Investigation of axial ligand effects on catalytic activity of manganese porphyrin, evidence for the importance of hydrogen bonding in cytochrome-P450 model reactions

Article abstract of DOI:10.1142/S1088424614500102

Various nitrogenous bases, such as imidazoles, pyridines and amines were employed as axial ligands in epoxidation reaction of cyclooctene bytetra-n-butylammonium hydrogen monopersulfate (n-Bu₄NHSO₅), in the presence of Mn(III)-tetrakis(2,3-dimethoxyphenyl)porphyrin-acetate (T(2,3-OMeP)PorMnOAc). T(2,3-OMeP)PorMnOAc is a fairly stable catalyst, with the ability of producing hydrogen bonding. High epoxidation yield of 85 ± 6% was obtained in the presence of imidazole axial ligand with 100% selectivity in 30 min. Higher conversion of around 100% was obtained by pyridine axial base, while selectivity was reduced to 69%. Further epoxidation reactions were also performed using Mn(III)-Tetrakis(2,3-dihydroxyphenyl)porphyrin-acetate (T(2,3-OHP)PorMnOAc) as catalyst. In addition to the usual electronic and steric effects, it is proposed that the catalytic activity depends on the existence and kind of hydrogen bonding between the axial base and the ortho-methoxy or hydroxy groups on the phenyl rings of manganese porphyrin. The cis to trans ratio of cis-stilbene oxide formed by imidazole and pyridine axial bases were obtained as 7.5 and 2.5 respectively. In addition GC-Ms and UV-vis studies were employed to find the nature of active species and product formation. Our DFT calculations disclosed that pyridine hydrogen bonding with moiety of the macrocycle rings strongly affects the relative energies of S/Q spin states in [T(2,3-OMeP)PorMn^V(O)(Py)]⁺, in that it results in the longer Mn-O bond and reactivity toward substrates.

Full text of DOI:10.1142/S1088424614500102

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2014; 18: 1–12
DOI: 10.1142/S1088424614500102
Published at http://www.worldscinet.com/jpp/
Investigation of axial ligand effects on catalytic activity
of manganese porphyrin, evidence for the importance of
hydrogen bonding in cytochrome-P450 model reactions
Elnaz Mesbahi, Nasser Safari* and Mahin Gheidi
Chemistry Department, Shahid Beheshti University, G.C, Evin, Tehran 19839-63113, Iran
Received 30 October 2013
Accepted 15 January 2014
ABSTRACT: Various nitrogenous bases, such as imidazoles, pyridines and amines were employed as
axial ligands in epoxidation reaction of cyclooctene by tetra-n-butylammonium hydrogen monopersulfate
(n-Bu4NHSO5), in the presence of Mn(III)-tetrakis(2,3-dimethoxyphenyl)porphyrin-acetate (T(2,3-
OMeP)PorMnOAc). T(2,3-OMeP)PorMnOAc is a fairly stable catalyst, with the ability of producing
hydrogen bonding. High epoxidation yield of 85 ± 6% was obtained in the presence of imidazole axial
ligand with 100% selectivity in 30 min. Higher conversion of around 100% was obtained by pyridine
axial base, while selectivity was reduced to 69%. Further epoxidation reactions were also performed
using Mn(III)-Tetrakis(2,3-dihydroxyphenyl)porphyrin-acetate (T(2,3-OHP)PorMnOAc) as catalyst. In
addition to the usual electronic and steric effects, it is proposed that the catalytic activity depends on
the existence and kind of hydrogen bonding between the axial base and the ortho-methoxy or hydroxy
groups on the phenyl rings of manganese porphyrin. The cis to trans ratio of cis-stilbene oxide formed
by imidazole and pyridine axial bases were obtained as 7.5 and 2.5 respectively. In addition GC-Ms
and UV-vis studies were employed to find the nature of active species and product formation. Our DFT
calculations disclosed that pyridine hydrogen bonding with moiety of the macrocycle rings strongly
affects the relative energies of S/Q spin states in [T(2,3-OMeP)PorMnV(O)(Py)]+, in that it results in the
longer Mn–O bond and reactivity toward substrates.
KEYWORDS: porphyrin, manganese porphyrin, epoxidation, axial ligand, hydrogen bonding.
hinder and affect axial ligand rotation and interaction by
INTRODUCTION
van der Waals or hydrogen bonding interactions [17–20].
Catalytic epoxidation of olefins have been paid much
In addition, the employment of pyridines and imidazoles
attention, using modified metalloporphyrins as models for
in metalloporphyrin systems for mimicking the axial
cytochrome-P450 [1–8]. Changing the substitution groups
coordination function of P450 and peroxidase has led to
on the periphery of the porphyrin caused in different elect-
marked improvement in the epoxidation of alkane [21–24].
ronic and steric properties on the active site of the catalyst
It is believed that co catalytic activity of nitrogenous bases
and on its reactivity [9–11]. Generation of porphyrins with
arises by accelerating the O–O bond cleavage through
electron withdrawing substituents leads to the more stability
push and pull effects preventing µ- oxo –dimer formation
of catalyst on the expense of the longer reaction time [3, 12,
and secondary interaction such as hydrogen bonding [25].
13]. Substitution of the bulky electron donating groups on
The unusual co-catalytic activity of 2,6-dimethylpyridine
the orthoposition of the phenyl substituted porphyrin result
in the presence of F-20TPPorMnOAc (F-20TPPor stand for
in the both stability and fast reactivity of the catalyst [14–
panta floro tetra phenyl porphyrin) was explained based
16]. In addition, the substitution near the metal center could
on the hydrogen bonding between the ortho-C–F groups
of Mn(Por) with the ortho methyls of pyridine [26, 27].
