Redox potentials of oxoiron(IV) porphyrin π-cation radical complexes: Participation of electron transfer process in oxygenation reactions

Article abstract of DOI:10.1021/ic102564e

The oxoiron(IV) porphyrin π-cation radical complex (compound I) has been identified as the key reactive intermediate of several heme enzymes and synthetic heme complexes. The redox properties of this reactive species are not yet well understood. Here, we report the results of a systematic study of the electrochemistry of oxoiron(IV) porphyrin π-cation radical complexes with various porphyrin structures and axial ligands in organic solvents at low temperatures. The cyclic voltammogram of (TMP)Fe^IVO, (TMP = 5,10,15,20-tetramesitylporphyrinate), exhibits two quasi-reversible redox waves at E_1/2 = 0.88 and 1.18 V vs SCE in dichloromethane at -60 °C. Absorption spectral measurements for electrochemical oxidation at controlled potential clearly indicated that the first redox wave results from the (TMP)Fe^IVO/[(TMP^+?)Fe^IVO]⁺ couple. The redox potential for the (TMP)Fe^IVO/[(TMP ^+?)Fe^IVO]⁺ couple undergoes a positive shift upon coordination of an anionic axial ligand but a negative shift upon coordination of a neutral axial ligand (imidazole). The negative shifts of the redox potential for the imidazole complexes are contrary to their high oxygenation activity. On the other hand, the electron-withdrawing effect of the meso-substituent shifts the redox potential in a positive direction. Comparison of the measured redox potentials and reaction rate constants for epoxidation of cyclooctene and demethylation of N,N-dimethylanilines enable us to discuss the details of the electron transfer process from substrates to the oxoiron(IV) porphyrin π-cation radical complex in the oxygenation mechanisms.

Full text of DOI:10.1021/ic102564e

ARTICLE
pubs.acs.org/IC
Redox Potentials of Oxoiron(IV) Porphyrin π-Cation Radical
Complexes: Participation of Electron Transfer Process in
Oxygenation Reactions
Akihiro Takahashi, Takuya Kurahashi, and Hiroshi Fujii*
Institute for Molecular Science and Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences,
Myodaiji, Okazaki 444-8787, Japan, and Department of Functional Molecular Science, The Graduate University for
Advanced Studies, Myodaiji, Okazaki 444-8787, Japan
S Supporting Information
b
ABSTRACT: The oxoiron(IV) porphyrin π-cation radical
complex (compound I) has been identified as the key reactive
intermediate of several heme enzymes and synthetic heme
complexes. The redox properties of this reactive species are
not yet well understood. Here, we report the results of a
systematic study of the electrochemistry of oxoiron(IV) por-
phyrin π-cation radical complexes with various porphyrin
structures and axial ligands in organic solvents at low tempera-
tures. The cyclic voltammogram of (TMP)FeIVO, (TMP =
5,10,15,20-tetramesitylporphyrinate), exhibits two quasi-rever-
sible redox waves at E1/2 = 0.88 and 1.18 V vs SCE in dichloromethane at À60 °C. Absorption spectral measurements for
electrochemical oxidation at controlled potential clearly indicated that the first redox wave results from the
(TMP)FeIVO/[(TMP+•)FeIVO]+ couple. The redox potential for the (TMP)FeIVO/[(TMP+•)FeIVO]+ couple undergoes a
positive shift upon coordination of an anionic axial ligand but a negative shift upon coordination of a neutral axial ligand (imidazole).
The negative shifts of the redox potential for the imidazole complexes are contrary to their high oxygenation activity. On the other
hand, the electron-withdrawing effect of the meso-substituent shifts the redox potential in a positive direction. Comparison of the
measured redox potentials and reaction rate constants for epoxidation of cyclooctene and demethylation of N,N-dimethylanilines
enable us to discuss the details of the electron transfer process from substrates to the oxoiron(IV) porphyrin π-cation radical
complex in the oxygenation mechanisms.
’ INTRODUCTION
Because compound I is reduced to a ferric heme complex in the
process of carrying out its oxidation reactions, the reactions of
compound I with substrates accompany electron transfer from the
substrates to compound I. Therefore, characterization of the
redox potential of compound I has been thought to be a key to
gaining an understanding of its reaction mechanism. Compound I
and compound II, which is one-electron reduced form of com-
pound I, of certain peroxidases are relatively stable at room
temperature. The redox potentials of compound I/compound
II of these peroxidases were measured using chemical and
electrochemical methods.23À30 The redox potentials for com-
pound I/compound II were reported to be in the range of
900À1150 mV versus NHE (660À910 mV vs SCE). These
redox potentials were unexpectedly lower than anticipated for
species with such high oxidizing activity toward various organic
substrates. However, the electrochemistry of the oxoiron(IV)
porphyrin π-cation radical complex in organic solvent has not
been well studied because of its instability at room temperature.
