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

Abstract

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.

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Interests in polyoxometalates can be traced as far back as the 19th century, with the synthesis of ammonium phosphomolybdate (NH₄)₃(PMo₁₂O₄₀) first reported in 1826.¹ Since then, polyoxometalates, POMs, have permeated into various other fields, ranging from drug delivery to microelectronics, catalysts, energy storage, and more.^2–6 POMs can also act as metal chelators and, while their chemistry is incredibly diverse, POMs are typically derived from a handful of major structures, with notably, the Peacock–Weakley⁷e.g., [Mⁿ⁺(W₅O₁₈)₂]¹⁵⁻ⁿ, the Lindqvist⁸ (e.g., Nb₆O₁₉⁸⁻), the Keggin⁹ (e.g., PMo₁₂O₄₀ⁿ⁻), the Well–Dawson¹⁰ (e.g., P₂W₁₈O₆₂⁶⁻), the Preyssler¹¹ (e.g., [Mⁿ⁺P₅W₃₀O₁₁₀]¹⁵⁻ⁿ) ions, etc.

Perhaps not surprisingly, compared to other fields, POM chemistry has had little overlap with actinide chemistry thus far, and particularly for heavy actinides (Am, Cm, Bk, Cf…). The first instance of a polyoxometalate compound containing an actinide was reported in 1914, with the synthesis of a thorium(IV) polymolybdate (NH₄)₈(ThMo₁₂O₄₂).¹² However, it is not until after the 1970's that a renewed interest in incorporating actinide elements into polyoxometalate chemistry yielded additional structures. Moreover, while the actinide group consists of fifteen members, most of the data published on actinide POMs is about thorium and uranium. 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). However, the number of known actinide POMs with transplutonium elements is still very limited: only one with Am(IV),¹⁵ one with americyl(VI),¹⁶ and three with Cm(III).¹³

Fig. 1 Reaction scheme (a) and equation (b) for forming the curium bis-pentatungstate complex [Cm(W₅O₁₈)₂]⁹⁻. The starting curium(III) aqua ion is considered to have 9 water molecules, as previously reported by Skanthakumar et al.²⁴

One class of POMs that has historically received strong interest is the bis-pentatungstate–metal complex [M^x(W₅O₁₈)₂]^(12+x)−. Intense luminescent and optical properties when complexing lanthanide (Ln) ions, in solution and solid-state,^17–20 provide strong motivation to pursue analogous compounds with transplutonium ions, notably Cm³⁺, as it can undergo fluorescence with a wide variety of ligands.^21–23 Unlike the other POM structures that can complex metal ions, such as the Keggin or the Well–Dawson anions, the pentatungstate anion [W₅O₁₈]⁶⁻ (W₅), is often considered as the smallest and simplest POM ligand, both in terms of structure and synthesis. Fluorescence studies on aqueous solutions of W₅ and Cm³⁺ previously showed¹³ a ∼500 times increase in the luminescence of Cm³⁺ upon complexation to W₅, with a shift and splitting of the emission peaks (598.4 to 604.6 nm with multiple Stark levels). By analogy to the lanthanides, the expected complex formed in solution was [Cm(W₅O₁₈)₂]⁹⁻ (CmW₅). CmW₅ solutions exhibit the longest fluorescence lifetime for a Cm³⁺ complex reported so far (780 μs), and we hypothesized¹³ it was due to the complete removal of water molecules from the starting [Cm(H₂O)₉]³⁺ aqua ion (Fig. 1b).²⁴ However, the crystal structure of this kind of heavy actinide complex has remained elusive.

Herein we report the single crystal XRD structure of CmW₅, which represents the first heavy actinide compound of the Peacock–Weakley POM class. The cesium salt of CmW₅, fully formulated as NaCs₈[Cm(W₅O₁₈)₂]·14H₂O, crystallizes in the anorthic space group P1̄ (V = 2711.36 (6) ų). We further report the first [Ln(W₅O₁₈)₂]⁹⁻ compound with cesium counterions, Na₃Cs₆[Nd(W₅O₁₈)₂]·18H₂O, which crystallizes in the anorthic space group P1̄ (V = 2811.36 (6) ų). While the only distinguishable feature between the heavy actinide and the lanthanide is a difference in the number of Na⁺ and Cs⁺ counterions, the apparent crystallographic divergence preludes the spectroscopic differences observed for these compounds both in solution and solid-state, as discussed below.

