Polystyrene anchored ruthenium(II) complex catalyzed carbonylation of nitrobenzene and amines for the synthesis of disubstituted ureas

Article abstract of DOI:10.1016/j.jorganchem.2014.08.017

The catalytically active complex [Ru(PS-imd)(CO)₂Cl₂] (PS-imd = polystyrene anchored imidazole) was synthesized and characterized using various spectroscopic techniques. The complex is well characterized and highly stable. The catalytic activity of the resulting species was investigated towards the synthesis of diphenyl urea and other disubstituted ureas. The experiments were carried out under high CO pressure, high temperature condition in mild coordinating media. The catalyst was found to produce excellent yields with high product selectivity. Variable yields are obtained depending on the substituent on nitrobenzene and aniline. The effects of co-solvent and co-catalyst were also studied. The catalyst was very stable and could be reused for more than five times without any noticeable loss of its catalytic activity.

Full text of DOI:10.1016/j.jorganchem.2014.08.017

Journal of Organometallic Chemistry 772-773 (2014) 152e160
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Journal of Organometallic Chemistry
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Polystyrene anchored ruthenium(II) complex catalyzed carbonylation
of nitrobenzene and amines for the synthesis of disubstituted ureas
,
Sk Manirul Islam a *, Kajari Ghosh a, Anupam Singha Roy a, Rostam Ali Molla a,
Noor Salam a, Tanmay Chatterjee b, Md. Asif Iqubal c
a Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, West Bengal, India
b Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
c Department of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytically active complex [Ru(PS-imd)(CO)2Cl2] (PS-imd ¼ polystyrene anchored imidazole) was
synthesized and characterized using various spectroscopic techniques. The complex is well characterized
and highly stable. The catalytic activity of the resulting species was investigated towards the synthesis of
diphenyl urea and other disubstituted ureas. The experiments were carried out under high CO pressure,
high temperature condition in mild coordinating media. The catalyst was found to produce excellent
yields with high product selectivity. Variable yields are obtained depending on the substituent on
nitrobenzene and aniline. The effects of co-solvent and co-catalyst were also studied. The catalyst was
very stable and could be reused for more than five times without any noticeable loss of its catalytic
activity.
Received 3 June 2014
Received in revised form
15 August 2014
Accepted 19 August 2014
Available online 16 September 2014
Keywords:
Carbonylation reaction
Polymer anchored
Ruthenium(II) complex
High CO pressure
Disubstituted ureas
© 2014 Elsevier B.V. All rights reserved.
Introduction
disubstituted urea is an important intermediate for the production
of carbamates, which are the raw materials for agrochemicals [13].
Among the different catalytic processes, carbonylation reactions
which make use of carbon monoxide are of great interest as it
represents industrial core technologies for converting various bulk
chemicals into a diverse set of useful products of our daily life
[1e3]. Carbonylation of alkenes to aldehydes [4] and carboxylic
acids [5], carbonylation of aryl halides to aryl carboxylic acids [6],
aldehydes [7], or amides [8] and the carbonylation of methanol to
acetic acid [9] are well known industrial processes with major
commercial significance. Although the use of CO as a reductant had
been in the past confined to few reactions, its use in organic syn-
thesis, especially in the reductive carbonylation of nitro aromatics
and the oxidative carbonylation of aromatic amines, has increased
dramatically [10].
Unsymmetrical N,N -diaryl ureas are found in a variety of biologi-
cally active molecules, and their efficient synthesis is of great
importance, especially to medicinal chemists [14].
