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 10⁸ -10¹⁰ ), in sharp contrast to the usual Irving-Williams behavior, and record Mn^II complex stabilities and inertness with logK_MnL close to 25.