Jean-Claude Bunzli - Supramolecular recognition of heteropairs of lanthanide ions: A step toward self-assembled bifunctional probes

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  Andre,N., Jensen,T.B., Scopelliti,R., Imbert,D., Elhabiri,M.,   Hopfgartner,G., Piguet,C., Bunzli,J.C.G. Inorganic   Chemistry 2004 43 (2) 515-529

  Abstract: Three unsymmetrical ditopic   hexadentate ligands coded for the recognition of trivalent   lanthanide ions have been synthesized, L-AB, L-AC, and L-BC,   where A represents a benzimidazole-pyridine-benzimidazole   coordination unit, B a benzimidazole-pyridine-carboxamide one,   and C a benzimidazole-pyridine-carboxylic acid moiety. Under   stoichiometric 2:3 (Ln:L) conditions, these ligands self-assemble   with lanthanide ions to yield triple-stranded bimetallic   helicates having a sizable stability in acetonitrile: log   beta(23) values for Eu are equal to 23.9 +/- 0.5 (L-AB), 23.3 +/-   0.7 (deprotonated L-AC), and 29.8 +/- 0.5 (deprotonated L-BC).   The crystal structure of the EuEu helicate with L-AB shows   9-coordinate metal ions and an HHH (H stands for head)   configuration of the helically wrapped ligand strands. In the   presence of equimolar quantities of Ln and Ln' ions, LAB displays   a remarkable predisposition to form HHH-heterobimetallic   edifices, as proved both in the solid state by the crystal   structures of the LaEu, LaTb, PrEr, and PrLu helicates and in   solution by NMR spectroscopy, In all cases, the   benzimidazole-pyridine-carboxamide units of the three ligands are   bound to the smaller lanthanide ion, a fact further ascertained   by high-resolution luminescence data on LaEu and by H-1 NMR.   Analysis of the lanthanide-induced H-1 NMR shifts and of the   spin-lattice relaxation times of the [LnLu(L-AB)(3)](6+) series   (Ln = Ce, Pr, No, Sm, Eu) demonstrates the isostructural nature   of the complexes in solution and that the crystal structure of   LaTb is a good model for the solution structure. The selectivity   of LAB for heteropairs of Ln(III) ions increases with increasing   difference in ionic radius, resulting in 70% of the   heterobimetallic species for Deltar(i) = 0.1 Angstrom and up to   90% for LaLu (Deltar(i) = 0.18 Angstrom), and corresponding to   Delta(DeltaG) in the range 3-10 kJ.mol(-1). The origins of this   stabilization are discussed in terms of the donor properties of   the coordinating units and of the preferential formation of HHH   isomers

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