The conventional method for preparing ureas is the reaction of
amines with carbon monoxide in the presence of non-metal cata-
lysts such as tert-amine [15]. The yield of urea is increased only if
excess sulfur is used for the reaction with amine. The use of sulfur
produces hydrogen sulfide as an undesired by-product, which rai-
ses the cost of the process because of its toxicity and its special
disposal requirements. The other method utilizes selenium, which
is a well-known toxic catalyst and which has to be precipitated by a
continuous flow of oxygen [16]. Therefore industry has always been
on the lookout for new catalyst systems to make symmetric ureas
and the more challenging unsymmetric ureas [10]. Among the
metal complex catalysts used for this purpose, the complexes of
palladium(II), rhodium(I) and ruthenium(II) show very good cata-
lytic activity for the preparation of ureas [17e19]. In homogeneous
reactions catalyzed by organometallic compounds, the catalyst gets
intimately involved with the reactants to form various in-
termediates during the course of the reaction [20]. The polymer
anchored metal complex catalysts which may be separated easily
from the product mixture and reused are expected to have higher
Phenyl ureas are a known class of commercially available her-
bicides. Since phosgene is toxic and expensive, hence there is al-
ways a search for comparatively mild non-phosgenation processes
[11,12]. These are based on the reductive carbonylation of nitro-
benzene to form N,N’-diphenylurea (DPU). The conventional N,N’-
* Corresponding author. Tel.: þ 91 33 2582 8750; fax: þ91 33 2582 8282.
E-mail address: manir65@rediffmail.com (S.M. Islam).
http://dx.doi.org/10.1016/j.jorganchem.2014.08.017
0022-328X/© 2014 Elsevier B.V. All rights reserved.

S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
153
Table 1
Chemical composition of polymer anchored ligand and polymer supported catalyst.
Compound
PS-imd
Color
C%
H%
Cl%
N%
9.39
Metal%
White
76.32
6.36
2.15
e
(78.82)
68.62
(69.26)
(6.57)
5.61
(5.76)
(14.62)
14.71
(14.94)
Ru(PS-imd)(CO)2Cl2
Marsh
green
4.60
5.30
(3 g) was stirred with a 1:1 mixture of acetonitrile and toluene for
30 min. Then imidazole (1.2 g) was added to the above mixture of
the polymer, and it was heated for 24 h at 80 ꢀC. The white polymer-
anchored ligand was filtered out, washed thoroughly with meth-
anol and dried under vacuum (Yield ¼ 60%).
Preparation polymer supported ruthenium(II) catalyst, [Ru(PS-
imd)(CO)2Cl2]
The polymer anchored imidazole ligand (2.0 g) was added to a
DMF solution (20 mL) of RuCl3.3H2O (1.9 mmol) and the mixture
was refluxed for 6 h. Carbon monoxide was bubbled through it
under reflux condition when pale green catalyst was formed. It was
filtered, washed with ethanol and then dried under high vacuum
(Yield ¼ 20%).
Scheme 1. Synthesis of polymer anchored Ru(II) complex.
chemical and thermal stabilities. These advantages make the
polymer anchored catalysts more attractive than their homoge-
neous counterpart [21].
In order to perform a new contribution to the field of carbon-
ylation reaction, we report here carbonylation reaction for the
synthesis of disubstituted ureas using polymer anchored ruth-
enium(II) catalyst. The effect of various reaction parameters such as
mole ratio of reactants, catalyst amount, and temperature was
studied to optimize the conditions for maximum conversion. Also
the catalyst was regenerated and reused up to five cycles.
Process for the synthesis of ureas
All carbonylation reactions were conducted in a 100 mL glass
lined stainless steel autoclave equipped with magnetic stirring. In
each reaction, 1.0 ꢁ 10ꢂ2 mmol of catalyst, 10 mmol of nitroben-
zene, 20 mmol of amines and carbon monoxide at 60 atm were
charged successively. The reaction proceeded at 120 ꢀC for 5 h, and
after the reaction 20 mL of methanol were added to the resulting
mixture which was qualitatively analyzed with Varian 3400 gas
chromatograph equipped with a 30 m CP-SIL8CB capillary column
and a Flame Ionization Detector by the external standard method.
All reaction products were identified by using Trace DSQ II GC-MS.
Experimental
Materials
Analytical grade reagents and freshly distilled solvents were
used throughout. All reagents and substrates were purchased from
Merck. Liquid substrates were predistilled and dried by molecular
sieve and solid substrates were recrystallized before use. Distilla-
tion and purification of the solvents and substrate were done by
standard procedures [22]. 5.5% crosslinked chloromethylated
polystyrene and ruthenium chloride was purchased from Aldrich
Chemical Company, U.S.A. and used without further purification.
