Stabilization of Plutonium(V) Within a Crown Ether Inclusion Complex

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Aug 2020Stabilization of Plutonium(V) Within a Crown Ether Inclusion Complex Yaxing Wang†, Shu-Xian Hu†, Liwei Cheng, Chengyu Liang, Xuemiao Yin, Hailong Zhang, Ao Li, Daopeng Sheng, Juan Diwu, Xiaolin Wang, Jun Li, Zhifang Chai and Shuao Wang Yaxing Wang† State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 †Y. Wang and S.-X. Hu contributed equally to this workGoogle Scholar More articles by this author , Shu-Xian Hu† School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083 †Y. Wang and S.-X. Hu contributed equally to this workGoogle Scholar More articles by this author , Liwei Cheng State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Chengyu Liang State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Xuemiao Yin State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Hailong Zhang State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Ao Li State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Daopeng Sheng State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Juan Diwu State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Xiaolin Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] China Academy of Engineering Physics, Mianyang, 621900 Google Scholar More articles by this author , Jun Li Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Zhifang Chai State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author and Shuao Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000152 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Crystalline coordination complexes of actinides, especially in atypical oxidation states, are not only fundamentally important for expanding the notably limited knowledge on the bonding nature of actinides but could also provide critical information toward the development of nuclear fuel cycle, waste management, and national security. Plutonium (Pu) is the only element in the periodic table that could exist in four oxidation states in aqueous solutions simultaneously. It represents a relatively blank gap in coordination chemistry study owing to the highly radiotoxic nature of its available isotopes and its unique position in the actinide series. Isolation of the pentavalent plutonium [Pu(V)] complex has been thought to be challenging because the highly reactive and dynamic surface redox behavior of the element, and/or the disproportionation trend of the Pu(V) state, make it thermodynamically unfavorable in aqueous solutions, which is even amplified upon strong coordination. Herein, we document a distinct Pu(V) crown ether inclusion complex, namely, PuVO2[18-crown-6]ClO4, which could be crystallized and subsequently, stabilized during a mild reduction reaction from a stock solution of Pu(VI) perchlorate. This compound was characterized fully, using X-ray crystallography and spectroscopic techniques, providing the first experimental spectroscopy data on a solid-state complex of Pu(V) with rich electronic transition information. Relativistic density functional theory (DFT) calculations confirmed the ground-state electronic configuration of Pu(V) being f3 ([core]5fδ25fФ1) and demonstrated further that a combination of cavity and steric repulsion effects assisted in the stabilization of the Pu(V) species within the 18-crown-6 ligand complex. Download figure Download PowerPoint Introduction Since the first discovery in 1940, plutonium (Pu) promptly became one of the most conspicuous elements in the periodic table owing to its irreplaceable role in the development of weapons, civil nuclear power, and the generation of radioisotope thermoelectric.1 However, the chemical research on Pu is hindered substantially by synthetic challenges originating from the extreme radio- and chemo-toxicity of its available radioisotopes.2 In fact, only a handful of chemical laboratories worldwide have attempted to manipulate Pu isotopes, thus, leading to limited knowledge on Pu chemistry, compared with those of transition metals, lanthanides, and even early actinides such as thorium and