Jean-Claude Bunzli - Lanthanide triple-stranded helicates: Controlling the yield of the heterobimetallic species

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  Jensen,T.B., Scopelliti,R., Bunzli,J.C.G. Inorganic   Chemistry 2006 45 (19) 7806-18

  Abstract: 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

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