Advancing Understanding of the +4 Metal Extractant

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

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Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA−); Synthesis and Structure of MIVTTA4 (MIV = Zr, Hf, Ce, Th, U, Np, Pu) and MIII(TTA)4− (MIII = Ce, Nd, Sm, Yb) Samantha K. Cary,† Maksim Livshits,‡ Justin N. Cross,† Maryline G. Ferrier,† Veronika Mocko,† Benjamin W. Stein,† Stosh A. Kozimor,*,† Brian L. Scott,† and Jeffrey J. Rack‡ †

Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States University of New Mexico, Albuquerque, New Mexico 87131, United States

‡

S Supporting Information *

ABSTRACT: Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating microscopic amounts of transuranic actinides (i.e., Np and Pu) from macroscopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extractionor find alternativesbecause little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (MIV = Zr, Hf, Ce, Th, U, Np, and Pu vs MIII = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula MIV(TTA)4. Meanwhile, +3 metals formed anionic MIII(TTA)4− species. Characterization of these M(TTA)4x− (x = 0, 1) compounds by UV−vis−NIR, IR, 1H and 19F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that NpIV(TTA)4 and PuIV(TTA)4 were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between MIV(TTA)4 and MIII(TTA)4− are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np IV to PuIV, HfIV, and ZrIV.

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INTRODUCTION

Of the many approaches for separating Pu, Np, and U,1,3,12,14,15 one particularly effective operational procedure involves an extraction step with thenoyltrifluoroacetone (HTTA), Figure 1.4,16−24 This method involves an oxidation

Advances in actinide separations science have had an impact in virtually every technologically relevant area associated with the f-elements. This spans from isotope production to advanced nuclear fuel cycle development, plutonium sustainment, and the national nuclear security administration’s (NNSA) missions in nuclear science.1−11 Among numerous actinide separations relevant to these operational efforts, those associated with separating Pu, Np, and U are particularly important. For instance, being able to cleanly isolate trace amounts of Np and Pu from bulk U matrices is critical for quantifying actinide mobility and abundances in the environment.12 These separations are also essential for making radiometric and mass-spectrometry measurements in support of environmental monitoring efforts.12 Similarly, certification of actinide metals across the Department of Energy (DOE) complex relies on isolation of small amounts of Np and Pu from bulk U sources as well as the ability to characterize trace amounts of Np from bulk Pu samples. Finally, the resumption of domestic 238Pu production for advanced long-term space missions requires separation of microscopic quantities of Pu from bulk Np and subsequent recycling of Np in follow-on production cycles.13 © XXXX American Chemical Society

Figure 1. A picture showing the HTTA and TTA− compounds.

state adjustment of the metal to +4 in acidic media (e.g., HCl, 1 M), followed by HTTA extraction into an aromatic solvent (e.g., o-xylene, toluene, benzene, etc.). The process selectively extracts +4 metals into the organic phase, while leaving behind metal ions (including actinides and lanthanides) in other oxidation states, such as +3, +5, and +6.25 Unfortunately, the entire process, especially the back-extraction into aqueous Received: December 12, 2017

A

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Comparison of pH50 values vs a wide variety of metal cations. The term pH50 is defined as the pH when 50% of the metal was extracted by 0.2 M TTA into benzene, adapted from Poskanze et al.24

and X-ray absorption spectroscopy (XAS). Finally, we characterized the neptunium and plutonium speciation within the organic phase of an actual HTTA extraction by 1H and 19F NMR and Np and Pu L3-edge XAS. Comparison of the extraction results with the M(TTA)4x− (x = 0, 1) coordination complexes provides insight into why HTTA is an effective extractant for separating +4 metals from metals in the +3 oxidation state.

media, can be complicated and unpredictable, such that success with the HTTA extraction requires highly skilled, experienced, and well-trained personnel. Despite widespread use of the HTTA extraction procedure for actinide processing in analytical chemistry and significant efforts to characterize the parameters for conducting successful HTTA extractions (separation factors, distribution coefficients, etc.4,16−24), little is understood about the interworking of the separation. Although it has been proposed that the +4 metals react with HTTA to form the M(TTA)4 extracted complex,25 the identities of the extracted species have not been characterized, and the driving force for the selectivity is mysterious. Consider the comprehensive study conducted by Poskanzer and Foreman, which we summarized in Figure 2. Here, the pH when 50% of the analyte is extracted (i.e., the pH50 parameter) systematically changes with metal oxidation statesuch that pH50 is achieved for +3 metals at higher pH values, intermediate values for AnO22+ species, and low values for the +4 metals. These subtle differences provide the basis for selectively extracting +4 metals in the HTTA extraction from +3, +5, and +6 cations. Furthermore, changing the metal from NpIV to PuIV, HfIV, and ZrIV systematically increases the M− TTA extraction ability (Figure 2).24 Because so little is known about the details of the HTTA extraction process it is difficult to propose credible hypotheses that explain these separation trends, let alone rationally improve the HTTA extraction or find more user-friendly alternative separation methods. Advancing understanding of the HTTA extraction (1st) has potential to aid analytical process chemistry and improve robustness of the separation and (2nd) provide an opportunity to identify other extractants that are equally effective but more reliable than HTTA. As such, we report rigorous characterization of the homoleptic MIV(TTA)4 (MIV = Zr, Hf, Ce, Th, U, Np, Pu) and MIII(TTA)4− (MIII = Ce, Nd, Sm, Yb) family of coordination complexes. Although Prasad and co-workers prepared many of these compounds in 1966,26 numerous chemical properties remain ill-defined, such as their solid-state structure and solution-phase behavior. Contained in this manuscript are detailed and convenient methods to prepare the M(TTA)4x− (x = 0, 1) compounds. Also included is the rigorous characterization of the M(TTA)4x− (x = 0, 1) coordination complexes by single-crystal X-ray diffraction (XRD) as well as by UV−vis−near-infrared (UV−vis−NIR), infrared (IR), 1H and 19F nuclear magnetic resonance (NMR),

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RESULTS AND DISCUSSION Synthesis of Ammonium Lanthanide(III) Tetrakis(thionyltrifluoroacetonate); (NH4)MIII(TTA)4 (MIII = Ce, Nd, Sm, Yb). The synthetic procedures used to generate (NH4)MIII(TTA)4 saltsreferred to hereafter as MIII(TTA)4− (MIII = Ce, Nd, Sm, Yb)were inspired from Cefola and coworker’s method for preparing praseodymium(III) tetrakis(thionyltrifluoroacetonate), PrIII(TTA)4−. The original procedure utilized inert atmospheres (nitrogen) and ammonium hydroxide (NH4OH) as a base.27 Our approach (Scheme 1) Scheme 1. Synthetic Scheme for the Formation of (NH4)MIII(TTA)4

deviated slightly, in that the reactions were conducted on the benchtop, using non-anhydrous solvents and reagents. The chemical transformations that occurred during the procedure involved deprotonation of HTTA, substitution of three Cl− ligands for three TTA− anions, and coordination of an additional TTA− ligand to the MIII cation. For experiments with these +3 metals, addition of NH4OH created a murky solution. However, over the course of 2 to 3 d, the solution cleared, and crystals of (NH4)MIII(TTA)4 emerged leaving behind the dissolved NH4Cl byproducts. In our laboratory, crystallizations with tetraphenyl phosphonium (PPh4+), tetrabutyl ammonium (NBu 4 + ), and tetramethylammonium (NMe4+) cations were unsuccessful suggesting that the B

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ammonium (NH 4 + ) cation was critical for isolating MIII(TTA)4− anions in crystalline form. All of the MIII(TTA)4− compounds were isolated with moderate-to-high crystalline yields (60 to 80%), albeit YbIII(TTA)4− and CeIII(TTA)4− were isolated in low (18%, 10%, respectively) crystalline yield. It is important to point out that success in preparing CeIII(TTA)4− required a fresh bottle of NH4OH; otherwise, oxidation to CeIV occurred. This unexpected oxidation likely resulted from carbonate present in the aged NH4OH bottles, which stabilizes CeIV.28 In comparison to many studies that have compared reactivity across the lanthanide series, the syntheses of the MIII(TTA)4− were somewhat unique. For example, often the early lanthanides react differently than the late lanthanides. This break in reactivity is usually attributed to increased steric crowding that results from the decreased lanthanide ionic radii. Surprisingly, with TTA− no reactivity break was observed. Hence, the MIII(TTA)4− anions formed as dominate products for the large, intermediate, and small lanthanides (MIII = Ce, Nd, Sm, and Yb).29−33 Synthesis of Metal(IV) Tetrakis(thionyltrifluoroacetonate); MIV(TTA)4·H2O (MIV = Zr, Hf, Ce, Th, U, Np, Pu). Emboldened by the formation of the (NH4)MIII(TTA)4 salts, the coordination chemistry between +4 metals and HTTA was subsequently investigated. We acknowledged that these reactions had a high probability for success for forming exclusively MIV(TTA)4 compounds, especially considering Baskin and Prasad isolated crystalline material for MIV(TTA)4 (MIV = Zr, Hf, Ce, Th, U, Pu) in 1966. Baskin and Prasad characterized these compounds by IR spectroscopy and indexed MIV(TTA)4 crystals using X-ray diffraction methods.26 Although these X-ray studies suggested that the six compounds were isomorphous, the exact space groups were unclear, and the full structures were not determined. Note for plutonium, the lattice parameters were not determined from the diffraction pattern but were instead assumed to be identical to that of CeIV(TTA)4. For thorium, years later, the full structure of ThIV(TTA)4 was reported.34 In light of these reports, we set out to more rigorously evaluate the MIV(TTA)4 structures and characterize their solidand solution-state properties using modern techniques, namely, NMR spectroscopy, UV−vis−NIR, XAS, and single-crystal Xray diffraction. Our preparative method was similar to the previous reports26,34 in that it could be performed without exclusion of air and moisture. We observed that the MIV cations reacted with HTTA, which was likely deprotonated by the solvent (either EtOH or trace H2O), to generate MIV(TTA)4· H2O. In the CeIV case, Ce(NH4)2(NO3)6 was used as the MIV starting material (Scheme 2), while MIVCl4 reagents were used for the other +4 metals (Zr, Hf, Th, and U; Scheme 2). Because these reactions generated crystalline products from ethanol, benzene, or mixtures of acetonitrile and ether solutions, it seemed likely that analogous reactions with NpIV and PuIV would generate single crystals as well. Indeed, scaled down reactions that mimicked reasonable quantities of NpIV and PuIV available for experimentation (5 to 10 mg of metal) were successful. In these experiments, either NpIV or PuIV aqueous stock solutions were used to generate hydrated chloride residues that were subsequently reacted with HTTA. The resulting NpIV(TTA)4 and PuIV(TTA)4 coordination complexes were isolated as single crystals. Structure Descriptions. Except for CeIII(TTA)4−, all of the MIII(TTA)4− (Nd, Sm, Yb) complexes, as well as the reported

Scheme 2. Synthetic Schemes for the Formation of MIV(TTA)4 Complexes

Figure 3. (left) A ball-and-stick representation of the crystal structure of (NH4)MIII(TTA)4 (MIII = Nd, Sm, Yb). (right) The inner oxygen coordination shell (dodecahedron) surrounding the MIII ion.

