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Experimental and Theoretical Approaches to Three Uranyl Coordination Polymers Constructed by Phthalic Acid and N,N′-Donor Bridging Ligands: Crystal Structures, Luminescence, and Photocatalytic Degradation of Tetracycline Hydrochloride Wei Xu, Zhen-Xiu Si, Miao Xie, Lin-Xia Zhou, and Yue-Qing Zheng* Joint Laboratory for Environmental Test and Photocatalytic Research of Ningbo University−Zhejiang Zhonghao Applied Engineering Technology Institute Co., Ltd., Research Center of Applied Solid State Chemistry, Chemistry Institute for Synthesis and Green Application, Ningbo University, Ningbo, Zhejiang 315211, P.R. China S Supporting Information *

ABSTRACT: Three uranyl coordination polymers have been synthesized by using phthalic acid (H2PHA) in the presence of N,N′-donor ligands, and namely, (UO2)(BPP)0.5(PHA) (1), (UO2)(BPE)0.5(PHA) (2), and [(UO2)(H2O)(PHA)](PYZ)0.5· H2O (3) [BPP = 1,3-di(4-pyridyl)- propane, BPE = 4,4′-vinylenedipyridine, and PYZ = pyrazine]. X-ray analysis of single crystal structure determined that compounds 1 and 2 exhibit a threedimentional (3D) framework, and compound 3 exhibits a twodimentional (2D) network. Compound 1 is linked by PHA2− anions to form a one-dimensional ribbon and bridged by BPP ligands to build a 3D framework of cds CdSO4 topological type. Compound 2 is bridged by PHA2− anions into layers of a 4-connected dinodal net topology of (44;62) and connected through BPE ligands to form a 3D framework of pcu alpha-Po primitive cubic topological type. Compound 3 is assembled into 2D layers via PHA2− anions; the PYZ ligands and lattice water molecules only stabilize the 3D supramolecular structure. Considering the effect of hydrogen bonding, compound 3 has the same topological type as compound 2. These complexes are characterized by elemental analysis, IR and UV−vis spectroscopy, thermal analysis, powder X-ray diffraction, and photoluminescence spectroscopy. The photocatalytic properties of 1 and 2 for degradation of tetracycline hydrochloride upon light emitting diode lamp irradiation have been examined. Moreover, the electronic structural and spectra properties of compounds 1−3 have been systematically explored by time-dependent density functional theory calculations. and porous adsorption.12−16 Though many factors, such as concentration, pH, temperature, solvent, etc., are involved in controlling uranyl coordination and speciation,17−19 organic ligands play a dominant role in structural diversity of uranyl coordination polymers, and a literature survey clearly demonstrates that carboxylic acid ligands containing more coordinated sites and bridging auxiliary ligands could control and adjust the final structures into a 3D framework.20−24 Previously studies found that poly(carboxylic acid) ligands are excellent linkers for the formation of coordination polymers.25,26 Among these polycarboxylate ligands, phthalic acid (H2PHA) has attracted much attention by virtue of its inherent chemical features:27,28 (i) the ortho carboxylate groups can chelate the metal ions safely to generate stable structures and are able to connect other metal centers to form high dimensional coordination frameworks; (ii) it can be deproto-

1. INTRODUCTION Metal−organic coordination polymers have advantages of metal ions and organic ligands and are becoming a research hot spot.1−3 Among them, uranyl-organic frameworks (UOFs) have attracted considerable research interest owing to their versatile intriguing architectures and potential applications in the functional materials field concerning luminescence, photoelectric conversion, and photocatalysis.4−8 The uranyl cation (UO22+) is the main form of the hexavalent uranium and shows linear structure, which is usually coordinated with four to six oxygen or nitrogen atoms at the equatorial region to yield square, pentagonal, and hexagonal bipyramids. The two oxygen atoms in uranyl cation restrict the ligand coordinated to the central U atom in the uranyl hybrid materials, and therefore it is difficult to construct a three-dimensional (3D) framework.9−11 However, the construction of 3D uranyl frameworks is relatively lacking but highly desirable, because 3D UOFs usually exhibit superior thermal stability to low-dimensional structures and many outstanding properties, such as water insolubility, photoelectric effects, nonlinear optical properties, © XXXX American Chemical Society

Received: January 20, 2017 Revised: February 22, 2017 Published: February 28, 2017 A

DOI: 10.1021/acs.cgd.7b00097 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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nated to HPHA− and PHA2− anions, the protonated or deprotonated carboxylate groups can easily behave as hydrogen bond donors or acceptors, which exhibit various pH-tuning coordination modes and hydrogen interactions to construct diverse supramolecular architectures; (iii) its centrosymmetric formation may be helpful for improving its coordination capability with metal ions and for enhancing the stability of coordination polymers; (iv) the benzene ring and carboxylate groups consist of a π-conjugated system, which behave as efficient antenna linkers that favor fluorescence enhancement of the complexes. In constrast to the abundant transition metal or lanthanide metal complexes, uranyl complexes with H2PNA ligands only using single rigid-type backbones PNA2− through covalent bonds as a linker to coordinate to uranyl ions are not only relatively rare but also structurally simple.29−33 The coordination modes of phthalic acid with uranyl ions are listed in Scheme 1. On the other hand, exo-bidentate rodlike N,N′-

