Bipyridine-Directed Syntheses of Uranyl Compounds Containing

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

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Bipyridine-Directed Syntheses of Uranyl Compounds Containing Semirigid Dicarboxylate Linkers: Diversity and Consistency in Uranyl Speciation Shu-wen An,†,‡,§ Lei Mei,‡,§ Kong-qiu Hu,‡ Fei-ze Li,‡ Chuan-qin Xia,*,† Zhi-fang Chai,‡,⊥ and Wei-qun Shi*,‡ †

College of Chemistry, Sichuan University, Chengdu 610064, China Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ⊥ Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo 315201, China

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S Supporting Information *

ABSTRACT: Bipyridine organic bases are beneficial to the synthesis of novel uranyl−organic hybrid materials, but the relationship between their molecular structures and specific roles as structure-directing agents, especially for the semirigid dicarboxylate systems, is still unclear. Here we demonstrate how the bipyridine ligands direct the coordination assembly of uranyl−organic compounds with a semirigid dicarboxylate linker, 4,4′-dicarboxybiphenyl sulfone (H2dbsf), by utilizing a series of bipyridine ligands, 1,10-phenanthroline (phen), 2,2′bipyridine (2,2′-bpy), 5,5′-dimethylbipyridine (5,5′-dmbpy), 4,4′-bipyridine (4,4′-bpy), or 1,3-di(4-pyridyl)propane (bpp). Under hydrothermal conditions, eight uranyl−organic coordination polymers (UCPs), four of which [[UO2(dbsf)(phen)] (1), [UO2(dbsf)(phen)]·H2O (1′), [U4O10(dbsf)3]2[H2bpp]2 (6), and [U4O10(dbsf)3]2[H2bpp] (6′)] were reported previously, were synthesized and divided into two types based on the chelate or template effect of these bipyridine ligands. 1, 1′, [UO2(dbsf)(2,2′-bpy)] (2), and [(UO2)2(dbsf)2(5,5′-dmbpy)2] (3) are springlike triple helices with bipyridine ligands (phen, 2,2′-bpy, or 5,5′-dmbpy) as chelate ligands, while [U4O10(dbsf)3][H2(4,4′-bpy)] (4), [U4O10(dbsf)3]2[H(4,4′-bpy)]2[Ni(H2O)6] (5), 6, and 6′ are tetranuclear uranyl-mediated 2-fold-interpenetrating networks with 4,4′-bpy or bpp as template ligands and charge-balancing agents. The participation or not in uranyl coordination of different bipyridine ligands promotes not only diversity in uranyl speciation and final topological structures among different classes of organic bases but also consistency for the same types of bipyridine ligands, which thus endows the possibility of the rational design of UCPs based on semirigid dicarboxylate ligands with the aid of cautiously selected bipyridine ligands.

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INTRODUCTION As a dominant group of actinide hybrid materials, uranyl− organic coordination polymers (UCPs) have drawn much attention in recent years because of their intriguing structures and properties1−11 as well as their close relevance to the nuclear fuel cycle. Because of the complicated hydrolysis mechanism of the uranyl cation in aqueous solutions, uranyl speciation always shows “hypersensitivity” to reaction conditions (such as the pH, temperature, additive reagents, and so on) and subsequently might produce different hydrolysis products. Such a condition-dependent sensitivity makes the reaction outcome of UCPs more or less unpredictable, which is in contrast to less variation for the synthesis of coordination compounds based on 3d or 4f block metal ions, and leads to difficulties in the controllable synthesis of targeted UCPs.12−14 On the other hand, the variability of uranyl species also contributes to the structural diversity of UCPs, thus providing © XXXX American Chemical Society

opportunities to explore new uranyl compounds with different nuclearities and novel coordination modes. Like the syntheses of transitional and lanthanide metal− organic frameworks, aromatic ligands with relatively rigid skeletons15−25 have been widely used for the construction of UCPs. Meanwhile, highly flexible organic ligands such as aliphatic carboxylic acids have also been reported to assemble with uranyl by Cahill’s26−28 and Thuery’s29−31 groups. In addition, other functionalized ligands, including macrocyclic molecules or polyrotaxane-type supramolecular compounds, have also been used to prepare actinide compounds,6,32−36 thus enriching the library of extending the potent functionality of UCPs. Besides, a third type of semirigid organic ligand37−40 with modest structural flexibility has drawn our attention Received: February 15, 2019

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DOI: 10.1021/acs.inorgchem.9b00452 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Nitrogen-Donor Bipyridine and H2dbsf Ligands Used in This Work

Table 1. Synthetic Conditions Used for Different UCPs H2dbsf (mmol) bipyridine (mmol) UO2(NO3)2 (mmol) Ni(NO3)2 (mmol) water (mL) pH(initial) pH(final) temperature (°C) time (h)