Figure 4. (left) A ball-and-stick representation of the crystal structure of (NH4)CeIII(TTA)4. (right) The inner coordination oxygen shell surrounding the MIII ion.

previously (NH4)PrIII(TTA)4 salt, were isomorphous and crystallized in the triclinic space group P1̅ (Figure 3, Table S1). For CeIII(TTA)4−, the salt crystallized in the monoclinic space group P21/n (Figure 4, Table S1). Two polymorphs were also observed for the neutral MIV(TTA)4 molecules (Figure 5, Tables S2 and S3). The ZrIV, HfIV, and UIV data were modeled as Pca21, as was data we obtained from ThIV(TTA)4, which consequently matched the crystal structure preciously reported by Lenner.34 Meanwhile, PuIV crystallized in the space group P2/n. For CeIV and NpIV we isolated crystals in both space groups. For the non-centrosymmetric orthorhombic (Pca21) structures, there was one crystallographically unique metal center. The arrangement of MIV(TTA)4 molecules in the orthorhombic (Pca21) structures were similar to that associated with the monoclinic (P2/n) crystalline solids, and it was difficult to determine which structure was thermodynamically C

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Figure 5. (left) A ball-and-stick representation of the crystal structure of MIV(TTA)4·(H2O) (MIV = Zr, Hf, Ce, Th, U, Np, and Pu). (right) Inner oxygen coordination shell (bicapped trigonal prism) around the MIV ion.

Figure 6. Average M−O bond lengths from MIV(TTA)4 (purple) and MIII(TTA)4− (orange) vs the Shannon ionic radii.38 Error bars represent standard deviation from the mean (1σ) for +4 species with a coordination number of 8.38

favored. For example, under a microscope (ZEISS, Stemi SV11) it was not possible for us to distinguish these polymorphs and selectively choose one over the other for Xray diffraction studies. However, we observedby X-ray diffractionthe Pca21 structure more often than the less symmetric P2/n polymorph. The extended structures for both of these polymorphs were distinct in comparison to the those observed for the +3 metals, MIII(TTA)4− (P1̅), where an intricate network of hydrogen bonds between water molecules and ammonium cations directed the arrangement of the MIII(TTA)4− anions. Despite the space group and long-range ordering differences, local geometries around the metal ions were nearly identical for all of the MIII(TTA)4− and MIV(TTA)4 structures. For instance, the inner-coordination sphere around each metal cation contained eight oxygen atoms associated with the four bidentate TTA− ligands. For the +3 metal cations in MIII(TTA)4−, Raymond and co-worker’s Shape8 analysis program revealed the eight oxygen atoms adopted a dodecahedron around each metal center with an approximate D2d symmetry (Figures 3 and 4).35,36 In contrast, geometries for +4 metal cations in MIV(TTA)4 were subtly different and best described as bicapped trigonal prisms with approximate C2v symmetries (Figure 5).35−37 For both the +3 and +4 structures, the average M−O interatomic distances became shorter from CeIII to HfIV (Figure 6), as expected based on the decreasing metal ionic radii.38 Surprisingly, Figure 6 also illustrated that, for metals of equivalent ionic radii, the average MIII−O distance was ∼0.4 Å shorter than the MIV−O distance. This difference was likely related to geometric difference between the +3 versus +4 coordination complexes; the ligand arrangement between the two structures was not equivalent. Perhaps the combination of the large +3 metal and the additional NH4+ cation enables flexibility for asymmetric TTA− binding, thereby enabling some of the TTA− oxygen atoms to approach the central MIII cation closely. Note the trend toward larger standard deviations from the mean (at 1σ) with increasing ionic radii for the MIII(TTA)4− complexes shown in Figure 6. Within all of the structures, the trifluoromethyl (CF3) and thiophene (SC4H3) groups of the TTA− ligands experienced varied degrees of disorder, which is common in many TTA− crystal structures reported previously.29,39,40 The most notable difference between MIV(TTA)4 and MIII(TTA)4− was associated with the orientation of the

TTA− ligands. Two isomers were observed, one that was specific to the +4 metals and another that was unique to the +3 metals. In MIV(TTA)4, the TTA− anions aligned themselves with all four thiophene (SC4H3) groups above the square plane created by the bicapped trigonal prism of TTA− oxygen atoms. We describe this TTA− arrangement as forming a pocket with the thiophene carbon backbone oriented outward and away from the MIV(TTA)4 molecular C4 rotation axis. This arrangement constrained the four sulfur atoms to face inward toward the C4 axis. In contrast, the MIII(TTA)4− structures (MIII = Nd, Sm, and Yb) had one TTA− ligand misaligned, such that three thiophenes substituents were orientated above the square plane of the oxygen dodecahedron and one pointed below. In this “three-up/one-down” isomer, there was no thiophene pocket. For CeIII, an intermediate structure was observed with a “two-up/two-down” TTA arrangement. The origins for the aligned TTA− ligands in MIV(TTA)4 versus the misaligned TTA− ligands in MIII(TTA)4 remain unclear. Ce L3-Edge XANES. Because the M(TTA)4x− (x = 0, 1) complexes were isolated for Ce in both the +3 and +4 oxidation states, and because it is difficult to unambiguously characterize CeIV by many spectroscopic techniques,35,41−47 we set out to characterize the oxidation state of Ce in the M(TTA)4x− compounds using Ce L3-edge X-ray absorption near edge structure (XANES) spectroscopy in the solid state. After Ce L3edge XANES spectra from CeIII(TTA)4− and CeIV(TTA)4 were obtained using fluorescence detection methods, the data were background-subtracted and normalized (see Figure 7 and the Supporting Information). For reference, spectra from Ce(TTA)4− (x = 1, 0) were compared with a CeIIICl3 standard, where the Ce oxidation state was unambiguously +3. For all of the analyzed compounds, no evidence for sample decomposition was observed due to radiolysis from the X-ray experiments over the course of the XANES measurement. This was confirmed by comparing spectra collected rapidly (five scans, 5 s/scan) with data collected at longer acquisition times (two scans, 30 to 40 min/scan). Additionally, there was no dependence on temperature, as spectra obtained at 10 K were superimposable on data obtained at 85 K. The Ce L3-edge XANES spectra from CeIII(TTA)4− and CeIIICl3 were dominated by large edge features superimposed on steplike absorption thresholds. The CeIII(TTA)4− inflection D

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Solid-state Ce L3-edge XANES spectra from CeIV(TTA)4 (top, blue trace), (NH4)CeIII(TTA)4 (middle, yellow), and CeCl3 (bottom, pink) acquired at 85 K.

point and peak maxima at 5724.2 and 5726.6 eV were nearly identical to that from CeIIICl3 reference standard; 5723.7 (inflection point) and 5725.8 eV (peak maxima). These values were similar to CeIII compounds characterized previously by Ce L3-edge XANES.45,48−57 In contrast, the spectrum from CeIV(TTA)4 differed significantly from those obtained from its +3 Ce counterpart. The CeIV(TTA)4 spectrum contained a “double white-line” feature with a pronounced peak at high energy (5737.4 eV) and a larger feature at low energy (5729.4 eV). This double white-line profile is a well-established diagnostic for Ce in the +4 oxidation state.48−50,52,56−61 1 H and 19 F NMR Spectra from M IV (TTA) 4 and MIII(TTA)4−. The solution behavior of the MIV(TTA)4 and MIII(TTA)4− complexes were interrogated by 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. In these room-temperature spectra, there was only one chemical environment for the TTA− ligands. We interpreted these results as suggesting only a single species present in solution, ostensibly the MIV(TTA)4 or MIII(TTA)4− compounds. In general, four distinct resonances dominated the 1H NMR spectra (Figure 8). These features included two doublets and a triplet associated with the thiophene group and a singlet associated with the methylene proton of the acetonate backbone. Resonances were present between 6 and 8 ppm for compounds containing diamagnetic metals (ZrIV, HfIV, CeIV, and ThIV). Among the diamagnetic series, the metal identity marginally influenced the peak positions for the thiophene resonances, and the MIV(TTA)4 chemical shifts were similar to those from HTTA. In contrast, the peak position for the methylene peak for the actetonate backbone shifted appreciably, providing a useful diagnostic for each compound. For example, in the ZrIV spectrum the methylene peak was bracketed by the thiophene doublets and triplet. Moving from ZrIV to HfIV to CeIV decreased the methylene peak position, such that for CeIV the methylene feature was further upfield than the thiophene triplet. On the basis of the ZrIV, HfIV, and CeIV data we anticipated that the ThIV methylene feature would be even further upfield than the analogous peak observed for CeIV. However, this was not the case, and the ThIV methylene features were shifted furthest downfield. Perhaps, the larger ThIV ion is accessing fluxional processes that the

Figure 8. Comparison of the 1H NMR spectra from MIV(TTA)4 (MIV = Zr, Hf, Ce, Pu) in benzene-d6 (7.16 ppm).

smaller ZrIV, HfIV, and CeIV ions cannot achieve in the MIV(TTA)4 framework. All of the compounds containing paramagnetic metals were similar to their diamagnetic counterparts, albeit the peaks were broadened and shifted outside of the 6 to 8 ppm window. An example is provided in Figure 8 for PuIV(TTA)4. Spectra from the lanthanide salts (NH4)MIII(TTA)4 could not be directly compared with the MIV(TTA)4 data, as the salts were not soluble in benzene. Instead, the NMR studies on (NH4)MIII(TTA)4 were performed in methanol-d4. Spectra from (NH4)MIII(TTA)4 salts were similar to the neutral MIV(TTA)4 compounds, in that the three thiophene responses were resolved. Unexpectedly, the acetonate methylene protons for the MIII(TTA)− anions and those from the NH4+ cations were not located between ±300 ppm, likely owing to exchange with the methanol-d4. For all the analytes, 19F NMR spectroscopic results were consistent with the 1H NMR data. For instance, the 19F NMR results suggested a single species was present in solution and showed only one chemical environment for the TTA− ligands, as all spectra contained a single resonance. Chemical shifts for these peaks were quite similar, spanning only −70.0 to −77.3 ppm. This tight range in CF3 peak positions was somewhat surprising, given that the solvent identity varied from experiment to experiment (benzene-d6 for +4 metals vs methanol-d4 for +3 metals) as did the metal electronic configurations (diamagnetic vs paramagnetic). To calibrate E