The thermal and luminescence properties of these complexes have also been examined. Moreover, spectral properties and electronic transitions are interpreted by density functional theory (DFT) calculations and the time-dependent density functional theory (TD-DFT) method.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals of reagent grade were commercially available and used without further purification. 2.2. Physical Methods. Single crystal X-ray diffraction data were collected by a Rigaku R-AXIS Rapid X-ray diffractometer. Powder Xray diffraction (PXRD) measurements were carried out with a Bruker D8 Focus X-ray diffractometer using Cu Kα1 radiation of wavelength 1.54012 Å with a scan speed of 5°·min−1 and a step size of 0.02° in 2θ to check the phase purity. C, H, and N microanalyses were performed with a PerkinElmer 2400II elemental analyzer. The infrared spectra were recorded from KBr pellets in the range 4000−400 cm−1 on a Shimadzu FTIR-8900 spectrometer. Thermogravimetric (TGA) was carried out from room temperature to 950 °C on preweighed samples in an air atmosphere using a Seiko Exstar 6000 TG/DTA apparatus with a heating rate of 10 °C·min−1. The photoluminescence properties were measured at room temperature using a Hitachi F-4600 Molecular fluorescence spectrometer. The ultraviolet absorption spectroscopy was recorded by using a DR-UV−vis spectrometer Lambda 950, and the wavelength range from 200 to 800 nm. The concentration of tetracycline hydrochloride (TC) was analyzed through a UV−vis spectrophotometer (Shimadzu, UV-2501) by checking the absorbance at 356 nm. 2.3. Theoretical Methods. In this work, the initial structures for the model fragments of the uranyl compounds 1−3 were acquired from the single X-ray crystal structures. The Gaussian09 suite of programs (Revision D.01)36 with a tight self-consistent field convergence threshold (10−8 a.u.) was employed for all calculations. All the DFT calculations were carried out using the B3LYP37,38 hybrid functional. For the uranium atom, the quasi-relativistic effective core potentials (RECP) were employed, and the 6-31G(d) basis set39 was used to describe C, H, O and N atoms. On the basis of such calculations, TD-DFT was used to obtain the electronic transitions of uranyl compounds 1−3. In addition, the solvent effects of aqua solution were simulated by the polarizable continuum model (PCM).40 At the B3LYP/RECP/6-31G(d) level of theory, infrared vibrational frequencies and UV−vis absorption were also calculated. In order to gain further insights into the natural charge distribution, the natural bond orbital (NBO) charge analysis was performed at the same level of theory by employing the NBO 3.1 program packages.41 2.4. Syntheses. Synthesis of (UO2)(BPP)0.5(PHA)(1). H2PHA (0.085 g, 0.5 mmol), BPP (0.101 g, 0.5 mmol), and UO2Ac2·2H2O (0.205 g, 0.5 mmol) were added in a mixed solvent (2 mL deionized water and 1 mL acetonitrile) and then 0.5 mL of 1 M tetramethylammonium hydroxide solution was added. After being stirred for 30 min, the reaction mixture was sealed into a 25 mL Teflon-lined stainless steel autoclave and heated at 140 °C. After 3 days, the autoclave was cooled to room temperature naturally. Yellow blocklike crystals of 1 were obtained after being washed with ultrapure water and ethanol, and air-drying at room temperature. Yield: 30.6% based on UO2Ac2·2H2O. Anal. Calc. for C29H22N2O12U2 (1): C, 32.63; H, 2.06; N, 2.63; O, 18.00 (%). Found: C, 33.57; H, 1.98; N, 2.57; O, 18.09 (%). IR (KBr pellet, cm−1): 3120(vw), 1617(vs), 1567(s), 1525(vs), 1441(m), 1401(s), 1368(s), 1071(vw), 1018(w), 933(s), 814(w), 736(w), 690(m), 597(w). The TG curve shows that compound 1 displays thermal stability up to around 400 °C, and the main weight loss (46.9%) appears between 420 and 460 °C (Figure S1). Synthesis of (UO2)(BPE)0.5(PHA) (2). Dropwise addition of a 1 M tetramethylammonium hydroxide solution (1 mL) to a stirred mixed solvent (5 mL of deionized water and 1 mL of acetonitrile) containing H2PHA (0.169 g, 1 mmol), BPE (0.095 g, 0.5 mmol), and UO2(NO3)2·6H2O (0.260 g, 0.5 mmol). After being stirred for 30 min, the reaction mixture was sealed in a 25 mL Teflon-lined stainless

Scheme 1. (a−f) Coordination Modes of Phthalic Acid with Uranyl Ion

donor building blocks, such as pyrazine (PYZ), 4,4′-bipyridine (BPY), 4,4′-vinylenedipyridine (BPE), and 1,3-di(4-pyridyl)propane (BPP), which have different spacers between N,N′donors can have a signficant influence on the assembly systems of multicarboxylate ligands and metal centers.34 It could lead to fascinating architectures with interesting properties. The use of these auxiliary ligands may also contribute to understanding of coordination-driven assembly and recognition processes.35 Therefore, in this contribution, we chose phthalic acid and three nitrogen-based bridging ligands to construct uranyl coordination polymers. Herein we present three new hydrothermally synthesized phthalic uranyl compounds incorporating N,N′-donor ligands: (UO2)(BPP)0.5(PHA) (1), (UO2)(BPE)0.5(PHA) (2), and [(UO2)(H2O)(PHA)](PYZ)0.5·H2O (3). The structures of compounds 1−3 have been determined and compared with the reported phthalic uranyl compounds, and the nitrogenbased ligands are also discussed. The diffuse-reflectance UV/vis spectra reveal that complexes 1−3 have similar absorption features in the UV and visible regions. Therefore, we investigated the photocatalytic performances of 1 and 2 under light-emitting diode (LED) lamps with simulated sunlight irradiation, and this is the first time that tetracycline antibiotics as the simulation of pollutants were applied to the photocatalytic degradation by uranyl-containing compounds. B



