Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical properties and catalytic activity for reduction of molecular oxygen in acid media

Article abstract of DOI:10.1016/j.jinorgbio.2013.12.014

Cobalt(III) triarylcorroles containing 0-3 nitro groups on the para-position of the three meso-phenyl rings of the macrocycle were synthesized and characterized by electrochemistry, mass spectrometry, (UV-vis) and ¹H NMR spectroscopy. The examined compounds are represented as (NO₂Ph)_nPh_{3 - n}CorCo(PPh₃), where n varies from 0 to 3 and Cor represents the core of the corrole. Each compound can undergo two metal-centered one-electron reductions leading to formation of Co(II) and Co(I) derivatives in CH₂Cl₂ or pyridine containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). A stepwise two electron reduction of each NO₂Ph group of the compound is also observed. The first is reversible and occurs in a single overlapping step at the same potential which involves an overall one-, two- or three-electron transfer process for compounds 2-4, respectively. This indicates the lack of an interaction between these redox active sites on the corroles. The second reduction of the NO₂Ph groups is irreversible and located at a potential which overlaps the Co(II)/Co(I) process of the compounds. Thin-layer UV-visible spectroelectrochemical measurements in CH₂Cl₂, 0.1 M TBAP demonstrate the occurrence of an equilibrium between a Co(III) π-anion radical and a Co(II) derivative with an uncharged macrocycle after the first controlled potential reduction of the nitro-substituted corroles. All four cobalt corroles were also examined as catalysts for the electroreduction of O₂ when coated on an edge-plane pyrrolytic graphite electrode in 1.0 M HClO₄. This study indicates that the larger the number of nitro-substituents on the cobalt corrole, the better the compound acts as a catalyst.

Full text of DOI:10.1016/j.jinorgbio.2013.12.014

JIB-09444; No of Pages 10
Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Cobalt triarylcorroles containing one, two or three nitro groups. Effect of
NO2 substitution on electrochemical properties and catalytic activity for
reduction of molecular oxygen in acid media
a
a
b
a
Bihong Li a, Zhongping Ou a, , Deying Meng , Jijun Tang , Yuanyuan Fang , Rui Liu , Karl M. Kadish
⁎
b,⁎⁎
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Cobalt(III) triarylcorroles containing 0–3 nitro groups on the para-position of the three meso-phenyl rings of the
macrocycle were synthesized and characterized by electrochemistry, mass spectrometry, (UV–vis) and 1H NMR
spectroscopy. The examined compounds are represented as (NO2Ph)nPh3 − nCorCo(PPh3), where n varies from 0
to 3 and Cor represents the core of the corrole. Each compound can undergo two metal-centered one-electron
reductions leading to formation of Co(II) and Co(I) derivatives in CH2Cl2 or pyridine containing 0.1 M tetra-n-
butylammonium perchlorate (TBAP). A stepwise two electron reduction of each NO2Ph group of the compound
is also observed. The first is reversible and occurs in a single overlapping step at the same potential which in-
volves an overall one-, two- or three-electron transfer process for compounds 2–4, respectively. This indicates
the lack of an interaction between these redox active sites on the corroles. The second reduction of the NO2Ph
groups is irreversible and located at a potential which overlaps the Co(II)/Co(I) process of the compounds.
Thin-layer UV–visible spectroelectrochemical measurements in CH2Cl2, 0.1 M TBAP demonstrate the occurrence
of an equilibrium between a Co(III) π-anion radical and a Co(II) derivative with an uncharged macrocycle after
the first controlled potential reduction of the nitro-substituted corroles. All four cobalt corroles were also exam-
ined as catalysts for the electroreduction of O2 when coated on an edge-plane pyrrolytic graphite electrode in 1.0
M HClO4. This study indicates that the larger the number of nitro-substituents on the cobalt corrole, the better the
compound acts as a catalyst.
Received 11 September 2013
Received in revised form 24 December 2013
Accepted 24 December 2013
Available online xxxx
Keywords:
Cobalt corroles
Nitro-substitution
Synthesis
Electrochemistry
Dioxygen reduction
© 2014 Elsevier Inc. All rights reserved.
1. Introduction
Previous electrochemical characterization of nitrocorroles has
shown that the presence of one or more electron-withdrawing NO2
Cobalt corroles are one of the most frequently studied groups of
metallocorroles because of their unique spectral and electrochemical
properties [1–22], as well as their potential applications as catalysts
for a variety of reactions [23–32]. In this regard, part of our own research
efforts have been directed toward the synthesis [4,7–9,28–30,33,34]
and characterization of substituted cobalt corroles as catalysts for the
electroreduction of O2 in acid media [30,33–36].