Carbon monoxide (99.9%) was purchased from IOL Bombay, India.
Results and discussion
Catalyst characterization
The polymer supported imidazole ligand was well characterized
and established earlier [23]. Due to the insolubilities of the polymer
supported metal complex in all common organic solvents, its
structural investigation was limited to their physicochemical
properties, chemical analysis, SEM, TGA, FT-IR, EPR and UVeVis
spectral data. Table 1 provides the data of elemental analysis of
polymer supported ligand and the polymer supported ruthenium
catalyst. The metal content of the polymer supported catalyst was
estimated by atomic absorption spectrometry.
The attachment of metal onto the support was confirmed by
comparing the FT-IR spectra (Fig. 1) of the polymer before and after
loading with metal. The IR spectrum of pure chloromethylated
polystyrene has an absorption band at 1261 cm-1 due to the CeCl
group, which was weak in the ligand and in the catalyst. IR spectra
show a stretching vibration for eCH2 at 2918 cm-1 for the polymer
bound ligand and its complex. The stretching vibration of C]N
bond appeared at 1613 cm-1 for the polymer anchored imidazole
ligand which is shifted to 1618 cm-1 in the metal complex, indi-
cating the coordination of azomethine nitrogen to the metal
[24,25]. Another band at 1319 cm-1 (in ligand) for CeN stretching is
shifted to lower region in the metal complex [26]. A new intense
peak at around 1984-2080 cm-1 due to the CO is observed in the
Physical measurements
The FT-IR spectra of the samples were recorded from 400 to
4000 cm-1 on a Perkin Elmer FT-IR 783 spectrophotometer using
KBr pellets. UVeVis spectra were taken using a Shimadzu UV-
2401PC doubled beam spectrophotometer having an integrating
sphere attachment for solid samples. Thermogravimetric analysis
(TGA) was carried out using a Mettler Toledo TGA/DTA 851e. Sur-
face morphology of the samples was measured using a scanning
electron microscope (SEM) (ZEISS EVO40, England) equipped with
EDX facility. Ruthenium content in the catalyst was determined
using a Varian AA240 atomic absorption spectrophotometer (AAS).
Preparation of the polymer-anchored imidazole ligand (PS-imd)
An outline of the preparation of polymer-anchored Ru(II) com-
plex catalyst is given in Scheme 1. Chloromethylated polystyrene

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S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
Fig. 1. FT-IR spectra of polymer anchored ligand (a) and polymer anchored ruthenium(II) catalyst (b).
metal complex confirming the metal carbonyl coordination [27]. In
the far-IR data, the bands are due to the RueN stretching vibrations
around at 470 cm-1, which established the formation of the metal
complex through RueN bonding [28].
The electronic spectrum (Fig. 2) of the polymer anchored metal
complex was recorded in diffuse reflectance spectrum mode as
MgCO3/BaSO4 disks. The polymer anchored ruthenium (II) complex is
in the þ2 oxidation state. The electronic spectra of the ruthenium
complex showed four bands in the region 680e230 nm. The bands
around 680e600 nm range have been assigned to the spin allowed
ded transition. The other high intensity bands around 309e492 nm
have been assigned to charge transfer transitions (LMCT) arising from
the excitation of electrons from the metal t2g level to the unfilled
molecular orbitals derived from the p* level of the ligands. The bands
below 300 nmwerecharacterized by intra-ligand charge transfer[29].
Field emission-scanning electron micrographs of polymer
anchored ligand (PS-imd) and its complex [Ru(PS-imd)(CO)2Cl2]
were recorded and it was observed that the morphological changes
occurred on the polystyrene beads at various stages of the
Fig. 2. DRS-UV-visible absorption spectra of polymer anchored Ru(II) complex.
Fig. 3. SEM images of polymer anchored ligand (A) and the immobilized ruthenium(II) complex (B).

S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
155
synthesis. The SEM images of polymer anchored ligand (A) and the
immobilized ruthenium(II) complex (B) on functionalized polymer
are shown in Fig. 3. The pure chloromethylated polystyrene bead
has a smooth surface (not shown) while polymer anchored ligand
and complex show roughening of the top layer of polymer beads.
