Hydrogenation of substituted nitroarenes by a polymer-bound palladium(II) Schiff base catalyst

Article abstract of DOI:10.1007/s11243-009-9320-y

The catalytic activity of a polymer-bound palladium Schiff base catalyst was investigated toward the reduction of aryl nitro compounds under ambient temperature and pressure. The dependence of the rate of hydro- genation of o-nitroaniline and o-nitrotoluene on substrate concentration, catalyst concentration and temperature has been determined. Based on the results obtained a plausible mechanism for the hydrogenation reaction is proposed and a rate expression is deduced. The energy and entropy of activation have been evaluated from the kinetic data. The polymer-bound catalyst was found to be better than its homogeneous analog PdCl₂(NSBA) [NSBA = N-salicy- lidene benzylamine] for both stability and reusability. Recycling studies revealed that the catalyst could be used six times without metal leaching or significant loss in activity. Springer Science+Business Media B.V. 2009.

Full text of DOI:10.1007/s11243-009-9320-y

Transition Met Chem (2010) 35:247–251
DOI 10.1007/s11243-009-9320-y
Hydrogenation of substituted nitroarenes by a polymer-bound
palladium(II) Schiff base catalyst
S. Alexander
V. Gayathri
•
V. Udayakumar
•
N. Nagaraju
•
Received: 27 October 2009 / Accepted: 1 December 2009 / Published online: 15 December 2009
Ó Springer Science+Business Media B.V. 2009
Abstract The catalytic activity of a polymer-bound
palladium Schiff base catalyst was investigated toward the
reduction of aryl nitro compounds under ambient temper-
ature and pressure. The dependence of the rate of hydro-
genation of o-nitroaniline and o-nitrotoluene on substrate
concentration, catalyst concentration and temperature has
been determined. Based on the results obtained a plausible
mechanism for the hydrogenation reaction is proposed and
a rate expression is deduced. The energy and entropy of
activation have been evaluated from the kinetic data. The
polymer-bound catalyst was found to be better than its
recent years [2]. Such complexes have been extensively
used as catalysts for polymerization, hydrogenation, hydro-
formylation and isomerisation of alkenes and also for
substitution reactions [3].
Homogeneous catalysts, in general, have the drawbacks
of high susceptibility to reaction conditions and difficult
separation from the product mixture. Low thermal and
chemical stabilities can lead to their slow decomposition
under stringent reaction conditions [4]. These problems can
be addressed by immobilizing them on a suitable polymer
matrix or on an inorganic support [5–8], which provides
chemical and thermal stabilities; in addition, the catalyst
can often be easily separated from the product mixture and
reused. Steric crowding around the metal atom in the
polymer matrix may also increase the substrate specificity
and product selectivity. All these qualities can make such
polymer-anchored catalysts more attractive than their
homogeneous counterparts [9].
homogeneous analog PdCl (NSBA) [NSBA = N-salicy-
2
lidene benzylamine] for both stability and reusability.
Recycling studies revealed that the catalyst could be used
six times without metal leaching or significant loss in
activity.
Introduction
Investigations on the reduction of nitroaromatics using
polymer-bound metal complexes are uncommon to date
and have usually required high pressure [10]. Aromatic
amines are important starting materials and intermediates
for the manufacture of a variety of chemicals such as dye
stuffs, pharmaceuticals, agricultural chemicals, surfactants
and polymers [11]. The conventional method for hydro-
genation involves the use of stoichiometric amounts of
finely divided iron in the presence of acid [12].
The generation of large amounts of inorganic waste due to
the use of stoichiometric reagents and multistep synthesis
are major drawbacks of many conventional processes for
fine chemicals and pharmaceuticals [1]. Environmentally
clean synthesis using transition metal complexes as catal-
ysis have therefore been the focus of much research in
Due to the importance of polymer-supported complexes,
we have used various polymer supports for the hydroge-
nation of olefins, nitro compounds and Schiff bases
[13–15]. In continuation of this work, we describe in this
article an efficient and mild method for the reduction of
aromatic nitro compounds using a palladium Schiff base
complex anchored to polystyrene cross-linked with divinyl-
benzene (DVB) as catalyst.
S. Alexander ꢀ V. Udayakumar ꢀ V. Gayathri (&)
Department of Chemistry, Central College Campus,
Bangalore University, Bangalore 560001, India
e-mail: gayathritvr@yahoo.co.in
N. Nagaraju
Department of Chemistry, St. Joseph’s College P.G. Center,
Bangalore 560027, India
123

