Protonation and lanthanide(iii) complexation equilibria of a new tripodal polyaza-polycatechol-amine

Abstract

The sulfonated tripodal polyaza-polycatechol-amine ligand tris(2,3-hydroxy-5-sulfobenzylamine) ethyl)amine (STRENCAT) was synthesized and its protonation constants determined by potentiometry at 25 °C and in 0.1 mol dm⁻³ NaClO₄. In the adopted experimental conditions (2.5 < p[H⁺] < 11.5), six of the virtual thirteen protonation constants of STRENCAT were determined. The deprotonation sequence of STRENCAT, identified by UV-vis and NMR studies, is characterized by the initial loss of three catechol protons, between p[H⁺] 5 and 8, followed by the loss of three secondary ammonium protons at higher p[H⁺] (9 < p[H⁺] < 11.5). No deprotonation of the three, more basic, catechol sites of STRENCAT was observed even at the highest p[H⁺] studied. Complexation measurements run on lanthanide(III)–STRENCAT systems (Ln = La, Gd and Yb), in the same ionic medium and temperature, show that the ligand (partly protonated or completely deprotonated) is able to form soluble 1 : 1 and 1 : 2 metal–ligand complexes over the whole p[H⁺] range investigated. Due to the high affinity of lanthanide(III) for catecholate units, even at p[H⁺] near 10 and before ammonium deprotonation, STRENCAT forms tri-chelate complexes, in which the metal ions are hosted in the cage formed by the six catechol oxygen atoms, completely deprotonated. Deprotonation of secondary ammonium groups, although not directly involved in complexation, increases the stability of the 1 : 1 complexes. They prevail in solution at p[H⁺] > 10, even in excess of ligand, in all the metal–ligand systems.