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the reader, these values were also reminiscent of the −75.4 resonance from HTTA (benzene-d6). Although these 1H and 19F NMR spectra were consistent with M(TTA)4x− (x = 0, 1) as the primary species present in solution, the data indicated that the solid-state structures were not retained once the compounds dissolved. For example, we would expect two TTA chemical environments for the threeup/one-down MIII(TTA)4− isomers, while only one chemical environment was spectroscopically observed. These results implied that TTA− fluxionalities were fast on the NMR time scale and highlighted the potential for rapid conformation changes between the three-up/one-down MIII(TTA)4− isomers with “two-up/two-down” structures and geometries where TTA ligands were completely aligned. Solid- and Solution-Phase UV−vis−NIR Spectra from M(TTA)4x− (x = 0, 1). All of the M(TTA)4x− (x = 0, 1) compounds were characterized by UV−vis−NIR spectroscopy, either in the solution-phase for CeIII and CeIV or (for all other compounds) as single crystals using a micro-spectrophotometer. Spectra from NdIII(TTA)4− displayed typical Laporteforbidden 4f → 4f transitions characteristic for NdIII (4I9/2 → 4 G5/2 and 4G7/2 around 18 949.3, 17 399.8, and 17 084.3 cm−1; 4 I9/2 → 4F7/2 and 4S3/4 around 13 610.8 and 13 335.9 cm−1; 4I9/2 → 2H9/2 and 4F5/2 around 12 390.9 cm−1).62 These 4f → 4f transitions were superimposed on a strong absorption peak centered near 25 125.3 cm−1 (398.01 nm; see Supporting Information). In contrast for SmIII(TTA)4−, crystals sufficiently transparent for the single-crystal UV−vis measurement only displayed a large high-energy absorbance near 25 239.1 cm−1 (396.21 nm). Meanwhile, SmIII 4f → 4f transitions could not be rigorously resolved from the spectral baseline. For the colorless YbIII(TTA)4−, no 4f → 4f transitions were detected within the window of our spectrometer (40 000 to 5882.35 cm−1); however, a ubiquitous absorption band was observed at energies similar to Nd and Sm; at 25 510.2 cm−1 (392 nm; see Supporting Information). As the feature near 25 000 cm−1 was common for NdIII, SmIII, and YbIII, its origin was reasonably attributed to electronic transitions associated solely with TTA−. However, at this stage we cannot discount the possibility of metal/ligand charge transfer.63 Efforts to characterize CeIV(TTA)4 and CeIII(TTA)4− by single-crystal (solid-state) UV−vis−NIR spectroscopy were only mildly successful because of very high extinction coefficients. Even extremely small crystals pushed absorption intensities near the saturation point of the detector. Hence, only solution-phase measurements were informative (Figure 9). Consistent with data reported previously for other CeIV compounds, the UV−vis−NIR spectrum from CeIV(TTA)4 contained three absorption bands near 22 166, 29 903, and 36 464 cm−1 (451, 334, and 274 nm, respectively). Reducing CeIV(TTA)4 to CeIII(TTA)4− increased the valence electron count by one, manifesting in a dramatic color change from dark red (CeIV) to light orange-yellow (CeIII). Spectroscopic comparisons revealed the third low-energy absorption band disappeared in the CeIII data, which is characteristic of Ce in the +3 oxidation state. Overall, these solution-phase results were consistent with the oxidation state assignments based on solidstate Ce L3-edge XANES spectroscopy, discussed above. Spectra from the closed-shell d0 transition-metal MIV(TTA)4 (MIV = Zr, Hf) and 5f0/6d0 ThIV(TTA)4 complexes contained only the ubiquitous TTA− absorption feature near 25 500 cm−1 (392 nm), described above for Nd, Sm, and Yb. In contrast, spectra from the open-shell actinide counterparts were quite

Figure 9. CeIII(TTA)4− (yellow) and CeIV(TTA)4 (red) UV−vis absorption spectrum in acetonitrile.

rich. These AnIV(TTA)4 (AnIV = U, Np, Pu) compounds displayed a broad absorption band that began near 17 500 cm−1 and bled into the visible region, where 5f → 5f transitions were observed (Figure 10). In all spectra, these 5f → 5f transitions were characteristic of actinides (U, Np, and Pu) in the +4 oxidation state. For example, from the perspective of the free ion, the UIV spectrum contained transitions between the 3H4 ground state to 3F3 and 1G4 excited states at 8727 and 9238

Figure 10. Single-crystal UV−vis−NIR absorption spectra from UIV(TTA)4 (top, red), NpIV(TTA)4 (middle, green), and PuIV(TTA)4 (bottom, orange) crystals. F

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Inorganic Chemistry cm−1 (1145.87 and 1082.49 nm).64 For NpIV(TTA)4, multiple transitions between 12 000 and 16 000 cm−1 (833 and 625 nm) were present that we loosely attributed to excitations involving the 4I9/2 ground state and 4I5/2, 4I3/2, 4I7/2, and 4I15/2 excited states.64 Additionally, transitions associated with the 4I11/2 (6412 and 5969 cm−1; 1559.58 and 1675.32 nm), 4F3/2 (8163 cm−1; 1225.04 nm), and 4I13/2 (10 140 cm−1; 986.19 nm) excited states were also identified.64 We refrained from interpreting the potential transitions for the remaining higher energy transitions (≥11 000 cm−1; 909.09 nm), as the excited states have multiple j-state contributions.64 For PuIV(TTA)4, although the UV−vis−NIR spectrum was broadened, characteristic transitions near 9091 cm−1 (1099.99 nm) from the PuIV 5I4 ground state to 5F2 and 5I6 states were easily resolved.64 Most notable for all three of these spectra was the absence of features from AnIII, AnV, and AnVI. These results were particularly surprising, given the assumed instability of uranium and neptunium in the +4 oxidation state combined with the fact that no effort was invested to exclude air and moisture from the experimental conditions. Extraction of NpIV and PuIV by HTTA. To evaluate the relevancy of the AnIV(TTA)4 (AnIV = Pu, Np) compounds in terms of the HTTA extraction of AnIV ions, a series of NMR and X-ray absorption spectroscopy experiments were conducted on the extracted Np and Pu species. This was achieved by preparing chemically and radiochemically purified NpIV and PuIV stock solutions in HCl (1 M) using the Np procedure described herein and a Pu procedure reported previously.65 Metal oxidation states were confirmed to be +4 by UV−vis− NIR spectroscopy prior to extraction (see Supporting Information). Subsequently, solutions of NpIV or PuIV (3 mL; 8 μM in metal or 1 mg/mL) were contacted with o-xylene-d10 (1.5 mL) solutions containing HTTA (0.5 M). During this time, solutions were aggressively agitated using a vortex mixer. After 10 min, the NpIV and PuIV ions were extracted into the oxylene-d10, and the now orange (PuIV) or green (NpIV) organic solutions were separated from the aqueous phase. These actinide extracts were analyzed by UV−vis−NIR spectroscopy, which showed Np and Pu in the +4 oxidation state. Then, the o-xylene-d10 solutions were characterized by NMR and XAS spectroscopies. NMR Studies of the NpIV and PuIV HTTA Extractions. To determine if the MIV(TTA)4 coordination complexes formed during HTTA extractions, the HTTA extraction products for Np and Pu (in o-xylene-d10) were characterized by 1H and 19F NMR spectroscopy. Each 19F NMR spectrum was dominated by an intense resonance of the HTTA extractant at −75.4 ppm (Figure 11), as HTTA was present in significant excess (0.5 M) in comparison to the Np and Pu concentrations (∼0.8 mM). However, shifted slightly downfield was a small single resonance of the actinide extraction product. This suggested that a single actinide species was present in solution on the NMR time scale. Despite solvent differences (oxylene-d10 vs benzene-d6), the chemical shifts for these species were similar to those observed for Np IV (TTA)4 and PuIV(TTA)4. The extracted plutonium species had a single peak at −74.4 ppm, which was only 0.1 ppm upfield from that observed for PuIV(TTA)4. Similarly, the chemical shift for the extracted neptunium species was −73.8 ppm, whereas that from NpIV(TTA)4 was −74.0 ppm. Results from the 19F NMR experiments were consistent with analyses by 1H NMR spectroscopy. The top pane of Figure 12 shows the 1H NMR spectrum from the HTTA extractant in o-

Figure 11. Comparing 19F NMR spectra from solutions of HTTA (0.5 M) in o-xylene-d10 with NpIV and PuIV extraction products. (top) HTTA (0.5 M) in o-xylene-d10, (middle) NpIV (8 mM) extracted from HCl (1 M) into HTTA (0.5 M) in o-xylene-d10, and (bottom) PuIV (8 mM) extracted from HCl (1 M) into HTTA (0.5 M) in o-xylene-d10.

Figure 12. Comparing 1H NMR spectra from solutions of HTTA (0.5 M) in o-xylene-d10 with NpIV and PuIV extraction products. (top) HTTA (0.5 M) in o-xylene-d10, (middle) NpIV (8 mM) extracted from HCl (1 M) into HTTA (0.5 M) in o-xylene-d10, and (bottom) PuIV (8 mM) extracted from HCl (1 M) into HTTA (0.5 M) in o-xylene-d10.

