Oxygen reduction catalyzed by a fluorinated tetraphenylporphyrin free base at Liquid/Liquid interfaces

Article abstract of DOI:10.1021/ja103460p

The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H₂FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichlor

Full text of DOI:10.1021/ja103460p

Published on Web 09/09/2010
Oxygen Reduction Catalyzed by a Fluorinated
Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces
Imren Hatay,†,‡ Bin Su,†,| Manuel A. Me´ndez,† Cle´mence Corminboeuf,§
Tony Khoury,⊥ Claude P. Gros,⊥ Me´lanie Bourdillon,⊥ Michel Meyer,⊥
Jean-Michel Barbe,⊥ Mustafa Ersoz,‡ Stanislav Zálisˇ,¶ Zdeneˇk Samec,¶ and
Hubert H. Girault*,†
Laboratoire d’Electrochimie Physique et Analytique, Ecole Polytechnique Fe´de´rale de Lausanne,
Station 6, CH-1015 Lausanne, Switzerland, Department of Chemistry, Selcuk UniVersity,
42031 Konya, Turkey, Laboratory for Computational Molecular Design, Ecole Polytechnique
Fe´de´rale de Lausanne, BCH-5312, CH-1015 Lausanne, Switzerland, Institut de Chimie
Mole´culaire de l’UniVersite´ de Bourgogne, ICMUB (UMR 5260 du CNRS), 9 aVenue Alain
SaVary, BP 47870, 21078 Dijon Cedex, France, and J. HeyroVsky´ Institute of Physical
Chemistry of ASCR, V.V.i, DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic
Received April 23, 2010; E-mail: hubert.girault@epfl.ch
Abstract: The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-
tris(pentafluorophenyl)porphyrin (H2FAP), can catalyze the reduction of oxygen by a weak electron donor,
namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H4FAP2+ is
observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H2FAP
at the imino nitrogen atoms in the tetrapyrrole ring. H4FAP2+ is shown to bind oxygen, and the complex in
the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase
reactions with the Galvani potential difference between the two phases being controlled by the partition of
a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly
evidence that reduction of oxygen by Fc catalyzed by H4FAP2+ only occurs in the presence of the
tetrakis(pentafluorophenyl)borate (TB-) counteranion in the organic phase. Finally, ab initio computations
support the catalytic activation of H4FAP2+ on oxygen.
evolved.1 All of them contain a binuclear heme-Cu catalytic
site at which O2 is reduced by forming a ferriheme/superoxide
intermediate. This biological pathway has stimulated a signifi-
cant effort to synthesize biomimetic analogues, e.g. various
metalloporphyrins, for better understanding the structure/activity
relationship at the catalytic site and developing useful catalysts
for energy conversion processes such as O2 reduction in fuel
cells.1-6 Electrocatalytic O2 reduction by metalloporphyrins has
been mostly studied by deposition of metalloporphyrins on an
electrode surface, but the deposition methods often induce
morphological heterogeneities, making quantitative analyses
rather difficult. Alternatively, Fukuzumi et al. have proposed a
homogeneous pathway by replacing the electrode with a
molecular electron donor, such as ferrocene compounds.7-9 In
this case, the ferrocene compounds function as electron donors
like physiological electron carriers such as quinols in the case
of oxidases. The presence of metalloporphyrin catalysts sig-
nificantly accelerates the O2 reduction by the molecular electron
donors.7-9
1. Introduction
Molecular oxygen (O2) reduction reaction (ORR) maintains
the life of an aerobic organism by the respiration process.1-5
Owing to the inertness of O2 arising from its low affinity for
proton and its triplet electronic state, biological ORR requires
catalysis for energy metabolism. Various classes of membrane-
bound enzymes called terminal oxidases that perform this
catalytic task, e.g. cytochrome c oxidases, have naturally
† Laboratoire d’Electrochimie Physique et Analytique, Ecole Polytech-
nique Fe´de´rale de Lausanne.
‡ Department of Chemistry, Selcuk University.
§ Laboratory for Computational Molecular Design, Ecole Polytechnique
Fe´de´rale de Lausanne.
⊥ Institut de Chimie Mole´culaire de l’Universite´ de Bourgogne, ICMUB
(UMR 5260 du CNRS).
¶ J. Heyrovsky Institute of Physical Chemistry of ASCR.
| Present address: Institute of Microanalytical Systems, Department of
Chemistry, Zhejiang University, 310058 Hangzhou, China.
(1) Boulatov, R. In N4-Macrocyclic Metal Complexes; Zagal, J. H.,
Bedioui, F., Dodelet, J.-P., Eds.; Springer: New York, 2006, pp 1-
36.
