Molecular and Crystal Structures of Uranyl Nitrate Complexes with N

CRYSTAL GROWTH & DESIGN

Molecular and Crystal Structures of Uranyl Nitrate Complexes with N-Alkylated 2-Pyrrolidone Derivatives: Design and Optimization of Promising Precipitant for Uranyl Ion

2008 VOL. 8, NO. 7 2364–2376

Koichiro Takao,†,‡ Kyoko Noda,† Yasuji Morita,§ Kenji Nishimura,| and Yasuhisa Ikeda*,† Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34, O-okayama, Meguro-ku, Tokyo 152-8550, Japan, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan, and Mitsubishi Materials Corporation, 1002-14, Mukohyama, Naka-city, Ibaraki 311-0102, Japan ReceiVed December 14, 2007; ReVised Manuscript ReceiVed February 8, 2008

ABSTRACT: Molecular and crystal structures of UO2(NO3)2(NRP)2 (NRP ) N-alkylated 2-pyrrolidone derivative) have been investigated by using single crystal X-ray analysis. All UO2(NO3)2(NRP)2 complexes have typical structural properties of UO2(NO2)2(L)2 (L ) unidentate ligand), i.e., hexagonal-bipyramidal geometry, two NRP and two NO3- located in trans positions in an equatorial plane of the uranyl moiety, UdOyl = 1.76 Å, U-ONRP ) 2.38-2.41 Å, U-ONO3 ) 2.50-2.54 Å, and a bond angle between the U-ONRP bond and the carbonyl group of NRP (ca. 135°). In the crystals of the uranyl nitrate complexes with N-npropyl-2-pyrrolidone (L3a) and N-iso-propyl-2-pyrrolidone (L3b), polymorphism between monoclinic (293 K) and triclinic forms (173 K) was observed, indicating the presence of significant voids in the crystal lattices of these compounds. From this result, an approach for construction of efficient packing of UO2(NO2)2(NRP)2 was proposed, i.e., the efficient packing would be built up when an alkyl chain of NRP fills the voids in the crystal lattice of UO2(NO3)2(L3a)2 or UO2(NO3)2(L3b)2 without significant deformation of their crystal structures. On this basis, the molecular and crystal structures of UO2(NO3)3(NRP)3 with C4- and C5-alkyl chains were examined. Consequently, it was found that N-iso-butyl-2-pyrrolidone (L4b) completely satisfies that requirement. The high packing efficiency of UO2(NO3)2(L4b)2 was demonstrated quantitatively by introducing “compactness parameter” (Cp) defined as a mean volume occupied by one carbon atom of the alkyl chain in the crystal of UO2(NO3)2(NRP)2. Chart 1a

1. Introduction Uranyl(VI) nitrate can form complexes with various O- and N-donating unidentate ligands (L) such as H2O,1 urea derivatives,2,3 amides,4–6 alkyl phosphate,7–9 phosphine and nitrogen oxides,10–13 pyridine,14 and so on.15 The uranyl nitrate complexes with L normally have a composition of UO2(NO3)2(L)2, which shows common structural features, i.e., the uranyl ion is surrounded by four oxygen atoms from two NO3- and two donating atoms of two L in its equatorial plane. These ligands are located to make trans arrangement. The coordination geometry around the uranium atom is hexagonal bipyramidal. Recently, we have reported that N-cyclohexyl-2-pyrrolidone (NCP) can selectively precipitate UO22+ from HNO3 aqueous solution.16 From single crystal X-ray analysis of this precipitate, it was clarified that this uranyl precipitate also has the formula of UO2(NO3)2(NCP)2, and shows the characteristic structural properties of UO2(NO3)2(L)2 mentioned above.17 Performance of other 2-pyrrolidone derivatives with methyl, ethyl, n-butyl, and cyclohexylmethyl groups as the precipitating agent were also examined.18,19 As a consequence, it was suggested that the trans coordination of two bidentate NO3- to the uranyl ion is an important factor for precipitation, and that the precipitation ability of NCP is originated from its miscibility with aqueous solution and relatively high hydrophobicity. On this basis, we have proposed a new simple reprocessing process for spent nuclear fuels.20 * To whom correspondence should be addressed. E-mail: yikeda@ nr.titech.ac.jp. Phone and fax: +81-3-5734-3061. † Tokyo Institute of Technology. ‡ This author’s last name has been changed from “Mizuoka”. Present address: Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf (FZD), Germany. E-mail: [email protected]. § Japan Atomic Energy Agency. | Mitsubishi Materials Corporation.

a

Abbreviation of each NRP is in the parentheses.

Nevertheless, it is still unclear how the capability of the N-alkylated 2-pyrrolidone derivatives (NRPs) as the precipitant for the uranyl ion should be evaluated. In the previous article, we considered that distribution ratio between 1-octanol/water system (log Po/w), which is used as a measure of hydrophobicity of compounds, could be adopted to estimate the performance of NRPs as the precipitant for the uranyl ion.20 This estimation is, however, just qualitative, because it is difficult to relate the precipitation phenomenon of UO2(NO3)2(NRP)2 complexes with log Po/w values of free NRPs directly. Therefore, it is required to find out quantitative guides for design and optimization of the molecular structure of NRP as a promising precipitant for the uranyl ion. In this study, we prepared the uranyl nitrate complexes with various NRPs having C3-, C4-, and C5-alkyl chains (see Chart 1), and determined the molecular and crystal structures of these complexes by means of single crystal X-ray analysis. On the basis of the structural information of a series of UO2(NO3)2(NRP)2 complexes, we discussed how to evaluate the capability of NRPs as the precipitant for the uranyl ion from a viewpoint of the structural chemistry. Since 2-pyrrolidone (L0a,

10.1021/cg7012254 CCC: $40.75  2008 American Chemical Society Published on Web 05/30/2008

Molecular and Crystal Structures of UO2(NO3)2(NRP)2

see Chart 1) has a common skeleton of all NRPs, the molecular structure of UO2(NO3)2(L0a)2 should be the basic framework of all UO2(NO3)2(NRP)2 complexes. Therefore, we also performed preparation and the structure analysis of the uranyl nitrate complex with L0a.

2. Results and Discussion 2.1. UO2(NO3)2(NRP)2 in C3-Family (3a and 3b). The molecular and crystal structures of UO2(NO3)2(L3a)2 (3a) were characterized by means of the single crystal X-ray analysis at different temperatures (293 and 173 K). The ORTEP views of complex 3a are depicted in Figure 1. The crystallographic data and selected structural parameters of UO2(NO3)2(L3a)2 are listed in Tables 1 and 2, respectively. With decreasing temperature from 293 to 173 K, it was found that the crystal system of 3a converts from a monoclinic form to a triclinic one. This means that complex 3a shows polymorphism. Reversibility of the phase transition between these polymorphs was confirmed by recovery of the diffraction peaks in the X-ray photos at 293 K after cooling to 173 K. In the monoclinic form, the asymmetric unit contains one site for the uranium atom of 3a, while that in the triclinic form includes two sites. Nevertheless, there seem to be no significant differences in the molecular structure of 3a between these polymorphs. In both forms, the uranium atom is surrounded by two oxygen atoms at the axial position and six oxygen atoms provided from NO3- and L3a in the equatorial plane. Thus, the coordination geometry around U of 3a is hexagonal bipyramidal. The ligands in the equatorial plane are placed in trans positions as shown in Figure 1. Bond distances in the uranyl moiety (UdOyl) of 3a are around 1.76 Å, which is usual for the uranyl compounds. In the equatorial plane of 3a, the bond distances between U and O of L3a (U-OL3a) and of NO3- (U-ONO3) are

