Substituent effects on BTP's basicity and complexation properties with LnIII lanthanide ions

Abstract

Bis(1,2-4 triazin-3-yl)pyridines “BTPs” represent a new class of N-donor extracting agents that separate trivalent actinides and lanthanides from nuclear waste solutions with high An/Ln separation factors. We report here QM calculations on the effect of R_para–BTP substituents on the protonation and complexation energies of these ligands (1 ∶ 1 and 1 ∶ 3 complexes with Ln^III lanthanides) in the gas phase. Both processes follow similar trends and are highly sensitive to the electron donor/acceptor character and polarizability of R. When compared to R–pyr analogues with pyridine, R–BTPs are found to be intrinsically much more basic, by ca. 20 kcal mol⁻¹. In aqueous solution, however (modelled by the continuum PCM model), BTPs and pyridines have a similar basicity, pointing to the importance of solvent environment on their protonation states. In the optimized Ln(R–BTP)₃³⁺ complexes with Ln = La, Eu, Yb, complexation energies E_c3 increase with the intrinsic basicity of the ligands, in the order R = NMe₂ > NH₂ > OMe > C₆H₅ > ^tButyl > Me > H > F > Cl. Furthermore, comparison of complexes with different Ln^III cations indicates that their stability increases in the order La^III < Eu^III < Yb^III, by the same amount with the different R-substituents. The relative contributions of central pyridinyl and lateral triazinyl nitrogens of BTPs are shown to depend on the stoichiometry of the complex and on the Ln^III size, possibly contributing to the subtle An^III/Ln^III discrimination by substituted BTPs.