Effect of substituents on the Mn(III)Salen catalyzed oxidation of styrene

Article abstract of DOI:10.1016/S1381-1169(01)00112-1

The influence of the electron-donating tert-butyl group both on the encapsulation of the Salen ligand and the catalytic activity of the catalyst produced was studied. Styrene oxidation at room temperature and 1 atm using molecular oxygen as oxidant and tert-butyl hydroperoxide as initiator showed that neat and encapsulated Mn(III)Salen complexes (Br₂Salen and (tert-butyl)₄Salen) were active. Using the intrazeolite ligand synthesis (or template synthesis) method, the increase in the size of the complexes improved their physical entrapping in the zeolite. The encapsulation produced more stable catalysts, especially in the case of (tert-butyl)₄Salen catalyst.

Full text of DOI:10.1016/S1381-1169(01)00112-1

Journal of Molecular Catalysis A: Chemical 171 (2001) 95–102
Effect of substituents on the Mn(III)Salen
catalyzed oxidation of styrene
Ágnes Zsigmond∗, Attila Horváth, Ferenc Notheisz
Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, University of Szeged,
H-6720 Szeged, Dóm tér 8, Hungary
Received 3 August 2000; accepted 12 January 2001
Abstract
Manganese(III)complexes of Salen, Br2Salen and (tert-butyl)4Salen ligands and their encapsulated analogues were pre-
pared by the intrazeolite ligand synthesis (template synthesis) method. The oxidation of styrene was studied on the above
catalysts using molecular oxygen as oxidant and tert-butyl hydroperoxide as initiator. The progress of the reactions was
followed both by gas chromatography and by the oxygen consumed. The effect of the substituents on the encapsulation, on
the catalytic activity and the effect of the encapsulation on the catalytic activity were studied and discussed. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Mn(III)Salen; Ship-in-a-bottle complexes; Intrazeolite synthesis; Aerobic oxidation; Styrene oxidation
1. Introduction
within the confines of the cage such that the solution
reactivity should be maintained in a heterogeneous
environment. The advantages of site isolation in the
pores coupled with the size and shape selectivity of
the zeolite support should be reflected in the catalyst
activity.
Zeolite encapsulated metal complexes or ship-in-
a-bottle complexes comprise a class of biomimetic
catalysts sometimes referred to as zeozymes [1–12].
The synthetic faujasite (FAU) type zeolites (Y or X)
have been the most widely studied host materials. The
FAU structure involves large cavities about 1.2 nm in
diameter but the openings to these supercages are only
0.74 nm in diameter. One of the possible strategies for
encapsulating metal complexes in such zeolites is the
use of the intrazeolite ligand synthesis (or template
synthesis) method [1,13]. This procedure involves the
in situ assembly of a ligand and the complex that be-
comes effectively larger than the 0.74 nm windows is
then physically entrapped. Homogeneous complexes
encapsulated in the pores should be free to move
The classic example of such encapsulated com-
pounds is the tetradentate Schiff base H2Salen
molecule
(2,2ꢀ-[ethane-1,2-diil-bis(nitrilo-methyli-
dyne)]diphenol). The frequently used common name
of this molecule is bissalicylidene-ethylene diamine.
Manganese complexes of the Salen ligand have been
used extensively in low-temperature oxidation reac-
tions. Ratnasamy and co-workers [14] studied the
effect of different electron-withdrawing substituents
(like chloro, bromo and nitro) on the encapsulation
and catalytic activity of Mn(III)Salen catalysts. They
synthesized the neat 4,4ꢀ-disubstituted molecules
(Fig. 1) and encapsulated them by the zeolite synthe-
sis method.
∗ Corresponding author.
E-mail address: azsig@chem.u-szeged.hu (A. Zsigmond).
´
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00112-1

