Enhanced O2 electrocatalysis by a highly conjugated cobalt(II) porphyrin

Article abstract of DOI:10.1016/j.inoche.2012.11.028

Cobalt(II) tetraphenyltetranaphthoporphyrin, with extended conjugation at the pyrrole units, has been synthesized and its spectroscopic and electrocatalytic behavior toward the reduction of oxygen has been investigated. The new complex shows a red shift for the Soret band of 75 nm compared to its cobalt(II) tetraphenylporphyrin analog. In addition, the new complex shows enhanced electrocatalytic ability for the reduction of oxygen in acidic media converting over 50% of the oxygen directly to water, a four electron process at relatively high potentials.

Full text of DOI:10.1016/j.inoche.2012.11.028

Inorganic Chemistry Communications 29 (2013) 14–17
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Enhanced O2 electrocatalysis by a highly conjugated cobalt(II) porphyrin
⁎
Shawn Swavey , Allison Eder
Department of Chemistry, University of Dayton, 300 College Park, Dayton, OH 45469-2357, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Cobalt(II) tetraphenyltetranaphthoporphyrin, with extended conjugation at the pyrrole units, has been
synthesized and its spectroscopic and electrocatalytic behavior toward the reduction of oxygen has been
investigated. The new complex shows a red shift for the Soret band of 75 nm compared to its cobalt(II)
tetraphenylporphyrin analog. In addition, the new complex shows enhanced electrocatalytic ability for the
reduction of oxygen in acidic media converting over 50% of the oxygen directly to water, a four electron
process at relatively high potentials.
Received 15 October 2012
Accepted 26 November 2012
Available online 16 December 2012
Keywords:
Electrocatalysis
Oxygen reduction
Fused-pyrrole
Cobalt porphyrin
© 2012 Elsevier B.V. All rights reserved.
Introduction: There has been a growing interest in the spectro-
scopic effects of extending the conjugation of porphyrins through fusing
of aromatic groups to the pyrrole units [1]. Red shifted porphyrin com-
plexes have found use in photodynamic therapy [2], non-linear optics
[3], and solar energy conversion [4]. Many of these highly conjugated
complexes have exhibited 100 to 150 nm red shifts in the Soret
(B band) band of the porphyrins when compared to their pyrrole
analog complexes [5]. We have recently become interested in looking
at the effects of these extended π-systems, containing cobalt(II) metal
centers, on the electrocatalysis of molecular oxygen in acidic media.
A
great deal of work on metallo-porphyrins, specifically cobalt
Results and discussion: Using an adaptation of Barton–Zard chem-
istry [8], 1-nitronaphthalene was reacted with ethyl isocyanoacetate in
the presence of a strong non-nucleophilic phosphazine base to give
naphtho[1,2-c]pyrrole in excellent yields [9]. Removal of the ester
group was accomplished by refluxing the pyrrole in ethylene glycol
containing potassium hydroxide under nitrogen for 30 min. The
resulting fused pyrrole was reacted, by a slightly altered procedure of
the Lindsey method [5b], with benzaldehyde (under nitrogen) in the
presence of catalytic amounts of BF3∙Et2O in dry methylene chloride at
−50 °C for 2 h followed by 48 h at room temperature.
Addition of excess tetrachloro 1,4-benzoquinone (p-chloranil)
followed by chromatography gave the tetraphenyltetranaphtho-
porphyrin in ca. 9% yield. The first band from the column was the par-
tially oxidized porphodimethene, H4TPTNP (MALDI TOF: found, m/z=
1017.3, M+, calc. for [C76H48N4] 1017.2), followed by the target
tetraphenyltetranaphthoporphyrin, H2TPTNP. Insertion of cobalt(II)
was accomplished by reacting a slight excess of cobalt(II) acetate with
the porphodimethene or porphyrin in refluxing DMF under nitrogen
for 30 min to give the cobalt(II) porphodimethene, CoH2TPTNP
(MALDI TOF: found, m/z=1074.2, M+, calc. for [C76H46N4Co]
1074.1) and the cobalt(II) tetraphenyltetranaphthoporphyrin,
CoTPTNP [10] in ca. 70% yield for both. The reaction is illustrated
in Scheme 1.
porphyrins, as electrocatalysts for oxygen reduction has seen numerous
examples of dicobalt cofacial and multinuclear cobalt porphyrins capable
of catalyzing the reduction of oxygen by four electrons to water [6]. But
for a few exceptions [7] mononuclear cobalt(II) porphyrins are known
to reduce oxygen at catalytic potentials by two electrons, to form hydro-
gen peroxide, rather than the desired four electron process to form
water. This, of course, has limited the use of cobalt porphyrins as
replacements for the currently used platinum catalysts found in
most fuel cells. In our first attempt at investigating the effects of
extended conjugation we chose one of the simplest known two
electron cobalt porphyrin oxygen reduction catalysts, namely,
cobalt(II) tetraphenylporphyrin (CoTPP). To extend the conjugation
we fused naphthylene groups to the pyrrole subunits and inserted
cobalt(II) to give the cobalt(II) tetraphenyltetranaphthoporphyrin
(CoTPTNP).
⁎
Corresponding author. Tel.: +1 937 229 3145; fax: +1 937 229 2635.
E-mail address: shawn.swavey@notes.udayton.edu (S. Swavey).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.11.028

