Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions

Article abstract of DOI:10.1021/acs.inorgchem.7b02405

Three series of cobalt tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds have the general formula (TpYPP)Co, butano(TpYPP)Co^II, and benzo(TpYPP)Co^II, where TpYPP represents the dianion of the meso-substituted porphyrin, Y is a CH₃, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the β- and β′-substituted groups on the four pyrrole rings of the compound. Each porphyrin undergoes one or two reductions depending upon the meso substituent and solvent utilized. Two irreversible reductions are observed for (TpYPP)Co^II and butano(TpYPP)Co^II in CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate; the first leads to the formation of a highly reactive cobalt(I) porphyrin, which can then rapidly react with a solvent to give a Co^IIICH₂Cl as the product. Only one reversible reduction is seen for benzo(TpYPP)Co^II under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CH₂Cl₂. The first oxidation is metal-centered for the (TpYPP)Co and benzo(TpYPP)Co^II derivatives, leading to generation of a cobalt(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano(TpYPP)Co^II and gives a Co^II π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs) when adsorbed on a graphite electrode in 1.0 M HClO₄. The number of electrons transferred (n) during ORRs is 2.0 for the butano(TpYPP)Co^II derivatives, consistent with only H₂O₂ being produced as a product for the reaction with O₂. However, the reduction of O₂ using the cobalt benzoporphyrins as catalysts gave n values between 2.6 and 3.1 under the same solution conditions, thus producing a mixture of H₂O and H₂O₂ as the reduction product. This result indicates that the β and β′ substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.

Full text of DOI:10.1021/acs.inorgchem.7b02405

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes:
Effect of Substituents on the Electrochemical Properties and
Catalytic Activity of Oxygen Reduction Reactions
Lina Ye,†,‡ Yuanyuan Fang,‡ Zhongping Ou,*,‡,§ Songlin Xue,‡ and Karl M. Kadish*,§
†School of Computer, Jilin Normal University, Siping 136000, China
‡School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
§Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
S
* Supporting Information
ABSTRACT: Three series of cobalt tetraarylporphyrins were
synthesized and characterized by electrochemistry and spec-
troelectrochemistry. The investigated compounds have the
general formula (TpYPP)Co, butano(TpYPP)CoII, and benzo-
(TpYPP)CoII, where TpYPP represents the dianion of the meso-
substituted porphyrin, Y is a CH3, H, or Cl substituent on the
para position of the four phenyl rings, and butano and benzo are
respectively the β- and β′-substituted groups on the four pyrrole
rings of the compound. Each porphyrin undergoes one or two
reductions depending upon the meso substituent and solvent
utilized. Two irreversible reductions are observed for (TpYPP)-
CoII and butano(TpYPP)CoII in CH2Cl2 containing 0.1 M tetra-n-butylammonium perchlorate; the first leads to the formation of
a highly reactive cobalt(I) porphyrin, which can then rapidly react with a solvent to give a CoIIICH2Cl as the product. Only one
reversible reduction is seen for benzo(TpYPP)CoII under the same solution conditions, and the one-electron-reduction product
is assigned as a cobalt(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CH2Cl2.
The first oxidation is metal-centered for the (TpYPP)Co and benzo(TpYPP)CoII derivatives, leading to generation of a
cobalt(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano(TpYPP)CoII and
gives a CoII π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs)
when adsorbed on a graphite electrode in 1.0 M HClO4. The number of electrons transferred (n) during ORRs is 2.0 for the
butano(TpYPP)CoII derivatives, consistent with only H2O2 being produced as a product for the reaction with O2. However, the
reduction of O2 using the cobalt benzoporphyrins as catalysts gave n values between 2.6 and 3.1 under the same solution
conditions, thus producing a mixture of H2O and H2O2 as the reduction product. This result indicates that the β and β′
substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.
In contrast, both cobalt tetramethylporphyrin23 and cobalt
INTRODUCTION
■
porphine25 have been shown to catalyze the reduction of O2 via
It has long been known that cobalt porphyrins can be used as
catalysts for oxygen reduction reactions (ORR) in acid media to
a four-electron-transfer pathway (n = 4), giving H2O as a final
reduction product. These two less sterically hindered metal-
generate H2O2 and/or H2O via a two- or four-electron-transfer
pathway,1−4 and a large number of derivatives have been
examined for their catalytic properties for ORRs over the last 3
decades.5−41 The catalytic activity and selectivity of the cobalt
porphyrins toward the formation of H2O2 or H2O as a product
of the ORR will depend upon the type and position of the
porphyrin ring substituents and the planarity of the macro-
cycle.21,39,42−44 For example, meso-ferrocenyl-substituted por-
phyrins have been shown to be highly selective catalysts toward
the two-electron reduction of O2 (n = 2) and lead almost
exclusively to the formation of H2O2 as a reaction product. The
low value of n is due to steric hindrance from the bulky
ferrocene groups, which prevent π−π interaction between
macrocycles, thus hindering the formation of porphyrin dimers
and making the four-electron reduction of O2 less favorable.6,21
lomacrocycles may exhibit strong π−π interactions between the
molecules in solution, leading to the formation of porphyrin
dimers on the electrode surface and thus enhancing the four-
electron electrocatalytic reduction of O2 as a result of the two
cobalt centers being in close proximity to each other.
Benzoporphyrins, which possess an extended π-conjugated
system,45−50 display high chemical stability, increased basicity,
and unique UV−vis spectral and electrochemical properties
compared to the parent porphyrins. A number of benzopor-
phyrins have been synthesized to date, and these compounds
may have enormous potential applications in both natural and
Received: September 19, 2017
© XXXX American Chemical Society
A
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applied sciences.50,51 Among the most studied of the
benzoporphyrins are the free-base derivatives and complexes
containing nickel, copper, zinc, or platinum central metal ions.
In contrast to these transition-metal derivatives, to date only a
few cobalt benzoporphyrins have been synthesized and
characterized.50,51
Scheme 1. Synthetic Routes of Butano(TpYPP)Co and
Benzo(TpYPP)Co
It is generally expected that the larger the π system of a
porphyrin, the stronger the π−π interaction between the
molecules in solution. With this in mind, we wished to know
how the catalytic activity of a cobalt porphyrin for the ORR
might be changed by expanding the π system of the molecule.
