Cyclopropanation Using Electrons Derived from Hydrogen: Reaction of Alkenes and Hydrogen without Hydrogenation.

Have you ever imagined reactions of alkenes with hydrogen that result in anything other than hydrogenation or hydrogenative C-C coupling? We have long sought to develop not only hydrogenation catalysts that activate H2 as hydride ions but also electron transfer catalysts that activate H2 as a direct electron donor. Here, we report the reductive cyclopropanation of alkenes using an iridium electron storage catalyst with H2 as the electron source without releasing metal waste from the reductant. We discuss the catalytic mechanism with selectivity to give the trans-isomer. These findings are based on the isolation of three complexes and density functional theory calculations.

Storing electrons from H2 for transfer to CO2, all at room temperature.

We present an Ir complex that extracts electrons from H2 at room temperature and stores them as a H2-derived energy carrier (H2EC) at room temperature. Furthermore, we demonstrate that this complex reduces CO2 to a metal-CO22- species at room temperature, and present the first electrospray ionisation mass spectrum for this compound.

Single-Step Synthesis of NiI from NiII with H2.

Chemists have long sought to regulate the reactivity of H2 , to yield hydride ions, hydrogen atoms, or electrons on demand. One source of inspiration for achieving this control is [NiFe]hydrogenase ([NiFe]H2 ase), which reacts with H2 to form various hydrogen active species such as NiIII hydride species, NiII hydride species, and NiI low-valent species. Chemists have attempted to synthesize these hydrogen active species not only as models for the active species of [NiFe]H2 ase, but also as electron transfer catalysts. However, the synthesis of NiI complex directly from H2 has not been reported. This paper reports the first example of a single-step synthesis of a NiI complex, via reaction of a NiII complex with H2 , stable for over 3 months at room temperature and we further demonstrate a reductive coupling of acridinium ions as part of a reaction cycle.

Safe, One-Pot, Homogeneous Direct Synthesis of H2O2.

Hydrogen peroxide is an environmentally friendly oxidizing agent but current synthetic methods are wasteful. This is a result of the high flammability of H2/O2 mixtures and/or the requirement for cocatalysts. In this paper, we report the synthesis of H2O2 by means of a homogeneous catalyst, which allows a safe, one-pot synthesis in water, using only H2 and O2. This catalyst is capable of removing electrons from H2, storing them for the reduction of O2, and then permitting the protonation of the reduced oxygen to H2O2. The turnover number (TON) is 910 under an H2/O2 (95/5) atmosphere (1.9 MPa) for 12 h at 23 °C, which is the highest of any homogeneous catalyst. Furthermore, we propose a reaction mechanism based on two crystal structures.

C-H Arylation of Benzene with Aryl Halides using H2 and a Water-Soluble Rh-Based Electron Storage Catalyst.

This paper reports the first example of C-H arylation of benzene under mild conditions, using H2 as an electron source {turnover numbers (TONs)=0.7-2.0 for 24 h}. The reaction depends on a Rh-based electron storage catalyst, and proceeds at room temperature and in aqueous solution. Furthermore, the H2 is inactive during the radical transfer step, greatly reducing unwanted side reactions.

Synthesis of acetic acid from CO2, CH3I and H2 using a water-soluble electron storage catalyst.

This paper reports a possible mechanism of acetic acid formation from CO2, CH3I and H2 in aqueous media and the central role played by a water-soluble Rh-based electron storage catalyst. In addition to water-solubility, we also report the crystal structures of two presumed intermediates. These findings together reveal (1) the advantage of water, not only as a green solvent, but also as a reactive Lewis base to extract H+ from H2, (2) the role of the metal (Rh) centre as a point for storing electrons from H2 and (3) the importance of an electron-withdrawing ligand (quaterpyridine, qpy) that supports electron storage.

Reductive C(sp3)-C(sp3) homo-coupling of benzyl or allyl halides with H2 using a water-soluble electron storage catalyst.

This paper reports the first example of a reductive C(sp3)-C(sp3) homo-coupling of benzyl/allyl halides in aqueous solution by using H2 as an electron source {turnover numbers (TONs) = 0.5-2.3 for 12 h}. This homo-coupling reaction, promoted by visible light, is catalysed by a water-soluble electron storage catalyst (ESC). The reaction mechanism, and four requirements to make it possible, are also described.

