Computationally-Guided Assignment of Unexpected Signals in the

Jan 11, 2017 - Synopsis. Density functional theory computations and experimental measurements, including 18O isotopic labeling, were combined to estab...
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Computationally-Guided Assignment of Unexpected Signals in the Raman Spectra of Uranyl Triperoxide Complexes Mateusz Dembowski,†,∥ Varinia Bernales,‡,∥ Jie Qiu,§ Sarah Hickam,§ Gabriel Gaspar,§ Laura Gagliardi,*,‡ and Peter C. Burns*,†,§ †

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States Department of Chemistry, University of Minnesota, Superconducting Institute, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States § Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡

S Supporting Information *

ABSTRACT: Combination of uranium, peroxide, and mono- (Na, K) or divalent (Mg, Ca, Sr) cations under alkaline aqueous conditions results in the rapid formation of anionic uranyl triperoxide monomers (UTs), (UO2(O2)3)4−, exhibiting unique Raman signatures. Electronic structure calculations were decisive for the interpretation of the spectra and assignment of unexpected signals associated with vibrations of the uranyl and peroxide ions. Assignments were verified by 18O isotopic labeling of the uranyl ions supporting the computational-based interpretation of the experimentally observed peaks and the assignment of a novel asymmetric vibration of the peroxide ligands, v2(O22−).

1. INTRODUCTION A goal in actinide chemistry is furthering the understanding of actinide speciation under diverse solution conditions. This is in part a reflection of the importance of radioactive waste generated by uranium mining, fuel reprocessing, energy production, and weapons-related activities.1 Several radionuclides combine very long half-lives with high environmental mobility under oxidizing conditions, making them important not only for environmental remediation but also for predicting the long-term performance of a geological repository for nuclear waste.2 Raman spectroscopy is a rapid and powerful tool for the detection and characterization of uranium and other actinides in solution and is widely applied in nuclear forensics, geology, and chemistry.3−8 The majority of reported Raman spectra of uranium compounds are for uranyl minerals, and these often yield complex spectra that in some cases may be impacted by impurities. The complexity associated with assignment of vibrational modes has demonstrated the importance of empirical correlations relating the length of the uranyl (U− Oyl) bond with its symmetric and asymmetric vibrations, as proposed in 1989 by Bartlett et al.9−11 Computational models, most commonly density functional theory (DFT), have improved the understanding of uranium speciation in aqueous solution and consequently the fate of uranium in the environment.12−18 The accuracy of these models, however, is contingent on comparisons of predictions © XXXX American Chemical Society

to experimentally determined parameters for structurally simple and pure phases. In this regard, uranyl peroxides present a unique and sometimes complex family of inorganic compounds spanning structures ranging from mono- and oligomeric species to nanoscale capsules containing as many as 124 uranyl polyhedra.19−23 These materials are of interest because of their potential application in an advanced nuclear fuel cycle and possible role in the transport of actinides in the environment.24−27 Their ease of preparation and high purity as well as the presence of Raman-active vibrational modes make these materials good candidates for evaluation of the aforementioned empirical and computational models. Recent studies of uranyl peroxide nanocluster self-assembly highlight the importance and persistence of uranyl triperoxide monomers (UTs) under highly alkaline conditions with excess cation concentrations.28,29 Furthermore, their structural simplicity, coupled with high spectral activity, provides a unique opportunity to bridge computational and experimental approaches. Herein we report the synthesis, structures, and computational characterization of five isostructural uranyl triperoxide monomers that display unexpected bands in their Raman spectra in the 800−900 cm−1 region. Informed by computations, we conclude that these bands are due to a combination of symmetric and asymmetric vibrations of peroxide ligands, Received: November 3, 2016

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DOI: 10.1021/acs.inorgchem.6b02666 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry denoted here as v1−3(O22−).30 The bands in the 600−800 cm−1 region, originally assigned to the vibration of terminally bound peroxide groups,28,29 are instead assigned as the symmetric vibrations of uranyl, v1(UO22+), as confirmed by isotopic labeling of the uranyl.

