Metal−oxyl (M n+ -O • ) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal−oxo (M (n+1)+ O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of M n+ -O • species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of M n+ -O • complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the M n+ -O • species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

Ag@TiO 2 nanoparticles (NPs) with Ag metal cores and TiO 2 semiconductor shells were prepared with a hydrothermal method and their structure was characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The core−shell Ag@TiO 2 NPs were deposited on a glass plate and employed as photocatalysts for CO 2 conversion by irradiation of solar simulator (AM1.5) under CO 2 atmosphere. Selective CH 4 evolution by CO 2 photoconversion was attained with the core−shell Ag@TiO 2 NP photocatalyst. The CH 4 evolution rate normalized by specific surface areas was 10 times higher than those of reference TiO 2 NPs and conventional TiO 2 NPs with accompanying Ag deposits. The role of the Ag core was also demonstrated in photodecomposition of a dye by the core−shell Ag@TiO 2 NPs. These results suggested that the high photocatalytic activity of the core−shell Ag@TiO 2 NPs was archived due to bandgap improvement of the TiO 2 shell and increase of photon flux to the TiO 2 shell by the plasmonic Ag core.

Novel heterodinuclear IrIII–MII complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis­(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and employed as catalysts for H2 evolution through formic acid dehydrogenation in water. A cooperative effect of the hetero-metal centers was observed in the H2 evolution in comparison with the corresponding mononuclear IrIII and MII complexes as the components of the IrIII–MII complexes. The H2 evolution rate for the IrIII–MII complexes was at most 350-fold higher than that of the mononuclear IrIII complex. The catalytic activity increased in the following order: IrIII–CuII complex < IrIII–CoII complex < IrIII–NiII complex . The IrIII–H intermediates of the IrIII–MII complexes were successfully detected by ultraviolet–visible, 1H nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H2 evolution by the IrIII–MII complexes indicates that the IrIII–H species formed in the IrIII moiety act as reactive species and the MII moieties act as acceleration sites by the electronic effect from the MII center to the IrIII center through the bridging bpp– ligand. The IrIII–MII complexes may also activate H2O at the 3d MII centers as a proton source to facilitate H2 evolution. In addition, the affinity of formate for the IrIII–MII complexes was investigated on the basis of Michaelis–Menten plots; the IrIII–CoII and IrIII–NiII complexes exhibited affinities that were relatively higher than that of the IrIII–CuII complex. The catalytic mechanism of H2 evolution by the IrIII–MII complexes was revealed on the basis of spectroscopic detection of reaction intermediates, kinetic analysis, and isotope labeling experiments.

Proton-coupled electron-transfer oxidation of a Ru -OH complex, having an N-heterocyclic carbene ligand, gives a Ru -O species, which has an electronically equivalent structure of the Ru =O species, in an acidic aqueous solution. The Ru -O complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732 cm and the Ru-O bond length was estimated to be 1.77(1) Å. The Ru -O complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the Ru -O complex bearing a strong radical character as the active species.