Synthesis, characterization and catalytic oxyfunctionalization of cyclohexene with tert-butylhydroperoxide and hydrogen peroxide in the presence of alumina-supported Mn(II), Co(II), Ni(II) and Cu(II) bis(2-hydroxyanil)benzil complexes

Article abstract of DOI:10.1016/j.molcata.2006.12.009

New square-planar Mn(II), Cu(II), Ni(II) and Co(II) complexes of a tetradentate Schiff-base ligand bis(2-hydroxyanil)benzil, H₂[habenzil]; 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 N₂O₂ donors derived from the phenolic oxygen and azomethine nitrogen. The formulae was found to be [M(habenzil)] for the 1:1 non-electrolytic complexes. Alumina-supported metal complexes (ASMC; [M(habenzil)/Al₂O₃]) catalyze the oxidation of cyclohexene with tert-buthylhydroperoxide (TBHP) and H₂O₂. Oxidation of cyclohexene with TBHP gave 2-cyclohexene-1-one, 2-cyclohexene-1-ol and 1-(tert-butylperoxy)-2-cyclohexene whereas, oxidation with H₂O₂ resulted in the formation of cyclohexene oxide and cyclohexene-1,2-diol. Manganese(II) complex supported on alumina [Mn(habenzil)]/Al₂O₃ shows significantly higher catalytic activity than other catalysts. The activity of the immobilized catalyst remains nearly the same after three cycles, suggesting the true heterogeneous nature of the catalyst.

Full text of DOI:10.1016/j.molcata.2006.12.009

Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
Synthesis, characterization and catalytic oxyfunctionalization of
cyclohexene with tert-butylhydroperoxide and hydrogen peroxide in the
presence of alumina-supported Mn(II), Co(II), Ni(II) and Cu(II)
bis(2-hydroxyanil)benzil complexes
Masoud Salavati-Niasaria,∗, Seyed Nezamoddin Mirsattarib
a Department of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
b Department of Chemistry, Science and Research Campus, Islamic Azad University, P.O. Box 14515-775, Tehran, Iran
Received 19 November 2006; received in revised form 5 December 2006; accepted 6 December 2006
Available online 10 December 2006
Abstract
New square-planar Mn(II), Cu(II), Ni(II) and Co(II) complexes of a tetradentate Schiff-base ligand “bis(2-hydroxyanil)benzil, H2[habenzil];
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 N2O2 donors derived from the phenolic oxygen and azomethine nitrogen. The formulae
was found to be [M(habenzil)] for the 1:1 non-electrolytic complexes. Alumina-supported metal complexes (ASMC; [M(habenzil)/Al2O3]) catalyze
the oxidation of cyclohexene with tert-buthylhydroperoxide (TBHP) and H2O2. Oxidation of cyclohexene with TBHP gave 2-cyclohexene-1-one,
2-cyclohexene-1-ol and 1-(tert-butylperoxy)-2-cyclohexene whereas, oxidation with H2O2 resulted in the formation of cyclohexene oxide and
cyclohexene-1,2-diol. Manganese(II) complex supported on alumina “[Mn(habenzil)]/Al2O3” shows significantly higher catalytic activity than
other catalysts. The activity of the immobilized catalyst remains nearly the same after three cycles, suggesting the true heterogeneous nature of the
catalyst.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Alumina; THBP; H2O2; Oxidation; Cyclohexene; Schiff-base
1. Introduction
process, easier separation of products from reactants and recy-
cling reagents or catalysts and thus eliminating the requirement
for laborious and inefficient extraction processes. One of the
major contributors to waste in a chemical process is the separa-
tion of product or catalyst from the reaction mixture. At the same
time during the extraction, the catalyst often gets destroyed.
Thus, elimination of these steps is highly desirable and this is
one of the major goals of green chemistry [11].
We have recently reported the activation of C–H bond
with TBHP and H2O2 in the presence of exchanged zeolite
NaY with transition metal elements [12c], alumina-supported
and zeolite-encapsulation of metal complexes [12,13]. We
showed that some complexes of Mn(II) included in zeolite
Y, catalyzed the oxygen transfer from TBHP to cyclohex-
ene and concluded that such simple systems mimic the
behavior of cytochrome P-450 type oxidation systems [12c].
