This account describes a current summary of our research on ruthenium complexes having tris(2-pyridylmethyl)amine (TPA) and its derivatives as ligands. Topics include their characteristics, catalytic oxygenations of organic compounds including alkanes, alkenes, and alcohols by peroxide activation and proton-coupled electron transfer to generate Ru(IV)=O complexes as reactive species, Ru-TPA complexes with redox-active heteroaromatic coenzymes such as pterins and flavin analogues to elucidate their characteristics, construction of specific coordination spheres by introducing functional moieties to the TPA ligand via amide linkage to develop non-covalent interactions in the first and second coordination spheres, and bistability in photochromic structural change. The important features of the Ru-TPA complexes are following: (1) Reliable stability due to the π-back bonding from 4d_π orbitals of the ruthenium center to π* orbitals of heteroaromatic ligands, (2) Very clear reversible redox behaviour, (3) Availability of a wide range of oxidation states and a variety of reactivity at the ruthenium center, (4) Feasibility of derivatization of TPA toward the construction of unique coordination environments, and (5) Accessibility to bistability in photochemical and thermal processes.

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In this review, a new method for the preparation of metal-assembled complexes based on copper(II) acetate type dinuclear systems has been reported. A variety of coordination modes of carboxylato group were classified into ten types of groups including bis-, tris-, and tetrakis-bridging based on our complexes and discussed in relation to mononuclear, dinuclear, oligonuclear, and polynuclear systems. An interdimer interaction could not be observed in temperature dependence of magnetic susceptibilities at 4.5—300 K region in case of one-dimensional chain systems of copper(II) pivalate with N,N'-bidentate ligands such as pyrazine, 4,4'-bipyridine, 1,4-diazabicyclo[2.2.2]octane. Adsorption property for nitrogen gas was found in a chain system of copper(II) benzoate with pyrazine. Mesomorphic properties were also found for chain systems of copper(II) carboxylates with long alkyl chains. By using copper(II) trichloroacetate, a chain molecule consisting of alternated arrangement of tetranuclear cubane and pyrazine was isolated. Two-dimensional and three-dimensional assembled complexes were obtained by reactions of copper(II) trifluoroacetate and copper(II) propionate, respectively, with pyrazine. Bis-adducts of copper(II) propionate with pyridyl nitronyl nitroxides, copper(II) acetate with a stable adamantanyloxyamino radical, and dinuclear copper(II) complexes with 4-carboxy-2,2,6,6-tetramethylpiperidinyloxy and 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy were prepared as radical-containing copper(II) acetate systems.

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The study in the area of luminescent d¹⁰ complexes has been developing rapidly. The metals with d¹⁰ electronic configuration have a closed d-shell, therefore the nature of the complexes are quite different from regular transition metal complexes. The complexes show a diversity of the electronic excited states. This paper reviews the characteristic nature of the d¹⁰ metal complexes with interesting examples. Copper(I), silver(I), gold(I), nickel(0),palladium(0) and platinum(0) complexes are discussed. Luminescent copper(I) complexes are classified into two groups; the first includes the complexes bearing halogen ligands, and the second have no halogen ligands. In the second group, the complexes with phosphine and diimine type ligands are mainly discussed. As for the silver(I) and gold(I) complexes, multinuclear complexes are mostly described. For the palladium(0) and platinum(0) complexes, although a small number of works have been published so far, some interesting studies including our works have been described. In addition to the recent studies on the photophysical properties of the complexes, some applications are described in this review.

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Since the successful isolation of N-heterocyclic carbenes (NHCs), NHCs are now fully established as an important class of ligands for the homogeneous catalysis. The most common method for the preparation of NHC complexes is direct complexation of the free NHC with the metal, in which the free NHC, either isolated or generated in situ, is produced by deprotonation of the corresponding salt as the NHC precursor. However manipulation of the free NHCs is often difficult due to their highly reactive nature toward air and moisture. The reaction of the imidazolium salt with metal precursors bearing the basic ligand such as acetate, acetylacetonate, or alkoxide is also effective for the preparation of NHC complexes, whereas this method is limited to the synthesis of the late transition metal complexes. Recently, the employment of NHC adducts as “protected” forms of the free carbene attracted much attention for new methods of the preparation of the NHC complexes. In this review, the recent developments of “protected” NHC adducts as versatile synthons for the synthesis of NHC complexes are described.

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Activation of atmospheric dinitrogen under mild conditions has remained a challenge to chemists for more than a century. In addition to the fixation of dinitrogen to ammonia, direct elaboration of inert atmospheric N₂ into more value-added organic molecules is also attractive for the evolution of ammonia-independent synthetic pathways. Chirik et al. have found the rich chemistry of substituted bis(cyclopentadienyl)zirconium and hafnium complexes (metallocenes) bearing side-on coordinated dinitrogen ligands. As extension of this chemistry, syntheses of substituted hydrazines from N₂ and CO₂, which are both abundant chemical feedstocks, promoted by hafnocene and zirconocene were discovered. Addition of 2 equiv of carbon dioxide to the hafnocene and the ansa-zirconocene dinitrogen complexes resulted in insertion into M-N bonds, forming dicarboxylated diazenido complexes, where their insertion patterns are different. Subsequent treatments with Me₃SiI liberated dicarboxylated silyl-substituted hydrazines. Moreover, for the zirconocene type, new N-C bonds were also assembled by addition of methyl triflate before liberation of hydrazines by treatments with electrophiles.

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In this paper, a topic about recent theoretical calculations for di-chromium(I) complex and its related complexes is introduced. In 2005, a synthesis of a novel complex which has a short Cr(I)-Cr(I) bond was reported by Power's group. In the paper, they also indicated that the complex had a quintuple Cr(I)-Cr(I) bond with DFT calculations. Interestingly, other complexes consisted of Fe and Co ions did not show such a short metal-metal bond even if one used same ligands. In order to explain the structures of those complexes, they optimized the geometries by theoretical calculations. First, they optimized the Cr(I)-Cr(I) bond with simple ligands and the results indicated that the multiple bond is the origin of the short Cr-Cr bond. They, next, suggested an importance of an interaction between metal and π electrons of phenyl ligands. Their calculations indicate that the interaction seems to elongate metal-metal bonds especially for Fe and Co complexes although it hardly affects to the Cr(I)-Cr(I) bond length.

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