Hiroaki Kotani

University of Tsukuba, Tsukuba, Ibaraki, Japan

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Publications (39)365.2 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A square-shaped tetranuclear ruthenium(II) complex, [Ru(II)4Cl5(bpmpm)2](3+) [1; bpmpm = 4,6-bis[[N,N-bis(2'-pyridylmethyl)amino]methyl]pyrimidine], exhibited four reversible and stepwise one-electron-oxidation processes: chemical oxidation of 1 formed three different mixed-valence states, in one of which the charge is partially delocalized on the two Ru centers, to be evidenced by observation of an intervalence charge-transfer absorption band, categorized into the Robin-Day class II.
    Inorganic Chemistry 11/2014; · 4.79 Impact Factor
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    ABSTRACT: A mononuclear Cr(V)-oxo complex, [CrV(O)(6-COO–-tpa)](BF4)2 (1; 6-COO–-tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was made by ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on the analysis of electron-transfer (ET) equilibrium between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2’-bipyridine). Reorganization energy (λ) of 1 was also determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e–-oxidized products in moderate yields as determined by 1H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by 1 and kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET–proton transfer (ET/PT), depending on the redox potentials of R-BA.
    Chemical Science 10/2014; · 8.60 Impact Factor
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    ABSTRACT: A mononuclear RuIII-OH complex oxidizes substrates such as hydroquinones in water through a pre-equilibrium process based on adduct formation by hydrogen bonding between the RuIII-OH complex and the substrates. The reaction mechanism switches from hydrogen atom transfer to electron transfer depending on the oxidation potentials of substrates.
    Chemical Communications 10/2014; · 6.38 Impact Factor
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    ABSTRACT: A porphyrin–flavin-linked dyad and its zinc and palladium complexes (MPorFl: 2M, M=2 H, Zn, and Pd) were newly synthesized and the X-ray crystal structure of 2Pd was determined. The photodynamics of 2M were examined by femto- and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2H2 occurred from the singlet excited state of the porphyrin moiety (H2Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state 1(H2Por.+Fl.−), which decayed through back ET (BET) to form 3[H2Por]*Fl with rate constants of 1.2×1010 and 1.2×109 s−1, respectively. Similarly, photoinduced ET in 2Pd afforded the singlet CS state, which decayed through BET to form 3[PdPor]*Fl with rate constants of 2.1×1011 and 6.0×1010 s−1, respectively. The rate constant of photoinduced ET and BET of 2M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc3+ ions bind to the flavin moiety to form the FlSc3+ and Fl(Sc3+)2 complexes with binding constants of K1=2.2×105 M−1 and K2=1.8×103 M−1, respectively. Other metal ions, such as Y3+, Zn2+, and Mg2+, form only 1:1 complexes with flavin. In contrast to 2M and the 1:1 complexes with metal ions, which afforded the short-lived singlet CS state, photoinduced ET in 2Pd⋅⋅⋅Sc3+ complexes afforded the triplet CS state (3[PdPor.+Fl.−(Sc3+)2]), which exhibited a remarkably long lifetime of τ=110 ms (kBET=9.1 s−1).
    Chemistry - A European Journal 10/2014; · 5.93 Impact Factor
  • Chemical Science 04/2014; 5(4):1429-1436. · 8.60 Impact Factor
  • 223th ECS Meeting; 05/2013
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    ABSTRACT: We have synthesized a mononuclear ruthenium(II) azido complex (1) and a dinuclear ruthenium(II) μ-azido complex (2) having N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine (N4Py) as a pentadentate ancillary ligand. In the crystal structure of 2, intramolecular π-π stacking was found between the pyridine rings of the two different N4Py ligands, contributing to stabilize the dinuclear μ-azido structure. π donation from the HOMO π* orbital of the μ-azido ligand to the Ru-N(pyr) bond increases the bond order between the terminal and central N atoms in the μ-azido ligand to strengthen the N-N bonds of the μ-azido ligand. The μ-azido complex 2 was revealed to exhibit a stepwise oxidation behavior in CH3CN to afford a Ru(II)-μ-azido-Ru(III) mixed-valence (MV) state upon one-electron oxidation. The MV state of one-electron-oxidized 2 was categorized in the Robin-Day class II with the electronic coupling constant (Hab) of 570 cm(-1).
