Potentiometry and NMR studies of 1,5,9-triazacyclododecane-N,N',N''-triacetic acid and its metal ion complexes

Abstract

A new chelating macrocycle, 1,5,9-triazacyclododecane-N,N’,N’’-triacetiacc id (DOTRA), has been synthesized, and its complexes with Zn2’, Cd2+, Ca2+, Mg2+, and Mn2+ have been examined by potentiometry and NMR spectroscopy. The first protonation constant of the free ligand (log K, = 12.8) was determined spectrophotometrically, while the remaining were evaluated from potentiometric data (log K2 = 7.55, log K, = 3.65, log K4 = 2.1). DOTRA forms complexes with Mn2+, Mg2’, and Ca2+ rather slowly but reacts much more rapidly with Zn2+ and Cd2+. As reported previously for the nine-membered-ring triaza analogue NOTA, DOTRA also forms a more stable complex with Mg2+ than with Ca2+ (log K,, = 7.1 versus 6.0). High-resolution NMR spectra of the Zn2+, Cd2+, and Mgzt complexes (log K,, = 19.0, 15.7, and 7.1, respectively) indicate the three six-membered chelate rings are symmetrical and quite rigid in aqueous solution, as evidenced by nonequivalence of all six chelate ring protons.