Uranyl Compounds Involving a Weakly Bonded Pseudorotaxane

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Uranyl Compounds Involving a Weakly Bonded Pseudorotaxane Linker: Combined Effect of pH and Competing Ligands on Uranyl Coordination and Speciation Fei-ze Li,†,‡ Lei Mei,‡ Kong-qiu Hu,‡ Shu-wen An,‡ Si Wu,‡ Ning Liu,*,‡ Zhi-fang Chai,‡,§ and Wei-qun Shi*,‡

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Key Laboratory of Radiation Physics and Technology (Sichuan University), Ministry of Education; Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, P. R. China ‡ Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China § Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo 315201, P. R. China S Supporting Information *

ABSTRACT: Pseudorotaxane-type ligands with tunable structural dynamics offer an opportunity in the exploration of new actinide hybrid materials. In this work, we utilized a weakly bonded pseudorotaxane ligand involving CB[6] and 1, 1′-(heptane-1, 7-diyl)bis(4-(ethoxycarbonyl)pyridin-1-ium) bromides ([C7BPCEt]Br2@CB[6]) to assemble with uranyl ion, and we systematically investigated the effect of different factors including pH and competing ligands on the hydrothermal synthesis of URCPs. Nine uranyl-rotaxane coordination polymers (URCPs) with diversity in coordination mode and topological structure were successfully prepared (two previously reported complexes, URCP1 and URCP2 are also included). The results indicate that sulfate, bromide, CB[6], and C7BPCA (the hydrolyzate of [C7BPCEt]Br2) show a combined influence on the obtained URCPs. At low pH, both CB[6] and C7BPCA can bond with uranyl centers and produce interwoven structures in URCP1, URCP2, and URCP6; at high pH, C7BPCA and competing anions (sulfate and bromide) have priority to coordinate with uranyl ions in URCP3−URCP5 and URCP7−URCP9. Notably, for the first time, bromide anion with lower affinity to uranyl ions is also observed in solid-state uranyl coordination polymer (URCP7−URCP9), which has been demonstrated by both energy dispersive X-ray spectroscopy and single-crystal X-ray structure analysis. In addition, a spontaneously single-crystal-to-single-crystal transformation from URCP3 to URCP4, which is driven by thermodynamics, was observed and explained by computational study. Moreover, it reveals that sulfate with stronger coordination ability can inhibit the hydrolysis of uranyl ion to some extent with only a rarely reported pentanuclear uranyl center found in URCP5 obtained at pH 5.67. These results indicate that the combined effect of competing ligands and pH has great significance in the formation of URCPs in terms of uranyl coordination and speciation and can be an alternative way to design and synthesize uranyl coordination polymers with new topologies.

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selective detection,13,14 etc. have also obtained significant breakthroughs recently. The rapid developments of UOCPs have not only enriched actinide coordination chemistry but also extended the study of metal−organic materials (MOMs). The incorporation of mechanically interlocked molecules, such as catenane and rotaxane, into the design and the synthesis of MOMs, has been regarded as an effective way to achieve a high level of molecular organization.15,16 This method provides a unique approach to regulate the structures and properties of the desirable materials at the molecular

INTRODUCTION During the past decades, actinide−organic coordination polymers have drawn extensive attention from chemists and material scientists, because of their unique 5f bonding features and abundant topological structures.1−4 As one of the most representative actinide species, uranyl cation (UO22+), marked with two axial oxygen atoms and an equatorial plane, has been employed as a building block to make actinide coordination compounds and hybrid materials with structural diversity varying from zero-dimensional clusters to three-dimensional frameworks.5,6 In addition to the characteristic of structural features, potential applications of uranyl−organic coordination polymers (UOCPs) in separation,7−10 chemical sensing,11,12 © XXXX American Chemical Society

Received: December 3, 2018

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DOI: 10.1021/acs.inorgchem.8b03353 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Synthetic Information for Different URCPs [C7BPCEt]Br2@CB[6] (mmol) UO2SO4·2.5H2O (mmol) UO2(NO3)2·6H2O (mmol) H2O (mL) pHia pHfb temp (°C) time (h) yield (%)

URCP1

URCP2

URCP3

URCP4

URCP5

URCP6

URCP7

URCP8

URCP9

0.035 0.035 0 2 1.28 1.51 150 72 48.8

0.035 0.035 0 2 2.62 2.82 150 72

0.035 0.035 0 2 3.45 3.80 150 72 65.8

0.035 0.035 0 2 4.46 4.63 150 72 58.6

0.035 0.035 0 2 5.67 5.54 150 72 47.8

0.035 0 0.035 2 1.37 1.62 150 72 55.8

0.035 0 0.035 2 3.50 3.25 150 72

0.035 0 0.035 2 4.20 4.63 150 72 48.8

0.035 0 0.035 2 6.16 6.37 150 72 56.8

pHi: initial pH values of aqueous solution. bpHf: final pH of aqueous solution after hydrothermal reaction.

