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 LnI(3) (Ln = La, Ce) or [UI3(py)(4)] with 3 equivalents of terpy in acetonitrile gave the tris( terpy) complexes [M(terpy)(3)]I-3. 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)]X-3. 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 H-1 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.