Proton-coupled electron-transfer reduction of dioxygen catalyzed by a saddle-distorted cobalt phthalocyanine

Article abstract of DOI:10.1021/ja209978q

Proton-coupled electron-transfer reduction of dioxygen (O₂) to afford hydrogen peroxide (H₂O₂) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α- octaphenylphthalocyaninato)cobalt(II) (Co^II(Ph₈Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O ₂ by dimethylferrocene (Me₂Fc) and decamethylferrocene (Me₁₀Fc) occurs to yield H₂O₂ and the corresponding ferrocenium ions (Me₂Fc⁺ and Me ₁₀Fc⁺, respectively). Mechanisms of the catalytic reduction of O₂ are discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The active species to react with O₂ in the catalytic reaction is switched from Co^II(Ph₈Pc) to protonated Co^I(Ph₈PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co^II(Ph ₈Pc) inhibits the direct reduction of O₂; however, the proton-coupled electron transfer from Me₁₀Fc to Co ^II(Ph₈Pc) and the protonated [Co^II(Ph ₈PcH)]⁺ occurs to produce Co^I(Ph ₈PcH) and [Co^I(Ph₈PcH₂)] ⁺, respectively, which react immediately with O₂. The rate-determining step is a proton-coupled electron-transfer reduction of O ₂ by Co^II(Ph₈Pc) in the Co^II(Ph ₈Pc)-catalyzed cycle with Me₂Fc, whereas it is changed to the electron-transfer reduction of [Co^II(Ph₈PcH)] ⁺ by Me₁₀Fc in the Co^I(Ph₈PcH)- catalyzed cycle with Me₁₀Fc. A single crystal of monoprotonated [Co^III(Ph₈Pc)]⁺, [Co^IIICl ₂(Ph₈PcH)], produced by the proton-coupled electron-transfer reduction of O₂ by Co^II(Ph ₈Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co^II(Ph₈Pc).

Full text of DOI:10.1021/ja209978q

Article
pubs.acs.org/JACS
Proton-Coupled Electron-Transfer Reduction of Dioxygen Catalyzed
by a Saddle-Distorted Cobalt Phthalocyanine
†
,‡
,†,§
Tatsuhiko Honda, Takahiko Kojima,* and Shunichi Fukuzumi*
†Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871, Japan
‡
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
§
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
*
S Supporting Information
ABSTRACT: Proton-coupled electron-transfer reduction of dioxygen
O ) to afford hydrogen peroxide (H O ) was investigated by
(
2
2
2
using ferrocene derivatives as reductants and saddle-distorted (α-
II
octaphenylphthalocyaninato)cobalt(II) (Co (Ph Pc)) as a catalyst
8
under acidic conditions. The selective two-electron reduction of O2
by dimethylferrocene (Me Fc) and decamethylferrocene (Me Fc)
2
10
occurs to yield H O and the corresponding ferrocenium ions
2
2
+
+
(
Me Fc and Me Fc , respectively). Mechanisms of the catalytic
2 10
reduction of O are discussed on the basis of detailed kinetics
2
studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The active species to react with O
2
II
I
in the catalytic reaction is switched from Co (Ph Pc) to protonated Co (Ph PcH), depending on the reducing ability of
8
8
II
ferrocene derivatives employed. The protonation of Co (Ph Pc) inhibits the direct reduction of O ; however, the proton-coupled
electron transfer from Me Fc to Co (Ph Pc) and the protonated [Co (Ph PcH)] occurs to produce Co (Ph PcH) and
8
2
II
II
+
I
1
0
8
8
8
I
+
[
Co (Ph PcH )] , respectively, which react immediately with O . The rate-determining step is a proton-coupled electron-transfer
8
2
2
II
II
reduction of O by Co (Ph Pc) in the Co (Ph Pc)-catalyzed cycle with Me Fc, whereas it is changed to the electron-transfer
reduction of [Co (Ph PcH)] by Me Fc in the Co (Ph PcH)-catalyzed cycle with Me Fc. A single crystal of monoprotonated
2
8
8
2
II
+
I
8
10
8
10
III
+
III
II
[
Co (Ph Pc)] , [Co Cl (Ph PcH)], produced by the proton-coupled electron-transfer reduction of O by Co (Ph Pc) with
8
2
8
2
8
II
HCl, was obtained, and the crystal structure was determined in comparison with that of Co (Ph Pc).
8
INTRODUCTION
peripheral substituents around the dioxygen molecule bound to
1
■
0−14
metal centers.
Electron-transfer reactions can be efficiently controlled by
proton uptake and release to manipulate redox potentials of an
electron donor and/or an electron acceptor. A typical example
is a proton-coupled electron transfer, which plays pivotal roles
in biological electron-transfer systems such as photosynthesis
Porphyrin analogues such as phthalocyanines (Pc) have also
been applied as ligands of metal complexes acting as cata-
lysts for dioxygen reduction, in particular as electrochemical
23−27
catalysts.
The presence of basic nitrogen atoms at meso
1
−7
positions of phthalocyanine provides the possibility of facile
modification of its chemical and electronic properties by the
protonation of the meso nitrogen atoms. Recently, we have
reported the high basicity of saddle-distorted α-octaphenylph-
and respiration.
Cytochrome c oxidases (CcO) are the ter-
minal enzymes of respiratory chains, catalyzing the reduction of
molecular oxygen to water and at the same time generating
a proton gradient across the membrane for proton pump-
1
−3,6,7
thalocyanine (Ph Pc) and the enhanced electron-accepting abil-
ing.
In the active site of this enzyme, the reduction of
8
+
28
ity of the resulting protonated phthalocyanine (Ph PcH ).
dioxygen is catalyzed by a bimetallic active site consisting of
heme and nonheme Cu moieties assisted by the hydroxy group
of the neighboring tyrosine connected to a Cu-coordinated
imidazole. A number of synthetic models of CcO active sites
have been reported by using metal complexes of porphyrins
and porphyrin analogues for the dioxygen reduction in elec-
trocatalytic and homogeneous systems, because the reduction
of dioxygen is not only of biological interest but also of
8
Protonated planar (phthalocyaninato)iron(III) has recently
been reported as an effective cathode material for a hydrogen
29
peroxide fuel cell operating under acidic conditions. Hydro-
gen peroxide has merited increasing attention as a promising
30−32
candidate as a sustainable energy carrier,
because the free
enthalpy change of the decomposition of hydrogen peroxide to
−1
33
water and dioxygen is as large as −210.71 kJ mol . Hydrogen
peroxide has also been used as a highly efficient and
competitive chemical in terms of delignification efficiency and
8
−27
technological importance as fundamentals of fuel cells.
To
gain mechanistic insights into the proton-coupled multielectron
redox process in CcO active sites, molecular systems such as
Pacman and Hangman porphyrins have been extensively studied
to control the effective O−O bond cleavage by modifying the
Received: October 24, 2011
Published: February 2, 2012
©
2012 American Chemical Society
4196
dx.doi.org/10.1021/ja209978q | J. Am. Chem. Soc. 2012, 134, 4196−4206

Journal of the American Chemical Society
Article
3
4,35
reduction of ecological impact.
