Synergistic Extraction of Rare Earth Ions with Acidic Chelating Reagents and a Metal (III)-Tropolone Chelat As a Neutral Ligand

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Effect of cobalt (III) chelates such as tris (4-isopropyltropolonato)cobalt (III)(Co (ipt)_3) and tris (acetylacetonato) cobalt (III)(Co (acac)_3) for comparison on the extraction of rare earth ions (RE) with β-diketones (HA) has been studied. The extraction of RE with thenoyltrifluoroacetone (Htta) or hexafluoroacetylacetone (Hhfa) in benzene or toluene was remarkably enhanced by the addition of those cobalt (III) chelates, especially Co (ipt)_3 in the Hhfa-toluene system. This is the synergistic effect caused by the cobalt (III) chelates. From the equilibrium analysis, the synergism was ascribed to the formation of a 1 : 1 adduct of RE (A)_3 with the cobalt (III) chelate in the organic phase and the adduct formation constant (β_*) was determined. The β_* values decreased with decrease in the ionic radius of RE and gave the large differentce between light and heavy RE.Some spectroscopic studies such as electronic absorption and infrared absorption were performed to explain such difference in β_* From the IR spectra, the displacement of the coordinated water molecules of the light RE chelates such as Nd (Eta)_3 with Co (ipt)_3 was observed ; therefore, the direct coordination of Co (ipt)_3, toward the central metal of the RE chelate. On the other hand, the adduct formation of heavy RE was ascribed to the hydrogen bonding between Co (ipt)_3 and the coordinated water molecules of the RE chelates. It was proved that the coordinately saturated cobalt (III)-ipt chelate acts as a complex ligands and causes the synergistic enhancement of the extraction of RE.