Polymer-supported palladium: A hybrid system for multifunctional catalytic application

Article abstract of DOI:10.1002/aoc.3898

Polymer-supported palladium was synthesized by applying a single-step wet chemical synthesis route and the resultant composite material was characterized by means of various techniques. Infrared and UV–visible spectra provided information on the chemical structure of the polymer. Microscopy techniques showed the general morphology of the polymer. The oxidation state of palladium was determined using the X-ray photoelectron spectroscopy method. The synthesized material was applied as a heterogeneous catalyst for the Heck coupling reaction and also as an electrocatalyst for the oxidation of cysteine.

Full text of DOI:10.1002/aoc.3898

Received: 11 March 2017
DOI: 10.1002/aoc.3898
Revised: 21 April 2017
Accepted: 28 April 2017
F U L L PA P E R
Polymer‐supported palladium: A hybrid system for
multifunctional catalytic application
Abu Taher | Meenakshi Choudhary | Debkumar Nandi
| Samarjeet Siwal | Kaushik Mallick
Department of Chemistry, University of
Johannesburg, PO Box 524, Auckland Park
2006, South Africa
Polymer‐supported palladium was synthesized by applying a single‐step wet chem-
ical synthesis route and the resultant composite material was characterized by means
of various techniques. Infrared and UV–visible spectra provided information on the
chemical structure of the polymer. Microscopy techniques showed the general mor-
phology of the polymer. The oxidation state of palladium was determined using the
X‐ray photoelectron spectroscopy method. The synthesized material was applied as
a heterogeneous catalyst for the Heck coupling reaction and also as an
electrocatalyst for the oxidation of cysteine.
Correspondence
Kaushik Mallick, Department of Chemistry,
University of Johannesburg, PO Box 524,
Auckland Park 2006, South Africa.
Email: kaushikm@uj.ac.za
Funding information
National Research Foundation (NRF),
South Africa
KEYWORDS
cysteine oxidation, Heck coupling reaction, polymer‐supported palladium, single‐step synthesis
1 | INTRODUCTION
the easy recycling of the catalyst, but in many cases the
surface area and pH value directly influence the catalytic per-
The evolution of innovative materials and new fabrication
techniques has become a matter of success for various areas
of research activities. Metal‐containing polymers[1] hold a
lot of promise as they exhibit novel properties, such as
magnetic[2] and semiconductor[3] properties, and have poten-
tial applications in sensors[4] and organocatalysis reactions.[5]
Among the various organocatalysis reactions, carbon–carbon
(C─C) coupling reactions, through transition metal catalysis
routes, have been developed because of their real‐world
applications for the synthesis of organic building blocks, fine
chemicals, pharmaceuticals and biologically active com-
pounds.[6] Palladium catalyses various coupling processes,
such as Suzuki, Heck and other common reactions, generally
performed in the presence of homogeneous palladium spe-
cies, which results undesirable palladium contamination in
products.[7] For that reason, a support is an important factor
in designing a heterogeneous catalyst system for practical
implementation. Efforts have been made towards the devel-
opment of palladium complex catalysts and palladium nano-
particle catalysts supported on metal oxides, carbon‐based
materials, zeolites, mesoporous materials, layered materials
and polymer composites for various C─C coupling
reactions.[8] In general, support materials provide a way for
formance. In addition, the robust nature of the supports and
their abilities in stabilizing the active centres are the crucial
requirements for long‐lasting catalytic performance.
Various types of inorganic material and organic mole-
cules or polymers have been applied as matrices to stabilize
both ionic and metallic palladium. Due to the unique
properties of polymers, they are often used as hosts for the
appropriate palladium species.