However, the catalytic
was washed several times with water and dried over Na SO . The
2 4
product was purified by column chromatography on silica gel using
CH Cl as an eluent and recrystallized from CH Cl /hexane (361 mg,
mechanism of protonated metallophthalocyanines has yet to be
clarified in the reduction of dioxygen in homogeneous solution.
The mechanistic study of the two-electron reduction of
dioxygen is essential to develop sustainable systems employing
hydrogen peroxide as an energy carrier.
We report herein the catalytic two-electron reduction of
dioxygen by using ferrocene derivatives as one-electron reduc-
tants with (α-octaphenylphthalocyaninato)cobalt (Co(Ph Pc))
as a catalyst under acidic conditions. It has been found that
the protonation of Co (Ph Pc) (Scheme 1) in the catalytic
2
2
2
2
4
−1
−1
6
7
6%). UV−vis (PhCN) λ (εmax): 683 nm (2.9 × 10 M cm ),
max
4 −1 −1 +
60 nm (9.1 × 10 M cm ). MALDI-TOF-MS: m/z 1179.3 (M ).
Anal. Calcd for C H CoN (Co(Ph Pc)): C, 81.41; H, 4.10; N, 9.49.
80
48
8
8
1
Found: C, 81.15; H, 4.37; N, 9.44. H NMR (CDCl . 300 MHz):
3
δ 10.5 (16H), 9.7 (8H), 8.8 (24H).
III
III
Preparation of [Co Cl (Ph PcH)]. [Co Cl (Ph PcH)] was
2
8
2
8
obtained by two-layered recrystallization with addition of CH OH
8
3
II
on the top of the toluene solution (2.0 mL) of Co (Ph
Pc) (20 mg,
8
II
0
.042 mmol) in the presence of excess hydrochloric acid. The obtained
8
crystals were dried in vacuo. Anal. calcd for C H N CoCl ·1.5H O
80
49
8
2
2
(
7
[CoCl (Ph PcH)]·1.5H O): C, 75.12; H, 4.10; N, 8.76. Found: C,
2 8 2
Scheme 1
5.31; H, 3.96; N, 8.84.
X-ray Crystallography. Single crystals of Co (Ph Pc) and
II
8
III
[
Co Cl (Ph PcH)] were mounted on a glass capillary with silicon
2 8
grease. All measurements were performed on a Rigaku Mercury CCD
area detector at −150 °C with graphite-monochromated Mo Kα
radiation (λ = 0.710 70 Å) up to 2θ = 54.7°. All calculations were
max
performed using the Crystal Structure crystallographic software
package, and structure refinements were made by a direct method
II
III
using SIR97 for Co (Ph Pc) and SIR2004 for [Co Cl (Ph PcH)],
respectively.
8
2
8
3
9,40
III
The absolute structure of [Co Cl (Ph PcH)] was
2 8
deduced on the basis of the Flack parameter, 0.010(8), using 9240
4
1
Friedel pairs. Crystallographic data are summarized in Table 1.
reduction of dioxygen plays dual roles, as discussed in this
II
paper: one is an inhibiting effect on the reaction of Co (Ph Pc)
8
II
Table 1. X-ray Crystallographic Data for Co (Ph Pc) and
8
with dioxygen in the case of the Co(II/III) cycle; the other is
an acceleration effect on the proton-coupled electron-transfer
III
[
Co Cl (Ph PcH)]
2
8
II
II
+
II
III
reduction of Co (Ph Pc) and [Co (Ph PcH)] by decamethyl-
Co (Ph Pc)·2CH Cl [Co Cl (Ph PcH)]·5C H
8
8
8
2
2
2
8
7
8
I
I
ferrocene (Me Fc) to produce Co (Ph PcH) and [Co (Ph -
1
0
8
8
formula
C H Cl CoN
C115H Cl CoN
8
8
2
52
4
8
89 12
+
PcH )] , which readily react with dioxygen in the case of the
2
formula wt
cryst syst
space group
T, K
1350.11
triclinic
1712.86
Co(II/I) cycle. In such a case, the reaction mechanism of
orthorhombic
the catalytic reduction of dioxygen is switched from the
P1
̅
(No. 2)
P2 2 2 (No. 19)
1
1 1
II
Co (Ph Pc)-catalyzed cycle (Co(II/III) cycle) to the proto-
8
123
123
I
nated Co (Ph PcH)-catalyzed cycle (Co(II/I) cycle) depending
8
a, Å
12.050(2)
15.207(3)
17.854(4)
102.894(3)
96.042(3)
100.331(3)
3101.3(11)
2
14.798(1)
21.317(2)
27.914(3)
on the reducing ability of ferrocene derivatives. In addition, the
b, Å
III
+
III
crystal structure of monoprotonated [Co (Ph Pc)] , [Co Cl -
8
2
c, Å
(
Ph PcH)], produced by the proton-coupled electron-transfer
8
α, deg
β, deg
γ, deg
V, Å3
II
reduction of O by Co (Ph Pc) in the presence of HCl,
2
8
was successfully determined in comparison with that of
II
Co (Ph Pc).
8
8805.7(12)
4
Z
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
measurements was distilled over phosphorus pentoxide prior to use.
,4,8,11,15,18,22,25-Octaphenylphthalocyanine (H Ph Pc) was syn-
no. of rflns measd
no. of observns
21 973
87 608
20 181
1136
■
11 960
no. of params refined
850
a
R1
0.0735 (I > 2σ(I))
0.0446
0.0949
1.108
3
6
,
bc
wR2
GOF
0.2006 (all data)
1
1.068
2
8
37
thesized according to the reported procedure. Tetra-n-butylammonium
a
c
R1 = ∑||F | − |F ||/∑|F |. wR2 = [∑(w(F − F ) )/∑w(F ) ]1/2.
b
2
o
2
2
2 2
o
c
o
c
o
hexafluorophosphate (TBAPF ) was twice recrystallized from ethanol
2
2
o
2
2
6
w = 1/[σ (F ) + (0.0500P) + 30.0000P, where P = (max(F , 0) +
o
and dried in vacuo prior to use. [Ru(bpy) ](ClO ) (bpy = 2,2′-
bipyridine) was prepared from tris(2,2′-bipyridyl)ruthenium(II)
chloride hexahydrate by oxidation with PbO2.
300 MHz) were recorded on a JEOL AL-300 spectrometer at room
temperature, and chemical shifts (ppm) were determined relative to
the residual solvent peaks. MALDI-TOF-MS measurements were
performed on a Kratos Compact MALDI I (Shimadzu) using dithranol
as a matrix. UV−vis spectroscopy was carried out on a Hewlett-
Packard 8453 diode array spectrophotometer at room temperature
using 1 cm cells. EPR measurements were performed on a JEOL JES-
RE1XE spectrometer.
