Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin-porphyrin dyad

Article abstract of DOI:10.1039/c9sc05041h

Pacman dinuclear Co^II triphenylporphyrin-tri(pentafluorophenyl)porphyrin 1 and dinuclear Co^II bis-tri(pentafluorophenyl)porphyrin 2, anchored at the two meso-positions of a benzene linker, are synthesized and examined as electrocatalysts for the oxygen reduction reaction (ORR). Both dinuclear Co bisporphyrins are more efficient and selective than corresponding mononuclear Co^II tetra(pentafluorophenyl)porphyrin 3 and Co^II tetraphenylporphyrin 4 for the four-electron electrocatalytic reduction of O₂ to water. Significantly, although the ORR selectivities of the two dinuclear Co bisporphyrins are similar to each other, 1 outperforms 2, in terms of larger catalytic ORR currents and lower overpotentials. Electrochemical studies showed different redox behaviors of the two Co sites of 1: the Co^III/Co^II reduction of the Co-TPP (TPP = triphenylporphyrin) site is well-behind that of the Co-TPFP (TPFP = tri(pentafluorophenyl)porphyrin) site by 440 mV. This difference indicated their different roles in the ORR: Co^II-TPFP is likely the O₂ binding and reduction site, while Co^III-TPP, which is generated by the oxidation of Co^II-TPP on electrodes, may function as a Lewis acid to assist the O₂ binding and activation. The positively charged Co^III-TPP will have through-space charge interactions with the negatively charged O₂-adduct unit, which will reduce the activation energy barrier for the ORR. This effect of Co-TPP closely resembles that of the Cu_B site of metalloenzyme cytochrome c oxidase (CcO), which catalyzes the biological reduction of O₂. This work represents a rare example of asymmetrical dinuclear metal catalysts, which can catalyze the 4e reduction of O₂ with high selectivity and significantly improved activity.

Full text of DOI:10.1039/c9sc05041h

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Significantly improved electrocatalytic oxygen
reduction by an asymmetrical Pacman dinuclear
cobalt(II) porphyrin–porphyrin dyad†
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have been paid for by the Royal Society
of Chemistry
ab
a
b
a
c
Yanju Liu, Guojun Zhou, Zongyao Zhang, Haitao Lei, Zhen Yao,
c
b
a
Jianfeng Li,
Jun Lin
and Rui Cao
*
II
II
Pacman dinuclear Co triphenylporphyrin-tri(pentafluorophenyl)porphyrin 1 and dinuclear Co bis-
tri(pentafluorophenyl)porphyrin 2, anchored at the two meso-positions of a benzene linker, are
synthesized and examined as electrocatalysts for the oxygen reduction reaction (ORR). Both dinuclear
II
Co bisporphyrins are more efficient and selective than corresponding mononuclear Co
II
tetra(pentafluorophenyl)porphyrin
electrocatalytic reduction of O
3
and Co
tetraphenylporphyrin
4
for the four-electron
2
to water. Significantly, although the ORR selectivities of the two
dinuclear Co bisporphyrins are similar to each other, 1 outperforms 2, in terms of larger catalytic ORR
currents and lower overpotentials. Electrochemical studies showed different redox behaviors of the two
III
II
Co sites of 1: the Co /Co reduction of the Co-TPP (TPP ¼ triphenylporphyrin) site is well-behind that
of the Co-TPFP (TPFP ¼ tri(pentafluorophenyl)porphyrin) site by 440 mV. This difference indicated their
II
III
different roles in the ORR: Co -TPFP is likely the O
2
binding and reduction site, while Co -TPP, which is
II
generated by the oxidation of Co -TPP on electrodes, may function as a Lewis acid to assist the O
2
III
binding and activation. The positively charged Co -TPP will have through-space charge interactions with
the negatively charged O
effect of Co-TPP closely resembles that of the Cu
which catalyzes the biological reduction of O . This work represents a rare example of asymmetrical
2
-adduct unit, which will reduce the activation energy barrier for the ORR. This
Received 7th October 2019
Accepted 3rd November 2019
B
site of metalloenzyme cytochrome c oxidase (CcO),
2
DOI: 10.1039/c9sc05041h
dinuclear metal catalysts, which can catalyze the 4e reduction of O
2
with high selectivity and
rsc.li/chemical-science
significantly improved activity.
Among these studies, synthetic modeling of cytochrome c
Introduction
12–16
oxidase (CcO) has attracted particular attention.
CcO
Efficient and selective catalysts for the oxygen reduction reac- belongs to a superfamily of heme/Cu oxidases and catalyzes the
tion (ORR) are required in many kinds of energy conversion and selective biological reduction of O to water at the dinuclear
storage devices, such as fuel cells and metal–air batteries, heme a and Cu site, which is the destination of electrons from
because the kinetically slow ORR is the key cathode reaction in the electron transport chain in respiration. The dinuclear
2
3
B
17
1–7
these systems. Research on molecular catalysts is signicant heme/Cu site, in cooperation with the surrounding structures,
to provide fundamental knowledge on reaction mechanisms including the tyrosine residue in the vicinity, the distal pocket,
8
–11
and structure–function relationships,
the development of new catalysts with improved performance. play crucial roles in minimizing the release of partially reduced
3
which is essential for and also the trans ligand of the heme a Fe ion, is suggested to
4,18
I
oxygen species. Signicantly, the Cu state of the Cu ion can
B
II
provide an electron to reduce O , and moreover, its Cu state
2
a
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an
can function as a Lewis acid to facilitate the O binding and
2
16
activation at the heme site (Scheme 1).
As inspired by nature, metal porphyrins have been exten-
7
b
10119, China. E-mail: ruicao@ruc.edu.cn
Department of Chemistry, Renmin University of China, Beijing 100872, China
College of Materials Science and Optoelectronic Technology, University of Chinese
4,19–33
c
sively studied as ORR catalysts.
In addition to biologically
Academy of Science, Beijing 101408, China
relevant Fe porphyrins, Co porphyrins have also received great
Electronic supplementary information (ESI) available: Fig. S1–S27, Table S1, and attention because of their high activity and stability.
19–26,32–34
†
crystallographic data in CIF format for complexes
2 and 5. The X-ray
However, mononuclear Co porphyrins are much less selective
crystallographic coordinates for reported structures in this article have been
deposited at the Cambridge Crystallographic Data Centre (CCDC), under
deposition number CCDC 1956352 and 1956353. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9sc05041h
than Fe analogues to catalyze the 4e reduction of O to water.
