Crystal structure of an unsymmetrical Schiff base, immobilization of its cobalt and manganese complexes on a silica support, and catalytic studies

Article abstract of DOI:10.1007/s11243-012-9678-0

An unsymmetrical tetradentate Schiff base (H₂L) was synthesized by the reaction of 3-methoxysalicylaldehyde, o-phenylenediamine, and salicylaldehyde in EtOH. H₂L was characterized by single-crystallographic X-ray analysis. Its Co(II)

Full text of DOI:10.1007/s11243-012-9678-0

Transition Met Chem (2013) 38:199–206
DOI 10.1007/s11243-012-9678-0
Crystal structure of an unsymmetrical Schiff base, immobilization
of its cobalt and manganese complexes on a silica support,
and catalytic studies
•
•
•
Damla Durak Anıl Delikanlı Cahit Demetgu¨l
˙
Ibrahim Kani Selahattin Serin
•
Received: 11 October 2012 / Accepted: 22 November 2012 / Published online: 7 December 2012
ꢀ Springer Science+Business Media Dordrecht 2012
Abstract An unsymmetrical tetradentate Schiff base
(H2L) was synthesized by the reaction of 3-methoxysali-
cylaldehyde, o-phenylenediamine, and salicylaldehyde in
EtOH. H2L was characterized by single-crystallographic
X-ray analysis. Its Co(II) and Mn(III) complexes ([CoL]
and [MnLCl]) were prepared and immobilized on 3-ami-
nopropyltriethoxysilane functionalized silica gel. The immo-
bilized materials were found to be efficient catalysts for
epoxidation of styrene in the presence of tert-BuOOH in
acetonitrile at 40 ꢁC. The catalysts can be reused several
times without significant loss of performance.
be tuned by introducing different electron donating and/or
withdrawing groups at the two imine units. In addition,
unsymmetrical Schiff bases have been reported to give
improved enantioselective activities in various organic trans-
formations compared to their symmetrical derivatives [3].
New catalysts derived from unsymmetrical tetradentate
Schiff base complexes seem to be more and more attractive
in catalysis [4].
Schiff bases derived from substituted salicylaldehydes
and various aromatic amines are common ligands in
coordination chemistry because of their ease of preparation
[5]. The complexes of Schiff bases containing O and N
donor atoms have proved to be of particular interest [6].
Therefore, Schiff base complexes have been amongst the
most widely studied coordination compounds and are
becoming increasingly important as analytical, biochemi-
cal, and antimicrobial reagents [7]. These compounds also
find catalytic applications, ranging from unsymmetrical
epoxidation [8] to polymerization [9].
Introduction
In recent years, there has been a resurgence of interest in
the synthesis and characterization of unsymmetrical Schiff
bases and their use as ligands [1, 2]. It is well known that
the electronic properties of unsymmetrical Schiff bases can
Transition metal complexes of Schiff bases have proved
to be useful catalysts in the oxidation/epoxidation of
organic compounds by various oxidants such as H2O2,
tertbutylhydroperoxide (TBHP) and PhIO [10]. Catalytic
oxidation/epoxidation of olefins is important for the syn-
thesis of fine chemicals. Schiff base complexes have been
used as homogeneous catalysts in epoxidation of olefins,
oxidation of alkanes, alcohols and thiols, and amine oxi-
dation [11, 12].
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-012-9678-0) contains supplementary
material, which is available to authorized users.
D. Durak Á A. Delikanlı Á S. Serin
Department of Chemistry, Faculty of Science and Letters,
C¸ ukurova University, 01330 Adana, Turkey
Heterogenization of homogeneous catalysts onto solid
supports brings advantages such as easy separation, ther-
mal stability, high selectivity, and recyclability [13].