O2 reduction by molecular electron donors has been studied
at the interface between two immiscible electrolyte solutions
(2) Zagal, J. H.; Paez, M. A.; Silva, J. F. In N4-Macrocyclic Metal
Complexes; Zagal, J. H., Bedioui, F., Dodelet, J.-P., Eds.; Springer:
New York, 2006; pp 41-75.
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(4) Collman, J. P.; Boulatov, R.; Sunderland, C. J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, 2003; Vol. 11, pp 1-49.
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2004, 104, 561–588.
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(7) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Chem. Lett. 1989, 27–30.
(8) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28, 2459–
2465.
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Soc. 2004, 126, 10441–10449.
9
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J. AM. CHEM. SOC. 2010, 132, 13733–13741 13733

A R T I C L E S
Hatay et al.
by recrystallization from acetone.22 All the aqueous solutions were
prepared with ultra pure water (18.2 MΩ cm). The pH of the
aqueous solutions was adjusted by addition of HCl. All the
chemicals used for synthesis of 5-(p-aminophenyl)-10,15,20-
tris(pentafluorophenyl)porphyrin (H2FAP) were bought from either
Acros or Aldrich. HTB was prepared by shaking x mM LiTB and
10 mM HCl in the water phase with pure DCE for 1 h. The
concentration of HTB extracted to DCE was approximately x mM,
as determined by the pH change of the water phase before and
after shaking.
2.2. Spectrophotometric Titration. H2FAP in DCE was titrated
in a small flask by addition of concentrated trifluoroacetic acid
(TFA) in DCE (1 mM to 1 M). The total volume was adjusted to
1 mL with DCE to give a concentration of H2FAP at 20 µM. The
flask was capped and shaken for a sufficient reaction, and then the
UV-visible spectrum was measured on an Ocean Optical
CHEM2000 spectrophotometer using a quartz cuvette (path length
10 mm).
(ITIES).10-19 This inherently soft interface has been character-
ized to be sharp at the molecular level with some corrugations
or so-called fingerings. The separation of ORR products is also
largely favored by the heterogeneous nature of this interface. It
has been shown recently that the interface can function as a
proton pump driven by the interfacial Galvani potential differ-
ence for the O2 reduction by decamethylferrocene (DMFc) to
produce hydrogen peroxide (H2O2).16 Various catalysts including
in situ deposited platinum particles20 and cobalt porphyrins have
been characterized by voltammetric studies.14,15,17,19 More
surprisingly, it was found also by voltammetric measurements
that the protonated forms of free base tetraphenylporphyrin
(H3TPP+ and H4TPP2+) could also facilitate O2 reduction by
DMFc.18 Since, in fact, organic acids such dodecylanilinium
can catalyze O2 reduction by DMFc as recently demonstrated,21
the role of H2TPP was not clear, acting simply as an acid or
activating molecular oxygen.
2.3. Electrochemical Measurements. Electrochemical measure-
ments at the water/DCE interface were performed in a four-electrode
configuration with a commercial potentiostat (PGSTAT 30, Eco-
Chemie, Netherlands) or on a custom-built four-electrode system
connected to a DS335 synthesized function generator (Stanford
Research System). A three-compartment cell featuring a cross-
section of 1.53 cm2 and two side Luggin capillaries was employed.
The external potential was applied by means of the two reference
electrodes, silver/silver chloride (Ag/AgCl) wires, placed in two
Luggin capillaries. The composition of the electrochemical cell is
displayed in Scheme 3. The Galvani potential difference (∆wo φ) was
estimated by taking the formal ion transfer potential of tetraethyl-
ammonium cation (TEA+) as 0.019 V.23 All the electrochemical
measurements were performed at ambient temperature (23 ( 2 °C)
with air-saturated solutions.
Here, we have synthesized a fluorinated free base porphyrin,
namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)por-
phyrin (H2FAP) that combines a lipophilicity suitable to be
investigated by ion transfer voltammetry, and a fluorination of
three phenyl rings to ensure a strong electron-withdrawing effect
to provide a weak basicity to the tetrapyrrole ring to favor
oxygen reduction. Electrochemical and spectroscopic measure-
ments show that the diprotonated form of this molecule acts as
a catalyst for the reduction of oxygen by a weak electron donor
in the organic phase, namely ferrocene, provided that the
counteranion is properly chosen.
2. Experimental Section
2.4. Two-Phase Reactions. The two-phase reactions were
performed in a small flask under stirring conditions. DCE solution
(2 mL) containing either 5 mM Fc or 5 mM Fc and 30 µM H2FAP
was added to the flask, followed by the addition of 2 mL of aqueous
solution containing 10 mM HCl on the top. The aqueous and organic
salts of common ion, LiTB and BATB, were added at the same
concentration of 5 mM. The two phases were stirred for 30 min
then left for phase separation, and the aqueous and organic solutions
were isolated from each other. The organic phase was directly
subjected to the UV-visible spectroscopic measurement, while the
aqueous phase was treated with excess NaI prior to the spectroscopic
measurement.
