Self-assembly Syntheses, Crystal Structures and Quantum Chemistry of Two UO22+ Complexes①

TAN Yu-Xing NAN Xio-Long TAN Yn-Ling ZHANG Zhi-Jin JIANG Wu-Jiu

a (Key Laboratory of Functional Metal-organic Compounds of Hunan Proνince, Key Laboratory of Functional Organometallic Materials, Uniνersity of Hunan Proνince, Hunan Proνincial Engineering Research Center for Monitoring and Treatment of Heaνy Metals Pollution in the Upper Reaches of Xiangjiang Riνer, College of Chemistry and Materials Science, Hengyang Normal Uniνersity, Hengyang, Hunan 421008, China)

b (Nuclear Bureau of Hunan Proνince Nuclear Industry Brigade 306, Hengyang, Hunan 421008, China)

c (Hunan Proνincial Engineering Research Center for Uranium Mineral Exploration Technology, College of Physics and Electronic Engineering, Hengyang Normal Uniνersity, Hengyang, Hunan 421002, China)

ABSTRACT Two UO22+ complexes {[C5H4N(O)C=N-N=C(Ph)-(Ph)C=N-N=C(O)-C5H4N]2UO2(CH3OH)} (I)and {[C5H4N(O)C=N-N=C(Ph)-(Ph)C=N-N=C(O)-C5H4N]2UO2(C5H4N(O)C=N-NH2)} (II) were synthesized and characterized by IR, elemental analysis and thermal stability analysis, and the crystal structures were determined by X-ray diffraction. The crystal of complex I belongs to monoclinic system, space group P21/n with a = 11.7678(4),b = 16.9667(6), c = 14.3051(5) Å, β = 98.918(3)°, Z = 4, V = 2821.64(17) Å3, Dc = 1.837 Mg·m-3, μ(MoKα) =5.805 mm-1, F(000) = 1504, R = 0.0346 and wR = 0.0688. The crystal of complex II is of triclinic system, space group P1 with a = 11.6417(5), b = 11.7297(5), c = 14.2197(5) Å, α = 71.697(4)°, β = 86.020(3)°, γ = 71.572(4)°,Z = 2, V = 1748.02(12) Å3, Dc = 1.742 Mg·m-3, μ(MoKα) = 4.704 mm-1, F(000) = 894, R = 0.0283 and wR =0.0537. The U1 is a seven-coordinate pentagonal bipyramidal configuration in I and an eight-coordinate hexagonal dipyramidal configuration in II. The thermal stability and quantum chemical calculations of I and II were also investigated.

Keywords: UO22+ complexes, synthesis, crystal structure, quantum chemistry;

1 INTRODUCTION

Uranium is a radioactive metal element, which is the most important nuclear fuel in nature, and an element that has attracted much attention in the development of nuclear energy[1-3]. At the beginning of the nuclear fuel cycle, the release of uranium was inevitable during the mining and purification of uranium; at the end of the nuclear fuel cycle,radioactive waste will also contain a large amount of unreacted uranium[4,5]. At present, many countries in the world are stepping up research on the disposal of nuclear waste to use chemical methods for the treatment and reuse of nuclear waste. Therefore, studying the coordination chemistry of uranium, understanding the bonding characteristics of uranium, and discussing the structures and properties of novel uranyl complexes can solve the safe storage problems of nuclear waste and radioactive pollution. And it can provide experimental accumulation and new ideas.

The electron shell of uranium is [Rn]5f36d17s2, and the neutrons of 5forbital have a shielding effect on the outer electrons, which makes uranium have a changeable oxidation state. Among them, the +6 valence is the most stable, the center ionic electrical properties of the high oxidation state are high, the ionic radius is large, and the attraction of the ligand is strong, and more ligands can be attracted to form a highly complement number of mating units[6-9]. Therefore, two unreported uranyl complexes have been designed to synthesize the multidentate organic ligand containing ONO and uranyl acetate by self-assembly reaction in this paper, and the studies on two complexes have been performed with quantum chemistry calculation. The stabilities, some frontier molecular orbital energies and composition characteristics of some frontier molecular orbitals of the compound have been investigated. It provides a certain theoretical significance for the research of nuclear waste treatment, catalysis, mineralogue and energy.

2 EXPERIMENTAL

2. 1 Instruments and reagents

Infrared spectrum (KBr) was recorded by the Prestige-21 infrared spectrometer (Japan Shimadzu, 4000～400 cm-1).The elemental analysis was determined by PE-2400(II)elemental analyzer. Crystallographic data of the complexes were collected on a Bruker SMART APEX II CCD diffractometer. Melting points were determined using an X4 digital microscopic melting point apparatus without correction(Beijing Tektronix Instrument Co. Ltd.). Thermogravimetric analyses (TGA) were recorded on a NETZSCH TG 209 F3 instrument at a heating rate of 20 ℃·min-1from 40 to 800 ℃under air. Powder X-ray diffraction (PXRD) pattern of the complex was collected on a Shimadzu X-ray diffractometer XRD6100 with the CuKαradiation (λ= 1.5406 Å) at room temperature and 2θranging from 5° to 50°.

