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Sep 28, 2016 - the synthesis and structural characterization of five novel uranyl-Cu(II)/Zn(II) .... 1′-Zn, 2-Cu, and 3-Zn were all collected on an Agilent SuperNova X- ray CCD ...... angles; additional figures of 2-Cu, 2-Cu′ (PDF). □ AUTHOR ...

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Copper/Zinc-Directed Heterometallic Uranyl-Organic Polycatenating Frameworks: Synthesis, Characterization, and Anion-Dependent Structural Regulation Ran Zhao,†,∥ Lei Mei,†,∥ Lin Wang,† Zhi-fang Chai,†,‡ and Wei-qun Shi*,† †

Laboratory of Nuclear Energy Chemistry and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ School of Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123,China S Supporting Information *

ABSTRACT: By employing a multidentate ligand, 2,2′-bipyridine5,5′-dicarboxylic acid (H2bpdc), with both O-donors and N-donors, five uranyl-Cu(II)/Zn(II) heterometallic coordination polymers, (UO2)Cu(μ4-bpdc)(μ3-bpdc) (1-Cu), (UO2)Zn(μ4-bpdc)(μ3-bpdc) (1′-Zn), (UO 2 )CuCl(μ 3 -bpdc)(μ 2 -Hbpdc)(H 2 O) (2-Cu), (UO2)2Cu2Cl2(μ3-bpdc)2(μ2-Hbpdc)2(H2O)3·2H2O (2-Cu′), and (UO2)2Zn(μ3-SO4)(μ4-bpdc)(μ3-bpdc)(H2O)3 (3-Zn), were prepared under hydrothermal conditions. Thermal stability and luminescent properties of 1-Cu, 1′-Zn, 2-Cu, and 3-Zn were also investigated. Isostructural compounds 1-Cu and 1′-Zn both have a three-dimensional (3D) framework built by polycatenating of two sets of paralleling two-dimensional (2D) grids with octahedral transition metal cations (Cu or Zn) as the cross-linking nodes. As far as we know, compounds 1-Cu and 1′-Zn are the first two cases that possess polycatenated networks in heterometallic uranylorganic coordination polymers. Compound 2-Cu contains 3-fold interpenetrated 2D networks which are built by the connection of [(UO2)2(bpdc)2(Hbpdc)2]2− secondary building units and Cu(II). A one-dimensional tilted ladder-like structure in 2-Cu′ is constructed by uranyl-bpdc chains connected by Cu(II) and Hbpdc−. Compound 3-Zn displays a layered-like 2D network contain an unusual [(UO2)2Zn(μ3-SO4)] unit. Interestingly, different anions could lead to the change of coordination sites of transition metal cations, resulting in structural diversity of heterometallic uranyl-organic frameworks.



INTRODUCTION Uranyl-bearing complexes have played an important role in the research of actinide chemistry due to their rich structures, intriguing chemical or physical properties, and potential application in waste management and separation.1−6 Uranyl coordination polymers usually possess one-dimensional (1D) or two-dimensional (2D) structures rather than three-dimensional (3D) frameworks because of the peculiar coordination sites of linear uranyl ion which only allows the binding of organic ligands in its equatorial plane.2,4,7−10 In order to construct 3D uranyl organic frameworks, one possible method is to add d-block transition metal cations which usually afford an octahedral coordination environment to the reaction systems. These cations can bind organic ligands or uranyl oxo groups through cation−cation interactions, which provide the feasibility to extend low dimensional structures into 3D frameworks.11−20 Considering the challenge of combining two different metal centers in one complex for single-functional organic ligands, a multifunctional ligand that possesses different coordination sites which can selectively coordinate with uranyl or transition metal ions are often proposed. According to the hard-soft acid-base (HSAB) concept, uranyl ions are normally © 2016 American Chemical Society

regarded to be hard acids and have high binding affinities toward O-donor groups such as carboxylate acid groups, while d-block transition metal ions are borderline acids, which prefer to bind some N-donor groups, for example, pyridyl groups.21,22 On the basis of this principle, the idea of using O/N mixed ligands seems to be promising for the synthesis of uranyl-3d transition metal heterometallic coordination polymers.23−31 Among these O/N mixed ligands, only N-containing monocarboxylic acid ligands, i.e., nicotinic or isonicotinic acid, have been attempted to assemble uranyl organic frameworks,26,27,32−35 while N-containing polycarboxylic acids, with more than one carboxylate group, have been rarely concerned. For example, 2,2′-bipyridine-5,5′-dicarboxylic acid (H2bpdc), which can be considered as an integration of 4,4′biphenyl dicarboxylic acid and 2,2′-bipyridine, has not been employed to prepare uranyl-organic compounds, though it had been widely used for the construction of mono- and bimetallic coordination polymers.36−41 Compared to the N-containing monocarboxylic acids, the rigid dicarboxylic acid moieties of Received: March 29, 2016 Published: September 28, 2016 10125

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinements for All the Compounds 1-Cu, 1′-Zn, 2-Cu, and 3-Zn formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g·cm−3) μ (Mo Kα) (mm−1) F(000) R1, wR2 [I ≥ 2σ(I)] R1, wR2 (all data)