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Table 1. Crystal Structure Data for Compounds 1−3

a

compounds

1

2

3

empirical formula formula weight description crystal size (mm) temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dcalc (g cm−3) F(000) μ (mm−1) θ range (deg) reflections collected unique reflections (Rint) data, restraints, parameters goodness of fit on F2 hkl range R1, wR2 [I ≥ 2σ(I)]a R1, wR2 (all data)a A, B values in wb δρmax, δρmin (e·Å−3)

C29H22N2O12U2 1066.55 yellow, block 0.48 × 0.45 × 0.41 293(2) monoclinic C2/c 12.398(3) 14.935(3) 17.078(3) 90 107.21(3) 90 3020.7 (1) 4 2.345 1960 10.776 3.00−27.48 14242 2963 3460, 0, 205 1.073 ±16, ±19, ±22 0.0443, 0.0960 0.0543, 0.1050 0.0314, 17.2939 2.073, −1.850

C14H9NO6U 525.25 yellow, block 0.34 × 0.24 × 0.19 293(2) monoclinic C2/c 16.407(3) 18.109(3) 9.291(3) 90 93.01(3) 90 2756.8 (1) 8 2.531 1920 11.805 3.07−27.48 13417 2811 3166, 0, 200 1.086 ±21, ±23, ±12 0.0348, 0.0826 0.0401, 0.0934 0.0145, 10.8457 2.008, −2.758

C10H10NO8U 510.22 yellow, block 0.26 × 0.24 × 0.15 293(2) monoclinic P21/n 10.849(2) 10.592(2) 12.222(2) 90 109.00(3) 90 1327.9(5) 4 2.345 932 12.260 3.05−27.48 12452 3018 2577, 0, 181 1.040 ±14, ±13, ±15 0.0458, 0.1013 0.0546, 0.1095 0.0474, 3.4525 3.758, −2.240

R1 = ∑(|F0| − |Fc|)/∑|F0|, wR2 = [∑w(F02 − Fc2)2/Σw(F02)2]1/2. bw = [σ2(F02) + (AP)2 + BP]

−1

with P = (F02 + 2Fc2)/3.

Table 2. Selected Interatomic Distances (Å) for (UO2)(BPP)0.5(PHA) (1), (UO2)(BPE)0.5(PHA) (2), and [(UO2)(H2O)(PHA)](PYZ)0.5·H2O (3) (UO2)(BPP)0.5(PHA) U1

O1 O2 O3 O4 O5 O6 N1

[(UO2)(H2O)(PHA)](PYZ)0.5·H2O

(UO2)(BPE)0.5(PHA) 2.377(5) 2.348(6) 2.405(5) 2.353(5) 1.756(5) 1.751(5) 2.566(6)

U1

O1 O2 O3 O4 O5 O6 N1

1.756(4) 1.761(4) 2.345(4) 2.374(4) 2.339(4) 2.394(4) 2.540(6)

steel autoclave and heated at 140 °C. After 5 days, the autoclave was cooled to room temperature naturally. Yellow blocklike crystals of 2 were obtained after being washed with ultrapure water and ethanol, and air-drying at room temperature. Yield: 28.6% based on UO2(NO3)2·6H2O. Anal. Calc. for C14H9NO6U (2): C, 31.98; H, 1.71; N, 2.67; O, 18.28 (%). Found: C, 32.39; H, 1.65; N, 2.59; O, 18.36 (%). IR (KBr pellet, cm−1): 3120(vw), 1610(m), 1567(s), 1520(vs), 1500(vs), 1433(m), 1407(s), 1375(vs), 1025(w), 926(s), 834(w), 735(w), 683(w), 551(w). Compound 2 exhibits thermal stability up to about 400 °C, and the main lost weight (48.8%) occurs at 390−430 °C (Figure S1). Synthesis of [(UO2)(H2O)(PHA)](PYZ)0.5·H2O (3). H2PHA (0.089 g, 0.5 mmol) and UO2(NO3)2·6H2O (0.254 g, 0.5 mmol) were successively dissolved in a stirred ethanolic aqueous solution consisting of 8 mL of H2O and 5 mL of EtOH. Under continuous stirring, PYZ (0.044 g, 0.5 mmol) was added to the above solution. Then 1 mL of NaOH (1M) was added dropwise, forming a yellow transparent solution, which was allowed to stand at room temperature for slow evaporation. About 2 weeks later, block-shaped yellow crystals of 3 (0.185 g, 36.3% based on UO2(NO3)2) appeared in solution and were

U1

O1 O2 O3 O4 O5 O6 O7

2.456(5) 2.464(5) 2.390(5) 2.320(5) 2.346(6) 1.757(6) 1.756(6)

isolated. Anal. Calc. for C10H10NO8U (3): C, 23.52; H, 1.96; N, 2.74; O, 25.09 (%). Found: C, 23.61; H, 1.90; N, 2.68; O, 25.18 (%). IR (KBr pellet, cm−1): 3627(w), 3423(w), 2982(w), 1618(s), 1553(vs), 1513(vs), 1427(s), 1394(vs), 1091(vw), 1045(vw), 926(s), 874(w), 748(m), 696(m). The weight loss of compound 3 begins at about 60 °C. There are three steps of decomposition between 60 and 435 °C, and total weight loss is 44.8% (Figure S1). As shown in Figures S2−S4, the experimental and simulated IR spectra for compounds 1−3 are displayed, which is in agreement with the X-ray structural analyses described above and analogous to another. Selected theoretical and experimental vibrational frequencies and their assignments are provided in Table S1. The powder X-ray diffration pattern of three compounds are in good agreement with the simulated ones from single-crystal data, indicating that three compounds are in pure phase (Supporting Information, Figures S5− S7). 2.5. X-ray Crystallography. Suitable single crystals were selected under a polarizing microscope and fixed with epoxy cement on respective fine glass fibers, and the structure of single crystals was determined by X-ray single crystal diffraction with graphiteC



