Oxalate

Article pubs.acs.org/IC

Mixed-Ligand Uranyl Polyrotaxanes Incorporating a Sulfate/Oxalate Coligand: Achieving Structural Diversity via pH-Dependent Competitive Effect Zhen-ni Xie,†,‡,∥ Lei Mei,†,∥ Kong-qiu Hu,† Liang-shu Xia,‡ Zhi-fang Chai,†,§ and Wei-qun Shi*,† †

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China School of Nuclear Science and Technology, University of South China, Hengyang 421001, Hunan Province, China § School of Radiological and Interdisciplinary Sciences and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China ‡

S Supporting Information *

ABSTRACT: A mixed-ligand system provides an alternative route to tune the structures and properties of metal−organic compounds by introducing functional organic or inorganic coligands. In this work, five new uranyl-based polyrotaxane compounds incorporating a sulfate or oxalate coligand have been hydrothermally synthesized via a mixed-ligand method. Based on C6BPCA@CB6 (C6BPCA = 1,1′(hexane-1,6-diyl)bis(4-(carbonyl)pyridin-1-ium), CB6 = cucurbit[6]uril) ligand, UPS1 (UO2(L)0.5(SO4)(H2O)·2H2O, L = C6BPCA@CB6) is formed by the alteration of initial aqueous solution pH to a higher acidity. The resulting 2D uranyl polyrotaxane sheet structure of UPS1 is based on uranyl-sulfate ribbons connected by the C6BPCA@CB6 pseudorotaxane linkers. By using oxalate ligand instead of sulfate, four oxalate-containing uranyl polyrotaxane compounds, UPO1−UPO4, have been acquired by tuning reaction pH and ligand concentration: UPO1 (UO2(L)0.5(C2O4)0.5(NO3)· 3H2O) in one-dimensional chain was obtained at a low pH value range (1.47−1.89) and UPO2 (UO2(L)(C2O4)(H2O)·7H2O)obtained at a higher pH value range (4.31−7.21). By lowering the amount of oxalate, another two uranyl polyrotaxane network UPO3 ((UO 2 ) 2 (L) 0.5 (C 2 O 4 ) 2 (H 2 O)) and UPO4 ((UO 2 ) 2 O(OH)(L)0.5(C2O4)0.5(H2O)) could be acquired at a low pH value of 1.98 and a higher pH value over 6, respectively. The UPO1− UPO4 compounds, which display structural diversity via pH-dependent competitive effect of oxalate, represent the first series of mixed-ligand uranyl polyrotaxanes with organic ligand as the coligand. Moreover, the self-assembly process and its internal mechanism concerning pH-dependent competitive effect and other related factors such as concentration of the reagents and coordination behaviors of the coligands were discussed in detail.

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INTRODUCTION Actinide coordination polymers, which show fascinating topologies and functions as well as the intriguing 5f bonding features, have drawn extensive attention from chemists and material specialists.1−3 As one of the key actinide elements in nuclear fuel cycle, uranium has been studied most extensively among all the actinides and displays enough ability to successfully construct a wide variety of frameworks.4−7 Uranyl cation (UO22+) is the most common form of hexavalent uranium, the unique structure of which with two axial oxygen atoms and an equatorial plane contributes to the richness of the structural topologies of such solid compounds.8−11 Among the large library of uranyl-organic compounds, various aliphatic or aromatic carboxylic acids have been used as organic linkers in several early studies.10,12−14 However, supramolecular linkers such as pseudorotaxane or rotaxane, one of the typical mechanically interlocked molecules (MIMs),15−17 were rarely utilized for the construction of actinide coordination polymers. As a special supramolecular motif with unique mechanical properties, MIM has been suggested to be organized inside the © XXXX American Chemical Society

metal−organic polymer materials to achieve a higher level of molecular organization and create functional materials,16,18−22 such as molecular switch, molecular machines, molecular piston, molecular pump, molecular electronic devices and molecular memories. To date, there have been numerous reports on the use of MIMs (almost exclusively using pseudorotaxane precursors) as linkers in coordination polymers to create interesting crystalline lattices,16,17,23−26 but few of these materials employ actinides as metal nodes. To this end, we have recently reported a series of interesting 1D, 2D, and 3D coordination polymers with rotaxane linkers and uranyl nodes,27−31 which encourage us to pursue more well-designed actinide polyrotaxanes with intriguing topologies and potential functions through the utilization of new synthesis strategies and methods. As a new synthesis methodology, a mixed-ligand system provides an alternative route to tune the structure and Received: October 16, 2016

A

DOI: 10.1021/acs.inorgchem.6b02515 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Synthetic Conditions for UPSs and UPOs UO2(NO3)2·6H2O (mmol) L1 or L1′ (mmol) Na2C2O4 (mmol) HNO3 (4M) (μL, mmol) NaOH (1M) (μL, mmol) pHia pHfb temperature (°C) time (h) other phases a