1-Cu

1′-Zn

2-Cu

2-Cu′

3-Zn

C24H12CuN4O10U 817.95 monoclinic C2/c 17.4935(9) 26.0116(8) 15.8081(8) 90 114.995(6) 90 6519.5(5) 8 1.667 5.665 3080 0.0513,0.1711 0.0639,0.1863

C24H12N4O10UZn 819.78 monoclinic C2/c 17.4301(3) 26.2700(3) 15.4596(3) 90 113.498(3) 90 6491.8(2) 8 1.678 5.773 3088 0.0291,0.0761 0.0353,0.0789

C24H15ClCuN4O11U 872.42 monoclinic P21/c 10.701(5) 7.686(3) 30.469(13) 90 91.121(17) 90 2505.53(180) 4 2.313 7.484 1652 0.0312,0.0650 0.0430,0.0687

C48H36Cl2Cu2N8O25U2 1798.89 triclinic P1̅ 13.859(3) 14.216(3) 15.202(3) 87.58(3) 72.22(3) 67.95(3) 2634.51(90) 2 2.268 7.125 1712 0.0526,0.1416 0.0592,0.1453

C24H18N4O19SU2Zn 1239.91 triclinic P1̅ 9.4771(3) 11.4559(4) 15.8438(5) 72.660(1) 83.447(1) 71.302(1) 1554.98(9) 2 2.648 11.315 1144 0.0200,0.0435 0.0231,0.0446

pH = 2.30). The autoclave was sealed and heated to 200 °C for 3 days and then cooled to room temperature naturally (final pH = 2.06). Yellow rod-like crystals were isolated from the white-yellow precipitate, with a yield of 11.0 mg (26.8% based on uranium). Using ZnCl2 instead of Zn(NO3)2 also produced compound 1′-Zn. Synthesis of (UO2)CuCl(μ3-bpdc)(μ2-Hbpdc)(H2O) (2-Cu) and (UO2)2Cu2Cl2(μ3-bpdc)2(μ2-Hbpdc)2(H2O)3·2H2O (2-Cu′). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (26 mg, 0.1 mmol), CuCl2·2H2O (8.5 mg, 0.05 mmol), and ultrapure water (1 mL) was loaded into a 20 mL Teflon-lined autoclave (Initial pH = 1.72). The autoclave was sealed and heated to 200 °C for 3 days and then cooled to room temperature naturally (final pH = 1.39). Deep green clusters consisting of needle-like crystal of 2-Cu were isolated, with a yield of 23.6 mg (26.2% based on uranium). A few needle-like crtstals of 2-Cu′ were also observed in one pot. Synthesis of (UO2)2Zn(μ3-SO4)(μ4-bpdc)(μ3-bpdc)(H2O)3 (3-Zn). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (26 mg, 0.1 mmol), ZnSO4 (8.1 mg, 0.05 mmol), 1 M NaOH (50 μL), and ultrapure water (950 μL) was loaded into a 20 mL Teflon-lined autoclave (initial pH = 2.49). The autoclave was sealed and heated to 200 °C for 3 days and then cooled to room temperature naturally (final pH = 2.00). Yellow block crystals were isolated from the whiteyellow precipitate, with a yield of 11.4 mg (18.3% based on uranium). X-ray Crystal Structure Determination. X-ray diffraction data of compound 2-Cu′ was performed with synchrotron radiation facility at BSRF (beamline 3W1A of Beijing Synchrotron Radiation Facility, λ = 0.71073 Å) using a MAR CCD detector. The crystal was mounted in nylon loops and cooled in a cold nitrogen-gas stream at 100 K. Data were indexed, integrated, and scaled using DENZO and SCALEPACK from the HKL program suite. Data collection of the compounds 1-Cu, 1′-Zn, 2-Cu, and 3-Zn were all collected on an Agilent SuperNova Xray CCD diffractometer with a Mo Kα X-ray source (λ = 0.71073 Å) at room temperature. Standard Agilent Crysalis software was used for the determination of the unit cells and data collection control. The crystal structures were solved by means of direct methods and refined with full-matrix least-squares on SHELXL-97. The crystal structures were solved by means of direct methods and refined with full-matrix leastsquares on SHELXL-97. The crystal data of all the compounds mentioned above are given in Table 1. CCDC-1471051 (1-Cu), CCDC-1471054 (1′-Zn), CCDC1484770 (2-Cu), CCDC-1471053 (2-Cu′), and CCDC-1502608 (3Zn) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif

H2bpdc ligand make it more suitable for the construction of MOF-like uranyl-organic frameworks. Meanwhile, two spare nitrogen atoms of pyridine groups can afford an additional cross-linking ability by coordination with other heterometallic ions (i.e., d-block transition metal ions). In this work, we report the synthesis and structural characterization of five novel uranyl-Cu(II)/Zn(II) heterometallic coordination polymers based on the H2bpdc ligand. Notably, two novel copper/ zinc-directed heterometallic uranyl-organic polycatenating frameworks have been acquired. Moreover, we found that employing different anions could tune the connection site of transition metal cations, leading to the structural regulation of heterometallic uranyl-organic frameworks and the formation of various frameworks. Thermal stability, luminescence properties, and IR spectra of these compounds are also studied.