S. Swavey, A. Eder / Inorganic Chemistry Communications 29 (2013) 14–17
15
Scheme 1. Synthesis of CoTPTNP.
Electronic spectroscopy of the free-base tetraphenylporphyrin
(H2TPP) and its cobalt(II) analog (CoTPP) in dichloromethane gave
Soret bands at 418 nm and 411 nm, respectively.
with CoTPTNP (blue line) at 1000 rpm in air saturated 1.0 M H2SO4.
The bare electrode shows the reduction of molecular oxygen with
an E1/2 =−0.35 V vs. SCE. Reduction of molecular oxygen at the
EPG electrode coated with CoTPP shows enhanced catalysis with an
Fig. 1 shows the electronic spectra of H4TPTNP, CoH2TPTNP,
H2TPTNP and CoTPTNP in dichloromethane. The peak absorbance
for the free-base naphtho-fused porphodimethene (blue line,
Fig. 1A) appears at 517 nm, nearly a 100 nm redshift compared to
the free-base tetraphenylporphyrin. Insertion of cobalt(II) shifts the
Soret band even farther to the red with a peak absorbance at
544 nm (green line, Fig. 1A), approximately a 150 nm shift compared
to the cobalt(II) tetraphenylporphyrin. The nearly 30 nm red shift
after insertion of a metal into the porphodimethene has been linked
to structural changes of the macrocycle after metalation [11]. A
broad Soret band (530 nm) with very weak Q-bands is observed for
H2TPTNP (Fig. 1B, blue line) in dichloromethane. Upon addition of
TFA, protonation of the porphyrin leads to formation of the cationic
porphyrin and a much sharper Soret band (549 nm, Fig. 1B red line)
and well defined Q-bands (657 nm and 686 nm) which has been
linked to increased distortions caused by the meso-substituents [5c].
Insertion of cobalt(II) into H2TPTNP results in a shift of the Soret
band and Q-bands to higher energy (486 nm, 626 nm, 671 nm) as
expected for a d7 metal. Molar absorptivities of the complexes are
in the tens of thousands; however, considerably less than the parent
tetraphenylporphyrin and its cobalt(II) analog due to saddling of
the ligand [5b].
E
1/2 =0.16 V vs. SCE, a catalytic shift of ca. 500 mV compared to the
bare electrode. When the EPG electrode is coated with the cobalt
porphodimethene (CoH2TPTNP), the E1/2 value for the reduction of
oxygen shifts to an even greater catalytic potential of 0.22 V vs. SCE;
60 mV more positive than the CoTPP complex. In addition the current
is higher than the current observed for the CoTPP coated electrode
suggesting that some of the molecular oxygen is being reduced to
water.
The EPG electrode coated with the fully oxidized CoTPTNP com-
plex displays an E1/2 value of 0.30 V vs. SCE, an additional shift of
80 mV compared to the partially oxidized CoH2TPTNP complex and
a 140 mV catalytic shift compared to CoTPP, with considerably higher
current values for the reduction of oxygen.
To determine the number of electrons transferred to oxygen by
the CoH2TPTNP and CoTPTNP coated electrodes, RDE experiments
were performed at various rotation rates (Fig. 3A and B, respectively)
and the diffusion limited current plotted according the Koutecky–
Levich equation (Eq. (1)) [12].
1=IL ¼ 1=Ik þ 1=Bω1=2
ð1Þ
B ¼ 0:2nFCv−1=6D2=3
The electrocatalytic reduction of molecular oxygen in 1.0 M H2SO4
was studied at a roughened edge plane pyrolytic graphite (EPG) elec-
trode coated with the Co(II) porphyrin complexes using rotating disk
electrode (RDE) voltammetry. Small aliquots, ca. 10 μL, of mM solu-
tions of the complexes in acetone were added to the surface of the
roughened EPG electrode and the solvent was allowed to evaporate.
Fig. 2 illustrates a comparison of the bare EPG electrode (red line),
an EPG electrode coated with CoTPP (green line), an EPG electrode
coated with CoH2TPTNP (black line), and an EPG electrode coated
Where IL is the current density (μA cm−2), n is the number of elec-
trons for the reaction, F is the Faraday constant (96,500 Cmol−1), D is
the diffusion coefficient for oxygen in solution (2.0×10−5 cm2 s−1),
v is the kinematic viscosity of the solution (0.01 cm2 s−1), C is the con-
centration of oxygen in the air-saturated solution (0.25 mM), and ω is
the rotation rate (rpm).
Fig. 3 illustrates the results of the RDE experiments and the
Koutecky–Levich plot of the limiting current. Results of RDE experiments