UPS1

UPS2

UPO1

UPO2

UPO3

UPO4

0.035 0.035 0 40, 0.16 0 1.30 1.53 150 48 UP

0.035 0.035 0 0 50, 0.05 3.46 2.93 150 48 pure

0.035 0.035 0.035 30, 0.12 0 1.47 1.85 150 48 UPO2

0.035 0.035 0.035 0 0 4.31 3.13 150 48 pure

0.035 0.035 0.07 10, 0.04 0 1.98 3.62 150 48 pure

0.035 0.035 0.07 0 30, 0.03 6.10 4.08 150 48 UPO2

pHi: initial pH of aqueous solution. bpHf: final pH of aqueous solution. Synthesis. All the uranyl compounds were synthesized hydrothermally under autogenous pressure by using a Teflon-lined stainlesssteel bomb from a mixture of uranyl nitrate hexahydrate, pseudorotaxane precursors L1 or L1′, and auxiliary ligands (sulfate ion or oxalate ion), where L1 or L1′ transformed in situ into the carboxylate form (C6BPCA). The corresponding synthetic conditions for UPSs and UPOs are shown in Table 1. UPS1 (UO2(C6BPCA@CB6)0.5(SO4)(H2O)·2H2O) from L1. A 0.5 M uranyl nitrate solution (70 μL, 0.035 mmol) was added to a suspension of the pseudorotaxane ligand of L1 (0.052 g, 0.035 mmol) in water (2 mL) in a stainless-steel bomb. After treating with HNO3 (40 μL, 0.16 mmol) to adjust the initial pH to about 1.30, the bomb was sealed, kept at 150 °C for 48 h, and gradually cooled to room temperature. The resulting product showing a mixture of UPS1 (brilliant yellow laminar crystallites) as well as another uranyl compound reported previously29 (pale yellow prismatic crystallites, namely as UP) was then filtered off respectively, washed with water, and dried at room temperature. The final pH of the reaction mixture was 2.7. IR (cm−1): 3074 (w, ν(CH2)); 2933 and 2848 (w, ν(CH2) and ν(CH)); 1734 (vs, ν(COO)C6BPCA); 1639−1616 (m, ν(C O)CB6); 1570 (w, ν(COO)C6BPCA); 1472 (s, ν(C−Npyridinium); 1379 (m, ν(NCN)CB6); 1325 (m, ν(NCN)CB6); 1240 (m, ν(C−N)CB6); 1190 (m, ν(C−N)CB6); 1049 (m, ν(SO4)); 960 (m, δ(CH)pyridinium); 924 (w, ν(UO22+));798 (s, γ(CH)pyridinium). When the hydrothermal reaction was conducted under similar conditions except the change of initial pH from ∼1.30 to a higher value in the range of 1.9 to 4.6, a previously reported sulfate-bearing uranyl compound29 (for the convenience of discussions that follow, namely as UPS2 here), were obtained, with the final pH of the reaction mixture turning out to be in the range of 2.5 to 3.9. Concentration-Dependent Synthesis of UPS1 from L1′. UPS1 could be synthesized by replacing L1 with L1′ in the presence of additional sulfate ion under similar hydrothermal conditions as described above. In order to find out the effect of sulfate ion on structural evolution of final products in sulfate-containing mixed-ligand system, a group of experiments with varying amount of sulfate ion were conducted as followed: seven reaction vessels with 0.5 M uranyl nitrate solution (70 μL, 0.035 mmol), L1′ (0.052 g, 0.035 mmol), and water (2 mL) were numbered from 1 to 7, and different amounts of (NH4)2SO42− (0 μL (0 mmol), 17.5 μL (0.00875 mmol), 35 μL (0.0175 mmol), 52.5 μL (0.02625 mmol), 70 μL (0.035 mmol), 140 μL (0.07 mmol), and 350 μL (0.175 mmol)) was added, respectively. Then HNO3 (40 μL, 0.08 mmol) was added to all the seven reaction kettles to adjust the initial pH value to about 1.30. The reaction kettles were sealed, kept at 150 °C for 48 h, and gradually cooled to room temperature. The resulting products of reaction kettles nos. 1−4 were pure UP (with the average yield of 54.2%), while the resulting products of reaction kettles no. 5 showing a mixture of UP and UPS1. The reaction kettles nos. 6 and 7 obtained UPS1 (with the average yield of 27.6%). UPO1 (UO2(C6BPCA@CB6)0.5(C2O4)0.5(NO3)·3H2O). A mixture of pseudorotaxane ligand L1′ (0.052 g, 0.035 mmol), Na2C2O4 (9.38 mg, 0.070 mmol), H2O (2 mL), and 0.5 M uranyl nitrate solution (70 μL,

properties of metal−organic compounds by introducing a second functional organic or inorganic ligand as the coligand.25,26 In our previous work, a similar strategy has been also used to assemble uranyl-organic rotaxane compounds with an inorganic sulfate ion involved.27−30 However, a combination of a pseudorotaxane ligand and simple organic ligands have never been used in uranyl polyrotaxanes. Oxalate, which has been intensively studied for their potential application in actinide separation reprocessing of spent nuclear fuel, could be a versatile ligand with small steric hindrance and different coordination possibilities.32 Herein, we introduced oxalate ligand as an organic coligand into the uranyl polyrotaxane system for the first time. Another important issue is the sensibility of uranyl hydrothermal system to pH. In fact, not only uranyl hydrolysis but also deprotonation of ligands could be largely affected by the aqueous pH values. In this work, pH-dependent competitive effect has been intensively investigated for both of the sulfate-containing and oxalate-containing mixed-ligand systems, and a series of sulfate/ oxalate-bearing uranyl polyrotaxanes with structural diversity have been obtained by change on reaction condition. Moreover, the self-assembly process and its internal mechanism concerning pH-dependent competitive effect and other related factors such as concentration of the reagents and coordination behaviors of the coligands were discussed in detail.

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EXPERIMENTAL SECTION

General Methods. Caution! Precautions with suitable care and protection for handling uranyl nitrate UO2(NO3)2·6H2O should be followed, while natural uranium is a radioactive and chemically toxic reactant. 1,1′-(Hexane-1,6-diyl)bis(4-(ethoxycarbonyl)pyridin-1-ium) bromide ([C6BPCEt]Br2) as well as cucurbit[6]uril (CB[6]) including CB[6](H2SO4) and CB[6](HCl) was synthesized according to ref 33. The corresponding pseudorotaxane precursors, [C6BPCEt@ CB6] Brx[SO4]1−0.5x (L1) and [C6BPCEt@CB6]Br2 (L1′), were synthesized according to the reported procedure previously.30 Commercially purchased uranyl nitrate UO2(NO3)2·6H2O (12.55g, 0.025 mol) was dissolved in deionized water (50 mL) to give 0.5 M uranyl nitrate solution. Other chemicals were commercially purchased and used without further purification. Powder X-ray diffraction measurements (PXRD) were obtained with a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range 5−70° (step size: 0.02°). Thermogravimetric analysis (TGA) was recorded from a TA Q500 analyzer over the temperature range of 25−800 °C in air atmosphere with a heating rate of 5 °C/min. The Fourier transform infrared (IR) spectra were performed on KBr pellets in the range of 4000−400 cm−1 on at Bruker Tensor 27 spectrometer. Solid-state fluorescence spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer. B

