Heterotrinuclear manganese(II) and vanadium(IV) Schiff base complexes as epoxidation catalysts

Article abstract of DOI:10.1007/s11243-011-9486-y

The catalytic epoxidation of styrene using urea-hydrogen peroxide and heterotrinuclear Cu(II) complexes with general formula (ML ⁿ ) ₂Cu(acac)₂, where n = 1-3 and M = VO²⁺ or Mn²⁺ is reported. Schiff

Full text of DOI:10.1007/s11243-011-9486-y

Transition Met Chem (2011) 36:425–431
DOI 10.1007/s11243-011-9486-y
Heterotrinuclear manganese(II) and vanadium(IV) Schiff base
complexes as epoxidation catalysts
Sajjad Mohebbi
Asad Shangaie
•
Shokoh Bahrami
•
Received: 20 January 2011 / Accepted: 21 March 2011 / Published online: 5 April 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The catalytic epoxidation of styrene using urea-
hydrogen peroxide and heterotrinuclear Cu(II) complexes
hydrogen peroxide [4], alkyl hydroperoxide [5, 6], molec-
ular oxygen [7], and more recently urea-hydrogen peroxide
(UHP) [8–11]. UHP is a readily available derivative of
H O that releases H O into solution in a controlled
n
with general formula (ML ) Cu(acac) , where n = 1–3 and
2
2
2?
2?
M = VO or Mn is reported. Schiff base complexes
2
2
2 2
n
ML involving a 3,4-diaminopyridine bridge with free
manner, and has been used as oxidant in the presence of a
transition metal complex catalyst [8].
n 2-
coordination site were used as the ligand, where (L ) is
2
(5-x-Sal) Py] and x = H, Br or NO . The complexes
[
Manganese complexes are less environmentally damag-
ing than those of other transition metals, and are also
involved in many biological processes. Therefore, many
manganese complexes of porphyrin, phthalocyanine, tri-
azamacrocycle, and Schiff base ligands have been synthe-
sized as mimics of enzymes in connection with oxidation
state, coordination number, and number of manganese sites
[12]. Also, the ability of the copper(II) to accommodate
different geometries and coordination numbers can lead to
structural diversity and variations in physico-chemical
properties of its heterometallic complexes.
2
2
were characterized by physico-chemical and spectroscopic
methods. The electrochemical properties of M were mod-
ified upon trinuclear complex formation. The trinuclear
complexes show high catalytic activity, with up to 86%
conversion and 93% selectivity, while no catalytic prop-
erties were observed for the monomeric complexes. The
catalyst could be reused with some loss of activity.
Introduction
Heteropolymetallic complexes are of current interest
due to their importance in the study of charge transfer
between metal centers [13], in catalysis and molecular
recognition [14], and in the area of bioinorganic chemistry
for their potential use as bio-mimetics for the active sites of
many metalloproteins [15–17], especially when the bridg-
ing group between the metal atoms is of biological rele-
vance, such as imidazolate, carboxylate, etc. A large
number of homo-metallic complexes have been described
[18–20], but hetero-metallic compounds with Schiff base
ligands are still limited in number [21–23]. To our
knowledge, no examples of trinuclear compounds ML–Cu–
LM, where M = V(IV) or Mn(II), involving pyridine-
bridges have been reported. A popular and successful two-
step approach for the preparation of hetero-metallic sys-
tems is the metallo-ligand strategy, based on the interaction
between a preformed complex possessing free coordination
sites with a secondary metal [24].
Controlled epoxidation of alkenes with stable and efficient
catalysts using environmental friendly oxidants is an
important goal. In recent years, styrene oxide has been used
as a starting material for epoxy resins, flavoring agents in
food, tobacco, soap, cosmetic essences, perfumes, and
pharmaceuticals [1, 2]. Since the first epoxidation of allylic
alcohols by Sharpless [3], various manganese Schiff base
catalysts have been developed using different oxygen
sources such as iodosylbenzene, sodium hypochlorite,
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-011-9486-y) contains supplementary
material, which is available to authorized users.
