A metallosalen-based microporous organic polymer as a heterogeneous carbon-carbon coupling catalyst

Article abstract of DOI:10.1039/c3ta13128a

A novel microporous organic polymer framework with built-in metal sites in the skeleton has been synthesized by the Sonogashira-Hagihara reaction using salen-palladium as building units. The new microporous organic polymer possesses a high BET surface area, large pore volume, good stability and acts as a highly efficient robust recyclable heterogeneous catalyst towards carbon-carbon coupling reactions with low catalyst loading.

Full text of DOI:10.1039/c3ta13128a

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Materials Chemistry A
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A metallosalen-based microporous organic polymer as a
heterogeneous carbon–carbon coupling catalyst†
Cite this: J. Mater. Chem. A, 2013, 1,
14108
a
a
a
a
a
b
a
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He Li, Bo Xu, Xiaoming Liu, Sigen A, Chuanyi He, Hong Xia and Ying Mu
A novel microporous organic polymer framework with built-in metal sites in the skeleton has been
synthesized by the Sonogashira–Hagihara reaction using salen-palladium as building units. The new
microporous organic polymer possesses a high BET surface area, large pore volume, good stability and
acts as a highly efficient robust recyclable heterogeneous catalyst towards carbon–carbon coupling
reactions with low catalyst loading.
Received 8th August 2013
Accepted 17th September 2013
DOI: 10.1039/c3ta13128a
www.rsc.org/MaterialsA
organic frameworks (EOFs),9 covalent triazine frameworks
(CTFs)10 and covalent organic frameworks (COFs)11 can be
Introduction
Transition metal-catalyzed cross-coupling reactions for the
formation of carbon–carbon bonds have been recognized as
powerful synthetic tools and play an important role in multiple
organic transformations and in the pharmaceutical industry.1
High activity for coupling reactions can be achieved by using a
constructed from well-functional organic precursors and they
represent a new class of organic porous materials with excellent
stability. Better than previously developed inorganic and inor-
ganic–organic hybrid porous materials, the MOPs not only
exhibit great promise in gas storage and separation, but they
can also serve as ideal platforms for incorporating catalytic
segments into highly stable, recyclable, and reusable hetero-
geneous catalyst systems by taking advantage of their perma-
nent porosity and the ability to tune their compositional
properties at the molecular level. Recently, a series of research
results demonstrated that the new class of porous organic
polymers serve as excellent candidates for supports for hetero-
geneous catalysts.12 For instance, the Cooper group recently
synthesized a series of metal-functionalized microporous pol-
y(aryleneethynylene) networks for reductive amination.13 Wang
et al. selected imine COFs as supramolecular ligands of sup-
ported palladium complexes which catalyzed Suzuki coupling
reactions under heterogeneous conditions.14 The Thomas group
immobilized palladium nanocrystals onto the surface of cova-
lent triazine frameworks and used them in liquid phase reac-
tions.15 In contrast to post-immobilized networks for loading
activated sites in the pores,16 however, exploration of hetero-
geneous catalysis systems based on MOPs with covalently
linked catalytic metal sites is very rare.17
With these considerations in mind, in this contribution, we
report the synthesis of a metallosalen-based microporous
organic polymer (MsMOP-1) with salen-palladium building
blocks by a bottom-up strategy. New MsMOP-1 as a heteroge-
neous catalyst has a distinct advantage over other known
porous materials that they could incorporate catalytic sites into
the skeleton, thus the skeleton itself serves as the solid catalyst;
the inherent pores provide space for the transformation,
and the strong interaction between the activated metal and
framework prevents catalytic site leaching. Therefore, MsMOP-1
homogeneous metal catalyst with
a variety of auxiliary
ligands.2–4 However, many noble metals used in homogeneous
systems are quite expensive and it is difficult to separate the
soluble catalyst from the reaction system. To address these
limitations, immobilization of the homogeneous catalyst is a
promising option. This was normally achieved by supporting
the active components on prefunctionalized solid materials
such as activated carbon, silicates, zeolites, organic polymers,
nanoparticles and metal–organic frameworks.5 However, the
reaction performance of the heterogeneous catalysts was
affected importantly by the intrinsic properties of the support as
well as the links between active components and the support. In
this context, an excellent support for a heterogeneous catalyst
not only needs easy synthesis but also should possess a high
surface area, large pore volume, and good chemical and thermal
stability. Therefore, the development of a new class of
outstanding supports for heterogeneous catalysis is also an
important challenge.