2.1. Chemicals. All solvents and chemicals were used as
received without further purification. Lithium tetrakis(pentafluo-
rophenyl)borate diethyl etherate (LiTB), ferrocene (Fc, g 98%) and
HClO4 (70%) were provided by Sigma-Aldrich. Trifluoroacetic acid
(TFA) was purchased from Acros. Hydrochloric acid (HCl),
anhydrous lithium chloride (LiCl, g 99%), 1,2-dichloroethane
(DCE, g 99.8%), sodium iodide (NaI, > 99.5%), tetraethylammo-
nium chloride (TEACl, g 98%), and bis(triphenylphosphoranylide-
ne)ammonium chloride (BACl, g 98%) were obtained from Fluka.
Bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophe-
nyl)borate (BATB) was prepared by metathesis of 1:1 mixtures of
BACl and LiTB in a methanol/water mixture (V:V ) 2:1), followed
2.5. Spectrophometric Study of Oxygen Reduction by
Ferrocene Catalyzed by H2FAP. The course of the oxidation
reaction of Fc by O2 was monitored by visible absorption spectro-
photometry at the maximum of the Fc+ band (λ ) 620 nm) on a
Cary 50 (Varian) instrument. The temperature was maintained
constant at 298.2(2) K by water circulation through the double-
walled cell holder connected to a RE106 (Lauda) bath. Stock
solutions of all reagents were prepared in DCE ([H2FAP] ) 500
µM, [Fc] ) 24.7 mM, [HTB] ) 10.6 mM). In a typical experiment,
0.2 mL of H2FAP and 0.4 mL of HTB solutions were diluted with
1.0 mL of pure DCE in a quartz cell (Hellma) of 1 cm path length.
After setting the optical density value of the mixture to zero, an
aliquot (0.4 mL) of Fc solution was injected. Reactants were rapidly
mixed by shaking vigorously the cuvette, after which the absorbance
changes as a function of time were recorded. Since TFA is a much
weaker proton donor in DCE than HTB, 15 µL of commercial acid
(13.3 M) diluted in 1.385 mL of DCE were introduced in the cell
before adding 0.4 mL of the Fc solution. For the control experiment
carried out in the absence of porphyrin, an equivalent amount of
solvent was added in order to keep the HTB and Fc concentrations
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9
13734 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Diprotonated H2FAP as Catalyst for ORR
A R T I C L E S
Scheme 1. Synthetic Pathway Leading to H2FAP
identical to those used in the catalyzed reaction. Final concentrations
were [H2FAP] ) 50 µM, [Fc] ) 4.94 mM, [HTB] ) 2.12 mM or
[TFA] ) 0.10 M. The oxygen concentration was limited by the
solubility (1.39 mM) and rapid diffusion of O2 in DCE. Air-
saturated solutions afforded the same kinetic traces whether or not
the headspace of the cuvette was maintained under a slight
overpressure of pure dioxygen.
triatomics in molecules (TRIM)30 approach, which reduces the basis
set superposition error (BSSE) and, more importantly, the compu-
tational cost.
3. Results and Discussion
3.1. Synthesis of H2FAP. The stepwise synthesis of the 5-(p-
aminophenyl)-10,15,20-tris(pentafluorophenyl) porphyrin
7
2.6. Computation. All the geometries were optimized at the
M05-2X24/cc-pVDZ25 level using Gaussian09.26 Zero-point cor-
rected relative energies are computed at the same level for the
protonated forms of the fluorinated free base porphyrin. Only the
lowest energy form of H3FAP+ protonated in the tetrapyrrole ring
was considered. The binding energies for the complexes ([H3FAP+
· · ·O2] and [H4FAP2+ · · ·O2]) correspond to the energy difference
between the complexes and its separate adducts (i.e., triplet oxygen
and singlet H3FAP+/H4FAP2+). Those binding energies were
computed using RI-LMP227,28/cc-pVDZ single-point energies in a
developmental version of Q-Chem.29 No symmetry was exploited
and the convergence criterion for the reference wave function was
set to 10-6, core electrons were kept frozen and the standard
auxiliary basis set was used for the resolution-of-identity (RI). For
all other settings the default values were adopted. Open-shell
structures (i.e. the complexes and the oxygen molecule) were treated
in the unrestricted formalism. LMP2 in Q-Chem refers to the
(H2FAP) was achieved as described in Scheme 1 and in the
Supporting Information. The preparation of the 5-(pentafluo-
rophenyl)dipyrromethane 3 was performed in 67% yield starting
from pyrrole 1 and pentafluorobenzaldehyde 2 (Scheme 1); the
dipyrromethane was used in order to prepare the bilane
intermediate 4 which was then condensed with 4-nitro-benzal-
dehyde 5 in a stirring dichloromethane/trifluoroacetic acid
medium to give the nitrophenyl-tris(pentafluorophenyl) porphy-
rin 6 in 8% yield. Tin(II) chloride mediated reduction of 6 gave
the expected free-base amino-porphyrin H2FAP 7 in 75% yield.
