Comparative study of the catalytic activity of a series of β-brominated Mn-porphyrins in the oxidation of olefins and organic sulfides: Better catalytic performance of the partially brominated ones

Article abstract of DOI:10.1016/j.poly.2011.12.021

The oxidation of olefins and organic sulfides with tetra-n-butylammonium periodate (TBAP) in the presence of a Mn(III) complex from a series of partially and fully brominated meso-tetraphenylporphyrins, MnTPPBr_x(OAc) (x = 0, 2, 4, 6 and 8) has been studied in a comparative manner. With the exception of MnTPPBr₂(OAc), the half-wave potential for the Mn(III)/Mn(II) metal-centered process was gradually shifted to higher values on going from MnTPP(OAc) to MnTPPBr₈(OAc). However, a complex order has been observed for the catalytic activity of the complexes; while the stability of the Mn-porphyrins towards oxidative degradation under the reaction conditions decreases in the order MnTPPBr₈(OAc) > MnTPP(OAc) > MnTPPBr₆(OAc) > MnTPPBr₂(OAc) > MnTPPBr ₄(OAc), their catalytic activity has been found to follow the order MnTPPBr₈(OAc) ? MnTPP(OAc) ≤ MnTPPBr₆(OAc) < MnTPPBr₂(OAc) ≤ MnTPPBr₄(OAc). The maximum conversion (42%) was obtained in the case of MnTPPBr₄(OAc) and the minimum conversion (14%) is related to MnTPPBr₈(OAc). Oxidation of methyl phenyl sulfide with TBAP in a 1:1 molar ratio leads to the following pattern of catalytic activity for the used Mn-porphyrins: MnTPPBr₄(OAc) > MnTPP(OAc) > MnTPPBr₂(OAc) ≥ MnTPPBr₆(OAc) ? MnTPPBr₈(OAc). The highest methyl phenyl sulfide conversion is 69% at room temperature.

Full text of DOI:10.1016/j.poly.2011.12.021

Polyhedron 34 (2012) 102–107
Contents lists available at SciVerse ScienceDirect
Polyhedron
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Comparative study of the catalytic activity of a series of b-brominated
Mn–porphyrins in the oxidation of olefins and organic sulfides: Better
catalytic performance of the partially brominated ones
Saeed Rayati , Saeed Zakavi b, , Elaheh Bohloulbandi , Majid Jafarian , Mehdi Rashvand avei
a,
⇑
⇑
a
a
a
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, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of olefins and organic sulfides with tetra-n-butylammonium periodate (TBAP) in the pres-
ence of a Mn(III) complex from a series of partially and fully brominated meso-tetraphenylporphyrins,
Received 3 October 2011
Accepted 15 December 2011
Available online 30 December 2011
MnTPPBr
x
(OAc) (x = 0, 2, 4, 6 and 8) has been studied in a comparative manner. With the exception of
(OAc), the half-wave potential for the Mn(III)/Mn(II) metal-centered process was gradually
(OAc). However, a complex order has been
observed for the catalytic activity of the complexes; while the stability of the Mn–porphyrins towards oxi-
dative degradation under the reaction conditions decreases in the order MnTPPBr (OAc) > MnTP-
P(OAc) > MnTPPBr (OAc) > MnTPPBr (OAc) > MnTPPBr (OAc), their catalytic activity has been found to
follow the order MnTPPBr (OAc) < MnTPPBr (OAc) 6 MnTPPBr (OAc).
(OAc) ꢀ MnTPP(OAc) 6 MnTPPBr
The maximum conversion (42%) was obtained in the case of MnTPPBr (OAc) and the minimum conversion
14%) is related to MnTPPBr (OAc). Oxidation of methyl phenyl sulfide with TBAP in a 1:1 molar ratio leads
to the following pattern of catalytic activity for the used Mn–porphyrins: MnTPPBr (OAc) > MnTP-
MnTPPBr
2
shifted to higher values on going from MnTPP(OAc) to MnTPPBr
8
Keywords:
Brominated porphyrins
Sulfide
Olefin
Homogeneous catalyst
Oxidation
8
6
2
4
8
6
2
4
4
(
8
4
P(OAc) > MnTPPBr
conversion is 69% at room temperature.