Balch et al. reported that the oxoiron(IV) TMP complex,
The oxoiron(IV) porphyrin π-cation radical species, which is
an oxidation state two-electron equiv higher than the resting
state, is a reactive intermediate known as compound I of heme-
containing peroxidases, catalases, and cytochrome P450.1À5
Peroxidases catalyze one-electron oxidations of amines, phenols,
and other aromatic substrates with compound I while catalases
oxidize hydrogen peroxide to oxygen molecule with compound
I.1,2 Compound I of cytochrome P450 can hydroxylate saturated
hydrocarbons.3,4 Because of its biological significance and ex-
tremely high reactivity, compound I has also received much
attention with respect to biomimetic monooxygenation catalysis
with synthetic heme complexes. After Groves et al. prepared the
first example of a synthetic oxoiron(IV) porphyrin π-cation
radical structure using an iron 5,10,15,20-tetramesitylporphyrin
(TMP) complex,6 various types of oxidation reactions of com-
pound I, such as epoxidations of olefins,6À11 hydroxylations of
hydrocarbons,9,11,12 sulfoxidations of sulfides,13 and demethyla-
tions of anisoles and N,N-dimethylanilines,14 were studied. The
reaction mechanisms of these oxidation reactions have been
studied with chemical and theoretical methods but remain the
subject of debate.12À22
Received: December 24, 2010
Published: June 29, 2011
r
2011 American Chemical Society
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Figure 1. Structures of the oxoiron(IV) porphyrin complexes used in
this study.
(TMP)FeIVO, was oxidized to its π-cation radical complex,
[(TMP+•)FeIVO]+ with an iron(III) porphyrin π-cation radical
species at À80 °C.31 This suggests that the redox potential for the
(TMP)FeIVO/[(TMP+•)FeIVO]+ couple was lower than that of
[(TMP)FeIII]+/[(TMP+•)FeIII] (∼1.1 V vs SCE). Sawyer et al.
reported that the electrochemistry of the oxoiron(IV) 5,10,15,20-
tetrakis(2,6-dichlorophenyl)porphyrin π-cation radical complex,
[(TDCP+•)FeIVO]+, prepared by ozone oxidation in acetonitrile
at À35 °C, showed a reversible redox peak at 1.27 V versus SCE.7
This was assigned as the (TDCP)FeIVO/[(TDCP+•)FeIVO]+
couple. The absence of comprehensive studies of the redox
potentials of oxoiron(IV) porphyrin π-cation radical complexes
has hampered the identification of electron transfer processes in
the reaction mechanisms.
In this article, we report a systematic low temperature study of
the electrochemistry of oxoiron(IV) porphyrin and oxoiron(IV)
porphyrin π-cation radical complexes having various porphyrin
structures and axial ligands as shown in Figure 1. We assign redox
potentials between oxoiron(IV) porphyrin and oxoiron(IV) por-
phyrin π-cation radical complexes, (P)FeIVO/[(P+•)FeIVO]+,
using a combination of cyclic voltammograms and thin-layer
spectroelectrochemistry. The redox potential for the (P)FeIV-
O/[(P+•)FeIVO]+ couple undergoes a positive shift upon coordi-
nation of an anionic axial ligand, but a negative shift upon
coordination of a neutral axial ligand (imidazole). However, the
electron-withdrawing effect of the meso-substituent shifts the
redox potentialin a positivedirection. Onthe basisof the measured
redox potentials, the participation of the electron transfer process
from substrates to the oxoiron(IV) porphyrin π-cation radical
complex in the oxygenation mechanism is discussed.
Figure 2. (a) Cyclic voltammogram and (b) differential pulse voltam-
mogram of (TMP)FeIVO in dichloromethane containing 0.1 M tetra-n-
butylammonium perchlorate at À60 °C. The scan rate was 100 mV/s.
tetra-n-butylammonium hydroxide and 3-fluoro4-nitrophenol and pur-
ified by recrystallization in dichloromethane-hexane.
Preparation of Oxoiron(IV) Porphyrin Complexes. Oxoiron-
(IV) porphyrin complexes were prepared as previously described.35
(TMP+•)FeIII(ClO4)2 or (TMTMP+•)FeIII(ClO4)2, which were pre-
pared from oxidation of (TMP)FeIIIOH or (TMTMP)FeIIIOH by
addition of solid ferric perchlorate in dichloromethane, was transferred
through a short basic alumina (20% water) column (0.5 Â 1 cm) at
ambient temperature directly into an electrochemical cell containing
anhydrous dichloromethane and 0.1 M n-Bu4NClO4, which was pre-
cooled in a temperature controlled cooling bath. For preparation of a
series of imidazole complexes, 1 equiv of 1-MeIm, 2-MeIm, or 4(5)-
MeIm was slowly added to the cooled solution. A series of complexes
with anionic axial ligands was prepared by addition of 1 equiv of tetra-n-
butylammonium salts, n-Bu4N(L), where L is fluoride, chloride, acetate,
trifluoroacetate, benzoate, nitrate, and 3-fluoro-4-nitrophenolate.
