Covalency-Driven Dimerization of Plutonium(IV) in a Hydroxamate

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Covalency-Driven Dimerization of Plutonium(IV) in a Hydroxamate Complex Mark A. Silver,† Samantha K. Cary,† Jared T. Stritzinger,† T. Gannon Parker,† Laurent Maron,‡ and Thomas E. Albrecht-Schmitt*,† †

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Laboratorie de Physique et Chimie des Nano-objets, Institut National des Sciences Appliquées, 31077 Toulouse Cedex 4, France

‡

S Supporting Information *

Single-crystal X-ray diffraction studies on these crystals revealed a dimeric structure with the molecular formula Pu2(FHA)8. A view of the molecule is shown in Figure 1. The

ABSTRACT: The reaction of formohydroxamic acid [NH(OH)CHO, FHA] with PuIII should result in stabilization of the trivalent oxidation state. However, slow oxidation to PuIV occurs, which leads to formation of the dimeric plutonium(IV) formohydroxamate complex Pu2(FHA)8. In addition to being reductants, hydroxamates are also strong π-donor ligands. Here we show that formation of the Pu2(FHA)8 dimer occurs via covalency between the 5f orbitals on plutonium and the π* orbitals of FHA− anions, which gives rise to a broad and intense ligand-to-metal charge-transfer feature. Time-dependent density functional theory calculations corroborate this assignment.

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dvanced nuclear fuel cycles require the use of selective reductants to control the speciation of actinides in solution and enable subsequent liquid−liquid extraction processes.1−4 Although an array of reagents have been employed to enact these redox reactions, hydroxamates have been shown to be particularly effective as both chelating agents and reductants for neptunium and plutonium to low oxidation states while leaving UVI unaltered.1−4 Although the utility of hydroxamates has been demonstrated on an industrial scale, a number of unusual observations have been made from these reactions that include the atypical red color of the putative uranium(VI) formohydroxamate complex UO2(FHA)2.4−7 We demonstrated that the origins of this coloration lie in significant distortions of the UVI coordination environment from what is normally observed owing to strong π donation by the FHA− ligands into uranium’s 5f and 6d orbitals.8 These observations, as well as the propensity of hydroxamates to stabilize CeIV,9,10 motivated the detailed study of the complexation of PuIII by FHA discussed herein. In order to mitigate the effects of radiolysis on the reaction of PuIII with FHA, we made use of the long-lived isotope of plutonium, 242Pu (t1/2 = 3.76 × 105 years). The reaction of 242 PuIII with excess FHA in aqueous media initially results in retention of the blue-purple color characteristic of PuIII. In fact, the color of these solutions remained unchanged over the course of several weeks as they evaporated. However, just prior to crystallization, the highly concentrated solutions suddenly became red. This color is typically associated with PuIV complexes, and red crystals with a tablet habit formed within 3−4 days of the color change. © 2016 American Chemical Society

Figure 1. (a) View of the dimeric structure of the plutonium(IV) formohydroxamate Pu2(FHA)8. Color code: Pu, purple; O, red; N, blue; C, black; H, gray.

dimer has a point of inversion between the two Pu centers, and therefore only half of the dimer is crystallographically unique. The overall molecule has Ci point symmetry. Each Pu ion is bound by four chelating FHA− anions, where the ligands utilize both the aldehyde and oxamate O atoms to bind Pu, which leads to the formation of five-membered rings. In the deprotonated state, the anionic charge is partially delocalized through the ligand. The dimer is created because one of the O atoms of an N−O moiety is a μ2-O atom and bridges between the Pu centers. Again because of the inversion symmetry, there are two such bridges. A similar coordination mode is found in UO2(FHA)2 and shows strong electron-donating capabilities of the hydroxamate moiety.8 This bridge represents the ninth site of the [PuO9] core, whose geometry is best described as a distorted muffin.11 PuIV bonding distances are typically ∼0.1 Å shorter than those found with PuIII. The Pu−O bond distances range from 2.283(3) to 2.450(4) Å and are characteristic of PuIV−O bond lengths in nine-coordinate geometries.12−15 If the bridging μ2-O distances of 2.402(3) and 2.435(3) Å (which are necessarily longer than Received: February 9, 2016 Published: May 26, 2016 5092

DOI: 10.1021/acs.inorgchem.6b00340 Inorg. Chem. 2016, 55, 5092−5094

Communication

Inorganic Chemistry the other contacts) are excluded from this list, then the average distance is 2.368(3) Å. This is considerably shorter than would be found with PuIII.16−20 Again this leads to the straightforward formula of PuIV2(FHA)8. The solid-state UV−vis−near-IR absorption spectrum acquired from a single crystal of Pu2(FHA)8 oriented along [010] is presented in Figure 2. At wavelengths longer than 600 nm, f−f

Figure 3. View of the role of 5f orbitals in bonding in Pu2(FHA)8. This figure shows the overlap of one 5f orbital on each PuIV center with π* orbitals of the bridging FHA− ligand.

