Molecular and Crystal Structures of Plutonyl(VI) Nitrate Complexes with

Apr 12, 2010 - Dresden-Rossendorf, P.O. Box 51 01 19, 01314 Dresden, Germany, and ... a bond angle between the Pu-ONRP bond and the carbonyl group of ...
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DOI: 10.1021/cg100015t

Molecular and Crystal Structures of Plutonyl(VI) Nitrate Complexes with N-Alkylated 2-Pyrrolidone Derivatives: Cocrystallization Potentiality of UVI and PuVI

2010, Vol. 10 2033–2036

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Seong-Yun Kim,†,# Koichiro Takao,‡,§ Yoshinori Haga,† Etsuji Yamamoto,† Yoshihisa Kawata,† Yasuji Morita,*,† Kenji Nishimura, and Yasuhisa Ikeda‡ Japan Atomic Energy Agency, Tokai-mura Naka-gun, Ibaraki 319-1195, Japan, #Cyclotron and Radioisotope Center, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan, ‡ Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34, O-okayama, Meguro-ku, Tokyo 152-8550, Japan, §Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, P.O. Box 51 01 19, 01314 Dresden, Germany, and Mitsubishi Materials Corporation, 1002-14, Mukohyama, Naka-city, Ibaraki 311-0102, Japan )

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Received January 5, 2010; Revised Manuscript Received March 22, 2010

ABSTRACT: Plutonyl(VI) nitrate complexes with N-cyclohexyl-2-pyrrolidone (NCP) and N-neopentyl-2-pyrrolidone (NNpP) were prepared, and their molecular and crystal structures were determined by single crystal X-ray analysis. The obtained compounds have a similar composition, PuO2(NO3)2(NRP)2 (NRP = NCP, NNpP), which are analogous to the corresponding UVI complexes. Both PuO2(NO3)2(NRP)2 complexes show typical structural properties of actinyl(VI) nitrates, that is, hexagonal-bipyramidal geometry consisting of two NRP molecules and two NO3- ions located in trans positions in the equatorial plane of the PuO22þ moiety, PudOax = 1.73 A˚, Pu-ONRP = 2.38 A˚, Pu-ONO3 = 2.50 A˚, and a bond angle between the Pu-ONRP bond and the carbonyl group of NRP ≈ 135°. PuO2(NO3)2(NNpP)2 is isostructural with the corresponding uranium compound, whereas PuO2(NO3)2(NCP)2 is not. These findings provide one of criteria in selection of suitable NRP as a precipitation agent for the spent nuclear fuel reprocessing based on the precipitation method from a viewpoint of crystal engineering. Understanding of the fundamental chemistry of actinides is very important to consider how these elements behave in a reprocessing processes of spent nuclear fuels.1 Recently, we have reported that N-cyclohexyl-2-pyrrolidone (NCP, Chart 1) selectively precipitates UO22þ in HNO3 aqueous solutions.2 The formula and structure of this precipitate were clarified to be UO2(NO3)2(NCP)2, in which the equatorial plane of UO22þ is surrounded by four oxygen atoms from two bidentate NO3- and two oxygen atoms of NCP.3,4 These ligands are located in trans position. The coordination geometry around the uranium atom is hexagonal bipyramidal. Furthermore, the UO2(NO3)2(NCP)2 complex was found to be easily converted to uranium oxides.5 On these results, we have proposed a new simple reprocessing process for spent nuclear fuels based on the precipitation method.6 In later works,4,7 we examined the precipitation ability of several N-alkylated 2-pyrolidone derivatives (NRPs). As a result, it was found that NRP with relatively high hydrophobicity may also act as a precipitant of UO22þ, and that the trans coordination of NO3- and NRP to UO22þ, the hydrophobicity of NRP, and its miscibility with aqueous solution are the essential factors for controlling the precipitation behavior. Our most recent crystallographic investigation of UO2(NO3)2(NRP)2 complexes with different alkyl chain lengths and branching suggested that the packing efficiency of UO2(NO3)2(NRP)2 molecules in their crystal lattices also plays an important role.8 In our proposed reprocessing processes, we intend to recover Pu from the parent solution dissolving the spent nuclear fuels as its admixture with U, which is favorable from a viewpoint of nuclear nonproliferation. Plutonium(VI) also forms a dioxo cation, PuO22þ, which is analogous of UO22þ. Therefore, it was predicted that the precipitation of PuO22þ with NRP is also possible as well as UO22þ. As a matter of fact, this prediction has been proven.9 However, it has not been examined whether PuO2(NO3)2(NRP)2 cocrystallizes with UO2(NO3)2(NRP)2. The formation of (U/Pu)O2(NO3)2(NRP)2 should facilitate