DOI: 10.1021/acs.inorgchem.6b02515 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Crystal Data and Structure Refinement for Uranyl-Based Compounds UPS1 and UPOs formula formula weight crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K F(000) Dc, g/cm3 μ (mm−1) Rint R1, wR2 (all data)

UPS1

UPO1

UPO2

UPO3

UPO4

C27H34N13O17SU 1082.76 monoclinic P21/c 14.1824(2) 21.4688(2) 12.25055(17) 90 110.6498(16) 90 3490.40(9) 4 296 2124.0 2.060 4.810 0.0266 0.0407, 0.0642

C28H28N14O17U 1070.67 triclinic P1̅ 12.3767(6) 12.4099(6) 12.8991(5) 64.776(2) 82.741(2) 79.506(2) 1759.68(14) 2 304 1044.0 2.021 4.713 0.0260 0.0229, 0.0560

C56H56N26O23U 1699.29 triclinic P1̅ 12.3634(7) 12.7236(8) 23.2444(14) 87.389(2) 87.387(2) 72.441(2) 3480.6(4) 2 291 1700.0 1.621 2.427 0.0356 0.0356, 0.0928a

C31H30N13O21U2 1396.74 triclinic P1̅ 11.5890(19) 12.452(2) 16.987(3) 97.075(8) 109.271(7) 104.827(7) 2178.4(7) 2 295 1318.0 2.129 7.521 0.0565 0.0510, 0.0964a

C28H28N13O17U2 1297.69 monoclinic P21/c 17.8932(11) 12.9624(8) 20.0984(14) 90 111.563(2) 90 4335.4(5) 4 291 2428.0 1.984 7.542 0.0622 0.0620, 0.0725a

a R1, wR2 after PLATON/SQUEEZE process. The corresponding R1 and wR2 values without PLATON/SQUEEZE are 0.0643, 0.1968 (UPO2); 0.0675, 0.1651 (UPO3); 0.0885, 0.1878 (UPO4).

(m, ν(C−N)CB6); 966 (m, δ(CH)pyridinium); 924 (w, ν(UO22+)); 798 (s, γ(CH)pyridinium). UPO4 ((UO2)2O(OH)(C2O4)0.5(C6BPCA@CB6)0.5(H2O)). A mixture of pseudorotaxane ligand L1′ (0.052 g, 0.035 mmol), Na2C2O4 (4.69 mg, 0.035 mmol), H2O (2 mL), and 0.5 M uranyl nitrate solution (70 μL, 0.035 mmol) was placed in a stainless-steel bomb. Then NaOH was added to adjust the initial pH value in the range of 6.10 to 6.84. The mixture was sealed, kept at 150 °C for 48 h, and gradually cooled to room temperature. The resulting product showed a mixture of UPO2 (light yellow prismatic crystallites) and a small amount of UPO4 (deep yellow block crystallites). The final pH of the reaction mixture was in the range of 4.08 to 4.11. IR (cm−1): 3074 (w, ν(CH2)); 2923 and 2853 (w, ν(CH2) and ν(CH)); 1737 (vs, ν(COO)C6BPCA); 1658 (m, ν(COO)oxalate and ν(CO)CB6); 1562 (m, ν(COO)C6BPCA); 1473 (s, ν(C−Npyridinium), 1381 (m, ν(NCN)CB6); 1325 (m, ν(NCN)CB6); 1236 (m, ν(C−N)CB6); 1188 (m, ν(C−N)CB6); 968 (m, δ(CH)pyridinium); 910 (w, ν(UO22+)); 798 (s, γ(CH)pyridinium). X-ray Single Crystal Structure Determination. Single crystals suitable for X-ray single crystal structure determination isolated from each bulk sample were mounted on micromounts. For UPS1, X-ray diffraction data were collected using 0.5° φ and ω scans on an Aglient Technologies SuperNova Atlas dual system diffractometer equipped with an Eos CCD detector using a Mo Kα X-ray microfocus sources (λ = 0.71073 Å) under room temperature (296 K). Data reductions were performed with the CrysAlisPro package, and an analytical absorption correction was performed. For UPO1, UPO2, UPO3, and UPO4, Xray diffraction data of each compound was performed on Bruker D8 VENTURE X-ray diffractometer equipped with a Photon CMOS detector using a Mo Kα X-ray source (λ = 0.71073 Å) at room temperature. All data was integrated using the SAINT software package, and an absorption correction was applied using SADABS. All the crystal structures were solved by means of direct methods (SHELXS-97) and refined with full-matrix least-squares on SHELXL2014 within APEX III software package.34,35 The aromatic and hydroxyl hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon or oxygen atoms. In UPO2, UPO3, and UPO4, PLATON/SQUEEZE36,37 was employed to calculate the diffraction contribution of the solvent water molecules and, thereby, to afford a new set of solvent-free diffraction intensities because the unit cell for each compound includes a large region of disordered solvent water molecules, which could not be modeled as discrete atomic sites. The crystal data of all these four compounds are given in Table 2. Crystallographic data for the structures in this paper