2
Unlike Fe complexes, Co complexes in general catalyze the 2e
reduction of O
2
because the peroxo intermediates of Co are
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the ORR with both high activity and selectivity (Scheme 1). The
II
presence of a positively charged Cu ion can decrease the acti-
vation energy barrier for the formation of the negatively charged
4,16,17
O -adduct unit at the heme site.
2
Similar through-space
charge interaction effects have been demonstrated for
synthetic model complexes to catalyze articial small molecule
54,55
activation reactions,
2 2
including O reduction and CO
reduction. In order to develop ORR catalysts with satisfactory
Scheme 1 Illustration of the O
heme a and Cu site of CcO.
3 B
2
binding and activation at the dinuclear activity and selectivity, we herein report the synthesis and
II
catalytic features of asymmetrical Pacman dinuclear Co
triphenylporphyrin-tri(pentauorophenyl)porphyrin 1. Our
results show that 1 is signicantly more efficient and selective
II
challenging to undergo heterolytic O–O bond cleavage to than corresponding mononuclear Co tetra(pentauorophenyl)
II
generate terminal Co-oxo species that have large d electron porphyrin 3 and Co tetraphenylporphyrin 4 to catalyze the 4e
3
5–37
counts and thus are high in energy.
Several molecular reduction of O2 to water, and more notably, it outperforms
II
design strategies have been reported to improve the selectivity symmetrical dinuclear Co bis-tri(pentauorophenyl)porphyrin
for the 4e ORR, including (1) formation of intramolecular 2, in terms of both larger catalytic ORR currents and lower
hydrogen bonding interactions to stabilize O
2
7,30,38,39
II
2
adducts,
overpotentials. Electrochemical studies indicated that in 1, Co -
1
9,28,29,40
III
(
2) appending acid groups to assist proton delivery,
and TPFP is likely the O binding and reduction site, while Co -TPP
2
(
3) introduction of functional groups to ensure rapid electron may function as a positively charged Lewis acid to assist the O2
34,41–44
transfer between catalyst molecules and electrodes.
binding and activation. This work represents a rare example of
Additionally, for complexes of late-transition metals, such as asymmetrical dinuclear metal catalysts with signicantly
Co, it is found that dinuclear complexes are typically more improved performance for electrocatalytic 4e reduction of O2.
efficient than mononuclear analogues to catalyze the 4e
20–26,45–53
reduction of O2.
the formation of peroxo-bridged dinuclear intermediates,
whose subsequent reduction can result in the 4e ORR. Despite
This improvement is likely caused by
Results
Synthesis and characterization
45,47
the improvement in the selectivity, however, from mononuclear We choose a benzene unit to link two porphyrin moieties at its
to dinuclear complexes, the enhancement of catalytic activity, in two meso-positions (Fig. 1a). The resulting Pacman structure
terms of both catalytic currents and overpotentials, is unsatis- will enable the two porphyrin moieties to have a face-to-face
factory. In general, asymmetrical dinuclear complexes are rarely conguration, which is considered to be suitable for the
48,50,51
20,21
reported,
while symmetrical dinuclear complexes are binding and activation of an O2 molecule.
The synthetic
usually designed and studied with the two metal ions bearing routes to bisporphyrins and corresponding metal complexes are
very similar coordination environments. This will lead to depicted in Fig. 2, and the details of synthetic conditions and
parallel redox behaviors of the two metal ions. In other words, characterizations are described in the ESI.† As shown in Fig. 2,
the two metal ions will be reduced at close potentials for O
2
methyl 2-formylbenzoate was used for the synthesis of inter-
binding and activation. This will weaken the Lewis acid role of mediate a to introduce the rst tri(pentauorophenyl)porphyrin
the second metal ion. Therefore, few dinuclear metal complexes macrocycle (Fig. S2 and S3†). In order to reduce methyl
can catalyze the ORR with both high activity and selectivity.
benzoate to benzyl alcohol, we rst treated a with lithium
As mentioned above, in nature, the Lewis acidic and redox aluminium hydride (LiAlH , route 1). However, in addition to
4
active features of the Cu site are essential for CcO to catalyze the reduction of methyl benzoate, we found that the uorine
B
Fig. 1 (a) Molecular structures of 1 and 2. (b) Thermal ellipsoid plot of the X-ray structure of 2 (30% probability).
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Fig. 2 Synthetic routes to dinuclear metal complexes of 1, 2, and 5.
1
0
atom at the para-position of the three pentauorophenyl units by using H NMR spectroscopy and HRMS. In HRMS, 1 displays
is also replaced by a hydrogen atom, leading to the formation of an ion with a mass-to-charge ratio of 1421.3137 (Fig. S16†),
1
intermediate b. This undesired reduction was conrmed by H matching the calculated value of 1421.3131 for the monocation
NMR spectroscopy and high-resolution mass spectrometry of [1 + H ] ; 2 displays an ion with a mass-to-charge ratio of
HRMS). In H NMR, these three hydrogen atoms can be found 1691.1699 (Fig. S19†), matching the calculated value of
0
+ +
0
1
(
0
+ +
with chemical shis d ¼ 7.61 (Fig. S4†). The HRMS of b shows 1691.1718 for the monocation of [2 + H ] .
0
an ion with a mass-to-charge ratio of 861.1532 (Fig. S5†). This
Reaction of 1 with cobalt chloride CoCl
2
gave dinuclear Co
value matches the calculated value of 861.1518 for the mono- bisporphyrin 1 with a yield of 87%. The UV-vis spectrum of 1 in
+
+
cation of [b + H ] with identical isotopic distribution. Subse- dimethylformamide shows a characteristic Soret band at
quent oxidation of benzyl alcohol to benzaldehyde (Fig. S6 and 412 nm and a Q band at 560 nm (Fig. S21†). These bands are in
S7†), and the next conversion to the second tri(penta- accordance with those from corresponding mononuclear Co

uorophenyl)porphyrin macrocycle resulted in bisporphyrin porphyrins 3 and 4. In HRMS, 1 shows an ion with a mass-to-
0
ligand 5 (Fig. S8 and S9†). Attempts to obtain high quality charge ratio of 1535.1461 (Fig. S17†). This value matches the
single crystals of the dinuclear Co complex of 5 were unsuc- calculated value of 1535.1437 for the monocation of [1 + H ]
cessful. However, we were able to get the single crystal X-ray with identical isotopic distribution. Reaction of 2 with CoCl
0
+ +
0
2
structure of its dinuclear Ni analogue (see below), which resulted in 2 with a yield of 85%. The UV-vis spectrum of 2 is
clearly shows the replacement of three para-F atoms by H similar to that of 1, showing a Soret band at 401 nm and a Q
atoms.
