Synthesis, characterisation of polymer-supported palladium-2- methylimidazole complex catalyst for the hydrogenation of aromatic nitro compounds

Article abstract of DOI:10.3184/174751911X556855

A polymer-supported palladium-2-methylimidazole complex was synthesised and characterised by various physicochemical methods. The complex was successfully used as a catalyst for the hydrogenation of nitrobenzene and a few of its derivatives under ambient conditions. Results reveal that the electronic as well as the steric effects of the substitutent control the rate of hydrogenation of the nitro group in the studied nitro compounds. The kinetics of hydrogenation and the reusability of the catalyst were also studied.

Full text of DOI:10.3184/174751911X556855

1
12 RESEARCH PAPER
FEBRUARY, 112–115
JOURNAL OF CHEMICAL RESEARCH 2011
Synthesis, characterisation of polymer-supported
palladium-2-methylimidazole complex catalyst for the hydrogenation of
aromatic nitro compounds
a
a
b
Udayakumar Velua, Alexander Stanislaus , Gayathri Virupaiah *, Shivakumaraiah ,
c
d
Kashinath Rangu Patil and Viswanathan Balasubramanian
Department of Chemistry, Central College Campus, Bangalore University, Ambedkar Veedi, Bangalore, 560 001, India
a
bSiddaganga Institute ofTechnology, Tumkur, Karnataka,572 103, India
cNational Chemical Laboratories, Pune, 411 008, India
dNational Centre for Catalysis Research, I.I.T. Madras, Chennai, 600 036, India
A polymer-supported palladium-2-methylimidazole complex was synthesised and characterised by various physico-
chemical methods. The complex was successfully used as a catalyst for the hydrogenation of nitrobenzene and a
few of its derivatives under ambient conditions. Results reveal that the electronic as well as the steric effects of the
substitutent control the rate of hydrogenation of the nitro group in the studied nitro compounds. The kinetics of
hydrogenation and the reusability of the catalyst were also studied.
Keywords: transition metal complexes, polymer-supported complex catalyst, hydrogenation, aromatic nitro compounds
Aromatic amines which are important intermediates for the
synthesis of various chemicals are generally prepared by the
hydrogenation of aromatic nitro compounds. Transition-metal
complexes have been found to catalyse the hydrogenation
FTIR spectrometer. Surface area was measured using a Nova 1000
instrument in a nitrogen atmosphere. TGA studies were carried out
using a Perkin Elmer-Diamond instrument under an inert atmosphere.
Scanning electron micrographs were recorded on a Leica S440i instru-
ment. HPLC data were obtained using a Shimadzu-Prominence 2A
with C18 column and UV detector. GC-MS was recorded using a
Shimadzu GC-MS QP5050A instrument with a BP5 column. C, H and
N analyses were carried out using a Thermo Finnegan Eager 300 and
an Elementar Vario EL III, Carlo Erba 1108 analyser. Chloride content
was estimated by Volhard’s method. Palladium content was deter-
mined using a GBC Avanta-PM atomic absorption spectrophotometer.
Photoelectron spectra were recorded on a ESCA-3000 electron spec-
trometer equipped with a hemispherical electron analyser using the
Al-Kα radiation (1486.6 eV) X-ray source.
1
–4
of various nitro compounds in a homogeneous medium. As
transition-metal complexes are often expensive to prepare,
recent interest in the development of environmentally benign
synthesis has evoked interest in the development of polymer-
supported metal complex catalysts. Anchored metal complexes
on polymer supports have been successfully used as catalysts
for hydrogenation, oxidation, dehalogenation, epoxidation and
5–17
polymerisation.
In this context, present work was focused
on the synthesis and characterisation of a polymer-supported
palladium 2-methylimidazole complex catalyst and its use as
a catalyst for the hydrogenation of various aromatic nitro
compounds under ambient conditions. The reactivity of vari-
ous nitro compounds for the hydrogenation reaction has been
examined. Kinetic studies were carried out and the catalyst
was recycled six times without any loss of efficiency or
leaching of metal from the support.
