Neptunyl Peroxide Chemistry: Synthesis and Spectroscopic

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Neptunyl Peroxide Chemistry: Synthesis and Spectroscopic Characterization of a Neptunyl Triperoxide Compound, Ca2[NpO2(O2)3]·9H2O Sarah Hickam,† Debmalya Ray,‡ Jennifer E. S. Szymanowski,† Ru-Ye Li,‡ Mateusz Dembowski,§ Philip Smith,† Laura Gagliardi,‡ and Peter C. Burns*,†,§

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†

Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, United States § Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Little is known about the crystal chemistry of neptunyl peroxide compounds compared to uranyl peroxide compounds, for which dozens of structures have been described. Uranyl peroxides are formed over a broad range of pH and solution conditions, but neptunyl peroxide chemistry is complicated by the ability of H2O2 to act as an oxidizing or reducing agent for Np, depending on the conditions present. The combination of Np(V) in 1 M HCl, H2O2, and CaCl2 under alkaline conditions leads to the immediate crystallization of a neptunyl triperoxide monomer, Ca2[NpO2(O2)3]·9H2O, which is the first Np(VI)-based peroxide compound to be characterized in the solid state and is isostructural to Ca2[UO2(O2)3]·9H2O. The crystal structure reveals bond distances of 1.842(7) Å that are the longest reported to date for nonbridging Np(VI)-Oyl bonds. Computational studies probe the oxidation state and bond distances of the monomer unit and differences in Raman spectra of the neptunyl and uranyl triperoxide compounds.

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INTRODUCTION Neptunium, the first transuranium element discovered, is chemically complex, with accessible oxidation states ranging from Np(II) through Np(VII). Neptunium has no stable isotopes, and the longest-lived, neptunium-237 with a half-life of 2.14 million years, contributes significantly to long-term radioactivity of nuclear waste. 237Np is used in production of 238 Pu that fuels thermoelectric generators for space and other applications, as well as in neutron detectors. Despite its importance and complexity, Np is one of the least-studied early actinides.1 Although Np(V) is the most common oxidation state in solution, the rich redox chemistry of Np is likely important in systems containing hydrogen peroxide produced by alpha radiolysis of water near or in contact with radioactive materials.2−8 Specifically, H2O2 oxidizes Np(IV) to Np(V/ VI) in solutions containing H2O2 and NaHCO3,9 and reduces Np(VI) to Np(V) in solutions containing Na2CO3 and H2O2.10 This leads to complex chemistry that is dependent on solution conditions and initial oxidation state of Np and warrants more in-depth studies aimed at elucidating Np redox activity as a function of peroxide and hydroxide concentration.11 © XXXX American Chemical Society

In addition to the complex redox chemistry, the complexation of Np with peroxide is not well-understood, and the descriptions of peroxide complexes of Np that have been isolated in the solid state are scarce. Cubic and hexagonal forms of Np(IV) peroxide precipitates were characterized by X-ray diffraction and have similarities to Pu(IV) peroxides.12 A Np(V) peroxide precipitate was reported, although its composition and structure are unknown.13,14 Only one neptunyl peroxide-based crystal structure is known and is likely mixed-valence Np(V/VI).15 This is surprising in light of the tremendous structural and chemical diversity of uranyl peroxide materials that encompass over 60 unique structures.16−23 The complexity of uranyl peroxide structures arises from the pliable nature of the uranyl−peroxide−uranyl binding interactions, which facilitate formation of nanometer-scale, hollow, and anionic cages in addition to the simpler zero- to two-dimensional structures.16,20,22 The structural unit of the only known neptunyl peroxide cluster, {(NpO2 )(O2)(OH)}24x‑ (Np24) has a sodalite-type cage topology analogous Received: June 9, 2019

A

DOI: 10.1021/acs.inorgchem.9b01712 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Polyhedral representation of Ca2[NpO2(O2)3]·9H2O (A) projected along [100] and (B) along [010]. The unit cell is represented by dotted lines. [NpO2(O2)3] polyhedra are in green, Ca atoms are blue, and O atoms corresponding to water molecules are in red.

