Characterization and Subsequent Reactivity of an Fe-Peroxo Porphyrin Generated by Electrochemical Reductive Activation of O2

Article abstract of DOI:10.1021/acs.inorgchem.6b01804

Reductive activation of O₂ is achieved by using the [Fe^III(F₂₀TPP)Cl] (F₂₀TPP = 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinate) porphyrin through electrochemical reduction of the [Fe^III(F_20<

Full text of DOI:10.1021/acs.inorgchem.6b01804

Article
pubs.acs.org/IC
Characterization and Subsequent Reactivity of an Fe-Peroxo
Porphyrin Generated by Electrochemical Reductive Activation of O2
Raquel Oliveira,† Wiem Zouari,† Christian Herrero,‡ Frederic Banse,‡ Bernd Schollhorn,† Claire Fave,*,†
́
́
̈
and Elodie Anxolabehere-Mallart*,†
́
̀
†Laboratoire d’Electrochimie Molec
Cedex 13, France
‡Institut de Chimie Molec
Orsay Cedex, France
́
ulaire, Universite
́
Paris Diderot, Universite
́
Sorbonne Paris Cite,
́
UMR CNRS 7591, 75205 PARIS
́
ulaire et des Materiaux d’Orsay, Universite
́
́
́
Paris Sud, Universite Paris Saclay, UMR CNRS 8182, 91405
S
* Supporting Information
ABSTRACT: Reductive activation of O2 is achieved by using the [FeIII(F20TPP)Cl] (F20TPP = 5,10,15,20-tetrakis-
(pentafluorophenyl) porphyrinate) porphyrin through electrochemical reduction of the [FeIII(F20TPP)(O2•−)] superoxo
complex. Formation of the [FeIII(F20TPP)(OO)]− peroxo species is monitored by using low-temperature electronic absorption
spectroscopy, electron paramagnetic resonance, and cyclic voltammetry. Its subsequent protonation to yield the
[FeIII(F20TPP)(OOH)] hydroperoxo intermediate is probed using low-temperature electronic absorption spectroscopy and
electron paramagnetic resonance.
intermediates have been obtained by reaction of the ferrous
complex with O2 in the presence of reductants.8−10 Only few
examples have been reported on electrochemical methods to
provide the necessary electrons.11−13 However, this method
could present several advantages such as the lack of side
products and a better control of the reaction steps by triggering
the input of electrons.
INTRODUCTION
■
The present economic, environmental, and climatic contexts
require the urgent development of new chemical processes that
are environmentally sound and economically viable. There is a
general interest in using dioxygen (O2) as an oxidant, since it is
inexpensive, abundant, and environmentally benign. However,
O2 is relatively inert due to its fundamental triplet state (S = 1).
Thus, reductive activation is necessary to overcome the kinetic
barrier of its reaction with diamagnetic species. A plethora of
crucial metabolic transformations catalyzed by heme-containing
enzymes, such as cytochromes P450, involve the binding of
dioxygen at the iron(II) heme center to yield an oxyheme
intermediate analogous to oxymyoglobin.1 Electron transfer
from iron(II) to bound a O2 molecule leads to a superoxo ferric
heme complex. Further reduction and protonation of this latter
species gives a hydroperoxo ferric heme complex that evolves
after a second proton transfer into the active Compound I
(Cpd I, formally oxo Fe(V)). Synthetic iron porphyrin
complexes have been developed as models of the enzymatic
heme intermediates for decades.2,3 In the specific case of ferric
peroxo complexes, they have generally been directly generated
from the ferrous complex and superoxide.4−7 More rarely, these
We have recently reported work on bioinspired Mn14 or Fe15
complexes highlighting the relevance of an electrochemical
approach coupled to spectroscopic measurements for the
identification and mechanistic studies of reactive intermediates.
The present article deals with the electrochemical activation of
O2 to yield a ferric peroxo complex and the study of its
reactivity in protic media. The formation and fate of peroxo
species were monitored using low-temperature electronic
absorption spectroscopy, electron paramagnetic resonance
(EPR), and cyclic voltammetry (CV). This work demonstrates
the potentialities of electrochemistry as a tool to control and
detect the formation of intermediates.