Preparation of Oxoiron(IV) Porphyrin π-Cation Radical
Complexes. Oxoiron(IV) porphyrin π-cation radical complexes were
prepared by ozone oxidation at low temperature.10,36 Iron(III) porphyrin
complexes were dissolved in anhydrous solvent containing 0.1 M
n-Bu4NClO4 in an electrochemical cell. The electrochemical cell was
cooled in a temperature-controlled cooling bath. Ozone gas was slowly
bubbled into the solution using a gastight syringe. With the formation of
oxoiron(IV) porphyrin π-cation radical complexes, the brown solution
changed to green. Excess ozone gas was removed by bubbling argon gas
using a gastight syringe.
’ EXPERIMENTAL SECTION
Materials. Anhydrous dichloromethane was commercially obtained
and stored in the presence of 4 Å molecular sieves. Other chemicals were
commercially obtained and used without further purification. 5,10,15,20-
Tetramesitylporphyrin (TMP), 5,10,15,20-tetrakis-pentafluorophenyl-
porphyrin (TPFP), and 2,7,12,17-teramethyl-3,8,13,18- tetramesitylpor-
phyrin (TMTMP) were prepared according to previously described
methods.32,33 (TMP)FeIIICl, (TPFP)FeIIICl, and (TMTMP)FeIIICl
were prepared by insertion of iron into porphyrins with FeCl2 and
sodium acetate in acetic acid and purified with a silica gel column using
CH2Cl2/CH3OH as an eluent.8 (TMP)FeIIIClO4, (TPFP)FeIIIClO4,
and (TMTMP)FeIIIClO4 were prepared by the reaction of iron(III)
porphyrin chloride complexes with silver(I) perchlorate in dichloro-
methane solution and purified by recrystallization from dichloro-
methane/n-hexane.34 (TMP)FeIIIOH and (TMTMP)FeIIIOH were
obtained from (TMP)FeIIICl and (TMTMP)FeIIICl respectively by
passing through a basic alumina (10% water) column using dichloro-
methane as an eluent.35 Tetra-n-butylammonium 3-fluoro-4-nitrophe-
nolate, n-Bu4N(3-F-4-NO2-PhO), was prepared from the reaction of
Instrumentation. UVÀvis absorption spectra were recorded on an
Agilent 8453 spectrometer (Agilent Technologies) equipped with a USP-
203 low-temperature chamber (UNISOKU). Cyclic voltammogram
(CV) and differential pulse voltammogram (DPV) were measured in a
conventional three-electrode electrochemical cell with an ALS612A
electrochemical analyzer. Tetra-n-butylammonium perchlorate, n-Bu4N-
ClO4, (0.1 M) was usedasa supportingelectrolyte. The workingelectrode
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result revealed a CV similar to that of (TMP)FeIVO (Figure S3 of
the Supporting Information). The first redox potential at E1/2
=
0.88 V in acetonitrile was same as that in dichloromethane, but the
second redox potential at E1/2 = 1.25 V in acetonitrile was slightly
higher than that in dichloromethane. This is most likely due to a
solvent effect. Moreover, the CV was different from that of a
thermally decomposed compound of (TMP+•)FeIVO(ClO4).
These results indicate that these peaks result from redox processes
of (TMP)FeIVO and (TMP+•)FeIVO(ClO4).
To assign the redox potential for the (TMP)FeIVO/
(TMP+•)FeIVO(ClO4) couple, we performed thin-layer spectro-
electrochemistry for (TMP)FeIVO in dichloromethane contain-
ing 0.1 M n-Bu4NClO4 at À60 °C. By applying 1.10 V versus
SCE, the voltage between the first and second peaks, the
absorption spectrum of (TMP)FeIVO changed to that of
(TMP+•)FeIVO(ClO4) with clear isosbestic points (Figure 3).
This clearly indicates that the redox peaks at Ea = 0.79 and
Ec = 0.97 V (E1/2 = 0.88 V) correspond to the redox potential of
the (TMP)FeIVO/[(TMP+•)FeIVO]+ process.
Figure 3. Absorption spectral change for electrochemical oxidation of
(TMP)FeIVO at 1.10 V vs SCE in dichloromethane containing 0.1 M
tetra-n-butylammonium perchlorate at À60 °C. Experimental condi-
tions are described in the Experimental Section.