1

1′

2

3

4

5

6/6′

0.1 phen, 0.1 0.1 0 2 5.44 6.01 180 72

0.1 phen, 0.1 0.1 0 2 3.97 5.42 180 72

0.1 2,2′-bpy, 0.1 0.1 0 2 4.52 5.68 180 72

0.1 5,5′-dmbpy, 0.1 0.1 0 2 4.02 5.57 180 72

0.05 4,4′-bpy, 0.05 0.05 0 1 6.77 4.86 180 72

0.05 4,4′-bpy, 0.05 0.05 0.1 1 6.60 4.34 180 72

0.1 bpp, 0.1 0.1 0 2 7.85 5.68 180 72

dmbpy), 6,6′-dimethylbipyridine (6,6′-dmbpy),14,44,45 and 4,4′-bipyridine (4,4′-bpy)46 (Scheme 1), are systematically selected and used with H2dbsf to react with uranyl to probe the possible relationship between the molecular structures of organic bases and their functions during the assembly process of UCPs with semirigid linkers. A total of eight UCPs have been hydrothermally synthesized and characterized from H2dbsf and bipyridine ligands. Crystal structure analyses of these UCPs reveal that the specific role of those structuredirecting bases can be classified exactly by using the chelate effect as a key criterion, in which bipyridine ligands with chelating sites possessing no hindrance serve as uranyl chelators to give final helical chains, while nonchelated linear 4,4′-bpy acts only as a template agent just like bpp ligands. Most interestingly, as revealed by the structures of the final uranyl compounds, the participation or not in uranyl coordination of different bipyridine ligands has an influence on uranyl speciation, which shows not only diversity among different classes of organic bases but also consistency for the same types of bipyridine ligands. Moreover, a detailed regulation mechanism of bipyridine ligands in terms of π−πstacking and hydrogen-bonding interactions involving pyridinyl moieties is proposed. The difference between the same catalogues of UCPs originated from different structural factors, including varied flexibility, hindrance molecular size, and supramolecular isomerism, is also discussed in detail.

recently. The construction of UCPs based on semirigid carboxylic acids is still scarcely explored, although semirigid ligands have been intensively used to assemble coordination polymers with different structures for transition or lanthanide metals.41 As previous studies have demonstrated, the uniqueness of such ligands can be exemplified by special triple helices42 and tetranuclear uranyl-mediated interpenetrating structures43 from 4,4′-dicarboxybiphenyl sulfone (H2dbsf). Most interestingly, these two types of assemblies have a pair of supramolecular isomers in terms of changes in the coordination orientation or interlayer spacing, revealing a structural variety of UCPs based on the modest flexibility of this kind of ligand. These preliminary results encourage us to conduct further studies on UCPs with semirigid ligands so as to seek more novel hybrid materials and provide insight into the underlying formation mechanisms. Nitrogen-donor organic bases are usually used as auxiliary ligands for the construction of UCPs. For example, in UCPs with triple-helix structures as mentioned above, the organic base 1,10-phenanthroline (phen) chelates to uranyl cations and promotes the construction of helical chains formed by H2dbsf and uranyl through π−π stacking.42 When phen is replaced with 1,3-di(4-pyridyl)propane (bpp), the bpp ligand acts only as a template and charge-balancing agent in the final interpenetrating structures.43 It seems that different structural-directing roles of organic bases greatly affect the outcomes of the final assembly of UCPs. Although some scattered reports like those above have been discussed about the roles of nitrogen-donor organic bases, the relationship between the molecular structures of organic bases and their specific roles as structure-directing agents is still unclear. In this work, in addition to phen and bpp, a series of new bipyridine ligands with gradual variation in the molecular structure, including 2,2′-bipyridine (2,2′-bpy), 4,4′-dimethylbipyridine (4,4′-dmbpy), 5,5′-dimethylbipyridine (5,5′-

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EXPERIMENTAL SECTION

General Methods. Caution! Because of the radioactive and chemically toxic nature of uranyl nitrate hexahydrate, UO2(NO3)2· 6H2O, suitable measures for precautions and protection should be taken, and all operations should follow the criteria during handling of such substances, although natural uranium was used in the experiment. All of the reagents used in the synthesis were obtained commercially and used as received. Powder X-ray diffraction (XRD) measurements were B


Article

Inorganic Chemistry Table 2. Crystal Data and Structure Refinement Details chemical formula M (g mol−1) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) Dcalcd (g cm−3) μ (mm−1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

2

3

4

5

C24H16N2O8SU 730.48 monoclinic C2/c 29.201(6) 8.4690(17) 18.935(4) 90 99.70(3) 90 4615.7(17) 8 153 2.102 7.178 0.0412 0.0275 0.0621

C52H40N4O16S2U2 1517.06 triclinic P1̅ 8.3648(11) 15.945(2) 20.972(3) 106.549(2) 93.875(2) 101.498(2) 2604.3(6) 2 153 1.935 6.365 0.0293 0.0364 0.0981

C52H34N2O28S3U4 2183.11 triclinic P1̅ 10.2086(2) 12.0001(2) 28.0552(5) 92.4530(10) 100.0480(10) 97.4760(10) 3347.84(11) 2 170 2.166 28.464 0.0472 0.0279 0.0639

C104H66N4O62S6U8Ni 4518.91 monoclinic P21/c 12.0104(4) 20.1349(6) 28.2209(9) 90 91.468(1) 90 6822.4(4) 2 294 2.200 9.776 0.0744 0.0410 0.0877