A general strategy for converting a homogeneous process
into a heterogeneous one is to anchor the catalyst onto a
large surface area inorganic support [14]. In addition,
silane-coupling reagents serve as a linker keeping the
C. Demetgu¨l (&)
Department of Chemistry, Faculty of Science and Letters,
Mustafa Kemal University, 31040 Hatay, Turkey
e-mail: cdemetgul@hotmail.com; demetgul@mku.edu.tr
˙
I. Kani
Department of Chemistry, Faculty of Science,
Anadolu University, 26470 Eskisehir, Turkey
123

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Transition Met Chem (2013) 38:199–206
catalyst center away from the support material so that it
behaves like a homogeneous catalyst.
(DTA) of the samples were performed with a Perkin–Elmer
Pyrris Thermal Analyzer in nitrogen medium, from 40 to
700 ꢁC at a heating rate of 10 ꢁC min-1. Molar conduc-
tances were determined in DMSO (*10-3 M) at room
temperature using a Jenway Model 4070 conductivity
meter. Magnetic measurements were carried out with a
Sherwood magnetic susceptibility balance (Model MK1)
with CuSO4.5H2O as the calibrant.
Various materials (e.g., alumina, mesoporous silica,
polymers, carbon materials, zeolites) can be used as sup-
ports for immobilized Mn(salen) catalysts [15, 16]. In
particular, mesoporous silica materials, particularly SBA-
15 and MCM-41 materials, have received much attention
because of their large surface areas, uniform hexagonally
ordered two-dimensional mesoporous channels, high hydro-
thermal stability, and tunable pore dimensions and channel
structure, allowing ready diffusion of reactants to the active
sites located in the nanopores. More importantly, the
nanopores of these mesoporous supports can influence the
mechanism of the epoxidation reaction [17], resulting in a
different product distribution than seen in homogeneous
reactions.
X-ray structural analysis
Diffraction data for H2L were collected with a Bruker
SMART APEX CCD diffractometer equipped with a
rotation anode at 296(2) K using graphite monochro-
˚
meted Mo Ka radiation (k 0.71073 A). Diffraction data
were collected over the full sphere and were corrected
for absorption. The data reduction was performed with
the Bruker SAINT [18] program package. For further
crystal and data collection details see Table 1. Structure
solution was carried out with the SHELXS-97 [19]
package using direct methods and refined with SHELXL-
97 [20] against F2 using first isotropic and later aniso-
tropic thermal parameters for all non-hydrogen atoms.
Hydrogen atoms were added to the structure model on
calculated positions. Geometric calculations were per-
formed with Platon [21].
In this article, the preparation and characterization of
Co(II) and Mn(III) complexes of an unsymmetrical Schiff
base supported on organoamine derivatized silica is reported.
Characterization of the compounds was performed by ana-
lytical, spectroscopic, and chromatographic methods. The
catalytic activities of the compounds were investigated for
the epoxidation of styrene.
Experimental
Materials and instrumentation
Preparation of H2L
Silica gel 60 (0.015–0.040 mm), o-phenylenediamine, and
salicylaldehyde derivatives were purchased from Merck.
3-aminopropyltriethoxysilane (APTS) and tert-Butyl hydro-
peroxide (TBHP) solutions (70 % in H2O) were purchased
from Sigma–Aldrich. Mn(AcO)2Á4H2O and Co(AcO)2Á4H2O
were supplied by Fluka and were used as received. Solvents
and all other reagents were technical grade and were purified
and dried by distillation from appropriate desiccants when
necessary.
H2L was prepared according to the modified method
described in the literature [1]. To a stirred, cooled solution
of 3-methoxysalicylaldehyde (4.56 g, 30 mmol) in EtOH
(100 cm3), a cold solution of o-phenylenediamine (3.24 g,
30 mmol) in EtOH (100 cm3) was added dropwise fol-
lowed by addition of a cold solution of salicylaldehyde
(3.66 g, 30 mmol) in EtOH ((20 cm3)) over a period of
10 min (Scheme 1). The mixture was then stirred and
refluxed for 12 h. After cooling, the ligand was obtained as
orange microcrystals. These were filtered off, washed with
cold absolute ethanol, and recrystallized several times from
ethanol–chloroform (1:1, v/v).
