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. 2023 May 23;62(22):8462–8466. doi: 10.1021/acs.inorgchem.3c00522

Chlorination of Pu and U Metal Using GaCl3

Stephanie H Carpenter †, Bonnie E Klamm ‡, Taylor V Fetrow ‡, Brian L Scott §, Andrew J Gaunt †, Nickolas H Anderson ‡,*, Aaron M Tondreau †,*
PMCID: PMC10246562  PMID: 37220066

Abstract

graphic file with name ic3c00522_0004.jpg

The oxidative chlorination of the plutonium metal was achieved through a reaction with gallium(III) chloride (GaCl3). In DME (DME = 1,2-dimethoxyethane) as the solvent, substoichiometric (2.8 equiv) amounts of GaCl3 were added, which consumed roughly 60% of the plutonium metal over the course of 10 days. The salt species [PuCl2(dme)3][GaCl4] was isolated as pale-purple crystals, and both solid-state and solution UV–vis–NIR spectroscopies were consistent with the formation of a trivalent plutonium complex. The analogous reaction was performed with uranium metal, generating a dicationic trivalent uranium complex crystallized as the [UCl(dme)3][GaCl4]2 salt. The extraction of [UCl(dme)3][GaCl4]2 in DME at 70 °C followed by crystallization produced [{U(dme)3}2(μ-Cl3)][GaCl4]3, a product arising from the loss of GaCl3. This method of halogenation worked on a small scale for plutonium and uranium, providing a route to cationic Pu3+ and dicationic U3+ complexes using GaCl3 in DME.

Short abstract

Gallium trichloride (GaCl3) can serve as a mild oxidant and chloride source for the generation of a plutonium(III) complex, [PuCl2(dme)3][GaCl4] (dme = 1,2-dimethoxyethane), directly from plutonium metal or for the generation of a dicationic uranium(III) complex, [UCl(dme)3][GaCl4]2.

Many recent advances in molecular actinide (An) reactivity studies can be attributed to growth in the availability of nonaqueous, organic-soluble, halide starting materials. For plutonium, anhydrous PuCl3 species are commonly isolated from acidic stock solutions or via chlorination of the metal using Cl2 gas.1−13 Plutonium iodide materials are typically generated from I2 addition to the metal, after which supporting solvent ligands can be exchanged.1 Recent studies demonstrated the isolation of PuI3 through I2 loss arising from the inherent instability of PuI4.14 Generating anhydrous Pu starting materials typically requires technically rigorous drying methods or the use of strongly oxidizing halogenating reagents. Furthermore, simple PuCl3 is insoluble and difficult to use with ensuing chemistry, and further development of soluble forms of Pu–Cl bonds is advantageous for subsequent experiments. Alternative methods may facilitate new access routes and encourage further investigations of Pu chemistry.

Here, we investigate gallium trichloride (GaCl3) as a chlorine source for the oxidation of plutonium and uranium metal to generate coordination complexes in the 3+ oxidation state. The conjugate reductant of GaCl3, gallium dichloride (Ga2Cl4), is a stable complex, suggesting that GaCl3 has the potential to serve as a mild oxidant and halide source for the formation of UCl3 and PuCl3 from the respective metal via chlorine transfer, generating Ga2Cl4 as a side product. The proposed thermodynamics are favorable, given the simplified equations:

graphic file with name ic3c00522_m001.jpg

These negative heats of formation bolster the hypothesis that chlorine transfer from GaCl3 to plutonium or uranium metal will proceed smoothly.15 This work explores the products arising from halogen transfer to plutonium and uranium metal using GaCl3 as the oxidizing chlorine transfer agent in DME (DME = 1,2-dimethoxyethane). It is well established that GaCl3 is found as [GaCl2(dme)2][GaCl4] in DME.16 The reactivity described herein provides an anhydrous synthetic route to a Pu3+ complex under mild reaction conditions, generating a cationic coordination complex. The same reactivity with uranium generated a U3+ dicationic complex.

The reaction with plutonium metal was used to generate a Pu3+ halide species under anhydrous conditions without the use of I2 or Br2. The 0.025 g piece of metal used was cut from a larger piece and was a low-surface-area block, resulting in slow reaction kinetics (Figure S1). DME was chosen as the reaction solvent based on its successful prior use in the synthesis of actinide starting materials.17,18 The reaction was performed over the course of 10 days at room temperature in the presence of substoichiometric quantities of GaCl3 (2.8 equiv) to prevent over-oxidation. The solution turned from colorless to purple, and a darker-purple/gray precipitate formed. The unreacted plutonium metal was then separated and weighed, confirming the consumption of approximately 60% of the starting metal. Following decanting of the mother liquor, a DME extraction of the pale-purple material was performed at 40 °C. A pentane diffusion into the mother liquor and extracted DME solutions at −35 °C gave two batches of purple crystals of [PuCl2(dme)3][GaCl4], although the batch from the mother liquor was contaminated with crystals of [GaCl2(dme)2][GaCl4].