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A. Zsigmond et al. / Journal of Molecular Catalysis A: Chemical 171 (2001) 95–102
96
solid products were dissolved in hot ethanol and
refluxed with manganese(II)acetate tetrahydrate.
Lithium chloride was used to produce Mn(III)Salen
complexes.
2.3. Preparation of encapsulated complexes
Fig. 1. The structure of the 4,4ꢀ,6,6ꢀ-substituted-{2,2ꢀ-[ethane-
1,2-diil-bis(nitrilo-methylidyne)]diphenolato}-manganese(III)-chlo-
ride complexes.
Intrazeolite metal complexes based on the Salen
type ligands are prepared normally by the flexible
ligand method [1]. However, when the substituents
on the aromatic rings of the Salen ligand are bulky
tert-butyl groups, diffusion of this ligand through the
pore openings is hindered. Therefore, this type of en-
capsulated Mn(III)Salen catalysts are synthesized by
the intrazeolite ligand synthesis [13] (or template syn-
thesis [1]) method using NaY zeolite. The unsubsti-
tuted and bromo-substituted complexes were prepared
in a similar manner for comparative purposes.
These catalysts were used for the oxidation of
styrene and it was concluded that the catalytic ef-
ficiency of the encapsulated complexes was much
higher than that of the neat complexes and the
electron-withdrawing substituents enhanced the rate
of oxidation.
The goal of the present work is to study the effect
of the electron-donating tert-butyl group both on the
encapsulation of the Salen ligand and the catalytic ac-
tivity of the catalyst produced. Furthermore, a compar-
ison between the results of the electron-withdrawing
and the electron-donating substituents.
2.4. General procedure for the encapsulation
1500 cm3 of 0.02 M sodium acetate — acetic
acid buffer was added to 30 g of NaY. 5.5 g man-
ganese(II)acetate tetrahydrate was dissolved in deion-
ized water and added to the buffered mixture slowly
with stirring. Then the solution was stirred for 24 h
at room temperature. The slurry was then filtered and
the solid was washed with deionized water and dried.
11.4 mmol (1.22 cm3) 2-hidroxybenzaldehyde (or
its substituted analogues) was dissolved in 15 cm3
ethanol and added to 8 g of Mn-exchanged zeolite. Af-
ter 4 h stirring 0.38 cm3 ethylenediamine was added
and the mixture was stirred at room temperature for
24 h. The solid was filtered, washed with ethanol and
stirred for another 24 h in 100 cm3 concentrated NaCl
solution to remove the uncomplexed manganese ions.
The solid material was filtered, washed with deionized
water and dried.
2. Experimental
2.1. Materials
2-Hydroxybenzaldehyde (salicylaldehyde), 3,5-di-
tert-butyl-2-hydroxybenzaldehyde, 5-bromo-2-hydro-
xybenzaldehyde, ethylenediamine (99%), manganese
(II)acetate tetrahydrate, lithium chloride (99%), ace-
tonitrile (99%), styrene (99%) and NaY zeolite were
purchased from Aldrich, while tert-butyl hydroper-
oxide (80% solution in di-tert-butyl peroxide) was
originated from Merck and used as received. Ethanol
(Reanal, 99%) was also used without any additional
purification.
2.5. Catalyst characterization
2.2. Preparation of neat metal complexes
The catalysts were characterized by FT-IR and the
metal content was determined by ICP. The FT-IR spec-
tra of the zeolite, the neat complexes and the heteroge-
nized samples were taken, as well. These spectra were
recorded in KBr pellets, using Bio-Rad FTS — 65 A
The neat metal complexes were synthesized by the
standard method [14,15]. 2-Hydroxybenzaldehyde or
its substituted derivatives (3,5-di-tert-butyl-2-hydrox-
ybenzaldehyde or 5-bromo-2-hydroxybenzaldehyde)
were reacted with ethylenediamine in ethanol. The
spectrophotometer, in the range of 400–4000 cm−1
.