Melting points were measured in sealed tubes using a
Gallenkamp digital melting point apparatus. IR samples
were prepared as KBr pellets, and spectra were recorded on
a Perkin–Elmer RX-1 FTIR spectrometer within the range
of 4000–400 cm-1. UV–vis spectra were measured using a
Perkin–Elmer Lambda 25 UV–vis spectrometer. Room
temperature diffuse reflectance spectra in the ultra-violet
and visible region (DR–UV–vis) were recorded on a Varian
Cary 100 model UV–vis spectrophotometer. C, H, and N
elemental microanalyses were performed with a LECO-
CHNS-932 elemental analyzer. Metal analysis was carried
out with an AAS Perkin–Elmer 3100 instrument in solu-
tions prepared by decomposing the complex in aqua regia
and then subsequently digesting in concentrated HCl.
Thermogravimetry (TG) and differential thermal analyses
[H2L]: Color: orange, m. p. 140–142 ꢁC, yield 65 %,
UV–vis; 245, 270 and 340 nm.
Preparation of [CoL]
A solution of Co(II) acetate (0.99 g, mmol) in methanol
(20 cm3) was added dropwise to a stirred warm solution of
H2L (1.38 g, 4 mmol) in methanol (20 cm3) under N2, and
the resulting solution was stirred for 2 h. The precipitate
was filtered off and washed with water, methanol, and
diethyl ether and finally dried in vacuum.
123

Transition Met Chem (2013) 38:199–206
201
Yield (55 %), color: brown, m.p. [250 ꢁC, leff: 4.31
BM, K: 0.5 X-1 cm2 mol-1, Anal. Calcd. for C21H16CoN2
O3 (%): C, 62.5; H, 4.0; N, 7.0; Co, 14.6. Found: C, 62.6;
H, 4.1; N, 6.9; Co, 14.5.
Table 1 Crystal data and structure refinement for H2L
Empirical formula
Formula weight
Temperature
C21 H18 N2 O3
346.37
110(2) K
Wavelength
0.71073 A
Crystal system, space
group
Orthorhombic, P bca
Preparation of [MnLCl]
˚
Unit cell dimensions (A, ꢁ) a = 36.9886 (19) alpha = 90
b = 14.1290 (8) beta = 90
This complex was prepared by a procedure adapted from
the literature. A mixture of H2L (0.69 g, 2.0 mmol) and
Mn(OAc)2Á4H2O (1.24 g, 5.0 mmol) in ethanol (30 cm3)
was refluxed for 1 h [11]. Then, LiCl (0.6 g, 15 mmol) was
then added, and the mixture was refluxed for an additional
1 h under exposure to the air. The mixture was concen-
trated under vacuum and then filtered. The residue was
washed with deionized water (3 9 25 cm3), then ethanol,
and dried as a dark brown solid.
c = 6.2938 (3) gamma = 90
3298.2(3) A3
Volume
Z, Calculated density
Absorption coefficient
F(000)
8, 1.399 Mg/m3
0.094 mm-1
1456
Crystal size
0.43 9 0.30 9 0.29 mm
1.10–22.79 deg.
Theta range for data
collection
Yield (53 %), color: brown, m.p. [250 ꢁC, leff: 4.46
BM, K: 0.8 X-1 cm2 mol-1, Anal. Calcd. for C21H16ClMn
N2O3 (%): C, 58.0; H, 3.7; N, 6.4; Mn, 12.7. Found: C,
58.0; H, 3.8; N, 6.4; Co, 12.6.
Limiting indices
-48\=h \=49, -18\=k \=17, -8\=
l\=8
4042/3411 [R(int) = 0.042]
Reflections collected/
unique
Max. and min. transmission 0.973 and 0.966
Refinement method
Full-matrix least-squares on F2
Modification of silica surface (SiO2–APTS)
Data/restraints/parameters
Goodness-of-fit on F2
1704/0/238
0.97
Silica gel was activated by the following procedure. Silica
gel 60 (40 g) was treated with and was activated by
refluxing it with 6 M HCl for 24 h. It was then filtered off,
washed with deionized water until the filtrate was neutral,
and dried at 120 ꢁC for 12 h [22].
