Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins

Article abstract of DOI:10.1021/ja048403c

The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co ₂(DPX)]. The other cofacial dicobalt porphyrins [Co₂(DPA), Co₂(DPB), and Co₂(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co₂(DPX). The μ-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting μ-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the μ-superoxo species. The superhyperfine coupling constant of the μ-superoxo species of Co₂(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co₂(DPX) for the four-electron reduction of dioxygen by Fe(C₅H₄Me)₂ results from the strong binding of the reduced oxygen with Co₂(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be 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 turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP) ⁺, whereas the turnover-determining step in the Co ₂(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co ₂(DPX), depending on the electron donor ability of ferrocene derivatives.

Full text of DOI:10.1021/ja048403c

Published on Web 07/29/2004
Mechanism of Four-Electron Reduction of Dioxygen to Water
by Ferrocene Derivatives in the Presence of Perchloric Acid
in Benzonitrile, Catalyzed by Cofacial Dicobalt Porphyrins
Shunichi Fukuzumi,*,† Ken Okamoto,† Claude P. Gros,‡ and Roger Guilard*,‡
Contribution from the Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, CREST, Japan Science and Technology Agency,
Suita, Osaka 565-0871, Japan, and LIMSAG, UMR 5633, Faculte´ des Sciences Gabriel,
UniVersite´ de Bourgogne, 6, bd Gabriel, 21100 Dijon, France
Received March 20, 2004; E-mail: fukuzumi@ap.chem.eng.osaka-u.ac.jp; roger.guilard@u-bourgogne.fr
Abstract: The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin
[Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt
porphyrin [Co2(DPX)]. The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB), and Co2(DPD)] also
catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the
case of Co2(DPX). The µ-superoxo species of cofacial dicobalt porphyrins were produced by the reactions
of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-
electron oxidation of the resulting µ-peroxo species by iodine. The superhyperfine structure due to two
equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the µ-superoxo species.
The superhyperfine coupling constant of the µ-superoxo species of Co2(DPX) is the largest among those
of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co2(DPX) for the four-electron
reduction of dioxygen by Fe(C5H4Me)2 results from the strong binding of the reduced oxygen with Co2(DPX)
which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic
two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be 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 turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an
electron transfer from ferrocene derivatives to Co(OEP)+, whereas the turnover-determining step in the
Co2(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O
bond cleavage of the peroxo species of Co2(DPX), depending on the electron donor ability of ferrocene
derivatives.
brane proton translocation.6,7 The resulting proton gradient is
used as the driving force for generation of ATP as protons flow
Introduction
The highly exergonic four-electron reduction of oxygen to
water is essential to maintain the life of an aerobic organism
by the respiration.1-3 Cytochrome c oxidases (CcOs) located
in the inner mitochondrial membrane are the terminal enzymes
of the respiratory chains, catalyzing the reduction of molecular
oxygen to water by the soluble electron carrier, cytochrome c.1-3
The X-ray structures of CcOs have revealed that the catalytic
site of CcOs consists of the bimetallic complex of heme a and
Cu (Fea3/CuB) where the distance between Fea3 and CuB has
been reported as 4.5 Å in the absence of O2.4-6 On the matrix
side of the membrane, four protons are consumed for each
oxygen molecule reduced, and this is coupled to the intramem-
back through the membrane via the enzyme ATP synthase.6,7
A number of synthetic Fea3/CuB analogues have been
synthesized to mimic the coordination environment of the Fe/
Cu core as well as the catalytic function of the four-electron
reduction of O2.8-13 Although the four-electron reduction of O2
is not only of great biological interest9-13 but also of techno-
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‡ Universite´ de Bourgogne.
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10.1021/ja048403c CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 10441-10449
10441

A R T I C L E S
Fukuzumi et al.
Scheme 1
logical significance such as fuel cells,14-17 electrocatalytic
reduction of O2 has been the only method to probe the catalytic
reactivity of synthetic CcO model complexes. The most
important question of how the CcO enzyme catalyzes the four-
electron reduction of O2 to water without releasing the two-
electron reduced species (H2O2) has yet to be well understood.
Comparison of the catalytic reactivities of the biomimetic
complexes in the FeCu and Cu-free forms indicate that Cu does
not significantly affect the turnover frequency or the stability
of the catalyst and do not improve catalysis.18,19 The O2 binding
to Fe at one end and to Cu at the other is not necessary, and
only the iron porphyrin itself has been reported to be essential
for the four-electron reduction of O2.18-20 In contrast, dicobalt
bisporphyrins efficiently electrocatalyze the direct four-electron
reduction of O2 to water,21-23 whereas single cobalt porphyrins
can act as the four-electron reduction catalysts only through
self-assembly on the electrode surface.24 The difficulty in
determining the self-assembled structure on the electrode surface
has precluded the actual role of the bimetallic systems in the
four-electron reduction of O2. Thus, it is highly desired to study
the catalytic four-electron reduction of O2 vs two-electron
reduction of O2 using a one-electron reductant in a homogeneous
solution. Although cytochrome c is used as a one-electron
reductant in CcOs, no catalytic systems of four-electron reduc-
tion of O2 by a one-electron reductant in a homogeneous system
has ever been clarified quantitatively.25,26
purchased from Tokyo Kasei Organic Chemicals, Japan, and distilled
over P2O5 prior to use.30 Ferrocene (Wako Pure Chemicals, Japan),
1,1′-dimethylferrocene (Aldrich Co., USA), and decamethylferrocene
(9) Collman, J. P.; Boulatov, R.; Sunderland, C. J. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds; Elsevier Science: USA,
2003; Vol. 11, pp 1-49.
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We report herein efficient four-electron reduction of dioxygen
by ferrocene derivatives which are one-electron reductants,
catalyzed by cofacial dicobalt porphyrins in the presence of
perchloric acid (HClO4) in benzonitrile (PhCN) as shown in
Scheme 1. The cofacial dicobalt porphyrins are chosen as
catalysts for the four-electron reduction of O2, because the
bimetallic system is indispensable for the four-electron reduction
of oxygen in the case of cobalt porphyrins, thus allowing for
the difference from the single metal system to be clarified.
