Molecular Hydroxo-Bridged Dimers of Uranium(VI), Neptunium(VI

Feb 12, 2019 - The synthesis of a series of molecular actinyl(VI), namely, uranium(VI), neptunium(VI), and plutonium(VI), hydroxo-bridged dimers is re...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Molecular Hydroxo-Bridged Dimers of Uranium(VI), Neptunium(VI), and Plutonium(VI): [Me4N]2[(AnO2)2(OH)2(NO3)4] Matthieu Autillo and Richard E. Wilson*

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Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: The synthesis of a series of molecular actinyl(VI), namely, uranium(VI), neptunium(VI), and plutonium(VI), hydroxo-bridged dimers is reported. These complexes were isolated from an aqueous nitrate solution by titration with tetramethylammonium hydroxide. The solid-state structures were determined using single-crystal X-ray diffraction, revealing molecular complexes with the formula [Me4N]2[(AnO2)2(μ2-OH)2(NO3)4], where An = UVI, NpVI, and PuVI. Spectroscopic dataUV−vis−near-IR absorption, IR, and Ramanwere collected on the solutions and solidstate complexes where available and compared to those of the aqueous solutions from which the crystals formed. These data provide structural evidence for higher-order polynuclear complexes of actinyl(VI) complexes upon a pH increase in the aqueous solution, confirming earlier thermodynamic models.

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INTRODUCTION The actinide elements occupy a unique position in the periodic table with chemical behaviors intermediate to the transition metals and the lanthanides. The filling of the 5f orbitals along with the increasing influences of relativistic effects imparts a rich and complex chemistry to the 5f elements that is not observed for the 4f lanthanides.1 Of the many consequences of these two effects are the rich redox chemistry of the early actinides and the existence of the linear dioxo cations, the actinyl cations AnO2x+, in both penta- and hexavalent oxidation states.2,3 The most common of these ions is the uranyl(VI) cation, whose chemistry is among the most widely explored for the actinide elements and also possesses a rich natural mineralogy.4 Less fully explored are its periodic homologues NpO22+, PuO22+, and AmO22+, which are manmade elements without any natural precedent. Considerable work has been conducted on the aqueous chemistry of these ions, focusing on their reactivity under environmental conditions and their behavior in separations for nuclear energy systems.5,6 Among the most fundamental reactions of metal ions in aqueous systems is the hydrolysis reaction. Thermodynamic studies have been conducted on the hydrolysis reactions of the actinide elements, resulting in volumes of tabulated and critically reviewed data.7−10 Less well-known are the structural properties of the proposed species and the trends in structure across the series. Prior studies of the hydrolysis of actinyl(VI) ions have postulated that polynuclear hydrolysis species exist in aqueous solution, particularly under neutral-to-alkaline conditions, and assignments of their thermodynamic stability constants have been tabulated.8,11−16 These studies have employed potentiometric titrations coupled with UV−vis−near-IR (NIR) absorption spectra and vibrational (IR and Raman) spectros© XXXX American Chemical Society

copy to determine the speciation of these ions in aqueous solution. For the particular cases of polynuclear complexes, potentiometric methods become less reliable because, in general, only an empirical formula for the species can be determined reliably. Nevertheless, the original studies and critically reviewed compendia of these data suggest that, from acidic to neutral pH, there are three principle species to be considered in the hydrolysis of the An(VI) ions: AnO2OH+, (AnO2)2(OH)22+, and (AnO2)3(OH)5+.9,12,16−19 Limited direct structural evidence exists for these species among the transuranic actinyl(VI) ions, although both solution-based Xray absorption spectroscopy and single-crystal X-ray diffraction studies have identified such species for uranyl(VI).20−25 Successful applications of solution-based high-energy X-ray scattering techniques have conclusively identified polynuclear hydrolysis species in aqueous solutions for a variety of tetravalent actinide ions, including their dimers and higherorder oligomers.26−29 Such experiments have been successful in correlating the solution speciation of these solutions with the resulting solid-state precipitates, thus providing direct structural correlations between the thermodynamics of these systems and their structural properties.30,31 Prior scattering experiments seeking similar correlations in the UVI-HBr(aq) system were less successful in identifying polynuclear species in the HBr system.32 In contrast to the tetravalent actinide ions, which show a periodic increase in their acidity as Z is increased, the opposite occurs for the hexavalent actinide(VI) ions, despite their contraction in size. As an example, if the first hydrolysis constant log β1,1 is converted to a pKa, then for the tetravalent Received: November 27, 2018

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DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Parameters for the Reported Crystal Structures

a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Å) μ (mm−1) ρ (g cm−3) R1 wR2 CCDC

(Me4N)2[(UO2)2 (OH)2(NO3)4], triclinic P1̅

(Me4N)2[(NpO2)2 (OH)2(NO3)4], triclinic P1̅

(Me4N)2[(PuO2)2(OH)2(NO3)4], triclinic P1̅

(Me4N)2[(PuO2)2(OH)2(NO3)4], monoclinic P21/c

(Me4N)2[(PuO2)2(OH)2(NO3)4], monoclinic P21/n

6.874(1) 9.88(1) 9.885(1) 79.522(1) 85.459(1) 71.902(1) 627.54(7) 1 100 0.71073 12.973 2.568 0.0152 0.0345 1870499