The observed high activity of the manganese porphyrins
bearing methoxy or hydroxy groups situated at the 2 and
*Correspondence to: Nasser safari, email: n-safari@sbu.ac.ir,
tel: +98 21-29902886, fax: +98 21-22403041
Copyright © 2013 World Scientific Publishing Company

2
E. MESBAHI ET AL.
and imidazole all have ability to make secondary
interaction such as hydrogen bonding. Effects of axial
ligands on catalytic activity of the manganese porphyrins
(Fig. 1) with capability of producing hydrogen bonding
is surveyed both by experimental and theoretical studies.
This study shows that changing substitution on the
periphery of the Mn(por) and nature of axial ligand
can dramatically influence the S/T and S/Q gaps and so
change the reactivity of [(Por)Mn(O)(X)]+.
Ar
R1
R5
R2
R3
OAc
N
N
N
N
Mn
Ar :
Ar
Ar
R4
Ar
R2
T(2,3-OMeP)PorMnOAc OCH3 OCH3
R3 R4
R5
H
R6
H
R1
RESULTS AND DISCUSSION
H
H
H
T(2,3-OHP)PorMnOAc
OH
OH
H
H
H
Epoxidation in the presence of
T(2,3-OMeP)PorMnOAc
Fig. 1. Structures of the substituted manganese(III)tetra-
phenylporphyrins used as catalyst in this work
The T(2,3-OMeP)PorMnOAc was used in alkene
epoxidation in the presence of n-Bu4NHSO5 as oxidant
in dichloromethane medium. The reactions were all
triplicated at room temperature in CH2Cl2 and the
average data are reported. It has been shown previously
[10] that substitution of methoxy groups at both ortho
and meta position of phenyls on the porphyrin ring brings
about fair stability, due to steric crowdedness around the
meso positions, and also fast and high activity, because
of electronic effects and hydrogen bonding. According
to the significant role of axial base in alkene epoxidation
reactions catalyzed with manganese porphyrins, various
nitrogenous bases were employed as axial ligands in
the mentioned reaction. The reactions were all done
at optimized time and conditions, obtained from our
previous report [10]. Conversion and epoxidation yield
percentage in the presence of some axial bases are briefly
shown in Fig. 2. The results for other axial ligands are
3 positions of the phenyls resulted from inter-molecular
and intra-molecular hydrogen bindings as well [10, 14].
The existence of hydrogen bonding in the active site
of natural systems has been proposed as an important
factor for heterolytic O–O bond cleavage, especially in
peroxidase [28].
Iron and manganese porphyrins were extensively used as
models for natural oxidative enzymes. Axial ligands could
alsoaffecttheelectronicconfigurationsoftheoxoporphyrins.
Changingoftheaxialligandbringsaboutdifferentelectronic
configurations in their corresponding ironoxo species as
reported [29].A common active oxidant, called compound 1
which is (Fe=O) p-adduct of the heme unit was extensively
adapted for heme unit in natural system and in model
compounds. Recently, the involvements of hydroperoxo
and alkylperoxo intermediates were reported by peracids
[5, 25, 30–33]. For mangenase porphyrins [(Por)MnIV=ꢀO],
[(Por)+MnIV=O]+,[(Por)+MnV=O]+,[(Por)+ꢀOꢀ=ꢀMnVI=ꢀO]and
[(Por)+Oꢀ=ꢀMnV=O] were proposed as active intermediates
by experimental evidences [34–37] and theoretical studies
[38–40]. Ghosh and coworker have shown by theoretical
studies that [(Por)MnIV=O] has triplet spin state, longer
metal–oxygen bond distances to that of [(Por)FeIV=O].
However, they have also shown that [(Por)Mn=O(py)]+
(where por stands for simple porphine), corresponds to a
Mn(V) configuration reflect formal triple bonding and
singlet ground state. Triplet and quintet states suggesting
[(Por)+MnIV=O(py)] are calculated to be a few tenths of an
electron volt higher than singlet [(Por)-MnV=O(py)]. Ghosh
and Eisenstein have also discussed that axial ligands can
change the S/Q and S/T energies reflecting Mn–O bond
distances and its reactivity. Accordingly it is believed that
high Mn- oxo intermediate gains its reactivity from low
lying T/Q spin states [38, 41].
Fig. 2. Conversion percentage and epoxidation yield percentage
in epoxidation of cyclooctene by n-Bu4NHSO5, in the presence
of T(2,3-OMeP)PorMnOAc and various axial bases, using
molar ratios of {catalyst/axial ligand/cyclooctene/oxidant}:
{1/80/83/164}, (Im: imidazole, 2-MeIm: 2-methylimidazole,
2-EtIm:2-ethylimidazole, 1-BzIm:1-benzylimidazole, 1-MeIm:
1-methylimidazole, Py: pyridine, 4-MePy: 4-methylpyridine, 2,-
6-DiMePy: 2,6-dimethylpyridine, NHEt2: diethylamine, NHEt3:
triethylamine)
In this work, cyclooctene epoxidations in the presence
of Mn(por) catalyst and different nitrogenous axial
bases have been examined. OMe and OH substituted in
the ortho and metha position of the porphyrin, tetra-n-
butylammoniummonopersulfate as oxidant and pyridine
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J. Porphyrins Phthalocyanines 2014; 18: 2–12

INVESTIGATION OF AXIAL LIGAND EFFECTS ON CATALYTIC ACTIVITY OF MANGANESE PORPHYRIN
3
Table 3. Conversion (yield of epoxy) percentage of cyclooctene
Table 1. Conversion (yield of epoxy) percentage of cyclooctene
epoxidationa in the presence of imidazoles
epoxidationa in the presence of amines
Conversion (yield), %
Conversion (yield), %
Entry
Axial ligand
30 min
24 h
Entry
Axial ligand
Imidazole
30 min
86 (85)
24 h
1
2
3
4
Piperidine
31 (28)
40 (31)
23 (15)
23 (12)
32 (25)
42 (39)
28 (22)
23 (14)
1
2
3
4
5
6
5
3
5
4
3
94 (94)
86 (74)
72 (68)
71 (61)
78 (77)
88 (85)
Diethylamine
Triethylamine
Diethylamineb
2-Methylimidazole
4(5)-Methylimidazole
2-Ethylimidazole
75 (64)
69 (62)
49 (43)
73 (70)
81 (76)
a The reactions were all triplicated at room temperature, in CH2Cl2
and the average data are reported 5. GC injections were carried
out after 30 min or 24 h. The molar ratio for {catalyst/axial
ligand/substrate/oxidant} is {1/80/83/164}. Internal standard
method was used. b A little amount of methanol was adjoined to
the default solvent.
