Unsymmetric Chiral Ligands for Large Metallo-Macrocycles: Selectivity of Orientational Self-Sorting

Nature uses various chiral and unsymmetric building blocks to form substantial and complex supramolecular assemblies. In contrast, the majority of organic ligands used in metallosupramolecular chemistry are symmetric and achiral. Here we extend the group of unsymmetric chiral bile acids used as a scaffold for organic bispyridyl ligands by employing chenodeoxycholic acid (CDCA), an epimer of the previously used ursodeoxycholic acid (UDCA). The epimerism, flexibility, and bulkiness of the ligands leads to large structural differences in coordination products upon reaction with Pd(NO₃)₂. The UDCA-bispyridyl ligand self-assembles quantitatively into a single crown-like Pd₃L₆ complex, whereas the CDCA ligand provides a mixture of coordination complexes of general formula Pd_nL_2n, i.e., Pd₂L₄, Pd₃L₆, Pd₄L₈, Pd₅L₁₀, and even Pd₆L₁₂ containing an impressive 120 chiral centers. The coordination products were studied by a combination of analytical methods, with ion-mobility mass spectrometry (IM-MS) providing valuable details on their structure and allowed an effective separation of m/z 1461 to individual signals according to the arrival time distribution, thereby revealing four different ions of [Pd₃L₆(NO₃)₃]³⁺, [Pd₄L₈(NO₃)₄]⁴⁺, [Pd₅L₁₀(NO₃)₅]⁵⁺, and [Pd₆L₁₂(NO₃)₆]⁶⁺. The structures of all the complexes were modelled using DFT calculations. Finally, the challenges and conclusions in determining the specific structural identity of these unsymmetric species are discussed.