Clarification of the oxidation state of cobalt corroles in heterogeneous and homogeneous catalytic reduction of dioxygen

Article abstract of DOI:10.1021/ic800458s

Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15- tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15- dimesitylcorrole)cobalt (F₅PhMes₂Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes₃Cor)Co, all of which contain bulky substituents at the three meso positions of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of 1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O₂ in benzonitrile containing 10^-2 M HClO₄. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E_1/2 values between 0.42 and 0.89 V versus SCE depending upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible one-electron abstraction at E_1/2 = 0.86 to 1.18 V in CH₂Cl₂, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E_1/2 for oxidation or reduction and the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect is compared to what is observed in the case of tetraphenylporphyrins containing meso-substituted C₆F₅ substituents. The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F ₅PhMes₂Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.

Full text of DOI:10.1021/ic800458s

Inorg. Chem. 2008, 47, 6726-6737
Clarification of the Oxidation State of Cobalt Corroles in Heterogeneous
and Homogeneous Catalytic Reduction of Dioxygen
Karl M. Kadish,*,† Jing Shen,† Laurent Fre´mond,† Ping Chen,† Maya El Ojaimi,‡
Mohammed Chkounda,‡ Claude P. Gros,‡ Jean-Michel Barbe,‡ Kei Ohkubo,§ Shunichi Fukuzumi,*,§
and Roger Guilard*,‡
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, UniVersite´ de
Bourgogne, ICMUB (UMR 5260), 9, AVenue Alain SaVary, BP 47870, 21078 Dijon Cedex France,
and Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan
Received March 12, 2008
Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric
acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15-tris(pentafluo-
rophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15-dimesitylcorrole)cobalt (F5PhMes2Cor)Co, and
(5,10,15-trismesitylcorrole)cobalt (Mes3Cor)Co, all of which contain bulky substituents at the three meso positions
of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine
the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of
1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O2 in
benzonitrile containing 10-2 M HClO4. The corroles were also investigated as to their redox properties in nonaqueous
media. A reversible one-electron oxidation occurs at E1/2 values between 0.42 and 0.89 V versus SCE depending
upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible
one-electron abstraction at E1/2 ) 0.86 to 1.18 V in CH2Cl2, THF, or PhCN. Two reductions of each corrole are
also observed in the three solvents. A linear relationship is observed between E1/2 for oxidation or reduction and
the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect
is compared to what is observed in the case of tetraphenylporphyrins containing meso-substituted C6F5 substituents.
The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and
the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis
spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F5PhMes2Cor)Co,
which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized
macrocyclic ring.
Introduction
but chloro-21 and phenyl-3 cobalt(IV) corroles have been
structurally characterized. The oxidized and reduced forms
of cobalt corroles can also be in situ generated and
spectroscopically characterized in nonaqueous media.1–7 The
Co(III) and Co(II) forms of the corroles will strongly
coordinate solvent molecules, nitrogenous bases, and
anions,2,21–26 whereas Co(III) corroles can axially bind a
Cobalt corroles are by far the most studied of any
metallocorrole complexes.1–20 The air-stable form of the
compound contains Co(III) in the absence of axial ligands,
* To whom correspondence should be addressed. E-mail: kkadish@uh.edu
(K.M.K.),
Roger.Guilard@u-bourgogne.fr
(R.G.),
fukuzumi@
chem.eng.osaka-u.ac.jp (S.F.).
†
University of Houston.
Universite´ de Bourgogne.
Osaka University.
‡
(2) Guilard, R.; Barbe, J.-M.; Stern, C.; Kadish, K. M. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R. Eds.; Academic
Press: Boston, 2003; Vol. 18, pp 303-349.
§
(1) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R. Eds.; Academic Press:
Boston, 2000; Vol. 2, pp 233-300.
(3) Will, S.; Lex, J.; Vogel, E.; Adamian, V. A.; Van Caemelbecke, E.;
Kadish, K. M. Inorg. Chem. 1996, 35, 5577–5583.
6726 Inorganic Chemistry, Vol. 47, No. 15, 2008
10.1021/ic800458s CCC: $40.75  2008 American Chemical Society
Published on Web 06/27/2008