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97
To determine the metal content of the heterogenized
3. Results and discussion
samples, the samples were dissolved in cc. HNO3 and
HF. The metal content was determined by JOBLIN
YVON 24 type ICP-AES instrument, at 257.61 nm.
The flow rate of the argon was 12 dm3 min−1 and
the sample dosing was carried out with the speed of
3.1. Catalyst characterization
The characteristics of the prepared disubstituted
(X2-Salen) or tetrasubstituted (X4-Salen) complexes
and the encapsulated (Salen-Y) derivatives are shown
in Table 1.
1.5 cm3 min−1
.
The IR spectra of all the neat complexes, the encap-
sulated samples were recorded. As an example, Fig. 2
shows the IR spectra of the Mn(III)tBu4Salen com-
plex, the encapsulated catalyst and the NaY zeolite.
The IR spectrum of the Mn(III)tBu4Salen complex
show major bands around 1631, 1553, 1535, 1462,
1437, 1411, 1389, 1361, 1306, 1217, 1252, 1200 and
1175 cm−1. Similar frequencies were observed in the
case of Mn(III)tBu4Salen-Y catalyst, indicating the in-
corporation of the complex. Some differences, how-
ever, can be observed between the spectra of the neat
and heterogenized complexes, which is even more em-
phasized on the extended spectrum in the range of
1200–1550 cm−1 (Fig. 2).
2.6. Catalytic reaction
Oxidation reactions were carried out in the liquid
phase under one atmosphere of oxygen at room tem-
perature. The apparatus consists of a glass reactor,
shaken in a vortex manner and a gas handling sys-
tem, for monitoring the volume of dioxygen reacted
at constant pressure [16,17].
0.1 mmol neat complex (or 0.5 g encapsulated cat-
alyst) was put into the glass reactor and the apparatus
was filled with 1 atm of dioxygen. 4 cm3 acetonitrile
and 0.54 cm3 (4.35 mmol) tert-butyl hydroperoxide
were added to the catalyst. After a short shaking
0.5 cm3 (4.35 mmol) styrene was injected into the re-
actor and the reaction was started with shaking. The
product mixture was analyzed at 6, 12, 24 and 48 h
by gas chromatography (Hewlett-Packard 5890 Series
II), using dichloromethane as internal standard. The
products were identified by authentic sample or by
Hewlett-Packard 5970 mass selective detector. The
amount of styrene, benzaldehyde, phenyloxirane and
phenylacetaldehyde was calculated from the chro-
matograms. In some cases, the reaction was repeated
in a higher scale, and an additional product was ex-
tracted from the reaction mixture and identified as
benzoic acid.
This difference could be an indication of the encap-
sulation and might be caused by the distortion of the
encapsulated complex.
3.2. Tert-butyl hydroperoxide as initiator
The oxidation of styrene using molecular oxygen
was chosen as a model reaction. It was found earlier
that styrene could be oxidized with dioxygen alone
but the conversion was low. Tert-butyl hydroperox-
ide (2 wt.% of styrene) as initiator, however, can
increase the oxidation efficiency of the molecular
oxygen [14]. At the same time in the absence of dioxy-
gen (under an argon atmosphere) the Mn(III)Salen
complexes did not show substantial catalytic activity
In addition to the gas chromatographic analysis the
oxygen consumption was also recorded during the re-
action.
Table 1
Characteristics of the prepared neat and encapsulated catalysts
Catalysts
X4 (Fig. 1)
X6 (Fig. 1)
Mn(III) complex (mmol/gram catalyst)
Mn(III)Salen
H
H
–
Mn(III)Salen-Y
Mn(III)tBu4Salen
Mn(III)tBu4Salen-Y
Mn(III)Br2Salen
Mn(III)Br2Salen-Y
H
H
0.027
–
0.115
–
tBu
tBu
Br
Br
tBu
tBu
H
H
0.040

