Polymer-stabilized palladium nanoparticles for the chemoselective transfer hydrogenation of α,β-unsaturated carbonyls: Single-step bottom-Up approach

Article abstract of DOI:10.1002/cctc.201300985

Polypyrrole stabilised palladium nanoparticles show good catalytic efficiency for the chemoselective transfer hydrogenation of α,β- unsaturated carbonyl compounds. The catalyst is very specific and selectively hydrogenates the olefins or acetylenes only,

Full text of DOI:10.1002/cctc.201300985

CHEMCATCHEM
FULL PAPERS
DOI: 10.1002/cctc.201300985
Polymer-Stabilized Palladium Nanoparticles for the
Chemoselective Transfer Hydrogenation of
a,b-Unsaturated Carbonyls: Single-Step Bottom-Up
Approach
[
a]
[a]
[b]
[c]
Sanjit K. Mahato, Rafique Ul Islam, Chiranjit Acharya, Michael J. Witcomb, and
Kaushik Mallick*
[
a]
Polypyrrole stabilised palladium nanoparticles show good cata-
lytic efficiency for the chemoselective transfer hydrogenation
of a,b-unsaturated carbonyl compounds. The catalyst is very
specific and selectively hydrogenates the olefins or acetylenes
only, without affecting the carbonyl moiety, with an excellent
yield of products for a wide range of substrates.
Introduction
Metal nanoparticles are of extensive interest in current re-
search activities as they exhibit unique electronic and optical
method is expected to provide a high degree of synthetic con-
trol over both the size of the nanoparticles and the morpholo-
gy of the polymer matrix, which in turn may be expected to
exert a strong influence on the metal–polymer interaction.
Metal nanoparticles of different sizes and shapes can be
combined with polymers to produce composites that have in-
teresting physical properties and important potential applica-
tions. The ultimate properties of the resulting composite can
be controlled by the synthesis techniques used and the charac-
[1–4]
and catalytic properties because of quantum size effects.
The application of nanoparticles in catalysis has undergone tre-
[
5–7]
mendous growth over the past few years,
and it has been
reported that metal nanoparticles with specific sizes and
shapes exhibit improved catalytic performance over conven-
[
8,9]
tional catalysts.
The incorporation of nanoparticles in poly-
mers has attracted attention because the composite architec-
tures of polymers and metal nanoparticles provide synergisti-
cally useful functionality and mechanical integrity as well as
stability. Many investigations have been published regarding
the development of the incorporation of the metal nanoparti-
cles into a polymer matrix. Three different approaches have
been used to make such a composite material: (a) the in situ
preparation of the nanoparticles in the polymer, which is ach-
ieved either by the reduction of metal salts dissolved in the
[16]
[15,17–20]
[21,22]
[23]
teristics of the inorganic materials. Au,
Cu,
Pt
[24–26]
and Pd
nanoparticles have been incorporated into polyan-
iline or substituted polyaniline successfully. Co and Ni particles
embedded in polyaniline matrices have also been prepared at
[27]
higher temperatures.
Polymer-stabilised noble-metal nanoparticles have attracted
much attention recently as a new research direction in cataly-
sis. The polymer matrix serves both as the support as well as
the stabiliser of the nanoparticles, which provides a mechanism
to prevent aggregation. Pd-based catalysts, in particular nano-
sized Pd particles, have recently drawn a great deal of atten-
[
10–12]
support matrix
or by the evaporation of metals on
[
13]
a heated polymer surface,
(b) the polymerisation of the
[
14]
matrix around the nanoparticles and (c) the in situ formation
of the polymer from its monomer and the formation of the
[28–33]
tion because of their versatile role in organic synthesis.
Pd-catalysed hydrogenation reactions are important for indus-
[
15]
metal nanoparticles from their ionic precursor.
The latter
[34–36]
trial applications.
In most cases, chemists use molecular
hydrogen for hydrogenation, and these reactions are either
[
a] Dr. S. K. Mahato, Dr. R. Ul Islam, Prof. K. Mallick
Department of Chemistry
[37–40]
less selective or offer low product yields.
Catalytic transfer
University of Johannesburg
P.O. Box 524, Auckland Park 2006 (South Africa)
Fax: (+27)11-559-2819
hydrogenation is an interesting reaction in organic chemistry
compared to the conventional hydrogenation reaction in sev-
eral respects, and a large number of hydrogen donors have
E-mail: kaushikm@uj.ac.za
[41]
been used for this kind of reaction. Catalytic transfer hydro-
genation offers the possibility to avoid the use of molecular
[
b] C. Acharya
Department of Chemistry
CSIR-Indian Institute of Chemical Biology
[42]
hydrogen and its associated hazards. Therefore, hydrogen-
donating agents need to be added to the reaction mixture.