[Pd(MeImz) Cl ] (MeImz = 2-methylimidazole): The complex
2
2
18
[Pd(MeImz)
PdCl
Cl
] was prepared according to the literature method.
2
2
(177 mg, 1 mmol) was treated with a solution of 2-methylimid-
2
azole (164 mg, 2 mmol) dissolved in acetone (20 mL) and refluxed on
a water bath for 4 h during which a yellow crystalline complex
[
Pd(MeImz) Cl ] separated. The product was filtered off, washed with
2 2
acetone and dried under reduced pressure.
Polymer-supported palladium-2-methylimidazole catalyst: Func-
tionalisation of PS-DVB with 2-methylimidazole was carried out by
Experimental
−1
treating PS-DVB (5 g, 5 mmol Cl g PS-DVB) with 2-methylimid-
−
1
Chloromethylated polystyrene beads cross-linked with 6.5% divinyl-
benzene (PS-DVB) was obtained as a gift from Thermax India Ltd,
Pune, India. Palladium chloride was procured from Arora Matthey
Ltd, India. All the substrates and solvents were purified before use.
The IR spectra were recorded using a Shimadzu FTIR-8500S spec-
trometer and far-IR spectra by using a Perkin Elmer-Spectrum 1000
azole (2.5 g, 3 mmol g PS-DVB) in a mixture of 1:1 toluene and
acetonitrile on a hot water bath maintained at 50 °C for 60 hours. 5 g
of the functionalised beads were treated with ethanolic solution con-
−1
taining palladium chloride (0.90 g, 1.2 mmol Pd g PS-DVB) at 50 °C
for 10 h. The beads were filtered off and finally washed by Soxhlet
19,20
extraction using ethanol (Scheme 1).
The polymer-supported
Scheme 1 Preparation of the polymer-supported palladium-2-methylimidazole complex.
Correspondent. E-mail: gayathritvr@yahoo.co.in
*

JOURNAL OF CHEMICAL RESEARCH 2011 113
complex beads were treated with an ethanolic solution of sodium
borohydride, washed several times with methanol, vacuum-dried and
21
then used as catalyst for the hydrogenation reactions.
Hydrogenation procedure: All the reactions were carried out using
methanol as a solvent at 596 mm Hg hydrogen pressure in a glass
reactor. In a typical experiment, the catalyst was added along with the
solvent to the reactor which was flushed with hydrogen and then evac-
uated. A known quantity of the substrate was injected and the system
was opened to hydrogen gas burette. The reaction was monitored by a
22
hydrogen uptake at different intervals of time. The products obtained
23
were identified using HPLC, IR spectra and GC-MS. A blank
reaction was also carried out for all the substrates without using the
catalyst.
Abbreviations: PS-DVB: Chloromethylated polystyrene beads
cross-linked with 6.5% divinylbenzene; PSMeimz: PS-DVB function-
alised with 2-methylimidazole; PSPdMeimz: PSMeimz treated with
palladium chloride; PSPdMeimz-actv : PSPdMeimz treated with
methanolic solution of sodium borohydride; PSPdMeimz–rec : Recy-
cled catalyst, MeImz = 2-methylimidazole; NB: nitrobenzene; PNA:
p-nitroaniline; PNP: p-nitrophenol; PNT: p-nitrotoluene; PNBA:
p-nitrobenzoic acid; PDNB: p-dinitrobenzene; PCNB: p-chloronitro-
benzene, SEM = Scanning electron micrograph.
Fig. 1 Overlay of IR -spectra of PS-DVB (spectrum A); PSMeimz
(
spectrum B); PSPdMeimz (spectrum C); [Pd(MeImz)
spectrum D).
2 2
Cl ]
(
Results and discussion
Elemental analysis: Elemental analysis at different stages of prepara-
tion of the catalyst was carried out and the data are given in Table 1.
The presence of nitrogen in PSMeimz determines the extent of func-
tionalisation of PS-DVB. The palladium content was determined
by stripping the bound palladium complex from the support. For
stripping, the polymer-supported complex (25 mg) was digested
with conc. sulfuric acid (about 3 mL) by heating followed by addition
of hydrogen peroxide (about 2 ml). The clear solution was heated
to decompose the excess of hydrogen peroxide and then cooled.