EXPERIMENTAL SECTION

Materials, Syntheses, and Characterization. Caution! Uranium is a radioactive and chemically toxic element. Standard procedures for handling radioactive materials should be followed. All chemical reagents were commercial available and used as received. Ligand 2,2′-bipyridine-5,5′-dicarboxylic acid (H2bpdc) was synthesized according to the previous report.42 Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418 Å). Thermogravimetric analyses (TGA) were performed on TA Instruments Q-500 thermogravimetric analyzer used in air with a heating rate of 5 °C· min−1. Solid-state fluorescence spectra were measured on a Hitachi F4600 fluorescence spectrophotometer with an excitation wavelength of 250 nm in all cases. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets in the range of 4000−400 cm−1 on a Bruker Tensor 27 spectrometer. Synthesis of (UO2)Cu(μ4-bpdc)(μ3-bpdc) (1-Cu). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (26 mg, 0.1 mmol), Cu(NO3)2·3H2O (12.1 mg, 0.05 mmol), 1 M NaOH (125 μL), and ultrapure water (875 μL) was loaded into a 20 mL Teflon-lined autoclave (initial pH = 2.11). The autoclave was sealed and heated to 200 °C for 3 days and then cooled to room temperature naturally (final pH = 1.81). Green block crystals were isolated from the whitegreen precipitate, with a yield of 10.4 mg (25.4% based on uranium). Using CuSO4 instead of Cu(NO3)2 also produced compound 1-Cu. Synthesis of (UO2)Zn(μ4-bpdc)(μ3-bpdc) (1′-Zn). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (26 mg, 0.1 mmol), Zn(NO3)2 (6.9 mg, 0.05 mmol), 1 M NaOH (50 μL), and ultrapure water (950 μL) was loaded into a 20 mL Teflon-lined autoclave (initial 10126

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Inorganic Chemistry Scheme 1. Possible Coordination Modes of bpdc Ligands in All Compounds



RESULTS AND DISCUSSION Synthesis. Because of the low solubility of the H2bpdc in water, a hydrothermal synthesis technique was employed. All compounds were synthesized by a hydrothermal method at 200 °C, and compounds 1-Cu, 1′-Zn, and 2-Cu can be easily reproduced in the same reaction process. Phase purity of compounds 1-Cu, 1′-Zn, 2-Cu, and 3-Zn were checked by PXRD experiments. As shown in PXRD patterns (Figures S1− S4 in the Supporting Information), the major peak positions of the PXRD patterns of these four compounds match well with that of the simulated patterns from their respective singlecrystal data, which means that there are no observable amounts of impurities in these compounds, indicating their high phase purities. As we know, the pH value is a crucial factor for the formation of uranyl complexes. Compounds 1-Cu, 1′-Zn, and 3-Zn were obtained with the pH value around 2, while 2-Cu as well as 2-Cu′ formed in a very acidic solution. However, we also found that the choice of different transition metal salts plays a significant role in the construction of different heterometallic complexes. At the beginning, 2-Cu and 2-Cu′ were successfully synthesized by using CuCl2 as the Cu(II) source. Considering the presence of Cl atoms in the structure of 2-Cu and 2-Cu′, we chose other Cu(II) salts such as Cu(NO3)2 and CuSO4 as the Cu(II) source, and both of them led to the formation of 1-Cu. Then we replaced the above three Cu(II) salts with their corresponding Zn(II) salts for the reason that Zn(II) might have coordination geometry similar to Cu(II). Interestingly, compound 1′-Zn, which is isomorphous to 1-Cu, was obtained by employing ZnCl2 or Zn(NO3)2, while 3-Zn was synthesized in the presence of ZnSO4 (Table S1). The influence of different anions on the synthesis of the above four heterometallic complexes will be discussed later. Crystal Structures. Prior to the discussions, all of the binding modes of the bpdc ligand in compounds 1-Cu,1′Zn,2Cu,2-Cu′, and 3-Zn are summarized in Scheme 1 for the convenience. Compounds 1-Cu and 1′-Zn. Compounds 1-Cu and 1′-Zn are isostructural, and both crystallize in monoclinic lattices with C2/c space group. Hence only the structure of 1-Cu will be discussed in detail as a representative. As depicted in Figure 1, there are one uranyl ion, one Cu(II) ion, and two bpdc2− anions in the asymmetric unit of 1-Cu. The coordination polyhedron around the uranyl ion can be visualized as pentagonal bipyramid geometry with a conventional [UO7] coordination mode, which contains five carboxylate oxygen

Figure 1. Coordination environment of uranyl and Cu(II) in 1-Cu. All H atoms are omitted for clarity. Symmetry codes: A: 0.5 + x, −0.5 + y, z; B: x, 1 − y, −0.5 + z; C: 1.5 − x, 1.5 − y, 1 − z; D: −0.5 + x, 0.5 + y, z.