Final R indices
[I [ 2sigma(I)]
R1 = 0.0474, wR2 = 0.1434
The activated silica gel (30 g) was suspended in
100 cm3 of a solution of 3-aminopropyltriethoxysilane
(APTS) in dry toluene (10 % v/v). The mixture was
refluxed with stirring for 48 h under nitrogen. The slurry
was filtered, and the resulting solid SiO2–APTS was
washed successively with copious amounts of toluene,
ethanol, and diethyl ether, then dried in a vacuum oven for
10 h.
O
O
+
+
OH
HO
H2N
NH2
OCH3
EtOH
White color powder, elemental analysis: C 9.0, H 2.3, N
3.4 %. FTIR (cm-1): 3454 (br, Si–OH); 2941 (w, R–H);
1096 (br, Si–O).
25OC (1h)
Reflux (6h)
Preparation of [CoL]@SiO2–APTS
The route for preparing the heterogeneous Co(II) catalyst
by coordination of the [CoL] catalyst to the amino groups
on the support surface is outlined in Scheme 2. [CoL]
(0.81 g, 2 mmol) was dissolved in acetonitrile (50 cm3).
The solution was added to a suspension of SiO2–APTS
(2.0 g) in dichloromethane (50 cm3). The resulting mixture
was stirred at room temperature for 48 h. After filtration,
the solid was washed with dichloromethane and acetoni-
trile until the eluate became colorless.
N
O
N
H
H
O
H3CO
Scheme 1 Synthesis pathway of H2L
123

202
Transition Met Chem (2013) 38:199–206
NH2
NH2
+
N
O
N
O
+
N
O
N
O
Co
Si
Si
Mn
Cl
O
OO
O
OO
CH3CN
CH3CN
48 h
AgClO4
48h
SiO2
SiO2
H3CO
r.t.
H3CO
Reflux
+ ClO4-
N
O
N
N
O
N
O
Mn
Co
O
NH2
H2N
H3CO
H3CO
Si
Si
O
OO
O
OO
SiO2
SiO2
Scheme 3 Immobilization of [MnLCl] onto SiO2–APTS
Scheme 2 Immobilization of [CoL] onto SiO2–APTS
or acetonitrile (10 cm3) as the solvent. The products of the
reactions were collected at different time intervals ranging
from 8 to 24 h. For repeated catalytic experiments, the
catalyst was separated using centrifugation followed by
decantation, twice washed with acetonitrile and dichloro-
methane, dried in air, and then used in a new run.
Preparation of [MnL]@SiO2–APTS
[MnLCl] (0.87 g, 2 mmol) was dissolved in acetonitrile
(50 cm3), and 0.62 g (3 mmol) of AgClO4 was added to
remove the chloride from the coordination sphere by pre-
cipitation. The suspension was stirred at room temperature
for 24 h and then filtered through a filter crucible. The
filtrate was then added to a suspension of SiO2–APTS
(2.0 g) in dichloromethane (50 cm3). The resulting sus-
pension was stirred at room temperature for 48 h (Scheme 3).
After filtration, the solid was washed with dichloromethane
and acetonitrile until the eluate became colorless.
Gas chromatography was used for the identification
and quantification of the reaction products, using a
Finnigan-Trace GC–MS equipped with an auto sampler.
Chromatographic separations were accomplished with a
Zebron ZB–35 column (30 m 0.2 mm i.d.; 0,25 lm film
thickness), with injections in the split mode with 50:1
split ratio. Analyses were carried out using helium as the
carrier gas, flow rate 1.0 cm3 min-1. Conditions used;
40 ꢁC (10 min), 10 ꢁC min-1 to 240 ꢁC (10 min), injector
temperature 250 ꢁC, detector temperature 250 ꢁC. The
ionization voltage was 70 eV, mass range m/z 41–400
a.m.u. The separated components were identified tenta-
tively by matching with GC–MS results of reference
samples. Chlorobenzene was used as an internal standard.
Quantitative determinations were carried out based on
peak area integration.