G

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry xylene-d10. Upon extraction of an AnIV ion, small resonances associated with AnIV(TTA)4 emerged. In both the NpIV and PuIV spectra, the two thionyl doublets were between ∼7.8 and 8.8 ppm, while the thionyl (SC4H3) triplets were between ∼6.3 and 7.0 ppm. There was substantially more variation in the position of the methylene singlet, likely because of paramagnetism differences between the NpIV 5f1 versus the PuIV 5f2 electronic configurations. For example, in the NpIV case the methylene singlet was shifted downfield to 8.5 ppm; meanwhile, for PuIV the analogous peak was shifted upfield to 6.0 ppm. The similarity of these spectra, as well as those obtained by 19F NMR, to those of the NpIV(TTA)4 and PuIV(TTA)4 reference compounds (collected in benzene-d6, Figure 8) suggested that neutral AnIV(TTA)4 coordination complexes were the dominant species extracted during the TTA/o-xylene liquid/liquid extraction procedure. XAS Studies of the NpIV and PuIV HTTA Extractions. To test our interpretations of the 1H and 19F NMR results and provide further support that AnIV(TTA)4 complexes were the primary species extracted by HTTA, a series of XAS experiments was conducted. Approximately 3 d after the NMR studies, the o-xylene-d10 solutions containing the NpIV and PuIV HTTA extraction products were interrogated by actinide L3-edge XAS at the Stanford Synchrotron Radiation Lightsource (SSRL). The extracted solutions were analyzed at room temperature, and the results were evaluated in comparison to low-temperature (85 K) solid-state measurements made on NpIV(TTA)4 and PuIV(TTA)4 reference samples. Each actinide L3-edge XANES spectrum (Figure 13) contained a single absorption peak (near 18 062 eV for Pu and 17 614 eV for Np) that was superimposed upon an absorption threshold. In both the Np and Pu cases, spectra from the organic phase of the HTTA extractions were nearly equivalent to the solid-state data from the AnIV(TTA)4 (AnIV = Pu, Np) references. This similarity was exemplified through inflection point comparisons, which were determined graphically as the point where the second derivative of the data equaled zero. For instance, the 18 062.3 eV inflection point from PuIV(TTA)4 obtained in the solid state was only 0.1 eV lower in energy than the 18 062.4 eV value measured for the PuIV in the organic phase of the HTTA extraction, which is within the energy resolution of the technique.66−69 A similar inflection point variation was observed in the NpIV case; NpIV(TTA)4 was 0.2 eV higher than the NpIV extracted species. The peak positions and postedge line shapes suggested that all samples contained metals in the +4 oxidation. The k3-weighted extended X-ray absorption fine structure (EXAFS) spectra from the extracted species were essentially identical to data obtained on the solid control samples of NpIV(TTA)4 and PuIV(TTA)4 (Figure 14), indicating that local geometries around Np and Pu in the HTTA extract were similar to that for the AnIV(TTA)4 control samples. Subtle amplitude variations were attributed to differences in the Debye−Waller factors, as the experimental conditions were quite distinct (solution phase vs solid state; room temperature vs low temperature, 85 K). Analysis of the EXAFS spectra from the AnIV(TTA)4 control samples (solid state) provided a basis for modeling the solution-phase AnIV L3-edge EXAFS data. The solid-state spectra were fit (Figure 15) by allowing the atomic distances (R), Debye−Waller factors (σ2), and amplitude reduction factors (S02) to converge to reasonable values (Table 1). In all of the fits, optimized Debye−Waller factors were in a normal range, spanning from 0.0031(8) to 0.0084(8) Å2. For

Figure 13. A comparison of the solid-state XANES spectra from PuIV(TTA)4 (top; solid red trace) and NpIV(TTA)4 (bottom; solid brown trace) acquired at 85 K with the solution-phase spectrum from organic phase (o-xylene-d10) of the HTTA extraction of PuIV (top; purple dashed trace) and NpIV (bottom; blue dashed trace) from HCl (1 M).

the control samples, coordination numbers were fixed based on the solid-state single-crystal X-ray diffraction measurements, and S02 values were allowed to converge to 0.93(5) for NpIV(TTA)4 and 0.97(6) for PuIV(TTA)4. Interpretations for each data set were similar. As a representative example, Figure 16 shows the individual scattering paths employed in the PuIV(TTA)4 fit. The shortest scattering pathway (pink trace near 1.9 Å in Figure 16) was attributable to a shell of eight oxygen atoms that was closely followed by a shell containing eight carbon atoms of the TTA− ketone functional groups, (lime-green trace at 2.9 Å). The experimental resolution (ΔR) was calculated to be 0.143 Å (π/ 2Δk; k = 2−13 Å). Hence, we modeled the oxygen and carbon scattering pathways as single shells with “averaged” M−O and M···Cketone distances of 2.33 ± 0.02 and 3.78 ± 0.03 Å for NpIV(TTA)4 and 2.32 ± 0.02 and 3.77 ± 0.02 Å for PuIV(TTA)4. These values were consistent with the singlecrystal measurements. These EXAFS analyses provided a foundation for evaluating the NpIV and PuIV speciation in the o-xylene-d10 fraction of the HTTA liquid/liquid extraction. The solution-phase EXAFS data were modeled by fixing S02 at 0.9 (based on the solid-state fits above) and allowing the coordination numbers to converge. Overall, there was excellent agreement between the fitted H

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Inorganic Chemistry

Figure 15. Room-temperature Fourier transform of k3-EXAFS spectra (purple traces) of the HTTA extraction of Pu (top) and Np (bottom) in o-xylene-d10. These data are compared with the Fourier transform of k3-EXAFS spectra from NpIV(TTA)4 and PuIV(TTA)4 measured in the solid state (85 K). In all four panes, the real part of the Fourier transform is shown as orange traces. Fits to the data were provided as black dashed traces.

Figure 14. EXAFS function k3 χ(k) of the organic o-xylene-d10 phase from the HTTA extraction of Pu (top; yellow trace) or Np (bottom; blue trace) from aqueous solutions (HCl, 1 M). Overlaid on these data are EXAFS functions k3 χ(k) from AnIV(TTA)4 (AnIV = Pu, top; Np, bottom; black dashed traces) acquired as solid-state samples at 85 K.

parameters for these solution-phase data with the analyses for the solid-state controls, which also agreed with the results from single-crystal X-ray diffraction models. The O and C coordination numbers in both NpIV(TTA)4 and PuIV(TTA)4 were close to 8 and consistent with the presences of four TTA− ligands coordinated to the actinide(IV) ions. Additionally, the solution-phase M−O and M···Cketone distances were equivalent (when uncertainties were considered) to those obtained via solid-state EXAFS and by single-crystal X-ray diffraction for the AnIV(TTA)4 control samples. Overall, these distances and coordination numbers indicated that the NpIV(TTA)4 and PuIV(TTA)4 coordination complexes were the dominant species extracted by HTTA into o-xylene-d10 from aqueous solutions (HCl; 1 M), consistent with our interpretation of the 1 H and 19F NMR results above.

neutral MIV(TTA)4 coordination compounds. The anions were only soluble in polar solvents, whereas the neutral compounds were soluble in nonpolar solvents. In both cases, 1H and 19F NMR experiments revealed only a single species in solution on the NMR time scale. The 1H and 19F NMR data collected from the lanthanides(III) (methanol-d4) and metal(IV) (benzene-d6) compounds suggested that the M(TTA)4x− (x = 0, 1) stoichiometries were retained once the samples were dissolved. Characterization of these well-defined M(TTA)4x− (x = 0, 1) compounds was critical for determining Np and Pu speciation in the organic phase of an actual HTTA extraction. Analysis of the Np and Pu species extracted into o-xylene-d10 from HCl (1 M) by 1H and 19F NMR spectroscopy provided nearly equivalent spectra to that of AnIV(TTA)4 dissolved in benzene-d6. These results showed that, during the separation, the HTTA extractant was deprotonated and formed TTA− monoanions that bound the Np and Pu metals. The XAS analyses of the HTTA-extracted products were consistent with the NMR results and suggested that AnIV(TTA)4 (AnIV = Np, Pu) compounds were the dominant species in the organic phase. Oxidation states for Np and Pu were determined to be +4 by a combination of UV−vis−NIR and An L3-edge XANES spectroscopy. Furthermore, the EXAFS data were (1st) nearly identical to that obtained from the NpIV (TTA) 4 and PuIV(TTA)4 standards and (2nd) fit with four TTA− molecules bound by the central actinide +4 ion. These coordination numbers, as well as the corresponding MIV−TTA distances,

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OUTLOOK A family of MIV(TTA)4 (MIV = Zr, Hf, Ce, Th, U, Np, Pu) and MIII(TTA)4− (MIII = Ce, Nd, Sm, Yb) compounds were rigorously characterized in both the solution phase and the solid state. While the structures for compounds containing +4 metals subtly differed from +3 analogues, all of the transition metal(IV), lanthanide(III), cerium(IV), and actinide(IV) molecules crystallized with four TTA− ligands. In contrast to the neutral MIV(TTA)4 molecules, +3 lanthanides formed MIII(TTA)4− anions. This difference in molecular charge MIII(TTA)− versus MIV(TTA)4imparted substantial solubility differences between the anionic MIII(TTA)4− and the I

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Table 1. Np and Pu L3-Edge EXAFS Fitting Parameters from the AnIV(TTA)4 (An = Pu, Np) Reference Standards and from the HTTA Extraction of Pu (top) and Np (bottom) in o-Xylene-d10 compound

ΔE0 [eV]

S02

M−O

O-σ2

CN O

M−C

C-σ2

CN C

Pu (TTA)4 solid PuIV HTTA extract NpIV(TTA)4 solid NpIV HTTA extract

1.2 ± 0.5 −0.1 ± 0.7 1.5 ± 0.4 −0.6 ± 0.8

0.97(6) 0.90 0.93(5) 0.90

2.316(6) 2.312(6) 2.337(6) 2.324(7)

0.0048(8) 0.0084(8) 0.0031(8) 0.0064(11)

8.0 10.8 ± 0.8 8.0 9.3 ± 0.7

3.36(3) 3.35(2) 3.42(2) 3.41(2)

0.008(4) 0.006(4) 0.006(3) 0.007(5)

