Complexation of Lanthanide(III) and Yttrium(III) cations with terpyridyl, dipyridoacridine, and 1,10-phenanthroline in aqueous solution. Importance of covalence in metal–ligand bonding in complexes of lanthanides with nitrogen donor ligands

Abstract

Absorbance titrations following intense π → π* transitions of the ligands were used to determine log K1 for TERPY (terpyridyl) and DPA (dipyrido acridine) in aqueous solution for lanthanide(III) (Ln(III)) complexes plus Y(III) at 25 °C. For TERPY the titrations of 2 × 10−5 M TERPY and 0.0167 M Ln(ClO4)3, giving μ (ionic strength) 0.1, were followed as a function of pH between pH 2.0–6.0 to give log K1. For DPA, absorbance as a function of [Ln3+] for 10−5 M DPA titrated with 0.01 M Ln(ClO4)3 yielded [Ln3+] ranging from 0 M to 10−4 M. These titrations were carried out effectively at μ = 0, to prevent salting out of the poorly soluble DPA ligand. For PHEN (1,10-phenanthroline) complexes changes in the absorbance spectrum on complexation with Ln(III) ions were too small to yield satisfactory values of log K1, so that the fluorescence spectra of 2 × 10−6 M PHEN with 0.0167 M Ln(ClO4)3 (μ → 0.1) were titrated between pH 2.0 and 6.0 to yield log K1 values. Log K1 for all three ligands showed an overall increase with decreasing ionic radius (r+) from La(III) to Lu(III), with local maxima at Sm(III) and local minima at Gd(III). Log K1 for the Y(III) complexes is consistently lower for TERPY, DPA, and PHEN than for the similarly sized Ho(III), as is also found for other tridentate and tetradentate polypyridyl ligands. This is rationalized as greater covalence in the Ln–N bonds than in Y–N bonds, due to stronger participation of the 5d orbitals of Ln(III) ions in bonding than for the analogous participation of the 4d orbitals of Y(III): as found by DFT calculations (Inorg. Chem., 2022 (61) 4627) there is a small but significant contribution from the 4f orbitals of Ln(III) ions in producing covalence in Ln–N bonds. It is also found that the sharpness of the local maxima at Sm(III) and minima at Gd(III) become more marked as the number of nitrogen donor atoms increases, and the level of preorganization of polypyridyl ligands is increased by bridging benzo groups in the ligand. Preorganization may be important in ligands for separating Am(III), similar in size to Sm(III), from the Ln(III) ions, particularly Gd(III), in treating nuclear waste.