Immobilized into montmorillonite Mn(II) complexes of novel pyridine schiff-base ligands and their catalytic reactivity in epoxidation of cyclohexene with O2

Article abstract of DOI:10.1007/s10562-013-1002-x

Mild temperature and atmospheric oxygen pressure assisted epoxidation of cyclohexene was achieved with a series of novel montmorillonite-supported Mn(II) complexes of pyridine Schiff-base ligands. The catalysts were characterized by IR, UV-Vis DRS, XRD, SEM and ICP. The heterogeneous catalysts were proved to promote the epoxidation of cyclohexene efficiently, affording the desired product epoxycyclohexane with a yield up to 92 %. The effects of various reaction conditions on the catalytic reaction were optimized. Repeated runs indicated that the catalysts were stable for 3 cycles without noticeable loss of their catalytic activity.

Full text of DOI:10.1007/s10562-013-1002-x

Catal Lett (2013) 143:592–599
DOI 10.1007/s10562-013-1002-x
Immobilized into Montmorillonite Mn(II) Complexes of Novel
Pyridine Schiff-Base Ligands and Their Catalytic Reactivity
in Epoxidation of Cyclohexene with O2
•
Zhongying Li Ruiren Tang Guiyin Liu
•
Received: 27 February 2013 / Accepted: 28 March 2013 / Published online: 4 April 2013
Ó Springer Science+Business Media New York 2013
Abstract Mild temperature and atmospheric oxygen
pressure assisted epoxidation of cyclohexene was achieved
with a series of novel montmorillonite-supported Mn(II)
complexes of pyridine Schiff-base ligands. The catalysts
were characterized by IR, UV–Vis DRS, XRD, SEM and
ICP. The heterogeneous catalysts were proved to promote
the epoxidation of cyclohexene efficiently, affording the
desired product epoxycyclohexane with a yield up to 92 %.
The effects of various reaction conditions on the catalytic
reaction were optimized. Repeated runs indicated that the
catalysts were stable for 3 cycles without noticeable loss of
their catalytic activity.
conditions. Employing oxidant reagents which maximize
atom efficiency is a key goal in oxidation reactions. Most
of the prior work is suffering from the drawback more or
less: low conversion or selectivity or employment of
environmental unfriendly oxidants such as PhIO, NaClO,
H2O2, t-BuOOH [3, 4]. Molecular oxygen as a cheap,
environmentally clean and convenient oxidant has attracted
significant attention, but it is relatively unreactive toward
the strong bond of C–H unless it is activated by highly
efficient catalysts[5] or severe reaction conditions are
required, i.e., high reaction temperature and pressure [6].
So it is more interesting from an academic and industrial
point of view to ameliorate the reaction condition (mild
reaction condition) maintaining high catalytic activity
through a clean and environmentally friendly process. It is
reported that many catalysts efficiently catalyze epoxida-
tion of cyclohexene with molecular oxygen, such as
metalloporphyrins, metal phthalocyanines, transition met-
als oxides, salen-metal complexes and heteropoly acid [6].
Although metalloporphyrins and metal phthalocyanines
display high catalytic activity and selectivity when applied
to the epoxidation of cyclohexene, the preparation of cat-
alysts are very complicated and need to consume large
amounts of toxic solvents. It is easy to operate when using
transition metals oxides as catalysts, but the conversion and
selectivity of products are lower than other catalysts and it
is difficult to purify the products [7].
Keywords Cyclohexene Á Epoxidation Á Mn(II)
complexes Á Pyridine Schiff-base ligands Á Molecular
oxygen Á Montmorillonite
1 Introduction
Selective oxidation of alkenes is considered as a funda-
mental and critical reaction for its extensive applications in
synthetic chemistry [1]. Epoxidation of cyclohexene is an
industrially important reaction, since its epoxide compound
is the vital ingredient for the manufacture of variety of
chemicals such as pharmaceutical products, agrochemicals
and food additives [2].