This roughening is relatively more in complex. Also the presence of
ruthenium metal can be further proved by energy dispersive
spectroscopy analysis of X-rays (EDAX) (Fig. 4) which suggests the
formation of metal complex with the polymer anchored ligand.
Thermal stability of complex was investigated using TGA-DTA at
a heating rate of 10 ꢀC/min in air over a temperature range of
30e600 ꢀC. TGA curve of polymer anchored Ru(II) complex is
shown in Fig. 5. Polymer anchored ruthenium(II) complex
decomposed at 360e400 ꢀC. So from the thermal stability, it con-
cludes that polymer anchored metal complex degraded at consid-
erably higher temperature.
The X-band EPR (Electron Paramagnetic Resonance) spectra of
the Ru-complex in the fresh and used catalyst were recorded in the
solid state at room temperature (Fig. 6). The EPR spectra of both the
fresh and used catalyst are quite similar and also matches with the
EPR spectra of the previously reported Ru(II) complexes [30,31].
Hence, we can predict that the Ru remains in the þ2 oxidation state
before and after the reaction.
Fig. 5. TGA plot for [Ru(p-imd)(CO)2Cl2] complex.
To find the effect of amount of nitrobenzene and aniline on the
carbonylation reaction, their molar ratio was varied. The results are
summarized in Table 2. The yield of DPU was highest (92%) when
the ratio was 1:2. Very low DPU yield was observed when no aniline
Catalytic activity of [Ru(PS-imd)(CO)2Cl2] for carbonylation reaction
In an autoclave, nitrobenzene, aniline, and the catalyst were
mixed with solvent. Then co-catalyst was added to it and the
autoclave was flushed with CO gas at 120 ꢀC. After 5 h, the for-
mation of diphenyl urea was analyzed by gas chromatograph. The
catalyst was found to be highly effective for carbonylation of
nitrobenzene and aniline to give N,N'-diphenyl urea (DPU) as the
major product. The above reaction was taken as model and the
effects of nitrobenzene and aniline molar ratio, carbon monoxide
pressure, temperature, reaction time, solvent, co-solvent, the
amount of catalyst and co-catalyst were investigated (Scheme 2).
Fig. 4. EDX images of polymer anchored ligand (A) and its Ru(II) complex (B).
Fig. 6. The X-band EPR spectra of Ru (II) for (a) fresh catalyst and (b) used catalyst.

156
S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
Scheme 2. Carbonylation of nitrobenzenes and anilines.
Table 2
Influence of PhNO2: PhNH2 molar ratio on the carbonylation reaction.a
Run
Molar ratio PhNO2: PhNH2
Conversion (%)
Yield (%)
DPU
PhNH2
MPCb
1
2
3
4
5
6
1:0
0.5:1
1:0.5
1:1
1:2
1:4
100
46
100
100
100
100
10
15
84
88
92
84
8
7
6
7
7
9
82
78
10
5
1
7
a
Conditions: PhNO2 þ PhNH2 ¼ 30 mmol, CH3OH ¼ 60 mmol, FeCl3 (20 mmol),
Catalyst ¼ 1.0 ꢁ 10ꢂ2 mmol, medium ¼ DMF, T ¼ 120 ꢀ C, t ¼ 5 h, PCO ¼ 60 atm.
Product identified by GC.
b
Methyl-N-phenyl carbamate.
Table 3
Effect of solvent on the carbonylation reaction in presence of methanol.a
Entry
Solvent
Conversion (%)
Yield of DPU (%)
1
2
3
4
5
6
DMF
100
98
84
56
54
53
92
88
74
35
34
34
DMSO
CH3CN
C6H6
C6H5CH3
Hexane
Fig. 8. Effect of PCO on the production of diphenyl urea catalyzed by polymer anchored
ruthenium complex. Reaction condition: [nitrobenzene] 10 mmol,
¼
[aniline] ¼ 20 mmol, CH3OH (60 mmol), FeCl3 (20 mmol), DMF (10 mL), catalyst
amount ¼ 1.0 ꢁ 10ꢂ2 mmol, time (5 h).
