A highly controllable, effective, and recyclable magnetic-nanoparticle-supported palladium catalyst for the Suzuki–Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.jcat.2021.03.017

In this work, a highly efficient palladium catalyst was facilely immobilized on the surfaces of magnetic nanoparticles with the coordination of N-heterocyclic carbene. The prepared Pd–NHC@NCPs were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma atomic emission spectrometry. With suitably designed emulsion polymerization and immobilization, the final palladium loading could reach 0.78 mmol/g. The catalyst showed outstanding catalytic performance in the Suzuki–Miyaura cross-coupling reaction among various substrates, with little catalyst usage (0.03 mol.%), short reaction time, and mild reaction conditions. In addition, the catalyst could be separated conveniently from the reacting system with an external magnet and show good catalytic performance even after being reused five times or more, indicating good recyclability.

Full text of DOI:10.1016/j.jcat.2021.03.017

Journal of Catalysis 397 (2021) 36–43
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
A highly controllable, effective, and recyclable
magnetic-nanoparticle-supported palladium catalyst
for the Suzuki–Miyaura cross-coupling reaction
1
1
⇑
Lu Ye , Xiaojing Liu , Yangcheng Lu
State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a highly efficient palladium catalyst was facilely immobilized on the surfaces of magnetic
nanoparticles with the coordination of N-heterocyclic carbene. The prepared Pd–NHC@NCPs were char-
acterized by transmission electron microscopy, Fourier transform infrared spectroscopy, energy disper-
sive spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma atomic emission
spectrometry. With suitably designed emulsion polymerization and immobilization, the final palladium
loading could reach 0.78 mmol/g. The catalyst showed outstanding catalytic performance in the Suzuki–
Miyaura cross-coupling reaction among various substrates, with little catalyst usage (0.03 mol.%), short
reaction time, and mild reaction conditions. In addition, the catalyst could be separated conveniently
from the reacting system with an external magnet and show good catalytic performance even after being
reused five times or more, indicating good recyclability.
Received 21 February 2021
Revised 8 March 2021
Accepted 9 March 2021
Available online 16 March 2021
Keywords:
Magnetic-nanoparticle-supported
palladium catalyst
Emulsion polymerization
N-heterocyclic carbine
Suzuki–Miyaura cross-coupling reaction
Recyclability
Ó 2021 Elsevier Inc. All rights reserved.
1. Introduction
separability of the carriers. Due to the larger specific surface, appli-
cation of nanomaterial could result in larger anchoring site and
Among different kinds of organic synthesis, the Suzuki–Miyaura
cross-coupling reaction has received considerable attention due to
its universality in industrial production and huge potential for syn-
thesizing hundreds of intermediates and compounds [1–3]. To
realize high operability and efficiency of the reaction, many transi-
tion metal catalysts and their complexes have been utilized in var-
ious systems [4]. N-heterocyclic carbene (NHC)–Pd complexes
have become the most prospective choice, which exhibit a perfect
combination of the catalytic activity of the noble metal and the low
lower steric hindrance than with bulky carriers. Nevertheless, nan-
odimension also brings about difficulties in separation. To handle
such problems, magnetic nanoparticles, especially Fe3O4, may pro-
vide a solution to fill the gap between homogeneity and hetero-
geneity [14–18]. This kind of nanoparticle is easy to separate
with an external magnetic field due to its superparamagnetism,
which is quite economical and time-saving compared with tradi-
tional separation methods such as centrifugation and filtration.
Recently, several works have focused on the preparation of
NHC–Pd complexes immobilized on magnetic nanoparticles (Pd–
NHC@NCPs) [19–25]. Laska [26] grafted the catalysis moiety onto
the surface of the Fe3O4 nanoparticles directly. However, in this
case, the agglomeration of magnetic nanoparticles happened
easily, which resulted in poor stability and a decrease in catalyst
activity after several cycles. Considering the instability of pure
Fe3O4 nanoparticles, especially under oxidizing and acidic condi-
tions [14], surface functionalization or encapsulation was paid
attention as a way to realize reliable immobilization of NHC–Pd
onto the surface of Fe3O4. In the existing literature, ways of func-
tionalization can generally be summarized into two kinds. In one
kind, magnetic nanoparticles were functionalized with silane
group agents. Deng [25] successfully immobilized acenaphthoimi-
dazolyildene palladacycle onto the surface of magnetic nanoparti-
cles, with a long-chain triethoxysilyl tail. The palladium loading
toxicity, strong r-donation, high dissociation energy, and environ-
mental stability of the N-heterocyclic carbine [5,6].
