Lanthanide Triple-Stranded Helicates: Controlling the Yield of the Heterobimetallic Species

Two unsymmetrical ditopic hexadentate ligands designed for the simultaneous recognition of two different trivalent
lanthanide ions have been synthesized, LAB2 and LAB3, where A represents a tridentate benzimidazole-pyridine-benzimidazole coordination unit, B2 a diethylamine-substituted benzimidazole-pyridine-carboxamide one, and B3 a
chlorine-substituted benzimidazole-pyridine-carboxamide moiety. Under stoichiometric 2:3 (Ln/L) conditions, these
ligands self-assemble with lanthanide ions to yield triple-stranded bimetallic helicates. The crystal structures of four
helicates with LAB3 of composition [LnLn‘(LAB3)3](ClO4)6·solv (CeCe, PrPr, PrLu, NdLu) show the metal ions embedded
into a helical structure with a pitch of about 13.2−13.4 Å. The metal ions lie at a distance of 9.1−9.2 Å and are
nine-coordinated by the three ligand strands, which are oriented in a HHH (head−head−head) fashion, where all
ligand strands are oriented in the same direction. In the presence of a pair of different lanthanide ions in acetonitrile
solution, the ligand LAB3 shows selectivity and gives high yields of heterobimetallic complexes. LAB2 displays less
selectivity, and this is shown to be directly related to the tendency of this ligand to form high yields of HHT
(head−head−tail) isomer. A fine-tuning of the HHH ⇆ HHT equilibrium and of the selectivity for heteropairs of LnIII
ions is therefore at hand.