Releasing Metal-Coordination Capacity of Cucurbit[6]uril Macrocycle

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Releasing Metal-Coordination Capacity of Cucurbit[6]uril Macrocycle in Pseudorotaxane Ligands for the Construction of Interwoven Uranyl−Rotaxane Coordination Polymers Fei-ze Li,†,‡,§ Lei Mei,†,§ Kong-qiu Hu,† Ji-pan Yu,† Shu-wen An,† Kang Liu,† Zhi-fang Chai,†,⊥ Ning Liu,*,‡ and Wei-qun Shi*,† Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/15/18. For personal use only.

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Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China ‡ 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 ⊥ School of Radiological and Interdisciplinary Sciences and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: As an emerging type of actinide hybrid material, uranyl−rotaxane coordination polymers (URCPs) with new coordination patterns and topological structures are still desired. In this work, we propose a new strategy to construct URCPs by promoting the simultaneous coordination of both the wheel and axle moieties in pseudorotaxane linkers with metal nodes. Starting from a series of cucurbit[6]uril (CB[6])based pseudorotaxane ligands, CnBPCA@CB[6] [CnBPCA = 1,1-(α,ω-diyl)bis[4-(ethoxycarbonyl)pyridin-1-ium] bromides, where n = 5−8] with slightly deformed CB[6], four new URCPs (URCP1, URCP3, URCP4, and URCP5) with interwoven network structures, as well as another noninterwoven polymer(URCP2), have been successfully prepared. According to single-crystal structure analysis, we attribute the interwoven structures of the URCPs to the distortion of CB[6] in pseudorotaxane ligands with shorter or longer spacers (C5, C7, and C8). This indicates that the deformation could effectively diminish the steric hindrance around the portals, thus endowing the “inert” CB[6] host with coordination ability like the string molecule. Besides, the participation of water molecules and sulfate anions in the uranyl coordination sphere is also found to have a great influence on the final structures of the obtained URCPs. The successful preparation of interwoven URCPs in this work gives some new insights into the metal coordination of supramolecular entities and could facilitate other new applications of CB[6]-based pseudorotaxane ligands. Most importantly, the strategy proposed in this work provides some hints in the controllable design of metal−organic rotaxane frameworks with unique topologies.

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INTRODUCTION In the past decades, metal−organic frameworks (MOFs) have drawn tremendous attention from chemists and material scientists1−3 because of their fascinating topological structures and potential applications in storage,4−6 separation,7−10 catalysis,11−13 drug delivery,14,15 chemical sensing,16−18 etc. Recently, the incorporation of mechanically interlocked molecules, mainly pseudorotaxanes or rotaxanes, into MOFs has brought in plenty of metal−organic rotaxane frameworks (MORFs) with new structures and features of rotational dynamics.19−23 Studies concerning MORFs have not only immensely enriched research about the design and synthesis of MOFs but also effectively extended the scope of coordination chemistry and supramolecular recognition. As the primary components of MORFs, pseudorotaxane/rotaxane linkers with macrocyclic hosts, such as crown ether,24−28 catenane,29 cucurbituril,30−34 and cyclodextrin,35,36 threading onto the © XXXX American Chemical Society

string molecules could endow the target materials with dynamic flexibility and switching properties. Among the various host−guest supramolecular entities, cucurbit[n]uril (CB[n])-based pseudorotaxanes have been well developed to be classical supramolecular linkers for the assembly of MORFs, and several categories of related metal−organic materials have been prepared and characterized.37−40 Despite of the recognized excellent coordination capacity of free cucurbituril to metal ions,41−46 only the string molecules with functional ends (carboxylate or pyridine) act as organic linkers in most of the previously CB[n]-based MORFs. On the other hand, the macrocyclic CB[n] wheels with high rigidity just thread onto the framework skeleton without any participation in the coordination with metal nodes, as Received: July 27, 2018

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DOI: 10.1021/acs.inorgchem.8b02126 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Different Coordination Behaviors Controlled by the Structural Deformation of the CB[6] Host in Pseudorotaxane Ligandsa

a

(a) The steric hindrance from the adjacent pyridine ring would prohibit metal ions from approaching the portal of regular CB[6], and only the end functional groups of the string molecules could coordinate to the metal cations. (b) The deformation of CB[6] could effectively relieve the hindrance around the portal, and the CB[6] host has the capability for metal coordination together with the string guest. The yellow polyhedra present the metal cations. Bottom: molecular structure of the CB[6] host and string guests.

that the simultaneous coordination of the whole complexes to the metal center would produce more complicated structures in the final coordination polymers. In this work, we employ a series of pseudorotaxanes L1−L4, involving 1,1′-(α,ω-diyl)bis[4-(ethoxycarbonyl)pyridin-1-ium] bromides with different lengths of alkyl spacers from pentylidene (C5) to octamethylene (C8) and slightly distorted CB[6] hosts, as organic ligands to probe their coordination modes with uranyl ions. By hydrothermal reactions of pseudorotaxanes containing deformed CB[6] (L1 with a C5 spacer, L3 with a C7 spacer, and L4 with a C8 spacer, respectively) with uranyl sulfate, four new interwoven URCPs (URCP1 from L1, URCP3 and URCP4 from L3, and URCP5 from L4) with coordination of both the string guest and CB[6] host to the uranyl center have been obtained. In contrast, in noninterwoven URCP2 derived from L2 with a C6 spacer, only the carboxyl groups from the guest molecules participate in the metal coordination. All of these URCPs have been fully characterized and analyzed, and the mechanism for the formation of interwoven structures has also been discussed in detail.

