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Journal of Porphyrins and Phthalocyanines
Published at http://www.worldscinet.com/jpp/
J. Porphyrins Phthalocyanines 2016; 20: 35–44
DOI: 10.1142/S1088424616300032
Thermal and photoinduced electron-transfer catalysis
of high-valent metal-oxo porphyrins in oxidation of substrates
Shunichi Fukuzumi*a,b◊ and Wonwoo Nam*a◊
a
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea
Faculty of Science and Engineering, ALCA and SENTAN, Japan Science and Technology Agency (JST),
Meijo University, Nagoya, Aichi 468-0073, Japan
b
Dedicated to Professor Kevin M. Smith on the occasion of his 70th birthday
Received 26 November 2015
Accepted 5 December 2015
ABSTRACT: In this manuscript, we have overviewed thermal and photoinduced electron transfer
catalysis of high-valent metal-oxo porphyrins in oxidation of various substrates. The high-valent ironoxo porphyrin in cytochrome P450 (P450) is produced by photoinduced electron transfer from electron
donors, such as triethanolamine (TEOA), to the excited state of a photosensitizer such as eosin Y, followed
by the reduction of the heme domain of P450 by the resulting radical anion of the photosensitizer and the
subsequent reaction of the reduced heme with dioxygen (O2). Various substrates were oxidized by O2 in
this visible light-driven electron-transfer catalytic reaction with several P450s from bacteria and humans.
A manganese(V)-oxo corrorazine was produced by photoinduced electron transfer from the excited
state of manganese(III) corrorazine to O2, followed by hydrogen abstraction from toluene derivatives,
catalyzing the oxidation of toluene derivatives with O2 in the presence of an acid via photoinduced
electron transfer catalysis. High-valent manganese-oxo porphyrins are also produced by photoinduced
electron transfer from the excited state of [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) to electron acceptors,
followed by electron transfer oxidation of manganese(III) porphyrins with [Ru(bpy)3]3+, catalyzing
oxidation of various substrates with O2. Finally photoinduced electron-transfer catalysis of cobalt
porphyrins is discussed for the photocatalytic water oxidation with persulfate.
KEYWORDS: electron-transfer catalysis, metal-oxo complexes, metalloporphyrins, photocatalytic
oxidation.
INTRODUCTION
Cytochromes P450 can catalyze various oxidative
metabolic reactions of endogenerous and exogenerous
compounds in living organisms [1–8]. NAD(P)H
provides electrons to the diflavins of the reductase,
followed by the transfer of electrons to the catalytic
P450 heme domain through an intermolecular electron
transfer, resulting in the generation of a reactive iron(IV)oxo porphyrin p-radical cation species (Compound I,
◊
SPP full member in good standing
*Correspondence to: Shunichi Fukuzumi, email: fukuzumi@chem.
eng.osaka-u.ac.jp; Wonwoo Nam, email: [email protected]
Copyright © 2016 World Scientific Publishing Company
Cpd I) via reductive activation of O2 [1–8]. Efficient
and continuous supplementation of electrons to P450s
is required to produce iron-oxo species for the catalytic
oxidation of substrates with O2 [1–8]. Alternatively,
iron-oxo species are produced by electron-transfer
oxidation of the P450 heme via photoinduced electron
transfer from the excited state of [Ru(bpy)3]2+ (bpy =
2,2′-bipyridine) to sacrificial electron acceptors such
as [Co(NH3)5Cl]2+ to produce [Ru(bpy)3]3+, which can
oxidize the heme to produce iron-oxo species in which
oxygen comes from water [9–11]. Thus, high-valent
metal-oxo porphyrins (e.g. (P)MV(O)) can be produced
either by reductive activation of O2 with one-electron
reductants that can reduce (P)MIII with O2 or oxidative
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S. FUKUZUMI AND W. NAM
PHOTOINDUCED ELECTRONTRANSFER CATALYSIS OF P450 VIA
REDUCTIVE ACTIVATION OF O2
Scheme 1. Generation of (P)MV(O) via reduction activation of
O2 and oxidative activation of H2O
activation of water with one-electron oxidants that can
oxidize (P)MIII with H2O (Scheme 1) [12, 13]. In both
cases, (P)MV(O) can oxidize substrates to produce
the oxidized products, accompanied by regeneration
of (P)MIII, acting as catalysts for efficient oxidation of
substrates [13].
