Autor(es):
Geraldes, C. F. G. C. 
; Alpoim, M. C. ; Marques, M. P. M. ; Sherry, A. Dean ; Singh, M.
Data: 1985
Identificador Persistente: http://hdl.handle.net/10316/10515
Origem: Estudo Geral - Universidade de Coimbra
Descrição
The protonation constants of the macrocyclic ligand 1,4,7-triazacyclononane-N,N’,N”-triacetiacc id (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. http://dx.doi.org/10.1021/ic00217a034
