Controlling the Oxygen Reduction Selectivity of Asymmetric Cobalt Porphyrins by Using Local Electrostatic Interactions

Article abstract of DOI:10.1021/jacs.0c03861

The development and improvement of electrocatalysts for the 4H+/4ea' reduction of O2 to H2O is an ongoing challenge. The addition of ancillary groups (e.g., hydrogen bonding, Br?nsted acid/base) near the active site of metal-containing catalysts is an effective way to improve selectivity and kinetics of the oxygen reduction reaction (ORR). In this regard, iron porphyrins are among the most researched ORR catalysts. Closely related cobalt porphyrin ORR catalysts can function closer to the O2/H2O thermodynamic potential, but they tend to be less selective and follow a different mechanism than for the iron porphyrins. Herein, we explore strategies to extend the ideas about ancillary groups that have been developed for iron porphyrin ORR electrocatalysts to improve the performance of the corresponding cobalt complexes. We describe a series of porphyrin electrocatalysts that are modified versions of Co(5,10,15,20-tetraphenylporphyrin), where the 2-position of one of the phenyl groups contains-NH2,-N(CH3)2, and-N(CH3)3 +. Investigations using cyclic voltammetry and hydrodynamic electrochemistry show that the presence of a cationic ancillary group gives rise to a catalyst that is selective for the conversion of O2 to H2O across a wide pH range. In contrast, the other catalysts are selective for reduction of O2 to H2O at pH 0, but produce H2O2 at higher pH. The ORR rate (～106 Ma'1 sa'1) and selectivity of the-N(CH3)3 +-modified catalyst are invariant between pH 0 and 7. Quantum chemical calculations support the hypothesis that the enhancement of selectivity can be attributed to the distinct mechanism of O2 reduction by Co-porphyrins. Specifically, the mechanism relies on anionic, peroxide-bound intermediates. While protic ancillary groups are important in the performance of iron porphyrin ORR catalysts, we suggest that electrostatic stabilizers of O2-bound intermediates are more crucial for cobalt porphyrin ORR catalysts.

Full text of DOI:10.1021/jacs.0c03861

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Article
Controlling the Oxygen Reduction Selectivity of Asymmetric Cobalt
Porphyrins by Using Local Electrostatic Interactions
Rui Zhang and Jeffrey J. Warren*
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ABSTRACT: The development and improvement of electrocatalysts for the
4H+/4e− reduction of O2 to H2O is an ongoing challenge. The addition of
ancillary groups (e.g., hydrogen bonding, Brønsted acid/base) near the active
site of metal-containing catalysts is an effective way to improve selectivity and
kinetics of the oxygen reduction reaction (ORR). In this regard, iron
porphyrins are among the most researched ORR catalysts. Closely related
cobalt porphyrin ORR catalysts can function closer to the O2/H2O
thermodynamic potential, but they tend to be less selective and follow a
different mechanism than for the iron porphyrins. Herein, we explore
strategies to extend the ideas about ancillary groups that have been developed
for iron porphyrin ORR electrocatalysts to improve the performance of the
corresponding cobalt complexes. We describe a series of porphyrin
electrocatalysts that are modified versions of Co(5,10,15,20-tetraphenylpor-
phyrin), where the 2-position of one of the phenyl groups contains -NH2,
+
-N(CH3)2, and -N(CH3)3 . Investigations using cyclic voltammetry and hydrodynamic electrochemistry show that the presence of a
cationic ancillary group gives rise to a catalyst that is selective for the conversion of O2 to H2O across a wide pH range. In contrast,
the other catalysts are selective for reduction of O2 to H2O at pH 0, but produce H2O2 at higher pH. The ORR rate (∼106 M−1 s−1)
+
and selectivity of the -N(CH3)3 -modified catalyst are invariant between pH 0 and 7. Quantum chemical calculations support the
hypothesis that the enhancement of selectivity can be attributed to the distinct mechanism of O2 reduction by Co-porphyrins.
Specifically, the mechanism relies on anionic, peroxide-bound intermediates. While protic ancillary groups are important in the
performance of iron porphyrin ORR catalysts, we suggest that electrostatic stabilizers of O2-bound intermediates are more crucial for
cobalt porphyrin ORR catalysts.
INTRODUCTION
■
The oxygen reduction reaction (ORR) has been studied for a
long time, with emphasis placed on biological reactions1−3 and
on cathodic reactions in fuel cells.4−6 While natural systems
use first row metals for ORR chemistry, platinum is primarily
used as the catalyst in fuel cells. Improving the adoption of fuel
cell technology is closely tied to the identification of an
inexpensive and selective catalyst for the ORR. For fuel cell
applications, it is particularly important that O2 reduction
involves the 4 electron + 4 proton (4H+/4e−) conversion of O2
Figure 1. Structures and abbreviations of the cobalt(II) 5-(2-R)-
10,15,20-triphenylporphyrins described in this work.
to H2O. Other pathways, such as those that make superoxide
(1e− reduction) or hydrogen peroxide (2H+/2e− reduction),
groups that support hydrogen bonding and those that carry a
permanent positive charge.
are undesirable and can lead to degradation of catalyst systems.
With these ideas in mind, the two ongoing challenges for the
development of ORR catalysts are (1) to identify nonprecious
Received: April 8, 2020
catalysts and (2) to favor selectivity for O2 reduction to
H2O.4,5 Herein, we describe the systematic investigation of a
series of cobalt porphyrin electrocatalysts (Figure 1) that
probes how the identity of different ancillary functional groups
affect the performance and selectivity for ORR. Of particular
interest are differences in the performance of catalysts with
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To address the two challenges identified above, many
different coordination complexes, and especially metallopor-
phyrin complexes, have been studied.7 First row metal-
loporphyrin ORR catalysts have garnered interest and are a
means to satisfy the first of the aforementioned challenges. In
particular, many investigations of homogeneous ORR of
tetraarylporphyrins have emerged, including work in organic
solvents8−11 and in aqueous buffers.12−14 A unifying theme of
many of these reports is that the management of protons and
electrons is crucial for selective ORR catalysis, which addresses
the second of the above challenges. One important design
feature that has received attention is “proton relays.” For ORR
catalysis, these are typically Brønsted acids (e.g., carboxylic
acids or pyridinium).9,13,15,16 Although the explicit roles of
these groups during catalysis is debated, and likely depends on
reaction conditions,7 they appear to play an important role in
proton transfer reactions. In addition, there are several
examples of catalysts that incorporate multiple metal centers
as a means to mediate multiple redox reactions.15,17−19 Early
multimetallic porphyrin O2 reduction catalysts were inspired
by biological systems, exemplified by cytochrome c oxidase
active site models.20−22 As noted above, a great many other
molecular catalyst systems have been developed that have
different levels of performance and comprehensive reviews are
available.7,23
active ancillary groups have been investigated in Co-
porphyrins, but those designs do not explicitly account for
the possibility of an anionic, peroxide-bound intermediates. In
this report, we show that a cationic group proximal to the
metal in Co-porphyrin ORR catalysts improves selectivity for
the 4H+/4e− reduction O2 across a wide pH range.