A modified starch, a
biopolymer, served as a support of Pd(0) and performed as
an active catalyst for Suzuki, Heck and Sonogashira coupling
reactions.[9] Polystyrene‐supported palladium nanoparticles
were also used as a potential catalyst for various C─C
coupling reactions, whereas a recyclability study showed a
considerable decrease of yield in successive reactions and
was due to the leaching of palladium species during the
reaction.[10] A relatively low‐leaching and moisture‐stable
polystyrene‐supported palladium complex catalyst was
reported for the Suzuki reactions at room temperature under
aerobic conditions.[11] Polystyrene containing functionalized
heterocycles can also be a good candidate for the
immobilization of Pd(II) species and showed good catalytic
activity and reusability for the Suzuki reaction.[12]
polyacrylamide‐based Pd(II) catalyst has been reported for
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Appl Organometal Chem. 2017;e3898.
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TAHER ET AL.
the Heck reaction with high catalytic recycling efficiency.[13]
Monodispersed palladium nanoparticles in poly(N‐vinyl‐2‐
pyrrolidone) showed activity towards the coupling reaction
between iodo−/bromobenzene and butyl acrylate.[14] Poly-
pyrrole is a conducting organic polymer that in combination
with appropriate noble metal nanoparticles produces
composites, which exhibit catalytic activity.[4] A ‘core–shell’
type of structure with ‘polystyrene in the core’ and
‘palladium–polypyrrole nanocomposite as the shell’ has been
employed for Suzuki and Heck cross‐coupling reactions in
water.[15] Polyaniline as well as its derivatives are conducting
organic polymers and have also been demonstrated as
supports for immobilizing palladium species for various
carbon–carbon bond formation reactions.[16, 5] In our previ-
ous study we found that polymer‐encapsulated palladium
nanoparticles served as an efficient catalyst for the Suzuki
coupling reaction in the absence of phosphine ligand.[17]
In this report, we describe the formation of a
supramolecular composite of mono‐valent palladium and
polymeric form of 4‐thiophen‐3‐ylaniline, Pd‐pAT, using
the in situ polymerization and composite formation
approach.[18] The composite system was characterized using
2.3 | Preparation of Pd‐pAT composite
catalyst
In a typical experiment, 3.5 ml of K2PdCl4 (from a stock
solution of 0.0326 g of K2PdCl4 in 10 ml of water) was added
dropwise to 4‐thiophen‐3‐ylaniline (0.350 g in 10 ml of
CH3OH). A yellow precipitate was formed at the bottom of
the reaction pot. The solid material was characterized using
various optical, surface and microscopic techniques and also
applied as a catalyst for Heck coupling reactions and electro-
chemical cysteine oxidation reaction.
2.4 | General procedure for Pd‐pAT‐catalysed
Heck coupling reaction
In a typical experiment, aryl halide (1.0 mmol), alkene
(1.5 mmol), KOAc (147 mg, 1.5 mmol) and the palladium–
polymer (Pd‐pAT) catalyst (5.0 mg, 0.050 mol% Pd) were
added to dimethylformamide (DMF; 2 ml) in a round‐bottom
flask at 100 °C for 8 h. The reaction was followed using the
TLC technique.
optical,
microscopic
and
non‐destructive
surface
3 | RESULTS AND DISCUSSION
3.1 | Characterization of compound
characterization techniques. The supramolecular material
was used as a catalyst for the Heck coupling reaction under
phosphine‐free reaction conditions and also used as an
electrocatalyst for the oxidation of cysteine.
The optical properties of the synthesized material were
characterized using UV–visible and Fourier transform
infrared (FTIR) spectroscopic methods. The UV–visible
spectrum (Figure 1a) for Pd‐pAT shows absorption peaks at
around 320 nm which is attributed to the π–π* transition
associated with benzenoid rings. A broad band within the
range 400–500 nm is accounted for by the polaron–bipolaron
transition. The presence of a quinoid unit in the polymer
composite is confirmed from the FTIR spectrum (Figure 1b)
2 | EXPERIMENTAL
2.1 | Materials
Analytical‐grade chemicals and solvents were used in the
experiments without further purification.
