Jean-Claude Bunzli - Structural, Spectroscopic, and Thermodynamic Consequences of Anti-Chelate Effect in Nine-Coordinate Lanthanide Podates

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      Ryan,P.E., Guenee,L., Canard,G., Gumy,F., Bunzli,J.C.G.,   Piguet,C. Inorganic Chemistry 2009  48 (6) 2549-2560

      Abstract: The connection of three tridentate   2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine binding units to an   extended sulfur-containing tripodal anchor in the ligand L9   yields nine-coordinate podates [Ln(L9)](3+) upon reaction with   trivalent lanthanides, Ln(III). Structural analysis of   [Eu(L9)](CIO(4))(3) in the solid state with the help of the bond   valence method shows that the peripheral ethyl groups are   responsible for a specific distortion of the triple-helical   structure, which allows a closer approach of the nitrogen donors   toward the central metal, while minimizing interstrand repulsion.   The consequences of this distortion on the Eu(III) luminescent   probe are investigated by high-resolution emission spectroscopy,   while paramagnetic NMR data collected in acetonitrile demonstrate   that [Ln(L9)](3+) adopts a single relaxed C(3)-symmetrical   structure along the complete lanthanide series. The persistence   of the triple-helical structure in solution is obtained at the   cost of severe constraints in the helically wrapped organic   tripod, which strongly disfavor intramolecular cyclization   processes. The resulting antichelate effect can be exploited for   the selective preparation of polynuclear complexes with tripodal   ligands

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