Ion-size recognition of Group 13 metals (Al3+, In3+) with modified β-diketones
Ion-size recognition of Group 13 metals (Al3+ and In3+) with modified β-diketones, 3-phenylpentane-2,4-dione (α-phenylacetylacetone, HL2) and 1,2-diphenylbutane-1,3-dione (α-phenylbenzoylacetone, HL4), has been studied by the liquid–liquid extraction method and compared with that of pentane-2,4-dione (acetylacetone, HL1) and 1-phenylbutane-1,3-dione (benzoylacetone, HL3). While HL1 and HL3 quantitatively extracted both ions, HL2 and HL4 greatly discriminated Al3+ from In3+: Al3+ was readily extracted into benzene at a pH below 5, while In3+ was entirely unextractable, leading to an effective extraction separation of Al3+ from In3+. This could be ascribed to the large ‘bite size’ (O · · · O distance in the chelate ring) in the indium β-diketonates. The steric repulsions between the α-phenyl group and the terminal methyl or phenyl groups prevent HL2 and HL4 from widening their bite size. In addition, the extraction of Al3+ was greatly influenced by the interligand contact due to its remarkably small ionic radius. The extraction order with HL1 and HL3 is Al3+ > In3+ obeying the common tendency that the metal ion with a smaller ionic radius is more readily extracted, while that with ligands having bulky terminal groups, such as 1,3-diphenylpropane-1,3-dione (dibenzoylmethane, HL5), is the opposite. These steric factors have been evaluated by the investigation of the crystal structures of the complexes of Al3+ and In3+ and by molecular mechanics calculations. The steric factors have been confirmed through extraction behaviors with various kinds of β-diketones.