Our previous work on Co-porphyrins for ORR26 built upon
the idea that proton delivery was crucial for catalyst selectivity,
as is the case for Fe-porphyrins.26 However, related work on
CO2 reduction has shown that electrostatic directing groups in
iron 5,15,15,20-tetra(trimethylanilinium)porphyrin led to a
more dramatic improvement in catalyst activity than did
Brønsted acid proton relays.39 That same iron porphyrin also is
an O2 reduction catalyst in MeCN solvent and its unique
catalytic properties are tied, in part, to the ability of the
cationic porphyrin to bind anions that shift the catalyst
reduction potentials.40 Given the above examples, the catalytic
improvements we observed26 for a Co-porphyrin with a 2-
pyridinium group could have been because of proton transfer
reactions (i.e., as a proton relay) or due to electrostatic
stabilization of anionic intermediates. In the present report, we
designed, prepared, and investigated a series of cobalt
porphyrins (Figure 1) to systematically probe the relative
importance of proton relay activity and electrostatic
interactions of pendant groups in ORR catalysis.
Iron porphyrin molecular ORR catalysts are among the most
widely investigated, in part because they tend to favor
reduction of O2 to H2O.7 The drawback is that they usually
function at high overpotentials. We found that, for Fe-
porphyrins, graphite adsorption of catalysts, and the incorpo-
ration of one proton relay, gave rise robust catalysts for O2 to
H2O conversion, albeit at overpotentials ≥1 V.24 In contrast,
Co-porphyrin ORR catalysts can function at lower over-
potentials, but favor production of H2O2.25 We recently
reported that the replacement of one phenyl group of
Co(5,10,15,20-tetraphenylporphyin) (CoTPP) with a 2-
pyridyl group dramatically shifted the ORR selectivity from
H2O2 to O2 in the case of graphite adsorbed catalysts.26 In
related work, Anson and co-workers carried out several
investigations of graphite-adsorbed Co-porphyrin ORR cata-
lysts,25,27,28 but only some of those molecules or catalyst
preparations were selective for reduction of O2 to H2O.28,29
Any design of a molecular catalysts must consider the key
intermediates in a catalytic cycle.30 The mechanism by which
O2 is reduced by Fe- and Co-porphyrins is likely different at
some key reaction steps. In both cases, a reduced metal
(M(II)) binds O2 to give a metal-superoxide complex
([M(III)-O2•−]n). For Fe, the pathway likely involves
reduction and protonation to yield the corresponding hydro-
peroxo complex ([M(III)-O2H]n). Further reduction and
protonation of a ferric-hydroperoxo can yield a formally
FeIV=O complex.31 Catalyst-control over proton transfer is
especially important for this reaction in iron porphyrin ORR
catalysts.32 Co(II) also binds O2 to form a superoxo
complex,33−35 but from this point, the mechanisms for Co
and Fe diverge. Cobalt porphyrins cannot form stable CoIV-oxo
complexes36 and reduction of a Co(III)-O2•− intermediate has
been proposed to give a [Co-OOH−] complex during aqueous
ORR.37 Protonation of that complex can yield H2O2. We note
that this reduction sequence is different than has been
proposed in homogeneous organic solutions,8 but such
medium effects can have substantial effects on ORR
chemistry.38 The incorporation of proton relays or redox-
EXPERIMENTAL SECTION
■
Materials and Instrumentation. Reagents were obtained from
Sigma-Aldrich unless otherwise noted and used without further
purification. Gases were from Praxair Canada. Basal plane and edge
plane graphite (BPG and EPG, respectively) electrodes were prepared
according to the literature.41 Mass spectra were collected by using a
Bruker microFlex MALDI-TOF mass spectrometer and electrospray
ionization mass spectrometry experiments used and Agilent 6210
instrument. UV−visible spectra were recorded using a Cary100Bio
spectrophotometer. Elemental analyses (C, H, N) were performed at
Simon Fraser University on a Carlo Erba EA 1110 CHN elemental
analyzer.
The ligands, 5,10,15,20-tetraphenylporphyrin (H2TPP) and 5-(2-
aminophenyl)-10,15,20-triphenylporphyrin (H2TPPNH2), were syn-
thesized using a literature procedure.42 Metalation of porphyrins with
Co(II)acetate was carried out according to the literature.26 The
ligand, 5-(2-N,N-dimethylphenyl)-10,15,20-triphenylporphyrin
(H2TPPNMe2) was synthesized using modified literature proce-
dures,39,42 and its metalation was carried out by refluxing the ligand
with excess Co(II)acetate in dimethylformamide (DMF) for 6 h.
Finally, CoTPPNMe2 was reacted with an excess of methyl triflate in
+
DMF in 24 h to yield CoTPPNMe3 . H2TPPNMe2, CoTPPNMe2,
+
and CoTPPNMe3 were characterized by NMR, mass spectrometry,
UV−vis spectroscopy and elemental analysis. Detailed experimental
descriptions and characterization data can be found in the Supporting
Information (Figures S1−S9).
Electrochemical Methods. A Pine Instruments WaveDriver 20
bipotentiostat was used for electrochemical measurements. Cyclic
voltammetry (CV) and controlled potential electrolysis (CPE)
measurements used a conventional three-electrode cell, with an
edge plane graphite (EPG) working electrode, basal plane graphite
(BPG) counter electrode, and a Ag/AgCl (saturated KCl) reference
electrode. Rotating disk electrochemistry (RDE) and rotating ring-
disk electrochemistry (RRDE) measurements used the Pine
Modulated Speed Rotator. Potassium ferricyanide was used as an
external standard for heterogeneous experiments and all potentials are
reported with respect to the normal hydrogen electrode (NHE).
Electrochemical experiments were carried out in solutions with pH
values of 0, 4, and 7. Solutions contained 1 M H2SO4 and the pH was
adjusted using 1 M NaOH.
B
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Computational Methodology. Calculations were performed
using the ORCA 4.2.1 ab initio quantum chemistry program.43,44
Geometry optimizations and single point calculations were carried out
using the TPSS functional,45 utilizing the RIJCOSX algorithm.46 The
basis set was def2-TZVP on Co, def2-SVP/def2/J on other atoms47,48
and the Becke-Johnson damping scheme was utilized.49,50 Metrical
parameters calculated for CoTPP are similar to those in experimental
X-ray structures.51,52 Optimized structures were characterized using
vibrational frequency calculations at the same level of theory to
confirm that the structures were located at a minimum on the
potential energy surface. Single-point energy calculations used the all
electron aug-cc-pVTZ basis set53,54 in the gas phase and in water
solvent using a dielectric continuum model (CPCM).55 The total free
energy was calculated following Carter et al.56 and included
thermochemical and entropic contributions. The explicit solvation
energy of H+ in water (−270.3 kcal mol−1)57−59 and an empirical
value for NHE (4.281 V)60 were used in reaction free energy
calculations.
RESULTS AND DISCUSSION
■
Figure 2. Cyclic voltammograms at pH 0 (A), pH 4 (B), and pH 7
(C) of CoTPPNMe3+ drop-cast on EPG. Blue traces are in Ar-sparged
solution and red are in O2-sparged solution. All scan rates are 100 mV
s−1. The small vertical arrow indicates the position of Ep,c.
