Characterization and Subsequent Reactivity of an Fe-Peroxo

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Characterization and Subsequent Reactivity of an Fe-Peroxo Porphyrin Generated by Electrochemical Reductive Activation of O2 Raquel Oliveira,† Wiem Zouari,† Christian Herrero,‡ Frédéric Banse,‡ Bernd Schöllhorn,† Claire Fave,*,† and Elodie Anxolabéhère-Mallart*,† †

Laboratoire d’Electrochimie Moléculaire, Université Paris Diderot, Université Sorbonne Paris Cité, UMR CNRS 7591, 75205 PARIS Cedex 13, France ‡ Institut de Chimie Moléculaire et des Matériaux d’Orsay, Université Paris Sud, Université Paris Saclay, UMR CNRS 8182, 91405 Orsay Cedex, France 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.

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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 © XXXX American Chemical Society

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. 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

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DOI: 10.1021/acs.inorgchem.6b01804 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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RESULTS AND DISCUSSION The complex [FeIII(F20TPP)Cl] (1) (Scheme 1; F20TPP = 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinate) was chos-

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 spectroelectrochemical 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. 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•− wave, even when considering diffusion-limited conditions (k = 1 × 1010 M−1 s−1) for the superoxide to Fe(II) coordination (Figure S4). 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, reduction of 1 was conducted at Eapp1 = −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

Scheme 1. Complex 1 ([FeIII(F20TPP)Cl]) and Schematic Representation of the Reaction of 2 ([FeII(F20TPP)]) with O2 and Subsequent Controlled Reductive Conditionsa

a

For simplification, the F20TPP ligand was omitted from the formula of the complexes.

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 solution.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 quasireversible single-electron wave (−0.85 V versus SCE) 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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Inorganic Chemistry

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, DO2 = 4 × 10−5 cm2 s−1 and DFe = 2.5 × 10−6 cm2 s−1 for all of the Fe 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 Fe I I I -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 Eapp2 = −0.60 V 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 (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 Eapp1 = −0.25 V under argon. The resulting [FeII(F20TPP)] 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.

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 ([O2] = 1 mM) to give 3, T = 243 K (blue line); Eapp2 = −0.60 V versus SCE, T = 243 K to give 4 (red line). Optical path 0.2 cm.

Table 1. Electrochemical and Spectroscopic Data λ (nm) (εa (mM cm−1)) III

[Fe Cl] (1) [FeII] (2)

c

[FeIII(O2•−)] (3)

[FeIII(OO)]− (4)

d

d

414 (10.8) 559 (1.0) 427d (18.3) 554d (1.0) 420f (15.8) 530f (1) 552(sh)f (0.6) 432f (25) 559f (6)

[FeIII(OOH)(meIm)] (5)

g valuesb 5.78

E (V versus SCE) E0,apFeIIICl/FeII = 0.02e

Epc(3) = −0.50g

4.27

Epa(4) = +0.50g

2.26, 2.14, 1.96

a ε values are calculated for each step when absorbance at λmax no longer varies. bEPR perpendicular mode, spectra recorded at T = 100 K. cFor simplification, the F20TPP ligand was omitted in the formula of the complexes. dValue at T = 293 K. eFrom simulation of the CV (see ESI). fValue at T = 243 K. gAnodic (Epa) or cathodic (Epc) peak potentials experimental values for irreversible waves.