atoms from four bpdc2− anions. The U−O bonds in the equatorial plane range from 2.301(7) to 2.488(7) Å, while the uranyl UO bonds are 1.738(9) and 1.746(8) Å, respectively. The Cu(II) ion is six-coordinated with four nitrogen (N1−N4, Cu−N bonds range from 1.995(8) to 2.119(9) Å) and two oxygen atoms (Cu1−O9B: 2.699(14), Cu1−O10B: 1.992(10) Å) from three bpdc2− anions in a distorted octahedral environment because of the Jahn−Teller effect (Figure 1, Table S2).43 There are also two diverse coordination modes in ligand bpdc2− in compound 1-Cu. In both modes, two nitrogen atoms in bpdc2− anion are coordinated to the Cu(II) ion in a chelated manner. Two carboxylate groups from bpdc2− in coordination 1-mode A possess μ2‑η1:η1 mode for two uranyl ions and μ1‑η1 mode for another one uranyl ion (Scheme 1a), while two μ1‑η2 mode carboxylate groups chelate one uranyl ion and one Cu(II) ion in coordination 1-mode B (Scheme 1b). Bridging by the carboxylate groups from bpdc2− anions in coordination 1mode A, a dimeric uranyl unit is formed with the distance of 5.203(3) Å between two symmetric uranyl ions. Each dimer can be viewed as a six-connected node with four 1-mode A and two 1-mode B bpdc2− anions. The dimers along the chain extending direction are connected alternatively by 1-mode A bpdc2− anions, which are further linked together by Cu(II) ions (Figure 2a). Notably, the Cu(II) ion is chelated by three bpdc2− ligands: one in 1-mode A with bis-N,N-chelating, another one in 1mode B with bis-N,N-chelating, and a third one in 1-mode B with bis-O,O-chelating (Figure 2b). The participation of the 10127

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Cu(II) ion connected the adjacent uranyl 1D chains in parallel through coordinating to two different bpdc2− ligands via bisN,N-chelating (1-mode A) and η2-(O,O)-chelating modes (1mode B) to form 2D grids which extend along the c-axis (Figure 2c,d). With the aid of Cu(II) coordination with another bpdc2− of 1-mode B by means of bis-N,N-chelating, these 2D grids further intercross with each other and assemble in a polycatenating mode, leading to a 3D uranyl-organic polycatenating framework as shown in Figure 3. Similar to 1Cu, uranyl compound 1′-Zn also gives an identical extended structure of a 3D polycatenating framework (Figure 3, Table S3). As an interesting type of entangled structure, polycatenating frameworks have been found in many metal−organic coordination polymers based on transition metal and lanthanide.44−50 Recently, several uranyl-organic polycatenating frameworks have been reported by Wang and Thuery.1,51 However, uranyl centers are the only metal center in these uranyl-organic polycatenating frameworks, without any other metal ions involved. As far as we know, our findings here represent the first report of heterometallic uranyl-organic polycatenating frameworks. Moreover, there are obvious distinctions between the polycatenating frameworks in this work and common polycatenating frameworks. Unlike the relatively independent 2D networks in common polycatenating frameworks, all the 2D grids from two different directions for the polycatenating frameworks here are cross-linked together and fixed firmly by heterometallic ions (Cu, Zn) to build up a true 3D framework. Referring to the nomenclature of catenane, we can hence call the polycatenating framework here [1]polycatenane, which indicates that it is really a whole entity. Compound 2-Cu. Crystal structure analyses show that compound 2-Cu crystallizes in the P1̅ space group. The asymmetric unit of 2-Cu contains a uranyl cation, a Cu(II) cation, a Cl− anion, a Hbpdc− ligand, a bpdc2− ligand, and an aqua ligand (Figure 4). The U1 atom adopts a conventional

Figure 2. (a) The 1D uranyl-bpdc chain in 1-Cu. (b) Coordination environment of Cu(II) and the three bpdc ligands which directly bind to Cu(II); the blue and purple arrows are directing the extending direction of the uranyl chains (along with 1-mode A of bpdc ligands) and 1-mode B of bpdc ligands, respectively; (c) the 2D grid constructed by the connection of 1D chain and Cu(II). Panel (d) is the side view of (c). Colors: U: yellow; Cu: turquiose and teal; 1-mode A bpdc in 1D chain: bright green; 1-mode B bpdc which connect the adjacent parallel 1D chains to form 2D grids: indigo; 1-mode B bpdc which does not belong to this set of parallel 2D grids: orange.

Cu(II) ion in this coordination pattern plays a vital role in the formation of its 3D polycatenating framework. First, each

Figure 3. (a) Polycatenating of two sets 2D grids along the a direction in 1-Cu. (b) Extending of 1 × 1 2D grids and the (c) polycatenated 1 × 3 2D grids, (d) Polycatenated 3 × 3 2D grids, and (e) the whole 3D framework of 1-Cu viewing along the c direction. Different 2D grids are shown in different colors. 10128

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Figure 4. Coordination environment of uranyl and Cu(II) in 2-Cu. All H atoms and noncoordinated water molecules are omitted for clarity. Symmetry codes: A: x, 1.5 − y, −0.5 + z; B: 1 − x, 3 − y, 1 − z; C: x, 1.5 − y, 0.5 + z.

