Syntheses, characterization and study of the use of cobalt (II) Schiff-Base complexes as catalysts for the oxidation of styrene by molecular oxygen

Article abstract of DOI:10.3390/10010302

Schiff-Base complexes of bis-5-phenylazosalicylaldehyde ethylenediimine and bis-5-phenylazosalicylaldehyde-O-phenylenediimine ligands with Co(II) (I and II) have been synthesized and characterized by their IR spectra and elemental analyses. These complexes catalyze the oxidation of styrene in the presence of dioxygen and excess pyridine. The effect of the reaction conditions on the oxidation of styrene was studied by varying solvent, nature and amount of the catalyst and substrate. The catalytic behavior of the studied complexes was shown to be dependent on the conditions applied. In all reactions, acetophenone and 1- phenylethanol were the only observed products.

Full text of DOI:10.3390/10010302

Molecules 2005, 10, 302-311
molecules
ISSN 1420-3049
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Syntheses, Characterization and Study of the Use of Cobalt (II)
Schiff–Base Complexes as Catalysts for the Oxidation of
Styrene by Molecular Oxygen
Ali Akbar Khandar , Kamellia Nejati and Zolfaghar Rezvani 3
1
2,*
1
Department of Inorganic Chemistry, Faculty of Chemistry, Tabriz University, Tabriz, Iran
Payam Noor University -Tabriz Center, Emamieh ,Hakimnezami Street, Tabriz, Iran
Department of Chemistry, Faculty of Science, Azarbaijan University of Tarbiat Moallem, Tabriz,
2
3
Iran
*
Author to whom correspondence should be addressed: Fax: (+98) 411 5412108; Tel: (+98) 411
412117; e-mail: nejati_k@yahoo.com.
5
Received: 13 August 2004 / Accepted: 14 September 2004 / Published: 31 January 2005
Abstract: Schiff-Base complexes of bis-5-phenylazosalicylaldehyde ethylenediimine and
bis-5-phenylazosalicylaldehyde-O-phenylenediimine ligands with Co(II) (I and II) have
been synthesized and characterized by their IR spectra and elemental analyses. These
complexes catalyze the oxidation of styrene in the presence of dioxygen and excess
pyridine. The effect of the reaction conditions on the oxidation of styrene was studied by
varying solvent, nature and amount of the catalyst and substrate. The catalytic behavior
of the studied complexes was shown to be dependent on the conditions applied. In all
reactions, acetophenone and 1- phenylethanol were the only observed products.
Keywords: Cobalt, Schiff base complexes, Salicylidene, Styrene, Oxidation
Introduction
The ability of certain cobalt (II) complexes to reversibly bind dioxygen was discovered many
decades ago [1,2]. Since then, many cobalt(II) dioxygen carriers have been discovered [3,4]. Some of
them have properties which make them good candidates for industrial and/or medicinal applications.
Oxidations of organic molecules with dioxygen transition metal complexes have been extensively
studied in view of the potential for exploitation of new synthetic methodologies and as an aid to the

Molecules 2005, 10
303
understanding of biological oxidation mechanisms [5,6]. On the other hand, controlled oxidation of
organic compounds with molecular oxygen is a hotly pursued approach for the development of
environmentally benign and ecologically sustainable chemical processes. These processes are
significant in the preparation of organic reagents [7], bleaching of paper pulp [8] or textiles [9],
chemicals raw materials industries [10] and environmental catalysis. We describe here the synthesis
and characterization of some complexes of cobalt (II) with tetracoordinate Schiff–base ligands (I and
II, Figure 1). We also report the oxidation of styrene by molecular oxygen in the presence of excess
aromatic base to the corresponding 2–ketone and 2–alcohol using the complexes I and II as catalysts.
Figure 1
X
N
O
N
O
Co
N
N
N
N
X= - CH CH - , - C H -
2
2
6
4
I
II
Results and Discussion
Synthesis
All ligands and complexes were obtained in good yield and purity. Cobalt complexes were
characterized by elemental analysis and IR spectroscopy. All characterization data for the complexes is
given in Table 1.
Table 1 Characterization data for complexes I and II
Complex
Yield (%)
I
90
II
89
-1
IR data(cm )
3040
υ
C-H(aromatic)
3050
-
υ
C-H(aliphatic)
2920
υ
υ
υ
C=N
C=C
N=N
1600
1590
1525
1600
1585
1520
Analyses, found (calc.)
62.81(63.04)
3.75(4.10)
15.48(15.75)
C
H
N
65.73(66.09)
3.59(3.78)
14.30(14.45)

