A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen Catalyzed by N,N’-Dimethylated Porphyrin Isomers

Article abstract of DOI:10.1002/chem.202000942

Selective two-electron reduction of dioxygen (O₂) to hydrogen peroxide (H₂O₂) has been achieved by two saddle-distorted N,N’-dimethylated porphyrin isomers, an N21,N’22-dimethylated porphyrin (anti-Me₂P) and an N21,N’23-dimethylated porphyrin (syn-Me₂P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me₂P with higher turnover number (TON=250 for 30 min) than that by syn-Me₂P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me₂Iph or syn-Me₂Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O₂ with isophlorins based on kinetic analysis. The ORR rate by anti-Me₂Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me₂Iph by external protons was observed. The different mechanisms in the O₂ reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O₂ with inner NH protons of the isophlorins.

Full text of DOI:10.1002/chem.202000942

Accepted Article
Accepted Article
Title: A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen
Catalyzed by N,N’-Dimethylated Porphyrin Isomers
Authors: Takahiko Kojima, Wataru Suzuki, Hiroaki Kotani, and Tomoya
ishizuka
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To be cited as: Chem. Eur. J. 10.1002/chem.202000942
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01/2020

10.1002/chem.202000942
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A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen
Catalyzed by N,N’-Dimethylated Porphyrin Isomers
Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, and Takahiko Kojima*
Abstract: Selective two-electron reduction of dioxygen (O2) to
hydrogen peroxide (H2O2) has been achieved by two saddle-distorted
N,N’-dimethylated porphyrin isomers, an N21,N’22-dimethylated
porphyrin (anti-Me2P) and an N21,N’23-dimethylated porphyrin (syn-
Me2P) as catalysts and ferrocene derivatives as electron donors in the
presence of protic acids in acetonitrile. The higher catalytic
performance in an oxygen reduction reaction (ORR) was achieved by
anti-Me2P with higher turnover number (TON = 250 for 30 min) than
that by syn-Me2P (TON = 218 for 60 min). The reactive intermediates
in the catalytic ORR were confirmed to be the corresponding
isophlorins (anti-Me2Iph or syn-Me2Iph) by spectroscopic
measurements. The rate-determining step in the catalytic ORRs was
concluded to be proton-coupled electron-transfer reduction of O2 with
isophlorins based on kinetic analysis. The ORR rate by anti-Me2Iph
was accelerated by external protons, judging from the dependence of
the observed initial rates on acid concentrations. In contrast, no
acceleration of the ORR rate with syn-Me2Iph by external protons was
observed. The different mechanisms in the O2 reduction by the two
isomers should be derived from that of the arrangement of hydrogen
bonding of a O2 with inner NH protons of the isophlorins.
Introduction
between NH protons of a porphyrinoid and O2 has been
suggested in ORRs catalyzed by metal-free porphyrinoids.[16,17]
While the hydrogen bonding also work as a factor to decrease the
activation barrier of ORR by trapping the O2 molecule, the
relationship between the ORR reactivity for H2O2 production and
the strength of hydrogen bonding is still unclear. This should be
mainly due to the difficulty of changing the arrangement of
hydrogen bonding with O2 without changing other physical
properties such as redox potentials.
