Structural, spectroscopic, and thermodynamic consequences of anti-chelate effect in nine-coordinate lanthanide...

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)](ClO(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.