Microporous organic polymers (MOPs) such as polymers of
intrinsic microporosity (PIMs),6 conjugated microporous poly-
mers (CMPs),7 hypercross-linked polymers (HCPs),8 element
aState Key Laboratory for Supramolecular Structure and Materials, College of
Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun, 130012, P.R.China.
E-mail: xm_liu@jlu.edu.cn; Fax: +86 431 85168376; Tel: +86 431 85168376
bState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
Technology, Jilin University, 2699 Qianjin Avenue, Changchun, 130012, P.R.China
† Electronic supplementary information (ESI) available: TGA, PXRD, UV, SEM
images of MsMOP-1 samples fresh and aer reaction, XPS spectra of MsMOP-1
samples fresh and aer reaction, some catalytic data, 1H-NMR data and GC-MS
data for catalyzed products. See DOI: 10.1039/c3ta13128a
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Scheme 1 Synthesis of MsMOP-1 using (i) DMAc–NEt3, Pd(PPh3)2Cl2, CuI, 80 ꢀC,
72 h.
can be used as a highly efficient recyclable heterogeneous
catalyst towards carbon–carbon coupling reactions (Scheme 1).
Fig. 1 SEM image of a typical MsMOP-1 sample (a), DLS profile of MsMOP-1 (b),
TEM image of MsMOP-1 (c), EDS elemental-mapping image of MsMOP-1 (d) and
the XPS spectrum of the Pd 3d orbital in MsMOP-1 (e).
Results and discussion
are consistent with the expected network structures. For
instance, a weak peak at 2253 cmꢂ1, which is attributed to the
band of C^C triple-bond stretching mode of disubstituted
acetylene groups (Fig. 2) is observed. Furthermore, the structure
of MsMOP-1 was characterized at the molecular level by solid
state 13C CP/MAS NMR. Fig. 3 shows the characteristic peaks at
60 ppm and 162 ppm, which is attributed to the carbon atom of
methylene and the imine bond, respectively. Furthermore, a
peak of the triple-bonded carbon resonates at around 90 ppm.
The minor resonances at 81 and 77 ppm are due to the terminal
–C^CH functionalities. The electronic adsorption spectra of
MsMOP-1 exhibited a long tailing in the low energy region,
which suggested an extended p-conjugation over the skeleton
(Fig. S3†).
In order to characterize the porosity parameters of MsMOP-1,
the nitrogen sorption isotherms were measured at 77 K. As
shown in Fig. 4a, the MsMOP-1 exhibits a combination of type-I
and II nitrogen sorption isotherms featured according to the
IUPAC classication and shows a steep nitrogen gas uptake at
low relative pressure, indicating that the material is micropo-
rous. The nitrogen sorption increases with increasing pressure,
The Schiff-base salen complexes are excellent catalysts for a lot
of molecular transformation and polymerization reactions. To
date, however, only a few examples of porous materials con-
taining metallosalen units have been reported.18 Along this line,
using salen-palladium and 1,3,5-triethynylbenzene as comono-
mers, MsMOP-1 was synthesized by using palladium-catalyzed
Sonogashira–Hagihara cross-coupling chemistry, which can be
used to prepare poly(arylene ethynylene) networks with high
microporosity.19 Different monomer ratios and solvent systems
including dioxane–triethylamine (TEA), N,N-dimethylforma-
mide (DMF)–TEA, tetrahydrofuran (THF)–TEA and N,N-dime-
thylacetamide (DMAc)–TEA in the polymerization reaction were
considered rst, in order to achieve high surface areas for the
resulting polymer frameworks. As an optimal condition, the
polymerization reaction is carried out using a 1.5 M excess of
the ethynyl functionality in DMAc–TEA (1/1 in volume) and
Pd(PPh3)2Cl2/CuI as a catalyst, MsMOP-1 with the largest
surface area can be prepared.