In the last decade, many efforts had been dedicated to
the development of routes for the preparation of epoxides
using efficient catalysts under environmental friendly
Recently, metal complexes of Schiff-base have been
widely applied to catalyze the oxidation alkenes under both
heterogeneous and homogeneous conditions [3, 4, 8] for
the reason they display high catalytic activity and selec-
tivity in the epoxidation of alkenes [4, 8]. Compared with
homogeneous catalysts, the heterogenization of the metal
complexes by chemical bonding to a support leads to an
increase of their stability and makes easier the separation of
Z. Li Á R. Tang (&) Á G. Liu
School of Chemistry and Chemical Engineering, Central South
University, Changsha 410083, Hunan, China
e-mail: trr@mail.csu.edu.cn
123

Novel Pyridine Schiff-Base Ligands and Their Catalytic Reactivity
593
the catalysts from the reaction mixture for recycling [3, 4,
6, 8]. For this purpose, a variety of supports have been
used, such as: zeolites [9], polymers [10], nanoporous
carbon [11] and Smectite clays [4, 8, 12, 13]. Smectite
clays, such as montmorillonite, have attracted interest as
supports for the immobilization of metal complexes in
recent years, because they combine cation exchange,
intercalation and swelling properties which make them
unique [9, 10]. Chemically speaking, it is a hydrated alu-
minosilicate of idealized formulation Al2Si4O10(OH)2
(mineral pyrophyllite) and its crystalline structure is gen-
erated by the encapsulation of a gibbsite layer (polymorph
of Al(OH)3) with two silica layers [12–16]. Montmoril-
lonite is a cheap support compared with the others, there-
fore, using montmorillonite-based material as catalyst is
more economical for industry.
catalytic performance of these catalysts were investigated
in the autoxidation of cyclohexene and the reaction
parameters in the system were optimized.
2 Experimental
2.1 Materials and Methods
Powder X-ray diffraction (XRD) patterns of the heteroge-
neous samples were obtained on Rigaku D/Max III VC
diffractometer with Cu KR radiation at 40 kV and 40 mA
in the range of 2h = 2 * 9°. Melting points were deter-
mined on a XR-4 apparatus (thermometer uncorrected).
Elemental analysis was carried out by a Perkin Elmer 2400
elemental analyzer. Infrared spectra were recorded on a
Nicolet NEXUS 670 FT-IR spectrophotometer using KBr
discs in the 400–4,000 cm-1 region. 1H-NMR spectra were
measured with a Bruker-400 MHz nuclear magnetic reso-
nance spectrometer with CDCl3 as solvent and TMS as
internal reference. The UV–vis spectra were recorded on an
UV-2450 spectrophotometer. ESI-MS were measured with
MicromassÒ ZQTM4000. The metal loading and leaching
of the reaction solution were determined by ICP with ICP-
AES 5300D.
The aim of the present work is to obtain a better catalyst
for the epoxidation of cyclohexene with a friendly process.
To pursue this goal, a series of novel montmorillonite-
supported Mn(II) complexes of pyridine Schiff-base
ligands were designed and prepared, then their activity and
selectivity as catalysts for the epoxidation of cyclohexene
were studied. Pyridine Schiff-base ligands were selected
for the work because of their easy preparation and ability to
form complexes with almost all metal ions [17]. The
montmorillonite-supported Mn(II) complexes of pyridine
Schiff-base ligands, which were first applied to catalyze the
epoxidation of cyclohexene under Mukaiyama conditions
using molecular oxygen as the oxidant, has gained attention
because of its high activity, stability and selectivity under
mild conditions (mild temperature and atmospheric pres-
sure) and recyclable properties. The effect of different
substituent Schiff-base catalysts on regulate the catalytic
activity of the complex through the introduction of elec-
tron-withdrawing (Cl, NO2) and electron-donating substit-
uents (CH3) were also studied. And then the stability and
2.2 Synthetic Procedure
2.2.1 Preparation of Schiff-Base Ligands
As shown in Scheme 1, 2,6-pyridinedicarbaldehyde was pre-
pared according to the literature method [18–20]. Schiff-base
ligands (LR) were synthesized by co-condensation method
mixing 2,6-pyridinedicarbaldehyde and substituted o-hydro-
xyl-aromatic amines in 1:1 molar ratio. In a typical experiment,
a mixture of 2,6-pyridinedicarbaldehyde (7.4 mmol) in 20 mL
Scheme 1 Preparation of montmorillonite-supported Mn(II) complexes of Schiff-base ligands
123