Molecules 2005, 10
304
–1
In the IR spectra of the complexes υC=N was shifted to a lower wavenumber by ca. 10-40 cm upon
coordination. In addition, the disappearance of the OH band of the free ligands in the cobalt (II)
-
complexes indicates that the OH group has become deprotonated and bonded to the metal ion as –O .
On the basis of these results it can be concluded that in the complexes I and II, the Schiff–bases are
coordinated to the cobalt atom as tetradentate ONNO ligands. The coordination spheres of our
complexes are similar to those of cobalt salen–type complexes that have been reported to be square–
planar [11-13], therefore we suggest that the title complexes I and II also possess similar square–
planar structures.
Catalytic Properties
The catalytic properties of complexes I and II were investigated in the oxidation reaction of styrene
in the presence of pyridine as axial base under different experimental conditions. The only products
formed were acetophenone and 1-phenylethanol. The salen–type complexes of cobalt(II) have been
known for a long time as dioxygen activators [10,14-18].
Four–coordinate square–planar cobalt salen–type complexes need an added base, which binds to the
axial position of the coordination sphere of the central metal [10,19,20]. The trans axial ligand
enhances the oxygen binding capability of the square–planar complexes and stabilizes the formed
dioxygen adducts [19]. The most commonly utilized axial base in cobalt salen–type catalytic oxidation
systems is pyridine [10,20]. Co(salen) and similar compounds react reversibly with oxygen to give an
equilibrium mixture of a superoxo complex (Co:O
2
ratio 1:1 ) and a dimeric peroxo complex (Co:O
2
ratio 2:1) [10,14,21]. The formation of these species (superoxo or µ-peroxo) depends on the reaction
conditions [17,22].
Many of the observed effects of reaction conditions can be explained by the altered equilibrium of
these two species. The formation of superoxo complex is favored in the most polar solvents [19]. On
the other hand, high temperatures favor the peroxo– bridged species, while low temperatures [for
example (-70)˚C-25˚C] favor 1:1 dioxygen adduct formation, which can be explained by the different
thermal parameters for 1:1 and 2:1 adduct formation [23-26]. Therefore it can be suggested that our
complexes also, with respect to the temperature (70°C), act as µ-peroxo dimers.
Several reaction conditions were varied to study their effect on the oxidation activity of the
catalytic system. These variables included the added solvent, axial base, nature and concentration of
catalyst and substrate. The different experimental conditions used, % yields of the products and
styrene conversions to these products, turnover numbers of the reactions and the [acetophenone]/[1-
phenylethanol] ratios (which are representative of reaction selectivity) after 8 hours are summarized in
Table 2 .