Crystal Growth & Design, Vol. 8, No. 7, 2008 2365

in the ranges of 2.38-2.39 Å and 2.50-2.54 Å, respectively. These structural properties are comparable with other UO2(NO3)2(NRP)2 complexes reported previously.6,17–19 In both forms, bond angles around O of L3a (134-137°) and flip angles of the pyrrolidone ring in L3a from the equatorial plane of the complex (73-79°) are also similar to each other. Further, all of n-propyl chains of L3a are close to anti conformation (dihedral angle δN(1)-C(5)-C(6)-C(7) ) 172.3° (monoclinic), δN(1)-C(5)-C(6)-C(7) ) 174.5° and δN(3)-C(12)-C(13)-C(14) ) 172.7° (triclinic)). The similarity between the molecular structures of 3a in the monoclinic and triclinic forms indicates that the polymorphism of this compound arises from rearrangement of the molecules in the crystal lattice. The crystal structures of 3a in the monoclinic and triclinic forms were compared. Schematic views of the crystal lattice of 3a in the monoclinic and triclinic forms are shown in Figure 2. In the monoclinic form at 293 K, the lattice constants a ) 7.089(2) Å, b ) 18.177(5) Å, c ) 8.849(2) Å, and β ) 101.95(3)° were obtained. On the other hand, those in the triclinic form at 173 K were evaluated as a ) 6.934(2) Å, b ) 9.000(3) Å, c ) 17.630 Å, R ) 89.64(3)°, β ) 83.90(3)°, γ ) 77.43(3)°. In a comparison between these constants, it is likely that a, b, and c axes in the monoclinic form correspond to a, c, and b ones in the triclinic form, respectively. Thus, R, β, and γ of the monoclinic form are converted to R, θ () 180° - γ), and β of the triclinic form, respectively, as shown in Figure 2. As described above, the molecular structures of 3a in these different crystal systems are similar to each other in spite of the difference in the number of the sites for U in the asymmetric unit. Therefore, it can be predicted that the polymorphism of 3a is mainly due to difference in a packing manner of the complexes in the crystal lattice. To examine this prediction, the packing views of 3a in the monoclinic and triclinic forms are illustrated in Figure 3. Because of the different definition of the axes, the packing view of the triclinic form along the b axis corresponds to that of the monoclinic one along the c axis. From these figures and the transition of the lattice constants, it is understandable that the polymorphism of 3a is mainly caused by the rearrangement of the molecules in the crystal lattice as predicted. With the phase transition of 3a from the monoclinic form to the triclinic one, the following changes in interplanar spacings (dhkl) and dihedral angles between the planes (δhkl-h′k′l′) specified by the Miller indices (hkl) were observed.

Figure 1. ORTEP views of UO2(NO3)2(L3a)2 (3a) in (a) monoclinic form at 293 K (30% probability level) and (b) triclinic form at 173 K (50% probability level). Hydrogen atoms are omitted for clarity.

Figure 2. Schematic views of crystal lattices of UO2(NO3)2(L3a)2 (3a) in monoclinic (black) and triclinic (red) forms.

UO2(NO3)2(L5c)2 5c

UO2(NO3)2(L5b)2 5b

UO2(NO3)2(L5a)2 5a

1316.0(8) 2 658.0(4) 173 1.778 6.220 3014 0.0221 0.0478 1.106 0.849 -0.573

1253.7(8) 2 626.9(4) 173 1.866 6.530 2859 0.0219 0.0443 1.026 0.612 -0.500

1299.3(8) 2 649.7(4) 173 1.801 6.301 2974 0.0323 0.0661 1.107 0.683 -0.958

88.21(3)

EtOH C18H34N4O10U 704.52 0.20 × 0.20 × 0.30 monoclinic P21/n (#14) 7.438(2) 15.180(6) 11.513(4)

EtOH C18H34N4O10U 704.52 0.15 × 0.15 × 0.35 triclinic P1j (#2) 7.272(2) 8.988(2) 10.467(3) 105.60(2) 102.70(2) 98.12(2) 628.0(3) 1 628.0(3) 173 1.863 6.518 2855 0.0218 0.0495 1.061 1.070 -1.156

UO2(NO3)2(L5d)2 5d


UO2(NO3)2(L3b)2 3b


UO2(NO3)2(L5e)2 5e

1134.7(7) 2 567.4(4) 173 1.980 7.210 2583 0.0161 0.0346 1.069 0.446 -0.720

100.97(3)

3 M HNO3 aq C16H30N4O10U 676.47 0.10 × 0.30 × 0.40 monoclinic P21/c (#14) 7.405(3) 18.100(5) 8.623(3)

UO2(NO3)2(L4b)2 4b


UO2(NO3)2(L5f)2 5f

2343(2) 4 586(1) 173 1.918 6.984 2683 0.0409 0.0939 1.054 0.878 -1.010

3 M HNO3 aq C16H30N4O10U 676.47 0.20 × 0.20 × 0.30 orthorhombic Pbca (#61) 10.284(6) 13.114(6) 17.371(8)

UO2(NO3)2(L4c)2 4c

750.1(4) 2 375.1(2) 173 2.498 10.879 1701 0.0187 0.0402 1.028 0.753 -0.740

90.07(3)

EtOH C8H14N4O10U 564.26 0.05 × 0.20 × 0.25 monoclinic P21/a (#14) 6.099(2) 15.420(5) 7.976(3)

UO2(NO3)2(L0a)2 0a

2312(2) 4 578(1) 173 1.943 7.076 5129 0.0326 0.0854 1.053 4.348 -0.769

CH2Cl2 C16H30N4O10U 676.47 0.10 × 0.40 × 0.40 orthorhombic Pna21 (#33) 17.364(3) 7.385(6) 18.031(9)

UO2(NO3)2(L4d)2 4d

wR ) [∑(w(Fo2 - Fc2)2)/∑(Fo2)2]1/2. c GOF ) [∑w(Fo2 - Fc2)2/(No - Nv)]1/2. Detail value of the weight (w) in each compound is given in the crystallographic information file as

100.04(3)

104.00(3)

b

EtOH C18H34N4O10U 704.52 0.22 × 0.37 × 0.42 monoclinic P21/n (#14) 10.306(3) 11.891(4) 10.905(3)

EtOH C18H34N4O10U 704.52 0.10 × 0.20 × 0.30 monoclinic P21/c (#14) 8.335(3) 14.600(5) 10.617(4)

61.17(3) 1114.2(7) 2 557.1(4) 293 1.933 7.338 2477 0.0419 0.0910 1.043 1.375 -1.126

1111.8(6) 2 555.9(3) 293 1.937 7.354 4622 0.0437 0.1121 1.051 0.820 -0.828

101.95(3)

EtOH C14H26N4O10U 648.42 0.10 × 0.20 × 0.35 monoclinic P21/a (#14) 10.002(4) 11.697(4) 10.870(5)


UO2(NO3)2(L3b)2 3b

Table 1. Crystallographic Data of UO2(NO3)2(NRP)2 Complexes UO2(NO3)2(L3a)2 3a

EtOH C14H26N4O10U 648.42 0.20 × 0.20 × 0.40 monoclinic P21 (#4) 7.089(2) 18.117(5) 8.849(2)

a R ) ∑||Fo| - |Fc||/∑|Fo|. Supporting Information.

solvent formula formula weight crystal size (mm) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å 3) Z V/Z (Å 3) T (K) Dcalcd (g · cm -3) µ (mm -1) obsd data (all) Ra (I > 2σ) wRb (all) GOFc ∆F (e- · Å-3) ∆F (e- · Å-3)

solvent formula formula weight crystal size (mm) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z V/Z (Å3) T (K) Dcalcd (g · cm -1) µ (mm -1) obsd data (all) Ra (I > 2σ) wRb (all) GOFc ∆Fmax (e- · Å-3) ∆Fmax (e- · Å-3)

