Synthesis of a carbene transfer organometallic polymer and application to forming a recyclable heterogeneous catalyst for the Suzuki reactions of aryl chlorides

Article abstract of DOI:10.1039/b904944b

An efficient and versatile synthetic approach to NHC-based organometallic polymers has been developed by a simple three-step synthesis. A novel brush polymer having imidazolium salt side chains (P2) was firstly synthesized through postpolymerization modification of P1, which permits access to Ag(i)-NHC-based side chain polymer (P3) at room temperature in high yield. P3 was applied as a carbene transfer agent to form a Pd-NHC-containing polymer (P4) by the transmetallation reaction of Ag(i)-NHCs in the side chains. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis indicated that 77% of Ag(i)-NHCs in the side chains of P3 were transmetallated. The resulting Pd-NHC-containing polymer (P4) showed high catalytic activity and reusability in the Suzuki reactions of aryl chlorides and aryl boronic acids. This novel Pd-NHC-containing polymeric catalyst was used five times and still remained active giving the desired biaryl products in 70% yield in the fifth run of the cross-coupling reaction of deactivated 4-chloroanisole with phenylboronic acid. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b904944b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of a carbene transfer organometallic polymer and application
to forming a recyclable heterogeneous catalyst for the Suzuki reactions
of aryl chlorides†
Xiaoming Zeng, Tongxin Zhang, Yuancheng Qin, Zhijun Wei and Meiming Luo*
Received 10th March 2009, Accepted 13th July 2009
First published as an Advance Article on the web 10th August 2009
DOI: 10.1039/b904944b
An efficient and versatile synthetic approach to NHC-based organometallic polymers has been
developed by a simple three-step synthesis. A novel brush polymer having imidazolium salt side chains
(P2) was firstly synthesized through postpolymerization modification of P1, which permits access to
Ag(I)–NHC-based side chain polymer (P3) at room temperature in high yield. P3 was applied as a
carbene transfer agent to form a Pd–NHC-containing polymer (P4) by the transmetallation reaction of
Ag(I)–NHCs in the side chains. Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
analysis indicated that 77% of Ag(I)–NHCs in the side chains of P3 were transmetallated. The resulting
Pd–NHC-containing polymer (P4) showed high catalytic activity and reusability in the Suzuki
reactions of aryl chlorides and aryl boronic acids. This novel Pd–NHC-containing polymeric catalyst
was used five times and still remained active giving the desired biaryl products in 70% yield in the fifth
run of the cross-coupling reaction of deactivated 4-chloroanisole with phenylboronic acid.
polymeric materials. Pioneering work introducing metal–NHCs
1. Introduction
into polymers was performed by Bertrand and co-workers, who
The incorporation of transition metals into polymers have received
described a polymeric silver–biscarbene compound based on
trialkylated 1,2,4-triazoles.13 Recently, Bielawski’s group carried
out a program on using NHCs as key components to synthesize
novel organometallic macromolecules,11,14 which were based on
linearly opposed bis(imidazolium) salts by incorporating tran-
sition metals into the main chains. Among those reported, the
metal–NHC moieties were all located in the backbone of the
polymers. Althoughmacromolecules withorganometallic moieties
in the side chains are an important class of transition metal-based
polymers,15–17 metal–NHC-based side chain polymers are scarce.