change to dark orange/red in less than 5 min. Addition of 200 μL of 0.25 M Ca(NO3)2 resulted in the formation of yellow diffractionquality crystals of CaUT within 24 h. The structure of CaUT was previously reported.34 Yield: 0.031 g (25% based on U). ICP-OES Ca/ U ratio, expcd. (calcd): 2.0 (1.98). 2.7. Sr2[UO2(O2)3]·9H2O (SrUT, 5). Compound 5 was obtained in the same way as 4 by substituting 0.25 M Sr(NO3)2 for 0.25 M Ca(NO3)2. Yellow diffraction-quality crystals formed within 24 h. Yield: 0.025 g (18% based on U). ICP-OES Sr/U ratio, expcd. (calcd): 2.0 (1.99). 2.8. Single-Crystal X-ray Diffraction. Crystals suitable for diffraction were isolated from their mother solution and mounted on a cryoloop in oil. A full sphere of diffraction data was collected at 100 K using a Bruker APEX II Quazar diffractometer equipped with Mo Kα X-radiation provided by a conventional sealed tube. Data were corrected for Lorentz, polarization, and background effects using the Bruker APEX III software package. Empirical absorption corrections were performed using the SADABS35 software package. Structure solutions and refinements were done using the SHELXTL36 software package. Typical refinements included anisotropic displacement parameters for all non-H atoms. 2.9. Raman Spectroscopy. Measurements were performed using a Bruker Sentinel system equipped with a thermoelectrically cooled CCD detector, a fiber-optic probe, and a 785 nm excitation source. Spectra of all UT samples were acquired in the 80−3200 cm−1 range using three 15 s exposures at a laser power of 148 mW. Suitable single crystals of MUTs were isolated under a Nikon optical microscope attached to the Raman probe. Spectral deconvolution of the 600−900 cm−1 region of KUT was performed using MagicPlot software. The spectrum was manually fitted using a set of five Lorentzian curves followed by an automated “Fit by Sum” procedure. Details pertaining to the resulting fit agreement are available in the Supporting Information. 2.10. ICP-OES. Elemental analyses were done using a PerkinElmer Optima 8000 inductively coupled plasma optical emission spectrometer (ICP-OES). Crystalline materials were dissolved in 5% nitric acid, and then aliquots of the solution were diluted to reach a volume of 10 mL, also in 5% nitric acid. Samples were prepared in triplicate with yttrium added as an internal standard in each case. Concentrations were determined by analysis against standard solutions. 2.11. Kohn−Sham Density Functional Theory Calculations. Models were optimized using the B3LYP37,38 density functional as implemented in the Gaussian 09 software package,39 together with the 6-31+G* basis set for O and Mn+ cations (m = Na, K, Mg, Ca, Sr, n = 1, 2). The Stuttgart small-core scalar-relativistic pseudopotential was used to describe the 60 core electrons of uranium atoms, while the remaining 32 electrons were represented by the ECP60MWB_SEG associated valence basis set.40 The nature of each stationary point was verified by analytical computation of vibrational frequencies, which were also used to compute thermochemical parameters at 298.15 K. Solvent effects were introduced using the SMD solvation model41 with water as the solvent for all geometry optimizations and vibrational frequency calculations because of the presence of water molecules in all of the crystal structures. The selected methodology was based on previous uranyl Raman studies and a benchmark study provided in this work.12 (For more details, see the Supporting Information.)

