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Correlation between intermolecular hydrogen bonds and melting points of uranyl nitrate complexes with cyclic urea derivatives
Polyhedron 96 (2015) 102–106
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Correlation between intermolecular hydrogen bonds and melting points of uranyl nitrate complexes with cyclic urea derivatives Tomoya Suzuki a,1, Koichiro Takao a, Takeshi Kawasaki a, Masayuki Harada a, Masanobu Nogami b, Yasuhisa Ikeda a,⇑ a b
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Department of Electric and Electronic Engineering, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashiosaka City, Osaka 577-8502, Japan
a r t i c l e
i n f o
Article history: Received 26 January 2015 Accepted 29 April 2015 Available online 5 May 2015 Keywords: Uranyl nitrate complex Cyclic urea derivatives Melting point Hydrogen bond Crystal structure
a b s t r a c t To clarify requirements for forming UO2(NO3)2(CU)2 (CU: cyclic urea derivative) and UO2(NO3)2(NRP)2 (NRP: N-alkylated pyrrolidone derivative) with high melting points (mps), molecular and crystal structures of UO2(NO3)2(0a)2 (0a: 2-imidazolidone), UO2(NO3)2(0b)2 (0b: tetrahydro-2-pyrimidone) and UO2(NO3)2(1a)2 (1a: 1-methyl-2-imidazolidone) were determined by means of single crystal X-ray analysis. Melting points (mps) of these complexes were compared with each other and those of other UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2. As a result, the uranyl nitrate complexes with 0a, 0b and 1a exhibit typical structural properties of UO2(NO3)2L2 (L: unidentate ligand); i.e., hexagonal bipyramidal coordination geometry, and two bidentate NO3 and two CUs located at trans positions in an equatorial plane of UO2+ 2 ion. Several intermolecular N–H O hydrogen bonds (HBs) were found in the crystal structures of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. In contrast, no intermolecular C–H O HBs were formed in these crystal structures. The intermolecular HBs were discussed in connection with mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 including those reported previously. The mp tends to increase as the number of N–H O HBs per molecule increases, whereas effects of C–H O HBs on the mps is not very significant. These results suggest that the intermolecular N–H O HBs strongly contribute to form a uranyl nitrate complex with high mps. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Uranyl nitrate complexes with unidentate ligands (L) such as H2O [1], ureas [2,3], amides [4–6], alkyl phosphates and phosphine oxides [7–12] generally have a composition, UO2(NO3)2L2, which exhibits hexagonal bipyramidal coordination geometry around U. Four oxygen atoms from two bidentate NO3 and two donating atoms from Ls are bound to U at the trans positions in the equatorial plane of the UO2+ 2 ion. This general formula is commonly found in both fundamental chemistry of uranium and nuclear engineering processes like spent nuclear fuel reprocessing [13–20]. In our previous study, we have found that N-cyclohexyl-2pyrolidone (NCP) selectively precipitates UO2+ from a HNO3 2 aqueous solution [21]. From the single crystal X-ray analysis, the ⇑ Corresponding author. Tel.: +81 3 5734 3061. E-mail addresses: [email protected] (T. Suzuki), [email protected] (Y. Ikeda). 1 Present address: Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. http://dx.doi.org/10.1016/j.poly.2015.04.034 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
chemical formula of precipitates was found to be UO2(NO3)2(NCP)2 with the typical structural properties of UO2(NO3)2L2 mentioned above [22]. On the basis of UO2+ 2 precipitation ability of NCP, we proposed a reprocessing method for the spent nuclear fuels produced from fast breeder reactors [23–26], and assessed various pyrrolidone derivatives (NRP) as UO2+ 2 precipitants in this process. As a result, it was clarified that the UO2+ 2 precipitation ability depends on hydrophobicity of NRP and packing efficiency of UO2(NO3)2(NRP)2 in the crystal structures [25,26]. In our recent works [24,27,28], hydrophilic cyclic urea derivatives (CUs, Fig. 1) such as 2-imidazolidone (0a), tetrahydro2-pyrimidone (0b), 1,3-dimethyl-2-imidazolidone (2a), and 1,3dimethyl-3,4,5,6-tetrahydro-2-pyrimidone (2b) were also found 2+ to precipitate UO2+ 2 from the HNO3 solution. Especially, the UO2 precipitation abilities of 0a and 0b are comparable to that of NCP in spite of their low hydrophobicity. Comparing precipitation behavior of UO2+ 2 through addition of CUs and NRPs together with other physical traits, we have inferred that the high precipitation ability of 0a and 0b is likely to arise from high melting points
T. Suzuki et al. / Polyhedron 96 (2015) 102–106
R2
R1 H CH3 CH3
R2 H H CH3
0a 1a 2a
R2
R1 H CH3
R2 H CH3
0b 2b
O R1
N
N
O R1
N
N
O
R N
R H CH3
103
2.2. Preparation of UO2(NO3)2(CU)2
0c 1c
Fig. 1. Schematic formulae of cyclic urea and pyrrolidone derivatives.
(mps) of UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2 [28]. Thus, there should be some correlation between the precipitation ability of L and mp. However, it is still unclear what determines mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2. In the previous studies [26,27], we have reported that UO2(NO3)2(0c)2 (see Fig. 1) involving the imino group shows higher mp than UO2(NO3)2(NRP)2 with N-alkylation despite the lower molecular weight. In the crystal structure of UO2(NO3)2(0c), intermolecular hydrogen bonds (HBs) are present in between the imino group and the carbonyl oxygen atom of the neighboring molecule, and therefore, presence of the N–H O HBs should be correlated with the high mp. On the basis of this insight, UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2, which have two imino groups as shown in Fig. 1, are expected to form more intermolecular N–H O HBs in their crystal structures and to realize the high mps. To examine this hypothesis, it is necessary to compare the molecular and crystal structures of the uranyl nitrate complexes with different Ls listed in Fig. 1. For this purpose, we performed the single crystal X-ray analyses for UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 (1a: 1-methyl-2-imidazolidone) in this study. Although crystal structures of UO2(NO3)2(2a)2, UO2(NO3)2(2b)2, UO2(NO3)2(0c)2 and UO2(NO3)2(1c)2 have already determined in our previous studies [23,24,26,29], they are revisited here to systematically discuss correlations between mps and intermolecular HBs present in UO2(NO3)2L2 (L = CU, NRP; Fig. 1).