This minireview focuses on thermal and photo­
induced electron transfer catalysis of high-valent metaloxo porphyrins which are produced either by reductive
activation of O2 with one-electron reductants or by
oxidative activation of H2O with one-electron oxidants
for the catalytic oxidation of various substrates.
Cofactor-free in vivo photoreduction of P450s was
achieved by using eosin Y [C20H6O5Br4Na2, 2,4,5,7tetrabromofluorescein (EY)] as a photosensitizer that
mediates direct electron transfer to the heme domain
of P450 [14]. EY has frequently been used for various
biomedical and diagnostic purposes [15–18]. EY can
transfer photoexcited electrons directly to the heme
domain of P450 for the photoactivation of P450 catalytic
cycle in Scheme 2 [14]. The reduced heme reacts with
O2 to produce the iron-oxo species, while the oxidized
EY was reduced by sacrificial electron donors such as
triethanolamine (TEOA). The iron-oxo species can
oxidize various substrates for catalytic conversions
of marketed drugs including simvastatin, lovastatin,
17b-estradiol, and omeprazole by different human P450s
[14]. This whole-cell P450 system, in which photoexcited
EY can reduce P450 heme domain directly without
the need for a redox partner and a cofactor, provides
a powerful and cost-effective tool for visible lightdriven P450-catalyzed reactions using an inexpensive
photosensitizer and without the need for an expensive
cofactor and a redox partner.
Scheme 2. Photocatalytic oxygenation of RH with cytochrome P450 and Eosin Y
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THERMAL AND PHOTOINDUCED ELECTRON-TRANSFER CATALYSIS OF HIGH-VALENT METAL-OXO PORPHYRINS37
Scheme 3. Photocatalytic oxygenation of RH with P450 and a soluble deazaflavin mediator in combination with a sacrificial
electron donor (EDTA)
Scheme 4. Photocatalytic hydroxylation of lauric acid using a hybrid P450 BM3 enzyme with a Ru(II)-diimine photosensitizer and
a sacrificial electron donor, sodium diethyldithiocarbamate (DTC)
The light-driven cytochrome P450 catalyzed monoxygenation was also made possible by using a soluble
deazaflavin mediator [19] in combination with an
inexpensive sacrificial electron donor (EDTA) as
shown in Scheme 3 [20]. By using deazaflavins as
electron mediators, instead of normal flavins, a lightdriven regeneration of flavin-dependent enzymes can
also proceed efficiently under aerobic conditions [20].
Under aerobic conditions a pronounced reduction of the
P450-BM3 heme domain is achieved only in the presence
of deazaflavin [20]. In contrast to this, FAD as a mediator
failed to yield notable reduction of the heme domain with
additional NADP+ or under positive control conditions
(NADPH) [19]. This result suggests the superior electron
mediator properties of deazaflavins compared to flavins
of the isoalloxazine-type (FAD, FMN, riboflavin) under
aerobic conditions [20]. The light-driven hydroxylation
Copyright © 2016 World Scientific Publishing Company
of lauric acid occurred by P450-BM3 with deazaflavin
and NADP+ with the same regioselectivity as the P450
catalyzed hydroxylation with NADPH [20].
Hybrid P450 BM3 enzymes consisting of a Ru(II)diimine photosensitizer covalently attached to non-native
single cysteine residues of P450 BM3 heme domain
mutants [21]. The hybrid enzymes (Ru-K97C-BM3 and
Ru-Q397C-BM3) were assembled by covalently attaching
the photosensitizer Ru(II)-diimine to non-native single
cysteine residues of P450 BM3 heme domain mutants
[21]. These enzymes are capable, upon light activation,
of selectively hydroxylating lauric acid using a sacrificial
electron donor, sodium diethyldithiocarbamate (DTC),
with a TON number that is 40 times higher than the
value of the wild type heme domain (BM3-WT) using
the peroxide shunt (Scheme 4) [21]. The hybrid enzymes
most likely utilize reactive oxygen species generated
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S. FUKUZUMI AND W. NAM
in situ under the photoreductive conditions to perform
the hydroxylation reaction for more than two hours
[21]. The family of hybrid enzymes by site directed
mutagenesis has been prepared by modifying the nature
of the Ru(II)-diimine photosensitizer, resulting in diverse
catalytic activities in the hydroxylation of lauric acid
upon light activation [22–24]. When 10-undecenoic acid
was employed as a substrate, the hydroxylation with
the hybrid enzyme using DTC occurs exclusively at the
9 position to yield the R enantiomer with 85% ee [25].