These can be inorganic hydrides or organic donor molecules
such as ammonium formate or formic acid.
4
, Raja S. C. Mullick Road, Kolkata—700 032 (India)
[
c] Prof. M. J. Witcomb
DST/NRF Centre of Excellence in Strong Materials
University of the Witwatersrand
In association with our ongoing research on the develop-
ment of effective catalysts for synthetic organic transforma-
Private Bag 3, WITS 2050 (South Africa)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300985.
[25,26,28–33]
tions,
we have found that a polypyrrole-immobilised
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1419

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Pd nanoparticle catalyst can be used for the chemoselective
transfer hydrogenation of a,b-unsaturated carbonyls in the
presence of NH OOCH or HCOOH under reflux conditions. The
4
interesting aspect of this catalyst is that it can selectively hy-
drogenate the olefins or acetylenes without affecting the car-
bonyl moiety and ArÀX (X=Cl, Br) bonds with an excellent
yield of products for a wide range of substrates. In the present
study, we critically examine the recyclability of the catalyst, the
possibility of the leaching of the metal nanoparticles from the
catalyst and the kinetics of the hydrogenation reaction.
Results and Discussion
The optical behaviour of the product was studied by FTIR
spectroscopy. A typical FTIR spectrum of the product obtained
during the reaction between pyrrole and palladium acetate is
À1
shown in Figure 1. The band that occurs at n˜ =1649 cm cor-
Figure 2. A and B) SEM images of the Pd polypyrrole composite. C) TEM
image of the Pd polypyrrole composite. The higher magnification TEM
images (D and E) clearly indicate that the 2–4 nm diameter Pd nanoparticles
are dispersed within the polymer matrix (some are circled).
Figure 1. FTIR spectra of the Pd polypyrrole composite.
responds to the CÀC stretching vibration in the polypyrrole
À1
ring, and the bands at n˜ =1469 and 1415 cm correspond to
the CÀN stretching vibration in the ring. The polypyrrole ring
À1
breathing vibration is located at n˜ =1250–1100 cm . The peak
À1
at n˜ =1111 cm is attributed to the breathing vibration of the
[
43]
pyrrole ring. The bands of the CÀH and NÀH in-plane defor-
À1
mation vibrations are located at n˜ =1021 cm , and the band
À1
at n˜ =744 cm corresponds to the CÀH out-of-plane bending
[
44]
À1
vibration. The bands above n˜ =2500 cm (Figure 1B) are at-
tributed to the aromatic CÀH and NÀH stretching vibrations.
The spectroscopic evidence confirms the formation of polypyr-
role.
The SEM images (Figure 2A and B) illustrate the chainlike
structure of the product. A similar kind of structure was also
obtained if the ratio of palladium acetate to pyrrole was al-
tered. Various diameters of the chain were seen on the grid
surface, which may be because of the overlap of the chains.
The TEM image (Figure 2C) of the product shows that the sur-
face of the nano-spheres is not smooth and, from the higher
magnification TEM images (Figure 2D and E), dark spots on
the surface can be seen that are distributed uniformly (some
are circled, Figure 1E). Energy-dispersive spectroscopy (EDS;
Figure 3A), obtained if the electron beam was focussed onto
a dark spot in the polymer, confirms that these spots are Pd
Figure 3. A) The EDS spectrum of the Pd polymer composite. The presence
of Pd is indicated clearly around 3.0 keV. The Cu peak originates from scat-
tering from the TEM copper mesh support grid. B) Pd3d XPS spectrum of
the Pd polypyrrole composite. The peaks at binding energies of 335.90 eV
for 3d5/2 and 340.80 eV for 3d3/2 are indicative of metallic Pd.
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1420

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
particles. If the beam was focussed between the dark spots
near the thin edge of the chain, no Pd X-ray peaks were yield-
ed. Stereo images show that the Pd particles are distributed
homogeneously within and throughout the sample. Thus, we
can envisage that the polymer acts as a cage and the Pd nano-
particles, with a particle diameter of 2–3 nm, are encapsulated
and highly dispersed inside that cage. To identify the chemical
state of the polymer-stabilised Pd particles, X-ray photoelec-
tron spectroscopy (XPS) measurements were performed. The
Pd3d region of the XPS spectrum of the Pd polypyrrole com-
posite is illustrated in Figure 3B, which reveals the presence of
Pd3d5/2 and 3d3/2 peaks at binding energies of 335.90 and
Scheme 1. Oxidative polymerization of pyrrole.
[48]
Pd ion to form a Pd atom. Subsequently, the coalescence of
the atoms results in the formation of clusters that stabilise the
polymer.
