Electrochemistry, spectroelectrochemistry, chloride binding, and O 2 catalytic reactions of free-base porphyrin - Cobalt corrole dyads

Article abstract of DOI:10.1021/ic050738n

Three face-to-face linked porphyrin-corrole dyads were investigated as to their electrochemistry, spectroelectrochemistry, and chloride-binding properties in dichloromethane or benzonitrile. The same three compounds were also investigated as to their ability to catalyze the electroreduction of dioxygen in aqueous 1 M HClO₄ or HCl when adsorbed on a graphite electrode. The characterized compounds are represented as (PCY)H₂Co, where P = a porphyrin dianion; C = a corrole trianion; and Y = a biphenylenyl, 9,9-dimethylxanthenyl, or anthracenyl spacer, which links the two macrocycles in a face-to-face arrangement. An axial binding of one or two Cl^- ligands to the cobalt center of the corrole is observed for singly and doubly oxidized (PCY)H₂Co, with the exact stoichiometry of the reaction depending upon the spacer size and the concentration of Cl^- added to solution. No Cl^- binding occurs for the neutral or reduced forms of the dyad, which contrasts with what is seen for the monocorrole, (Me ₄-Ph₅COr)Co, where a single Cl^- ligand is added to the Co(III) corrole in PhCN. The Co(III) form of the corrole in (PCY)H ₂Co also appears to be the catalytically active species in the electroreduction of dioxygen, which occurs at potentials associated with the Co(IV)/Co(III) reaction, that is, 0.35 V in 1 M HClO₄ as compared to 0.31-0.42 V for the same three dyads in PhCN and 0.1 M TBAP. The potential for the catalytic electroreduction of O₂ in HCl shifts negatively by 60 to 70 mV as compared to E_1/2 values in 1 M HClO₄, consistent with the binding of Cl^- to the Co(IV) form of the corrole and its rapid dissociation after electroreduction to Co(III) at the electrode surface.

Full text of DOI:10.1021/ic050738n

Inorg. Chem. 2005, 44, 6744−6754
Electrochemistry, Spectroelectrochemistry, Chloride Binding, and O2
Catalytic Reactions of Free-Base Porphyrin Cobalt Corrole Dyads
−
Karl M. Kadish,*,† Jianguo Shao,† Zhongping Ou,† Laurent Fre´mond,† Riqiang Zhan,† Fabien Burdet,‡
Jean-Michel Barbe,‡ Claude P. Gros,‡ and Roger Guilard*,‡
UniVersity of Houston, Department of Chemistry, Houston, Texas 77204-5003, and UniVersite´ de
Bourgogne, LIMSAG UMR 5633, Faculte´ des Sciences Gabriel, 6 BouleVard Gabriel,
21000 Dijon, France
Received May 10, 2005
Three face-to-face linked porphyrin−corrole dyads were investigated as to their electrochemistry, spectroelectro-
chemistry, and chloride-binding properties in dichloromethane or benzonitrile. The same three compounds were
also investigated as to their ability to catalyze the electroreduction of dioxygen in aqueous 1 M HClO4 or HCl when
adsorbed on a graphite electrode. The characterized compounds are represented as (PCY)H2Co, where P
) a
porphyrin dianion; C a corrole trianion; and Y a biphenylenyl, 9,9-dimethylxanthenyl, or anthracenyl spacer,
)
)
which links the two macrocycles in a face-to-face arrangement. An axial binding of one or two Cl- ligands to the
cobalt center of the corrole is observed for singly and doubly oxidized (PCY)H2Co, with the exact stoichiometry of
the reaction depending upon the spacer size and the concentration of Cl- added to solution. No Cl- binding
occurs for the neutral or reduced forms of the dyad, which contrasts with what is seen for the monocorrole, (Me4-
Ph5Cor)Co, where a single Cl- ligand is added to the Co(III) corrole in PhCN. The Co(III) form of the corrole in
(PCY)H2Co also appears to be the catalytically active species in the electroreduction of dioxygen, which occurs at
potentials associated with the Co(IV)/Co(III) reaction, that is, 0.35 V in 1 M HClO4 as compared to 0.31−0.42 V for
the same three dyads in PhCN and 0.1 M TBAP. The potential for the catalytic electroreduction of O2 in HCl shifts
negatively by 60 to 70 mV as compared to E1 values in 1 M HClO4, consistent with the binding of Cl- to the
/
2
Co(IV) form of the corrole and its rapid dissociation after electroreduction to Co(III) at the electrode surface.
Introduction
corrole dyads. We have also examined the use of face-to-
face bisCo(III) corroles or dyads containing one Co(III)
corrole and one Co(II) porphyrin as catalysts in the elec-
troreduction of O2 when adsorbed on a graphite electrode in
acid media.8
This present paper expands upon our earlier electrochemi-
cal and spectroelectrochemical studies in several respects.
For one, we here changed the examined species from a
In previous studies, we have examined the electrochemistry
and axial ligand binding ability of cobalt(III) corroles both
in their monomeric form1-3 and as part of a face-to-face
linked dyad with a second metallomacrocycle, either another
cobalt(III) corrole in the case of biscorroles4,5 or a Co(II),5,6
Fe(III),7 or Mn(III)7 porphyrin in the case of porphyrin-
* To whom correspondence should be addressed. E-mail: kkadish@uh.edu
(K.K.); Roger.Guilard@u-bourgogne.fr (R.G.).
(4) 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.
(5) 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.
(6) 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, 245-254.
(7) 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.
(8) 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.
† University of Houston.
‡ Universite´ de Bourgogne.
(1) (a) Guilard, R.; Barbe, J.-M.; Stern, C.; Kadish, K. M. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Elsevier:
New York, 2003; Vol. 18, pp 303-349. (b) Erben, C.; Will, S.; Kadish,
K. M. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard. R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp
233-300.
(2) 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.
(3) Kadish, K. M.; Shao, J.; Ou, Z.; Gros, C. P.; Bolze, F.; Barbe, J.-M.;
Guilard, R. Inorg. Chem. 2003, 42, 4062-4070.
6744 Inorganic Chemistry, Vol. 44, No. 19, 2005
10.1021/ic050738n CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/17/2005

Free-Base Porphyrin-Cobalt Corrole Dyads
Chart 1
Several face-to-face bis macrocycles with one cobalt
porphyrin and one free-base porphyrin have been examined
as to their efficiency in the electrocatalysis of O2 when
adsorbed on an electrode surface in acid media.11,12 These
monocobalt bisporphyrins exhibit four electron reduction
pathways, as evidenced by napp values much greater than 2.0,
and it was, therefore, of interest to examine the three (PCY)-
H2Co dyads’ reactivities toward O2 electroreduction in 1 M
HClO4 as well in 1 M HCl where Cl- anions were expected,
on the basis of previous studies,7 to bind to the Co(IV) and
maybe Co(III) forms of the complex.