MsMOP-1 is insoluble in water and common organic
solvents and it is stable in a nitrogen atmosphere up to 300 ꢀC,
as revealed by TGA (Fig. S1†). The Pd metal loading for MsMOP-
1 was determined by ICP to be 5.01%.20 The polymer framework
is basically amorphous with a low degree of crystallinity as
indicated by PXRD (Fig. S2†). SEM images also showed that
MsMOP-1 adopted a ake morphology with a size of 200–
400 nm (Fig. 1a). The size distribution of the polymer samples at
room temperature in a methanol suspension was 220.2 ꢁ
25.0 nm (Fig. 1b). TEM revealed the presence of a porous texture
(Fig. 1c). The elemental mapping by energy-dispersive X-ray
spectroscopy (EDS) analysis displayed a homogeneous distri-
bution of palladium (Fig. 1d). X-ray photoelectron spectroscopy
(XPS) studies were conducted for MsMOP-1 to characterize the
chemical state of palladium. As shown in Fig. 1e, the binding
energies at 338.3 and 343.5 eV correspond to Pd 3d5/2 and 3d3/2
orbitals, which indicated that the palladium species is present
in the +2 state.14,21 Compared with IR spectra of monomers,
Fig. 2 The IR spectra of MsMOP-1 (black line) and its monomers: 1,3,5-triethynyl-
MsMOP-1 also retains its many feature peaks, and other peaks benzene (green line) and salen palladium (red line).
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As one of the most versatile methods for the synthesis of
pharmaceutical, intermediate and conjugated polymers, the
Suzuki cross-coupling reaction has attracted extensive attention
in recent years. To evaluate the catalytic activity of MsMOP-1 as
a robust heterogeneous catalyst, we accordingly tested the
Suzuki–Miyaura coupling reactions of arylboronic acids with
various types of aryl halides. The inuences of different
parameters including bases, solvents and reaction temperature
on the yield in the coupling reaction of iodobenzene with
phenylboronic acid were rst researched. As could be seen in
Table S1,† the DMF and DMAc proved to be poor solvents
(31 and 35%, Table S1,† entries 1 and 2), whereas toluene and
DMF–H2O at a 1 : 1 volume ratio afforded moderate yield
(50 and 71%, Table S1,† entries 3 and 5) under the same
conditions. In sharp contrast, high yields were obtained in
either EtOH or a mixture of EtOH–H2O at a 1 : 1 volume ratio,
which could be attributed to the good solubility of both organic
reactants and inorganic bases in those solvents. Among the
various bases screened in EtOH–H2O, K2CO3 gives the highest
Fig. 3 13C CP/MAS NMR spectroscopy of MsMOP-1. Asterisks (*) indicate peaks
arising from spinning side bands.
ꢀ
yield of 99% in 1 h at 80 C (Table S1,† entry 6). However, low
ꢀ
catalytic activity was obtained at 60 C and a product yield of
94% could be achieved in more than 2 h (Fig. 5a). For a reaction
to be feasible for industrial application, a short reaction time
would be highly attractive. Under the optimized reaction system
EtOH–H2O, K2CO3 and 80 ꢀC, we also found that MsMOP-1 can
catalyze the coupling reaction of a variety of iodo and bromo
Fig. 4 Nitrogen adsorption–desorption isotherms (filled circles: adsorption,
open circles: desorption) (a) and pore size distribution (b) of MsMOP-1.
which indicates a large external surface area owing to the
presence of small particles. The sharp increase in the nitrogen
uptake at relatively high pressure may arise in part due to
interparticulate porosity associated with the meso/macrostruc-
tures of the samples. The specic surface areas were calculated
in the relative pressure range from 0.01 to 0.1, and the results
showed that the Brunauer–Emmett–Teller (BET) specic surface
area of MsMOP-1 was 554 m2 gꢂ1. The MsMOP-1 has a wide pore
size distribution, with the pore widths centering around 0.65,
1.15 and 2.18 nm, respectively, as calculated by the Saito–Flory
method (Fig. 4b).7d–f The total volume calculated with nitrogen
gas adsorbed at P/P0 ¼ 0.99 was 0.547 cm3 gꢂ1 for MsMOP-1.
Fig. 5 Comparison of the catalytic activity of MsMOP-1 (a) at 60 ꢀC ( ) and 80 ꢀ
C
( ) and reusability of MsMOP-1 (b) in the Suzuki–Miyaura coupling reaction (entry
1 in Table 1).
