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

## TL;DR

Scientists created a new type of coordination polymer crystal that melts at a surprisingly low temperature and can transform back to its original form when exposed to water vapor.

## Contribution

A novel 3D coordination polymer with an unusually low melting point was synthesized and shown to reversibly transform upon hydration.

## Key findings

- A melting composite of KCd[Cu(CN)2]3 and K2Cu3(CN)5 was synthesized via dehydration.
- The composite melts at 559 K due to surface effects and low-coordinate Cu centers.
- The material reversibly transforms between dehydrated and hydrated forms via water vapor exposure.

## Abstract

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.

Melting crystals containing separated domains of KCd[Cu(CN)2]3 and K2Cu3(CN)5 were obtained via simple dehydration of K2Cd(H2O)Cu4(CN)8·1.5H2O. Their low melting point of 559 K resulted from the combined effects of strong crystal‐interface interactions and low‐coordinate Cu nodes.