The ligand H2TPPNMe2 and the complex CoTPPNMe3+ were
prepared following literature procedures for closely related
complexes.39,42 H2TPP and H2TPPNH2 are known ligands.
Full characterization data are given in the Supporting
Information. The optical spectra of the amine-substituted Co
complexes show Soret bands (412 nm) and Q-bands (521 nm)
that are similar to those for the parent CoTPP (410 and 527,
respectively).61 We conclude that the inclusion of the amine
group does not dramatically affect the electronic structure of
the Co-porphyrins with respect to CoTPP. This series of
complexes allows for a systematic test of the role of
electrostatic interactions versus proton relay effects. At pH 0,
CoTPPNH2 and CoTPPNMe2 will be protonated (i.e., the
dependence of the CoIII/II couple is roughly consistent with the
behavior of a soluble, related analog cobalt(5,10,15,20-tetra(4-
N-methlypyridyl)porphyrin65,66 where the pH-dependence is
attributed to pronation/deprotonation of an axially ligated
H2O. For CoTPPNH2 and CoTPPNMe2, Ep.c are observed at
0.32 and 0.38 V, respectively. CoTPPNMe3+ has Ep,c at 0.37 V.
+
Notably, the Ep,c values for CoTPPNMe3 are comparatively
weakly dependent on pH, shifting ∼50 mV more negative
between pH 0 and 7. In cases where Ep,a is clearly observed, the
large peak separations are consistent with the large
reorganization energy associated with the CoIII/II couple.
Furthermore, weak current responses for CO-porphyrins can
attributed to the slow addition of axial ligands to CoIII sites in
heterogeneous films.25 We also note that the amount of
catalyst that we deposit is higher than reports for other EPG-
deposited Co-porphyrins,18,25,28 to enable direct comparison
with our previous work.26 Additional CVs for CoTPPNMe3+ at
50% less loading are shown in the Supporting Information for
comparison (Figure S16).
When the electrolyte solutions are sparged with O2, CVs for
all of the drop-cast Co-porphyrins show a definite increase in
current near the CoIII/II couple (Figure 3 and Supporting
Information Figures S11−S13). The increase in current is
known for CoTPP, which is an established O2-to-H2O2
reduction catalyst.25,27 For CoTPP (100 mV s−1 scan rate),
+
conjugate acids, CoTPPNH3 and CoTPPNMe2H+), but at
higher pH they will exist in the neutral form. In contrast, the
charge of the trimethylanilinium group in CoTPPNMe3 is
independent of pH.
+
Homogeneous cyclic voltammetry experiments were first
carried out in MeCN solvent (see Supporting Information,
Figure S10). For CoTPP a reversible CoIII/II couple was
observed at 0.67 V (versus ferrocenium/ferrocene, Cp2Fe+/0),
consistent with the reported value in propionitrile.62 The
CoIII/II potential for CoTPPNH2 (0.70 V versus Cp2Fe+/0) is
similar to that of CoTPP, but the potentials for CoTPPNMe2
(0.75 V versus Cp2Fe+/0) and CoTPPNMe3 (0.78 V versus
+
Cp2Fe+/0) are somewhat higher. This is qualitatively consistent
+
with the shift in potential observed for an NMe3 -substituted
iron porphyrin.39 The observed values are similar to the
reported potentials for a series of Co-tetraarylporphyrin
derivatives.63
Next, each of the Co-porphyrins were drop cast on edge
plane graphite (EPG) electrodes. CVs were first collected in
Ar-sparged electrolyte at pH values of 0, 4, and 7. CVs for all
complexes can be found in the Supporting Information. As
expected, from studies of other deposited Co-porphyrins the
responses are weak.25,27,64 The data for CoTPPNMe3 are
+
shown in Figure 2. CoTPP exhibits a weak, broad wave at 0.55
V versus NHE, consistent with a report for a similar electrode
preparation of CoTPP.27 The cathodic peak (Ep.c) is observed
at 0.47 V (pH 0). This wave is pH-dependent, shifting to more
negative values as the pH is increased.
A definitive assignment of formal potentials is complicated
by the background response from the EPG electrode and
poorly defined anodic waves (Ep,a). Overall, the observed pH
Figure 3. Comparison of cyclic voltammograms of EPG-adsorbed
+
CoTPPNH2 (black), CoTPPNMe2 (blue), and CoTPPNMe3 (red)
at pH 0 (A) and pH 7 (B). All scan rates are 100 mV s−1.
C
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the peak of the catalytic wave, or Ecat, appears at 0.25 V (pH
0), 0.11 V (pH 4) and −0.03 V (pH 7), or 40 mV per pH unit.
For CoTPPNH2, the Ecat is at 0.31 V (pH 0), 0.05 V (pH 4), −
0.16 V (pH 7), or 67 mV per pH unit. The corresponding
methylated complex, CoTPPNMe2, shifts slightly more (ca. 75
mV per pH). Again, the CV behavior of CoTPPNMe3+ (Figure
2) is distinct from the other Co-porphyrins. Catalytic currents
are observed, but the maximum current values and Ecat
positions are more weakly dependent on pH. From pH 0 to
pH 7, the catalytic wave position moves about 0.18 V, or 26
mV per pH unit. In all cases, the observed current density is
not systematically dependent on pH.
The pH dependence of the O2/H2O couple is 59 mV per
pH, but the speciation of the Co-porphyrins will introduce
additional pH dependences. First, axially ligated waters will
introduce a pH dependence to the CoIII/II reduction
potentials.65,66 In addition, the protonation state of
CoTPPNH2 and CoTPPNMe2 will depend on pH. Using
the pKa values of aniline (pKa = 4.6) and N,N-dimethylaniline
(pKa = 5.2) as models,67 it is expected that CoTPPNH2 and
CoTPPNMe2 are fully protonated at pH 0 and deprotonated at
pH 7. Finally, changing the pH can affect the protonation
states of surface groups on the graphite electrode,68 which also
could affect the CoIII/II couple.
The above results indicate that all of the newly tested Co-
porphyrins are active ORR catalysts. Next, the products of O2
reduction by each catalyst were analyzed using a series of
hydrodynamic electrochemistry experiments. First, rotating
ring-disk electrochemistry (RRDE) was used as a direct probe
of the selectivity of ORR for each of the cobalt catalysts. As for
the CV experiments, pH values of 0, 4, and 7 were used and
the results are set out in Table 1. ORR selectivity was assayed
+
Figure 4. RRDE linear sweep voltammograms for CoTPPNMe3
deposited on an EPG disk electrode under 1 atm O2. The rotation
rate was 900 rpm. The scan rate was 20 mV s−1. The potential at the
Pt ring was 1.2 V.
reaction to yield H2O2. The addition of a protonatable
nitrogen in CoTPPNH2 and CoTPPNMe2 somewhat increases
the selectivity for reduction of O2 to H2O at low pH, but the
selectivity begins to favor production of H2O2 as the pH is
+
raised. In contrast, CoTPPNMe3 exhibits a greater degree of
4H+/4e− O2 reduction at all pH values tested.
