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Oct 24, 2017 - ... (DHNRP) was designed as a novel building block to connect UO2(NO3)2 units infinitely. ... and DHNRPs, were successfully connected t...
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Article Cite This: Inorg. Chem. 2017, 56, 13530-13534

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Molecular and Crystal Structures of Uranyl Nitrate Coordination Polymers with Double-Headed 2‑Pyrrolidone Derivatives Hiroyuki Kazama,† Satoru Tsushima,†,‡ Yasuhisa Ikeda,† and Koichiro Takao*,† †

Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1-N1-32 O-okayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Institute of Resource Ecology, Helmholtz Zentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany S Supporting Information *

ABSTRACT: Double-headed 2-pyrrolidone derivatives (DHNRPs) were designed and synthesized as bridging ligands for the efficient and selective separation of UO22+ from a HNO3 solution by precipitation. The building blocks, UO2(NO3)2 and DHNRPs, were successfully connected to form an infinite 1D coordination polymer. The solubility of [UO2(NO3)2(DHNRP)]n is no longer correlated to the hydrophobicity of the ligand but is exclusively governed by the ligand symmetry and packing efficiency. The newly designed DHNRP family can be used to establish a new spent nuclear fuel reprocessing scheme.

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INTRODUCTION The chemistry of actinides is highly relevant to various chemical processes in the nuclear fuel cycle.1 Especially in the reprocessing of spent nuclear fuels, coordination chemistry governs the selective separation and efficient recovery of fertile or fissile materials like Th, U, and Pu. Although the most popular reprocessing method commonly employed in the current U/Pu fuel cycle is a solvent extraction like PUREX involving tri-n-butyl phosphate as an extractant, there is still enough space to explore a diversity of separation methods for attaining more simplicity and versatility. Particularly, a U/Th fuel cycle is attracting special attention as one of the alternative nuclear energy systems to overcome the upcoming scarcity of uranium resources, while the reprocessing method of spent ThO2 fuels is not developed and experienced very well.2,3 Previously, we found that several N-alkylated 2-pyrrolidone derivatives (NRPs; Figure 1, left) are able to selectively and efficiently precipitate UVI and PuVI from a HNO3 solution.4−8 These hexavalent actinides are usually present as actinyl ions (AnO22+, where An = U and Pu), which form sparingly soluble

bis(nitrato) complexes with NRPs like AnO2(NO3)2(NRP)2 (Figure 1, right). In contrast, tetravalent actinides (An4+) and other simulated fission products (FPs) remain dissolved.9−11 On the basis of these findings, we have proposed a precipitationbased reprocessing process for spent nuclear fuels, where the most important aim is the efficient and selective recovery of U and Pu through precipitation with appropriately selected NRPs.9,10 A similar concept for nuclear fuel recycling was also proposed by Burns and Moyer.12 This principle may also be directly applied to a wet reprocessing process of spent ThO2 fuels in the U/Th fuel cycle because its main purpose is the separation of UO22+ of fissile 233U from Th4+ and other FPs. In our former developments, we focused on optimization of the hydrophobicity of NRPs to control the solubility of AnO2(NO3)2(NRP)2. Higher hydrophobicity generally affords lower solubility, while the selectivity for AnO22+ is also getting worse at the same time. Furthermore, the solubility is also affected by the packing efficiency of AnO2(NO3)2(NRP)2 in the crystal structure, which was assessed with a compactness parameter (Cp).6 This quantity was defined as a mean volume occupied by a single C atom of an N substituent on NRP. However, there is a limit to improving the packing efficiency (i.e., to reducing Cp) because the closest contact between the neighboring complexes is simply defined by its collision radius unless any specific interactions are formed. Therefore, it is necessary to innovate the concept of the molecular design of NRP as a selective and efficient precipitant for AnO22+.

