Metal – Aminopolycarboxylic Acid Complexes. II. Studies of Triethylenetetraaminehexaacetic Acid and its Lead(II) Complexes in Aqueous Solution by Proton Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (n.m.r.) spectroscopy was used to determine the preferred protonation sites in TTHA. For its 1:1 complex with Pb(II) the following equilibrium constants for protonation were obtained (triethylenetetraaminehexaacetic acid ≡ H6A)[Formula: see text]The non-protonated complex is considered to have four coplanar (or nearly coplanar) metal–nitrogen bonds with the center carboxylate groups coordinated above and below this plane, and with the terminal carboxylate groups playing only a small part in the coordinate bonding. The first and second protonations of the complex occur preferentially at the terminal and center nitrogen atoms, respectively, on the same side of the complex, accompanied by breaking of the respective metal–nitrogen bonds. This causes partial unwrapping of the complex from one side. Rapid interconversion between configurations in which unwrapping and rewrapping occurs first from one side of the molecule and then from the other leads to simplified n.m.r. spectra.