Crystallographic and Spectroscopic Characterization of Americium

Feb 6, 2018 - For over 30 years, separations chemists have sought the molecular structures of minor actinide-containing species relevant to nuclear re...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Crystallographic and Spectroscopic Characterization of Americium Complexes Containing the Bis[(phosphino)methyl]pyridine-1-oxide (NOPOPO) Ligand Platform Jordan F. Corbey,* Brian M. Rapko, Zheming Wang, Bruce K. McNamara, Robert G. Surbella, III, Kristi L. Pellegrini, and Jon M. Schwantes Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *

ABSTRACT: The crystal structures of americium species containing a common multifunctional phosphine oxide ligand, reported for its ability to extract f elements from acidic solutions, namely, 2,6-[Ph2P(O)CH2]2C5H3−NO, L, were finally determined after over three decades of separations studies involving these species and their surrogates. The molecular compounds Am(L)(NO3)3, Am 1:1, and [Am(L)2(NO3)][2(NO3)], Am 2:1, along with their neodymium and europium analogues, were synthesized and characterized using single-crystal X-ray crystallography, attenuated total reflectance Fourier transform infrared spectroscopy, and luminescence spectroscopy to provide a comprehensive comparison with new and known analogous complexes.

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INTRODUCTION

part due to their strong affinity for trivalent lanthanides and actinides,13 Figure 1. Structural characterization of several f element species possessing ligand L has been reported in the past for compounds containing Yb,13 Eu18 (Figure 1), Th,13 and Pu20 as well as post-transition metal Bi.21 However, never before has a crystal structure containing Am been reported with any of the ligand series listed above, merely hypothesized.22,23 Trivalent f element compounds containing L in crystalline form have been found to be air-sensitive likely due to the rapid desolvation of cocrystallized solvent molecules that are often trapped in the crystalline lattice.13,18 While L has proven to be a powerful extractant of trivalent f elements, including Am(III),18,20,22−24 from concentrated acid solutions, it has not been demonstrated as practical for large-scale separations, as it tends to only be soluble in polar, volatile, and so process unfriendly, solvents such as chloroform.24 Nevertheless, the NOPOPO ligand platform has led to a variety of derivatives containing the same 2,6-bis[(phosphino)methyl]pyridine N,P,P′-trioxide backbone,1,25−31 some including additional alkyl chains to enhance ligand solubility in nonpolar diluent.24 Recently, our team has successfully produced and characterized single crystals of americium-containing species featuring the NOPOPO ligand L, despite caveats such as desolvation of the crystals outside of their mother liquor and destruction of the crystal lattice due to α bombardment from the Am-243 nuclide. The data presented here provide a rare opportunity to directly compare trivalent

Although several nuclear reprocessing methods have been devised over the past few decades, highly efficient 4f and 5f element separation schemes still pose a significant challenge to researchers.1,2 So-called designer reprocessing routes have featured a collection of ligand platforms intended to sequester f elements from aqueous solutions and/or separate minor actinides, such as americium and curium, from the major actinides, namely, uranium and plutonium.1,3−6 Of these, organophosphorus extractants have proven to be some of the most useful and are implemented in solvent extraction processes such as TALSPEAK (Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes),5,7 Advanced TALSPEAK,8−10 PUREX (Plutonium and Uranium Reduction Extraction),11 and TRUEX (Trans Uranium Extraction).12 For over 30 years, separations chemists have sought the molecular structures of minor actinide-containing species relevant to nuclear reprocessing for comparison with their more easily accessible lanthanide surrogates in various systems. Some of the earlier ligand series analyzed in separation studies of f elements have been the bifunctional chelating ligands (carbamoylmethyl)phosphonates (CMP),13 (carbamoylmethyl)phosphine oxides (CMPO),13,14 used in the TRUEX process,12,15 and di(2-ethylhexyl)phosphoric acid (HDEHP), the extractant used in the TALSPEAK process.16 More recently, (phosphino)pyridine oxides (NOPO and NOPOPO)13,17 such as 2,6-bis[(diphenylphosphino)methyl]pyridine N,P,P′-trioxide, L, have attracted research interest, in © XXXX American Chemical Society