16
S. Swavey, A. Eder / Inorganic Chemistry Communications 29 (2013) 14–17
Fig. 2. Comparison of the reduction of oxygen in 1.0 M H2SO4 at an EPG electrode,
bare (red line), coated with CoTPP (green line), coated with CoH2TPTNP (black
line), coated with CoTPTNP (blue line), and coated with CoTPTNP in nitrogen saturat-
ed 1.0 M H2SO4 at 1000 rpm, scan rate=10 mV/s.
reduction are occurring, with one of those pathways being the direct four
electron reduction of oxygen to water.
The intercept of the Koutecky–Levich plot gives the kinetic current
density, Ik, for the reduction of oxygen. This has been related to the
rate of formation of the Co\O2 bond, k [13] (Eq. (2)).
Ik ¼ nFkΓCO2
ð2Þ
Where n is the number of electrons transferred and Γ is the surface
complex concentration. It is; however, difficult to determine the true
surface concentration by the method used in this study to adsorb the
complex onto the roughened EPG electrode. Assuming that the
surface concentration is the same for each of the complexes studied
(which is reasonable since the same amount of each complex was
added to the EPG electrode) and that all of the cobalt porphyrin
molecules added to the EPG surface are electroactive toward the
reduction of oxygen, a more difficult assumption, then the following
preliminary analysis of the kinetic current is conjectured. Ik for CoTPP
from the Koutecky–Levich plot is ca. 900 μA/cm2, it is 1500 μA/cm2
for CoH2TPTNP, and 2300 μA/cm2 for CoTPTNP. Dividing by their
respective
n values reveals that k(CoH2TPTNP)=1.23 k(CoTPP),
suggesting that formation of the Co\O2 bond is 1.23 times faster for
CoH2TPTNP compared to its analog complex. The ratio of k(CoTPTNP)/
k(CoTPP) is 1.70 indicating that the formation of the Co\O2 bond for
CoTPTNP is nearly twice as fast than for CoTPP and 1.36 times faster
than Co\O2 bond formation for the partially oxidized CoH2TPTNP
cobalt(II) porphodimethene.
Fig. 1. Electronic spectra: A) of H4TPTNP (blue) and CoH2TPTNP (green) in dichloromethane.
B) H2TPTNP (blue) and in the presence of less than 1% TFA (red). C) H2TPTNP (blue) and
CoTPTNP (green) in dichloromethane.
Conclusions: In summary, a new conjugated cobalt(II) porphy-
rin (CoTPTNP) has been synthesized and characterized. A signifi-
cant red shift in the Soret band is observed for this new complex
compared to its analog cobalt(II) tetraphenylporphyrin complex.
In addition, enhanced reduction of oxygen in acidic media is ob-
served for the conjugated system with a shift to more catalytic po-
tentials and greater current density upon reduction of oxygen, as
determined by analysis of the Koutecky–Levich plot. This opens a
new avenue for investigating other conjugated porphyrin com-
plexes as potential electrocatalysts for oxygen reduction. We are
currently looking at computational methods to determine the ef-
fect of the extended π-system on the d-orbitals of the cobalt(II)
metal center.
run on an EPG electrode coated with CoTPP gave (from the slope of the
Koutecky–Levich plot) an n value of 2. As expected the CoTPP complex
reduces oxygen by two electrons to hydrogen peroxide (data not
shown). From the data in Fig. 3 the slope of the Koutecky–Levich plot
for the EPG electrode coated with CoH2TPTNP (red line Fig. 3C) gives
an n value of 2.6 electrons for each molecule of oxygen; while the
value for the CoTPTNP coated electrode gave an n value of 3.2 electrons
for each molecule of oxygen indicating two parallel pathways for oxygen

S. Swavey, A. Eder / Inorganic Chemistry Communications 29 (2013) 14–17
17
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Fig. 3. Reduction of oxygen at a rotating disk electrode coated with CoH2TPTNP (A) and
CoTPTNP (B); current–potential curves in air-saturated 1.0 M H2SO4, scan rate=10 mV/s.
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