Studies of Cl- binding to the (PCY)H2Co dyads in
nonaqueous media were especially of interest since hetero-
bimetallic dyads containing CoIV corroles were isolated in
the case of (PCY)MIIIClCoIVCl, where M ) Fe or Mn7 and
some of the compounds could be converted to bischloride
Co(IV) derivatives in the presence of excess Cl-. Especially
of importance, in this regard, is the fact that the binding of
Cl- to the metal centers of (TPP)CoII and (TPP)CoIIICl leads
to substantially easier oxidations than observed for the same
species in the presence of coordinated perchlorate anions,13-15
which is often added to solutions as a supporting electrolyte
when doing electrochemistry in nonaqueous media. Thus,
an additional goal of this study was to quantitatively measure
the changes in E1/2 for electrooxidation of (PCY)H2CoIII to
its Co(IV) form since these data might prove useful in the
synthesis or electrosynthesis of new Co(IV) corroles as well
as provide information as to how the chemical reactivity of
the chloride-containing (PCY)H2Co dyads would change as
a function of the type of bridge (A, X, and B), cobalt corrole
oxidation state, and number of axially coordinated chloride
ligands.
biscobalt complex with two corrole macrocycles4,5 or one
corrole and one porphyrin macrocycle5,6,8 to a series of face-
to-face linked dyads where one macrocycle is a cobalt(III)
corrole and the other a free-base porphyrin.9,10 This “re-
moval” of a second cobalt center greatly simplifies the
prevailing redox behavior and leads to a clearer understand-
ing of how the electron-transfer and chloride-binding reac-
tions of these face-to-face dyads are influenced in nonaque-
ous media by the type of spacer, size of the cavity, and
solution conditions. We recently demonstrated7 that the
Co(IV) form of corrole dyads linked in a face-to-face
arrangement with Mn(III) or Fe(III) porphyrins could be
stabilized in a high oxidation state by the binding of Cl-,
and the current study now presents quantitative data on the
binding of Cl- to both the Co(IV) and Co(III) forms of the
related monocorrole as well as to three free-base porphyrin-
Co(III) corrole dyads having the two macrocycles in a face-
to-face orientation. We have also measured the ability of
the same three dyads to catalyze the electroreduction of
dioxygen in 1 M HClO4 or 1 M HCl when adsorbed on a
graphite electrode, and these results are compared to what
has been reported for a series of biscobalt porphyrin-corrole
dyads8 with the same linker groups. The examined com-
pounds are illustrated in Chart 1 and are represented as
(PCY)H2Co, where P ) a porphyrin dianion; C ) a corrole
trianion; and Y is a biphenylenyl (B), 9,9-dimethylxanthenyl
(X), or anthracenyl (A) spacer, which links the two macro-
cycles.
Experimental Section
Instrumentation. Cyclic voltammetry was carried out with an
EG&G model 173 potentiostat. A three-electrode system was used
and consisted of a glassy carbon or 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 mixture. Half-wave
potentials were calculated as E1/2 ) (Epa + Epc)/2 and are referenced
to SCE.
Rotating disk experiments 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
monitored by an IBM-compatible PC microcomputer controlled by
the software M270 (EG&G PARC). An RDE4 bipotentiostat (Pine
Instrument) was employed with an HP 7090A three-channel digital
plotter for rotating ring-disk electrode experiments. The rotating
ring-disk electrode, purchased from the Pine Instrument Co.,
(11) Durand, R. R.; Bencosme, S. C.; Collman, J. P.; Anson, F. C. J. Am.
Chem. Soc. 1993, 105, 2710-2718.
(12) Liu, H.-Y.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J. Phys. Chem.
1985, 89, 665-670.
(13) Huet, J.; Gaudemer, A.; Boucly-Goester, C.; Boucly, P. Inorg. Chem.
1982, 21, 3413-3419.
(9) Barbe, J.-M.; Stern, C.; Pacholska, E.; Espinosa, E.; Guilard, R. J.
Porphyrins Phthalocyanines 2004, 8, 301-312.
(10) Barbe, J.-M.; Burdet, F.; Espinosa, E.; Gros, C. P.; Guilard, R. J.
Porphyrins Phthalocyanines 2003, 7, 365-374.
(14) Kadish, K. M.; Lin, X. Q.; Han, B. C. Inorg. Chem. 1987, 26, 4161-
4167.
(15) Constant, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2253-2260.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6745

Kadish et al.
Table 1. Half-Wave Potentials (V vs SCE) for Porphyrin-Corrole Dyads, (PCY)H2Co in Nonaqueous Media Containing 0.1 M TBAP
oxidation
reduction
compound
solvent
PhCN
porphyrin
corrole
0.72
corrole
-1.67
porphyrin
(Me4Ph5Cor)Coa
(Et2Me6PhPor)H2
(PCA)H2Co
(PCX)H2Co
(PCB)H2Co
1.45b
0.47
-0.16
1.06b
0.92b
-1.42
-1.79
-1.77
-1.73
-1.88b
1.22b
0.85
0.83
0.72
0.65
0.69
(0.42, 0.32)c
(0.41, 0.31)c
0.40
-0.26
-0.38
-0.37
-1.67
-1.55
-1.59
-1.41
-1.41
-1.41
1.09
(Me4Ph5Cor)Coa
(Et2Me6PhPor)H2
(PCA)H2Co
(PCX)H2Co
(PCB)H2Co
CH2Cl2
1.26
0.87
(0.62, 0.45)c
-0.15
-1.69
1.08b
0.90b
1.22b
0.87
0.87
-1.39
-1.42
-1.44b
-1.41
-1.84b
0.74
0.68
0.71
(0.41, 0.26)c
(0.41, 0.24)c
0.39
-0.20
-0.39
-0.38
-1.74b
-1.62b
-1.74b
-1.80b
-1.74b
1.10
a Ref 3. b Peak potential at a scan rate of 0.1 V/s. c Potentials in parentheses correspond to oxidations of a dimer formed between two dyads in solution.
consisted of a platinum ring and a removable edge-plane pyrolytic
graphite disk (A ) 0.282 cm2).
of E1/2 versus log [Cl-] from which both the stoichiometry and
binding constants can be determined.
Chemicals and Reagents. Absolute dichloromethane (CH2Cl2)
was obtained from Fluka Chemical Co. and used as received.
Pyridine (py) was distilled over KOH under argon prior to use.
Benzonitrile (PhCN) was purchased from Aldrich Chemical Co.
and distilled over P2O5 under a vacuum 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. Tetrabutylammonium chloride
(TBACl) was purchased from Sigma-Aldrich and used as received.
Perchloric acid (HClO4, 70%) was purchased from Mallinckrodt,
while hydrochloric acid (HCl, 36.5-38.0%) was purchased from
EMD and used as received.
