Confinement of Cd(II) by 2,2′ -Bipyridyl: Control of Structural Transformations and Porosity in Perfluorinated Biphenyldicarboxylate MOFs

Abstract

Cadmium(II) can form complexes with coordination numbers 6 and 7, which makes them labile, especially with relatively weak ligands like perfluorinated carboxylates. Confining the coordination environment with chelating ligands such as 2,2'-bipyridyl (2,2'-bpy) is a promising approach to obtain complexes with more predictable geometry and stability. Using this strategy, MOFs with anions of octafluorobiphenyldicarboxylic acid (H₂oFbpdc) were successfully synthesized in different alcohols. Compound [Cd(2,2'-bpy)(oFbpdc)] · CH₃OH (1 * MeOH) exhibits a 3D framework, whereas the other phases [Cd(2,2'-bpy)(oFbpdc)] · G} (2 · G), G = EtOH, iPrOH, tBuOH) adopt a layered structure. Thermal activation of all phases results in guest removal, yielding the non-porous phase [Cd(2,2'-bpy)(oFbpdc)] (2). Despite its non-porous nature, 2 undergoes a structural transformation upon CO₂ adsorption into an open form with a specific surface area of 237 m²·g⁻¹. This transformed phase shows good adsorption selectivity for binary C₂H₂/CH₄ and CO₂/CH₄ mixtures.