Chart 1

*To whom correspondence should be addressed. E-mail: morita.yasuji@ jaea.go.jp.

production of the uniform U/Pu admixture at the molecular-level as a precursor of mixed oxide (MOX) fuels for thermal/fast nuclear reactors. In this case, the molecular structure and lattice constants of PuO2(NO3)2(NRP)2 must be the same as those of UO2(NO3)2(NRP)2. Therefore, the molecular and crystal structures of PuO2(NO3)2(NRP)2 are of great interest from a viewpoint of crystal engineering in the spent nuclear fuel reprocessing we proposed. In this study, we selected NCP and N-neopentyl-2pyrrolidone (NNpP, Chart 1) as model precipitants. Herein, we report the molecular and crystal structures of PuO2(NO3)2(NCP)2 and PuO2(NO3)2(NNpP)2 determined by the single crystal X-ray diffraction method and discuss the cocrystallization potentiality of (U/Pu)O2(NO3)2(NRP)2. Plutonium(IV) oxide10 (238Pu: 0.0427%, 239Pu: 85.5928%, 240 Pu: 13.7114%, 241Pu: 0.3659%, 242Pu: 0.2873 in May, 2005) stored in Japan Atomic Energy Agency was dissolved in aqueous solution containing 3 M HNO3 and H2O2. This crude solution was purified through anion exchange resin (Dowex 1X4, 200-400 mesh) and evaporated to dryness under heating. During this heating process with HNO3, PuIV was oxidized to PuVI, and the purity of the oxidation state was spectrophotometrically confirmed.9a The residue was dissolved in 3 M HNO3 aq and used as a stock solution of PuO22þ (0.095 M) in 3 M HNO3 aq. N-Cyclohexyl-2-pyrrolidone (NCP, 99%) was purchased from Tokyo Chemical Industry, Co., Ltd. and used without further purification. N-Neopentyl-2-pyrrolidone (NNpP) was synthesized and purified by the reported method.8 Twice molar-equivalent of NRP was added dropwise to a portion of the PuVI stock solution (3 mL) with vigorous stirring at room temperature, followed by additional agitation for 1 h. As a result, an orangecolored powder of PuO2(NO3)2(NRP)2 precipitated. Yields of

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Table 1. Crystallographic Data of PuO2(NO3)2(NRP)2 formula formula weighta crystal size (mm) crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z T (K) Dcalcd (g 3 cm-3) μ (mm-1) no. unique reflns no. ls parameters Rb [I > 2σ(I)] wRc (all) GOFd ΔFmax,min (e- A˚-3)

PuO2(NO3)2(NCP)2

PuO2(NO3)2(NNpP)2

C20H34N4O10Pu 729.66 0.53  0.40  0.30 monoclinic P21/c 7.0236(5) 18.431(1) 10.5097(7)

C18H34N4O10Pu 705.64 0.50  0.50  0.40 triclinic P1 9.8161(3) 10.3389(3) 14.5174(4) 100.204(1) 98.402(1) 112.979(1) 1296.74(7) 2 296 1.807 2.587 5931 300 0.0366 0.0907 1.023 2.064, -2.039

102.255(2) 1329.5(2) 2 296 1.830 2.535 2424 162 0.0443 0.0852 1.201 0.918, -0.574

Table 2. Selected Structural Parameters in PuO2(NO3)2(NRP)2 PuO2(NO3)2(NCP)2

Pu(1)-O(1) Pu(1)-O(2) Pu(1)-O(3) Pu(1)-O(4)

C(1)-O(2)-Pu(1)

PuO2(NO3)2(NNpP)2 Bond Distances (A˚) 1.727(5) Pu(1)-O(1) Pu(1)-O(2) 2.377(5) Pu(1)-O(3) Pu(1)-O(4) 2.490(7) Pu(1)-O(5) 2.506(6) Pu(1)-O(6) Pu(1)-O(8) Pu(1)-O(9) Bond Angles (deg) 133.7(5) C(1)-O(2)-Pu(1) C(10)-O(4)-Pu(1)

1.730(4) 1.731(4) 2.382(3) 2.390(3) 2.503(4) 2.490(4) 2.507(4) 2.494(4) 138.6(4) 140.0(4)

Flip Angles of Pyrrolidone Ring from Equatorial Plane (deg) 82.7 60.7, 79.6

a Averaged atomic mass of Pu used: 239.15 g 3 mol-1 (238Pu: 0.0417%, 242 Pu: 85.5854%, 240Pu: 13.7071%, 241Pu: 0.3167%, Pu: 0.2873%, P P calcd from Pu decay fromP 2005 to 2008). b R = P Fo| - |Fc / |Fo|. P 2 2 2 2 2 1/2 d 2 2 2 c wR = [ (w(Fo - Fc ) )/ (Fo ) ] . GOF = [ w(Fo - Fc ) /(No Nv)]1/2. Detailed value of the weight (w) in each compound is given in the crystallographic information file.