UO2(NO3)2(L3a)2 3a

2366 Crystal Growth & Design, Vol. 8, No. 7, 2008 Takao et al.

a

a

UO2(NO3)2(L5c)2 5c

UO2(NO3)2(L5b)2 5b

U(1)-O(1) 1.765(2)

U(1)-O(2) 2.372(2)

U(1)-O(3) 2.532(2) U(1)-O(4) 2.513(2)

∠C(1)-O(2)-U(1) 136.7(2)

59.3

UO2(NO3)2(L5a)2 5a

U(1)-O(1) 1.763(2)

U(1)-O(2) 2.365(2)

U(1)-O(3) 2.530(3) U(1)-O(4) 2.524(3)

∠C(1)-O(2)-U(1) 139.5(2)

56.5

1.758(4) 1.756(4) 2.399(4) 2.377(4) 2.530(4) 2.531(4) 2.523(4) 2.503(5)

∠C(1)-O(2)-U(1) 135.7(4) ∠C(8)-O(7)-U(2) 135.4(4)

Bond Angles (°)

U(1)-O(1) U(2)-O(6) U(1)-O(2) U(2)-O(7) U(1)-O(3) U(1)-O(4) U(2)-O(8) U(2)-O(9)

3b

∠C(1)-O(2)-U(1) 138.3(2)

U(1)-O(3) 2.518(2) U(1)-O(4) 2.522(2)

U(1)-O(2) 2.380(2)

U(1)-O(1) 1.769(2)

UO2(NO3)2(L4b)2 4b

∠C(1)-O(2)-U(1) 138.0(2)

Bond Angles (°)

U(1)-O(9)

U(1)-O(3) 2.525(3) U(1)-O(4) 2.513(3)

U(1)-O(2) 2.369(2)

U(1)-O(1) 1.761(2)

Bond Distances (Å)

UO2(NO3)2(L5d)2 5d

68.4 86.4

72.3

1.761(3) 1.761(3) 2.389(3) 2.382(3) 2.507(3) 2.515(3) 2.522(3)

∠C(1)-O(3)-U(1) 139.7(2) ∠C(10)-O(4)-U(1) 137.9(2)

U(1)-O(1) U(1)-O(2) U(1)-O(3) U(1)-O(4) U(1)-O(5) U(1)-O(6) U(1)-O(8) 2.513(3)

UO2(NO3)2(L5e)2 5e

73.3

61.9 77.8

Flip Angles of Pyrrolidone Ring from Equatorial Plane (°)

∠C(1)-O(2)-U(1) 137.8(3)

U(1)-O(3) 2.523(4) U(1)-O(4) 2.517(4)

U(1)-O(2) 2.358(3)

U(1)-O(1) 1.757(3)

74.6

b

Bond Distances (Å)

UO2(NO3)2(L3b)2

Flip Angles of Pyrrolidone Ring from Equatorial Plane (°)

∠C(1)-O(2)-U(1) 136.9(5)

U(1)-O(3) 2.519(7) U(1)-O(4) 2.513(6)

U(1)-O(2) 2.381(5)

86.1

Monoclinic form at 293 K. b Triclinic form at 173 K.

3b

U(1)-O(1) 1.753(6)

UO2(NO3)2(L3b)2

78.6 73.7

1.755(6) 1.764(6) 2.385(5) 2.394(5) 2.501(6) 2.535(6) 2.519(6) 2.519(5)

3a

76.7 78.0


UO2(NO3)2(L3a)2

Table 2. Selected Structural Parameters in UO2(NO3)2(NRP)2

∠C(1)-O(2)-U(1) 134.7(5) ∠C(8)-O(7)-U(2) 134.0(5)

1.754(8) 1.757(8) 2.386(8) 2.387(8) 2.50(1) 2.53(1) 2.49(1) 2.51(1)

3a

b

∠C(1)-O(3)-U(1) 137.3(9) ∠C(8)-O(4)-U(1) 135.3(9)


UO2(NO3)2(L3a)2

a

71.2

∠C(1)-O(2)-U(1) 136.4(6)

U(1)-O(3) 2.517(7) U(1)-O(4) 2.514(7)

U(1)-O(2) 2.367(6)

U(1)-O(1) 1.749(6)

UO2(NO3)2(L5f)2 5f

61.6

∠C(1)-O(2)-U(1) 137.5(4)

U(1)-O(3) 2.539(5) U(1)-O(4) 2.522(5)

U(1)-O(2) 2.372(4)

U(1)-O(1) 1.764(4)

UO2(NO3)2(L4c)2 4c

75.8

∠C(1)-O(2)-U(1) 134.2(2)

U(1)-O(3) 2.502(3) U(1)-O(4) 2.501(3)

U(1)-O(2) 2.414(3)

U(1)-O(1) 1.764(3)

UO2(NO3)2(L0a)2 0a

80.3 81.4

∠C(1)-O(3)-U(1) 138.7(4) ∠C(9)-O(4)-U(1) 138.9(4)

U(1)-O(1) 1.766(4) U(1)-O(2)1.767(4) U(1)-O(3) 2.383(5) U(1)-O(4) 2.371(5) U(1)-O(5) 2.513(5) U(1)-O(6) 2.496(5) U(1)-O(8) 2.513(5) U(1)-O(9) 2.536(5)

UO2(NO3)2(L4d)2 4d

Molecular and Crystal Structures of UO2(NO3)2(NRP)2 Crystal Growth & Design, Vol. 8, No. 7, 2008 2367

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Takao et al.

Figure 3. Packing views of UO2(NO3)2(L3a)2 (3a) in monoclinic form at 293 K (left) and triclinic form at 173 K (right) along the c and b axes, respectively. Dotted circles represent the presence of the void in these crystal lattice. Hydrogen atoms are omitted for clarity. monoclinic (293 K) d100 ) 6.935 Å d010 ) 18.117 Å d001 ) 8.657 Å δ100-010 ) 90° δ100-001 ) 78.05° δ010-001 ) 90°

f f f f f f f

triclinic (173 K) d100 ) 6.729 Å d001 ) 17.528 Å d010 ) 8.783 Å δ100-001 ) 83.83° δ100-010 ) 77.40° δ101-001 ) 89.01°

dev -0.206 Å -0.5.89 Å +0.126 Å -6.17° -0.65° -0.99°

From these results, it was confirmed that the deviation of d010 of the monoclinic form to d001 of the triclinic one (-0.589 Å) is most remarkable. In δhkl-h′k′l′, the difference between δ100-010 of the monoclinic form and δ100-001 of the triclinic one (-6.17°) is most significant. Thus, the distortion from the monoclinic form to the triclinic one in 3a occurs anisotropically. From the packing views shown in Figures 3, S1, and S2 (Supporting Information), it is convincing that there is a void between the propyl chains and NO3- moiety of the neighboring molecules in the monoclinic lattice, and that this void is shrunk by closer packing of the molecules in the triclinic lattice. From the lattice constants in Table 1, d020 of the monoclinic form and d002 of the triclinic one were evaluated as 9.059 Å and 8.764 Å, respectively, that is, each void shrinks in the direction of the b axis of the monoclinic form by 0.295 Å. Furthermore, with the phase transition, each molecule of 3a was found to slide along the a and c axes of the monoclinic form by 0.947 and 0.151 Å, respectively. Consequently, it can be concluded that the shrink of the void in the b axis direction and the sliding of the molecules to the a axis direction predominantly take place in the phase transition in 3a. Molecular and crystal structures of the uranyl nitrate complex with L3b, which is the structural isomer of L3a, were also studied at different temperatures (293 and 173 K) by using the single crystal X-ray analysis. The ORTEP views of UO2(NO3)2(L3b)2 (3b) are displayed in Figure 4. The crystallographic data and selected structural parameters of 3b are summarized in Tables 1 and 2, respectively.

