Kinetics and mechanistic studies on the formation and reactivity of high valent MnO porphyrin species: Mono-: ortho or para -substituted porphyrins versus a di- ortho -substituted one

Article abstract of DOI:10.1039/c7nj04233g

The high valent Mn^V(O) (λ_max ≈ 407 nm) and Mn^IV(O) (λ_max ≈ 421 nm) species of a series of electron-rich and electron-deficient meso-tetra(aryl)porphyrins (aryl = phenyl, 2-chlorophenyl, 2-nitrophenyl, 2-methylphenyl, 2-bromophenyl, 2,6-dichlorophenyl 4-methoxyphenyl, 4-methylphenyl, 4-chlorophenyl and 4-pyridyl) were prepared through the reaction of their corresponding manganese porphyrins with oxone in dichloromethane at a relatively low temperature (273 K). While the high valent Mn(O) intermediates of the para-substituted porphyrins were significantly degraded under reaction conditions, the corresponding species of the ortho substituted porphyrins showed high oxidative stability up to 20 min. Interestingly, the high valent Mn(O) of the mono-ortho-substituted meso-tetra(phenyl)porphyrins were as stable as that of meso-tetra(2,6-dichlorophenyl)porphyrin. The presence of imidazole (ImH) was found to play crucial roles in the formation and reactivity of the high valent Mn(O) species. The kinetics and mechanism of the oxidation of olefins with the high valent Mn^V(O) porphyrin intermediate were studied and the second order rate constants were evaluated under pseudo-first-order conditions. Also, the Mn^IV(O) species showed a reactivity comparable with that of the corresponding Mn^V(O) species. The observation of a Hammet constant of ρ = -0.48 in the oxidation of para-substituted styrenes is in accord with an electrophilic mechanism.

Full text of DOI:10.1039/c7nj04233g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Kinetics and mechanistic studies on the formation and reactivity
of high valent MnO porphyrin species: mono-ortho or para-
substituted porphyrins versus a di-ortho-substituted one
Received 00th January 20xx,
Accepted 00th January 20xx
Rahele Nasrollahi and Saeed Zakavi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The high valent MnV(O) (λmax ≈ 407 nm) and MnIV(O) (λmax ≈ 421 nm) species of a series of electron-rich and electron-
deficient meso-tetra(aryl)porphyrins (aryl = phenyl, 2-chlorophenyl, 2-nitrophenyl, 2-methylphenyl, 2-bromophenyl, 2,6-
dichlorophenyl 4-methoxyphenyl, 4-methylphenyl, 4-chlorophenyl and 4-pyridyl) were prepared through the reaction of
their corresponding manganese porphyrins with Oxone in dichloromethane at a relatively low temperature (273 K). While
the high valent Mn(O) intermediates of the para-substituted porphyrins were significantly degraded under reaction
conditions, the corresponding species of the ortho substituted porphyrins showed high oxidative stability up to 20 min.
Interestingly, the high valent Mn(O) of the mono-ortho-substituted meso-tetra(phenyl)porphyrins were as stable as that of
meso-tetra(2,6-dichlorophenyl)porphyrins. The presence of imidazole (ImH) was found to play crucial roles in the
formation and reactivity of the high valent Mn(O) species. The kinetics and mechanism of the oxidation of olefins with the
high valent MnV(O) porphyrin intermediate were studied and the second order rate constants were evaluated under
pseudo-first-order conditions. Also, the MnIV(O) species showed a reactivity comparable with that of the corresponding
MnV(O) species. The observation of a Hammet constant of ρ = -0.48 in the oxidation of para-substituted styrenes is in
accord with an electrophilic mechanism.
.
the high valent iron oxo porphyrin intermediates, the nature of
manganese(V)–oxo porphyrins has been debatable until
Groves et al. reported the UV-Vis and 1H NMR spectra of
Introduction
Catalytic, electron-transfer and structural properties of heme
and non-heme high valent metal-oxo species have been the
subject of extensive computational and experimental
investigations over the past decades. From the catalytic point
of view, these species are used to mimic the active site of
cytochrome P450 (CYP) enzymes.1-5 A high valent oxo-iron(IV)
porphyrin π cation radical is known to be involved in the
catalytic cycle of CYP 3,6-7. While there are simple alkyl and
alkenyl substituents at the porphyrin periphery of CYP
enzymes, the active site is surrounded by protein residues
which can efficiently protect the substrate-free ferric resting
state of CYP from the oxidative degradation by the high valent
iron-oxo intermediate 8. Over the past decades, synthetic
metalloporphyrins with different meso and/or β substituents
have been used to mimic the active site of CYP.8 In contrast to
Mn(O) porphyrins in aqueous solution.9,10 After this report, the
high valent manganese oxo porphyrin intermediate of a wide
range of electron-deficient manganese porphyrins was
characterized by Nam et al. by Raman and X-ray absorption
spectroscopy/extended X-ray absorption fine structure
spectroscopy (XAS/EXAFS).11 Also, the latter was used in
different biomimetic organic transformations 12. A literature
survey reveals that the high valent metal-oxo complexes of
metalloporphyrins with electron demanding or sterically
hindered substituents at the porphyrin periphery are
sufficiently stable which allows the study of their catalytic
activity at relatively high temperatures. Accordingly, most of
the previous studies on the high valent metal-oxo species
devoted to this family of metalloporphyrins1,2,5,9-14
.