1
H2FAP 7 was fully characterized by HRMS, H NMR, and
UV-vis spectroscopy (see Supporting Information for details).
3.2. Spectrophotometric Titration of H2FAP. When consider-
ing the molecular structure of H2FAP, both the two imino
nitrogen atoms in the tetrapyrrole ring and the peripheral amine
can be protonated. To verify that the amino group, used to confer
some amphiphilic character to the molecule, does not interfere
with the protonation of the ring, a spectrophotometric titration
of H2FAP with an organic soluble acid, TFA, was performed
in DCE. Figure 1 illustrates the absorption spectra of successive
acid titration products of H2FAP, which show that protonation
occurs in two well-separated stages with good sets of isosbestic
(24) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory Comput.
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3183.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13735

A R T I C L E S
Hatay et al.
previously for a free base tetraphenylporphyrin bearing one
aminophenyl group at one meso-position.31 The protonation sites
at the first stage can therefore be assigned to both imino nitrogen
atoms in the tetrapyrrole ring leading to the formation of
H4FAP2+, while the second stage can be assigned to the
protonation of the single peripheral amine. These conclusions
are also supported by density functional theory computations
as illustrated in Scheme 2. The M05-2X/cc-pVDZ relative
energies of the protonated forms of H2FAP indeed demonstrate
that the protonation first occurs on the imino nitrogen atoms of
the porphyrin ring. The protonation of the peripheral amine is
disfavored by ∼30 kcal mol-1. The binding energy of a second
proton on the ring is found to be rather easy:
H2FAP + H3O+ f H3FAP+ + H2O - 68.9 kcal · mol-1
H3FAP+ + H3O+ f H4FAP2+ + H2O - 5.1 kcal · mol-1
3.3. Ion-Transfer Voltammetry of H2FAP and Determination
of pKaDCE. Electrochemical measurements at the water/DCE
interface were carried out so as to characterize the acid-base
properties of the fluorinated porphyrin. Figure 2a shows the
cyclic voltammograms of the water/DCE interface obtained in
the absence (dotted line) and presence (full line) of 100 µM
H2FAP. As can be seen, the presence of H2FAP in DCE resulted
in a single voltammetric wave at positive potentials. The absence
of a clear and well-defined wave for the second protonation of
the free-base could be due either to its extreme hydrophobicity,
so that it cannot be observed within the potential window, or
to the close proximity between the two pKa values. Considering
that the monoprotonated species cannot be observed by spec-
trophotometric titration, it is very likely that two successive
protonations occur in a narrow potential range, which can be
equivalent to two successive EE reactions in the classical cyclic
voltammetry of electron transfer reactions (E) on solid elec-
trodes. If the second protonation would have been easier than
the first one, the voltammetric signal should correspond to the
simultaneous transfer of two protons, and the peak-to-peak
separation would be 29 mV. If the two protonations have exactly
Figure 1. Spectrophotometric titration of 20 µM H2FAP in DCE by TFA:
(a) at TFA concentrations lower than 50 mM; (b) at TFA concentrations
higher than 50 mM.
points. At the first stage of protonation (Figure 1a), when the
TFA concentration is lower than 50 mM, adding TFA to a
H2FAP solution led to a slight decrease and red-shift (from 415
to 419 nm) of the Soret band, as well as the disappearance of
the four Q bands (λmax ) 513, 548, 587, and 644 nm) and the
appearance of two new but broad Q bands (λmax ) 505 and
665 nm) that increase with increasing TFA concentration. The
second stage at much higher TFA concentrations (Figure 1b)
resulted in a sharp Soret band (λmax ) 429 nm), the intensity of
which increases with increasing concentrations of acid. More-
over, the two Q bands at 505 and 665 nm are damped and finally
replaced by two new bands with λmax ) 578 and 628 nm. All
these experimental results are indeed similar to those observed
Scheme 2. Structures and Relative Energies of H3FAP+ Protonated Either on the Ring (left) or on the Amino Group (right); (Top) Top View;
(Bottom) Side View
9
13736 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Diprotonated H2FAP as Catalyst for ORR
A R T I C L E S
Figure 2. (a) Cyclic voltammograms in the absence (dotted line) and
presence (full line) of H2FAP in DCE at a scan rate of 50 mV s-1 (Scheme
3: x ) 100 µM, y ) 0, z ) 100 mM). (b) Background subtracted
voltammograms for 100 µM H2FAP facilitated proton transfer reactions at
various scan rates (Scheme 3: x ) 100 µM, y ) 0, z ) 100 mM): 10, 20,
50, 80, and 100 mV s-1 from inner to outer.