Starting Compounds. The following examined compounds were
synthesized according to previously described procedures:10 1-(13,17-
diethyl-2,3,7,8,12,18-hexamethylporphyrin-5-yl)-8-[cobalt(III)-
2,3,17,18-tetraphenyl-7,8,12,13-tetramethylcorrol-10-yl]biphen-
ylene, (PCB)H2Co; 1-(13,17-diethyl-2,3,7,8,12,13,18-hexamethyl-
porphyrin-5-yl)-5-[cobalt(III)-2,3,17,18-tetraphenyl-7,8,12,13-tetra-
methylcorrol-10-yl]-9,9-dimethylxanthene, (PCX)H2Co; 1-(13,17-di-
ethyl-2,3,7,8,12,18-hexamethylporphyrin-5-yl)-8-[cobalt(III)-7,8,12,13-
tetramethyl-2,3,17,18-tetraphenylcorrol-10-yl]anthracene, (PCA)-
H2Co; 13,17-diethyl-2,3,7,8,12,18-hexamethyl-5-phenylporphyrin,
(Et2Me6PhPor)H2;4 and cobalt(III)-7,8,12,13-tetramethyl-2,3,10,17,18-
pentaphenylcorrole, (Me4Ph5Cor)Co.2
UV-visible spectroelectrochemical experiments were performed
with an optically transparent platinum thin-layer electrode whose
construction is described in the literature.16 Potentials were applied
with an EG&G model 173 potentiostat. Time-resolved UV-visible
spectra were recorded with a Hewlett-Packard model 8453 diode
array rapid-scanning spectrophotometer. UV-visible spectra of the
neutral complexes were taken with the HP model 8453 spectro-
photometer or with a Varian Cary 50 spectrophotometer.
Electrode Surface Preparation for O2 Catalysis. Before use,
the graphite disk was polished separately from the platinum ring
with 600 grit SiC paper, rinsed with water, and wiped off to remove
any free graphite particles. The molecular catalyst was irreversibly
adsorbed on the electrode surface by means of a dip-coating
procedure described previously.17 The platinum ring was succes-
sively cleaned with a 1 µm diamond paste and a 5 µm alumina
slurry and rinsed thoroughly with water after each polishing step.
After sonication in water for 1 min, the ring-disk electrode was
introduced into air-saturated aqueous 1 M HClO4 or 1 M HCl and
the platinum ring was activated by cycling between 1.20 and -0.24
V until reproducible voltammograms were obtained.
Chloride-Binding Reactions Monitored by UV-Visible Spec-
troscopy. The binding of chloride ions to the neutral Co(III) corrole
was monitored by UV-visible spectroscopy in PhCN at room
temperature (296 K). The absorbance data were then fitted to the
Hill equation (eq 1).18
Results and Discussion
log[(Ai - A0)/(Af - Ai)] ) log K + p log [L]
(1)
Electrochemistry in Nonaqueous Media. The electro-
chemistry of the three dyads in CH2Cl2 and PhCN is similar
to that of the related monocorrole and monoporphyrin under
the same solution conditions. The half-wave potentials are
listed in Table 1, and cyclic voltammograms of (PCX)H2Co
and (PCB)H2Co in the two solvents are shown in Figure 1.
The occurrence of dimerization upon oxidation of cobalt
monocorroles such as (Me4Ph5Cor)Co2,3 and (OEC)Co20 in
CH2Cl2 or PhCN has been well-documented in the literature.
This behavior is electrochemically characterized by a splitting
of the first oxidation into two processes with equal current
heights, each of which is half as high as the other redox
reactions of the same compound. This is seen in the present
study for both (PCX)H2Co (Figure 1) and (PCA)H2Co in
CH2Cl2 or PhCN but not for (PCB)H2Co in either solvent.
where Ai ) absorbance at a specific concentration of ligand L, A0
) initial absorbance where [L] ) 0, and Af ) final absorbance
where the fully ligated complex is the only species presented. Values
of log K were obtained from the intercept of the regression line in
a plot of log[(Ai - A0)/(Af - Ai)] versus log [py]. The slope, p, is
equal to the number of axially coordinated ligands.
Chloride-Binding Reactions Monitored by Electrochemistry.
The binding of chloride ions to the neutral and oxidized forms of
the corroles was also monitored electrochemically by measuring
the reversible half-wave potentials in CH2Cl2 or PhCN containing
added Cl- in the form of TBACl. The relevant equations are given
in the literature,19 and the data analysis involves diagnostic plots
(16) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1489-1501.
(17) Shi, C.; Anson, F. C. Inorg. Chem. 1998, 37, 1037-1043.
(18) Ellis, J. P. E.; Linard, J. E.; Szymanski, T.; Jones, R. D.; Budge, J.
R.; Basolo, F. J. Am. Chem. Soc. 1980, 102, 1889-1896.
(19) Crow, D. R. Polarography of Metal Complexes; Academic Press:
London, 1969.
(20) Kadish, K. M.; Adamian, V. A.; Van Caemelbecke, E.; Gueletii, E.;
Will, S.; Erben, C.; Vogel, E. J. Am. Chem. Soc. 1998, 120, 11986-
11993.
6746 Inorganic Chemistry, Vol. 44, No. 19, 2005

Free-Base Porphyrin-Cobalt Corrole Dyads
absolute potential difference in E1/2 between the two split
oxidations of (PCX)H2Co (Figure 1a; ∆E1/2) is equal to 170
mV in CH2Cl2 and 100 mV in PhCN, while ∆E1/2 for
(PCA)H2Co is 150 mV in CH2Cl2 and 100 mV in PhCN
(see Table 1). The separations in half-wave potentials for
(PCA)H2Co and (PCX)H2Co in CH2Cl2 can be compared to
a ∆E1/2 of 170 mV for the monocorrole, (Me4Ph5Cor)Co,
under the same solution conditions,2,3 but no dimerization
occurs for the monocorrole in PhCN.3 The fact that no
dimerization is observed for (PCB)H2Co in PhCN or CH2Cl2
(that is, no splitting of the first oxidation) can be attributed
to a stronger internal π-π interaction between the two
macrocycles of the dyad, which would tend to weaken any
π-π interaction between the corrole macrocycles of two
separate (PCB)H2Co complexes.
Further differences between current-voltage curves of the
three dyads in CH2Cl2 or PhCN involve the Co(III)/Co(II)
process. This reaction occurs at E1/2 ) -0.37 to -0.39 V
for (PCX)H2Co or (PCB)H2Co (see Table 1), while the same
metal-centered electrode reaction of (PCA)H2Co is located
at E1/2 ) -0.20 V in CH2Cl2 and -0.26 V in PhCN. The
half-wave potentials for the Co(III)/Co(II) reactions of the
three dyads are all more negative than E1/2 values for
reduction of the monocorrole (-0.15 to -0.16 V) in the same
nonaqueous solvents (Table 1). This parallels what has been
observed for other related biscorroles and porphyrin-corrole
dyads having B or A bridges1a,4,5 and can be accounted for
by an increased interaction between the two face-to-face π
systems of the dyads, which are close to each other and,
thus, have a more negative reduction potential. The larger
the internal interaction between the two π systems of the
dyad, the harder the reduction, and this leads to the following
proposed order of internal π-π interaction between the
porphyrin and corrole macrocycles in the currently investi-
gated series of compounds: (PCA)H2Co < (PCX)H2Co ≈
(PCB)H2Co.
