Acylpyrazolone derivatives of high selectivity for lanthanide metal ions: effect of the distance between the two donating oxygens

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Abstract
Four 4-acyl-3-methyl-1-phenyl-5-pyrazolone derivatives [4-acyl = acetyl (HL1), propionyl (HL2), isobutyryl (HL3) and pivaloyl (HL4)] have been synthesized. Their distribution behaviors between cyclohexane and aqueous 0.1 mol dm−3 sodium perchlorate solution were examined, and the partition constants and the acid dissociation constants obtained. The structures of the acylpyrazolone derivatives were evaluated by semi-empirical MNDO/H molecular orbital calculation and 1H NMR. The solvent extraction of lanthanide metal ions (La3+, Pr3+, Eu3+, Ho3+ and Yb3+) into chloroform was examined, and it was found that the selectivity improves as the 4-acyl groups become bulkier. This improvement could be attributed to the narrowed distance between the two donating oxygens (“bite size”), which is caused by the steric repulsion between the 3-methyl and bulky 4-acyl groups. The extraction selectivity for lanthanide metal ions including 4-benzoyl-(HL5) and 4-trifluoroacetyl-pyrazolone (HL6), 4-benzoyl-3-phenyl-5-isoxazolone (HL7), and other β-diketones could be elucidated by considering the effect of the bite size, suggesting that this is one of the most significant factors that govern the selectivity in the extraction of lanthanide metal ions. The separability of the 4-pivaloyl derivative (HL4) is comparable to that of bis(2-ethylhexyl) hydrogenphosphate, one of the most selective extractants for lanthanides.
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