6.854(2) 9.839(1) 9.888(1) 79.255(1) 71.556(1) 84.758(1) 621.08(8) 1 100 0.71073 8.406 2.589 0.0138 0.0324 1870500

7.379(1) 9.731(2) 9.775(2) 85.183(3) 76.054(3) 74.966(3) 657.7 1 100 0.71073 5.050 2.465 0.0712 0.1453 1870501

15.430(1) 12.845(1) 12.432(1) 90.00 97.111(1) 90.00 2445.0(3) 4 100 0.71073 5.433 2.652 0.0370 0.0849 1870502

9.321(1) 12.744(1) 10.878(1) 90.00 102.098(1) 90.00 1263.38(16) 2 100 0.71073 5.257 2.566 0.0241 0.0589 1870503

Table 2. Selected Bond Lengths (Å) and Angles (deg) for the Reported Actinyl(VI) Dimers (Me4N)2[(UO2)2(OH)2(NO3)4] An−Oyl An−OH An−O2NO An−NO3 An−An Oyl−An−Oyl An−O2NO deviation from the plane

1.776(1), 1.784(1) 2.325(1), 2.340(1) 2.533(2), 2.554(2), 2.559(1), 2.563(1) 2.987(2), 2.993(2) 3.879(1) 177.04(6) 13.05°, 11.11°, 3.01°, 3.97°

(Me4N)2[(NpO2)2(OH)2(NO3)4] 1.753(2), 1.760(2) 2.312(2), 2.315(2) 2.526(2), 2.541(2), 2.558(2), 2.562(2) 2.981(2), 2.985(2) 3.832(1) 178.28(8) 11.90°, 10.40°, 5.17°, 5.43°

(Me4N)2[(PuO2)2 (OH)2(NO3)4] 1.710(12), 1.717(10) 2.303(9), 2.316(8) 2.496(10), 2.509(10), 2.509(10), 2.515(9) 2.931(12), 2.945(13) 3.897(1) 177.8(5) 3.54°, 3.54°, 2.60°, 1.99°

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RESULTS Structural Description. The hydroxo-bridged dimers were crystallized from aqueous solutions of the actinyl(VI) nitrate salts after the addition of 1 equiv of nitric acid and 2 equiv (mol/mol) of tetramethylammonium hydroxide and subsequent evaporation under air. Single crystals were selected for X-ray diffraction studies at 100 K; the crystallographic parameters for each of the complexes are presented in Table 1, with selected bond distances and angles presented in Tables 2 and 3. A representative model of the complexes is presented

actinide ions, thorium is the least acidic with a pKa of 2.2, while plutonium(IV) is the most acidic with a pKa of −0.6. Conversely, for the hexavalent actinides, the pKa of UO22+ is 5.2, while PuO22+ is less acidic at 5.5, the degree to which this trend changes depends on the chosen data.8 Such a trend is contrary to our chemical intuition and has been hypothesized previously to be indicative of the changing 5f and 6d character to the bonding in these complexes, making investigations into their structures and spectroscopic properties useful for understanding periodic trends in both speciation and electronic structure across the entire actinide series.12 Herein, we report the synthesis and structural and spectroscopic characterization of an isostructural series of hexavalent actinide hydroxo-bridged dimers that crystallize in the triclinic space group P1̅. In addition to these complexes, two additional phases of the plutonium(VI) compound have been synthesized. All of these complexes have been crystallized by evaporation from aqueous solution and their solid-state structures determined by single-crystal X-ray diffraction measurements. Spectroscopic studies of these molecules including UV−vis−NIR, Raman, and Fourier transform infrared (FT-IR) spectroscopy were conducted on the solids and solutions from which they were produced in order to correlate the crystallized molecular complex in the solid state and the solution speciation prior to crystallization. The utility of studying the chemistry of the actinyl(VI) ions as a periodic series allows us to understand the chemical effects that come with filling of the 5f electronic shell and how it influences the structure, reactivity, and energetics and also serves to inform computational and theoretical chemists investigating the electronic structure of these complex elements.

Table 3. Selected Bond Lengths (Å) and Angles (deg) for the Reported Monoclinic (Me4N)2[(PuO2)2(OH)2(NO3)4] Complexes An−Oyl An−OH An−O2NO An−NO3 An−An Oyl−An−Oyl

P21/c

P21/n

1.744(5), 1.745(5) 2.324(5), 2.325(4) 2.491(6), 2.510(5), 2.520(5) , 2.520(5) 2.931(7), 2.945(6) 3.9298(4) 179.0(3)

1.745(3), 1.745(3) 2.314(3), 2.333(2) 2.490(3), 2.500(3), 2.507(3) , 2.510(3) 2.935(4), 2.945(4) 3.9297(3) 178.92(14)

in Figure 1. Unlike the uranium(VI) and neptunium(VI) syntheses, three different unit cells were identified for plutonium(VI) from the same synthesis (Table 1). In general, the structures of the complexes contain two nearly linear actinyl(VI) ions bridged by two μ2-OH ligands to form the dimeric molecule. The remainder of the equatorial coordination sphere of the actinyl(VI) ions is occupied by κ2NO3− ligands, resulting in a dimer of two edge-sharing B