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PuO2(NO3)2(NRP)2 were roughly calculated by the UVvis-NIR absorption spectra of the supernatants; PuO2(NO3)2(NCP)2: 93%, PuO2(NO3)2(NNpP)2: 90%. The resulting compounds were filtered off and washed with a small volume of water. For the single crystal X-ray diffraction measurement, each PuO2(NO3)2(NRP)2 precipitate was recrystallized from ethanol. Characterizations of the obtained PuO2(NO3)2(NCP)2 and PuO2(NO3)2(NNpP)2 were performed by using a Rigaku RAXIS RAPID single crystal X-ray diffractometer.11 A single crystal of each complex was glued on top of a glass capillary and confined in a quartz tube. Intensity data were collected at 296 K by using the imaging plate area detector in the diffractometer with graphite monochromated Mo KR radiation (λ = 0.71073 A˚). The structures of the PuVI complexes were solved by direct method SIR 9212 and expanded using Fourier techniques.13 An empirical absorption correction was applied. All non-hydrogen atoms were anisotropically refined by SHELXL-97.14 Hydrogen atoms were refined as riding on their parent atoms with Uiso(H) = 1.2Ueq(C). The crystallographic data and selected structural parameters of PuO2(NO3)2(NCP)2 and PuO2(NO3)2(NNpP)2 are summarized in Tables 1 and 2, respectively. The ORTEP view of PuO2(NO3)2(NCP)2 is shown in Figure 1. In the molecular structure, the Pu atom is surrounded by two O(1) atoms at the axial position (Oax) and six O(2-4) atoms provided from NO3- and NCP in the equatorial plane. Thus, the coordination geometry around Pu is hexagonal bipyramidal. The ligands in the equatorial plane are placed in trans positions. A bond distance between Pu and Oax (PudOax) is 1.727(5) A˚, which is usual for the actinyl(VI) compounds. In the equatorial plane, the bond distances between Pu and O of NCP (Pu-ONCP) and of NO3- (Pu-ONO3) are 2.377(5) and 2.50 A˚ (mean), respectively. These structural properties are comparable with UO2(NO3)2(NRP)2 complexes reported previously.3,4,7,8 Bond angles around ONCP (133.7°) and flip angles of the pyrrolidone ring of NCP from the mean equatorial plane of the complex (82.7°) are also commonly found in UO2(NO3)2(NRP)2.8 Conformation of the cyclohexyl group in PuO2(NO3)2(NCP)2 is “chair”-shaped, which is more stable than “boat”-shaped. This may imply that neither significant steric hindrance nor specific interactions

Figure 1. ORTEP drawing of PuO2(NO3)2(NCP)2 showing 40% probability displacement ellipsoids. Hydrogen atoms are omitted for clarity. Symmetry code (i) -x þ 1, y, -z þ 1.

between PuO2(NO3)2(NCP)2 molecules occur in its crystal structure. The crystal system of PuO2(NO3)2(NCP)2 is monoclinic P21/c, and its lattice constants are a = 7.0236(5) A˚, b = 18.431(1) A˚, c = 10.5097(7) A˚, and β = 102.255(2)°. These crystallographic data are completely different from those of UO2(NO3)2(NCP)2 [triclinic P1, a = 8.627(1) A˚, b = 8.748(1) A˚, c = 9.707(1) A˚, R = 113.61(1)°, β = 102.255(2)°, and γ = 108.74(1)°] in spite of their analogy in chemical composition.3,4 The same solvent (ethanol) was used in the crystallization experiments of both compounds. Furthermore, no solvent molecules are incorporated in the crystal structures of UO2(NO3)2(NCP)2 and PuO2(NO3)2(NCP)2. Therefore, it is hard to consider that the solvent used for recrystallization resulted in the difference. Although the reasons for such a significant difference are not clarified yet, this finding gives an important insight that PuO2(NO3)2(NCP)2 may not cocrystallize with UO2(NO3)2(NCP)2. In contrast, the crystal system and lattice constants of PuO2(NO3)2(NCP)2 are quite similar to those of UO2(NO3)2(NProP)2 [NProP: N-n-propyl-2-pyrrolidone, monoclinic at 293 K, a = 7.089(2) A˚, b = 18.117(5) A˚, c = 8.849(2) A˚, and β = 101.95(3)°] and UO2(NO3)2(NiBP)2 [NiBP: N-isobutyl-2-pyrrolidone, monoclinic, a = 7.405(3) A˚, b = 18.100(5) A˚, c = 8.623(3) A˚, and β = 100.97(3)°].8 The remarkable difference in the lattice constants of PuO2(NO3)2(NCP)2 from those of the above UVI compounds is only the longer c axis by 1.7-1.9 A˚. Comparing the packing views along the c axis of these compounds, molecular arrangement of PuO2(NO3)2(NCP)2 (Figure 2a) is very similar to those of UO2(NO3)2(NProP)2 and UO2(NO3)2(NiBP)2.8 The NCP ligand has the additional distal carbon atom, C(8), which

