Synthesis and characterisation of a polymer-bound rhodium-benzimidazole complex as catalyst for the hydrogenation of nitroarenes

Article abstract of DOI:10.1007/s11243-011-9542-7

A polymer-anchored rhodium complex was synthesised by sequential attachment of benzimidazole (BzlH) and RhCl₃ to chloromethylated poly(styrene-divinylbenzene) co-polymer (PSDVB) with 6.5% cross-linking. The catalyst was characterised by X-ray photoelectron spectroscopy, far-IR, UV-Vis, FTIR, SEM and thermogravimetric analysis. Various physico-chemical properties such as bulk density, surface area and swelling behaviour in different solvents were also studied. The polymer-anchored complex was tested as a catalyst for reduction of nitroarenes, namely o,m,p-nitrobenzoic acid, nitroaniline, nitrophenol and nitrotoluene. Kinetic measurements were carried for o-nitroaniline and p-nitrophenol by varying temperature, catalyst concentration and concentration of substrates. The rate of the reaction was found to be first order with respect to catalyst concentration and also with substrate concentration at low concentrations, becoming independent of substrate at higher concentrations. A plausible mechanism for the reaction is proposed. The energy and entropy of activation calculated from Arrhenius plots indicate high activity of the catalystonthe support.Therecycling efficiencyof the catalyst has been studied and there was no leaching of metal from the catalyst surface. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9542-7

Transition Met Chem (2012) 37:1–6
DOI 10.1007/s11243-011-9542-7
Synthesis and characterisation of a polymer-bound
rhodium-benzimidazole complex as catalyst
for the hydrogenation of nitroarenes
S. Alexander
•
V. Udayakumar V. Gayathri
•
Received: 10 June 2011 / Accepted: 19 September 2011 / Published online: 20 October 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A polymer-anchored rhodium complex was
synthesised by sequential attachment of benzimidazole
Introduction
(
BzlH) and RhCl to chloromethylated poly(styrene–divi-
3
Transition metal complexes have been used extensively as
catalysts for homogeneous hydrogenation reactions due to
their high selectivity under mild operating conditions
[1–4]. However, homogeneous catalysis is associated with
the problems of product contamination and catalyst loss
when the products are not readily separated from the
catalyst.
nylbenzene) co-polymer (PSDVB) with 6.5% cross-linking.
The catalyst was characterised by X-ray photoelectron
spectroscopy, far-IR, UV–Vis, FTIR, SEM and thermo-
gravimetric analysis. Various physico-chemical properties
such as bulk density, surface area and swelling behaviour in
different solvents were also studied. The polymer-anchored
complex was tested as a catalyst for reduction of nitroarenes,
namely o,m,p-nitrobenzoic acid, nitroaniline, nitrophenol
and nitrotoluene. Kinetic measurements were carried for
o-nitroaniline and p-nitrophenol by varying temperature,
catalyst concentration and concentration of substrates. The
rate of the reaction was found to be first order with respect to
catalyst concentration and also with substrate concentration
at low concentrations, becoming independent of substrate
at higher concentrations. A plausible mechanism for the
reaction is proposed. The energy and entropy of activation
calculated from Arrhenius plots indicate high activity of the
catalystonthesupport.Therecyclingefficiencyofthecatalyst
has been studied and there was no leaching of metal from the
catalyst surface.
Immobilisations of metal complexes onto polymer
supports for heterogeneous catalysis have been reported by
many workers [5–7]. This has remained an area of con-
siderable interest with a major objective of designing
polymer supports with appropriate functional groups for
attachment to the active metal centre. Poly(styrene–
divinylbenzene) modified chemically into a ligand having a
variety of coordination sites is the most commonly
employed support. The preparation and characterisation of
polymers incorporating N-heterocycles and their reactions
with metal salts and complexes have been studied by
Mahadevan et al. [8]. Magdalene et al. [9] and Frechet and
et al. [10] have used polybenzimidazole-supported rhodium
and palladium complexes respectively, for catalytic
hydrogenation of nitro compounds. Recently, we have
reported the hydrogenation of olefins, nitro compounds and
Schiff bases using polymer-supported complexes [11–13].