Scheme 3. Composition of the Electrochemical Cell
Figure 3. (a) Cyclic voltammograms in the presence of H2FAP in DCE at
different pH values (Scheme 3: x ) 100 µM, y ) 0), scan rate 50 mV s-1
.
the same pKa values, only one wave would be observed but
with a peak-to-peak separation of 41 mV (see Figure S1,
Supporting Information), as calculated by Digisim software.
Here, the peak-to-peak separation after background subtraction
and extrapolation to zero scan rate was measured to be 60 mV
(see Figure 2b), what is classically obtained for a single ion
transfer. However, the same peak-to-peak separation can be
obtained for two successive reactions for which the half-wave
potentials are separated by 38 mV, as calculated by Digisim
software. In other words, these voltammetric data suggest a
double assisted proton transfer reaction for which the two pKa
values are separated by at least 0.65 pH unit. Digisim fits of
experimental data gives a diffusion coefficient value of 8 ×
10-7 cm2 ·s-1 compared to a value of 3 × 10-6 cm2 ·s-1 for
H2TPP measured in the same conditions. (see Figures S2 and
S3, respectively (Supporting Information)).
Furthermore, the magnitude of this current wave does not
change with the acidity of the aqueous phase, whereas the wave
continuously moves to positive potentials with the half-wave
potential shifting by approximately 60 mV/pH unit, as displayed
in a and b of Figure 3. These data indicate that the voltammetric
wave corresponds to the assisted transfer of two successive
protons from water to DCE by H2FAP, i.e. the diprotonation of
H2FAP at the interface.
(b) pH dependence of the half-wave potentials.
D
H2FAP
RT
RT
F
∆wo φ1/2
) ∆wo φ0H
+
ln
+
ln Ka2 +
+
+
H3FAP
(
)
2F DH FAP
+
3
2.303RT
pHw (3)
F
with the second acidity constant in the organic phase defined
as:
aH FAP aH
+
2
Ka2
)
(4)
a
+
H3FAP
where ∆wo φ0H+ is the standard transfer potential of H+. DH FAP
2
+
and DH FAP represent the diffusion coefficients of the proton
3
+
acceptor ligand and its protonated form, and DH FAP ≈ DH FAP
2
3
can be assumed for simplicity. pHw is the aqueous pH and Ka2
is the dissociation constant of H3FAP+ in DCE. aH FAP, aH , and
+
2
+
+
aH FAP are the activities of H2FAP, proton, and H3FAP in DCE,
3
respectively. As shown in Figure 3b, the relationship between
1/2
∆wo φ
taken as a first approximation as the midpeak potential
+
H3FAP
and the pHw is found to be linear with a slope of 63.8 mV, and
Ka2 value for H3FAP+ was determined to be equal to 5.5 ×
10-7. Since the two half-wave potential values are separated
by at least 0.65 pH unit, the first Ka1 value can be estimated to
The interfacial reaction is limited by the linear diffusion of
H2FAP in the organic phase toward the interface. Moreover,
ion-transfer voltammetry allows the estimation of dissociation
constant if one takes into account the following protonation
reactions:
be at least 2.5 × 10-6
.
As reported previously and shown in Figure S3 (Supporting
Information), in the case of 5,10,15,20-tetraphenylporphyrin
(H2TPP) it was possible to observe the two successive proto-
nations of the two imino nitrogen atoms of the porphyrin ring
with dissociation constants of 1.6 × 10-10 and 1.0 × 10-6 for
H3TPP+ and H4TPP2+ by ion-transfer voltammetry.32 Compared
to the structure of H2TPP, H2FAP has three pentafluorinated
phenyl groups that exhibit a strong electron-withdrawing effect,
leading to the electron deficiency in the tetrapyrrole ring and
thus to a weaker basicity of the imino nitrogen atoms. It means
2+
H4FAP
a H3FAP+(DCE) + H+(DCE)Ka1
(1)
(2)
(DCE)
H3FAP+(DCE) a H2FAP(DCE) + H(+DCE)Ka2
Thus, the half-wave potential of the first protonation
1/2
3
∆wo φH FAP depends on the pH according to the following
+
equation:
(31) Ojadi, E. C. A.; Linschitz, H.; Gouterman, M.; Walter, R. I.; Lindsey,
J. S.; Wagner, R. W.; Droupadi, P. R.; Wang, W. J. Phys. Chem.
1993, 97, 13192–13197.
(32) Su, B.; Li, F.; Partovi-Nia, R.; Gros, C. P.; Barbe, J.-M.; Samec, Z.;
Girault, H. H. Chem. Commun. 2008, 5037–5038.