Hitherto only a few examples of polymer-supported catalysts that
have metal–NHC moieties in parts of the side chains have been
reported.16
Ag(I)–NHC complexes, which are readily generated by
treatment of the imidazolium salt with silver oxide under mild
conditions, have proved to be effective carbene transfer agents18 for
the preparation of various NHC complexes of other metals, such
as palladium(II),19 rhodium(I),20 rhodium(III),21 iridium(III),21
copper(I),22 copper(II),23 ruthenium(II),24 ruthenium(III),25
ruthenium(IV),26 nickel(II),27 and platinum(II).28 Notably, carbene
transfer reactions proceed rapidly at ambient temperature,
afford excellent yields, and are especially suitable for preparing
carbene complexes, which are hard to synthesize by conventional
methods.19 However, to the best of our knowledge, only
one example has been reported on the application of the
transmetallation of Ag–NHC units for the construction of
a great amount of attention due to the unusual and attractive
physical and chemical properties that these organometallic ma-
terials possess.1 Recent investigations show that transition metal-
containing macromolecules may be useful as catalysts,2 sensors,3
electroactive films,4 non-linear optical devices5 and liquid crystals.6
The production of these kinds of polymers has been encouraged
by supplying new materials with desirable properties. Significant
advances have been achieved in recent years in methodologies
for the preparation of transition metal-containing polymers, as
exemplified by the olefin and alkyne polymerization, and ring-
opening and coordination polymerizations.7 However, further
developments of novel versatile synthetic strategies giving high
yields and employing mild reaction conditions are still highly
desirable.
Since the discovery of stable carbenes by the groups of Bertrand8
and Arduengo,9 N-heterocyclic carbenes (NHCs) have attracted
considerable interest in coordination chemistry and catalysis.10
However, comparatively little attention has been directed to-
wards developing effective synthetic approaches to functional
organometallic macromolecular materials that contain NHC
moieties.11 Considering their structural diversity and capability to
form stable complexes with a large number of transition metals,12
NHCs are interesting components to construct organometallic
Key Laboratory of Green Chemistry and Technology of Ministry of
Education at Sichuan University, College of Chemistry, Sichuan University,
Chengdu 610064, P. R. China. E-mail: luomm@scu.edu.cn; Fax: +86-28-
85462021; Tel: +86-28-85462021
† Electronic supplementary information (ESI) available: IR spectra of P1,
P2, P3, P4 and recovered P4; Pd3d, Ag3d, C 1s, N 1s, O 1s and Cl 2p
XPS spectra of P4; analytical data for products of the Suzuki cross-
coupling reactions of aryl chlorides and aryl boronic acids. See DOI:
10.1039/b904944b
a
tailored organometallic–inorganic hybrid mesostructured
material.29 We envisioned that if a soluble Ag(I)–NHC-based side
chain polymer could be synthesized, then it might be further
utilized as a carbene transfer agent to prepare a functional
NHC-based organometallic polymer with higher metal loading.
This would be a general and versatile synthetic approach to
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NHC-based organometallic polymers (Scheme 1). Herein we
describe the synthesis of a novel soluble Ag(I)–NHC-based
side chain polymer by postpolymerization modification of
polychloromethylstyrene, and its application in the synthesis
of the Pd–NHC-containing polymer by the transmetallation
reaction of the Ag(I)–NHC moieties. The resulting Pd–NHC-
containing polymer showed high catalytic activity and reusability
in the Suzuki reactions of the challenging, relatively inactive aryl
chlorides and aryl boronic acids.
Scheme 1 General depiction of a versatile synthetic approach for prepar-
ing novel imidazolium salts containing brush polymer and NHC-based
organometallic polymers.
2. Results and discussion
Fig. 1 1H NMR spectra of P1, P2 measured in CDCl3 and P3 measured
in DMSO-d6.
2.1. Postpolymerization modification for preparation of brush
polymer (P2) and the Ag(I)–NHC based side chain polymer (P3)
CDCl3) was shifted downfield to 5.3 ppm (CH2 unit of the
benzyl imidazolium). This is corroborated by the absence of the
characteristic frequency of the CH2–Cl wag of P1 at 1264 cm-1 in
the IR spectrum of P2 (see ESI†).35 The appearance of new bands
One of the most appealing synthetic approaches to polymers
containing side chains connected to a linear backbone (brush
polymer)30 is postpolymerization modification, in which the syn-
thetic difficulties are greatly reduced by using common polyolefins
as starting materials, and then altering the existing functional side
chain groups to achieve the desired function.31 The inexpensive,
readily available, chemically inert polychloromethylstyrene (P1),
can be easily modified through the benzyl chloride group by
nucleophilic substitution with various nucleophilic reagents, and
is therefore an ideal functional linear polymer for postpolymeriza-
tion modification.32
For the preparation of the brush polymer having imidazolium
salt side chains (P2), the “grafting onto” method,33 one of the post-
polymerization modification strategies, was applied to modify the
side chains of P1 (Scheme 2). Following a literature procedure,34 P1
(Mn = 2.8 ¥ 104 Da, PDI = 2.1; relative to polystyrene standards)
was prepared by radical polymerization of p-chloromethylstyrene.
