Electrochemistry and catalytic properties for dioxygen reduction using ferrocene-substituted cobalt porphyrins

Article abstract of DOI:10.1021/ic501210t

Cobalt porphyrins having 0-4 meso-substituted ferrocenyl groups were synthesized and examined as to their electrochemical properties in N,N'-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte. The examined compounds are represented as (Fc) _n(CH₃Ph)_4-nPorCo, where Por is a dianion of the substituted porphyrin, Fc and CH₃Ph represent ferrocenyl and/or p-CH₃C₆H₄ groups linked at the four meso-positions of the macrocycle, and n varies from 0 to 4. Each porphyrin undergoes two reversible one-electron reductions and two to six one-electron oxidations in DMF, with the exact number depending upon the number of Fc groups on the compound. The first electron addition is metal-centered to generate a Co(I) porphyrin. The second is porphyrin ring-centered and leads to formation of a Co(I) π-anion radical. The first oxidation of each Co(II) porphyrin is metal-centered to generate a Co(III) derivative under the given solution conditions. Each ferrocenyl substituent can also be oxidized by one electron, and this occurs at more positive potentials. Each compound was investigated as a catalyst for the electoreduction of dioxygen when adsorbed on a graphite electrode in 1.0 M HClO₄. The number of electrons transferred (n) during the catalytic reduction was 2.0 for the three ferrocenyl substituted compounds, consistent with only H₂O₂ being produced as a product of the reaction. Most monomeric cobalt porphyrins exhibit n values between 2.6 and 3.1 under the same solution conditions, giving a mixture of H₂O and H₂O₂ as a reduction product, although some monomeric porphyrins can give an n value of 4.0. Our results in the current study indicate that appending ferrocene groups directly to the meso positions of a porphyrin macrocycle will increase the selectivity of the oxygen reduction, resulting in formation of only H₂O₂ as a reaction product. This selectivity of the electrocatalytic oxygen reduction reaction is explained on the basis of steric hindrance by the ferrocene substituents which prevent dimerization.

Full text of DOI:10.1021/ic501210t

Article
pubs.acs.org/IC
Electrochemistry and Catalytic Properties for Dioxygen Reduction
Using Ferrocene-Substituted Cobalt Porphyrins
†
,†
†
‡
‡
†
Bin Sun, Zhongping Ou,* Deying Meng, Yuanyuan Fang, Yang Song, Weihua Zhu,
§
,§
,‡
Pavlo V. Solntsev, Victor N. Nemykin,* and Karl M. Kadish*
†
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
‡
§
Department of Chemistry & Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812-2496, United States
S Supporting Information
*
ABSTRACT: Cobalt porphyrins having 0−4 meso-substituted ferrocenyl
groups were synthesized and examined as to their electrochemical properties
in N,N′-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium
perchlorate as a supporting electrolyte. The examined compounds are
represented as (Fc) (CH Ph)4−nPorCo, where Por is a dianion of the
n
3
substituted porphyrin, Fc and CH Ph represent ferrocenyl and/or p-CH C H
3
3
6
4
groups linked at the four meso-positions of the macrocycle, and n varies from 0
to 4. Each porphyrin undergoes two reversible one-electron reductions and
two to six one-electron oxidations in DMF, with the exact number depending
upon the number of Fc groups on the compound. The first electron addition is
metal-centered to generate a Co(I) porphyrin. The second is porphyrin ring-
centered and leads to formation of a Co(I) π-anion radical. The first oxidation
of each Co(II) porphyrin is metal-centered to generate a Co(III) derivative
under the given solution conditions. Each ferrocenyl substituent can also be
oxidized by one electron, and this occurs at more positive potentials. Each compound was investigated as a catalyst for the
electoreduction of dioxygen when adsorbed on a graphite electrode in 1.0 M HClO . The number of electrons transferred (n)
4
during the catalytic reduction was 2.0 for the three ferrocenyl substituted compounds, consistent with only H O being produced
2
2
as a product of the reaction. Most monomeric cobalt porphyrins exhibit n values between 2.6 and 3.1 under the same solution
conditions, giving a mixture of H O and H O as a reduction product, although some monomeric porphyrins can give an n value
2
2
2
of 4.0. Our results in the current study indicate that appending ferrocene groups directly to the meso positions of a porphyrin
macrocycle will increase the selectivity of the oxygen reduction, resulting in formation of only H O as a reaction product. This
2
2
selectivity of the electrocatalytic oxygen reduction reaction is explained on the basis of steric hindrance by the ferrocene
substituents which prevent dimerization.