We also showed the [Mn(haacac)]/Al2O3; (haacac = bis(2-
hydroxyanil)acetylacetone), catalyzed cyclohexene oxidation
The catalytic oxyfunctionalization of hydrocarbons, the main
crude oil and natural gas constituents, into value-added oxy-
genated derivatives such as alcohols, aldehydes, ketones or
carboxylic acids, using peroxides as oxidants have been exten-
sively studied over the last few decades, primarily because
such products are important intermediates in many industrial
processes [1–9]. More environment-friendly processes for the
oxidative transformation of organics have gained considerable
momentum [10]. The main area of concern is the large vol-
ume of effluents produced by a variety of chemical processes.
Improvement can be made in several ways, such as using alter-
native reagents and catalysts, increasing the efficiency of the
∗
Corresponding author. Tel.: +98 361 555 333; fax: +98 361 555 29 30.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.12.009

M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
51
Scheme 1.
with the highest reactivity and selectivity and 2-cyclohexene-
2. Experimental
1-one was formed as the main product [13g], and we showed
a simple catalyst system of alumina-supported Mn(II) com-
plexes with a number of bidentate ligands of N,N; N,O and
O,O donor atoms in the oxidation of cyclohexene [13]. Since,
alumina-supported metal systems exhibit catalytic activity in a
wide ranging of the industrially important processes and have
been extensively studied, we decided to investigate the effect
of transition metal complexes with a tetradentate Schiff-base
ligand; bis(2-hydroxyanil)benzil, H2[habenzil]; supported on
acidic alumina in the oxidation of cyclohexene with TBHP. In
this paper, we report the synthesis and characterization of transi-
tion metal (cobalt(II), manganese(II), nickel(II) and copper(II))
complexes of the Schiff-base ligand; bis(2-hydroxyanil)benzil,
H2[habenzil]; and these complexes were immobilized on acidic
alumina (Scheme 1); [M(habenzil)]/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 synthesis; due to the presence
of a highly reactive carbonyl group, it is also used in cycload-
dition reactions. We have studied different 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.
2.1. Materials
All the solvents were purchased from Merck (pro analysi)
and were distilled and dried using molecular sieves (Linde
˚
4 A) [14a]. Manganese(II) acetate, copper(II) acetate, nickel(II)
acetate, cobalt(II) acetate, benzil, 2-aminophenol, hydrogen
peroxide and tert-butlhydroperoxide (solution 80% in di-tert-
butylperoxide) were obtained from Merck Co. Cyclohexene was
˚
distilled under nitrogen and stored over molecular sieves (4 A).
Cyclohexanone was used as an internal standard for the quantita-
tiveanalysisoftheproductusinggaschromatography. Reference
samples of cyclohexene oxide, 2-cyclohexene-1-ol and 2-
cyclohexene-1-one (Aldrich) were distilled and stored in the
refrigerator. 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 [13d].
2.2. Physical measurements
XRD patterns were recorded by a Rigaku D-max C III,
X-ray diffractometer using Ni-filtered Cu K␣ radiation. Ele-
mental analyses were obtained from Carlo ERBA Model EA

52
M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
1108 analyzer. The transition metal contents of the samples
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 Chro-
matograph (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 spec-
trophotometer the range 1500–200 nm, using MgO as reference.
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 CD3NO2 solutions and referenced to the solvent
signals.
sphere. Bis(2-hydroxyanil)benzil (6.28, 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(habenzil)] which was purified by
recrystallization from chloroform (yield: 54%). Anal. calcd. for
C26H18N2O2Co: C, 69.52; H, 4.01; N, 6.23; Co, 13.12; C/N,
11.16; C/Co, 5.30. Found: C, 69.30; H, 3.88; N, 6.34; Co, 13.02;
C/N, 10.93;C/Co, 5.32%;υC N, 1620 cm−1;d ↔ d, 420 nm;μB,
1.74 B.M; ΛM, 17 ꢀ−1 cm2 mol−1
.
