W. Zhang et al. / Molecular Catalysis 445 (2018) 170–178
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(725 mg, 5 mmol). In a beakerflask, 1,4-diaminobenzene (504 mg,
5 mmol) was dissolved in 40 mL dimethyl sulfoxide (DMSO). After
the solutions in the three glasswares were mixed together, the reac-
tion mixture was stirred for 48 h at room temperature. Then, the
product (PImCl) was filtered out as the brown precipitate (690 mg,
77%), which was successively washed with THF, distilled water and
methanol. 1H NMR (500 MHz, DCOOD [ppm]): ı = 8.29 (s, 1H); 8.16
(br, 2H); 7.90 (br, 4H) (Fig. S1).
Fig. 1. General supporting materials for heterogeneous NHC-metal complex cata-
lysts.
2.2.2. Preparation of NHC-Pd polymer
between the NHC compounds and supporters greatly limited its
immobilization.
To PImCl (500 mg) solution, an excess of potassium tert-
butoxide (8.4 mmol) was added. After stirring for 48 hours,
3-chloropydine (12 mL) and an excess of palladium chloride
(504 mg) were added into the solution. Then, the reaction mix-
ture was heated to 60 ◦C and kept stirring for 48 h under argon
atmosphere. The dark precipitate (NHC-Pd polymer) was isolated
by filtration. The careful washing was carried out with THF, distilled
water and methanol.
Of particular interest are the recent attempts at preparing NHC-
based metal organic frameworks (MOFs) for use in heterogeneous
Suzuki-Miyaura reaction [40–42]. Wu reported NHC-Pd-based
supporting materials, MOFs could extremely improve the load-
ing capacity of NHC units. In recent years, significant efforts have
(COF) materials as the solid supporters [44–47]. In the structure
of imidazolium-based COF materials, the main chains are linked
with covalent bonds, resulting in its high stability even under rig-
orous conditions [48,49]. Meanwhile, the loading of NHC units
was effectively improved because the responding imidazolium
monomers were directly synthesized into the solid supporter.
These advantages highlight their potential as the supporters of
heterogeneous catalysts. However, the synthetic pathways of the
imidazolium-based polymers were rather tedious and complicated,
which greatly limited the popularization of imidazolium-based
polymers.
In order to increase the loading of NHC units on solid sup-
porter, we intended to prepare imidazolium-based polymer, which
contained the NHC units on the responding polymeric backbone.
While being enlightened by the previously reported one-step
method of main-chain imidazolium-containing polymers [50,51],
we knew that it remained challenging to select starting materi-
als for the formation of solid supporter. It was anticipated that
the rigid part of aromatic amine might improve the insolubility
of imidazolium-based polymer compared with polymer ionic liq-
uid (PIL). Therefore, 1,4-diaminobenzene was made the focus of
our study. In the purpose of precisely controlling the monomer
stoichiometry ratio, paraformaldehyde was employed in one-step
method to prepare imidazolium-based polymer. Inspired by this
good result, we prepared NHC-Pd polymer and studied its applica-
tion in heterogeneous Suzuki-Miyaura reaction.
2.3. Catalysis of NHC-Pd polymer in Suzuki–Miyaura reaction
A
pyrex tube was charged with aryl bromide (1 mmol),
phenylboronic acid (1.2 mmol),K2CO3 (2 mmol), palladium cata-
lyst (1 mmol%) and mixture of ethanol and distilled water (4 mL,
v:v = 1:1). The reaction mixture was stirred at 70 ◦C for 12 h under
argon atmosphere. After cooled to room temperature, the result-
ing reaction mixture was added with chloromethane (3 mL) and
internal standard substance (20 L n-hexadecane). The mixture
was completely shook for 10 min, and then the organic layer was
collected. The organic layer was dried over anhydrous Na2SO4. The
product yield was determined by gas chromatography (GC).
2.4. Recyclability test of NHC-Pd polymer
4-bromoanisole (1 mmol), phenylboronic acid (1.2 mmol), pal-
ladium catalyst (5 mmol%), K2CO3 (2 mmol) in C2H5OH/H2O (4 mL,
v:v = 1:1) was heated at 70 ◦C for 12 h under argon atmosphere.
The catalyst was then collected by centrifugal separation and then
washed with distilled water, ethanol, CHCl3 and dried at 40 ◦C
overnight under air atmosphere. The dried catalyst was reused
again to catalyze the reaction.
2.5. Characterization
1H NMR and 13C NMR spectra of PImCl were recorded on a
Bruker Avance III 500 MHz spectrometer. The structure of PImCl
was analyzed by the attenuated total reflection-infrared (Nico-
let iS50). The intrinsic viscosity of PImCl was measured with an
Ubbelohde-type capillary viscometer (IVS300-6). The morphology
of NHC-Pd polymer was observed by scanning electron microscopy
(Hitachi S-4800). The content of palladium in NHC-Pd polymer was
determined by energy dispersive spectrometer (Hitachi S-4800)
and atomic absorption spectroscopy (Perkin-Elm 1100B). X-ray
photoelectron spectra (XPS) of NHC-Pd polymer was measured on
the spectrometer (Thermo Scientific Escalab 250Xi). The XRD pat-
tern was carried out from 20◦ to 80◦ at the scanning rate of 10◦/min
on the spectrometer (Rigaku SmartLab). Size distribution of NHC-
Pd polymer was analyzed on a laser particle analyzer (Beckman
LS13320). Thermal gravimetric analysis (TGA) was carried out from
308 K to 1073 K at the heating rate of 10 ◦C/min using TGA-50H.
N2 adsorption experiment was carried out on Autosorb-iQ at 77 K.
Yields were determined by gas chromatography (Agilent 7890B).
2. Experimental section
2.1. Materials
Dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) were
obtained from Sinopharm Chemical Reagent Co. Ltd. Paraformalde-
hyde was obtained from XiLong Chemical. 1,4-diaminobenzene,
glyoxal solution, palladium chloride, 3-chloropyridine and potas-
sium tert-butoxide were purchased from Aladdin Industrial
Corporation.
2.2. Experimental protocols for the synthesis of PImCl and
NHC-Pd polymer
2.2.1. Preparation of PImCl
In one vial, paraformaldehyde (195 mg, 6.5 mmol) was dissolved
with stirring in a solution of 4 M HCl in 1,4-dioxane (2 mL). Another
vial was charged with the corresponding glyoxal of 40 wt% in H2O