Cyclohexene oxidation with tert-butylhydroperoxide and hydrogen peroxide catalyzed by new square-planar manganese(II), cobalt(II), nickel(II) and copper(II) bis(2-mercaptoanil)benzil complexes supported on alumina

Article abstract of DOI:10.1016/j.molcata.2007.04.031

New square-planar manganese(II), copper(II), nickel(II) and cobalt(II) complexes of a tetradentate Schiff-base ligand bis(2-mercaptoanil)benzil, H₂[mabenzil] have been prepared and characterized by elemental analyses, IR, UV-vis, conductometric and magnetic measurements. The results suggest that the symmetrical Schiff-base is a bivalent anion with tetradentate N₂S₂ donors derived from the thiophenole groups and azomethine nitrogen. The formulae was found to be [M(mabenzil)] for the 1:1 non-electrolytic complexes. Alumina-supported metal complexes (ASMC; [M(mabenzil)/Al₂O₃]) catalyze the oxidation of cyclohexene with tert-buthylhydroperoxide (TBHP) and H₂O₂.

Full text of DOI:10.1016/j.molcata.2007.04.031

Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
Cyclohexene oxidation with tert-butylhydroperoxide and hydrogen
peroxide catalyzed by new square-planar manganese(II), cobalt(II),
nickel(II) and copper(II) bis(2-mercaptoanil)benzil
complexes supported on alumina
Masoud Salavati-Niasari∗, Hassan Babazadeh-Arani
Department of Chemistry, University of Kashan, Kashan P.O. Box. 87317-51167, Iran
Received 28 March 2007; received in revised form 17 April 2007; accepted 18 April 2007
Available online 25 April 2007
Abstract
New square-planar manganese(II), copper(II), nickel(II) and cobalt(II) complexes of a tetradentate Schiff-base ligand “bis(2-mercaptoanil)benzil,
H2[mabenzil]” have been prepared and characterized by elemental analyses, IR, UV–vis, conductometric and magnetic measurements. The results
suggest that the symmetrical Schiff-base is a bivalent anion with tetradentate N2S2 donors derived from the thiophenole groups and azomethine
nitrogen. The formulae was found to be [M(mabenzil)] for the 1:1 non-electrolytic complexes. Alumina-supported metal complexes (ASMC;
[M(mabenzil)/Al2O3]) catalyze the oxidation of cyclohexene with tert-buthylhydroperoxide (TBHP) and H2O2.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Alumina; Oxidation; Cyclohexene; Schiff-base
1. Introduction
life, various strategies can be employed. The encapsulation in
zeolites [3–10], the grafting on polymers [11–14] or MCM-41
silica [15] and the immobilization in polysiloxane membranes
[16] have been used as supporting methods, with moderate to
excellent results. Application of alumina-supported catalysis in
organic transformation has been receiving attention in recent
years [17–27]. Immobilization of homogeneous transition metal
catalysts to alumina carriers offers several practical benefits
of heterogeneous catalysis, while retaining the advantages of
homogeneous catalytic reactions [17–29]. Some of the attractive
features of alumina-supported catalysis include: (1) easy separa-
tion of the catalysts from the reagents and reaction products; (2)
simplificationofmethodstorecycleexpensivecatalysts;(3)non-
volatile and nontoxic characteristics to high molecular weight
alumina backbones; (4) minimization of certain catalyst deac-
tivation pathways by site isolation. These attractive features of
these catalysts could possibly help in developing high through-
put discovery applications as well as in developing continues
catalytic processes for industrial scale synthesis.