Received: July 26, 2016
© XXXX American Chemical Society
A
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concentration of 1 (Figure S2). This new wave is attributed to
the reduction of a ferric superoxo complex (3), (Scheme 1) that
forms upon coordination of O2 to the [FeII(F20TPP)] complex
generated at the surface of the electrode.20 The present
voltammetric signal does not allow to identify the electronic
structure of this species. Nevertheless the UV−vis spectro-
electrochemical experiments described below, provide further
evidence of the ferric superoxo nature of 3. Accordingly the
wave at ca. −0.50 V disappears upon addition of large amounts
of Cl− to an oxygenated solution of 1 (Figure S3), due to
complexation of Cl− ions to Fe(II), preventing coordination of
O2.
RESULTS AND DISCUSSION
■
The complex [FeIII(F20TPP)Cl] (1) (Scheme 1; F20TPP =
5,10,15,20-tetrakis(pentafluorophenyl) porphyrinate) was chos-
Scheme 1. Complex 1 ([FeIII(F20TPP)Cl]) and Schematic
Representation of the Reaction of 2 ([FeII(F20TPP)]) with
O2 and Subsequent Controlled Reductive Conditions
a
To further support this attribution, the experimental CV can
be simulated according to a chemical−electrochemical (CE)
mechanism 1 in which C correspond to O2 complexation to
Fe(II) leading to 3 and E corresponds to its reduction (Figure 2
and Table S1). In contrast the change in the CV cannot be
attributed to the reaction of Fe(II) with superoxide anion at the
electrode via an electrochemical−chemical (EC) mechanism
(mechanism 2 in Supporting Information, Scheme S1 and
Figure S4), as it was reported in the case of a Mn complex.14
Simulation of the CV response using a simple EC mechanism
•−
leads to the appearance of a prepeak at the foot of the O2/O2
a
For simplification, the F20TPP ligand was omitted from the formula
wave, even when considering diffusion-limited conditions (k =
1 × 1010 M−1 s−1) for the superoxide to Fe(II) coordination
(Figure S4).
of the complexes.
In the following paragraphs we will show that selective
electrochemical reduction of 3 leads to the FeIII(OO) peroxo
species 4.11 The nature of this latter intermediate is first
evidenced by UV−vis spectro-electrochemistry experiments
using a Pt grid working electrode in a thin-layer cell.22,23 The
evolution of the UV−vis spectra upon successive controlled
potential electrolyses is shown in Figure 3, and the
corresponding data are reported in Table 1. In a first step,
en due to its electron-withdrawing fluoro groups shifting its
reduction potential (280 mV toward positive values) compared
to the parent [FeIII(TPP)Cl] complex (Figure S1; TPP =
5,10,15,20-tetraphenylporphyrinate), as well as for its inability
to form diferric μ-oxo complexes in O2-containing solu-
tion.16−18 At 293 K the cyclic voltammogram (CV) of an
electrolytic dimethylformamide (DMF) solution purged with
O2 (in the absence of the Fe complex) exhibits a quasi-
reversible single-electron wave (−0.85 V versus SCE)
1
reduction of 1 was conducted at Eapp = −0.25 V versus SCE
•−
under argon. Under these conditions, the initial spectrum of 1
(Soret band at λ = 414 nm; Q-band at λ = 559 nm, Figure 3,
dashed black line) changes to yield a new one showing a Soret
band at λ = 427 nm and one major Q-band at λ = 554 nm
(black line in Figure 3). These latter values are similar to those
reported for the ferrous form [FeII(TPP)] generated from
[FeIII(TPP)Cl],24,25 and can therefore be ascribed to the
analogous [FeII(F20TPP)] (2). Lowering the temperature to
−30 °C and purging the solution with O2 ([O2] = 1 mM, air
saturated)26 results in a different spectrum (Figure 3, blue
corresponding to the O2/O2 redox couple (Figure 1, dashed
red line) and confirming the extremely low proton content of
the solution. CV of a 1 mM solution of 1 in DMF under argon
displays three typical and well-defined waves corresponding to
the successive Fe(III)/Fe(II), Fe(II)/Fe(I), and Fe(I)/Fe(0)
reduction processes +0.025, −0.805, and −1.315 V versus
saturated calomel electrode (SCE), respectively (Figure 1, black
trace and Figure S1).19 When the solution is purged with O2,
the CV is modified, and a new wave appears at ca. −0.50 V
(Figure 1, red trace), the intensity of which increases with the
Figure 1. (left) CV in DMF + 0.1 M TBAPF6 of 1 mM solution of 1 under argon (black), under O2 (1 mM, air saturated) (red), and of O2 alone (1
mM, air saturated) (dashed red). (right) Same as left panel, without CV of O2 alone, with narrower potential windows.21 Scan rate is 0.1 V s−1 at a
glassy carbon disk electrode (0.07 cm2), T = 293 K.
