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.

Disassembly and reassembly of the non-conventional thermophilic C-phycocyanin.

C-phycocyanin (CPC), which contains open-chain tetrapyrroles, is a major light-harvesting red-fluorescent protein with an important role in aquatic photosynthesis. Recently, we reported a non-conventional CPC from Thermoleptolyngbya sp. O-77 (CPCO77) that contains two different structures, i.e., a hexameric structure and a non-conventional octameric structure. However, the assembly and disassembly mechanisms of the non-conventional octameric form of CPC remain unclear. To understand this assembly mechanism, we performed an in vitro experiment to study the disassembly and reassembly behaviors of CPC using isolated CPC subunits. The dissociation of the CPCO77 subunit was performed using a Phenyl-Sepharose column in 20 mM potassium phosphate buffer (pH 6.0) containing 7.0 M urea. For the first time, crystals of isolated CPC subunits were obtained and analyzed after separation. After the removal of urea from the purified α and ß subunits, we performed an in vitro reassembly experiment for CPC and analyzed the reconstructed CPC using spectrophotometric and X-ray crystal structure analyses. The crystal structure of the reassembled CPC was nearly identical to that of the original CPCO77. The findings of this study indicate that the octameric CPCO77 is a naturally occurring form in the thermophilic cyanobacterium Thermoleptolyngbya sp. O-77.

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.

H2-driven reduction of CO2 to formate using bacterial plasma membranes.

Bacterial membranes shield the intracellular compartment by selectively allowing unwanted substances to enter in, which in turn reduces overall catalytic efficiency. This report presents a model system using the isolated plasma membranes of Citrobacter sp. S-77 that harbor oxygen-stable [NiFe]hydrogenase and [Mo]formate dehydrogenase, which are integrated into a natural catalytic nanodevice through an electron transfer relay. This naturally occurring nanodevice exhibited selectivity and efficiency in catalyzing the H2-driven conversion of CO2 to formate with the rate of 817 mmol·L-1·gprotein-1·h-1 under mild conditions of 30 °C, pH 7.0, and 0.1 MPa. When the isolated plasma membranes of Citrobacter sp. S-77 was immobilized with multi-walled carbon nanotubes and encapsulated in hydrogel beads of gellan-gum cross-linked with calcium ions, the catalyst for formate production remained stable over 10 repeated uses. This paper reports the first case of efficient and selective formate production from H2 and CO2 using bacterial plasma membranes.

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.

Selective formate production from H2 and CO2 using encapsulated whole-cells under mild reaction conditions.

Biocatalytic CO2 reduction into formate is a crucial strategy for developing clean energy because formate is considered as one of the promising hydrogen storage materials for achieving net-zero carbon emissions. Here, we developed an efficient biocatalytic system to produce formate selectively by coupling two enzymatic activities of H2 oxidation and CO2 reduction using encapsulated bacterial cells of Citrobacter sp. S-77. The encapsulated whole-cell catalyst was made by living cells depositing into polyvinyl alcohol and gellan gum cross-linked by calcium ions to form hydrogel beads. Formate production using encapsulated cells was carried out under the resting state conditions in the gas mixture of H2/CO2 (70:30, v/v%). The whole-cell biocatalyst showed highly efficient and selective catalytic production of formate, reaching the specific rate of formate production of 110 mmol L-1· gprotein-1·h-1 at 30 °C, pH 7.0, and 0.1 MPa. The encapsulated cells can be reused at least 8 times while keeping their high catalytic activities for formate production under mild reaction conditions.

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.

Light-driven oxidation of CH4 to C1 chemicals catalysed by an organometallic Ru complex with O2.

CH4 conversion is one of the most challenging chemical reactions due to its inertness in terms of physical and chemical properties. We have achieved photo-induced C-H bond breaking of CH4 and successive C-O bond formation to form CH3OH concomitant with HCHO by an organometallic Ru complex with O2.

Hydrogen evolution, electron-transfer, and hydride-transfer reactions in a nickel-iron hydrogenase model complex: a theoretical study of the distinctive reactivities for the conformational isomers of nickel-iron hydride.