exemplified by the case of uranyl−rotaxane coordination polymers (URCPs).47 Virtually, the apparent chemical inertness of this rigid macrocyclic molecule in pseudorotaxane is mainly attributed to the hindrance around the portals given by the guest molecules, which would hinder metal ions from approaching the CB[6] host. Meanwhile, noncovalent interactions such as ion−dipole interaction between the host and guest molecules may also slightly diminish the electron density of the macrocycle,48 thus further weakening the coordination ability of the host molecule. As a result, only the string molecules with functional end groups could coordinate to the metal cations (Scheme 1a). Herein, we intend to construct new metal−rotaxane coordination polymers by improving the connectivity of the pseudorotaxane linkers, i.e., promoting the coordination of macrocyclic cucurbituril with the metal nodes in the presence of guest molecules. We realized this would bring MORFs with unrecognized coordination patterns, new topological structures, and distinctive physicochemical properties. Generally, the most common method for increasing the coordination availability of the macrocyclic host is to functionalize the macrocycle on the side chains, such as for the MORFs based on crown ethers.27 However, the chemical modification of CB[n] seems to be a challenge because of their chemical inertness.49 Another available approach is to release the portal carbonyl groups of the CB[n] host through a prudent choice of guest molecules with different binding affinities.50 Recently, we found that the highly rigid CB[6] would adopt structural deformation in response to the “ill-suited” guests in both the host−guest recognition process and the ground state of pseudorotaxanes.51 We inferred that, unlike those systems with regular macrocycles (Scheme 1a), this adaptive deformation of CB[6] might effectively relieve the steric hindrance at both portals of the macrocylic host and endow the CB[6] host of [2]pseudorotaxane with a potential capacity for metal coordination together with the string molecule (Scheme 1b). Considering the special host−guest interlocked structures of pseudorotaxanes/rotaxanes, it can be expected

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EXPERIMENTAL SECTION

General Methods. Caution! Strict care including radiation protection (protective suit or lead glass, when necessary) and chemical protection (mask, goggle, glove, laboratory gown, etc.) are prerequisites in handing uranyl compounds because of the radioactivity and chemical toxicity of uranium. 1,1′-(α,ω-Diyl)bis[4-(ethoxycarbonyl)pyridin-1ium] bromides [CnBPCEt]Br2, where n = 5−8) and CB[6] were synthesized as previously reported.52 The pseudorotaxane ligands L1−L4 (Scheme S1), i.e., [CnBPCEt]Br2@CB[6], were preassembly products of [CnBPCEt]Br2 and CB[6] in aqueous solution. Commercially purchased uranyl sulfate (UO2SO4·2.5H2O) was dissolved in deionized water to give a 0.5 M uranyl solution. 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). A TA Q500 analyzer was used to perform thermogravimetric analysis (TGA) of all uranyl compounds. The Fourier transform B

DOI: 10.1021/acs.inorgchem.8b02126 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for URCP1−URCP5 formula fw cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 T, K F (000) Dc (g cm−3) μ (mm−1) Rint R1, wR2

URCP1

URCP2a

URCP3

URCP4

URCP5

C53H54N26O22SU 1767.39 monoclinic P21/c 12.2592(14) 19.783(2) 13.1287(16) 90 94.197(4) 90 3175.4(7) 295 1776.0 1.748 2.700b 0.0304 0.0265, 0.0673

C54H56N26O22SU 1691.34 triclinic P1̅ 15.4905(4) 15.7934(5) 16.7184(6) 68.423(3) 89.602(3) 83.048(3) 3772.4(2) 293 1692.0 1.489 7.054c 0.0318 0.0354, 0.1004

C55H60N26O24UBr2 1867.14 monoclinic P21/c 12.4117(11) 22.9721(18) 24.8619(17) 90 91.746(2) 90 7085.4(1) 297 3704.0 1.750 3.515b 0.0609 0.0596, 0.1734

C55H68N26O36S2U2 2209.53 triclinic P1̅ 12.416(2) 13.880(3) 21.639(4) 89.751(7) 82.849(7) 85.998(7) 3691.0(12) 295 2168.0 1.988 4.553b 0.0541 0.0350, 0.0980

C28H42N13O24SU 1214.83 triclinic P1̅ 12.3485(5) 12.8164(6) 14.5041(6) 95.197(2) 110.1710(10) 98.578(2) 2105.49(16) 170 1202.0 1.916 4.010b 0.0301 0.0235, 0.06543

The data of URCP2 are referred to the reference reported previously.53 bμ(Mo Kα). cμ(Cu Kα).

