Polyoxometalate Ligands Reveal Different Coordination Chemistries Among Lanthanides and Heavy Actinides

Experimental studies involving actinide compounds are inherently limited in scope due to the radioactive nature of these elements and the scarcity and cost of their research isotopes. Now, ∼80 years after the introduction of the actinide concept by Glenn Seaborg, we still only have a limited understanding of the coordination chemistry of f-block metals when compared to more common elements such as the s-, p-, and d-blocks. This is particularly true for transplutonium actinides (Am, Cm, Bk, etc.) whose chemistry is often considered similar to trivalent lanthanides-mainly because of the lack of experimental data. We here report a metal-ligand system for which lanthanide and heavy actinide coordination compounds can be synthesized efficiently (i.e., requiring only a few micrograms) under identical conditions. Seventeen single crystal XRD structures of trivalent f-elements complexed to the polyoxometalate (POM) PW₁₁O₃₉ ^7- were obtained, including the full lanthanide series (Cs₁₁Ln(PW₁₁O₃₉)₂·nH₂O, Ln = La to Lu, except Pm), the equivalent yttrium compound, a curium-POM compound (α₂-Cs₁₁Cm(PW₁₁O₃₉)₂·33H₂O), and the first two Am³⁺-POM compounds structurally characterized (α₁-Cs₁₁Am(PW₁₁O₃₉)₂·6H₂O and α₂-Cs₁₁Am(PW₁₁O₃₉)₂·21H₂O). Importantly, this represents a unique series of compounds built on the same 1:2 metal:ligand unit and where all the f-elements are 8-coordinated and squared antiprismatic, thus providing a consistent platform for intra- and inter-series comparison. Despite a similar first coordination sphere environment, significant crystallographic and spectroscopic differences were observed among early and late lanthanides, as well as lanthanides and actinides, and even between americium and curium. These results show that even within the same coordination chemistry framework, 4f and 5f elements exhibit fundamental chemical differences that cannot be explained by simple size-match arguments. This study offers a versatile coordination platform to magnify differences within the f-block that have remained difficult to observe with traditional ligand systems.