2.2.1. UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2 2+ A stock solution of UO2+ 2 ([UO2 ] = 0.1 M for UO2(NO3)2(0a)2 or 0.05 M for UO2(NO3)2(0b)2, M: mol/dm3) was prepared by dissolving UO2(NO3)26H2O in 3.0 M HNO3 aq. After addition of twice molar amount of 0a or 0b to [UO2+ 2 ], CH3OH (1 mL) was added to these reaction mixtures. Each of the mixtures gave crystals suitable for X-ray diffraction after standing for a week in the refrigerator. IR Data (KBr, cm 1): UO2(NO3)2(0a)2: 920, 928 (O@U@O asymmetric stretching, m3), 1647 (C@O stretching in CU, mC@O), 3383, 3418 (N–H stretching in CU, mN–H). UO2(NO3)2(0b)2: 906, 926 (m3), 1623 (mC@O), 3404 (mN–H). 2.2.2. UO2(NO3)2(1a)2 A solution containing UO2+ 2 (0.2 M) and 1a (0.4 M) was prepared by dissolving UO2(NO3)26H2O and 1a in CH3OH (1 mL). After addition of diethyl ether (1 mL), this solution was stood in the refrigerator. Crystals suitable for X-ray diffraction deposited within 1 week. UO2(NO3)2(1a)2: 917 (m3), 1636 (mC@O), 3436 (mN–H). Although these uranyl nitrate complexes are newly reported here, their elemental analyses have not been performed yet because of unavailability of such an instrument in our control area dedicated for treatment of nuclear fuel materials like uranium. 2.3. X-ray crystallography Molecular and crystal structures of uranyl nitrate complexes of 0a, 0b and 1a were determined at 223 K by means of single crystal X-ray diffraction. The yellow crystal of each compound was mounted on a glass fiber. Intensity measurements were carried out on a Rigaku RAXIS RAPID diffractometer with Mo Ka radiation (k = 0.71075 Å). The structures were solved by direct method (SIR92) and expanded by using Fourier techniques [31]. Nonhydrogen atoms were anisotropically refined by SHELXL-97 [32]. Hydrogen atoms were refined as riding on their parent atoms with Uiso(H) = 1.2 Ueq(C, N). The final cycle of full-matrix least-squares refinements on F2 were based on observed reflections and variable parameters, and converged with unweighted and weighted agreement factors (R and wR, respectively). All calculations were performed by the CrystalStructure crystallographic software package [33]. The number of HBs in each uranyl complex was determined by using PLATON software using CIF files of UO2(NO3)2(CU)2 or UO2(NO3)2(NRP)2 [34]. The following three criteria were utilized to assign HBs: (1) if distance between hydrogen donor (D) and P acceptor (A) atoms (D) is less than ( rvdw + 0.5) Å (rvdw: van der Waals radius), (2) if an A H distance (d) is less than P ( rvdw 0.12) Å, and (3) if D–H A angle (h) is larger than 100° [35]. 2.4. Infrared spectroscopy
2. Experimental 2.1. Chemicals 2-Imidazolidone (0a), tetrahydro-2-pyrimidone (0b) and 1,3dimethyl-2-imidazolidone (2a) were purchased from Tokyo Chemical Industry, Kanto Chemical or Sigma–Aldrich Co., Ltd. All chemicals were used without further purification. 1-Methyl-2imidazolidone (1a) was prepared as described elsewhere [30], and characterized by 1H NMR [(CDCl3, TMS, ppm): 2.79 (s, 3H, N-CH3), 3.42 (t, 4H, –(CH2)2–), 4.80 (s, 1H, N-H)]. UO2(NO3)2(2a)2 was synthesized according to the reported methods [24].
IR samples were prepared by mixing each uranyl nitrate complex with dry KBr, and their IR spectra were measured by using a diffuse reflectance method with Shimadzu FTIR-8400S spectrometer. The data collection for each sample was performed in the range of 4000–400 cm 1 with a resolution of 2 cm 1, and repeated 36 times and merged. 2.5. Determination of melting point Melting points of UO2(NO3)2(1a)2 and UO2(NO3)2(2a)2 were recorded by ASONE ATM-02 mp apparatus. For this experiment, powdered uranyl complexes (ca. 1 mg) were loaded in a glass capillary filled with Ar gas, followed by sealing with hot melting. The
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T. Suzuki et al. / Polyhedron 96 (2015) 102–106
sample heating was performed at a raising rate approximate 1– 2 °C/min. The mp data of other compounds discussed here were brought from our previous publications [27,28]. 3. Results and discussion 3.1. UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 Molecular and crystal structures of UO2(NO3)2(0a)2 were determined by means of single crystal X-ray analysis. The ORTEP views of single molecule and packing diagram of UO2(NO3)2(0a)2 are shown in Fig. 2. Crystallographic data, and selected structural parameters are listed in Tables 1 and 2, respectively. The with UO2(NO3)2(0a)2 complex crystallizes in space group P1 Z = 1. An equatorial position of uranyl ion is occupied with two oxygen atoms from carbonyl groups of two 0a and four oxygen atoms from two nitrate groups. The mean bond distances between U and O at apical positions (Oyl), of carbonyl groups (OCU) and of
NO3 (ONO3 ) are 1.776, 2.389 and mean 2.52 Å, respectively. A bond angle around OCU is 132.2°. This uranyl complex with 0a has typical molecular structure of UO2(NO3)2L2 determined in the former time [1–12,22–24,26,29]. Along the abovementioned criteria for HBs assignment in PLATON [35], intermolecular short contacts suggesting N–H O HBs were found in UO2(NO3)2(0a)2 as depicted by dashed lines in Fig. 2. The related quantities (d, D and h) in UO2(NO3)2(0a)2 are listed in Table 3. As seen from Table 3, d, D and h are in the ranges of 2.26–2.43 Å, 3.08–3.14 Å and 137– 165°, respectively, and comparable with those of reported N– H O HBs [36–38]. As seen from Fig. 2, one of the imino hydrogen atoms in the UO2(NO3)2(0a)2 forms bifurcated HBs with the oxygen atoms of carbonyl and nitrate groups of the neighboring molecule (N3–H3 O2 ii and N3–H3 O4 iii [symmetry code: (ii) 2 x, 1 y, 2 z and (iii) x, 1 + y, z]). The other imino hydrogen interacts with uranyl oxygen of the neighboring UO2(NO3)2(0a)2 (N2– H2 O1 i [symmetry code: (i) 2 x, y, 2 z]). The bond parameters of N3–H3 O2 and N3–H3 O4 are similar to each other.
