DFT modeling of the relative affinity of nitrogen ligands for trivalent f elements: an energetic point of view

In many theoretical studies dealing with the selective complexation of trivalent actinides with respect to trivalent lanthanides, the method of calculation is assessed by comparing computed geometries with crystal structures that are often available. Yet, the selectivity is better rationalized through thermodynamic data, as enthalpy and entropy terms. In this article, we have theoretically modeled competing complexation reactions of [Ce(terpY)(3)](3+), [U(terpy)(3)](3+), [Ce(MeBTP)(3)](3+) and [U(MeBTP)(3)](3+) systems (terpy = 2,2':62-terpyridine; MeBTP = methyl-2,6-di(1,2,4-triazin3-yl)pyridine) within the framework of the Density Functional Theory. Our calculations manage to qualitatively account for the experimental relative stabilities of terpy and MeBTP complexes, and in particular for the better coordinating strength of MeBTP for trivalent uranium. We also show by comparing the MeBTP ligand with its non-alkylated form (HBTP) that model systems often used in quantum chemistry must be carefully chosen when energetic comparisons are undertaken.