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

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 one-dimensional (1D) ³¹P NMR, variable-temperature NMR, and spin-lattice relaxation time (T₁) 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 (La³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Yb³⁺, and Lu³⁺), in the presence of the model POM ligand PW₁₁O₃₉^7- was elucidated and revealed the concurrent formation of two stable complexes, [M^III(PW₁₁O₃₉)(H₂O)]^4- and [M^III(PW₁₁O₃₉)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 Am³⁺ and Cm³⁺ ions in solution. Furthermore, the study reveals a previously unnoticed periodicity break along the f-element series with the reversal of T₁ relaxation times of the 1:1 and 1:2 complexes and the preferential formation of the long T₁ species for the early lanthanides versus the short T₁ 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 difficult-to-study elements such as heavy actinides and other radionuclides.