Mn(III) and Mn(II) complexes of tridentate Schiff base ligands; synthesis, characterization, structure, electrochemistry and catalytic activity

Article abstract of DOI:10.1016/j.ica.2018.01.027

Three Mn(III) complexes [Mn(L¹)₂] (ClO₄) 1, [Mn(L²)₂] (ClO₄) 2, [Mn(L³)₂] (ClO₄) 3, and one Mn(II) complex [Mn(L⁴)₂] 4 were studied fo

Full text of DOI:10.1016/j.ica.2018.01.027

Accepted Manuscript
Research paper
Mn(III) and Mn(II) complexes of tridentate Schiff base Ligands; Synthesis,
Characterization, Structure, Electrochemistry and Catalytic activity.
Rita N. Egekenze, Yilma Gultneh, Ray Butcher
PII:
S0020-1693(17)30785-5
https://doi.org/10.1016/j.ica.2018.01.027
ICA 18101
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
17 May 2017
19 January 2018
26 January 2018
Please cite this article as: R.N. Egekenze, Y. Gultneh, R. Butcher, Mn(III) and Mn(II) complexes of tridentate Schiff
base Ligands; Synthesis, Characterization, Structure, Electrochemistry and Catalytic activity., Inorganica Chimica
Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.01.027
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Mn(III) and Mn(II) complexes of tridentate Schiff base Ligands; Synthesis,
Characterization, Structure, Electrochemistry and Catalytic activity.
Rita N. Egekenze *, Yilma Gultneh, Ray Butcher.
Department of Chemistry, Howard University, Washington DC, 20059, USA
Abstract:
Three Mn(III) complexes [Mn(L1)2] (ClO4) 1, [Mn(L2)2] (ClO4) 2, [Mn(L3)2]
(ClO4) 3, and one Mn(II) complex [Mn(L4)2] 4 were studied for their efficiency as
catalysts for epoxidation of olefins with H2O2 at room temperature and 0°C in the
presence of ammonium acetate-acetic acid or triethylamine-perchloric acid system as
co-catalyst/buffer. The complexes were obtained from the reaction of Mn(ClO4)2.6H20
with NNO tridentate Schiff base ligands HL1-HL4 synthesized from the condensation
reaction
of
2-(2-aminoethyl)pyridine
and
2-hydroxybenzaldehyde
(HL1),
5-chloro-2-hydroxybenzaldehyde (HL2), 5-methoxy-2-hydroxybenzaldehyde (HL3), or
5-nitro-2-hydroxybenzaldehyde (HL4) respectively. The complexes were characterized by
spectroscopic techniques as well as by single crystal X-ray diffraction analysis.
Electronic effect of the substituents on epoxide yield was also investigated. The crystal
structures of complexes confirm coordination of the manganese ion to the ligands through
the NNO atoms of the ligands. The spectral changes observed as a function of time for
the reaction of the complexes with aqueous (30%) hydrogen peroxide indicates possible
formation of an intermediate product; hydroperoxo-complex implicated in the
epoxidation reaction. The complexes catalyzed epoxidation of cyclohexene with low
yield at room temperature but higher yield at 0°C in the order complex 3 lower than 1, 1
lower than 2, with the Mn(II) complex 4 recording highest epoxide yield of 9% with
turnover of 2.25 at room temperature and yield of 58% with turnover of 14.5 at 0°C in the
presence of ammonium acetate-acetic acid system/ buffer. With the triethylamine-
perchloric acid system/ buffer, epoxide yield of 46% and turnover of 11.5 was recorded at
room temperature while 44% with turnover of 11.0 was obtained at 0°C for complex 4.
1

Graphical Abstract:
Mn(III) and Mn(II) complexes of Schiff base ligands were used as catalyst in
epoxidation of cyclohexene with H2O2 in the presence of ammonium acetate-acetic acid
or triethylamine-perchloric acid systems as cocatalyst/buffer in a mixture of 1:1:2 of
CH2Cl2/CH3OH/CH3COCH3 solvent at room temperature and 0°C. The Mn(II)complex
with –NO2 substituent on the Schiff base ligands gave epoxide yield of 9% at room
temperature and 58% at 0°C with ammonium acetate-acetic acid system but 46% at room
temperature and 44% at 0°C in the presence of triethylamine-perchloric acid system.
Epoxide yield depended on the electronic effect of the catalyst, as well as temperature
and co-catalyst. The triethylamine-perchloric acid system/buffer is a promising co-
catalyst for epoxidation of olefin at room temperature as well as 0°C
H2O2 + Mn catalyst
+ TEA/HClO4 co-catalyst
O
CH2Cl2/CH3OH/CH3COCH3
℃
r.t or 0
46% at r.t
℃
44% at 0
Keywords: Manganese complexes, Schiff base, Electronic effect, Triethyamine-Perchloric acid
co-catalyst, H2O2, Epoxidation, Catalysis
2

1.0
Introduction:
Manganese complexes are known for their catalytic role in the active site of
enzymes [1, 2, 3] where the redox active manganese ion catalyzes the metabolism of O2
-
2-
and its reduced forms superoxide (O2 ) and peroxide (O2 ) [1-5]. The mononuclear
manganese enzyme, superoxide dismutase (Mn-SOD), and dinuclear manganese enzyme,
catalase (Mn-catalase), catalyze the dismutation of the harmful reduction products of
-
oxygen (O2), superoxide (O2 ) and H2O2 respectively, as a biologically defensive
mechanism to protect living cells [3-13], while the tetranuclear manganese oxygen
evolving complex (OEC) in photosystem II (PSII) of green plants catalyzes the oxidation
of H2O to O2 [1, 4, 6, 7, 14, 15]. Motivated by the role of manganese in biological
systems, synthetic manganese complexes with various ligands, such as Schiff bases, have
been prepared [16-19].
Schiff base ligands are good choice for complex formation with manganese ion
since they possess N and O donor atoms observed in the coordination environment of
manganese enzymes [20], could be electronically and sterically modified by introducing
substituents on the aromatic ring to enhance the catalytic role of the Mn complex [21, 22,
23]. They also stabilize the metal ions in their various oxidation states [1, 24, 25, 26,] and
control their activity in many catalytic reactions [24, 25].
Epoxides are among the resourceful intermediates in organic chemistry [27] and
important starting materials for many finished chemical products [28-32]. They are
traditionally synthesized from olefins using strong oxidants such as iodosyl benzene
(PhIO), m-chloroperbenzoic acid (m-CPBA) and sodium hypochlorite (NaOCl) [33-35].
These oxygen donors produce wastes that are harmful to the environment [34]. Use of
milder oxidant such as hydrogen peroxide is preferred for epoxidation reactions due to its
high oxygen content; it is environmentally friendly and produces water as the only waste
product [36-40].
H2O2 requires a metal catalyst to activate its oxidation reaction with olefins.
Manganese complexes have been shown to be efficient catalyst for this purpose
especially since the catalase-like reaction of H2O2 in the presence of manganese complex
could be suppressed with acetone [41-43] and at 0°C [21].
3