a

were synthesized according to a previous report.39 CB[6] was allowed to preassemble with [C7BPCEt]Br2 in water to form the pseudorotaxane ligand, i.e., [C7BPCEt]Br2@CB[6]. Commercially purchased uranyl sulfate (UO2SO4·2.5H2O) and uranyl nitrate (UO2(NO3)2·6H2O) were dissolved in deionized water to give 0.5 M uranyl solutions. All other chemicals were commercially purchased and used without any further purification. Powder X-ray diffraction (PXRD) spectra were measured on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å). The thermogravimetric analysis (TGA) of all uranyl compounds was carried out on a TA Q500 analyzer. A Bruker Tensor 27 spectrometer was used to perform Fourier transform infrared (FTIR) spectra. Solid-state fluorescence spectra of all uranyl polyrataxanes were obtained using a Hitachi F-4600 fluorescence spectrophotometer. Synthesis. All URCPs were synthesized by hydrothermal reaction of [C7BPCEt]Br2@CB[6] with uranyl ions using a Teflon lined stainless-steel reaction vessel. To investigate the effect of pH and competing ligands on the final structures of obtained URCPs, UO2SO4·2.5H2O and UO2(NO3)2·6H2O were respectively employed as metal sources to carry out the reactions at different acidities. The pseudorotaxane ligand was demonstrated to transform in situ into the corresponding carboxylate form, namely as C7BPCA@CB[6]. The specific synthetic conditions of pH and competing ligands of the occurrence of each phase are gathered in Table 1. Single-Crystal X-ray Structure Determination. Single crystal X-ray diffraction was carried on a Bruker D8 VENTURE X-ray CMOS diffractometer with a Mo Kα (λ = 0.71073 Å) X-ray source at room temperature or low temperature (120 or 170 K). All data were integrated using the SAINT software package, and the absorption correction was achieved using SADABS. The crystal structures were solved by means of direct methods (SHELXS-9740) and refined with full-matrix least-squares techniques on F2 using the SHELXL201840,41 and Olex242 software packages. The C−H and water molecule hydrogen atoms were added at calculated positions, and all of them were treated as riding atoms with an isotropic displacement parameter 1.2 times higher than that of the parent atoms. The bromide anions in URCP1, URCP6, and URCP7−URCP9 were first revealed by energy dispersive X-ray spectroscopy (EDS) (Figure S1− S5) before their final attributions in single-crystal X-ray diffraction analyses. Considering the large region of disordered water molecules in URCP4 and URCP7−URCP9, we employed PLATON/ SQUEEZE43 to calculate the diffraction contribution of the solvent molecules to obtain new sets of diffraction intensities. The bond lengths of nitrate anion in URCP6 were constrained to be equidistant (∼1.2 Å). Due to the disordering situations for the solvent water and the string molecule (C7BPCA) in URCP8, relevant restraints were taken to restrict the bond lengths to reasonable ranges (C−C ≈ 1.45 Å and CO ≈ 1.25 Å). The aromatic ring in this compound is also slightly twisted because of the disordered structure, which has been constrained to be flat during the refinement process. Because of the disordering situations for the guest anions, bromide atoms in URCP6, URCP8, and URCP9 were given calculated occupancy parameters for disorder over several sites to gain acceptable displacement parameters.

level16,17 and has led to plenty of so-called metal−organic rotaxane frameworks (MORFs).18−23 Typically, to construct crystalline coordination polymers of MORFs, a pseudorotaxane precursor is first preassembled in situ or ex situ by threading a linear axle into a macrocyclic wheel such as a crown ether or cucurbituril24−27 and then allowed to assemble with specific metal ions. Due to the intrinsically interlocked features of pseudorotaxane precursors, MORFs are regarded as a class of soft porous crystals with intriguing topologies.17 Besides, recent reports also reveal that this kind of materials can show interesting dynamic motions in solid state16,28 and some potential applications in guest exchange.29 Attracted by the fascinating features of both MORFs and actinide−organic hybrid materials, we are especially interested in the construction of uranyl−rotaxane coordination polymers (URCPs) from cucubit[n]uril (CB[n])-based pseudorotaxane precursors.30−34 To this end, different series of URCPs have been designed and assembled, encouraging us to pursue welldefined actinide polyrotaxanes with special properties and potential applications by the utilization of new strategies and approaches.35 Actually, the weak intermolecular interactions between the wheel and the axle molecules give more dynamics to pseudorotaxane ligands,36 thus offering us an opportunity in the exploration of new actinide polyrotaxanes as exemplified by those obtained through tuning the binding affinity via weakly bonded pseudorotaxane ligands with “ill-suited” guests and releasing the coordination capability of host molecules.37,38 In this work, we systemically investigated the effect of different factors including pH and competing ligands on the hydrothermal synthesis of URCPs based on a weakly bonded CB[6]bipyridinium pseudorotaxane ligand which involves CB[6] and 1, 1′-(heptane-1, 7-diyl)bis[4-ethoxycarbonyl)pyridin-1-ium] bromides ([C7BPCEt]Br2@CB[6]). Nine URCPs (URCP1 and URCP2 have been previously reported37) were obtained by changing reaction pH or introducing competing ligands, revealing the great influence of reaction reagents or conditions on the coordination patterns of uranyl centers, uranyl speciation, and the final topological structures of URCPs. Those compounds with high phase purity have been further characterized by thermogravimetric analysis, infrared spectroscopy, and luminescence spectroscopy, and the underlying mechanism for uranyl speciation and the diversity of coordination modes has been discussed in detail.

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EXPERIMENTAL SECTION

General Methods. Caution! Because of the radioactive and chemical toxicity of uranium, strict care and protection should be adopted in the process involving uranyl compounds. [C7BPCEt]Br2 and CB[6] B


Article

Inorganic Chemistry Table 2. Crystal Data and Structure Refinement for URCP3−URCP9 formula fw crystal sys space group a, Å b, Å c, Å α, degree β, degree γ, degree V, Å3 T, K F (000) Dc (g cm−3) μ (mm−1) Rint R1, wR2 formula fw crystal sys space group a, Å b, Å c, Å α, degree β, degree γ, degree V, Å3 T, K F (000) Dc (g cm−3) μ (mm−1) Rint R1, wR2

URCP3

URCP4

C55H64N26O29SU 1823.41 triclinic P1̅ 12.4304(5) 16.4919(7) 17.9856(7) 94.617(2) 94.330(2) 107.449(2) 3486.7(2) 294 1832.0 1.737 2.464 0.0575 0.0559, 0.1534 URCP7