Detailed kinetic comparison of catalytic reactivities of cofacial
dicobalt porphyrins and a single cobalt porphyrin and detection
of the reactive intermediates by ESR provide valuable insights
into the question how the bimetallic system catalyzes the four-
electron reduction of O2 to water without releasing the two-
electron reduced species (H2O2).
(18) Cu suppresses superoxide-releasing autoxidation of oxygenated catalyst and
accelerates O2 binding and minimizes O-O bond homolysis in the reduction
of H2O2. See: (a) Collman, J. P.; Fudickar, W.; Shiryaeva, I. Inorg. Chem.
2003, 42, 3384. (b) Reference 10d.
(19) Cu might be in a suitable electronic environment to interact properly with
the iron-bound O2 in such a way that the cleavage of the O-O bond occurs.
See: (a) Ricard, D.; Andrioletti, B.; L’Her, M.; Boitrel, B. Chem. Commun.
1999, 1523. (b) Ricard, D.; L’Her, M.; Richard, P.; Boitrel, B. Chem.s
Eur. J. 2001, 7, 3291. (c) Didier, A.; L’Her, M.; Boitrel, B. Org. Biomol.
Chem. 2003, 1, 1274.
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Y.; Reinhard, R. Electroanalysis 1997, 9, 1288. (d) Wan, G.-X.; Shigehara,
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102, 6027.
Experimental Section
Materials. All solvents and chemicals were of reagent grade quality,
obtained commercially and used without further purification except as
noted below. 2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine cobalt-
(II) [Co(OEP)] was purchased by Aldrich Co., USA. The PhCN solution
of Co(III)(OEP) was produced by addition of the trace of perchloric
acid. Details on the synthesis and characterization of each cofacial
dicobalt porphyrin [Co2(DPB), Co2(DPA), Co2(DPX), and Co2(DPD)]
have been reported elsewhere.22a,23a,27,28 Preparation of 1-tert-butyl-5-
phenylimidazole has been described.29 Benzonitrile (PhCN) was
(23) (a) Deng, Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122,
410. (b) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.; Nocera,
D. G. Chem. Commun. 2000, 1355. (c) Le Mest, Y.; Inisan, M.; Laoue´nan,
A.; L’Her, M.; Talarmin, J.; El Khalifa, M. E.; Saillard, J.-Y. J. Am. Chem.
Soc. 1997, 119, 6095. (d) Steiger, B.; Anson, F. C. Inorg. Chem. 2000, 39,
4579. (e) Zou, S.; Clegg, R. S.; Anson, F. C. Langmuir 2002, 18, 3241.
(24) D’Souza, F.; Hsieh, Y.-Y.; Deviprasad, G. R. Chem. Commun. 1998, 1027.
(25) For the catalytic two-electron reduction of O2 in a homogeneous solution,
see: (a) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28,
2459. (b) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1990, 29,
653. (c) Anson, F. C.; Ni, C.-L.; Saveant, J.-M. J. Am. Chem. Soc. 1985,
107, 3442.
9
10442 J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004

Four-Electron Reduction of Dioxygen to Water
A R T I C L E S
(Wako Pure Chemicals) were obtained commercially and purified by
sublimation or recrystallization from ethanol. Perchloric acid (70%),
hydrogen peroxide, and iodine were obtained from Wako Pure
Chemicals. Tetra-n-butylammonium perchlorate (TBAP) was purchased
from Fluka Chemical Co., twice recrystallized from absolute ethanol
and dried in a vacuum at 45 °C prior to use. Tris(2,2′-bipyridyl)-
ruthenium(III) hexafluorophosphate [Ru(bpy)3(PF6)3] was prepared
according to the literature.31
derivative was transferred by means of a glass syringe to the reactant
reservoirs of the stopped flow apparatus which were already purged
with a stream of argon. Co(III) porphyrins were prepared by the
oxidation of Co(II) porphyrins by O2 in the presence of an acid.35,36
The kinetic measurements of electron transfer were carried out under
the pseudo-first-order conditions where concentrations of ferrocene
derivatives were maintained at an excess of more than 10 times the
Co(III) porphyrin concentration.
Spectral Measurements. The amount of hydrogen peroxide (H2O2)
was determined by titration by iodide ion.32 The aliquots of the product
mixture in PhCN was treated with excess NaI, and the amount of I3
ESR Measurements. The ESR spectra of µ-superoxo (Co-O2-
Co) species in Co2(DPA), Co2(DPB), Co2(DPX), and Co2(DPD) were
produced by the chemical oxidation of the dicobalt porphyrins (∼1 ×
10-3 M) containing the bulky base, i.e., 1-tert-butyl-5-phenylimidazole
(5 × 10-3 M), with a trace of iodine in deaerated PhCN under an
atmospheric pressure of oxygen according to the literature procedure.37
The solution containing the µ-superoxo species was transferred to an
ESR tube under an atmospheric pressure of oxygen. The ESR spectra
were taken on a JEOL X-band spectrometer (JES-RE1XE) with a quartz
ESR tube (1.2 mm i.d.). The ESR spectra were recorded under
nonsaturating microwave power conditions. The magnitude of modula-
tion 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 super hyperfine coupling constants were
determined by computer simulation using a Calleo ESR Version 1.2
program coded by Calleo Scientific on a personal computer.
-
formed was determined by the UV-visible spectrum (λmax ) 365 nm,
ꢀmax ) 28 000 M-1 cm-1 33 using a Hewlett-Packard 8453 diode array
)
spectrophotometer with a quartz cuvette (path length ) 10 mm) at 298
K.
Kinetic Measurements. All kinetic measurements were performed
on a UNISOKU RSP-601 stopped-flow spectrophotometer with the
MOS-type high selective photodiode array at 298 K using a Unisoku
thermostated cell holder. Rates of oxidation of ferrocene derivatives
by O2 in the presence of a catalytic amount of Co(OEP) or cofacial
dicobalt porphyrin and HClO4 in aerated PhCN at 298 K were
determined by monitoring the appearance of the absorption band due
to ferricenium ions (Fe(C5H5)2+: λmax ) 620 nm, ꢀmax ) 330 M-1 cm-1
.
