Isolation of a new polyoxometalate complex of plutonium

Abstract

We report the synthesis, plus structural and spectroscopic characterization, of a plutonium(IV) complex with the polyoxometalate W₅O₁₈⁶⁻. The complex was isolated as Cs₈[Pu(W₅O₁₈)₂]·CsCl·6.5H₂O, with the tetravalent actinide cation sandwiched between two lacunary polyoxotungstate anions. It represents a rare case of a POM compound with plutonium and also the first Peacock–Weakley type complex of plutonium.

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Thus far, a few hundred plutonium (Pu) compounds have been synthesized and characterized, encompassing different categories of materials, from metallic phases and oxides^1,2 to complexes with organic ligands,^3–5 inorganic compounds,^6–10 MOFs,^11,12 oxo/hydroxo-clusters,^13,14 and more. However, only a small number of Pu complexes with polyoxometalates (POM) are currently known. In fact, only two crystal structures of POM complexes with the Pu⁴⁺ ion^15,16 and two crystal structures with the PuO₂²⁺ ion^17,18 have been published to date.

Sokolova et al.¹⁵ reported in 2009 the first successful synthesis and structural characterization of a complex of Pu⁴⁺ with a POM ligand. The authors obtained K₁₂H₄Pu(P₂W₁₇O₆₁)₂·19H₂O, which consists of two P₂W₁₇O₆₁¹⁰⁻ ligands bound to one Pu⁴⁺ ion. This complex belongs to the Wells–Dawson POM category. In 2016, Charushnikova et al.¹⁶ expanded the chemistry of Pu–POM to polyoxomolybdates, with the structural characterization of Pu⁴⁺ encapsulated in Mo₁₂O₄₂¹²⁻, which belongs to the Dexter-Silverton POM category. Another major category of POMs¹⁹ is the Peacock–Weakley complexes²⁰ which typically features one cation coordinated to two W₅O₁₈⁶⁻ (W5) ligands (Fig. 1). However, despite the large body of literature on W5 and on Pu chemistries, no plutonium compound with this type of POM has been reported yet.

Fig. 1 Reaction scheme and equation for forming the water-soluble plutonium(IV) complex with the POM ligand W₅O₁₈⁶⁻, formulated [Pu(W₅O₁₈)₂]⁸⁻. The complex was isolated in the solid-state as Cs₈[Pu(W₅O₁₈)₂]·CsCl·6.5H₂O.

The W5 anion has been has the subject of numerous studies due to its ability to complex a wide range of cations in aqueous and non-aqueous media, as well as the versatility of the compounds for a broad range of applications. For example, the W5 complexes with lanthanides can be used as tuneable luminescent materials.²¹ The magnetic properties of W5 compounds have also been the subject of many experimental and theoretical studies.^22,23 Some lanthanide–W5 complexes function as molecular nanomagnets with long coherence times, and have been proposed for quantum applications.^24,25

In terms of tetravalent cations, the W5 complexes with Zr⁴⁺, Ce⁴⁺, Th⁴⁺, U⁴⁺, and Np⁴⁺ have been crystallized and their structures determined. Na₈[U(W₅O₁₈)₂]·30H₂O was first reported by Golubev et al.²⁶ in 1975, Na₈[Ce(W₅O₁₈)₂]·31H₂O by Rosu & Weakley²⁷ in 1998, Na₈[Th(W₅O₁₈)₂]·28H₂O by Griffith et al.²⁸ in 2000, (Me₄N)₂[ZrW₅O₁₈(H₂O)₂(DMSO)] by Carabineiro et al.²⁹ in 2006, K₄Na₃H[Np(W₅O₁₈)₂]·16H₂O by Villars et al.³⁰ in 2012. We note that none of the protocols used in these prior studies would be directly transposable to plutonium chemistry due to the large amounts of materials required, with tens to hundreds of milligrams of the target element engaged in the reactions (Table S1, ESI†).

Herein, we describe efforts to overcome the radiolytic constraints associated with plutonium isotopes by leveraging a microscale technique that we previously used for other actinide–POM systems,^31–34 and completed the series of tetravalent W5 complexes. Using only 3.4 micrograms of weapons-grade Pu (Fig. 1 and Table S1, ESI†), we managed to crystalize the compound as its caesium salt, Cs₈[Pu(W₅O₁₈)₂]·CsCl·6.5H₂O (Cs₈Pu(W₅)₂).

