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

Regueiro-Figueroa, Martín and Ruscsák, Erika and Fra, Laura and Tircsó, Gyula and Tóth, Imre and de Blas, Andrés and Rodríguez-Blas, Teresa and Carlos, Platas-Iglesias and David, Esteban-Gómez (2014) Highly Stable Complexes of Divalent Metal Ions (Mg2+, Ca2+, Cu2+, Zn2+, Cd2+ and Pb2+) with a dota-like Ligand Containing a Picolinate Pendant. EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, 36. pp. 6165-6173. ISSN 1434-1948

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Abstract

The stability constants of complexes of the macrocyclic ligand do3a-pic4- (H4do3a-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 °C). The stability of these complexes follows the trend Cu2+>Cd2+~Pb2+~Zn2+>>Ca2+>>Mg2+. A particularly high stability constant has been determined for the Cu2+ complex [logKCuL = 23.20(4)]. The analysis of the titration curves indicates the presence of protonated forms of the complexes in solution with protonation constants of logKM(HxL) = 6.9 – 2.0 (x=1, 2 or 3). The structure of the complexes in solution has been investigated by using 1H and 13C 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-coordinated, their coordination polyhedra being best described as capped twisted square antiprisms. The Cd2+ and Mg2+ complexes are seven-coordinated, 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.

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