Palladium nanoparticles supported on triazine functionalised mesoporous covalent organic polymers as efficient catalysts for Mizoroki-Heck cross coupling reaction

Article abstract of DOI:10.1039/c4gc00412d

A novel class of mesoporous covalent organic polymer (MCOP) was synthesised by the nucleophilic substitution of cyanuric chloride with 4,4′- dihydroxybiphenyl. The MCOP was fully characterized using powder X-ray diffraction analysis, Fourier transform infrared spectroscopy, ¹³C-solid state NMR spectroscopy, field emission scanning electron microscopy and thermogravimetric analysis. These nitrogen rich materials act as good supports for palladium nanoparticles (Pd NPs) and exhibit excellent catalytic activity towards Mizoroki-Heck cross coupling between aryl bromides and alkenes. Hot filtration tests demonstrate that the presence of the triazine rings on the polymers is beneficial for enhancing the stability of Pd NPs. The polymers are also cheap, easy to synthesise and can be recycled up to five times with only a minor loss of activity. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00412d

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Palladium nanoparticles supported on triazine
functionalised mesoporous covalent organic
polymers as efficient catalysts for Mizoroki–Heck
cross coupling reaction†
Cite this: DOI: 10.1039/c4gc00412d
Pillaiyar Puthiaraja and Kasi Pitchumani*a,b
A novel class of mesoporous covalent organic polymer (MCOP) was synthesised by the nucleophilic
substitution of cyanuric chloride with 4,4’-dihydroxybiphenyl. The MCOP was fully characterized
using powder X-ray diffraction analysis, Fourier transform infrared spectroscopy, 13C-solid state
NMR spectroscopy, field emission scanning electron microscopy and thermogravimetric analysis. These
nitrogen rich materials act as good supports for palladium nanoparticles (Pd NPs) and exhibit excellent
catalytic activity towards Mizoroki–Heck cross coupling between aryl bromides and alkenes. Hot filtration
tests demonstrate that the presence of the triazine rings on the polymers is beneficial for enhancing the
stability of Pd NPs. The polymers are also cheap, easy to synthesise and can be recycled up to five times
with only a minor loss of activity.
Received 8th March 2014,
Accepted 9th June 2014
DOI: 10.1039/c4gc00412d
www.rsc.org/greenchem
Recently, we have reported triazine based mesoporous covalent
imine polymers as novel solid supports for copper mediated
Introduction
Covalent organic polymers (COPs) are a new class of emerging Chan–Lam cross coupling of the N-arylation reaction.5b Some
light weight micro/meso porous materials ingeniously formed porous organic frameworks (POFs) and nanoparticles sup-
by strong covalent linkages between C, Si, B, N and O.1 These ported POFs materials have been used as catalysts in organic
materials contain well-defined, predictable two dimensional reactions.6 As some of the porous materials based supports
(2D) or three dimensional (3D) ordered porous architectures,2 used to include the nanoparticles are unstable in aqueous and
similar to those of metal–organic frameworks (MOFs).3 These most organic solvents, tedious synthetic protocols are needed
materials have advantages, such as light weight, low density, for their synthesis as well as the usage of these catalysts.7 In
cheap and easy synthesis, tenability, high stability to thermal order to overcome these issues, we believe that the synthesis is
treatment with water and most of the organic solvents, high desirable of new highly stable porous materials in which there is
thermal stability and permanent porosity.1 Based on these a strong interaction between the support and the loaded metals.