Fe(C5H4Me)2+: λmax ) 650 nm, ꢀmax ) 290 M-1 cm-1. Fe(C5Me5)2
:
+
λmax ) 700 nm, ꢀmax ) 240 M-1 cm-1). The ꢀmax values of ferricenium
ions were determined by the electron-transfer oxidation of ferrocene
derivatives with Ru(bpy)3(PF6)3. At the monitoring wavelengths, there
is no spectral overlap with cobalt porphyrins (Co(II)(OEP): λmax ) 396
nm (107 000 M-1 cm-1), 518 nm (9200 M-1 cm-1), 552 nm (15 000
M-1 cm-1). Co(III)(OEP)+: λmax ) 417 nm (150 000 M-1 cm-1), 526
nm (9100 M-1 cm-1), 558 nm (10 000 M-1 cm-1). Co(II)Co(II)-
(DPD): λmax ) 397 nm (212 000 M-1 cm-1), 526 nm (10 200 M-1
Cyclic Voltammetry. Cyclic voltammetry measurements were
performed at 298 K on a BAS 100 W electrochemical analyzer in
deaerated PhCN containing 0.1 M tetra-n-butylammonium perchlorate
(TBAP) as supporting electrolyte. A conventional three-electrode cell
was used with a platinum working electrode (surface area of 0.3 mm2)
and a platinum wire as the counter electrode. The Pt working electrode
(BAS) was routinely polished with a BAS polishing alumina suspension
and rinsed with acetone before use. The measured potentials were
recorded with respect to the Ag/AgNO3 (0.01 M) reference electrode.
All potentials (vs Ag/Ag+) were converted to values vs SCE by adding
0.29 V.38 All electrochemical measurements were carried out under an
atmospheric pressure of argon.
cm-1), 555 nm (16 100 M-1 cm-1). Co(III)Co(III)(DPD)2+: λmax
)
420 nm (243 000 M-1 cm-1), 533 nm (14 900 M-1 cm-1), 563 nm
(10 000 M-1 cm-1)). An air-saturated PhCN solution was used for the
catalytic reduction of oxygen by ferrocene derivatives. The O2
concentration in an air-saturated PhCN solution (1.7 × 10-3 M) was
determined by the spectroscopic titration for the photooxidation of 10-
methyl-9,10-dihydroacridine by O2 described as reported previously.34
The concentrations of ferrocene derivatives larger than the O2 concen-
tration were used for the catalytic reduction of O2 by ferrocene
derivatives, when oxygen is the limiting reagent in the reaction cell
which is filled with the reactant solution. In contrast, the concentrations
of ferrocene derivatives smaller than the O2 concentration were used
in O2-saturated PhCN, when ferrocene derivatives are the limiting
reagents.
Results and Discussion
Catalytic Two-Electron vs Four-Electron Reduction of O2
by Ferrocene Derivatives. No oxidation of ferrocene deriva-
tives occurs by O2 in the presence of HClO4 in benzonitrile
(PhCN) at 298 K. The addition of cobalt porphyrin catalysts
and HClO4 to air-saturated PhCN solutions of ferrocene
derivatives results in efficient oxidation of ferrocene derivatives
by O2. The formation of ferrocene derivatives was monitored
by a rise in absorbance at 620-700 nm due to ferricenium ions
(see Experimental Section, e.g., ꢀ(650 nm) ) 290 M-1 cm-1
for Fe(C5H4Me)2+). Figure 1a shows the time course of
formation of 1,1′-dimethylferricenium ion Fe(C5H4Me)2+ in the
reduction of O2 (1.7 × 10-3 M)34 by a large excess of 1,1′-
dimethylferrocene [Fe(C5H4Me)2] (0.10 M) in the presence of
catalytic amounts of Co(OEP) (7.2 × 10-6-1.7 × 10-5 M) and
(35) Formation of Co(III)(OEP)+ was confirmed by the appearance of Co(III)-
(OEP)+ (λmax ) 417 nm) together with the disappearance of Co(II)(OEP)
(λmax ) 395 nm) upon addition of 5.0 × 10-3 M HClO4 to a air-saturated
PhCN solution of Co(II)(OEP). See: Setsune, J.; Saito, Y.; Ishimaru, M.;
Ikeda, M.; Kitao, T. Bull. Chem. Soc. Jpn. 1992, 65, 639.
(36) For the electron-transfer oxidation of Co(II) porphyrins to Co(III) porphy-
rins, see: (a) Fukuzumi, S.; Ohkubo, K. Chem.sEur. J. 2000, 6, 4532. (b)
Fukuzumi, S.; Kitaguchi, H.; Suenobu, T.; Ogo, S. Chem. Commun. 2002,
1984.
Rates of electron transfer from ferrocene derivatives to Co(III)
porphyrins in deaerated PhCN were determined by the decay of the
spectrum at 417 nm due to Co(III)(OEP)+ or at 420 nm due to Co2-
(DPX)+ using the stopped-flow apparatus mentioned above. A deaerated
PhCN solution containing a Co(III) porphyrin and that of a ferrocene
(26) Redox titrations with O2 and cobaltocene were reported using a cobalt(II)
porphyrin with a copper(I) triazacyclononane macrocycle as a cytochrome
c oxidase model, and the O2 adduct was fully reduced to the deoxygenated
form with 4 equiv of cobaltocene.10a See also: Collman, J. P.; Herrmann,
P. C.; Boitrel, B.; Zhang, X.; Eberspacher, T. A.; Fu, L.; Wang, J. L.;
Rousseau, D. L.; Williams, E. R. J. Am. Chem. Soc. 1994, 116, 9783.
(27) Guilard, R.; Lopez, M. A.; Tabard, A.; Richard, P.; Lecomte, C.; Brande`s,
S.; Hutchison, J. E.; Collman, J. P. J. Am. Chem. Soc. 1992, 114, 9877.
(28) Chang, C. J.; Deng, Y.; Heyduk, A. F.; Chang, C. K.; Nocera, D. G. Inorg.
