Synthesis, characterization and catalytic activity of polymer anchored transition metal complexes toward oxidation reactions

Article abstract of DOI:10.1007/s11243-010-9409-3

The polymer-anchored Schiff base complexes of Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) were prepared by reacting polystyrene amine with 2-pyridinecarbaldehyde followed by loading of metal atom in methanol. These complexes were characterized by using different physico-chemical and spectroscopic methods. The catalytic activity of these polymer-supported metal catalysts was tested for the oxidation of various olefins and alcohols. Influence of various reaction parameters, such as reaction temperature, reaction time, oxidant, substrate-to-oxidant mole ratio and nature of solvent, was studied for the oxidation of cyclohexene with these catalysts. Among the catalysts studied, Cu-Cat showed higher catalytic activity toward oxidation reactions than the other catalysts. Moreover, hot filtration experiments proved that these catalysts are truly heterogeneous and can be reused a number of times without significant loss of activity.

Full text of DOI:10.1007/s11243-010-9409-3

Transition Met Chem (2010) 35:891–901
DOI 10.1007/s11243-010-9409-3
Synthesis, characterization and catalytic activity of polymer
anchored transition metal complexes toward oxidation reactions
S. M. Islam
Anupam Singha Roy
Sanchita Mondal
•
Paramita Mondal
•
•
Kazi Tuhina
•
•
Dilder Hossain
Received: 4 May 2010 / Accepted: 30 July 2010 / Published online: 25 August 2010
Ó Springer Science+Business Media B.V. 2010
Abstract The polymer-anchored Schiff base complexes of
Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) were prepared by
reacting polystyrene amine with 2-pyridinecarbaldehyde
followed by loading of metal atom in methanol. These
complexes were characterized by using different physico-
chemical and spectroscopic methods. The catalytic activity
of these polymer-supported metal catalysts was tested for the
oxidation of various olefins and alcohols. Influence of vari-
ous reaction parameters, such as reaction temperature, reac-
tion time, oxidant, substrate-to-oxidant mole ratio and nature
of solvent, was studied for the oxidation of cyclohexene with
these catalysts. Among the catalysts studied, Cu-Cat showed
higher catalytic activity toward oxidation reactions than the
other catalysts. Moreover, hot filtration experiments proved
that these catalysts are truly heterogeneous and can be reused
a number of times without significant loss of activity.
chemical and pharmaceutical industries [3]. Now, from both
economic and environmental points of view, much attention
has recently been focused on the aerobic catalytic oxidation
of hydrocarbons to oxygenic compounds using metal cata-
lysts. Transition metal complexes are widely used in
homogeneous and heterogeneous catalytic oxidations of
different alkenes, alkynes, alcohols, halides, phenols etc
[4, 5]. In the recent past, there has been an increasing interest
in developing environmental friendly greener processes,
which are also economically viable. Homogeneous transi-
tion metal catalyst systems suffer from a major drawback of
the catalyst recovery and/or reuse affecting the overall
economics of the process [6, 7]. In the past few decades,
there have been significant developments in the application
of heterogeneous catalysts for the industrial production of
organic chemicals. Heterogeneous catalysts, which are
widely used in industry, have good thermal stability, can be
easily separated from the reaction mixture and can be often
regenerated and reused. Therefore, heterogenizing of a
homogeneous metal complex by supporting it on an insol-
uble support has attracted a lot of interest as a suitable
method for solving many practical problems including
recovery of the catalyst from reaction mixture and recycling.
Transition metals supported on materials such as alumina
[8], amorphous silicates [9], polymers [10] and zeolites [11]
are commonly employed in heterogeneous catalysis.
Introduction
The oxidation of hydrocarbons to oxygenic compounds is a
pivotal reaction in organic chemistry, both for fundamental
research and industrial manufacturing [1, 2]. Oxidation of
olefins and alkenes to give oxygen containing value-added
products like alcohols, aldehydes, ketones, acids, epoxides,
etc. is an extremely important and useful reaction in both
Polystyrene is one of the most widely studied hetero-
geneous supports due to its environmental stability and
good catalytic activity. Polymer-supported transition metal
catalysts derived from functionalized polystyrene resin
have been employed in various organic reactions [12, 13]
and have shown lower leaching of palladium during cross-
coupling. This encouraged us to investigate the function-
alized polystyrene resin-supported transition metal com-
plexes for the oxidation reactions. We have already
SS .. MM .o nI sd l aa lm (&) Á P. Mondal Á A. S. Roy Á D. Hossain Á
Department of Chemistry, University of Kalyani,
Kalyani, Nadia, WB 741235, India
e-mail: manir65@rediffmail.com
K. Tuhina
Department of Chemistry, B.S. College, S-24 P.G.S.,
Canning, WB 743329, India
123

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Transition Met Chem (2010) 35:891–901
reported the use of a polystyrene-anchored transition metal
complex as an air-stable, active, and reusable catalyst in
various organic reactions [14–16].
