Saburo Hosokawa

An oxyhydride of BaTiO3 exhibiting hydride exchange and electronic conductivity

In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

A Doping Technique that Suppresses Undesirable H2 Evolution Derived from Overall Water Splitting in the Highly Selective Photocatalytic Conversion of CO2 in and by Water

Photocatalytic conversion of CO2 to reduction products, such as CO, HCOOH, HCHO, CH3OH, and CH4, is one of the most attractive propositions for producing green energy by artificial photosynthesis. Herein, we found that Ga2O3 photocatalysts exhibit high conversion of CO2. Doping of Zn species into Ga2O3 suppresses the H2 evolution derived from overall water splitting and, consequently, Zn-doped, Ag-modified Ga2O3 exhibits higher selectivity toward CO evolution than bare, Ag-modified Ga2O3. We observed stoichiometric amounts of evolved O2 together with CO. Mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. Doping of the photocatalyst with Zn is expected to ease the adsorption of CO2 on the catalyst surface.

Recyclable solid ruthenium catalysts for the direct arylation of aromatic C-H bonds.

The development of new environmentally benign processes for synthesizing organic compounds is one of the most important issues currently facing synthetic chemists. If efficiencies comparable to those achieved with homogeneous catalysis can be realized, heterogeneous catalysis is quite attractive, because the catalysts can be readily separated from the reaction media and re-used. On the other hand, the formation of C C bonds by the direct activation of less reactive hydrocarbon C H bonds by using homogeneous, transition-metal catalysts has also attracted considerable attention in modern synthetic chemistry. Several solid catalysts have been reported to be effective for use in conventional C C bond-forming reactions; the arylation of heteroarenes or phenols over heterogeneous Ni, Pd, or Ru catalysts have been reported. However, to the best of our knowledge, there have been no previous reports of C C bond formation by using chelation-assisted activation of stable aromatic C H bonds by solid ruthenium catalysts. Recently, we reported a heterogeneous Ru/CeO2-catalyzed transfer-allylation of homoallyl alcohols into aldehydes, in which the active species was considered to be a low-valent Ru species generated in situ from a Ru oxide species on CeO2. In this communication, we report the first example of Ru/ CeO2-catalyzed nitrogen-directed arylation of aromatic C H bonds with aryl halides. This reaction, for which lowvalent Ru-complex catalysts are quite effective, is one of the most ecological and economical methods available for preparing unsymmetrical biaryls. The results in this communication imply that solid Ru/CeO2 can be a good alternative to homogeneous Ru catalysts, which are known to be effective for a wide range of C C bond-forming reactions. Supported-Ru catalysts were prepared as follows; an oxide support was added to a solution of a Ru-catalyst precursor in THF at room temperature. After impregnation by Ru, the resulting powder was calcined in air at 400 8C for 30 min to afford Ru (2.0 wt %)/support. The reaction of benzo[h]quinoline (1 a) with chlorobenzene (2 a) in N-methyl-2-pyrrolidone (NMP) at 170 8C for 18 h in the presence of a Ru/CeO2 catalyst prepared by using [Ru3(CO)12] as a catalyst precursor gave the arylated product 3 a selectively in a yield of 61 % (Table 1, entry 1). The addition of a small amount of PPh3 to the reaction mixture facilitated the reaction and gave 3 a quantitatively (entry 2). The reactions of 1 a with bromobenzene (2 a’) or iodobenzene (2 a’’) also produced 3 a quantitatively (Table 1, entries 3 and 4). The catalytic activity was greatly affected by the use of different Ru precursors. Catalysts prepared from Cl-containing Ru precursors such as RuCl3·nH2O, [{RuCl2(CO)3}2], or [{RuCl2ACHTUNGTRENNUNG(p-cymene)}2] gave 3 a in higher yields than that prepared from [Ru ACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] (cod =1,5-cyclooctadiene; cot=1,3,5-cyclooctatriene; Table 1, entries 5– 8). The reaction that was carried out at 150 8C gave only a

Highly efficient photocatalytic conversion of CO2 into solid CO using H2O as a reductant over Ag-modified ZnGa2O4

