Exceptional Manganese(II) Stability and Manganese(II)/Zinc(II) Selectivity with Rigid Polydentate Ligands.

While Mn complexes meet increasing interest in biomedical applications, ligands are lacking that enable high Mn complex stability and selectivity vs. Zn , the most relevant biological competitor. We report here two new bispidine derivatives, which provide rigid and large coordination cavities that perfectly match the size of Mn , yielding eight-coordinate Mn complexes with record stabilities. In contrast, the smaller Zn ion cannot accommodate all ligand donors, resulting in highly strained and less stable six-coordinate complexes. Combined theoretical and experimental data (X-ray crystallography, potentiometry, relaxometry and H NMR spectroscopy) demonstrate unprecedented selectivity for Mn vs. Zn (K /K of 10 -10 ), in sharp contrast to the usual Irving-Williams behavior, and record Mn complex stabilities and inertness with logK close to 25.