2
3
4 3
2
F )/3.
c
3
8
1
H NMR spectra
Spectroscopic Measurements. The formation constants (log K)
(
+
of [Co(Ph PcH )] were determined by using the Hill equation, which
8
analyzes changes of absorption spectra during the titration as a func-
tion of the concentration of added acids. The amount of hydrogen
42
peroxide (H O ) formed was determined by titration with iodide ion:
2
2
a dilute CH CN solution (2.0 mL) of the product mixture (20 μL)
3
−
was treated with an excess amount of NaI and the amount of I3
formed was determined by the absorption spectrum (λ 361 nm, ε =
max
4
−1
−1 43
2
.5 × 10 M cm ).
Preparation of (1,4,8,11,15,18,22,25-Octaphenyl-
II
phthalocyaninato)cobalt(II) (Co (Ph Pc)). A mixture of H Ph Pc
Kinetic Measurements. Kinetic measurements for fast reactions
8
2
8
(
500 mg, 0.45 mmol) and Co(CH CO ) ·4H O (500 mg, 2.0 mmol)
with short half-lifetimes (within 10 s) were performed on a UNISOKU
RSP-601 stopped-flow spectrophotometer with an MOS-type high
selective photodiode array at 298 K using a Unisoku thermostated cell
3
2
2
2
in 300 mL of CHCl /CH OH (2/1 v/v) was refluxed under nitrogen
for 1 h. The residue was dissolved into CH Cl , and then the solution
3
3
2
2
4
197
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Journal of the American Chemical Society
Article
II
III
Figure 1. Views of the molecular structures of (a) Co (Ph Pc) and (b) [Co Cl (Ph PcH)] from different directions: (gray) carbon; (blue)
8
2
8
nitrogen; (red) cobalt; (green) chlorine. Hydrogen atoms and solvent molecules of crystallization are omitted for clarity. Selected bond lengths (Å)
II
of Co (Ph Pc): Co1−N2, 1.881(3); Co1−N4, 1.879(2); Co1−N6, 1.890(3); Co1−N8, 1.891(3). Selected bond lengths (Å) and angles (deg) of
8
III
[
Co Cl (Ph PcH)]: Co1−N2, 1.909(2); Co1−N4, 1.914(2); Co1−N6, 1.918(2); Co1−N8, 1.910(2); Co1−Cl1, 2.2622(6); Co1−Cl2, 2.2666(6);
2
8
N2−Co1−Cl1, 87.00(5); N4−Co1−Cl1, 91.29(5); N6−Co1−Cl1, 88.40(5); N8−Co1−Cl1, 93.73(5); N2−Co1−Cl2, 92.15(5); N4−Co1−Cl2,
8.56(5); N6−Co1−Cl2, 92.45(5); N8−Co1−Cl2, 86.42(5).
8
holder. When stopped-flow measurements were carried out under
deaerated or O -saturated conditions, a deaerated or O -saturated
four-coordinate planar structure, as shown in Figure 1a.
Selected bond lengths (Å) are given in the figure caption. A
2
2
PhCN solution with a stream of argon or O was transferred by means
III
+
III
2
single crystal of monoprotonated [Co (Ph Pc)] , [Co Cl -
8 2
of a glass syringe to a spectrometer cell that was already purged with a
(
Ph PcH)], was also obtained by two-layered recrystallization
8
stream of argon or O . Rate constants of oxidation of ferrocene
2
with addition of CH OH on top of a toluene solution of
II
3
derivatives by O in the presence of a catalytic amount of Co (Ph Pc)
2
8
II
Co (Ph Pc) in the presence of an excess amount of HCl. This
indicates that Co (Ph Pc) is oxidized by dioxygen in the
presence of HCl to produce [Co Cl (Ph PcH)]. The crystal
8
and an excess amount of formic acid in PhCN at 298 K were
determined by monitoring the appearance of the absorption band due
II
8
III
+
to the corresponding ferrocenium ions (Fc , λ
620 nm, ε
=
2
8
max
max
III
−1
−1
+
−1
−1
+
3
30 M cm ; Me Fc , λ 650 nm, ε = 290 M cm ; Me Fc ,
cm ; Me Fc , λ 700 nm, εmax
40 M cm ). At the wavelengths monitored, spectral overlap was
structure of [Co Cl (Ph PcH)] was also determined as
2 8
2
max
max
8
−1
−1
+
λmax 750 nm, ε = 410 M
2
=
depicted in Figure 1b. Selected bond lengths (Å) and angles
(deg) are given in the figure caption. In the crystal structure of
max
−1 13
10
max
−
1
II
3
−1
−1
III
observed with Co (Ph Pc) (λ 620 nm (6.7 × 10 M cm ), 650 nm
8
octahedral [Co Cl (Ph PcH)], two chloride ions axially
2
8
4
−1
−1
4
−1
−1
(
1
1.4 × 10 M cm ), 700 nm (2.9 × 10 M cm ), 750 nm (8.3 ×
5
coordinate to the central cobalt ion at distances of 2.2622(6)
and 2.2666(6) Å. As shown in Figure 1, both of the crystal
−1
−1
0 M cm )).
Electrochemical Measurements. Cyclic voltammetry (CV)
II
III
structures of Co (Ph Pc) and [Co Cl (Ph PcH)] show an
8
2
8
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
out-of-plane distortion because of the steric hindrance among
45
peripheral phenyl groups as well as Zn(Ph Pc). To verify
8
the extent of deformation of the phthalocyanine ring, the
displacement of each atom from the least-squares mean plane
2
platinum working electrode (surface area of 0.3 mm ) and a platinum
II
wire as the counter electrode. The Pt working electrode (BAS) was
routinely polished with BAS polishing alumina suspension and rinsed
with acetone before use. The potentials were measured with respect to
of 24 atoms of the phthalocyanine core (Δrms) in Co (Ph Pc)
8
was calculated to be 0.60 Å, which is nearly the same as that of
2
8,45
Zn(Ph Pc) (0.59 Å).
A small relaxation of the ring
distortion in [Co Cl (Ph PcH)] (Δrms = 0.48 Å) is observed,
probably due to elongation of the Co−N bonds as compared to
8
−
2
the Ag/AgNO (1.0 × 10 M) reference electrode. All potentials (vs
III
3
2
8
+
44
Ag/Ag ) were converted to values vs SCE by adding 0.29 V. All
electrochemical measurements were carried out under an atmospheric
pressure of nitrogen.
II
those of Co (Ph Pc).
