Iron(III) aroylhydrazone complexes: Structure, electrochemical studies and catalytic activity in oxidation of olefins

Article abstract of DOI:10.1016/j.molcata.2009.02.004

Tridentate Schiff base ligands derived from aromatic aldehydes and benzhydrazide, and their iron complexes [Fe(L¹)(HL¹)] 1, [Fe(HL¹)Cl₂(CH₃OH)]·(CH₃OH) 2 and [Fe(HL²)Cl₂

Full text of DOI:10.1016/j.molcata.2009.02.004

Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Iron(III) aroylhydrazone complexes: Structure, electrochemical studies and
catalytic activity in oxidation of olefins
Hassan Hosseini Monfareda,∗, Somayeh Sadighiana, Mohammad-Ali Kamyabia, Peter Mayerb
a Department of Chemistry, Zanjan University 45195-313, Zanjan, Islamic Republic of Iran
b Fakultät für Chemie und Pharmazie, Ludwig-Maximilians-Universität, München, Butenandtstr. 5-13, Haus D, D-81377 München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Tridentate Schiff base ligands derived from aromatic aldehydes and benzhydrazide, and their iron com-
plexes [Fe(L1)(HL1)] 1, [Fe(HL1)Cl2(CH3OH)]·(CH3OH) 2 and [Fe(HL2)Cl2(H2O)] 3 have been prepared
and characterized (H2L1 = (E)-Nꢀ-(2-hydroxy-3-methoxybenzylidene)benzohydrazide, H2L2 = (E)-Nꢀ-(5-
bromo-2-hydroxybenzylidene)benzohydrazide). The crystal structure of 2 has been determined. The
electrochemical properties of these complexes have been investigated by cyclic voltammetric technique
in acetonitrile solutions. Electrochemical studies have revealed quasi-reversibility for these compounds.
The catalytic potential of these complexes has been tested for the oxidation of cyclooctene using
tert-butylhydroperoxide (TBHP) as oxidant. The effects of the molar ratio of oxidant to substrate, the tem-
perature, the co-catalyst concentration and the solvent have been studied. Excellent selectivities have
been obtained for the epoxidation of cyclohexene, cyclooctene, norbornene, cis- and trans-stilbene.
© 2009 Elsevier B.V. All rights reserved.
Received 16 December 2008
Received in revised form 3 February 2009
Accepted 3 February 2009
Available online 13 February 2009
Keywords:
Crystal structure
Aroylhydrazone
Olefins
Epoxidation
Iron catalyst
Cyclic voltammetry
1. Introduction
aroylhydrazones, R–CO–NH–N CH–Rꢀ, and the dependence of
their mode of chelation with transition metal ions present in
The design of metal catalysts to mimic bio-oxidative activity
of P-450 has continued to be an active area of research [1]. There
have been numerous reports on the catalytic oxidation of organic
substrates by porphyrin complexes [2,3]. Due to the difficulties in
modification, the preparation of other planar tetradentate ligands,
especially Schiff base [4,5] and amido ligands [6], is highly desir-
able. The oxidation of hydrocarbons with Schiff base complexes
has been a field of academic and industrial interest to analyze the
catalytic activity of various metal complexes [7]. Schiff bases play
an important role in inorganic chemistry as they easily form sta-
ble complexes with most transition metal ions. The development
in the field of bioinorganic chemistry has increased the interest
in Schiff base complexes, since it has been recognized that many
of these complexes may serve as models for biologically impor-
tant species [8]. Iron-catalyzed systems for C–H oxidation (Gif8
and Fenton chemistry [9], nonheme mimic systems [10–12], olefin
epoxidation [5], and the chemistry of Fe-porphyrins [13]) have been
summarized in various reports.
the living system have been of significant interest [14–17]. The
coordination compounds of aroylhydrazones have been reported
to act as enzyme inhibitors [18] and are useful due to their phar-
macological applications [19]. Aroylhydrazones complexes, on the
other hand, seem to be a good candidate for catalytic oxidation
studies because of their stability to resist oxidation. Investigation
into the iron-binding potential of a range of hydrazone derivatives
[20–22], as drugs for genetic disorders such as thalassemia, led
to the discovery that salicylaldehyde benzoylhydrazone inhibits
DNA synthesis and cell growth [23]. This class of diprotic ligand
typically acts as tridentate, planar chelate ligand coordinating
through the phenolic and amide oxygens and the imine nitrogen.
