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Pioneering and Developing Studies on Chemistry od Coordination Space
The hallmark of the work accomplished by Prof. Susumu Kitagawa is the pioneering and developing studies of new porous materials. Prof. Kitagawa noticed in the early 1990s that space surrounded and partitioned by atoms and molecules could constitute another world of science in addition to the framework entity, and defined coordination space as the space where the coordination bond plays an important role in the formation of the spatial structures and where various physical properties are exhibited. He developed the field of gChemistry of coordination spaceh by rational synthesis of porous crystalline compounds
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Prof. Susumu Kitagawa
Kyoto University
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named porous coordination polymers (PCPs) or metal-organic frameworks (MOFs). Because of the chemistry of coordination space, porous coordination polymers have advanced extensively, affording various functional architectures, which are constructed from a variety of molecular building blocks with different interactions between them. Some of his outstanding achievements are listed below.
1. Prof. Kitagawa developed new synthetic routes to build the desired nanosized space effectively and on a large scale by using self-assembly processes, where coordination bonds are the key to the development of the required new synthetic technologies. He has succeeded in achieving pore surface control of porous coordination polymers, by which he discovered exciting phenomena based on molecular coagulation, molecular stress, and activation of molecules.
2. Early on in this field, he noticed the importance of the chemical and physical properties of these architectures and their application through the porous properties of the frameworks, and achieved new chemistry and physics of and in the micropores of porous coordination polymers. He is the first in the world to have synthesized a novel PCP relevant to gas storage of supercritical gases such as methane at ambient temperature, and developed its functions not only for gas storage but also for separation and catalysis with higher capacity than conventional materials. Low molecular weight molecules, such as O2, N2, CO2, CH4, and alkanes (C2-C3) are important gases for human life, since they are associated not only with energy sources, but also with global environment issues.
3. A one-dimensional (1D) regular assembly of dioxygen molecules, that cannot be realized under normal conditions, has long been one of the most fascinating targets in chemistry and physics because of the keen interest in its magnetic and photo-physical properties that are characteristics of low dimensionality. One approach to the formation of a 1D-specific assembly of dioxygen molecules is to use a uniform nanosized channel in a microporous compound. Prof. Kitagawa has made the first observation of a detached oxygen molecule in a solid, a one-dimensional ladder structure aligned to the host channel, which provided a better understanding of the adsorption phenomena in a nano-channel and led to novel nano-technology. Molecular arrays for other gases, namely, N2, H2, Ar, and CH4, have also been determined, indicating that guest molecules are confined to forming crystalline-like regular ordered arrays, in a linear fashion in contrast to the situation in the gas and liquid states, even at temperatures above the boiling point.
4. Low molecular weight molecules, such as carbon dioxide (CO2), methane(CH4), acetylene (C2H2), and alkanes (C2 - C3) are gases important for human life, since they are associated with the global issues of energy and environment. Acetylene is one of the key molecules used as a starting material for many chemical and electric materials. In order to obtain highly pure C2H2 for the preparation of these materials, the separation of C2H2 from a mixture gas containing carbon dioxide (CO2) impurities without a large expenditure of energy is an important subject. In addition, acetylene is well known to be a highly reactive molecule and, therefore, it cannot be compressed above 0.2 MPa, otherwise, it explodes without oxygen, even at room temperature. With this background, more feasible and safe materials for C2H2 separation/storage were required. Compared with a very similar molecules, carbon dioxide, Prof. Kitagawa attained extremely high levels of selective sorption of acetylene molecules onto the functionalized surface of porous coordination polymers. This permits stable storage of acetylene at a density 200 times higher than the safe compression limit of free acetylene at room temperature. Thanks to this discovery, we are now at the stage of on-demand synthesis of functional pores by tuning size/shape and chemical properties.
5. In 1998, Prof. Kitagawa classified the porous coordination polymers into the three categories?first, second and third generations?predicting the presence and importance of flexible porous frameworks. Among these, the first-generation materials have frameworks whose porosity collapses irreversibly after guest removal, that is, there is no permanent porosity. The second-generation materials have stable and robust frameworks, which maintain the original porous structures before and after guest sorption. The second-generation compounds can be used as an adsorbent, and are considered to be analogous to zeolites. For the third-generation compounds, he emphasized the flexible or dynamic porous frameworks, which reversibly respond to external stimuli, not only chemical but also physical. He discovered many flexible crystalline porous frameworks with a specific gate-opening pressure for guest gases, in particular super critical gases N2, O2, CH4, CO2 at ambient temperature, which will find applications in gas separation and sensors. He has established a new field of porous materials by introducing the third generation-compounds named soft porous crystals and developing their rational design/synthesis, thereby displaying an in-depth understanding of sorption phenomena.
Moreover, he has opened the door for the application of porous materials to polymerization reactions.
Because of these achievements, many researchers have become involved in this new field and have synthesized various types of PCPs with sorption properties ranging from H2, CH4, and other organic and inorganic compounds. As a world leader in the field, he has published many instructive reviews on PCPs and MOFs, and in particular, his article in Angewandte Chemie, which is one of the most cited reviews in this field as well as his original papers during the past five years. To date, the number of published articles including coordination polymers amounts to more than 2000 per year worldwide. Currently PCPs attract much attention among porous materials, and consequently, the chemistry of coordination polymers has developed markedly. Therefore, Kitagawa blazed a trail to porous materials from gchance findingh to a tailor-made synthesis toward the way for the future of gas storage/separation technology, and he deserves the award of the Chemical Society of Japan.
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