It is known that the addition of one or more highly electron-
withdrawing NO2 groups to a corrole will significantly modify its chem-
ical and physical properties, with the type and magnitude of the effect
being dependent upon the position of the substituents. Three types of
nitro-substituted corroles have been reported in the literature. These
are: (i) meso-substituted nitrocorroles [37], (ii) β-pyrrole substituted
nitrocorroles [38–45] and (iii) meso-phenyl substituted nitrocorroles
[38,46–50].
groups at the meso-phenyl or β-pyrrole positions of the conjugated
macrocycle will have a large effect on the E1/2 for electroreduction,
shifting the potentials in a positive direction by up to 200 mV per
nitro group in the case of β-pyrrole substituted (TPC)M where M =
Cu or Fe, TPC = triphenylcorrole. This compares to a 70–80 mV shift
in potentials in the case of meso-nitrophenyl substituted corroles [38]. A
direct electroreduction of the NO2 group does not occur in nonaqueous
media when this electron-withdrawing group is attached at the β-pyrrole
position of a porphyrin or corrole, but this is not the case when the NO2
group is located on the phenyl ring of a TPC type complex. Under these
conditions a reversible one-electron reduction is observed at a potential
of about −1.1 V vs SCE (saturated calomel electrode) [38,50].
We have earlier demonstrated that the type of substituents located
at the meso-phenyl positions of a triarylcorrole or tetraarylporphyrin
not only will affect the redox potentials, but also significantly influence
the ability of the compound to act as an electrocatalyst in the reduction
of O2 when adsorbed at an electrode surface in acid media [21,36]. How-
ever, no studies of these types have been carried out with corroles or
porphyrins having NO2 substituents and it is therefore not known
how the addition of one or more strongly electron-withdrawing NO2
⁎
Corresponding author. Tel.: +86 511 88791800.
⁎⁎ Corresponding author. Tel.: +1 713 7432740.
E-mail addresses: zpou2003@yahoo.com (Z. Ou), kkadish@uh.edu (K.M. Kadish).
0162-0134/$ – see front matter © 2014 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

2
B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
groups on the macrocycle will affect the compounds' electrocatalytic ac-
tivity for the reduction of dioxygen. This is examined in the present
study where a series of cobalt corroles containing zero, one, two or
three para-nitrophenyl substituents were synthesized and then charac-
terized by electrochemistry and spectroscopic techniques. The investi-
gated compounds are represented as (NO2Ph)nPh3 − nCorCo(PPh3),
where n varies from 0 to 3 and Cor is the core of the corrole and the
structure of compounds is shown in Chart 1.
Reduction/oxidation potentials and UV-visible (UV-vis) spectra of
each neutral, oxidized and reduced species were measured in dichloro-
methane (CH2Cl2) containing 0.1 M tetra-n-butylammonium perchlorate
(TBAP). The effect of the NO2 substituents on redox potentials and the site
of electron transfer are examined and an overall electroreduction/
electrooxidation mechanism is proposed.
The catalytic activity of the corroles in Chart 1 was also examined as
to their ability to reduce O2 when adsorbed on an electrode surface. The
numbers of electrons transferred and the percentage of H2O2 produced
in the catalytic reduction of oxygen were calculated from cyclic and lin-
ear sweep voltammetry measurements coupled with measurements at
a rotating disk electrode (RDE).
solution stirred at room temperature for 1 h. The same procedure was
then followed as described above for the A3 type compounds.
2.3.1. Ph3CorCo(PPh3)
22 mg 68.7% yield. UV–vis λmax/nm in CH2Cl2: 387, 562. 1H NMR
(400 MHz CDCl3) 8.63 (s, 2H, β-H), 8.34 (s, 2H, β-H), 8.12–8.06
(m, 7H, β and Ph-H), 7.65 (s, 11H, Ph-H), 7.42 (s,1H, Ph-H),7.08
(s, 3H, p-H of PPh3), 6.73(s, 6H, m-H of PPh3), 4.76 (t, 6H, o-H of
PPh3). m/z observed 582.52 [M-PPh3]+, calcd 582.54 for C37H23N4Co.
2.3.2. (NO2Ph)Ph2CorCo(PPh3)
21 mg 67.5% yield. UV–vis λmax/nm in CH2Cl2: 384, 559. 1H NMR
(400 MHz CDCl3) 8.70 (s, 2H, β-H), 8.44 (m, 4H, β-H), 8.19 (s, 1H, β-H),
8.09–8.05 (m, 6H, β and Ph-H), 7.67 (s, 8H, Ph-H), 7.45 (s, 1H, Ph-H),
7.09 (s, 3H, p-H of PPh3), 6.73 (s, 6H, m-H of PPh3), 4.69 (t, 6H, o-H of
PPh3). m/z observed 628.36 [M-PPh3]+, ca1cd 629.13 for C37H24N5O2Co.
