Catalytic activity of carbon nanotube supported iron(III) and manganese(III) porphyrins in oxidation of olefins with tert-butyl hydroperoxide: Higher activity of the iron(III) porphyrin

Article abstract of DOI:10.1016/j.inoche.2012.11.017

Oxidation of olefins with tert-butyl hydroperoxide (TBHP) catalyzed by carbon nanotube supported iron(III) and manganese(III) complexes of meso-tetrakis(4-hydroxyphenyl)porphyrin is reported. The results show higher catalytic activity of the Fe(III) complex compared to the Mn(III) one. With the exception of cyclooctene which gives cyclooctene oxide as the sole product, oxidation of styrene, α-methyl styrene and cyclohexene leads to the formation of benzaldehyde, acetophenone and cyclohexene-1-one as the major product, respectively. In this study, acetonitrile has been found to be the best solvent for reaction performed in the presence of the manganese and iron porphyrin. In comparison of the two metalloporphyrins, catalytic activity of the manganese porphyrin is more sensitive to the type of solvent than the iron one. In spite of the crucial role of imidazole (ImH) on catalytic performance of the manganese porphyrin, the presence of ImH has a negative effect on the activity of the iron porphyrin. The nano-tube supported iron porphyrin may be recovered and reused at least four times without significant loss of the catalytic activity.

Full text of DOI:10.1016/j.inoche.2012.11.017

Inorganic Chemistry Communications 29 (2013) 40–44
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Catalytic activity of carbon nanotube supported iron(III) and manganese(III)
porphyrins in oxidation of olefins with tert-butyl hydroperoxide: Higher activity of
the iron(III) porphyrin
a
b,
Saeed Rayati a, , Parisa Jafarzadeh , Saeed Zakavi
⁎
⁎
a
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxidation of olefins with tert-butyl hydroperoxide (TBHP) catalyzed by carbon nanotube supported iron(III)
and manganese(III) complexes of meso-tetrakis(4-hydroxyphenyl)porphyrin is reported. The results show
higher catalytic activity of the Fe(III) complex compared to the Mn(III) one. With the exception of
cyclooctene which gives cyclooctene oxide as the sole product, oxidation of styrene, α-methyl styrene and
cyclohexene leads to the formation of benzaldehyde, acetophenone and cyclohexene-1-one as the major
product, respectively. In this study, acetonitrile has been found to be the best solvent for reaction performed
in the presence of the manganese and iron porphyrin. In comparison of the two metalloporphyrins, catalytic
activity of the manganese porphyrin is more sensitive to the type of solvent than the iron one. In spite of the
crucial role of imidazole (ImH) on catalytic performance of the manganese porphyrin, the presence of ImH
has a negative effect on the activity of the iron porphyrin. The nano-tube supported iron porphyrin may be
recovered and reused at least four times without significant loss of the catalytic activity.
Received 12 October 2012
Accepted 15 November 2012
Available online 16 December 2012
Keywords:
Porphyrin
Heterogeneous catalyst
Carbon nanotube
Olefin
Oxidation
© 2012 Elsevier B.V. All rights reserved.
Considerable insights into the nature of the enzymatic processes
have been obtained from experimental and theoretical studies on the
synthetic metalloporphyrins as models for cytochrome P450 [1]. The
synthesis of metalloporphyrins resistant enough to oxidation is the
first step in mimicking the oxidation reactions mediated by heme-
enzymes of the cytochrome P450 family [1]. The introduction of bulky
and/or electron-withdrawing substituents at the meso positions of the
synthetic metalloporphyrins has been among the main strategies com-
monly employed with the aim of increasing the stability of the aromatic
heterocycle towards oxidative degradation in reaction condition.