2
(OAc) P MnTPPBr
6
(OAc) ꢁ MnTPPBr
8
(OAc). The highest methyl phenyl sulfide
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
the energy level of this orbital. In contrast, both the a1u and a2u
orbitals are stabilized by the electron-withdrawing effects of a
substituent at the b positions [7]. In other words, substitution of
electron-withdrawing groups at the b positions may also be used
to enhance the oxidative stability of metalloporphyrins. The
catalytic activity of partially and fully b-brominated analogs of meso-
tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chlo-
Biomimetic oxidation of organic compounds in the presence of
metalloporphyrins has been studied from different points of view
1]. According to the four-orbital model of Gouterman for metal
[
porphyrins, the electron densities of the HOMOs are largest on
the nitrogen and meso-carbon atoms for the a2u orbital and on the
pyrrole carbon atoms in the case of the a1u orbital (Fig. 1) [2]. The
ride for the hydroxylation of cyclohexane with PhIO and PhI(OAc)
2
positions with highest electron densities, i.e. the
a
and meso ones,
has been investigated by Idemori and co-workers in a comparative
study [8]. We have previously reported the oxidation of olefins and
organic sulfides with tetrabutylammonium oxone (TBAO) catalyzed
by Mn(III) complexes of partially brominated meso-tetraphenylpor-
phyrins [9–13]. Very recently we have studied the catalytic perfor-
mance of two partially brominated Mn–porphyrins in comparison
with that of a range of electron-rich and electron-deficient ones
for the oxidation of olefins with tetra-n-butylammonium periodate
(TBAP) and PhIO. The partially brominated Mn–porphyrins were
shown to have higher or lower activities compared to their non-
brominated analogs and other electron-rich Mn–porphyrins
[14,15]. In the present work, the catalytic activity of a series of
meso-tetraphenylporphyrins with 0, 2, 4, 6 and 8 bromine atoms
at the pyrrole rings in the oxidation of olefins and organic sulfides
with TBAP was studied in a comparative manner.
are expected to be involved in the oxidative degradation of metal-
loporphyrins [3]. One of the most common strategies employed
with the aim of reducing the oxidative degradation ofmetallopor-
phyrins during oxidation reactions is the use of electron-deficient
substituents on the aryl groups of meso-tetraarylmetalloporphyrins
[
4,5]. It has been shown previously that electron-donating meso-
substituents will raise the energy of the a2u orbital by increasing
its electron density [6]. Since the meso-carbon atoms lie on the
nodal planes of the a1u orbital [2], introduction of electron-
withdrawing or electron-donating groups at the appropriate posi-
tions of the meso-aryl substituents seems to have little effect on
⇑
Corresponding authors. Tel.: +98 21 22850266; fax: +98 21 22853650.
E-mail addresses: srayati@yahoo.com, rayati@kntu.ac.ir (S. Rayati).
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.12.021

S. Rayati et al. / Polyhedron 34 (2012) 102–107
103
(
2 Â 20 ml) to remove any soluble succinimide impurities. b-Di-
bromo-meso-tetraphenylporphyrin (H TPPBr ), UV–Vis in CH Cl
max (nm): 423, 520, 595, 650. H NMR (300 MHz, CDCl ) d (ppm):
.78 (m, 6H, b-pyrrole), 8.20–8.24 (m, 8H, o-phenyl), 7.73–7.78
2
2
2
2
,
1
k
8
3
(
m, 12H, m- and p-phenyl), À2.83 (s, 2H, NH). b-Tetra-bromo-
meso-tetraphenylporphyrin (H TPPBr ), UV–Vis in CH Cl
nm): 436, 533, 612, 683. H NMR (300 MHz, CDCl ) d (ppm): 8.69
m, 4H, b-pyrrole), 8.16–8.19 (m, 8H, o-phenyl), 7.70–7.90 (m,
2H, m- and p-phenyl), À2.82 (s, 2H, NH).
2
4
2
2
, kmax
1
(
(
1
3
2
.2.2. MnTPPBr (OAc) and MnTPPBr (OAc)
2
4
MnTPPBr (OAc) and MnTPPBr (OAc) were prepared as follows
20]. H TPPBr or H TPPBr (0.13 mmol) was dissolved in a mini-
mum amount of methanol and then 30 ml chloroform was added.
Mn(OAc) O (250 mg, 1.04 mmol) was added to the reaction
Á4H
mixture and refluxed for a period of 2 h. MnTPPBr (OAc), UV–Vis
(OAc), UV–Vis
2
4
[
2
2
2
4
2
2
2
in CH
in CH
2
Cl
Cl
2
, kmax (nm): 375, 475, 583, 619. MnTPPBr
, kmax (nm): 380, 483, 590.
4
2
2
x
2.2.3. MnTPPBr (OAc) (x = 6 or 8)
MnTPPBr (OAc) and MnTPPBr (OAc) were prepared from H TPP
6
8
2
by the following four step procedure [21].
Fig. 1. Gouterman four orbital model.
