Efficient two-electron reduction of dioxygen to hydrogen peroxide with one-electron reductants with a small overpotential catalyzed by a cobalt chlorin complex

Article abstract of DOI:10.1021/ja312199h

A cobalt chlorin complex (Co^II(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O₂) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H ₂O₂) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K. The catalytic reactivity of Co ^II(Ch) was much higher than that of a cobalt porphyrin complex (Co^II(OEP), OEP^2- = octaethylporphyrin dianion), which is a typical porphyrinoid complex. The two-electron reduction of O₂ by 1,1′-dibromoferrocene (Br₂Fc) was catalyzed by Co ^II(Ch), whereas virtually no reduction of O₂ occurred with Co^II(OEP). In addition, Co^II(Ch) is more stable than Co^II(OEP), where the catalytic turnover number (TON) of the two-electron reduction of O₂ catalyzed by Co^II(Ch) exceeded 30000. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O₂ with the protonated Co^II(Ch) ([Co^II(ChH)]⁺) that is produced by facile electron-transfer reduction of [Co^III(ChH)]²⁺ by ferrocene derivative in the presence of HClO₄. The one-electron-reduction potential of [Co^III(Ch)]⁺ was positively shifted from 0.37 V (vs SCE) to 0.48 V by the addition of HClO₄ due to the protonation of [Co^III(Ch)]⁺. Such a positive shift of [Co ^III(Ch)]⁺ by protonation resulted in enhancement of the catalytic reactivity of [Co^III(ChH)]²⁺ for the two-electron reduction of O₂ with a lower overpotential as compared with that of [Co^III(OEP)]⁺.

Full text of DOI:10.1021/ja312199h

Article
pubs.acs.org/JACS
Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide
with One-Electron Reductants with a Small Overpotential Catalyzed
by a Cobalt Chlorin Complex
†
†
,†,‡
Kentaro Mase, Kei Ohkubo, and Shunichi Fukuzumi*
†Department of Material and Life Science, Graduate School of Engineering, ALCA, Japan Science and Technology Agency (JST),
Osaka University, Suita, Osaka 565-0871, Japan
‡
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
*
S Supporting Information
II
ABSTRACT: A cobalt chlorin complex (Co (Ch)) efficiently and selectively
catalyzed two-electron reduction of dioxygen (O ) by one-electron reductants
2
(
ferrocene derivatives) to produce hydrogen peroxide (H O ) in the presence of
2 2
perchloric acid (HClO ) in benzonitrile (PhCN) at 298 K. The catalytic
reactivity of Co (Ch) was much higher than that of a cobalt porphyrin complex
4
II
II
2−
(Co (OEP), OEP = octaethylporphyrin dianion), which is a typical porphyrinoid
complex. The two-electron reduction of O by 1,1′-dibromoferrocene (Br Fc) was
2
2
II
catalyzed by Co (Ch), whereas virtually no reduction of O occurred with
Co (OEP). In addition, Co (Ch) is more stable than Co (OEP), where the
2
II
II
II
II
catalytic turnover number (TON) of the two-electron reduction of O catalyzed by Co (Ch) exceeded 30000. The detailed kinetic
2
studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O with
2
II
II
+
III
2+
the protonated Co (Ch) ([Co (ChH)] ) that is produced by facile electron-transfer reduction of [Co (ChH)] by ferrocene
derivative in the presence of HClO . The one-electron-reduction potential of [Co (Ch)] was positively shifted from 0.37 V (vs SCE)
to 0.48 V by the addition of HClO due to the protonation of [Co (Ch)] . Such a positive shift of [Co (Ch)] by protonation
resulted in enhancement of the catalytic reactivity of [Co (ChH)] for the two-electron reduction of O with a lower overpotential as
III
+
4
III
+
III
+
4
III
2+
2
III
+
compared with that of [Co (OEP)] .
INTRODUCTION
phthalocyanines act as efficient catalysts for the selective two-
■
electron reduction of O by ferrocene derivatives in the presence
2
Hydrogen peroxide is one of the most versatile and environ-
mentally benign oxidizing reagents produced in large scale in
industry, with various applications including pulp and paper
1
1,19,21
of an acid.
An increase of the one-electron-reduction
potential of cobalt(III) complexes is required to reduce the
1
,2
overpotential for the two-electron reduction of O . However,
2
bleaching. Hydrogen peroxide is also a promising candidate as
3
,4
when the one-electron reduction potentials of cobalt(III)
complexes are too positive, the Co(II) complexes may not be
a sustainable energy carrier, because it has high energy density
and emits no CO , which is regarded as a greenhouse gas causing
2
able to reduce O . The overpotential of the two-electron
2
serious environmental issues. Hydrogen peroxide fuel cells,
which emit only water and oxygen after power generation, have
recently been developed, and the cell performance has been
improved significantly.
manufactured in industry by the autoxidation of a 2-
alkylanthrahydroquinone to the corresponding 2-alkylanthra-
quinone (the so-called anthraquinone process). This process
requires hydrogen as a reductant and a noble metal such as
palladium to regenerate the anthrahydroquinone. The cost of the
anthraquinone process depends heavily on effective recycling of
the anthraquinone, extraction solvents, and the noble-metal
hydrogenation catalyst, which are all quite expensive.
reduction of O with cobalt complexes has not been optimized
2
yet. The stability of the catalyst under the acidic conditions
should also be improved because demetalation from a macro-
cyclic ligand results in inactivation of the catalyst.
4
−9
Currently hydrogen peroxide is
In general, the stability of a metal complex with a porphyrinoid
40−43
1
0
ligand in the presence of acid depends on two factors.
One is
the core size and rigidity of a macrocyclic ligand, because a low-
valent metal ion, which is produced in a catalytic cycle, is difficult
to accommodate in a macrocyclic ligand due to its large ionic
radius and low electrostatic interaction. The core size of the
chlorin ligand is larger than that of porphyrin because the chlorin
ligand is more flexible than the porphyrin in accommodating the
low-valent metal ion. The other is the nucleophilicity of the core
nitrogen atoms, which is responsible for electrophilic attack of an
Alternatively, production of hydrogen peroxide has been
extensively studied by the electrocatalytic two-electron reduction
4
,11−18
of oxygen, which is abundant in air.