0.035 mmol) was placed in a stainless-steel bomb. Then HNO3 (30 μL, 0.12 mmol) was added to adjust the initial pH value to about 1.47. The mixture was sealed, kept at 150 °C for 48 h, and gradually cooled to room temperature. The resulting product showed a mixture of UPO1 (a small amount of light yellow block crystallites) and UPO2 (a large number of light yellow prismatic crystallites, described as followed). The final pH of the reaction mixture was 1.85. IR (cm−1): 3074 (w, ν(CH2)); 2923 and 2849 (w, ν(CH2) and ν(CH)); 1734 (vs, ν(COO)C6BPCA); 1682 (w, ν(COO)oxalate); 1637 (m, ν(CO)CB6); 1568 (w, ν(COO)C6BPCA); 1473 (s, ν(C−Npyridinium); 1375 (m, ν(NCN)CB6); 1323 (m, ν(NCN)CB6); 1236 (m, ν(C−N)CB6); 1188 (m, ν(C−N)CB6); 966 (m, δ(CH)pyridinium); 924 (w, ν(UO22+)); 798 (s, γ(CH)pyridinium). UPO2 (UO2(C6BPCA@CB6)(C2O4)(H2O)). A mixture of pseudorotaxane ligand L1′ (0.052 g, 0.035 mmol), Na2C2O4 (9.38 mg, 0.070 mmol), H2O (2 mL), and 0.5 M uranyl nitrate solution (70 μL, 0.035 mmol) was placed in a stainless-steel bomb. Then NaOH was added to adjust the initial pH value in the range of 4.31 to 7.21. The mixture was sealed, kept at 150 °C for 48 h, and gradually cooled to room temperature. The final pH of the reaction mixture was in the range of 3.13 to 3.24. The obtained light yellow prismatic crystals were washed with water and ethanol and dried in air to afford compound UPO2 (with the average yield of 50.5%). IR (cm−1): 3074 (w, ν(CH2)); 2919 and 2853 (w, ν(CH2) and ν(CH)); 1736 (vs, ν(COO)C6BPCA); 1682 (w, ν(COO) oxalate ); 1633 (m, ν(CO) CB6 ); 1570 (w, ν(COO)C6BPCA); 1473 (s, ν(C−Npyridinium); 1375 (m, ν(NCN)CB6); 1323 (m, ν(NCN)CB6); 1236 (m, ν(C−N)CB6); 1188 (m, ν(C− N)CB6); 966 (m, δ(CH)pyridinium); 924 (w, ν(UO22+)); 798 (s, γ(CH)pyridinium). UPO3 ((UO2)2(C6BPCA@CB6)0.5 (C2O4 )2 (H 2O)). A mixture of pseudorotaxane ligand L1′ (0.052 g, 0.035 mmol), Na2C2O4 (4.69 mg, 0.035 mmol), H2O (2 mL), and 0.5 M uranyl nitrate solution (70 μL, 0.035 mmol) was placed in a stainless-steel bomb. Then HNO3 (10 μL, 0.04 mmol) was added to adjust the initial pH value to 1.98. The mixture was sealed, kept at 150 °C for 48 h, and gradually cooled to room temperature. The obtained yellow prismatic crystals were washed with water and ethanol and dried in air to afford compound UPO3. The final pH of the reaction mixture was 3.62. IR (cm−1): 3074 (w, ν(CH2)); 2923 and 2849 (w, ν(CH2) and ν(CH)); 1736 (vs, ν(COO)C6BPCA); 1678 (vw, ν(COO)oxalate); 1631 (s, ν(CO)CB6); 1570 (w, ν(COO)C6BPCA); 1473 (s, ν(C−Npyridinium), 1382 (m, ν(NCN)CB6); 1323 (m, ν(NCN)CB6); 1238 (m, ν(C−N)CB6); 1190 C

DOI: 10.1021/acs.inorgchem.6b02515 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) 2D polyrotaxane network of compound UPS1 with CB[6] omitted for clarity (insert: coordination spheres of uranyl and sulfate ions). (b) 2D polyrotaxane network of compound UPS1 showing uranyl-sulfate ribbons. (c) The rhomboid, three-membered molecular necklace in the 2D polyrotaxane network of UPS1. have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-1502448 (UPO1), CCDC1502449 (UPO2), CCDC-1502450 (UPO3), CCDC-1502451 (UPO4), and CCDC-1502452 (UPS1).

47.153(1)°. Structural analysis indicates that the 2D uranyl polyrotaxane sheet are packing through three kinds of C−H···O hydrogen bonds: the first type is between the uncoordinated carboxylate oxygen and the carbon of cucurbituril macrocycles, the other one is between the uncoordinated oxygen of sulfate ion and the carbon of cucurbituril macrocycles, and the last one is between the axial oxygen of uranyl motifs and the carbon of cucurbituril macrocycles. The widely distributed C−H···O hydrogen bonding between two adjacent 2D uranyl polyrotaxane sheets define a hydrogen-bonded three-dimensional framework (Figure S3). The crystal structure of UPO1 is composed of a mononuclear uranyl center, a nitrate ion, half an oxalate ion and half a pseudorotaxane C6BPCA@CB6 ligand (Figures 2a and S4). Besides the carboxyl groups, the uranium atom is

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RESULTS AND DISCUSSION Structure Description. Crystallographic analysis of UPS1 shows that it crystallizes in the monoclinic space group of P21/ c, containing a uranyl-sulfate ribbon connected by the C6BPCA@CB6 pseudorotaxane carboxylate linker originated from in situ hydrolysis of C6BPCEt@CB6 (Figures 1 and S1). Each uranyl cation is 7-fold coordinated and forms a pentagonal bipyramidal environment with an OUO angle of 178.7(1)° and axial UO distances of 1.763(2) and 1.760(2) Å (Figure 1a). The coordination sphere of uranyl is composed of a η2-mode and a η1-mode sulfate ion, a η1-mode carboxylic group of pseudorotaxane and a water molecule in the equatorial plane (Figure 1a), with the U−O bond lengths ranging from 2.284(3) to 2.456(3) Å (U−O3 = 2.285(3) Å, U−O5 = 2.456(3) Å, U−O6 = 2.434(3) Å, U−O8 = 2.399(3) Å, U−O9 = 2.416(3) Å) (Table S1). Each tetrahedral sulfate anion in the uranyl-sulfate ribbon is monodentate-coordinated with a uranyl cation on one side and bidentate-coordinated with another uranyl cation on the other side. All the C6BPCA@CB6 pseudorotaxane ligands here present similar coordination modes, where both the carboxyl groups of C6BPCA@CB6 linkers are coordinated to the uranyl cations from two adjacent uranyl-sulfate ribbons by a monodentate pattern, which leads to an extended 2D wave-shaped sheet (Figure 1b). With the continuous linkage of uranyl-sulfate ribbon between two uranyl nodes instead of pseudorotaxane linkers, the minimum building block in the 2D network of UPS1 is interestingly a three-membered molecular necklace ([3]MN) rather than the [5]MN (Figure 1c). The [3]MN presents a rhomboid shape with a cavity in the center of the ring, and the two internal side lengths are 12.251(3) and 22.428(3) Å, respectively, with an intersection angle of

