Contrasting Trivalent Lanthanide and Actinide Complexation by Polyoxometalates via Solution-State NMR

Abstract

Deciphering the solution chemistry and speciation of actinides is inherently difficult due to radioactivity, rarity, and cost constraints, especially for transplutonium elements. In this context, the development of new chelating platforms for actinides and associated spectroscopic techniques is particularly important. In this study, we investigate a relatively overlooked class of chelators for actinide binding, namely, polyoxometalates (POMs). We provide the first NMR measurements on americium−POM and curium−POM complexes, using onedimensional (1D) 31 P NMR, variable-temperature NMR, and spin-lattice relaxation time (T 1 ) experiments. The proposed POM−NMR approach allows for the study of trivalent f-elements even when only microgram amounts are available and in phosphate-containing solutions where f-elements are typically insoluble. The solution-state speciation of trivalent americium, curium, plus multiple lanthanide ions (La 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Yb 3+ , and Lu 3+ ), in the presence of the model POM ligand PW 11 O 39 7− was elucidated and revealed the concurrent formation of two stable complexes, [M III (PW 11 O 39 )(H 2 O) x ] 4− and [M III (PW 11 O 39 ) 2 ] 11− . Interconversion reaction constants, reaction enthalpies, and reaction entropies were derived from the NMR data. The NMR results also provide experimental evidence of the weakly paramagnetic nature of the Am 3+ and Cm 3+ ions in solution. Furthermore, the study reveals a previously unnoticed periodicity break along the f-element series with the reversal of T 1 relaxation times of the 1:1 and 1:2 complexes and the preferential formation of the long T 1 species for the early lanthanides versus the short T 1 species for the late lanthanides, americium, and curium. Given the broad variety of POM ligands that exist, with many of them containing NMR-active nuclei, the combined POM−NMR approach reported here opens a new avenue to investigate difficultto-study elements such as heavy actinides and other radionuclides.

Chemical diversity and versatility of polyoxometalate ligands: homologous series of twenty-five new structures with polyoxometalates binding to alkali, alkaline earth, lanthanide, and actinide ions

“…If cations are present in solution, then XW11 can essentially act as a ligand, forming complexes such as [M(XW11O39)(H2O)n] xand [M(XW11O39)2] y-. [14,42,52] In prior work, first row transition metals and trivalent lanthanides have been bound to XW11 (X = B, Si, P, Ga, and Ge) (Figure 2a). More recently, BW11 and PW11 have also been shown, through micro-scale crystallization techniques, to bind and crystalize with trivalent heavy actinides Am/Cm(PW11)2.…”

“…More recently, BW11 and PW11 have also been shown, through micro-scale crystallization techniques, to bind and crystalize with trivalent heavy actinides Am/Cm(PW11)2. [14,42,43] Complexes of the Keggin ions Zr 4+ , Ce 4+ , and U 4+ have also been reported [53][54][55] but those compounds often contain different counterions or have been obtained under readily different conditions, impeding comparison. Thus far, there has been no systematic study of the chelating ability of the Keggin ion for cations and heteroelements across the periodic table.…”

“…We here focused on the Keggin structure as a potential universal chelating platform that provides a consistent coordination framework, with the goal of directly comparing the coordination chemistry of a broad range of cations across the periodic table, going from monovalent alkalis to tetravalent actinides. Based on our prior work on POM complexes with heavy trivalent actinides [14,[42][43][44] , we selected tungsten-based Keggin ligands. The Keggin ions used in the present study for cation complexation are derived from the parent Keggin structure XW12O40 n-(with X = P, B, Al, etc.).…”

Anticancer Drug Extraction from Plasma Samples Using Three-Dimensional Polyoxometalate-Based Supramolecular Frameworks as Sorbents

“…In this regard, the reactants (POMs, metal ions, and organic ligands) must be carefully chosen when constructing inorganic–organic hybrids based on POMs. Also, the structures of these hybrids are extremely dependent on the synthesis conditions like the molar ratio of the starting materials, pH, temperature, and time of the reaction due to the high sensitivity of POMs and coordination modes of ligands toward these factors. − …”

Characterization of the first Peacock–Weakley polyoxometalate containing a transplutonium element: curium bis-pentatungstate [Cm(W5O18)2]9−

“…The high specific activity and isotope scarcity are only some of the constraints that have limited studies with transuranic elements. Recently, certain POMs have been leveraged to crystallize their complexes in microgram quantities, 13,14 opening the avenue to studying their interactions with elements beyond plutonium (Fig. 1a).…”