UV-Visible Spectra of Initial Compounds. UV-visible
spectral data for the three dyads in PhCN and CH2Cl2 are
shown in Table 2. The Soret band of the 9,9-dimethylxan-
thene (X)- and biphenylene (B)-bridged compounds are at
similar wavelengths and blue-shifted by 10-13 nm as
compared to λmax of the anthracene (A)-bridged compound
in the same two solvents. In addition, all three dyads show
an additional weak band at 623-630 nm, which is also seen
for (Et2Me6PhPor)H2 but not for (Me4Ph5Cor)Co, thus,
providing evidence that this latter band is associated with
the free-base porphyrin macrocycle of the dyad. The Soret
region of the spectrum is also dominated by porphyrin
transitions in the case of the dyads, and this is most evident
in Table 2 by the similarity in λmax and ꢀ values for bands in
the spectra of the three dyads and the monoporphyrin,
(Et2Me6PhPor)H2.
Figure 1. Cyclic voltammograms of (a) (PCX)H2Co and (b) (PCB)H2Co
in CH2Cl2 or PhCN containing 0.1 M TBAP.
The electronic interaction between equivalent redox sites
has been investigated for oxidation and reduction of free-
base face-to-face bisporphyrins as well as bisporphyrin
derivatives containing Co(II), Zn(II), Cu(II), or Ni(II) metal
ions.21-24 The interaction between two equivalent redox sites
on Co(III)2 or Cu(III)25 biscorroles has also been electro-
chemically examined for dyads with the A and B bridges
linking the two macrocycles and gave separations between
the two “split” E1/2 values of 130-400 mV, depending upon
the solvent and the size of the linking group.
The interaction between cobalt centers of two different
(PCY)H2Co dyads in solution also depends on the solvent
and the linking group (Y) as shown by the cyclic voltam-
mograms in Figure 1 and the electrochemical data in Table
1. Here, the two interacting electroactive units are proposed
to be part of a π-π dimer between two identical corrole
units of the dyad, either (PCA)H2Co or (PCX)H2Co. The
larger the interaction between the two cobalt corrole units
of the “dimerized” (PCY)H2Co, the larger will be the
separation between the first two oxidation processes. The
(21) Le Mest, Y.; L’Her, M.; Hendricks, N. H.; Kim, K.; Collman, J. P.
Inorg. Chem. 1992, 31, 835-847.
(22) Le Mest, Y.; L’Her, M.; Saillard, J.-Y. Inorg. Chim. Acta 1996, 248,
181-191.
(23) Le Mest, Y.; L’Her, M.; Courtot-Coupez, J.; Collman, J. P.; Evitt, E.
R.; Bencosme, C. S. J. Electroanal. Chem. 1985, 184, 331-346.
(24) Le Mest, Y.; L’Her, M.; Collman, J. P.; Kim, K.; Hendricks, N. H.;
Helm, S. J. Electroanal. Chem. 1987, 234, 277-295.
(25) Guilard, R.; Gros, C. P.; Barbe, J.-M.; Espinosa, E.; Je´roˆme, F.; Tabard,
A.; Latour, J.-M.; Shao, J.; Ou, Z.; Kadish, K. M. Inorg. Chem. 2004,
43, 7441-7455.
Chloride Binding to the Cobalt Center of the Mono-
corrole. To investigate the effect of Cl- on the electrode
reactions of (PCY)H2Co, we first characterized the electro-
chemistry of the monocorrole, (Me4Ph5Cor)Co, in PhCN
containing 0.1 M TBAP and excess Cl-. These results are
discussed below and provide clear evidence for the binding
Inorganic Chemistry, Vol. 44, No. 19, 2005 6747

Kadish et al.
Table 2. UV-Visible Spectral Data (λmax, ꢀ × 10-4 L mol-1 cm-1) for (PCY)H2Co Dyads in Different Solvents
solvent
compound
Soret region
visible regiona
PhCN
(Me4Ph5Cor)Co
(Et2Me6PhPor)H2
(PCA)H2Co
(PCX)H2Co
(PCB)H2Co
399 (8.9)
535 (2.3)
503 (1.8)
507 (2.4)
507 (2.4)
507 (1.7)
407 (19.4)
405 (15.2)
394 (21.8)
392 (14.3)
535 (0.9)
538 (1.9)
538 (1.9)
530 (1.6)sh
572 (0.9)
625 (0.4)
627 (0.8)
625 (0.7)
626 (0.5)sh
574 (1.5)
572 (1.5)
575 (1.0)sh
CH2Cl2
(Me4Ph5Cor)Co
(Et2Me6PhPor)H2
(PCA)H2Co
(PCX)H2Co
(PCB)H2Co
398 (4.5)
529 (1.1)
501 (1.3)
507 (2.2)
507 (2.1)
505 (1.8)
402 (15.7)
397 (14.7)
387 (16.6)
387 (15.8)
534 (0.7)
535 (1.9)
536 (1.8)
528 (1.6)sh
571 (0.6)
623 (0.3)
627 (0.5)
630 (0.4)
629 (0.4)sh
570 (1.1)sh
571 (1.0)sh
573 (1.0)sh
a sh ) shoulder.
of Cl- to the electrogenerated Co(IV) form of the mono-
corrole as well as to the initial Co(III) species under
conditions of higher chloride concentration.
in PhCN containing the highest concentrations of added
TBACl.
The data in Figure 2 are consistent with the binding
of Cl- to (Me4Ph5Cor)CoIII, [(Me4Ph5Cor)CoIV]+, and
[(Me4Ph5Cor•+)CoIV]2+, which results in a shift of E1/2 values
for all three processes toward more negative potentials. The
magnitude of the shift between solutions containing 0.1 M
TBAP and those with 0.1 M TBAP plus 60 equiv of Cl-
amounts to 80 mV in the case of the Co(III)/Co(II) reaction
(-0.16 f -0.24 V), 410 mV in the case of the Co(IV)/
Co(III) reaction (0.47 f 0.06 V), and 450 mV in the case
of the Co(IV) radical/Co(IV) process (0.72 f 0.27 V).
A linear relationship is obtained between half-wave
potentials for the first oxidation of (Me4Ph5Cor)Co to its
Co(IV) form and log [TBACl] in PhCN (the middle process
in Figure 2), and the slope of -56 mV (Figure 3a) is close
to the predicted theoretical Nernstian value of -59 mV for
an electrode reaction involving the loss of one axially bound
Cl- ligand upon conversion of Co(IV) to Co(III).