As expected from the work of Balch et al,31 the redox potential
for the (TMP)FeIVO/[(TMP+•)FeIVO]+ couple was un-
expectedly low relative to that of (TMP)FeIIIX/[(TMP+•)-
FeIIIX]+ (∼1.1 V vs SCE). This is due to the strong electron
donor effect of the oxo-ligand, which increases the electron
density on the porphyrin ring via the ferryl iron. A similar effect
of the oxo-ligand was also identified in the redox potentials of
other oxo-metalloporphyrin complexes, such as (TMP)CrIVO
(0.76 V vs Ag/AgCl) and (TMP)RuVIO2 (0.61 V vs Ag/AgCl).37
We also examined thin-layer spectroelectrochemistry for
(TMP+•)FeIVO(ClO4) to assign the second redox process at
À80 °C (Figure S4 of the Supporting Information). With
applying 1.45 V versus SCE, the absorption spectrum of
(TMP+•)FeIVO(ClO4) changed to a new one having Soret band
at 368 nm and weak bands from 900 to 500 nm, and then a final
one having absorption peaks at around 353 and 730 nm. The final
compound was not reduced to (TMP+•)FeIVO(ClO4) even when
0.30 V versus SCE was applied. Moreover, the similar spectral
change was observed when trifluoroacetic acid was titrated to
(TMP+•)FeIVO(NO3) (data not shown). These results indicate
that the final compound is not one-electron oxidized species of
(TMP+•)FeIVO(ClO4) but probably its decomposed compound.
The absorption spectrum of the final compound is unique and
further spectroscopic studies are required to characterize it.
Porphyrin and Axial Ligand Effects. To investigate the effect
of an anionic axial ligand on the redox potential of oxoiron(IV)
porphyrin π-cation radical complex, we measured a series of CVs
of (TMP)FeIVO in the presence of 1 equiv of n-Bu4N(L), where L
is fluoride, chloride, acetate, benzoate, trifluoroacetate, and nitrate.
The CVs in the presence of n-Bu4N(L) were similar to those in the
absence of n-Bu4N(L), but the redox peaks were shifted to the
more positive region (Figure S5 of the Supporting Information).
To assign these redox peaks, we examined the thin-layer spectro-
electrochemistry of (TMP)FeIVO in the presence of 1 equiv of
n-Bu4NF in dichloromethane containing 0.1 M n-Bu4NClO4 at
À60 °C (Figure S6 of the Supporting Information). The absorp-
tion spectrum of (TMP)FeIVO in the presence of 1 equiv of
n-Bu4NF was similar to that in the absence of n-Bu4NF. By
applying 1.20 V versus SCE, the absorption spectrum of
(TMP)FeIVO changed to that of (TMP+•)FeIVO(F), indicating
that the first redox peaks were due to the (TMP)FeIVO/
[(TMP+•)FeIVO]+ couple. In addition, these results indicate that
fluoride ion binds as an axial ligand for [(TMP+•)FeIVO]+, but the
was a glassy carbon electrode and the counter electrode was a platinum-
wire electrode. The potentials were recorded with respect to a saturated
calomel electrode (SCE) as a reference electrode. Temperature of
electrochemical cell was controlled by a PSL-1800 low temperature
cryostat bath (EYELA, Tokyo).
Spectroelectrochemistry. Spectroelectrochemsitry was carried
out in a handmade thin-layer quartz cell (optical path = 0.5 mm) with a
gold-mesh (100 mesh) working electrode, a platinum-wire counter
electrode, and an Ag reference electrode (Figure S1 of the Supporting
Information), which were connected to a HA-151 potentiostat-galvano-
stat (HOKUTO DENKO). The E1/2 value of ferrocene in this spectro-
electrochemical cell was 0.370 V at À40 °C, which was 0.100 V lower
than that measured in the electrochemical cell with SCE reference
electrode described above. Therefore, to apply the potential versus SCE,
the applied potentials to the spectroelectrochemical cell were 100 mV
lower than the desired potentials versus SCE. The spectroelectrochem-
ical cell was placed in a low-temperature chamber set on a spectrometer
and cooled to low temperature (213 or 193 K). Samples were prepared
as described above. After bubbling Ar gas, a constant potential was
applied while monitoring UVÀvis absorption spectral changes.