In this context, saddle-distorted porphyrins[19] should be a
good candidate for the evaluation of ORR reactivity owing to well-
known synthetic procedure,[20] redox properties,[21-24] and
availability of pyrrolic NH protons as hydrogen-bonding sites.[25]
Recently, we have synthesized N,N’-dimethylated saddle-
distorted porphyrin isomers, syn-Me2P and anti-Me2P (Scheme
1), having two methyl groups at the different position on inner
nitrogen atoms of the porphyrin framework, while the molecular
conformation and redox potentials of two isomers were almost the
same.[26] Moreover, we established reversible O2/H2O2
conversion by using syn-Me2P and the corresponding isophlorin
derivative (syn-Me2Iph) as a two-electron reduced species,
through the two-point hydrogen-bonding interaction between the
porphyrinoids and H2O2 or O2.[26] In stark contrast, anti-Me2P or
the corresponding isophlorin derivative (anti-Me2Iph) were
Oxygen reduction reaction (ORR) is recognized as one of
the most fundamental reactions in biological processes for cellular
respiration and chemical technologies such as fuel cells.[1] In
addition, hydrogen peroxide (H2O2), a two-electron-reduced
product of dioxygen (O2), is widely required for the industrial
usage for paper bleaching, a liquid fuel in the component of fuel
cells, and an environmentally benign oxidant.[2] H2O2 has been
industrially supplied through the “anthraquinone process”, which
is achieved by two-electron ORR. However, there are serious
issues to be solved in this process: Rare-metal catalysts have
been employed to proceed catalytic hydrogenation of an
anthraquinone derivative by a hydrogen gas.[3] Thus, alternative
catalysts for efficient and selective H2O2 production have been
highly demanded without using precious metals.
So far, electrocatalytic,[4] photocatalytic,[5-8] and thermal[9-14]
O2 reduction to afford H2O2 have been achieved by redox-active
metal complexes having Co[9-12] and Cu[13] centers. Instead of
transition metal catalysts, redox-active organic molecules
including flavin derivatives,[15] porphyrinoids,[16,17] and carbon-
based materials[18] also have been reported as ORR catalysts for
selective two-electron reduction of O2. Compared with metal
catalysts, redox-active organic molecules are able to reduce O2
to H2O2 selectively because side-reactions such as
disproportionation or decomposition of H2O2 by metal catalysts[8e]
can be suppressed. In addition, compared with coordinate bonds
in metal complexes, the weaker non-covalent interaction of
organocatalysts with O2 would prevent the O–O bond cleavage to
form H2O as a four-electron reduced product of O2 and other
radical species such as hydroxyl (•OH) and hydroperoxyl (•OOH)
radicals, showing high selectivity in the formation of H2O2. For
example, the importance of hydrogen-bonding interaction
Department of Chemistry, Faculty of Pure and Applied Sciences, University of
Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: kojima@chem.tsukuba.ac.jp.
Scheme 1. Overview of formation of N,N’-dimethylated isophlorin derivatives
for selective two-electron reduction of O2 to H2O2.
Electronic Supplementary Information (ESI) available: Crystallographic data of syn-
H2Me2P2+(CF3COO–)2 in the CIF format, 1H NMR, UV-vis, and electrochemical data.
CCDC-1954353. See DOI: 10.1039/x0xx00000x
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revealed to be ineffective for the reversible conversion of O2/H2O2,
suggesting just one-point hydrogen-bonding interaction of the
porphyrinoids with H2O2 or O2.[26] Based on the observation, we
have hypothesized that the structural difference in the porphyrin
isomers would contribute to the catalytic performance in ORR.
Herein, we report catalytic and selective two-electron reduction of
O2 to H2O2 catalyzed by two kinds of N,N’-dimethylated
porphyrins (syn-Me2P or anti-Me2P) as catalysts to gain
mechanistic insights into ORRs and the controlling factors of the
reactivity of the porphyrins as catalysts in ORRs (Scheme 1).
(Table 1, Entry 1). The amounts of H2O2 formed in the catalytic
reactions were quantified by iodometry upon addition of excess
amounts of potassium iodide (KI) to diluted reaction mixtures after
the catalytic reactions.[9b,27] As shown in Table 1, both syn-Me2P
and anti-Me2P acted as catalysts for two-electron-reduction of O2
to form H2O2 in CH3CN. While the turnover number (TON) of the
H2O2 production by syn-Me2P was determined to be 32 with 64%
yield based on the amount of Me8Fc used (Entry 2), TON for anti-
Me2P was reached to 50 in 100% yield (Entry 3). Upon addition of
1% H2O in CH3CN, TON of catalytic H2O2 production with use of
syn-Me2P was improved to be 45 and the yield of H2O2 was 90%
based on the amount of Me8Fc used (Entry 4). When the water
contents increased up to 10% in CH3CN, maximum TONs were
reached to 218 (87% yield) for 1 h for syn-Me2P (Entry 5) and 250
(100% yield) for 30 min for anti-Me2P (Entry 6), although the yield
of H2O2 was negligible in the absence of the catalysts (Entry 7).
