Jean-Claude Bunzli - Lanthanide podates with predetermined structural and photophysical properties: Strongly luminescent self-assembled heterodinuclear d-f complexes with a segmental ligand containing heterocyclic imines and carboxamide binding units

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  Piguet,C., Bunzli,J.C.G., Bernardinelli,G., Hopfgartner,G.,   Petoud,S., Schaad,O. Journal of the American Chemical   Society 1996 118 (28) 6681-6697

  Abstract: The segmental ligand   2-{6-[N,N-diethylcarbamoyl]pyridin-2-yl}-1,1'-dimethyl-5,5'-methylene-2'-(5-met hylpyridin-2-yl)bis[1H-benzimidazole]   (L(2)) reacts with stoichiometric amounts of Ln(III) (Ln = La-Nd,   Sm-Tb, Tm-Lu, Y) and Zn(II) in acetonitrile to yield   quantitatively and selectively the heterodinuclear triple-helical   complexes [LnZn(L(2))(3)](5+) under thermodynamic control. The   crystal structure of [EuZn(L(2))(3)] (ClO4)(CF3SO3)(4)(CH3-CN)(4)   (13; EuZnC111H111N25O19F12S4Cl, monoclinic, C2/c, Z = 8) shows   the wrapping of the three ligands L(2) about a pseudo-C-3 axis   passing through the metal ions. Zn(II) occupies the distorted   pseudooctahedral capping coordination site defined by the three   bidentate binding units while Eu(III) lies in the resulting   ''facial'' pseudotricapped trigonal prismatic site produced by   the three remaining tridentate units as exemplified by   luminescence measurements using the Eu(III) structural probe. The   separation of contact and pseudocontact contributions to the   H-1-NMR paramagnetic shifts of the axial complexes   [LnZn(L(2))(3)](5+) (Ln = Ce, Pr, Nd, Sm, Eu, Tm, Yb) establishes   that the triple helical structure is maintained in solution.   Photophysical measurements and quantum yields in acetonitrile   indicate that the terminal N,N-diethylcarbamoyl group in L(2)   favors efficient intramolecular L(2) --> Eu(III) energy   transfers leading to strong Eu-centered red luminescence.   Improved resistance toward hydrolysis also results from the use   of carboxamide groups, and no change in luminescence is observed   for [EuZn(L(-2))(3)](5+) in moist acetonitrile. The preparation   of the segmental ligand L(2) from the new asymmetric synthon   6-(N,N-diethylcarbamoyl)pyridine-2-carboxylic acid is described   together with its crystal and molecular structure (C33H33N7O,   monoclinic, P2(1)/c, Z = 4). The use of 3d metal ions as a   noncovalent tripodal spacer for lanthanide podates is discussed   together with the crucial role played by carboxamide groups for   the control of structural, electronic, and photophysical   properties

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