An analysis of the crystalline material by UV–vis–NIR spectroscopy was performed in both the solution and solid state, where both spectra contained bands that are consistent with those typically observed for f–f transitions in 5f5 Pu3+ species (Figure 1; Figures S2 and S3); notably, the spectra lacked features attributable to simple ions of Pu4+.19−22[PuCl2(dme)3][GaCl4] displayed poor solubility in DME once crystallized, and only approximate concentrations for solution UV–vis–NIR were obtained.

Figure 1.

Figure 1

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Solid-state UV–vis–NIR spectra of [PuCl2(dme)3][GaCl4]; three crystals were measured (Figures S3 and S12).

Single-crystal X-ray diffraction (SC-XRD) of the pale-purple crystals confirmed the structure as the eight-coordinate plutonium complex [PuCl2(dme)3][GaCl4], with three DME molecules arranged equatorially and two axial chloride ligands best described as a trigonal dodecahedron (Figure 2). The [GaCl4]− counterion is noncoordinating with no close-contact interactions. The general structural motif of [PuX2]+ (X = Cl or I) has been previously reported.1 In the case of X = Cl, a mixed-valent salt [PuCl2(thf)5][PuCl5(thf)] (thf = tetrahydrofuran) was isolated, where the cationic plutonium species bears a structure similar to that of trans-chloride ligands and equatorially coordinated solvent molecules.1 For X = I, the cationic plutonium species [PuI2(thf)4(py)]+ (py = pyridine) was isolated with an [I3]− counterion,1 with the equatorial positions of the plutonium center occupied by four THF molecules and one pyridine molecule. The most notable geometrical differences between the previously reported structures can be found in a more linear bond angle of X–Pu–X (176.36(8)°, X = Cl; 179.79(2)°, X = I)1 when compared to the 161.15(1)° found in [PuCl2(dme)3][GaCl4] (Table S3). The difference between the X–Pu–X angles in these complexes likely arises from the increased equatorial coordination number. [PuCl2(dme)3][GaCl4] is a rare example of a structurally characterized, molecular Pu3+ possessing chloride ligands.23−25

Figure 2.

Figure 2

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Two views of the solid-state structure of [PuCl2(dme)3]+ are presented with 50% probability of thermal ellipsoids. Hydrogen atoms and [GaCl4]− are omitted for clarity.

Chloride complexes of U3+ are less common than those of U4+ and usually arise from [UCl4] reductions.18,26−28 Given the cationic nature of the Pu3+ species, [PuCl2(dme)3]+, the uranium product, was expected to show attenuated oxidation reactivity if its product is also cationic. The analogous reaction with uranium metal was performed over the course of 3 days at room temperature using thin, cleaned turnings of uranium metal. Within an hour, the solution turned from colorless to pale pink, with a darker, bright-pink color eventually forming along with a dark-green precipitate. Measuring the remaining metal was difficult as the metal was found as an amorphous gray material following the reaction. The isolation of dark-green crystalline blocks was made challenging by the presence and coprecipitation of [GaCl2(dme)2][GaCl4]. Careful layering or vapor diffusion of small quantities of n-hexane onto the pink solution generated large blocks, which were washed and isolated in low yields. In this way, gallium salts can be removed from the product. The UV–vis–NIR spectra comprised a broad absorption band centered at 550 nm, a rapid rise of absorption in the UV region, and many broad absorption bands from 980 to 1300 nm in the NIR region, consistent with a U3+ ion (Figures S4–S6).29−31

SC-XRD experiments, combined with elemental analysis, led to the assignment of the green crystalline material as [UCl(dme)3][GaCl4]2 (Figure 3). Initial ambiguity in assignment from SC-XRD experiments arose from the coordinated DME molecules, which were marked by the strong disorder of DME freely rotating about the uranium center. The resulting 3+ oxidation state is atypical for oxidative chlorination reactions with uranium that often result in U4+ complexes, which is attributed to the stability imparted by the Cl– ligand on the U4+ center. We hypothesize that the presence of GaCl3 and the subsequent formation of cationic uranium species during the synthesis remove electron density from the uranium center, stabilizing the U3+ product. This dicationic U3+ species represents a unique coordination mode in nonaqueous uranium chemistry.

Figure 3.