To answer this question, three series of cobalt tetraarylpor-
phyrins were synthesized and their catalytic activity toward the
ORR was evaluated in the present study by cyclic voltammetry
and linear-sweep voltammetry using a rotating disk electrode
(RDE) and a rotating ring-disk electrode (RRDE). The
structures of the examined benzoporphyrin catalysts are
shown in Chart 1, which also includes structures of the two
Chart 1. Structures of the Investigated Cobalt Porphyrins
mide (DMF) mixture [1:1 (v/v)] to give the desired
benzo(TpYPP)Co derivatives 3a−3c, as shown in Scheme 1b.
1H NMR and UV−Vis Spectra. The 1H NMR spectrum of
each porphyrin was measured in CDCl3 at 298 K. Examples of
the obtained spectra are shown in Figure 1 for (TPP)Co 1b,
butano(TPP)Co 2b, and benzo(TPP)Co 3b. In the case of
(TPP)Co 1b, the pyrrole proton resonances are located at
15.96 ppm, while the meso-phenyl proton resonances are
observed at 13.16, 9.95, and 9.74 ppm (o-H, m-H, and p-H,
respectively). Resonances of the butano protons of 2b are seen
at 12.28 and 3.15 ppm, while the meso-phenyl proton
resonances are located at 15.75, 10.49, and 10.23 ppm (o-H,
m-H, and p-H, respectively). The benzo proton resonances of
3b are seen at 10.11 and 8.79 ppm, while the phenyl proton
resonances are observed at 14.00, 12.97, and 9.80 ppm (o-H, m-
H, and p-H, respectively).
UV−vis spectra in CH2Cl2 for compounds in the three series
are illustrated in Figure 2. The (TpYPP)CoII complexes 1a−1c
are each characterized by a sharp Soret band at 416−418 nm
and a low-intensity Q band at 534 nm (Figure 2a), while the
butano(TpYPP)CoII derivatives 2a−2c exhibit red-shifted Soret
and Q bands compared to the three (TpYPP)CoII compounds
with the same meso substituents as seen in Figure 2b. In
addition, the single Q band at 534 nm for 1a−1c is replaced by
two Q bands for 2a−2c at 548−581 nm, as seen in the figure.
A different spectral pattern is seen for the three benzo-
(TpYPP)CoII complexes 3a−3c. These porphyrins with four β-
and β′-fused benzo rings are characterized by a sharp Soret
band at 442−446 nm and two Q bands, one of which is
relatively intense and located at 638−642 nm. The Soret bands
are red-shifted by 16−20 nm compared to (TpYPP)Co, while
the highest-intensity Q band is red-shifted by ∼105 nm from
the related Q-band absorption of the porphyrin lacking the
fused benzo rings. The relatively high intensity of the longest-
related cobalt tetraarylporphyrin macrocycles lacking the four
β- and β′-fused rings, i.e., (TpYPP)Co and butano(TpYPP)Co,
the latter of which possesses four β- and β′-fused butano rings.
The effects of the benzo substituents on the UV−vis spectra,
reduction/oxidation potentials, and catalytic activity for the
reduction of O2 in acid media were examined, and comparisons
were made with data for compounds in the two series of
porphyrins lacking the fused benzo rings.
RESULTS AND DISCUSSION
■
Synthesis. The (TpYPP)Co complexes 1a−1c were
synthesized according to procedures described in the
literature.6,44,51 The butano(TpYPP)Co complexes 2a−2c
were prepared via a reaction of the free-base butanotetraar-
ylporphyrins with Co(OAc)2 in a mixed CHCl3 and CH3OH
solvent [4:1 (v/v)] at room temperature (Scheme 1a). The
reaction is facile, and the final products were obtained in yields
ranging from 80 to 85%.
Zinc(II) and copper(II) benzotetraarylporphyrins have been
synthesized by oxidizing the corresponding butano derivatives
using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, as described
in the literature.45,52 However, the cobalt benzotetraarylpor-
phyrins 3a−3c could not be obtained by this method starting
from the corresponding cobalt porphyrins 2a−2c because of
decomposition of the butanoporphyrins, which occurred during
the reaction, and a different synthetic procedure was therefore
utilized. In this method, the benzo(TpYPP)Zn derivatives were
first prepared according to literature methods45,52 and then
demetalated to give the free-base benzotetraporphyrins, which
were reacted with Co(OAc)2 in a CH2Cl2/N,N-dimethylforma-
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1
Figure 1. H NMR spectra of (a) (TPP)Co 1b, (b) butano(TPP)Co 2b, and (c) benzo(TPP)Co 3b in CDCl3 at 298 K.
Figure 2. UV−vis spectra of (a) (TpYPP)Co, (b) butano(TpYPP)Co, and (c) benzo(TpYPP)Co in CH2Cl2.
wavelength Q band in Figure 2c results from the increased
conjugation between the four benzo groups and macrocycle.
This result is consistent with what has been reported for other
benzoporphyrins having different central metal ions.33−38
Electrochemistry. The redox properties of (TpYPP)CoII
1a−1c, butano(TpYPP)CoII 2a−2c, and benzo(TpYPP)CoII
3a−3c were examined in CH2Cl2 and PhCN containing 0.1 M
tetra-n-butylammonium perchlorate (TBAP). The measured
potentials are summarized in Table 1, while examples of the
cyclic voltammograms are given in Figures 3 and 4.