2.2.4. NiTPP
2
H TPP (614 mg, 1 mmol) was dissolved in DMF (10 ml).
Ni(OAc) O (800 mg, 3 mmol) was added to the reaction mix-
2
Á4H
2
2
. Experimental
ture and refluxed for 4 h. At the end of reaction, the solvent was
evaporated under reduced pressure and the product was washed
twice with distilled water to remove the excess of Ni(OAc) Á4H O.
2.1. Materials and methods
2
2
1
3
UV–Vis in DMF, kmax (nm): 414, 527. H NMR (300 MHz, CDCl ) d
1H NMR spectra were obtained in CDCl
solutions with a Bruker
in con-
gives a signal at d = 7.26 ppm, which
3
(ppm): 8.74 (s, 8H, b-pyrrole), 8.13–8.20 (m, 8H, o-phenyl), 7.68
(broad, 12H, m- and p-phenyl).
FT-NMR 300 (300 MHz) spectrometer. The residual CHCl
ventional 99.8 atom% CDCl
3
3
was used for the calibration of the chemical shift scale. The elec-
tronic absorption spectra were recorded on a single beam spectro-
2.2.5. NiTPPBr (x = 6 or 8)
x
NiTPPBr and NiTPPBr were prepared from H TPP and freshly
6
8
2
photometer (Camspect, UV-M330) in CH
2
Cl
2
. Electrochemical
recrystallized NBS according to the literature [22] with some mod-
studies were carried out in a conventional three electrode cell
powered by an electrochemical system comprising of an EG&G
model 273A potentiostat/galvanostat. The system is run by a PC
through M270 commercial software via a GPIB interface. A coiled
platinum wire served as an auxiliary electrode and an Ag wire con-
ventional reference electrode was employed. All measurements
were carried out at the ambient temperature of 25 ± 2 °C. Electro-
ifications. NBS (1.58 g, 9 mmol for NiTPPBr ; or 3.20 g, 18 mmol for
6
NiTPPBr ) was added with stirring to a solution of NiTPP (1.0 g,
8
1.5 mmol) in 1,2-dichlorobenzene (60 ml). The solution was
brought to reflux for 2–3 h, and the reaction was monitored using
UV–Vis spectroscopy (the Soret band shifted from 414 to 440 nm
for NiTPPBr6 and to 445 nm in the case of NiTPPBr ). After the
8
end of reaction, the solvent was evaporated under reduced pres-
chemical measurements were conducted in CH
taining 0.1 M of tetrabutylammonium perchlorate (TBAPC) and
2
Cl
2
solutions con-
sure and the crude residue was washed using a mixture of solvents,
30 ml of CH Cl /MeOH (1:10 v/v), to remove any soluble succini-
2
2
À3
2
 10 M of manganese porphyrin complexes. Gas chromato-
mide impurities. NiTPPBr , UV–Vis in CH Cl , k
max
(nm): 440,
6
2
2
1
graphic analyses were performed on a Shimadzu GC-14B flame
ionization detector (FID) with a SAB-5 capillary column (phenyl
methyl siloxane 30 Â 320 Â 0.25 mm).
554. H NMR (CDCl , 300 MHz) d (ppm): 8.3 (m, 2H, b-pyrrole),
3
7.94 (m, 8H, o-phenyl), 7.71–7.72 (m, 12H, m- and p-phenyl). NiT-
1
PPBr , UV–Vis in CH Cl , k
8
2
2
max
(nm): 445, 555. H NMR (CDCl3,
The free base meso-tetraphenylporphyrin was prepared and
purified as reported previously [16]. Chemicals were purchased
from Merck chemical Co. Tetra-n-butylammonium periodate
300 MHz) d (ppm): 8.00–8.10 (m, 8H, o-phenyl), 7.75–7.90 (m,
12H, m- and p-phenyl).
(
TBAP) was prepared according to the literature [17,18].
2.2.6. H TPPBr (x = 6 or 8)
2
x
H
2
TPPBr
8
and H
2
TPPBr
SO
(x = 6 or 8) was suspended in 100 ml of CH
30 ml of H SO (98%) was added. The mixture was rigorously stir-
8
were prepared by demetallation of the
and NH in CH Cl . About 400 mg of
Cl and
2
2
.2. Synthesis
Ni(II) complexes with H
NiTPPBr
2
4
3
2
2
x
2
2
2
.2.1. H TPPBr
x
(x = 2 or 4)
2
4
b-Di-bromo-meso-tetraphenylporphyrin and b-tetra-bromo-
red for 3–4 h at room temperature. The progress of the reaction
was followed by UV–Vis spectroscopy. At the end of the reaction,
the flask was transferred to an ice bath, and NH solution (25%)
3
was added dropwise, with great precaution to avoid overheating
and violent boiling/evaporation of the organic solvent. The mixture
meso-tetraphenylporphyrin were synthesized independently
according to the method reported by Bhyrappa and co-workers,
with some modifications [19]. Freshly recrystallized N-bromosuc-
cinimide (NBS) (180 mg, 0.98 mmol for H
.96 mmol for H TPPBr ) was added to a solution of H
300 mg, 0.49 mmol) in CHCl (80 ml) under stirring conditions.