Received: December 14, 2017

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DOI: 10.1021/acs.inorgchem.7b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Known synthetic routes to produce trivalent f element compounds containing ligand L. The products shown are those that have been previously crystallographically characterized and reported in the literature.13,18,19 resulting solution was allowed to stir for 40 min. Upon slow evaporation of the solvent, colorless X-ray quality crystals of [Eu(L)2(NO3)][2(NO3)], Eu 2:1, were obtained after 2 d (52 mg after drying, 70% crystalline yield). IR: 3079w, 3057w, 2991w, 2957w, 2908m, 1610w, 1589w, 1573w, 1484w, 1467m, 1437m, 1390m, 1352s, 1337s, 1304s, 1212m, 1186w, 1159s, 1121s, 1104m, 1094m, 1071m, 1031m, 995w, 969w, 858m, 827m, 797w, 751m, 725m, 688m, 675m, 635m, 562w, 528w, 515w, 503m, 492m, 439w cm−1. Anal. Calcd for C62H54N5O15P4Eu: C, 53.77; H, 3.93; N, 5.06. Found: C, 53.64; H, 3.81; N, 4.99. Note that these colorless, transparent crystals become opaque within minutes once removed from their mother liquor. [Nd(L)2(NO3)][2(NO3)], Nd 2:1. Following the procedure above for [Eu(L)2(NO3)][2(NO3)], Eu 2:1, Nd(NO3)3·6(H2O) (31 mg, 71 μmol) was combined with 2,6-[Ph2P(O)CH2]2C5H3−NO, L, (78 mg, 149 μmol) to make a pale blue solution. Upon slow evaporation of the solvent, pale blue X-ray quality crystals of [Nd(L)2(NO3)][2(NO3)], Nd 2:1, were obtained after 2 d (82 mg after drying, 84% crystalline yield). IR: 3078w, 3055w, 3020w, 2990w, 2956w, 2908m, 1610m, 1589m, 1572m, 1484w, 1466m, 1437m, 1390m, 1352s, 1300s, 1223m, 1213m, 1185w, 1157s, 1121s, 1103m, 1092m, 1071m, 1031m, 995w, 968w, 859m, 827m, 797w, 766w, 751m, 725m, 688m, 674m, 635m, 562w, 528w, 515w, 503m, 492m, 438w cm−1. Anal. Calcd for C62H54N5O15P4Nd: C, 54.07; H, 3.95; N, 5.09. Found: C, 53.91; H, 3.97; N, 5.04. Crystals become opaque within minutes once removed from mother liquor. [Am(L)2(NO3)][2(NO3)], Am 2:1. In a radiological glovebox, 79 μL (0.8 mg of 243Am(NO3)3, 2 μmol) of a pale pink 243Am(NO3)3 stock solution (23 mM) was transferred to a 1 dram vial, which was then heated in a radiological fume hood to remove excess nitric acid nearly to dryness producing a yellow/pink residue. The 243Am(NO3)3 residue was dissolved in EtOH (1 mL) after it cooled, and a solution of 2,6[Ph2P(O)CH2]2C5H3−NO, L, (8.2 mg L, 15 μmol) in CHCl3 (1 mL) was added to the americium solution dropwise with stirring. The resulting yellow solution was allowed to stir 10 min. Upon slow evaporation of the solvent, yellow X-ray quality crystals of [Am(L)2(NO3)][2(NO3)], Am 2:1, formed within a week. IR: 3081w, 3058w, 2960w, 2909m, 1590w, 1573w, 1484m, 1466m, 1436s, 1388m, 1350s, 1337s, 1319s, 1299s, 1216m, 1187w, 1148s, 1122s, 1095s, 1071m, 1041w, 1027w, 996m, 967m, 859m, 825m, 797w, 746m, 725m, 689m, 675m, 633m, 562w, 527m, 503m, 493m cm−1. When kept in mother solution, crystals become opaque within a month. Nd(L)(NO3)3, Nd 1:1. Following the literature procedure for Yb(L)(NO3)3,13 Nd(NO3)3·6(H2O) (438 mg, 1 mmol) was dissolved in EtOH (25 mL), and a white slurry of 2,6-[Ph2P(O)CH2]2C5H3− NO, L, (523 mg, 1 mmol) in a 4:1 EtOH/CHCl3 mixture (10 mL) was added to form a pale blue precipitate. The suspension was left to stir overnight. Additional EtOH (100 mL), CHCl3 (20 mL), and water (10 mL) were added until most pale blue solids had dissolved. The remaining reaction suspension was passed through a syringe filter to

and previously reported tetravalent actinide analogues containing identical organophosphorous ligands.20 Other studies have shown that within phosphonate32 and phosphite33 ligand environments, Am and Pu can take on significantly different structural forms, highlighting the importance of crystallographic analysis toward these efforts. In addition, optical measurements obtained on the bulk samples reported here connect and support the conclusions from single-crystal analysis. To date, there are only 21 crystal structures containing americium reported in the Cambridge Structural Database (CSD 2016 Version 1.19), 34 and only one contains phosphorus.35 Of the 111 entries in the Inorganic Crystal Structural Database (ICSD 2016 Version 1.9.8)36 containing diffraction data for americium compounds, only two of these were also found to be phosphorus-containing,37,38 illustrating a clear lack of structural information available to help elucidate the interactions between minor actinides and phosphoruscontaining groups, which play important roles in nuclear reprocessing. With this study, we are able to directly compare the molecular structures of trivalent and tetravalent lanthanide and actinide analogues in their organophosphorous environments with the aim of supporting the recent impetus to transition common structural assumptions in extraction chemistry regarding f element surrogates for highly radioactive elements to the realm of established fact.32,39−42

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EXPERIMENTAL SECTION

Synthesis. Ln(NO3)3·6(H2O) (Ln = Nd or Eu), EtOH, and CHCl3 were purchased from Sigma-Aldrich and used as received. The ligand 2,6-[Ph2P(O)CH2]2C5H3−NO, L, was synthesized according to the literature procedure.13 Americium nitrate stock solutions were prepared from a stock of 243AmO2 at the Pacific Northwest National Laboratory using deionized water (18 MΩ cm) and Optima grade nitric acid purchased from Fisher Scientific. Caution! Americium-243 is an α-emitter (specif ic activity = 7.03 × 109 Bq/g) that presents both radioactivity and toxicity hazards. Manipulation and handling of these materials were performed only by qualif ied personnel in radiological facilities. Attenuated total reflectance (ATR) spectra were collected for dried crystals on a Bruker ALPHA-P ATR-FTIR spectrometer. CHN elemental analyses were performed by Galbraith Laboratories, Inc. [Eu(L)2(NO3)][2(NO3)], Eu 2:1.19 Modifying a previously reported procedure for analogous compounds,13,19 Eu(NO3)3·6(H2O) (24 mg, 54 μmol) was dissolved in EtOH (4 mL) to make a colorless solution. In a separate vial, 2,6-[Ph2P(O)CH2]2C5H3−NO, L, (61 mg, 117 μmol) was dissolved in CHCl3 (4 mL) to make a colorless solution that was added to the europium solution dropwise with stirring. The B

DOI: 10.1021/acs.inorgchem.7b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

Eu 2:1 1504.31 room temperature 0.56086 (Ag) P1̅ 13.531(1) 15.797(2) 19.790(2) 84.116(3) 77.263(3) 80.558(3) 4060.4(7) 2 1.230 0.535 0.0674 0.2003

Nd 2:1 1561.49 room temperature 0.71073 (Mo) P1̅ 13.533(2) 15.671(2) 19.644(3) 85.189(5) 77.361(5) 80.731(5) 4006.8(9) 2 1.294 0.794 0.0822 0.2234

[Nd(L)2(NO3)]-[2(NO3)]·4(EtOH) Am 2:1 1475.98 room temperature 0.56086 (Ag) P1̅ 13.720(4) 17.293(5) 17.989(5) 67.013(6) 81.140(6) 80.468(7) 3856(2) 2 1.271 1.382 0.1253 0.2906