band at 545 nm (Fig. S21†). The HRMS of 2 shows an ion with
In order to prevent this undesired replacement, we chose a mass-to-charge ratio of 1805.0038 (Fig. S20†), matching the
+
+
diisobutylaluminum hydride (DIBAL-H) to reduce intermediate calculated value of 1805.0023 for the monocation of [2 + H ] . All
a. With the use of this relatively weak hydride-donating reagent, these results are in agreement with the expected structures for 1
we were able to selectively convert methyl benzoate to benzyl and 2. Notably, we were able to characterize the structure of 2 by
alcohol, giving intermediate d. Subsequent oxidation of benzyl using the single crystal X-ray diffraction method (see below),
alcohol to benzaldehyde leads to the formation of intermediate which conrmed the dinuclear Co bisporphyrin structure.
1
e. Both d and e are fully characterized by H NMR (Fig. S11 and
S13,† respectively) and HRMS (Fig. S12 and S14,† respectively),
conrming the preservation of the three pentauorophenyl
Crystallographic studies
units during the treatment with DIBAL-H. Starting from e, we Although we were not able to structurally characterize complex
0
can synthesize the asymmetrical bisporphyrin ligand
1
1, its key structural information could be obtained from crys-
0
(
Fig. S15,† route 2) and symmetrical analogue 2 (Fig. S18,† tallographic studies of its analogues 2 and 5. Single crystals of 2
route 3). The identity and purity of both 1 and 2 are conrmed that are suitable for X-ray structure analysis were obtained by
0
0
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slow vapor diffusion of hexane to a toluene solution of 2 at room mentioned above. Each Ni ion is located at the center of
temperature for two weeks. Complex 2 crystallized in the a porphyrin ligand with Ni–N bond distances ranging from
ꢀ
˚
˚
triclinic space group P1 with Z ¼ 2. Crystal data and structure 1.931(4) A to 1.947(4) A. The distorted square-planar coordina-
8
renement details can be found in Table S1.† In the X-ray tion geometry of Ni and the BVS calculation all suggest the d
structure of 2, two symmetrical TPFP moieties are connected Ni electronic conguration. The Ni/Ni distance is 6.182 A. In
II
˚
together at the two meso-positions of a benzene linker, the Pacman cle of 5, there is a pentane solvent molecule,
providing a Pacman conguration (Fig. 1b). In each TPFP which is located at the angle bisector of the two porphyrin
moiety, the three pentauorophenyl substituents are well macrocycles.
resolved. Particularly, the three para-F atoms can be well
dened. This result further conrms the preservation of the Electrochemical and electrocatalytic ORR studies
three pentauorophenyl units during synthesis. Each TPFP
We next measured the cyclic voltammograms (CVs) of dinuclear
ligand coordinates a Co ion at the center of the porphyrin
and mononuclear Co complexes in benzonitrile using a glassy
macrocycle through four N atoms. The Co–N bond distances are
carbon (GC) working electrode. The CV of mononuclear Co
˚
˚
in the range of 1.908(2) A to 2.007(2) A. There are no axial
ligands on the Co ions, leading to a distorted square-planar
coordination geometry.
II
III
porphyrin 3 shows a Co -to-Co oxidation wave at Ep,a ¼ 0.43 V
versus ferrocene (all potentials reported in benzonitrile in this
work are referenced to ferrocene) and its corresponding
7
The bond valence sum (BVS) calculation suggested the d
Co oxidation state. This result is consistent with the charge
balance calculation because the porphyrin ligands are doubly
negatively charged and no more counter anions are found in the
X-ray structure of 2. In addition, we measured the electron
paramagnetic resonance (EPR) spectrum of 1 and 2 in toluene
reduction wave at E ¼ ꢀ0.29 V (Fig. 3b). This irreversibility of
II
p,c
III
II
the Co /Co redox couple is indicative of the binding and
dissociation of solvent benzonitrile on the Co ion during the
III
II
56
Co /Co conversion. Additionally, there is a reversible 1e
II
reduction wave at E1/2 ¼ ꢀ1.13 V, which is assigned to the Co /
I
II
III
Co redox couple. For mononuclear complex 4, the Co -to-Co
(
Fig. S22† and 3a, respectively). The axial spectra conrmed the
oxidation wave is at ꢀ0.03 V, and its two corresponding
II
S ¼ 1/2 low spin states of the four-coordinated Co porphyrin
complexes with g ¼ 2.26. The parallel components display
a septet superhyperne splitting pattern, which is due to the
II
reduction waves are at ꢀ0.15 and ꢀ0.56 V. The reversible Co /
I
Co redox couple is found at E ¼ ꢀ1.38 V. As compared to 3,
1
/2
II
III
the Co -to-Co oxidation wave of 4 is shied to the cathodic
direction by 460 mV. This large cathodic shi is caused by the
replacement of four strong electron-withdrawing penta-
II
interaction of Co with N atoms. The Co/Co distance is 5.762
A˚ . Interestingly, in the Pacman cle of 2, there is a toluene
solvent molecule, which is located at and is parallel to the angle
bisector of the two porphyrin macrocycles.
Moreover, we obtained high-quality single crystals of dinu-
clear Ni bisporphyrin 5 by slow vapor diffusion of pentane to
a chloroform solution of 5 at room temperature for two weeks.
56

uorophenyl units by four phenyl units.
Dinuclear Co complex 1 displays two Co -to-Co oxidation
waves at Ep,a ¼ 0.43 and ꢀ0.03 V using the GC working electrode
in benzonitrile. These values are identical to those represented
by mononuclear complexes 3 and 4, respectively. On the basis of
II
III
ꢀ
Complex 5 crystallized in the triclinic space group P1 with Z ¼ 2
this comparison, we can assign the oxidation wave at E
¼
p,a
(
see Table S1† for crystal data and structure renement details).
II
III
0
.43 V to the Co -to-Co process of Co-TPFP, and assign the
The structure of 5 closely resembles that of 2 (Fig. S23†), except
that, in one TPFP moiety, the para-F atom of the three penta-
uorophenyl substituents is each replaced by a para-H atom.