Clarification of the Oxidation State of Cobalt Corroles
single CO molecule22–24,27–30 or act as a catalyst for the
electroreduction of O2 in acid media.21,31–33 Doubly reduced
Co(III) corroles, which formally contain Co(I), have been
spectroscopically characterized in acetonitrile. The Co(I)
form of the corrole is stable in polar organic solvents but
has been shown to catalytically reduce CO2 in nonaqueous
media.20
There have been extensive studies on the catalytic reduc-
tion of O2 using cobalt porphyrins,34–39 cobalt corroles,31
and dyads containing one porphyrin and one cobalt corrole
linked in a face-to-face arrangement.21,31–33 The main
difference between the cobalt corrole and cobalt porphyrin
catalysts is the oxidation state of the central metal ion in the
neutral compound. Uncharged cobalt porphyrins generally
contain a Co(II) ion in their air stable form, whereas cobalt
corroles contain Co(III) as a result of the -3 charge on the
macrocycle. In the case of the Co(II) porphyrins, the oxidized
species are well established to be Co(III) porphyrins rather
than a radical cation of the porphyrin with the same metal
oxidation state, that is, Co(II). In the case of Co(III) corroles,
however, it has not been clearly established whether the
oxidized species is in all cases a Co(IV) corrole as has been
previously observed4,5 or a radical cation of the corrole
containing Co(III).
We clarify this point in the present study by examining
the electrochemistry and spectroscopic properties of the three
meso-substituted corroles shown in Chart 1. The compounds
electrogenerated in each redox reaction can exist with Co(I),
Co(II), Co(III), or Co(IV) central metal ions, and the site of
each electron addition or abstraction was examined by
UV-vis spectroelectrochemistry. These compounds differ
from a previously examined compound, (Me4Ph5Cor)Co,7,31
in that they contain bulky substituents at the three meso-
positions of the macrocycle, which influence not only the
half-wave potentials for oxidation and reduction but also
diminish the ability of the corroles to dimerize in solution
or at an electrode surface. The ability of 1–3 to act as
homogeneous and heterogeneous catalysts in the reduction
of O2 is also investigated in acid media, and the singly
oxidized cobalt corrole products are characterized by UV-vis
and/or ESR spectroscopy.
(4) Adamian, V. A.; D’Souza, F.; Licoccia, S.; Di Vona, M. L.; Tassoni,
E.; Paolesse, R.; Boschi, T.; Kadish, K. M. Inorg. Chem. 1995, 34,
532–540.
(5) Kadish, K. M.; Adamian, V. A.; Van Caemelbecke, E.; Gueletti, E.;
Will, S.; Erben, C.; Vogel, E. J. Am. Chem. Soc. 1998, 120, 11986–
11993.
(6) Kadish, K. M.; Koh, W.; Tagliatesta, P.; D’Souza, F.; Paolesse, R.;
Licoccia, S.; Boschi, T. Inorg. Chem. 1992, 31, 2305–2313.
(7) Kadish, K. M.; Shao, J.; Ou, Z.; Gros, C. P.; Bolze, F.; Barbe, J.-M.;
Guilard, R. Inorg. Chem. 2003, 42, 4062–4070.
(8) Johnson, A. W. Pure Appl. Chem. 1970, 23, 375–389.
(9) Hush, N. S.; Dyke, J. M. J. Inorg. Nucl. Chem. 1973, 35, 4341–4347.
(10) Hush, N. S.; Dyke, J. M.; Williams, M. L.; S., W. I. J. Chem. Soc.,
Dalton Trans. 1974, 395–399.
(11) Hush, N. S.; Woolsey, I. S. J. Chem. Soc., Dalton Trans. 1974, 24–
34.
(12) Paolesse, R.; Licoccia, S.; Fanciullo, M.; Morgante, E.; Boschi, T.
Inorg. Chim. Acta 1993, 203, 107–114.
(13) Paolesse, R.; Licoccia, S.; Bandoli, G.; Dolmella, A.; Boschi, T. Inorg.
Chem. 1994, 33, 1171–1176.
(14) Licoccia, S.; Paolesse, R. Struct. Bonding (Berlin) 1995, 84, 71–133.
(15) Genokhova, N. S.; Melent’eva, T. A.; Berezovskii, V. M. Russ. Chem.
ReV. (Engl. Transl.) 1980, 49, 2132–2158.
(16) Melent’eva, T. A. Russ. Chem. ReV. (Engl. Transl.) 1983, 52, 1136–
1172.
Experimental Section
(17) Murakami, Y.; Yamada, S.; Matsuda, Y.; Sakata, K. Bull. Chem. Soc.
Jpn. 1978, 51, 123–129.
Chemicals and Compounds. Neutral alumina (Merck; usually
Brockmann grade III, i.e. deactivated with 6% water) was used for
column chromatography. Reactions were monitored by thin-layer
chromatography and UV-vis spectroscopy. Pyridine (py, g99.8%),
tetrahydrofuran (THF, g99.9%), and tetra-n-butylammonium chlo-
ride (TBACl, g99.5%) were obtained from Sigma-Aldrich Chemi-
cal Co. and used as received. Absolute dichloromethane (CH2Cl2)
was obtained from EMD Chemical Inc. and used without further
purification. Benzonitrile (PhCN) was purchased from Sigma-
Aldrich Chemical Co. and distilled over P2O5 under reduced
pressure prior to use. Tetra-n-butylammonium perchlorate (TBAP,
Fluka Chemical Co.) was twice recrystallized from absolute ethanol
and dried in a vacuum oven at 40 °C for 1 week prior to use.
Perchloric acid (HClO4, 70%) was purchased from Sigma-Aldrich
Chemical Co. Aqueous solutions of 1 M HClO4 were prepared with
deionized water of resistivity not less than 18 MΩ cm. High-purity
N2 gas was purchased from Matheson-Trigas.
(18) Murakami, Y.; Matsuda, Y.; Sakata, K.; Yamada, S.; Tanaka, Y.;
Aoyama, Y. Bull. Chem. Soc. Jpn. 1981, 54, 163–169.
(19) Conlon, M.; Johnson, A. W.; Overend, W. R.; Rajapaksa, D.; Elson,
C. M. J. Chem. Soc., Perkin Trans. I 1973, 2281–2288.
(20) Grodkowski, J.; Neta, P.; Fujita, E.; Mahammed, A.; Simkhovich, L.;
Gross, Z. J. Phys. Chem A 2002, 106, 4772–4778.
(21) Kadish, K. M.; Shao, J.; Ou, Z.; Fre´mond, L.; Zhan, R.; Burdet, F.;
Barbe, J.-M.; Gros, C. P.; Guilard, R. Inorg. Chem. 2005, 44, 6744–
6754.
(22) Guilard, R.; Je´roˆme, F.; Barbe, J.-M.; Gros, C. P.; Ou, Z.; Shao, J.;
Fischer, J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4856–
4865.
(23) Kadish, K. M.; Ou, Z.; Shao, J.; Gros, C. P.; Barbe, J.-M.; Je´roˆme,
F.; Bolze, F.; Burdet, F.; Guilard, R. Inorg. Chem. 2002, 41, 3990–
4005.
(24) Guilard, R.; Gros, C. P.; Bolze, F.; Je´roˆme, F.; Ou, Z.; Shao, J.; Fischer,
J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4845–4855.
(25) Guilard, R.; Burdet, F.; Barbe, J.-M.; Gros, C. P.; Espinosa, E.; Shao,
J.; Ou, Z.; Zhan, R.; Kadish, K. M. Inorg. Chem. 2005, 44, 3972–
3983.
(26) Kadish, K. M.; Shao, J.; Ou, Z.; Zhan, R.; Burdet, F.; Barbe, J.-M.;
Gros, C. P.; Guilard, R. Inorg. Chem. 2005, 44, 9023–9038.
(27) Barbe, J.-M.; Burdet, F.; Espinosa, E.; Guilard, R. Eur. J. Inorg. Chem.
2005, 6, 1032–1041.
10-(pentafluorophenyl)-5,15-bis(2,4,6-trimethylphenyl)corrole, (F5-
PhMes2Cor)H3, 5,10,15-tris(pentafluorophenyl)corrole (TPFCor)H3,
(28) Barbe, J.-M.; Canard, G.; Brande`s, S.; Guilard, R. Angew. Chem., Int.
Ed. 2005, 44, 3103–3106.
(34) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.;
Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027–6036.
(35) Collman, J. P.; Kaplun, M.; Decre´au, R. A. Dalton Trans. 2006, 554–
559.
(29) Barbe, J.-M.; Canard, G.; Brande`s, S.; Guilard, R. Chem. Eur. J. 2007,
13, 2118–2129.
(30) Barbe, J.-M.; Canard, G.; Brande`s, S.; Je´roˆme, F.; Dubois, G.; Guilard,
R. Dalton Trans. 2004, 1208–1214.
(36) Durand, R. R.; Bencosme, C. S.; Collman, J. P.; Anson, F. C. J. Am.
Chem. Soc. 1983, 105, 2710–2718.
(31) Kadish, K. M.; Fre´mond, L.; Ou, Z.; Shao, J.; Shi, C.; Anson, F. C.;
Burdet, F.; Gros, C. P.; Barbe, J.-M.; Guilard, R. J. Am. Chem. Soc.
2005, 127, 5625–5631.
(37) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28, 2459–
2465.
(32) Kadish, K. M.; Fre´mond, L.; Burdet, F.; Barbe, J.-M.; Gros, C. P.;
Guilard, R. J. Inorg. Biochem. 2006, 100, 858–868.
(33) Guillard, R.; Jérôme, F.; Gros, C. P.; Barbe, J.-M.; Ou, Z.; Shao, J.;
Kadish, K. M. C. R. Acad. Sci. Series IIC: Chimie 2001, 4, 245–254.
(38) Fukuzumi, S.; Okamoto, K.; Gros, C. P.; Guilard, R. J. Am. Chem.
Soc. 2004, 126, 10441–10449.
(39) Fukuzumi, S.; Okamoto, K.; Tokuda, Y.; Gros, C. P.; Guilard, R. J. Am.
Chem. Soc. 2004, 126, 17059–17066.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6727

