Abstract
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.