Jean-Claude Bunzli - Nine-coordinate lanthanide podates with predetermined structural and electronic properties: Facial organization of unsymmetrical tridentate binding units by a protonated covalent tripod

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  Renaud,F., Piguet,C., Bernardinelli,G., Bunzli,J.C.G.,   Hopfgartner,G. Journal of the American Chemical Society  1999 121 (40) 9326-9342

  Abstract: Three unsymmetrical tridentate   pyridine-2,6-dicarboxamide binding units have been connected to   the tris(2-(N-methyl)aminoethyl)amine tripod to give the podand   L-10 that exists as a statistical mixture of four conformers in   solution. In aqueous acidic medium, the protonated apical   nitrogen atom of the tripod (pK(a)([L-10+H](+)) = 4.66(2)) adopts   an endo conformation compatible with the formation of bi- and   trifurcated hydrogen bonds with the oxygen atoms of the proximal   carboxamide groups, thus producing a clipped conformation   preorganized for the complexation of lanthanide metal ions.   Reactions of L-10 and [L-10+H](+) with Ln(ClO4)(3) (Ln = La-Lu)   in acetonitrile provide stable nine-coordinate podates   [Ln(L-10)](3+) and [Ln(L-10+H)](4+). Thermodynamic investigations   indicate that the increased electrostatic repulsion associated   with the complexation of the protonated podand is compensated by   preorganization leading to only minor effects on the stability of   the final podates. A structural characterization in solution   using paramagnetic NMR concludes that a weak interaction between   Ln(III) and the lone pair of the apical nitrogen atom of the   tripod in [Ln(L-10)](3+) is removed in [Ln(L-10+H)](4+) leading   to a distortion of the coordination site. The crystal structure   of the complex [Eu(L-10+H)](CF3SO3)(3)(PF6)(CH3CN)(0.5) (12,   EuC46H62.5N10.5O15F15PS3, trigonal, R (3) over bar, Z = 6)   reveals a cationic conical triple-helical podate [Eu(L-10+H)](4+)   resulting from the wrapping of the three meridionally   tricoordinated chelating units about the metal ion. A remarkable   trifurcated hydrogen bond (N-H ...(O=C)(3)) rigidifies the tripod   and forces Eu(III) Co lie at the center of the pseudo-tricapped   trigonal prismatic cavity. High-resolution emission spectroscopy   demonstrates that Eu(III) is efficiently protected within the   pedate whose resistance toward hydrolysis is significantly   improved compared to related nonclipped triple-helical complexes.   The implications of covalent tripod for the design of   nine-coordinate lanthanide building blocks with predetermined   structural, thermodynamic, and electronic properties is discussed

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