2.2 | Material characterization
Microscopy studies of the synthesized material were
performed using a JEOL JEM‐2100 analytical electron
microscope. Surface images were obtained using a JEOL
JSM‐840 scanning electron microscopy (SEM) instrument
operating at an accelerating voltage of 20 kV. Surface proper-
ties of the material were characterized using a Shimadzu XD‐
3A X‐ray diffractometer and X‐ray photoelectron spectra
(XPS) were collected using a Physical Electronics 560
ESCA/SAM instrument. Optical properties of the
synthesized material were measured using Shimadzu
IRAffinity‐1 and Shimadzu UV‐1800 spectrophotometers.
Electrochemical studies was carried out with a potentiostat
(Bio‐Logic SP‐200) connected to a data controller, with a
glassy carbon electrode (GCE) as the working electrode. A
1
Bruker UltraShield‐400 was used to record H NMR and
FIGURE 1 (a) UV–visible and (b) FTIR spectra of Pd‐pAT
composite
13C NMR spectra.

TAHER ET AL.
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and which is associated with the vibrational peak at 1658 cm
−1. A doublet band in the range between 1410 and 1450 cm−1
can be attributed to the ν3 vibration of thiophene segment.[19]
Figure 2(a), an SEM image, shows the general morphology
of the synthesized supramolecular composite, whereas
Figure 2(b) shows a transmission electron microscopy
(TEM) image for the same material. In the TEM image, a
smooth surface is noticed without evidence of the formation
of palladium nanoparticles. Energy‐dispersive X‐ray spec-
troscopy (EDS) was conducted by focusing the electron beam
on different areas of the investigated material and the charac-
teristic palladium X‐ray peaks are observed around 3.0 keV
(Figure 3a). XPS was employed to investigate the chemical
state of palladium. A high‐resolution XPS Pd 3d spectrum,
for the Pd‐pAT sample, with the core level split into 3d5/2
and 3d3/2 components due to a spin–orbit interaction is
shown in Figure 3(b). Here, the binding energy value associ-
ated with the Pd 3d5/2 line was estimated to determine the
valence state of palladium. In general, the peaks at 335.7
and 337.75 eV correspond to metallic and bi‐valent state of
palladium, respectively.[20] In Figure 3(b), after
FIGURE 3 (a) EDS spectrum of Pd‐pAT composite. The presence of
palladium is evident from the signal AT ca 3.0 keV. (b) XPS signal of
Pd‐pAT composite. The peak at 337.56 eV is due to the presence of
unreacted Pd(II) whereas the peak AT 336.74 eV indicates the presence
of Pd(I) within the sample
deconvolution of the spectrum within the range
335.0–340.0 eV, two separate peaks at 337.56 and
336.74 eV become visible and those correspond to bi‐valent
and mono‐valent state of palladium, respectively. The inten-
sity and area of the band with the peak positioned at
336.74 eV are higher than those of the band at 337.56 eV,
indicating the presence of higher concentration of Pd(I) in
the Pd‐pAT sample.
The detailed mechanism of composite formation has been
reported elsewhere.[18] The chemistry of mono‐valent
palladium‐containing complexes is potentially important in
various catalytic process applications.[21]
3.2 | Catalytic property of Pd‐pAT
The catalytic property of the synthesized material was inves-
tigated in terms of a carbon–carbon bond formation reaction.
Among the various C─C bond formation reactions, Heck
coupling is one of the important for its diverse applications
in the fine chemical and pharmaceutical industries. The same
composite material, Pd‐pAT, was also used as an
electrocatalyst for the voltammetric detection of L‐cysteine
FIGURE 2 (a) SEM and (b) TEM images of Pd‐pAT composite

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TAHER ET AL.
(CySH), an important amino acid performing a critical role in
biological systems.
(3ca) and 1‐nitro‐4‐styrylbenzene (3da) with yields of 98
and 97%, respectively. A similar trend is noticed for the
coupling reactions when styrene is replaced with 2b
(Table 1, products 3ab–3db). In our current study, when
acrylonitrile is used as one of the coupling partners with
various substituted aryl halides a moderate to good yield is
observed from 77 to 85% for the desired coupling products
3 cc, 3 dc and 3ec (Table 1). The reaction between 2‐
methyliodobenzene (1e) and acrylonitrile (2c) produces the
coupled product 3‐o‐tolylacrylonitrile (3ec) with a yield of
83%. Similarly, 1c and 1d when coupled with 2c produce
the compounds 3‐(4‐methoxyphenyl) acrylonitrile (3 cc)
and 3‐(4‐nitrophenyl) acrylonitrile (3 dc) with yields of 80
and 85%, respectively. In the current Heck coupling reaction,
for the aryl halide component, when iodo group is replaced
with bromo group the yield of coupled product decreases
because of the presence of stronger C─Br bond compared
with C─I bond (Table 1).