Oxidation of 10-undecenoic acid by the light-activated
hybrid WT/L407C-Ru1 enzyme leads directly to a highly
enantiomerically enriched monohydroxylated product,
highlighting the advantages of biophotocatalysis over
traditional methods.
Reductive activation of O2 in a nonenzymatic
system was reported for a diiron(III) complex and
a ruthenium(II)-polypyridine-type complex, which
acts as a photosensitizer, and triethylamine (TEA),
which functions as a sacrificial electron donor [26].
Exposure of the photogenerated diiron(II) complex
to O2 leads to the formation of the diiron(III)–peroxo
intermediate responsible for oxygen-atom transfer to
triphenylphosphine to produce triphenylphosphine oxide.
PHOTOINDUCED ELECTRONTRANSFER CATALYSIS OF
MANGANESE CORRORZAINES
AND PORPHYRINS VIA REDUCTIVE
ACTIVATION OF O2
A well-characterized manganese(V)-oxo complex was
produced by visible light irradiation of a manganese(III)
corrolazine [MnIII(TBP8Cz): TBP8Cz3- = octakis(p-tertbutylphenyl)corrolazinato3-] in the presence of toluene
derivatives with O2 [27, 28]. MnIII(TBP8Cz) is converted
to the monoprotonated complex MnIII(OTf)(TBP8Cz(H))
in the presence of triflic acid (HOTf). The crystal structure
of MnIII(OTf)(TBP8Cz(H)) revealed that monoprotonation
occurs at one of the meso-N-atoms (N5(H)) adjacent to
the direct pyrrole–pyrrole bond of the corrolazine ligand,
and that the manganese center is five-coordinate with
an axially-ligated triflate (MnIII–O = 2.115(3) Å) [28,
29]. The photodynamics of MnIII(OTf)(TBP8Cz(H)) in
the presence of hexamethylbenzene (HMB) and O2 is
shown in Scheme 5 [29]. Photoexcitation of MnIII(OTf)
(TBP8Cz(H)) resulted in formation of the tripquintet state
{MnIII(OTf)(TBP8Cz(H))}* (5T1) upon photoirradiation
as revealed by femtosecond laser transient absorption
measurements [29]. The 5T1 of MnIII(OTf)(TBP8Cz(H))
is converted rapidly by intersystem crossing (ISC) to the
tripseptet excited state (7T1). Electron transfer from 7T1 to
O2 occurs to produce the superoxo complex MnIV(O2·-)
(OTf)(TBP8Cz(H)), which abstracts a hydrogen atom from
HMB to produce the hydroperoxo complex, MnIV(OOH)
(OTf)(TBP8Cz(H)), and pentamethylbenzyl radical,
in competition with the back electron-transfer (k–et)
to regenerate the ground state MnIII(OTf)(TBP8Cz(H))
and O2. The subsequent homolytic O–O bond cleavage
and combination with benzyl radical to yield PMB–OH
is accompanied by generation of high-valent Mn(V)-oxo
species [MnV(O)(OTf)(TBP8Cz(H))]. This species is
proposed to react with another equivalent of substrate
to produce PMB–OH, accompanied by regeneration of
MnIII(OTf)(TBP8Cz(H)) in the catalytic cycle (Scheme 5)
[30]. The oxidation of HMB by [MnV(O)(OTf)
(TBP8Cz(H))] is accelerated by the further addition of
Scheme 5. Photocatalytic cycle of oxygenation of hexamethylbenzene by O2 with the monoprotonated complex [MnIII(OTf)
(TBP8Cz(H))] in the presence of HOTf
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THERMAL AND PHOTOINDUCED ELECTRON-TRANSFER CATALYSIS OF HIGH-VALENT METAL-OXO PORPHYRINS39
HOTf as reported for acid-promoted oxidation reactions