A NiRhS fuel cell catalyst - lessons from hydrogenase.

We present a novel fuel cell heterogeneous catalyst based on rhodium, nickel and sulfur with power densities 5-28% that of platinum. The NiRhS heterogeneous catalyst was developed via a homogeneous model complex of the [NiFe]hydrogenases (H2ases) and can act as both the cathode and anode of a fuel cell.

[NiFe], [FeFe], and [Fe] hydrogenase models from isomers.

The study of hydrogenase enzymes (H2ases) is necessary because of their importance to a future hydrogen energy economy. These enzymes come in three distinct classes: [NiFe] H2ases, which have a propensity toward H2 oxidation; [FeFe] H2ases, which have a propensity toward H2 evolution; and [Fe] H2ases, which catalyze H- transfer. Modeling these enzymes has so far treated them as different species, which is understandable given the different cores and ligand sets of the natural molecules. Here, we demonstrate, using x-ray analysis and nuclear magnetic resonance, infrared, Mössbauer spectroscopies, and electrochemical measurement, that the catalytic properties of all three enzymes can be mimicked with only three isomers of the same NiFe complex.

Multifunctional Catalysts for H2 O2 -Resistant Hydrogen Fuel Cells.

The development of hydrogen fuel cells is greatly hindered by the unwanted generation of H2 O2 at the cathode. A non-Pt cathode catalyst is now shown to be capable of simultaneously reducing both O2 and H2 O2 , thus rendering H2 O2 a useful part of the feed stream. The applicability of this unique catalyst is demonstrated by employing it in a fuel cell running on H2 /CO and O2 /H2 O2 .

Oxidation of Guanosine Monophosphate with O2 via a Ru-peroxo Complex in Water.

Oxidative damage of DNA by reactive oxygen species (ROS) is responsible for aging and cancer. Although many studies of DNA damage by ROS have been conducted, there have been no reports of the oxidation of RNA components, such as guanosine monophosphate, by metal-based species in water. Here, we report the first case of oxidation of guanosine monophosphate to 8-oxoguanosine monophosphate by a metal-based oxygen bound species, derived from O2 and in water.

Mechanistic investigation of the formation of H2 from HCOOH with a dinuclear Ru model complex for formate hydrogen lyase.

We report the mechanistic investigation of catalytic H2 evolution from formic acid in water using a formate-bridged dinuclear Ru complex as a formate hydrogen lyase model. The mechanistic study is based on isotope-labeling experiments involving hydrogen isotope exchange reaction.

One Model, Two Enzymes: Activation of Hydrogen and Carbon Monoxide.

The ability to catalyze the oxidation of both H2 and CO in one reaction pot would be a major boon to hydrogen technology since CO is a consistent contaminant of H2 supplies. Here, we report just such a catalyst, with the ability to catalyze the oxidation of either or both H2 and CO, based on the pH value. This catalyst is based on a NiIr core that mimics the chemical function of [NiFe]hydrogenase in acidic media (pH 4-7) and carbon monoxide dehydrogenase in basic media (pH 7-10). We have applied this catalyst in a demonstration fuel cell using H2 , CO, and H2 /CO (1/1) feeds as fuels for oxidation at the anode. The power density of the fuel cell depends on the pH value in the media of the fuel cell and shows a similar pH dependence in a flask. We have isolated and characterized all intermediates in our proposed catalytic cycles.

Inorganic clusters with a [Fe2MoOS3] core-a functional model for acetylene reduction by nitrogenases.

We report the first example of a wholly inorganic mimic of a part of the FeMoco active centre of nitrogenases. We detail the synthesis, characterisation and reactivity of two related, transient hydride-containing inorganic clusters, a dihydride complex and a vinyl monohydride complex, which bear the [Fe2MoOS3] portion of FeMoco. The dihydride complex is capable of reducing acetylene to ethylene via the vinyl monohydride complex. In the reaction cycle, a transient low-valent complex was generated by the reductive elimination of H2 or ethylene from dihydride or vinyl monohydride complexes, respectively.

An IrSi oxide film as a highly active water-oxidation catalyst in acidic media.