3.3 | Performance of Pd‐pAT in the Heck
reaction
The presence of a base in the reaction media is very important for
the Heck reaction and a series of bases, namely K2CO3, Na2CO3,
KOAc, KOtBu, Cs2CO3 and K3PO4, were used to find the
optimum conditions for the reaction. We find that KOAc acts
as a highly efficient base for the current coupling reaction when
Pd‐pAT is applied as a catalyst. Again, the selection of a suitable
solvent is also an important issue for organic reactions. Among
the various solvents, namely Dimethylformamide (DMF),
Dimethylacetamide (DMA), N‐methyl‐2‐pyrrolidone, toluene,
dimethylsulfoxide and EtOH–H2O (1.0:1.0), used for this
experiment, we find that DMF performs efficiently when
combined with KOAc and the combination of DMF and KOAc
produces the highest yield of desired product 3cb for the current
Heck reaction between 4‐iodoanisole (1c) and methyl acrylate
(2b) in the presence of 0.050 mol% Pd from the Pd‐pAT catalyst
(supporting information, Table 1S, entry 4). We also find that by
decreasing or increasing the amount of catalyst no further
improvement of the product, in terms of yield, is obtained. Based
on the above optimized reaction conditions, we explored the
versatility of the catalyst with diverse examples of aryl halides
and activated alkenes (Table 1). All the substrates produce the
expected products with very good to excellent yields and
selectivity.
3.4 | Mechanism, recyclability and metal
leaching studies for Heck reaction
The Heck reaction is a cross‐coupling reaction of an aryl
halide with an alkene to make a substituted alkene using pal-
ladium as a catalyst in the presence of a base.[22] The reaction
begins by oxidative addition of the aryl halide to the
palladium, followed by the coordination of alkene segment
to the palladium and that finally produces the aryl–alkene
coupled product through the reductive elimination of Pd(0).
The recyclability as well as the scaling up of the Heck
reaction using the Pd‐pAT composite material as a catalyst
was investigated for the coupling of 1d with 2a (1d:
10.0 mmol; 2a: 15.0 mmol; catalyst (Pd‐pAT): 50 mg
(0.05 mol% Pd); temperature: 100 °C; time: 8 h). After the
first cycle, a product yield of 95% was achieved and the
recovered catalyst was used for another four times under the
same reaction conditions. At the end of the fifth cycle, a yield
An almost identical yield is obtained when styrene is
coupled with iodobenzene and 4‐methyliodobenzene to
produce the desired products 3aa and 3ba with 90 and 92%
yields, respectively (Table 1). It is important to mention that
the presence of electron‐donating and electron‐withdrawing
group does not have any effect on yield for the reactions since
4‐methoxyiodobenzene (1c) or 4‐nitroiodobenzene (1d) cou-
ple with styrene (2a) to produce 1‐methoxy‐4‐styrylbenzene
TABLE 1 Heck coupling reaction of olefins with aryl halidesa
aReaction conditions: aryl halide (1.0 mmol), alkene (1.5 mmol), KOAc (147 mg, 1.5 mmol), DMF (2 ml). For aryl halides (X = Br−): catalyst concentration of
0.075 mol% Pd. All reactions were carried out at 100 °C for 8 h. Isolated yields are presented.

TAHER ET AL.
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of 81% of 1‐nitro‐4‐styrylbenzene was achieved (Figure 4a).
