Solution versus solid-state structure of ytterbium heterobimetallic catalysts

The solution structures of the ytterbium heterobimetallic complexes Na-3[Yb((S)-BINOL)(3)] (1), K-3[Yb((S)-BINOL)(3)] (2), and Li-3[Yb((S)-BINOL)(3)] (3), belonging to a family of well-known enantioselective catalysts, are studied by means of NMR and circular dichroism (CD) in the UV and near-IR regions. The experimental NMR paramagnetic shifts were employed to obtain a refined solution structure of 1. NMR analysis demonstrated that 1, 2, and 3 have the same solution geometry but different magnetic susceptibility anisotropy D factors. By comparing XRD and NMR structures of 1, we demonstrate that, upon dissolution, this complex experiences a rearrangement from the crystalline C-3 symmetry into the solution D symmetry. Remarkably, Yb is not bound to water in solution, and Ln-BINOL bonds are labile as demonstrated through EXSY. NIR-CD is confirmed especially sensitive to changes in the ytterbium coordination sphere.