Heterogenizing homogeneous catalysts may also be used to increase
the oxidative stability of metalloporphyrins [2]. The heme center of
cytochrome p450 enzymes may be considered as an immobilized iron
porphyrin attached through the iron to the cysteine residue of the pro-
tein part of the enzyme as the support [3,4]. In addition to the effects of
protein structure and dynamic on the spin state of the metal center, the
oxidative stability of heme is significantly determined by the steric
effects of the protein part of the enzyme [3]. Iron and manganese
porphyrins have been extensively studied as model complexes of cyto-
chrome p450 enzymes [5–7]. The difference between catalytic activity
of porphyrin complexes of metals such as iron, manganese, chromium,
cobalt, osmium and ruthenium in homogeneous condition have been
investigated in some studies [5]. Recently, the catalytic activity of iron
and cobalt porphyrins and phthalocyanines supported on carbon
nanotubes towards oxygen reduction has been studied in a comparative
manner [8]. In the present work, the catalytic activity of a nano-tube
supported iron porphyrin in oxidation of olefins with tert-butyl hydro-
peroxide (TBHP) has been studied and compared with the correspond-
ing manganese counterpart. Also, the influence of different parameters
on the catalytic activity of the iron and manganese porphyrins has been
investigated.
MWNTs containing ~2 mol% carboxylic acid groups were used
for the study (Table 1).
Meso-tetrakis(4-hydroxyphenyl)porphyrin (H2THPP) was prepared
as described by Adler et al. [9,10]. Iron(III) porphyrin (FeTHPPCl) and
manganese(III) porphyrin (MnTHPPOAc) were obtained by metallation
of the H2THPP with ferrous chloride tetrahydrate and manganese acetate
tetrahydrate respectively in DMF with the method described by Buchler
et al. [11,12]. In order to support metalloporphyrins on the multi-wall car-
bon nanotubes, to a mixture of 250 mg of MWCNT-COOH in 25 ml DMF,
FeTHPPCl or MnTHPPOAc (0.3 mmol), 2-(1H-benzotriazole-1-yl)-1,1,3,3-
Table 1
The structural characteristics of MWCNT-COOH used in this study.
Outside
diameter (nm)
Inside
diameter
(nm)
Length
(μm)
COOH content
(wt.%)
Specific surface
area (m2/g)
⁎
Corresponding authors. Tel.: +98 21 22850266; fax: +98 21 22853650.
E-mail addresses: rayati@kntu.ac.ir, srayati@yahoo.com (S. Rayati).
10–20
5–10
~30
2
>200
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.11.017

S. Rayati et al. / Inorganic Chemistry Communications 29 (2013) 40–44
41
PhOH
The covalent bonding between the metalloporphyrins and MWCNT-
COOH was carried out by acid–base reaction between the porphyrin
hydroxyl groups and carboxylic acid of MWCNT-COOH using TBTU as
a highly effective uranium salt in the presence of DIPEA [13]. The
prepared catalyst was characterized by IR spectroscopy and scanning
electron microscopy (SEM). The metal content of the catalyst was deter-
mined by atomic absorption spectroscopy. Accordingly, the metal con-
tent of the catalysts was found to be ca. 821 and 355 μmol per gram of
the catalysts, in the case of the iron and manganese porphyrins res-
pectively. The nitrogen content of the catalysts was determined by
CHN analysis, which showed a value of 2% and 0.8% for the Fe and
Mn-catalyst respectively. Based on these values, the manganese por-
phyrin content of the catalyst was about 357 μmol per gram of the
Mn-catalyst and the iron porphyrin one was ca. 815 μmol per gram of
the Fe-catalyst. These results are in good agreement with those
obtained by atomic absorption spectroscopy. The scanning electron mi-
crograph of MWCNT-COOH (Fig. 2) shows the morphology of MWCNT-
COOH used in this study. The comparison of SEM images of FeTHPPCl-
MWCNT and MnTHPPOAc-MWCNT and the carbon nanotube, indicates
that the metalloporphyrins have been supported on the nanotubes.