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Figure 1. (a) ORTEP view of the structure of components of the compound 1 with thermal ellipsoids at the 45% probability level (#1 = −x, −y+1, −z+1; #2 = −x−1/2, −y+1/2, −z+1); (b) ORTEP view of the structure of components of the compound 2 with thermal ellipsoids at 45% probability level (#1 = x, −y, z+1/2; #2 = −x+1/2, y+1/2, −z+3/2); (c) ORTEP view of the structure of components of the compound 3 with thermal ellipsoids at 45% probability level (#1 = −x+3/2, y+1/2, −z+1/2; #2 = x−1/2, −y+1/2, z−1/2; #3 = −x+2, −y+1, −z+1; #4 = −x+3/2, y +1/2, −z+1/2). Hydrogen atoms are omitted for clarity. monochromated Mo Kα radiation (λ = 0.71073 Å). The reflection intensity in the θ range 3.00−27.48°, 3.07−27.48°, and 3.05−27.48° for 1, 2, and 3 were collected at 293(2) K using the ω scans technique. The employed single crystals exhibit no detectable decay during the data collection. The data were corrected for Lp and absorption effects. The direct method employing the SHELXS-97 program gave the initial positions for part of non-hydrogen atoms, and the subsequent difference Fourier syntheses using SHELXL-9742,43 program resulted in initial positions for the rest of the non-hydrogen atoms. The hydrogen atoms on the organic ligands were geometrically generated, while those of water molecules were located from the successive difference Fourier syntheses. The full-matrix least-squares technique was applied for refinement of positions and anisotropic displacement parameters of all the non-hydrogen atoms, as well as the positions of the hydrogen atoms using the riding mode with isotropic displacement parameters set to 1.2 times and 1.5 times of the values for C atoms and O atoms, respectively. Detailed information about the crystal data and structure determination is summarized in Table 1. Selected interatomic distances and bond angles are given in Table 2 and Tables S2−S4. 2.6. Photocatalytic Activity Measurements. The photocatalytic properties of compounds 1 and 2 were assessed by the degradation of TC under 120 W LED lamps (simulated sunlight, λ > 420 nm). 36 mg, 48 mg, 60 mg, 72 mg, and 90 mg of compound 1 were suspended in 60 mL of 40 mg·L−1 TC aqueous solution in a 100 mL beaker, respectively. To establish an adsorption−desorption equilibrium of TC on the sample surface, the suspension solution was magnetically stirred in the dark for at least 30 min in the beaker before irradiation. After that, the solution was continuously stirred under the irradiation of

LED lamp. A sample was continually taken from the beaker and collected by centrifugation at 30 min intervals during the irradiation, and suspensions were sampled and centrifuged with a HC-3518 high speed centrifuge at 9000 rpm for 5 min to remove the uranyl sample and then analyzed by using a Shimadzu UV−vis 2501PC recording spectrophotometer. The photocatalyst obtained from the first run was filtered, washed several times with water and ethanol, dried at room temperature, and then subsequently reused for the next run. To evaluate the stability of the as-prepared photocatalysts, the TC degradation experiments of compound 1 were repeated up to four times under the same conditions.

3. RESULTS AND DISCUSSION 3.1. Description of the Crystal Structure. Crystal Structure of (UO2)(BPP)0.5(PHA) (1). X-ray crystallography reveals that compound 1 crystallizes in the centrosymmetric monoclinic space group C2/c, and it exhibits a 3D framework with its asymmetric unit consisting of one crystallographically unique uranyl cation (U1, O5, O6), one phthalic acid anion (PHA2−), and half a BPP (1,3-di(4-pyridyl)propane) ligand with the other half being generated by the symmetry center. As depicted in Figure 1a, the PHA2− anion is in μ3η4 coordination mode (a in Scheme 1), and the U atom only occupies one crystallographic metallic center and adopts a 7-fold coordination environment. The UO22+ is surrounded by four oxygen atoms (O1#1, O2, O3#1, O4; #1 = −x, −y + 1, −z + 1) coming from two distinct PHA2− and a nitrogen atom (N1) of BPP D



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Figure 2. (a) A ribbon-like chain containing polyhedron in 1; for clarity, the majority of hydrogens were omitted; (b) the building block is linked to form a 3D framework by the BPP ligand. (c) A schematic view of (65;8) 3D topological representation of 1.