C110H142N52O57S2U2 3644.93 triclinic P1̅ 16.5957(7) 18.0817(7) 24.9915(11) 86.131(2) 70.777(2) 85.463(2) 7052.2(5) 120 3676.0 1.716 2.436 0.0355 0.0502, 0.1141 URCP8

C55H66BrN27O29U2 2125.31 monoclinic P21/n 14.1790(5) 20.5851(7) 24.3073(8) 90 94.854(1) 90 7069.3(4) 170 4144.0 1.997 5.248 0.0557 0.0536, 0.1524

URCP5

C55H66Br2N26O29U3 2429.24 monoclinic P2/n 14.1474(13) 17.5831(18) 17.2716(16) 90 112.124(3) 90 3980.1(7) 295 2312.0 2.027 7.187 0.0908 0.1110, 0.2961

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For URCP9 crystallizing in the chiral space group of P212121, BASF/ TWIN refinements were adopted to handle the observed inversion twinning. Afterward refinements indicate 55.7(8)% of domain I and consequently 44.3(8)% of domain II. The crystal data of all these compounds are listed in Table 2, and the selected bond lengths are listed in Table S1. All the crystallographic data for the newly reported structures in this work have been deposited with the Cambridge Crystallographic Data Centre, and the CCDC numbers for URCP3− URCP9 are 1882547−1882553. Single-Crystal-to-Single-Crystal Transformation. To investigate the single-crystal-to-single-crystal (SCSC) transformation of URCP3 into URCP4, we first compared the PXRD patterns of URCP3 and URCP4 obtained from hydrothermal reaction. After confirming both compounds have high purity, we began to pick out crystals of URCP3 from the mother solution for relevant single crystal measurement every 2 days, until it has transformed into URCP4 (about 2 weeks later). The PXRD patterns of the final products transformed from URCP3 were compared with that of URCP4 directly obtained from hydrothermal reaction, to further confirm the transformation. In addition, to investigate whether the SCSC transformation could also occur in solid-state, we isolated the single crystals of URCP3 from the mother solution and heated them at 60 °C for 12 h. After that, the PXRD patterns of the heated products was also performed and compared with that of as-synthesized URCP4 (more detailed results are presented in the Supporting Information).

URCP6

C55H90N26O55S2U5 3249.79 monoclinic C2/c 15.2632(14) 19.961(2) 30.025(3) 90 91.754(3) 90 9143.6(16) 294 6136.0 2.361 8.992 0.0525 0.0368, 0.0826 URCP9

C55H72BrN27O29U 1893.33 monoclinic P21/c 12.4185(4) 22.7522(8) 24.7356(8) 90 95.271(1) 90 6959.4(4) 170 3800.0 1.807 3.015 0.0675 0.0431, 0.1032

C220H264Br8N104O132U16 10925.10 orthorhombic P212121 12.767(3) 22.808(6) 31.300(8) 90 90 90 9114(4) 295 5120.0 1.990 8.051 0.0447 0.0447, 0.1118

RESULTS AND DISCUSSION

Structure Description. URCP1 and URCP2 have been previously reported,37 and only a brief description of their structures was given here to facilitate the discussion in the following section. URCP1 crystallizes in the monoclinic space group of P21/c (Table 1). The seven-coordinated uranyl center is mononuclear in pentagonal bipyramidal geometry (Figure S6 and S7). Two different modes of carboxylate groups (η1 and η2) from two string molecules (guests encapsulated in the cavities of CB[6] hosts) and a water molecule coordinate to the equatorial plane of uranyl ion. The coordination sphere is completed by another oxygen atom from the carbonyl group of CB[6] (Figure S7a). The string linkers with dicarboxylate groups connect the mononuclear uranyl centers to a zigzag chain (Figure S7b) with CB[6] hosts hanging on the inflection points, which interweave with each other by the interlocked structure of pseudorotaxane and finally result in a 2D network (Figure S7c). URCP2 crystallizes in the triclinic space group of P1̅, in which two uranyl ions are bridged by two η2-mode sulfate ions (Figure S8 and S9). Also, both CB[6] host and string molecules have participated in the coordination sphere of the uranyl center (Figure S9a). The binuclear uranyl centers are further connected by the string guest and CB[6] host to C


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Inorganic Chemistry form a zigzag chain. Finally, two 1D chains interweave with each other to form a 1D daisy chain (Figure S9d). URCP3 crystallizes in the triclinic space group of P1̅ and shows a 1D polyrotaxane chain. The uranyl ion is sevencoordinated with an OUO angle of 177.3(2)° and axial UO bond lengths of 1.751(4) and 1.770(4) Å, respectively (Figure 1a and S10). Besides the two axial oxygen atoms, two

Figure 2. (a) Coordination environment of the uranyl cation in URCP4, (b) extended 1D chain of URCP4 with uranyl center in a stick mode and (c) in a pentagonal bipyramid polyhedron. All hydrogen atoms are omitted for clarity.

