Neodymium(III) Complexes of Dialkylphosphoric and

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Neodymium(III) Complexes of Dialkylphosphoric and Dialkylphosphonic Acids Relevant to Liquid−Liquid Extraction Systems Gregg J. Lumetta,*,† Sergey I. Sinkov,† Jeanette A. Krause,‡ and Lucas E. Sweet§ †

Nuclear Chemistry and Engineering Group and §Radiochemical Analysis Group, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡ The Richard C. Elder X-ray Crystallography Facility, Chemistry Department, University of Cincinnati, Cincinnati, Ohio 45221, United States S Supporting Information *

ABSTRACT: The complexes formed during the extraction of neodymium(III) into hydrophobic solvents containing acidic organophosphorus extractants were probed by single-crystal Xray diffractometry, visible spectrophotometry, and Fouriertransform infrared spectroscopy. The crystal structure of the compound Nd(DMP)3 (1, DMP = dimethyl phosphate) revealed a polymeric arrangement in which each Nd(III) center is surrounded by six DMP oxygen atoms in a pseudooctahedral environment. Adjacent Nd(III) ions are bridged by (MeO)2POO− anions, forming the polymeric network. The diffuse reflectance visible spectrum of 1 is nearly identical to that of the solid that is formed when an n-dodecane solution of di(2-ethylhexyl)phosphoric acid (HA) is saturated with Nd(III), indicating a similar coordination environment around the Nd center in the NdA3 solid. The visible spectrum of the HA solution fully loaded with Nd(III) is very similar to that of the NdA3 material, both displaying hypersensitive bands characteristic of an pseudo-octahedral coordination environment around Nd. These spectral characteristics persisted across a wide range of organic Nd concentrations, suggesting that the pseudo-octahedral coordination environment is maintained from dilute to saturated conditions.

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INTRODUCTION The extraction of trivalent f-block elements by lipophilic organophosphorus ligands is of great industrial importance. In particular, such systems can be used for the separation and purification of the lanthanide elements,1 which are increasing in strategic importance.2 Acidic organophosphorus extractants are also being aggressively investigated for separating americium and curium from irradiated nuclear fuel, so that these radiotoxic elements can be transmuted to shorter-lived or stable nuclides.3 Much of the recent development in this area has been directed at simplifying these separations either through developing more robust chemical systems (e.g., less dependent upon changes in pH)4 or through combining multiple process steps.5 This chemistry has been known since the 1950s, and a substantial body of literature has been accumulated on the subject since that time. However, despite this effort, uncertainties remain regarding the species formed in the extractant phase, especially with respect to the structural features of the complexes formed. On the basis of graphical analysis of the distribution ratios for the extraction of trivalent actinides and lanthanides by di(2ethylhexyl)phosphoric acid (HDEHP; Figure 1), Peppard et al. proposed that at low metal ion concentrations, the extracted © XXXX American Chemical Society

Figure 1. Chemical structures for the phosphorus-based acidic compounds used in this study.

species formed in the organic phase can be formulated as M(AHA)3, according to the reaction6 M3 +(aq) + 3(HA)2 (org) ⇌ M(AHA)3 (org) + 3H+(aq) (1) Received: November 6, 2015

A

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Figure 2. Possible structures for M(AHA)3 and M2A6 species; M = trivalent actinide or lanthanide, A = DEHP.

The complex is stabilized by hydrogen bonding between the water molecules and two of the uncoordinated oxygen atoms in the DHP− ligands. However, if this interpretation is correct, it would be surprising given earlier laser-induced fluorescence spectroscopic investigations of the analogous Eu(III)/ HDEHP11 and Eu(III)/Cyanex 27212 systems, which revealed complete dehydration of the Eu(III) ion upon extraction into the dialkylphosphoric acid solvent (although direct comparison of these results should be made with caution since different diluentsn-dodecane vs toluenewere used). Although reaction 1 is operable at low metal-to-HA concentrations, other species are formed as the metal loading in the organic phase increases. On the basis of small-angle neutron scattering (SANS) investigations, Jensen et al. proposed the formation of the dimeric species Nd2A6 when the HDEHP/Nd molar ratio is 8−9 in toluene,13 and the extraction mechanism is described by the equilibrium in reaction 2.