Figure 2. (a) First coordination sphere of uranium atoms in UPO1. (b) A front view of the extended 1D sidestep-shaped chain of UPO1. (c) A top view of the extended 1D sidestep-shaped chain of UPO1. D

DOI: 10.1021/acs.inorgchem.6b02515 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

present in the crystal lattice define a hydrogen-bonded threedimensional framework. The crystal structure of UPO3 is composed of half a pseudorotaxane ligand and two crystallographically nonequivalent uranyl centers, U1 and U2, which connect with each other through an oxalate ion (Figures 4 and S7). Both of the uranyl centers are seven-coordinated with apical UO distances in the range of 1.738(7) to 1.767(6) Å and OU O angles of 178.5(3)° and 179.3(3)°. The equatorial plane of U1 contains five oxygen atoms: four carboxyl oxygen atoms from two oxalate ions (U−O1= 2.414(5) Å, U−O2 = 2.379(5) Å, U−O3 = 2.423(6) Å, U−O4 = 2.414(5) Å), and a terminal aquo ligand (U1−O5 = 2.389(6) Å), while that of U2 contains four carboxyl oxygen atoms of two oxalate ions, (U−O8= 2.437(5) Å, U−O9= 2.437(5) Å, U−O10= 2.449(5) Å, U− O11= 2.427(5) Å), and one η1-carboxyl oxygen atom of the pseudorotaxane ligand (U1−O12 = 2.306(5) Å) bridging two adjacent uranyl-oxalate ribbons. Notably, the equatorial planes of U1 and U2 show an intersection angle of 16.912°, leading to a slight bend of the polyrotaxane ribbons (Figure 4d,e). As usually observed, each oxalate ligand chelates two uranyl cations symmetrically from both sides by a conventional μ2bridging mode. Similar to UPS1, UPO3 contains uranyl-oxalate ribbons and the C6BPCA@CB6 ligands acting as linkers between two adjacent ribbons (Figure 4a,b). Figure 4c presents the repeating unit in the 2D network, which is a three-membered molecular necklace ([3]MN) exhibiting a rhomboid shape. The two internal side lengths are 22.470(3) and 23.336(4) Å, respectively, with an inclined angle of 75.711(2)°. Moreover, we compared the molecular structure of UPO3 with a previously reported mixed-ligand uranyl-organic compound, (UO2)4(C2O4)3(NO3)(H2O)6·CB6·2H2O),40 using simple oxalate and CB[6] ligands. Since CB[6], not C6BPCA@CB6 pseudorotaxane ligand, used in this uranyl-organic compound, only oxalate ligand as well as nitrate and water coordinated to uranyl center to form a similar short uranyl-oxalate ribbon, while CB[6] was involved in the crystal lattice as a structuredirecting agent by hydrogen bonds. Further, even CB[6] molecule is coordinated with uranyl in other mixed-ligand uranyl-organic compound,41−46 the final coordination modes are also totally different from C6BPCA@CB6 pseudorotaxane ligand. The remarkable disparity between them suggests that C6BPCA@CB6 pseudorotaxane ligand also play a vital role in mediating the coordination assembly of uranyl nodes by its characteristic terminal carboxylate groups. The crystal structure of UPO4 contains two crystallographically nonequivalent uranyl centers in a tetranuclear motif (Figures 5 and S8). Both of the uranyl centers are sevencoordinated with apical UO distances in the range of 1.791(4)-1.776(4) Å and OUO angles of 174.6(2) and 175.2(2)°. The equatorial plane of U1 contains five oxygen atoms: two oxygen atoms from a μ2-bonded oxalate ion (U1− O5 = 2.464 (4) Å, U1−O6 = 2.484(4) Å), one oxygen atom from the bridging carboxyl group of C6BPCA@CB6 (U1−O7 = 2.391(4) Å), one μ3-oxo atom (U1−O11 = 2.197(4) Å), and a μ2-hydroxo atom, while that of U2 contains the other oxygen atom from the bridging carboxyl group (U2−O8 = 2.506(4) Å), two μ3-oxo atoms (U2−O11= 2.242(4) Å, U2−O11B = 2.342(4) Å), one terminal aquo ligand (U2−O9 = 2.542(5) Å), and a μ2-hydroxo atom (Figure 5a). Each oxalate ligand chelates two neighboring tetranuclear uranyl motieties from both sides, leading to a 1D tetranuclear uranyl-oxalate ribbon.

coordinated with a bidentate nitrate ion (U−O1 = 2.509(2) Å, U−O2 = 2.522(2) Å), a chelate oxalate ion (U−O3 = 2.442(2) Å, U−O4 = 2.428(2) Å) and a η2-mode carboxylic group of pseudorotaxane (U−O5 = 2.579(2) Å, U−O6 = 2.439(2) Å) to form typical hexagonal-bipyramidal geometries with axial UO distance of 1.753(7) on average and OUO angle of 179.2(1)° (Table S1). Specially, the case that nitrate ion is present in the first coordination sphere of uranyl in solid-state coordination polymer has been rarely reported,38 since the weak coordination ability of NO3− relative to other counteranions.39 Each oxalate ligand chelates two uranyl cations symmetrically from both sides by a conventional μ2-bridging mode. The pseudorotaxane carboxylate ligand C6BPCA@CB6 is originated from in situ hydrolysis of C6BPCEt@CB6, of which the two carboxyl groups on each side coordinated to uranyl cation in a bidentate pattern. The “pseudorotaxaneuranyl-oxalate-uranyl-pseudorotaxane···” mode produces a onedimensional sidestep-like polyrotaxane structure (Figure 2b,c) and links to adjacent chains through hydrogen bonds between the polyrotaxane linkers and the water molecules present in the crystal lattice. The crystal structure of UPO2 contains one uranium center in its independent unit (Figures 3a and S5), which is 7-fold