2. EXPERIMENTAL METHODS Caution! All experiments were conducted using isotopically depleted uranium (238U, α = 4.267 MeV), and it is essential that such experiments be conducted by trained personnel in a laboratory designed for the use of radioactive isotopes. 2.1. General Considerations and Syntheses. Commercially obtained uranyl nitrate hexahydrate (UN, UO2(NO3)2·6H2O; International Bio-Analytical Industries, Inc.) was purified and recrystallized according to previously published procedures.30 All other reagents were used as received from commercial suppliers. Isotopic labeling of the uranyl ion (U16O22+ → U18O22+) was conducted according to previously described procedures31 using 97% H218O (Cambridge Isotope Laboratories, Inc.) and confirmed via solution-mode Raman spectroscopy (see the Supporting Information). Isotopically labeled sodium uranyl triperoxide monomer (designated NaUT) was synthesized using a sodium hydroxide solution prepared from anhydrous sodium hydroxide dissolved in isotopically enriched water (97% H218O). The uranyl triperoxide monomer compounds are abbreviated as MUT (MxUO2(O2)3·nH2O), where M = Na, K, Mg, Sr, or Ca, x = 4 (Na, K) or 2 (Mg, Sr, or Ca), and n = 4−13. 2.2. Na4[UO2(O2)3]·9H2O (NaUT, 1a). Synthesis of 1a was performed according to a modified version of a previously published method32 by combining 400 μL of 0.5 M UO2(NO3)2, 100 μL of 30% H2O2, and 600 μL of 10 M NaOH. Light-orange diffraction-quality crystals of NaUT resulted in less than 5 min. Yield: 0.116 g (94% based on U). ICP-OES Na/U ratio, expcd. (calcd): 4.0 (4.01). 2.3. Na4[U18O2(O2)3]·9H2O (NaUT, 1b). Isotopically labeled NaUT was obtained in the same way as 1a using photochemically labeled 0.5 M U18O2(NO3)2 and isotopically enriched 10 M NaOH solution. Yield: 0.111 g (90% based on U). 2.4. K4[UO2(O2)3]·4H2O (KUT, 2). Combination of 2 mL of 0.5 M UO2(NO3)2, 2 mL of 30% H2O2, and 2 mL of 8 M KOH in a 20 mL scintillation vial resulted in the formation of a clear, dark-red effervescent solution. Slow addition of 1 mL of 95% methanol (BDH) to the reaction mixture resulted in rapid precipitation of a bright-yellow microcrystalline solid that was isolated via vacuum filtration and rinsed with copious methanol. The resulting solid was promptly transferred to a 23 mL Parr reaction vessel, which was sealed and subsequently stored at room temperature. The bright-yellow microcrystalline solid obtained initially via the KUT synthesis showed different spectral features than the final product, presumably representing a different species. Because of its microcrystalline form, structural determination via single-crystal X-ray diffraction was not possible. Conversion from the bright-yellow microcrystalline solid to the light-orange crystalline form of 2 takes approximately 1 week. Yield: 0.511 g (86% based on U). ICP-OES K/U ratio, expcd. (calcd): 4.0 (3.99). 2.5. Mg2[UO2(O2)3]·13H2O (MgUT, 3). Combination of 0.075 g of [(UO2)(O2)(H2O)2]·(H2O)2 (studtite), 400 μL of H2O, and 400 μL of 30% H2O2 in a 5 mL glass vial resulted in a pale-yellow suspension. Addition of 450 μL of 25% N(CH3)4OH (TMAOH) solution resulted in gradual dissolution of the solid accompanied by effervescence and a color change to dark orange/red in less than 5 min. Addition of 200 μL of 0.25 M Mg(NO3)2 caused precipitation of a bright-yellow solid that was consequently dissolved by vortexing. The clear, dark-red solution yielded yellow diffraction-quality crystals of MgUT within 24 h. Yield: 0.028 g (21% based on U). ICP-OES Mg/U ratio, expcd. (calcd): 2.0 (2.00). 2.6. Ca2[UO2(O2)3]·9H2O (CaUT, 4). Combination of 400 μL of 0.5 M UO2(NO3)2 and 400 μL of 30% H2O2 in a 5 mL glass vial resulted in rapid precipitation of the pale-yellow mineral studtite.33 Addition of 450 μL of 25% TMAOH solution resulted in gradual dissolution of the solid accompanied by effervescence and a color

3. RESULTS AND DISCUSSION Among all of the UT salts considered in this study, two (Na and Ca) have been previously described.32,34 For the sake of comparison and to improve upon the quality of the previously published structure of the Na salt, a higher-resolution structure was obtained and is provided in the Supporting Information. The structures of all of the UTs consist of a nearly linear uranyl ion (UO22+) that is equatorially coordinated by three η2 peroxide (O22−) ligands, resulting in a series of species exhibiting approximate D3h molecular symmetry (Figure 1). The bond lengths and angles of each UT are largely insensitive B

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Figure 2. (left) Raman spectra of the UTs in the 600−900 cm−1 region. (right) Assignments of bands observed in the 600−900 cm−1 region of the Raman spectra of the UTs.