The actual ionization state is dependent upon the conditions and
metal employed [24]. With copper(II) in alkaline solution, both
the phenolic and amide protons are ionized; in neutral and mild
acidic solution the ligands are monoanionic, whereas strongly
acidic conditions are necessary to form compounds formulated
with a neutral ligand. A binuclear complex of iron(III) with a Schiff
base hydrazone derived from 7-chloro-4-hydrazinoquinoline and
bridging chloride has been reported [25].
The remarkable biological activity of acid hydrazides
R–CO–NH–NH2,
a class of Schiff base, their corresponding
Continuing our studies on catalysis by hydrazone Schiff base
manganese complexes [26], we report herein on the synthesis
of a series of iron(III) complexes containing substituted aroylhy-
drazones ligands (Fig. 1) as well as their applications in catalytic
epoxidation of olefins with tert-butylhydroperoxide.
∗
Corresponding author. Tel.: +98 241 5152576; fax: +98 241 2283203.
E-mail addresses: monfared@znu.ac.ir, monfared 2@yahoo.com
(H.H. Monfared).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.02.004

140
H.H. Monfared et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
Fig. 1. In situ formed H2L1 and H2L2 ligands.
2. Experimental
hydroxybenzaldehyde, tert-butylhydroperoxide (TBHP) (solution
80% in di-tert-butylperoxide) and the olefins were purchased from
Merck and Fluka and used as received. Solvents of the highest grade
commercially available (Merck) were used without further purifi-
cation.
2.1. Equipments
The reaction products of oxidation were determined and
analyzed by HP Agilent 6890 gas chromatograph equipped
with
a
HP-5 capillary column (phenyl methyl siloxane
2.4. Synthesis of [FeIII(L1)(HL1)], 1 (H2L1 = (E)-Nꢀ-(2-hydroxy-3-
methoxybenzylidene)benzohydrazide)
30 m × 320 ␮m × 0.25 ␮m) with flame-ionization detector and
gas chromatograph–mass spectrometry (Hewlett-Packard 5973
Series MS-HP gas chromatograph with a mass-selective detector).
1H NMR spectra were recorded by use of a Bruker 250 MHz
spectrometer. Atomic absorptions were measured by a Varian
220 FS spectrophotometer. Microanalyses were carried out using
a Heraeus CHN–O– Rapid analyzer. IR spectra were recorded
using PerkinElmer 597 and Nicolet 510P spectrophotometers. A
Thermospectronic model ␣, UV–vis spectrometer with 1 cm quartz
cells was used for recording and storage of UV–vis absorbance
spectra.
Benzhydrazide
(0.20 g,
1.47 mmol)
and
2-hydroxy-3-
methoxybenzaldehyde (0.22 g, 1.47 mmol) were dissolved in
methanol (100 ml). A solution of FeCl3·6H2O (0.40 g, 0.148 mmol)
and NaN3 (0.147 g, 2.26 mmol) in methanol (50 ml) was added to
the above solution and refluxed for 3 h. The resulting black-brown
solid 1 was filtered off, washed with methanol and dried under air
(0.319 g, yield 73%).
IR (KBr, cm−1): 3371 (s), 3170 (s, br), 2785 (w), 1682 (vs), 1620 (s,
br), 1396 (s), 1365 (m), 1203 (m), 1026 (m), 779 (m), 702 (m), 609
(s), 509 (m), 409 (m).