We also carried out rotating disk electrochemistry (RDE)
experiments and analyzed the results using Koutecky−Levich
(K-L) analysis (eq 2, Figure 5) as an independent measure of
the number of electrons involved in O2 reduction. eq 2 is given
as
i−1 = iK−1 + (0.62nFAD2/3ν−1/6C)−1 × ω−1/2
(2)
Table 1. Summary of the Number of Electrons (n) Involved
in ORR Catalysis for Each Cobalt Porphyrin
RRDE
RDE
nRDE
Catalyst
CoTPP
pH
nRRDE
H2O%
0
4
7
0
4
7
0
4
7
0
4
7
2.6 0.1
2.8 0.2
3.0 0.1
3.1 0.1
2.7 0.2
3.0 0.1
3.4 0.1
3.0 0.1
3.1 0.1
3.8 0.2
3.5 0.2
3.2 0.2
31
40
48
57
33
49
68
51
54
91
74
62
5
2.2 0.3
2.3 0.2
2.6 0.1
3.5 0.5
3.1 0.4
2.2 0.2
3.0 0.5
3.2 0.4
2.2 0.2
3.5 0.2
3.2 0.3
3.4 0.4
5
5
4
5
6
3
6
7
4
5
8
CoTPPNH2
CoTPPNMe2
CoTPPNMe3
+
by calculating the number of electrons (n) involved in ORR
using eq 1
n = 4Id × (Id + Ir/N)−1
(1)
In eq 1, Id is the EPG disk current, Ir is the Pt ring current, and
N is the ring collection efficiency (0.24, as determined using
potassium ferricyanide as a standard, see Supporting
Information). The percent H2O was calculated using equation
S1 (see Supporting Information). RRDE traces for
+
CoTPPNMe3 are set out in Figure 4 and are in available
Figure 5. (A) Linear sweep voltammograms from rotating disk
electrochemistry experiments for CoTPPNMe3+ in pH 0 solution and
(B) Koutecky−Levich (K-L plots) derived from the data in panel (A).
the Supporting Information for the other porphyrins. As
expected, CoTPP catalyzes primarily a 2e− O2 reduction
D
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where I is the observed current, n is the number of electrons
passed, F is the Faraday constant (96485 C mol−1), A is the
electrode surface area (0.075 cm2), D is the diffusion
coefficient of O2 (1.9 × 105 cm2 s−1), ν is the kinematic
viscosity of water (at 22 °C = 0.01 cm2 s−1), C is the
concentration of O2 (2.6 × 10−7 mol cm3),69,70 and ω is the
electrode rotation rate in radians per second. K-L plots (e.g.,
Figure 5) were linear and the values calculated for n from the
slopes of the linear fits are set out in Table 1. Additional K-L
plots are shown in the Supporting Information.
The results for analyses using K-L plots are, in general,
similar to those from RRDE. However, there are some
differences, especially as the pH is increased for CoTPP,
CoTPPNH2 and CoTPPNMe2. We are not the first to notice
such behavior when comparing between RDE and RRDE
experiments for Co-porphyrin catalyzed ORR.64 In addition,
RRDE experiments are sensitive to the state of the Pt ring
electrode, especially when significant H2O2 is produced.25 For
CoTPP, n is less than 4 at all pH, consistent with other
reports.25,27 In contrast, CoTPPNMe3+ is much more selective
for the reduction of O2 to H2O at all pH. For CoTPPNH2 and
CoTPPNMe2, more H2O is produced at pH 0 (when the
ancillary group is protonated) than at pH 4 or 7. Based on
marked differences in the shape of RDE traces as the pH is
increased, these two catalysts appear to be less stable when
more H2O2 is produced. One possibility is reactions of free or
metal-catalyzed oxidations of the amine groups by the H2O2
produced during O2 reduction.71−73 This may contribute to
the disagreement between RDE and RRDE results at higher
pH (Table 1). Based on these results, aniline and dimethylani-
line groups appear to be satisfactory proton relays at low pH
(i.e., when they are protonated), but their susceptibility to
oxidation makes them unsuitable for systems where strong
oxidants can be produced.
+
Figure 6. Turnover−overpotential relationships for CoTPPNMe3 at
different pH values. TOF values were determined from K−L plots
from RDE data collected in pH 0, 4, and 7 solutions under 1 atm O2.
selectivity (H2O versus H2O2) changes with pH. Likewise,
inspection of K-L plots for overpotentials lower than 0.6 for
CoTPPMe3+ suggest an increase in H2O2 production, so those
data are not shown in Figure 6.
Among the catalysts described here, and those in our
previous work,26 CoTPPNMe3 is unique. It lacks an explicit
+
proton relay, yet catalyzes the 4H+/4e− reduction of O2 with
good rates and selectivity from pH 0 to 7. For CoTPPNH2,
CoTPPNMe2, and two previously reported complexes
CoTPOH and CoTPPy (5-(2-hydroxyphenyl)-10,15,20-tri-
phenylporphyrinato cobalt(II) and 5-(2-pyridyl)-10,15,20-
triphenylporphyrinato cobalt(II), respectively), we can begin
by comparing relative pKa values of the different proton relays.
Again taking the aqueous pKa values as models (i.e., pyridine
(5.3), aniline (4.6), N,N-dimethylaniline (5.2) and phenol
(10)67) it is straightforward to see one reason why CoTPOH is
the least effective ORR catalyst; the 2-hydroxypheyl group is a
much weaker acid and also cannot form a cation. Consistent
with this idea is the behavior of the three Co-porphyrins with
nitrogen bases, all of pKa ∼ 5. These catalysts have n = 3−4 for
ORR at pH 0 (i.e., H2O is the main product), but n ∼ 2 at
higher pH values. Notably, at pH 0 the selectivity of CoTPPy
is quite similar to CoTPPNH2 and CoTPPNMe2, despite the
different distance between the ionizable nitrogen and the
metal. This has been commented on, and modeled, for a
related iron porphyrin.13
The kinetics of O2 reduction by CoTPPNMe3+ drop-cast on
EPG electrodes in different pH solutions were extracted from
+
K-L plots. The electroactive CoTPPNMe3 concentration on
EPG electrodes were calculated from the total charge (QCV
155, 119, and 90 nC at pH 0, 4, and 7, respectively) passed at
the reductive CoIII/II in argon-sparged solutions, i.e., Γcat
=
=
QCV/nFA, where n was the number of electrons is 1, F was
Faraday constant, and A was electrode surface area. The above
charges yields values of Γcat of 2.1 × 10−11 mol cm−2, 1.6 ×
10−11 mol cm−2 and 1.2 × 10−11 mol cm−2, respectively, at pH
0, 4, and 7. The intercepts form K-L plots were used to
calculate the catalytic rate constants. The second-order
+
Next, we consider reasons why CoTPPNMe3 shows better
selectivity for reduction of O2 to H2O in comparison to the
three other Co-porphyrins. As noted in the Introduction, the
mechanism of ORR by Co-porphyrins has been proposed to
procced via reduction of a peroxide- or hydroperoxide-bound
intermediates.37 Our initial hypothesis was that the cationic
trimethylaniline group would stabilize these intermediates and
promote selectivity for reduction of O2 to H2O. To gain a
greater level of insight into this proposal, we carried out
quantum chemical calculations using the series of intermedi-
ates proposed by Anson.37 The complexes considered were
+
catalytic rate constant for CoTPPNMe3 drop-cast on EPG
were 1.1 × 106, 9.3 × 105, and 9.1 × 105 M−1 s−1 at pH 0, 4
and 7, respectively. These rates are very similar to those
reported for other heterogeneous Co-porphyrin catalysts.64,74
Strikingly, the rate constants are nearly insensitive to pH
between pH 0 and 7.
