Covalent Post-assembly Modification Triggers Structural Transformations of Borromean Rings

A series
of supramolecular transformation cascades are presented
here, employing ligand exchange reactions, concentration-dependent
Borromean ring (BR) linking and unlinking, and inverse electron-demand
Diels–Alder (IEDDA) reactions. The new family of template-free,
tetrazine-edged BRs are constructed by using ligand exchange reactions,
and their concentration-dependent linking and unlinking reactions
have been observed. Moreover, Borromean precursors have been demonstrated
to further undergo supramolecular structural transformations induced
by rapid and efficient IEDDA reactions to afford corresponding post-assembly
modified architectures. Remarkably, subtle steric increases of the
pyridazine fragments obtained by IEDDA reactions is regarded to induce
controlled topological transformations in the cascade, unlinking the
Borromean structures by using electron-rich dienophiles as triggering
signals.