Synthesis and electrochemistry of β-pyrrole nitro-substituted cobalt(ii) porphyrins. the effect of the NO2 group on redox potentials, the electron transfer mechanism and catalytic reduction of molecular oxygen in acidic media

Article abstract of DOI:10.1039/c4dt01072h

Four cobalt(ii) porphyrins, two of which contain a β-pyrrole nitro substituent, were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds are represented as (TRPP)Co and (NO₂TRPP)Co, where TRPP is the dianion of a substituted tetraphenylporphyrin and R is a CH₃ or OCH₃ substituent on the four phenyl rings of the macrocycle. Two reductions and three oxidations are observed for each compound in CH₂Cl₂ containing 0.10 M tetra-n-butylammonium perchlorate. The first reduction of the compounds without a nitro substituent is metal-centered and leads to formation of a Co(i) porphyrin which then reacts with the CH₂Cl₂ solvent to generate a carbon σ-bonded Co^III-R complex. A further reduction then occurs at more negative potentials to generate an unstable Co(ii) σ-bonded compound. In contrast to these reactions, the first reduction of the nitro-substituted porphyrins is macrocycle-centered under the same solution conditions and gives a Co(ii) porphyrin π-anion radical product. This reversible electron transfer is then followed at more negative potentials by a second reversible one-electron addition to give a Co(ii) dianion. Three reversible one-electron oxidations are also seen for each compound. The first is metal-centered and the next two involve the conjugated π-system of the macrocycle. Each neutral Co(ii) porphyrin was also examined as to its catalytic activity for electroreduction of molecular oxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO_4. The β-pyrrole nitro-substituted derivatives were shown to be better catalysts than the non-nitro substituted compounds under the utilized experimental conditions.

Full text of DOI:10.1039/c4dt01072h

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Synthesis and electrochemistry of β-pyrrole
nitro-substituted cobalt(II) porphyrins. The effect
of the NO2 group on redox potentials, the
electron transfer mechanism and catalytic
reduction of molecular oxygen in acidic media
Cite this: Dalton Trans., 2014, 43,
10809
Bin Sun,a Zhongping Ou,*a Shuibo Yang,a Deying Meng,a Guifen Lu,a
Yuanyuan Fangb and Karl M. Kadish*b
Four cobalt(II) porphyrins, two of which contain a β-pyrrole nitro substituent, were synthesized and
characterized by electrochemistry and spectroelectrochemistry. The investigated compounds are rep-
resented as (TRPP)Co and (NO2TRPP)Co, where TRPP is the dianion of a substituted tetraphenylporphyrin
and R is a CH3 or OCH3 substituent on the four phenyl rings of the macrocycle. Two reductions and three
oxidations are observed for each compound in CH2Cl2 containing 0.10 M tetra-n-butylammonium per-
chlorate. The first reduction of the compounds without a nitro substituent is metal-centered and leads to
formation of a Co(I) porphyrin which then reacts with the CH2Cl2 solvent to generate a carbon σ-bonded
CoIII–R complex. A further reduction then occurs at more negative potentials to generate an unstable Co(II)
σ-bonded compound. In contrast to these reactions, the first reduction of the nitro-substituted por-
phyrins is macrocycle-centered under the same solution conditions and gives a Co(II) porphyrin π-anion
radical product. This reversible electron transfer is then followed at more negative potentials by a second
reversible one-electron addition to give a Co(II) dianion. Three reversible one-electron oxidations are also
seen for each compound. The first is metal-centered and the next two involve the conjugated π-system
of the macrocycle. Each neutral Co(II) porphyrin was also examined as to its catalytic activity for electro-
reduction of molecular oxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M
HClO4. The β-pyrrole nitro-substituted derivatives were shown to be better catalysts than the non-nitro
substituted compounds under the utilized experimental conditions.