The coordination of axial ligands in the trans position of the
metal-oxo group of heme enzymes plays crucial roles in the
reactivity of the enzymes15,16. In this study, the presence of
imidazole (ImH) was found to play crucial roles in the
stabilization and reactivity of the high valent manganese(IV
a. Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Zanjan 45137-66731, Iran. E-mail:zakavi@iasbs.ac.ir
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See and V)-oxo porphyrin intermediates. The oxidation of organic
DOI: 10.1039/x0xx00000x
compounds with Oxone and its organic derivatives catalyzed
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by manganese and iron porphyrins have been studied in chlorophenyl)porphyrin,
details 17-24. However, there are limited reports on the kinetics methylphenyl)porphyrin,
of these reactions. Very recently, we have reported kinetic and bromophenyl)porphyrin,
mechanistic studies of the oxidation of alcohols with high- nitrophenyl)porphyrin,
H2T(2-Cl)PP
meso-tetrakis(2-
H2T(2-Me)PP
H2T(2-Br)PP
meso-tetrakis(2-
meso-tetrakis(2-
Meso-tetrakis(2,6-
H2T(2-NO2)PP,
valent manganese oxo species formed by the reaction of dichlorophenyl)porphyrin, H2TDClPP and the corresponding
meso-tetrakis(2,6-dichlorophenyl)porphyrinatomanganese(III)
manganese porphyrins have been prepared according to the
acetate, [MnT(DCl)PP(OAc)] and tetra-n-butylammonium Literature 25-28. TBAO was prepared by a previously published
oxone (TBAO)32. In the present study, the formation of high method modified by Meunier et al.
valent metal-oxo species for the manganese complexes of Information for more experimental details).
meso-tetra(aryl)porphyrins (aryl = phenyl, 2-chlorophenyl, 2-
(See Supporting
29
methylphenyl,
2-bromophenyl,
4-chlorophenyl,
4- Results and discussion
methoxyphenyl and 4-pyridyl) was evidenced by UV-vis High valent metal-oxo porphyrins complexes are strong active
spectroscopy studies at a relatively low temperature. The main oxidants that are involved in the oxidation reactions
objective of the present work is to study the formation and performed by strong terminal oxidants such as
reactivity of the high valent manganese oxo intermediates of iodosylbenzene, meta-chloroperbenzoic acid and TBAO
structurally simple, low cost and easily synthesized porphyrins. catalyzed by metalloporphyrins 2,30-37. However, due to the
Furthermore, the latter will be compared with the well known high reactivity of these species at room or near room
high valent MnO porphyrin species of the sterically hindered temperature, they should be studied at low temperatures. In
and electron-deficient porphyrin, H2T(DCl)PP. The results of the case of para-substituted porphyrins, the high valent metal-
the present study show that mono-ortho-substituted meso- oxo intermediates are stable only at very low temperatures.
tetra(phenyl)porphyrins can readily form stable high valent Moreover, the extensive oxidative degradation of the catalyst
manganese(IV) and manganese(V)-oxo intermediates through precursor by high valent metal-oxo complex may be prevented
reaction with TBAO at 273 K. Furthermore, the kinetics of at low temperatures. On the other hand, the presence of
oxidation of styrenes with the high valent Mn(O) porphyrin sterically hindered and/or electron-withdrawing substituents
species of different Mn-porphyrins were investigated to at the meso positions may be utilized to overcome the
elucidate the influence of the meso substituents on the oxidative degradation of the porphyrinic catalysts. It is
reactivity of the high valent Mn(O) species. Also, the high noteworthy that the presence of bulky substituents at the
valent species were used for the oxidation of a wide range of ortho position increases the dihedral angle between the aryl
organic compounds including indene, cyclohexene, substituents and the porphyrins mean plane 38. This in turn
cyclohexane, toluene, methyl phenyl sulfide and aniline.
decreases the resonance interaction between the aryl group
and porphyrins π systems and leads to an increase in the
electron-deficient character of the aromatic macrocycle.
Accordingly, the introduction of bulky electron-releasing
groups at the ortho positions may be also employed to
increase the electron-deficiency of Mn-porphyrins. Meot-Ner
et al. showed that the presence of methyl group at the ortho
position of meso-tetra(phenyl)porphyrin makes it as electron-
deficient as meso-tetra(4-cyanophenyl)porphyrin 39. Also, the
electron-deficient character of ortho substituted meso-
tetra(aryl)porphyrins was revealed by UV-vis spectroscopy of
40,41
their dications
.