where (HA)2 refers to the dimerized form of HDEHP, which predominates in nonpolar diluents.7 This mechanism for extraction of trivalent f-elements at low metal loading is still widely accepted.3a However, the exact structure of the M(AHA)3 species remains poorly defined. It is generally envisioned that the “AHA” moiety consists of one DEHP− anion (A−) bridged to HDEHP through a hydrogen bond, in which case the AHA moiety essentially acts as a bidentate ligand (Figure 2a).6,8 Despite the widespread acceptance of the existence of the M(AHA)3 complexes, limited spectral evidence exists that supports the existence of a structure such as that depicted in Figure 2a. Recent applications of electrospray ionization mass spectrometry (ESI-MS) have supported the existence of species of stoichiometry M(AHA)3.8a,9 However, a number of other species were also observed, and these investigations are complicated by the fact that the samples must be diluted into a more polar solvent for the ESI-MS measurement which could lead to formation of species not present in the liquid−liquid extraction system. Also, verification of the M(AHA) 3 stoichiometry does not necessarily support the structure assigned in Figure 2a. For example, the species could be formulated as M(AH)3(A)3 with no hydrogen bonding between the protonated and deprotonated DEHP− ligands. Grimes et al. examined the Eu(III)/HDEHP system by time-resolved fluorescence spectroscopy (TRLFS).10 The absence of the 5 D0 → 7F0 transition in the fluorescence spectrum of Eu(III)/ HDEHP at low Eu(III) loading provided evidence for a highly symmetric coordination environment around the Eu(III) center, which is consistent with a structure such as that indicated in Figure 2a. The fluorescence spectra suggested the possible formation of bridged metal species at high metal loading in this system. Extended X-ray absorption fine structure spectra of solutions generated by the extraction of Nd(III), Eu(III), Yb(III), and Am(III) into n-dodecane solutions of di-n-hexylphosphoric acid (HDHP) were best fit by considering a pseudo-octahedral arrangement of six oxygen atoms around the metal center.8b Consideration of the secondary shell suggested that the Nd(III) and Yb(III) complexes contained six phosphorus atoms around the metal center (consistent to what would be expected from Figure 2a), whereas the Eu(III) and Am(III) complexes contained only three phosphorus atoms in the secondary sphere around the metal center. To explain the latter, a structure was proposed in which three DHP− anions are coordinated in a monodentate fashion to the Eu(III) or Am(III) cation, along with three coordinated water molecules.

2M3 +(aq) + 3(HA)2 (org) ⇌ M 2A 6(org) + 6H+(aq) (2)

Note that slope analysis of equilibrium distribution data cannot distinguish between extraction mechanisms 1 and 2 based on the dependence of the distribution ratio on the extractant or acid concentrations. The existence of the M2A6 dimeric species is also suggested by SANS investigations of the analogous La, Gd, and Yb systems14 and by TRLFS of the Eu + La system.10 It is known that increased metal loading of aliphatic phases containing HDEHP eventually leads to formation of a gelatinous precipitate that gathers at the interface between the aqueous and the organic phases (such precipitates are referred to as interfacial crud).9 Independently prepared materials of stoichiometry LnA3 (Ln = lanthanide) are generally believed to be polymeric.15 It can reasonably be hypothesized that as the concentration of metal ion in the organic phase increases relative to the total amount of HDEHP present, increasingly larger oligomeric species are formed, which eventually solidify into a polymeric material. We report here the crystal structure of Nd(DMP)3 (DMP = the deprotonated form of dimethylphosphoric acid HDMP; Figure 1), which can be viewed as an analog of the polymeric species formed by lanthanide ions and HDEHP. We also compare the diffuse reflectance electronic spectrum and the Fourier-transform infrared (FTIR) spectrum of this compound to those obtained both in solution and in the solid state for the Nd(III)−HDEHP complexes and to those obtained in solution for the closely related HEH[EHP]−Nd−n-dodecane system, B