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Figure 2. Packing views of PuO2(NO3)2(NCP)2 along the c axis (a) and a axis (b). Dotted circles in panel a represent the presence of the void in the crystal lattice. Hydrogen atoms are omitted for clarity.

Figure 3. ORTEP drawing of one of the enantiomers of racemic PuO2(NO3)2(NNpP)2 showing 40% probability displacement ellipsoids. Left: a bird’s-eye view, right: side view along O(4)-Pu(1)-O(3). Hydrogen atoms are omitted for clarity. Both tert-butyl groups point in the same direction, leading the chirality.

prevents closer packing between neighboring molecules in the direction of the c axis (Figure 2b) and results in the longer axis. The single crystal X-ray diffraction experiment in this study was performed only at room temperature (296 K). Hence, it is still unclear if PuO2(NO3)2(NCP)2 shows polymorphism with changing temperature in a manner similar to UO2(NO3)2(NProP)2.8 However, a possibility of such polymorphism in PuO2(NO3)2(NCP)2 still remains, because the voids similar to those in UO2(NO3)2(NProP)2 are found between the cyclohexyl group and NO3- of the neighboring PuO2(NO3)2(NCP)2 molecules as shown in Figure 2a. The ORTEP view of PuO2(NO3)2(NNpP)2 is shown in Figure 3. The molecular structure of this complex shows the common properties to PuO2(NO3)2(NCP)2 and other UO2(NO3)2(NRP)2,3,4,7,8 that is, the hexagonal bipyramidal coordination geometry around Pu, the trans configuration of NNpP and NO3- in the equatorial plane, PudOax [Pu-O(1,2), mean: 1.73 A˚], Pu-ONNpP [Pu-O(3,4), mean: 2.39 A˚], Pu-ONO3 [Pu-O(5,6,8,9), mean: 2.50 A˚], the angles between Pu-ONNpP and the carbonyl group (mean: 139°), and the flip angles of the pyrrolidone ring from the mean equatorial plane (60.7°, 79.6°). Both terminal tert-butyl groups face the same direction as shown in the right panelof Figure 3. This

character leads to stereoisomerism of the complex. In the crystal structure of PuO2(NO3)2(NNpP)2, both enantiomers occur by symmetric operation; that is, this compound is racemic. This is also the case for UO2(NO3)2(NNpP)2 reported previously.8 The crystal system of PuO2(NO3)2(NNpP)2 is triclinic, and its lattice constants are a = 9.9161(3) A˚, b = 10.3389(3) A˚, c = 14.5174(4) A˚, R = 100.204(1)°, β = 98.402(1)°, and γ = 112.979(1)°, which are almost identical with those of UO2(NO3)2(NNpP)2 [a = 9.764(3) A˚, b = 10.170(2) A˚, c = 14.481(5) A˚, R = 100.12(2)°, β = 98.90(2)°, and γ = 112.86(2)°].8 This agreement is reasonable, because the chemical formulas are the same except for the center metal and PuVI is expected to resemble UVI in the chemical behavior. Figure 4 shows the packing views of PuO2(NO3)2(NNpP)2 along the c and a axes. The molecular arrangement of PuO2(NO3)2(NNpP)2 in its crystal lattice is very similar to that of UO2(NO3)2(NNpP)2 (Figure S1, Supporting Information), indicating that the crystal structures of both compounds are the same. This facilitates replacement of UVI with PuVI in MO2(NO3)2(NNpP)2 with any Pu/U ratios. Accordingly, the cocrystallization of (U/Pu)O2(NO3)2(NNpP)2 will be feasible. From the viewpoint of the crystal engineering in the reprocessing of spent nuclear fuels, use of NNpP can be suggested to be

