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 Pm) were determined by potentiometry. Six protonation sites and three metal-ligand complexes [ML, MHL, and MHL(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 ∼10 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 Am 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.

Influence of a Pre‐organized N‐Donor Group on the Coordination of Trivalent Actinides and Lanthanides by an Aminopolycarboxylate Complexant

“…Figure shows the correlation between log β 101 for trivalent gadolinium and the total ligand acidity (Σp K a ) for aminopolycarboxylates forming five‐membered chelates upon metal coordination. Structures with variable denticity and nature of the substituents adhere to this linear relationship, including ligands recently investigated by our group . Figure shows a consistent departure from this linearity when the aminopolycarboxylate reagents contain pre‐organized motifs …”

“… Linear free‐energy correlation for log β 101 for GdL complexes with total ligand acidity (Σp K a ) for aminopolycarboxylates forming five‐membered chelates upon metal coordination. In order of increasing chelate stability, the complexants adhering to the linear relationship are: (1) nitrilotriacetate, (2) ethylenediamine‐ N , N′ ‐di(acetylglycine)‐ N , N′ ‐diacetate, (3) N ‐hydroxyethane‐ N , N′ , N′ ‐triacetate, (4) ethylenediamine‐ N , N , N′ , N′ ‐tetraacetate, (5) diethylenetriamine‐ N , N′′ ‐di(acetylglycine)‐ N , N′ , N′′ ‐triacetate, (6) N ‐butyl‐2‐acetamide‐diethylenetriamine‐ N , N′ , N′′ , N′′ ‐tetraacetic acid, (7) diethylenetriamine‐ N , N , N′ , N′′ , N′′ ‐pentaacetic acid . Rigid complexants deviating from the linear correlation are: N ‐2‐methylpicolinate‐ethylenediamine‐ N , N′ , N′ ‐triacetic acid (EDTA‐Mpic), trans ‐1,2‐diaminocyclohexane‐ N , N , N′ , N′ ‐tetraacetate (CDTA), 4‐azido‐2,6‐{[bis(ethylcarboxymethyl)amino]methyl}pyridine (Py), N , N′ ‐bis(2‐methylpicolinate)‐ethylenediamine‐ N , N′ ‐diacetic acid (H 4 octapa), and 1,4,7,10‐tetraazacyclodecane (H 4 dota) …”

“…The preparation of lanthanide and actinide stock solutions followed procedures described previously . The concentrated actinide stock solutions in 0.01 m HClO 4 were standardized by using spectroscopy.…”

“…We recently reported several structural modifications of DTPA, replacing one and two acetate groups with amide pendant arms. The introduction of the electron‐withdrawing influence of the amide group(s) reduced the total ligand acidity (Σp K a ) of these complexants.…”

“…The modifications inhibited ligand protonation, while promoting metal‐ion complexation in an aqueous environment of increased acidity. The first‐order dependence of the dissociation of the metal complex on the concentration of the hydrogen ion was reflected in faster equilibration rates of the investigated liquid/liquid metal‐ion partitioning systems …”