[Am(L)2(NO3)]-[2(NO3)] N/A 1438.99 room temperature 0.71073 (Mo) P21 13.8222(6) 16.6815(8) 15.1765(7) 90 113.762(2) 90 3202.7(3) 2 1.492 1.154 0.0664 0.1432

[Eu(L)2(H2O)2]-[3(NO3)]·H2O N/A 1487.37 room temperature 0.56086 (Ag) C2/c 17.397(1) 20.176(1) 21.380(1) 90 95.506(2) 90 7470.2(8) 4 1.323 0.454 0.0344 0.1013

[Nd(L)2(H2O)2]-[3(NO3)]·OC4H10

a

An analogue of this compound containing cocrystallized waters and ethanol molecules in place of chloroform has been previously reported.19 bR1 = Σ||Fo| − |Fc||/Σ|Fo|. cwR2 = [Σ[w(Fo2 − Fc2)2]/ Σ[w(Fo2)2]]1/2.

abbreviation formula weight temperature source λ (Å) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcald (mg/m3) μ (mm−1) R1b [I > 2σ(I)] wR2c

[Eu(L)2(NO3)]-[2(NO3)]·CHCl3a

Table 1. X-ray Data Collection and Refinement Parameters for [M(L)2(NO3)][2(NO3)], M 2:1, (where M = Eu, Nd, or Am and L = 2,6-[Ph2P(O)CH2]2C5H3−NO) and [M(L)2(H2O)2][3(NO3)] (where M = Eu or Nd and L = 2,6-[Ph2P(O)CH2]2C5H3−NO) Obtained at Room Temperature

Inorganic Chemistry Article

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DOI: 10.1021/acs.inorgchem.7b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. X-ray Data Collection and Refinement Parameters for M(L)(NO3)3, M 1:1, (where M = Nd or Am and L = 2,6[Ph2P(O)CH2]2C5H3−NO) and [M(L)2(NO3)][2(NO3)], M 2:1, (where M = Eu, Nd or Am and L = 2,6-[Ph2P(O)CH2]2C5H3− NO) Obtained at 80 K Nd(L)(NO3)3 abbreviation formula weight temperature

Nd 1:1 853.74 room temperature source λ (Å) 0.71073 (Mo) space group Cc a (Å) 12.1971(6) b (Å) 19.1392(9) c (Å) 14.8031(8) α (deg) 90 β (deg) 91.7723(18) γ (deg) 90 V (Å3) 3454.0(3) Z 4 ρcald (mg/m3) 1.642 μ (mm−1) 1.663 R1b [I > 2σ(I)] 0.0493 wR2d 0.0762

Am(L)(NO3)3 Am 1:1 952.50 room temperature 0.56086 (Ag) Cc 12.216(5) 19.158(6) 14.838(5) 90 91.680(11) 90 3471(2) 4 1.823 2.987 0.0485 0.1112

[Eu(L)2(NO3)]- [2(NO3)]·4(CHCl3)a

[Nd(L)2(NO3)]- [2(NO3)]·4(CHCl3)

[Am(L)2(NO3)][2(NO3)]

Eu 2:1 1862.41 83(2) K

Nd 2:1 1854.69 80(2) K

Am 2:1 1475.98 84(2) K

0.56086 (Ag) P1̅ 13.2172(7) 15.2557(9) 19.8448(12) 83.232(2) 77.334(2) 82.073(2) 3850.9(4) 2 1.606 0.725 0.0874 0.2352

0.56086 (Ag) P1̅ 13.2192(6) 15.2343(8) 19.8616(10) 83.252(2) 77.417(2) 82.345(2) 3852.7(3) 2 1.599 0.650 0.0832 0.2199

0.56086 (Ag) P1̅ 13.577(3) 17.631(3) 17.658(3) 62.010(4) 77.074(5) 80.082(5) 3626.7(12) 2 1.352 1.470 0.0975c 0.2411c

a

An analogue of this compound containing cocrystallized waters and ethanol molecules in place of chloroform has been previously reported.19 bR1 = ∑||Fo| − |Fc||/∑|Fo|. cBecause of low crystal quality, these data were used only to confirm connectivity. A satisfactory model could not be achieved for the outer-sphere nitrate counteranions, and they were not included. dwR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. produce a pale blue solution. Slow evaporation over three weeks produced pale blue X-ray quality crystals of Nd(L)(NO3)3, Nd 1:1, and [Nd(L)2(NO3)][Nd(NO3)5]; in the latter case, only connectivity structural data could be obtained (see Figure S4 in the Supporting Information). Subsequent attempts to synthesize pure crystalline batches of Nd 1:1 using 0.9 or 0.5 equiv of L to 1 equiv of Nd(NO3)3· 6(H2O), and diethyl ether diffusion to promote crystal formation, resulted only in isolation of [Nd(L)2(H2O)2][3(NO3)] as determined by X-ray crystallography. Am(L)(NO3)3, Am 1:1. In a radiological glovebox, 90 μL (5.2 mg 243 Am(NO3)3, 12 μmol) of a pale pink 243Am(NO3)3 stock solution (135 mM) were transferred to a 1 dram vial that was then heated to remove excess nitric acid and reduce the solution volume by half. After it cooled, EtOH (1 mL) was added, and a solution of 2,6[Ph2P(O)CH2]2C5H3−NO, L, (8.4 mg L, 15 μmol) in CHCl3 (1 mL) was added to the americium solution dropwise with stirring. The resulting yellow solution was allowed to stir 2 min. Upon slow evaporation of the solvent, yellow X-ray quality crystals of Am(L)(NO3)3, Am 1:1, formed within 4 d. [Eu(L)2(H2O)2][3(NO3)]. In an attempt to synthesize Eu(L)(NO3)3 analogously to the monoligated Nd 1:1 species above, Eu(NO3)3· 6(H2O) (446 mg, 1 mmol) was dissolved in EtOH (40 mL), and a solution of 2,6-[Ph2P(O)CH2]2C5H3−NO, L, (523 mg, 1 mmol) in CHCl3 (10 mL) was added dropwise to form a white precipitate. Additional EtOH (100 mL) and water (10 mL) were added to the reaction vessel to dissolve most of the solids present. The remaining reaction suspension was passed through a syringe filter to produce a colorless solution. Slow evaporation over 6 d produced colorless crystals of [Eu(L)2(H2O)2][3(NO3)], identified by X-ray crystallography. In a previous study, a crystal structure of the chloride analogue of this species was obtained by reacting 2 equiv of L with 1 equiv of EuCl3·6(H2O) using a 2:1 CHCl3/EtOH mix.18 Subsequent attempts in the present study to form crystals of Eu(L)(NO3)3 using 0.9 equiv of L to 1 equiv of Eu(NO3)3·6(H2O) and diethyl ether diffusion to promote crystal formation resulted only in isolation of Eu 2:1 above. Luminescence Measurements. The luminescence spectroscopic measurements for Eu 2:1 and Am 2:1 were performed by excitation of the samples, in capped and sealed 2 mm × 4 mm × 25 mm fused quartz cuvettes, at 503 and 250 nm for Am 2:1 and Eu 2:1, respectively, using a Spectra-Physics Nd:YAG laser pumped Laser-