´
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A. Zsigmond et al. / Journal of Molecular Catalysis A: Chemical 171 (2001) 95–102
Fig. 2. FTIR spectra of NaY zeolite, Mn(III)tBu4Salen and Mn(III)tBu4Salen-Y catalysts.
[18]. It means that the above effect is a synergetic
one.
3.3. Oxidation with neat complexes
In the oxidation reaction acetonitrile was used
as solvent, because this is the best solvent for the
Mn(III)Salen type complexes. However, the complete
dissolution of the complexes is difficult, especially in
the case of the Mn(III)Br2Salen complex. We found
that adding a certain amount of tert-butyl hydroper-
oxide to the reaction mixture before the reaction,
makes easier to dissolve the complex. Since the cata-
lyst and the tert-butyl hydroperoxide form an adduct
which is the active intermediate in the oxidation
reaction [18]. This adduct has a much better solubil-
ity than the original compound, a sufficient amount
of tert-butyl hydroperoxide may dissolve the whole
amount of complex. This is a crucial point, because
if we want to compare the specific catalytic activity
of the homogeneous complexes to the encapsulated
ones, the neat complexes must be completely dis-
solved. Since in the homogeneous circumstances only
those part of the neat complex is active, which is
dissolved.
The major product of the oxidation of styrene is
benzaldehyde together with phenyloxirane (styrene
oxide). Fig. 3 shows the conversion values of styrene
and Table 2 shows the selectivity data based on the
gas chromatographic analysis.
Fig. 3 and Table 2 show that, all of the catalysts
were active in the oxidation of styrene. The shape of
the curves based on the dioxygen consumption data
We have found that 4 mmol tert-butyl hydroper-
oxide seems to be enough to dissolve 0.1 mmol neat
complex used in the reaction. Since the relatively
high amount of tert-butyl hydroperoxide, the reaction
progress was followed both by gas chromatography
and by the oxygen consumption. We also wanted
to check, whether the molecular oxygen is the real
oxidant or not.
Fig. 3. Percent styrene conversion values on different Mn(III)Salen
type catalysts.

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A. Zsigmond et al. / Journal of Molecular Catalysis A: Chemical 171 (2001) 95–102
99
Table 2
The electron-donating tert-butyl groups increased
the initial rate significantly. With a small amount
of tert-butyl hydroperoxide (2 wt.% of styrene) the
bromo-substituent increased the catalytic activity, too
[14]. In this work, however, large amount of tert-butyl
hydroperoxide was used and the bromo-substituent
had a reverse effect. Namely, the substituted deriva-
tive had a lower initial rate than the none substituted
one. The highest initial rate was observed in the case
of the tert-butyl substituents, but the deactivation of
this catalyst was also found. After 24 h the rate on this
catalyst was lower than the rate on Mn(III)Salen. It
was also observed, that the Mn(III)tBu4Salen catalyst
decomposed the tert-butyl hydroperoxide molecules,
meanwhile dioxygen evolved. The observed deactiva-
tion may be attributed to this decomposition.
Product selectivities (%) in oxidation of styrene on different
Mn(III)Salen type catalysts (substrate to catalyst ratio = 43.5)
Catalysts
Products
6 h
12 h 24 h 48 h
Mn(III)Salen
Benzaldehyde 35.5 26.2 18.3 16.0
Phenyloxirane
Benzoic acid
5.2
5.3
4.9
6.0
59.3 68.5 76.6 78.0
Mn(III)tBu4Salen Benzaldehyde 35.0 31.4 26.4 22.6
Phenyloxirane
Benzoic acid
7.9
8.3
7.6
7.1
57.1 60.2 66.0 70.3
Mn(III)Br2Salen
Benzaldehyde 60.3 67.3 70.1 76.5
Phenyloxirane
Benzoic acid
5.5
4.3
3.8
3.8
34.2 28.4 26.1 19.6
3.4. The effect of substituents on the encapsulation
In this work, the intrazeolite ligand synthesis
method was used for the preparation of the encap-
sulated catalysts. This method produced the lowest
complex concentration in the case of Mn(III)Salen-Y
catalyst (Table 1). Both of the two other derivatives
had a higher complex concentration in the zeolite.
Since the highest complex concentration was observed
in the case of Mn(III)tBu4Salen-Y, we believe that the
increase in the size of the complex, to be encapsulated
increases its encapsulation efficiency. This fact seems
to be independent from the electron-withdrawing or
electron-donating nature of the substituents. It seems,
that the increase in size of the complex enhances its
physical entrapping in the zeolite supercage.
Fig. 4. Oxygen consumption during the oxidation of styrene on
different Mn(III)Salen catalysts.
3.5. Oxidation with encapsulated complexes
(Fig. 4) were very similar to the gas chromatographic
curves in Fig. 3, supporting the assumption [18] that,
dioxygen is the real oxidant in this reaction.
The initial specific rate values were calculated from
the experimental data (Table 3).
The encapsulated analogues of the three homoge-
neous Mn(III)Salen complexes were also used in the
oxidation of styrene. According to the literature (e.g.
Table 3
Initial specific rate values (mol styrene converted/mol Mn(III)complex) at different Mn(III)Salen catalysts and the ratio of encapsulated
rate values to homogeneous ones
Catalysts
Initial rates
Catalysts
Initial rates
Ratio
Mn(III)Salen
Mn(III)tBu4Salen
Mn(III)Br2Salen
1.8
4.0
0.6
Mn(III)Salen-Y
Mn(III)tBu4Salen-Y
Mn(III)Br2Salen-Y
7.3
3.9
5.0
4.0
0.98
8.3