594
Z. Li et al.
2.2.3 Immobilization of Mn(II) Complex of Schiff-base
Ligands into Montmorillonite
ethanol and substituted o-hydroxyl-aromatic amines
(14.8 mmol) was refluxed for 6 h. Then the reacting mixture
was cooled and allowed to crystallize. Finally, the ligands were
recrystallized from ethanol. These Schiff-base ligands were
characterized by IR and 1H NMR.
To 40 mL of methanol, 0.3 mmol of the Mn(II) complex
was dissolved. To the solution, a suspension of montmo-
rillonite (2.0 g) in water (100 mL) was added and the
mixture was refluxed for 5 h. The solid was filtered,
washed with water, and dried at room temperature to form
the montmorillonite-supported Mn(II) complex of pyridine
Schiff-base ligand. It was characterized by IR, diffuse
reflectance ultraviolet visible spectra, powder XRD and
scanning electron microscopy (SEM) measurements. The
concentration of the filtrate was measured, from which the
amount of complex adsorbed into clay was calculated.
2.2.1.1 L1 N,N-(2,6-Pyridinediyldimethylidyne)bis(2-hy-
droxybenzenamine) Yield: yellow solid (2.02 g, 86 %).
mp 200–202 °C; IR (KBr, cm-1): 3,388 (O–H), 1,623
1
(C=N); H NMR (400 MHz, CDCl3): d = 8.90 (s, 1H),
8.31–8.29 (d, J = 8.0 Hz, 1H), 8.04–8.02 (t, J = 8.0 Hz,
1H), 7.46–7.44 (s, J = 8.5 Hz, 2H), 6.96–7.46 ppm (m,
8.0H).
2.2.1.2 L2 N,N-(2,6-Pyridinediyldimethylidyne)bis(2-hydroxy-
4-methylben-zenamine) Yield: yellow solid (2.09 g, 82 %).
mp 214–216 °C; IR (KBr, cm-1): 3,373 (O–H), 1,624
2.3 Catalytic Epoxidation Reaction
The epoxidation of cyclohexene with molecular oxygen was
carried out in a 50 mL three-necked bottle at 40 °C. In a
typical run, 20 mg of catalyst, 10 mmol of cyclohexene,
20 mL of solvent (acetonitrile) and 10 mmol of isobutyral-
dehyde were added into the reaction. Then the flask equipped
with an water condenser was kept in a constant temperature
oil bath with the temperature maintained at 40 2 °C. The
reaction started by bubbling molecular oxygen into the stir-
red reaction mixture at atmospheric pressure with a rate of
10 mL/min. The mixture was dried by anhydrous Na2SO4
and then analyzed by GC–MS with DB-5 MS column
(30 m 9 0.25 mm 9 0.25 lm). The conversion and selec-
tivity were determined by GC equipped with OV-1701 col-
umn (50 m 9 0.25 mm 9 0.25 lm), a programmed oven
(temperature increased from 60 to 200 °C at a rate of 10 °C/
min, and then kept at 200 °C for 2 min), with N2 as the carrier
gas. n-Heptane was used as an internal standard. The selec-
tivity of the epoxide (cyclohexene oxide) was a measure of
the reactivity of the catalyst.