2.3.3. (NO2Ph)2PhCorCo(PPh3)
18 mg 59.4% yield. UV–vis λmax/nm in CH2Cl2: 375,576. 1H NMR
(400 MHz CDCl3) 8.77 (s, 2H, β-H), 8.51 (s, 4H, β-H), 8.36–8.25
(m, 5H, β and Ph-H), 8.04–7.99 (m, Ph-H), 7.69–7.89 (m, 6H, Ph-H),
7.39 (s, 1H, Ph-H), 7.11 (s, 3H, p-H of PPh3), 6.73 (s, 6H, m-H of PPh3),
4.65 (t, 6H, o-H of PPh3). m/z observed 673.43 [M-PPh3]+, calcd
674.11 for C37H23N6O4Co.
2. Experimental
2.1. Chemicals
CH2Cl2 was purchased from Sinopharm Chemical Reagent Co. or
Aldrich Chemical Co. and used as received for electrochemistry and
spectroelectrochemistry experiments. TBAP was purchased from
Sigma Chemical or Fluka Chemika Co., recrystallized from ethyl alcohol,
and dried under vacuum at 40 °C for at least one week prior to use.
2.3.4. (NO2Ph)3CorCo(PPh3)
23 mg 67.6% yield. UV–vis λmax/nm in CH2Cl2: 372, 575. 1H NMR
(400 MHz CDCl3) 8.84 (s, 2H, β-H), 8.54–8.44 (m, 8H, β and Ph-H),
8.28–8.08 (m, 7H, Ph-H), 7.72 (s, 2H, Ph-H), 7.45 (s, 1H, Ph-H), 7.13
(s,3H, p-H of PPh3), 6.73 (s, 6H, m-H of PPh3), 4.61 (t, 6H, o-H of
PPh3). m/z observed 718.62 [M-PPh3]+ cacld 719.10 for C37H22N7O6Co.
2.2. Synthesis of A3 type compounds 1 and 4 [51]
2.4. Instrumentation
Aldehyde (2.5 mmol) and pyrrole (0.35 mL, 5 mmol) were dis-
solved in MeOH (100 mL) followed by H2O (100 mL) and HCl (36%,
2.1 mL) being added to solution. After stirring at room temperature
for 3 h the compound was extracted with CHCl3. The organic layer
was washed twice with H2O, then dried with MgSO4, filtered and dilut-
ed to 300 mL with CHCl3. After the addition of p-chloranil (615 mg,
2.5 mmol) the mixture was refluxed for 1 h and then chromatographed
on a silica column using CH2Cl2/hexane as eluent. The free-base corroles
were obtained by evaporating the solvent, after which ~20 mg of the
compound was dissolved in 30 mL MeOH containing 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 starting free-base corrole was consumed. The sample was
evaporated to dryness and flash chromatographed (neutral alumina,
200–300 mesh, CH2Cl2/hexane as eluent). The red fraction was collect-
ed and evaporated to dryness to yield the desired cobalt corrole product.
Thin-layer UV–visible spectroelectrochemical experiments were
performed with a home-built thin-layer cell which has a light trans-
parent platinum net working electrode. Potentials were applied and
monitored with an EG&G PAR Model 173 potentiostat or a BiStat
electrochemistry station. Time-resolved UV–visible spectra were re-
corded with a Hewlett-Packard Model 8453 diode array spectropho-
tometer. High purity N2 from Trigas was used to deoxygenate the
solution and kept over the solution during each electrochemical and
spectroelectrochemical experiment.
All reported 1H NMR spectra were recorded on a Bruker Avanc II 400
MHz instrument. Chemical shifts (δ ppm) were determined with TMS
(tetramethylsilane) as the internal reference. MALDI-TOF (matrix-
assisted laser desorption/ionization) mass spectra were taken on a
Bruker BIFLEX III ultra-high resolution instrument using alpha-cyano-
4-hydroxy-cinnamic acid as the matrix.
All electrochemical measurements were carried out at 298 K using an
EG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat or
a Chi-730C Electrochemistry Work Station. A three-electrode system was
used and consisted of a graphite working electrode (Model MT134, Pine
Instrument Co.) for cyclic voltammetry and rotating disk voltammetry. A
2.3. Synthesis of A2B type compounds 2 and 3 [52,53]
Dipyrromethane (1 mmol) and aldehyde (0.5 mmol) were dissolved
in MeOH (50 mL). HCl (36%, 2.5 mL) in H2O (50 mL) was added and the
Chart 1. Structure of investigated cobalt meso-substituted triphenylcorroles, where the L is the PPh3 axial ligand.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
3
platinum wire served as the auxiliary electrode and a home-made
SCE as the reference electrode, which was separated from the
bulk of the solution by means of a salt bridge. The RRDE was pur-
chased 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 rotating disk electrode (RDE) and rotating ring disk electrode
(RRDE) experiments. The Pt ring was first polished with 0.05 μm
α-alumina powder and then rinsed successively with water and ac-
etone before being activated by cycling the potential between 1.20
and −0.20 V in 1.0 M HClO4 until reproducible voltammograms are
obtained [54,55].