Aromatic amines are important in the manufacture of a
variety of chemicals such as dyes, pharmaceuticals, agri-
cultural chemicals, surfactants and polymers [14]. Con-
ventional hydrogenation involves the use of stoichiometric
amounts of finely divided iron in the presence of acid [15].
Reports on the reduction of nitroaromatics using polymer-
bound metal complexes are relatively scarce and usually
involve high pressure reactions [16]. In the present paper,
we report the synthesis and characterisation of rhodium
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-011-9542-7) contains supplementary
material, which is available to authorized users.
S. Alexander ꢀ V. Udayakumar ꢀ V. Gayathri (&)
Department of Chemistry, Bangalore University,
Central College Campus, Bangalore 560 001, India
e-mail: gayathritvr@yahoo.co.in
123

2
Transition Met Chem (2012) 37:1–6
trichloride supported on polystyrene cross-linked with
6
.5% divinylbenzene functionalised with benzimidazole
CH2
N
CH2
and its catalytic activity towards reduction of nitroarenes at
ambient temperature and pressure.
N
N
N
Rh
Experimental
Cl
S
Chloromethylated poly(styrene–divinylbenzene) co-poly-
mer with 6.5% cross-linking was obtained as a gift from
Fig. 1 Structure of polymer complex active towards hydrogenation
of nitro compounds, S solvent
Thermax India Ltd, Pune, India. RhCl was obtained from
3
Arora Matthey Ltd. Nitro compounds were purified before
use. Superdry methanol was prepared according to a lit-
erature method [17].
again saturated with H gas. A known quantity of the nitro
2
compound was dissolved in superdry MeOH and injected
into the system, which was connected to a gas burette filled
Elemental analyses were carried out using a Thermo
Finnegan Eager 300 analyser at RRI, Bangalore. Rhodium
was estimated spectrophotometrically by the tin chloride
method [18] and chloride volumetrically by Volhard’s
method [19]. The FTIR spectra (in KBr) were recorded
using a Shimadzu FT-IR–8400S spectrometer. Surface
areas were measured using a Nova 1000 analyzer in N2
atmosphere. TGA studies were carried out using a Perkin-
Elmer 7 series instrument. Scanning electron micrographs
were recorded on a Leica S440i. HPLC was carried out
with a Shimadzu Prominence, 2A series instrument using a
C-18 column.
with H . The reaction was monitored by the volume of H
2 2
absorbed with time. The reaction mixture was then filtered
and the filtrate was evaporated to dryness. The residue was
recrystallised, and the product was identified by IR, melt-
ing point and HPLC.
Results and discussion
Characterisation of the catalyst
Physico-chemical properties of the supported catalyst are
given in Table 1. A decrease in the surface area of the
beads after loading of the metal ions may be due to
blocking of pores in the polymer support [20, 21].
The elemental analyses at different stages of preparation of
the catalyst indicate successful functionalisation of the
polymer. Rhodium content in the functionalised beads was
found to be 1.6%; on activation with sodium borohydride
this value reduced to 1.4%.
Preparation of the catalyst
Beads of chloromethylated polystyrene cross-linked with
6
.5% divinylbenzene polymer beads were first washed with
a mixture of THF and H O (4:1) using a Soxhlet extractor
2
for 10 h, then vacuum dried. The polymer was then func-
tionalised with benzimidazole by treating the beads (5 g)
with benzimidazole (2.845 g) in MeCN and toluene mix-
ture (1:1) at 60 °C for 30 h. The beads were then filtered
off. Soxhlet extracted with EtOH and dried.
In order to ascertain the mode of attachment of benz-
imidazole and rhodium on the polymer support, IR spectra
were recorded. The intensity of the peak due to tC–Cl of the
The functionalised beads (5 g) were then treated with
-
1
RhCl (0.350 g) in 130 mL of MeOH, acetone and MeCN
3
–CH
Cl group at 1,261 cm was found to decrease after
2
mixture (20:5:1). The mixture was refluxed for 60 h.