The similar polymorphism to 3a was also found out in 3b. The crystal system of 3b at 293 K is monoclinic, and converted to a triclinic form with decreasing temperature to 173 K. The lattice constants of the monoclinic form of 3b were evaluated as a ) 10.002(4) Å, b ) 11.697(4) Å, c ) 10.870(5) Å, and β ) 61.17(13)°, and those of the triclinic form were obtained as a ) 9.860(4) Å, b ) 10.474(3) Å, c ) 11.713(4) Å, R ) 87.60(3)°, β ) 88.11(3)°, and γ ) 62.59(3)°. The reversibility of these polymorphs was confirmed by the same method as described above. The asymmetric unit of the monoclinic form contains one site for U of complex 3b, while that in the triclinic form comprises two sites for U. The coordination geometry, structural arrangement of the ligands, bond distances and angles of 3b in both forms seem to be similar to each other, that is, hexagonalbipyramidal geometry, L3b and NO3- placed in trans positions of the equatorial plane, UdOyl (1.75-1.76 Å), U-OL3b (2.37-2.40 Å), U-ONO3 (2.50-2.54 Å), and bond angles around O of L3b (ca. 135°), respectively. These structural features are typical of those of other UO2(NO3) 2(NRP)2.6,17–19 Additionally, the terminal methyl groups of L3b are placed at the outside of the complex in each form. On the other hand, difference between the molecular structures of 3b in the monoclinic and triclinic forms can be found in the flip angles of the pyrrolidone ring of L3b from the equatorial plane of the complex. The flip angle in the monoclinic form is 86.1°, while the corresponding angles in the triclinic form are 68.4° for U(1) and 86.4° for U(2). The former is smaller than that in the monoclinic form by 17.7°, in contrast the latter is comparable with it. Thus, it can be considered that the molecular structure of 3b relates with the phase transition between the monoclinic and triclinic forms. Schematic views of the crystal lattices of 3b are depicted in Figure 5. The dhkl and dhkl-h′k′l′ values of both polymorphs of 3b were obtained as follows.



Figure 5. Schematic views of crystal lattices of UO2(NO3)2(L3b)2 (3b) in monoclinic (black) and triclinic (red) forms.

Figure 4. ORTEP views of UO2(NO3)2(L3b)2 (3b) in (a) monoclinic form at 293 K (30% probability level) and (b) triclinic form at 173 K (50% probability level). Hydrogen atoms are omitted for clarity. monoclinic (293 K) d100 ) 8.762 Å d010 ) 11.697 Å d001 ) 9.523 Å d100-010 ) 90° d100-001 ) 61.17° d010-001 ) 90°

f f f f f f f

triclinic (173 K) d100 ) 8.752 Å d001 ) 11.701 Å d010 ) 9.294 Å d100-001 ) 89.11° d100-010 ) 62.64° d010-001 ) 88.28°

dev -0.010 Å -0.004 Å +0.229 Å -0.89° +1.47° -1.72°

By using these data of dhkl and δhkl-h′k′l′, the shifts of each molecule of 3b along the a and c axes of the monoclinic form were calculated as 0.090 and 0.176 Å, respectively. To discuss the relationship between the molecular and crystal structures of 3b, the packing views of this complex in the monoclinic and triclinic forms are illustrated in Figures 6, S3, and S4 (Supporting Information). In a comparison between these packing views, the molecular structure and the spatial arrangement of 3b containing U(2) in the triclinic form seem to be quite similar to those in the monoclinic form. As mentioned above, the molecular structure of 3b with U(2) in the triclinic form resembles that in the monoclinic form even in the flip angle of the pyrrolidone ring of L3b. In contrast, the structural feature of the 3b molecule with U(1) in the triclinic form is different from that with U(2), especially in the flip angle of the pyrrolidone ring (68.4°). Nevertheless, the spacial arrangement of the UO2(NO3)2 moieties of 3b with U(1) in the triclinic lattice is very similar to that at the corresponding position of the monoclinic one. Therefore, the shift of the 3b molecules along the c axis and the rotation of the coordinated L3b in the (020) plane of the monoclinic lattice mainly occur in the phase transition between the monoclinic and triclinic forms. The significant rotation of L3b in the crystal lattice indicates that complex 3b in the (020) plane of the monoclinic form has flexibility for the modification of the molecular structure, i.e.,

Figure 6. Packing views of UO2(NO3)2(L3b)2 (3b) in monoclinic form at 293 K (upper) and triclinic form at 173 K (lower) along the directions perpendicular to the c and b axes, respectively. Hydrogen atoms are omitted for clarity.

the crystal lattice of 3b has space to allow the structural change of the complex. In summary, it was clarified that both complexes 3a and 3b have relatively large voids in their crystal lattices. Because of such a void, the molecular and/or crystal structures of these compounds are not so rigid. As a result, the phase transition between the monoclinic and triclinic forms was observed in 3a and 3b. If the voids in the crystal lattices of 3a and 3b are filled by introducing other alkyl chain instead of n-propyl or iso-propyl group and the crystal structures still remain, it is expected to


Takao et al.

Scheme 1. Alkyl Chain Lengthening and Branching in NRPs

build up more efficient packing of UO2(NO3)2(NRP)2. On this basis, we constructed a scheme for lengthening and branching of the alkyl chain of NRP (Scheme 1). In this scheme, all of isomers of NRPs with -C4H9 (C4-family) are covered. On the other hand, the NRP isomers with -C5H11 (C5-family) from commercially available primary amines are only included. 2.2. UO2(NO3)2(NRP)2 in C4-Family (4b-d). At the first step of Scheme 1, we examined the molecular and crystal structures of UO2(NO3)2(NRP)2 in C4-family. Previously, the structure analysis of UO2(NO3)2(L4a)2 (4a) has been reported.19 Hence, in the present study, we prepared the uranyl nitrate complexes, UO2(NO3)2(L4b)2 (4b), UO2(NO3)2(L4c)2 (4c), and UO2(NO3)2(L4d)2 (4d), and analyzed their structures by using the single crystal X-ray analysis. The ORTEP views of these complexes are shown in Figure 7. The crystallographic data and selected structural parameters are listed in Tables 1 and 2, respectively. It should be noted that any polymorphism such as complexes 3a and 3b was not observed for 4b, 4c, and 4d in the range from 173 K to room temperature. The molecular structures of 4b-d are quite similar to each other and show the features common to other UO2(NO3)2(NRP)2 complexes,6,17–19 i.e., the hexagonal-bipyramidal coordination geometry around U, the trans arrangements of NRP and NO3in the equatorial plane, UdOyl (ca. 1.76 Å), U-ONRP (2.36-2.37 Å), U-ONO3 2.51-2.54 Å), and the bond angles between U-ONRP bond and the carbonyl group of L4b-L4d (ca. 138°). On the other hand, differences in the flip angles of the pyrrolidone rings of the coordinated L4b-L4d are found to be 73.3° for 4b, 61.6° for 4c, and 81.1° and 82.0° for 4d. In the molecular structure of 4b (Figure 7(a)), the main propyl chain (C(5)-C(6)-C(7)) of the coordinated L4b has anti conformation (δN(1)-C(5)-C(6)-C(7) ) 176.3°). This is similar to that of 3a shown in Figure 1. The additional C(8) is attached on C(6) at the outside position of the complex. The crystal system of 4b is monoclinic, and its lattice constants are a ) 7.405(3) Å, b ) 18.100(5) Å, c ) 8.623 Å, and β ) 100.97(3)°. Surprisingly, these crystallographic data of 4b are quite similar to those of 3a in the monoclinic form. These constants of 4b differ from 3a only by +0.316(5) Å in the a axis, -0.02(1) Å in the b axis, -0.226(5) Å in the c axis, and -0.98(6)° in the β angle. To discuss how the branch expansion from n-propyl of L3a to iso-butyl of L4b influences the crystal structures of 3a and 4b, the packing views of 4b are displayed in Figures 8 and S5 (Supporting Information). Comparing Figure 8 with the corresponding view of the monoclinic 3a (left one of Figure 3), it was found that the molecular structure and the spatial arrangement of the UO2(NO3)2(L3a)2 skeleton in 4b are almost consistent with those of the monoclinic 3a. This means that the crystal structure of the monoclinic 3a still remains in that of 4b. It must be noted that the additional C(8) branch of 4b plays an important role to fill the voids in the monoclinic 3a without any large distortion or destruction of its fundamental crystal

