ALKALINE-EARTH METAL AND LANTHANIDE(III) COMPLEXES OF LIGANDS BASED UPON 1,4,7,10-TETRAAZACYCLODODECANE-1,7-BIS(ACETIC ACID)

The macrocyclic ligand DO2A (1,4,7,10-tetraazacyclododecane-1,7-bis(ac etic acid)) was prepared and used as a building block for four new mac rocyclic ligands having mixed side-chain chelating groups. These ligan ds and their complexes with Mg-II, Ca-II, and Ln(III) were studied ext ensively by potentiometry, high-resolution NMR, and water proton relax ivity measurements. The protonation constants of all compounds compare d well with those of other cyclen-based macrocyclic ligands. All Ca-II complexes were found to be more stable than the corresponding Mg-II c omplexes. Trends for the stabilities of the Ln(III) complexes are disc ussed and compared with literature data, incorporating the effects of water coordination numbers, Ln(III) contraction, and the nature of the side chains and the steric hindrance between them. H-1 NMR titrations of DO2A revealed that the first and second protonations take place pr eferentially at the secondary ring nitrogens, while the third and four th involved protonation of the acetates. O-17 NMR shifts showed that t he DyDO2A(+) complex had two inner-sphere water molecules. Water proto n spin-lattice relaxation rates for the GdDO2A(+) complex were also co nsistent with water exchange between bulk water and two inner-sphere G d-III coordination positions. Upon formation of the diamagnetic comple xes of DO2A (Ca-II, Mg-II, La-III, and Lu-III), all of the macrocyclic ring protons became nonequivalent due to slow conformational rearrang ements, while the signals for the acetate CH2 protons remained a singl et.