Imidazolyl-Functionalized Ordered Mesoporous Polymer from Nanocasting as an Effective Support for Highly Dispersed Palladium Nanoparticles in the Heck Reaction

Article abstract of DOI:10.1002/cctc.201600630

New imidazolyl-functionalized ordered mesoporous cross-linked polymers were prepared by the copolymerization of the ionic liquid 3-benzyl-1-vinyl-1H-imidazolium bromide with divinylbenzene as the cross-linker and azobisisobutyronitrile as the radical initiator in the presence of O-silylated SBA-15 as the hard template. The materials were characterized by N₂ adsorption–desorption analysis, TEM, thermogravimetric analysis, elemental analysis, and FTIR spectroscopy. The material, which benefits from the use of entrapped ionic liquid in the prepared polymer matrix in combination with its ordered mesoporous structure, is an excellent environment for the stabilization of highly dispersed Pd nanoparticles to result in a recyclable catalyst system with a significant activity in the Heck coupling reaction of aryl halides. The presence of well-distributed imidazolium functionalities in the polymeric framework might be responsible for the relatively uniform and nearly atomic scale distribution of Pd nanoparticles throughout the mesoporous structure and the prevention of Pd agglomeration during the reaction, which results in high durability, high stability, and good recycling characteristics of the catalyst. Although our catalyst system operates in a homogeneous pathway, it is also very stable and recyclable.

Full text of DOI:10.1002/cctc.201600630

DOI: 10.1002/cctc.201600630
Full Papers
Imidazolyl-Functionalized Ordered Mesoporous Polymer
from Nanocasting as an Effective Support for Highly
Dispersed Palladium Nanoparticles in the Heck Reaction
Babak Karimi,*[a] Mohammad Reza Marefat,[a] Maliheh Hasannia,[a] Pari Fadavi Akhavan,[a]
Fariborz Mansouri,[a] Zahra Artelli,[a] Fariba Mohammadi,[a] and Hojatollah Vali[b]
New imidazolyl-functionalized ordered mesoporous cross-
linked polymers were prepared by the copolymerization of the
ionic liquid 3-benzyl-1-vinyl-1H-imidazolium bromide with di-
vinylbenzene as the cross-linker and azobisisobutyronitrile as
the radical initiator in the presence of O-silylated SBA-15 as the
hard template. The materials were characterized by N2 adsorp-
tion–desorption analysis, TEM, thermogravimetric analysis, ele-
mental analysis, and FTIR spectroscopy. The material, which
benefits from the use of entrapped ionic liquid in the prepared
polymer matrix in combination with its ordered mesoporous
structure, is an excellent environment for the stabilization of
highly dispersed Pd nanoparticles to result in a recyclable cata-
lyst system with a significant activity in the Heck coupling re-
action of aryl halides. The presence of well-distributed imidazo-
lium functionalities in the polymeric framework might be re-
sponsible for the relatively uniform and nearly atomic scale dis-
tribution of Pd nanoparticles throughout the mesoporous
structure and the prevention of Pd agglomeration during the
reaction, which results in high durability, high stability, and
good recycling characteristics of the catalyst. Although our cat-
alyst system operates in a homogeneous pathway, it is also
very stable and recyclable.
Introduction
Recent years have witnessed a substantial increase in environ-
mental concerns associated with organic transformations.
