Network-Forming Liquids from Metal–Bis(acetamide) Frameworks with Low Melting Temperatures

Abstract

Molten phases of metal−organic networks offer exciting opportunities for using coordination chemistry principles to access liquids and glasses with unique and tunable structures and properties. Here, we discuss general thermodynamic strategies to provide an increased enthalpic and entropic driving force for reversible, low-temperature melting transitions in extended coordination solids and illustrate this approach through a systematic study of a series of bis(acetamide)-based networks with record-low melting temperatures. The low melting temperatures of these compounds are the result of weak coordination bonds, conformationally flexible bridging ligands, and weak electrostatic interactions between spatially separated cations and anions, which collectively reduce the enthalpy and increase the entropy of fusion. Through a combination of crystallography, spectroscopy, and calorimetry, enthalpic trends are found to be dictated by the strength of coordination bonds and hydrogen bonds within each compound, while entropic trends are strongly influenced by the degree to which residual motion and positional disorder are restricted in the crystalline state. Extended X-ray absorption fine structure (EXAFS) and pair distribution function (PDF) analysis of Co(bba) 3 [CoCl 4 ] [bba = N,N′-1,4-butylenebis(acetamide)], which features a record-low melting temperature for a three-dimensional metal−organic network of 124 °C, provide direct evidence of metal−ligand coordination in the liquid phase, as well as intermediate-and extended-range order that support its network-forming nature. In addition, rheological measurements are used to rationalize differences in glass-forming ability and relaxation dynamics. These results provide new insights into the structural and chemical factors that influence the thermodynamics of melting transitions of extended coordination solids, as well as the structure and properties of coordination network-forming liquids.

Exploration of glassy state in Prussian blue analogues

“…S27). The presence of extendedrange structural correlations mimics density fluctuation caused by topological and chemical ordering, which is also found in structurally disordered glassy AX2 inorganic systems, such as ZnCl2 and GeSe2 67-69 , and network-forming liquid/glassy MOFs/CPs 20,25 . Lack of sufficient void space and lower degree of freedom in Fe[Fe]3/4-g also affects the recrystallization process.…”

“…In higher r-space (Fig. S26), detailed examination reveals quasiperiodic oscillations 25 that continue to at least 35 Å even after 144 h of mechanical milling, much larger than that observed in Cu[Fe]2/3-g with 2 h of identical treatment (Fig. S27).…”

“…Thus far, only a few CPs and MOFs have shown the capability to undergo the glass transformation process. Most examples are melt-quenched glass, where the crystal-liquid-glass transition is mainly governed by the counterbalance between entropic/enthalpic driving forces and the kinetics of crystallization 24,25 . In contrast, a direct crystal-glass transformation of the mechanicalinduced glass has been recognized as another path to prepare the glass and is initiated by plastic deformation under mechanical stress 26,27 .…”

Principles of Melting in Hybrid Organic-Inorganic Perovskite and Polymorphic ABX3 Structures

“…This is consistent with the rational adopted by Mason et al in their network-forming bis(acetamides). 31,32 Any trends in ΔHf and alkyl chain length are less clear, though are not perhaps expected given the proposal that melting does not directly involve bond breaking within the alkylammonium cation itself, but rather the M-N coordination bonds in the surrounding framework (Fig. 3).…”

Shining Light on Porous Liquids: From Fundamentals to Syntheses, Applications and Future Challenges

“…In addition, the accessible abundance of known sterically hindered solvents [45][46][47][48] (e.g., ionic liquids and oligomers), together with their endless combinations flourish the accelerative development. As portrayed in Figure 5, a couple of novel PLs based on APMs were reported in rapid succession especially during the past 5 years, such as zeolite-based PLs, [49] mesoporous MOF MIL-101(Cr) based PLs, [32] MOP-based PLs, [50] and the more recent COFs-based PLs.…”