Co-catalyst is also needed for manganese complex catalyzed epoxidation of
olefins with H2O2 to ensure heterolytic cleavage of the O-O bond leading to fast transfer
of the oxygen atom to the olefin to produce epoxide [32, 37, 39, 44]. Nitrogen
heterocycle including imidazole and pyridine have been used as co-catalysts acting as
bases or as axial ligands on the metal catalysts for alkene epoxidation with H2O2 to
enhance heterolytic bond cleavage to produce reactive metal Oxo species and goods
yields were obtained [37, 44]. Nitrogenous bases are known to activate manganese
complexes and accelerate oxidation reaction in epoxidation of alkenes with H2O2 [33]. It
was reported [33b] that some olefins such as cyclohexene, which can be easily oxidized
in the allylic position, their low epoxide yield could be enhanced by addition of
nitrogenous bases. Ammonium acetate or ammonium acetate with acetic acid [32, 39]
have been used efficiently as co-catalyst to achieve high epoxide yield. However, to date,
there is no literature report on the application of triethylamine-perchloric acid system as
co-catalyst for epoxidation of olefin.
Manganese species Mn(V)=O, Mn(IV)=O, Mn(III)=OO, and Mn(III)-OOH are
implicated as active intermediates in epoxidation of alkene by hydrogen peroxide
catalyzed by manganese complexes including manganese Schiff base complexes [3, 44,
45-51].
Herein, mononuclear manganese complexes of tridentate Schiff base ligands
containing phenolate, pyridine and imine N donors are described. Synthesis,
spectroscopic characterization, Electrochemistry, structure and catalytic activity of the
complexes in the presence of triethylamine-perchloric acid system as a novel co-catalyst
for epoxidation of cyclohexene with H2O2 catalyzed by manganese complexes of Schiff
base ligands are also reported. Electronic effect of the substituents is also investigated.
2. Experimental
2.1. Materials
Manganese(II) perchlorate hexahydrate {Mn(ClO4)2.6H2O} analytical grade, the
olefin (cyclohexene), 2(2-aminoethyl) pyridine, salicylaldehyde and the substituted
derivatives were purchased from Sigma-Aldrich and used as received. Aqueous hydrogen
4

peroxide (30% solution in water) obtained from Sigma-Aldrich was stored at 5°C.
Solvents used were either analytical (99%) or reagent grade and used without further
purification.
2.2. Physical measurements
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 Voltammetry
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. GC-MS
data were obtained on Agilent 6890N Network GC system coupled to Agilent 5973 inert
mass Selective Detector. Crystal X-ray structures were determined with Oxford
Diffraction Gemini-R diffraction instrument. The complexes were dissolved in
appropriate solvents and layered with diethyl ether to obtain pure crystals used for the x-
ray crystallography experiment.
GC-MS data for epoxides were obtained on Agilent 6890N Network GC system
coupled to Agilent 5973 inert mass Selective Detector.
Crystal X-ray structures were determined with Oxford Diffraction Gemini-R
diffraction instrument. The complexes were dissolved in appropriate solvents and layered
with diethyl ether to obtain pure crystals used for the x-ray crystallography experiment.
5

2.3. Synthesis of the Ligands
2.3.1. 2- {N- (2- [2-Pyridyl]ethyl)iminomethyl}Phenol [C14H14N2O] (HL1):
HL1 was synthesized as described in [52, 53] with modification as follows;
2-(2-aminoethyl)pyridine [C7H10N2], (2.1052 g, 17.23 mmol) in 15 mL of methanol was
reacted with salicylaldehyde [C7H6O2], (2.1180 g, 17.34 mmol) also in 15 mL under
reflux for five hours and cooled to room temperature. Removal of water under pressure
gave the yellow oil product. This was purified by extraction with diethyl ether and
subsequently removing the ether to obtain a yield of 6.29 g (87%). Characterization data
for [C14H14N2O] HL1; are as follows; Molecular mass: calculated for [C14H15N2O]+ =
227.1184; ESI-MS determined m/z = 227.1135. IR (LiTaO3, KBr) (cm-1): 3053 (w) OH;
1630 (s) υ(HC=N), {1588 (m), 1569 (m) C=Npy}, 1277 (s C-O), UV-Vis, {λmax, (nm),
1
(MeOH)}: 213 (191.44), 255 (131.30), 317 (37.39), 406 (10.82). HNMR {CDCl3}: δ
(ppm) 13.35 (s, 1H, ArO-H), 8.55, (s, 1H, -CH=N), 6.83, 6.93, 7.28, 7.58, 8.25, (s, 1H,
ArH); 7.15 (t, 3H); 3.15, 4.0 (d, 2H, CH2)
2.3.2. 2-{N-(2-[2-Pyridyl]ethyl)iminomethyl}5-chlorophenol [C14H13ClN2O] (HL2):
Ligand HL2 was synthesized by a modified method of [20, 54] as follows;
5-chlorosalicylaldehyde [C7H5ClO2] (3.5908 g 23.0 mmol) was dispersed in 25 mL
methanol and heated on a hot water bath maintained at 72°C to dissolve. To this solution
was added the solution of 2-(2-aminoethyl)pyridine (2.9500 g, 24 mmol) in 25 mL
methanol to obtain yellow solution that was refluxed for 5 hours and cooled to room
temperature. The yellow solid obtained was recrystallized from dichloromethane. The
yield was 5.26 g (93 %). Analysis of [C14H13ClN2O] gave the following data; Molecular
mass: calculated for [C14H14ClN2O]+ = 261.7265, ESI-MS determined m/z = 261.0851.
IR (LiTaO3, KBr) (cm-1): 2904 (w) OH), 1635 (m HC=N), 1600 (w) C=C), 1585 (m
6

