Complexation of lanthanides and other metal ions by the polypyridyl ligand quaterpyridine: Relation between metal ion size, chelate ring size, and complex stability

Metal ion complexing properties of the ligand QPY (2,2′:6′,2′’:6′’,2′’’-quaterpyridine) are reported in relation to its possible use as a functional group for reagents in the extraction and separation of Ln(III) (lanthanide) ions. QPY is almost totally insoluble in water, but variation of the intense π-π* transitions in the UV spectrum of 1 × 10^–5 M QPY solutions in 50% CH₃OH/H₂O (v/v) 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_a values of QPY are 4.54 ± 0.02 and 3.47 ± 0.02 in 50% CH₃OH/H₂O. The log K₁ values determined by UV–visible spectroscopy for the Ln(III) ions by titration of 1:1 solutions of 1.0 × 10^­5 M Ln(III) ion and QPY between pH 2 and 6 were: La(III), 3.79; Pr(III), 4.34; Nd(III) 4.57; Sm(III), 4.78; Gd(III) 4.69; Dy(III), 4.76, Ho(III), 4.94; Er(III), 5.36, Tm(III), 5.84, and Lu(III) 5.54, all at 25 °C and ionic strength zero. The log K₁ values for the Ln(III) ions show quite strong increases from La(III) to Lu(III), with a local maximum at Sm(III), and a local minimum at Gd(III). The local maxium at Sm(III) is attributed to this ion being the best-fit size for coordinating with polypyridyl ligands. The effect of preorganization of polypyridyls such as QPY with bridging benzo groups in the ligand backbone to yield DPP (2,9-dipyrid-2-yl-1,10-phenanthroline) is to produce significant increases in log K₁ for large metal ions such as Ln(III) ions, but for small metal ions such as Zn(II), there is a large decrease in log K₁ in passing from QPY to DPP: this shows how rigidification of the ligand can enhance the selectivity against small metal ions of polypyridyls forming five-membered chelate rings. DFT calculations are used to analyze the variation in log K₁ in passing from the La(III) complexes of QPY to DPP compared to those for the smaller Lu(III) ion.