Biological functions in metallo-enzymes are induced by the unique first and secondary coordination spheres around the metal ions of active centers. We designed and prepared some structural and functional model complexes with their unique coordination spheres, such as oxygenases, oxidases, tyrosinase, hemocyanine, hemerythrin, methane monooxygenase, nitrile hydratase, photosynthesis, nitrogenase, siderophore, and so on, and tried not only basic elucidations of their structure-function relationships but also industrial development of their functions. Here we describe some examples as follows; (i) from the stable capture of active oxygen species to their activations, (ii) from the selective preparations of bis((μ-oxo)dicopper(III) and μ-η ²:η²-peroxo dicopper(II) complexes to clarification of tyrosinase function, (iii) from hemerythrin functional model (reversible dioxygen binding) to cathode of fuel cell (four-electron reduction of dioxygen), and (iv) from the construction of nitrile hydratase functional model to NO sensor.

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This Account will focus on some recent aspects of the research performed in the author’s laboratory. Selected examples of complexes in which the ligands play an essential role in determining the molecular properties will be examined. Concerning the applications of these complexes, we will limit ourselves to some aspects related to the catalytic oligomerization of ethylene. In the search for such catalysts, we found that deprotonation of N,OH ligands chelated to Ni(II) with NaH can give rise to a range of heterometallic Na-Ni polynuclear complexes. In addition to their catalytic relevance, their magnetic properties were also investigated and have revealed in some cases Single Molecule Magnet behaviour. The diverse functions that co-catalysts may exert will be illustrated by the characterization of Cr(III) complexes obtained by reaction of fac-[Cr(N_pyPN_py)Cl₃] with typical co-catalysts such as MAO, EtAlCl₂, AlMe₃ and AlEt₃. We will also discuss some polynuclear and cluster chemistry where a prediction of the nuclearity and composition of the metal cores remains difficult or impossible. This emphasizes the importance of basic research and the need to recognize the contribution of serendipity. The relevance of cluster chemistry to various fields of molecular synthesis and activation, catalysis and nanosciences, will also be mentioned.

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This review describes a newborn method for the single crystal X-ray analysis of non-crystalline or trace amount of compounds in which single crystal samples are prepared without recrystallization of the target compounds. In this method, porous coordination networks are employed as ‘crystalline sponges’ that can absorb target compounds from a contacting solution and orient them in the pores so that the structures of incorporated guest molecules can be analyzed by X-ray diffraction. Since the target compounds are delivered as a solution, they are needed to be neither crystal nor solid. Furthermore, the single crystal sample can be reproducibly prepared using only a 100 μm-sized host crystal, which allows nano-to-microgram scale X-ray analysis of non-crystalline compounds. With this technique, the crystal structure of guaiazulene, a natural hydrocarbon, has been determined with only 50 ng an amorphous sample. Combination with analytical HPLC system resulted in the development of a prototypical LC-SCD (liquid chromatography-single crystal diffraction) analysis. Here the principle, scope, and limitation of this ‘crystalline sponge method’ are shown with examples of the X-ray analysis including natural and synthetic compounds.

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Crystalline coordination frameworks often have intrinsic molecular dynamics such as rotation, libration in wide temperature range. The cooperative motions in the frameworks are controllable by tuning of structural dimensionality, density, strength of coordination bond, etc. We focus on the motions of local structures in coordination frameworks, and apply them to construct solid state ion conductor and CO₂ separation material. In case of the synthesis of ion conductors, pinning of proton carriers such as imidazole and phosphate in coordination frameworks provides regular alignment for H⁺ hopping and it results high H⁺ conductivity under anhydrous condition. In case of CO₂ separation, the CO₂ molecule-responsive structural rearrangement of coordination framework is feasible by use of the local dynamics of coordination bonds. It enables specific molecular recognition of CO₂ and provides high CO₂ separation property over other gases (CH₄, C₂H₄, C₂H₆) without strong interaction. The gentle and selective capture of CO₂ via structural transformation does not require large energy consumption for CO₂ regeneration. In this review, we discuss the design and utilization of molecular motion in coordination framework solids for the energy-related functions.

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Recent research trends on the photophysical properties of transition metal complexes under rigid environments are overviewed. Since the transition metal complexes are often introduced into rigid environments in photochemical applications/devices such as organic light-emitting diodes (OLEDs) and dye-sensitized solar cells (DSSCs), the rigid medium effects on the excited-state characteristics of the complexes are still worth studying. Crystalline solids, frozen glasses and polymer matrices are picked up as examples of rigid environments. Under these conditions, the metal complexes show characteristic photophysical properties which differ from those in fluid solution owing to the rigidity of the surrounding environments.

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Molecular oxygen is a highly atom-economical, environmentally benign, and abundant oxidant. However, the large activation energies in the reactions of oxygen and formation of undesirable intermediates such as superoxide, hydroperoxide, and hydroxyl radical have prevented utilization of molecular oxygen as oxidant. Among recently reported systems that employ dioxygen as the terminal oxidant, noteworthy is alcohol oxidation method reported by Stahl and co-workers. The system consisting of copper complex, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl), and base is suited for widespread use in synthetic chemistry and practically applicable level. Muldoon and co-workers developed one-pot synthesis of cyanide or N-heterocycles from alcohols based on Stalh’s reports. In parallel with reports from synthetic chemistry field, Hayton group and other groups reported isolation of TEMPO transition-metal complexes which can react with organic substrates. Those works suggested that the reactivity of TEMPO toward alcohols enhanced by the complexation with metal ion, where the nitrogen atom of TEMPO works as an acceptor of proton rather than oxygen atom in C-H bond cleavage of substrates.

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