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

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because one would expect more crowding about the smaller plutonium(VI) ion and thus more puckering, as evidenced by the shorter average Pu−O2NO bond distances of 2.51(1) Å compared to 2.55(1) Å for uranium(VI) and 2.55(1) Å for neptunium(VI). In the case of PuVIO22+, two additional phases were synthesized using the same synthetic protocol. These phases share the same chemical formula as the triclinic structures but crystallize in the monoclinic space groups P21/c and P21/n. The structure that crystallizes in P21/n is very similar to those in the triclinic symmetry except for the elongated plutonyl bond distances of 1.745(3) Å compared to the triclinic structure with an average Pu−Oyl bond distance of 1.71(1) Å. The same elongated plutonyl bond is observed in the P21/c phase with an average Pu−Oyl bond of 1.75(1) Å. In both monoclinic phases, the Pu−Pu distances are also slightly longer than those observed in the triclinic phase, ∼3.93 Å compared to ∼3.90 Å. Notably, the P21/c structure is considerably bent across Pu−OH−Pu, deviating approximately 15° out of the Pu−Pu plane (Figure 1b). The relevant bond distances describing these two structures are provided in Table 3. Spectroscopic Characterization. The actinyl(VI) dimers were further characterized using Raman, FT-IR, and UV−vis− NIR spectroscopy with the goal of understanding whether the dimeric species were present in the mother solution that produced the crystals and also the properties of this set of molecules as a periodic series within the actinides. Trends in the vibrational data have been established, aimed at correlating the vibrational frequencies of actinyl ions with their bonding properties, and a complete set of homologous actinyl(VI) complexes across the series are useful for developing these trends and properties.33−38 The Raman spectra of single crystals of the actinyl(VI) dimers are presented in Figure 3 along with the FT-IR spectra of their powders dispersed and pressed into Teflon films and tabulated in Table 4. In the case of the triclinic structures, the symmetric stretching of the actinyl moiety appears as a dominant single peak on the Raman spectra and red shifts across the actinide series: uranium, 846 cm−1; neptunium, 837 cm−1; plutonium, 805 cm−1. The two monoclinic phases characterized for PuVIO22+ show two peaks in their Raman spectra located at 813 and 819 cm−1 in both phases. There are two crystallographic plutonium sites in the P21/c phase and only one crystallographic plutonium site in the P21/n phase with two formula units per unit cell. The magnitude of the splitting is 6 cm−1, leading us to assign this to an intermolecular interaction, or Davydov splitting in the P21/n phase as a result of the interacting vibrations of the two

Figure 1. (a) Ball-and-stick representation of the molecular AnVIO22+ dimers crystallized from aqueous solution. (b) Ball-and-stick representation of the PuVIO22+ dimer crystallized in P21/c highlighting the distortion across the Pu−OH−Pu bridge compared to that in 1a. The metal ions are magenta, oxygen atoms red, and nitrogen atoms blue. Hydrogen atoms are omitted for clarity.

hexagonal bipyramids. The anionic charge of the resulting molecular species is compensated for by two tetramethylammonium cations in the crystal lattice. The packing diagrams of each of the triclinic structures are presented in Figure 2. The observed actinyl metal−oxygen bond distances are consistent with those previously reported for uranium(VI) [1.776(1) and 1.784(1) Å], neptunium(VI) [1.753(2) and 1.760(2) Å], and plutonium(VI) [1.710(12) and 1.717(1) Å] complexes; their decreasing bond distances highlight the actinide contraction as Z is increased across the series. The actinyl bonds are slightly asymmetric and nonlinear, and the crystallographic inversion centers lie between the actinyl(VI) centers in the dimers and not at the metal site. The actinyl(VI) ions form a plane with the hydroxide bridges in the triclinic structures with average AnVI−OH distances of 2.33(4) Å for uranium(VI), 2.31(1) Å for neptunium(VI), and 2.31(1) Å for plutonium(VI). The coordinating nitrate groups are not coplanar with the actinide− hydroxide plane; instead, they are slightly puckered above and below the plane with a maximum deviation of ∼13° for the uranium(VI) dimer versus only ∼4° out-of-plane for the plutonium(VI) dimer. The origin of this distortion is not clear

Figure 2. Packing diagrams of the triclinic structures of (a) [(CH3)4N]2[(UO2)2(OH)2(NO3)4], (b) [(CH3)4N]2[(NpO2)2(OH)2(NO3)4], and (c) [(CH3)4N]2[(PuO2)2(OH)2(NO3)4]. The a axis of each unit cell is projected into the page. C

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Raman and IR spectra of the (Me4N)2[(AnO2(OH)2(NO3)4] complexes in the range of the symmetric and asymmetric vibrational modes of the actinyl cations (700−1100 cm−1).

