Manganese(II) complexes meet increasing interest in biomedical applications, in particular as potential Magnetic Resonance Imaging contrast agents with a better biocompatibility and safety profile than the currently used gadolinium(III) chelates. However, we severely lack chelating ligands that enable high MnII complex stability and especially good selectivity for manganese(II) versus zinc(II), the most relevant biological competitor. In an article published in Angewandte Chemie Int. Ed., the “Metal complexes and MRI” group in collaboration with the group of Peter Comba at the University of Heidelberg, has identified novel cage molecules which, for the first time, satisfy these criteria. The bispidine derivative ligands provide rigid and large coordination cavities that perfectly match the slightly larger size of MnII, yielding eight-coordinate MnII complexes with record stabilities. In contrast, the smaller ZnII ion cannot accommodate all ligand donors, resulting in highly strained and less stable six-coordinate complexes.
Combined theoretical and experimental data demonstrate unprecedented selectivity, with up to 10 orders of magnitude higher stability for the manganese(II) than for the zinc(II) analogues, in sharp contrast to usual coordination chemistry rules.