A R T I C L E S
Hatay et al.
Scheme 1. Molecular Structure of CoP
anisms as recently reviewed by Huynh and Meyer and
Costentin.14,15
The interface between two immiscible electrolyte solutions
is formed between two solvents of a low mutual miscibility,
such as water and 1,2-dichloroethane (DCE), each containing
an electrolyte. An ITIES can be polarized with the formation
of two back-to-back Gouy-Chapman diffuse layers, and it is
possible to use classical methodologies to study charge transfer
reactions, as well as adsorption phenomena. This type of
electrochemistry without a solid working electrode provides a
suitable model for investigating heterogeneous processes oc-
curring in biological systems, which are often PCET reactions
such as the proton-coupled oxygen reduction reactions within
aerobic living organisms. Oxygen reduction at the ITIES has
been studied by using various lipophilic electron donors,
decamethylferrocene (DMFc),16-21 reduced flavin mononucle-
otide (FMN),22 tetrachlorohydroquinone (CQH2)23 and fullerene
monoanion (C60-).24 In the case of DMFc, the oxygen reduction
produces H2O2, as evidenced by two-phase reactions19 and
in situ detection of H2O2 using scanning electrochemical
microscopy.20 Furthermore, the catalytic effect of various por-
phyrin compounds including cobalt tetraphenylporphyrin,18,21
as well as electrodeposited platinum particles,17 on the oxygen
reduction by DMFc at the water-DCE interface has also been
investigated.
Scheme 2. Three-Compartment Glass Cell for Liquid-Liquid
Electrochemistry
range as that of the PCET reaction and the two processes
can be observed.
Herein, we report that cobalt porphine (CoP) catalyzes the
reduction of O2 in biphasic systems where the aqueous phase
is acidic and where the organic phase contains electron donors.
In particular, we show by voltammetry at ITIES that this
interfacial catalytic process occurs through a proton-coupled
electron transfer (PCET) reaction that depends on the interfacial
polarization, i.e. on the potential difference applied between the
two phases. Indeed, no reaction takes place in the absence of
either CoP, ferrocene (Fc) or O2. In this biphasic system, the
reactants are separated by the interface with the protons in the
aqueous phase, the catalyst and the electron donor in the organic
phase. Voltammetry at ITIES records the current, i.e. the
passage of charges through the interface. In the case of the
interfacial catalytic reduction of O2, the current records
the transport of H+ from the aqueous phase to the interfacial
reaction site. This is the main difference with voltammetry
at solid electrodes that only records electron transfer steps.
Furthermore, the oxidation product of ferrocene, namely
ferrocenium, does not cross the interface in the same potential
Experimental Section
Chemicals and Reagents. All solvents and chemicals were used
as received without further purification. Ferrocene (Fc, 98%), 1,1′-
dimethylferrocene (DFc, 97%), ferrocene carboxaldehyde (FcCA,
98%), and lithium tetrakis(pentafluorophenyl)borate diethyl etherate
(LiTB) were purchased from Aldrich. Lithium chloride anhydrous
(LiCl, g99%), hydrochloric acid (HCl, 37%), 1,2-dichloroethane
(DCE, g99.8%), sodium iodide (NaI, >99.5%), tetraethylammo-
nium chloride (TEACl, g98%), and bis(triphenylphosphoranylide-
ne)ammonium chloride (BACl, g98%) were obtained from Fluka.
Hydrogen peroxide (H2O2, 30%) was ordered from Reactolab SA.
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
by recrystallization from acetone.25 All the aqueous solutions were
prepared with ultra pure water (18.2 MΩ cm-1). The pH of the
acidic aqueous solutions was adjusted by addition of HCl. The
synthesis of CoP is detailed in the Supporting Information, and its
molecular structure is illustrated in Scheme 1.
Voltammetric Measurements. Voltammetry measurements at
the water-DCE interface were performed in a four-electrode
configuration with two reference electrodes to polarize the interface
and two counter electrodes to provide the current. A commercial
potentiostat (PGSTAT 30, Eco-Chemie, Netherlands) or a custom-
built system connected to a Stanford Research System DS335
synthesized function generator was used. A three-compartment glass
cell featuring a cylindrical vessel was used, where the water-DCE
interface with a geometric area of 1.53 cm2 was formed as illustrated
in Scheme 2. Two platinum counter electrodes were positioned in
the aqueous and DCE phases, respectively, to supply the current
flow. The external potential was applied by means of two silver/
silver chloride (Ag/AgCl) reference electrodes, which were con-
nected to the aqueous and DCE phases, respectively, by means of
a Luggin capillary. The electrochemical cell composition is
schematically illustrated in Scheme 3. The Galvani potential
difference (∆wo φ) was estimated by taking the formal ion transfer
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13454 J. AM. CHEM. SOC. VOL. 131, NO. 37, 2009