of a variety of substrates with high-valent metal-oxo
complexes [31–37]. When MnIII(OTf)(TBP8Cz(H)) is
further protonated to [MnIII(OTf)(H2O)(TBP8Cz(H)2)]
[OTf] in the presence of a large excess of HOTf, however,
no catalytic activity is observed, because the tripseptet
excited state (7T1) of [MnIII(OTf)(H2O)(TBP8Cz(H)2)]
[OTf] cannot react with O2, possibly due to the higher
oxidation potential for [MnIII(OTf)(H2O)(TBP8Cz(H)2)]
[OTf] as compared with that of MnIII(OTf)(TBP8Cz(H))
[30]. When HMB was replaced by thioanisole in the
photocatalytic reaction with MnIII(OTf)(TBP8Cz(H)),
a single S-oxygenated product, methylphenylsulfoxide
(PhS(O)Me), was obtained with an excellent TON (902)
and conversion (98%) of PhSMe to PhS(O)Me [30]. In
this case as well, the triplet excited state of MnIII(OTf)
(TBP8Cz(H)) can react with O2 to produce the superoxo
complex MnIV(O2·-)(OTf)(TBP8Cz(H)), which reacts
with PhSMe to produce PhS(O)Me and high-valent
[MnV(O)(OTf)(TBP8Cz(H))] that can also oxygenate
PhSMe to produce another PhS(O)Me, accompanied by
regeneration of MnIII(OTf)(TBP8Cz(H)) [30].
Manganese porphyrins [(P)MnIII: 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphinatomanganese(III) hydroxide [(TMP)MnIII(OH)] and 5,10,15,20-tetrakis pentafluoro
phenyl)porphyrinatomanganese(III) acetate [(TPFPP)MnIII(CH3COO)]] also act as efficient photocatalysts for
oxygenation of 10-methyl-9,10-dihydro­
acridine (AcrH2)
by O2 to yield 10-methyl-(9,10H)-acridone (Acr=O) in
an oxygen-saturated benzonitrile (PhCN) solution under
visible light irradiation [38]. The mechanism of photo­
catalytic oxidation of AcrH2 by O2 with (P)MnIII to give
Acr=O as the product is shown in Scheme 6 [38]. The
photoexcitation of (P)MnIII results in formation of the
tripquintet excited state ([(P)MnIII]* (5T1)), which is
converted rapidly to the triplet excited state ([(P)MnIII]*
(7T1)) by intersystem crossing (ISC). Electron transfer from
[(P)MnIII]* (7T1) to O2 (ket) occurs to produce the superoxo
complex [(P)MnIV(O2•–)], followed by hydrogen-atom
transfer (HAT) from AcrH2 to (P)MnIV(O2·-) (kH) to afford
the hydroperoxo complex (P)MnIV(OOH) and acridinyl
radical (AcrH•) in competition with the back electron
transfer from AcrH• to (P)MnIV(O2·-) (k-et) to regenerate the
ground state of (P)MnIII and O2. The subsequent O–O bond
cleavage by AcrH• occurs rapidly to yield (P)MnV(O) and
9-hydroxy-10-methyl-9,10-dihydroacridine [AcrH(OH)].
This is followed by subsequent hydride transfer from
AcrH(OH) to (P)MnV(O) to yield the four-electron oxidized
product (Acr=O), accompanied by regeneration of (P)
MnIII (Scheme 6). The photocatalytic reactivity of (P)MnIII
agrees with the rate constants of electron transfer from [(P)
MnIII]* (7T1) to O2 in order (TMP)MnIII(OH) > (TBP8Cz)
MnIII > (TPFPP)MnIII(CH3COO) [38]. In both Schemes 5
and 6, MnV(O) species are produced via electron transfer
from the triplet excited state (MIII*) to O2, followed by
hydrogen atom transfer from HMB or oxygen atom transfer
to PhSMe to produce the oxygenated products, acting as
efficient catalysts for oxygenation of the substrates.