C=Npy), 1285 (m C-O); UV-Vis {λmax, (nm), (MeOH)}: 222 (194.82), 256 (97.27), 327
1
(29.88), 417 (96.9). HNMR {CDCl3}: δ (ppm) 13.35 (s, 1H, ArO-H), 8.58 (s 1H, -
CH=N), 6.88, 7.21, 7.28, 7.60, 8.20, (s, 1H ArH); 7.18 ppm (t, 3H); 3.18, 4.05 (d, 2H,
CH2).
2.3.3. 2- {N-[2-(2-Pyridyl)ethyl]iminomethyl}5-methoxyPhenol [C15H16N2O2] (HL3):
HL3 was synthesized as described in [54] according a modified procedure;
2-(2-aminoethyl)pyridine (0.9169 g, 7.5 mmol) in 15 mL of methanol was reacted with
5-methoxysalicylaldehyde (1.1500 g, 7.6 mmol) also in 15 ml of methanol to obtain a
brownish yellow solution that was refluxed for 4 hours. The dark yellow solid that
resulted was recrystallized from methanol to obtain a yellow solid with a yield of 1.17 g
(57%). Analysis data for [C15H16N2O2] are; Molecular mass: calculated for
[C15H17N2O2]+ = 257.3078; ESI-MS determined m/z = 257.1296. IR (LiTaO3, KBr)
(cm-1): 3005 (w OH), 1635 (m HC=N), {1589 (m), 1569 (w), C=Npy}1267 (s C-O),
UV-Vis {λmax (nm), (MeOH)}: 209 (151.73), 234 (197.07), 261 (106.91), 345 (41.03).
1HNMR {CDCl3}: δ (ppm) 12.80 (s, 1H, ArO-H), 8.58 (s, 1H, -CH=N), 6.7, 7.28, 7.40,
8.26, (s, 1H, ArH); 6.9, 7.18 (d, 2H); 3.18, 4.20 (d, 2H, CH2); 3.75 (t, 3H, O-CH3).
2.3.4. 2- {N- (2-[2-Pyridyl]ethyl)iminomethyl}5-nitrophenol [C14H13N3O3] (HL4)
HL4 was synthesized according to a modified procedure reported in [20, 54] as
follows; 5-nitrosalicylaldehyde (2-hydroxyl-5-nitrobenzaldehyde) [C7H5NO4] (1.6204 g
9.7 mmol) was dispersed in 25 mL of methanol and heated on a hot water bath to
dissolve. This solution was reacted with 2-(2-aminoethyl)pyridine (1.1805 g, 9.7 mmol)
in 10 mL of methanol under reflux for 5 hours. The solid obtained was recrystallized
from dichloromethane and dried in a desiccator over anhydrous calcium chloride. The
yield was 2.72 g (97%). Characterization data for [C14H13N3O3] are; Molecular mass:
calculated for [C14H14N3O3]+ = 272.1035, ESI-MS determined m/z = 272.1136. IR
(LiTaO3, KBr)
(cm-1): 3048 (br OH), 1658 (m HC=N), 1587 (m C=N py), 1290 (m
C-O), UV-Vis {λmax, (nm), (MeOH)}: 204 (129.90), 262 (202.31), 345 (104.77), 392
7

1
(94.12). HNMR {CDCl3}: δ (ppm) 14.75 (s, 1H ArO-H); 8.60, (s, 1H –CH=N); 7.62,
8.28, (s, 1H); 7.19, 7.28, 8.18 ppm (d, 2H); 3.22, 4.14 (d, 2H, CH2); 7.18 (t, 3H); 3.18,
4.05 (d, 2H CH2).
2.4
Synthesis of the of Mn(III) complexes:
2.4.1. bis{[2-(2-pyridyl)ethyl]-iminomethyl-phenoxy}manganese(llI)perchlorate}
{Mn(III)(L1)2}(ClO4) [C28H26MnClN4O6] (1)
{Mn(III)(L1)2}(ClO4) was synthesized as described in [52-54) with modification
according to the following procedure; HL1 (1.9147 g, 8.5 mmol) and triethylamine
[C6H15N] (1.3 ml, 9.3 mmol) in 25 mL of methanol was added to Mn(ClO4)2. 6H20
(1.5710 g, 4.3mmol) also in 25 mL a methanol. A dark green solution that resulted was
refluxed for 4 hours and cooled to room temperature. Brownish precipitate obtained was
recrystallized from methanol and the yield was 1.95 g (56%). Analysis data for
[C28H26MnClN4O6] are; Molecular mass: calculated for [C28H26MnN4O2]+ = 505.1436,
ESI-MS determined m/z = 505.1444. IR (LiTaO3, KBr) (cm-1); 3026 (w OH), 1610
(m HC=N), 1598 (s C=Npy), 1289 (s C-O), UV-Vis {λmax (nm), (MeOH)}: 214
(19102), 262 (12401), 383 (1807).
2.4.2. bis{[2-(2-pyridyl)ethyl]-iminomethyl-5-chloro-phenoxy}manganese(llI)-
perchlorate {Mn(III)(L2)2}(ClO4) [C28H24MnCl3N4O6] (2)
{Mn(III)(L2)2}(ClO4) was prepared following the procedures reported in [20, 54]
with amendment. Ligand HL2 (2.4793 g, 9.5 mmol) and equivalent mole of triethylamine
[C6H15N] (1.3 ml, 9.5 mmol) both in methanol 25 mL, was mixed with methanol solution
25 mL of Mn(ClO4)2 .6H2O (1.7276 g, 4.8 mmol). The mixture was refluxed for 4 hours
before it was cooled to room temperature. The precipitate that formed was filtered and
recrystallized from methanol and dichloromethane to obtain a yield of 2.60 g, (62%).
8