S. Mohebbi (&) ꢀ S. Bahrami ꢀ A. Shangaie
Department of Chemistry, University of Kurdistan,
PO. Box 66179-416, Sanandaj, Iran
e-mail: smohebbi@uok.ac.ir; sajadmohebi@yahoo.com
123

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Transition Met Chem (2011) 36:425–431
-
1
In this paper, the synthesis of new heterotrinuclear
n
complexes 1–6 (Fig. 1), namely (ML ) Cu(acac) , n = 1–3
A strong broad band observed at 3,447–3,135 cm in the
spectrum of the free ligand is assigned to mOH. A broad band
in this range is also seen in the spectra of the manganese
complexes, andinthiscase is attributed to watersofhydration
or coordinated ethanol. The Schiff bases show the C=N imine
2
2
2
?
and M = VO or Mn , involving unsymmetrical Schiff
2?
n 2- 2-
base ligands where (L ) is [(5-x-Sal) Py] , x = H, Br
2
or NO are reported. Preformed Schiff base complexes
2
n
ML , 1a–6a with a 3,4-diaminopyridine bridge and free
-1
band in the range 1,603–1,609 cm . Upon coordination to
-1
coordination site were used. Compounds 1–6 have been
studied as catalysts for epoxidation of styrene with UHP as
primary oxidant at low temperature.
the metal, a shift to lower wavenumber (1,594–1,606 cm )
was observed, consistent with coordination of the nitrogen.
The phenolic C–O of the free ligands is observed around
-
,273–1,285 cm . Upon coordination, this band is shifted to
1
1
lower wavenumber, suggesting coordination of the phenolic
oxygen [27]. In the free ligand, a band observed at
Results and discussion
-1
980–993 cm is ascribed to mC–H(CH=N). This band shif-
Selective epoxidation reactions were typically performed
according to the established procedure [25] using complexes
tedtohigherwave numberuponcomplexation. NewIR bands
-1
at 431–455 and 470–560 cm in the spectra of the com-
plexes are assigned to m(M / O) and m(M / N) vibrations
[28], respectively. Hence, the IR spectra indicate that the
Schiff bases act as dibasic tetradentate ligands towards
the central metal atoms. The m(C=N) bands of the pyridine
moiety were unshifted in the spectra of the complexes.
The assignments of the important IR bands of the tri-
nuclear complexes are recorded in Table 1. It can be seen
that these Schiff base complexes exhibit an absorption
1–6 as catalysts, styrene as substrate, 1-methyl imidazole
as co-catalyst, and UHP as primary oxidant. Mononuclear
Schiff base complexes {Mn[(5-R-Sal) Py]}, {VO[(5-R-
2
Sal) Py]}, (R = H, Br, NO ) and the corresponding ligands
2
2
were obtained according to previous procedures [5–7].
Trinuclear Schiff base complexes M–Cu–M, {M[(5-R-
Sal) Py] Cu(acac) }, 1–6, were prepared by mixing two
2
2
2
equivalents of mononuclear Schiff base complexes {M[(5-
R-Sal) Py]} with one equivalent of Cu(acac) in refluxing
-
1
at around 1,596–1,615 cm , which is ascribed to the
stretching vibration of the C=N(imine) band. The fre-
quencies of this band are considerably lower than these
of the C=N(pyridine) vibrations, and the C=N(imine) band
also shows a stronger intensity. In the IR spectra of the
trinuclear Schiff base complexes 1–6, the pyridine C=N
2
2
ethanol. Similar treatment of three or four equivalent of
M[(5-R-Sal) Py]} afforded the same trinuclear com-
{
2
pounds (Fig. 1). The products 1, 2, and 3 were isolated as
dark brown, brown, and orange powders, respectively.
-
1
Spectroscopic and physico-chemical studies
stretching vibrations at 1,632–1,680 cm , are shifted by
-
1
about 5–30 cm compared to 1a–6a [7], due to the strong
inductive effect of the copper atom. Our efforts to prepare
single crystals of these complexes for X-ray diffraction
measurements have been unsuccessful.
In order to give an idea about the structures of the mononu-
clear metal complexes, {M[(5-R-Sal) Py]}, 1a–6a, the main
2
IR bands were compared with those of the free ligands [26].