Kadish et al.
Chart 1. Investigated Monomeric Cobalt corroles
and (5,10,15-tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co (1)
were synthesized according to literature procedures.27,40–42 5,10,15-
trismesitylcorrole (Mes3Cor)H3 was synthesized according a method
reported in ref 41 for trans-A2B-corroles.
tion of the Co(III) corrole in solution and [red] is the concentra-
tion of the singly reduced species. The latter value was
determined from measurements of UV-vis spectra obtained
during the electroreduction as previously described in the
literature.7
[10-Pentafluorophenyl-5,15-dimesitylcorrole]cobalt (F5PhMes2-
Cor)Co (2). Under argon, a solution of 100 mg (0.143 mmol, 1 equiv)
of the free-base corrole (F5PhMes2Cor)H3, 52.5 mg (0.217 mmol, 1.5
equiv) of Co(II) acetate tetrahydrate in 10 mL of CHCl3/methanol (70/
30) was stirred and refluxed for 1 h 15 min, the metalation reaction
being monitored by UV-vis spectroscopy and MALDI-TOF mass
spectrometry. The solution was then poured into water (100 mL)
and extracted with dichloromethane. The combined organic layers
were dried over MgSO4. After removal of the solvent in vacuo,
the residue so obtained was redissolved in dichloromethane and
chromatographed on alumina using CH2Cl2 as eluent. The title
product 2 was isolated in 48% yield (52 mg, 0.069 mmol). MS
Electrochemical measurements for the reduction of O2 using the
cobalt corroles as catalysts were carried out using a three-electrode
system consisting of a platinum ring-graphite disk working electrode
(Model MTI34, Pine Instruments Co.), a platinum wire as the auxiliary
electrode, and an SCE reference electrode, which was separated
from the bulk of the solution by means of a salt bridge. The rotating
ring-disk electrode, purchased from the Pine Instrument Co.,
consisted of a platinum ring and a removable edge-plane pyrolytic
graphite (EPPG) disk (A ) 0.282 cm2). Cyclic and rotating disk
voltammograms were carried out using a Pine model AFMSR
rotator linked to an EG&G Princeton Applied Research (PAR)
model 263A potentiostat/galvanostat. The potentiostat was con-
trolled by an IBM compatible PC microcomputer having an M270
(EG&G PARC) software package.
UV-vis spectroelectrochemical experiments were performed
with an optically transparent platinum thin-layer electrode of the
type described in the literature.44 Potentials were applied with an
EG&G PAR model 173 potentiostat. Time-resolved UV-vis spectra
were recorded with a Hewlett-Packard Model 8453 diode array
rapid-scanning spectrophotometer.
(MALDI/TOF): m/z
)
756.53 [M]+•; 756.17 Calcd for
C43H30F5N4Co. UV-vis (CH2Cl2): λmax (ε × 10-3 L mol-1 cm-1
)
) 389.0 (59.8), 544.0 (7.6) nm. Anal. Calcd for C43H30F5N4Co: C,
68.26; H, 4.00; N, 7.40. Found: C, 67.97; H, 3.71; N, 7.06.
[5,10,15-trismesitylcorrole]cobalt (Mes3Cor)Co (3). The tris-
mesitylcorrole complex was synthesized as described above for 2,
starting from (Mes3Cor)H3 (170 mg, 0.261 mmol, 1 equiv) and 94.7
mg (0.392 mmol, 1.5 equiv) of Co(II) acetate tetrahydrate in 10%
yield (18 mg, 0.025 mmol). MS (MALDI/TOF): m/z ) 708.07
[M]+•; 708.27 Calcd for C46H41N4Co. HR-MS (MALDI/TOF):
708.2613 [M]+•; 708.2657 Calcd for C46H41N4Co. UV-vis
(CH2Cl2): λmax (ε × 10-3 L mol-1 cm-1) ) 390.1 (26.2) nm.
Instrumentation. Microanalyses were performed at the Univer-
site´ de Bourgogne on a Fisons EA 1108 CHNS instrument. Mass
spectra and accurate mass measurements (HR-MS) were obtained
on a Bruker Daltonics Ultraflex II spectrometer of the “Centre de
Spectrome´trie Mole´culaire” in the MALDI/TOF reflectron mode
using dithranol as a matrix. Cyclic voltammetry was carried out
with an EG&G model 173 potentiostat/galvanostat. A three-
electrode system was used and consisted of a glassy carbon or a
platinum disk working electrode, a platinum wire counter electrode,
and a saturated calomel reference electrode (SCE). The SCE
electrode was separated from the bulk of the solution by a fritted-
glass bridge of low porosity, which contained the solvent/supporting
electrolyte. Half-wave potentials for reversible redox processes were
Homogeneous reduction of oxygen by 1,1′-dimethylferrocene
Fe(C5H4Me)2 in the presence of HClO4 in PhCN was examined by
monitoring the appearance of an absorption band due to the
38
[Fe(C5H4Me)2]+ product (λmax ) 650 nm, εmax ) 290 M-1 cm-1
)
using a Hewlett-Packard 8453 diode array spectrophotometer with a
quartz cuvette (path length ) 10 mm) at 298 K. An air-saturated PhCN
solution was used for the catalytic reduction of oxygen by
Fe(C5H4Me)2. The O2 concentration in an air-saturated PhCN solution
(1.7 × 10-3 M) was determined as reported in the literature38,45 and
involved a spectroscopic titration for the photooxidation of 10-methyl-
9,10-dihydroacridine by O2. The concentrations of Fe(C5H4Me)2 used
for the catalytic reduction of O2 were larger than the O2 concentration
in solution so that oxygen was the limiting reagent in the reaction cell,
which was filled with the reactant solution.
Electrode Surface Preparation for O2 Catalysis. Before use,
the edge-plane pyrolytic graphite with the edges of the graphitic
planes exposed was polished with 600 grit SiC paper, rinsed with
purified water, and wiped off with clean tissue before it was coated
with the catalytic film. The catalysts were adsorbed onto the graphite
disk by transferring aliquots of a solution in CHCl3 directly onto
determined by cyclic voltammetry and were calculated as E1/2
)
(Epa + Epc)/2 where Epa and Epc represent the anodic and cathodic
peak potentials, respectively. The E1/2 values for irreversible redox
processes in pyridine were obtained by the use of spectroelec-
trochemistry and the modified Nernst equation Eapp ) E1/2
+
(0.059/n)/log([ox]/[red]),7,43 where [ox] represents the concentra-
(40) Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed. 2000,
39, 4045–4047.
(41) Gryko, D. T.; Jadach, K. J. Org. Chem. 2001, 66, 4267–4275.
(42) Gros, C. P.; Brisach, F.; Meristoudi, A.; Espinosa, E.; Guilard, R.;
Harvey, P. D. Inorg. Chem. 2007, 46, 125–135.
(43) Rohrbach, D. F.; Deutsch, E.; Heineman, W. R.; Pasternack, R. F.
Inorg. Chem. 1977, 16, 2650–2652.
(44) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498–1501.
(45) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. J. Chem. Soc., Perkin Trans.
I 1989, 1037–1045.
6728 Inorganic Chemistry, Vol. 47, No. 15, 2008

