Catalysis of alkene epoxidation by manganese(II) and (III) complexes of both Schiff base and reduced Schiff base ligands utilizing environmentally benign H2O2

Article abstract of DOI:10.1016/j.poly.2018.01.008

Mono and disubstituted tridentate Schiff base ligands were synthesized from the reaction of 2(2-aminoethyl)pyridine with 5-bromosalicylaldehyde (H-L¹) and 3,5-dibromosalicylaldehyde (H-L²) respectively, which were subsequently reacte

Full text of DOI:10.1016/j.poly.2018.01.008

Accepted Manuscript
Catalysis of Alkene Epoxidation by Manganese (II) and (III) Complexes of both
Schiff base and Reduced Schiff base Ligands utilizing Environmentally Benign
H2O2
Rita Egekenze, Yilma Gultneh, Ray Butcher
PII:
S0277-5387(18)30016-0
https://doi.org/10.1016/j.poly.2018.01.008
POLY 13002
DOI:
Reference:
Polyhedron
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Accepted Date:
29 October 2017
8 January 2018
Please cite this article as: R. Egekenze, Y. Gultneh, R. Butcher, Catalysis of Alkene Epoxidation by Manganese (II)
and (III) Complexes of both Schiff base and Reduced Schiff base Ligands utilizing Environmentally Benign
H2O2, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.01.008
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Catalysis of Alkene Epoxidation by Manganese (II) and (III)
Complexes of both Schiff base and Reduced Schiff base Ligands
utilizing Environmentally Benign H2O2.
Rita Egekenze, * Yilma Gultneh, Ray Butcher
Department of Chemistry, Howard University, Washington Dc, 20059, USA
Email Address of Corresponding Author: rnegekenze@gmail.com
Abstract:
Mono and disubstituted tridentate Schiff base ligands were synthesized from the reaction
2(2-aminoethyl)pyridine with 5-bromosalicylaldehyde and
(H-L1)
of
3, 5-dibromosalicylaldehyde (H-L2) respectively, which were subsequently reacted with NaBH4
to yield the corresponding reduced Schiff base ligands H-rL1 and H-rL2. The complexes;
MnIII-(L1)2 (1), MnIII-(rL1)2 (3), and MnII-(L2)2 (2), MnII-(rL2)2 (4) were obtained by the reaction
of the ligands with {Mn(ClO4)2.6H2O}. The ligands and complexes were characterized by
UV-Visible, ESI-MS and FT-IR spectroscopy. The 1H NMR spectra of the ligands as well as the
crystal structures, and electrochemical properties of the complexes were obtained. The crystal
structures of MnIII-(L1)2 and MnII-(L2)2 show coordination of the ligands in tridentate mode in
octahedral geometry. Cyclic voltammetric studies of Mn-(L1)2 and Mn-(L2)2 in acetonitrile
solution show two prominent reversible peaks attributed to the redox processes: Mn(II)/Mn(III),
(E½ = 0.203 - (-0.204 V) and Mn(III)/Mn(IV), (E½ = 1.103 - 0.195 V) vs. Ag/AgCl while that of
Mn-(rL4)2 showed three quasi reversible peaks suggesting three redox couples: Mn(II)/Mn(III)
(E1/2 = -0.109); Mn(III)/Mn(IV), (E1/2 = 0.125 and (Mn(IV)/Mn(V), (E1/2 = 0.649) vs Ag/AgCl.
Epoxidation of cyclohexene and 1-hexene by hydrogen peroxide catalyzed by the manganese
complexes gave epoxide yield of 42-53% with a turnover of 10.50-13.25 after 20 hours of
reaction for the reduced Schiff base complexes {MnIII-(rL1)2 and MnII-(rL2)2} at 0°C.
Keywords: Schiff base, Reduced Schiff base, Manganese complexes, Co-catalysts, Epoxidation, Catalysis
1. Introduction
1

The epoxidation of (carbon-carbon double bond) alkenes plays an important role in
fundamental chemistry and industrial processes as it leads to the generation of the reactive three
membered epoxide ring [1-3]. Epoxides are important reaction intermediates and starting
materials for many products [4-10]. Reduction, rearrangement or ring opening reactions of
epoxides with various nucleophiles convert epoxides to diols, amino alcohols, allylic alcohols,
ketones and polyether’s. [11, 12]. These functionalized products are in turn used as raw materials
for the manufacture of pharmaceutical products, pesticides, epoxy paints, agrochemicals, food
additives, flavor and fragrance compounds [13 -17]. There is a great need therefore, to develop a
highly efficient catalyst for the epoxidation reactions [2, 9, 24, 25, 29].
The traditional epoxidation reactions use strong oxidants such as peroxy acids and
peroxides and mineral acids to promote oxygen transfer to the double bond [4, 18, 19]. These
oxidants are hazardous and produce organic by-products and wastes that are not environmentally
friendly [18-21]. In a bid to overcome the disadvantages of using peracids, many other oxygen
donors such as molecular oxygen, iodosyl benzene (PhIO), alkylhydroperoxides, hypochlorites
and hydrogen peroxide have been employed as oxidants for epoxidation of olefins [6, 11, 18, 22,
23]. Hydrogen peroxide is a preferred oxidant for both laboratory and industrial production
because of its low cost and safety when used at low concentration. In addition, it is a mild
environmentally benign reagent with water as the sole by-product and has high and effective
oxygen content [6, 20, 24-27].
A metal complex catalyst is required to enhance the activity of hydrogen peroxide as
oxidant for olefin epoxidation [20, 28]. Many metal complexes have been synthesized and
studied for epoxidation of olefins. Extensive research has been conducted on manganese
complexes of synthetic ligands, particularly Schiff base ligands, prompted by the desire to
achieve biomimetic epoxidation catalysts [24, 28, 30] and these manganese complexes have been
found to be efficient catalysts. The choice of manganese among other transition metals is
particularly important considering its presence and role in many metalloenzymes [12, 31-40], its
low toxicity and its commercial availability [1]. Activation of H2O2 by Mn(II) or Mn(III)
complexes requires a co-catalyst for high epoxide yield. Co-catalysts such as pyridines, pyridine
N-oxides, imidazole, and tertiary amines [41, 42] have been used alone or in combination with
carboxylic acids [42]. It has also been reported that simple ammonium salts such as ammonium
acetate alone can be used as efficient co-catalysts for alkene epoxidation by H2O2 [42].
2

