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

The segmental ligand ene-2'-(5-methylpyridin-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, mo noclinic, C2/c, Z = 8) shows the wrapping of the three ligands L(2) ab out a pseudo-C-3 axis passing through the metal ions. Zn(II) occupies the distorted pseudooctahedral capping coordination site defined by th e 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 measurement s using the Eu(III) structural probe. The separation of contact and ps eudocontact contributions to the H-1-NMR paramagnetic shifts of the ax ial complexes [LnZn(L(2))(3)](5+) (Ln = Ce, Pr, Nd, Sm, Eu, Tm, Yb) es tablishes that the triple helical structure is maintained in solution. Photophysical measurements and quantum yields in acetonitrile indicat e that the terminal N,N-diethylcarbamoyl group in L(2) favors efficien t intramolecular L(2) --> Eu(III) energy transfers leading to strong E u-centered red luminescence. Improved resistance toward hydrolysis als o results from the use of carboxamide groups, and no change in lumines cence is observed for [EuZn(L(-2))(3)](5+) in moist acetonitrile. The preparation of the segmental ligand L(2) from the new asymmetric synth on 6-(N,N-diethylcarbamoyl)pyridine-2-carboxylic acid is described tog ether with its crystal and molecular structure (C33H33N7O, monoclinic, P2(1)/c, Z = 4). The use of 3d metal ions as a noncovalent tripodal s pacer for lanthanide podates is discussed together with the crucial ro le played by carboxamide groups for the control of structural, electro nic, and photophysical properties.