Synthesis and electrochemistry of A2B type mono-and bis-cobalt triarylcorroles and their electrocatalytic properties for reduction of dioxygen in acid media

Article abstract of DOI:10.1142/S1088424616501030

Two A₂B mono-cobalt corroles and the corresponding bis-cobalt derivatives linked by a xanthene acyl group were synthesized, characterized as to their electrochemistry and spectroelectrochemistry in nonaqueous media and then examined as catalysts for the electroreduction of O₂ in 1.0 M HClO₄. The investigated mono-and bis-corroles are represented as (NH₂Ph)(YPh)₂CorCo(PPh₃) and [(YPh)₂CorCo(PPh₃)]₂Xac, respectively, where Cor is the trianion of the corrole, Y is a CH₃ or Cl substituent on the para-position of the 5-and 15-meso-phenyl rings of the macrocycle and Xac is the xanthene acyl group linking the two macrocycles in a face-to-face arrangement. The neutral compounds were characterized by cyclic voltammetry while thin-layer spectroelectrochemistry was used to elucidate UV-visible spectra of the compounds in their electrooxidized or electroreduced forms. Rotating disk and rotating ring-disk electrode measurements were also utilized to examine the electrocatalytic activity for reduction of O₂ when the face-to-face corroles were adsorbed on the electrode surface in 1.0 M HClO₄. The number of electrons transferred in the catalytic electroreduction of dioxygen was calculated from currents at the disk electrode using Koutecky-Levich plots. The effect of linking the two cobalt corrole macrocycles on UV-visible spectra, redox potentials and electrocatalytic activity for the reduction of dioxygen is discussed.

Full text of DOI:10.1142/S1088424616501030

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2016; 20: 1–12
DOI: 10.1142/S1088424616501030
Published at http://www.worldscinet.com/jpp/
Synthesis and electrochemistry of A2B type mono- and
bis-cobalt triarylcorroles and their electrocatalytic properties
for reduction of dioxygen in acid media
Yue Wanga, Zhongping Ou*a◊, Yuanyuan Fanga,b◊, Rui Guoa, Jijun Tanga,
Yang Songb◊◊ and Karl M. Kadish*b◊
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P.R. China
b Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Dedicated to Professor Tomás Torres on the occasion of his 65th birthday
Received 3 March 2016
Accepted 26 April 2016
ABSTRACT: Two A2B mono-cobalt corroles and the corresponding bis-cobalt derivatives linked
by a xanthene acyl group were synthesized, characterized as to their electrochemistry and spectro-
electrochemistry in nonaqueous media and then examined as catalysts for the electroreduction of O2 in
1.0 M HClO4. The investigated mono- and bis-corroles are represented as (NH2Ph)(YPh)2CorCo(PPh3)
and [(YPh)2CorCo(PPh3)]2Xac, respectively, where Cor is the trianion of the corrole, Y is a CH3 or Cl
substituent on the para-position of the 5- and 15-meso-phenyl rings of the macrocycle and Xac is the
xanthene acyl group linking the two macrocycles in a face-to-face arrangement. The neutral compounds
were characterized by cyclic voltammetry while thin-layer spectroelectrochemistry was used to elucidate
UV-visible spectra of the compounds in their electrooxidized or electroreduced forms. Rotating disk and
rotating ring-disk electrode measurements were also utilized to examine the electrocatalytic activity for
reduction of O2 when the face-to-face corroles were adsorbed on the electrode surface in 1.0 M HClO4.
The number of electrons transferred in the catalytic electroreduction of dioxygen was calculated from
currents at the disk electrode using Koutecky–Levich plots. The effect of linking the two cobalt corrole
macrocycles on UV-visible spectra, redox potentials and electrocatalytic activity for the reduction of
dioxygen is discussed.
KEYWORDS: A2B type of cobalt triarylcorrole, bis-cobalt corrole, synthesis, electrochemistry,
electrocatalytic reduction of oxygen.
linked face-to-face corroles have been synthesized and
INTRODUCTION
electrochemically characterized by our groups over the
Cobalt corroles have attracted a great deal of
last two decades [13–26], with some of these compounds
interest, in part because of their unique spectral and
being examined as catalysts for the electroreduction of
electrochemical properties [1, 2] and in part because
O2 in acid media [13–17, 22–24].
they can be utilized as catalysts for a variety of reactions
Bis-copper and bis-manganese face-to-face corroles
[3–12]. In this regard, a number of mono- and bis-cobalt
connected by a xanthene Xac linker were synthesized
and electrochemically characterized by Sun, Liu and
coworkers [27]. The bis-manganese corrole was also
◊ SPP full and ◊◊ student member in good standing
examined as to its catalytic activity for the oxidation of
*Correspondence to: Zhongping Ou, email: zpou2003@yahoo.