detector. All reaction products were identified by using an
Agilent, USA, GC-MS (QP-5050). Surface morphology of
functionalized polystyrene ligand and metal complex was
analyzed using a scanning electron microscope (SEM)
(ZEISS EVO40, England) equipped with EDX facility. The
thermal stability of the immobilized catalyst was determined
using a Mettler Toledo, Switzerland, TGA/DTA 851. Mag-
netic moment of the complexes was determined at room
temperature using Guoy’s method. The FTIR spectra of the
immobilized catalyst were recorded on a Perkin-Elmer,
USA, FTIR 783 spectrophotometer using KBr pellets. Dif-
fuse reflectance UV–Vis spectra were taken using a Shima-
dzu, Japan, UV-2401PC double beam spectrophotometer
having an integrating sphere attachment for solid samples.
Herein, we report the synthesis of new functionalized
transition metal complexes anchored to polystyrene via
nitration and then reduction to amino functionality fol-
lowed by Schiff base condensation. The Schiff base
anchored in the polymer matrix has been designed in such a
way that it could interact covalently with the metal centers
and thus generate a suitable heterogeneous catalyst. These
polymer-anchored complexes showed high catalytic activity
in the oxidation reactions of olefins and alcohols. Further,
this polymer-anchored complex offers easy catalyst recov-
ery and excellent recycling efficiency.
Experimental
Catalytic oxidation reaction
All the reagents were of analytical grade and used as such
without further purification. Solvents were purified and dried
according to standard procedures [17]. The liquid substrates
were predistilled and dried by appropriate molecular sieve,
and the solid substrates were recrystallized before use.
Macroporous polystyrene beads cross-linked with divinyl-
benzene were purchased from Aldrich Chemical Company,
USA. Other reagents were purchased from Merck.
The catalytic oxidation reaction was carried out in a 50-mL
flask keeping in a thermostatted oil bath. In a typical reaction,
substrate (5 mmol) and oxidant (10 mmol) were taken in
10 mL solvent in the flask, and then the catalyst (0.88 mmol)
was added. When the reaction was carried out under oxygen
atmosphere, oxygen gas was purged into the flask continu-
ously. After the reaction was completed, the catalyst was
separated by filtration. The product was analyzed by gas
chromatography (Varian 3400) using a FID. The formation
of product was confirmed by GC-MS (Agilant QP-5050).
A Perkin-Elmer, USA, 2400C elemental analyzer was
used to collect microanalytical data (C, H and N). The metal
content in the polymer-anchored complex was analyzed
using a Varian, USA, AA240 atomic absorption spectro-
photometer (AAS). The reaction products were analyzed by
Varian, USA, 3400 gas chromatograph equipped with a30 m
CP-SIL8CB capillary column and a flame ionization
Preparation of catalyst
The outline for the preparation of polymer-anchored Schiff
base metal complexes is given in Scheme 1.
Scheme 1 Synthesis of the
polymer-anchored metal
complexes. P = Polystyrene
frame work, M = Cu (Cat.1),
Co (Cat.2), Ni (Cat.3), Mn
HNO + CH COOH
3
3
SnCl + HCl
2
P
NO2
P
NH Cl
3
P
CH COOH
3
5
0 °C, 6 h
polystyrene
polystyrene
amino hydrochloride
2)
p-nitropolystyrene
1)
(
2
(Cat.4), Fe (Cat.5); X = H O or
Cl
(
O
C
H
N
NaOH
P
N
P
NH2
p-aminopolystyrene
3)
Toluene, reflux
N
(4)
MeOH
(
M(Cl)2
MeOH M(Cl)2
or M(Cl)3
P
N
P
N
N
N
Cl
X
M
M
Cl
Cl
X
Cl
5)
Cat.4, Cat. 5)
(
5)
(
(
Cat.1, Cat. 2, Cat. 3)
(
1
23

Transition Met Chem (2010) 35:891–901
893
Preparation of the polymer-anchored catalyst (5)
(not shown in Fig. 1). The introduction of the ligand on to
the polystyrene beads causes a roughening on the top layer.
