Polymer-supported bis (2-hydroxyanyl) acetylacetonato molybdenyl Schiff base catalyst as effective, selective and highly reusable catalyst in epoxidation of alkenes

Article abstract of DOI:10.1016/j.inoche.2012.11.015

Polymer-supported bis (2- hydroxylanyl) acetylacetone Schiff base ligand was prepared by reaction of bis (2- hydroxylanyl) acetylacetone with chloromethylated polystyrene in DMF. In the subsequent reaction of this polymer-supported Schiff base ligand with

Full text of DOI:10.1016/j.inoche.2012.11.015

Inorganic Chemistry Communications 28 (2013) 90–93
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Inorganic Chemistry Communications
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Polymer-supported bis (2-hydroxyanyl) acetylacetonato molybdenyl Schiff base
catalyst as effective, selective and highly reusable catalyst in epoxidation of alkenes
⁎
Gholamhossein Grivani , Akram Akherati
School of Chemistry, Damghan University, Damghan, 36715-364, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Polymer-supported bis (2- hydroxylanyl) acetylacetone Schiff base ligand was prepared by reaction of bis
(2- hydroxylanyl) acetylacetone with chloromethylated polystyrene in DMF. In the subsequent reaction of
this polymer-supported Schiff base ligand with MoO2(acac)2 the polymer-supported bis (2- hydroxylanyl)
acetylacetonato MoO2 Schiff base complex was prepared. This polymer-supported molybdenum Schiff base cat-
alyst was highly active and selective in epoxidation of various alkenes in the presence of tert-buthyl hydroperox-
ide (TBHP) in CCl4. It can be recovered and reused for 8 times without any loss in its activity.
© 2012 Elsevier B.V. All rights reserved.
Received 13 October 2012
Accepted 15 November 2012
Available online 23 November 2012
Keywords:
Polymer-support
Molybdenyl dication
Schiff base complex
Diimine complex
Epoxidation
In recent years much effort has been devoted to heterogenization of
homogeneous catalysts on insoluble supports because homogeneous
catalysts suffer from drawback of poor catalyst recovery and product
separation [1–3]. Furthermore in homogeneous catalyst systems the for-
mation of μ-oxo dimmers or other polymeric species lead to irreversible
catalyst deactivation process. In order to overcome to this shortcoming
and combine the advantages of homogeneous and heterogeneous cata-
lyst, homogeneous active catalysts have been immobilized in several
heterogeneous organic and inorganic supports [4,5]. In addition to inor-
ganic supports, polymeric counterparts have gained attention because
they are inert, nontoxic, nonvolatile, insoluble and recyclable [6,7].
Among the polymeric supports, chloromethylated polystyrene cross-
linked with divinylbenzene is one of the most widely used supports
[8]. Different approaches like encapsulation, intercalation, ion exchange
and covalent attachment of homogeneous catalyst on functionalized
polymeric surfaces or inorganic solids were used for this strategy
[9–13]. Recently immobilization of transition metal complex on poly-
meric supports through covalent grafting has received wide attention
and they were used extensively in organic reactions [14,15].
hand, they play an important role in the formation of various biological-
ly active compounds [16].
High-valent d0 transition metal complexes such as MoVI, VV and TiIV
are good catalysis for olefin epoxidation reactions, usually by employing
tert-butyl hydroperoxide (TBHP) as mono-oxygenated oxidant but
among these metal complexes molybdenum complexes are considered
to be very effective catalysts [17–20]. Numerous attempts have been
made to immobilize homogeneous Schiff base complexes, such as, an-
choring of them on a polymeric matrix for oxidation reactions [21–23].
Here we report the preparation of polymer-supported molybdenum-
based catalyst as efficient heterogeneous catalyst by coordination of
MoVI on anchored Schiff base ligand onto polystyrene.
We have developed convenient catalyst preparation procedures by
two simple reactions. Scheme 1 shows the preparation procedures of
polymer-supported bis (2- hydroxylanyl) acetylacetonato molybdenyl
Schiff base catalyst. In this approach the MoO2 Schiff base complex is
attached via a covalent bond to pendant chloromethyl group on the sur-
face of polymer. Chloromethylated polystyrene resin was reacted with
bis (2- hydroxylanyl) acetylacetone Schiff base ligand in DMF [24(a)].
The polymer-supported Schiff base ligand of 1 was used to the immobi-
lization of molybdenyl dication (MoO22+) by reaction of this supported
Schiff base ligand and MoO2(acac)2 complex in DMF as a reaction sol-
vent [24(b)]. The principal evidence for anchoring of Schiff base ligand
onto polymer (1) is appearance and loss of IR peaks that correspond
to replace Schiff base ligand on chlorine of chloromethylated polysty-
rene. In the free Schiff base ligand of H2L the imine vibration frequencies
appeared in lower frequencies at 1606 cm−1 because of existence of
strong hydrogen bonding between protons of phenolic OH and nitro-
gens of imine. In the FT-IR spectrum of polymer-bound Schiff base li-
gand in similar to unsupported Schiff base ligand (H2L) a broad band
Oxidation of alkenes to their corresponding epoxides is one of the
most important reactions in organic synthesis and it is both of academic
and industrial interest. The reaction products–epoxides are used widely
as intermediates in organic synthesis, pharmaceuticals as well as poly-
mer production. They are used to provide industrially important prod-
ucts such as surfactants, detergents, antistatic agents and corrosion
protection agents, lubricating oils, textiles and cosmetics. On the other
⁎
Corresponding author. Fax: +98 232 5235431.
E-mail address: grivani@dubs.ac.ir (G. Grivani).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.11.015