The metal content in the solution was determined using an atomic
absorption spectrophotometer.
at 3304 cm−1 (Spectrum D), whereas this peak was not found in the
polymer-supported complex catalyst, since the complex has bonded to
the polymer support through nitrogen. No significant changes in the
spectroscopic features were observed for PSPdMeimz (Spectrum C),
2
4
−1
PSPdMeimz-actv and PSPdMeimz-rec. An intense peak at 326 cm
in the far-IR spectrum recorded for PSPdMeimz-actv was assigned to
Pd–Cl stretching.
X-ray photoelectron spectroscopy studies: The oxidation state of
the palladium in PSPdMeimz, PSPdMeimz-actv and the recycled
catalyst was ascertained by X-ray photoelectron spectroscopy (XPS)
studies (Fig. 2). PSPdMeimz exhibited two peaks with binding ener-
gies 337.7 eV (3d5/2) and 342.9 eV (3d3/2) indicating the +2 oxidation
state of palladium (Spectrum A). PSPdMeimz-actv did not show
Surface area, porosity, thermo-gravimetric, SEM analysis: Solid
catalysts are generally porous in nature and the pores play a very
important role in their adsorption behaviour and catalytic activity.
Surface area and porosity was determined by the BET (Bruanauer,
25
26,27
Emmette, Teller) method in a nitrogen atmosphere.
PSPdMeimz-
2
−1
actv had surface area 22 (m g ), pore diameter 54.2 Å and pore
3
−1
volume 0.030 (cm g ). No significant changes in surface area and
pore diameter were observed after recycling; the surface area remained
2
−1
the same 22 (m g ) whereas pore diameter and pore volume were
3
−1
found to be 49.4 Å and 0.035 (cm g ) respectively. Thermo-gravi-
metric analyses of PSPdMeimz-actv in an inert atmosphere with a
heating rate of 10 °C min− up to 500 °C revealed that the catalyst was
stable without significant weight loss up to 225 °C. Scanning electron
micrographs revealed that PSPdMeimz-actv has a relatively rough
surface when compared PS-DVB. This change in surface could be due
to reconstruction of the surface during catalyst preparation.
1
Infrared spectroscopic studies: The IR spectra of PS-DVB,
PSMeimz, PSPdMeimz, PSPdMeimz-actv, PSPdMeimz-rec and
[
Pd(MeImz) Cl ] were recorded in nujol and a comparative study was
2 2
1
−
made (Fig. 1). The peak at 1265 cm for PS-DVB due to – CH Cl
2
(
Spectrum A) decreased in intensity on functionalisation (Spectrum
28
−1
B). The peak at 3170 cm corresponding to υN-H of 2-methylimid-
azole disappeared on functionalisation indicating the bonding of
2
-methylimidazole to the support through –CH –. The decrease in
2
1
−
intensity of the band at 833 cm corresponding to υC-Cl of PS-DVB
after functionalisation further confirms the bonding of 2-methylimid-
29
azole with PS-DVB (Spectrum B). [Pd(MeImz) Cl ] exhibited υ
2
2
N-H
Table 1 Elemental analysis of polymer-support and polymer-
supported catalyst
%
C
% H
% N
% Pd
a
PS-DVB
69.50
57.86
54.59
55.54
55.28
5.78
5.37
5.64
5.22
6.43
–
–
PSMeimz b
PSPdMeimz
PSPdMeimz-actv
PSPdMeimz-rec
8.02
6.01
7.01
7.11
–
12.29
12.27
12.00
c
a
b
4
.7 mmol Cl/g PS-DVB,
2.8 mmol MeImz/g PS-DVB,
Fig. 2 XPS spectra of PSPdMeimz (spectrum A); PSPdMeimz-
actv (spectrum B); recycled catalyst (spectrum C).
c
1
.14 mmol pd/g PS-DVB.

1
14 JOURNAL OF CHEMICAL RESEARCH 2011
2
1,30
any significant changes in the XPS results (Spectrum B).