Figure 1. (a) Coordination environment of the uranyl cation in URCP3, (b) extended 1D chain of URCP3 with a uranyl center in a stick mode and (c) in a pentagonal bipyramid polyhedron. All hydrogen atoms are omitted for clarity.

monodentate carboxylate groups from different guests of C7BPCA@CB[6] contribute two oxygen atoms to the coordination plane. A bidentate sulfate anion together with a water molecule finally complete the equatorial plane of the mononuclear uranyl center (Figure 1a). The end carboxylate groups of the guest molecules of the pseudorotaxane ligands assemble the uranyl centers into a 1D chain, while the CB[6] molecules hang on the string like beads without participating in the coordination sphere of any uranyl center (Figure 1b and 1c). URCP4 crystallizes in the triclinic space group of P1̅ and contains two mononuclear uranyl centers with different coordination environments, which are bridged by carboxylate groups of the string molecule from C7BPCA@CB[6] (Figures 2a and S11). Both uranyl centers are seven-coordinated with the distances of UO varying in the range of 1.770(3) ∼ 1.784(3) Å, and the angles of OUO are 175.30(15)° (U1) and 178.1(2)° (U2). Specifically, U1 further coordinates with three oxygen atoms from the bidentate and monodentate carboxylate groups of the string molecules, one oxygen atom from a water molecule, and one oxygen atom from a monodetate sulfate anion. For U2, the equatorial plane consists of two oxygen atoms from two monodentate carboxylate groups, two oxygen atoms from a bidentate sulfate anion, and one oxygen atom from a water molecule. These two kinds of uranyl centers are alternatively connected by the end groups of the guest molecules and give a 1D chain to URCP4, while the CB[6] hosts still hang on the string just like beads (Figure 2b and 2c). Crystal structure analysis reveals that URCP5 crystallizes in the monoclinic space group of C2/c and contains three crystallographically nonequivalent uranyl centers in a pentanuclear motif (Figures 3a and S12). Interestingly, although

Figure 3. (a) Coordination environment of the uranyl cation in URCP5, (b) extended 1D chain of URCP5 with uranyl center in a stick mode and (c) in a bipyramid polyhedron. All hydrogen atoms are omitted for clarity.

higher uranyl nuclearity such as hexamer44−46 and octamer47,48 are known, the pentanuclear one has rarely been reported.49,50 Moreover, it is the first time that a pentanuclear uranyl unit is observed in URCPs. The UO lengths of the two sevencoordinated uranyl ions (U1 and U2) vary in the range of 1.772(5) ∼ 1.786(5) Å, while the angles of OUO are 175.6(2)° (U1) and 173.4(2)° (U2), respectively. The coordination plane of U1 consists of one oxygen atom from a monodentate carboxylate group, one oxygen atom from a bidentate sulfate anion, one μ2-hydroxo atom, one μ3-oxo atom, and a water molecule. The equatorial plane of U2 is occupied by one oxygen atom from a bidentate sulfate anion, one μ2-hydroxo atom, one μ3-oxo atom, and two water molecules. Different from U1 and U2, U3 is 8-coordinated and presents a hexagonal-bipyramidal geometry. The coordination sphere is filled with two axial atoms (on the mirror plane), four oxygen atoms (on the mirror plane) from two bidentate sulfate ions, and two μ3-oxo atoms (on the mirror plane). Connected by the string molecules with trans-configuration, the D


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Inorganic Chemistry pentanuclear uranyl centers finally assemble to a 1D zigzag chain (Figure 3b and 3c). URCP6 is very similar to URCP1 and also crystallizes in the monoclinic space group of P21/c. The mononuclear uranyl center is seven-coordinated and forms a pentagonal bipyramid (Figure 4a and S13). The angle of OUO is found to be

Figure 5. (a) Coordination environment of the uranyl cation in URCP7, (b) extended zigzag 1D chain of URCP7 with uranyl center in a stick mode, (c) and in a pentagonal bipyramid polyhedron. All hydrogen atoms are omitted for clarity.

2.7710(17) Å. Although U−Br bonds have been reported in some simple coordination complexes,51−53 this seems to be the first case that the bromide anion presents in the first coordination sphere of uranyl ion in the solid-state polymer, since bromide anion is a weaker Lewis base relative to other counteranions.53 As the two adjacent pseudorotaxanes of one uranyl center anti-parallelly locate at the terminals, an S-shaped 1D chain can be found in URCP7 (Figure 5b and 5c). URCP8 crystallizes in the space group of monoclinic P2/n and contains two crystallographically nonequivalent uranyl centers in a trimeric motif (Figure 6a and S15). The U1 atom

Figure 4. (a) Coordination environment of the uranyl cation in URCP6, (b) simultaneous coordination of both CB[6] and the string guest molecules (top: stick mode, bottom: space-filling mode), (c) top: zigzag 1D chain constructed by CB[6] and the string molecule; bottom: the side view of the interwoven network given by the interlocked feature of pesudorotaxane, and (d) the nitrate and the bromide anions (the pink atoms) in the cavities of the 2D polyrotaxane network of URCP6 (front view). All hydrogen atoms have been omitted for clarity.

178.76(15)°, with UO lengths of 1.769(3) and 1.755(3) Å, respectively. The equatorial coordination plane is occupied by three oxygen atoms of two carboxylate groups (η1 and η2) from different C7BPCA@CB[6] pseudorotaxanes, one oxygen atom from a carbonyl group of CB[6] host, and a water molecule (Figure 4a). Like URCP1 and URCP2, both the CB[6] host and the string guest molecules among C7BPCA@CB[6] are observed to coordinate with the uranyl center, resulting in a local interlocked structure (Figure 4b). The string molecules connect the uranyl centers to a zigzag chain with a series of CB[6] molecules located at the inflection points (Figure 4c). By the intrinsic host−guest encapsulation of pseudorotaxane, the string molecules and CB[6] hosts interweave different 1D chains to a 2D network (Figure 4d). Different from URCP1, both nitrate and bromide anions acting as guest molecules are found in the cavities of the coordination polymers. The space group of URCP7 is P21/n, of which the coordination environment is shown as Figure 5 and S14. It indicates that the dimer metal center consists of two sevencoordinated uranyl cations, with the axial UO lengths varying from 1.771(5) to 1.778(5) Å. U1 is coordinated by two axial oxygen atoms with an OUO angle of 178.4(2)°, two oxygen atoms from a bidentate carboxylate groups of a string guest, a water molecule, and two μ2-hydroxo atoms (Figure 5a and S14). For U2, besides the two axial oxygen atoms, there are two oxygen atoms from a bidentate carboxylate group, and two μ2-hydroxo atoms coordinating in the equatorial plane. Especially, a bromide anion from the pseudorotaxane ligand also coordinates to the U2 center, with a U−Br bond length of