to four carboxyl oxygen atoms (O3, O4#1, O5#1, O6#2; #1 = x, −y, z+1/2; #2 = −x+1/2, y+1/2, −z+3/2) coming from two distinct PHA2− and one nitrogen atom (N1) of BPE ligand resulting in UO6N pentagonal bipyramid geometry. The distance of short UO bonding is 1.756(4) Å and 1.761(4) Å, forming a nearly 180° (176.7(2)°) angle in axial. The U−O bond distances fall in the range of 2.339(4)−2.374(4) Å, and the distance of U1−N1 is 2.540(6) Å in the equatorial plane (Table 2). The corresponding bond angles is in the region 67.8(2)−145.8(1)° (Table S3). The uranyl ion is connected by PHA2− (O3) completing a (UO2)(PHA) unit, and the (UO2)(PHA) units are bridged to each other via carboxyl oxygen atoms (O4, O5, O6) from PHA2− to form a 2D layer 2 ∞{(UO2) (PHA)} parallel to (100). The 2D layers are further extended into a 3D 3∞{2∞{(UO2)(PHA)}·BPE} framework pillared through BPE ligand (via N1 atom) (Figure 3a). From a topological viewpoint, if two uranyl anions are bound together, defined as a simplified metal-symbol U*. As far as the 2D network alone is concerned, the simplified metal-symbol U* connected four U* via PHA2− anion, which could be referred to as the 4-connected nodes, leading to the topology (44;62) with vertex symbols of 4·4·4·4·62·62. The 2D network above is further pillared by the BPE ligand to give rise to a 3D framework structure as shown in Figure 3b. The BPE ligand linked two U* and as a pillar in the 3D framework structure. As a result, the overall topology of (UO2)(BPE)0.5(PHA) can be described as a 6-connected dinodal net topology of (412;63) with the vertex symbol 4·4·4·4·4·4·4·4·4·4·4·4·64·64·64·64 and belongs to pcu alpha-Po primitive cubic topological type (Figure 3c). Crystal Structure of [(UO2)(H2O)(PHA)](PYZ)0.5·H2O (3). The asymmetric unit of compound 3 is made up of one independent UO22+ ion, a phthalic acid anion (PHA2−), half a

ligand resulting pentagonal bipyramidal primary building unit. The distance of UO bond is 1.756(5) Å and 1.751(5) Å and forming a nearly 180° (177.1(2)°) angle in axial (Table S2). The U1−O bond distances fall in the range of 2.348(6)− 2.405(5) Å, and the distance of U1−N1 is 2.566(6) Å in the equatorial plane (Table 2). As in Figure 2a shown, the uranyl units graphically unique are connected by two carboxyl oxygen atoms (O2, O4) from PHA2− completing a (UO2)(PHA) unit and the units are further connected to one another through carboxyl oxygen (O1, O3) forming a ribbon-like chain along the [110] direction in terms of 1∞{(UO2)(PHA)}. The chain as the unique repeating unit cross-linking by the nitrogen atom (N1) of the BPP ligand to constitute 3D 3∞{1∞{(UO2)(PHA)}· BPP} (Figure 2b). Topologically, two uranyl anions are bound together, defined as a simplified metal-symbol U*. The PHA2− anion and BPP ligand connected two simplified metal-symbols U* respectively, which could be simplified as a bond. The simplified metal-symbol U* linked four U* through PHA2− anion and BPP ligand, which could be regarded as a 4-fold connector. As shown in Figure 2c, the 3D framework can be described as a 4-connected dinodal net topology of (65;8) with the vertex symbol 6·6·6·6·62·82 and belongs to cds CdSO4 topological type. Crystal Structure of(UO2)(BPE)0.5(PHA) (2). The asymmetric unit of compound 2 consists of one crystallographically unique UO22+ ion, a phthalic acid anion (PHA2−), and half a BPE (4,4′-vinylenedipyridine) ligand with the other half being generated by the symmetry center. The PHA2− anion displays a μ3η4 coordination mode (a in Scheme 1) with two carboxylate oxygens of different carboxy groups chelating the U atom and another two carboxylate oxygens bridging two U atoms, respectively. As depicted in Figure 1b, the U atom adopts a 7-fold coordination environment. The UO22+ ion is connected E



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Figure 3. (a) 2D layer generated from uranyl ion bridged by phthalic acid in 2; for clarity, the majority of hydrogens were omitted; (b) a schematic view of (44;62) 2D topological net of 2; (c) the building block is linked to form a 3D framework by the BPE ligand. (d) A schematic view of (412;63) 3D topological representation of 2.

simplified metal-symbol U*. From a topological viewpoint, if the interlayer hydrogen bonding interactions are taken into account, the simplified U* can be defined as six-connected nodes to link six U* through four PHA2− anions as well as hydrogen bonding interactions with a vertex symbol of (412;63) (Figure 4d). Therefore, compound 3 has the same topology as 2, belonging to pcu alpha-Po primitive cubic topological type. Comparison of the Structures of 1−3 with Those of Related Compounds: Significant Aspects. As described above, three phthalic acid uranyl compounds with different N,N′donor ligands were successfully synthesized and characterized. Compounds 1 and 2 were synthesized by a solvent-thermal method, while compound 3 by room temperature solution. It just so happens that the N,N′-donor ligands of compounds 1 and 2 coordinated with the U atom, and the N,N′-donor ligand of compound 3 is free. We suspect that the difference of the N,N′-donor ligands is caused by the reaction conditions. In addition, there are three uranyl-phthalate compounds (not including heterometallic complexes) that have been reported, namely, (UO2)2(PHA)2(H2O)2,46 UO2(H2O)(PHA)·0.32H2O, and (NH4)(UO2)3O(OH)(H2O)(PHA)2.47 U central atoms in these compounds are all situated in pentagonal bipyramidal geometry. Except for compounds 1 and 2, aqua ligands in other four compounds all coordinate with U central atoms. All bond lengths and angles are similar to the range of normally accepted values. The PHA2− ligands in compounds 1 and 2 have the same coordination mode with μ3η4 (a in Scheme 1), while UO2(H2O)(PHA)·0.32H2O and (NH4)(UO2)3O(OH)(H2O)-

