Metal ion selectivities of the highly preorganized tetradentate ligand 1,10-phenanthroline-2,9-dicarboxamide with lanthanide(III) ions and some actinide ions

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 2×10⁻⁵ M PDAM solutions as a function of pH or metal ion concentration allowed for the determination of the protonation constant (pK) and logK₁ 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 °C. The logK₁ values for the Ln(III) ions show only small changes from La(III) to Lu(III) (both have logK₁ = 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. A reviewer has pointed out that the amide groups would also stabilize the complexes of PDAM by virtue of the chelate effect, in that PDAM is tetradentate, while phen is only bidentate. The small change in complex stability for PDAM complexes in passing from La(III) to Lu(III) 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 logK₁ values slightly higher than those of the Ln(III) cations, while Y(III) forms a slightly less stable complex. The best-fit size of metal ion for coordination with PDAM is analyzed using molecular mechanics calculations. A fluorescence study shows that the free ligand PDAM fluoresces very strongly, but that the Ln(III) cations quench the fluorescence of PDAM rather than produce a chelation enhanced fluorescence (CHEF) effect as Ln(III) ions do with other phen-based tetradentate ligands. Possible reasons for the lack of a CHEF effect with PDAM are discussed.