Confinement of Cd(II) by 2,2'-bipyridyl: control of structural transformations and porosity in perfluorinated biphenyldicarboxylate MOFs.

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.