8
II
Protonation Equilibrium of Co (Ph Pc). To examine the
protonation of Co (Ph Pc) (Scheme 1), we measured
8
II
8
absorption spectral changes upon addition of trifluoromethane-
RESULTS AND DISCUSSION
■
sulfonic acid (CF SO H), trifluoroacetic acid (TFA), and
3
3
II
Synthesis and Characterization of Co (Ph Pc) and
formic acid (HCOOH) in deaerated PhCN. The one-step
spectral change was induced by adding 1 equiv of CF SO H,
8
III
II
[
Co Cl (Ph PcH)]. Co (Ph Pc) was obtained by a reaction of
2 8 8
3
3
H Ph Pc with Co(CH CO ) . The two-layered recrystallization
clearly showing that monoprotonation occurred to form
2
8
3
2 2
II
+
with addition of n-hexane on top of a CH Cl solution of
[Co (Ph PcH)] (red line in Figure 2b, see also Figure S1 in
2
2
8
II
Co (Ph Pc) gave a single crystal sufficient for X-ray diffraction.
the Supporting Information). This was also evidenced by the
observation of EPR spectral changes upon addition of 1 equiv
8
II
The crystal structure of Co (Ph Pc) was determined to reveal a
8
4
198
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Journal of the American Chemical Society
Article
I
−
II
III
+
I
Figure 2. Absorption spectra of (a) [Co (Ph Pc)] (black), Co (Ph Pc) (red), and [Co (Ph Pc)] (blue) and (b) Co (Ph PcH) (black),
[
8
8
8
8
II
+
III
2+
Co (Ph PcH)] (red), and [Co (Ph PcH)] (blue) in PhCN at 298 K.
8 8
II
of CF SO H in frozen PhCN at 77 K; Co (Ph Pc) exhibited
oxidations and one reversible reduction in PhCN. On the basis
of redox potentials of previously characterized Co(II)
3
3
8
well-resolved signals at g = 2.2896 and g = 1.9603 (A = 58
and A = 125 G, respectively), which are typical for a low-spin
S = / ) cobalt(II) complex, whereas [Co (Ph PcH)]
⊥
∥
⊥
4
6,47
phthalocyanines,
the first oxidation and reduction processes
∥
1
II
+
(
are assigned as Co(II)/Co(III) and Co(II)/Co(I) couples,
respectively. Thus, the second and third oxidation processes are
ascribed to one- and two-electron ligand oxidations. Upon
addition of 2 equiv of CF SO H, the redox potential of Co(II)/
2
8
exhibited broader signals at g = 1.9996 (A = 120 G) (Figure
∥
∥
S2a,b in the Supporting Information). The addition of TFA and
II
HCOOH to a PhCN solution of Co (Ph Pc) afforded the
8
3
3
same spectra; however, greater concentrations were required
for HCOOH to complete the protonation as compared with
those of CF SO H and TFA. The protonation constants of
Co(I) was positively shifted from −0.31 to 0.02 V (vs SCE).
This result clearly shows that the ligand protonation causes the
positive shift of the metal-centered redox potential, because the
electron-donating ability of the phthalocyanine ligand decreases
3
3
II
+
[
Co (Ph PcH)] with TFA and HCOOH (log K) were deter-
8
28,48
mined by absorption spectral changes to be 2.7 and 0.56,
upon protonation.
On the other hand, the Co(II)/Co(III)
respectively (Figure S1 in the Supporting Information). The
potential was also positively shifted (+0.31 V) by protonation
II
II
+
lowered basicity of Co (Ph Pc) as compared with that of
(Figure 3c). The oxidation process of [Co (Ph PcH)] be-
8
8
II
28
III
+
Zn (Ph Pc) (log K = 5.1, TFA, PhCN) can be explained by a
8
comes irreversible due to the deprotonation of [Co (Ph PcH)] ,
8
decrease in the electron density of the phthalocyanine ring,
because the basicity of the Pc ligand bound to Co(III) is lower
than that of the ligand bound to Co(II) due to an increase in
which is caused by the electronic interaction between the
2
2
49
unoccupied d
orbital of the low-spin Co(II) ion and the
x −y
the net charge of the metal center. This is consistent with the
lone pairs of the phthalocyanine ligand.
Electrochemical measurements on Co (Ph Pc) were per-
observation that the ligand oxidation remains unchanged upon
addition of 2 equiv of CF SO H.
II
8
3
3
III
+
formed in deaerated PhCN containing 0.10 M TBAPF , as
6
The deprotonation of [Co (Ph PcH)] is also clarified by
the chemical oxidation of Co (Ph Pc) and [Co (Ph PcH)] by
8 8
8
II
II
II
+
shown in Figure 3a. Co (Ph Pc) undergoes three reversible
8
III
using [Ru (bpy) ](ClO ) as a one-electron oxidant. The one-
3
4 3
II
II
+
electron-oxidized species of Co (Ph Pc) and [Co (Ph PcH)]
8
8
exhibited the same spectra, as shown by the blue line in Figure 2a,
II
+
indicating that the one-electron oxidation of [Co (Ph PcH)]
8
III
+
produces [Co (Ph Pc)] accompanied by deprotonation (see
8
also Figure S3 in the Supporting Information). The oxidation of
II
the Co(II) center in Co (Ph Pc) by the addition of 1 equiv of
8
III
[
Ru (bpy) ](ClO ) was evidenced by disappearance of the
3
4 3
EPR signal derived from the low-spin Co(II) center of
II
Co (Ph Pc) in PhCN at 77 K and also by the observation of
8
1
a diamagnetic H NMR spectrum in CDCl (Figures S3c and 4
3
in the Supporting Information). We also examined the one-
II
electron reduction of Co (Ph Pc) by using cobaltocene
8
(
CoCp ) as a one-electron reductant (Figure S2 in the
2
I
−
Supporting Information). The formation of [Co (Ph Pc)]
8
was confirmed by observing a characteristic absorption around
5
00 nm associated with metal-to-ligand charge transfer, as
50−55
shown by the black line in Figure 2a.
II
II
The protonation equilibrium between different oxidation
Figure 3. Cyclic voltammograms of (a) Co (Ph Pc), (b) Co (Ph Pc)
8
8
II
states of [Co(Ph Pc)] was examined by using TFA in deaerated
with 1 equiv of CF SO H, and (c) Co (Ph Pc) with 2 equiv of
8
3
3
8
CF SO H in PhCN containing 0.10 M TBAPF . The concentration of
PhCN, as shown in Figures 2b and 4 (see also Figure S5 in the
Supporting Information). In all cases, the protonated species
3
3
6
II
−3
Co (Ph Pc) is 1.0 × 10 M.
8
4
199
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Journal of the American Chemical Society
Article
much as the concentration of O in air-saturated PhCN (1.7 ×
2
−3
1
0
M). This result clearly indicates that the two-electron
reduction of O occurs to produce 2 equiv of Me Fc (eq 1),
+
2
2
+
+
2
Me Fc + O + 2H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ ⎯→ 2Me Fc + H O
2
2
2
2 2
II
Co (Ph Pc)
8
(1)
which is consistent with the case for the other mononuclear
1
3,17
Co(II) complexes.
