Dicobalt diporphyrin complex with hyperfine EPR spectrum mediates only a two-electron reduction of O2

Article abstract of DOI:10.1016/j.inoche.2005.06.004

A dicobalt diporphyrin complex can form μ-superoxo dimer with O ₂ in chloroform in the absence of organic base to give 15-line highly symmetrical EPR spectrum. This complex can only catalyze O₂ reduction by a two-electron pathway, suggesting that there is no correlation between the shape of the Co-O₂-Co EPR signal and the electrocatalytical ability of dicobalt diporphyrin complexs to mediate the four-electron reduction of O₂.

Full text of DOI:10.1016/j.inoche.2005.06.004

Inorganic Chemistry Communications 8 (2005) 789–791
www.elsevier.com/locate/inoche
Dicobalt diporphyrin complex with hyperfine EPR spectrum
mediates only a two-electron reduction of O2
*
Wei-Bing Lu, Xiao-Hai Zhou , Jian-Guo Ren
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
Received 17 May 2005; accepted 31 May 2005
Available online 18 July 2005
Abstract
A dicobalt diporphyrin complex can form l-superoxo dimer with O2 in chloroform in the absence of organic base to give 15-line
highly symmetrical EPR spectrum. This complex can only catalyze O2 reduction by a two-electron pathway, suggesting that there is
no correlation between the shape of the Co–O2–Co EPR signal and the electrocatalytical ability of dicobalt diporphyrin complexs to
mediate the four-electron reduction of O2.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Dicobalt diporphyrin complex; EPR; O2 reduction; Electrocatalytic reduction
Considerable attempts have been made in searching
for 4eÀ catalysts towards O2 reduction over the past
years. It is known that O2 can reduce on the chemically
modified electrodes by a 2eÀ process to H2O2 or via a
direct 4eÀ pathway to H2O. Metalloporphyrins are
known as effective catalysts for O2 reduction, but almost
all of the reported monomeric porphyrins can catalyze
only the reduction of O2 to H2O2. Metallodiporphyrins,
because of the control they provide over the geometrical
and electronic properties of the reaction center, are
potential 4eÀ catalysts for O2 reduction [1]. Several
cofacial diporphyrins of cobalt [2] and iridium [3] have
been synthesized and identified as catalysts for the 4eÀ
reduction of dioxygen. Herein, we report a dicobalt
diporphyrin complex (Fig. 1) which can form l-super-
oxo dimer with O2 in CHCl3 in the absence of organic
base to give 15-line highly symmetrical EPR spectrum
but can only catalyze O2 reduction by a two-electron
pathway. The results suggest that there is no correlation
between the shape of the Co–O2–Co EPR signal and the
electrocatalytical ability of dicobalt diporphyrin com-
plexs to mediate the four-electron reduction of O2.
The diporphyrin 1 was prepared by the reaction of
5-(3-aminophenyl)-10,15,20-triphenylporphyrin (ATPP)
with carbonyl chloride in benzene in the presence of
anhydrous pyridine in 49% yield. Then metallation of
1 with cobalt acetate in refluxed propionic acid afforded
the dicobalt diporphyrin complex 2 [4].
A solution of complex 2 in chloroform reacts with air
to give a highly symmetrical 15-line EPR spectrum typ-
ical of a l-superoxo dicobalt(III) complex (Fig. 2, g =
2.019). This isotropic spectrum is consistent with the
presence of a single unpaired electron, localized on the
superoxo bridge, interacting equally with the two I =
7/2 cobalt nuclei.
The Soret band of the free base 1 at 416 nm as a
sharp peak in CHCl3, and the pyrrole NH signal at
À2.98 ppm in CDCl3 are both very adjacent to that of
the corresponding monomer ATPP (kmax = 417 nm
and d = À2.70 ppm for ATPP in the same solvent,
respectively). Following the criteria given by Collman
et al. [1] for a typical cofacial diporphyrin that a blue
shift and a broadening of the Soret band and a upfield
shift of the pyrrole NH proton signal relative to the
*
Corresponding author. Fax: +86 27 87218534.
E-mail address: lwblqy@163.com (X.-H. Zhou).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.06.004