Base-metal catalysts
1
1,13−32
33−36
37−39
such as cobalt,
iron,
and copper
complexes
have been employed for the two-electron and four-electron
Received: December 13, 2012
reduction of O . Monomeric cobalt porphyrins and cobalt
2
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja312199h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
incoming proton. The nucleophilicity of a macrocyclic ligand
diminishes, accompanied by the electron density of the
macrocyclic ligand, which is associated with saturation of C−C
bonds in the pyrrole rings (porphyrin > chlorin). In this regard,
Co (Ch) possesses advantages for the stability in an acidic
solution, leading to efficiency and durability for the catalytic two-
were much larger than that of O
the reaction solution. In contrast, the small amount of a ferrocene derivative
for the two-electron reduction of O was employed in an O -saturated PhCN
solution, where a ferrocene derivative is the reaction-limiting reagent.
Electrochemical Measurements. Cyclic voltammetry (CV)
measurements were performed on an ALS 630B electrochemical
analyzer, and voltammograms were measured in deaerated PhCN
containing 0.10 M TBAPF6 as a supporting electrolyte at room
temperature. A conventional three-electrode cell was used with a glassy-
carbon working electrode (surface area of 0.3 mm ) and a platinum wire
as the counter electrode. The glassy-carbon working electrode (BAS)
was routinely polished with BAS polishing alumina suspension and
rinsed with acetone before use. The potentials were measured with
2
, when O
2
is the reaction-limiting reagent in
2
2
II
electron reduction of O2.
We report herein that a cobalt(II) chlorin complex (Co (Ch))
II
2
catalyzes efficiently the two-electron reduction of O with a series
2
of ferrocene derivatives as an electron donor in the presence of
perchloric acid (HClO ) in PhCN. The electrocatalytic
4
II
reduction of O with Co (Ch) occurs with high durability in
2
−2
respect to the Ag/AgNO (1.0 × 10 M) reference electrode. All
3
the acidic solution and a small overpotential in comparison with
4
,13
potentials (vs Ag/AgNO
3
) were converted to values vs SCE by adding
macrocyclic ligands already reported.
The catalytic mecha-
50
0
.29 V. Redox potentials were determined using the relation E1/2 =
nism for the selective two-electron reduction of O by ferrocene
2
(Epa + E )/2.
pc
derivatives is clarified on the basis of a detailed kinetic study.
EPR Measurements. The EPR spectra were measured on a JEOL X-
band EPR spectrometer (JES-ME-LX) using a quartz EPR tube containing
a deaerated sample frozen solution at 80 K. The internal diameter of the
EPR tube is 4.5 mm, which is small enough to fill the EPR cavity but large
enough to obtain good signal-to-noise ratios during the EPR
measurements at low temperature (at 80 K). The EPR spectra were
measured under nonsaturating microwave power conditions. The
amplitude of modulation was chosen to optimize the resolution and
the signal-to-noise (S/N) ratio of the observed spectra. The g values
were calibrated with a Mn2 marker, and the hyperfine coupling
EXPERIMENTAL SECTION
General Procedure. Chemicals were purchased from commercial
sources and used without further purification, unless otherwise noted.
Benzonitrile (PhCN) used for spectroscopic and electrochemical
■
4
4
measurements was distilled over phosphorus pentoxide prior to use.
II
Cobalt chlorin (Co (Ch)) was synthesized by the published method
4
5,46
(
see the Supporting Information for details).
Ferrocene (Fc), 1,1′-
+
dimethylferrocene (Me Fc), octamethylferrocene (Me Fc), bromofer-
2
8
(
hfc) constants were determined by computer simulation using Calleo
rocene (BrFc), and 1,1′-dibromoferrocene (Br Fc) were purchased
2
EPR Version 1.2 program coded by Calleo Scientific Software
commercially and purified by sublimation or recrystallization from
Publishers.
ethanol. Tetra-n-butylammonium hexafluorophosphate (TBAPF ) was
6
1
twice recrystallized from ethanol and dried in vacuo prior to use. H
RESULTS AND DISCUSSION
Protonation of Co (Ch). Protonation of Co (Ch) was
NMR spectra (300 MHz) were recorded on a JEOL AL-300
spectrometer at room temperature, and chemical shifts (ppm) were
determined relative to tetramethylsilane (TMS). MALDI-TOF-MS
measurements were performed on a Kratos Compact MALDI I
■
II
II
studied by the UV−vis absorption spectral change upon addition
II
of HClO to a deaerated PhCN solution of Co (Ch). The
4
(
Shimadzu) using dithranol as a matrix. UV−vis spectroscopy was
II
characteristic absorption bands of Co (Ch) at 424 and 646 nm
disappeared by the addition of HClO , as shown in Figure 1a.
carried out on a Hewlett-Packard 8453 diode array spectrophotometer
at room temperature using 1 cm cells.
4
Spectroscopic Measurements. The protonation equilibrium
II
II
+
constants between Co (Ch) and [Co (ChH)] and between
[
III
+
III
2+
Co (Ch)] and [Co (ChH)] were determined by using the Hill
equation, which analyzes changes in absorption spectra during the
4
7
titration as a function of the concentration of an added acid. The
amount of hydrogen peroxide (H O ) formed was determined by
2
2
titration with iodide ion: a dilute CH CN solution (2.0 mL) of the
3
product mixture (40 μL) was treated with an excess amount of NaI, and
−
the amount of I3 formed was determined by the absorption spectrum
4
−1
−1 48
(
λmax 361 nm, ε = 2.8 × 10 M cm ).
Kinetic Measurements. Kinetic measurements for fast reactions
with short half-lifetimes were performed on a UNISOKU RSP-601
stopped-flow spectrophotometer with an MOS-type highly selective
photodiode array at 298 K using a Unisoku thermostated cell holder.
