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

While Mn(II) complexes meet increasing interest in biomedical applications, ligands are lacking that enable high Mn(II) complex stability and selectivity vs. Zn(II), 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(II), yielding eight‐coordinate Mn(II) complexes with record stabilities. In contrast, the smaller Zn(II) 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 (1)H NMR spectroscopy) demonstrate unprecedented selectivity for Mn(II) vs. Zn(II) (K (MnL)/K (ZnL) of 10(8)–10(10)), in sharp contrast to the usual Irving–Williams behavior, and record Mn(II) complex stabilities and inertness with logK (MnL) close to 25.