Initial studies of the manganese salen complex catalyzed epoxidation proved that steric
bulk at the C3 position of the phenol ring and variation of electronic properties of the
substituents at the C5 position has great effect in the selectivity and efficiency of the epoxidation
reaction [43, 45]. The presence of an electron-withdrawing group at the C5 position enhances the
catalytic activity of the Mn catalyst [46-48] and was also proposed to result in a more reactive
oxo intermediate to yield the epoxide [43].
Cu(II) complexes of reduced salen ligands also known as salan or tetrahydrosalen ligands
have been used as catalysts for epoxidation reactions. While this group of ligands have received
less attention, they possess benefits that include increased ligand flexibility because of the
reduction of the imine bond (C=N) and stability [49, 50].
In this study, we present the synthesis and characterization of monobromo and dibromo
substituted salicylaldehyde Schiff bases and their reduced forms with NNO coordination. We
have studied here, Mn complexes of two Schiff bases and the reduced forms as homogeneous
catalysts for epoxidation of olefins with H2O2. The yields of epoxides by complexes of reduced
Schiff base ligands were found to be higher than those of the complexes of Schiff bases.
2.
2.1. Materials.
Analytical grade Mn(ClO4)2.6H2O, olefins (cyclohexene and 1-hexene), hydrogen
Experimental
peroxide (aqueous, 30%), ammonium acetate, acetic acid, perchloric acid, triethylamine, and
tetrabutylammonium perchlorate were purchased from Sigma-Aldrich and used as received.
Solvents (dichloromethane, methanol, acetone, diethyl ether and acetonitrile) used were either
99% or purified by refluxing and distillation under a nitrogen or argon atmosphere as follows;
dichloromethane over CaCl2 and CaH2, diethyl ether over CaCl2 anhydrous 20 mesh granular,
Methanol over CaO, Acetonitrile over P2O5 and CaH2. The distillates were stored over 3 Å or 4
Å molecular sieves.
2.2. Physical measurements
3

1
FT-IR spectra were recorded with Perkin-Elmer 100 FT-IR, H NMR spectra of the
ligands were obtained on a Bruker Avance 400 MHz spectrometer using CDCl3 solvent.
Chemical shifts were reported using CHCl3 (δ 7.26) as internal reference. The UV-Vis
absorption spectra were recorded on a HP 8453 (Agilent 8453) Spectrophotometer using
methanol and dichloromethane as solvents. Cyclic Voltammetric studies of the complexes were
performed in the potential range +1.5 to -1.5 V with a Bio-Analytical system (BAS) 100 in
acetonitrile or N,N-dimethylformamide (DMF) containing 0.1 M tetrabutylammonium
perchlorate (TBAClO4 or TBAP) as the supporting electrolyte under an argon atmosphere at
room temperature and scan rate of 100mVs-1. The electrochemical cell consists of a standard
three-electrode system with a glassy carbon working electrode, platinum-wire as auxiliary
electrode, and Ag/AgCl reference electrode.
ESI-(+)-MS {high resolution mass spectra (HRMS)} were carried out on Agilent 6224
Accurate-mass (ESI) Q-TOF mass spectrometer. Electrospray ionization technique in the
positive ion mode (ESI-(+)-MS) was used. The sample solutions were made in methanol or
acetonitrile and auto injection method was used with a 2.0 mL vial.
2.3. Syntheses
2.3.1. Synthesis of the Ligands;
2.3.1. Synthesis of Ligands H-L1 and H-rL1
H-L1: Ligand H-L1 was prepared as described in (20b) according to the following
procedure; 5.6250 g (28 mmole) of 5-bromosalicylaldehyde [C7H5BrO2] in 30 mL of methanol
was added to a solution of 3.4048 g (28 mmole) of 2-(2-aminoethyl)pyridine in 10 mL of
methanol and stirred under reflux for 5 hours forming an orange yellow solution. The solvent
was then removed by rotary evaporation and the yellow solid obtained dissolved in
dichloromethane and methanol (1:1 v/v) and recrystallized by slow evaporation of solvent
mixture yielding the ligand H-L1 [C14H13BrN2O] in 98% yield. Molecular mass: calculated for
[C14H14BrN2O]+ = 305.0289, ESI-MS determined m/z = 305.0303. IR (LiTaO3, KBr)
(cm-1) imine; 1634 (m), 1HNMR {CDCl3}: δ (ppm) 13.45 (s, 1H, ArO-H).
4

H-rL1: 8.8438 g (29 mmole) of the Schiff base H-L1 in 40 mL of methanol was reacted by
incremental addition of a solution of sodium borohydride (NaBH4) (3.2240 g; 85 mmole) in
methanol until a colorless solution resulted. Refluxing the solution for 3 hours and cooling to
room temperature produced the ligand H-rL1 in 77% yield (6.81 g). Molecular mass: calculated
for [C14H16BrN2O]+ = 307.0446, ESI-MS determined = 307.0468. IR (LiTaO3, KBr) (cm-1);
1
N-H stretch reduced imine bond 3235 (m), HNMR {CDCl3}: δ (ppm): reduced imine
bond; 3.0 (d, 2H CH2)
2.3.1.2. Synthesis of Ligands H-L2 and H-rL2.
H-L2: Schiff base ligand H-L2 was obtained as described in (20b) with modification by
dispersing 3, 5-dibromosalicylaldehyde [C7H4Br2O2] 3.5480 g (12.7mmole), in 25 mL of hot
methanol and mixing with solution of 2-(2-aminoethyl)pyridine (1.5431 g; 12.6 mmole) in
15 mL of methanol which produced a yellow solution. The solution was refluxed for 4 hours and
cooled to room temperature to obtain a yellow solid, ligand H-L2 with a yield of 4.63 g (91%).
Molecular mass calculated for [C14H13Br2N2O]+ = 382.9595, ESI-MS determined
m/z = 382.9412. IR (LiTaO3, KBr) (cm-1); imine bond stretch; 1632 (s), 1HNMR {CDCl3}: δ
(ppm) 14.60 (s, 1H ArO-H).
H-rL2: H-rL2 was prepared by reacting the solution of H-L2 (3.3888 g; 8.2 mmole) in
30 mL of methanol with 1.1056 g (29.2 mmole) of sodium borohydride (NaBH4) powder added
in small portions until the change of the orange yellow solution to near colorless. The solution
was refluxed for 4 hours and cooled to room temperature to obtain the colorless solid by gravity
filtration, ligand H-rL2 in 79% yield (2.07 g). Molecular mass: calculated for [C14H15Br2N2O]+
= 384.9551, ESI-MS determined = 384.9548. IR (LiTaO3, KBr) (cm-1): NH from reduced
1
imine bond; 3055 (w), HNMR {CDCl3}: δ (ppm): 3.02 (d, 2H CH2) from reduced imine
bond.
5

