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.