Images of the metal complexes show further roughening on
the top layer, which may be due to interaction of the metal
atoms with the ligand to accommodate the fixed geometry
of the complexes. Energy dispersive spectroscopy X-ray
analyses (EDX analysis) of the complexes show the metal
content along with C and Cl, suggesting the formation of
metal complex with the anchored ligand at various sites
(Fig. 2). Cl/M ratio obtained from EDX analysis is quite
similar to the data obtained from elemental analysis
(Table 1). The attachment of metal on the polymer matrix
is confirmed from these SEM images and EDX data.
The IR bands that can provide structural evidence for the
coordination of the ligand to the central metal atom are given
in Table 2. The FT–IR spectra of various catalysts prepared
The polymer-anchored 2-pyridinecarbaldehyde ligand (4)
was synthesized by the procedure similar to that described
in our previous report [18].
Polymer-anchored 2-pyridinecarbaldehyde ligand (1 g)
was taken in methanol (10 mL). In the reaction mixture, 1%
(
w/v) methanolic solution of metal chloride was added
dropwise over a period of ca. 30 min under constant stirring.
The mixture was refluxed for 24 h, and the complex was
filtered off, washed thoroughly with methanol until the
washings were colorless and dried in room temperature
under vacuum.
Results and discussion
-1
in this study were recorded in the ranges of 4,000–400 cm
-1
and 500–100 cm . The polystyrene-anchored Schiff base
Characterization of the polymer-anchored Schiff base
complexes
-1
ligand (4) exhibits a peak at 1,640 cm due to (C=N)azomethene
stretching vibration. Coordination of azomethene nitrogen
in the complexes is suggested by the shift of m(C=N)azomethene
to lower frequencies in the IR spectra of the complexes
Due to insolubilities of the polymer-anchored metal com-
plexes in all common organic solvents, their characteriza-
tion was limited to their physicochemical properties,
chemical analysis, SEM, TGA, IR and UV–Vis spectral
data. Elemental analyses of the ligand and metal complexes
-1
compared to the ligand [19]. Another band at 1,594 cm in
the free ligand is due to m(C=N)pyridine group and is also
shifted toward lower frequency on complexation [20]. This
indicates that the nitrogen atom of the pyridine group is
also involved in complex formation. Coordination of imine
(
Table 1) support the formulation of the complex as pro-
posed. Metal content of catalysts determined by AAS
suggests 1.4 wt% Cu, 1.8 wt% of Co, 2.3 wt% of Ni, 1.7
wt% of Mn and 2.7 wt% of Fe in the metal complexes,
respectively. In Fig. 1, the SEM images of polymer-
anchored Schiff base ligand (Fig. 1a) and the immobilized
metal complexes on modified polystyrene (Fig. 1b, f) are
shown. Pure polymer beads have a smooth and flat surface
nitrogen is consistent with the presence of a band at 460,
-1
467, 470, 465 and 470 cm in Cat. 1, Cat. 2, Cat. 3, Cat. 4
and Cat. 5, respectively, which is due to the formation of
M–N bond between the metal and azomethine nitrogen
-1
[
21]. Another band at 545, 536, 540, 542 and 542 cm in
the Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) metal com-
plexes due to the formation of metal bond with pyridine
.
nitrogen (M–N) is also observed [22]. The far IR spectrum
Table 1 Physical and analytical data of polymer-anchored ligand and
complexes
of these complexes exhibits a broad intense band around
-
1
15–336 cm , which may be assigned to M–Cl [23].