During this time, the colour of the beads turned pink and
the solution turned almost colourless. The beads were then
filtered off, Soxhlet extracted and dried (Scheme 1). The
supported Rh complex was activated by suspending in
functionalisation with benzimidazole (Fig. S-1 of Supple-
-
mentary Material). A peak at 1,612 cm was assigned to
1
t
C=C and tC=N in the functionalised polymer. The rhodium
contents of the catalyst and recycled beads imply that there
is no leaching of the metal ions even after recycling them
six times.
EtOH (25 mL) and NaBH (2.64 mmol) was added. After
4
6
0 min the beads were filtered off, washed with MeOH,
Reduction using NaBH usually changes the oxidation
4
dried and used for hydrogenation reactions (Fig. 1).
state of rhodium from ?3 to ?1 and in some cases from ?3
to zero [8, 22, 23]. To ascertain the oxidation state of the
polymer anchored catalyst before and after treatment with
Hydrogenation procedure
NaBH , Far-IR, diffuse reflectance and XPS studies were
4
The polymer-supported Rh complex was added to 30 mL
of deaerated superdry MeOH and allowed to saturate under
carried out. The far-IR spectrum of the catalyst before
-
1
activation shows a band at 340 cm assigned to tRh-Cl
which on activation, shifted towards lower frequency by
atmosphere of H for 2 h. The system was evacuated and
2
1
23

Transition Met Chem (2012) 37:1–6
3
Table 1 Elemental analysis and physical properties of polymer and polymer-bound complex
Metal
%)
C
(%)
H
(%) (%)
N
Pore volume Surface
3
(cm /g)
Apparent bulk
area (m /g) density (g/cm ) (A)
Pore diameter
2
3
´
˚
(
Polymer support (PSDVB)
–
–
69.5 6.0
68.0 5.5
–
0.045
0.039
35.5
35.4
0.42
0.45
84.79
43.44
Benzimidazole functionalised beads
(
6.3
PSDVB-BzlH)
PSDVB-BzlH-RhCl
PSDVB-BzlH-RhCl
Recycled beads
3
3
1.6
1.4
1.4
64.0 6.0
67.0 5.0
69.7 5.9
6.0
6.0
0.051
0.043
30.7
30.2
35.6
0.46
0.46
0.45
66.26
45.20
44.30
activated with NaBH
4
5.30 0.039
-
1
1
6 cm
(Fig. 2). The diffuse reflectance spectrum of
PSDVB-Bzl-RhCl exhibited a weak band at ca. 400 nm
3
assigned to a d–d transition. This was absent after peak on
treatment of the beads with NaBH , indicating a change in
4
oxidation state of rhodium. For rhodium(I) the d–d transi-
tions may be obscured in the intense bands of the polymer
and benzimidazole [24].
The XPS spectrum of the catalyst prior to activation
showed peaks at 309.7 and 315.4 eV corresponding to 3d5/2
and 3d3/2 respectively, confirming the ?3 oxidation state for
rhodium (Fig. 3). Upon activation with NaBH these peaks
4
shift to 308.9 and 311.3 eV, corresponding to rhodium(I).
A similar change in oxidation state of rhodium from ?3 to
Fig. 3 XPS spectra of A PSDVB-BzlH-RhCl
BzlH-RhCl . C Recycled beads
3
B Activated PSDVB-
?