A prior attempt to isolate this compound as a rubidium or cesium salt failed,¹³ but a cesium salt of CmW₅ was here successfully isolated as single crystals (Fig. 3) by modifying our recently developed microgram-level POM–metal complex synthesis.¹³ In this optimized synthesis, we only used 1.2 μg of ^247/246/248Cm³⁺, starting from a HCl solution (see Experimental section in the ESI†). Note that XRD-quality single crystals of CmW₅ could only be grown with Cs⁺ counterions (Fig. 3c and d). Yet, the structure is isomorphic with previously reported sodium and potassium salts of [Ln^III(W₅O₁₈)₂]⁹⁻ (Table S1, ESI†) and [An^IV(W₅O₁₈)₂]⁸⁻ (An = Th, U, and Np).^17,25 Briefly, the structure is comprised of two [W₅O₁₈]⁶⁻ ligands coordinating a central Cm³⁺. The W₅ ligand is built upon five edge-sharing octahedral tungstates, four of which corner-share to the central Cm³⁺ ion, forming an eight-coordinate square antiprismatic coordination environment for curium, with a nearly ideal D_4d symmetry. Bond lengths range 2.459(9)–2.508(9) Å for Cm–O, 1.715(6)–1.740(10) Å for W–O^terminal, 2.348–1.776 Å for W–O^bridging, and 2.967(10)–3.762(10) Å for Cs–O^Cm's. See ESI† for additional information on the synthesis and structure of CmW₅.

Interestingly, no water molecule is present in the first coordination sphere of the Cm³⁺ ion, which contrasts with previously studied heavy actinide complexes with organic ligands which are typically 9-coordinated and often have actinide-bound water molecules.^24,26,27 Moreover, in all the other W₅ reported structures with trivalent lanthanides (Table S1, ESI†), the Na⁺ counterions bind to several water molecules instead of direct ion-pairing with the POM observed for the Cs–CmW₅ structure. The lanthanide structures mainly featured hydrogen bonding interactions between the [Ln^III(W₅O₁₈)₂]⁹⁻ and the [Na(H₂O)_n]⁺ counterions. On the other hand, the CmW₅ salt reported herein is the only instance thus far of the W₅ complex crystallizing with Cs counterions.

The most noticeable effect of the Cs counterions is the complete removal of the 9 water molecules from the Cm³⁺ aqua ion,²⁴ and it manifests as increased bonding interactions among Cs⁺ ions, the POM, and the central actinide ion. Of the eight Cs counterions in the CmW₅ structure, five are close to curium (Cs–Cm distance of 4.48–4.54 Å) and bind directly to the bridging O in Cm–O–W (Cs–O^Cm distance of 2.967(10)–3.762(10) Å). The lone Na⁺ counterion found in the structure at ∼7 Å from Cm³⁺ favors binding with solvated water molecules, similar to what is observed in the lanthanide structures, and it only binds with the tungstate POMs via one yl oxygen between two complexes (Fig. 2). The interplay among Cm³⁺, W₅, and Cs⁺ ion leads to a unique geometry where Cm³⁺ is completely shielded from solvent molecules and surrounded by eight oxygens at ∼2.48 Å, and five Cs⁺ at ∼4.51 Å (Fig. 2).

Fig. 2 Polyhedral representations of the CmW₅ and NdW₅ compounds. (a) The [Cm(W₅O₁₈)₂]⁹⁻ complex, with counterions and solvated water molecules for clarity. (b) Asymmetric unit of the Cs–CmW₅ crystal, fully formulated as NaCs₈[Cm(W₅O₁₈)₂]·14H₂O. (c) Closeup view of the [CmO₈] square antiprismatic coordination, with five Cs counterions binding to all eight oxygens. (d) Asymmetric unit of the Na₉NdW₅ structure,¹³ showing no connectivity between complexes, (e) connectivity between complexes for Na₃Cs₆[Nd(W₅O₁₈)₂]. (f) Connectivity between complexes for NaCs₈[Cm(W₅O₁₈)₂]·nH₂O. Color code: Cm (orange), Cs (light blue), Na (green), W (maroon octahedra), O (red).