to U24, {(UO2)24(O2)24(OH)24}24−.15,24 This suggests that other neptunyl peroxide complexes should be attainable. Despite advances in understanding uranyl peroxide cluster chemistry, similar studies aimed at exploring the formation of Np24 are lacking.25−28 Several salts of uranyl triperoxide monomers, XmUO2(O2)3 (X = Na+, K+, Mg2+, Ca2+, or Sr2+, and m = 4 for X+, and m = 2 for X2+) have been described.16,29,30 Their importance in the assembly of uranyl peroxide cage clusters is a subject of ongoing research, and they have been identified as intermediate species in some cases.16,25,27,28 In the current study, a neptunyl peroxide monomer, Ca2[NpO2(O2)3]·9H2O, is described for the first time. Its facile synthesis allows for characterization using powder X-ray diffraction and Raman spectroscopy, and computational methods provide insight into the spectroscopic differences between uranyl and neptunyl peroxide compounds.

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absorption correction were accomplished using SADABS in the Bruker APEX III software package.31,32 SHELXTL was used for structure solution and refinement, and PLATON was further used to refine and verify the structure.33,34 Powder Diffraction. Using a pipet, crystals of 1 were transferred with a small quantity of mother liquor onto a glass slide and were allowed to dry. The slide was placed into an airtight holder with a domed lid and mounted onto a Bruker D8 Advance Davinci diffractometer equipped with Cu Kα radiation, collected over 5 to 50° 2θ with a 0.02° step increment and 2 s spent counting per step. The powder pattern was simulated using Mercury (CCDC) and the crystallographic data obtained from single crystal X-ray diffraction of 1. Raman Spectroscopy. Crystals of 1 were placed onto a glass slide with a welled-out center, and then a coverslip was placed over the well. The slide was then placed into a clean glass Petri dish that was tested for radioactive contamination before it was placed directly onto the stage of a Nikon optical microscope connected to a Bruker Sentinel system via a fiber optic probe. Spectra were acquired using a 785 nm excitation source over the 80 to 3200 cm−1 region with 100 mW laser power. Elemental Analysis. The solid was characterized for molar ratios of Ca:Np using inductively coupled plasma mass spectrometry (ICPMS), inductively couple plasma optical emission spectrometry (ICPOES), and liquid scintillation counting (LSC). Crystals were dissolved in 2% nitric acid and counted using a PerkinElmer tricarb 3110 TR liquid scintillation analyzer to determine Np concentration. Samples were counted for 20 min with background subtraction over beta-A region 1000, beta-B region 600, and alpha region 600, with a coincidence time of 18 ns, delay before burst of 75 ns, and tSIE quench indicator. After determining Np concentration by LSC, the sample was analyzed for Np and Ca concentrations using ICP-MS (Np/Ca) and ICP-OES (Ca). For ICP-MS, samples were diluted using 2% HNO3 and spiked with 1 ppb Tl and In and analyzed using a Nu Instruments Attom ICP-MS. For ICP-OES, samples were diluted using 2% nitric acid with internally spiked standards of 0.5 ppm Y and analyzed using a PerkinElmer Avio 200. Thermogravimetric Analysis. Water content was determined by heating approximately 3 mg of 1 in a TA Instruments TGA Q50 thermogravimetric analyzer. Crystals were loaded into an alumina crucible with a lid and heated at 5 °C/min to 900 °C under nitrogen gas flowing at 60 mL/min. Computational Characterization. A cluster model of Ca2[M(VI)O2(O2)3] where M = Np or U was extracted from the experimental crystal structure of 1 and was optimized using the Gaussian 09 package.35 Six different positions of Ca ions were chosen for initial starting points, and the entire model was optimized using the PBE,36,37 PBE0,36−38 B3LYP,39,40 and M06-L41 density functionals. Further, Ca2[Np(V)O2(O2)3]− geometries were optimized using the same functionals in order to compare Np(VI) and Np(V) structures with experimental values. The SMD solvation model42 with

