Mechanisms of Two-Electron versus Four-Electron Reduction of Dioxygen Catalyzed by Earth-Abundant Metal Complexes

Fukuzumi, Shunichi,Lee, Yong-Min,Nam, Wonwoo Wiley Blackwell (John Wiley Sons) 2018 ChemCatChem Vol.10 No.1

Mechanisms of metal ion-coupled electron transfer

Fukuzumi, Shunichi,Ohkubo, Kei,Morimoto, Yuma The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.24

<P>Redox inactive metal ions acting as Lewis acids can control electron transfer from electron donors (D) to electron acceptors (A) by binding to radical anions of electron acceptors which act as Lewis bases. Such electron transfer is defined as metal ion-coupled electron transfer (MCET). Mechanisms of metal ion-coupled electron transfer are classified mainly into two pathways, <I>i.e.</I>, metal ion binding to electron acceptors followed by electron transfer (MB/ET) and electron transfer followed by metal ion binding to the resulting radical anions of electron acceptors (ET/MB). In the former case, electron transfer and the stronger binding of metal ions to the radical anions occur in a concerted manner. Examples are shown in each case to clarify the factors to control MCET reactions in both thermal and photoinduced electron-transfer reactions including back electron-transfer reactions.</P> <P>Graphic Abstract</P><P>This article clarifies mechanisms of metal ion-coupled electron-transfer (MCET) reactions, which provide valuable insights into fine control of electron-transfer reactions by binding metal ions to radical anions of various electron acceptors. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp40459a'> </P>

Contrasting Effects of Axial Ligands on Electron-Transfer Versus Proton-Coupled Electron-Transfer Reactions of Nonheme Oxoiron(IV) Complexes

Fukuzumi, Shunichi,Kotani, Hiroaki,Suenobu, Tomoyoshi,Hong, Seungwoo,Lee, Yong-Min,Nam, Wonwoo WILEY-VCH Verlag 2010 Chemistry Vol.16 No.1

<P>The effects of axial ligands on electron-transfer and proton-coupled electron-transfer reactions of mononuclear nonheme oxoiron(IV) complexes were investigated by using [Fe<SUP>IV</SUP>(O)(tmc)(X)]<SUP>n+</SUP> (1-X) with various axial ligands, in which tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is CH<SUB>3</SUB>CN (1-NCCH<SUB>3</SUB>), CF<SUB>3</SUB>COO<SUP>−</SUP> (1-OOCCF<SUB>3</SUB>), or N<SUB>3</SUB><SUP>−</SUP> (1-N<SUB>3</SUB>), and ferrocene derivatives as electron donors. As the binding strength of the axial ligands increases, the one-electron reduction potentials of 1-X (E<SUB>red</SUB>, V vs. saturated calomel electrode (SCE)) are more negatively shifted by the binding of the more electron-donating axial ligands in the order of 1-NCCH<SUB>3</SUB> (0.39) > 1-OOCCF<SUB>3</SUB> (0.13) > 1-N<SUB>3</SUB> (−0.05 V). Rate constants of electron transfer from ferrocene derivatives to 1-X were analyzed in light of the Marcus theory of electron transfer to determine reorganization energies (λ) of electron transfer. The λ values decrease in the order of 1-NCCH<SUB>3</SUB> (2.37) > 1-OOCCF<SUB>3</SUB> (2.12) > 1-N<SUB>3</SUB> (1.97 eV). Thus, the electron-transfer reduction becomes less favorable thermodynamically but more favorable kinetically with increasing donor ability of the axial ligands. The net effect of the axial ligands is the deceleration of the electron-transfer rate in the order of 1-NCCH<SUB>3</SUB> > 1-OOCCF<SUB>3</SUB> > 1-N<SUB>3</SUB>. In sharp contrast to this, the rates of the proton-coupled electron-transfer reactions of 1-X are markedly accelerated in the presence of an acid in the opposite order: 1-NCCH<SUB>3</SUB> < 1-OOCCF<SUB>3</SUB> < 1-N<SUB>3</SUB>. Such contrasting effects of the axial ligands on the electron-transfer and proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes are discussed in light of the counterintuitive reactivity patterns observed in the oxo transfer and hydrogen-atom abstraction reactions by nonheme oxoiron(IV) complexes (Sastri et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19 181–19 186).</P> <B>Graphic Abstract</B> <P>Counterintuitive reactivities: The rates of electron transfer (ET) and proton-coupled electron-transfer (PCET) in the reactions of with ferrocene derivatives are markedly affected by the electron-donating ability of the axial ligands (X) in opposite directions (see figure); the electron-donating axial ligand decelerates the ET rate in the reactions, but enhances the PCET reactivity of 1-X in the presence of acid. <img src='wiley_img/09476539-2010-16-1-CHEM200901163-content.gif' alt='wiley_img/09476539-2010-16-1-CHEM200901163-content'> </P>

