Hiroaki Kotani

Crystal structure of a metal ion-bound oxoiron( IV ) complex and implications for biological electron transfer

Critical biological electron-transfer processes involving high-valent oxometal chemistry occur widely, for example in haem proteins [oxoiron(IV); Fe(IV)(O)] and in photosystem II. Photosystem II involves Ca(2+) as well as high-valent oxomanganese cluster species. However, there is no example of an interaction between metal ions and oxoiron(IV) complexes. Here, we report new findings concerning the binding of the redox-inactive metal ions Ca(2+) and Sc(3+) to a non-haem oxoiron(IV) complex, [(TMC)Fe(IV)(O)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). As determined by X-ray diffraction analysis, an oxo-Sc(3+) interaction leads to a structural distortion of the oxoiron(IV) moiety. More importantly, this interaction facilitates a two-electron reduction by ferrocene, whereas only a one-electron reduction process occurs without the metal ions. This control of redox behaviour provides valuable mechanistic insights into oxometal redox chemistry, and suggests a possible key role that an auxiliary Lewis acid metal ion could play in nature, as in photosystem II.

Catalytic mechanisms of hydrogen evolution with homogeneous and heterogeneous catalysts

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 Pt nanoparticles as a catalyst is also discussed based on the kinetic analysis of the electron-transfer rates from a photogenerated electron donor to Pt nanoparticles, 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 Pt nanoparticles on the rates of hydrogen evolution and the electron-transfer reaction are examined to optimize the catalytic efficiency. Finally, catalytic H2 evolution systems from H2storage molecules are described including shape dependent catalytic activity of Co3O4 particles for ammonia borane hydrolysis and a large tunneling effect observed in decomposition of formic acid with [IrIII(Cp*)(H2O)(bpm)RuII(bpy)2](SO4)2.

Metal Ion-Coupled Electron Transfer of a Nonheme Oxoiron(IV) Complex: Remarkable Enhancement of Electron-Transfer Rates by Sc3+

Rates of electron transfer from a series of one-electron reductants to a nonheme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+), are enhanced as much as 10(8)-fold by addition of metal ions such as Sc(3+), Zn(2+), Mg(2+), and Ca(2+); the metal ion effect follows the Lewis acidity of metal ions. The one-electron reduction potential of [(N4Py)Fe(IV)(O)](2+) is shifted to a positive direction by 0.84 V in the presence of Sc(3+) ion (0.20 M).

Cupric superoxo-mediated intermolecular C-H activation chemistry.

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.

Mononuclear Copper Complex Catalyzed Four-Electron Reduction of Oxygen

A mononuclear Cu(II) complex acts as an efficient catalyst for four-electron reduction of O(2) to H(2)O. Its reduction by a ferrocene derivative (Fc*) and reaction with O(2) leads to the formation of a peroxodicopper(II) complex; this is subsequently reduced by Fc* in the presence of protons to regenerate the Cu(II) complex.

Photocatalytic Hydrogen Evolution under Highly Basic Conditions by Using Ru Nanoparticles and 2-Phenyl-4-(1-naphthyl)quinolinium Ion

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.

Homogeneous catalytic O2 reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

An efficient and selective four-electron plus four-proton (4e-/4H+) reduction of O2 to water by decamethylferrocene and trifluoroacetic acid can be catalyzed by a synthetic analog of the heme a3/CuB site in cytochrome c oxidase (6LFeCu) or its Cu-free version (6LFe) 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 O2-binding process at 25 °C room temperature (RT) to the O-O bond cleavage of a newly observed FeIII-OOH species at lower temperature (-60 °C). At RT, the rate of O2-binding to 6LFeCu is significantly faster than that for 6LFe, whereas the rates of the O-O bond cleavage of the FeIII-OOH species observed (-60 °C) with either the 6LFeCu or 6LFe catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O2-binding at RT. However, the proximate Cu ion has no effect on the O-O bond cleavage of the FeIII-OOH species at low temperature.

Size- and Shape-Dependent Activity of Metal Nanoparticles as Hydrogen-Evolution Catalysts: Mechanistic Insights into Photocatalytic Hydrogen Evolution

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.

Photocatalytic hydrogen evolution with Ni nanoparticles by using 2-phenyl-4-(1-naphthyl)quinolinium ion as a photocatalyst

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 nickel nanoparticles (NiNPs) as a non-precious metal catalyst. The hydrogen evolution rate with the most active Ni nanoparticles (hexagonal close-packed (hcp) structure, 6.6 nm) examined here was 40% of that with commercially available Pt nanoparticles (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 proton reduction or desorption of hydrogen.

Efficient photocatalytic hydrogen evolution without an electron mediator using a simple electron donor–acceptor dyad

A highly efficient photocatalytic hydrogen evolution system without an electron mediator such as methyl viologen (MV(2+)) has been constructed using 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt-PVP) and NADH (beta-nicotinamide adenine dinucleotide, reduced form) as the photocatalyst, hydrogen evolution catalyst and electron donor, respectively. The photocatalyst (Acr(+)-Mes) undergoes photoinduced electron transfer (ET) from the Mes moiety to the singlet excited state of the Acr(+) moiety to produce an extremely long-lived ET state, which is capable of oxidizing NADH and reducing Pt-PVP, leading to efficient hydrogen evolution. The hydrogen evolution efficiency is 300 times higher than that in the presence of MV(2+) because of the much faster reduction rate of Pt-PVP by Acr(*)-Mes compared with that by MV(*+). When the electron donor (NADH) is replaced by ethanol in the presence of an alcohol dehydrogenase (ADH), NADH is regenerated during the photocatalytic hydrogen evolution.