water. However, to the best of our knowledge, bis-cobalt
face-to-face corroles containing the same Xac linker have
never been synthesized and it was therefore of interest to
com, tel: +86 511-88791800, fax: +86 0511-88791800; Karl
M. Kadish, email: kkadish@uh.edu, tel: +1 (713) 743-2740,
fax: +1 (713) 743-2745
Copyright © 2016 World Scientific Publishing Company

2
Y. WANG ET AL.
(a) Mono-cobalt corrole
(b) Bis-cobalt corrole
Y
N
NH2
L
N
HN
Co
N
N
L
O
O
Y
N
N
O
Co
Y
Y
Y
N
N
L
N
N
HN
Co
N
N
Y = CH3 (1a)
Y = Cl (1b)
L = PPh3
Y = CH3 (2a)
Y = Cl (2b)
L = PPh3
Y
Chart 1. Structures of examined mono- and bis-cobalt corroles
Y
know if these compounds with two closely spaced redox
active metal centers might function as efficient catalysts
for the electroreduction of dioxygen in acid media.
This is addressed in the present manuscript where two
mono-cobalt A2B corroles and the corresponding bis-
cobalt derivatives linked by a xanthene acyl group were
synthesized, andcharacterizedastotheirelectrochemistry
and spectroelectrochemistry in nonaqueous media and
then examined as catalysts for the electroreduction of O2
in 1.0 M HClO4.
The structures of the investigated mono- and bis-
corroles are shown in Chart 1 and represented as (NH2Ph)
(YPh)2CorCo(PPh3) and [(YPh)2CorCo(PPh3)]2Xac, res-
pectively, where Cor is the trianion of the corrole macro-
cycle, Y is a CH3 or Cl substituent on the para-position
of the 5- and 15-meso-phenyl rings of the compound and
Xac is a xanthene acyl group linking the two macrocycles
in a face-to-face arrangement. The number of electrons
transferred and the percentage of H2O2 produced in the
catalytic reduction of O2 was calculated from measure-
ments performed at a rotating disk and/or rotating-ring
disk electrode in 0.1 M HClO4.
NO2
+
HN
CHO
NH
H2O, CH3OH
HCl
p-Chloranil CHCl3, reflux
NO2
N
HN
HN
Y
Y
NH
.
SnCl2. 2H2O
NH2
RESULTS AND DISCUSSION
Synthesis and characterization
N
HN
Y
Y
The synthetic routes for obtaining the two types of
A2B corroles are described in Scheme 1a. The procedure
for the mono-corroles follows a procedure reported in
the literature [28, 29] while synthesis of the bis-cobalt
corroles was according to the route shown in Scheme 1b.
The progress of the reaction was monitored by thin-
layer chromatography and the purity of the final corrole
product was confirmed by UV-visible spectroscopy and
mass spectrometry.
NH
HN
.
Co(OAc)2 4H2O
PPh3
CH3OH, reflux
1a (Y = CH3)
1b (Y = Cl)
Scheme 1a. Synthesis routes for mono-corroles 1a and 1b
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SYNTHESIS AND ELECTROCHEMISTRY OF A2B TYPE MONO- AND BIS-COBALT TRIARYLCORROLES
3
HO
Cl
O
O
O
O
OH
Cl
SOCl2
reflux
(i)
O
O
NH2
(ii)
Y
N
NH
HN
HN
NH
N
HN
HN
HN
O
Y
Y
.
Co(OAc)2 4H2O, PPh3
TEA, 0 °C
Y
O
2a, 2b
Y
CH3OH, reflux
O
Cl
O
O
Cl
NH
HN
HN
HN
N
O
Y = CH3 (2a)
Y = Cl (2b)
Y
Scheme 1b. Synthesis routes for bis-corroles 2a and 2b
The 1H NMR spectrum of each compound was
oxidations of this mono-cobalt corrole are located at
E1/2 = 1.02 and 1.48 V, respectively (Fig. S3). The first
two oxidations of 1b, which contains two electron-
withdrawing meso-ClPh substituents, are positively
shifted by 80 and 30 mV as compared to that of compound
1a. Similar oxidation behavior has been reported for
other cobalt triarylcorroles containing a variety of
electron-donating or electron-withdrawing substituents
on the meso and/or b-pyrrole positions of the macrocycle
[13, 16, 17, 26].
measured in CDCl3 or CD2Cl2 and examples of the spectra
for (NH2Ph)(CH3Ph)2CorCo(PPh3) 1a and [(CH3Ph)2-
CorCo(PPh3)]2Xac 2a are shown in Fig. S1. The protons
of the para-NH2 group on 1a appear as a broad resonance
at 3.57 ppm while resonances of the two NH protons of
2a are sharp and shifted to 9.20 ppm. Resonances of
the six para-CH3 protons of 1a are located at 2.59 ppm
while the 12 para-CH3 protons of 2a give peaks at
2.02 ppm and are characterized by a stronger absorption
as compared to 1a.