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Figure 3. Structural representation of Nd(OOP(OCH3)2)3 showing the coordination environment of Nd (upper left), the layers formed in the bc plane (upper right), and the stacking of the layers through translation along the a axis (lower center). range 348−1001 nm at intervals of 0.36 nm. Ground samples (2−3 mg) were loaded into a 3 mm diameter round-bottom nuclear magnetic resonance tube (Wilmad LabGlass Company, Vineland, NJ) and measured against a reference sample of aluminum oxide powder in the same geometry. FTIR spectra were recorded using a Bruker Alpha spectrometer. Solid-phase spectra were obtained using a diamond attenuated total reflectance (ATR) cell. Solution spectra were recorded using a solution cell with 0.025 mm path length, equipped with NaCl windows. For each FTIR spectrum recorded, 23 scans were made at a resolution of 4 cm−1. Elemental analyses were performed by Columbia Analytical Services (Tucson, AZ). Synthesis of Tris(μ2-dimethylphosphato)neodymium(III) (1). NdCl3·6H2O (0.3365 g, 0.938 mmol) and dimethylphosphoric acid (0.3570 g, 2.832 mmol) were stirred together in 2 mL of CH3CN (Sigma−Aldrich) for 2.5 h. The solution was filtered through a 0.2 μm nylon membrane. A portion of the clarified solution was diluted 3-fold with CH3CN in preparation for measurement of the visible (vis) absorption spectrum. After the spectrum was recorded, the solution was allowed to evaporate over a period of approximately 3 months, resulting in crystal formation. One crystal was retrieved from the vial, washed with 2 × 0.5 mL of 2-propanol, and dried under vacuum. This crystal was subjected to single-crystal X-ray diffraction analysis as described below. A separate portion of the crystalline material was washed with CH3CN and dried under vacuum. This portion of compound 1 was used for examination by FTIR spectroscopy and diffuse reflectance vis spectroscopy. Determination of the Crystal Structure of 1. For X-ray examination and data collection, a plate-shaped crystal of approximate dimensions 0.02 × 0.02 × 0.005 mm was mounted in a loop with Paratone-N oil and transferred to the goniostat bathed in a cold

where HEH[EHP] is 2-ethylhexylphosphonic acid mono-2ethylhexyl ester (Figure 1). To our knowledge, this is the first example of a direct link between the structure of a Nd−HA complex definitively determined by single-crystal X-ray diffraction and solution-phase species present in HA extraction systems. As such, this work provides an important step toward the full understanding of these extraction systems.

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EXPERIMENTAL METHODS

Dimethylphosphoric acid was procured from Epsilon Chemie (BrestGuipavas, France). HDEHP and NdCl3·6H2O were obtained from Sigma-Aldrich (St. Louis, MO). HEH[EHP] was purchased from YickVic Chemicals & Pharmaceuticals, Ltd. (Hong Kong). HDEHP and HEH[EHP] were purified according to the method reported in the literature.16 n-Dodecane (99+%) was obtained from Alfa-Aesar (Ward Hill, MA). Spectrophotometric measurements of Nd-containing solutions were made using a 400-series charge-coupled-device (CCD) array spectrophotometer (Spectral Instruments, Inc., Tucson, AZ) in the scanning range 350−950 nm at intervals of 1.16 nm. The solutions were held in a narrow window quartz cell of 0.8 mL capacity with an optical path length of 10.00 mm. Spectra were taken as 10 replicate scans for low-millimolar solutions of Nd and as 25 replicate scans for solutions with submillimolar concentrations of the metal in order to improve the signal-to-noise ratio in the detection of weak spectral signatures. For spectrophotometric measurements of solid compounds of Nd with HDEHP and DMP, a single-lamp diffuse reflectance probe from SI Photonics, Inc. (Tucson, AZ) was coupled with an LS-1 tungsten halogen light source and a USB2000-VIS-NIR spectrophotometer (both from Ocean Optics, Dunedin, FL). Spectra were recorded in the C