Figure 6. (a) Coordination environment of the uranyl cation in URCP8, (b) extended 1D chain of URCP8 with uranyl center in a stick mode, and (c) in a pentagonal bipyramid polyhedron. All hydrogen atoms are omitted for clarity.

adopts pentagonal-bipyramidal geometry, which is composed of two axial oxygen atoms [the U = O bond lengths are 1.77(2) and 1.789(18) Å, as well as an angle of 174.5(10)° for O UO, respectively], one oxygen atom from a mononuclear carboxylate group, one μ2-hydroxo atom, one μ3-oxo atom, one water molecule, and one μ2-bromide anion (Figure 6a). The U2 atom also possesses a pentagonal-bipyramidal geometry E


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Inorganic Chemistry

2.33−2.53 Å. Compared with variable uranyl coordination by free CB[6] without any guest, U−O bond lengths in URCP1, URCP2, and URCP6 only slightly vary from 2.40 to 2.45 Å, which indicates that the metal coordination behavior of CB[6] in a pseudorotaxane ligand is largely restricted by its concomitant guest molecules coordinating with the metal center. Besides, since there is no steric hindrance around the portals, both sides of guest-free CB[6] are prone to bond with uranyl centers (Figure 8a) and consequently act as bidentate

containing two axial oxygen atoms (at symmetric position) with a U2O bond length of 1.790(15) Å, two μ2-hydroxo atoms, one μ3-oxo atom, and a water molecule. Different from the polypseudorotaxane based on uranyl-oxalate nodes in our previous work,54 the trimeric uranyl centers here are directly connected by the end carboxylate groups of C7BPCA@CB[6] precursors, resulting in a 1D polyrotaxane chain as shown in Figure 6b and 6c. Uniquely, URCP9 crystallizes in a space group with higher symmetry, i.e., orthorhombic P212121. Four uranyl ions with pentagonal-bipyramidal geometries construct a tetranuclear motif (Figure 7 and S16). These four uranyl ions are

Figure 8. (a) Typical coordination behavior of guest-free CB[6],56 (b) topological structure of prepared reported uranyl-CB[6] complexes,57 (c) host−guest coordination behavior of C7BPCA@ CB[6], and (d) interwoven structure derived from pseudorotaxane ligand. Figure 7. (a) Coordination environment of the uranyl cation in URCP9, (b) extended 1D chain of URCP9 with uranyl center in a stick mode, and (c) in a pentagonal bipyramid polyhedron. All hydrogen atoms are omitted for clarity.

organic linker to construct a uranyl−organic coordination polymer (Figure 8b). However, only one portal of CB[6] in C7BPCA@CB[6] has coordination ability due to the existence of the string molecule (Figure 8c). Furthermore, because of the intrinsically host−guest interaction of pseudorotaxane, interwoven structure is easy to construct in uranyl−organic complexes derived from pseudorotaxane precursors (Figure 8d). Moreover, as illustrated above, both CB[6] and the string molecule in C7BPCA@CB[6] can coordinate with the uranyl centers; URCPs thereby have shown much more abundant diversity in coordination mode and topology. From URCP1 to URCP9, the coordination behavior of C7BPCA@CB[6] has shown great difference: in URCP1, URCP2, and URCP6, both components of pseudorotaxane, i.e., CB[6] and C7BPCA simultaneously coordinate to the uranyl centers while only C7BPCA bonds to the uranyl center in URCP3−URCP5 and URCP7−URCP9 (Figure 9a). On the other hand, as Figure 9b illustrates, uranyl centers in URCP1−URCP9 also show various nuclearity and coordination modes, accompanied by varying modes of the competing anions in related URCPs [sulfate: μ2‑(η2, η2) in URCP2 and URCP5, η2 in URCP3, and η2/η1 in URCP4; bromide: η1 in URCP7, μ2 in URCP8 and URCP9]. As a result, the topologies of URCPs are abundantly varied and show as 2D interwoven networks in URCP1 and URCP6, 1D interwoven chain in URCP2, and simple 1D chains in URCP3−URCP5 and URCP7−URCP9 (Figure 9c). Most interestingly, similar uranyl coordination modes show a tendency to transform from one to another. For example, a SCSC transformation from URCP3 to URCP4 can be observed, after either aging URCP3 within mother solution for 2 weeks at room temperature or heating the crystals of

crystallographically nonequivalent, of which the UO lengths vary from 1.761(9) to 1.803(8) Å and the angles of OUO in the range of 172.0(4)° ∼ 175.2(5)°. To complete the coordination environment of U1, one oxygen atom from a bidentate carboxylate group, one μ3-oxo atom, one μ2-hydroxo atom, and two water molecules coordinate to the equatorial plane besides two axial oxygen atoms (Figure 7a). U2 atom is coordinated by two axial oxygen atoms, one oxygen atom from a bidentate group, two μ3-oxo atoms, one water molecule, and one μ2-bromide anion [U2−Br1 = 3.064(3) Å]. U3 atom coordinates with two axial oxygen atoms, two μ3-oxo atoms, two μ2-hydroxo atoms, and one water molecule. The bipyramid of U4 atom is filled with two axial oxygen atoms, one μ3-oxo atom, one μ2-hydroxo atom, one oxygen atom from a monodentate carboxylate group, one water molecule, and one μ2-bromide anion [U4−Br1 = 2.958(3) Å]. The uranyl centers are connected by the carboxylate groups to be a 1D chain, with all of the CB[6] hosts hanging on the string molecules (Figure 7b and 7c). It is notable that, though the P212121 space group is often observed in compounds with helical structures,55 URCP9 exhibits no helix here and it should be racemic as a single crystal. This might be ascribed to the irregular uranyl oligomers and slightly twisted C7BPCA@ CB[6] linkers. Structural Diversity in Coordination and Uranyl Speciation. In previously reported coordination complexes of which guest-free CB[6] macrocycles directly coordinate with the uranyl center,56,57 the bond lengths of U−O between the uranyl ion and CB[6] are found to vary in the range of F