Reaction of P1 with excess 1-(mesityl)imidazole in chloroform
at 60 ◦C for two days gave P2. The pendent benzyl chloride
moieties along the backbone of P1 appeared to be completely
consumed as confirmed by the 1H NMR spectrum of P2, where the
characteristic peak of the 2H-imidazolium proton at 10.59 ppm
(Fig. 1) appeared, and the signal typically found at d 4.51 ppm
(attributable to the CH2 unit of the benzyl chloride of P1 in
at 1545 and 1449 cm-1 frequencies can be assigned to v(C N) of
=
imidazolium.36
Ag(I)–NHC-based side chain polymer P3 was readily pre-
pared by the reaction of P2 with 1/2 equivalent of Ag2O in
dichloromethane using Wang and Lin’s procedure.37 After stirring
for three days at room temperature, the black silver(I) oxide
completely disappeared, and workup of the solution gave P3 in
93% yield as a white air-stable solid. The formation of P3 was
established by spectroscopic techniques. The signal (10.59 ppm)
corresponding to the 2H-imidazolium proton was absent in the 1H
NMR spectrum of P3 (Fig. 1), which demonstrates that silver was
attached quantitatively and specifically at the C2 position of the
imidazolium moieties on P2. This was further supported by the
13C NMR spectra of P3 (Fig. 2), where a broad single resonance
around 180 ppm was assigned diagnostically to C2 of the imidazol-
2-ylidene carbon.38 To our great delight, P3 was soluble in solvents
such as CH2Cl2, CHCl3, methanol, DMF and DMSO. It should
be noted that we can not rule out the possibility of cross-linked
structures in P3 such as (NHC)2Ag–Cl and (NHC)2Ag–Ag–Cl2,
although it is less possible due to its solubility in many organic
solvents. As a result of its good solubility, a general approach for
preparing other NHC-based side chain organometallic polymers
can be developed by the transmetallation reaction of Ag(I)–NHCs
in the side chains of P3.
2.2. Transfer strategy for preparing palladium–NHC-containing
polymer (P4)
We first investigated the direct metallation reaction of P2 with
Pd(OAc)2 in DMSO.39 Inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES) showed that the Pd content in the
Scheme 2 Synthetic route to the imidazolium salts containing brush
polymer (P2) and the Ag(I)–NHC based side chain polymer (P3).
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Fig. 2 13C NMR spectra of P1, P2 measured in CDCl3 and P3 measured in DMSO-d6, and the solid 13C NMR spectra of P3 and P4, those of the solvent
are marked with asterisks (*).
resulting product was only 0.04 mmol g-1. We also performed the
reaction in the presence of excess NaOAc at 110 ◦C, 0.51 mmol g-1
palladium loading was found in the final polymer by ICP-AES
analysis. These results showed that the direct metallation is not
very effective in this case. We then turned our attention to
the transmetallation strategy using Ag(I)–NHCs for synthesizing
the desired Pd–NHC-containing polymer with higher palladium
loading (Scheme 3). Treatment of P3 with an excess amount of
Pd(OAc)2 in CH2Cl2 at room temperature resulted in immediate
formation of white precipitate. After stirring for four days, the
precipitates were filtered off. Removal of solvent from the filtrate
and washing with ether afforded P4 as an orange-brown powder.
ICP-AES analysis indicated that the Ag and Pd contents were
0.52 and 1.49 mmol g-1 respectively in P4, which means that 77%
Scheme 3 Transfer strategy for preparing functional palladium–NHC–
of Ag(I)–NHCs in the side chains of P3 were transmetallated.
