Metal-polymer hybrid material as a catalyst for the heck coupling reaction under phosphine-free conditions

Article abstract of DOI:10.1080/00397911.2010.519202

A method is described for making a poly-aminophenol-based hybrid material by in situ polymerization of aminophenol using palladium acetate as the oxidant. The oxidative polymerization of aminophenol leads to the formation of poly-aminophenol, while the reduction of palladium acetate results in the formation of palladium nanoparticles. The palladium nanoparticles were found to be highly dispersed and stabilized throughout the polymer matrix. The hybrid nanocomposite material showed excellent catalytic efficiency with respect to a Heck coupling reaction at a relatively low temperature and under flexible conditions.

Full text of DOI:10.1080/00397911.2010.519202

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Metal–Polymer Hybrid Material as a
Catalyst for the Heck Coupling Reaction
Under Phosphine-Free Conditions
a
b
c
Rafique Ul Islam , Michael J. Witcomb , Michael S. Scurrell
,
c
d
d
Willem van Otterlo , Elma van der Lingen & Kaushik Mallick
a
Department of Applied Chemistry, Birla Institute of Technology,
Mesra, Ranchi, India
b
Microscopy and Microanalysis Unit, University of the
Witwatersrand, Johannesburg, South Africa
c
Molecular Sciences Institute, School of Chemistry, University of the
Witwatersrand, Johannesburg, South Africa
d
Advanced Materials Division, Mintek, Randburg, South Africa
Accepted author version posted online: 30 Jun 2011. Version of
record first published: 09 Jun 2011
To cite this article: Rafique Ul Islam, Michael J. Witcomb, Michael S. Scurrell, Willem van Otterlo,
Elma van der Lingen & Kaushik Mallick (2011): Metal–Polymer Hybrid Material as a Catalyst for the
Heck Coupling Reaction Under Phosphine-Free Conditions, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:23, 3561-3572
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1
Synthetic Communications , 41: 3561–3572, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.519202
METAL–POLYMER HYBRID MATERIAL AS A CATALYST
FOR THE HECK COUPLING REACTION UNDER
PHOSPHINE-FREE CONDITIONS
1
Rafique Ul Islam, Michael J. Witcomb, Michael S. Scurrell,
2
3
3
Willem van Otterlo, Elma van der Lingen, and
4
4
Kaushik Mallick
1
Department of Applied Chemistry, Birla Institute of Technology, Mesra,
Ranchi, India
2
Microscopy and Microanalysis Unit, University of the Witwatersrand,
Johannesburg, South Africa
3
Molecular Sciences Institute, School of Chemistry, University of the
Witwatersrand, Johannesburg, South Africa
4
Advanced Materials Division, Mintek, Randburg, South Africa
GRAPHICAL ABSTRACT
Abstract A method is described for making a poly-aminophenol-based hybrid material by in
situ polymerization of aminophenol using palladium acetate as the oxidant. The oxidative
polymerization of aminophenol leads to the formation of poly-aminophenol, while the
reduction of palladium acetate results in the formation of palladium nanoparticles.
The palladium nanoparticles were found to be highly dispersed and stabilized throughout
the polymer matrix. The hybrid nanocomposite material showed excellent catalytic
efficiency with respect to a Heck coupling reaction at a relatively low temperature and
under flexible conditions.
Keywords Carbon–carbon coupling reaction; IPCF; metal–polymer composite;
palladium nanoparticle; TEM
Received May 28, 2010.
Address correspondence to Kaushik Mallick, Advanced Materials Division, Mintek, Private Bag
X3015, Randburg 2125, South Africa. E-mail: kaushikm@mintek.co.za
3
561