2.2. Experimental
2.5. Synthesis of [Fe(HL1)Cl2(CH3OH)]·CH3OH, 2
Voltammetric experiments were performed using a Metrohm
computrace voltammetric analyzer model 757 VA. A conventional
three-electrode system was used with a polished glassy carbon
electrode (area 3.14 mm2) as working electrode and a platinum
wire counter electrode. The reference was an aqueous Ag/AgCl
saturated electrode, separated from the bulk of the solution by a
bridge with solvent and supporting electrolyte. The solutions in
the bridge were changed periodically to avoid aqueous contam-
ination from entering the cell via the Ag/AgCl electrode. Before
each experiment the working electrode was cleaned by polishing
with alumina 0.05 mm and rinsed thoroughly with distilled water
and acetone. The electrolytic medium consisted of 0.1 mol/l tetra-
butylammonium perchlorate (TBAP) as supporting electrolyte in
acetonitrile and all experiments were carried out at room temper-
ature. The solutions were freshly prepared before use, and were
purged with N2 saturated with solvent for ca. 15 min prior to
taking measurements in order to remove dissolved O2. Voltam-
mograms were recorded in the range from −0.7 to +0.7 V vs.
Ag/AgCl.
Benzhydrazide
(0.20 g,
1.47 mmol)
and
2-hydroxy-3-
methoxybenzaldehyde (0.22 g, 1.47 mmol) were dissolved in
methanol (100 ml). A solution of FeCl3·6H2O (0.40 g, 1.48 mmol) in
methanol (50 ml) was added to the above solution and refluxed for
3 h. The resulting black-green solid 3 was filtered off, washed with
methanol and dried under air (0.636 g, yield 94%).
IR (KBr, cm−1): 3431 (vs, br), 3231 (s), 2931 (m), 2862 (w), 1600
(vs), 1546 (vs), 1438 (s), 1392 (s), 1315 (m), 1262 (m), 1223 (s), 1092
(s), 746 (w), 708 (m).
2.6. Synthesis of [Fe(HL2)Cl2(H2O)], 3
(H2L2 = (E)-Nꢀ-(5-bromo-2-hydroxybenzylidene)benzohydrazide)
Benzhydrazide
(0.20 g,
1.47 mmol)
and
5-bromo-2-
hydroxybenzaldehyde (0.30 g, 1.49 mmol) were dissolved in
methanol (100 ml). A solution of FeCl3·6H2O (0.40 g, 1.48) in
methanol (50 ml) was added to the above solution and refluxed
for 3 h. The resulting black-brown solid 2 was filtered off, washed
with methanol and dried under air (0.480 g, yield 69%).
2.3. Materials
IR (KBr, cm−1): 3431 (s, br), 3215 (m), 2923 (m), 2862 (w), 1608
(vs), 1562 (m), 1531 (m), 1500 (m), 1462 (s) 1374 (s), 1346 (m), 1300
(s), 1185 (vs, br), 1092 (s), 715 (m), 654 (m), 600 (m).
Iron(III) chloride hexahydrate, anhydrous iron(II) chloride,
benzhydrazide, 2-hydroxy-3-methoxybenzaldehyde, 5-bromo-2-

H.H. Monfared et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
141
Table 1
Crystallographic data of 2.
bonded hydrogen atoms were located from the difference map with
U(H) = 1.5 × Uiso(O) and U(H) = 1.2Uiso(N). The molecular structure
plot was prepared using ORTEPIII [29].
CCDC 711373 contains the supplementary crystallographic data
for 2. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data
request/cif.