free pyrazine (PYZ) with the other half being generated by the inversion center, one coordinated aqua, and one free aqua. The coordination mode of PHA2− is μ3η4 (b in Scheme 1), which of one carboxy group chelating one U atom and another carboxy group bridging two U atoms. As illustrated in Figure 1c, the uranyl ion is connected to four carboxylate oxygen atoms (O1, O2, O3#1, O4#2; #1 = −x+3/2, y+1/2, −z+1/2; #2 = x−1/2, −y +1/2, z−1/2) from one PHA2− anion and one aqua oxygen atom (O5) in the equatorial plane, which results in a 7-fold UO7 pentagonal bipyramidal primary building unit. The distance of short UO bonding is 1.756(5) Å and 1.751(5) Å, forming a nearly 180° (178.7(2)°) angle in axial. The U−O bond length ranges from 2.353(5) Å to 2.405(5) Å, and the U− O bond length is 2.566(6) Å in the equatorial plane (Table 2). The corresponding bond angles are in the region of 52.4(2)− 161.2(1)° (Table S4). The above values correspond to normal values reported in the literature.44,45 The PHA2− bidentate chelate one uranyl unit via O1, O2 and bridge one uranyl unit via O4, forming 1D ripple chains 1∞{(UO2)(PHA)} (Figure 4a) that propagate along the [101] direction. The adjacent 1D chains are further interconnected through carboxylate oxygen (O3) to form a 2D net structure 2∞{(UO2)(PHA)} parallel to (110) (Figure 4b). Between layers are connected with the help of the free PYZ and aqua via hydrogen bond (O5−H51···N1; O5−H52···O8; O8−H81···O1#4; O8−H82···O7#3. #3 = −x+3/ 2, y+1/2, −z+1/2; #4 = x−1/2, −y+1/2, z−1/2) to build a 3D supramolecular structure (Figure 4c). Topologically, two adjacent UO7 pentagonal bipyramids could be regarded as a F



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Figure 4. (a) A ribbon-like chain containing polyhedron in 3; for clarity, the majority of hydrogens were omitted; (b) 2D layer generated from uranyl ion bridged by phthalic acid in 3. (c) the 3D supermolecular structure is linked by hydrogen bond. (d) A schematic view of (65;8) 3D supermolecular topological representation of 3.

Figure 5. Lowest unoccupied molecular orbitals (LUMO) and the highest occupied MO (HOMO) orbitals of compounds 1−3.

(PHA)2 are in μ4η4 (c in Scheme 1) and μ5η3 (d in Scheme 1), respectively. Two different coordination modes of PHA2− are μ4η4 (c in Scheme 1) and μ3η4 (b in Scheme 1) in compound (UO2)2(PHA)2(H2O)2. The results of X-ray analysis demonstrate that compounds 1 and 2 feature 3D framework structures, compound 3 features 2D layers, and the other three compounds feature 1D chains. Compounds 2 and 3 form into a 2D layer with PHA2− anion and other compounds

forming into a 1D chain, and layers in compound 2 are linked via BPE ligands to form a 3D construction. Compound 1 constitutes a 1D chain by PHA2− anion, BPP as a bridging ligand connecting chains to form a 3D framework structure. The 1D uranyl compounds are also structured in different ways to construct the chains. Compound UO2(H2O)(PHA)· 0.32H2O exhibits a 1D structure built up from the connection of the uranium-centered polyhedra linked to each other G



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length. The above calculation results correspond to the bond length from single X-ray structure analysis. 3.3. Fluorescence Property. As shown in Figure 6, all of the photoluminescence of the title compounds and free H2PHA

through the phthalate ligands. The 1D chains are connected to each other through two carboxylate groups from two distinct phthalate ligands in order to form corrugated six-membered rings. The uranyl ion in compound (NH4)(UO2)3O(OH)(H2O)(PHA)2 is connected by oxygen atoms from phthalate ligands and hydroxyl groups to form tetranuclear and dinuclear uranyl units, which are bridged to each other through the phthalate ligands to form a 1D chain. The uranyl ions in the structure of (UO2)2(PHA)2(H2O)2 are linked through two different PHA2− and constitute infinite chains directed along [100]. From the crystal structures described above, different coordination modes of the PHA2− ligands resulted in the different uranyl coordination polymers assembly. The results also imply that N,N′-donor based ligands have a great impact on the structure of uranyl compounds. 3.2. Electronic Spectra. On the basis of the single X-ray crystal structures, electronic properties of three uranyl compounds were analyzed by frontier molecular orbitals (FMOs) and natural bond orbital (NBO) methods. The distribution of the molecular orbital provides us helpful information for a detailed framework of the electronic structure. The lowest unoccupied molecular orbitals (LUMO) and the highest occupied MO (HOMO) orbitals of three uranyl compounds are shown in Figure 5. The energy gap between HOMO and LUMO is found to be 2.64, 2.22, and 2.16 eV for 1, 2, and 3, which is in accordance to the experimental results of 2.32, 2.35, and 2.29 for 1, 2, and 3 (see Figures S8−S10). Our calculation data are similar to experimental data and further illustrate the reliability of the experimental data. Energy of some selected molecular orbitals of compounds 1−3 is recorded in Table S5. For compounds 1 and 3, the LUMOs can be classified as similar character, which is labeled as a contribution coming from the d orbital of central UO22+. While the LUMO of compound 2 can be labeled as d orbitals of (UO22+) but substantially mixed with the BPE ligand with π* character. And the HOMOs of three compounds have their entire contribution from π (H2PHA) characters. The NBO analysis investigated the type and the strength of interactions of the UO22+ cation with the PHA2− anion and nitrogen-based ligands. Table 3 shows the

Figure 6. Photoluminescence spectrum of phthalic acid and compounds 1−3 (the excitation wavelength is 384 nm).