uranium.3 Besides, Pu is one of the most chemically complicated elements in the periodic table, predominately due to its unique position in the actinide series where the degeneracy of 5f and 6d orbitals emerges, while 5f electrons are at the transition border between being delocalized and localized.4–9 This leads to distinct chemical behaviors that could not be mimicked by any other element. For example, Pu metal could have up to six different isomers at ambient pressure (1 atm),10 and generally displays five oxidation states in aqueous solutions. Four of these states, [Pu(III), Pu(IV), Pu(V), Pu(VI)], could exist simultaneously. Until now, only 197 entries containing Pu complexes have been deposited in the Cambridge Structural Database (CSD) with the majority containing Pu(IV) or Pu(VI) (Figure 1). In sharp contrast, only four Pu(V) compounds have been reported,11–13 which is not surprising because of the relatively facile disproportionation trend of Pu(V) in aqueous solutions. (The standard redox potentials of Pu(V)/Pu(IV) and Pu(VI)/Pu(V) are 1.04 and 0.94 V, respectively.) This trend is amplified further when strong Pu coordination is achieved, given the notably lower effective charge of Pu(V) in comparison with those of other valence states. Accordingly, a significant synthetic challenge arises in an effort to isolate a stable Pu(V) compound, leading to an almost blank characterization of this metal cation coordination (PuVO2+), especially its electronic structure in its +5 oxidation state. Figure 1 | Statistical data on the deposited entries of iron, uranium, and plutonium in the Cambridge structural database. Download figure Download PowerPoint Pu(V) is an environmentally relevant valence state as it is chemically accessible as an intermediate species during the reduction from Pu(VI) to Pu(IV).14–16 From a synthetic point of view, the critical issue pertaining to the isolated Pu(V) complex is the prevention of Pu(V) disproportionation or further reduction to Pu(IV) in the reduction process. It is possible to capture Pu(V) by taking advantage of the vast distinction between the coordination geometries of high-valent (V, VI) and low-valent (III, IV) actinides, where high-valent actinides favor the equatorial coordination given its linear di-oxo configuration, and low-valent actinides tend to adopt the relatively isotropic coordination.17 Recently, cryptand-deriving ligands were extensively involved in the preparation of early actinide complexes in which multiple donors and cavity showed selectivity and recognition toward specific oxidation states. For example, Sessler et al.18–21 synthesized a series of expanded porphyrin ligands with a cavity matching the coordination geometry of high-valent uranyl neptunyl ions to introduce a high affinity to high-valent actinides specifically. Also, tetravalent actinide ions could be encapsulated selectively by employing expanded porphyrin ligands.22 Moreover, crown ethers, members of cryptand-deriving ligand family with tunable cavity size and multidentate oxygen donors, also exhibited selective coordination of actinide ions through the formation of inclusion complexes. Specifically, when uranyl(VI) and neptunyl(V) were inserted into the cavity of 18-crown-6 through equatorial coordination by the multidentate oxygen donors, they formed several unique actinyl crown ether inclusion complexes.23,24 Meanwhile, there was no report on single tetravalent actinide crown ether inclusion complexes, suggesting that 18-crown-6 might be a promising candidate for the recognition of high-valent actinides. Therefore, we envisioned that, with Pu, selective coordination of Pu(V) might potentially weaken the reduction trend from Pu(V) to Pu(IV). More importantly, the formation of the inclusion complex could reduce the chance of Pu–Pu interaction through steric effects, which could, in turn, diminish the disproportionation of Pu(V) dramatically. Following this simple idea, we isolated a Pu(V) inclusion complex successfully, namely PuVO2[18-crown-6]ClO4, which featured a PuVO2+ cation coordinated by an 18-crown-6 ligand. The combined X-ray