The mixture was stirred for 24 h in darkness and then the CHCl
was evaporated to dryness. The residue was washed with methanol
2 2
TPPBr ; or 360 mg,
1
(
2
4
2
TPP
was extracted several times with H
solution of NaHCO until the pH reached a value of 7.0. The organic
phase was over Na SO . H TPPBr , UV–Vis in CH Cl , kmax (nm):
445, 555, 607, 705. H NMR (CDCl , 300 MHz) d (ppm): 8.18–8.51
2
O and washed with a saturated
3
3
3
2
1
4
2
6
2
2
3

1
04
S. Rayati et al. / Polyhedron 34 (2012) 102–107
(
m, 2H, b-pyrrole), 8.18 (m, 8H, o-phenyl), 7.79–7.92 (m, 12H, m-
and p-phenyl), À2.29 (broad, 2H, NH). H TPPBr , UV–Vis in CH Cl
max (nm): 368, 469, 569, 625, 745. 1H NMR (CDCl
, 300 MHz) d
ppm): 8.13–8.15 (m, 8H, o-phenyl), 7.72–7.75 (m, 12H, m- and
p-phenyl), À1.25 (broad, 2H, NH).
were not significantly different from those of H
The signal of the b-protons of pyrrole rings for H
d 8.85. The chemical shifts of the b-protons of H TPPBr
and H TPPBr are respectively as follows: 8.78, 8.69 and 8.18–8.51
(Table 2). The upfield shift of the b-protons of the brominated por-
phyrins may be attributed to the ring current decrease generated
by the out-of-plane deformation of the macrocycle [29]. However,
due to the planar conformation of the macrocycle in the case of the
di- and tetra-brominated porphyrins, the shielding of the b protons
cannot be explained according to the ring current effects. The up-
2
TPP (Table 2).
TPP appears at
, H TPPBr
2
8
2
2
,
2
k
(
3
2
2
2
4
2
6
2
x
.2.7. MnTPPBr (OAc) (x = 6 or 8)
MnTPPBr (OAc) and MnTPPBr
6
8
(OAc) were prepared and puri-
(OAc), UV–Vis in
(OAc), UV–Vis in
fied according to the literature [20]. MnTPPBr
CH
CH
6
2
Cl
Cl
2
, kmax (nm): 499, 605, 653. MnTPPBr
, kmax (nm): 496, 610.
8
2 2 2 4
field shift of the b protons of H TPPBr and H TPPBr seems to be
2
2
due to the shielding of the b protons resulting from the electron-
withdrawing effects of the bromine atoms; although the elec-
tron-withdrawing effects of the bromine atoms is expected to
decrease the total electron density on the porphyrin core, the
2
.3. General oxidation procedure
Stock solutions of the catalyst (0.003 M) and nitrogenous bases
0.5 M) were prepared in CH Cl . The reagents were added to a test
tube in the following order: substrate (0.25 mmol), catalyst
0.003 mmol, 1.0 ml), nitrogenous bases (0.03 mmol, 60 l), chlo-
(
2
2
inductive effects of the bromine atoms seems to increase the elec-
1
tron densities of the
spectra of H TPPBr
protons relative to that of H
ening of the interamolecular N–HÁ Á ÁN hydrogen bonds [30]. Inter-
estingly, the NH resonance of H TPPBr and H TPPBr is downfield
relative to H TPP (Table 2). This downfield shift may be attributed
p
system at the b positions [15]. The H NMR
(
l
2
2
and H
2 4
TPPBr shows upfield shifts of the NH
robenzene (1 mmol) as an internal standard. TBAP (167 mmol)
was then added to the reaction solution at 25 °C. The mixture
was stirred thoroughly for 4 h at ambient temperature. The reac-
tion solutions were analyzed by GC.
2
TPP, which may be due to some weak-
2
6
2
8
2
to the ring current decrease resulting from the out-of-plane defor-
mation of the porphyrin core, the electron-withdrawing effects of
the bromine atoms and the strengthening of N–HÁ Á ÁN hydrogen
bond in the case of the two porphyrins [31].