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Transition Met Chem (2010) 35:247–251
Table 1 Initial rate of hydrogenation of o-nitroaniline with various
-3
solvents (30 mL), [catalyst] = 14.41 9 10 M and [substrate] =
0.033 M at 30 °C
CH2
CH2
-
5
3
Initial rate 9 10 (mol/dm /s)
N
Solvent
H
N
Cl
Cl
H
Pd
Pd
Methanol
Ethanol
Acetone
Acetonitrile
THF
5.06
4.65
1.70
2.66
0.03
0.29
0.81
HO
OH
Cl
Cl
Fig. 1 Structure of polymer-bound palladium Schiff base complex
Benzene
Toluene
Experimental
Chloromethylated polystyrene divinyl benzene copolymer
with 6.5% cross-linking (PSDVB) was obtained as a gift
from Thermax India Ltd; Pune, India. Palladium chloride
was obtained from Arora Matthey Ltd, India. Salicylalde-
hyde, ortho- and meta-nitro toluene were distilled under
reduced pressure, while all the other nitroarenes were re-
crystallized before use. Super dry methanol was prepared
according to the literature method [16]. The synthesis and
characterization of the polymer-bound Schiff base (Fig. 1)
catalyst has been reported elsewhere [13].
aminomethylated beads. These beads were functionalized
with salicylaldehyde to form the Schiff base as described
previously [13]. The functionalized beads were then reac-
ted with palladium chloride to form the supported palla-
dium Schiff base complex.
The interaction of the solvent with the polymer is a
complicated process. The processes of diffusion, absorp-
tion and swelling of the polymer matrix take place simul-
taneously [17]. The nature of the solvent influences the
flexibility of the polymers and the swollen polymer seg-
ments change the physical state. This contributes to the rate
of catalytic reactions. In order to study the effect of vari-
ation in solvent on the hydrogenation of nitro compounds,
the rate of hydrogenation of o-nitroaniline in various sol-
vents was measured and the results are tabulated in
Table 1. It can be seen that polar protic solvents are more
favorable as a reaction medium than aprotic polar solvents
and nonpolar solvents. Since the solubility of hydrogen and
miscibility of substrates are optimal in methanol, further
studies of the reduction of the nitro compounds were car-
ried out in methanol.
Catalyst activation
The supported palladium Schiff base complex (0.50 g) was
suspended in ethanol (25 mL), and NaBH4 (0.10 g,
2
.64 mmol) was added during which the supported com-
plex changed from beige color to gray [13]. After 60 min,
the beads were filtered off, washed with ethanol, dried and
used for hydrogenation reactions. The activated catalyst
was effective for the reduction of various nitro aromatics at
3
0 °C and 1 Atm pressure.
Catalytic hydrogenation
Hydrogenation of various nitro compounds produced the
corresponding anilines in good yields. Table 2 shows the
initial rates, relative rates of hydrogenation of nitro com-
pounds and their turnover numbers (TON). In general, the
rate of reduction of nitro groups is enhanced by the pres-
ence of electron donating groups [17]. Our results indicate
a similar trend. The rate of reduction follows the order:
o-nitrotoluene[ m-nitrotoluene[ p-nitrotoluene[ o-nitro-
aniline [ o-nitrophenol [ p-nitrophenol [ o-nitrobenzoic
acid [ m-nitrophenol [ p-nitrobenzoic acid [ p-nitroani-
line [ m-nitroaniline [ m-nitrobenzoic acid.
Among the o/m/p isomers, the ortho isomer is reduced
much faster than the other isomers. Saha and Islam [4]
reported the hydrogenation of nitro compounds with a
polymer-supported Ru(II) complex. They observed that
nitrobenzene and its p-substituted derivatives are reduced at
a higher rate, followed by the meta- and ortho-substituted
compounds. They attributed this to dominant steric factors.
The activated catalyst (0.50 g) was added to methanol
(
30 mL) and the mixture was saturated in an atmosphere of
hydrogen for 1 h. The system was evacuated and again
saturated with hydrogen gas for 15 min. A known quantity
of nitro compound was injected into the system followed
by releasing the system to a gas burette filled with hydro-
gen. The reaction was monitored by the volume of
hydrogen absorbed at different intervals of time. The
products were identified by IR, melting point, TLC and
HPLC.
Results and discussion
Chloromethylated polystyrene (crosslinked with 6.5%
divinyl benzene) beads were chemically modified into
1
23