a

on uranium). FTIR (cm−1): 3070 [w, ν(CH2)], 2995 and 2933 [w, ν(CH2) and ν(CH)], 1740 [vs, ν(COO)C7BPCA], 1643 [m, ν(C O)CB[6]], 1570 [w, ν(COO)C7BPCA], 1470 and 1417 [s, ν(C− N)pyridinium], 1376 and 1325 [m, ν(NCN)CB[6]], 1295 and 1234 [m, ν(C−N)CB[6]], 1188 [m, ν(C−O)], 964 [m, δ(CH)pyridinium], 921 [w, ν(UO22+)], 875, 818, 795, and 760 [s, γ(CH)pyridinium]. (UO2)2(C7BPCA@CB[6])(H2O)3(SO4)2·5H2O (URCP4). Typically, 0.070 mmol of L3 was dissolved in 2 mL of deionized water in a stainless-steel bomb, and then 70 μL (0.035 mmol) of uranyl sulfate was added. The pH of the suspension was adjusted to ∼3.0. After that, the bomb was sealed, kept at 150 °C for 72 h, and gradually cooled to room temperature. The obtained yellow crystals were washed with water and ethanol and dried at 50 °C overnight (yield: 45.9% based on uranium). FTIR (cm−1): 3080 [w, ν(CH2)], 2976 and 2927 [w, ν(CH2) and ν(CH)], 1741 [vs, ν(COO)C7BPCA], 1645 [m, ν(C O)CB[6]], 1573 [w, ν(COO)C7BPCA], 1472 and 1418 [s, ν(C− N)pyridinium], 1383 and 1324 [m, ν(NCN)CB[6]], 1281 and 1236 [m, ν(C−N)CB[6]], 1189 [m, ν(C−O)], 1142, 1090, and 1050 [ν(SO4)], 967 [m, δ(CH)pyridinium], 915 [w, ν(UO22+)], 880, 820, 798, and 760 [s, γ(CH)pyridinium]. UO2(C8BPCA@CB[6])0.5(H2O)(SO4)·9H2O (URCP5). Typically, 0.070 mmol of L4 was dissolved in 2 mL of deionized water in a stainless-steel bomb; subsequently, 70 μL (0.035 mmol) of a uranyl sulfate solution was added. The pH of the suspension was adjusted to ∼2.5. After that, the bomb was sealed, kept at 150 °C for 72 h, and gradually cooled to room temperature. The obtained yellow crystals were washed with water and ethanol and dried at 50 °C overnight (yield: 37.8% based on uranium). FTIR (cm−1): 3073 [w, ν(CH2)], 2924 and 2852 [w, ν(CH2) and ν(CH)], 1747 [vs, ν(COO)C8BPCA], 1642 and 1612 [m, ν(CO)CB[6]], 1570 and 1153 [w, ν(COO)C8BPCA], 1472 and 1418 [s, ν(C−N)pyridinium], 1378 and 1324 [m, ν(NCN)CB[6]], 1237 [m, ν(C−N)CB[6]], 1189 [m, ν(C− O)], 1120, 1170, and 1023 [ν(SO4)], 967 [m, δ(CH)pyridinium], 926 [w, ν(UO22+)], 870, 819, 797, and 760 [s, γ(CH)pyridinium]. Single-Crystal X-ray Structure Determination. Single-crystal X-ray diffraction was performed on a Bruker D8 VENTURE X-ray CMOS diffractometer with a Mo Kα (λ = 0.71073 Å) or a Cu Kα (λ = 1.54184 Å) X-ray source at room temperature or 170 K according to the crystal qualities. All data were integrated using the SAINT software package, and absorption correction was achieved using SADABS. The crystal structures were solved by means of direct methods (SHELXL-9754) and refined with full-matrix least-squares techniques on F2 using the SHELXL-201854,55 and Olex256 software packages. Bromide anions in URCP3 were given calculated occupancy parameters for disorder over multiple sites to obtain acceptable displacement parameters. The C−H and water molecule hydrogen

infrared (FTIR) spectra were obtained using a Bruker Tensor 27 spectrometer. Solid-state fluorescence spectra of all uranyl polyrotaxanes were obtained using a Hitachi F-4600 fluorescence spectrophotometer. Synthesis. All uranyl−rotaxane coordination polymers (URCPs) were synthesized by the hydrothermal reaction of L1−L4 with uranyl ions under autogenous pressure in a Teflon-lined stainless-steel bomb. The pseudorotaxane ligands were demonstrated to transform in situ into the corresponding carboxylate forms (CnBPCA@CB[6], where n = 5−8), namely, as L1′−L4′, respectively (Scheme S1). UO2(C5BPCA@CB[6])(SO4)·5H2O (URCP1). Typically, 0.070 mmol of L1 was dissolved in 2 mL of deionized water in a stainless-steel bomb, and then 70 μL (0.035 mmol) of uranyl sulfate was added. The pH of the suspension was adjusted to ∼2.5. After that, the bomb was sealed, kept at 150 °C for 72 h, and gradually cooled to room temperature. The obtained yellow crystals were washed with water and ethanol and dried at 50 °C overnight (yield: 58.5% based on uranium). FTIR (cm−1): 3088 [w, ν(CH2)], 2998 and 2929 [w, ν(CH2) and ν(CH)], 1738 [vs, ν(COO)C5BPCA], 1662 and 1639 [m, ν(CO)CB[6]], 1581 [w, ν(COO)C5BPCA], 1475 [s, ν(C−N)pyridinium], 1373 and 1322 [m, ν(NCN)CB[6]], 1279 and 1234 [m, ν(C− N)CB[6])], 1186 [m, ν(C−O)], 1128 [ν(SO4)], 963 [m, δ(CH)pyridinium], 920 [w, ν(UO22+)], 876, 818, 795, and 760 [s, γ(CH)pyridinium]. UO2(C6BPCA@CB[6])(SO4) (URCP2). URCP2 was synthesized according to our previous work.53 Typically, 0.070 mmol of L2 was dissolved in 2 mL of deionized water in a stainless-steel bomb, and then 70 μL (0.035 mmol) of uranyl sulfate was added. The pH of the suspension was adjusted to ∼2.5. After that, the bomb was sealed, kept at 150 °C for 72 h, and gradually cooled to room temperature. The obtained yellow crystals were washed with water and ethanol and dried at 50 °C overnight (yield: 54.5% based on uranium). FTIR (cm−1): 3082 [w, ν(CH2)], 2975 and 2923 [w, ν(CH2) and ν(CH)], 1731 [vs, ν(COO)C6BPCA], 1631 [m, ν(CO)CB[6]], 1573 [w, ν(COO)C6BPCA], 1470 and 1411 [s, ν(C−N)pyridinium], 1384 and 1327 [m, ν(NCN)CB[6]], 1280, 1256, and 1237 [m, ν(C−N)CB[6]], 1187 [m, ν(C−O)], 1129, 1085, and 1047 [ν(SO 4 )], 966 [m, δ(CH)pyridinium], 922 [m, ν(UO22+)], 880, 819, 798, and 760 [s, γ(CH)pyridinium]. UO2(C7BPCA@CB[6])(H2O)Br2·5H2O (URCP3). Typically, 0.070 mmol of L3 was dissolved in 2 mL of deionized water in a stainless-steel bomb, and then 70 μL (0.035 mmol) of uranyl sulfate was added. The pH of the suspension was adjusted to ∼2.5. After that, the bomb was sealed, kept at 150 °C for 72 h, and gradually cooled to room temperature. The obtained yellow crystals were washed with water and ethanol and dried at 50 °C overnight (yield: 54.3% based C

