Screening of the Structure of Americium Extractants Based on a 2,2 '-Bipyridyl Scaffold: a Simple Way to a N2,O2-Tetradentate Ligands Library for Rational Design of An/Ln Extractants

A library of N‐ethylanilides of 2,2′‐bipyridine‐6,6′‐dicarboxylic acids were synthesized to evaluate the influence of the side chains on the extraction properties. The structure of the various diamides is studied in solid state by X‐ray and in solution by 2D NMR. In the solid‐state, the diamides featured flat Cs geometry irrespective of the nature and position of the substituents. Carboxylic groups in the crystals shifted from the residing planes of both pyridine and aromatic anilide rings indicating disruption of the conjugated system. The solution behaviour is substituent dependent. Only three types of relative orientations of amidic fragments and pyridine rings were found in solutions of diamides depending on the nature of the amidic side chain substituents. The conformations of the diamides mostly depended on the size of the substituent and less on their electronic properties. Solubility of the diamides and their metal extraction properties in relation to americium and europium ions were studied the diamides, bearing substituents in pyridine rings and in phenyl ring of diamide moiety (one or two alkyl‐, alkoxy‐, fluorine‐, carboxylic‐groups). The symmetrical substitution in the pyridine moiety critically diminished both solubility and metal extraction. Metal ions recovery strongly depended on the substitution in the anilidic moiety; distribution coefficients depended more on the spatial arrangement of the substituents and to a lesser extent on their electronic properties.