technik-GWU MOPO laser. The emitted light was collected at 85° to the excitation beam, dispersed through an Acton SpectroPro 300i double monochromator spectrograph, and detected with a thermoelectrically cooled Princeton Instruments PIMAX intensified CCD camera that was triggered by the delayed output of the laser pulse and controlled by the WinSpec data acquisition software. The photoluminescence decay curves were constructed by plotting the spectral intensity of a series of time-delayed luminescence spectra as a function of the corresponding delay time. The emission spectra and decay data were analyzed using commercial software, IGOR, from Wavematrix, Inc. X-ray Crystallography. As previously reported, chemical systems containing L or similar ligands tend to produce crystals that retain solvent in their lattices and readily desolvate in ambient air and other environments.13 To avoid destruction of the crystal lattice prior to or during data collection, the wet crystals were sealed in glass capillaries containing some of their mother liquor. In cases with americium, an additional barrier comprised of heat-shrink tubing was fixed over the capillary to ensure a completely nondispersible configuration (see Figure S1 in the Supporting Information). To obtain low-temperature data, the entire sample configuration was placed within a nitrogen cold stream and maintained at 80 K. Diffraction data were collected using a Bruker D8 Venture diffractometer and either AgKα or MoKα radiation. For some structures, such as those containing Am, the AgKα X-ray source produced cleaner data. In other cases, the MoKα source was preferred. Unit cell determination was performed using the APEX343 program package. Data integration was performed using SAINT,44 and SADABS45 was used to determine the absorption correction. Subsequent data reduction, structure solution, and refinement were performed using the SHELXL46 program. Low-temperature data were not collected in all cases, and consistent temperatures were needed to make structural comparisons. In those cases for which room-temperature and low-temperature data could be collected for the same compound, slight differences in structure are noted. For example, additional chloroform molecules of solvation can be located in data collected for Eu 2:1 at low temperature versus the room-temperature data. We also note that the unit cells determined for Am 2:1 using room-temperature or low-temperature data are more different than expected. This could be due to a labile inner-sphere nitrate ligand that was disordered between bidentate and monodentate D

DOI: 10.1021/acs.inorgchem.7b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (above) Crystal structure of the cation in [Am(L)2(NO3)][2(NO3)], Am 2:1, where L = 2,6-[Ph2P(O)CH2]2C5H3−NO. (below) Perspective showing orientation of nitrate ligands in neighboring molecular units of Am 2:1. Black, red, blue, orange, and green atoms are carbon, oxygen, nitrogen, phosphorus, and americium, respectively. Hydrogen atoms were omitted for clarity.

Table 3. Comparison of Select Bond Lengths (Å) from Room-Temperature Data for Known Bis-Ligated Species Containing L, Where the Metal Center Is Either Trivalent (M 2:1 where M = Yb, Eu, Nd, or Am) or Tetravalent ([M(L)2(NO3)2][2(NO3)], M(IV) 2:1, where M = Pu or Th)a ionic radius (Å, coord)

M−O(N)L

M−O(P)

O−NL

OP

Yb 2:1·2(H2O), 2.5(MeCN)

compound 13

ref

0.985 (VIII)

Eu 2:1·2(H2O), 0.5(EtOH) Eu 2:1·CHCl3

19 this work

1.066 (VIII) 1.066 (VIII)

Nd 2:1·4(EtOH)

this work

1.109 (VIII)

Am 2:1

this work

1.108 ± 0.00433 (VIII)

Pu(IV) 2:1·1.5(H2O), 0.5(MeOH)

20

1.03349 (X)

Th(IV) 2:1·2(H2O)

13

1.13 (X)

2.372(5) 2.379(4) 2.443(6)b 2.427(4) 2.458(4) 2.463(4) 2.482(4) 2.506(10) 2.506(9) 2.338(4) 2.346(4) 2.391(1) 2.400(1)

2.222(5), 2.269(5) 2.232(6), 2.259(6) 2.330(5)b 2.333(4), 2.345(4) 2.330(4), 2.333(4) 2.374(4), 2.377(5) 2.368(4), 2.356(4) 2.387(10), 2.344(11) 2.380(11), 2.392(12) 2.310(4), 2.357(4) 2.339(4), 2.382(4) 2.394(1), 2.398(1) 2.386(1), 2.431(1)

1.343(8) 1.353(8) 1.335(7)b 1.328(6) 1.328(6) 1.335(7) 1.323(6) 1.317(14) 1.346(13) 1.334(6) 1.343(6) 1.297(1) 1.296(1)

1.488(6), 1.506(5) 1.484(6), 1.494(5) 1.494(5)b 1.504(4), 1.481(5) 1.497(4), 1.491(4) 1.500(5), 1.490(5) 1.499(5), 1.500(5) 1.468(12), 1.500(12) 1.456(12), 1.486(12) 1.502(4), 1.499(4) 1.504(4), 1.490(4) 1.491(1), 1.509(1) 1.500(1), 1.484(1)

a

NL refers to nitrogen atoms in ligand L. Ionic radii48 are listed with respect to the coordination numbers indicated as Roman numerals. bReported as average value.