Figure 3. First coordination sphere of uranium atom in UPO2 (a), and the extended 1D zigzag-shaped chain of UPO2 with uranium center in a stick mode (b) and in a pentagonal bipyramid polyhedron (c).

coordinated with two carboxyl oxygen atoms from two pseudorotaxane ligands (U−O1 = 2.356(3) Å, U−O2 = 2.389(3) Å), two carboxyl oxygen atoms from a chelate oxalate ion (U−O3 = 2.347(2) Å, U−O4 = 2.340(3) Å), two oxygen atoms from the typical uranyl bonds (U = O5 = 1.763(3) Å, U = O6 = 1.767(3) Å) with the OUO bond angel of 178.10(15)°, and a water molecule in terminal position (U−O7 = 2.476(3) Å). Different from the coordination mode of oxalate found in UPO1, each oxalate here chelates only one uranyl cation from one side, and the C6BPCA@CB6 linker connect uranyl nodes on both terminals in a monodentate pattern, which forms an extended 1D zigzag-shaped chain (Figure 3b). The widely distributed C−H···O hydrogen bonding between uranyl polyrotaxane chains and adjacent water molecules E

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Figure 4. (a) 2D polyrotaxane network of compound UPO3 contains uranyl-oxalate ribbons. (b) The rhomboid, three-membered molecular necklace in the 2D polyrotaxane network. (c) The coordination environment of uranium atoms. (d) The 2D network shows a slight wave-like sheet. (e) The angle between the two equatorial plant of U1 and U2 is 16.912°.

membered molecular necklace ([3]MN) with the two internal side lengths of 12.962(2) and 25.056(1) Å, respectively, and an inclined angle of 12.962(1)°. The comparison of UP, UPS1, UPO3, and UPO4 shows that the [3]MN of UPO4 is close to that of UPS1, and the length of C6BPCA@CB6 linkers in UPO4 are the longest among the four compounds, which could have resulted by the η2-mode of the carboxyl group than unique η1-mode. pH-Dependent Competitive Effect in Sulfate-Containing Mixed-Ligand System. Although the sulfate-containing mixed-ligand system was established in our previous work,29 a systematic study concerning pH-dependent structural variance has not been explored yet. Detailed studies on sulfate-bearing uranyl polyrotaxanes will be helpful to guide other mixed-ligand systems such as oxalate-containing system followed. Based on our previous work about UPS2, we made a minor modification on the hydrothermal conditions by the alteration of initial aqueous solution pH to a higher acidity so as to explore the effect of pH on the hydrothermal products of L1 and uranyl. Interestingly, it is a totally new sulfate-containing uranyl compound UPS1 rather than previously reported UPS2 that could be found in the final products, though another known crystal phase, UP,29 was also formed together (Scheme 1 and Figure S9a-b). Unlike the seven-coordinated sulfate-free mononuclear uranyl node in UP and the symmetric η2-sulfate coordinated carboxylate-bridging monomeric uranyl node in UPS2, a trident sulfate linker in a (μ2-η2, η1) mode is found in UPS1. The distinctly different structures of UPS1 and UPS2, especially different coordination modes of the sulfate group,

Figure 5. (a) Coordination environment of uranium atoms. (b) 2D polyrotaxane network of compound UPO4 contains tetranuclear uranyl-oxalateribbons. (c) The rhomboid, three-membered molecular necklace in the 2D polyrotaxane network.

Similar to UPS1, C6BPCA@CB6 moieties here exhibit a unique coordination mode, of which both ends are in η2-mode, connecting the adjacent uranyl-ocalate ribbons and finally form a 2D uranyl polyrotaxane network (Figure 5b). The minimum building blocks were depicted in Figure 5c, which is a threeF

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Table 4. Stability constants (logK) of uranyl complexes with different ligands and dissociation constants (pKa) of some weak acids log Ka H2O NO3‑ Ph−COOHc SO42‑ C2O42‑

pKab

−5.20 ± 0.30 0.30 ± 0.1550 2.57d 3.15 ± 0.0250 4.63 ± 0.02e

nature of acid form

50

4.2 1.2, 4.2

strong acid weak acid strong acid weak acid

a

log K: stability constants of uranyl complexes with the corresponding ligand. bpKa: dissociation constant of weak acid. cPh−COOH was selected as the model compound of C6BPCA@CB6. dDetermined at pH value of 1.5−3.5.49 eCalculated from (430 ± 25)*100,which was obtained at pH value of 1.7−3.5.51

acid (pKa ≈ 4.2) is more difficult. When the acidity of aqueous solution is high as for the preparation of UPS1, the protonated form of benzoic acid dominates in a large amount and decreases its coordination capacity of C6BPCA@CB6 to uranyl. On the other hand, there is no obvious change of coordination capacity for sulfate ion as well as water molecule as the pH changes. Thus, in order to complete the coordination sphere of the uranyl center, a sulfate group as well as a water molecule show a higher degree of participation into uranyl coordination through increased connectivity or coordination number. According to the tuning mechanism proposed above, the important role of pH on the sulfate-containing mixed-ligand system of uranyl polyrotaxane is closely related to the metalcoordination competition between sulfate coligand and pseudorotaxane ligand. Then we further investigated the effect of concentration of sulfate ions on the hydrothermal process. It should be noticed that, given the ambiguous and hard-tocontrol concentration of residual sulfate in sulfate-containing pseudorataxane precursor L1, another feasible way by mixing controllable sulfate ions with sulfate-free pseudorataxane L1′, was used for the study. A group of experiments starting from uranyl nitrate and L1′ with varying amount of sulfate ion added were conducted under similar hydrothermal conditions (aqueous pH were kept at about 1.30, see the Experimental Section). As the PXRD patterns showed (Figure 6), when the ratio of sulfate/uranyl was below 0.75, UP will dominate in the final products (Figure S9c). As the ratio was increased to 1.00, the resulting products turned out to be a mixture of UP and UPS1 (Figure S9d). When the ratio of sulfate/uranyl further grew to 2.00 and above, UPS1 be the major product by increasing the amount of sulfate ion (Figure S9e). The above results suggest that only a high amount of sulfate ion can afford UPS1 in the pure phase, proving the important role of the concentration of sulfate ions serving as auxiliary ligands on the formation process and compositions of uranyl polyrotaxane based on mixed-ligand system. pH-Dependent Competitive Effect in Oxalate-Containing Mixed-Ligand System. As a typical organic ligand, oxalate has been intensively studied for its application in actinide separation of spent nuclear fuel. On the basis of sulfatebearing uranyl polyrotaxanes with sulfate ion as the inorganic coligand as mentioned above, we turned to studying this new catalogue of oxalate-containing mixed-ligand system. Herein we employ oxalate as the prototype to explore the possibility of introducing an organic coligand into the skeletons of uranyl polyrotaxanes. Fortunately, four new oxalate-bearing uranyl polyroraxane compounds, UPO1-UPO4, were obtained under