Figure 1. (left) Ball-and-stick top and side views of the uranyl triperoxide monomer. (right) Polyhedron-and-ball views of the crystal packing for different UT salts down the 100 plane. Yellow, red, orange, purple, teal, and green spheres/polyhedra represent uranium, oxygen, sodium, potassium, magnesium, and strontium/calcium atoms, respectively. Red boxes represent the unit cells.

good agreement with values reported in the literature, including those for uranyl peroxide nanoclusters.30,42,45 UTs, which are basic building blocks of some uranyl peroxide clusters, have an average U−Oyl bond length of 1.855 Å, and the signals corresponding to the symmetric vibration of the uranyl ion are observed in the 677−738 cm−1 region. These results are in stark contrast to the empirically predicted value of 759 cm−1, seemingly emphasizing shortcomings in the empirical relationship, particularly where the U−Oyl bond length exceeds 1.8 Å (Figure 3). In contrast, the quantum-chemical approach developed by Vallet et al.46 relating the U−Oyl bond length of discrete species with the symmetric vibration of the uranyl ion affords good agreement between the predicted (717 cm−1) and experimental (avg. v1(UO22+): 710 cm−1) values (Figure 3). The assignment of v1(UO22+) for UTs has been experimentally confirmed here via isotopic substitution of the uranyl oxygens in the structure of NaUT with 18O (Figure S5), which results in a blue shift of signals related to the symmetric vibration of the uranyl from 680 and 727 cm−1 (U16O2) to 665 and 690 cm−1 (U18O2) (Figure 4). The lack of response to isotopic labeling of bands observed in the 800−900 cm−1 region is consistent with their assignment to peroxide, as the peroxide contained no 18O. To support the assignment of the Raman spectra, a benchmark study was performed for UO22+ and UT comparing bond lengths, bond angles, and available vibrational data. On the basis of these results we decided to employ the B3LYP/631+G* level of theory for the rest of the study. Four different models were designed in order to explore the influence of the counterions and their positions (Figures S11 and S12). The predicted Raman spectra obtained from models containing both mono and divalent cations display five distinguishable peaks in the 600−900 cm−1 region (Table S4). Overall, there is good agreement between the vibrations predicted in the 800−900 cm−1 region and the experimental spectra. Figure 5 compares the Raman spectra predicted using the B3LYP functional with those measured for KUT. The only vibrations appearing in the 800−900 cm−1 region were those due to the symmetric v1(O22−), asymmetric v2(O22−), and mixed symmetric/asymmetric v3(O22−) intramolecular vibrations of the peroxides (Figure 6). However, the peak positions present

to the identity of the counterion. The uranyl (U−Oyl) bond distances range from 1.840(6) to 1.873(12) Å (avg. 1.855 Å), with Oyl−U−Oyl bond angles ranging from 176.7(3) to 179.7(2)° (avg. 178.4°). The uranyl−peroxide oxygen (U− Oeq) bond distances range from 2.277(10) to 2.334(4) Å (avg. 2.305 Å). The O−O bond distances of the peroxide ligands range from 1.493(6) to 1.532(13) Å (avg. 1.503 Å). The oxidation state of U present in all of the UT salts was confirmed to be +6 on the basis of bond valence calculations (see the Supporting Information). The Raman spectra of the UTs consist of three basic regions. The first, from 100 to 600 cm−1, is attributed to combinations of bending vibrations of the uranyl, v2(UO22+), the U−Oeq vibrations, and M−O vibrations. The second region, 600−800 cm−1, consists of symmetric vibrations of the uranyl, v1(UO22+). Bands observed in the third region, 800−900 cm−1, are attributed to the intramolecular peroxide vibrations v1−3(O22−). Deconvolution analysis and assignment of bands present in the first region are not addressed here. The Raman spectra and a summary of the band positions, their nature, and their assignments are presented in Figure 2. DFT calculations reveal the Raman-active symmetric stretch of the uranyl ion around 614 cm−1 (for more information, see Table S4). Moreover, up to three peroxide-related vibrational modes are observed in the 802−867 cm−1 region for the most stable structures. A summary of the calculated bands, their assignments, and the corresponding vibrational modes is available in Table S4. The Raman-active symmetric vibration of uranyl, v1(UO22+), is typically observed in the 800−900 cm−1 region.42,43 An empirical formulation based on this assignment has been used to correlate the region of experimentally observed band positions with U−Oyl bond lengths ranging from 1.717 to 1.811 Å.9 A review of the crystal chemistry of the uranyl ion provided the average U−Oyl bond length of 1.79 Å, and for this the predicted v1(UO22+) position is 820 cm−1.44 This result is in C