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Inorganic Chemistry stream. Intensity data were collected at 150 K on a Bruker APEX II CCD detector at Beamline 11.3.1 at the Advanced Light Source (Lawrence Berkeley National Laboratory) using synchrotron radiation tuned to λ = 0.77490 Å. For data collection, frames were measured for a duration of 1 s at 0.3° intervals of ω with a maximum 2θ value of ∼60°. The data frames were collected using the program APEX2 and processed using the SAINT routine within APEX2. The data were corrected for absorption and beam corrections based on the multiscan technique as implemented in the SADABS v2008/1 semiempirical absorption and beam correction program (Bruker Analytical X-ray Instruments, Inc., Madison, WI). The structure was solved by a combination of direct methods in SHELXTL v6.14 (G.M. Sheldrick, University of Gö ttingen, Germany) and the difference Fourier technique and refined by full-matrix least-squares on F2. Nonhydrogen atoms were refined with anisotropic displacement parameters. The remaining methyl H atoms were calculated based on geometric criteria after the first was located directly from the difference map; a riding model was applied in subsequent refinement cycles. The H-atom isotropic displacement parameters were defined as 1.5Ueq of the adjacent atom. The refinement converged with crystallographic agreement factors of R1 = 3.35%, wR2 = 8.36% for 3582 reflections with I > 2σ(I) (R1 = 4.08%, wR2 = 8.71% for all data) and 205 variable parameters. Preparation of Neodymium/HDEHP Interfacial Crud Material. An n-dodecane solution of HDEHP (2 mol/L; 0.5 mL) was contacted with aqueous NdCl3 solution (0.31 mol/L; 1.0 mL) by vortex mixing for 1 min. The mixture was centrifuged, and the aqueous phase was withdrawn and discarded. The HDEHP solution was contacted with a fresh 1 mL portion of 0.31 mol/L NdCl3 in the same manner. After the second contact, the organic phase was a semisolid mass. The aqueous phase again was removed and discarded. The organic phase was suspended in a mixture of deionized water (2 mL) and 2-propanol (2 mL). The mixture was centrifuged and the liquid phase decanted. The solid material was washed with several portions of CH3OH and dried under vacuum. Hereafter, this solid will be referred to as the Nd/HDEHP crud material.17 Anal. Calculated for NdC48H102O12P3: 52.01, C; 9.28, H. Found: 53.95, C; 9.13, H. Spectroscopic Study of Nd-Loaded HDEHP Solutions. NdSaturated HDEHP Solution. An n-dodecane solution of HDEHP (0.2 mol/L; 1.5 mL) was contacted with aqueous NdCl3 solution (0.0654 mol/L; 1.5 mL) by vortex mixing for 2 min. The mixture was centrifuged. Visual observation of a solid film (interfacial crud) at the interface between the aqueous and the organic phases verified that the organic phase was fully saturated with Nd. To determine the Nd concentration in the fully loaded HDEHP solution, a 1.0 mL portion was contacted three successive times with fresh 1.0 mL portions of 6 mol/L HCl. The three portions of HCl solution were combined and diluted to a total volume of 10 mL using deionized water. The resulting solution was analyzed for Nd by vis spectrophotometry. The organic phase was also examined by vis spectrophotometry to verify complete removal of Nd from the HDEHP solution. Nonsaturated Nd HDEHP Solutions. An n-dodecane solution of HDEHP (0.2 mol/L; 1.5 mL) was contacted with aqueous NdCl3 solution (0.0327 mol/L; 1.5 mL) by vortex mixing for 2 min. The mixture was centrifuged. In this case, no interfacial crud was observed, indicating the HDEHP solution was not saturated with Nd. The organic phase was collected. For the vis spectrophotometric examinations, dilutions of this Nd/HDEHP solution were made using 0.2 mol/L HDEHP in n-dodecane as diluent.

3. The layers of cross-linked [Nd(OP(OCH3)2O)3]n stack by simple translation along the a axis. Compound 1 is isostructural with Sm(DMP)318 and Eu(DMP)3.19 The structure is also similar to that of La(DMP)320 and Nd(DEP)3 (where DEP− is the deprotonated form of diethylphosphoric acid).21 In the latter cases, the local environment around the lanthanide ion also consists of six (RO)2POO− groups bridging to adjacent lanthanide ions to form polymeric networks. Distortion of the octahedral environment around Nd(III) is evidenced by the cis O−Nd−O bond angles varying from 84.7(1)° to 95.4(1)° and trans O−Nd−O bond angles ranging from 171.0(1)° to 176.3(1)°. The Nd−O bond lengths range from 2.310(3) to 2.337(3) Å. The localized environments around the P atoms are distorted tetrahedrons of four O atoms: two from the methoxy groups and two from the Nd-bridging O atoms. The angles between the two bridging O atoms approach the theoretical tetrahedral angle and ranged from 116.9(2)° to 118.9(2)°. On the other hand, the MeO−P−OMe angles are compressed to the range of 100.7(2)−105.5(2)°. Table S1 in the Supporting Information presents the crystal data and refinement information, and Table S2 presents selected interatomic angles and distances. Figure S1 in the Supporting Information illustrates the asymmetric unit and atomic labeling scheme. Visible Spectrophotometry. Figure 4 displays the Nd hypersensitive band region of the diffuse reflectance spectrum

Figure 4. Diffuse reflectance spectra for (a) compound 1 and (b) interfacial crud formed during extraction of Nd(III) into HDEHP in ndodecane, (c) absorption spectrum for an organic phase obtained by extraction of Nd(III) into n-dodecane solution of HDEHP until the point of saturation (HDEHP/Nd ≈ 8.6), and (d) absorption spectrum for an organic phase obtained by six successive extractions of Nd(III) into n-dodecane solution of 1 mol/L HEH[EHP] (HEH[EHP]/Nd ≈ 14). Spectra are arbitrarily offset for clarity.

