Conformational 2-Fold Interpenetrated Uranyl Supramolecular

Article Cite This: Inorg. Chem. 2018, 57, 15370−15378

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Conformational 2‑Fold Interpenetrated Uranyl Supramolecular Isomers Based on (6,3) Sheet Topology: Structure, Luminescence, and Ion Exchange Chao Liu,†,‡,∥ Chao Wang,‡,§,∥ and Zhong-Ming Sun*,†,‡ †

Inorg. Chem. 2018.57:15370-15378. Downloaded from pubs.acs.org by YORK UNIV on 12/18/18. For personal use only.

School of Materials Science and Engineering, Research Center of Rare Earth and Inorganic Functional Materials, State Key Laboratory of Elemento-Organic Chemistry and College of Chemistry, Nankai University, Tianjin 300350, China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Six new 2-fold interpenetrated uranyl coordination polymers with two distinct types of (6,3)-net layers, (H3O)[(UO2)(L)]·H2O (1), (Hbipy)[(UO2)(L)]·H2O (2), (Hbib)[(UO2)(L)]· H 2 O (3), (H 2 dib)[(UO 2 ) 2 (L) 2 ]·H 2 O (4), [Zn(H 2 O) 6 ][(UO2)2(L)2]·5H2O (5), and (NH4)[(UO2)(L)]·H2O (6), (bipy = 2,2′-bipyridine, bib = 4,4′-di(1H-imidazol-1-yl)-1,1′-biphenyl, and dib = 1,4-di(1H-imidazol-1-yl)benzene), were hydrothermally prepared from a tripodal polycarboxylate ligand, tri(4carboxyphenyl)phenylsilane (H3L), with different N-bearing organic templates as the stacking templates and charge compensators. Structural analyses indicate that these compounds comprise two sets of conformational supramolecular isomers because of the same framework compositions but different conformations of the carboxylate ligands. The solid-state emission spectra of compounds 1−6 were recorded. Ion-exchange studies revealed that the ammonia hydrate in 6 can be selectively substituted by alkali metal cations with appropriate ionic radii, and that the resulting structures remain stable, as demonstrated by crystallographic characterization.

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INTRODUCTION Research advances in the coordination chemistry of actinides hold a promising potential for application in advanced nuclear fuel cycles.1−3 Compared with other actinide-based materials, uranyl coordination polymers (UCPs) have gained particular attention because of their diversified structures and intriguing architectures.4−7 Typically, uranium species most commonly exist in a linear dioxocation (UO22+) state, which can further coordinate in the equatorial plane with O-donor ligands, thus forming varied architectures with different network dimensionalities, including clusters, chains, layers, nanotubes, and 3dimensional frameworks.8−11 Uranium-based CPs also display many outstanding properties that include dye adsorption,12 photoelectronic effects,13 selective ion-exchange,14 aggregation-induced emission,15 and ionizing radiation detection.16 Supramolecular isomerism is widely encountered in the area of CPs and is applied when the structures of compounds are distinct but the whole coordination networks possess an identical formula.17 Supramolecular isomerism can be divided into several types, including structural isomerism,18 conformational isomerism,19 catenane isomerism,20 and optical isomerism.21 Conformational isomerism is applied to depict the © 2018 American Chemical Society

superstructural diversity arising from the difference in ligand conformation. Supramolecular isomerism has become a hotspot in material chemistry research, not only because the isomers may have different chemical and physical characteristics, but also because the control of isomerism truly embodies the core idea of coordination chemistry. Supramolecular isomers in uranyl polymers have been investigated.22 For example, An et al. recently synthesized two identical tetranuclear uranyl-based two-fold interpenetrated frameworks, in which conformational isomerism occurs because of the different arrangements of flexible template molecules.22b However, it is interesting to find that there is no report on uranyl conformational isomers that have the same topology but different network geometries. In fact, considering the influence of a variety of uncertainty factors on the crystal growth processes, the design and realization of supramolecular isomers, even the simplest one, are still full of difficulties. Therefore, to make a thorough inquiry into the spontaneous self-assembly behavior in uranyl coordination chemistry, a Received: September 21, 2018 Published: November 27, 2018 15370