INTRODUCTION
The well-known electron-donor properties of ferrocene have
■
1
,2
been effectively utilized to enhance the catalytic activity of
Ferrocenyl-containing compounds, such as phthalocyanines,
5
3,54
55−57
3
−6
7−9
cobalt
and copper
complexes in the electrocatalytic
tetraazaporphyrins,
corroles,
have received considerable attention, in part because
of their rich redox activity which is of fundamental importance
and especially porphyr-
1
0−41
four-electron reduction of oxygen to water. It has been reported
that an iron porphyrin linked to four ferrocene groups with
ins
4
2−45
short conjugated spacers in an α -FeFc4 arrangement can also
4
for the development of molecular-based electronic devices
27
46
catalyze the four-electron reduction of O to H O. However,
2 2
or molecular electrogenic sensors. The ability of these types
of compounds to reversibly accept and/or release multiple
electrons at distinct potentials is particularly important in the
it was not known if a similar enhancement of the catalyst might
occur when Fc groups are directly connected at the meso
position of a porphyrin. This is investigated in the present
paper where we have characterized four Co(II) porphyrins
having zero, one, three, and four ferrocenyl groups directly
connected on the meso positions of the macrocycle. The
structures of the examined cobalt compounds are shown in
Chart 1.
47
area of multielectron redox catalysis and information storage
4
8,49
at the molecular level.
The addition of one or more electron-donating or electron-
withdrawing substituents onto the meso- or β-pyrrole positions
of a porphyrin, corrole, or related macrocycle will have a
significant effect on the compound’s UV−visible spectra and
5
0
redox potentials. This is also the case when ferrocene groups
are connected to the meso positions of a porphyrin macro-
cycle.
Received: May 23, 2014
Published: July 28, 2014
1
3−19,28,51,52
©
2014 American Chemical Society
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Article
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 cm ). A Pine Instrument MSR speed
Chart 1. Structures of Examined meso-Substituted Cobalt(II)
Porphyrins
2
controller was used for the RDE and RRDE experiments. The Pt ring
was first polished with 0.05 μm α-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 HClO until
4
6
0,61
reproducible voltammograms were obtained.
The catalysts were irreversibly adsorbed on the electrode surface by
6
2,63
a dip-coating procedure described in the literature.
The freshly
polished electrode was dipped in a 1.0 mM catalyst solution of CH Cl
2
2
for 5 s, transferred rapidly to pure CH Cl for 1−2 s, and then exposed
2
2
to air where the adhering solvent rapidly evaporated, leaving the
porphyrin catalyst adsorbed on the electrode surface. All experiments
were carried out under room temperature. The average number of
electrons transferred (n) and the amount of H O formed in the
2
2
catalytic reduction of O were determined by the Koutecky−Levich
2
6
4
plots and calculated with eqs 1 and 2.
n = 4I /(I + I /N)
(1)
(2)
D
D
R
%
H O = 100(2I /N)/(I + I /N)
2
2
R
D
R
where ID and IR are the Faradaic currents at the disk and ring
electrodes, respectively. The intrinsic value of the collection efficiency
3
−
4−
(
N) was determined to be 0.24 using the [Fe(CN) ] /[Fe(CN) ]
6 6
redox couple in 1.0 M KCl.
Chemicals. Reagents and solvents (Sigma-Aldrich, Fluka or
Sinopharm Chemical Reagent Co.) for synthesis and purification
were of analytical grade and used as received. Dichloromethane
It has been demonstrated that monomeric cobalt porphyrins,
when adsorbed on a carbon electrode, can catalyze two- or
(
CH Cl 99.8%) was purchased from EMD Chemicals Inc. and used
2 2,
four-electron reduction of O in an acid media with the
2
as received. Tetra-n-butylammonium perchlorate (TBAP) was
purchased from Sigma Chemical or Fluka Chemika Co., recrystallized
from ethyl alcohol, and dried under a vacuum at 40 °C for at least 1
week prior to use.
formation of a H O and/or H O product. The catalytic activity
2
2
2
and selectivity of the cobalt porphyrins (i.e., two- versus four-
electron pathways) should be dependent on the type and
location of the macrocycle substituents as well as on the
T h e e x a m i n e d c o m p o u n d s a r e r e p r e s e n t e d a s
5
8,59
(
Fc) (CH Ph)4−nPorCo, where Por is a dianion of the substituted
planarity of the macrocycle.
