Crystal Structures, Melting Behavior, and Glass Formation of Ionic Liquid-containing Coordination Polymers

Abstract

Recently, meltable coordination polymers (CPs) have attracted attention because of their unusual melting behavior, glass-forming ability, and processability. Herein, we describe the crystal structures and thermal properties of CPs with the formula {[cation][MX2]}n, prepared by reacting metal salts MX with ionic liquids (ILs) [cation]X, where M is an alkali metal and X is a polycyano anion. These materials were designed to lower the melting points of CPs by incorporating IL components. Their crystal structures consist of two- or three-dimensional anionic frameworks, [MX2]n−, that accommodate the cations. Upon heating, most of the CPs undergo incongruent melting with precipitation of the metal salt MX. The melting points of the CPs decrease with decreasing melting points of the parent salts. Upon further heating above the melting point, the precipitated metal salt gradually dissolves to form a homogeneous liquid. This liquid recrystallizes upon cooling, whereas rapid cooling leads to glass formation. CPs containing more rigid cations require higher cooling rates to achieve vitrification.