When stopped-flow measurements were carried out under deaerated or
O -saturated conditions, a deaerated or O -saturated PhCN solution
II
−5
Figure 1. (a) Absorption spectral changes of Co (Ch) (2.0 × 10 M)
upon addition of HClO in deaerated PhCN at 298 K. (b) Absorbance
4
change at 646 nm upon addition of HClO4.
2
2
with a stream of argon or O was transferred by means of a glass syringe
2
to a spectrometer cell that was already purged with a stream of argon or
O . Rate constants of oxidation of ferrocene derivatives by O in the
presence of a catalytic amount of Co (Ch) and an excess amount of
2
2
II
The change in the absorption spectra can be ascribed to the
protonation of Co (Ch) at the carbonyl group, which is
II
HClO in PhCN at 298 K were determined by monitoring the appearance
4
+
conjugated to the π system of the chlorin ligand to form
of an absorption band due to the corresponding ferrocenium ions (Fc , λ
max
II
+
43
51
−
1
−1
+
−1
−1
6
20 nm, ε = 330 M cm ; Me Fc , λ 650 nm, ε = 290 M cm ;
[Co (ChH)] , as shown in Scheme 1.
max
2
max
max
+
−1
−1
+
Me Fc , λ 750 nm, ε = 410 M cm ; BrFc , λ 620 nm, ε = 320
The protonation induces the decrease of the electron density
8
max
−1
max
max
−1
max
−1 21
−
1
+
M
cm ; Br Fc , λ
700 nm, εmax = 180 M cm ). At the
on the macrocyclic ligand, leading to the low nucleophilicity of
2
max
II
+
43,52,53
wavelengths monitored, spectral overlap was observed with [Co (ChH)]
the pyrrole nitrogen atoms.
The protonation equilibrium
4
−1
−1
4
−1
−1
(
(
λ 620 nm (1.6 × 10 M cm ), 650 nm (1.4 × 10 M cm ), 700 nm
II
constant of Co (Ch) with HClO (K) was determined by the
absorption spectral change at 646 nm to be 2.2 × 10 M in
deaerated PhCN at 298 K, as shown in Figure1b (eq 1).
3
−1
−1
3
−1
−1
4
7.0 × 10 M cm ), 750 nm (2.0 × 10 M cm )). An air-saturated
4
−1
PhCN solution was used for the catalytic reduction of O by ferrocene
47
2
derivatives. The concentration of O in an air-saturated PhCN solution
2
−3
49
(
1.7 × 10 M) was determined as reported previously. The con-
centrations of ferrocene derivatives employed for the catalytic reduction of O2
II
+
II
+
Co (Ch) + H ⇄ [Co (ChH)]
(1)
B
dx.doi.org/10.1021/ja312199h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
II
II
+
Scheme 1
a deaerated PhCN solution of Co (Ch) to form [Co (ChH)] ,
only small changes in the EPR parameters were observed, as
shown in Figure 2b. The g value remained nearly the same,
∥
whereas the g value became slightly smaller on protonation.
⊥
II
The O binding equilibrium of Co (Ch) was also monitored
by the EPR spectral changes. When a PhCN solution of Co (Ch)
2
II
is exposed to air, large changes in g values and coupling constant
are observed, as shown in Figure 2c, which shows signals at g =
∥
5
9
2
.091 and g = 1.990 with hyperfine splitting due to Co
⊥
59
59
(
A ( Co) = 22 G, A ( Co) = 17 G). The g value is smaller than
⊥ ∥ ∥
•
−
the reported value for free O (g = 2.102), but it is larger than
those of O2 when it is bound to metal cations and NH .
has been reported that the g value of O2 −metal ion complexes
becomes smaller when the interaction between O and metal
2
∥
•
−
+
55,56
It
4
•−
II
The protonation of Co (Ch) at the carbonyl group of the
∥
•−
chlorin ligand was evidenced by the observation of the EPR
2
56
II
ions is increased. However, the observation of superhyperfine
spectrum of a deaerated PhCN solution of Co (Ch) at 80 K
due to 59Co (Figure 2c) clearly indicates the existence of an
(
Figure 2). The obtained signal exhibited a well-resolved signals
•−
interaction between O2 and the Co(III) center. Thus, the
observed EPR spectrum in Figure 2c is assigned to the
III
+
•−
[
Co (Ch)] −O complex, in which the interaction between
2
•
−
the Co(III) center and O is relatively weak. The weak binding
2
III
+
•−
in the [Co (Ch)] −O
complex is confirmed by the
2
III
+
•−
reversibility of the formation of the [Co (Ch)] −O complex,
2
II
which reverts to Co (Ch) by evacuation of O .
2
II
The reversible binding of O to Co (Ch) was also confirmed
2
by the UV−vis absorption spectral change of an air-saturated
II
propionitrile solution of Co (Ch) at various temperatures. As
II
shown in Figure 3, the absorption bands of Co (Ch) at 424 and
II
−5
Figure 3. Absorption spectral changes of Co (Ch) (1.0 × 10 M) in
air-saturated propionitrile at various temperatures.
6
46 nm were changed to those at 436 and 656 nm on a decrease
in temperature. The changes in the absorption spectra are
ascribed due to the O binding of Co (Ch) to form
II
2
III
+
•−
[Co (Ch)] −O (eq 2).
2
II
III
+
•−
Co (Ch) + O ⇄ [Co (Ch)] −O
(2)
2
2
II
III
+
The reversible change between Co (Ch) and [Co (Ch)] −
II
−3
•−
Figure 2. EPR spectra of (a) Co (Ch) (1.0 × 10 M) in deaerated
O2 was observed by heating or cooling and also by evacuation
or introduction of O2.