’ RESULTS AND DISCUSSION
Electrochemistry. We measured the cyclic voltammogram
(CV) of (TMP)FeIVO in dichloromethane containing 0.1 M
n-Bu4NClO4 at À60 °C. As shown in Figure 2, (TMP)FeIVO
exhibits two quasi-reversible redox waves at E1/2 = 0.88 and 1.18 V
versus SCE. The obtained CV was similar to the CV reported for
(TDCP+•)FeIVO(ClO4) in acetonitrile at À35 °C,7 except for
∼300 mV negative shifts of the redox peaks. The differential pulse
voltammogram of (TMP)FeIVO showed two peaks at 0.91 and
1.20 V versus SCE. The small peak at 1.03 V was due to a
decomposed iron(III) porphyrin complex because the peak
became strong with thermal decomposition of the sample.
(TMP)FeIVO did not show significant peaks from 0.5 V to
À0.5 V versus SCE, but a strong catalytic current below À0.75
V, probably due to reductive decomposition of (TMP)FeIVO
(Figure S2 of the Supporting Information). To further confirm
whether these peaks resulted from redox processes of
(TMP)FeIVO, we measured the CV of (TMP+•)FeIVO(ClO4)
in acetonitrile containing 0.1 M n-Bu4NClO4 at À35 °C. The
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Table 1. Redox Potentials (vs SCEa) of Iron(III) Porphyrin and Oxoiron(IV) Porphyrin π-Cation Radical Complexes in
Dichloromethane at À60 °C
porphyrin
axial ligand (L)
first: (P)FeIVO/ (P+•)FeIVOb
secondb
(P)FeIII/ (P+•)FeIIIb
ref
Epa
Epc
E1/2
E1/2
E1/2
TMP
perchlorate
perchlorate c
fluoride
0.97
0.91
1.03
1.06
1.10
1.02
1.09
1.03
1.03
0.84
0.81
0.78
1.30
1.43
0.98
1.01
0.87
0.79
0.84
0.89
0.88
0.89
0.90
0.89
0.88
0.89
0.76
0.73
0.73
1.25
1.35
0.83
0.89
0.80
0.88 (0.91)
0.88 (0.90)
0.96 (0.95)
0.97 (0.94)
1.04 (1.04)
0.96 (0.96)
0.99 (1.04)
0.95 (0.94)
0.96 (0.96)
0.80 (0.76)
0.77 (0.75)
0.76 (0.72)
1.275
1.18 (1.20)
1.25 (1.28)
1.25 (1.22)
1.26 (1.23)
1.26 (1.24)
1.25 (1.22)
1.25 (1.23)
1.21 (1.19)
1.25 (1.23)
1.01 (0.98)
1.02 (1.00)
0.98 0.96)
1.59
1.09 (1.08)
1.12(1.15)
1.10 (1.09)
1.10 (1.04)
1.06 (1.03)
1.09 (1.08)
1.05
chloride
acetate
trifluoroacetate
benzoate
nitrate
1.11
3-fluoro-4-nitrophenolate
imidazole
1.16
nd
2-methylimidazole
5-methylimidazole
perchloratec
perchlorated
perchlorate
nitrate
nd
nd
7
TDCP
TPFP
∼1.46
(1.56)
1.39 (1.39)
0.90 (0.92)
0.95 (0.94)
0.84
nd
TMTMP
1.22 (1.19)
1.30 (1.28)
0.99
1.07(1.06)
1.09(1.08)
nd
imidazole
a In our system, the redox potential (E1/2) of ferrocene was 0.460 V at À60 °C and 0.528 V at 20 °C. b Values in parentheses were obtained from
differential pulse voltammetry. c In acetonitrile at À35 °C. d In dichloromethane/acetonitrile (1:1).
binding of fluoride ion is not sure for (TMP)FeIVO. To study the
binding ability of anionic ligands to (TMP)FeIVO, (TMP)FeIVO
was titrated with n-Bu4N(L), where L is fluoride, chloride, and
benzoate, at À60 °C (Figure S7 of the Supporting Information).
With increase in amount of fluoride ion, the absorption at 545 nm
for (TMP)FeIVO slightly decreased its intensity and shifted to
554 nm. The peak intensity was also decreased with addition of
chloride and benzoate, but the peak did not shift even with
addition of excess amount. These results indicated that anionic
ligands bind with (TMP)FeIVO, but their affinities seems to be
low. The E1/2 values forthe (TMP)FeIVO(L)/(TMP+•)FeIVO(L)
couple are summarized in Table 1. Interestingly, the redox
potential showed a positive shift upon coordination of the anionic
axial ligand but was not changed significantly by the identity of
the axial ligand. This is the same as the redox potential for
(TMP)FeIIIX/[(TMP+•)FeIIIX]+, and the insensitivity of the
redox potential to the axial anion would be due to that the redox
process occurs at porphyrin macrocycle but not iron center.