Turnover frequency (TOF)75,76 values were determined
using the kcat values given above and the concentration of O2 in
electrolytes under 1 atm of O2. A summary of different logTOF
for each pH studied as a function of overpotential are shown in
Figure 6. The logTOF values come from the different intercept
values from K-L plots (see Supporting Information). The
maximum values of TOF were observed between 0.8 and 1 V
overpotential for all pH values. Again, it is noteworthy that the
trends of logTOF versus overpotential are very similar across
pH values of 0 to 7. Analogous plots for CoTPP, CoTPPNH2,
and CoTPPNMe2 are not as informative since the product
+
2−
CoTPP, CoTPPNH2 and CoTPPNMe3 with O2, O2 and
,
HOO− ligands. Optimized structures are given in the
Supporting Information. Bond metrical parameters for
CoTPP were in accord with experiment,51,52 and those for
the CoTPP-O2 complex were similar to other computational
studies.77,78 Optimized structures and some key bond lengths
are shown in Figure 7. For simplicity, only CoTPPNH2 and
+
CoTPPNMe3 are shown in Figure 7. The results for CoTPP
are very similar to those for CoTPPNH2 (see Supporting
E
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complex. The subsequent protonation (3 → 4) of the
dianionic CoTPP-peroxide complex, to give the corresponding
Co-hydroperoxide complex, is more favorable for CoTPP and
CoTPPNH2, resulting in an energy that is relatively close to
+
that for the CoTPPNMe3 hydroperoxide complex. Proto-
+
nation of the CoTPPNMe3 -hydroperoxide complex to give
H2O2 is predicted to be unfavorable (4 → 5), while the
pathway to yield H2O is favored (4 → 6).
CONCLUSIONS
■
The development of catalysts for the oxygen reduction reaction
(ORR) continues to be a challenge. The results presented here
suggest that stabilization of intermediates through electrostatic
interactions can play distinct roles in determining catalyst
selectivity for reduction of O2. The ORR selectivity of iron
porphyrin catalysts can be improved by introduction of
hydrogen bonding interactions that influence the chemistry
of Fe(III)-OOH intermediates or O2 binding events.16,17,32
One important example is the ORR selectivity over a wide
range of pH values observed in a triazole-ferrocene modified
Fe-porphyrin, which likely arises from an interplay of local
hydrogen bonds and ferrocene-mediated ET.17 In contrast,
cobalt porphyrin ORR catalysts cannot access high valent
intermediates and therefore must proceed via a different
mechanism. This leads to our proposal that ORR selectivity of
Co-porphyrins can be improved by accounting for the
mechanistic steps that are distinct from Fe-porphyrins. Instead
of hydrogen bonding, electrostatic stabilization of anionic
intermediates (likely peroxide-bound Co(II)) by conjugate
acids of nitrogen bases, or trimethylanilinium (cationic) groups
is important. We demonstrated that the use of a
trimethylanilinium group proximal to the catalytic porphyrin-
cobalt site results in pH-independence of ORR rates and
overpotentials. We suggest that this could be an overlooked
design element for ORR catalysts, and potentially for other
redox catalysts. One example comes from the CO2 reduction
literature, where a cobalt phthalocyanine with a trimethylani-
linium group is an effective catalyst,79 though the mechanism
there is less clear because the NMe3+ group is not proximal to
the Co. In sum, the unique physical properties of cobalt
porphyrins mean that ORR occurs by a mechanism that is
different than for iron porphyrins. This, in turn, demands
different catalyst design elements. Ultimately, catalysts that
operate over a wider range of reaction conditions, such as
different pH ranges, can be envisioned by considering
electrostatic interactions of activated intermediates.
Figure 7. Optimized structures for CoTPPNH2 (left column) and
CoTPPNMe3+ right column. The top row shows dioxygen complexes,
the middle row shows peroxide complexes, and the bottom row show
hydroperoxide complexes. The O−O distances are given above the
oxygen ligands and the NH-O bond distances are given from
CoTPNH2.
Information). The CoTPPNMe3+ complex shows longer O−O
bond lengths in each of the complexes in Figure 7. Notably, the
O−O bond length in the hydroperoxide complex [HOO-
CoTPPNMe3]0 is longer than we calculated for H2O2 (1.472 Å
versus 1.466 Å). This greater degree of activation is consistent
+
with the observation the CoTPPNMe3 reduction of O2 to
H2O rather than H2O2.
Free energies were calculated for different reaction steps
(Figure 8) following methods described by Carter and co-
ASSOCIATED CONTENT
■
sı
* Supporting Information
Figure 8. Calculated relative free energy changes for O2 reduction by
CoTPPNH2 (red) and CoTPPNMe3 (blue).
The Supporting Information is available free of charge on the
ACS Publications Web site. The Supporting Information is
available free of charge at https://pubs.acs.org/doi/10.1021/
jacs.0c03861.
+
workers.56 The calculations suggest that hydrogen bond
between bound O2 and the anilino group slightly promotes
+
Additional experimental details and characterization
data, additional cyclic voltammograms and correspond-
ing analyses, controlled potential electrolysis traces,
details of all calculations, and comparisons of catalyst
performance under difference conditions (PDF)
the O2 binding step (1 → 2). Interestingly, CoTPPNMe3 is
calculated to have a higher affinity for O2, likely via ionic
•−
stabilization of bound superoxide (i.e., a formally Co(III)-O2
complex33−35). Two electron reduction (2 → 3) to give a
+
Co(II)-peroxide is more favorable for the CoTPPNMe3
F
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Article
́
(12) Costentin, C.; Dridi, H.; Saveant, J.-M. Molecular Catalysis of
O2 Reduction by Iron Porphyrins in Water: Heterogeneous versus
Homogeneous Pathways. J. Am. Chem. Soc. 2015, 137 (42), 13535−
13544.
AUTHOR INFORMATION
■
Corresponding Author
Jeffrey J. Warren − Simon Fraser University, Department of
Chemistry, Burnaby, BC V5A 1S6, Canada; orcid.org/
0000-0002-1747-3029; Email: j.warren@sfu.ca
(13) Matson, B. D.; Carver, C. T.; Von Ruden, A.; Yang, J. Y.;
Raugei, S.; Mayer, J. M. Distant Protonated Pyridine Groups in
Water-Soluble Iron Porphyrin Electrocatalysts Promote Selective
Oxygen Reduction to Water. Chem. Commun. 2012, 48 (90), 11100−
11102.
(14) Forshey, P. A.; Kuwana, T. Electrochemistry of Oxygen
Reduction. 4. Oxygen to Water Conversion by Iron(II)(Tetrakis(N-
Methyl-4-Pyridyl)Porphyrin) via Hydrogen Peroxide. Inorg. Chem.
1983, 22 (5), 699−707.