DOI: 10.1021/acs.inorgchem.8b02126 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry atoms were added at calculated positions, and all of them were treated as riding atoms with an isotropic displacement parameter 1.2 times high than that of the parent atoms, whereas some of the water solvent molecules in URCP3, URCP4, and URCP5 were not included in the mode. The crystal data of all of these three compounds are listed in Table 1, and the selected bond lengths and angles are listed in Table S1. Crystallographic data for the structures in this work have been deposited with the Cambridge Crystallographic Data Centre, and the numbers for URCP1, URCP3, URCP4, and URCP5 are CCDC 1820934, 1857849, 1820935, and 1820936, respectively.

intrinsic host−guest encapsulation of pseudorotaxane, the string molecules and CB[6]s would well interweave the 1D chain to a 2D network (Figure 1d). From this point of view, the final structure of URCP1 could be regarded as an interwoven topology built by host−guest interactions between the macrocycle and string molecules. URCP2 crystallizes in the P1̅ space group and possesses a 2D network structure, which has been reported in our previous work.53 The metal center is a dimer with two sevencoordinated uranyl ions bridged by two μ2-carboxylate groups of the string molecule, and coordination of a η2-mode of pseudorotaxane and a η2-connected sulfate anion would complete the pentagonal bipyramid (Figure 2a). A five-

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RESULTS AND DISCUSSION Structure Description. URCP1 crystallizes in the space group P21/c and has a 2D interwoven network structure. The uranyl ion is 7-fold-coordinated (Figure 1a), and the angle of

Figure 2. (a) Coordination environment of the uranyl cation, (b) fivemembered rhomboid molecular necklace ([5]MN) resulting from the uranyl SBUs and pseudorotaxane ligands, (c) 2D network given by the string molecules, (d) and 2D polyrotaxane network in URCP2. Figure 1. (a) Coordination environment of the uranyl cation, (b) locally interlocked structure given by the simultaneous coordination of both CB[6] and the string guest molecules, (c) zigzag 1D chain constructed by CB[6] and the string molecules, (d) and 2D polyrotaxane network interwoven by the interlocked feature of the pseudorotaxane ligand in URCP1. All hydrogen atoms have been omitted for clarity.

membered rhomboid molecular necklace ([5]MN) containing a cavity in the center of the rings (Figure 2b) could be observed in this polyrotaxane compound. As pointed out by Figure 2c, the string molecules with functional carboxylate groups act as building struts to connect the uranyl dimers together. In contrast, CB[6] molecules only hang on the skeleton like beads (Figure 2d). URCP3 also displays a 2D interwoven network structure and crystallizes in the space group P21/c. The uranyl ion is 7fold-coordinated, and the angle of OUO is determined to be 178.8(3)°, while the distances of the two axial UO bonds are 1.753(6) and 1.753(5) Å, respectively. Two modes of carboxylic groups (η1 and η2) from the string guests of the pseudorotaxanes and a water molecule coordinate to the equatorial plane of the uranyl ion (Figure 3a), while the U−O bond lengths vary in the 2.278(6)−2.456(5) Å range [U−O3 = 2.277(6) Å, U−O4 = 2.456(5) Å, U−O5 = 2.431(6) Å, and U−O6 = 2.431(5) Å]. The coordination sphere is completed by another oxygen atom from the carbonyl group of CB[6] with a U−O7 length of 2.403(5) Å. Although a uranyl source with sulfate is used, no sulfate anion participates in coordination to the uranyl center in URCP3. Two uranyl ions and two pseudorotaxane ligands form a local interlocked structure (Figure 3b), which is given by the simultaneous coordination of both the host and guest to the metal cation. Similar to URCP1, the string molecules with dicarboxylate groups connect the mononuclear uranyl centers to a zigzag chain (Figure 3c), which also interweave with each other to form a 2D network (Figure 3d). URCP4 is also derived from L3 but at a higher pH value. It crystallizes in the space group P1̅ and presents a 1D

