Jean-Claude Bunzli - Lanthanide helicates self-assembled in water: A new class of highly stable and luminescent dimetallic carboxylates

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  Elhabiri,M., Scopelliti,R., Bunzli,J.C.G., Piguet,C. Journal   of the American Chemical Society 1999  121 (46) 10747-10762

  Abstract: The segmental ligand   bis{1-ethyl-2-[6'-carboxy)pyridin-2'-yl]benzimidazol-5-yl}methane   (L-C) reacts with the entire series of Ln(III) ions to yield a   new class of neutral lanthanide carboxylates, the triple-stranded   dimetallic helicates [Ln(2)(L-C-2H)(3)], which are stable in   water in the pH range 4-13. A competitive titration with   1,4,7,10-tetraazacyclododecane-N,N',N",N'''-tetraacetic acid   (dota) shows the Eu-III helicate to have a stability comparable   to that of [Eu(dota)](-). According to the X-ray structures of   isotypical [Ln(2)(L-C-2H)(3)].20.5H(2)O (Ln = Eu, Tb)   (C(93)H(72)Ln(2)N(18)O(12).20.5H(2)O, monoclinic, P2(1)/c, Z =   8), the helicates are comprised of two types of slightly   different molecules (A, B) within the asymmetric unit. The three   ligand strands are helically wrapped around the two   nine-coordinate Ln(III) ions, leading to pseudo-D-3 symmetries   around the metals. The Ln(III) distances lie in the ranges   8.81--8.83 Angstrom for molecules A and 9.04-9.07 Angstrom for   molecules B, while the helical pitch amounts to 23.7--23.8   Angstrom (A) and 24.5-24.7 Angstrom (B). In solution, the   helicates retain time-averaged D-3 symmetries along the entire   Ln(III) series, as shown by H-1 and C-13 NMR. The derivation of   equations adapted to heterodimetallic complexes allows the   separation of contact and dipolar contributions to the isotropic   paramagnetic shift. Somewhat different structures are evidenced   between larger (Ln = Ce-Tb) and smaller (Ln = Er--Yb) Ln(III)   ions, attributed to a lengthening of the helix on going toward   heavier lanthanides. The photophysical properties of the   helicates depend on the nature of the Ln(III) ion, and LC is   revealed to be a good sensitizer of Eu-III. The flexible route   used to synthesize LC opens interesting perspectives for the   fine-tuning of the chemical and photophysical properties of   lanthanide-containing homodimetallic helicates with potential   applications in labeling and sensing technologies

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