Complexation of a macrocyclic ligand, 2,6-di (N-methyl)formamide-calix[4]pyridine, with Eu(III) and extraction of Eu(III) and Am(III)

Complexation of a new macrocyclic compound, 2,6-dimethylformamide-calix[4]pyridine (L¹), with Eu(III) was studied by spectrophotometry. Stability constants of the Eu(III)/L¹ complex in different solvents were determined. The results reveal that L¹ forms moderately strong complexes with Eu(III) and other lanthanides in aprotic solvents and shows little binding ability with transition metals. Moreover, the binding strength of L¹ weakens significantly in protic solvents. Using 2-bromodecanoic acid as the synergistic reagent, L¹ extracts Am(III) and Eu(III) successfully with a separation factor of SF_Am/Eu=1.3, and the distribution ratios of Am(III) and Eu(III) increases as the aqueous acidity is decreased. DFT computational studies were conducted to corroborate the solvent extraction data, and compare the coordination properties of Am(III)/Eu(III) complexes with L¹ and a related, 2,6-diformamide-calix[4]pyridine (L²). The computational results suggest that L² could form stable complexes [ML]³⁺ and ML(NO₃)₃ [where M represent Am(III) or Eu(III)] in aqueous phase, in sharp contrast to the case of L¹ where such complexes in aqueous phase are not stable.