Figure 3

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SC-XRD data of [UCl(dme)3][GaCl4]2 established its atom connectivity. U, Ga, and Cl atoms are clearly refined, but the coordinated solvent (DME) is highly disordered.

Attempted recrystallization of [UCl(dme)3][GaCl4]2 for purification purposes led to the formation of a secondary product. Dissolution of [UCl(dme)3][GaCl4]2 in DME was performed with difficulty, with approximately 0.020 g soluble in 5 mL of DME at 70 °C. Extended heating times resulted in the crystal formation of several different uranium complexes. The different crystal morphologies include trace quantities of red blocks comprising a complex with diglyme bound to the uranium (Figure S14), pale-green plates of UCl4(dme)3 (Figure S15), and the predominant crystal morphology of thin, green overlapping plates.

These plates were found to be [{U(dme)3}2(μ-Cl3)][GaCl4]3 as determined by SC-XRD (Figure S17). Reducing the heating time to 10 min allowed for the isolation of [{U(dme)3}2(μ-Cl3)][GaCl4]3 in good yields without evidence of secondary product formation. Coprecipitation of [GaCl2(dme)2][GaCl4] was not observed, and [{U(dme)3}2(μ-Cl3)][GaCl4]3 was easily isolated with slow vapor diffusion under dilute conditions. UV–vis–NIR measurements of [{U(dme)3}2(μ-Cl3)][GaCl4]3 produced spectra indistinguishable from those obtained from [UCl(dme)3][GaCl4]2, suggestive of a common intermediate of the two complexes in a DME solution related by an equilibrium of GaCl3 coordination and decoordination (Figure S10).

SC-XRD experiments led to the assignment of the dimeric complex. However, strong disorder and twinning coupled with weak diffraction data limited the SC-XRD data. The obtained connectivity structure proved to be valuable for determining the chemical composition of this complex (Figure S17). The asymmetric unit cell of [{U(dme)3}2(μ-Cl3)][GaCl4]3 comprises two full and two half molecules of [{U(dme)3}2(μ-Cl3)] and nine [GaCl4] units. Several examples of related bridged diuranium U2(μ-Cl3) compounds have been reported and are typically found in the 4+ oxidation state,32−35 including those of [AlCl4] salts.36,37 Fewer U3+ species are reported with three bridging chloride ligands.38

This work describes a new, mild method of plutonium and uranium metal oxidation using GaCl3. When plutonium metal is employed, the monocationic species [PuCl2(dme)3]+ is formed. The analogous uranium reactions led to the isolation of a dicationic species, [UCl(dme)3]2+. The divergent products isolated from the plutonium and uranium reactions can be attributed to the difference in the Lewis acidities between the two metals. Uranium 3+ is a softer ion and is tolerant of the abstraction of two of the three chloride atoms. Pu3+ is a stronger Lewis acid and outcompetes the excess GaCl3 for an additional Cl– ligand, and the product is observed as the monocation. [UCl(dme)3][GaCl4]2 exhibited limited stability as a monochloride; simple dissolution of [UCl(dme)3][GaCl4]2 in DME resulted in the generation of [{U(dme)3}2(μ-Cl3)][GaCl4]3, and heating for extended periods led to a mixture of products. Further investigation will focus on methods for gallium removal for the purpose of generating PuCl3/UCl3 starting materials; preliminary results employing pyridine are encouraging for the formation of both plutonium and uranium trihalide complexes. Alternatively, heating [UCl(dme)3][GaCl4]2 demonstrates potential as a method of generating UCl4(dme)3. These results will be communicated in subsequent reports. The reactivity, structural motifs, and potential use as synthetic precursors presented for plutonium and uranium encourage long-term investigations using GaCl3 as an oxidant for f-block metals.

Acknowledgments

This work was conceived and executed at Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (contract no. 89233218CNA000001). A.M.T., B.L.S., and A.J.G. acknowledge the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program (2020LANLE372, contract number AC52-06NA25396). A.M.T., S.H.C., T.V.F., B.E.K., and N.H.A. were supported by the U.S. Department of Energy, NNSA, Plutonium Maturation Program (NA-191). A.M.T. thanks Sarah K. Tondreau for assistance with the organization and editing of this manuscript. S.H.C. and T.V.F. are grateful for additional postdoctoral support provided by the Glenn T. Seaborg Institute of Los Alamos National Laboratory.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c00522.

  • Experimental descriptions; photographs of crystals and product solutions; UV–vis–NIR characterization spectra; and crystallographic data including refinement parameters and metrical data for all complexes (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic3c00522_si_001.pdf (2.4MB, pdf)

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Supplementary Materials

ic3c00522_si_001.pdf (2.4MB, pdf)

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