The coordination chemistry between inorganic and organic species can be optimally exemplified by metal–organic frameworks (MOFs), whose structures and functionalities can be rationally designed from these highly tunable building blocks. The high porosity, stability, and versatile functionalities of MOFs have attracted wide-spread attention from energy-related research and pollution remediation to biomedical applications. A unique and underexplored subset of these materials are MOFs based on actinide nodes; these MOFs have distinguished themselves as a unique platform for investigating the versatile oxidation states, reactivity, and coordination chemistry of actinides. Herein, we will focus on the rational design and synthesis of actinide-based MOFs under the general guidelines of coordination chemistry for their structural and functional explorations. The dimensionality, topology, and structures of actinide-based MOFs can be controlled by selecting pre-designed building blocks of actinide-based nodes and organic linkers with certain desired coordination geometries and functionalities. These unique actinide-based MOFs have shown promise for applications in nuclear waste mitigation, pollution control, and catalysis.

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　Metal complexes are generally dynamic due to the reversibility of formation/cleavage of the coordination bonds. Therefore, metal complexes could be used as platforms for switchable functional molecules with dynamic structural changes. In order to achieve structural conversions as intended, finely designed organic frameworks as well as metal centers with appropriate lability are necessary. For this purpose, the author has focused on macrocyclic, acyclic, and cage-like oligosalen structures, which can incorporate multiple metal centers at desired positions. In this review, various kinds of dynamic structural conversions, such as helicity inversion of helical complexes and open/close functions of molecular cage structures, which were developed by the author and coworkers, are described.

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　Flexible and soft molecules are at the focus of materials research for the construction of functional devices for polymer, LB film, gel, liquid crystal and a number of biological materials. These flexible and soft metal complexes can easily be produced by adding long alkyl chains to their periphery. The soft metal complexes are very interesting from the point of view not only of functional materials, but also for phase transitions involving a synchronicity between the central metal complex and the long alkyl chains. Spin-crossover (SCO) compounds exhibiting conversion between high-spin (HS) and low-spin (LS) states are phase transition compounds, and the SCO phenomenon is very often found in iron(II), iron(III), and cobalt(II) compounds. In the solid state, The cooperativity is considered if intermolecular interactions are sufficiently strong. The cooperativity induces abrupt spin transitions and hysteresis loops in SCO compounds, which is the reason why the role of intermolecular interactions in spin transitions has been extensively studied. In the present review, we focus our attention on the flexible and soft SCO iron(II), iron(III) and cobalt(II) compounds with long alkyl chains. The flexible and soft SCO molecules can produce soft materials, i.e. LB films, liquid crystals, self-assembled molecules. Here we attempt to illustrate the topics about multi-functional SCO compounds with long alkyl chains.

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　Stable radicals possessing open-shell electronic states exhibit vivid physical properties such as electric conductivity and magnetism attributed to unpaired electrons. Their luminescent properties, however, have been poorly investigated due to the rarity and low chemical stability in their photoexcited states. I have developed highly-photostable luminescent organic radicals as promising candidates for establishing novel photofunctions based on the doublet (or multiplet) state. I have shown that the coordination to metal ions can increase the emission quantum yield and photostability of the radicals. The absence of heavy atom effect has been suggested as a unique character of doublet-based fluorescence. These findings show that coordination chemistry is an efficient strategy to enhance, vary, or elucidate the photofunctions of radicals, and enables to extend the scope of this class of radicals toward molecular photonics.

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　Porous materials, including porous carbons, zeolites, and porous coordination polymers (PCPs), are generally recognized as rigid, hard, and fragile substances. However, focusing on their structure and surface at the molecular level, there are various dynamic phenomena and processes such as deformation and reconstruction of the crystal structure as well as molecular motion such as vibration and rotation occurring in the materials. In this account, recent developments of functional PCPs based on a new design approach making use of such dynamic nature of molecular entities are described. A dynamic molecular functionality incorporated into the PCP nanochannel affords precise control of gas diffusion process in the PCP, thus enabling outstanding gas separation and storage capability. In order to visualize the dynamic behavior of PCP, atomic force microscopy was used to observe native surfaces of a PCP crystal, which enabled real-time imaging of the dynamic response of the PCP with a molecular-level resolution. In addition, an interdisciplinary approach that combines porous material chemistry and soft material chemistry, which gives novel porous soft materials with solution/thermal processable feature, is described. This bottom-up design concept that connects dynamic molecular properties to the material functions offers a promising way to the next-generation porous materials.

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　Heme (iron-porphyrin complex) is present in biological system as an active site of several proteins and enzymes, and is involved in many biological conversion processes of bio-energy, bio-compounds, and bio-information. We have elucidated molecular mechanisms of these processes on a basis of their molecular structures by combinational uses of crystallographic, spectroscopic, kinetic, biochemical, and molecular biological techniques. In addition, we are recently interested in dynamics (transport and sensing) of iron and heme in biological system on the structural basis. In this review, I present recent scientific achievement of our group.

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　Quantum chemical studies on methane C–H bond activation and hydroxylation by methane monooxygenase (iron and copper enzyme species), related metal-oxo species such as FeO+, metal-exchanged zeolites, and metal-oxide surfaces are reviewed. The tetrahedral Td structure of methane should be deformed into a C3v or D2d structure at coordinatively unsaturated metal-oxo species. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ions such as FeO+, NiO+, and CuO+ are analyzed in detail by using density functional theory calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process, the energy barrier of which is significantly decreased by the spin inversion. These mechanistic insights are reasonably extended to Fe, Co, Ni, and Cuexchanged zeolites and IrO2 and β-PtO2 (110) surfaces.

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　Nickel is one of the first-row transition metals used as alternatives in homogeneous catalysts using precious metals for organic transformations. Nickel(0) and (II) complexes are generally involved as intermediates in catalytic cross-coupling reactions. However, a handful of recent studies showed that mechanisms can exist involving other oxidation states of nickel, such as nickel(I) and (III). Different pathways could provide different selectivity in products and/or starting materials. However, it is still not clear how these reactions proceed, so it is not possible to predict the reaction pathways from the structure of nickel complexes and/or reaction conditions. Unfortunately, it is not easy to clarify the true active species, such as “paramagnetic” monovalent and trivalent nickel species, so elucidation of the reaction mechanism needs various spectroscopic and/or theoretical techniques. In this account, the possibility of nickel(I) and/or nickel(III) active species involving in catalysis is discussed, based on our recent results on the experimental and theoretical mechanistic studies on N-heterocyclic carbene(NHC)-ligated nickel system. The characteristics and future perspectives of cross-coupling reactions catalyzed by the NHC-nickel(I) complexes are also described.

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