Clarification of the Oxidation State of Cobalt Corroles
Figure 1. Cyclic voltammograms of (F5PhMes2Cor)Co (2) in (a) CH2Cl2, (b) THF, (c) PhCN, and (d) pyridine containing 0.1 M TBAP.
Table 1. Half-Wave Potentials (V vs SCE) of Co(III) Corroles in
the electrode surface followed by evaporation of the solvent.
Potentials were measured with respect to a saturated calomel
reference electrode (SCE).
Different Solvents Containing 0.1 M TBAP
ox
red
solvent
CH2Cl2
compd
Co(III/II)
Co(II/I)
The slopes of the diagnostic Koutecky-Levich plots were
determined by linear regression analysis of data acquired at 100,
300, 600, 900, 1600, 2500, and 3600 rpm (rotation per minute).
The diffusion-limiting currents for the reduction of O2 in aqueous
solution at the rotating disk electrode were calculated using the
following parameters: kinematic viscosity of H2O at 25 °C, 0.01
1
2
3
1.18
0.98
0.94
0.89
0.66
0.56
0.24
-0.03
-0.18
-1.37
1.54
1.45
-1.58a
b
PhCN
1
2
3
1.26
1.00
0.87
0.74
0.57
0.48
0.13
-0.07
-0.21
-1.40
-1.68
-1.74
1.70
1.64
cm2 s-1; solubility of O2 in air-saturated 1 M HClO4, 0.24 mM;
46
diffusion coefficient of O2, 1.7 × 10-5 cm2 s-1
.
THF
py
2
0.86
0.59
-0.08
-1.60
ESR Measurements. ESR measurements were performed on a
JEOL JES-FA100 ESR spectrometer and spectra were recorded
under nonsaturating microwave power conditions. The magnitude
of modulation was chosen to optimize the resolution and signal-
to-noise (S/N) ratio of the observed spectra. The g values were
calibrated with a Mn2+ marker.
2
0.42
-0.77c
-1.64
a Peak potentials at a scan rate of 0.1 V/s. b Reaction not observed within
cathodic potential window of solvent. c Reduction potential determined by
monitoring changes in the spectra during electron addition and using the
equation: Eapp ) E1/2 + (0.059/n) log([ox]/[red]).
Theoretical Calculations. Density-functional theory (DFT)
calculations were performed on an 8CPU workstation (PQS,
Quantum Cube QS8-2400C-064). Geometry optimizations were
carried out using the Becke3LYP functional and 6-31G(d) basis
set47–50 with the restricted Hartree-Fock (RHF) formalism and as
implemented in Gaussian 03, Revision C.02.50 Graphical outputs
of the computational results were generated with the GaussView,
ver. 3.09, developed by Semichem, Inc.51
Results and Discussion
(46) Shi, C.; Anson, F. C. Inorg. Chem. 1998, 37, 1037–1043.
(47) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
Electrochemistry in Nonaqueous Media. Each com-
pound undergoes up to three oxidations and two reductions
depending upon the solvent. Cyclic voltammograms illustrat-
ing the reduction and oxidation of 2 in four different
nonaqueous solvents (CH2Cl2, THF, PhCN, and pyridine)
containing 0.1 M TBAP are shown in Figure 1, and half-
wave potentials for each redox reaction are summarized in
Table 1, which also includes data for the other two
investigated compounds. The electrochemical behavior of
1-3 in these solvents is similar to what has been earlier
reported for other Co(III) corroles with different substitu-
ents,1,2 namely each corrole undergoes a facile reduction to
generate the Co(II) form of the compound, an additional one-
electron reduction at negative potentials to generate the
(48) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.
(49) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(50) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J., T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; ReV. C.02
Gaussian, Inc.: Wallingford, CT, 2004.
(51) Dennington, R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.;
Gilliland, R. Semichem, Inc.: Shawnee Mission, KS, 2003.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6729

Kadish et al.
Figure 3. The relationship between the half-wave potentials and number
of fluorine groups for 1-3 during the first and second oxidations and first
reduction in PhCN.
(E1/2 ) -0.21 V in PCN) is more difficult to reduce than
(Me4Ph5Cor)Co (E1/2 ) -0.16 V7), but E1/2 values for the
first oxidation of the meso-substituted corrole and the
ꢀ-pyrrole-substituted (Me4Ph5Cor)Co are similar to each
other, being 0.48 and 0.47 V, respectively. There is also a
linear trend between E1/2 values and the number of fluorine
groups on the compound for the first two oxidations and first
reduction of 1-3 in PhCN. This is graphically shown in
Figure 3 where the slope of the line amounts to 17 mV/
fluorine group for the first oxidation, 26 mV/fluorine group
for the second, and 22 mV/fluorine group for the first
reduction. The substituent effect for the first oxidation and
first reduction of the three corroles is similar to what has
been reported in the case of tetraphenylporphyrins containing
C6F5 groups at the meso positions of the macrocycle.52
As seen in Figure 1, the reduction of 2 to its formal Co(I)
form is irreversible in CH2Cl2 and is located at Ep ) -1.58
V for a scan rate of 0.1 V/s. This process is, however,
Figure 2. UV-vis spectral changes of (F5PhMes2Cor)Co (2) in CH2Cl2
upon addition of pyridine to solution. The inset of each figure shows the
Hill plot used to analyze the data. The pyridine concentration for the plots
ranges from (a) 6.24 × 10-6 to 2.29 × 10-5 M and (b) 4.78 × 10-5 to 1.67
× 10-3 M.
reversible in the other three solvents and occurs at E1/2
)
-1.60 to -1.68 V versus SCE for the same compound in
PhCN, THF, and pyridine. The fact that the second reduction
of the Co(III) corroles is not strongly affected by a change
in solvent is consistent with a lack of axial ligand binding
to the Co(II) and Co(I) forms of the compound.
The first reversible oxidation of 1-3 in CH2Cl2 and PhCN
ranges from E1/2 ) 0.48 to 0.89 V depending upon the solvent
and macrocycle and this reaction is followed by up to two
additional electron abstractions as seen in Figure 1 and Table
formal Co(I) corrole, and multiple one-electron oxidations,
the exact number of which will depend upon the compound
substituents, type of axial ligands, and positive potential limit
of the nonaqueous solvent.4,7,21 The Co(III)/Co(II) reactions
of 1-3 are all reversible in CH2Cl2, THF, or PhCN but
irreversible and shifted to more-negative potentials in pyri-
dine. The ill-defined cathodic reduction peak in py and
negative shift of E1/2 from the Co(III)/Co(II) processes in
the other solutions is due to the large pyridine binding
constants for the Co(III) form of the corrole and a coupled
chemical reaction associated with the electron transfer.7,22
This was verified in the present study by spectrally monitor-
ing the binding of pyridine in a CH2Cl2 solution containing
the cobalt corrole 2. The spectral changes are shown in Figure
2, and the pyridine binding constants were calculated as log
K1 ) 4.90 and log K2 ) 3.54 by analysis of the data.
In comparison to previously characterized cobalt corroles
with four or five phenyl substituents at the ꢀ-positions of
the macrocycle, such as (Me4Ph5Cor)Co,7 the Co(III)/Co(II)
redox potential of 1 and 2 in CH2Cl2, PhCN, or THF are
shifted to more positive values due to the strong electron-
withdrawing effects of the pentafluorophenyl group at the
meso-positions of the corrole macrocycle. (Mes3Cor)Co 3
1. The second oxidation of the corroles is located at E1/2
)
0.86 to 1.18 V in CH2Cl2, THF, or PhCN (Table 1) but is
not observed in pyridine due to the limited positive potential
range of this solvent. The meso-substituted corroles 1-3 do
not dimerize in CH2Cl2 as reported for the ꢀ-aryl-substituted
corroles,7 and this can be attributed to the presence of the
bulky meso-aryl substituents on the macrocycle.
UV-vis Spectroelectrochemistry of (F5PhMes2Cor)Co
(2). The neutral, singly oxidized, and singly or doubly reduced
corroles were spectrally characterized in the different solvents
and a summary of the data for 2 is given in Table 2. Similar
sets of spectral changes were obtained for the other two
compounds.
(52) Kadish, K. M.; Royal, G.; Van Caemelbecke, E.; Gueletti, L. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R. Eds.;
Academic Press: Boston, 2000; Vol. 9, pp 1-220.
6730 Inorganic Chemistry, Vol. 47, No. 15, 2008

