In recent years, significant research efforts have been dedicated to finding efficient
and safe alternatives to the currently used gadolinium (Gd)-based MRI contrast agents.
Among the most explored alternatives are paramagnetic chelates of the Earth-abundant
Mn2+, which form a prominent class of metal complexes. The design of Mn2+ complexes
with enhanced relaxation properties and improved safety profiles hinges on a delicate
balance between thermodynamic and kinetic stability, as well as the presence of coordinated
water molecules. In this study, we present a comprehensive investigation into the
coordination chemistry of three structurally related polyetheraminocarboxylic chelating
agents. Our aim is to elucidate the structural features, paramagnetic properties,
and thermodynamic and kinetic inertness of the corresponding Mn2+ complexes. The most
significant finding is the considerable difference in the dissociation rates of the
complexes, with the octadentate EGTA complex being the most labile. The observed dissociation
rates correlate well with the nitrogen inversion dynamics, as assessed through NMR
spectral analysis of the analogous Zn2+ complexes.