Table 4. Vibrational Frequencies and Bond Force Constants for the Reported Actinyl(VI) Dimers (Me4N)2[(UO2)2(OH)2(NO3)4] (Me4N)2[(NpO2)2(OH)2(NO3)4] (Me4N)2[(PuO2)2(OH)2(NO3)4] P1̅ P21/c P21/n

ν1 (cm−1)

ν3 (cm−1)

846(1) 837

928 928

6.95 6.88

−0.20 −0.28

82 91

805 813, 819 813, 819

929, 939

6.70, 6.82

−0.47, −0.50

116, 120

crystallographic plutonium centers.39 Pure phases of the triclinic and P21/n phase of the plutonium dimers were not isolated, while sufficient material of the P21/c phase could be isolated for FT-IR sample preparation. In the FT-IR spectra of these complexes dispersed in Teflon to prevent reaction with KBr or KCl during pressing, the asymmetric stretching vibration of the actinyl ions appears as an intense band: uranium (P1̅), 928 cm−1; neptunium (P1̅), 928 cm−1; plutonium (P21/n), 929 and 939 cm−1. The Davydov coupling observed in the Raman spectrum of a single crystal of the P21/n phase of PuVIO22+ is reproduced in its IR spectrum, with the two bands separated by 10 cm−1. Upon comparison of the vibrational spectra of all of the triclinic phases, it is noted that while the actinyl symmetric vibrational modes decrease in frequency from uranium to plutonium (41 cm−1), the asymmetric frequencies remain constant. This is interesting because often the asymmetric vibrational frequencies are used as indicators of the actinyl bond strength and length.34,35 We have observed this phenomenon for several isostructural actinyl complexes, and further investigation will be necessary to identify the origin of this behavior.36−38 The remainder of the bands in the Raman and IR spectra are attributable to the vibrational modes of the coordinated NO3− and charge-compensating Me4N+ cations. The symmetric and

k1 (mdyn/Å)

k12 (mdyn/Å)

Δ(ν3 − ν1) (cm−1)

asymmetric C−N stretching frequencies of the cation are observed around 750 and 950 cm−1, respectively. The symmetric C−N stretching is split into two components for the triclinic phase, which may be an indication of a deviation from the tetrahedral geometry of this cation although this is not supported by the crystallography; the symmetric frequency is not degenerate in its vibration. Bands associated with the symmetric vibration of the bound nitrate ions appear as two bands between 1025 and 1060 cm−1 for the triclinic structures and the monoclinic P21/n phase for PuVIO22+ as expected from the two crystallographic nitrate ions in the unit cell. There are four different crystallographic positions for the nitrate molecules in the monoclinic P21/c phase for plutonium(VI) although only two bands are observed in the Raman spectrum. Lowering of the D3h symmetry of the free nitrate ions in the crystal structure allows the six vibrational modes to be Raman- and IR-active. Consequently, the asymmetric vibration of the bound nitrate ions can be observed in the Raman spectrum around 743 cm−1 as a shoulder of the symmetric C−N stretching of Me4N+. The corresponding IR spectra agree with the symmetric and asymmetric C−N stretching modes of Me4N+ and the bands associated with the symmetric and asymmetric vibrations of the bound nitrate ions observed on the Raman spectra. D

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry In order to determine if these dimers were present in solution prior to crystallization, Raman spectra were collected on the original mother solutions that produced the crystals before and after the addition of Me4NOH. The Raman spectra of the actinyl(VI) solutions created by the dissolution of AnO2(NO3)2(H2O)2 dissolved in water were recorded and are dominated by the symmetric stretching of the actinyl moiety (uranium, 871 cm−1; neptunium, 857 cm−1; plutonium, 833 cm−1) and the symmetric vibration of the nitrate ions (1048 cm−1; Figures 4 and 5).

Figure 5. Raman spectra of the mother solutions that produced the plutonium(VI) dimers (a) and their corresponding UV−vis−NIR spectra (b).

Upon the addition of nitric acid and Me4NOH (HNO3/ AnO22+ = 1 mol/mol and Me4NOH/AnO22+ = 2 mol/mol) to the UVIO22+ solution, multiple bands appear simultaneously in the Raman spectrum (Figure 4a). Deconvolution of this spectrum shows two new species located at 854 cm−1 (21 cm−1 fwhm) and 836 cm−1 (21 cm−1 fwhm) (Figure 6). These species were assigned in previous work to the dimeric [(UO2)2(μ2-OH)2(H2O)4]2+ and trimeric [(UO2)3(μ3-O)(μ2OH)3(H2O)6]+ species in solution, respectively.40 The uranium(VI) dimer appears to be the dominant species in the mother solution (Figure 4a). There is a small difference of 8 cm−1 between the uranyl symmetric stretching frequency assigned to the dimer in solution (854 cm−1) versus that measured in the crystallized phase (846 cm−1) reported here, providing some confidence in the prior assignment. For NpVIO22+, we have observed a broadening of the neptunium(VI) stretching mode in both the solid state and aqueous solution. This broadening makes identification of the individual species formed in solution, as was done for uranium(VI), using Raman spectroscopy alone more difficult. In the solution titrated with Me4NOH, a shift of 9 cm−1 is observed versus the untitrated solution in water (ν1, 848 cm−1; Figure 4b). To aid in the identification of the neptunyl

Figure 4. Raman spectra of the mother solutions that produced the actinyl(VI) dimers with An = U (a) and Np (b) associated with the spectrum of AnO2(NO3)(H2O)2·H2O dissolved in water. The Raman spectra of the neptunium(VI) solutions are complemented by the corresponding UV−vis−NIR spectra in part c. E

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Deconvolution of the Raman spectra of the mother solutions that produced the AnVI dimers (with An = U and Np). The shoulder in the neptunium Raman spectra at 890 cm−1 is from the sample cuvette.