Clarification of the Oxidation State of Cobalt Corroles
Table 2. UV-vis Spectral Data of Neutral and Electroxidized or Electroreduced (F5PhMes2Cor)Co (2) in Different Solvents
central metal
electrode rxn
solvent
λ nm (ε × 10-4 M-1 cm-1
)
Co(III)
none
CH2Cl2
THF
PhCN
pyridine
CH2Cl2
THF
PhCN
pyridine
CH2Cl2
THF
PhCN
pyridine
pyridine
PhCN
THF
389 (5.98)
404 (4.63)
388 (5.09)
442 (4.22)
421 (5.17)
419 (4.66)
426 (4.87)
544 (0.76)
532 (0.71)
450 (4.28)
537 (0.55)
577 (0.80)
569 (0.89)
572 (0.74)
574 (0.83)
595 (0.75)
607 (1.58)
Co(II)
Co(I)
reduction
reduction
oxidation
419 (4.60)
435a (2.93)
b
419 (5.71)
421 (5.45)
419 (5.45)
414 (3.86)
413 (5.04)
411 (3.99)
387 (3.76)
574 (0.70)
Co(IV)
427 (3.82)
424 (4.81)
424 (3.55)
424a (2.29)
687 (0.32)
684 (0.39)
CH2Cl2
a Shoulder. b Electroreduced compound reacts with solvent.
Figure 4. UV-vis spectral changes of (F5PhMes2Cor)Co (2) during the
(a) first and (b) second reductions in PhCN containing 0.1 M TBAP.
Figure 5. UV-vis spectral changes of (F5PhMes2Cor)Co (2) during the
first oxidation in (a) PhCN and (b) CH2Cl2 containing 0.1 M TBAP.
The neutral Co(III) corrole 2 has a well-defined Soret band
at 388 nm in PhCN, and this band decreases in intensity
and red-shifts to 426 nm as the Co(II) form of the corrole is
electrogenerated (part a of Figure 4). There are no major
visible bands for the neutral corrole in PhCN, but a weak
band grows in at 574 nm when the first reduction proceeds
at an applied potential of -0.20 V. No radical marker bands
are observed between 600∼800 nm during the first reduction,
and this is consistent with the first process involving a
Co(III)/Co(II) metal-centered reaction as previously described
in the literature.1,2
The controlled reducing potential in the thin-layer cell was
then switched to -1.80 V and the resulting spectral changes
are shown in part b of Figure 4. As the reaction proceeded,
the 426 nm band assigned to the Co(II) corrole increased in
intensity and was shifted to 421 nm at the complete of the
reaction. These types of spectral changes can be interpreted
in terms of a metal-centered electron addition rather than a
ring-centered process where a decreased intensity Soret band
and broad absorption in the near-IR region of the spectrum
would be expected. The assignment of a Co(II)/Co(I) process
was also made in an earlier study, which reported the
electrocatalytic reduction of CO2 in CH3CN by the doubly
reduced corrole 1.20
UV-vis spectral changes during the first oxidation of 2
in PhCN are shown in part a of Figure 5. The 388 nm band
of the neutral Co(III) corrole red-shifts during the oxidation,
giving a product with a split Soret band at 413 and 424 nm
and a broad band in the near-IR at 684 nm. There are multiple
well-defined isosbestic points in the spectra, indicating
formation of a single oxidation product. The lack of a
Inorganic Chemistry, Vol. 47, No. 15, 2008 6731

