High Nuclearity Uranyl Cages Using Rigid Aryl Phosphonate Ligands

Two high nuclearity uranyl cages were crystallized in high yield under mild solvothermal conditions using rigid aryl phosphonate ligands. These compounds are prepared either through an in situ phosphonate ligand condensation reaction or via serendipitous assembly with a bulky phosphonate ligand. Firstly, the flexibility of the phosphonate ligands enhances the curvature of the uranyl polyhedra in the assembly of both compounds. Secondly, the formation of bent pyrophosphonate ligands through temperature and solvent-driven in situ ligand synthesis is crucial for the assembly of UPhPO3. Thirdly, the serendipitous assembly of UPh2CPO3 is favored by the use of sterically hindered bulky phosphonate ligands. The overall structure of UPhPO3 consists of 16 uranyl cations that are coordinated by phenyl phosphonate and pyrophosphonate ligands while UPh2CPO3 structure is built from uranyl dimers and bulky fluorenyl phosphonate ligand to yield a 12 membered cage.