Oxidation. As seen in Figure 3a, benzo(TPP)CoII 3b
undergoes three reversible one-electron oxidations at E1/2
=
0.44, 0.70, and 0.89 V in CH2Cl2. These processes are all easier
by 260−270 mV compared to (TPP)CoII 1b, which exhibits
three reversible one-electron oxidations at E1/2 = 0.70, 0.97, and
1.15 V under the same solution conditions (Figure 3b). The
cyclic voltammogram for oxidation of butano(TPP)CoII 2b
(Figure 3c) differs from that of both compounds 1b and 3b in
that the first one-electron abstraction is quasi-reversible and
located at E1/2 = 0.39 V, while the second and third oxidations
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Table 1. Half-Wave Potentials (V vs SCE) of (TpYPP)Co, Butano(TpYPP)Co, and Benzo(TpYPP)Co in CH2Cl2 and PhCN
Containing 0.1 M TBAP
solvent
CH2Cl2
β and β′ substituents
Y
cpd
oxidation
reduction
a
a
a
a
a
a
a
a
a
a
a
a
none
CH3
H
1a
1b
1c
2a
2b
2c
3b
3c
1a
1b
1c
2a
2b
2c
3b
3c
1.14
1.15
1.22
0.70
0.80
0.83
0.89
0.88
0.96
0.97
0.99
0.72
0.70
0.74
0.37
0.39
0.44
0.44
0.47
0.57
0.58
0.75
0.34
0.36
0.39
0.45
0.46
−0.94
−0.87
−0.83
−1.08
−1.02
−1.00
−0.61
−0.55
−0.80
−0.85
−0.76
−1.02
−1.01
−0.93
−0.70
−0.65
−1.48
−1.38
−1.33
−1.72
−1.70
−1.60
>−2.0
−2.00
−1.88
−1.98
−1.79
Cl
b
b
b
b
c
c
c
butano
CH3
H
0.70
b
0.80
b
Cl
0.83
benzo
none
H
0.70
0.71
a
d
Cl
d
d
a
d
d
d
PhCN
CH3
H
1.16
1.37
1.19
d
d
Cl
1.26
butano
benzo
CH3
H
0.94
0.94
0.92
0.94
0.93
0.76
0.87
b
b
Cl
0.92
H
0.76
0.74
Cl
a
b
c
d
Irreversible peak potential at a scan rate of 0.10 V/s. Two overlapped one-electron oxidations. Quasi-reversible oxidation. Data measured in
DMF and taken from ref 53.
Figure 3. Cyclic voltammograms showing the oxidation of (a)
benzo(TPP)CoII 3b, (b) (TPP)CoII 1b, and (c) butano(TPP)CoII 2b
in CH2Cl2 containing 0.1 M TBAP.
Figure 4. Cyclic voltammograms showing the oxidation of (a)
butano(TpCH3PP)CoII 2a, (b) butano(TPP)CoII 2b, and (c)
butano(TpClPP)CoII 2c in PhCN containing 0.1 M TBAP.
are overlapped in potential and located at E1/2 = 0.80 V in
CH2Cl2. Overlapping of the second and third oxidations is also
seen for the butano derivatives 2a (E1/2 = 0.70 V) and 2c (E1/2
= 0.83 V) in CH2Cl2 (Table 1), but an overlapping of the last
two oxidations occurs only for compound 2c in PhCN (Figure
4).
It is known that the type of meso substituents on a porphyrin
will affect the measured redox potentials, with the magnitude of
the effect on E1/2 being dependent in many cases on both the
site of oxidation or reduction and the properties of the solvent.
Generally, porphyrins having electron-withdrawing substituents
are harder to oxidize and easier to reduce, while those having
electron-donating substituents are easier to oxidize and harder
to reduce.53 As shown in Figure 4, the first two oxidations of
the butano derivatives are easiest for the meso-CH3Ph derivative
2a (E1/2 = 0.34 and 0.76 V), while the hardest oxidations are
seen for the meso-ClPh porphyrin 2c (E1/2 = 0.39 and 0.92 V).
The magnitude of the substituent effect on the redox potential
is not the same for these two oxidations, with the difference in
E1/2 amounting to 50 mV in the case of the first electron
abstraction and 150 mV in the case of the second. This
contrasts with the third oxidation of the three butano
derivatives, where a virtually identical E1/2 value of 0.92−0.94
V is observed for the three compounds having different
substituents on the meso-phenyl rings. Because this third
oxidation changes little with the substituent and the second
shifts positively upon going from Y = CH3 to Y = Cl, the
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second and third oxidations of 2c become overlapped in this
solvent.
Figure 5 shows the plots of the measured oxidation potentials
(E1/2) for butanoporphyrins in CH2Cl2 and PhCN versus the
Figure 6. Cyclic voltammograms showing the reduction of compounds
1b, 2b, and 3b in (a) CH2Cl2 and (b) PhCN containing 0.1 M TBAP.
more negative potentials.44,54,55 A chemical reaction with
CH2Cl2 also occurs for the one-electron-reduced (TpYPP)CoII
1a−1c and butano(TpYPP)CoII 2a−2c, but the first one-
electron reduction of the benzoporphyrins 3a−3c are reversible
in both CH2Cl2 and PhCN, as seen from the cyclic
voltammogram of 3b in Figure 6a. This indicates the lack of
a chemical reaction following the electron-transfer process and
the formation of stable reduction products on the cyclic
voltammetric time scale. Finally, it should be noted that
reversible reductions are seen for each investigated porphyrin in
PhCN, as shown in Figure 6b for 1b, 2b, and 3b.
UV−Vis Spectroelectrochemistry. It has long been
known that the site of oxidation for cobalt(II) porphyrins can
occur either at the central metal ion to give cobalt(III)
derivatives or at the porphyrin macrocycle to give cobalt(II)
porphyrin π-cation radicals, with the exact site of electron
transfer dependent on the structure of the compound and the
solvent utilized.53,56−60 In order to determine the exact site of
the initial oxidation in the currently investigated cobalt(II)
butanoporphyrins and benzoporphyrins, thin-layer UV−vis
spectroelectrochemistry of (TpYPP)CoII 1a−1c, butano-
(TpYPP)CoII 2a−2c, and benzo(TpYPP)CoII 3a−3c was
carried out in CH2Cl2 containing 0.1 M TBAP, and examples
of the spectral changes that occurred during the first controlled
potential oxidation are shown in Figure 7.
