Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers

Article abstract of DOI:10.1002/anie.202102523

Achieving a selective 2 e^? or 4 e^? oxygen reduction reaction (ORR) is critical but challenging. Herein, we report controlling ORR selectivity of Co porphyrins by tuning only steric effects. We designed Co porphyrin 1 with meso-phenyls each bearing a bulky ortho-amido group. Due to the resulted steric hinderance, 1 has four atropisomers with similar electronic structures but dissimilar steric effects. Isomers αβαβ and αααα catalyze ORR with n=2.10 and 3.75 (n is the electron number transferred per O₂), respectively, but ααββ and αααβ show poor selectivity with n=2.89–3.10. Isomer αβαβ catalyzes 2 e^? ORR by preventing a bimolecular O₂ activation path, while αααα improves 4 e^? ORR selectivity by improving O₂ binding at its pocket, a feature confirmed by spectroscopy methods, including O K-edge near-edge X-ray absorption fine structure. This work represents an unparalleled example to improve 2 e^? and 4 e^? ORR by tuning only steric effects without changing molecular and electronic structures.

Full text of DOI:10.1002/anie.202102523

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International Edition: doi.org/10.1002/anie.202102523
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Electrocatalysis
Hot Paper
Controlling Oxygen Reduction Selectivity through Steric Effects:
Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction
with Cobalt Porphyrin Atropisomers
Bin Lv+, Xialiang Li+, Kai Guo, Jun Ma, Yanzhi Wang, Haitao Lei, Fang Wang, Xiaotong Jin,
Qingxin Zhang, Wei Zhang, Ran Long,* Yujie Xiong, Ulf-Peter Apfel, and Rui Cao*
Abstract: Achieving a selective 2eꢀ or 4eꢀ oxygen reduction
reaction (ORR) is critical but challenging. Herein, we report
controlling ORR selectivity of Co porphyrins by tuning only
steric effects. We designed Co porphyrin 1 with meso-phenyls
each bearing a bulky ortho-amido group. Due to the resulted
steric hinderance, 1 has four atropisomers with similar
electronic structures but dissimilar steric effects. Isomers
abab and aaaa catalyze ORR with n = 2.10 and 3.75 (n is
the electron number transferred per O2), respectively, but aabb
and aaab show poor selectivity with n = 2.89–3.10. Isomer
abab catalyzes 2eꢀ ORR by preventing a bimolecular O2
activation path, while aaaa improves 4eꢀ ORR selectivity by
improving O2 binding at its pocket, a feature confirmed by
spectroscopy methods, including O K-edge near-edge X-ray
absorption fine structure. This work represents an unparalleled
example to improve 2eꢀ and 4eꢀ ORR by tuning only steric
effects without changing molecular and electronic structures.
a promising fuel.[8–10] Recently, many molecular ORR cata-
lysts have been reported.[1–5,11–28] Although achievements have
been made in improving the 4eꢀ ORR by providing rapid
electron and proton transfers,[17,29,30] introducing hydrogen-
bonding[31,32] and electrostatic interactions,[33] and using
dinuclear cooperation,[34–38] realizing these functionalities is
challenging from both design and synthesis points of view.
Unlike 4eꢀ ORR, few studies have been dedicated to
improving the 2eꢀ ORR.[27,39,40] Thus, achieving 2eꢀ and 4eꢀ
ORR in a controllable manner is very rare, especially using
one catalyst system.
In principle, O2 tends to be reduced by 2eꢀ at single Co
sites (Scheme 1a), because one Co ion is not efficient to
ꢀ
mediate O O bond cleavage to form Co-oxo species, which is
high in energy.[41] However, Co complexes can also catalyze
4eꢀ ORR through a bimolecular path (Scheme 1a).[42,43] Due
to the presence of both paths, Co complexes usually show
poor or moderate selectivity for either 2eꢀ or 4eꢀ ORR. For
examples, Co porphyrins have been largely studied as ORR
catalysts by displaying high activity but poor selectivity.[36] To
improve selectivity, extensive efforts have been made to
synthesize Co porphyrin cages with isolated Co sites[40] for
2eꢀ ORR and dinuclear Co porphyrins[34–38] for 4eꢀ ORR, in
which peroxo-bridged Co porphyrins are proposed as inter-
mediates (Scheme 1b).