However, considering the price of and foreseeable pollution by
the noble metal palladium, recycling the catalysts has become a
significant issue [7]. The traditional homogeneous systems could
hardly break through this limitation even by exploitation of com-
plicated separation methods. As a result, numerous efforts have
recently been put into the improvement of heterogeneous catalyst
systems, including introducing various kinds of carriers, such as
polyethylene glycol, polystyrene, MOFs, Al2O3, and SiO2 [8–13].
However, there exists a contradiction between the size and
⇑
Corresponding author.
E-mail address: luyc@tsinghua.edu.cn (Y. Lu).
These authors contributed equally.
1
https://doi.org/10.1016/j.jcat.2021.03.017
0021-9517/Ó 2021 Elsevier Inc. All rights reserved.

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
was around 0.01 mmol/g and the usage of catalyst in the reaction
was around 0.5 mol.%. Wilczewska [22] utilized magnetic nanopar-
ticles with aminosilane shells as carriers; NHC ligand precursor
and palladium acetate were subsequently introduced onto the sur-
face of the carrier. The palladium loading ratio reached 6 wt%. Nev-
ertheless, in this research, complicated organic synthesis steps
were common and the huge steric hindrance brought about by
the functional group limited the regulation of palladium loading
on each carrier, which led to a large dosage of the catalyst. In
another kind of functionalization, magnetic nanoparticles were
encapsulated with polymer shell. Liao [27] fabricated a networked
metal–organic gel on the basis of PEG-modified magnetic nanopar-
ticles, in which palladium was buried in the gel with organic
ligands through self-assembly. In Suzuki–Miyaura cross-coupling,
the catalyst amount was 1 mol.% Pd and the catalyst could be
reused up to four times. However, the performance of the catalyst
was hindered to some extent by the buried catalytic active sites.
Yang and co-workers [28] immobilized palladium on magnetic
nanoparticles that were coated with poly(undecylenic acid-co-N-i
sopropylacrylamide-co-potassium 4-acryloxyoylpyridine-2,6-dicar
boxylate). The yield of the following Suzuki reaction could reach
97% with 0.1 mol.% catalyst addition, but the reaction conditions
and rate were not satisfactory. The same situation could also be
found in Wang’s work [23], where bulky NHC ligands were grafted
to the surface of poly(DVB-co-VBC)@ Fe3O4. Although the catalysts
were at the nanoscale, it still took 12 h to reach a high yield. A
more effective way to prepare Pd–NHC@NCPs with controllable
composition, structure, and catalytic efficiency at the same time
is lacking.
In our previous work [29], our research group successfully
developed an efficient and flexible method to get water-
dispersed Fe3O4 nanoclusters with co-precipitation and micromix-
ing, which was available to produce well-dispersed and well-
modified clusters of sizes ranging from 30 to 200 nm with
decreased demand for surfactants. Screening the reaction condi-
tions in a controlled and expanded window associated with these
nanoclusters, in this work, we successfully prepared a protective
polymer shell with evenly distributed functional sites via emulsion
copolymerization for each nanocluster. Through the full and care-
ful utilization of the nanoparticle surface, the capacity of the palla-
dium was controllably improved and the catalyst addition in the
following cross-coupling reaction was greatly decreased due to
the controllable dispersion of the catalytic sites. With the prepared
Pd–NHC@NCPs, the Suzuki–Miyaura reaction could be conducted
with higher reactivity, milder conditions, and shorter reaction
time. Moreover, thanks to the reliable chemical bonding, the inhi-
bition of palladium leaching and excellent recovery of catalysts
were as expected.
sulfate (SDS, C12H25SO4Na, 99%), N-methylpyrolidone (C5H9NO,
99%), 1-methylimidazole (C4H6N2, 99%), sodium carbonate (Na2-
CO3, 99.5%), N, N-dimethylformamide (DMF, C3H7NO, 99.9%),
potassium carbonate (K2CO3, 99%), isopropyl alcohol (i-PrOH,
C3H8O, 99.9%), and dimethyl phthalate (C10H10O4, 99%) were
purchased from J&K Scientific, China. Ethanol (C2H6O, >99.8%)
and 1-octane (C8H18, >99%) were purchased from Aladdin, China.