II
III
oxidation wave at Ep,a ¼ ꢀ0.03 V to the Co -to-Co process of
Co-TPP. There are two reversible 1e reduction waves at E1/2
¼

II
I
ꢀ
1.25 and ꢀ1.43 V, which can be assigned to the Co /Co redox
This result is consistent with the ndings in synthesis as
couple of Co-TPFP and Co-TPP, respectively. For dinuclear Co
Fig. 3 (a) EPR spectrum of 2 in toluene at 90 K. (b) CVs of 0.5 mM benzonitrile solutions of 1 and 2, and 1.0 mM benzonitrile solutions of 3 and 4.
ꢀ1
4 6
Conditions: GC working electrode, 0.1 M (Bu N)PF benzonitrile solution, scan rate 50 mV s .
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II
III
complex 2, there are two close Co -to-Co oxidation waves at improved activity, dinuclear Co complexes 1 and 2 are also more
Ep,a ¼ 0.45 and 0.30 V. The two reversible 1e reduction waves of selective than mononuclear analogues 3 and 4 to catalyze the 4e
2
are overlapped with E ¼ ꢀ1.25 V. These results suggest that ORR. Moreover, the n value can be evaluated using the Kou-
1
/2
the electrochemical behaviors of the two Co porphyrin moieties tecky–Levich (K–L) method in RDE measurements (Fig. 5c, S26
in dinuclear complexes 1 and 2 are mainly determined by the and S27†), giving n ¼ 3.70 for 1, 3.60 for 2, 2.60 for 3, and 2.60
electronic structures of the porphyrin macrocycles and that the for 4. These values are in good agreement with those calculated
interaction between the two porphyrin moieties is weak, which from RRDE experiments. Under applied potentials of 0.10–
ꢀ
1
is consistent with the large Co/Co distance as revealed by 0.40 V with catalyst 1, the linear plots of j (j ¼ current density)
ꢀ1/2
crystallographic studies.
on u
(u ¼ the rotation rate) and the near-parallelism of
The electrocatalytic ORR features of complexes 1–4 loaded these tting lines suggested a rst-order reaction dependence
on carbon nanotubes were examined in 0.50 M H SO aqueous on dissolved O concentrations and similar ORR n values under
2
4
2
solutions at room temperature. Their CVs under N2 and O2 different potentials (Fig. 5d).
clearly show that all four complexes are active for the ORR The improved selectivity of dinuclear Co complexes for the
Fig. 4 and S24†). Signicantly, the CV of 1 displayed a large 4e ORR shows that the Pacman structure constructed at the two
(
catalytic wave with the half-wave potential E1/2 ¼ 0.72 V versus meso-positions of a benzene linker enables efficient cooperation
the reversible hydrogen electrode (RHE, all potentials reported between the two Co ions. For late-transition metals, such as Co,
in aqueous solutions in this work are referenced to RHE) the electrostatic repulsion between their d electrons and the p
(Fig. 4a). This value suggests that 1 represents the state-of-the- electrons of the oxo ligand will make terminal late-transition
art electrocatalytic ORR performance among reported molec- metal-oxo species unfavorable from an energy point of view.
ular catalysts operating under similar acidic conditions (see As a consequence, mononuclear Co complexes in general cata-
Table 1 for details). For complex 2, the ORR half-wave potential lyze the 2e ORR, because the key peroxo intermediates are
E
0
1/2 is 0.55 V (Fig. 4b), while for 3 and 4, these values are E1/2
.46 V and 0.41 V, respectively. Two conclusions can be made generate high-energy terminal Co-oxo species. Nevertheless, the
from these results. First, mononuclear complex 3 is superior to bimetallic mechanism with the formation of peroxo-bridged
for ORR catalysis, which is a result of the strong electron- dinuclear Co complexes is likely to happen. Subsequent
¼
challenging to undergo heterolytic O–O bond cleavage to
4
withdrawing pentauorophenyl substituents in 3. Second, homolytic O–O bond cleavage can lead to the improved 4e ORR.
dinuclear Co catalysts are more efficient than mononuclear
analogues, and 1 outperforms all other Co catalysts examined in
this work. Considering that complex 2 contains more penta-
Discussion

uorophenyl substituents than 1, the much higher efficiency of
is noteworthy.
The ORR selectivity was then evaluated by determining the
Despite that dinuclear Co complexes 1 and 2 are both highly
selective for the 4e reduction of O , the superior activity of 1 is
noteworthy. Strong electron-withdrawing groups, such as the
1
2
number of electrons n transferred per O
rotating disk electrode (RDE) and rotating ring-disk electrode
RRDE) measurements. In the RRDE, the partially reduced
oxygen species (i.e., H O ) that is possibly produced during the
2
molecule using
meso-pentauorophenyl substituents, can cause large shis of
the redox couples to the anodic direction and thus are usually
(
58,59
favored for ORR catalysis,
because this effect will lead to the
2
2
formation of catalytically active species at relatively smaller
ORR at the GC disk electrode can be detected at the Pt ring
electrode (Fig. 5a and S25†). According to the current densities
detected at the disk electrode and the ring electrode, the n value
can be determined to be 3.90 for 1, 3.80 for 2, 2.90 for 3, and
overpotentials. Complex 2 has two Co-TPFP units, while
III
complex 1 has one Co-TPFP unit and one Co-TPP unit. The Co /
II
Co reduction of the Co-TPP site of 1 is well behind that of the
Co-TPFP site by 440 mV, which is in agreement with the
replacement of three pentauorophenyl groups by three phenyl
2.90 for 4 (Fig. 5b). This result shows that in addition to much
Fig. 4 CVs of GC electrodes loaded with 5 mL ink of 1 (a) and 2 (b) under N
electrode, 50 mV s scan rate, 20 C.
2
(black) and O
2
(red) in 0.5 M H SO
2 4
solution. Conditions: GC working
ꢀ1
ꢁ
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Table 1 Comparison of catalytic performances of Co-porphyrin ORR electrocatalysts
a
Catalyst category
Catalyst
Solution
0.5 M H
h at E1/2 (V)
n
Ref.