B
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Figure 2. (left) CE Mechanism 1. (right) CV in DMF + 0.1 M TBAPF6 of 1 mM of 1 under [O2] (1 mM, air saturated) at 0.1 V s−1 at a glassy
carbon disk electrode, T = 293 K (plain and dotted black traces); simulation of the CV according to mechanism 1 (red trace), with parameters from
Table S1 and the following parameters: α = 0.5 for all the electron transfers, DO = 4 × 10−5 cm2 s−1 and DFe = 2.5 × 10−6 cm2 s−1 for all of the Fe
2
species.
trace) with a Soret band at λ = 420 nm and a Q-band at λ = 530
nm. These spectral changes are consistent with those reported
for the formation of the FeIII-superoxo complex
[(FeIII(F8TPP)(O2•−)] in tetrahydrofuran upon oxygenation
of an [FeII(F8TPP)] (F8TPP = tetrakis(2,6-difluorophenyl)-
porphyrinate) solution at low temperature.8 We thus attribute
the new spectrum to the formation of FeIII-superoxo,
FeIII(O2•−) (3).27,28 Upon application of Eapp = −0.60 V
2
versus SCE to this oxygenated solution, the Soret band at 420
nm and the Q-band at 530 faded while an intense Soret band
centered at λ = 432 nm appeared simultaneously with a Q-band
centered at 559 nm (Figure 3, red trace). These bands are
identical to the ones reported in the literature for the
chemically prepared [FeIII(F20TPP)(OO)]− peroxo complex
Figure 3. UV−vis spectra ((A) Soret band and (B) Q bands) recorded
in the course of the spectro-electrochemical experiment: 0.025 mM
solution of 1 in DMF + 0.2 M TBAPF6, (dashed black line) and
spectral evolution upon applying different reductive potentials under
various conditions: Eapp1 = −0.25 V versus SCE, under argon, T = 293
K to give 2 (black line) and after saturation of the solution with air
(4) in acetonitrile.6
The electrochemical formation of the peroxo species 4 is also
evidenced using low-temperature bulk electrolysis coupled to
EPR spectroscopy. The ferric precursor 1 was first reduced at
1
Eapp = −0.25 V under argon. The resulting [FeII(F20TPP)] 2
2
solution was saturated with air and the temperature lowered to
−30 °C. The CV recorded on this solution (Figure 4, blue
trace) shows a cathodic peak at Epc(3) = −0.50 V corresponding
to the reduction of the [FeIII(F20TPP)(O2•−)] adduct.
([O2] = 1 mM) to give 3, T = 243 K (blue line); Eapp = −0.60 V
versus SCE, T = 243 K to give 4 (red line). Optical path 0.2 cm.
Table 1. Electrochemical and Spectroscopic Data
a
b
λ (nm) (ε (mM cm−1))
g values
5.78
E (V versus SCE)
c
d
d
e
III
II
[FeIIICl] (1)
[FeII] (2)
414 (10.8) 559 (1.0)
E0,ap
= 0.02
Fe Cl/Fe
d
427 (18.3)
d
554 (1.0)
f
g
[FeIII(O2•−)] (3)
420 (15.8)
Epc = −0.50
(3)
f
530 (1)
f
552(sh) (0.6)
f
g
[FeIII(OO)]− (4)
432 (25)
4.27
Epa = +0.50
(4)
f
559 (6)
[FeIII(OOH)(meIm)] (5)
2.26, 2.14, 1.96
a
b
c
ε values are calculated for each step when absorbance at λmax no longer varies. EPR perpendicular mode, spectra recorded at T = 100 K. For
d
e
f
simplification, the F20TPP ligand was omitted in the formula of the complexes. Value at T = 293 K. From simulation of the CV (see ESI). Value at
g
T = 243 K. Anodic (Epa) or cathodic (Epc) peak potentials experimental values for irreversible waves.