ligand was measured in the solid state at room temperature, and reflecting the expected green fluororescence. All of the photoluminescent spectra were studied at an excitation wavelength of 384 nm. The free H2PHA ligand exhibits a strong emission at 400 nm, which can probably be attributed to π*−n or π*−π transitions of the ligand. The photoluminescent spectra peak type of three compounds is similar. Four obvious peaks at 497, 517, 538, and 563 nm are observed for 1 and 491, 512, 535, and 560 nm for 3, which are the symmetric and antisymmetric vibrational modes of the uranyl cation and corresponding to the electronic and vibronic transitions S11− S00 and S10−S0ν (ν = 0−4).48−51 It is found that compound 2 displays weak broad peak at 470 nm, 489 nm, and a broad signal in the range of 500−600 nm. This should be due to the coordination of UO22+ and BPE ligand, leading to the overlap of the energy levels and the change of electronic configuration. In addition, the uranyl ion in compound 2 is linked by PHA2− anion to the lamellar structure, which in compound 1 forming a chain structure. The different surroundings around the uranyl center result in variations of the fluorescence peak. It is obvious that three compounds all show a characteristic peak of the uranyl cation, while there is a difference between peak intensity. The spectrum intensity of compound 1 is stronger than 3 and exhibits a small number of red shifts, which may be attributed to the coordination mode of uranium (UO7 and UO6N polyhedral environments) and the effect of N-donor ancillary ligands (free PYZ ligand and coordinated BPP ligand).52,53 The uranyl ion in compoud 3 is surrounded by carboxyl oxygen atoms from the PHA2− anion and oxygen atom of coordinated aqua, while the uranyl ion in compound 1 also coordinated with the nitrogen atom from the BPP ligand. The electron-donating BPP ligand stengthened emission from the uranyl compound, resulting in a red shift, and the long conjugation of n-π* from coordinated BPP ligand may enhance the fluorescence intensity of complex 1.54,55 3.4. UV−vis Absorption Spectra. In an aim to found out the photoresponse regions, UV−vis absorption spectroscopy of compounds 1−3 was measured in the solid state (Figure S11) and calculated in aqua solution (Figure S12). Compared with experimental data, the main TD-DFT calculation absorption is

Table 3. Natural Bond Order for the Uranyl Compounds 1− 3 Compound 1

Compound 2

Compound 3

bond

bond order

bond

bond order

bond

bond order

U1−O5 U1−O6 U1−O3 U1−O4 U1−O1 U1−O2 U1−N1

2.1617 2.1600 0.6275 0.6247 0.5937 0.5770 0.3307

U1−O2 U1−O1 U1−O6 U1−O4 U1−O3 U1−O5 U1−N1

2.1801 2.1542 0.6713 0.6428 0.5689 0.5660 0.3603

U1−O6 U1−O7 U1−O2 U1−O1 U1−O3 U1−O5 U1−O4

2.1996 2.1488 0.7395 0.6092 0.4603 0.4399 0.4261

partial bond order of three uranyl compounds. According to the results of NBO for three compounds, the strongest donor− acceptor interactions occur between the UO double bond. For compounds 1 and 2, the interaction between the U and O atom is much stronger than that between the U and N atom. In addition, the interaction between U and O (PHA2−) is stronger than between the U and O atom (H2O) for compound 3. Meanwhile, as we known, the bond order is related to bond H



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Table 4. Calculated Absorptions of Compounds 1−3 in Aqueous Solution compound

cal (λ)/nm

oscillator strength

E (eV)

major contribution (%)

1

586.6 510.3 438.5 643.8 593.6 501.9 468.8 736.3 659.7 532.1 493.7

0.0001 0.0015 0.0044 0.0001 0.0044 0.0001 0.0007 0.0003 0.0001 0.0001 0.0013

2.11 2.43 2.83 1.93 2.09 2.47 2.64 1.68 1.88 2.33 2.51

H-1 → LUMO (62%), HOMO → LUMO (32%) H-1 → L+3 (55%), HOMO → L+3 (36%) H-2 → L+2 (80%), H-7 → L+2 (−6%) H-1 → LUMO (99%) HOMO → LUMO+2 (92%), HOMO−2 → L+2 (−5%) H-1 → L+4 (89%), H-1 → L+3 (9%) H-3 → LUMO (97%), HOMO−4 → LUMO (−3%) H-1 → L+1 (99%) HOMO → L+2 (100%) H-2 → L+2 (98%) H-2 → L+3 (98%)

2

3

slightly red shifted. The absorption emerged in the region of 380−500 nm, and absorption peaks of three compounds were nearly the same. Absorption at 290 nm should be ascribed to the electronic transition between the UO double bond, and absorption at 420 nm due to the metal-to-ligand charge transfer transition (MLCT) between unoccupied orbital of uranium and the surrounding ligands.51,56 The assignment for each electronic transition of three compounds is provided in Table. 4. The major contribution to the strongest absorption band of three compounds is attributable to H-12 → L+3, H-12 → LUMO and H-1 → L+7 transition, which is due to the character of π → f from the PHA2− anion to the UO22+ anion,57 intra ligand charge transfer (ILCT), ILCT and ligand-to-ligand charge transfer (LLCT). 3.5. Photocatalytic Performance. Previous investigations have already reported that many uranyl-based coordination polymers could exhibit promising photocatalytic degradation of dye pollutants (such as rhodamine B, methylene blue, and methyl orange) under UV/visible/UV−visible irradiation.58−62 Because of the richness of uranyl nodes and organic bridging linkers, as well as the controllability of the synthesis, it is easy to construct uranyl-organic frameworks with tailorable capacity to absorb light and enhance the activities for the photocatalytic degradation of organic contaminants in water. The study of the application of UOFs has a bright future in this meaning, even though it has not been so widely explored to date, in contrast to the conventional photocatalysts of metal oxides and sulfides. However, the reported photocatalytic performance of uranyl complexes is limited in organic dye as simulated pollutants up to now. To the best of our knowledge, this is the first time for the application of uranyl complexes degrading antibiotics pollutants. The photocatalytic activities of the samples were evaluated by the degradation of TC antibiotics in aqueous solution. Because compound 3 is water-soluble, we only investigated the photocatalytic performance of complexes 1 and 2. A total of 60 mL of TC aqueous solution with a concentration of 40 mg·L−1 was mixed with different contents of the catalysts. As Figure 7 illustrates, the TC degradation rate was 61.70%, 69.85%, 88.82%, 75.88%, and 49.62% degraded by different doses of 0.6 mg/mL, 0.8 mg/mL, 1.0 mg/mL, 1.2 and 1.5 mg/mL compound 1 for 3 h under an LED lamp. It can be seen that the TC solution containing 1.0 mg/L of 1 exhibits the highest degeneration efficiency. According to the above results, the photodegradation experiments were carried out under the optimum catalyst dosage at four cyclic runs. As shown in Figure 8, there is no apparent deactivation of the photocatalytic performance. It suggests that the catalyst is relatively stable in the process of photocatalysis. Moreover, the PXRD patterns of