Dinuclear Co bisporphyrin dyads
1
2
2
SO
4
0.51
0.70
0.65
0.66
0.75
0.74
0.84
0.70
0.87
0.93
0.94
0.77
0.81
0.60
0.61
0.61
0.64
0.64
0.52
0.53
0.54
0.58
0.59
0.67
0.67
0.66
0.60
0.64
0.66
0.69
0.66
0.63
0.67
0.73
0.83
3.9
3.8
3.7
3.8
3.6
3.5
3.5
3.9
3.2
2.3
2.2
2.0
1.9
3.4
2.4
2.9
3.4
3.1
3.9
3.7
3.7
3.5
3.1
2.5
2.4
2.5
2.8
2.8
2.6
2.6
2.6
2.5
2.8
2.9
2.9
This work
21 and 22
20
Co
2
Co
2
Co
2
Co
2
(DPX)
(DPD)
(DPXM)
(DPDM)
0.5 M HClO
4
4
and 1.5 M CF
and 1.5 M CF
3
3
COOH
COOH
0.5 M HClO
Ox-Co
Oxa-Co
Benzo-Co
0.5 M H
2
SO
4
25
Co
2
Co
2
Co
2
Co
2
PP
0
PP
2
PP
5
PP
8
pH 6.8 phosphate buffer
57
48
Dinuclear Co biscorrole dyads
(BCA)Co
2
1 M HClO
4
4
(
(
(
(
BCB)Co
BCX)Co
BCO)Co
BCS)Co
2
2
2
2
Dinuclear Co porphyrin-corrole dyads
(PCA)Co
2
1 M HClO
48
50
51
(
(
(
(
(
(
(
PCB)Co
PCX)Co
2
2
PCO)Co
PCO )Co
2
X
2
PMes
PMes
PMes
2
2
2
CX)Co
CO)Co
CO )Co
2
2
X
2
Heterobimetallic Fe/Mn porphyrin-Co
corrole dyads
(PCA)FeClCoCl
(PCO)FeClCoCl
1 M HClO
4
(
(
(
(
(
PCX)FeClCoCl
PCB)FeClCoCl
PCX)MnClCoCl
PCO)MnClCoCl
PCB)MnClCoCl
Mononuclear Co porphyrin
3
4
0.5 M H
2
SO
4
This work
a
Half-wave overpotential. Although the overpotentials for the 2e and 4e ORR are different, for simplicity, we treated all entries as the 4e ORR for
overpotential calculation.
groups. Although the two Co sites in 2 are more easily reduced reaction with an O molecule will generate the peroxo-bridged
2
II
III
to the catalytically active Co state, the half-wave overpotential dinuclear Co intermediate. The different redox behaviors of
of 2 is larger than that of 1 by 165 mV. Additionally, to reach the 1 and 2 indicated that the Co -TPP unit is likely responsible for
III
plateau currents, the potential required by 2 is larger than that the stabilization of the O
of 1 by >300 mV. These results are in sharp contrast to the fact ORR performance of 1.
2
adduct and thus for the improved
III
ꢀ
that 2 has more electron withdrawing groups.
In addition to assisting the formation of formally Co –O c
2
III
On the basis of these results, we propose that, upon reduc- species, the positively charged Co -TPP unit in the vicinity will
II
tion to the Co state, the Co-TPFP site of 1 becomes catalytically also benet the subsequent 1e reduction of the superoxo unit to
4
,48,50
III
2ꢀ
III
active for binding and activating the O molecule,
while at generate peroxo, leading to the formation of Co –O2 –Co
2
III
this stage, the Co-TPP site of 1 still maintains the Co oxidation species. Note that there are two possible pathways for this 1e
II
III
state (Fig. 6). The reaction of Co -TPFP and O
2
leads to the reduction: the superoxo may be reduced directly, or the Co -
This negatively TPP is rst reduced to its Co oxidation state and then transfers
-adduct unit is likely to interact with the positively this electron to the superoxo. Aer the formation of peroxo, for
charged Co -TPP unit. Such through-space charge interaction both 1 and 2, the peroxo-bridged dinuclear Co intermediates
effects have been demonstrated to assist the formation and will be further reduced by two electrons to break the O–O bonds.
III
ꢀ
60,61
II
formation of a formally Co –O
charged O
2
c
species.
2
III
III
54,55
stabilization of charged intermediates.
As a consequence,
Comparing complex 1 with reported dinuclear Co bispor-
III
the Co -TPP unit is suggested to be able to decrease the acti- phyrins shows the state-of-the-art ORR performance of 1 (Table
vation energy barrier for the binding and activation of O in the 1). The n value catalyzed with 1 is 3.90. This value is larger than
2
Pacman cle of 1. However, for complex 2, its two Co-TPFP sites the n values obtained with previous dinuclear Co bisporphyrins.
II
are reduced to the Co state at close potentials. Subsequent In addition to the increased selectivity, the ORR half-wave
92 | Chem. Sci., 2020, 11, 87–96
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2 2 4
Fig. 5 (a) RRDE measurements for the ORR at the GC disk electrode coated with 1 (black), 2 (red), and 3 (blue) in an O -saturated 0.5 M H SO
solution at 2500 rpm. The ring electrode was polarized at 1.0 V. (b) n value of the ORR with 1 (black), 2 (red), and 3 (blue). (c) RRDE measurements
for the ORR with 1 in O -saturated 0.5 M H SO solution at various rotation rates. (d) K–L plots for the ORR with 1. Conditions: GC disk electrode
area 0.125 cm ), Pt ring electrode (area 0.188 cm ), 10 mV s scan rate, 20 C.
2
2
4
2
2
ꢀ1
ꢁ
(
overpotential of 510 mV with 1 is also smaller than the half-wave relatively larger half-wave overpotentials for the ORR. By using
overpotentials obtained with previous dinuclear Co bispor- electrochemical and spectroscopic methods, Kadish and co-
phyrins. These results clearly demonstrate that 1 is superior to workers established that the catalytically active species of Co
other dinuclear Co bisporphyrins in catalyzing the 4e ORR with corroles for the ORR are generated at relatively more anodic
48–50
higher activity and selectivity.
potentials than Co porphyrins with similar substituents.