speciation, UV−vis−NIR spectra of the NpVIO22+ solutions were recorded to characterize the influence of hydroxide complexation on the f−f electronic transition at 1222 nm for neptunium(VI). Upon the addition of Me4NOH, the intensity of this absorption band slightly decreases with the appearance of a shoulder at lower wavelengths indicative of a new species in solution (Figure 4c). A Raman study of the neptunium(VI) hydrolysis performed by Madic et al. using NaOH in the pH range 1.75−3.65 identified only two species in solution, 854 cm−1 for the NpO22+ aqua ion and 834 cm−1 identified as a dimer (NpO2)2(OH)22+.13,14 On the basis of their interpretation and analysis, deconvolution of our Raman spectra in the titrated neptunium(VI) solutions shows a second species located at 837 cm−1 (38 cm−1 fwhm) and the aquated ion at 856 cm−1 (33 cm−1 fwhm) (Figure 6). This frequency is in good agreement with the symmetric stretching frequency measured for the crystallized neptunium(VI) dimer (837 cm−1) and seems to support the presence of this species in aqueous solution prior to crystallization. The origin of the plutonium(VI) dimer species in aqueous solution has been studied using Raman spectroscopy and complementary UV−vis−NIR spectrophotometry for which the intense band at 830 nm of the plutonyl aqua ion is followed as an indicator of complex formation (Figure 5). To investigate the origin of the formation of different phases and optimize the synthesis, various experimental conditions were explored: (a) 1.5 equiv of HNO3 and 2 equiv of (CH3)4NOH; (b) 1 equiv of HNO3 and 2 equiv of (CH3)4NOH; (c) 1 equiv of HNO3 and 3 equiv of (CH3)4NOH. Only the first two solutions lead to crystallization of the plutonium(VI) dimer. The monoclinic P21/n phase was identified in a majority of crystals isolated from the first solution, while a mixture of the three different phases was present in the second. No crystals suitable for diffraction measurements were found in the third solution. Upon the addition of hydroxide, both solution Raman and UV−vis spectra show the simultaneous appearance of several species without isosbestic points, making deconvolution of both sets of spectra difficult (Figure 5). In the UV−vis−NIR spectrum of the plutonium(VI) solution containing 1.5 equiv of HNO3 and 2 equiv of Me4NOH, the band at 830 nm is asymmetrically broadened and a new band appears around 843 nm. Spectra of the solution containing 1 equiv of HNO3 with 2 and 3 equiv of Me4NOH show that this band shifted to 847 nm with the appearance of an additional shoulder at longer wavelengths. This modification of the spectrum suggests the emergence of at least two species between 840 and 850 nm. An earlier study using UV−vis spectrophotometry with potentio-

metric titrations in a more dilute solution ([PuO22+] = 1−10 mM) described the same behavior and assigned these hydrolysis products to the dimeric (PuO2)2(OH)22+ and (PuO2)2(OH)40 complexes in solution,15 while a more recent study considered the successive formation of PuO2OH+, (PuO2)2(OH)22+, and (PuO2)3(OH)5+.16 A monomeric species, PuO2(OH)3−, was assigned to the shoulder centered at higher wavelengths, with the dimer being the dominant hydrolysis product.15 We also observed this same behavior in our UV−vis spectra at higher plutonium concentrations as well as in more dilute samples, which may suggest the formation of other polymeric complexes of plutonium(VI) and not only the mononuclear hydroxo complex, as was previously assigned.15 A similar complexity is encountered in the Raman spectra, leading us to conclude that there is a coexistence of multiple species in the plutonium(VI) system and that the plutonium(VI) speciation is likely more complex than either the uranium(VI) or neptunium(VI) cases studied here, despite prior studies using only three species to fit all three systems.12 A prior Raman study of plutonium(VI) hydrolysis at higher concentration (([PuO22+] = 0.1 M) also shows the coexistence of multiple polymeric species.13,14 The first hydroxo complex observed at 817 cm −1 was assigned to the dimeric (PuO2)2(OH)22+ species. This frequency is in good agreement with the symmetric stretching frequency measured for the crystallized plutonium(VI) dimer in the monoclinic phases (≈816 cm−1) and seems to support their assignment. In our Raman spectra (Figure 5a), a band is observed at 823 cm−1 in the solution containing 1.5 equiv of HNO3 and 2 equiv of Me4NOH. Increasing the hydroxide concentration leads to the appearance of a new band centered at 817 cm−1 in addition to the band at 823 cm−1. While the Raman spectrum of the solution with the larger quantity of hydroxide (2 equiv) supports the presence of the plutonium(VI) dimer in aqueous solution prior to crystallization, we are unable to correlate the band observed at 823 cm−1 in our solution Raman measurements to any monomeric or oligomeric crystallized compounds. In the solution with 3 equiv of hydroxide, two new bands are observed at 826 and 805 cm−1, as observed in previous work at higher plutonium concentrations. Madic and co-workers assigned this signature to the polymeric complex (PuO2)4(OH)7+,13,14 which is in opposition with the common hydrolysis products considered for the actinyl(VI) ions. While it is difficult from our results to identify exactly what the species is, our Raman data show the appearance of these new bands and no plutonium(VI) dimers were crystallized from this solution. On the basis of these data and the UV−vis F