The IR spectrum (see ESI†) was used to identify the functional
groups in P4. The two major peaks centred at 1577 and 1414 cm-1,
could be attributed to symmetric and unsymmetric stretching
vibrations of the COO- group,35 demonstrating the presence of
acetate in the final transmetallated product. The wide-survey
containing polymer (P4).
X-ray photoelectron spectroscopy (XPS) of P4 (Fig. 3) reveals
the predominant presence of carbon, oxygen, nitrogen, chlorine,
palladium and silver. The observed Pd 3d5/2 and Pd 3d3/2 at 337.61
and 343.03 eV are characteristic of Pd2+ species (see ESI†).40
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Fig. 5 Thermogravimetric curves of P2, P3 and P4 in a nitrogen
atmosphere.
Fig. 3 The wide-survey XPS spectrum of P4.
2.3. Catalysis of the Suzuki–Miyaura cross-coupling reactions of
aryl halides with aryl boronic acids
The binding energy of Ag3d is located at 368.28 eV.41 The 2p
photoelectron peak appearing at 198.52 eV is characteristic of
Cl-1 (see ESI†).42 Based on these features, the coordination modes
of Pd to NHC and carboxylate,43 and the low solubility, a cross-
linked structure might be proposed for P4 (Scheme 3). The solid
state 13C NMR spectrum of P4 gives valuable information for
confirming the structure, in which the boader signal around
181 ppm demonstrates the multiple carbene–metal environments
in P4 (Fig. 2).
The size and morphology of P4 were examined by scanning
electron microscopy (SEM). Fig. 4(d) displays SEM images of
the sandwich. Fig. 4(a) shows micron solid tubes of a sample
of P2 obtained through slowly volatilizing the solvent CH2Cl2
and dragging the rest of the material. Furthermore, membrane
structures having a velvet surface are observed (Fig. 4(b)).
Such properties make brush polymer P2 a promising candidate
for advanced materials for membranes. Thermal properties of
the polymers were investigated using thermogravimetric analysis
(TGA) at a heating rate of 10 ◦C min-1 under nitrogen atmosphere
(Fig. 5). Thermal degradation analysis of P2 showed that weight
loss mainly started at 239 ◦C, and P3 at 275 ◦C. Obviously, P2
and P3 have higher thermal stability than P4 (173 ◦C). The weight
residue of P2 at 550 ◦C was 9.6%, whereas that of P3 was 28%. P4
showed a small weight loss (31%) below 550 ◦C.
The Suzuki reactions of aryl halides with aryl boronic acids
has become an attractive standard method for the preparation
of biaryl compounds in modern organic synthesis.44 Due to the
advantages of heterogeneous over homogeneous catalysis, which
include the ease of product separation from the reaction mixture,
the purity of the crude product and the recycling of the catalyst,
numerous heterogeneous Pd catalysts have been described for the
Suzuki reactions involving aryl bromides or iodides.45 However,
very limited success has been reported for the Suzuki reactions of
aryl chlorides,46 which are more challenging due to their relatively
inertness. To our knowledge, examples of recyclable NHC-based
heterogeneous catalytic systems for the Suzuki reactions of
unreactive aryl chlorides have not been reported. Lee et al. recently
reported a macroporous polystyrene-supported NHC–palladium
catalyst for the Suzuki reactions of aryl chlorides, which only
shows high reusability for the Suzuki reactions of aryl bromides.47
In light of that, P4 possesses much higher Pd loading and contains
Ag which has been shown to enhance the catalytic activity of some
Pd-catalyzed Suzuki reactions,48 we are, thus, intrigued by the
possibility of applying our Pd–NHC-containing polymer (P4) as a
novel, active, recyclable heterogeneous catalyst for the challenging
Suzuki reactions of less reactive aryl chlorides.
