Catalytic four-electron reduction of O2 via rate-determining proton-coupled electron transfer to a dinuclear cobalt-μ-1,2-peroxo complex

Article abstract of DOI:10.1021/ja303674n

Four-electron reduction of O₂ by octamethylferrocene (Me ₈Fc) occurs efficiently with a dinuclear cobalt-μ-1,2-peroxo complex, 1, in the presence of trifluoroacetic acid in acetonitrile. Kinetic investigations of the overall catalytic reaction and each step in the catalytic cycle showed that proton-coupled electron transfer from Me₈Fc to 1 is the rate-determining step in the catalytic cycle.

Full text of DOI:10.1021/ja303674n

Communication
pubs.acs.org/JACS
Catalytic Four-Electron Reduction of O2 via Rate-Determining
Proton-Coupled Electron Transfer to a Dinuclear Cobalt-μ-1,2-peroxo
Complex
Shunichi Fukuzumi,*,†,‡ Sukanta Mandal,‡,§ Kentaro Mase,† Kei Ohkubo,† Hyejin Park,‡
Jordi Benet-Buchholz,§ Wonwoo Nam,*,‡ and Antoni Llobet*,‡,§
†Department of Material and Life Science, Graduate School of Engineering, Osaka University, and ALCA, Japan Science and
Technology Agency, Suita, Osaka 565-0871, Japan
‡Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
§Institute of Chemical Research of Catalonia, Avinguda Països Catalans 16, E-43007 Tarragona, Spain
S
* Supporting Information
Complex 1 was synthesized by the reaction of Co(trpy)Cl2
with bpp under O2 in methanol [Figure 1; also see the
ABSTRACT: Four-electron reduction of O2 by octame-
thylferrocene (Me8Fc) occurs efficiently with a dinuclear
cobalt-μ-1,2-peroxo complex, 1, in the presence of
trifluoroacetic acid in acetonitrile. Kinetic investigations
of the overall catalytic reaction and each step in the
catalytic cycle showed that proton-coupled electron
transfer from Me8Fc to 1 is the rate-determining step in
the catalytic cycle.
he catalytic four-electron (4e−) reduction of O2 to water
T
has merited increasing attention because it is intrinsic to
respiration1 as well as fuel cell technology.2 In fuel cells, the 4e−
reduction of O2 is catalyzed at the cathode by Pt impregnated
in carbon.2,3 The high loadings of this precious metal that are
required to achieve appreciable activity have prompted the
development of catalysts based on nonprecious metals such as
Co, Cu, and Fe.4−7 In addition to electrocatalytic reduction of
O2 with nonprecious metal complexes, homogeneous catalytic
reduction of O2 by one-electron (1e−) reductants such as
ferrocene (Fc) derivatives has provided mechanistic insight into
the key question: How can the two-electron (2e−) versus 4e−
reduction of O2 be controlled by metal complexes?8−13 The key
intermediate for the catalytic 4e− reduction of O2 is usually
regarded to be a dinuclear metal−peroxo complex, which is
further reduced to water. However, there has been no study of
the electron-transfer (ET) reduction of isolated dinuclear
metal−peroxo complexes with structures established by X-ray
crystallography. Thus, the exact role of the dinuclear complex in
the catalytic 4e− reduction of O2 has yet to be uncovered.
We report that a dinuclear cobalt-μ-1,2-peroxo complex with
Figure 1. Syntheses of complexes 1 and 2.