crystallography structure, absorption spectrum, electronic structure, and chemical bonding analyses provided the first comprehensive characterization of this atypical oxidation state of Pu. Results and Discussion Since pentavalent plutonium Pu(VI) is not stable in aqueous solutions, we designed a synthetic strategy of selective crystallization of a Pu(V) complex during the in situ chemical reduction process of Pu(VI).11–13 A stock solution of PuO2(ClO4)2 (0.1 M) was prepared by dissolving high-purity 242PuO2 powder in concentrated nitric acid and heated in an autoclave at 200 °C, followed by distillation at ambient pressure (1 atm). The solution is concentrated HNO3 at this stage, therefore, Pu(NO3)4 and NO2 are correct here. HClO4 was added later to form a PuO2(ClO4) solution ( Supporting Information Scheme S1). Ultraviolet–visible (UV–vis) spectroscopy was used to monitor the oxidation state of Pu in the stock solution. A pronounced absorption peak at 833 nm was observed, indicating that the chemical species of Pu was PuVIO22+ ( Supporting Information Figure S1). To 73 μL of this stock solution in a scintillation vial, 500 μL distilled water, 18-crown 6-ether solution (5.6 μL, 2.6 mol/L), and hydrazine solution (53.5 μL, 0.15 mol/L) were added in sequence. It should be noted that the hydrazine solution had to be added dropwise to achieve a mild reduction process to avoid the direct reduction to Pu(IV) at locally high hydrazine concentration. We observed in our preliminary experiments that prompt addition of hydrazine resulted in the formation of Pu(IV) in greenish color. However, the process of slow addition of hydrazine, resulted in a gradual change of the color of the reaction solution from pink to pale purple, suggesting the partial reduction of Pu(VI) to Pu(V) (Figure 2a). Subsequently, the solution was allowed to stand still to enable slow water evaporation under ambient conditions, with the formation of pale yellow block crystals of PuVO2[18-crown-6]ClO4 after 3 days. Figure 2 | (a) Reduction process and solution color change from PuVIO22+ to PuVO2+; (b) the molecular structure of PuVO2[18-crown-6]ClO4, ClO4− ion is omitted for clarity; (c) and (d) hexagonal bipyramidal coordination geometry of PuO8 from the top view and side view. Color codes: blue, plutonium; black, carbon; red, oxygen; hydrogen is omitted for clarity. Download figure Download PowerPoint Single-crystal X-ray diffraction revealed that PuVO2[18-crown-6]ClO4 crystallized in the tetragonal space group I41/a ( Supporting Information Table S1), which is isotypic to the pentavalent neptunium compound [NpO2([18]Crown-6)]ClO4,23 initially suggesting a parallel pentavalent assignment of Pu atoms in the Pu(V) complex structure. Notably, that Np(V) is the most stable and common oxidation state of neptunium, whereas Pu(V) is an atypical oxidation state that represents a challenge to be stabilized. Interestingly, in the crystal structure, the Pu atoms were found within a strictly linear dioxo unit of PuO2+, which was encapsulated entirely by a disordered crown ether ligand (Figure 2b). In the crown ether, the C–O–C–C units had a trans conformation, and the O–C–C–O units adopted a gauche conformation, which had been observed in uranyl and neptunyl inclusion complexes.23,24 The six oxygen atoms in the crown ether coordinated equatorially to PuO2+, forming a distorted hexagonal bipyramidal coordination geometry (Figures 2c and 2d). The axial bond length of the plutonyl ions was 1.784(6) X 2 Å, while the equatorial Pu–O distances ranged from 2.574(9) to 2.574(9) Å. Compared with the bond distances in previously reported Pu(V) compound, the steric effect and cavity effect from 18-crown-6 ligand led to a slight change in bond distances in PuVO2[18-crown-6]ClO4. The axial bond lengths were slightly shorter than the values reported in Pu(V) compounds, ranging from 1.78 to 1.82 Å. Consequently, the equatorial bond distances were longer than those of the reported values (2.43–2.49 Å).11–13 Notably, the axial bond lengths of plutonyl in PuVO2[18-crown-6]ClO4 were significantly longer than the statistical