DOI: 10.1021/acs.inorgchem.8b02696 Inorg. Chem. 2018, 57, 15370−15378

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Inorganic Chemistry

tightly closed autoclave. NEt4OH (30 μL) was added into the mixture. The mixture was heated at 180 °C for 48 h. After slow cooling to 25 °C, crystals of 2 were formed. Yield: ∼ca. 35%. Anal. Calcd (wt%) for C37H28O9N2SiU: C, 48.75; H, 3.07; N, 3.07. Found: C, 46.36; H, 2.89; N, 3.30. Synthesis of Compound (H 2 bib) 0.5 [(UO 2 )L]·H 2 O 3. UO2(NO3)2·6H2O (50 mg, 0.1 mmol), 30 mg H3L (0.064 mmol), 28 mg bib (0.100 mmol), and 1.0 mL deionized H2O were added into a 20 mL tightly closed autoclave. Then, 30 μL NEt4OH was added into the mixture. The solution was heated at 180 °C for 48 h. Crystals of 3 were formed after cooling to 25 °C. Yield: ∼ca. 28%. Anal. Calcd (wt%) for C36H23O9N2SiU: C, 48.33; H, 2.57; N, 3.13. Found: C, 45.86; H, 2.29; N, 3.83. Synthesis of Compound (H 2 dib)[(UO 2 ) 2 L 2 ]·H 2 O 4. UO2(NO3)2·6H2O (50 mg, 0.1 mmol), 30 mg H3L (0.064 mmol), 21 mg dib (0.100 mmol), and 1.0 mL deionized H2O were added into a 20 mL autoclave. Then, 30 μL NEt4OH was added into the mixture. The solution was heated at 180 °C for 48 h. Crystals of 4 were isolated after slow cooling to 25 °C. Yield: ∼ca. 25%. Anal. Calcd (wt %) for C54H56O27SiU2Zn: C, 37.35; H, 3.32. Found: C, 35.21; H, 3.96. Synthesis of Compound [Zn(H2O)6][(UO2)2L2]·5H2O 5. H3L (30 mg, 0.064 mmol) and 50 mg Zn(UO2)(OAc)4·7H2O (0.05 mmol) were added to 1.5 mL deionized H2O in a 20 mL tightly closed autoclave. NEt4OH (30 μL) was added into the mixture. The resulting solution was heated at 180 °C for 48 h. Crystals of 5 were formed after cooling to 25 °C. Yield: ∼ca. 35%. Anal. Calcd (wt%) for C66H48N4O17Si2U2: C, 46.55; H, 2.82; N, 3.29. Found: C, 44.69; H, 2.48; N, 3.70. Synthesis of Compound (NH4)[(UO2)L]·H2O 6. H3L (30 mg, 0.064 mmol) and 50 mg Zn(UO2)(OAc)4·7H2O (0.05 mmol) were added to 1.5 mL deionized H2O in a 20 mL tightly closed autoclave. Ammonia hydrate (10 μL, NH3·H2O) was added into the mixture. The solution was heated at 180 °C for 48 h. Compound 6 was formed after cooling to 25 °C. Yield: ∼ca. 45%. Anal. Calcd (wt %) for C27H22NO9.5SiU: C, 41.61; H, 2.82; N, 1.79. Found: C, 40.36; H, 2.49; N, 1.85. Ion Exchange Experiment. Single crystals of compound 6 were placed in vials (10 mL), and 5 mL of a 100 ppm aqueous solution of alkali metal salts (LiNO3, NaNO3, KNO3, or CsCl) was also placed. The mixture was allowed to stand at 25 °C for 24 h without disturbance. The exchanged crystals (H3O)0.5[Na(H2O)2]0.5[(UO2)L]·0.5H2O (6-Na), (K)[(UO2)L] (6-K) and (Cs)(CsH2 O)[(UO2)2L2] (6-Cs) were then picked out and rinsed with deionized water, further characterized by single-crystal XRD and EDXS spectra. Anal. Calcd (wt %) for C27H21O10SiUNa0.5 (6-Na): C, 41.37; H, 2.68. Found: C, 41.36; H, 2.595. Anal. Calcd (wt %) for C27H17O8SiUK (6K): C, 45.31; H, 2.19; Found: C, 45.01; H, 2.10. Anal. Calcd (wt %) for C54H36O17Si2U2Cs2 (6-Cs): C, 36.92; H, 2.05. Found: C, 35.89; H, 1.98. Crystallography. Single crystals of the title compounds were immobilized on glass fibers for the crystallographic determinations. Reflection data were collected using a Bruker Apex II CCD diffractometer (Mo K radiation, λ = 0.71073 Å) at 150 K. SAINT program was used for data processing. The initial structures of the compounds were solved using direct methods with the ShelXle program and then refined with the ShelXle refinement package to convergence.25 Hydrogen atoms on carbon and nitrogen atoms were fixed by geometrical considerations with isotropic displacement parameters. Table 1 lists the crystallographic data of all compounds and U−O bond distances are summarized in Tables S1−S9. Further crystallographic data for this paper are available from the Cambridge Structural Database with numbers 1868061−1868069.