indicate the great influence of pH on the assembly process under hydrothermal conditions. Generally, pH values of aqueous solution could exert influence on uranyl-ligand hydrothermal system through two aspects. One is controlling the hydrolysis of the uranyl cation as described by uranyl hydrolysis equation: mUO22+ + nH2O ↔ [(UO2)m(OH)n]2m‑n + nH+, and the other is affecting the deprotonation of weak acid ligand, such as inorganic phosphoric acid, organic carboxylic acid, et al., and subsequently its coordination ability with the uranyl center.47,48 For UPS1 and UPS2 here, no significant effect of pH on the uranyl hydrolysis was observed when pH changes within this range (UPS2: 3.46, UPS1: 1.30). However, detailed structural analysis reveals that UPS1 obtained at lower aqueous pH has lower uranyl/C6BPCA@CB6 ratio (0.5/1) than UPS2 from higher pH (1/1), while the sulfate/uranyl ratios (1/1) are identical as listed in Table 3. This decreased degree of Table 3. Amounts of Coordination Groups in Uranyl Coordination Spheres of UPSs and UPOs [UO2]

a

[LR]a

[SO4]

[H2O]

[UO2]/[LR]b/[SO4] 1/2/0 1/0.5/1 1/1/1 [UO2]/[LR]a/ [C2O4] 1/0.5/0.5 1/1/1 1/0.25/1 1/0.25/0.25 (1/1/1 b )

UP UPS1 UPS2

1 1 1 [UO2]

2 0.5 1 [LR]a

0 1 1 [C2O4]

0 1 0 [H2O]

UPO1 UPO2 UPO3 UPO4

1 1 2 2 (0.5b)

0.5 1 0.5 0.5

0.5 1 2 0.5

0 1 1 1

[LR]:C6BPCA@CB6 ligand motif (U4)

b

Calculated using tetrameric uranyl

participation into uranyl coordination sphere for C6BPCA@ CB6 reflects to some extent the weakening of coordination capacity of C6BPCA@CB6 to uranyl at lower pH. Considering the nature of weak acid for the organic carboxylic acid form of C6BPCA@CB6, a possible origin of this phenomenon should be related with pH-dependent coordination capacity of C6BPCA@CB6 as well as competitive effect from other coligand. For the ease of discussion, benzoic acid (Ph−COOH) was selected as the model compound of C6BPCA@CB6. As shown in Table 4, although the stability constant of uranyl complexes49 with benzoic acid (log K ≈ 2.57) is comparable to that with sulfate (log K ≈ 3.15),50 the deprotonation of benzoic G

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Scheme 2. pH-Tuning and Concentration-Related Structural Diversity of Oxalate-Based Uranyl Polyrotaxanes

of the concentrations of free C6BPCA@CB6 and C2O42− and their coordination ability to uranyl via enhanced protonation, and meanwhile increased the total concentration of nitrate in the aqueous solution. It is interesting to find that, when compared with UPSs, no participation of nitrate group into the uranyl coordination has been observed for UPS1 under similar conditions, even more amount of HNO3 was added. Apparently, this disparity is closely related with different nature of sulfuric acid and oxalic acid, where the former is a strong acid, but the latter is a weak acid. Moreover, the formation of UPO3 with a less amount of C6BPCA@CB6 in the uranyl coordination sphere (uranyl/ C6BPCA@CB6/C2O42−: 1/0.25/1) than that of UPO2 could be attributed to the improved connectivity of oxalate from oneside to two-side chelate mode when the amount of oxalate was reduced. Unlike UPO3, the formation of tetra-nuclear uranylbearing UPO4 formed underwent a typical process of uranyl hydrolysis. Although oligomeric uranyl is only observed in UPO4 for both of the UPSs and UPOs, it is not a surprising product in consideration of high pH used for the synthesis of UPO4.52 In term of competitive effect between oxalate and C6BPCA@CB6 (maybe also water molecules), the ratio of uranyl/C6BPCA@CB6/C2O42− in UPO4, that is 1/1/1 (the tetrauranyl center is considered as a total motif, see Table 3), is identical to that of UPO2, which is consistent with higher degrees of deprotonation of C6BPCA@CB6 and C2O42− at relatively basic solution. As demonstrated above, similar to the sulfate-containing UPSs, the pH-dependent competitive effect between different ligands also contributes to the difference of UPO1-UPO4 for the oxalate-containing uranyl polyrotaxane system. Another important issue is the difference for both of the systems. Besides the difference in the nature of their corresponding acid forms, the coordination modes and geometric structures of sulfate and oxalate are also different. As shown in Figure 7, both two coordination modes (η2 and μ2−bridging) can be found for sulfate (SO42−) or oxalate (C2O42−) groups. Due to different molecular configurations for them, where the oxalate ion is approximately in a planar configuration but the sulfate ion exhibits as a tetrahedron, the linking modes in the crystal packing are also different. For the μ2−bridging mode, two adjacent uranyl centers connected by an oxalate group are

Figure 6. Powder X-ray diffraction patterns of the products starting from uranyl nitrate and L1′ with different amounts of sulfate ion. The top (black) and bottom (black) parts for the diagram are the simulated spectra from single crystal data of UPS1 and UP, respectively.

