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

Abstract

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