Strong Enantiomeric Preference on the Macroion-Counterion Interaction Induced by Weakly Associated Chiral Counterions

The role of chiral counterions on the attraction and self-assembly of chiral Pd₁₂L₂₄ metal organic cages (MOCs) with NO₃^- being the original counterion is studied by laser light scattering and isothermal titration calorimetry. Nitrates can trigger the self-assembly of macrocationic Pd₁₂L₂₄ into hollow spherical blackberry-type supramolecular structures via counterion-mediated attraction. Although chiral counteranions, such as N-(tert-butoxycarbonyl)-alanine (Boc-Ala), have weaker interaction with the MOCs compared to NO₃^-, they can induce different assembly behaviors between two enantiomeric MOCs by inhibiting the MOC-nitrate binding and weakening the interaction between them. The d-counterions are capable of selectively suppressing and slowing down the assembly of l-MOCs and also considerably decreasing their assembly size due to the much weaker MOC-nitrate interaction. The same scenario is observed for l-counterions when interacting with the d-MOCs. This study unveils the role of weakly associated chiral counterions on the central chiral macroions, especially their supramolecular structure formation, and provides additional evidence on the mechanism of the homochirality phenomenon.