Complexation of trivalent lanthanides with planar tridentate aromatic ligands tuned by counteranions and steric constraints.

Among the plethora of parameters controlling the stability and structures of lanthanide coordination complexes, it is often difficult to decipher their relative importance in the global complexation processes. The combination of the bond valence method (for analyzing solid state structures) with the thermodynamic site binding model (for unravelling complexation reactions occurring in solution) appears to be an efficient tool for specifically addressing interligand effects, which affect the output of the coordination process. When applied to the reaction of the tridentate aromatic scaffolds 2,2':6',2''-terpyridine (L1) and 2,6-bis(benzimidazol-2-yl)pyridine (L2) with trivalent lanthanides, Ln(III), we demonstrate that the successive fixation of ligands, eventually leading to the triple-helical complexes [Ln(Lk)3]3+, is anticooperative both in the solid state and in solution, with a special sensitivity to the nature of the counteranion and to the peripheral substitution for L2. Consequently, in addition to the classical entropic driving forces resulting from the use of specific metal/ligand ratio, the stoichiometry of the final complex can be tuned by a judicious choice of interligand interactions, as exemplified by the unusual isolation of stable complexes with Ln/L = 2:3 ratios.