Figure 5. Plots of the oxidation potentials (E1/2) versus the sum of the
Hammett substituent constants (4σ) for butano(TpYPP)Co 2a−2c in
(a) CH2Cl2 and (b) PhCN.
sum of the Hammett substituent constant (4σ) for the para
substituents on the meso-phenyl rings of the compounds. A
linear relationship is seen for each oxidation in both solvents,
but the slopes (ρ) of the ΔE1/2/Δ(4σ) plots differ from each
other, indicating a different substituent effect on each oxidation
process in the two solvents. The ρ value is 44 mV for the first
one-electron oxidation, but it is 78 mV for the second and third
overlapped oxidations in CH2Cl2 (see Figure 5a). The ρ values
are +31, +97, and −13 mV for the three oxidations in PhCN
(see Figure 5b). Thus, in both solvents, the meso substituents
have a smaller effect on the first oxidation than the second,
independent of the solvent utilized.
Reduction. Cyclic voltammograms showing the reductions
of compounds 1b, 2b, and 3b in CH2Cl2 and PhCN are
illustrated in Figure 6. Like (TPP)CoII 1b, which exhibits two
irreversible reductions at Epc = −0.87 and −1.38 V in CH2Cl2,
the first of which involves reduction of the original cobalt
porphyrin and the second a product of a chemical reaction with
CH2Cl2.44 Butanoporphyrin 2b also undergoes two irreversible
reductions (Epc = −1.02 and −1.70 V) in this solvent, the
second of which is also a product of the first reaction, but the
potentials for these processes are shifted negatively by 150−320
mV as compared to potentials for the related redox reactions of
compound 1b (Figure 6a). Similar irreversible reductions have
been reported for most previously examined meso- and/or β-
pyrrole-substituted cobalt porphyrins in a CH2Cl2 solvent,53
and this is related to a reaction involving the electrogenerated
cobalt(I) and the CH2Cl2 solvent to form a transient σ-bonded
cobalt(III) porphyrin, which is then irreversibly reduced at
Two different types of spectral changes are observed in
Figure 7. One is for (TPP)CoII 1b and benzo(TPP)CoII 3b,
and the other is for butano(TPP)CoII 2b. The spectral changes
during oxidation of 1b and 3b are diagnostic of a CoII/CoIII
conversion in that the Soret and Q bands of the singly oxidized
compound are well-defined (at 432 and 539 nm for 1b and at
471 and 651 nm for 3b) and red-shifted with respect to their
neutral form in CH2Cl2 containing 0.1 M TBAP. Similar
spectral changes are observed for benzo(TpClPP)CoII 3c under
the same solution conditions (Figure S1).
E
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cobalt(II) porphyrins are only partially converted to their
cobalt(III) form and that a cobalt(II) porphyrin π-cation radical
might also be generated in solution and is in equilibrium with
the cobalt(III) porphyrin when the oxidation is carried out in
PhCN.
The Soret band of (TPP)CoII 1b decreases only slightly in
intensity during the first one-electron reduction of this
porphyrin in CH2Cl2 containing 0.1 M TBAP (Figure 9a).
Similar spectral changes are seen for butano(TPP)CoII 2b in
the same solvent (Figure 9b), indicating that the first reduction
of 2b is metal-centered to give cobalt(I) porphyrin, which then
reacts with CH2Cl2 to generate cobalt(III) porphyrin having a
Co−C σ band, i.e., (TPP)Co(CH2Cl). This result is consistent
with what has been previously reported for other cobalt(II)
tetraarylporphyrins under the same solution conditions.44
Different spectral changes were observed for the benzopor-
phyrin derivatives during the first controlled potential reduction
in CH2Cl2. Unlike the changes for 1a−1c and 2a−2c, the initial
Soret band at 442−444 nm and the Q band at 638−642 nm
both substantially decreased in intensity, while a new Soret
band grew in at 416−418 nm (Figures 9c and S2). It should be
pointed out that the new Soret band for the singly reduced 3b
has an intense band at 416 nm, the same value as that of the
neutral 1b (Figure 9), suggesting the formation of a cobalt(II)
porphyrin π-anion radical, with the added electron being
localized on the fused rings. A similar result was obtained after
the first one-electron reduction of cobalt(II) quinoxalinopor-
phyrins having a fused quinoxaline group.60 The spectrum of
the initial benzoporphyrin could be recovered by setting the
controlled potential back to 0.0 V, a result consistent with the
observed reversible one-electron reduction on the cyclic
voltammetric time scale (Figure 6a). This suggests that the
singly reduced product of cobalt(II) benzoporphyrin is a
cobalt(II) porphyrin π-anion radical, which does not react with
the CH2Cl2 solvent under the given experimental conditions.
In summary, the butano(TpYPP)CoII complexes undergo
metal-centered reductions to give cobalt(I) porphyrins in
CH2Cl2, but the electrogenerated products rapidly react with
CH2Cl2 to generate cobalt(III) porphyrins with bound CH2Cl,
which is irreversibly reduced at more negative potentials. This is
consistent with that previously been reported for other cobalt
tetraarylporphyrins44 and cobalt triarylcorroles61,62 under the
same solution conditions. In the case of the benzo(TpYPP)CoII
compounds, the first one-electron reduction is proposed to
generate a stable cobalt(II) porphyrin π-anion radical in the
CH2Cl2 solvent (Scheme 2).
Figure 7. Thin-layer UV−vis spectral changes of (a) (TPP)CoII 1b,
(b) butano(TPP)CoII 2b, and (c) benzo(TPP)CoII 3b during the first
controlled potential oxidation at an applied potential in CH2Cl2
containing 0.1 M TBAP.
In contrast to the above, the spectral changes during
oxidation of butano(TPP)CoII 2b are diagnostic of cobalt(II)
porphyrin π-cation radical generation in that the spectrum of
the singly oxidized species exhibits a broad low-intensity Soret
band with a broad Q band (Figure 7b). The same types of
spectral changes are also observed for the butanoporphyrins 2a
and 2c (Figure 8a), indicating formation of the cobalt(II)
porphyrin π-cation radical during the first controlled potential
oxidation of all three compounds under the given solution
conditions. Similar spectral changes have been previously
observed for β-pyrrole brominated cobalt(II) porphyrins,
(BrxTPP)CoII, where x = 6, 7, or 8, during the first oxidation
to generate cobalt(II) porphyrin π-cation radicals in CH2Cl2
containing 0.1 M TBAPF6.56
The spectral changes that occur after the first one-electron
oxidation of the cobalt butanoporphyrins in PhCN differ from
that observed for oxidation of the same compounds in CH2Cl2.