The corrole catalysts were irreversibly adsorbed on the elec-
trode surface by means of a dip-coating procedure described in
the literature [56]. The freshly polished electrode was dipped in a
1.0 mM catalyst solution of CH2Cl2 for 5 s, transferred rapidly to
pure CH2Cl2 for 1–2 s, and then exposed to air where the adhering
solvent rapidly evaporated leaving the corrole catalyst adsorbed on
the electrode surface. All experiments were carried out at room
temperature.
a) Route for A type compounds 1 and 4
3
b) Route for A B type compounds 2 and 3
2
Scheme 1. Synthesis routes for compounds 1–4.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

4
B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
a) (Ph) CorCo(PPh )
3
3
b) (NO Ph)(Ph) CorCo(PPh )
2
2
3
c) (NO Ph) (Ph)CorCo(PPh )
2
2
3
d) (NO Ph) CorCo(PPh )
2
3
3
Fig. 1. Cyclic voltammograms of compounds 1–4 in CH2Cl2 containing 0.1 M TBAP at a scan rate of 0.1 V/s.
3. Results and discussions
In order to elucidate these points, electrochemistry and spectro-
electrochemistry experiments were carried out in the nonbonding
solvent CH2Cl2. Cyclic voltammograms of compounds 1–4 in CH2Cl2 are
shown in Fig. 1 and a summary of measured half-wave and peak poten-
tials is given in Table 1.
As seen from the cyclic voltammograms and table of potentials, Ph3-
CorCo(PPh3) 1, which lacks an NO2 substituent, undergoes two irrevers-
ible reductions at Epc = −0.73 and −1.62 V in CH2Cl2 containing 0.1 M
TBAP. These two reductions have been shown to generate the Co(II) and
Co(I) forms of the corrole, respectively [34]. There are also two
macrocycle-centered oxidations at E1/2 = 0.51 and 0.91 V. Similar
3.1. Synthesis
The A3 type of compounds (Ph)3CorCo(PPh3) 1 and (NO2Ph)3
CorCo(PPh3) 4 were synthesized according to the synthetic route de-
scribed in Scheme 1a which follows a procedure reported in the litera-
ture [51]. The A2B type of compounds, (NO2Ph) Ph2CorCo(PPh3) 2 and
(NO2Ph)2PhCorCo(PPh3) 3, were synthesized according to the route
shown in Scheme 1b [52,53]. The formula of the compound was verified
by 1H NMR and mass spectrometry.
3.2. Electrochemistry in CH2Cl2
Table 1
Half-wave potential (V vs SCE) of (4-NO2Ph)xPh3 − xCorCo(PPh3) 1–4 in CH2Cl2 containing
0.1 M TBAP.
The effect of nitration on the overall oxidation/reduction mechanism
and the site of each electron transfer were investigated by cyclic
voltammetry and thin-layer UV–visible spectroelectrochemistry. The
addition of one or more NO2 groups on the three meso-phenyl rings of
the triarylcorrole would be expected to shift all redox potentials toward
more positive values [57,58], but it was not clear what would be the
magnitude of the potential shift for the addition of one, two or three
electron-withdrawing NO2 groups on the three meso-phenyl rings of
the cobalt corroles, nor was it clear how the addition of multiple NO2
groups might effect the site of electron transfer which could occur at
the central metal ion, at the conjugated π-ring system of the macrocycle,
and/or at the NO2Ph groups of compounds 2, 3 and 4.
cpd #NO2
groups
Oxidation
Reduction
E1/2 (NO2Ph) Ep (2)
E1/2 (2) E1/2 (1) ΔE (2 − 1) Ep (1)
1
2
3
4
None
One
Two
0.94
0.99
1.03
1.05
0.51
0.57
0.62
0.67
0.40
0.42
0.41
0.38
−0.73a
–
−1.62a
−0.65a −1.10
−0.63a −1.09c
−0.56a −1.06d
−1.63a,b
−1.68a,b
−1.71a,b
Three
a
b
c
Irreversible peak potential at a scan rate of 0.1 V/s.
Multiple overlapping electron transfer processes.