advantages, COPs based materials have found various appli-
The palladium-catalyzed Heck reaction of aryl halides with
cations of technical relevance in separation, gas storage, gas alkenes is one of the most important and versatile tool for C–C
purification, drug delivery, sensors, semiconductive and bond forming processes in synthetic and medicinal chemistry
photoconductive devices, charge carriers and catalysis.4 Conse- in the search for new derivatives with wide ranging therapeutic
quently the inclusion of multifaceted functionality into porous potentials.8,9 Over the decades, various homogeneous catalytic
networks is one of the frontline areas of research, which could systems, which always exhibit better activity and selectivity,
lead to the synthesis of new materials with diverse appli- have been developed for this transformation.10 Although many
cations. For example, the incorporation of an active functional- organometallic compounds with special ligands have been
ity, namely, nitrogen on the surfaces of the supported used as catalysts for this reaction, most of them suffer from
materials may be beneficial to enhance the catalytic perform- drawbacks such as high ligand sensitivity towards air and
ance as well as the stability of the supported materials.5 moisture, tedious multistep synthesis and work-up, high cost
of the ligands and use of various additives and harmful
organic solvents. Furthermore, the potential application of
aSchool of Chemistry, Madurai Kamaraj University, Madurai-625 021, India
homogeneous catalytic systems in industry is limited due to
bCentre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj
difficulties in separation, recovery and reusability and metal
University, Madurai-625021, India. E-mail: pit12399@yahoo.com
leaching of these expensive catalysts.11 Thus the current
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4gc00412d
challenge for palladium catalyzed coupling reactions is the
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development of high performance catalysts using sustainable thoroughly characterized by Fourier transform infrared spectro-
and environmentally benign reaction conditions.12 Thus, the scopy (FT-IR), 13C cross polarization-magic angle spinning
search for heterogeneous technologies that are greener, safer, (CP-MAS) NMR spectroscopy, field emission-scanning electron
environmentally friendly and reusable is a research priority, microscopy (FE-SEM), powder X-ray diffraction analysis
particular for the chemical and pharmaceutical industries. (PXRD), thermogravimetric analysis (TGA) and nitrogen gas
Recently, our group reported the use of a palladium amino- adsorption studies. The synthesised MCOP material is insolu-
cyclodextrin complex as an efficient catalyst for Heck cross ble in water and common organic solvents such as DMF, THF,
coupling.13 However, palladium nanoparticles (Pd NPs), which are DMSO, acetone, etc.
important heterogeneous catalysts, are prone to aggregation
The bands at 1249 cm−1 and 1562 cm−1 in the FT-IR spec-
and lose their catalytic activity without the use of a stabilizer.14 trum of MCOP (Fig. 1) indicate the presence of C–O–C bonds
Therefore, they need to be supported on solid materials (such and triazine units. The C–Cl stretching at 851 cm−1 has
as carbon frameworks, silica, silica carbon composites and completely diminished, suggesting that the starting material,
metal–organic frameworks) to be useful as catalysts for Heck cyanuric chloride, has been converted.
reactions.15 In addition, in many cases, the catalytic active
The CP-MAS NMR spectral data (Fig. 2) clearly confirmed
sites may be lost due to the leaching of palladium in sup- the chemical structure of the synthesised MCOP material. Five
ported systems which makes the catalyst difficult to recover peaks at 122.2, 127.1, 137.4, 151.4, 173.5 ppm can be seen in
and reuse.12 Therefore, the need for suitable supports for Pd the solid state NMR spectrum. The peak at 173.5 ppm corres-
NPs catalysts is an important goal in heterogeneous catalysis.
Recently, Zhang et al. have reported the use of mesoporous
silica supported Pd NPs as an efficient and reusable catalyst
for the Ullmann reaction in water-medium.16 In this context,
nitrogen rich triazine based POFs may incorporate a large
number of metal binding sites. Herein, we first report a novel
strategy for the synthesis of new mesoporous covalent organic
polymer supported Pd NPs for use in the Heck reaction under
eco-friendly conditions. The porous organic polymer materials
have been synthesised by means of the simple nucleophilic
substitution of relatively cheap, industrially important and
readily accessible chemicals.
Results and discussion
The triazine based mesoporous covalent organic polymer
(MCOP) was synthesised by the nucleophilic substitution
between cyanuric chloride and 4,4′-dihydroxybiphenyl as
Fig. 1 FT-IR spectra of cyanuric chloride (CNC), 4,4’-dihydroxybiphenyl
(DBP) and MCOP.
depicted in Scheme 1. The synthesised material was
Scheme 1 Schematic representation of MCOP.
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cauliflower-like morphology. These observations show that the
substitution of cyanuric chloride and 4,4′-dihydroxybiphenyl
leads to a uniform morphology, porosity and a certain degree
of structural regularity. Fig. 3d in the HR-TEM image clearly
indicates the porosity of the structure.
