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, ceff, 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, ceff, 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.