3
40.80 eV, respectively. These binding energy values are in
[
45]
accord with those reported for metallic Pd.
The UV/Vis spectrum of the product is shown in Figure 4. A
shoulder-like feature at lꢀ400 nm corresponds to the bipolar-
Catalytic performance of the Pd polypyrrole composite
To verify the catalytic activity of the Pd polypyrrole composite,
1,3-diphenylpropenone (1a) was chosen as a model substrate
for the chemoselective transfer hydrogenation by using
NH OOCH or HCOOH, a hydrogen storage material, under dif-
4
ferent solvents (Table 1). In the absence of the Pd catalyst,
Table 1. Optimization of the reaction under different solvent condi-
[
a]
tions. Chemoselective transfer hydrogenation of 1a in the presence of
NH OOCH or HCOOH.
4
Figure 4. UV/Vis spectrum of the Pd polypyrrole composite, in which
a shoulder-like feature that appears at lꢀ400 nm corresponds to the bipo-
laron transition, a feature of the oxidised state of polypyrrole. The second
absorption band at l=330 nm represents the p*–p transition.
Entry Catalyst (concentration [mol%]) Solvent
t [h] Yield [%]
[
b]
1
2
3
4
5
6
7
8
9
1
Pd polypyrrole (0.0)
Pd polypyrrole (0.05)
Pd polypyrrole (0.05)
Pd polypyrrole (0.05)
Pd polypyrrole (0.05)
Pd polypyrrole (0.05)
Pd polypyrrole (0.05)
Pd polypyrrole (0.1)
Pd polypyrrole (1.0)
Pd polypyrrole (0.05)
Pd polypyrrole (0.05)
Pd C (10.0)
CH
CH
CHCl
DCE
toluene
iPrOH
MeOH
MeOH
MeOH
MeOH/H
THF
2
Cl
Cl
2
2
48
n.r.
69
61
73
81
78
96
96
96
n.r.
48
19
23
2
18
20
3
on transition, which is a feature of the oxidised state of poly-
18
24
pyrrole. The second absorption band at l=330 nm results
[
46]
from the p*–p transition. It is well known that metal nano-
particles absorb photons in the UV/Vis region because of the
coherent oscillation of the conduction-band electrons induced
by the interacting electromagnetic fields. However, Pd nano-
particles do not show a pronounced peak as a result of the
surface plasmons.
10
8
8
8
0
2
O (8:2) 48
11
1
1
48
24
24
2
3
MeOH
MeOH
2
Pd SiO (10.0)
A wide variety of methods have been applied for the prepa-
ration of polypyrrole by the oxidative polymerisation of the
[
a] Reaction conditions: 1a (1.0 mmol), NH
4
OOCH (3.0 mmol) or HCOOH
[
47]
(3.0 mmol), under reflux, [b] n.r.=No reaction.
monomer. Oxidative polymerisation of pyrrole to polypyrrole
proceeds through the oxidation of pyrrole to a radical cation,
which subsequently couples with another radical cation to
form bipyrrole. This process is then repeated to form longer
chains (Scheme 1). The final form of polypyrrole is that of
a long conjugated backbone. The polymer has resonance
structures that resemble the aromatic or quinoid forms. In this
neutral state, the polymer is not conducting and only becomes
conducting if it is oxidised. The charge associated with the oxi-
dised state is typically delocalised over several pyrrole units
and can form a radical cation (polaron) or a dication (bipolar-
there was no sign of the formation of the reduced product 2a.
However, the addition of a Pd-based catalyst afforded the de-
sired reduced product with varying yields (Table 1). Further op-
timisation of the reaction was performed by variation of the
catalyst loading and solvent (Table 1). The best result was ach-
ieved if the catalyst concentration was 0.05 mol% of Pd in
methanol as the solvent (Table 1, entry 7). The product yield
was not improved if we doubled the catalyst concentration
(Table 1, entry 8). Methanol was the best solvent out of those
tested (Table 1, entries 2–7), although toluene also gave
[
47]
on). Each polymerisation step is associated with the release
[
47]
of an electron, and that electron is then used to reduce the
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1421

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
À1
a good yield (Table 1, entry 5). Notably, the presence of water
in the reaction mixture hampered the formation of the product
1170 h was recorded for the hydrogenation of 1,4-diphenyl-
but-2-ene-1,4-dione (Table 2, entry 2), which is because of the
presence of two carbonyl groups that decrease the electron
density at the double bond and decrease the rate of hydroge-
nation. If the phenyl group was substituted by an electron-do-
(
(
Table 1, entry 10). The commercially available catalysts Pd/C
10 mol%) and Pd/SiO (10 mol%) provided much lower yields
2
even with a prolonged reaction time (Table 1, entries 12 and
3).