ELECTRON-TRANSFER CATALYSIS
OF MANGANESE PORPHYRINS BY
OXIDATIVE ACTIVATION OF H2O
Various high-valent metal-oxo species can also be pro­
duced by oxidative activation of H2O in which the oxygen
of the oxo group comes form H2O accompanied by twoelectron oxidation of the metal complexes [39–45]. Examples
of formation of manganese(V)-oxo porphyrins ([(P)MnV(O)]+) via electron-transfer oxidation of manganese(III)
Scheme 6. Photocatalytic cycle of oxygenation of AcrH2 by O2 with (P)MnIII
Copyright © 2016 World Scientific Publishing Company
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Scheme 7. Electron-transfer catalytic cycle of oxygenation of a
substrate (S) with H2O and [Ru(bpy)3]3+, catalyzed by (P)MnIII
epoxides, diols and aldehydes. Epoxides were converted
to the corresponding diols by hydrolysis, and the diols
were further oxidized to the corresponding aldehydes
which are four-electron oxidized products of olefins [45].
Hydroxylation of alkanes (ethylbenzene) also occurs by
the manganese porphyrin-catalyzed electron-transfer
oxidation with [Ru(bpy)3]3+ to yield the corresponding
alcohols [44]. The epoxidation of styrene and the
hydroxylation of ethylbenzene with [Ru(bpy)3]3+ in the
presence of 95% 18O-water containing a catalytic amount
of (P)MnIII afforded the corresponding epoxide and
alcohol with 90% incorporation of 18O in the oxygenated
products [45]. Thus, the oxygen source in the oxygenated
products in the manganese porphyrin-catalyzed electrontransfer oxygenation of substrates with [Ru(bpy)3]3+ has
been confirmed to be water in the mixed solvent (MeCN/
H2O) [45]. When [(TMP)MnIII(H2O)2](PF6) was used
as the catalyst, oxygenation of cyclohexene occurred
more efficiently than that of styrene probably due to the
stronger steric repulsion of styrene against the bulky
TMP ligand of [(TMP)MnV(O)]+ as compared with
that of cyclohexene. In the case of TDCPP ligand, the
oxygenation of cyclohexene and styrene is more reactive
than TMP ligand because of the less steric effect of the
TDCPP ligand as compared with the TMP ligand and
high redox potentials [44]. In the case of [(DTMPD)Mn2]
Cl2, the TON values are significantly smaller as compared
with those with the corresponding monomer, [(TMP)Mn]
Cl, because the MnV(O) species derived from [(DTMPD)
Mn2]Cl2 may be produced inside the diporphyrin cavity
to prevent the interaction with substrates [45].
The dependence of log ket of electron-transfer oxidation
of (P)MnIII with one-electron oxidants on the free energy
change of electron transfer (DG0et) is shown in Fig. 1,
where the log ket value increases with increasing the
driving force of electron transfer (–DG0et) in accordance
with the Marcus theory of adiabatic outer-sphere electron
transfer (Equation 1), where Z is the collision frequency
taken as 1 × 1011 M-1s-1 and l is the reorganization energy
of electron transfer [46, 47].
ket = Zexp[–(l/4)(l + DG0et/l)2/RT](1)
porphyrins ([(P)MnIII]+) are shown in Scheme 7, where [(P)
MnIII]+ is oxidized by electron transfer with two equivalents
of [Ru(bpy)3]2+ accompanied by deprotonation to produce
[(P)MnV(O)]+, which can oxygenate substrates (S) to SO,
accompanied by regeneration of [(P)MnIII]+ [45]. Manganese
porphyrins employed as catalysts in this study are [(TMP)
MnIII]+ (TMP2- = 5,10,15,20-tetrakis(2,4,6-tris-metylphenyl)porphyrin dianion), [(TDCPP)Mn]+ (TDCPP2- = 5,
10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin dianion),
[(TMOPP)MnIII]+, (TMOPP2- = 5,10,15,20-tetrakis-(2,4,6trimethoxyphenyl)porphyrin dianion), and [(DTMPD)
Mn2]2+, (DTMPD4- = di-trimesitylporphyrin dibenzofuran
tetraanion). Addition of [Ru(bpy)3]3+ to an MeCN solution
of olefins (e.g., styrene and cyclohexene) containing water
in the presence of a catalytic amount of (P)MnIII afforded
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The best-fit value of l in Fig. 1a, which includes
both electron transfer from (TMP)MnIII(OH) and (TMP)
MnIV(O) to one-electron oxidants, is determined to
be 24 kcal mol-1. The l values for the first and second
electron transfer from (TMP)MnIII(Cl) (34 kcal mol-1)
and (DTMPD)MnIII2(Cl)2 (40 kcal mol-1) to [Ru(bpy)3]3+
(Fig. 1b) are larger than that for electron transfer from
(TMP)MnIII(OH) due to the large reorganization of the Cl
ligand associated with the electron transfer [45].