C
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(O2•−)] 3. For simplicity and based on UV−visible absorption
spectra, this intermediate has been denoted as a superoxo
entity, however it is known that the correct electronic structure
of such a species is not trivial and implies several limiting
valence tautomers, such as the ferric superoxo FeIII(O2•−) and
Fe(II)O2 adduct.31,32 Upon reduction to yield [FeIII(F20TPP)
(OO)]− 4 a significant electronic reshuffling occurs as well as a
change of the coordination mode of the bound O2 from end-on
in 3 to side-on in 4 (Scheme 2). Consequently, the
Scheme 2. Proposed Mechanism for the Protonation of the
Iron(III)-Peroxo Complex 4 in the Presence of 1-
Methylimidazole
Figure 4. CV of 0.5 mM solution of 1 in DMF + 0.2 M TBAPF6, scan
rate is 0.1 V s−1: under argon, T = 293 K (black dotted line), after bulk
electrolysis at −0.25 V versus SCE, under O2 (air saturated, 1 mM), T
2
= 243 K (blue line), after bulk electrolysis at Eapp = −0.60 V versus
SCE, under O2 (air saturated, 1 mM), T = 243 K (red lines).
The solution was then electrolyzed at Eapp2 = −0.60 V. In the
course of the experiment aliquots were collected and
immediately frozen and EPR spectra recorded on the samples.
The EPR signature of the initial solution of 1 (Figure 5, trace a)
is characteristic of a five coordinate high spin Fe(III) state (S =
5/2) with an anionic ligand such as Cl−.29,30 Upon polarization
electrochemical irreversibility of the interconversion between
3 and 4 is due to the concomitant structural and electronic
reorganizations occurring upon electron transfer. Such
irreversibility, although less pronounced, has been observed in
the case of copper complexes.33−35 The assignment of the
anodic wave to the oxidation of 4 is confirmed by recording the
CV of a chemically prepared [FeIII(F20TPP) (OO)]− (Figure
S6). To our knowledge, this is the first oxidation potential value
ever reported for a porphyrin supported Fe(III) peroxo. This
value is notably lower than those reported in the case of Fe(III)
peroxo complexes using 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-
cyclotetradecane (+0.92 V versus SCE)36 or N,N,N′,N′-
tetrakis(2- pyridylmethyl)ethane-1,2-diamine (+0.74 V versus
SCE)15 ligands. In summary, the obtained data (UV−vis, EPR,
CV) strongly support the formation of the peroxo complex 4
from the electrochemical reduction of 3 at a moderate potential
(−0.60 V versus SCE). These results clearly mimic the O2
reductive activation process that is encountered in natural heme
systems such as cyt P450. We shall emphasize that the present
1
at Eapp = −0.25 V under argon, the signal at g = 5.78
disappears, in agreement with the conversion of 1 into 2
(Figure 5, trace b). After purging with O22(air saturated 1 mM)
and electrolysis of the solution at Eapp = −0.60 V at low
temperature (T = 243 K) a new signal at g = 4.27 appears
(Figure 5, trace c). This signal is identical to the one previously
reported for the rhombic high-spin (S = 5/2) side-on
[FeIII(F20TPP) (OO)]− complex in acetonitrile6 thus confirm-
ing the formation of 4 following reduction of the
[FeIII(F20TPP)(O2•−)] superoxo. The CV recorded on this
solution shows a large irreversible signal at Epa = +0.50 V
(4)
(Figure 4, red trace) attributed to the oxidation of 4. The large
potential difference between reduction of 3 and oxidation of 4
illustrates the irreversibility of the reduction of [FeIII(F20TPP)-
Figure 5. (left) X-band EPR spectra of starting solution of 0.5 mM of 1 in DMF + 0.2 M TBAPF6 (black line (a), three scans); after electrolysis at
Eapp1 = −0.25 V under argon (black line (b), two scans); after electrolysis at Eapp2 = −0.60 V versus SCE under [O2] (air saturated, 1 mM; red line
(c), two scans); upon addition of 10 equiv of (CF3)3C−OH in presence of 5 equiv of 1-meIm, no polarization (blue line (d), five scans), and after
decomposition at room temperature during 30 s (green line (e), five scans). (right) Trace (d) enlarged. Conditions: T = 100 K, microwave
frequency 9.38 GHz, mod. amp. 10 G, microwave power 2.00 mW. Mod. frequency 100 MHz, gain 60 db, time constant 40 ms, samp. time. 163 ms,
1024 points.
D
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electrochemical approach provides a clean method to generate
reactive peroxo species without the involvement of chemical
oxidants or reductants.