Different redox behavior is observed for the bis-cobalt
corroles 2a and 2b, as compared to the related mono-
cobalt corroles under the same solution conditions. As
seen in Figs 1 and S3, compounds 2a and 2b exhibit
four reversible oxidations, the first two of which are
located at E1/2 = 0.44 and 0.53 V for 2a and 0.50 and
0.63 V for 2b. The first two oxidations of the bis-cobalt
corroles have a peak to peak separation consistent with
one-electron transfer but each process has a current
which is reduced by about 50% as compared to that
of the other electron abstractions. This suggests the
stepwise formation of {[(YPh)2CorCo(PPh3)]2Xac}+ and
{[(YPh)2CorCo(PPh3)]2Xac}2+ in the first two oxidations,
followed by the formation of {[(YPh)2CorCo(PPh3)]2-
Xac}4+ and {[(YPh)2CorCo(PPh3)]2Xac}6+ in the last two
redox processes. The difference in E1/2 between the first
two oxidations of 2b is 130 mV (see Figs 1 and S3),
consistentwithaweaktomoderateinteractionbetweenthe
two face-to-face macrocycles of this bis-corrole.A smaller
DE1/2 of 90 mV is seen between the first two one-electron
oxidations of 2a under the same solution conditions
(Fig. S3), indicating a weaker interaction between the
UV-visible spectra
UV-visible spectra of the four neutral corroles in
CH2Cl2 are illustrated in Fig. S2. As seen in the figure,
all four compounds are characterized by a Soret band
at 392–394 nm and a single Q-band absorption at 562–
566 nm. Almost identical UV-visible spectra are seen
for the mono- and bis-corroles, indicating that neither
the substituents on the para-positions of the meso-
phenyl rings nor the face-to-face arrangement of the
two metallomacrocycles has a significant effect on the
UV-visible spectra.
Electrochemistry
Three reversible one-electron oxidations are observed
for the mono-corroles 1a and 1b in CH2Cl2 containing
0.10 M TBAP. The first oxidation of compound 1a, which
contains two electron-donating CH3Ph substituents,
is located at E1/2 = 0.48 V while the second and third
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Y. WANG ET AL.
two face-to-face corroles in this compound. However,
large values of DE1/2 (170–280 mV) have been reported
for other previously examined face-to-face cobalt corrole
dyads [19, 21], which are consistent with a stronger
interaction between the two corrole macrocycles of these
dyads.
The third oxidations of 2a and 2b are negatively
shifted by 80–90 mV in potential as compared to the
corresponding E1/2 values of 1a and 1b, while the fourth
oxidation of the two bis-corroles occurs at an E1/2 which
is positively shifted by 110–130 mV from E1/2 values for
oxidation of the structurally related mono-corroles under
the same solution conditions (see Fig. S3).
Examples of the UV-visible spectral changes obtained
during controlled potential oxidation of 1b and 2b in
CH2Cl2 are illustrated in Fig. 2. The Soret and Q-bands of
1b both decrease in intensity upon the first one-electron
oxidation at 0.80 V (top left illustrations in Fig. 2),
suggesting formation of a Co(III) p-cation radical rather
than a Co(IV) corrole in the thin-layer cell. The Soret band
further decreases in intensity when the applied oxidizing
potential is set at 1.25 V (lower left illustration in Fig. 2)
and this is consistent with generation of a Co(III) dication
under the given solution conditions. Similar spectral
changes were recorded for the bis-cobalt corrole 2b and
theseareshownintherighthandsideofFig. 2.Thespectral
changes accompanying oxidation of 1b and 2b indicate
Fig. 1. Cyclic voltammograms showing stepwise oxidations of
the bis-cobalt corrole 2b in CH2Cl2 containing 0.1 M TBAP
Fig. 2. Thin-layer UV-vis spectral changes of (a) mono-cobalt corrole 1b and (b) bis-cobalt corrole 2b during the oxidations in
CH2Cl2 containing 0.1 M TBAP
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SYNTHESIS AND ELECTROCHEMISTRY OF A2B TYPE MONO- AND BIS-COBALT TRIARYLCORROLES
5
Fig. 3. Cyclic voltammograms showing the reductions of (NH2Ph)(CH3Ph)2CorCo(PPh3) 1a and [(CH3Ph)2CorCo(PPh3)]2Xac 2a in
CH2Cl2 containing 0.1 M TBAP
+e
that the first three redox processes are all macrocycle-
centered to give stepwise, {(ClPh)2CorCoIII(PPh3)]2Xac}+,
{[(ClPh)2CorCoIII(PPh3)]2Xac}2+ and {[(ClPh)2CorCoIII-
(PPh3)]2Xac}4+ in CH2Cl2.