Received 10th April 2014,
Accepted 1st May 2014
DOI: 10.1039/c4dt01072h
www.rsc.org/dalton
gated macrocycle will have a large effect on the half-wave
potentials for electroreduction. The reversible E1/2 values of
Introduction
Porphyrins and porphyrin analogues have attracted consider- metallated tetraphenylporphyrins, (TPP)M, have been shown
able attention for use in a broad range of applications1,2 to shift in a positive direction by 230–380 mV per nitro group
because of their versatile properties, derived in part from their in the case of β-pyrrole substituted nitro derivatives and by
stable π-ring system, in part from the nature of the substitu- 450–550 mV in the case of meso-substituted nitro derivatives of
ents in the meso and/or β-positions of the macrocycle and in octaethylporphyrins, (OEP)M.3 When a NO2 substituent is
part from the coordinating ability of the central metal ion located on the phenyl ring of a TPP type porphyrin,4,5 on the
which can exist in a variety of oxidation states.
quinoxaline group of a quinoxalino–porphyrin,6 or on the
Previous electrochemical characterization of porphyrins phenyl group of a triphenylcorrole,7–9 it can be reversibly
with redox inactive M(II) and M(III) central metal ions has reduced by one electron in non-aqueous media. However, a
shown that the presence of one or more electron-withdrawing direct reduction of this electron-withdrawing group does not
NO2 groups at the meso- or β-pyrrole positions of the conju- occur in non-aqueous media when this group is directly
attached at the meso or β-pyrrole positions of a porphyrin
macrocycle; the main effect of NO2 in this case is to positively
aSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,
shift E1/2 for the oxidation and reduction.
P. R. China. E-mail: zpou2003@yahoo.com; Fax: +86 511 88791800;
In a few cases, the incorporation of NO2 into the porphyrin
Tel: +86 511 88791800
bDepartment of Chemistry, University of Houston, Houston, Texas 77204-5003, USA.
E-mail: kkadish@uh.edu; Fax: +1 713 7432745; Tel: +1 713 7432740
macrocycle has been shown to change the site of electron
transfer from metal-centered to macrocycle-centered
a
a
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reduction, two examples being given for [(NO2)P]AuIIIPF6
10
Experimental
and [(NO2)TRPP]FeIIICl,11 where P and TRPP are the dianions
Materials and instrumentation
of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and a
substituted tetraphenylporphyrin, respectively, and R rep- Reagents and solvents (Sigma-Aldrich, Fluka or Sinopharm
resents one CH3 or two OCH3 groups on each phenyl ring of Chemical Reagent Co.) for synthesis and purification were of
the TRPP macrocycle. Finally, it has been well-documented in analytical grade and used as received. Dichloromethane
the literature that NO2 substitution at the β-pyrrole or meso- (CH2Cl2, 99.8%) was purchased from Sinopharm Chemical
position of a TPP or OEP type complex has a much smaller Reagent Co. and distilled before use. Tetra-n-butylammonium
effect on E1/2 for oxidation than on the half-wave potentials for perchlorate (TBAP) was purchased from Sigma Chemical or
reduction.4
Our own laboratories have recently demonstrated that the dried under vacuum at 40 °C for at least one week prior to use.
type of substituent on the meso-phenyl rings of a porphyrin or Thin-layer UV-visible spectroelectrochemical experiments
Fluka Chemika Co., recrystallized from ethyl alcohol, and
related corrole macrocycle can have a significant effect on the were performed with a home-built thin-layer cell which has a
compound’s catalytic properties for reduction of molecular light transparent platinum net working electrode. Potentials
oxygen in acidic media.7,12,13 However, to the best of our were applied and monitored with the Chi-730C Electrochemi-
knowledge, it is still not known how a single strong electron- cal Work Station. Time-resolved UV-visible spectra were
withdrawing NO2 substituent on the β-pyrrole position of a recorded with a Hewlett-Packard Model 8453 diode array
cobalt porphyrin will affect the electrocatalytic activity of these spectrophotometer. High purity N2 was used to deoxygenate the
types of compounds when adsorbed on an electrode surface. solution and kept over the solution during each electrochemi-
This is investigated in the present paper where we have syn- cal and spectroelectrochemical experiment. Infrared spectra
thesized and characterized two β-pyrrole NO2-substituted Co(II) were recorded with a Nicolet Nexus 470. MALDI-TOF mass
porphyrins and the related non-nitro substituted Co(II) spectra were recorded on a Bruker BIFLEX III with ultrahigh
porphyrins (see Chart 1). The electrochemical properties of resolution.
