Steric Effect of 3,5-Bis(trifluoromethyl)phenol as Hydrogen-Bond Donor on Outer-Sphere Complexation in the Synergistic Extraction of Aluminium(III) and Gallium(III) with 2,4-Pentanedione

The synergistic extraction of aluminium(III) and gallium(III) with 2,4-pentanedione (Hacac) in heptane from a weakly acidic solution was investigated in the presence of 3,5-bis(trifluoromethyl)phenol (BTMP). A large enhancement of the extraction of metal(III) was ascribed to the formation of outer-sphere complexes between metal(III) chelates and BTMP in the organic phase. Furthermore, an IR study demonstrated that the outer-sphere complexes were formed by the hydrogen bond between the hydroxy hydrogen atom of BTMP and the oxygen atoms of metal(III) chelates. The formation constants of the outer-sphere complexes with BTMP were determined and compared with those with 3,5-dichlorophenol (DCP) to understand the steric effect of the phenol derivative in this synergistic extraction. The bulky trifluoromethyl-group of BTMP and the short oxygen–oxygen nonbonded distance of Al(acac)3 resulted in a steric repulsion between two BTMP molecules to prevent the formation of Al(acac)3·3BTMP, although Al(acac)3·3DCP, Ga(acac)3·3BTMP, and Ga(acac)3·3DCP could be formed. Such a steric effect on outer-sphere complexation improves the separation efficiency between aluminium(III) and gallium(III) in the present synergistic extraction.