1 upon treatment with NaBH was observed by Selvaraj
4
3
and Mahadevan [8] when a copolymer containing amino
and heterocyclic N-vinylpyrrolidinone (N-VP) loaded with
Table 2 List of 3d5/2 and 3d3/2 values for various rhodium catalysts
with reference to C1s peak at 285 eV
rhodium trichloride was activated using NaBH . The XPS
4
data for some rhodium catalysts found by other workers are
listed in Table 2. Based on these values and from our data,
we assign the ?3 and ?1 oxidation states to rhodium for the
Species
Binding energy (eV)
References
3d5/2
3d3/2
polymer anchored PSDVB-BzlH-RhCl before and after
3
Rh(0) (metal)
307.1
308.1
308.2
309.7
308.5
308.5
310.1
308.1
308.1
310.2
307.6
310.0
308.0
311.8
312.9
312.7
314.4
313.4
313.3
315.0
313.0
312.1
314.0
312.5
314.1
312.2
[25]
[26]
[8]
activation, respectively. The recycled catalyst exhibited
3
Rh(CO)PPh (H)
3 3 2
RhCl (PPh )
RhCl
3
ꢀ3H
2
O
[27]
[22]
[22]
[8]
Rh (OAc)
2
4
2(DMSO)
2(Py)
Rh (OAc)
2
4
-1
3
24 cm
PS-Rh(III)
PS-Rh(II)
PS-Rh(I)
νRh-Cl
[8]
B
[28]
[8]
4
4
-VP-Rh(III)
-VP-Rh(I)
[8]
-1
N-VP-Rh(III)
N-VP-Rh(I)
[8]
3
40 cm
[8]
νRh-Cl
A
peaks at 308.9 and 311.4 eV assignable to 3d5/2 and 3d3/2
indicating ?1 oxidation state and there was no change in the
oxidation state of the metal after six reaction cycles.
Scanning electron micrographs (SEM) were recorded
to understand the morphological changes occurring on the
surface of the polymer at various stages. The initially
1
00
200
300
400
-1
Wavenumber (cm )
Fig. 2 Far-IR spectra of
PSDVB-BzlH-RhCl
A
PSDVB-BzlH-RhCl
3
B
Activated
3
123

4
Transition Met Chem (2012) 37:1–6
smooth surface of the polymer bead was roughened on
functionalisation with benzimidazole and anchoring of the
metal. TGA of the activated catalyst indicated that it was
stable up to 250 °C. Some weight loss (5.0%) was also
observed below 100 °C due to loosely bound water mole-
cules. Swelling was fond to increase with polar solvents
compared to nonpolar solvents. Though maximum swelling
Kinetic measurements were carried out for hydrogenation
of o-nitroaniline and p-nitrophenol. The reduced products
were characterised by melting points, IR spectra and HPLC.
The free complex RhCl(BzlH) (H O) was synthesised by
2
2
the literature method [30] and also used for reduction of nitro
compounds under similar conditions. However, as soon as
the methanolic solution of the complex was saturated with
hydrogen, metal precipitated out making it unsuitable for the
reaction. In contrast, the polymer-supported rhodium-
benzimidazole catalyst retained its activity over six cycles
with no loss of metal.
of the beads is in H O which may be attributed to hydrogen
2
bonding, methanol appears to be better suited for carrying
out hydrogenation reactions since the solubilities of both
substrates and hydrogen are greater in the latter solvent.
The influence of substrate concentration [S] on the rate
of hydrogenation was studied for o-nitroaniline and
p-nitrophenol over a range of concentration from 0.03 to
Kinetics of hydrogenation
3
0
.07 mol/dm at 30 °C and 1 atmosphere pressure of H at a
2
-
4
3
Hydrogenation of various nitro compounds produced the
corresponding amines in good yields. Table 3 shows the
initial rate and relative rates of hydrogenation of nitro
compounds. Our earlier results with polymer-supported
palladium Schiff base catalysts indicated that the rate of
hydrogenation of nitro groups is enhanced by the presence
of electron donating substituents [13] and a similar trend
has been observed in the present study. The ortho and para
isomers are generally reduced much faster than the meta
isomers. Saha and Islam reported the hydrogenation of
nitro compounds with a polymer-supported Ru(II) complex
constant catalyst concentration of 22.35 9 10 mol/dm .
The rate of hydrogenation (R) increases with increase in
substrate concentration at lower concentrations, becoming
independent at higher concentration (Fig. S-2 of Supple-
mentary Material). The catalyst concentration was varied
-
4
-4
3
between 4.47 9 10 and 22.35 9 10 mol/dm of rho-
dium at 30 °C, 1 atmosphere pressure and a constant sub-
strate concentration of 0.033 mol/dm . The initial rate
3
showed a direct dependence on catalyst concentration.