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derivatives with phenylboronic acid. As shown in Table 1, all of substrates. The MsMOP-1 polymer network was separated from
the reactions were completed very efficiently and gave corre- the reaction mixtures by centrifugation, washed and reused in
sponding biaryl compounds. For aryl iodides, the coupling the subsequent round of coupling reaction. The reaction was
reactions were rapidly completed with high yields, even for repeated ve times without any signicant loss of the original
substrates with a large steric hindrance (Table 1, entry 5). shape and catalytic activity (Fig. 5b and S5†), which is attributed
Similarly, high to excellent catalytic activity was observed for the to the built-in character of the covalently linked catalytic sites of
aryl bromides, which was slightly affected by the electronic MsMOP-1. This is supported by very low metal leaching before
properties of the substituents on the phenyl groups. Compared (5.01%) and aer (4.94%) the reaction as shown by ICP analysis.
with electron-donating groups, the electron-withdrawing In addition, we ltered the catalyst from the hot solution at 50%
substituents exhibited a positive effect on the coupling reaction conversion, the isolated solution did not exhibit any further
in the same reaction time. (Table 1, entries 8 and 10).
reactivity, which indicated clearly that the heterogeneous cata-
The recycling of the catalyst is the most important advantage lytic property of MsMOP-1. In any case, our present evidence
in the heterogeneous reaction for commercial application. Thus indicates that leaching of the active palladium species during
we investigated the possibility of recycling MsMOP-1 in the the reaction was negligible. At present, the most reasonable
model reaction using iodobenzene and phenylboronic acid as proposal for the catalytic mechanism is based on the process for
homogeneous palladium complexes and Pd0 oxidation state
species are considered as active intermediates during the cata-
Table 1 The Suzuki–Miyaura coupling reaction of various aryl halides with
phenylboronic acid in the presence of MsMOP-1a
lytic cycle. However, XPS of the used MsMOP-1 samples was
almost identical to those of fresh samples (Fig. S6†). Therefore,
we speculate that the catalytic process of heterogeneous
Yieldb (%)
Table 2 Heck coupling reaction of various aryl halides with olefins in the pres-
ence of MsMOP-1a
Entry
Arylhalide
Time (h)
1
2
1
1
99
99
Entry
Aryl halide
Olen
Time (h)
Yieldb (%)
3
4
1
1
98
96
1
2
1
1
99
99
5
6
1
1
77
99
3
4
1
1
90
90
7
8
3
3
90
71
5
6
7
1
1
95
25
76
9
3
92
12
10
11
3
3
99
99
8
3
77
9
3
3
80
92
12
3
3
75
80
10
11c
1
94
13
a
a
Reaction conditions: arylhalide (0.5 mmol), phenylboronic acid
(0.75 mmol), K2CO3 b(1 mmol), MsMOP-1 (1.0 mg), EtOH–H2O (1.5 mL/
1.5 mL) and 80 ꢀC. Determined by GC yields using undecane as the
internal standard.
Reaction conditions: arylhalide (0.5 mmol), olen (1.5 mmol),
Et3N (1.5 mmol), DMAc (3.0 mL), MsMOP-1 (1.0 mg) and 130 ꢀC.
b
Determined by GC yields using undecane as the internal standard.
The h cycle.
c
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MsMOP-1 may be similar to those of the reported metal–organic atmosphere. Aer cooling to room temperature, the precipitate
frameworks.22
was ltered and washed three times with water, chloroform,
Because the Heck coupling reaction is another powerful and methanol and acetone to remove any unreacted monomer and
versatile tool to prepare arylated alkenes in ne chemical catalyst. Further purication of the target polymer was carried
synthesis, the reactions between olens and aryl halides were out by Soxhlet extraction with water, methanol, acetone and
carried out to further evaluate the catalytic performance of tetrahydrofuran for 48 h. The target product was dried at 120 ꢀC
MsMOP-1. As expected, MsMOP-1 also exhibits high catalytic under vacuum for 24 h to give a brown powder. Yield: 55 mg
efficiency for the Heck coupling reaction of various aryl halides (71%). The elemental analysis result of MsMOP-1 found: C,
and olens in DMAc in the presence of Et3N at 130 ꢀC with 80.9; H, 2.92; N, 1.72%. We inferred that the probable chemical
excellent yields (Table 2 and S2†). The recovered heterogeneous formula for MsMOP-1 was C110H48N2O8Pd. The value can be
catalyst MsMOP-1 retains high catalytic activity in subsequent affected by the incomplete combustion or the sorption of small
runs (Table 2, entry 11).