H2A C2
H2
H3B C3
N2
H2B
C1 O2
U1
N3
H3A
N1 H3
O4
O3 O5
O1
UO2(NO3)2(0a)2
H2B
H3B H2A
C2 C3 H4B
H3A C4 N2
H2 C1
H4A N3
O2
U1
H3 O4 O5
N1
O3 O1
UO2(NO3)2(0b)2
H2B C2 H3A
C3
H3B
N2
H2
H2A C1
U1
O2
H4C
N3 N1 H4B
H4A
O4
O3 O1
O5
UO2(NO3)2(1a)2 Fig. 2. ORTEP views of single molecules of UO2(NO3)2(CU)2 [left, CU = 0a (top), 0b (middle), 1a (bottom)], and their packing diagrams (right) 50% probability displacement ellipsoids. Dashed lines indicate intermolecular N–H O HBs.
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T. Suzuki et al. / Polyhedron 96 (2015) 102–106 Table 1 Crystallographic parameters of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. UO2(NO3)2(1a)2
C6H12N6O10U 566.25 triclinic P1
C8H16N6O10U 594.30 triclinic P1
C8H16N6O10U 594.30 triclinic P1
6.1680(5) 7.8036(6) 7.9142(6) 87.334(2) 67.919(2) 85.961(2) 352.03(5) 1 223 2.671 0.0364 0.1093
6.4188(4) 7.7157(5) 8.4920(6) 92.624(2) 110.613(2) 94.523(2) 391.20(4) 1 223 2.523 0.0232 0.0615
6.8952(6) 7.7266(7) 8.4964(8) 95.377(2) 100.321(2) 113.490(2) 401.55(7) 1 223 2.458 0.0381 0.1005
300 250
2a 2b
200
100
0
1a
1
2
3
4
the number of N-H···O HBs Fig. 3. A plot of mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 vs. the number of N–H O HBs.
350 300
P P R = | |F0| |Fc| |/ |F0|. P 2 2 2 P wR = [ (w (F0 Fc ) )/ w(F20)2]1/2.
0a (dec) 0b (dec)
0c
150 1c
mp/°C
a b
UO2(NO3)2(0b)2
mp/°C
Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) Dcalc (g cm 1) Ra wRb
UO2(NO3)2(0a)2
350
0a (dec) 0b (dec)
250
2b 2a
1a
200
0c 1a
150 Table 2 Selected bond lengths (Å) and angles (°).
U1–O1 (Å) U1–O2 (Å) U1–O3 (Å) U1–O4 (Å) U1–O2–C1 (°)
100
UO2(NO3)2(0a)2
UO2(NO3)2(0b)2
UO2(NO3)2(1a)2
1.776(6) 2.389(6) 2.506(7) 2.539(7) 132.2(6)
1.784(3) 2.354(3) 2.522(4) 2.538(4) 135.2(3)
1.771(6) 2.390(5) 2.510(6) 2.520(6) 131.6(5)
d (Å)
D (Å)
h (°)
Symmetry code
UO2(NO3)2(0a)2 N2–H2 O1 i N3–H3 O2 ii N3–H3 O4 iii
2.26 2.39 2.43
3.096 3.076 3.143
165 137 140
(i) 2 x, y, 2 z (ii) 2 x, 1 y, 2 z (iii) x, 1 + y, z
UO2(NO3)2(0b)2 N2–H2 O1 i N3–H3 O2 ii N3–H3 O4 iii
2.39 2.60 2.34
3.071 3.329 3.041
136 144 139
(i) 1 x, y, z (ii) x, 1 y, z (iii) x, 1 + y, z
UO2(NO3)2(1a)2 N2–H2 O1 i
2.42
3.069
132
(i) 1
y,
1
2
3
4
the number of C-H···O HBs Fig. 4. A plot of mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 vs. the number of C–H O HBs.
Table 3 Structural parameters and symmetry code of intermolecular N–H O HBs in UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2.
x,
0
z
This is a typical property for bifurcated HBs [36]. In the previous study [26], intermolecular C–H O HBs have been found in several crystal structures of UO2(NO3)2(NRP)2. In contrast, such an interaction was not formed in the crystal structure of UO2(NO3)2(0a)2. Molecular and crystal structures of UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 were also studied by using the single crystal X-ray analysis. The ORTEP views of single molecules and packing diagrams of them are shown in Fig. 2. The crystallographic data, and selected structural parameters are listed in Tables 1 and 2, respectively. Both UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 crystallize with Z = 1. The coordination geometries, and in space group P1 structural parameters of them are similar to those of UO2(NO3)2(0a)2 and other UO2(NO3)2L2 reported so far [1–12, 22–24,26,29]. Intermolecular short contacts attributable to N–H O HBs were also found in the UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 (dashed lines in Fig. 2). The d, D and h values listed in Table 3 are similar to those of reported N–H O HBs [36–38]. As seen from Fig. 2, the types of three HBs formed in the
UO2(NO3)2(0b)2 (N3–H3 O2 ii and N3–H3 O4 iii [symmetry code: (ii) x, 1 y, z and (iii) x, 1 + y, z] and N2–H2 O1 i [symmetry code: (i) 1 x, y, z]) are the same with those of UO2(NO3)2(0a)2. An interaction of imino hydrogen with uranyl oxygen of the neighboring UO2(NO3)2(1a)2 (N2–H2 O1 i [symmetry code: (i) 1 x, y, z]) was observed in its crystal structure. Although d of N3–H3 O2 in the UO2(NO3)2(0b)2 is relatively long, the bond parameters of N3–H3 O2 and N3–H3 O4 show the common properties for bifurcated HBs; i.e., 1.6 6 d 6 2.4 Å and 100 6 h 6 160° [36]. Intermolecular C–H O HBs were not found in the structures of UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 as well as UO2(NO3)2(0a)2 described above. Consequently, it was found that the intermolecular N–H O HBs involving the imino hydrogen atoms are formed in UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2, whereas the intermolecular C–H O HBs were not observed in those uranyl complexes.
