Supramolecular Chelation Based on Folding

Crystallographic analysis revealed that pyridine−palladium complexation is a good geometric
match to the m-phenylene ethynylene (mPE) repeat unit and thus could serve as a reversible linking group
to join oligomer segments together. A series of pyridine-terminated mPE oligomers were then synthesized
and found to coordinate with palladium dichloride to give complexes effectively twice the length of the free
oligomers. A quantitative analysis of these coordination equilibria by isothermal calorimetry found the ability
of the pyridine end-group to form a coordination complex corresponded with their ability to fold. Oligomers
that were able to form complexes of sufficient length to fold showed positive cooperativity based on
experimental determination of their association constants with a palladium ion. We suggest that the additional
free energy of complexation for the folded oligomers is analogous to chelation by multidentate ligands, but
here the “multidentate ligand” is held together by supramolecular rather than covalent bonds.