When Me Fc was replaced by a stronger
2
13
one-electron reductant, Me Fc (E = −0.08 V), the clean
two-electron reduction of O also occurred to afford Me Fc ,
10
ox
+
2
10
as shown by the red line in Figure 5. The amount of H O
2
2
formed was determined by the reaction of an excess amount of
Figure 4. Absorbance changes at 860 (black and red) and 1000 nm
NaI in CH CN (2.0 mL) with the reaction mixture (20 μL:
3
(
blue) upon addition of TFA to a deaerated PhCN solution of
II
−3
I
−
−5
II
−5
[Co (Ph Pc)] = 1.0 × 10 M, [HCOOH] = 0.50 M, and
8 0
3
[
Co (Ph Pc)] (2.0 × 10 M, black), Co (Ph Pc) (1.0 × 10 M,
8
8
−
III
+
−5
[
Me Fc] or [Me Fc] = 2.0 × 10 M in O -saturated PhCN).
red), and [Co (Ph Pc)] (1.0 × 10 M, blue) at 298 K.
2 10 2
8
Under the above conditions, 97 and 74% of H O were formed,
2
2
showed red-shifted spectra. The formation constants of mono-
respectively.
Catalytic Mechanism of the Two-Electron Reduction
+
protonated Co(I), Co(II), and Co(III) Ph PcH complexes
8
II
of O by Me Fc with Co (Ph Pc) in the Presence of an
(
2
[
log K) were determined by spectroscopic titration to be 4.3,
.7, and 0.26, respectively, clearly indicating that the basicity of
Co(Ph Pc)] decreases on elevating the oxidation states of the
2
2
8
+
II
Acid. The rate of formation of Me Fc in the Co (Ph Pc)-
catalyzed two-electron reduction of O by Me Fc (1.0 × 10
M) in the presence of 0.50 M HCOOH in O -saturated PhCN
2
8
−3
2
2
8
4
9
cobalt center. This is consistent with the deprotonation of
2
II
+
obeyed zero-order kinetics, as shown in Figures 6a (see also
Figure S6 in the Supporting Information). The zero-order rate
constant showed a saturation behavior with increasing con-
centration of HCOOH (black circles in Figure 6b), which
agreed with the absorbance change at 860 nm due to the
[
Co (Ph PcH)] upon one-electron oxidation observed in
8
electrochemical measurements.
Catalytic Two-Electron Reduction of O by Ferrocene
2
II
Derivatives with Co (Ph Pc) in the Presence of an Acid.
The addition of Co (Ph Pc) and HCOOH (0.50 M) to an O -
8
II
8
2
II
+
formation of [Co (Ph PcH)] (red circles in Figure 6b). The
saturated PhCN solution of Me Fc resulted in efficient
8
2
zero-order rate constant increased linearly with increasing con-
oxidation of Me Fc by O . It should be noted that no oxidation
2
2
II
II
centrations of Co (Ph Pc) and O , as shown in Figure 6c,d,
of Me Fc occurred by O in the absence of Co (Ph Pc). The
formation of dimethylferrocenium ion (Me Fc ) was moni-
8
2
2
2
8
+
respectively. When Me Fc was replaced by a weaker one-
2
2
13
electron reductant, Fc (Eox = 0.37 V), the formation of Fc+
also obeyed zero-order kinetics at the initial stage and the
tored by a rise in absorbance at 650 nm. The black line in
+
Figure 5 shows the time course of formation of Me Fc in the
2
obtained rate constant was virtually the same as that of Me Fc
2
(
Figure S7a in the Supporting Information). The observed
zero-order kinetics indicates that an electron-transfer process
involving Me Fc is not the rate-determining step in the catalytic
2
cycle of the two-electron reduction of O by Me Fc. Instead,
2
2
II
proton-coupled electron transfer from Co (Ph Pc), which is in
a protonation equilibrium with [Co (Ph PcH)] , to O to
produce [Co (Ph Pc)] and HO2 may be the rate-
8
II
+
8
2
III
+
•
8
determining step in the catalytic cycle, as shown in Scheme 2.
•
The produced HO2 may be rapidly reduced by Me Fc with
2
+
III
+
H to afford H O . On the other hand, [Co (Ph Pc)] can
also be rapidly reduced by Me Fc to regenerate Co (Ph Pc),
2
2
8
II
2
8
because Me Fc (E = 0.26 V vs SCE) lies at a 240 mV
2
ox
overpotential relative to the Co(III/II) couple of Co(Ph Pc)
8
(
Ered = 0.50 V vs SCE), as shown in Scheme 3.
According to Scheme 2, the rate of formation of Me Fc is
+
2
given by eq 2, because electron transfer from Me Fc to
+
+
2
Figure 5. Time profiles of formation of Me Fc (black) and Me Fc
2
10
−1
−1
(
2
(
red) monitored at 650 nm (ε = 290 M cm ) and 700 nm (ε =
+
II
−
1
−1
d[Me Fc ]/dt = 2k [Co (Ph Pc)][O ][HCOOH]
40 M cm ) in electron-transfer oxidation of Me Fc or Me Fc
2.0 × 10 M), respectively, with O (1.7 × 10 M) catalyzed by
Co (Ph Pc) (5.0 and 1.0 × 10 M, respectively) in the presence of
formic acid (0.50 M) in air-saturated PhCN at 298 K.
2
et
8
2
(2)
2
10
−2
−3
2
II
−5
8
III
+
[
Co (Ph Pc)] and proton-coupled electron transfer from
8
•
Me Fc to HO2 are much faster than proton-coupled electron
2
−3
II
+
reduction of O (1.7 × 10 M) in the presence of an excess
amount of Me Fc (2.0 × 10 M), a catalytic amount of
Co (Ph Pc) (5.0 × 10 M), and a large excess amount of
formic acid (0.50 M). The concentration of Me Fc (3.4 × 10
transfer (k ) from Co (Ph Pc) to O , when 2 equiv of Me Fc
2
et 8 2 2
−2
is formed by the rate-determining proton-coupled electron
2
II
−5
II
transfer from Co (Ph Pc) to O . From the protonation
8
8
2
+
−3
II
equilibrium in Scheme 1, the concentration of Co (Ph Pc) is
given by eq 3, where [Co (Ph Pc)] is the initial concentration
2
8
II
M) formed in the catalytic reduction of O by Me Fc is twice as
2
2
8
0
4
200
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+
−1
−1
Figure 6. (a) Time profile of the formation of Me Fc monitored by absorbance at 650 nm (ε = 290 M cm ) in electron-transfer oxidation of
2
−3
−3
II
−5
Me Fc (1.0 × 10 M) by O (8.5 × 10 M) catalyzed by Co (Ph Pc) (1.0 × 10 M) in the presence of 0.50 M formic acid in PhCN. (b) Plots of
kobs (black) and absorbance changes at 860 nm due to [Co (Ph PcH)] (red) vs [HCOOH]. (c) Plot of k vs [Co (Ph Pc)] ; [HCOOH] =
2
2
8
II
+
II
8
obs
8
0
0.50 M. (d) Plot of kobs vs [O2].
II
Scheme 2
[Co (Ph Pc)] /(1 + K[HCOOH]) in eq 2 corresponds to the
8 0
II
concentration of unprotonated Co (Ph Pc), which acts as the
8
actual electron donor. Equation 4 agreed well with the experi-
mental observations in Figure 6, where the rate of formation of
+
Me Fc was independent of concentration of Me Fc (zero
2
2
order), and the zero-order rate constant was proportional to the
II
initial concentrations of Co (Ph Pc) and O . The saturation
8
2
behavior of the zero-order rate constant with respect to the
concentration of HCOOH (Figure 6b) results from the
II
protonation equilibrium of Co (Ph Pc), when only unproton-
8
II
ated Co (Ph Pc) acts as an electron donor in the proton-
8
coupled electron transfer to O , because the Co(II)/Co(III)
2
potential is positively shifted (+0.31 V) by the protonation
Scheme 3
(
Figure 3c). From the results in Figure 6c,d and eq 4, the k
et
II
value of proton-coupled electron transfer from Co (Ph Pc) to
8
O in the presence of 0.50 M HCOOH is determined to be
2
2
−2 −1
(
1.4 ± 0.1) × 10 M
s
at 298 K.
