Reversible and Massive Structural Transformation in Meltable Cyanido-bridged Coordination Polymer Crystals

Cyanido (CN⁻)-bridged coordination polymers (CP) have been extensively studied as molecular-based functional materials. However, synthesizing 3D compounds composed only of metal ions and CN⁻—without bulky organic groups—and that melt before decomposing remains a considerable challenge. This difficulty arises because CN⁻ strongly interconnect metal ions, forming rigid, dense frameworks with high melting points. In this study, we successfully synthesized a melting composite consisting of 3D KCd[Cu(CN)₂]₃ and 2D K₂Cu₃(CN)₅ by dehydrating K₂Cd(H₂O)Cu₄(CN)₈·1.5H₂O. Remarkably, nanodomains of these two compounds coexisted within single particles, allowing their crystal structures to be independently determined by 3D electron diffraction (MicroED) of the resulting powders. Each compound melted at its respective melting point, around 559 K. Notably, the melting point of KCd[Cu(CN)₂]₃ is unusually low for a 3D dense coordination framework. This atypically low melting point results from a combination of crystalline surface effects, and the entropy contribution of the dynamic, labile two-coordinate Cu centers in the framework. Additionally, we demonstrated a reversible transformation between the dehydrated mixture and the hydrated parent compound through exposure to water vapor, highlighting the dynamic and responsive nature of these CN⁻-based solid-state materials.