Characterization data for [C28H24MnCl3N4O6] are: Molecular mass: calculated for
[C28H24MnClNO]+ = 573.0657, ESI-MS determined m/z = 573.0379. IR (LiTaO3, KBr)
(cm-1); 3073 (w OH), 2942 (w), 1616 (m HC=N)), 1601 (m), 1567 (w C=Npy), 1280
(s C-O), UV-Vis {λmax (nm), (MeOH)}: 231 (21007), 285 (12847), 361 (4269), 421
(1691).
2.4.3.
bis{[2-(2-pyridyl)ethyl]-iminomethyl-5-methoxy-phenoxy}manganese(llI)-
perchlorate {Mn(III)(L3)2}(ClO4) [C30H30MnClN4O8] (3)
{Mn(III)(L3)2}(ClO4) was prepared using a modified method of the procedure
reported in [20, 54]. Methanol solution (25 mL) of ligand HL3 (1.6554 g, 6.5 mmol) and
triethylamine [C6H15N] (1.34 ml, 6.5 mmol) was reacted with Mn(ClO4)2.6H2O
(1.1777g, 3.3 mmol) in 20 mL methanol under reflux for 4 hours. The precipitate formed
was recrystallized from methanol and diethylether to obtain a yield of 1.93 g (67%).
Characterization data for [C30H30MnClN4O8] are: Molecular mass: calculated for
[C30H30MnN4O4]+ = 565.1648, ESI-MS determined m/z = 565.1620. IR (LiTaO3, KBr)
(cm-1); 3481 (w OH), 1614 (s HC=N), 1597 (m C=Npy), UV-Vis, {λmax (nm),
(MeOH)}; 207 (20311), 227 (21311), 257 (15831), 342 (4346), 427 (1805).
2.4.4. bis{2-(2-pyridyl)ethyl]-iminomethyl-5-nitro-phenoxy}manganese(ll)
{Mn(II)(L4)2} [C28H24MnN6O6] (4)
{Mn(II)(L4)2} was synthesized according to the procedure reported in [20, 54]
with modification; Ligand HL4 (2.1776 g, 8.0 mmol) in 40 mL of methanol was heated
on a water bath to dissolve and Triethylamine [C6H15N] (1.16 ml, 8.1 mmol) was added
followed by addition of Mn(ClO4)2.6H2O (1.5151 g, 4.2 mmol) in 25 mL methanol. The
entire mixture was refluxed for 4 hours and cooled to room temperature. The orange
precipitate was recrystallized from dichloromethane and diethyl ether to obtain a yield of
2.30 g (62%). Analysis data for [C28H24MnN6O6] are; Molecular mass: calculated for
[C28H24MnN6O6]+ = 596.1216, ESI-MS determined m/z = 596.1199). IR (LiTaO3, KBr);
9

3067 (w OH), 1619 (s HC=N), 1595 (s (C=N py), 1287 (s C-O), UV-Vis {λmax (nm),
(MeOH)}: 201 (21959), 235 (10321), 255 (10863), 361 (8438).
10

Complex
R
HL1: R= H;
HL2: R=Cl;
HL3: R= OMe,
HL4: R= NO2
H
1
2
3
4
Cl
OMe
NO2
Scheme;
1a: General structure of the Ligands.
1b: Proposed structure of the
complexes
2.5. X-ray structural determination
The crystal structures of complexes 2 and 3 were determined using Oxford
Diffraction Gemini R.CCD diffractometer with Mo Kα radiation graphite-
monochromatized (λ= 0.71073Å) with 1° scans. In 2, there are two similar cations both
with Mn on a center of inversion and one perchlorate anion in a general position but
disordered over two equivalent positions, thus giving an overall stoichiometry of
[Mn(L2)2](ClO4). The structure of 3 also contains two cations on centers of inversion and
one perchlorate anion in a general position. However, in this case the two cations are not
identical. One of these cations was similar to the cations in 2 while the other cation for
ligand L3 was bidentate, coordinating through the phenolic oxygen and imine nitrogen
11

with the coordination sphere completed with two methanol ligands giving an overall
stoichiometry of {[Mn(L3)2][Mn(L3)2(CH3OH)](ClO4)2. An Oxford Diffraction Cryojet
XL low temperature device was used to keep the crystals at constant temperature of 123
(2) K during data collection.
2.6. 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 or triethylamine-perchloric acid was the
co-catalyst, cyclohexene was used as substrate and H2O2 was the oxidant. In a typical
reaction, the catalyst, co-catalyst and substrate were mixed in a round bottom flask and
while stirring, the internal standard was added before the oxidant was added drop wise
over a period of 1 hour 58 minutes at room temperature or at 0°C. The solution mixture
was reacted for 3 hours at first and then for 20 hours. Samples were extracted with
dichloromethane, dried with anhydrous magnesium sulfate 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 samples were extracted to remove H2O2 and prevent its
corrosive effect on the GC-MS. A plot of the ratio of peak area of standard epoxide to the
peak area of bromobenzene versus the concentration ratio of the standard epoxide to that
of bromobenzene (Figure S44b) was used to calculate the concentration of the epoxide in
the sample. Turn-over numbers were obtained by dividing moles of substrate by moles of
catalyst times percent yield. That is;
ΛΝΚΓΡ ΝΔ ΡΣΐΡ΢ΠΏ΢Γ
TON =
ͬ ͙͙ͤͦ͗ͨ͢ ͙ͭ͘͝͠
ΛΝΚΓΡ ΝΔ ΑΏ΢ΏΚΧΡ΢
12

3. Results and discussion
3.1. Synthesis and characterization of ligands and complexes
Although the ligands and complexes have been synthesized previously according
to references [20, 52-55], their method of synthesis varied slightly from the procedure
employed in this work. Mabad et al; [55] employed the same aldehydes used here but
with diaminopropanol or Tri or tetraamines and ethanol as solvent with Manganese
acetate tetrahydrate as the metal ion. Frederick el; did not isolate or characterize the
ligands as they were reacted together with Mn(OAc)2.4H2O, added solid NH4NCS and
KOH and used isopropyl alcohol/tert-butyl alcohol, heptane and 95% ethanol as solvent.
Luaces et al; [20] synthesized the ligands with ethanol as solvent and electrochemical
synthesis for the complexes employing manganese as the platelets. Using our method of
preparation, the ligands were synthesized successfully as evidenced by the
characterization techniques before they were reacted with equimolar amounts of
Mn(ClO4)2.6H2O in methanol to give mononuclear Mn(III) and Mn(II) complexes in
good yields. Complexes 1, 2 and 3 have been formulated as [Mn(L)2]ClO4 and complex 4
as Mn(L)2 as confirmed by the crystal X-ray structures of these complexes.
The azomethine (-CH=N-) characteristic band for the free ligand appeared
between 1630--1658 cm-1 and decrease to 1610-1619 cm-1 in the complexes indicating
coordination of the azomethine nitrogen to the manganese ion (formation of metal-ligand
bond) [3, 5]. Possible drift of lone pair of electron density from the imine nitrogen to the
Mn(III) on coordination resulted in shift of –CH=N- stretch frequency to lower values
[5]. The bands at 1255-1285 cm-1 are assigned to C-O stretch of the phenolate group in
the ligands. These bands occurred at higher frequencies (1280-1288 cm-1) in the
complexes conforming deprotonation and coordination of the phenolate oxygen to the
metal [4, 5]. IR spectra for the ligands are in Figures S13 –S16 while the complexes as
shown in Figures S17- S20.
In the UV-Visible spectra of the ligands (Figures S21-S24), the observed set of
bands 204 – 233 nm and 256-262 nm are assigned to π → π* electronic transition of
phenyl-ring [56]; 318 – 345 nm are due to n → π* transitions of the imine (C=N) bond
[57]. The azomethine bonds are shifted to lower frequencies 285-342 nm in the
13