OUO is measured to be 179.8(3)°, while the distances of the two axial UO bonds are 1.769(5) and 1.774(5) Å, respectively. The equatorial coordination plane consists of two oxygen atoms from two monodentate carboxylic groups of the string guests, two oxygen atoms from a bidentate sulfate ion, and one oxygen atom from the carbonyl group of the CB[6] host, with the U−O bond lengths varying from 2.282(5) to 2.462(5) Å [U−O3 = 2.462(4) Å, U−O4 = 2.450(5) Å, U− O5 = 2.449(5) Å, U−O6 = 2.348(5) Å, and U−O7 = 2.282(5) Å]. The simultaneous coordination of both host and guest molecules with metal nodes found here is rarely observed in cucurbituril-based MORFs.57,58 Commonly, in most of these, only the string guests with functional end groups coordinate with the metal ions, and the rigid host molecules hang on the skeleton just like beads. To the best of our knowledge, coordination of CB[6] in the presence of guest molecules with metal centers is found in uranyl-based MORFs for the first time. As illustrated in Figure 1b, the simultaneous coordination mode gives a locally interlocked structure around the metal center, establishing the foundation for the final interwoven topological structure of URCP1. Specifically, the string molecules connect the uranyl ions to a zigzag chain, and a series of CB[6] molecules locate at the inflection points by coordinating to the uranyl cations (Figure 1c). Because of the D

DOI: 10.1021/acs.inorgchem.8b02126 Inorg. Chem. XXXX, XXX, XXX−XXX

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bond lengths of U−O range from 2.300(4) to 2.489(4) Å [U2−O10 = 2.300(4) Å, U2−O11 = 2.313(4) Å, U2−O12 = 2.297(4) Å, U2−O13 = 2.489(4) Å, and U2−O14= 2.479(3) Å]. U1 and U2 are bridged by two η2-mode sulfate ions to form a dimer serving as the secondary building unit (SBU) in the construction of URCP4. The binuclear uranyl centers are connected by the string molecules as a 1D zigzag chain, and all of the CB[6] molecules locate on the same side (Figure 4b). As Figure 4d illustrates, two 1D chains interweave with each other and form a 1D daisy chain. URCP5 also possesses a 2D interwoven structure and crystallizes in the P1̅ space group. Two equivalent uranyl centers locate on the mirror plane and adopt a conventional pentagonal-bipyramidal geometry (Figure 5a). The OUO

Figure 3. (a) Coordination environment of the uranyl cation, (b) locally interlocked structure given by the simultaneous coordination of both CB[6] and the string guest molecules, (c) zigzag 1D chain constructed by CB[6] and the string molecules, (d) and 2D polyrotaxane network interwoven by the interlocked feature of the pesudorotaxane ligand in URCP3. All hydrogen atoms have been omitted for clarity.

intertwined polyrotaxane structure. Two crystallographically nonequivalent uranyl centers could be observed in a dimeric motif (Figure 4a). The OUO angles are 179.09(18)° and

Figure 5. (a) Coordination environment of the uranyl cation, (b) locally interlocked structure given by the simultaneous coordination of both CB[6] and the string guest molecules, (c) 2D network constructed by CB[6] and the string molecules, and (d) 2D polyrotaxane network interwoven by the interlocked feature of the pseudorotaxane ligand in URCP5. All hydrogen atoms have been omitted for clarity.

angle is 178.64(1)°, and the UO bond lengths are 1.772(2) and 1.773(2) Å, respectively. Each uranyl center is coordinated by two μ2-oxo atoms from two bidentate sulfate ions, one oxygen atom from a monodentate carboxylate group, one oxygen atom from a carbonyl group contributed by CB[6], and a water molecule [U−O3 = 2.481(4) Å, U−O4 = 2.4274(19) Å, U−O5 = 2.502(2) Å, U−O6 = 2.340(2) Å, and U−O7 = 2.330(2) Å)]. Two sulfate ions, which centrosymmetrically locate on the mirror plane, bridge the two uranyl centers from a dimeric SBU. The uranyl dimer and pseudorotaxane ligand also assemble as an interlocked structure just like those in URCP1, URCP3, and URCP4 (Figure 5b). More strikingly, two carbonyl groups on both portals of each CB[6] have participated in coordination with the uranyl ions (Figure 5b). This coordination mode is quite different from the other three compounds and suggests that CB[6] in L4′ might have a stronger coordination ability. The bidentate CB[6] and the bipyridinium with functional carboxylate groups connect the bridged uranyl dimers to a 2D network, as shown in Figure 5c. Furthermore, two networks interweave with each other to form a 2-fold interwoven 2D network (Figure 5d). Interwoven Coordination Networks Constructed by CB[6]-Based Pseudorotaxane Ligands with Varying Guest Linkers. As demonstrated above, the pseudorotaxane linkers L1′−L4′ could assemble with the uranyl centers under hydrothermal conditions to give different rotaxane coordina-

Figure 4. (a) Coordination environment of the uranyl cation, (b) locally interlocked structure given by the simultaneous coordination of both CB[6] and the string guest molecules, (c) zigzag 1D chain constructed by CB[6] and the string molecules, and (d) 1D interwined polyrotaxane chain in URCP4. All hydrogen atoms have been omitted for clarity.