As
Comparison of 1 with other dinuclear catalysts, including a consequence, dinuclear catalysts, which bear Co corrole
dinuclear Co biscorrole dyads, dinuclear Co porphyrin-corrole moieties, in general will display smaller half-wave over-
dyads, and heterobimetallic Fe/Mn porphyrin-Co corrole potentials than dinuclear Co bisporphyrins. Therefore, it is
dyads, is also made (Table 1). Three conclusions can be ob- remarkable that the ORR performance of dinuclear Co bispor-
tained from this table. First, heterobimetallic Fe/Mn porphyrin- phyrin 1 (hE1/2 ¼ 510 mV, n ¼ 3.9) is almost identical to that of
Co corrole dyads show quite poor selectivity for the 4e ORR, the dinuclear Co porphyrin-corrole dyad (PCA)Co
2
(h
E
¼
1/2
although their half-wave overpotentials are modest among all 520 mV, n ¼ 3.9, Table 1), in terms of both activity and
catalysts. Second, dinuclear Co biscorroles show moderate selectivity.
selectivity for the 4e ORR, indicating the presence of parallel 4e
In summary, we report the synthetic procedures and elec-
and 2e ORR pathways. Third, dinuclear Co porphyrin-corrole trocatalytic ORR features of asymmetrical Pacman dinuclear Co
dyads display both high activity and selectivity for the 4e ORR. porphyrin–porphyrin dyad 1. Complex 1 is highly efficient and
For dinuclear Co bisporphyrins, only complex 1 can show selective to catalyze the 4e ORR in aqueous solutions, and its
comparable performance to the dinuclear Co porphyrin-corrole performance is superior to those of corresponding mono-
dyads, while other dinuclear Co bisporphyrins all show nuclear Co porphyrins and also all other symmetrical dinuclear
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nitrogen cooling system. The EPR spectra were measured at 90
K with a modulation amplitude of 0.6 mT, a modulation
frequency of 100 kHz, and a microwave power of 6.325 mW.
Typically, ꢂ5 mg of sample of complex 1 or 2 was transferred
into an EPR tube in a dry box. A toluene solution with
a concentration of 1.63 mM for 1 and 1.39 mM for 2 is used for
measurements.
Crystallographic studies
Complete datasets for 2 (CCDC 1956352) and 5 (CCDC 1956353)
were collected. Single crystals suitable for X-ray analysis were
each coated with Paratone-N oil, suspended in a small ber
loop, and placed at 150(2) K on a Bruker D8 Venture X-ray
diffractometer. Diffraction intensities were measured using
˚
graphite monochromated Mo Ka radiation (l ¼ 0.71073 A). Data
collection, indexing, data reduction, and nal unit cell rene-
62
ments were carried out using APEX2; absorption corrections
63
were applied using the program SADABS. The structure was
6
4
2
solved by direct methods using SHELXS and rened against F
65
Fig. 6 Proposed catalytic cycle for the ORR with 1. Complex 1 will be on all data by full-matrix least-squares using SHELXL,
III
III
instantly oxidized to the Co Co oxidation state on the electrode
following established renement strategies. In the X-ray struc-
ture of 2 and 5, all non-hydrogen atoms were rened aniso-
tropically. All hydrogen atoms binding to carbon were included
in the model at geometrically calculated positions and rened
using a riding model. The isotropic displacement parameters of
all hydrogen atoms were xed to 1.2 times the U value of the
atoms they are linked to (1.5 times for methyl groups). Details of
the data quality and a summary of the residual values of the
renements are listed in ESI Table S1.†
upon applying large positive potentials, and this 2e-oxidized species
highlighted with a red box) is the real starting catalyst.
(
Co bisporphyrins reported in this work and in the literature.
The signicantly improved performance is due to the asym-
metrical structure, in which one Co porphyrin moiety functions
as a positively charged Lewis acid to assist the binding and
activation of the negatively charged O
man cle. This work demonstrates the benet of asymmetrical
dinuclear metal catalysts in electrocatalytic reduction of O , and
2
-adduct unit at its Pac-
Electrochemical studies
2
this catalyst design strategy may be valuable to be explored in All electrochemical experiments were carried out using a CH
other small molecule activation reactions.
Instruments electrochemical analyzer (model CHI660E) at room
temperature. The solution was bubbled with N
2
gas for at least
3
Bu
0 min before analysis. CVs were recorded in benzonitrile (0.1 M
2
4
NPF
6
) using a three-compartment cell possessing a 0.07 cm
Experimental section
General materials and methods
GC electrode as the working electrode, graphite rod as the
auxiliary electrode, and Ag/AgNO3 as the reference electrode
Mononuclear Co porphyrins 3 and 4 were synthesized according (BASi, 10 mM AgNO
3
, 0.1 M Bu
4 6
NPF in acetonitrile). Ferrocene
19,24
to reported procedures.
Synthetic details and characteriza- was added at the end of the measurement as an internal stan-
tion of dinuclear Co porphyrins 1 and 2 and also dinuclear Ni dard. In aqueous solutions, an Ag/AgCl (KCl saturated) elec-
porphyrin 5 are described in the ESI.† All reagents were trode was used as the reference electrode. All potentials
purchased from commercial suppliers and were used as measured in aqueous solutions are referenced to the reversible
received. Dry solvents, including dimethylformamide, hydrogen electrode (RHE). The GC working electrode was pol-
dichloromethane, acetonitrile, and tetrahydrofuran, were puri- ished with a-Al O of decreasing sizes (1.0 mm to 50 nm) and
2
3

ed by passage through activated alumina. Manipulations of air washed with distilled water and absolute ethanol. For rotating
and moisture-sensitive materials were performed under ring-disk electrode measurements, a bipotentiostat (Model CHI
nitrogen gas using standard Schlenk line techniques. All 832 Electrochemical Analyzer) and a rotating ring-disk electrode
aqueous solutions were prepared freshly using Milli-Q water. with a rotating GC disk electrode and a platinum ring electrode
NMR measurements were made on a Br u¨ ker spectrometer (ALS RRDE-2) were used. The disk electrode was scanned from
operating at 400 MHz. High-resolution mass spectra were 1.20 to 0 V, while the ring potential was held at 1.15 V to detect
acquired using a Br u¨ ker Fourier transform ion cyclotron reso-
2 2
H O that was possibly generated during the ORR. The collec-
nance mass spectrometer APEX IV. UV-vis spectra were recorded tion efficiency of the ring-disk electrode was evaluated with the
3
ꢀ/4ꢀ
using a Cary 50 spectrophotometer. EPR measurements were [Fe(CN)6]
redox couple and was calculated to be 0.47. Stock
carried out on a Bruker EMX plus 10/12 CW X-band EPR spec- dimethylformamide solutions (2 mL) were prepared by mixing
trometer at a microwave frequency of 9.43 GHz using a liquid multi-walled carbon nanotubes (2 mg), Naon (20 mL 5%), and
94 | Chem. Sci., 2020, 11, 87–96
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Co porphyrin catalysts with the nal concentration of 0.1 mM 16 T. Chishiro, Y. Shimazaki, F. Tani, Y. Tachi, Y. Naruta,
for 1 and 2 and 0.2 mM for 3 and 4. The mixture solution was
sonicated for 30 min, and then well-mixed solutions were
S. Karasawa, S. Hayami and Y. Maeda, Angew. Chem., Int.
Ed., 2003, 42, 2788–2791.
applied to the surface of the GC (5 mL) and the GC disk (10 mL) 17 S. Yoshikawa and A. Shimada, Chem. Rev., 2015, 115, 1936–
electrodes. Evaporation of the solvents at room temperature
1989.
produced thin catalyst lms.