4-(Chloromethyl)styrene (VBC, C9H9Cl, 90%, stabilized with p-tert-
butylcatechol), 4-bromotoluene (C7H7Br, 99%), 4-bromoanisole
(C7H7BrO, 97%), p-bromoacetophenones (C8H8Br2O, 98%),
4-nitrobromobenzene (C6H4BrNO2, >99%), phenylboronic acid
(C6H7BO2, 99%), and bromobenzene (C6H5Br, 99%) were purchased
from TCI, China. Hydrochloric acid (HCl, 37%) and concentrated
nitric acid (HNO3, 68%) were purchased from Tongguang
Fine Chemicals Company, China. Palladium (II) acetate
(Pd(OAc)2, 99%) was purchased from 3A Chemical, China. 4-
Methoxyphenylboronic acid (C7H9BO3, 98%) and 4-tolylboronic
acid (C7H9BO2, 99.5%) were purchased from Amethyst Chemicals,
China. Among all the reagents, St, DVB, and VBC were washed with
5% NaOH solution and deionized water three times, respectively,
following by reduced pressure distillation. AIBN was recrystallized
with ethanol twice. Water used throughout the experiment was
prepared by an ultrapure water system (Center 120FV-S).
2.2. Characterization
The morphology of the immobilized NCP was observed with a
transmission electron microscope (TEM, JEM2010, JEOL), with an
energy-dispersive spectroscopy (EDS, HORIBA X-manN) attach-
ment to analyze elemental distribution. The composition of the
particles was analyzed with Fourier transform infrared spec-
troscopy (FT-IR, Nexus670, Nicolet) with wavelengths ranging
from 4000 cmꢀ1 to 400 cmꢀ1; samples were pressed to tablets with
KBr before the measurement. The surface chemical properties were
analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250
Xi, Thermo Fisher Scientific Inc., USA), where AlKa radiation (72 W,
12 kV) at a pressure of 10ꢀ9 Torr was applied. The diameter of the
analyzed area was 400 mm. Inductively coupled plasma-atomic
emission spectrometry (ICP-AES, iCAP6300, Thermo Fisher Scien-
tific) was applied to measure the amount of immobilized palla-
dium. The specific surface area of the sample was measured with
a chemisorption analyzer in liquid nitrogen and calculated with
the BET equation. X-ray powder diffraction (XRD) patterns were
collected on an X-ray powder diffractometer (D8-Advance, Bruker)
operating at 40 kV and 40 mA using CuKa radiation at a scanning
rate of 5 minꢀ1. The yield of the Suzuki–Miyaura cross-coupling
reaction was measured by gas chromatography (GC, Agilent Tech-
nologies, 8860) with an HP-5MS capillary column (5% phenyl, 95%
methylpolysiloxane; 30 m ꢁ 0.25 mm i.d. ꢁ 0.25
lm film thick-
ness). The parameters of the gas chromatography were as follows:
oven temperature was programmed to be maintained at 100 °C for
0.5 min, heated at a rate of 15 °C/min, and maintained at 250 °C for
10 min; inlet temperature, 280 °C; detector temperature, 280 °C;
2. Experimental
2.1. Materials
carrier gas, nitrogen, 1 mL/min; injection volume, 0.2
lL; split
Fe3O4 magnetic nanoparticles were prepared and dispersed fol-
lowing the procedure introduced in our previous work [29].