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RESULTS AND DISCUSSION Crystal Structure of 1. Single-crystal X-ray diffraction analysis revealed that 1 consists of a polymeric network of Nd(III) ions connected by bridging (MeO)2POO− anions. This structural motif is quite different than that suggested in Figure 2a, although the bridging (MeO)2POO− anions would be analogous to the bridging (RO)2POO− suggested in Figure 2b. The monomeric [Nd(OP(OCH3)2O)3]n units cross-link to form two-dimensional sheets in the bc plane as shown in Figure

of compound 1. The transitions in this region can be attributed to the 4I9/2 → 4G5/2, 2G7/2 transition, and this is known to be sensitive to the coordination environment around the Nd(III) center.22 The splitting of the 4I9/2 → 4G5/2, 2G7/2 transition into six bands when Nd(III) is complexed in an octahedral environment (e.g., [NdCl6]3−) has been reported.23 The diffuse reflectance spectrum of 1 indicates six bands in the hyperD

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Inorganic Chemistry sensitive region, consistent with the distorted octahedral structure indicated by the crystal structure. Figure 4 also presents the diffuse reflectance spectrum of the interfacial crud material formed when Nd(III) is extracted from aqueous NdCl3 solution into an n-dodecane solution of HDEHP until the point of saturation. The pattern shown in the hypersensitive band of the interfacial crud is nearly identical to that displayed by compound 1, indicating the coordination environment around Nd(III) in the interfacial crud is probably a pseudo-octahedral arrangement of six oxygen atoms, similar to that in 1. Furthermore, a very similar spectral pattern is displayed by ndodecane solutions of HDEHP or HEH[EHP] that have been saturated with Nd(III) (Figure 4c and 4d). The SANS investigations of Jensen et al. indicated that the aggregates formed near saturation in toluene media contained only two Nd(III) centers and corresponded to a stoichiometry of [Nd2(DEHP)6].13 Those authors suggested a structure of the type shown in Figure 2b for this species. Drawn as such, the coordination environment around the Nd(III) centers should approach octahedral. The visible spectrum reported for the highly Nd-loaded HDEHP solution in toluene displayed a spectral pattern similar to what we observed in n-dodecane (Figure 4c), with the peak at 570 nm having the strongest intensity.13 This supports the assignment of near-octahedral symmetry around the Nd(III) centers in the [Nd2(DEHP)6] species. The [Nd2(DEHP)6] complex indicated by this previous study represents an intermediate step on the way to the polymeric [Nd(DEHP)3]n material formed at full saturation of the HDEHP solution with Nd(III). We hypothesized that at low Nd loading of the HDEHP solvent (i.e., at high HDEHP/Nd ratio) the hypersensitive band of Nd(III) would resemble that seen at high loading because, if the structure shown in Figure 2a is correct, in both cases the coordination environment around Nd(III) is near octahedral with an approximate inversion center. Thus, at low loading a six-peak hypersensitive band should be observed, with the peak at 570 nm being predominant. On the other hand, at intermediate loading of the HDEHP solvent with Nd(III), other species with intermediate stoichiometries, such as Nd(AHA)2A24 or NdA2(AHA),25 would be formed. These species would presumably be of lower symmetry and should thus display different spectral patterns. Indeed, such spectral changes were suggested in the previously reported spectrum of the n-dodecane solution of HDEHP at relatively low Nd(III) concentration.13 To test this hypothesis, a portion of 0.5 mol/L HDEHP in ndodecane was contacted with an equal volume of aqueous solution consisting of 50 mmol/L NdCl3 and 1.0 mol/L NaCl. The resulting extractant phase contained 37.3 mmol/L Nd(III). Two dilutions of the extractant phase were made using 0.5 mol/L HDEHP in n-dodecane, giving organic phases with 10.1 and 2.0 mmol/L Nd(III). Figure 5 shows the absorption spectra of the variably Nd-loaded HDEHP solutions. Inconsistent with the hypothesis proposed above, the spectral pattern in the hypersensitive region for Nd remains virtually unchanged in going from 2.0 to 37.3 mmol/L. This is evidenced by the ratio of the absorbance of the band at 570 nm (A570) to that at 583 nm (A583) remaining constant at 1.06−1.07 for these solutions. This suggests that a pseudooctahedron of six oxygen atoms around the Nd(III) center persists across conditions with HDEHP/Nd molar ratios from 250 to 13.4 or less (e.g., Figure 4c). The persistence of high symmetry around the Nd center is consistent with the