Highly crystalline spinel phase ZnGa2O4 modified with a Ag cocatalyst exhibited high activity and selectivity toward CO evolution in the photocatalytic conversion of CO2 using H2O as a reductant under UV light irradiation. The stoichiometric evolution of CO, H2, and O2 clearly indicated that H2O worked as an electron donor for the photoreduction of CO2. Highly crystalline ZnGa2O4 was synthesized by a solid-state reaction method at a calcination temperature as low as 973 K. Upon optimizing the fabrication conditions, such as calcination temperature and duration, the photocatalytic activity of ZnGa2O4 was maximized because of the optimum balance between crystallinity and surface area in the catalyst material. Furthermore, the formation of metallic Ag particles with different sizes and dispersions on the surface of ZnGa2O4 influenced the evolution of CO. When Ag nanoparticles were loaded onto the ZnGa2O4 calcined at 1123 K for 40 h using a chemical reduction method, the highest formation rates of CO, H2, and O2 (155, 8.5, and 74.3 μmol h−1, respectively) were obtained, and the selectivity toward CO evolution reached 95.0%. An isotope-labeling experiment using 13CO2 confirmed that the origin of the evolved CO was not from organic contamination but from the CO2 gas introduced during the reaction process.

Tuning the selectivity toward CO evolution in the photocatalytic conversion of CO2 with H2O through the modification of Ag-loaded Ga2O3 with a ZnGa2O4 layer

Stoichiometric evolutions of CO, H2, and O2 were achieved for the photocatalytic conversion of CO2 with H2O as an electron donor using Ag-loaded Zn-modified Ga2O3. The selectivity toward the evolution of CO over H2 can be controlled by varying the amount of Zn species added in the Ag-loaded Zn-modified Ga2O3 photocatalyst. The production of H2 gradually decreased with increasing amounts of Zn species from 0.1 to 10.0 mol%, whereas the evolution of CO was almost unchanged. The XRD, XAFS, and XPS measurements revealed that a ZnGa2O4 layer was generated on the surface of Ga2O3 by modification with Zn species. The formation of the ZnGa2O4 layer eliminated the proton reduction sites on Ga2O3, although the crystallinity, surface area, and morphology of Ga2O3 as well as the particle size and chemical state of Ag did not change. In conclusion, we designed a highly selective photocatalyst for the conversion of CO2 with H2O as an electron donor using Ag (the cocatalyst for the CO evolution), ZnGa2O4 (the inhibitor of the H2 production), and Ga2O3 (the photocatalyst).

Ruthenium‐Catalyzed Intermolecular Hydroacylation of Internal Alkynes: The Use of Ceria‐Supported Catalyst Facilitates the Catalyst Recycling

Versatile and practical: Intermolecular hydroacylation of internal alkynes takes place in the presence of Ru catalysts together with HCO(2)Na and Xantphos to give the corresponding conjugated enones. Aromatic aldehydes with or without coordinating groups could be used in the present catalytic system. The solid Ru/CeO(2) catalysts can be recycled for several times without significant decreases in yield (see scheme).

Catalytic Addition of Aromatic CH Bonds to Vinylsilanes in the Presence of Ru/CeO2

Low-valent ruthenium species show unique catalytic properties and enable various organic transformations, such as C C bond formation through cleavage of stable aromatic C H bonds. In particular, the [RuH2(CO)(PPh3)3]-catalyzed selective addition of aromatic C H bonds to unsaturated compounds has made it possible to rapidly construct carbon skeletons with excellent atom efficiency. However, in preparing and handling such low-valent complexes, difficulties are often encountered because they are highly sensitive toward air and moisture. As an alternative approach, the catalytically active species, generated in situ from catalytically inactive but airstable Ru complexes, has been adapted for these reactions, although it is essential to create an optimized coordination environment with suitable additives. However, problems remain in the separation of the catalysts from the products. In contrast, the use of solid catalysts, in particular simple solid oxides, are advantageous from environmental and industrial perspectives because they are thermally and chemically quite stable and can be readily separated from the products. However, there have been few previous examples of reactions involving C H bond activation in the presence of solid catalysts, and the development of new strategies for the preparation of active low-valent Ru species on solid surfaces remains a challenge. Recently, we reported C C bond-forming reactions through the activation of C H or C C bonds in the presence of simple metal oxide catalysts Ru/CeO2 and Ru/ZrO2, whereas other Ru catalysts supported on SiO2, Al2O3, MgO, and TiO2 showed no catalytic activities. 9] The catalytically active species in these reactions were considered to be low-valent Ru species generated from Ru species on CeO2. From these backgrounds, various selective C C bond forming reactions are expected to be realized on solid catalysts, if the coordination sites of low-valent Ru species can be controlled by an appropriate choice of additives and pretreatment conditions. Herein, we present an example of the catalytic addition of C H bonds of aromatic ketones to vinylsilanes in the presence of Ru/CeO2. The reactions proceeded smoothly in association with appropriate phosphines to selectively give alkylated compounds in high yields. Furthermore, the pretreatment of Ru/CeO2 under appropriate conditions increased the catalytic activities so markedly that the reaction rapidly went to completion at lower temperatures. Ru/CeO2 was prepared by the impregnation method. CeO2, prepared by the treatment of cerium(III) nitrate hexahydrate with aqueous ammonia, was added to a THF solution of [Ru3(CO)12] at room temperature. After impregnation, the resulting light yellow powder was calcined in air at 400 8C for 30 min, to afford the solid oxide, Ru (2.0 wt %)/CeO2. The reaction of aromatic ketone 1 a with vinylsilane 2 a in the presence of Ru (2.0 wt %)/CeO2 (125 mg) and PPh3 in mesitylene at 170 8C gave the product 3 a quantitatively (Table 1,

