Synthesis and characterization of NaY zeolite-encapsulated Mn-hydrazone Schiff base: An efficient and reusable catalyst for oxidation of olefins

Article abstract of DOI:10.1080/00958972.2012.731595

The Mn-hydrazone Schiff base has been prepared, characterized, and encapsulated into NaY to prepare a new heterogeneous catalyst. Elemental analysis, UV-Vis, infrared spectroscopic analysis, diffuse reflectance spectroscopy, thermal analysis, small angle X-ray diffraction, and N₂ sorption indicate the presence of Mn-hydrazone Schiff base within the nanocavity pores of zeolite-Y. The catalysts showed excellent catalytic efficiency in epoxidation with various olefinic compounds including cyclooctene, using tert-BuOOH as oxidant. Cyclooctene showed high conversion (97%) as well as epoxide selectivity (89%) with tert-BuOOH. Moreover, the encapsulated complex showed good recoverability without significant loss of activity and selectivity within successive runs.

Full text of DOI:10.1080/00958972.2012.731595

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Synthesis and characterization of NaY
zeolite-encapsulated Mn-hydrazone
Schiff base: an efficient and reusable
catalyst for oxidation of olefins
Mojtaba Bagherzadeh a & Maryam Zare a
a Chemistry Department , Sharif University of Technology , P.O.
Box 11155-3615, Tehran , Iran
Accepted author version posted online: 18 Sep 2012.Published
online: 09 Oct 2012.
To cite this article: Mojtaba Bagherzadeh & Maryam Zare (2012) Synthesis and characterization
of NaY zeolite-encapsulated Mn-hydrazone Schiff base: an efficient and reusable catalyst
for oxidation of olefins, Journal of Coordination Chemistry, 65:22, 4054-4066, DOI:
10.1080/00958972.2012.731595
To link to this article: http://dx.doi.org/10.1080/00958972.2012.731595
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Journal of Coordination Chemistry
Vol. 65, No. 22, 20 November 2012, 4054–4066
Synthesis and characterization of NaY zeolite-encapsulated
Mn-hydrazone Schiff base: an efficient and reusable
catalyst for oxidation of olefins
MOJTABA BAGHERZADEH* and MARYAM ZARE
Chemistry Department, Sharif University of Technology,
P.O. Box 11155-3615, Tehran, Iran
(Received 6 June 2012; in final form 27 August 2012)
The Mn-hydrazone Schiff base has been prepared, characterized, and encapsulated into NaY
to prepare a new heterogeneous catalyst. Elemental analysis, UV-Vis, infrared spectroscopic
analysis, diffuse reflectance spectroscopy, thermal analysis, small angle X-ray diffraction, and
N2 sorption indicate the presence of Mn-hydrazone Schiff base within the nanocavity pores of
zeolite-Y. The catalysts showed excellent catalytic efficiency in epoxidation with various olefinic
compounds including cyclooctene, using tert-BuOOH as oxidant. Cyclooctene showed high
conversion (97%) as well as epoxide selectivity (89%) with tert-BuOOH. Moreover,
the encapsulated complex showed good recoverability without significant loss of activity and
selectivity within successive runs.
Keywords: Hydrazone Schiff base; Zeolite; Epoxidation; tert-Butyl hydroperoxide
1. Introduction
Heterogenization of homogeneous catalysts on supports through non-covalent immo-
bilization in zeolite [1–4], tethering of a ligand to support, covalent grafting on to
inorganic supports such as silica and MCM-41 [5–11], and co-polymerization of a
functionalized Salen monomer into an organic polymer [12–17] provide easy separation
of products from reaction medium, thermal stability, recovery, recycling of the
catalysts, etc.
Zeolites are excellent candidates for encapsulation of homogeneous catalysts in that
the metal complex can be physically trapped in nanopores of the zeolite and not bound
to the oxide surface. Synthesis of zeolite-entrapped metal complexes has followed three
approaches: template synthesis [18, 19], a zeolite synthesis method [20–23], and a
flexible-ligand method [24–26].
Schiff-base complexes play important roles in biological, clinical, analytical, and
industrial areas in addition to important roles in catalysis and organic synthesis [27–31].
Their properties depend on the metal ion and nature of the ligand. Hydrazone Schiff-
base metal complexes have biological and pharmacological applications [32–34].
*Corresponding author. Email: bagherzadeh@sharif.edu
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.731595