a
conditions: [nitrobenzene] ¼ 10 mmol, [aniline] ¼ 20 mmol, CH3OH (60 mmol),
FeCl3 (20 mmol), catalyst amount ¼ 1.0 ꢁ 10ꢂ2 mmol, T ¼ 120 ꢀC, time (5 h),
PCO ¼ 60 atm.
as co-solvent. Weakly coordinating solvents were found to be more
efficient and most of the catalytic runs were taken in DMF and
DMSO. Very sluggish reaction occurs in strong coordinating sol-
vents like CH3CN or in non-polar solvents like C6H6, C6H5CH3, and
hexane (Table 3). The highest conversion (100%) with DPU selec-
tivity of 92% observed in DMF medium.
was added to the reaction mixture i.e when nitrobenzene: aniline
molar ratio was 1:0.
The carbonylation reaction using the present catalyst was
effective in mild coordinating solvents and in presence of methanol
Table 4
Effect of co-catalyst and co-solvent on the carbonylation reaction.a
Run
1
Co-catalyst
FeCl3
Co-solvent
Conversion (%)
Yield of DPU (%)
e
94
100
100
92
98
97
98
99
97
67
70
65
66
68
67
67
72
70
70
76
72
72
78
78
8
92
88
5
90
87
6
90
88
1
24
22
3
23
21
2
27
25
4
30
27
5
32
29
MeOH
EtOH
e
2
3
4
5
6
7
8
SnCl4
PTS
MeOH
EtOH
e
MeOH
EtOH
e
Py
MeOH
EtOH
e
Et3N
MeOH
EtOH
e
KOH
MeOH
EtOH
e
CH3CONa
C2H5CONa
MeOH
EtOH
e
MeOH
EtOH
Fig. 7. Effect of temperature on the production of diphenyl urea catalyzed by polymer
anchored ruthenium complex. Reaction condition: [nitrobenzene]
¼
10 mmol,
a
[aniline] ¼ 20 mmol, CH3OH (60 mmol), FeCl3 (20 mmol), DMF (10 mL), catalyst
Conditions: PhNO2 þ PhNH2 ¼ 30 mmol, co-solvent ¼ 60 mmol, co-catalyst
(20 mmol), Cat ¼ 1.0 ꢁ 10ꢂ2 mmol, medium ¼ DMF, T ¼ 120 ꢀ C, t ¼ 5 h, PCO ¼ 60 atm.
amount ¼ 1.0 ꢁ 10ꢂ2 mmol, time (5 h), PCO ¼ 60 atm.

S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
157
Effect of carbon monoxide pressure and temperature on the
carbonylation of nitrobenzene and aniline was therefore studied
to optimize the yield of DPU. Reaction at room temperature and
low carbon monoxide pressure (20 atm) even in presence of
methanol did not yield any product. Highest conversion (100%)
was achieved at 60 atm and 120 ꢀC and at a methanol concen-
tration of 60 mmol. The products were majorly DPU (92%) with a
little amount of aniline (6%) and MPC (2%) as the by-product. No
product formation occurred at low temperature (less than 35 ꢀC)
and at carbon monoxide pressure maintained below 20 atm. The
concentration of DPU increased gradually when the temperature
was increased from 35 ꢀC to 50 ꢀC and finally to 100 ꢀC. The DPU
yield obtained 85% at 100 ꢀC. Finally, the optimized temperature
for the carbonylation reaction was 120 ꢀC when the yield of DPU
was 92% (Fig. 7).
At a carbon monoxide pressure of 20 atm or below, no
carbonylation reaction takes place, as nitrobenzene and aniline
concentration remained unchanged and no reaction products were
observed even after 16 h of a catalytic run (Fig. 8). As the pressure of
carbon monoxide was slowly raised from as low as 20e80 atm, the
percentage of diphenylurea increased gradually and reached the
optimum value of 92% at Pco ¼ 60 atm.
Fig. 9. Effect of amount of catalyst on the production of diphenyl urea catalyzed by
polymer anchored ruthenium complex. Reaction condition: [nitrobenzene] ¼ 10 mmol,
[aniline] ¼ 20 mmol, DMF (10 mL), temperature (120 ꢀC), time (5 h), PCO ¼ 60atm.