Hydrocarbon oxidation to give oxygen-containing com-
pounds (alcohols, aldehydes, ketones, acids, etc.) is an extremely
important and useful reaction in the chemical industry. Normally
morethanoneoxygenateandallproductsaresusceptibletocom-
plete combustion to give CO2 [1]. The search for new selective
oxidation catalysts is one of the most important current top-
ics, connected with both industrial and academic research. In
this field, the use of homogeneous catalysts has received great
attention in the last few years, given that they have shown to be
useful in the oxidation of olefins [2]. Despite its interest, this
method of oxidation suffers from some drawbacks from a prac-
tical point of view. The low conversion, the catalyst deactivation
by ␮-dimerization through the formation of oxygen bridges,
the high cost of the complex and the lack of recycling meth-
ods make it difficult for application of this system on a large
scale. To avoid these problems, to improve the separation of
the catalyst from the reaction medium and to increase its active
In this paper, we report the synthesis and characteriza-
tion of transition metal (cobalt(II), manganese(II), nickel(II)
and copper(II)) complexes of the Schiff-base ligand; bis
(2-mercaptoanil)benzil, H2[mabenzil]; and these complexes
∗
Corresponding author. Tel.: +98 361 5555333; fax: +98 361 5552930.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.04.031

M. Salavati-Niasari, H. Babazadeh-Arani / Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
59
Scheme 1.
were immobilized on acidic alumina (Scheme 1); [M(mab-
enzil)]/Al2O3; and used in the oxidation of cyclohexene with
tert-butylhydroperoxide and hydrogen peroxide as oxygen
donors. The ␣,␤-unsaturated ketone, which is obtained by allylic
oxidation, is an important intermediate in natural product syn-
thesis; due to the presence of a highly reactive carbonyl group,
it is also used in cycloaddition reactions. We have studied dif-
ferent catalytic reaction mediums, especially water, to make the
process more cost effective and environment-friendly. Modified
alumina has been selected as surface for immobilization due to
its chemical and thermal stability and its strong ability to bind
to the metal complex.
nickel(II) acetate, cobalt(II) acetate, benzil, 2-aminothiophenol,
hydrogen peroxide and tert-butlhydroperoxide (solution 80% in
di-tert-butylperoxide) were obtained from Merck Co. Cyclohex-
ene was distilled under nitrogen and stored over molecular sieves
˚
(4 A). Cyclohexanone was used as an internal standard for the
quantitative analysis of the product using gas chromatography.
Reference samples of cyclohexene oxide, 2-cyclohexene-1-ol
and 2-cyclohexene-1-one (Aldrich) were distilled and stored in
the refrigerator.
The stability of the supported catalyst was checked after the
reaction by UV–vis and possible leaching of the complex was
investigated by UV–vis in the reaction solution after filtration
of the alumina. Magnetic moments were calculated from mag-
neticsusceptibilitydataobtainedusingaJohnsonMattheyMK-1
magnetic susceptibility balance and conductance measurements
with a Metrohm Herisau conductometer E 518. Chlorine was
determined gravimetrically. H NMR (400 MHz) spectra were
measured in CDCl3 solutions and referenced to the solvent
signals. XRD patterns were recorded by a Rigaku D-max C
III, X-ray diffractometer using Ni-filtered Cu K␣ radiation.
Elemental analyses were obtained from Carlo ERBA Model
EA 1108 analyzer. The transition metal contents of the sam-
ples were measured by Atomic Absorption Spectrophotometer
(AAS-Perkin-Elmer 4100–1319) using a flame approach. The
products were analyzed by GC–MS, using a Philips Pu 4400
2. Experimental
2.1. Materials and physical measurements
Acidic alumina was purchased from Merck (Art. No. 1078,
aluminum oxide 90 active acidic, 0.063–0.200 mm). It was
activated at 500 ◦C for 8 h before use. The alumina-supported
metal(II) chloride (MCl2/Al2O3; M = Mn(II), Co(II), Ni(II),
Cu(II)) was prepared according to the procedure described pre-
viously [13]. All the solvents were purchased from Merck (pro
analysi) and were distilled and dried using molecular sieves
˚
(Linde 4 A) [30]. Manganese(II) acetate, copper(II) acetate,

60
M. Salavati-Niasari, H. Babazadeh-Arani / Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
Chromatograph (capillary column: DB5MS, 30 m), Varian 3400
Chromatograph (15 m capillary column of HP-5; FID) coupled
with a QP Finnegan MAT INCOF 50, 70 eV. Diffuse reflectance
spectra (DRS) was registered on a Shimadzu UV/3101 PC
spectrophotometer the range 1500–200 nm, using MgO as
reference.