Net formula
C17 H21 Cl2FeN2O5·(CH3OH)
460.110
200(2)
Monoclinic
P21/c
18.8683(6)
9.0534(5)
12.0460(5)
90
107.673(3)
90
1960.61(15)
4
1.55878(12)
1.072
None
11947
0.0507
0.0448
3.19–25.25
2779
0.0529, 19.3334
Mixed
3534
257
3
Mr (g mol−1
T (K)
)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
2.8. General oxidation procedure
˛ (◦)
ˇ (◦)
ꢀ (◦)
Oxidation reactions were performed in a stirred round-bottom
flask fitted with a water-cooled condenser. The reactions were car-
ried out under atmospheric pressure in air in an oil bath at 60 1 ◦C
with acetonitrile as a solvent and TBHP as an oxidant. In a typi-
cal experiment a mixture of 0.0022 mmol catalyst, 5.0 ml solvent,
1.0 mmol olefin, 0.073 mmol imidazole, and 1.0 mmol n-octane as
internal standard was prepared in a round-bottom flask. After the
mixture was heated to 60 ◦C, 2.0 mmol TBHP was added. At appro-
priate intervals, aliquots were removed and analyzed immediately
by GC. Oxidation products yields based on the starting substrate
were quantified by comparison with n-octane.
V (Å3)
Z
Calc. density (g cm−3
)
ꢁ (mm−1
)
Absorption correction
Refls. measured
Rint
Mean ꢂ(I)/I
ꢃ range
Observed refls.
x, y (weighting scheme)
Hydrogen refinement
Refls. in refinement
Parameters
3. Results and discussion
Restraints
R(Fobs
)
0.0710
0.2048
Rw(F2)
3.1. Characterization of the Fe(III) complexes
S
1.132
Shift/errormax
0.001
0.700
−0.622
The H2L1 and H2L2 are formed in situ when equimolar
quantities of 2-hydroxy-3-methoxybenzaldehyde or 5-bromo-2-
hydroxybenzaldehyde and benzohydrazide are reacted with iron
chloride in methanol, respectively [30]. The in situ formed ligand
could be in equilibrium with mono- (HL)− and dianionic (L)2− forms
(Fig. 1). Depending on the reaction conditions, H2L may coordi-
nate to a metal center as a monoanionic (HL)− [26] or a dianionic
(L)2− ligand [31]. The pKa1 and pKa2 values obtained for the acid-
ity constants of H2L1 are 9.03 and 10.92, respectively [32]. So, under
neutral or less basic conditions of FeCl3·6H2O in methanol, H2L acts
as a monoanionic tridentate chelating ligand by phenolic oxygen
deprotonation.
Max electron density/e (Å−3
)
Min electron density/e (Å−3
)
Table 2
IR spectral data of the ligands and the iron complexes.
Compound
Selected IR bands (cm−1
)
ꢄ¯(C N) + ı(NH)
ꢄ¯(C O)
ꢄ¯(NH)
ꢄ¯(OH)
H2L1
H2L2
1
2
3
1607, 1577
1615, 1546
1620
1546
1562, 1531
1653
1646
1682
1600
1608
3215
3215
3170
3231
3215
3569, 3376
3415
3371
3431(broad)
3431 (broad)
A practical list of IR spectral data is presented in Table 2. A com-
parison of the spectra of the complexes with the ligands provides
evidence for the coordination mode of the ligands in the catalysts.
The hydrazone Schiff base ligand (H2L1) exhibits three bands on
3215–3569 cm−1 due to phenolic and –NH-vibrations. The pres-
2.7. X-ray crystallography
A dark green crystal of 2 (0.01 mm × 0.08 mm × 0.16 mm) was
investigated in a diffraction experiment at 200(2) K on a Nonius
Kappa CCD diffractometer with monochromated Mo K␣ radiation
(ꢅ = 0.71073) obtained from a graded multilayer X-ray optics. The
structure was solved by Direct Methods with SIR97 [27], and refined
with full-matrix least-squares techniques on F2 with SHELXL-97
[28]. The crystal data and refinement parameters are presented
in Table 1. The C-bonded hydrogen atoms were calculated in
idealized geometry riding on their parent atoms. The O- and N-
ence of a NH band, red shifts in azomethine (–C N–, 61 cm−1
)
and carbonyl bands (53 cm−1) of the H2L1 ligand indicate coordina-
tion of H2L1 through the deprotonated hydroxyl group, azomethine
nitrogen and amide oxygen groups in the complex 2 (Table 2). The
analytical and physical data of the complexes are given in Table 3.