data obtained for the hexavalent plutonyl compounds, where the Pu=O distances ranged from 1.72 to 1.75 Å.25,26 Furthermore, we calculated the bond valence sum (BVS) for the Pu site in PuVO2[18-crown-6]ClO4, as shown in Supporting Information Table S2, to be equal to 5.1, again, confirming the pentavalent assignment of Pu.23 In addition, the crystallographic results indicated that the actinide contraction effect dominated the An=O axial bond distances and An–O equatorial bond distances in Np and Pu isotopic compounds. In the series of AnVO2[18-crown-6]ClO4 (An = Np, Pu), these experimental values transited axial: from 1.80 to 1.773(8) Å and equatorial: from 2.602–2.576 Å to 2.574(9)–2.574(9) Å,23 respectively. Further, we analyzed the molecular stacking in the crystal structure of PuVO2[18-crown-6]ClO4. As shown in Supporting Information Figure S2, the neighboring PuVO2[18-crown-6]+ units were well separated by counterions of ClO4−. The Pu–Pu distances were longer than 10 Å, which contributed to the weakening of Pu(V) disproportionation, as proposed. We used scalar-relativistic density functional theory (DFT) to elucidate the nature of the chemical bonding and electronic structure of the PuVO2[18-crown-6]ClO4 compound by optimizing the geometry of the PuVO2[18-crown-6]+ cation, where the ClO4− counterion was not included.27,28 Subsequently, time-dependent DFT (TDDFT) calculation was performed to determine the electronic ground state and to explore the electronic spectra ( Supporting Information Tables S3–S5). Initially, we explored the electronic structures of free PuVO2+ ions, and PuVO2[18-crown-6]ClO4 explored using two different exchange-correlation functionals (PBE and B3LYP) for the S = 3/2 high-spin configuration state ( Supporting Information Tables S3 and S4). In free PuVO2+ with linear D∞h symmetry, the Pu 5f orbitals would transform as a1u [z3], e1u [xz2, yz2], e2u [xyz, z(x2 − y2)], and e3u [x(x2 − 3y2), y(3x2 − y2)], respectively, denoted as fσ, fπ, fδ, and fϕ, hereafter. As in the uranyl and neptunyl ions,29,30 the fσ and fπ orbitals of the PuVO2+ ion were destabilized by bonding with Oyl, whereas the fδ and fϕ orbitals remained as nonbonding orbitals (here, Oyl denotes the plutonyl coordinated oxygen atoms). Inasmuch as the fϕ orbitals lied mainly in the equatorial plane, it would split into a lower-fϕ orbital and an upper-fϕ orbital and destabilize upon equatorial coordination. As a result, the high-spin 4Φ state with an electronic configuration of Pu(V) 5fδ25fϕ1 became the lowest energy state for the equatorially coordinated PuVO2+ ion, consistent with previous theoretical reports.31 For PuVO2[18-crown-6]ClO4, the unpaired electrons remained located in the 5f orbitals, and the electronic configuration of PuVO2+ ions in PuVO2[18-crown-6]ClO4 still kept 5fδ25fϕ1 either in the gas phase or with implicit water solvation. In addition, the computed bond lengths of the crystal structure were consistent with experimental data ( Supporting Information Table S5). Further, we extended our DFT analyses to reveal the intrinsic interaction between PuVO2+ and 18-crown-6 ligand by performing chemical bonding calculations ( Supporting Information Tables S6–S9). For PuVO2[18-crown-6]ClO4, the σ bonding between equatorial oxygen atoms (Oe) and PuVO2+ ion involved weak dative electron donation, which originates from Oe lone pairs to the 5f and 6d orbitals of plutonyl(V) moiety. Weinhold’s natural bond orbital (NBO) analysis reveals that the weak dative bond was composed of 4% Pu and 96% Oe in character ( Supporting Information Table S8), implying its predominately ionic nature. More evidence from Mulliken charge analysis confirmed charge donation from the ligands. We observed that the charge of Oe units in PuO2[18-crown-6]ClO4 was −0.65|e|, whereas, for the free 18-crown-6, the value was only −0.57|e| ( Supporting Information Table S7). Additionally, we investigated how the interaction between PuVO2+ and 18-crown-6 ligand was corroborated further by electron localization functions, as shown in Supporting Information Figure S3. Quantitatively, the spin densities