Chem. 2000, 39, 959.
(29) Van Leusen, A. M.; Schaart, F. J.; Van Leusen, D. Recl. J. R. Neth. Chem.
Soc. 1979, 98, 258.
(30) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
(31) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
(32) Mair, R. D.; Graupner, A. J. Anal. Chem. 1964, 36, 194.
(33) Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Am. Chem. Soc. 1985, 107, 3020.
(34) (a) Fukuzumi, S.; Imahori, H.; Yamada, H.; El-Khouly, M. E.; Fujitsuka,
M.; Ito, O.; Guldi, D. M. J. Am. Chem. Soc. 2001, 123, 2571. (b) Fukuzumi,
S.; Ishikawa, M.; Tanaka, T. J. Chem. Soc., Perkin Trans. 2 1989, 1037.
(37) (a) Chang, C. K. J. Chem. Soc., Chem. Commun. 1977, 800. (b) Le Mest,
Y.; L’Her, M.; Courtot-Coupez, J.; Collman, J. P.; Evitt, E. R.; Bencosme,
C. S. J. Chem. Soc., Chem. Commun. 1983, 1286. (c) Guilard, R.; Je´roˆme,
F.; Gros, C. P.; Barbe, J.-M.; Ou, Z.; Shao, J.; Kadish, K. M. C. R. Acad.
Sci., Ser. IIc: Chim. 2001, 4, 245.
(38) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous
Systems; Marcel Dekker: New York, 1990.
9
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A R T I C L E S
Fukuzumi et al.
Figure 2. Time profiles of formation of Fe(C5H4Me)2+ monitored at 650
nm (ꢀ ) 290 M-1 cm-1) in electron-transfer oxidation of Fe(C5H4Me)2
(1.0 × 10-1 M) by O2 (1.7 × 10-3 M), catalyzed by Co2(DPX) (2.0 ×
10-5 M), Co2(DPA) (2.0 × 10-5 M), Co2(DPB) (2.0 × 10-5 M), Co2(DPD)
(2.0 × 10-5 M), and Co(OEP) (3.0 × 10-5 M) in the presence of HClO4
(2.0 × 10-2 M) in PhCN at 298 K.
Figure 1. Time profiles of formation of Fe(C5H4Me)2+ monitored at 650
nm (ꢀ ) 290 M-1 cm-1) in electron-transfer oxidation of Fe(C5H4Me)2
(1.0 × 10-1 M) by O2 (1.7 × 10-3 M), catalyzed by (a) Co(OEP) and (b)
Co2(DPX) in the presence of HClO4 (2.0 × 10-2 M) in PhCN at 298 K.
HClO4 (2.0 × 10-2 M). As soon as the reaction is started,
Co(OEP) is oxidized to Co(III)(OEP)+ which remains virtually
the same during the reaction. This indicates that the catalytic
steady state is established during the reaction. The concentration
+
Figure 3. Time profiles of formation of Fe(C5H5)2 monitored by
absorbance at 620 nm (ꢀ ) 330 M-1 cm-1) in electron-transfer oxidation
of Fe(C5H5)2 (1.0 × 10-1 M) by O2 (1.7 × 10-3 M), catalyzed by Co2(DPX)
(2.0 × 10-5 M), Co2(DPA) (2.0 × 10-5 M), Co2(DPB) (2.0 × 10-5 M),
Co2(DPD) (2.0 × 10-5 M), and Co(OEP) (3.0 × 10-5 M), in the presence
of HClO4 (2.0 × 10-2 M) in PhCN at 298 K.
+
of Fe(C5H4Me)2 (3.4 × 10-3 M) formed in the Co(OEP)-
catalyzed reduction of O2 by Fe(C5H4Me)2 is twice that of the
O2 concentration (1.7 × 10-3 M). Thus, only two-electron
reduction of O2 occurs and no further reduction occurs to
The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB),
and Co2(DPD)] also catalyze the reduction of O2 by Fe(C5H4-
Me)2, but the amount of Fe(C5H4Me)2 formed is less than 4
+
+
produce more than 2 equiv of Fe(C5H4Me)2 (eq 1).
equiv of O2 (Figure 2). Thus, the clean four-electron reduction
of O2 by Fe(C5H4Me)2 occurs only in the case of Co2(DPX)
used as a catalyst.
2Fe(C5H4Me)2 + O2 + 2H+ Co(OEP)8
2Fe(C5H4Me)2+ + H2O2 (1)
When Fe(C5H4Me)2 (E°ox ) 0.26 V vs SCE) is replaced by
a weaker one-electron reductant, Fe(C5H5)2 (E°ox ) 0.37 V),
and a stronger one-electron reductant, Fe(C5Me5)2 (E°ox ) -0.08
V), the clean four-electron reduction of O2 also occurs only in
the case of Co2(DPX) used as a catalyst as shown in Figure 3.
It was confirmed that no H2O2 was formed in the Co2(DPX)-
catalyzed reduction of O2 and that the rate of reduction of H2O2
is much slower than the rate of reduction of O2. This indicates
that H2O2 is not an intermediate for the Co2(DPX)-catalyzed
four-electron reduction of O2 by ferrocene derivatives.
Electron Transfer from Ferrocene Derivatives to Cata-
lysts. As described above, electron transfer from ferrocene
derivatives to the oxidized form of the catalyst is the key step
in the catalytic reduction of O2 by ferrocene derivatives. To
determine the free energy change of electron transfer, the one-
electron reduction potentials (E°red vs SCE) of Co(OEP) are
determined by the cyclic voltammetry measurements as shown
in Figure 4.39 The E°red value of Co(OEP) is determined at 0.31
V which corresponds to the Co(II)/Co(III) couple.40,41 In the
It was confirmed that H2O2 (1.6 × 10-3 M) is formed in the
two-electron reduction of O2 by iodometric measurements (see
Experimental Section).