1
(C=N); H NMR (400 MHz, CDCl3): d = 8.88 (s, 1H),
8.27–8.26 (d, J = 8.0 Hz, 1H), 7.97–7.94 (t, J = 8.0 Hz,
1H), 7.36–7.35 (d, J = 8.0 Hz, 2H), 7.26–6.75 (m, 6H),
2.36 ppm (s, 6H).
2.2.1.3 L3 N,N-(2,6-Pyridinediyldimethylidyne)bis(4-chloro-2-
hydroxyben-zenamine) Yield: yellow solid (2.27 g, 80 %).
mp 225–227 °C; IR (KBr, cm-1): 3,394 (O–H), 1,623
1
(C=N); H NMR (400 MHz, CDCl3): d = 8.90 (s, 1H),
8.29–8.27 (d, J = 8.0 Hz, 1H), 8.0–7.97 (t, J = 8.0 Hz,
1H), 7.37–7.35 (s, J = 8.5 Hz, 2H), 7.26–6.93 ppm (m,
6H).
2.2.1.4 L4 N,N-(2,6-Pyridinediyldimethylidyne)bis(4-nitro-
2-hydroxyben-zenamine) Yield: yellow solid (2.25 g,
75 %). mp 237–239 °C; IR (KBr, cm-1): 3,386 (O–H),
1
1,593 (C=N); H NMR (400 MHz, CDCl3): d = 8.96 (s,
1H), 8.34–8.32 (d, J = 8.0 Hz, 1H), 8.05–8.02 (t,
J = 8.0 Hz, 1H), 7.44–7.42 (s, J = 8.5 Hz, 2H),
7.31–7.0 ppm (m, 6H).
3 Results and Discussion
3.1 The Structures of the Complexes
2.2.2 Preparation of Mn(II) Complexes of Schiff-Base
Ligands
The elemental analytical data for the complexes are pre-
sented in Table 1, which is in good agreement with the
calculated values, indicating that the composition of the
complexes are confirmed to be [MnLRCln]Á3H2O(n1 = 1,
n2 = 1, n3 = 3, n4 = 1). From the electron spray ioniza-
tion mass spectrometry data (ESI-MS) given in supple-
mentary Material for the complexes Mn–L1, we can find
the molecular ion (M) m/z 459.9, the strong peak at
m/z 371 is formed due to the molecular ion (M) lost 3H2O
and Cl. The isotope peaks at m/z 459.9 and m/z 461.9
(isotope ratio m/z 459.9 to m/z 461.9 is 3:1) show the
Mn(II) complexes were synthesized by mixing
MnCl2Á4H2O and ligands (metal to ligand ratio 1:1) in
ethanol. The resulting mixture was stirred under refluxed
for 5 h, where upon the complex was precipitated. It was
collected by filtration, washed with 1:1 ethanol: water
mixture, then dried in vacuum at room temperature for
24 h. These complexes were characterized by elemental
analyses, IR, and ESI-MS.
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Novel Pyridine Schiff-Base Ligands and Their Catalytic Reactivity
595
Table 1 Analytical and physical data of Mn–L1, Mn–L2, Mn–L3 and Mn–L4 complexes
Compound
Color (yield)
m.p. (°C)
Found (calcd.) %
C
ESI-MS (m/z)
M
H
N
Mn
[MnL1Cl]Á3H2O
Yellow (84)
[300
49.16 (49.56)
51.45 (51.63)
43.55 (43.18)
41.25 (41.50)
4.54 (4.13)
9.01 (9.13)
11.63 (11.95)
459.9
488.0
527.9
549.9
C19H19ClMnN3O2
[MnL2Cl]Á3H2O
Bright-yellow (85) [300
4.82 (4.71)
3.46 (3.21)
3.15 (3.09)
8.50 (8.60)
7.32 (7.95)
11.36 (11.27)
11.21 (10.41)
10.25 (10.01)
C21H23ClMnN3O2
[MnL3Cl]Á3H2O
Red yellow (80)
Red yellow (75)
[300
[300
C19H17Cl3MnN3O5
[MnL4Cl]Á3H2O
12.38 (12.75)
C19H17ClMnN5O9
complex has one Cl atom. The ESI-MS analytical data for
the complex indicates that the composition of the complex
is confirmed to be [MnL1Cl]Á3H2O. The ESI-MS analytical
data for the other three complexes are also confirmed to be
[MnLRCln]Á3H2O(n2 = 1, n3 = 3, n4 = 1). The molecular
ions (M) are listed in Table 1.