2.6. Preparation of [Ni(habenzil)], [Cu(habenzil)] and
[Zn(habenzil)]
[Ni(habenzil)] (yield: 57%), [Cu(habenzil)] (yield: 59%)
and [Zn(habenzil)] (yield: 42%) were prepared similarly.
H2[habenzil] (5.49 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
was added. Refluxing continued for 8 h. Upon cooling the solu-
tion, a solid crystalline was obtained which was filtered, washed
with ethanol, and dried in vacuum and purified by recrystal-
lization from chloroform. Anal. calcd. for C26H18N2O2Ni: C,
69.55; H, 4.01; N, 6.24; Ni, 13.07; C/N, 11.14; C/Ni, 5.32.
Found: C, 69.36; H, 3.87; N, 6.35; Ni, 12.89; C/N, 10.92; C/Ni,
5.38%; υC N, 1618 cm−1; d ↔ d, 471 nm; μB, −0.017 B.M;
ΛM, 15 ꢀ−1 cm2 mol−1. Anal. calcd. for C26H18N2O2Cu: C,
68.81; H, 3.97; N, 6.17; Cu, 14.00; C/N, 11.15; C/Cu, 4.92.
Found: C, 68.70; H, 3.81; N, 6.30; Cu, 13.85; C/N, 10.90; C/Cu,
4.96%; υC N, 1615 cm−1; d ↔ d, 478, 550 nm; μB, 1.73 B.M;
ΛM, 21 ꢀ−1 cm2 mol−1. Anal. calcd. for C26H18N2O2Zn: C,
68.53; H, 3.95; N, 6.14; Zn, 14.35; C/N, 11.16; C/Zn, 4.78.
Found: C, 68.40; H, 3.86; N, 6.29; Zn, 14.23; C/N, 10.87; C/Zn,
2.3. Preparation of bis(2-hydroxyanil)benzil; H2[habenzil]
2-Aminophenol (2.18 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 reducing the volume of the solution to ca.
50 ml, the flask was kept at ambient temperature for 4 h. On cool-
ing 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%. Anal. calcd. for C26H20N2O2: C, 79.57; H,
5.14; N, 7.14; C/N, 11.14. Found: C, 79.30; H, 5.01; N, 7.29;
C/N, 10.88%; υC N, 1630 cm−1
.
4.81%; υC N, 1613 cm−1; ΛM, 14 ꢀ−1 cm2 mol−1
.
2.4. Preparation of [Mn(habenzil)]
2.7. Preparation of alumina-supported metal complexes
(ASMC)
The Schiff-base ligand (4.32 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 contain-
ing 2.69 g (0.011 mol) of manganese(II) acetate tetrahydrate in
water was added dropwise to the deoxygenated ligand solu-
tion. 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(habenzil)] which purified
by recrystallization from chloroform (yield: 51%). Anal. calcd.
for C26H18N2O2Mn: C, 70.14; H, 4.04; N, 6.29; Mn, 12.34;
C/N, 11.15; C/Mn, 5.68. Found: C, 69.93; H, 3.91; N, 6.41; Mn,
12.22; C/N, 10.91; C/Mn, 5.72%; υC N, 1624 cm−1; μB, 5.92
A solution of the [M(habenzil)], (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(habenzil)]/Al2O3 catalyst was dried at
40 ◦C under vacuum overnight prior to use. Anal. calcd. for
[Mn(habenzil)]/Al2O3: C, 6.84; H, 1.56; N, 0.63; Mn, 1.25;
C/N, 10.84; C/Mn, 5.50%; υC N, 1620 cm−1. Anal. calcd. for
[Co(habenzil)]/Al2O3: C, 6.81; H, 1.54; N, 0.63; Co, 1.30; C/N,
10.81; C/Co, 5.24%; υC N, 1615 cm−1; d ↔ d, 418 nm. Anal.
calcd. for [Ni(habenzil)]/Al2O3: C, 6.79; H, 1.52; N, 0.62;
Ni, 1.29; C/N, 10.87; C/Ni, 5.27%; υC N, 1616 cm−1; d ↔ d,
470 nm. Anal. calcd. for [Cu(habenzil)]/Al2O3: C, 6.78; H, 1.51;
B.M; ΛM, 21 ꢀ−1 cm2 mol−1
.