3
Compound Color
Cl
%
C % H
%
N
%
M
%
Cl/M
The magnetic moment measurements of these com-
plexes were carried out in the solid state at room temper-
ature. The magnetic moment (l) of the complexes of
Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) is found to be
PS
Colorless
Light yellow
Cl Yellow
Pale yellow
–
–
92.5 7.6
–
–
–
–
–
–
PS-NO
PS-NH
PS-NH
2
3
2
74.1 5.7 6.0
13.3 72.0 6.7 5.8
1.88, 2.76, 0.00, 5.84 and 5.71, respectively (Table 2). The
–
–
84.1 7.3 6.7
83.5 6.5 8.8
room temperature magnetic moment of the polymeric
Cu(II), Fe(III), Co(II) and Mn(II) complexes are para-
magnetic in nature. On the other hand, the polymeric Ni(II)
complex has been found to be diamagnetic due to spin-
PS-Ligand Brown
Cat. 1
Cat. 2
Cat. 3
Brown
Brown
6.5 73.3 5.7 7.7 1.4 4.6 (4.5)
7.1 72.9 5.7 7.6 1.8 3.9 (3.8)
5.9 73.9 5.9 7.8 2.3 2.5 (2.3)
8
Yellowish
brown
paired d system [24]. This diamagnetic nature supports the
formation of a low-spin square-planar geometry around
Ni(II). The Cu(II) complex shows a magnetic moment
value of 1.88 B.M. [25], a value close to the spin-only
value of 1.73 B.M., expected for S = ‘ system, while the
magnetic moment value of the Co(II) complex is 2.76
B.M., which is generally arising from one unpaired
Cat. 4
Cat. 5
Yellowish
brown
5.2 73.2 6.1 7.7 1.7 3.1 (3.1)
11.5 66.8 5.5 7.0 2.7 4.2 (4.0)
Dark brown
PS Polystyrene
Cl/M ratio obtained from EDX analysis is given in parentheses
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Transition Met Chem (2010) 35:891–901
Fig. 1 FE SEM image of
polymer-anchored Schiff base
ligand (a) and polymer-
anchored Cu (b), Co (c), Ni (d),
Mn (e) and Fe (f) complexes,
respectively
electron and an apparently large orbital contribution and
also diagnostic of the geometry around the low-spin tetra-
coordinated Co(II) atom [26]. The magnetic moment val-
ues of the Fe(III) and Mn(II) complexes support the for-
mation of high-spin octahedral geometry [27].
the absorption bands in the region 580–630 nm can be
2
2
assigned to A ? B transition [28]. The Ni(II) com-
1
g
1g
plex shows two peaks at *425 nm and 532 nm that are
characteristic of square planar diamagnetic Ni(II) [32]. The
manganese complex exhibits a broad band around
382–452 nm, which might be due to the d–d transition of
Mn(II) [33]. The electronic spectrum of the Fe(III) com-
plex exhibits four bands at 312, 358, 440 and 627 nm. The
first two peak in the UV region may be assigned to n–p*
and p–p* transition [34, 35]. The medium intensity band at
The electronic spectra of all the polymeric metal cata-
lysts have been recorded in diffuse reflectance spectrum
mode as BaSO disks due to their solubility limitations in
4
common organic solvents. The electronic spectral bands for
the Schiff base ligand and metal complexes have been
given in Table 2 and Fig. 3. The polymer-anchored Schiff
base ligand shows two bands at 330 nm and 375 nm that
may be assigned to intraligand n–p* and p–p* transitions,
respectively. These bands show blue shift on complexation.
The reflectance spectrum of the Cu(II) catalyst shows two
bands at 405 and 418 nm, which may be assigned to charge
transfer transitions, while the shoulder at around 490 nm
6
4
440 nm may be assigned as A ? T (G) transitions in
1
g
2g
octahedral symmetry [28] around Fe(III). The intensities of
the bands in the spectra are affected possibly due to the low
loading of the metal onto the polymer backbone.
Thermal analysis of the metal complexes was carried out
to find the stability of the complexes. Thermogravimetric
analyses of these complexes were performed in air atmo-
2
2
-1
may be attributed to B1g ? A transition in a dis-
sphere at a heating rate of 10 °C min . The TG-data of the
1g
torted tetrahedral stereochemistry [28, 29]. The electronic
Cat. 1–Cat. 5 are depicted in Fig. 4. From these results, the
decomposition temperatures of Cat. 1–Cat. 5 are 323, 315,
308, 313 and 295, respectively. From the TG profile, it is
spectrum of the Co(II) complex showed a sharp peak at
4
95, which is attributed to LMCT transition [30, 31], while
1
23

Transition Met Chem (2010) 35:891–901
895
Fig. 2 EDX images of polymer-anchored Schiff base ligand (a) and polymer-anchored Cu (b), Co (c), Ni (d), Mn (e) and Fe (f) complexes,
respectively
seen that the Cu(II) complex is more stable than the other
complexes.
Effect of solvent
Solvent plays a crucial role in the yield and product dis-
tribution of oxidation reactions. In order to study the effect
of solvents, several solvents were employed in the oxida-
tion reaction of cyclohexene (5 mmol) with TBHP
(10 mmol) at 60 °C keeping other parameters constant.
The oxidation reaction was carried out in several polar and
non-polar solvents. The selected solvent should possess
certain criteria such as it should be stable and it should
dissolve the substrate and oxidant. As shown in Table 3,
quantitative yields were observed with polar solvents such
as acetonitrile (ACN), methanol and dichloromethane
(DCM). On the contrary, unsatisfactory yields were
obtained with non-polar solvents like toluene and benzene.
