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

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)(3)(3+) 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 > (t)Butyl > 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.