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 2-{6-[N,N-diethylcarbamoyl]pyridin-2-yl}-1,1‘-dimethyl-5,5‘-methylene-2‘-(5-methylpyridin-2-yl)bis[1H-benzimidazole] (L2) 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(L2)3]5+ under thermodynamic control. The crystal structure of [EuZn(L2)3](ClO4)(CF3SO3)4(CH3CN)4 (13; EuZnC111H111N25O19F12S4Cl, monoclinic, C2/c, Z = 8) shows the wrapping of the three ligands L2 about a pseudo-C3 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 1H-NMR paramagnetic shifts of the axial complexes [LnZn(L2)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 L2 favors efficient intramolecular L2 → 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(L2)3]5+ in moist acetonitrile. The preparation of the segmental ligand L2 from the new asymmetric synthon 6-(N,N-diethylcarbamoyl)pyridine-2-carboxylic acid is described together with its crystal and molecular structure (C33H33N7O, monoclinic, P21/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.