The reagents used in the experiment were all analytical reagents, and used directly without further purification.

2. 2 Synthesis of the complexes

A mixture of 4-pyridoylhydrazine (2.0 mmol), benzil (1.0 mmol), uranyl acetate (1 mmol) and CH3OH (10.0 mL) was added in a Teflon-lined stainless-vessel (20.0 mL), and heated at 120 °C for 10.0 h, then cooled to room temperature at a rate of 5 °C·h-1. The crystals of I were collected. Complex I was a red block crystal. Yield: 63%. m.p.: 116～118 ℃ (dec.). Anal.Calcd. (C28H26N6O6U): C, 43.08; H, 3.36; N 10.77%. Found:C, 43.14; H, 3.41; N, 10.69%. FT-IR (KBr, cm-1): 3090, 3065,3034, 2931, 2819, 1607, 1570, 1528, 1501, 1472, 1373, 1296,1155, 1061, 926, 878, 843, 783, 754, 696, 687, 615, 538.

Complex II was prepared in a similar procedure (Fig. 1) as I by 4-pyridoylhydrazine (2.0 mmol) in place of 3-pyridoylhydrazine (2.0 mmol). The product was a bronze block crystal with the yield of 61% (based on 3-pyridoylhydrazine).m.p.: 105～107 ℃ (dec.). Anal. Calcd. (C34H32N9O7U): C,44.55; H, 3.52; N, 13.75%. Found: C, 44.54; H, 3.58; N,13.82%. FT-IR (KBr, cm-1): 3291, 3198, 3173, 3057, 1657,1593, 1541, 1504, 1485, 1474, 1406, 1381, 1335, 1319, 1211,1165, 1111, 1059, 1026, 907, 881, 827, 731, 698, 538, 459.

Fig. 1. Syntheses of the complexes

2. 3 Crystal structure determination

Suitable single crystals with dimensions of 0.13mm ×0.11mm × 0.10mm (I) and 0.13mm × 0.12mm × 0.09mm (II)were selected for data collection at 100 K on a Bruker SMART APEX II CCD diffractometer equipped with graphite-monochromated MoKαradiation (λ= 0.71073 Å)using aφ-ωmode. All the data were corrected byLpfactors and empirical absorbance. The structures were solved by direct methods. All non-hydrogen atoms were determined in successive difference Fourier synthesis, and hydrogen atoms were added according to theoretical models or located from the Fourier maps. All hydrogen and non-hydrogen atoms were refined by their isotropic and anisotropic thermal parameters through full-matrix least-squares techniques. All calculations were completed by the SHELXTL-97[10]program. For complex I, a total of 13993 reflections were obtained in the range of 2.09＜θ＜26.00° with 5557 unique ones (Rint= 0.0405),S= 1.030, (Δρ)max= 1.631 and (Δρ)min= -1.484 e/Å3, max transmission was 1.00000, min transmission was 0.68535, and the completeness was 100.0%. For complex II, a total of 14220 reflections were obtained in the range of 1.92＜θ＜26.00° with 6876 unique ones (Rint= 0.0346),S=1.023, (Δρ)max= 1.114, (Δρ)min= -1.017 e/Å3, max transmission was 1.00000, min transmission was 0.72112, and the completeness was 100.0%. The selected bond lengths and bond angles for I and II are listed in Table 1.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°) for I and II

3 RESULTS AND DISCUSSION

3. 1 Synthesis

The solvothermal synthesis method was used to prepare complexes in this paper. Under a certain temperature,self-assembly of the reactants can form the final product, and the solvent heat method has an advantage over the ordinary synthetic method[11-13]. For example, (1) Under high temperature, the solvent gasification in the reactor has generated pressure such that some ligands dissolving in difficulty at room temperature can be dissolved, and the high temperature condition causes the solvent viscosity to decrease,thus facilitating the transfer between the substances; (2) The solvent reaction conditions are simple, fast and efficient and easy to control with better reproducibility; (3) Under this condition, a novel compound that has an unexpected structure can be obtained by the self-assembly of the organic ligand.

Compared to the reactant ratio and conditions of the two reactions, only the position of the nitrogen atom on the pyridine ring was different, and the other is the same.However, two complexes are obtained by self-assembly reactions. It can be seen that the self-assembly reaction of the organic ligand does get a lot of novel compound molecules.

3. 2 Spectral analyses

In the infrared spectra of complexes I and II, the strong peaks at 925 and 906 cm-1and the weak ones at 842 and 827 cm-1are attributed to the symmetric and asymmetric stretching vibration peaks of UO22+[14,15]. It is a characteristic peak of UO22+complex, which is consistent with the position of the absorption peak reported in literature. The absorption peaks at 3090, 3065, 3034 cm-1in complex I and 3198, 3173,3057 cm-1in II are assigned to the C-H stretching vibration absorption peaks on the aromatic ring of the complex. The absorption peaks at 2931, 2819 cm-1in I are due to the saturated C-H stretching vibration absorption peak, while the absence of a peak in II indicates no saturated C-H bond in this complex. This conclusion is consistent with the X-ray single-crystal diffraction results.