In the present study, the
n 3
porphyrin, Fc and CH Ph represent ferrocenyl and/or p-CH C H
3
3
6
4
catalytic activity of the compounds in Chart 1 toward the
reduction of molecular oxygen was evaluated in acid media by
utilizing cyclic voltammetry and linear sweep voltammetry with
a rotating disk electrode (RDE) and a rotating ring-disk
electrode (RRDE). The effect of the ferrocenyl groups on the
UV−visible spectra, reduction/oxidation potentials, and the
catalytic activity of the currently examined compounds are
reported with comparisons made to nonferrocenyl substituted
cobalt porphyrins previously examined under similar exper-
imental conditions.
groups linked at the four meso positions of the macrocycle, and n varies
from 0 to 4. Details on the synthesis and properties of the investigated
porphyrins are given below.
(
CH Ph) PorCo, 1. This compound was synthesized according to a
3 4
65
procedure described in the literaure.
Fc(CH Ph) PorCo, 2. A mixture of the free-base porphyrin,
3
3
5
1
Fc(CH
Ph)
PorH
(30 mg, 0.036 mmol), and cobalt acetate
3
3
2
tetrahydrate (30 mg, 0.12 mmol) in 20 mL of DMF was heated for
h at 140 °C. The solvent was removed when the reaction was
complete, and the residue was then dissolved in CH Cl and subjected
4
2
2
to chromatography on silica-gel using a mixed solvent of CH Cl /n-
2
2
hexcene = 1:1 as an eluent. The brown-red solution was collected and
the solvent evaporated to dryness at 40−50 °C for 12 h. Yield: 60%.
UV−vis (λ /nm, CH Cl ): 412, 532. Elem. Anal. Calcd for
EXPERIMENTAL SECTION
■
Instrumentation. Thin-layer UV−visible spectroelectrochemical
experiments were performed with a home-built thin-layer cell which
has a light transparent platinum net working electrode. Potentials were
applied and monitored with an EG&G PAR Model 173 potentiostat.
Time-resolved UV−visible spectra were recorded with a Hewlett-
max
2
2
C H N FeCo: C, 74.55; H, 4.66; N, 6.82%. Found: C, 74.36; H,
5
1
38
4
+
4
.52; N, 6.79%. MS: m/z 821.18. Calcd. [M − H] : 821.17.
Fc (CH Ph)PorCo, 3. A mixture of the free-base porphyrin,
3
3
5
1
Fc (CH Ph)PorH
3
(30 mg, 0.04 mmol), and cobalt acetate
3
2
Packard Model 8453 diode array spectrophotometer. High purity N2
was used to deoxygenate the solution and kept over the solution
tetrahydrate (25 mg, 0.10 mmol) in 20 mL of DMF was heated 6 h
at 140 °C. The solvent was removed when the reaction was complete,
during each electrochemical and spectroelectrochemical experiment.
Cyclic voltammetry was carried out at 298 K 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 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 a salt bridge of low porosity which
contained the solvent/supporting electrolyte mixture. The rotating
and the residue was then dissolved in CH Cl2 and subjected to
2
chromatography on silica-gel using a mixed solvent of CH Cl /n-
2
2
hexcene = 1:2 as an eluent. The brown-red solution was collected and
the solvent evaporated to dryness at 40−50 °C for 12 h. Yield: 50%.
UV−vis (λ /nm, CH Cl ): 414, 538. MS: m/z 1009.68. Calcd. for
max
2
2
+
C H N Fe Co [M − 2H] : 1010.08.
57
45
4
3
Fc PorCo, 4. This porphyrin was synthesized according to a
4
1
5,18
procedure described in the literaure.
RESULTS AND DISCUSSION
Electrochemistry and Spectroelectrochemistry. Each
cobalt porphyrin undergoes two reversible one-electron
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reductions as seen in Figure 1. The first is located at E1/2 values
between −0.81 and −0.92 V and the second at E1/2 between
reduction of 1 and 3 are shown in Figure 2. As the first one-
electron reduction proceeds at a controlled potential of −1.00
V, the intensity of the Soret band at 416 nm (cpd 1) or 420 nm
(
cpd 3) decreases in intensity, and a new split Soret band
appears at 365 and 426 nm for 1 and 376 and 450 nm for 3.
Both sets of spectral changes are consistent with an initial
reduction at the central metal ion to give a Co(I) porphyrin
66
product rather than a Co(II) π-anion radical. The second
controlled potential reduction at −2.20 V gives a product which
is then assigned to a Co(I) porphyrin π-anion radical. Similar
spectral changes are observed for compounds 2 and 4, and
these are also consistent with the stepwise generation of a
Co(I) porphyrin and a Co(I) porphyrin π-anion radical.
Each linked Fc group on 2, 3, and 4 can be reversibly
1
5,18
oxidized by one electron,
and it was necessary to determine
which redox process comes first upon scanning the potential in
a positive directionoxidation of the porphyrin macrocycle,
oxidation of the central metal ion, or oxidation of the linked
ferrocene group(s). The site of electron transfer for these
oxidations was determined both by comparison with literature
50
data for the oxidation of similar cobalt porphyrins and
confirmed as well as by the spectroelectrochemistry described
as follows in this article.