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Figure 5. (a) Two-phase reaction controlled by TB- partition after 30 min
shaking: the top aqueous phase contains 5 mM LiTB + 10 mM HCl in all
three flasks; the bottom DCE phase contains 5 mM Fc, 5 mM Fc + 30 µM
H2FAP and 30 µM H2FAP in the flask 1, 2, and 3, respectively; the inset
shows the flasks before adding water phase. (b) A two-phase reaction under
anaerobic conditions, the concentrations are the same as those for flask 2
in Figure 5a.
Figure 4. Cyclic voltammograms of Fc obtained at a water/DCE interface
with the cell illustrated in Scheme 3. (a) x ) 0 and y ) 0 (black), x ) 100
µM and y ) 0 (green), x ) 0 and y ) 5 mM (blue) and x ) 100 µM and
y ) 5 mM (red). (b). x ) 100 µM and y ) 5 mM under air-saturated (black),
anaerobic (red) and oxygen-saturated (blue) conditions. The pH was kept
constant, z ) 100 mM. The curves show the first scans.
assisted proton transfer step represents a faradaic process with
an ionic flux across the interface, thus being equivalent to the
EE step. The oxygen reduction reaction involving H4FAP2+
,
that H2FAP is more difficult to protonate than H2TPP. Therefore,
what is observed in the ion-transfer voltammetry corresponds
to the successive protonations of the imino nitrogen atoms to
form the diacid H4FAP2+. It should be mentioned that no
protonation of the amino group was observed during this
experiment.
Fc, and O2 is the following C step to produce H2O2 and
regenerate the catalyst H2FAP. Unfortunately, the chemical step
is too slow with respect to the ion transfer reaction to affect the
voltammetric signal for the double protonation of H2FAP. In
this case, voltammetry can only be used to observe the formation
of ferrocenium in the organic phase after the formation of
H4FAP2+ in aerobic conditions.
3.4. H4FAP2+ Catalysis of Oxygen Reduction by Fc. Fc does
not react with O2 in the absence of catalyst at least on the time
scale of a day. Figure 4a compares the cyclic voltammograms
in the presence of only Fc (blue line), in the presence of only
H2FAP (green line) and in the presence of both (red line) under
experimental conditions given in Scheme 3. The current wave
with a half-wave potential close to 0 V when adding only Fc in
DCE corresponds to the transfer of ferrocenium (Fc+) across
the water/DCE interface. Fc+ stems from a slow oxidation of
Fc in the stock solution, and is used here as an internal potential
reference. In the presence of both Fc and H2FAP in DCE, the
current wave originating from the double assisted proton transfer
by H2FAP seems not to be affected, being only shifted by the
back transfer of Fc+ from water to DCE. However, the current
wave corresponding to the transfer of Fc+ from DCE to water
is significantly increased indicating that Fc+ is generated upon
polarizing the interface to positive values. Moreover, Figure 4b
shows that the generation of Fc+ does not occur under anaerobic
conditions but increases under oxygen-saturated conditions.
These data suggest that the transfer of protons facilitated by
H2FAP induces an oxygen reduction by Fc, thereby forming
Fc+. Although H2FAP is a free base porphyrin, it exhibits a
catalytic behavior similar to that of cobalt tetraphenylporphyrin
(CoTPP) toward oxygen reduction by Fc.15
To further study this catalytic process, we have used biphasic
reaction conditions to investigate the role of H4FAP2+ and to
identify the reaction products. A chemical way to drive aqueous
protons to the organic phase is to add a very lipophilic anion to
the aqueous phase. Here, we have used tetrakis(pentafluorophe-
nyl)borate (TB-) to form, by extraction, the organic acid HTB.
As illustrated in Figure 5a, 10 mM HCl was present in the top
aqueous phase in all three flasks, while the DCE phase contained
5 mM Fc in flask 1, 30 µM H2FAP in flask 3, and both reagents
in flask 2. The color of the DCE phases is shown in the inset of
Figure 5a before addition of the aqueous phase. After adding
the water phase and stirring for 30 min, the color of the DCE
phase in flask 1 did not change, indicating that no reaction took
place. When only H2FAP is present in the DCE phase (flask
3), the color change of the organic layer from orange to pink is
indicative of the protonation of H2FAP. In the presence of both
Fc and H2FAP (flask 2), the color results from a mixture of
green Fc+ and pink H2FAP. Moreover, the experiment per-
formed in the absence of oxygen showed that the DCE solution
containing Fc and H2FAP did not change its color even after
stirring for 3 h as shown in Figure 5b.
After separation of both phases, the UV-visible absorption
spectra of DCE layers were recorded, whereas the aqueous
phases were titrated with NaI. When both Fc and H2FAP were
present in the DCE phase, the formation of Fc+ could be clearly
observed, as shown by the characteristic absorption band with
a maximum at 620 nm (red curve in Figure 6a). In contrast, a
fresh Fc solution only displays a shallow absorption band at
This process is equivalent to a catalytic EEC reaction scheme
in the classical electrode/solution electrochemistry, involving
two successive electron transfer reactions (EE) at the electrode/
solution interface followed by a chemical reaction (C) in the
solution regenerating the starting reactant. Here, the double-
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Diprotonated H2FAP as Catalyst for ORR
A R T I C L E S
Figure 7. Absorption spectra of 20 µM H2FAP (full line), in the presence
of H2FAP + 20 mM HClO4 (dotted) and in the presence of H2FAP +20
mM HClO4+ 1 mM Fc (dashed).