Two overlapping one-electron processes.
d
Three overlapping one-electron processes.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
5
1.5
redox processes are seen for compounds 2–4 but with the potentials
shifted positively due to the electron-withdrawing nitro groups. In addi-
tion, two new reduction processes are observed for compounds 2, 3 and
4 which involve the electroactive NO2Ph groups on the meso positions of
the triarylcorrole.
The reduction of nitrobenzene in CH2Cl2 occurs in two steps, the first
of which is reversible and located at E1/2 = −1.08 V [59]. As seen in
Fig. 1, almost identical half-wave potentials are seen for reduction of
1
0.5
0
the meso-NO2Ph groups on compounds 2 (E1/2 = −1.10 V), 3 (E1/2
=
−1.09 V) and 4 (E1/2 = −1.06 V) under the same solution conditions.
The peak current for the NO2Ph reduction of compound 2 at −1.10 V
-0.5
-1
is the same as that of the preceding CoIII/CoII reaction at Epc
=
−0.65 V, consistent with a one-electron transfer in each step, while
the peak current for reduction of NO2Ph in compound 3 is located at
Epc = −1.09 V and is twice high as that of the first reduction of the
same compound at Epc = −0.63 V. Likewise, the process for reduction
of NO2Ph in compound 4 is located at Epc = −1.06 V and is three
times as high as the current for the first reduction at Epc = −0.56 V
(see voltammograms in Fig. 1). These results are consistent with a
two-electron addition to compound 3 and a three-electron addition to
compound 4 in the second reduction step. A single multielectron reduc-
tion of meso-NO2Ph groups has previously been reported for iron [38]
and copper triarylcorroles [50] as well as for tetraphenylporphyrins
having NO2 substituents at the four meso-positions of the macrocycle
[60]. The fact that only a single overlapping process is seen for the
second reduction of compounds 3 and 4 is consistent with a lack of
any interaction between the meso-NO2Ph groups of the corrole, each
of which is reduced at essentially the same potential.
As indicated above, nitrobenzene undergoes two one-electron
reductions in CH2Cl2 [59] and the same is seen for the NO2Ph groups
of compounds 2, 3 and 4. The second reduction of the NO2Ph group is ir-
reversible as seen in Fig. 1 and located at a peak potential of −1.63 V
(cpd 2) to −1.71 V (cpd 4). These redox processes are overlapped
with the Co(II)/Co(I) reduction of the corrole as evidenced by the rela-
tive peak currents for the three reductions of the nitrocorroles.
-1.5
-2
0
1
2
3
Fig. 2. Plots of redox potentials of the cobalt corroles 1–4 in CH2Cl2 containing 0.1 M TBAP
vs the number of NO2 groups.
Correlations were also made between redox potentials of the
currently investigated nitrocorroles and five previously examined
triarylcorroles containing electron-withdrawing or electron-donating
substituents at the meso-phenyl positions of the macrocycle [34]. This
comparison is shown in Fig. 3 which plots E1/2 or Epc vs the sum of the
Hammett substituent constants (Σσ) taken from Ref. [63].
Linear relationships are seen for the first and second oxidations of
all the compounds, with the slopes being +58 and +63 mV, respec-
tively (see Fig. 3). This suggests that the three nitro-substituted
corroles examined in the present study and the five previously inves-
tigated non-nitro-substituted corroles all undergo the same two
stepwise one-electron macrocycle-centered oxidations as described
3.3. Substituent effect on the redox potentials and the site of
electron transfer
The addition of one to three electron-withdrawing nitro groups to
the phenyl rings of the macrocycles leads to a systematic positive shift
in potential for the first two reductions and first two oxidations of com-
pounds 2–4. For example, the CoIII/CoII transition of 1 is located at Epc
=
−0.73 V in CH2Cl2, 0.1 M TBAP and the potential shifts to −0.65 V for
(NO2Ph)Ph2CorCo(PPh3) 2, −0.63 V for (NO2Ph)2PhCorCo(PPh3) 3
and −0.56 V for (NO2Ph)3CorCo(PPh3) 4 (see Table 1 and Fig. 1). Plots
of E1/2 or Ep for each redox reaction of 1–4 vs the number of NO2 groups
on the compound were constructed and give a linear relationship for
each electron addition and electron abstraction as shown in Fig. 2. The
first and second oxidations in the plots of Fig. 2 have a slope of +53
and +46 mV per each added NO2 group while the slopes of ΔEp/
Δ#NO2 for the reductions are +53, +20 and −32 mV, respectively.
The change from a positive slope for the first two reductions to a nega-
tive slope for the third reduction is perhaps expected, because of the
negative charge on the reaction site after the first one-electron reduc-
tion of the NO2Ph group.