1-Methylimidazole
1-Benzylimidazole
6
a
The reactions were all triplicated at room temperature, in
CH2Cl2 and the average data are reported. GC injections were
carried out after 30 min or 24 h. The molar ratios for {catalyst/
axial ligand/substrate/oxidant} are {1/80/83/164}. Internal
standard method was used.
Table 2. Conversion (yield of epoxy) percentage of cyclooctene
epoxidationa in the presence of pyridines
Conversion (yield), %
30 min
97 (69)
24 h
Entry
Axial ligand
Pyridine
1
2
3
4
5
6
7
3
6
5
8
100 (69)
100 (75)
77 (62)
59 (42)
50 (43)
73 (60)
53 (26)
Fig. 3. Cyclooctene conversion percentage (after 30 min) vs.
pKa values of nitrogen donors, in catalytic epoxidation by
T(2,3-OMeP)PorMnOAc
4-Methylpyridine
68 (60)
46 (33)
55 (46)
15 (7)
2,6-DiMethylpyridine
4-DiMethylaminopyridine
4-Aminopyridineb
Nicotineamideb
6
3
caused by replacement of any hydrogen on the imidazole
ring, with a larger group of atoms, lower conversion
(yield) percentage is anticipated (Table 1, Entry 1, 2, 3
and 4, Fig. 2). Furthermore, it appears that within this
group the dominant factor in determining the cocatalytic
activity, depends on the substituent position. In other
words the importance of electronic and steric factors on
the substituents are not the same in different positions of
the imidazole ring in that the steric effects in position 2
are more considerable comparing to position 1 whereas
electronic ones are almost the same in both positions.
Hence the larger a substituent is, the lower conversion
(yield) percentage is inevitable. Besides, the more
conversion (yield) percentage for replacement of the
methyl with the larger group of benzyl discloses that the
steric effects are of low importance in the 1 position that
could be simply ignored. The conversion (yield of epoxy)
percentage in the reactions using imidazoles are detailed
in Table 1.
20 (9)
4-Cyanopyridine
20 (10)
4
a The reactions were all triplicated at room temperature, in CH2Cl2
and the average data are reported. GC injections were carried out
after 30 min or 24 h. The molar ratio for {catalyst/axial ligand/
substrate/oxidant} is {1/80/83/164}. Internal standard method
was used. b A little amount of methanol was added to the default
solvent in order to make the axial base soluble.
also presented in Tables 1, 2 and 3 followed by the pKa
diagram (Fig. 3).
On the basis of the results obtained, a general trend
is revealed as follows: imidazoles with the ability
of s-donating and weak p-donating, mainly operate
better as a cocatalyst, compared to other nitrogenous
bases. Among the R-substituted imidazoles, imidazole
itself has the best co catalytic effect. Apparently the
conversion and epoxide selectivity would be reduced,
wherever each of the hydrogen atoms is replaced with
a substituent. In addition to the hydrogen bonding, it is
clear that the steric effects are also operative, owing to
the existence of methoxy groups on the meso-situated
phenyl rings. Therefore, in a more hindered situation,
Among nitrogen donors possessing relatively lower
p-donating ability compare to that of imidazole, pyridine
and 4-methylpyridine showed unexpectedly quite good
cocatalytic activity for our catalyst (Table 2, Entry 1 and 2)
controversy to that of tetraphenyl porphyrin manganese
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4
E. MESBAHI ET AL.
for various nitrogen donors vs. pKa values is plotted in
Fig. 3. As it can be seen in Fig. 3, it seems that the pKa
values between 5–8 is required for high cocatalytic effect
of axial bases.
CH3
OCH3
O
H
N
n
M
The aforementioned trends also remained opera-
tive after 24h, since the variations of the conver-
sions and the yields after 30min and 24h are not
conspicuous, except for those cases where the coordi-
nating atom of the axial base is accompanied by
steric hindrance, as it is in 2-ethylimidazole and 2,6-
dimethylpyridine. This is in line with the slower reaction
rate, less destruction and more stability of the catalyst in
these situations.
Fig. 4. Possible hydrogen bonding interaction between pyridine
and the ortho-methoxy group of a meso-positioned phenyl ring
on a schematic T(2,3-OMeP)PorMnOAc
catalyst. Mohajeretal. reported35%yieldforcyclooctene
epoxidation in the presence of TPPMnOAc with pyridine,
while reporting the number of 90 for the epoxidation
yield with T(2,3,4,5,6-FP)PorMnOAc and the same axial
base [26]. The observed difference in yield was proposed
to be related to the C–H…F–C interactions between
the ortho-C–F bonds of T(2,3,4,5,6-FP)PorMnOAc and
the vicinal C–H bonds of nitrogen in pyridine. With
regard to all these points, a kind of hydrogen bonding is
suggested to be, due to the presence of methoxy groups
which facilidate pyridine coordination to the manganese
(Fig. 4). Also this hydrogen bonding with moiety of the
macrocycle rings affects the relative energies of S/Q spin
states in [T(2,3-OMeP)PorMnV(O)(Py)]+ and reactivity
of this species according to our theoretical calculations
(see the following paragraphs in theoretical part). The
previous theoretical studies in literature has shown
that the singlet ground state (S) is unreactive and only
the higher energy triplet (T) or quintet (Q) states have the
oxo character needed for reactivity [41]. Thus, a species
such as [(TPP)Mn(O)(Py)]+ with a single ground state
(S) is unreactive with regard to our theoretical studies
while [T(2,3-OMeP)PorMnV(O)(Py)]+ is reactive with a
quintet state. Note that these are all in a good accordance
with the experimental results explained above. It seems
the proposed hydrogen bonding is no more existed for
2,6-dimethylpyridine; a substituted pyridine where the
hydrogen atoms situated near the nitrogen are displaced
by the massive methyl groups (Table 2, Entry 3). As a
result of this displacement, it is evident that the pyridine
coordination to the metal center would be dealt with
significant hindrance and the hydrogen bonding will not
exist anymore. Within this group, 4-cyanopyridine with
the electron withdrawing CN substituent and less s or
p-donating operates as a poor cocatalyst due to probably
the less coordination ability. Another remarkable point
about pyridines is that the electron donation enhancement
on the pyridine ring and so the more p-donating of it, did
not show the higher activity. This could be explained by
the less polarization of the C–H bonds in the vicinity of
the coordinating nitrogen. The less partial positive charge
on hydrogens proning to participate in the hydrogen
bonds (as mentioned with the oxygen atom of the
methoxy groups) would result in their weakening.
