A Schiff base is formed through the condensation of an amine and an aldehyde. The Schiff base is commonly found in the living system. For example, the amine functional group of a lysine residue of an enzyme intermediate reversibly reacts with a cofactor or substrate. Here, the enzyme cofactor PLP forms a Schiff base with a lysine residue, and is involved in the transaldimidation reaction of a substrate. Through a similar mechanism, a retinal cofactor forms a Schiff base with a lysine residue of rhodopsin.
Further, the Schiff base is one of the representative ligands in coordination chemistry. The nitrogen donor atom has both nucleophilicity and Lewis basicity, and could simultaneously exhibit a 7-acceptor property as well. Representative multidentate ligands based on the Schiff base motif include salen-type ligands. In particular, chiral Schiff bases have been used extensively as ligands for asymmetric catalysis. When the Schiff base is used as the backbone of multidentate ligands to support metal centers of planar coordination geometry, porous network materials can constructed, which have unobstructed channels to facilitate the access of substrates to the metal sites.
A Schiff base involved in π-conjugation also exhibits various photoelectronic properties. Based on this property, many composite materials having Schiff base functional groups are being used as key electronic components of organic solar cells or perovskite solar cells. Further, covalent organic frameworks (COFs) of Schiff bases are emerging as next-generation molecule-based materials that exploit the reversibility of amine-aldehyde condensation reaction.
To take full advantage of the unique structural, chemical, and photoelectronic properties of the Schiff base, studies for preparing a new framework connected as a two-dimensional or three-dimensional network structure are actively in progress. For example, according to a recently reported document (R. Banerjee et al. J. Am. Chem. Soc. 2016, 138, 2823-2828), the aldehyde groups of 1,3,5-triformylphloroglucinol, which is a molecule of D3h symmetry, are condensed with guanidinium having amine groups positioned in three directions to form an ionic covalent organic framework having Schiff base linkages. According to another document (P. Sun et al. Cryst. Growth. Des. 2015, 15, 5360-5367), a ligand prepared using Schiff base condensation is reacted with gold to form a metallogel.
The present inventors designed Schiff base-derived multidirectional ligands to maximize the number of metal-ligand bonds, confirmed that metal-organic hybrid structures with network topology are formed through supramolecular interaction with multinuclear repeating units growing bidirectionally, and completed the present invention.