Figure 5. Absorption spectra for 0.5 mol/L HDEHP/n-dodecane solution containing variable concentrations of Nd(III).

observations of Grimes et al. regarding the analogous Eu system,10 although this does not necessarily mean that the specific species present are the same across this range of concentrations. It should be noted that the absorbance A570/A583 ratio is less than 1 in some Nd/HDEHP solutions reported in the literature.13,26 We believe that this observation can be attributed to the presence of the mono-2-ethylhexylphosphoric acid (H2MEHP) impurity present in the HDEHP used. Even though it was reported that the HDEHP was purified in the previous studies, a small fraction of H2MEHP impurity will skew the band at 583 nm to higher intensity relative to that at 570 nm,27 especially as the Nd concentration decreases relative to the HDEHP ratio becoming low (which can lead to a situation in which Nd:H2MEHP can approach stoichiometric). We observed this to be the case with HDEHP solutions that have not been purified (see Figure S2). The Nd−HEH[EHP]−n-dodecane system was also examined by visible spectrophotometry. After contacting three successive times with fresh portions of aqueous 1 mol/L NdCl3 at an organic-to-aqueous phase ratio of 3, the 1 mol/L ndodecane solution of HEH[EHP] contained 48.2 mmol/L Nd. This solution, along with several dilutions into 1 mol/L HEH[EHP], was subjected to visible spectrophotometric analysis. In addition, a separate portion of 1 mol/L HEH[EHP] was contacted six successive times with fresh portions of aqueous 1 mol/L NdCl3 at an organic-to-aqueous phase ratio of 3 to give an organic-phase Nd concentration of 70.3 mmol/L. Figure 6 presents the hypersensitive region of the visible spectra from this series of Nd-containing 1 mol/L HEH[EHP] solutions. Similar to the Nd−HDEHP system (comparing Figures 5 and 6), the Nd−HEH[EHP] system displays little change in the spectral features of the Nd hypersensitive band over the wide HEH[EHP]-to-Nd range examined. FTIR Spectroscopy. The Nd(III) complexes of DMP and DEHP were examined by FTIR spectroscopy. Figure 7 presents the FTIR spectra for HDMP, 1, HDEHP, and the Nd/HDEHP crud in the P−O stretching region. The assignment of the individual bands is not entirely straightforward. Thomas and Chittenden assigned the characteristic infrared absorption frequencies for organophosphorus acids of the type E

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Figure 6. Vis absorption spectra for 1.0 mol/L HEH[EHP]/n-dodecane solution containing variable concentrations of Nd(III).

Figure 7. FTIR spectra of neat HDMP and compound 1 (left) and neat HDEHP and the Nd/HDEHP interfacial crud material (right).

Table 1. FTIR Band Assignments for Nd Complexes of HDMP and HDEHPa Shown in Figure 7

a

band assignment

HDMP

PO νa(PO2) νs(PO2) P−O−(C) P−O−(H) H−O−(P)

1215(sh), 1184

1031 993 1684

Nd(DMP)3 (1)

HDEHP

Nd/HDEHP crud

Nd/HDEHP solutionb

1183/1163 1095 1048/1032/1020

1201 1094 1033

1222 1173 1094 1032

1013 not assigned 1682

Spectra recorded using a diamond ATR cell; all values reported in wavenumbers (cm−1). bUnder high loading conditions (see text).

(RO)2POOH as follows: PO, 1210−1250 cm−1;28 P−O− (H), 1000−1031 cm−1; P−O−(C), 987−1042 or 1015−1060 cm−1 when R is a methyl group.29 Furthermore, it has been reported that the PO band often appears as a doublet in the infrared spectrum.30 On this basis the following assignments can be made for neat HDMP (Table 1): PO, 1215(sh)/1184 cm−1; P−O−(C), 1031 cm−1; P−O−(H), 993 cm−1. Although there are similarities in the FTIR spectrum of neat HDEHP, the band associated with the P−O−(H) group that appears at 993

cm−1 in HDMP is not obvious in the HDEHP spectrum. However, both HDMP and HDEHP display broad bands at ∼1680 cm−1 which are associated with the P−O−H group.29 The characteristic band assignments for the HDEHP spectrum are (Table 1) PO, 1222 cm−1 and P−O−(C), 1013 cm−1. Grimes et al. reported little change in the FTIR spectrum of HDEHP upon variable loading with La(III) by extraction from dilute HNO3 media.10 In contrast, we observed substantial changes in the FTIR spectrum upon deprotonation and F