3
. Results and discussion
.1. UV–Vis spectra of H TPPBr
The UV–Vis spectra of H TPPBr
3
2
x
(x = 0, 2, 4, 6 and 8)
(x = 0, 2, 4, 6 and 8) are given in
2
x
3.3. Electrochemical studies
Table 1. The Soret and Q bands of the perhalogenated porphyrins
are reportedly red shifted relative to those of the non-brominated
analogs [23,24], and this is also observed in this work (Table 1).
Large shifts of the Soret and Q (visible) bands of porphyrins have
been attributed to the enhanced co-planarity of the phenyl groups
with the porphyrin mean plane and/or the out-of-plane deforma-
tion of the macro cycle and/or the inductive effects of substituents
at meso and b-positions [25–27]. According to the X-ray crystallo-
graphic studies, substitution of more than 4 bromine atoms at the
b positions of meso-tetraphenylporphyrins cause the non-planarity
of the macrocycle [28]. We have previously attributed the large red
shifted bands of b-tetrabrominated-meso-tetraphenylporphyrin to
the electron-withdrawing effects of the bromine atoms. In the case
The cyclic voltammograms for MnTPP(OAc), MnTPPBr
2
(OAc),
Cl
0.1 M TBAP) are presented in Fig. 2, and the redox potential data
for the one-electron process associated with the Mn(III)/Mn(II)
couple are summarized in Table 3. The half-wave potentials (E1/2
for the first reduction of MnTPP(OAc), MnTPPBr (OAc),
MnTPPBr (OAc), MnTPPBr (OAc) and MnTPPBr (OAc) correspond-
ing to the Mn /Mn couple were located, respectively, at E1/2
À342, À380, À161, À141 and À87 mV in CH Cl . With the excep-
tion of MnTPPBr (OAc), the potentials were positively shifted (ano-
dic shift) with the progressive increase in the number of bromine
atoms. MnTPPBr (OAc) exhibited an irreversible voltammogram
MnTPPBr
4
(OAc), MnTPPBr
6
(OAc) and MnTPPBr
8
(OAc) in CH
2
2
(
)
2
4
6
8
III
II
2
2
2
2
of H
2
TPPBr
6
and H
2 8
TPPBr , the out-of-plane deformations of the
III
II
corresponding to the Mn /Mn reduction, while the other com-
plexes showed a quasi-reversible one-electron reduction under
the same conditions. The observed anodic shift on going from
MnTPP(OAc) to the octabrominated counterpart can be ascribed
to the electron-withdrawing and steric effects of the bromine sub-
stituents. As reported in the literature [31–38], the electron-with-
drawing character and steric effects of the bromine substituents
modify the redox properties of the brominated complexes with re-
spect to their precursors due to a decrease in the HOMO–LUMO en-
ergy gap, in accordance with the UV–Vis studies.
porphyrin core as well as the electronic effects of the bromine
atoms may be used to explain the red shifts of the Soret and the
Q(0,0) bands [7].
1
3
.2. H NMR spectra of H
2
TPPBr
x
(x = 0, 2, 4, 6 and 8)
TPPBr (x = 0, 2, 4, 6 and 8) are
1
The H NMR spectral data of H
2
x
summarized in Table 2. The chemical shifts of the phenyl protons
Table 1
The Soret and Q(0,0) bands of H
bands of H TPP.
2 x
TPPBr (x = 2, 4, 6 and 8) in comparison with the
3.4. Oxidation of olefins
2
Porphyrins
Soret band
k (nm)
Q(0,0) band
k (nm)
The oxidation of olefins with TBAP has been carried out in the
presence of MnTPPBr (OAc) (x = 0, 2, 4, 6 and 8). As has been pre-
x
viously reported by Mohajer et al. [40], no reaction was observed
in the absence of the catalyst. Different reaction parameters were
optimized as follows.
H
H
D
2
TPP
418
423
282
440
1207
445
1415
469
646
651
118
683
838
705
1295
745
2057
2
TPPBr
2
(cmÀ1)a
m
H
D
2
TPPBr
4
À1
m
(cm
)
)
)
H
D
2
TPPBr
6
À1
m
(cm
3.4.1. Effect of the TBAP/olefin molar ratio
Different molar ratios of cyclooctene to the oxidant were used
Table 4) and a 1:2 molar ratio has been found to be the optimized
H
2
TPPBr
8
À1
D
m
(cm
2601
(
a
7
D
m
= 10 (1/k
1
À 1/k
2 2
) relative to the corresponding band of H TPP.
one.