The first oxidation of each cobalt(II) porphyrin involves an
II
III
irreversible Co /Co process at a scan rate of 0.1 V/s. These
processes are located at E = 0.45 V for 1, 0.30 V for 2, 0.25 V
pa
for 3, and 0.15 V for 4 in DMF containing 0.10 M TBAP (see
III
II
Figure 3 and Table 1). The reverse Co /Co rereduction
process is also irreversible at a scan rate of 0.1 V/s and occurs at
more negative potentials of −0.08, −0.22, +0.02, and −0.37 V
for compounds 1−4.
Figure 1. Cyclic voltammograms showing reductions of (a)
II
II
(
CH Ph) PorCo , 1; (b) Fc(CH Ph) PorCo , 2; (c) Fc (CH Ph)-
3 4 3 3 3 3
II
II
PorCo , 3; and (d) Fc PorCo , 4, in DMF containing 0.1 M TBAP.
4
Scan rate = 0.10 V/s.
It is known that the neutral Co(II) porphyrins can axially
bind one DMF solvent molecule to form five-coordinate
complexes while Co(III) porphyrins will strongly bind two
−
1.95 and −1.98 V vs SCE. Similar redox behavior has been
50
reported for other structurally related cobalt(II) porphyrins in
DMF molecules and are six coordinate. The irreversibility of
50
II
III
nonaqueous media.
the Co /Co process is known to occur in all coordinating
solvents (such as DMF), and this is due to a change of
coordination number from 5 to 6 upon electron transfer. The
combined oxidation and rereduction mechanism for this
Thin-layer UV−visible spectroelectrochemistry was utilized
to elucidate the site of electron transfer, and examples of the
spectral changes which occurred during controlled potential
II
II
Figure 2. Thin-layer UV−visible spectral changes of (a) (CH Ph) PorCo , 1, and (b) Fc (CH Ph)PorCo , 3, during the controlled potential
3
3
3
3
reductions in DMF containing 0.1 M TBAP.
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Scheme 1. Proposed Mechanism for the First Oxidation of
Compounds 1−4 in DMF
It should be noted that the potentials for oxidation of the
ferrocenyl group on the porphyrins 2−4 are shifted positively
by 60−230 mV as compared to the potential for oxidation of
II
free ferrocene (E = 0.50 V) while at the same time the Co /
1/2
III
Co process of 2−4 is negatively shifted by 150−300 mV
under the same solution conditions. This indicates that an
interaction occurs between the electroactive ferrocenyl
substituent(s) which become(s) harder to oxidize and the
electroactive porphyrin macrocycle which becomes easier to
oxidize as compared to compound 1 which lacks a Fc group. An
interaction also occurs between the multiple ferrocenyl groups
on compounds 3 and 4 which are nonequivalent as
demonstrated by the different redox potentials for the Fc
groups in the two porphyrins (see Figure 3 and Table 1). It
should be noted that the degree of metal−metal coupling in
compounds 3 and 4 is smaller than that for the bis(ferrocenyl)-
Figure 3. Cyclic voltammograms showing oxidations of (a)
II
II
(
CH Ph) PorCo , 1; (b) Fc(CH Ph) PorCo , 2; (c) Fc (CH Ph)-
3 4 3 3 3 3
II
II
PorCo , 3; and (d) Fc PorCo , 4, in DMF containing 0.1 M TBAP.
4
Scan rate = 0.10 V/s.
50
process has previously been elucidated and is shown by the
“square mechanism” in Scheme 1.
The site of electron transfers in the first oxidation of
compounds 1−4 was confirmed by in situ thin-layer UV−visible
spectroelectrochemical measurements. As seen in Figure 4a, the
original Soret band of the neutral Co(II) porphyrin at 416 nm
decreases in intensity, and a new sharp Soret band for Co(III)
grows at 432 nm during the first one-electron oxidation of
compound 1 at an applied potential of 0.60 V. Similar Co(III)
spectra are observed after the first oxidation of the ferrocenyl
substituted compounds 2, 3 (Figure 4b,c), and 4 (Figure S1),
although the direction of λmax shifts from 432 nm of 1 to 448
nm of 4 due to the effect of the electron-donating ferrocene
group (see Table 2).
In the case of compound 1, which lacks an Fc group, only
metal and porphyrin ring-centered oxidations are possible, and
the macrocycle-centered process occurs at 1.08 V in DMF (see
Figure 3a). This is consistent with the UV−visible data in
Figure 4a, which show a loss of the Soret band intensity as a
Co(III) π-cation radical is generated upon abstraction of a
second electron from compound 1.