Experimental Section in the Supporting Information (SI)]. The
aqua−hydroxo complex 2 was obtained by oxidation of
[CoII (trpy)2(μ-bpp)(μ-Cl)](PF6)2 with cerium(IV) ammo-
2
nium nitrate (Figure 1; also see the SI). Both 1 and 2 were
well-characterized by spectroscopic methods (Figures S3−S6 in
the SI) and X-ray crystallography (Figure 2). The X-ray
structure of 1 (Figure 2a) shows that the peroxo ligand is
coordinated to the two CoIII ions in a μ-1,2 fashion. The O−O
bond distance, 1.397(2) Å, is typical for dinuclear Co−peroxo
complexes.14 The structure of 2 (Figure 2b) shows that the
aqua and hydroxo ligands are coordinated to the two CoIII ions
and share a H atom. The O−O distance of 2.415 Å is long
enough to accommodate the H atom.
bis(pyridyl)pyrazolate (bpp) and terpyridine (trpy) ligands,
[CoIII2(trpy)2(μ-bpp)(μ-1,2-O2)]3+ (1), catalyzes the 4e−
reduction of O2 by Fc derivatives in the presence of
trifluoroacetic acid (TFA) in MeCN at 298 K. The structure
of 1 was successfully determined by X-ray crystallography.
Kinetic studies of the overall catalytic cycle and each step in the
catalytic cycle enabled us to show for the first time that proton-
coupled electron transfer (PCET) reduction of 1 is the rate-
determining step in the overall catalytic 4e− reduction of O2.
The 4e− reduction of O2 by octamethylferrocene (Me8Fc)
occurred with a catalytic amount of 1 in the presence of TFA in
MeCN at 298 K (Figure 3a), with complete conversion of
Received: April 17, 2012
Published: June 1, 2012
© 2012 American Chemical Society
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Figure 2. ORTEP plots (50% probability) of the crystal structures of
the cationic parts of (a) 1 and (b) 2. H atoms, counteranions, and
solvent molecules of crystallization have been omitted for clarity.
Figure 4. Plots of (a) kobs vs [1] in O2-saturated MeCN and (b) kobs vs
[O2] with [1] = 25 μM for the 4e− reduction of O2 by Me8Fc (1.5
mM) in the presence of TFA (50 mM) in MeCN.
Figure 3. (a) Absorption spectral changes in the 4e− reduction of O2
by Me8Fc (2 mM) catalyzed by 1 (25 μM) in the presence of TFA (50
mM) in O2-saturated MeCN at 298 K. The black and red lines show
the spectra before and after addition of TFA, respectively. The dotted
line is the absorbance at 750 nm due to 2 mM Me8Fc+. (b) Time
profile of the absorbance at 750 nm due to Me8Fc+.
Figure 5. (a) Plot of kcat vs TFA concentration for the catalytic 4e−
reduction of O2 by Me8Fc (1.5 mM) in O2-saturated MeCN at 298 K.
(b) Plot of kET vs TFA concentration for the reaction involving ET
from Me8Fc (1.5 mM) to 1 (25 μM) in deaerated MeCN at 298 K.
Me8Fc to Me8Fc+ (λmax = 750 nm, ε = 410 M−1 cm−1).
Iodometric titrations confirmed that no H2O2 was formed after
the reaction was complete (Figure S7). Thus, 1 catalyzes the
4e− reduction of O2 by Me8Fc in the presence of TFA in
MeCN:
1
4Me8Fc + O2 + 4H+ → 4Me8Fc+ + H2O
(1)
The time profile in Figure 3b indicates that this process
proceeds in a single step rather than by stepwise reduction of
O2 to H2O2 and then to H2O. When 1 was replaced by 2, the
4e− reduction of O2 by Me8Fc occurred at the same rate
(Figure S8).
Figure 6. (a) Absorption spectral changes in the reduction of 1 (50
μM) by Me8Fc (1.5 mM) in the presence of TFA (50 mM) in
deaerated MeCN. (b) Time profiles of the absorbance at 545 nm due
to decreasing 1 (black) and 750 nm due to increasing Me8Fc+ (red).
The formation of Me8Fc+ obeyed pseudo-first-order kinetics
(Figure 3b). The pseudo-first-order rate constant (kobs
)
increased linearly with increasing concentration of 1 (Figure
4a) but did not depend on the O2 concentration (Figure 4b).
