The hydrogen-bonding ability of metal chelates plays an important role in their solution behavior, such as solubility and liquid-liquid partition. In coordinately saturated chelates, especially tris(pentane-2,4-dionato)metal(III), specific solvation with chloroform, methanol, and water has long been studied by using various techniques, such as the solubility, X-ray analysis, IR, and NMR spectroscopy, and has been ascribed to outersphere coordination, principally via hydrogen bonding between the chelate and the solvent molecules. 1 Recently, one of the present authors demonstrated by the liquid-liquid partition technique that tris(pentane-2,4-dionato)cobalt(III) forms stable hydrogen-bonded complexes with chlorinated phenols, which are much stronger proton donors than the solvent molecules stated above.2 The partition coefficient of the cobalt(III) chelate remarkably increased owing to the formation of hydrogenbonded complexes. This finding led us to a new synergistic extraction system for aluminium(III), 3 scandium(III), 4 iron(III), 5 gallium(III), 6 and indium(III) 6 with pentane-2,4-dione (Hacac) and 3,5-dichlorophenol. In another work, Katsuta and Suzuki have reported on the extraction of iron(III) with 8-quinolinol in the presence of various chlorinated phenols as the synergist.
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