The Co(IV) center of [(Me4Ph5Cor)CoIV]+ can axially bind
one or two Cl- ligands, and thus, either of the two electrode
reactions given by eqs 2 and 3 could account for the -56
mV slope in Figure 3a.
Figure 2 illustrates cyclic voltammograms of the cobalt
corrole monomer, (Me4Ph5Cor)Co, in PhCN and 0.1 M
TBAP containing different concentrations of added TBACl.
The examined potential range was from -0.8 to 1.4 V in
PhCN containing 0.1 M TBAP, but the anodic (positive) limit
was not extended beyond +0.60 V in solutions of TBACl
in order to avoid the oxidation of Cl- that occurs at about
0.8 V. Three well-defined processes are seen under all
solution conditions, with the most positive E1/2 values being
observed in PhCN and 0.1 M TBAP and the most negative
(Me4Ph5Cor)CoIVCl + e- h (Me4Ph5Cor)CoIII + Cl- (2)
[(Me4Ph5Cor)CoIVCl2]- + e- h
[(Me4Ph5Cor)CoIIICl]- + Cl- (3)
On the basis of results for the related (PCY)MClCoCl
complexes [M ) Fe(III) or Mn(III)],7 the electrochemical
reduction shown by eq 2 most likely occurs in PhCN
solutions containing 1 equiv of Cl-, but at higher concentra-
tions of chloride, the prevailing reaction in PhCN involves
reduction of a bis-Cl Co(IV) complex to a mono-Cl Co(III)
species, as shown in eq 3. Electrochemistry alone cannot
differentiate the above two possibilities in solutions contain-
ing greater than 2 equiv of Cl-, but the prevailing electron
transfer mechanism can easily be determined by indepen-
dently measuring the binding of Cl- to the Co(III) form of
the monocorrole using UV-visible spectroscopy. These
results are shown in Figure 3b, which illustrates the changes
observed during a spectrally monitored titration of
(Me4Ph5Cor)Co in PhCN with increasing amounts of TBACl.
The spectral changes at 534 and 566 nm were analyzed by
Figure 2. Cyclic voltammograms of (Me4Ph5Cor)Co in PhCN containing
0.1 M TBAP and 0-60 equiv of TBACl.
6748 Inorganic Chemistry, Vol. 44, No. 19, 2005

Free-Base Porphyrin-Cobalt Corrole Dyads
In summary, the addition of one Cl- ligand to
(Me4Ph5Cor)CoIII generates [(Me4Ph5Cor)CoIIICl]-, which is
electrochemically oxidized to [(Me4Ph5Cor)CoIVCl2]- in the
presence of excess Cl- according to the electrode reaction
given by eq 3. A knowledge of this stoichiometry, when
combined with the log K ) 2.88 for reaction 4, the Nernstian
slope of the line in Figure 3a, and the classical Lingane
equation19 then enables a calculation of the binding constant
for the addition of two Cl- ligands to [(Cor)CoIV]+. This
ligand addition reaction is shown in eq 5, where log â2 )
11.4.
[(Me4Ph5Cor)CoIV]+ + 2Cl- h [(Me4Ph5Cor)CoIVCl2]- (5)
In the same manner, the 450 mV negative shift in E1/2
(from 0.72 to 0.27 V) for the second oxidation in PhCN,
0.1 M TBAP upon the addition of g60 equiv of Cl- enables
calculation of the Cl- binding constant for the doubly
oxidized complex, [(Me4Ph5Cor•+)CoIV]2+, which is a Co(IV)
corrole π-cation radical. This reaction is shown in eq 6,
where log â2 ) 19.0.
[(Me4Ph5Cor•+)CoIV]2+ + 2Cl- h (Me4Ph5Cor)CoIVCl2 (6)
The above calculation was done by inserting the value of
log â2 ) 11.4 for Cl- binding to Co(IV) (eq 5) into eq 7,
where â2,Co(IV) describes Cl- binding as seen in eq 5 and
â2,Co(IV)
the Cl- binding shown in eq 6.19 A similar
radical
method has been used in the past for elucidating pyridine
binding constants to a number of oxidized porphyrins, one
of which is (TPP)Zn.26,27
â2,Co(IV)radical
0.059
n
∆E1/2
)
log
(7)
â2,Co(IV)
Figure 3. (a) Plot of E1/2 for the [CoIVCl2]-/[CoIIICl]- process of
(Me4Ph5Cor)Co in PhCN and 0.1 M TBAP vs log [TBACl] added to solution
and (b) UV-visible spectral changes of 2.5 × 10-5 M (Me4Ph5Cor)Co in
PhCN with increasing TBACl concentrations from 1.5 × 10-4 to 8.1 ×
10-3 M. Inset shows the Hill plot.
The Co(II) corrole in electrogenerated [(Me4Ph5Cor)Co]-
does not bind Cl-, as clearly seen by a comparison of the
UV-visible spectrum for the singly reduced complex in
PhCN with and without added Cl- (Figure 4b and d). The
anionic Co(II) corrole in PhCN and 0.1 M TBAP exhibits a
spectrum with the Soret band at 424 nm and two visible
bands at 561 and 642 nm (Figure 4d). The same spectrum is
obtained in PhCN containing 0.1 M TBAP plus 100 equiv
of added Cl- (Figure 4b).
The UV-visible spectral changes obtained during the first
oxidation of (Me4Ph5Cor)Co [the Co(III)/Co(IV) process]
with and without added Cl- are also shown in Figure 4a
and c. The spectra of the singly oxidized species are different
from each other under the different solution conditions.
[(Cor)CoIV(Cl)2]-, generated in a PhCN solution containing
0.1 M TBAP and 100 equiv of TBACl (Figure 4a) has a
broad Soret band with a maximum at 448 nm and a weak
visible band located at 731 nm. In contrast, the [(Cor)CoIV]+
species generated in PhCN solutions with no added TBACl
(Figure 4c) has a split Soret band at 383 and 442 nm and a
weak visible band at 680 nm.
plotting log[(Ai - Ao)/(Af - Ai)] versus log [TBACl], and
the latter correlation is shown in the inset of Figure 3b where
the slope of the line is 1.0, thus demonstrating that only one
Cl- ligand is bound to (Me4Ph5Cor)CoIII with a log K )
2.88. The prevailing axial ligand binding reaction to Co(III)
is given by eq 4 and, when combined with the electrochemi-
cal data in Figure 3a, eliminates the reaction in eq 2 as a
possible electron transfer mechanism in PhCN solutions
containing high Cl- concentrations.