Figure 7. ORTEP views of (a) UO2(NO3)2(L4b)2 (4b), (b) mesoUO2(NO3)2(L4c)2 (4c) and (c) UO2(NO3)2(L4d)2 (4d) at the 50% probability level. Hydrogen atoms are omitted for clarity.

structure. In Figure 9, the packing view of 4b along the a axis is compared with that of the monoclinic 3a. It is clearly found that the additional methyl group with C(8) of 4b fits into the voids in the crystal lattice of the monoclinic 3b. As seen from Scheme 1, L4c shows optical isomerism due to the sec-butyl group on N. In this study, the racemic compound of L4c was used to prepare complex 4c. As a consequence, the obtained crystal of 4c contains both R- and S- isomers of L4c in the 4c molecule as shown in Figure 7(b). Thus, complex 4c has a meso form. We attempted the X-ray structure analyses for the several crystals of 4c. As a result, all of crystals were the meso compound. Hence, it seems that 4c prefers the meso form rather than (R, R) and (S, S). Since the L4c ligand arises from both L3a and L3b, it was expected that the crystal structure of 4c is similar to that of 3a or 3b. However, the crystal system and the lattice constants of 4c (orthorhombic, a ) 10.284(6) Å, b ) 13.114(6) Å, and c ) 17.371(8) Å) are completely different from those of 3a and 3b as shown in Table 1. Another pyrrolidone derivative with tert-butyl group, L4d, is induced by branching from the secondary carbon of iso-propyl group in L3b. Therefore, complex 4d was expected to reproduce the crystal structure of 3b. From Figure 7(c) and Table 2, the structural properties of the 4d molecule are almost similar to those of 3b and other UO2(NO3)2(NRP)2. However, the positions of the terminal methyl groups in 4d do not match with those in 3b. Therefore, the molecular skeleton of 3b does not remain in 4d. Additionally, the crystal system and the lattice constants of


Figure 8. Packing view of UO2(NO3)2(L4b)2 (4b) along the c axis. Hydrogen atoms are omitted for clarity.

4d (orthorhombic, a ) 17.364(3) Å, b ) 7.385(6) Å, and c ) 18.031(9) Å) are not comparable with those of 3b. N-n-Butyl-2-pyrrolidone (L4a) is the alternative NRP in the C4-family, and derived by lengthening of the n-propyl chain of L3a to n-butyl. In our previous article,19 the crystal system and lattice constants of UO2(NO3)2(L4a)2 (4a) at 117 K are reported to be triclinic, a ) 7.601(1) Å, b ) 8.188(2) Å, c ) 10.030 Å, R ) 72.679(9)°, β ) 77.43(1)°, and γ ) 78.28(28)°, respectively, which are also different from those of 3a. In addition, the packing manner of 4a cannot be compared with that of 3a and 3b. Consequently, it was confirmed that the uranyl nitrate complexes with L4a, L4c, and L4d do not reproduce the crystal


structures of 3a and 3b. This may imply that the alkyl chains in L4a, L4c, and L4d do not fill the voids in the crystal structures of 3a and 3b, and/or complexes 4a, 4c, and 4d are stabilized more largely in their own crystal structures rather than those of 3a and 3b. 2.3. UO2(NO3) 2(NRP)2 in the C5-Family (5a-f). At the second step of Scheme 1, NRPs with the C5 alkyl chain, L5a-L5f were also prepared, and the structural characterization for the UO2(NO3)2(NRP)2 complexes in the C5-family was performed by means of the single crystal X-ray analysis. The ORTEP views of UO2(NO3)2(L5a)2 (5a), UO2(NO3)2(L5b)2 (5b), UO2(NO3)2(L5c)2 (5c), UO2(NO3)2(L5d)2 (5d), UO2(NO3)2(L5e)2 (5e), and UO2(NO3)2(L5f)2 (5f) are depicted in Figure 10(a)-(f), respectively. The crystallographic data and selected structural parameters of these complexes are summarized in Tables 1 and 2, respectively. In these complexes, any polymorphism as found in complexes 3a and 3b was not observed in the range from 173 K to room temperature. The molecular structures of 5a-f also show the common properties to other UO2(NO3)2(NRP)2, i.e., the hexagonal bipyramidal coordination geometry around U, the trans arrangements of NRP and NO3- in the equatorial plane, UdOyl (ca. 1.76 Å), U-ONRP (2.35-2.39 Å), U-ONO3 (2.50-2.53 Å), and the bond angles between U-O NRP bond and the carbonyl group of L5a-L5f (ca. 138°). The ligand L5a is derived from L3a via L4a as shown in Scheme 1. In the molecular structure of 5a (Figure 10(a)), the main butyl chain (N(1)-C(5)-C(6)-C(7)-C(9)) of L5a has anti conformation (δN(1)-C(5)-C(6)-C(7) ) 179.6°, δC(5)-C(6)-C(7)-C(9) ) 176.7°). This is similar to 4a reported previously.19 The additional C(8) on C(7) is placed at the inner side of the molecule of 5a. The crystal system of 5a is monoclinic, and its lattice constants are a ) 8.335(3) Å, b ) 14.600(5) Å, c ) 10.617(4) Å, and β ) 104.00(3)°. These parameters are not similar to those of 3a and 4a. As seen from Figure 10(b), the alkyl group of L5b with the branches C(7) and C(9) on C(6) and C(8) shows a “W”-shape. The molecular structure of 5b can be regarded to be based on that of 3b. The crystal system and the lattice constants of 5b are monoclinic, a ) 10.306(3)Å, b ) 11.891(4) Å, c ) 10.905(3) Å, and β ) 100.04(3)°. These parameters of 5b are comparable with those of the monoclinic 3b, except for β. However, the packing manner of molecules in 5b does not

Figure 9. Packing views of monoclinic 3a (left) and 4b (right) drawn by van der Waals radii along the a axis. Carbon atoms in pyrrolidone ring and additional methyl group of 4b are represented by purple and green spheres, respectively.


Takao et al.