Similar
Table 3 Percentage conversion of various nitro compounds
2
4
observations are reported by Mathew and Mahadevan and Mani
with supported palladium-2-methylimidazole complex catalyst
31
et al. The XPS spectrum of the recycled catalyst (Spectrum C)
revealed that, palladium continues to be in the +2 oxidation state even
after recycling the catalyst for six times although the broadening of
the peaks observed in spectrum C could be due to the presence of
palladium in zero oxidation state formed during usage.
Substrates
Products
Percentage Time
a
conversion /min
Nitrobenzene
p-Nitroaniline
o-Nitroaniline
m-Nitroaniline
p-Nitrophenol
m-Nitrophenol
p-Nitrotoluene
Aniline
72
70
70
82
79
80
72
75
80
80
84
100
80
p-Phenylenediamine
o-Phenylenediamine
m-Phenylenediamine
p-Aminophenol
m-Aminophenol
p-Toluidine
Hydrogenation reaction: When the homogeneous analogue of the
polymer-supported catalyst; [Pd(MeImz) Cl ] was used as catalyst for
120
130
100
120
100
110
110
120
180
2
2
the hydrogenation of nitro compounds in methanol, it resulted in the
separation of metal from the reaction mixture during the course of
the reaction, making it unsuitable for reaction. On the other hand,
when the polymer-supported complex was used for the hydrogenation
reaction it was found that metal does not leach out from the support.
The reaction was monitored by hydrogen uptake at different intervals
of time. The rate of hydrogenation reaction was calculated from the
slope after plotting the volume of hydrogen uptake by the substrate as
a function of time.
p-Nitrobenzoic acid p-Aminobenzoic acidb
o-Nitrobenzoic acid o-Aminobenzoic acid
m-Nitrobenzoic acid m-Aminobenzoic acid
m-Dinitrobenzene
m-Phenylenediamine
a
Conversion based on the HPLC analysis. b Isolated yield 72%.
Reactivity of aromatic nitro compounds: The reactivity of the nitro
group in substituted nitro aromatics may depend both on the presence
of electron-releasing or electron-donating groups attached to the aro-
matic ring and the steric effect contributed by the substituted group
MDNB sequential hydrogenation of nitro groups was observed.
MDNB was first converted into m-nitroaniline before m- phenylenedi-
1,36,37
amine was formed.
(
Scheme 2). When the hydrogenation of nitrobenzene was studied by
The percentage conversions of various nitro compounds are listed
in Table 3. All the reactions were carried out with a constant 5.92 ×
10 mol dm Pd and 33.3 × 10 mol dm substrate concentration at
a temperature of 30 °C with 596 mm Hg hydrogen pressure using
methanol as solvent.
varying the para substituents with electro-donating groups like –NH2,
–
–
with electron-donating groups > nitrobenzene > nitro compounds with
electron-withdrawing groups. Initial rates and relative reactivities are
tabulated in Table 2. Different trends are reported by Baralt and Holy ,
and Werner. Baralt and Holy reported that the hydrogenation reac-
tion of nitro compounds was not highly dependent on electronic
effects, whereas Werner observed the order of initial rate of hydroge-
nation of nitro compounds as nitrobenzene > electron-donating group
substituted >> electron-withdrawing group substituted nitrobenzenes.
