fe3O4/o-PDA–Pd) as a heteroge-
neous nanocatalyst with superparamagnetic behavior, and then we
investigate its activity in catalyzing Suzuki–Miyaura coupling reactions
under mild conditions. Among the various types of heterogeneous
catalysis systems, Fe3O4-based systems are the most applicable because
of the ease of the separation process and recycling. From a physical
aspect, their magnetic property provides a substantial opportunity to
recycle and reuse them several times with a low amount of waste. Also, a
larger active surface area is provided through dispersion of these NPs in
comparison with integrated polymeric systems. Thus, lower ratios of the
catalyst work as well as relatively large quantities of other types of these
catalytic systems. From a chemical aspect, there are two advantages of
this catalytic system. First, the effective electronic interactions between
the loaded Pd(II) and heteroatoms in the structure of the aryl halides and
phenylboronic acid increase the probability of contact between the
molecules. Second, the convenient conversion of Pd(II) to Pd(0) on the
NP surfaces leads to good execution of the coupling reactions. Addi-
tionally, the chance of producing byproducts is increased because of the
longer reaction times. Thus, this is considered a important benefit of the
Fe3O4/o-PDA–Pd nanocatalyst, which catalyzes the coupling reactions
in a short reaction time (10 min). Here we report that a high reaction
yield (98%) is obtained in a very short reaction time (10 min) through
use of the Fe3O4/o-PDA–Pd nanocatalyst (0.01 g) at room temperature
(Fig. 1).
with a DRON-8 X-ray diffractometer. Kα X-ray photoelectron spectros-
copy (XPS) was used for identification of reduced palladium, and the
loading value of palladium in the catalyst was estimated by inductively
coupled plasma (ICP) analysis with
instrument.
a Varian ICP-OES 730-ES
2.2. Synthesis
2.2.1. Preparation of chloropropyl-functionalized silica-coated Fe3O4
MNPs
In a round-bottomed flask (100 mL), as-prepared Fe3O4/SiO2 MNPs
(0.4 g) (prepared according to our previous reports [11–13]) were well
dispersed in tetrahydrofuran (8.0 mL) (ultrasonication for 10 min). Then
NaH (0.2 g, 8.33 mmol) was added to the flask, CPTES (3.0 mL) was
added drop by drop at room temperature, and the mixture were well
stirred for 12 h at 60 �C. Ultimately, Fe3O4/SiO2/CPTES MNPs were
collected (with an external magnet), washed with ethanol and deionized
water, and then dried at 60 �C.
2.2.2. Preparation of o-PDA-functionalized Fe3O4 MNPs
In a round-bottomed flask (100 mL), Fe3O4/SiO2/CPTES MNPs
(0.3 g) were dispersed in ethanol (10 mL) (ultrasonication for 10 min).
Then o-PDA (0.03 g, 0.277 mmol) was added and the mixture were
stirred for 1 h at room temperature. The stirring was continued for an
additional 12 h under reflux. Finally, the Fe3O4/o-PDA MNPs prepared
were collected magnetically, washed with ethanol and deionized water,
and then dried in a vacuum oven for 24 h.
2.2.3. Preparation of the Pd-supported o-PDA-functionalized Fe3O4
magnetic nanocatalyst
In a round-bottomed flask (100 mL), Fe3O4/o-PDA MNPs (0.2 g)
were well dispersed in dry acetone (20 mL) and mixed with palladium
acetate (0.23 g, 1.28 mmol) by vigorous stirring for 12 h at room tem-
perature. The fabricated Fe3O4/o-PDA–Pd magnetic nanocatalyst was
then magnetically isolated from the reaction mixture, washed three
times with acetone and deionized water, and dried at 50 �C.
2. Materials and methods
2.2.4. General procedure for the synthesis of compounds 3a–3l via a
Suzuki–Miyaura cross-coupling reaction catalyzed by the Fe3O4/o-PDA–Pd
nanocatalyst
2.1. Materials and equipment
In a round-bottomed flask (100 mL), Fe3O4/o-PDA–Pd MNPs (0.01)
were dispersed in dichloromethane (5.0 mL) and ethanol (2.0 mL) by
ultrasonication for 20 min at room temperature. Then aryl halides 1
(1 mmol), phenylboronic acid 2 (1.2 mmol), NaBH4 (0.005 g, 0.1 mmol),
K2CO3 (0.21 g, 1.5 mmol), and PPh3 (0.0026 g, 0.1 mmol) were added to
the flask and the mixture were stirred for 10 min at room temperature.
After completion of the reaction, the magnetic nanocatalyst was
conveniently isolated by use of an external magnet, and excess
All solvents, chemicals, and reagents were purchased from Merck,
Sigma, and Aldrich. Melting points were measured with an Electro-
thermal 9100 apparatus and are uncorrected. Fourier-transform infrared
(FT-IR) spectra were recorded with a Shimadzu IR-470 spectrometer by
the KBr pellet method. 1H and 13C nuclear magnetic resonance (NMR)
spectra were recorded with a Bruker DRX-500 Avance spectrometer at
500 and 125 MHz, respectively. Scanning electron microscopy (SEM)
Fig. 1. Schematic of a Suzuki–Miyaura coupling reaction catalyzed by Fe3O4/o-PDA–Pd nanoscale system (r.t.: room temperature).
2