The regularity and crystallinity of the structure of the MCOP
material was further confirmed by PXRD (Fig. 4). The obtained
PXRD pattern of MCOP indicates the partial crystallinity of the
material, furthermore, the set of peaks in the range of 8 to 35°
Fig. 2 13C CP-MAS NMR spectrum of MCOP.
ponds to the triazine carbon. The peak at 151.4 ppm corres-
ponds to the carbon atom of the oxygen attached to biphenyl.
The signals at 122.2, 127.1 and 137.4 ppm can be assigned to
the carbon atoms of the phenyl rings. Thus, the NMR data
clearly confirm that the polymer was formed from cyanuric
chloride and 4,4′-dihydroxybiphenyl.
The morphology and porosity of MCOP can be observed
from the FE-SEM and high resolution-transmission electron
microscopy (HR-TEM) images presented in Fig. 3. The images
presented in Fig. 3a–3c clearly indicate that MCOP adopts a
Fig. 4 Powder XRD pattern of MCOP.
Fig. 3 Images of the synthesised MCOP (a, b and c are FE-SEM images and d is an HR-TEM image of MCOP).
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suggests that the framework of the material has a certain
degree of order.
As seen from the structure of MCOP, which contains large
amounts of nitrogen and oxygen atoms, the incorporation of
The thermal stability of MCOP was studied by TGA under a suitable metal ions in its frameworks can be anticipated.
nitrogen atmosphere (Fig. 5). As shown in Fig. 5, MCOP exhi- Recently amorphous triazine based frameworks were success-
bits a 3% weight loss at around 80 °C due to the loss of solvent fully employed as heterogeneous supports for noble metal
molecules and the second weight loss takes place above catalysts.2a,5,12d With this in mind, we incorporated the Pd NPs
250 °C. A gradual weight loss (up to 60%) of MCOP was on MCOP as a support (Pd@MCOP) for use as an excellent
observed above 250 °C. The TGA of Pd@MCOP demonstrated heterogeneous catalyst for Mizoroki–Heck cross coupling.
that it also had good thermal stability up to 300 °C.
Through the simple treatment of MCOP with palladium
The porosity and surface area of the above materials were acetate, a Pd@MCOP material was synthesised in a facile
measured by nitrogen adsorption–desorption analysis at 77 K manner (see ESI†). The synthesised Pd@MCOP was also
(Fig. 6). These isotherms are closely related to a type IV iso- characterized by PXRD, HR-TEM, X-ray photoelectron spec-
therm, which is characteristic of mesoporous materials.4d,17 troscopy (XPS) analysis and the stability of the material was
From the BET isotherms, the surface area was found to be confirmed by TGA. The palladium loading level of the fresh
146 m2 g−1 for MCOP. Pore size distribution plots of MCOP catalyst was found to be ∼2.04 wt% from SEM-energy disper-
(insets of Fig. 6) display a pore width of 23.4 Å.
sive X-ray spectroscopy (EDX) and 2.42% from inductively
coupled plasma-optical emission spectroscopy (ICP-OES).
From BET isotherm analysis of Pd@MCOP the surface area
was found to be 63 m2 g−1 and there was almost no change in
the pore size (23.2 Å).
The crystalline nature of the Pd NPs was confirmed from
PXRD data. Fig. 7 shows the PXRD pattern of vacuum dried
Pd@MCOP NPs obtained from MCOP. A comparison of the
PXRD patterns of MCOP (Fig. 4) and Pd@MCOP (Fig. 7) shows
that the structure of MCOP was well maintained after the syn-
thesis of Pd NPs. Bragg reflections with 2θ values of 40.10°,
46.42° and 68.32° corresponding to the (111), (200) and (220)
sets of lattice planes are observed which may be indexed as the
band for face centred cubic structures of palladium (JCPDS no.
7440-05-3). The PXRD pattern thus clearly illustrates that the
Pd NPs synthesized by the present method are crystalline in
nature. The electronic state of palladium and the metallic pal-
ladium which was supported strongly on the nitrogen in the
MCOP materials were further confirmed by XPS analysis
(Fig. 8). Two strong peaks with binding energies at around
335.5 and 340.6 eV for the 3d5/2 and 3d3/2 core levels can be
Fig. 5 TGA of MCOP (blue), Pd@MCOP (red).