CorCoIII(PPh3)
+PPh3
[CorCoII(PPh3)]-
-PPh3
-0.77 V
-0.21 V
-e
+e
CorCoIII
[CorCoII]-
[CorCoI]2-
CH2Cl2
-1.69 V
Two irreversible reductions are observed for the
investigated corroles in CH2Cl2 containing 0.1 M TBAP
and examples of cyclic voltammograms for 1a and 2a
are illustrated in Fig. 3. The irreversibility of the first
reduction is due to the known dissociation of the bound
triphenylphosphine (PPh3) axial ligand upon generation of
the Co(II) corrole [13, 26] and this four coordinate species
can then be further reduced to its Co(I) form at a more
negative potential.The electrogenerated Co(II) corroles can
also be irreversiblely reoxidized at anodic peak potentials
of -0.04 to -0.21 V (see Table 1) to give short lived Co(III)
four-coordinate corroles which then coordinate again the
PPh3 ligand to give back the original species.
The second irreversible reduction of 1a–2b is assigned
as a CoII/CoI process in CH2Cl2 and is associated with
a following coupled chemical reaction leading to the
proposed formation of an unstable s-bonded product,
[CorCoIII(CH2Cl)]-, as previously described in the
literature for Co(I) porphyrins and corroles [26, 30].
Cl-
-0.01 V
-e
[CorCoIV(CH2Cl)]
[CorCoIII(CH2Cl)]-
CH2Cl
Scheme 2. Proposed reduction mechanism for the examined
corroles in CH2Cl2 containing 0.1 M TBAP. The given potentials
are for 1a
It should be noted that a second reoxidation peak
is observed at -0.01~0.06 V for compounds 1a–2b in
CH2Cl2 when the potential scan is reversed after initially
scanning beyond the second reduction potential of the
neutral compounds (see Table 1 and Fig. 3). These “new”
reactions are assigned to a re-oxidation of the proposed
s-bonded corrole as shown in Scheme 2.
A similar reduction mechanism as shown in Scheme 2
is also proposed to occur in PhCN containing 0.1 M
TBAP and with added CH3I. Examples of the cyclic
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Y. WANG ET AL.
Table 1. Redox potentials (V vs. SCE) of the mono-(1a, 1b) and bis-cobalt corroles (2a, 2b)
in CH2Cl2, 0.1 M TBAP
Solvent
Cpd
Oxidation
2nd
Re-oxidation
Reduction
3rd
1st
0.48
2nd
1st
1st
2nd
CH2Cl2
1a
2a
1b
2b
1a
2a
1.48
1.57
1.47
1.60
1.50
1.57
1.02
0.94
1.05
0.96
1.01
0.93
-0.01
0.01
0.03
0.06
0.10
—
-0.21a
-0.09a
-0.13a
-0.04a
-0.16a
-0.10a
-0.77a
-0.88a
-0.73a
-0.81a
-0.73a
-0.87a
-1.69a
-1.78a
-1.64a
-1.69a
-1.67
-1.75
0.53, 0.44
0.56
0.63, 0.50
0.61
PhCN
0.56, 0.50
a Irreversible peak potentials at a scan rate of 0.10 V/s.
Fig. 4. Cyclic voltammograms of (a) (NH2Ph)(CH3Ph)2CorCo(PPh3) 1a and (b) [(CH3Ph)2CorCo(PPh3)]2Xac 2a in PhCN containing
0.1 M TBAP (a) without and (b) with added CH3I
voltammograms for 1a and 2a under these conditions
are illustrated in Fig. 4. The first reduction is irreversible
for both corroles in PhCN (again due to the loss of the
PPh3 ligand after generation of Co(II)), but the second
reduction becomes reversible when the solvent is changed
from CH2Cl2 to PhCN, indicating that the doubly reduced
form of the compound is stable in PhCN which does not
react with the electrogenerated Co(I) corrole. However,
after adding 1–8 equivalents CH3I to the PhCN solution
the second reduction becomes irreversible and a new
re-oxidation peak is observed, as is also the case in
CH2Cl2. This new oxidation peak is assigned to a reaction
of the transient s-bonded cobalt corrole as shown in
Scheme 2. The peak currents for the re-oxidation in PhCN
containing CH3I significantly increase when the potential
is held at a negative potential (-1.80 for 1a and -1.95 V
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SYNTHESIS AND ELECTROCHEMISTRY OF A2B TYPE MONO- AND BIS-COBALT TRIARYLCORROLES
7
values located beyond -0.2 V, as described in earlier
publications [13, 14].