8.0 6.4 ± 3 8.0 8.3 ± 3

IV

proof, Fisher), o-xylenes-d10 (C8D10, 99 atom % D, Sigma-Aldrich), benzene-d6 (C6D6, 99.96 atom % D, Sigma-Aldrich), methanol-d4 (CD3OD, 99.96 atom % D, Sigma-Aldrich), acetonitrile (MeCN, HPLC grade; Fisher), toluene (MeC6H5, certified ACS; Fisher), ammonium hydroxide (NH4OH, trace metal basis; Sigma-Aldrich), hydrochloric acid (trace metal basis, HCl; Fisher), HF (trace metal basis, Fisher), HNO3 (trace metal basis, Fisher), NH2OH·HCl (99.995%; Sigma-Aldrich), H3BO3 (99.5%; Sigma-Aldrich), NaNO2 (99.99%; Sigma-Aldrich), and diethyl ether (ether, 99.7%; SigmaAldrich) reagents and solvents were obtained commercially and used as received. The 2-thenoyltrifluoroacetone (HTTA, 99%; SigmaAldrich) reagent was also obtained commercially and used as received. This regent should be used fresh and stored in the dark at low temperature. If the color of the HTTA reagent is yellow, it should be purified before use. The UCl 4 was prepared as previously described.70,71 All water used in these experiments was deionized and passed through a Barnstead water purification system, until a resistivity of 18 MΩ was achieved. The plutonium (MT52) and neptunium-237 starting materials were used as oxidation state pure starting solutions in HCl (6 M). The plutonium stock solution was prepared as previously described from plutonium residues recovered from various experimental campaigns.72 The 237Np stock solution was prepared from a residue, used previously in a separate experimental campaign, as described below. Elemental analyses were obtained on all samples (excluding Np and Pu) in one of two ways. Some samples were washed with hexanes and dried thoroughly before they were shipped to the Midwest Microlab LLC, where elemental analysis was performed. For (NH4)NdIII(TTA)4, (NH4)SmIII(TTA)4, and (NH4)YbIII(TTA)4 the percent metals was determined by direct complexometric titration of rare-earth ethanol solutions with ethylenediaminetetraacetic acid (EDTA).73−75 Caution! 237Np (t1/2 = 2.14 million y)76 and MT52which contains 239 Pu (t1/2 = 24 065 y),76 240Pu (t1/2 = 6537 y),76 241Pu (t1/2 = 14 y),76 and 242Pu (t1/2 = 373 300 y)76are serious health threats, due to their direct α-emissions and the α-, β-, γ-gamma emissions of their radioactive daughters. Hence, all studies with neptunium and plutonium were conducted in a radiation laboratory equipped with high-ef f iciency particulate air (HEPA) f iltered hoods, continuous air monitors, and gloveboxes. All f ree-f lowing solids were handled within negative-pressure gloveboxes equipped with HEPA f ilters. Single-crystal UV−vis−NIR measurements were made on single crystals mounted on a quartz slide under low-absorbance oil, and absorbance data were collected from 33 333 to 8333 cm−1 (300 to 1200 nm) using a Craic Technologies micro-spectrophotometer. Infrared spectra were obtained using an iD7 ATR crystal window (inside of a negative pressure glovebox), and transmission spectra were collected from 410 to 3300 cm−1 on a Nicolet iS 5N FT-NIR Spectrometer. All NMR spectra were collected using a Bruker Avance 400 MHz NMR and processed with MestReNova v10. Single-Crystal X-ray Diffraction. Single crystals of AnIV(TTA)4 (AnIV = Np, Pu) were mounted with three appropriate layers of containment prior to single-crystal X-ray diffraction studies, as previously described.72 All other single crystals were mounted on nylon loops with mineral oil (Hampton Research) and placed on a D8 Bruker QUEST diffractometer. No corrections for crystal decay were necessary. Standard Apex II software was used for determination of the unit cells and data collection control. The intensities of the reflections of a sphere were collected by combining four sets of exposures (frames), which totaled to 1464 frames with an exposure time of 10− 60 s per frame, depending on the crystal. Apex II software was used for

Figure 16. An interpretation of the Fourier transform of k3-EXAFS spectra of the PuIV(TTA)4 (purple trace). The fit is shown as a dashed black trace, while contributions from the TTA− oxygen atoms and the ketone carbon atoms are shown as pink and green traces, respectively.

were similar to those observed from the single-crystal AnIV(TTA)4 standards. Moreover these results confirm possible extraction mechanisms invoked previously to rationalize why the HTTA extraction works so well, such as the presumed HTTA extraction of BkIV presented by Moore in 1966.25 Insight into the effectiveness of the HTTA/TTA− extraction for removing +4 cations from +3 contaminants was inferred by considering these results in the context of the solid-state and solutions-phase data described above. Overall, the data suggested that MIV(TTA)4 formation provides neutral molecules that can effortlessly pass from the aqueous HCl (1 M) solutions into the organic diluent during the liquid/liquid extraction process. In contrast, MIII(TTA)4− anion formation is not favorable, because it generates charged species that are insoluble in nonpolar hydrocarbon solvents. As such, under the experimental conditions, the +3 metals remain in the aqueous phase, whose speciation is likely quite complicated. Given the effectiveness of this unusually specific +4 metal extraction procedure and the importance of the HTTA/TTA− extraction process to the nuclear complex, we are motivated by these results to investigate further the electronic structure and bonding characteristics of these M(TTA)4x− (x = 0, 1) coordination complexes. It is our hope that these efforts will provide the necessary understanding to better control the HTTA/TTA− extraction procedure and help explain the subtle differences in MIV distribution coefficient trends reported previously.

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EXPERIMENTAL SECTION

General Considerations. The Th(NO3)4·xH2O (99%; SigmaAldrich), HfCl4 (99.9+%; Strem), ZrCl4 (99.5%; Strem), Ce(SO4)2· xH2O (98%; Sigma-Aldrich), NdCl3·xH2O (99.9% Sigma-Aldrich), YbCl3·xH2O (99.9%; Sigma-Aldrich), SmCl3·xH2O (99%; SigmaAldrich), boron nitride (BN, 99.5%; Fisher), ethanol (EtOH; 200 J

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Article

Inorganic Chemistry data integration including Lorentz and polarization corrections. All crystal structures were solved using SHELX software, and PLATON was used to check the Crystallographic Information Files (CIFs) for missed symmetry and twinning.77 All CIF files used in this manuscript are available through the Cambridge Crystal Data Centre (CCDC 1572745−1572748, 1572754, 1572756, 1572757, 1572759−1572761). L3-Edge XANES. Cerium samples were prepared on the benchtop, while the solid-state NpIV and PuIV samples were prepared in a negative-pressure glovebox filled with air, in the following manner. A mixture of the analyte and boron nitride (BN) was weighed, such that the edge jump for the absorbing atom was calculated to be at one absorption length in transmission (∼10 mg for Ce samples, 60 mg for NpIV and PuIV). The samples were diluted with BN (∼10 mg) that had been dried at elevated temperature (200 °C) under vacuum (1 × 10−3 Torr) for 24 h prior to use. Samples were ground in a polycarbonate capsule containing a Teflon bead using a Wig-L-Bug grinder. Solid-state sample holders for the Ce samples consisted of an aluminum plate with a 3 × 15 mm oval window and screw holes. One side of the plate was covered with Kapton tape, and the sample was evenly painted in the window. The powder was then secured by covering the sample with a second piece of Kapton tape. A second layer of compound was painted onto a third piece of Kapton tape, which was subsequently fixed to the backside of the sample holder. The sample holder was then loaded onto a sample rod, taken out of the glovebox, and transported to the beamline while submerged within a N2(liq)-cooling bath. Once at the beam, the rod with the sample was placed at 45° inside the Oxford He(liq) cryostat, which was precooled at 85 K and attached to the SSRL Beamline 11−2 rail. When the cryostat was closed, the system was put under vacuum and flushed with helium three times. The valve was closed, and the measurements were performed in the cryostat at 85 and 10 K. Solid-state NpIV and PuIV samples were loaded without excluding air or moisture at Los Alamos National Laboratory into two nested aluminum sample holders equipped with Kapton windows. One set of windows was glued on, and one set was sealed with indium wire. The sample holder was shipped to SSRL. Upon arrival, the samples were opened at the beamline and attached to the coldfinger of a liquid N2 cryostat, which was also attached to the SSRL’s Beamline 11−2 rail. The cryostat was evacuated (1 × 10−6 Torr) and cooled with liquid N2. The Np and Pu solution-phase samples were obtained on solutions that contained 0.5 mg of metal. Analyte solutions were loaded into XAS cells that were triply contained. The XAS holder consisted of a Teflon body with a 2 mm well equipped with a set of Teflon windows (1 mil) and a Kapton window (1 mil). Solutions were introduced into the holder through an injection hole sealed with a Teflon gasket that was held in place by an aluminum plate. The sample cell holder was nested within secondary and tertiary aluminum holders that were equipped with a set of Kapton windows (2 mil). The windows were sealed with Viton gaskets and held in place with stainless steel brackets. A kinematic mount was fixed to the bottom of the tertiary holder, which magnetically held the sample holder at 45° to the incident beam on the SSRL’s Beamline 11−2. The solid-state and solution-phase CeCl3, (NH4)CeIII(TTA)4, and MIV(TTA)4 (MIV = Ce, Np, Pu) compounds were characterized by metal L3-edge X-ray measurements. The X-ray absorption measurements were made at SSRL, under dedicated operating conditions (3.0 GeV, 5%, 500 mA using continuous topoff injections) on end station 11−2. This beamline was equipped with a 26-pole and a 2.0 T wiggler. With the use of a liquid nitrogen-cooled double-crystal Si(220) (ϕ = 0) monochromator that employed collimating and focusing mirrors, a single energy was selected from the incident white beam. For Ce measurements, harmonic rejection was achieved by detuning the second crystal of the monochromator by 70% at 6323 eV for Ce. For plutonium and neptunium measurements, crystals were run fully tuned, and higher harmonics from the monochromatic light were removed using a 370 mm Rh-coated harmonic rejection mirror. The Rh coating was 50 nm with 20 nm seed coating, and the substrate was Zerodur. Vertical acceptance was controlled by slits positioned before the monochromator. The harmonic rejection cutoff was set by the