2.3.2. Synthesis of the complexes
2.3.2.1. MnIII-(L1)2 (1); {Mn(III)-(L1)2}(ClO4) [C28H24 Br2MnClN4O6]: Solution mixture of
Ligand H-L1 [C14H13MnBrN2O] (1.803 g, 5.9 mmole) in 25 mL methanol and triethylamine
[C6H15N] (988 µL, 7.08 mmole) was reacted with Mn(ClO4)2.6H2O (1.072 g, 2.96 mmole) in
methanol solution (25 mL) under reflux for 4 hours. When cooled to room temperature, the
solvent volume was reduced with rotary evaporator; the precipitate was filtered, washed with
diethyl ether and dried in the desiccator following recrystallization from methanol/
dichloromethane, and layering with diethyl ether. The yield was 1.85 g (64%). Characterization
data for [C28H24MnBr2ClN4O6] are as follows; Molecular mass: calculated for
[C28H24MnBr2N4O2]+ = 660.9646, ESI-MS determined m/z = 660.9641. IR (LiTaO3, KBr)
(cm-1): imine bond; 1614 (s).
2.3.2.2. MnIII-(rL1)2 (3), {MnIII-(rL1)2}(ClO4) [C28H28 Br2MnClN4O6]:
To a methanol solution (20 mL) of Mn(ClO4)2.6H2O (1.8695 g, 5.2 mmole), the ligand
H-rL2 (3.1678 g, 10.4 mmole) in 20 mL methanol was added followed by the addition of
triethylamine (1.58 mL, 11.4 mmole) with constant stirring and refluxed for 4 hours. When
cooled to room temperature, the solvent was reduced by rotary evaporation; the precipitate was
filtered, washed with diethyl ether and recrystallized from acetonitrile and diethyl ether. A yield
of 2.02 g (40%) was obtained. ESI-MS analysis gave molecular mass more than twice the
calculated mass indicating presence of a dimer or multimetal centers. Molecular mass:
calculated for the structure = 667.3008; {for the proposed molecular formula of the dimer
[C56H52Br4Mn4N8O10]+} = 1536.4505, ESI-MS determined m/z = 1509.8334.
2.3.2.3. MnII-(L2)2 (2), {MnII-(L2)2} [C28H22Br4MnN4O2]:
Methanol solution (25 mL) of the Mn(ClO4)2.6H2O (1.2710 g, 3.5 mmole) was mixed
with the methanol solution (15 mL) of the ligand HL2 (2.5918 g, 6.7 mmole) and triethylamine
[C6H15N] (867µL, 6.8 mmole) was added with stirring and refluxing for 4 hours. Solvent was
reduced by rotary evaporation after cooling to room temperature; the precipitate was filtered,
washed with diethyl ether followed by recrystallization from dichloromethane and diethyl ether.
The yield was 1.82 g (56%). Molecular mas: calculated for [C28H22Br4MnN4O2]+ = 816.7857,
ESI-MS determined m/z = 816.7854.
6

2.3.2.4. MnII-(rL2)2 (4), {MnII-(rL2)2} [C28H26Br4MnN4O2]:
Mn(ClO4)2.6H2O (0.5120 g, 1.4 mmole) in 20 mL of methanol was added to a solution
20 mL of ligand H-rL4 (1.0594 g, 2.7 mmole) and triethylamine [C6H15N] solution (706 µL, 2.7
mmole) in 5 mL methanol was added. The brown solution that resulted was refluxed for 4 hours
and cooled to room temperature. The precipitate was filtered, washed with diethyl ether and
dried in the desiccator. Recrystallization was achieved from dichloromethane and diethyl ether.
The yield was 1.11 g (71%). Characterization data for [C28H26Br4MnN4O2] are; Molecular
mass: calculated for the structure = 825.0930; (for the proposed molecular formula for the
multimetal centered complex [C56H52Br8Mn4N8O10]+} = 1856.09, ESI-MS determined m/z =
1827.4829.
2.4. X-ray structural determination
The crystal structures of complexes 1, 2 and 4 were determined using Oxford Diffraction
Gemini R.CCD diffractometer with Mo K radiation graphite-monochromatized (= 0.71073Å)
with 1° scans. A low-quality structure of a manganese complex of H-(rL2)2 could be determined.
It shows a cubane structure with one missing corner made up of four manganese centers bridged
by three oxo-ligands. The coordination sphere for each manganese ion is completed by
coordination to the tridentate reduced Schiff base ligand. The data for 4 was of very low-quality
due to extensive and multiple twinning and so apart from a diagram, details of this structure are
only contained in the supplementary material. An Oxford Diffraction Cryojet XL low
temperature device was used to keep the crystals at constant temperature of 123 (2) K during
data collection.
2.5. Epoxidation reactions
The catalysis experiments for the manganese complexes 1, 2, 3 and 4 were carried out in
a mixture of CH3OH/CH2Cl2/CH3COCH3 (volume ratio 1:1:2); using mole ratio of
catalyst/co-catalyst/substrate/oxidant of 1:10:25:1000. The manganese complexes were the
catalysts, ammonium acetate/acetic acid (NH4COOCH3/CH3COOH or triethylamine/perchloric
7

acid (TEA/HClO4) was the co-catalyst, cyclohexene or 1-hexene was used as substrate and H2O2
was the oxidant. In a typical reaction, the catalyst, co-catalyst, substrate and oxidant was
continuously stirred at room temperature or at 0°C for 20 hours. Samples were extracted with
dichloromethane, dried with anhydrous magnesium sulphate and analyzed for epoxide with a
GC-MS Agilent 6890N Network GC system coupled with Agilent 5973 inert mass selective
detector. The epoxides were identified by comparison with the retention time of an internal
standard, bromobenzene. Dichloromethane was preferred because of its low temperature
volatility. The sample was extracted to remove H2O2 and prevent its corrosive effect on the
GC-MS. A plot of the ratio of peak area of standard oxide of the substrate to the peak area of
bromobenzene versus the concentration ratio of the epoxide to that of bromobenzene was used to
calculate the concentration of the epoxide in the sample. The turn over number (TON) was
calculated by diving moles of substrate by moles of catalyst times % yield of product.
Br
Br
NH
N
OH
N
OH
N
H-L1
H-rL1
Br
Br
N
NH
OH
N
OH
N
Br
Br
H-L2
H-rL2
Scheme 1. Structure of the Ligands employed for complexation
8