Kadish et al.
reactions are observed for 2 as seen in part a of Figure 7
which shows the UV-vis spectral changes upon addition of
TBACl to solution. The changes in absorbance of the 389
nm band were analyzed as a function of added Cl-
concentration and a plot of log ((Ai - Ao)/(Af - Ai)) versus
log[TBACl] gives a slope of 1.0. This means that only one
chloride ion binds to the Co(III) center of the uncharged
neutral compound. The intercept from the Hill plot is log
K1 ) 2.3 which is similar to the log K ) 2.88 for the chloride
binding reaction of (Me4Ph5Cor)Co.21 This is not surprising
because 3 and (Me4Ph5Cor)Co have virtually the same E1/2
values for the first oxidation, indicating a similar electron
density on the metal center.
A stabilization of the higher oxidation states of 2 by the
binding of Cl- is also observed and this results in a negative
shift of E1/2 for the first two oxidations of the corrole with
increasing concentration of chloride ion in solution. This is
shown in part b of Figure 7 where the addition of excess
Cl- shifts both oxidation processes toward more-negative
potentials. The E1/2 for the first electron abstraction in CH2Cl2
shifts by 63 mV per 10-fold change in log [TBACl] (part b
of Figure 7), and this indicates that the singly oxidized
corrole contains one more bound chloride ion than the neutral
form of the compound. Thus, the electrode reaction for the
first oxidation in chloride-containing media can be described
as shown in eq 2.
Figure 6. UV-vis spectral changes of (F5PhMes2Cor)Co (2) by addition
of PhCN in CH2Cl2 at a controlled potential of 0.80 V.
decreased intensity Soret band for the singly oxidized corrole
strongly suggests Co(IV) formation,21 but the presence of a
band at 684 nm suggests a corrole radical cation, which is
also the conclusion from ESR spectra of the electroxidized
species presented on the following pages.
Surprisingly, quite different spectral changes are observed
during the first oxidation of 2 in CH2Cl2 (part b of Figure
5). In this solvent, the one-electron oxidized corrole has a
decreased-intensity Soret band at 387 nm, suggesting forma-
tion of a corrole radical cation, but the expected radical
marker band between 600 and 800 nm is not observed. The
fact that the UV-vis spectra in both solvents have dual
characteristics of a metal- and macrocycle-centered oxidation
might be interpreted as a solvent-dependent change in the
site of the electron transfer or it could simply be a difference
in the degree of coordination, changing from four-coordinate
in CH2Cl2 to five- or six-coordinate in PhCN. This was
investigated in the present study by monitoring changes in
the UV-vis spectrum of a CH2Cl2 solution containing singly
oxidized 2 with increasing amounts of added PhCN. The
results of this titration are shown in Figure 6, where the
PhCN was added stepwise to 2+ in CH2Cl2 while maintaining
a constant controlled potential of 0.80 V to keep the oxidized
species in solution. As seen in this figure, the 387 nm Soret
band of the singly oxidized corrole red-shifts during the
titration and leads to a species that has a split band at 413
and 424 nm. This spectrum is exactly the same as the
spectrum obtained after a one-electron oxidation of 2 in neat
PhCN (part a of Figure 5). A Hill plot was constructed (inset
of Figure 6) and gives a straight line with a slope of 1.0,
indicating that one PhCN molecule binds to the cobalt center
of the singly oxidized species under the given experimental
conditions. The equilibrium constant for this ligand addition
reaction was calculated as log K ) 1.3 from the zero intercept
of the plot, and the reaction can be described by eq 1.
(F PhMes Cor)CoCl - + CI- h
[
]
5
2
[(F5PhMes2Cor)CoCl2]- + e (2)
The second oxidation of 2 is also negatively shifted with
increasing concentration of Cl-, and this is shown by the
cyclic voltammograms in part b of Figure 7, where the E1/2
goes from 0.98 V in CH2Cl2, 0.1 M TBAP to 0.34 V after
addition of 51.3 equiv of TBACl and then remains at this
potential with further increases in [Cl-].
Similar, but not identical UV-vis spectra, are obtained
for the singly oxidized corrole 2 in CH2Cl2 and PhCN
solutions containing 0.1 M TBACl. As the first oxidation
proceeds at an applied potential of 0.35 V, the Soret band at
397 nm (CH2Cl2) or 410 nm (PhCN) decreases in intensity
and shifts to 415 nm in CH2Cl2 and 427 in PhCN. The 553
nm band (in CH2Cl2) or 557 nm band (in PhCN) disappears,
and a new band at 716 nm (CH2Cl2) or 722 nm (PhCN)
appears as the electron abstraction proceeds (Figure 8). The
final UV-vis spectrum in both cases is assigned to a
bis-Cl– complex, [(F5PhMes2Cor)CoCl2]-.
ESR Characterization of the Singly Oxidized Cobalt
Corrole. The site of the electron-transfer oxidation was
examined by ESR characterization of the singly oxidized
cobalt corrole 2. These spectra, obtained by the chemical
oxidation of 2 with 1 equiv of [Fe(bpy)3]3+ (bpy ) 2,2′-
bipyridine) in CH2Cl2 and PhCN are shown in parts a and b
of Figure 9, respectively. The observed g value (2.0032) is
the same under both solution conditions and is characteristic
of an organic radical; it is quite different from the large g
[(F5PhMes2Cor)Co]+ + PhCN h
[(F5PhMes2Cor)Co(PhCN)]+ (1)
We earlier demonstrated that chloride ions strongly bind
7
to the unoxidized (Me4Ph5Cor)Co in CH2Cl2 and similar
6732 Inorganic Chemistry, Vol. 47, No. 15, 2008

Clarification of the Oxidation State of Cobalt Corroles
Figure 7. (a) UV-vis spectral changes for (F5PhMes2Cor)Co (2) in CH2Cl2 upon addition of Cl- to solution and (b) dependence of E1/2 on the concentration
of TBACl added to solutions of CH2Cl2 containing 0.1 M TBAP.
value (2.037) observed for Co(IV) porphyrin complexes.53
Thus, the singly oxidized species is assigned as a Co(III)
corrole radical cation rather than as a Co(IV) corrole. The
same ESR spectrum is obtained by the oxidation of 2 by O2
in the presence of HClO4 (part c of Figure 9). Similar radical
cation ESR spectra were also obtained for 1+ (g ) 2.0033)
and 3+ (g ) 2.0034). Figure 10 shows HOMO orbitals
calculated by DFT at the B3LYP/6-31G(d) basis set. The
macrocycle-centered electron-transfer oxidation of 1-3 is
consistent with the calculated HOMO, where no metal orbital
is involved in the HOMO.
Redox Properties and Catalytic Reduction of O2
with HClO4. Because the highest activity of adsorbed cobalt
porphyrins and corroles toward oxygen reduction is obtained
in acid media,32–39 we examined all compounds in aqueous
solutions containing 1 M HClO4. Each examined complex
adsorbs strongly and irreversibly on an EPPG electrode.
Figure 11 illustrates cyclic voltammograms for (F5PhMes2-
Cor)Co (2) adsorbed on an EPPG electrode. In the absence
of dioxygen (curves a), the corrole exhibits a single redox
process centered at E1/2 ) 0.33 V (2). When the same
solutions are saturated with air (curves b), the responses of
(F5PhMes2Cor)Co (2)-coated graphite electrodes are similar
to each other and are characterized by an irreversible
reduction peak located at Epc ) 0.24-0.25 V for a scan rate
(53) Fukuzumi, S.; Miyamoto, K.; Suenobu, T.; Van Caemelbecke, E.;
Kadish, K. M. J. Am. Chem. Soc. 1998, 120, 2880–2889.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6733