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Inorganic Chemistry complexation to the Nd(III) center, indicating a significant change in the (RO)2POO− moiety. Specifically, there is no longer a phosphoryl (PO) group per se, but rather the ligand adopts an approximately C2v symmetry around the P center, that is, the two oxygen atoms not bound to alkyl groups become equivalent, bridging between adjacent Nd(III) ions as in the structure of 1. This results in the appearance of a sharp symmetric (O−P−O) stretching band, which is seen at 1094 cm−1 in 1 and at 1095 cm−1 in the Nd/HDEHP interfacial crud material. The positioning of the νs(PO2) band is consistent with that for H2PO4−, which is located at 1078 cm−1.31 In addition, the asymmetric (O−P−O) stretching band appears at 1173 cm−1 in 1 and as a split band at 1183 and 1163 cm−1 in the Nd/HDEHP interfacial crud material. These can be compared to the νas(PO2) band at 1151 cm−1 for H2PO4−.31 Concurrent with the appearance of the symmetric and asymmetric (O−P−O) bands, the broad ν(PO) bands disappear from the spectra of 1 and the crud material (Figure 7). In the FTIR spectrum of 1, the P−O−(C) band is not significantly shifted from that in neat HDMP, both occurring at ∼1030 cm−1. On the other hand, the band at 993 cm−1 attributed to P−O−(H) in HDMP is not seen in the spectrum of 1, as would be expected upon deprotonation of the ligand. Likewise, the broad band at 1684 cm−1 assigned to the P−O− H group of HDMP is not present in the FTIR spectrum of 1. The same is true for the Nd/HDEHP system. The HDEHP P− O−(C) band is shifted to higher energy upon complexation to Nd(III), and it splits into three bands. This splitting can be attributed to different rotational isomers of the 2-ethylhexyl group within the solid-state structure.30 Rotational isomers are not possible for the methyl groups in HDMP; therefore, the P− O−(C) band remains a single peak in the FTIR spectrum of 1. The infrared spectrum of the Nd/HDEHP crud can be compared to that for Nd(DEHP)3 reported in the literature.32 The Nd(DEHP)3 described in the literature was prepared by reaction of the sodium salt of HDEHP with Nd(NO3)3 in acetone. There are some substantial differences between the infrared spectra of the Nd/HDEHP crud and Nd(DEHP)3. The νa(PO2) bands are similar in the two Nd/DEHP complexes, with both displaying split peaks: at 1183 and 1163 cm−1 in the Nd/HDEHP crud material and at 1186 and 1167 cm−1 in Nd(DEHP)3. However, the νs(PO2) band in Nd(DEHP)3 is reported to split into two bands at 1102 and 1077 cm−1. Only a single νs(PO2) band at 1095 cm−1 is present in the spectrum of the Nd/HDEHP crud. The literature infrared spectrum of Nd(DEHP)3 is less resolved than the Nd/ HDEHP crud spectrum shown in Figure 7, but deconvolution reveals only the presence of a single P−O−(C) band at 1036 cm−1, which contrasts with the three bands reported here for the Nd/HDEHP crud. These results suggest that in the case of the Nd/HDEHP system the specific coordination environment around the Nd(III) center is sensitive to the manner in which the complex is formed. The complex formed upon loading an n-dodecane solution of HDEHP with Nd(III) until saturation was also examined by FTIR spectroscopy. To isolate the spectrum of the Nd/ HDEHP complex, the spectrum of the loaded solvent was adjusted by first subtracting the spectral contribution of the ndodecane solvent and then subtracting the spectral contribution of HDEHP. The resulting spectrum of the Nd/HDEHP complex in solution is presented in Figure 8, along with the solution spectrum of HDEHP (with the contribution of n-

Figure 8. FTIR spectra of (a) 0.2 mol/L HDEHP in n-dodecane and (b) Nd/HDEHP complex in the Nd-saturated HDEHP solution.

