Guest-Induced Reversible Transformation between an Azulene-Based Pd₂L₄ Lantern-Shaped Cage and a Pd₄L₈ Tetrahedron

Azulene, a blue structural isomer of naphthalene, is introduced as the backbone for a new family of Pd(II)-based self-assemblies. Three organic ligands, equipped with varying donor groups, produce three [Pd₂L₄] cages of different cavity dimensions. Unexpectedly, the addition of organic disulfonate guests to the smallest lantern-shaped cage (featuring pyridine donors) led to a rapid and quantitative transformation to a distorted-tetrahedral [Pd₄L₈] species. On the contrary, [Pd₂L₄] cages formed from ligands with isoquinoline donors either just encapsulated the guests or showed no interaction. The tetrahedral species could be fully reverted back to its original [Pd₂L₄] topology by capturing the guest by another, stronger binding [Pd₂L'₄] coordination cage, narcissistically self-sorting from the first cage. The azulenes, serving as colored hydrocarbon backbones of minimal atom count, allow one to follow cage assembly and guest-induced transformation by the naked eye. Furthermore, we propose that their peculiar electronic structure influences the system's assembly behavior.