(based on X-ray structural data) reported that the similar
Schiff-base ligands always coordinated to metals via the
pyridine-N, two azomethine-N and two hydroxy–O. Thus,
we suspect that L1, L2, L3 and L4 ligands coordinated to
Mn(II) though the pyridine-N, two azomethine-N and two
hydroxy–O forming a five binding chelating sites. The
octahedral Mn(II) complexes, the one extra position is
provided by the one-halide.
The IR spectra of L1, L2, L3 and L4 and their chelates are
found to be very similar to each other. The main stretching
frequencies of the IR spectra of the L1 and Mn–L1 are
listed in Fig. 2. Upon comparison, it is found that the
azomethine (CH=N) stretching vibration in the free ligand
at 1,623 cm-1. The band which disappears in the com-
plexes indicate the participation of the azomethine nitrogen
in coordination (Mn–N). The strong bands located at
1,586 cm-1 are assigned to the t(C=N) stretching vibrations
of pyridyl nitrogen of the L1. Coordination of pyridyl
nitrogen is indicated by a 34 cm-1 shift to a lower wave-
number in the chelate. The other series of weak bands
between 3,100 and 2,800 cm-1 are related to (C–H) modes
of vibrations. Also, some weak bands are located between
2,000 cm-1 and 1,750 cm-1 can be assigned to overtones
of the aromatic rings. The t(OH) and t(C–O) vibrations are
observed at 3,388, 1,359 cm-1 for L1, the existence of
water of hydration and/or water of coordination in the
spectra of the complexes render it difficult to get conclu-
sion from the t(OH) group of the ligand, which will be
overlapped by those of the water molecules. The partici-
pation of the OH group is further confirmed by clarifying
the effect of chelation on the t(C–O) and in-plane bending,
t(OH) vibrations. The participation of the phenolic O atom
in the complex formation is proved by the shift in position
of the band to 1,363 cm-1. New bands are found in the
3.2 Characterization of the Heterogeneous Catalyst
All the characterization of the montmorillonite-supported
Mn(II) complexes of pyridine Schiff-base ligands are
similar to each other, herein, the characterization of
montmorillonite and Mn–L1–mont is given.
3.2.1 Powder XRD Analysis
The characteristic XRD (5 * 70°) of mont and Mn–L1–
mont is given in Fig. 1. The X-ray powder diffraction
(XRD) patterns of the montmorillonite and Mn–L1–mont
reveal that Mn–L1–mont has identical structural charac-
teristics to that of the montmorillonite. The corresponding
spectra of the complexes in the regions 517–521 cm-1
,
which are assigned to t(M–O) stretching vibrations. The
bands at 455–490 cm-1 have been assigned to t(M–N) mode
[21].
From all of the above of EA, ESI-MS and IR, the
structure of these complexes can be interpreted in accor-
dance with complexes of ligands with a similar distribution
Fig. 1 Characteristic XRD (5 * 70o) of a mont and b Mn–L1–mont
´
of like coordination sites [7, 21]. Gonzalez et al. [22]
123

596
Z. Li et al.
basal spacing value of Mn–L1–mont is slightly lower than
that of montmorillonite which suggests that, more likely,
the Mn(II) complexes covered preferentially the outer
surface and in a very low amount the interlayer space,
wherein they adopt more likely a flat-lying position.
