SYNTHESIS, CHARACTERIZATION, AND CRYSTAL-STRUCTURES OF M(DO3A) (M = FE, GD) AND NA[M(DOTA)] (M = FE, Y, GD)

Macrocyclic amino carboxylate (DOTA and DO3A, where DOTA is 1,4,7,10-tetrakis(carboxymethyl)- 1,4,7,10-tetraazacyclododecane and DO3A is 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane) complexes of Gd-(III) and Fe(III) have been synthesized, characterized, and evaluated for use as potential contrast agents for magnetic resonance imaging (MRI). X-ray single-crystal analyses reveal four nitrogens and three oxygens coordinated to the iron in both Na[Fe(DOTA)]-5H2O and Fe(DO3A).3H2O. None of the waters are coordinated to iron atoms; the former iron complex has an uncoordinated carboxylate arm. Magnetic susceptibility and Mossbauer data confirm that both of the Fe(III) complexes are high spin (S = 5/2) compounds. Enneacoordination about the metal in the crystallographically isostructural series Na[M(DOTA)(H2O)].4H2O (M = Gd, Y, Eu) involves the four nitrogen atoms, an oxygen from each carboxyl arm, and one water molecule. Gd(DO3A) has been crystallized as a novel hydrated complex [{Gd(DO3A)}3.Na2CO3].17H2O in which each enneacoordinate Gd atom is coordinated to the four nitrogens and an oxygen from each carboxyl arm of the ligand, as well as two oxygens of the carbonate ion. No water molecule is coordinated to the Gd atoms in the crystal structure. The Gd(DO3A) and Gd(DOTA)-species are more effective water proton relaxation agents than the Fe(III) chelates.