Cs₈Pu(W₅)₂ was prepared by adding 1 eq. of Pu⁴⁺ to an aqueous solution containing ∼50 eq. of sodium tungstate (Na₂WO₄) and buffered at pH 6 by 0.5 M sodium acetate. At the pH of the reaction, the WO₄²⁻ anions are unstable and readily undergo hydrolysis and condensation to form POMs. In the absence of cations, non-lacunary paratungstate salts can be obtained. In the presence of cations, like Pu⁴⁺, the speciation is driven towards the complexation of the cation by W5 anions, resulting in a 1 : 2 complex, [Pu(W₅O₁₈)]⁸⁻. With subsequent addition of excess counterions (e.g., alkali), the W5 complexes often precipitate or, under suitable conditions, crystallize. Upon addition of excess Cs⁺ ions (Cs : W5 : Pu ratio 1.3 × 10⁵ : 10 : 1), several crystals of Cs₈Pu(W₅)₂ were obtained (Fig. 1 and Fig. S1, ESI†). Due to the small scale of the reaction and low Pu and W5 concentrations (i.e., [Pu]_initial = 24 μM, [W5]_initial = 240 μM), the Cs₈Pu(W₅)₂ crystals only appeared after about 3 months, and no by-product was co-crystallized. The synthesis was done at room temperature, in water, was one-pot, and without any complex steps like filtrations that can be detrimental to experiments with Pu.

For other elements, the obtention of some XRD-quality crystals can often be solved by scaling-up the synthesis to several milligrams of more (Table S1, ESI†). In the case of Pu, this kind of approach cannot be used due to its radiotoxicity. Even at the milligram scale, reactions with Pu represent a significant radiological hazard, plus logistical and financial burden. Syntheses at this scale can also result in radiolytic damage to the compound, so that scientists often resort to using longer-lived isotopes, i.e., ²⁴²Pu or ²⁴⁴Pu (Half-lives = 3.73 × 10⁵ and 8.13 × 10⁷ years, respectively), which are only made in minute quantities for research and are highly expensive. We overcame these issues by minimizing the reaction scale, down to a few micrograms of the target element. The small scale allowed us to directly use weapons-grade Pu (i.e., 94% ²³⁹Pu, 6% ²⁴⁰Pu, see ESI† for details), an isotope mixture that is less costly to produce and isolate than research-grade ²⁴²Pu or ²⁴⁴Pu. The reaction was done with 3.4 μg of ^WGPu, corresponding to 8.6 kBq (or 233 nCi). At this scale, the use of ²³⁸Pu (Half-life = 87.7 years), which is another Pu isotope that is easier to produce than ²⁴²Pu or ²⁴⁴Pu, could also be considered knowing that 3 μg of ²³⁸Pu corresponds to ∼1.9 MBq (∼50 μCi), which is still a manageable activity level. In future studies, we will leverage our microscale POM synthesis to probe isotopic effects on the properties of actinide coordination compounds.

The crystals obtained were first analyzed via Raman microscopy. Fig. 2 shows the solid-state spectrum of the compound. The Raman features confirmed that the crystal contained a W5 complex. The Raman spectrum of Cs₈Pu(W₅)₂ is readily different from that of its starting material (Na₂WO₄·2H₂O, Fig. S2, ESI†). The Raman peaks pattern for Cs₈Pu(W₅)₂ is consistent with that of Na₈[Th(W₅O₁₈)₂]·28H₂O, Na₈[Ce(W₅O₁₈)₂]·31H₂O and Cs₈[U(W₅O₁₈)₂]·12H₂O previously reported.^28,35,36 By analogy to an experimental and TD-DFT study performed on the equivalent complex with Ce⁴⁺ by Roy et al.,³⁶ the features in the 950–1000 cm⁻¹ region are characteristic of terminal ν(W = O_t) stretching modes, the bands at 500–850 cm⁻¹ correspond to bridging ν(W–O–W) and ν(W–O–Pu) stretches. The multiple bands around 400 cm⁻¹ correspond to the bending, rocking, and twisting modes of the complex, while the lower energy bands around 200 cm⁻¹ represent Pu–O stretches.³⁶

Fig. 2 Solid-state Raman spectrum of the plutonium(IV) complex with the POM ligand W₅O₁₈⁶⁻ (purple), and background from the sample holder (grey).

Crystals of the sample were also analysed by solid-state UV-visible absorbance microspectrophotometry. As the crystal obtained were thin plates, their absorbance was relatively faint, but they appeared slightly red-orange under an optical microscope (Fig. 3a). This colour is often a characteristic of Pu⁴⁺ compounds.^1,37 The UV-visible spectrum confirmed the presence of Pu⁴⁺ in the compound, with no sign of other oxidation states of Pu. The Cs₈Pu(W₅)₂ crystals exhibit main absorbance bands at 459, 472, 519, and 650 nm (Fig. 3b). These bands are consistent with the characteristic absorbance features observed for our reference solution of Pu⁴⁺ (Fig. S3, ESI†) and other reference spectra for the +IV oxidation state of Pu.^1,38 As expected, the ligand field of the two POM ligands induces some minor shifts relative to aqueous Pu(IV). Similar shifts have been reported for Pu⁴⁺ complexes with donating oxygens, such as oxalates,³⁹ peroxides,⁹ or organic ligands.³

Fig. 3 Solid-state absorbance spectrum of the plutonium(IV) complex with the POM W₅O₁₈⁶⁻. (a) Raw spectrum. Inset: A cluster of crystals used for the analysis. (b) Baseline subtracted spectrum, highlighting the transitions that correspond to Pu⁴⁺.

