Radiochim. Acta 92, 31–38 (2004)  by Oldenbourg Wissenschaftsverlag, München Crown ethers as synergists in the extraction of trivalent lanthanoids with 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone By Rani Pavithran and M.L.P. Reddy ∗ Separation Science and Technology Group, Regional Research Laboratory (CSIR), Thiruvananthapuram – 695019, India (Received June 5, 2003; accepted in revised form August 21, 2003) Synergistic extraction / Lanthanoids / 3-phenyl-4-acyl-5-isoxazolones / Crown ether Summary. This paper highlights the results of investiga- tions carried out on the extraction of lanthanoids such as Nd(III), Eu(III) and Tm(III) from nitrate solutions into chloroform with 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI) in the presence and absence of various crown ethers (CE); 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DC18C6), benzo-18-crown-6 (B18C6) and dibenzo-18- crown-6 (DB18C6). The results demonstrated that these trivalent metal ions were extracted into chloroform as Ln(FBPI) 3 with HFBPI alone and as Ln(FBPI) 3 · CE in the presence of a CE. The equilibrium constants of the above extracted complexes deduced by a non-linear regression an- alysis were found to increase monotonically with a decrease in ionic radii of these metal ions. The addition of a CE to the metal chelate system significantly improves the extraction efficiency of these metal ions. The complexation strength of trivalent lanthanoids with various CEs follows the order: DC18C6 > 18C6 > B18C6 > DB18C6. Solid complexes of Eu(III) with HFBPI alone and with mixtures of HFBPI and various crown ethers have been isolated and characterised by IR and 1 H NMR spectral data to further clarify the nature of the extracted complexes. Introduction 4-Acyl-3-phenyl-5-isoxazolones (1), especially, 3-phenyl- 4-benzoyl-5-isoxazolone (HPBI) has come to occupy a spe- cial place in the extraction of metal ions due to its lower p K a value [1–4]. *Author for correspondence (E-mail: mlpreddy@yahoo.co.in). The acid dissociation constant of HPBI (p K a = 1.23) is lower than that of corresponding 4-acyl pyrazolones (p K a = 2.56–4.01) due to the electron delocalization induced by the isoxazolone group. This property makes 4-acyl-3-phenyl- 5-isoxazolone an interesting class of β-diketones with poten- tial application as reagents for the extraction and separation of metal ions from complexing strong acidic media [5–8]. From our laboratory, the extraction behaviour of trivalent lanthanoids with HPBI in the presence of various CEs have been investigated and reported a significant synergistic en- hancement in the extraction of these metal ions [9]. Further, a sharp decrease in the complexation strength of these metal chelates from 18C6 to DB18C6 and 15-crown-5 (15C5) to Benzo-15-crown-5 (B15C5) has been attributed to the steric effects. Recently, a highly acidic β-diketone, 3-phenyl- 4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI, p K a = 0.65), was synthesized and utilized for the extraction of trivalent lanthanoids from dilute nitric acid solutions in the pres- ence and absence of DC18C6 into 1,2-dichloroethane [10]. The results clearly demonstrated that the substitution of fluorine in the fourth position of the benzoyl moiety in the 3-phenyl-4-benzoyl-5-isoxazolone, significantly im- proved the extraction efficiency as compared to 3-phenyl- 4-benzoyl-5-isoxazolone (HPBI) or 3-phenyl-4-(4-toluoyl)- 5-isoxazolone (HTPI). Further, the addition of DC18C6 to the metal chelate system significantly enhanced the extrac- tion efficiency and also improved the selectivity among these metal ions. This prompted us to extend the above studies for the extraction of trivalent lanthanoids from ni- tric acid solutions with HFBPI in the presence and absence of various crown ethers (CEs) with a view to elucidate the nature of the complexes extracted into the organic phase and also to investigate the selectivity among these trivalent metal ions. For comparison, studies have also been per- formed with 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) and 3-phenyl-4-(4-toluoyl)-5-isoxazolone (HTPI). Experimental Chemicals 3-Phenyl-4-benzoyl-5-isoxazolone (HPBI), 3-phenyl-4-(4- fluorobenzoyl)-5-isoxazolone (HFBPI) and 3-phenyl-4-(4- toluoyl)-5-isoxazolone (HTPI) were synthesized according to the method of Korte and Storiko [11] from 3-phenyl-