Evaluation of substituent effects using Taft
parameters
According to the data obtained, it is obvious that the
electronic and steric effects of substituents in different
positions are of diverse significance. Hence, imidizoles
are classified as class (I) and (II), in terms of their
substituted position shown in Fig. 5.
In order to get insight into substituent electronic and
steric influences, the contribution percentage of each
have been determined individually, using Taft parameters
[42–44], approximations and Equations 1 and 2 [45, 46].
The results are represented in Table 4.
Electronic effect (%) = [|r*| ∕ (|r*| +ꢀ|d|)] × 100 (1)
Steric effect (%) = [1 – electronic effect] × 100
(2)
The negative values of d for both classes, is expressive
of the reaction rate reduction in the presence of a larger
substituent. The contribution of electronic effects is
predominantlymorethanthestericonesinbothcategories,
but it revealed that the steric factor at the “2” position
(group (II)) plays a more significant role rather than
position “1” (group (I)); however, it is still less important
than the electronic ones. This is in accordance with the
N
N
H
Me
Et
N
H
Me
Bz
N
H
(I)
(II)
Fig. 5. Imidazole classes in terms of substituent position
Table 4. Electronic and steric effect percentage of substituents
and their classes
Class
r*
d
Electronic
effect, %
Steric
effect, %
And finally amines, the pure s-donors, work as
relatively poor cocatalysts, though they have higher pKa
values compared to the other nitrogenous bases, Table 3.
The diagram of cyclooctene conversions in epoxidation
(I) 1.71 × 10-1 -1.47 × 10-2
(II) 2.5
-9.43 × 10-1
92
72
8
27
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INVESTIGATION OF AXIAL LIGAND EFFECTS ON CATALYTIC ACTIVITY OF MANGANESE PORPHYRIN
5
CN
results obtained experimentally represented in Table 1
H
N
H
and Fig. 2. As the substituent at the “2” position gets
enlarged, the conversion (epoxidation yield) percentage
would be decreased clearly in the second category. On
the contrary, the replacement of “methyl” group at the
“1” position, with a more satirically hindered substituent
group, such as “benzyl,” results in higher conversion
(epoxidation yield) percentage, whereas the electronic
factors are of great importance and the steric contribution
could be simply ignored. Moreover, considering the
positive values for r* it could be concluded that the
reaction would progress more slowly, whenever an
electron donating substituent (such as methyl, ethyl and
benzyl) existed (Table 1). In other words, the reaction
would have a better progress when electron donating
groups are not present (Table 1).
C O
C
OH
OH
OH
OH
OH
O
O
H2
H
N
N
H
Mn
Mn
Mn
(c)
(a)
(b)
Fig. 7. Feasible orientation of (a) 2-methylimidazole, (b) 4-
cyanopyridine and (c) acetate to the ortho-hydroxy groups of
phenyl rings in the effective isomer of T(2,3-OHP)PorMnOAc
less possibility of hydrogen bonding between the C–H of
the methyl group located at the 4(5) position of imidazole
and the hydroxies.
Observed data for pyridines also express that they are
prone to produce interaction between the C–H (in the
vicinity of the coordinating nitrogen) and the oxygen
of hydroxy group (as it is shown for 4-cyanopyridine in
Fig. 7).Another evidence supporting the idea of hydrogen
bonding is the relatively better result of 4-cyanopyridine
(4-CNPy). 4-CNPy is weaker coordinating ligand
compared to that of pyridine and therefore mainly did
not act as good axial ligand. We assume that due to the
polarization of hydrogen C–H bond by CN substitution,
a better hydrogen bonding is resulted and so coordination
of the 4-CNPy to T(2,3-OHP)PorMnOAc is facilitated.
Therefore we propose that the better conversion of
cyclooctene in the presence of 4-CNPy is another
indication of the importance of hydrogen bonding, see
Fig. 7b. Although the conversion is relatively high in this
case, it was low for T(2,3-OMeP)PorMnOAc. We cannot
fully explain this phenomenon in this stage.
Epoxidation in the presence of
T(2,3-OHP)PorMnOAc
To further explore hydrogen bonding effects,
examination of cyclooctene epoxidation, catalyzed by
T(2,3-OHP)PorMnOAc (T(2,3-OHP)Por stands for
the substituted tetraphenylporphyrin ligand with two
hydroxy groups at the ortho and meta position of the
phenyl groups) in the presence of different axial bases
were also studied. The results are shown in Fig. 6.
