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, L-AB2 and L-AB3, 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 L-AB3 of composition [ LnLn'(L-AB3)(3)](ClO4)(6)center dot solv (CeCe, PrPr, PrLu, NdLu) show the metal ions embedded into a helical structure with a pitch of about 13.2-13.4 angstrom. The metal ions lie at a distance of 9.1-9.2 angstrom 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 L-AB3 shows selectivity and gives high yields of heterobimetallic complexes. L-AB2 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 reversible arrow HHT equilibrium and of the selectivity for heteropairs of Ln(III) ions is therefore at hand.