790
W.-B. Lu et al. / Inorganic Chemistry Communications 8 (2005) 789–791
A
20
N
M
N
a
N
N
b
c
N
M
NH
C
N
N
N
O
NH
1
2
M = 2H
M = CoII
-1.1
-0.3 -0.5 -0.7
0.1
-0.9
-0.1
E/Vvs SCE
Fig. 1. The structure of the free base 1 and its cobalt complex 2.
Fig. 4. Cyclic voltammograms for dioxygen reduction on glassy
carbon electrode modified with 2 (curve a) and bare glassy carbon
electrode (curve b) in O2 saturated 0.5 M CF3COOH. Curve c
represents the cyclic voltammogram of complex 2 modified electrode
in the absence of O2. Scan rate = 100 mV sÀ1
.
dicobalt complex with oxygen. The formation of l-
superoxo dimer by complex 2 with O2 in the absence
of any organic base seems to suggest a very high affinity
of complex 2 for dioxygen. To our knowledge, this is the
first porphyrin dimer that can form l-superoxo dimer
with O2 in solution without addition of organic base.
We then use cyclic voltammetry to check its electrocata-
lytical activity for O2 reduction.
Fig. 4 shows the cyclic voltammograms for O2 reduc-
tion on the complex 2 adsorbed glassy carbon electrode
(GCE) (curve a) and on the bare GCE (curve b) in 0.5 M
CF3COOH saturated with O2. Curve c shows the cyclic
voltammogram for complex 2 adsorbed electrode in
the absence of O2. The peak potential for O2 reduction
100G
g = 2.019
Fig. 2. EPR spectrum of l-superoxo complex of 2. The spectrum was
obtained with reaction of a solution of 2 in chloroform with air at
room temperature.
corresponding monomer, it can be easily concluded that
this diporphyrin is not a true cofacial diporphyrin. In
the presence of dioxygen in chloroform, complex 2
is supposed to form a l-superoxo dicobalt complex
(Co–O2–Co) via a geometry change from slipped
conformation to a cofacial conformation (Fig. 3).
Chang et al. [2g,2h] have reported that several cofa-
cial dicobalt diporphyrins formed l-superoxo dicobalt
complex with dioxygen in the presence of excess organic
base as axial ligand. Similar face-to-face dicobalt por-
phyrin dimmers have been reported by Chang [5] and
Collman et al. [2d] to form l-peroxo dicobalt complex
with dioxygen in the presence of organic base as axial li-
gand and molecular iodine as oxidant. Collman et al. [6]
have also reported a mixed valence CoIICoIII cofacial
porphyrin produced by electrolysis to form l-superoxo
on complex 2 adsorbed electrode is located at Epc
=
À0.45 V vs. SCE. This peak potential is positively
shifted by 190 mV when compared to the peak potential
for O2 reduction on the bare electrode. At the same time,
the peak current for O2 reduction on complex 2 ad-
sorbed electrode increases relative to that on the bare
electrode. The decrease of overpotential for O2 reduc-
tion and the increase of O2 reduction current demon-
strate that complex 2 shows electrocatalytical activity
towards O2 reduction. In order to quantify this result,
voltammetry at a rotating disk electrode has been per-
formed. In Fig. 5, voltammetric curves recorded with
the use of a rotating GCE disk electrode for O2 reduc-
tion are shown. The Koutecky–Levich plot in Fig. 5
indicates that reduction of O2 occur via a two-electron
pathway on the complex 2 modified electrode [7].
It has been noted that the precise geometry of dipor-
phyrins is a crucial factor that determines two- or four-
electron reduction of dioxygen occurs [1,8]. The single
bond connections in complex 2 are not expected to af-
ford a well-defined cofacial structure. Although in solu-
tion in the presence of dioxygen, complex 2 may change
into a cofacial conformation (Fig. 3). But when modified
Co
Co
O2
O
.
inCHCl3
O
Co
Co
Fig. 3. The geometry change of complex 2 in the presence of dioxygen
in chloroform.

W.-B. Lu et al. / Inorganic Chemistry Communications 8 (2005) 789–791
791
alysts for O2 with ‘‘metallodiporphyrin approach’’, more
attention should be paid to the geometrical conformation
of the designed metallodiporphyrins.
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(629 mg 1 mmol), anhydrous pyridine (0.2 mL) in anhydrous
benzene (250 mL), carbonyl chloride (COCl2) was added to the
mixture and stirred for 2 h then the excess COCl2 was displaced
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recrystallized from CHCl3/CH3OH to afford 1 (315 mg, 49% yield).
1: IR(cmÀ1): 3360, 3051, 1680, 1599, 1533, 1515, 1438, 1350;
MS(FAB, relative intensity): 1285[(M + 1)+, 55.8%], 1284[M+,
48.8%], 671[M À TPP, 32.6%], 656[M À TPPNH, 65.6%],
628[M À TPPNHCO, 100%], 614[M À TPPNHCONH, 20.9%];
UV–vis(CHCl3): 416, 517, 551, 586, 642 nm; 1H NMR (CDCl3,
300 M): 8.86–9.04(m, 16H), 8.24(m, 12H), 7.90(s, 2H), 7.78(m,
18H), 7.61(d, 2H, J = 7.2 Hz), 7.18(m, 2H), 7.10(d, 2H, J = 7.2 Hz),
4.68–5.42(br, s, 2H), À2.98(s, 4H); Anal. calcd. for C89H60N10O: C,
83.18, H, 4.67, N, 10.90; Found: C, 82.77, H, 5.05, N, 11.20%. 2,
IR(cmÀ1): 3349, 3053, 1680, 1578, 1533, 1515, 1446, 1350, 1000;
UV–vis(CHCl3): 435, 545 nm; Anal. calcd. for CO2C89H56N10O: C,
76.39, H, 4.01, N, 10.01, Co, 8.44; Found: C, 76.67, H, 4.45, N,
9.76, Co, 8.07%.
Fig. 5. Reduction of O2 at a rotating glassy carbon disk electrode with
different rotation speeds, cyclic voltammograms obtained for an O2
saturated solution in 0.5 M CF3COOH on glassy carbon disk electrode
coated with 2; All potentials are referenced against SCE. The below
part is the Koutecky–Levich plot for the reduction of O2, n = 2 and
n = 4 lines are the calculated responses for reduction of O2 by two and
four electrons, respectively.
onto electrode surface, the two porphyrin rings in com-
plex 2 may be confined to in a largely slipped conforma-
tion. So it can only catalyze a 2eÀ reduction of O2
because the two rings would not act in concert when
binding and reducing O2, like most of monomeric por-
phyrin complexes.
Chang et al. [2f] have noticed a curious correlation be-
tween the shape of the Co–O2–Co EPR signal and the
electrocatalytical ability of dicobalt diporphyrin complex
to mediate the four-electron reduction of O2. Although
complex 2 can form l-superoxo dimer with O2 to give a
15-line hyperfine EPR spectrum, it can only catalyze O2
reduction by a 2eÀ pathway. The current results suggest
that there is no always correlation between the EPR spec-
trum and the catalytical activity of dicobalt diporphyrin
complex for 4eÀ reduction of O2. In the design of 4eÀ cat-
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