To widen the scope of this method, a variety of alkene and
1
nating ÀCH group at the para position, a further decrease in
3
the TOF value for the rate of hydrogenation of 1,4-di-p-tolyl-
but-2-ene-1,4-dione (Table 2, entry 3) was again a result of the
presence of a carbonyl group that decreased the electron den-
sity at the double bond. Compounds that contain
two double bonds, such as 1,5-diphenylpenta-1,4-
dien-3-one (Table 2, entry 4), showed lower TOF
values because of the longer time required for the
hydrogenation to occur at the two unsaturated cen-
alkyne substrates was screened under the optimised condi-
tions (Table 2). It was found that the alkene and alkyne seg-
[
a]
Table 2. Chemoselective transfer hydrogenation of olefins/acetylenes.
Entry Olefin
Product
t [h] Yield [%] TOF
tres. The carbonyl group of the substrates does not
have any effect on the hydrogenation. Alkynes or
acetylenes are compounds that contain a CꢁC bond,
which is stronger than a double bond and a higher
energy is needed to reduce it. Therefore, compounds
that contain a triple bond (Table 2, entries 5–7) took
a longer time for hydrogenation and, consequently,
had lower TOF values than the compounds that con-
tain double bonds. The TOF value for the hydrogena-
tion of 1,3-diphenylpropynone (Table 2, entry 5) was
8
96
96
1463
1
1
1
a
2a
1
0
1170
2
b
2b
1
1
1
0
8
4
94
77
81
1146
552
706
3
4
5
À1
À1
706 h . Again, a decrease in the TOF value (603 h )
1
1
c, R=4-Me-C
6
H
4
2c
in the hydrogenation of 1,4-diphenylbut-2-yne-1,4-
dione (Table 2, entry 6) was because of the presence
of two carbonyl groups that decrease the electron
density at the triple bond and slow the rate of hydro-
d
2d
genation. A further decrease in the TOF value
À1
(
583 h ) for the hydrogenation reaction of 1,4-di-p-
tolylbut-2-yne-1,4-dione (Table 2, entry 7) is because
of the combined effect of two carbonyl groups and
1
1
e
f
2a
2b
the presence of an electron-donating ÀCH group at
3
the para position of the phenyl group that facilitates
the decrease of the electron density at the triple
bond. We also observed that the reactivity for the hy-
drogenation of 4-phenylbut-3-en-2-one further de-
creased if the aromatic group was replaced by an ali-
phatic group (Table 2, entry 8). The low reactivity to-
1
8
89
603
6
1
2
8
4
86
88
583
447
7
8
wards the chemoselective hydrogenation (TOF=
1
1
g, R=4-Me-C
6
H
4
2c
À1
4
47 h ) is because of the decrease of the electron
density at the triple bond as a result of the presence
of ÀCH CO, a moderately strong electron-withdraw-
3
h
2h
ing group.
[a] Olefins/acetylenes (1.0 mmol), NH
4
OOCH (3.0 mmol) or HCOOH (3.0 mmol). Catalyst
It has been reported that a Pd-based catalyst can
(0.5 mol%), MeOH (5 mL), under reflux.
[49]
be used for a dehalogenation reaction, whereas in
our catalytic system, the hydrodehalogenation of the
haloarene segment of the substrate was not affected
under the optimised conditions (Table 3). In the pres-
ments of the substrates are reduced, whereas the catalyst did
not reduce the carbonyl group. No semi-hydrogenation prod-
uct was observed for the alkynes if the Pd polypyrrole catalyst
was used.
ent catalytic chemoselective hydrogenation reaction, a similar
activation trend has been observed for both the halogenated
compounds (Table 3) and those compounds without a halogen
segment (Table 2). We also used a variety of substrates that
contain ArÀX (X=Cl, Br) segments under the optimised condi-
tions (Table 3). A decrease of the TOF value was observed for
the hydrogenation of 3-(4-chlorophenyl)-1-phenylpropenone
À1
The turnover frequency (TOF) was as high as 1463 h for
the hydrogenation of 1a (Table 2, entry 1) if 0.05 mol% of Pd
was used as the catalyst. A decrease in the TOF value to
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1422

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
group and a triple bond, further decreased with
[
a]
Table 3. Chemoselective transfer hydrogenation of halogenated olefins/acetylenes.
À1
a TOF value of 442 h (Table 3, entry 7).