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DOI: 10.1021/acs.inorgchem.7b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry binding modes, which was modeled in the low-temperature data set and not observed for any other structure reported here (see the Supporting Information for additional discussion). This difference in unit cell values could also be due to potential cocrystallized solvent, which could not be identified in either data set. Crystallographic details for all fully structurally characterized compounds can be found in Tables 1 and 2 in the Results and Discussion section.

Attenuated total reflectance Fourier transform infrared (ATR) spectroscopy was performed on dried crystals of Eu 2:1, Nd 2:1, and Am 2:1 as well as the ligand starting material L. These spectra are included in the Supporting Information (Figure S6). L displays peaks at 1232 and 1186 cm−1, which are consistent with bands previously assigned to νNO and νPO, respectively.13 As observed for analogous compounds,13,19 upon complexation, these characteristic stretching frequencies shift to lower energy and are observed at 1212 and 1159 cm−1 (Eu 2:1), 1213 and 1157 cm−1 (Nd 2:1), and 1216 and 1148 cm−1 (Am 2:1), respectively. M 1:1 Species. In our hands, M(L)(NO3)3, M 1:1, for M = Eu and Nd could not be consistently crystallized even when 0.5−1 equiv of L was used with respect to the f element precursor; the supernatants were not characterized for these reactions. The more consistent products that crystallized under these conditions were bis-ligated species such as [M(L)2(H2O)2][3(NO3)] and [M(L)2(NO3)][2(NO3)], M 2:1. This tendency to favor 2:1 complexes has been observed with similar NOPOPO ligands in the case of Nd,29 and we are currently pursuing computational studies that may provide an explanation as to why this occurs, at least in the crystalline state. Nevertheless, monoligated complexes were isolated and crystallographically characterized in the cases of Nd(L)(NO3)3, Nd 1:1, and Am(L)(NO3)3, Am 1:1, for the first time, Figure 3,

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RESULTS AND DISCUSSION M 2:1 Species. Although [M(L)2(NO3)][2(NO3)], M 2:1, have likely been synthesized for M = Am in the past,19,22 single crystals have never been isolated and characterized by X-ray diffraction until now. Similar compounds containing 2,6[Ph2P(O)CH2]2C5H3−NO, L, or analogous ligands with para alkyl groups,29 trifluoromethyl groups,26 tolyl groups,28 benzyl groups,28 ethyl groups,28 cyclohexyl groups,31 and more recently diethylcarbamoyl groups1,30 have been structurally characterized when the metal center is a lanthanide. Here, we were able to consistently produce pure crystalline batches of M 2:1, for M = Eu, Eu 2:1; Nd, Nd 2:1; and Am, Am 2:1, (Figure 2) by reacting 2 or more equiv of L with 1 equiv of the f element precursor in a 1:1 CHCl3/EtOH solution, as confirmed by CHN elemental analysis for all except the Amcontaining species. Select metrical parameters for known M 2:1 species are listed in Table 3. For the structures reported here and analyzed at room temperature, significant thermal motion was present that prevented anisotropic refinement of all atoms in the structure, although heteroatoms could be anisotropically refined. It was desirable to compare data collected at the same temperature for all cases in Table 3, since atomic displacement factors are known to change with respect to temperature.47 Experimental details and metrical parameters obtained from data collected at 80 K for Eu 2:1, Nd 2:1, and Am 2:1 can be found in the Supporting Information. For the bis-ligated trivalent lanthanide species Yb 2:1, Eu 2:1, and Nd 2:1, listed in Table 3, the average M−O(N)L distances of 2.376,13 2.443,19 and 2.473 Å, respectively, increase as expected with respect to the increase in ionic radii. The same is true for the average M−O(P) distances of 2.246 (Yb),13 2.334 (Eu),19 and 2.369 Å (Nd), respectively. When compared with the lanthanides, Am 2:1 displays very similar M−O(N)L and M−O(P) distances, taking into account the difference in ionic radii for the smaller elements, as has been previously observed for Am−S bonds35 and other Am−O(P) interactions.32 The 8-coordinate ionic radius of Am(III) (1.108 Å)33 falls between the 10-coordinate radii of Pu(IV) (1.033 Å)49 and Th(IV) (1.13 Å).48 For the tetravalent actinide species, Pu(IV) 2:1 has an average Pu−O(N)L distance of 2.342 Å, slightly shorter than that of the smallest metal listed in Table 3, Yb,13 as well as Am 2:1. This is likely due to the greater charge-to-radius ratio of Pu(IV) (3.87) versus Yb(III) (3.05) and Am(III) (2.71) and the stronger affinity of the tetravalent metal center for the partial negative charge of O(N)L. As previously noted,20 the average Pu−O(N)L and Pu−O(P) bonds in Pu(IV) 2:1 are significantly shorter than those in Th(IV) 2:1, but not unexpectedly, once the difference in ionic radii is taken into account. While the average OP distances for all f element species in Table 3 are equivalent within error, a modest trend can be observed in the average O−NL distances, as they tend to increase with decreasing ionic radii: 1.297 (Th(IV) 2:1), 1.329 (Nd 2:1), 1.332 (Am 2:1), 1.330 (Eu 2:1), 1.339 (Pu(IV) 2:1), 1.348 Å (Yb 2:1).

Figure 3. Thermal ellipsoid plot of Am(L)(NO3)3, Am 1:1, drawn at the 30% probability level, where L = 2,6-[Ph2P(O)CH2]2C5H3−NO. Black, red, blue, orange, and green atoms are carbon, oxygen, nitrogen, phosphorus, and americium, respectively. Hydrogen atoms were omitted for clarity. The Nd analogue Nd(L)(NO3)3, Nd 1:1, is isomorphous.