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Figure 3. Empirically (green dotted line) and quantum-chemically (black dotted line) derived relationship between U−Oyl and v1(UO22+). Blue circles represent experimentally measured values. Error bars correspond to standard deviation of observed bond lengths (clusters) or standard uncertainties of measured bond lengths (monomers).

Figure 4. Raman spectra of isotopically neutral (black) and enriched (red) NaUT solids. Figure 6. Ball-and-stick representations of Raman (and IR)-active uranyl and peroxide vibrational modes. Yellow and red spheres correspond to uranium and oxygen, respectively.

model from the crystal structure. To maintain charge balance, four and six cations of the first coordination shell were added to the UT building blocks, respectively. Extension from the single UT monomer to a system containing two and three units induces a red shift of the predicted peak positions. The observed shift of the vibrations appearing in the 600−800 cm−1 region relative to the monomeric peak position is approximately 28 cm−1 for both of the Ca and Sr dimers, whereas shifts of 40 and 32 cm−1 are noted for the Ca and Sr trimers, respectively. These results reflect a transition from the aqueous solution Raman spectra extrapolated to the infinite solid Raman spectra (Table S5 and Figure S13). Deconvolution of the 600−900 cm−1 region of the KUT spectrum (Figure 7) revealed four strong bands located at 715, 814, 825, and 843 cm−1 and one weak band located at 760 cm−1. That at 715 cm−1 is assigned to v1(UO22+), which is consistent with trends observed in calculated models and the predicted value of 719 cm−1 (for U−Oyl = 1.8540 Å).46 The 760 cm−1 band is assigned to activation of the asymmetric vibration of uranyl, v3(UO22+) (predicted 778 cm−1), on the basis of its weak intensity and broad profile compared with the

Figure 5. Comparison of the experimental Raman spectrum of KUT (black) with the spectrum predicted using the B3LYP functional (red).

in the 600−800 cm−1 region associated with the stretching of the uranyl were underestimated (blue-shifted) for most cases. This is due to the slight overestimation of the U−Oyl bond length in the DFT-optimized models containing only one monomeric unit. The alkali and alkaline-earth cation series studied for the monomeric units present a similar behavior (see the Supporting Information). To address this problem, we modeled some systems containing two and three monomers (Figure S13) using Ca2+ and Sr2+ cations by crafting the neutral D

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The lack of literature reports concerning the asymmetric vibrations of peroxide ligands, v2(O22−), may relate to the frequency range of these bands, as these often coincide with the typical v1(UO22+) of uranyl peroxide nanoclusters (Figure 9).

Figure 7. Deconvolution of the 600−900 cm−1 region of the KUT spectrum. The red and black dotted lines correspond to the fitted sum and residual, respectively.

other signals. The signals at 814, 825, and 843 cm−1 are assigned in accordance with the trends observed in the calculated models to v2(O22−), v3(O22−), and v1(O22−), respectively. Comparison of Raman spectra arising from different UT salts reveals discrepancies that cannot be explained solely on the basis of structural differences of the cations. For example, the uranyl region (Figure 2) contains one major band for the K and Mg salts and multiple bands for the Na, Ca, and Sr salts. Although the ab initio calculations show activation of the asymmetric vibration of the uranyl, v3(UO22+), in models where the uranyl bond angle departs from its typically linear conformation,46 this is likely not the case here. The assignment of the signals at 680 and 727 cm−1 for NaUT to v1(UO22+) and 2+ v3(UO2 ), respectively, is inconsistent with their predicted values of 710 and 769 cm−1.46 We propose that both of these signals arise from the symmetric vibration of the uranyl as a result of perturbations of the idealized D3h symmetry, longrange cation-mediated interactions, crystal packing effects, or a combination of these. Assignment of all of the signals appearing in the 600−800 cm−1 region to v1(UO22+) is consistent with the Raman spectra obtained by dissolution of NaUT in ultrapure water, which resulted in convergence and red shifts of the uranyl and some of the peroxide bands (Figure 8). Consequently, the bands observed upon dissolution of NaUT at 723, 813, and 842 cm−1 are assigned to v1(UO22+), v2(O22−), and v1(O22−), respectively.