An electronically rich, deactivated arylchloride, 4-chloroanisole,
and phenylboronic acid were employed as substrates in the screen-
ing of the Suzuki reaction parameters (Table 1). In anhydrous THF
or DMF, the conversion of 4-chloroanisole was <5% (Table 1,
entries 1 and 2). Gratifyingly, P4 (at 3.0 mol% loading of Pd)
exhibited high catalytic activity with 2-propanol (IPA) as the
reaction solvent. For example, switching the solvent to IPA
increased the product yield to 38% using K2CO3 as a base (Table 1,
entry 3). The yield of cross-coupling product was further increased
to 72% when NaOtBu was used as a base. It should be noted
that the concentration of the reaction mixture was an important
factor in affecting the yield. When the catalyst concentration
increased from 0.01 mol L-1 to 0.015 mol L-1, the yield of the
desired coupling product increased from 72% to 93% (Table 1,
entry 8). The catalyst prepared by direct metallation of P2 with
Pd(OAc)2 in the presence of excess of NaOAc at 110 ◦C gave the
product under the same reaction conditions in 57% yield (Table 1,
entry 9). Neither Pd(OAc)2 nor AgOAc alone promoted the cross-
coupling reaction in the absence of P4 (Table 1, entries 10 and
11). However, a mixture of Pd(OAc)2 and AgOAc did produce
Fig. 4 SEM images of (a) micron solid tubes of P2, (b) membrane
structures having velvet surface of P2, (c) the solid size of P4, (d) sandwich
of P4.
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Table 1 Reaction condition optimization on the coupling of
optimized conditions. After six hours, the catalyst (P4) was
removed from the reaction mixture by hot filtration and the desired
biaryl product was obtained in 51% yield. Further treatment of
the hot filtrate under the same reaction conditions did not give
increased conversion. Only 0.13% of the loaded Pd leached into the
hot filtrate based on elemental analysis using inductively coupled
plasma mass spectrometry (ICP-MS). We also analyzed the Pd-
leaching in the recycling experiments by ICP-MS. There were very
low levels of Pd-leaching (about 0.33% for the first run and less
than 0.1% for the fifth run) from the polymeric catalyst. To rule out
the possibility that the real catalyst was the leached Pd, in addition
to the “hot filtration experiment”, the mercury poisoning test was
carried out.16c,49 Addition of mercury (the molar ratio of Pd to
Hg was 1 : 500) did not reduce the yields of the cross-coupling
product. These results confirmed that P4 was the reactive catalyst
rather than a boomerang system. The XPS spectrum showed that
Pd in the recovered P4 was still in the +2 oxidation state. The IR
spectrum of the recovered P4 indicated the absence of acetates. It
is not clear what the anionic ligand is in the recovered P4. It may
be chloride, alkoxide or hydroxide.
The Pd–NHC-containing polymeric catalyst P4 has been suc-
cessfully applied to the Suzuki reactions of various aryl chlorides
and a series of aryl boronic acids under the optimized conditions
(Table 3). Both electron-poor (Table 3, entries 1–4, 9) and electron-
rich (Table 3, entries 5–7 and 10–15) aryl chlorides coupled in high
yields with aryl boronic acids in the presence of P4 (3.0 mol% Pd).
Remarkably, even ortho-substituted and electronically deactivated
2-chloroanisole can be coupled with aryl boronic acids in good
yields (Table 3, entries 7 and 14). In addition, the less reactive
aryl boronic acids containing nitryl groups undergo the Suzuki
reactions smoothly with this procedure, though the yield decreased
(Table 3, entries 15 and 16).