To note tetrabuthylammoniumacetate (TBAA) and 2-
methylimidazole (2-MeIm) have the highest epoxidation
yield probably due to the same hydrogen bonding between
the mentioned hydroxy groups oriented appropriately to
these ligands as shown in Fig. 7. Lack of such a hydrogen
bondingcouldbeconsideredasanexplanationforthelower
conversions of other imidazoles [14]. The lower conversion
percentage in the presence of 4(5)-methylimidazole
(4(5)-MeIm) compared to 2-methylimidazole is simply
expressed by the fluxional hydrogen movement between
the two nitrogen atoms. This fluxionality brings about the
Active oxidizing species and mechanism
UV-visible spectra. UV-visible spectra of catalyst
solutions (10-5 M) in the presence of the reaction
components were investigated in order to get information
about catalyst stability and the active
oxidizing species. The UV-vis spectra were
surveyed using imidazole and pyridine
axial ligands. The results of each are
depicted in Figs 8, 9 and 10 respectively.
Imidazole and UV-vis spectra. UV-vis
spectra of T(2,3-OMeP)PorMnOAc with
imidazole in CH2Cl2 in the absence of
substrate followed by addition of oxidant
were investigated. As the oxidant was
added into the catalyst solution containing
imidazole, a new wide peak appeared at
408 nm accompanied by occurrence of a
Soret band red shift (6 nm, lmax: 478 nm)
(Fig. 8).
The same results observed while
examining the catalyst system under
identicalconditionsinCH2Cl2,butthistime
in the presence of substrate (see Fig. S1
Fig. 6. Conversion percentage and epoxidation yield percentage in
epoxidation of cyclooctene by n-Bu4NHSO5, in the presence of T(2,3-OHP)-
PorMnOAc and various axial bases, using molar ratios of {catalyst/axial ligand/
cyclooctene/oxidant}:{1/80/83/164}, after 10 h, (TBAA: tetrabuthylammonium-
acetate, 2-MeIm: 2-methylimidazole, 4(5)-MeIm: 4(5)-methylimidazole, 1-BzIm:
1-benzylimidazole, 1-MeIm: 1-methylimidazole, Im: imidazole, Py: pyridine,
4-MePy: 4-methylpyridine, 4-CNPy: 4-cyanopyridine, Pipe: piperidine)
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6
E. MESBAHI ET AL.
Fig. 8. The UV-vis spectra of T(2,3-OMeP)PorMnOAc in
CH2Cl2 (1) solution of 10-5 M cat., (2) solution of 10-5 M cat.
with imidazole; {cat/im}:{1/20}, (3) solution of 10-5 M cat.
with imidazole and oxidant; {cat/im/ox}:{1/20/164} (after
Fig. 10. The UV-vis spectra of T(2,3-OMeP)PorMnOAc in
CH2Cl2 (1) solution of 10-5 M cat., (2) solution of 10-5 M cat.
with pyridine; {cat/py}:{1/80}, (3) solution of 10-5 M cat.
with pyridine and oxidant; {cat/py/ox}:{1/80/164} (after the
moment poured into the cell), (4) solution of 10-5 M cat. with
pyridine and oxidant; {cat/py/ox}:{1/80/164} (after 3 min), and
(5) solution of 10-5 M cat. with pyridine and oxidant; {cat/py/
ox}:{1/80/164} (after 2h stirring in the presence of Na2S2O4).
The spectra were all recorded at ambient temperature
the moment poured into the cell), and (4) solution of 10-5
M
cat. with imidazole and oxidant; {cat/im/ox}:{1/20/164} (after
5 min). The spectra were all recorded at ambient temperature
[(Por)MnIV=O] is reported to be seen at room temperature
at the same positions in Figs 8 and 9 [34]. Hence, with
regard to the points mentioned above we suppose that
the intermediate species observed at 408 nm is relative
to an active oxidizing species or an intermediate formed,
containing MnIV, such as [T(2,3-OMeP)PorMnIV(O)(Im)].
Pyridine and UV-vis spectra. The UV-vis spectra of
T(2,3-OMeP)PorMnOAc with pyridine in CH2Cl2, and
the oxidant effects on this catalytic system is followed
(Figs 10 and S3). It is observed that addition of pyridine
to the catalyst solution is attended by a Soret shift to the
higher wavelengths. The red shift is supposed to be taken
place due to the production of a six coordinated complex
by pyridine, such as [Mn(por)(py)(OAc)] species. Note
that the aformentioned Soret shift could be considered as
a hint showing the presence of a more effective hydrogen
bonding with pyridine in comparison to the imidazole
case where there wasn’t observed such a red shift
(Fig. 8). Hydrogen bonding between C–H of pyridines
with OMe of the phenyl ring caused shortening of Mn–N
bond from 2.42 Å in (2,3-OMeP)PorMn (O)(Im) to that
of 2.22 Å in (2,3-OMeP)PorMn (O)(Py) confirming the
result obtained in UV-vis studies. Moreover, as it is clear
in Fig. 10, addition of oxidant to the catalyst solution
containing pyridine, caused a sharp reduction in the Soret
band intensity in that it led to be totally disappeared.
Adjoining of a reductant such as Na2S2O4 to this system
was noteworthy since it did not have any effect on the
appearance of the vanished Soret band. This could
indicate that the catalyst is highly reactive and had been
instantly destroyed, upon addition of the oxidant.
Fig. 9. The UV-vis spectra of T(2,3-OMeP)PorMnOAc in
CH3CN, (1) is the solution of 10-5 M cat., (2) is the solution
of 10-5 M cat. with imidazole; {cat/im}:{1/20}, (3) relates to
the solution of 10-5 M cat. with imidazole and oxidant; {cat/im/
ox}:{1/20/164} (after the moment poured into the cell). (4) is
the solution of 10-5 M cat. with imidazole and oxidant; {cat/im/
ox}:{1/20/164} (after 15 min). All the spectra were recorded at
room temperature
in the Supplementary material). The more stability of the
catalyst against catalytic degradation was also clear in
these series of spectra (Fig. S1). The subsequent spectra
in Fig. 9 show more investigations of the intermediate
peak in acetonitrile (appearing at 407nm) with the
conditions alike to the formers in Fig. 8. The increase
in intensity and stability of the intermediate peak was
evident in acetonitrile which is more polar (see Fig. 9
and S2).