R4
O
R4
NH2
R3
R2
N
R3
MeOH
H
N
+
OH
N
Reflux 4hrs
OH
R2
R1
R1
General structure for Schiff base Ligands (H-L)
(a) MeOH
(b) NaBH4
(c) Reflux 4hrs
(H-L1), (H-rL1); R1, R2, R4 = H, R3 = Br
(H-L2), (H-rL2); R2, R4 = H; R1, R3 = Br
R4
R3
N
H
R2
OH
N
R1
General structure for Reduced Schiff base Ligands (H-rL)
Scheme 2: Synthetic rout to the Ligands
R4
R4
R3
R2
R3
N
H
N
2
2
OH
N
R2
OH
N
R1
R1
Ligands (H-rL)
Ligands (H-L)
(a) MeOH
(b) Triethylamine
(c) Reflux 4hrs
(a) MeOH
(b) Triethylamine
(c) Reflux 4hrs
+ Mn(ClO4)26H2O
+ Mn(ClO4)26H2O
4
R4
R3
H
N
R3
R2
N
R2
O
N
O
N
R1
R1
Mn
Mn
R1
R1
O
R2
R3
N
O
R2
R3
N
NH
N
R4
R4
General structure for
Complexes (Mn-rL)
General structure for
Complexes (Mn-L)
Scheme 3: Synthetic rout to the complexes
9

H2O2 + Mn complex (catalyst)
+ co-catalyst
O
CH3OH + CH2Cl2 + CH3COCH3
r.t or 0OC
Epoxide yield %: NH4COOCH3/CH3COOH;
TEA/ClO4
36 (r.t), 42 (0OC
21 (r.t), 45 (0OC
Mn-(rL1)2
Mn-(rL2)2.
43 (r.t), 53 (0OC);
18 (r.t), 52 (0OC).
)
)
Scheme 4: General scheme for epoxidation of cyclohexene
H3C
H3C
H2O2 + Mn complex (catalyst)
+ co-catalyst
CH2
O
CH3OH + CH2Cl2 + CH3COCH3
r.t or 0OC
Epoxide yield %: NH4COOCH3/CH3COOH;
TEA/ClO4
13 (r.t), 13 (0OC
9 ( r.t), 19 (0OC
Mn-(rL1)2
Mn-(rL2)2.
13 (r.t), 20 (0OC);
9 (r.t), 24 (0OC).
)
)
Scheme 5: General scheme for epoxidation of 1-hexene
10

3. Results and Discussion
3.1. Elemental (Combustion) Analysis
The results of elemental analysis for the new complexes; MnIII-(rL1)2 and MnII-(rL2)2
indicate that the observed micro analytical data (C, H and N) of the complexes compare
favorably with the theoretically calculated C, H and N values. For complex MnIII-(rL1)2,
calculated: C (43.78%), H (3.41%), N (7.29%); found/observed C (43.72%), H (3.58%), N
(7.18%) and complex MnII-(rL2)2; Calculated: C (36.23%), H (2.82%), N (6.04%);
found/observed: C (35.82%), H (2.60%), N (6.03%). The results of the micro analysis further
confirm the molecular formula and molecular weight of the complex MnII-(rL2)2 as obtained
from x-ray crystallography and ESI-MS analysis. It also confirms the ESI-MS result for the
complex MnIII-(rL1)2 and suggests possible molecular formula since the x-ray structure could not
be determined.
3.2. NMR spectra
1
The HNMR spectra are displayed in Figures S1-S4. Aromatic protons appear as singlets and
multiplets between 5.50 ppm and 8.6 ppm. The doublets for the methylene proton (CH2-CH2)
was observed for H-rL1 at 3.08 and 3.97 ppm; H-rL2 at 3.18 and 4.0 ppm. The peak at 3.00 ppm
for H-rL1 and H-rL2 is assigned to the proton of the methylene (-ArCH2) resulting from reduction
of the imine bond. The appearance of the ArCH2–N signal at 3.0 ppm for H-rL1 and H-rL2
simultaneously with the disappearance of the C=NH signal around 8.12 -- 8.28 ppm for these
ligands confirm reduction of the imine bond [52, 55, 57]. The results obtained compare very
well with δ = 13.42 obtained for –OH, δ = 3.86 ppm for CH2CH2-, 3.64 ppm for -NCH2, and the
results for aromatic protons previously reported [58-60].
3.3. Infrared spectra (FT-IR)
Infrared spectra of the complexes were analyzed and compared with those of the
corresponding free ligands. The absorption peaks are listed in Table S1. Complexes 1 and 2
11

show strong bands at 1614 cm-1 and 1617 cm-1 respectively, assigned to imine (C=NH)
stretching vibration which shifted to lower frequencies than 1634cm-1 and 1632 cm-1 observed
for the free ligands H-L1 and H-L2. The shift to lower wave number in the complexes indicates
coordination of the azomethine nitrogen to the metal ion [34, 35]. The phenolic C-O stretching
frequency occurred between 1264 – 1289 cm-1 in the complexes and slightly lower in the free
ligands as seen between 1261 – 1280 cm-1 indicative of coordination of the metal ion to the
phenolic oxygen [36]. Absence of C=NH band and appearance of the N-H band at 3238 cm-1 and
3230 cm-1 for complexes 3 and 4, and at 3236 cm-1 and 3055 cm-1 recorded for ligands 3 and 4
confirm reduction of imine bond [49]. The sharp, strong and high intensity of the bands at 1068-
-
1095cm-1 indicates the perchlorate (ClO4 ) in the complexes [36]. FT-IR spectra for all the
ligands and complexes are shown in the supporting information Figures S5-S12.
3.4. Electronic spectra
The UV-Vis absorption spectra for the ligands are shown in Figure S13 while those of the
complexes are displayed in the supporting document Figures S14 – S17. All the ligands and complexes
showed absorption bands in the range 202 – 298 nm assigned to π→ π* resulting from transitions
involving the aromatic ring [51, 61]. The bands at 328 nm for ligand H-L1 and 332 nm for H-L2
are due to n → π* transitions of the imine (C=NH) bond, while the bands at 413 nm and 422 nm
are assigned to intramolecular charge transfer (CT transition) involving the whole ligand
molecule [61-64]. These results are similar to those previously reported [60, 65-67]. The n → π*
transitions of the imine (C=NH) bond were observed at 334 nm for complex 1 and 339 nm for
complex 2 while the bands at 425 nm (complex 1) and 431 nm (complex 2) are assigned to
transition resulting from intramolecular charge transfer in the entire complex molecule [61 68].
3.5. ESI mass spectra
The calculated molecular mass, molecular ion peaks (m/z) and corresponding species
responsible for the signals for the ligands and complexes are displayed on Table S2. The spectra
for complex 1 and complex 3 are shown in Figures 1 and 2. The rest of the spectra are displayed
12

