Influence of a Pre-organized N-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by an Aminopolycarboxylate Complexant

Thermodynamic influence of a pre-organized N-donor group on the coordination of trivalent actinides and lanthanides by aminopolycarboxylate aqueous complexant was investigated. The synthesized complexant N-2-methylpicolinate-ethylenediamine-N,N',N'-triacetic acid, EDTA-Mpic, resembles ethylenediamine-N,N,N',N'-tetraacetic acid, EDTA, with single acetate pendant arm replaced by 6-carboxypyridin-2-yl-methyl group. The rigidified N-donor picolinate functionality shows profound impact on the ligand protonation, and trivalent f-element complexation equilibria as demonstrated using potentiometric, spectroscopic, liquid-liquid metal partitioning studies and molecular dynamics calculations. While the coordination pocket of EDTA-Mpic is somewhat hardened by the N-methylpicolinate the substantial amplification of its total acidity offers the advantage of sustaining metal complexation in aqueous mixtures of unprecedented acidity for this class of reagents. As a result the EDTA-Mpic complexant may be considered the best option for efficient f-element recovery and An3+/Ln3+ differentiation in acidic media.