Quyen T. H. Le

Liquid-liquid extraction of lanthanides with a highly acidic extractant, 3-phenyl-4-benzoyl-5-isoxazolone, in the presence and absence of tri-n-octylphosphine oxide

A highly acidic β-diketone, 3-phenyl-4-benzoyl-5-isoxazolone (HPBI), was synthesized and the acid dissociation constant and the partition constant between an organic solvent and 0.1 M sodium perchlorate solution were determined by a liquid-liquid distribution method. The extraction of lanthanides with HPBI was investigated in the presence and absence of tri-n-octylphosphine oxide. HPBI was found to be a very powerful extractant for lanthanides owing to its strong acidity. The acidity and extractability are discussed on the basis of the molecular structure optimized by a semi-empirical MNDO/H calculation.

Improved separation in the solvent extraction of lanthanides by β-diketones through a controlled separation of the two donating oxygens

The extractability and separability of lanthanides with β-diketones is shown to be governed by the separation of the two donating oxygens.

Role of the intramolecular hydrogen bond and ligand rigidity in the complexation of trifluoroacetylcycloalkanones with lanthanides: novel strategy for the design of organic ligands of high selectivity

Trifluoroacetylcyclopentanone (1), -cyclohexanone (2) and -cycloheptanone (3) have been synthesized and the complexation with lanthanides (La3+, Pr3+, Eu3+, Ho3+ and Yb3+) has been investigated via the solvent extraction technique. The strength of the intramolecular hydrogen bond was found to depend on the distance between the two donating oxygens in the enol form estimated by semi-empirical MNDO/H and ab initio calculations. The OO distance for 1 is larger than those for 2 and 3 as predicted from their cyclic structures. The strength of the intramolecular hydrogen bond increases in the order, 1 < 2 ≈ 3, which is also supported by 1H NMR and IR spectra. The acid dissociation constants (pKa) were measured by potentiometric titration in 30% dioxane-water. The pKa values increase in the order, 1 < 2 ≤ 3, reflecting the strength of the intramolecular hydrogen bond. Lanthanides were readily extracted by 1 owing to its strong acidity. However, better separation for lanthanides was achieved with 2 and 3. The extractability and separability have been discussed in terms of the ligand rigidity and the intramolecular hydrogen bond originating from the distance between the two donating oxygens.

Improved extraction-separation of alkaline earths and lanthanides with the aid of an ion size-selective masking reagent

A water-soluble macrocyclic compound, 4′-sulfobenzo-18-crown-6 (SB18C6), was prepared and utilized as an ion size-selective masking reagent with a view to establishing a highly selective extraction–separation system. In the synergistic extraction of alkaline earths with 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone and trioctylphosphine oxide in cyclohexane or benzene, the extraction of those with larger ionic radii shifted to a higher pH region when SB18C6 was added to the aqueous phase, giving improved separations. A similar result was achieved in the extraction of lanthanides into chloroform with bis(2-ethylhexyl)phosphoric acid. The distribution of SB18C6 into cyclohexane, benzene or chloroform was so low in comparison with that of 18-crown-6 (18C6) and cryptand[2.2.2] that the synergistic extraction with 18C6, which usually reduces the selectivity, was not observed.

Decisive role of a phenyl group at the α-position of benzoylacetone in the solvent extraction of Al3+ and In3+

Al3+ is readily extracted into benzene with α-phenylbenzoylacetone, while In3+ is entirely unextractable, which leads to an efficient extraction-separation of Al3+ from In3+.

α-Substituted β-diketones: effect of the α substituent on the complexation and selectivity for lanthanides

Some β-diketones having a substituent at the α position have been prepared and their keto–enol tautomerism, acid dissociation reaction and complexation with lanthanides have been studied. The tautomerism was examined by 1 H NMR spectroscopy, revealing that substitution of a methyl and bromo group at the α position of PhC(O)CH 2 C(O)Ph or PhC(O)CH 2 C(O)CF 3 decreases the percentage of the enol form drastically, whereas the parent diketones exist quantitatively in the enol form in CDCl 3 . The pK a value of PhC(O)CHMeC(O)CF 3 determined in 75% (v/v) 1,4-dioxane–water containing 0.1 mol dm -3 NMe 4 ClO 4 at 25 °C was 10.36, being much larger than that of PhC(O)CH 2 C(O)CF 3 (8.10). This weak acidity could be attributed to the strong intramolecular hydrogen bond which results upon decreasing the separation between the two donating oxygens through the steric repulsion between the α-methyl substituent and the terminal phenyl and trifluoromethyl groups. The diketone structures and the intramolecular hydrogen bond were evaluated using semiempirical molecular orbital calculations (MNDO/H), taking into account the hydrogen bond. The complexation with lanthanides has been examined via the solvent-extraction method. The lanthanides were extracted from a lower pH region with PhC(O)CH 2 C(O)CF 3 than with PhC(O)CHMeC(O)CF 3 owing to its higher acidity. On the contrary, the separation with the latter was found to be better than that with the former. The separability is discussed with regard to the structures of the diketones.