Figure 5. (a) A [(UO2)2(bpdc)2(Hbpdc)2]2− SBU in 2-Cu and (b) its connection with four other SBUs. (c) 2D framework in 2-Cu and (d) its topological representation.

forming a 2D grid structure (Figure 5b,c). On the basis of the point of the extension to metal atoms, both of the U(VI) atoms and Cu(II) atoms can be treated as a three-connected node. Therefore, this 2D network can be represented as a (3,3)connected network (Figure 5d). Moreover, three sets of 2D networks are densely packed and interpenetrated with each other, forming an interpenetrating network structure (Figure S5), which makes almost no voids in the 2-Cu structure. Compound 2-Cu′. Single-crystal X-ray analysis revealed that compound 2-Cu′ crystallizes in the triclinic system with P1̅ space group and possesses 1D tilted ladder-like structure. As illustrated in Figure 6, the asymmetric unit of compound 2-Cu′ contains two symmetrically distinct uranyl cations, two Cu(II) cations, two Cl− anions, two Hbpdc− ligands, two bpdc2− ligands, and three aqua ligands. The local coordination geometry of two uranyl cations is similar. Each uranyl cation is coordinated by four oxygen atoms from two carboxyl groups of bpdc2−, an oxygen atom from Hbpdc−, and an oxygen atom from one terminal water molecule residing in the usual hexagonal bipyramidal geometry for uranyl cations. The U− O bonds in the equatorial plane range from 2.343(7) to 2.585(8) Å, while the uranyl UO bonds are 1.769(8) and

pentagonal bipyramidal coordination geometry, which is formed by two axial oxygen atoms and five equatorial oxygen atoms from one Hbpdc− ligand, one bpdc2− ligand, and an aqua ligand [U1−Oequatorial: 2.277(3)−2.492(3) Å], respectively. Cu1 atom adpots a distorted square pyramidal geometry, formed by four nitrogen atoms from Hbpdc− ligand and bpdc2− ligand [Cu1−N: 2.004(4)−2.229(4) Å] and a Cl− anion [Cu1−Cl1: 2.242(2) Å] (Table S4). Interestingly, the H2bpdc ligands in compound 2-Cu adopt two different coordination modes. In 2-mode A (Scheme 1c) bpdc2− ligands, two carboxylic groups are all deprotonated and bridge two uranyl ions in monodentate fashion. For 2-mode B (Scheme 1d) with one of its carboxylate oxygen atom (O9) protonated, the Hbpdc− ligand bridges one uranyl ion in bisO,O-chelating mode. Futhermore, each ligand in 2-mode A or 2-mode B also coordinate with a Cu(II) cation in a bis-N,Nchelating pattern using its bipyridine nitrogen atoms. The fully deprotonated bpdc2− ligands and monodeprotonated Hbpdc− ligands link the U1 atoms through carboxylic groups, forming a [(UO2)2(bpdc)2(Hbpdc)2]2− secondary building unit (SBU) (Figure 5a). This SBU in 2-Cu is connected to four other SBUs from four directions by Cu(II) atom through Cu−N bonds, 10129

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O,O-chelating and one Cu(II) cation in bis-N,N-chelating, while for 2′-mode B (Scheme 1f), the Hbpdc− ligand, with one of its carboxylate oxygen atoms protonated, bridges one uranyl ion in monodentate fashion and one Cu(II) cation in bis-N,Nchelating. The uranyl [UO8] units which are linked by the bpdc in 2′-mode A create two sets of uranyl-bpdc chains that are parallel to each other and extend to the opposite directions, which are further joined together by the 2′-mode B Hbpdc− ligand and Cu(II) cations, yielding a tilted ladder-like structure, where the uranyl-bpdc chains act as the armrests, Hbpdc− as rungs, and Cu(II) ions as the wedges used for fastening (Figure 7). Another such 1D-ladder structure can insert from the “back” to form a “ladder pair” through π−π stacking interaction which comes from the face-to-face of pyridine ring with an average distance of about 3.5 Å from the Hbpdc− ligands in different “ladders” (Figure S6). Furthermore, these “ladder pairs” assemble through hydrogen bonds of Hbpdc− ligands and pack along the a-axis (Figure S7). According to the previous literature, these supramolecular interactions are useful to the formation and stabilization of compound 2-Cu′.38 Compound 3-Zn. In the asymmetric unit of 3-Zn, there are two crystallographically independent uranyl ions, two bpdc2− ligands, one Zn(II) atom, a sulfate ion, and three aqua ligands (Figure 8). U1 is coordinated by two axial O atoms, three carboxylate O atoms from two bpdc2− ligands, one O atom of SO42− group, and one aqua ligand; U2 is also coordinated by coordinated by two axial O atoms, three carboxylate O atoms from three bpdc2− ligands, one O atom of SO42− group, and one aqua ligand. Both of them possess pentagonal bipyramid geometry. Zn1 is coordinated by four N atoms from two bpdc2− ligands, one O atom of SO42− group, and one aqua ligand, possessing a distorted octahedral environment.