67
containing porphyrins reported by Officer and co-workers but
larger than that of the ferrocenyl-containing iron porphyrins
examined by Dey and co-workers, who did not observe any
68
metal−metal coupling.
Consistent with the different sites of electron transfer,
significant differences are seen in the spectral changes which
occur during the second oxidation of the nonferrocenyl
substituted compound 1 and the ferrocenyl substituted
compounds 2−4. As shown in Figure 4a, the Soret band at
432 nm of the Co(III) porphyrin for 1 decreases significantly in
intensity during controlled potential oxidation at 1.30 V, while
only minimal spectral changes are seen for the second oxidation
of the ferrocenyl substituted compounds 2−4 where the
conjugated π-system of the porphyrin macrocyle remains
unchanged (see Figure 4b and c for spectral changes of
compounds 2 and 3).
The oxidation potential of the parent ferrocene in DMF
containing the 0.1 M TBAP system is 0.50 V, and similar
oxidation potentials are observed at 0.56−0.59 V for
compounds 2, 3, and 4 (Figure 3b−d). Additional oxidations
are observed at 0.72 V for 3 and 0.73 V for 4 (Figure 3c,d).
These later processes at more positive potentials are all
proposed to occur on the meso-linked Fc groups. Thus, two
different oxidation routes can be proposed for the second
oxidation of 1−4 as shown in Scheme 2.
Electrocatalytic Reduction of O . Electroreduction of
each cobalt porphyrin was carried out in 1.0 M HClO4
2
saturated with N or air using an EPPG disk where the
2
porphyrin catalyst was adsorbed on the surface of the electrode.
Examples of the cyclic voltammograms obtained for com-
pounds 1, 2, and 3 under N or air are illustrated in Figure 5.
2
The cyclic voltammograms under N are characterized by an
2
Co(III)/Co(II) reduction peak at E = 0.07 V (1), 0.20 V (2),
pc
and 0.14 V (3) at a scan rate of 50 mV/s, but when the solution
Table 1. Half-Wave Potentials (V vs SCE) of the Cobalt Porphyrins 1−4 in DMF, 0.1 M TBAP
oxidation
reduction
ring-centered
II
III
a
II
I
compound
ring-centered
1.08
Fc-centered
Co /Co
Co /Co
(
CH Ph) PorCo, 1
0.45
0.30
0.25
0.15
−0.81
−0.83
−0.88
−0.92
−1.97
−1.95
−1.98
−1.97
3
4
a
Fc(CH Ph) PorCo, 2
1.13
0.57
0.59
0.56
3
3
b
c
Fc (CH Ph)PorCo, 3
1.25
0.72
0.73
3
3
Fc PorCo, 4
4
a
b
c
Irreversible peak potential at a scan rate of 0.10 V/s. Two overlapping one-electron oxidation processes. Three overlapping one-electron
oxidation processes.
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II
II
II
Figure 4. UV−visible spectral changes of (a) (CH Ph) PorCo , 1; (b) Fc(CH Ph) PorCo , 2; and (c) Fc (CH Ph)PorCo , 3, during the controlled
3
3
3
3
3
3
potential oxidations in DMF containing 0.1 M TBAP.
Table 2. UV−Visible Spectral Data (λmax, nm) and the
Corresponding Energy (eV) of the Bands for Compounds
1−4 in DMF
Soret band (nm)
E (eV)
Co(III)
compound
Co(II)
Co(III)
Co(II)
(
CH Ph) PorCo, 1
416
418
420
421
432
439
445
448
2.9827
2.9685
2.9543
2.9473
2.8720
2.8262
2.7881
2.7694
3
4
Fc(CH Ph) PorCo, 2
3
3
Fc (CH Ph)PorCo, 3
3
3
Fc PorCo, 4
4
Scheme 2. Proposed Mechanism of the Second Oxidation of
Compounds 1−4
is saturated with air, the peak potential shifts to Epc = 0.05 V
(
1), 0.19 V (2), and 0.13 V (3) and the cathodic peak current
Figure 5. Cyclic voltammorgams of cobalt porphyrins 1, 2, and 3
becomes larger (see Figure 5). The large increase in peak
current under air combined with the lack of a reverse anodic
peak on the cyclic voltammograms is characteristic of a catalytic
process, which in this case would be the catalytic reduction of
O to generate H O and/or H O at the surface of the EPPG
adsorbed on an EPPG electrode in 1.0 M HClO saturated with N
4
2
(-----) or with air (). Scan rate = 50 mV/s.