Figure 1. Crystal structures of 2: (a) asymmetric unit of compound 2; (b) single helical chain in compound 2; (c) triple helix in compound 2; (d) π−π-stacking interactions between the adjacent pyridine groups of the 2,2′-bpy ligands in compound 2 and the twist of the bpp ligands. Hydrogen atoms are omitted in all views. Yellow polyhedra are UO22+ centers, whereas spheres represent sulfur (pink), nitrogen (green), carbon (gray), and oxygen (red). [U4O10(dbsf)3][H2(4,4′-bpy)] (4). In a 12 mL Teflon cup, the reagents UO2(NO3)2·6H2O (100 μL, 0.05 mmol, 1 equiv), H2dbsf (15.3 mg, 0.05 mmol, 1 equiv), and 4,4′-bpy (0.05 mmol, 1 equiv) were added to a solution of 1 mL of water. The pH of the solution was adjusted to 6.77 by NaOH. The Teflon cup was then placed in a stainless-steel Parr bomb, heated statically at 180 °C for 72 h, and slowly cooled to room temperature. Yellow block crystals were washed with water. Single crystals were then isolated and characterized by single-crystal XRD. [U4O10(dbsf)3]2[H(4,4′-bpy)]2[Ni(H2O)6] (5). In a 12 mL Teflon cup, the reagents UO2(NO3)2·6H2O (100 μL, 0.05 mmol, 1 equiv), Ni(NO3)2·6H2O (0.1 mmol, 2 equiv), H2dbsf (15.3 mg, 0.05 mmol, 1 equiv), and 4,4′-bpy (0.05 mmol, 1 equiv) were added to a solution of 1 mL of water. The pH of the solution was adjusted to 6.60 by NaOH. The Teflon cup was then placed in a stainless-steel Parr bomb, heated statically at 180 °C for 72 h, and slowly cooled to room temperature. Yellow-green block crystals were washed with water. Single crystals were then isolated and characterized by single-crystal XRD.

recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range of 5−50° (step size: 0.02°). Solid-state fluorescence spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer. Synthesis. All UCPs were synthesized by a hydrothermal reaction of the H2dbsf ligand with uranyl ions in the presence of different bipyridine ligands using a Teflon-lined stainless-steel reaction vessel. The detailed synthetic conditions are gathered in Table 1. [UO2(dbsf)(phen)] (1) and [UO2(dbsf)(phen)]·H2O (1′). Compounds 1 and 1′ were synthesized as reported before.42 [UO2(dbsf)(2,2′-bpy)] (2). Compound 2 was synthesized the same way as compound 1 except that the phen ligand was replaced by 2,2′-bpy and the pH of the solution was adjusted to 4.52 by NaOH. Yellow block crystals (yield: 50.3 mg, 68.5%) were washed with water. [(UO2)2(dbsf)2(5,5′-dmbpy)2] (3). Compound 3 was synthesized the same way as compound 1 except that the phen ligand was replaced by 5,5′-dmbpy and the pH of the solution was adjusted to 4.02 by NaOH. Yellow block crystals (yield: 54.7 mg, 72.1%) were washed with water. C


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Inorganic Chemistry

Figure 2. Crystal structures of 3: (a) asymmetric unit of compound 3; (b) triple helix in compound 3; (c and d) two sets of π−π-stacking interactions between the adjacent pyridine groups of 5,5′-dmbpy ligands in compound 3 and the twist of the 5,5′-dmbpy ligands. Hydrogen atoms are omitted in all views. Yellow polyhedra are UO22+ centers, whereas spheres represent sulfur (pink), nitrogen (green), carbon (gray), and oxygen (red). [U4O10(dbsf)3]2[H2bpp]2 (6) and [U4O10(dbsf)3]2[H2bpp] (6′). Compounds 6 and 6′ were synthesized as reported.43 Crystal Structure Determination. XRD data collection of compounds 2−5 was performed on a Bruker D8 VENTURE X-ray CMOS diffractometer with a Mo Kα (λ = 0.71073 Å for 2, 3, and 5) or a Cu Kα (λ = 1.54178 Å for 4) X-ray source at room temperature or 153 or 170 K. With Olex2,47 all crystal structures were solved by means of direct methods and refined with full-matrix least squares on SHELXL-97.48 The crystal data of all of these compounds are listed in Table 2, and the selected bond lengths are listed in Table S1. All of the crystallographic data for the newly reported structures in this work have been deposited with Cambridge Crystallographic Data Centre as CCDC 1894908 (2), 1894909 (3), 1894910 (4), and 1894911 (5).

crystallizes in the monoclinic space group C2/c. The asymmetric unit corresponds to the formula unit, which consists of one uranyl unit, one dbsf2− ligand, and one bpy ligand (Figure 1a). The uranyl center is primarily coordinated by two bidentate η2-coordinated carboxyl groups from two dbsf2− ligands to give a one-dimensional (1D) chain, and two nitrogen atoms of the bpy ligand complete the coordination spheres of the uranyl centers to form a distorted hexagonalbipyramidal geometry (Figure 1b). Because the V-shaped bidentate dbsf2− ligands are flexible and have a large torsion angle of 107.16(2)°, the 1D chain can further twist to generate a chiral helix along the b axis with a large helical pitch of 25.41 Å. The helical pitch of compound 1 is sufficiently large to accommodate another two identical helical chains, and a unique uranyl triple-stranded helix can be formed by these three independent polymeric chains intertwining with each other (Figure 1c). The 2,2′-bpy ligand coordinates to the uranyl centers at an out-of-the-equatorial-plane angle of about 34.4(1)° and points up at an angle of 34.2(6)° to the helical axis. Also, this special orientation affords a face-to-face detachment between the adjacent 2,2′-bpy ligands in the triple-stranded helices. In contrast to the structures of compounds 1 and 1′, two pyridinyl groups of 2,2′-bpy molecularly twist along the C−C single bond and lead to a dihedral angle of 13.238°, which makes the two pyridine groups parallel to their corresponding adjacent pyridine group, and the two different distances of face-to-face detachment between the pyridine groups are 7.46 and 6.54 Å. Both of these distances are enabling an adjacent triple-stranded helix to be inserted into the space between the 2,2′-bpy motifs. Therefore, the pyridine groups of the 2,2′-bpy motifs from adjacent triplestranded helices are alternately arranged with two different distances between them of 3.40 and 3.43 Å, which are both within the range for π−π-stacking interactions (Figure 1d). Another difference compared to compound 1′ is that there are no lattice water molecules in the structure of compound 2. Apart from those above, C−H···O hydrogen bonds between neighboring strands also make a stacking mode of compound 2 different from that of compound 1′ (Figure S3).