Entry
Olefins
Product
t [h]
Yield [%]
97
TOF
Recyclability study
1
1
0
0
1182
1
A recyclability study of the Pd polypyrrole composite
catalyst was investigated for the reduction of 1a by
using 0.05 mol% of Pd catalyst in methanol as a sol-
vent. The recovery of the catalyst was a tedious task
because of the minute quantity of the material used
for the reaction and the nature of the support of the
catalyst. At the end of the each reaction cycle, the
product was extracted with ethyl acetate and the re-
sidual catalyst was reused under the same reaction
conditions. The results indicated that the used mate-
rial was active as a catalyst without a significant loss
of catalytic performance. At the end of the third
cycle, a yield of 84% of the hydrogenated product
was achieved, which is 12% less than the fresh cata-
lyst. The TEM image of the used catalyst (Figure 5)
shows the enlargement of the Pd particles, but no
evidence was found for the formation of Pd black.
The size of the Pd nanoparticles in the fresh sample
was 2–3 nm (Figure 2D and E), whereas Ostwald rip-
ening of the particles was observed at the end of the
third cycle, and the particles were up to 5–15 nm in
size (Figure 5).
1
i
2i
6 4
R=R=4-Cl-C H
96
97
90
92
94
87
1170
1182
610
623
637
442
2
3
4
5
6
7
1
j
2j
2k
2l
2j
2k
2l
6 4
R=4-Cl-C H
1
1
1
1
2
0
8
8
8
4
1
k
6 4
R=4-Br-C H
1
l
6 3
R=3-Cl-4-Me-C H
1
m
6 4
R=4-Cl-C H
Importantly, inductively coupled plasma mass
spectrometry (ICP-MS) analysis did not provide any
evidence for the leaching of Pd during the recovery
of the catalyst. The gradual decrease of the yield was
mainly a result of the loss of catalyst during washing.
In every cycle, the fresh reaction mixture had a lower
amount of Pd in the catalyst, which thus affected the
yield, but it may not have influenced the TOF value,
which is difficult to calculate at that stage of the pro-
cess. However, to confirm the degree of Pd leaching,
we increased the catalyst loading and maintained the
catalyst-to-reactant ratio used for the experiments
summarised in Table 2 for the reduction of 1a
1
n
6 4
R=4-Br-C H
1
o
R=3-Cl-4-Me-C
6 3
H
[
(
a] Olefins/acetylenes (1.0 mmol), NH OOCH (3.0 mmol) or HCOOH (3.0 mmol). Catalyst
4
0.5 mol%), MeOH (5 mL), under reflux.
(
Table 2, entry 1). At the end the first, second and
À1
(
Table 3, entry 1, TOF=1182 h ) in comparison with that of 1a
third cycles of the reaction, a small fraction of the filtrate was
withdrawn for ICP-MS analysis. The concentration of leached
À1
(
Table 2, entry 1, TOF=1463 h ) as the phenyl group substi-
tuted by Cl (an electron-withdrawing group) was responsible
for the diminution of the electron density of the olefin seg-
ment. Chloro or bromo substitution on the phenyl group has
a similar effect on hydrogenation in terms of TOF values
À1
(
Table 3, entry 2, TOF=1170 h and Table 3, entry 3, TOF=
À1
1
182 h ). However, the presence of the electron-donating
ÀCH3 group causes a retarded hydrogenation for 1,4-bis-(3-
chloro-4-methylphenyl)but-2-ene-1,4-dione, which has a dimin-
À1
ished TOF value (610 h ; Table 3, entry 4). Close TOF values
have been achieved for the hydrogenation of 1m and 1n
À1
(
Table 3) with a chloro (TOF=623 h ) and bromo (TOF=
À1
6
37 h ) substitution on the phenyl group. The rate of the hy-
drogenation reaction of 1,4-bis-(3-chloro-4-methylphenyl)but-
-yne-1,4-dione, which contains the electron-donating ÀCH3
Figure 5. TEM image of the Pd nanoparticles at the end of the third cycle,
which reveal particle enlargement as a result of Ostwald ripening.
2
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1423

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Pd in the solution was found to
be 3.0, 7.0 and 15.0 ppm after
the first, second and third cycles,
respectively, in the presence of
methanol. In the presence of di-
chloromethane and chloroform,
Pd leaching was more than
7.0 ppm after the first cycle,
whereas toluene showed a simi-
lar leaching behaviour to metha-
nol after the first cycle.
Scheme 2. Proposed mechanism for the chemoselective reduction of 1a by using a Pd polypyrrole composite cat-
alyst.