Molecules 2005, 10
Table 2. The % of the products and styrene conversion to the products, turnover number
305
of reactions and [acetophenone]/[1- phenylethanol] ratio after 8h under different
experimental conditions.
Styrene
Products(%)
Experiment
No.
Substrate
(ml )
Turnover
number
[ketone] /
[alcohol]
Catalyst
Solvent
conversion
Ketone
Alcohol
(%)
1
2
3
4
5
6
7
8
9
II(0.25g)
II(0.25g)
II(0.25g)
II(0.25g)
II(0.5g)
II(0.25g)
II(0. 5g)
I(0.25g)
I(0.25g)
I(0.25g)
I(0.25g)
I(0.5g)
3
3
3
6
3
6
3
3
3
3
6
3
EtOH
81.78
93.72
61.40
84.90
100.00
54.12
100.00
84.13
83.10
86.82
80.79
82.00
18.22
6.28
19.75
13.54
2.53
5.73
5.80
4.18
2.69
10.42
2.66
12.14
1.07
2.92
11.94
8.19
1.53
5.12
1.75
5.06
0.81
5.78
1.47
6.73
1.19
0.81
4.49
14.93
1.78
5.63
∞
1-PrOH
2-PrOH
1-PrOH
1-PrOH
EtOH
35.90
15.10
__
45.88
__
1.18
∞
EtOH
EtOH
15.87
16.90
13.18
19.21
18.00
5.30
5.00
6.59
4.58
4.55
1-PrOH
2-PrOH
2-PrOH
2-PrOH
1
1
1
0
1
2
P=1atm, T=70°C, substrate=styrene, internal standard=2-octanone, alcohol = 1-phenylethanol,
ketone = acetophenone, turnover numbers of reactions are calculated as the number of moles of
substrate oxidized by 1 mol of the catalyst. The axial base is pyridine.
The effect of the solvent
In Figures 1 and 2 functional changes of the [acetophenone]/[1- phenylethanol] ratios in terms of
the time for experiments 1, 2, 3 and 8, 9, 10 are shown for complexes II and I, respectively.
Figure 1. Variation of the [acetophenone]/[1-phenylethanol] ratio vs.
time for experiments 1, 2 and 3

Molecules 2005, 10
306
Figure 2. Variation of the [acetophenone]/[1-phenylethanol] ratio vs.
time for experiments 8, 9 and 10
When the [acetophenone] /[1-phenylethanol] ratio is ∞ this means that we have only one product
acetophenone) and the reaction is completely selective. The functional changes of the 1-
(
phenylethanol and acetophenone concentration in terms of the time for experiments 1, 2, 3 and 8, 9, 10
are shown in Figures 3, 4, 5 and 6 respectively. From Figures 1 and 2 it can be concluded that complex
II in 1-propanol has more selectivity than in the other two solvents. But in the case of complex I, in
1
-propanol the reaction is completely selective for the first 4 hours. The yields of 1-phenylethanol and
acetophenone production in the presence of complex II as catalyst in ethanol are greater than the
yields of the reaction in the other solvents (Figures 3 and 4) and for complex I, in 1-propanol the
yields are less than in the other two solvents (Figures 5 and 6) after 8 hours.
Figure 3. Variation of the 1-phenylethanol concentration vs. time for
experiments 1, 2 and 3. The concentration unit in axial axis is
4
M×10 .
1
0
8
6
4
2
0
ethanol
-propanol
2-propanol
1
0
2
4
6
8
10
Time(h)

Molecules 2005, 10
307
Figure 4. Variation of [acetophenone] vs. time for experiments 1, 2 and
3.
50
40
30
20
10
0
ethanol
1
2
-propanol
-propanol
0
2
4
6
8
10
Time(h)
Figure 5. Variation of [1-phenylethanol] vs. time for experiments 8, 9
and 10.
5
4
3
ethanol
1
2
-propanol
-propanol
2
1
0
0
2
4
6
8
10
Time(h)
Figure 6. Variation of [acetophenone] vs. time for experiments 8, 9 and
10
30
25
20
15
10
5
0
ethanol
1
2
-propanol
-propanol
0
2
4
6
8
10
Time(h)

Molecules 2005, 10
308
The effect of the catalyst
In experiments 1 and 8, 2 and 9 and 3 and 10 the selectivity of the reactions varies as the complex
is changed. In Figures 7, 8 and 9 the changes of selectivity of the reactions in terms of the time for
these experiments are shown.
Figure 7. Variation of the [acetophenone]/[1-phenylethanol] ratio vs.
time for experiments 1 and 8.
From Figures 7, 8 and 9 it can be concluded that complex I has more selectivity than complex II in
ethanol and 2-propanol but in 1-propanol complex I reacts with completely selectivity up to 4 hours
and then its selectivity decreases and eventually becomes less than that seen with complex II.
Figure 8. Variation of the [acetophenone]/[1- phenylethanol] ratio vs.
time for experiments 2 and 9.
Figure 9. Variation of the [acetophenone] / [1- phenylethanol ] ratio vs.
time for experiments 3 and 10