Tetradentate Schiff-base ligand (bis(2-mercaptoanil)benzil,
H2[mabenzil]) was prepared by following the procedures
reported in Ref. [31]. 2-Aminothiophenol (2.50 g, 0.02 mol) was
dissolved in 75 ml ethanol, and a solution of benzil (2.10 g,
0.01 mol) in 25 ml ethanol was added to it. The mixture was
refluxed on a water bath for 10 h. After reducting the volume of
the solution to ca. 50 ml, the flask was kept at ambient tempera-
ture for 4 h. On cooling the white-yellow crystalline Schiff-base
ligand was collected by filtration, washed with ethanol twice
(2 × 20 ml) and dried. Finally the ligand was recrystallized from
ethanol to give pure crystals, yield 60%.
2.5. Preparation of alumina-supported metal complexes
(ASMC)
A solution of the [M(mabenzil)] (2.0 g) in CHCl3 was added
to a suspension of alumina (10.0 g) in CHCl3 and stirred at
50 ◦C under Ar atmosphere. The solid was filtered, washed with
CHCl3. The [M(mabenzil)]/Al2O3 catalyst was dried at 40 ◦C
under vacuum overnight prior to use.
2.6. Heterogeneous oxidation of cyclohexene
A mixture of 1.02 × 10−5 mol catalyst, 25 ml solvent and
10 mmol cyclohexene was stirred under nitrogen atmosphere
in a 50 ml round-bottom flask equipped with a condenser and a
dropping funnel at room temperature for 30 min. Then 16 mmol
of tert-buthylhydroperoxide (TBHP) (solution 80% in di-tert-
butylperoxide) or hydrogen peroxide (30% in water) was added.
The resulting mixture was then refluxed for 8 h under N2 atmo-
sphere. After filtration and washing with solvent, the filtrate was
concentrated and then subjected to GC analysis. The concentra-
tion of products was determined using cyclohexanone as internal
standard.
2.2. Preparation of [Mn(mabenzil)]
Bis(2-mercaptoanil)benzil (4.67 g, 0.011 mol) was dissolved
in 100 ml of refluxing ethanol and a stream of nitrogen was
purged for 4 h to eliminate the oxygen. A solution containing
2.69 g(0.011 mol)ofmanganese(II)acetatetetrahydrateinwater
was added dropwise to the deoxygenated ligand solution. The
resulting mixture was agitated and refluxed under nitrogen with
5 ml of ethanol followed by 5 ml of water. The mixture was then
cooled and filtered under reduced pressure. The collected solid
was washed with diethyl ether and dried in air to give yellow
crystalline [Mn(mabenzil)] which purified by recrystallization
from chloroform (yield: 51%).
2.7. Homogeneous oxidation of cyclohexene
Neat metal complex (1.02 × 10−5 mol) in dichloromethane
(10 ml), TBHP (2 ml) was added to a solution of cyclohexene
(1 ml). The resulting mixture was then refluxed for 8 h under N2
atmosphere, the solvent was evaporated under reduced pressure
and the crude analyzed by GC and GC–MS. The concentrations
of products were determined using cyclohexanone as internal
standard.
2.3. Preparation of [Co(mabenzil)]
3. Results and discussion
The flask containing a stirred suspension of cobalt(II) acetate
tetrahydrate (4.0 g, 0.016 mol) in propanol (100 ml) was purged
with nitrogen, and then warmed to 50 ◦C under a nitrogen atmo-
sphere. Bis(2-mercaptoanil)benzil (6.79, 0.016 mol) was added
in one portion, and the resulting black suspension was then
stirred and reflux under nitrogen atmosphere for 8 h. Then the
mixture was cooled and filtered under reduced pressure. The
collected solid was washed with diethyl ether and dried in air
to give black crystalline [Co(mabenzil)] which was purified by
recrystallization from chloroform (yield: 54%).
3.1. Synthesis and characterization
Synthesis of the metal complexes was essentially the same
and involved heating and stirring of stoichiometric amounts of
H2[mabenzil] and metal acetate in ethanol. Elemental analysis
indicates that all of the complexes are formed by coordination
of 1 mol of the metal ion and 1 mol [mabenzil]. All of the metal
chelates in this study are insoluble in water but soluble in most
organic solvents. Electrical conductivity measurements of the
metal complexes give ΛM values of 12–25 ꢀ−1 cm2 mol−1 and
confirm that they are non-electrolytes.
The metal content of the ASMC catalysts was estimated
by dissolving the known amounts of the heterogeneous cata-
lyst in concentrated HCl and from these solutions, transition
metal contents were estimated by atomic absorption spectrom-
eter. The metal content of the different catalysts synthesized
was almost the same in all the supported system and was
0.012 mol g−1. The chemical composition confirmed the purity
and stoichiometry of the neat and alumina-supported com-
plexes. The chemical analysis of the samples reveals the
presence of organic matter with a C/N ratio roughly similar to
neat complexes. In the case of the immobilized catalyst, the
2.4. Preparation of [Ni(mabenzil)], [Cu(mabenzil)] and
[Zn(mabenzil)]
[Ni(mabenzil)] (yield: 57%), [Cu(mabenzil)] (yield: 59%)
and [Zn(mabenzil)] (yield: 42%) were prepared similarly.
H2[mabenzil] (5.94 g, 0.014 mol) was dissolved in 100 ml of
ethanol, and the solution was refluxed. Metal acetate (0.014 mol)
dissolved in 100 ml of ethanol was added to this hot solution
and refluxing continued for 8 h. Upon cooling the solution, a
solid crystalline was obtained which was filtered, washed with
ethanol, and dried in vacuum and purified by recrystallization
from chloroform.

M. Salavati-Niasari, H. Babazadeh-Arani / Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
61
ratios of carbon to metal and carbon to nitrogen have been
provided to ensure the immobilization of complex on the alu-
mina.
The IRbands of all catalystsareweak dueto theirlowconcen-
tration of the complex on alumina. The IR spectra of supported
complexes are essentially similar to that of the neat complexes.
The adsorbing tendency of the acidic alumina might arise from
the presence of oxygen groups on the surface in order to coor-
dinate to the metal ion center (Scheme 1). Electronic spectral
data for the neat complexes have been measured in chloroform
(Table 1). The square-planar geometry of the [M(mabenzil)]
is strongly indicated by similarities in the visible spectra of
this chelate with known square-planar complexes containing
sulfur–nitrogen donor atoms [31–41].
The X-ray diffractograms of Al2O3, NiCl2/Al2O3 and
[Ni(mabenzil)]/Al2O3 were recorded to study their crystallinity
and to ensure supporting. After careful comparison of XRD pat-
ternsofAl2O3 andNiCl2/Al2O3, itwasobservedthatthereisone
˚
new peak with a d value of 17.29 A in NiCl2/Al2O3. This peak
was also observed in [Ni(mabenzil)]/Al2O3 and [Ni(mabenzil)]
at the same position. In addition, the [Ni(mabenzil)]/Al2O3
˚
exhibits one new signal with value of 82.68 A, which is a part
of the ligand as this signal was also observed in [Ni(mabenzil)]
but not observed in Al2O3 or NiCl2/Al2O3. This information
clearly indicates the support of [Ni(mabenzil)] on alumina.
Very low intensity of other peaks made it difficult to distin-
guish them from the other peaks in the XRD pattern of the
ASMC.
1
Comparison of H NMR spectral data of the H2[mabenzil]
and the neat diamagnetic [Zn(mabenzil)] and [Ni(mabenzil)])
complexes recorded in DMSO-d6 further supplements the
conclusion drawn from IR data. The 1H NMR spectrum of
H2[mabenzil] exhibits the following signals: 12.70 (s, 2H, SH),
6.75–7.10 (m, 10H, C6H5) and 7.20–7.35 (m, 8H, C6H4).
The disappearance of thiophenolic signal and down field shift
(7.70 ppm) of C6H4 signal indicates of the coordination of
thiophenole groups after deportation and azomethine nitrogen
atoms. 1H NMR spectrum of other neat complexes could not be
recorded due to its partial paramagnetic nature of complex as
noticed earlier [33,35].