N, 0.62; Cu, 1.43; C/N, 10.89; C/Cu, 4.73%; υC N, 1610 cm−1
d ↔ d, 476 nm.
;
2.5. Preparation of [Co(habenzil)]
2.8. Heterogeneous oxidation of cyclohexene
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-
A mixture of 1.02 × 10−5 mol catalyst, 25 ml solvent and
10 mmol cyclohexenewasstirredunder nitrogenatmosphereina

M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
53
50 ml round-bottom flask equipped with a condenser and a drop-
ping funnel at room temperature for 30 min. Then 16 mmol of
TBHP (solution 80% in di-tert-butylperoxide) or hydrogen per-
oxide (30% in water) was added. The resulting mixture was then
refluxed for 8 h under N2 atmosphere. After filtration and wash-
ing with solvent, the filtrate was concentrated and then subjected
to GC analysis. The concentration of products was determined
using cyclohexanone as internal standard.
the aromatic skeletal vibration. In the immobilized catalyst, the
presence of Schiff-base moiety was indicated by the appearance
of an absorption band at ∼1625 cm−1 due to C N stretching
vibration. The intensity of the alumina-supported metal com-
plexes (ASMC) are though, weak due to low concentration of the
complex, the IR spectra of supported complexes are essentially
similar to the free metal complexes. The adsorbing tendency
of the acidic alumina might arise from the presence of oxygen
groups on the surface in order to coordinate to metal ion center
(Scheme 1).
2.9. Homogeneous oxidation of cyclohexene
The X-ray diffractograms of Al2O3, NiCl2/Al2O3 and
[Ni(habenzil)]/Al2O3 were recorded to study their crystallinity
and to ensure supporting. After careful comparison of XRD pat-
ternsofAl2O3 andNiCl2/Al2O3, itwasobservedthatthereisone
To a solution of cyclohexene (1 ml), neat metal complex
(1.02 × 10−5 mol) in dichloromethane (10 ml), TBHP (2 ml)
was added. The resulting mixture was then refluxed for 8 h under
N2 atmosphere, the solvent evaporated under reduced pressure
and the crude analyzed by GC and GC–MS. The concentrations
of products were determined using cyclohexanone as internal
standard.
˚
new peak with a d value of 17.30 A in NiCl2/Al2O3. This peak
was also observed in [Ni(habenzil)]/Al2O3 and [Ni(habenzil)] at
the same position. In addition, the [Ni(habenzil)]/Al2O3 exhibits
˚
one new signal with value of 82.70 A, which is a part of the lig-
and as this signal was also observed in [Ni(habenzil)] but not
observed in Al2O3 or NiCl2/Al2O3. This information clearly
indicates the supported of [Ni(habenzil)] on alumina. Very low
intensity of other peaks made it difficult to distinguish them from
the other peaks in the XRD pattern of the ASMC.
3. Results and discussion
The metal content of the ASMC catalysts was estimated by
dissolving the known amounts of the heterogeneous catalyst in
concentrated HCl and from these solutions, transition metal con-
tents were estimated by atomic absorption spectrometer. 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 complexes. The chemical anal-
ysis 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 ratios of carbon to metal (C/M)
and carbon to nitrogen (C/N) have been provided to ensure the
immobilization of complex on the alumina.
Synthesis of the metal complexes was essentially the same
and involved heating and stirring of stoichiometric amounts of
H2[habenzil] 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 [habenzil]. 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 structural information of the transition metal complexes
and its immobilized analogue over alumina was obtained from
the FT-IR spectroscopy. The presence of Schiff-base moiety was
indicated by the appearance of absorption band at 1630 cm−1due
to C N stretching vibration, which shifted to lower frequency
and appears at 1613–1625 cm−1 indicating the coordination of
azomethine nitrogen to the metal. The phenolic (C–O) stretching
frequency was observed in the region of 1275 cm−1 (of ligand),
which shifted to higher frequency region of 1297 cm−1 in com-
plex, indicating coordination through phenolic oxygen [14b].