Transition Met Chem (2010) 35:247–251
249
Table 2 Rates, relative rates and turnover numbers for various nitro
-3
compounds with [catalyst] = 14.41 9 10 M and [substrate] =
.033 M at 30 °C in 30 mL methanol
0
Substrate
Initial rate 9
0
Relative rate
TON
-
5
3
mol/dm /s
1
o-Nitrotoluene
m-Nitrotoluene
p-Nitrotoluene
o-Nitrobenzoic acid
m-Nitrobenzoic acid
p-Nitrobenzoic acid
o-Nitrophenol
12.62
11.74
7.52
3.93
0.88
2.98
4.68
3.51
4.03
5.06
1.12
2.40
14.34
13.34
8.55
4.47
1.00
3.39
5.32
3.99
4.58
5.75
1.27
2.73
31.5
29.3
18.8
9.8
2.1
7.4
11.7
8.8
m-Nitrophenol
p-Nitrophenol
10.1
12.6
2.8
o-Nitroaniline
m-Nitroaniline
p-Nitroaniline
Fig. 2 Linear plot of log(initial rate) versus log(substrate)
6.0
-
1
TON = Initial rate/[catalyst] 9 60 9 60 M (g atom of Pd) h
The influence of the amount of catalyst on the rate of the
-
3
reaction was studied over the range of 2.88 9 10 to
-
4.41 9 10 M at a constant substrate concentration of
3
1
0
3
.033 mol/dm at 30 °C keeping the pressure of H at
2
Our observation is that the reduction rate follows the order
ortho [ para [ meta in most cases. This may be due to
dominance of electronic factors over steric factors in our
system.
1 Atm. The slope of the linear plot of log(initial rate) against
log(catalyst) was first order for both the substrates (Fig. 3).
Similar results for the hydrogenation of substituted nitro-
benzenes with poly(vinyl pyridine) complexes of palla-
dium(II) were observed by Saha and Bhattacharya [21].
From the experimental data, it is concluded that the rate
of hydrogenation of nitroarenes is first order with respect to
catalyst concentration and fractional order with respect to
substrate concentration that leads to the experimental rate
equation as follows:
Among the ortho isomers, o-nitrotoulene is reduced at a
much greater rate than o-nitroaniline and o-nitrophenol.
The –CH group is well known as an electron donating
3
group, while the –OH and the –NH groups are also elec-
2
tron donating by resonance [18]. The lower rates of
hydrogenation for o-nitrophenol and o-nitroaniline may be
due to partial blocking of coordination of the –NO group
2
x
Rate ¼ k½Catalystꢁ½substrateꢁ
where x = 0.23 for o-nitroaniline and 0.26 for o-nitrotoluene.
on the available sites [4].
Kinetics of hydrogenation
The kinetics for the reduction of o-nitroaniline and
o-nitrotoluene was studied in methanol medium. All reac-
tion parameters except one were kept constant during any
set of runs. The initial rate in each case was determined
from a graph of hydrogen uptake versus time.
The concentration of o-nitroaniline or o-nitrotoluene was
3
varied in the range 0.033–0.167 mol/dm at a constant
temperature of 30 °C and hydrogen pressure of 1 Atm using
-
3
a fixed amount of palladium catalyst (14.41 9 10 M).
The order of the reaction was calculated from the slope of
the linear plot of log(initial rate) versus log(substrate) and
was found to be fractional (Fig. 2). This indicates that an
intermediate complex is formed during the reaction which
could be strongly adsorbed on the catalyst surface [19, 20].
Fig. 3 Linear plot of log(initial rate) versus log(catalyst)
123