O
2 can be reduced by two electrons (2eꢀ) to H2O2 or by four
electrons (4eꢀ) to H2O.[1–3] The 4 eꢀ ORR is required in many
energy conversion techniques,[4–7] while the 2eꢀ ORR makes
H2O2, which is a widely used industrial oxidant and also
[*] B. Lv,[+] Dr. X. L. Li,[+] K. Guo, Y. Z. Wang, Dr. H. T. Lei, F. Wang,
X. T. Jin, Q. X. Zhang, Prof. Dr. W. Zhang, Prof. Dr. R. Cao
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of
Education, School of Chemistry and Chemical Engineering,
Shaanxi Normal University
Xi’an 710119 (China)
E-mail: ruicao@snnu.edu.cn
J. Ma, Prof. Dr. R. Long, Prof. Dr. Y. J. Xiong
Hefei National Laboratory for Physical Sciences at the Microscale,
Collaborative Innovative Center of Chemistry for Energy Materials
(iChEM), School of Chemistry and Materials Science, and National
Synchrotron Radiation Laboratory,
Scheme 1. a) Earlier proposed 2eꢀ and 4eꢀ ORR paths with Co
catalysts. b) Proposed peroxo-bridged bimolecular Co porphyrin
species.
University of Science and Technology of China
Hefei, Anhui 230026 (China)
E-mail: longran@ustc.edu.cn
Prof. Dr. U.-P. Apfel
Ruhr-Universitꢀt Bochum, Fakultꢀt fꢁr Chemie und Biochemie,
Anorganische Chemie I
Universitꢀtsstrasse 150, 44801 Bochum (Germany)
and
Fraunhofer UMSICHT
Herein, we report achieving 2eꢀ and 4eꢀ ORR in
a controllable manner by tuning only steric features of
a catalyst system. Co porphyrin 1 was used (Figure 1). Due to
the steric hinderance of the bulky ortho-amido group at each
meso-phenyl substituent, 1 has four atropisomers with similar
electronic structures but dissimilar steric effects (Fig-
ure 1b).[44] Isomers abab and aaaa display significantly
improved 2eꢀ and 4eꢀ ORR, respectively, but aabb and
aaab have poor ORR selectivity. Note that aaaa has been
Osterfelder Strasse 3, 46047 Oberhausen (Germany)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.202102523.
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ratio of 1067.4363 in high-resolution mass spectrometry
(HRMS, Figure S4) and displayed characteristic Soret
(419 nm) and Q (535 nm) bands in its UV-vis spectrum in
tetrahydrofuran (THF, Figure S10a).[48] These results con-
firmed its identity and purity.
Significantly, the abab configuration was confirmed (Fig-
ure 2a). X-ray studies showed that abab crystallized in the
tetragonal space group I41/a. Each Co is incorporated by
a porphyrin ring through four N atoms. An aqueous molecule
axially binds to Co with a long Co···O distance of 2.331(15) ꢀ.
ꢀ
The four Co N bond distances are identical 1.959(3) ꢀ. The
CoII state was suggested based on bond valence sum and
charge balance calculations. The abab configuration with
alternating up and down orientations of the four bulky amido
groups creates large steric encumbrance on both sides of the
porphyrin ring.
For isomers aabb, aaab, and aaaa, their identity and
purity were confirmed by HRMS (Figures S5–S7) and UV-vis
spectra (Figures S10b–S10d). Identical UV-vis spectra of
these atropisomers suggest their very similar electronic
structures. We structurally characterized the Co-free forms
of aabb (Figure 2b) and aaaa (Figure 2c). In the structure of
Co-free aabb, two close amido groups orientate upward,
while the other two orientate downward. In the structure of
Co-free aaaa, all four amido groups orientate upward,
creating a pocket. In addition, we made Co porphyrin 2
(Scheme S2, Figures S9 and S10e). As the position isomer of
1, the four amido groups of 2 are located at the para-position
of its meso-phenyls, leading to tiny steric hinderance on both
porphyrin sides.
Figure 1. a) Co porphyrins 1 and 2, showing their different substitution
positions at meso-phenyls. b) Molecular structures of the four atrop-
isomers of 1, namely abab, aabb, aaab and aaaa.
The redox features of these Co porphyrins were first
studied in dimethylformamide (DMF, 0.1 M Bu4NPF6) under
N2. As shown in Figure 3a and Table 1, all five Co porphyrins
displayed two well-defined reversible redox waves, which
were attributed to formal CoII/I and CoI/0 redox couples,
respectively.[48,49] Importantly, their redox potentials for CoII/I
(ꢀ1.06 to ꢀ1.08 V versus ferrocene) and CoI/0 (ꢀ2.16 to
ꢀ2.18 V versus ferrocene) are very close to each other,
confirming their very similar electronic structures.
Next, we loaded Co porphyrins on carbon black (CB) for
ORR studies. The resulted hybrids were analyzed by scanning
electron microscopy and transmission electron microscope
(Figure S11), showing no aggregated particles of Co porphyr-
ins. The cyclic voltammogram (CV) of abab@CB on a glassy
carbon (GC) electrode dis-
studied previously for O2 binding,[45–47] but its electrocatalytic
ORR has been rarely studied. In addition, we made Co
porphyrin 2 as a substituent position isomer of 1, which also
displayed poor ORR selectivity. Our results underline the
critical role of steric effects in controlling ORR selectivity.