3.2.2 FT-IR Spectra
The FT-IR spectra of L1, Mn–L1, Mn–L1–mont and
montmorillonite are given in Fig. 2. The band emerging at
about 3,388 and 1623 cm-1 in the FT-IR spectrum of
Schiff-base (L1) can be assigned to the vibration of t(O–H)
and t(C=N). The Schiff-base Mn(II) complex shows typi-
cal absorption bands at 3,344 (O–H stretching vibration of
H2O), 1,590 (C=N stretching vibration) and 1,360 cm-1
(C–O stretching vibration). The montmorillonite mainly
exhibits Al–O–H and Si–O–Si stretching vibration bands at
3,594 and 1,034 cm-1, respectively, and H–O–H stretching
and bending vibration bands at 3,416 and 1,636 cm-1. As
can be seen in Fig. 2, the FT-IR spectra of montmor-
illonite-supported Mn(II) complex both display the typical
absorption bands of the complex and montmorillonite,
showing the direct evidence of immobilization of Mn(II)
complex of Schiff-base into montmorillonite.
Fig. 3 Characteristic UV–Vis DRS bands (200–700 cm-1) of a
mont, b Mn–L1–mont, c Mn–L1 and d Mn–L1
500 nm. The broad band is from the absorption of C=N of
ligand and also from the absorption of metal–ligand and
ligand–metal charge transfer processes. This characteriza-
tion further provided the evidence for immobilization of
Mn(II) complex of Schiff-base into montmorillonite.
3.2.4 SEM
3.2.3 UV–Vis DRS
Figure 4 shows the SEM of the montmorillonite and Mn–
L1–mont. A clear change in the morphology of Mn–L1–
mont compared with the montmorillonite observed by
SEM confirms that the Mn(II) complex of Schiff-base was
immobilized into montmorillonite.
The diffuse reflectance UV–Vis spectra characterization of
mont, L1, Mn–L1 and Mn–L1–mont samples were mea-
sured. The spectrum of the Schiff-base ligand shows a
p–p* band at 325–375 nm due to the absorption of pyridyl
and imine (Fig. 3), which does not significantly alter in
case of Mn(II) complex of Schiff-base. Compared with the
spectrum of the montmorillonite, the spectrum of Mn–L1–
mont exhibits an additional broad absorption from 300 to
3.2.5 ICP Analysis
As revealed by ICP analysis, immobilization of the Mn(II)
complexes of Schiff-base ligands into montmorillonite led to
1.64 9 10-2 mmol of Mn loading/g of clay. This corre-
sponds to global anchoring efficiencies (amount of adsorbed
complex/amount in original solution 9 100) of 80 %.
3.3 Analysis of Catalytic Epoxidation Reaction
3.3.1 Catalytic Study
The effect of different substituent Schiff-base catalysts on
epoxidation of cyclohexene was investigated. These results
were summarized in Table 2, the conversion was only
6.0 % in the blank reaction carried out under the same
conditions when the reaction time was 5 h. The result
which was presented in Entry 3 indicated that Mn2? ion did
not act in the epoxidation reaction. When only catalytic
amount (0.02 mol%) of the Mn–L1 was added in reaction,
the conversion reached 82 % rapidly. It confirmed that the
Fig. 2 Characteristic IR bands (4000–400 cm-1) of a mont, b Mn–
L1–mont, c Mn–L1 and d L1
123

Novel Pyridine Schiff-Base Ligands and Their Catalytic Reactivity
597
Fig. 4 SEM images of the samples a Montmorillonite and b Mn–L1–mont
catalytic activity and selectivity of Mn–L3 and Mn–L4
decreased in light of electron-withdrawing effect of chloro
and nitro atoms. The conversion reached 100 % when we
used montmorillonite-supported Mn(II) complexes of
Schiff-base ligands as catalysts (Entries 8-11), the result
confirmed that the immobilization of the complex into
montmorillonite appeared to be particularly effective in the
activation due to the oxidation of the active complex spe-
cies. The reaction proceeded more rapidly, affording
epoxycyclohexane in good yield. Increased yield in heter-
ogeneous medium may be attributed as the interlayer space
localizes the substrate and active species in close proxim-
ity, which was not the case in solution reaction [12].