(Me4Ph5Cor)CoIII + Cl- h [(Me4Ph5Cor)CoIIICl]- (4)
It is interesting to note that the UV-visible spectrum for
[(Me4Ph5Cor)CoIIICl]- in PhCN is virtually identical in its
Q-band region to the spectrum of the mono-CO adduct,
(Me4Ph5Cor)Co(CO), which has a band at 567 nm and a
shoulder at 548 nm in CH2Cl2.2 This similarity is shown in
Figure S1 (Supporting Information) where the major Q band
of [(Me4Ph5Cor)CoCl]- is at 566 nm and a shoulder is at
546 nm.
(26) Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.; Bottomley, L. A. Inorg.
Chem. 1981, 20, 1274-1277.
(27) Kadish, K. M.; Rhodes, R. K. Inorg. Chem. 1981, 20, 2961-296.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6749

Kadish et al.
Figure 4. UV-visible spectral changes for (a) oxidation and (b) reduction of (Me4Ph5Cor)Co in PhCN and 0.1 M TBAP containing 100 equiv of TBACl
as compared to (c) oxidation and (d) reduction of the same solution not containing TBACl.
Scheme 1
TBAP is characterized by a Co(III)/Co(II) reduction at E1/2
) -0.26 V and two oxidations, the first of which is split
into two overlapping processes indicative of weak dimer-
ization of the two dyads as compared to what is observed in
the case of (PCX)H2Co (see Figure 1).
The cyclic voltammograms of (PCA)H2Co in PhCN
containing TBACl are similar to what is observed for the
monocorrole, (Me4Ph5Cor)Co, under similar solution condi-
tions (see Figure 2). The stepwise abstraction of two electrons
from the corrole unit of (PCA)H2Co occurs at E1/2 values
that, in the presence of added Cl-, are substantially shifted
The spectral and electrochemical data for the three
investigated cobalt corrole redox processes of the monocor-
role, namely, CoIV radical/CoIV, CoIV/CoIII, and CoIII/CoII,
toward negative potentials as compared to the oxidations in
are self-consistent in the presence and absence of added Cl-,
PhCN and 0.1 M TBAP, consistent with a strong binding of
and the overall electron transfer and Cl- addition processes
Cl- to the singly and doubly oxidized forms of the dyad.
are presented in Scheme 1, where the upper pathway
No shift in E1/2 is observed for the Co(III)/Co(II) process of
describes the redox reactions in PhCN, 0.1 M TBAP and
the lower one the reactions in the same solutions containing
60 equiv of added TBACl. The chloride binding constants
(PCA)H2Co upon Cl- addition nor are any UV-visible
spectral changes observed for neutral (PCA)H2Co upon
addition of Cl-, thus indicating the lack of Cl- binding to
the Co(III) or electrogenerated Co(II) centers of the PCA
complex.
for each oxidation state of the monocorrole are shown in
Scheme 1 and range from log â2 ) 19.0 for the CoIV corrole
π-cation radical to log K1 ) 2.88 for the neutral CoIII form
The number of Cl- ligands bound to the Co(IV) unit of
of the compound.
Chloride Binding to (PCY)H2Co. Binding of Cl- to the
cobalt center of the three dyads in their various oxidation
states was also electrochemically investigated in PhCN
containing 0.1 M TBAP and Cl- added stepwise in the form
of TBACl. Examples of the resulting cyclic voltammograms
are shown in Figure 5a for (PCA)H2Co in PhCN containing
0-160 equiv of added TBACl, and a plot of E1/2 for the
Co(IV)/Co(III) process versus log [TBACl] is shown in
Figure 5b. The initial voltammogram in PhCN and 0.1 M
singly oxidized (PCA)H2Co was obtained by analyzing the
potential shift of the Co(IV)/Co(III) process as a function
of increasing Cl- concentration, and the relevant diagnostic
plot is shown in Figure 5b. A straight line slope of -57 mV
is obtained, consistent with the predicted theoretical slope
of -59 mV for a reaction involving the loss of one Cl- ligand
upon going from the singly oxidized Co(IV) corrole to its
neutral Co(III) form. Combining this result with the fact that
the Co(III) in (PCA)H2Co does not bind Cl- (see above
6750 Inorganic Chemistry, Vol. 44, No. 19, 2005

Free-Base Porphyrin-Cobalt Corrole Dyads
Table 3. Half-Wave Potentials (V vs SCE) of (PCY)H2Co in PhCN
and 0.1 M TBAP
PhCN + excess Cl-
PhCN
(160 equiv)
compound second ox first ox first red second ox first ox first red
(PCA)H2Co
(PCX)H2Co
(PCB)H2Co
(Me4Ph5Cor)Co
0.72
0.65
0.69
0.72
0.42, 0.32 -0.26
0.41, 0.31 -0.38
0.33
a
0.09 -0.25
0.05 -0.38
0.07 -0.37
0.06b -0.25b
0.40
0.47
-0.37
-0.16
a
0.27b
a Reaction occurs at E1/2 values beyond the positive potential limit of
the solvent with 160 equiv of Cl- (∼0.60 V vs SCE). b PhCN + 120 equiv
of Cl-.
Scheme 2
h CoIII process of the same corrole linked in a face-to-face
orientation to FeIII or MnIII porphyrins.7
A further oxidation of (PCA)H2CoIVCl to its Co(IV)
π-cation radical form in PhCN solutions containing 160 equiv
of Cl- gives (PCA)H2CoIVCl2 at E1/2 ) 0.33 V, and this half-
wave potential, which remains invariant with further addi-
tions of Cl- to solution, can be compared to the measured
E1/2 ) 0.27 V for the same redox process of monomeric (Me4-
Ph5Cor)Co under similar solution conditions (see Figure 2
and Table 3). The E1/2 ) 0.33 V for the above process can
also be combined with the E1/2 ) 0.72 V for the same
electrode reaction in the absence of TBACl (see Figure 5a),
and using eq 7 leads to a log â2 ) 13.1 for the binding of
2Cl- to the doubly oxidized Co(IV) π-cation radical.
The proposed mechanism for the oxidation and reduction
of the cobalt center in (PCA)H2Co is given in Scheme 2
and differs from the monocorrole in two aspects. The first
is that Cl- does not coordinate to Co(III) in the dyad, and
the second is that only one Cl- ligand binds to Co(IV) in
PhCN solutions containing up to 160 equiv of TBACl.
Figure 5. (a) Cyclic voltammograms of (PCA)H2Co in PhCN and 0.1 M
TBAP with added TBACl and (b) plot of E1/2 for the CoIVCl/CoIII process
vs log [TBACl].
discussion) leads to the conclusion that the Co(IV) corrole
in [(PCA)H2CoIV]+ binds only one Cl- axial ligand to give
(PCA)H2CoIVCl in solutions containing added Cl-.
The formation constant for the binding of one Cl- ligand
to [(PCA)H2CoIV]+ was calculated using the Lingane equa-
tion19 combined with the data in Figure 5b and gives a log
K1 ) 6.5 for the reaction shown in eq 8.