We also examined CVs of a series of (TMP+•)FeIVO(L)
complexes. However, (TMP+•)FeIVO(L) could not be formed
from (TMP)FeIII(L) in acetonitrile at À40 °C by ozone oxida-
tion when L is other than perchlorate. (TMP+•)FeIVO(L) were
prepared and measured CVs in dichloromethane containing
0.1 M n-Bu4NClO4 at À60 °C (Figure S8 of the Supporting
Information). The CVs of (TMP+•)FeIVO(L) were different
from those of (TMP)FeIVO in the presence of 1 equiv of
n-Bu4N(L). (TMP+•)FeIVO(L) exhibited quasi-reversible redox
peaks at around 1.0 V, but the E1/2 values of the redox peaks were
different from those of (TMP)FeIVO. Moreover, irreversible
peaks were observed at around 0.6 V versus SCE. The compli-
cated CVs of (TMP+•)FeIVO(L) do not seem to show redox
potential for (TMP)FeIVO/(TMP+•)FeIVO(L) couple correctly.
These differences would be induced by contamination of hydro-
chloric acid, which is formed from the ozone-driven oxidation
of dichloromethane. The decomposition of unstable (TMP+•)-
FeIVO(L) and participation of proton in the redox processes
make assignments of these redox peaks difficult.
Imidazole and phenolate are the axial ligands of peroxidases and
catalases, respectively. To investigate the axial ligand effects of
imidazole and phenolate, we measured CVs of (TMP)FeIVO in
the presence of 1 equiv of imidazole, 2-methylimidazole, 5-methyl-
imidazole, and n-Bu4N(3-F-4-NO2ÀPhO) (part f of Figure S5,
and Figure S9 of the Supporting Information). The CV of
(TMP)FeIVO(Im) was very close to that of (TMP)FeIVO, but
the redox potential for (TMP)FeIVO(Im)/[(TMP+•)FeIV-
O(Im)]+ was found to be lower than that of (TMP)-
FeIVO/[(TMP+•)FeIVO]+ (Table 1). (TMP)FeIVO(2-MeIm)
and (TMP)FeIVO(5-MeIm) were found to exhibit similar CVs
and the redox potential was increased in the order of 5-MeIm <
2-MeIm < Im < none. The redox potentials (0.76À0.80 V vs SCE)
for these imidazolecomplexes wereinthe range of redox potentials
reported for compound II/compound I of peroxidases (0.66À
0.91 V vs SCE).23À30 However, the redox potential for the
3-fluoro-4-nitrophenolate complex was higher than that for
(TMP)FeIVO and close to those of the other anionic ligands.
In a previous article, we showed that the reactivity of the
oxoiron(IV) porphyrin π-cation radical complex for cyclo-
octene epoxidation is drastically changed by the axial ligand
and increases in the order of nitrate < chloride < 3-fluoro-4-
nitrophenolate e imidazole.8b This study clearly showed that the
redox potentials of these complexes do not match this reactivity
order. This indicates that the axial ligand effect on the reactivity
of oxoiron(IV) porphyrin π-cation radical complex cannot be
rationalized by the change of the redox potential. In fact, as
discussed in later section, the direct electron transfer process
from cyclooctene to oxoiron(IV) porphyrin π-cation radical
complex may not be involved in its rate-limiting step. The axial
ligand modulates reactivity of oxoiron(IV) porphyrin π-cation
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radical complex by changing other factors than its redox poten-
tial. The axial ligand effect must be further studied from various
spectroscopic methods.
Electron-withdrawing porphyrin substituents have been
shown to increase the oxygenation reactivity of the oxoiron(IV)
porphyrin π-cation radical complex.8a,38 We measured the redox
potential of (TPFP+•)FeIVO(ClO4) in dichloromethane/aceto-
nitrile at À60 °C. The CV of (TPFP+•)FeIVO(ClO4) was found
to be similar to that of (TMP+•)FeIVO(ClO4), but the redox
peaks showed significant positive shifts. The E1/2 for
(TPFP)FeIVO/[(TPFP+•)FeIVO]+ is 1.39 V versus SCE. This
is 510 mV higher than that of (TMP+•)FeIVO(ClO4). It is clear
that the E1/2 value for the (P)FeIVO/[(P+•)FeIVO]+ couple
increases with an increase in the electron-withdrawing effect of
the meso substituent.
We measured CVs of (TMTMP)FeIVO, which has mesityl
groups at the pyrrole β-position, to compare the redox potentials
of (TMP)FeIVO (Figure S10 of the Supporting Information).
Previously, we showed that the porphyrin π-cation radical of
(TMTMP+•)FeIVO is in the a1u orbital, whereas that of
(TMP+•)FeIVO is in the a2u orbital.8a The data obtained from
the CV of (TMTMP)FeIVO are listed in Table 1. In spite of the
radical occupying different porphyrin π-cation orbitals, the redox
potentials of (TMTMP)FeIVO were found to be close to that of
(TMP)FeIVO. This was also observed for the nitrate and
imidazole complexes.