177.5(2)° for U1 and U2, with the UO distances ranging from 1.762(4) to 1.772(4) Å, respectively. Both uranyl centers still adopt pentagonal-bipyramidal geometry but differ in their coordination environment. The equatorial coordination plane of U1 is occupied by five oxygen atoms: two from bidentate sulfate ions, one from the carboxylate group of a string molecule, one from the carbonyl group of CB[6], and one from a water molecule (Figure 4a). The U−O distances vary in the range of 2.311(4)−2.481(4) Å [U1−O5 = 2.481(4) Å, U1−O6 = 2.462(3) Å, U1−O7 = 2.311(4) Å, U1−O8 = 2.326(4) Å, and U1−O9 = 2.354(3) Å]. For U2, the equatorial coordination situation is similar to that of U1, but the oxygen atom from CB[6] has been replaced by a water molecule. The E

DOI: 10.1021/acs.inorgchem.8b02126 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) 2D network of URCP2 derived from L2′, (b) 2D interwoven network of URCP1 derived from L1′, (c) 2D interwoven network of URCP3 derived from L3′, and (d) 2D interwoven network of URCP5 derived from L4′.

Figure 7. Comparison of the deformation of CB[6] between URCP1 (a), URCP3 (b), URCP4 (c), URCP5 (d), and URCP2 (e), using guest-free CB[6] in a nearly perfect configuration (f) as a control.62

tion networks with distinctive topologies, which are dependent on the coordination ability of macrocyclic CB[6] and the configurations of the flexible guest linkers. As shown in Figure 6a, without any participation of the portal carbonyl groups in metal−organic coordination, only the string molecule (C6BPCA) with functional end groups can connect the uranyl SBU to a 2D network. As a result, CB[6] only hang on the skeleton like beads, and URCP2 is a simple 2D polyrotaxane network. In contrast, the uranyl centers in URCP1 and URCP3 would first respectively assemble with CB[6] and the string molecules to produce 1D zigzag chains serving as building blocks. The 2D network is completed by the inherent host−guest interaction of pseudorotaxane: the coordinated CB[6] molecules would encapsulate the relevant string molecules, while the string molecule would thread into the relevant CB[6] molecules, which gradually assemble the 1D chains to the 2D networks (Figure 6b,c). This process could be

regarded as that in which the 1D building blocks selfinterweave one by one to produce multiple interwoven structures in the prepared coordination polymers. Because of the stronger capacity resulting from the ultralong guest length as well as trans configuration in L4′, the uranyl SBU could first use CB[6] and C8BPCA from different sets of pseudorotaxanes as building struts to form a prismatic building block, which further extends to be a 2D network. Also, the interlocked feature of L4′ can produce an interwoven structure consisting of two 2D networks in URCP5 (Figure 6d). Role of Coordination of Cucurbituril in Pseudorotaxane with Uranyl. With the aid of the carbonyls at both ports, the guest-free cucurbiturils exhibit good metal−organic coordination ability in the previous reports59−61 and have been developed to be a kind of macrocyclic ligand to design and synthesize various coordination polymers. However, when it comes to MORFs, the cucurbituril macrocycle seems to be F

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Inorganic Chemistry Table 2. Deformation of CB[6] in URCP1−URCP5 and L1′−L4′ (Units: Å, SD ⩽ 0.005 Å, and 0.07%) portal 1

portal 2

compound

dmax

dmin

dmax/dmin

dmax

dmin

dmax/dmin

deformation (dmax − dmin)

deformation %

URCP1 URCP2 URCP3 URCP4 URCP5 L1′ L2′ L3′ L4′

7.62 7.25 7.30 7.34 7.57 7.81 7.14 7.25 7.51

6.84 6.82 6.79 6.98 6.59 6.23 7.02 6.97 6.62

1.11 1.06 1.07 1.05 1.15 1.25 1.02 1.04 1.13

7.71 7.25 7.64 7.46 7.57 7.78 7.14 7.12 7.51

6.41 6.82 6.58 6.85 6.59 6.41 7.02 7.11 6.62

1.20 1.06 1.16 1.09 1.15 1.37 1.02 1.00 1.13

1.30 0.43 1.06 0.61 0.98 1.58 0.12 0.28 0.89

18.5 6.13 15.1 8.70 14.0 22.5 1.70 3.99 12.7

Figure 8. Deformation of CB[6] with the dmax/dmin for each opposite portal of CB[6] in (a) L1′, (b) L2′, (c) L3′, (d) and L4′.51

(6.13%). Besides, the values of dmax/dmin for each opposite portal are 1.11 and 1.20 in URCP1, 1.16 and 1.07 in URCP3, 1.09 and 1.05 in URCP4; i.e., CB[6]s in these three polyrotaxanes possess two portals with different radii. Also, the two portals exhibit the same dmax/dmin value (1.15) in URCP5, suggesting that this CB[6] is elliptical (Figure S10). Meanwhile, both dmax/dmin values of the CB[6] portals are as low as 1.06 in URCP2. A combination of crystallographic analysis for the pseudorotaxane ligands L1′−L4′ identifies the deformation of CB[6] in pseudorotaxane as an intrinsic feature of the host−guest complexes but not purely a consequence of metal coordination. Actually, CB[6] macrocycles in L1′−L4′ have already adopted different deformations in the process of self-assembly before binding to uranyl: CB[6] molecules in L1′, L3′, and L4′ have adopted ellipsoidal configurations like these in the relevant uranyl−organic compounds (Figure S11), but CB[6] in L2′ possesses virtually no deformation and the portal shows a perfect geometry like that of the guest-free CB[6] (Figure 8). As shown in Table 2, the deformations of CB[6]s in L1′, L3′, and L4′ have already reached 22.5%, 3.99%, and 12.7%, respectively, but could be ignored in L2′ (1.71%). The values of d max /d min for each opposite portal of CB[6] in pseudorotaxane ligands keep the same trend as that in the relevant URCPs (1.37 and 1.25 in L1′, 1.02 in L2′, 1.04 and 1.00 in L3′, and 1.13 in L4′; Figure S13). In our previous work,51 we measured the binding constants of L1−L4, which revealed that the binding affinities of host CB[6] to the different guests follow the trend of C6 (9.65 × 104 M−1) > C7 (5.04 × 104 M−1) > C8 (2.71 × 104 M−1) > C5 (1.71 × 104 M−1). The deformations of CB[6] are demonstrated to be adaptive responses of the CB[6] hosts to the ill-suited guests