Figure 6. Coordination environment of uranyl and Cu(II) in 2-Cu′. All H atoms and noncoordinated water molecules are omitted for clarity. Symmetry codes: A: −1 + x, 1 + y, z; B: 1 + x, −1 + y, z.

1.783(8) Å, respectively. The Cu1 cation is founded as a [CuN4Cl] distorted square pyramid, which is bound by one bpdc2− ligand in 2-mode A with two N atoms, one Hbpdc− ligand in 2-mode B with two N atoms, and a Cl− anion. The Cu2 cation occupies [CuN4 OCl] elongated octahedral geometry, which is similar to Cu1 by just adding a terminal aqua ligand [Cu2−O3W: 2.606(10) Å]. This aqua ligand is between Cu1 and Cu2 cation, while the distance between Cu1 and O3W is 3.226 Å. The Cu2−N7 bond, which occupies the opposite position of the Cu2−O3W bond, is longer than other Cu−N bonds due to the Jahn−Teller effect (Table S5). Similar to 2-Cu, the H2bpdc ligands in compound 2-Cu′ adopt two different coordination modes. In the 2′-mode A (Scheme 1e) bpdc2− linker, in which both carboxyl groups are all deprotonated, simultaneously bridges two uranyl ions in bis-

Figure 7. Viewing the tilted ladder-like structure in 2-Cu′ along (a) c, (b) a directions, and (c) viewing from the top of the “ladder” along the uranylbpdc chain. 10130

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transition metal or lanthanide.52−57 However, to the best of our knowledge, uranium-bearing compounds constructed from this multifunctional ligand have not been reported. Considering the feasibility of a similar organic ligand, 4,4′-biphenyldicarboxylic acid, in developing uranium-bearing structures,51,58,59 the H2bpdc ligand should be promising when used to construct novel uranyl-organic compounds, especially with the aid of other heterometallic ions. Because of the restricted coordination sites of linear uranyl ions, uranyl-ligand bonds mainly occur in the equatorial plane of uranyl bipyramid, which prefer to produce 1D or 2D structures. It was reported that introducing 3d metal ions could bring about uranyl-heterometallic complexes with higher dimensional frameworks for the reason that d-block metal ions usually have an octahedral coordination environment, which can form bonds with their ligands through three directions.4 In our work, all the five uranyl compounds have been acquired in the presence of transition metal ions Cu(II) or Zn(II), and our efforts to produce uranyl-bpdc compounds without other metal ions turned out to be a failure, meaning that heterometallic ions play an important role in the formation of uranyl-organic compounds based on the H2bpdc ligand. In compounds 1-Cu and 1′-Zn, uranyl ions in neighborhood are first connected by the bpdc linkers in 1-mode A to form 1D chains, which are further joined together via the bpdc linkers in 1-mode B through the coordination bonds of Cu(II) or Zn(II) cations to generate the final 3D frameworks. Interestingly, the degree of connectivity in the 3D frameworks of 1-Cu or 1′-Zn is even higher than those in the Cu(II)-bpdc and Zn(II)-bpdc complexes with only single Cu(II) or Zn(II) metal center, which were synthesized under similar hydrothermal conditions.36,38,60 As demonstrated above, introducing a second metal ion may be helpful for constructing coordination polymers with higher dimensional structures. H2bpdc has two different types of donor atoms: N atoms from the pyridine ring and O atoms from the carboxylate group. According to the hard-soft acid-base (HSAB) theory, the uranyl ion has a higher binding affinity to O atoms than N atoms, thus often being considered as hard acid. On the other hand, d-block transition metal ions are borderline acids, which

Figure 8. Coordination environment of uranyl, Zn(II), and sulfate anion in 3-Zn. All H atoms are omitted for clarity. Symmetry codes: A: x, 1 + y, z; B: 1 + x, −1 + y, z; C: 2 + x, −2 + y, z; D: 4 − x, −2 − y, 1 − z. E: −1 + x, 1 + y, z; F: −2 + x, 2 + y, z.

Interestingly, there are still two types of bpdc2− ligands in compound 3-Zn. In both modes, two nitrogen atoms in bpdc2− anion are coordinated with the Zn(II) ion in a bis-N,Nchelating manner. Two carboxylate groups from bpdc2− in 3mode A possess a μ1‑η2 mode for one uranyl ion, and a monodentate mode for another uranyl ion, respectively (Scheme 1g), while two carboxylate groups in 3-mode B possess a μ2‑η1:η1 mode and a monodentate fashion, binding three uranyl ions in all (Scheme 1f). Moreover, the SO42− anion acts as a tridentate connector, binding two uranyl ions and one Zn(II) ion together. The bpdc2− ligand in 3-mode A coordinates with the dinuclear uranyl unit [(UO2)2(SO4)], forming a pair of 1D uranyl chains which extend to the opposite directions respectively (Figure 9a). Given an 1D uranyl chain as a layer, these layers are piled and connected by the 3-mode B bpdc2− ligand, along with the assistance of Zn(II) and SO42− anions, as shown in Figure 9b,d. This type of connection produces 3-Zn possessing a layered 2D structure.