dodecane subtracted). The solution-phase FTIR spectrum of HDEHP was very similar to that for the spectrum of neat HDEHP (compare Figures 7 and 8). The FTIR spectrum for the Nd/HDEHP complex in the saturated HDEHP solution displays features that are consistent with those seen for the solid interfacial crud material. The νs(PO2) band is located at 1094 cm−1, consistent with what was observed in the solid phase. The P−O−(C) band appears as a single peak at 1033 cm−1. The fact that this band is a single peak supports the notion that its being split into three bands in the solid state is due to the existence of rotational conformers. In solution, the 2ethylhexyl group can freely rotate so that the splitting of this band is eliminated. The νa(PO2) stretch appears as a single band at 1201 cm−1 in the solution spectrum of the Nd/ HDEHP complex, in contrast with the two peaks observed in Nd/HDEHP crud material. Finally, the broad H−O−(P) band at 1684 cm−1 in the HDEHP solution spectrum is not observed in the solution spectrum of the Nd/HDEHP complex at high solvent loading. This too is consistent with the observations regarding the FTIR spectra of 1 and the Nd/HDEHP interfacial crud material. The Nd−HEH[EHP]−n-dodecane system was also examined by FTIR spectroscopy. The FTIR spectral data was manipulated in a manner similar to that described above for the HDEHP system. Specifically, the spectrum of the Nd-loaded HEH[EHP] solution was adjusted by (1) subtracting the spectral contribution of the n-dodecane solvent and (2) subtracting the spectral contribution of HEH[EHP]. The resulting spectrum of the Nd−HEH[EHP] complex in solution is presented in Figure S3, along with the solution spectrum of HEH[EHP] (with the contribution of n-dodecane subtracted). The PO band of HEH[EHP] appears at 1219 cm−1. This band shifts to 1164 cm−1 upon complexation to Nd in the highly loaded HEH[EHP] solvent. The P−O−(C) stretch at 1038 cm−1 in HEH[EHP] is split into three bands at 1064, 1042, and 1036 cm−1. This splitting is probably due to contributions from the various stereochemical arrangements around the chiral P atoms. Unlike HDEHP, HEH[EHP] displays a strong band at 986/982 cm−1, attributable to the P− O−(H) stretch. The intensity of this band decreases upon complexation to Nd and shifts slightly to ∼976 cm−1, indicating G

DOI: 10.1021/acs.inorgchem.5b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Advanced Light Laboratory. The Science, Office Department of 05CH11231.

disruption of the hydrogen bonding network in the HEH[EHP] dimer. The broad band centered at 1689 cm−1 for the HEH[EHP] dimer is not present in the Nd complex, but the broad feature centered at ∼1380 cm−1 would be consistent with breaking of the strong hydrogen bonding network in the dimer and replacing it with the single hydrogen bond in the monodeprotonated [A → H−A]− ion. This feature supports the notion that the Nd(AHA)3 species persists to high loading in the HEH[EHP] system. It should be noted that a similar broad underlying feature is seen in the Nd/HDEHP spectrum shown in Figure 8.

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CONCLUSION Spectrophotometric and FTIR spectral investigations of Nd(III) extracted into n-dodecane solutions of HDEHP or HEH[EHP] have been combined with X-ray crystallographic and diffuse reflectance spectroscopy of NdA3 compounds to provide insight into the structural features of f-block metal species present in liquid−liquid separation systems. The spectral features of the polymeric NdA3 solid phases correlate closely to the solution spectral features at near saturated conditions. Furthermore, these spectral features persist as the Nd concentration decreases in the HDEHP or HEH[EHP] media, suggesting the primary coordination environment around Nd remains a pseudo-octahedral arrangement of oxygen donors across a wide range of Nd concentrations in the extraction phases. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02524. Table of the crystallographic and refinement data for compound 1, table of selected bond lengths and bond angles for compound 1, illustration of the asymmetric unit and atomic labeling scheme for compound 1, absorption spectra of Nd-loaded HDEHP solutions illustrating the effects of the H2MEHP impurity, and FTIR spectrum for the Nd/HEH[EHP] complex in ndodecane solution (PDF) (CIF)

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Source (ALS), Lawrence Berkeley National ALS is supported by the Director, Office of of Basic Energy Sciences, of the U.S. Energy under Contract No. DE-AC02-

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the U.S. Department of Energy, Office of Nuclear Energy, through the Fuel Cycle Research and Development Program. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under contract DE-AC05-76RL01830. The authors thank Aaron Johnson of Washington State University for supplying the purified HDEHP and Heather Culley for her editorial review of the manuscript. Samples for crystallographic analysis at the synchrotron were submitted through the SCrALS (Service Crystallography at Advanced Light Source) program. Crystallographic data were collected at the Small-Crystal Crystallography Beamline 11.3.1 at the H

DOI: 10.1021/acs.inorgchem.5b02524 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02524 Inorg. Chem. XXXX, XXX, XXX−XXX