Structural phase transition and dielectric responses in two novel cyano-bridged coordination polymers synthesized by sealing the incomplete cyano-bridged cage

Abstract

Three-dimensional anion frameworks loaded with small cationic molecules are often reasonable models for the design of switching materials. However, due to limitations of the bond length and structural stability, it is difficult for the framework to carry larger molecules, so it is still a challenge to construct frameworks with larger capacities. In this article, two coordination polymers (C₇N₂H₁₈)[KFe(CN)₆]·H₂O (1) and (C₇N₂H₁₈)[KCo(CN)₆]·H₂O (2) with three-dimensional anion frameworks were synthesized and characterized. The cyano-group bridged with metals formed this three-dimensional anion framework and coordinated water molecules were introduced in the lattice, resulting in the formation of this novel three-dimensional skeleton with a larger capacity. In this novel framework, water molecules can easily escape from the lattice, and form two new coordination polymers (C₇N₂H₁₈)[KFe(CN)₆] (3) and (C₇N₂H₁₈)[KCo(CN)₆] (4) with a more regular structure. It is notable that 3 and 4 experienced reversible phase transition with a dielectric response (phase change temperatures, T_c = 153 K and 159 K for 3 and 4, respectively). In addition, the crystal structures, dielectric properties, thermal properties and optical properties of 3 and 4 are discussed in detail. This work will provide guidance for designing novel phase transition materials with three-dimensional cyano-bridge structures.