molecules.24 Different monomer ratios (ethynyl–bromine group
¼ 1.5/1, 1/1, 1/1.5) and solvent systems (dioxane–triethylamine
(TEA), N,N-dimethylformamide (DMF)–TEA, tetrahydrofuran
(THF)–TEA and N,N-dimethylacetamide (DMAc)–TEA) in the
polymerization reaction were investigated, in order to achieve
high surface areas for the resulting polymer frameworks. The
isolation yields for MsMOP-1 obtained under various reaction
conditions are 68% to 85%. The nitrogen sorption isotherms
and pore distributions of MsMOP-1 under different reaction
conditions are shown in Fig. S4.†
Conclusions
In summary, we reported a powerful strategy for the construc-
tion of an excellent heterogeneous catalyst based on micropo-
rous organic polymers and demonstrated that a new MsMOP-1
as a robust solid catalyst for carbon–carbon coupling reactions.
Owing to the presence of inherent pores, covalently linked
catalytic sites and its good stability, MsMOP-1 displays high
activity, wide substrate adaptability and good recyclability in
carbon–carbon coupling reactions. Especially, using this
synthetic strategy we can easily prepare homochiral MsMOP-1
with a single catalytic center using chiral metallosalen as
building units. Currently, research into the asymmetric catalytic
properties of homochiral MsMOPs as solid catalysts is
underway in our laboratory.
A typical procedure for the Suzuki–Miyaura coupling reaction
catalyzed by MsMOP-1
In a round-bottom ask catalyst MsMOP-1 (1.0 mg), phenyl-
boronic acid (0.75 mmol), K2CO3 (138 mg, 1.0 mmol), aryl
halide (0.5 mmol), ethanol (1.5 mL) and water (1.5 mL) were
ꢀ
placed. The reaction mixture was stirred at 80 C for a certain
time under nitrogen atmosphere and the progress of the reac-
tion was monitored by GC. Aer the reaction was complete, the
reaction mixture was cooled down to room temperature,
centrifuged, and extracted with ethyl acetate–water. The organic
layer was separated, dried over anhydrous MgSO4, and
concentrated in a vacuum. The coupling product was puried
by column chromatography.
Experimental section
Materials
5-Bromo-2-hydrobenzaldehyde, Pd(OAc)2, ethylenediamine,
and bis-(triphenylphosphine)palladium(II) dichloride were
purchased from Aldrich and used as received. Other organic
solvents for reactions were distilled over appropriate drying
reagents under nitrogen. Salen ligand and salen palladium
complex were all prepared using the literature procedure.23
Deuterated solvents for NMR measurement were obtained from
Aldrich.
A typical procedure for the Heck coupling reaction catalyzed
by MsMOP-1
In a round-bottom ask catalyst MsMOP-1 (1.0 mg), aryl halide
(0.5 mmol), olen (1.5 mmol), Et3N (1.5 mmol) and DMAc
(3.0 mL) were placed. The reaction mixture was stirred at 130 ꢀC
for 1 h under nitrogen atmosphere and the progress of the
reaction was monitored by GC. Aer the reaction was complete,
the reaction mixture was cooled down to room temperature,
centrifuged, and extracted with ethyl acetate–water. The organic
layer was separated, dried over anhydrous MgSO4, and
concentrated in a vacuum. The coupling product was puried
by column chromatography.