In this study, a series of Mn-porphyrins
with bulky substituents at the para and ortho position were
used to prepare the corresponding high valent Mn(O)
porphyrin species at
a
relatively low temperature.
Interestingly, the high valent Mn(O) intermediates of the ortho
substituted porphyrins were remarkably stable regardless of
the electron-withdrawing or electron-releasing properties of
the ortho position. The formation of high-valent Mn(IV or V)(O)
porphyrin species from their manganese(III) counterparts is
accompanied with the appearance of an absorption band at
lower wavelengths and therefore UV-vis spectroscopy has
been extensively employed to provide evidence on the
formation of these species 42-44. According to the literature, the
position of the blue shifted band in the UV-vis spectrum of
these species depends on the meso substituents and therefore
the band due to the high valent species was observed at 407 to
Figure 1. Manganese porphyrins used in this study.
Experimental
Preparation of porphyrins and metalloporphyrins
Meso-tetraphenylporphyrin,
H2TPP,
meso-tetrakis(4-
methoxyphenyl)porphyrin, H2T(4-OMe)PP meso-tetrakis(4-
methylphenyl)porphyrin,
chlorophenyl)porphyrin,
pyridyl)porphyrin,
H2T(4-Me)PP
H2T(4-Cl)PP
meso-tetrakis(4-
meso-tetrakis(4-
meso-tetrakis(2-
H2T(4-Py)PP
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Figure 2. UV-vis spectra of the Mn-porphyrins, their ImH (1:15 molar ratio to the catalyst) complexes and the corresponding MnIV(O) (≈1:500 ratio to the catalyst) and MnV(O)
species (≈ 1:1000 ratio to the catalyst) species in dichloromethane at 273 K.
413 and 420-425 nm for the MnV(O) and MnIV(O) species, excess amounts (1:1000), led to the disappearance of the
respectively34,45-53
absorption bands at 474 and 421 nm and appearance of a
The UV-vis spectra of the high valent Mn(O) porphyrins band at 407 nm due to [(ImH)MnV(O)T(DCl)PP]+(orange curve).
compexes (MnIV(O) or MnV(O)) of different Mn-porphyrins at It is noteworthy that significantly less amounts of TBAO were
273 K are shown in Figure 2. It should be noted that the needed for the completeness of the reaction in the case of the
formation of two types of high valent manganese oxo more electron-rich Mn-porphyrins such as MnTPP(OAc). This
porphyrins, MnIV(O) and MnV(O), occurred in the presence of observation is probably due to the higher Mn(V)/Mn(III)
different excess amounts of TBAO; as seen in Figure 2 , When reduction potential (Scheme 1) of the more electron-deficient
TBAO was added to this solution in 1:500 molar ratio with Mn-porphyrins54,55
respect to the catalyst, an absorption band appeared at ≈ 421
nm that was attributed to the (ImH)MnIV(O)porphyrin; The
addition of larger excess amounts of TBAO (1:1000), led to
disappearance of the bands at ≈ 421 nm and the appearance
of a band at ≈ 407 nm. The latter band belongs to the
(ImH)MnV(O)porphyrin. Decay of this species gives an
absorption band at ca. 477 nm that is attributed to the six
coordinate (ImH)MnIII(P)(OOSO3H) intermediate. In the case of
(ImH)MnT(DCl)PP(OAc) (Supporting information, Figure S1) the
.
K= kf / kr
∆Gº = ∆Hº-T∆Sº = -RTlnK
∆G = -nFEº
Eº = Eº (HSO5- /HSO4 ) - Eº (MnV/MnIV)
-
addition of TBAO to this solution in CH2Cl2 in 1:500 molar ratio
with respect to the catalyst, two absorption bands appeared at
Scheme 1. The presence of an equilibrium between the Mn(III) and Mn(V) species of
various Mn-porphyrins caused by the difference between their Mn(V)/Mn(III) reduction
potential.
474
and
421
nm
that
and
are
attributed
to
(ImH)MnIIIT(DCl)PP(OOSO3H)
(ImH)MnIV(O)T(DCl)PP
species, respectively (blue curve). The use of TBAO in larger
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UV-vis spectral data of Mn-porphyrins, their ImH complexes solution of the Mn-porphyrins in CH2Cl2 and its coordination to
and the corresponding high valent Mn(O) complexes in the Mn(III) centre led to a little or no shift in the absoprtion
dichloromethane at 273 K are collected in Table 1. As seen in bands. The use of large eccess amount of TBAO (≈1:1000 molar
Table 1, the position of the Soret and Q bands 56,57 of the used ratio) resulted in the formation of high valent MnV(O)
Mn-porphyrins only slightly depends on the type of meso complexes with the absorption maxima in the range of 406-
substituents. This observation is in accord with the nearly 413 nm. When TBAO was added in ca
. 1:500 molar ratio with
perpendicular orientation of the meso aryl substituents with respect to the catalyst, two absorption bands appeared in the
the porphyrins mean plane in metalloporphyrins40,58,59
.