The most informative spectroscopic data, confirming the covalent an-
choring of FeTHPPCl and MnTHPPOAc on MWCNT-COOH, were obtained
by comparison of the IR spectra of MWCNT-COOH and FeTHPPCl-
MWCNT (Fig. 3). The C_O stretching band of the carboxylic acid group
of MWCNT-COOH appears at 1642 cm−1. When the metalloporphyrin
was attached to MWCNT-COOH, a strong band was observed at
1714 cm−1 in the case of FeTHPPCl-MWCNT and at 1709 cm−1 in the
case of MnTHPPOAc-MWCNT, respectively which were assigned to the
formed esteric bond between the metalloporphyrins and the nanotubes.
The prepared heterogeneous catalyst was used for the oxidation
of olefins with TBHP at room temperature. The effect of various
OAc
N
N
N
N
M
PhOH
COOH
HOPh
+
PhOH
PhOH
TBTU
DIPEA
OAc
N
O
N
N
M
PhOH
C
O
Ph
N
PhOH
M = Fe(III) or Mn(III)
Fig. 1. Preparation of multi-wall carbon nanotubes supported metalloporphyrins.
tetramethyluronium tetrafluoroborate (TBTU) (90 mg, 0.2 mmol) and N,
N′-diisopropylamine (DIPEA) (70 mg, 0.5 mmol) were added and the
reaction mixture was stirred for 2 days at room temperature [13]. The
dark solid product was then filtered, and washed with DMF to remove
the weakly adsorbed metalloporphyrins. The solid residue was dried at
room temperature (Fig. 1).
Fig. 2. SEM images of MWCNT-COOH (a), FeTHPPCl-MWCNT (b) and MnTCPPOAc-MWCNT (c).

42
S. Rayati et al. / Inorganic Chemistry Communications 29 (2013) 40–44
Table 4
Oxidation of olefins with TBHP catalyzed by FeTHPPCl-MWCNT in acetonitrile at room
temperature.a,b
Entry
Olefine
Conversion (%)
56 (22)c
Selectivity (%)
Epoxide
Others
1
2
3
16 (10)
84 (90)d
CH3
96 (78)
2 (4)
98 (96)e
20 (17)
54 (23)
100 (100)
6 (4)
–
4
81 (74)f
Fig. 3. The IR spectra of: (a) FeTHPPCl-MWCNT and (b) MnTHPPOAc-MWCNT.
a
The molar ratios for catalyst:olefin:oxidant are 1:25:50, respectively.
For a reaction time of 24 h.
The values in the parentheses are due to a 4 h reaction time.
Benzaldehyde is the main product.
Acetophenone is the main product.
2-Cyclohexene-1-one is the major product.
b
c
d
e
f
parameters such as catalyst loading, nature of solvent, type of oxidant
and axial base was investigated to optimize the oxidation conditions.
In reactions catalyzed by metalloporphyrins, the conversion values,
product yields and product distribution are usually influenced by
the type of solvent, especially due to its effects on the nature and
stability of the intermediate species [14]. Methanol, dichloromethane,
chloroform and acetonitrile were chosen as reaction media for oxida-
tion of cyclooctene with TBHP in the presence of FeTHPPCl-MWCNT
and MnTHPPOAc-MWCNT (Table 2) and acetonitrile was found to
be the best one, apparently due to its high dielectric constant.
Since the catalytic activity of Mn-porphyrins usually increases in
the presence of different nitrogenous bases as co-catalyst [15–17],
the effect of imidazole as co-catalyst on the oxidation of cyclooctene
has been studied (Table 3). The presence of imidazole led to a de-
crease in the catalytic activity of the iron catalyst. This observation
may be due to the formation of the inactive six coordinate species
i.e. FeTHPP(ImH)2 [7]. Otherwise, the existence of imidazole as co-
catalyst is essential for oxidation of cyclooctene in the presence of
manganese catalyst.
FeTHPPCl-MWCNT/TBHP and MnTHPPOAc-MWCNT are efficient
heterogeneous catalytic systems for the oxidation of various alkenes
under the optimized reaction conditions (Tables 4 and 5). The sig-
nificant yields of allylic oxidation products obtained in the catalytic
oxidation of cyclohexene, styrene and α-methyl styrene with may
be due to the H-abstraction from the substrate by a high valent iron
or manganese oxo species (see the proposed mechanism).