Catalysis studies and product identification
The immobilized [CoL] and [MnLCl] complexes were
investigated as catalysts for the epoxidation of styrene with
tert-butyl hydroperoxide (TBHP) as an oxidant. The typical
reaction mixture was composed of styrene (5.0 mmol),
TBHP (5.0 mmol), catalyst (0.05 mmol of active centers;
0.098 g [CoL] and 0.11 g [MnLCl]), and dichloromethane
123

Transition Met Chem (2013) 38:199–206
Results and discussion
203
the three bands from the azomethine ligand (p–p*), charge
transfer (MLCT), and d–d transitions are observed at 373
and 378, 419 and 425 and 500 and 520 nm, for [CoL] and
[MnLCl], respectively.
Structural description of H2L
An ORTEP view of the unsymmetrical Schiff base mole-
cule, which crystallized in orthorhombic P bca space
group, is shown in Fig. 1. The crystal data and selected
bond lengths and angles are listed in Tables 1 and S1,
respectively. The packing diagram is also shown in Fig. S1.
The aromatic ring systems are in almost planar confor-
mation, with endocyclic torsion angles of 1.9 (3)ꢁ, -0.9
(2)ꢁ, and -0.6 (2)ꢁ. The mean plane between the rings
(C9–C10–C11–C16–C17 and C2–C3–C4–C14–C15) is
55.19ꢁ. All the bond lengths are comparable to those
observed in similar compounds [1–4]. The bond lengths of
In the FTIR spectrum of the free ligand H2L, a broad
band at 3428 cm-1 might be attributed to O–H vibration
(Fig. S4a). On the other hand, in the FTIR spectra of [CoL]
and [MnLCl], the bands due to the O–H modes are no
longer observed, suggesting that the hydroxyl protons are
displaced by the metal atoms leading to M–O bonding (Fig.
S4b, S4c). In addition, the band due to the CH=N groups of
H2L was shifted from 1613 to 1606 and 1605 cm-1 for
[CoL] and [MnLCl], respectively, consistent with involve-
ment of the azomethine nitrogen atoms in the coordination
[28].
˚
˚
C8=N2 [1.285(2) A] and C5=N1 [1.284 (2) A] conform to
the values for double bonds and are comparable to those in
other Schiff bases [23, 24]. In addition, the bond lengths
The bands assigned to the intramolecular H-bonding
vibration (O–HÁÁÁN) (2650–2700 cm-1) of H2L are absent
from the spectra of complexes, as a result of deprotonation
and coordination of the oxygen atoms [29]. The bonding of
the Co(II) and Mn(III) atoms to the ligand through the
nitrogen and oxygen atoms is further supported by the
presence of new bands at 520 (Co) and 541 (Mn) cm-1 and
450 (Co) and 445 (Mn) cm-1 assigned to the M–O and
M–N stretching, respectively [30].
˚
˚
(C6—N1) = 1.410(2) A and (C7—N2) = 1.408(2) A are
typical for the single bonds. Three torsion angles, C8–N2–
C7–C18 [43.4 (2)ꢁ], C21–N1–C6–C5 [-10.3 (2)ꢁ], N2–C7
–C6–N1 [6.7 (2)ꢁ], are responsible for mutual orientation
of both aromatic ring systems. In the crystal structure, the
˚
NÁÁÁN distance is equal to 2.732 (2) A.
The hydrogens attached to O2 and O3 form asymmetric
weak intramolecular hydrogen bonds to N2 and N1,
Immobilization of [CoL] and [MnLCl] onto
SiO2–APTS
˚
respectively: O2–H2ÁÁÁN2 1.875(1), O2ÁÁÁN2 2.608(2) A,
˚
145.1(8)ꢁ; O3–H3ÁÁÁN1 1.871(1), O3ÁÁÁN1 2.610(1) A,
146.0(7)ꢁ. There are columns of molecules arranged in a
2D network connected by intermolecular hydrogen bonds
along the crystallographic c direction (Fig. S2): C8–H8…
Modification of the silica support through surface func-
tionalization with APTS and immobilization of [CoL] and
[MnLCl] onto the modified silica was carried out by using
the procedure shown in Schemes 2 and 3. On the basis of
nitrogen elemental analysis, the amount of APTS anchored
to the silica was determined as 2.42 mmol g-1. The
amounts of [CoL] and [MnLCl] immobilized onto SiO2–
APTS were 0.51 and 0.46 mmol g-1, respectively, deter-
mined from metal analyses by AAS.