3.2. Melting points for UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 We determined the molecular and crystal structures of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. As expected, the intermolecular N–H O HB networks were formed in the structures of UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2. The HB was also observed in the crystal structure of UO2(NO3)2(1a)2. In the next step, we tried to find a correlation between mps and the number of intermolecular interactions in the related uranyl nitrate complexes with Ls listed in Fig. 1. The relevant data of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 are summarized in Table S1 (Supplementary data). Three C–H O HBs were observed in trimethylene groups of UO2(NO3)2(2b)2 [29], while these moieties are disordered. Therefore, the net number of C–H O HBs in the UO2(NO3)2(2b)2 were calculated by using the occupancies of the disordered parts (0.786 and 0.214); i.e., (2 of C–H O HBs) 0.786 + (1 of C–H O HBs) 0.214 = 1.79. In Figs. 3 and 4,
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T. Suzuki et al. / Polyhedron 96 (2015) 102–106
mp was plotted as functions of the number of N–H O HBs and that of C–H O HBs, respectively. As seen from Fig. 3, the larger number of N–H O HBs tends to give the higher mp. Although thermal decomposition of the UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2 at 280 °C were observed in our previous study [28], these uranyl nitrate complexes should have the higher mps. In contrast, as shown in Fig. 4, little effect of the C–H O HBs on the mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 seems to be present. However, the mps of UO2(NO3)2(2a)2 (211 °C) and UO2(NO3)2(2b)2 (211 °C) display similar values to those of UO2(NO3)2(1a)2 (202 °C) and UO2(NO3)2(0c)2 (216 °C) in spite of absence of N–H O HBs. This result indicate that mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 are also affected by factors other than N–H O HBs. Melting points of organic compounds are generally known to depend on their molecular weights (MWs) [39]. In Fig. S1 (Supplementary data), mp was plotted as a function of MW. As seen from Fig. S1, despite the fact that UO2(NO3)2(2b)2 (650.38 g mol 1) is heaviest in the uranyl complexes compared here, its mp (211 °C) is the same with that of UO2(NO3)2(2a)2 (622.35 g mol 1). Furthermore, the UO2(NO3)2(1a)2 (594.30 g mol 1) and UO2(NO3)2(0c)2 (564.26 g mol 1) also exhibit the similar mps. Therefore, the MWs of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 seem not to largely affect their mps. Desiraju et al. have suggested that mps of oxalate esters depend on the symmetry of molecule along with the intermolecular HBs [40]; i.e., molecules with asymmetric structures have relatively low mps. The ligand symmetries may possess an importance to determine mps of UO2(NO3)2L2, although the whole symmetries of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 should also be taken into account.
4. Summary In this study, the molecular and crystal structures of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 have been determined, and correlations between mps and intermolecular HBs of UO2(NO3)2(CU)2 (CU: 0a, 1a, 2a, 0b and 2b) and UO2(NO3)2(NRP)2 (NRP: 0c and 1c) have been examined. As a result, UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 have similar structures to each other; i.e., two NO3 and two CUs coordinate to UO2+ 2 at the trans positions, U@Oyl (1.77–1.78 Å), U–OCU (2.35–2.39 Å), U–OONO3 (2.52–2.54 Å), and the bond angles around the carbonyl oxygen atoms (132–135°). Three intermolecular N–H O HB were found in the UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2. One of the imino groups forms bifurcated HBs with oxygen of carbonyl and nitrate group. The other imino group attracts uranyl oxygen. An intermolecular N–H Oyl@U HB were also observed in the UO2(NO3)2(1a)2. Furthermore, the intermolecular N–H O HBs largely affects mps of the UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2, while any clear influences of the intermolecular C–H O HBs and MWs on mps were not demonstrated.
Acknowledgements We thank Dr. Ki Chul Park, Prof. Takehiko Tsukahara and Prof. Kunihiko Mizumachi for their useful comments.
Appendix A. Supplementary data CCDC 1042684, 1042685 and 1042686 contain the supplementary crystallographic data for compounds UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving. html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.poly.2015.04.034. References [1] D. Hall, A.D. Rae, T.N. Waters, Acta Crystallogr. 19 (1965) 389. [2] N.W. Alcock, T.J. Kemp, J. Leciejewicz, Z. Trzaska-Durski, Acta Crystallogr. C 46 (1990) 981. [3] Z. Cao, T. Qi, L. Zhu, D.-C. Zhang, R. Zhou, K.-B. Yu, Acta Crystallogr. C 55 (1990) 1270. [4] Z. Cao, H. Wang, J. Gu, L. Zhu, Acta Crystallogr. C 49 (1993) 1942. [5] P.P. Charpin, M. Lance, M. Nierlich, D. Vigner, Acta Crystallogr. C 42 (1986) 560. [6] L.-M. Zhu, Z.-B. Cao, B.-L. Li, Y.-L. Wang, Chin. J. Struct. Chem. 21 (2002) 501. [7] G. Agostini, G. Giacometti, D.A. Clemente, M. Vicentini, Inorg. Chim. Acta 62 (1982) 237. [8] J.H. Burns, G.M. Brown, R.R. Ryan, Acta Crystallogr. C 41 (1985) 1446. [9] C.D. Auwer, R. Revel, M.C. Charbonnel, M.T. Presson, S.D. Conradson, E. Simoni, J.F.L. Du, C.J. Madic, Synchrotron. Radiat. 6 (1999) 101–104. [10] J.H. Burns, Inorg. Chem. 20 (1981) 3868. [11] A.A. Russell, R.L. Meline, E.N. Duesler, R.T. Paine, Inorg. Chim. Acta 231 (1995) 1. [12] J.H. Burns, Inorg. Chem. 22 (1983) 1174. [13] E.K. Dukes, T.H. Siddall III, J. Inorg. Nucl. Chem. 28 (1966) 2307. [14] T.H. Siddall III, J. Phys. Chem. 64 (1960) 1863. [15] S. Kannan, S.B. Deb, J.S. Gamare, M.G.B. Drew, Polyhedron 27 (2008) 2557. [16] L.L.J. Burger, J. Phys. Chem. 62 (1958) 590. [17] T.H. Siddall III, Ind. Eng. Chem. 51 (1959) 41. [18] Y. Meguro, S. Iso, Z. Yoshida, Anal. Chem. 70 (1998) 1262. [19] C.A. Horton, J.C. White, Anal. Chem. 30 (1958) 1779. [20] C. Blake, C. Baes, K. Brown, Ind. Eng. Chem. 50 (1958) 1763. [21] T.R. Varga, M. Sato, Z. Fazekas, Y. Ikeda, H. Tomiyasu, Inorg. Chem. Commun. 3 (2000) 637. [22] T.R. Varga, A.C. Benyei, Z. Fazekas, H. Tomiyasu, Y. Ikeda, Inorg. Chim. Acta 342 (2003) 291. [23] Y. Ikeda, E. Wada, M. Harada, T. Chikazawa, T. Kikuchi, Y. Ikeda, J. Alloys Compd. 