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RESULTS AND DISCUSSION Structural Description. Compounds 1and 1′ have been previously reported,42,43 so we only describe these structures briefly here. 1 crystallizes in the orthorhombic space group P212121. The uranyl cation is coordinated by two dbsf2− ligands with two bidentate η2-coordinated carboxyl groups and one phen ligand, which leads to a helical chain (Figure S1a). The helical pitch of the helix is 25.31 Å, which is large enough to accommodate another two helical chains, and three identical chains can intertwine with each other to form a neutral triple-stranded helix (Figure S1b). The phen ligands in compound 1′ point up to the helical axis with an angle of 35.9(1)° (Figure S2a), and a face-to-face detachment of 3.15 Å between the interstrand phen ligands is formed. Then a threedimensional (3D) stacking can be formed (Figure S1c). Compound 1′ is an analogous neutral uranyl compound and crystallizes in the monoclinic space group P21/c. The uranyl center is also coordinated by two dbsf2− ligands and one phen ligand (Figure S1d). The phen ligands in compound 1′ point up to the helical axis with an angle of 35.9(1)°, which leads to a larger distance of 7.03 Å between the interstrand phen motifs (Figures S1e and S2b). As a result, the neutral triple-stranded helix can be further inserted into the adjacent triple helices and form a short distance between the adjacent phen ligands of 3.50 Å (Figure S1f), and a two-dimensional (2D) network can be formed through interchain π−π interactions (Figure S1g). Compound 2 comprises neutral triple-stranded helices. Xray crystallographic analysis revealed that compound 2 D


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Inorganic Chemistry

Figure 3. Crystal structures of 4: (a) asymmetric unit of compound 4; (b) coordination environments of the uranyl centers in compound 4; (c) 2fold-interpenetrating frameworks of compound 4 viewed down the a axis; (d) topological structure corresponding to the 2-fold-interpenetrating 2D network of compound 4; (e) 3D packing mode of compound 4; (f) weak interaction of the 4,4′-bpy ligands with the adjacent tetranuclear uranium units. Yellow polyhedra are UO22+ centers, whereas spheres represent sulfur (pink), nitrogen (green), carbon (gray), and oxygen (red).

kinds of uranyl centers according to their coordination environments (U1 and U3, U2 and U4; Figure 3b). U1 and U3 are coordinated by three carboxylate groups in a bidentate pattern, a bidentate bridging pattern, and a bridging tridentate pattern [U−O bond lengths in the range of 2.381(4)− 2.485(4) Å, average 2.433 Å]. The axial oxygen atoms are bonded to the uranium center with distances of 1.776(4) and 1.781(4) Å in U1 [1.772(5) and 1.771(6) Å in U3], and the OUO bond angle of 176.21° in U1 (176.85° in U3) is nearly linear. The pentagonal-bipyramidal environment of U1 and U3 is completed by two μ3-oxygen atoms with U−O (μ3) bond lengths of 2.202(4) and 2.229(3) Å. The U2 and U4 atoms also possess a [UO7] coordination polyhedron but five oxygen atoms at the equatorial plane of the uranium atom from two dbsf2− ligands (in a bidentate bridging pattern and a tridentate bridging pattern) and two μ3-oxygen atoms, with the bond distances ranging from 2.240(4) to 2.593(5) Å. The two axial UO bonds of U2 with bond distances of 1.772(4) and 1.766(4) Å [1.785(6) and 1.778(6) Å in U4] and a OUO bond angle of 174.57° (173.99° in U4) are very common. The tetranuclear uranium units were connected by two dbsf2− ligands to form a small U8(dbsf)2 loop and further extended to a 1D chain with the two V-shaped ligands stretching in similar deflection angles of 102.01(27)° and 103.27(27)° (Figure S5a). Another single-stranded dbsf2− ligand in the bidentate bridging pattern further connected these chains from the vertical direction with a C−S−C deflection angle of 108.81(40)° to form the final wavelike 2D network (Figure S5b). Two of these 2D networks can weave in a 2D-to-2D parallel manner to form an interpenetrated structure (Figure 3c). One network penetrates through the other from the big