Fig. 6 Nitrogen adsorption–desorption isotherm of MCOP (adsorption –
solid square, desorption – empty circle). Pore size distribution of MCOP (inset).
Fig. 7 Powder XRD pattern of Pd@MCOP.
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Fig. 8 XPS spectra of fresh Pd@MCOP catalyst (a) metallic Pd species and (b) N 1s core levels.
seen in the XPS spectrum presented in Fig. 8a,16,18 which This strong binding energy shift in the N 1s core level clearly
clearly indicates that all of the palladium species in demonstrates that the metallic palladium strongly binds with
Pd@MCOP catalyst exist in the metallic state. Fig. 8b is an XPS nitrogen of MCOP, thereby stabilising the metallic palladium
spectrum describing the N 1s core level of Pd@MCOP catalyst. and inducing the catalytic performance of Pd@MCOP.
A binding energy of 398.9 eV has been previously reported for
HR-TEM analysis was used to determine the morphology
the triazine unit (N 1s core level).19 The N 1s core level and shape of Pd NPs supported on MCOP. The HR-TEM image
binding energy of Pd@MCOP catalyst appeared at 397.5 eV. analysis of the Pd@MCOP catalyst (Fig. 9) suggested that the
Fig. 9 HR-TEM image of the synthesised Pd@MCOP.
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Table 1 Optimization of reaction conditionsa
entries 13–16) used for this coupling reaction, K2CO3 gave
excellent results.
Under these conditions, the effect of other parameters such
as temperature, amount of bases, catalyst and reaction time
was also recorded. At a temperature below 90 °C, the yield of
3a decreased, whereas an increase of the temperature to above
90 °C (Table 1, entries 17–20) resulted in no marked change in
the overall yield. The base concentration plays an important
role in this heterogeneous palladium catalysed Mizoroki–Heck
cross coupling reaction. In the absence of base, the reaction
failed completely (Table 1, entry 4) and when the amount of
base was reduced to 1 mmol, the yield of 3a was reduced to
92% (Table 1, entry 21). When the reaction was conducted in
an oxygen atmosphere, the Heck coupled product was
observed with an 86% yield along with a 12% yield of the
Ullmann coupled product (Table 1, entry 22). As the amount of
the solid (Pd@MCOP) catalyst was increased to 30 mg, the
product was obtained in excellent yield and further addition of
catalyst had no positive effect on the overall yield of the
product (Fig. 10). An increase of the reaction time from 2 to
10 h increased the overall yield (Fig. 11). These observations
show that the optimum conditions for this palladium cata-
lysed Mizoroki–Heck coupling are use of Pd@MCOP (30 mg)
as a catalyst in DMSO–water (1 : 1, v/v) at 90 °C with 1 mmol of
base for 10 h.
Temp.
(°C)
Yieldb
(%)
Entry
Catalyst
Base
Solvent
1
2
3
4
5
6
7
8
—
MCOP
K2CO3
K2CO3
K2CO3
—
DMF
DMF
DMF
DMF
DMAc
DMSO
Water
MeOH
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
100
80
70
RT
90
90
NRf
NRf
94
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd(OAc)2
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
Pd@MCOP
NRf
90
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
K3PO4
NEt3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
97
57
62
60
68
98
57
92
80
83
26
98
86
79
NR
92
86
9
EtOH
ACN
10
11
12c
13
14
15
16
17
18
19
20
21d
22e
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
DMSO–waterg
The heterogeneity of the Pd@MCOP catalyst was checked
by carrying out a hot filtration test with bromobenzene and
styrene as substrates, to find out whether Pd is leaching out
from the solid catalyst to the solution or whether the catalyst is
truly heterogeneous in nature. After continuing the reaction
under optimized conditions for about 6 h, the catalyst was fil-
tered under hot conditions from the reaction mixture with
62% formation of 3a (Table 2). After removal of the solid cata-
lyst, the filtrate was then subjected to the reaction conditions
for an additional 4 h. After 10 h, no further yield of 3a was
observed. The absence of any significant metal leaching was
also confirmed by EDX. The EDX results suggested ∼2.04 wt%
and ∼2.00 wt% of palladium in fresh and fifth reused
Pd@MCOP (see ESI Fig. S1, S2 and Tables S1, S2†). ICP
a Bromobenzene (1 mmol), styrene (1.2 mmol), base (1.5 mmol),
solvent (2 mL), catalyst (30 mg, 0.006 mol%), 14 h. b Isolated yield
based on bromobenzene. c 5 mol% of Pd(OAc)2. d Yield of the reaction
carried out with 1 mmol of K2CO3. e Oxygen atmosphere. f NR = no
reaction. g 1 : 1 (v/v) ratio.