3562
R. UL ISLAM ET AL.
INTRODUCTION
With the advancement of nanotechnology research, almost all the fields in
science and engineering have achieved great momentum in both research and develop-
ment. There has been great enthusiasm by scientists in the area of nanotechnology
with regard to finding new materials with improved behavior and superior perform-
ance. Nanoparticles are of considerable interest in catalysis, and this subject has
[
1–5]
undergone tremendous growth during the past few years.
Palladium nanoparticles
have drawn considerable attention because of their catalytic properties. The use of
palladium nanoparticles in catalysis is not only industrially important but also scien-
tifically interesting, as a result of the sensitive relationship between catalytic activity
and the nanoparticle size and shape as well as the nature of the surrounding media.
Palladium-catalyzed carbon–carbon bond-formation chemistry occupies a
special place in fundamental academic research as well as in industrial research
and development. The CꢀC bond-forming reactions are very promising for the syn-
thesis of fine chemicals, agrochemicals, and many complex pharmaceutical inter-
mediates. This chemistry has been stimulated by the discovery of a new generation
[
6,7]
of catalysts, for example, palladacycles and palladium–carbene complexes.
Nevertheless, several factors, such as the use of toxic, easily oxidable phosphines
and volatile organic solvents, have hampered broad industrial application. In
addition, a homogeneous palladium catalyst is usually precipitated from a solution
and thus recycling or recovery of the expensive catalyst can be difficult. To overcome
[
8]
[9]
this drawback, polymer-supported palladium, palladium on carbon, and pal-
ladium on different metal oxides
[10]
have been utilized by several groups for the
carbon–carbon bond-formation reaction. Metal–polymer nanocomposite materials
have the unique combination of good stability, easy processing ability, and light
weight. We have shown previously that such a composite material can be used
[
11]
successfully as a catalyst for a gas-phase hydrogenation reaction.
In the present study, we have synthesized a metal–polymer composite material
in which the palladium metal nanoparticles have been immobilized in a
poly-aminophenol matrix. We have paid particular attention to both the polymer
and the metal nanoparticles, which have been characterized by optical and micro-
scopic techniques. Optical characterization techniques have been used to determine
the chemical structure and oxidation states of the polymer. Scanning electron
microscopy (SEM) and transmission electron microscopy (TEM) techniques were
used to explore both the morphology of the composite material and the size of the
nanoparticles while x-ray photoelectron spectroscopy (XPS) was utilized to deter-
mine the ionic state of the palladium. The catalytic activity of the polymer-based
composite material was investigated for a Heck coupling reaction with different aryl
halides and substituted alkenes under phosphene-free conditions.
RESULTS AND DISCUSSION
Optical Characterization
We first examined the spectroscopic behavior of the synthesized material by
infrared (IR) analysis (Fig. 1). In the IR spectra (Fig. 1), the bands at 1590 and
ꢀ
1
1
496 cm can be assigned to the C=C stretching vibration of the quinoid and

METAL–POLYMER HYBRID FOR HECK REACTION
3563
ꢀ1
Figure 1. IR spectrum of the polymeric compound. The bands at 1572 and 1499 cm indicate that the
polymer is composed of quinoid and benzenoid units.
ꢀ1
benzenoid rings respectively. The peak at 1390 cm can be attributed to the ꢀCꢀOH
deformation vibration. The presence of a C-N stretching vibration can be confirmed
ꢀ1
through the presence of the band at 1250 cm . The peaks at 1150, 1100, and
1
ꢀ1
015 cm correspond to the aromatic C-H in-plane bending vibration. The band at
ꢀ
1
1150 cm has been explained as a vibrational band of nitrogen quinine.
high-intensity band at 830 cm is due to an aromatic out-of-plane C-H deformation
vibration and is related to the 1,2,4-trisubstituted benzene ring. The bands at the
[12]
ꢁ
A
ꢀ1
ꢀ
1
higher wavenumber region correspond to the NꢀH stretching (3500–3200 cm
)
ꢀ
1
and the aromatic CꢀH stretching vibration (3000–2850 cm ).
The Raman spectrum (Fig. 2B) reveals the CꢀC deformation bands of benze-
ꢀ1
noid ring at 1612 and 1589 cm , which are characteristic of the semiquinone
[
13,14]
ꢀ1
rings.
structure. An overlapping feature of bands observed between 1300 and 1400 cm
The band at 1521 cm corresponds to the NꢀH bending deformation
ꢀ
ꢀ1
1
,
,
ꢀ
1
with peak positions at 1316 and 1347 cm as well as a shoulder at 1367 cm
Figure 2. Raman spectrum of the resultant material revealing the presence of both quinoid and benzenoid
units in the polymer backbone. (Figure is provided in color online.)

