Unexpected Self-Sorting Self-Assembly Formation of a [4:4] Sulfate: Ligand Cage from a Preorganized Tripodal Urea Ligand

The design and synthesis of tripodal ligands 1-3 based upon the N-methyl-1,3,5-benzenetricarboxamide platform appended with three aryl urea arms is reported. This ligand platform gives rise to highly preorganized structures and is ideally suited for binding SO42- and H2PO4- ions through multiple hydrogen-bonding interactions. The solid-state crystal structures of 1-3 with SO42- show the encapsulation of a single anion within a cage structure, whereas the crystal structure of 1 with H2PO4- showed that two anions are encapsulated. We further demonstrate that ligand 4, based on the same platform but consisting of two bis-urea moieties and a single ammonium moiety, also recognizes SO42- to form a self-assembled capsule with [4:4] SO42-:4 stoichiometry in which the anions are clustered within a cavity formed by the four ligands. This is the first example of a self-sorting self-assembled capsule where four tetrahedrally arranged SO42- ions are embedded within a hydrophobic cavity.