3.2. Catalytic activity
The selectivity and activity results of alumina-supported
and homogeneous catalysts on the oxidation of cyclohex-
ene with TBHP have been given in Table 2. Comparing
between neat and ASMC as catalyst evidence that neat catalysts
gave higher conversion of cyclohexene than their correspond-
ing alumina-supported complexes. The effect of transition
metal complexes supported on alumina was studied on the
oxidation of cyclohexene with TBHP in dichloromethane
and the results have been shown in Table 2. As shown
in Table 2, only allylic oxidation has occurred with the
formation of 2-cyclohexene-1-one, 2-cyclohexene-1-ol and 1-
(tert-butylperoxy)-2-cyclohexene. Oxidation with the same
oxidant in the presence of MnCl2/Al2O3 was 29.4% [42].
The increase of conversion from 29.4 to 87.6% compared to

62
M. Salavati-Niasari, H. Babazadeh-Arani / Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
Table 2
Effect of various solvents, catalyst weight, percent selectivity of 2-cyclohexene-1-one, 2-cyclohexene-1-ol and 1-(tert-butylperoxy)-2-cyclohexene formation and
percent conversion of cyclohexene oxidation (solvent = 10 ml; duration = 8 h, at reflux; cyclohexene = 1 ml, TBHP = 2 ml)
Entry
Catalyst
Amount of catalyst (mol)
Solvent
Conversion (%)
Selectivity (%)
Ketonea
Alcoholb
Peroxidec
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
[Mn(mabenzil)]
[Mn(mabenzil)]
[Mn(mabenzil)]
[Mn(mabenzil)]
[Co(mabenzil)]
[Ni(mabenzil)]
[Cu(mabenzil)]
1.02 × 10−5
0.50 × 10−5
2.04 × 10−5
4.08 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
0.50 × 10−5
2.04 × 10−5
4.08 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
1.02 × 10−5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3Cl
92.8
67.5
86.3
40.7
83.5
52.7
63.9
87.6
85.3
84.1
83.5
63.5
89.4
91.3
76.4
45.7
57.3
82.6
74.5
45.7
56.4
75.6
70.2
40.4
49.9
53.4
48.1
25.2
27.4
63.2
58.5
66.7
69.1
53.6
36.1
40.2
85.3
84.5
81.7
80.3
71.7
87.1
87.6
78.3
68.5
72.6
81.3
76.2
65.6
69.5
71.8
66.4
57.5
58.3
68.5
62.3
54.1
53.2
19.1
24.2
20.6
19.7
26.5
37.6
28.9
10.5
11.6
13.0
14.6
22.9
9.6
10.6
18.0
23.1
18.2
13.4
19.2
24.7
20.1
17.5
24.0
28.2
24.5
19.3
26.0
29.2
28.3
17.7
17.3
12.7
11.2
19.9
26.3
30.9
4.2
3.9
5.3
5.1
5.4
3.3
1.8
3.7
8.4
9.2
5.3
4.6
9.7
10.4
10.7
9.6
14.3
17.2
12.2
11.7
16.7
18.5
[Mn(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Co(mabenzil)]/Al2O3
[Ni(mabenzil)]/Al2O3
[Cu(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Co(mabenzil)]/Al2O3
[Ni(mabenzil)]/Al2O3
[Cu(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Co(mabenzil)]/Al2O3
[Ni(mabenzil)]/Al2O3
[Cu(mabenzil)]/Al2O3
[Mn(mabenzil)]/Al2O3
[Co(mabenzil)]/Al2O3
[Ni(mabenzil)]/Al2O3
[Cu(mabenzil)]/Al2O3
d
e
f
CH3Cl
CH3Cl
CH3Cl
CH3OH
CH3OH
CH3OH
CH3OH
CH3CN
CH3CN
CH3CN
CH3CN
a
2-cyclohexene-1-one.
b
c
d
e
f
2-cyclohexene-1-ol.
1-(tert-butylperoxy)-2-cyclohexene.
First reuse.
Second reuse.
Third reuse.
MnCl2/Al2O3 with [Mn(mabenzil)]/Al2O3 indicates that the
existence of ligand has increased the activity of the catalyst by a
factor of 2.98. From the indicated results in Table 2 it is evident
that cyclohexene-2-one is selectively formed in the presence of
all catalysts.
destructive oxidation of alkenes via epoxidation pathway with
H2O2 under the catalytic effect of alumina-supported Mn(II)
complexes seem interesting [47–52]. Although the two systems
are alike, it is the oxidant structure that has changed the fate of
the reaction.