Figure 1. Schematic structures of NRP (left) and AnO2(NO3)2(NRP)2 (right). © 2017 American Chemical Society

Received: September 1, 2017 Published: October 24, 2017 13530

DOI: 10.1021/acs.inorgchem.7b02250 Inorg. Chem. 2017, 56, 13530−13534

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Inorganic Chemistry

was performed also for L3, while no deposits were observed. After concentration of the mixture through spontaneous evaporation, yellow crystals were finally obtained (64% yield). The results of the characterization of these compounds are described in ref 16 and the SI. Elemental analysis revealed that the molecular formulas of these compounds are [UO 2 (NO 3 ) 2 (L1)] (1), [UO 2 (NO 3 ) 2 (L2)] (2), and [UO2(NO3)2(L3)] (3). The stoichiometry of UO22+, NO3−, and DHNRP is always 1:2:1, implying formation of the coordination polymers. In the Raman and IR spectra, the characteristic vibration modes of UO22+ and DHNRPs were observed. A low-energy shift of the CO stretching vibration compared with free DHNRP indicates that its carbonyl O atom certainly binds to UO22+. The molecular and crystal structures of compounds 1−3 were determined by single-crystal X-ray diffraction. As shown in Figure 3, the infinite 1D coordination polymers of UO2(NO3)2 units and bridging DHNRPs were successfully confirmed in all of the compounds studied here. In accordance with this, the formulas of these compounds are written more correctly as [UO2(NO3)2(DHNRP)]n. The coordination structures around the U center in these compounds are quite similar to each other. The interatomic distances between U and axial O are 1.769(2) Å in 1, 1.772(4) and 1.776(4) Å in 2, and 1.763(4) and 1.766(4) Å in 3. The OUO bond angles in UO22+ are 180.0(2)° in 1, 179.3(2)° in 2, and 180.0(2)° in 3, showing that the linearity of UO22+ in these compounds is retained. Furthermore, UO22+ is surrounded by two bidentate NO3− and two monodentate pyrrolidone rings of DHNRP to form a hexagonal equatorial plane in trans geometry. In Figure 3b, only the R,R enantiomer of L2 is displayed, while the S,S isomer is also present in the crystal structure of 2 to make it racemic. The mean U−ONO3 bond lengths are 2.55 Å in 1, 2.51 Å in 2, and 2.54 Å in 3. The U− ODHNRP bond distances are 2.362(2) Å in 1, 2.395(4) and 2.402(4) Å in 2, and 2.356(3) and 2.365(4) Å in 3. The CO− U bond angles are 134.4(2)° in 1, 145.4(3) and 135.1(4)° in 2, and 134.4(3) and 143.0(4)° in 3. The structural parameters discussed here are also commonly found in various UO2(NO3)2L2 (L = monodentate ligand).5,6,17−20 Looking at these structural parameters, one can find that the U−ONO3 lengths in 2 are shorter than those of the other compounds, while U−ODHNRP in 2 is longer than others at the same time. This trend can be ascribed to steric effects between NO3− and the C−H moiety vicinal to N in DHNRP. In fact, the interatomic distance from ONO3 to the spatially closest H is 2.264 Å in 2, which is much shorter than those in 1 (2.378 Å) and 3 (2.528 Å). Although the H···ONO3 distance shorter than the sum of the van der Waals radii of O and H (1.52 + 1.20 = 2.72 Å)21 suggests that a short contact is actually present between NO3− and C−H in these complexes, its extent in 2 seems to be stronger than those in 1 and 3. As a matter of fact, the C−H···ONO3 angle in 2 (172.4°) also implies the presence of the C−H···O hydrogen bond.22,23 In contrast, such a discussion cannot be easily applied to 1 and 3 because of the narrower angles (143.9° for 1 and 133.8° for 3). In the molecular structure of 2, the pyrrolidone rings are forced to stack with their faces toward each other because of the trans-1,2-cyclohexylene group. As a result, this bridging moiety tends to lean over NO3− to form the C−H··· ONO3 hydrogen bond. Although the molecular arrangements in 1−3 are similar to each other at a glance, several differences are also observable. The

Figure 2 shows one of the promising solutions, where two 2pyrrolidone moieties are cross-linked by a bridging moiety, R′.

Figure 2. General structure of the DHNRP studied here.

This double-headed NRP (DHNRP) allows one to connect AnO2(NO3)2 units to form a 1D chain coordination polymer, which can be much less soluble than the AnO2(NO3)2(NRP)2 reported so far. Covalent bonds connecting the 2-pyrrolidone groups may afford a closer approach of the AnO2(NO3)2 moieties beyond the limit arising from the van der Waals radii. Furthermore, having two amide groups in a single molecule is beneficial to decreasing the hydrophobicity of DHNRP. As a result, both of efficiency and selectivity of the AnVI precipitation can be achieved simultaneously. While the ethylene-bridged structure has been reported previously,13,14 any further developments have not been achieved at all. In this article, the synthesis and characterization of UO2(NO3)2 complexes with several DHNRPs are described together with their solubility to demonstrate the effectiveness of our new molecular design for NRP toward the simple and versatile reprocessing for spent nuclear fuels.

