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 (M z + = 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.