Metal – Aminopolycarboxylic Acid Complexes. III. Studies of Lead(II) – Tetraethylenepentaamineheptaacetic Acid in Aqueous Solution by Proton Nuclear Magnetic Resonance Spectroscopy

Proton nuclear magnetic resonance (n.m.r.) spectroscopy and the pH dependence of the chemical shifts of the nonlabile protons have been used to determine the preferred protonation sites in tetraethylenepentaamineheptaacetic acid (TPHA). The nitrogen atoms are protonated more readily than the carboxylate groups with the sequence of protonation dependent on electrostatic interactions. The 1:1 Pb(II)–TPHA complex which is not protonated for solution conditions from pH 10 to 14, has five metal–nitrogen bonds. The coordinate bonds are labile so that rapid interconversion between nonequivalent configurations produces an average configuration in which the protons of the acetate groups exhibit single n.m.r. peaks. Protonation of the complex probably occurs in three stages. From pH 10 to pH 8 the preferred protonation sites are the terminal nitrogen atoms with the attendant elimination of the metal–nitrogen bonds. Increasing the acidity to pH 4 causes all but the central nitrogen site to be protonated. Below pH 4 the central nitrogen atom becomes protonated and causes further unwrapping of the complex.