these compounds were examined in non-aqueous media and
All electrochemical measurements were carried out with a
the effect of the β-nitro group on the UV-visible spectra and Chi-730C Electrochemical Work Station. A three-electrode
reduction/oxidation properties is discussed with comparisons system was used and consisted of a glassy carbon working elec-
made with the non-nitro substituted porphyrins containing trode for regular cyclic voltammetry, an edge-plane pyrolytic
the same central metal ions. The catalytic activity of these por- graphite (EPPG) disk working electrode (Model MT134, Pine
phyrins towards the reduction of molecular oxygen in acidic Instrument Co.) for cyclic voltammetry and linear sweep volta-
media was also evaluated by utilizing cyclic voltammetry and mmetry at an RDE and a platinum net working electrode for
linear sweep voltammetry at a rotating disk electrode (RDE).
in situ spectroelectrochemistry. A platinum wire served as the
auxiliary electrode and a saturated calomel electrode (SCE)
served as the reference electrode, which was separated from
the bulk of the solution by means of a salt bridge. The poten-
tial of the SCE is 0.241 V vs. NHE.
A Pine Instrument MSR speed controller was used for the
RDE experiments. The catalysts were irreversibly adsorbed on
the electrode surface by means of a dip-coating procedure.14
The freshly polished electrode was dipped in a 1.0 × 10−3
M
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 porphyrin catalyst
adsorbed on the electrode surface.
Synthesis
Free-base porphyrins. (TRPP)H2 and (NO2TRPP)H2 were pre-
pared according to previously described procedures.11,15
Cobalt porphyrins. The corresponding free-base porphyrin
(∼30 mg) and 10 equivalents of cobalt acetate were dissolved
in N,N′-dimethylformamide. The solution was then stirred and
heated to reflux. After 1.5 h the solvents were removed and the
crude products were chromatographed using neutral alumina
(200–300 mesh) and CH2Cl2–hexane as the eluent. The red
fraction was collected and evaporated to dryness, yielding the
pure desired compounds, (TmPP)Co, (NO2TmPP)Co, (TdmPP)
Co and (NO2TdmPP)Co, where TmPP and TdmPP are dianions
Chart 1 Structures of investigated cobalt porphyrins.
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of a substituted tetraphenylporphyrin, m represents a CH3 CH3 to OCH3 has only a small effect on the spectra. In con-
group and dm represents two OCH3 substituents on each of trast, significant spectral differences are seen between the
the four phenyl rings of the macrocycle.
β-pyrrole nitro-substituted porphyrins 3 and 4 and their non-
(TmPP)Co 1. Yield 95%. UV-vis (CH2Cl2): λmax, nm (log ε) nitro substituted parent derivatives 1 and 2. The nitro-substi-
410 (5.51), 529 (4.48). IR (KBr): (cm−1) 1000 (νN–Co). MS tuted porphyrins 3 and 4 are characterized by a broad Soret
(MALDI-TOF): m/z 729.35, calcd for [M + 2H]+ 728.77.
band at 424–427 nm and a shoulder at 380–382 nm. There are
(TdmPP)Co 2. Yield 90%. UV-vis (CH2Cl2): λmax, nm (log ε) also two Q bands between 539 and 580 nm as shown in Fig. 1.
412 (5.48), 527 (4.23). IR (KBr): (cm−1) 1004 (νN–Co). MS The broadening of the bands and the 12–15 nm red-shift for
(MALDI-TOF): m/z 913.26, calcd for [M + 2H]+ 912.87.
compounds 3 and 4 as compared to 1 and 2 are as expected
(NO2TmPP)Co 3. Yield 86%. UV-vis (CH2Cl2): λmax, nm due to addition of the strong electron-withdrawing NO2 group
(log ε) 380 (4.43), 424 (4.91), 542 (3.89), 575 (3.79). IR (KBr): at the β-pyrrole position of the porphyrin macrocycle.
