Selectivity of heterocyclic β-ketoenols toward trivalent lanthanoids: Semi-empirical molecular orbital calculations and solvent extraction from nitrate aqueous medium

The solvent extraction of lanthanum(III) and lutetium(III) from 0.1 M nitrate medium with monoacidic chelating extractants of various acidities and lipophilicities, in chloroform, at 25 °C, has been studied and compared with previously reported extraction data for europium. The extractants, “HL”, are either the 3-phenyl-4-p-tert-butylbenzoyl-isoxazol-5-one, “HPtbBI”, or the substituted 4-acyl-5-hydroxypyrazoles 1-(p-nitro-phenyl)-3-phenyl-4-lauroyl-5-hydroxypyrazole, “HNPPLP”, and 1-phenyl-3-methyl-4-(p-tert-butylbenzoyl)-5-hydroxy-pyrazole, “HPMtbBP”. Like europium, lanthanum and lutetium are extracted by cation exchange as LnL3,org. Because of the higher acidity of the former ones, extractions performed with 4-acylisoxazol-5-ones occur at lower pH values than with 4-acyl-5-hydroxypyrazoles. The order of extraction efficiencies is: HPMtbBP (pKa = 4.2) < HNPPLP (pKa = 3.2) < HPtbBI (pKa = 1.3) < HPBI (pKa = 1.2) (HPBI: 3-phenyl-4-benzoyl-isoxazol-5-one). Moreover, for all these chelators, the extraction efficiency follows the order lanthanum < europium < lutetium, while their separation factors decrease with increasing extractant acidity. The extractions of other trivalent lanthanoids (Ln3+ = Ce3+, Nd3+, Sm3+, Tb3+, Ho3+ and Tm3+) and yttrium(III) from nitrate medium with HNPPLP have also been investigated. The equilibrium constants and the intra-lanthanoidic separation factors show the same trends as those observed for other β-ketoenol extractants, HPMBP, 1-phenyl-3-methyl-4-benzoyl-5-hydroxypyrazole, and HTTA, thenoyltrifluoroacetone. The overall selectivity, SF(Lu/La), is discussed with respect to the extraction thermodynamics, the d(O⋯O)HL structural parameters, measured or calculated by semi-empirical methods, and the inner- or outer-sphere nature of the extracted complexes.