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100
[14]) encapsulation generally increases the specific
activity of the catalysts. The amount of the complex
in the encapsulated catalysts used in the catalytic
reactions is usually much lower (in some cases by
two magnitudes) than in the homogeneous reaction.
For the better comparison, in this work the amount of
complexes were almost the same in the homogeneous
and in the heterogeneous case.
Using encapsulated catalysts the product distri-
bution was similar to the homogeneous reactions,
except phenylacetaldehyde was observed on the het-
erogenized catalysts. It is probable, that this product
is formed via ring opening of phenyloxirane on the
acidic sites of the zeolite. It is well known, that ben-
zyl oxiranes are very sensitive to the presence of even
a very small amount of acidic sites.
Fig. 5 shows the conversion values of styrene and
Table 4 shows the selectivity data based on the gas
chromatographic analysis. The initial respective rate
values were also calculated (Table 3).
In this case the Mn(III)Salen-Y was the most active
catalyst, while both substituents decreased the respec-
tive reaction rate in the case of encapsulated catalysts.
Fig. 5. Percent styrene conversion values on different
Mn(III)Salen-Y catalysts.
The encapsulation increased the catalytic activity ac-
cording to the following trend: Mn(III)Br2Salen >
Mn(III)Salen > Mn(III)tBu4Salen. This trend is
opposite to the solubility trend: Mn(III)Br2Salen <
Mn(III)Salen < Mn(III)tBu4Salen. The rate increas-
ing effect of the encapsulation may be attributed to
the fact that the neat complexes are not completely
dispersed molecularly in the reaction mixture, while
the encapsulated ones are molecularly dispersed in the
zeolite. The Mn(III)Br2Salen has the lowest solubility
in the reaction mixture, consequently, this complex
produced the highest rate increase by encapsulation.
In the case of the Mn(III)tBu4Salen, however, which
3.6. The effect of encapsulation on the catalytic
activity
The encapsulation increased the respective cat-
alytic activity in the case of the Mn(III)Salen-Y
and Mn(III)Br2Salen-Y catalysts, while the neat and
encapsulated Mn(III)tBu4Salen complexes had al-
most the same specific catalytic activity (Table 3).
Table 4
Product selectivities (%) in oxidation of styrene on different encapsulated Mn(III)Salen type catalysts
Catalysts
Products
6 h
12 h
24 h
48 h
Mn(III)Salen-Y (substrate to catalyst ratio = 322)
Benzaldehyde
Phenyloxirane
Phenylacetaldehyde
Benzoic acid
Benzaldehyde
Phenyloxirane
Phenylacetaldehyde
Benzoic acid
Benzaldehyde
Phenyloxirane
Phenylacetaldehyde
Benzoic acid
29.7
0.8
1.6
67.9
28.7
1.2
30.4
1.0
2.6
66.0
34.2
3.5
19.0
43.5
30.4
0.5
30.8
1.2
3.6
64.8
38.2
4.6
21.6
35.6
42.9
1.2
30.9
1.3
5.8
62.2
35.2
5.4
16.5
42.8
51.6
1.6
Mn(III)tBu4Salen-Y (substrate to catalyst ratio = 75)
Mn(III)Br2Salen-Y (substrate to catalyst ratio = 217)
6.0
64.1
24.2
0.0
0.8
75.0
1.0
68.1
1.6
54.3
2.6
44.2