Suggested structures of complexes 1 and 3 could be rationalized in
the same way (Fig. 2).
Caused by complexation, the UV–vis spectra of 1, 2 and 3 in
methanol show absorbance bands at 362, 305, 233 for complex
Fig. 2. Proposed structures of octahedral geometry of 1, 2, and 3.

142
H.H. Monfared et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
Table 3
Analytical and physical data of the complexes.
Empirical formula
Formula weight (g mol−1
)
Yield (%)
Color
Analyses found (calc.) (%)
%C
%H
%N
%Fe
1, C30H25FeN4O6
2, C17 H21 Cl2FeN2O5
3, C14 H12 BrCl2FeN2O3
593.39
460.11
462.91
74
94
69
Black-brown
Black-green
Black-brown
60.50 (60.72)
44.00 (44.38)
36.2 (36.32)
4.50 (4.25)
4.55 (4.60)
2.60 (2.61)
9.40 (9.44)
6.00 (6.09)
6.30 (6.05)
10.00 (9.41)
12.11 (12.14)
12.25 (12.06)
1, at 304, 226 for complex 2, and at 362, 292, 227 for complex 3,
slightly different from those at 338, 262, 222 of the corresponding
free ligand H2L1 (all absorbances are given in nm).
potential range −0.7 to +0.7 V vs. Ag/AgCl. Typical CVs of 1–3 are
shown in Fig. 5. The redox parameters from CVs of 1 and 2 are nearly
identical with E0ꢀ = +0.049 V (vs. Ag/AgCl), which can be ascribed
to the redox reaction FeIII/FeII. However, complex 3 shows a very
weak redox peak at the same condition. The cyclic voltammogram
of the complex 2 in acetonitrile displays a quasi-reversible peak at
E0ꢀ = +0.049 V (vs. Ag/AgCl). The ꢆEp = (Epa − Epc) value was 125 mV
and the ipa/ipc = 0.6. This finding shows a quasi-reversibility of this
redox process. Electrochemical studies have also revealed quasi-
reversibility for 1 and 3.
3.2. X-ray structure of [Fe(HL1)Cl2(CH3OH)]·(CH3OH), 2
The X-ray study reveals that 2 crystallizes in the monoclinic
space group P21/c with four formula units in the unit cell. The
molecular structure of the complex and the labelling of the atoms
are shown in Fig. 3. Crystallographic data and structure refinement
parameters are given in Table 3. The Fe(III) atom is meridionally
coordinated by O, N, O donor set of the (E)-Nꢀ-(2-hydroxy-3-
methoxybenzylidene)benzohydrazide ligand. Two cis-coordinated
chloro and a methanol ligand complete the distorted pseudo-
octahedral metal coordination sphere. The metal center in 2 is
displaced by 0.143 Å out of the least-squares plane defined by the
donor atoms O2, N2, Cl2 and O1.
The cyclic voltammograms of complex 2 at different scan rates
from 10 to 1000 mV s−1 are illustrated in Fig. 6. Obviously the sep-
Table 4
Hydrogen bonding interactions for 2.
D–H· · ·A
D–H (Å)
H· · ·A (Å)
D· · ·A (Å)
D–H· · ·A (◦)
N1–H71· · ·O2a
0.879(18)
0.879(18)
2.16(3)
2.39(7)
3.022(7)
2.941(8)
165(7)
121(6)
Adjacent molecules of 2 are connected together by strong hydro-
gen bonds N1-H71.O2 and N1-H71.O3 which leads to an infinite 1-D
chain along [0 0 1] (Fig. 4, Table 4).
N1–H71· · ·O3a
a
Symmetry transformation: x, 1/2 − y, −1/2 + z.