Table 4. The reaction to form Am 1:1 was successful on the first attempt and was not repeated in this study, so consistency of formation cannot be commented on here. Although M 1:1 have never been previously reported for M = Eu, Nd, or Am, analogous 1:1 compounds containing NOPOPO ligands similar to L that include trifluoromethyl groups,26 diethylcarbamoyl groups,1,30 ethyl groups,28 cyclohexyl groups,31 and benzyl groups28 have been structurally characterized for Pr, Nd, Eu, Dy, Er, and Yb species, in respective instances. As mentioned above, only one attempt was made to synthesize Am 1:1, and, fortuitously, only single crystals of the target complex were isolated and characterized by X-ray crystallography from this batch. However, it would have been very difficult to ascertain the purity of the crystalline batch as a whole, and this was not pursued. The initial attempt to F

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Table 4. Comparison of Select Bond Lengths (Å) from Room-Temperature Data for Known Monoligated Species Containing L, M 1:1, Where M = Yb,13 Nd, or Ama

a

compound

ref

ionic radius (Å, coord)

M−O(N)L

M−O(P)

O−NL

OP

Yb 1:1·(MeOH) Nd 1:1 Am 1:1

13 this work this work

1.042 (IX) 1.163 (IX) 1.162 ± 0.00233 (IX)

2.260(4) 2.419(5) 2.417(9)

2.235(4), 2.252(5) 2.373(5), 2.362(6) 2.379(10), 2.385(10)

1.333(7) 1.328(7) 1.334(14)

1.500(5), 1.509(4) 1.502(5), 1.499(5) 1.501(10), 1.499(10)

NL refers to the nitrogen atom in ligand L. Ionic radii48 are listed with respect to the coordination numbers indicated as Roman numerals.

dominate the metal−oxygen bonds in these complexes. Indeed, their M−O(N)L bonds lengths are the same within error at 2.419(5) (Nd 2:1) and 2.417(9) Å (Am 2:1). Their average M−O(P) distances display the same similarities when compared (2.368 and 2.382 Å for Nd 2:1 and Am 2:1, respectively). Interestingly, all the trivalent f element M 1:1 and M 2:1 complexes display longer M−O(N)L distances than M− O(P), respectively, which could be due to the zwitterionic nature of the O−NL bond in which the partial positive charge on the nitrogen competes with the f element cation for oxygen interaction. The analogous M−O(N)L and M−O(P) distances in the tetravalent species Pu(IV) 2:1 and Th(IV) 2:1 are much more similar to each other, which can be seen in Table 3 and attributed to the greater positive charge of the metal center in these cases. Despite the significant difference in ionic radii between Yb, Nd, and Am and the fact that Am is an actinide, all three M 1:1 species show identical O−NL and OP distances within error, as can be seen in Table 4. Unlike the bis-ligated species M 2:1 in this study, which display nitrate ligands oriented toward one another, each nitrate ligand in the monoligated complexes M 1:1, when M = Nd or Am, points more toward the hydrogens of the nearest aromatic rings in the neighboring molecular units. The shortest O···H distances for each nitrate ligand in Am 1:1 are 2.501 (O6 to H2A), 2.774 (O9 to H28A), and 2.479 Å (O12 to H22A), short enough to suggest hydrogen bonding-type interactions. Since the nitrate ligands are oriented more toward each other in the M 2:1 cases, it follows that the polar portions of the complexes are better encapsulated by the hydrophobic phenyl rings of L, which could promote more efficient separation of the f element complex into an organic phase. Indeed, previously reported extraction studies of Am(III) from nitric acid solutions using L as the extractant suggest two L ligands bind the f element to form the extraction complex.22,24 An analogous study analyzing extraction of Am(III) from hydrochloric acid solutions predicts three L ligands bind the f element to form the extraction complex,23 although none of the solid-state structural data for Am or lanthanide analogues with L support formation of a tris-ligated solution-state species. Luminescence. Luminescence spectra and lifetimes of both Eu 2:1 and Am 2:1 were measured for dried crystals and crystals contained in their mother solution, respectively, by time-resolved luminescence with laser excitations at 250 nm (Eu 2:1) and 503 nm (Am 2:1), respectively. Both compounds showed strong luminescence with emission spectral maxima located at 616.6 nm for Eu 2:1 and 690.5 nm for Am 2:1 (Figure 5A,B). These spectral positions are similar to those observed in other Eu(III) and Am(III) complexes.51,52 The profiles of the emission spectra recorded at different delay times (data not shown) remained the same for both Eu 2:1 and Am 2:1, suggesting a single Eu(III)/Am(III) coordination environment in these compounds. Consistent with these observations, the luminescence decays of both Eu 2:1 and Am 2:1 showed single exponential decays (Figure S5A,B in the Supporting

synthesize Nd 1:1 was successful in that crystals of the target complex could be isolated and characterized by X-ray crystallography. However, as mentioned in the Experimental section, crystals of another species, namely, [Nd(L)2(NO3)][Nd(NO3)5], were also isolated from the same crystalline batch and crystallographically identified (see the Supporting Information), having very similar color and crystal shape as Nd 1:1. Complexes containing f element nitrate counteranions are certainly not uncommon, and a plutonium NOPO compound, containing plutonium hexanitrate counterions, [Pu{2-[Ph2P(O)CH2]C5H4NO}2(NO3)3][Pu(NO3)6]0.5, has been structurally characterized.50 Subsequent attempts to synthesize pure crystalline samples of Nd 1:1 using 0.9 or 0.5 equiv of L resulted only in isolation of [Nd(L)2(H2O)2][3(NO3)], as determined by X-ray crystallography, Figure 4.

Figure 4. Thermal ellipsoid plot of the cation in [Nd(L)2(H2O)2][3(NO3)] drawn at the 30% probability level, where L = 2,6[Ph2P(O)CH2]2C5H3−NO. Black, red, blue, orange, and magenta atoms are carbon, oxygen, nitrogen, phosphorus, and neodymium, respectively. Hydrogen atoms and cocrystallized solvent molecules were omitted for clarity.