S. Rayati et al. / Polyhedron 34 (2012) 102–107
105
Table 2
Table 5
The 1H NMR data of H
(x = 0, 2, 4, 6 and 8).
Effect of various ImH/catalyst molar ratios on the cyclooctene epoxidation rate and
selectivity with TBAP in CH
2
TPPBr
x
a
2 2
Cl at room temperature.
Porphyrins b-Pyrrole
ppm)
o-Phenyl
m,p-Phenyl
N–H (ppm)
(
(ppm)
(ppm)
ImH/catalyst
Conversion %b
H
H
H
H
H
2
2
2
2
2
TPP
8.85 (s, 8H)
8.78 (s, 6H)
8.69 (s, 4H)
8.18–8.51
8.20–8.24
7.73–7.77 (m,
12H)
7.73–7.78 (m,
12H)
7.70–7.90 (m,
12H)
7.79–7.92 (m,
12H)
À2.77 (s, 2H)
À2.83 (s, 2H)
À2.82 (s, 2H)
MnTPP
MnTPPBr
2
MnTPPBr
4
MnTPPBr
6
MnTPPBr
8
(
m, 8H)
8.20–8.24
m, 8H)
8.16–8.19
m, 8H)
1
5
0
35
31
41
41
38
42
34
33
3
6
TPPBr
TPPBr
TPPBr
TPPBr
2
4
6
8
(
a
The molar ratios for catalyst:ImH:cyclooctene:oxidant are 1:x:83:167/4.
For a 4 h reaction time.
(
b
8.18 (m, 8H)
À2.29
(m, 2H)
(broad, 2H)
À1.25
8.13–8.15
7.72–7.75 (m,
12H)
3
.4.2. The effect of the co-catalyst/catalyst ratio
Effects of different molar ratios of imidazole (ImH) to catalyst
on the performance of MnTPPBr (OAc) (x = 0, 2, 4, 6 and 8) are pre-
(
m, 8H)
(broad, 2H)
x
sented in Table 5. The ratio of 5:1 has been used for the oxidation
of cyclooctene.
3.4.3. Effect of the number of bromine atoms at the b positions
Oxidation of cyclooctene and cyclohexene with TBAP was
carried out in the presence of MnTPPBr
x
(OAc) (x = 0, 2, 4, 6 and
8
) (Table 6). According to the data of Table 6, with the exception
8
of MnTPPBr (OAc), there is no significant meaningful difference be-
tween the catalytic activity of the brominated Mn–porphyrins and
the non-brominated one. Interestingly, MnTPP(OAc) and the par-
tially brominated counterparts are clearly more efficient than
MnTPPBr (OAc). The stability of the metalloporphyrins towards
8
oxidative degradation under the reaction conditions is one of the
main factors influencing their catalytic performance. According to
8
the UV–Vis studies, MnTPPBr (OAc) is the most stable metallopor-
phyrin of the series (vide infra) under the reaction conditions.
Accordingly, the decreased catalytic performance of the fully b bro-
minated Mn–porphyrin with respect to the partially brominated
ones and the non-brominated counterpart cannot be explained
on the basis of the stability of the Mn–porphyrins.
MnP = 2 Â 10À3 M,
3.4.4. Catalyst stability towards the oxidant or the active intermediate
The stability of metalloporphyrins towards oxidative degrada-
tion with the active oxidation intermediates or oxidant under the
reaction conditions is a major factor influencing their catalytic
activity [7,41]. The relative degradation of the used metalloporphy-
rins has been studied in the presence and the absence of cyclooc-
Fig. 2. Cyclic voltammograms of Mn(III)(Br
TBAP = 0.1 M, scan rate 50 mV s ).
x
TPP) (CH
2
Cl
2
,
À1
Table 3
4
tene. With the exception of MnTPPBr (OAc), the stability of the
Half-wave potential data for the metal-centered one-electron reduction of Mn(III)–
porphyrin complexes.a
catalysts increased with the increase in number of halogen atoms
substituted at the b positions, there is no correlation between the
number of bromine atoms and the stability of the brominated
Mn–porphyrins relative to MnTPP(OAc); in the case of the di-,
tetra and hexa-brominated Mn–porphyrins the catalyst stability
decreased relative to the non-brominated one. In contrast,
b
Mn–porphyrins
Mn(III)(TPP)
E
pa
E
pc
D
E
p
E
1/2
À198
À145
À120
À86
À486
À616
À202
À196
À160
288
471
82
110
165
À342
À380
À161
À141
À87
Mn(III)(Br
Mn(III)(Br
Mn(III)(Br
Mn(III)(Br
2
TPP)
4
TPP)
6
TPP)
8
TPP)
À13
MnTPPBr
MnTPP(OAc). However, MnTPPBr
(OAc) showed
a
higher stability compared to
(OAc) is only slightly more stable
8
a
À4
À1
8
Conditions: CH
Potential difference (Epa À Epc).