Figure 10. ORTEP views of (a) UO2(NO3)2(L5a)2 (5a), (b) UO2(NO3)2(L5b)2 (5b), (c) meso-UO2(NO3)2(L5c)2 (5c), (d) UO2(NO3)2(L5d)2 (5d), (e) UO2(NO3)2(L5e)2 (5e) and (f) meso-UO2(NO3)2(L5f)2 (5f) at the 50% probability level. Hydrogen atoms are omitted for clarity.

reproduce that in 3b as shown in Figure S6 (Supporting Information). The pyrrolidone derivative with the (1-methyl)butyl group (L5c) contains an asymmetric carbon, C(5) of 5c in Figure 10(c). In the preparation of complex 5c, the racemic L5c was used. The obtained crystals of 5c were in a meso form. In the molecular structure of complex 5c, the conformation of the main butyl chain (C(5)-C(6)-C(7)-C(8)) is gauche for N(1)-C(5)-C(6)-C(7) (δ ) 56.2°) and anti for C(5)-C(6)-C(7)-C(8) (δ ) 178.2°). The crystal system and the lattice constants of 5c are monoclinic, a ) 7.438(2) Å, b ) 15.180(6) Å, c ) 11.513(4) Å, and β ) 88.21(3)°, which are not similar to those of 3a and 3b. The molecular structure of 5d (Figure 10(d)) seems to be similar to that of 4d (Figure 7(c)). The arrangement of C(6), C(8), and C(9) traces that of the tert-butyl group in 4d. The additional C(7) is attached on C(6), and placed to form a slightly distorted gauche conformation for C(8)-C(5)-C(6)-C(7) (δ ) 70.4°). The crystal system and the lattice constants of 5d are

triclinic, a ) 7.272(2) Å, b ) 8.988(2) Å, c ) 10.467(3) Å, R ) 105.60(2)°, β ) 102.70(2)°, and γ ) 98.12(2)°, which are different from those of 3b. In the molecular structure of 5e (Figure 10(e)), there is no inversion center. The main propyl chains C(5)-C(6)-C(7) and C(14)-C(15)-C(18) show anti conformation (δN(1)-C(5)-C(6)-C(7) ) 179.0°, δN(2)-C(14)-C(15)-C(18) ) 177.2°). Both terminal tertbutyl groups in 5e face in the same direction. The crystal system and the lattice constants of 5e are triclinic, a ) 9.764(3) Å, b ) 10.170(2) Å, c ) 14.481(5) Å, R ) 100.12(2)°, β ) 98.90(2)°, and γ ) 112.86(2)°, which are not comparable with those of 3a. The ligand L5f can be derived from both L3a and L3b through L4b or L4c, and has an asymmetric carbon, which corresponds to C(5) of 5f in Figure 10(f). When the racemic L5f was used in the preparation of 5f, the obtained compound was in a meso form. The main propyl chain of the 1,2-dimethylpropyl group on N shows a slightly distorted anti conformation (δN(1)-C(5)-C(6)-C(7) ) 166.6°).



Figure 11. ORTEP view of UO2(NO3)2(L0a)2 (0a) at the 50% probability level.

Figure 12. Dependence of V/Z on Nc. Table 3. Compactness Parameters (Cp) of UO2(NO3)2(NRP)2 complex 0a 1a 2a 3ab,c 3ad 3bb,c 3bd

Cp/Å3a 27.9 27.4 28.7 26.4 28.9 26.9

ref this 18 18 this this this this

work work work work work

complex

Cp/Å3a

4a 4b 4c 4d 5a 5b 5c 5d 5e 5f

25.0 24.0 26.3 25.4 25.2 28.3 27.5 25.3 25.7 24.9

ref 19 this this this this this this this this this

work work work work work work work work work

a Calculated by eq 1. b Monoclinic form at 293 K. c Calculated using V0a/Z0a, at 293 K [383.9(1) Å3, ref 23]. d Triclinic form at 173 K.

The shape of a butyl chain C(8)-C(5)-C(6)-C(9) is very similar to that in UO2(NO3)2(L4c)2 (Figure 7(b)). The crystal system and the lattice constants of 5f are triclinic, a ) 7.001(2) Å, b ) 9.841(3) Å, c ) 10.152(3) Å, R ) 109.64(2)°, β ) 102.15(2)°, and γ ) 98.93(3)°, which are not consistent with those of 3a and 3b. As a consequence, it was clarified that the uranyl nitrate complexes with L5a-L5f cannot reproduce the crystal structures of 3a and 3b. This indicates that the alkyl chains in L5a-L5f

are too large to fill the voids found in 3a and 3b, and/or complexes 5a-f prefer their own crystal structures. 2.4. Packing Efficiency of UO2(NO3)2(NRP)2 in Crystal Lattice. In the UO2(NO3)2(NRP)2 complexes studied above, only complex 4b keeps the crystal structure of the monoclinic 3a. Therefore, it is likely that complex 4b forms the most efficient packing in the UO2(NO3)2(NRP)2 complexes studied here. However, this conclusion is qualitative. It is necessary to evaluate the packing efficiency of UO2(NO3)2(NRP)2 in its crystal lattice quantitatively. It is clear that all NRP ligands have a basic framework of 2-pyrrolidone (L0a). Thus, a molecular structure of UO2(NO3)2(L0a)2 (0a) would be a common molecular skeleton to all UO2(NO3)2(NRP)2 complexes. In this section, we first performed the structure analysis of complex 0a using the single crystal X-ray diffraction method, and then estimated the packing efficiency of each UO2(NO3)2(NRP)2. The ORTEP view of complex 0a is shown in Figure 11. The crystallographic data and the selected structural parameters of 0a are listed in Tables 1 and 2, respectively. As shown in Figure 11 and Table 2, the structural properties of the 0a molecule (Figure 11) are also typical of a series of UO2(NO3)2(NRP)2,6,17–19 that is, the hexagonal-bipyramidal geometry around U, the trans arrangements of L0a and NO3- in the equatorial plane, UdOyl (1.764(3) Å), U-OL0a (2.414(3) Å), U-ONO3 (mean 2.50 Å), ∠C(1)-O(2)-U(1) (134.2(2)°), and the flip angle of the pyrrolidone ring from the equatorial plane (75.8°). In addition, hydrogen bonds and short contacts between N(1)-H(1) and oxygen atoms in the neighboring molecule [O(1), O(2), and O(4)] were found. Their geometric parameters are shown in Table S1 in Supporting Information. As shown in Figure 11, it was confirmed that the molecular structure of 0a is the basic skeleton of all UO2(NO3)2(NRP)2 as predicted above. Furthermore, from the packing views of 0a along all axes (Figure S7 in Supporting Information), it is acceptable that there are no significant voids in the crystal structure of 0a. Assuming that the voids due to the molecular skeleton of 0a are similar among all UO2(NO3)2(NRP)2 complexes, the difference between the volume of UO2(NO3)2(NRP)2 and 0a should arise from the alkyl chain length of NRP and packing efficiency of the complex. On this basis, 0a is used as the reference in the estimation of the packing efficiency of UO2(NO3)2(NRP)2. Intermolecular interaction may also affect the packing of the UO2(NO3)2(NRP)2 molecules. The short contacts between the molecules in 3a, 3b, 4b-d, 5a-f, and 0a were examined. As a result, the short contacts supposed to be meaningful21 were found in 3a at 173 K [C(2)-H(2A) · · · O(1)], 5a [C(5)-H(5B) · · · O(3)], 5b [C(4)-H(4A) · · · O(5)], 5c [C(2)-H(2B) · · · O(1)], 5d [C(4)-H(4A) · · · O(5)], and 5e [C(11)H(11B) · · · O(1)]. The geometric parameters of these short contacts are summarized in Table S1. It is known that Oyl of the uranyl ion have very weak Lewis-basic character.22 Furthermore, partial negative charge would occur on the coordinating and terminal O of NO3-. On the other hand, the H atoms involved in the above contacts are in the methylene groups adjacent to the amido group of NRP, and hence, these H atoms would be activated to have positive partial charge. Since the H · · · O distances and the C-H · · · O angles of these short contacts listed in Table S1 are comparable with those of reported C-H · · · O hydrogen bonds,21 these short contacts might be regarded as weak C-H · · · O hydrogen bonds. In the crystal structure of each compound, no significant interactions other than the weak C-H · · · O hydrogen bonds were, however, found


Takao et al.

out. Hence, it is unlikely that the packing of UO2(NO3)2(NRP)2 is remarkably affected by the short contacts between the molecules. Mean volume occupied by a UO2(NO3)2(NRP)2 molecule in the crystal lattice can be expressed by V/Z, which is listed in Table 1. In Figure 12, V/Z of 0a, UO2(NO3)2(L1a)2 (1a, R ) Me), UO2(NO3)2(L2a)2 (2a, R ) Et), 3a, 3b, 4a-d and 5a-f were plotted against the number of carbon atoms in the alkyl chain of NRP (NC). As a result, the value of V/Z certainly increases with an increase in NC. Particularly, V/Z linearly increases in 0 e NC e 2. On the other hand, in NC g 3, there seem to be differences in V/Z between the UO2(NO3)2(NRP)2 isomers of each NC. The variation of V/Z should be caused by the difference in the packing efficiency of each UO2(NO3)2(NRP)2. To evaluate the packing efficiency of UO2(NO3)2(NRP)2, we propose “compactness parameter” (Cp) defined by the following equation.