Kinetics study
formato complex 1ay. This is followed by decarboxylation to
a,b-Unsaturated carbonyl compounds, such as (a) 1a (Table 2,
entry 1) and (b) 1,4-bis-(3-chloro-4-methylphenyl)but-2-yne-1,4-
dione (1o, Table 3, entry 7) were chosen to study the kinetics
of hydrogenation reactions in the presence of the Pd polypyr-
role hybrid catalyst. To achieve the maximum yields of the re-
action products, a relatively long reaction time (25 h) was
chosen. The hydrogenation reactions were performed at 658C
in the presence of 0.05 mol% Pd in methanol. The overall ki-
netics data (Figure 6) indicates that the conversion of 1o
form a hydropalladium complex 1az, which undergoes the re-
ductive elimination of Pd to form Pd nanoparticles and enol
2ax furnishes the product 2a (Scheme 2).
Notably, for the substrates that contain ÀC=O, ÀC=CÀ and
ÀCꢁCÀ groups without extended conjugation (Table 4), the
Table 4. Unsuccessful examples of the chemoselective transfer of hydro-
[
a]
genation of olefins/acetylenes.
Entry
1
Olefins
Product
–
t [h]
Yield [%]
n.r.
48
1
p
–
–
48
48
n.r.
n.r.
2
3
1
1
q
r
–
48
n.r.
4
1
s
Figure 6. Comparative kinetic study of the chemoselective transfer hydroge-
nation of a) 1a and b) 1,4-bis-(3-chloro-4-methylphenyl)but-2-yne-1,4-dione
in the presence of formic acid and a Pd polypyrrole hybrid catalyst
[
4
a] Reaction conditions: Olefins (1.0 mmol), NH OOCH (3.0 mmol) or
HCOOH (3.0 mmol). Catalyst (0.5 mol%), MeOH (5 mL), under reflux.
(0.05 mol% Pd).
Pd polypyrrole composite was not a successful catalyst to per-
form the transfer hydrogenation reaction even if the reaction
time was prolonged, the temperature was increased and the
catalyst loading was doubled.
(
graph b) occurred at a slower rate than 1a (graph a). At the
beginning of the reaction, the conversion of both the com-
pounds was relatively slow, but, with time, the hydrogenation
of 1a took place at a faster rate than that of 1o. Overall,
a steady conversion was achieved in both cases. For 1o, 60%
conversion was achieved in approximately 25 h, after which
the conversion rate plateaued because of the deposition the
product on the surface of the catalyst that hindered the access
of the reactants.
Conclusions
The synthesis of a Pd polypyrrole composite material by using
the in situ polymerisation and composite formation method is
reported. The composite was used as a catalyst for the chemo-
selective transfer hydrogenation of a wide range of substrates,
and the results revealed that the catalyst was highly active for
this reaction. No semi-hydrogenated product was observed if
this catalyst was applied to alkyne systems. Hydrodehalogena-
tion of the haloarene segment of the substrate was not en-
countered under this catalytic regime.
Mechanistic pathway
A mechanism for the reduction of 1a is presented in
Scheme 2. The first step consists of the oxidative addition of
Pd nanoparticles to the substrate 1a to form the organopalla-
dium species 1ax, which reacts with formic acid to give the
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1424

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Supporting Information
Experimental Section
1
13
Representative H and C NMR spectra of the compounds: 2a, 2b,
d and 2k.
TEM studies of the particles were performed at an acceleration
voltage of 197 kV by using a Philips CM200 TEM instrument
2
equipped with a LaB source. An ultra-thin windowed energy-dis-
6
persive X-ray spectrometer (EDS) and a Gatan Imaging Filter (GIF)
attached to the TEM were used to determine the chemical compo-
sition of the samples. The samples were prepared by depositing
a small amount of a dilute toluene solution of the metal sample
onto a TEM grid (200 mesh size Cu grid) coated with a lacy carbon
film. The SEM studies were performed at 5 kV by using an FEI
Quanta 400 instrument. As a precaution to prevent possible charg-
ing, the samples were sputter-coated with a thin, uniform layer of
Au-Pd. XPS spectra were collected by using an ultra-high vacuum
chamber attached to a Physical Electronics 560 ESCA/SAM instru-
ment. The XRD patterns were recorded by using a Shimadzu XD-
Acknowledgements
We thank the University Research Council (URC) and Faculty of
Science, University of Johannesburg, South Africa for financial
support. M.J.W. gratefully acknowledges the financial support of
the National Research Foundation, South Africa.
Keywords: hydrogenation
polymers · supported catalysts
·
nanoparticles
·
palladium
·
3
A
diffractometer operating at 20 kV using CuKa radiation
0.1542 nm). The measurements were performed over the range of
q=20–808. For UV/Vis spectroscopy, a small portion of the solid
sample was dissolved in methanol and scanned within the range
(
2
[
[
1] G. Schmid, Clusters and Colloids, VCH, Weinheim, 1994.
2] L. N. Lewis, Chem. Rev. 1993, 93, 2693–2730.