The X‐ray diffraction pattern of the recovered catalyst is
shown in Figure 4(b). The observed peaks located at
40.10°, 45.60°, 67.50° and 81.0° representing the (111),
(200), (220) and (311) Bragg reflection planes of a face‐
centred cubic lattice, respectively, confirm the presence of
metallic palladium nanoparticles in the recovered catalyst.[23]
The TEM image (Figure 4c) shows the formation of
palladium nanoparticles with an average size of 20 nm. The
gradual decrease of yield with the number of cycles is
possibly due to the leaching of the palladium species from
the support and also due to the loss of composite catalyst dur-
ing the filtration process in each cycle. The total amount of
metal leached was assessed using the inductively coupled
plasma mass spectrometry technique where the estimated
concentration of the leached palladium is found to be 3.5
and 23.0 ppm, during the first and fifth cycle, respectively.
In this work, we find that the Heck coupling reaction is feasi-
ble between olefins and both aryl iodide and bromide in the
presence of 0.050 and 0.075 mol% Pd, respectively, at
100 °C for 8 h. For aryl chloride, we observe that a higher
concentration of catalyst (0.5 mol% Pd) is required for the
coupling reaction at 120 °C for a period of 12 h with a yield
of 25%. We also find that at 120 °C the polymer‐based cata-
lyst loses its performance after the first cycle of the reaction.
3.5 | Electrocatalytic performance of Pd‐pAT
for detection of CySH
CySH is a thiol group‐containing amino acid, and has
been extensively used in the medicine and food industries.
On oxidation, it forms cystine (CyS─SCy) with a disul-
fide bond. The cysteine–cystine couple can act as a redox
switch in a variety of biological systems[24] and therefore
the quantitative detection of CySH and an understanding
of the oxidation mechanism are important in protein
chemistry[25] and also has drawn considerable attention.[26]
The electrochemical technique is a sensitive and conve-
nient method for the quantitative detection of many bio-
logically active molecules. CySH oxidation and detection
on the surface of an unmodified electrode suffer because
of electrode corrosion, lack of sensitivity and high value
of oxidation potential. These problems can be potentially
overcome using a suitable electrocatalyst as an electrode
modifier.
Transition metals have an important role as catalysts for
various reactions including electrochemical reactions.[27] In
this current study, we applied Pd‐pAT as an electrocatalyst
for the electrochemical recognition of CySH.
Cyclic voltammetry was used to study the
electrochemical properties of a Pd‐pAT‐modified working
electrode. Shown in Figure 5(a) are voltammograms for bare
GCE in the absence (curve ‘a’) and in the presence
(curve ‘b’) of 40 μM CySH. The intensity of the current for
curve ‘b’ is increased, which indicates the oxidation of CySH
on the bare electrode. To check the performance of the com-
posite system, we used Pd‐pAT‐modified working electrode
as a sensing device for CySH detection. Figure 5(b) shows
the voltammograms of Pd‐pAT‐modified working electrode
in the absence (curve ‘a’) and in the presence (curve ‘b’) of
40 μM CySH (analyte). The voltammogram in the presence
of analyte shows higher current value at lower potential
range. The differential pulse voltammetry technique was
applied, using different concentrations of analyte, for further
confirmation of the cyclic voltammetry results. With increas-
ing concentration of CySH (40, 80, 120, 160 and 200 μM),
the current signal (curves ‘b’–‘f’, respectively) increases
gradually as evidenced by differential pulse voltammograms
FIGURE 4 (a) histogram generated from recyclability study for Heck
reaction. (b) typical X‐ray diffraction pattern of the used catalyst
(recovered after the fifth cycle of reaction). (c) TEM image of the used
catalyst at the end of the fifth cycle for the Heck coupling reaction
reveals the formation of palladium nanoparticles with an average size of
20 nm

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TAHER ET AL.
FIGURE 7 Voltammograms of chronoamperometry study for Pd‐
pAT‐modified GCE with a fixed potential 0.5 V for a period of 20 s.