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

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slowly evaporated under a N2 flow until the formation of PuO2(NO3)2(H2O)2·H2O. The solid compound was dissolved in water, and then 1 M HNO3 and Me4NOH were added to reach the desired concentration described as follows: (1) HNO3/PuO22+ = 1.5 and OH−/PuO22+ = 2; (2) HNO3/PuO22+ = 1 and OH−/PuO22+ = 2; (3) HNO3/PuO22+ = 1 and OH−/PuO22+ = 3. The resulting solutions ([PuO22+] ≈ 0.05 M; liquid scintillation counting) were slowly evaporated under air until the formation of multiple phases of (Me4N)2(PuO2)2(OH)2(NO3)4. The monoclinic P21/n phases were exclusively identified in solution 1, while the monoclinic P21/c and P21/n and triclinic P1̅ phases were found in solution 2. No crystals suitable for single-crystal X-ray diffraction measurements were obtained from solution 3. Vibrational Spectroscopy. Raman data were collected on single crystals and in solution using a Renishaw inVia Raman microscope with excitation lines of 532 and 785 nm. Solid samples were placed on a microscope slide with a concave cavity and covered with a glass coverslip affixed with epoxy. Liquid samples were contained in a plastic cuvette and sealed with a paraffin film. Spectra were collected using circularly polarized radiation between Δν 100 and 4000 cm−1. IR samples were collected on ground crystals (2.5 wt %) pressed into Teflon powder with a Nicolet Nexus 870 FT-IR system. Data were collected over 400−4000 cm−1 with a resolution of 2 cm−1. Spectrophotometry. UV−vis−NIR spectra of the solution samples were collected using an Ocean Optics spectrophotometer with fiber optics inserted in a sample holder located inside a fume hood. Absorption spectra of the samples were recorded in a plastic cuvette with 0.2 and 1 cm path lengths from 350 to 1600 nm. Single-Crystal X-ray Diffraction. Crystals suitable for singlecrystal X-ray diffraction measurements were affixed to glass micropipettes using a quick-drying two-part epoxy, and their diffraction patterns were collected on a Bruker SMART diffractometer equipped with an APEX II CCD detector using Mo Kα radiation. An Oxford Cryosystems 700 series cryostat was used for controlling the sample temperature between 100 and 300 K. Data were corrected for absorption using SADABS. Structure solutions and structure refinement on F2 were carried out using SHELXS and SHELXL, respectively.42

spectra, we suggest that the plutonium(VI) speciation at higher hydroxide concentrations is more complex than that observed in the uranium(VI) and neptunium(VI) systems.

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CONCLUSION We have isolated the series of molecular actinyl(VI) hydroxo bridged dimers from aqueous solution. Our Raman and UV− vis−NIR studies have also allowed us to correlate these structures with the solution speciation, providing evidence for the existence of these molecules in solution prior to their crystallization. The complexes isolated here are in contrast to those that have been isolated for uranium(VI) by the evaporation of acidic solutions using HCl and HBr.32,41 In those studies, there was not any evidence of molecular uranium(VI) dimers in the solution supernatant, indicating that the hydrolysis was an effect of concentrating the uranyl solution and the necessity of maintaining charge neutrality in the system via the formation of hydroxide upon evaporation.32 The synthesis of the complexes reported here was aided by the previous thermodynamic studies and our own spectroscopic studies of the solution speciation, wherein we were able to target areas of the solution speciation where we believed the dimers to be the majority species; this is particularly true in the case of plutonium(VI). The success of this approach in targeting the desired molecular dimers reinforces our assertions that these species are structural representatives of the solution speciation.

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EXPERIMENTAL METHODS

Caution! Actinide elements (238U, 237Np, and 242Pu) are radioactive materials and must be handled in an appropriate facility. Solutions other than sealed samples were handled in dedicated fume hoods or gloveboxes. All other reagents were purchased f rom Sigma-Aldrich and used as received. Synthesis of (Me4 N)2(UO 2)2(OH)2(NO3 )4. To a solution containing 25 mg of UO2(NO3)2·6H2O (0.05 mmol) dissolved in water were added 1 M HNO3 (HNO3/UO22+ = 1) and Me4NOH (OH−/UO22+ = 2). The resulting solution ([UO22+] = 0.1 M) was slowly evaporated under air to form large elongated prisms identified as (Me4N)2(UO2)2(OH)2(NO3)4. The stoichiometric nature of the reaction and the phase purity suggested by the resulting FT-IR spectra suggest a near-quantitative yield upon evaporation. Synthesis of (Me4N)2(NpO2)2(OH)2(NO3)4. A stock solution of NpO22+ was prepared by dissolving the NpO2OH compound in 0.5 M HNO3. Oxidation of the neptunium was then performed by bubbling the solution with O3 for 12 h, and the oxidation state of the solution was checked by UV−vis spectroscopy. The solution was slowly evaporated under a N 2 flow until the formation of NpO2(NO3)2(H2O)2·H2O. The solid compound was dissolved in water, followed by the addition of 1 M HNO3 (HNO3/NpO22+ = 1) and Me4NOH (OH−/NpO22+ = 2). The resulting solution ([NpO22+] = 0.14 M; liquid scintillation counting) was centrifuged to eliminate the precipitate formed by the addition of Me 4NOH. After evaporation, large plates were formed and identified as (Me4N)2(NpO2)2(OH)2(NO3)4. The stoichiometric nature of the reaction and the phase purity suggested by the resulting FT-IR spectra suggest a near-quantitative yield upon evaporation. Synthesis of (Me4N)2(PuO2)2(OH)2(NO3)4. A solution of plutonium(IV) in concentrated nitric acid was purified using an anion-exchange resin, DOWEX 1X8. The green plutonium(IV) solution was loaded onto the resin, washed with 7.5 M HNO3, and then eluted with 0.5 M HCl. A medium change was performed by evaporating the resulting solution several times, followed by the addition of concentrated nitric acid. After dilution in 0.5 M HNO3, oxidation of plutonium was performed by bubbling O3 for 12 h to ensure the complete oxidation of plutonium(IV), and the solution was