3
1
00–800 nm by using a digital spectrophotometer (Shimadzu UV-
[3] A. P. Alivisatos, Science 1996, 271, 933–937.
[4] A. Henglein, Chem. Rev. 1989, 89, 1861–1873.
800). The other fraction of the dried solid was used for IR spec-
À1
[
5] A. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed. 2005, 44, 7852–
7872; Angew. Chem. 2005, 117, 8062–8083.
troscopy in the range 4000–700 cm by using an FTIR spectrome-
ter (Shimadzu, IRAffinity-1) operating at a resolution of 4 cm . The
sample was deposited as a thin film on a NaCl disk.
À1
[
[
[
6] E. Moreno-MaÇas, R. Pleixats, Acc. Chem. Res. 2003, 36, 638–643.
7] Y. Li, E. Boone, M. A. El-Sayed, Langmuir 2002, 18, 4921–4925.
8] K. R. McCrea, J. S. Parker, G. A. Somorjai, J. Phys. Chem. B 2002, 106,
10854–10863.
[
9] M. Valden, X. Lai, D. W. Goodman, Science 1998, 281, 1647–1650.
Materials
[10] A. B. R. Mayer, Mater. Sci. Eng. C 1998, 6, 155–166.
11] S. T. Selvan, J. P. Spatz, H.-A. Klok, M. Moller, Adv. Mater. 1998, 10, 132–
34.
[12] J. J. Watkins, T. J. McCarthy, Chem. Mater. 1995, 7, 1991–1994.
[13] K. Sayo, S. Deki, S. Hayashi, Eur. Phys. J. D, 1999, 9, 429–432.
[14] J. Lee, V. C. Sundar, J. R. Heine, M. G. Bawendi, K. F. Jensen, Adv. Mater.
[
All the solvents used for this experiment were purchased from
Merck, and all other chemicals were purchased from Aldrich and
used without further purification. All reactions were performed
under an Ar atmosphere.
1
2
000, 12, 1102–1105.
15] K. Mallick, M. J. Witcomb, R. Erasmus, A. Strydom, J. Appl. Phys. 2009,
06, 074303–074306.
[
1
[
[
16] R. Gangopadhyay, A. De, Chem. Mater. 2000, 12, 608–622.
17] K. Mallick, M. Witcomb, M. Scurrell, A. Strydom, J. Phys. D 2009, 42,
Preparation of Pd polypyrrole nanoparticles
0
95409.
The whole process was performed at RT. Pyrrole (0.202 g, 3 mmol)
was added to toluene (10 mL) under continuous stirring. Palladium
acetate (0.225 g, 1 mmol) in toluene (5 mL) was added dropwise to
the pyrrole solution over 1 h. The reaction mixture was allowed to
stand for 30 min, at which point a dark brown cloudy precipitate
developed. The solvent was allowed to evaporate under an inert
atmosphere. The solid product was a Pd polypyrrole composite
material, which was confirmed by TEM, SEM, XPS and IR spectros-
copy.
[
[
18] K. Mallick, M. J. Witcomb, A. Dinsmore, M. S. Scurrell, Macromol. Rapid
Commun. 2005, 26, 232–235.
19] K. Mallick, M. J. Witcomb, M. S. Scurrell, J. Phys. Condens. Matter 2007,
1
9, 196225.
[20] K. Mallick, M. J. Witcomb, M. S. Scurrell, A. M. Strydom, Gold Bull. 2008,
41, 246–250.
[21] K. Mallick, M. J. Witcomb, M. S. Scurrell, Eur. Polym. J. 2006, 42, 670–
6
75.
22] K. Mallick, M. J. Witcomb, M. S. Scurrell, Appl. Phys. A 2005, 123, 181–
86.
[
[
[
1
23] A. P. O’Mullane, S. E. Dale, J. V. Macpherson, P. R. Unwin, Chem.
Commun. 2004, 1606–1607.
24] K. Mallick, M. Witcomb, A. Dinsmore, M. Scurrell, Langmuir 2005, 21,
7964–7967.
[25] S. Scalzullo, K. Mondal, M. Witcomb, A. Deshmukh, M. Scurrell, K. Mal-
lick, Nanotechnology 2008, 19, 075708.
General procedure for the hydrogenation reaction catalysed
by Pd polypyrrole composite
For
a
general reaction, a,b-unsaturated carbonyl analogue
[26] K. Mallick, M. J. Witcomb, A. Dinsmore, M. S. Scurrell, J. Mater. Sci. 2006,
41, 1733–1737.
(
1.0 mmol), NH OOCH (3.0 mmol) or HCOOH (3.0 mmol) and meth-
4
anol (5.0 mL) were placed in a small round-bottomed flask. The
catalyst was added to the reaction mixture in the required amount.