complexes (Figures S25-S28) indicating coordination of its nitrogen to the metal ion [57].
The bands at the longer wavelength, 391-427 nm are assigned to charge transfer, Ligand
to metal (L⇒M) electronic transitions [57, 58].
3.2. UV-Vis spectra of the reaction of complex 4 with H2O2 and cyclohexene:
The reactions of complex 4 with H2O2, then complexes 1, 2 and 4 with H2O2 and
different concentrations of cyclohexene were monitored with UV-Visible spectral studies
(Figures 1 and 2 for complex 4, Figures S38 and S39 for complexes 1 and 2 respectively).
Reaction of 2.7 x 10-5 M solution of complex 4 with 250 mol equivalence of H2O2
produced two peaks at 240 nm and 390 nm. The peak at 240 nm increased slightly before
decaying while the peak at 390 nm decreased continuously. Two new peaks were
observed at 258 nm and 325 nm between 20 to 30 minutes into the reaction. These new
peaks continued to grow (increase in intensity) indicating formation of new species. The
same observation was made for complexes 1 and 2. The new peaks observed are similar
to those reported in literature [59, 60] and have been assigned to formation of Mn(V)=O
through heterolytic cleavage of O-O bond [59]. Addition of cyclohexene to the reaction
mixture of the complexes with H2O2 resulted to an instant sharp decrease of absorption
intensity. The decrease subsequently continued over 1½ hours reaction period. The rate
of the decrease in intensity is dependent on the concentration of cyclohexene used. From
the observed reaction, it could be suggested that the intermediate formed by reaction of
the complex with H2O2 is involved in transfer of oxygen to the cyclohexene [59, 61].
14

Figure 1. UV-Visible absorption spectra of the reaction of Complex 4 with
250 mol equivalent of H2O2 recorded every 3 mins for 1 hour 15 min.
The Insert shows reading 1, 2, 5, 10, 15, 20 and 25
15

Figure 2: UV-Vis. absorption spectra of the reaction of 4 + H2O2 + cyclohexene
{2.7 x 10-5 M solution of complex 4 with (a) 500 mole of H2O2 and
(b) 500; (c) 1000; (d) 1500 mole equivalent of cyclohexene
The possible mechanism for the formation of Mn(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(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(III)salen-OOH]−. Heterolytic
cleavage of the O-O bond in the hydroperoxo intermediate activated by the co-catalyst,
16

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; [44].
Figure 2b: Proposed mechanism for Mn-Schiff base catalyzed epoxidation reaction
with H2O2 in the presence of TEA/HClO4 co-catalyst.
3.3. Redox Properties (cyclic voltammetry):
The free ligands did not show any voltammogram in the potential window
-1.5 to +1.5 V investigated in this study, hence all the redox waves were attributed to the
redox activity at the metal center. 1 mM (1.0 x 10-3 M) solution of the complexes in
supporting electrolyte (0.1 M solution of tetrabutylammonium perchlorate in HPLC grade
acetonitrile) was used at scan rate of 100 mVs-1. The redox potentials for all the
complexes are described in Table 1 while the voltammograms are shown in Figure 3. All
the complexes showed similar shapes of cyclic voltammograms with two redox peaks
that are quasi reversible for 1, 2 and 4, but 3 shows weak peaks. The cyclic voltammetry
17

data (Table 1) reveals that one electron is involved in the redox process. The wave
detected at E½ = 0.068-0.479 V vs Ag/AgCl is attributed to the oxidation of [MnIIL2] to
[MnIIIL2] indicating Mn(II)/Mn(III) redox couple while redox peaks observed at E½ =
0.924-1.234 V vs Ag/AgCl are ascribed to the Mn(III)/Mn(IV) redox couple.
All the samples showed values that match with results reported in literature
[4, 5,62, 64, 67] for manganese complexes with similar ligand frameworks. The weak
waves observed for 3 may be due to the electron donating (-OMe) group on the ligand
that increases electron density on the metal ion and make coordination unstable [5].
The highest redox potential was recorded for complex 4, followed by complex 2,
then complex 1. Complex 3 recorded the least redox potential. The observed variations in
redox potential as a function of substituents (-NO2, –Cl and -OMe] on the ligand conform
to the report that the presence of the substituents in the ligands significantly affects the
redox potential [5, 67]. Presence of electron- withdrawing substituents on the phenyl ring
of the ligands shift the reduction potentials to more positive values (higher potential)
[63, 65, 68] and electron-donating substituents on the ligand shift the reduction potentials
to more negative values (lower potential) [25, 62, 68]. Kuma et al; observed the same
trend; [67] and suggested that the observation is due to electronic effect of the
substituents, which is highest for -F followed by -Cl, -Br, and -OMe substituents in their
work. It can be concluded for this study that, the redox process is metal centered and the
variation in redox potential is due to electronic effect which increased from -OMe to -Cl,
to -Br and to -NO2. This effect is reflected in the epoxide yield with the –NO2 giving
higher yield than the
-Cl, and -OMe gave the lowest yield.
18

