Symmetry breaking in self-assembled M4L6 cage complexes

Here we describe the phenomenon of symmetry breaking within a series of M₄L₆ 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 S₄-symmetric, achiral configuration. Reduction of symmetry from S₄ to C₁ is achieved by the spatial offset between each ligand’s pair of binding sites, which breaks the S₄ 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 S₄ symmetry owing to rapid interconversion between energetically degenerate, enantiomeric C₁-symmetric conformations.