    Inorganic Chemistry 04/2013; · 4.79 Impact Factor
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    ABSTRACT: Photocatalytic hydrogen evolution with 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+–NA) as a photocatalyst and dihydronicotinamide adenine dinucleotide (NADH) as a sacrificial electron donor has been made possible for the first time by using nickelnanoparticles (NiNPs) as a non-precious metal catalyst. The hydrogen evolution rate with the most active Ninanoparticles (hexagonal close-packed (hcp) structure, 6.6 nm) examined here was 40% of that with commercially available Ptnanoparticles (2 nm) using the same catalyst weight. The catalytic activity of NiNPs depends not only on their sizes but also on their crystal phases. The hydrogen-evolution rate normalized by the catalyst weight increased as the size of NiNPs becomes smaller, with regard to the crystal phase, the hydrogen-evolution rate of the NiNPs with hcp structure is more than 4 times higher than the rate of the NiNPs with face-centred cubic (fcc) structure of similar size. NiNPs act as the hydrogen-evolution catalyst under the pH conditions between 4.5 and 8.0, although the hydrogen-evolution rate at pH > 7.0 was much lower as compared with the hydrogen-evolution rate at pH 4.5. A kinetic study revealed that the rate of electron transfer from photogenerated QuPh˙–NA to NiNPs was much higher than the rate of hydrogen evolution, indicating that the rate-determining step may be protonreduction or desorption of hydrogen.
    Energy & Environmental Science 03/2012; 5(3):6111-6118. · 15.49 Impact Factor
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    ABSTRACT: Extensive efforts have been devoted to developing electron donor-acceptor systems that mimic the utilization of solar energy that occurs in photosynthesis. X-ray crystallographic analysis shows how absorbed photon energy is stabilized in those compounds by structural changes upon photoinduced electron transfer (ET). In this study, structural changes of a simple electron donor-acceptor dyad, 9-mesityl-10-methylacridinium cation (Acr(+)-Mes), upon photoinduced ET were directly observed by laser pump and X-ray probe crystallographic analysis. The N-methyl group in Acr(+) was bent, and a weak electrostatic interaction between Mes and a counteranion in the crystal (ClO(4)) was generated by photoinduced ET. These structural changes correspond to reduction and oxidation due to photoinduced ET and directly elucidate the mechanism in Acr(+)-Mes for mimicking photosynthesis efficiently.
    Journal of the American Chemical Society 03/2012; 134(10):4569-72. · 11.44 Impact Factor
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    ABSTRACT: The four-electron reduction of dioxygen by decamethylferrocene (Fc*) to water is efficiently catalyzed by a binuclear copper(II) complex (1) and a mononuclear copper(II) complex (2) in the presence of trifluoroacetic acid in acetone at 298 K. Fast electron transfer from Fc* to 1 and 2 affords the corresponding Cu(I) complexes, which react at low temperature (193 K) with dioxygen to afford the η(2):η(2)-peroxo dicopper(II) (3) and bis-μ-oxo dicopper(III) (4) intermediates, respectively. The rate constants for electron transfer from Fc* and octamethylferrocene (Me(8)Fc) to 1 as well as electron transfer from Fc* and Me(8)Fc to 3 were determined at various temperatures, leading to activation enthalpies and entropies. The activation entropies of electron transfer from Fc* and Me(8)Fc to 1 were determined to be close to zero, as expected for outer-sphere electron-transfer reactions without formation of any intermediates. For electron transfer from Fc* and Me(8)Fc to 3, the activation entropies were also found to be close to zero. Such agreement indicates that the η(2):η(2)-peroxo complex (3) is directly reduced by Fc* rather than via the conversion to the corresponding bis-μ-oxo complex, followed by the electron-transfer reduction by Fc* leading to the four-electron reduction of dioxygen to water. The bis-μ-oxo species (4) is reduced by Fc* with a much faster rate than the η(2):η(2)-peroxo complex (3), but this also leads to the four-electron reduction of dioxygen to water.