mirror angle. This controls which photons experience total external reflection. The horizontal slit sizes were 8 mm for CeIII, 7 mm for CeIV, 2.8 mm for solution NpIV, 3.0 mm for solution PuIV, and 9 mm for the solid-state NpIV and PuIV measurements. Vertical slit sizes were 1 mm in all measurements. The cryostat (for solids) and tertiary sample holder (for solutions) were attached to the beamline 11−2 XAS rail (SSRL), which was equipped with three ionization chambers, through which nitrogen gas was continually flowed. One chamber was positioned before the helium beam pass and the cryostat (10 cm for Ce, 30 cm for Np and Pu) to monitor the incident radiation (I0). The second chamber was positioned after the cryostat (30 cm) so that sample transmission (I1) could be evaluated against I0 and so that the absorption coefficient (μ) could be calculated as ln(I0/I1). The third chamber (I2; 30 cm) was positioned downstream from I1 so that the XANES of a calibration foil could be measured against I1. A potential of 1600 V was applied in series to the ionization chambers. A Lytle detector under argon was placed on one side of the cryostat (4 cm) to detect the fluorescence from the samples. The Ce samples were calibrated in situ to the energy of the first inflection point of the K-edge of chromium foil (5989 eV), while the Np and Pu samples were calibrated to the first inflection point of the K-edge of an yttrium foil (17 038.4 eV). Ammonium Cerium(III) Tetrakis(thionyltrifluoroacetonate); (NH4)CeIII(TTA)4. A mixture of CeCl3·(H2O)x (0.0384 g, 0.1031 mmol) and HTTA (0.0939 g, 0.4226 mmol) contained within a glass scintillation vial were suspended in ethanol (EtOH; 1 mL) and water (H2O, 0.5 mL). The mixture was swirled by hand, until the solids dissolved. Subsequently, ammonium hydroxide (NH4OH; 14.5 M; 50 μL) was added dropwise, at which point red oil formed and yellow solid precipitated. The red oil was separated from the solid by decanting and dissolved in benzene, and the yellow solution was left to sit undisturbed. After 2 d yellow tablet-shaped crystals suitable for single-crystal X-ray diffraction formed, and (NH4)CeIII(TTA)4 was isolated in low crystalline yield; typical reactions provide 10.7 mg (10% yield). 1H NMR (400 MHz, methanol-d4 δ 3.34 ppm) 7.3 (t, J = 5 Hz, 1H), (C4H3S)C2O2CHCF3; 8.8 (d, J = 11 Hz, 1H), (C4H3S)C2O2CHCF3; 7.8 (d, fwhh = 5 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. The methylene proton resonance was not found between ±300 ppm. 19 F NMR δ −74.7 ppm. UV−vis−NIR [cm−1]: 25 941.01, CT. IR [cm−1]: 1593.43 s, 1575.34 s, 1534.96 s, 1407.49 s, 1294.79 s, 1174.53 s, 1120.86 s, 1060.33 s, 787. 79 s, 716.18 s, 641.43 s, 1504.84 m, 1353.41 m, 1247.24 m, 1230.58 m, 1079.30 m, 1034.82 m, 931.99 m, 577.07 m, 3032.18 w, 1010.48 w, 858.69 w, 769.00 w, 605.52 w, 521.83 w, 487.71 w. Anal. Calcd [%]: C 36.2, H 2.1, F 21.5. Found: C 36, H 2.16, F 21.8. Ammonium Neodymium(III) Tetrakis(thionyltrifluoroacetonate); (NH4)NdIII(TTA)4. As described above for (NH4)CeIII(TTA)4, the air and moisture-stable (NH4)NdIII(TTA)4 salt was isolated as single pale blue plate-shaped crystals in 75.9% (85.9 mg) crystalline yield from the reaction between NdCl3·(H2O)x (0.0386 g, 0.108 mmol), HTTA (0.0949 g, 0.427 mmol), and NH4OH (14.5 M; 50 μL) in a mixture of ethanol and water. UV−vis−NIR [cm−1]: 25 125.33, 18 949.44, 17 059.02, 13 611, 13 236.79, 12 611.9. 1H NMR (400 MHz, methanol-d4 δ 3.34 ppm) 7.1 (t, J = 4.1 Hz, 1H), (C4H3S)C2O2CHCF3; 8.0 (d, J = 4.8 Hz, 1H), (C4H3S)C2O2CHCF3; 9.1 (d, fwhh = 11.6 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. The methylene proton resonance was not found between ±300 ppm. 19F NMR δ −70 ppm. IR [cm−1]: 1576.24 s, 1534.85 s, 1406.78 s, 1298.0 s, 1281.27 s, 1181.73 s, 1131.52 s, 1058.31 s, 789.97 s, 717.48 s, 578.94 s, 454.17 s, 1509.16 m, 1352.67 m, 1246.31 m, 1229.55 m, 928.71 m, 859.08 m, 682.25 m, 603.82 m, 492.0 m, 3334.13 w, 1458.27 w, 1036.32 w, 1012.36 w, 767.64 w, 603.82 w. Anal. Calcd [%]: Nd 13.5. Found: Nd 13.46. Ammonium Samarium(III) Tetrakis(thionyltrifluoroacetonate); (NH4)SmIII(TTA)4. As described above for (NH4)CeIII(TTA)4, yellow block crystals of the air and moisture-stable (NH4)SmIII(TTA)4 were obtained in 64.5% (98.0 mg) crystalline yield from a reaction between SmCl3(H2O)x (0.0567 g, 0.155 mmol) and HTTA (0.1283 g, 0.577 mmol). 1H NMR (400 MHz, methanol-d4 δ 3.34 ppm) 7.2 (t, J = 4.4 Hz, 1H), (C4H3S)C2O2CHCF3; 7.6 (d, J = 4.9 Hz, 1H), (C4H3S)K

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry C2O2CHCF3; 7.9 (s, J = 3.8 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. The methylene proton resonance was not found between ±300 ppm. 19F NMR δ −72.7 ppm. UV−vis−NIR [cm−1]: 25 239.14. IR [cm−1]: 1576.15 s, 1535.72 s, 1407.04 s, 1299.98 s, 1282.51 s, 1181.92 s, 1134.4 s, 788.91 s, 717.08 s, 579.64 s, 1508.78 m, 1354.40 m, 1246.55 m, 1059.10 m, 640.53 m, 3368.27 w, 1459.40 w, 1037.13 w, 1014.01 w, 929.51 w, 859.42 w, 767.84 w, 694.11 w, 519.20 w, 494.87 w, 455.63 w. Anal. Calcd [%]: Sm 14.0. Found: Sm 13.9. Ammonium Ytterbium(III) Tetrakis(thionyltrifluoroacetonate); (NH4)YbIII(TTA)4. As described above for (NH4)CeIII(TTA)4, yellow rod crystals of the air and moisture-stable (NH4)YbIII(TTA)4 were obtained in 18.9% (19.6 mg) crystalline yield from a reaction between YbCl3(H2O)x (0.0374 g, 0.0965 mmol) and HTTA (0.0947 g, 0.426 mmol). 1H NMR (400 MHz, methanol-d4 δ 3.34 ppm) 7.1 (t, J = 4.5 Hz, 1H), (C4H3S)C2O2CHCF3; 7.7 (d, J = 4.7 Hz, 1H), (C4H3S)C2O2CHCF3; 7.6 (s, J = 3.5 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. The methylene proton resonance was not found between ±300 ppm. 19F NMR δ −77.0 ppm. IR [cm−1]: 1598.64 s, 1537.13 s, 1297.87 s, 1248.18 s, 1230.66 s, 1180.45 s, 1125.26 s, 785.98 s, 720.68 s, 642.10 s, 581.60 s, 1579.59 m, 1510.72 m, 1410.99 m, 1058.0 m, 933.02 m, 681.41 m, 2921.9 w, 1463.97 w, 1354.34 w, 1009.54 w, 860.72 w, 520.28 w, 495.81 w, 461.40 w. Anal. Calcd [%]: Yb 15.8. Found: 15.8. Cerium(IV) Tetrakis(thionyltrifluoroacetonate); CeIV(TTA)4·(H2O). A mixture of Ce(NH4)2(NO3)6 (0.0597 g, 0.1089 mmol) and HTTA (0.0929 g, 0.4181 mmol) was suspended in ethanol (EtOH; 2 mL) within a Pyrex vial (20 mL). The vial was agitated using a vortex mixer, until all of the reagents dissolved. Within seconds, the color of the solution turned dark red. After 2 d, the solvent evaporated leaving a red residue, which was reconstituted in benzene (2 mL). After two undisturbed days, dark red block crystals suitable for single-crystal Xray diffraction formed, and CeIV(TTA)4 was isolated in 65.7% crystalline yield (73.3 mg). 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 6.3 (s, 1H), (C4H3S)C2O2CHCF3; 6.4 (t, J = 4.4 Hz, 1H), (C4H3S)C2O2CHCF3; 6.8 (d, J = 4.9 Hz, 1H), (C4H3S)C2O2CHCF3; 7.1 (d, J = 3.9 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −74.7 ppm. UV−vis−NIR [cm−1]: 18 452.57, CT. IR [cm−1]: 1563.89 s, 1532.92 s, 1301.22 s, 1250.49 s, 1191.83 s, 1129.68 s, 1062.38 s, 796.09 s, 730.30 s, 683.10 s, 635.56 s, 581.26 s, 498.83 s, 461.41 s, 1505.83 m, 1403.20 m, 1352,02 m, 1039.02 m, 1015.40 m, 931.89 m, 863.16 m, 748.92 m, 3091.34 w, 1597.83 w. Anal. Calcd [%]: C 37.5, H 1.57. Found: C 37.48, H 1.6. Zirconium(IV) Tetrakis(thionyltrifluoroacetonate); ZrIV(TTA)4· (H2O). In an argon-filled glovebox, ZrCl4 (0.0247 g, 0.0810 mmol) and HTTA (0.0879 g, 0.396 mmol) were combined in a glass scintillation vial (20 mL). The vial was capped and brought out of the glovebox, where ethanol (EtOH; 1 mL) was added. After 4 d, the air and moisture-stable ZrIV(TTA)4 complex was isolated in 61.2% (49.3 mg) yield as colorless blocks that were suitable for single-crystal X-ray diffraction. 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 6.5 (s, 1H), (C4H3S)C2O2CHCF3; 6.4 (t, J = 4.4 Hz, 1H), (C4H3S)C2O2CHCF3; 6.8 (d, J = 4.9 Hz, 1H), (C4H3S)C2O2CHCF3; 7.1 (d, J = 3.9 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −74.9 ppm. UV−vis−NIR [cm−1]: 26 092.19 CT. IR [cm−1]: 1573.04 s, 1540.57 s, 1404.92 s, 1319.89 s, 1252.04 s, 1194.0 s, 1131.89 s, 1065.75 s, 938.52 s, 739.69 s, 729.50 s, 640.49 s, 586.25 s, 512.70 s, 1510.15 m, 1361.26 m, 1354.53 m, 1233.26 m, 1086.76 m, 1041.33 m, 1021.19 m, 718.13 m, 686.09 m, 473.14 m, 462.64 m, 1630.22 w, 1599.09 w, 1458.50 w, 773.10 w, 750.59 w, 562.03 w. Anal. Calcd [%]: C 39.38, H 1.65, F 23.36, S 13.14. Found: C 39.35, H 1.62, F 23.19, S 12.98. Hafnium(IV) Tetrakis(thionyltrifluoroacetonate); HfIV(TTA)4·(H2O). As described above for ZrIV(TTA)4, colorless block-shaped crystals of the air and moisture-stable HfIV(TTA)4 were obtained in 66.8% (71.7 mg) crystalline yield from a reaction between HfCl4 (0.0324 g, 0.101 mmol) and HTTA (0.0882 g, 0.397 mmol) in an ethanolic (EtOH, 1 mL) solution. 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 6.5 (s, 1H), (C4H3S)C2O2CHCF3; 6.4 (t, J = 4.5 Hz, 1H), (C4H3S)C2O2CHCF3; 6.8 (d, J = 4.8 Hz, 1H), (C4H3S)C2O2CHCF3; 6.1 (d, J = 3.7 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −75.1 ppm. UV−vis−NIR [cm−1]: 26 747.29 CT. IR [cm−1]: 1575.63 s, 1541.53 s, 1404.56 s, 1322.65 s, 1252.59 s, 1194.75 s, 1132.32 s, 1066.19 s, 793.64

