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.