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

The solid state structure of the isostructural complexes [M(tpa)I-3(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)]I-3 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-D-3 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 (K-2/K-1=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 pi-pi 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.