systematic investigation on uranium-based supramolecular isomerism is necessary. Our group has recently been interested in the synthesis of UCPs with polycarboxylic acids.23 Tri(4-carboxyphenyl)phenylsilane, which features a tripodal geometry with a tetrahedral center, attracted our attention. In this study, through the reaction of uranyl nitrate/zinc uranyl acetate and tri(4-carboxyphenyl)phenylsilane in the presence of different organic templates, we successfully constructed six new UCPs that possess an identical two-dimensional (2D) 2-fold interpenetrated framework with (6,3) nets. Interestingly, such materials have identical framework compositions, but their single (6,3) nets possess two sets of configurations, and hence, can be classified as supramolecular isomers. Ion-exchange studies reveal that the NH4+ in 6 is active and can be easily substituted by monovalent alkali metal ions such as Na+, K+, and Cs+ without degrading the crystal structure; this was confirmed by crystallographic measurements and energydispersive X-ray spectroscopy (EDXS).

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EXPERIMENTAL SECTION

Caution! Although uranyl materials used here contained depleted uranium, standard procedures for handling toxic and radioactive materials should be followed. Materials. All reagents purchased commercially (reagent grade or better). Uranyl nitrate (UO2(NO3)2·6H2O, 99.5%), zinc uranyl acetate (ZnUO2(OAc)4·7H2O, 99.8%), and N-donor ligands were obtained from Sinopharm Chemical Reagent. Tri(4-carboxyphenyl)phenylsilane was synthesized by using a modified literature procedure.24 Scheme 1 lists the structure of the carboxylate ligand and N-donor organic templates.

Scheme 1. Schematic Representation of the SiliconCentered Polycarboxylate Ligand as Well as N-Bearing Organic Templates

Measurements. Powder X-ray diffraction (PXRD) patterns were recorded from 3° to 50°, with an increment of 0.02 on a D8 Focus diffractometer with Cu Kα radiation (40 kV, 30 mA and λ = 1.5405 Å) (Bruker) (Figure S28). The emission spectra were performed by using an F-7000 luminescence spectrometer, and the excitation light source is the 450 W xenon lamp. The Nicolet 6700 FT-IR spectrometer was used to perform infrared spectra (IR), which were recorded with a diamond ATR objective in the range of 500−4000 cm−1 using a single crystal. Elemental analyses were collected by using a PerkinElmer 2400 elemental analyzer. Synthesis of Compound (H 3 O)[(UO 2 )(H 3 O)L]·H 2 O 1. UO2(NO3)2·6H2O (50 mg, 0.1 mmol), 30 mg H3L (0.064 mmol), 30 μL tetraethyl ammonium hydroxide (NEt4OH), and 1.0 mL deionized H2O were mixed in a 20 mL tightly closed autoclave and heated at 180 °C for 48 h. Crystals of 1 were collected after cooling to 25 °C. Yield: ∼ca. 50% (based on uranium source). Anal. Calcd (wt %) for C27H19O9SiU: C, 42.99; H, 2.52. Found: C, 41.03; H, 2.39. Synthesis of Compound (Hbipy)[(UO2)L]·H2O 2. UO2(NO3)2· 6H2O (50 mg, 0.1 mmol), 30 mg H3L (0.064 mmol), and 18 mg bipy (0.100 mmol) were placed into 1.0 mL deionized H2O in a 20 mL

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RESULTS AND DISCUSSION

Compound 1 was hydrothermally synthesized by heating a mixture of the polycarboxylate ligand H3L, uranyl nitrate, and N(Et)4OH in deionized water at 180 °C for two days; the complex crystallized in the space group P21/c. The structure of 15371