The mixture was heated to reflux in an oil bath. At the end of the
reaction (monitored by TLC), the reaction mixture was cooled, di-
luted with methanol and then centrifuged to recover the catalyst.
The methanol solution was evaporated. The residue was extracted
with ethyl acetate, washed with water and brine and dried over
[27] A. Deschamps, J. P. Lagier, F. Fievert, S. Aeiyach, P. C. Lacaze, J. Mater.
Chem. 1992, 2, 1213–1214.
[
[
[
28] R. Ul Islam, M. Witcomb, M. Scurrell, W. Otterlo, E. Lingen, K. Mallick,
Synth. Commun. 2011, 41, 3561–3572.
29] R. Islam, M. Witcomb, M. Scurrell, E. Lingen, W. Otterlo, K. Mallick, Catal.
Sci. Technol. 2011, 1, 308–315.
30] R. Islam, M. Witcomb, E. Lingen, M. Scurrell, W. Otterlo, K. Mallick, J. Or-
ganomet. Chem. 2011, 696, 2206–2210.
MgSO . After the evaporation of the solvents, the residue was puri-
fied by flash chromatography.
[31] R. Islam, M. Witcomb, M. Scurrell, W. Otterlo, K. Mallick, Catal. Commun.
2010, 12, 116–121.
4
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1425

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
[
[
32] R. Ul Islam, S. Mahato, S. Shukla, M. Witcomb, K. Mallick, ChemCatChem
013, 5, 2453–2461.
33] M. Choudhary, R. Islam, M. Witcomb, M. Phali, K. Mallick, Appl. Organo-
met. Chem. 2013, 27, 523–528.
RSC Adv. 2013, 3, 2112–2153; c) C. M. Moore, N. K. Szymczak, Chem.
Commun. 2013, 49, 400–402; d) S. Gladiali, E. Alberico, Chem. Soc. Rev.
2006, 35, 226–236; e) J. S. M. Samec, J. E. Bꢁckvall, P. G. Andersson, P.
Brandt, Chem. Soc. Rev. 2006, 35, 237–248.
2
[
[
[
[
34] J. Tsuji, Synthesis 1990, 739–749.
[43] L. Bansal, M. A. El-Sherif, Int. J. Smart Sens. Intell. Syst. 2010, 3, 230–252.
[44] L. J. Bellamy, The Infrared Spectra of Complex Molecules, Chapman&Hall,
London, 1980.
[45] D. Bera, S. C. Kuiry, M. McCutchen, A. Kruize, H. Heinrich, M. Meyyappan,
S. Seal, Chem. Phys. Lett. 2004, 386, 364–368.
35] T. Beebe, J. Yates, J. Am. Chem. Soc. 1986, 108, 663–671.
36] Y. Niu, L. Yeung, R. Crooks, J. Am. Chem. Soc. 2001, 123, 6840–6846.
37] T. Mitsudome, Y. Takahashi, S. Ichikawa, T. Mizugaki, K. Jitsukawa, K.
Kaneda, Angew. Chem. Int. Ed. 2012, 51, 136–139; Angew. Chem. 2012,
1
24, 140–143.
[46] D. N. Huyen, N. T. Tung, T. D. Vinh, N. D. Thien, Sensors 2012, 12, 7965–
7974.
[
[
[
[
[
38] K. Paredis, L. K. Ono, F. Behafarid, Z. Zhang, J. C. Yang, A. I. Frenkel, B. R.
Cuenya, J. Am. Chem. Soc. 2011, 133, 13455–13464.
39] B. S. P. Chauhan, J. S. Rathore, T. Bandoo, J. Am. Chem. Soc. 2004, 126,
[47] T. V. Vernitskaya, O. N. Efimov, Russ. Chem. Rev. 1997, 66, 443–457.
[48] K. Mallick, M. J. Witcomb, M. S. Scurrell, Eur. Phys. J. E 2006, 19, 149–
154.
8
493–8500.
40] S. Bhattacharjee, D. M. Dotzauer, M. L. Bruening, J. Am. Chem. Soc. 2009,
31, 3601–3610.
41] H. Yagi, D. Thakker, R. Lehr, D. Jerina, J. Org. Chem. 1979, 44, 3439–
442.
42] a) A. Koziol, A. Lendzion-Paluch, A. Manikowskind, Org. Process Res. Dev.
013, 17, 869–875; b) A. Stolle, T. Gallert, C. Schmoeger, B. Ondruschka,
[49] R. Nakao, H. Rhee, Y. Uozumi, Org. Lett. 2005, 7, 163–165.
1
3
Received: November 16, 2013
Revised: December 13, 2013
2
Published online on March 19, 2014
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 1419 – 1426 1426