range of 417-424 and 474-479 nm, due to
It is noteworthy that the HOMO-LUMO gap in porphyrins and (ImH)MnIV(O)porphyrin and (ImH)MnIII(P)(OOSO3H) species,
metalloporphyrins is determined by the dihedral angle respectively.
between the meso substituents and porphyrin mean plane and The absorption maxima of the six coordinate species of the
the out-of-plane deformation of porphyrins macrocycle 38. For used Mn-porphyrins is observed at slightly higher wavelengths
a series of Mn-porphyrins with different meso substituents the relative to that of the corresponding Mn-porphyrin. The
extent of coplanarity of meso substituents and porphyrin π presence of ImH was crucial for the formation of the high
systems influencing the resonance interaction between the valent metal-oxo species. Also, the high valent metal-oxo
two π systems determines the electron-rich or electron- intermediates was completely stable up to 20 min at 273 K.
deficient nature of the metalloporphyrin 39
.
Interestingly at the solution of the Mn-porphyrins with ortho
According to the literature, high valent Mn(O) porphyrin substituents such as MnT(2-NO2)PP(OAc) the band for MnV(O)
intermediate has been prepared using different oxidants such or MnIV(O) was rapidly and completely replaced with the band
as m-chloroperbenzoic acid, iodosylarenes, hydrogen peroxide, of six coordinated compound. In other word, no catalyst
Oxone and iodobenzene diacetate 2,31-36 under various reaction degradation was observed. On the other hand, the addition of
conditions. However, most of these species are highly reactive TBAO to the solution of the Mn-porphyrins with no ortho
toward oxidizable organic and inorganic substrate and solvents substituents, MnT(4-Cl)PP(OAc), MnT(4-Me)PP(OAc), MnT(4-
and therefore have been prepared and studied in situ. In this OMe)PP(OAc) and MnT(4-Py)PP(OAc) in dichloromethane led
regard, the introduction of electron-withdrawing and/or to disappearance of the Soret band at ca. 470 nm and the
sterically hindered substituents at the ortho position may be appearance of a blue shifted band at ca. 410 nm.
utilized to enhance their stability so that the species may be
studied at high temperatures as 273 K. Also, the catalyst
precursor of electron deficient and sterically hindered
metalloporphyrins would be more stable with respect to the
oxidative attack by the corresponding high valent metal-oxo.
As seen from Table 1, the UV-vis spectra of the high valent
Mn(O) species consists of two absorption bands at the Soret
and Q bands region. First of all, the addition of ImH to the
Table 1. UV-vis spectral data of Mn-porphyrins, their ImH complexes and the corresponding high valent Mn(O) porphyrin complexes in dichloromethane at 273 K.
Soret band
(λ/nm)
Q(0,1)band
(λ/nm)
Q(0,0) band
(λ/nm)
Q(0,0)band
(λ/nm)
Mn- Porphyrins
Catalyst
precursor
ImH
complex
MnV(O)
MnIV(O)
species
Six Coordinate
Catalyst precursor
MnIV(O) and
MnV(O)b
species
Peroxymonosulfate
complexa
MnTPP(OAc)
470
473
470
470
470
476
471
472
472
471
470
473
470
470
470
475
468
472
473
471
410
411
406
406
412
410
407
409
413
409
---
---
477
479
477
477
477
476
474
478
478
478
577
580
576
576
577
580
576
576
579
579
612
614
611
607
611
611
607
609
618
614
526
524
523
518
527
520
518
518
530
529
MnT(2-NO2)PP(OAc)
MnT(2-Me)PP(OAc)
MnT(2-Cl)PP(OAc)
MnT(4-Cl)PP(OAc)
MnT(4-Py)PP(OAc)
MnT(DCl)PP(OAc)
MnT(2-Br)PP(OAc)
MnT(4-OMe)PP(OAc)
MnT(4-Me)PP(OAc)
417
417
422
424
421
423
---
---
[a] (ImH)MnIII(Porphyrin)(OOSO3H). [b] The Q(0,0) bands of MnIV(O) and MnV(O) species appear at nearly the same wavelengths.
4 | J. Name., 2012, 00, 1-3
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Figure 2, no absorption band due to the catalyst precursor was
observed after disappearance of the band at 410 nm that
confirms the oxidative decomposition of the catalyst precursor
that is In accord with that Mn-porphyrins with no electron-
withdrawing or bulky substituents are readily decomposed in
the presence of TBAO by an intermolecular oxidative
degradation mechanism and therefore the oxidative
degradation of MnT(4-OMe)PP(OAc) as the most electron-rich
metalloporphyrins of the series occurred much faster than the
other para-substituted Mn-porphyrins by oxidative attack by
the corresponding high valent metal-oxospecies. It is
noteworthy that due to the remarkably high reactivity of the
metal-oxo species, these species are often studied at low
temperature to prevent their reaction with oxidizable
substrates such as the catalyst precursor and water impurity of
commonly used solvents. However, the band for MnV(O) or
MnIV(O) nm was rapidly replaced with the band of six
coordinated compound. The experiments were also conducted
in acetonitrile and very similar results were obtained.