The efficiency of the catalysts for oxidation of cyclohexene
in different solvents decreased in the order ACN [ meth-
anol [ DCM [ toluene [ benzene. The higher catalytic
activity in acetonitrile is attributed to high dielectric con-
stant, polarity of solvent and better solubility of substrate
Catalytic activity
To explore the catalytic activity of the present catalysts, we
performed the oxidation reaction with these catalysts. In
search of suitable reaction conditions to achieve the max-
imum oxidation products, initial catalytic test was carried
out for the oxidation of cyclohexene with polymer-
anchored Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) catalysts
(
Scheme 2). The oxidation of cyclohexene produced
-cyclohexene-1-one and 2-cyclohexene-1-ol as major
2
product and also cyclohexane epoxide in minor yield. Since
the performance of a successful metal-catalyzed oxidation
reaction is known to be governed by a number of factors, at
first, the effects of various solvents, oxidants, reaction time,
reaction temperature, substrate-to-oxidant ratio and catalyst
amount on the oxidation reaction were surveyed to opti-
mize the reaction conditions for yield of product.
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Transition Met Chem (2010) 35:891–901
Table 2 Magnetic, IR (in
cm ) and electronic spectral
data of the compounds
-1
Compound mNO
2
mNH
3
Cl mNH
2
mC=N vM–N vM–Cl
k
max (nm)
leff (B.M.)
PS-NO
PS-NH
PS-NH
2
3
2
1520(s)
350(s)
Cl 1520(w) 2545
350(w)
1520(w)
350(w)
PS-Ligand 1520(w)
340(w)
1520(w)
350(w)
1520(w)
340(w)
1520(w)
340(w)
1
1
1625
(sh)
1
a
1640
330, 375
b
1
1594
a
1614 460
c
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
336
315
327
325
335
328, 375, 405, 418, 490 1.88
309, 373, 495, 580–630 2.76
b
1588 545
d
1
a
1622 467
c
b
1572 536
d
1
a
1605 470
c
312, 368, 425, 532
327, 350, 382–452
312, 358, 440, 627
0.00
5.84
5.71
b
1576 540
d
c
1
a
1520(w)
1340(w)
1520(w)
1618 465
a
b
1570 542
d
m(C=N)azomethene
;
b
c
d
a
1626 470
c
m(C=N)pyridine
m(M–Nazomethene);
m(M–Npyridine
;
b
1583 542
d
1
340(w)
)
Fig. 4 Thermogravimetric weight loss plots for polymer-anchored
metal complexes
O
OH
Catalyst
Fig. 3 DRS-UV-visible absorption spectra of polymer-anchored
metal complexes
+
+
O
TBHP ACN
and oxidant in the solvent so that the substrates and oxidant
can easily approach the active sites of the catalyst.
Scheme 2 Various oxidation products of cyclohexene
hydroperoxide, iodosyl benzene (PhIO) and molecular
oxygen was examined in the oxidation of cyclohexene. In
the absence of oxidant, the reaction did not proceed. As
shown in Table 4, TBHP in acetonitrile medium was most
efficient oxidant source to oxidize cyclohexene because of
its unreactiveness in the absence of catalyst and good
Effect of oxidant
In order to design the best catalytic system, the ability of
different oxidants such as tert-butylhydroperoxide (TBHP)
(
70% aq.), hydrogen peroxide (H O ) (30% aq.), cumene
2 2
1
23

Transition Met Chem (2010) 35:891–901
897
Table 3 Effect of solvents on the catalytic oxidation of cyclohexene
decomposition of peroxide, conversely lowering down the
conversion. The optimum temperature was 60 °C.
Entry
Solvent
Conversion (%)
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
Effect of reaction time
1
2
3
4
5
.
.
.
.
.
ACN
86
69
48
20
07
80
69
62
83
The time dependence of the catalytic oxidation of cyclo-
hexene was studied by performing the reaction of cyclo-
hexene (5 mmol) with TBHP (10 mmol) in the presence of
MeOH
DCM
58
56
49
65
39
26
22
40
Toluene
Benzene
16
12
07
14
0.88 mmol catalyst at 60 °C with constant stirring. The
Trace
Trace
Trace
Trace
results are presented in Fig. 5. The reaction hardly occur-
red during the first 1 h, which might be the initiation per-
iod. After the initiation period, the cyclohexene conversion
increased for every catalyst as reaction time was prolonged.
Comparison of the results during 2–10 h exhibited in Fig. 5
showed that highest conversion of cyclohexene was
obtained after 8 h and then remained stable. Selectivity of
the products was not significantly changed with increasing
time.