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Inorganic Chemistry Table 1. X-ray Measurements and Structure Solution of the Compounds compound

1

2

3

4

formula Fw crystal system space group a, Å b, Å c, Å α/deg β/deg γ/deg V, Å3 Z 2Θ range/deg reflections collected/unique ρcalcd (g cm−3) μ (mm−1) F(000) R1/wR2 (I > 2σ(I))a R1/wR2 (all data) compound

C27H19O9SiU 753.5488 monoclinic P2/c 15.684(3) 9.9882(19) 21.050(4) 90 106.692(4) 90 3158.7(11) 4 2.71−50 17637/5554 1.564 1.369 1444 0.0564/0.1603 0.0928/0.1754 6

C37H28N2O9SiU 910.7407 monoclinic P21/c 10.5954(10) 20.837(2) 15.4980(15) 90 92.8528(18) 90 3417.4(6) 4 3.278−52.354 20763/6829 1.766 4.84 1760 0.0324/0.0746 0.0472/0.0811 6-Na

C36H23N2O9SiU 893.6903 monoclinic P21/c 11.7816(6) 20.2191(10) 15.3411(8) 90 92.4380(10) 90 3651.1(3) 4 3.334−52.224 22542/7252 1.615 4.532 1700.0 0.0370/0.0832 0.0545/0.0906

C54H56O27Si2U2Zn 1734.6441 orthorhombic Pbca 17.3736(10) 25.6273(14) 28.4477(15) 90 90 90 12666.0(12) 8 3.174−50 69570/11145 1.796 5.596 6528 0.0648/0.1584 0.0926/0.1712

formula Fw crystal system space group a, Å b, Å c, Å β/deg V, Å3 Z 2Θ range/deg reflections collected/unique ρcalcd (g cm−3) μ (mm−1) F(000) R1/wR2 (I > 2σ(I)) R1/wR2 (all data)

C27H22O9.5NSiU 778.5790 orthorhombic Ibca 16.764(2) 25.764(4) 27.763(4) 90 11991(3) 16 3.162−52.34 35648/5961 1.718 5.507 5904 0.0580/0.1557 0.1057/0.1865

C27H21O10Na0.5SiU 783.0589 orthorhombic Ibca 16.580(4) 25.835(4) 27.730(5) 90 11878(4) 16 2.938−50 31886/5237 1.743 5.565 5928 0.0514/0.1103 0.1222/0.1305

5

6-K

C66H48N4O17Si2U2 1701.3329 orthorhombic Iba2 18.0846(14) 25.5564(19) 28.648(2) 90 90 90 13240.5(18) 8 3.188−52.15 40245/12367 1.696 4.992 6500 0.0488/0.1109 0.1048/0.1439 6-Cs

KC27H17O8SiU 774.6318 orthorhombic Ibca 17.0188(14) 25.881(2) 27.620(2) 90 12165.5(17) 16 3.148−50 33449/5352 5888 5.556 5888 0.0548/0.1416 0.1030/0.1564

Cs2C54H36O17Si2U2 1754.8932 orthorhombic Pcca 26.046(3) 15.9688(18) 27.908(3) 90 11608(2) 8 2.918−50 64025/10223 2.004 6.909 6528 0.0577/0.1116 0.1347/0.1308

R1 = Σ(ΔF/Σ(Fo)); wR2 = (Σ[w(Fo2 − Fc2)])/Σ[w(Fo2)2]1/2, w = 1/σ2(Fo2).

a

1 comprises a crystallographically unique silicon-centered ligand and a uranyl center (Figure S1). The uranyl is coordinated equatorially by six O atoms from three chelating carboxylate groups in three silicon-centered ligands, thus affording a hexagonal bipyramidal geometry, where the U−Oeq bond contacts are between 2.416(5) and 2.497(5) Å, and the U−Oyl bond lengths are 1.731(8)−1.776(8) Å. Every L ligand links three uranyl ions. Such an assembly of uranyl units and L ligands produces a 2D (6,3)-net layered structure (Figure 1a). It is noteworthy that although the (6,3) topologic 2D layer in uranyl organic materials has been well-described,26 the layer in 1 is distinctive because of its wave-like configuration (Figure 1b). Viewed down the a axis, the infinite 2D layer exhibits a periodic fluctuation with a folding angle of 97.6°, in which the crest and trough of the wave are constituted by the siliconcentered ligands and the wave nodes are formed by the uranium oxide polyhedrons. The distance between the adjacent crests in 1 is 19.9 Å (Figure S5). Two equivalent monolayers entangled with each other, resulting in a two-fold interpenetrated double layer (Figure 1c), which exhibits a standing-wave shape along the b axis (Figure 1d). Although