When a cofacial dicobalt porphyrin [Co2(DPX)] is used as a
catalyst instead of a single cobalt porphyrin Co(OEP), the
+
concentration of Fe(C5H4Me)2 formed in the Co2(DPX)-
catalyzed reduction of O2 by Fe(C5H4Me)2 (6.8 × 10-3 M in
the case of Co2(DPX) compared to 3.4 × 10-3 M in the case
of Co(OEP)) is 4 times the O2 concentration (1.7 × 10-3 M) as
shown in Figure 1b. Thus, the four-electron reduction of O2 by
Fe(C5H4Me)2 occurs efficiently in the presence of a catalytic
amount of Co2(DPX) and HClO4 (2.0 × 10-2 M) in PhCN (eq
2).
4Fe(C5H4Me)2 + O2 + 4H+ Co (DPX)8
2
4Fe(C5H4Me)2+ + 2H2O (2)
It was confirmed that no H2O2 was formed in the catalytic
reduction of O2 by Fe(C5H4Me)2. The rate of four-electron
reduction of O2 increases with increasing the catalyst concentra-
tion (Figure 1b).
(39) Cyclic voltammograms for Co2(DPB), Co2(DPA), Co2(DPX), and Co2(DPD)
have been reported previously in different conditions. See: ref 23b and c.
(40) (a) Dolphin, D.; Forman, A.; Borg, D. C.; Fajer, J.; Felton, R. H. Proc.
Natl. Acad. Sci. U.S.A. 1971, 68, 614. (b) Felton, R. H.; Dolphin, D.; Borg,
D. C.; Fajer, J. J. Am. Chem. Soc. 1969, 91, 196.
9
10444 J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004

Four-Electron Reduction of Dioxygen to Water
A R T I C L E S
Table 1. Free Energy Change (∆G°et) and Rate Constants (ket) of Electron Transfer from Ferrocene Derivatives to Co(III) Porphyrins in
Deaerated PhCN at 298 K and Rate Constants (kcat) of Co(III) Porphyrin-Catalyzed Reduction of O2 (8.5 × 10-3 M) by Ferrocene
Derrivatives in the Presence of HClO4 (5.0 × 10-2 M)
cobalt porphyrin
ferrocene
∆G°et(1), eV
ket(1), M-1 s-1
∆G°et(2), eV
ket(2), M-1 s-1
kcat, M-1 s-1
Co2(DPX)
Fe(C5H5)2
-0.16
-0.27
-0.61
0.06
-0.05
-0.39
4.0 × 106
-0.02
-0.13
-0.47
2.8 × 105
1.0 × 106
a
3.6 × 105 b
8.0 × 105 b
320 s-1 c
9.8 × 104
2.1 × 105
a
Fe(C5H4Me)2
Fe(C5Me5)2
Fe(C5H5)2
Fe(C5H4Me)2
Fe(C5Me5)2
a
a
Co(OEP)
4.8 × 104
1.0 × 105
a
c
a Too fast to be determined accurately. b In the case of 0.05 M HClO4 and 8.5 × 10-3 M O2. kcat determined from (zero-order rate)/[Co2(DPX)] in
Figure 10, which corresponds to the rate of O-O bond cleavage.
Figure 5. Plots of log ket vs -∆G°et in electron transfer from ferrocene
derivatives [Fe(C5H5)2, Fe(C5H4Me)2, and Fe(C5Me5)2] to Co(III) porphyrins
[Co2(DPX) and Co(OEP)] in deaerated PhCN at 298 K.
+
d[Fe(C5H5)2 ]/dt )
Figure 4. Cyclic voltammograms of (a) Co2(DPX) and (b) Co(OEP) (1.0
× 10-3 M) in PhCN containing 0.1 M TBAP; scan rate 100 mV s-1
.
ket(1)[Fe(C5H5)2][Co(III)(OEP)+] in the absence of O2 (4)
case of cofacial dicobalt porphyrins [Co2(DPX)], two reversible
redox waves (E°red) are observed at 0.53 and 0.39 V (vs SCE)
(Figure 4). The two reversible waves correspond to the first
and second one-electron reduction potentials of two Co(III)
moieties.42
From the slope is determined the second-order rate constant of
electron transfer (ket(1)). The ket(1) values of ferrocene and other
ferrocene derivatives are listed in Table 1.
Electron transfer from ferrocene derivatives to Co(III)2-
(DPX)2+ occurs via two step processes which correspond to
electron transfer from ferrocene derivatives to Co(III)2(DPX)2+
and the subsequent electron transfer from ferrocene derivatives
to Co(III)Co(II)(DPX)+. Both the first and second steps obeyed
pseudo-first-order kinetics. The pseudo-first-order rate constants
increased linearly with increasing concentrations of Co(III)2-
(DPX)2+. Thus, the kinetic formulation is given by eq 5.
The free energy change (∆G°et in eV) of electron transfer
from Fe(C5H5)2 (E°ox ) 0.37 V), Fe(C5H4Me)2 (E°ox ) 0.26
V), or Fe(C5Me5)2 (E0 ) -0.08 V) to Co(III)(OEP)+ (E°red
ox
) 0.31 V) or Co(III)2(DPX)2+ (E°red ) 0.53, 0.39 V) is obtained
from the E°ox values of Fe(C5H5)2 and the E°red values of Co(III)
porphyrins using eq 3, where e is elementary charge.
+
∆G°et ) e(E°ox - E°red)
(3)
d[Fe(C5H5)2 ]/dt ) ket(1)[Fe(C5H5)2][Co(III)2] +
ket(2)[Fe(C5H5)2][Co(III)Co(II)] (5)
The ∆G°et values are described in Table 1. In most cases, the
free energy change of electron transfer is negative (exergonic),
and thereby the electron transfer is expected to occur thermally.
The second-order rate constants of the first-step and second-
step electron transfer (ket(1) and ket(2), respectively) were
determined from the slopes of linear plots of the pseudo-first-
order rate constant vs [Fe(C5H5)2], and the results are sum-
marized in Table 1.
The ket(1) values of electron transfer from ferrocene derivatives
to Co(III)2(DPX)2+ are larger than the ket(2) values of the
subsequent electron transfer to Co(III)Co(II)(DPX)+ because
of the more positive one-electron reduction potential of Co(III)2-
(DPX)2+ than that of Co(III)Co(II)(DPX)+. In the case of
electron transfer from Fe(C5Me5)2 to Co(III)Co(II)(DPX)+, the
initial electron transfer was too fast to determine the rate constant
accurately.
