Jean-Claude Bunzli - The first self-assembled trimetallic lanthanide helicates driven by positive cooperativity

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  Floquet,S., Ouali,N., Bocquet,B., Bernardinelli,G., Imbert,D.,   Bunzli,J.C.G., Hopfgartner,G., Piguet,C. Chemistry-A European   Journal 2003 9 (8) 1860-1875

  Abstract: The segmental tris-tridentate ligand   L7 reacts with stoichiometric quantities of Ln(III) (Ln = La-Lu)   in acetonitrile to give the complexes [Ln(2)(L7)(3)](6+) and   [Ln(3)(L7)(3)](9+). Formation constants point to negligible   size-discriminating effects along the lanthanide series, but   Scatchard plots suggest that the self-assembly of the trimetallic   triple-stranded helicates [Ln(3)(L7)(3)](9+) is driven to   completion by positive cooperativity, despite strong   intermetallic electrostatic repulsions. Crystallization provides   quantitatively [Ln(3)(L7)(3)](CF3SO3)(9) (Ln = La, Eu, Gd, Tb,   Lu) and the X-ray crystal structure of [Eu-3(L7)(3)](CF3SO3)(9)   (.) (CH3CN)(9) (.) (H2O)(2) (Eu3C216H226N48O35F27S9, triclinic, P   (1) over bar, Z = 2) shows the three ligand strands wrapped   around a pseudo-threefold axis defined by the three metal ions   rigidly held at about 9 Angstrom. Each metal ion is coordinated   by nine donor atoms in a pseudo-trigonal prismatic arrangement,   but the existence of terminal carboxamide units in the ligand   strands differentiates the electronic properties of the terminal   and the central metallic sites. Photophysical data confirm that   the three coordination sites possess comparable pseudo-trigonal   symmetries in the solid state and in solution. High-resolution   luminescence analyses evidence a low-lying LMCT state affecting   the central EuN9 site, so that multi-metal-centered luminescence   is essentially dominated by the emission from the two terminal   EuN6O3 sites in [Eu-3(L7)(3) ](9+). New multicenter equations   have been developed for investigating the solution structure of   [Ln(3)(L7)(3)](9+) by paramagnetic NMR spectroscopy and linear   correlations for Ln = Ce-Tb imply isostructurality for these   larger lanthanides. NMR spectra point to the triple helical   structure being maintained in solution, but an inversion of the   magnitude of the second-rank crystal-field parameters, obtained   by LIS analysis, for the LnN(6)O(3) and LnN(9) sites with respect   to the parameters extracted for Eu-III from luminescence data,   suggests that the geometry of the central LnN(9) site is somewhat   relaxed in solution

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