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RESULTS AND DISCUSSION Here, we selected DHNRPs including trans-1,4-cyclohexylene (L1), trans-1,2-cyclohexylene (L2), and propylene (L3) groups as a bridging moiety R′. Compounds L1−L3 were prepared through Scheme 1.6,15 Consequently, L1 and L2 were obtained Scheme 1. Preparation of DHNRP

as colorless solids, while L3 was an oily product. All DHNRPs prepared here were identified by 1H and 13C NMR and IR spectroscopy. Furthermore, L1 and L2 were also characterized by elemental analysis and single-crystal X-ray diffraction. The details are described in the Supporting Information (SI). The hydrophobicity of DHNRPs was assessed in terms of the logarithmic partition coefficients in a 1-octanol/water biphasic system (log Po/w). As a result, the log Po/w values of L1−L3 were −0.07, 0.35, and −0.74, respectively. These values are smaller than those of the single-headed NRPs [Figure 1; R = n-butyl (0.70), isobutyl (0.59), and cyclohexyl (1.10)] reported previously,8 demonstrating the lower hydrophobicity of the DHNRPs prepared here. In 1.0 M HNO3(aq), DHNRP (0.25 M) and UO22+(0.25 M) were slowly mixed in a glass tube (see the SI). As a result, yellow crystals deposited in the samples of L1 and L2 within several hours (99% and 92% yields, respectively). The same experiment 13531

DOI: 10.1021/acs.inorgchem.7b02250 Inorg. Chem. 2017, 56, 13530−13534

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Figure 3. ORTEP drawings of (a) 1, (b) 2, and (c) 3) at the 50% probability level. H atoms were omitted for clarity.

most critical one is the size of an asymmetric unit. Compound 1 consists of only one U, one axial O, one NO3−, and half of a L1 molecule. This asymmetric unit is the smallest in the compounds studied here. The asymmetric unit of compound 2 comprises the larger group of components, i.e., UO22+, two NO3−, and a whole molecule of L2. Compound 3 shows the largest asymmetric unit containing two U, two axial O, two NO3−, and a whole molecule of L3. Interestingly, compounds 1−3 belong to P1̅ (No. 2), where only two kinds of symmetry operators, (x, y, z) and (−x, −y, −z), are available. Therefore, their crystallographic symmetries are actually the same. On the other hand, the difference in the size of the asymmetric unit is clearly observed as mentioned above. Such a difference most likely arises from the symmetry of DHNRPs connecting the UO2(NO3)2 units. Thus, L1 in C2h is the most symmetric in the tested DHNRPs, while both L2 and L3 in C2 are lower in symmetry. The compactness parameters (Cp) of 1−3 were estimated as 15.6, 18.8, and 21.3 Å3, respectively. These values are much smaller than those of UO2(NO3)2(NRP)2 (Figure 1; 24.0−28.9 Å3),6 indicating that connecting 2-pyrrolidone moieties through covalent bonds efficiently helps to improve the packing efficiency as expected. The recovery efficiency of AnO22+ (An = U and Pu) is highly important in the recycle of nuclear fuel materials through spent fuel reprocessing. In the precipitation-based reprocessing method, we proposed that the solubility of [UO2(NO3)2(DHNRP)]n is the most essential. In this context, compounds 1−3 were soaked in 3.0 M HNO3(aq) at 298 K to determine their solubility. The obtained results were summarized in Table 1, together with several quantities that may affect the solubility of [UO2(NO3)2(DHNRP)]n. As seen from Table 1, the hydrophobicity referring to log Po/w is not directly correlated to the solubility of compounds 1−3. This situation is much different from the former series of UO2(NO3)2(NRP)2 (Figure 1) that we studied previously. In contrast, [UO2(NO3)2(DHNRP)]n tends to become more soluble with increasing Cp. This means that the