[FeII(F20TPP) (OH)(1-meIm)]−. The approach developed by
Naruta7 using a covalently linked axial imidazole ligand led to
stabilization of the chemically prepared ferric peroxo as well as
the ferric hydroperoxo species, while avoiding unwanted side
reduction reactions. In the present work, we used more than
one stoichiometric amount of 1-meIm to ensure its
coordination to the Fe(III) peroxo. Doing so, a significant
amount of Fe(II) species is obtained. However, our work shows
that we can activate O2 using a [FeIII(F20TPP)Cl] porphyrin
using an electrochemical device as a clean electron supply for
the reduction of the [FeII(F20TPP)(O2)] adduct. The present
method may be easily improved by using an Fe(III) complex
displaying a covalently linked axial ligand with the hope to
increase the yield of hydroperoxo formation and, ultimately,
with the aim to develop electrochemically assisted oxidation
reactions.
Whereas side-on peroxo complexes have been frequently
characterized and reported in the literature, examples of
porphyrinic hydroperoxo iron species are scarce.7,37,38 We
first probed the protonation of 4 with perfluoro-tert-butanol
((CF3)3C−OH, pKa = 11.8 in DMF39). All our attempts
resulted in the formation of a species that we identified as
[FeIII(F20TPP) (OH)] based on EPR and UV−vis data (see
Supporting Information, Figures S7 and S8). Following studies
reported by the group of Naruta,7 stressing the role of
exogenous axial ligation to the heme for the generation of the
hydroperoxo species, we decided to probe the protonation of
the electrochemically prepared peroxo complex 4 in the
presence of 1-methylimidazole (1-meIm) as sixth ligand.
The EPR spectrum recorded on electrochemically prepared
solution of 4 to which 5 equiv of 1-meIm and 20 equiv of
perfluoro-tert-butanol ((CF3)3C−OH) were added shows the
disappearance of the g = 4.27 signal and the appearance of a
new set of signals at g = 2.26, 2.14, and 1.96 (Figure 5, blue
trace). Naruta et al.7 have reported the protonation of a heme
Fe(III)-peroxo complex (g = 4.27) into a low-spin Fe(III)-
hydroperoxo intermediate associated with a set of signals at g =
2.31, 2.19, and 1.95. Tajima38 also reported the EPR signature
of a low-spin Fe(III)-hydroperoxo [FeIII(TMP) (OOH) (Im)]
(TMP = tetramesityl porphyrinate) complex at g = 2.32, 2.19,
and 1.94. Consequently, in the present work the signals
detected at g = 2.26, 2.14, and 1.96 can be unambiguously
attributed to the hydroperoxo [FeIII(F20TPP) (OOH) (meIm)]
5 formed upon protonation of 4 by (CF3)3C−OH in the
presence of 1-meIm. Quantification of this signal against a
range of CuII standards indicates an overall conversion of
FeIIICl (1) to FeIIIOOH (5) of 10% (0.049 mM), with the
remaining FeIII centers being transformed into EPR-silent
species. When left at room temperature, the hydroperoxo 5
degrades as reflected by the EPR spectrum of the resulting
solution, which shows the decline of the signal of the low-spin
Fe(III)-hydroperoxo 5 and the appearance of a new one at g =
5.8 (Figure 5, trace (e)), attributed to a high-spin Fe(III)
species similar to the starting complex 1.
Conclusion. In conclusion, using [FeIII(F20TPP)Cl] 1 as
the starting material, formation of the [FeIII(F20TPP)OO]−
peroxo complex 4 resulting from the electrochemical reduction
of [FeIII(F20TPP)(O2•−)] adduct 3 at a fairly cathodic potential
2
(Eapp = −0.60 V) was evidenced by CV, UV−vis, and EPR
spectroscopies. In presence of 1-meIm, the reactive and
seldom-characterized intermediate [FeIII(F20TPP)OOH] hy-
droperoxo 5 is detected upon protonation of 4. It is
nevertheless remarkable that axial ligation of the Fe(III) center
is crucial for the relative stabilization of compound 5. The
present electrochemical approach provides an elegant way for
the stepwise preparation of important and biologically relevant
heme intermediates. Our system, which can easily be
transferred to other relevant systems, offers further oppor-
tunities to develop novel electrocatalytic devices for challenging
reactivity studies such as hydroxylation of robust C−H bonds.
