An unusual gadolinium ten-coordinated dimeric complex in the series of MRI contrast agents: Na[Gd(H2O)AAZTA]∙3H2O

The crystal structures of the ligand AAZTA (6-amino-6-methylperhydro-1,4-diazepine tetraacetic acid) and of its Gd complex Na[Gd(H2O)AAZTA] · 3H2O have been determined by single crystal X-ray diffraction. The AAZTA ligand crystallizes in a zwitterion form with two deprotonated carboxylic groups and two protonated tertiary nitrogen atoms. Two independent molecules of about a similar conformation and three water molecules as asymmetric units, are present in the crystal. In the solid state the gadolinium complex is a centrosymmetric dimer with a bicapped square antiprismatic coordination geometry around each metal ion. Two symmetric bridging carboxylate groups determine the dimer formation with Gd–O(2) and Gd–O(2)′ bond distances rather comparable of 2.526(4) and 2.548(4) Å, respectively, while the Gd–O(1) (inner sphere water) bond distance is 2.443(5) Å. A network of hydrogen bonds between the water of inner and outer spheres and the AAZTA carboxylic groups is present in the crystal structure.