2
Cl
2
, MnP = 2.8 Â 10 M, TBAP = 0.1 M, scan rate 100 mV s
.
b
than MnTPP(OAc). Apparently, the bromine atoms act as electron
donor groups rather than electron acceptors. The enhanced stabil-
8 6
ity of MnTPPBr (OAc) and MnTPPBr (OAc) with respect to the di-
and tetra-brominated Mn–porphyrins is suggested to be due to
the steric effects of the bulky bromine atoms.
Table 4
Effect of various n-Bu
in the presence of ImH in CH Cl at room temperature.
2 2
4
NIO
4
/cyclooctene molar ratios on the cyclooctene epoxidation
a
3.5. Oxidation of sulfides
nBuNIO
4
/
Conversion %b
cyclooctene
MnTPP MnTPPBr
2
MnTPPBr
4
MnTPPBr
6
MnTPPBr
8
Oxidation of methyl phenyl sulfide with TBAP catalyzed by
1
1
2
:2
:1
:1
35
58
90
41
55
96
38
50
87
34
43
51
3
14
14
x
Mn(TPPBr )OAc (x = 0, 2, 4, 6 and 8) gave methyl phenyl sulfoxide
as the major product. In a search for suitable reaction conditions to
achieve the highest selectivity for sulfoxide, the effect of different
parameters including solvent, amount of co-catalyst and amount
of TBAP were studied in detail. Running the reaction in the absence
a
The molar ratios for catalyst:ImH:cyclooctene:oxidant are 1:10:83:x.
For a 4 h reaction time.
b

1
06
S. Rayati et al. / Polyhedron 34 (2012) 102–107
Table 6
Oxidation of cyclooctene and cyclohexene with TBAP catalyzed by MnTPPBr
Table 8
x
(OAc)
Effect of various TBAP/sulfide molar ratios on the oxidation of methyl phenyl sulfide
in the presence of ImH in CH
a
a
(x = 0, 2, 4, 6 and 8) in the presence of ImH in CH
2
Cl
2
.
2
Cl
2
at room temperature.
Catalyst
Cyclohexene
Yield %
Cyclooctene
TBAP/
sulfide
Catalyst
Conversion Yield %
Yield %
(sulfone)
Sulfoxide/
sulfone
b
Conversion %b Yield % Conversion %
%
(sulfoxide)
1
:2
MnTPP
22
21
29
20
3
33
41
44
28
4
56
69
65
46
11
19
19
26
18
3
26
30
33
23
6
30
35
41
27
7
3
2
3
1
6.3
9.5
8.6
18
3
3.7
3
3
Epoxide 1-ol 1-on
Epoxide
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
MnTPP
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
MnTPP
2
4
6
8
MnTPP(OAc)
32
1
1
2
2
2
5
5
5
4
1
38
41
42
34
14
38
42
38
34
13
38
42
38
34
13
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
2
4
6
8
(OAc) 35
(OAc) 31
(OAc) 28
(OAc) 11
1:1
7
2
4
6
8
10
11
5
a
The molar ratios for catalyst:ImH:alkene:oxidant are 1:5:83:167/4.
For a 4 h reaction time.
4.6
6
b
1
2:1
26
34
24
18
3
1.1
ꢂ1
1.7
1.5
2.3
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
2
4
6
8
of metalloporphyrin gives the sulfoxide and sulfone in substan-
tially lower yields (0.64% and 0.87%, respectively).
a
The molar ratios for catalyst:ImH:sulfide:oxidant are 1:10:83:x.
For a 4 h reaction time.
3
.5.1. Solvent effect
In order to obtain the best reaction media, solvents with differ-
b
ent dielectric constants [41] have been used (Table 7). According to
Table 7, the maximum conversion and highest selectivity for sulf-
oxide formation was obtained in dichloromethane. Consequently,
dichloromethane had the highest ability for sulfoxide formation
and was selected for the oxidation of methyl phenyl sulfide.
Table 9
Effect of various ImH/catalyst molar ratios on the oxidation of methyl phenyl sulfide
a
selectivity with TBAP in CH
2
Cl
2
at room temperature.