The addition of H2O is effective in enhancing the catalytic
selectivity and yield of H2O2 catalyzed by syn-Me2P. These TONs
were higher than those of previously reported metal-free O2
reduction catalysts.[16,17]
Results and Discussion
Catalytic Reduction of O2 to H2O2 with Me8Fc as a Reductant
using syn-Me2P or anti-Me2P as a Catalyst
By using syn-Me2P or anti-Me2P as a catalyst, we
developed catalytic ORR systems to afford H2O2 selectively in
acetonitrile (CH3CN) (Scheme 2). Octamethylferrocene (Me8Fc;
Eox = –0.40 V vs. Fc+/Fc in CH3CN)[14f] and trifluoroacetic acid
(TFA) were selected as an electron donor and a proton source,
respectively. Note that no H2O2 was formed without the catalysts
Characterization of Reduced Species of syn-Me2P and anti-
Me2P in the Presence of Protons
To reveal the origin of the difference in the reactivity
between syn-Me2P and anti-Me2P in the catalytic ORR,
electrochemical measurements of syn-Me2P and anti-Me2P were
performed for the comparison of their redox potentials (E1/2) in
deaerated CH3CN solutions in the presence of TFA and [(n-
butyl)4N](PF6) (TBAPF6) as an electrolyte (Figure S1). In the
presence of excess amount of acids, saddle-distorted porphyrins
such as dodecaphenylporphyrin (H2DPP) were easily protonated
to form diprotonated ones such as H4DPP2+.[19b] Since syn-Me2P
and anti-Me2P also were confirmed to show highly saddle-
distorted conformations,[26] they were protonated by TFA to form
the corresponding diprotonated species (syn-H2Me2P2+ and anti-
H2Me2P2+), as confirmed by UV-vis spectroscopic and X-ray
crystallographic analysis (Figure S2 and S3). Thus, we observed
redox waves of syn-H2Me2P2+ and anti-H2Me2P2+ under the
electrochemical conditions, as described below. Electrochemical
data of syn-H2Me2P2+ showed one reversible two-electron
reduction at –0.38 V vs. Fc+/Fc (Figure S1a) due to the
disproportionation of one-electron reduced species as observed
for H4DPP2+.[28] On the other hand, two reversible redox waves
were observed at –0.33 V and –0.44 V vs. Fc+/Fc in the case of
anti-H2Me2P2+ (Figure S1b), suggesting the stepwise one-
electron reduction of anti-H2Me2P2+ to form two-electron reduced
species. The reason why the stepwise redox processes of anti-
H2Me2P2+ is probably the slow disproportionation of one-electron
reduced species of anti-H2Me2P2+ (anti-H2Me2P•+). Thus, such
small difference of redox potentials between syn-H2Me2P2+ and
anti-H2Me2P2+ should not affect the difference of reactivity in
catalytic ORR between syn-Me2P and anti-Me2P (Table 1).
Next, to characterize two-electron-reduced species of syn-Me2P
or anti-Me2P in the presence of excess TFA (pKa = 12.6 in
CH3CN)[29,30] under Ar, chemical reduction by Me8Fc was
cat.
O2
+
2H+
+
2Me8Fc
H2O2 +
2Me8Fc+
cat. = syn-Me2P or anti-Me2P
Scheme 2. Catalytic O2 reduction to afford H2O2 with Me8Fc by N,N’-
dimethylated porphyrins in CH3CN.