3564
R. UL ISLAM ET AL.
Figure 3. A typical UV-vis spectrum of the composite material, which shows three characteristic absorption
ꢂ
bands at 315, 420, and 610 nm. The absorption peak at 315 nm is due to the p-p transition of the benzenoid
rings. The high intensity peak at 420 nm results from the polaron=bipolaron transition. A broad band with
an absorption maximum at ꢁ610 nm corresponds to the benzenoid to quinoid excitonic transition.
correspond to CꢀN stretching modes. Using the Gaussian fit from the 1140 to
ꢀ
1
ꢀ
1
1
300 cm region (Fig. 2A), four peaks at 1167, 1182, 1218, and 1270 cm are
ꢀ1
clearly visible. The band at 1270 cm can be assigned to the CꢀN stretching mode
ꢀ
1
of the polaronic units. The band at 1218 cm corresponds to the CꢀN stretching
mode of a single bond. The positions of the C–H benzene deformation modes fall
ꢀ
1
ꢀ1
at 1167 cm and 1189 cm , indicating the presence of the quinoid rings.
Both the IR and the Raman analyses suggest the formation of poly-(o-amino-
phenol) with both benzenoid and quinoid segments in the polymer backbone.
The electronic absorption spectrum (Fig. 3) of the resultant material shows
three characteristic absorption bands at 315, 420, and 610 nm. The absorption peak
ꢂ
at 315 nm is due to the p-p transition of the benzenoid rings, while the high intensity
peak at 420 nm results from the polaron=bipolaron transition. A broad band with an
absorption maximum at ꢁ610 nm corresponds to the transition from a localized
benzenoid highest occupied molecular orbital to a quinonoid lowest unoccupied
[
15]
molecular orbital, that is, a benzenoid to quinoid excitonic transition.
Mechanism of Formation Pd–Poly-(o-Aminophenol) Composite
The formation of the o-aminophenol polymer suggests the operation of the follow-
ing mechanism. The presence of an electron-donating group (-OH) in aminophenol makes
2þ
the amino group electron rich and forms an electrostatic bond with Pd cations, and the
aminophenol, under acidic condition undergoes polymerization, an oxidation process.
Each step of the polymerization is associated with the release of one electron, which is then
used in the reduction of palladium ions to form palladium atoms. These atoms subse-
quently coalesce and form clusters, which are stabilized within the growing polymer.
Microscopic Characterization
The TEM image in Fig. 4 illustrates the formation of polymer. The chain
length of the polymer was found to be about 1 mm while the diameter was between

METAL–POLYMER HYBRID FOR HECK REACTION
3565
1
00 and 150 nm. TEM images (Figs. 5A and 5B) show the surface morphology and
internal structure of the polymer. From such images, it is clear that the surface is not
smooth. Both on the rough surfaces and in the interior of the polymer as shown by
these and stereo images, there are seen to be highly distributed dark regions within
and throughout the polymer; these regions have a diameter of about 2–3 nm. A typi-
cal energy dispersive X-ray analysis (EDX) (Fig. 5C) obtained from the electron
beam being focused onto a dark spot in the polymer confirms that these spots are
palladium particles. Focusing the beam between the dark spots near the edge of
the nanocomposite yielded no palladium x-ray peaks. Thus, we can envisage that
the polymer acts as a cage, and the palladium nanoparticles are encapsulated and
highly dispersed inside that cage.
X-Ray Photoelectron Spectroscopy (XPS) Analysis
XPS analysis was undertaken to determine the chemical state of the palladium
in the composite material. The Pd 3d core lines of the XPS spectrums are seen in
Fig. 6, which reveals the presence of Pd 3d5=2 and 3d3=2 peaks at binding energies
of 335.5 and 341.0 eV respectively. These binding energy values are in accordance
[
16]
with those reported for metallic palladium.
Catalytic Application of the Synthesized Pd–Polymer
Nanocomposite
The palladium-based polymer composite material was used as a catalyst for the
Heck coupling reaction of an unsaturated halide with an alkene in the presence of a
strong base to form a substituted alkene (Scheme 1).
Figure 4. SEM image of the composite material synthesized during the reaction between Pd-acetate and
ortho-aminophenol.

3566
R. UL ISLAM ET AL.
Figure 5. TEM micrograph of the palladium-(poly-aminophenol) composite. (A) and (B) show the surface
morphology and internal microstructure of the composite. The ꢁ2 nm sized dark spots are the palladium
nanoparticles, which showed a high dispersion throughout the polymer. An EDX spectrum (C) derived
from placing the focused electron beam onto a single dark spot in Figure 3B clearly shows the presence
of palladium. The copper signal results from electron scattering onto the sample support grid. (Figure
is provided in color online.)
Heck reaction of aryl iodide with olefin. We utilized the poly-(o-
aminophenol)-palladium composite as a catalyst for the Heck reaction of iodoben-
zene and n-butyl acrylate in dimethylformamide (DMF) as solvent for the test
ꢃ
reaction. The reaction mixture was heated at 80 C for about 8 h, and the reac-
tion was monitored by thin-layer chromatography (TLC) from time to time.
The reaction showed an excellent yield with the formation of the coupled pro-
duct (Table 1, entry 1). The other aryl iodides were also coupled with different
olefins, producing very good yields under the same reaction conditions (see
Table 1).
Heck reaction of aryl bromide with olefin. Application of the same cata-
lyst for the coupling between activated and deactivated aryl bromides with acrylic
acid was also found to be effective, producing substituted olefins in very good yields
(Table 2). Activated aryl bromides afforded the product in slightly greater conver-
sions rather than deactivated ones, which displayed a lower reaction rate. A com-
parative study of different aryl halides (iodo, bromo, and chloro) with acrylic acid