Entry ImH/
Conversion Yield %
Yield %
Sulfoxide/
b
MnTPPBr
4
(OAc)
%
(sulfoxide) (sulfone) sulfone
ratio
3.5.2. Effect of the TBAP/sulfide molar ratio
The selective oxidation of sulfides to sulfoxides has been an
1
2
3
0
5
10
6
24
29
5
22
26
1
2
3
5
11
8.6
important challenge in synthetic organic chemistry. While higher
ratios of TBAP to the organic sulfide increase the total conversion
of the reaction (Table 8), the over-oxidation of sulfoxide to sulfone
decreases the chemoselectivity of the reaction. Using different mo-
lar ratios of methyl phenyl sulfide to the oxidant (Table 8) shows
that the 1:2 molar ratio has been found to be the optimized one
to obtain the highest conversion. However, the use of a 2:1 molar
ratio leads to the selective oxidation of methyl phenyl sulfide to
the corresponding sulfoxide.
a
The molar ratios for catalyst:ImH:sulfide:oxidant are 1:x:83:167/4.
For a 4 h reaction time.
b
phenyl sulfide. The effect of ImH concentration on the oxidation
of methyl phenyl sulfide has also been studied (Table 9). Addition
of ImH up to 5:1 relative to the catalyst led to an increase in the
conversion of the reaction with an increase in the ratio of sulfoxide
to sulfone. Beyond this ratio, a significant decrease in the ratio of
sulfoxide to sulfone was observed.
Whilst the choice of periodate as oxidant, in comparison with
oxone used in our previous works [9–13], leads to lower yield of
products in the oxidation of olefins and methyl phenyl sulfide,
the selectivity towards sulfoxide increased significantly in the oxi-
dation of sulfides. In the case of olefins, using periodate as an
oxidant gives some ketone and alcohol in addition to the corre-
sponding epoxide as the major product.
3
.5.3. Comparison of the Mn–porphyrins
As has been observed in the case of the olefin oxidation, the par-
tially brominated Mn–porphyrins show remarkably higher cata-
lytic activities compared to the fully b brominated one. On the
other hand, there is no significant difference between the efficiency
of the partially brominated Mn–porphyrins with 2, 4 and 6 bro-
mine atoms at the b positions. Also, the activity of MnTPPBr (OAc)
6
is comparable with the non-brominated analog. While high ratios
of sulfoxide to sulfone have been observed in the presence of
MnTPPBr
tion catalyzed by MnTPPBr
8
(OAc), the conversion significantly decreased in the reac-
(OAc). Interestingly, the catalytic per-
8
formance of MnTPP(OAc) is much higher than MnTPPBr
but is comparable with MnTPPBr6.
8
(OAc),
4. Conclusions
In summary, the catalytic activity of the Mn(III) complexes of a
series of b brominated meso-tetraphenylporphyrins for the oxida-
tion of olefins and organic sulfides with TBAP has been studied
and compared. The results show that: (i) there is no correlation
between the number of bromine atoms and the stability of the
3
.5.4. The effect of the co-catalyst/catalyst ratio
The co-catalytic activities of ImH in the presence of
MnTPPBr (OAc) at various ImH/catalyst ratios are presented in
4
Table 9 and a 10:1 ratio was selected for the oxidation of methyl
Table 7
Oxidation of methyl phenyl sulfide with TBAP catalyzed by MnTPPBr
4
(OAc) in the presence of ImH in different solvents at room temperature.a
Solvent
Dipole moment
Conversion %b
Yield % (sulfoxide)
Yield % (sulfone)
Sulfoxide/sulfone
Dichloromethane
Tetrahydrofuran
1.60
1.63
29
7
26
6
3
1
8.6
6
a
The molar ratios for catalyst:ImH:sulfide:oxidant are 1:10:83:167/4.
For a 4 h reaction time.
b

S. Rayati et al. / Polyhedron 34 (2012) 102–107
[6] M. Meot-Ner, A.D. Adler, J. Am. Chem. Soc. 97 (1975) 5107.
107
Mn–porphyrins relative to the non-brominated one; (ii) the par-
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pared to MnTPPBr (OAc); (iii) the catalytic performance of
MnTPP(OAc) is comparable with the hexabrominated Mn–porphy-
rin; (iv) in spite of the higher stability of MnTPPBr (OAc) with
respect to the other Mn–porphyrins, its catalytic activity is signif-
icantly lower than that of partially brominated and non-bromi-
2
nated counterparts; (v) with the exception of MnTPPBr (OAc),
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[
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MnTPPBr (OAc); (vi) oxidation of cyclohexene and cyclooctene
8
[
[
gave the corresponding epoxides as the major product and some
alcohol and ketone as the minor products; (vii) cyclooctene and
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