The comparative study for hydrogenation of different isomers of
nitroaniline and nitrobenzoic acid showed that the hydrogenation rate
was in the order of para- > ortho- > meta- substituted isomers. Wan
et al. have reported the order of reactivity as ortho and meta isomers
higher than the para one. These observations reveal that the type
of catalyst used, the electronic effect as well as the steric effect of
the substitutent controls the rate of hydrogenation of the nitro group
in the substrates studied. When PCNB was used as a substrate, dechlo-
rination was observed during hydrogenation giving aniline as the
−
3
−3
−3
−3
OH and –CH3, and electron-withdrawing groups like –COOH and
NO , rate of hydrogenation followed the order: nitro compounds
2
Kinetic studies: The influence of catalyst concentration on rate of
hydrogenation was investigated for NB, PNA and PNT over a range of
14
32
−3
−3
−3
1.46 × 10 to 5.92 × 10 mol dm Pd at constant substrate concentra-
tion of 33.3 × 10 mol dm−3 at a temperature of 30 °C with 596 mm
Hg hydrogen pressure using methanol as solvent. The order of reac-
tion determined from the slope of the plot of log(initial rate) against
log[Catalyst] showed that all the substrates followed first order
−
3
3
8
kinetics. The influence of substrate concentration on the rate of the
−
3
−3
reaction was studied in the range of 3.33 × 10 to 33.3 × 10 mol
dm of substrate at 30 °C and at 596 mm Hg hydrogen pressure with
a constant catalyst concentration of 5.92 × 10 mol dm Pd for all
the substrates in methanol. The order of reaction found by plot of
log(initial rate) against log[substrate] was first order. Based on the
results obtained, a probable reaction pathway is proposed for the
−
3
−3
−3
33
39
21
hydrogenation of nitro compounds (Scheme 3).
According to the proposed reaction pathway, after oxidative addi-
tion of a hydrogen molecule to the catalyst, a nitro group is hydroge-
nated to produce a hydroxylamine intermediate in two steps which
finally undergoes reductive elimination to yield the corresponding
amine. Krathy et al. have also proposed a similar plausible reaction
mechanism for the hydrogenation of aromatic nitro compounds using
a polymer supported palladium catalyst where the reaction route is via
34,35
product.
be determined.
Hence the effect of the chloro group on the rate could not
Sequential hydrogenation of m-dinitrobenzene: When the hydroge-
nation of m-dinitrobenzene (MDNB) was carried out with catalyst
concentration 5.93 × 10− mol dm Pd and 33.3 × 10 mol dm
3
−3
−3
−3
40
formation of a hydroxylamine intermediate. A rate law proposed for
the reaction pathway (Scheme 3) is given in equation (1).
Rate = K k [Catalyst] [Substrate] [H ]
(1)
1
2
Where K is the equilibrium constant for addition of hydrogen to the
1
catalyst, and k is the velocity constant for the rate determining step
(Step 2) Since the reactions were carried out at constant hydrogen
pressure, equation (1) can be modified as given in equation (2).
Scheme 2 Hydrogenation of aromatic nitro compounds.
Rate = K′k [Catalyst] [Substrate]
(2)
Where K′ = K [H ]. The rate equation was found to be in agreement
1
2
Table 2 Initial rates of hydrogenation and relative rates for
with the experimental results where the reaction was first order in the
catalyst and first order in the substrate concentration at constant
hydrogen pressure.
various nitro compounds
Reaction condition: 5.92 × 10−3 mol dm−3 Pd, 33.3 × 10−3 mol dm−3
substrate concentration, 30 °C temperature, 30 mL methanol,
Test for metal leaching from the polymer support and the recycling
efficiency of the catalyst
596 mm Hg hydrogen pressure.
−
3
One of the main objectives of synthesising polymer-supported metal
complex catalysts is to recycle the catalyst efficiently. The catalyst
was reused for six cycles at a constant catalyst concentration of 59.2 ×
Substrate
Initial rate x10
−1
Relative rate
−
3
/
mol dm min
p-Nitroaniline
1.33
1.26
1.25
1.15
0.93
0.85
1.15
1.09
1.08
1.00
0.80
0.73
−4
−3
−3
−3
1
0
mol dm Pd, PNBA concentration of 33.3 × 10 mol dm at
p-Nitrophenol
p-Nitrotoluene
Nitrobenzene
30 °C with 596 mm Hg of hydrogen pressure. The initial rate remained
almost constant for over six cycles without any loss in the efficiency
of the catalyst. There was no change in the percentage of palladium
in the catalyst when metal estimation was carried out at the end of
first and sixth cycle. No traces of palladium were found in the
p-Nitrobenzoic acid
p-Dinitrobenzene

JOURNAL OF CHEMICAL RESEARCH 2011 115
Received 19 September 2010; accepted 15 December 2010
Paper 1000365 doi: 10.3184/174751911X556855
Published online: 10 February 2011
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to their corresponding amines. Recycling experiments showed
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