As a comparison, the UV−vis spectral changes of butano-
(TpYPP)CoII 2a−2c during the first controlled potential
oxidation at an applied potential in PhCN are shown in Figure
8b. As seen in the figure, a new Soret band grows in at ∼450
nm upon the first one-electron oxidation in PhCN, but the
intensity of this new Soret band, which is assigned to the
cobalt(III) derivative, is much lower than that of the initial
cobalt(II) porphyrin. The result indicates that the initial
The benzo(TpYPP)CoII 3b and 3c show oxidation behavior
similar to that of (TpYPP)CoII 1a−1c, which undergo three
reversible oxidations in CH2Cl2. The first of those is a metal-
centered electron-transfer process to give the cobalt(III)
porphyrins described in eqs 1 and 2. Thin-layer spectroelec-
trochemical data suggest that cobalt(II) porphyrin π-cation
radicals are produced after the first one-electron oxidation of
the butano(TpYPP)CoII 2a and 2b in CH2Cl2, but a mixture of
cobalt(III) porphyrins and cobalt(II) porphyrin π-cation radical
is generated when PhCN is the electrochemical solvent, and the
prevailing reaction is then given as shown in Scheme 3.
−e
(TpYPP)CoII ⇌ [(TpYPP)CoIII]+
(1)
−e
benzo(TpYPP)CoII ⇌ [benzo(TpYPP)CoIII]+
(2)
F
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Figure 8. Thin-layer UV−vis spectral changes of butano(TpYPP)CoII 2a−2c during the controlled potential oxidations at an applied potential in (a)
CH2Cl2 and (b) PhCN containing 0.1 M TBAP.
Electrocatalytic Reduction of O2. Each cobalt porphyrin
was examined for its catalytic activity for the ORR in 1.0 M
HClO4. Figure 10 illustrates the cyclic voltammograms
obtained for compounds 1c, 2c, and 3c when adsorbed on
the surface of an edge-plane pyrolytic graphite (EPPG) disk
electrode in acid media. No reduction is seen under N2, but an
irreversible reduction is observed at Epc = 0.15 V for 1c and at
0.20 V for compounds 2c and 3c at a scan rate of 50 mV/s. The
voltammetric data indicate that a catalytic ORR has occurred on
the surface of the electrode.
Each porphyrin was also examined by linear-sweep
voltammetry in air-saturated 1.0 M HClO4, and the steady-
state limiting currents taken at 0.10 V on the plateau of the
current−voltage curve were utilized to determine the number
of electrons transferred (n) during the ORR. Examples of the
obtained current−voltage curves under these conditions are
shown in the top of Figure 11 for compounds 1c, 2c, and 3c.
The diagnostic Koutecky−Levich plots,63 which were analyzed
on the basis of eq 3, are given in the bottom of Figure 11,
where jlim is the measured limiting current density (mA/cm2), jk
is the kinetic current, and jlev is the Levich current, which is
used to measure the rate of the current-limiting chemical
reaction as defined by eq 4.
is the kinematic viscosity of H2O, and ω is the angular rotation
speed (rad/s) of the electrode.
The value of n and the percentage of H2O2 product in the
ORR process using the cobalt porphyrins as catalysts are given
in Table 2. Generally, a two-electron-transfer process (n = 2)
would correspond to the generation of 100% H2O2, while a
four-electron process (n = 4) would give 0% H2O2 and 100%
H2O. The Koutecky−Levich plots in Figure 11 showed that the
number of electrons transferred (n) is 2.4 for 1c, 2.0 for 2c, and
3.1 for 3c, with the corresponding percentage of H2O2
produced ranging from 45 to 100% (Table 2).
It should be pointed out that the values of n for the butano-
substituted cobalt porphyrins 2a−2c are 2.0 in each case (see
Table 2), indicating that the ORR catalyzed by the butano-
substituted porphyrins is mainly a two-electron-transfer
process, giving exclusively 100% H2O2. However, when the
benzo-substituted porphyrins 3b and 3c were used as the
catalysts, the number of electrons transferred increased to 2.6
and 3.1, respectively. This is consistent with generating a
mixture of H2O2 and H2O as the products under the given
solution conditions.
The RRDE measurements were also carried out in 1.0 M
HClO4 for the ORR, with the disk potential being scanned
from +0.5 to −0.1 V at a rotation rate of 400 rpm while the ring
potential was held at 1.0 V versus saturated calomel electrode
(SCE). Examples of the current−voltage curves observed under
these conditions are given in Figure 12 for compounds 1c, 2c,
and 3c. As seen in the figure, the disk current begins to increase
at about 0.40 V for 1c and 0.30 V for both 2c and 3c, and a
current plateau is reached at about 0.20 V for 1c and 0.15 V for
2c and 3c, respectively. The amount of H2O2 generated from
1/jlim = 1/jlev + 1/jk
(3)
jlev = 0.62nFD2/3cv−1/6ω1/2
(4)
Where n is the number of electrons transferred in the overall
electrode reaction, F is the Faraday constant (96485 C/mol), D
and c are the diffusion coefficient (cm2/s) and bulk
concentration of O2 (mol/L) in 1.0 M HClO4, respectively, v
G
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Figure 10. Cyclic voltammograms of (a) (TpClPP)CoII 1c, (b)
butano(TpClPP)CoII 2c, and (c) benzo(TpClPP)CoII 3c absorbed on
an EPPG electrode in 1.0 M HClO4 saturated with N2 (- - -) or air
(). Scan rate = 50 mV/s.
the ORR was calculated using eq 5, where ID and IR are the
Faradaic currents at the disk and ring electrodes, respectively,
and N is the collection efficiency (0.24) of the ring electrode.
The calculated value of % H2O2 is 78% for 1c, 100% for 2c, and
54% for 3c under the given experimental conditions.