Figure 6. (a) UV-visible absorption spectra of the DCE phases separated
from the flask 1 (black), flask 2 (red), and flask 3 (blue) after the two-
phase reaction illustrated in Figure 5a. The spectra of freshly prepared 5
mM Fc (dotted black) and 30 µM H2FAP (dotted blue) are also displayed
for comparison. (b) UV-visible spectra of the aqueous phases separated
from the flask 1 (black), flask 2 (red), and flask 3 (blue) after treatment
with an excess of NaI.
Figure 8. Absorbance at λ ) 620 nm to monitor ferrocenium concentration.
Initial conditions: 0 or 50 µM H2FAP, 2.12 mM HTB as shown, 0.1 M
TFA as shown and 4.94 mM ferrocene.
the reduction of oxygen in a single phase of DCE by ferrocene
in the presence of HTB or TFA as acids and catalytic amounts
of H2FAP. The catalytic reaction was carried out using 4.94
mM Fc, 50 µM H2FAP, and 2.12 mM of HTB or 0.1 M TFA
in a O2-saturated DCE solution. Figure 8 reproduces the
absorbance changes at 620 nm (absorption band of Fc+) as a
function of time. It clearly shows the catalytic role of H2FAP
in the oxidation of ferrocene in the presence of oxygen and
HTB as acid. In contrast, Figure 8 also evidence the absence of
a catalytic effect when TFA is used instead of HTB or in the
absence of porphyrin (Fc and HTB alone). These data prove
that the catalytic reaction can only proceed through direct
interaction of O2 with H4FAP2+ (Vide infra). This can only
happen if the counteranion does not interact with the protons
inside the porphyrin core. In a solvent possessing a low dielectric
constant such as 1,2-dichloroethane (ε ) 10.42 at 293 K), one
can logically expect an increased stability of ion pairs of the
type H4FAP· · ·X- (X- being a coordinating anion). Tetrak-
is(pentafluorophenyl)borate (TB-) is a noncoordinating anion
compared to CF3COO- and should not interact with the inner
protons. This assumption is further corroborated by marked
differences in the Q-band region of the absorption spectra
obtained by protonating H2FAP either with HTB (λmax ) 544
nm, ε ) 8150 M-1 cm-1; λmax ) 574 nm, ε ) 10900 M-1 cm-1;
λmax ) 628 nm, ε ) 5550 M-1 cm-1) or TFA (λmax ) 579 nm,
ε ) 11000 M-1 cm-1; λmax ) 628 nm, ε ) 12900 M-1 cm-1).
Moreover, a close examination of several X-ray crystal structures
deposited in the Cambridge Structural Database (CCDC) for
diprotonated free-base porphyrins with CF3COO- as counter-
ions, clearly shows that for each structure the CF3COO- anions
are interacting with the inner protons through NH · · ·OOCCF3
439 nm (dotted black curve in Figure 6a). The formation of
Fc+ in the presence of only Fc in the DCE phase could not be
observed (full black curve in Figure 6a). When only H2FAP is
present in the DCE phase, protonation of H2FAP to form
H4FAP2+ is clearly evidenced by the UV-visible spectral
changes undergone by the Soret and Q bands (dotted blue curve
shifted to the full blue curve in Figure 6a).
Addition of sodium iodide (NaI) to the different aqueous
phases was used to detect hydrogen peroxide by formation of
tri-iodide, displaying two absorption bands at 286 and 352 nm
as shown by the red curve in Figure 6b for flask 2. In contrast,
no H2O2 was detected in flask 1 and flask 3 (blue and black
curve in Figure 6b), indicating that O2 reduction in the presence
of only Fc does not occur at this time scale. All these
spectroscopic data suggest that H2O2 is formed during the
biphasic reaction in the presence of both Fc and H2FAP in DCE.
To further prove these results, the titration of H4FAP2+ with
Fc in DCE was performed, as shown in Figure 7. Adding Fc in
large excess to a strongly acidic solution (HClO4) of H2FAP
resulted in the disappearance of the absorption features of
H4FAP2+ and the appearance of those of H2FAP, resolving the
two broad Q bands at λmax ) 505 and 665 nm to four Q bands
at λmax ) 513, 548, 587, and 644 nm and the blue shift of the
Soret band from 419 to 415 nm. This titration shows that
H4FAP2+ catalyzes the oxygen reduction by Fc to generate H2O2,
although this electron donor cannot reduce O2 directly even in
the presence of an acid.