The trend observed in Table 2 can be explained by the
donor ability of ligand available in the complex catalysts. As
Wang et al. have pointed out recently the key point in the
conversion of cyclohexene to the products is the reduction of
L-Mn3+ to L-Mn2+. This reduction to L-Mn2+ is facilitated
with the ligands available around the metal cation [43]. The
formation of the allylic oxidation products 2-cyclohexene-1-
one and 2-cyclohexene-1-ol shows the preferential attack of the
activated C-H bond over the C C bond. The formation of 1-
(tret-butylperoxy)-2-cyclohexene shows the presence of radical
reactions [44]. TBHP as oxidant promotes the allylic oxida-
tion pathway and epoxidation is minimized, especially under the
highly acidic properties of alumina-supported with divalent and
trivalent transition metal ions and complexes, has been observed
by us and others [45,47–52]. It should be emphasized that the
When the oxidant was changed to hydrogen peroxide (Fig. 1),
the oxidation occurred on the double bond and cyclohexene
epoxide obtained as the sole product. It seems that the diol
resulted from the epoxide ring opening under the aqueous acidic
conditions. AlthoughbothH2O2 andTBHPoxidizecyclohexene
in the presence of alumina-supported metal complexes, but only
H2O2 and not TBHP gave epoxidation of cyclohexene under
the similar conditions leads us to conclude that the two types of
reactions do not occur via a common intermediate. As Valen-
tine and co-workers have pointed out one possible explanation
is that the species responsible for the cyclohexene oxidation are
the products formed from cleavage of the O–O bond, whereas,
the epoxidation reaction occurs by a direct reaction of olefin
with coordinated HOO radical. Since the O–O bond of HOOH
is 5 kcal/mol stronger than TBHP, an HOO complex is expected

M. Salavati-Niasari, H. Babazadeh-Arani / Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
63
Fig. 2. Oxidation of cyclohexene with TBHP in various solvent with
[Mn(mabenzil)]-Al2O3.
At the end of the heterogeneous reaction, the catalyst was
separated by filtration, thoroughly washed with solvent and
reused under similar conditions. Although the analysis of
the recovered catalysts by AAS showed no reduction in the
amount of transition metal ions, they showed a slightly lower
catalytic activity (∼2%) (Table 2). The results indicate that
[Mn(mabenzil)]/Al2O3 are almost stable to be recycled for
the oxidation of cyclohexene without much loss in activity
(Table2). Thus, thealumina-supportedmetalcomplexesisfound
to increase the life of the catalyst by reducing dimerization due
to the site isolation and restriction of internal framework struc-
ture. IR spectrum of the recycled sample is quite similar to that
of fresh sample indicating little changes in the coordination of
mabenzil after the oxidation reactions.
The results clearly suggest that [Mn(mabenzil)]/Al2O3 effi-
ciently catalyzes conversion of cyclohexene to 2-cyclohexene-
1-one with 85.3% selectivity and conversion 87.6%. The more
activity of mabenzil system has clearly arisen from the exis-
tence of electron donating ligand which facilitates the electron
transfer rate, a process that has previously observed by us in
other oxidation reactions. All conversion efficiency with high
selectivity obtained in this study is significantly higher than that
obtained using manganese(II) complexes supported on alumina
(Table 3).
Fig.
1. Oxidation
products
distribution
in
acetonitrile
with
[M(mabenzil)]/Al2O3/H2O2.
to have a higher activation energy for O–O bond cleavage than
a TBOO complex and therefore, to have a longer lifetime [46].
