We found the following new photochemical reactions and photophysical properties of transition metal complexes: (1) photochemical formation of Ru(II) hydride complexes, (2) photochemical ligand substitution reactions of the Re(I)diiminetricarbonyl complexes, and (3) control of photophysical and electrochemical properties of metal complexes by introducing p-p interaction between ligands. Application of these reactions and photophysical properties of transition metal complexes enabled us to synthesize new photofunctional metal-complex oligomers and polymers， and to develop efficient， durable， and highly selective photocatalysts for hydride reduction of NAD(P)⁺ models and CO₂ reduction. We also succeeded construction of new photofunctional hybrids of the metal-complex photocatalysts with various solid materials， i.e. semiconductor-particle photocatalysts， p-type semiconductor electrodes， and mesoporous organosilica， which can drive photocatalytic reduction of CO₂ via Z-scheme electron transfer， photocatalytic CO₂ reduction by using water as a reductant and visible light as energy， and lightenergy accumulation for photocatalytic CO₂ reduction， respectively.

Structurally constrained multinuclear transition metal assemblies are fascinating building blocks for atomically precise nanostructured molecular devices due to a variety of functions with electronic, magnetic, photophysical, and catalytic properties originating from cooperative effects of proximate metal centers. Their viable syntheses are in debt to design of multidentate supporting ligands, and to develop low valent metal assemblies as molecular miniatures of metallic materials, those with a pair of soft donors connected by a single atom are highly desired. The authorʼs group synthesized a series of linear polyphosphine ligands with short bite distances, and has systematically studied such multinuclear metal clusters. This review describes the recent developments of low-valent metal clusters supported by linear tri-, tetra- and hexadentate phosphine ligands, Ph₂PCH₂[P(Ph)CH₂]_nPPh₂ (n = 1, 2, 4), Ph₂PCH₂P(Ph) [CH₂]_mP(Ph)CH₂PPh₂ (m = 1-4), and Ph₂PCH₂P(Ph) N(Ph)P(Ph)CH₂PPh₂, mainly focusing on self-aligned Pt₆, Pt₂Pd₂Pt₂, Pd₈ chains, strongly luminous Au₄ and Au₆ chains and {Au₂AgCu}₂ rings, and trigonal, rhombic, square planar, and cage-type copper hydride clusters with their reactivity and catalytic activity.

Numerous coordination compounds have recently been identified exhibit the technologically important property of bulk magnetic ordering as ferro-, ferri- and antiferromagnets. These compounds range from the ionic [Fe(C₅Me₅)₂]⁺[TCNE]^.- (TCNE = tetracyanoethylene) ferromagnet to 3-D extended coordination network ferrimagnets. Additionally, many magnetically ordered percyanometallates are summarized. This Award presentation covers new magnetically ordered coordination compounds with an emphasis on new coordination chemistry that were discovered and studied in our laboratory.

The studies on dynamic changes in metal-organic frameworks (MOFs) induced by various external stimuli have attracted great attention in various fields and have been continuously investigated since the appearance of MOFs, porous crystalline materials. Many interesting phenomena occurred in frameworks, for example, breathing, swelling, linker rotation and subnetwork displacement, as well as solid-state transformation accompanying with bond formation/deformation are observed and investigated through advanced techniques. In this account, we will summarize our efforts to expand the related research field, focusing on the dynamic changes induced by specific stimuli, including guest molecules (gas, solvent, and other organic molecules) and UV irradiation, as well as an elucidation for interesting applications achieved the responsive MOFs.

Homo- and heteropolynuclear transition-metal complexes are expected to show specific interactions and cooperative effects between metal atoms, providing novel functions that are unobtainable from mononuclear complexes. For example, Pt(III) and Pd(III), unusual oxidation states of Pt and Pd, are stabilized by forming polynuclear complexes through Pt−Pt and Pd−Pd bond formation. Pyrazole and its derivatives are interesting ligands which often act as bridging ligands, and a variety of coordination modes are known for them. We have developed various types of Pt-based heteropolynuclear complexes with pyrazolate bridges, whose emission energies can be controlled by the change of incorporated group 11 metal ions. Luminescence is one of the best indices to investigate the metal-metal interactions in heteropolynuclear complexes, because it is sensitive to the change of energies of ground and excited states. The pyrazolate bridged Pt2Ag2 complexes having N^C and C^C chelate ligands show isomerization between U- and Z-shaped geometrical isomers in solution. These complexes can be converted to the corresponding Pt₂Ag₃ complexes, which are stabilized by Pt→Ag dative bond. The Pt₂Ag₃ complex having Ph-NHC chelating ligands, identified to be “Chiral-at-cluster”, was optically resolved by the formation of a diastereomeric salt with a chiral anion. The enantiomers show circular dichroism (CD) and circularly polarized luminescence (CPL) properties.