3. 3 Structure description

The molecular structures of complexes I and II are shown in Fig. 2. Both of them contain a mononuclear UO22+complex molecule, but the coordination modes of Th (IV) are different.

In complex I, the U1 adopts a seven-coordinate pentagonal bipyramidal configuration. UO22+forms a pentagonal bipyramidal configuration with the two nitrogen atoms (N(3),N(4)) and two oxygen atoms (O(1), O(2)) from the diacylhydrazone ligand and the one oxygen atom O(3) from the methanol. These five atoms form the equatorial plane of the pentagonal bipyramid, the axis of which is occupied by the two oxygen atoms on UO22+. The bonds between oxygen or nitrogen atoms on the equatorial plane and U are similar to the reports[16,17].dU1-O1= 2.325(3),dU1-O2= 2.340(3),dU1-O3=2.378(4),dU1-N3= 2.529(4) anddU1-N4= 2.514(4) Å. The two O atoms are bonded from axial and uranium, with the U=O bonds to be 1.777(3) and 1.776(3) Å, respectively, and the O=U=O angle of 177.95(15) °, so it can be approximately considered to be on a straight line.

Fig. 2. Molecular structures of I (a) and II (b)

In complex II, the U1 is an eight-coordinate hexagonal bipyramidal configuration. The structures of complexes II and I are slightly different. Comparing complex I to II, another 3-pyridoylhydrazine molecule was involved in coordination by bidentate. Thereby, the U1 is eight-coordinated in complex II. Other parameters are similar to the literature[18,19].

3. 4 XRD and TGA

To verify the purity of complexes, XRD of complexes I and II was performed[20,21]. As shown in Fig. 3, the relevant positions of diffraction peaks in experimental patterns match well with those in the simulated ones, indicating good purity for complexes I and II.

Fig. 3. XRD of complexes I (a) and II (b)

Thermal stabilities of both complexes are carried out using a NETZSCH TG 209 F3 thermogravimetric analyzer from 40 to 800 ℃ at a rate of 20 ℃·min-1under an air atmosphere at a flowing rate of 20.0 mL·min-1. As shown in Fig. 4, with the increase of temperature, complexes I and II have a similar weight loss process. In the first stages, complexes I and II display a small weight loss at around 110 ℃, corresponding to the departure of methanol molecule. The results were consistent with X-ray single-crystal diffraction data. It shows that the molecule of the complex contains methanol. In the next stages, both complexes suffer complete decomposition until about 600 ℃, corresponding to the removal of ligand.The remaining weight (35.6% (I) and 30.8% (II)) indicates the final products are UO2(34.5% (I) and 29.4% (II)). In summary, I and II are stable up to 100 ℃.

Fig. 4. TG-DTG curves for I (a) and II (b)

3. 5 Quantum chemical

According to the atomic coordinates of the crystal structure,the total energy of the molecule and the energy of the frontier molecular orbital were calculated by the Gaussian 09W program at the B3lyp/mwb basis group level.

Complex I:ET= -782.639337051 a.u.,EHOMO= -0.22101 a.u.,ELUMO= -0.10866 a.u. and ΔELUMO-HOMO= 0.11235 a.u..Complex II:ET= -841.810598184 a.u.,EHOMO= -0.1944 a.u,ELUMO= -0.1106 a.u. and ΔELUMO-HOMO= 0.0838 a.u. It can be seen that the total energy and occupied orbital energy of the two complexes are both low, and the energy gap between the highest occupied and lowest unoccupied orbitals is small.It shows that complexes I and II are more difficult to lose electrons and be oxidized.

In order to explore the electronic structure and bonding characteristics of both complexes, the molecular orbitals of I and II were analyzed. The squares sum of various atomic orbital coefficients participating in combination is used to express the contribution of this part in the molecular orbital,which is normalized. The atoms of compounds were divided into five parts. For I or II: (a) U atom; (b) O atom; (c) N atom;(d) C atom; (e) H atom. Five frontier occupied and unoccupied orbitals are taken respectively, and the calculated results are shown in Tables 2 and 3 as well as Figs. 5 and 6.By comparing the components of atomic orbitals of HOMO and LUMO in I, it can be seen that when excited from HOMO to LUMO orbitals, the electrons are mainly transferred from ligands to U atoms, so that the contributions of U atom are 97.03186%. When electrons are excited from HOMO to LUMO orbitals in II, they mainly transfer between the ligands,and some are transferred from the ligand to the U atom.

Fig. 5. Schematic diagram of the frontier MO for I

Fig. 6. Schematic diagram of the frontier MO for II

Table 2. Some Calculated Frontier Molecular Orbitals Composition of Complex I (%)

Table 3. Some Calculated Frontier Molecular Orbitals Composition of Complex II (%)

4 CONCLUSION

Two UO22+complexes have been synthesized and characterized. In complex I, the U1 is a seven-coordinate pentagonal bipyramidal configuration. In II, the U1 is an eight-coordinate hexagonal bipyramidal configuration. I and II are stable up to 100 ℃. The quantum chemical has indicated that complexes I and II are more difficult to lose electrons and be oxidized.