s, 729.73 s, 641.38 s, 1511.62 m, 1362.04 m, 1354.19 m, 1233.61 m, 1087.13 m, 1041.43 m, 1022.25 m, 940.43 m, 864.07 m, 751.0 m, 686.66 m, 587.42 m, 513.54 m, 488.62 m, 479.97 m, 1653.26 w, 1461.57 w, 551.87 w, 548.25 w. Anal. Calcd [%]: C 36.15, H 1.52, F 21.44, S 12.06. Found: C 36.19, H 1.47, F 21.43, S 12.15. Thorium(IV) Tetrakis(thionyltrifluoroacetonate); ThIV(TTA)4·(H2O). In an open-front hood, ethanol (EtOH, 1 mL) was added to a glass scintillation vial (20 mL) charged with ThCl4 (0.0359 g, 0.0960 mmol) and HTTA (0.0867 g, 0.3905 mmol). Within seconds, opaque oil formed. The lighter (top) aqueous layer was decanted from the oil on the bottom. The oil was then dissolved in acetonitrile (MeCN; 2 mL) and the light yellow solution was allowed to sit undisturbed. After 2 d light yellow block-shaped crystals suitable for single-crystal X-ray diffraction formed, and ThIV(TTA)4 (70.6 mg) was isolated in 65.8% crystalline yield. 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 6.6 (s, 1H), (C4H3S)C2O2CHCF3; 6.4 (t, J = 4.3 Hz, 1H), (C4H3S)C2O2CHCF3; 6.7 (d, J = 4.9 Hz, 1H), (C4H3S)C2O2CHCF3; 7.1 (d, J = 3.8 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −75.2 ppm. UV−vis−NIR [cm−1]: 25 555.84. IR [cm−1]: 1568.25 s, 1538.93 s, 1403.52 s, 1304.82 s, 1252.99 s, 1193.12 s, 1130.92 s, 1063.44 s, 933.71 s, 797.11 s, 731.13 s, 720.78 s, 684.37 s, 636.28 s, 582.54 s, 498.71 s, 1510.70 m, 1352.22 m, 1230.90 m, 1083.88 m, 1039.21 m, 1016.93 m, 464.71 m, 3093.74 w, 1619.77 w, 1446.73 w, 862.01 w, 771.53 w, 751.15 w, 582.54 w. Anal. Calcd [%]: C 33.87, H 1.6, F 20. Found: C 33.9, H 1.64, F 19.7. Uranium(IV) Tetrakis(thionyltrifluoroacetonate); UIV(TTA)4·(H2O). In a glovebox filled with argon, UCl4 (0.0395 g, 0.1040 mmol) was added to a glass scintillation vial. The vial was brought out of the glovebox and placed into an open-front hood, where it was charged with ethanol (EtOH, 2 mL) and HTTA (0.0894 g, 0.4024 mmol). The resulting vibrant orange solution was allowed to slowly evaporate. After 3 d, orange block crystals formed. To obtain crystals suitable for X-ray diffraction the small orange crystals were dissolved in acetonitrile and layered with ether, which was allowed to diffuse and evaporate over 2 d. The UIV(TTA)4 (83.4 mg) product was isolated in 73.9% crystalline yield. 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 12.4 (s, fwhm = 8.7 Hz, 1H), (C4H3S)C2O2CHCF3; 5.2 (t, fwhm = 11.6 Hz, J = 4.8 Hz, 1H), (C4H3S)C2O2CHCF3; 6.9 (d, J = 4.9 Hz, 1H), (C4H3S)C2O2CHCF3; 7.5 (d, fwhm = 34.8 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −77.3 ppm. UV−vis−NIR [cm−1]: 8790.2 m, 9308.47 m, 10 488.89 w, 12 293.32 w, 15 289.82 sh, 16 010.5 w, 18 543.93 m, 21 445.42 CT. IR[cm−1]: 1564.28 s, 1537.73 s, 1305.11 s, 1192.97 s, 1128.78 s, 1063.0 s, 796.69 s, 730.26 s, 1510.02 m, 1403.54 m, 1252.28 m, 1039.37 m, 1016.06 m, 933.48 m, 718.44 m, 636.29 m, 581.71 m, 499.68 m, 462.66 m, 3094.37 w, 1618.20 w, 1352.94 w, 771.76 w, 749.45 w. Anal. Calcd [%]: C 33.7, H 1.59, F 19.9. Found: C 33.7, H 1.62, F 19.2. Preparation of Np(IV) Stock Solution. In a fume hood, a +4 Np stock solution was prepared as shown in Figure 17. First, nitric acid (HNO3, 16 M) was added to a Teflon beaker charged with a neptunium residue used previously in other research campaigns. The solution was heated to a soft dryness, reconstituted in nitric acid (16 M), and boiled to a residue again. This process was repeated twice. The neptunium nitrate residue was converted to the chloride by adding concentrated hydrochloric acid (10 mL), and the solution was boiled to near dryness. This process was repeated three times. The residue was taken into a minimal amount of hydrochloric acid (2 M) and quantitatively transferred to a falcon tube (polypropylene, 50 mL) with hydrochloric acid (2 M). Analysis by UV−vis−NIR spectroscopy suggested that this solution contained ∼350 mg of Np (1.28 mmol); however, it is difficult to quantify the exact amount of Np present by UV−vis−NIR, as the matrix was not an exact match of published extinction coefficients.12 Reduction to NpIV was achieved by adding an excess of a saturated solution of hydroxylamine hydrochloride (NH2OH·HCl). After 30 min, UV−vis−NIR analysis showed that the solution contained a mixture of NpV and NpIV. Hence another aliquot of NH2OH·HCl (1 mL) was added, and the solution was heated at ∼80 °C in a hot block. After 30 min, complete conversion from NpV to NpIV was observed by UV−vis−NIR. An additional aliquot of NH2OH·HCl (1 mL) was L

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

solution (0.250 mL, 0.0297 M) in HCl (6 M) was added to a scintillation vial (20 mL). The solution was brought to a green residue by heating to a soft dryness. This residue was allowed to cool to room temperature before the addition of HTTA (0.0529 g, 0.2381 mmol) in EtOH (1 mL). Almost immediately, the solids dissolved, and the color changed from green to dark brown. The vial was covered with perforated parafilm allowing for slow evaporation of the solvent. Over the course of 48 h green/brown hexagonal plates suitable for X-ray crystallography formed. 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 8.3 (s, fwhm = 27.8 Hz, 1H), (C4H3S)C2O2CHCF3; 7.9 (t, fwhm = 35 Hz, 1H), (C4H3S)C2O2CHCF3; 7.6 (d, fwhm = 28.7 Hz, 1H), (C4H3S)C2O2CHCF3; 6.2 (d, fwhm = 30 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −74.0 ppm. UV−vis−NIR [cm−1]: 19 905.07 CT, 17 769.33 w, 16 547.48 sh, 14 531.48 sh, 14 085.44 m, 13 562.9 m, 13 251.91 m, 12 523.74 w, 11 955.2 w, 11 665.49 sh, 11 032.45 sh, 10 173.99 sh, 8570.55 w, 8185.23 sh, 6426.08 sh, 6426.08 sh, 6061.28 sh. IR [cm−1]: 1567.02 s, 1538.27 s, 1304.66 s, 1252.14 s, 1192.42 s, 1127.68 s, 1083.73 s, 1062.64 s, 933.55 s, 795.95 s, 731.70 s, 718.72 s, 684.12 s, 636.20 s, 582.52 s, 500.38, 464.97 s, 1510.70 m, 1403.52 m, 1352.22 m, 1230.90 m, 1083.88 m, 1016.93 m, 797.11 m, 3093.74 w, 1619.77 w, 1446.73 w, 1039.21 w, 603.83 w, 519.67 w. Plutonium(IV) Tetrakis(thionyltrifluoroacetonate); PuIV(TTA)4· (H2O). An aqueous plutonium solution in HCl (6 M) was prepared as described previously from recovered plutonium residues.72 In an open-front fume hood, an aliquot of the PuIV stock solution (1 mL, 0.047 M) in HCl (6 M) was evaporated to a soft dryness. The resulting red PuIV residue was dissolved in ethanol (EtOH; 1 mL) and HTTA (0.0418 g, 0.187 mmol). Evaporation of the resulting brown/ orange solution afforded an orange solid. The solid was dissolved in benzene (1 mL). After 3 d of slow evaporation orange blocks suitable for single-crystal X-ray diffraction formed. 1H NMR (400 MHz, benzene-d6 δ 7.16 ppm) δ 5.7 (s, fwhm = 7.9 Hz, 1H), (C4H3S)C2O2CHCF3; 6.5 (t, fwhm = 10.8 Hz, J = 4.2 Hz, 1H), (C4H3S)C2O2CHCF3; 6.5 (d, fwhm = 9.7 Hz, J = 4.9 Hz, 1H), (C4H3S)C2O2CHCF3; 8.0 (d, fwhm = 8.7 Hz, J = 3.8 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ −74.5 ppm. UV−vis−NIR [cm−1]: 20 712.93 CT, 14 448.99 sh, 15 777.10 sh, 9675.30 sh, 18 467.9 w, 8777.70 w. IR [cm−1]: 1566.59 s, 1537.52 s, 1404.21 s, 1304.36 s, 1251.86 s, 1192.56 s, 1131.34 s, 1063.17 s, 796.03 s, 730.08 s, 637.25 s, 583.58 s, 1509.37 m, 1353.13 m, 1231.16 m, 1084.07 m, 1039.72 m, 1016.78 m, 933.51 m, 862.29 m, 684.18 m, 500.88 m, 466.74 m, 1619.73 w, 772.0 w, 749.87 w. Neptunium and Plutonium TTA Extraction. A liquid−liquid biphasic extraction was performed using HTTA (0.5 M) as the extractant. The organic phase was prepared by adding HTTA (0.555 g) to o-xylenes-d10 (5 mL). The aqueous phase consisted of either Np (1 mg) or Pu (1 mg) in 1 mL of HCl (1 M). Prior to use, the +4 oxidation states were confirmed for Np and Pu via UV−vis−NIR spectroscopy. After oxidation state confirmation, 1 mL of the aqueous phase and 1 mL of the organic phase were added to a 50 mL centrifuge tube. The tube was capped and placed in a zip lock bag for secondary containment. The contents of the tube were agitated using a votex mixer (10 min). Subsequently, the flask was left undisturbed (2 min) to let the phases fully separate. The organic phase was pipetted off into a clean centrifuge tube, and then the organic phase was washed with an additional 1 mL of HCl (1 M). This mixture was vigorously agitated using a vortex mixer (10 min), the solution was left undisturbed for 2 min, and the aqueous phase was discarded. After the extraction was complete, the organic phases were brightly colored orange or green for Np and Pu, respectively. UV−vis−NIR spectra were collected on the organic phases; then they were loaded into a Teflon NMR-tube liner (capped with a Teflon plug). The liner was placed in an NMR tube for analysis by NMR spectroscopy. For Pu: 1H NMR (400 MHz, oxylenes-d10 δ 2.20 ppm) δ 6.0 (s, fwhm = 6 Hz, 1H), (C4H3S)C2O2CHCF3; 6.8 (t, fwhm = 11 Hz, J = 4 Hz, 1H), (C4H3S)C2O2CHCF3; 8.3 (d, fwhm = 8 Hz, J = 3 Hz, 1H), (C4H3S)C2O2CHCF3; 7.85 (d, fwhm = 9 Hz, J = 5 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ 74.4 ppm. For Np: H1 NMR (400 MHz, o-xylenes-d10 δ 2.20 ppm) δ 8.2 (s, fwhm = 7 Hz, 1H),