Curve ‘a’ represents the voltammogram in the absence of cysteine and
curves ‘b’–‘e’ represent the voltammograms in the presence of cysteine
with the concentrations of 40, 80, 120 and 150 μM, respectively. Curves
‘f’–‘i’ represent the voltammograms in the presence of tryptophan,
glycine, histidine and leucine, respectively, with concentrations of 40
and 150 μM, and indicate the irresponsive nature towards the current
signal
FIGURE 5 Cyclic voltammograms for (a) bare working electrode
and (b) Pd‐pAT‐modified working electrode in the absence (curve ‘a’)
and presence (curve ‘b’) of 40 μM cysteine. (electrolyte: 10 mM
phosphate buffer; scan rate: 50 mV s−1
)
chronoamperometry voltammograms (Figure 7), as repre-
sented by curves ‘b’–‘e’, are for different concentrations of
CySH, namely 40, 80, 120 and 150 μM, respectively, and
which indicate that current intensity is directly proportional
to the CySH concentration for the Pd‐pAT electrocatalyst.
We also investigated the universality of the Pd‐pAT catalyst
by the addition of some potential interfering species, like
tryptophan, glycine, histidine and leucine. A passive nature
is found for the catalyst towards all the biomolecules, with
various concentrations (curves ‘f’–‘i’), used as the interfering
species in this experiment.
The mechanism of CySH detection can be represented by
the following equations, where the first step corresponds to
the oxidation of univalent palladium and the second step
involves CySH oxidation with the formation of cystine
(CyS─SCy) followed by the subsequent reduction of
palladium ion to zero‐valent palladium:
FIGURE 6 (a) Voltammograms of differential pulse voltammetry
study for Pd‐pAT‐modified GCE with increasing concentrations of
cysteine: Curves ‘b’–‘f’ for 40, 80, 120, 160 and 200 μM cysteine,
respectively. Curve ‘a’ is the voltammogram in the absence of cysteine.
Differential pulse voltammetry was conducted with a pulse width of
50 ms and scan rate of 0.05 V s−1. (b) regression coefficient plot
PdðIÞ→PdðIIÞþe‐
(1)
(2)
‐−‐‐‐½2‐Hꢀ
PdðIIÞ þ 2CySH → CyS─SCy þ Pdð0Þ
(Figure 6a) and the current signal shows a linear correlation,
with a regression coefficient value of 0.9965, for CySH from
40 to 200 μM (Figure 6b). The sensitivity of
Pd‐pAT‐modified GCE for the detection of CySH is found
to be 174 μA mM−1 cm−2. A chronoamperometry study
was also performed using Pd‐pAT‐modified working
electrode in the absence of CySH (Figure 7, curve ‘a’). The
The occurrence of the above mechanism can be evi-
denced from an electron microscope image (Figure 8) of
the material recovered from the working electrode, where tiny
particles, within the range 2–5 nm in size, are visible on the
polymer matrix (some representative particles are shown
within circles).

TAHER ET AL.
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4 | CONCLUSIONS
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The present article reports the synthesis of a functional
polymer‐based palladium composite for application in the
Heck coupling reaction under phosphine‐free conditions.
The synthesized material was also used as an electrocatalyst
for the detection of cysteine using the differential pulse
voltammetry technique and the sensitivity of the palladium–
polymer catalyst for the detection of cysteine was found to
be 174 μA mM−1 cm−2. A chronoamperometry study
simultaneously showed the active behaviour towards the
detection of cysteine with various concentrations and passive
behaviour with some biomolecules like tryptophan, glycine,
histidine and leucine. The mechanism for both Heck coupling
and cysteine oxidation indicates the formation of palladium
nanoparticles, which corroborates the experiment results
and is evidenced by the microscopic techniques.
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ACKNOWLEDGEMENTS
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The authors acknowledge financial support from the
University of Johannesburg and the National Research
Foundation (NRF), South Africa.
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TAHER ET AL.
SUPPORTING INFORMATION
How to cite this article: Taher A, Choudhary M,
Additional Supporting Information may be found online in
the supporting information tab for this article.
Nandi D, Siwal S, Mallick K. Polymer‐supported
palladium: A hybrid system for multifunctional
catalytic application. Appl Organometal Chem. 2017;
e3898. https://doi.org/10.1002/aoc.3898