Synthesis and characterization of a novel macrocyclic ligand containing nine donor atoms

The synthesis of a novel macrocyclic ligand containing nine potential coordination sites (six nitrogens and three oxygens) formed by a triazacyclododecane ring trisubstituted with -CH2CH2-N(CH(2)Ph)-CH2COOH groups is reported. The protonation scheme of this ligand has been elucidated by measuring the proton NMR shifts of the various methylenic resonances upon changing the pH of the solution. This ligand affords stable metal complexes with Cd2+ ion, whose H-1- and C-13-NMR spectra suggest a remarkable kinetic inertia and stereochemical rigidity.