We report an acid-stable Si oxide-doped Ir oxide film (IrSi oxide film), made by metal organic chemical vapour deposition (MOCVD) of an Ir(V) complex for electrochemical water-oxidation. This is a successful improvement of catalytic ability and stability depending upon the pH of Ir oxide by doping of Si oxide. The turnover frequency (TOF) of the electrochemical water-oxidation by the IrSi oxide film is the highest of any Si oxide-doped Ir oxide materials and higher even than that of Ir oxide in acidic media.

Synthesis and crystal structure of a dinuclear, monomeric Mn(II) p-semiquinonato complex.

Herein, we report the first crystal structure of a monomeric p-semiquinonato d-block complex and its reactivity toward dioxygen, closely associated with a biological system of an oxygen evolving centre of photosystem II.

A [NiFe]hydrogenase model that catalyses the release of hydrogen from formic acid.

We report the decomposition of formic acid to hydrogen and carbon dioxide, catalysed by a NiRu complex originally developed as a [NiFe]hydrogenase model. This is the first example of H2 evolution, catalysed by a [NiFe]hydrogenase model, which does not require additional energy.

Molybdenum-containing membrane-bound formate dehydrogenase isolated from Citrobacter sp. S-77 having high stability against oxygen, pH, and temperature.

Membrane-bound formate dehydrogenase (FDH) was purified to homogeneity from a facultative anaerobic bacterium Citrobacter sp. S-77. The FDH from Citrobacter sp. S-77 (FDHS77) was a monomer with molecular mass of approximately 150 kDa. On SDS-PAGE, the purified FDHS77 showed as three different protein bands with molecular mass of approximately 95, 87, and 32 kDa, respectively. Based on the N-terminal amino acid sequence analysis, the sequence alignments observed for the 87 kDa protein band were identical to that of the large subunit of 95 kDa, indicating that the purified FDHS77 consisted of two subunits; a 95 kDa large subunit and a 32 kDa small subunit. The purified FDHS77 in this purification did not contain a heme b subunit, but the FDHS77 showed significant activity for formate oxidation, determined by the Vmax of 30.4 U/mg using benzyl viologen as an electron acceptor. The EPR and ICP-MS spectra indicate that the FDHS77 is a molybdenum-containing enzyme, displaying a remarkable O2-stability along with thermostability and pH resistance. This is the first report of the purification and characterization of a FDH from Citrobacter species.

Purification and characterization of an oxygen-evolving photosystem II from Leptolyngbya sp. strain O-77.

A new cyanobacterium of strain O-77 was isolated from a hot spring at Aso-Kuju National Park, Kumamoto, Japan. According to the phylogenetic analysis determined by 16S rRNA gene sequence, the strain O-77 belongs to the genus Leptolyngbya, classifying into filamentous non-heterocystous cyanobacteria. The strain O-77 showed the thermophilic behavior with optimal growth temperature of 55°C. Moreover, we have purified and characterized the oxygen-evolving photosystem II (PSII) from the strain O-77. The O2-evolving activity of the purified PSII from strain O-77 (PSIIO77) was 1275 ± 255 µmol O2 (mg Chl a)(-1) h(-1). Based on the results of MALDI-TOF mass spectrometry and urea-SDS-PAGE analysis, the purified PSIIO77 was composite of the typical PSII components of CP47, CP43, PsbO, D2, D1, PsbV, PsbQ, PsbU, and several low molecular mass subunits. Visible absorption and 77 K fluorescence spectra of the purified PSIIO77 were almost identical to those of other purified PSIIs from cyanobacteria. This report provides the successful example for the purification and characterization of an active PSII from thermophilic, filamentous non-heterocystous cyanobacteria.

A model for the water-oxidation and recovery systems of the oxygen-evolving complex.

We propose a model for the water-oxidation and recovery systems of the oxygen-evolving complex (OEC) of the photosystem II (PSII) enzyme. The whole system is constructed from two catalytic cycles, conducted as a tandem reaction: (i) a water-oxidation loop uses cerium(IV) ammonium nitrate as an oxidant to activate a dimanganese complex for water-oxidation and thereby liberate a molecule of O2 and (ii) a recovery loop begins with photoinhibition of the dimanganese complex but then uses O2 to reactivate the manganese centre. The net result is a catalytic water-oxidation catalyst that can use self-generated O2 for recovery.