in Figures S18 to S21 for the Ligands and Figures S22-S23 for the complexes. The peaks
observed at m/z 305, 382, 307 and 382 for the ligands H-L1, H-L2, H-rL1 and H-rL2 respectively
are assigned to [M + 1]+1 ion and the peaks at m/z 663, 817, 1509 and 1827 for the complexes
Mn-(L1)2, Mn-(L2)2, Mn-(rL1)2 and Mn-(rL2)2 respectively are assigned to molecular ion M+. The
m/z of the ligands indicate that they are neutral compounds and required positive ion to achieve
ionization. The molecular ion peaks (m/z) for complexes 1 and 2 are consistent with the
calculated molecular mass (as shown on Table S1) confirming that they are charged molecules.
However, m/z for complexes 3 and 4 (complexes of the reduced Schiff bases) show masses
consistent with dimerization of the complexes or presence of more than one manganese centers
as their molecular masses determined by ESI-MS are more than twice the calculated molecular
masses. The crystal structure for complex 4 confirms presence of multi manganese center as
possible explanation for the high molecular mass. Result of combustion analysis also suggests
similar structure for complex 3 and accountable for the high molecular mass for complex 3.
There could be possible hydrogen bonding of the type O-H- - -O and N-H- - -O resulting from
amine hydrogens-bonding to N or O of neighboring ligand molecules to form dimerization and
oligomerization as reported in the structure of Cu (II) salan type complex [69]. It demonstrates
that the secondary amine in the reduced complexes makes the ligand more flexible in
coordination, but also capable of forming hydrogen bonding structures that lead to dimerization
and oligomerization in the solid states, previous [49]. This effect of dimerization and
oligomerization is tantamount to the formation of dinuclear/multinuclear metal ions centers that
make cooperative reactivity leading to more effective catalytic center compared to mononuclear
metal ion centers. This is borne out in the higher epoxide yield obtained with complexes 3 and 4
(with the amine ligands) compared with those of their Schiff base complexes 1 and 2 (see tables
4 and 5 on data on experiment for the catalytic epoxidation efficiencies of the Mn(III) and Mn
(II) complexes studied in this work).
13

Figure 1: ESI-MS spectrum showing molecular ion (m/z) of complex MnIII-(L1)2 at
662.9625 assigned to M+ which is in agreement with the calculated molecular mass of
663.2692
Figure 2: ESI-MS of complex MnIII-(rL1)2 showing the m/z at 1509.8334 assigned to M+ for
the specie [C56H52Br4Mn4N8O10]+ which is more than twice the calculated mass of
667.3008 shown in Table S2, suggesting presence of more than one manganese center.
14

3.6. Cyclic voltammetry
The cyclic voltammetric potentials (Ag vs Ag/AgCl) of complexes 1, 2 and 4 are shown on
Table 1. Figure 3 shows the voltammetric plots of complexes 1 and 2 each showing two
quasi-reversible Mn(II)/Mn(III) and Mn(III)/Mn(IV) redox processes. The half way potential
calculated as E½ = ½ (Epa + Epc) and Ep = Epa - Epc) are listed in Table 1. Complex 1 redox
potentials are observed at E½ = 0.203 V (∆Ep = 92 mV) and E½ = 1.103 V (∆Ep = 79 mV).
Complex 2 shows E½ = 0.0381 V (∆Ep = 95 mV) for Mn(II)/Mn(III) redox couple and a
quasi-reversible reaction at E½ = 1.194 V (∆Ep = 196 mV) for the Mn(III)/Mn(IV) redox
reactions. Due to poor solubility of 3 in both acetonitrile and DMF, very weak peaks are
observed in the voltammogram. Complex 4 shows three redox couple peak at E½ = -0.109 V
(∆Ep = 22 mV), 0.125 V, (∆Ep = 26 mV) and 0.649 V (∆Ep = 39 mV) respectively. The
voltammetry behavior of this complex suggests the presence of multi-manganese ion as also
indicated that in the ESI-MS, a peak corresponding to more than twice the molecular weight of
the monomeric species is observed. It was reported that hydrogen-bonding feature in such
compounds enable formation of multi-metal complexes [49].
15

Table 1. Redox potentials vs. Ag/AgCl in the Cyclic Voltammetry of the Complexes 1-4 in
Acetonitrile at 25OC (298K)
Compl Mn(II)/(Mn(III)
exes
Mn(III)/Mn(IV)
∆E Epa Epc
Mn(IV)/Mn(V)
∆E Epa Epc
Epa Epc
(V) (V)
E
E
E
½
∆E
p
½
½
p
p
(V) (V)
(V) (V)
(V)
(V)
(V)
(Ep
(Ep
(Ep
-
-
-
a
a
a
Epc
)
Epc
)
Epc
)
Mn-
(L1)2
0.2 0.1 0.2 0.0 1.1 1.0 1.1 0.0
49 57 03 92 42 63 03 79
Mn-
(L2)2
0.4 0.3 0.3 0.0 1.2 1.0 1.1 0.1
28
33
81
95
92
96
94
96
Mn-
-
-
-
0.3 0.3 0.0 0.1 0.2
1 28 61 95 67
(rL1)2
0.0 0.3 0.2
49
59
04
Mn-
-
-
-
0.0 0.1 0.1 0.1 0.0 0.6 0.6 0.6 0.0
38 12 25 26 68 29 49 39
(rL2)2
0.0 0.1 0.1 22
98 09
2
Measured in acetonitrile with 0.1 M tetrabutylammonium perchlorate (TBAP)
a Data from cyclic Voltammetric measurements: E½ was calculated as average of anodic (Epa) and
cathodic (Epc) peak potentials E½ = ½ (Epa + Epc)
b ∆Ep
E
E
=
- pc at scan rate 0.1 V/s
pa
16