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Inorganic Chemistry

Figure 9. (a) Different coordination behaviors of C7BPCA@CB[6] in URCP1 to URCP9, (b) different nuclearity and coordination modes of uranyl nodes observed in the nine uranyl polyrotaxanes, and (c) three main kinds of topologies of URCPs.

Figure 10. (a) SCSC transformation from URCP3 to URCP4: one sulfate anion of the two adjacent uranyl centers turns out to be the η1-mode while the carboxylate groups are the η2-mode. (b) Difference for single point energy of the simplified geometrical structures in the aqueous phase at the BP86//ECP60MWB-SEG/6-31G* level of theory. All the density functional theory (DFT)59,60 calculations were carried out the using B3LYP61,62 hybrid functional through the Gaussian 09 program package.

10b) has a higher energy of 11.80 kcal mol−1 than that of URCP4, i.e., once one SO42− of the two uranyl centers turns out to be monodentate, the compound will transform to be the most stable state, and there is no extra driving force for it to take further transformation (Figure 10b). In addition, since the energy of URCP3 is much higher than that of URCP4, we observed no reversible transformation throughout the system. Combined Effect of pH and Competing Ligands on Uranyl Coordination and Topological Structures. The above results indicate that CB[6] macrocycle hosts threaded on the string linkers as well as other competing ligands such as sulfate and bromide anions exert a non-negligibly competitive effect on the formation of URCPs. Moreover, it should be noted that the regulation of competing ligands is pH-sensitive. Thus, the diversity of URCPs can be ascribed to a combined

URCP3 at 60 °C for 12 h (Figure S17 and S25). During the SCSC transformation, one of the carboxylate groups from the string molecule in URCP3 turns out to be η2-mode, while one SO42− anion turns from η2- to η1-mode (Figure 10a). Inspired by a previous work of our group dealing with coordination variability of the uranyl center in uranyl-rotaxane compounds,58 we speculate that URCP4 might be more thermodynamically stable than URCP3. This speculation has been further verified by theoretical simulation using simplified modes. As Figure 10b illustrates, when all SO42− anions adopt the η2 mode in URCP3, the energy is 115.09 kcal mol−1 higher than that of URCP4. Especially, no polyrotaxane with both adjacent SO42− anions adopting η1 modes is obtained in the whole process of SCSC transformation. As the computational result points out, this mode (named as URCP-inter in Figure G


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Inorganic Chemistry

sphere of the uranyl centers in URCP1−URCP5. That is to say, in this situation, the main competitive components are SO42−, CB[6], and C7BPCA. For the ease of discussion, we chose benzoic acid (BA) as the model compound of C7BPCA. For BA, SO42−, and CB[6], dissociation constants (pKa) are ∼4.01, 3.02, and 1.92, respectively (Table 3), which means all these three components should get protonated when pH is lower than 1.50. However, since its 12 carbonyl groups on both portals are not completely protonated, CB[6] can show stronger coordination ability than sulfate and C7BPCA in this situation. Actually, even C7BPCA with two end carboxyl groups has quantitative superiority than SO42− to coordinate with uranyl ion. As a result, in the formation of URCP1, one CB[6] host and two C7BPCA guests occupied the coordination sites of the mononuclear uranyl center and consequently introduce steric hindrance to prevent SO42− from coordinating with the metal node (Scheme 1a). As Table 3 shows, when pH is in the range of 2.0−3.5, the stability constants (log K) of BA, SO42−, and CB[6] with UO22+ are comparable (∼3.11 for BA, ∼3.15 for SO42−, and ∼3.14 for CB[6], respectively). Hence, in the formation of URCP2 obtained at pH 2.62, all these three components are observed in the first coordination sphere of the uranyl center (Scheme 1b). With the increasing of solution pH values, deprotonated BA shows stronger coordination capability (log K ≈ 4.48) than CB[6] and prefers to form a 1:2 complex with uranyl ion.70 This means C7BPCA is easier to coordinate with UO22+ and can produce steric hindrance to prevent CB[6] from participating in meta-organic coordination (Scheme 1c). On the other hand, there is no obvious change of coordination capability for SO42− and water molecule as the pH changes. Thus, to complete the coordination sphere, they show a higher

effect of pH and competing ligands, which is similar to that observed in previously reported URCPs derived from C6BPCA@CB[6]),34 but become more complicated for the weakly bonded C7BPCA@CB[6]. Specifically, C7BPCA@ CB[6] shows a significant difference in coordination capability toward the uranyl center, i.e., tunable coordination patterns of CB[6] (Figure 9a). In order to figure out how the pHdependent effect functions in the formation of URCP1− URCP9, the competition of sulfate, nitrate, bromide, CB[6], and C7BPCA is discussed in detail. In the uranyl sulfate system, the competing anions in the formation of URCP1−URCP5 should be SO42− and Br−. Undoubtedly, Br− comes from the ([C7BPCEt]Br2@CB[6]) precursor, serving as counteranion. However, due to the stability constant (log K) of Br− with UO22+ being as low as ∼1.79 (Table 3), this anion is absent in the first coordination Table 3. Stability Constants (log K) of Uranyl Complexes with Different Ligands and Dissociation Constants (pKa) of Related Acids logK H2O NO3− Br− Ph−COOHa CB[6] SO42− Ph−COO−

