Steering the Shape-shifting of Bullvalene-Pd^II Complexes Through Steric and Geometric Strain

Di-substituted bullvalenes are fluxional molecules possessing up to fifteen possible isomers. Their shape-shifting nature has been harnessed in host-guest chemistry, drug design, polymers, and electromechanical systems. Despite this, principles for controlling the fluxional pathways of bullvalene, including access to isomers that are not normally populated, remain poorly understood. In this study, we elucidate the restricted shapeshifting of bis-pyridyl bullvalene ligands in ethylenediamine (en) cis-capped Pd^II complexes. The energetic landscape of 3-substituted bis-pyridyl bullvalene remains open to A and B isomers in [Pd₂(en)L₂] complexes, whilst the coordination of 4- and 2-substituted bis-pyridyl derivatives significantly restrict bullvalene's isomerization pathway. In one case, chelation of bis-2-pyridyl bullvalene enables access to an otherwise energetically inaccessible D-isomer. Exclusive formation of this isomer is achieved by increasing the sterics of the cis-capping ligand.