Figure 3. The addition of a large excess amount of TBAO to the solution of MnT(2-
NO2)PP(OAc) in dichloromethane in the absence of ImH (red curve). The spectrum after
the addition of ImH to this solution (green curve).
Furthermore, the hydrogen bond formation between ImH and
the leaving group can also be involved in the formation of the
high valent Mn(O) species, due to the stabilization of the
anionic leaving group in non-polar solvents such as
dichloromethane. (Scheme 2). Accordingly, the presence of
ImH led to the blue shift of the Soret and Q bands. It is
noteworthy that the high valent Mn(O) intermediate prepared
under alkaline conditions was not able to oxidize olefins,
evidenced by no change in the UV-vis spectra upon the
addition of substrate 60. However, the high valent Mn(O)
complexes prepared in the presence of ImH readily oxidized
indene and other olefins. Very recently we have shown that
the presence of ImH is crucial for the oxidation of electron-rich
Interestingly, the other ortho substituted Mn-porphyrins
MnT(2-NO2)PP(OAc), MnT(2-Br)PP(OAc), MnT(2-Cl)PP(OAc)
and MnT(2-Me)PP(OAc) were more stable than that of the
para substituted Mn-porphyrins. Furthermore, the high valent
metal-oxo complexes of the ortho mono-substituted Mn-
porphyrins showed an oxidative stability comparable with that
of MnT(DCl)PP(OAc). Accordingly, the previous may be used
instead of the latter in kinetic and mechanistic studies on the
high valent Mn(O) porphyrin species. It should be noted that
the porphyrins free base of the former may be synthesized and
purified and metallated with more simple and efficient
procedures as well as low cost reagents.
and
electron-
deficient
alcohols
with
the
[(ImH)MnV(O)T(DCl)PP]+. These observations in combination
with the spectral changes upon the addition of ImH to the
solution of MnT(DCl)PP(OAc) provide evidence for the
coordination of ImH to the Mn(V) centre of the active oxidant
(Scheme 3).
Spectral changes upon the formation of high valent
intermediates
The high valent Mn(O) intermediate of MnT(DCl)PP(OAc) has
been also synthesized by the oxidation of this complex with m-
CPBA under alkaline conditions. The Soret and Q bands of
[(ImH)MnIV(O)T(DCl)PP]+ and [(ImH)MnV(O)T(DCl)PP]+ were
observed at 444 and 560 nm, respectively60. In the present
study, the corresponding absorption bands of the high valent
intermediates formed in the presence of ImH appeared at 407
and 520 nm, respectively (Table 1). The presence of ImH was
crucial for the formation of the latter; the addition of a large
excess amount of TBAO to the solution of MnT(2-NO2)PP(OAc)
in dichloromethane. in the absence of ImH gave no band due
to the high valent metal-oxo complex (Figure 3 ). Interestingly,
the addition of ImH to this solution caused the appearance of
an absorption band at 411 nm. This observation was attributed
to the multiple roles played by ImH in the formation and
reactivity of the active oxidants upon the activation of
different terminal oxidants by Mn-porphyrins. It should be
noted that the coordination of ImH to the metal centre
H
N
N
N
N
H
H
H
N
OH
S
N
O
O
N
N
N
N
H
N
H
N
N
N
O
H
O
O
OH
S
O
O
MnIII
N
MnV
+
O
N
H
N
H
N
N
N
N
N
H
H
Scheme 2. Coordination of ImH to metal centre of metalloporphyrins and hydrogen
bond formation between the leaving group and ImH facilitate the formation of high
valent Mn(O) species.
-
facilitates departure of the leaving group i.e. HSO4 .
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Effect of catalyst:ImH molar ratio
pseudo-first order conditions (Supporting Information, Figure
The rate of formation of the high-valent Mn(O) species 5, 6 and Figure S2).
determined by the initial rate method, was found to be
dependent on the catalyst:ImH molar ratio (Figure 4). The
maximum rate was obtained in the ca. 1:15 molar ratio.
a
Figure 4. Initial rate of [(ImH)MnV(O)T(DCl)PP]+ formation against ImH to catalyst molar
ratio.
b
Although the catalytic activity of Mn-porphyrins strongly
enhances in the presence of ImH31,61-63, MnIII(Porphyrin)(ImH)2
species can readily form in the case of high concentrations of
ImH64 (Scheme 3). In other words, an optimum molar ratio of
catalyst:ImH should be used to prevent the formation of the
latter.
Scheme 3. Formation of inactive species formed by occupation the sixth coordination
Figure 5. UV−vis spectral changes upon the oxidaꢀon of a) cyclohexene and b) indene
(2.25 × 10-3 M) with [(ImH)MnV(O)T(2-Me)PP]+( 4.5 × 10-5 M) in ≈ 5 min.
site of Mn(III) centre.