Table 1. Summary of reaction conditions, TONs and yields of H2O2 production
in catalytic reduction of O2 catalyzed by syn-Me2P or anti-Me2P.
Entry
1
[cat.] / µM
Solv. (v/v)
CH3CN
TON
Yield
0%
0
–
2
3
10 (syn-Me2P)
10 (anti-Me2P)
10 (syn-Me2P)
2 (syn-Me2P)
2 (anti-Me2P)
0
CH3CN
32
50
64%
100%
90%
87%
100%
6%
CH3CN
4
CH3CN/H2O = 99/1
CH3CN/H2O = 90/10
CH3CN/H2O = 90/10
CH3CN/H2O = 90/10
45
5[a]
218
250
–
6
7[a]
Catalytic conditions: [TFA] = 3 mM, [Me8Fc] = 1 mM, under O2, at room
temperature for 30 min. [a] For 60 min. TON = [H2O2 ]/ [cat.]. Yield (%) = (100 ´
[H2O2]) / (2 ´ [Me8Fc]). [H2O2] was determined by iodometry.
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scrutinized by UV-vis and NMR measurements at 298 K
(Schemes 3 and 4). Upon addition of Me8Fc (1.0 mM) to a
deaerated CH3CN solution containing syn-Me2P (0.010 mM) with
TFA (3.0 mM), absorption bands at 503 and 730 nm derived from
syn-H2Me2P2+, which was formed by diprotonation of syn-Me2P
with TFA (Figure S2), disappeared with simultaneous appearance
of an absorption band at 450 nm as shown in Figure 1. The
observed UV-vis spectral change indicated the formation of an
isophlorin derivative (syn-Me2Iph) because the spectral change
was comparable with that observed in the reaction of syn-Me2P
with an aqueous solution of Na2S2O4 in deaerated DMF.[22] In
addition, a 19F NMR spectrum of syn-Me2P in the presence of TFA
and Me8Fc in CD3CN showed a single peak at –63.4 ppm,
reflecting the C2v symmetric structure of syn-Me2Iph (Figure S4).
Thus, even in the presence of excess TFA, no further
transformation or isomerization of syn-Me2Iph occurred.
Scheme 3. Chemical reduction of syn-H2Me2P2+ to form the corresponding
isophlorin derivative (syn-Me2Iph) in the presence of an acid.
Figure 2. a) UV-vis spectral changes of anti-Me2P (0.010 mM) in the course of
the reduction with Me8Fc (0.50 mM) in the presence of TFA (1.0 mM) in
deaerated CH3CN at 298 K. Inset: Time-course of absorbance at 980 nm with
curve fitting. b) Dependence of kobs on [TFA] in the presence of Me8Fc (0.50
mM). c) Dependence of kobs on [Me8Fc] in the presence of TFA (1.0 mM).
In contrast to the case of syn-Me2P, chemical reduction of
anti-Me2P by addition of Me8Fc in CH3CN containing excess TFA
afforded initially the corresponding isophlorin derivative (anti-
Me2Iph) through the two-electron reduction of anti-H2Me2P2+;
however, anti-Me2Iph underwent subsequent isomerization to a
protonated phlorin (Phlo-H+)[23] as shown in Scheme 4. UV-vis
spectrum of anti-Me2Iph showing the absorption maxima at 466
nm changed to that with absorption bands at 507 nm and around
980 nm derived from Phlo-H+ (Figure 2a).[23,31] The pseudo-first-
order rate constant (kobs) of the protonation-induced isomerization
from anti-Me2Iph to Phlo-H+ was determined at 298 K to be (4.09
± 0.02) ´ 10–3 s–1 in the presence of TFA (1.0 mM) and Me8Fc
(0.50 mM) on the basis of the time course of absorbance at 980
nm. The rate constant was consistent with that of absorbance at
466 nm (kobs = (3.94 ± 0.02) ´ 10–3 s–1, Figure 2a inset). The kobs
values showed linear dependence on the concentration of TFA
(Figure 2b), while no dependence on that of Me8Fc was observed
(Figure 2c). Thus, the rate of the protonation-induced
isomerization of anti-Me2Iph to afford Phlo-H+ is given by
Equation (1):
Figure 1. UV-vis spectral change of syn-Me2P (0.010 mM) before (red) and
after (purple) the reduction with Me8Fc (1.0 mM) in the presence of TFA (3.0
mM) in deaerated CH3CN at 298 K.