374 (2004) 420. [24] N. Koshino, M. Harada, M. Nogami, Y. Morita, T. Kikuchi, Y. Ikeda, Inorg. Chim. Acta 358 (2005) 1857. [25] Y. Morita, K. Takao, S.Y. Kim, Y. Kawata, M. Harada, M. Nogami, K. Nishimura, Y. Ikeda, J. Nucl. Sci. Technol. 46 (2009) 1129. [26] K. Takao, K. Noda, Y. Morita, K. Nishimura, Y. Ikeda, Cryst. Growth Des. 8 (2008) 2364. [27] T. Suzuki, T. Kawasaki, K. Takao, M. Harada, M. Nogami, Y. Ikeda, J. Nucl. Sci. Technol. 49 (2012) 1010. [28] T. Suzuki, K. Takao, T. Kawasaki, M. Harada, M. Nogami, Y. Ikeda, J. Nucl. Sci. Technol. 51 (2014) 514. [29] T. Suzuki, T. Kawasaki, Y. Ikeda, Acta Crystallogr. E67 (2011) m18. [30] H. Belghith, C. Blonski, A. Klaebe, J. Perie, J. Roussel, Org. Mass Spectrom. 24 (1989) 718. [31] A. Altomare, M.C. Burla, M. Camalli, G. Cascarano, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32 (1999) 115. [32] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Gottingen, Germany, 1997. [33] Crystal Structure 3.10, Crystal Structure Analysis Package, Rigaku and Rigaku/ MSC, 2000–2002. [34] A.L. Spek, PLATON; Bijvoet Center for Biochemical Research, Vakgroep Krisral-en Structure-Chemie, Unibersity of UtreCht, The Netherlands. [35] [36] R. Taylor, O. Kennard, W. Versichel, J. Am. Chem. Soc. 106 (1984) 244. [37] P. Vishweshwar, A. Nangia, V.M. Lynch, Cryst. Growth Des. 3 (2003) 783. [38] L.S. Reddy, S. Basavoju, V.R. Vangala, A. Nangia, Cryst. Growth Des. 6 (2006) 161. [39] J.B. Austin, J. Am. Chem. Soc. 52 (1930) 1049. [40] S. Joseph, R. Sathishkumar, S. Mahapatra, G.R. Desiraju, Acta Crystallogr. B 67 (2011) 525.
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Correlation between intermolecular hydrogen bonds and melting points of uranyl nitrate complexes with cyclic urea derivatives Tomoya Suzuki a,1, Koichiro Takao a, Takeshi Kawasaki a, Masayuki Harada a, Masanobu Nogami b, Yasuhisa Ikeda a,⇑ a b
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Department of Electric and Electronic Engineering, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashiosaka City, Osaka 577-8502, Japan
a r t i c l e
i n f o
Article history: Received 26 January 2015 Accepted 29 April 2015 Available online 5 May 2015 Keywords: Uranyl nitrate complex Cyclic urea derivatives Melting point Hydrogen bond Crystal structure
a b s t r a c t To clarify requirements for forming UO2(NO3)2(CU)2 (CU: cyclic urea derivative) and UO2(NO3)2(NRP)2 (NRP: N-alkylated pyrrolidone derivative) with high melting points (mps), molecular and crystal structures of UO2(NO3)2(0a)2 (0a: 2-imidazolidone), UO2(NO3)2(0b)2 (0b: tetrahydro-2-pyrimidone) and UO2(NO3)2(1a)2 (1a: 1-methyl-2-imidazolidone) were determined by means of single crystal X-ray analysis. Melting points (mps) of these complexes were compared with each other and those of other UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2. As a result, the uranyl nitrate complexes with 0a, 0b and 1a exhibit typical structural properties of UO2(NO3)2L2 (L: unidentate ligand); i.e., hexagonal bipyramidal coordination geometry, and two bidentate NO3 and two CUs located at trans positions in an equatorial plane of UO2+ 2 ion. Several intermolecular N–H O hydrogen bonds (HBs) were found in the crystal structures of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. In contrast, no intermolecular C–H O HBs were formed in these crystal structures. The intermolecular HBs were discussed in connection with mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 including those reported previously. The mp tends to increase as the number of N–H O HBs per molecule increases, whereas effects of C–H O HBs on the mps is not very significant. These results suggest that the intermolecular N–H O HBs strongly contribute to form a uranyl nitrate complex with high mps. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Uranyl nitrate complexes with unidentate ligands (L) such as H2O [1], ureas [2,3], amides [4–6], alkyl phosphates and phosphine oxides [7–12] generally have a composition, UO2(NO3)2L2, which exhibits hexagonal bipyramidal coordination geometry around U. Four oxygen atoms from two bidentate NO3 and two donating atoms from Ls are bound to U at the trans positions in the equatorial plane of the UO2+ 2 ion. This general formula is commonly found in both fundamental chemistry of uranium and nuclear engineering processes like spent nuclear fuel reprocessing [13–20]. In our previous study, we have found that N-cyclohexyl-2pyrolidone (NCP) selectively precipitates UO2+ from a HNO3 2 aqueous solution [21]. From the single crystal X-ray analysis, the ⇑ Corresponding author. Tel.: +81 3 5734 3061. E-mail addresses: [email protected] (T. Suzuki), [email protected] (Y. Ikeda). 1 Present address: Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. http://dx.doi.org/10.1016/j.poly.2015.04.034 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
chemical formula of precipitates was found to be UO2(NO3)2(NCP)2 with the typical structural properties of UO2(NO3)2L2 mentioned above [22]. On the basis of UO2+ 2 precipitation ability of NCP, we proposed a reprocessing method for the spent nuclear fuels produced from fast breeder reactors [23–26], and assessed various pyrrolidone derivatives (NRP) as UO2+ 2 precipitants in this process. As a result, it was clarified that the UO2+ 2 precipitation ability depends on hydrophobicity of NRP and packing efficiency of UO2(NO3)2(NRP)2 in the crystal structures [25,26]. In our recent works [24,27,28], hydrophilic cyclic urea derivatives (CUs, Fig. 1) such as 2-imidazolidone (0a), tetrahydro2-pyrimidone (0b), 1,3-dimethyl-2-imidazolidone (2a), and 1,3dimethyl-3,4,5,6-tetrahydro-2-pyrimidone (2b) were also found 2+ to precipitate UO2+ 2 from the HNO3 solution. Especially, the UO2 precipitation abilities of 0a and 0b are comparable to that of NCP in spite of their low hydrophobicity. Comparing precipitation behavior of UO2+ 2 through addition of CUs and NRPs together with other physical traits, we have inferred that the high precipitation ability of 0a and 0b is likely to arise from high melting points
T. Suzuki et al. / Polyhedron 96 (2015) 102–106
R2
R1 H CH3 CH3
R2 H H CH3
0a 1a 2a
R2
R1 H CH3
R2 H CH3
0b 2b
O R1
N
N
O R1
N
N
O
R N
R H CH3
103
2.2. Preparation of UO2(NO3)2(CU)2
0c 1c
Fig. 1. Schematic formulae of cyclic urea and pyrrolidone derivatives.