    Chemistry - A European Journal 01/2012; 18(4):1084-93. · 5.93 Impact Factor
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    Hiroaki Kotani, Kei Ohkubo, Shunichi Fukuzumi
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    ABSTRACT: An electron donor-quinolinium ion dyad, 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+-NA), has been synthesized based on a rational design. The X-ray crystal structure of QuPh+-NA indicates that the dihedral angle between the NA and QuPh+ moieties of QuPh+-NA is nearly perpendicular. The one-electron reduction potential (E(red)) was observed as a well-defined reversible wave at -0.90 V versus SCE. The one-electron reduced species (QuPh*-NA) was detected by ESR. The electron self-exchange rate constant (k(ex)) between QuPh+-NA and QuPh*-NA has been determined from the ESR linewidth alternation. The reorganization energy (lambda) of the electron self-exchange was determined to be 0.42 eV from the k(ex) value. Femtosecond laser irradiation of QuPh+-NA at 355 nm results in formation of the ET state (QuPh*-NA*+) within 0.5 ps via photoinduced ET from NA to the singlet-excited state of QuPh+. The transient absorption bands at 420 nm and 700 nm are assigned to the QuPh and NA*+ moieties, respectively. The nanosecond laser excitation of QuPh+-NA affords the broad absorption band at 1000 nm and is due to the pi-dimer radical cation formed between QuPh*-NA*+ and QuPh+-NA. The intramolecular back electron-transfer process was too slow to compete with the intermolecular back electron-transfer reaction judging from the decay time profile of QuPh*-NA*+, which obeyed second-order kinetics.
    Faraday Discussions 01/2012; 155:89-102; discussion 103-14. · 4.19 Impact Factor
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    ABSTRACT: Photocatalytic hydrogen evolution with a ruthenium metal catalyst under basic conditions (pH 10) has been made possible for the first time by using 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA), dihydronicotinamide adenine dinucleotide (NADH), and Ru nanoparticles (RuNPs) as the photocatalyst, electron donor, and hydrogen-evolution catalyst, respectively. The catalytic reactivity of RuNPs was virtually the same as that of commercially available PtNPs. Nanosecond laser flash photolysis measurements were performed to examine the photodynamics of QuPh(+)-NA in the presence of NADH. Upon photoexcitation of QuPh(+)-NA, the electron-transfer state of QuPh(+)-NA (QuPh(•)-NA(•+)) is produced, followed by formation of the π-dimer radical cation with QuPh(+)-NA, [(QuPh(•)-NA(•+))(QuPh(+)-NA)]. Electron transfer from NADH to the π-dimer radical cation leads to the production of 2 equiv of QuPh(•)-NA via deprotonation of NADH(•+) and subsequent electron transfer from NAD(•) to QuPh(+)-NA. Electron transfer from the photogenerated QuPh(•)-NA to RuNPs results in hydrogen evolution even under basic conditions. The rate of electron transfer from QuPh(•)-NA to RuNPs is much higher than the rate of hydrogen evolution. The effect of the size of the RuNPs on the catalytic reactivity for hydrogen evolution was also examined by using size-controlled RuNPs. RuNPs with a size of 4.1 nm exhibited the highest hydrogen-evolution rate normalized by the weight of RuNPs.
