Thermal and photoinduced electron-transfer …cbs.ewha.ac.kr/pub/data/2016_20_JPP_20(1-4)_35_44.pdfcatalyzed hydroxylation with NADPH [20]. Hybrid P450 BM3 enzymes consisting of a

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Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines 2016; 20: 35–44

DOI: 10.1142/S1088424616300032

Published at http://www.worldscinet.com/jpp/

Copyright © 2016 World Scientific Publishing Company

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,

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

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 b Faculty of Science and Engineering, ALCA and SENTAN, Japan Science and Technology Agency (JST), Meijo University, Nagoya, Aichi 468-0073, Japan

Dedicated to Professor Kevin M. Smith on the occasion of his 70th birthday

Received 26 November 2015Accepted 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 iron-oxo 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.

◊ SPP full member in good standing

*Correspondence to: Shunichi Fukuzumi, email: [email protected]; Wonwoo Nam, email: [email protected]

http://dx.doi.org/10.1142/S1088424616300032

http://dx.doi.org/10.1142/S1088424616300032

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Copyright © 2016 World Scientific Publishing Company J. Porphyrins Phthalocyanines 2016; 20: 36–44

36 S. FUKUZUMI AND W. NAM

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 metal-oxo 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.

PHOTOINDUCED ELECTRON-TRANSFER CATALYSIS OF P450 VIA REDUCTIVE ACTIVATION OF O2

Cofactor-free in vivo photoreduction of P450s was achieved by using eosin Y [C20H6O5Br4Na2, 2,4,5,7- tetrabromofluorescein (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 light-driven P450-catalyzed reactions using an inexpensive photosensitizer and without the need for an expensive cofactor and a redox partner.

Scheme 1. Generation of (P)MV(O) via reduction activation of O2 and oxidative activation of H2O

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 PORPHYRINS 37

The light-driven cytochrome P450 catalyzed monoxy- genation 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 light-driven 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

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

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)

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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 ELECTRON-TRANSFER 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-tert-butylphenyl)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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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 two-electron 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

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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-metyl-phenyl)porphyrin dianion), [(TDCPP)Mn]+ (TDCPP2- = 5, 10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin dianion), [(TMOPP)MnIII]+, (TMOPP2- = 5,10,15,20-tetrakis-(2,4,6-trimethoxyphenyl)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

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 electron-transfer 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 (DG0

et) is shown in Fig. 1, where the log ket value increases with increasing the driving force of electron transfer (–DG0

et) 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)

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)MnIII

2(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

Scheme 7. Electron-transfer catalytic cycle of oxygenation of a substrate (S) with H2O and [Ru(bpy)3]

3+, catalyzed by (P)MnIII

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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-sul fonato

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 H2

18O (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 two-electron 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)]+

Fig. 1. Dependence of log(ket, M-1s-1) on DG0

et for the electron-transfer oxidation of (TMP)MnIII(OH) with various oxidants ((D) [Fe(5-NO2phen)3](PF6)3, (u) [Ru(bpy)3](PF6)3, (s) [Fe(5-Cl-phen)3](PF6)3, (¨) [Fe(bpy)3](PF6)3) and ((u) (TMP)MnIII(OH), (·) [(TMP)MnIII(H2O)2]

+, () (TMP)MnIII(Cl) (p) (DTMPD)MnIII

2(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

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-methyl- N,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

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 light-driven water oxidation by persulfate, no nanoparticulate materials ware formed during the photocatalytic oxidation [60]. The kinetic study suggested that the rate-determining 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)]3

3+ 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.

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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 (NRF-2012R1A3A2048842 to W.N.) and GRL (NRF-2010-00353 to W.N.).

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