Fig. 1 Heterotrinuclear
copper(II) complexes {M[(5-R-
R
O
N
O
N
R
M
2 2 2
Sal) Py]} Cu(acac) , 1–6 where
IV
II
M = V O or Mn . 1, 1a,
M = Mn, R = H; 2, 2a,
M = Mn, R = Br; 3, 3a,
2
M = Mn, R = NO ; 4, 4a,
N
R
O
N
O
N
R
M = VO, R = H; 5, 5a,
M = VO, R = Br; 6, 6a,
M = VO, R = NO
M
Cu(acac)2
O
O
O
2
Cu
N
Reflux, 2 h
O
N
1
a - 3a
N
O
N
M
R
O
R
1
- 6
1
23

Transition Met Chem (2011) 36:425–431
427
-
Table 1 Selected IR bonds (cm ) and UV–Vis data (nm) for trinuclear complexes 1–6
1
a
-1
-1
No. Complex
m(C = N)
m(M–N) m(M = O) k, nm (e, M Cm )
Imine Pyridine
p ? p*
n ? p*
d ? d
1
2
3
4
5
6
a
{Mn[(5-H-Sal)
{Mn[(5-Br-Sal)
{Mn[(5-NO -Sal)
{VO[(5-H-Sal) Py]}
{VO[(5-Br-Sal) Py]}
{VO[(5-NO -Sal) Py]}
2
Py]}
Py]}
Py]}
Cu(acac)
Cu(acac)
Cu(acac)
2
Cu(acac)
Cu(acac)
Cu(acac)
2
1,603 1,632
1,596 1,647
1,601 1,680
1,602 1,648
1,615 1,648
1,608 1680
549
585
562
543
547
519
–
233 (4,129)
–
386 (402)
465 (187), 742 (16)
743 (49)
2
2
2
–
237 (4,430) 294 (1,449) 438 (290)
291 (1,207) 363 (668)
2
2
2
2
–
–
408 (803), 725 (28)
2
2
2
978
960
950
238 (4,288) 312 (2,025) 413 (1,369) 742 (49)
2
2
2
232 (9,430)
232 (3,958
–
–
396 (385)
379 (416)
741 (18)
739 (19)
2
2
2
2
In DMF solution
The V=O stretching frequencies of complexes 4–6 were
1
are non-electrolytes [33]. As such, these complexes are
likely neutral overall.
-
observed between 950 and 978 cm (Table 1). It has been
reported that oxovanadium complexes with coordination
Electrochemical study of the complexes was performed
using DMF as solvent at 25 °C under a nitrogen atmosphere
using glassy carbon as the working electrode, Ag/AgCl as
reference electrode, and tetrabutylammonium hexafluoro-
phosphate as the supporting electrolyte at a scan rate of
100 mV/s. The results are summarized in Tables 2 and 3.
The cyclic voltammogram of compound 3 shows two
reductive peaks at 320 and 482 mV due to the Mn(II)/
Mn(III) redox couple, both of which are irreversible in
nature. The observed potential gap (150 mV) between the
two reductions indicates efficient communication between
the Mn(III) centers through the Cu(II) bridge. Two oxida-
tion responses are also expected for such a system, but only
one such oxidation is observed at 612 mV, due to oxidation
of Mn(III) to Mn(IV).
-
1
number 5 have m(V=O) values at about 950–1,000 cm
29]. Thus, the three vanadyl complexes should exhibit
[
monomeric square-pyramidal geometries with no ligand in
the axial position.
-
1
Bands between 434 and 498 cm in the spectra of 1–6
can be assigned to m(Cu–N). It is apparent that chelation of
the divalent copper to the ligand occurs through the pyri-
dine nitrogen atom. The acetylacetonate ligands occupy
the other coordination sites to complete the coordination
sphere of the copper. Figure 1 shows the proposed struc-
ture of the trinuclear complexes. In the UV–Vis spectra, the
Cu(II) complex has a band at 630–650 nm and a shoulder
at 425–455 nm that could be assigned to LMCT.
The electronic spectral data of the manganese(II) and
vanadyl complexes in DMF are summarized in Table 1. The
transitions due to the organic ligands have higher emax values
than the d–d transitions of the atoms. The bands observed for
all the metal complexes at 233–291 nm and the shoulder at
Catalytic epoxidation of styrene
Selective epoxidation reactions were investigated using the
trinuclear complexes 1–6 as catalysts, styrene as substrate,
UHP as oxidant, and in the case of manganese catalysts
1–3, 1-methyl imidazole as co-catalyst. The oxidation
2
94–312 nm were assigned to p ? p* and n ? p* transi-
tions, respectively, within the aromatic rings in the ligands.
The absorption bands in the range 363–441 nm may be
attributed to ligand-to-metal charge transfer by analogy with
similar manganese complexes. The bands at 408–744 nm
can be assigned to d–d transitions on the basis of their low
extinction coefficients [30, 31].
Table 2 Electrochemical results for mononuclear complexes 1a–6a
Entry
Complex
Ep, mVD
E
pa, mV
E
pc, mV
Although the electronic spectra of the manganese
complexes with multidentate Schiff base ligands are not in
general a good indicator of their geometry, they can pro-
vide supporting information about the structure [32].