EXPERIMENTAL SECTION
■
Chemicals. All reagents and solvents were obtained commercially
(Acros Organics and Aldrich). [FeIII(F20TPP)Cl] (1), tetrabutylam-
monium hexafluorophosphate (TBAPF6) supporting electrolyte, DMF
(anhydrous, 99.8%), and perfluoro-tert-butanol were used without
further purification. 1-Methylimidazole was predried with sodium
metal and then distilled under vacuum at 110 °C (vapor temperature:
70 °C). Distilled 1-methylimidazole was then stored at room
temperature under argon.
A summary of the reactivity of 4 toward protonation in the
presence of 1-meIm is proposed in Scheme 2. Its conversion
into the Fe(III)-hydroperoxo complex 5 is associated with a
high-spin (S = 5/2) to low-spin (S = 1/2) conversion, which
can be related to a coordination change of the peroxo ligand
from a side-on to an end-on conformation in a similar manner
to what is proposed in the literature.8,9 Most of this reaction
pathway yields EPR-silent FeII complexe 6 whose detailed
formation remains unclear. Nonetheless, we shall underline that
reduction of ferric porphyrin peroxo complexes by 1-meIm has
been previously observed in the literature.40 Proof of this
Cyclic Voltammetry and Bulk Electrolysis. Electrochemical
experiments were run under argon or O2 atmosphere. Dry O2
atmosphere was obtained by purging the solution with compressed
air via a glass tube filled with CaCl2. Cyclic voltammograms and
electrolyses were recorded on a Metrohm potentiostat (AUTOLAB
PGSTAT302N model). For cyclic voltammetry, the counter electrode
used was a Pt wire, and the working electrode was a glassy carbon disk
(3 mm diameter) carefully polished before each voltammogram with a
1 μm diamond paste, sonicated in an ethanol bath and then washed
with ethanol. For bulk electrolysis the working electrode was a glassy
carbon cylinder, and the counter electrode used was a piece of Pt,
separated from the rest of the solution with a fritted bridge. The
reference electrode used was an SCE, isolated from the rest of the
solution with a fritted bridge. Supporting electrolyte was 0.1 M (293
K) or a 0.2 M (243 K). Low-temperature regulation was ensured by a
Julabo circulation cryostat.
Low-Temperature Ultraviolet−Visible Spectro-Electrochem-
istry. Thin-cell spectro-electrochemical data were obtained using a
combination of three electrodes in a thin cell (optical length = 0.2 cm)
mounted on a UV/visVarian Cary 60 Spectrophotometer, equipped
with a transparent dewar.22 It consists of a 0.2 cm quartz UV−vis−
NIR cell surmounted by a glass compartment. The entire solution was
saturated with air (1 mM O2), and the cell was cooled to 243 K by a
Julabo circulation cryostat.
hypothesis is the emergence of a wave at E0 = +0.15 V after
(6)
reaction of 4 with 1-meIm and (CF3)3C−OH (Figures S9 and
S10 and Table S2), which strongly supports the presence of an
Fe(II) species in solution. Additionally, upon addition of 1-
meIm and (CF3)3C−OH the UV−vis spectrum of complex 4
(Figure S11 red line) shows significant changes: the Soret band
increases and sharpens while being shifted to λ = 422 nm, and
two Q bands appear at λ = 530 and 555 nm (Figures S11 and
S12 green line). On the basis of the similarities of these spectral
features with the ones reported for [FeII(F20TPP) (OH)2]2−
(or [FeII(F20TPP) (OH) (OH2)]−)41 we formulate 6 as
E
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Electron Paramagnetic Resonance Spectroscopy. X-band
EPR spectra were recorded on a Bruker ELEXSYS 500 spectrometer
equipped with a Bruker ER4119HS X band resonator, an Oxford
Instrument continuous flow ESR 900cryostat, and a temperature
control system. EPR samples were collected during the course of
electrolysis and immediately frozen in liquid N2. The EPR tubes were
put through five cycles of vacuum/helium to purge oxygen from the
samples before being entered into the EPR spectrometer.
iron porphyrin peroxide complex. Peroxoferrioctaethylporphyrin(1-).
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Maeda, Y.; Naruta, Y. Spectroscopic Characterization of a Hydro-
peroxo−Heme Intermediate: Conversion of a Side-On Peroxo to an
End-On Hydroperoxo Complex. Angew. Chem., Int. Ed. 2009, 48 (49),
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ASSOCIATED CONTENT
* Supporting Information
■
S
(8) Chufan, E. E.; Karlin, K. D. An Iron−Peroxo Porphyrin Complex:
́
New Synthesis and Reactivity Toward a Cu(II) Complex Giving a
Heme−Peroxo−Copper Adduct. J. Am. Chem. Soc. 2003, 125 (52),
16160−16161.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01804.
