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High-valent cobalt–oxo complexes are reactive transient intermediates in a number of oxidative transformation processes e.g., water oxidation and oxygen atom transfer reactions. Studies of cobalt–oxo complexes are very important for understanding the mechanism of the oxygen evolution center in natural photosynthesis, and helpful to replicate enzyme catalysis in artificial systems. This review summarizes the development of identification of high-valent cobalt–oxo species of tetrapyrrolic macrocycles and N-based ligands in oxidation of organic substrates, water oxidation reaction and in the preparation of cobalt–oxo complexes.

This review presents an overview of the biological applications of coordinative compounds based on unsaturated carboxylates accompanied by other ligands, usually N-based heterocyclic species. The interest in these compounds arises from the valuable antimicrobial and antitumor activities evidenced by some species, as well as from their ability to generate metal-containing polymers suitable for various medical purposes. Therefore, we describe the recently discovered aspects related to the synthesis, structure, and biological activity of a wide range of unsaturated carboxylate-containing species and metal ions, originating mostly from 3d series. The unsaturated carboxylates encountered in coordinative compounds are acrylate, methacrylate, fumarate, maleate, cinnamate, ferulate, coumarate, and itaconate. Regarding the properties of the investigated compounds, it is worth mentioning the good ability of some to inhibit the development of resistant strains or microbial biofilms on inert surfaces or, even more, exert antitumor activity against resistant cells. The ability of some species to intercalate into DNA strands as well as to scavenge ROS species is also addressed.

Herein, the synthesis and the structural as well as the photophysical characterization of five transition metal complexes bearing a neutral pyridine-pyrazole-based N^N*N^N ligand (L) acting as a tetradentate chelator are reported. The luminophore can be synthesized via two different pathways. An alkyl chain with a terminal tert-butyl moiety was inserted on the bridging nitrogen atom to enhance the solubility of the complexes in organic solvents. Due to the neutral character of L, metal ions with different charges and electronic configurations can be chelated. Thus, complexes with Pt(II) (C1), Ag(I) (C2), Zn(II) (C3), Co(II) (C4) and Fe(II) (C5) were synthesized. Single-crystal X-ray diffraction experiments showed that complex C2 exhibits a completely different structure in the crystalline state if compared with C3 and C5, i.e., depending on the chelated cation. The UV-vis absorption and the NMR spectra showed that the complexes dissociate in liquid solutions, except for the Pt(II)-based coordination compound. Therefore, the photophysical properties of the complexes and of the ligand were studied in the solid state. For the Pt(II)-based species, a characteristic metal-perturbed ligand-centered phosphorescence was traceable, both in dilute solutions as well as in the solid state.

Ammonia is an essential compound that can be used as an ingredient for various chemicals and a next‐generation energy carrier for a carbon‐neutral society. Although the Haber‐Bosch process is the widely used industrial production method for ammonia, this method consumes a large amount of energy to conduct the catalysis under high pressure and temperature; therefore, the development of efficient catalysts for ammonia production under ambient reaction conditions should be one of the most important research topics for attaining carbon neutrality. In this study, we have developed a new series of molybdenum complexes bearing dihydroimidazole‐based PCP‐type pincer ligands, which can catalyze ammonia production from dinitrogen and water using samarium diiodide as a reductant under ambient conditions. Although the nonsubstituted complex did not exhibit efficient catalytic activity, introducing phenyl groups onto the dihydroimidazole skeleton substantially enhanced its catalytic activity. According to our electrochemical studies and density functional theory calculations, the phenyl groups effectively increased the electron‐accepting ability of the molybdenum complex, resulting in an increase in the strength of NH bonds (i.e., bond dissociation free energy (BDFENH)) in the corresponding imide complex, which is an important intermediate in the catalytic cycle for ammonia production, and its catalytic activity. A new series of molybdenum complexes bearing dihydroimidazole‐based PCP‐type pincer ligands has been developed, which can catalyze ammonia production from dinitrogen and water using samarium diiodide as a reductant under ambient reaction conditions. The introduction of phenyl groups onto the dihydroimidazole skeleton significantly enhanced its catalytic activity.