(cm−1) 1523, 1345 (νNO ), 1003 (νN–Co). MS (MALDI-TOF): m/z
2
774.03, calcd for [M + 2H]+ 773.77.
Electrochemistry
(NO2TdmPP)Co 4. Yield 86%. UV-vis (CH2Cl2): λmax, nm (log
The electrochemical properties of each complex were examined
ε) 382 (4.55), 427 (5.06), 539 (4.00), 580 (3.91). IR (KBr): (cm−1
)
in CH2Cl2 containing 0.10 M TBAP. Examples of cyclic
voltammograms for compounds 1 and 3 are illustrated in
Fig. 2 and a summary of the half-wave or peak potentials for
all four cobalt porphyrins is given in Table 1.
1520, 1351 (νNO ), 1008 (νN–Co). MS (MALDI-TOF): m/z 958.19,
2
calcd for [M + 2H]+ 957.87.
Three reversible one-electron oxidations are shown for each
porphyrin in CH2Cl2 containing 0.10 M TBAP. This redox be-
havior is similar to that for other cobalt tetraphenylporphyrins
containing electron-donating or electron-withdrawing substitu-
ents in the meso and/or β-pyrrole positions of the macrocycle
and is unique to CH2Cl2 where all three processes are
reversible.3
Results and discussion
Synthesis
Cobalt(II) porphyrins were synthesized by reaction of the
corresponding free-base porphyrin and cobalt acetate. The pro-
gress of the reaction was monitored by thin-layer chromato-
graphy (tlc). A high yield of the porphyrin was obtained
(86–95%) and the purity was confirmed by UV-visible
spectroscopy and mass spectrometry.
The first oxidation of compound 1 is located at E1/2 = 0.72 V
while the second and third oxidations are located at E1/2
=
0.96 V and 1.15 V, respectively. Compound 3 is harder to
oxidize than compound 1 and the E1/2 values are shifted to
0.80, 1.05 and 1.28 V. Thus the positive shift in half-wave
potentials due to the addition of one nitro group to the conju-
gated π-system of the macrocycle is 80 mV for the first oxi-
dation, 90 mV for the second and 130 mV for the third. This is
consistent with the first oxidation being a metal-centered
process and leading to a Co(III) porphyrin and the last two pro-
cesses being porphyrin ring-centered to give a Co(III) π-cation
radical and dication, respectively.3 The same relative magni-
tude of the shift in E1/2 is seen when comparing E1/2 values for
compounds 2 and 4 and the same sites of electron transfer are
UV-visible spectra
UV-visible spectra of the cobalt porphyrins were measured in
CH2Cl2 and are illustrated in Fig. 1. As shown in this figure,
the non-nitro substituted porphyrins, (TmPP)Co
(TdmPP)Co 2, exhibit a sharp Soret band at 410–412 nm and a
weak Q band at 527–529 nm. Almost identical UV-visible
spectra are seen for 1 and 2, indicating that the change of sub-
stituents on the four meso-phenyl rings of the macrocycle from
1 and
Fig. 1 UV-visible spectra of (a) the non-nitro substituted porphyrins 1,
2 and (b) the β-pyrrole NO2-substituted cobalt porphyrins 3 and 4 in
CH2Cl2.
Fig. 2 Cyclic voltammograms of (a) (TmPP)Co 1 and (b) (NO2TmPP)Co
3 in CH2Cl2 containing 0.1 M TBAP. Scan rate = 0.10 V s−1
.