EXPERIMENTAL SECTION

Caution: 237Np is an alpha emitter and it decays to 233Pa, which undergoes beta decay. Precautions must be taken to prevent area or personnel contamination, including working in a glovebox or radiological fume hood and the use of multiple layers of containment when analyzing and transporting samples. Synthesis. 237Np solids recovered from previous experiments were dissolved in 12 M HCl, followed by separation of Np from impurities using ion exchange columns containing Dowex 1-X8 resin and precipitation steps. A solution was then made by distilling Np in 6 M HCl to dryness, followed by washing with concentrated HNO3 and drying again. The precipitate was then dissolved into 4 M HNO3 and NaNO2 was added to reduce the Np to Np(V), followed by precipitation by adding NaOH. The resulting precipitate was washed using ultrapure water and was finally dissolved in 1 M HCl to produce a 100 mM Np(V) stock. The oxidation state of Np was confirmed by UV−vis−NIR spectroscopy, and the concentration of Np was determined by liquid scintillation counting (Supporting Information). Ca2[NpO2(O2)3]·9H2O (1). In a glass vial, 100 μL of the turquoise-green 0.1 M 237Np in 1 M HCl stock was combined with 100 μL of 30% H 2O 2, 75 μL of CaCl 2, and 100 μL of tetramethylammonium hydroxide (TMAOH). Introduction of H2O2 and CaCl2 to the stock Np solution did not cause a visible reaction, but upon addition of TMAOH, dark red acicular crystals of 1 formed immediately. Single Crystal X-ray Diffraction. A crystal of 1 was epoxymounted onto a tapered glass fiber. Diffraction data was collected on a Bruker APEX II single-crystal diffractometer using graphite monochromated Mo Kα X-ray radiation and a ω−θ scanning mode. During collection, flowing nitrogen gas was used to cool the crystal to 120 K. Data integration and correction for background, Lorentz, and polarization was done using SAINT, and scaling and an B

DOI: 10.1021/acs.inorgchem.9b01712 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry water as solvent was used for optimization. SDD pseudopotentials were used for Np and U centers, and the def3-TZVP basis set was used for Ca and O centers. Vibrational frequencies were computed to confirm that the optimized structures are local minima.

Np(VI) neptunyl hexagonal bipyramids. In April 2019, the Inorganic Crystal Structure Database (ICSD) contained welldetermined structures for 38 compounds containing 49 symmetrically distinct Np(VI) sites. Eleven compounds contain 13 Np(VI) neptunyl hexagonal bipyramids with average neptunyl and equatorial bond lengths of 1.747 (σ = 0.015) and 2.471 (σ = 0.072) Å, respectively. Twenty-nine examples of Np(VI) neptunyl pentagonal bipyramids occur in 22 compounds and have average neptunyl and equatorial bond lengths of 1.749 (σ = 0.020) and 2.378 (σ = 0.054) Å, respectively. There are seven examples of the less-common Np(VI) neptunyl square bipyramids in five compounds with average neptunyl and equatorial bond lengths of 1.767 (σ = 0.026) and 2.274 (σ = 0.023) Å, respectively. Across all 38 compounds, the maximum and minimum neptunyl bond lengths are 1.819 and 1.706 Å, and only two compounds have neptunyl bond lengths greater than 1.79 Å. A powder X-ray diffraction pattern collected for powdered crystals (Figure 3) is in good agreement with the pattern

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RESULTS AND DISCUSSION Crystal Structure. Ca2[Np(VI)O2(O2)3]·9H2O (1) crystallizes in Pbcn (Figure 1) with a = 9.5263(17), b = 12.022(2), and c = 12.224(2) Å and is isostructural with Ca2[UO2(O2)3]· 9H2O (Pbcn, a = 9.576(3), b = 12.172(3), c = 12.314(2) Å).16 The single unique Np site in the structure of 1 is strongly bonded to two oxygen atoms (Oyl) with bond lengths of 1.843(7) Å, forming a neptunyl ion (NpO22+) with a O NpO bond angle of 177.1(4)°. The neptunyl ion is coordinated by three bidentate peroxo ligands, with Np−Oeq bond lengths in the range of 2.278(8) to 2.290(7) Å, resulting in a neptunyl hexagonal bipyramid (Figure 2).

Figure 2. Ball and stick representation of the neptunyl coordination environment in 1, with Np (green) coordination to O (red) atoms. Average bond distances to yl and equatorial O atoms are indicated.

