Silica supported Ru(salophen)Cl: An efficient and robust heterogeneous catalyst for epoxidation of alkenes with sodium periodate

Article abstract of DOI:10.1016/j.poly.2010.07.019

In the present work, heterogenization of Ru(salophen)Cl via its axial ligation to silica-bound imidazole, SiIm, is reported. The heterogeneous catalyst, [Ru(salophen)Cl-SiIm], was characterized by elemental analysis, SEM, TEM, FT-IR and diffuse reflectance UV-Vis spectroscopic techniques. The catalyst, which is not soluble in water and common organic solvents, was used for efficient epoxidation of cyclic and linear alkenes with NaIO₄ under agitation with magnetic stirring. This new heterogenized catalyst is of high stability and reusability in the oxidation reactions. The effect of reaction parameters such as solvent and oxidant in the epoxidation of cis-cyclooctene were also investigated.

Full text of DOI:10.1016/j.poly.2010.07.019

Polyhedron 29 (2010) 2953–2958
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Silica supported Ru(salophen)Cl: An efficient and robust heterogeneous
catalyst for epoxidation of alkenes with sodium periodate
b,
a
Mehdi Hatefi a, Majid Moghadam b,c, , Valiollah Mirkhani
, Iran Sheikhshoaei
*
**
a Department of Chemistry, Shahid Bahonar University, Kerman, Iran
b Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
c Department of Nanotechnology Engineering, University of Isfahan, Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, heterogenization of Ru(salophen)Cl via its axial ligation to silica-bound imidazole,
SiIm, is reported. The heterogeneous catalyst, [Ru(salophen)Cl–SiIm], was characterized by elemental
analysis, SEM, TEM, FT-IR and diffuse reflectance UV–Vis spectroscopic techniques. The catalyst, which
is not soluble in water and common organic solvents, was used for efficient epoxidation of cyclic and lin-
ear alkenes with NaIO4 under agitation with magnetic stirring. This new heterogenized catalyst is of high
stability and reusability in the oxidation reactions. The effect of reaction parameters such as solvent and
oxidant in the epoxidation of cis-cyclooctene were also investigated.
Received 4 April 2010
Accepted 20 July 2010
Available online 24 July 2010
Dedicated to Prof. Shahram Tangestaninejad
and Prof. Iraj Mohammadpoor-Baltork for
their 50th annual birthday
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Silica
Ru(III) salophen
Alkene epoxidation
Periodate
Supported catalyst
1. Introduction
ganic supports such as silica or MCM-41 [33–37], co-polymeriza-
tion of a functionalized salen monomer into an organic polymer
Ruthenium complexes have been known as versatile
electron-transferandenergy-transfercompounds[1–22].Duetothese
uniquecharacteristics, rutheniumcomplexes exhibita wide range of
applicationsinvariousresearchareassuchasartificialphotosynthesis,
photomoleculardevices,probesforbiologicalmacromolecules,oxida-
tioncatalystsandorganicsynthesis[8–22].
In the last two decades, salen and salophen ligands bearing O
and N donor atoms and their complexes have received much atten-
tion, mainly because of their extensive applications in the fields of
synthesis and catalysis [23–26]. Homogeneous catalysts are not
recoverable and degrade in the reaction media and therefore
contaminate the products. But their heterogeneous counterparts
present many advantages such as easy separation and recovery
of the catalyst, and higher stability of the catalytic species. Differ-
ent approaches such as non-covalent immobilization in zeolites,
clay or siloxane membranes [27–32], covalent grafting onto inor-
[38], attachment or build-up of a salen structure to a performed
polymer [39–44], and axial ligation to surface bound ligands
[45–47] have been reported for immobilization of Schiff base
complexes (salen or salophen) on the supports.
Co(salen) has been used by Jacobsen group as a highly efficient
catalyst for hydrolytic kinetic resolution of racemic epoxides and
enantioselective parallel synthesis [48,49].
In comparison with modified resins, modified silica exhibits
some advantages such as higher surface area, higher thermal and
chemical stability.
In continuation of our research on the preparation of supported
Schiff base catalysts [46,47,50–54], here, we report the prepara-
tion, characterization and investigation of catalytic activity of
[Ru(salophen)Cl] supported on silica-bound imidazole in the
epoxidation of alkene with sodium periodate at room temperature
(Scheme 1).
2. Experimental
* Corresponding author at: Department of Chemistry, Catalysis Division, Univer-
sity of Isfahan, Isfahan 81746-73441, Iran. Tel.: +98 311 7932705; fax: +98 311
6689732.
** Corresponding author. Tel.: +98 311 7932705; fax: +98 311 6689732.
E-mail address: moghadamm@sci.ui.ac.ir (M. Moghadam).
All materials were commercial reagent grade. 3-Chloropropyl-
functionlized silica gel (230–400 mesh, 2.5% Cl, 0.714 mmol/g)
was purchased from Aldrich. Alkenes obtained from Merck or Fluka
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.07.019