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Table 1 Half-wave potentials (V vs. SCE) of cobalt porphyrins in CH2Cl2, 0.10 M TBAP
Compound
Ring-ox
CoIII/CoII
CoII/CoI
CoIII–R reda
Ring-red
(TmPP)Co 1
(TdmPP)Co 2
(NO2TmPP)Co 3
(NO2TdmPP)Co 4
1.15
1.16
1.28
1.33
0.96
0.96
1.05
1.11
0.72
0.74
0.80
0.83
−0.90b
−0.92b
−1.44b
−1.42b
−0.66
−0.64
−1.32
−1.30
a Reduction of the product, (TRPP)CoIII(CH2Cl), generated from the reaction between the cobalt(I) porphyrin and CH2Cl2. b The peak potential for
irreversible reduction at a scan rate of 0.10 V s−1
.
proposed to occur upon oxidation as described above. This UV-visible spectroelectrochemistry
was confirmed by in situ thin-layer UV-visible spectroelectro-
In order to elucidate the site of electron transfer, the reduced
chemical measurements described in the following pages.
Although two reductions are seen for the nitro and non-
nitro substituted porphyrins, the reversibility and sites of elec-
tron transfer differ between the two types of porphyrins. As
shown in Fig. 2, both reductions of (TmPP)Co 1 are irreversible
and located at Epc = −0.90 and −1.44 V in CH2Cl2 containing
0.10 M TBAP, while both reductions of (NO2TmPP)Co 3 are
reversible and located at E1/2 = −0.66 and −1.32 V.
and oxidized forms of compounds 1–4 were monitored by
thin-layer UV-visible spectroelectrochemistry. Examples of the
spectral changes which occurred during controlled potential
reduction are shown in Fig. 3 while the changes during oxi-
dation are shown in Fig. 4.
As shown in Fig. 3a, the intensity of the Soret and Q bands
of compound 1 decreases only slightly upon the first one-elec-
tron reduction at −1.00 V in CH2Cl2 containing 0.1 M TBAP.
The spectral changes are irreversible and the initial spectrum
could not be generated when the potential was set back to
0.0 V for re-oxidation. This result is consistent with the first
reduction occurring at the metal center to give a Co(I) por-
phyrin which then reacts with CH2Cl2 to generate a σ-bonded
porphyrin, (TmPP)CoIII(CH2Cl). Thus, the initial spectrum
before reduction is that of a Co(II) porphyrin and that after
reduction is that of a Co(III) porphyrin. In contrast to com-
pound 1, the spectral changes during reduction of compound
3 are more significant (Fig. 3b). The Soret band initially at
424 nm decreases in intensity as the reduction proceeds, con-
sistent with formation of a Co(II) porphyrin π-anion radical.
Further reduction at −1.60 V leads to an additional loss of the
Soret band intensity as the Co(II) dianion is generated in
solution.
It is well-documented in the literature that Co(II) porphyrins
can undergo either a metal-centered reduction to give a Co(I)
porphyrin or reduction at the conjugated π-system to give a
Co(II) porphyrin π-anion radical, with the exact site depending
upon the structure of the compound and solution conditions.3
Most electrogenerated Co(I) porphyrins will react with CH2Cl2,
which lead to an irreversible Co(II)/Co(I) reduction in this
solvent.3 This is the case for (TmPP)CoII 1 and (TdmPP)CoII
2
which are assigned to metal-centered processes, leading to for-
mation of a transient Co(I) product after the first one-electron
reduction in CH2Cl2. This low valent complex is highly reactive
and reacts with CH2Cl2 to generate a σ-bonded Co(III) product,
(TmPP)CoIII(CH2Cl), as earlier described in the literature.16,17
The in situ generated σ-bonded porphyrin product is then
further reduced at −1.44 V by one electron. Again, this is
similar to what has been reported for other chemically and
electrochemically generated σ-bonded (TPP)CoIII(R) complexes,
where R = CH3, C2H5, C3H7 or CH2Cl.16,17
The fact that a chemical reaction with CH2Cl2 does not
occur for the nitro-substituted porphyrin 3 is consistent with
the fact that the two reversible reductions both involve the con-
jugated porphyrin π-ring system and lead to generation of a
Co(II) π-anion radical rather than a Co(I) porphyrin in the first
step. Macrocycle ring-centered reductions of Co(II) porphyrins
are not common but have previously been reported for a cobalt
tetraphenylporphyrin with four CN groups at the β-pyrrole
positions of the macrocycle.18
The easier reduction of 3 as compared to compound 1 is
due in part to the substituent effect of the added nitro group
and in part to the different sites of electron transfer. The
absolute difference in peak potential between the first
reduction of 1 and 3 is about 200 mV and a similar separation
in peak potential is seen between the first reduction of com-
pounds 2 and 4. These values are in agreement with what has
been reported for other nitro substituted porphyrins.3
Fig. 3 Thin-layer UV-visible spectral changes of (a) (TmPP)CoII 1 and (b)
(NO2TmPP)CoII 3 during controlled potential reductions in CH2Cl2 con-
taining 0.10 M TBAP.