Thus, the kinetic equation is
band at 750 nm due to Me8Fc+. The decay rate of 1 coincided
with the rate of formation of Me8Fc+ (Figure 6b). The
stoichiometry of the PCET reduction of 1 by Me8Fc was
determined to be
d[Me8Fc+]
= kcat[Me8Fc][1]
(2)
dt
where kcat is the second-order rate constant for the catalytic 4e−
reduction of O2 by Me8Fc. kcat increased with increasing TFA
concentration with a nonzero intercept (Figure 5a). The 4e−
reduction of O2 by decamethylferrocene (Fc*) catalyzed by 1
in the presence of TFA in MeCN at 298 K occurred more
efficiently than that by Me8Fc (Figure S9).15
4Me8Fc + [CoIII(O2)CoIII]3+ + 4H+
→ 4Me8Fc+ + [CoII−CoII]3+ + 2H2O
(3)
The ET from Me8Fc to 1 in the presence of TFA obeyed
pseudo-first-order kinetics (Figure S10a), and kobs increased
linearly with increasing [Me8Fc] concentration (Figure S10b).
Thus, the decay rate of 1 is given by
The 4e− reduction of O2 requires the reduction of 1 by
Me8Fc. Thus, we examined the reduction of 1 by Me8Fc in the
absence and presence of TFA in deaerated MeCN. In the
absence of TFA, no reduction of 1 by Me8Fc was observed. In
the presence of TFA, however, 1 was efficiently reduced by
Me8Fc (Figure 6a), as the absorption band at 545 nm due to 1
disappeared, accompanied by the appearance of the absorption
d[Me Fc+]
d[1]
dt
1
4
8
−
=
= kET[1][Me8Fc]
(4)
dt
where kET is the second-order rate constant for ET from Me8Fc
to 1 in the presence of TFA. Because 4 equiv of Me8Fc+ are
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produced by ET from Me8Fc to 1 in the presence of TFA, the
decay rate of 1 is equal to one-fourth the formation rate of
Me8Fc+ (eq 4). The kET value increased linearly with increasing
TFA concentration with a nonzero intercept (Figure 5b). The
intercept corresponds to ET from Me8Fc to 1, which is
followed by much faster PCET, and the linear dependence of
kET on [TFA] suggests that initial ET from Me8Fc to 1 is
accelerated by the acid. Although no protonation of 1 by TFA
was observed, as indicated by the absence of absorption spectral
changes for 1 in the presence of TFA (Figure S11a), absorption
spectral changes due to demetalation of 1 were observed in the
presence of HClO4, which is a much stronger acid than TFA
(Figure S11b). Since kET increased linearly with increasing TFA
concentration (Figure 5b), ET from Me8Fc to protonated 1,
whose concentration is proportional to the TFA concentration,
may be much faster than ET from Me8Fc to 1.
Figure 8. CV of 2 in MeCN (0.25 mM) in the presence of 0.10 M
TBAPF6. Sweep rate = 100 mV s−1.
respectively. On the basis of the Ered values for 2, ET from
Me8Fc to 2 to produce 2 equiv of Me8Fc+ and the CoII
2
Comparison of kcat and kET at various TFA concentrations
(Figure 5) showed kcat always to be 4 times as large as kET. This
indicates that the PCET reduction of 1 by Me8Fc is the rate-
determining step in the overall catalytic cycle of the 4e−
reduction of O2 by Me8Fc in the presence of TFA in MeCN.
The kET values for ET from Fc* to 1 in the presence of TFA
in MeCN were also determined at 298 K and found to be ∼10
times larger than those for Me8Fc, as expected given the lower
1e− oxidation potential of Fc* (Eox = −0.08 V vs SCE) than of
Me8Fc (Eox = −0.04 V vs SCE).9
complex is thermodynamically feasible. In fact, ET from Me8Fc
to 2 occurs in two steps with rate constants of 3.2 × 105 and 1.6
× 103 M−1 s−1 for the first and second steps, respectively
(Figures S12 and S13).