In the absence of O2, efficient electron transfer from ferrocene
to Co(III)(OEP)+ occurs under pseudo-first-order conditions
(large excess ferrocene derivatives). The rates obeyed pseudo-
first-order kinetics, and the observed pseudo-first-order rate
constant increased linearly with increasing the [Fe(C5H5)2] (eq
4).
(41) The first oxidation of a Co(II) porphyrin in a noncoordinating solvent
involves formation of a Co(II) porphyrin radical cation, whereas the first
oxidation in potentially coordinating solvents invariably involves the metal-
centered oxidation to produce a Co(III) porphyrin. See: (a) Salehi, A.;
Oertling, W. A.; Babcock, G. T.; Chang, C. K. J. Am. Chem. Soc. 1986,
108, 5630. (b) Kadish, K. M.; Lin, X. Q.; Han, B. C. Inorg. Chem. 1987,
26, 4161.
The driving force dependence of the ket(1) and ket(2) values in
Table 1 is shown in Figure 5, where the ket values increase with
increasing the driving force of electron transfer (-∆G°et). The
(42) (a) Le Mest, Y.; L’Her, M.; Collman, J. P.; Kim, K.; Hendricks, N. H.;
Helm, S. J. Electroanal. Chem. 1987, 234, 277. (b) Kadish, K. M.; Guo,
N.; Van Caemelbecke, E.; Paolesse, R.; Monti, D.; Tagliatesta, P. J.
Porphyrins Phthalocyanines 1998, 2, 439.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10445

A R T I C L E S
Fukuzumi et al.
Scheme 2
driving force dependence of ket is evaluated in light of the
Marcus theory of adiabatic outersphere electron transfer (eq 6),43
ket ) Z exp[-(λ/4kBT)(1 + ∆G°et/λ)2]
(6)
where Z is the frequency factor (1 × 1011 M-1 s-1), λ is the
reorganization energy of electron transfer, and kB is the
Boltzmann constant. The best fit of the data to eq 6 (the solid
line in Figure 5) affords the λ value 1.4 eV.
Catalytic Mechanism of Two-Electron Reduction of O2.
The rate of formation of Fe(C5H5)2+ in the Co(OEP)-catalyzed
electron-transfer oxidation of Fe(C5H5)2 by O2 (8.5 × 10-3 M)
in the presence of 0.05 M HClO4 in O2-saturated PhCN at 298
K obeyed pseudo-first-order kinetics. A typical time course is
shown in Figure 6a with the pseudo-first-order plot (inset of
Figure 6a). The pseudo-first-order rate constant (kobs) increases
linearly with an increase in the catalyst concentration (Figure
6b). From the slope is determined the second-order rate constant
of the catalytic electron transfer (kcat). The kcat values remain
constant with the change in O2 and HClO4 concentrations as
shown in Figure 6c and d, respectively. The kcat values thus
determined are also listed in Table 1. The kcat values determined
+
+
from formation of Fe(C5H5)2 and Fe(C5H4Me)2 in the
Co(OEP)-catalyzed electron-transfer oxidation of Fe(C5H5)2 and
Fe(C5H4Me)2 by O2 are approximately 2 times the ket values of
electron transfer from Fe(C5H5)2 and Fe(C5H4Me)2 to Co(III)-
(OEP)+ in the absence of O2, respectively (Table 1). This
indicates that the turnover-determining step (t.d.s.) for the
catalytic two-electron reduction of O2 is the electron-transfer
step from ferrocene derivatives to Co(III)(OEP)+ as shown in
Scheme 2. In such a case, the rate of formation of ferricenium
ions is given by eq 7, where the catalytic rate constant (kcat)
corresponds to 2ket.
+
Figure 6. (a) Time profile of formation of Fe(C5H5)2 monitored by
absorbance at 620 nm (ꢀ ) 330 M-1 cm-1) in electron-transfer oxidation
of Fe(C5H5)2 (3.0 × 10-4 M) by O2 (8.5 × 10-3 M), catalyzed by Co(OEP)
(2.5 × 10-5 M) in the presence of 0.05 M HClO4 in PhCN at 298 K. Inset:
First-order plots. (b) Plot of kobs vs [Co(OEP)]. (c) Plot of kcat vs [HClO4]
in the presence of 8.5 × 10-3 M O2. (d) Plot of kcat vs [O2] in the presence
of 0.05 M HClO4.
+
d[Fe(C5H5)2 ]/dt )
2ket[Fe(C5H5)2][Co(OEP)] in the presence of O2 (7)
that reported for Co(TPP) (TPP ) tetraphenylporphyrin dian-
ion)-catalyzed two-electron reduction of O2 by ferrocene
derivatives.25a
Catalytic Mechanism of Four-Electron Reduction of O2.
Monomeric cobalt porphyrin catalyzes only two-electron reduc-
tion of O2 by ferrocene derivatives (eq 1) and a cofacial dicobalt
porphyrin is required for the four-electron reduction of O2 by
ferrocene derivatives (eq 2). Thus, the interaction of two cobalt
nuclei with an active form of oxygen seems essential for the
The initial fast electron transfer from Fe(C5H5)2 to Co(III)-
(OEP)+ is followed by the fast electron transfer from Co(II)-
(OEP) to O2 in the presence of an acid to produce the
Co(III)(OEP)O2H+, which is further reduced by Fe(C5H5)2 in
the presence of an acid to produce H2O2, accompanied by
regeneration of Co(III)(OEP)+. The catalytic mechanism of two-
electron reduction of O2 in Scheme 2 is virtually the same as
9
10446 J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004

Four-Electron Reduction of Dioxygen to Water
A R T I C L E S
Figure 8. Selected distance (Å) in Co2(DPB),22a Co2(DPA),46 Co2(DPX),23b,47
and Co2(DPD)(2MeOH).23b,47
The reported crystal structures of Co2(DPB),22a Co2(DPA),46
Co2(DPX),23b,47 and Co2(DPD)(2MeOH)23b,47 indicate that the
lengths between cofacial porphyrins are quite different depend-
ing on the spacer as shown in Figure 8. The metal-metal
separations in Co2(DPA) (4.53 Å) and Co2(DPX) (4.58 Å) are
virtually the same. Flexibility by itself would not favor stronger
binding; it may accommodate a shorter Co-Co distance that
favors O2 binding. The metal-metal separation in Co2(DPB)
(3.73 Å) may be too short, whereas the separation in Co2(DPD)-
(2MeOH) (8.62 Å) is too long to achieve efficient four-electron
reduction of O2 by ferrocene derivatives. The metal-metal
separation in Zn2(DPD) is also too long (7.78 Å).47,48 However,
the definitive structures of the µ-superoxo species of cofacial
dicobalt porphyrins in solution have yet to be determined.