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Figure 4. Packing views of racemic PuO2(NO3)2(NNpP)2 along the c axis (a) and a axis (b). Hydrogen atoms are omitted for clarity.

preferred to that of NCP. In selection of a promising precipitant for our nuclear fuel reprocessing based on the precipitation method, it is also necessary to consider thermodynamics on coordination of NRP to MO22þ (M = U, Pu) and precipitation phenomena of MO2(NO3)2(NRP)2 (e.g., solubility),15 properties of the MO2(NO3)2(NRP)2 precipitates (e.g., grain size, filtration rate, thermal decomposition profile, etc.), decontamination factors of other metal ions including neptunium and transplutonic elements and fission products, miscibility of NRP with aqueous solution, and so on. Acknowledgment. K.T. thanks Prof. Dr. G. Bernhard (FZD) for giving the opportunity to work there and to prepare this communication. The present study is the result of “Development of Advanced Reprocessing System Based on Use of Pyrrolidone Derivatives as Novel Precipitants with High Selectivity and Controllability” entrusted to Tokyo Institute of Technology by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). Supporting Information Available: Packing views of UO2(NO3)2(NNpP)2, crystallographic information files of PuO2(NO3)2(NCP)2 and PuO2(NO3)2(NNpP)2. This material is available free of charge via the Internet at http://pubs.acs.org.

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(4) Ikeda, Y.; Wada, E.; Harada, M.; Chikazawa, T.; Kikuchi, T.; Mineo, H.; Morita, Y.; Nogami, M.; Suzuki, K. J. Alloys Compd. 2004, 374, 420–425. (5) (a) Koshino, N.; Harada, M.; Nogami, M.; Suzuki, K.; Mineo, H.; Morita, Y.; Yamasaki, K.; Chikazawa, T.; Tamaki, Y.; Kikuchi, T.; Ikeda, Y. GLOBAL 2003: Advanced Nuclear Fuel Cycles and Systems, New Orleans, November 16-20, 2003. (b) Kim, S.-Y.; Kawata, Y.; Morita, Y.; Nogami, M.; Harada, M.; Ikeda, Y.; Kikuchi, T.; Nishimura, K. International Workshop for Asian Nuclear Prospect, Kobe, Japan, October 19-22, 2008. (6) Koshino, N.; Harada, M.; Morita, Y.; Kikuchi, T.; Ikeda, Y. Prog. Nucl. Energy 2005, 47, 406–413. (7) Koshino, N.; Harada, M.; Nogami, M.; Morita, Y.; Kikuchi, T.; Ikeda, Y. Inorg. Chim. Acta 2005, 358, 1857–1864. (8) Takao, K.; Noda, K.; Morita, Y.; Nishimura, K.; Ikeda, Y. Cryst. Growth Des. 2008, 8, 2364–2376. (9) (a) Morita, Y.; Kawata, Y.; Mineo, H.; Koshino, N.; Asanuma, N.; Ikeda, Y.; Yamasaki, K.; Chikazawa, T.; Tamaki, Y.; Kikuchi, T. J. Nucl. Sci. Technol. 2007, 44, 354–360. (b) Morita, Y.; Takao, K.; Kim, S.-Y.; Kawata, Y.; Harada, M.; Nogami, M.; Nishimura, K.; Ikeda, Y. J. Nucl. Sci. Technol. 2009, 46, 1129–1136. (10) Caution! All plutonium isotopes are strongly radioactive and an Remitter. It has to be handled in dedicated facilities with appropriate equipment for radioactive materials to avoid health risks caused by radiation exposure. (11) The structural characterization of a crystalline compound of interest should be done using other structure-sensitive methods such as IR and Raman spectroscopy. In our case, we however have to overcome many experimental difficulties arising from the high radioactivity of our plutonium compounds, and at the moment cannot perform such experiments virtually in our institute. UV-vis-NIR absorption spectroscopy might also be of interest. But, we already know that the complexation equilibria of UO22þ with NO3- and NRP in aqueous solutions seem to be complicated15 and that NO3- may dissociate even in a noncoordinating organic solvent such as dichloromethane.3 These could be the case for PuO22þ and complicate understanding of the characterization. (12) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343–350. (13) Beurskens, P. T.; Admiraal, G.; Beurkens, G.; Bosman, W. P.; Gelder, de, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99 Program System. Technical Report of the Crystallography Laboratory; University of Nijmengen: The Netherlands, 1999. (14) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122. (15) Takao, K.; Noda, K.; Nogami, M.; Sugiyama, Y.; Harada, M.; Morita, Y.; Nishimura, K.; Ikeda, Y. J. Nucl. Sci. Technol. 2009, 46, 995–999.