Figure 1. (a) Monolayer 2D network of 1; (b) monolayer 2D network of 1 viewed down the α axis; (c) two-fold interpenetrating 2D networks of 1 along the c axis. Two different single layers are marked in green and red colors, respectively; (d) two-fold interpenetrated 2D networks of 1.

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that in 1, in which the 2D (6,3) nets also exhibit a wave-type configuration (Figures S7 and S10). The folding angles of the single layer in 2 and 3 are 106.2° and 123.6°, respectively, and the corresponding distances between adjacent crests extend to 21.9 and 23.6 Å (Figure 3). This subtle change in the monolayer subsequently increases the void space of the framework, thus making the channels in the structures large enough to accommodate the organic template. For 2, monoprotonated bipy molecules are located in the voids between the interpenetrated layers (Figure 4a), whereas protonated linear bib molecules in 3 appear in the channels between the adjacent double layers (Figure 4b).

two identical networks entangled with each other, there also exist large channels along the c axis, where protonated solvent water interacts with the layers through electrostatic cation/ anion interactions, acting as charge compensators (Figure 2a). The whole framework of 1 is architected by the stacking of such interpenetrated double layers (Figure 2b).

Figure 2. (a) Structural view of compound 1. H3O+ existed in the channels as charge compensators. (b) Neighboring interpenetrated layers are extended to three-dimensional structures adopting the ABAB stacking mode.

Figure 4. Structural view of 2 and 3, respectively, where protonated bipy and bib molecules are located in different channels of compounds.

When using the dib molecule as the N-donor template, however, we observed another type of (6,3) layer in 4, which possesses an identical framework formula to that of the layer in 1−3 but adopts a distinctly different geometry (Figure 5a). Compound 4 crystallizes in the orthorhombic space group Pna2. Structural analysis reveals that there exist two unique uranyl centers, two carboxylate ligands, and a dib molecule in the asymmetric unit (Figure S11). The U1 and U2 atoms are

Generally, N-donor templates with different sizes and configurations have a non-negligible effect on the structural expansion of the uranyl network, which can change the interaction between the carboxylate group and uranyl ion, thus generating different structures. The synthesis of 2 and 3 followed the same protocol described for 1, except the addition of bipy or bib in the reaction. Interestingly, compounds 2 and 3 possess an identical 2-fold interpenetrated (6,3) network to

Figure 3. Subtle structural difference between compounds 1−3. 15373


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Figure 6. View of the structure of 5 along the c axis. Zn(H2O)62+ existed in the channels between the adjacent double layer.

Figure 5. (a) Space-filling view of the monolayer in 4; (b) interpenetrated double layers in 4; (c) single layer in 4, viewed along the b axis; (d) structural view of 4 along the c axis. Protonated dib molecules existed in the channels between the adjacent double layer.

