Efficient long-range stereochemical communication and cooperative effects in self-assembled Fe ₄L ₆ cages

A series of large, optically active Fe ₄L ₆ cages was prepared from linear 5,5′-bis(2-formylpyridines) incorporating varying numbers (n = 0-3) of oligo-p-xylene spacers, chiral amines, and Fe ^II. When a cage was constructed from the ligand bridged by one p-xylene spacer (n = 1) and a bulky chiral amine, both a homochiral Fe ₂L ₃ helicate and Fe ₄L ₆ cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe ₄L ₆ cage was observed. In the case of larger cages (n = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands.