Planned Failures from the Principle of Maximum Site Occupancy in Lanthanide Helicates

Despite the recent emergence of a toolbox fitted with microscopic thermodynamic descriptors for predicting the stabilities and speciations of polynuclear complexes in solution, the discovery of novel or unusual type of metal−ligand assemblies in metallosupramolecular chemistry still often relies on serendipity. In order to highlight the novel perspectives offered by a rational exploitation of these thermodynamic parameters, the segmental bis-tridentate ligands L7 and L8 have been designed for providing effective molarities upon reaction with trivalent lanthanides, Ln(III), so small that the saturated binuclear triple-stranded helicates [Ln2(Lk)3]6+, which obey the well-respected principle of maximum site occupancy, cannot be detected in solution because of their deliberately planned instabilities. The hierarchical evolution of the effective molarities with an increasing number of ligand strands in these complexes indeed favors the formation of the alternative unsaturated single-stranded [Ln2(Lk)]6+ and double-stranded [Ln2(Lk)2]6+ complexes, whose relative speciations in solution depend on the nature of the binding sites introduced into the segmental ligand.