The structure of compound 3 is also an analogous triplestranded helical structure with a shorter helix pitch of 25.09 Å, but single-crystal analysis revealed that compound 3 crystallizes in the triclinic space group P1̅, and the asymmetric unit of 3 is composed of two crystallographically unique uranyl centers, which are connected with the carboxyl groups of two dbsf2− ligands to perform a helical chain (Figure 2a,b). Besides the carboxyl groups, two nitrogen atoms from the 5,5′-dmbpy ligand coordinate to the uranyl centers to form a distorted hexagonal-bipyramidal geometry with apical UO distances in the range of 1.737(7)−1.781(7) Å and OUO angles of 178.8(3)° and 178.5(3)°. Specifically, as shown in Figure 2c,d, the two pyridine groups of the 5,5′-dmbpy ligand take a dihedral angle of 9.043(3)° or 11.762(3)°, while after the adjacent triple-stranded helix is inserted into the space between the 5,5′-dmbpy motifs, the two adjacent pyridine groups of the 5,5′-dmbpy ligand afford face-to-face detachments of 3.74 and 3.53 Å (3.51 and 3.43 Å for another 5,5′-dmbpy), which are all larger than those in compound 2. This means that the 5,5′dmbpy ligand performs four different kinds of π−π-stacking interactions in one helical chain, which is rarely seen. Subsequently, a 2D uranyl coordination network is generated through interchain π−π interactions, which are further crosslinked by weak C−H···O hydrogen bonds to form a 3D supramolecular framework (Figure S4a). Also, because of the presence of a methyl group in the base ligand, more hydrogen bonds can be found in the structure (Figure S4b). Single-crystal XRD revealed that compound 4 crystallizes in the triclinic space group P1̅ and has an asymmetric unit consisting of four uranium atoms and three dbsf2− ligands, as well as a protonated 4,4′-bpy ligand (Figure 3a). There are two E


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Inorganic Chemistry

Figure 4. Crystal structures of 5: (a) asymmetric unit of compound 5; (b) coordination environments of the uranyl center in compound 5; (c) 3D packing mode of compound 5; (f) weak interaction of the 4,4′-bpy ligands and Ni(H2O)62+ with the adjacent tetranuclear uranium units. Yellow polyhedra are UO22+ centers, whereas spheres represent sulfur (pink), nitrogen (green), carbon (gray), and oxygen (red).

Compounds 6 and 6′ are the analogous structures of compounds 4 and 5 and were reported in our previous work.43 Here we only give a brief description. Compound 6 crystallizes in the monoclinic space group P21/c. The coordination environment of the tetranuclear unit in compound 6 is the same as that in compound 4 (Figure S6a), and the distance between the adjacent networks is 10.9 Å. The bpp ligands are located between the networks as 4,4′-bpy ligands in compound 4, but the bpp ligands are connected with the 2D networks through π−π interactions and hydrogen bonds (Figure S6b). Compound 6′ is the supramolecular isomer of compound 6. The space group and asymmetric unit of compound 6′ are the same as those of compound 4 except that the 4,4′-bpy ligand and [Ni(H2O)6]2+ cation were replaced by bpp ligands (Figure S6c). Compared to compound 6, the distance between the adjacent 2D networks in compound 6′ is reduced to 10.4 Å, which is still bigger than those in compounds 4 and 5 (Figure S6d). The bpp ligands in compound 6′ interacted with the 2D networks by two strong π−π interactions with the face-to-face distances of 3.23 and 3.22 Å. Discussion on the Synthesis. Resembling those of 1 and 1′ reported previously,42 2 and 3 are also triple-helical structures. Because 1 and 1′ are supramolecular isomers, we also attempted to synthesize supramolecular isomers of 2 or 3 by tuning the solution pH but failed, which may have been due to the close relationship of the reduced π−π-stacking areas for 2,2′-bpy and 5,5′-dmbpy compared to phen. Additionally, the twist of two pyridine groups along the C−C bond in 2,2′-bpy and 5,5′-dmbpy may also prevent the formation of their supramolecular isomers. Moreover, 4,4′-dmbpy and 6,6′dmbpy were also used as base ligands in the assembly process but did not produce any crystals suitable for diffraction. The failure in preparing UCPs from 4,4′-dmbpy or 6,6′-dmbpy might to be attributed to the steric effect. Moreover, 4 was tetrameric uranyl-mediated interpenetrating structures with 4,4′-bpy as a charge-balancing agent and a stacking template, which resembles the case of compounds 6 and 6′. Because compounds 6 and 6′ are supramolecular isomers, we also tried to get the supramolecular isomer of

