A donor–acceptor model of Ln(III) complexation with terdentate nitrogen planar ligands

Abstract

The mechanism of the formation of a complex between several planar terdentate nitrogen ligands and trivalent lanthanides (Ln³⁺) in mixed MeOH–H₂O solutions is studied and a model is proposed for the thermodynamic properties of the complexation reactions. The objective is not to cover all aspects of complexation between Ln³⁺ and planar terdendate nitrogen ligands, but rather to provide insight into the factors that govern the complexation mechanism. It is shown that the complexation of Ln³⁺ in solution with similar ligands does not follow the classical trends in change of thermodynamic properties across the Ln series, which have long been known. Ab initio calculations show that the stability of complexes increases with the increasing electron donor ability of the coordinating central nitrogen atom (N_c) of the ligand and the electron acceptor ability of the coordinating lateral ones (N_l). The effect of covalence for the stability of complexes is analysed. The complexes with high stability are characterised by distinct covalence of the bonds between Ln³⁺ and the lateral coordinating nitrogen atoms of the ligand. The thermodynamic properties of complexation of Ln³⁺ with 2,6-bis-(pyridin-2-yl)-4-amino-1,3,5-triazine (Adptz) and 2,6-bis-(1,2,4-triazin-yl)-pyridine (Btp) are compared. The central ring is negatively charged in the Adptz ligand, but positive in Btp. The strong donor–acceptor interaction and the net covalence result in an enthalpy mechanism of complexation of Ln³⁺ with Adptz. Owing to the repulsion between the cation and the central nitrogen atom in the [Ln(Btp)]³⁺_solv complexes, the cation is pushed away from the nitrogen cavity, the Ln–N_c interatomic distances become larger and the electron donor ability of N_c is weak. Entropy promotes the formation of the [Ln(Btp)]³⁺_solv complex.