Abstract
Mn(2+) has five unpaired d electrons, a long electronic relaxation time, and labile water exchange, which make it an attractive alternative to Gd(3+) in the design of contrast agents for medical Magnetic Resonance Imaging. In order to ensure in vivo safety and high contrast agent efficiency, the Mn(2+) ion has to be chelated by a ligand that provides high thermodynamic stability and kinetic inertness of the complex and has to have at least one free coordination site for a water molecule.
Unfortunately, these two requirements are contradictory, as lower denticity of the ligands, which leads to more inner-sphere water molecules often implies a decreased stability of the complex, and, therefore, it is necessary to find a balance between both requirements. In the last decade, a large amount of experimental data has been collected to characterize the physico-chemical properties of Mn2+ chelates with variable ligand structures.
They now allow for establishing trends of how the ligand structure, the rigidity of the ligand scaffold, and its donor-acceptor properties influence the thermodynamic, kinetic, and redox stability of the Mn(2+) complex. This microreview surveys the current literature in this field.