Control experiments were performed with Nd³⁺ (Fig. 2 and 3), as its size²⁸ is similar to Cm³⁺, by synthesizing both a cesium salt (Cs–NdW₅) and a sodium salt (Na–NdW₅). For Cm, a solid sample was also prepared with Na⁺ counterions (Na–CsW₅) but only micro to sub-microcrystals could be obtained (Fig. 3d) so that no single crystal XRD characterization could be performed. Raman microscopy experiments on Na₉[Ln^III(W₅O₁₈)₂] previously assigned the characteristics bands^13,17: 980–920 cm⁻¹ for the terminal yl’O stretching (W=O), 850–500 cm⁻¹ for the bridging O stretching in W–O–W, <350 cm⁻¹ for various vibrational modes for the entire complex, and lastly peaks at 890 and 200 cm⁻¹ correspond to the bridging O stretching in Ln–O–W. The Raman spectrum of Na–NdW₅ is consistent with previously reported spectra.^13,17 Interestingly, we observed a clear effect of the Cs⁺ counterions (Fig. 3b) with combined changes in Raman peak intensities and peak shifting. Although Raman intensities can typically vary slightly from crystal to crystal, significant differences were observed between CmW₅ and NdW₅. For CmW₅, the intensities of the terminal yl oxygens (W=O) increase, which could be due to increased Cs-complex interactions. For Nd–O–W, there is a decrease in intensity for the 890 cm⁻¹ peak and an increase for the 211 cm⁻¹ peak, which also shifts to 223 cm⁻¹. For CmW₅, an increase in Raman intensity for the terminal O and bridging O and peak shifting from 211 to 223 cm⁻¹ is also observed (Fig. 3a). However, the 890 cm⁻¹ peak corresponding to Cm–O–W is shifted to higher energy by 35 cm⁻¹ to 925 cm⁻¹, i.e., the opposite of what is observed for the Na/Cs–NdW₅ compounds. The blue shifting caused by the Cs on the bridging O in Cm–O–W is a clear indicator of the outsized role Cs⁺ ions on the [Cm(W₅O₁₈)₂]⁹⁻ complex as opposed to its lanthanide analogs. To the best of our knowledge, this represents the first observation of cesium–actinide interactions within a molecular complex.

Fig. 3 Solid-state Raman microscopy of CmW₅ and Nd–W₅ (a) [Cm(W₅O₁₈)₂]⁹⁻ with sodium (in orange) and cesium (in blue) counterions. (b) Similar experiments with [Nd(W₅O₁₈)₂]⁹. (c) CCDC image capture of Cs–CmW₅ single crystals. This picture was taken under UV-light – the pink-orange fluorescence highlights the presence of Cm³⁺. (d) Na–CmW₅ solution evaporated onto a microscopy slide. The material was crystalline, but the crystals were too small for XRD analysis. See Fig. S1 (ESI†) for additional crystal pictures.

The impact of Cs counter ions on CmW₅ was further investigated via fluorescence spectroscopy (Fig. 4 and 5). We recently reported¹³ solution-state fluorescence experiments on Na–CmW₅. The Cm³⁺ emission peak is centered at 604.6 nm and the fluorescence lifetime decay of 780 μs is the longest observed for a Cm³⁺ complex in solution. We performed similar Cm³⁺ fluorescence experiments with and without cesium counterions, and both in the solid-state (Fig. 4) and solution-state (Fig. 5). Consistent with the Raman spectroscopy results, the fluorescence properties of CmW₅ were also found to be impacted by the nature of the counterions. Fig. 4a shows the emission spectrum of single crystals of Cs–CmW₅ and a crystalline precipitate of Na–CmW₅. Switching the counterions from Na to Cs, the Cm³⁺ emission peak shifts from 604.6 to 608.0 nm. The structure of the emission peak of Cs–CmW₅ is also more pronounced, likely indicative of less disorder in the structure. We also measured the emission spectrum of Na–CmW₅ in solution at room temperature and as a flash-frozen solution at liquid-nitrogen temperature (−196 °C). The spectrum of the Na–CmW₅ solution (Fig. 4b) at room temperature is consistent with that of the solid (Fig. 4a). Upon cooling, the Na–CmW₅ emission peak becomes more structured, and we could confirm the main emission band is at 604.6 nm, consistent with the solid-state data. The results clearly indicate that the nature of the counterion impacts the energy levels of CmW₅.