The reaction conditions including catalyst amount,
reaction time, reaction temperature and molar ratio of
isobutyraldehyde to cyclohexene were investigated. As can
be seen from Table 3, the variation in catalyst amount from
0.01 to 0.03 g at 40 °C and atmospheric pressure showed
an increase of 17 % yield (Entries 6–8). The conversion
increased from 15.5 to 80.7 % between 1 and 3 h and then
the conversion and epoxide selectivity reach the highest at
5 h (Entries 9–12). Taking both conversion and selectivity
into consideration, 40 °C might be the suitable reaction
temperature, high temperature may accelerate the rate of
chemical reaction, but the conversion decreased a little
when the temperature exceeded 40 °C because isobutyral-
dehyde with low boiling point (64 °C) may evaporate more
at the relatively higher temperature (Entries 1–5). The
molar ratio of isobutyraldehyde to cyclohexene plays a key
role in the epoxidation of cyclohexene by molecular oxy-
gen because the conversion was only 60 % at the low
molar ratio (isobutyraldehyde to cyclohexene) of 0.1. The
conversion reached the maximum at the molar ratio of 1.0
(Entries 13–16).
Table 2 Effect of different substituent Schiff-base catalysts on
epoxidation of cyclohexene with molecular oxygen
Run Catalyst
Conversion (%) Epoxide selectivity (%)
A
B
C
1
None
6.0
7.2
68.1
71.2
69.2
90.2
91.2
86.2
80.2
88.3
92.0
87.2
91.0
9.5
9.2
9.7
3.0
2.2
3.6
4.6
3.5
2.0
3.7
2.8
21.5
20.0
20.2
7.8
2
Mont
3
MnCl2
7.3
4
Mn–L1
Mn–L2
Mn–L3
Mn–L4
Mn–L1–mont
82.3
85.6
80.1
78.0
95.5
5
7.2
6
11.1
14.2
8.2
7
8
9
Mn–L2–mont 100
6.6
10
11
Mn–L3–mont
Mn–L4–mont
97.7
99.2
10.3
6.7
Reaction conditions: 10 mmol cyclohexene; 0.02 mol% (20 mg)
catalyst; 20 mL acetonitrile, 10 mmol isobutyraldehyde, temperature
40 °C; molecular oxygen rate 10 mL/min
Schiff-base catalyst was necessary for this oxidation reac-
tion. The result (Entries 4-7) confirmed that Mn–L2 dis-
played the highest catalytic activity and selectivity when
applied to the epoxidation of cyclohexene, because of
steric and electron effects of substituents in the Schiff-base
ligands. The electron density on N atoms was increased due
to providing electronic of methyl. The coordination ability
of metal Mn2? and Schiff-base were enhanced, as with the
ability of activating oxygen. But quite on the contrary, the
123

598
Z. Li et al.
Table 3 Epoxidation of cyclohexene catalyzed by the Mn–L2–mont under different conditions
Entry
Parameters
Temperature (°C)
Catalyst weight (g)
Time (h)
Molar ratio
Conversion (%)
Epoxide selectivity (%)
1
Temperature
25
30
35
40
45
40
40
40
40
40
40
40
40
40
40
40
0.02
0.02
0.02
0.02
0.02
0.01
0.02
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
5
5
5
5
5
5
5
5
1
3
5
7
5
5
5
5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.5
1.0
1.5
40.5
65.8
80.2
100
78.6
81.5
85.6
92.0
90.0
89.2
92
2
3
4
5
95.4
85
6
Catalyst
Time
7
100
8
98.2
15.5
80.7
100
90
9
85.6
90.3
92.0
91.0
85.2
90.2
92.0
94.0
10
11
12
13
14
15
16
100
Molar ratio
60.7
75.8
100
99.7
Reaction conditions: 10 mmol cyclohexene; 20 mL solvent, molecular oxygen rate 10 mL/min
indicated almost no change in catalytic activity even after
the third recycle. That meant the direct interaction of the
complex and the clay was stable in the catalytic reaction
media.