2.50E+01
2.00E+01
1.50E+01
1.00E+01
5.00E+00
0.00E+00
-5.00E+00
-1.00E+01
-1.50E+01
-2.00E+01
3.50E+01
3.00E+01
2.50E+01
2.00E+01
1.50E+01
1.00E+01
5.00E+00
0.00E+00
-5.00E+00
-1.00E+01
-1.50E+01
-2.00E+01
1.142 V
1.292 V
Mn-(L1)2
Mn-(L2)2
0.428 V
0.249 V
1.096 V
1.063 V
0.333 V
0.157 V
-1.5
-0.5
0.5
1.5
2.5
-1.5
-0.5
0.5
1.5
2.5
Potential V
Potential V
2.00E+01
1.50E+01
1.00E+01
5.00E+00
0.00E+00
-5.00E+00
-1.00E+01
-1.50E+01
-2.00E+01
3.00E+01
2.50E+01
2.00E+01
1.50E+01
1.00E+01
5.00E+00
0.00E+00
-5.00E+00
-1.00E+01
-1.50E+01
-2.00E+01
Mn-(rL1)2
Mn-(rL2)2
0.328 V
0.668 V
-0.049 V
0.138 V
-0.098 V
0.061 V
-0.359 V
0.629 V
0.112 V
-0.12 V
-1.5
-0.5
0.5
1.5
-1.5
-0.5
0.5
Potential V
1.5
2.5
Potential V
Figure 3.
Cyclic voltammograms of a 10-3 M solution of the complexes 1, 2, 4 in acetonitrile
and 3 (in DMF) with tetrabutylammonium perchlorate (TBAP) supporting
electrolyte (0.1 M), glassy-carbon working electrode, Ag/AgCl reference electrode,
platinum wire auxiliary electrode and at scan rate of 100 mv/s.
3.7. X-ray Diffraction crystallographic structure of complexes 1, 2 and 4
Although all four complexes were synthesized by reaction of the ligand and
Mn(II)(ClO4)2 6H2O, the Schiff bases 1 and 3 produced Mn(III) complexes, whereas, the reduced
forms (amine ligands), 2 and 4 produced Mn(II) complexes. This demonstrates the stark
difference in the stabilizing of different oxidation states of the metal complexes by these two
types of ligands. The Schiff bases stabilize higher oxidation states of the complex while the
reduced form stabilizes the lower oxidation state. The crystal of Mn-(L1)2 (1) contains two
17

crystallography non-equivalent complex cations (labeled 1A and 1B atoms with respective labels
as Mn1 and Mn2 and A and B for the other atoms), in both structures Mn ions are in
centrosymmetric coordination. While both 1 and 2 have crystallized in the same triclinic system
with similar cell constants, complex 2, in contrast to complex 1, has crystallized with one
complete cation in the asymmetric unit with no crystallographically imposed symmetry.
In both the Mn(II) and Mn(III) complexes, the two ONN tridentate ligand molecules are
coordinated to the metal ion with the manganese ion in an axially elongated octahedral
coordination by two molecules of the tridentate ligands. Each of the Mn(III) in complex 1 is
coordinated to four nitrogen atoms {2N1A, 2N2A for Mn1 and 2N1B, 2N2B for Mn2} and two
oxygen atoms {2O1A for Mn1 and 2O1B for Mn2} from the two tridentate Schiff bases.
However, there are significant differences in their metrical parameters due to their difference in
oxidation state and due to the fact that Mn(III), being a high spin d4 system, is subject to
Jahn-Teller distortion.
The metal-donor atom bond length for the complexes (Table 3) show that the Mn-O bond
lengths for the Mn(II) compared to the Mn(III) complex are significantly longer as is expected
for the change on oxidation state. For complex Mn-(L1)2 (1) a Jahn-Teller axial elongation is
expected and this is seen from a comparison of the Mn-N1 [(2.032(8) Å and 2.049(8) Å] and
Mn-N2 distances [2.348(8) Å and 2.345(8) Å].
In general, for the complexes the Mn-O bond length is shorter than Mnazomethine-N1 which
is shorter than the Mnpy-N2. Oxygen is more electronegative and has smaller radius than nitrogen
hence the bond Mn-O is shorter than Mn-N. This shows that the Mn-O bond is shorter than
Mn-N1 (azomethine) bond and the Mn-N1 is shorter than Mn-N2 for the pyridine [33]. Mn-O
bond lengths are consistent with those reported for similar Mn(II) and Mn(III) phenolic oxygen
donors [71-76], and Mn-Nazomethine bond lengths are similar to those of Mn complexes of Schiff
base ligands previously reported [77-79].
The bond angles around the manganese atom in complexes 1 and 2 reveal significant
distortion from the ideal octahedral geometry probably because of ligand rigidity due to the
imine bond [80]. The bond angles in complex 1 {Mn-(L1)2} are slightly distorted. Complex 2
{Mn-(L2)2} showed greater distortion than complex 1. The cis- bond angles O(1A)-Mn-N(1B)
=100.48, and the trans- bond angles O(1B)-Mn-N(2A) = 166.93, N(1A)-Mn-N(1B) =174.05 in
complex 2 {Mn-(L2)2} deviate from trans bond angle of 180 and cis bond angle of 90 for a
18

regular octahedron hence, showing the extent of distortion in this complex. These bond lengths
and bond angles observed for complexes 1 and 2 are consistent with that observed in other
structurally characterized complexes containing manganese central atom and Schiff base ligands
[33, 64, 71, 72, 80].
While the data for complex 4 is not of high quality due to extensive and multiple
twinning for which no suitable twin law could be found, nevertheless the structure could be
determined and is of interest since it was the only derivative from the reduced Schiff base ligands
for which crystals could be obtained. The crystal structure of complex 4 (Figure 6) shows a
cubane structure with one missing corner made up of four manganese centers bridged by three
oxo-ligands. The coordination sphere for each manganese ion is completed by coordination to
the tridentate reduced Schiff base ligand. The molecular formula and formula weight of complex
4 determined by x-ray crystallography are close to the molecular mass of 1824. 4879 determined
by ESI-MS. The structure obtained for complex 4 supports literature report that reduced Schiff
base ligands have the potentials to form multinuclear complexes through a bridging phenolate
group [55, 56].
19