Reaction conditions [cyclohexene] = 5 mmol; catalyst = 0.88 mmol;
[TBHP] = 10 mmol; solvent = 10 mL; temperature = 60 °C; reac-
tion time = 8 h
Conversion refers to GC & GC-MS analysis
Table 4 Effect of oxidant on the catalytic oxidation of cyclohexene
Entry Oxidant
Conversion (%)
Effect of substrate-to-oxidant ratio
Cat. 1 Cat. 2 Cat. 3 Cat. 4 Cat. 5
1
2
3
4
5
.
.
.
.
.
TBHP
86
54
45
80
45
34
–
69
32
27
–
62
16
12
–
83
43
35
–
The substrate-to-oxidant ratio is another important param-
eter influencing the results of this oxidation reaction. Effect
of mole ratio of cyclohexene/TBHP was investigated in the
range of 1.0–3.0, and the results are given in Table 5. A
lower conversion of cyclohexene was observed with
1:1 mol ratio of cyclohexene/TBHP. Increasing the sub-
strate-to-oxidant molar ratio from 1:1 to 1:2, the conversion
of cyclohexene increased markedly. Further increasing this
ratio to 1:3 hardly affected the conversion but decreased
the selectivity for 2-cyclohexene-1-one. Based on the
results, the mole ratio of 1:2 was chosen as the optimal
ratio of cyclohexene/TBHP.
2 2
H O
PhIO
Cumene hydroperoxide –
Molecular O 14
2
09
Trace Trace 16
Reaction conditions [cyclohexene] = 5 mmol; catalyst = 0.88 mmol;
[oxidant] = 10 mmol; acetonitrile = 10 mL; temperature = 60 °C;
reaction time = 8 h
Conversion refers to GC & GC-MS analysis
solubility in acetonitrile medium. Oxidants like H O and
2
2
PhIO were found to be less effective than TBHP and gave
moderate conversion of cyclohexene. Using cumene
hydroperoxide as oxidant, no conversion was obtained. In
the presence of molecular oxygen, cyclohexene was oxi-
dized but lower yield of products was obtained.
Table 5 Effect of reaction temperature, substrate-to-oxidant ratio
and catalyst concentration on the catalytic oxidation of cyclohexene
Reaction parameters Conversion (%)
Cat. 1 Cat. 2 Cat. 3 Cat. 4 Cat. 5
Effect of reaction temperature
Temperature (°C)
4
6
8
0
0
0
49
86
80
43
80
75
32
69
62
29
62
58
47
83
76
The reaction temperature has a strong influence on the
progress of the cyclohexene oxidation. The effect of
reaction temperature on the progress of oxidation of
cyclohexene was studied at different temperatures with the
present catalysts. The oxidation reaction was carried out
using 1:2 mmol ratio of cyclohexene-to-TBHP in acetoni-
trile medium for 8 h with different reaction temperatures
from 40 °C to 80 °C, and results are shown in Table 5. As
expected, cyclohexene conversion was increased with
increasing reaction temperature, although no conversion
was detected when the reaction temperature was below
Substrate: oxidant
1
1
1
:1
:2
:3
37
86
86
32
80
80
23
69
69
18
62
62
24
83
83
Catalyst concentration (g)
0
0
0
.02
.04
.06
52
86
86
44
80
80
40
69
69
31
62
62
43
83
83
4
0 °C. However, high temperature led to a quick
Conversion refers to GC & GC-MS analysis
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Transition Met Chem (2010) 35:891–901
1
00
summarized in Table 6. Results of Table 6 also show the
catalytic activity of the neat metal chlorides. Comparing
the catalytic activity between neat and polymer-anchored
complexes proves that polymer-supported catalyst gave
higher conversion of cyclohexene than their corresponding
metal chlorides. The higher activity of supported com-
plexes is because of site isolation of the complexes. Many
efficient heterogeneous catalysts have been reported for the
oxidation of cyclohexene with TBHP [36–38]. The results
are summarized in Table 7. Comparison of these catalysts
with previously reported systems reveals that the present
systems give better yields than other catalysts.