In the complex strong to medium intensity bands are observed
in ∼400 and ∼530 cm−1 attributed to υM–N and υM–O stretch-
ing frequencies, respectively. The presence of several bands of
medium intensity at 1599 and 1468 cm−1 have been assigned to
1
Comparison of H NMR spectral data of the H2[habenzil]
and the neat diamagnetic [Zn(habenzil)] and [Ni(habenzil)])
complexes recorded in DMSO-d6 further supplements the
conclusion drawn from IR data. The 1H NMR spectrum of
H2[habenzil] exhibits the following signals: 13.85 (s, 2H, OH),
6.80–7.12 (m, 10H, C6H5) and 7.25–7.40 (m, 8H, C6H4). The
disappearance of phenolic signal and down field shift (7.70 ppm)
of C6H4 signal indicates of the coordination of phenolic oxy-
gen after deportation and azomethine nitrogen atoms. 1H NMR
spectrum of [Cu(habenzil)], [Co(habenzil)] and [Mn(habenzil)]
complexes could not be recorded due to its partial paramagnetic
nature of complex as noticed earlier [15].
Electronic spectra of [Cu(habenzil)] complexes were
recorded in CHCl3 solution over the range 400–700 nm (see
Section 2). The visible spectra of the [Cu(habenzil)] com-
plexes consists of a shoulders at ∼478 nm and a maximum
or a broad shoulder around 550 nm, which can be assigned
to the dxz,yz ↔ dxy and dx2–y2 ↔ dxy transitions in D2h sym-
metry [15]. The room temperature magnetic moments of
[Cu(habenzil)] fall in the range 1.74 μB which are typical for
square-planar (D4h) and tetrahedrally distorted (D2h) mononu-
clear copper(II) complexes with a S = 1/2 spin state and did
not indicate any antiferromagnetic coupling of spines at this
temperature. The tetrahedral geometry of the [Mn(habenzil)] is
strongly indicated by similarities in the visible spectra of this
chelate with those of known tetrahedral complexes containing
oxygen–nitrogen donor atoms [16]. The electronic spectrum of
the [Ni(habenzil)] exhibit one band at 471 nm which can be
assigned to a d ↔ d transition of the metal ion. The average
energy of this absorption is comparable to d ↔ d transitions
of other square-planar Schiff-base of nickel(II) chelates with
nitrogen and oxygen donor atoms [17,18], which have reported
values in the range of 465–485 nm. The electronic spectrum of
[Co(habenzil)] is very similar to that reported for [Co(salen)].

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M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
Table 1
Table 3
Oxidation of cyclohexene with TBHP catalyzed by metal complexes in CH2Cl2
(solvent = 10 ml; catalyst = 1.02 × 10−5 mol; duration = 8 h, at reflux; cyclohex-
ene = 1 ml, TBHP = 2 ml)
Oxidation of cyclohexene with TBHP catalyzed by metal complexes on alumina
in CH3Cl
Catalyst
Conversion
(%)
Selectivity (%)
Catalyst
Conversion (%)
Selectivity (%)
Ketonea
Alcoholb
Peroxidec
Ketonea
Alcoholb
Peroxidec
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Co(habenzil)]-Al2O3
[Ni(habenzil)]-Al2O3
[Cu(habenzil)]-Al2O3
80.7
78.4
77.1
76.2
72.4
42.6
53.2
79.9
79.0
78.2
76.5
74.1
64.9
67.1
15.2
15.8
15.9
17.2
22.4
26.5
23.7
4.9
5.2
5.9
6.3
3.5
8.6
9.2
d
e
f
[Mn(habenzil)]
[Mn(habenzil)]d
[Mn(habenzil)]e
[Mn(habenzil)]f
[Co(habenzil)]
[Ni(habenzil)]
[Cu(habenzil)]
90.6
64.3
85.7
51.4
81.4
49.7
61.5
60.1
56.4
64.8
67.6
51.5
33.7
38.5
21.3
26.3
22.8
21.4
28.3
40.8
30.8
18.6
17.3
12.4
11.0
20.2
25.5
30.7
a
2-Cyclohexene-1-one.