Zeolite-encapsulated Mn-hydrazone
4055
Hydrazones have been shown to possess antimicrobial, anticonvulsant, analgesic, anti-
inflammatory, and antitumor properties [35–37]. Due to tautomerism in hydrazones,
the amide oxygen can be in neutral keto or enolic forms and can coordinate in neutral,
mono-anionic, di-anionic, or tetra-anionic forms to metal ions depending on the metal
ion concentration, the pH, and the nature of the hydrazones [38–40].
Here, we describe the synthesis and characterization of neat and zeolite-
Y-encapsulated Mn(II) complexes and test the catalytic activities of these complexes
on epoxidation reactions in different media using tert-butyl-hydroperoxide as oxidant.
The heterogenized catalyst has comparable activity to the corresponding homoge-
neous one. The lifetime of the heterogenized catalyst has been examined by repeated use
of the complex leading to similar yields of cyclooctene oxide without noticeable loss
of metal.
2. Experimental
2.1. Materials and methods
All reagents were supplied by Merck and employed without purification. NaY zeolite
with Si : Al ratio of 2.53 was purchased from Aldrich. Hydrazone (H2sal-bhz) was
synthesized according to published procedures [41]. FT-IR spectra were recorded on a
Unicam Matson 1000 FT-IR paragon 1000 spectrophotometer. 1H and 13C NMR
spectra of the ligand and products of epoxidation were recorded on a Bruker FT-NMR
500 MHz spectrometer. Electronic spectra of the neat complex in the UV-Vis region
were recorded in methanol using a Shimadzu UV-Vis scanning spectrophotometer.
Diffuse reflectance spectra (DRS) were taken on a Shimadzu UV/3101 PC spectro-
photometer from 1500 to 200 nm using MgO as reference. The loading of complex
encapsulated in zeolite was determined by atomic absorption spectroscopy (AAS)
recorded on a Perkin-Elmer AA-300 spectrophotometer. X-ray diffraction (XRD)
patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered
Cu-Ka radiation. Thermogravimetric analysis (TGA) was carried out using a Perkin-
Elmer TGA-7 instrument at a heating rate of 10ꢀC minꢁ1 under air. Nitrogen
adsorption measurements were performed at 77 K using a Coulter Omnisorb 100CX
instrument. The samples were degassed at 150ꢀC until a vacuum better than 10ꢁ3 Pa
was obtained. Gas chromatographic (GC) analyses were performed on an Agilent
Technologies 6890N, 19 019J-413 HP-5, 5% phenyl methyl siloxane, capillary
60.0 m ꢂ 250 mm ꢂ 1.00 mm.
2.2. Sample preparation
2.2.1. Preparation of Mn-hydrazone Schiff base, MnII(Hsal-bhz)2. The ligand was
prepared in a manner similar to reported procedures [41]. A solution of salicylaldehyde
(1 mmol) was added dropwise to a stirred solution of benzhydrazide (2 mmol) in
methanol (10 mL) and the mixture was refluxed for 2 h. The solvent was removed under
vacuum to produce yellow crystals, which were washed with 5 mL of cooled methanol
and then dried in air (97% yield).