The single crystallographically unique Ca site in 1 is coordinated by nine ligands consisting of yl and peroxo O atoms and four water molecules with an average Ca−O bond length of 2.474 Å. Each neptunyl hexagonal bipyramid is coordinated to four Ca atoms, which form linkages between Np polyhedra. Each linkage consists of two Ca atoms, which have side-on interactions with shared peroxo ligands, and results in chains that propagate along [001]. Bond valence calculations43 and charge balance requirements indicate the Np in 1 is Np(VI), with a bond valence sum of 6.45 valence units (vu).44 In Ca2[U(VI)O2(O2)3]·9H2O the bond valence sum at the U(VI) site is 6.60 vu.29 Both bond valence sums are higher than the expected values of the formal valences, and this arises largely from the equatorial O atom bond distances in 1 (2.285(5) Å) that are short due to the strong donor contribution from peroxide ligands to the Np center. Such contributions for uranyl−peroxo interactions, which have some covalent character,45,46 result in elongation of UOyl bond distances from the typical average value of 1.783 Å to an average of 1.855 Å in uranyl triperoxide monomers.29,47 The volume of the Voronoi−Dirichlet polyhedron of the Np site (VVDP = 8.76 Å3) as well as the radius of a sphere of the volume equal to VVDP (Rsd = 1.279 Å), calculated using ToposPro software,48 also confirm the presence of Np(VI).49 Np(V) was oxidized to Np(VI) prior to or during formation of 1. The bond lengths about the Np cation in 1 are extraordinary in that it has the longest Np(VI) neptunyl bonds reported for an inorganic crystal structure that is not a NpOyl bond involved in a cation−cation interaction, and equatorial Np−O bond lengths that are much shorter than in other reported

Figure 3. Powder X-ray diffraction patterns of Ca2[NpO2(O2)3]· 9H2O (1) crystals and the simulated pattern from the crystal structure.

simulated from the refined structure parameters, with no additional phases detected. Elemental analysis confirmed the presence of Ca and Np in the crystals of 1, although widely varying Ca concentrations in different samples indicate the presence of a Ca-bearing impurity that may be X-ray amorphous. DFT Simulations of Np Coordination Environments. DFT simulations provided the relative stabilities and optimized geometries of six model clusters (Figure 4) of Ca2[M(VI)O2(O2)3], M = Np or U, that differ in the positions of the Ca cations. Models I, III, and V are the most energetically favorable and are within 20 kJ/mol among each other, for both neptunium and uranium triperoxide and are discussed here, with more details in the Supporting Information. The optimized NpOyl bond distances in the Ca2[Np(VI)O2(O2)3] models range between 1.844 and 1.920 Å depending on the model and level of theory (Table 1). The average PBE0 predicted NpOyl bond lengths of 1.845 Å for Models I, III, and V are indistinguishable from the experimentally determined bond length of 1.843 Å, whereas those derived at other levels of theory are all larger than the experimental value. The predicted Np−Oeq bond lengths range between C

DOI: 10.1021/acs.inorgchem.9b01712 Inorg. Chem. XXXX, XXX, XXX−XXX

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pH of this solution is lower than that of uranyl nitrate dissolved in water) and observed the immediate precipitation of studtite. Incorporation of Np into the structure of [UO2(O2)(H2O)2]· 2H2O has been postulated, although subsequent incongruent dissolution of Np from the solid in those studies suggests that Np was not incorporated.51 An alternative interpretation for this result was that the Np was instead reduced and may have formed a Np(IV)-based precipitate.52 The Np analog of studtite (i.e., [Np(VI)O2(O2)(H2O)2]·2H2O) remains unknown. The Raman spectrum of a solution created by addition of 30% H2O2 to the Np(V) stock in 1 M HCl solution (Figure 5)

Figure 4. Six different initial models of Ca2[(M(VI)O2)(O2)3], where M = U and Np. Green, red, and blue spheres represent actinide, oxygen, and calcium, respectively.

2.304 Å and 2.361 Å for the different levels of theory and are slightly longer than the experimentally determined value of 2.278 Å. The calculated Oeq−Oeq bond lengths of the peroxo ligands are between 1.445 and 1.474 Å. The computed bond lengths of hypothetical Ca2[Np(V)O2(O2)3]− species are summarized in Table 1. The calculated Np(V)Oyl bond lengths range from 1.953 to 2.018 Å, which are all much longer than the experimentally determined bond distance in 1. The calculated equatorial Np−Oeq bond lengths range from 2.364 to 2.428 Å, which are also longer than the experimental and predicted distances for Np(VI). The DFT calculated Ca2[Np(VI)O2(O2)3] geometries are in much better agreement with the experimentally determined geometries of 1 than those of Ca2[Np(V)O2(O2)3]−, which supports our assignment of Np(VI) in 1. Neptunium Oxidation and Speciation during Synthesis of Ca2[Np(VI)O2(O2)3]·9H2O. During the preparation of 1, it is notable that there is no visible reaction upon addition of H2O2 to the Np(V) in 1 M HCl stock, whereas addition of H2O2 to a uranyl nitrate solution results in immediate precipitation of studtite, [UO2(O2)(H2O)2]·2H2O.20,25,50 We also dissolved uranyl nitrate in 1 M HCl and added peroxide in the same proportion as for the reaction that produced 1 (as the