Pd NPs are in the 6–10 nm range and have almost spherical
shapes. The catalytic activity of Pd@MCOP was examined in
the Mizoroki–Heck coupling reaction.
The potential activity of Pd@MCOP was optimized for the
Mizoroki–Heck coupling reaction using the less reactive
bromobenzene (1a) and styrene (2a) as model substrates and
the results are summarized in Table 1. In the absence of catalyst
and in the presence of MCOP at 90 °C, no product was
obtained (Table 1, entries 1 and 2) and these results clearly
highlight the specific role of palladium NPs as catalysts in the
Mizoroki–Heck cross coupling reaction. Interestingly, when
the reaction was carried out with Pd@MCOP in DMF, it was
faster with a 94% yield of 3a, which indicates the importance
of Pd NPs (Table 1, entry 3). The reaction was also conducted
in N,N′-dimethyl acetamide (DMAc) and 90% yield of 3a was
obtained (Table 1, entry 5). The reaction was studied in various
solvents such as water, methanol, ethanol and acetonitrile,
only moderate yields were afforded (Table 1, entries 5 and
7–10). But with dimethyl sulfoxide as a solvent, the reaction
proceeded smoothly with 97% yield (Table 1, entry 6). On the
other hand, use of a mixture of solvents like DMSO–water
(1 : 1), afforded 98% yield (Table 1, entry 11). When the reac-
tion was carried out with Pd(OAc)2 instead of Pd@MCOP, a
57% yield of the product was observed (Table 1, entry 12).
Among the various inorganic and organic bases (Table 1,
Fig. 10 Dependence of yield on amount of catalyst.
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Table 3 Mizoroki–Heck reaction catalyzed by Pd@MCOP with different
substituentsa
Yieldb
(3) (%)
Entry Aryl bromides (1)
Alkenes (2)
1
2
3
4
5
6
7
8
9
10
Bromobenzene
4-Bromoanisole
4-Bromotoluene
4-Bromonitrobenzene
4′-Acetylbromobenzene
4-Bromobenzonitrile
4-Fluorobromobenzene
4-Chlorobromobenzene
3-Bromoanisole
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
3a 98
3b 95
3c 94
3d 91
3e 90
3f 89
3g 93
3h 94
3i 91
3j 93
Fig. 11 Effect of reaction time on the percentage conversion in the
Pd@MCOP catalysed Mizoroki–Heck reaction.
3,5-Dimethoxy-
bromobenzene
11
12
13
14c
15c
16c
17
18
19
20c
21
22
23
24
25
26c
3,5-Dichlorobromobenzene Styrene
3k 86
3l 90
3m 91
3n 87
3o 84
3p 80
3q 93
3r 96
3s 95
3t 82
3b 92
3c 91
3u 90
3v 93
3w 87
3x 88
Table 2 Hot filtration testa
1-Bromonaphthalene
2-Fluoro-4-bromoanisole
2-Bromo-4-nitrotoluene
2-Bromonitrobenzene
1,3,5-Tribromobenzene
3-Bromopyridine
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
GC conversion (%)
Catalyst
6 h
(6 + 4) h
62
3-Bromothiophene
2-Bromothiophene
Pd@MCOP
62
a Bromobenzene (1 mmol), Styrene (1.2 mmol, 1.2 equiv.), K2CO3
(1.5 equiv.), DMSO–water (2 mL, 1 : 1, v/v), Pd@MCOP (30 mg,
0.006 mol%), 90 °C, 10 h.