d[Phlo-H+]/dt = kobs[anti-Me2Iph] = k[TFA][anti-Me2Iph]
(1)
On the basis of Equation (1), the second-order rate constant (k)
was determined to be (4.0 ± 0.2) M–1 s–1 from the slope in Figure
2b. When dichloroacetic acid (DCA, pKa = 15.9 in CH3CN)[29,30]
was used as acids instead of TFA, the second-order rate constant
Scheme 4. Chemical reduction of anti-Me2P in the presence of an acid to form
protonated phlorins.
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(k) decreased to (1.6 ± 0.4) M–1 s–1 due to the lower acidity of DCA
than that of TFA (Figure S5). This result indicates that the
isomerization rates of anti-Me2Iph depends on not only the
concentration of acids but also the pKa values of acids used as
shown in Figure 2b and Figure S5.
Then, we also performed 1H and 19F NMR measurements of
Phlo-H+ in CD3CN. A 1H NMR spectrum of Phlo-H+ showed two
sets of 1H NMR signals derived from non-equivalent N-Me groups
at 4.20, 3.82 ppm and 4.52, 4.83 ppm with the intensity ratio of
2:1, accompanied by the observation of singlet signals due to the
methine proton on a meso-carbon atom at 6.27 ppm and 6.50
ppm with the same integration ratio (Figure S6). In the 19F NMR
spectrum, two sets of four non-equivalent signals derived from the
trifluoromethyl (CF3) groups were also observed with the 2:1
integration ratio as depicted in Figure S7, in consistent with the
result of H NMR measurement. The H and 19F NMR spectra
reflected the low symmetrical structure of Phlo-H+, which should
have one imine nitrogen atom and one sp3 meso-carbon atom in
the porphyrinoid core.[23] Because the meso-carbon atoms of a
protonated phlorin bear non-equivalent nature, there are several
isomers of Phlo-H+ depending on the position of an sp3 meso-
carbon atom. To compare the thermodynamic stability among
structural isomers of the protonated phlorins, DFT calculations on
possible three structural isomers (Phlo(n)-H+, n = 1-3) were
conducted at the B3LYP/6-31G** level of theory. As shown in
Figure S8, Phlo1-H+ having an sp3 meso-carbon atom between
two non-methylated NH pyrrole moieties was the most stable
among the three isomers, while Phlo3-H+ having an sp3 meso-
carbon atom between two N-Me pyrrole moieties was the most
unstable. Then, Phlo2-H+ having an sp3 meso-carbon atom
between one NH pyrrole moieties and N-Me pyrrole moieties was
relatively unstable in comparison with Phlo1-H+ by 4.2 kcal mol–1.
Considering that two kinds of methine protons were observed in
1H NMR measurements of the reduced species (Figure S6),
Phlo1-H+ and Phlo2-H+ should be formed in the 2:1 ratio by
chemical reduction of anti-Me2P in the presence of external
protons (Scheme 4). Hereafter “Phlo-H+” dictates a mixture of
Phlo1-H+ and Phlo2-H+.
1
1
Figure 3. a) UV-vis spectral change in the course of catalytic O2 reduction
catalyzed by syn-Me2P (0.010 mM) in air-saturated CH3CN. [TFA] = 1.0 mM,
[Me10Fc] = 0.10 mM. Inset: Time profile of [Me10Fc+]. b) [Me10Fc] dependence
of v. c) [O2] dependence of v. d) [syn-Me2P] dependence of v. e) [TFA]
dependence of v.