Polysorbate-80 (Tween 80, 98%), styrene (St, C8H8, >99%, stabilized
with p-tert-butylcatechol), sodium hydroxide (NaOH, >96%),
methyl alcohol (CH3OH, 99.9%), and 4-nitrobromobenzene (C6H4-
BrNO2, 99%) were purchased from Sinopharm, China. Tetrahydro-
furan (THF, C4H8O, >99.9%) was purchased from Fisher Chemical,
USA. Divinylbenzene (DVB, C10H10, containing ethylvinylbenzene
and diethylbenzene, stabilized with p-tert-butylcatechol), 2,20-azo
bis(2-methylpropionitrile) (AIBN, C8H12N4, >99%), sodium dodecyl
ratio: 100:1. An isopropanol solution of dibutyl phthalate
(3.7 wt%) was used as an internal standard. Turnover number
(TON) and turnover frequency (TOF) calculated through the results
of GC were applied to evaluate the performance of the catalyst:
TON = mole number of the product/mole number of the catalyst
ð2:1Þ
TOF = TON/reaction time (h).
ð2-2Þ
37

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
2.3. Chloromethylation of magnetic polymer nanocomposite particles
As introduced in our previous work, the prepared modified
Fe3O4 magnetic nanoparticles (100 mg) were dispersed in THF
(10 mL) with Tween 80 (10 mg) and mixed with ultrapure water
(40 mL) to prepare magnetic nanoclusters [29]. Then the dispersion
was shifted into a three-necked flask with supplemental Tween 80
to reach a total amount of 40 mg of the surfactant. Another surfac-
tant, SDS (2.2 mg), was added to the system to reach a mixing mole
ratio of 4 to 1 between Tween 80 and SDS. Three kinds of mono-
mers, St (250 lL), DVB (200 lL), and VBC (50 lL), were mixed with
initiator AIBN (15 mg) to obtain a homogeneous oil phase. Then the
mixture was added slowly into the solution containing nanoclus-
ters, with stirring at a speed of 700 rpm. The mixture was stirred
for 1 h to get a stable dispersion. Finally, the flask was put into a
60 °C water bath to initiate polymerization with stirring
(400 rpm). This reaction was maintained for 5 h to obtain magnetic
polymer nanocomposite particles. The product could easily be sep-
arated with an external magnetic field. After being washed three
times each with water and ethanol, the product was dried for
12 h in a vacuum oven at 40 °C.
Scheme 1. Preparation of Pd–NHC complex immobilized magnetic polymer
nanocomposite particles.
3. Results and discussion
3.1. Analysis of Pd–NHC@NCPs
During the chloromethylation, the functional monomer VBC,
containing a chloromethyl group, was added into the emulsion
polymerization system. After 5 h of co-polymerization, a brown
latex was obtained, which could be separated completely from
the reaction medium. Also, due to the precise addition of the sur-
factant, no pure polymer microspheres could be found in the clear
water. As can be seen from Fig. 1a, the product NCPs possessed uni-
form morphology with a size around 60 nm and Fe3O4 nanoclusters
were embedded in the center. XPS analysis confirm the existence
of the elements of Cl, C, and O on the surface of the prepared
NCPs-Cl, as shown in Fig. 2. FT-IR was utilized to analyze the pro-
duction of each step during preparation of Pd–NHC@NCPs. From
Fig. 3, with the comparison between 3a and 3b, the characteristic
peak at 1265 cmꢀ1 in 3b also indicated the existence of the
chloromethyl unit in the polymer by adding VBC. The chloromethyl
unit was assumed to connect with NHC precursors through alkaliza-
tion with N-imidazole, especially 1-methylimidazole in this work,
which is the simplest and most universal N-imidazole. As shown
in Fig. 3c, the peak at 1635 cmꢀ1 was attributed to C@N in 1-
methylimidazole [30,31]. This proved that the NHC precursor had
been successfully grafted onto the surface of the nanocomposite.
After the reaction between NHC@NCPs and Pd(OAc)2, it could be
seen from Fig. 1b that the final particles were almost the same size
as NCPs-Cl, indicating that the immobilization did not make any
difference in the morphology. Besides, in Fig. 3d, the characteristic
peak assigned to C@N at 1635 cmꢀ1 disappeared and the peak at
1685 cmꢀ1 might indicate the successful immobilization of palla-
dium onto the surface of NHC@NCPs particles, which caused the
shift. Similar evidence could also be found in the energy spectrum
in Fig. 4, where an obvious peak of Pd could be found in Fig. 4b.