Complexes 1–3 display strong absorption bands at ca.
1a
2a
3a
4a
5a
6a
Mn[(5-H-Sal)
2
Py]
Py]
-Sal) Py]
VO[(5-H-Sal) Py]
VO[(5-Br-Sal) Py]
VO[(5-NO -Sal) Py]
–
–
1,039
1,005
1,104
598
–
–
–
Mn[(5-Br-Sal)
Mn[(5-NO
2
2
2
–
2
78
59
69
520
688
919
3
86 nm, which can be assigned to the charge transfer from
2
747
the ligands to the Mn. The spectrum of compound 1 shows
bands associated with d–d transitions at ca. 465 nm.
Molar conductivities of the complexes were measured in
2
2
988
Electrochemical data were measured for 0.1 M of sample in DMF
solution, 0.1 M of tetrabutylammonium hexaflourophosphate as sup-
porting electrolyte. Potentials are reported versus AgCl/Ag as refer-
ence electrode at 100 mV/S Scan rate
-
3
DMF (10 M solution) at room temperature and lie in the
-
1
2
-1
range of 7.23–14.02 X cm mol , indicating that they
123

4
28
Transition Met Chem (2011) 36:425–431
Table 3 Electrochemical data for the trinuclear complexes 1–6
No.
Complex
E
pc, mV
E
pa, mV
Ep, mVD
E1/2, mV
1
2
3
4
5
6
{Mn[(5-H-Sal)
2
Py]}
Py]}
-Sal) Py]}
{VO[(5-H-Sal) Py]} Cu(acac)
{VO[(5-Br-Sal) Py]} Cu(acac)
{VO[(5-NO -Sal) Py]} Cu(acac)
2
Cu(acac)
Cu(acac)
Cu(acac)
2
2
520
480
482
450
695
707
610
592
612
625
790
802
90
112
140
175
95
565
536
552
527
752
760
{Mn[(5-Br-Sal)
{Mn[(5-NO
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
95
Electrochemical data were measured for 0.1 M of sample in DMF solution, 0.1 M of tetrabutylammonium hexaflourophosphate as supporting
electrolyte. Potentials are reported versus AgCl/Ag as reference electrode at 100 mV/S Scan rate
reactions were performed under previously optimized
conditions, specifically catalyst/ImH/styrene/UHP (1:30:30:50)
in a (1:1) mixture of CH OH/CH Cl . The yields remained
Using of H O , within 2 h, the conversion of styrene
2 2
reached a maximum of 42% for 2. When the reaction time
was increased to 8 h, the conversion was a little changed
and finally reached 50%. The vanadyl complexes 4–6
showed lower catalytic activity than the manganese com-
plexes 1–3. Figure 2 shows the correlation between the
conversion and the reaction time, where the reaction tem-
perature was fixed at 4 °C for 1–3, using UHP or H O
3
2
2
constant upon further increasing of ImH up to 50
equivalents.
In addition, H O was used as an oxidant for comparison
2
2
with UHP (Scheme 1). For control experiments, epoxida-
tions without any catalyst and with mononuclear metal
complexes were run. GC analysis showed extremely low
yields of styrene oxide in these control experiments; within
2
2
as oxidants. Maximum styrene epoxide selectivity was 73
and 69% for 2 and 5, respectively, for 8 h reaction time.
Increasing the reaction time to 24 h gave 86% conversion
of styrene for 2.
8
h almost no conversion occurred. This is not surprising
for the mononuclear complexes, given their totally irre-
versible redox behavior.
Maximum conversion of styrene with UHP catalyzed by
complex 2 was achieved at about 8 h with 48% conversion
and 93% selectivity. In the case of the vanadyl complexes,
both the conversion and selectivity were lower than the
manganese complexes for a reaction temperature of 4 °C.
Using a co-catalyst such as imidazole is necessary for
efficient alkene epoxidation; the imidazole acts as a prox-
imal ligand to Mn in the presence of H O or UHP [8–10,
As expected, on the basis of their CVs, the trinuclear
complexes 1–6 exhibited catalytic activity toward conver-
sion of styrene to styrene oxide. These catalytic activities
are associated with the binding of copper between two
metal complex centers, via the pyridine nitrogen atoms.