Synthesis procedure for MsMOP-1 networks
All of the metallosalen microporous organic polymers were
synthesized by palladium catalyzed Sonogashira–Hagihara
cross-coupling polycondensation of 1,3,5-triethynylbenzene
and the salen-palladium monomer. The reaction was carried
out using a 1.5 M excess of ethynyl groups in DMAc–TEA, and it
was found to maximize the surface area in the microporous
polymer frameworks. A typical experimental procedure for
MsMOP-1 is given in detail as follows: 1,3,5-triethynylbenzene
(30.0 mg, 0.20 mmol) and salen-Pd (69 mg, 0.13 mmol) were
dissolved in a mixture of DMAc (3.0 mL) and triethylamine
Typical recycling procedure for the carbon–carbon coupling
reaction catalyzed by MsMOP-1
(3.0 mL), and then the resulting mixture was degassed by the Aer the completion of the carbon–carbon coupling reaction,
freeze–pump–thaw cycles. Bis-(triphenylphosphine)palladium(II) the MsMOP-1 catalyst was recovered by centrifugation, and then
dichloride (9.2 mg, 0.013 mmol) and copper(I) iodide (3.8 mg, washed thoroughly with tetrahydrofuran, water, ethanol and
0.02 mmol) were added to the mixture. The resulting mixture acetone to remove the residual reactant and base. The recovered
was heated to 80 ꢀC and stirred for 72 h under a nitrogen catalyst was dried under vacuum at 100 ꢀC overnight. This used
14112 | J. Mater. Chem. A, 2013, 1, 14108–14114
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catalyst was re-employed in the subsequent cycle under iden- X. Liu is also grateful for support by the Frontiers of Science and
tical conditions.
Interdisciplinary Innovation Project of Jilin University
(no. 450060481015).
Physical characterization
Notes and references
1H spectra were recorded on a Varian Mercury-300 NMR spec-
trometer, where chemical shis (d in ppm) were determined
with a residual proton of the solvent as the standard. Solid-state
13C CP/MAS NMR measurements were recorded on a Bruker
AVANCE III 400 WB spectrometer at a MAS rate of 5 kHz and a
CP contact time of 2 ms. All 13C CP MAS chemical shis are
referenced to the resonances of an adamantane (C10H16) stan-
dard (d CH2 ¼ 38.5). The infrared spectra were recorded from
400 to 4000 cmꢂ1 on a Nicolet FT-IR 360 spectrometer by using
KBr pellets. UV-vis absorption spectra were recorded on a Shi-
madzu UV-2550 spectrophotometer. Elemental analyses were
carried out on an Elementar model vario EL cube analyzer. Field
emission scanning electron microscopy was performed on a
SU8020 model HITACHI microscope. Transmission electron
microscopy was performed on a JEOL model JEM-2100 micro-
scope equipped with an EDS detector. The sample was prepared
by drop-casting a methanol suspension of MsMOP-1 onto a
copper grid. The size distribution of MsMOP-1 was determined
by a dynamic light scattering method using a NICOMP 380 ZLS
(Particle Sizing System Co.). The Pd contents in polymer
frameworks were determined by Perkin-Elmer ICP-OES Optima
3300DV spectroscopy. X-ray photoelectron spectroscopy (XPS)
was performed using an ESCALAB 250 spectrometer. Powder
X-ray diffraction data were recorded on a PANalytical BV
Empyrean diffractometer by depositing powder on a glass
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ꢀ
substrate, from 2q ¼ 4.0ꢀ to 60ꢀ with 0.02ꢀ increment at 25 C.
Thermogravimetric analysis (TGA) was performed on a TA Q500
thermogravimeter by mꢂe1asuring the weight loss while heating
at a rate of 10 ꢀC min from 25 to 800 ꢀC under nitrogen.
Nitrogen sorption isotherms were measured at 77 K with a
JW-BK 132F analyzer. Before measurement, the samples were
ꢀ
degassed in a vacuum at 150 C for more than 10 h. The Bru-
nauer–Emmett–Teller (BET) method was utilized to calculate
the specic surface areas and pore volume, and the Saito–Foley
(SF) method was applied for the estimation of pore size
distribution.
The catalytic products were quantied by GC analysis (Shi-
madzu GC-2014C) using an Rtx-5 column (30 m ꢃ 0.25 mm ꢃ
0.25 mm, Restek). The melting point was recorded on a melting
point apparatus (MPA100, Stanford Research Systems, Inc.).
GC-MS was performed using a QP2010 gas chromatography
mass spectrometer (GC-2010 coupled with a GC-MS QP-2010,
Shimadzu) equipped with a DB-5MS column (30 m ꢃ 0.25 mm
ꢃ 0.25 mm, Agilent). The column temperature was programmed
from 150 ꢀC (3 min hold) to 280 ꢀC at 50 ꢀC minꢂ1. Mass spectra
were obtained by the electron-impact (EI) at 70 eV using the full-
scan mode.
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Acknowledgements
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