4056
M. Bagherzadeh and M. Zare
To prepare the complex, 20 mL of a methanolic solution of H2sal-bhz (2 mmol) was
added dropwise to a methanolic solution of Mn(CH3COO)2 ꢃ 4H2O (1 mmol) at room
temperature. A yellow precipitate formed immediately upon addition and stirring was
continued for 30 min, after which the yellow solid was filtered, washed with methanol,
and dried under vacuum (yield: 93%). Anal. Calcd for C28H22N4O4Mn (%): C, 62.98;
H, 4.12; N, 10.49. Found (%): C, 62.95; H, 3.93; N, 10.47. IR (KBr): ꢀ(C¼N),
1614 cmꢁ1. UV-Vis (CH3OH): ꢁmax: 240, 297, 395 nm.
2.2.2. Preparation of the metal-exchanged zeolite (Mn(II)-Y). Mn(II)@NaY was
prepared by ion-exchange. Typically, 1.1 mmol of Mn(CH3COO)2 salt was first
dissolved in 150 mL deionized water. Then, 1 g NaY was added to the solution and
further stirred for 24 h at ambient temperature. Then, the solid fraction was filtered,
twice washed with deionized water, and dried at 120ꢀC for 12 h [42].
2.2.3. Preparation
of
zeolite-encapsulated
Mn(II)
complex,
MnII(Hsal-
bhz)2/Y. Mn(II)@NaY was mixed with an excess of H2sal-bhz and the reaction
mixture was refluxed for 24 h on an oil bath with stirring. Uncomplexed ligand and the
complex adsorbed on the exterior surface were removed by full Soxhlet extraction with
DMF and acetonitrile. The unreacted Mn(II) present in the zeolite was removed by
stirring with aqueous 0.1 mol Lꢁ1 NaCl solution (200 mL) for 8 h. The resulting solid
was washed with deionized water until no Clꢁ could be detected with AgNO3 aqueous
solution. Finally, it was dried at 120ꢀC in an air oven for several hours.
2.3. Catalytic reaction
Catalytic reactions were carried out at 75ꢀC in a 25 mL glass reactor. In a typical
procedure, to a solution of cyclooctene (1 mmol), either MnII(Hsal-bhz)2 (0.001 mmol)
or MnII(Hsal-bhz)2/Y (0.001 mmol, 0.005 g) and chlorobenzene (1 mmol) as internal
standard in acetonitrile (1 mL) was added 1 mmol tert-butyl hydroperoxide as oxidant.
The reaction was followed by GC and results are summarized in table 1; assignments of
products were made by comparison with authentic samples. All the reactions were run
at least in duplicate.
3. Results and discussion
3.1. Synthesis and characterization
The neat complex was prepared in good yield by condensation of H2sal-bhz with metal
salt. It was slightly soluble in common solvents such as ethanol, acetone, acetonitrile,
and chloroform, but soluble in DMF and DMSO. Elemental analysis confirmed that
two molecules of hydrazone are chelated to metal as mono-anionic ligand with
carbonyl-oxygen, azomethine-nitrogen and phenolic-oxygen as ONO, coordination
forming non-electrolytic octahedral metal complexes (scheme 1).
The encapsulation was achieved by a flexible ligand method in which ligand enters
slowly into the cavity through the zeolite pores due to its flexible nature and reacts with

Zeolite-encapsulated Mn-hydrazone
4057
Table 1. Optimized reaction condition in oxidation of cyclooctene with t-BuOOH catalyzed by neat and
encapsulated complexes.a
Catalyst
(mmol)
Temperature
(ꢀC)
Conversionc
(%)d
Selectivity to
epoxidec (%)d
Turnover
numberb, c ( )d
Run
Solvent
1
2
3
4
5
6
7
8
9
0.005
0.005
0.005
0.005
0.005
0.01
0.001
0.0005
0.005
0.005
0.005
0.005
CH3OH
DMF
CH2CL2
DCE
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
75
75
75
75
75
75
75
75
65
55
40
R.T
58 (43)
33 (32)
76 (69)
86 (85)
95 (98)
97 (98)
91 (89)
88 (88)
71 (65)
69 (57)
34 (21)
20 (18)
88 (87)
93 (93)
80 (89)
89 (75)
87 (87)
88 (87)
89 (90)
89 (89)
89 (89)
90 (90)
98 (98)
98 (98)
116 (86)
66 (64)
152 (138)
172 (170)
182 (196)
97 (98)
910 (890)
1760 (1760)
142 (130)
138 (114)
68 (42)
10
11
12
40 (36)
aReaction condition: 1 mmol cyclooctene, 1 mmol t-BuOOH, time: 14 h.
bTurnover number is moles of products per mole catalyst. GC conversion based on starting cyclooctene.
cNeat complex as catalyst.
dEncapsulated complex as catalyst.
Scheme 1. Preparation of neat and encapsulated Mn-hydrazone Schiff-base complexes.
previously exchanged metal ions. The complexation of Mn(II) with H2sal-bhz was
accompanied by a color change from light pink of exchanged zeolite to pale yellow.
Soxhlet extraction using DMF and acetonitrile purified the impure materials. The
remaining uncomplexed metal ions in the zeolite were removed by exchanging back
with aqueous 0.1 mol Lꢁ1 NaCl solution (scheme 1). The analytical analysis of solid
catalyst sample showed that it contained 2.72 wt% of manganese, which is similar to
that estimated by atomic absorption. Analytical and physical data of the MnII(Hsal-
bhz)2 complex and isolated zeolite-encapsulated complex as well as the parent NaY
zeolite are listed in table 2. To characterize the resulting materials and to assess the
efficiency of the encapsulation process, the parent zeolite, neat complex, and sample of