Figure 4. Reaction coordinate diagram showing the reaction profile for
the epoxidation reaction of cyclooctene by (TMP+•)FeIVO(L). The
values are for nitrate complex. The relative energies of bond-formation
complex and the final product are uncertain.
The reaction mechanisms of amine N-demethylation cata-
lyzed by heme-containing peroxidases and cytochrome P450
have been studied over the past four decades.14,41À48 The
mechanisms were also studied with synthetic model
complexes.49À52 Whereas the overall reaction consists of hydro-
xylation of the N-methyl group followed by hydrolysis to afford
N-demethylated amine and formaldehyde, the mechanism of
the hydroxylation remains controversial. These enzymatic and
model studies proposed two different mechanisms: a hydrogen
atom transfer (HAT) mechanism and consecutive electron-
transfer proton-transfer (ET/PT) mechanism. Previous studies
of catalytic demethylation reactions with iron(III) porphyrins
supported ET/PT mechanism.49,50 The demethylation reaction
by (TMP+•)FeIVO(L) was proposed to proceed via electron
transfer followed by H-atom transfer, which competes with back
electron transfer.14 To further investigate the reaction mechan-
ism of the demethylation reaction of aniline with the oxoiron(IV)
porphyrin π-cation radical complex, we calculated the change of
free energies (ΔG°ET) for the electron transfer from p-substi-
tuted N,N-dimethylanilines to (TMP+•)FeIVO(L) using the
relationship ΔG° = nF(P1 À P2) and the free energies of acti-
vation (ΔG‡DMA) for these demethylation reactions at 220 K
using Eyring’s equation. The calculated energies are summarized
in Figure 5. When the substrate is N,N-dimethyl-p-nitroaniline,
which has a strong electron-withdrawing substituent and high
oxidation potential (E1/2 = 1.38 V vs SCE),14 the ΔG°ET value is
calculated to be +38 kJ/mol. Therefore, the direct electron
transfer process from N,N-dimethyl-p-nitroaniline to (TMP+•)-
FeIVO(L) is energetically uphill. The ΔG°ET value is smaller
than the ΔG‡N-DMA value (∼49 kJ/mol) calculated from Eyring’s
equation with k2 = 2.2 Â 10 MÀ1 sÀ1 at 223 K.14 Moreover,
previous study showed large kinetic isotope effect and agreement
of kinetic and product isotope effects for N,N-dimethyl-p-
nitroaniline.14 These results clearly indicate that the H-atom
transfer step from the methyl group of the formed amine radical
would be the rate-limiting step of the overall reaction and the
electron transfer process may occur before the H-atom transfer.
The energy barrier for the H-atom transfer process can be
estimated to be ∼11 kJ/mol. Back electron transfer easily occur
because the activation energy for the back electron transfer
process is smaller than that for the subsequent H-atom transfer
process from the methyl group of the formed amine radical
Electron Transfer in the Reaction Mechanism. The redox
potentials of oxoiron(IV) porphyrin π-cation radical complexes
provide useful information for discussions of the participation of
electron transfer processes in oxygenation mechanisms. Epox-
idations of olefins are one of the best studied reactions for
oxoiron(IV) porphyrin π-cation radical complexes and there
have been several reaction intermediates proposed.15À19 Here,
we discuss the participation of electron transfer processes in an
epoxidation mechanism with the measured redox potentials and
the reaction rate constants for epoxidation of cyclooctene with
(TMP+•)FeIVO(L).8b The free energy of activation (ΔG‡CO) for
cyclooctene epoxidation calculated from Eyring’s equation,
ΔG‡ = ÀRTlnk2h/kT (where R is the gas constant, k2 is reaction
rate constant, h is the Plank constant, k is Boltzmann constant,
and T is temperature) is approximately 60 kJ/mol at 213 K when
L is nitrate (k2 = 8.21 Â 10À4
M
À1sÀ1). A similar value was
obtained when ΔG‡ values were calculated from the enthalpy of
activation (ΔH‡) and entropy of activation (ΔS‡), estimated
from an Eyring plot (data not shown). However, the change of
free energy (ΔG°ET) for the electron transfer from cyclooctene
to (TMP+•)FeIVO(L) is calculated to be ∼100 kJ/mol from the
relationship, ΔG° = nF(P1 À P2), where n is number of electrons
(n = 1), F is Faraday’s constant, P1 is the redox potential of
cyclooctene (2.03 V vs SCE),39 and P2 is the redox potential of
(TMP+•)FeIVO(L) (0.95 V vs SCE for L = nitrate). Obviously,
the ΔG‡ value is much smaller than the ΔG° value. Therefore,
the results of these calculations enable us to rule out direct
electron transfer from cyclooctene to (TMP+•)FeIVO(L) in the
reaction mechanism of cyclooctene epoxidation. As previously
suggested for epoxidation by the oxochromium(V) porphyrin
complex,39,40 weak orbital interactions between cyclooctene and
the FedO moiety of the oxoiron(IV) porphyrin π-cation radical
complex may be formed at the transition state and the electron
transfer may occur in the bond formation process between
the oxo-ligand of FedO and CdC moiety of cyclooctene
(Figure 4).
