Yttrium(III) and lanthanide(III) metal complexes of an 18-membered hexaaza tetraimine macrocycle. Crystal structure of the gadolinium(III) complex

Abstract

Complexes of Y^III, La^III, Gd^III and Dy^III with an 18-membered hexaaza tetraimine macrocyclic ligand (L¹) containing backbone cyclohexyl units have been prepared and characterized. The crystal structure of [GdL¹(H₂O)₃]Cl₃·3H₂O shows a nine-co-ordinate gadolinium ion bound to three water oxygens and six nitrogen donors from the macrocyclic ligand. Solution studies with radiolabelled ⁹⁰Y indicated that, in competition with the very strong chelator diethylenetriamine-<i>N,N,N′,N″,N″</i>-pentaacetic acid (H₅dtpa), a ⁹⁰Y^IIIL¹ macrocyclic unit is maintained for 1 h. The relaxivity of the gadolinium(III) complex, [GdL¹(H₂O)₃]³⁺, is higher than those of [Gd(dtpa)(H₂O)]²– and [GdL(H₂O)]–(H₄L = 1,4,7,10-tetraazacyclododecane-<i>N,N′,N″,N‴</i>-tetraacetic acid), both of which are currently used as contrast agents in magnetic resonance imaging. A tetramethyl analogue of L¹, <i>i.e.</i> L², has been synthesized for the first time and a crystal structure determination showed that it adopts a stepped conformation, the direction of folding being dictated by the racemic conformation of the cyclohexane.