4-chloroanisole with phenylboronic acida
Entry
Base
Time/h
Solvent
T/◦C
Yield (%)b
1
2
3
4
K2CO3
K2CO3
K2CO3
K2CO3
24
24
24
24
DMF
THF
IPA
IPA–H2O
(v/v = 1/1)
IPA
IPA
IPA
IPA
IPA
100
70
80
<5
<5
38
80
27
5
6
7
8
9
10
11
12
Na2CO3
K3PO4
24
24
12
12
12
12
12
12
80
80
80
80
80
80
80
80
30
57
62
Cs2CO3
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
72 (93)c
57cd
0e
IPA
IPA
IPA
0f
8g
a Reaction conditions: 4-chloroanisole (1.0 mmol), PhB(OH)2 (2.5 mmol),
P4 (3.0 mol% Pd), base (2.5 mmol), solvent (3.0 mL). b Isolated yields
are based on 4-chloroanisole. c 2.0 mL of solvent was used. d The catalyst
(3.0 mol% Pd) was prepared by direct metallation of P2 with Pd(OAc)2 in
the presence of excess NaOAc at 110 ◦C. The Pd loading of this catalyst is
0.51 mmol g-1. e Pd(OAc)2 (4.0 mol%) was used as catalyst. f P3 (4.0 mol%
Ag) was used as catalyst. g AgOAc (1.0 mol%) and Pd(OAc)2 (3.0 mol%)
were used as catalyst.
Table 2 Catalyst recycling experiments for the Suzuki reaction of 4-
chloroanisole with phenylboronic acida
Cycle
1
2
3
4
5
Yield (%)b
94
82
75
74
70
a Reaction conditions: 4-chloroanisole (2.0 mmol), boronic acid
(5.0 mmol), P4 (3.0 mol% Pd), NaOtBu (5.0 mmol), IPA (4.0 mL), 80 ◦C,
12 h. b Isolated yield.
the cross-coupling product, albeit the yield was low (Table 1, entry
12), which may be attributed to the presence of AgOAc.48 Thus, an
optimized reaction condition using 1 equivalent of arylchloride,
P4 (3.0 mol% of palladium, 0.015 mol L-1), 2.5 equivalent ◦of
aryl boronic acid and 2.5 equivalent of NaOtBu in IPA at 80 C
was concluded as a standard condition for the Suzuki coupling
reactions of aryl chlorides. Utilizing technical grade IPA as a
solvent is very attractive since it is cheap and environmentally
friendly.
From an economic and industrial point of view, the recyclability
of a heterogeneous catalyst is very important. Taking advantage
of the insolubility of P4 in IPA, a simple filtration was sufficient
to recover the catalyst. The recyclability of the catalyst was then
studied in the cross-coupling reaction of 4-chloroanisole with
phenylboronic acid. The results of the recycling experiments are
shown in Table 2. The catalyst could be reused under the same
reaction conditions after separation, washing with water, ethanol
and ether and drying in vacuo. It was notable that the polymer
still remained catalytically active after being reused four times.
The coupling reaction at the fourth and fifth runs gave the desired
biaryl product in 74% and 70% yields.
3. Experimental
3.1. Reagents and materials
All manipulations were performed under an inert atmosphere of
argon using standard Schlenk techniques. DMF, DMSO and IPA
were used directly without further disposal. Ag2O, Pd(OAc)2,
Na2CO3, K2CO3 and Cs2CO3 were obtained from commercial
sources and directly used without further disposal. CHCl3 was
purified according to the method described elsewhere.50 CH2Cl2
was dried over calcium hydride and freshly distilled before
use. THF was dried over sodium and freshly distilled before
use. 1-(Mesityl)imidazole,51 NaOtBu was prepared by literature
methods.52
3.2. Synthesis of P2
In a 25 mL dried Schlenk flask, P1 (0.305 g, 2 mmol) was added to
a solution of 1-(mesityl)imidazole (0.428 g, 2.3 mmol) in CHCl3
(5 mL). The mixture was heated to 60 ◦C and stirred for 4 d.
After cooling to room temperature, the solution was concentrated
to 2 mL under vacuum. The white precipitate formed after the
addition of 15 mL of ether, was filtered, washed several times with
ether (6 ¥ 20 mL) and dried in vacuo to give P2 as a white powder
(0.65 g, 96%).