Table 5
Synthesis of di-substituted ureas using nitrobenzene and different amines.
H
H
Ru catalyst
60 atm, 120 oC
N
N
R
NO2
+
RNH2
DMF, MeOH, FeCl3
O
Entry
1
Amine
Product
Yield (%)
92
TON
H
N
H
N
2760
NH2
O
H
H
N
N
2
94
2820
NH2
O
H
H
N
N
3
4
96
97
2880
2910
NH2
O
H
N
H
N
NH2
O
H
H
N
N
5
6
89
91
2670
2730
NH2
O
H
H
N
N
MeO
NH2
O
OMe
NO2
NH2
NO2
H
H
N
N
7
4
120
O
Conditions: substrates ¼ 30 mmol, MeOH ¼ 60 mmol, FeCl3 (20 mmol), Cat ¼ 1.0 ꢁ 10ꢂ2 mmol, medium ¼ DMF, T ¼ 120 ꢀC, t ¼ 5 h, PCO ¼ 60 atm.

158
S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
In absence of methanol, low DPU yield was recorded even when
nitrobenzene and substituted aniline. The substrates and their
corresponding products under optimized reaction conditions are
given in Tables 5 and 6. Syntheses of ureas were also tested when
aniline was replaced with n-butylamine, n-hexylamine and cyclo-
hexylamine (Table 5, entries 2e4). The corresponding ureas were
also formed with high conversion and selectivity (94e97%). Finally,
several other aromatic amines, i.e., p-methylaniline and p-
methoxyaniline, were subjected to the reaction (Table 5, entries 5
and 6), with sufficient high catalytic performance. No reaction
occurred on using o-nitroaniline as substrate (Table 5, entry 7),
suggesting that the presence of the electron-attracting eNO2
severely weakened the activity of the NH2.
the carbon monoxide pressure was increased to 80 atm and tem-
perature raised to 100 ꢀC. The presence of a co-solvent such as ROH
(R ¼ eCH3, eC2H5) was therefore necessary for the reaction be-
tween PhNO2, PhNH2 and CO to proceed under high pressure and
high temperature conditions. Presence of moisture in the system
greatly hinders the catalytic conversion to DPU and almost 90%
aniline was recorded. High aniline formation may also be due to
hydrolysis of any DPU formed.
(PhNHCONHPh) þ H2O / 2PhNH2 þ CO2
Carbonylation reactions in presence of both co-solvent and co-
catalyst were tried out to optimize the yield of DPU (Table 4). In-
vestigations were made with both acidic and basic co-catalysts in
the presence of alcohol in the reaction medium. In presence of
methanol concentration (60 mmol), addition of acid co-catalysts
such as FeCl3, AlCl3 and p-toluene sulphonic acid (PTS) increased
the yield of DPU. The selectivity of the co-catalysts for the conver-
sion of nitrobenzene and aniline to DPU is comparable, though
highest yield of DPU was obtained using FeCl3. Again, it is seen that
the addition of basic co-catalysts like KOH, Et3N, Pyridine, RONa
(R ¼ eCH3, eC2H5) not only lowers the total conversion, it also
reduces the yield of DPU and increases the yield of carbamates.
The effect of amount of catalyst was studied for the carbonyla-
tion reaction. It was found that the highest yield of DPU was ob-
tained when 1.0 ꢁ 10ꢂ2 mmol of the catalyst was taken (Fig. 9).
The successful yield of DPU using the polymer anchored Ru(II)
catalyst encouraged us to study the efficiency of the present cata-
lyst to obtain other disubstituted urea from substituted
When substituted nitro aromatic compounds and aniline were
subjected to react in 60 atm pressure of CO at 120 ꢀC in the presence
of [Ru(PS-imd)(CO)2Cl2] using DMF as solvent, disubstituted ureas
were produced in variable yields depending on the substituents
(Table 6). For methyl substitution in the para position, the yield of
disubstituted urea was dramatically decreased to 30% (Table 6,
entry 1). The Fluoro, cyano, and methoxycarbonyl groups in the
para position led to the formation of the corresponding disubsti-
tuted ureas in 60%, 88%, and 76% yield, respectively (Table 6, entry
2e4).