C


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Inorganic Chemistry

(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 1Methylimidazole 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 = 243 K (blue line), after bulk electrolysis at Eapp2 = −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 at Eapp1 = −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 O2 (air saturated 1 mM) and electrolysis of the solution at Eapp2 = −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 confirming 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(4) = +0.50 V (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)-

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-tetraazacyclotetradecane (+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

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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Inorganic Chemistry

[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. 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 (Eapp2 = −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] hydroperoxo 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 opportunities to develop novel electrocatalytic devices for challenging reactivity studies such as hydroxylation of robust C−H bonds.

electrochemical approach provides a clean method to generate reactive peroxo species without the involvement of chemical oxidants or reductants. 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. 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 hypothesis is the emergence of a wave at E0(6) = +0.15 V after 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 1meIm 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

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EXPERIMENTAL SECTION

Chemicals. All reagents and solvents were obtained commercially (Acros Organics and Aldrich). [FeIII(F20TPP)Cl] (1), tetrabutylammonium 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. 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-Electrochemistry. 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. E


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Inorganic Chemistry 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.

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iron porphyrin peroxide complex. Peroxoferrioctaethylporphyrin(1-). J. Am. Chem. Soc. 1988, 110 (5), 1382−1388. (6) Selke, M.; Sisemore, M. F.; Valentine, J. S. The Diverse Reactivity of Peroxy Ferric Porphyrin Complexes of Electron-Rich and ElectronPoor Porphyrins. J. Am. Chem. Soc. 1996, 118 (8), 2008−2012. (7) Liu, J.-G.; Ohta, T.; Yamaguchi, S.; Ogura, T.; Sakamoto, S.; Maeda, Y.; Naruta, Y. Spectroscopic Characterization of a Hydroperoxo−Heme Intermediate: Conversion of a Side-On Peroxo to an End-On Hydroperoxo Complex. Angew. Chem., Int. Ed. 2009, 48 (49), 9262−9267. (8) Chufán, 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. (9) Liu, J.-G.; Shimizu, Y.; Ohta, T.; Naruta, Y. Formation of an EndOn Ferric Peroxo Intermediate upon One-Electron Reduction of a Ferric Superoxo Heme. J. Am. Chem. Soc. 2010, 132 (11), 3672−3673. (10) Schappacher, M.; Weiss, R.; Montiel-Montoya, R.; Trautwein, 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 electrochemical reduction of a ferrous porphyrin dioxygen complex. J. Am. Chem. Soc. 1981, 103 (10), 2869−2871. (12) Sengupta, K.; Chatterjee, S.; Samanta, S.; Dey, A. Direct observation of intermediates formed during steady-state electrocatalytic O2 reduction by iron porphyrins. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (21), 8431−8436. (13) Sengupta, K.; Chatterjee, S.; Dey, A. Catalytic H 2 O 2 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.; Anxolabéhère-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. (15) Ségaud, N.; Anxolabéhère-Mallart, E.; Sénéchal-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. (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. (19) Gueutin, C.; Lexa, D.; Savéant, J. M.; Wang, D. L. σ−Alkyl iron porphyrins from sterically encumbered alkyl halides and iron(0) porphyrins. Stabilities of the four accessible oxidation states. Organometallics 1989, 8 (7), 1607−1613. (20) In the present experimental conditions the concentration of [FeII(F20TPP)Cl] is negligible. (21) The small intensity at −0.5 V on the black trace is due to small amount of adduct resulting from trace of O2 in solution. (22) Gueutin, C.; Lexa, D. Low temperature spectroelectrochemistry for the characterization of highly reduced σ-alkyl iron halogenated porphyrins. Electroanalysis 1996, 8 (11), 1029−1033. (23) It was checked that the CV response of a 1 mM solution of 1 in oxygenated DMF is identical using a Pt working electrode (see Figure S5).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01804. 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 1methylimidazole; characterization of species resulting from protonation of [FeIII(F20TPP) (OO)]− (4) in the absence of 1-methyl-imidazol (addition of nonafluorotert-butyl alcohol ((CF3)3C−OH)) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (C.F.) *E-mail: [email protected]. (E.A.-M.) ORCID

Elodie Anxolabéhère-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. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the ANR (Project ANR Blanc Cathymethoxy). The EU-COST Networks for Bioinorganic 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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REFERENCES

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Article

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Characterization and Subsequent Reactivity of an Fe-Peroxo

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