Also, with the EDS elemental dot-mapping of the catalyst, it was
clear that the distributions of Fe, O, and Pd were quite consistent
and the outline of each distribution was similar to the morphology
of the nanocomposite, which indicated the uniform distribution of
Pd on the particle surface (see Fig. 5). Through the measurement of
ICP-AES of the final catalyst, palladium loading was up to
0.78 mmol/g in Pd–NHC@NCPs, proving excellent loading effi-
ciency of palladium on the nanoparticle. The loading efficiency
was much higher than for catalysts using polystyrene resin as car-
rier [32,33]. Also, it offered conditions for flexible adjustment in
palladium loading with different feed ratios. The adjustable load-
ing was owing to the considerable specific surface area of the
nanoscale carrier, which was analyzed to be 74.6 m2/g with BET.
Through the aforementioned characterization, by the method of
emulsion polymerization and solid-phase synthesis, Pd–
2.4. Preparation of 1-methylimidazole-mobilized nanocomposite
particles–Cl (NHC@NCPs)
To realize successful immobilization, the above-prepared NCPs–
Cl (150 mg) were mixed with 1-methylimidazole (0.45 mL) and N-
methyl pyrolidone (25 mL). The mixture was dispersed uniformly
with ultrasonic and shifted into an 80 °C water bath with stirring
(100 rpm) for 12 h. After the reaction, the product was washed
three times with N-methylpyrolidone (20 mL), 0.1 M HCl solution
(20 mL), and methyl alcohol (20 mL) in turn and finally dried for
12 h in a vacuum oven at 40 °C.
2.5. Preparation of palladium-mobilized NHC@NCPs (Pd–NHC@NCPs)
With ultrasound, the dried NHC@NCPs (100 mg) and Pd(OAc)2
(22.5 mg) were uniformly dispersed into 2 mL DMF. Then Na2CO3
solution (2 mL, 53 mg/mL) was added into the system. The mixture
was put under ultrasound at 0 °C for 30 min and shifted to a shak-
ing bed with a temperature of 50 °C for another 2 h. The product
was separated from the system with an external magnetic field.
After being washed five times each with water (20 mL) and methyl
alcohol (20 mL), the product was dried for 12 h in a vacuum oven at
40 °C. The general synthesis method for the Pd–NHC@NCPs is
shown in Scheme 1.
2.6. General procedure for Suzuki–Miyaura cross-coupling reaction
catalyzed by Pd–NHC@NCPs
A certain amount of prepared Pd–NHC@NCPs was mixed with
1 mmol aryl bromides, 1.2 mmol arylboronic acid, 2 mmol
K2CO3, and 4 mL solvent. The mixture was sealed in a 10-mL
round-bottomed flask and put into a shaking bed to react at a con-
stant temperature. After the reaction, the flask was shifted to an
ice-water bath at 0 °C to terminate the reaction. The product was
extracted with n-hexane, followed by weighing and analysis. To
recycle the catalyst and avoid the loss of catalyst that remained
on the oil–water interface, the hexane phase was first evaporated
by nitrogen blowing. In this way, the catalyst could return to the
water phase and be separated quickly with an external magnetic
field. Then the separated catalyst was washed alternately with
ethanol and ultrapure water five times each and put in a 40 °C vac-
uum oven for 12 h for recycling.
NHC@NCPs were successfully prepared with
a high loading
38

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
Fig. 1. TEM images of (a) NCPs-Cl and (b) Pd–NHC@NCPs.
Fig. 2. XPS spectra of NCPs-Cl (a) full-spectrum scanning and (b) high solution scanning of chlorine.
3.2. Optimization of the reaction conditions
To optimize the solvents and bases in applying the prepared
Pd–NHC@NCPs in Suzuki–Miyaura cross-coupling reaction, the
most typical cross-coupling of bromobenzene and phenylboronic
acid was utilized. As shown in Table 1, when i-PrOH or DMF was
applied as the solvent, the yield of target product biphenyl could
only reach 16% or 15% with 0.5 mol.% (mole ratio of the immobi-
lized palladium to bromobenzene in the substrate) of the afore-
mentioned catalyst, even when reacting under 70 °C for 12 h.