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
8
6
4
2
0
0
0
0
0
2
4
6
8
10
Time (h)
Heterogeneity test
Fig. 5 Effect of reaction time on oxidation of cyclohexene using
present catalysts. Reaction condition: cyclohexene (5 mmol), aceto-
nitrile (10 mL), TBHP (10 mmol), catalyst (0.88 mmol) and temp. =
An important criterion in the commercial applications of
transition metal catalysts is to obtain metal-free end products,
since metal contamination is of great concern for food and
pharmaceutical industries. The heterogeneity of the catalyst
was verified by performing the typical hot-filtration experi-
ment. Hot-filtration test was performed in the oxidation
reaction of cyclohexene with Cat. 1. During catalytic reaction,
the solid catalyst was separated from the reaction mixture by
filtration after 4 h and the determined conversion was 38%.
The reaction was carried out for a further 6 h. The gas chro-
matographic analysis showed no noticeable increment in the
conversion. This confirms that the reaction did not proceed
upon the removal of the solid catalyst. Further, atomic
absorption spectrometric analysis of the liquid phase of the
reaction mixtures collected by filtration confirms that Cu is
absent in the filtrate. Metal content of the catalyst also
remained unaltered, indicating no leaching of the metal from
the polymer support. These resultssuggest thatthemetal is not
being leached out from the catalyst during catalytic reactions.
6
0 °C
Effect of amount of catalyst
The influence of amount of catalyst on the oxidation of
cyclohexene was carried out over the range 0.4–1.4 mmol,
while the cyclohexene conc. (5 mmol), TBHP conc.
(
10 mmol), temp (60 °C) and time (8 h) were kept constant.
The effect of amount of catalyst on the oxidation of cyclo-
hexene as a function of time is illustrated in Table 5. A
control experiment was also carried out with cyclohexene as
a representative case, in the presence of TBHP, without the
addition of catalyst. It was observed that in the absence of
any added catalyst, very poor conversion of cyclohexene
(
7%) was obtained. At low catalyst concentration (0.44
mmol catalyst), moderate conversion of cyclohexene was
obtained. With the increase in catalyst amount from 0.44 to
0
.88 mmol, the conversion of cyclohexene increased. This
may be due to the availability of more active sites of the
catalyst that favors the accessibility of larger number of
molecules of substrates and oxidant to the catalyst. Cyclo-
hexene conversion remained almost constant upon further
increase in catalyst amount to 1.32 mmol.
Catalyst reuse and stability
To assess long-term stability and reusability of the polymer-
anchored metal complexes, recycling experiments were
carried out for oxidation of cyclohexene. After each exper-
iment, the catalyst was separated by simple filtration,
washed, dried under vacuum and then subjected to the next
run under the same experimental conditions. The catalytic
run was repeated with further addition of substrates in
appropriate amount under optimum reaction conditions and
the nature and yield of the final product were comparable
with those of the original run. From the Table 8, it is seen that
after four recycles, there is a small decrease in the conver-
sion. Selectivity of the products remained almost the same
after the fourth recycle. The nature of the recovered catalyst
has been followed by IR and UV–vis spectra. The results
indicated that the catalysts after reusing several times,
showed no change in their IR and UV–vis spectra. From
Thus, the optimum reaction conditions for the oxidation
of cyclohexene were as follows: cyclohexene (5 mmol),
7
0% aqueous TBHP (10 mmol), catalyst (0.88 mmol),
ACN (10 mL) time (8 h) and temperature (60 °C). Among
these catalysts, Cat 1 was found to be the most active,
while the least active catalyst was Cat. 4. The following
order of catalytic activities toward oxidation of cyclohex-
ene was obtained for the complexes: Cat. 1 [ Cat. 5 [ Cat.
2
[ Cat. 3 [ Cat. 4. The selectivity and activity results of
these catalysts on the oxidation of cyclohexene are given in
Table 6. These catalysts were also tested for the catalytic
oxidation of other olefins and alcohols under optimized
reaction conditions, and the obtained results are
1
23

Transition Met Chem (2010) 35:891–901
899
Table 6 Oxidation of various substrates with polymer-anchored Schiff base metal complexes
a
Conversion (%)
-1
TOF (h )
Entry
Substrates
Catalyst
Selectivity (%)
1
2
3
4
5
6
.
.
.
.
.
.