A different situation occurs when the corrole-coated
electrode is placed in air-saturated 1.0 M HClO4. This
is shown by the solid lines in Fig. 5 where irreversible
but well-defined cathodic reduction peaks are observed at
Epc = 0.25V for 1a, 0.24V for 1b, 0.19V for 2a and 0.22V
for 2b, at a scan rate of 50 mV/s. As will be shown, these
irreversible peaks in the air-saturated HClO4 solutions
correspond to the catalytic reduction of dissolved O2 to
give only H2O2.
Peak potentials for the catalytic reduction of oxygen
at the corrole-coated electrodes are similar for the two
mono-corroles 1a and 1b and they are also similar for the
two bis-corroles 2a and 2b. However, the reduction of O2
occurs at more negative potentials when the bis-corroles
are used as catalysts (Fig. 5).
Measurements were also performed at a rotating disk
electrode (RDE). The RDE response in air-saturated
1.0 M HClO4 is similar for all four examined corroles
(Fig. 6a) and is characterized by a half-wave potential
located at 0.24–0.29 V.
The number of electrons transferred during the
reduction of oxygen was calculated from the magnitude
of the steady-state limiting currents that were taken at a
fixed potential (-0.1 V) on plateau of the current–voltage
curves for the catalytic waves in Fig. 6a. When the
amount of O2 reduction at the corrole-modified electrode
is controlled by mass transport alone, the relationship
between the limiting current and rotation rate should
obey the Levich equation given in Equation 1.
jlev = 0.62nFAD2/3cv-1/6w1/2
(1)
where jlev is the Levich current, n is the number of
electrons transferred in the overall electrode reaction, F is
the Faraday constant (96485 C.mol-1), A is the electrode
area (cm2), D is the dioxygen diffusion coefficient
(cm2.s-1), c is the bulk concentration of O2 in 1.0 M
HClO4, v is the kinematic viscosity of the solution, and w
is the angular rotation rate of the electrode (rad.s-1).
When the reciprocal of the limiting current density is
plotted against the reciprocal of the square root of the
rotation rate, the resulting straight line (Fig. 6b) obeys the
Koutecky−Levich equation (Equation 2).
Fig. 5. Cyclic voltammograms of the mono- and bis-cobalt
corroles coated on EPPG electrode in 1.0 M HClO4 under air
(
____) and N2 (------). Scan rate = 50 mV/s
for 2a) for 20 s before reversing the scan and sweeping
in a positive direction. The resulting voltammograms
under this condition is illustrated in the lower part of
Fig. 4 and is consistent with larger quantities of the
s-bonded corrole being generated at the electrode surface
under the given experimental conditions.
1/j = 1/jlev + 1/jk
(2)
Electrocatalytic reduction of dioxygen
where j is the measured limiting current density
(mA.cm-2) and jk is the kinetic current which can be
obtained experimentally from the intercept of the
Koutecky−Levich plot in Fig. 6b.
Figure 5 illustrates cyclic voltammograms for the four
corroles adsorbed on an EPPG disk electrode in 1.0 M
HClO4 under N2 (dashed line) and under air (solid
line). A surface reaction is seen at about 0.4 V in the
N2-saturated solution, but no peaks assigned to the
electrode reactions of cobalt corrole can be detected
from 0.6 to -0.2 V. The lack of a cobalt corrole electrode
reaction between 0.6 and -0.2 V under N2 is consistent
with the fact that PPh3 coordination to Co(III) shifts
the potential for the CoIII/CoII process to more negative
jk = 103knFΓc
(3)
The value of k (M-1.s-1) in Equation 3 is the second-
order rate constant of the reaction that limits the plateau
current, Γ (mol.cm-2) is the surface concentration of the
corrole catalyst, and the other terms have their usual
significance as described previously.
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Y. WANG ET AL.