II
−3
PhCN at 80 K, (b) Co (Ch) (1.0 × 10 M) upon addition of HClO4
−3
II
(
5.0 × 10 M) in deaerated PhCN at 80 K, and (c) Co (Ch) (1.0 ×
4
The protonation behavior of the one-electron-oxidized species
−
10
M) in air-saturated PhCN at 80 K. The black and red lines show the
III
+
[
[
Co (Ch)] was also monitored by UV−vis titration.
experimental and simulated spectra, respectively. Experimental
parameters: microwave frequency 9.0 GHz, microwave power 1.0 mW,
modulation frequency 100 kHz, and modulation width 10 G.
III + II
Co (Ch)] was prepared by oxidation of Co (Ch) by oxygen
19
dissolved in PhCN containing a small amount of HClO .
4
III
+
[
Co (Ch)] exhibited a characteristic spectrum with two peak
tops at 436 and 656 nm, which is identical with that of
5
9
III
+
II
at g = 2.490 and g = 2.026 with hyperfine splitting due to Co
[Co (Ch)] prepared by the electron-transfer oxidation of Co (Ch)
⊥
∥
59
1
•+
−
(
A ( Co) = 102 G), which are typical for a low-spin (S = / )
by the one-electron-oxidizing reagent (p-BrC H ) N SbCl (E =
∥
2
6 4 3 6 red
2
1,54
five-coordinate cobalt(II) complex.
By addition of HClO to
1.05 V vs SCE), as shown in Figure S2 (Supporting Information).
4
C
dx.doi.org/10.1021/ja312199h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
The addition of HClO to an air-saturated PhCN solution of
with chlorin ligand protonation, which causes the positive shift of
the center-metal redox potential due to a decrease in the electron
4
III
+
III
2+
[
Co (Ch)] showed a spectral change to afford [Co (ChH)] , as
III
+
52,53
shown in Figure 4a. In the case of [Co (Ch)] , however, a large
density of the chlorin ligand,
as described above.
II
When Co (Ch) is replaced by a cobalt(II) porphyrin
Co (OEP); OEP = octaethylporphyrin), the reversible couples
Co (OEP)] /Co (OEP) and [Co (OEP )]
II
(
[
[
III
+
II
III
•+
2+
/
III
+
II
Co (OEP)] similar to those of Co (Ch) were observed at
14
E1/2 = 0.31, 0.91 V (vs SCE), respectively, as shown in Figure
II
5
b (black). In contrast to the case for Co (Ch), the addition of
HClO to a deaerated PhCN solution of Co (OEP) resulted in a
II
4
decrease in current due to demetalation of the center-metal ion
from the macrocyclic ligand (blue line in Figure 5b) (vide infra).
II
The oxidation peak for demetalated Co (OEP) by HClO
4
2+
(
[H (OEP)] ) was observed at 1.5 V vs SCE in the CV
measurement of Co (OEP) with HClO (Figure S3a, Supporting
4
II
III
+
−5
Figure 4. (a) Absorption spectral changes of [Co (Ch)] (1.0 × 10 M)
4
Information). This value agrees with the one-electron oxidation
upon addition of HClO in air-saturated PhCN at 298 K. (b) Absorbance
4
2
+
change at 656 nm upon addition of HClO4.
peak of [H (OEP)] (Figure S3b, Supporting Information).
4
II
When Co (Ch) is employed as a catalyst for the electro-
chemical reduction of O
TBAPF , catalytic currents corresponding to the reduction of O
2
II
excess of HClO was required, in comparison to the case of Co (Ch),
in O -saturated PhCN containing 0.10 M
2 2
4
to complete the protonation (Figure 4b). The protonation
6
III
+
equilibrium constant of [Co (Ch)] with HClO (K) was
with an onset potential of 0.6 V (vs SCE) were observed, as shown
4
determined by the absorption spectral change at 656 nm to be
in Figure 6a. The onset potential for the reduction of O with
Co (Ch) is more positive than in the case of Co (OEP) (Figure 6).
2
2
−1
47
II
II
1.1 × 10 M in PhCN at 298 K (eq 3).
III
+
+
III
2+
[
Co (Ch)] + H ⇄ [Co (ChH)]
(3)
The smaller protonation equilibrium constant between
Co (Ch)] and [Co (ChH)] results from a small electron
III
+
III
2+
[
density on the chlorin ligand. This is caused by the electronic
interaction between the high-valent Co(III) ion and the lone
5
2,53
pairs of the nitrogen atoms on chlorin ligand.
II
II
Electrochemical measurements on Co (Ch) and Co (OEP)
were performed in deaerated PhCN containing 0.10 M TBAPF6
to determine the catalytic activity of cobalt complexes toward the
II
reduction of O , as shown in Figure 5. In the case of Co (Ch),
2
Figure 6. Cyclic voltammograms of O -saturated PhCN solutions of (a)
2
II
−3
Co (Ch) (1.0 × 10 M) recorded in the presence of 0.10 M TBAPF
without HClO (black line), with HClO (5.0 × 10 M) (green line),
with HClO (1.0 × 10 M) (red line), and with HClO (2.5 × 10 M)
blue line) and (b) Co (OEP) (1.0 × 10 M) recorded in the presence
6
−3
4
4
−
2
−2
4
4
II
−3
(
−3
of 0.10 M TBAPF without HClO (black line), with HClO (5.0 × 10 M)
6
4
4
−
2
(
(
green line), with HClO (1.0 × 10 M) (red line), and with HClO
4 4
2
−
−1
2.5 × 10 M) (blue line). The sweep rate was 10 mV s .
As far as we know, this value of onset potential for the two-
electron reduction of O is the largest (the least overpotential), in
2
Figure 5. Cyclic voltammograms of deaerated PhCN solutions of (a)
comparison to those of other metal complexes for the two-
electron reduction of O2.