Attempts to synthesize and crystallize Eu(L)(N3O)3, Eu 1:1, resulted only in isolation of [Eu(L)2(H2O)2][3(NO3)] or Eu 2:1, as determined by X-ray crystallography. Conversely, hydrated species were not observed crystallographically or spectroscopically during synthesis of the M 2:1 species, which is corroborated by the luminescence data discussed below and further supports the consistent formation of pure crystalline batches of M 2:1 in this study. Nd 1:1 and Am 1:1 are isomorphous with one another and crystallize in a different space group (Cc) than their previously reported Yb analogue (P21/n), which contains cocrystallized methanol molecules of solvation.13 Since the 9-coordinate ionic radii of Nd(III) (1.163 Å)48 and Am(III) (1.162 ± 0.002 Å)33 are identical within error, we expect their M−O(N)L and M− O(P) distances to be very comparable if ionic interactions G

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Figure 5. Luminescence spectra of Eu 2:1 (A) and Am 2:1 (B) at room temperature. The excitation wavelengths are 250 nm (Eu) and 503 nm (Am), respectively.

Information) with luminescence lifetimes of 749 ± 70 μs for Eu 2:1 and 228 ± 6 ns for Am 2:1, calculated for data from triplicate measurements. It is well-documented that the intensity ratio of the 5D0→7F2 band (∼616 nm) to the 5D0→7F1 band (∼592 nm), RE/M, reflects the strength of the ligand field and thus the strength of Eu-ligand complexation.53,54 RE/M is ∼0.3 for fully hydrated Eu(III) but greatly increases in strong complexes. For Eu 2:1, RE/M reached 9.6, among the highest that have been reported for Eu(III) complexes, indicating the formation of strong metal−ligand complexes, in this case. Previous work by Horrocks and Sudnick,55 Kimura and Kato,56 and several other groups have shown that, in chemical systems of aqueous origin, Eu(III)/Am(III) luminescence quenching is primarily due to the presence of hydration waters in the inner coordination sphere,57 and linear relationships between the number of inner-sphere hydration waters, NH2O, and the measured luminescence decay constants, k, have been derived. With the equations by Barthelemy and Choppin for Eu(III)57 and Kimura and Kato for Am(III)56 NH2O,Eu = 1.05kH2O,Eu − 0.70

(1)

NH2O,Am = 2.56 × 10−7kH2O,Am − 1.43

(2)

crystallographically identified and do not support the presence of f element bound water molecules.

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CONCLUSION Here, we report the synthesis and structural characterization of Am, Nd, and Eu complexes featuring the well-studied multifunctional phosphine oxide ligand 2,6-[Ph 2 P(O)CH2]2C5H3−NO, L. Although this ligand and its derivatives have been studied with americium and other f elements over the past 30 years as effective extractants of f elements from acidic aqueous solutions and are still being studied today, this is the first time its Am(III) products have been crystallographically characterized for comparison with new and known trivalent lanthanide and tetravalent actinide analogues. From a structural standpoint, the bis-ligated species [Am(L)2(NO3)][2(NO3)], Am 2:1, and the monoligated species Am(L)(NO3)3, Am 1:1, possess cations that interact very similarly with their oxygen coordination environments as their trivalent lanthanide analogues, which is expected. A relationship can be observed between the pyridine-N-oxide bond of the ligand and the charge-to-radius ratio of the actinides when comparing them in the +3 and +4 oxidation states. Luminescence measurements of [M(L)2(NO3)][2(NO3)], when M = Eu or Am, indicate spectral maxima that are similar to those previously observed for these elements, and the calculated intensity ratios indicate strong complexation. Synthetically, the bis-ligated complexes [M(L)2(NO3)][2(NO3)], M 2:1, (where M = Eu, Nd, or Am) were much more straightforward to consistently isolate in crystalline form affording large crystals. Estimates of the number of innersphere water molecules using the measured luminescence decay constants support the pure formation of M 2:1 as structurally identified. In contrast, although Am(L)(NO3)3, Am 1:1, and Nd(L)(NO3)3, Nd 1:1, were synthesized and crystallographically characterized for the first time, Nd 1:1 could not be consistently isolated, and subsequent reactions often produced mixed crystalline batches dominated by bis-ligated species such as [Nd(L)2(H2O)2][3(NO3)]. Computational modeling efforts are currently underway to gain insight into the preferential crystallization of similar bis-ligated complexes under these conditions in the solid state. In closing, the results presented here open the door to further structural exploration of minor actinide-containing species formed during nuclear

where kH2O, Eu and kH2O, Am are the measured luminescence decay constants for Eu(III) (in ms−1) and Am (III) (in s−1), respectively. With the measured luminescence decay constants in this work, the number of inner-sphere water molecules in Eu 2:1 and Am 2:1 were calculated to be 0.7 ± 0.5 and −0.3 ± 0.5, respectively, where the standard deviations were calculated based on triplicate measurements, while it was known that those methods themselves carry errors of ±0.5 as well. While the cause of any small difference in the calculated number of inner-sphere water molecules in Eu 2:1 and Am 2:1 is unknown, clearly the inner-sphere hydration waters are almost completely replaced by the multidentate ligands in both compounds, supporting the formation of strong complexes. These luminescence measurements on bulk samples from the 2:1 stoichiometric reactions corroborate observations from single-crystal analysis. Unlike the 1:1 reactions above, which, in some cases, produced crystalline species with two inner-sphere water ligands, the luminescence results for the 2:1 reactions are consistent with pure formation of the M 2:1 species H