3-Hexyl-2-bromothiophene Styrene
Bromobenzene
Bromobenzene
4-Bromotoluene
4-Bromoanisole
Bromobenzene
4-Bromotoluene
4-Methoxystyrene
4-Methylstyrene
4-Methoxystyrene
4-Methoxystyrene
4-Vinylbenzoic acid
4-Methyl-
5-vinylthiazole
Methylacrylate
analysis of the filtrate from the hot mixture showed the pres-
ence of 0.003% of the palladium in the solution and in the
reused Pd@MCOP (after fifth reuse), the palladium content
27
Bromobenzene
3y 94
was found to be 2.41%. These results clearly demonstrate that a Aryl bromides (1 mmol), alkenes (1.2 mmol, 1.2 equiv.), K2CO3
(1.5 equiv.), DMSO–water (2 mL, 1 : 1, v/v), Pd@MCOP (30 mg,
Pd@MCOP is truly heterogeneous in nature.
0.006 mol%), 90 °C, 10 h. b Isolated yield based on aryl bromides. c 16 h.
After identifying the optimal conditions for the Mizoroki–
Heck reactions catalysed by Pd@MCOP, the activity of a range
of alkenes and substituted aryl bromides was also evaluated
and the results are summarized in Table 3. It was observed
that the aryl bromides containing electron-withdrawing groups
coupled readily with styrene to give excellent yields. Examples
include 4-bromonitrobenzene, 4′-acetylbromobenzene, 4-bromo-
benzonitrile, 4-fluoro-1-bromobenzene, 4-chloro-1-bromo-
benzene and 3,5-dichloro-1-bromobenzene (Table 3, entries
4–8 and 11). Interestingly, coupling of aryl bromides bearing a
methyl or methoxy group with styrene also gave higher yields
(Table 3, entries 3, 4 and 9). The reaction was also facile with
sterically hindered substrates such as 2-bromo-4-nitrotoluene,
2-bromonitrobenzene (Table 3, entries 14 and 15), although a
longer reaction time (16 h) was required. It should also be
noted that aryl bromides can be selectively coupled in the pres-
ence of aryl chlorides. When the substrate contains both
bromo and chloro substituents, styrene coupled readily only
with the bromo side (Table 3, entry 8). 1,3,5-Tribromobenzene
was also successfully coupled with styrene in good yield
(Table 3, entry 16), although a longer reaction time (16 h) was
required. Heteroaromatic and bicyclic bromides also gave the
corresponding coupled products in excellent yields (Table 3,
entries 12, 17–20). Different substituted aromatic alkenes
(Table 3, entries 21–25) and heteroaromatic alkenes (Table 3,
entry 26) also afforded good to excellent yields.
One of the main advantages of using heterogeneous cata-
lysts such as Pd@MCOP from an industrial perspective is that
they can be recovered and reused efficiently up to eight con-
secutive runs. After completion of the reaction, the solid cata-
lyst was recovered by filtration and extensively washed with
ethyl acetate and dried at room temperature in a vacuum desic-
cator for 2 h. Only a marginal loss in the activity of the catalyst
was observed for up to five consecutive reactions, and from the
sixth to eighth runs, the decrease in activity was small, yet sig-
nificant (Fig. 12).
On the basis of the above results, and also in accordance
with previous literature reports,11–13 a plausible reaction
pathway is proposed as shown in Scheme 2. In the first step,
oxidative addition of aryl bromides to Pd(0) leads to a Pd(II)
species. In the second step, the alkene coordinates to Pd(II)
species, which then undergoes an insertion reaction to form
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Experimental section
General methods
All of the chemicals were purchased from commercial suppli-
ers and used without further purification as commercially
available unless otherwise noted. Solvents were dried and dis-
tilled following a standard procedure. NMR spectra were
recorded at 400 and 300 MHz (mentioned in the respective
1
NMR data) using a Brucker spectrometer. All of the H NMR
and 13C NMR spectra were measured in CDCl3 with TMS as the
internal standard. The PXRD pattern of the catalyst sample
was measured with a SHIMADZU XD-D1 instrument using
Cu Kα radiation at room temperature. CP-MAS NMR measure-
ments were carried out using a Bruker Avance 300 spectro-
meter operating at 75 MHz for carbon NMR. Brunauer Emmett
Teller (BET) surface area analysis was carried out using a
Quantachrome-Autosorb instrument at 77 K. Surface mor-
phology and microstructures of the materials were analysed by
FESEM on a SFG (Quanta-250) instrument. HRTEM images
were recorded with a FEI TECNAI 30 G2 S-TWIN instrument.
