Ionothermal Route to Layered Two-Dimensional Polymer-Frameworks Based on Heptazine Linkers

The synthesis of three molecular building blocks based on 1,4-aryl, 4,40-biphenyl and 1,3-aryl each containing a triazine and a carbonitrile functionality and the thermally induced autocondensation of these into three respective heptazine-based, ordered, 2D, layered polymer frameworks (HBFs) in an eutectic saltmelt of lithium and potassium was reported. 1,4-Dicyanobenzene and dicyandiamide were heated to 105°C in 2-propanol. KOH was dissolved in 2-propanol and added dropwise to the reaction mixture which was then refluxed at 105°C for 1 h. The solution was poured into cold water and the white precipitate collected and washed with cold water until neutral. A quartz ampule of the dimensions was charged with 4-(4,6-diamino-1,3,5-triazin-2-yl) benzonitrile and a mixture of lithium bromide and potassium bromide under inert atmosphere. After cooling, the brown-black block of salt and product was stirred for overnight in distilled water to dissolve the salt, and was subsequently washed with water, THF and acetone to remove residual salt and poorly condensed residues. Scanning electron microscopy and transmission electron microscopy reveal indicators of the atomic, molecular and supramolecular order of the frameworks.