II
−3
4,13
Co (Ch) (1.0 × 10 M) recorded in the presence of TBAPF (0.10 M)
6
−2
without HClO (black line), with HClO (1.0 × 10 M) (red line), and
with HClO (2.5 × 10 M) (blue line) and (b) Co (OEP) (1.0 × 10 M)
recorded in the presence of TBAPF (0.10 M) without HClO (black line),
Catalytic Two-Electron Reduction of O by Me Fc with
4
4
2
2
−
2
II
−3
II
4
Co (Ch) in the Presence of HClO . The addition of HClO to
4
4
II
6
4
an air-saturated PhCN solution of Co (Ch) and 1,1′-
−
2
−2
with HClO (1.0 × 10 M) (red line), and with HClO (2.5 × 10 M)
4
4
dimethylferrocene (Me Fc) resulted in the efficient oxidation
−1
2
(
blue line). The sweep rate was 100 mV s .
of Me Fc by O to produce 1,1′-dimethylferrocenium ion
2
2
+
(
Me Fc ). It should be noted that no oxidation of Me Fc
2
2
III
+
II
II
the reversible redox couples [Co (Ch)] /Co (Ch) and
occurred by O in the absence of Co (Ch) under the present
2
III
•+ 2+
III
+
[
Co (Ch )] /[Co (Ch)] were observed at E = 0.37,
.82 V (vs SCE), respectively, as shown in Figure 5a (black). In
acidic conditions. The stoichiometry of the catalytic oxygen
1
/2
1
4
0
reduction was confirmed under the reaction conditions using air-
−
3
the presence of HClO (Figure 5a, blue), the redox wave for
saturated O (concentration 1.7 × 10 M) in PhCN. The
4
2
III
•+ 2+
III
+
+
[
Co (Ch )] /[Co (Ch)] was observed at 0.82 V, which is
formation of Me Fc was monitored by a rise in absorbance at
2
the virtually the same as that in the absence of HClO , whereas
the redox potential for [Co (Ch)] /Co (Ch) was positively
shifted from E1/2 = 0.37 to 0.48 V (vs SCE). This is consistent
650 nm, as shown in Figure 7a. The rising curved line in Figure 7b
4
III
+
II
+
shows the time course of the formation of Me Fc in the
2
−3
reduction of O (1.7 × 10 M) in the presence of a catalytic
2
D
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Journal of the American Chemical Society
Article
Scheme 2
Kinetics and Mechanism of Two-Electron Reduction of
Figure 7. (a) Absorption spectral changes in the two-electron reduction
−3
−2
II
II
of O (1.7 × 10 M) by Me Fc (2.5 × 10 M) with Co (Ch) (1.0 ×
O by Me Fc with Co (Ch). The catalytic rate constant (k )
2
2
cat
2
2
−
5
−2
10
M) in the presence of HClO (2.5 × 10 M) in air-saturated
was determined from the dependence of the observed rate
4
+
PhCN at 298 K. The black and red lines show the spectra before and
constant (k ) for formation of Me Fc on the concentrations of
obs
2
II
after addition of HClO , respectively. The blue line shows the spectrum
4
Co (Ch), HClO , Me Fc, and O (Figure 8). The k value was
4 2 2 obs
of the intermediate in the catalytic reaction. The dotted line is the
−
3
+
absorbance at 650 nm due to 3.4 × 10 M Me Fc . (b) Time profile of
2
+
absorbance at 650 nm due to Me Fc .
2
II
−5
amount of Co (Ch) (1.0 × 10 M), a large excess of Me Fc
2
2
−2
(
2.5 × 10− M) and HClO (2.5 × 10 M). At the end of the
catalytic reaction, the concentration of Me Fc (3.4 × 10 M)
formed in the catalytic reduction of O by Me Fc is twice the
concentration of O (1.7 × 10 M) in air-saturated PhCN. This
result clearly indicates that the two-electron reduction of O occurs
to produce 2 equiv of Me Fc and there is no further reduction to
produce more than 2 equiv of Me Fc , which is in agreement with
results for other mononuclear Co(II) complexes (eq 4).
The stoichiometric production of H O was ascertained by
performing the iodometric titration, which is the reaction of an
4
+
−3
2
2
2
−
3
2
2
+
2
+
2
11,19,21
2
2
48
+
+
O + 2H + 2Me Fc ⎯⎯⎯⎯⎯⎯⎯ ⎯→ H O + 2Me Fc
2
2
II
2
2
2
Co (Ch)
(4)
excess amount of NaI in deaerated CH CN (2.0 mL) with the 50
3
II
−7
times diluted reaction mixture of Co (Ch) (2.0 × 10 M),
Me Fc (6.8 × 10 M), and HClO (5.0 × 10 M) (Figure S4,
+
−5
−4
2
4
Supporting Information). The catalytic turnover number
TON = mol of H O as the product of the two-electron reduction
+
Figure 8. (a) Time profiles of absorbance at 650 nm due to Me Fc in
2
−3
−2
(
2
2
the two-electron reduction of O (1.7 × 10 M) by Me Fc (2.5 × 10 M)
2 2
II
II
of O /mol of Co (Ch)) was determined from the concentration of
with various concentrations of Co (Ch) in the presence of HClO
4
2
+
−2
Me Fc to be more than 30000, when the concentrations of
(2.5 × 10 M) in air-saturated PhCN at 298 K. (b) Time profiles of
absorbance at 650 nm due to Me Fc in the two-electron reduction of
2
O (1.7 × 10 M) by Me Fc (2.5 × 10 M) with Co (Ch) (1.0 × 10
M) in the presence of various concentrations of HClO in PhCN at
98 K. (c) Time profiles of absorbance at 650 nm due to Me Fc in the
(1.7 × 10 M) by various concentrations
of Me Fc with Co (Ch) (1.0 × 10 M) in the presence of HClO4
2.5 × 10 M) in PhCN at 298 K. (d) Time profiles of absorbance at 650
nm due to Me Fc in the two-electron reduction of various concentrations
of O by Me Fc (2.5 × 10 M) with Co (Ch) (1.0 × 10 M) in the
2
II
+
Co (Ch), Me Fc, and HClO in O -saturated PhCN were 2.0 ×
2
4
2
−3
−2
II
−5
−7
−2
−2
1
0 , 2.5 × 10 , and 2.5 × 10 M, respectively.
