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

The synergistic extraction of 14 trivalent lanthanoids (Ln(3+)) into 1,2-dichloroethane with a linear polyether (DEO6), HO(C(2)H(4)O)(6)C(12)H(25), 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 Ln(3+) with β-diketone alone, the extraction constants of the neutral LnA(3) complex, [Formula: see text] , were determined. The intrinsic extraction constants, [Formula: see text] , were evaluated by employing the regular solution theory. Results indicate that the extractability of LnA(3) 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 Ln(3+) by the formation of LnA(3)(DEO6). The ternary complex formation constants, β(add), were determined for all the Ln(3+) and HA. The β(add) 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 β(add) 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 LnA(3)(DEO6) among HAs and across the series of Ln(3+). The structures thus obtained were ascertained by the molecular models created by MM2 calculation.