2
50
Transition Met Chem (2010) 35:247–251
S
S
Pd Cl
Cl
+
H2
Pd H
Cl
+ HCl
S
S
OH
N
K
Pd H
Cl
+
RNO2
Pd O
Cl
R
S
OH
N
S
H
N
k1
Pd O
Cl
R
+
+
H
2
Pd O
Cl
R
+
+ H O
2
S
S
H
N
k2
RNH2
Pd O
Cl
R
+
H
2
Pd OH
Cl
S
S
k3
Pd OH
Cl
H2
Pd H
Cl
+ H O
2
2
Fig. 4 Plot of 1/[rate] versus 1/[RNO ]
Scheme 1 Plausible mechanism for the hydrogenation of nitroarenes
by polymer-bound Schiff base catalyst. S solvent
Based on the results obtained and evidence from the
literature [22–24], a plausible mechanism can be proposed
as in Scheme 1.
The rate equation derived from Scheme 1 is as follows;
Rate
k k k K½RNO ꢁ½Cat ꢁ½H ꢁ
1
2
3
2
tot
2
¼
k k f1 þ K½RNO ꢁg þ k k K½RNO ꢁ þ k k K½RNO ꢁ
2
3
2
1
3
2
1
2
2
ð1Þ
Since all reactions were carried out at a fixed hydrogen
pressure of 1 atmosphere, Eq. 1 reduces to
Rate
Fig. 5 Arrhenius plot: 1/T versus log(initial rate)
k1k2k3K½Cattotꢁ½RNO2ꢁ
¼
k2k3f1 þ K½RNO2ꢁg þ k1k3K½RNO2ꢁ þ k1k2K½RNO2ꢁ
energy was calculated from the slope of the straight line
plot of log(initial rate) versus 1/T as 20.5 and 28.2 kJ/mol
for o-nitroaniline and o-nitrotoluene respectively (Fig. 5).
These low energies of activation indicate high activity of
the catalyst [25]. The entropy of activation calculated from
ð2Þ
Taking the reciprocal of Eq. 2
ꢀ
ꢁ
1
1
1
1
1
¼
þ
þ þ
ð3Þ
Rate k1K½RNO2ꢁ
k1 k2 k3
-
1
-1
the kinetic data was -268.3 and -234.6 JK mol for
o-nitroaniline and o-nitrotoluene, respectively. The decrease
in entropy suggests a considerable loss of freedom due to
fixation of the catalyst molecule on the polymer support
during the reaction.
According to Eq. 3, if the order of the reaction is
fractional with respect to substrate concentration, then a
plot of 1/[Rate] versus 1/[RNO ] should be a straight line
2
with a positive intercept on the y axis (Fig. 4). This has
been realized, supporting the proposed scheme.
Arrhenius plot
Recycling efficiency of the catalyst
Hydrogenation of the nitro compounds was carried out
using the polymer-supported palladium complex at various
temperatures in the range of 25–45 °C. The activation
In order to find the recycling efficiency of the catalyst, a
known quantity of o-nitroaniline was reduced and its initial
rate was determined. The beads were then filtered off,
1
23

Transition Met Chem (2010) 35:247–251
251
Table 3 Recycling efficiency of the catalyst over six cycles for the
hydrogenation of o-nitroaniline at 30 °C in 30 mL methanol and
hydrogen pressure of 1 Atm with a catalyst concentration of 14.41 9
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