II
Proton-Coupled Electron Transfer from Co (Ph Pc) to
8
O . The rate-determining step in the catalytic cycle for the two-
2
electron reduction of O by Me Fc was examined indepen-
2
2
dently (vide infra). In the presence of an excess amount of
II
HCOOH, proton-coupled electron transfer from Co (Ph Pc)
8
II
III
+
of Co (Ph Pc) and K is the protonation constant with
to O
2
occurs to produce [Co (Ph Pc)] , as shown in Figure 7a,
8
8
−1
where the final absorption spectrum (red) is the same as that of
HCOOH (K = 3.6 M at 298 K, vide supra). By combining
III
+
[
[
Co (Ph Pc)] in Figure 2a (blue). The formation of
8
eqs 2 and 3, eq 4 is obtained. It should be noted that
III
+
Co (Ph Pc)] was also confirmed by the disappearance of
8
II
II
II
[
Co (Ph Pc)] = [Co (Ph Pc)] /(1 + K[HCOOH])
the EPR signal derived from Co (Ph Pc) after the reaction with
O (Figure S2d in the Supporting Information). This suggests
that the two-electron reduction of O is achieved by 2 equiv of
Co (Ph Pc). As shown in Scheme 4, the uphill nature of the
proton-coupled electron transfer from Co (Ph Pc) to O (k )
implies that the complex of [Co (Ph Pc)] and hydroperoxyl
8
8
0
8
2
(3)
2
II
+
II
2k [Co (Ph Pc)] [O ][HCOOH]
8
d[Me Fc ]
2
et
8
0
2
II
=
8
2
et
III
+
dt
1 + K[HCOOH]
(4)
8
4
201
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Journal of the American Chemical Society
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II
−5
Figure 7. (a) Absorption spectral changes of Co (Ph Pc) (1.0 × 10 M) in the presence of formic acid (0.50 M) in O -saturated PhCN. (b) Time
8
2
profiles of absorbance at 760 nm. Inset: first-order plot.
Scheme 4
eq 9, the decay rate does not obey simple first-order kinetics, as
shown in Figure 7b.
II
d[Co (Ph Pc)]
8
−
dt
II
2
2
= 2
k k′ [Co (Ph Pc)] [O ][HCOOH]
et et
8
II
2
k
+ k′ [Co (Ph Pc)][HCOOH]
et
(9)
−
et
8
•
III
+
•
radical (HO ) ([Co (Ph Pc)] HO ) undergoes the reverse
2
8
2
reaction (k ) in competition with the subsequent proton-
−et
The first-order rate constant (k ) taken from the initial
slope in the first-order plots is given from eqs 3 and 9 by eq 10.
obs
II
coupled electron transfer from Co (Ph Pc) to yield H O and
8
2
2
III
+
2
equiv of [Co (Ph Pc)] .
8
II
II
2
In such a case, the decay rate of Co (Ph Pc) is given by eq 5,
k
obs
= {2k k′ [Co (Ph Pc)] [O ][HCOOH]
8
et et
8
0
2
where k is the rateconstant of proton-coupled electron transfer
2
et
/
(1 + K[HCOOH]) }
II
II
−
d[Co (Ph Pc)]/dt
8
/{k
+ k′ [Co (Ph Pc)] [HCOOH]
−
et
et
8
0
II
=
k [Co (Ph Pc)][O ][HCOOH] − k [C] + k′et
et
8
2
−et
/(1 + K[HCOOH])}
(10)
II
[
Co (Ph Pc)][HCOOH][C]
(5)
8
The kobs value increased linearly with increasing concentration
of O (Figure 8a) as expected by eq 10. The kobs values
exhibited saturation behaviors with increasing concentrations of
II
2
from Co (Ph Pc) to O , k is the rate constant of the reverse
8
2
−et
reaction, k′ is the rate constant of proton-coupled electron
et
II
II
•
HCOOH and Co (Ph Pc), as shown in Figure 8b,c,
8
transfer from Co (Ph Pc) to HO , K is the protonation
constant of Co (Ph Pc) with HCOOH, and [C] corresponds
to the concentration of the complex ([Co (Ph Pc)] HO ) in
8
2
II
respectively, which were consistent with eq 4. In order to
determine the ket value, eq 10 is rewritten by eq 11, which
8
III
+
•
8
2
Scheme 4. The rate of formation of the complex
−1
−
1
II
= {k [Co (Ph Pc)]
0
III
+
•
k
+ k′et
obs
−et
8
(
[Co (Ph Pc)] HO ) is given by eq 6. By subtracting eq 6
8 2
[
HCOOH](1 + K[HCOOH])}
II
d[C]/dt = k [Co (Ph Pc)][O ][HCOOH] − k [C]
et −et
8
2
2
/
/
{2k k′ [O ][HCOOH]
II
et et
2
−
k′ [Co (Ph Pc)][HCOOH][C]
et
(6)
8
(1 + K[HCOOH])2}
(11)
from eq 5, eq 7 is obtained. Because [C] remains nearly
constant during the reaction, [C] is given by eq 8 by applying
−
1
predicts a linear correlation between k
and
and
obs
II
−1
−1
(
[Co (Ph Pc)] ) . The linear correlation between k
8 0 obs
([Co (Ph Pc)] ) was obtained as shown in Figure S8 in the
Supporting Information. The inverse of the intercept is given
by eq 12, which corresponds to the formation rate of Me Fc
II
II
−1
−
d[Co (Ph Pc)]/dt − d[C]/dt
8 0
8
II
=
2k′ [Co (Ph Pc)][HCOOH][C]
(7)
(8)
+
et
8
2
II
C] = k [Co (Ph Pc)][O ][HCOOH]
2k [O ][HCOOH]
−
1
et
2
[
et
(k
8
2
(intercept)
=
1
+ K[HCOOH]
(12)
II
/
+ k′ [Co (Ph Pc)][HCOOH])
−
et
et
8
under the catalytic conditions (eq 4). The ket value was
II
−1
the steady-state approximation. The decay rate of Co (Ph Pc)
determined from the intercept of the linear plot of kobs vs
8
II
II
−1
2
−2 −1
in the proton-coupled electron transfer from Co (Ph Pc) to O
is derived from eqs 7 and 8 as shown in eq 9. As expected from
([Co (Ph Pc)] ) to be (1.6 ± 0.3) × 10 M s , which
8 0
8
2
2
−2 −1
agreed well with the k value ((1.4 ± 0.1) × 10 M s )
et
4
202
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Journal of the American Chemical Society
Article
II
−5
Figure 8. (a) Plot of kobs vs [O ]; [Co (Ph Pc)] = 1.0 × 10 M and [HCOOH] = 0.50 M. (b) Plots of k (black) and absorbance changes at
2
8
obs
II
+
II
−5
−3
II
8
60 nm due to [Co (Ph PcH)] (red) vs [HCOOH]; [Co (Ph Pc)] = 1.0 × 10 M and [O ] = 8.5 × 10 M. (c) Plot of kobs vs [Co (Ph Pc)] ;
8 8 2 8 0
3
−
[
HCOOH] = 0.50 M and [O ] = 8.5 × 10 M.