2
2
The intermediate observed during the catalytic reaction shown
4
+
2
2
by the broad blue line around 620 nm in Figure 7a was identified
−3
II
+
two-electron reduction of O
2
as [Co (ChH)] by comparison with the absorption spectrum of
II
−5
II
+
2
[
Co (ChH)] produced by the addition of HClO to a deaerated
PhCN solution of Co (Ch) in Figure 1. This suggests that
proton-coupled electron transfer (PCET) from [Co (ChH)] to
−2
4
(
II
+
2
II
+
−
2
II
−5
2
2
−
2
O is the rate-determining step under the catalytic conditions
2
presence of HClO (2.5 × 10 M) in PhCN at 298 K.
4
(
vide infra).
II
When Co (OEP) was employed as a catalyst under the same
II
II
catalytic conditions as the case of Co (Ch), Co (OEP) was
oxidized by O to form [Co (OEP)] as soon as the two-
determined from the increase in absorbance at 650 nm, obeying
III
+
pseudo-first-order kinetics under the reaction conditions limited
2
electron reduction of O was initiated (Figure S5, Supporting
by air-saturated O concentration in PhCN (Figure S8, Supporting
2
2
1
9
57
Information). During the catalytic two-electron reduction of
Information). This indicates that the reduction of O is involved in
2
III
+
O , the absorption bands associated with [Co (OEP)]
the rate-determining step.
2
decreased rapidly, accompanied by an increase of new absorption
The pseudo-first-order rate constant (k ) is proportional to
the concentration of Co (Ch) without intercept (Figure 9a).
obs
2
+
II
bands due to [H (OEP)] formed by demetalation and
4
diprotonation, as shown in Scheme 2 (Figures S6 and S7,
Supporting Information). These results indicate that fast
The kobs values increased linearly with increasing concentration
of HClO with an intercept (Figure 9b), whereas the k values
4
obs
II
demetalation of Co (OEP) occurred, followed by the proto-
remained constant irrespective of the concentrations of Me Fc
2
2
+
nation of H (OEP) to form [H (OEP)] (vide supra).
(Figure 9c) or O (Figure 9d). The observed first-order kinetics
2
4
2
E
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Journal of the American Chemical Society
Article
+
II
+
d[Me Fc ]/dt = (k
+ kcat(2)[HClO ])[[Co (ChH)] ][O ]
2
cat(1)
4
2
(5)
The kinetic equation (eq 5) was also confirmed under reaction
conditions where the concentrations of O were in large excess to
2
Me Fc (Figure 10). In such a case, the concentration of a large
2
Figure 10. (a) Absorption spectral changes in the two-electron
−
3
−4
reduction of O (8.5 × 10 M) by Me Fc (6.5 × 10 M) with
2
2
II
−5
−2
Co (Ch) (1.0 × 10 M) in the presence of HClO (5.0 × 10 M) in
O -saturated PhCN at 298 K. The black and red lines show the spectra
II
4
Figure 9. (a) Plot of k vs [Co (Ch)] for the two-electron reduction of
obs
−3
−2
2
O (1.7 × 10 M) by Me Fc (2.5 × 10 M) in the presence of HClO4
2.5 × 10 M) in air-saturated PhCN. (b) Plot of k vs [HClO ] for
the two-electron reduction of O (1.7 × 10 M) by Me Fc (2.5 × 10 M)
with Co (Ch) (1.0 × 10 M) in the presence of various concentrations
of HClO in PhCN at 298 K. (c) Plot of k vs [Me Fc] for the two-
electron reduction of O (1.7 × 10 M) by various concentrations of
Me Fc with Co (Ch) (1.0 × 10 M) in the presence of HClO4
2.5 × 10 M) in PhCN at 298 K. (d) Plot of kobs vs [O ] for the two-
electron reduction of various concentrations of O by Me Fc (2.5 × 10 M)
with Co (Ch) (1.0 × 10 M) in the presence of HClO (2.5 × 10 M) in
2
2
before and after addition of HClO , respectively. The dotted line is the
−2
4
(
−4
+
obs
4
absorbance at 650 nm due to 6.5 × 10 M Me Fc . (b) Time profile of
absorbance at 650 nm due to Me Fc .
−3
−2
2
+
2
2
II
−5
2
4
obs
2
−
3
excess of O remained constant during the catalytic reduction of
2
2
II
−5
+
O by Me Fc, where the rate of formation of Me Fc exhibited
2
2
2
2
−2
(
zero-order kinetics, as shown in Figure 10b. When the catalytic
reaction was complete, a steep rise in absorbance at 650 nm due
2
−2
2
2
II
−5
−2
III
2+
4
to [Co (ChH)] was observed at around 1.3 s (Figure 10b),
PhCN at 298 K.
III
2+
because Me Fc was consumed not to reduce [Co (ChH)] .
2
This result suggests that the catalytic reduction of O with
Co (Ch) proceeds efficiently without decomposition of the
catalyst because of its high durability under the acidic conditions,
in contrast with the case of Co(OEP). The pseudo-zero-order
2
II
II
depending on the concentrations of Co (Ch), HClO , and O
4
2
indicates that proton-coupled electron transfer (PCET) from
II
+
III
2+
•
[
Co (ChH)] to O to produce [Co (ChH)] and HO is the
2
2
rate-determining step in the catalytic cycle, as shown in Scheme 3.
rate constant (kobs(0)) was proportional to concentration of
II
Co (Ch) (Figure S9a, Supporting Information). The k
values
obs(0)
Scheme 3
increased linearly with increasing concentration of HClO with an
4
intercept (Figure S9b, Supporting Information), whereas the kobs(0)
values remained constant irrespective of the concentration of Me Fc
2
(Figure S9c, Supporting Information). These results are totally
consistent with the kinetic formation in eq 5. In order to further
confirm the catalytic mechanism in Scheme 3, each step in the
catalytic cycle was examined separately (vide infra).