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ASSOCIATED CONTENT

Accession Codes

CCDC 1870499−1870503 contain 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]. ORCID

Richard E. Wilson: 0000-0001-8618-5680 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed at Argonne National Laboratory, operated by UChicagoArgonne LLC for the United States Department of Energy, Office of Science, Basic Energy Sciences, Heavy Elements Chemistry, program under Contract DE-AC02-06CH11357. G

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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(22) Moll, H.; Reich, T.; Szabo, Z. The hydrolysis of dioxouranium (VI) investigated using EXAFS and 17O-NMR. Radiochim. Acta 2000, 88, 411−416. (23) Basile, M.; Unruh, D. K.; Streicher, L.; Forbes, T. Z. Spectral analysis of the uranyl squarate and croconate system: Evaluating differences between the solution and solid-state phases. Cryst. Growth Des. 2017, 17, 5330−5341. (24) Charushnikova, I. A.; Den Auwer, C. Crystal structures of molecular adducts between uranyl nitrate and 2,2 ’: 6 ’,2 ″ -terpyridine or 2,2 ’-bipyridyl. Russ. J. Coord. Chem. 2004, 30, 511−519. (25) Saravani, H.; Mozafaripoor, F. Synthesis, characterization, and crystal structure of new complex, based on proton transfer to uranium(VI) complex. Synth. React. Inorg., Met.-Org., Nano-Met. Chem. 2015, 45, 1717−1722. (26) Hu, Y. J.; Knope, K. E.; Skanthakumar, S.; Soderholm, L. Understanding the ligand-directed assembly of a hexanuclear Th(IV) molecular cluster in aqueous solution. Eur. J. Inorg. Chem. 2013, 2013, 4159−4163. (27) Soderholm, L.; Almond, P. M.; Skanthakumar, S.; Wilson, R. E.; Burns, P. C. The structure of the plutonium oxide nanocluster [Pu38O56Cl54(H2O)8] 14−. Angew. Chem., Int. Ed. 2008, 47, 298−302. (28) Wilson, R. E.; Skanthakumar, S.; Sigmon, G.; Burns, P. C.; Soderholm, L. Structures of dimeric hydrolysis products of thorium. Inorg. Chem. 2007, 46, 2368−2372. (29) Wilson, R. E.; Skanthakumar, S.; Soderholm, L. Separation of plutonium oxide nanoparticles and colloids. Angew. Chem., Int. Ed. 2011, 50, 11234−11237. (30) Soderholm, L.; Skanthakumar, S.; Wilson, R. E. Structures and energetics of erbium chloride complexes in aqueous solution. J. Phys. Chem. A 2009, 113, 6391−6397. (31) Soderholm, L.; Skanthakumar, S.; Wilson, R. E. Structural correspondence between uranyl chloride complexes in solution and their stability constants. J. Phys. Chem. A 2011, 115, 4959−4967. (32) Wilson, R. E.; Skanthakumar, S.; Cahill, C.; Soderholm, L. Structural studies coupling X-ray diffraction and high-energy X-ray scattering in the UO22+−HBraq system. Inorg. Chem. 2011, 50, 10748−10754. (33) Basile, L. J.; Sullivan, J. C.; Ferraro, J. R.; LaBonville, P. The Raman scattering of uranyl and transuranium V, VI, and VII ions. Appl. Spectrosc. 1974, 28, 142−145. (34) Jones, L. H. Infrared spectra and structure of the crystalline sodium acetate complexes of U(VI), Np(VI), Pu(VI), and Am(VI). A comparison of metal-oxygen bond distance and bond force constant in this series. J. Chem. Phys. 1955, 23, 2105−2107. (35) Jones, L. H.; Penneman, R. A. Infrared spectra and structure of uranyl and transuranium (V) and (VI) ions in aqueous perchloric acid solution. J. Chem. Phys. 1953, 21, 542−544. (36) Schnaars, D. D.; Wilson, R. E. Structural and vibrational properties of U(VI)O2Cl42‑ and Pu(VI)O2Cl42‑ complexes. Inorg. Chem. 2013, 52, 14138−14147. (37) Schnaars, D. D.; Wilson, R. E. Lattice Solvent and Crystal Phase Effects on the Vibrational Spectra of UO2Cl42‑. Inorg. Chem. 2014, 53, 11036−11045. (38) Schnaars, D. D.; Wilson, R. E. Synthesis, structure, and vibrational properties of [Ph4P] 2NpO2Cl4 and [Ph4P] 2PuO2Cl4 complexes. Inorg. Chem. 2018, 57, 3008−3016. (39) Ferraro, J. R. Introductory Raman Spectroscopy; Elsevier, 2003. (40) Toth, L.; Begun, G. Raman spectra of uranyl ion and its hydrolysis products in aqueous nitric acid. J. Phys. Chem. 1981, 85, 547−549. (41) Â berg, M. The Crystal Structure of [(UO2)2(OH)2Cl2(H2O)4]. Acta Chem. Scand. 1969, 23, 6. (42) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

