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

Abstract

Abstract 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 (Pb 2+ , Cd 2+ , Zn 2+ , Cu 2+ , Ca 2+ , and Mg 2+ ) have been determined by using pH‐potentiometric titrations ( I = 0.1 M KCl, 25 °C). The stability of these complexes follows the trend Cu 2+ > Cd 2+ ≈ Pb 2+ ≈ Zn 2+ >> Ca 2+ >> Mg 2+ . A particularly high stability constant has been determined for the Cu 2+ complex [log K CuL = 23.20(4)]. Analysis of the titration curves indicate the presence of protonated forms of the complexes in solution, with protonation constants of log K M(HxL) = 6.9–2.0 ( x = 1, 2, or 3). The structure of the complexes in solution has been investigated by using 1 H and 13 C NMR spectroscopy and DFT calculations performed in aqueous solution at the TPSSh/6‐31G(d) level. In the case of the Pb 2+ and Cd 2+ complexes, relativistic effects were considered with the use of relativistic effective core potentials. Calculations show that the complexes with the largest metal ions (Pb 2+ and Ca 2+ ) are nine‐coordinate, with their coordination polyhedra being best described as capped twisted square antiprisms. The Cd 2+ and Mg 2+ 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 Cu 2+ and Zn 2+ 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.