This suitable metal-metal separation may be the reason
Co2(DPX), which has the largest superhyperfine coupling
constant of the µ-superoxo species (Figure 7), acts as the most
efficient catalyst for the selective four-electron reduction of O2
by ferrocene derivatives (Figure 2,3).
Figure 7. ESR spectra of the µ-superoxo complex (∼10-3 M) produced
by adding iodine (∼10-3 M) to an air-saturated PhCN solution and ESR
simulation of (a) Co2(DPB), (b) Co2(DPA), and (c) Co2(DPX) in the
presence of 1-tert-butyl-5-phenylimidazole (5 × 10-3 M) at 298 K.
The proposed mechanism of four-electron reduction of O2
by ferrocene derivatives is summarized as shown in Scheme 3
by combining with the mechanism of the two-electron reduction
of O2 in Scheme 2. The initial two-electron reduction of the
Co(III)2 complex by ferrocene derivatives gives the Co(II)2
complex, which reacts with O2 to produce the µ-peroxo Co(III)-
O2-Co(III) complex. The heterolytic O-O bond cleavage of
the Co(III)-O2-Co(III) complex affords the high valent
Co(IV)oxo species which is reduced by ferrocene derivatives
in the presence of proton to yield H2O (Scheme 3).49 Alterna-
tively the homolytic O-O bond cleavage affords two Co(III)-
oxyl species which are reduced by ferrocene derivatives to H2O
in the presence of proton. The critical point to distinguish
between the two-electron and four- electron reduction pathways
is the competition between the O-O bond cleavage and the
protonation of the Co(III)-O2-Co(III) complex. The O-O
bond cleavage of the Co(III)-O2-Co(III) complex leads to the
four-electron reduction of O2, whereas the protonation leads to
four-electron reduction of O2. In fact, the µ-superoxo species
of cofacial dicobalt porphyrins are produced by the reactions
of cofacial dicobalt(II) porphyrins with O2 in the presence of a
bulky base (1-tert-butyl-5-phenylimidazole) and the subsequent
one-electron oxidation of the resulting peroxo species by iodine
(see Experimental Section).37 The superhyperfine structure due
to two equivalent cobalt nuclei is observed at room temperature
in the ESR spectra of the µ-superoxo species as shown in Figure
7.44 The superhyperfine coupling constant of the µ-superoxo
species of Co2(DPX) determined from the computer simulation
(Figure 7) is the largest among those of cofacial dicobalt
porphyrins. This suggests that the efficient catalysis of Co2(DPX)
for the four-electron reduction of O2 by Fe(C5H4Me)2 (Figure
1b) results from the strong binding of the reduced oxygen with
Co2(DPX) which may have the most suitable distance between
two cobalt nuclei for the oxygen binding.45
(43) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus, R.
A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (c) Eberson, L. AdV. Phys.
Org. Chem. 1982, 18, 79.
(46) Bolze, F.; Gros, C. P.; Drouin, M.; Espinosa, E.; Harvey, P. D.; Guilard,
R. J. Organomet. Chem. 2002, 643-644, 89.
(44) The ESR spectra of µ-superoxo species in the case of Co2(DPD) and
Co(OEP) was not observed at room temperature due to less stability as
compared with those in the case of Co2(DPB), Co2(DPA), and Co2(DPX).
Without a bulky base (1-tert-butyl-5-phenylimidazole), the µ-superoxo
species could not be detected either. This indicates that the detected
µ-superoxo species are formed by the intramolecular binding of O2 between
the two cobalt atoms of cofacial dicobalt porphyrins with an appropriate
cobalt-cobalt separation.
(47) Chang, C. J.; Baker, E. A.; Pistorio, B. J.; Deng, Y.; Loh, Z.-H.; Miller, S.
E.; Carpenter, S. D.; Nocera, D. G. Inorg. Chem. 2002, 41, 3102.
(48) The flexibility of the DPD spacer allows us to shorten the metal-metal
separation, resulting in forming the µ-oxo species; see ref 47.
(49) Although the formation of the Co(III)-Co(IV) intermediate is energetically
uphill, the followup rapid electron transfer from ferrocene derivatives makes
the process energetically feasible. Formation of a high-valent cobalt-oxo
complex has been suggested. See: Nam, W.; Kim, I.; Kim, Y.; Kim, C.
Chem. Commun. 2001, 1296.
(45) However, it should be noted that the thermodynamic stability is not
necessarily related directly with the kinetic reactivity.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10447

A R T I C L E S
Fukuzumi et al.
Scheme 3
the two-electron reduction of O2 to produce H2O2 which is not
reduced further under the present experimental conditions
(Scheme 3).