Therefore, without changing molecular and electronic struc-
tures, this work represents an unparalleled example to
improve 2eꢀ and 4eꢀ ORR by tuning only steric effects of
a catalyst system.
The Co-free forms of the four atropisomers of 1 were first
synthesized (Scheme S1) and characterized (Figures S1–S3).
Next, their corresponding Co complexes were each isolated
and purified. Isomer abab showed an ion at a mass-to-charge
played a catalytic wave in
0.1 M KOH solution under
O2 (Figure 3b). Similar
electrocatalytic behaviors
were observed with the
other three atropisomers
of 1 and with 2 (Figur-
es S13–S16). Note that cat-
alytic ORR waves of all
these Co porphyrins have
identical onset potentials,
which is consistent with
their similar electronic
structures.
Figure 2. Thermal ellipsoid plots (50% probability) of the X-ray structures of abab (a), Co-free aabb (b), and
Co-free aaaa (c).
2
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Figure 3. a) CVs of 0.5 mM abab, aabb, aaab, aaaa, and 2 in DMF
with a scan rate of 50 mVsꢀ1. b) CVs of GC electrode loaded with
abab@CB under O2 and N2 in 0.1 M KOH solution at a scan rate of
50 mVsꢀ1. c) LSVs of RRDE disk electrode loaded with abab@CB,
aabb@CB, aaab@CB, aaaa@CB, and 2@CB under O2 in 0.1 M
KOH with a scan rate of 5 mVsꢀ1
.
Table 1: Co porphyrin redox couple E1/2 potentials (versus ferrocene)
under N2.
Figure 4. ORR RRDE data with abab@CB (a), aabb@CB (b),
aaab@CB (c), and aaaa@CB (d). The ORR n value of the five Co
porphyrins loaded on CB (e) and on CNT (f) as determined from
RRDE. Conditions: GC disk electrode (area 0.125 cm2), Pt ring
electrode (area 0.188 cm2), scan rate: 10 mVsꢀ1, 208C.
Complex
abab
aabb
aaab
aaaa
2
E1/2 of CoII/I
E1/2 of CoI/0
ꢀ1.08
ꢀ2.18
ꢀ1.07
ꢀ2.16
ꢀ1.08
ꢀ2.17
ꢀ1.07
ꢀ2.18
ꢀ1.06
ꢀ2.17
aabb, aaab, and 2, they displayed poor selectivity for either
2eꢀ or 4eꢀ ORR with n = 2.89–3.10, which is commonly
observed for Co porphyrins without tethered functional
groups to assist electron and proton transfers and to stabilize
O2-adduct through hydrogen-bonding and electrostatic inter-
actions. As discussed above, due to the presence of both 2eꢀ
and 4eꢀ paths (Scheme 1a), Co porphyrins usually display
poor or moderate ORR selectivity.
For abab, due to large steric encumbrance on both
porphyrin sides, the bimolecular path is blocked. As a result,
abab displays high 2eꢀ ORR selectivity. For aaaa, its
improved 4eꢀ ORR is likely due to improved O2 binding at
the Co site. In principle, O2 can bind in and out of the pocket
of aaaa. Previous studies showed that aaaa could bind an O2
molecule in the pocket, when the out-of-the-pocket site was
occupied by an imidazole group.[45–47] This axial imidazole can
improve O2 binding and activation through a so-called “push
effect”.
However, no imidazole axially binds to Co of aaaa in our
case. We therefore determined the O2-binding site. Upon
reacting with O2 in THF, abab showed slight changes in UV-
vis (Figure 5a). Unlike abab, the UV-vis spectrum of aaaa
changed significantly after reacting with O2 (Figure 5b),
showing the disappearance of the bands at 419 and 535 nm
and the appearance of new bands at 428 and 544 nm. These
results indicate the binding and activation of O2 at CoII sites to
give CoIII-superoxo species.[50–52] Electron paramagnetic res-
The number (n) of electrons transferred per O2 molecue
was determined by rotating-ring disk electrode (RRDE)
measurements. As shown in Figure 3c, the linear sweep
voltammogram (LSV) of abab@CB displayed large catalytic
ORR currents with the half-wave potential E1/2 = 0.73 V
versus reversible hydrogen electrode (RHE, all potentials
reported in aqueous solutions are referenced to RHE). For
the other three atropisomers and 2, their ORR waves had
similar E1/2 values of 0.72–0.73 V. However, aaaa@CB and
abab@CB displayed the largest and the smallest ORR
current, respectively, while the currents with aabb@CB,
aaab@CB and 2@CB are moderate and are close to each
other. This result indicates that these Co porphyrins have
different ORR selectivity.