Table 2. The observed rate constants for oxidation of Indene and cyclohexene by Mn-
porphyrins and TBAO.
Mn-porphyrins
Observed
rate constants
(× 103 M-1s-1)a
6.85
Mn-porphyrins
Observed
rate constants
(× 103 M-1s-1)b
0.81
a
MnT(2-Cl)PP
MnT(2-CH3)PP
MnT(2-Br)PP
MnT(2-Cl)PP
MnT(2-CH3)PP
MnT(2-Br)PP
3.05
2.15
1.83
1.49
a and b are for the oxidation of Indene and cyclohexene, respectively.
Kinetic and mechanistic studies
Reactivity of different high valent manganese oxo species in
the oxidation of indene and cyclohexene
In order to study the influence of the groups substituted at the
phenyl substituents of meso-tetra(phenyl)porphyrins on the
reactivity of the corresponding high valent Mn(O) porphyrins
species, the rate of oxidation of indene and cyclohexene to the
corresponding epoxides was studied by these species through
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a
b
b
Figure 6. a) The results of observed rate constants for the oxidation of indene (a) and
cyclohexene (b) with (ImH)MnV(O)P in dichloromethane at 273 K.
It is noteworthy that due to high degree of catalyst
degradation in the case of MnTPP(OAc) and the para-
substituted analogues, the oxidation reactions were studied
using MnT(2-Cl)PP(OAc), MnT(2-Me)PP(OAc) and MnT(2-
Br)PP(OAc) as catalyst (Table 2).
As seen in Table 2, the order of reactivity of the manganese
oxo species decreased as MnT(2-Cl)PP(OAc)
> MnT(2-
Me)PP(OAc) > MnT(2-Br)PP(OAc) and MnT(2-Me)PP(OAc) >
MnT(2-Br)PP(OAc) > MnT(2-Cl)PP(OAc) for the oxidation of
indene and cyclohexene, respectively.
Figure 7. a) UV-vis spectrum of MnT(DCl)PP(OAc) in CH2Cl2 (λmax = 471 nm, black curve).
The spectrum after the addition of ImH in 1:15 molar ratio (ImH)(MnIIIT(DCl)PP, λmax
=
The observed order of reactivity may be explained on the basis
of the electronic and steric effects of the substrate and high
valent Mn(O) porphyrins species; the formation of high valent
Mn(O) species leads to a significant decrease in the steric
effects around the reaction centre in comparison with the
468 nm, red curve), and that after the addition of excess amount of TBAO on
(ImH)MnIII(DCl)PP (λmax = 407 nm, blue curve); b)UV-vis spectral changes monitored
upon the reaction of [(ImH)MnV(O)(TDClPP)]+ (4.5 × 10-5 M) with styrene (2.25 × 10-3 M).
The Inset demonstrates the plot of absorbance vs time at 407 and 474 nm for
[(ImH)MnV(O)T(DCl)PP]+ and (ImH)MnIII(OOSO3H)T(DCl)PP]+.
(Porphyrin)Mn(HSO5)(ImH) species. However,
a
steric
In order to investigate the Kinetic and mechanism of the
reactions of [(ImH)MnV(O)T(DCl)PP]+, the kinetics of oxidation
of styrene was studied under pseudo-first-order conditions. As
was observed in the case of the ortho mono-substituted Mn-
porphyrins the addition of ImH led to slight shift in the position
of absorption maxima of MnT(DCl)PP(OAc). (Figure 7a),
Also, the formation of high valent Mn(O) intermediate was
evident from the large blue shift of the Soret band from 468 to
407 nm (Figure 7a). The reaction of styrene with the high
valent metal-oxocomplexes obeyed the second order rate
equation.
hindrance should be considered due to the presence of bulky
meso-substituents65,66. Also, the presence of phenyl adjacent
to the reaction centre of indene increases the steric hindrance
around the C=C bond of indene compared to that of
cyclohexene. In the case of cyclohexene, the reverse order of
reactivity of the high valent Mn(O) species cannot be explained
on the basis of the steric hindrance at the meso positions. This
observation may be due to the decreased reactivity of the
active oxidants of the electron-deficient manganese
porphyrins in comparison with those of the electron-rich
ones67-69
.
Mohajer et al. showed that manganese meso-
tetra(aryl)porphyrins with perfluorophenyl, 2,6-dichlorophenyl
and 2,4,6-trimethylphenyl substituents are remarkably less
reactive than MnTPP(OAc) for the oxidation a wide range of
olefins31.