Hydrogen fuel is a promising alternative to fossil fuel. Therefore, efficient hydrogen production is crucial to elucidate the distinctive reactivities of metal hydride species, the intermediates formed during hydrogen activation/evolution in the presence of organometallic catalysts. This study uses density functional theory (DFT) to investigate the isomerizations and reactivities of three nickel-iron (NiFe) hydride isomers synthesized by mimicking the active center of NiFe hydrogenase. Hydride transfer within these complexes, rather than a chemical reaction between the complexes, converts the three hydrides internally. Their reactivities, including their electron-transfer, hydride-transfer and proton-transfer reactions, are investigated. The bridging hydride complex exhibits a higher energy level for the highest occupied molecular orbital (HOMO) than the terminal hydride during the electron-transfer reaction. This energy level indicates that the bridging hydride is more easily oxidized and is more susceptible to electron transfer than the terminal hydride. Regarding the hydride-transfer reaction between the NiFe hydride complex and methylene blue, the terminal hydrides exhibit larger hydricity and lower reaction barriers than the bridging hydride complexes. The results of energy decomposition analysis indicate that the structural deformation energy of the terminal hydride in the transition state is smaller than that of the bridging hydride complex, which lowers the reaction barrier of hydride transfer in the terminal hydride. To produce hydrogen, the rate-determining step is represented by the protonation of the hydride, and the terminal hydrides are thermodynamically and kinetically superior to the bridging ones. The differences in the reactivities of the hydride isomers ensure the precise control of hydrogen, and the theoretical calculations can be applied to design catalysts for hydrogen activation/production.

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.

Non-conventional octameric structure of C-phycocyanin.

C-phycocyanin (CPC), a blue pigment protein, is an indispensable component of giant phycobilisomes, which are light-harvesting antenna complexes in cyanobacteria that transfer energy efficiently to photosystems I and II. X-ray crystallographic and electron microscopy (EM) analyses have revealed the structure of CPC to be a closed toroidal hexamer by assembling two trimers. In this study, the structural characterization of non-conventional octameric CPC is reported for the first time. Analyses of the crystal and cryogenic EM structures of the native CPC from filamentous thermophilic cyanobacterium Thermoleptolyngbya sp. O-77 unexpectedly illustrated the coexistence of conventional hexamer and novel octamer. In addition, an unusual dimeric state, observed via analytical ultracentrifugation, was postulated to be a key intermediate structure in the assemble of the previously unobserved octamer. These observations provide new insights into the assembly processes of CPCs and the mechanism of energy transfer in the light-harvesting complexes.

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.

H2 activation by hydrogenase-inspired NiFe catalyst using frustrated Lewis pair: effect of buffer and halide ion in the heterolytic H-H bond cleavage.

Hydrogen is a clean fuel alternative to fossil fuels, and it is vital to develop catalysts for its efficient activation and production. We investigate the reaction mechanism of H2 activation in an aqueous solution by the recently developed NiFe complex (Ogo et al. Sci. Adv. 2020, 6, eaaz8181) using density functional theory (DFT) calculation. Our computational results showed that H2 is activated using frustrated Lewis pair. That is, H2 binds to the Fe site of the NiFe complex, acting as a Lewis acid, while the added buffer, acting as Lewis base, abstracts protons to form a hydride complex. Furthermore, the higher basicity in the proton abstraction reaction characterises reaction more exergonic and lowers the reaction barrier. In addition, in the proton abstraction by the water molecule, the reaction barrier was lowered when anion such as Cl- is in the vicinity of the water. Understanding the chemical species that contribute to the catalytic process in cooperation with the metal catalyst at the atomic level should help to maximise the function of the catalyst.

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.

Biochemical and structural characterization of a thermostable Dps protein with His-type ferroxidase centers and outer metal-binding sites.

The DNA-binding protein from starved cells (Dps) is found in a wide range of microorganisms, and it has been well characterized. However, little is known about Dps proteins from nonheterocystous filamentous cyanobacteria. In this study, a Dps protein from the thermophilic nonheterocystous filamentous cyanobacterium Thermoleptolyngbya sp. O-77 (TlDps1) was purified and characterized. PAGE and CD analyses of TlDps1 demonstrated that it had higher thermostability than previously reported Dps proteins. X-ray crystallographic analysis revealed that TlDps1 possessed His-type ferroxidase centers within the cavity and unique metal-binding sites located on the surface of the protein, which presumably contributed to its exceedingly high thermostability.

Selective Oxidation of H2 and CO by NiIr Catalyst in Aqueous Solution: A DFT Mechanistic Study.

