Exceptional Manganese(II) Stability and Manganese(II)/Zinc(II) Selectivity with Rigid Polydentate Ligands**
Patrick Cieslik, Peter Comba, Benedikt Dittmar, Daouda Ndiaye, Éva Tóth, Gunasekaran Velmurugan, Hubert Wadepohl
Abstract
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 log K MnL close to 25.