METAL–POLYMER HYBRID FOR HECK REACTION
3567
Figure 6. The Pd 3d core line XPS spectrum of palladium-(poly-aminophenol) composite. The binding
energy values obtained from the peaks identify the presence of metallic palladium in the composite.
were carried out under the same reaction conditions, and the yields of the C-C coup-
ling products were determined. Aryl iodide produced a better yield (95%) than the
aryl bromide (82%), while aryl chloride afforded only a very low conversion (8%)
to the coupled product.
We also carried out a comparative kinetic study between two coupling reac-
tions: 1 aryl iodide with n-butyl acrylate (Table 1, entry 1) and 2 3-substituted aryl
iodide with n-butyl acrylate (Table 1, entry 4) in the presence of the same amount of
catalyst (0.006 mol% Pd). The experimental progress was monitored by gas chroma-
tographic (GC) analyses, and to minimize experimental errors, we repeated both the
experimental procedures twice. From Fig. 7, it can be seen that for reaction 1, 96% of
the conversion was achieved after 7 h, and after that no further improvement in
conversion was detected. For reaction 2, 91% conversion was achieved after 8 h.
For reaction 1, a small conversion (ꢁ9%) was measured after 1 h, whereas, for reac-
tion 2, an almost comparable level of conversion was achieved only after 2 h.
Because of the increasing aryl-halogen bond strengths of the different halides,
the reactivity of aryl halides decreases in the order I > Br > Cl. Accordingly, acti-
vation of aryl iodides and aryl bromides is feasible in the presence of almost any kind
of palladium catalyst. In contrast, activation of aryl chlorides remains a major chal-
lenge, requiring a highly active catalyst system. A comparative study (Table 3) was
undertaken to show the efficiency of the Pd–poly-(o-aminophenol) composite cata-
lyst in comparison with some of the active Pd-based catalysts toward the Heck coup-
[
17–21]
ling of aryl bromides and terminal olefins from the literature.
Some of the
Scheme 1. Heck coupling reaction of an unsaturated halide with an alkene.

3568
R. UL ISLAM ET AL.
Table 1. Heck coupling reaction of aryl iodides with different olefins
Entry
1
Aryl iodide
Olefin
Product
Yield (%)
9
8
6
9
2
3
8
9
3
1
4
5
9
3
6
7
8
8
5
1
Notes. Reagents and reaction conditions: Aryl iodides (1.0 mmol), alkenes (1.5 mmol), triethyl amine
ꢃ
(
1.5 mmol), DMF (5 ml), and catalyst [Pd–poly-(o-aminophenol)] (4 mg), for 8 h at 80 C.
Table 2. Heck coupling reaction of aryl bromides with acrylic acid
Entry
Ary1 halide
Olefin
Product
Yield (%)
78
1
2
3
81
75
Notes. Reagents and reaction conditions: Aryl bromides (1.0 mmol), alkenes (1.5 mmol), triethyl amine
ꢃ
(
1.5 mmol), DMF (5 ml), and catalyst [Pd–poly-(o-aminophenol)] (4 mg), for 8 h at 80 C.

METAL–POLYMER HYBRID FOR HECK REACTION
3569
Figure 7. A comparative kinetic study between two coupling reactions: (I) aryl iodide with n-butyl acrylate
entry: 1) and (II) 3-substituted aryl iodide with n-butyl acrylate (entry: 4) in the presence of the same
amount of catalyst (0.006 mol% Pd).
(
catalysts (entry 2 and 4) show higher turnover number (TON) when the reaction
took place for longer time period and at higher temperature than the Pd-poly(o-
aminophenol) composite catalyst, but our composite catalyst shows greater turnover
frequency (TOF) value than others mentioned in Table 3.
The recyclability of the palladium–(poly-aminophenol) catalyst was also inves-
tigated (Table 1, entry 1). The recovery of the catalyst required a tedious filtration
technique because of the minute quantity of the material used for the reaction.
The results indicated that the used material was active without significant loss of
catalytic performance, the yield loss being within 5% of the original after three
cycles. At the end of the third cycle, a slight enlargement of the particles was
observed, which was found to be due to interparticle agglomeration within the cata-
lyst. All the products were purified by chromatography and identified from their
Table 3. Catalytic performance of the Pd–poly-(o-aminophenol) composite catalyst as compared to some
other Pd-based catalysts reported for the Heck coupling reaction of aryl bromides and styrene
ꢀ1
Entry [Pd] (mol%)
Base
Solvent
Condition
Yield (%) TON TOF (h
)
Reference
ꢃ
ꢃ
ꢃ
1
2
3
4
5
6
0.006
0.00247
1.5
TEA
PO
DMF
DMA
80 C, 8 h
140 C, 44 h
72
95
70
56
76
77
12,000
38,500
47
1500
875
—
This work
K
3
4
17
18
19
20
21
NaOAc NMP
NaOAc DMA
NaOAc DMA
NaOAc DMA
150 C, 30 h
140 C, 28 h
ꢃ
ꢃ
0.002
0.1
0.1
28,000
760
770
1000
—
150 C, 24 h
140 C, 26 h
ꢃ
—
Notes. Dimethylformamide (DMF), dimethylacetamide (DMA), and N-methylpyrrolidone (NMP).