A plot of -log(initial rate) versus -log(catalyst) showed
first order kinetics (Fig. 4).
[
29]. They observed that nitrobenzene and p-substituted
Based on the experimental data and the literature
[31–33], the tentative mechanism shown in Scheme 1 is
suggested for the reaction.
derivatives are reduced at a higher rate, followed by meta
and ortho-substituted ones in order and they attributed this
to the steric factors. Our observation that the reduction rate
follows the order ortho [ para [ meta in most cases may
be due to the dominance of electronic factors over steric
factors.
Cl
N
N
Cl
S
Ko
N
N
H
H
Rh
+
H2
Rh
S
Cl
Rh
S
Cl
Rh
S
N
H
H
N
N
H
O
OH
N
K
Table 3 Initial rate of hydrogenation of various substituted nitro
-
+
RNO2
4
N
R
compounds with [catalyst] = 22.35 9 10
0
M and [substrate] =
.033 M at 30 °C in 30 mL methanol
-4
Substrate
Initial rate 9 10
(
Relative rate
Cl
Rh
S
Cl
Rh
S
3
mol/dm /s)
N
H
O
OH
N
N
N
H
O
H
N
k1 ( slow)
+
H
+
R
H O
2
2
N
R
o-nitrophenol
9.78
5.29
8.33
2.24
1.92
2.08
8.08
1.05
3.18
3.89
4.94
6.66
9.3
5.0
7.9
2.1
1.8
2.0
7.7
1.0
3.0
3.7
4.7
6.3
m-nitrophenol
p-nitrophenol
Cl
Rh
S
Cl
Rh
S
o-nitrobenzoicacid
m-nitrobenzoicacid
p-nitrobenzoicacid
o-nitroaniline
N
N
H
O
H
N
N
N
H
Fast
+
H2
+
RNH
2
R
OH
Cl
Rh
S
m-nitroaniline
p-nitroaniline
N
N
H
N
N
Cl
S
Fast
+
H O
2
Rh
OH
o-nitrotoluene
m-nitrotoluene
p-nitrotoluene
Scheme 1 Proposed mechanism for the reduction of nitro com-
pounds using polymer-bound rhodium catalyst, S solvent
1
23

Transition Met Chem (2012) 37:1–6
5
4
4
3
3
3
3
3
.2
.0
.8
.6
.4
.2
.0
-
-
2.8
2.9
o-nitroaniline
o-nitroaniline
p-nitrophenol
-3.0
p-nitrophenol
-
3.1
-3.2
-
3.3
2.6
2.8
3.0
3.2
3.4
3.15
3.20
3.25
3.30
3.35
-3
-
log[Catalyst]
1/Tx10 K
Fig. 4 Logarithmic plot of initial rate versus catalyst concentration
Fig. 5 Arrhenius plot:logarithmic plot of initial rate versus reciprocal
of temperature
and reused for six cycles. The activity of the catalyst
remained essentially unaltered during these experiments.
The rate law derived for the above mechanism is as
follows:
2
K0Kk1½H2ꢁ ½RNO2ꢁ½Catalystꢁ
Acknowledgments The authors would like to thank UGC for DRS
programme, Prof. Sadashiva, Liquid Crystal Lab. RRI for CHN
analysis and DST, Central Facility, Physics Department, IISc
Bangalore for recording far-IR spectra.
Rate ¼ 1
þ K0½H2ꢁ þ K0K½H2ꢁ½RNO2ꢁ
ð1Þ
Since all reactions were carried out at fixed hydrogen
pressure of 1 atmosphere, Eq. 1 reduces to
K0Kk1½RNO2ꢁ½Catalystꢁ
Rate ¼
ð2Þ
1
þ K0 þ K0K½RNO2ꢁ
At low concentrations of the substrate, the rate equation
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