Table 1. Solubility of [UO2(NO3)2(DHNRP)]n in 3.0 M HNO3(aq) at 298 K and Related Parameters 1 2 3

solubility/mM

log Po/w

Cp/Å3

ligand symmetry

2.49 14.8 102

−0.07 0.35 −0.74

15.6 18.8 21.3

C2h C2 C2

packing efficiency is a dominant factor to govern the solubility of [UO2(NO3)2(DHNRP)]n. It is worth noting that compound 1 (2.49 mM) is much less soluble compared with any of UO2(NO3)2(NRP)2 reported so far (18−137 mM)8 despite the much lower hydrophobicity of the ligand (log Po/w = −0.07 for L1; log Po/w = 0.20−1.10 for NRPs; R = propyl, n-butyl, isobutyl, cyclohexyl). The solubility of uranyl oxalate, a wellknown uranyl precipitate, is ca. 50 mM in 3 M HNO3(aq),24 which is even higher than 1 and 2. This means that L1 and L2 are superior to oxalate as effective precipitants for UO22+ in HNO3(aq).

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CONCLUSION In conclusion, the formation of 1D chain coordination polymers of uranyl nitrate complexes was successfully observed. The solubility of these complexes was liberated from the hydrophobicity of additional monodentate ligands by adopting the new concept of molecular design, DHNRP, to form a coordination polymer and closer packing. Further investigations are currently ongoing, e.g., design and optimization of DHNRP structure, synthesis and characterization of their UO2(NO3)2 complexes, interaction of DHNRPs with tetravalent metal ions like Th4+, U4+, Ce4+, and Zr4+, and selective separation of UO22+ from An4+ and other FPs to give a prospect to a simple and versatile reprocessing method for spent nuclear fuels in both U/Pu and U/Th fuel cycles. Recently, sophisticated polyactinide complexes like oxo/hydroxo clusters,25−28 metal−(in)organic frame13532

DOI: 10.1021/acs.inorgchem.7b02250 Inorg. Chem. 2017, 56, 13530−13534

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Inorganic Chemistry works,29−33 and topological clusters34−36 are extensively studied. A series of DHNRPs that we designed here is a promising building block to exploring a new aspect of the actinide coordination chemistry exclusively oriented to spent fuel reprocessing.