Figure 5. Solution Raman spectra of Np(V) in 1 M HCl combined with 30% H2O2.

contains a strong mode at 875 cm−1 due to unbound peroxide and a mode at 766 cm−1 due to the symmetric vibrational mode of the Np(V)O2+ ion. A Raman mode at 766 cm−1 is observed for the Np(V)O2+ in 1 M HCl stock solution with no added H2O2.53 Actinyl vibrational modes relate to AnOyl bond lengths and the number and type of coordinating ligands. Complexation of Np by peroxide should red-shift the symmetric neptunyl vibrational mode.45,54,55 The lack of shift of the symmetric neptunyl vibrational mode and the presence

Table 1. Comparison of Np(VI)−O and Np(V)−O Predicted Bond Lengths and Angles Using PBE, PBE0, M06-L, and B3LYP Density Functionals for Model Va Ca2[Np(V)O2(O2)3]−

Ca2[Np(VI)O2(O2)3] Density Functional

Np−Oyl(Å)

Np−Oeq(Å)

Oyl−Np-Oyl(deg)

Np−Oyl(Å)

Np−Oeq(Å)

Oyl−Np-Oyl(deg)

PBE PBE0 M06-L B3LYP PBE PBE0 M06-L B3LYP PBE PBE0 M06-L B3LYP

1.920 1.846 1.890 1.877 1.910 1.844 1.892 1.883 1.913 1.844 1.886 1.887 1.842(7)

2.346 2.323 2.361 2.359 2.343 2.309 2.361 2.344 2.335 2.304 2.349 2.338 2.285(5)

172.1 175.9 172.6 174.1 176.3 177.4 176.2 175.7 174.8 176.8 175.0 174.9 177.2(4)

2.018 1.976 1.998 1.997 2.005

2.382 2.364 2.399 2.428 2.398

169.3 167.6 169.5 166.4 175.0

1.974 1.977 1.996 1.953 1.976 1.975

2.422 2.419 2.392 2.364 2.411 2.419

176.2 176.2 174.8 172.5 175.4 174.1

Model I

Model III

Model V

Experimental Result a

The average of all DFT results for those models is also shown. D

DOI: 10.1021/acs.inorgchem.9b01712 Inorg. Chem. XXXX, XXX, XXX−XXX

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(AnO22+), however, differ with an ca. 20 cm−1 red shift of the neptunyl signal as compared to those of uranyl (Table 2).29

of free peroxide indicate that neither oxidation of Np(V) nor complexation of neptunyl with peroxide occurs during the synthesis of 1 prior to the addition of CaCl2 and TMAOH, in stark contrast to the analogous reaction involving U(VI). Upon addition of tetramethylammonium hydroxide (TMAOH) to a mixture containing stock Np(V) in 1 M HCl, H2O2, and CaCl2, the solution color changes from green to red, and 1 immediately begins to crystallize from the solution with pH ∼ 12. The oxidation of Np and complexation of neptunyl with peroxide occurs quickly upon addition of TMAOH. Studies of systems containing Np(V) and H2O2 are limited. Oxidation of Np(III) and Np(IV) by hydrogen peroxide has been observed,9,56 and studies of Np(VI) interaction with H2O2 mostly indicate that Np(VI) is reduced to Np(V), and some Np(IV) is produced at high H2O2 concentrations.10,57 One study postulated the formation of a Np(VI) peroxo complex in alkaline conditions on the basis of reaction rates.10,58 A study of Np(V) in solutions containing NaHCO3 and H2O2 indicated Np(V) was oxidized to Np(VI).59 As observed for 1, Np(V/VI)24 clusters formed when hydroxide base and excess H2O2 were added to a Np(V) stock solution. Again, oxidation of Np(V) to Np(VI) occurs rapidly in alkaline, peroxide-rich systems. For uranyl peroxide systems, isolation and characterization of compounds has aided identification of corresponding solution species. Falaise et al. identified monomeric peroxide species in solutions that later contained uranyl peroxide clusters. A potassium salt of a uranyl triperoxide monomer assembled into pentameric species and uranyl peroxide clusters over time.28 Isolation of a neptunyl triperoxo compound in the solid state here is hopefully a significant step toward understanding neptunyl speciation in conditions that may lead to the formation of neptunyl peroxide clusters, such as Np24. Spectroscopic Comparisons of Neptunyl and Uranyl Compounds. The observed Raman spectra of 1 and Ca2[UO2(O2)3]·9H2O are shown in Figure 6. Both contain broad signals between 650 and 750 cm−1 and sharper signals at 808 and ∼834 cm−1.29 The modes above 800 cm−1 are bound peroxide vibrational modes and are similar for the uranyl and neptunyl compounds. The broad signals between 650 and 750 cm−1 assigned to symmetric vibrations of the actinyl ions