It is worth noting that the average ~50 mV positive shift in redox
potentials of the first reduction and first two oxidations for each
added NO2 group is similar to the shift of potential for iron corroles
with the same meso-NO2Ph substituents [38]. It is however, less
than the ~200 mV shift in E1/2 observed when NO2-substituents are
added to the β-pyrrole positions of a triarylcorrole [38,41,42]. The
smaller effect of the meso-phenyl substituents as compared to the
β-pyrrole substituents is consistent with data reported in the literature
for other corroles, porphyrins and related macrocycles [39,58–61].
Fig. 3. Plots of the redox potentials of cobalt corroles containing OMe, Me, H, F and Cl (solid
points) or NO2 groups (open points or asterisk) on the para-positions of the phenyl rings
of the macrocycle vs the sum of Hammett substituent constants (Σσ). The potentials were
measured in CH2Cl2 containing 0.1 M TBAP and are summarized in Table 1. The Hammett
substituent constants are taken from Ref. [62].
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

6
B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
in the literature [34]. In contrast, two linear plots with different
slopes are observed for the three reductions in the plots in Fig. 3
and this suggests that compounds in the series of non-nitrocorroles
and the nitrocorroles might undergo two different electron transfer
mechanisms or, alternately, two different sites of electron transfer.
It is known that in the case of non-nitrocorroles, the first reduction
of the compound is a metal-centered electron transfer to give a
Co(II) derivative in CH2Cl2 [34]. Thus, the nitrocorroles might under-
go a macrocycle-centered reduction to generate a Co(III) π-anion
radical under the same solution conditions.
The site of electron transfer for each redox reaction was confirmed
by thin-layer UV–visible spectroelectrochemistry. Examples of the
spectral changes obtained for (Ph)3CorCoIII(PPh3) 1 and (NO2Ph)
Ph2CorCoIII(PPh3) 2 during reduction in CH2Cl2, 0.1 M TBAP are illus-
trated in Fig. 4 and the final spectrum of each electrogenerated Co(II)
and Co(I) complex is shown in Fig. 5 which also includes the UV–vis-
ible spectra of compounds 1–4 in their neutral form.
slightly decreases in intensity and a newly formed band of the prod-
uct at 414 nm is present only as a shoulder. Interestingly, the spectral
changes of 2 during the second reduction at −1.40 V (Fig. 4b) are al-
most the same as the spectral changes obtained for 1 during the first
reduction of this compound. This result suggests that the second re-
duction of 2 involves a complete CoIII/CoII conversion after the addi-
tion of two electrons, one of which is added to the NO2Ph substituent.
Similar spectroelectrochemical behavior is seen for compounds 3
and 4 and the electron transfer mechanism for the first two reduc-
tions of the three nitrocorroles is proposed to occur as shown in
Scheme 2.
The final UV–visible spectra after the second one-electron reduction
of 1 at −1.80 V and the third two-electron reduction of 2 at −1.70 V are
quite similar. Both spectra are characterized by a single Soret band at
414–415 nm and a single Q band at 575–580 nm (see Figs. 4 and 5).
These spectra are assigned to a Co(I) corrole in each case.
The Soret (378 nm) and Q bands (562 nm) of the neutral corrole 1
are both red-shifted during the first controlled potential reduction at
−1.20 V to give a final product with an intense Soret band at 421 nm
and a weak Q band at 575 nm. This type of spectrum has previously
been assigned to a Co(II) corrole [34]. The UV–visible spectrum of
unreduced 2 is almost identical to that of unreduced 1 (see Table 2
and Fig. 5a), but the spectral changes for 2 during the first one-
electron reduction at −0.95 V in CH2Cl2 differ from that of 1 (Fig. 4b).
In the case of 2, the Soret band of the neutral corrole at 384 nm only
3.4. Electrocatalytic reduction of O2
Fig. 6 illustrates the examples of cyclic voltammograms for 1 and 4
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 for both
corroles at about 0.4 V, but no peaks assigned to oxidation or reduction
of the cobalt corrole can be observed from 0.60 to −0.20 V under N2.
The lack of a Co(III) corrole reduction at the electrode surface is consis-
tent with the fact that PPh3 coordination to Co(III) shifts the half-wave
a) (Ph) CorCo(PPh )
b) (NO Ph)(Ph) CorCo(PPh )
3
3
2
2
3
Fig. 4. UV–vis spectral changes during reduction of compounds 1 and 2 in CH2Cl2 containing 0.1 M TBAP. Cyclic voltammograms under the same solution conditions are shown in Fig. 1.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
7
II
I
a) neutral form
b) Co form
c) Co form
Fig. 5. UV–vis spectra of compounds 1–4 in CH2Cl2 containing 0.1 M TBAP under the following conditions: (a) in their neutral Co(III) form, (b) after reduction to Co(II) and (c) after re-
duction to Co(I) in a thin-layer cell. Examples of the spectral changes as a function of time under a controlled reducing potential are shown in Fig. 4.
potential for the CoIII/CoII process to potential located more negative
than −0.20 V under the given experimental conditions [56]. A similar
lack of a cobalt corrole redox process was earlier reported for other
five-coordinate cobalt corroles containing a bound PPh3 in acid media
under N2 [32,36].