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(4) Choppin, G. R.; Liljenzin, J.; Rydberg, J. Radiochemistry and nuclear chemistry, 3rd ed.; Butterworth-Heinemann: Oxford, U.K., 2001. (5) Nilsson, M.; Nash, K. L. Review article: A review of the development and operational characteristics of the TALSPEAK process,. Solvent Extr. Ion Exch. 2007, 25, 665−701. (6) Philip Horwitz, E.; Kalina, D. C.; Diamond, H.; Vandegrift, G. F.; Schulz, W. W. The TRUEX process - a process for the extraction of the transuranic elements from nitric acid wastes utilizing modified PUREX solvent. Solvent Extr. Ion Exch. 1985, 3, 75−109. (7) Nash, K. L. The chemistry of TALSPEAK: A review of the science,. Solvent Extr. Ion Exch. 2015, 33, 1−55. (8) Lumetta, G. J.; Casella, A. J.; Rapko, B. M.; Levitskaia, T. G.; Pence, N. K.; Carter, J. C.; Niver, C. M.; Smoot, M. R. An advanced TALSPEAK concept using 2-ethylhexylphosphonic acid mono-2ethylhexyl ester as the extractant. Solvent Extr. Ion Exch. 2015, 33, 211−223. (9) Lumetta, G. J.; Levitskaia, T. G.; Wilden, A.; Casella, A. J.; Hall, G. B.; Lin, L.; Sinkov, S. I.; Law, J. D.; Modolo, G. An advanced TALSPEAK concept for separating minor actinides. Part 1. Process optimization and flowsheet development. Solvent Extr. Ion Exch. 2017, 35, 377−395. (10) Wilden, A.; Lumetta, G. J.; Sadowski, F.; Schmidt, H.; Schneider, D.; Gerdes, M.; Law, J. D.; Geist, A.; Bosbach, D.; Modolo, G. An advanced TALSPEAK concept for separating minor actinides. Part 2. Flowsheet test with actinide-spiked simulant. Solvent Extr. Ion Exch. 2017, 35, 396−407. (11) Paiva, A. P.; Malik, P. Recent advances on the chemistry of solvent extraction applied to the reprocessing of spent nuclear fuels and radioactive wastes. J. Radioanal. Nucl. Chem. 2004, 261, 485−496. (12) Horwitz, E. P.; Schulz, W. W. The TRUEX Process: A Vital Tool for the Disposal of U.S. Defense Waste; Elsevier: London, U.K., 1991. (13) Rapko, B. M.; Duesler, E. N.; Smith, P. H.; Paine, R. T.; Ryan, R. R. Chelating properties of 2-((diphenylphosphino)methyl)pyridine N,P-dioxide and 2,6-bis((diphenylphosphino)methyl)pyridine N, P,P’trioxide toward f-element ions. Inorg. Chem. 1993, 32, 2164−2174. (14) Navratil, J. D.; Schulz, W. W.; Talbot, A. E. Actinide recovery from waste and low-grade sources; Hardwood Academic: New York, 1982. (15) Nash, K. L. A review of the basic chemistry and recent developments in trivalent f-elements separations,. Solvent Extr. Ion Exch. 1993, 11, 729−768. (16) Weaver, B.; Kappelmann, F. A. Talspeak: A new method of separating americium and curium from the lanthanides by extraction from an aqueous solution of an aminopolyacetic acid complex with a monoacidic organophosphate or phosphonate; Technical Report ORNL-3559; Oak Ridge National Laboratory, 1964. (17) Conary, G. S.; Russell, A. A.; Paine, R. T.; Hall, J. H.; Ryan, R. R. Synthesis and coordination chemistry of 2-(diisopropoxyphosphino)pyridine N,P-dioxide. Crystal and molecular structure of bis[2(diisopropoxyphosphino)pyridine N,P-dioxide]lanthanum nitrate. Inorg. Chem. 1988, 27, 3242−3245. (18) Bond, E. M.; Duesler, E. N.; Paine, R. T.; Nöth, H. Isolation and structure of a europium(III) chloride complex of 2,6-bis[(diphenylphosphino)methyl]pyridine N, P,P′-trioxide. Polyhedron 2000, 19, 2135−2140. (19) Bond, E. M.; Gan, X.; Fitzpatrick, J. R.; Paine, R. T. Coordination chemistry and extraction properties of phosphonopyridyl N, P oxides. J. Alloys Compd. 1998, 271−273, 172−175. (20) Bond, E. M.; Duesler, E. N.; Paine, R. T.; Neu, M. P.; Matonic, J. H.; Scott, B. L. Synthesis and molecular structure of a plutonium(IV) coordination complex: [Pu(NO3)2{2,6-[(C6H5)2P(O)CH2]2C5H3NO}2](NO3)2·1.5H2O·0.5MeOH. Inorg. Chem. 2000, 39, 4152−4155. (21) Engelhardt, U.; Rapko, B. M.; Duesler, E. N.; Frutos, D.; Paine, R. T.; Smith, P. H. Synthesis and molecular structures of complexes of bismuth(III) nitrate with tridentate ligands: 2,6-bis(-CH2-P((=O)R2) substituted pyridine-N-oxides. Polyhedron 1995, 14, 2361−2369. (22) Bond, E. M.; Engelhardt, U.; Deere, T. P.; Rapko, B. M.; Paine, R. T.; FitzPatrick, J. R. The solvent extraction of americium(III) in

reprocessing, and we plan to pursue the crystallographic analysis of separation schemes utilizing other extractant ligands whose minor actinide products have been previously concealed from the scientific community.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03154. Experimental details, full refinement data, and spectroscopic data (PDF) Accession Codes

CCDC 1587603−1587611 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jordan F. Corbey: 0000-0002-3273-3044 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was conducted at the U.S. Department of Energy (DOE) Pacific Northwest National Laboratory (PNNL), which is operated for the DOE by Battelle Memorial Institute under Contract No. DE-AC05-76RL1830. We thank and acknowledge the Department of Homeland Security’s Nuclear Forensics Postdoctoral Fellowship Program run by the National Technical Nuclear Forensics Center (NTNFC) within the Domestic Nuclear Detection Office for providing support for J.F.C., while R.G.S. acknowledges an internship through the National Security Internship Program (NSIP) at PNNL and funding from the NTNFC. J.F.C. thanks Profs. T. AlbrechtSchmitt and M. J. Polinski for their assistance with crystallographic refinement of americium compounds. J.F.C. also thanks Dr. D. A. Penchoff for computational chemistry assistance and helpful discussion. The laser luminescence measurements were performed at the William R. Wiley Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the U.S. DOE Office of Biological and Environmental Research and located at PNNL.

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b03154 Inorg. Chem. XXXX, XXX, XXX−XXX