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Derivatives: Cocrystallization Potentiality of UVI and PuVI. Cryst. Growth Des. 2010, 10, 2033−2036. (8) Takao, K.; Noda, K.; Nogami, M.; Sugiyama, Y.; Harada, M.; Morita, Y.; Nishimura, K.; Ikeda, Y. Solubility of Uranyl Nitrate Precipitates with N-Alkyl-2-pyrrolidone Derivatives (Alkyl = n-Propyl, n-Butyl, iso-Butyl, and Cyclohexyl). J. Nucl. Sci. Technol. 2009, 46, 995− 999. (9) Koshino, N.; Harada, M.; Morita, Y.; Kiikuchi, T.; Ikeda, Y. Development of a simple reprocessing process using selective precipitant for uranyl ions: Fundamental studies for evaluating the precipitant performance. Prog. Nucl. Energy 2005, 47, 406−413. (10) Morita, Y.; Takao, K.; Kim, S.-Y.; Kawata, Y.; Harada, M.; Nogami, M.; Nishimura, K.; Ikeda, Y. Development of Advanced Reprocessing System Based on Precipitation Method Using Pyrrolidone Derivatives as Precipitants -Precipitation Behavior of U(VI), Pu(IV), and Pu(VI) by Pyrrlidone Derivatives with Low Hydrophobicity-. J. Nucl. Sci. Technol. 2009, 46, 1129−1136. (11) Suzuki, T.; Kawasaki, T.; Takao, K.; Harada, M.; Nogami, M.; Ikeda, Y. A study on selective precipitation ability of cyclic urea to U(VI) for developing reprocessing system based on precipitation method. J. Nucl. Sci. Technol. 2012, 49, 1010−1017. (12) Burns, J. D.; Moyer, B. A. Group Hexavalent Actinide Separations: A New Approach to Used Nuclear Fuel Recycling. Inorg. Chem. 2016, 55, 8913−8919. (13) Doyle, G. A.; Goodgame, D. M. L.; Sinden, A.; Williams, D. J. Conversion of atmospheric dioxygen to a μ-η2,η2-peroxo bridge in a dinuclear uranium(VI) complex. J. Chem. Soc., Chem. Commun. 1993, 1170. (14) Doyle, G. A.; Goodgame, D. M. L.; Hill, S. P. W.; Menzer, S.; Sinden, A.; Williams, D. J. Chain and Large-Ring Polymeric Transition Metal and Lanthanide Complexes Formed by N,N′-Ethylenebis(pyrrolidin-2-one). Inorg. Chem. 1995, 34, 2850−2860. (15) Wang, E.-C.; Lin, H.-J. Reaction of β-, γ-, and δ-Chloroalkanamides with Potassium tert-Butoxide in Tetrahydrofuran: Elimination, and Lactamization. Heterocycles 1998, 48, 481−489. (16) Caution! 238U is an emitter, and therefore standard precautions for handling radioactive materials should be followed. Characterization of 1. Anal. Calcd for C14H22N4O10U: C, 26.10; H, 3.44; N, 8.69. Found: C, 26.30; H, 3.34; N, 8.65. Crystallographic data for 1: fw = 644.38, 0.400 × 0.300 × 0.200 mm3, triclinic, P1̅ (No. 2), a = 5.8962(4) Å, b = 7.6330(6) Å, c = 11.0279(9) Å, α = 76.841(5)°, β = 84.480(6)°, γ = 76.229(5)°, V = 468.93(6) Å3, Z = 1, T = 93 K, Dcalcd = 2.282 g cm−3, μ = 87.199 cm−1, GOF = 1.042, R (I > 2σ) = 0.0174, wR (all) = 0.0393. IR (ATR, cm−1): 1589 (>CO), 927 (UO22+, asymmetric). Raman (cm−1): 852 (UO22+, symmetric). Characterization of 2. Anal. Calcd for C14H22N4O10U: C, 26.10; H, 3.44; N, 8.69. Found: C, 26.03; H, 3.31; N, 8.45. Crystallographic data for 2: fw = 644.38, 0.400 × 0.300 × 0.300 mm3, triclinic, P1̅ (No. 2), a = 9.9552(7) Å, b = 10.0507(7) Å, c = 10.1794(7) Å, α = 90.900(6)°, β = 106.750(8)°, γ = 90.412(6)°, V = 975.12(12) Å3, Z = 2, T = 93 K, Dcalcd = 2.194 g cm−3, μ = 83.868 cm−1, GOF = 1.073, R (I > 2σ) = 0.0344, wR (all) = 0.0792. IR (ATR, cm−1): 1597 (>CO), 931 (UO22+, asymmetric). Raman (cm−1): 854 (UO22+, symmetric). Characterization of 3. Anal. Calcd for C11H18N4O10U: C, 21.86; H, 2.94; N, 9.27. Found: C, 21.71; H, 3.11; N, 8.87. Crystallographic data for 3: fw = 604.31, 0.300 × 0.200 × 0.200 mm3, triclinic, P1̅ (No. 2), a = 5.9980(10) Å, b = 11.534(2) Å, c = 13.224(2) Å, α = 100.169(7)°, β = 95.142(7)°, γ = 100.614(7)°, V = 878.0(3) Å3, Z = 2, T = 183 K, Dcalcd = 2.286 g cm−3, μ = 93.065 cm−1, GOF = 1.057, R (I > 2σ) = 0.0298, wR (all) = 0.0831. IR (ATR, cm−1): 1609 (>CO), 929 (UO22+, asymmetric). Raman (cm−1): 852 (UO22+, symmetric). (17) Ikeda, Y.; Wada, E.; Harada, M.; Chikazawa, T.; Kikuchi, T.; Mineo, H.; Morita, Y.; Nogami, M.; Suzuki, K. A study on pyrrolidone derivatives as selective precipitant for uranyl ion in HNO3. J. Alloys Compd. 2004, 374, 420−425. (18) Koshino, N.; Harada, M.; Nogami, M.; Morita, Y.; Kikuchi, T.; Ikeda, Y. A structural study on uranyl(VI) nitrate complexes with cyclic amides: N-n-butyl-2-pyrrolidone, N-cyclohexylmethyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidone. Inorg. Chim. Acta 2005, 358, 1857−1864.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02250. Details of the synthesis and characterization of L1−L3 and 1−3 (PDF) Accession Codes

CCDC 1573161−1573163 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]. Tel and Fax: +81 3 5734 2968. ORCID

Koichiro Takao: 0000-0002-0952-1334 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Emer. Hirotake Moriyama for stimulating discussion and his helpful advice. This work is the result of the “Fundamental Study on Simple Reprocessing Method for Spent Thorium Fuels by Using Uranium-Selective Precipitant” (Project 271501) entrusted to the Tokyo Institute of Technology by MEXT, Japan.

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