Synthesis, potentiometric and H-1 NMR study of protonation and complex formation of 1,4,7-triazacyclononane-1,4-diacetate

Protonation constants and the protonation scheme of 1,4,7-triazacyclononone -1,4-diacetate (NO2A) have been determined by pH potentiometry and H-1 NMR techniques; shielding constants valid for the entire pH range have been cal culated. It has been pointed out that the most basic site in the molecule i s the unsubstituted secondary amino group. The first two protonation steps belong to ring nitrogens, the third and fourth ones to the carboxylates; th e last nitrogen is protonated in very acidic solutions only. Stability cons tants of complexes of NO2A with selected divalent and trivalent metal ions were determined; with them no indication of kinetic inertness was found. In NO2A complexes the relative contribution of the triazacyclononane ring to the log K-ML values is greater for soft than for hard metal ions, compared to corresponding values for 1,4,7-triazacyclononane-1,4,7-triacetate.