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Fig. 5 Cyclic voltammograms of (a) unsubstituted compound 2 and (b)
β-pyrrole NO2-substituted compound 4 absorbed on an EPPG electrode
in 1.0 M HClO4 under N2 (---) and air (—). The scan rate is 50 mV s−1
.
nation with the Co(III) central metal ion and the irreversibility
of the process is due to coupled chemical reactions after elec-
tron transfer. The chemical reactions involve changes in
coordination which has been demonstrated for many cobalt
porphyrins in many different solvents.3
The magnitudes of the reduction peak currents under O2
are much larger in the acidic solutions which are saturated
with air. The significant increase in the cathodic peak current
in the presence of oxygen and the lack of a return peak on the
reverse sweep of the cyclic voltammogram in air are character-
istic of a catalytic reaction, which, in this case, is the catalytic
reduction of O2, either via a two-electron transfer process to
give H2O2 or via a four-electron transfer process to give H2O.
As shown in Fig. 5, both the unsubstituted compound 2
and the β-NO2 substituted compound 4 act as catalysts for the
reduction of O2. The Co(III)/Co(II) reduction peak potential of
the β-pyrrole NO2 substituted compound 4 is located at +0.18 V
under N2 and is positively shifted by 80 mV as compared to
the peak potential of +0.10 V for compound 2 (see dashed
lines in Fig. 5). The peak potential of 4 (+0.14 V) in air satu-
rated solution is also positively shifted by 60 mV as compared
to the Ep (+0.08 V) for the unsubstituted compound 2.
The product formed in the catalytic reduction of O2 (H2O2
or H2O) and the number of electrons transferred in the
reduction of dioxygen were calculated from the magnitude of
the steady-state limiting currents that were taken at a fixed
potential (−0.10 V) on the plateau of the catalytic wave of the
current–voltage curve. Examples of the current–voltage curves
are shown in Fig. 6 along with the related Koutecky–Levich
plots. The slope of the plot obtained by linear regression was
then used to estimate the average number of electrons (n)
involved in the catalytic reduction of O2.22 The Koutecky–
Levich plot is interpreted on the basis of eqn (1),23 where j is
the measured limiting current density (mA cm−2), jlev is the
Levich current, and jk is the kinetic current which are defined
by eqn (2) and (3).24
Fig. 4 Thin-layer UV-visible spectral changes of (NO2TdmPP)CoII
during controlled potential oxidations in CH2Cl2 containing 0.10 M
TBAP.
4
The first one-electron oxidation of (NO2TdmPP)CoII 4 at
0.95 V results in a decreased intensity Soret band at 427 nm as
a new Soret band grows at 445 nm. These spectral changes are
shown in Fig. 4a and are consistent with a metal-centered elec-
tron transfer to give a Co(III) porphyrin during the first electron
abstraction. In the second one-electron oxidation process, a
new broad Q band appears at 675 nm while the Soret band at
445 nm decreases in intensity and is shifted to 438 nm, indi-
cating generation of a Co(III) porphyrin π-cation radical in
CH2Cl2. Similar spectral changes occur for compounds 1–3.
Similar spectral changes have also been reported for related
porphyrins in non-aqueous media.19–21
Electrocatalytic reduction of oxygen
Each compound was examined as a catalyst for the electrore-
duction of oxygen when adsorbed on an EPPG disk in 1.0 M
HClO4. Examples of cyclic voltammograms showing the Co(III)/
Co(II) reduction process in 1.0 M HClO4 saturated with N2 (-----)
and saturated with air (—) are illustrated in Fig. 5.