One of the challenges in utilizing hydrogen gas (H2) as a sustainable fossil fuel alternative is the inhibition of H2 oxidation by carbon monoxide (CO), which is involved in the industrial production of H2 sources. To solve this problem, a catalyst that selectively oxidizes either CO or H2 or one that co-oxidizes H2 and CO is needed. Recently, a NiIr catalyst [NiIICl(X)IrIIICl(η5-C5Me5)], (X = N,N'-dimethyl-3,7-diazanonane-1,9-dithiolate), which efficiently and selectively oxidizes either H2 or CO depending on the pH, has been developed (Angew. Chem. Int. Ed. 2017, 56, 9723-9726). In the present work, density functional theory (DFT) calculations are employed to elucidate the pH-dependent reaction mechanisms of H2 and CO oxidation catalyzed by this NiIr catalyst. During H2 oxidation, our calculations suggest that dihydrogen binds to the Ir center and generates an Ir(III)-dihydrogen complex, followed by subsequent isomerization to an Ir(V)-dihydride species. Then, a proton is abstracted by a buffer base, CH3COO-, resulting in the formation of a hydride complex. The catalytic cycle completes with electron transfer from the hydride complex to a protonated 2,6-dichlorobenzeneindophenol (DCIP) and a proton transfer from the oxidized hydride complex to a buffer base. The CO oxidation mechanism involves three distinct steps, i.e., (1) formation of a metal carbonyl complex, (2) formation of a metallocarboxylic acid, and (3) conversion of the metallocarboxylic acid to a hydride complex. The formation of the metallocarboxylic acid involves nucleophilic attack of OH- to the carbonyl-C followed by a large structural change with concomitant cleavage of the Ir-S bond and rotation of the COOH group along the NiIr axis. During the conversion of the metallocarboxylic acid to the hydride complex, intramolecular proton transfer followed by removal of CO2 leads to the formation of the hydride complexes. In addition, the barrier heights for the binding of small molecules (H2, OH-, H2O, and CO) to Ir were calculated, and the results indicated that dissociation from Ir is a faster process than the binding of H2O and H2. These calculations indicate that H2 oxidation is inhibited by CO and OH- and thus prefers acidic conditions. In contrast, the CO oxidation reactions occur more favorably under basic conditions, as the formation of the metallocarboxylic acid involves OH- attack to a carbonyl-C and the binding of OH- to Ni largely stabilizes the triplet spin state of the complex. Taken together, these calculations provide a rationale for the experimentally observed pH-dependent, selective oxidations of H2 and CO.

Computational Study on the Light-Induced Oxidation of Iridium-Aqua Complex to Iridium-Oxo Complex over WO3(001) Surface.

An iridium aqua complex [IrIII(η5-C5Me5){bpy(COOH)2}(H2O)]2+ under visible light irradiation has been experimentally reported to form an iridium-oxo (Ir-oxo) complex [IrV(η5-C5Me5){bpy(COOH)2}(O)]2+, which oxidizes H2O to O2. However, the mechanism for the formation of this Ir-oxo complex remains unclear, due to the difficulties in observing the unstable Ir-oxo complex and computing light-induced systems having different numbers of electrons. In this study, we perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to investigate more in detail our previously proposed deprotonation and light-induced oxidation reactions composing the formation of the Ir-oxo complex. In particular, we discuss effects of light irradiation and WO3 support on the formation of the Ir-oxo complex. We suggest two distinct mechanisms, that is, direct and indirect for the light-induced oxidation. In the direct mechanism electrons are directly transferred from the occupied π* orbitals of IrIII-OH or IrIV=O• to the conduction band of the WO3 surface, whereas in the indirect mechanism electrons are first excited from the valence band to the conduction band of the WO3 surface due to the UV light, and then the resultant electron hole oxidizes the Ir complex. In the direct mechanism, in particular, we found that the lowest energy of the anode's conduction band determines the adsorption wavelength of the light irradiation, enabling us to predict alternative semiconductor anodes for more efficient formation of the Ir-oxo complex.

Dual Catalytic Cycle of H2 and H2O Oxidations by a Half-Sandwich Iridium Complex: A Theoretical Study.

While hydrogenase and photosystem II enzymes are known to oxidize H2 and H2O, respectively, a recently reported iridium aqua complex [IrIII(η5-C5Me5){bpy(COOH)2}(H2O)]2+ is able to oxidize both of the molecules and generate energies as in the fuel and solar cells ( Ogo ChemCatChem 2017 , 9 , 4024 - 4028 ). To understand the mechanism behind such an interesting bifunctional catalyst, in the present study, we perform density functional theory (DFT) calculations on the dual catalytic cycle of H2 and H2O oxidations by the iridium aqua complex. In the H2 oxidation, we found that the H-H bond is easily cleaved in a heterolytic fashion, and the resultant iridium hydride complex is significantly stabilized by the presence of H2O molecules, due to dihydrogen bond. The rate-determining step of this reaction is found to be the H2O → H2 ligand substitution with an activation energy of 10.7 kcal/mol. In the H2O oxidation, an iridium oxo complex originating from an oxidation of the iridium aqua complex forms a hydroperoxide complex, where an O-O bond is formed with an activation energy of 21.0 kcal/mol. Such a relatively low activation barrier is possible only when at least two H2O molecules are present in the reaction, allowing the water nucleophilic attack (WNA) mechanism to take place. The present study suggests and discusses in detail six reaction steps required for the dual catalytic cycle to complete.