2
2
2
2
disk electrode.
in HClO (Figure 6c). No increase in the currents for the
4
+
The cyclic voltammogram obtained at a bare EPPG electrode
FcH /FcH process are seen upon going from N to air as the
2
under N or air exhibits a reversible surface peak at ∼0.40 V
gas above the solution, and this indicates that ferrocene is not
involved in the catalytic reduction of molecular oxygen. This
result is also consistent with what has been reported for other
ferrocenyl substituted metalloporphyrins.
Linear sweep voltammetry of each cobalt porphyrin was
2
(
Figure 6a). This surface process is labled by an asterisk and
was previously characterized in the literature as due to
69−71
68
oxidation of the carbon electrode.
0.40 V is also seen at an EPPG electrode coated with
ferrocene (Figure 6b), but a second reversible process is seen at
A surface process at
∼
carried out in air-saturated 1.0 M HClO in order to determine
4
+
0.17 V and is assigned to the surface oxidation of ferrocene.
The potential and currents for the surface FcH /FcH process
the electrocatalytic activity of the four target cobalt complexes.
The numbers of electrons transferred in the reduction were
then determined from the magnitude of the steady-state
limiting currents that were taken at a fixed potential (−0.10 V)
on the catalytic wave plateau of the current−voltage curve.
+
of ferrocene are the same under air and under N (Figure 6b).
The value of E1/2 at the surface is 0.17 V and a similar E1/2 of
2
0.14 V is seen for the solution oxidaion/reduction of ferrocene
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Representative current−voltage curves obtained for com-
pounds 2−4 are presented in Figure 7 and Figure S2a. The
64
diagnostic Koutecky−Levich plots are also illustrated in these
figures, and the slope of a plot obtained by linear regression was
then used to estimate the average number of electrons (n)
involved in the catalytic reduction of O2.
The Koutecky−Levich plots are interpreted on the basis of
eq 3, where j is the measured limiting current density (mA·
lim
−2
cm ), j is the kinetic current, and j is the Levich current
k
lev
which is used to measure the rate of the current-limiting
chemical reaction as defined by eq 4.
1
/jlim = 1/jlev + 1/jk
(3)
jlev = 0.62nFD2/3cv−1/6w1/2
(4)
The value of n in eq 4 is the number of electrons transferred
in the overall electrode reaction, F is the Faraday constant
−
1
2
(
96485 C·mol ), D and c are the diffusion coefficient (cm /s)
and bulk concentration of O (mol/L) in 1.0 M HClO , v is the
2
4
kinematic viscosity of water, and ω is the angular rotation speed
(
rad/s) of the electrode.
The calculated number of electrons transferred (n) to O in
2
the electroreduction process using compounds 1−4 as catalysts
and the corresponding percentage of the H O product are
2
2
Figure 6. Cyclic voltammorgams of (a) bare EPPG electrode only, (b)
given in Table 3. A two electron transfer (n = 2) would
generate 100% H O while a four electron transfer (n = 4)
ferrocene adsorbed on an EPPG electrode in 1.0 M HClO , and (c)
4
ferrocene in 1.0 M HClO solution saturated with N (-----) or with air
2
2
4
2
would give 0% H O and 100% H O. The Koutecky−Levich
(
). Scan rate of 50 mV/s for a and c, and 100 mV for b. * = surface
2
2
2
process due to carbon oxidation.
plots in Figures 7 and S2b showed that the number of electrons
transferred (n) for compounds 2, 3, and 4 to be 2.1, 2.0, and
2
.1, which corresponds to 95−100% H O produced (Table 3),
2
2
indicating that the catalytic electroreduction of O by 2, 3, and
2
Figure 7. Current−voltage curves and Koutecky−Levich plots for catalyzed reduction of O at a rotating EPPG disk electrode coated with the cobalt
2
porphyrins 2 and 3 in 1.0 M HClO saturated with air. Values of the electrode rotation rates (ω) are indicated on each curve. Potential scan rate = 50
4
mV/s.
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Inorganic Chemistry
Article
Table 3. Peak Potentials (V vs SCE) of Cobalt(II)
measured wavelengths in nanometers and the calculated values
of energy in electronvolts are given in Table 2, and a plot of
Soret band energy vs number of Fc groups is shown in Figure
9a. From this figure, it is clear that the Soret band undergoes a
Porphyrins for Catalytic Reduction of O in 1.0 M HClO ,
2
4
the Number of Electron Transferred (n), and the Amount of
H O Produced during the Reaction
2
2
catalyst
CH Ph) PorCo, 1
Epc without O2 Epc with O2
n
H O %
2
2
60
95
(
0.07
0.20
0.14
0.30
0.05
0.19
0.13
0.29
2.8
2.1
2.0
2.1
3
4
Fc(CH Ph) PorCo, 2
3
3
Fc (CH Ph)PorCo, 3
100
95
3
3
Fc PorCo, 4
4
−
4
is mainly a 2e transfer process, giving almost exclusively
−
H O rather than a 4e transfer process to produce H O.
