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 K1) 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 K1(TPTZ) plotted against ionic radius (r+) in reverse order shows a novel set of log K1 values with log K1 rising from La(III) to peak at Sm(III) and then dropping down steadily to Lu(III) with log K1 lower than for La(III). Y(III) and Am(III) show log K1 values relative to their Ln(III) size analogues, with a small drop for Y(III) and little difference in log K1 for Am(III). The variation of log K1 along the Ln(III) series, as well as log K1 for Am(III) and Y(III) compared to their Ln(III) size analogues, is discussed for several ligands. The patterns in variation of log K1 along the Ln(III) series depend on increasing covalence, which contributes to a rise in log K1 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 K1 to Lu(III), resulting in the unusual log K1 vs r+ diagram for the Ln(III) cations with TPTZ.