Me2Iph was maintained after adding Me10Fc as an electron donor
(t = 0 s). In addition, the formation rate of Me10Fc+ obeyed zero-
order kinetics during catalytic ORR reactions (Figure 3a, inset).
After the consumption of Me10Fc (t = 260 s), syn-Me2P was
recovered as the diprotonated species (syn-H2Me2P2+) owing to
the protonation by excess amount of remained TFA (Figure S9).
It should be noted that no decomposition of syn-Me2P was
observed after the catalytic reaction, judging from the comparison
of the UV-vis spectra before and after the catalytic reactions
(Figure S9). Then, dependence of the zero-order rate (v, mM s–1)
of formation of Me10Fc+ on [Me10Fc], [O2], [syn-Me2P], and [TFA]
were investigated by changing the concentration of each
component. First-order dependence of v on [O2] and [syn-Me2P]
was observed, while no dependence of v on [Me10Fc] and [TFA]
was observed (Figure 3b-e). These results indicate the reaction
between syn-Me2Iph and O2 is the rate-determining step in the
catalytic ORR cycle. Thus, the v value in the catalytic ORR by
syn-Me2P is given by Equation (2):
Revealing the Reaction Mechanisms in ORR Catalyzed by
Dimethylated Porphyrins
To gain mechanistic insights into the catalytic ORR catalyzed by
syn-Me2P and anti-Me2P, kinetic analyses on ORR were
conducted under the catalytic conditions. Thus, for revealing
reaction mechanisms of the catalytic H2O2 production,
decamethylferrocene (Me10Fc, Eox = –0.49 V vs. Fc+/Fc in
CH3CN)[14f] was employed as an electron donor to reduce syn-
H2Me2P2+ and anti-H2Me2P2+ in the presence of excess TFA. At
first, catalytic ORR by syn-Me2P (0.010 mM) was conducted in
CH3CN containing TFA (1.0 mM) and Me10Fc (0.10 mM) in air-
saturated CH3CN ([O2] = 2.6 mM, Figure 3).[14b] As shown in
Figure 3a, UV-vis spectral change corresponding to the formation
of a decamethylferrocenium ion (Me10Fc+) showing the absorption
around 800 nm was observed in the course of the catalytic
reaction,[16] whereas the absorption band at 450 nm owing to syn-
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In the case of catalytic ORR by anti-Me2P, anti-Me2Iph or
Phlo-H+ can be possible reactive intermediates for catalytic H2O2
production as described above. Thus, the reactivity of anti-
Me2Iph and Phlo-H+ toward O2 was qualitatively compared by
changing the reaction conditions in CH3CN (Figure S11 and S12).
When slightly excess amounts of Me10Fc was used for the
reduction of anti-Me2P under O2 in the presence of excess
amounts of TFA, anti-Me2Iph was formed and smoothly reacted
with O2 to form anti-H2Me2P2+ within 5 seconds, while the
formation of Phlo-H+ was hardly observed under the conditions
(Figure S11). On the other hand, the reaction of Phlo-H+ with O2
was significantly slow when O2 was introduced after the formation
of Phlo-H+, which was generated by reduction of anti-Me2P with
Me10Fc in the presence of excess amounts of TFA under Ar
(Figure S12). Therefore, the reactivity of anti-Me2Iph with O2 is
much higher than that of Phlo-H+ in the catalytic reduction of O2,
suggesting the reaction intermediate in ORR catalyzed by anti-
Me2P should be anti-Me2Iph, not Phlo-H+. Thus, in the presence
of O2, ORR by anti-Me2Iph should proceed much faster than the
isomerization under the catalytic conditions without suffering from
the isomerization.
d[Me10Fc+]/dt = v = k1[syn-Me2P][O2]
(2)
Interestingly, no dependence of v on acid concentrations was
observed (Figure 3e) in spite of the fact that two protons should
be needed to form H2O2 from O2. Furthermore, the averaged v
value (v = 2.5 ´ 10–4 mM s–1) in the presence of TFA was almost
the same as that in the presence of DCA. (Figure 3e, 4, and S10).