Fig. 6. (a) Linear sweep voltammograms at different rotating rate and (b) Koutecky–Levich plots for catalytic reduction of O2 in
air-saturated 1.0 M HClO4 at a rotating EPPG disk electrode coated with compounds 1a, 1b and 2a, 2b at a scan rate of 50 mV/s
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SYNTHESIS AND ELECTROCHEMISTRY OF A2B TYPE MONO- AND BIS-COBALT TRIARYLCORROLES
9
Fig. 7. Rotating ring-disk electrode voltammograms of (a) 1a, (b) 1b, (c) 2a and (d) 2b in air-saturated 1.0 M HClO4 and the
potential on the ring electrode maintained at 1.0 V. Rotation rate = 400 rpm and scan rate = 10 mV/s
The slope of a plot obtained by linear regression can
then be used to estimate the average number of electrons
(n) involved in the catalytic reduction of O2. As seen
in Fig. 6b, the slope of the Koutecky−Levich plots
indicates that the number of electrons transferred in the
electroreduction process of O2 is 2.0.
as the catalyst in 1.0 M HClO4. This could be attributed
to the lack of a strong face-to face interaction between the
two corrole macrocycles on the EPPG electrode surface.
EXPERIMENTAL
When the products of O2 reduction consist of a mixture
of H2O2 and H2O, values of n between 2 and 4 will be
calculated. In the current study, the Koutecky−Levich
plots in Fig. 6b show that n = 2.0 for all four investigated
compounds, indicating that the catalytic electroreduction
of O2 is a simple 2e- transfer process to give only H2O2
in acidic media.
The catalytic reduction of O2 was also examined at
an RRDE under the same solution conditions. The disk
potential was scanned from 0.5 to -0.2 V at a rotating rate
of 400 rpm while holding the ring potential constant at
1.0 V. This data is shown in Fig. 7 for 1a, 1b and 2a, 2b,
where the disk current begins increasing at about 0.4 V
and a plateau is reached at about 0.1 V. The ring current
increases throughout the range of the disk potentials
where the disk current rises. The number of electrons
transferred (n) calculated, on the basis of Equation 4 (see
Experimental section), is 2.2 for mono-cobalt corroles
(1a and 1b) and 2.0 for bis-cobalt derivatives (2a and
2b). Based on Equation 5 (see Experimental section), the
calculated amount of H2O2 formed upon the reduction of
dioxygen is 90–92% for 1a and 1b, and 100% for 2a and
2b under the given experimental conditions.
Instrumentation
Thin-layer UV-visible spectroelectrochemical experi-
ments were performed with a home-built thin-layer
cell which has a light transparent platinum net working
electrode. Potentials were applied and monitored with
Chi-730C Electrochemical Work Station. Time-resolved
UV-visible spectra were recorded with a Hewlett-
Packard Model 8453 diode array spectrophotometer.
High purity N2 was used to deoxygenate the solution
and kept over the solution during each electrochemical
and spectroelectrochemical experiment. Infrared spectra
were recorded with Nicolet Nexus 470. MALDI-TOF
mass spectra were carried out on a Bruker BIFLEX III
ultrahigh resolution.
Cyclic voltammetry was carried out at 298K by
using an EG&G Princeton Applied Research (PAR) 173
potentiostat/galvanostat or CHI-730C Electrochemical
Workstation. A homemade three-electrode cell was
used for all electrochemical measurements. A three-
electrode system was used in each case and consisted of
a glassy carbon or graphite working electrode (Model
MT134, Pine Instrument Co.) for cyclic voltammetry and
voltammetry at an RDE and a platinum-ring, graphite-
disk electrode combination for voltammetry at the
The RDE and RRDE results both indicate that a two-
electron reduction occurres and only H2O2 is formed as
the final products when the bis-cobalt corrole is utilized
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Y. WANG ET AL.
RRDE. A platinum wire served as the auxiliary electrode
and a saturated calomel electrode (SCE) as the reference
electrode, which was separated from the bulk of the
solution by means of a salt bridge of low porosity which
contained the solvent/supporting electrolyte mixture. The
potential of the SCE is 0.241 V vs. NHE.
collected and washed twice with H2O, then dried with
Na2SO4, filtered and diluted to 200 mL with CHCl3.
After the addition of p-chloranil (490 mg, 2 mmol), the
mixture was refluxed for 1 h and then chromatographed
on a silica column using CH2Cl2/hexane as eluent. The
free-base nitrocorrole was obtained by evaporating the
solvent to dryness.
The rotating ring-disk electrode (RRDE) was
purchased from Pine Instrument Co. and consisted of
a platinum ring and a removable edge-plane pyrolytic
graphite (EPPG) disk (A = 0.196 cm2). A Pine Instrument
MSR speed controller was used for the RDE and RRDE
experiments. The Pt ring was first polished with 0.05
micron a-alumina powder and then rinsed successively
with water and acetone before being activated by cycling
the potential between 1.20 and -0.20 V in 1.0 M HClO4
until reproducible voltammograms were obtained [31, 32].
