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Haipeng Fan et al. / Chinese Journal of Catalysis 38 (2017) 589–596
actions of aryl chlorides and bromides with phenols and
K4Fe(CN)6 using heterogeneous and homogeneous catalyst
systems [35–37]. Moreover, Pd nanoparticles formed in situ in
ionic solid polymers have been reported as highly active het‐
erogeneous catalysts for the Suzuki‐Miyaura reaction; only 10−7
Pd catalysts were required for the Suzuki‐Miyaura reaction of
aryl bromides [38]. In addition, heterogeneous catalysts with
highly active Pd nanoparticles immobilised in situ have been
designed and used in Suzuki‐Miyaura reactions; these catalysts
can be reused several times without evident deactivation [39].
It was found that electron‐rich and bulky phosphorus (P) lig‐
ands enhanced the activity of these Pd catalysts. Besides, Pd
nanoparticles immobilised in N‐containing polymers by
Pd‐catalysed C‐N coupling of tris(4‐bromophenyl)amine with
piperazine have been applied as catalysts, achieving a TON and
turnover frequency (TOF) as high as 250000 and 41666 h−1,
respectively, in Suzuki‐Miyaura reactions [40]. We are still in‐
terested in the development of highly active Pd catalysts for
Pd‐catalysed cross‐coupling reactions.
In homogeneous Pd catalyst systems, good ligands for
cross‐coupling reactions combine both favourable electronic
and steric properties. On the one hand, electron‐rich ligands
can aid oxidative addition to the Pd center to activate aryl hal‐
ides. On the other hand, bulky ligands can improve the reduc‐
tive elimination from the Pd center to form products. Moreover,
Pd nanoparticles supported on polyaniline (PAN) have been
reported as highly active and reusable catalysts for Suzu‐
ki‐Miyaura reactions, but the recycling process was not easy
[41–44]. Inspired by these advances, have we attempted to
combine a PAN support and bulky ligands with Pd nanoparti‐
cles to form highly active, reusable Pd catalysts. Pd nanoparti‐
cles are encapsulated in situ in cross‐linked PAN by
Pd‐catalysed C‐N coupling of tris(4‐iodophenyl)amine with
p‐phenylenediamine as highly active catalysts for Suzu‐
ki‐Miyaura reactions (see Scheme 1). The resulting Pd catalysts
exhibit high efficiency for the Suzuki‐Miyaura coupling of aryl
bromides and chlorides with aryl boronic acids. No Pd leaching
is detected in the reaction solution after filtration of the Pd
catalysts, revealing that we attained active, clean Pd catalysts
for Suzuki‐Miyaura reactions.
All chemicals used in this work were purchased from Alfa
Aesar, Aladdin Reagent Company and Sigma‐Aldrich and used
without further purification. H NMR spectra were measured
1
with a Bruker AVANCE 400D spectrometer in CDCl3 using tet‐
ramethylsilane as an internal reference. Thermogravimetric
analysis (TGA) was performed with a STA409 instrument un‐
der dry nitrogen at a heating rate of 20 °C/min. Gas sorp‐
tion/desorption analysis was performed on a Micromeritics
ASAP2010 analyser at −196 °C with liquid nitrogen. Samples
were pretreated at 140 °C under vacuum before analysis. The
amount of Pd was measured with a Jarrell‐Ash 1100 inductive‐
ly coupled plasma‐atomic emission spectrometer (ICP‐AES).
Transmission electron microscope (TEM) images were cap‐
tured using a JEOL JEM‐2010 (200 kV) TEM and scanning elec‐
tron microscope (SEM) images were obtained using a Hitachi
S‐4800 field‐emission SEM. Fourier transform infrared (FT‐IR)
spectra were recorded in the 500–4000 cm−1 region using a
Nicolet 360 FT‐IR spectrometer with a scan rate of 0.4747
cm/s. X‐ray photoelectron spectroscopy (XPS) was conducted
with an ESCALab 220i‐XL electron spectrometer from VG Sci‐
entific using 300‐W Al Kα radiation. Binding energies were cal‐
ibrated using the C1s peak at 284.6 eV. X‐ray diffraction (XRD)
patterns were collected on a Bruker D8 Advance powder dif‐
fractometer using a Ni‐filtered Cu Kα radiation source at 40 kV
and 20 mA from 5° to 80° with a scan rate of 0.5°/min.
2.2. Preparation of heterogeneous Pd catalysts
Synthesis of Pd@PAN‐Ad‐0.5 catalyst: The C‐P coupling re‐
action was conducted according to a reported method with
modification [45]. Di‐1‐adamantylphosphine (HPad2, 77.2 mg,
0.25 mmol), tris(4‐iodophenyl)amine (1.25 mmol, 778.8 mg),
Pd(OAc)2
(32.7
mg,
0.145
mmol),
1,1’‐bis(diisopropylphosphino)ferrocene (dippf, 6.4 mg, 0.015
mmol) and NaOtBu (36 mg, 0.37 mmol) were added into a
100‐mL Schlenk tube containing toluene (15 mL) under argon.
After the Schlenk tube was heated at 100 °C with stirring for 22
h, the reaction mixture was cooled to room temperature. Then,
p‐phenylenediamine (189.3 mg, 1.75 mmol), NaOtBu (673.0
mg, 7 mmol) and toluene (20 mL) were added to the tube un‐
der argon. After heating at 100 °C with stirring for another 24 h
under argon, the reaction mixture was again cooled to room
temperature. The dark blue solid catalyst was obtained via
centrifugation, and then washed with water and ethanol three
times. The Pd@PAN‐Ad‐0.5 catalyst was obtained after drying
under vacuum for 24 h at room temperature. The Pd content of
the Pd@PAN‐Ad‐0.5 catalyst measured by ICP‐AES was 0.58
wt%. A Pd@PAN‐Cy‐0.5 catalyst was prepared similarly using
dicyclohexylphosphine instead of di‐1‐adamantylphosphine.
The amount of Pd in the Pd@PAN‐Cy‐0.5 catalyst was 0.53
wt%.
2. Experimental
2.1. General methods and reagents
NH
NH2
N
P
H
NH
HN
I
NH2
NH
HN
The catalyst Pd@PAN‐Ad‐0.2 was prepared similarly to the
Pd@PAN‐Ad‐0.5 catalyst, but 16.3 mg of Pd(OAc)2 was added
instead of 32.7 mg. The Pd content in the Pd@PAN‐Ad‐0.2 cat‐
alyst measured by ICP‐AES was 0.25 wt%.
Pd@PAN-Ad
N
N
Pd(OAC)2
N
P
P
I
I
I
Scheme 1. Synthesis of the palladium nanoparticle/ polyani‐
line/di‐1‐adamantylphosphine (Pd@PAN‐Ad) catalysts.
2.3. Typical procedure for Suzuki‐Miyaura coupling of aryl