% H2O2 = 100(2IR/N)/(ID + IR/N)
(5)
Electrocatalytic Reduction of H2O2. The electrocatalytic
reduction of H2O2 was also examined in 1.0 M HClO4 using a
benzo(TPP)CoII 3b or benzo(TpClPP)CoII 3c coated EPPG
electrode; the cyclic and linear-sweep voltammograms as well as
the corresponding Koutecky−Levich plots under these
conditions are shown in Figure 13. The measured reduction
potential of H2O2 is located at Epc = −0.03 V for 3b and 0.03 V
for 3c and is negatively shifted by 0.17−0.20 V compared to the
reduction potential of O2 using the same porphyrin as the
catalyst under the same experimental conditions. The cathodic
current for reduction of H2O2 is much lower than that of O2,
indicating that the catalytic activity of the benzo(TpYPP)CoII
derivatives toward the reduction of H2O2 in 1.0 M HClO4 is
weaker than that for the reduction of O2. The cathodic currents
shown in Figure 13 might be due to the two-electron reduction
of H2O2 to generate of H2O, and this is confirmed by the
Koutecky−Levich plots shown in Figure 13, where the number
of electrons transferred (n = 2.0) is consistent with that
previously reported for the reduction of H2O2 catalyzed by
structurally related cobalt corroles64 and cobalt porphyrins.6,44
It is worth pointing out that no cathodic current is observed
for the reduction of H2O2 when butano(TpYPP)CoII 2a−2c is
used as the catalyst in 1.0 M HClO4. As seen in Figure S3,
identical cyclic voltammograms are obtained with or without
added H2O2 for each of the butano-substituted porphyrin
catalysts in N2-saturated 1.0 M HClO4 solutions. This result is
Figure 9. Thin-layer UV−vis spectral changes of (a) (TPP)CoII 1b,
(b) butano(TPP)CoII 2b, and (c) benzo(TPP)CoII 3b during the first
controlled potential reduction at an applied potential in CH2Cl2
containing 0.1 M TBAP.
Scheme 2. Proposed First Reduction Mechanism of the
Investigated Cobalt Porphyrins in CH2Cl2 Containing 0.1 M
TBAP
Scheme 3. Proposed First Oxidation Mechanism of
Butano(TpYPP)Co in CH2Cl2 and PhCN Containing 0.1 M
TBAP
H
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Figure 11. Current−voltage curves and Koutecky−Levich plots for catalyzed reduction of O2 at a rotating EPPG disk electrode coated with the
cobalt porphyrins 1c, 2c, and 3c in 1.0 M HClO4 saturated with air. Values of the electrode rotation rate (ω) are indicated on each curve. Potential
scan rate = 50 mV/s.
Table 2. Electrocatalytic Reduction of O2 by Cobalt
Porphyrins in 1.0 M HClO4
2a−2c could be used as selective electrocatalysts for the two-
electron reduction of O2 in acid media.
The benzo-substituted porphyrins have an extended π
system, which could lead to stronger π−π interactions between
the porphyrin macrocycles in solution than is observed for the
non-benzo-substituted compounds, thus enhancing the electro-
catalytic activity for the reduction of O2 and leading to a higher
n value than that using butanoporphyrins under the same
solution conditions. On other hand, the benzoporphyrin
macrocycle is nonplanar,52,67−69 leading to a decreased π−π
interaction on the electrode surface and n = 2.6 for
benzo(TPP)Co 3b and 3.1 for benzo(TpClPP)Co 3c rather
than the desired n = 4.0.
a
b
compound
(TPP)Co 1b
Ep, with O2
E1/2
% H2O2
n
0.13
0.15
0.15
0.17
0.20
0.17
0.20
0.15
0.20
0.20
0.20
0.22
0.20
0.22
100
80
2.0
2.4
2.0
2.0
2.0
2.6
3.1
(TpClPP)Co 1c
butano(TpCH3PP)Co 2a
butano(TPP)Co 2b
butano(TpClPP)Co 2c
benzo(TPP)Co 3b
100
100
100
70
benzo(TpClPP)Co 3c
45
a
Half-wave potential at i = 0.5imax, where imax is the limiting current
measured at 400 rpm of the RDE. Calculated based on the value of n;
b
see the discussion in the text.
CONCLUSION
■
Three series of cobalt tetraarylporphyrins with the general
formulas (TpYPP)Co, butano(TpYPP)CoII, and benzo-
(TpYPP)CoII have been prepared and investigated using
UV−vis spectroscopy as well as electrochemical and
spectroelectrochemical approaches. The β- and β′-butano and
-benzo groups have a significant effect on the UV−vis spectra,
redox potentials, and site of electron transfer of the compounds.
The first one-electron oxidation of the butano(TpYPP)CoII
complexes is porphyrin-ring-centered and generates a cobalt(II)
porphyrin π-cation radical in CH2Cl2 rather than the expected
cobalt(III) porphyrin, which is generated upon oxidation of
(TpYPP)CoII or benzo(TpYPP)CoII under the same solution
conditions. However, a mixture of the cobalt(III) porphyrin
and cobalt(II) porphyrin cation radical is proposed for the first
oxidation of butano(TpYPP)CoII in PhCN. Cobalt(I)
porphyrin is initially generated for (TpYPP)CoII and
butano(TpYPP)CoII after the first one-electron reduction in
CH2Cl2 and can rapidly react with the solvent to give a CoIII−C
σ-bonded species as the final product. In contrast, the
electrochemical and spectroelectrochemical data suggest that
a cobalt(II) porphyrin π-anion radical is generated after the first
consistent with the fact that the value of n is 2.0 and the
product is 100% H2O2 for the ORR when butano(TpYPP)CoII
2a−2c is used as the catalyst (see the above discussion). This
indicates that compounds 2a−2c cannot catalyze the reduction
of H2O2 under the given experimental conditions.