Therefore, many attempts have been directed toward the de-
velopment of chemical synthesis through safer, more economi-
cal, and cleaner processes.[1] In this regard, ionic liquids (ILs)
have received great attention in recent years as environmental-
ly acceptable solvents for organic transformations because of
their unique properties such as negligible vapor pressure, non-
flammable behavior, and reasonable thermal stability.[2–4] In ad-
dition, ILs exhibit an excellent ability to generate and trap tran-
sition-metal nanoparticles without the need for extra
protecting molecules, which allow their use as reaction media
in wide range of metal-catalyzed chemical reactions.[5,6] Howev-
er, crucial problems of ILs such as their high cost, high viscosi-
ty, and potential toxicity even at low concentrations and their
efficient recycling and separation are still unresolved.[7,8] The
concomitant heterogenization of ILs along with catalysts yields
supported IL catalyst systems that integrate some of the
unique characteristics of ILs with the common physiochemical
features of the supported materials, a versatile technology that
has emerged from the pioneering work of Mehnert and co-
workers.[9,10] This approach minimizes the amounts of IL media
and allows the facile separation and recycling of both the pre-
cious metal catalyst and the IL in a single operation. Based on
this strategy, several ILs supported on inorganic porous materi-
als[11,12] and polystyrene-based polymers[13] were developed
and employed successfully in transition-metal catalysis with
good recyclability and performance. However, the poor acces-
sibility of the catalytic sites caused by thee imbalanced hydro-
philicity of the inorganic supports with organic substrates and
the nonporous nature of the polymeric materials can potential-
ly limit the catalytic performance of these supported IL catalyst
systems. Additionally, a major drawback associated with most
inorganic supports, especially silica-based materials, is their rel-
atively low stability under harsh alkaline conditions, which
causes serious support decomposition and subsequent catalyst
deactivation. To circumvent this issue, the use of porous poly-
meric ILs could provide an alternative mode of operation,
which combines the benefits of an immobilized IL and a purifi-
cation strategy with the inherently high surface area and hy-
drophobicity suitable for the favorable exposure of active sites
to the starting materials. Accordingly, various porous organic
polymers with amorphous morphologies that support the IL
(mainly imidazolium) functional group at the surface of the
nanopores have been prepared and applied as innovative ma-
terials in a wide range of applications such as catalysis,[14–16]
catalyst supports,[17,18] and materials for the controlled capture
or fixation of CO2.[19–22] However, to the best of our knowledge,
[a] Prof. Dr. B. Karimi, M. R. Marefat, M. Hasannia, P. F. Akhavan, F. Mansouri,
Z. Artelli, F. Mohammadi
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
P.O.Box 45195-1159, Gava Zang, Zanjan 45137-66731 (Iran)
Fax: (+98)24-33153232
E-mail: karimi@iasbs.ac.ir
[b] Prof. H. Vali
Anatomy and Cell Biology and Facility for Electron Microscopy Research
McGill University
Montreal, Quebec, H3A 2A7 (Canada)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600630.
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there is no example of the preparation of ordered mesoporous
ionic-liquid-based organic polymers (OMILP) by nanocasting in
the presence of ordered mesoporous solid materials (e.g., SBA-
15) and their application as supports in catalytic transforma-
tions. To date, there have only been a few reports on the
direct preparation of OMILP using a block copolymer self-as-
sembly approach.[23–26] If we consider the promising potential
of mesoporous polymeric ILs developed recently in various ap-
plications, OMILP, which have well-controlled pore structures,
high surface areas, and large and tunable pore sizes, would be
a step forward in the preparation of functional materials with
improved properties. Here we wish to report the preparation
of a new ordered mesoporous organic polymer that possesses
large pore sizes and IL (imidazolium) functional groups (OMP-
IL-Bn) from nanocasting using trimethylsilylated SBA-15 as
a hard template. In addition, we show that the chemical
nature of the imidazolium groups within the network, together
with the ordered mesoporous structure of OMP-IL-Bn not only
provides a means to generate and stabilize a high dispersion
of Pd nanoparticles (NPs; 3–4 nm) but also improves the cata-
lytic activity and recyclability considerably in the Heck reaction
of various aryl halides.
