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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Coordination Chemistry

Background:

• Cyanido-bridged coordination polymers (CPs) are functional materials, but synthesizing dense, 3D metal-cyanide frameworks that melt before decomposition is challenging.
• High melting points of CPs are attributed to strong metal-cyanide interactions forming rigid structures.

Purpose of the Study:

• To synthesize a novel melting composite of 3D and 2D cyanido-bridged coordination polymers without bulky organic groups.
• To investigate the structural, thermal, and responsive properties of the synthesized composite.

Main Methods:

• Dehydration of a hydrated precursor K2Cd(H2O)Cu4(CN)8·1.5H2O to form a composite of 3D KCd[Cu(CN)2]3 and 2D K2Cu3(CN)5.
• 3D electron diffraction (MicroED) for crystal structure determination of coexisting nanodomains.
• Differential scanning calorimetry (DSC) to determine melting points.

Main Results:

• Successfully synthesized a composite of 3D KCd[Cu(CN)2]3 and 2D K2Cu3(CN)5.
• Nanodomains of both compounds were observed within single particles, with structures determined by MicroED.
• Both compounds exhibited melting points around 559 K, with KCd[Cu(CN)2]3 showing an unusually low melting point for a dense 3D framework.
• Reversible transformation between the dehydrated composite and the hydrated precursor was demonstrated upon exposure to water vapor.

Conclusions:

• The synthesized composite offers a rare example of a melting 3D/2D cyanido-bridged coordination polymer.
• The low melting point of the 3D framework is attributed to surface effects and dynamic Cu centers.
• The material demonstrates dynamic and responsive behavior, highlighting potential for novel solid-state applications.