Effect of the Butano and Benzo Groups on the
Catalytic Activity. It is known that the nature of the
substituents on the meso and/or β positions of metallocorroles
and metalloporphyrins can have a significant effect on the
catalytic activity of the compound.6,44,64−66 The same appears
to be true for the currently examined cobalt porphyrins with
different macrocycles. As discussed above, the non-β-
substituted cobalt porphyrins can catalyze the ORR with a
value of n ranging from 2.0 for (TPP)Co to 2.4 for
(TpClPP)Co. However, the calculated n is 2.0 for each
butano(TpYPP)Co, independent of the meso substituents on
the porphyrin macrocycle. This result suggests that the butano
substituents may change the planarity of the porphyrin
macrocycle, resulting in a change in the site of electron transfer
for oxidation. Thus, the butano-substituted cobalt porphyrins
I
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desorption/ionization time-of-flight (MALDI-TOF) mass spectrome-
try (MS) spectra were taken on a Bruker BIFLEX III ultrahigh-
resolution instrument using α-cyano-4-hydroxycinnamic acid as the
matrix.
Cyclic voltammetry was carried out at 298 K by using an EG&G
Princeton Applied Research (PAR) 173 potentiostat/galvanostat or a
CHI-730C electrochemical workstation. A homemade three-electrode
cell was used for all electrochemical measurements. A three-electrode
system was used in each case and consisted of a glassy carbon or
graphite working electrode (model MT134, Pine Instrument Co.) for
cyclic voltammetry and voltammetry at an RDE and a platinum ring−
graphite disk electrode combination for voltammetry at the RRDE. A
platinum wire served as the auxiliary electrode and SCE as the
reference electrode, which was separated from the bulk of the solution
by means of a salt bridge of low porosity that contained the solvent/
supporting electrolyte mixture. The RRDE was purchased from Pine
Instrument Co. and consisted of a platinum ring and a removable
EPPG disk (A = 0.196 cm2). A Pine Instrument MSR speed controller
was used for the RDE and RRDE experiments. The platinum ring was
first polished with 0.05 μm α-alumina powder and then rinsed
successively with H2O and acetone before being activated by cycling
the potential between +1.20 and −0.20 V in 1.0 M HClO4 until
reproducible voltammograms were obtained.
The catalysts were irreversibly adsorbed on the electrode surface by
means of a dip-coating procedure described in the literature.23 The
freshly polished electrode was dipped in a 1.0 mM catalyst solution of
CH2Cl2 for 5 s, transferred rapidly to pure CH2Cl2 for 1−2 s, and then
exposed to air, where the adhering solvent rapidly evaporated, leaving
the porphyrin catalyst adsorbed on the electrode surface. All
experiments were carried out under room temperature.
Chemicals. Reagents and solvents (Sigma-Aldrich, Fluka, or
Sinopharm Chemical Reagent Co.) for the synthesis and purification
were of analytical grade and were used as received. Dichloromethane
(CH2Cl2; 99.8%) was purchased from EMD Chemicals Inc. and used
as received. Tetra-n-butylammonium perchlorate (TBAP) was
purchased from Sigma Chemical or Fluka Chemika Co., recrystallized
from ethyl alcohol, and dried under vacuum at 40 °C for at least 1
week prior to use.
Synthesis of (TpYPP)Co. About 50 mg of free-base tetraar-
ylporphyrins, (TpYPP)H2, were dissolved in 100 mL of CHCl3/
MeOH [4/1 (v/v)], which contains excess Co(OAc)2·2H2O (100
mg), the solution was stirred for 2 h at room temperature, and the
progress of the reaction was monitored by TLC and UV−vis spectra.
The reaction solution was then evaporated to dryness and freshly
chromatographed with neutral alumina (200−300 mesh) using
CH2Cl2 as the eluent. The red fraction was collected and evaporated
to dryness to obtain the product with a yield of 80−85%.
Figure 12. RRDE voltammograms of (a) (TpClPP)CoII 1c, (b)
butano(TpClPP)CoII 2c, and (c) benzo(TpClPP)CoII 3c in HClO4
saturated with air. The potential of the disk electrode was scanned in a
negative direction from +0.5 to −0.1 V, while the potential of the ring
electrode was held at 1.0 V. Rotation rate = 400 rpm, and scan rate =
10 mV/s.
reduction of benzo(TpYPP)CoII in CH2Cl2, with the proposed
site of electron addition being the fused rings. The electro-
catalytic properties of the investigated butano-substituted
cobalt porphyrins are highly selective in the two-electron
electrocatalytic reduction of O2 and lead exclusively to the
formation of H2O2 as a reaction product. However, the number
of electrons transferred for the ORR using cobalt benzopor-
phyrins ranges from 2.6 to 3.1 in acid media, probably because
an extension of the conjugated π system in the benzoporphyr-
ins may lead to stronger π−π interactions between the
porphyrin macrocycles on the electrode surface.
(TpCH3PP)Co 1a. Yield: 46.1 mg, 85%. UV−vis (CH2Cl2): λmax
=
1
418, 534 nm. H NMR (400 MHz, CDCl3, 298 K): δ 16.00 (s, 8H,
pyrrole H), 13.09 (s, 8H, ph-ortho H), 9.76 (s, 8H, ph-meta H), 4.17
(s, 12H, group H). MS (MALDI-TOF). Calcd for C48H36CoN4: m/z
727.227. Obsd: m/z 727.339.
(TPP)Co 1b. Yield: 44.8 mg, 82%. UV−vis (CH2Cl2): λmax = 416,
534 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.96 (s, 8H, pyrrole
H), 13.16 (s, 8H, ph-ortho H), 9.95 (s, 8H, ph-meta H), 9.74 (s, 4H,
ph-para H). MS (MALDI-TOF). Calcd for C44H28CoN4: m/z
671.165. Obsd: m/z 671.243.
(TpClPP)Co 1c. Yield: 43.0 mg, 80%. UV−vis (CH2Cl2): λmax = 416,
534 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.75 (s, 8H, pyrrole
H), 12.88 (s, 8H, ph-ortho H), 9.82 (s, 8H, ph-meta H). MS (MALDI-
TOF). Calcd for C44H24Cl4CoN4: m/z 809.006. Obsd: m/z 809.343.
Synthesis of Butano(TpYPP)Co. Butano(TpYPP)H2 (50 mg)
was dissolved in 150 mL of CHCl3/MeOH [4:1 (v/v)] containing
excess Co(OAc)2·2H2O (100 mg). The mixture was stirred for 2 h at
room temperature, and the progress of the reaction was monitored by
thin-layer and UV−vis spectra. After the starting compound was
completely consumed, the mixture was evaporated to dryness and then
freshly chromatographed with neutral alumina (200−300 mesh) using
CH2Cl2 as the eluent. The red fraction was collected and evaporated to
dryness.
