Effect of substituent of β-diketones on the synergistic extraction of lanthanoids with linear polyether

The synergistic extraction of 14 trivalent lanthanoids (Ln3+) into 1,2-dichloroethane with a linear polyether (DEO6), HO(C2H4O)6C12H25, and β-diketones (HA) having different substituents was investigated at 25.0 °C. The HAs used were trifluoroacetylacetone (Htfa), thenoyltrifluoroacetone (Htta), benzoyltrifluoroacetone (Hbta), naphthoyltrifluoroacetone (Hnta), and pivaloyltrifluoroacetone (Hpta). By the extraction of Ln3+ with β-diketone alone, the extraction constants of the neutral LnA3 complex, K ex = [ LnA 3 ] org [ H + ] aq 3 / [ L n 3 + ] aq [ HA ] org 3 , were determined. The intrinsic extraction constants, K ex * = [ LnA 3 ] org / [ L n 3 + ] aq [ A − ] aq 3 , were evaluated by employing the regular solution theory. Results indicate that the extractability of LnA3 is dependent on the lipophilicity of the ligand, and the planar aromatic rings do not cause steric hindrance in the formation of the binary complex. Addition of DEO6 significantly enhanced the extraction of Ln3+ by the formation of LnA3(DEO6). The ternary complex formation constants, βadd, were determined for all the Ln3+ and HA. The βadd of bta− and nta− complexes is similar with those of tfa− complexes, indicating that planar aromatic rings do not sterically hinder even the formation of the ternary complex. The higher values of βadd for the complexes of tta−, which has a slightly dipolar thenoyl moiety, can be accounted for the presence of ligand–ligand interaction. The formation constants of the ternary complexes of pta− were lower compared to complexes of other β-diketones because of steric hindrance due to the bulky t-butyl moiety. The detailed structures of the ternary complexes in solution were elucidated by NMR spectroscopy. Estimated structures sufficiently explain the variation in stability constants of LnA3(DEO6) among HAs and across the series of Ln3+. The structures thus obtained were ascertained by the molecular models created by MM2 calculation.