Figure 7b shows the UV-vis spectral changes monitored upon
the reaction of [(ImH)MnV(O)T(DCl)PP]+ with styrene. The
absorption changes at the λmax of the high valent Mn(O)
species were used to study the kinetics of the oxidation of
para-substituted styrenes to the corresponding epoxides
(Figure 7b The UV-vis spectral changes for other para
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Table 3. Second order rate constants (k2) for alkenes
substituted styrenes are shown in Supporting information,
Figure S3. The non-linear curve fitting, [At = A∞ - (A∞ - A0)exp(-
kobst)] plotted versus the time gave the kobs values (Supporting
information, Figure S4). As seen in Figures S4 and S5 in the
supporting information, an induction period was observed for
the oxidation of styrene and other olefins (Supporting
information, Figure S5). This observation seems to be due to
the presence of hydrogen bond forming molecules such as ImH
and water adjacent to the active site of the metal-oxo species;
previous studies demonstrate that the coordination of ImH to
the metal centre is crucial for the reactivity of the high valent
metal-oxo intermediate. However, ImH can also facilitate the
formation of the high valent metal-oxo complex through the
formation of hydrogen bond with the leaving group of the
Olefins
Second order rate constants
k2 (× 103 M-1s-1)
Styrene
2.25
4.94
4.27
3.08
4-Methoxystyrene
4-Methylstyrene
4-Chlorostyrene
-
oxidant assisting leaving group (HSO4 ) departure. On the other
hand, ImH and other hydrogen bond donor molecules such as
water and alcohols stabilize the high valent metal-oxo species
through hydrogen bond formation with the formed
manganese oxo intermediate (Scheme S1) 31, 70-72
.
Accordingly, these molecules should be removed from the
active site to allow substrates access the electrophilic oxygen
atom of the active oxidant. The latter is similar to the
departure of water molecules from the active site of CYP
following the arrival of substrate in the active site8, 73, 74
.
Figure 9. Hammett plot for the oxidation of para substituted styrenes with
[(ImH)MnV(O)T(DCl)PP]+.
The kinetics of reaction was studied using different
concentrations of styrene (Figure 8.) to obtain the second
order rate constants (K2). As seen from Figure 8, the increase
in the concentration of styrene led to no change in the second
order kinetics of the reaction. However, continuous increase in
the Kobs values was observed.
As seen from Figure 2, high valent Mn(IV)O porphyrin (Soret
band ~ 417 nm) may be also prepared by using TBAO in lesser
molar ratios of catalyst to TBAO. Interestingly, the Mn(IV)oxo
species was also able to oxidize olefins with a rate comparable
with that of the Mn(V)oxo counterpart. The changes in the
Soret band of the Mn(IV)oxo species of MnT(2-Cl)PP(OAc)
upon the addition of indene are shown in Supporting
information, Figure S6. It should be noted that the latter was
much less reactive than the Mn(V)oxo species towards the
oxidation of alcohols. Also, the Mn(IV)Oxo species was
completely unreactive for with respect to the oxidation of
electron-deficient alcohols25.
In all the experiments relevant to the study of the kinetics of
oxidation of olefins, the substrate was added to the reaction
mixture after the formation of the high valent MnV(O) or
MnIV(O) complexes. In a control experiment, TBAO was added
to a solution containing MnTPP(OAc), ImH and styrene and the
changes in the absorption spectrum were recorded.
Interestingly, no absorption band due to the formation of a
high valent Mn(O) was observed. Furthermore, the band due
to (ImH)Mn(Porphyrin)(HSO5) at 477 nm (Supporting
information, Figure S7) was gradually disappeared confirming
degradation of the catalyst precursor. In other words, the high
reactivity of the Mn(O) species led to the rapid oxidation of
styrene and oxidative degradation of the catalyst.
Figure 8. Pseudo-first-order rate constants (kobs / s-1) against the concentration of para
substituted styrenes in reaction with [(ImH)MnV(O)T(DCl)PP]+.
Table 3 shows the second order rate constant values for the
oxidation
of
para-substituted
styrenes
with
[(ImH)MnV(O)T(DCl)PP)]+.
Also the Hammett treatment of the oxidation data is shown in
Figure 9. A slope (ρ) of -0.48 is consistent with an electrophilic
mechanism75-77 that shows the remarkable dependence of the
rate of the reaction on the electronic properties of the
substrate.
Competitive oxidation of stilbenes has been used to provide
indirect evidence on the nature of the active oxidant in
metalloporphyrin catalyzed oxidation of olefins; for the Mn-
porphyrins with no steric hindrance at the meso positions, a cis
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to trans stilbene oxide ratio of ca. 1 has been observed in the various substrates with [(ImH)MnV(O)(TDCl)PP]+, and
case of the involvement of a high valent Mn(O) intermediate MnV(O)T(2-Me)PP(OAc) was studied (Supporting information
,
as the active oxidant. In the presence of a six coordinate Figures S8 and S9). The results show that the high valent
(ImH)Mn(Porphyrin)(HSO5) species, the steric hindrance Mn(O) intermediates of these Mn-porphyrins are able to
around the active oxidant leads to the higher reactivity of cis- oxidize a wide range of organic compounds at 273 K.
stilbene relative to trans stilbene and therefore deviation from Furthermore, the meso substituents has no significant effect
ca. 1 is observed for the competitive reaction.
on the reactivity of the high valent Mn(O) complexes.
