Dioxygen binding and activation by non-heme type transition metal complexes are of current interest due to its importance in biological system and industrial processes. Since active-oxygen species generated in catalytic cycles are reactive and unstable, it is difficult to identify those active-oxygen species and elucidate reaction mechanisms. To overcome such difficulties, it is essential to synthesize model complexes which can stabilize active-oxygen species by constructing proper coordination environments around metal centers using the stereoelectronic effect of the supporting ligand. Those models allow us to observe a variety of active-oxygen species and to elucidate their reactivity and oxidation reaction mechanisms. In this account, we report syntheses of non-heme type transition metal complexes having a variety of observable active-oxygen species, which are capable of performing the following functionalities: 1) reversible dioxygen binding by diiron(II), cobalt(II), and copper(I) complexes, 2) reversible breaking and making the O-O bond by dicopper and mononuclear iron complexes, 3) a variety of oxidation reactions mediated by high-valent bis(μ-oxo)dicopper(III) and dinickel(III) complexes, cis-(μ-1,2-peroxo)diiron(III) and (μ-η²:η²-peroxo)Cu(II)₂ complexes as functional models for the dioxygen activating dimetalloenzymes such as methane monooxygenase, toluene monooxygenase, etc.

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Better understanding of materials should be achieved by complementarily employing experimental techniques which lead to either microscopic or macroscopic aspect. The specific characteristic of thermodynamics is the absence of any selection rule or selectivity. This makes a sharp contrast with various spectroscopies in which particular nuclide and/or particular modes are selectively sensed. This review demonstrates calorimetric investigations of spin-spin interaction in transition-metal complexes and the phase transitions occurring in molecule-based materials in which electrons are directly involved. Important roles played by molecular thermodynamics are illustrated for the following subjects: (i) Spin-spin interactions in tri-nuclear metal-complex, (ii) ferromagnetic phase transition in molecule-based magnets, (iii) determination of structure in a low-dimensional assembled-metal complex, (iv) change of electronic state due to molecular motion in an organometallic compound, (v) phase transitions due to spin-crossover phenomena, (vi) phase transition due to intramolecular electron transfers in mixed-valence complexes, and (vii) phase transition due to thermochromic phenomena. Since a change in the electronic state is strongly coupled with a change in the lattice, the transition mechanisms always include interplays with various molecular motions in the lattice.

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Host materials based on metal complexes are of great interest because of their high structural and functional diversities. To obtain unique and applicative metal complex hosts, we attempted new combination of building blocks, a divalent Cu(II) ion and a PF₆^- anion. A PF₆^- anion has been used as a noncoordinated anion in coordination chemistry due to its very weak Lewis basicity. However, a Cu(II) ion makes it possible to catch a variety of inorganic anions with weak Lewis-base properties at its axial sites using electrostatic interaction, resulting in the formation of Cu(II) complexes with weakly coordinated inorganic anions showing unprecedented host properties. In this accounts, we report (1) syntheses and crystallographic characterization of Cu(II) complex hosts with weakly-coordinated, fluorinated inorganic anions such as PF₆^-, BF₄^-, and CF₃SO₃^- at their axial sites, (2) a design and fabrication of flexible Cu(II) complex hosts using fluorinated inorganic anions, (3) selective adsorption of Lewis base guests utilizing latent Lewis acid properties, and (4) rational syntheses of monoanion-bridged Cu(II) complexes using an anion-mixing method.

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Proton conductors are applied as electrolytes in fuel cells and act as key material in it. We introduced acidic functional groups into the porous coordination polymer (PCP), or metal-organic framework (MOF), and construct proton conductive PCP/MOFs. Novel synthetic method for introducing acidic group in PCP/MOF was invented. Proton conductivity of various PCP/MOF was investigated and some of them showed high proton conductivity up to 8 × 10^-3 S cm^-1 at ambient temperature. We also investigate the dependency of functional groups onto proton conductivity and found the relationship between proton conductivity, acidity of the functional groups and hydrogen bond network formed inside the pore of PCP/MOF. These PCP/MOF materials have high crystallinity and the frameworks and arrangement of guests in the inner pore were clearly determined by X-ray crystallographic method, and the relationship between proton conductivity and hydrogen bond networks were investigated. This study will provide the novel field for investigating highly proton-conductive materials.

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