Higher reactivity of the conjugated double bonds (Table 4, entries 1, 2)
in comparison with the unconjugated ones may be interpreted on the
basis of the π-electron donor ability of phenyl-group to the double
bond. Also α-methyl styrene was found to be more reactive than
styrene towards the allylic oxidation reaction. It leads to the formation
of acetophenone as the product. On the other hand, apparently due to
the steric hindrance of the methyl group, α-methyl styrene is less
reactive than styrene towards epoxidation reaction.
The reaction was also carried out using two other peroxides. 30%
aqueous hydrogen peroxide and urea hydrogen peroxide (UHP)
were used as oxidant; almost no product was observed in the case
of UHP and hydrogen peroxide.
In this catalytic system, the reactivity of cyclohexene, styrene and
α-methyl styrene is higher than that of cyclooctene (Tables 4 and 5).
This finding seems to be due to the involvement of different reaction
mechanisms and active oxidants in the catalytic cycles of the used olefins.
Table 2
Table 5
Solvent effect on the oxidation of cyclooctene with TBHP at room temperature, for a
reaction time of 4 h.
Oxidation of olefins with TBHP catalyzed by MnTHPPOAc-MWCNT in acetonitrile at
room temperature.a,b
Entry
Solvent
Dielectric
constant
Epoxide yield (%)
Epoxide yield (%)
Entry
1
Olefin
Conversion (%)
19 (15)c
Selectivity (%)
Epoxide
FeTHPPCl-MWCNTa
MnTHPPOAc-MWCNTb
Others
1
2
3
4
CH3CN
CHCl3
CH2Cl2
MeOH
37.5
4.8
9.1
33
17
7
8
5
–
–
–
21 (4)
79 (96)d
6
CH3
a
The molar ratios for catalyst:cyclooctene:TBHP are 1:25:50.
The molar ratios for catalyst:ImH:cyclooctene:TBHP are 1:10:50:50.
2
3
38 (24)
13 (16)
87 (84)e
b
10 (5)
100 (100)
12 (9)
–
Table 3
Effect the molar ratio of the catalyst/ImH on oxidation of cyclooctene with TBHP in
acetonitrile at room temperature.a,b
4
33 (11)
70 (64)f
FeTHPPCl-MWCNT/
ImH
Epoxide yield
(%)
MnTHPPOAc-MWCNT/
ImH
Epoxide yield
(%)
a
The molar ratios for catalyst:olefin:oxidant are 1:25:50, respectively.
For a reaction time of 24 h.
The values in the parentheses are due to a 4 h reaction time.
Benzaldehyde is the main product.
Acetophenone is the main product.
2-Cyclohexene-1-one is the major product.
b
c
d
e
f
1:0
1:10
17
4
1:0
1:10
–
5
a
The molar ratios for catalyst:ImH:cyclooctene:oxidant are 1:X:25:50, respectively.
For a reaction time of 4 h.
b

S. Rayati et al. / Inorganic Chemistry Communications 29 (2013) 40–44
43
M III
TBHP
M=Fe, Mn
M
O
O
t-Bu
OH
H
t-Bu
MV
MIV
O
I
O
II
E
n
o
i
p
o
xi
t
a
d
i
d
a
t
i
x
o
o
ic
l
n
y
l
l
A
O
M V
OH
M IV
M III
OH
O
O
M V
O
M III
Fig. 4. Proposed mechanism for oxidation of olefins with TBHP.
The involvement of a high-valent metal–oxo species (Fig. 4, I) may be
considered for oxidation of cyclooctene with TBHP. In the case of styrene,
α-methyl styrene and cyclohexene the formation of allylic products sug-
gests that a radical pathway is involved. In the first step, TBHP coordinates
to the metal center of FeTHPPCl-MWCNT or MnTHPP-MWCNT to form a
high valent metal–oxo species (Fig. 4, I) or an oxo–iron/manganese(IV)
porphyrin π-cation radical (Fig. 4, II) as the active oxidant species. The
active oxidant species may transfer an oxygen atom to the double bond
to form epoxide product or cause the allylic oxidation via the H-
abstraction form the allylic positions of the substrates.