EXPERIMENTAL SECTION
■
Instrumentation. Thin-layer UV−vis spectroelectrochemical
experiments were performed with a home-built thin-layer cell that
has a light transparent platinum net working electrode. Potentials were
applied and monitored with an EG&G PAR model 173 potentiostat.
Time-resolved UV−vis spectra were recorded with a Hewlett-Packard
model 8453 diode-array spectrophotometer. High-purity N2 was used
to deoxygenate the solution and kept over the solution during each
electrochemical and spectroelectrochemical experiment.
1H NMR spectra were recorded on a Bruker Avanc II 400 MHz
instrument. Chemical shifts (δ ppm) were determined with
tetramethylsilane as the internal reference. Matrix-assisted laser
J
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Figure 13. (i) Cyclic voltammograms under N2 without (- - -) or with 0.2 mM H2O2 (), (ii) current−voltage curve for catalyzed H2O2 in 1.0 M
HClO4, and (iii) a Koutecky−Levich plot. A rotating EPPG disk electrode coated with (a) benzo(TPP)CoII 3b and (b) benzo(TpClPP)CoII 3c was
employed. Scan rate = 50 mV/s.
Butano(TpCH3PP)Co 2a. Yield: 42.6 mg, 80%. UV−vis (CH2Cl2):
λmax = 426, 548, 582 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.69
(s, 8H, ph-ortho H), 12.70 (s, 16H, butano H), 10.27 (s, 8H, ph-meta
H), 4.35 (s, 12H, group H), 3.11 (s, 16H, butano H). MS (MALDI-
TOF). Calcd for C64H60CoN4: m/z 943.415. Obsd: m/z 943.817.
Benzo(TpCH3PP)Co 3a. Yield: 9.6 mg, 45%. UV−vis (CH2Cl2): λmax
= 446, 587, 638 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 13.89 (s,
8H, ph-ortho H), 13.03 (s, 8H, ph-meta H), 9.87 (s, 8H, benzo H),
8.81 (s, 8H, benzo H), 4.16 (s, 12H, group H). MS (MALDI-TOF).
Calcd for C64H44CoN4: m/z 927.290. Obsd: m/z 927.406.
Benzo(TPP)Co 3b. Yield: 11.3 mg, 53%. UV−vis (CH2Cl2): λmax
=
Butano(TPP)Co 2b. Yield: 43.8 mg, 82%. UV−vis (CH2Cl2): λmax
=
1
1
442, 586, 638 nm. H NMR (400 MHz, CDCl3, 298 K): δ 14.00 (s,
8H, ph-ortho H), 12.97 (s, 8H, ph-meta H), 10.11 (s, 8H, benzo H),
9.80 (s, 4H, ph-para H), 8.79 (s, 8H, benzo H). MS (MALDI-TOF).
Calcd for C60H36CoN4: m/z 871.227. Obsd: m/z 871.377.
Benzo(TpClPP)Co 3c. Yield: 10.0 mg, 47%. UV−vis (CH2Cl2): λmax
= 444, 586, 642 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 13.91 (s,
8H, ph-ortho H), 12.92 (s, 8H, ph-meta H), 10.06 (s, 8H, benzo H),
8.82 (s, 8H, benzo H). MS (MALDI-TOF). calcd for C60H32Cl4CoN4:
m/z 1007.071. Obsd: m/z 1006.649.
426, 548, 580 nm. H NMR (400 MHz, CDCl3, 298 K): δ 15.75 (s,
8H, ph-ortho H), 12.28 (s, 16H, butano H), 10.49 (s, 8H, ph-meta H),
10.23 (m, 4H, ph-para H), 3.15 (s, 16H, butano H). MS (MALDI-
TOF). Calcd for C60H52CoN4: m/z 887.352. Obsd: m/z 887.774.
Butano(TpClPP)Co 2c. Yield: 45.0 mg, 85%. UV−vis (CH2Cl2):
λmax = 424, 550, 581 nm. 1H NMR (400 MHz, CDCl3, 298 K): δ 15.63
(s, 8H, ph-ortho H), 11.55 (s, 16H, butano H), 10.49 (s, 8H, ph-meta
H), 3.16 (s, 16H, butano H). MS (MALDI-TOF). Calcd for
C60H48Cl4CoN4: m/z 1025.194. Obsd: m/z 1025.760.
Synthesis of Benzo(TpYPP)Co. The benzo(TpYPP)Zn com-
plexes (50 mg) were dissolved in 100 mL of CHCl3 containing 3 mL
of concentrated HCl. The mixture was stirred for 1 h at room
temperature, then washed with H2O, and dried over Na2SO4. After
that, the mixture was freshly chromatographed with neutral alumina
(200−300 mesh) using CHCl3 as the eluent. The green fraction was
collected and evaporated to dryness, and benzo(TpYPP)H2 was
obtained with a yield of ∼80%.
Benzo(TpYPP)H2 (20 mg) was dissolved in 80 mL of 1:1 CH2Cl2/
DMF containing Co(OAc)2·2H2O (50 mg). The reaction mixture was
refluxed for 2 h, and the progress of the reaction was monitored by
UV−vis spectra. After the starting compound was completely
consumed, the mixture was evaporated to remove CH2Cl2 and
washed with H2O. After filtration, the solid was dissolved in CHCl3,
dried over Na2SO4, and evaporated to dryness. The green fraction was
crystallized from CHCl3/hexanes to obtain benzo(TpYPP)Co.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02405.
UV−vis spectral changes and cyclic voltammograms
(PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: zpou2003@yahoo.com.
*E-mail: kkadish@uh.edu.
K
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ORCID
Karl M. Kadish: 0000-0003-4586-6732
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support from the National Natural
Science Foundation of China (Grants 21501070 and
21071067), Jiangsu University Foundation (Grants 15JDG131
and 05JDG051), China Postdoctoral Science Foundation
(Grant 2017M611707), and Robert A. Welch Foundation (to
K.M.K.; Grant E-680).
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