SBA-15 were protected by trimethylsilyl groups to facilitate the
diffusion of the hydrophobic reactants (monomers) inside the
pores. From the N2 sorption analysis, the decrease of the BET
surface area from 1042 to 547 m2 gÀ1, the pore volume from
1.20 to 0.82 cm3 gÀ1, and the pore size from 10.5 to 7.0 nm
confirmed the successful coating of trimethylsilyl groups onto
the channel surfaces of pristine SBA-15 (Figures S1 and S2 and
Table S1). The material still exhibited open pores with a narrow
pore size distribution as well as a large pore volume, which is
expected to allow the entry of starting monomers into the
channels to ensure their high concentration in the pores
before subsequent polymerization. In the next step, the chan-
nels of silylated SBA-15 were charged with mixtures of divinyl-
benzene (DVB), azobisisobutyronitrile (AIBN), and 3-benzyl-1-
vinyl-1H-imidazolium bromide (IL-Bn) and polymerization was
performed under programmed reaction conditions (see Experi-
mental Section and Supporting Information for details). After
the removal of the SBA-15 template using a HF solution, the
ordered mesoporous polymer that bears hydrophobic IL-Bn
was obtained as a pale yellow powder. Notably, the OMP-IL-Bn
was prepared under various conditions to investigate the
impact of several experimental parameters on the structure of
the prepared polymer. The experiments indicated that many
factors such as the kind and amount of monomer used, ratio
of monomer to template, scale of the reaction, method em-
ployed to remove the template, and washing of the final poly-
mer have a crucial influence on the structure and uniformity of
the pores in the final polymeric materials (Table S2 and Figur-
es S5–S11). For example, the use of styrene in addition to DVB
in the presence of IL-Bn under otherwise identical reaction
conditions led to a significant decrease in the surface area (111
to 17 m2 gÀ1; Table S2, entries 1–4) and the structural order of
the resulting polymer. This can be understood simply by the
fact that a high degree of cross-linking is necessary to maintain
the mesoporous structure of the polymer against collapse after
the removal of the template.[20]
Results and Discussion
The synthetic strategy employed to produce OMP-IL-Bn and
the Pd@OMP-IL-Bn nanocomposite is shown in Scheme 1. We
chose SBA-15[27] as the hard template because of its highly
ordered structure and 3D microporous connectivity, which is
a prerequisite for the successful implementation of the nano-
casting strategy.[28,29] Given the inherent high hydrophilicity of
the mesoporous silica network, the surface hydroxyl groups in
In addition, it was found that template removal using HF so-
lution is more efficient than that of NaOH in this case.
Although, the major reason for this observation is unclear to
us at this stage, it might be because the hydrophobic nature
of the polymer prevents the penetration of NaOH solution into
the composite and the subsequent dissolution of SBA-15. Fi-
nally, the material was prepared with the desired structural uni-
formity under the optimized conditions according to Table S2,
entry 7 (OMP-IL-Bn-5).
The prepared OMP-IL-Bn was characterized by using various
techniques. N2 adsorption–desorption analysis of OMP-IL-Bn
showed a type IV isotherm with a capillary condensation at
P/P0 =0.4–0.8, which is characteristic of mesoporous materials
based on the IUPAC classification. This analysis indicated that
the BET surface area and primary pore volume of the OMP-IL-
Bn are 289 m2 gÀ1 and 0.34 cm3 gÀ1, respectively. In addition,
the Barrett–Joyner–Halenda (BJH) pore size distribution of
OMP-IL-Bn calculated from the adsorption branch of the N2
sorption isotherm demonstrated a relatively narrow pore size
distribution with a maxima centered at ꢀ2.4 nm, which corre-
sponds roughly to the wall thickness of pristine SBA-15
Scheme 1. Schematic route for the preparation of OMP-IL-Bn and Pd@OMP-
IL-Bn by nanocasting using SBA-15 as the template.
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Figure 1. A) N2 adsorption–desorption isotherms and B) BJH pore size distributions of OMP-IL-Bn, Pd@OMP-IL-Bn, and recovered Pd@OMP-IL-Bn.
(Figure 1, Table S1, Figure S12). A sharp increase in almost all
of the isotherms of the prepared samples occurred at P/P0
>0.9, which indicates the presence of secondary mesoporous
structures in the samples. FTIR spectra of OMP-IL-Bn demon-
strated the characteristic bands associated with the polymer
backbone and the functional IL (Figure S13). The absorption
bands at n˜ =2921 cmÀ1 (aromatic CÀH stretching), 1601 (aro-
matic C=C stretching), 901 and 714 (aromatic C=C bending),
and 1157 cmÀ1 (CÀN stretching) confirmed the presence of imi-
dazolium IL and the aromatic rings in the structure of the pre-
pared polymer.
OMP-IL-Bn was measured to be 81.49% C, 7.14% H, and
1.78% N by elemental analysis. The imidazolium IL loading in
OMP-IL-Bn was 0.7 mmolgÀ1 based on the N content obtained
from elemental analysis, which is in good agreement with that
obtained from TGA. The structure and morphology of the pre-
pared OMP-IL-Bn were analyzed by using TEM and high-resolu-
tion transmission electron microscopy (HRTEM). OMP-IL-Bn has
a 2D hexagonal porous structure (Figure 2a and b). If we look
at the TEM image of the parent SBA-15 (Figure S3), it is clear
that the structure of OMP-IL-Bn is a negative copy of SBA-15.