These kinetic analyses indicate that syn-Me2Iph reacts with O2
without acceleration by external protons in the reaction of syn-
Me2Iph with O2. Therefore, it is considered that syn-Me2Iph can
form a two-point hydrogen bonding with O2 and reduce O2 to
produce H2O2 through proton-coupled electron transfer (PCET)
from syn-Me2Iph to O2 (Scheme 5a).[26]
Next, kinetic analysis in the catalytic reduction of O2 by anti-
Me2P was examined. To suppress the transformation of anti-
Me2Iph to Phlo-H+ by external protons, DCA was selected as a
proton source (Figure S5). In the case of the reduction of O2
catalyzed by anti-Me2P in the presence of excess DCA, formation
of anti-Me2Iph was observed in UV-vis measurements during the
catalytic reaction in CH3CN containing DCA (1.0 mM) and Me10Fc
(0.10 mM), indicating anti-Me2Iph should be
a reactive
intermediate (Figure S13). Due to the partial formation of Phlo-H+,
the yield of anti-Me2P after the catalytic reaction was slightly
decreased on the basis of the comparison of absorbance at 494
nm before and after the catalytic reaction (Figure S14). The
formation rate of Me10Fc+ obeyed zero-order kinetics (Figure S13,
inset). Furthermore, linear correlation was observed between v
and [O2] as shown in Figure S15. Therefore, the rate-determining
step in the catalytic cycle was also concluded to be the reaction
of anti-Me2Iph with O2. In contrast to the case of syn-Me2P, first-
order dependence of v of formation of Me10Fc+ on [DCA] was
observed when using anti-Me2P as a catalyst (Figure 4, blue
circle). In addition, v of the ORR catalyzed by anti-Me2P was
higher than that by syn-Me2P in the presence of more than 1 mM
of DCA as shown in Figure 4. Thus, the v value in the catalytic
ORR by anti-Me2P is given by Equation (3):
Scheme 5. Comparison of reaction mechanisms in two-electron-reduction of O2
by a) syn-Me2Iph or b) anti-Me2Iph.
d[Me10Fc+]/dt = v = k2[anti-Me2P][O2][DCA]
(3)
The first-order dependence of the zero-order rate constant on
[DCA] indicates that an external proton is involved in the rate-
determining step in the catalytic cycle in use of anti-Me2P as the
catalyst.[32] Since NH protons of anti-Me2Iph should be oriented
to different directions relative to the mean plane of the saddle-
distorted macrocycle, only one-point hydrogen bonding should be
available between anti-Me2Iph and O2 in the course of ORR
(Scheme 5b). This assumption was supported by DFT
Figure 4. Plots of zero-order rate (v) vs. [DCA] in the catalytic H2O2 production
by syn-Me2P (red circle) and anti-Me2P (blue circle).
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Conclusions
In summary, we have developed catalytic systems for
selective two-electron reduction of O2 to afford H2O2 by N,N’-
dimethylated saddle-distorted porphyrin isomers (syn-Me2P and
anti-Me2P) as metal-free organocatalysts. The high selectivity
and the highest TON for H2O2 production among homogeneous
metal-free catalysts were achieved by using these two structural
isomers and anti-Me2P showed higher reactivity than syn-Me2P.
