Adjustment of twist angles in pseudo-helical lanthanide complexes by the size of metal ions

Control of self-assembled nanostructures is a promising technique for nanotechnology. We have examined as to whether nanostructures could be controlled by the size of the central metal ion. Lanthanides are a natural choice for such a study as the size of their trivalent ions changes with atomic number gradually. For this investigation, a series of rare earth complexes ([LaL¹], [CeL¹], [SmL¹], [TUL¹], [YL¹], and [LuL¹]) with a tripodal heptadentate ligand L¹ were synthesized, and their X-ray crystallographic analysis was performed. Although the structures of the ligand (H₃L¹) and of the metal complex ([ML¹]) were quite different, all complexes were almost isostructural pseudohelices. The result of the crystallographic studies demonstrated that the twist angles of helices in the complexes depend on the ionic size of the central metal. A detailed analysis helped determine which portion of the helical strand contributed to the total helicity, and the major cause for the difference in helicity among the lanthanides is discussed. Moreover, this result is the first example showing that La^III and Lu^III complexes with the same tripodal heptadentate ligand are isostractural.