II
+
PCET from [Co (ChH)] to O . As described above,
2
II
III
+
Co (Ch) reacts with O reversibly to produce [Co (Ch)] −
2
•
−
O
(eq 2) at low temperature. At 298 K the equilibrium lies
2
II
largely to the Co (Ch) side when no appreciable oxidation of
Co Ch by O was observed. The addition of HClO to an air-
saturated PhCN solution of Co (Ch), however, resulted in rapid
PCET from [Co (ChH)] to O to afford [Co (ChH)] , as
shown in Figure 11a. The rate of formation of [Co (ChH)]
was determined from the rise in absorbance at 656 nm due to
[Co (ChH)] , obeying first-order kinetics. The pseudo-first-
The produced HO • is rapidly reduced by Me Fc with H to
+
II
2
2
2
4
II
produce H O . The PCET process as the rate-determining step is
2
2
II
+
II
+
III
2+
also supported by the steady-state appearance of [Co (ChH)] as
the intermediate in the catalytic cycle (vide supra). On the other
2
III
2+
III
2+
hand, [Co (ChH)] , which is in the protonation equilibrium with
III
+
III
2+
[
Co (Ch)] , as shown in Figure 4, is also rapidly reduced by Me Fc
to regenerate [Co (ChH)] . Thus, the kinetic equation is given by
2
II
+
order-rate constant (k ) increased linearly with increasing
obs
eq 5, where the kcat(1) value was determined from the intercept of the
concentration of HClO with an intercept as shown in Figure 11b.
4
3
−1 −1
linear plot of kobs vs [HClO ] to be (1.2 ± 0.2) × 10 M
s
In this case, the kinetic equation is given by eq 6. From the intercept of
4
(
Figure 9b). The observation of the intercept indicates that proton-
the linear plot of kobs vs [HClO ], the second-order rate constant k ,
4
(1)
II
+
coupled electron transfer from [Co (ChH)] to O proceeds even
2
III
2+
in the absence of excess HClO . The k
value was determined
4
cat(2)
d[[Co (ChH)] ]/dt
= (k + k [HClO ])[[Co (ChH)] ][O ]
II
from the slopes of the linear plot of k vs [Co (Ch)] and [HClO ]
obs
4
II
+
5
−2 −1
(6)
to be (1.9 ± 0.3) × 10 M
s
(Figure 9a,b, respectively).
1
2
4
2
F
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Journal of the American Chemical Society
Article
second-order rate constant was determined from the second-
7
−1 −1
order plot to be (4.0 ± 0.3) × 10 M
noted that the electron-transfer reduction of [Co (ChH)]
Ered = 0.48 V vs SCE) by Me Fc should be faster than the
s
(Figure 12c). It is
III
2+
(
2
III
+
electron-transfer reduction of [Co (Ch)] (E = 0.37 V vs
red
SCE) by Me Fc. This result confirmed that the electron-transfer
2
III
2+
reduction of [Co (ChH)] by Me Fc was much faster than the
PCET reduction of O by [Co (ChH)] .
Because electron transfer from Me Fc to [Co (ChH)] is
2
II
+
2
III
2+
2
fast enough not to be the rate-determining step, the replacement
of Me Fc by a stronger one-electron reductant would give the
2
Figure 11. (a) Absorption spectral changes in the two-electron
−3
II
−5
same catalytic rate constant. This was confirmed by using
octamethylferrocene (Me Fc: E = −0.04 V vs SCE) for the
reduction of O (1.7 × 10 M) with Co (Ch) (2.0 × 10 M) in the
2
presence of various concentrations of HClO in air-saturated PhCN at
8
ox
4
−3
298 K. (b) Plot of kobs vs [HClO4].
catalytic two-electron reduction of O (1.7 × 10 M) with
2
Co (Ch) in the presence of HClO (2.5 × 10 M) in PhCN.
II
−2
4
The observed first-order rate constant remained the same irrespective
which is independent of the concentration of [HClO ], was
4
−2 −1
3
−1 −1
of concentration of Me Fc to be (4.3 ± 0.3) × 10 s (Figure S10a,
8
determined to be (1.0 ± 0.3) × 10 M s .
Supporting Information), which is about the same as the value
On the other hand, the third-order rate constant k was
(
2)
−2 −1
5
−2 −1
for Me Fc (5.0 ± 0.3) × 10 s (Figure 9c). When Me Fc (E =
2
2
o
x
determined from the slope to be (2.1 ± 0.2) × 10 M s . The
k(1) and k(2) values agreed within experimental errors with the
0.28 V) was replaced by weaker one-electron reductants, ferrocene
(
Fc: E = 0.37 V) and bromoferrocene (BrFc: E = 0.54 V), the
rate constants derived under catalytic conditions (k
= (1.2 ±
ox
ox
cat(1)
3
−1 −1
5
−2 −1
observed first-order rate constants also remained the same as those
for Me Fc and Me Fc irrespective of concentrations of Fc and BrFc,
0.2) × 10 M
s
and kcat(2) = (1.9 ± 0.3) × 10 M s ,
respectively). Such agreements confirm that the rate-determin-
2
8
ing step in the catalytic cycle in Scheme 3 is indeed the PCET
as shown in Figure S10b,c (Supporting Information), respectively.
II
+
III
2+
from [Co (ChH)] to O .
This suggests that electron transfer from BrFc to [Co (ChH)] is
not the rate-determining step in the catalytic cycle, although electron
2
III
2+
Reduction of [Co (ChH)] . The aforementioned results
indicate that the electron-transfer reduction of [Co (ChH)]
by Me Fc is much faster than the PCET oxidation of [Co (ChH)] .
III
2+
+
III
2+
transfer from BrFc to [Co (ChH)] (Ered = 0.48 V) is slightly
II
2
endergonic, as shown in Scheme 4.
This was independently confirmed by examining electron transfer
III
+
from Me Fc to [Co (Ch)] , which was prepared by the oxidation
2
Scheme 4
II
•+
−
of Co (Ch) with (p-BrC H ) N SbCl as a one-electron oxidant,
6
4 3
6
as shown in Figure 12a.