In the literature, are some reports characterizing
such an intermediate as [(Por)MnV=O]+ species [35,
36]. However, Nam et al. have indicated that [(Por)-
MnV=O]+ species is not stable enough to be seen at
room temperature and has a Soret band above 430 nm
observed at low temperature (> -30°C). On the other hand,
Further UV-vis studies were also pursued (Fig. S4). It is
clear that the Soret band has slowly declined in the presence
of substrate. Moreover with respect to the spectra shown in
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INVESTIGATION OF AXIAL LIGAND EFFECTS ON CATALYTIC ACTIVITY OF MANGANESE PORPHYRIN
7
Table 5. cis to trans product ratios of cis-stilbene
epoxidation for varied axial ligands
Table 6. cis to trans product ratios of cis-stilbene epoxidation
for varied axial ligands
Axial ligand
Im
2-MeIm
5.5
Py
4-MePy
2.5
Product
No.
Yield, %
cis epoxide/
trans epoxide
7.5
2.5
Structure
Name
(I)
cis epoxide
8.10
3.94
Fig. 10, cyclooctene epoxidation using pyridine, apparently
goes along with completely catalyst destruction. The active
oxidizing species is highly unstable and different from the
one in the systems containing imidazole with regard to
the spectra demonstrated in the previous section (Figs 8
and 9). This active species is supposed to be the one such
as [T(2,3-OMeP)PorMnV(O)(Py)]+ or any other similar
active species, not stable at room temperature. The higher
stability of the catalyst in the presence of substrate was
evidently seen compared to Fig. 10 (Fig. S4).
(II)
trans epoxide
(III)
(IV)
aldehyde
31.29
11.31
ketone
Cis-stilbene epoxidation. Cis-stilbene epoxidation
was carried out under the conditions similar to those used
in the cyclooctene reactions. The results in form of the cis
to trans epoxide ratios are represented in Table 5.
[28, 47]. These include the formation of species such as
an alkene drived p-cation radical and a carbon radical
as shown in Fig. 11. Apparently whenever the reaction
goes through each of the radical pathways, a variety of
products may be readily yielded.
The notable difference in cis to trans epoxide ratios,
high conversion and lower selectivity for epoxide yield,
the production of aldehyde and ketone by-products
all confirm the radical pathway in the case of pyridine
axial base. On the contrary, with regard to the negligible
difference of conversion and epoxidation yield, in the
presence of imidazole, epoxide seems to be the sole main
product. Note that this would not happen if the reaction
goes through the same pathway proposed for pyridine.
Therefore a concerted mechanism may play a role here
for the reaction progress (Fig. 12).
As it can be interpreted from the data shown in Table 5,
the cis to trans ratios and the selectivity in the presence
of imidazoles and pyridines are not the same. This means
that the active oxidizing species, the reactivity or the
reaction mechanism might be distinctive depending on
the kind of axial base that is used. Therefore concerning
the relatively small amount of cis/trans-stilbene oxide
in epoxidation with pyridine (2.5), the active oxidizing
species may be dealt with more radical nature in the
pyridine derivate in comparison to imidazole (with
the cis/trans-stilbene oxide of 7.5). This concept is in
agreement with the data obtained for 4-methylpyridine
and 2-methylimidazole with the cis/trans-stilbene oxide
of 2.5 and 5.5 respectively. The cis to trans ratios in
Table 5 express that the reaction has probably proceeded
through different mechanisms as the free rotation around
C=C seems to have occurred more readily in the presence
of pyridine or 4-methylpyridine. The idea of radical
nature for the reaction with pyridine and concerted one
with imidazole is explained further in the next paragraph
(Fig. 12).
Concerning the notable difference of conversion and
epoxy yield values for the epoxidation reactions using
pyridine or 4-methylpyridine, the GC-MS spectrum
of the cis-stilbene epoxidation with 4-methyl pyridine
has been studied as a sample in order to find out more
information about its mechanism and products. The data
taken out of GC-MS chromatogram revealed that there
are a kind of aldehyde and ketone product other than
the expected epoxides; cis and trans-stilbene oxide. The
products and the yield percentage corresponding to each
are shown in Table 6.
Theoretical studies
The structures of 1,3,5[(TPP)Mn(O)(Im)]+; where TPP
is tetraphenylporphyrin dianion, were fully optimized and
computed in all possible spin states. OPBE/6-311+G**
shows that all spin states potential energy surfaces are
close in this species, for example the singlet state is only
0.89 and 1.14 kcal/mol more stable than triplet and quintet
states respectively (see Table 7). Thus calculations show
that the species [(TPP)Mn(O)(Im)]+ has a singlet ground
2
2
state with dxy a2u electronic configuration with closely
lying triplet and quintet states, 3,5[(TPP·)MnIV(O·)(Im)]+.
The electronic configuration for both triplet and quintet
1
states is dxy a2u1 p*xz1 p*yz1 in which the unpaired electrons
are coupled antiferromagnetically and ferromagnetically
respectively (Fig. 13). In the triplet and quintet states,
three unpaired electrons are localized on Mn (2.45) and
O (0.65) and the other unpaired electron is on the macro
cycle (0.90). This is a result of the higher participation of
the p*xz and p*yz (Mn=O) orbitals in the singly-occupied
There are some mechanism reports of parallel reactions
occurring besides the concerted epoxidation leading to
different intermediates proposed along reaction path
Copyright © 2013 World Scientific Publishing Company
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8
E. MESBAHI ET AL.
1-electron
oxidation
concerted
"oxene"
O
MnV
+
insertion
+
O
MnIV
O
MnIV
O
O
epoxide
radical
chain products
+
MnV
MnIV
rearranged
products
rearranged
products
epoxide
rearranged
products
+
epoxide
radical
chain products
Fig. 11. Intermediates proposed along different oxidation mechanisms of alkenes by [(Por)MnV=O]+, and their corresponding
products expected
[MnO]3+
orbitals, compared to the singlet state. This is
also consistent with the elongation of the Mn–O
+
imidazole
pyridine
(manganese-oxygen distance) in the quintet
state (1.64Å) relative to singlet state (1.51Å).