2
2
2
The catalytic reduction of O was also examined at an RRDE
2
under the same solution conditions, and examples of the
relevant current−voltage curves are shown in Figure 8 for
Figure 9. Plots of number of ferrocenyl groups of the porphyrin vs (a)
energy of the Soret band of Co(II) and Co(III) porphyrins and (b)
redox potentials of the porphyrin ring-centered and metal-centered
electron transfer.
low-energy shift as the number of Fc groups is increased from 0
to 4. The slope of the plot of Soret band energy vs the number
of Fc is −0.085 for Co(II) and −0.108 for Co(III). Thus, the
Co(III) porphyrins are slightly more sensitive to the electron-
donating Fc groups than the Co(II) porphyrins.
Figure 8. Rotating ring-disk electrode voltammograms of cobalt
porphyrins 2 and 3 in 1.0 M HClO saturated with air. The potential
4
of the disk electrode was scanned in a negative direction from 0.6 V to
−
0.2 V while the potential of the ring electrode was held at 1.0 V.
Rotation rate = 400 rpm and scan rate = 10 mV/s.
Linear relationships are also seen between the measured
redox potentials and the number of ferrocenyl groups on
compounds 1−4. The largest substituent effect is seen for the
II
III
compounds 2 and 3. The disk potential was scanned from 0.6
to −0.2 V at a rotation speed of 400 rpm while holding the ring
potential constant at 1.0 V. As seen in the figure, the disk
current begins to increase at about 0.40 V for compounds 2 and
Co /Co process and the smallest for the second reduction,
which is almost totally insensitive to the electron-donating
substituent. The first oxidation potential shifts negatively by
300 mV upon going from compound 1 (Epa = 0.45 V) to
0
0
.30 V for 3, and a plateau is reached at about 0.20 V for 2 and
.15 V for 3. The anodic ring current increases throughout the
compound 4 (E = 0.15 V), and the slope of the plot in Figure
9b shows an average shift in peak potential for the Co /Co
pa
II
III
range of the disk potentials where the disk current rises. From
the Faradaic currents at the disk (I ) and at the ring (I ), the
amount of H O generated upon the reduction of dioxygen was
process of about 65 mV per added Fc group on the compound.
The first reduction of the Co(II) porphyrins shifts negatively by
an average of 27 mV per added Fc group on the compounds,
while the second reduction has a slope of almost zero as seen in
Figure 9b. In contrast, the porphyrin ring-centered oxidation
shifts positively with an increase in the number of linked Fc
groups, but it should be noted that the overall charge on the
electroactive species is different in each case, being +1 in the
case of 1, +2 in the case of 2, and +4 in the case of compound 3
where the three Fc groups have been oxidized. Thus, the harder
oxidation of the porphyrin ring upon going from compound 1
D
R
2
2
calculated using eq 2 as 88% and 92% for compounds 2 and 3,
respectively, under the given experimental conditions. These
values are smaller than those calculated using the Koutecky−
Levich plots in Figure 7.
Effect of Ferrocenyl Substituents on Spectra and
Redox Potentials. A linear relationship exists between the
energy of the Soret band for the Co(II) and Co(III) porphyrins
and the number of ferrocenyl groups on compounds 1−4. The
8
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Inorganic Chemistry
Article
Table 4. Number of Electron Transferred (n) for Catalytic Reduction of O by Different Cobalt Porphyrins Used as Catalyst in
2
a
Acid Media
catalyst
CH ) PorCo
β-group
meso-group
n
acid media
ref
63
(
(
none
none
NO2
NO2
C H
CH3
4.0
4.0
3.1
3.1
2.8
2.8
2.6
2.6
2.0
2.0
2.1
2.0
2.1
1.0 M HClO4
1.0 M HClO4
1.0 M HClO4
1.0 M HClO4
3
4
Por)Co
none
73
NO (CH Ph) PorCo
CH Ph
58
2
3
4
3
NO (3,5-diCH OPh) PorCo
CH OPh
58
2
3
4
3
(
(
(
(
(
(
OEP)Co
none
59
2
5
CH Ph) PorCo
none
CH Ph
1.0 M HClO4
1.0 M HClO4
1.0 M HClO4
1.0 M HClO4
tw
3
4
3
3,5-diCH OPh) PorCo
none
CH OPh
58
3
4
3
PPIX)Co
TPP)Co
CH , CHCH , C H COOH
none
Ph
59
3
2
2
4
none
59, 63
59, 74
tw
TPyP)Co
none
none
none
none
Py
0.05 M H SO4
2
Fc(CH Ph) PorCo
Fc, CH Ph
1.0 M HClO4
1.0 M HClO4
1.0 M HClO4
3
3
3
Fc (CH Ph)PorCo
Fc, CH Ph
tw
3
3
3
Fc PorCo
4
Fc, CH Ph
tw
3
a
tw = this work.
to compound 4 may be due to the increased positive charge on
the compound rather than to the effect of increasing the
number of Fc groups.