Author
Rui Zhang − Simon Fraser University, Department of Chemistry,
Burnaby, BC V5A 1S6, Canada
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c03861
(15) Rosenthal, J.; Nocera, D. G. Role of Proton-Coupled Electron
Transfer in O-O Bond Activation. Acc. Chem. Res. 2007, 40, 543−553.
(16) Ghatak, A.; Bhakta, S.; Bhunia, S.; Dey, A. Influence of the
Distal Guanidine Group on the Rate and Selectivity of O2 Reduction
by Iron Porphyrin. Chem. Sci. 2019, 10 (42), 9692−9698.
(17) Mittra, K.; Chatterjee, S.; Samanta, S.; Dey, A. Selective 4e−/
4H+ O2 Reduction by an Iron(Tetraferrocenyl)Porphyrin Complex:
From Proton Transfer Followed by Electron Transfer in Organic
Solvent to Proton Coupled Electron Transfer in Aqueous Medium.
Inorg. Chem. 2013, 52 (24), 14317−14325.
(18) Shi, C.; Anson, F. C. Cobalt Meso- Tetrakis(N -Methyl-4-
Pyridiniumyl)Porphyrin Becomes a Catalyst for the Electroreduction
of O2 by Four Electrons When [(NH3)5Os]n+ (n = 2, 3) Groups Are
Coordinated to the Porphyrin Ring. Inorg. Chem. 1996, 35 (26),
7928−7931.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Natural Sciences and Engineering Research Council
(NSERC, RGPIN 05559 to J.J.W.) and Simon Fraser
University are acknowledged for support. R.Z. is grateful for
Ph.D. scholarship support from the China Scholarship Council.
We are grateful for helpful discussions with Soumalya Sinha
and Tim Storr. Calculations were enabled in part by support
provided by (WestGrid) (www.westgrid.ca) and Compute
Canada (www.computecanada.ca)
(19) Shi, C.; Anson, F. C. Electrocatalysis of the Reduction of
Molecular Oxygen to Water by Tetraruthenated Cobalt Meso-
Tetrakis(4-Pyridyl)Porphyrin Adsorbed on Graphite Electrodes.
Inorg. Chem. 1992, 31 (24), 5078−5083.
(20) Collman, J. P. Synthetic Models for the Oxygen-Binding
Hemoproteins. Acc. Chem. Res. 1977, 10 (7), 265−272.
(21) Collman, J. P.; Fu, L. Synthetic Models for Hemoglobin and
Myoglobin. Acc. Chem. Res. 1999, 32, 455−463.
(22) Collman, J. P.; Devaraj, N. K.; Decreau, R. A.; Yang, Y.; Yan, Y.-
L.; Ebina, W.; Eberspacher, T. A.; Chidsey, C. E. D. A Cytochrome c
Oxidase Model Catalyzes Oxygen to Water Reduction Under Rate-
Limiting Electron Flux. Science 2007, 315 (5818), 1565−1568.
(23) Zhang, W.; Lai, W.; Cao, R. Energy-Related Small Molecule
Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen
Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based
Systems. Chem. Rev. 2017, 117 (4), 3717−3797.
(24) Sinha, S.; Aaron, M. S.; Blagojevic, J.; Warren, J. J.
Electrocatalytic Dioxygen Reduction by Carbon Electrodes Non-
covalently Modified with Iron Porphyrin Complexes: Enhancements
from a Single Proton Relay. Chem. - Eur. J. 2015, 21 (50), 18072−
18075.
(25) Song, E.; Shi, C.; Anson, F. C. Comparison of the Behavior of
Several Cobalt Porphyrins as Electrocatalysts for the Reduction of O 2
at Graphite Electrodes. Langmuir 1998, 14 (15), 4315−4321.
(26) Sinha, S.; Ghosh, M.; Warren, J. J. Changing the Selectivity of
O2 Reduction Catalysis with One Ligand Heteroatom. ACS Catal.
2019, 9 (3), 2685−2691.
(27) Yuasa, M.; Steiger, B.; Anson, F. C. Hydroxy-Substituted
Cobalt Tetraphenylporphyrins as Electrocatalysts for the Reduction of
O2. J. Porphyrins Phthalocyanines 1997, 1, 181−188.
REFERENCES
■
́
(1) Rich, P. R.; Marechal, A. The Mitochondrial Respiratory Chain.
Essays Biochem. 2010, 47, 1−23.
(2) Yoshikawa, S.; Shimada, A. Reaction Mechanism of Cytochrome
c Oxidase. Chem. Rev. 2015, 115 (4), 1936−1989.
̈
(3) Wikstrom, M.; Krab, K.; Sharma, V. Oxygen Activation and
Energy Conservation by Cytochrome c Oxidase. Chem. Rev. 2018,
118 (5), 2469−2490.
(4) Debe, M. K. Electrocatalyst Approaches and Challenges for
Automotive Fuel Cells. Nature 2012, 486 (7401), 43−51.
̀
(5) Jaouen, F.; Proietti, E.; Lefevre, M.; Chenitz, R.; Dodelet, J.-P.;
Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P. Recent Advances
in Non-Precious Metal Catalysis for Oxygen-Reduction Reaction in
Polymer Electrolyte Fuelcells. Energy Environ. Sci. 2011, 4 (1), 114−
130.
(6) Gewirth, A. A.; Varnell, J. A.; DiAscro, A. M. Nonprecious Metal
Catalysts for Oxygen Reduction in Heterogeneous Aqueous Systems.
Chem. Rev. 2018, 118 (5), 2313−2339.
(7) Pegis, M. L.; Wise, C. F.; Martin, D. J.; Mayer, J. M. Oxygen
Reduction by Homogeneous Molecular Catalysts and Electrocatalysts.
Chem. Rev. 2018, 118 (5), 2340−2391.
(8) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Efficient Reduction of
Dioxygen with Ferrocene Derivatives, Catalyzed by Metalloporphyr-
ins in the Presence of Perchloric Acid. Inorg. Chem. 1989, 28 (12),
2459−2465.
(9) Carver, C. T.; Matson, B. D.; Mayer, J. M. Electrocatalytic
Oxygen Reduction by Iron Tetra-Arylporphyrins Bearing Pendant
Proton Relays. J. Am. Chem. Soc. 2012, 134 (12), 5444−5447.
(10) Passard, G.; Dogutan, D. K.; Qiu, M.; Costentin, C.; Nocera, D.
G. Oxygen Reduction Reaction Promoted by Manganese Porphyrins.
ACS Catal. 2018, 8 (9), 8671−8679.
(11) Pegis, M. L.; Martin, D. J.; Wise, C. F.; Brezny, A. C.; Johnson,
S. I.; Johnson, L. E.; Kumar, N.; Raugei, S.; Mayer, J. M. Mechanism
of Catalytic O2 Reduction by Iron Tetraphenylporphyrin. J. Am.
Chem. Soc. 2019, 141 (20), 8315−8326.
(28) Yuasa, M.; Yamaguchi, A.; Oyaizu, K.; Fujito, Y.; Kitao, M.;
Sato, T. Cobaltporphyrin-Adsorbed Carbon Black: Highly Efficient
Electrocatalysts for Oxygen Reduction. Polym. Adv. Technol. 2005, 16
(9), 702−705.