Table 1. Electrochemical data for the redox complexes Mn(II)/Mn(III) and Mn(III)/Mn(IV)
25 OC vs Ag/AgCl
Complexes
Mn(II)/(Mn(III)
Mn(III)/Mn(IV)
b
a
b
a
Epa (V)
Epc (V)
E
∆Ep (V)
Epa (V)
Epc (V)
E
∆Ep (V)
½
½
1
2
3
4
0.121
0.233
0.136
0.515
0.015
0.149
0.103
0.443
0.068
0.191
0.120
0.479
0.106
0.084
0.033
0.072
1.077
1.127
0.963
1.312
0.986
1.059
0.884
1.156
1.032 0.091
1.093 0.068
0.924 0.079
1.234 0.156
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 Epa
E
=
-
pc at scan rate 0.1 V/s
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
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
1.077 V
1.127 V
Mn(L1)2
Mn(L2)2
0.233 V
0.149 V
0.121 V
1.059 V
0.986 V
0.015 V
-1.5
-0.5
0.5
Potential V
1.5
2.5
-1.5
-0.5
Mn(L3)2
-0.5
0.5
Potential in V
1.5
2.5
5.00E+01
4.00E+01
3.00E+01
2.00E+01
1.00E+01
0.00E+00
-1.00E+01
-2.00E+01
4.00E+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
-2.50E+01
-3.00E+01
1.312
0.515 V
0.963 V
Mn(L4)2
0.136 V
1.156 V
0.884 V
0.443 V
0.103 V
-1.5
0.5
1.5
2.5
-1.5
-0.5
0.5
1.5
2.5
Potential V
Potential in V
Figure 3. Cyclic voltammograms of a 10-3 M (1mM) solution of the complexes in acetonitrile
in the presence of supporting electrolyte tetrabutylammonium perchlorate (TBAP) using
glassy-carbon working electrode, Ag/AgCl reference electrode, platinum wire auxiliary
electrode and scan rate of 100 mv/s
19

3.4. Description of the Crystal Structures of the Complexes 1-4:
The crystal structures of complexes 1 and 4 have been reported elsewhere ([20]
for complex 4 and [53] for complex 1.
The crystal data and structure refinement of the complexes 2 and 3 are tabulated
in Table 2 while Table 3 lists the selected bond lengths and angles. The ORTEP diagrams
of the coordination of the metal ion to the ligands and the unit cell parking are depicted in
Figures 4a and 4b.
Complex 2 crystallized in the space group P-1 in the triclinic crystal system and
consist of two cations in the asymmetric unit (labeled as complex A and complex B) as
well as a perchlorate anion in a general position thus the overall stoichiometry was
[Mn(L2)2](ClO4). Complex 3 also crystallized in the space group P-1 with two different
cations located on crystallographic centers of inversion. One of these cations was similar
to the cations in 2 while the other cation for ligand L3 was bidentate, coordinating
through the phenolic oxygen and imine nitrogen with the coordination sphere completed
with two methanol ligands giving an overall stoichiometry of {[Mn(L3)2][Mn(L3)2
(CH3OH)](ClO4)2. Two ONN tridentate ligand molecules are involved in coordination to
the metal ion for both complexes 2 and 3, (in addition for 3 there is a cation with a
different coordination sphere). Thus, all the cations in 2, and 3 were located on centers of
inversion. For complex 2 and one of the cations of 3, the Mn(III) is in an O2N4 octahedral
environment provided by the phenolic oxygen, imine nitrogen and pyridine nitrogen
donor atoms of both ligands. In the other cation for 3, the Mn(III) ion is bonded to the
oxygen of the solvent methanol (Mn-O1S), nitrogen of imine and oxygen of phenol ring.
The hydrogen of the OH in the methanol solvent is hydrogen bonded to the nitrogen of
the pyridine ring.
The bond lengths for complex Mn(L2)2 are Mn (1)-O (1A) = 1.8815(9) Å,
Mn (1)-N(1A) = 2.0229(9) Å, Mn (1)-N(2A) = 2.3200(11) Å, Mn (2)-O(1B) = 1.8734(9)
Å, Mn (2)-N (1B) = 2.0413(11) and Mn (2)-N(2B) = 2.2852(11) Å. Bond length in Mn-
(L3)2 are Mn (1)-O(1A) = 1.8712(14), Mn (1)-N(1A) = 2.0234(15), Mn (1)-N(2A) =
20

2.3766(15), Mn (2)-O(1B) = 1.8668(13), Mn (2)-N(1B) = 2.0496(14), and Mn (2)-O(1S)
= 2.2401(16).
The bond lengths (Table 3) observed MnII-O (2.10) greater than MnIII-O (1.87-
1.89) are consistent with the differences in oxidation states of the metal ions. Ionic size is
in the order MnII in complex 4 smaller than MnIII in the other complexes hence complex
4 forms slightly longer bonds. Mn-O bond lengths are similar to those reported for
similar
Mn(II)-phenolic Oxygen [1, 5, 64, 69-71], and Mn-Nazomethine bond lengths
(Table 3) are similar to those of Mn complexes of Schiff base ligands previously reported
[72-74]. In all the Mn(III) cations there is evidence for elongation of one pair of trans
bonds due to the Jahn-Teller effect. The Mn ion being a hard acid, preferred complexing
to the hard-base oxygen of the methanol solvent in the axial position than the soft pyridyl
nitrogen thus, leaving the pyridyl arm dangling. In both complexes, there are slipped
π…π stacking interactions involving some of the pyridyl rings.
Complex Mn(L2)2 showed some selected bond angles of O(1A)#1-Mn(1)- O(1A)
= 180.0°, O(1A)#1-Mn(1)-N(1A)#1 = 88.72(4)°, O(1A)-Mn(1)-N(1A)#1 = 91.28(4)°,
N(1A)#1-Mn(1)-N(1A) = 180.0°, O(1A)#1-Mn(1)-N(2A)= 88.06(4)°, O(1A)-Mn(1)-
N(2A) = 91.94(4)°, N(1A)#1-Mn(1)-N(2A) = 97.39(4)°, N(1A)-Mn(1)-N(2A) =
82.61(4)°, O(1A)#1-Mn(1)-N(2A)#1 =91.94(4)°, O(1A)-Mn(1)-N(2A)#1= 88.06(4)°,
N(1A)#1-Mn(1)-N(2A)#1 = 82.61(4)°, N(1A)-Mn(1)-N(2A)#1 = 97.39(4)°, N(2A)-
Mn(1)-N(2A)#1 = 180.00(5)°.
Some selected bond angles for complex Mn(L3)2 are; O(1A)#1-Mn(1)-O(1A) =
180.0°, O(1A)#1-Mn(1)-N(1A) = 92.00(6)°, O(1A)-Mn(1)-N(1A) = 88.00(6)°, O(1A)#1-
Mn(1)-N(1A)#1 = 88.00(6)°, O(1A)-Mn(1)-N(1A)#1= 92.00(6)°, N(1A)-Mn(1)-
N(1A)#1 = 180.00(9)°, O(1A)#1-Mn(1)-N(2A)#1= 92.21(6)°, O(1A)-Mn(1)-N(2A)#1 =
87.79(6)°, N(1A)-Mn(1)-N(2A)#1 = 98.69(6)°, N(1A)#1-Mn(1)-N(2A)#1= 81.31(6)°,
O(1A)#1-Mn(1)-N(2A)= 87.79(6)°, O(1A)-Mn(1)-N(2A) = 92.21(6)°, N(1A)-Mn(1)-
N(2A) = 81.31(6)°, N(1A)#1-Mn(1)-N(2A) = 98.69(6)°, N(2A)#1-Mn(1)-N(2A)
180.00(8)°.
=
21

