Oxidation reactions utilizing reactive species generated on metal ions are important processes in biological reactions and various catalytic reactions. A typical example is dioxygen (O₂) activation on iron and copper metal ions in the enzyme active sites. Although it appears to be a simple reaction, the diversity in structure, physicochemical properties, and reactivity of the generated reactive oxygen species makes it not so easy to control them. In metalloenzyme active centers, this is accomplished by skillfully exploiting the coordination geometry of the metal centers, electronic interactions with the coordinating atoms, and weak interactions in the secondary coordination sphere (hydrogen bonds, hydrophobic interactions, electrostatic interactions, etc.). Metalloenzymes are also known to cooperate with redox-active amino acid side chains (phenol group of tyrosine, thiol group of cysteine, indole group of tryptophan, etc.), coenzymes, or organic cofactors in the enzyme active site to perform the various enzymatic functions. Inspired by the essence of such metalloenzyme functions, we have been trying to create various metal-oxidizing active species and exploring their functions and catalytic applications. These studies are important not only for the elucidation of enzyme functions but also for the development of new catalysts. In this account, an overview of some of the research we have conducted is introduced.

Intermetallics (M¹/M²) of abundant metals of very different properties can substitute rare and precious metals for industrial catalysis. A challenge in this context is the atom-precise and -efficient catalyst design at the size regime of ≤ 1 nm, where every metal atom counts. Along these lines of thought we have been developing a novel way to generate libraries of all-hydrocarbon (R^*) ligated clusters [M¹_aM¹_b](R^*)_c. These organometallic clusters and superatom complexes are molecular counterparts of the solid state M¹/M² materials. The electronic structure, e.g. open vs. closed ‘superatom shell’, controls stability and reactivity. The libraries are metastable and the hypothesis is that they are loaded with interrelated clusters, elusive and highly reactive ones as well as more accessible but less reactive ones. The chemical control parameters relate to fundamental organometallic reaction networks of nucleation, growth and degradation, to ligand and/or additive association, dissociation, to cluster surface metal-atom site activation by ligand deprotection and to reactions with small molecules. The article is a personal account of a long-time research enterprise on the coordination chemistry of group12/13 metallo-ligands at transition metal centers, and the related mixed-metal cluster chemistry to the emergence of “living libraries” of such clusters.

We have investigated cyanide-based metal complexes, which were synthesized by combining [M(CN)₄]^2- (M = Pt, Pd, Ni) or [MN(CN)₄]^2- (M = Mn, Re) with metal complex cations, metal ions, and organic cations, by focusing on their dynamic structures such as thermal expansion (TE) and phase transition. For example, FePd(CN)₄ is a three dimensional framework consisting of four-coordinate iron(II) nodes. This metal cyanide demonstrates large anisotropic TE involving negative TE (NTE) and colossal positive TE (PTE) during 100 K-500 K. Other examples are two-dimensional (2D) undulating layer-type coordination polymers of type [M(salen)]₂[Ḿ (CN)₄] (M = Mn, Fe; Ḿ = MnN, ReN, Pt, Pt(I₂)_x; x = 0.18, 0.45, 0.85, 1.0). Uniquely, they demonstrate composition-dependent anisotropic TE including 2D-PTE and 2D-NTE with wide range of linear TE coefficient values −45~+63 × 10^-6 K^-1. Furthermore, the combination of [MN(CN)₄]^2-(M = Mn, Re) with an ionic liquid cation, 1-ethyl-3-methylimidazolium (EmIm⁺), provided melting systems of (EmIm)₂[MN(CN)₄] involving cyanide-bridged architectures. Moreover, we also developing molecular probes incorporating the metal complex moieties, that can work in complex environments such as cell membranes, to investigate and promote membrane functions.

Transition metal-sulfur clusters are ubiquitous as enzymatic cofactors and play critical roles in various molecular transformations. Their synthetic counterparts have been studied with a significant focus on structural and spectroscopic modeling in the past 50 years. However, applications of this class of compounds for other purposes remain limited. In this context, we have been working on applying synthetic metal-sulfur clusters to biochemical analysis and catalysis with great interest in the enzymatic N2-reducing cluster (FeMoco). Synthetic clusters function as artificial cofactors of enzymes. The resultant reconstituted proteins can perform catalytic reduction using a natural electron transfer system or help analyze an unidentified intermediate involved in the biosynthesis of FeMoco. In addition, we developed catalytic reduction reactions of inert small molecules (e.g. CO, CO₂, and N₂) promoted by synthetic clusters. This account overviews the series of research to deliver a brief perspective of the functional aspect of the synthetic metal-sulfur clusters.

In recent years, hydrogen energy has attracted increasing attention in relation to climate change and energy issues. However, the hydrogen used must be produced without carbon dioxide emissions. Water is the only raw material that can be obtained cheaply and in large quantities to produce such hydrogen. The decomposition of water into hydrogen and oxygen requires Gibbs energy of 237 kJ/mol at room temperature and ambient pressure. The energy is usually supplied as electrical or light energy. This account focuses on the development of particulate photocatalysts that use the energy of sunlight to split water. After explaining the basic conditions required for semiconductor particulate photocatalysts for water splitting, the development of several photocatalysts will be described in detail. The development of a total system to produce solar hydrogen from sunlight and water using particulate photocatalysts is then described. Finally, the remaining challenges for the practical application of this novel technology are described.

The chemistry of coordination polymers and metal-organic frameworks, MOFs, has dominantly progressed with the development of crystallography. The study of "disordered systems" such as liquid and glassy states of coordination polymers and MOFs has been developed in recent years, and various methods have been recognized to produce glasses from crystals. The structures of the glass and liquid phases are investigated using synchrotron radiation X-rays and spectroscopies. Many of them have the coordination bond-based networked structure found in the crystal structures. This suggests that the structural design principles of coordination chemistry can be applied to glasses and liquids of coordination polymers and MOFs. The inherent properties of the disordered systems, such as wide compositions and high internal degrees of freedom, high materials' formability, and softness contribute to the coupled physicochemical properties. In this review, the recent development of coordination polymer liquids and glasses is presented with a focus on the authors' group research.