Kadish et al.
and 0.35 V for 3, respectively), thus indicating that the
neutral Co(III) form of the cobalt corrole obtained on the
electrode surface when sweeping the potential positively from
+0.5 to 0.0 V catalyzes the electroreduction of O2 in acid
media as previously reported for related compounds.31,32
The half-wave potentials for O2 reduction at a rotating disk
electrode (Table 3) are similar for the corrole catalysts, 1,
2, and 3 (E1/2 ) 0.30 V) and indicate that the catalytic
reduction of O2 is not strongly influenced by differences in
the substitution pattern between the two compounds. How-
ever, in both cases the E1/2 for catalytic reduction of O2 occurs
at significantly lower potential than what has been reported
for previously examined cobalt monocorroles with ꢀ-pyrrole
substituents, that is (Me4Ph5Cor)Co (E1/2 ) 0.38 V);31 the
potentials are also lower than when the catalysts are
(PCY)H2Co,21 (PCY)FeClCoCl,32 or (PCY)MnClCoCl,32
which show an average E1/2 of 0.34 V for O2 electroreduction.
The shift in redox potential for the oxidation process of
(TPFCor)Co (1), (F5PhMes2Cor)Co (2), and (Mes3Cor)Co
(3) may be explained by the greater bulkiness of the meso-
substituents on the compounds. In both cases, the methyl
and fluoro groups at the ortho positions of meso-phenyl group
would prevent their rotation due to a strong repulsion by
the pyrrole hydrogens.42,54 As pointed out by Yamazaki et
al.,54 the bulkiness and orientation of these substituents
should decrease π-π interactions between the macrocycles
and the electrode surface, and this would result in a negative
shift of the half-wave potential for O2 reduction.
Figure 8. UV-vis spectral changes of (F5PhMes2Cor)Co (2) during the
first controlled potential oxidation at 0.35 V in (a) CH2Cl2 and (b) PhCN
containing 0.1 M TBACl.
To calculate the number of electrons transferred in the
catalytic electroreduction of dioxygen, voltammetric mea-
surements were also performed at a rotating disk electrode
(RDE). The RDE responses of (F5PhMes2Cor)Co (2) ad-
sorbed on the graphite electrode in 1 M HClO4 (part a of
Figure 12) show a single wave for the reduction of O2.
Similar behavior is observed for the tris(pentafluorophenyl)
derivative 1 and trimesityl derivative 3 (data not shown).
The half-wave potentials are located at 0.30 V for both
compounds (at i ) 0.5imax, where imax is the limiting current
measured at 100 rpm), and both E1/2 values become more
negative at higher rotation rates. The number of electrons
transferred during oxygen reduction was calculated from the
magnitude of the steady-state limiting current values, which
were taken at a fixed potential on the catalytic wave plateaus
of the different current-voltage curves in part a of Figure
12 (-0.1 V). If mass transport alone controls the reduction
of dioxygen at the 2-modified electrode, then the relationship
between the limiting current and rotation rate should obey
the Levich equation:55
Figure 9. ESR spectra of singly oxidized 2 produced by the chemical
oxidation of 2 (3.0 × 10-4 M) with (a) Fe(bpy)3(PF6)3 (3.0 × 10-4 M) in
CH2Cl2 at 183 K, (b) Fe(bpy)3(PF6)3 (3.0 × 10-4 M) in PhCN at 298 K,
and (c) HClO4 (2.0 × 10-2 M) in aerated PhCN at 298 K.
iLev ) 0.62nFAυ-1⁄6D2⁄3[O2]ω1⁄2
(3)
of 50 mV/s. There is also a larger current than under an argon
atmosphere as seen in Figure 11. In comparison, the reduction
of O2 at an uncoated graphite electrode occurs at a much
more-negative potential (Ep ) -0.41 V), which clearly
indicates that the reactivity of the electrode upon modification
with 2 must be attributed to the adsorbed cobalt corroles.
The catalyzed electroreduction of O2 by 1, 2, or 3 occurs at
potentials similar to those for the electrode reaction of just
the corrole under argon (E1/2 ) 0.34 V for 1, 0.33 V for 2,
where n is the number of electrons transferred, F is the
Faraday constant (96 485 C mol-1), A is the electrode area
(cm2), ν is the kinematic viscosity of the solution (cm2 s-1),
D is the dioxygen diffusion constant (cm2 s-1), [O2] is the
(54) Yamazaki, S.-I.; Yamda, Y.; Ioroi, T.; Fujiwara, N.; Siroma, Z.;
Yasuda, K.; Miyazaki, Y. J. Electroanal. Chem. 2005, 576, 253–259.
(55) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications, 2nd ed.; John Wiley & Sons, Inc: New York, 2001.
6734 Inorganic Chemistry, Vol. 47, No. 15, 2008

Clarification of the Oxidation State of Cobalt Corroles
Figure 10. HOMO orbitals of (a) (TPFCor)Co (1), (b) (F5PhMes2Cor)Co (2), and (c) (Mes3Cor)Co (3) calculated at the B3LYP/6-31G(d) level.
value, jk (mA cm-2), which can be obtained experimentally
from the intercept of the Koutecky-Levich line in part c of
Figure 12,
jk ) 103nFΓk[O2]
(5)
where k is the second-order rate constant of the reaction (M-1
s-1), which limits the plateau current, and Γ (mol cm-2) is
the surface concentration of 2 and the other terms have their
usual significance as described previously. The slope of the
Koutecky-Levich plot shows that the catalytic electrore-
duction of O2 by 2 is a two-electron process. The Koutecky-
Levich plot for 1 and 3 (not shown) also indicates a two-
electron reduction of O2 to H2O2 (Table 3) as previously
reported in the literature.35 In contrast, the number of
electrons transferred is 2.9 for (Me4Ph5Cor)Co,31 indicating
that the catalytic electroreduction of O2 by the cobalt complex
leads to the formation of H2O and H2O2 through processes
involving both two-electron and four-electron reactions. The
catalytic activity of this ꢀ-pyrrole-substituted corrole was
attributed to the spontaneous dimerization (and higher
aggregation) of the complex on the electrode surface.31 Such
a van der Waals-driven association has also been reported
for unsubstituted cobalt porphyrins46,61 and is believed to
create cobalt-cobalt separations that are small enough to
permit O2 molecules to coordinate simultaneously to two
cobalt centers. One result of such an orientation at the
electrode surface is a weakening of the O-O bond, which
would favor its breaking during the four-electron reduction
of O2 to H2O.46,62 In the case of 1-3, the two-electron
reduction of O2 clearly indicates that a dimerization (or
oligomerization) does not occur for either complex, and this
is consistent with the expected steric interactions involving
the bulky phenyl meso-substituents.61
Figure 11. Cyclic voltammograms of (F5PhMes2Cor)Co (2) adsorbed on
EPPG electrode. Supporting electrolyte: 1 M HClO4 (a) saturated with argon
and (b) saturated with air. Scan rate: 50 mV/s.
Table 3. Electroreduction of Dioxygen by Cobalt Corroles in
Air-Saturated 1 M HClO4
a
b
compound
Ep
E1/2
nc
(TPFCor)Co (1)
(F5PhMes2Cor)Co (2)
(Mes3Cor)Co (3)
0.25
0.24
0.25
0.30
0.30
0.30
2.0
2.0
2.2
a Peak potential of the dioxygen reduction wave (V vs SCE). b Half-wave
potential (V vs SCE) for dioxygen reduction at rotating disk electrode (ω
) 100 rpm). c Electrons consumed in the reduction of O2 as estimated from
the slope of the Koutecky-Levich plots.
bulk concentration of oxygen (mol dm-3), and ω is the rate
of rotation (rad s-1).
From eq 3, the plot of the limiting current density j (j )
i/A) as a function of the square root of the electrode rotation
rate ω1/2 should be a straight line intersecting the origin. Part
b of Figure 12 shows that the Levich plot56 of the plateau
currents versus ω1/2 is curved as is typical for the electrore-
duction of O2 catalyzed by adsorbed cobalt complexes.57 The
clear lack of linearity in the Levich plot suggests that the
catalytic reaction is limited by kinetics, not by mass transport.
A chemical step, previously assigned to the formation of a
cobalt(III)-O2 adduct,58 precedes the electron transfer and
limits the current to values below the convection-diffusion
limit.
The jk value obtained from the intercept of part c of Figure
12 was used to calculate the second-order rate constant k
for the formation of the (F5PhMes2Cor)CoIIIO2 complex on
the electrode surface. The value calculated at pH 0 using eq
5 is k ) 1.7 × 105 M-1 s-1, which is lower than that for the
catalytic electroreduction of dioxygen at the (Me4Ph5Cor)Co
The reciprocal of the limiting current density was plotted
against the reciprocal of the square root of the rotation rate
(Figure 12c). The straight line obeys the Koutecky-Levich
equation:59,60
(56) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall, Inc.:
Englewood Cliffs, N. J., 1962.
(57) Liu, H.-Y.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J. Phys. Chem.
1985, 89, 665–670.
(58) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.; Nocera, D. G. J. Am.
Chem. Soc. 2004, 126, 10013–10020.
1
j
1
1
)
+
(4)
(59) Koutecky, J.; Levich, V. G. Zh. Fiz. Khim. 1958, 32, 1565–1575.
(60) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192–1198.
(61) Song, E.; Shi, C.; Anson, F. C. Langmuir 1998, 14, 4315–4321.
(62) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36,
4294–4295.
0.62nFυ-1⁄6D2⁄3[O2]ω1⁄2
nFk[O2]
For large values of ω1/2 (ω f ∝), the current is controlled
by the kinetic process and reaches an asymptotic maximum
Inorganic Chemistry, Vol. 47, No. 15, 2008 6735