To check the hydrophobicity of the prepared material, the con-
tact angle was measured. The experiment showed that the
contact angle of a thin film of OMP-IL-Bn with a drop of water
is around 123.58 (Figure S15), which confirms its hydrophobic
character (a contact angle >908 is characteristic of hydropho-
bic materials).[14]
Thermogravimetric analysis (TGA) of OMP-IL-Bn obtained
under a N2 atmosphere indicated two major weight losses (Fig-
ure S14). The first weight loss at 3008C is attributed to decom-
position of functional IL, and the second one at 4608C is
assigned to the polymer decomposition. This analysis also con-
firmed the presence of the organic backbone and the success-
ful removal of the silica template. The chemical composition of
In the next stage, the capability of OMP-IL-Bn for the prepa-
ration and stabilization of Pd NPs was investigated. Notably,
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in homogeneous catalysis, the Heck coupling reaction of non-
activated substrates such as aryl chlorides, which are industrial-
ly relevant and inexpensive haloarenes, using a recyclable cata-
lyst system has still not been addressed properly.[6]
As the synthesis of a stable, highly dispersed Pd NPs with
a narrow size distribution is a key factor in the catalytic activity
of Pd-based catalysts,[39] and if we consider that Pd@OMP-IL-Bn
has such characteristics, we hypothesized that this catalyst
may indeed demonstrate a high activity toward Heck coupling
reactions even in the case of challenging substrates. To evalu-
ate this hypothesis, we optimized the reaction conditions in
the Heck coupling reactions of iodobenzene (Table S3) and
bromobenzene (Table S4) with methyl acrylate. Our next inves-
tigations demonstrated that Pd@OMP-IL-Bn is a highly active
catalyst for the reactions of iodobenzene and 4-iodoanisol
with methyl-, ethyl-, and n-butyl acrylates using sodium ace-
tate as a base and DMF as a solvent in the presence of as low
as 0.01–0.03 mol% catalyst within relatively short reaction
times (Table 1, entries 1–6). The catalyst could also promote
the coupling reaction of aryl bromides and even aryl chlorides
with varied acrylates successfully by changing the solvent to
N-methyl-2-pyrrolidone (NMP) and the base to the mixture of
K2CO3/Et3N (Table 1). Under these conditions, activated aryl
bromides provided excellent yields in the coupling with
acrylates using as low as 0.01–0.05 mol% catalyst (Table 1, en-
tries 7–18). Interestingly, the catalyst is quite effective for bro-
mobenzene and deactivated aryl bromides such as 4-bromoa-
nisol and 4-bromoethylbenzene in the coupling with a variety
of acrylates to furnish excellent yields of the corresponding
cinnamate esters within short reaction times of 4–10 h (Table 1,
entries 19–27). We were also pleased to find that with the use
of 0.1 mol% Pd@OMP-IL-Bn, the procedure could be applied
effectively for the reaction of highly challenging aryl chlorides
although specific additives were necessary to achieve high
yields (Table 1, entries 28–30).
Figure 2. a, b) TEM images of OMP-IL-Bn (Scale bar: 100 nm), c) TEM and
d) HRTEM image of Pd@OMP-IL-Bn (Scale bar: 20 nm).
because of the presence of imidazolium moieties in the poly-
mer matrix, Pd species can be immobilized by a simple ion ex-
change with the counter ion of the IL under ambient condi-
tions (Scheme 1). TEM images of the prepared catalyst
(Pd@OMP-IL-Bn) indicated that Pd NPs are generated and well
distributed throughout the polymer network (Figure 2c).
HRTEM images of Pd@OMP-IL-Bn demonstrate that Pd NPs are
mostly in the range of 2–4 nm (Figure 2d). Notably, Pd NPs
were formed without the use of a reducing agent as the sup-
port is highly electron rich because of the presence of aromat-
ic rings. Leftover water inside the polymer backbone or the
residue of reductive solvents might be also responsible for the
formation of Pd NPs. The N2 adsorption–desorption isotherm
of Pd@OMP-IL-Bn indicated that the porous structure of OMP-
IL-Bn is well preserved even after the immobilization of Pd NPs
(Figure 1). However, a significant decrease in the BET surface
area and pore volume occurred with the increasing metal load-
ing for Pd@OMP-IL-Bn, which suggests that Pd NPs are mostly
formed inside the nanospaces of the mesostructured polymer
(Figure S16, Table S1). Notably, a decrease in the BET surface
area higher than that expected from the amount of supported
metal might indicate that some pore blockage occurred in
Pd@OMP-IL-Bn. In addition, the Pd content of Pd@OMP-IL-Bn
was 0.09 mmolgÀ1 from inductively coupled plasma atomic
emission spectroscopy (ICP-AES).