Figure 12. (a) Absorption spectral changes in the electron-transfer
Figure 13. (a) Absorption spectral change in the two-electron reduction
III
+
−5
−5
reduction of [Co (Ch)] (1.0 × 10 M) with Me Fc (1.0 × 10 M) in
−3
−2
II
2
of O (1.7 × 10 M) by Br Fc (2.0 × 10 M) with Co (Ch) (1.0 ×
2 2
M) in the presence of HClO (2.5 × 10 M) in air-saturated
4
deaerated PhCN at 298 K. (b) Time profile monitored by absorbance
−5
−2
1
0
(
Abs) at 656 nm. (c) Second-order plot.
PhCN at 298 K. The black and red lines show the spectra before and
after addition of Br Fc, respectively. The dotted line is the absorbance at
2
−3
+
7
00 nm due to 3.4 × 10 M Br Fc . (b) Plot of k vs [Br Fc] for the
2 obs 2
3 II
III
+
−
The rate of the reduction of [Co (Ch)] by 1 equiv of Me Fc
2
two-electron reduction of O (1.7 × 10 M) by Br Fc with Co (Ch)
2
2
−5
−2
was determined from a decrease in absorbance at 656 nm due to
(1.0 × 10 M) in the presence of HClO (2.5 × 10 M) in PhCN at
4
III
+
[
Co (Ch)] using a stopped-flow technique (Figure 12b). The
298 K.
G
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Journal of the American Chemical Society
Article
When 1,1′-dibromoferrocene (Br Fc: E = 0.72 V) was
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2
ox
■
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shown in Figure 13a, where the absorption band due to
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(
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2
(
determining step is now changed from the PCET reduction of O to
2
III
2+
the ET reduction of [Co (ChH)] because electron transfer from
Br Fc to [Co (ChH)] becomes thermodynamically more
(
8
III
2+
2
unfavorable (Scheme 4). At large concentrations of Br Fc, however,
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II
We have demonstrated that a cobalt chlorin complex (Co (Ch))
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4
produce H O with high catalyst stability, where the TON
2
2
reached more than 30000. This remarkable stability can be
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core size and greater flexibility of the ligand. The other is the
lower nucleophilicity of the core nitrogen atoms due to the
protonation of the ligand at the carbonyl group (position C-13 )
in the presence of HClO . When Co (Ch) is employed as a
catalyst for an electrochemical reduction of O , catalytic currents
correspond to the two-electron reduction of O with the highest
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2
2
́
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2
(
́
determining step is proton-coupled electron transfer (PCET)
II
+
Chkounda, M.; Gros, C. P.; Barbe, J.-M.; Ohkubo, K.; Fukuzumi, S.;
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2
Guilard, R. Inorg. Chem. 2008, 47, 6726.
III
2+
reductants to [Co (ChH)] . When Br Fc is employed as a
2
(
15) (a) Partovi-Nia, R.; Su, B.; Men
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much weaker reductant than other ferrocene derivatives under
the catalytic conditions, the rate-determining step is changed to
the ET reduction of [Co (ChH)] . In summary, the present
study provides valuable insights toward the development of more
2
́
III
2+
P.; Barbe, J.-M.; Daina, A.; Carrupt, P.-A.; Girault, H. H. J. Am. Chem.
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(
4
b) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36,
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produce H O , which is a promising candidate for a renewable
2
2
energy source.
(
ASSOCIATED CONTENT
■
* Supporting Information
Hanaizumi, N. ChemPhysChem 2008, 1, 988.
(18) Fellinger, T.-P.; Hasche, F.; Strasser, P.; Antonietti, M. J. Am.
Chem. Soc. 2012, 134, 4072.
S
Text and figures giving spectroscopic and kinetic data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
(19) (a) Fukuzumi, S.; Okamoto, K.; Gros, C. P.; Guilard, R. J. Am.
Chem. Soc. 2004, 126, 10441. (b) Fukuzumi, S.; Okamoto, K.; Tokuda,
Y.; Gros, C. P.; Guilard, R. J. Am. Chem. Soc. 2004, 126, 17059.
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AUTHOR INFORMATION
Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28, 2459. (e) Fukuzumi, S.;
Mochizuki, S.; Tanaka, T. Inorg. Chem. 1990, 29, 653. (f) Fukuzumi, S.;
Mochizuki, S.; Tanaka, T. J. Chem. Soc., Chem. Commun. 1989, 391.
■
Corresponding Author
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp.
*
(20) Fukuzumi, S.; Mandal, S.; Mase, K.; Ohkubo, K.; Park, H.; Benet-
Notes
Buchholz, J.; Nam, W.; Llobet, A. J. Am. Chem. Soc. 2012, 134, 9906.
(21) Honda, T.; Kojima, T.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134,
4
The authors declare no competing financial interest.
196.
22) (a) Chang, C. J.; Deng, Y.; Shi, C.; Anson, F. C.; Nocera, D. G.
(
ACKNOWLEDGMENTS
■
Chem. Commun. 2010, 46, 7334. (b) Chang, C. J.; Loh, Z.-H.; Shi, C.;
Anson, F. C.; Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 10013.
(c) McGuire, R.; Dogutan, D. K.; Teets, T. S.; Suntivich, J.; Shao-Horn,
Y. Chem. Sci. 2010, 1, 411. (d) Dogutan, D. K.; Stoian, S. A; McGuire, R.;
This work was supported by Grants-in-Aid (Nos. 20108010 to
S.F. and 23750014 to K.O.) from MEXT, Japan, and KOSEF/
MEST through WCU project (R31-2008-000-10010-0), Korea.
H
dx.doi.org/10.1021/ja312199h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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̈
(
(
(
(
́
II
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II
[
Co (ChH)] . However, no peak due to the protonated Co (Ch) was
II
III
+
observed because oxidation of Co (Ch) to [Co (Ch)] with HClO
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1
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(
2
(58) The saturated kobs value is much smaller than the kobs value of
Me Fc in Figure 9. In such a case, the saturation behavior in Figure 13b
2
(
may result from the complex formation between Br Fc and
2
(
III
2+
[
Co (ChH)] prior to electron transfer.
(
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2
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I
dx.doi.org/10.1021/ja312199h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX