Effect of Ligand Substitution on the Formation of the Meltable Fe-ZIF

Meltable metal–organic frameworks (MOF) are essential for the formation of MOF glasses, which have emerged as a new family of functional materials offering promising potential for applications in gas separation, luminescence, energy storage, and beyond. Herein, the synthesis of iron-based zeolitic imidazolate framework (ZIF) crystals, specifically Fe3(Im)6(HIm)2, where Im is imidazolate, is reported. Upon the substitution of some Im linkers with a secondary ligand, 5,6-dimethylbenzimidazole (dmbIm), it was found that such substitution induces the formation of new phases: one phase exhibits meltability and subsequent glass formation, while another phase [Fe3(Im)1.56(dmbIm)4.44(HIm)2] is non-meltable. Through structural characterizations, the configuration of the tetrahedral [Fe-linkers] units was revealed to be crucial in determining the meltability of Fe-ZIF. The incorporation of a large secondary ligand hinders the occurrence of melting. This work provides an insight into how ligands affect the accessibility of the liquid state of MOFs, showing a practical strategy for designing meltable MOFs.