Manganese(II) Complexes as Potential Contrast Agents for MRI

Mn2+ has five unpaired d electrons, a long electronic relaxation time, and labile water exchange, which make it an attractive alternative to Gd3+ in the design of contrast agents for medical Magnetic Resonance Imaging. In order to ensure in vivo safety and high contrast agent efficiency, the Mn2+ 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 donoracceptor properties influence the thermodynamic, kinetic, and redox stability of the Mn2+ complex. This microreview surveys the current literature in this field.