Figure 9. Raman spectra of crystalline LiU24 (black) and NaUT (red).

These vibrations may be present in the spectra for uranyl peroxide nanoclusters but obscured by v1(UO22+), resulting in characteristically asymmetric peaks.29,30 The disparity of the observed v1(UO22+) of the UTs relative to those of uranyl peroxide nanoclusters (e.g., 716 and 816 cm−1 for KUT and LiU24, respectively) is thought to be related to their U−Oyl bond lengths (1.854(6) and 1.789(45) Å for KUT and LiU24, respectively). Despite the similarity of the electronic environments about the U centers, the nonbridging nature of the peroxide ligands in UTs results in stronger equatorial bonds. This is reflected by the shortening of the U−Oeq bonds from 2.404(74) Å for LiU 24 to 2.314(16) for KUT and, consequently, elongation of the uranyl bonds.

4. CONCLUSIONS We have studied the nature of the Raman signals observed for the family of isostructural MUTs using a combination of experimental and computational approaches. This revealed a previously unobserved asymmetric vibration of the peroxide ligands, v2(O22−), and led to a reassignment of the identity of the signals appearing in the 600−800 cm−1 region to the symmetric vibration of uranyl, v1(UO22+). Furthermore, certain salts of UTs (Na, Ca, and Sr) exhibit splitting of the v1(UO22+) signal in the solid state, resulting in spectral complexities exceeding those expected from compounds with D3h molecular symmetry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02666. Additional details of materials synthesis, spectroscopic characterization data, electronic structure characterization of UT and MUT complexes, and relative energies, Raman spectra, and Cartesian coordinates of the computed structures (PDF) Crystallographic data for NaUT, KUT, MgUT, and SrUT (CIF)