Cp )

V ⁄ Z - V0a ⁄ Z0a 2NC

(1)

where V0a/Z0a denotes V/Z of complex 0a, and is equal to 375.1(2) Å3 at 173 K and 383.9 Å3 at 293 K.23 The division of (V/Z - V0a/Z0a) by 2NC normalizes the dependence of V/Z on NC. Hence, Cp corresponds to the mean volume occupied by one carbon atom of an alkyl chain of NRP. One should note that contribution of the voids in the crystal lattice is also included in Cp. If the packing efficiency of all UO2(NO3)2(NRP)2 is same, no difference in Cp will be detected. On the other hand, if the packing of UO2(NO3)2(NRP)2 is more efficient than others, Cp will decrease, and vice versa. Thus, Cp can be used as a measure of the packing efficiency of UO2(NO3)2(NRP)2. The calculated Cp values of UO2(NO3)2(NRP)2 are summarized in Table 3. In this table, the Cp values of 1a and 2a are 27.9 Å2 and 27.4 Å3, respectively, and are almost constant. Hence, their average value, Cp ) 27.6 Å3, can be adopted as a criterion of the packing efficiency of other UO2(NO3)2(NRP)2, i.e., if Cp of UO2(NO3)2(NRP)2 is smaller than 27.6 Å 3, the packing of the complex of interest is more efficient, and vice versa. For the C3-family, the Cp values of 3a (28.7 Å3) and 3b (28.9 3 Å ) in the monoclinic forms are larger than 27.6 Å 3, indicating their loose packing. Actually, the relatively large voids in the crystal lattice and the flexibility of the molecular structure were found in these compounds. With the phase transition from the monoclinic form to the triclinic one, the Cp values of both compounds decrease (3a, 26.4 Å3; 3b, 26.9 Å3). Such a decrease in Cp is related to the shrink of the voids in the crystal lattice as observed in Figures 3 and 6. All of the Cp values of UO2(NO3)2(NRP)2 in the C4-family are smaller than 27.6 Å3 (4b < 4a CdO). N- (()sec-Butyl-2-pyrrolidone (L4c). 1H NMR (CDCl3, TMS, ppm): 0.76 (t, 3H, N-CH(CH3)CH2CH3), 1.03 (d, 3H, N-CH(CH3)CH2CH3), 1.39 (quintet, 2H, N-CH(CH3)CH2CH3), 1.92 (quintet, 2H, 4-CH2), 2.33 (t × d, 2H, 5-CH2), 3.21 (m, 2H, 3-CH2), 4.04 (sextet, 1H, N-CH(CH3)CH2CH3). 13C NMR (CDCl3, TMS, ppm): 11.03, 17.92, 18.19, 27.00, 31.67, 41.76, 48.16, 174.80 (>CdO). N-tert-Butyl-2-pyrrolidone (L4d). 1H NMR (CDCl3, TMS, ppm): 1.31 (s, 9H, N-C(CH3)3), 1.84 (quintet, 2H, 4-CH2), 2.25 (t, 2H, 5-CH2), 3.38 (t, 2H, 3-CH2). 13C NMR (CDCl3, TMS, ppm): 18.00, 27.73, 33.23, 45.95, 53.84, 175.51 (>CdO). N-iso-Amyl-2-pyrrolidone (L5a). 1H NMR (CDCl3, TMS, ppm): 0.93 (d, 6H, N-CH2CH2CH(CH3)2), 1.40 (quartet, 2H, N-CH2CH2CH(CH3)2), 1.57 (septet, 1H, N-CH2CH2CH(CH3)2), 2.02 (quintet, 2H, 4-CH2), 2.40 (t, 2H, 5-CH2), 3.30 (t, 2H, N-CH2CH2CH(CH3)2), 3.38 (t, 2H, 3-CH2). 13C NMR (CDCl3, TMS, ppm): 17.98, 22.56, 26.00, 31.21, 36.08, 41.01, 47.14, 174.94 (>CdO). N-(1-Ethyl)propyl-2-pyrrolidone (L5b). 1H NMR (CDCl3, TMS, ppm): 0.78 (t, 6H, N-CH(CH2CH3)2), 1.41 (m, 4H, N-CH(CH2CH3)2), 1.95 (quintet, 2H, 4-CH2), 2.37 (t, 2H, 5-CH2), 3.16 (t, 2H, 3-CH2), 3.84 (septet, 1H, N-CH(CH2CH3)2). 13C NMR (CDCl3, TMS, ppm): 10.90, 18.38, 25.30, 31.64, 41.69, 54.19, 175.57 (>CdO). N-(()(1-Methyl)butyl-2-pyrrolidone (L5c). 1H NMR (CDCl3, TMS, ppm): 0.77 (t, 3H, N-CH(CH3)CH2CH2CH3), 0.97 (d, 3H, N-CH(CH3)CH2CH2CH3), 1.12 (m, 2H, N-CH(CH3)CH2CH2CH3), 1.28 (m 2H, N-CH(CH3)CH2CH2CH3), 1.86 (quintet, 2H, 4-CH2), 2.25 (t × d, 2H, 5-CH2), 3.15 (m, 2H, 3-CH2), 4.08 (m, 1H, N-CH(CH3)CH2CH2CH3). 13C NMR (CDCl3, TMS, ppm): 13.87, 18.11, 18.13, 19.64, 31.59, 36.09, 41.70, 46.22, 174.34 (>CdO). N-tert-Amyl-2-pyrrolidone (L5d). 1H NMR (CDCl2, TMS, ppm): 0.77 (t, 3H, N-C(CH3)2CH2CH3), 1.28 (s, 6H, N-C(CH3)2CH2CH3), 1.81 (quartet, 2H, N-C(CH3)2CH2CH3), 1.86 (quintet, 2H, 4-CH2), 2.29 (t, 2H, 5-CH2), 3.38 (t, 2H, 3-CH2). 13C NMR (CDCl3, TMS, ppm): 8.74, 18.26, 25.79, 31.81, 33.27, 46.78, 56.88, 175.42 (>CdO). N-Neopentyl-2-pyrrolidone (L5e). 1H NMR (CDCl3, TMS, ppm): 0.91 (s, 9H, N-CH2C(CH3)3), 1.97 (quintet, 2H, 4-CH2), 2.35 (t, 2H, 5-CH2), 3.02 (s, 2H, N-CH2C(CH3)3), 3.45 (t, 2H, 3-CH2). 13C NMR (CDCl3, TMS, ppm): 18.56, 28.38, 30.95, 33.74, 50.69, 55.23, 176.02 (>CdO). N-(()(1,2-Dimethyl)prolyl-2-pyrrolidone (L5f). 1H NMR (CDCl3, TMS, ppm): 0.84, 0.95 (d, 3H × 2, N-CH(CH3)CH(CH3)2), 1.12 (d, 3H, N-CH(CH3)CH(CH3)2), 1.66 (m, 1H, N-CH(CH3)CH(CH3)2), 2.00 (quintet, 2H, 4-CH2), 2.39 (m, 2H, 5-CH2), 3.29 (m, 2H, 3-CH3), 3.82 (m, 1H, N-CH(CH3)CH(CH3)2). 13C NMR (CDCl3, TMS, ppm): 16.22, 18.36, 19.79, 19.96, 31.56, 42.40, 52.75, 174.76 (>CdO). Preparation of UO2(NO3)2(NRP)2 Complexes. A stock solution of UO22+ (1.5 M) was prepared by dissolving UO2(NO3)2 · 6H2O in 3 M nitric acid solution. After addition of 2 equiv of NRP (except for L0a) to this stock solution (1 mL) with vigorous stirring, yellowish powder of UO2(NO3)2(NRP)2 precipitated. The resulting precipitate in each sample was filtered off, and washed with a small amount of water and ethanol. For the single crystal X-ray analysis, the UO2(NO3)2(NRP)2 precipitates were recrystallized from ethanol or dichloromethane. In the cases of L4b and L4c, their UO2(NO3)2(NRP)2 crystals grew up by storing the precipitates in the 3 M HNO3 aq mother liquors for 2 days and 1 month, respectively, and these crystals were directly used in the structural characterization. The crystals of UO2(NO3)2(L0a)2 were prepared from a mixture of UO2(NO3)2 · 6H2O and L0a (1:2 mixing ratio) in ethanol, followed by slow evaporation of the solvent. The obtained