Kadish et al.
Figure 12. Catalyzed reduction of O2 in 1 M HClO4 at a rotating graphite disk electrode coated with (F5PhMes2Cor)Co (2). (a) Values of the rotation rates
of the electrode (ω) are indicated on each curve. The disk potential was scanned at 5 mV s-1. (b) Levich plots of the plateau currents of (a) vs (rotation
rate)1/2. The dashed line refers to the theoretical curve expected for the diffusion-convection limited reduction of O2 by 2e-. (c) Koutecky-Levich plots of
the reciprocal plateau currents vs (rotation rates)-1/2. Supporting electrolyte: 1 M HClO4 saturated with air.
(F5PhMes2Cor)Co (2) to an air-saturated MeCN solution
of Fe(C5H4Me)2 and HClO4 resulted in efficient oxidation
of ferrocene by O2. Figure 13 shows formation of the 1,1′-
dimethylferricenium ion [Fe(C5H4Me)2]+ during reduction
of O2 (1.7 × 10-3 M) by a large excess of Fe(C5H4Me)2
(2.0 × 10-2 M) in the presence of a catalytic amount of 2
(2.0 × 10-5 M) and HClO4 (2.0 × 10-2 M). The concentra-
tion of [Fe(C5H4Me)2]+ (3.4 × 10-3 M) formed in the
catalytic reduction of O2 by Fe(C5H4Me)2 is twice that of
the O2 concentration (1.7 × 10-3 M). Thus, only a two-
electron reduction of O2 occurs, and there is no further
reduction to produce more than 2 equiv of [Fe(C5H4Me)2]+
(eq 6).
Figure 13. Visible absorption spectral changes in the catalytic reduction
of O2 (1.7 × 10-3 M) by Fe(C5H4Me)2 (2.0 × 10-2 M) in the presence of
2Fe(C5H4Me)2 + O2 + 2H+
HClO4 (2.0 × 10-2 M) and 2 (2.0 × 10-5 M) in PhCN at 298 K.
(F5PhMes2Cor)Co (2)
2[Fe(C5H4Me)2)]+ + H2O2 (6)
modified electrode (k ) 5.7 × 105 M-1 s-1) but is identical
to a value reported for the electrocatalytic reduction of O2
by the cobalt porphyrin (diethylesterMe2Et2P)Co (k ) 1.4
× 105 M-1 s-1).36
In a similar manner, 1 and 3 also catalyze the two-electron
reduction of O2 by Fe(C5H4Me)2 in the presence of HClO4
and give as a corrole oxidation product the Co(III) π-cation
radical, [(·Cor)CoIII]+, as indicated in an earlier section of
the manuscript.
Catalytic Reduction of O2 in the Homogeneous Phase. The
catalytic two-electron reduction of O2 with cobalt corroles
was confirmed for the homogeneous phase using 1,1′-
dimethylferrocene Fe(C5H4Me)2 as a reductant in PhCN.
No oxidation of Fe(C5H4Me)2 by O2 occurred in the
presence of HClO4 in PhCN at 298 K but the addition of
Summary and Conclusions
In summary, we have shown that cobalt corroles can
be used as effective catalysts in the reduction of O2 with
HClO4 via the redox couple between Co(III) corroles and
6736 Inorganic Chemistry, Vol. 47, No. 15, 2008

Clarification of the Oxidation State of Cobalt Corroles
Co(III) corrole radical cations. The site of electron-transfer
oxidation is clearly identified as macrocycle centered by
detection of the ESR signal for the Co(III) corrole radical
cation. Koutecky-Levich plots for the meso-substituted
corroles 1-3 indicate a two-electron reduction of O2 to
H2O2. The catalytic activity of the previously studied
ꢀ-pyrrole-substituted complex (Me4Ph5Cor)Co involves
both two-electron and four-electron transfer reactions, and
this is attributed to spontaneous dimerization (and higher
aggregation) of the complex on the electrode surface.
Dimerization does not occur for the meso-substituted
corroles, and here only the two-electron transfer pathway
is observed.
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., grant E-680), the French Ministry
of Research (MENRT), CNRS (UMR 5260) and the
Ministry of Education, Culture, Sports, Science and
Technology, Japan (grant 19205019 and a Global COE
program, the Global Education and Research Center for
Bio-Environmental Chemistry) are gratefully acknowl-
edged. We also acknowledge the assistance of Wiroaj
Kajonkijya for collection of the O2 electrocatalytic reduc-
tion data using 3 as a catalyst.
IC800458S
Inorganic Chemistry, Vol. 47, No. 15, 2008 6737