Molecules 2005, 10
309
The effect of catalyst concentration
For complex II, the changes of selectivity of the reactions in terms of the time in experiments 1
and 7 and 2 and 5 are shown in Figures 10 and 11, respectively. These figures show that the selectivity
of complex II increases with an increase in the catalyst concentration in ethanol and 1-propanol,
whereas the selectivity of complex I in 2-propanol (experiments 10 and 12) decreases when the
catalyst concentration is increased (Figure 12).
Figure 10. Variation of the [acetophenone]/[1-phenylethanol] ratio vs.
time in experiments 1 and 7
Figure 11. Variation of the [acetophenone]/[1-phenylethanol] ratio vs.
time in experiments 2 and 5.
Figure 12. Variation of the [acetophenone]/[1-phenylethanol] ratio vs.
time in experiments 10 and 12

Molecules 2005, 10
310
Conclusions
The Co(II) complexes I and II catalyze the oxidation of styrene in the presence of O
2
and excess
pyridine. In all reactions, acetophenone and 1- phenylethanol were the only observed products. The
catalytic activities were shown to depend on the conditions used, as complex II was more selective in
1
1
-propanol than in other solvents, whereas in the case of complex I, the reaction was 100% selective in
-propanol for the first 4 hours. With complex II the reaction becomes 100% selective upon increasing
the catalyst concentration in ethanol and 1-propanol. On the other hand, in the case of complex I, the
selectivity of the reaction in 2-propanol decreases when the catalyst concentration increases.
Acknowledgements
We thank Professor M.T. Zaaferani, Professor J. Manzouri and Dr A. Shahrisa for their useful
suggestions and Azarbaijan University of Tarbiat Moalem, for GC spectra.
Experimental
General
All reagents and solvents were used as supplied by Merck. Absolute ethanol, anhydrous 1–
propanol and 2–propanol were stored over 3 Å and 4 Å molecular sieves and used without further
purification. Styrene and pyridine were distilled and stored over 4Å molecular sieves. 5–Phenylazo
salicylaldehyde and the ligands were obtained as described elsewhere [27]. Elemental (C, H and N)
analyses were done on a Perkin – Elmer model 240B automatic instrument. Infrared spectra were
recorded on IR 408 Shimadzu 568 instrument. GC spectra were recorded on a Philips model P44410
gas chromatograph equipped with a flame ionization detector, using a stainless steel column packed
with OPEG 20 M % diethylene glycol adipate on Chromosorb P.
Complexes
The complexes were prepared in similar manner. The ligand (0.004 mol) was mixed with
Co(acac)
2
.4H O (0.004 mol) and refluxed in ethanol (100 mL) for 1h. The mixture was then cooled,
2
filtered and resulting complex was washed with ethanol and ether and dried under vacuum at 100˚C.
Oxidation Reactions
The oxidation reactions were carried out in 250 mL flask containing a magnetic stir bar, fitted with
a pressure gauge and placed in a temperature controlled oil bath inside a steel explosion shield. For a
typical oxidation, solvent (25 mL), substrate (3 mL), pyridine (5 mL), 2-octanone (0.28 mL, internal
GC standard) and a plastic cap containing the catalyst (0.25 g) were added to the flask, which was
attached to the pressure head and placed in the oil bath at 70°C (the complexes are insoluble below
this temperature). The system was then purged several times with oxygen. Stirring was begun to
capsize the catalyst-containing cap and initiate the oxidation. The progress of the reactions was
monitored by injecting an aliquot of sample (0.4 – 0.5 µL) into the GC for analysis. The amounts of
products present were calculated from calibration curves based on the 2- octanone standard.

Molecules 2005, 10
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