G. Grivani, A. Akherati / Inorganic Chemistry Communications 28 (2013) 90–93
91
OH
OH
N
OH
OH
NaI/ DMF
N
N
HCl
Cl
N
110 oC/ 5days
1
MoO2(acac)2/
DMF/ 4h
H2L
O
O
O
N
N
Mo
O
2
Scheme 1. Preparation procedures of polymer-supported bis (2-hydroxyanyl) acetylacetonato molybdenyl Schiff base catalyst.
appeared in the region of 1620–1560 cm−1 that overlapped with C_C
stretching vibration frequencies of phenyls of polystyrene support. The
N content of resin 1 was also determined by elemental analysis that
obtained 2.06% (1.5 mmol/g) which is another evidence of this reaction
performance on the resin surface. Further evidence for this reaction on
the chloromethylated polystyrene resin noted by decreasing in intensity
of the peak at 1266 cm−1 in the FT-IR spectrum of the 1, corresponded
to the H\C\Cl wagging modes of the original chloromethylated
resin [25].
and 1, 2, dichloroethane [29–32] as we also observed in this research.
This can be explained by hard-soft acid–base theory. The solvents
such as methanol, acetone, THF, and acetonitrile containing hard coor-
dinate donating atoms (O and N as hard bases) tend strongly to link
to the hard metal centers such as Mo(VI) (as hard acid). On the other
hand all of chlorinated solvents do not have any coordination ability
to the metal (molybdenum(VI)) center. As the coordination ability of
the solvent increases, the solvent competes with TBHP for coordination
to the molybdenum(VI) and inhibits the TBHP coordination and retards
the reaction progress, resulting in decreasing the epoxide yield. The
effect of other different reaction media such as NaIO4 and H2O2 on the
epoxidation of cyclooctene under various conditions was also investi-
gated (Table 2). It was clear that in CCl4 in the presence of TBHP high
epoxide yield was observed. This may be related to the ability of TBHP
and the inability of the H2O2 and NaIO4 reagents to mix with the organic
substrate phase.
In the subsequent reaction of MoO2(acac)2 with polymer-supported
Schiff base ligand the molybdenyl dication (MoO22+) was coordinated to
the supported Schiff base ligand by attracting of two hydrogen bonded
protons of resin 1 by two acac−1 of MoO2(acac)2. This coordination
has considerable effect on the stretching vibration frequencies of C_N
bonds of Schiff base ligand because of the elimination of protons and
consequently deleting of strong hydrogen bondings and substitution of
MoO22+ dication instead of two protons. Thus the stretching vibration
frequencies of C_N bonds of coordinated Schiff base (resin 2) shifts to
higher frequencies and lies at 1654 cm−1 in similar to supported bis
(acetylacetone) ethylene diimine Co(II) complex [26]. In the FT-IR spec-
trum of polymer-supported molybdenyl Schiff base complex (2) two
new peaks appeared at 908 cm−1 and 950 cm−1 that corresponded to
the Mo_O stretching vibrations. The molybdenum content of the
resin 2 was also determined by NAA (neutron activation analysis) that
obtained 5.5%. The supported molybdenyl Schiff base complex was
firstly tested in epoxidation of cylooctene as a model substrate. The
epoxidation reaction was investigated in different solvents and oxidants
(Tables 1 and 2, respectively) in the presence of supported molybdenyl
Schiff base catalyst 2 [27]. The results showed that in chlorinated sol-
vents such as CCl4, CHCl3 and CH2Cl2 in the presence of TBHP high epox-
ide yields were observed. The high epoxide yield for CCl4 in comparison
to CHCl3 and CH2Cl2 can be related to high boiling point of CCl4.
In the recent years many researchers have investigated the mecha-
nistic aspects of molybdenum(VI)-catalyzed epoxidation reaction in
the presence of TBHP [28–31]. Generally they believed the TBHP coordi-
nation to the molybdenum(VI)-center, activation by it and then oxygen
transition to substrate. In addition, the high epoxide yields obtained in
the chlorinated solvents such as CCl4, CHCl3, 1,1,2,2-tetrachloroethane,
Thus the activity of polymer-supported molybdenyl Schiff base
catalyst (2) in epoxidation of different alkenes was investigated in CCl4
as a reaction solvent in the presence of TBHP as an oxidant (Table 3).
These results showed that the supported molybdenyl Schiff base catalyst
(2) was very active and selective in epoxidation of different (cyclic and
linear) alkenes in the presence of TBHP in CCl4. It was more active in
Table 1
Epoxidation of cis-cyclooctene catalyzed by polymer-supported bis (2- hydroxylanyl)
acetylacetonato MoO2 Schiff base catalyst in different solvents in the presence of
tert-buthyl hydro peroxide under reflux conditionsa.
Solvent
Time (h)
Epoxide (%)b
CH3COCH3
THF
3
3
No reaction
No reaction
MeOH
CH3CN
CH2Cl2
CHCl3
CCl4
3
3
3
3
3
10
65
84
97
1.5
a
Reaction conditions: cis-cyclooctene (0.5 mmol), TBHP (1.5 mmol), catalyst (0.05 g
as 0.029 mmol/Mo), reaction solvent (5 mL).
b
GLC yield based on the starting cyclooctene.