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M. Hatefi et al. / Polyhedron 29 (2010) 2953–2958
200 or Carbowax 20m and n-decane was used as internal standard.
O
[Ru(salophen)-SiIm]/ NaIO
4
1H NMR spectra were recorded with
300 MHz.
a Bruker-Arance AQS
CH3CN/ H2O
The Schiff base ligand was prepared by the standard procedure
of refluxing ethanolic solutions of the 1,2-phenylenediamine and
Scheme 1.
salicylaldehyde in
a 1:2 molar ratio and metallated with
and were passed through a short column containing active alumina
to remove the peroxidic impurities. Diffuse reflectance spectra
were recorded on a Shimadzu UV-265 instrument using optical
grade BaSO4 as the reference. FT-IR spectra obtained as potassium
bromide pellets in the range 400–4000 cmÀ1 with a Nicolet-Impact
400D spectrometer. Scanning electron micrographs of the catalyst
and silica were taken on a SEM Philips XL 30 instrument. Gas chro-
matography experiments (GC) were performed with a Shimadzu
GC-16A instrument using a 2 m column packed with silicon DC-
RuCl3Á3H2O [55,56]. Silica surface modification with imidazole to
give the SiIm was carried out through the method described by
Lindsay and co-workers [57].
2.1. Preparation of silica-supported ruthenium salophen,
[Ru(salophen)Cl–SiIm]
To a solution of ruthenium(III) salophen (0.45 g, 1 mmol) in
CH3CN (50 ml) was added silica-bound imidazole (3 g). The mix-
Fig. 1. UV–Vis spectrum of: (A) non-supported catalyst and (B) [Ru(salophen)Cl–SiIm].
N
O
N
O
Ru
Cl
H
N
CH3CN
+
Cl
N
N
reflux (48 h)
reflux (48 h)
N
N
N
N
O
N
O
Ru
Cl
Scheme 2. Preparation of silica supported [Ru(salophen)Cl].

M. Hatefi et al. / Polyhedron 29 (2010) 2953–2958
2955
Fig. 2. FT-IR spectrum of: (A) chloropropylated silica; (B) SiIm and (C) [Ru(salophen)Cl–SiIm].