To the best of our knowledge, there is no example in the
literature of catalytic activity for pyridine bridged manga-
nese or vanadyl Schiff base complexes. As mentioned
2
2
34, 35]. Also, in the current catalytic system, the proximal
effect of imidazole is much more important than its distal
effect.
above, the monomeric complexes 1a–6a show no catalytic
,
activity. Interestingly, however, the analogous salophen
complexes, in which pyridine is replaced by a simple
benzene ring, do show catalytic activity. Using hydrogen
peroxide as oxidant, conversions of 27 and 18% were
obtained for manganese and vanadyl salophen complexes,
respectively. Therefore, the results for trinuclear catalysts
The catalyst solution was reused under the same epox-
idation conditions, using the same amounts of styrene and
UHP or H O . In all cases, conversion was decreased at
2
2
about 25–36%, probably due to degradation of the catalyst.
Further investigations are in progress to define the reaction
mechanism, role of 1-methylimidazole and the reasons for
loss of activity of the catalyst on reuse.
1
–6 suggest that the two metal centers are not working in
isolation but have some cooperative interaction through the
copper, and further, the pyridine bridge has lost its inhibitor
activity due to coordination to copper.
Experimental
O
Chemicals and solvents were obtained from Merck, apart
Catalyst, ImH
from VO(acac) and 2-hydroxybenzaldehyde which were
2
°
UHP, 4 C, 8 h
obtained from Aldrich.
Up to 93%
Infrared spectra were recorded on a Shimadzu IR 460
spectrophotometer using KBr pellets. Electronic absorption
spectra were recorded on a Varian UV–Vis Cary 100E
in CH Cl /CH OH
2
2
3
Scheme 1 Catalytic oxidation of styrene
1
23

Transition Met Chem (2011) 36:425–431
429
Preparation of the mononuclear Schiff bases complexes
Mn[(5-R-Sal) Py]} and {VO[(5-R-Sal) Py]}
{
2
2
Schiff base complexes 1a–6a were obtained by previously
reported methods [4–7, 25]. Synthetic procedures and char-
acterization data are provided in the supporting information.
Synthesis of {Mn[(5-R-Sal) Py](EtOH)} Cu(acac)
2
2
2
To a stirred and hot solution of Cu(acac)2 (0.261 g,
mmol) in ethanol (40 ml) was added a hot solution of
1
Mn[(5-R-Sal) Py] (1a–3a) (2 mmol), in ethanol (30 ml).
2
The color of the solution changed after a few minutes.
The reaction mixture was then refluxed for 120 min. The
solution was concentrated and cooled to yield a brown
precipitate. This was filtered off, washed with diethyl ether,
dried at 60 °C overnight, and then recrystallized from
ethanol until a pure product was obtained. Yields 70–75%.
Elemental analysis results for 1: found(%) C, 56.8; H, 4.7;
N, 8.0; Cu, 5.4; Mn, 10.3. Calc. for C H CuMn N O :
5
2
54
2 6 10
C, 57.0; H, 5.0; N, 7.7; Cu, 5.8; Mn, 10.0%. For 2: found C,
4.0; H, 3.4; N, 6.2; Cu, 4.7; Mn, 8.0. Calc. for
C H Br CuMn N O : C, 44.2; H, 3.6; N, 6.0; Cu, 4.5;
4
5
2
50
4
2 6 10
Mn, 7.8%. For 3: found C, 48.6; H, 3.7; N, 11.2; Cu, 5.3;
Mn, 9.0. Calc. for C H CuMn N O : C, 48.9; H, 4.0;
5
2
50
2 10 18
Fig. 2 Relationship between the conversion of styrene and the
reaction time, where the reaction temperature was fixed at 4 °C and
N, 11.0; Cu, 5.0; Mn, 8.6%.
Synthesis of {VO[(5-R-Sal) Py]} Cu(acac)
2
1
-methyl imidazole was used as co-catalyst for 1–3 using a UHP
b H as oxidant. Triangles, complex 1; cross, complex 2; diamond,
complex 3
2 2
O
2
2
To a stirred and hot solution of Cu(acac)2 (0.261 g,
mmol) in ethanol (40 ml) was added a hot solution of
VO[(5-R-Sal) Py] (2 mmol), in ethanol (30 ml). The color
1
1
13
spectrophotometer. H{ C} NMR spectra were obtained
on a Bruker FT-NMR AC-250 (250 MHz) spectropho-
2
of the solution changed after a few minutes. The reaction
mixture was then refluxed for 120 min. The solution was
concentrated and cooled to yield a light brown precipitate,
which was filtered off, washed with diethyl ether and dried
at 60 °C overnight. The product was recrystallized from
ethanol. Yields 70–75%. Elemental analysis results for 4:
found (%) C, 56.2; H, 4.4; N, 7.9; Cu, 6.4; V, 9.7. Calc. for
tometer using TMS as internal standard and CDCl and
3
DMSO-d as solvents. Elemental analyses (C, H, N) were
6
obtained using a Heraeus Elemental Analyzer CHN-O-
Rapid (Elemental Analyses system, GmbH-West Germany).
