Hideyuki Higashimura

Triangular Trinuclear Metal-N4 Complexes with High Electrocatalytic Activity for Oxygen Reduction

A new class of macrocyclic metal-N(4) complexes [MN(4)](n) (M = Co and Fe) were designed and synthesized based on a triangular ligand. Their unique triangular trinuclear structure provides a high density of active sites and facilitates the reduction of dioxygen via a four-electron pathway. Among them, a [CoN(4)](3)/C catalyst (20 wt %) exhibits high catalytic activity and long-time stability for the oxygen reduction reaction (ORR) in alkaline conditions, superior to the commercial Pt/C catalyst. Such structurally well-defined [MN(4)](n) complexes provide a platform for a new generation of nonprecious metal catalysts (NPMCs) for fuel cell applications.

Efficient hydrogen production and photocatalytic reduction of nitrobenzene over a visible-light-responsive metal–organic framework photocatalyst

Efficient hydrogen production and photocatalytic reduction of nitrobenzene were achieved by using a Pt-deposited amino-functionalised Ti(IV) metal–organic framework (Pt/Ti-MOF-NH2) under visible-light irradiation. XRD and N2 adsorption measurements revealed that crystalline microporous structures were formed and maintained even after the Pt deposition. The photocatalytic activity for the visible-light-promoted hydrogen production was improved through the optimization of the deposition amount of Pt as a cocatalyst. The optimised amount of Pt was determined to be 1.5 wt%. The results of in situ ESR measurements clearly indicate that the reaction proceeds through the electron transfer from the organic linker to deposited Pt as a cocatalyst by way of titanium-oxo clusters. In addition, the Pt/Ti-MOF-NH2 photocatalyst was found to catalyse photocatalytic reduction of nitrobenzene under visible-light irradiation. It was also confirmed that the catalyst can be reused at least three times without significant loss of its catalytic activity.

One-dimensional alignment of strong Lewis acid sites in a porous coordination polymer

A new lanthanoid porous coordination polymer, La-BTTc (BTTc = benzene-1,3,5-tris(2-thiophene-5-carboxylate)), was synthesized and structurally characterized to have densely aligned one-dimensional open metal sites, which were found to act as strong Lewis acid sites after the removal of the coordinated solvent.

New crystalline polymers: poly(2,5‐dialkyl‐1,4‐phenylene oxide)s

New heat-reversibly crystalline poly-(alkylated phenylene oxide)s are described, the oxidative polymerization of 2.5-dimethylphenol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane) copper dichloride produced poly(2,5-dimethyl-1,4-phenylene oxide), which showed heat-reversible crystallinity with a high melting point at ca. 300°C, although the isomeric polymer, poly(2,6-dimethyl-1,4-phenylene oxide), never recrystallizes once melted. The polymerization of 2,5-diethylphenol and 2,5-dipropylphenol gave the polymers consisting of 1,4-phenylene oxide units; the latter polymer possessed heat-reversible crystallinity, however, the former one did not.

Coupling selectivity in the radical-controlled oxidative polymerization of 4-phenoxyphenol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane)copper(II) complex

Various effects on the coupling selectivity of the oxidative polymerization of 4-phenoxyphenol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane)copper(II) halogeno complex [Cu(tacn)X 2 ] are described. With respect to the amount of the catalyst and the nature of the halide ion (X) of Cu(tacn)X 2 , the coupling selectivity hardly changed. The Cu(tacn) catalyst possessed a turnover number greater than 1860. As the temperature of the reaction and the polarity of the reaction solvent were elevated, the C-O coupling at the o-position increased, but the C-C coupling was not involved. For the polymerization in toluene at 80 °C, poly(1,4-phenylene oxide), obtained as a methanol-insoluble part, showed the highest number-average molecular weight of 4000 with a melting point (T m ) of 195 °C. Only a slight change in the coupling selectivity was observed in the presence or absence of hindered amines as the base. Surprisingly, however, the C-O selectivity decreased from 100 to 24% with less hindered amines, indicating that the selectivity drastically changed from a preference for C-O coupling to a preference for C-C coupling.