Notably, the catalyst could be recovered easily by simple
centrifugation and employed efficiently in six consecutive Heck
couplings of 4-iodoanisol with methyl acrylate without any re-
markable loss of activity (Figure S17). As the long-term durabil-
ity of the catalyst in the Heck reaction is an important issue
because of the harsh reaction conditions typically used for the
Heck coupling, after six cycles the recovered catalyst was ana-
lyzed to investigate any structural changes. N2 sorption analysis
of the recovered catalyst showed an isotherm and pore size di-
ameter very similar to that obtained for the fresh catalyst
(Figure 1, Table S1). Furthermore, HRTEM images of the recov-
ered catalyst showed that the Pd NPs still are highly dispersed
over the polymer support, and their mean diameters were in-
creased to some extent compared to that of the fresh catalyst
(5–11 nm; Figure 3).
The catalytic performance of Pd@OMP-IL-Bn was evaluated
in the Heck cross-coupling reaction. Heck cross-coupling is
a challenging Pd-catalyzed CÀC coupling that is performed
typically under homogeneous conditions using Pd complexes
with a variety of ligands[30,31] or immobilized Pd catalysts on
solid supports, such as magnetic nanoparticles,[32] carbon ma-
terials,[33] silica-based supports,[34,35] and polymers.[36–38] Howev-
er, although significant results have been achieved, particularly
Notably, the Pd loading of the recovered catalyst was found
to be 0.08 mmolgÀ1 by using ICP-AES, which corresponds to
an 89% Pd survival. All of these observations revealed that the
Pd@OMP-IL-Bn is not only a highly active but also extremely
stable and recyclable catalyst under the described reaction
conditions. Although we found that Pd@OMP-IL-Bn is recycla-
ble, a hot-filtration test after the reaction of 4-iodooanisole
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Table 1. Heck cross-coupling reactions catalyzed by Pd@OMP-IL-Bn.
Entry R1
X
R2
Catalyst
t [h] Yield [%][a] TOF [hÀ1
]
loading (y)
1
2
3
4
5
6
7[b]
8
H
H
H
I
I
I
I
I
I
Me
Et
nBu 0.03
Me
Et
nBu 0.03
0.01
0.01
0.5 >99
0.5 >99
20000
20000
1667
10000
1429
2222
6060
1333
667
2
1
7
>99
>99
>99
4-OMe
4-OMe
4-OMe
3-COMe Br Me
3-COMe Br Me
3-COMe Br Et
0.01
0.01
1.5 >99
0.3 >99
1.5 >99
0.05
0.05
0.05
9
10
11
3
3
10
>99
>99
>99
Figure 3. TEM image of recovered Pd@OMP-IL-Bn.
3-COMe Br nBu 0.05
3-COMe Br Me
667
0.01
0.05
0.05
1000
4000
800
800
1111
606
400
2222
500
500
485
667
667
485
571
480
176
112
155
12
13
14
15
16
17
18
19[b]
20[b]
21[b]
22[b]
23[b]
24[b]
25[b]
26[b]
27[b]
3-CHO
3-CHO
3-CHO
3-CHO
4-CN
4-CN
4-CHO
H
H
H
4-Et
4-Et
Br Me
Br Et
Br nBu 0.05
Br Me
Br Me
Br Et
Br Me
Br Me
Br Et
Br nBu 0.05
Br Me
Br Et
Br nBu 0.05
Br Me
Br Et
0.5 >99
2.5 >99
2.5 >99
sites.[40] To conduct the test, a flask was charged with 4-iodoa-
nisole (0.5 mmol), methyl acrylate (1 mmol), Pd@OMP-IL-Bn
(0.05 mol%), and NaOAc (1 mmol) in DMF (2 mL) as the sol-
vent. The mixture was then heated to 1008C, and Hg0 was
added to the flask in a ratio of 400 equivalents to the initial Pd
content of the catalyst. Kinetic studies in the presence and the
absence Hg0 showed that the reaction was quenched com-
pletely upon the addition of the poison (Figure 4).