(29) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Electroreduction of
O
to H O at Unusually Positive Potentials Catalyzed by the
2
2
Simplest of the Cobalt Porphyrins. Inorg. Chem. 1997, 36 (20), 4294−
4295.
́
(30) Costentin, C.; Saveant, J.-M. Towards an Intelligent Design of
Molecular Electrocatalysts. Nat. Rev. Chem. 2017, 1 (11), 0087.
G
https://dx.doi.org/10.1021/jacs.0c03861
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(31) Sengupta, K.; Chatterjee, S.; Samanta, S.; Dey, A. Direct
Observation of Intermediates Formed during Steady-State Electro-
catalytic O2 Reduction by Iron Porphyrins. Proc. Natl. Acad. Sci. U. S.
A. 2013, 110 (21), 8431−8436.
(32) Bhunia, S.; Rana, A.; Roy, P.; Martin, D. J.; Pegis, M. L.; Roy,
B.; Dey, A. Rational Design of Mononuclear Iron Porphyrins for
Facile and Selective 4e−/4H+ O2 Reduction: Activation of O-O Bond
by 2nd Sphere Hydrogen Bonding. J. Am. Chem. Soc. 2018, 140 (30),
9444−9457.
(33) Walker, F. A. Electron Spin Resonance Study of Coordination
to the Fifth and Sixth Positions of α,β,γ,δ-Tetra(p-Methoxyphenyl)-
Porphinatocobalt(II). J. Am. Chem. Soc. 1970, 92 (14), 4235−4244.
(34) Walker, F. Ann. Reactions of Monomeric Cobalt-Oxygen
Complexes. I. Thermodynamics of Reaction of Molecular Oxygen
with Five- and Six-Coordinate Amine Complexes of a Cobalt
Porphyrin. J. Am. Chem. Soc. 1973, 95 (4), 1154−1159.
(35) Collman, J. P.; Yan, Y.-L.; Eberspacher, T.; Xie, X.; Solomon, E.
I. Oxygen Binding of Water-Soluble Cobalt Porphyrins in Aqueous
Solution. Inorg. Chem. 2005, 44 (26), 9628−9630.
(50) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping
Function in Dispersion Corrected Density Functional Theory. J.
Comput. Chem. 2011, 32 (7), 1456−1465.
(51) Nascimento, B. F. O.; Pineiro, M.; Rocha Gonsalves, A. M.
d’A.; Ramos Silva, M.; Matos Beja, A.; Paixao, J. A. Microwave-
̃
Assisted Synthesis of Porphyrins and Metalloporphyrins: A Rapid and
Efficient Synthetic Method. J. Porphyrins Phthalocyanines 2007, 11
(02), 77−84.
(52) Stevens, E. D. Electronic Structure of Metalloporphyrins. 1.
Experimental Electron Density Distribution of (Meso-
Tetraphenylporphinato)Cobalt(II). J. Am. Chem. Soc. 1981, 103
(17), 5087−5095.
(53) Balabanov, N. B.; Peterson, K. A. Systematically Convergent
Basis Sets for Transition Metals. I. All-Electron Correlation
Consistent Basis Sets for the 3d Elements Sc-Zn. J. Chem. Phys.
2005, 123 (6), 064107.
(54) Balabanov, N. B.; Peterson, K. A. Basis Set Limit Electronic
Excitation Energies, Ionization Potentials, and Electron Affinities for
the 3d Transition Metal Atoms: Coupled Cluster and Multireference
Methods. J. Chem. Phys. 2006, 125 (7), 074110.
(55) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies,
Structures, and Electronic Properties of Molecules in Solution with
the C-PCM Solvation Model. J. Comput. Chem. 2003, 24 (6), 669−
681.
(56) Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.;
Carter, E. A. Mechanistic Contrasts between Manganese and
Rhenium Bipyridine Electrocatalysts for the Reduction of Carbon
Dioxide. J. Am. Chem. Soc. 2014, 136, 16285−16298.
(57) Keith, J. A.; Carter, E. A. Theoretical Insights into Pyridinium-
Based Photoelectrocatalytic Reduction of CO2. J. Am. Chem. Soc.
2012, 134 (18), 7580−7583.
(58) Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.;
Cohen, M. H.; Earhart, A. D.; Coe, J. V.; Tuttle, T. R. The Proton’s
Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from
Cluster-Ion Solvation Data. J. Phys. Chem. A 1998, 102 (40), 7787−
7794.
(59) Fifen, J. J.; Dhaouadi, Z.; Nsangou, M. Revision of the
Thermodynamics of the Proton in Gas Phase. J. Phys. Chem. A 2014,
118 (46), 11090−11097.
(60) Isse, A. A.; Gennaro, A. Absolute Potential of the Standard
Hydrogen Electrode and the Problem of Interconversion of Potentials
in Different Solvents. J. Phys. Chem. B 2010, 114 (23), 7894−7899.
(61) Mu, X. H.; Kadish, K. M. Oxidative Electrochemistry of Cobalt
Tetraphenylporphyrin under a CO Atmosphere. Interaction between
Carbon Monoxide and Electrogenerated [(TPP)Co]+ in Nonbonding
Media. Inorg. Chem. 1989, 28 (19), 3743−3747.
(62) Truxillo, L. A.; Davis, D. G. Electrochemistry of Cobalt
Tetraphenylporphyrin in Aprotic Media. Anal. Chem. 1975, 47 (13),
2260−2267.
(63) Zhu, X.-Q.; Li, Q.; Hao, W.-F.; Cheng, J.-P. Dissociation
Energies and Charge Distribution of the Co-NO Bond for Nitrosyl-
α,β,γ,δ-Tetraphenylporphinatocobalt(II) and Nitrosyl-α,β,γ,δ-
Tetraphenylporphinatocobalt(III) in Benzonitrile Solution. J. Am.
Chem. Soc. 2002, 124 (33), 9887−9893.
(64) Zhang, W.; Shaikh, A. U.; Tsui, E. Y.; Swager, T. M. Cobalt
Porphyrin Functionalized Carbon Nanotubes for Oxygen Reduction.
Chem. Mater. 2009, 21 (14), 3234−3241.
(65) Pasternack, R. F.; Cobb, M. A. The Substitution Reactions of a
Water Soluble Cobalt(III) Porphyrin with Thiocyanate as a Function
of PH. J. Inorg. Nucl. Chem. 1973, 35 (12), 4327−4339.
(66) Chan, R. J. H.; Su, Y. O.; Kuwana, T. Electrocatalysis of Oxygen
Reduction. 5. Oxygen to Hydrogen Peroxide Conversion by
Cobalt(II) Tetrakis(N-Methyl-4-Pyridyl)Porphyrin. Inorg. Chem.
1985, 24 (23), 3777−3784.
(67) March, J. Advanced Organic Chemistry; John Wiley and sons:
1985; Vol. 3rd Ed.
(68) Jackson, M. N.; Pegis, M. L.; Surendranath, Y. Graphite-
(36) Winkler, J. R.; Gray, H. B. Electronic Structures of Oxo-Metal
Ions. In Molecular Electronic Structures of Transition Metal Complexes I;
Mingos, D. M. P., Day, P., Dahl, J. P., Eds.; Structure and Bonding;
Springer: Berlin, Heidelberg, 2012; pp 17−28. DOI: 10.1007/
430_2011_55.