Electrochemically monitored chloride titrations were also
carried out for (PCX)H2Co (Figure 6) and (PCB)H2Co
(Figure S2, Supporting Information). The (PCX)H2Co com-
plex in CH2Cl2 or PhCN containing 0.1 M TBAP has a first
oxidation that is split into two processes of equal current
height, indicating a π-π dimerization between corrole
macrocycles of two different (PCX)H2Co species. The dimer
is broken up upon addition of 1.0 equiv of Cl- to the solution
(see Figure 6c), and under these conditions, the two reversible
oxidations in Figure 6a are replaced by an irreversible
[(PCA)H2CoIV]+ + Cl- h (PCA)H2CoIVCl
(8)
The Cl- axial ligand dissociates during the
Co(IV) h Co(III) reaction, and the relevant redox process
is then given by eq 9.
Co(IV)/Co(III) process with a coupled reduction at Epc
)
(PCA)H2CoIVCl + e- h (PCA)H2CoIII + Cl-
(9)
0.03 V and an oxidation at Epa ) 0.28 V for a scan rate of
0.1 V/s. The split peaks then merge into a single reversible
oxidation/reduction couple at higher Cl- concentrations, and
this is shown by the cyclic voltammogram in Figure 6d. The
second oxidation of the corrole unit is not seen under these
conditions.
The E1/2 for the above reaction is 0.20 V in PhCN solutions
containing 2 equiv of Cl- and shifts to 0.09 V in solutions
containing 160 equiv of Cl- (Figure 5a). Both half-wave
potentials are within the range of E1/2 values for the CoIVCl
Inorganic Chemistry, Vol. 44, No. 19, 2005 6751

Kadish et al.
Redox Properties of (PCA)H2Co, (PCB)H2Co, and
(PCX)H2Co and the Catalytic Reduction of O2 in 1 M
HClO4. The catalytic activity of the (PCY)H2Co dyads
toward the reduction of O2 was examined by cyclic volta-
mmetry and rotating ring-disk electrode voltammetry in
aqueous solutions of 1 M HClO4. Current-potential curves
recorded at a graphite disk coated with (PCA)H2Co and
(PCB)H2Co are illustrated in Figure 7. In the absence of
dioxygen, the response of the graphite electrode coated with
(PCA)H2Co resembles that obtained with the (PCB)H2Co-
coated electrode (Figure 7a). The cyclic voltammogram of
(PCB)H2Co shows two reversible processes located at E1/2
) 0.37 and -0.02 V in 1 M HClO4, whereas the (PCA)H2Co
system is characterized by a reversible process at E1/2 ) 0.36
V and an irreversible reduction peak at Epc ) -0.12 V. On
the basis of comparisons with the electrochemical response
of (Me4Ph5Cor)Co adsorbed on a graphite electrode (E1/2
)
0.38, 0.20, and -0.08 V),8 the two processes of (PCB)H2Co
at E1/2 ) 0.37 and -0.02 V can be assigned to the formal
potentials of the Co(IV)/Co(III) and Co(III)/Co(II) couples,
respectively. The presence of an additional reversible process
at E1/2 ) 0.20 V for (Me4Ph5Cor)Co was earlier explained
by the tendency of the corrole to spontaneously dimerize8
(or to form higher aggregates) on the graphite surface. This
electrochemical behavior is not observed for the three
investigated free-base porphyrin-cobalt corrole dyads.
When the solution is saturated with air (Figure 7b), the
response of the (PCY)H2Co-coated electrodes is characterized
by a large reduction peak located at Epc ) 0.29-0.31 V,
depending upon the spacer (see exact values in Table 4).
The electroreduction of O2 by the three (PCY)H2Co dyads
occurs at slightly less positive potentials than that of the
related cobalt(III) corrole, (Me4Ph5Cor)Co (Epc ) 0.36 V),8
but leaves no doubt that the electrocatalysis proceeds at the
potential of the Co(IV)/Co(III) process, with the cobalt(III)
center of the dyad being the active site in the electroreduction
of O2.
The rotating ring-disk electrode responses obtained for
(PCA)H2Co and (PCB)H2Co adsorbed on the graphite
electrode in 1 M HClO4 (Figure 7c) exhibit a single wave
for the reduction of O2 (E1/2 ) 0.35 V) as does (PCX)H2Co,
and the average number of electrons transferred (n) is higher
than 2 (Table 4) for all three investigated porphyrin-corrole
dyads. The number of electrons transferred in the O2
electroreduction process ranges from 2.5 to 2.9 for (PCY)H2Co
and (Me4Ph5Cor)Co,8 indicating that the electrocatalytic
reduction of O2 leads to formation of H2O2 and H2O through
processes involving both 2e- and 4e- reactions, as was
previously reported for monocobalt cofacial bisporphyrins.12
This is clearly different from the porphyrin-corrole dyads
with two cobalt centers, (PCY)Co2 (Y ) O, A, X, and B),8
where n ) 3.5-3.9 and O2 is mainly reduced to H2O through
a 4e- process.
Figure 6. Cyclic voltammograms of (PCX)H2Co in the following solutions
containing 0.1 M TBAP: (a) CH2Cl2, (b) PhCN, (c) PhCN + 1 equiv of
TBACl, and (d) PhCN + 160 equiv of TBACl.
A dimerization between the two oxidized dyads does not
occur for (PCB)H2Co in CH2Cl2 or PhCN. Here, a reversible
one-electron oxidation is seen at E1/2 ) 0.40 V in PhCN
and 0.1 M TBAP (Figure S2, Supporting Information), but
the Co(IV)/Co(III) reaction becomes irreversible in PhCN
solutions containing 1.2-40 equiv of added Cl-. Like
(PCX)H2Co, this electrode reaction is reversible in PhCN
solutions containing 160 equiv of TBACl, but no second
oxidation of the corrole unit could be seen up to the anodic
potential limit of the solvent where the added excess Cl- is
oxidized (∼0.6 V vs SCE).
The half-wave potentials for the Co(III)/Co(II) reductions
of (PCX)H2Co, (PCB)H2Co, and (PCA)H2Co in PhCN occur
at E1/2 values that are invariant with the addition of Cl- (see
Figures 5, 6, and S2), indicating that neither Co(III) nor
Co(II) axially binds Cl- in solutions of PhCN. Half-wave
potentials for the oxidation and reduction of the three dyads
in PhCN containing 0.1 M TBAP and 160 equiv Cl- are
summarized in Table 3, which also includes data on
(Me4Ph5Cor)Co.
Attempts were made to calculate Cl- binding constants
by the Co(IV) ion of the PCX and PCB derivatives, but plots
of E1/2 versus log [TBACl] gave non-Nernstian slopes due
to slow kinetics, and therefore, no reliable thermodynamic
data could be obtained.