The rate of formation of Fe(C5H5)2+ in Co2(DPX)-catalyzed
electron-transfer oxidation of Fe(C5H5)2 by O2 (8.5 × 10-3 M)
in the presence of 0.05 M HClO4 in O2-saturated PhCN at 298
K also obeyed pseudo-first-order kinetics. The pseudo-first-order
rate constant (kobs) increases with increasing the catalyst
concentration of Co2(DPX) (Figure 9b). The kcat values increase
linearly with increasing concentrations of HClO4 and O2 as
shown in Figure 9c and d, respectively. Such a linear depen-
dence of kcat on [HClO4] and [O2] shows sharp contrast with
the case of the Co(OEP)-catalyzed two-electron reduction of
O2 by Fe(C5H5)2 in Figure 6 where the kcat values remain
constant irrespective of HClO4 or O2 concentration. The kcat
values of the Co2(DPX)-catalyzed four-electron reduction of O2
by Fe(C5H4Me)2 also increase linearly with increasing concen-
trations of HClO4 and O2. This indicates that the proton-coupled
electron transfer from Co(III)Co(II)(DPX)+, which is produced
in the initial electron transfer from Fe(C5H5)2 and Fe(C5H4Me)2
to Co(III)2(DPX)2+, to O2 is the turnover-determining step (t.d.s.)
in the catalytic four-electron reduction of O2 in Scheme 3.
When Fe(C5H5)2 is replaced by a much stronger reductant
than Fe(C5H5)2 or Fe(C5H4Me)2, that is Fe(C5Me5)2, the kinetics
of formation of Fe(C5Me5)2+ changes drastically from pseudo-
first-order kinetics in the case of Fe(C5H5)2 (Figure 6a) to zero-
order kinetics as shown in Figure 10a, where the rate remains
constant irrespective of concentration of Fe(C5Me5)2. Further-
more the zero-order rate constant in the case of Fe(C5Me5)2
remains constant with variation of concentrations of HClO4 and
O2 as shown in Figure 10c and d, respectively. In contrast, the
kcat values in the case of Fe(C5H5)2 increase linearly with
increasing concentrations of HClO4 and O2 (Figure 9b and c,
respectively). This indicates that the turnover-determining step
changes from the proton-coupled electron transfer from Co(III)-
Co(II)(DPX)+ to O2 in the case of Fe(C5H5)2 or Fe(C5H4Me)2
+
Figure 9. (a) Time profile of formation of Fe(C5H5)2 monitored by
absorbance at 620 nm (ꢀ ) 330 M-1 cm-1) in electron-transfer oxidation
of Fe(C5H5)2 (3.0 × 10-4 M) by catalytic dioxygen (8.5 × 10-3 M)
reduction, catalyzed by Co2(DPX) (1.0 × 10-5 M) in the presence of 0.05
M HClO4 in PhCN at 298 K. Inset: First-order plots. (b) Plot of kobs vs
[Co2(DPX)]. (c) Plot of kcat vs [HClO4] in the presence of 8.5 × 10-3
M
O2. (d) Plot of kcat vs [O2] in the presence of 0.05 M HClO4.
to the reaction step which has nothing to do with Fe(C5Me5)2,
HClO4, or O2. Such a process which does not involve any
intermolecular electron transfer step is most likely to be O-O
9
10448 J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004

Four-Electron Reduction of Dioxygen to Water
A R T I C L E S
Summary and Conclusions
The present study has demonstrated that the O-O bond
cleavage of the Co(III)-O2-Co(III) complex plays a critical
role for the four-electron reduction of O2 by strong one-electron
reductants (Fe(C5Me5)2). No such a pathway is available for
the monomeric cobalt(III) porphyrin which catalyzes only the
two-electron reduction of O2 (Figure 1a). In the case of iron
porphyrins, however, monomeric iron porphyrins act efficiently
in the electrochemical four-electron reduction of O2.18-20 This
indicates that the formation of µ-peroxo derivative (Fe-O2-
Fe) is not necessary for the four-electron reduction of O2 in
contrast to the case of cobalt porphyrins demonstrated herein.
In the case of iron porphyrins, the heterolytic O-O bond
cleavage of the hydroperoxo species (Fe-OOH), in which the
proton acts as an acid instead of a metal Lewis acid in Fe-
O2-Fe, may be essential for the four-electron reduction of O2
as the case of the Co-O2-Co complex in which two Co(III)
nuclei act as Lewis acids to bind O2. Even though its catalytic
site is bimetallic, the four-electron reduction of O2 by cyto-
chrome c oxidase has been indicated to proceed via intermedi-
ates that are similar, with respect to the binding modes of O2,
to those observed in monometallic heme enzymes such as
oxygenases, peroxidases, and catalases.50-53 However, the actual
role of ferric hydroperoxo species for the four-electron reduction
of O2 has yet to be clarified.
Acknowledgment. This work was supported by the Develop-
ment of Innovative Technology (No.12310) from the Ministry
of Education, Science, Sports and Culture, Japan and the French
Ministry of Research and CNRS (UMR 5633).
+
Figure 10. (a) Time profiles of formation of Fe(C5Me5)2 monitored by
absorbance at 700 nm (ꢀ ) 240 M-1 cm-1) in electron-transfer oxidation
of Fe(C5Me5)2 [2.5 × 10-4 M (O), 4.0 × 10-4 M (b)] by O2 (8.5 × 10-3
M), catalyzed by Co2(DPX) (8.0 × 10-5 M) in the presence of 0.05 M
HClO4 in PhCN at 298 K. (b) Plot of the zero-order rate constant vs
[Co2(DPX)]. (c) Plot of the zero-order rate vs [HClO4] in the presence of
8.5 × 10-3 M O2. (d) Plot of the zero-order rate vs [O2] in the presence of
0.05 M HClO4.
JA048403C
(50) (a) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV.
1996, 96, 2841. (b) Poulos, T. L. In The Porphyrin Handbook; Kadish, K.
M.; Smith, K. M., Guilard, R., Ed.; Academic Press: San Diego, CA, 2000;
Vol. 4, pp 189-218.
(51) Loew, G. H.; Harris, D. L. Chem. ReV. 2000, 100, 407.
(52) Nicholls, P.; Fita, I.; Loewen, P. C. AdV. Inorg. Chem. 2001, 51, 51.
(53) de Momtellano, P. R. O. Cytochrome P450, 2nd ed.; Plenum: New York,
1995.
bond cleavage of the Co(III)-O2-Co(III) complex in Scheme
3. The O-O bond cleavage rate has been determined as 320
s-1 from the slope in Figure 10b.
9
J. AM. CHEM. SOC. VOL. 126, NO. 33, 2004 10449