Symmetry breaking in self-assembled M 4 L 6 cage complexes

Here we describe the phenomenon of symmetry breaking within a series of M 4 L 6 container molecules. These containers were synthesized using planar rigid bis-bidentate ligands based on 2,6-substituted naphthalene, anthracene, or anthraquinone spacers and Fe II ions. The planarity of the ligand spacer favors a stereochemical configuration in which each cage contains two metal centers of opposite handedness to the other two, which would ordinarily result in an S 4 -symmetric, achiral configuration. Reduction of symmetry from S 4 to C 1 is achieved by the spatial offset between each ligand’s pair of binding sites, which breaks the S 4 symmetry axis. Using larger Cd II or Co II ions instead of Fe II resulted, in some cases, in the observation of dynamic motion of the symmetry-breaking ligands in solution. NMR spectra of these dynamic complexes thus reflected apparent S 4 symmetry owing to rapid interconversion between energetically degenerate, enantiomeric C 1 -symmetric conformations.