18 S. M. Adam, G. B. Wijeratne, P. J. Rogler, D. E. Diaz,
D. A. Quist, J. J. Liu and K. D. Karlin, Chem. Rev., 2018,
1
18, 10840–11022.
Conflicts of interest
1
9 R. McGuire Jr, D. K. Dogutan, T. S. Teets, J. Suntivich,
Y. Shao-Horn and D. G. Nocera, Chem. Sci., 2010, 1, 411–414.
0 C. J. Chang, Z. H. Loh, C. N. Shi, F. C. Anson and
D. G. Nocera, J. Am. Chem. Soc., 2004, 126, 10013–10020.
The authors declare no competing nancial interests.
2
Acknowledgements
21 C. J. Chang, Y. Q. Deng, C. N. Shi, C. K. Chang, F. C. Anson
and D. G. Nocera, Chem. Commun., 2000, 1355–1356.
We are grateful for support from the “Thousand Talents
Program” of China, Fok Ying-Tong Education Foundation for
Outstanding Young Teachers in University, National Natural
Science Foundation of China (Grants 21101170, 21573139, and
2
2
2
2
2
2 S. Fukuzumi, K. Okamoto, C. P. Gros and R. Guilard, J. Am.
Chem. Soc., 2004, 126, 10441–10449.
3 S. Fukuzumi, K. Okamoto, Y. Tokuda, C. P. Gros and
R. Guilard, J. Am. Chem. Soc., 2004, 126, 17059–17066.
4 A. N. Oldacre, A. E. Friedman and T. R. Cook, J. Am. Chem.
Soc., 2017, 139, 1424–1427.
5 A. N. Oldacre, M. R. Crawley, A. E. Friedman and T. R. Cook,
Chem.–Eur. J., 2018, 24, 10984–10987.
6 P. Peljo, L. Murtom ¨a ki, T. Kallio, H. J. Xu, M. Meyer,
C. P. Gros, J. M. Barbe, H. H. Girault, K. Laasonen and
K. Kontturi, J. Am. Chem. Soc., 2012, 134, 5974–5984.
21773146), Fundamental Research Funds for the Central
Universities, and Research Funds of Shaanxi Normal University.
References
1
2
3
4
5
M. L. Pegis, C. F. Wise, D. J. Martin and J. M. Mayer, Chem.
Rev., 2018, 118, 2340–2391.
S. Dey, B. Mondal, S. Chatterjee, A. Rana, S. Amanullah and 27 B. D. Matson, C. T. Carver, A. Von Ruden, J. Y. Yang,
A. Dey, Nat. Rev. Chem., 2017, 1, 0098.
Y. Nie, L. Li and Z. D. Wei, Chem. Soc. Rev., 2015, 44, 2168–
S. Raugei and J. M. Mayer, Chem. Commun., 2012, 48,
11100–11102.
28 C. T. Carver, B. D. Matson and J. M. Mayer, J. Am. Chem. Soc.,
2012, 134, 5444–5447.
29 M. L. Rigsby, D. J. Wasylenko, M. L. Pegis and J. M. Mayer, J.
Am. Chem. Soc., 2015, 137, 4296–4299.
2
201.
W. Zhang, W. Z. Lai and R. Cao, Chem. Rev., 2017, 117, 3717–
797.
Z. Z. Liang, X. Fan, H. T. Lei, J. Qi, Y. Y. Li, J. P. Gao,
3
M. L. Huo, H. T. Yuan, W. Zhang, H. P. Lin, H. Q. Zheng 30 K. Mittra, S. Chatterjee, S. Samanta, K. Sengupta,
and R. Cao, Angew. Chem., Int. Ed., 2018, 57, 13187–13191.
K. Mase, M. Yoneda, Y. Yamada and S. Fukuzumi, Nat.
Commun., 2016, 7, 11470.
J. W. Han, J. Jung, Y.-M. Lee, W. Nam and S. Fukuzumi,
Chem. Sci., 2017, 8, 7119–7125.
H. Bhattacharjee and A. Dey, Chem. Commun., 2012, 48,
10535–10537.
6
7
8
9
31 G. Passard, D. K. Dogutan, M. T. Qiu, C. Costentin and
D. G. Nocera, ACS Catal., 2018, 8, 8671–8679.
32 I. Hijazi, T. Bourgeteau, R. Cornut, A. Morozan, A. Filoramo,
J. Leroy, V. Derycke, B. Jousselme and S. Campidelli, J. Am.
Chem. Soc., 2014, 136, 6348–6354.
H. T. Lei, X. L. Li, J. Meng, H. Q. Zheng, W. Zhang and R. Cao,
ACS Catal., 2019, 9, 4320–4344.
M. Guo, Y.-M. Lee, R. Gupta, M. S. Seo, T. Ohta, H.-H. Wang, 33 Q. Zuo, G. Z. Cheng and W. Luo, Dalton Trans., 2017, 46,
H.-Y. Liu, S. N. Dhuri, R. Sarangi, S. Fukuzumi and W. Nam,
9344–9348.
J. Am. Chem. Soc., 2017, 139, 15858–15867.
0 S. L. Hooe, A. L. Rheingold and C. W. Machan, J. Am. Chem.
Soc., 2018, 140, 3232–3241.
1 Y. J. Liu, Y. Z. Han, Z. Y. Zhang, W. Zhang, W. Z. Lai, Y. Wang
and R. Cao, Chem. Sci., 2019, 10, 2613–2622.
2 J. P. Collman, N. K. Devaraj, R. A. Decr ´e au, Y. Yang, Y. L. Yan,
34 L. N. Ye, Y. Y. Fang, Z. P. Ou, S. L. Xue and K. M. Kadish,
Inorg. Chem., 2017, 56, 13613–13626.
35 B. Wang, Y.-M. Lee, W.-Y. Tcho, S. Tussupbayev, S.-T. Kim,
Y. Kim, M. S. Seo, K.-B. Cho, Y. Dede, B. C. Keegan,
T. Ogura, S. H. Kim, T. Ohta, M.-H. Baik, K. Ray, J. Shearer
and W. Nam, Nat. Commun., 2017, 8, 14839.
1
1
1
W. Ebina, T. A. Eberspacher and C. E. D. Chidsey, Science, 36 S. Liu, K. Mase, C. Bougher, S. D. Hicks, M. M. Abu-Omar
007, 315, 1565–1568.
and S. Fukuzumi, Inorg. Chem., 2014, 53, 7780–7788.
3 J. P. Collman, R. A. Decr ´e au, H. W. Lin, A. Hosseini, Y. Yang, 37 S. Fukuzumi, Y.-M. Lee and W. Nam, ChemCatChem, 2018,
2
1
A. Dey and T. A. Eberspacher, Proc. Natl. Acad. Sci. U. S. A.,
009, 106, 7320–7323.
10, 9–28.
2
38 S. Bhunia, A. Rana, P. Roy, D. J. Martin, M. L. Pegis, B. Roy
and A. Dey, J. Am. Chem. Soc., 2018, 140, 9444–9457.
39 K. Sengupta, S. Chatterjee, S. Samanta and A. Dey, Proc. Natl.
Acad. Sci. U. S. A., 2013, 110, 8431–8436.
1
1
4 J. P. Collman, L. Fu, P. C. Herrmann and X. M. Zhang,
Science, 1997, 275, 949–951.
5 S. Chatterjee, K. Sengupta, S. Hematian, K. D. Karlin and
A. Dey, J. Am. Chem. Soc., 2015, 137, 12897–12905.
This journal is © The Royal Society of Chemistry 2020
Chem. Sci., 2020, 11, 87–96 | 95

View Article Online
Chemical Science
Edge Article
40 D. K. Dogutan, S. A. Stoian, R. McGuire Jr, M. Schwalbe, 52 M. Quernheim, H. W. Liang, Q. Su, M. Baumgarten,
T. S. Teets and D. G. Nocera, J. Am. Chem. Soc., 2011, 133,
31–140.
N. Koshino, H. Higashimura and K. M u¨ llen, Chem.–Eur. J.,
2014, 20, 14178–14183.
1
41 H. T. Lei, C. Y. Liu, Z. J. Wang, Z. Y. Zhang, M. N. Zhang, 53 C. Y. Liu, H. T. Lei, Z. Y. Zhang, F. F. Chen and R. Cao, Chem.
X. M. Chang, W. Zhang and R. Cao, ACS Catal., 2016, 6,
429–6437.
2 J. Meng, H. T. Lei, X. L. Li, J. Qi, W. Zhang and R. Cao, ACS
Catal., 2019, 9, 4551–4560.
Commun., 2017, 53, 3189–3192.
6
54 S. Bang, Y.-M. Lee, S. Hong, K.-B. Cho, Y. Nishida, M. S. Seo,
R. Sarangi, S. Fukuzumi and W. Nam, Nat. Chem., 2014, 6,
934–940.
4
4
3 J. J. Tang, Z. P. Ou, R. Guo, Y. Y. Fang, D. Huang, J. Zhang, 55 I. Azcarate, C. Costentin, M. Robert and J. M. Sav ´e ant, J. Am.
J. X. Zhang, S. Guo, F. M. McFarland and K. M. Kadish,
Inorg. Chem., 2017, 56, 8954–8963.
4 A. Friedman, L. Landau, S. Gonen, Z. Gross and L. Elbaz, ACS
Catal., 2018, 8, 5024–5031.
Chem. Soc., 2016, 138, 16639–16644.
56 Y. Y. Fang, Z. P. Ou and K. M. Kadish, Chem. Rev., 2017, 117,
3377–3419.
57 W. B. Lu, C. X. Wang, Q. Y. Lv and X. H. Zhou, J. Electroanal.
Chem., 2003, 558, 59–63.
4
4
5 S. Fukuzumi, S. Mandal, K. Mase, K. Ohkubo, H. Park,
J. Benet-Buchholz, W. Nam and A. Llobet, J. Am. Chem. 58 A. Schechter, M. Stanevsky, A. Mahammed and Z. Gross,
Soc., 2012, 134, 9906–9909. Inorg. Chem., 2012, 51, 22–24.
6 G. Passard, A. M. Ullman, C. N. Brodsky and D. G. Nocera, J. 59 N. Levy, A. Mahammed, M. Kosa, D. T. Major, Z. Gross and
Am. Chem. Soc., 2016, 138, 2925–2928. L. Elbaz, Angew. Chem., Int. Ed., 2015, 54, 14080–14084.
7 C. Di Giovanni, C. Gimbert-Suri n˜ ach, M. Nippe, J. Benet- 60 Y.-H. Wang, P. E. Schneider, Z. K. Goldsmith, B. Mondal,
4
4
Buchholz, J. R. Long, X. Sala and A. Llobet, Chem.–Eur. J.,
016, 22, 361–369.
S. Hammes-Schiffer and S. S. Stahl, ACS Cent. Sci., 2019, 5,
1024–1034.
2
4
4
5
5
8 K. M. Kadish, L. Fr ´e mond, Z. P. Ou, J. G. Shao, C. N. Shi, 61 J. F. Li, B. C. Noll, A. G. Oliver and W. R. Scheidt, J. Am. Chem.
F. C. Anson, F. Burdet, C. P. Gros, J. M. Barbe and
Soc., 2012, 134, 10595–10606.
R. Guilard, J. Am. Chem. Soc., 2005, 127, 5625–5631.
62 APEX2 v2009, Bruker AXS., Madison, WI, 2009.
9 K. M. Kadish, J. G. Shao, Z. P. Ou, L. Fr ´e mond, R. Q. Zhan, 63 G. M. Sheldrick, SADABS, 2008/1, University of G ¨o ttingen,
F. Burdet, J. M. Barbe, C. P. Gros and R. Guilard, Inorg.
Chem., 2005, 44, 6744–6754.
0 K. M. Kadish, L. Fr ´e mond, J. Shen, P. Chen, K. Ohkubo,
G ¨o ttingen, Germany, 2008.
64 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
1990, 46, 467–473.
S. Fukuzumi, M. El Ojaimi, C. P. Gros, J. M. Barbe and 65 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
R. Guilard, Inorg. Chem., 2009, 48, 2571–2582.
2008, 64, 112–122.
1 K. M. Kadish, L. Fr ´e mond, F. Burdet, J. M. Barbe, C. P. Gros
and R. Guilard, J. Inorg. Biochem., 2006, 100, 858–868.
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