3570
R. UL ISLAM ET AL.
spectroscopic data. These recovered compounds were found to be identical with the
reported ones. For the Heck cross-coupling reactions, Pd(0) is the catalytic species,
1
and the possible mechanism is the interaction of aryl halide (R X) and Pd(0) to form
2
þ
2
the aryl–palladium halide complex [R1(Pd )X], which then couples with aryl (R )
1
alkenes in the presence of a base to produce the [R –(Pd )–R ] intermediate, which
2þ
2
1
2
2þ
finally produces the biaryl product (R –R ) via the reductive elimination of Pd to
Pd(0).
CONCLUSIONS
A metal–polymer composite material has been synthesized in which palladium
nanoparticles were stabilized by the poly-(o-aminophenol) matrix. The composite
material showed excellent activity towards the Heck coupling reaction under
phosphine-free conditions. This is quite advantageous from both economic and
environmental points of view. Other advantages such as good yield and reusability
make this material potentially useful in synthetic organic chemistry as well as for
industrial applications.
EXPERIMENTAL
Materials
Ortho-aminophenol was purchased from BDH (London). Toluene was
obtained from Merck. Palladium acetate from Next Chimica was used to prepare
ꢀ3
ꢀ3
a stock solution having a concentration of 10 mol dm in toluene. All other
chemicals were obtained from commercial sources (Aldrich or Alfa-Aesar) and were
used as received without further purification.
Synthesis of the Metal–Polymer Nanocomposite
In a typical experiment, 1.005 g of o-aminophenol was dissolved in 30mL of
toluene under continuous stirring conditions. A previously prepared stock solution
of palladium acetate in toluene (15 mL) was added drop by drop to the first solution.
A slow precipitation occurred, during which time the solution turned brown in color.
Samples were collected onto lacy carbon-coated copper grids for TEM analysis. After
viewing, the TEM grids were sputter coated with a conducting layer a few nanometers
thick of Au-Pd and viewed in the SEM. The rest of the solution was immediately fil-
tered and dried under vacuum. A small portion of the dried sample was used for ultra-
violet–visible (UV-vis), IR, and Raman analyses. The remaining portion of the sample
ꢃ
was dried in an oven at 80 C and used as the catalyst for the Heck coupling reaction.
General Procedure for the Heck Coupling Reactions
The catalyst (4 mg), aryl halide (1.0 mmol), alkenes (1.5 mmol), triethyl amine
1.5 mmol), and DMF (5 ml) were placed in a round-bottom flask containing a small
stirring bar. The reaction mixture was placed in an 80 C oil bath and stirred for 8 h.
The reaction was monitored by TLC, and after completion of the reaction, the
(
ꢃ

METAL–POLYMER HYBRID FOR HECK REACTION
3571
mixture was extracted three times with ethyl acetate. Subsequently, the ethyl acetate
layer was washed with water (2 ꢄ 25 mL) and brine (25 mL). The extract was dried
over MgSO . After the evaporation of the solvents under reduced pressure, the resi-
4
due was purified by flash chromatography over silica gel to obtain the product.
Characterization of Materials
The samples were characterized using a FEI FEG Nova 600 Nanolab SEM at
kV and a Philips CM200 TEM operating at 197 kV equipped with a Gatan Tridiem
GIF. IR spectra were collected in the region 4000–700 cm with a Perkin-Elmer
000, while Raman spectra were acquired using the green (514.5 nm) line of an argon
5
ꢀ
1
2
ion laser as the excitation source using a Jobin-Yvon T64000 Raman spectrometer.
UV-vis spectral analyses were obtained within the range 300–700 nm using a Varian
Cary 1E digital spectrophotometer. XPS were collected in a ultra-high vacuum
(UHV) chamber attached to a Physical Electronics 560 ESCA=SAM instrument.
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