0.01
0.03
0.05
0.03
0.05
0.05
9
>99
5.5 >99
>99
1.5 >99
5
4
4
4
3
3
4
>99
>99
97
>99
>99
97
0.05
0.05
4-Et
4-OMe
4-OMe
4-OMe
0.05
0.05
3.5 >99
4
10
7.5
6
8
96
88
84
93
98
–
Br nBu 0.05
28[b,c] 4-CHO
29[b,c] 4-NO2
30[b,c] 4-NO2
31[b,c]
32[d]
Cl Me
Cl Me
Cl Et
Cl Me
Br Me
0.1
0.1
0.1
0.1
0.05
123
–
240
H
24
4-OMe
3.5
12
Reaction conditions for aryl iodides: aryl iodide (0.5 mmol), acrylate
(1 mmol), NaCH3COO·3H2O (2 equiv.), and DMF (2 mL) at 1008C; Reaction
conditions for aryl bromides and chlorides: aryl halide (0.5 mmol), acrylate
(1 mmol), K2CO3 (2 equiv.)/Et3N (2 equiv.), and NMP (2 mL) at 1408C.
[a] GC yields. [b] Tetra-n-butyl ammonium bromide (1 equiv.) was used.
[c] KI (10 mol%) was used. [d] The nonfunctionalized catalyst Pd@OMP-
DVB was used as the catalyst under otherwise the same reaction condi-
tions for aryl bromides.
Figure 4. Reaction profile of the Heck reaction of 4-iodoanisol and methyl
acrylate catalyzed by Pd@OMP-IL-Bn in a) the absence and b) the presence
of Hg.
with methyl acrylate was conducted to investigate the role of
homogeneous catalysis in our protocol. For this reason, the re-
action was stopped before the substrates were consumed
completely (30 min, 56%), and we found that the reaction pro-
gressed significantly to higher conversions of 93% upon the
heating of the catalyst-free solution for 24 h at 1008C. The pos-
itive hot-filtration test highlights that the Pd@OMP-IL-Bn cata-
lyst system is a precatalyst for the soluble (homogeneous) Pd
species in the described Heck coupling reaction. Although
a positive hot-filtration test is a strong indication for the forma-
tion of soluble Pd species, it does not provide information
with regard to the presence of Pd NPs. It is well known that
naked metallic Pd species that are not protected tightly with
effective stabilizing agents can be poisoned by homogeneous
Hg0, which blocks the access of substrates to the active
These results suggest that for the present reaction, the sup-
ported Pd catalyst serves as a reservoir for active Pd species,
most probably in the form of Pd NPs that are dissolved from
the solid catalyst under the described reaction conditions. As
the catalytic activity of Pd@OMP-IL-Bn stems mostly from
leached (soluble) Pd species in DMF, a critical question at this
stage is: what are the major reasons for the high observed re-
cyclability and durability of the present catalyst system? The
high stability and reactivity of the catalyst are most likely at-
tributed to mesoporous organic matrix of the support com-
bined with its IL framework that provides highly dispersed Pd
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NPs in the mesochannels and the possibility to stabilize the
NPs.
ty of Pd@OMP-IL-Bn, a new catalyst system that comprised Pd
supported on an ordered mesoporous DVB polymer without
imidazolium moieties (Pd@OMP-DVB) was prepared using the
same approach (Scheme 1, see Figures S20–S22 for characteri-
zation). N2 adsorption–desorption isotherms of both OMP-DVB
and Pd@OMP-DVB are of type IV and feature capillary conden-
sation in the mesopores, which indicates the presence of
a well-ordered mesoporous structure and uniform pore sizes in
both samples (Figure 5). The BJH desorption of OMP-IL-Bn and
In addition, the IL-like mesostructure of the polymer may
also operate as a nanoenvironment to recapture the leached
(soluble) Pd species rapidly after catalysis, which thereby pre-
vents them from aggregation even during repeated use under
harsh reaction conditions.
To ascertain the beneficial role of functionalized imidazolium
on the polymer surface to stabilize Pd NPs and the high activi-
Figure 5. A) N2 adsorption–desorption isotherms and B) BJH pore size distributions of OMP-DVB and Pd@OMP-DVB.