also in typical hexagonal-bipyramidal geometries defined by six O-donor atoms from three sets of carboxylate groups, and two “yl” atoms. The U−Oyl and U−Oeq contacts are in the ranges of 1.724(5)−1.747(5) Å and 2.405(5)−2.508(5) Å, respectively, which are comparable to the typical U−O bond lengths in uranyl-based materials.27 The uranyl sites and carboxylate ligands in 4 are all 3-connected nodes, thus finally affording the same interpenetrating 2D network with (6,3) nets as that in compounds 1−3 (Figure 5a). The main difference between them is the (6,3) nets they possess. As shown in Figure 5a, the building elements in 4 are bridged to create hexagonal open motifs, where the opposite orientation silicon-centered ligands are arranged alternately projected along the c axis, thus forming a significantly distorted spiderweb-type 2D layer instead of the wave shape in 1. Two equivalent twisty monolayers interpenetrated each other via the hexagonal hole, resulting in a 2fold interpenetrating double layer (Figure 5b,c). The interpenetrated layer is anionic overall, stacking along the c axis in an ABAB packing model. Protonated dib molecules exist in the pseudochannels between the adjacent double layers (Figure 5d). To better understand the behavior of the self-assembly of actinide CPs, we further investigated the impact of different uranium sources on the resulting structures. Interestingly, when Zn(UO2)2(OAc)6·7H2O replaced UO2(NO3)2·6H2O, a distorted spiderweb-like layer nearly identical to that in 4 again appeared in 5. The asymmetric unit of 5 also comprises two unique uranyl cations, two L ligands, and a hexahydrated zinc(II)-ion Zn(H2O)62+ (Figure S16). The building units in 5 have the same coordination arrangement as that in 4, further extending the structure to a similar 2D interpenetrated net. Zn(H2O)62+ ions that replaced protonated dib exist in the pseudochannels between the adjacent double layers along the c axis (Figure 6). In addition, when the pH was adjusted to be more basic, by introducing ammonia hydrate into the reaction system, an analogous compound 6 was exclusively obtained; in this case, the ammonium ions acted as charge compensators located in the hole within the framework (Figure 7a). In the structure of 6, it is notable that in addition to the large oval-

Figure 7. View of the structure of 6 along the c axis (a) and b axis (b), respectively.

shaped channel between the adjacent double layers, there also exist other smaller channels within the interpenetrated layer; some ammonium ions and solvent water occupied these channels and were arranged along the b axis (Figure 7b). Topologically, the resulting structures can all be simplified as two-fold interpenetrated single-nodal 3-connected networks, which exhibit two of the four topologically different modes of (6,3) parallel interpenetration, as discussed by Batten and Robson.28 Compounds 1−3 are topologically equivalent and can be attributed to type I. The crooked and stair-shaped monolayer in 1−3 allows the equivalent neighboring (6,3) layers to interpenetrate each other (Figure 8a). The interpenetration mode in type II is distinct; the significantly twisty

Figure 8. Two topologically different modes of parallel interpenetration of (6,3) nets. 15374


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Inorganic Chemistry spiderweb net allows part of the framework to extend into the hole of the adjacent neighboring hexagon (Figure 8b). Compounds 4−6 belong to this type. The essential differences between the isomers are triggered by the change in the arrangement of tripodal ligands around the uranium center, which exhibits different conformations in those polymers. As shown in Figure 9, the three p-carboxyphenyl groups acting as

Figure 10. Luminescent spectra of compounds 1−6 (excited at 420 nm).

explore its ion-exchange behaviors. Notably, by immersing single crystals of 6 in a neutral aqueous solution with an appropriate M+ concentration (Li+, Na+, K+, and Cs+; 100 ppm), nearly 100% of NH4+ in the pseudochannels can be quickly replaced with K+ and Cs+ cations, giving the formula K[(UO2)L] (6-K) and Cs(CsH2O)[(UO2)2L2] (6-Cs), whereas only 50% of NH4+ cations are substituted by Na+ cations, resulting in the formula (H 3 O) 0 · 5 [Na(H2O)2]0·5[(UO2)L] 0.5H2O (6-Na) (Figure 11). Structural analyses for the exchanged crystals reveal that 50% of K+ and Cs+ cations are transferred to the channel between the adjacent double layers, and the remaining K+ and Cs+ cations are

Figure 9. Different stereoisomers of ligands existed in 1−6.

