SYNTHESES OF NEW 15-MEMBERED AND 16-MEMBERED MACROCYCLIC LIGANDS WITH 3 PENDANT ACETATO GROUPS AND THE STRUCTURES OF THE GADOLINIUM(III) COMPLEXES

A condensation of diethylenetriaminepentaacetic dianhydride with ethylenediamine gave a 15-membered macrocyclic ligand with three pendant acetato groups, (15-dtpa-en)H-3=C10H18N5O2(CH2CO2H)3; a 16-membered analogue, (16-dtpa-pn)H-3 = C11H20N5O2(CH2CO2H)3, was obtained by the use of propanediamine instead of ethylenediamine. The structures of their gadolinium(III) complexes, Gd2(15-dtpa-en)2.16H2O and Gd(16-dtpa-pn).4H2O, were determined by X-ray analyses. Gd2(15-dtpa-en)2.16H2O crystallized in the orthorhombic space group Pbca with: a=18.205(1), b=18.930(1) and c=15.609(1) angstrom. Two Gd(III) ions are located between two ligand molecules, forming a binuclear metal chelate molecule with a center of inversion. The coordination geometry around a metal ion is described as a distorted tricapped trigonal prism that consists of nine coordinated atoms. Gd(16-dtpa-pn).4H2O crystallized in the monoclinic space group P2(1)/c with: a=8.246(2), b=14.995(3), c=19.367(4) angstrom and beta=90.258(2)degrees. In this compound, a water molecule and a single ligand molecule are coordinated to a Gd(III) ion, forming a mononuclear chelate with a tricapped trigonal prism. The structural differences between the two Gd(III) complexes are a result of the differences in the favorable conformations assumed by the two macrocyclic ligands.