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Containers and Vessels for Supramolecular Catalysis
Published in Jubaraj Bikash Baruah, Principles and Advances in Supramolecular Catalysis, 2019
Molecular flasks are compounds that mediate or catalyse chemical transformations inside their cavities. From an inorganic complex point of view, many polyhedral coordination cages act as hollow containers for guest encapsulation. These molecules activate, modulate and guide reactivity of substrates for efficient catalytic reactions to form selective products. Since the sizes and shapes of containers are important, there is great interest among synthetic chemists to generate hollow containers from inorganic complexes. In one approach, synthetic chemists modify container molecules, such as in the case of cages in external surfaces, edges or corners, or in the interiors to perform various functional activities and selective guest binding. Alternatively, as per the choices and requirements, ligands are also modified and reacted with metal salts or complexes to form container molecules. Though such syntheses are apparently simple, in actual practice, selective synthesis of container molecules is a challenge. This is because of the necessity of seeking one among several options to form container capable of performing the desired catalytic reaction. The environment in the interiors of the cages, or rather the surroundings of the cavities and their binding selectivity of cage-like complexes, provides an advantage to using them in catalytic reactions. There may be single or multiple types of cavities in cage-like structures, but the beauty is that they are uniformly distributed. Hence, single or multiple types of cavities can differentiate molecules of different sizes and shapes. The process of differentiation enables one to perform size-selective catalysis. Furthermore, certain cavities’ shape and size change by solvation, desolvation or application of external stimuli. The changes in cavity sizes and the intrinsic properties of the complex cause differences in binding abilities to substrates in each state. Due to the difference, the reaction rate of a catalytic reaction can be controlled in addition to obtaining a specific product. For example, crystalline molecular flasks are built with trigonal linker 2,4,6-tris(4-pyridyl)-1,3,5-triazine (tpt), coupled with zinc iodide, which shows dynamic behaviour. The framework structure formed by including nitrobenzene has the composition [(ZnI2)3(tpt)2]·6C6H5NO2. The complex loses nitrobenzene molecules easily on heating, but the parent framework is retained. After losing nitrobenzene, the structure shrinks and the original structure is brought back after reuptake of nitrobenzene. The complex also shows flexibility of structure upon exchange of included solvent molecules without crystal degradation. Metal organic frameworks provide ideal examples of crystalline molecular flasks. The catalytic aspects of MOFs are discussed in Chapter 2. In the next section, container molecules or molecular flasks of polyhedral discrete units are discussed.
Self-assembly as a key player for materials nanoarchitectonics
Published in Science and Technology of Advanced Materials, 2019
Katsuhiko Ariga, Michihiro Nishikawa, Taizo Mori, Jun Takeya, Lok Kumar Shrestha, Jonathan P. Hill
As compared with some molecular interactions such as hydrophobic effects and electrostatic interaction, coordination provides specific and strict molecular pairing. Self-assembly processes based on coordination interactions play unique roles in self-assembly science. For example, Severin and co-workers systematically investigated effects of size and geometry of the ligand on the self-assembled structures of Pt-based coordination self-assembly [150]. In some cases, formation of barrel- or gyrobifastigium-like structures with a molecular weight of more than 23 kDa and a diameter of 4.5 nm can be assembled. Generally speaking, coordination interaction promotes both continuous regular structures, such as metal–organic frameworks and coordination polymers, and isolated objects with strict size and shape including molecular capsules and coordination cages. Especially, the latter characteristic is useful for precise structural regulations and structural shifts/tuning.