(mps) of UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2 [28]. Thus, there should be some correlation between the precipitation ability of L and mp. However, it is still unclear what determines mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2. In the previous studies [26,27], we have reported that UO2(NO3)2(0c)2 (see Fig. 1) involving the imino group shows higher mp than UO2(NO3)2(NRP)2 with N-alkylation despite the lower molecular weight. In the crystal structure of UO2(NO3)2(0c), intermolecular hydrogen bonds (HBs) are present in between the imino group and the carbonyl oxygen atom of the neighboring molecule, and therefore, presence of the N–H O HBs should be correlated with the high mp. On the basis of this insight, UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2, which have two imino groups as shown in Fig. 1, are expected to form more intermolecular N–H O HBs in their crystal structures and to realize the high mps. To examine this hypothesis, it is necessary to compare the molecular and crystal structures of the uranyl nitrate complexes with different Ls listed in Fig. 1. For this purpose, we performed the single crystal X-ray analyses for UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 (1a: 1-methyl-2-imidazolidone) in this study. Although crystal structures of UO2(NO3)2(2a)2, UO2(NO3)2(2b)2, UO2(NO3)2(0c)2 and UO2(NO3)2(1c)2 have already determined in our previous studies [23,24,26,29], they are revisited here to systematically discuss correlations between mps and intermolecular HBs present in UO2(NO3)2L2 (L = CU, NRP; Fig. 1).
2.2.1. UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2 2+ A stock solution of UO2+ 2 ([UO2 ] = 0.1 M for UO2(NO3)2(0a)2 or 0.05 M for UO2(NO3)2(0b)2, M: mol/dm3) was prepared by dissolving UO2(NO3)26H2O in 3.0 M HNO3 aq. After addition of twice molar amount of 0a or 0b to [UO2+ 2 ], CH3OH (1 mL) was added to these reaction mixtures. Each of the mixtures gave crystals suitable for X-ray diffraction after standing for a week in the refrigerator. IR Data (KBr, cm 1): UO2(NO3)2(0a)2: 920, 928 (O@U@O asymmetric stretching, m3), 1647 (C@O stretching in CU, mC@O), 3383, 3418 (N–H stretching in CU, mN–H). UO2(NO3)2(0b)2: 906, 926 (m3), 1623 (mC@O), 3404 (mN–H). 2.2.2. UO2(NO3)2(1a)2 A solution containing UO2+ 2 (0.2 M) and 1a (0.4 M) was prepared by dissolving UO2(NO3)26H2O and 1a in CH3OH (1 mL). After addition of diethyl ether (1 mL), this solution was stood in the refrigerator. Crystals suitable for X-ray diffraction deposited within 1 week. UO2(NO3)2(1a)2: 917 (m3), 1636 (mC@O), 3436 (mN–H). Although these uranyl nitrate complexes are newly reported here, their elemental analyses have not been performed yet because of unavailability of such an instrument in our control area dedicated for treatment of nuclear fuel materials like uranium. 2.3. X-ray crystallography Molecular and crystal structures of uranyl nitrate complexes of 0a, 0b and 1a were determined at 223 K by means of single crystal X-ray diffraction. The yellow crystal of each compound was mounted on a glass fiber. Intensity measurements were carried out on a Rigaku RAXIS RAPID diffractometer with Mo Ka radiation (k = 0.71075 Å). The structures were solved by direct method (SIR92) and expanded by using Fourier techniques [31]. Nonhydrogen atoms were anisotropically refined by SHELXL-97 [32]. Hydrogen atoms were refined as riding on their parent atoms with Uiso(H) = 1.2 Ueq(C, N). The final cycle of full-matrix least-squares refinements on F2 were based on observed reflections and variable parameters, and converged with unweighted and weighted agreement factors (R and wR, respectively). All calculations were performed by the CrystalStructure crystallographic software package [33]. The number of HBs in each uranyl complex was determined by using PLATON software using CIF files of UO2(NO3)2(CU)2 or UO2(NO3)2(NRP)2 [34]. The following three criteria were utilized to assign HBs: (1) if distance between hydrogen donor (D) and P acceptor (A) atoms (D) is less than ( rvdw + 0.5) Å (rvdw: van der Waals radius), (2) if an A H distance (d) is less than P ( rvdw 0.12) Å, and (3) if D–H A angle (h) is larger than 100° [35]. 2.4. Infrared spectroscopy
2. Experimental 2.1. Chemicals 2-Imidazolidone (0a), tetrahydro-2-pyrimidone (0b) and 1,3dimethyl-2-imidazolidone (2a) were purchased from Tokyo Chemical Industry, Kanto Chemical or Sigma–Aldrich Co., Ltd. All chemicals were used without further purification. 1-Methyl-2imidazolidone (1a) was prepared as described elsewhere [30], and characterized by 1H NMR [(CDCl3, TMS, ppm): 2.79 (s, 3H, N-CH3), 3.42 (t, 4H, –(CH2)2–), 4.80 (s, 1H, N-H)]. UO2(NO3)2(2a)2 was synthesized according to the reported methods [24].