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M. Hatefi et al. / Polyhedron 29 (2010) 2953–2958
Fig. 3. Scanning electron micrograph of: (A) SiIm and (B) [Ru(salophen)–SiIm].
Table 2
Effect of solvent on epoxidation of cyclooctene with NaIO4 catalyzed by [Ru(salo-
phen)Cl–SiIm] at room temperature.a
Row
Solvent
Epoxide yield (%)
Time (h)
1
2
3
4
5
6
7
CH3CN/H2O (1:1)
CH3CN/H2O (2:1)b
CH3COCH3/H2O
CH3OH/H2O
CH3CH2OH/H2O
CH2Cl2/H2O
95
70
47
75
55
26
12
3
3
3
3
3
3
3
CCl4/H2O
a
Cyclooctene (0.5 mmol), NaIO4 (1 mmol), catalyst (0.025 mmol), solvent (5 ml),
H2O (5 ml).
b
CH3CN (5 ml) and H2O (2.5 ml).
2.2. General procedure for catalytic epoxidation of alkenes with
sodium periodate catalyzed by [Ru(salophen)Cl–SiIm]
In a 25 ml flask equipped with a magnetic stirrer bar, a mixture
of alkene (0.5 mmol), [Ru(salophen)Cl–SiIm] (250 mg, 0.026 mmol)
in CH3CN (5 ml) and NaIO4 (1 mmol) in H2O (5 ml) was prepared
and stirred at room temperature. The progress of the reaction
was monitored by GC. At the end of the reaction, the reaction mix-
ture was diluted with Et2O (20 ml). The catalyst was filtered,
washed thoroughly with Et2O and combined washings and filtrates
were purified on a silica-gel plates or a silica-gel column. IR and 1H
NMR spectral data confirmed the identities of the products. Blank
experiments with the same experimental conditions in the absence
of catalyst were also performed.
Fig. 4. Transmission electron micrograph of [Ru(salophen)–SiIm].
Table 1
Effect of oxidant on the epoxidation of cyclooctene catalyzed by [Ru(salophen)Cl–
a
SiIm] at room temperature.
Row
Oxidant
Solvent
Epoxide yield (%)
Time (h)
1
2
3
4
5
6
NaIO4
H2O2
H2O2/urea
NaOCl
tert-BuOOH
Bu4NIO4
CH3CN/H2O
CH3CN
CH3CN
CH3CN
CH3CN
95
43
19
33
64
40
4
4
4
4
4
4
CH3CN
3. Results and discussion
a
Cyclooctene (0.5 mmol), oxidant (1 mmol), catalyst (300 mg, 0.034 mmol),
3.1. Preparation and characterization of silica-bound ruthenium(III)
salophen catalyst, [Ru(salophen)Cl–SiIm]
CH3CN (5 ml), H2O (5 ml).
ture was vigorously stirred at 80 °C for 24 h. After cooling, the
brown silica particles were collected by filtration, washed thor-
oughly with acetonitrile, methanol, and ether successively, and
dried at room temperature for several hours.
The supported ruthenium catalyst, [Ru(salophen)–SiIm], was
obtained by stirring a suspension of SiIm in a solution of the
[Ru(salophen)Cl] in dichloromethane (Scheme 2). The silica-bound
imidazole was characterized by elemental analysis, SEM, TEM, dif-