Based on earlier reported mechanistic studies of catalytic car-
bonylations of nitobenzene [20,32], the reaction mechanism of this
DPU synthesis is proposed in Scheme 3.
Heterogeneity test
To determine whether the catalyst actually functions in a
heterogeneous manner,
a
test was performed on the
Table 6
Synthesis of di-substituted ureas using substituted nitrobenzene and aniline.
Ru catalyst
60 atm, 120 oC
H
H
N
N
X
NO2
+
NH2
DMF, MeOH, FeCl3
O
X
X= CH3, F, CN, COOCH3
Entry
1
X
Product
Yield (%)
24
TON
720
H
N
H
N
CH3
O
O
H
N
H
N
2
F
60
1800
F
H
H
N
N
3
4
CN
88
76
2640
2280
O
NC
H
N
H
N
COOCH3
O
H3COOC
Conditions: substrates ¼ 30 mmol, MeOH ¼ 60 mmol, FeCl3 (20 mmol), Cat ¼ 1.0 ꢁ 10ꢂ2 mmol, medium ¼ DMF, T ¼ 120 ꢀC, t ¼ 5 h, PCO ¼ 60 atm.

S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
159
Scheme 3. Proposed reaction mechanism of ruthenium catalyzed carbonylation nitrobenzene.
carbonylation of nitrobenzene and aniline. During the catalytic
Recycling of the catalyst
carbonylation of nitrobenzene and aniline, the solid catalyst was
separated from the reaction mixture after 3 h. The obtained DPU
at this point was 78%. The reaction was carried out for a further
3 h without the catalyst. The gas chromatographic analysis
showed no increase in the conversion, whereas there was an
increase in conversion in the uninterrupted experiment. AAS
analyses of the liquid phase of the reaction mixtures collected by
filtration confirmed that Ru is absent in the reaction mixture.
These results suggest that the Ru is not being leached out from
the catalyst during the carbonylation reaction. Furthermore,
when the reaction was carried out in the absence of catalyst
(blank reaction), no product was observed. This result suggested
the catalytic role is played by the Ru centre in this carbonylation
reaction.
One of the main advantages of a heterogeneous catalyst is to
enhance the lifetime of the catalyst. To investigate the reusability
of the polymer anchored Ru(II) complex, the catalyst was sepa-
rated by filtration after the first run, dried under vacuum and then
subjected to a second run under the same reaction conditions.
Recycling efficiency was tested for the carbonylation of nitro-
benzene and aniline with further addition of substrates in the
appropriate amount under optimum reaction conditions, and the
nature and yield of the final products were found to be compa-
rable to that of the original one. As seen in Fig. 10, the catalyst can
be reused up to five times without any appreciable loss of its
catalytic activity.
Conclusion
A versatile polymer anchored ruthenium catalyst has been
introduced for carbonylation reaction. The reaction conditions
and catalyst system employed in the present study were found to
be exceptionally efficient in N,N’ediphenylurea synthesis from
aniline, nitrobenzene, and carbon monoxide. The catalyst is also
effective for the selective production of other disubstituted ureas.
The system allows facile product separation and catalyst recov-
ery. The polystyrene-supported ruthenium complex can be
recycled several times without substantial loss of catalytic
efficiency.
Acknowledgments
SMI acknowledges the Council of Scientific and Industrial
Research (CSIR), 02(0041)/11/EMR-II, dated: 19-12-2011 and
Department of Science and Technology (DST), New Delhi, India,
SB/S1/PC-107/2012; dated 10.06.2013 for funding. RAM acknowl-
edges UGC, New Delhi, India for his Maulana Azad National
Fellowship.
Fig. 10. Recycling efficiency for the production of diphenyl urea catalyzed by polymer
anchored ruthenium complex.

160
S.M. Islam et al. / Journal of Organometallic Chemistry 772-773 (2014) 152e160
[17] N. D Ca, P. Bottarelli, A. Dibenedetto, M. Aresta, B. Gabriele, G. Salerno,
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