Abstract
The thermodynamic influence of a pre‐organized N‐donor group on the coordination of trivalent actinides and lanthanides by an aqueous aminopolycarboxylate complexant has been investigated. The synthesized reagent, N‐2‐methylpicolinate‐ethylenediamine‐N,N′,N′‐triacetic acid (EDTA‐Mpic), resembles ethylenediamine‐N,N,N′,N′‐tetraacetic acid (EDTA) with a single acetate pendant arm replaced by a 6‐carboxypyridin‐2‐ylmethyl group. The rigid N‐donor picolinate functionality has a profound impact on ligand protonation and trivalent f element complexation equilibria, as demonstrated by potentiometric, spectroscopic, and liquid/liquid metal‐partitioning studies as well as by molecular dynamics calculations. Relative to diethylenetriamine‐N,N,N′,N′′,N′′‐pentaacetic acid (DTPA), the ability to preferentially bind trivalent actinides over trivalent lanthanides was moderately lowered due to the presence of the N‐(6‐carboxypyridin‐2‐ylmethyl) substituent. The structural modification substantially amplifies the total ligand acidity of EDTA‐Mpic. As a result the complexant sustains the metal complexation and efficient An3+/Ln3+ differentiation in aqueous mixtures of unprecedented acidity for this class of reagents.