Conclusion
The high valent Mn(O) species of mono-ortho substituted
meso-tetra(phenyl)porphyrins with Cl, methyl, nitro or Br at
the ortho position were prepared using TBAO as the terminal
oxidant and ImH as co-catalyst at 273 K. The high valent
MnV(O) and MnIV(O) intermediates were stable up to ca. 20
min. in the absence of organic substrate. In other words, the
presence of bulky ortho substituents with a size equal to or
greater than that of chlorine atom leads to the formation of a
stable high valent Mn(O) complex. Also, the addition of water
to the dichloromethane solution of these species led to no
detectable change in the intensity of the absorption band due
to the high valent MnV(O) and MnIV(O) intermediates at ca.
407 and 421 nm, respectively. Moreover, the formation,
Scheme 4. Different species formed by the addition of ImH and TBAO to Mn-porphyrins reactivity and stability of the high valent Mn(O) species
and the formation of high valent Mn(V) or Mn(IV) oxo species in the presence of
remarkably depended on the presence of ImH. The kinetic
different equivalents of TBAO.
studies on the oxidation of olefins with the former showed
that the kobs. (k2[olefin]) values were only slightly dependent
Furthermore, the presence of sterically hindered substituents
on the nature of the ortho substituents. Interestingly, the high
at the meso position also lead to a cis to trans stilbene oxide
valent Mn(O) complexes of the mono-ortho-substituted Mn-
ratio greater than 1; in the case of MnT(2,4,6-
porphyrins showed
corresponding
a
stability similar to that of the
trimethyl)phenylporphyrin acetate a value of ca. 3.5 was
observed for the oxidation of stilbenes with high valent Mn(O)
complex. 31,78,79. In this study, the use of MnT(TDCl)PP(OAc)
and TBAO in 1:200 molar ratio gave no band due to the
formation of a high valent Mn(O) species. In other words, the
six coordinate (ImH)MnT(DCl)PP(HSO5) was the exclusive
active oxidant. The oxidation of cis and trans-stilbenes under
this condition gave a value of 6 for the molar ratio of cis to
trans-stilbene oxides. On the other hand, when the reactions
were repeated using the catalyst and TBAO in 1:1000 molar
ratio, a cis to trans-stilbene oxide value of 3 was observed. It
should be noted that under this molar ratio,
[(ImH)MnV(O)T(DCl)PP]+ (λmax = 407 nm) was the sole active
oxidant, evident from the UV-vis spectrum of the reaction
mixture. Accordingly, the results confirm that the competitive
oxidation may be used to distinguish between the high valent
Mn(O) intermediate and six coordinate intermediates as the
active oxidants.
species of meso-tetra(2,6-
dichloro)phenylporphyrin. Also, the oxidative stability of the
catalyst precursor of the former was very similar to the latter
within a reaction time of ca. 20 min. It is noteworthy that the
kinetics of the oxidation reaction conducted with the MnV(O)
of meso-tetra(2,6-dichloro)phenylporphyrin was comparable
with that of the mono-ortho-substituted counterparts.
Furthermore, the MnIV(O) species were as reactive as the
MnV(O) analogues. On the other hand, the Soret band due to
the high valent Mn(O) species of para-substituted meso-
tetra(phenyl)porphyrins was observed only at the first seconds
of the reaction at 273 K and quickly disappeared without the
apperance of the absorption band of the catalyst precursor.
Also, high degress of catalyst degradation were observed for
their catalyst precursor. It should be noted that the formation
of the high valent intermediates of the latter occurred at lower
concentrations of TBAO, in comparison with the ortho
substituted manganse porphyrins. The high valent Mn(O)
complexes were also sufficiently reactive toward the oxidation
of a wide range of organic substrates including methyl phenyl
sulfide, aniline, benzyl alcohol, toluene and cyclohexane.
The formation of different intermediates upon the addition of
ImH and TBAO to the solution of Mn-porphyrins in
dichloromethane is summarized in Scheme 4.
Reactivity of the MnV(O) species towards the oxidation of
other substrates
Conflicts of interest
There are no conflicts to declare.
The high valent metal-oxo complexes are usually employed for
the oxidation of different organic substrates36, 83. In order to
study the reactivity of the high valent MnV(O) species towards
the oxidation of other organic compounds, the oxidation of
Acknowledgements
Financial support of this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) is acknowledged.
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New Journal of Chemistry
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DOI: 10.1039/C7NJ04233G
High valent manganese oxo species of a series of electron-rich and electron-deficient meso-
tetra(aryl)porphyrins (aryl = phenyl, 2-chlorophenyl, 2-nitrophenyl, 2-methylphenyl, 2-bromophenyl,
2,6-dichlorophenyl 4-methoxyphenyl, 4-methylphenyl, 4-chlorophenyl and 4-pyridyl) were prepared at
273 K.