The effect of various solvents for the oxidation of cyclo-
hexene with [Mn(mabenzil)]/Al2O3 catalysts was also studied
(Fig. 2). The oxidation reactions were carried out in protic
and aprotic solvents. The results have been given in Table 2
and Fig. 2. In all the oxidation reaction, 2-cyclohexene-1-
one was formed as the major product. When the reaction
was carried out in a coordinating solvent like CH3CN the
conversion decreased by a factor of ∼1.56 (Table 2). This
might be attributed to donor number of acetonitrile (14.1)
and therefore, its higher ability to occupy the vacant spaces
around the metal center and prevent the approaching of oxi-
dant molecules. In dichloromethane and chloroform the yields of
2-cyclohexene-1-o1 and 2-cyclohexene-1-one were higher and
lower yield of the peroxy species were obtained as compared
to the other solvents. The efficiency of the catalysts for oxida-
tion of cyclohexene in different solvents decreases in the order:
dichloromethane > chloroform > methanol > acetonitrile.
Table 3
Oxidation of cyclohexene catalyzed by alumina-supported metal complexes in CH2Cl2 with TBHP
Catalyst
Conversion (%)
Selectivity (%)
Ketonea
Ref.
Alcoholb
Peroxidec
MnCl2/Al2O3
[Mn(acac)2]/Al2O3
[Mn(en)2]/Al2O3
[Mn(salen)]/Al2O3
[Mn(bpy)2]/Al2O3
[Mn(haacac)]/Al2O3
[Mn(Me2salpnMe2)]/Al2O3
[Mn(salpnMe2)]/Al2O3
[Mn(habenzil)]/Al2O3
[M(mabenzil)/Al2O3
29.4
44.3
52.1
65.8
73.4
76.8
82.8
78.4
84.5
87.6
52.3
48.6
56.1
71.6
75.4
74.5
80.5
77.2
83.2
85.3
39.6
28.2
26.5
18.6
17.4
16.6
14.2
16.5
13.1
10.5
8.1
23.2
17.4
9.8
7.2
8.9
5.3
6.3
3.7
4.2
[42]
[42]
[42]
[42]
[42]
[51]
[49]
[53]
[52]
This work
acac = acetylacetonato. en = ethylenediamine. salen = N,N-bis(salicylidene)ethylene-1,2-diamine. bpy = 2,2-bipyridine. haacac = bis(2-hydroxyanil)acetylacetone.
Me2salpnMe2 = N,Nꢀ-bis-(␣-methylsalicylidene)-2,2-dimethylpropane-1,3-diamine. salpnMe2 = N,Nꢀ-bis(salicylidene)-2,2-dimethylpropane-1,3-diamine. haben-
zil = bis(2-hydroxyanil)benzil.
a
2-cyclohexene-1-one.
2-cyclohexene-1-ol.
1-(tert-butylperoxy)-2-cyclohexene.
b
c

64
M. Salavati-Niasari, H. Babazadeh-Arani / Journal of Molecular Catalysis A: Chemical 274 (2007) 58–64
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In this study, we have used a rather simple catalysis sys-
tem of alumina-supported manganese(II), cobalt(II), nickel(II)
and copper(II) complexes with a Schiff-base ligand “bis(2-mer-
captoanil)benzil, H2[mabenzil]” in the oxidation of cyclohex-
ene. Oxidation of allylic site and double bond were resulted
with the oxidants of TBHP and H2O2, respectively. Oxidation
of cyclohexene with TBHP gave 2-cyclohexene-1-one, 2-cyclo-
hexene-2-ol and 1-(tert-butylperoxy)-2-cyclohexene whereas,
oxidation with H2O2 resulted in the formation of cyclohexene
oxide and cyclohexene-1,2-diol. The high percentage yield
of reactions especially in the presence of alumina-supported
manganese(II) complex seems promising. The prepared cata-
lysts are also highly stable and reusable, as varied by the un-
changed activity within three successive reaction runs and no
further increase in the cyclohexene conversion with increas-
ing reaction time after removing the catalyst. The activity of
cyclohexene oxidation decreases in the series [Mn
(mabenzil)]/Al2O3 > [Co(mabenzil)]/Al2O3 > [Cu(mabenzil)]/
Al2O3 > [Ni(mabenzil)]/Al2O3. The efficiency of the catalysts
for cyclohexene oxidation in different solvents decreases in
the order: CH2Cl2 > CH3Cl > MeOH > MeCN. Studies on other
olefins are currently under investigation in our laboratory and
we believe that the observation of similar results on other
olefins is not unexpected.
´
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Authors are grateful to Council of University of Kashan for
providing financial support to undertake this work.
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