Single crystal XRD analysis definitively confirmed the isolation of a new POM complex containing Pu⁴⁺. The isolated compound follows the same general speciation as the previous cases with other tetravalent metals,^{26–28,30,35} with formation of 1 : 2 species, i.e., [Pu(W₅O₁₈)]⁸⁻. The obtained compound is fully formulated as Cs₈[Pu(W₅O₁₈)₂]·CsCl·6.5H₂O. It crystallizes in anorthic space group P1̄, with a cell volume of 5056.8(1) A³ (see Table S2 for full crystallographic details, ESI†). The asymmetric unit that describes half the unit cell is comprised of two unique [Pu(W₅O₁₈)₂]⁸⁻ complexes (Fig. 4). Cs⁺ counterions then connect the [Pu(W₅O₁₈)₂]⁸⁻ units together creating an extended three-dimensional network. Excess solvent water molecules and chloride ions can also be found connected throughout the Cs–Pu–W5 network (Fig. S4–S6, ESI†).

Fig. 4 Structure of the Cs₈Pu(W₅)₂ compound showing (a) polyhedral representation of the core complex [Pu(W₅O₁₈)₂]⁸⁻; (b) polyhedral representation with Cs, Cl, and solvent waters surrounding the asymmetric unit, and (c) top-down view showing only the equatorial Cs⁺ counterions and their proximity to the Pu⁴⁺ cation. Colour code: Pu in blue, W in marron, Cs in cyan, Cl in green, O in red.

As illustrated in Fig. 4, the Cs⁺ counterions are tightly packed around the Pu–POM complex. Indeed, each unique [Pu(W₅O₁₈)₂]⁸⁻ has five Cs⁺ located around the equatorial plane of the complex, at 4.35–4.67 Å away from Pu⁴⁺, plus a sixth one from the neighbouring complex, with Cs–Pu distance of ∼6.7 Å (Fig. 4c). These are remarkably short distances for two cations in aqueous environment. No such interaction is seen in the previous structures reported for tetravalent complexes of W5 (Table S1, ESI†). The positions of Cs⁺ in Cs₈Pu(W₅)₂ contrast sharply with all the equivalent structures with Th⁴⁺, U⁴⁺, or Np⁴⁺ and W5 previously reported.^26,28,35 In a recent study from Subintoro and Carter,³⁵ the authors found alkali-U distances of 5.8–7.2 Å in A_x[U(W₅O₁₈)₂]·nH₂O, regardless of the alkali counterion (A = Li, Na, K, Rb, or Cs)—a difference of 1.5–2.5 Å compared to Cs₈Pu(W₅)₂. Interestingly, we previously observed³² a somewhat similar interaction between multiple Cs⁺ counterions and the analogous complex with trivalent curium ([Cm(W₅O₁₈)₂]⁹⁻), with Cm–Cs distances of 4.48–4.54 Å. This hints that the interactions between counterions, the two POM ligands, and the central f-element maybe be specific in the case of Pu⁴⁺ and Cm³⁺, with an outsized impact of the Cs⁺ counterions on the overall structure. Efforts are on-going to expand the experimental dataset on transuranic–POM complexes and to rationalize these seemingly transuranic-specific effects via computation.

The average Pu–O bond distance in Cs₈Pu(W₅)₂ is 2.378 Å, which is consistent with the oxidation state +IV of Pu. However as shown in Fig. S7 and Table S3 (ESI†), this is slightly longer than what would be extrapolated from the related complexes with Zr⁴⁺, Th⁴⁺, U⁴⁺, and Np⁴⁺. It appears that the tightly packed Cs⁺ counterions in the equatorial plane of the complex lead to a slight elongation of the Pu–O bonds. Bond Valence Summation calculations reveal a calculated valency of 3.6 valence units, which is consistent with the slightly longer bond lengths measured for Pu–O resulting in a underbonded Pu(IV). If the Cs counterions are positioned close to Pu⁴⁺, in the equatorial plane, then we presume that the structure compensates with a slight bond elongation along the perpendicular axis (i.e., stretching the complex along its long axis – Fig. 4).

In conclusion, our microscale synthesis approach allowed for the extension of the chemistry of W5 POM to plutonium, providing an important step toward better understanding actinide coordination requirements and informing ongoing efforts to develop novel actinide-based materials. The isolated compound, Cs₈[Pu(W₅O₁₈)₂]·CsCl·6.5H₂O, is the first Pu complex of the Peacock–Weakley category. The structure features elongated Pu–O bonds and closely packed Cs⁺ counterions around the complex, which is different from the complexes of W5 with other tetravalent cations but reminiscent of what is observed for the analogous compound with trivalent curium. Work is underway to extend this chemistry to other cation-POM-counterions systems.

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. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration (DOE/NNSA) under Contract DE-NA0003525. Release number: LLNL-JRNL-2007860.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article has been included as part of the ESI.† Crystallographic data for the plutonium compound has been deposited at the CCDC (Deposition number 2465152).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2465152. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5cc03481g

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