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

Abstract

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(1I)-TPHA complex which is not protonated for solution conditions from p H 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 p H 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.La spectroscopie r.m.n. et la dCpendance du p H avec les dCplacements chimiques des protons non labiles ont Ct C utiliskes pour dtterminer les sites de protonation prefirentiels dans I'acide tttraCthyltnepentaamineheptaacktique (TPHA). Les atomes d'azote sont protones plus rapidement que les groupes carboxylates; la sCquence de protonation dtpendant des interactions tlectrostatiques. Le complexe 1 :1 Pb(I1)-TPHA qui n'est pas proton6 pour des solutions ayant un p H compris entre 10 ef 14, possede cinq liaisons mCtal-azote. Les liaisons coordonnees sont labiles et de ce fait, I'interconversion rapide entre des configurations non Cquivalente, produit une configuration moyenne dans laquelle les protons des groupes acCtates ne montrent que des singulets. La protonation du complexe se fait probablement en trois Ctapes. Entre pH 10 et p H 8 les sites de protonation prCfCrentiels sont les atomes d'azote terminaux; ceci se faisant avec rupture substquente des liaisons mttal-azote. L'augrnentation de l'aciditt jusqu'i pH 4 provoque la protonation de tous les sites A l'exception de I'atome d'azote central. En dessous de p H 4 I'atome d'azote central se protone et provoque le dtroulement ultkrieur du complexe.

An nmr protonation study of metal diethylenetriaminepentaacetic acid complexes: An integrated senior chemistry experiment

“…As one adds acid to the fully basic form of DTPA, each site (carboxyls and nitrogens) is protonated for a certain average fraction of the time at any given pH. The pmr spectra indicate that the methylenic protons (type a, b, c, and d) are deshielded by protonation of a nearby basic site and by an amount which depends upon the nature of the basic site, its proximity to the particular methylene protons, and the fraction of time protonated (10)(11)(12)(13)(14). Since there are three basic nitrogen sites, two of which are equivalent by symmetry, a student might ask readily which sites are protonated at one, two, and three equivalents of acid per equivalent of the basic DTPA anion.…”

Nuclear magnetic resonance spectrometry

“…Among the organolead compounds investigated by proton NMR, a study of tetra(2-furyl)-and tetra-(2-thienyl)lead (2040), acceptor properties of some alkynyllead compounds (2041), the magnetic moment of lead-207 (2042), studies of lead(II)-tetraethylenepentamine heptaacetic acid (2043,2044), some aryllead tricarboxylates (2045), and the complexing of diethylzinc (2046) have been presented. Lead-207 satellite spectra and lead-207 proton long range coupling constants of the symmetrical isomers of tetrafuryl-and tetrathienyllead (2047) have been exhibited.…”