When 2 was reduced by 2 equiv of Me8Fc, the peroxo
complex was produced by the reaction of the reduced 2 with O2
(Figure 9a), as the absorption band due to 1 (λmax = 545 nm)
To confirm the thermodynamics of PCET from Me8Fc to 1,
cyclic voltammograms (CVs) of 1 were measured in the
absence and presence of TFA in MeCN (Figure 7). In the
Figure 9. (a) Transient absorption spectral changes in the reaction of
2 (50 μM) with Me8Fc (100 μM) in the presence of TFA (30 mM) in
air-saturated MeCN. (b) Time profiles of the absorbance at 545 nm
due to 1 in the reaction of 2 with Me8Fc in the presence of various
concentrations of TFA (100 μM to 30 mM) in air-saturated MeCN.
appeared together with that of Me8Fc+ (λmax = 750 nm). The
rate of formation of 1 increased with increasing TFA
concentration (Figure 9b). Because the formation of 1 is
much faster than the PCET reduction of 1,17 the formation of 1
in the catalytic cycle is not the rate-determining step.
Figure 7. CVs of 1 in MeCN (1 mM) without TFA in the presence of
0.10 M TBAPF6 (black) and with 2 mM TFA (red). Sweep rate = 100
mV s−1.
The overall catalytic cycle is summarized in Scheme 1. When
the reaction is started from peroxo complex 1, 1 is reduced by
PCET from Me8Fc. Because the decay rate of 1 coincides with
the rate of formation of 4 equiv of Me8Fc+, once 1 is reduced by
PCET, the subsequent PCET processes should be much faster
absence of TFA, the reversible CoIII2/CoIIICoII and CoIVCoIII/
CoIII couples were observed at E1/2 = −0.27 and +1.47 V vs
2
SCE, respectively. The cathodic peak current observed at −0.75
V corresponds to the reduction of CoIIICoII to CoII . However,
2
this reduction was irreversible, probably because of the O−O
bond cleavage in the dissociative ET reduction. In the presence
Scheme 1
of TFA, the reversible CoIVCoIII/CoIII couple remained the
2
same as that in the absence of TFA, whereas the CoIII
/
2
CoIIICoII reduction peak was shifted in the positive direction.16
This is consistent with the previous results for ET from Me8Fc
to 1, as ET from Me8Fc (Eox = −0.04 V vs SCE) to 1 (Ered
=
−0.27 V vs SCE) is thermodynamically unfavorable but
becomes energetically feasible in the presence of TFA.
Figure 8 shows a CV for aqua−hydroxo complex 2. The two
reversible couples observed at +0.31 and −0.04 V vs SCE
correspond to the first 1e− reduction of 2 to the CoIIICoII
complex and the second 1e− reduction to the CoII complex,
2
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(5) (a) Collman, J. P.; Devaraj, N. K.; Decrea
́
u, R. A.; Yang, Y.; Yan,
than the initial PCET process. The reduction of 1 by Me8Fc
can proceed via two pathways. One involves ET from Me8Fc to
1 followed by protonation of the reduced 1, which results in
O−O bond cleavage. This is followed by rapid PCET to
produce 2. The other pathway involves coupling of the ET and
the protonation, which occur in a concerted manner, since the
PCET rate constant increases linearly with increasing TFA
concentration (Figure 5). After 1 is converted to 2, a fast ET
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́
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reduction of 2 by Me8Fc occurs, producing the CoII complex,
Chen, P.; Ojaimi, M. E.; Chkounda, M.; Gros, C. P.; Barbe, J.-M.;
2
which reacts with O2 to regenerate 2. When the reaction is
started from 2, 2 is converted rapidly to 1 by ET reduction of 2
and the reaction of the CoII complex with O2, both of which are
much faster than the PCET reduction of 1. Thus, the same
catalytic rate is observed starting from 1 or 2. Because the rate
of formation of 1 from 2 with Me8Fc increases with increasing
TFA concentration (Figure 9), this process may also proceed
via a PCET pathway, although the detailed mechanism has yet
to be clarified.18
Ohkubo, K.; Fukuzumi, S.; Guilard, R. Inorg. Chem. 2008, 47, 6726.