The reaction intermediates in the ORR catalyzed by syn-Me2P
and anti-Me2P were the corresponding isophlorin derivatives
(syn-Me2Iph and anti-Me2Iph, respectively), which are two-
electron reduced porphyrins having two inner NH protons at the
macrocycle core. On the other hand, protonated phlorins (Phlo-
H+), formed through the reaction of anti-Me2Iph with external
protons, showed much less reactivity toward O2 than that of
isophlorin derivatives. The ORR mechanisms are drastically
changed by the positions of NH protons in syn-Me2Iph and anti-
Me2Iph: Single-point hydrogen bonding between anti-Me2Iph and
O2 play an important role to promote O2 reduction by virtue of
further strong interactions with external protons. Thus, the
arrangement and hydrogen-bonding abilities of NH protons of
reduced porphyrinoids with O2 should be crucial to control their
reactivity in metal-free ORR catalysis.
Figure 5. Proposed catalytic cycles of two-electron reduction of O2 to H2O2
catalyzed by a) syn-Me2P and b) anti-Me2P. D = Electron donor. r.d.s. = rate-
determining step.
Acknowledgements
calculations on the O2 adduct of anti-Me2Iph (anti-Me2Iph-O2,
Figure S16) to indicate the formation of single-point hydrogen
bonding between an oxygen atom of O2 and a proton on one of
the pyrrole nitrogen atoms. Thus, the distal oxygen atom of O2 is
able to accept an external proton from an acid such as DCA,
accelerating the proton-coupled reduction of O2 by anti-Me2Iph.
Therefore, higher catalytic reactivity of anti-Me2P than that of
syn-Me2P could be observed under catalytic conditions where
excess proton source existed (Scheme 5). Proposed catalytic
cycles of two-electron reduction of O2 to H2O2 catalyzed by syn-
Me2P and anti-Me2P are summarized in Figure 5. In the case of
syn-Me2P, as depicted in Figure 5a, thermal ET from an electron
donor such as Me8Fc to syn-H2Me2P2+ affords the two-electron-
reduced species, syn-Me2Iph, as the responsible intermediate for
the ORR in the presence of a proton source such as TFA and
DCA. The reduced species, syn-Me2Iph, interacts directly with O2
through two-point hydrogen bonding and reduces O2 to form H2O2
without the aid of external protons.[26] After releasing H2O2, the
syn-Me2P is recovered and diprotonated by the proton source to
regenerate syn-H2Me2P2+. On the other hand, in the case of anti-
Me2P (Figure 5b), after forming anti-Me2Iph via thermal ET from
an electron donor to anti-H2Me2P2+, the two-electron reduced
species interacts with O2 through single-point hydrogen bonding.
The captured O2 molecule picks up one external proton from an
external proton source such as TFA and DCA to be reduced by
anti-Me2Iph, affording H2O2. After releasing H2O2, the resultant
anti-Me2P is diprotonated by the proton source to recover anti-
H2Me2P2+.
This work was supported by a Grant-in-Aid (17H03027) from the
Japan Society of Promotion of Science (JSPS, MEXT) of Japan
and the Cooperative Research Program of “Network Joint
Research Center for Materials and Devices”. Financial support
through CREST (JST) is also appreciated (JPMJCR16P1). W.S.
appreciates a support from JSPS Research Fellowship for Young
Scientists (18J12184).
Keywords: Oxygen reduction reaction (ORR) • Non-planar
porphyrin • Isophlorin • Hydrogen bonding • Organocatalysts
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Wataru Suzuki, Hiroaki Kotani, Tomoya
Ishizuka, and Takahiko Kojima*
Selective two-electron reduction of
dioxygen to hydrogen peroxide has
been achieved using two N,N’-
dimethylated porphyrin isomers as
catalysts. Based on kinetic analysis,
we have revealed the difference of
position on pyrrolic protons in reduced
porphyrinoids causes difference in
reaction mechanisms of dioxygen
reduction with regard to the proton
source of H2O2, whether pyrrole N-H
protons or external protons.
Page No. – Page No.
A Mechanistic Dichotomy in Two-
Electron Reduction of Dioxygen
Catalyzed by N,N’-Dimethylated
Porphyrin Isomers
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