Effect of Ferrocenyl Groups on the Catalytic Electro-
reduction of Dioxygen. As discussed above, free ferrocene
CONCLUSION
■
Several cobalt meso-ferrocenyl porphyrins of general formula
Fc (CH Ph) PorCo (n = 0−4) with direct ferrocene-to-
n
3
4−n
porphyrin bonds have been prepared and investigated using
UV−vis spectroscopy as well as electrochemical and
spectroelectrochemical approaches. The electron-donating
ferrocene substituent(s) have a significant effect on the
compound’s UV−visible spectra as well as on their cobalt-
centered and porphyrin ring-centered redox potentials. The
electrocatalytic properties of the investigated ferrocenyl-
containing cobalt porphyrins are highly selective in the
electrocatalytic reduction of dioxygen and lead almost
exclusively to formation of hydrogen peroxide as a reaction
product. Such a high selectivity can be explained on the basis of
steric hindrance resulting from the bulky ferrocene groups
which prevents dimerization of the porphyrin on the electrode
surface.
shows no catalytic activity for the reduction of O in acid media.
2
However, ferrocene substituents in complexes 2−4 also have
no effect on the four-electron electrocatalytic activity of the
target cobalt porphyrins, although it should be noted that
adding ferrocene groups directly to the meso positions of the
porphyrin will change the planarity of the porphyrin macro-
1
5
cycle. The steric hindrance resulting from addition of these
groups at the meso positions of the macrocycle will also prevent
the formation of dimers on the electrode surface, thus
preventing the occurrence of a four-electron reduction to give
63
an H O reduction product and an n value of 4.
2
We have previously reported that the electroreduction of O2
catalized by triphenylcorroles which contain substituents on the
ortho positions of the meso-phenyl rings give, in most cases, a
two-electron transfer process (n = 2) and only H O as the
ASSOCIATED CONTENT
■
2
2
* Supporting Information
S
product. This is because ortho positions of the meso-phenyl
Spectroelectrochemistry, current−voltage curves, and Kou-
rings of the triphenyl corrole can lead to steric hindrance and
tecky−Levich plot of Fc PorCo in 1.0 M HClO saturated
4
4
72
block the π−π interaction of the macrocycles.
with air. This material is available free of charge via the Internet
at http://pubs.acs.org.
It is also known that the catalytic activity of the monomeric
cobalt porphyrins will depend not only on the nature of the
substituents but also on steric interactions involving bulky
substituents on the meso-phenyl rings. As can be seen from
Table 4, cobalt porphine and cobalt tetramethylporphyrin,
AUTHOR INFORMATION
■
*
*
*
Corresponding Authors
E-mail: zpou2003@yahoo.com.
E-mail: vnemykin@d.umn.edu.
E-mail: kkadish@uh.edu.
(
CH ) PorCo, which lack bulky meso substituents, will both
3 4
catalyze the reduction of O via a four-electron pathway (n = 4)
2
6
3,73
to give H O as a final product.
The reason for such
2
Notes
selectivity is the formation of porphyrin dimers, which can carry
out the four-electron electrocatalytic reduction of dioxygen.
The authors declare no competing financial interest.
(
TPP)Co and (TpyP)Co can only catalyze a two-electron
5
9,63,74
ACKNOWLEDGMENTS
reduction of dioxygen (n = 2),
from 2.6 to 3.1 when other substituted monocobalt porphyrins
were utilized as the catlysts.
current investigated ferrocene substituted compounds are
consistent with a steric hindrance of ferrocene substituents,
which prevent formation of porphyrin dimers. Thus, the cobalt
meso-ferrocenyl porphyrins discussed in this paper can be
proposed as selective electrocatalysts for two-electron reduction
of oxygen.
while the values of n range
■
We gratefully acknowledge support from the Natural Science
Foundation of China (No. 21071067, 21171076), the Robert
A. Welch Foundation (K.M.K., Grant E-680), and National
Science Foundation (V.N.N. CHE-1110455 and CHE-
58,59
The values of n = 2 for the
1
401375).
REFERENCES
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