DISCUSSION As an integration of 4,4′-biphenyl dicarboxylic acid and 2,2′bipyridine, 2,2′-bipyridine-5,5′-dicarboxylic acid (H2bpdc) is a good candidate for a multifunctional ligand, which has been demonstrated in various coordination polymers based on

Figure 9. 1D uranyl chains in 3-Zn (a) and viewing 2d plywood-like structure of 3-Zn in three different directions: (b) top view (along the b axis), (c) side view, and (d) main view (in c direction). Different layers of 1D uranyl chains in (c) and (d) have been labeled. The 3-mode B bpdc ligands are drawn in green. 10131

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Inorganic Chemistry have a strong tendency to coordinate both N atoms and O atoms. In all these five compounds discussed above, uranyl ions show exclusive binding affinity for O atoms, leaving all N atoms from pyridine rings occupied by transition metal ions. Moreover, besides four atoms from two bpdc2− ligands, the coordination sphere of each six-coordinated transition metal ion is further completed by other coexisting counteranions or aqua molecules. It is very interesting to find that these anions also have an important effect on the formation of heterometallic uranyl-organic compounds from the H2bpdc ligand. For example, in compounds 1-Cu and 1′-Zn, each coordination sphere of the Cu(II) or Zn(II) center is finished by a bis-O,Ochelated carboxylate group of the bpdc ligands in 1-mode B with Cu(II) or Zn(II) (Scheme 1b). The participation of another bpdc ligand by a bis-O,O-chelated carboxylate group makes every transition metal ion directly bind three bpdc ligands and promote it to extend in three different directions (Figure 2b). Finally, a 3D framework is constructed in compound 1-Cu or 1′-Zn. For compound 2-Cu or 2-Cu′, on the other hand, partial coordination sites of Cu(II) are occupied by Cl− and water molecules, which block the coordination of another bpdc ligand from a third direction. For this reason, the bpdc ligands around Cu(II) can only extend in two directions (Figure 10a,b), finally forming a 2D framework and 1D tilted ladder-like structures with the Hbpdc− ligand bearing a free carboxylic group that can be observed in 2Cu or 2-Cu′, which prevents structural extension along this site. This is similar to the Cu-bpdc compound [Cu2(Hbpdc)2Cl2]2· 2H2O which has a Cu−Cl bond and also possesses a 1D structure.60 In compound 3-Zn, the SO42− ion acts as a tridentate μ3-bridge ligand, linking two 1D uranyl chains in the same direction by binding with two uranyl ions and a Zn(II) ion. It reveals that this sulfate ion (along with H2O molecule) also takes over the coordination site of Zn(II) and prevents a third bpdc ligand coordinating with Zn(II) by using its carboxylate group. As a result, given [(UO2)2Zn(μ3-SO4)] as a whole, both the bpdc ligands in 3-mode A and 3-mode B only extend in the ab plane (Figure 10c), and eventually, a 2D layered structure is formed. As discussed above, all transition metal ions in compounds 1-Cu, 1′-Zn, 2-Cu, 2-Cu′, and 3-Zn can coordinate with four N atoms from two bpdc2− ligands. Since H2bpdc is linear, if a third bpdc ligand can coordinate with transition metal ions with O atoms to achieve an octahedral coordination environment, these three bpdc ligands around transition metal ion will extend in three different directions, finally resulting in 3D structures, i.e., 1-Cu and 1′Zn. If coordination sites on transition metal ions are partially occupied by terminal or bridging ligands, such as water (in compound 2-Cu and 3-Zn), Cl− (in compound 2-Cu and 2Cu′), or SO42− (in compound 3-Zn) anions, these ligands will block the bpdc ligand binding from a third direction and prevent formation of three-dimensional frameworks, for example, 2-Cu, 2-Cu′, and 3-Zn. IR Spectra. In the high-frequency regions of the IR spectra (Figure S8), 1-Cu, 1′-Zn, 2-Cu, and 3-Zn exhibit weak absorption bands at 3128−3022 cm−1, which can be attributed to the νC−H vibration of the pyridine groups.60 In the lowfrequency regions, a multiplet which can be attributed to the combined vibration modes of carboxylic groups and pyridine rings deformation is observed. In general, the carboxylic groups are expected to give intense bands from asymmetric (1630− 1500 cm−1) and symmetric (1460−1350 cm−1) stretching vibrations.41,61 For this reason, the strong absorption bands at

Figure 10. Coordination environment of (a) Cu(II) in 2-Cu, (b) Cu(II) in 2-Cu′, and (c) Zn(II) in 3-Zn with the ligands which directly bind to Cu(II) or Zn(II). Blue arrow: the extending direction to the uranyl ions (mode A bpdc ligands); purple arrow: the extending direction of 2-mode B Hbpdc− ligands in 2-Cu; purple solid line: 2′mode B Hbpdc− ligands in 2-Cu′; purple dotted arrow: the extending direction of 3-mode B bpdc ligands in 3-Zn; red cross: blocked coordination sites of transition metal ions.

