Coordination Chemistry and f-Element Complexation by Diethylenetriamine-N,N″-bis(acetylglycine)-N,N',N″-triacetic Acid.

Abstract

Potentiometric and spectroscopic techniques were used to evaluate the coordination behavior and thermodynamic features of trivalent f-element complexation by diethylenetriamine-N,N″-bis(acetylglycine)-N,N',N″-triacetic acid (DTTA-DAG) and its di(acetylglycine ethyl ester) analogue [diethylenetriamine-N,N″-bis(acetylglycine ethyl ester)-N,N',N″-triacetic acid (DTTA-DAGEE)]. Protonation constants and stability constants of trivalent lanthanide complexes (except Pm3+) were determined by potentiometry. Six protonation sites and three metal-ligand complexes [ML2-, MHL-, and MH2L(aq)] were quantified for DTTA-DAG. Four protonation sites and one metal-ligand complex [ML(aq)] were observed for DTTA-DAGEE, consistent with the presence of two ester groups. Absorption spectroscopy was utilized to measure the stability constants for complexation of trivalent neodymium and americium by DTTA-DAG and trivalent neodymium by DTTA-DAGEE. The coordination environment of trivalent europium in the presence of DTTA-DAG was investigated at various acidities by luminescence lifetime measurements. Decay constants indicate one water molecule in the inner coordination sphere across the 1.0 < pH < 5.5 range, presumably due to octadentate coordination by DTTA-DAG. A trans-lanthanide pattern of complex stabilities for DTTA-DAG was found to be analogous to that observed for DTPA, with a ∼106 reduction of the complex stability. The lessened strength of complexation, relative to DTPA, was attributed to significant reduction of the total ligand basicity for DTTA-DAG due to the electronic influence of amide functionalization. When DTTA-DAG is used as an aqueous holdback complexant in liquid-liquid distribution experiments, the preferential coordination of Am3+ in the aqueous environment offers efficient An/Ln differentiation. The separation extends to pH 2 conditions, where the kinetics of phase transfer in such liquid-liquid systems are aided by the acid-catalyzed dissociation of a metal/aminopolycarboxylate complex.