Tuning the coordination number of hydroxypyridonate-based gadolinium complexes: Implications for MRI contrast agents

Eight-coordinate hydroxypyridinone/terephthalamide Gd^III complexes display high relaxivities due to their two inner sphere water molecules. This relaxivity can be further increased by functionalizing the terephthalamide moiety with an amine. A significant hydrogen bonding interaction between the amine and another water molecule close to the Gd^III apparently facilitates its coordination on the open site of the metal. The resulting nine-coordinate complex has three inner sphere water molecules, while maintaining high stability and fast ligand exchange rates.