Figure 8. Raman spectra of crystalline (black) and solvated (red) NaUT. E

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(12) Odoh, S. O.; Schreckenbach, G. DFT Study of uranyl peroxo complexes with H2O, F−, OH−, CO32‑, and NO3−. Inorg. Chem. 2013, 52, 5590−5602. (13) Soto, C. A. T.; Carauta, A. N. M.; Carneiro, J. W. D. M. DFT studies of structure and vibrational frequencies of isotopically substituted Diamin uranyl nitrate using relativistic effective core potentials. Spectrochim. Acta, Part A 2008, 71, 1140−1145. (14) Shundalau, M. B.; Zazhogin, A. A.; Zazhogin, A. P.; Komyak, A. I.; Umreiko, D. S. Structure and vibrational spectra of uranyl dinitrate complexes with water and DMSO. J. Appl. Spectrosc. 2015, 82, 25−32. (15) Oda, Y.; Aoshima, A. Ab initio quantum chemical study on charge distribution and molecular structure of uranyl (VI) species with Raman frequency. J. Nucl. Sci. Technol. 2002, 39, 647−654. (16) Garcia-Hernandez, M.; Willnauer, C.; Kruger, S.; Moskaleva, L. V.; Rosch, N. Systematic DFT study of gas phase and solvated uranyl and neptunyl complexes [AnO2X4]n (An = U, Np; X = F, Cl, OH, n = −2; X = H2O, n = + 2). Inorg. Chem. 2006, 45, 1356−1366. (17) Hattori, T.; Saito, T.; Ishida, K.; Scheinost, A. C.; Tsuneda, T.; Nagasaki, S.; Tanaka, S. The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study. Geochim. Cosmochim. Acta 2009, 73, 5975−5988. (18) Vazquez, J.; Bo, C.; Poblet, J. M.; de Pablo, J.; Bruno, J. DFT studies of uranyl acetate, carbonate, and malonate, complexes in solution. Inorg. Chem. 2003, 42, 6136−6141. (19) Ling, J.; Hobbs, F.; Prendergast, S.; Adelani, P. O.; Babo, J. M.; Qiu, J.; Weng, Z. H.; Burns, P. C. Hybrid uranium-transition-metal oxide cage clusters. Inorg. Chem. 2014, 53, 12877−12884. (20) Qiu, J.; Burns, P. C. Clusters of actinides with oxide, peroxide, or hydroxide bridges. Chem. Rev. 2013, 113, 1097−1120. (21) Qiu, J.; Ling, J.; Jouffret, L.; Thomas, R.; Szymanowski, J. E. S.; Burns, P. C. Water-soluble multi-cage super tetrahedral uranyl peroxide phosphate clusters. Chem. Sci. 2014, 5, 303−310. (22) Qiu, J.; Vlaisavljevich, B.; Jouffret, L.; Nguyen, L.; Szymanowski, J. E. S.; Gagliardi, L.; Burns, P. C. Cation templating and electronic structure effects in uranyl cage clusters probed by the isolation of peroxide-bridged uranyl dimers. Inorg. Chem. 2015, 54, 4445−4455. (23) Burns, P. C.; Kubatko, K. A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L. Actinyl peroxide nanospheres. Angew. Chem., Int. Ed. 2005, 44, 2135−2139. (24) Wylie, E. M.; Peruski, K. M.; Weidman, J. L.; Phillip, W. A.; Burns, P. C. Ultrafiltration of uranyl peroxide nanoclusters for the separation of uranium from aqueous solution. ACS Appl. Mater. Interfaces 2014, 6, 473−479. (25) Wylie, E. M.; Peruski, K. M.; Prizio, S. E.; Bridges, A. N. A.; Rudisill, T. S.; Hobbs, D. T.; Phillip, W. A.; Burns, P. C. Processing used nuclear fuel with nanoscale control of uranium and ultrafiltration. J. Nucl. Mater. 2016, 473, 125−130. (26) Burns, P. C.; Ewing, R. C.; Navrotsky, A. Nuclear fuel in a reactor accident. Science 2012, 335, 1184−1188. (27) Armstrong, C. R.; Nyman, M.; Shvareva, T.; Sigmon, G. E.; Burns, P. C.; Navrotsky, A. Uranyl peroxide enhanced nuclear fuel corrosion in seawater. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 1874− 1877. (28) Liao, Z. L.; Deb, T.; Nyman, M. Elucidating self-assembly mechanisms of uranyl peroxide capsules from monomers. Inorg. Chem. 2014, 53, 10506−10513. (29) Falaise, C.; Nyman, M. The key role of U28 in the aqueous selfassembly of uranyl peroxide nanocages. Chem. - Eur. J. 2016, 22, 14678−14687. (30) McGrail, B. T.; Sigmon, G. E.; Jouffret, L. J.; Andrews, C. R.; Burns, P. C. Raman Spectroscopic and ESI-MS characterization of uranyl peroxide cage clusters. Inorg. Chem. 2014, 53, 1562−1569. (31) Gaziev, S. A.; Gorshkov, N. G.; Mashirov, L. G.; Suglobov, D. N. Photostimulated oxygen-exchange and photochemical properties of uranyl ions. Inorg. Chim. Acta 1987, 139, 345−351. (32) Alcock, N. W. Crystal and molecular structure of sodium uranyl triperoxide. J. Chem. Soc. A 1968, 7, 1588−1594.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.C.B.). *E-mail: [email protected] (L.G.). ORCID

Laura Gagliardi: 0000-0001-5227-1396 Peter C. Burns: 0000-0002-2319-9628 Author Contributions ∥

M.D. and V.B. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontiers Center (DESC0001089). Raman spectroscopy measurements were conducted at the Materials Characterization Facility of the Center for Sustainable Energy at the University of Notre Dame. ICPOES measurements were conducted at the Center for Environmental Science and Technology at the University of Notre Dame.

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DOI: 10.1021/acs.inorgchem.6b02666 Inorg. Chem. XXXX, XXX, XXX−XXX