Studies on Deprotonation from OH Groups of Alditols by Heavy Rare Earth Ions

This study investigates single-crystal structures of Lu³⁺/erythritol and nine rare earth-glycerol complexes, revealing deprotonation of normally inert OH groups via μ₂-bridging (so-called DIOμ₂ phenomenon). The pH drops upon alditol addition correlate with the deprotonation extent, showing increased deprotonation with decreasing lanthanide ionic radius (lanthanide contraction) and exceptionally high deprotonation for Sc³⁺. In all the rare earth/alditol complexes with the DIOμ₂ phenomenon, two μ₂-bridges merge into an M₂O₂ cluster (M is a rare earth ion) that is not very stable due to the Coulombic repulsion between two M³⁺ and between two O^-. To stabilize the M₂O₂ cluster, the formation of more auxiliary five- and six-membered chelating rings between rare earth ions and alditol ligands is needed. Increment of the number of auxiliary five- and six-membered chelating rings favors the DIOμ₂ reaction. This is the reason why the DIOμ₂ reaction shows a progressively enhanced tendency when the alditol ligands change from myo-inositol to glycerol to erythritol and galactitol. The reaction is endothermic, as an increased temperature enhances deprotonation. The above results may be useful in the rational design of catalysts that efficiently activate inert OH bonds and facilitate related chemical processes.