obtained on Pu and Oyl unit were 3.33|e| and −0.16|e|, respectively, which agreed with the formal Pu(V) 5f3 configuration. The calculated Pu–Oe bond order was 0.16, showing a rather weak Pu–Oe dative bond, contrary to the Pu−Oyl bond order of 2.30, signifying strong triple bonds. The weak dative bonding in the PuVO2[18-crown-6]+ could be interpreted by donor–acceptor orbital interactions between PuVO2+ and 18-crown-6 fragments through fragment molecular orbital (FMO) analysis. Figure 3 shows the energy levels of the PuVO2[18-crown-6]ClO4 in a C2 symmetry and their correlation to the levels of PuVO2+ in C2v subgroup of D∞h symmetry, where no spin–orbit coupling effects were considered. As shown in Figure 3a, among the various orbital interactions between the 18-crown-6 fragment and Pu 5f6d7s hybrid orbitals, the major contribution to the bonding came from the direct overlap of the O 2p-based lone-pair molecular orbitals (MOs) of 18-crown-6 and 6d/5f-based orbitals of PuVO2+. The interaction with the 18-crown-6 fragment caused the Pu 6d5f-based MOs to destabilize and the 1a–3b group orbitals of 18-crown-6 ligands to stabilize (Figure 3b and Supporting Information Table S9). To probe the energetics further, we calculated the gas-phase Gibbs free-energy differences (DG) for the disproportionation reaction (2PuVO2[18-crown-6]+ → PuIV[18-crown-6] + PuVIO2[18-crown-6]2+) and ionization process {PuVO2[18-crown-6]+ → PuVIO2[18-crown-6]2+ + e−(g)}. The calculated ΔG values were 369.2 and 673.7 kJ/mol, respectively, in the gas phase, which demonstrated fair stability of PuVO2+ than either PuIV or PuVIO22+ in this system. Further evaluation of lattice or solvent effects on these energies could aid in understanding the stability of Pu(V) in our system. Figure 3 | (a) Kohn–Sham molecular orbital analysis of PuVO2[18-crown-6]+, where 7b and 9b orbitals are the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively. Only the HOMOs and SOMOs (singly occupied molecular orbitals) are shown with electrons (dots); (b) the major in-plane orbital interaction between Pu 6d- and 5f-orbitals and the orbitals of the 18-crown-6 ligand. Download figure Download PowerPoint Furthermore, the intrinsic interaction energy between the ligand and Pu ions were assessed by energy decomposition analysis (EDA) data for PuVO2+ + 18-crown-6 → PuVO2[18-crown-6]+ ( Supporting Information Table S9). The results showed that electrostatic interaction accounted for twice as much contribution as that of orbital interaction, consistent with the predominately ionic bonding character of the Pu−Oe bonding. An inspection of the additional EDA-NOCV (energy decomposition analysis with natural orbitals for chemical valence) analysis substantiated that the key bonding interactions between the Pu and Oe atoms were due to plutonium 5f and 6d orbitals, as to the in-plane bonding molecular orbitals, in with the results obtained from bond where the Pu of hybrid in character (Figure Supporting Information Tables and S9). the solid-state absorption was from a single crystal of PuVO2[18-crown-6]ClO4. As shown in Figure the ranging from to nm of a strong peak and a series of weak The absorption of PuVO2+ in the solid-state were complicated than the data in due to the that the ligand a on the electronic Furthermore, the of absorption data for PuVO2+ in the state to the electronic by preliminary DFT calculations scalar-relativistic calculation results indicated that a strong peak from to nm was with charge from ligand orbitals to metal orbitals The at nm were to the state due to electronic from fδ and to fπ and The additional electronic from the upper-fϕ to fπ resulted in absorption at and The at even lower energy from electronic from fδ to fϕ and to The states in but they weak due to and especially when spin–orbit coupling was considered. This of the Pu(V) f3 is of the of with electron configuration, has a and complicated electronic the of the in a it was to further assignment electron correlation spin–orbit coupling effects, due to the complicated Pu(V) crystal structure. 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