Table 2. Crystallographic data for complexes 1 and 2
Identification code
1
C28 H24 Br2 Cl Mn N4 O6
762.72
2
C30 H26 Br4 Cl4 Mn N4 O2
990.93
Formula
Formula weight (g mol-1)
123(2)
123(2)
Temperature (K)
1.54178
1.54184
Wavelength (Å)
Triclinic
Triclinic
Crystal system
Space group
P-1
P-1
10.3014(16)
10.846(2)
14.697(2)
75.124(14)
77.922(13)
64.531(16)
1423.6(4)
8.2563(5)
11.3194(6)
19.7151(9)
102.113(4)
90.804(5)
101.518(5)
1762.20(17)
a (Å)
b (Å)
c (Å)
α (°)
β(°)
γ (°)
Volume (Å3)
Z
2
1.779
8.358
2
1.868
11.437
966
Dcalc Mg/m3
Absorption coef. (mm-1)
F(000)
760
Crystal size (mm3)
Index ranges
0.3919 x 0.3000 x 0.1311
0.6958 x 0.5233 x 0.2279
-9<=h<=10, -14<=k<=11, -20<=l<=24
-12<=h<=12, -12<=k<=13, 0<=l<=18
5519
96.7
12072
99.9
Reflections collected
Data completeness (%)
Absorption correction
Tmin & Tmax
Analytical
0.491 and 0.154
Analytical
0.239 and 0.028
Full-matrix least-squares on F2
5519 / 68 / 396
Full-matrix least-squares on F2
7074 / 27 / 416
Refinement method
Data/restraints/parameters
Goodness of fit
1.029
1.037
R1 = 0.0492, wR2 = 0.1313
Final R indices [I>2σ(I)]
R1 = 0.0982, wR2 = 0.2490
R1 = 0.1386, wR2 = 0.2821
1.126 and -0.930 e.Å-3
R1 = 0.0543, wR2 = 0.1352
0.704 and -1.122 e.Å-3
R indices (all data)
Residual difference peaks
20

Table 3.
Selected bond distances (Å) and bond angles (O) at the manganese center for
complexes 1 {MnIII-(L1),} and 2 {MnII-(L2)2}.
1
2
Bond length Å
Bond length Å
1.878(7)
2.078(3)
Mn1-O1A
Mn1-N1A
Mn-O1A
Mn-N1A
2.032(8)
2.348(8)
1.895(7)
2.049(8)
2.212(3)
2.332(4)
2.113(3)
2.236(3)
2.288(4)
Mn1-N2A
Mn2-O1B
Mn2-N1B
Mn2-N2B
Mn-N2A
Mn-O1B
Mn-N1B
Mn-N2B
2.345(8)
Bond Angle (°)
Bond Angle (°)
O1A-Mn-O1A*
O1A-Mn-N1A
O1B-Mn-N1A
O1A-Mn1-O1A*
O1A-Mn1-N1A
O1A-Mn1-N1A*
180
87.6(3)
92.4(3)
85.15(12)
94.85(12)
N1A-Mn-N1A*
O1A-Mn-N2A
N1A-Mn1-N1A*
O1A-Mn1-N2A
180
93.4(3)
86.6(3)
167.73(12)
O1B-Mn-N2A
N1A-Mn-N2A
N1B-Mn-N2A
N2B-Mn-N2A
O1A-Mn1-N2A*
N1A-Mn1-N2A
N1A-Mn1-N2A*
N2A-Mn1-N2A*
88.59(13)
83.10(13)
82.7(3)
97.3(3)
91.39(12)
86.66(13)
180
O1A-Mn-O1B
O1B-Mn-N1B
O1B-Mn2-O1B*
O1B-Mn2-N1B
180
86.9(3)
95.95(12)
82.73(12)
O1A-Mn-N1B
O1B-Mn2-N1B*
100.48(12)
93.1(3)
180
N1A-Mn-N1B
O1B-Mn-N2B
N1B-Mn2-N1B*
O1B-Mn2-N2B
174.05(13)
91.1(3)
166.93 (11)
O1A-Mn-N2B
O1B-Mn2-N2B*
91.20(12)
88.9(3)
83.1(3)
N1B-Mn-N2B
N1A-Mn-N2B
85.21(12)
N1B-Mn2-N2B
N1B-Mn2-N2B*
96.65(13)
96.9(3)
N2B-Mn-N2B*
N2B-Mn2-N2B*
180
21

(a)
(c)
(b)
Figure 4. ORTEP representation of the crystal structure of the cations (a); (b) and (c) packing in
the unit cell for MnIII-(L1)2 {complex 1}
(a)
(b)
Figure 5. ORTEP representation of the crystal structure of the cation (a), and (b) packing in the
unit cell for MnII-(L2)2 {complex 2}
22

Figure 6. ORTEP representation of the crystal structure of the cation of complex MnII-(rL2)2
(complex 4).
23

3.8. Interaction of 2 with H2O2; and with H2O2 and cyclohexene.
Prior to the catalytic studies, the behavior of 2 {MnII-(L2)2} alone (Figure 6), with H2O2
(Figure 7), then with H2O2 and varying concentrations of cyclohexene (Figure 8), was monitored
using UV-Visible absorption spectroscopy. Initially, the absorbance behavior of the solution of
complex 2 (2.7 x 10-5) in dichloromethane was recorded every three minutes over a period of
1hour 15 minutes and absorbance peaks were observed at 239 nm and 389 nm.
Addition of 250 mol equivalence of H2O2 to the reaction solution of the complex
(2.7 x 10-5 M) enhanced the absorption intensity at 239 nm and 389 nm. The significant increase
in intensity signifies interaction of the H2O2 with the complex.
Reaction of the complexes with H2O2 and cyclohexene resulted in a sharp decrease of
absorption intensities. The extent of the decreased intensity is dependent on the concentration of
cyclohexene used. The observed reaction indicates formation of an intermediate that is involved
in transfer of oxygen to the cyclohexene [2, 29]. The intermediate is highly reactive that it could
not be isolated as noted by Kochi et al; [46]. It was suggested that the intermediate may be
MnV=O formed through heterolytic cleavage of O-O bond [27], [Mn(III)(O2)(L)]+
peroxomanganese (III) adducts [39] or MnIV-oxo [81-84] which were able to catalyze some
organic compounds to their corresponding oxide. In another report [80], it was shown that
addition of H2O2 to [LMnII]2+ generates a mixture of Mn(II) mononuclear species, a [LMnIIIOO]+
peroxo complex, homovalent (Mn(III) or Mn(IV) dinuclear species, and mixed-valent di-µ-oxo
Mn(III)Mn(IV) complexes.
24