The rate constant of the oxidation of cyclohexene
by [(TMP)MnV(O)]+ was determined by monitoring an
increase in the absorption band at 470 nm due to [(TMP)
MnIII]+ to be 1.6 × 105 M-1s-1 in MeCN/H2O (9:1 v/v) at
298 K. In such a case, the rate-determining step of the
electron-transfer catalytic oxidation of cyclohexene
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THERMAL AND PHOTOINDUCED ELECTRON-TRANSFER CATALYSIS OF HIGH-VALENT METAL-OXO PORPHYRINS41
Fig. 1. Dependence of log(ket, M-1s-1) on DG0et for the electrontransfer oxidation of (TMP)MnIII(OH) with various oxidants ((D)
[Fe(5-NO2phen)3](PF6)3, (u) [Ru(bpy)3](PF6)3, (s) [Fe(5-Clphen)3](PF6)3, (¨) [Fe(bpy)3](PF6)3) and ((u) (TMP)MnIII(OH),
(•) [(TMP)MnIII(H2O)2]+, () (TMP)MnIII(Cl) (p) (DTMPD)
MnIII2(Cl)2) by [Ru(bpy)3]3+ in MeCN containing [H2O] =
6.5 mM. The solid line represents the fits to Equation 1 with (a)
l = 24 kcal mol-1 (b) l = 34 kcal mol-1 and (c) l = 40 kcal mol-1
with [(TMP)MnIII(H2O)2]+ is electron transfer from
[(TMP)MnIII(H2O)2]+ to [Ru(bpy)3]3+ to produce [(TMP)
MnV(O)]+ rather than the oxidation of cyclohexene by
[(TMP)MnV(O)]+ [45].
PHOTOCATALYTIC OXIDATION
OF SUBSTRATES WITH
METALLOPORPHYRINS
Photocatalytic oxygenation of olefins and an alkane
by [CoIII(NH3)5Cl]2+, that is a weak one-electron oxidant,
occurs using 5,10,15,20-tetrakis(2,4,6-trimethyl-3-sul­fonato­
phenyl)porphinatomanganese(III) hydroxide ([(TMPS)
MnIII(OH)]) as an oxygenation catalyst and [Ru(bpy)3]2+
as a photosensitizer in the presence of water
which is shown to act as an oxygen source by
labeling experiments using H218O (Scheme 8) [48].
The epoxides initially formed are converted to the
corresponding diols by the hydrolysis because of the
acid produced in the catalytic oxygenation of olefins.