2-Cyclohexene-1-ol.
a
b
c
d
e
f
2-Cyclohexene-1-one.
b
c
d
e
f
2-Cyclohexene-1-ol.
1-(tert-Butylperoxy)-2-cyclohexene.
First reuse.
1-(tert-Butylperoxy)-2-cyclohexene.
Catalyst = 0.5 × 10−5 mol.
Catalyst = 2.04 × 10−5 mol.
Catalyst = 4.08 × 10−5 mol.
Second reuse.
Third reuse.
The spectrum of the [Co(habenzil)] exhibit two bands at
555–660 nm which are assigned to d ↔ d transitions. In addi-
tion, a lower energy absorption at 420 nm has been observed
such low energy bands which have been shown to be char-
acteristic of square-planar cobalt(II) chelates [17,19,20]. This
geometry is confirmed by the values of the effective magnetic
moment.
The selectivity and activity results of alumina-supported
and homogeneous catalysts on the oxidation of cyclohexene
with TBHP have been given in Tables 1–3 and Figs. 1–3.
Comparing between neat and ASMC as catalyst evidence that
neat catalysts gave higher conversion of cyclohexene than their
corresponding alumina-supported complexes. The effect of tran-
sition metal complexes supported on alumina was studied on
Fig. 1. Oxidation products distribution in dichloromethane with [M(habenzil)]
/Al2O3/TBHP.
the oxidation of cyclohexene with TBHP in dichloromethane
and the results have been shown in Table 2 and Fig. 1. As
shown in Tables 1–3, 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% [21].
Table 2
Oxidation of cyclohexene with TBHP catalyzed by metal complexes on alumina
in CH2Cl2
Catalyst
Conversion
(%)
Selectivity (%)
Ketonea
Alcoholb
Peroxidec
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Mn(habenzil)]-Al2O3
[Co(habenzil)]-Al2O3
[Ni(habenzil)]-Al2O3
[Cu(habenzil)]-Al2O3
84.5
83.7
83.1
82.4
62.4
88.5
90.3
75.2
43.5
55.2
83.2
82.5
80.4
79.6
70.3
85.6
87.8
76.9
67.4
70.6
13.1
13.9
14.5
15.6
24.8
11.6
10.8
20.2
25.1
20.5
3.7
3.6
5.1
4.8
4.9
2.8
1.4
2.9
7.5
8.9
d
e
f
g
h
i
a
2-Cyclohexene-1-one.
2-Cyclohexene-1-ol.
1-(tert-Butylperoxy)-2-cyclohexene.
First reuse.
Second reuse.
Third reuse.
Catalyst = 0.5 × 10−5 mol.
Catalyst = 2.04 × 10−5 mol.
Catalyst = 4.08 × 10−5 mol.
b
c
d
e
f
g
h
i
Fig. 2. Oxidation products distribution in methanol with [M(habenzil)]
/Al2O3/TBHP.

M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
55
Table 4
Oxidation of cyclohexene catalyzed by alumina-supported metal complexes in CH2Cl2 with TBHP
Catalyst
Conversion (%)
Selectivity (%)
Ketonea
Ref.
[21]
Alcoholb
39.6
Peroxidec
8.1
MnCl2/Al2O3
29.4
52.3
44.3
52.1
65.8
48.6
56.1
71.6
28.2
26.5
18.6
23.2
17.4
9.8
[21]
[21]
[21]
73.4
75.4
17.4
7.2
[21]
76.8
82.8
74.5
80.5
16.6
14.2
8.9
5.3
[13g]
[13c]
78.4
84.5
77.2
83.2
16.5
13.1
6.3
3.7
[13h]
This work
a
2-Cyclohexene-1-one.
2-Cyclohexene-1-ol.
