Angewandte
Communications
Chemie
Homogeneous Electrocatalysis
Fast Oxygen Reduction Catalyzed by a Copper(II) Tris(2-
I
+
Abstract: Catalytic pathways for the reduction of dioxygen can
either lead to the formation of water or peroxide as the reaction
product. We demonstrate that the electrocatalytic reduction of
dioxygen to [Cu (tmpa)] leads to fast formation of an end-on
II
Cu superoxo complex, followed by a slower dimerization
step to form a dinuclear copper peroxo complex. Addition-
ally, Fukuzumi, Karlin, and co-workers have studied the ORR
activity of Cu–tmpa in acetone, using decamethylferrocene as
a sacrificial reductant; this reaction involves a dinuclear
2
+
O by the pyridylalkylamine copper complex [Cu(tmpa)(L)]
in a neutral aqueous solution follows a stepwise 4e /4H
2
ꢀ
+
pathway, in which H O is formed as a detectable intermediate
2
2
[
6a,d]
and subsequently reduced to H O in two separate catalytic
intermediate.
It was shown that Cu–tmpa and several
2
reactions. These homogeneous catalytic reactions are shown to
derivatives, adsorbed on carbon black, catalyze the electro-
chemical ORR in aqueous buffer solutions. The ORR
I
[9]
be first order in catalyst. Coordination of O to Cu was found
2
to be the rate-determining step in the formation of the peroxide
intermediate. Furthermore, electrochemical studies of the
reaction kinetics revealed a high turnover frequency of 1.5 ꢀ
activity of Cu–tmpa in solution has also been investigated, as
[10]
well as pH effects on the redox chemistry. However, thus
far catalytic rates have not been reported, and the mechanism
wherein ORR occurs has not been solved.
5
ꢀ1
1
0 s , the highest reported for any molecular copper catalyst.
The field of homogeneous electrocatalysis for the con-
version of small molecules (O , CO , H O, H , etc.) is
W
ith the shift in the energy landscape from fossil fuels
2
2
2
2
towards sustainable sources of energy, storage and conversion
of fuels, such as hydrogen, is expected to play an important
role. It is therefore important that efficient fuel cells are
available to minimize energy loss during fuel-to-energy
interconversion. However, the cathodic oxygen reduction
reaction (ORR) is a significant limiting factor in the efficiency
expanding rapidly, and great strides have been made to
develop new methods to be able to study their reaction
kinetics and to allow for benchmarking of different cata-
[
11]
lysts. Foot-of-the-wave analysis (FOWA) has become an
important tool to determine the catalytic performance of
homogeneous electrocatalysts, as it allows for the determi-
[
1]
[11a,d,12]
of fuel cells. In nature, multicopper oxidases, such as laccase,
nation of rate constants under limiting conditions.
are known to catalyze the four-electron reduction of O to
Using these methods, we have quantified the fast electro-
catalytic ORR by homogeneous Cu–tmpa in neutral aqueous
solution. Additionally, a comprehensive study of the product
formation using R(R)DE techniques has provided important
new insight into the electrocatalytic ORR mechanism, and
shows that catalysis occurs by a stepwise mechanism at
a single copper center.
2
[2]
H O efficiently. Immobilization of laccase on electrodes has
2
shown that the ORR can be performed close to the
[
3]
thermodynamic equilibrium potential of water. In an
effort to create synthetic mimics of these copper enzymes,
a wide range of model copper systems have been studied for
[4]
their oxygen activation reactivity. While some early exam-
ples of copper complexes have been studied for their activity
The redox and catalytic behavior of Cu–tmpa in a phos-
phate buffer (PB) solution at pH 7, containing 100 mm
phosphate salts (NaH PO and Na HPO ), was investigated.
[
5]
towards the ORR, only in the last decade have the first
molecular copper model catalysts been evaluated for their
ORR activity, either by means of sacrificial reductants or in
2
4
2
4
Cyclic voltammograms (CVs) of Cu–tmpa were recorded
using glassy carbon (GC) working electrode (A =
0.0707 cm ). In the presence of argon (1 atm), a well-defined
[6]
2+
electrochemical studies. [Cu(tmpa)(L)]
(tmpa = tris(2-
a
2
pyridylmethyl)amine, L = solvent), as well as many deriva-
tives of the pyridylalkylamine template, has been studied as
a mimic for active sites in redox-active metalloenzymes for its
nonplanar and flexible coordination sphere and its reactivity
I
II
reversible Cu /Cu redox couple was visible at E1/2 = 0.21 V
versus RHE (Figure 1). In the presence of O (1 atm), a peak-
2
shaped catalytic wave appeared with an onset potential at
0.5 V versus RHE. This peak-shaped catalytic wave is
characteristic of substrate depletion, thus demonstrating the
fast catalysis by Cu–tmpa. The homogeneity of the catalyst
was established by electrochemical quartz crystal micro-
balance (EQCM) experiments, both under noncatalytic and
[4e,7]
towards dioxygen.
tmpa has been thoroughly studied by Karlin and co-workers.
It was shown that in a range of solvents, the binding of
The dioxygen binding chemistry of Cu–
[8]
[
*] M. Langerman, D. G. H. Hetterscheid
Leiden Institute of Chemistry, Leiden University
Gorlaeus Laboratories
[13]
catalytic conditions (see the Supporting Information).
Determination of the relationship between the catalytic
current and the catalyst concentration would provide useful
insight into the possible mechanism of the ORR. Owing to the
P.O Box 9502, 2300 RA Leiden (The Netherlands)
E-mail: d.g.h.hetterscheid@chem.leidenuniv.nl
low solubility of O in most solvents, aqueous or otherwise,
Supporting information and the ORCID identification number(s) for
2
either very high O pressures or low catalyst concentrations
2
must be used to avoid the O mass-transport limitation. By
2
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!