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

Consolidated Curium Reprocessing Procedure Inspires Molecular Design for Sensitized Curium Circularly Polarized Luminescence

Curium's stable redox chemistry and ability to emit strong metal-based luminescence make it uniquely suitable for spectroscopic studies among the actinide series. Targeted ligand and coordination compound design can support both fundamental electronic structure studies and industrial safeguards with the identification of unique spectroscopic signatures. However, limited availability, inherent radioactive hazards, and arduous purifications have long inhibited such investigations of this element. A consolidated reprocessing procedure for curium has been developed for the milligram scale. The recovery of not only standard legacy curium samples but also hazardous legacy perchlorate containing curium samples was achieved, culminating in column chromatography utilizing the extraction resin DGA (N,N,N',N'-tetra-2-ethylhexyldiglycolamide, branched). Surprisingly, controlled elution of the Cm band from the extraction resin was followed through bright pink luminescence triggered by an inexpensive hand-held UV-vis lamp (380-400 nm). This observation inspired the design of an enantiopure, C2-symmetrical ligand bearing a chiral (trans-1,2-diaminocyclohexane) backbone with achiral DGA moieties (N,N,N',N'-tetra-n-octylacetamide), that enabled rarely observed curium circularly polarized luminescence upon metal chelation. These combined achievements should unlock more luminescence and circularly polarized luminescence studies of curium, and enable the recovery of many curium and other trivalent actinide samples.

From +I to +IV, Alkalis to Actinides: Capturing Cations across the Periodic Table with Keggin Polyoxometalate Ligands

Coordination chemistry trends across the periodic table are often difficult to probe experimentally due to limitations in finding a versatile but consistent chelating platform that can accommodate various elements without changing its coordination mode. Herein, we present new metal/ligand systems covering a wide range of ionic radii, charges, and elements. Five different ligands derived from the Keggin structure (HBW11O398-, PW11O397-, SiW11O398-, GeW11O398-, and GaW11O399-) were successfully crystallized with six different cations (Na+, Sr2+, Ba2+, La3+, Ce4+, and Th4+) and characterized by single-crystal X-ray diffraction. Twenty-five new compounds were obtained by using Cs+ as the counterion, yielding a consistent base formula of Csx[M(XW11O39)2]·nH2O. Despite having a similar first-coordination sphere geometry (i.e., 8-coordinated), the nature of the central cation was found to impact the long-range geometry of the complexes. This unique crystallographic data set shows that, despite the traditional consensus, the local geometry of the cation (i.e., metal-oxygen bond distance) is not enough to depict the full impact of the complexed metal ion. The bending and twisting of the complexes, as well as ligand-ligand distances, were all impacted by the nature of the central cation. We also observed that counterions play a critical role by stabilizing the geometry of the M(XW11)2 complex and directing complex-complex interactions in the lattice. We also define certain structural limits for this type of complex, with the large Ba2+ ion seemingly approaching those limits. This study thus lays the foundation for capturing the coordination chemistry of other rarer elements across the periodic table such as Ra2+, Ac3+, Bk4+, Cf3+, etc.

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.

Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(v) sandwich complex

The synthesis and characterization of a series of (TBA)2[M{Mo5O13(OMe)4NO}2] (M = Zr, Hf, Th, and U) sandwich complexes is reported. A preformed lacunary, Lindqvist-type, polyoxomolybdate-alkoxide cluster provides access to first examples of actinide-polyoxomolybdate sandwich complexes isolated under non-aqueous conditions. Incorporation of metal(iv) cations into this framework was found to "switch on" reversible redox chemistry at the {Mo5} ligands, with the Zr and Hf containing complexes accepting up to two electrons, while the Th and U derivates accommodate as many as four additional electrons. The enhancement of the redox properties of the cluster upon actinide incorporation is an exciting observation, presenting actinide "doping" as a novel approach for accessing functional redox-active materials. Oxidation of the uranium containing sandwich complex (TBA)2[U{Mo5O13(OMe)4NO}2], chemically or electrochemically, allows access to the U(v) centered species, which was characterized both spectroscopically and by single crystal X-ray diffraction. This represents the first example of a U(v)-polyoxometalate sandwich complex to be isolated and structurally characterized.

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

Leveraging microgram-level techniques, we here present the first transplutonium bis-pentatungstate complex: NaCs8Cm(W5O18)2·14H2O (CmW5). Single crystal XRD, Raman, and fluorescence characterization show significant differences relative to analogous lanthanide compounds. The study reveals the unsuspected impact of counterions on fluorescence and vibrational modes of the curium complex and its lanthanide counterparts.

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

Four self-assembled inorganic-organic hybrid materials, namely, H{Na(H2O)3[Gd(PDA)(H2O)2]3[BW12O40]}·4H2O (1), H{Na(H2O)3[Tb(PDA)(H2O)2]3[BW12O40]}·3H2O (2), H{Na(H2O)3[Er(PDA)(H2O)3]3[BW12O40]}·H2O (3) (PDA = 1,10-phenanthroline-2,9-dicarboxylate), and [Pr3(H2O)13(pydc-OH)2][BW12O40]·12H2O (4) (pydc-OH = 4-hydroxy-2,6-pyridinedicarboxylate), were hydrothermally synthesized and structurally characterized. Hybrids 1-3 are isostructural and contain a Keggin unit, which is linked to lanthanoids to produce distinct trinuclear lanthanoid building blocks. The fragments are connected by anion-π and hydrogen bonding interactions to create 3D networks. In hybrid 4, a trimeric Pr-organic species bearing a Keggin unit forms a 2D coordination polymer, and then hydrogen bonding interactions between 2D layers lead to the formation of a 3D structure. These polyoxometalate-based frameworks were used as sorbents for the dispersive microsolid-phase extraction (D-μSPE) of two anticancer drugs (doxorubicin and epirubicin) in human plasma samples. Analytes were quantified and separated using high-performance liquid chromatography with fluorescence detection (HPLC-FLD). The method's linearity was between 0.8-500 ng mL-1 and 1.0-500 ng mL-1 for the antineoplastic drugs doxorubicin and epirubicin, respectively. The limits of detection (S/N = 3) were in the range of 0.2-0.3 ng mL-1, while the precision was in the range of 3.5-4.3%. Finally, human plasma samples from patients treated with doxorubicin or epirubicin were analyzed by using the D-μSPE-HPLC-FLD method.