The affinity and selectivity of terdentate nitrogen ligands towards trivalent lanthanide and uranium ions viewed from the crystal structures of the 1 ∶ 3 complexes

Treatment of LnI3 (Ln = La, Ce) or [UI3(py)4] with 3 equivalents of terpy in acetonitrile gave the tris(terpy) complexes [M(terpy)3]I3. Addition of 3 equivalents of Rbtp (2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine) to MX3 (X = I or OSO2CF3) in pyridine or acetonitrile afforded the tris(Rbtp) compounds [M(Rbtp)3]X3. By comparison with terpy, the Rbtp ligand has a better affinity for the 4f and 5f ions and is more selective for U(III) than for Ce(III) or La(III). This trend has been revealed by 1H NMR competition experiments and X-ray crystallographic studies which show that in the [M(terpy)3]3+ and [M(Rbtp)3]3+ cations, the M–N(Rbtp) bond lengths are shorter than the M–N(terpy) bond lengths, and the U–N bond lengths are shorter than the corresponding Ce–N or La–N bond distances.