4058
M. Bagherzadeh and M. Zare
Table 2. Physical and analytical data.
C
(%)
H
(%)
N
(%)
C/N
(%)
Mn
(%)
SBET
(m2 gꢁ1
VP
(cm3 gꢁ1
)
Compound
Color
)
MnII(Hsal-bhz)2
Zeolite
Yellow
White
Pale yellow
62.98
–
16.93
4.12
–
1.11
10.49
–
2.76
6.00
–
6.13
10.11
–
–
–
438.16
356.36
0.1765
0.1514
MnII(Hsal-bhz)2/Y
2.72 (2.68)a
aDetermined by ICP.
MnII(Hsal-bhz)2/Y were studied by several techniques and the obtained results
compared.
The textural properties (surface area (SBET), pore volume (VP)) of the encapsulated
complex were determined from N2 absorption carried out at 77 K (table 2). A noticeable
reduction in surface area, pore volume, and adsorption capacity of the zeolite was
observed upon encapsulation of metal complexes in the supercages of zeolite-Y,
indicating the presence of the MnII(Hsal-bhz)2 complex within the zeolite cages and not
on the external surface.
FT-IR spectra of zeolite, Hsal-bhz, MnII(Hsal-bhz)2, and zeolite-encapsulated
complex are presented in the ‘‘Supplementary material’’ section. The IR spectra of
the complex show a significant change in some bands from free Schiff base. For
example the free ligand exhibits ꢀ(C¼N) at 1673 cmꢁ1. In the complex, this band shifts
to 1614 cmꢁ1, indicating coordination of azomethine nitrogen. The IR spectrum of the
hydrazone contains a strong C–O absorption at 1621 cmꢁ1 and an N–H absorption at
3005 and 3050 cmꢁ1. Both of these bands appear on complexation, indicating that the
hydrazone has not undergone deprotonation on complexation. The appearance of two
or three bands between 420 and 540 cmꢁ1 indicates coordination of phenolic oxygen in
addition to azomethine nitrogen. The presence of several bands of medium intensity at
2700–2950 cmꢁ1 indicates C–H of the tridentate Schiff-base ligand. The ꢀ O–H as a
medium band at 3270 cmꢁ1 disappears in spectra of the neat complex. These data give
evidence for coordination of H2sal-bhz to Mn(II) via two nitrogen atoms and one
oxygen atom. The intensities of the peaks of the encapsulated complex are weak due to
low concentration of the complex in zeolite. Spectra of encapsulated complex and neat
complex show similar bands, indicating encapsulating of the complex in NaY zeolite.
The zeolite-Y-encapsulated metal complex is confirmed by UV-Vis spectra shown in
figure 1. The complex shows bands at ca 230, 283, 296, 325 corresponding to the ligand
and a band at 396 nm having a molar extinction coefficient of 9277 (mol L )
ꢁ1 ꢁ1 cmꢁ1
that can be attributed to ligand-to-metal charge transfer originating from the
azomethine [43]. For the encapsulated complex, bands corresponding to the intraligand
ꢂ–ꢂ*, n–ꢂ* transitions and ligand-to-metal charge transfer are visible at 230, 283, 296,
325, and 396, respectively.
The powder XRD patterns of Na-Y and encapsulated metal complex were recorded
at 2ꢃ values between 4ꢀ and 70ꢀ. The XRD pattern of zeolite-encapsulated metal
complex along with Na-Y is presented in the ‘‘Supplementary material’’ section. The
diffraction patterns of MnII(Hsal-bhz)2 and Na-Y are similar except slight changes in
the intensity of the bands in the encapsulated complex. These observations indicate that
the framework of the zeolite has not undergone significant structural change during
encapsulation. This is expected as the flexible ligand enters slowly through pores of the

