Hybrid Uranyl-Phosphonate Coordination Nanocage

We report herein a general synthetic approach for designing uranyl coordination cages. Compounds 1 and 2 are constructed through a temperature-dependent and solvent-driven self-assembly. In both cases, the synthetic strategy involves in situ phosphonate ligand condensation into a flexible pyrophosphonate ligand. This pyrophosphonate ligand formation is essential for the introduction of curvature into these compounds. In the presence of PF₆ ^- ions that are derived from hydrofluoric acid, a macrocyclic uranyl-phosphonate discrete compound, 1, whose cavity contains PF₆ ^- ions, hydronium ions, and water molecules, is obtained. When Cs⁺ cations are used in the synthesis, a remarkable uranyl coordination nanocage, 2, resulted. The macrocycle (1) is approximately 10.9 × 10.9 Ų in diameter while the nanocage (2) is approximately 15.0 × 11.3 Ų in diameter, as measured from the outer oxygen atoms of the uranyl centers. Both compounds are constructed from a UO₂ ²⁺ moiety, coordinated by an additional four oxygen atoms from the phosphonate group to form pentagonal bipyramidal geometry. All the compounds fluoresce at room temperature, showing characteristic vibronically coupled charge-transfer based emission.