3.3. Electrochemical studies
Cyclic voltammograms (CVs) of the complexes 1–3 were
recorded in acetonitrile with TBAP as supporting electrolyte in the
Fig. 3. The molecular structure of 2. Ellipsoids are drawn at the 50% probability
level. Selected distances (Å) and angles (◦): Fe-Cl1 2.320(2), Fe-Cl2 2.2610(19), Fe-O1
2.077(6), Fe-O2 1.935(4), Fe-O4 2.104(6), Fe-N2 2.130(6) Cl1-Fe-Cl2 96.06(8), Cl1-Fe-
O1 93.10(17), Cl1-Fe-O2 94.74(14), Cl1-Fe-O4 175.93(17), Cl1-Fe-N2 90.75(16), Cl2-
Fe-O1 93.64(15), Cl2-Fe-O2 105.90(15), Cl2-Fe-O4 87.84(17), Cl2-Fe-N2 166.15(17),
O1-Fe-O2 158.0(2), O1-Fe-O4 85.5(2), O1-Fe-N2 73.9(2), O2-Fe-O4 85.3(2), O2-Fe-
N2 85.5(2), O4-Fe-N2 85.2(2).
Fig. 4. One-dimensional chain along [0 0 1] formed by intermolecular hydrogen
bonds (viewing direction along [0 1 0]).
Fig. 5. Cyclic voltammogram of 1, 2 and 3 (10−3 mol/l) in acetonitrile and TBAP
(0.1 mol/l); scan rate 100 mV s−1
.

H.H. Monfared et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
143
Table 5
Control experiments for catalytic oxidation of cyclooctene.
No.
Catalyst
Co-catalyst
Oxidant
%Conversion
Time (h)
1
2
3
4
2
Imidazole
Imidazole
None
None
TBHP
TBHP
TBHP
0 (0)
0 (6)
14 (19)
55 (70)
4 (24)
4 (24)
4 (24)
4 (24)
None
2
2
Imidazole
Reaction conditions: cyclooctene (1.0 mmol), TBHP (2.0 mmol), catalyst
(0.0022 mmol), acetonitrile (5 ml), imidazole (0.073 mmol) and temperature
(60
◦C).
2
1
does not occur (Table 5, no. 1) and in the absence of catalyst the oxi-
dation proceeds only up to 6% after 24 h (Table 5, no. 2). Addition
of a co-catalyst (imidazole) increases the cyclooctene conversion
from 19 to 70% (Table 5, no. 3 and 4).
In search of suitable reaction conditions to achieve the maxi-
mum oxidation of cyclooctene, the effect of oxidant concentration
(moles of TBHP per moles of cyclooctene), solvent and tempera-
ture of the reaction were studied. The effect of TBHP concentration
on the oxidation of cyclooctene is illustrated in Fig. 8. Different
TBHP/cyclooctene molar ratios (0.5:1, 1:1, 2:1 and 3:1) were con-
sidered while keeping the fixed amount of cyclooctene (1.0 mmol)
and catalyst 2 (0.0022 mmol) in 5 ml of ethylacetate at 60 1 ◦C.
Increasing the TBHP/cyclooctene ratio from 0.5:1 to 2:1 increases
the conversion from 3 to 11%. Further increment results to reduction
of conversion. The maximum cyclooctene conversion is obtained
with a molar ratio of 2:1.
Fig. 9 illustrates the influence of the solvent nature in the
catalytic epoxidation of cyclooctene by 2. Carbon tetrachlo-
ride, methanol, ethanol, acetonitrile, chloroform, n-hexane,
tetrahydrofurane (THF) and ethyl acetate were used as sol-
vents. The highest conversion was obtained in acetonitrile,
14% in the absence of imidazole and 38% in the presence
of imidazole. It was observed that the catalytic activity
Fig. 6. Cyclic voltammogram of 2 (10−3 mol/l) in acetonitrile and TBAP (0.1 mol/l);
scan rate 10, 20, 30, 40, 60, 80, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700,
800, 900 and 1000 mV s−1
.
of the complex
2
decreases in order acetonitrile > ethyl
aration between Ep increases as the scan rate increases and this is
characteristic of a quasi-reversible system. There is a linear relation-
ship betweenthecathodicand anodic peaks currents andthe square
root of the scan rate (ꢇ1/2) in the 10–1000 mV s−1 range (Fig. 7).