(37) Durand, R. R.; Anson, F. C. Catalysis of Dioxygen Reduction at
Graphite Electrodes by an Adsorbed Cobalt(Ii) Porphyrin. J.
Electroanal. Chem. Interfacial Electrochem. 1982, 134 (2), 273−289.
(38) Rigsby, M. L.; Wasylenko, D. J.; Pegis, M. L.; Mayer, J. M.
Medium Effects Are as Important as Catalyst Design for Selectivity in
Electrocatalytic Oxygen Reduction by Iron-Porphyrin Complexes. J.
Am. Chem. Soc. 2015, 137 (13), 4296−4299.
́
(39) Azcarate, I.; Costentin, C.; Robert, M.; Saveant, J.-M. Through-
Space Charge Interaction Substituent Effects in Molecular Catalysis
Leading to the Design of the Most Efficient Catalyst of CO2 -to-CO
Electrochemical Conversion. J. Am. Chem. Soc. 2016, 138 (51),
16639−16644.
(40) Martin, D. J.; Mercado, B. Q.; Mayer, J. M. Combining Scaling
Relationships Overcomes Rate versus Overpotential Trade-Offs in O2
Molecular Electrocatalysis. Sci. Adv. 2020, 6 (11), No. eaaz3318.
(41) Hanson, S. S.; Warren, J. J. Syntheses, Characterization, and
Electrochemical Behavior of Alkylated 2-(2′-Quinolylbenzimidazole)
Complexes of Rhenium (I). Can. J. Chem. 2018, 96 (2), 119−123.
(42) Nichols, E. M.; Derrick, J. S.; Nistanaki, S. K.; Smith, P. T.;
Chang, C. J. Positional Effects of Second-Sphere Amide Pendants on
Electrochemical CO
Reduction Catalyzed by Iron Porphyrins.
2
Chem. Sci. 2018, 9 (11), 2952−2960.
(43) Neese, F. The ORCA Program System. Wiley Interdiscip. Rev.:
Comput. Mol. Sci. 2012, 2, 73−78.
(44) Neese, F. Software Update: The ORCA Program System,
Version 4.0. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2018, 8 (1),
No. e1327.
(45) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E.
Climbing the Density Functional Ladder: Nonempirical Meta-
Generalized Gradient Approximation Designed for Molecules and
Solids. Phys. Rev. Lett. 2003, 91 (14), 146401.
(46) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient,
Approximate and Parallel Hartree-Fock and Hybrid DFT Calcu-
lations. A ‘Chain-of-Spheres’ Algorithm for the Hartree-Fock
Exchange. Chem. Phys. 2009, 356 (1), 98−109.
(47) Stoychev, G. L.; Auer, A. A.; Neese, F. Automatic Generation of
Auxiliary Basis Sets. J. Chem. Theory Comput. 2017, 13 (2), 554−562.
(48) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn.
Phys. Chem. Chem. Phys. 2006, 8 (9), 1057−1065.
(49) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and
Accurate Ab Initio Parametrization of Density Functional Dispersion
Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010,
132 (15), 154104.
Conjugated Acids Reveal a Molecular Framework for Proton-Coupled
H
https://dx.doi.org/10.1021/jacs.0c03861
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Electron Transfer at Electrode Surfaces. ACS Cent. Sci. 2019,
acscentsci.9b00114. DOI: 10.1021/acscentsci.9b00114.
̈
(69) Schofberger, W.; Faschinger, F.; Chattopadhyay, S.; Bhakta, S.;
Mondal, B.; Elemans, J. A. A. W.; Mullegger, S.; Tebi, S.; Koch, R.;
̈
Klappenberger, F.; Paszkiewicz, M.; Barth, J. V.; Rauls, E.; Aldahhak,
H.; Schmidt, W. G.; Dey, A. A Bifunctional Electrocatalyst for Oxygen
Evolution and Oxygen Reduction Reactions in Water. Angew. Chem.,
Int. Ed. 2016, 55 (7), 2350−2355.
(70) Chatterjee, S.; Sengupta, K.; Hematian, S.; Karlin, K. D.; Dey,
A. Electrocatalytic O
-Reduction by Synthetic Cytochrome c
2
Oxidase Mimics: Identification of a “Bridging Peroxo” Intermediate
Involved in Facile 4e−/4H+ O2-Reduction. J. Am. Chem. Soc. 2015,
137 (40), 12897−12905.
(71) Cunningham, I. D.; Bachelor, J. L.; Pratt, J. M. Kinetic Study of
the H2O2 Oxidation of Phenols, Naphthols and Anilines Catalysed by
the Haem Octapeptide Microperoxidase-8. J. Chem. Soc., Perkin Trans.
2 1991, 0 (11), 1839−1843.
(72) Howe, G. R.; Hiatt, R. R. Metal-Catalyzed Hydroperoxide
Reactions. I. Substituent Effects in the Oxidation of Aniline. J. Org.
Chem. 1970, 35 (12), 4007−4012.
(73) Sheng, M. N.; Zajacek, J. G. Hydroperoxide Oxidations
Catalyzed by Metals. II. Oxidation of Tertiary Amines to Amine
Oxides. J. Org. Chem. 1968, 33 (2), 588−590.
́
(74) Kadish, K. M.; Fremond, L.; Ou, Z.; Shao, J.; Shi, C.; Anson, F.
C.; Burdet, F.; Gros, C. P.; Barbe, J.-M.; Guilard, R. Cobalt(III)
Corroles as Electrocatalysts for the Reduction of Dioxygen: Reactivity
of a Monocorrole, Biscorroles, and Porphyrin-Corrole Dyads. J. Am.
Chem. Soc. 2005, 127 (15), 5625−5631.
́
(75) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M. Turnover
Numbers, Turnover Frequencies, and Overpotential in Molecular
Catalysis of Electrochemical Reactions. Cyclic Voltammetry and
Preparative-Scale Electrolysis. J. Am. Chem. Soc. 2012, 134 (27),
11235−11242.
(76) Boudart, M. Turnover Rates in Heterogeneous Catalysis. Chem.
Rev. 1995, 95 (3), 661−666.
(77) Shi, Z.; Zhang, J. Density Functional Theory Study of
Transitional Metal Macrocyclic Complexes’ Dioxygen-Binding
Abilities and Their Catalytic Activities toward Oxygen Reduction
Reaction. J. Phys. Chem. C 2007, 111 (19), 7084−7090.
(78) Sun, S.; Jiang, N.; Xia, D. Density Functional Theory Study of
the Oxygen Reduction Reaction on Metalloporphyrins and Metal-
lophthalocyanines. J. Phys. Chem. C 2011, 115 (19), 9511−9517.
́
(79) Wang, M.; Torbensen, K.; Salvatore, D.; Ren, S.; Joulie, D.;
Dumoulin, F.; Mendoza, D.; Lassalle-Kaiser, B.; Isci, U.; Berlinguette,
̧
C. P.; Robert, M. CO2 Electrochemical Catalytic Reduction with a
Highly Active Cobalt Phthalocyanine. Nat. Commun. 2019, 10 (1),
1−8.
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