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Pd@OPM-IL-Bn showed a narrow pore size distribution (Fig-
ure 5b). The BET surface area and total pore volume only de-
creased slightly from 556 m2 gÀ1 and 0.48 cm3 gÀ1, respectively,
for pristine OMP-DVB to 499 m2 gÀ1 and 0.42 cm3 gÀ1, respec-
tively, for Pd@OMP-DVB. The absence of a significant change
in the pore volume and surface area highlights the notion that
the supported NPs are mostly outside the pores and most
likely are larger than the mesochannels of OMP-DVB. The Heck
Conclusions
A new ordered mesoporous polymer that comprises a “built-
in” hydrophobic imidazolium ionic liquid was prepared by the
nanocasting method using SBA-15 as the hard template. The
material was a multifunctional support for highly dispersed Pd
nanoparticles. The catalyst demonstrated an extremely high ac-
tivity, stability, and recyclability in Heck coupling reactions
even for aryl chlorides. Although our catalyst system operates
in a homogeneous pathway, it is also very stable and recycla-
ble. It is suggested that the presence of imidazolium moieties
in the size-restricted mesopores in the parent polymer provide
a high nanoparticle stability, presumably through a combina-
tion of the electrostatic stabilization of the functionalized ionic
liquid and the confined environment dedicated by the inert
polymeric materials. This was confirmed by the fact that the
Heck reaction of 4-bromoanisol using a catalyst system that
comprises Pd supported on an ordered mesoporous polymer
without an imidazolium moiety under the optimized reaction
condition was remarkably inefficient because of the extensive
agglomeration of unprotected Pd nanoparticles.
reaction of 4-bromoanisole as
a model substrate using
Pd@OMP-DVB (0.05 mol%) was then studied under the opti-
mized reaction conditions demonstrated in entry 25 of Table 1.
The catalyst was remarkably inefficient (Table 1, entry 32) be-
cause the Pd NPs were unstable under the described condi-
tions, which led to the formation of large metal clusters (Pd
black) presumably as a consequence of the extensive agglom-
eration of the unprotected Pd NPs. Therefore, it may be con-
cluded that the presence of imidazolium in the size-restricted
mesopores in the parent OMP-IL-Bn polymer provide improved
NP stability, presumably through the combination of the elec-
trostatic stabilization of the functionalized IL and the confined
environment dedicated by the inert polymeric material.
We compared the reactivity of Pd@OMP-IL-Bn with that of
some of supported Pd catalysts reported previously in the
Heck reaction of bromobenzene with acrylate esters. Clearly,
Pd@OMP-IL-Bn demonstrated much higher catalytic activities
and turnover frequencies (TOFs) than most of the well-known
supported Pd catalyst systems (Table 2). Notably, in the case of
commercially available Pd/C catalysts, only aryl iodides resulted
in satisfactory yields, and the reactions are mostly conducted
in the presence of large amounts of catalyst or IL as the reac-
tion medium (Table 2, entries 1 and 7).
Experimental Section
General procedure for the synthesis of OMP-IL-Bn
Detailed experimental procedures for the preparation of all precur-
sors and all polymer samples are collected in the Supporting Infor-
mation. Here, a brief description for the synthesis of OMP-IL-Bn
under the optimized conditions is given. Typically, DVB (0.8 mL,
5.7 mmol), 3-benzyl-1-vinyl-1H-imidazolium bromide (0.343 g,
1.3 mmol), and AIBN (0.035 g, 3% relative to the total vinyl groups)
were dissolved in anhydrous dichloromethane (DCM; 1.0 mL). This
solution was injected into a flask that contained silylat-
ed SBA-15 (1 g) under an Ar atmosphere, which was
used after vacuum degassing at 1008C for 1.5 h. After
Table 2. Comparison of supported Pd-based catalyst systems with Pd@OMP-IL-Bn in
impregnation with the solution, the powder was dried
under vacuum at À788C for 1 h to help the diffusion of
monomers into the hard template and then subjected
to a freeze–vacuum–thaw cycle with a trap to remove
dissolved air and residual DCM. The mixture was al-
lowed to settle for 6 h at 358C to achieve a uniform dis-
tribution of organic monomers before polymerization.