Zeolite-encapsulated Mn-hydrazone
4059
Figure 1. UV-Vis spectra of the samples: (a) NaY, (b) encapsulated complex, (c) neat complex, and
(d) hydrazone ligand.
zeolite and fits nicely in the cavity upon coordination with the metal ions. Our data
show the relative peak intensities of the (220) and (311) reflections in the XRD pattern
vary upon inserting the complexes. I220 at 2ꢃ ¼ 10ꢀ is greater than I311 at 12ꢀ for NaY,
but I220 is lower than I311 for zeolite-encapsulated metal complex, clear evidence for
formation of a complex within the supercage of zeolite [44, 45].
The TG patterns of MnII(Hsal-bhz)2 and MnII(Hsal-bhz)2/Y are presented in the
‘‘Supplementary material’’ section. The thermograms of the neat complex indicate
decomposition in two steps. The first at 300–350ꢀC and the second step between 400 and
470ꢀC are associated with decomposition of the complex. There is no peak at 100–120ꢀC,
indicating the absence of water or solvent in the structure. The encapsulated complex
exhibits an exothermic weight loss of ca 20% attributed to removal of intra-zeolite water
up to 150ꢀC. Weight loss of ca 2.7% extends to 560ꢀC, indicating that thermal stability is
greatly enhanced. This very small percentage of weight loss is in agreement with small
amounts of metal content obtained for encapsulated complex, as estimated by AAS.
3.2. Catalytic oxidation of cyclooctene with t-BuOOH
Catalysis of the neat and encapsulated complexes was studied in oxidation
of cyclooctene, a model substrate, with t-BuOOH as the oxygen donor (table 1).
The results show a similarity between the catalytic activity of the encapsulated complex
and that of the dissolved complex in solution. Control experiments show that zeolite-Y

4060
M. Bagherzadeh and M. Zare
Table 3. Recycling studies of catalyst in oxidation of cyclooctene with t-BuOOH.a
Run
Conversion (%)b
Selectivity to epoxide
Leaching of Mn (%)c
1
2
3
4
5
98
97
91
91
90
87
90
91
88
91
0.039
0.004
0.007
–
0.004
aReaction condition: The molar ratio for catalyst: cyclooctene: t-BuOOH are 1 : 200 : 200.
The reactions were run for 14 h at 75ꢀC temperature in 1 mL acetonitrile.
bGC conversion based on starting cyclooctene.
cDetermined by AAS.
is inactive, under the operating conditions. Also, use of metal/NaY zeolite showed
lower conversion than MnII(Hsal-bhz)2/Y, confirming that the Schiff-base complex
played a role. The effect of various reaction media on the oxidation of cyclooctene has
been studied. Efficiency of the catalysts in different solvents decreases in the order,
acetonitrile 4 dichloroethane 4 dichloromethane 4 methanol 4 DMF. The effect of
catalyst amount on the conversion and selectivity is illustrated in table 1. Four different
amounts of catalyst, namely 0.01, 0.005, 0.001, 0.0005 mmol, were examined while
keeping a fixed amount of cyclooctene (1 mmol) and t-BuOOH (1 mmol). Increasing of
catalyst from 0.005 to 0.01 mmol does not have a significant effect on the percent of
conversion under the reaction conditions (table 1, entries 5, 6). With a further decrease
in the catalyst amount from 0.005 to 0.0005 mmol, the conversion shows a small
reduction from 98 to 88 (table 1, entries 5, 8). The effect of reaction temperature on
cyclooctene oxidation was also investigated. As expected, conversion of cyclooctene
increased with increasing reaction temperature to 75ꢀC for neat and encapsulated
complexes, while low conversion was detected when the reaction temperature was below
40ꢀC (table 1, entries 5, 9–12).
The major advantages of heterogeneous catalysis over its homogeneous counterpart
are ease of the recovering and reusability of the catalyst. The stability and recyclability
of the catalyst were examined by using it in several reaction cycles. Before reuse, the
solid was separated from the reaction medium by filtration, washed several times with
diethyl ether, dried, and subjected to catalytic reactions under similar conditions as in
the first cycle. The heterogenized catalyst could be reused several times with a
conversion ranging from 98% at the beginning to 90% in the fifth run, with unchanged
selectivity (table 3).
This was also supported by the AAS analysis and by using UV-Vis in the reaction
solution after filtration of the encapsulated complex that a trace amount of Mn
complex was observed in the reaction liquid, indicating slight leaching of the complex
from the interior of the zeolite host to the solution. To verify further that the observed
catalysis was truly heterogeneous, the solid was filtered from the reaction mixture and
the fresh alkene and oxidant was added to the reaction mixture. The conversion of
oxidation is comparable to the blank experiment, in accord with the leaching data.
Moreover, heterogeneity can be confirmed by filtering the catalysts at the reaction
temperature before completion of the reaction and performing the AAS analysis of the
hot filtration. The results show the amounts of leached Mn are 0.025, 0.030, 0.039, and
0.039 wt% at 2, 6, 10, and 14 h, respectively. Thus the observed catalysis is truly
heterogeneous in nature.