    Journal of the American Chemical Society 08/2011; 133(40):16136-45. · 11.44 Impact Factor
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    ABSTRACT: This perspective focuses on reaction mechanisms of hydrogen (H2) evolution with homogeneous and heterogeneous catalysts. First, photocatalytic H2 evolution systems with homogeneous catalysts are discussed from the viewpoint of how to increase the efficiency of the two-electron process for the H2 evolution via photoinduced electron-transfer reactions of metal complexes. Two molecules of the one-electron reduced species of [RhIII(Cp*)(bpy)(H2O)](SO4) (bpy = 2,2′-bipyridine) and [IrIII(Cp*)(H2O)(bpm)RuII(bpy)2](SO4)2 (bpm = 2,2′-bipyrimidine) produced by photoinduced electron-transfer reactions are converted to the two-electron reduced complexes suitable for H2 generation by disproportionation. The photocatalytic mechanism of H2 evolution using Ptnanoparticles as a catalyst is also discussed based on the kinetic analysis of the electron-transfer rates from a photogenerated electron donor to Ptnanoparticles, which are comparable to the overall H2 evolution rates. The electron-transfer rates become faster with increasing proton concentrations with an inverse kinetic isotope effect, when H+ is replaced by D+. The size and shape effects of Ptnanoparticles on the rates of hydrogen evolution and the electron-transfer reaction are examined to optimize the catalytic efficiency. Finally, catalytic H2 evolution systems from H2 storage molecules are described including shape dependent catalytic activity of Co3O4 particles for ammoniaboranehydrolysis and a large tunneling effect observed in decomposition of formic acid with [IrIII(Cp*)(H2O)(bpm)RuII(bpy)2](SO4)2.
    Energy & Environmental Science 08/2011; 4(8):2754-2766. · 15.49 Impact Factor
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    ABSTRACT: An efficient and selective four-electron plus four-proton (4e(-)/4H(+)) reduction of O(2) to water by decamethylferrocene and trifluoroacetic acid can be catalyzed by a synthetic analog of the heme a(3)/Cu(B) site in cytochrome c oxidase ((6)LFeCu) or its Cu-free version ((6)LFe) in acetone. A detailed mechanistic-kinetic study on the homogeneous catalytic system reveals spectroscopically detectable intermediates and that the rate-determining step changes from the O(2)-binding process at 25 °C room temperature (RT) to the O-O bond cleavage of a newly observed Fe(III)-OOH species at lower temperature (-60 °C). At RT, the rate of O(2)-binding to (6)LFeCu is significantly faster than that for (6)LFe, whereas the rates of the O-O bond cleavage of the Fe(III)-OOH species observed (-60 °C) with either the (6)LFeCu or (6)LFe catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O(2)-binding at RT. However, the proximate Cu ion has no effect on the O-O bond cleavage of the Fe(III)-OOH species at low temperature.
    Proceedings of the National Academy of Sciences 08/2011; 108(34):13990-4. · 9.81 Impact Factor
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    ABSTRACT: Addition of potassium superoxide with 18-crown-6 ether (KO(2)(•-)-18-crown-6) to a toluene solution of an acridinium ion-linked porphyrin triad (Acr(+)-H(2)P-Acr(+)) resulted in a remarkable enhancement of the fluorescence intensity. Thus, Acr(+)-H(2)P-Acr(+) acts as an efficient fluorescence sensor for superoxide. Electron transfer from KO(2)(•-)-18-crown-6 to the Acr(+) moiety to produce the two-electron-reduced species (Acr(•)-H(2)P-Acr(•)) results in inhibition of the fluorescence quenching via photoinduced electron transfer, as revealed by laser flash photolysis measurements.
    Journal of the American Chemical Society 06/2011; 133(29):11092-5. · 11.44 Impact Factor
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    ABSTRACT: The new cupric superoxo complex [LCu(II)(O(2)(•-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.
    Journal of the American Chemical Society 02/2011; 133(6):1702-5. · 11.44 Impact Factor
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    ABSTRACT: The electron-transfer and hydride-transfer properties of an isolated manganese(V)−oxo complex, (TBP8Cz)Mn(V)(O) (1) (TBP8Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)−oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C5H4Me)2], [Fe(C5HMe4)2], and ([Fe(C5Me5)2] = Fc*) to give the direct formation of [(TBP8Cz)Mn(III)(OH)]− ([2-OH]−), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)−oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn(IV) complex, was not observed during the reduction of 1. However, a Mn(IV) complex was successfully generated via one-electron oxidation of the Mn(III) precursor complex 2 to give [(TBP8Cz)Mn(IV)]+ (3). Complex 3 exhibits a characteristic absorption band at λ(max) = 722 nm and an EPR spectrum at 15 K with g(max)′ = 4.68, g(mid)′ = 3.28, and g(min)′ = 1.94, with well-resolved 55Mn hyperfine coupling, indicative of a d3 Mn(IV)S = 3/2 ground state. Although electron transfer from [Fe(C5H4Me)2] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH3CO2H, CF3CH2OH, and CH3OH). In the case of CH3CO2H, saturation behavior for the rate constants of electron transfer (k(et)) versus acid concentration was observed, providing insight into the critical involvement of H+ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD+ analogues and [2-OH]−. The logarithms of the observed second-order rate constants of hydride transfer (k(H)) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil.