The following temperature scheme was used for poly-
merization: 6 h at 608C, 2 h at 808C, and finally 6 h at
1008C. After polymerization, the resulting material was
washed with HF (2m) four times. The powder was
washed with ethanol (50 mL) and water (50 mL) for 6 h
and, subsequently, with DCM for 20 min. A white
powder was collected as OMP-IL-Bn and dried at 808C
overnight.
the Heck reaction of bromobenzene.
[h]
Entry Catalyst (mol%)
T [8C]
R
t [h] Yield [%] TOF [hÀ1
]
Ref.
1
2
3
4
5
6
7
Pd/C (3)
140
170
140
135
130
100
100
Et
12
40
67
90
0
1.1
13
375
0
[41]
[42]
[35]
[43,44]
[45]
[46]
[47]
[48]
[49]
Pd/TMS11 (0.1)[a]
Pd/SiO2-IL (0.01)[b]
Pd/PS-IL (0.2)[c]
Pd/FAP (0.1)[d]
Pd0/MCM-4[e]
Pd/C (5)
nBu 48
Me 24
Me 24
Et
24
89
56
55
5
35
96
>99
32
167
nBu 24
Et
nBu 51
Me 20
nBu
1.5
7.3
0.78
13
548
500
8
9
10
11
Pd/medenite (0.125) 130
Pd/HT[f]
150
Pd@PMO-IL-I (0.05)[g] 140
Pd@OMP-IL-Bn (0.05) 140
3.5
4
[34]
this work
Me
[a] PdII grafted onto mesoporous silica MCM-41 by the vapor deposition method.
[b] N-Heterocyclic carbene (NHC) Pd complex/IL matrix immobilized on silica as a pre-
catalyst. [c] Palladium N-methylimidazolium supported on polystyrene-divinylbenzene
copolymer (Merrifield resin). [d] Fluorapatite-supported Pd. [e] Pd NPs supported on
mesoporous silica MCM-41. [f] PdII contained in hydrotalcite. [g] Pd supported on peri-
odic mesoporous organosilica with an IL framework. [h] TOF values are calculated by
TOF=(total conversion/total Pd loading)ꢁtime [h]. As a result of Pd leaching, ag-
glomeration, and redeposition during reactions, this single point yield to calculate the
TOF might sometimes result in imprecise conclusions.
Preparation of Pd@OMP-IL-Bn catalyst
The ion exchange method was used for the preparation
of Pd@OMP-IL-Bn. OMP-IL-Bn (0.3 g) was added to de-
ionized water (30 mL) and sonicated for 1 h to obtain
a uniform dispersion of copolymer. Na2PdCl4 (15 mg,
0.4 mmol) was dissolved in deionized water (5 mL) and
added to the above-mentioned mixture (dispersion)
under vigorous stirring and an Ar atmosphere. The mix-
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Typically, in a two-necked 25 mL Schlenk flask, aryl halide (1 mmol),
olefin (1.5–2 mmol depending on the olefin), base (2 mmol), NMP
(4 mL), and catalyst (0.01–0.1 mol% related to aryl halides) were
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acetate (3ꢁ10 mL). The combined ethyl acetate solution was
washed with water (3ꢁ5 mL) and dried over Na2SO4. A sample was
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Acknowledgements
The authors acknowledge the IASBS Research Councils and Irani-
an National Science Foundation (INSF) through a grant no. G014
for support of this work.
Keywords: ionic liquids
·
mesoporous materials
·
nanoparticles · palladium · polymers
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Published online on && &&, 0000
&
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ÝÝ These are not the final page numbers!

FULL PAPERS
Order, order! An ordered mesoporous
polymer, which benefits from the use of
entrapped ionic liquid in a new polymer
matrix in combination with its ordered
mesoporous structure, is an excellent
environment for the stabilization of
highly dispersed Pd nanoparticles in the
Heck reaction of various aryl halides.
B. Karimi,* M. R. Marefat, M. Hasannia,
P. F. Akhavan, F. Mansouri, Z. Artelli,
F. Mohammadi, H. Vali
&& – &&
Imidazolyl-Functionalized Ordered
Mesoporous Polymer from
Nanocasting as an Effective Support
for Highly Dispersed Palladium
Nanoparticles in the Heck Reaction
ChemCatChem 2016, 8, 1 – 9
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