˚
O3, 2.716 (2) A, 142.3ꢁ (3), x, ?y, ?z?1; C12–H12ÁÁÁO1,
˚
2.405 (1) A, 161.3ꢁ (1), -x?/2, -y?1, ?z-1/2). These are
reinforced by C–HÁÁÁp interactions formed between rings.
Moreover, there is a weak face-to-face stacking interaction
˚
[3.632 (2) A] between the rings C2–C3–C4–C13–C14–C15
and C6–C7–C18–C19–C20–C21.
The DR–UV–vis spectra of SiO2–APTS, CoL@SiO2–
APTS, and MnL@SiO2–APTS are given in Fig. S5. There
are important differences between the spectrum of SiO2–
APTS and the spectra of CoL@SiO2–APTS and MnL@
SiO2–APTS. In the spectrum of SiO2–APTS, no peaks are
observed in the range from 300 to 500 nm. In contrast,
there are two intense peaks in this range at the spectra of
both CoL@SiO2–APTS and MnL@SiO2–APTS,consistent
with the presence of the metal complexes on the surface of
SiO2–APTS.
Characterization of the complexes
The results of elemental analysis are in good agreement
with the suggested compositions of the metal complexes.
The effective room temperature magnetic moments of
[CoL] and [MnLCl] were calculated as 4.31 BM and 4.46
BM, respectively, indicating distorted tetrahedral [25] and
square pyramidal [26] geometries, respectively.
The solution conductivities of [CoL] and [MnLCl] in
DMSO were measured as 0.5 and 0.8 X-1 cm2 mol-1
respectively, showing that the complexes are non-electro-
,
The FTIR spectra of SiO2, APTS functionalized silica
(SiO2–APTS), CoL@SiO2–APTS, and MnL@SiO2–APTS
are shown in Fig. S6. The spectrum of SiO2 shows a broad
band in the range 3430–3080 cm-1, attributed to the sila-
nol (–OH) groups, and a broad absorption at 1089 cm-1
assigned to the Si–O–Si bonds. After functionalization of
lyte as expected [27].
The electronic spectra of H2L, [CoL] and [MnLCl] were
measured in chloroform. The UV–vis spectra of H2L and
[CoL] are given in Fig. S3. In the spectra of the complexes,
123

204
Transition Met Chem (2013) 38:199–206
Fig. 1 A view of H2L, showing
50 % probability displacement
ellipsoids and the atom-
numbering scheme. Dashed
lines are intramolecular
hydrogen bonding in the
structure
silica with APTS, the intensity of the 3400 cm-1 band is
decreased, indicating successful grafting of the organosi-
lane onto the silica surface via Si–O–Si bond formation. In
addition, SiO2-APTS also shows the characteristic bands
In addition to the loss of adsorbed water in the range
50–150 ꢁC, SiO2–APTS also displays other significant
weight losses (7–8 %) without distinct steps in the region
220–580 ꢁC, attributable to the loss of organic moieties
attached to the APTS unit. The TG–DTA thermogram of
MnL@SiO2–APTS is given in Fig. S7. Upon immobiliza-
tion of the complexes onto the organofunctionalized silica
gel, the decomposition of the organic parts of the com-
plexes starts at ca. 300 ꢁC and continues up to ca. 700 ꢁC,
with weight loss of about 20 %.
for C–H and N–H stretching from 2800 to 3200 cm-1
.
Upon immobilization of the metal complexes onto SiO2–
APTS, there are no significant changes in the FTIR spectra,
as the quantities of the complexes were so low that their
absorption bands were obscured by the strong background
peaks of SiO2–APTS [31].