This behavior is typical for an electron transfer process controlled
by diffusion.
acetate > methanol > ethanol > chloroform > tetrahydrofuran > n-
hexane > carbontetrachloride at a temperature of 60 1 ◦C. The
highest conversion in acetonitrile possibly is caused by its dielectric
constant (ε/ε0 = 35.94) and dipole moment (ꢁ = 3.90 D) which are
the highest of all the solvents used.
In order to get the best reaction temperature, the reaction mix-
ture was stirred at various temperatures (for results see Fig. 10).
With 2 as catalyst a maximum conversion of cyclooctene (55%) is
obtained after 4 h at 60 ◦C. A temperature rise up to 80 ◦C lowers
the cyclooctene conversion. The highest conversion of cyclooctene
3.4. Catalytic activity studies
3.4.1. Oxidation of cyclooctene
The oxidation of cyclooctene catalyzed by 1, 2, and 3 was carried
out with tert-butylhydroperoxide as an oxidant to give cyclooctene
oxide as the sole product. The results of control experiments
revealed that the presence of catalyst and oxidant are essential for
the oxidation. The oxidation of cyclooctene in the absence of TBHP
Fig. 8. Effect of TBHP concentration on oxidation of cyclooctene by 2. Reaction
conditions: cyclooctene (1.0 mmol), n-octane (1.0 mmol), catalyst (0.0022 mmol),
imidazole (0.10 mmol), ethylacetate (5.0 ml) and temperature (60
Fig. 7. Plot of cathodic and anodic currents vs. the square root of sweep rate (ꢇ1/2
for 2.
)
1
◦C).

144
H.H. Monfared et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
Fig. 9. Effect of solvent on oxidation of cyclooctene by 2. Reaction conditions:
cyclooctene (1.0 mmol), n-octane (1.0 mmol), catalyst (0.0022 mmol), imidazole
(0.10 mmol), solvent (5.0 mmol), TBHP (2.0 mmol), reaction times (4 h) and tem-
perature (60
1
◦C) in the absence imidazol and presence of imidazole (0.10 mmol).
Fig. 11. Effects of the catalyst nature on the oxidation of cyclooctene. Reaction
conditions: cyclooctene (1.0 mmol), n-octane (1.0 mmol), catalyst (0.0022 mmol),
imidazole (0.10 mmol), acetonitrile (5.0 ml), TBHP (2.0 mmol), and temperature
was also obtained at 60 ◦C when the reaction continued until
24 h.
(60
1
◦C).
In addition to the catalyst, the presence of a nitrogenous base like
imidazole as a co-catalyst increased the conversion of cyclooctene.
It is well established that imidazole has more influence than pyri-
dine as a fifth ligand in heme compounds [33]. It has been suggested
that, in addition to ␲ basicity (sideways basicity), unusually large ␲
basicity of imidazole compared to other ␲ bases leads to a stronger
3.4.2. Oxidation of various olefins
Finally, in order to explore further the oxidation potential of
the hydrazone Schiff base catalyst 2, oxidation of various olefins
was performed under the same reaction conditions which proved
to be the best for cyclooctene. We have studied linear, cyclic and
phenyl substituted olefins as substrates. Epoxidation of various
olefins by TBHP in the presence of 2 and imidazole proceeded
quantitatively in CH3CN. This catalytic system led to the epoxida-
tion of various olefins (Table 6) with good conversions (31–80%)
(except for indene with 9% conversion) and excellent epoxide
selectivity. The only exception was the oxidation of allyl benzene,
where 8% 3-phenyl-propenal, 5% benzaldehyde and 60% 2-benzyl-
oxirane were obtained (Table 6). Electron-rich cyclohexene and
cyclooctene displayed a greater reactivity than terminal olefins. The
relatively electron-poor 1,3-cyclooctadiene shows lower reactivity
than cyclooctene. This reflects the electrophilic nature of the oxy-
gen transfer from the plausible iron–oxo intermediate to the olefinic
double bond. Allylic oxidation or formation of by-products, which
is seen in the metalloporphyrin systems [34,35] was not observed
in the epoxidation of alkenes such as cyclohexene. The higher reac-
tivity of cyclohexene relative to cyclooctene might be due to the
greater angle strain in the former.