REFERENCES

(1) Seth, M.; Dolg, M.; Fulde, P.; Schwerdtfeger, P. Lanthanide and actinide contractions: relativistic and shell structure effects. J. Am. Chem. Soc. 1995, 117, 6597−6598. (2) Morss, L. R.; Edelstein, N. M.; Fuger, J. The Chemistry of the Actinide and Transactinide Elements; Springer, 2010; Vols. 1−6. (3) Seaborg, G. T. Electronic Structure of the Heaviest Elements. The Transuranium Elements: Research Papers; McGraw-Hill Book Co. Inc.: New York, 1949; Vol. 2; Paper 21.1. (4) Burns, P. C.; Finch, R. J. Uranium: mineralogy, geochemistry and the environment; Mineralogical Society of America, 1999. (5) Nash, K. L.; Madic, C.; Mathur, J. N.; Lacquement, J. Actinide Separation Science and Technology. In The Chemistry of the Actinide and Transactinide Elements; Morss, L. R., Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, The Netherlands, 2006; Vol. 4, pp 2622− 2798. (6) Silva, R.; Nitsche, H. Actinide environmental chemistry. Radiochim. Acta 1995, 70-71, 377−396. (7) Grenthe, I.; Fuger, J.; Konings, R. J.; Lemire, R. J.; Muller, A. B.; Nguyen-Trung, C.; Wanner, H. Chemical Thermodynamics of Uranium; North-Holland: Amsterdam, The Netherlands, 1992; Vol. 1. (8) Guillamont, R. Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium, and Technetium; Elsevier, 2003; p 919. (9) Lemire, R. J. Chemical Thermodynamics of Neptunium and Plutonium; Elsevier, 2001; Vol. 4. (10) Silva, R. J.; Bidoglio, G.; Robouch, P.; Puigdomènech, I.; Wanner, H.; Rand, M. Chemical Thermodynamics of Americium; Newnes, 2012; Vol. 2. (11) Cassol, A.; Magon, L.; Portanova, R.; Tondello, E. Hydrolysis of plutonium (VI): acidity measurements in perchlorate solutions. Radiochim. Acta 1972, 17, 28−32. (12) Cassol, A.; Magon, L.; Tomat, G.; Portanova, R. Soluble intermediates in the hydrolysis of neptunium (VI) and comparison with other actinides (VI). Inorg. Chem. 1972, 11, 515−519. (13) Madic, C.; Begun, G.; Hobart, D.; Hahn, R. Raman spectroscopy of neptunyl and plutonyl in aqueous solutions: hydrolysis of Np (VI) and Pu (VI) and disproportionation of Pu (V). Inorg. Chim. Acta 1984, 94, 100−102. (14) Madic, C.; Begun, G.; Hobart, D.; Hahn, R. L. Raman spectroscopy of neptunyl and plutonyl ions in aqueous solution: hydrolysis of neptunium (VI) and plutonium (VI) and disproportionation of plutonium (V). Inorg. Chem. 1984, 23, 1914−1921. (15) Reilly, S. D.; Neu, M. P. Pu (VI) hydrolysis: Further evidence for a dimeric plutonyl hydroxide and contrasts with U (VI) chemistry. Inorg. Chem. 2006, 45, 1839−1846. (16) Rao, L.; Tian, G.; Di Bernardo, P.; Zanonato, P. Hydrolysis of plutonium (VI) at variable temperatures (283−343 K). Chem. - Eur. J. 2011, 17, 10985−10993. (17) Nguyen-Trung, C.; Palmer, D.; Begun, G.; Peiffert, C.; Mesmer, R. Aqueous uranyl complexes 1. Raman spectroscopic study of the hydrolysis of uranyl (VI) in solutions of trifluoromethanesulfonic acid and/or tetramethylammonium hydroxide at 25 C and 0.1 MPa. J. Solution Chem. 2000, 29, 101−129. (18) Quiles, F.; Burneau, A. Infrared and Raman spectra of uranyl (VI) oxo-hydroxo complexes in acid aqueous solutions: a chemometric study. Vib. Spectrosc. 2000, 23, 231−241. (19) Quiles, F.; Nguyen-Trung, C.; Carteret, C. d.; Humbert, B. Hydrolysis of uranyl (VI) in acidic and basic aqueous solutions using a noncomplexing organic base: a multivariate spectroscopic and statistical study. Inorg. Chem. 2011, 50, 2811−2823. (20) Rowland, C. E.; Cahill, C. L. Capturing hydrolysis products in the solid state: effects of pH on uranyl squarates under ambient conditions. Inorg. Chem. 2010, 49, 8668−8673. (21) Tsushima, S.; Rossberg, A.; Ikeda, A.; Mueller, K.; Scheinost, A. C. Stoichiometry and structure of uranyl (VI) hydroxo dimer and trimer complexes in aqueous solution. Inorg. Chem. 2007, 46, 10819− 10826. H

DOI: 10.1021/acs.inorgchem.8b03304 Inorg. Chem. XXXX, XXX, XXX−XXX