Cyclohexene
2-cyclohexene-1-one
2-cyclohexene-1-ol
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
86
80
69
62
83
77
18
24
21
27
14
610.8
568.2
490.1
369.3
589.5
74
63
60
79
1-hexene
1-hexanone
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
65
53
42
27
55
100
461.6
411.9
298.3
191.7
390.6
100
100
100
100
Styrene
Benzaldehyde
Styrene epoxide
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
79
70
53
47
73
79
21
28
32
30
25
561.1
497.2
376.4
333.8
518.5
78
68
70
75
Benzylalcohol
Ethylbenzene
Cyclohexene
Benzaldehyde
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
83
75
66
54
73
100
589.5
532.7
468.7
383.5
518.5
100
100
100
100
Benzaldehyde
Others
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
72
67
54
32
68
84
16
511.4
475.8
383.5
227.3
483.0
80
20
82
18
80
20
86
14
2-cyclohexene-1-one
2-cyclohexene-1-ol
b
2
CuCl
CoCl
46
34
20
17
44
57
63
50
59
60
31
35
36
40
32
95.8
70.8
41.7
35.4
91.7
b
2
b
2
NiCl
b
2
MnCl
b
FeCl
3
Reaction conditions [substrate] = 5 mmol; catalyst = 0.88 mmol; [TBHP] = 10 mmol; acetonitrile = 10 mL; Temperature = 60 °C; reaction
time = 8 h
a
Conversion refers to GC & GC-MS analysis
b
Catalyst amount = 3.0 mmol
Turn over frequency (TOF) = moles of substrate converted per mole of metal per hour
these results, we may conclude that these catalysts remain
unaltered after use in the reaction.
characterized by different analytical and spectroscopic
techniques. These complexes showed high catalytic activ-
ity in the oxidation of olefins and alcohols. Under opti-
mized reaction conditions, oxidation of cyclohexene gave
three major products namely 2-cyclohexene-1-one,
Conclusion
2
-cyclohexene-1-ol and cyclohexene epoxide and the per-
Polymer-anchored Schiff base complexes of Cu(II), Co(II),
Ni(II), Mn(II) and Fe(III) have been synthesized and
centage of conversion of cyclohexene varied in the order:
123

9
00
Transition Met Chem (2010) 35:891–901
Table 7 Comparison of the
catalytic activity of present
catalytic systems with other
reported systems toward
oxidation of cyclohexene
Catalyst
Conversions (%)
Reaction conditions
Reference
Present
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
86
ACN, TBHP, 60 °C, 8 h
80
69
62
83
[
[
[
[
Mn(SAH)H
2
O]-Al
O]-Al
O]-Al
O]-Al
2
O
3
44.7
36.7
28.9
34.5
4.36
5.17
18.65
11.90
32.4
57.4
38.4
61.3
ACN, TBHP, reflux, 8 h
ACN, TBHP, 80 °C, 4 h
ACN, TBHP, 323 K, 12 h
Salavati-Niasari
and Amiri [36]
Co(SAH)H
2
2
O
3
Ni (SAH)H
2
2
O
3
Cu(SAH)H
2
2 3
O
a-TiCuAs
a-TiCoAs
a-TiMnAs
a-TiFeAs
Khare and
Shrivastava [37]
Cu-S-AmSiO
Cu-S-AM(PS)
Co-S-AmSiO
2
Karandikar et al. [38]
2
Co-S-AM(PS)
Table 8 Activity of the
regenerated catalyst
Catalyst
Cat. 1
Cycle
Cyclohexene
conversion (%)
Product selectivity (%)
2
-cyclohexene-1-one
2-cyclohexene-1-ol
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
86
85
85
83
80
79
78
78
69
67
65
65
62
62
60
60
83
83
82
81
77
77
77
78
74
74
73
74
63
63
63
63
60
59
57
57
79
77
79
79
18
18
18
16
24
24
24
24
21
21
21
21
27
28
26
26
14
13
14
14
Cat. 2
Cat. 3
Cat. 4
Cat. 5
Reaction conditions
[
cyclohexene] = 5 mmol;
catalyst = 0.88 mmol;
TBHP] = 10 mmol;
[
acetonitrile = 10 mL;
Temperature = 60 °C; reaction
time = 8 h
Conversion refers to GC & GC-
MS analysis
Cat. 1 [ Cat. 5 [ Cat. 2 [ Cat. 3 [ Cat. 4. These sup-
ported catalysts are obtained by a simple, facile and cheap
synthesis methodology. The main advantages of the present
catalysts are ready accessibility, high product yield,
robustness, high thermal stability and applicability for a
variety of substrates. These catalysts are reused several
times without appreciable decrease in their initial activities.
All these facts allow us to view them as prospective het-
erogeneous catalysts for practical applications.
Acknowledgments We acknowledge the Department of Science
and Technology (DST), the Council of Scientific and Industrial
1
23

Transition Met Chem (2010) 35:891–901
901
Research (CSIR) and the University Grants Commission (UGC), New
Delhi, India, for funding. One of the authors, K.T., is thankful to the
University Grants Commission (Eastern Region), India, for financial
support.
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