Melting points were determined with a B-540 Buchi melting
point apparatus. Cyclic voltammograms (CVs) were
obtained using an electrochemical system (Palm Sense, The
Netherlands) in conjunction with a three-electrode system
and a personal computer for data storage and processing. An
Ag/AgCl (3 M KCl) reference electrode, a Pt wire (counter
electrode) and a glassy carbon working electrode were
employed for the electrochemical studies. Voltammetric
measurements were performed at room temperature in DMF
solution with 0.1 M tetrabutylammonium hexaflourophos-
phate as the supporting electrolyte. Gas chromatography
analyses were obtained on an Agilent instrument 6,890 N
equipped with an FID detector and a capillary column
C
H
42CuN
9.9%. For 5: found C, 42.7; H, 3.1; N, 6.0; Cu, 5.0; V, 7.3.
Calc. for C48 CuN : C, 42.9; H, 2.8; N, 6.2;
Cu, 4.7; V, 7.6%. For 6: found C, 47.9; H, 2.9; N, 11.9; Cu,
5.4; V, 8.1. Calc. for C48 ; C, 47.7; H, 3.2;
N, 11.6; Cu, 5.3; V, 8.4%.
O V : C, 56.1; H, 4.1; N, 8.2; Cu, 6.2; V,
48
6 10 2
H38Br
O V
6 10 2
4
H
38CuN10O V
18 2
Catalytic epoxidation of styrene
(a) Using H
O
2 2
: To a stirred solution of 0.01 mmol cata-
and
lysts, 1–6, in DMF, 3 mL of 0.1 M NaHCO
0.3 mmol styrene was added at 2–4 °C under argon. For
the manganese catalysts, 0.03 mol 1-methylimidazole
3
(
30 m 9 0.25 mm 9 0.01 mm), 5% phenyl methyl silox-
ane, and hydrogen as carrier gas.
123

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30
Transition Met Chem (2011) 36:425–431
Table 4 Epoxidation of styrene using complexes 1–6 as catalyst
Complex
UHP/conversion (%)
Selectivity to epoxide
H
2 2
O /conversion (%)
Selectivity to epoxide
2
h
4 h
8 h
2 h
4 h
8 h
1
31
33
18
–
36
39
22
–
43
48
27
30
34
18
\5
89
93
71
64
75
43
\1
37
42
20
–
42
47
25
–
45
50
28
29
30
20
\5
68
73
53
60
69
2
3
4
5
–
–
–
–
a
42
6
–
–
–
–
Without catalyst
–
–
–
–
\1
For the manganese catalysts, 1-methyl imidazole was added as co-catalyst
The selectivity was calculated on the basis of epoxide yield/conversion ratio
a
Other major styrene oxides, benzaldehyde [36], and phenylacetaldehyde [37], were determined qualitatively as by products
was also added. After 15 min stirring, 1 mmol hydrogen
peroxide30%wasaddeddropwise.Thereactionmixture
was stirred for 8 h at room temperature. The progress of
the reaction was monitored by periodic sampling for GC
determination. Chlorobenzene was used as standard.
b) Using urea-hydrogen peroxide (UHP): To a stirring
solution of 0.01 mmol catalyst, a solution of 0.3 mmol
styrene in 1:1 dichloromethane: methanol (2 mL) was
added at 2–4 °C under argon. In the case of the
manganese catalysts, 0.3 mmol 1-methylimidazole
was also added. After 10 min stirring, 0.5 mmol
UHP was added in three portions at a reaction
temperature of 4 °C. The reaction mixture was stirred
for 8 h at room temperature. The progress of the
reaction was monitored by periodic sampling for GC
determination. Chlorobenzene was used as standard
for quantitative determination of product.
activities, as shown in Table 4. It is evident that the vanadyl
catalysts 4–6 are less active for the epoxidation of styrene
than the Mn catalysts 1–3. Similar selectivities could be
reached at about 8 h for both catalysts; however, the highest
conversion and selectivity for catalyst 5 were only 34 and
75%, respectively, but those for catalyst 2 were 48 and 93%,
respectively. These results could be due to several factors,
such as the nature of the metals, their ability to coordinate the
reactants, the solubility of the complexes, etc.
(
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