The bond angles around the manganese atom in all the complexes (2 and 3) reveal
significant distortion from the ideal octahedral geometry probably because of ligand
rigidity [4]. The bond angles 180, 88, 91 or 92 (Table 3) in complexes 2 and 3 are similar
and are slightly distorted. In addition, of course, the molecular symmetry is the same for
2, and 3 as seen in Table 3. These structural features are consistent with that observed in
other structurally characterized complexes containing a manganese central atom and
Schiff base ligands [4, 5, 64, 75, 76].
Table 2. Crystallographic data for complexes 2 and 3
Identification code
Formula
2
C28H30Cl2MnN4O6
644.40
3
C62H68Cl2Mn2N8O18
1394.02
Formula weight (g mol-1)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
123(2)
0.71073
123(2)
1.54184
Triclinic
Triclinic
P-1
P-1
11.1069(6)
11.3663(4)
12.7282(5)
104.706(3)
101.848(4)
108.962(4)
1395.24(10)
10.8119(7)
11.4175(6)
13.2606(9)
91.907(5)
97.714(6)
103.530(5)
1573.56(17)
b (Å)
c (Å)
α (°)
β(°)
γ (°)
Volume (Å3)
Z
2
1
Dcalc Mg/m3
Absorption coef. (mm-1)
F(000)
1.534
0.715
1.471
4.713
666
0.5387 x 0.3749 x 0.0393
724
0.7525 x 0.2114 x 0.1901
Crystal size (mm)
Index ranges
-20<=h<=19, -20<=k<=20, -
23<=l<=16
-13<=h<=10, -12<=k<=14, -
16<=l<=16-
Reflections collected
Data completeness
Absorption correction
Tmin & Tmax
32604
99.7
10911
99.9
Gaussian
Analytical
0.588 and 0.264
Full-matrix least-squares on F2
0.972 and 0.755
Full-matrix least-squares on F2
17941 / 10 / 398
Refinement method
Data/restraints/parameters
Goodness of fit
6489 / 0 / 425
1.022
R1 = 0.0516, wR2 = 0.1017
1.027
R1 = 0.0368, wR2 = 0.0895
Final R indices [I>2σ(I)]
22

R1 = 0.0806, wR2 = 0.1148
1.002 and -0.643 e.Å-3
R1 = 0.0438, wR2 = 0.0950
0.392 and -0.442 e.Å-3
R indices (all data)
Residual difference peaks
Table 3: Selected bond distances (Å) and angles (°) about the manganese center for
complexes 2 and 3
2
Bond length Å
3
Bond length Å
Mn-O1A
Mn-N1A
Mn-N2A
Mn-O1B
Mn-N1B
Mn-N2B
1.8815(9)
Mn-O1A
Mn-N1A
Mn-N2A
Mn-O1B
Mn-N1B
Mn-O1S
1.871(4)
2.0229(9)
2.320(1)
1.8734(9)
2.041(1)
2.285(1)
2.023(1)
2.377(1)
1.867(1)
2.050(1)
2.240(1)
Bond Angle (°)
180
Bond Angle (°)
180
O1A-Mn-O1A*
O1A-Mn-N1A
O1A-Mn-O1A*
O1A-Mn-N1A
88.72(4)
88.00(6)
O1A-Mn-N1A*
N1A-Mn-N1A*
O1A-Mn-N1A*
N1A-Mn-N1A*
91.28(4)
180
92.00(6)
180
O1A-Mn-N2A
O1A-Mn-N2A*
N1A-Mn-N2A
N1A-Mn-N2A*
N2A-Mn-N2A*
O1B-Mn-O1B*
O1B-Mn-N1B
O1B-Mn-N1B*
N1B-Mn-N1B*
O1B-Mn-N2B
O1B-Mn-N2B*
O1A-Mn-N2A
O1A-Mn-N2A*
N1A-Mn-N2A
N1A-Mn-N2A*
N2A-Mn-N2A*
O1B-Mn-O1B*
O1B-Mn-N1B
O1B-Mn-N1B*
N1B-Mn-N1B*
O1B-Mn-O1S
O1B-Mn-O1S*
91.94(4)
88.06(4)
82.61(4)
97.39(4)
180
92.21(6)
87.79(6)
81.31(6)
98.69(6)
180
180
180
89.35(4)
90.65(4)
180
90.68(6)
89.33(6)
180
89.54(4)
90.46(4)
87.36(7)
92.64(7)
23

N1B-Mn-N2B
N1B-Mn-N2B*
N2B-Mn-N2B*
N1B-Mn-O1S
N1B-Mn-O1S*
N2B-Mn-O1S*
81.46(4)
98.54(4)
180
92.40(6)
87.60(6)
180
(a)
(b)
24

(c)
Figure 4a. ORTEP representation of the crystal structure of the cations (a and b),
unit cell packing (c) of the complex [Mn(L2)2] (ClO4), (2).
(a)
(b)
25