A well-defined current–voltage curve is seen when the corrole coat-
ed electrode is placed in 1.0 M HClO4 under air. This is shown by the
solid line in Fig. 6 where an irreversible cathodic reduction peak is
seen at Epc = 0.13 V (1) and 0.17 V (4) for a scan rate of 50 mV/s. Com-
pounds 2 and 3 also exhibit a similar reduction peak at 0.15 and 0.14 V,
respectively (see Table 3). As will be demonstrated, the irreversible
peaks obtained in HClO4 under air correspond to the catalytic reduction
of dissolved O2 to give a mixture of H2O2 and H2O.
It should be pointed out that when the corresponding free-base
corrole was used as the catalyst under the same solution conditions
(data not shown), a reduction peak with very a small current was ob-
served at a more negative potential. The more negative potential and
the much reduced current indicates that cobalt ions are necessary for
the catalytic activity of the cobalt corrole to electroreduce oxygen in
acid solution.
Peak potentials for the catalytic reduction of oxygen at the corrole
coated electrode are similar for compounds 1 to 4, which indicates
that the potential for this reaction is not strongly influenced by differ-
ences in the number of NO2 substituents on the corrole. The O2 reduc-
tion peak varies from Ep = 0.13 for 1 to 0.17 V for 4 (see Table 3) and
a similar potential (~0.14 V) has been reported when other cobalt
corroles with a bound PPh3 axial ligand are used as catalysts for the
electroreduction of O2 [32,36]. These peak potentials are all signifi-
cantly lower than the potential reported for O2 reduction using a
β-pyrrole substituted mono-cobalt corrole such as 7,8,12,13-
tetramethyl-2,3,10,17,18-pental- phenylcorrole cobalt(III), [(Me4
Ph5Cor)Co] (0.38 V) [30] or 2,3,7,8,12,13,17,18-octalbromine-
5,10,15-trispentafluorophenylcorrole cobalt(III), [(Br8TF5PCor)Co
(0.56 V)] [25] as the catalyst. The O2 reduction potentials in
Table 2
UV–visible spectral data, λmax, nm (ε × 10−4, cm−1 M−1) of compounds (Cpd) 1–4 in
CH2Cl2.
Compound
Soret region
Q region
Ph3CorCo(PPh3) 1
387 (4.3)
384 (4.6)
375 (4.3)
372 (4.2)
562 (0.8)
559 (1.0)
576 (1.3)
575 (1.1)
(NO2Ph)Ph2CorCo(PPh3) 2
(NO2Ph)2PhCorCo(PPh3) 3
(NO2Ph)3CorCo(PPh3) 4
417 (2.9)a
419 (3.1)a
a
Shoulder peak.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

8
B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
Scheme 2. Proposed reduction mechanism of Co(III) corroles 2–4 in CH2Cl2 containing 0.1 M TBAP.
Fig. 6 are also lower than when the catalyst is a hangman corrole
(0.43–0.48 V) [26] or a meso-substituted corrole, such as 5,10,
15-trispentafluorophenyl corrole cobalt(III), [(TPFCor)Co], 10-
pentafluorophenyl-5,15-dimesitylcorrole cobalt(III), [(F5PhMes2Cor)
Co] or 5,10,15-trismesitylcorrole cobalt(III), [(Mes3Cor)Co] [28].
Measurements were also performed at a rotating disk electrode
(RDE) to calculate the number of electrons transferred in the catalytic
electroreduction of dioxygen. The RDE response was similar for all
four corroles in air-saturated 1.0 M HClO4 (see Fig. 7a) and is character-
ized by a half-wave potential located at 0.28 to 0.29 V for 1–4, where
imax is the limiting current measured at 400 rpm and E1/2 is the potential
O2 reduction at the corrole modified electrode is controlled by mass
transport alone, the relationship between the limiting current and rota-
tion rate should obey the Levich equation given in Eq. (1) [63].
jlev ¼ 0:62nFAD2=3cv−1=6ω1=2
ð1Þ
where n is the number of electrons transferred in the overall electrode
reaction, F is the Faraday constant (96,485 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 ω 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 (see Fig. 7) obeys the Koutecky–Levich equation [64].
when i = 0.5imax
.
