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

Abstract

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 ¹H NMR spectroscopy, revealing that substitution of a methyl and bromo group at the α position of PhC(O)CH₂C(O)Ph or PhC(O)CH₂C(O)CF₃ decreases the percentage of the enol form drastically, whereas the parent diketones exist quantitatively in the enol form in CDCl₃. The pK_a value of PhC(O)CHMeC(O)CF₃ determined in 75% (v/v) 1,4-dioxane–water containing 0.1 mol dm^-3 NMe₄ClO₄ at 25 °C was 10.36, being much larger than that of PhC(O)CH₂C(O)CF₃ (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)CH₂C(O)CF₃ than with PhC(O)CHMeC(O)CF₃ 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.

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