Highly Stable Complexes of Divalent Metal Ions (Mg2+, Ca2+, Cu2+, Zn2+, Cd2+, and Pb2+) with a Dota-Like Ligand Containing a Picolinate Pendant

The stability constants of complexes of the macrocyclic ligand do3a-pic(4-) (H(4)do3a-pic = 2,2,2-{10-[(6-carboxypyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triyl}triacetic acid) with several divalent metal ions (Pb2+, Cd2+, Zn2+, Cu2+, Ca2+, and Mg2+) have been determined by using pH-potentiometric titrations (I = 0.1 M KCl, 25 degrees C). The stability of these complexes follows the trend Cu2+> Cd2+approximate to Pb2+approximate to Zn2+>>Ca2+>>Mg2+. A particularly high stability constant has been determined for the Cu2+ complex [logK(CuL) = 23.20(4)]. Analysis of the titration curves indicate the presence of protonated forms of the complexes in solution, with protonation constants of logK(M(HxL)) = 6.9-2.0 (x = 1, 2, or 3). The structure of the complexes in solution has been investigated by using H-1 and C-13 NMR spectroscopy and DFT calculations performed in aqueous solution at the TPSSh/6-31G(d) level. In the case of the Pb2+ and Cd2+ complexes, relativistic effects were considered with the use of relativistic effective core potentials. Calculations show that the complexes with the largest metal ions (Pb2+ and Ca2+) are nine-coordinate, with their coordination polyhedra being best described as capped twisted square antiprisms. The Cd2+ and Mg2+ complexes are seven-coordinate, with the metal ions being bound to the four nitrogen atoms of the cyclen unit and the three acetate pendant arms. Finally, in the Cu2+ and Zn2+ complexes, the metal ions are six-coordinated, with the metal ions being asymmetrically placed inside the macrocyclic cavity of the ligand, and the coordination polyhedra can be described as an octahedron and a trigonal prism, respectively.