Finally, because it is experimentally difficult to follow the
kinetics of ferrocene oxidation in the biphasic system due to
the time required to separate the two phases, we have studied
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A R T I C L E S
Hatay et al.
Scheme 4. Structures of the [H3FAP· · ·O2]+ (left) and [H4FAP· · ·O2]2+ Adducts; Unrestricted LMP2/cc-pVDZ Binding Energies at the M05-2X/
cc-pVDZ Optimized Geometriesa
a Closest O-H distances are given in Å.
H4FAP2+ + O2 f [H4FAP2+ · · ·O2]
(5)
(6)
hydrogen bonds (H · · ·O ≈ 1.9 Å).33-38 Even though no X-ray
crystal structure has been reported so far with TB- counteran-
ions, Rayati et al. have described the structure of meso-
tetraphenylporphyrin protonated with tetrafluoroboric acid,
HBF4.39 Noteworthy, two water molecules are ligated to the
[H4FAP2+ · · ·O2] + 2Fc f H2FAP + H2O2 + 2Fc+
The double proton coupled electron transfer reaction 6 is
currently under study (see also Scheme 5). As shown in the
Supporting Information (Figure S4), the oxidation potential of
H2FAP is rather high for this molecule to operate as a redox
catalyst.
-
inner protons of H4TPP2+ and further to the BF4 counterions
-
but no direct interaction between the inner protons with BF4
can be evidenced. In this report, the authors argue that fluorine
is not efficient to induce hydrogen bonding simultaneously with
the pair of inner protons of the porphyrin, although F · · ·H bonds
are generally stronger bonds than O · · ·H ones. It is reasonable
to assume a similar situation in the case of TB- anions.
Therefore, by using HTB as the proton source in the catalytic
experiments, the inner protons of H4FAP2+ are probably free
for coordination of oxygen as shown below.
4. Conclusions
The catalytic activation of the diprotonated form of a free-
base porphyrin, 5-(p-aminophenyl)-10,15,20-tris(pentafluorophe-
nyl)porphyrin (H2FAP), on the molecular oxygen (O2) reduction
by ferrocene (Fc) has been investigated at the polarized water/
1,2-dichloroethane (DCE) interface. Ion-transfer voltammetry
has revealed that H2FAP can assist a double proton transfer from
water to DCE by the formation of H4FAP2+ in DCE, which
thereby activates O2 reduction by Fc. The catalytic role has been
verified by using a weak reductant, ferrocene (Fc) that is quasi-
inert to O2 in the absence of a catalyst. Two-phase reactions
with the Galvani potential difference between the two phases
All the data presented above demonstrate unambiguously that
H4FAP2+ can promote oxygen reduction by Fc. LMP2/cc-pVDZ
single-point computations corroborates the formation of a triplet
[H4FAP2+ · · ·O2] complex that is stabilized by 4.65 kcal ·mol-1
as compared to its separate adducts (i.e., triplet oxygen and
H4FAP2+). [H3FAP+ · · ·O2] is slightly less stabilized (3.77
kcal·mol-1 in Scheme 4) and consequently expected to play a
minor role in the catalytic activation of O2. The intramolecular
production of HO2 from the [H4FAP2+ · · ·O2] complex was
•
Scheme 5. Mechanism of O2 Reduction with Fc Catalyzed by
H2FAP
found to be strongly disfavored. The data of Figure 7 show that
we can globally describe the mechanism as:
(33) Berlicka, A.; Latos-Grazynski, L.; Lis, T. Inorg. Chem. 2005, 44, 4522.
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Birge, R. R.; Crundwell, G. Phys. Chem. Chem. Phys. 2006, 8, 2402–
2412.
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61, 3489–3495.
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Chem., Int. Ed. Engl. 1995, 33, 2485–2487.
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13740 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Diprotonated H2FAP as Catalyst for ORR
A R T I C L E S
I. H. also gratefully acknowledges the Scientific and Research
Council of Turkey (TUBITAK). C.C. thanks Stephan N. Steinmann
for the single point computations within Q-Chem.
controlled by a common ion partition were performed, which
not only prove the catalytic activation of H4FAP2+ on O2
reduction but also suggest a two-electron reduction pathway to
produce H2O2.
Supporting Information Available: Complete refs 26 and 29,
synthetic and characterization details of H2FAP, Cartesian
coordinates of the M05-2X/cc-pVDZ optimized geometries, and
electrochemistry data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by Ecole Poly-
technique Fe´de´rale de Lausanne (EPFL), European Cooperation in
the field of Scientific and Technical Research (COST Action, D36/
007/06), the Centre National de la Recherche Scientifique (CNRS,
UMR 5260), the Agence Nationale pour la Recherche (ANR project
“Chelan”), Grant Agency of the Czech Republic (no. 203/07/1257).
JA103460P
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