Melt-quenched carboxylate metal–organic framework glasses

Although carboxylate-based frameworks are commonly used architectures in metal-organic frameworks (MOFs), liquid/glass MOFs have thus far mainly been obtained from azole- or weakly coordinating ligand-based frameworks. This is because strong coordination bonds of carboxylate ligands to metals block the thermal vitrification pathways of carboxylate-based MOFs. In this study, we present the example of carboxylate-based melt-quenched MOF glasses comprising Mg²⁺ or Mn²⁺ with an aliphatic carboxylate ligand, adipate. These MOFs have a low melting temperature (T_m) of 284 °C and 238 °C, respectively, compared to zeolitic-imidazolate framework (ZIF) glasses, and superior mechanical properties in terms of hardness and elastic modulus. The low T_m may be attributed to the flexibility and low symmetry of the aliphatic carboxylate ligand, which raises the entropy of fusion (ΔS_fus), and the lack of crystal field stabilization energy on metal ions, reducing enthalpy of fusion (ΔH_fus). This research will serve as a cornerstone for the integration of numerous carboxylate-based MOFs into MOF glasses.