Photoirradiation (l > 430 nm) of phosphate buffer
(pH 7.4) containing sodium p-styrene sulfonate (NaSS),
(TMPS)MnIII(OH), [Ru(bpy)3]2+, and [CoIII(NH3)5Cl]2+ led to
formation of the epoxide, two-electron oxidation product in
a good yield (90%) based on the amount of [CoIII(NH3)5Cl]2+
as a one-electron oxidant. The photocatalytic hydroxylation
of sodium 4-ethylbenzene sulfonate also occurs efficiently
to yield the secondary alcohol [48]. The oxygenation of
these substrates has been achieved by the oxidation with a
high-valent manganese-oxo species, [(TMPS)MnV(O)]+,
produced by the two-electron oxidation of (TMPS)MnIII(OH)
with two equivalents of [Ru(bpy)3]3+, which was obtained by
the oxidative electron-transfer quenching of the excited state
of [Ru(bpy)3]2+ by [CoIII(NH3)5Cl]2+ as shown in Scheme 8
[48]. It was confirmed that no oxygenated product was
obtained in the absence of (TMPS)MnIII(OH) under otherwise
the same experimental conditions [48]. The quantum yields
(F) of the photocatalytic epoxidation of NaSS with water
increased with increasing concentration of [CoIII(NH3)5Cl]2+
to reach 50%, which is the maximum quantum efficiency
expected from the stoichiometry of the photocatalytic
oxygenation, because two photons are required for the twoelectron oxidation of the substrate [48]. When DClO4 (55 mM)
was added to the (TMPS)MnIII(OH)–[Ru(bpy)3]3+ system, no
catalytic oxygenation of cyclohexene was observed in D2O/
CD3CN (1:9, v/v) [48]. This indicates that the formation
of [(TMPS)MnV(O)]+ by the two-electron oxidation of
(TMPS)MnIII(OH) with two equivalents of [Ru(bpy)3]3+,
which requires the deprotonation process in Scheme 8,
is retarded by a strong acid. The rate constant of the oxidation
of NaSS with [(TMPS)MnV(O)]+ in H2O/CH3CN (1:3, v/v)
at 298 K was determined to be 4.0 × 104 M-1s-1, which is 67
times larger than the rate constant with [(TMPS)MnIV(O)]+
Scheme 8. Photocatalytic cycle of oxygenation of substrates (S) with H2O and [CoIII(NH3)5Cl]2+, catalyzed by [Ru(bpy)3]2+ and MnIII
porphyrins
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[48]. The reaction of (TMPS)MnIV(O) with NaSS occurs via
the disproportionation of (TMPS)MnIV(O) to give (TMPS)
MnIII(OH) and[(TMPS)MnV(O)]+, the latter of which is the
actual oxidant for the oxygenation reaction [48, 49].
The photosensitizer ([Ru(bpy)3]2+) was recently
connected to an Fe(II) complex with a pentadentate
amine/pyridine ligand and the RuII–FeII linked complex
was used as a photocatalyst for oxygenation of triphenyl­
phosphine with H2O to yield triphenylphosphine
oxide via formation of the FeIV(O) complex with
[CoIII(NH3)5Cl]2+ [50]. A mononuclear nonheme [MnIV(O)
(BQCN)]2+ complex (BQCN = N,N′-dimethyl-N,N′bis(8-quinolyl)cyclohexanediamine) was produced by
the electron-transfer oxidation of [MnII(BQCN)]2+ by
[Ru(bpy)3]3+ produced by photoinduced electron transfer
from the excited state of [Ru(bpy)3]2+ to [CoIII(NH3)5Cl]2+,
acting as a catalyst for oxidation of benzyl alcohol and
thioanisole with [CoIII(NH3)5Cl]2+ [51]. A mononuclear
nonheme [FeIV(O)(MePy2tacn)]2+ (MePy2tacn = N-methylN,N-bis(2-picolyl)-1,4,7-triazacyclononane) was also
produced by the electron-transfer oxidation of [FeII(MePy2­
tacn)(solvent)]2+ by [Ru(bpy)3]3+ produced by photo­
induced electron transfer from the excited state of
[Ru(bpy)3]2+ to persulfate (Na2S2O8), acting as a catalyst for
oxidation of p-methozylthioanisole with persulfate [52].
PHOTOCATALYTIC OXIDATION
OF WATER WITH
METALLOPORPHYRINS
High-valent metal-oxo complexes are known to be
active species for four-electron oxidation of water to
evolve O2 including the oxygen evolving complex (OEC)
in photosystem II (PSII) [53–58]. Sakai and coworkers
reported that water-soluble cobalt porphyrins act as
good catalysts for photocatalytic oxidation of water
with Na2S2O8 using [Ru(bpy)3]2+ as a photocatalyst. The
maximum TON (121) was obtained by photoirradiation
of CoTPPS (10 mM) in phosphate buffer (0.1 M, pH =
11) containing Na2S2O8 (5 mM) and [RuII(bpy)3](NO3)2
(1 mM) under Ar [59]. When CoTPPS was replaced by
a fluorinated CoFPS, TON was increased to 570 [60].