chemically inert and plays no other role beyond acting as a rigid host to encapsulate the string molecules in most cases. A detailed investigation indicates that the coordination behavior of CB[6] in the pseudorotaxane ligand (i.e., L1′, L2′, and L4′) reported here is rarely reported. Hence, the unusual phenomenon observed here indicates that there might be some other unrecognized driving force to endow the cucurbituril in pseudorotaxane with coordination ability. Further crystallographic analysis suggests that the CB[6] molecules have adopted various deformations in URCP1− URCP5. As Figure 7 indicates, no matter how we rotate the CB[6] molecules in URCP1, URCP3, URCP4, and URCP5 along any axis, no proper direction could be found to place all atoms of CB[6] on a relatively symmetric location. Especially, the CB[6] molecules in these compounds have assumed ellipsoidal shapes (Figure 7a−d) compared to the round cycle shown as guest-free CB[6] (Figure 7f). Distinctively, no obvious deformation of the uncoordinated CB[6] could be observed in URCP2, and the macrocyclic host still possesses a symmetric configuration (Figure 7e). To quantify the deformation, the difference in the distance between the two opposite oxygen atoms of the CB[6] carbonyl groups is evaluated. Deformed CB[6] hosts in the relevant URCPs were compared with the nearly perfect guest-free CB[6] molecule in an ideal compound of [InCl2(H2O)4]Cl3· (C36H36N24O12)·4H2O with the possibly highest symmetry D6h, of which the distance is 7.01 Å.62 As illustrated in Figure 7 and Table 2, CB[6] molecules in URCP1, URCP3, URCP4, and URCP5 allow considerable deformation higher than 0.5 Å. The deformation reaches a maximum (∼18.5%) in URCP1 and is also noticeable in URCP3−URCP5 (15.1%, 8.70%, and 14.0%, respectively). In contrast, CB[6] in URCP2 without coordination to the uranyl ion causes a slight deformation G

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Inorganic Chemistry and are anticorrelated with the binding constants between the host CB[6] and string molecules. In L2′, because of the good bonding ability of the regular CB[6] toward the C6 alkyl chain, tight inclusion makes the pyridinium groups produce huge steric hindrance around the portal planes of CB[6], which would prevent metal ions from approaching the carbonyl groups. However, the considerable adaptive deformation of CB[6] in L1′ and L3′ would effectively diminish the steric hindrance and offer the metal ions the chance to coordinate with the portal carbonyl groups. More strikingly, two carbonyl groups from both sides of CB[6] are available for metal coordination in URCP5. This might be due to the ultralong alkyl chain of the string molecule in L4′, which could not only induce the deformation of CB[6] but also give sufficient isolation of the CB[6] portals from the two pyridinium rings, further diminishing the steric hindrance. Actually, isolation of the CB[6] portals from the pyridinium rings of the guest linkers is also clearly evidenced by the liquid 1 H NMR spectral analysis of host−guest complexes L1−L4 (Figure S12). All Ha protons protruding outside of the cavity suffer deshielding effects from the portal carbonyl groups and exhibit different degrees of guest-sensitive downfield shifts (0.38 ppm for C5, 0.50 ppm for C6, 0.25 ppm for C7, and 0.24 ppm for C8) upon binding to the CB[6] hosts (Figure S12). Specifically, in contrast to the highest downshift of the Ha proton of pyridinium in L2, the Ha proton of the pyridinium group in L4 exhibits the smallest downfield shift upon binding to the CB[6] hosts among all four of these supramolecular complexes. This suggests that the longest distance of the alkyl chain can release CB[6] portals from the bulky pyridinium rings as potential uranyl binding sites as discussed above. Furthermore, we can see that the deformation of CB[6] in URCP1 decreases slightly in reference to L1′ but obviously increases in the other compounds compared to that in the relevant ligands. All of these results suggest that the intrinsic adaptive deformation of cucurbiturils in pseudorotaxanes with ill-suited guests plays a vital role in releasing the steric hindrance around the portals of CB[6] and promoting their coordination with uranyl ions. Subsequently, the metal− organic coordination would also further intensify the deformation of the CB[6] host in pesudorotaxane. Activation of the CB[6] molecule in the pseudorotaxane ligand has a great effect on the mechanism for the formation of designed MORFs. In URCP1, URCP3, URCP4, and URCP5, the pseudorotaxane ligands would first bond to uranyl centers by the carboxylate groups, resulting in 1D zigzag chains (Figures 9 and S13 and S14). Different from the CB[6] molecules hung on the skeleton in URCP2, the macrocyclic hosts in these 1D chains have been activated with coordination ability by the adaptive deformation in the process of supramolecular preassembly. Hence, the CB[6] molecules hung on these 1D chains would further bind to the uranyl centers of the adjacent building blocks, thus completing the unique structures of the interwoven URCPs. Distinctively, URCP1, URCP3, and URCP5 finally exhibit as 2D interwoven networks, while URCP4 is found to be a 1D intertwined chain (Figure S13b). Effect of a Coordinated Anion and Water on the Final Structures of URCPs. Besides the availability of both the axle and wheel molecules for metal coordination, the coordinated anions and water molecules also have a great influence on the final topology of the interwoven URCPs. Comparing URCP3 and URCP4, we can see that, although these two uranyl−