The n values were then determined (Figures 4a to d),
giving n = 2.10 for abab@CB, n = 3.75 for aaaa@CB, n =
2.89–3.10 for aabb@CB, aaab@CB, and 2@CB (Figures 4e).
By using carbon nanotubes (CNTs) as supports, the same
n values were obtained (Figure 4 f), indicating that the ORR
selectivity of these Co porphyrins does not change when
loaded on different carbon materials. In addition, Koutecky–
Levich (K–L) analysis gave similar n values as RRDE
obtained for all Co porphyrins loaded on CB and CNTs
(Figures S18 to S27).
Our results show that all five Co porphyrins have similar
ORR activities, but they display different selectivities. For
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peptide (-CONH-), including the 1s!1p* transition of the
carbonyl O at 534.2 eV and the s* resonance at 538–542 eV.
Importantly, the spectrum under O2 showed two additional
shoulder peaks at 531 and 545.4 eV for p* and s* peak,
respectivley. This result suggests the effective chemisorption
of O2 in aaaa.[57] The small but significant shoulder peak next
to the chemisorpted O2 shifts to lower energy, indicating an
ꢀ
enlarged O O bond distance, which is consistent with
electron transfer from CoII to O2 to form superoxo species.
In conclusion, we report controlling ORR selectivity by
tuning only steric effects. Five Co porphyrins show identical
ORR activity but different selectivity. With large steric
encumbrance to prevent bimolecular path, abab catalyzes
2eꢀ ORR; with improved O2 binding in the pocket, aaaa
catalyzes 4eꢀ ORR; without these steric effects, aabb, aaab
and 2 showed poor selectivity for either 2eꢀ or 4eꢀ ORR. This
work is significant to achieve 2eꢀ and 4eꢀ ORR by tuning
only steric effects without changing molecular and electronic
structures and is valuable for catalyst design with the aim to
understand ORR mechanisms and to further improve catalyst
performance.
Acknowledgements
We are grateful for support from National Natural Science
Foundation of China (21773146), Fok Ying-Tong Education
Foundation for Outstanding Young Teachers in University,
Fundamental Research Funds for the Central Universities,
and Research Funds of Shaanxi Normal University. U.-P.A. is
grateful for financial support from Fraunhofer Internal
Programs (Attract 097-602175) and Deutsche Forschungs-
gemeinschaft (Emmy Noether grant AP242/2-1; under Ger-
Figure 5. UV-vis spectra of abab (a) and aaaa (b) under N2 and O2
in THF at room temperature. EPR spectra of abab (c) and aaaa (d)
under N2 and O2 in THF at 100 K. Infrared (e) and NEXAFS (f) spectra
of aaaa under N2 and O2.
onance (EPR) spectra of abab (Figure 5c) and aaaa (Fig-
ure 5d) under N2 are identical, further supporting the same
coordination and electronic structures of their CoII
sites.[27,53,54] However, abab under O2 showed both CoII and
CoIII-superoxo signals in EPR,[53,55,56] while aaaa under O2
showed only CoIII-superoxo signals in EPR. These results
suggest that aaaa has a stronger O2-binding affinity than
abab. Moreover, the infrared spectrum of abab under N2 and
O2 showed tiny difference (Figure S28), but significant
changes were observed for aaaa under N2 and O2 at amide-
manyꢁs Excellence Strategy – EXC 2033–390677874 –
RESOLV). NEXAFS measurements were performed at the
beamline MCD-A and MCD-B of NSRL in Hefei, China.
Conflict of interest
The authors declare no conflict of interest.
Keywords: atropisomers · molecular electrocatalysis ·
ꢀ
related resonances (Figure 5e). The changes of amide N H
oxygen reduction · selectivity control · steric effect
=
and C O resonances indicated potential hydrogen-bonding
interactions between O2-adduct and the amide groups in the
pocket of aaaa.
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4
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Manuscript received: February 18, 2021
Accepted manuscript online: March 19, 2021
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Electrocatalysis
Cobalt porphyrin atropisomers with sim-
ilar electronic structures but dissimilar
steric effects were synthesized. One
isomer is selective for the 2eꢀ ORR, one
is selective for the 4eꢀ ORR, while the
other two show poor selectivity for either
2eꢀ or 4eꢀ ORR.
B. Lv, X. L. Li, K. Guo, J. Ma, Y. Z. Wang,
H. T. Lei, F. Wang, X. T. Jin, Q. X. Zhang,
W. Zhang, R. Long,* Y. J. Xiong,
U.-P. Apfel, R. Cao*
&&&& — &&&&
Controlling Oxygen Reduction Selectivity
through Steric Effects: Electrocatalytic
Two-Electron and Four-Electron Oxygen
Reduction with Cobalt Porphyrin
Atropisomers
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!