(9) Liu, J.-G.; Shimizu, Y.; Ohta, T.; Naruta, Y. Formation of an End-
On Ferric Peroxo Intermediate upon One-Electron Reduction of a
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A.; Tabard, A. Formation of an iron(IV)-oxo ″picket-fence″ porphyrin
derivative via reduction of the ferrous dioxygen adduct and reaction
with carbon dioxide. J. Am. Chem. Soc. 1985, 107 (12), 3736−3738.
(11) Welborn, C. H.; Dolphin, D.; James, B. R. One-electron
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observation of intermediates formed during steady-state electro-
catalytic O2 reduction by iron porphyrins. Proc. Natl. Acad. Sci. U. S.
A. 2013, 110 (21), 8431−8436.
CV analysis of a simulation of [FeIII(F20TPP)Cl] (1) in
DMF under O2 atmosphere; CV of [FeIII(F20TPP)Cl]
(1) in DMF under O2 atmosphere using Pt as working
electrode. CV of chemically prepared [FeIII(F20TPP)
(OO)]−. Additional characterizations of [FeII(F20TPP)-
(O2)] (2) and species resulting from protonation of
[FeIII(F20TPP) (OO)]− (4) in the presence of 1-
methylimidazole; characterization of species resulting
from protonation of [FeIII(F20TPP) (OO)]− (4) in the
absence of 1-methyl-imidazol (addition of nonafluoro-
tert-butyl alcohol ((CF3)3C−OH)) (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: claire.fave@univ-paris-diderot.fr. (C.F.)
*E-mail: elodie.anxolabehere@univ-paris-diderot.fr. (E.A.-M.)
ORCID
■
(13) Sengupta, K.; Chatterjee, S.; Dey, A. Catalytic H2O2
Disproportionation and Electrocatalytic O2 Reduction by a Functional
Mimic of Heme Catalase: Direct Observation of Compound 0 and
Compound I in Situ. ACS Catal. 2016, 6, 1382−1388.
́ ̀
(14) Ching, H. Y. V.; Anxolabehere-Mallart, E.; Colmer, H. E.;
Costentin, C.; Dorlet, P.; Jackson, T. A.; Policar, C.; Robert, M.
Electrochemical formation and reactivity of a manganese peroxo
complex: acid driven H2O2 generation vs O-O bond cleavage. Chem.
Sci. 2014, 5 (6), 2304−2310.
Elodie Anxolabehere-Mallart: 0000-0002-8708-802X
́
̀
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
́ ́ ̀ ́ ́
(15) Segaud, N.; Anxolabehere-Mallart, E.; Senechal-David, K.;
Acosta-Rueda, L.; Robert, M.; Banse, F. Electrochemical study of a
nonheme Fe(II) complex in the presence of dioxygen. Insights into the
reductive activation of O2 at Fe(II) centers. Chem. Sci. 2015, 6 (1),
639−647.
Notes
The authors declare no competing financial interest.
(16) In the applied conditions formation of the μ-oxo dimer was not
detected. See references for μ-oxo characterization.
(17) Kadish, K. M.; Autret, M.; Ou, Z.; Tagliatesta, P.; Boschi, T.;
Fares, V. Synthesis, Structure, and Electrochemistry of an Electron
Deficient μ-Oxo Porphyrin Dimer, [(TPPBr4)Fe]2O. Inorg. Chem.
1997, 36 (2), 204−207.
(18) Helms, J. H.; Ter Haar, L. W.; Hatfield, W. E.; Harris, D. L.;
Jayaraj, K.; Toney, G. E.; Gold, A.; Mewborn, T. D.; Pemberton, J. E.
Effect of meso substituents on exchange-coupling interactions in μ−
oxo iron(III) porphyrin dimers. Inorg. Chem. 1986, 25 (14), 2334−
2337.
ACKNOWLEDGMENTS
■
This work was financially supported by the ANR (Project ANR
Blanc Cathymethoxy). The EU-COST Networks for Bio-
inorganic Reaction Mechanisms (CM1003) and Explicit
Control Over Spin-states in Technology and Biochemistry
(ECOSTBio, CM1305) are acknowledged for support to F.B.,
while C. Achaibou, C. Cometto, and S. Groni are thanked for
running last additional experiments.
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