Zeolite-encapsulated Mn-hydrazone
4061
Table 4. Catalytic oxidation of various alkenes with t-BuOOH by neat and encapsulated catalysts.a
Catalyst
Substrate
Conversion (%)b Product Product selectivity (%)b
MnII(Hsal-bhz)2
97
Epoxide
32
54
8
Benzaldehyde
Benzoic acid
Other
6
95
92
Epoxide
Other
87
12
Ketone
Other
91
9
56
98
Epoxide
Alcohol
Ketone
20
36
41
3
33
28
35
4
Other
MnII(Hsal-bhz)2/Y
Epoxide
Benzaldehyde
Benzoic acid
Other
98
Epoxide
Other
87
13
94
57
Ketone
Other
93
7
Epoxide
Alcohol
Ketone
Other
21
35
40
4
aReaction condition: The molar ratio for catalyst: alkene: t-BuOOH are 1 : 200 : 200. The reactions were run for 14 h at 75ꢀC
in 1 mL acetonitrile.
bGC conversion based on starting alkene.
Manganese(V)-oxo, manganese(IV)-oxo, and/or radical species have been known as
reactive intermediates in reactions mediated by compounds of manganese [46–52].
Formation of oxomanganese(V) species in the Mn(III)Salen catalysis of cyclohexene
oxidation with iodosylbenzene (PhIO) was reported by Kochi et al. [46]. In this system
cyclohexene is oxygenated to cyclohexene oxide with high selectivity. A typical radical
reaction has been demonstrated by Busch [47] in which two distinct reactive
intermediates, the alkyl peroxy radical, ROO. , and the Mn(IV)/alkyl peroxo adduct,

4062
M. Bagherzadeh and M. Zare
Mn(III)-OH
Mn(II) + t-BuOO
+
t-BuO
t-BuOOH
+ Mn(II)
Mn(III)-OH + t-BuOOH
+
H2O
t-BuOH
+
t-BuO
+
O2-t-Bu
+
t-BuOO
decomposition
O2-t-Bu
t-BuOH +
O
Scheme 2. Proposed mechanism for oxidation of cyclohexene with Mn-hydrazone Schiff-base complex in
the presence of t-BuOOH.
Mn(IV)(Me2-EBC)(O)(OOR)þ, serve as epoxidizing reagents in Mn(Me2-EBC)Cl2-
mediated oxygen-transfer reaction using alkyl hydroperoxide as the terminal oxidant.
Epoxidation of cyclohexene provided a 32.4% yield of cyclohexen-1-one and a 1.4%
yield of cyclohexene oxide as the minor product. In the cyclohexene oxidation with
t-butyl hydroperoxide-pyridine catalyzed by Mn(III)Salen complexes, Kochi et al.
suggested dual pathways leading to allylic substitution (cyclohexenyl t-butyl peroxide)
and olefin oxidation (cyclohexene oxide) [48]. Allylic peroxidation corresponded to a
hemolytic chain process in which free radicals and one-electron redox changes of
MnSalen played important roles. Olefin epoxidation on the other hand represented the
two-electron interconversion of manganese(III) and oxomanganese(V) species.
In the present catalytic system, the epoxidation of different substrates has been
examined and the results are given in table 4. The cyclohexene oxidation in table 4
shows excellent conversion of 94% to form 2-cyclohexen-1-one as the major product
with 93% selectivity. These results strongly suggest a radical pathway involving alkoxy
(t-BuO. ) or alkylperoxy (t-BuOO. ) radicals which are produced from redox reactions
(Haber-Weiss process, Mn(II)/Mn(III)) as expected on the basis of earlier literature
reports [49, 53]. The oxidation of cyclohexene in the presence of MnII(Hsal-bhz)2 with
tert-butyl hydroperoxide is proposed to occur as in scheme 2.
Epoxidation of styrene with tert-BuOOH gives styrene oxide, benzaldehyde, 1-
phenylethane-1,2-diol, and benzoic acid. The selectivity of various reaction products in
the homogeneous system follows the order, benzaldehyde 4 styrene oxide 4 benzoic
acid, whereas selectivity of the reaction products in the presence of encapsulated
complex varies in the order, benzoic acid 4 styrene oxide 4 benzaldehyde. The
conversion of benzaldehyde to benzoic acid was probably catalyzed by the Bronsted
acid sites originating from Si–OH–Al in the zeolite framework.
The catalytic potential of the encapsulated complex presented here compares well
with similar encapsulated complexes (table 5). For example, [Mn(H4C6N6S2)]-Y
exhibited as high as 90.3% conversion of cyclohexene with tert-BuOOH, where no