It is important to verify that the observed catalytic conversion
is caused by the Pd–NHC-containing polymer P4 rather than
the leached palladium species. Therefore the following catalytic
transformation of 4-chloroanisole was carried out under the
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Table 3 Suzuki reactions of aryl chlorides with aryl boronic acidsa
Entry
1
Aryl chloride
Aryl boronic acid
Time/h
10
Product
Yield (%)b
88
R1 = p-NO2
R2 = H
2
3
o-NO2
H
H
10
10
87
90
m-NO2
4
5
6
p-Cl
H
H
H
8
12
12
94c
93
94
p-OMe
m-OMe
7
o-OMe
H
18
91
8
9
H
p-OMe
p-OMe
p-OMe
p-OMe
10
10
10
12
98
81
84
77
p-CN
p-Me
o-Me
10
11
12
13
p-OMe
o-OMe
p-Me
p-Me
12
12
90
90
14
m-OMe
p-Me
18
84
15
16
p-Me
m-NO2
m-NO2
12
12
72d
75d
H
◦
a Reaction conditions: aryl chloride (1.0 mmol), aryl boronic acid (2.5 mmol), P4 (3.0 mol% Pd), NaOtBu (2.5 mmol), IPA (2.0 mL), 80 C. b Isolated
yields are based on arylchloride. c Aryl chloride (1.0 mmol), PhB(OH)2 (5.0 mmol), catalyst P4 (4.0 mol% Pd), NaOtBu (5.0 mmol), IPA (2.0 mL).
d Homocoupling products of aryl chlorides and aryl boronic acids were detected by GC-MS.
n(KBr)/cm-1: 2924, 1609, 1545, 1487, 1449, 1373, 1197, 1158,
1067, 855, 762, 672, 553. dH (600 MHz; CDCl3; ppm): 10.59 (br, s,
N(CH)N), 6.46–7.44 (br, m, Ar-H and NCHCHN), 5.31 (br, s,
NCH2), 2.30 (br, s, p-mesityl CH3), 2.00–2.16 (br, m, CH2CH
and o-mesityl CH3), 1.15–1.53 (br, m, CH2CH). dC (100 MHz;
CDCl3; ppm): 140.8–141.1 (NCHN), 127.0–139.3 (Ar-C), 123.6
(NCHCHN), 52.2–53.4 (ArCH2N), 38.8–41.0 (CH2CH), 31.5
(CH2CH), 17.6–22.9 (mesityl CH3).
3.3. Synthesis of P3
A mixture of P2 (0.677 g, 2.0 mmol) and silver(I) oxide (0.232 g,
1.0 mmol) in 40 mL of CH2Cl2 was stirred for 3 d at room
temperature. The black silver(I) oxide completely disappeared and
the reaction mixture turned to a clear solution. The volume of
the solution was reduced to 3 mL under reduced pressure, and
20 mL of ether was then added to produce a white solid. The solid
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was washed with ether (6 ¥ 20 mL) and dried in vacuo to give P3
(0.829 g, 93%).
temperature. FT-IR measurements were recorded on a 17OSX
Fourier transform infrared spectrometer with KBr pellets. ICP-
AES analysis was carried out by a IRIS Adv. XPS was performed
on a XSAM 800 spectrometer using Al Ka as the excitation source
(hm = 1486.6 eV) and operating at 12 kV and 15 mA. SEM
was performed using a S-450 scanning electron microscope. TGA
was conducted using a Netzsch Sta 449C thermal analyzer at a
heating rate of 10 ◦C min-1 under a nitrogen flow. Pd-leaching was
determined by ICP-MS on a VG PQ Exceu. Elemental analyses
was performed on a Carlo Erba 1106 analyzer.
n(KBr)/cm-1: 2920, 1609, 1556, 1489, 1448, 1411, 1236,1191,
1162, 1030, 852, 737, 681, 584. dH (600 MHz; DMSO-d6; ppm):
6.99–7.47 (br, m, Ar-H and NCHCHN), 5.32 (br, s, NCH2),
2.31–2.32 (br, s, p-mesityl CH3), 1.79–2.14 (br, s, CH2CH
and o-mesityl CH3), 1.13–1.68 (br, s, CH2CH). dC (100 MHz;
DMSO-d6; ppm): 180.6 (C-carbene), 144.6, 138.7–140.4, 134.3–
135.8, 128.3–131.2 (Ar-C), 123.0 (NCHCHN), 55.3 (ArCH2N),
38.8–40.4 (CH2CH), 31.1 (CH2CH), 21.1 (p-mesity CH3), 17.6
(o-mesityl CH3). Elemental analysis: calcd: C, 56.46; H, 5.19; N,
6.27. Found: C, 56.56; H, 5.29; N, 5.74%. ICP-AES analysis: calcd:
Ag, 24.15. Found: Ag, 23.43%.