Table 2. Comparison of Symmetric (v1) Uranyl and Neptunyl Vibrational Modes from Experimental Raman Spectra and Computed Spectra for Model V and Different Density Functionals (cm−1) Functional

v1(UO22+)

v1(NpO22+)

Difference in v1

PBE PBE0 M06-L B3LYP Experimental Result

603 679 609 646 696, 739

597 672 600 637 683, 718

6 7 9 9 13, 21

Although wavenumbers less than 800 cm−1 are unusual for symmetric uranyl vibrational modes, theoretical and empirical predictions based on the observed UOyl bond distances confirmed this assignment.45 We have calculated Raman spectra for Ca2[Np(VI)O2(O2)3] and emphasize the region between 550 and 900 cm−1. Simulations at different levels of theory all support assignment of neptunyl vibrational modes for the signals in the region of 700 cm−1 (Figure 7).

Figure 7. Experimentally determined Raman spectrum of 1 and the predicted Raman spectra from DFT functionals of Model V.

The Raman spectra of the Ca salts of the neptunyl and uranyl triperoxide monomers show multiple signals in the v(NpO22+) region that were assigned to the symmetric vibrational mode of the uranyl in the previous study.29 Although asymmetric modes, v3(UO22+), may become Raman active when the symmetry of the uranyl moiety is reduced, the multiple actinyl signals in Raman observed here are likely not asymmetric in origin.60 Such signals in Raman would be weaker in intensity than v1(AnO22+), which is not our observation,61 and, although computational predictions of the Raman spectra include asymmetric signals, v3(NpO22+), they are usually of very low intensity and arise due to the distortion of the Oyl−Np−Oyl angle in the computational models. The Raman spectra of various salts of the uranyl triperoxide monomers show differing uranyl signals, including one or more peaks, despite their similar UOyl bond distances.29 The averages of the signals, however, are similar, and indicate splitting of v1(UO22+). Further, the infrared spectrum of

Figure 6. Raman spectra of crystals of the Ca salts of the uranyl and neptunyl triperoxide monomer crystals. E