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(11) Halime, Z.; Kotani, H.; Li, Y.; Fukuzumi, S.; Karlin, K. D. Proc.
Natl. Acad. Sci. U.S.A. 2011, 108, 13990.
In conclusion, the 4e− reduction of O2 by Me8Fc in the
presence of TFA is efficiently catalyzed by the dinuclear CoIII−
peroxo complex 1 as well as the aqua−hydroxo complex 2. The
X-ray structures of both 1 and 2 were successfully determined.
Kinetic analyses of the overall catalytic process and each
catalytic step revealed that PCET reduction of 1 is the rate-
determining step in the overall catalytic cycle, which may not be
the same as the natural process in respiration, however.
ASSOCIATED CONTENT
■
(12) (a) Peljo, P.; Murtomaki, L.; Kallio, T.; Xu, H.-J.; Meyer, M.;
̈
S
* Supporting Information
Gros, C. P.; Barbe, J.-M.; Girault, H. H.; Laasonen, K.; Kontturi, K. J.
Experimental section, additional data, and complete ref 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
́
Am. Chem. Soc. 2012, 134, 5974. (b) Su, B.; Hatay, I.; Trojanek, A.;
Samec, Z.; Khoury, T.; Gros, C. P.; Barbe, J.-M.; Daina, A.; Carrupt,
P.-A.; Girault, H. H. J. Am. Chem. Soc. 2010, 132, 2655. (c) Hatay, I.;
́
Su, B.; Li, F.; Mendez, M. A.; Khoury, T.; Gros, C. P.; Barbe, J.-M.;
AUTHOR INFORMATION
Ersoz, M.; Samec, Z.; Girault, H. H. J. Am. Chem. Soc. 2009, 131,
13453.
■
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp; wwnam@ewha.ac.kr;
allobet@ICIQ.ES
(13) (a) Devoille, A. M. J.; Love, J. B. Dalton Trans. 2012, 41, 65.
(b) Askarizadeh, E.; Yaghoob, S. B.; Boghaei, D. M.; Slawin, A. M. Z.;
Love, J. B. Chem. Commun. 2010, 46, 710.
Notes
(14) (a) Givaja, G.; Volpe, M.; Edwards, M. A.; Blake, A. J.; Wilson,
C.; Schroder, M.; Love, J. B. Angew. Chem., Int. Ed. 2007, 46, 584.
The authors declare no competing financial interest.
̈
(b) Volpe, M.; Hartnett, H.; Leeland, J. W.; Wills, K.; Ogunshun, M.;
Duncombe, B. J.; Wilson, C.; Blake, A. J.; McMaster, J.; Love, J. B.
Inorg. Chem. 2009, 48, 5195.
(15) When Me8Fc was replaced by 1,1′-dimethylferrocene (Me2Fc),
which is a weaker electron donor, no catalytic 4e− reduction of O2
occurred in the presence of TFA in MeCN because the ET reduction
of 1 by Me2Fc is thermodynamically unfavorable.
(16) Although detailed analysis of the CV at negative potentials (ca.
−1 V) has yet to be performed, these peaks may correspond to the 1e−
and 2e− reduction processes of CoII2 complex 2, which were observed
in the CV of 2 (Figure 8), as the ET reduction of 1 in the presence of
TFA results in O−O bond cleavage (eq 3).
ACKNOWLEDGMENTS
■
This work was supported in part by Grants-in-Aid (20108010
to S.F. and 23750014 to K.O.) and a Global COE Program (to
S.F.) from MEXT of Japan, by NRF/MEST of Korea through
WCU Project R31-2008-000-10010-0 (to W.N.), and by
MICINN of Spain (CTQ2010-21497 to A.L.).
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