Figure 17. A flowchart of the NpIV reprocessing procedure. added, and the solution was heated at ∼80 °C in a hot block for posterity. We remind the reader that this step should be performed using secondary containment, as the solution can bubble vigorously after ∼10 to 15 min. After it cooled to room temperature, hydrofluoric acid (5 mL; 29 M) was added to this solution resulting in an immediate green precipitate. After 15 min, the suspension was centrifuged, and the yellow supernatant (clear) was removed leaving behind a green pellet. The pellet was washed with water (2×) and dissolved by adding H3BO3 (2 mL) to the mixture while agitating the pellet with a glass stir rod. After the mixture was heated (75 °C; 5 min), HCl (12 M, 1 mL) was added, and the mixture was heated for another 5 min. While the temperature was maintained near 75 °C, aliquots of H3BO3 (1 mL) and HCl (1 mL) were added sequentially with periodic stirring. When this procedure is performed on more than 50 mg of Np, the presumed NpF4·(H2O)x typically dissolves by this point. However, when conducting the procedure on larger scales, addition of alternating aliquots of H3BO3 (1 mL) and HCl (1 mL) combined with heating will be necessary. At times it can be convenient to remove the supernate by centrifugation and restart the H3BO3/HCl additions on the remaining solid. After complete dissolution of the green solid, fractions were combined, and the Np oxidation state was confirmed to be +4 by UV−vis−NIR spectroscopy.78,79 During this time an anion-exchange column was prepared by charging a BioRad column (20 mL) with AG MP-1 anion resin (17 mL, 50−100 mesh). The resin was conditioned with water (2 × 15 mL) and HCl (12 M; 3 × 20 mL). The NpIV solution was loaded onto the column. The initial small stationary green band rapidly changed color to faint blue. The column was washed with HCl (12 M; 4 × 20 mL). The NpIV was then eluted from the column with slightly acidic HCl (50 mL of H2O to five drops of HCl; 12 M). The elution was accomplished by adding 1 mL of dilute HCl at a time. The first addition turned the light blue band green, the column colorless, and the band began moving down the column. After 13 mL the band was near the end of the column, and the neptunium fraction was collected in the smallest possible volume (∼7 mL). To the green colored solution, HCl (12 M) was added, until the final HCl concentration was ∼6 M. An aliquot (100 μL) was added to a quartz cuvette charged with HCl (2 M, 2.5 mL), and the concentration of the sample determined by UV−vis−NIR spectroscopy, e960nm = 162 and e723nm = 127 L mol−1 cm−1.12 Neptunium(IV) Tetrakis(thionyltrifluoroacetonate); NpIV(TTA)4· (H2O). In an open-front fume hood, an aliquot of the NpIV stock M

DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (C4H3S)C2O2CHCF3; 6.5 (t, fwhm = 10 Hz, J = 3 Hz, 1H), (C4H3S)C2O2CHCF3; 8.5 (broad d, fwhm = 11 Hz, 1H), (C4H3S)C2O2CHCF3; 7.87 (d, fwhm = 8 Hz, J = 4 Hz, 1H), (C4H3S)C2O2CHCF3 ppm. 19F NMR δ 73.7 ppm. Subsequently, the contents were loaded into the solution-phase XAS holder (described above) for XAS analysis.

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(2) Braley, J. C.; Carter, J. C.; Sinkov, S. I.; Nash, K. L.; Lumetta, G. J. The Role of Carboxylic Acids in TALSQuEAK Separations. J. Coord. Chem. 2012, 65, 2862−2876. (3) Jianchen, W.; Jing, C. Separation of Plutonium and Neptunium from Uranium by N,N-Dimethyl-3-Oxa-Glutaramic Acid (DMOGA) as a Complexant in the PUREX Process. Hydrometallurgy 2015, 157, 44−49. (4) Raut, D. R.; Mohapatra, P. K. Extraction of Uranyl Ion Using 2Thenoyltrifluoro Acetone (HTTA) in Room Temperature Ionic Liquids. Sep. Sci. Technol. 2015, 50, 380−386. (5) Nash, K. L. The Chemistry of TALSPEAK: A Review of the Science. Solvent Extr. Ion Exch. 2015, 33, 1−55. (6) Veliscek-Carolan, J. Separation of Actinides from Spent Nuclear Fuel: A Review. J. Hazard. Mater. 2016, 318, 266−281. (7) Nash, K. L.; Choppin, G. R. Separations Chemistry for Actinide Elements: Recent Developments and Historical Perspective. Sep. Sci. Technol. 1997, 32, 255−274. (8) Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M.; Denecke, M. A.; Geckeis, H. The Role of the 5f Valence Orbitals of Early Actinides in Chemical Bonding. Nat. Commun. 2017, 8, 16053. (9) Karmazin, L.; Mazzanti, M.; Gateau, C.; Hill, C.; Pécaut, J. The Important Effect of Ligand Architecture on the Selectivity of Metal Ion Recognition in An(III)/Ln(III) Separation with N-Donor Extractants. Chem. Commun. 2002, 23, 2892−2893. (10) Mayer, K.; Wallenius, M.; Fanghanel, T. Nuclear Forensic Science  From Cradle to Maturity. J. Alloys Compd. 2007, 444−445, 50−56. (11) Antonio, M. R.; Demars, T. J.; Audras, M.; Ellis, R. J. Third Phase Inversion, Red Oil Formation, and Multinuclear Speciation of Tetravalent Cerium in the Tri-N-Butyl Phosphate−n-Dodecane Solvent Extraction System System. Sep. Sci. Technol. 2017, 1−14. (12) Clark, D. L.; Hecker, S. S.; Jarvinen, G. D.; Neu, M. P. The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, L., Edelstein, N. M., Guger, J. E., Eds.; Springer: Berlin, Germany, 2006. (13) Poe, W. L.; Joyce, A. W.; Martens, R. I. Np237 and Pu238 Separation at the Savannah River Plant. Ind. Eng. Chem. Process Des. Dev. 1964, 3, 314−322. (14) Kolarik, Z. J.; Horwitz, E. P. EXTRACTION OF NEPTUNIUM AND PLUTONIUM NITRATES WITH n-OCTYL(PHENYL)-N,NDIISOBUTYL-CARBAMOYLMETHYLPHOSPHINE OXIDE∗. Solvent Extr. Ion Exch. 1988, 6, 247−263. (15) Mincher, B. J. The Separation of Neptunium and Plutonium From Nitric Acid Using N-Octyl (Phenyl)-N, N Diisobutylcarbamoylmethylphosphine Oxide Extraction and Selective Stripping. Solvent Extr. Ion Exch. 1989, 7, 645−654. (16) Moore, F. L. Separation and Determination of Neptunium by Liquid-Liquid Extraction. Anal. Chem. 1957, 29, 941−944. (17) Choppin, G. R.; Saito, A. Reduction of Neptunium and Plutonium in Photolyzed TTA-Organic Solvent Systems. Radiochim. Acta 1984, 35, 149−154. (18) Efurd, D. W.; Roensch, F. R.; Drake, J.; Perrin, R. E. Production, Separation, and Purification of 236 Np and 236Pu. Radiochim. Acta 1991, 54, 159−161. (19) Ghish Mazumdar, A. S.; Sivaramakrishnan, C. K. A Study of the Nitrate and the Chloride Complexes of plutonium(VI) by Solvent Extraction Technique Using TTA as the Chelating Agent. J. Inorg. Nucl. Chem. 1965, 27, 2423−2427. (20) Connick, R. E.; McVey, W. H. The Aqueous Chemistry of Zirconium. J. Am. Chem. Soc. 1949, 71, 3182−3191. (21) Day, R. A., Jr.; Stoughton, R. W. Chemistry of Thorium in Aqueous Solutions. I. Some Organic and Inorganic Complexes. J. Am. Chem. Soc. 1950, 72, 5662−5666. (22) Zebroski, E. L.; Alter, H. W.; Heumann, F. K. Thorium Complexes with Chloride, Fluoride, Nitrate, Phosphate and Sulfate. J. Am. Chem. Soc. 1951, 73, 5646−5650. (23) Cuninghame, J. G.; Miles, G. L. The Separation Of Plutonium By Extraction With Thenoyltrifluoracetone (TTA). J. Inorg. Nucl. Chem. 1956, 3, 54−63.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03089. Additional results including UV−vis, X-ray diffraction, Xray absorption spectroscopy data (PDF) Accession Codes

CCDC 1572745−1572748, 1572754, 1572756−1572757, and 1572759−1572761 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 data_ [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]; LA-UR-17-25117. ORCID

Samantha K. Cary: 0000-0003-0398-7106 Maryline G. Ferrier: 0000-0003-0081-279X Jeffrey J. Rack: 0000-0001-6121-879X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully recognize the Heavy Element Chemistry Program at Los Alamos National Laboratory (LANL) by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) (S.A.K., V.M., B.L.S.) and the U.S. Department of Energy. LANL is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of U.S. DOE (Contract No. DE-AC52-06NA25396). Portions of this work were supported by postdoctoral fellowships from the Glenn T. Seaborg Institute (M.G.F., B.W.S.) and the LDRD office, named fellowship program; Marie Curie Distinguished Postdoctoral Fellowship (S.K.C.). J.J.R. acknowledges National Science Foundation (Grant No. CHE 1602240) for financial support. Use of the Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, was supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03089 Inorg. Chem. XXXX, XXX, XXX−XXX