Figure 2. Solid-state absorption spectrum of Pu2(FHA)8 obtained from a single crystal oriented along [010].

transitions that are consistent with PuIV are evident, and these features are labeled for guidance. Examples of these fingerprint transitions include the 5F2 and 5I6 states near 1100 nm.15,21 Notably the spectrum is featureless near 900 nm, where PuIII exhibits distinct transitions. Given that the metal centers are not located on the inversion centers, transitions near 900 nm should be observed if the formal oxidation state is 3+.22−24 A broad and intense ligand-to-metal charge-transfer (LMCT) band is observed that extends from ∼600 nm into the deep UV. Regardless of the contributions of the Laporte-forbidden f−f transitions, the compound should be red because of this feature. This band is predicted to be LMCT in nature given the strong πdonor capabilities of FHA− and the high charge on Pu (vide infra). In order to understand the role of the frontier orbitals on Pu within the Pu2(FHA)8 dimer, calculations were carried out at the density functional theory level (see the Supporting Information for computational details). First, the B3PW91-optimized geometry was found to be in good agreement with that determined using X-ray diffraction. Among the other distances, the Pu−O ones are reproduced reasonably well by calculation with computed distances between 2.35 and 2.49 Å. Molecular orbital analysis indicates that the highest occupied molecular orbitals (HOMOs) describe the interaction between the PuIV ions and the FHA− ligands. Among others, HOMO−6 (Figure 3) plays a key role in the dimeric structure of the complex. As shown in Figure 3, the bridging structure is ensured by an interaction between two f-type orbitals (one on each Pu) and an antibonding π orbital of the FHA− ligand located mainly on the O atoms, which is in line with the π character of the ligand. The latter π-donor character is computationally demonstrated by analysis of the nature of the HOMO and lowest unoccupied molecular orbital (LUMO) as well as with time-dependent density functional theory methods that provide the energy region of the HOMO−LUMO transitions (see the Supporting Information). The frontier orbitals located on the ligand and on the metal are shown in Figure 4. The HOMO−LUMO transition appears to be LMCT in nature, in line with what would

Figure 4. Frontier orbitals of Pu2(FHA)8 showing the origin of the LMCT transition in the high-energy region of the UV−vis spectrum (left, HOMO; right, LUMO).

be expected from a highly charged metal bond to a strong donor ligand. Moreover, these charge-transfer features are found to occur in the same region of the UV−vis spectrum as that measured from the sample experimentally. PuIV nominally has a nonmagnetic, 5I4 singlet ground state. While some archetypal compounds like PuO2 are nonmagnetic when close to being fully stoichiometric, 25 other Pu IV compounds show complex temperature-dependent and -independent magnetic susceptibility that reflects either changes between the singlet and triplet states and/or changes in the population of a multiconfigurational ground state. For example, Cs2PuCl6 exhibits a non-Kramers doublet as its lowest state and is paramagnetic.26 Pu(SeO3)2 exhibits particularly complex magnetism, shows an abrupt change in its magnetism at 35 K, and is paramagnetic above this temperature with an effective moment of 2.71(5) μB and a large Weiss constant of −500(5) K.27 Plutonocene, Pu(C8H8)2, exhibits only temperatureindependent paramagnetism.28−30 While nonmagnetic plutonium compounds are ostensibly less interesting than those that undergo long-range magnetic coupling, uncovering nonmagnetic systems plays a key role in the expansion of 239Pu NMR spectroscopy.25 Magnetic susceptibility measurements on Pu2(FHA)8 indicate that the sample is nonmagnetic over the entire temperature range studied (1.8−300 K) at 100 Oe. This result may render the molecule useful for 239Pu NMR studies, and replicating these results with the more radiologically challenging 239Pu isotope (t1/2 = 24170 years; I = 1/2) is in progress. In conclusion, this study expands our understanding of bonding between actinides and π-donor ligands, such as hydroxamates. These complexes are both of fundamental interest 5093

DOI: 10.1021/acs.inorgchem.6b00340 Inorg. Chem. 2016, 55, 5092−5094

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Inorganic Chemistry in terms of the unusually strong π-donor/acceptor capabilities of hydroxamates and of practical concern owing to their use in the recycling of used nuclear fuel. In the uranium(VI) complex of FHA, the 5f and 6d orbitals are utilized in the formation of covalent bonds, and this causes massive distortion of the local coordination environment.8 Here we show that dimerization in a prototypical plutonium(IV) hydroxamate complex occurs via the use of 5f orbitals. This bonding has notable effects on the electronic properties of the resultant dimer by creating strong LMCT features.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00340. Crystallographic data, materials and synthesis, and computational details, data, and references (PDF) X-ray crystallographic file in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program, under Award DE-FG02-13ER16414. The 242Pu used in this research was supplied by the U.S. Department of Energy, Office of Science, via the Isotope Program in the Office of Nuclear Physics and was provided to Florida State University via the Isotope Development and Production for Research and Applications Program through the Radiochemical Engineering and Development Center at Oak Ridge National Laboratory. L.M. is a member of the Institut Universitaire de France. CalMip is acknowledged for a generous grant of computing time. The ANR, Humboldt Foundation, and Chinese Academy of Sciences are also acknowledged for financial support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00340 Inorg. Chem. 2016, 55, 5092−5094