−5.25 ± 0.2463 0.30 ± 0.1563 1.7964 3.11 ± 0.05b 3.14 ± 0.01c 3.15 ± 0.0263 4.48 ± 0.24d

pKa

nature of acid form

4.01 ± 0.0165 3.0266 1.9267 4.01 ± 0.0165

strong acid strong acid weak acid weak acid strong acid weak acid

a Ph−COOH was selected as the model compound of C7BPCA@ CB[6]. bDetermined at pH value of 1.5−3.5.68 cEstimated by CB[5] with better solubility at pH value of ∼2.0.69 dDetermined at pH value of 3.0−5.0.70

Scheme 1. Mechanism for the Formation of Different Coordination Modes in URCPs Driven by the Combined Effect of pH and Competing Ligandsa

a

(a) At low pH, more competitive CB[6] and C7BPCA can preferentially take the coordination sphere of UO22+ in URCP1 and prevent SO42− from approaching the uranyl center. (b) In URCP2, the available coordination sites allow SO42− anions to join the coordination with UO22+. (c) At high pH, two deprotonated C7BPCA guests with stronger coordination ability can preferentially coordinate to UO22+, giving steric hindrance around the uranyl centers; consequently, SO42− and water molecule but not CB[6] with more suitable sizes can enter the first coordination sphere in URCP3−URCP5. All hydrogen atoms are omitted for clarity. H


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Inorganic Chemistry

During the assembly process of coordination polymers, the nucleatity of uranyl centers is a comprehensive result of the competition between the uranyl hydrolysis and uranyl−ligand coordination. Obviously, the low nuclearity in URCP1 and URCP6 results from the inhibition of uranyl hydrolysis by H+ at low pH (600 °C) obtained from uranyl sulfate are prone to forming UO2SO4, while those from uranyl nitrate are demonstrated to be U3O8. For instance, the residual percentages of URCP1 and URCP6 are 18.80% and 14.21%, which are in good agreement with those calculated as UO2SO4 (19.60%) and U3O8 (14.82%), respectively. As shown in Figure 12, the fluorescence spectra of all URCPs recorded at excitation wavelength of 420 nm illustrate that all the obtained uranyl compounds exhibit different features. URCP1−URCP5 retain five or six distinct peaks between 470 and 650 nm, which represent the vibronic structure of the uranyl cation given by the symmetric and antisymmertric oscillations of the U−O bonds.72 Compared to UO2SO4·2.5H2O, all peaks of URCP1 show blue shifts, while those of URCP3−URCP5 are red-shifted and exhibit some peak broadenings. URCP6 remains the characteristic peak of UO2(NO3)2·6H2O but exhibits a slight red shift. Relatively, URCP8 and URCP9 possess broaden emission bands in range of 500−580 nm. This may be attributed to the interplay of

degree of participation into uranyl coordination through increased connectivity or coordination number (URCP3− URCP5). For the uranyl nitrate system, the competing anions to construct URCPs are NO3− and Br− anions. As shown in Table 3, the log K of NO3− with UO22+ (∼0.30) is much lower that of Br− (∼1.79), well illustrating the reason why this anion is rarely reported in the solid-state uranyl coordination polymers. With weaker coordination affinity to UO22+ than SO42−, the effect of Br− on the formation for URCP6 should remain consistent with URCP1 (Scheme 1a). Hence, no more discussion is performed here. Similar to URCP1, there are no anions in the first coordination sphere of the uranyl center in URCP6 obtained at pH 1.37, but the counteranions are NO3− and Br−. This makes the β interaxial angle of URCP6 ∼4° higher than that of URCP1 (Table 2), although these two compounds have almost the same coordination environment. In URCP7−URCP8 obtained at higher pH values, the coordination ability of Br− is much weaker than that of SO 4 2− , but the absence of CB[6] in metal−organic coordination can reduce the steric hindrance around the equatorial plane and give this anion the chance to coordination with uranyl centers in URCP7−URCP9. Combined Effect of pH and Competing Ligands on Uranyl Speciation. Another aspect of the combined effect of pH and competing ligands is on the uranyl nuclearity in obtained URCPs. In fact, uranyl nuclearity in solid URCPs reflects uranyl speciation in aqueous solution, which is mainly controlled by the hydrolysis of uranyl cation illustrated as the following reaction: m[UO2]2+ + nH2O → [(UO2)m(OH)n](2m−n) + nH+.71 Generally, the low pH value can inhibit the hydrolysis of uranyl cation, and the dominant species of uranium in the solution is UO22+. Correspondingly, higher pH value is beneficial for the hydrolysis of uranyl cation, and as a consequence, multimeric uranyl centers become the main species in the solution. For the uranyl nitrate system in this work, the metal node becomes sensitive to pH, and the uranyl centers are found to be monomeric at pH 1.37 (URCP6), dimeric at pH 3.50 (URCP7), trimeric at pH 4.20 (URCP8), and tertameric at pH 6.16 (URCP9), respectively (Figure 11). In contrast, no significant effect of pH on the uranyl hydrolysis can be observed when pH changes in the range of 1.28−4.46 for the sulfate system, as exemplified by the fact that all uranyl centers are mononuclear in URCP1− URCP4 (Figure 11). This difference between the two systems suggests that the competing anions also play an important role in promoting uranyl speciation of obtained URCPs.