The reusability of a heterogeneous catalyst is of prime importance in
catalyst design. Under homogeneous conditions, FeTHPPCl is readily
degraded but the carbon-nanotube supported iron-porphyrin catalyst
can be filtered and reused several times without detectable catalyst
leaching or a significant loss of the catalytic efficiency (Table 6). The re-
usability of FeTHPPCl-MWCNT was investigated in epoxidation of
cyclooctene with TBHP. The catalyst was separated from the reaction
mixture after each experiment by filtration, washed with acetonitrile
or methanol dried before use. After using the catalyst five consecutive
times, the epoxide yield was 11% (Table 6). Furthermore, the IR
spectrum of the recovered catalyst was prepared and no detectable
change was observed.
The observed increase in the epoxide yield in the 5th run may be
caused by the ability of alcoholic solvent to break the weaker covalent
bonds between the Fe-porphyrin and multi-wall the nanotubes
i.e. slight leaching of the catalyst.
Carbon nano-tube supported iron meso-tetrakis(4-hydroxyphenyl)
porphyrin has been found to show higher catalytic activity in oxidation
of olefins with tert-butyl hydroperoxide compared to the manganese
counterpart. While the oxidation of cyclooctene gives cyclooctene
oxide as the sole product, oxidation of styrene, 2-methylstyrene and
cyclohexene leads to the formation of benzaldehyde, acetophenone and
cyclohexene-1-one as the major product, respectively. Catalytic activity
Table 6
The recovery and reuse of FeTHPPCl-MWCNT in the oxidation of cyclooctene with
TBHP for a reaction time of 4 h.
Run
Rinsing solvent
Epoxide yield (%)
1
2
3
4
5
–
17
13
10
7
CH3CN
CH3CN
CH3CN
MeOH
11
The molar ratios for catalyst:alkene:oxidant are 1:25:50, respectively.

44
S. Rayati et al. / Inorganic Chemistry Communications 29 (2013) 40–44
presence of various manganese porphyrins and nitrogen donors: significant
axial ligand π-bonding effects, New J. Chem. 28 (2004) 740–747.
of the manganese porphyrin is more sensitive to the type of solvent than
the iron one. Although the presence of ImH is necessary for efficient cat-
alytic performance of the manganese porphyrin, the presence of ImH
has a negative effect on the activity of the iron porphyrin. The nano-
tube supported iron porphyrin may be recovered and reused at least
four times without significant loss of the catalytic activity. In this study,
acetonitrile has been found to be the best solvent for reaction performed
in the presence of the manganese and iron porphyrin.
[8] A. Morozan, S. Campidelli, A. Filoramo, B. Jousselme, S. Palacin, Catalytic activity
of cobalt and iron phthalocyanines or porphyrins supported on different carbon
nanotubes towards oxygen reduction reaction, Carbon 49 (2011) 4839–4847.
[9] A.D. Adler, F.R. Longo, W. Shergalis, Mechanistic investigations of porphyrin
syntheses, I. Preliminary studies on ms-tetraphenylporphin, J. Am. Chem. Soc.
86 (1964) 3145–3149.
[10] H2THPP: UV–vis (DMF): λmax (nm): 422, 517, 555, 594, 647. 1H NMR (MeOD,
500 MHz,
δ (ppm)): 8.88 (s, 8H, β-pyrrole), 7.99–8.02 (d, 8, o-phenyl),
7.19–7.22 (d, 8H, m-phenyl).
[11] J.W. Buchler, G. Eikellmann, J. Puppe, K. Rohback, H. Schneehage, D. Weck,
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[12] FeTHPPCl: UV–vis (DMF): λmax (nm): 422, 506. MnTCPPOAc: UV–vis (DMF):
λmax (nm): 477, 577, 609.
Acknowledgments
The financial support of this work by K.N. Toosi University of
Technology research council is acknowledged.
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