4 Conclusion
In summary, we have developed a novel method to the
epoxidation of cyclohexene under mild conditions using
molecular oxygen as oxidant. The Mn(II) complexes of
Schiff-base ligands have been supported on montmoril-
lonite by ion exchange method. The intercalation of the
complexes were confirmed by XRD, FT-IR, UV–Vis DRS,
SEM and ICP. The montmorillonite catalysts performed
high activity and epoxide selectivity for aerobic epoxida-
tion of cyclohexene under Mukaiyama conditions. The
catalysts could be reused three times and its high activity
and epoxide selectivity almost did not change.
Fig. 5 Recycling study of the catalyst. Reaction conditions: Cyclo-
hexene 10 mmol; oxidant (O2); catalyst, Mn–L2–mont 0.02 mol%
(20mg) catalyst; acetonitrile 20 mL; temperature 40 °C; Isobutyral-
dehyde 10 mmol; reaction time 5 h. Yield refers to GC
3.3.2 Reuse of Catalyst
Acknowledgments Financial support from Basic Science Founda-
tion of China national universities (No.1177-721500193) of Central
South University is gratefully acknowledged.
In the case of the Mn–L2–mont catalyst the persistence of
the catalytic activity was checked for three consecutive
runs in the epoxidation of cyclohexene under the optimal
conditions as described above. From the results of recy-
cling tests shown in Fig. 5, the catalyst was separated from
the reaction mixture after each experiment by filtration,
washed with the solvent (CH3CN), and dried carefully
before using it in the subsequent run. The catalyst gave
epoxycyclohexane in 93, 92, 90, and 87 % yield for the
first, second, third and fourth runs, respectively, which
References
1. Murphy EF, Mallat T, Baiker A (2000) Catal Today 57:115
2. Gao J, Chen Y, BoHan, Feng Z (2004) J Mol Catal A Chem
210:197
3. Pouralimardan O, Chamayou A-C (2007) Inorg Chim Acta
360:1599
4. Jiang Jian, Ma Kun (2009) Appl Clay Sci 45:117
123

Novel Pyridine Schiff-Base Ligands and Their Catalytic Reactivity
599
5. Chen L, Tang R (2012) Bull Korean Chem Soc 33:459
6. Angelescua E, Ionescua R, Pavela OD (2010) J Mol Catal A
Chem 315:178
7. Xia Q, Ge H (2005) Chem Rev 105:1603
8. Jiang J, Ma K, Zhen Y, Cai S (2009) Catal Lett 131:105
9. Seelan S, Sinha AK (2003) Appl Catal A 238:201
10. Blaz E, Pielichowski J (2006) Molecules 11:115
11. Lim SY, Kang M, Kim JM, Lee IM (2005) Bull Korean Chem
Soc 26:887
14. Gournis D, Louloudi M (2002) Mater Sci Eng C 22:113
15. Salavati-Niasari M (2007) J Mol Catal A Chem 263:247
16. Salavati-Niasari M, Zamani E (2007) Appl Clay Sci 38:9
17. Gupta KC, Sutar AK (2008) Coord Chem Rev 252:1420
18. Shelkov R, Melman A (2005) Eur. J Org Chem 2005:1397
19. Xinyan Wu, Yongbin He (1996) Wuhan University J Nat Sci
1105:106
20. Lu¨ning U, Baumstarka R (1990) Liebigs Ann Chem 129:143
21. Abdallah SM, Mohamed GG (2009) Spectrochimica Acta Part A
73:833
12. Parida K, Varadwaj GBB (2011) Ind Eng Chem Res 50:7849
13. Varadwaj GBB, Sahu S, Parida K (2011) Ind Eng Chem Res
50:8973
´ ´
22. Gonzalez A, Gomez E (2009) Chem Pharm Bull 57:5
123