Guest-Dependent Isomer Convergence of a Permanently Fluxional Coordination Cage

A fluxional bis-monodentate ligand, based on the archetypal shape-shifting molecule bullvalene, self-assembles with M²⁺ (M=Pd²⁺ or Pt²⁺ ) to produce a highly complex ensemble of permanently fluxional coordination cages. Metal-mediated self-assembly selects for an M₂ L₄ architecture while maintaining shape-shifting ligand complexity. A second level of simplification is achieved with guest-exchange; the binding of halides within the M₂ L₄ cage mixture results in a convergence to a cage species with all four ligands present as the "B isomer". Within this confine, the reaction graph of the bullvalene is greatly restricted, but gives rise to a mixture of 38 possible diastereoisomers in rapid exchange. X-ray crystallography reveals a preference for an achiral form consisting of both ligand enantiomers. Through a combination of NMR spectroscopy and DFT calculations, we elucidate the restricted isomerisation pathway of the permanently fluxional M₂ L₄ assembly.