The Weakly Basic Triazine Nitrogen Donor in TPTZ Promotes Selectivity against More Covalently Binding Metal Ions, Resulting in a Remarkable Stability Sequence along the Lanthanide Series

Formation constants (log <i>K</i>₁) for TPTZ (2,4,6-tris(2-pyridyl)-1,3,5-triazine) with a variety of metal ions in aqueous solution are reported at ionic strength 0.1 and at 25 °C. The Ln(III) series with log <i>K</i>₁(TPTZ) plotted against ionic radius (<i>r</i>⁺) in reverse order shows a novel set of log <i>K</i>₁ values with log <i>K</i>₁ rising from La(III) to peak at Sm(III) and then dropping down steadily to Lu(III) with log <i>K</i>₁ lower than for La(III). Y(III) and Am(III) show log <i>K</i>₁ values relative to their Ln(III) size analogues, with a small drop for Y(III) and little difference in log <i>K</i>₁ for Am(III). The variation of log <i>K</i>₁ along the Ln(III) series, as well as log <i>K</i>₁ for Am(III) and Y(III) compared to their Ln(III) size analogues, is discussed for several ligands. The patterns in variation of log <i>K</i>₁ along the Ln(III) series depend on increasing covalence, which contributes to a rise in log <i>K</i>₁ toward Lu(III). The coordinating nitrogen of the triazine moiety of TPTZ is shown by density functional theory calculation to be of very low basicity and therefore unable to cause a rise in log <i>K</i>₁ to Lu(III), resulting in the unusual log <i>K</i>₁ vs <i>r</i>⁺ diagram for the Ln(III) cations with TPTZ.