Complexes of the highly preorganized ligand PDALC (2,9-bis(hydroxymethyl)-1,10-phenanthroline) with trivalent lanthanides. A thermodynamic and crystallographic study

Metal ion complexing properties of the highly preorganized tetradentate ligand PDALC (2,9-bis(hydroxymethyl)-1,10-phenanthroline) are presented. The structure of Gd(PDALC)(NO₃)₃]·H₂O (1) is reported: triclinic, , a = 7.545(12), b = 10.811(17), c = 11.909(18) Å, α = 97.71(2)°, β = 91.56(2)°, γ = 109.06(2)°, V = 907(2) Å³, Z = 2, R = 0.0354. The Gd is 10-coordinate, with the coordination sphere comprising the four donor atoms of the PDALC plus the six O-donors of three chelated nitrates. Comparison with structures in the literature suggests that the Gd-L (L = ligand) bond lengths, particularly those to the alcoholic O-donors of PDALC, are a little short. It was suggested that the short Gd-L bond lengths in 1 were due to the efficiency of packing of the nitrates around the Gd, with the short ‘bite’ distances of the nitrate ligand. Formation constants (log K₁) were measured spectroscopically in 0.1 M NaClO₄ at 25 °C by monitoring the variation of the π-π∗ transitions of 2 × 10⁻⁵ M PDALC in the range 200-350 nm as a function of pH, in the presence of 1:1 concentrations of the lanthanide(III) (Ln(III)) metal ion. The measured log K₁ values varied from 5.34 (La(III)) to 6.40 (Lu(III), which is an unusually small variation across the series of Ln(III) ions. Values of log K₁ with PDALC were also measured for Y(III) (5.85) and Sc(III) (6.02). The small amount of variation in log K₁ for PDALC across the series of Ln(III) ions was rationalised in terms of the effect of neutral oxygen donors on complex stability, which promotes selectivity for larger metal ions such as La(III). It was discussed how the small amount of variation in log K₁ across the Ln(III) series might lead to optimal selectivity for the Am(III) ion relative to the Ln(III) ions as a group.