DOI: 10.1021/acs.inorgchem.9b01712 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Ca2[UO2(O2)3]·9H2O contains a peak at 782 cm−1 that is assigned to v3(UO22+) on the basis of the observed relationship between bond length and v3(UO22+) as well as several equations relating symmetric and asymmetric uranyl modes (Supporting Information).60 Although the cause of multiple signals is not well-understood, we also attribute the multiple signals here to v1(NpO22+). In the 800 to 850 cm−1 region of the Raman spectrum of 1, the peaks assigned to symmetric and asymmetric stretching of peroxo groups (v1(O22−), v2(O22−), and v3(O22−)) were better reproduced by the M06-L and PBE functionals. Use of PBE0 results in an underestimation of the Oeq−Oeq bond lengths, and thus the Raman peaks obtained from the PBE0 functional in this region are red-shifted relative to experimental Raman peaks. Detailed assignments of Raman spectra are in Table S3. Computed Raman spectra of Ca2[Np(VI)O2(O2)3] and Ca2[U(VI)O2(O2)3] are compared using new calculations for Ca2[U(VI)O2(O2)3] to minimize differences of models and density functionals. As in the experimental Raman spectra, a red shift of the v1(NpO22+) peak compared to the v1(UO22+) peak was evident in the simulated spectra (Table 2). For the v1(O22−) peak, a blue shift for neptunyl triperoxide relative to the uranyl triperoxide is seen both experimentally and computationally. The literature contains only a few examples of spectroscopic studies of isostructural uranyl and neptunyl compounds. Uranyl and neptunyl iodates AnO2(IO3)2(H2O) (An = U, Np) produced Raman spectra displaying symmetric stretching modes at 879 and 872 cm −1 , respectively. 62 Raman spectroscopic studies of (Me4N)2[(AnO2)2(OH)2)(NO3)4] showed v1(UO22+) signals at 846−1 and v1(NpO22+) signals at 837 cm−1.63 Crystals of [Co(NH3)6]2[AnO2(OH)4]3·H2O produce Raman spectra that exhibit symmetric actinyl stretching modes at 796 and 742 cm−1 for v1(UO22+) and v1(NpO2+), respectively, although the spectra of the Np solids sometimes displayed second peaks at 769 cm−1.64,65 In solution, the NpO22+ and UO22+ ions produce Raman symmetric vibrational modes at 854 and 869 cm −1 , respectively. 66,67 The differences in v 1 (NpO 2 2+ ) and v1(UO22+) range from 7 to 54 cm−1, with the Ca salts of the uranyl and neptunyl triperoxide monomers falling in the middle of this range. Based on the available data for isostructural compounds, the v1(NpO22+) signals consistently appear at lower wavenumbers relative to v1(UO22+). Plotting the AnOyl bonds against the frequency of their vibrations yields nearly parallel linear trends in agreement with prior studies of uranium systems (Supporting Information).45,54 Comparison of individual data points, however, shows no direct correlation, preventing reliable prediction of a neptunyl vibration on the basis of a spectrum of an isostructural uranyl complex, necessitating further studies.

solution of Np(V) in 1 M HCl did not cause a reaction, in contrast to the analogous system containing U(VI) in which studtite rapidly precipitates upon addition of H2O2. Addition of base to the Np(V) solution increased the pH to 12 and triggered the rapid precipitation of Ca2[NpO2(O2)3]·9H2O as well as a color change for the solution from green to red. Computational studies have provided further insight into the energetics of Ca2[NpO2(O2)3]·9H2O and its spectra. We have expanded on the Raman studies of the uranyl peroxide monomers to demonstrate that neptunyl vibrational modes can be predicted for similar structures, which may be useful when characterizing Np-containing phases. In isostructural neptunyland uranyl-based compounds, neptunyl vibrational modes are red-shifted with respect to uranyl by 7 to 54 cm−1. The library of vibrational spectra of neptunyl compounds is small compared to uranyl compounds; thus, further studies are needed to develop empirical data concerning bond strengths and formulas that describe spectra of neptunyl compounds. Future studies of Np peroxide compounds, both experimental and computational, may reveal additional information about the relative energetics of phases and Np peroxide chemistry.

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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.9b01712. Bond valence calculations, thermogravimetric, UV−vis− NIR, and FT-IR results, computational results, crystallographic tables, and coordinates for DFT-optimized models (PDF) Accession Codes

CCDC 1921800 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Debmalya Ray: 0000-0002-8309-8183 Mateusz Dembowski: 0000-0002-6665-8417 Laura Gagliardi: 0000-0001-5227-1396 Peter C. Burns: 0000-0002-2319-9628 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the U.S. Department of Energy, National Nuclear Security Administration, under Award Number DE-NA0003763. The authors thank Dr. Sergey M. Aksenov for his help with the VDP analysis. We thank the Materials Characterization Facility for some of the instrumentation used in this work.

CONCLUSIONS A new neptunyl peroxide compound, Ca2[NpO2(O2)3]·9H2O, has been synthesized and characterized in the solid state. This appears to be the first instance in which a Np(VI) peroxide compound has been isolated. As for uranyl triperoxide monomers, the interaction of peroxide with the actinide center results in lengthening of the NpOyl bond. The structure described here contains the longest nonbridging Np(VI)Oyl bond distance reported to date. The synthesis of 1 adds to knowledge of Np chemistry in alkaline, peroxide-rich conditions. Addition of H2O2 to a 0.1 M

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