4. Conclusion
In summary, we have developed an efficient approach for
organometallic polymer synthesis. A brush polymer containing
imidazolium salts is prepared by a postpolymerization mod-
ification strategy, and permits access to a Ag(I)–NHC-based
side chain polymer at room temperature with high yield. The
Ag(I)–NHC-based side chain organometallic polymer was applied
as carbene transfer agents to the synthesis of other metal–
NHC-based organometallic polymers. This development offers
a new methodology for constructing novel functional NHC-
based organometallic polymers by a simple three-step synthesis
under mild conditions. The Pd–NHC-containing macromolecule
(P4) acts as an effective, recyclable heterogeneous metal–NHC
catalyst for the Suzuki reactions of deactivated aryl chlorides
and aryl boronic acids under mild conditions. This novel Pd–
NHC-containing polymeric catalyst was used five times and still
remained catalytically active. Further applications of Ag(I)–NHC-
based side chain polymers as carbene transfer agents for preparing
functional NHC-based organometallic polymers other than pal-
ladium organometallic polymer are under way in our group.
3.4. Synthesis of P4
In a 100 mL dried Schlenk flask, Pd(OAc)2 (0.336 g, 1.5 mmol) was
added to a solution of P3 (0.535 g, 1.2 mmol) dissolved in CH2Cl2
(50 mL). White precipitate immediately formed. After stirring for
4 d at room temperature, the precipitates were filtered off over
Celite. An orange-brown solid was obtained after the solution was
evaporated under vacuum. The product was washed with ether
(6 ¥ 20 mL) and dried in vacuum to yield 0.607 g of P4.
n(KBr)/cm-1: 2922, 1607, 1577, 1488, 1414, 1328, 1195, 1163,
1018, 850, 731, 690, 585 cm-1. Solid 13C NMR d (75 MHz; ppm):
183.8–181.4 (br, C-carbenes), 145.5 (br), 135.2 (br), 128.7 (br),
54.1 (br), 40.2 (br), 20.3 (br, p-mesityl CH3), 17.6 (br, o-mesityl
CH3). ICP-AES analysis: Ag, 5.61; Pd, 15.85%.
3.5. General procedure for the catalytic Suzuki reactions of aryl
chlorides and aryl boronic acids
Aryl chlorides (1.0 mmol), aryl boronic acids (2.5 mmol), NaOtBu
(2.5 mmol, 240 mg) and P4 (19.8 mg, 3.0 mol% of Pd) were
placed in a 15 mL dried Schlenk flask. IPA (2 mL) was added
via syringe. The reaction mixture was stirred at 80 ◦C until
complete consumption of aryl chlorides as monitored by TLC.
After cooling to room temperature, the mixture was diluted with
ether (5 mL), the catalyst was filtered, and washed with ether (3 ¥
5 mL). The filtrate was concentrated in vacuo and then purified
through column chromatography. The filtered catalyst was rinsed
with water (3 ¥ 8 mL), ethanol (3 ¥ 8 mL) and ether (4 ¥ 8 mL),
and then dried in vacuo for the next run.
Acknowledgements
We thank the National Natural Science Foundation of China, the
Natural Science Foundation of Sichuan Province and Chengdu
Key Technologies R&D Program for financial support. We are also
grateful to the Analytical & Testing Center of Sichuan University
for supports in NMR, IR, SEM, XPS and ICP analyses.
Notes and references
3.6. Mercury poisoning test for the Suzuki reaction
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