The multifunctional roles of the ionic liquid [Bmim][BF4] in the creation of cadmium metal–organic frameworks

Abstract

This work addresses the application of ionic liquid [Bmim][BF₄] with multiple functions, including as a solvent, structure-directing agent, fluoride source and catalyst promoter, for the creation of F-center Cd₃F metal–organic frameworks (MOFs) via an in situ ionothermal oxidation and hydrolysis process. We thus report the syntheses, structures and characterizations of three novel three-dimensional MOFs based on the Cd₃F cluster, namely, [Cd₃F(ina)₄(4-pic)₃]·BF₄ (1), [Cd₃F(ina)₃(4,4′-bpy)₂(4-pic)₂]·2BF₄·(4,4′-bpy)·2H₂O (2) and [Cd₃F(ina)₃(4,4′-bpy)₃]·2BF₄·(4,4′-bpy)·2H₂O (3) (ina = isonicotinate, 4,4′-bpy = 4,4′-bipyridine, 4-pic = 4-methylpyridine), in which the ina ligand was produced in situ from ligand precursors, such as 4-pic and 4,4′-bpy. Compounds 1, 2 and 3 present cationic frameworks, in which the BF₄⁻ anions and the guest molecules, including the non-coordinating 4,4′-bpy and the lattice water, fill in the holes and/or channels. A thermal analysis shows that 1–3 are stable up to 200 °C. A study on the luminescence property indicates that 1–3 exhibit strong emission bands in the blue region. The current study opens an avenue for the use of ionic liquids in the formation of crystalline materials.