However, when pure water was utilized as the solvent, rapid phase
separation happened between the solvent and the other compo-
nents, along with sedimentation of catalysts and substrates in
the bottom. Due to the low polarity of bromobenzene, effective
contact with the other reactants could not be guaranteed unless
a polar organic solvent was introduced, whereas for the base
K2CO3, only in water could it accelerate the reaction. Based on
the above, mixed water and organic solvent would be the best
choice of solvent. In entry 3, mixed solvents, DMF, and water were
prepared with equal volume. Under this condition, with 0.5 mol.%
catalyst, the yield of biphenyl could reach 99% after 6 h reaction
under 70 °C. The yield decreased with decreasing temperature
and reaction time, as shown in entry 4. In comparison, as shown
in entries 5, 6, and 7, when ethanol and water were mixed with
equal volume, the reaction was speeded up obviously. Even when
the concentration of catalyst was cut down to 0.03 mol.%, the yield
Fig. 3. FT-IR spectra of the prepared (a) NCPs; (b) NCPs-Cl; (c) NHC@NCPs; (d) Pd–
NHC@NCPs.
efficiency. For the traditional bulk resin carrier, the functional sites
were always trapped in the network of the polymer, which brought
huge challenges for the mass transfer of palladium ions and limited
the loading ratio as a result. However, in this work, palladium was
immobilized efficiently due to the finely designed and distributed
functional sites on the surface.
39

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
Fig. 4. EDS energy spectra of (a) NCPs-Cl and (b) Pd–NHC@NCPs.
Fig. 5. TEM image of Pd–NHC@NCPs and its EDS elemental mappings.
of the product could still reach 93% in 5 min and 100% in 10 min,
while without catalyst, the reaction could hardly proceed. In brief,
equal volumes of ethanol and water were most appropriate for the
prepared catalyst in our work. Under this condition, the cross-
coupling reaction could be completed with little catalyst, mild
reaction conditions, and an extremely short time.
the yield of all the reactions could exceed 92% within 30 min.
When a strong electron-withdrawing group such as acetyl was
connected to aryl bromide, the reaction could be accomplished
swiftly in 10 min, with TOF of 2.000 ꢁ 104 (entry 14). Also, thanks
to the nitro group in 4-nitrobromobenzene, which is a strong
electron-withdrawing group, the yield could reach 94% in 10 min
(entry 13). However, due to the low solubility of 4-
nitrobromobenzene in the EtOH–H2O mixture, the DMF–H2O mix-
ture was chosen as the solvent instead in order to attain sufficient
interaction between the substrates in this entry. In contrast, when
the electron-donating group was connected to aryl bromide,
methyl or methoxy for instance (in entry 9 or 10), the reaction rate
was relatively low due to the low activity of the substrates.
3.3. Application of Pd–NHC@NCPs in different derivatives
Inspired by the outstanding catalytic activity of Pd–NHC@NCPs
in cross-coupling reaction between bromobenzene and phenyl-
boronic acid, Suzuki–Miyaura reactions among their derivatives
were conducted under similar conditions. As shown in Table 2,
40

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
Table 1
Suzuki–Miyaura cross-coupling reaction of bromobenzene and phenylboronic acid under different reaction conditions:
Entry
Solvent
Temperature/°C
Time
Pd/mol.%a
Yield/%b
1
2
3
4
5
6
7
8
i-PrOH
DMF
70
70
70
50
50
50
50
50
12 h
12 h
6 h
0.50
0.50
0.50
0.50
0.03
0.03
0.03
0
16
15
99
84
52
93
100
0
DMF/H2O (1:1)
DMF/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
5 h
2 min
5 min
10 min
5 h
Note: Reaction conditions: bromobenzene (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 base (2 mmol), solvent (4 mL).
a
The concentration of the catalyst was the mole ratio of the immobilized palladium to bromobenzene in the substrate.