Zeolite-encapsulated Mn-hydrazone
4063
Table 5. Comparison of the results obtained for oxidation of olefins catalyzed by MnII(Hsal-bhz)2/Y with
some of those reported in the literature.
Catalyst
Oxidant
Substrate
Conversion (%)
Ref.
MnII(Hsal-bhz)2/Y
t-BuOOH
Cyclohexene
Cyclooctene
Cyclohexene
Cyclohexene
Cyclooctene
Cyclohexene
Cyclooctene
Cyclohexene
Cyclooctene
94
98
90.3
8
6.8
25
7
This work
[Mn(H4C6N6S2)]-Y
Mn(salen)þ-Y
t-BuOOH
KHSO3
[54]
[55]
Mn(salen)þ-clay
t-BuOOH
[50]
[Mn(sal-1,3-phen)]-NaY
[Ni(L)]@Mont
t-BuOOH
t-BuOOH
84.6
69
[56]
[57]
Table 6. Oxidation of olefins catalyzed by neat and encapsulated complexes at the presence of ionol
(2,6-di-tert-butyl-p-cresol) as radical scavenger.a
Catalyst
Substrate
Cyclooctene
Styrene
Conversion (%)b
49 (495)c
Product
Epoxide
Alcohol
Yield to product (%)
MnII(Hsal-bhz)2
2
–
–
1
–
–
–
–
–
–
4 (497)c
Epoxide
Benzaldehyde
Benzoic acid
Epoxide
Alcohol
Epoxide
MnII(Hsal-bhz)2/Y
Cyclooctene
Styrene
46 (98)c
4 (498)c
Benzaldehyde
Benzoic acid
aReaction condition: The molar ratio for catalyst: alkene: TBHP: ionol are 1 : 200 : 200 : 200. The reactions were run for 14 h
at 75ꢀC in 1 mL acetonitrile.
bGC conversion based on starting alkene.
cGC conversion in the absence of the radical scavenger.
formation of cyclohexene epoxide was observed and the selectivity of the other
oxidation products varied in the order, 2-cyclohexene-1-one (87.5%) 4 2-cyclohexene-
1-ol (11.5%) 4 1-(tert-butylperoxy)-2-cyclohexene (1%) [54]. Cyclohexene and cyclooc-
tene oxidation catalyzed by the Mn(salen)þ-Y proceeded with 8% and 6.8% conversion
and 100% selectivity toward the corresponding epoxides, respectively, with KHSO3
[55]; catalytic oxidation of cyclohexane by Mn(salen)þ-clay with tert-BuOOH in the
presence of imidazole was 25% with selectivity of products, 1-(tert-butylperoxy)-
2-cyclohexene (14%) 4 cyclohexene epoxide (6.8%) 4 cyclohexenone(1.2%) 4 cyclo-
hexenol (1%). Cyclooctene was oxidized by TBHP and only 7% conversion was
obtained with selectivity to cyclooctene epoxide [50]. Cyclohexene was oxidized by
tert-butylhydroperoxide using similar catalyst [Mn(sal-1,3-phen)]-NaY with 84.6%
conversion obtained with selectivity of products, 2-cyclohexene-1-one (84.2%) 4 2-
cyclohexene-1-ol (14.7%) 4 1-(tert-butylperoxy)-2-cyclohexene (1.1%) [56]. Oxidation
of cyclooctene with tert-butylhydroperoxide in the presence of [Ni(L)]@Mont catalyst
proceeds with 62.3% selectivity for epoxidation with 69% conversion [57].
To explore in situ formation of radicals in the oxidation reaction, ionol (2,6-di-
tertbutyl-p-cresol) was used as a radical trap in oxidation of styrene and cyclooctene

