Metal Ion Selectivities of the Highly preorganized tetradentate ligand 1,10-phenanthroline-2,9-dicarboxamide with Lanthanide(III) ions and some Actinide ions
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Abstract
Metal ion complexing properties of the ligand PDAM (1,10-phenanthroline-2,9-dicarboxamide) are reported in relation to its possible use as a functional group for solvent extractants in the separation of Am(III) from Ln(III) (lanthanide) ions. PDAM is only slightly water soluble, but variation of the intense π-π* transitions in the UV spectrum of 2 x 10 5 M PDAM solutions as a function of pH or metal ion concentration allowed for the determination of the protonation constant (pK) and log K1 values with metal ions. The pK of PDAM is 0.6 ± 0.1 in 1.0 M NaClO4, the lowest for any 1,10-phenanthroline (phen) derivative (in contrast, pK phen = 5.1), which is attributed to the electron-withdrawing properties of the amide substituents of PDAM. The weak proton basicity of PDAM may be an important factor in its use as the functional group of a solvent extractant from acidic solutions. The formation constants are determined by UV-visible spectroscopy for the Ln(III) ions from La(III) to Lu(III) (excluding Pm(III)), as well as for Y(III), Sc(III), Th(IV), and the UO22+ cation in 0.1 M NaClO4 at 25 oC. The log K1 values for the Ln(III) ions show only small changes from La(III) to Lu(III) (both have log K1 = 3.80). The amide O-donors (oxygen donors) of the amide groups of PDAM appear to cause considerable stabilization of the complexes of PDAM as compared to those of phen, consistent with the idea that the neutral O-donor is a strong Lewis base towards large metal ions such as the Ln(III) ions. The small change in complex stability is rationalized in terms of the idea that neutral O-donors stabilize the complexes of the large La(III) ion more than the smaller Lu(III) ion, offsetting the greater affinity of Lu(III) than La(III) for N-donor ligands. The complexes of Th(IV), Sc(III), and the UO22+ cation with PDAM have log K1 values slightly higher than those of the Ln(III)