The number of electrons transferred during oxygen reduction was
calculated from the magnitude of the steady-state limiting currents
which were taken at a fixed potential on the catalytic wave plateau of
the current–voltage curves in Fig. 7b (−0.10 V). When the amount of
1=j ¼ 1=jlev þ 1=jk
ð2Þ
a) (Ph) CorCo(PPh )
b) (NO Ph) CorCo(PPh )
3
3
2
3
3
Fig. 6. Cyclic voltammograms of 1 and 4 adsorbed on EPPG electrode in 1.0 M HClO4 saturated with nitrogen (———) and saturated with air (____) at a scan rate of 50 mV/s.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
9
Dioxygen can be cathodically reduced by 4e− to give H2O or via 2e−
to give H2O2 after which the resulting H2O2 might be further reduced to
H2O, catalytically decomposed on the electrode surface or removed into
the bulk of the solution [25,32]. When the products of O2 reduction con-
sist 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 show that
the number of electrons transferred (n) ranges from 2.6 to 3.0 for com-
pounds 1–4, indicating that the catalytic electroreduction of O2 is nei-
ther a simple 2e− transfer process to give H2O2 nor a 4e− process to
produce H2O, but consistent with the formation of both H2O and H2O2
using compounds 1–4 as catalysts in acidic media.
As seen in Table 3, compound 1, which doesn't contain any substitu-
ent on the phenyl rings, produces the largest amount of H2O2 as indicat-
ed by the lower number of electron transferred (n = 2.6), while
compound 4, which contains three strong electron-withdrawing NO2
groups, produced the smallest amount of H2O2 and more H2O as indicated
by the higher n values of 3.0, under the same experimental conditions.
These results indicate that cobalt corroles substituted with the strongly
electron-withdrawing NO2 groups may have a higher dioxygen-binding
ability as was proposed for other substituted cobalt corroles and porphy-
rins having different structures [65].
Table 3
Data of catalytic reduction of O2 in 1.0 M HClO4.
a
catalyst
Ep with O2
E1/2
n
H2O2%
(Ph)3CorCo(PPh3) 1
0.13
0.15
0.14
0.17
0.28
0.28
0.29
0.28
2.6
2.7
2.9
3.0
70
65
55
50
(NO2Ph)Ph2CorCo(PPh3) 2
(NO2Ph)2PhCorCo(PPh3) 3
(NO2Ph)3CorCo(PPh3) 4
a
Half-wave potential at i = 0.5imax, where imax is the limiting current measured at
400 rpm of the RDE.
where j is the measured limiting current density (mA·cm−2), jlev is the
Levich current, and jk is the kinetic current which can be obtained ex-
perimentally from the intercept of the Koutecky–Levich line in Fig. 7b.
jk ¼ 103knFGc:
ð3Þ
In Eq. (3), the value of k (M−1 s−1) is the second-order rate constant
of the reaction which limits the plateau current and Γ (mol·cm−2) is the
surface concentration of corroles. The other terms have their usual
significance, as described above.
The slope of a plot in Fig. 7 obtained by linear regression can then be
used to estimate the average number of electrons (n) involved in the
catalytic reduction of O2. This analysis was carried out and the number
of electrons transferred per dioxygen molecule (n) during the catalytic
reduction of O2 by corroles 1–4 was calculated. These values are
summarized in Table 3 which also includes the reduction potentials for
1–4 in air saturated HClO4. The Koutecky–Levich plots for 1–4 indicate
that the number of electrons transferred to O2 in the electroreduction
process ranges from 2.6 to 3.0 (Table 3) with the largest value being
obtained for 4 (n = 3.0) and the smallest for 1 (n = 2.6).
4. Conclusion
Thin-layer UV-vis spectroelectrochemistry indicate that the first one-
electron reduction of the nitro-substituted cobalt(III) triarylcorroles is
delocalized, leading to the formation of a mixture of a Co(III) π-anion rad-
ical and a Co(II) corrole in CH2Cl2 containing 0.1 M TBAP. Each NO2Ph
group of the nitrocorrole can be reduced at the same potential before a
Co(I) corrole is generated at a more negative potential. The cobalt
corroles may catalyze the reduction of O2 to produce a mixture of H2O
a) (Ph) CorCo(PPh )
b) (NO Ph) CorCo(PPh )
3
3
2
3
3
Fig. 7. Current–voltage curve and the Koutecky–Levich plots for catalytic reduction of O2 in 1.0 M HClO4 saturated with air at a rotating EPG disk electrode coated with compounds 1 and 4.
Electrode rotating rates (ω) are indicated on each curve. Potential scan rate = 50 mV/s.
Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

10
B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx
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This work was supported by grants from the Natural Science Foun-
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(KMK, Grant E-680).
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Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical
properties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1016/j.jinorgbio.2013.12.014