SEM-EDX analysis of the materials was carried out using an
HITACHI S-3400N instrument. IR spectral analyses were per-
formed using a JASCO FT/IR-410 instrument in the range of
4000–500 cm−1 and the KBr pellet technique. TGA was per-
formed with a TGA/Shimadzu Thermal Analyser under a nitro-
gen atmosphere in the temperature range 30–800 °C at a
heating rate of 10 °C min−1. XPS measurements were per-
formed using a Kratos AXIS Ultra DLD X-ray photoelectron
spectrometer with a monochromatic Al Kα X-ray source and a
hemispherical analyser. Base pressure in the analysis chamber
was maintained in the range of 1 × 10−8 Torr. The Pd contents
of the Pd@COF samples were determined by ICP-OES analysis
with a Perkin Elmer Optima 5300 DV instrument.
Fig. 12 Recycling of the Pd@MCOP catalyst.
Synthesis of MCOP
Cyanuric chloride (0.8 mmol) was dissolved in 10 mL of
dioxane at 0 °C in a round-bottomed flask fitted with a con-
denser. Then 4,4′-dihydroxybiphenyl (1.2 mmol) and N,N-diiso-
propyl ethylamine (1.5 mmol) were added to the round-
bottomed flask. After stirring at 0 °C for 2 h, the temperature
was raised to room temperature and then refluxed for 1 day.
After 1 day, the reaction mixture was cooled and the solid was
removed by filtration and washed with hot ethyl acetate and
20 mL of 10% Na2CO3 (twice) and then the solid was
immersed in ethyl acetate for 3 days.
Scheme 2 Plausible mechanistic pathway forthe Mizoroki–Heck
reaction.
the Pd(II) complex. The coupled aryl and alkenic compound of
the palladium(II) complex is then eliminated via a β-hydride
removal followed by reductive elimination with base to retain
the original Pd(0).
Synthesis of Pd@MCOP
Details of the reaction conditions, activity and efficiency of
the various other catalysts employed earlier for the Mizoroki–
Heck cross coupling of aryl bromides with styrene are given in
Table 4. Comparison of the results indicates that our catalytic
system (entry 22) exhibits better catalytic activity compared to
other reported catalysts. These systems require external addi-
tives (entries 4, 13, 16 and 21), an inert atmosphere (entries 1,
3 and 13), longer reaction time (entries 2, 6, 7, 11 and 16–20)
and higher temperature (entries 1–21).
In a typical experiment, a mixture of palladium acetate (30 mg)
in acetone and MCOP material (500 mg) was stirred at room
temperature for 48 h. After 48 h, the solid was filtered and
washed with acetone and methanol to remove any unreacted
palladium acetate. The freshly prepared Pd/MCOP (500 mg)
was added to 50 mL of water, and the mixture was stirred for
10 min. Then hydrazine hydrate (2 mL, excess) was added
dropwise to the suspension under vigorous stirring, and the
mixture turned gray immediately. After stirring for 10 min, the
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Table 4 Comparison with previously reported catalytic systems for the Mizoroki–Heck cross coupling of aryl bromides with styrene
Temp. Time Yield
Entry Catalyst
Additive Base
Solvent
DMF
Atm. (°C)
(h)
(%)
References
1
2
[Pd{P(o-Tol)3}2]
CB[6]–Pd NPs
—
NaOAc
N2
—
N2
—
—
—
130 °C 10 h 94
140 °C 24 h 82
120 °C 6 h 59
140 °C 5 h 96
120 °C 6 h 93
140 °C 24 h 84
140 °C 24 h 84
90 °C 6 h 82
130 °C 1 h 92
160 °C 4 h 85
130 °C 24 h 90
Reflux 5 h 92