The unmetalated porphyrin unit of the (PCY)H2Co dyads
may increase the selectivity for the four-electron reduction
of O2 to H2O over the two-electron pathway by helping to
stabilize the partially reduced dioxygen species. A similar
effect has been reported for face-to-face bisporphyrin systems
6752 Inorganic Chemistry, Vol. 44, No. 19, 2005

Free-Base Porphyrin-Cobalt Corrole Dyads
Figure 7. (a) Cyclic voltammograms of (PCA)H2Co (left) and (PCB)H2Co (right) adsorbed on an EPG electrode. Supporting electrolyte: 1 M HClO4
saturated with argon. Scan rate: 50 mV/s. (b) Cyclic voltammograms of (PCA)H2Co (left) and (PCB)H2Co (right) adsorbed on an EPG electrode. Supporting
electrolyte: 1 M HClO4 saturated with O2. [O2] ) 0.24 mM. Scan rate: 50 mV/s. (c) Reduction of O2 at a rotating ring (pt)-disk (EPG) electrode coated
with (PCA)H2Co (left) and (PCB)H2Co (right) in air-saturated 1 M HClO4. The potential of the ring electrode was maintained at 1.1 V. Rotating rate: 100
rpm. Scan rate: 5 mV/s.
containing one free-base porphyrin and one cobalt porphy-
rin.28 Alternatively, the increased value of n above 2.0 may
be due to the presence of aggregates or dimers of the dyads
on the electrode surface, as was postulated to occur in the
case of (Me4Ph5Cor)Co(III),8 (OEP)Co(II),29 and Co(II) por-
phine.30 A dimerization is not detected by electrochemistry
of the dyads in HCl or HClO4, but it is clearly evident from
the cyclic voltammograms of (PCX)H2Co and (PCA)H2Co
in CH2Cl2 or PhCN (see Figures 1 and 6 and Table 4).
Table 4. Electroreduction of Dioxygen by Adsorbed Free-Base
Porphyrin-Cobalt Corrole Dyads in Air-Saturated 1 M HClO4 or 1 M
HCl
1 M HClO4
1 M HCl
∆E1/2
(HClO4-HCl)
a
b
a
b
compound
Ep
E1/2
nc
Ep
E1/2
(PCA)H2Co
(PCX)H2Co
(PCB)H2Co
0.30
0.31
0.29
0.35
0.35
0.35
2.8
2.5
2.9
0.21 0.28
0.23 0.28
0.24 0.29
0.07
0.07
0.06
0.05
(Me4Ph5Cor)Co 0.36d 0.38d 2.9d 0.28 0.33
a Peak potential of the dioxygen reduction wave (V vs SCE). b Half-
wave potential (V vs SCE) for dioxygen reduction at a rotating disk electrode
(ω ) 100 rpm). c The apparent number of electrons transferred per dioxygen
molecule (n) at E1/2 is calculated from n ) 4ID/(ID + IR/N) where ID and IR
are disk and ring currents, respectively, and N () 0.24) is the collection
efficiency of the ring-disk electrode. d Data taken from ref 8.
(28) Ni, C.-L.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J. Phys. Chem.
1987, 91, 1158-1166.
(29) Song, E.; Shi, C.; Anson, F. C. Langmuir 1998, 14, 4315-4321.
(30) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36,
4294-4295.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6753

Kadish et al.
Cl- axial ligand in PhCN, but Cl- binding by Co(III) or
Co(II) does not occur for any of the three investigated dyads
under the same solution conditions. The E1/2 for electroca-
talysis of O2 in HClO4 and HCl parallels changes in potential
for the Co(IV)/Co(III) reaction of the dyads adsorbed on a
graphite electrode. The half-wave potential for electroca-
talysis also shifts negatively upon going from 1 M HClO4
to 1 M HCl, and this is consistent with the binding of Cl-
to the electrooxidized Co(IV) center of the adsorbed corrole.
Although the electrocatalytic reduction of O2 by the
(PCY)H2Co dyads is not enhanced by the binding of Cl-,
the large formation constants for the addition of Cl- to the
neutral and electrooxidized forms of (Me4Ph5Cor)Co in
CH2Cl2 or PhCN suggest the possibly that the investigated
monocorrole and related porphyrin-corrole dyads might find
application in the area of sensors and anion recognition, as
has earlier been explored in the case of porphyrins.31-34 The
binding of other anions to Co(III) and Co(IV) corrole centers
is also possible, and studies of these ligand binding reactions
are now in progress.
Figure 8. Reduction of O2 at a rotating disk electrode coated with (PCB)-
H2Co in air-saturated 1 M HClO4 or 1 M HCl. Rotating rate: 100 rpm.
Scan rate: 5 mV/s.
Catalytic Reduction of O2 in 1 M HCl. The catalytic
reduction of dioxygen by (PCY)H2Co and the monocorrole,
(Me4Ph5Cor)Co, was also investigated by cyclic voltammetry
and rotating disk electrode voltammetry in 1 M HCl, where
Cl- from the acid would be expected to coordinate to the
Co(IV) center of the corrole adsorbed on the electrode
surface. As shown in Figure 8, the electrocatalytic reduction
wave of dioxygen recorded in air-saturated 1 M HCl at a
graphite disk coated with (PCB)H2Co is shifted by 60 mV
toward more negative potentials, consistent with Cl- coor-
dinating to the Co(IV) center of the porphyrin-corrole dyad.
A similar negative shift is seen for the other two (PCY)H2Co
complexes as well as for (Me4Ph5Cor)Co in HCl, and the
magnitude of the shift in E1/2 is listed in Table 4 as ∆E1/2,
which ranges from 50 to 70 mV. The fact that (PCY)H2Co
and (Me4Ph5Cor)Co both catalyze the reduction of dioxygen
in the presence of Cl- (see Table 4) suggests that the anionic
axial ligand is rapidly released when the cobalt(IV) center
is reduced to Co(III) in the dyad.
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., Grant E-680) and the French Ministry
of Research (MENRT), CNRS (UMR 5633), is gratefully
acknowledged. The “Re´gion Bourgogne” and “Air Liquide”
company are acknowledged for scholarships (F.B.), and the
authors are also grateful to M. Soustelle for the synthesis of
pyrrole and dipyrromethane precursors.
Supporting Information Available: Additional figures (Figures
S1 and S2). Figure S1 illustrates the UV-visible spectra of (Me4-
Ph5Cor)CoIII(CO) in CH2Cl2 and [(Me4Ph5Cor)CoIIICl]- in PhCN,
and Figure S2 shows cyclic voltammograms of (PCB)H2Co in
PhCN and 0.1 M TBAP with 0-160 equiv of TBACl. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC050738N
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Summary. A strong binding of Cl- to the corrole cobalt
center is shown to occur for all of the Co(IV) derivatives as
well as for the doubly charged Co(IV) π-cation radical of
(PCA)H2Co and (Me4Ph5Cor)Co in nonaqueous media. The
Co(III) monocorrole in its neutral form also binds a single
6754 Inorganic Chemistry, Vol. 44, No. 19, 2005