Therefore the ring in 3,5[(TPP·)MnIV(O·)(Im)]+
has radical nature, since the charge density on the
porphyrin is about 0.00 and the spin distribution
]
[
]
[
Mn
Mn
1
is about 0.90 while in [(TPP)Mn(O)(Im)]+, the
[MnO]2+
[Mn]
O
O
charge on the porphyrin is -0.38 and the spin
density is 0.00. Due to the electronegativity of
Mn(V), in quintet state, one electron is readily
shifted from the macrocycle ring to manganese
that results in the Mn (IV) p cation radical species
3,5[(TPP·)MnIV(O·)(Im)]+ with O·-character. Due
to this electron transfer mainly on the manganese,
the charge density on the manganese decreases
from around 1.62 in 1[(TPP)Mn(O)(Im)]+ to
1.48 in 3,5[(TPP·)MnIV(O·)(Im)]+ which is better
viewed as Mn(IV) d3 with the macrocycle as a p
cation radical. The radical nature of macrocycles,
oxygen atom and the elongation of the Mn–O
(manganese–oxygen distance) in these species
makes complexes of 3,5[(TPP·)MnIV(O·)(Im)]+ a
highly reactive species. While the ground state
O
+
[
]
Mn
O
O
cis-stilbeneoxide
and
trans-stilbeneoxide
N
N
1
N
N
singlet is unreactive and in principle, [(TPP)-
CHO
aldehyde
MnV(O)(Im)]+ could also be reactive by transfer
to the intermediate and high-spin potential
energy surfaces, that these surfaces are easily
accessible.
HO
The structures of 1,3,5[(TPP)Mn(O)(Py)]+
were also fully optimized and computed in all
possible spin states with the aforementioned
method. In this complex, the singlet spin state
(ground state) is 3.50 kcal/mol more stable than
the triplet spin state and 4.62kcal/mol more
stable than the quintet spin state and both are
in very good agreement with data reported by
O
ketone
HO
Fig. 12. Possible mechanisms of yielding diverse products in cis-stilbene
epoxidation reaction, with 4-MePy and T(2,3-OMeP)PorMnOAc
Copyright © 2013 World Scientific Publishing Company
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INVESTIGATION OF AXIAL LIGAND EFFECTS ON CATALYTIC ACTIVITY OF MANGANESE PORPHYRIN
9
Table 7. Calculated relative energies for manganese porphyrin
in all possible spin states using OPBE/6-311+G** method
Compounds
Singlet Triplet Quintet
[(TPP)Mn(O)(Py)]+
[T(2,3-OMeP)PorMn(O)(Py)]+
[(TPP)Mn(O)(Im)]+
0.00
13.40
0.00
3.50
—
4.62
0.00
1.14
0.00
0.89
—
[T(2,3-OMeP)PorMn(O)(Im)]+
7.05
Fig. 14. Correlation diagram for the d-block orbitals of Mn
porphyrin complex with the angle q varying from 92° to
about 100°
by two hydrogen bond interactions with methoxy groups
(Fig. 14). Due to this hydrogen bonding, the Mn–Nax
distance reduces to 2.22 Å (see Fig. 14). The presence
5
of the axial pyridine with distance of 2.22 Å in [T(2,3-
OMeP)Por·MnIV(O·)(Py)]+ as the sixth axial ligand
caused the elongation of the Mn–O double bond distance
to 1.67 Å, which is unexpectedly longer than that of the
Fig. 13. High-lying occupied and low-lying virtual orbitals of
1,5[(TPP)Mn(O)(Im)]+ (a) singlet ground state and (b) closely
lying triplet and quintet states
1
Mn–O triple bond distance (1.51Å) in [(TPP)MnV(O)-
(Py)]+ (with Mn–Nax = 2.47Å). The Mn–O bond is
influenced by the Mn–Nax bond, because both use the Mn
pz, and dz2 orbitals. When the Mn–Npy s-bond is strong, it
uses a larger part of these orbitals and leaves less for the
Mn–O bond. As a result, a strong s-bonding between Mn
and Npy leads to weaken the Mn–O bond. Thus, the axial
ligand as the pyridine and its interaction with the O–CH3
moiety cause the shift in spin density and increase in
reactivity of Mn porphyrin. The effect of the O–CH3
groups on axial pyridine changes the high-lying occupied
and low-lying virtual orbital energy levels.
Ghosh and Eisenstein [38, 41] (see Table 7). The better
p-accepting ability of pyridine ligand is also shown to
increase the singlet/triplet and singlet/quintet energy gaps
[48], leading to a slower reaction relative to imidazole
ligand. These theoretical results are consistent with
the experimental results showing that pyridine is not a
proper axial ligand for TPPMnOAc in the cyclooctene
epoxidation (yield: 35%) despite the imidazole working
as a good one (yield: 87%) [26].
Figure 14 shows that with the elongation of the
Mn–O (double bond), in the quintet energy surface,
The structures of [T(2,3-OMeP)PorMnV(O)(Im)]+ and
[T(2,3-OMeP)PorMnV(O)(Py)]+ were optimized by DFT
methods. In both these complexes, the quintet spin state
is ground state that in [T(2,3-OMeP)PorMnV(O)(Im)]+,
the quintet spin is only 7.05 kcal/mol more stable than
the singlet spin state while in [T(2,3-OMeP)PorMnV(O)-
(Py)]+, quintet spin is 13.40 kcal/mol more stable than
the singlet spin state (see Table 7). This result shows
that the complex [T(2,3-OMeP)PorMnV(O)(Im)]+ has an
admixture of singlet-quintet, leading to a slower reaction
relative to the complex [T(2,3-OMeP)PorMnV(O)(Py)]+.