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101
has the best solubility, the catalytic activity did not
increased by the encapsulation at all.
3.7. The effect of encapsulation on the catalyst
stability
We have also observed an interesting difference
between the neat and the encapsulated catalysts, con-
cerning their stability. The initial catalytic activity
of the Mn(III)tBu4Salen catalyst is the highest, but
its activity decreases during the reaction and after
24 h the rate is lower than on Mn(III)Salen. Similar
effect, however, was not observed in the case of the
encapsulated catalyst. We assumed earlier, that this
deactivation may be attributed to the decomposition
of the tert-butyl hydroperoxide. In the case of the
encapsulated complexes this decomposition cannot
affect the solubility of the complexes, consequently
has no effect on the reaction rate.
Fig. 6. Production of phenyloxirane on Mn(III)tBu4Salen (ꢀ) or
phenyloxirane + phenylacetaldehyde on Mn(III)tBu4Salen-Y (᭢)
catalysts.
3.8. The effect of encapsulation on the selectivity
The Mn(III)tBu4Salen was the most active complex
for the oxidation of styrene and, of course, this cat-
alyst produced the highest amount of phenyloxirane,
too. On the Mn(III)tBu4Salen-Y catalyst large amount
of phenylacetaldehyde was also observed. It is very
likely that the phenylacetaldeyde is produced by the
acidic cleavage of phenyloxirane. Therefore, the sum
of the phenylacetaldehyde and phenyloxirane was
presented on Fig. 6 in the case of Mn(III)tBu4Salen-Y
catalyst. As can be seen the encapsulation made this
catalyst much more active for the epoxidation.
[14]. Since the amount of catalysts, used in the reac-
tion was almost the same in both works, we believe
that their method produced lower complex concen-
tration in the zeolite, consequently higher specific
rate.
Our experimental data show that the electron-
withdrawing or electron-donating ability of the
substituents can affect the solubility of the neat
complexes; consequently, its catalytic activity. How-
ever, in the case of the encapsulated catalysts the
solubility difference does not have any effect on the
activity of the catalysts. Ratnasamy and co-workers
[14] have also observed a significant rate increase in
the case of the bromo-substituted derivative in the
homogeneous condition, while in the case of the en-
capsulated catalyst only a small effect was observed.
3.9. The effect of substituents on the catalytic activity
We found that in the case of the neat complexes the
activity trend agrees well with the solubility trend.
So, we believe that the substituents influence mainly
the solubility, consequently the catalytic activity.
In the case of encapsulated catalysts the
activity trend: Mn(III)Salen > Mn(III)Br2Salen >
Mn(III)tBu4Salen is opposite to the concentration of
the complexes in the zeolite. Comparing the hetero-
genized and the homogeneous catalysts, the rate in-
crease (four to seven-fold) is much smaller than it was
observed by Ratnasamy and co-workers (20–35-fold)
4. Conclusions
1. Both the neat and encapsulated Mn(III)Salen com-
plexes were active in the oxidation of styrene

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102
at room temperature using 1 atm of molecular
Acknowledgements
oxygen, as oxidant and tert-butyl hydroperoxide,
as initiator.
2. Dioxygen is the real oxidant in this reaction, in
spite of the relatively large amount of tert-butyl
hydroperoxide used.
Financial support from the Hungarian National Re-
search Fund (OTKA, project T O26382) and from
US-Hungarian Joint Fund through grant No. 669 is
gratefully acknowledged.
3. The higher amount of tert-butyl hydroperoxide
(4–1 mmol complex) can promote the reaction by
increasing the solubility of the complexes.
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Br2Salen < Mn(III)Salen < Mn(III)tBu4Salen,
while for the encapsulated catalysts the trend
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the zeolites.
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