Fig. 4 (a) Solid-state emission spectra of [Cm(W₅O₁₈)₂]⁹⁻ with Na and Cs counterions (b) fluorescence emission of aqueous solutions of Na–CmW₅ at room temperature (22 °C – pink curve) and after flash-freezing in liquid nitrogen (−196 °C – blue curve). Solvent: 1 : 4 vol/vol glycerol : H₂O. Excitation wavelength: 271.0 nm. See Fig. S2 and S3 (ESI†) for corresponding excitation spectra.

Fig. 5 Solution and time-resolved fluorescence. (a) and (b) Emission spectra of solutions of CmW₅ with sodium and cesium counterions, showing fluorescence quenching and lifetime shortening. (c) and (d) Similar experiments with NdW₅, showing opposite behavior. Both CmW₅ and EuW₅ complexes are under 100 mM acetate buffer at a pH of 5.5. [EuW₅] = 1 mM. [CmW₅] = 100 μM (Cs–CmW₅ spectra was adjusted to compensate for the dilution for the addition of 3 M CsCl, final concentration of Cs–CmW₅ was 50 μM). See Fig. S4 and S5 (ESI†) for corresponding excitation spectra.

To further confirm the results, Fig. 4a shows the emission spectra of a Na–CmW₅ solution (consistent with previously reported data¹³) and the same sample after addition of Cs⁺ counterions. A significant quenching of the luminescence is observed, with the emission intensity decreasing by an order of magnitude. A shortening of the lifetime to about 515 μs and a peak shifting from 604.6 to 606.0 nm were also observed (Fig. 5a and b). This strongly indicates that Cs⁺ counterions, typically considered spectator ions, actually influence bonding and energy levels within the CmW₅ complex.

The relatively close proximity between Cm³⁺ and Cs⁺ ions in the crystal structure (Fig. 2) suggests that Cs⁺ may provide a relaxation pathway for Cm³⁺ upon excitation, akin to water molecules, hence leading to shortening of the luminescence lifetime. Another hypothesis is that ion-pairing formation between Cs and W₅ is strong enough to change the excited state energy levels of the POM, leading to less efficient fluorescence sensitization of Cm³⁺ luminescence.

Similar solution-state experiments were performed with Eu³⁺, due to its relatively similar size to Cm³⁺ and fluorescence properties (Fig. 5c and d). Solutions of Na–EuW₅ exhibit the typical multi-peak emission band of a Eu³⁺ complex, with main peaks at ∼680, ∼620, ∼650, and ∼700 nm (Fig. 5c). The measured fluorescence lifetime of 2385 μs indicates that no water molecule is present.²⁹ However, contrary to curium, when an excess of Cs⁺ ions is added, the emission intensity of EuW₅ increases significantly, by a factor of ∼10, and the lifetime increases from 2385 to 3431 μs. This, combined with the Cm results indicate the Cs ions impact the energy levels of the POM.

In conclusion, this study reports the first example of a Weakley–Peacock-type polyoxometalate complex with a transplutonium element, namely the curium bis-pentatungstate NaCs₈Cm(W₅O₁₈)₂·14H₂O. This compound was synthesized at the microscale and characterized via single crystal XRD, Raman microscopy, as well as solid-state and solution-state fluorescence. Comparative experiments with Nd³⁺ and Eu³⁺ analogous compounds revealed a divergence between lanthanide and actinide coordination chemistries. This study further highlights the unsuspected but critical impact that counterions, particularly cesium, have on the fluorescence and vibrational modes of [Cm(W₅O₁₈)₂]⁹⁻. These results show that, even within the same coordination chemistry framework (i.e., 8-coordinated complexes with square antiprismatic geometry), 4f and 5f cations exhibit fundamental chemical differences that can be manifested at long-range and even via interactions with other cations present in the structure. Given the high number of heavy tungsten atoms in the POM compounds, in addition to many oxygens and presence of the heavy actinide, computational work (e.g., density functional theory) to explain the fundamental origin of these experimental observations are currently too expensive. Further compounding this, the results presented herein indicate that computational treatment of f-element POM complexes will require considering the full lattice (not just [Cm(W₅O₁₈)₂]⁹⁻, for example), including all cesium or sodium counterions and water molecules. Future work will focus on expanding the experimental space for actinide–POM compounds in order to provide a consistent framework for future theoretical work on this, so far, overlooked class of actinide compounds.

This material is based upon work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry program at Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Release number: LLNL-JRNL-861963.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2333195 and 2326544. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc01381f

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