The yield was calculated from gas chromatography.
b
Table 2
Suzuki-Miyaura cross-coupling reaction of derivatives of aryl bromides and arylboronic acid:
Entry
R1
R2
Time/min
Yield/%
TON
TOF
7
9
H
H
H
H
10
30
30
30
30
10
10
100
93
98
95
100
94
3333
3100
3267
3167
3333
3133
3333
2.000 ꢁ 104
4-CH3
4-OCH3
H
6200
10
11
12
13a
14
6534
6334
4-CH3
4-OCH3
H
H
6666
4-NO2
4-COCH3
1.880 ꢁ 104
H
100
2.000 ꢁ 104
Notes: Reaction conditions: aryl bromide (1 mmol), arylboronic acids (1.2 mmol), K2CO3 base (2 mmol), solvent: EtOH:H2O = 1:1 (volume) (4 mL) (except entry 13),
temperature 50 °C. Concentration of the catalyst: 0.03 mol.% (mole ratio of catalyst to aryl bromide).
a
Solvent: DMF:H2O = 1:1 (volume) (4 mL).
Nevertheless, a satisfactory yield could still be obtained when the
reaction time was extended to 30 min. In conclusion, with the pre-
pared Pd–NHC@NCPs as catalysts, Suzuki–Miyaura cross-coupling
reactions of various derivatives of aryl bromides and arylboronic
acids could be realized with high reaction rates and mild reaction
conditions.
fringes with d spacing of 0.23 nm observed by HRTEM both corre-
sponded to the (111) crystal plane of palladium nanoparticle. It
has already been proved in Fig. 5 that palladium was distributed
evenly on the surface of the nanocomposite in the form of com-
plexes instead of nanoparticles. As for the difference in Fig. 7 before
and after the recycling, it could be speculated that the partial pal-
ladium complex transformed into palladium nanoparticles during
3.4. Recyclability of Pd–NHC@NCPs
Owing to the magnetic separability of Pd–NHC@NCPs, the cata-
lysts could be separated from the reacting system with an external
magnet after each reaction; reusing the catalysts became possible.
To measure the recyclability of the aforementioned Pd–NHC@NCPs
in this work, recycling experiments of cross-coupling reaction
between bromobenzene and phenylboronic acid were conducted.
As shown in Fig. 6, in the fifth cycle, the yield of biphenyl could still
reach 92% within 10 min under 70 °C. However, a decline in the ini-
tial reaction rate could be found in each cycle. It seemed that with
different cycles, it took longer to activate the reaction, although
high yield could still be obtained within 10 min. Through ICP-
AES measurement, it was observed that 83.2% of the palladium
remained in the system. On one hand, due to the small catalyst
usage, the trace loss during washing and separating might have a
great influence. On the other hand, it could be found from the
TEM that after recycling, a small quantity of palladium nanoparti-
cles (4 nm in size) could be found in the view, as shown in Fig. 7b
and 7c. The formation of palladium nanoparticles was further
proved by X-ray diffraction and HRTEM. As shown in Fig. 8, the
characteristic peak at 2 = 39.4° of the XRD pattern and the lattice
Fig. 6. The yield and initial reaction rate of recycling the Suzuki–Miyaura cross-
coupling reaction of bromobenzene and phenylboronic acid with prepared Pd–
NHC@NCPs. Reaction conditions: aryl bromide (1 mmol), arylboronic acids
(1.2 mmol), K2CO3 base (2 mmol), solvent: EtOH:H2O = 1:1 (volume) (4 mL)
(except entry 15), temperature 70 °C. Concentration of the catalyst: 0.03 mol.%
(mole ratio of catalyst to aryl bromide)).
41

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
Fig. 7. TEM images of Pd–NHC@NCPs (a) before and (b, c) after five times recycling.