The corrole catalysts were adsorbed on the electrode
surface by means of a dip-coating procedure described
in the literature [15, 33]. In this procedure, the freshly
polished electrode was dipped in a 1.0 mM catalyst
solution of CH2Cl2 for 5s, transferred rapidly to pure
CH2Cl2 for 1–2 s, and then exposed to air where the
adhering solvent was rapidly evaporated, leaving the
porphyrin catalyst adsorbed on the electrode surface.
All experiments were carried out at room temperature.
The average number of electrons transferred (n) and the
amount of H2O2 formed in the catalytic reduction of O2
were determined by the Koutecky–Levich plot [34] and
calculated with Equations 4 and 5.
The nitrocorrole was added into a flask containing
SnCl2·2H2O (450 mg, 2 mmol) in concentrated HCl
(50 mL), after which the mixture was stirred at 75°C for 2
h, then neutralized with ammonia solution (25%) to pH 7.
The product was extracted with ethyl acetate and the
organic layer was collected and washed three times with
water, dried with Na2SO4, and then chromatographed on
basic alumina column to afford the free-base amino corrole.
The amino-substituted free-base corrole was dissolved
in 30 mL CH3OH containing excess Co(OAc)2·4H2O and
triphenylphosphine. The mixture was heated to reflux for
1 h and the progress of the reaction monitored by thin-
layer chromatography until the amino containing corrole
was consumed. The sample was evaporated to dryness
and chromatographed on a basic alumina column using
ethyl acetate/hexane as an eluent. The red fraction was
collected and the solvent evaporated to dryness.
(NH2Ph)(CH3Ph)2CorCo(PPh3) 1a. Yield 33.8 mg,
1
5.4%. UV-vis (CH2Cl2): lmax, nm 394, 564. H NMR
(400 MHz, CDCl3): d, ppm 8.61 (d, J = 12.4 Hz, 2H),
8.36 (m, 2H), 8.16 (s, 2H), 8.04 (s, 4H), 7.85 (s, 2H),
7.70 (m, 2H), 7.44 (s, 6H), 7.05 (d, J = 6.6 Hz, 3H), 6.71
(m, 6H), 4.75 (m, 6H), 3.57 (m, 2H), 2.59 (s, 6H). MS
(MALDI-TOF): m/z calcd. for C57H43CoN5P 887.26,
C39H28CoN5 625.61; found [M – PPh3]+ 625.10.
n = 4ID/(ID+IR/N)
(4)
(5)
%H2O2 = 100(2IR/N)/(ID+IR/N)
(NH2Ph)(ClPh)2CorCo(PPh3) 1b. Yield 30.0 mg,
4.5%. UV-vis (CH2Cl2): lmax, nm 396, 565. H NMR
where ID and IR are the Faradic currents at the disk and
ring electrodes, respectively. The intrinsic value of the
collection efficiency (N) was determined to be 0.24 using
the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple in 1.0 M KCl.
1
(400 MHz, CDCl3): d, ppm 8.62 (d, J = 4.4 Hz, 2H),
8.29 (d, J = 4.8 Hz, 2H), 8.20 (d, J = 4.7 Hz, 2H), 7.99
(d, J = 4.2 Hz, 2H), 767 (m, 4H), 7.54 (m, 4H), 7.47
(m, 6H), 7.04 (m, 3H), 6.68 (m, 6H), 4.66 (m, 6H). MS
(MALDI-TOF): m/z calcd. for C57H37Cl2CoN5P 927.15,
C37H22Cl2CoN5 665.06; found [M – PPh3]+ 665.01.
Chemicals
Reagents and solvents (Sigma-Aldrich or Sinopharm
Chemical Reagent Co.) for synthesis and purification are
of analytical grade and used as received. Dichloromethane
(CH2Cl2, 99.8%) was purchased from Sinopharm
Chemical Reagent Co. and freshly distilled prior to
use. Tetra-n-butylammonium perchlorate (TBAP) was
purchased from Sigma-Aldrich Chemical, recrystallized
and used as received.
Synthesis of bis-cobalt corroles
A mixture of SOCl2 (2.5 mL) and 2,7-di-tert-butyl-
9,9-dimethylxanthenes-4,5-dicarboxylic acid (20 mg,
0.05 mmol) was refluxed for 2 h. After removing excess
SOCl2 by distillation, the corresponding acid chloride
was obtained. The amino-substituted free-base mono-
corrole (57 mg, 0.10 mmol), which was dissolved in
CH2Cl2 (20 mL), was added to a flask containing Xac and
2 drops of TEA. The mixture was stirred for 2 h at room
temperature and then purified by column chromatography
on basic alumina with a mixture of ethyl acetate/hexane
as eluent to give the final free-base bis-corrole product.