92
G. Grivani, A. Akherati / Inorganic Chemistry Communications 28 (2013) 90–93
Table 2
Table 4
Epoxidation of cis-cyclooctene catalyzed by polymer-supported bis (2- hydroxylanyl)
Reusability and leaching of polymer-supported bis (2- hydroxylanyl) acetylacetonato
acetylacetonato MoO2 Schiff base catalyst with different oxidants under reflux
MoO2 Schiff base catalyst in epoxidation of cis-cyclooctene with TBHP under reflux
a
a
conditions
.
conditions
.
Solvent
Oxidant
Time (h)
Epoxide (%)b
Run
Cyclooctene epoxide (%)b
Time (min)
Leaching of Mo (%)c
CCl4
CCl4/H2O
CCl4/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
TBHP
H2O2
NaIO4
TBHP
H2O2
NaIO4
1.5
3
3
3
3
97
6
2
8
25
4
1
2
3
4
5
6
7
8
97
97
97
97
97
97
97
97
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.3
0.70
0.43
0.12
–
c
c
3
–
–
a
Reaction conditions: cis-cyclooctene (0.5 mmol), TBHP (1.5 mmol), catalyst (0.05 g
–
as 0.029 mmol/Mo), reaction solvent (5 mL).
b
a
GLC yield based on the starting cyclooctene.
Tetrabuthylphosphoniom bromaid (0.01 g) was used.
Reaction conditions: cis-cyclooctene (0.5 mmol), TBHP (1.5 mmol), catalyst (0.05 g
c
as 0.029 mmol/Mo), reaction solvent (5 mL).
b
GLC yield based on the starting cyclooctene.
Determined by ICP analysis.
c
conversion of cyclic than linear alkenes. Furthermore it converts both cis
and trans stylbene to only cis stylbene epoxide isomer.
The reusability of the titled catalyst was also investigated because it
can be very important economical criteria in industrial application of
catalysts especially in the case of supported one. In Table 4 the results
of the reusability experiments of polymer-supported molybdenyl Schiff
base catalyst (2) in epoxidation of cyclooctene in the presence of TBHP
have been reported. These results showed that the reusability and sta-
bility of supported catalyst 2 are very high as it converts cyclooctene
to its epoxide in 8 times without any decrees in its activity. This stability
can be related to the strong binding of catalyst to the polystyrene
support via covalent attachment.
The homogeneous liquid phase process yielding propylene oxide
(the Halcon process) operated by the Arco company utilizes a soluble
Mo(VI) catalyst [20,33]. A heterogeneous catalyst involving a Ti spe-
cies immobilized on silica particles has also been reported by the
Shell company [34] but it is unclear how extensively this catalyst sys-
tem is used. After that a numerous academic works was published on
polymer-supported metal complex alkene epoxidation catalysts with
a number of Mo(VI) based systems looking particularly attractive
for scale-up and commercial exploitation [35–40]. In comparison to
these works the polymer-supported molybdenyl Schiff base catalyst
(2) have considerable activity, selectivity and reusability.
support as highly active, selective and reusable polymer-supported
molybdenyl Schiff base catalyst.
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c

G. Grivani, A. Akherati / Inorganic Chemistry Communications 28 (2013) 90–93
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