Figure 11. Occurrence of the four distinct URCP1−URCP9 as a function of reaction pH (range 1−7) after hydrothermal treatment at 150 °C. I


Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lei Mei: 0000-0002-2926-7265 Wei-qun Shi: 0000-0001-9929-9732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the Science Challenge Project (TZ2016004). The National Natural Science Foundation of China (21671191, 21577144, 21876122, and 11405186) and the Major Program of National Natural Science Foundation of China (21790373) are also acknowledged.

Figure 12. Fluorescence spectra of URCP1−URCP9 and uranyl sulfate/nitrate compounds (pure URCP2 and URCP7 were not isolated in enough yield).

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multiple luminescence centers, which is also observed in previous work.44,73 The distinct coordination modes of uranyl cation should be responsible for the difference in luminescence properties of all obtained URCPs.74

(1) Li, P.; Vermeulen, N. A.; Malliakas, C. D.; Gómez-Gualdrón, D. A.; Howarth, A. J.; Mehdi, B. L.; Dohnalkova, A.; Browning, N. D.; O’keeffe, M.; Farha, O. K. Bottom-up construction of a superstructure in a porous uranium-organic crystal. Science 2017, 356, 624−627. (2) Li, P.; Vermeulen, N. A.; Gong, X.; Malliakas, C. D.; Stoddart, J. F.; Hupp, J. T.; Farha, O. K. Design and Synthesis of a Water-Stable Anionic Uranium-Based Metal-Organic Framework (MOF) with Ultra Large Pores. Angew. Chem., Int. Ed. 2016, 55, 10358−10362. (3) Li, Y.; Yang, Z.; Wang, Y.; Bai, Z.; Zheng, T.; Dai, X.; Liu, S.; Gui, D.; Liu, W.; Chen, M.; Chen, L.; Diwu, J.; Zhu, L.; Zhou, R.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. A mesoporous cationic thorium-organic framework that rapidly traps anionic persistent organic pollutants. Nat. Commun. 2017, 8, 1354. (4) Wang, Y.; Liu, W.; Bai, Z.; Zheng, T.; Silver, M. A.; Li, Y.; Wang, Y.; Wang, X.; Diwu, J.; Chai, Z. Employing a Unique Unsaturated Th4+ Site in a Mesoporous Thorium-Organic Framework for Kr/Xe Uptake and Separation. Angew. Chem., Int. Ed. 2018, 57, 5783−5787. (5) Andrews, M. B.; Cahill, C. L. Uranyl bearing hybrid materials: synthesis, speciation, and solid-state structures. Chem. Rev. 2013, 113, 1121−1136. (6) Loiseau, T.; Mihalcea, I.; Henry, N.; Volkringer, C. The crystal chemistry of uranium carboxylates. Coord. Chem. Rev. 2014, 266, 69− 109. (7) Wang, Y.; Liu, Z.; Li, Y.; Bai, Z.; Liu, W.; Wang, Y.; Xu, X.; Xiao, C.; Sheng, D.; Diwu, J.; Su, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. Umbellate distortions of the uranyl coordination environment result in a stable and porous polycatenated framework that can effectively remove cesium from aqueous solutions. J. Am. Chem. Soc. 2015, 137, 6144−6147. (8) Hu, F. L.; Di, Z. Y.; Lin, P.; Huang, P.; Wu, M. Y.; Jiang, F. L.; Hong, M. C. An Anionic Uranium-Based Metal−Organic Framework with Ultralarge Nanocages for Selective Dye Adsorption. Cryst. Growth Des. 2018, 18, 576−580. (9) Ai, J.; Chen, F. Y.; Gao, C. Y.; Tian, H. R.; Pan, Q. J.; Sun, Z. M. Porous Anionic Uranyl-Organic Networks for Highly Efficient Cs+ Adsorption and Investigation of the Mechanism. Inorg. Chem. 2018, 57, 4419−4426. (10) Wang, Y.; Li, Y.; Bai, Z.; Xiao, C.; Liu, Z.; Liu, W.; Chen, L.; He, W.; Diwu, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. Design and synthesis of a chiral uranium-based microporous metal organic framework with high SHG efficiency and sequestration potential for low-valent actinides. Dalton Trans 2015, 44, 18810−18814. (11) Liu, W.; Xie, J.; Zhang, L.; Silver, M. A.; Wang, S. A hydrolytically stable uranyl organic framework for highly sensitive and selective detection of Fe3+ in aqueous media. Dalton Trans 2018, 47, 649−653.

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CONCLUSION We have presented nine different URCPs derived from the hydrothermal assembly of uranyl sulfate or uranyl nitate with a weakly bonded pseudorotaxane linker C7BPCA@CB[6]. The results indicate URCPs show abundant diversity in coordination modes and topological structures, which is demonstrated to be a combined effect of pH and competing ligands. Simultaneously host−guest coordination of C7BPCA@CB[6] pseudorotaxane to uranyl ion occurs at low pH, while the coordination of competing anion to UO22+ is observed at high pH together with C7BPCA guest. Besides, competing anions with different coordination ability can also adjust uranyl speciation and the nuclearity in URCPs through the competitive coordination at different pH values, where SO42− with stronger coordination ability is able to inhibit the hydrolysis of uranyl ion, while the nonsulfate hydrothermal system URCPs are prone to form multimeric centers in URCPs. The outcome of this work can enrich the family of actinide polyrotaxanes and provide a new approach to achieve designed synthesis of URCPs with specific coordination modes and topologies.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03353. Typical figures including characterization data (PXRD, TGA, and FTIR) (PDF) Accession Codes

CCDC 1882547−1882553 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. J


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