loop but not the small U8(dbsf)2 loop at all. The network has also a Schlafli symbol (22.48.65) with the tetranuclear uranium units as nodes and dbsf2− as rods (Figure 3d).43 The 4,4′-bpy ligand located in the interval of two layers of interpenetrating networks in the form of protonated 4,4′-bpyH22+ molecules to balance the negative charges of the main networks of compound 4 (Figure 3e). Notably, the 4,4′-bpy ligands interact with the two adjacent 2D sheets by four hydrogen bonds [2.714(5), 2.801(4), 3.360(6), and 3.261(8) Å], resulting from interaction of the pyridine hydrogen atoms and the uranyl and sulfone oxygen atoms (Figure 3f). The weak interactions between the template base ligands and adjacent networks of compound 4 have resulted in a small distance (10.0 Å) between the two adjacent networks. Compound 5 crystallizes in the monoclinic space group P21/ c with the same network topology as compound 4. Its asymmetric unit corresponds to a formula unit that consists of only one type of uranyl tetranuclear unit as well as one [H(4, 4′-bpy)]+ cation, three dbsf2− ligands, and half a [Ni(H2O)6]2+ cation (Figure 4a). The tetrameric uranyl unit in compound 5 was coordinated by six dbsf2− ligands (Figure 4b), and all four uranium atoms possess a [UO7] coordination polyhedron and U−O bond lengths in the range of 2.217(5)−2.593(6) Å, corresponding to the most common type. The axial UO bonds of the four uranium atoms (U1−U4) reside at an average distance of 1.763 Å. The H(4,4′-bpy)+ and [Ni(H2O)6]2+ cations are located between the layers as template and charge-balancing agents (Figure 4c). Interestingly, the weak interactions between the H(4,4′-bpy)+ cation and adjacent layers are three hydrogen bonds [2.572(7), 2.666(8), and 3.125(8) Å] shorter than those in compound 4, but compound 5 has a larger distance between layers (10.1 Å) than that of compound 4, which means that the weak interactions between the template ligands and layers are not the main factors for the change of the distances between the adjacent layers (Figure 4d). Additionally, one hydrogen bond [1.901(12) Å] between the [H(4,4′-bpy)]+ and [Ni(H2O)6]2+ cations was also found in the structures. F


Article

Inorganic Chemistry Scheme 2. Role of Bipyridine Ligands in the Self-Assembly System of Uranyl Cations and Dicarboxylic Ligandse

a This systematic work can be seen in the reference reported by Thuery et al.49 bThe structures refer to those in the reference reported by Cahill et al.14,27,28 cThe structures refer to those in the reference reported by Cahill et al.14,27,28 dThis work refers to the reference reported previously by our group. eThe green planes were bipyridine ligands which acted as coligands in the structures, and the space-filling were bipyridine ligands which acted as template ligands in the structures.

(Scheme 2).14,27,28,34,49 We compared the behaviors of these bipyridine ligands in different uranyl compounds based on varying carboxylic acids with different backbone flexibility. As seen from Scheme 2, bipyridine ligands with a chelate effect prefer to coordinate to the uranyl cations when the dicarboxylic ligands were flexible enough.38 As the rigidity of the dicarboxylic ligands increases, the coordination of these bipyridine ligands would be greatly influenced by the steric effect of the bipyridine ligands, and thus bipyridine ligands would act as chelate and/or template ligands in these systems.14 While bipyridine ligands without a chelate effect were used, these ligands would always act as template ligands, especially in the case that the dicarboxylic ligands were rigid.26 Also, with the flexibility of the dicarboxylic ligands increasing, bipyridine ligands would possibly coordinate to the uranyl cations once the length and rigidity of the bipyridine ligands were appropriate.19,20 As a result, increasing the flexibility of the dicarboxylic ligands favors coordination of bipyridine ligands with the uranyl cations, while the steric effect would be more important when the dicarboxylic ligands are rigid; thus, the final topologies of these compounds change with the bipyridine ligands as mentioned above. Aslo, in the chemical systems of this work, both the triple-helical structures and 2fold-interpenetrating networks are retained when the chelated or templated bipyridine ligands are used, respectively, which are closely related with the flexible skeletons based on the semirigid linkers. Actually, the most important factor affecting the final topological structures is uranyl speciation, which is closely related to the bipyridine ligands. As demonstrated above, the participation or not in uranyl coordination of different bipyridine ligands promotes not only diversity among different

compound 4, and, interestingly, we obtained compound 5 in the presence of Ni(NO3)2·6H2O, which is similar to compound 4 except the presence of [Ni(H2O)6]2+ cations as additional guest molecules. The specific synthetic conditions for the syntheses of these UCPs are provided in Table 2. The effects of the initial and final pH of the self-assembly systems are also considered in the construction of UCPs. As seen from Table 1, the initial pH values of the systems that resulted in helical structures with mononuclear uranyl as nodes were lower than those that resulted in interpenetrating structures with tetranuclear uranyl as nodes. Also, the final pH of systems that resulted in interpenetrating structures with tetranuclear uranyl as nodes became higher. All of these can be related to the fact that the oligomeric species of uranyl become more prevalent in a higher pH and the formation of these oligomeric species would form more H+ cations, which further led to a lower pH. Structure-Directing Role of Bipyridine Ligands. Bipyridine ligands have been widely used in the construction of UCPs.14,27,28,46 Here we describe the structure-directing effect of different bipyridine ligands in the self-assembly systems of uranyl cations and a semirigid H2dbsf ligand. Three bipyridine ligands (phen, 2,2′-bpy, and 5,5′-dmbpy) with the chelate effect used in this system acted as coligands, while another two bipyridine ligands (4,4′-bpy and bpp) acted as template ligands and charge-balancing agents in the final structures. In the coordination assembly systems of uranyl cations and dicarboxylic acid ligands, the role of bipyridine ligands can be classified into two groups by their ability to chelate with uranyl cations. Additionally, the dicarboxylic acid ligands can also be divided into two groups based on their rigidity, and thus four different chemical systems of UCPs can be marked out, all of which have been systematically studied G


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Inorganic Chemistry

Figure 5. π−π-stacking distances (purple line) and the displacement angles (red) in compounds 1 (a), 1′ (b), 2 (c), and 3 (d and e). The displacement angle is the angle between the ring normal (red line) of the pyridine plane and the centroid vector (black line).