The CoII/CoIII (and CoIII/CoII) processes of compounds 1–4
are reversible in CH2Cl2 and located at E1/2 = 0.72–0.83 V (see
Fig. 2). In contrast, the Co(III)/Co(II) reduction peaks of 1–4 are
irreversible and located at similar potentials of +0.05 to +0.18
V in 1.0 M HClO4 under N2 or under O2 (air). The negative
shift in the Co(III) reduction potential is due to strong coordi-
1=j ¼ 1=jlev þ 1=jk
jlev ¼ 0:62nFD2=3c ꢀ vꢁ1=6ω1=2
jk ¼ 103knFΓcꢀ
ð1Þ
ð2Þ
ð3Þ
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range from 2.6 to 3.1 which correspond to 70–45% H2O2 (and
30–55% H2O) produced. The values of n for compounds 3 and
4 (n = 3.1) are larger than that for compounds 1 and 2 (n =
2.6–2.8), indicating that the β-pyrrole nitro-substituted com-
pounds are better catalysts than the non-nitro substituted por-
phyrins for the electroreduction of oxygen in acidic media.
Conclusions
In summary, four substituted cobalt(II) porphyrins were syn-
thesized and examined as to their electrochemistry and cata-
lytic properties for the reduction of molecular oxygen. The
strong electron-withdrawing nitro-substituent in a β-pyrrole
position of the porphyrin macrocycle is shown to have a sig-
nificant effect on the UV-visible spectra, redox potentials and
reduction mechanism in non-aqueous media. As shown in this
study, the nitro group also affects the catalytic efficiency of the
compounds. The β-pyrrole NO2-substituted cobalt porphyrins
are better electrocatalysts than the non-nitro substituted
derivatives for the reduction of molecular oxygen in acidic
media.
Fig. 6 Current–voltage curves and Koutecky–Levich plots for catalytic
reduction of O2 in 1.0 M HClO4 saturated with air at a rotating EPPG disk
electrode coated with (a) unsubstituted compound 2 and (b) β-pyrrole
NO2-substituted compound 4. Electrode rotating rates (ω) are indicated
on each curve. Potential scan rate = 50 mV s−1
.
Acknowledgements
The n in eqn (3) represents the number of electrons trans-
ferred in the overall electrode reaction, F is the Faraday con-
stant (96 485 C mol−1), D and c* are the diffusion coefficient
(cm2 s−1) and the bulk concentration of O2, respectively, in
1.0 M HClO4, v is the kinematic viscosity of water, ω is the
angular rotation speed (rad s−1) of the electrode, k is the
second order rate constant of the reaction (mol−1 L s−1) and
Γ is the surface concentration of porphyrins (mol cm−2).
The catalytic reduction of oxygen by metalloporphyrins or
metallocorroles adsorbed on an electrode surface can proceed
via a two-electron transfer pathway to generate hydrogen per-
oxide, or via a direct four-electron pathway to give H2O. Any
H2O2 produced can be further reduced to H2O and, when this
occurs, the number of electrons transferred (n) during the
reduction of oxygen could be between 2 and 4.12,13,25,26
We gratefully acknowledge support from the Natural Science
Foundation of China (no. 21071067) and the Robert A. Welch
Foundation (K.M.K., grant E-680).
Notes and references
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6 K. M. Kadish, E. Wenbo, P. J. Sintic, Z. P. Ou, J. G. Shao,
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8 S. Nardis, M. Stefanelli, P. Mohite, G. Pomarico, L. Tortora,
M. Manowong, P. Chen, K. M. Kadish, F. R. Fronczek,
Compound
Epc without O2
Epc with O2
n
H2O2%
(TmPP)Co 1
(TdmPP)Co 2
(NO2TmPP)Co 3
(NO2TdmPP)Co 4
0.07
0.10
0.16
0.18
0.05
0.08
0.14
0.14
2.8
2.6
3.1
3.1
60
70
45
45
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