1614−1530 cm−1 and 1380−1370 cm−1 in these compounds can be assigned to the asymmetric and symmetric streching vibration of carboxylic groups, respectively. However, it is difficult to distinguish the bands of bipyridine rings from this region due to the coexistence of carboxylic groups and pyridine rings. Additionally, a strong absorption peak at 1719 cm−1 in compound 2-Cu indicates the presence of the −COOH groups of the Hbpdc− ligands.38 These results are in accordance with the single-crystal structural analyses. Thermal Behavior. Thermal stability of 1-Cu, 1′-Zn, 2-Cu, and 3-Zn was investigated by a TGA measurement in the temperature range of 25−800 °C under constant air flow with a heating rate of 5 °C·min−1 (Figure 11). Compounds 1-Cu, 1′Zn, and 2-Cu show a fair plateau before 240 °C. Above 250 °C, a weight loss in the temperature range of 250−300 °C for 1-Cu and 2-Cu and 250−380 °C for 1′-Zn may be assigned to the loss of coordination water (for 2-Cu) and partial decomposition of carboxylate groups in the bpdc ligand (for 1-Cu, 1′Zn, and 2-Cu). This is followed by a sharp weight decrease 10132

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attributed to the electronic and vibronic transitions S11−S00 and S10−S0v (v = 0−4).62,63



CONCLUSIONS In summary, five uranyl-contained heterometallic coordination polymers assembled from H2bpdc were synthesized under hydrothermal conditions. Compounds 1-Cu and 1′-Zn have 3D frameworks constructed from the polycatenation of two sets of parallel 2D grids by Cu(II) or Zn(II). Compound 2-Cu contains a 3-fold interpenetrated 2D network which is built by the connection of [(UO2)2(bpdc)2(Hbpdc)2]2− SBUs and Cu(II). Compound 2-Cu′ has a tilted ladder-like structure containing a 1D uranyl-bpdc polymer chains as an armrest, which is connected by Cu(II) nodes and Hbpdc− which act as a rung. Compound 3-Zn has a layered-like 2D structure based on the linkages of the [(UO2)2Zn(μ3-SO4)] unit and bpdc ligands. To the best of our knowledge, our findings for 1-Cu and 1′-Zn here represent the first reports of heterometallic uranyl-organic polycatenating frameworks. Moreover, it reveals that different anions play crucial rules in tuning the structure of Cu(II)/ Zn(II)-uranyl heterometallic coordination polymers. Cl− in 2Cu, 2-Cu′, and SO42− in 3-Zn occupy the coordination site of transition metal ions, which block bpdc ligand binding from the third direction and prevent the formation of three-dimensional frameworks. Our results not only provide a new method to construct uranyl-contained heterometallic coordination polymers, but also demonstrate the possibility of tailoring the structures of uranyl compounds by introducing different anions.

Figure 11. TGA curves of all compounds.

(from 320 to 480 °C for 1-Cu and 2-Cu, from 380 to 550 °C for 1′-Zn) corresponding the fully thermolysis of bpdc ligand.38 For compound 3-Zn, two small fractions of weight loss which are attributed to the loss of three coordination waters (observed: 4.4%, calculated: 4.7%) can be observed in the range of 150−220 °C and 250−310 °C, respectively. The decomposition of bpdc ligand was observed in the range of 370−560 °C. The following weight loss above 600 °C could be attributed to the release of SO2 due to the decomposition of SO42− groups.37 The expected remaining weight losses observed in these four compounds are close to the calculated values (based on the stoichiometric chemical formula by treating Cu to CuO, Zn to ZnO, and U to 1/3(U3O8)).59 The remaining weight value of these compounds were 39.0% for 1Cu (calculated: 44.0%), 45.2% for 1′-Zn (calculated: 44.1%), 43.0% for 2-Cu (calculated: 41.2%), and 53.7% for 3-Zn (calculated: 51.8%), respectively. Photoluminescence Properties. The photoluminescent spectra of 1-Cu, 1′-Zn, 2-Cu, and 3-Zn are illustrated in Figure 12. Compounds 1-Cu and 2-Cu display no characterized uranyl emission, probably due to the quenching effect of Cu(II) in uranyl compounds.14,25 For uranyl-Zn(II) compounds 3-Zn, five emission peaks are well-resolved at 474, 503, 525, 549, 575 nm. In compound 1′-Zn, some ill-resolved peaks around 474, 504, 529, and 555 nm are also observed. These peaks can be



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00786. Structure factor information of the crystallographic information file (ZIP) Crystallographic information file (CIF) Typical figures including powder X-ray diffraction and FT-IR analysis; tables of selected bond distances and angles; additional figures of 2-Cu, 2-Cu′ (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Author Contributions ∥

R.Z. and L.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We acknowledge the support of this work by the National Natural Science Foundation of China (21577144 and 11405186) and the Major Research Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of the Natural Science Foundation of China (91426302 and 91326202), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA030104), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Figure 12. Solid-state emission spectra of all compounds at room temperature upon excitation at 250 nm. 10133

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