This award account focuses on metallo-supramolecular polymers (MSPs), which are composed of metal ions and multitopic ligands, as a new branch of coordination polymers. Their high solubility in polar solvents, such as methanol or water and their amorphous nature in the solid state, enabled the formation of thin layers of MSPs with a uniform thickness on an electrode surface by spin- or spray-coating the polymer solution, resulting in reversible electrochromic (EC) properties triggered by the electrochemical redox reactions of the metal. The EC properties were characterized in terms of contrast, switching time, charge consumption, and coloration efficiency (η). Dimensionally controlled (one-, two-, and three-dimensional) MSPs were designed, synthesized, and evaluated for their EC properties, finding that the three-dimensional hyperbranched structures enhanced the η value up to 689 cm²/C. The introduction of multimetal species in MSPs led to multicolor EC function owing to the different redox potentials of the metals. An MSP with Fe(II), Ru(II), and Os(II) exhibited quad-color EC behavior. The insolubility of MSPs in weak polar solvents, such as acetonitrile or propylene carbonate, enabled the fabrication of EC devices (ECDs) by combining MSPs with electrolytes dissolved in such solvents. The selection of the appropriate counter material in ECDs successfully decreased the operation voltage down to 0.8 V. Furthermore, the fabrication of ECDs driven at 0.6 V was achieved by modifying the ligands in MSPs.

Hemoprotein containing heme, an iron porphyrin complex, as a cofactor is a promising scaffold toward artificial metalloenzymes and functional materials due to the unique characteristics derived from the synergetic combination of the metal cofactor and protein matrix. The obtained catalysts and materials have been expected to contribute the environmental-friendly and/or biocompatible systems in the future. In this context, our group has demonstrated artificial metalloenzymes constructed by insertion of an artificial metal cofactor into the heme-binding site of a simple hemoprotein. The artificial metalloenzymes catalyze C–H bond hydroxylation and olefin cyclopropanation and replicate the reaction of cobalamin- or F430-dependent native enzymes. The catalytic mechanisms including the intermediate detection were investigated by the spectroscopic techniques. Furthermore, hemoprotein is a useful protein to extend to the functional materials by artificial assembling forms. Especially, light harvesting systems have been constructed by hemoprotein assembly containing photosensitizers. The arrayed photosensitizers in the protein assembly are found to show energy migration, successive energy transfer among the same-type of chromophores, to collect the excited energy. A series of findings in these engineered hemoprotein systems will contribute to the materials transformation as well as artificial photosynthesis.

Recently, metal–organic frameworks (MOFs) or coordination polymers (CPs) have attracted much attention because of their potential applications such as gas storage, separation, and delivery, regarding their designable porous structures and high materials variety. We have focused on development of energy-related functions of MOFs, such as ionic conduction and heterogeneous catalysis. In this paper, our recent achievements in three different research regions on proton conduction, hydroxide ion conduction, and heterogeneous catalysis with MOFs have been summarized. First, we succeeded in visualization of the change in crystalline hydrogen-bonding networks in a super proton-conductive MOF and clarification of relationship between the protonconducting pathways and proton conductivity in the MOF. Second, we succeeded in demonstrating design and synthesis of a hydroxide ion-conductive MOF. Third, we succeeded in clarifying support effects of MOFs in heterogeneous catalysis. Although the support effect of MOFs regarding molecular sieving has been already reported by many researches, we newly demonstrated that MOFs could show the support effects derived from charge transfer and substrate adsorption, and that they could overcome traditional oxide-based supports in catalytic alcohol synthesis reaction.

Since 2002, the structural science via Maximum Entropy Method based on synchrotron radiation(SR) diffraction data has driven an innovation of structural chemistry in Coordination Chemistry at SPring-8. The first successful contribution of the MEM Structural Science was a discovery of the peculiar structure of absorbed gas molecules in the nanopore of coordination polymer. The revealed one-dimensional array structure of absorbed gas molecules with no charge transfer made chemists recognize the power of Synchrotron Radiation as a probe of electrons in materials. The development of the MEM/Rietveld Method is described in terms of absorbed molecular site prediction as well as finding charge transfer among host-guest molecules. The expected role of the Next Generation Synchrotron Radiation facility under construction at Tohoku University Campus is also noted.

Polyoxometalates (POMs) are discrete metal–oxo cluster anions comprising high-valence transition metals (Mo, W, V, Nb, etc.) and have been researched in broad areas of sciences. POMs have attracted considerable attention as an important class of protonconductive materials since POMs can efficiently transport protons (H⁺) due to their small effective negative surface charge density. However, the acid salts of POMs suffer from low structural stability and practical applications are limited. A promising approach to overcome the drawback is hybridizing POMs with appropriate counter cations and/or polymers to achieve high structural stability and proton conductivity.