Highly efficient photocatalytic oxygenation reactions using water as an oxygen source

Fukuzumi, Shunichi,Kishi, Takashi,Kotani, Hiroaki,Lee, Yong-Min,Nam, Wonwoo Nature Publishing Group 2011 Nature chemistry Vol.3 No.1

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<SUP>II</SUP>(bpy)<SUB>3</SUB>]<SUP>2+</SUP> (bpy?=?2,2??bipyridine) as a photosensitized electron-transfer catalyst, [Co<SUP>III</SUP>(NH<SUB>3</SUB>)<SUB>5</SUB>Cl]<SUP>2+</SUP> 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.

Unusually Large Tunneling Effect on Highly Efficient Generation of Hydrogen and Hydrogen Isotopes in pH-Selective Decomposition of Formic Acid Catalyzed by a Heterodinuclear Iridium−Ruthenium Complex in Water

Fukuzumi, Shunichi,Kobayashi, Takeshi,Suenobu, Tomoyoshi American Chemical Society 2010 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.132 No.5

<P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2010/jacsat.2010.132.issue-5/ja910349w/production/images/medium/ja-2009-10349w_0005.gif'> <P>A heterodinuclear iridium−ruthenium complex [Ir<SUP>III</SUP>(Cp*)(H<SUB>2</SUB>O)(bpm)Ru<SUP>II</SUP>(bpy)<SUB>2</SUB>](SO<SUB>4</SUB>)<SUB>2</SUB> {<B>1</B>(SO<SUB>4</SUB>)<SUB>2</SUB>, Cp* = η<SUP>5</SUP>-pentamethylcyclopentadienyl, bpm = 2,2′-bipyrimidine, bpy = 2,2′-bipyridine} acts as the most effective catalyst for selective production of hydrogen from formic acid in an aqueous solution at ambient temperature among catalysts reported so far. An unusually large tunneling effect was observed for the first time for the catalytic hydrogen production in H<SUB>2</SUB>O vs D<SUB>2</SUB>O.</P></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja910349w'>ACS Electronic Supporting Info</A></P>

Fuel Production from Seawater and Fuel Cells Using Seawater

Fukuzumi, Shunichi,Lee, Yong-Min,Nam, Wonwoo Wiley (John WileySons) 2017 ChemSusChem Vol.10 No.22

<P>Seawater is the most abundant resource on our planet and fuel production from seawater has the notable advantage that it would not compete with growing demands for pure water. This Review focuses on the production of fuels from seawater and their direct use in fuel cells. Electrolysis of seawater under appropriate conditions affords hydrogen and dioxygen with 100% faradaic efficiency without oxidation of chloride. Photo-electrocatalytic production of hydrogen from seawater provides a promising way to produce hydrogen with low cost and high efficiency. Microbial solar cells (MSCs) that use biofilms produced in seawater can generate electricity from sunlight without additional fuel because the products of photosynthesis can be utilized as electrode reactants, whereas the electrode products can be utilized as photosynthetic reactants. Another important source for hydrogen is hydrogen sulfide, which is abundantly found in Black Sea deep water. Hydrogen produced by electrolysis of Black Sea deep water can also be used in hydrogen fuel cells. Production of a fuel and its direct use in a fuel cell has been made possible for the first time by a combination of photocatalytic production of hydrogen peroxide from seawater and dioxygen in the air and its direct use in one-compartment hydrogen peroxide fuel cells to obtain electric power.</P>

Mononuclear Copper Complex-Catalyzed Four-Electron Reduction of Oxygen

Fukuzumi, Shunichi,Kotani, Hiroaki,Lucas, Heather R.,Doi, Kaoru,Suenobu, Tomoyoshi,Peterson, Ryan L.,Karlin, Kenneth D. American Chemical Society 2010 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.132 No.20

<P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2010/jacsat.2010.132.issue-20/ja100538x/production/images/medium/ja-2010-00538x_0002.gif'> <P>A mononuclear Cu<SUP>II</SUP> complex acts as an efficient catalyst for four-electron reduction of O<SUB>2</SUB> to H<SUB>2</SUB>O. Its reduction by a ferrocene derivative (Fc*) and reaction with O<SUB>2</SUB> leads to the formation of a peroxodicopper(II) complex; this is subsequently reduced by Fc* in the presence of protons to regenerate the Cu<SUP>II</SUP> complex.</P></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja100538x'>ACS Electronic Supporting Info</A></P>

