The First Enantiomerically Pure Helical Noncovalent Tripod for Assembling Nine-Coordinate Lanthanide(III) Podates

Decomplexation of the trivalent lanthanide, Ln(III), from the racemic bimetallic triple-stranded helicates [LnCr(L8)3]6+ provides the inert chiral tripodal nonadentate receptor [Cr(L8)3]3+. Elution of the latter podand with Na2Sb2[(+)-C4O6H2]2·5H2O through a cation exchange column allows its separation into its inert helical enantiomers
M-(+)589-[Cr(L8)3]3+ and P-(−)589-[Cr(L8)3]3+, whose absolute configurations are assigned by using CD spectroscopy
and exciton theory. Recombination with Ln(III) restores the original triple-stranded helicates [LnCr(L8)3]6+, and the
associated thermodynamic parameters unravel the contribution of electrostatic repulsion and preorganization to the
complexation process. Combining M-(+)589-[Cr(L8)3]3+ with Eu(III) produces the enantiomerically pure d−f helicate
MM-(−)589-[EuCr(L8)3](CF3SO3)6·4CH3CN, whose X-ray crystal structure (EuCrC113H111N25O21S6F18, monoclinic, P21,
Z = 2) unambiguously confirms the absolute left-handed configuration for the final helix. The associated ligand-centered and metal-centered chiro-optical properties recorded for the complexes MM-[LnCr(L8)3]6+ and PP-[LnCr(L8)3]6+ (Ln = Eu, Gd, Tb) show a strong effect of helicity on specific rotary dispersions, CD and CPL spectra.