YTTRIUM(III) AND LANTHANIDE(III) METAL-COMPLEXES OF AN 18-MEMBERED HEXAAZA TETRAIMINE MACROCYCLE - CRYSTAL-STRUCTURE OF THE GADOLINIUM(III)COMPLEX

Complexes of Y-III, La-III Gd-III and Dy-III with an 18-membered hexaa za tetraimine macrocyclic ligand (L(1)) containing backbone cyclohexyl units have been prepared and characterized. The crystal structure of [GdL(1)(H2O)(3)]Cl.3.3H(2)O shows a nine-co-ordinate gadolinium ion bo und to three water oxygens and six nitrogen donors from the macrocycli c ligand. Solution studies with radiolabelled Y-90 indicated that, in competition with the very strong chelator diethylenetriamine-N.N,N',N' ',N''-pentaacetic acid (H,dtpa), a (90)Y(III)L(1) macrocyclic unit is maintained for 1 h. The relaxivity of the gadolinium(III) complex, [Gd L(H2O)(3)](3+), is higher than those of [Gd(dtpa)(H2O)]2(-) and [GdL(H 2O)](-) (H(4)L = ,10-tetraazacyclododecane-N,N',N'',N'''tetraacetic ac id), both of which are currently used as contrast agents in magnetic r esonance imaging. A tetramethyl analogue of L(1), ie. L(2), has been s ynthesized for the first time and a crystal structure determination sh owed that it adopts a stepped conformation, the direction of folding b eing dictated by the racemic conformation of the cyclohexane.