Figure 6. Views of the helical chain of compounds 1′ (a), 2 (b), and 3 (c) along the axis direction (left) and a sketch map of change on the helical chain (right). Spheres represent sulfur (pink), nitrogen (green), carbon (gray), and oxygen (red).

assembly systems. First, the systems containing 4,4′-dmbpy and 6,6′-dmbpy did not produce any crystals suitable for diffraction. Second, both helical pitches in the triple helices (from compound 1 to 3) are found to be influenced by the steric effect of the bipyridine ligands. To determine the mechanism that allows this compression, the π−π-stacking interactions between bipyridine ligands were analyzed first. The face-to-face π-stacking interactions in the

classes of organic bases but also consistency for the same types of bipyridine ligands. Steric Effect of Bipyridine Chelators on Helical Chains. The different helical pitches of the four triple helices (from compound 1 to 3) have revealed the flexibility of the helical chains and the structure-directing effect of chelate bipyridine ligands (phen, 2,2′-bpy, and 5,5′-dmbpy). It can be found that the steric effect plays an important role in these selfH


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Inorganic Chemistry

we attributed the narrowing of the distances between layers from compound 6 to 6′ to the translation of guest molecules (bpp ligands), and the interaction between the bpp ligands and networks is also considered to be the main reason for this kind of evolution. However, with the two new interpenetrating structures synthesizing (compounds 4 and 5), we can find that the interaction between the template ligands and networks is not so important, so is the translation of guest molecules. Instead, we observed that the narrowing of both distances between the layers is accompanied by changes in the space group (from P21/c to P1̅). When the wavelike interpenetrating networks are simplified to wavy lines, it is clearly to see that the relative translation of the networks, which resulted in a closer packing from compound 5 to 4 (or from compound 6 to 6′), is the main reason for the change of the distances between networks (Figure 7). Additionally, as shown in Figures 3e and

four triple helical structures (Figure 5) were simplified to the interactions between the paralleled moieties. Usually, the π−πstacking distances depend on the π-electron density of the faceto-face π-stacking moieties and the displacement angle between them.50 According to what we have reported, the supramolecular isomers from 1 to 1′ are the result of different orientations of phen coordinated to the uranyl cations, and the π−π-stacking distances have changed from 3.15 Å (in compound 1; Figure 5a) to 3.50 Å (average in compound 1′; Figure 5b).42 While the phen ligands were substituted with 2,2′-bpy ligands, the π−π-stacking distances in the triple helices have reduced from 3.50 Å (average in compound 1′; Figure 5b) to 3.40 and 3.43 Å (in compound 2; Figure 5c). This reduction of distances should be caused by two factors. First, the decrease of the π-electron density in 2,2′-bpy ligands reduces their π-electron repulsion which can further increase the stability of the face-to-face π stacking in compound 2.50 Second, the displacement angles in compound 2 are bigger than those in compound 1′, meaning reduced π−π repulsion and increased π−σ attraction, both of which can increase the stability of the face-to-face π stacking and lead to a reduced distance between the pyridine planes.51−53 When we replaced the 2,2′-bpy ligands with 5,5′-dmbpy, the π-electron density was increased by the electron donors (methyl groups) and so was the π-electron repulsion. As a result, the face-to-face πstacking distances in compound 3 increased to 3.43−3.74 Å (Figure 5d,e). The different face-to-face π-stacking behaviors between these bipyridine ligands can further result in compression of the chains along the helical axis. As mentioned above, the reduced π-electron density in the 2,2′-bpy ligand results in a closer π−π-stacking distance. An adjacent 2,2′-bpy ligand can also move to a position with a larger overlap area because of their reduced π−π stacking versus the phen ligands. The distance between two adjacent helical chains in compound 1′ was 8.42 Å (Figure 6a), while in compound 2, this distance has reduced to 6.87 Å (Figure 6b), which would further result in expansion of the helical chain in the horizontal direction (Figures 6a,b and S7). Because the absolute length of the helical chains is almost constant, its expansion in a direction perpendicular to the axis of the screw can lead to contraction of the helical pitch in the final structures (from 26.03 Å in compound 1′ to 25.41 Å in 2). When the structure changed from compound 2 to 3, the conjugated plane of 5,5′-dmbpy was still smaller than that of phen, and the distance between two adjacent helical chains in compound 3 was reduced to 6.80 Å (Figure 6c), which would keep expansion of the chains similar to that of compound 2. In addition, the methyl groups in 5,5′-dmbpy can induce expansion of the helical chains in the vertical direction (Figure 6c) by the formation of more hydrogen bonds between adjacent networks (Figure S8). As a result, the simultaneous expansion of the helix in compound 3 has led to the smallest helical pitch in the final structures (25.09 Å). Steric Effect of Templating Bipyridines on Crystal Stacking. Structural analysis reveals that a steric effect also exerts influence on the final structures of these self-assembly systems from compound 4 to 6′. In contrast to evolution of the helical pitches in the triple helix (from compound 1 to 3), the four interpenetrating structures (from compound 4 to 6′) have changed in two ways. One is that the distances between layers reduced from compound 5 (10.1 Å) to 4 (10.0 Å) [or from compound 6 (10.9 Å) to 6′ (10.4 Å)]. In our previous work,

Figure 7. Supramolecular isomers induced by shifting of the layers. The wavy networks of compounds 5 (a) and 4 (b) were simplified to wavy lines (c and d).