Figure 9. 2D interwoven network of URCP1 resulting from the simultaneous coordination of both CB[6] and bipyridinium string molecules from L1′. All hydrogen atoms are omitted for clarity.

organic compounds both derive from L3′, they have obvious differences in the specific coordination modes and final topologies. Because no sulfate ions participate in the coordination sphere, the uranyl centers are monomeric in URCP3. Furthermore, each metal node connects with one CB[6] molecule and two string molecules from three different pseudorotaxanes (Figure 10a). An abundant coordination

Figure 10. (a) Coordination sphere of the uranyl center in URCP3, (b) coordination sphere of the uranyl center in URCP4, (c) 2D interwoven network of URCP3 in the side (at left) and front (at right) views, and (d) 1D intertwined chain of URCP4 in the side (at left) and front (at right) views.

environment endows the uranyl SBUs with the chance to exhibit a complicated 2D interwoven structure. More interestingly, because the whole network of URCP3 is positively charged, there are plenty of bromide counteranions in the cavities (Figure 10c). This unique characteristic might make URCP3 have some potential in the applications of anion exchange and separation. At a higher pH value, two bidentate sulfate anions bridge the uranyl ions to a dimer in URCP4, while three water molecules further coordinate to the metal center (Figure 10b). Because the anions and solvent molecules occupy the coordination sites, the dimeric uranyl centers only bond to one CB[6] and two string molecules (Figure 10b). In addition, the two axle molecules adopt an antiparallel location at the terminal site of the uranyl dimer and form a zigzag chain with the CB[6] hosts hung on the inflection points (Figure 10d). As a result, a lower dimensionality naturally is obtained in URCP4: two 1D chains get intertwined with each other by the interlocked feature of the pseudorotaxane ligand, constructing a unique antiparallel intertwined structure (Figure 10d). PXRD, FTIR, TGA, and Fluorescence Spectroscopy. The high purity of the prepared URCP1−URCP4 (pure H

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Inorganic Chemistry

controllable manner. The strategy will be helpful in promoting the development of MORFs with complicated structures and valuable properties. Meanwhile, more studies on the actinide coordination chemistry of the newly explored functionality of CB[6]-based pseudorotaxane ligands can be expected in the future.

URCP5 could not be isolated in enough yield) is confirmed by the good match between the simulated and experimental PXRD patterns (shown in the SI). TGA demonstrates their high thermal stability, and all of them show no obvious weight loss up to 300 °C (Figure S15). Especially, URCP1, URCP3, and URCP4 show obviously higher thermal stability than URCP2. This improved thermal stability might be explained as densely packed structures resulting from the interlocked feature of pseudorotaxane ligands. Fluorescence spectra of URCPs are shown in Figure 11, and they illustrate that all of

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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.8b02126. Typical figures including characterization data (PXRD, TGA, and FTIR) (PDF) Accession Codes

CCDC 1820934−1820936 and 1857849 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.

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AUTHOR INFORMATION

Corresponding Authors

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

Figure 11. Fluorescence spectra of URCP1−URCP4 and uranyl sulfate/nitrate compounds (pure URCP5 cannot be separated in sufficient yield).

ORCID

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

the uranyl compounds exhibit five or six distinct peaks between 470 and 650 nm, which represent the symmetric and antisymmertric oscillations of the U−O bonds.63 Compared to the control compound, URCP1 remains the intrinsic luminescence property of UO2SO4·2.5H2O, all peaks of URCP2 and URCP4 show red shifts, and all peaks of URCP3 show slight blue shifts. This difference in the luminescence properties of the obtained URCPs is certainly caused by the distinct coordination environment of the uranyl cation.64 In the FTIR spectra of URCP1−URCP5, the UO stretching vibrations could be observed in the region of 930− 915 cm−1 (Figure S16). For URCP1, URCP2, URCP4, and URCP5, the existence of SO42− is demonstrated by the adsorption bands around 1130−1040 cm−1.

Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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

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CONCLUSION In this work, we employed pseudorotaxane ligands with adaptively deformed CB[6] macrocyclic hosts to construct complicated MORFs. A series of URCPs with a new coordination mode of CB[6]-based pseudorotaxane ligands were obtained, through the hydrothermal reaction of selected pseudorotaxanes with uranyl sulfate. Structural analysis demonstrates that the intrinsic adaptive deformation of pseudorotaxane with “ill-suited” guest linkers could effectively activate CB[6] and endow it with the ability for metal coordination. As a result, simultaneous coordination of both the host and guest components to the metal centers was observed for the first time in URCPs. In addition to the participation of CB[6] in uranyl coordination, the coordinated sulfate anions and water molecules also have a great effect on the final structures of URCPs. This new strategy, i.e., releasing the metal-coordination capacity of the CB[6] macrocycle in pseudorotaxane, is demonstrated to be effective in building interwoven structures of MORFs in a predictable and

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DOI: 10.1021/acs.inorgchem.8b02126 Inorg. Chem. XXXX, XXX, XXX−XXX