1.2
1
0.8
0.6
0.4
0.2
0
-0.2
228
278
328
378
428
478
Wavelength nm
Figure 7:
UV-Visible spectra of reaction of MnII-(L2)2 without H2O2
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
220
320
420
Wavelength nm
Figure 8:
UV-Visible spectra of reaction of MnII-(L2)2 with 250 mol equivalent of H2O2
25

Figure 9. UV-Vis. absorption spectra of the reaction of MnII-(L2)2 + H2O2 + cyclohexene
{2.7 X10-5 M solution of complex MnII-(L2)2 with (a) 500 mole equiv. of H2O2 and
(b) 500; (c) 1000; (d) 1500 mole equivalent of cyclohexene}
26

For this study, we propose that the possible mechanism for the formation of
Mn(IV)/(V)=O intermediate could be reaction of H2O2 with the Mn catalyst to form
[Mn(II/III)salen-H2O2] adduct. Dehydration of this adduct will lead to formation of the
Mn(IV)/(V)=O intermediate that subsequently transfers oxygen to the olefin. Deprotonation of
the adduct promoted by the co-catalyst could also give hydroperoxo intermediate,
[Mn(II)/(III)salen-OOH]−. Heterolytic cleavage of the O-O bond in the hydroperoxo intermediate
activated by the co-catalyst, produces the oxo-species [Mn(IV/V)salen = O] that transfers oxygen
to the olefin. This mechanism is similar to that proposed by Rocha et al; [41].
Figure 9b: Proposed mechanism for Mn-complexes Catalyzed epoxidation reaction with
H2O2 in the presence of NH4COOCH3/COOH or TEA/HClO4 co-catalysts
27

3.9. Catalytic Epoxidation of Cyclohexene and 1-hexene
A screening study of the catalytic performance of the synthesized complexes was
performed in the epoxidation of cyclohexene and 1-hexene with 30% H2O2 as the terminal
oxidant and at different pH conditions. Control experiment performed with cyclohexene and
H2O2 without the manganese complexes did not show any cyclohexene-oxide peak in the
GC-MS.
Epoxidation was performed at room temperature or 0°C in a 1:1:2 solution mixture of
CH3OH/CH2Cl2/CH3COCH3 that contained the manganese complex; cyclohexene or
1-hexene, and the oxidant in 1:25:1000 molar ratios respectively in the presence of
CH3COOH / NH4OOCCH3 (0.1: 0.2 mmol) or HClO4 /TEA (0.2:0.4 mmol) as co-catalyst/buffer
and bromobenzene as internal standard.
Epoxidation of cyclohexene to cyclohexene oxide gave yield of 5-43 % at room
temperature and 11-53 % at 0°C when CH3COOH/NH4OOCCH3 pH 7.52-8.0. was used
(Table 5). Use of the HClO4/TEA system, pH 8.09-9.02 gave cyclohexene oxide yield of 8-36 %
and 21-45 % at room temperature and 0°C respectively (Table 5). For cyclohexene oxidation,
complexes 3 and 4 gave the highest yield. In addition, higher epoxide yields were obtained at
0°C than at room temperature and the system of CH3COOH/NH4OOCCH3 gave slightly higher
yield than that of HClO4/TEA.
Epoxidation of 1-hexene in the presence of CH3COOH/NH4OOCCH3 (pH 7.72-8.36)
gave epoxide yield of 2-13 % and 14-24 % at room temperature and 0°C respectively. When
HClO4 / TEA (pH = 7.96-9.34) was used, the yield was 2-13 % at room temperature and 11-19 %
at 0°C (Table 6). Generally, complexes with the reduced Schiff base ligands, 3 and 4, gave
higher yield than those of the unreduced Schiff base ligands, 1 and 2. Even though 1 and 2
contain Mn(III) and Mn(II), respectively there is very little difference in their catalytic activity.
This makes sense as it has been suggested that the catalytically active intermediate may be
MnV=O formed through heterolytic cleavage of O-O bond [27], and both 1 and 2 will thus give
very similar active species. A higher yield of 1-hexene oxide was obtained at 0°C and with
CH3COOH/NH4OOCCH3 than at room temperature and with HClO4 / TEA. Epoxide yields
obtained after 3 hours of reaction were the same as that obtained after reacting for 20 hours.
Epoxide yield obtained from complexes 3 and 4 is within the range 49.1--59.9 % reported for
28

epoxidation of cyclohexene using biomimetic tris-imidazole and Mn system [86] and those
reported for cyclohexene and 1-hexene catalyzed by binuclear manganese complexes [30]. The
high yield also supports the dimerization or multimetal center observed in the molecular mass of
these reduced Schiff base complexes (as confirmed by the x-ray crystal structure of complex 4).
Thus, the dimer or presence of the multimetal center may be responsible for the higher epoxide
yield than those obtained with complexes 1 and 2. In certain cases, the yields exceed any other
reported in literature. For example, even in the case of electron-poor 1-hexene, the epoxide yield
obtained is higher compared to those reported previously [70].
Presence of acetic acid may be responsible for the higher yield obtained with the
CH3COOH/NH4OOCCH3 system as reported in [87] that CH3COOH suppresses
disproportionation of H2O2 by the manganese complexes resulting in high yield of epoxide. The
result also confirms the report that in the presence of carboxylic acids high yield of epoxide was
obtained using Mn-complex and H2O2 [88]. Performing the reaction at 0°C further suppressed
disproportionation of H2O2 as reported in [30, 89].
It is believed that use of NH4OOCCH3 is required in enhancing the epoxide yield
compared to the yield obtained in its absence as notated by [30, 90]. Comparing the yield
obtained using the CH3COOH/NH4OOCCH3 and HClO4/TEA, it could be suggested that the
reaction is pH dependent and higher yield is obtained at neutral pH than in alkaline medium.
This observation is supported by the report that NH4OAc-HOAc mixture (pH 7.13) in
water-acetone medium gave the highest amount of epoxide [29].
The results obtained for cyclohexene and 1-hexene parallel the report that alkene
epoxidation increase with the electron density of the double bond; 1-hexene is less reactive than
cyclohexene [30].
29