Cobalt porphyrins are also reported to act catalysts for
electrochemical oxidation of water [61, 62]. On the
other hand, the photocatalytic water oxidation to evolve
O2 was also reported to occur by photoirradiation (l >
420 nm) of an aqueous solution containing [Ru(bpy)3]2+,
Na2S2O8 and water-soluble cobalt complexes with
various organic ligands as precatalysts in the pH range
of 6.0–10.0 [63]. The turnover numbers (TONs) based
on the amount of Co for the photocatalytic O2 evolution
with [CoII(Me6tren)(OH2)]2+ and [CoIII(Cp*)(bpy)
(OH2)]2+ [Me6tren = tris(N,N′-dimethylaminoethyl)
amine, Cp* = h5-pentamethylcyclopentadienyl] at pH 9.0
reached 420 and 320, respectively [63]. Dynamic light
Copyright © 2016 World Scientific Publishing Company
scattering (DLS) experiments indicated the formation
of particles with diameters of around 20 × 10 nm
and 200 × 100 nm during the photocatalytic water
oxidation with [CoII(Me6tren)(OH2)]2+ and [CoIII(Cp*)
(bpy)(OH2)]2+, respectively [63]. The particle sizes
determined by DLS agreed with those of the secondary
particles observed by TEM [63]. The XPS measurements
of the formed particles suggest that the surface of the
particles is covered with cobalt hydroxides, which
could be converted to high-valent cobalt ions during
the photocatalytic water oxidation [63]. There are
many examples for metal complexes acting as only
precatalysts, which are converted to actual catalysts
that are metal nanoparticles during the water oxidation
reaction [64–70]. In the case of CoFPS-catalyzed lightdriven water oxidation by persulfate, no nanoparticulate
materials ware formed during the photocatalytic
oxidation [60]. The kinetic study suggested that the ratedetermining step of the photocatalytic water oxidation
with CoIIIFPS is the nucleophilic attack of water to the
two-electron oxidized CoIIIFPS (formally CoV(O)FPS),
which is produced by the electron-transfer oxidation
of CoIIIFPS by [Ru(bpy)]33+ following electron transfer
from the excited state of [Ru(bpy)3]2+ to persulfate [60].
However, the formation of CoV(O)FPS has yet to be
clarified.
CONCLUSION
High-valent metal-oxo porphyrins are produced
by the reductive activation of O2 with electron and
proton source or by the oxidative activation of H2O
with oxidants. The reductive activation of O2 results
in the catalytic oxidation of substrates by O2 with
reductants, via the formation of high-valent metal-oxo
species as active oxidants. Relatively strong reductants
required for the thermal reactions can be replaced by
weak reductants for the photocatalytic reactions via
photoinduced electron transfer from weak reductants
to the photocatalyst. On the other hand, the oxidative
activation of H2O results in the catalytic oxidation of
substrates with oxidants and H2O as an oxygen source,
also via the formation of high-valent metal-oxo species
as active oxidants. In this case as well, relatively
strong oxidants required for the thermal reactions can
be replaced by weak oxidants for the photocatalytic
reactions via photoinduced electron transfer from
the photocatalyst to weak oxidants. Cofactor-free
light-driven whole-cell cytochrome P450 catalysis
has been achieved via reductive activation of O2. The
alternative pathway of the oxidative activation of water
for cofactor-free P450 photocatalysis has yet to be
developed. It is hoped that the more efficient water
oxidation catalysts will be developed by optimizing the
oxidative activation of H2O.
J. Porphyrins Phthalocyanines 2016; 20: 42–44
1st Reading
THERMAL AND PHOTOINDUCED ELECTRON-TRANSFER CATALYSIS OF HIGH-VALENT METAL-OXO PORPHYRINS43
Acknowledgements
The authors gratefully acknowledge the contributions
of collaborators and co-workers mentioned in the
references. The authors also appreciate financial support
provided by Japan Science and Technology Agency
(JST to S.F.) and the NRF of Korea through CRI (NRF2012R1A3A2048842 to W.N.) and GRL (NRF-201000353 to W.N.).
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