    Journal of the American Chemical Society 02/2011; 133(6):1859-69. · 11.44 Impact Factor
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    ABSTRACT: The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen-evolution system composed of the 9-mesityl-10-methylacridinium ion (Acr(+)-Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one-electron-reduced species of Acr(+)-Mes (Acr·-Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen-evolution rate was virtually the same as the rate of electron transfer from Acr·-Mes to PtNPs. The rate constant of electron transfer (k(et)) increased linearly with increasing proton concentration. When H(+) was replaced by D(+), the inverse kinetic isotope effect was observed for the electron-transfer rate constant (k(et)(H)/k(et)(D)=0.47). The linear dependence of k(et) on proton concentration together with the observed inverse kinetic isotope effect suggests that proton-coupled electron transfer from Acr·-Mes to PtNPs to form the Pt-H bond is the rate-determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen-evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr·-Mes to FeNPs.
    Chemistry - A European Journal 02/2011; 17(9):2777-85. · 5.93 Impact Factor
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    ABSTRACT: The photocatalytic formation of a non-heme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+) [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine], efficiently proceeds via electron transfer from the excited state of a ruthenium complex, [Ru(II)(bpy)(3)](2+)* (bpy = 2,2'-bipyridine) to [Co(III)(NH(3))(5)Cl](2+) and stepwise electron-transfer oxidation of [(N4Py)Fe(II)](2+) with 2 equiv of [Ru(III)(bpy)(3)](3+) and H(2)O as an oxygen source. The oxoiron(IV) complex was independently generated by both chemical oxidation of [(N4Py)Fe(II)](2+) with [Ru(III)(bpy)(3)](3+) and electrochemical oxidation of [(N4Py)Fe(II)](2+).
    Journal of the American Chemical Society 02/2011; 133(10):3249-51. · 11.44 Impact Factor
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    ABSTRACT: The effective utilization of solar energy requires photocatalytic reactions with high quantum efficiency. Water is the most abundant reactant that can be used as an oxygen source in efficient photocatalytic reactions, just as nature uses water in an oxygenic photosynthesis. We report that photocatalytic oxygenation of organic substrates such as sodium p-styrene sulfonate occurs with nearly 100% quantum efficiency using manganese(III) porphyrins as an oxygenation catalyst, [Ru(II)(bpy)(3)](2+) (bpy = 2,2'-bipyridine) as a photosensitized electron-transfer catalyst, [Co(III)(NH(3))(5)Cl](2+) as a low-cost and weak one-electron oxidant, and water as an oxygen source in a phosphate buffer solution (pH 7.4). A high-valent manganese-oxo porphyrin is proposed as an active oxidant that effects the oxygenation reactions.
    Nature Chemistry 01/2011; 3(1):38-41. · 21.76 Impact Factor

Publication Stats

673 Citations
365.20 Total Impact Points


  • 2013–2014
    • University of Tsukuba
      • Graduate School of Pure and Applied Sciences
      Tsukuba, Ibaraki, Japan
  • 2012
    • Tokyo Institute of Technology
      • Department of Chemistry and Materials Science
      Tokyo, Tokyo-to, Japan
  • 2008–2012
    • Ewha Womans University
      • • Department of Bioinspired Science
      • • Department of Chemistry Nano Science
      Sŏul, Seoul, South Korea
  • 2004–2012
    • Osaka University
      • • Department of Beam Materials Science
      • • Division of Advanced Science and Biotechnology
      Ibaraki, Osaka-fu, Japan
  • 2011
    • Johns Hopkins University
      • Department of Chemistry
      Baltimore, MD, United States