Ligand-Based Phase Control in Porous Zirconium Coordination Cages

Zirconium-based coordination cages have received considerable recent attention as a result of their structural tunability and amenability to postsynthetic modification. Although these structures have adopted tetrahedral (4-vertex) or cigar (2-vertex) geometries depending on the shape and nuclearity of the ligand used in their assembly, their structures have not previously been shown to be either tunable or dynamic. Here, we examine the speciation of a series of zirconium-based cages where the geometry and nuclearity of the product cage can be tuned via judicious functionalization of dicarboxylate ligands. We further show that many of these materials exist as both cage structures in solution or the solid state. With appropriate solvent exchange and activation conditions, the cigar structures can display Brunauer–Emmett–Teller (BET) surface areas as high as 123 m2/g. As a result of their expanded pore structures, the tetrahedra display significantly increased BET surface areas approaching 700 m2/g. These results show that by appropriate ligand functionalization the structures of zirconium-based cages can be modified to tune their phase and surface area. Finally, the isolation of phase-pure cages allows for the utilization of anion metathesis reactions to tune their solubility.