Ursodeoxycholic Bile Acid as Building Unit of Unprecedented Supramolecular Complexes

Large, non-symmetrical, inherently chiral bispyridyl ligand (L) derived from natural ursodeoxycholic bile acid was used in coordination to square-planar tetravalent Pd(II) yielding the cationic enantiomer of Pd3L6 coordination complex (1) containing 60 well-defined chiral centers in its flower-like structure. Complex 1 can be readily transformed by addition of chloride into a smaller enantiomerically pure metallacycle Pd3L3Cl6 (2). This transformation is reversible and can be restored by the addition of silver cations. Furthermore, a mixture of two constitutional isomers of the trimer, 2 and 2’, can be obtained by direct coordination of L to trans-N-pyridyl-coordinating Pd(II). These intriguing supramolecular assemblies have been thoroughly described by combination of various methods. The interconversion between particular species was followed by 1H NMR and mass spectrometry. The dimensions of complexes determined by 1H DOSY NMR spectroscopy and drift tube ion-mobility mass spectrometry agree well with molecular models. Variable temperature 1H NMR experiment was carried out to study the thermodynamic equilibria between 2 and 2’ complexes. The thermodynamic data obtained from the van’t Hoff plot agree well with computational calculations.