(c)
Figure 4b. ORTEP representation of the crystal structure of the cations (a and b),
unit cell packing (c) of the complex [Mn(L3)2] (ClO4), (3).
3.5. Epoxidation Reaction:
The complexes were used as synthesized for catalysis. Before the epoxidation
reaction, control experiment performed with cyclohexene and H2O2 without the
manganese complexes did not show any cyclohexene-oxide peak in the GC-MS (Figures
S52 and S53).
Epoxidation of cyclohexene by H2O2 catalyzed by the complexes in the presence
of CH3COOH/NH4OOCCH3, gave epoxide yield of 2-9 % at room temperature. At 0OC,
the yield ranged from 1–58 %. Reaction in the presence of Triethyl amine-perchloric acid
medium, gave yield of 8-46 % and 12-44 % at room temperature and at 0OC respectively.
Generally, all the complexes gave higher epoxide yield in the presence of
Triethylamine-perchloric acid system at room temperature than was obtained with the
ammonium acetate-acetic acid system. Apart from complex 4 that gave higher epoxide at
0°C with the ammonium acetate-acetic acid system, the other complexes also yielded
better result at 0°C with the triethylamine-perchloric acid system. Epoxide yields
obtained after 3 hours of reaction were the same as that obtained after reacting for 20
hours for complexes 1, 2 and 3. For complex 4, the optimum epoxide yield was obtained
26

between 4-6 hours of reaction. Epoxide yield decreased after 6 hours of reaction
signifying conversion of the epoxide to other products with time. Overall, complex 4
gave the best epoxide yield in both the ammonium acetate-acetic acid and Triethyamine-
perchloric acid system. It was reported that performing the catalytic reaction at 0OC
increases the catalytic efficiency of hydrogen peroxide [21, 82]. This was observed in the
epoxide yield for complex 4 when ammonium acetate-acetic acid was used as co-catalyst.
The epoxide yield at room temperature was only 9% while a yield of 58% was obtained
at 0°C. When the triethyl amine-perchloric acid was used, the epoxide yield for complex
4 at both room temperature and 0°C were 46% and 44% respectively. This indicates that
the triethylamine-perchloric acid co-catalyst offer additional advantage of stabilizing the
oxidant (H2O2) at both room temperature and at 0°C, thereby making it efficient.
Epoxide yield obtained with complex 4 is within the range 49.1--59.9 % reported
for epoxidation of cyclohexene using complexes of Mn [40] and those reported for
cyclohexene catalyzed by binuclear manganese complexes [77]. The result is also
consistent with that of epoxidation of cyclohexene using 5, 5’-dinitro-(salen)MnIII
system and iodosylbenzene as oxidant at 25OC that gave a yield of 57 % cyclohexene
oxide after 5 hours [78]. In certain cases, the yields exceed any other reported in literature
[79].
The yield obtained with the triethyamine-perchloric acid system at both room
temperature and at 0°C confirm the need for a nitrogenous base to enhance the rate of
reaction and epoxide yield reported previous [33, 33b, 37]. Triethylamine being a
stronger nitrogenous base than ammonium acetate may have enhanced reaction rate by
coordinating to and interacting with the manganese complex during the catalytic reaction
[33b].
Generally, epoxide yield obtained with complex 4 was greater than that for
complex 2, which was higher than that for complex 1, with complex 3 giving the least
epoxide yield. This trend and the high epoxide yield obtained with complex 4 may be
explained by the observation of Cavallo and Jacobsen [80]. They studied electronic
effects in (salen)Mn-based epoxidation catalysts and used Hammett plot to establish a
correlation between substituent effect and some properties (explained below) of the
Mn=O intermediate. They reported that electron-donating substituents such as –OMe
27

cause an increase of negative charge at the oxo center and a stronger Mn=O bond.
Electron withdrawing substituents such as –NO2 cause increase of positive charge on the
oxo center, makes it a better electrophile to attack the nucleophilic alkene double bond
and give rise to a weaker Mn=O bond. A weaker bond has the tendency to release the
oxygen atom faster than the stronger Mn=O bond hence resulting in faster reaction and
higher yield of epoxide.
The low epoxide yield obtained with the Mn(III) complexes may be a result of
disproportionation of the H2O2 by the manganese complexes or destruction of the
manganese complexes by the H2O2 as reported in [37].
The highest yield of epoxide obtained with 4 and lowest yield observed with 3
confirm the report that electron withdrawing substituents such as –NO2 increase the
reactivity of the catalyst, while substituents such as –OMe decrease the reactivity of the
oxo-species [81]. Electron withdrawing substituent on the phenyl ring of the ligand
cleaves the complex-oxidant intermediate heterolytically to produce oxo species that
transfers oxygen to the olefin while electro donating species cleave the intermediate
homolytically to form a hydroxo radical that will oxidize the allylic carbon of the olefin
to form other products such as ketone [68].
Table 4: Epoxide yield from epoxidation of cyclohexene and the turnover in
percentage. The Mn(III) complex of the Schiff base and reduced Schiff base
ligands were used as catalysts and H2O2 was used as oxidant. Bromobenzene
was used as internal standard and peak area was used for calculation.
Reaction was monitored for 20 hours
Complexes With 1ml of CH3OH + 1 ml of CH2Cl2 + 2 ml of
With 1ml of CH3OH + 1 ml of CH2Cl2 + 2ml of
acetone,
acetone, 0.1mmol of CH3COOH and 0.2mmol of
NH4OOCCH3
0.2mmol of HClO4 and 0.4mmol of Triethylamine
(C6H15N)
At Room
At 0OC
pH
At Room
At 0OC
pH
Temperature
Temperature
28

%
Turnover
%
Turnover pH1 pH2
%
Turnover
%
Turnover pH1 pH2
Yield number
(TON)
Yield number
(TON)
Yield number
(TON)
Yield number
(TON)
Mn(L1)2
Mn(L2)2
Mn(L3)2
Mn(L4)2
6
8
2
9
1.5
2.0
0.5
2.25
9
2.25
3
7.72 4.85
7.79 4.63
7.85 4.75
8
9
7
2.0
12
20
11
44
3.0
5.0
2.8
11.0
8.27 4.81
8.57 6.05
8.60 6.84
8.91 6.03
12
1
2.25
1.75
11.5
0.25
14.5
58
7.90 3.93 46
*pH1 was the pH of the medium after adding the buffer while pH2 was the pH of the medium
after adding the oxidant (H2O2)
Reaction condition; catalyst: 0.027mmol; substrate:0.675mmol; oxidant: 2.75 mmol; solvent
mixture: 4ml;
TON is calculated by expression of mol of substrate divided by mol of catalyst times % yield
of product
4. Conclusions
Manganese complexes of Schiff base ligands were synthesized successfully as
confirmed by characterization methods. The strong electron withdrawing (–NO2)
substituted ligands reacted with the Mn(ClO4)2.6H2O to form Mn(II) while the other
ligands formed Mn(III) complexes. Cyclic voltammetry study indicates that electron-
withdrawing substituents shift the redox potential to more positive value while electron-
donating substituents shift the redox potential to more negative value. The complexes
particularly the electron withdrawing substituted ones effectively catalyzed epoxidation
of cyclohexene. Nitrogenous base is necessary for efficient catalytic activity of the
complexes. The co-catalyst triethylamine-perchloric acid stabilized the oxidant at both
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