Cobalt acetate was reacted with the free-base bis-corrole
according to the above described procedure for the
synthesis of 1a and 1b to yield the bis-cobalt corroles.
Synthesis of mono-cobalt corroles
5-(4-Methylphenyl)dipyrromethane (475 mg, 2 mmol)
or 5-(4-chlorophenyl)dipyrromethane (500 mg, 2 mmol)
and 4-nitrobenzaldehyde (150 mg, 1 mmol) were dis-
solved in CH3OH (100 mL) and, after which 100 mL
HCl (0.9%) were added to the solution which was stirred
at room temperature for 1 h, after which the products
were then extracted with CHCl3. The organic layer was
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 10–12

SYNTHESIS AND ELECTROCHEMISTRY OF A2B TYPE MONO- AND BIS-COBALT TRIARYLCORROLES
11
[(CH3Ph)2CorCo(PPh3)]2Xac 2a. Yield 41.8 mg,
51.5%. UV-vis (CH2Cl2): lmax, nm 396, 563. H NMR
REFERENCES
1
1. Paolesse R. In The Porphyrin Handbook, Vol. 2,
Kadish KM, Smith KM and Guilard R. (Eds.)
Academic Press: New York, 2000; Chap. 11,
pp. 201–232.
2. Erben C, Will S and Kadish KM. In The Porphy-
rin Handbook, Vol. 2, Kadish KM, Smith KM and
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(400 MHz, CD2Cl2): d, ppm 9.20 (s, 2H), 8.51 (d, J =
4.1 Hz, 4H), 8.25 (m, 4H), 8.03 (m, 6H), 7.70 (m, 8H),
7.53 (d, J = 4.3 Hz, 4H), 7.20 (d, J = 7.9 Hz, 2H), 7.02 (d,
J = 28.3 Hz, 8H), 6.81 (m, 6H), 6.46 (m, 12H), 6.28 (s, 4H),
5.64 (s, 4H), 4.51 (m, 12H), 2.02 (d, J = 31.0 Hz, 12H),
1.82 (s, 6H), 1.46 ppm (s, 18H). MS (MALDI-TOF): m/z
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45.9%. UV-vis (CH2Cl2): lmax, nm 396, 567. H NMR
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4.3 Hz, 4H), 8.32 (d, J = 8.1 Hz, 2H), 8.23 (d, J = 7.9 Hz,
2H), 8.14 (d, J = 2.0 Hz, 2H), 8.00 (d, J = 4.6 Hz, 4H),
7.77 (d, J = 2.2 Hz, 2H), 7.68 (m, 6H), 7.38 (d, J =
4.7 Hz, 4H), 7.21 (d, J = 8.2 Hz, 2H), 7.13 (s, 4H), 7.00
(d, J = 12.3 Hz, 4H), 6.82 (m, 6H), 6.46 (m, 12H), 5.96
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CONCLUSION
Two irreversible one-electron reductions were
observed for the investigated mono- and bis-cobalt
corroles, leading to the formation of a four-coordinate
Co(II) corrole and a Co(I) corrole or a transient s-bonded
Co(III) derivatives in CH2Cl2 and PhCN containing added
CH3I. The potentials for oxidation of the two bis-cobalt
corroles suggest a weak interaction exists between the
two corrole macrocycles in their neutral form and each of
which undergoes a one-electron abstraction at a different
half-wave potential. RDE and RRDE data indicate
that the bis-cobalt corroles can be utilized as selective
catalysts for the two-electron reduction of dioxygen to
give H2O2 but no H2O product is observed either with
the mono-cobalt corrole or the face-to-face bis-cobalt
derivatives.
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Support from the Natural Science Foundation of China
(21501070), Jiangsu University Foundation (05JDG051,
15JDG131) and the Robert A. Welch Foundation
(K.M.K., E-680) are gratefully acknowledged.
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Supporting information
1H NMR spectra for compounds 1a and 2a, UV-visible
spectra of the four corroles measured in CH2Cl2, cyclic
voltammograms showing the oxidations of investigated
compounds in CH2Cl2 containing 0.1 M TBAP (Figs
S1–S3) are given in the supplementary material. This
material is available free of charge via the Internet at
http://www.worldscinet.com/jpp/jpp.shtml.
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 11–12

12
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J. Porphyrins Phthalocyanines 2016; 20: 12–12