The only difference noticed between the spectra of SiO2–
APTS and CoL@SiO2–APTS, and MnL@SiO2–APTS is the
appearance of low-intensity bands at the range of 1350–
1650 cm-1 and especially at 1600 cm-1, assigned to C=N
groups of the complexes.
The vibrational bands of the counter ion (ClO4-) of
MnL@SiO2–APTS, expected at 1100 cm-1, might be
overlapped by the broad Si–O–Si band of the support.
Schematic representations of the proposed structures of
CoL@SiO2–APTS and MnL@SiO2–APTS [32] are given
in Schemes 2 and 3.
Catalytic studies
Styrene was used as substrate to investigate the behavior of
the immobilized complexes as heterogeneous catalysts for
olefin epoxidation, using TBHP as an oxidant in dichlo-
romethane or acetonitrile as solvent. The results are sum-
marized in Table 2.
The MnL@SiO2–APTS complex gave higher conver-
sion and selectivity compared to SiO2–APTS and CoL@
SiO2–APTS. Analysis of the products with a Finnigan-
Trace GC–MS instrument found that benzaldehyde was the
main by-product. The proposed mechanism of the catalytic
reaction is given Scheme S1.
These materials were also investigated by thermo-
gravimetric analysis. The TGA weight loss of the silica gel
shows only one peak with a corresponding weight loss of
about 4–5 % in the region of 50–150 ꢁC, attributed to the
loss of surface adsorbed water [31].
The influence of reaction time (8, 16 or 24 h) in the
epoxidation of styrene catalyzed by CoL@SiO2–APTS and
123

Transition Met Chem (2013) 38:199–206
205
immobilized onto silica through coordination of the amine
from APTS which was covalently bound to the silica
support. The immobilized Co(II) and Mn(III) complexes
are moderately efficient catalysts for epoxidation of styrene
with tert-BuOOH. The MnL@SiO2–APTS complex showed
higher conversion and selectivity compared to SiO2–APTS
and CoL@SiO2–APTS. The catalysts could be easily sepa-
rated from the reaction system and reused. They have
potential for further improvement as well as use heteroge-
neous catalysts, for other reactions.
Table 2 Epoxidation of styrene with TBHP catalyzed by a variety of
catalysts based on immobilized Schiff base-metal complexes
Catalyst
Styrene conversion (%) Selectivity (%)
SE BA Other
SiO2–APTS
12
52
58
61
70
71
34
30
24
22
8
9
6
7
CoL@SiO2–APTS
MnL@SiO2–APTS 73
MnL@SiO2–APTS 70 (after 5 recycling)
Reaction conditions: amount of catalyst: 0.05 mmol active center;
reaction temperature: 40 ꢁC; Styrene (5 mmol); acetonitrile (10 ml)
as a solvent; reaction time 24 h
Supplementary data
SE Styrene oxide, BA Benzaldehyde
MnL@SiO2–APTS was investigated. The highest conver-
sion of substrate was achieved for 24-h reaction times.
Next, the epoxidation of styrene was studied by varying the
reaction temperature from 25 to 40 ꢁC in various solvents.
These experiments showed that 40 ꢁC was the best tem-
perature in both solvents.
Crystallographic data can be obtained from the Cambridge
Crystallographic Data Center, by quoting the reference
number CCDC-863564. The data can be obtained free of
charge at www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments The authors thank the Scientific and Techno-
¨
˙
logical Research Council (TUBITAK) for financial support (Project
Number 109T416). Also, the authors are grateful to Anadolu Uni-
versity and the Medicinal Plants and Medicine research Centre of
Anadolu University, Eskis¸ehir, Turkey, for the use of X-ray
Diffractometer.
The influence of solvents (dichloromethane and aceto-
nitrile) on these catalytic epoxidations was explored for
both CoL@SiO2–APTS and MnL@SiO2–APTS. In both
cases, the highest conversion of substrate was achieved in
acetonitrile.
Using the optimum conditions, epoxide yields of 52 and
73 % were achieved for CoL@SiO2–APTS and MnL@
SiO2–APTS, respectively.
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