Substituent effects on BTP's basicity and complexation properties with LnIII lanthanide ionsElectronic supplementary information (ESI) available: Total energies of neutral, protonated and complexed ligands and structural parameters of the complexes. See DOI: 10.1039/c0nj00527d

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 = NMe 2 > NH 2 > OMe > C 6 H 5 > 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. R-substituents of R-BTP's can markedly modulate their basicity and their complexation properties towards Ln III (and presumably An III ) ions.