4c, the template ligands are always located in the peak position of the wavy networks, which also illustrates that the translation of the template ligands is not the main reason for structure evolution. Besides, the locations of the channels in the frameworks are also influenced by shifting of the layers. The arrangement of the 1D channels varies from a rhombus pattern to a ribbon one (Figure S9). This kind of decrease in the interlayer distance along with shifting of the layers is more common in the pressurecontrolled polytypism observed in hydrous-layered clay minerals.54,55 In addition, there is an example from Cahill about the uranium(VI) carboxyphosphonates, (UO 2 )(O3PCH2CO2H).56 The layer shift is evaluated by highpressure XRD analysis and undergoes no phase transformation (0−3 GPa). In contrast, the supramolecular isomers (4 and 5 and also 6 and 6′) in this work are synthesized together without high-pressure induction. Also, the coexistence of isomers can be rationalized in terms of the layers, adopting two distinct but energetically favorable layer stacking arrangement modes, which may be closely related to the flexibility of the template bipyridine ligands and networks. Another evolution is the distances between layers increased from compounds 4 (10.0 Å) and 5 (10.1 Å) to compounds 6 (10.9 Å) and 6′ (10.4 Å). This kind of evolution can be contributed to the increased steric effect from 4,4′-bpy to bpp ligands. Also, this kind of steric effect of bipyridine ligands has also led to expansion of the networks from compounds 4 and 5 to compounds 6 and 6′. As can be seen in Figure 8, the distances between the tetranuclear uranium nodes are 18.07 and 18.85 Å in compound 4 (Figure 8a) and change to 18.31 and 18.78 Å in compound 5 (Figure 8b), while in compounds I


Article

Inorganic Chemistry

487, 508, 531, and 555 nm in compound 2 and 475, 489, 509, 532, and 556 nm in compound 3. The spectra of compounds 2 and 3 are thus nearly identical, although that of compound 3 displays a red shift of ∼1 nm. In contrast, the emissions of compounds 2 and 3 are blue-shifted with respect to those of the uranyl nitrate compounds. These kinds of luminescence properties of compounds 2 and 3 are certainly caused by the distinct coordination environment of uranyl cations.20 Also, an enhanced uranyl emission around 475 nm in compounds 2 and 3 has also been found, which may result from energy transfer from bipyridine ligands to uranyl cations.14

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CONCLUSION In summary, we have demonstrated the structure-directing effect of bipyridine ligands as chelators or template agents in the construction of UCPs based on the semirigid H2dbsf ligands, and the participation or not in uranyl coordination of different bipyridine ligands promotes not only diversity in uranyl speciation and final topological structures among different classes of organic bases but also consistency for the same types of bipyridine ligands. The inherent features of semirigid skeletons combining both flexibility and rigidity endow them the ability of self-adjusting to adapt to different bipyridine ligands, which have also made the chemical systems of uranyl cations and semirigid dicarboxylic acid ligands distinct from other systems. These novel UCPs synthesized here not only enrich the unique structures from the selfassembly of uranyl cations and semirigid linkers but also promote the possibility of controllable synthesis of UCPs with novel structures and properties.

Figure 8. Network in compounds 4 (a), 5 (b), 6 (c), and 6′ (d). Green lines represent the distances between the tetranuclear nodes in the networks. Yellow polyhedra are UO22+ centers, whereas spheres represent sulfur (pink), nitrogen (green), carbon (gray), and oxygen (red).

6 and 6′, the networks are expanded and the distances between tetranuclear uranium nodes are from 18.87 and 19.12 (in 6) to 18.72 and 18.98 Å (in 6′) (Figure 8c,d). All of these changes in the structures have shown the flexibility of their skeletons. However, the rigidity of the skeletons has also played an important role in the final structures. For example, the skeletons that are based on aliphatic dicarboxylic acid ligands, flexible enough but lacking rigidity, cannot maintain their topology with different bipyridine ligands. As a result, the combination of flexibility and rigidity of the skeletons induced by the semirigid H2dbsf ligand is the key for structure evolution and is also the foundation of the structure-directing effect of bipyridine ligands. Fluorescence Properties. The purity of compounds 2 and 3 is confirmed by the good match between the experimental and simulated powder XRD patterns (Figure S10). Pure compounds 4 and 5 could not be isolated in enough yield. The emission spectra for compounds 2 and 3 and uranyl nitrate compounds were recorded under excitation at a wavelength of 420 nm and exhibited the characteristic vibronic structure of the uranyl cation, with five peaks ranging from approximately 460 to 600 nm (Figure 9). The maxima positions are at 475,

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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.9b00452. Typical crystal and topological structures of all compounds and typical figures including characterization data (powder XRD) (PDF) Accession Codes

CCDC 1894908−1894911 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]. ORCID

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

These authors contributed equally to this work.

Notes

Figure 9. Emission spectra of compounds 2 and 3 and uranyl nitrate as a control. The excitation wavelength was 420 nm.

The authors declare no competing financial interest. J


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Inorganic Chemistry

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ACKNOWLEDGMENTS We thank the support from the Science Challenge Project (Grant TZ2016004). The National Natural Science Foundation of China (Grants 21671191, 21876122, 11405186, and 21790373) is also acknowledged.

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Bipyridine-Directed Syntheses of Uranyl Compounds Containing

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