*
iron back-bond with the empty electronegative ␲ orbitals of oxy-
gen [33].
The catalytic activities of 1, 2, and 3 were examined under the
optimized conditions for catalyst 2 (TBHP/cyclooctene ratio = 2, ace-
tonitrile, reaction temperature 60 ◦C). Catalyst 3 shows the lowest
(10%)andcatalyst2 shows thehighest activity (38%)inthe oxidation
of cyclooctene with TBHP (Fig. 11).
The effect of the imidazole concentration on the oxidation of
cyclooctene is illustrated in Fig. 12. Different imidazole/catalyst
molar ratios (7:1, 33:1, 45:1 and 68:1) were considered while
keeping the fixed amount of cyclooctene (1 mmol) and catalyst 2
(0.0022 mmol) in 5 ml of acetonitrile at 60 1 ◦C. Increasing the
imidazole/catalyst ratio from 7:1 to 33:1 increases the conversion
from 19 to 55%. A higher ratio reduces the conversion. The max-
imum cyclooctene conversion is obtained with a molar ratio of
33:1.
Fig. 10. Effect of temperature on conversion of cyclooctene by 2. Reaction conditions:
cyclooctene (1.0 mmol), n-octane (1.0 mmol), catalyst (0.0022 mmol), imidazole
(0.073 mmol), acetonitrile (5.0 mmol) and TBHP (2.0 mmol).
Fig. 12. Effect of imidazole concentration on oxidation of cyclooctene by 2. Reaction
conditions: cyclooctene (1.0 mmol), n-octane (1.0 mmol), catalyst (0.0022 mmol),
acetonitrile (5.0 ml), TBHP (2.0 mmol) and temperature (60
1
◦C).

H.H. Monfared et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 139–146
145
Table 6
exo/endo ratio has been reported with iodosylarenes using sev-
eral substituted tetraphenylporphyrin and teterapyridylporphyrin
iron(III) salts as catalysts [39]. A comparison of this catalytic sys-
tem with previously reported systems shows that the conversion
and the selectivity are higher than in the other systems [40].
Oxidation of various olefins catalyzed by 2.
No.
Olefin
Product(s)
% Yield
Time 4 h (24 h)
%Conversion
Time 24 h
1
2
53 (80)
55 (70)
80
70
4. Conclusion
Our work has revealed that coordination complexes of Fe(III)
and tridentate hydrazone Schiff base ligands obtained by reaction
of benzhydrazide and 2-hydroxy-3-methoxybenzaldehyde deriva-
tives afford a new class of Fe(III) catalysts for the oxidation of olefins.
Three Fe(III) complexes were prepared and it was demonstrated
that these dissymmetric hydrazone Schiff base iron(III) complexes
(1–3) are highly selective catalysts for the oxidation of various
olefins by TBHP under mild conditions. The catalytic activity is fur-
ther increased by the addition of imidazole. Electron-rich alkenes
display higher reactivity than electron-poor ones. The potential
of these catalysts in the oxidation of various olefins was proved
by converting most of the investigated olefins to the correspond-
ing epoxides by 100% selectivities. Both steric effects of the olefins
and electronic effects of the catalyst ligands have an impact on the
selectivity of the oxidation reactions.
3
4
12 (47)
47
73
33 (60)
– (8)
– (5)
8 (15)a
31
Acknowledgments
5
We are grateful to the Zanjan University, the Faculty of Chem-
istry and Biochemistry of the Ludwig-Maximilians-Universität
München, and the School of Chemistry for financial support of this
study.
6
7
– (9)
9
1-octene
1-octene oxide
2-octanone
4 (35)
– (38)
73
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