With structural correlations reminiscent of their parent crystals, molten phases of metal–organic frameworks present the intriguing possibility of using coordination chemistry and reticular synthesis to access network-forming liquids and glasses with unique and tunable structure and properties. To promote liquid-state thermal stability and preserve structural correlations, we develop general thermodynamics strategies that maximize the enthalpy and minimize the entropy of fusion for reversible, low-temperature melting. Guided by this design principle, we discover several new series of two- and three-dimensional metal–bis(acetamide) frameworks with reversible melting transitions at record low temperatures, where weak metal–amide coordination bond and electrostatic interactions between spatially separated cations and anions reduce enthalpy of fusion, and conformationally flexible polymethylene group increases entropy of fusion. Interestingly, melting temperatures within each series are strongly depedent on the framework metal. This is found to be the result of coordination and hydrogen bond strength within each compound dictating enthalpic trend, and crystalline-state residual motion and positional disorder influencing entropic trend via a systematic study combining crystallography, spectroscopy, and calorimetry. Network-forming nature of molten Co(bba)3[CoCl4] [bba = N, N’-1,4-butylenebisacetamide], the lowest melting three-dimensional metal–organic network, is supported by extended X-ray fine structure (EXAFS) indicating metal–ligand coordination in the liquid and pair distribution function (PDF) showing intermediate- and long-range ordering reminiscent of the parent crystal. Glassy phases of multiple metal–bis(acetamide) frameworks are isolated upon quenching from their liquid states and viscosity measurements provide preliminary explanation for variations in glass-forming abilities. These results provide new insights into structural and chemical factors governing the thermophysical properties of melt–organic frameworks, informing the design of future functional disordered materials with tunable structural ordering on various length scales.

Network-forming liquids and glasses from metal–bis(acetamide) frameworks with low melting temperatures

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.