2
+
−1
−1
Figure 9. (a) Time profile of formation of Me Fc monitored by absorbance at 700 nm (ε = 240 M cm ) in electron-transfer oxidation of
10
−3
−3
II
−5
Me Fc (1.0 × 10 M) by O (8.5 × 10 M) catalyzed by Co (Ph Pc) (1.0 × 10 M) in the presence of formic acid (0.50 M) in PhCN. Inset:
first-order plot. (b) Plot of kobs vs [O ]. (c) Plot of k vs [Co (Ph Pc)] . (d) Plot of k vs [HCOOH].
10
2
8
II
2
obs
8
0
obs
determined from the catalytic conditions (Figure 6c,d). Such
Catalytic Mechanism of the Two-Electron Reduction
II
agreement of the k values determined from the single-turnover
of O
2
by Me10Fc with Co (Ph
8
Pc) in the Presence of an
et
Acid. When Me Fc was used as a one-electron reductant
conditions and catalytic conditions supports strongly that the
10
+
instead of Me Fc, the formation of Me Fc in the catalytic two-
electron reduction of O by Me Fc with Co (Ph Pc) in the
rate-determining step in the catalytic cycle in Scheme 2 is
2
10
II
II
indeed the proton-coupled electron transfer from Co (Ph Pc)
2 10 8
8
presence of HCOOH was completed within a few seconds.
Thus, we performed stopped-flow measurements to analyze the
to O2.
In the presence of TFA (pK = 0) instead of formic acid
a
+
kinetics. The rate of formation of Me Fc obeyed pseudo-
10
(
pK = 3.77), the two-electron reduction of O occurred in a way
a
2
first-order kinetics (Figure 9a), and the pseudo-first-order rate
similar to that for the case of formic acid. However, no electron
constant remained constant irrespective of O concentration
2
transfer occurred in the presence of acetic acid (pK = 4.76)
a
(
Figure 9b). Such independence of the pseudo-first-order rate
56
(
Figure S9 in the Supporting Information). This sharp con-
constant (k ) of the concentration of O is in sharp contrast to
obs
2
trast may stem from the relation among the pK values of super-
a
the case of Me Fc described above, in which the zero-order rate
2
•−
57
oxide anion (O2 , pK = 4.69) and those of acids mentioned
a
constant shows a linear dependence on the concentration of O2
(Figure 7d). This indicates that the proton-coupled electron
•
−
above, suggesting that the protonation of O is indispensable
2
II
II
to promote the electron transfer from Co (Ph Pc) to O .
transfer from Co (Ph Pc) to O is not the rate-determining
8
2
8
2
4
203
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Journal of the American Chemical Society
Article
+
step but that the proton-coupled one-electron reduction of
Me Fc obeyed pseudo-first-order kinetics (first order with
10
II
II
+
Co (Ph Pc) and [Co (Ph PcH)] by Me Fc may be the rate-
respect to concentration of Me Fc; Figure 9a), the k value
8
8
10
10 obs
determining step, as shown in Scheme 5, because Me Fc
was independent of concentration of O (Figure 9b), but
10
2
II
proportional to the initial concentration of Co (Ph Pc) (Figure 9c).
8
Scheme 5
The kobs value increased with increasing concentration of
HCOOH without exhibiting a saturation behavior, as expected
from eq 15. From Figure 9b, c, the second-order rate constant
of the proton-coupled electron transfer from Me Fc to
1
0
II
II
+
Co (Ph Pc) and [Co (Ph PcH)] in the presence of 0.50 M
8
8
5
−1 −1
HCOOH was determined to be (1.6 ± 0.1) × 10 M
98 K.
Proton-Coupled Electron Transfer from Me Fc to
s
at
2
10
II
+
[
Co (Ph PcH)] . The rate-determining step in the catalytic
8
cycle for the two-electron reduction of O by Me Fc with
2
10
II
+
[
Co (Ph PcH)] in Scheme 5 was studied independently
8
without O (vide infra). Because the electrochemical study
2
possess only 100 mV overpotential relative to the Co(II/I)
couple (Scheme 3). The rate-determining one-electron
II
revealed that the reduction potential of Co (Ph Pc) was
8
II
+
positively shifted by the protonation to 0.02 V vs SCE,
reduction of [Co (Ph PcH)] by Me Fc may be followed by
8
10
I
I
+
electron transfer from Me10Fc (Eox = −0.08 V vs SCE) to
rapid reduction of O by Co (Ph PcH) and [Co (Ph PcH )]
2
8
8
2
II
+
[
Co (Ph PcH)] is energetically feasible (Scheme 3). Indeed,
8
and the subsequent rapid proton-coupled one-electron
II
+
reduction of HO2• by Me Fc to yield Me Fc and H O .
+
electron transfer from Me10Fc to [Co (Ph PcH)] occurs in
8
10
10
2
2
the presence of HCOOH (0.50 M) in PhCN at 298 K. It
should be noted that no electron transfer from Me10Fc to
When Me Fc was replaced by a weaker one-electron reductant,
1
0
+
Me Fc, the rate constant of the formation of Me Fc also
8
8
II
Co (Ph Pc) (E = −0.31 V, Figure 3a) occurred without
8
red
obeyed first-order kinetics, but the kobs value was smaller as
+
HCOOH. The absorption spectral change in the electron-
transfer reaction was monitored by using a stopped-flow
compared with the value of Me Fc under the same
1
0
experimental conditions (Figure S7b in the Supporting
Information), as expected from the less negative one-electron
oxidation potential of Me Fc (E = −0.04 V) as compared with
technique as shown in Figure 10a, where the absorption band at
I
5
05 nm due to Co (Ph PcH) appeared, accompanied by a
8
8
ox
II
decrease in the absorption band at 760 nm due to Co (Ph Pc)
8
Me Fc (E = −0.08 V vs SCE). This also indicates that the
10
ox
II
+
and [Co (Ph PcH)] . The observed rate constant was deter-
8
rate-determining step is the proton-coupled electron-transfer
II
II
II
+
mined by the decay of Co (Ph
Pc) monitored from a decrease
8
reduction of Co (Ph Pc) and [Co (Ph PcH)] by Me Fc to
8
8
10
I
I
+
in absorbance at 760 nm, which coincides with formation
produce Co (Ph PcH) and [Co (Ph PcH )] , respectively.