M. Hatefi et al. / Polyhedron 29 (2010) 2953–2958
2957
fuse reflectance UV–Vis (DR UV–Vis) and FT-IR spectroscopic tech-
niques. The nitrogen content of this support was 1.7%. According to
this value, the degree of imidazole, introduced into the silica, was
0.607 mmol/g of support. This shows that 85% of total chlorines
have been substituted by imidazole.
the yield of cyclooctene oxide was 95% with 100% selectivity. Then,
under the optimized conditions, various alkenes such linear, cyclic
and phenyl substituted ones were epoxidized in the presence of
[Ru(salophen)Cl–SiIm] (Table 3). Cyclohexene was oxidized in
100% yield and 94% epoxide selectivity. The by-product was cyclo-
The Ru content of the supported catalyst, determined by ICP,
showed a value of about 0.102 mmol per gram of supported
catalyst.
The UV–Vis spectra in the diffuse reflectance mode provided
further evidence for the presence of [Ru(salophen)Cl] on the silica.
Absorption spectrum of homogeneous [Ru(salophen)Cl] showed
peaks at 292 and 318 nm (Fig. 1A). In the diffuse reflectance spec-
trum of the [Ru(salophen)Cl–SiIm], these peaks were appeared at
290 and 320 nm, which clearly showed the presence of metallosa-
len on the surface (Fig. 1B).
hexene-1-one. In the case of styrene and a-methyl styrene, the
conversions were 98% and 95% and the epoxide selectivities were
93% and 92%, respectively. In these systems benzaldehyde and ace-
topheneone were produced as by-products. This catalytic system
showed a high catalytic activity in the epoxidation of linear alkenes
such as 1-heptene and 1-dodecene with 100% epoxide selectivity.
Finally,
product was
a
-pinene was efficiently oxidized. In this system, the major
-pinene oxide (with 87% epoxide selectivity), and
a
allylic oxidation products, verbenone and verbenol were produced
as minor products.
Fig. 2 shows the FT-IR spectra of Silica, SiIm and [Ru(salo-
phen)Cl–SiIm]. After introducing the imidazole to silica, the imid-
azole stretching band was appeared at 1442 cmÀ1 (Fig. 2A and B).
Upon coordination of [Ru(salophen)Cl] to SiIm, a characteristic
Blank experiments in the presence of oxidant and using the
same experimental conditions in the absence of catalyst were also
investigated in the epoxidation of cyclooctene. The obtained re-
sults showed that NaIO4 has poor ability to oxidize the cyclooctene.
Also, no cyclooctene oxide was detected in the absence of oxidant.
band at 1604 cmÀ1 (belong to
tC@N in the salophen ligand) was ob-
served. On the other hand, the band at 1529 cmÀ1 can be attributed
to the aromatic rings in the salophen (Fig. 2C).
Scanning electron micrograph, (SEM), was recorded to under-
stand the morphological changes on the surface of silica. The image
of chloropropylated silica showed that silica particles have sizes in
Table 3
Epoxidation of alkenes with NaIO4 catalyzed by [Ru(salophen)–SiIm].
the micrometer range (2–10 lm). While after supporting of
Entry Alkene
Conversion Time Epoxide
Ru(salophen)Cl on silica, the particle sizes reduce and smaller par-
ticles are obtained (Fig. 3). The TEM image of the prepared catalyst
shows how the Ru(salophen)Cl has been distributed on the silica
particles (Fig. 4). Since the image has been taken in the nanometer
scale therefore the crystalline planes of silica has not been ob-
served in the image. But it is clear that the catalytic species have
a nanometer size.
(%)a/
95
(h)
selectivity
(%)a
1
3
100
94
2
100b
3
3.2. Catalytic activity
3
4
98c
95d
3
3
93
92
First, in order to attain the highest catalytic activity, the reac-
tion parameters were optimized in the epoxidation of cyclooctene.
3.2.1. Effect of terminal oxidants on the epoxidation of cyclooctene
catalyzed by [Ru(salophen)Cl–SiIm]
5
6
85
73
4
8
100
100
Different single oxygen donors such as NaIO4, H2O2, urea-H2O2
(UHP), NaOCl, tert-BuOOH and (n-Bu)4NIO4 were used in the epox-
idation of cyclooctene in the presence of [Ru(salophen)Cl–SiIm].
The results showed that sodium periodate is the best oxygen
source because of its inertness in the absence of catalyst and its
high solubility in CH3CN/H2O mixture (Table 1).
7
8
58
8
3
100
87
93e
3.2.2. Effect of solvent on the epoxidation of cyclooctene catalyzed by
[Ru(salophen)Cl–SiIm]
a
b
c
GLC yield based on starting alkene.
The by product is allylic ketone.
The by product is benzaldehyde.
The by product is acetophenone.
The effect of different solvents on the epoxidation of cyclooc-
tene with NaIO4 catalyzed by [Ru(salophen)Cl–SiIm] was also
investigated (Table 2). Among the mixture of methanol, ethanol,
acetone, and acetonitrile (single phase systems), chloroform,
dichloromethane and carbon tetrachloride (two phase systems
with Bu4NBr as phase transfer catalyst) with water, the 1:1 mix-
ture of acetonitrile/ water was chosen as the reaction medium, be-
cause the higher epoxide yield was observed. The higher catalytic
activity in aqueous acetonitrile is attributed to polarity of solvent
and high solubility of NaIO4 in it.
d
e
The by products are 7% verbenone and 6% verbenol.
Table 4
The results of [Ru(salophen)Cl–SiIm] catalyst recovery and the ruthenium leached in
the epoxidation of cyclooctene with sodium periodate.
Run
Epoxide yield (%)a
Time (h)a
Ru leached (%)b
1
2
3
4
95
91
88
82
3
3
3
3
2
1
0
0
3.2.3. Catalytic Alkene epoxidation with NaIO4 catalyzed by
[Ru(salophen)Cl–SiIm]
First, the catalytic activity of this catalyst was investigated in
the epoxidation of cyclooctene with NaIO4. The reaction was per-
formed at room temperature under air in CH3CN/H2O. After 3 h,
a
GLC yield based on starting alkene.
Determined by ICP.
b

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Since the reusability of a catalyst is important from economical
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the reusability and stability of [Ru(salophen)Cl–SiIm] in the oxida-
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4. Conclusion
In conclusion, Ru(salophen)Cl was successfully supported on
silica-bound imidazole via axial ligation. The catalyst, [Ru(salo-
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epoxidation with sodium periodate at room temperature. This cat-
alyst has advantages such as easy preparation and handling of the
catalyst, commercially available of the support, and facile and
effective recovery and recycling of the supported catalyst.
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Acknowledgements
The support of this work by University of Isfahan and Shahid
Bahonar University of Kerman is acknowledged.
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