Para‐Substituted 1‐Phenyl‐3‐methyl‐4‐aroyl‐5‐pyrazolones as Chelating Agents for the Synergistic Extraction of Thorium(IV) and Uranium(VI) in the Presence of Various Crown Ethers

Abstract

Para‐substituted 1‐phenyl‐3‐methyl‐4‐aroyl‐5‐pyrazolones, namely, 1‐phenyl‐3‐methyl‐4‐(4‐fluorobenzoyl)‐5‐pyrazolone (HPMFBP) and 1‐phenyl‐3‐methyl‐4‐(4‐toluoyl)‐5‐pyrazolone (HPMTP) were synthesized and utilized for the extraction of thorium(IV) and uranium(VI) from nitric acid solutions. For comparison, studies have also been carried out with 1‐phenyl‐3‐methyl‐4‐benzoyl‐5‐pyrazolone (HPMBP). The results demonstrate that thorium(IV) and uranium(VI) are extracted into chloroform according to: where HX refers to 1‐phenyl‐3‐methyl‐4‐aroyl‐5‐pyrazolone and K ex,0 denotes the equilibrium constant. The equilibrium constants of the extracted complexes have been calculated by nonlinear regression analysis taking into account the aqueous‐phase complexation of the metal ion with inorganic ligands and all plausible complexes extracted into the organic phase. The equilibrium constants of thorium(IV) and uranium(VI) with various 4‐aroyl‐5‐pyrazolones follow the order HPMFBP > HPMBP > HPMTP, which is in accordance with their pK a values. The synergistic solvent extraction of thorium(IV) and uranium(VI) was also investigated with mixtures of HPMFBP and 18‐crown‐6 (18C6), dicyclohexano‐18‐crown‐6 (DC18C6), dibenzo‐18‐crown‐6 (DB18C6), and benzo‐15‐crown‐5 (B15C5). The complexation strength of Th(PMFBP)4 chelate with crown ethers follows the order B15C5 > 18C6 > DC18C6 > DB18C6. The high extraction efficiency of thorium(IV) with B15C5 can be explained on the basis of “size‐fitting effect.” On the other hand, the complexation strength of UO2(PMFBP)2 chelate with various crown ethers follows the order 18C6 > DC18C6 > B15C5 > DB18C6, which can be explained on the basis of the crown ether basicity sequence and steric effects. Solid complexes of these metal ions with HPMFBP and crown ethers have been isolated and characterized by elemental analyses, IR and 1H NMR spectroscopic techniques to further clarify the nature of the extracted complexes.