Influence of a Heterocyclic Nitrogen-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by Aminopolycarboxylate Complexants

Abstract

The novel metal chelator N-2-(pyridylmethyl)diethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-PyM) was designed to replace a single oxygen-donor acetate group of the well-known aminopolycarboxylate complexant diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA) with a nitrogen-donor 2-pyridylmethyl. Potentiometric, spectroscopic, computational, and radioisotope distribution methods show distinct differences for the 4f and 5f coordination environments and enhanced actinide binding due to the nitrogen-bearing heterocyclic moiety. The Am, Cm, and Ln complexation studies for DTTA-PyM reveal an enhanced preference, relative to DTPA, for trivalent actinide binding. Fluorescence studies indicate no changes to the octadentate coordination of trivalent curium, while evidence of heptadentate complexation of trivalent europium is found in mixtures containing EuHL complexes at the same aqueous acidity. The denticity change observed for Eu suggests that complex protonation occurs on the pyridyl nitrogen. Formation of the CmHL complex is likely due to the protonation of an available carboxylate group because the carbonyl oxygen can maintain octadentate coordination through a rotation. The observed suppressed protonation of the pyridyl nitrogen in the curium complexes may be attributed to stronger trivalent actinide binding by DTTA-PyM. Density functional theory calculations indicate that added stabilization of the actinide complexes with DTTA-PyM may originate from π-back-bonding interactions between singly occupied 5f orbitals of Am and the pyridyl nitrogen. The differences between the stabilities of trivalent actinide chelates (Am, Cm) and trivalent lanthanide chelates (La-Lu) are observed in liquid-liquid extraction systems, yielding unprecedented 4f/5f differentiation when using DTTA-PyM as an aqueous holdback reagent. In addition, the enhanced nitrogen-donor softness of the new DTTA-PyM chelator was perturbed by adding a fluorine onto the pyridine group. The comparative characterization of N-(3-fluoro-2-pyridylmethyl)diethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-3-F-PyM) showed subdued 4f/5f differentiation due to the presence of this electron-withdrawing group.

Origin of Selectivity of a Triazinyl Ligand for Americium(III) over Neodymium(III)

“…In this regard, it has been noted that π‐back bonding occurs between a singly occupied 5f orbital from Am III and a pyridyl nitrogen atom in N ‐2‐pyridylmethyl‐diethylenetriamine‐ N , N′,N′ ′, N ′′‐tetraacetic acid (DTTA‐PyM) that is absent in lanthanide complexes . These studies augment recent work by Adam et al.…”

“…[12][13][14][15] In this regard, it has been noted that p-back bonding occurs betweenasinglyo ccupied 5f orbitalf rom Am III and ap yridyl nitrogen atom in N-2-pyridylmethyl-diethylenetriamine-N,N',N'',N''-tetraacetic acid (DTTA-PyM)t hat is absenti nl anthanide complexes. [16] These studies augmentr ecent work by Adam et al that made use of a 15 N-labeled BTP derivative to probet he solution complexation of Am III . [17] Considerably larger 15 NNMR shiftsw ere noted in the americium complex than with the corresponding lanthanides.…”

“…Similar observations have been reported in other N‐donor systems with Nd III and Am III , as well as in chalcogen‐based ligands systems when trivalent lanthanide and actinides are compared . We show that the contraction of the Am−N bonds is the result of back donation between 5f orbitals the EtBTP ligands similar to that observed in the Am III DTTA–PyM complex …”

“…[24,25] We show that the contraction of the AmÀNb onds is the result of back donation between 5f orbitals the EtBTP ligands similart ot hat observed in the Am III DTTA-PyM complex. [16] The solid-state absorption spectra of both compounds revealabroad band that spansf rom approximately 350 to 475 nm as shown in Figure2 [27] Calculated TD DFT absorption spectra for [Nd(EtBTP) 3 ] 3 + and [Am(EtBTP) 3 ] 3 + showeds trong similarities with respectt ot he experimental results provided in Figure3.H owever,t he f-f transition region is not predicted well by TD DFT,w hich is expectedi ft hese transitions involveachange in the multiplicity. Furthermore, the broad band in the 300-500nmr ange was slightly adjusted for ab etter fit in the Nd analogue( ca.…”

“…Electronic structure calculations and QTAIM analysis of these complexes supports that the shortened AmÀNb onds may be ac onsequence of back bonding between 5f orbitals and the EtBTP ligandsa sh as been observed in other pyridyl ligands. [16,34]…”