different hydrothermal conditions. As expected, compared with UP without oxalate group involved, the participation of oxalate into uranyl coordination in UPOs alters the coordination sphere of the uranyl center and decreases the number of coordinated C6BPCA@CB6 pseudorotaxane group around uranyl, indicating a competitive effect of oxalate coligand with C6BPCA@CB6. Furthermore, although the oxalate groups are involved in all these four polyrotaxane compounds, coordination modes of oxalate as well as total topological structures vary from UPO1 to UPO4, where both the UPO1 and UPO2 exhibit uranylbased 1D structures, while the UPO3 and UPO4 afford 2D sheets containing uranyl-oxalate ribbons. As indicated by the stability constants in Table 4, the successful preparation of oxalate-containing uranyl polyrotaxanes might be attributed to high binding affinity of the oxalate to uranyl (log K ≈ 4.63). The most amazing is pH-dependent structural variance for oxalate-containing uranyl polyrotaxanes. UPO1 was obtained at a low pH value range (1.47−1.89), while UPO2 was obtained at a higher pH value range (4.31−7.21; Scheme 2). When reducing the amount of oxalate added, another two uranyl polyrotaxane compounds UPO3 and UPO4, though UPO2 remained the main product, could be acquired at low pH value of about 1.98 and high pH value over 6.10, respectively (Scheme 2). We make a detailed analysis on the compositions of all the UPOs (Table 3) so as to recognize the formation mechanism of them. For UPO2, the uranyl/C6BPCA@CB6/ C2O42− ratio of 1/1/1 suggests that the binding affinity of oxalate to uranyl is also comparable to that of C6BPCA@CB6 within a large range of pH, which promotes UPOs to be almost the main crystal phase. With pH decreasing, the amounts of C6BPCA@CB6 and C2O42− in coordination sphere lower to 0.5 and 0.5 in UPO1, whereas the weakly coordinated nitrate group (log K ≈ 0.30) took part in the uranyl coordination. A reasonable explanation is that, a significant amount of nitrate acid (HNO3) added to adjust the aqueous pH decreased both H

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uranyl fluorescence through coordination of oxalate to uranyl in a η2 side-on mode followed by an intramolecular electron transfer process. These previous results indicate that the fluorescence quenching observed in UPO2 might be related with the intramolecular electron transfer between uranium center and oxalate group. Similar phenomenon can be found in a uranyl-oxalate compound reported recently.56 Infrared spectra were collected for all the compounds and the corresponding peaks were assigned by a comparison with pseudorotaxane precursor. The IR spectra of UPS1 and UPOs (Figure S12) show the expected UO stretching ribbons in the region of 910 and 924 cm−1. The typical vibrations corresponding to carboxylate groups of C6BPCA@CB6 and oxalate groups covers the range 1741−1562 cm−1, within which the vibration peaks corresponding to carbonyl groups of CB6 are also involved.

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CONCLUSION We have presented five novel uranyl-based polyrotaxane structures with the sulfate or oxalate ion as the coligand. The sulfate-bearing uranyl-based polyrotaxane, UPS1, represent a new type of actinide pseudorotaxane with uranyl-sulfate ribbons linked by pseudorotaxane ligands. All four compounds, UPO1−UPO4, are oxalate-bearing uranyl polyrotaxane, which can be obtained by tuning reaction pH and ligand concentration. The successful preparation of all these mixedligand uranyl-based polyrotaxanes with varied coordination modes and topological structures reveals the utilization of pHdependent competitive effect to be a feasible method to influence the results of the self-assembly process systematically. These newly synthesized uranyl-based polyrotaxane compounds found here not only enriches the family of actinide polyrotaxane comounds but also shed light on the way to explore new structures of actinide supramolecular systems by the mix-ligand method.

Figure 7. Different linking modes found in mixed-ligand uranyl polyrotaxane systems with sulfate (SO42−) (a) or oxalate (C2O42−) (b) ions.

nearly coplanar, while those connecting through a sulfate ion are not. On the other side, when they are coordinated in a η2mode, the difference between them is insignificant. Powder X-ray Diffraction (PXRD), Infrared Spectroscopy (IR), Thermogravimetric Analysis (TGA), and Fluorescence Spectroscopy. The PXRD pattern (Figure S2) of UPS1 matches well with the simulated pattern, which indicates its high purity as a single phase. The crystal sample of UPO2 also showed good phase purity (Figure S6). TGA analysis of UPS1 and UPO2 (Figures S11) demonstrated their similar high thermal stability. They showed no weight loss up to 300 °C, above which total collapse of the molecular structure occurs. Fluorescence spectrum of UPS1 shows a typical double uranyl bonding featured by five broad emission peaks, which is found to be a slight red-shift compared to the benchmark compound UO2(NO3)2·6H2O (Figure 8). Moreover, unlike

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02515. Typical figures including characterization data (PXRD, TGA, and IR) (PDF) Typical crystal and topological structures of all the compounds (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei-qun Shi: 0000-0001-9929-9732

Figure 8. Fluorescence spectra of UPS1 and UPO2 with uranyl nitrate as the benchmark compound, UO2(NO3)2·6H2O.

Author Contributions ∥

Z.-n.X. and L.M. contributed equally.

Notes

UPS1 exhibiting a typical ligand−metal charge transfer (LCMT) emission, there is a fluorescence quenching for UPO2. Considering the significant effect of coordination groups in the coordination sphere on the fluorescence of the uranyl compound, the quenching phenomenon here might be related with the coordinated oxalate group. Several early studies on the quenching of uranyl fluorescence by oxalate ion in solution53−55 show that oxalate has the capacity to quench

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support of this work by the National Natural Science Foundation of China (21671191 and 11405186) and the Major Resarch Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of I

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the Natural Science Foundation of China (91426302 and 91326202) and the Science Challenge Project (JCKY2016212A504). We appreciate the help from Prof. Daofeng Sun and Dr. Liangliang Zhang for X-ray single crystal measurements.

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DOI: 10.1021/acs.inorgchem.6b02515 Inorg. Chem. XXXX, XXX, XXX−XXX