100 °C 14 h 92
105 °C 10 h 85
100 °C 2 h 49
80 °C 24 h 99
135 °C
20
12f
21
22
12a
23
24
25
26
27
28
29
30
31
32
33
—
Na2CO3 DMF
3
4
5
6
Pd-TPA/ZrO2
—
TBAB
—
K2CO3
K3PO4
TEA
DMF
DMF
DMF
Cyclopalladated ferrocenyl imine
Pd/MIL-53(Al) and 50% NH2
K-Pd-Me10-CB
—
Na2CO3 DMF
7
8
9
Pd/C
—
—
—
NaOAc
K2CO3
K2CO3
DMAc
DMF
NMP
SPIONs-bis(NHC)–palladium(II)
[Pd{C6H2(CH2CH2NH2)-(OMe)2,3,4}Br(PPh3)]
[Pd(PhP(C6H4-2-S)2)(PAr3)]
Seven membered phosphine ylide palladacycle
PNP–SSS
Palladium dichloro-bis(aminophosphine) complexes TBAB
NHC–palladium complexes
POM-IL-Pd
Air
—
—
Air
—
N2
Air
—
—
Ar
—
—
—
—
—
10
11
12
13
14
15
16
17
18
19
20
21
22
—
Cs2CO3 DMF
—
—
K2CO3
K2CO3
K2CO3
K2CO3
TEA
NMP
Water
NMP
NMP
DMF
—
—
PdCl2(DBPF)2
TBAC
—
Cy2NMe DMAc
Na2CO3 DMAc
Pd0/SiO2
34
35
36
37
Pd(0)-MCM-41
NHCs–Pd
Pd(OAc)2
PdCl2
Pd@MCOP
—
—
—
GIL2
—
NaOAc
KOAc
K3PO4
—
DMF
DMAc
DMAc
—
100 °C 24 h 56
140 °C 24 h 87
140 °C 25 h 98
140 °C 1 h 96
90 °C 10 h 98
38
K2CO3
DMSO–water (v/v)
Present work
CB = cucurbituril; TPA = 12-tungstophosphoric acid; MIL = Materials of the Institute Lavoisier; SPIONs = superparamagnetic iron oxide
nanoparticles; SSS silica starch substrate; POM-IL polyoxometalate-ionic liquid; DBPF di-bisphosphenylferrocene; TBAB
tetrabutylammonium bromide; TBAC = tetrabutylammonium chloride; MCM = mesoporous silica matrix; GIL = guanidine based ionic liquids.
=
=
=
=
solid was isolated by filtration, washed with water, Pd@MCOP alkenes under mild conditions. A variety of aryl bromides and
was obtained as brown powder, and kept under a nitrogen alkenes were reacted in good to excellent yields. In addition,
atmosphere.
the Pd@MCOP was also attractive in view of its low cost, easy
synthesis, it is also environmentally friendly, highly stable,
shows negligible metal leaching and can be reused under
optimal conditions with only a minor loss of its catalytic
activity. Thus the present nitrogen rich functional material can
serve as an excellent mesoporous support and can potentially
find extensive catalytic applications in industry.
General procedure for the Mizoroki–Heck cross coupling
reaction between aryl bromides and alkenes
Into a reaction tube equipped with a magnetic stirring bar
were added Pd@MCOP (0.006 mol% of palladium loaded in
30 mg), aryl bromides (1.0 mmol), alkenes (1.2 mmol), K2CO3
(1.5 mmol) and DMSO–water (2.0 mL, 1 : 1, v/v). The mixture
was stirred at 90 °C for a certain amount of time. After com-
pletion of the reaction, the reaction mixture was cooled down
to room temperature, treated with ethyl acetate (10 mL), the
catalyst was filtered and the filtrate was extracted with distilled
water, and the organic layer was dried over anhydrous sodium
sulfate. The organic solvent was evaporated under vacuum, the
residues were purified by passing through a column of silica
gel (60–120 mesh) affording the corresponding pure coupled
product. The recovered catalyst was thoroughly washed with
ethyl acetate and activated under vacuum at room temperature
for 1 h, and subsequently reused.
Acknowledgements
PP gratefully acknowledges financial support from the Univer-
sity Grants Commission (UGC), New Delhi, for UGC-BSR-SRF.
KP thanks DST, New Delhi for financial support.
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