4064
M. Bagherzadeh and M. Zare
O2-t-Bu
t-BuOO
O
t-BuO
Scheme 3. Proposed mechanism for oxidation of cyclooctene with Mn-hydrazone Schiff-base complex in the
presence of t-BuOOH.
Figure 2. Spectral changes observed during titration of solution of Mn(II)(Hsal-bhz)2 in dichloroethane
(2.5 ꢂ 10ꢁ3 mol Lꢁ1) with t-BuOOH dissolved in dichloroethane.
with t-BuOOH catalyzed by MnII(Hsal-bhz)2 and MnII(Hsal-bhz)2/Y. The results are
shown in table 6. Oxidation of styrene is strongly inhibited in the presence of ionol. The
effective inhibition of oxidation by ionol is achieved by interception of the radical
intermediate. In the case of cyclooctene, substrate is consumed, but no oxidation
products can be detected. The findings with cyclooctene reveal that cyclooctene oxide
formation may occur via a free-radical pathway, shown in scheme 3 [50].
To understand the mechanism of the catalytic reaction, we have studied the
interaction of MnII(Hsal-bhz)2 with t-BuOOH in dichloroethane by electronic
absorption spectroscopy (figure 2). The addition of one-drop portions of t-BuOOH
dissolved in dichloroethane to 2 mL of ca 2.5 ꢂ 10ꢁ3 1 mol Lꢁ1 solution of MnII(Hsal-
bhz)2 in dichloroethane causes a band and shoulder to appear at 450 and 380 nm. This
change in spectra may correspond to formation of [MnIII(Hsal-bhz)2]þ [58]. In UV-Vis
spectrophotometer, no new absorption band appeared at 550 nm, showing the existence
of Mn(V) ¼ O species [46]. So the reaction does not proceed via the oxometal route.
4. Conclusion
We have synthesized Mn-hydrazone Schiff base and zeolite-encapsulated
Mn-hydrazone Schiff base. The nature of the encapsulated manganese complex in the
supercages of zeolite-Y was confirmed with various characterization techniques such as
FT-IR, DRS, UV–Vis, and TGA. The results showed that the homogeneous and
heterogeneous complexes are active and highly selective catalysts for cyclooctene
epoxidation with the oxidizing agent t-BuOOH in CH3CN. The oxidation of styrene

Zeolite-encapsulated Mn-hydrazone
4065
gives at least four different products, benzaldehyde, styrene oxide, benzoic acid, and 1-
phenylethane-1,2-diol. The reaction of cyclohexene with tert-butyl hydroperoxide in the
presence of both catalysts gives 2-cyclohexene-1-one formed by the decomposition of
cyclohexenyl-tert-butyl peroxide. The reaction products suggest the catalytic reactions
proceed through a radical mechanism in which t-BuOO. and t-BuO. are the reactive
intermediates. Addition of ionol (2,6-di-tertbutyl-p-cresol) as a radical-scavenging
species quenched formation of oxidation product, confirming the proposed radical
mechanism. The encapsulated catalyst was reusable for epoxidation of cyclooctene with
t-BuOOH with a conversion ranging from 98% at the beginning to 90% in the fifth run.
Acknowledgments
Financial support of this work by the Research Council of Sharif University of
Technology is greatly appreciated.
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