The axial ligands as pyridine and imidazole are anchored
5
the stabilization of p-radical resonance form, [T(2,3-
OMeP)Por·MnIV(O·)(Py)]+, is increased because the
energy surfaces of singly occupied p*xz and p*yz orbitals
are decreased. While in the singlet energy surface, the
stabilization of 1[(TPP)MnV(O)(Py)]+ with the shortening
of Mn–O (triple bond) is increased because the energy
surfaces of p*xz and p*yz LUMOs are increased. In Fig. 14
we present a schematic correlation diagram, which shows
thechangestotheshapeandtheenergyofthed-blockorbi-
tals during this geometrical deformation, as the angle q
1
varies from 92° to about 100°. In [(TPP)MnV(O)(Py)]+
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 9–12

10
E. MESBAHI ET AL.
idea has been also supported once before according to
our UV-vis spectra showing the red shift upon addition of
pyridine while not showing it in the presence of imidazole
(Figs 8 and 10).
EXPERIMENTAL
Materials
Synthesis and metalation of T(2,3-OMeP)PH2 were
performed according to the Lindsey and Adler methods
respectively [49, 50]. T(2,3-OHP)PorMnOAc was pre-
pared on the basis of the porphyrin demethylation method
of James [51] with some modifications in purification
process. The green mixture containing BBr3 in CH2Cl2
was exposed to heat up to 50°C so that the extra acid
not participating in reaction and the solvent would be
evaporated. The corresponding soluble salt of oxon (the
commercial triple potassium salt, which is insoluble in
organic solvents), n-Bu4NHSO5 was prepared and used
instead [10, 52]. All nitrogenous bases and also any
other chemical substances used were purchased from
the Merck, Fluka and Alfa Aesar companies. Tetra-
n-butylammonium acetate (TBAA) was prepared on
the basis of the literature method used for the oxidant
synthesis [10], utilizing sodium acetate (NaOAc) as the
starting salt instead of potassium monopersulfate.
Fig. 15. Energy diagram for conversion of [T(2,3-OMeP)-
PorMnV(O)(Py)]+ (with hydrogen bonding) to [T(2,3-OMeP)-
PorMnV(O)(Py)]+ (without hydrogen bonding) on the quintet
surface
complex p*xz and p*yz orbitals are destabilized (with
q:100°, Mn–O:1.51Å and Mn–Npy: 2.47Å), as the overlap
with the ligand orbital is increased. It must be noted that
a crossing between the energy levels of the orbitals that
are destabilized (xz and yz), can occur if q and Mn–O
(manganese–oxygen distance) become sufficiently large
and small, respectively.
Instrumentation
The radical nature of the macrocycle and O atom in
5[T(2,3-OMeP)Por·MnIV(O·)(Py)]+ and the elongation of
the Mn–O (double bond) distance possibly make it more
reactive in the oxidation process. This is in accordance
with our experimental data, and also could be regarded
as a resonable proof for the radical mechanism suggested
for pyridine as explained previously (Fig. 12).
GC (Gas chromatography) analyses were done using
a Trace GC ultra from the Thermo Company equipped
with Rtx®-1 capillary column and FID detector. UV-vis
spectra surveys were followed by a Shimadzu 2100
spectrophotometer. Further investigation of products in
cis-stilbene epoxidation was performed by means of a
Shimadzu Qp-2010 plus GC-Mass system.
Figure 15 shows an image of the structures from the
data obtained by the OPBE method at a quintet spin state.
It demonstrates that, in [T(2,3-OMeP)PorMnV(O)(Py)]+
with hydrogen bonding, the pyridine rotates around the
z axis to produce the [T(2,3-OMeP)PorMnV(O)(Py)]+
without hydrogen bonding that has been destabilized
by about 4.27kcal/mol. Our calculations show that a
large barrier of rotation, 9.05kcal/mol, is needed for
the rotation of pyridine around the z axis on the quintet
surface.
Epoxidation reactions
Epoxidation of alkenes by n-Bu4NHSO5 was investi-
gated in the presence of T(2,3-OMeP)PorMnOAc and
various nitrogenous bases using the optimized molar
ratios and time [10]. The reactions were all performed
after preparing the stock solutions in CH2Cl2 as follows:
substrate solution (1 mL, 0.05 M) was poured in a 5mL
volumetric flask. The substrate solution also contained
n-octane (0.03 M) as the inner standard. n-Bu4NHSO5
(0. 043g) was adjoined after adding the nitrogenous bases
solution(0.48mL,0.1M),(liquidbaseswereaddeddirectly
to the reaction flask). Finally epoxidation was started
by adding the catalyst solution (0.2mL, 3 × 10-3 M,
in CH2Cl2), and proceeded in a closed system stirring
by a magnet stirrer for 24h at room temperature. The
reaction solution (0.2mL) was injected into GC system
after 30min and 24h. Note that a small amount of
methanol was added to the default solvent, whereas the
In [T(2,3-OMeP)PorMnV(O)(Im)]+ with hydrogen
bonding, the imidazole rotates around the z axis to
produce the [T(2,3-OMeP)PorMnV(O)(Im)]+ without
hydrogen bonding that has been destabilized by about
2.20 kcal/mol and the barrier of rotation is only 5.62 kcal/
mol (see Fig. S5).
Therefore it could be concluded that in the [T(2,3-
OMeP)PorMnV(O)(Py)]+ species, the hydrogen bonding
has a more significant role in the fixation of the axial base,
and so making a larger rotation barier. The aformentioned
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INVESTIGATION OF AXIAL LIGAND EFFECTS ON CATALYTIC ACTIVITY OF MANGANESE PORPHYRIN
11
nitrogenous base was insoluble in CH2Cl2 (< 0.03 of the
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