Crystal Growth & Design, Vol. 8, No. 7, 2008 2375 crystals of UO2(NO3)2(NRP)2 were characterized by using single crystal X-ray analysis, IR and Raman spectroscopies. Characterization of UO2(NO3)2(NRP)2. Characterizations of the uranyl nitrate complexes with NRP were performed by means of single crystal X-ray diffraction (Rigaku RAXIS RAPID), IR (SHIMADZU FTIR-8400S) and Raman (JASCO RMP-200) spectroscopies. Single crystal X-ray analyses for the crystals of the uranyl nitrate complexes with NRP were performed by the following procedure. A single crystal of each complex was mounted on a glass fiber, and put into a temperature-controlled nitrogen gas flow. Intensity data were collected by using imaging plate area detector in Rigaku RAXIS RAPID with graphite monochromated Mo KR radiation (λ ) 0.71075 Å). The structures of the uranyl complexes were solved by direct (SIR 92,25 SIR 9726) or heavy atom Patterson methods27 and expanded using Fourier techniques.28 A numerical absorption correction was applied which resulted in transmission factors described in the crystallographic information file of each compound.29 All non-hydrogen atoms were anisotropically refined by SHELXL-97.30 Hydrogen atoms were refined as riding on their parent atoms with Uiso(H) ) 1.2Ueq(C, N). The final cycle of full-matrix least-squares refinement on F2 was based on observed reflections and parameters, and converged with unweighted and weighted agreement facters, R and wR. In the final Fourier map, relatively large peaks remained in 3a at 173 K (3.177 e- Å-3), 4d (4.348 e- Å-3), and 5f (2.899 e- Å-3). Since these peaks were found at the position close to the uranium atom, they are the ghost peaks in the Fourier map. All calculations were performed by the CrystalStructure crystallographic software package.31 Crystal data and other data collection parameters are summarized in Table 1. The interplanar spacings dhkl and the angles between the planes δhkl-h′k′l′ were calculated by Mercury 1.4.2 software.32 IR Data (KBr, cm-1). UO2(NO3)2(L3a)2: 930 (OdUdO asymmetric stretching, ν3), 1615 (CdO stretching in NRP, νCdO). UO2(NO3)2(L3b)2: 933 (ν3), 1607 (νCdO). UO2(NO3)2(L4b)2: 930 (ν3), 1615 (νCdO). UO2(NO3)2(L4c)2: 938 (ν3), 1606 (νCdO). UO2(NO3)2(L4d)2: 932 (ν3), 1608 (νCdO). UO2(NO3)2(L5a)2: 932 (ν3), 1616 (νCdO). UO2(NO3)2(L5b)2: 931 (ν3), 1612 (νCdO). UO3(NO3)2(L5c)2: 930 (ν3), 1609 (νCdO). UO2(NO3)2(L5d)2: 931 (ν3), 1608 (νCdO). UO2(NO3)2(L5e)2: 931 (ν3), 1616 (νCdO). UO2(NO3)2(L5f)2: 932 (ν3), 1610 (νCdO); UO2 (NO2)2 (L0a)2: 931 (ν3), 1644 (νCdO). Raman Data. (OdUdO symmetric stretching (ν1), Ex: 532 nm, cm -1.) UO2(NO3)2(L3a)2: 855. UO2(NO3)2(L3b)2: 855. UO2(NO3)2(L4b)2: 850. UO2(NO3)2(L4c)2: 856. UO2(NO3)2(L4d)2: 854. UO2(NO3)2(L5a)2: 853. UO2(NO3)2(L5b)2: 852. UO2(NO3)2(L5c)2: 853. UO2(NO3)2(L5d)2: 854. UO2(NO3)2(L5e)2: 854. UO2(NO3)2(L5f)2: 855. UO2(NO3)2(L0a)2: 858.

Acknowledgment. We thank Dr. Shinobu Takao for stimulated discussion about the polymorphism of UO2(NO3)2(NRP)2 complexes. Present study is the result of “Development of Advanced Reprocessing Systen 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 monoclinic 3a, triclinic 3a, monoclinic 3b, triclinic 3b, 4b, 5b, and 0a, table of geometric parameters of hydrogen bonds and short contacts, and crystallographic information files of 3a (both monoclinic and triclinic), 3b (both monoclinic and triclinic), 4b-d, 5a-f, and 0a. This material is available free of charge via the Internet at http://pubs.acs.org.

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Takao et al. (23) In the calculation of Cp, V0a/Z0a at the same temperature with a sample should be used as the reference. Since the structure analysis for the monoclinic crystals of 3a and 3b were performed at 293 K, the single crystal X-ray analysis for 0a was also carried out at the same temperature. Crystallographic data for 0a at 293 K. C8H14N4O10U, M ) 564.26, crystal size 0.05 × 0.15 × 0.18 mm, monoclinic, P21/a (#14), a ) 6.2047(9), b ) 15.540(2), c ) 7.964(1) Å, β ) 90.703(3)°, V ) 767.8(2) Å3, Z ) 2, Dcalcd ) 2.441 g · cm-3, F000 ) 524, µ(Mo KR) ) 10.629 mm-1, 7182 reflections collected, 956 unique [I > 2σ(I)], R value 0.0422, wR ) 0.0688 (all data R ) 0.0940, wR ) 0.0819), GOF ) 0.884. (24) Wang, E. C.; Lin, H.-J. Heterocycles 1998, 48, 481–489. (25) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343–350. (26) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (27) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; GarciaGranda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. PATTY; The DIRDIF program system, Technical Report of the Crystallography Laboratory; University of Nijmegen: The Netherlands, 1992. (28) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Gelder, de R.; Israel, R.; Smits, J. M. M. DIRDIF99; The DIRDIF-99 program system, Technical Report of the Crystallography Laboratory; University of Nijmegen: The Netherlands, 1999. (29) Higashi, T. NUMABS; Rigaku Corporation: Tokyo, Japan, 1999. (30) Sheldrick, G. M. SHELXL-97; Program for Crystal Structure Refinement; University of Go¨ttingen: Germany, 1997. (31) CrystalStructure 3.10; Crystal Structure Analysis Package; Rigaku and Rigaku/MSC; 2000-2002. (32) Mercury Molecular Graphics Program, version 1.4.2; Cambridge Crystallographic Data Centre: Cambridge, U.K., 2007.

CG7012254

Molecular and Crystal Structures of Uranyl Nitrate Complexes with N

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