Solution and solid state structures of uranium(III) and lanthanum(III) iodide complexes of tetradentate tripodal neutral N-donor ligands

Abstract

The solid state structure of the isostructural complexes [M(tpa)I₃(py)] (M = La^III or U^III) containing the tripodal ligand tris[(2-pyridyl)methyl]amine (tpa) has been determined. The metal ions are eight-co-ordinated by the tetradentate tpa, three iodide ions and a pyridine molecule. The [U(Mentb)₂]I₃ complex was prepared by treating [UI₃(thf)₄] with two equivalents of tris(N-methylbenzimidazol-2-ylmethyl)amine (Mentb). Crystallographic studies show that the U^III is eight-co-ordinated by two tetradentate Mentb with the ligand arms wrapped around the metal in a pseudo-D₃ symmetric arrangement. Solution NMR studies in pyridine show a large difference in the behaviour of ntb and tpa towards complexation, and a greater stability of the bis(ligand) complexes for ntb than for tpa. At a ntb∶metal ratio of 0.5∶1 the bis(ligand) complex is already present in solution in greater quantity than the mono(ligand) complex for both La and U and at a ratio of 2∶1 only the bis(ligand) species exists in solution. On the other hand the bis(tpa) complexes of La and U appear in pyridine solution only at tpa∶metal ratios larger than 1∶1, and even at ratios larger than 3∶1 both the bis(tpa) and the mono(tpa) species are still present in solution. The preferential formation of the bis(ntb) complexes with respect to the mono(ntb) complexes (K₂/K₁ = 8 ± 1 for U and =35 ± 4 for La) has been related to the presence in solution, as well as in the solid state, of strong π–π interaction between benzimidazole rings. The absence of such interactions in the bis(tpa) complexes can explain their lower stability with respect to the bis(ntb) complexes.