1-(tert-Butylperoxy)-2-cyclohexene.
b
c
The increase of conversion from 29.4 to 84.5% compared to
MnCl2/Al2O3 with [Mn(habenzil)]/Al2O3 indicates that the
existence of ligand has increased the activity of the catalyst by
a factor of 2.90. From the indicated results in Tables 2–4 and
Figs. 1 and 2 it is evident that cyclohexene-2-one is selectively
formed in the presence of all catalysts.
The trend observed in Tables 1–3 can be explained by
the donor ability of ligand available in the complex catalysts.

56
M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
Fig. 3. Oxidation products distribution in acetonitrile with [M(habenzil)]
/Al2O3/TBHP.
Fig. 5. Effect of solvent on the conversion by heterogeneous catalyst.
As Wang et al. have pointed out, the key point in the con-
version 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 [22]. The
formation of the allylic oxidation products 2-cyclohexene-1-
one and 2-clohexene-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 [23]. TBHP as oxidant promotes the allylic oxida-
tion pathway and epoxidation is minimized, especially under
the highly acidic properties of alumina supported with diva-
lent and trivalent transition metal ions and complexes, has been
observed by us and others [12,13,24]. It should be emphasized
that the destructive oxidation of alkenes via epoxidation path-
way with H2O2 under the catalytic effect of alumina-supported
Mn(II) complexes seem interesting [13]. Although the two sys-
tems are alike, it is the oxidant structure that has changed the
fate of the reaction.
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
to have a higher activation energy for O–O bond cleavage than
a TBOO complex and therefore, to have a longer lifetime [25].
The effect of various solvents for the oxidation of cyclo-
hexene with [M(habenzil)]/Al2O3 catalysts was also studied
(Fig. 5). The oxidation reactions were carried out in protic and
aprotic solvents. The results have been given in Tables 1–3
and Figs. 1–5. 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 (Fig. 3). 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.
When the oxidant was changed to hydrogen peroxide (Fig. 4),
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
At the end of the heterogeneous reaction, the catalyst was sep-
arated by filtration, thoroughly washed with solvent and reused
under similar conditions. Although the analysis of the recovered
catalysts by Atomic Absorption Spectroscopy showed no reduc-
tion in the amount of transition metal ions, they showed a slightly
lower catalytic activity (∼2%) (Tables 1–3). The results indi-
cate that [M(habenzil)]/Al2O3 are almost stable to be recycled
for the oxidation of cyclohexene without much loss in activ-
ity (Table 3). Thus, the alumina-supported metal complexes is
found to increase the life of the catalyst by reducing dimeriza-
tion due to the site isolation and restriction of internal framework
Fig. 4. Oxidation products distribution in acetonitrile with [M(habenzil)]
/Al2O3/H2O2.

M. Salavati-Niasari, S.N. Mirsattari / Journal of Molecular Catalysis A: Chemical 268 (2007) 50–58
57
structure. IR spectrum of the recycled sample is quite similar to
that of fresh sample indicating little changes in the coordination
of habenzil after the oxidation reactions.
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The results clearly suggest that [Mn(habenzil)]/Al2O3 effi-
ciently catalyses conversion of cyclohexene to 2-cyclohexene-
1-one with 83.2% selectivity and conversion 84.5%. The more
activity of habenzil system has clearly arisen from the existence
of electron donating ligand which facilitates the electron transfer
rate, a process that has previously observed by us in other oxi-
dation reactions (Table 4). All conversion efficiency with high
selectivity obtained in this study is significantly higher than that
obtained using manganese(II) complexes supported on alumina
(Table 4).
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4. Conclusions
In this study, we have used a rather simple catalysis system
of alumina-supported manganese(II), cobalt(II), nickel(II) and
copper(II) complexes with a Schiff-base ligand “H2[habenzil]”
in the oxidation of cyclohexene. Oxidation of allylic site
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[Mn(habenzil)]/Al2O3 efficiently catalyses conversion of cyclo-
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conversion 84.5%. The efficiency of the catalysts for oxi-
dation of cyclohexene in different solvents decreases in the
order: dichloromethane > chloroform > methanol > acetonitrile.
The extension of the method to different olefins is currently
under investigation in our laboratory.
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Acknowledgment
Authors are grateful to Council of University of Kashan for
providing financial support to undertake this work.
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