'Radical-controlled' oxidative polymerization of m-cresol catalyzed by μ-η2 :η2-peroxo dicopper(II) complex

Abstract Described is an oxidative polymerization of m-cresol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane)copper complex, from which a μ-η2:η2-peroxo dicopper(II) complex is formed under dioxygen and reacts with m-cresol to give ‘controlled’ phenoxy radical–copper(I) complex exclusively without formation of ‘free’ phenoxy radical. The present catalyst showed the highest activity in the metal complex catalysts reported for the polymerization of m-cresol with dioxygen. The resulting polymer consisted of oxyphenylene units and showed high molecular weight and high thermal stability. The high selectivity toward CO linkage would be due to the coupling from the phenoxy radical species controlled by this catalyst.

A systematic study on the stability of porous coordination polymers against ammonia.

To establish a strategy for designing porous coordination polymers (PCPs) for ammonia capture, the first systematic study on the stability of PCPs against ammonia was conducted. Various types of PCPs were investigated by comparing their powder XRD patterns before and after treatment with ammonia. Among the PCPs tested, ZIF-8, MIL-53(Al), Al-BTB, MOF-76(M) (M=Y or Yb), MIL-101(Cr), and MOF-74(Mg) were stable up to 350 °C under an ammonia atmosphere at ambient pressure. The origin of the stability of PCPs is discussed from the viewpoint of their components, metal cations, and organic linkers. Furthermore, adsorption isotherm measurements show that the adsorptive behavior of PCPs is independent of their stability.

Regio‐ and chemo‐selective polymerization of phenols catalyzed by oxidoreductase enzyme and its model complexes

Oxidative polymerizations of phenol derivatives have been performed using an oxidoreductase enzyme and its model complexes as catalyst to produce new functional polymers. Soluble polyphenols were synthesized using peroxidase catalyst in an aqueous methanol. Enzymatic polymerization of syringic acid involved elimination of carbon dioxide and hydrogen from the monomer to give poly(1,4-oxyphenylene) (PPO). Tyrosinase-model complexes catalyzed highly regioselective oxidative polymerization of a 2,6-unsubstituted phenol, 4-phenoxyphenol, to produce unsubstituted PPO showing crystallinity with a melting point. Chemoselective polymerization of phenols having an unsaturated group took place through peroxidase catalysis, yielding crosslinkable polyphenols.

Radical-controlled oxidative polymerization of 4-phenoxyphenol catalyzed by a dicopper complex of a dinucleating ligand

Abstract The oxidative polymerization of 4-phenoxyphenol (PPL) catalyzed by a dicopper complex with a dinucleating ligand, 1,2-bis[2-(bis(2-pyridyl)methyl)-6-pyridyl]ethane is described. The polymerization proceeded regioselectively to give crystalline unsubstituted poly(1,4-phenylene oxide) (PPO), in which μ-η 2 :η 2 -peroxo dicopper(II) species is probably involved as the active dioxygen intermediate. A copper complex with a mononucleating ligand of the similar structure, 1,1,1-tris(6-methylpyrid-2-yl)ethane, also catalyzed the oxidative coupling of PPL. The coupling selectivity for the dicopper/dinucleating ligand complex was close to that for the copper/mononucleating ligand complex. However, the initial reaction rate for the former was independent on the catalyst amount in a certain range, whereas that for the latter decreased with the decrease of the catalyst amount.

Cobalt Phenanthroline–Indole Macrocycles as Highly Active Electrocatalysts for Oxygen Reduction

The replacement of scarce and expensive platinum species poses a challenge in fuel-cell development. The design and synthesis of a novel type of Co(II) -N4 macrocyclic complex, [CoN4 ], based on the phenanthroline-indole macrocyclic ligand (PIM) is reported. This unique ligand allows the formation of mono- and dinuclear complexes with defined active sites that facilitate the direct four-electron reduction of oxygen. Electrochemical measurements revealed that the [CoN4 ]/C (20 wt %) catalysts have a high activity and long-term stability for the oxygen-reduction reaction (ORR) under alkaline conditions, similar to the Pt/C catalyst. These structurally well-defined complexes represent a nonprecious alternative to platinum species for future fuel-cell applications.