Nuclear magnetic resonance and potentiometric studies of the protonation scheme of a triaza triacetic macrocycle and its complexes with lanthanum and lutetium

The protonation constants of the macrocyclic ligand 1,4,7-triazacyclononane-N,N’,N”-triacetic acid (NOTA) have been measured by potentiometry, and the protonation sequence of the various amino and carboxylate groups of NOTA has been studied in DzO as a function of pD from the chemical shifts of the nonlabile protons. Shielding constants for protonation of the amino groups were determined in a NMR study of the triaza macrocyclic amine, its trimethylated analogue, and NOTA and compared with values reported for linear polyamino polycarboxylates and cyclic tetraaza tetracarboxylate ligands. The results indicate that two nitrogens of NOTA are protonated at higher pH than the carboxylate groups. The last nitrogen is protonated only at very low pH. The sequence of protonation of NOTA supports the formation of hydrogen bonds between two protonated nitrogens and the adjacent two nonprotonated carboxylates. The ‘H and I3C spectra of the La(N0TA) and Lu(N0TA) species were studied as a function of pH and temperature. The aqueous complexes show spectra characteristic of flexible triaza macrocycles, displaying fast interconversion between the two staggered 6 and X conformations of the ethylenediamine rings even at room temperature. Above pH 9.5, the La(N0TA) and Lu(N0TA) hydroxo complexes start to form and their spectra show evidence of greater rigidity as at room temperature the 6 /X ethylenediamine ring conformational interconversions in the Lu(N0TA) hydroxo complex are slow on the NMR time scale.