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Inorganic Chemistry
ARTICLE
Figure 5. Reaction coordinate diagram showing the reaction profile for the demethylation of N,N-dimethyl-p-nitroaniline and N,N-dimethyl-p-
methoxyaniline with (TMP+•)FeIVO(L). The structure of the H-atom transferred complex and the relative energies of the H-atom transferred complex
and the final products are uncertain.
(Figure 5). However, when the substrate is N,N-dimethyl-p-
methoxyaniline, which has a strong electron-donating substitu-
ent and low oxidation potential (E1/2 = 0.60 V vs SCE),14 the
calculated ΔG°ET value (À38 kJ/mol) was also found to be much
the meso-substituent shifts the redox potential in a positive
direction. The measured redox potentials in this study provide
useful information to analyze electron transfer processes in
oxygenation mechanisms of various organic substrates. The
direct electron transfer process from cyclooctene to
(TMP+•)FeIVO(L) is not involved in the reaction mechanism
of cyclooctene epoxidation. On the other hand, demethylation
reaction of N,N-dimethylanilines may proceed via electron
transfer process followed by H-atom transfer. The rate-limiting
step is the electron transfer process from aniline to oxoiron(IV)
porphyrin π-cation radical for electron-rich aniline, but is the
H-atom transfer process from the methyl group of the formed
amine radical for electron-deficient aniline.
smaller than the ΔG‡
value (21.0 kJ/mol) calculated
M-DMA
from Eyring’s equation with k2 = 5.6 Â 107
M
À1sÀ1 at
223 K.14 The direct electron transfer process from N,N-dimeth-
yl-p-methoxyaniline to (TMP+•)FeIVO(L) is energetically down-
hill. The energy barrier of the following H-atom transfer
process for N,N-dimethyl-p-methoxyaniline may be close that
(∼11 kJ/mol) for N,N-dimethyl-p-nitroaniline. These results
indicate that the first electron transfer step becomes the rate-
limiting step of the overall reaction (Figure 5). This is also
consistent to a very small kinetic isotope effect for N,N-dimethyl-
p-methoxyaniline, as previously reported.14 The transition state
of the rate-limiting step would be the formation of the complex
between (TMP+•)FeIVO(L) and N,N-dimethyl-p-methoxyani-
line for electron transfer. Back electron transfer is prevented from
occurring because the activation energy for the subsequent
H-atom transfer from the methyl group of the formed amine
radical is lower than the activation energy for the back electron
transfer (Figure 5). Finally, it is obvious that the stability of the
electron transferred complex, which is determined by the differ-
ence of the redox potential between oxoiron(IV) porphyrin
π-cation radical complex and N,N-dimethylaniline, controls the
character of the rate-limiting step.
’ ASSOCIATED CONTENT
S
Supporting Information. Scheme of spectroelectrochem-
b
ical cell, cyclic voltammograms of (TMP)FeIVO(L) and
(TMP+•)FeIVO(L), spectroelectrochemistry of (TMP+•)FeIVO-
(ClO4) and (TMP)FeIVO in the presence of 1 equiv of n-Bu4NF,
and titration of (TMP)FeIVO with n-Bu4N(L). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hiro@ims.ac.jp.
In summary, we reported herein electrochemistry of oxoiron-
(IV) porphyrin and oxoiron(IV) porphyrin π-cation radical
complexes having various porphyrin structures and axial ligands
in organic solvents at low temperatures. The redox potential for
the (TMP)FeIVO/[(TMP+•)FeIVO]+ couple was observed at
0.88 V versus SCE, which showed a positive shift upon coordina-
tion of an anionic axial ligand but a negative shift upon
coordination of imidazole. The electron-withdrawing effect of
’ ACKNOWLEDGMENT
This study was supported by grants from JSPS (Grant-in-Aid
for Science Research, Grant No. 22350030), JST (CREST), and
MEXT (Global COE Program).
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Inorganic Chemistry
ARTICLE
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’ NOTE ADDED AFTER ASAP PUBLICATION
This paper was published on the Web on June 29, 2011, with
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