the bracket can face different directions when coordinated with the uranyl unit. However, because of the Si−C bond-angle flexibility, which has a certain rotational degree of freedom, and the π−π interactions between the benzenes, the tetrahedral silicon-centered linkers eventually form the two types of the most stable conformation. The difference in the conformations of the ligand can be directed by the species of the N-bearing templates and the uranium sources; the reason for such a phenomenon is under further investigation. Infrared Spectra. The Fourier transform infrared spectra of compounds 1−6 were characterized (Figure S29). In the spectrum, the low-wavenumber regions (680−720 cm−1) are occupied by the out-of-plane vibration of the aromatic rings. The symmetric stretching vibrations of the uranyl unit were observed in the band of 850−858 cm−1, and the antisymmetric stretching mode of UO22+ ions appears in the area of 910−918 cm−1.29 The Si−C characteristic peaks dominate the band of 1082−1097 cm−1. The stretching vibrations of the aromatic ring and the carboxylate group are shown between 1580−1400 cm−1. The stretching vibrations of the hydroxyl group from lattice H2O were observed at 3570−3450 cm−1. Photoluminescence Properties. Uranyl materials typically show green light emission at 520−530 nm because of the symmetric and asymmetric vibration of UO22+ ions. Therefore, the photoluminescence of compounds 1−6 was investigated (Figure 10). Prototypical “five-finger” peaks were observed in the spectra for 1 (469, 488, 506, 528, and 5750 nm), 2 (469, 487, 506, 527, and 553 nm), 3 (471, 488, 508, 530, and 554 nm), 4 (470, 486, 506, 527, and 551 nm), 5 (470, 502, 524, 548, and 574 nm), and 6 (472, 488, 508, 527, 550 nm). These peaks originate from the vibrational and electronic transition of S11−S00 and S10−S0v (v = 0−4) of the uranyl unit.30 Overall, the emission peaks for these compounds are very similar and are also comparable to those found for uranyl nitrate (Figure S30). For example, the emission peaks for 1 are only redshifted by 6 nm compared with the peaks for UO2(NO3)2· 6H2O. Subtle differences may originate from the differences in the coordination environment. Ion-Exchange Studies. The elliptical pseudochannels in compound 6 and NH4+ occupied there stimulated us to

Figure 11. EDXS spectra and the exchanged sites of the Na+ (a), K+ (b), and Cs+ (c) in the structure of 6. 15375


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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

located at the site within the interpenetrated layer (Figure 11b,c). In contrast, all the Na+ cations are located in the site within the interpenetrated layer, whereas the channel between the double layers in 6-Na are stilled partially occupied by the solvent molecules (Figure 11a). The coordination numbers of Na+, K+, and Cs+ cations in the structure range 4−8, which are defined by framework O atoms and lattice H2O (Figure S27). It is notable that the inert “yl” oxo atoms in the uranyl unit interact with K+ and Cs+ cations but not with Na. Li+ was not successfully substituted into the structure, which may be attributed to its relatively small ionic radius. Further efforts made to substitute bivalent Zn2+ and N-donor molecules failed, despite the large oval-shaped channel within the adjacent double layers being sufficient to accommodate them, as indicated by the structure of 4 and 5. The ion-exchange experiments showed that 6 can undergo selective substitution by alkali-metal cations with appropriate ionic radii. More interestingly, alkali-metal ions with different radii can be substituted into different sites in the framework. The ion exchange of most uranium materials, to our knowledge, were conducted by heating and/or stirring in highly concentrated and high-pH solutions, and in many cases, the crystallinity of the substituted structure was too poor to perform single-crystal XRD characterization. However, although 6 is a layered structure, the NH4+ within the channels can easily get substituted at room temperature, without stirring, and the framework is strong enough that the crystal does not degrade or become damaged after ion exchange.

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Corresponding Author

*E-mail: [email protected], [email protected]. Web: http:// zhongmingsun.weebly.com/. ORCID

Zhong-Ming Sun: 0000-0003-2894-6327 Author Contributions ∥

C.L. and C.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21722106, and 21571171) and Jilin Province innovative research program (20160519004JH).

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REFERENCES

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CONCLUSIONS In summary, six interpenetrated uranyl-based coordination polymers based on a tripodal polycarboxylate ligand were synthesized and are rare examples of conformational uranyl supramolecular isomers. The structures feature two distinct types of (6,3)-net layers, which can be controlled through the regulation of the organic templates and uranium source. It should be noted that the building units in these compounds have an identical coordination mode, and their minor structural differences are due only to the conformation and arrangement of the polycarboxylate ligand. Photoluminescence analysis indicated that all the compounds show a characteristic emission from the uranyl center. In addition, the NH4+ in the framework of 6 possess sufficient activity to allow substitution by monovalent cations with appropriate ionic radii; Na+, K+, and Cs+ cations can be substituted into different sites in the compounds. The supramolecular isomers presented herein should represent an important step to devise and control the structures of uranyl organic materials.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02696. X-ray crystallographic cif files, PXRD pattern, and IR spectroscopy (PDF) Accession Codes

CCDC 1868061−1868069 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 15376


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Inorganic Chemistry

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Conformational 2-Fold Interpenetrated Uranyl Supramolecular

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