Ceria-supported ruthenium catalysts for the synthesis of indole via dehydrogenative N-heterocyclization

Simple heterogeneous Ru/CeO2 catalysts as well as Ru/ZrO2 catalysts were found to be quite effective for the selective direct synthesis of indole via intramolecular dehydrogenative N-heterocyclization of 2-(2-aminophenyl)ethanol, while catalysts supported on SiO2, Al2O3, TiO2, and MgO were less effective. Ru/CeO2 catalysts that were calcined at a relatively low temperature, 200 °C, showed excellent activity and gave indole in a yield over 99% by the reaction at 140 °C for 24 h (Ru catalyst 2.5 mol%). Spectroscopic studies of the Ru/CeO2 catalysts indicated the formation of RuIVO species on the surface, which is considered to be transformed into the catalytically-active species at the initial stage of the reaction. Hot filtration tests and an ICP-AES analysis indicated that these Ru/CeO2 catalysts act heterogeneously and that the leaching of ruthenium species into the solution is negligible. These catalysts could be recycled without a significant loss of activity, which suggests that the present oxide-supported catalysts are promising alternatives to conventional homogeneous catalysts.

A ZnTa2O6 photocatalyst synthesized via solid state reaction for conversion of CO2 into CO in water

Because of the environmental problems and the resulting exigent demand for CO2 recycling processes, great attention is being paid to the photocatalytic conversion of CO2 into useful chemicals such as CO, HCOOH, HCHO, CH3OH, and CH4. We have previously reported that the Ag-loaded, Zn-modified Ga2O3 photocatalyst exhibits excellent photocatalytic activity required for the conversion of CO2 into CO by using H2O as a reductant and that the Ag particles that exist together with the Zn species act as good cocatalysts for the selective formation of CO. In this study, we demonstrated the photocatalytic activity of ZnTa2O6 under UV light irradiation, which was prepared via solid-state reaction, for the conversion of CO2 in an aqueous NaHCO3 solution. A Ag cocatalyst-loaded ZnTa2O6 photocatalyst evolved CO as a reduction product of CO2 with 46% selectivity toward CO evolution among the reduction products. In contrast, when Pt and Au were introduced as cocatalysts, the ZnTa2O6 photocatalyst evolved H2 with high selectivity (>99.9%).

Visible-light-assisted selective catalytic reduction of NO with NH[3] on porphyrin derivative-modified TiO[2] photocatalysts

Porphyrin-derivative-modified TiO2 photocatalysts showed high photocatalytic activity for the selective catalytic reduction of NO with NH3 in the presence of O2 under visible-light irradiation. Tetra(p-carboxyphenyl)porphyrin (TCPP) was the most effective photosensitizer among the five porphyrin derivatives investigated. NO conversion and N2 selectivity of 79.0% and 100%, respectively, were achieved at a gas hourly space velocity of 50 000 h−1. UV–Vis and photoluminescence spectroscopies revealed the presence of two species of TCPP on the TiO2 surface; one was a TCPP monomer and the other was an H-aggregate of the TCPP molecules. It was concluded that the TCPP monomer is an active species for the photo-assisted selective catalytic reduction (photo-SCR). Moreover, an increase in the fraction of H-aggregates with increasing TCPP loading amount resulted in a decrease in the photocatalytic activity of the photo-SCR.