Synthesis and Photodynamics of 9-Mesitylacridinium Ion-Modified Gold Nanoclusters

Fukuzumi, Shunichi,Hanazaki, Ryo,Kotani, Hiroaki,Ohkubo, Kei American Chemical Society 2010 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.132 No.32

<P>Photoexcitation of gold nanoclusters covalently functionalized with 9-mesityl-10-methylacridinium ion (Mes-Acr<SUP>+</SUP>) resulted in the formation of the electron-transfer state (Mes<SUP>•+</SUP>-Acr<SUP>•</SUP>), which forms a π-dimer radical cation with the neighboring Mes-Acr<SUP>+</SUP> via an intramolecular π−π interaction.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2010/jacsat.2010.132.issue-32/ja105314x/production/images/medium/ja-2010-05314x_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja105314x'>ACS Electronic Supporting Info</A></P>

Photocatalytic Production of Hydrogen by Disproportionation of One‐Electron‐Reduced Rhodium and Iridium–Ruthenium Complexes in Water

Fukuzumi, Shunichi,Kobayashi, Takeshi,Suenobu, Tomoyoshi WILEY‐VCH Verlag 2011 Angewandte Chemie Vol.123 No.3

<P><B>3 Metalle, 2 Elektronen, 1 H</B><SUB><B>2</B></SUB><B>:</B> Ein Aquarhodiumkomplex und ein Iridium‐Ruthenium‐Komplex (siehe Bild) wirken als effektive Katalysatoren für die photokatalytische Zwei‐Elektronen‐Reduktion von Protonen. Wasserstoff wird mithilfe eines Photosensibilisators und eines Elektronendonors erzeugt, indem die um ein Elektron reduzierten Metallkomplexe disproportionieren.</P>

Hydrogen storage and evolution catalysed by metal hydride complexes

Fukuzumi, Shunichi,Suenobu, Tomoyoshi The Royal Society of Chemistry 2013 Dalton transactions Vol.42 No.1

<P>The storage and evolution of hydrogen are catalysed by appropriate metal hydride complexes. Hydrogenation of carbon dioxide by hydrogen is catalysed by a [C,N] cyclometalated organoiridium complex, [Ir<SUP>III</SUP>(Cp*)(4-(1<I>H</I>-pyrazol-1-yl-κ<I>N</I><SUP>2</SUP>)benzoic acid-κ<I>C</I><SUP>3</SUP>)(OH<SUB>2</SUB>)]<SUB>2</SUB>SO<SUB>4</SUB> [<B>Ir</B>–OH<SUB>2</SUB>]<SUB>2</SUB>SO<SUB>4</SUB>, under atmospheric pressure of H<SUB>2</SUB> and CO<SUB>2</SUB> in weakly basic water (pH 7.5) at room temperature. The reverse reaction, <I>i.e.</I>, hydrogen evolution from formate, is also catalysed by [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> in acidic water (pH 2.8) at room temperature. Thus, interconversion between hydrogen and formic acid in water at ambient temperature and pressure has been achieved by using [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> as an efficient catalyst in both directions depending on pH. The Ir complex [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> also catalyses regioselective hydrogenation of the oxidised form of β-nicotinamide adenine dinucleotide (NAD<SUP>+</SUP>) to produce the 1,4-reduced form (NADH) under atmospheric pressure of H<SUB>2</SUB> at room temperature in weakly basic water. In weakly acidic water, the complex [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> also catalyses the reverse reaction, <I>i.e.</I>, hydrogen evolution from NADH to produce NAD<SUP>+</SUP> at room temperature. Thus, interconversion between NADH (and H<SUP>+</SUP>) and NAD<SUP>+</SUP> (and H<SUB>2</SUB>) has also been achieved by using [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> as an efficient catalyst and by changing pH. The iridium hydride complex formed by the reduction of [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> by H<SUB>2</SUB> and NADH is responsible for the hydrogen evolution. Photoirradiation (<I>λ</I> > 330 nm) of an aqueous solution of the Ir–hydride complex produced by the reduction of [<B>Ir</B>–OH<SUB>2</SUB>]<SUP>+</SUP> with alcohols resulted in the quantitative conversion to a unique [C,C] cyclometalated Ir–hydride complex, which can catalyse hydrogen evolution from alcohols in a basic aqueous solution (pH 11.9). The catalytic mechanisms of the hydrogen storage and evolution are discussed by focusing on the reactivity of Ir–hydride complexes.</P> <P>Graphic Abstract</P><P>Hydrogen storage and evolution were catalysed by [C,N] and [C,C] cyclometalated iridium hydride complexes in water at ambient temperature under atmospheric pressure. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2dt31823g'> </P>