Effective concentration as a tool for quantitatively addressing preorganization in multicomponent assemblies: Application to the selective complexation of lanthanide cations

The beneficial entropic effect, which may be expected from the connection of three tridentate binding units to a strain-free covalent tripod for complexing nine-coordinate cations (Mz(+) = Ca2+, La3+, Eu3+, Lu3+), is quantitatively analyzed by using a simple thermodynamic additive model. The switch from pure intermolecular binding processes, characterizing the formation of the triple-helical complexes [M(L2)(3)](z+), to a combination of inter- and intramolecular complexation events in [M(L8)](z+) shows that the ideal structural fit observed in [M(L8)](z+) indeed masks large energetic constraints. This limitation is evidenced by the faint effective concentrations, c(eff), which control the intramolecular ring-closing reactions operating in [M(L8)](z+). This predominence of the thermodynamic approach over the usual structural analysis agrees with the hierarchical relationships linking energetics and structures. Its simple estimation by using a single microscopic parameter, c(eff), opens novel perspectives for the molecular tuning of specific receptors for the recognition of large cations, a crucial point for the programming of heterometallic f-f complexes under thermodynamic control.