Fig. 8. (a) XRD spectra and (b) HRTEM image of Pd–NHC@NCPs after five times recycling.
recycling, which might weaken the catalytic activity to some
extent. The N-imidazole, 1-methylimidazole in this work, despite
its simplicity, could only provide limited steric hindrance and elec-
tronic effect, which could not maintain excellent catalytic activity
in the long term. Such problems are expected to be solved by
changing the NHC ligands into ones possessing large steric hin-
drance in our future research to combine stabilization with our
advantages of controllable immobilization.
between bromobenzene and phenylboronic acid in different works
was carried out. From Table 3, it could be seen that the catalyst
used in this work had excellent catalytic activity, whether com-
pared with polymer or silane-based palladium catalysts. High yield
could be obtained within minutes with a very small amount of cat-
alyst even under mild reaction conditions. The outstanding perfor-
mance could be attributed to the nanoparticle carriers, which
possessed large specific surface area, uniform size distribution,
and controllable functional sites on their surfaces, leading to con-
trollable palladium immobilization. What is more, the surfactant
Tween 80 was applied during the polymerization, which was usu-
ally supposed to provide more effective interaction among oil-
soluble aryl halides, water-soluble arylboronic acids, and base, pro-
moting the reaction to some extent.
3.5. Comparison of the catalytic effect of Pd–NHC@NCPs
For the evaluation of the catalytic performance of the prepared
Pd–NHC@NCPs catalyst in our work, a comparison among palla-
dium catalysts in the Suzuki–Miyaura cross-coupling reaction
Table 3
Comparison among palladium catalysts in the Suzuki–Miyaura cross-coupling reaction between bromobenzene and phenylboronic acid.
Catalyst
[Cat.] mol.%
Solvent
Temperature/°C
Time/h
Yield/%
TON
TOF
Ref.
Pd@PS-Metc
NO2–NHC–Pd@Fe3O4
NHC–Pd@ p-(DVB-co-VBC)Fe3O4
NHC–Pd@silane-
Pd@ IPr-Fe3O4
Pd@gel-Fe3O4
0.1
0.15
0.02
7.3
0.16
1
0.1
1
0.03
H2O
80
50
70
50
80
60
90
60
50
4
1
12
12
2
5
1
4
96
95
99
93
90
89
95
73
100
960
633
4950
12.7
562.5
89
950
73
3333
240
633
412.5
1.10
281.2
18
[33]
[24]
[23]
[19]
[21]
[27]
[28]
[34]
c
EtOH/H2O (1:1)
EtOH/H2O (3:1)
DMF
i-PrOH
MeOH
H2O
EtOH/H2O (2:1)
EtOH/H2O (1:1)
a
b
c
-Fe2O3
c
c
c
Pd–PUNP@Fe3O4
950
18
GO–NH2–Pdc
Pd–NHC@NCPsc
10 min
2.000 ꢁ 104
Present work
a
b
c
Suzuki–Miyaura cross-coupling reaction between 40-bromoacetophenone and phenylboronic acid.
Suzuki–Miyaura cross-coupling reaction between 1-bromo-2-methylbenzene and phenylboronic acid.
Suzuki–Miyaura cross-coupling reaction between chlorobenzene and phenylboronic acid.
42

L. Ye, X. Liu and Y. Lu
Journal of Catalysis 397 (2021) 36–43
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4. Conclusions
A kind of effective Pd–NHC@NCPs was successfully synthesized
and was proved to possess outstanding performance in the Suzuki–
Miyaura cross-coupling reaction. The methods were facile, includ-
ing co-polymerization of the magnetic nanocomposite with the
functional group chloromethyl on the surface, preparation of cova-
lent compounds NHC@NCPs, and solid-phase coordination of palla-
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grafted to the surface of the nanoparticles, which had a surface
area of 74.6 m2/g. The palladium loading could reach 0.78 mmol/
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reported. Also, in the series of Suzuki–Miyaura cross-coupling
reactions, the prepared catalysts showed excellent catalytic perfor-
mance. Especially in the reaction between bromobenzene and
phenylboronic acid, complete cross-coupling could happen only
within 10 min, with 0.03 mol% Pd–NHC@NCPs and under 50 °C.
After five times recycling, the catalytic activity remained high. All
the phenomena proved the effective and controllable immobiliza-
tion of palladium on the carrier, which gave high catalytic activity
rivaling that of conventional homogeneous palladium catalysts and
outstanding recyclability at the same time. In such circumstances,
the consumption of the expensive palladium was avoided to the
greatest extent, which will be promising in industrial production.
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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supported on magnetite for Suzuki-Miyaura and Mizoroki-Heck cross-
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Acknowledgment
This work was supported by the National Natural Science Foun-
dation of China under Grant (21422603, U1662120).
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43