8
8
2
I
+
of Co (Ph PcH), monitored by an increase in absorbance at
8
According to Scheme 5, the rate of formation of Me Fc is
given by eq 13, because electron transfer from Co (Ph PcH)
10
I
505 nm (inset of Figure 10a). The observed rate constant
obeyed pseudo-first-order kinetics in the presence of a
large excess of Me Fc, and the observed rate constants (k )
8
+
d[Me Fc ]/dt = 2(k + ket2[HCOOH])
10
obs
10
et1
increased with increasing concentration of Me Fc (Figure 10c).
10
II
+
[
Me Fc][[Co (Ph PcH)] ]
(13)
1
0
8
The kobs value also increased with increasing concentration
of HCOOH without exhibiting a saturation behavior, as
II
I
+
expected from the protonation equilibrium of Co (Ph Pc)
and [Co (Ph PcH )] to O2 and proton-coupled electron
8
8
2
−1
58
•
(
K = 3.6 M ), as shown in Figure 10b. This is attributed to
transfer from Me Fc to HO are much faster than electron
transfer from Me Fc to [Co (Ph PcH)] to produce
Co (Ph PcH) (k ) and proton-coupled electron transfer
1
0
2
I
+
II
+
further protonation to produce diprotonated [Co (Ph PcH )]
8
2
1
0
8
I
in the presence of large concentrations of HCOOH, as shown
in Scheme 6 (vide supra, Figure S10 in the Supporting
Information).
8
et1
II
+
I
+
from Me Fc to [Co (Ph PcH)] to produce [Co (Ph PcH )]
10
8
8
2
+
(
k [HCOOH]), when 2 equiv of Me Fc is formed.
et2 10
II
+
According to Scheme 6, the decay rate of [Co (Ph PcH)] is
According to the protonation equilibrium in Scheme 1, the
8
II
+
given by eq 16. Thus, the second-order rate constant of the
concentration of [Co (Ph PcH)] in eq 13 is given by eq 14.
8
II
+
II
+
−
d[[Co (Ph PcH)] ]/dt
[
[Co (Ph PcH)] ]
8
8
II
+
II
=
(k + ket2[HCOOH])[[Co (Ph PcH)] ]
=
K[Co (Ph Pc)] [HCOOH]/(1 + K[HCOOH])
et1
[Me Fc]
8
8
0
(14)
(16)
1
0
II
proton-coupled electron transfer from Me Fc to Co (Ph Pc)
10
8
+
II
+
d[Me Fc ]/dt = {2(k + ket2[HCOOH])K[HCOOH]
et1
10
and [Co (Ph PcH)] in the presence of 0.50 M HCOOH was
8
II
determined from the slope of the linear plot of k vs [Me Fc]
obs
10
[
Me Fc][Co (Ph Pc)] }
1
0
8
0
5
−1 −1
to be (1.1 ± 0.2) × 10 M s . By using this value, the
observed second-order rate constant under the catalytic
conditions with 0.50 M HCOOH in the presence of O2,
which corresponds to 2(ket1 + ket2[HCOOH])K[HCOOH]/
(1 + K[HCOOH]) in eq 15, was determined to be (1.7 ± 0.3) ×
/
{1 + K[HCOOH]}
(15)
From eqs 13 and 14, eq 15 is obtained. The kinetic formulation
derived from Scheme 5 (eq 15) agrees with the experimental
observations in Figure 9, where the rate of formation of
5
−1 −1
10 M s , which agreed well with the experimental value
4
204
dx.doi.org/10.1021/ja209978q | J. Am. Chem. Soc. 2012, 134, 4196−4206

Journal of the American Chemical Society
Article
II
−5
−4
Figure 10. (a) Absorption spectral changes of Co (Ph Pc) (1.0 × 10 M) in the presence of formic acid (0.50 M) and Me Fc (2.0 × 10 M) in
8
10
I
II
deaerated PhCN. Inset: time profiles of absorbance at 505 (black) and 760 nm (red) to Co (Ph PcH) and Co (Ph Pc), respectively. (b) Plot of k
vs [HCOOH]; [Co (Ph Pc)] = 1.0 × 10 M and [Me Fc] = 2.0 × 10 M. (c) Plot of kobs vs [Me Fc]; [Co (Ph Pc)] = 1.0 × 10 M and
8
8
obs
II
−5
−4
II
−5
8
10
10
8
[
HCOOH] = 0.50 M.
Scheme 6
ASSOCIATED CONTENT
Supporting Information
■
*
S
Text, figures, and CIF files giving absorption titration, time
course of kinetic measurements, and electrochemical measure-
1
ment data, H NMR and EPR spectra, and crystal structure
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
5
−1
AUTHOR INFORMATION
Corresponding Author
*E-mail: kojima@chem.tsukuba.ac.jp (T.K.); fukuzumi@
chem.eng.osaka-u.ac.jp (S.F.).
(
9
(1.6 ± 0.1) × 10 M s−1) obtained from the slopes in Figure
■
b,c (vide supra). Such agreement strongly indicates that the
rate-determining step in the catalytic cycle in Scheme 5 is
indeed the proton-coupled electron transfer from Me Fc to
10
II
II
+
Co (Ph Pc) and [Co (Ph PcH)] , respectively (Scheme 6).
Notes
8
8
The authors declare no competing financial interest.
SUMMARY AND CONCLUSION
■
ACKNOWLEDGMENTS
■
The present study has demonstrated the protonation of
This work was supported by a Grant-in-Aid (No. 20108010)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan (to S.F.), NRF/MEST of Korea through
WCU (R31-2008-000-10010-0) and GRL (2010-00353)
Programs (to S.F.). T.H. appreciates support from the Global
COE program “Global Education and Research Center for Bio-
Environmental Chemistry” of Osaka University and a JSPS
fellowship for young scientists.
II
Co (Ph Pc) significantly enhances the catalytic two-electron
8
reduction of O by a ferrocene derivative in the presence of an
2
II
acid. The X-ray crystal structure of the protonated Co (Ph Pc)
8
II
+
(
[Co (Ph PcH)] ) was successfully determined in comparison
8
II
II
with that of Co (Ph Pc). The protonation of Co (Ph Pc) has
8
8
been found to inhibit the direct reduction of O due to the
2
elevation of the Co(II)/Co(III) redox potential of the Co(II)
center, whereas the proton-coupled electron transfer from
II
II
+
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2
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obs
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−2 −1
0
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