IR samples were prepared by mixing each uranyl nitrate complex with dry KBr, and their IR spectra were measured by using a diffuse reflectance method with Shimadzu FTIR-8400S spectrometer. The data collection for each sample was performed in the range of 4000–400 cm 1 with a resolution of 2 cm 1, and repeated 36 times and merged. 2.5. Determination of melting point Melting points of UO2(NO3)2(1a)2 and UO2(NO3)2(2a)2 were recorded by ASONE ATM-02 mp apparatus. For this experiment, powdered uranyl complexes (ca. 1 mg) were loaded in a glass capillary filled with Ar gas, followed by sealing with hot melting. The
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T. Suzuki et al. / Polyhedron 96 (2015) 102–106
sample heating was performed at a raising rate approximate 1– 2 °C/min. The mp data of other compounds discussed here were brought from our previous publications [27,28]. 3. Results and discussion 3.1. UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 Molecular and crystal structures of UO2(NO3)2(0a)2 were determined by means of single crystal X-ray analysis. The ORTEP views of single molecule and packing diagram of UO2(NO3)2(0a)2 are shown in Fig. 2. Crystallographic data, and selected structural parameters are listed in Tables 1 and 2, respectively. The with UO2(NO3)2(0a)2 complex crystallizes in space group P1 Z = 1. An equatorial position of uranyl ion is occupied with two oxygen atoms from carbonyl groups of two 0a and four oxygen atoms from two nitrate groups. The mean bond distances between U and O at apical positions (Oyl), of carbonyl groups (OCU) and of
NO3 (ONO3 ) are 1.776, 2.389 and mean 2.52 Å, respectively. A bond angle around OCU is 132.2°. This uranyl complex with 0a has typical molecular structure of UO2(NO3)2L2 determined in the former time [1–12,22–24,26,29]. Along the abovementioned criteria for HBs assignment in PLATON [35], intermolecular short contacts suggesting N–H O HBs were found in UO2(NO3)2(0a)2 as depicted by dashed lines in Fig. 2. The related quantities (d, D and h) in UO2(NO3)2(0a)2 are listed in Table 3. As seen from Table 3, d, D and h are in the ranges of 2.26–2.43 Å, 3.08–3.14 Å and 137– 165°, respectively, and comparable with those of reported N– H O HBs [36–38]. As seen from Fig. 2, one of the imino hydrogen atoms in the UO2(NO3)2(0a)2 forms bifurcated HBs with the oxygen atoms of carbonyl and nitrate groups of the neighboring molecule (N3–H3 O2 ii and N3–H3 O4 iii [symmetry code: (ii) 2 x, 1 y, 2 z and (iii) x, 1 + y, z]). The other imino hydrogen interacts with uranyl oxygen of the neighboring UO2(NO3)2(0a)2 (N2– H2 O1 i [symmetry code: (i) 2 x, y, 2 z]). The bond parameters of N3–H3 O2 and N3–H3 O4 are similar to each other.
H2A C2
H2
H3B C3
N2
H2B
C1 O2
U1
N3
H3A
N1 H3
O4
O3 O5
O1
UO2(NO3)2(0a)2
H2B
H3B H2A
C2 C3 H4B
H3A C4 N2
H2 C1
H4A N3
O2
U1
H3 O4 O5
N1
O3 O1
UO2(NO3)2(0b)2
H2B C2 H3A
C3
H3B
N2
H2
H2A C1
U1
O2
H4C
N3 N1 H4B
H4A
O4
O3 O1
O5
UO2(NO3)2(1a)2 Fig. 2. ORTEP views of single molecules of UO2(NO3)2(CU)2 [left, CU = 0a (top), 0b (middle), 1a (bottom)], and their packing diagrams (right) 50% probability displacement ellipsoids. Dashed lines indicate intermolecular N–H O HBs.
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T. Suzuki et al. / Polyhedron 96 (2015) 102–106 Table 1 Crystallographic parameters of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. UO2(NO3)2(1a)2
C6H12N6O10U 566.25 triclinic P1
C8H16N6O10U 594.30 triclinic P1
C8H16N6O10U 594.30 triclinic P1
6.1680(5) 7.8036(6) 7.9142(6) 87.334(2) 67.919(2) 85.961(2) 352.03(5) 1 223 2.671 0.0364 0.1093
6.4188(4) 7.7157(5) 8.4920(6) 92.624(2) 110.613(2) 94.523(2) 391.20(4) 1 223 2.523 0.0232 0.0615
6.8952(6) 7.7266(7) 8.4964(8) 95.377(2) 100.321(2) 113.490(2) 401.55(7) 1 223 2.458 0.0381 0.1005
300 250
2a 2b
200
100
0
1a
1
2
3
4
the number of N-H···O HBs Fig. 3. A plot of mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 vs. the number of N–H O HBs.
350 300
P P R = | |F0| |Fc| |/ |F0|. P 2 2 2 P wR = [ (w (F0 Fc ) )/ w(F20)2]1/2.
0a (dec) 0b (dec)
0c
150 1c
mp/°C
a b
UO2(NO3)2(0b)2
mp/°C
Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) Dcalc (g cm 1) Ra wRb
UO2(NO3)2(0a)2
350
0a (dec) 0b (dec)
250
2b 2a
1a
200
0c 1a
150 Table 2 Selected bond lengths (Å) and angles (°).
U1–O1 (Å) U1–O2 (Å) U1–O3 (Å) U1–O4 (Å) U1–O2–C1 (°)
100
UO2(NO3)2(0a)2
UO2(NO3)2(0b)2
UO2(NO3)2(1a)2
1.776(6) 2.389(6) 2.506(7) 2.539(7) 132.2(6)
1.784(3) 2.354(3) 2.522(4) 2.538(4) 135.2(3)
1.771(6) 2.390(5) 2.510(6) 2.520(6) 131.6(5)
d (Å)
D (Å)
h (°)
Symmetry code
UO2(NO3)2(0a)2 N2–H2 O1 i N3–H3 O2 ii N3–H3 O4 iii
2.26 2.39 2.43
3.096 3.076 3.143
165 137 140
(i) 2 x, y, 2 z (ii) 2 x, 1 y, 2 z (iii) x, 1 + y, z
UO2(NO3)2(0b)2 N2–H2 O1 i N3–H3 O2 ii N3–H3 O4 iii
2.39 2.60 2.34
3.071 3.329 3.041
136 144 139
(i) 1 x, y, z (ii) x, 1 y, z (iii) x, 1 + y, z
UO2(NO3)2(1a)2 N2–H2 O1 i
2.42
3.069
132
(i) 1
y,
1
2
3
4
the number of C-H···O HBs Fig. 4. A plot of mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 vs. the number of C–H O HBs.
Table 3 Structural parameters and symmetry code of intermolecular N–H O HBs in UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2.
x,
0
z
This is a typical property for bifurcated HBs [36]. In the previous study [26], intermolecular C–H O HBs have been found in several crystal structures of UO2(NO3)2(NRP)2. In contrast, such an interaction was not formed in the crystal structure of UO2(NO3)2(0a)2. Molecular and crystal structures of UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 were also studied by using the single crystal X-ray analysis. The ORTEP views of single molecules and packing diagrams of them are shown in Fig. 2. The crystallographic data, and selected structural parameters are listed in Tables 1 and 2, respectively. Both UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 crystallize with Z = 1. The coordination geometries, and in space group P1 structural parameters of them are similar to those of UO2(NO3)2(0a)2 and other UO2(NO3)2L2 reported so far [1–12, 22–24,26,29]. Intermolecular short contacts attributable to N–H O HBs were also found in the UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 (dashed lines in Fig. 2). The d, D and h values listed in Table 3 are similar to those of reported N–H O HBs [36–38]. As seen from Fig. 2, the types of three HBs formed in the
UO2(NO3)2(0b)2 (N3–H3 O2 ii and N3–H3 O4 iii [symmetry code: (ii) x, 1 y, z and (iii) x, 1 + y, z] and N2–H2 O1 i [symmetry code: (i) 1 x, y, z]) are the same with those of UO2(NO3)2(0a)2. An interaction of imino hydrogen with uranyl oxygen of the neighboring UO2(NO3)2(1a)2 (N2–H2 O1 i [symmetry code: (i) 1 x, y, z]) was observed in its crystal structure. Although d of N3–H3 O2 in the UO2(NO3)2(0b)2 is relatively long, the bond parameters of N3–H3 O2 and N3–H3 O4 show the common properties for bifurcated HBs; i.e., 1.6 6 d 6 2.4 Å and 100 6 h 6 160° [36]. Intermolecular C–H O HBs were not found in the structures of UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 as well as UO2(NO3)2(0a)2 described above. Consequently, it was found that the intermolecular N–H O HBs involving the imino hydrogen atoms are formed in UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2, whereas the intermolecular C–H O HBs were not observed in those uranyl complexes.
3.2. Melting points for UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 We determined the molecular and crystal structures of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2. As expected, the intermolecular N–H O HB networks were formed in the structures of UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2. The HB was also observed in the crystal structure of UO2(NO3)2(1a)2. In the next step, we tried to find a correlation between mps and the number of intermolecular interactions in the related uranyl nitrate complexes with Ls listed in Fig. 1. The relevant data of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 are summarized in Table S1 (Supplementary data). Three C–H O HBs were observed in trimethylene groups of UO2(NO3)2(2b)2 [29], while these moieties are disordered. Therefore, the net number of C–H O HBs in the UO2(NO3)2(2b)2 were calculated by using the occupancies of the disordered parts (0.786 and 0.214); i.e., (2 of C–H O HBs) 0.786 + (1 of C–H O HBs) 0.214 = 1.79. In Figs. 3 and 4,
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mp was plotted as functions of the number of N–H O HBs and that of C–H O HBs, respectively. As seen from Fig. 3, the larger number of N–H O HBs tends to give the higher mp. Although thermal decomposition of the UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2 at 280 °C were observed in our previous study [28], these uranyl nitrate complexes should have the higher mps. In contrast, as shown in Fig. 4, little effect of the C–H O HBs on the mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 seems to be present. However, the mps of UO2(NO3)2(2a)2 (211 °C) and UO2(NO3)2(2b)2 (211 °C) display similar values to those of UO2(NO3)2(1a)2 (202 °C) and UO2(NO3)2(0c)2 (216 °C) in spite of absence of N–H O HBs. This result indicate that mps of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 are also affected by factors other than N–H O HBs. Melting points of organic compounds are generally known to depend on their molecular weights (MWs) [39]. In Fig. S1 (Supplementary data), mp was plotted as a function of MW. As seen from Fig. S1, despite the fact that UO2(NO3)2(2b)2 (650.38 g mol 1) is heaviest in the uranyl complexes compared here, its mp (211 °C) is the same with that of UO2(NO3)2(2a)2 (622.35 g mol 1). Furthermore, the UO2(NO3)2(1a)2 (594.30 g mol 1) and UO2(NO3)2(0c)2 (564.26 g mol 1) also exhibit the similar mps. Therefore, the MWs of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 seem not to largely affect their mps. Desiraju et al. have suggested that mps of oxalate esters depend on the symmetry of molecule along with the intermolecular HBs [40]; i.e., molecules with asymmetric structures have relatively low mps. The ligand symmetries may possess an importance to determine mps of UO2(NO3)2L2, although the whole symmetries of UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2 should also be taken into account.
4. Summary In this study, the molecular and crystal structures of UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 have been determined, and correlations between mps and intermolecular HBs of UO2(NO3)2(CU)2 (CU: 0a, 1a, 2a, 0b and 2b) and UO2(NO3)2(NRP)2 (NRP: 0c and 1c) have been examined. As a result, UO2(NO3)2(0a)2, UO2(NO3)2(0b)2 and UO2(NO3)2(1a)2 have similar structures to each other; i.e., two NO3 and two CUs coordinate to UO2+ 2 at the trans positions, U@Oyl (1.77–1.78 Å), U–OCU (2.35–2.39 Å), U–OONO3 (2.52–2.54 Å), and the bond angles around the carbonyl oxygen atoms (132–135°). Three intermolecular N–H O HB were found in the UO2(NO3)2(0a)2 and UO2(NO3)2(0b)2. One of the imino groups forms bifurcated HBs with oxygen of carbonyl and nitrate group. The other imino group attracts uranyl oxygen. An intermolecular N–H Oyl@U HB were also observed in the UO2(NO3)2(1a)2. Furthermore, the intermolecular N–H O HBs largely affects mps of the UO2(NO3)2(CU)2 and UO2(NO3)2(NRP)2, while any clear influences of the intermolecular C–H O HBs and MWs on mps were not demonstrated.
Acknowledgements We thank Dr. Ki Chul Park, Prof. Takehiko Tsukahara and Prof. Kunihiko Mizumachi for their useful comments.
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