Dr. Mitsuhiko Shionoya has established the principle of construction of various types of supramolecular metal complexes based on the self-assembly of precisely designed molecules and metal ions chosen purposefully, which has been directed toward creative science of array, space, and motion. He has made remarkable achievements in internationally leading the fields of supramolecular chemistry, coordination chemistry, and bioinorganic chemistry. His recent major achievements are summarized as follows.
1. Metal array methods using one atom to biopolymer as a template
It is a fundamental issue in chemistry to establish a method to precisely arrange chemical species for creating functional molecules and materials. Metal array requires a template ligand that determines the type, number, and sequence of metal ions. Dr. Shionoya has found that a single atom, a synthetic cage molecule, or a synthetic biopolymer can be used as a template. For example, the metal array method using artificial metallo-DNAs he first developed was extended to array of lanthanide ions with a newly designed ligand-type nucleobases, thus leading to remarkable increase in the number of combinations of metal arrays. Moreover, the reversible bonding nature of the metal-mediated base paring has been applied to DNA strand exchange reactions with significant structural changes. The most notable biological applications is the finding that natural enzymes enable elongation of DNA including artificial base pairs to obtain DNAzymes that have a catalytic activity only when a metal-mediated base pair is formed. This switching function was successfully applied to a logic gate system. He has also reported the synthesis of metal clusters using synthetic molecules such as disc-shaped polydentate ligands, metal-assembled complexes with a cage shape, and the synthesis of gold clusters centered on a carbon atom. The construction principle of these metal sequence motifs is of great significance because it has the potential to create a unique electronic structure and reactivity specific to the type, number, and/or sequence of metal ions, ligand characteristics, and chemical environments.
2. Nanospace construction and elucidation of space-specific molecular behaviors
In a biological system, a given molecule is included in a confined space such as an enzyme pocket, and its relative position, orientation, and reactivity are highly controlled so that the ultimate molecular functions can be expressed. Inspired by the principle of supramolecular spatial control in biological systems, Dr. Shionoya has developed functionalized supramolecular nano-sized spaces that enable highly efficient and selective molecular recognition and space-specific reactions. Specifically, by expanding the concept of supramolecular space, he has focused on the design and synthesis of supramolecular spaces with different sizes, shapes, and symmetries, control of molecular arrangement, in-situ observation of molecular arrangement processes, and space-specific reactions. He has developed porous supramolecular crystals (Metal-Macrocycle Frameworks) that has both molecular recognition abilities and reaction fields to realize molecular arrangement and dynamic functions in the supramolecular nanospaces. MMF was constructed by self-assembly of four types of structural and stereoisomers of a cyclic, trinuclear PdII complex, and has a nanochannel in which five enantiomerically paired molecular binding pockets arranged in a unit structure. Soaking experiments revealed that many types of organic molecules, metal complexes, catalysts, and biomolecules can be included in the channel of MMF and site-selectively arranged on the inner surface, which provided a spatial map useful for prediction of molecular adsorption positions. As many of these inclusion complexes can be analyzed by single-crystal X-ray diffraction, this method enabled the first structure determination of a biomolecule and multiple different molecules arranged on the inner surface, and the world's first in-situ observation of a molecular adsorption/arrangement process. This observation method is also extremely useful for in-situ observation of the reaction process and identification of reaction products. The nanochannel space can also be used for substrate-size-specific acid-catalyzed reactions, photocatalytic olefin migration reactions, and gold nanoparticle synthesis. Thus, MMF provides a supramolecular space that enables tailored molecular arrangement, in-situ observation of molecular behaviors, and space-specific reactions. Dr. Shionoya also made a significant contribution to the development of self-assembled molecular containers capable of molecular inclusion and structural switching. Typical examples are molecular inclusion by self-assembled capsule-shaped complexes consisting of disk ligands with three monodentate ligands and metal ions, reversible structural conversion between cage structures, cyclic host complexes capable of molecular inclusion by metal-π interactions, and molecular inclusion and structural transformation of a low-symmetric cage complex composed of zinc porphyrin-type ligands and metal ions.
3. Supramolecular metal complexes with a dynamic feature
The nature of coordination bonding such as coordination structure diversity, dynamic ligand exchange, metal centered reactivity allows for the dynamic behaviors of molecules that are completely different from organic molecules. Dr. Shionoya has developed different types of multi-gear molecules consisting of two, four, or six triptycene gears, and successfully analyzed the interlocking movement of the gears. For example, in a platinum complex with two triptycene rotators as phosphine ligands, the two rotators are in mesh with each other triptycene in the cis form, but become disengaged due to the photo-induced cis to trans isomerization of the platinum complex. This switching behavior is reversible via the back reaction on heating. These results provide new structural motifs and operating principles for molecular motions utilizing the inherent properties of metal complexes, and thus are highly evaluated in Japan and overseas as contributing to the development of molecular machines.
As highlighted above, Dr. Shionoya has provided new insights into supramolecular metal complexes oriented to the array, space, and dynamic functions that form the basis of chemistry. Therefore, his contribution is highly valued in the advancement of supramolecular chemistry and related scientific fields, and he certainly merits the Chemical Society of Japan Award.