Figure 7. Photocatalytic decomposition of TC solution with the change in Ct/C0 under the use of 0.6 mg/mL, 0.8 mg/mL, 1.0 mg/mL, 1.2 mg/mL, and 1.5 mg/mL compound 1.

compound 1 simulated and after the photocatalytic reaction are shown in Figure S13, revealing that there is no difference in the structure and phase of the catalyst. For compound 2, a 1 mg/ mL catalyst was dispersed in 60 mL of 40 mg/L TC aqua solution, and the mixture was stirred about 30 min in the dark. As shown in Figure 9, in the presence of compound 2, the degradation rate of the TC solution is 78.02% after exposure for 3 h under LED light. The PXRD patterns of recovered compound 2 are nearly identical with the simulated peaks (Figure S14). Compounds 1 and 2 are good photocatalytic catalysts for the degradation of TC solution, and the catalytic efficiency of compound 1 is better than 2 at the same condition. The results above show that compounds 1 and 2 have potential applications in photodegrading TC. Therefore, the mechanism of photodegrading TC by uranyl compounds was discussed. Generally, hydrogen abstraction and electron transfer have been proposed for uranyl catalyzed photo-oxidation of organic pollutants.63,64, The uranyl ion would be excited under visible light illumination, and the resulting *UO22+ is highly active and leading to a series of redox reactions. There is a double bond between uranium and oxygen, and oxygen 2p bonding orbitals contributed to HOMO and empty uranium (U5f) orbitals contributed to LUMO. On photoinduced excitation, the electrons in the oxygen atom transfer from HOMO orbit to LUMO orbit, resulting uranium in the +5 and oxygen in the −1 oxidation state. The excited electron in the LUMO is very unstable, which will immediately return to the ground state. However, if the TC molecule is in a reasonable concentration range and suitable orientation, transitional active complexes may be found. Once a hydrogen atom of TC I



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Figure 8. Cycle performance of compound 1 in photodegrading TC solution.

refractory antibiotic contaminants. To obtain insights into the structure and spectral properties of three uranyl compounds, DFT and TD-DFT calculations have been implemented. The molecular contribution of the H2PHA ligand of three compounds is more toward the HOMO level but LUMO is mainly stabilized by uranium. The energy gap between HOMO and LUMO is consistent with the experimental data, which provides a theoretical basis for the good photocatalytic effect.

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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.cgd.7b00097. Tables of bond angles and energy of some selected molecular orbitals, figures of IR spectrum, thermogravimetric curve, PXRD patterns, UV−vis absorption spectroscopy and energy gap (PDF)

Figure 9. Photocatalytic decomposition of TC solution with the change in Ct/C0 of complex 2.

Accession Codes

molecules occupied the excited uranyl ion center (HOMO), the excited electrons in the uranium units will be permanently retained in the LUMO unless they are captured by electronegative substances. The captured electrons are transferred to highly active peroxide anion, which further oxidizing TC molecules.

CCDC 1482131−1482133 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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4. CONCLUSION A systematic study was undertaken in which differently sized N,N′-donor linkers (BPP, BPE, and PYZ) were used to construct uranyl compounds containing phthalic acid. These compounds were synthesized and characterized using single crystal X-ray diffraction, PXRD, IR, TG, UV−vis spectroscopy, and luminescence spectroscopy. Structure analysis shows that compounds 1 and 2 have a 3D framework and 3 is a 2D layer structure, N,N′-donor ancillary ligands are vital in enriching the diversity of uranyl compounds. The luminescent properties in 1−3 suggest that they are potentially fluorescent materials. The UV−vis absorption peaks of three compounds emerge at the visible region, which exposes their photoresponsive region. Therefore, compounds 1 and 2 display good photocatalytic activities in degrading TC under 120 W LED lamps (simulated sunlight). The TC solution containing 1.0 mg/L of compound 1 exhibits the highest degeneration efficiency. Under the optimal conditions, the TC solutions are degraded by about 88.82% and 78.02% for 1 and 2 in 3 h, respectively, which illustrates that they have potential application in degrading

AUTHOR INFORMATION

Corresponding Author

*Telefax: Int. +574/87600747; e-mail: [email protected]. cn. ORCID

Yue-Qing Zheng: 0000-0002-8216-1072 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Public Projects of Zhejiang Province (2017C33008). Honest thanks are also extended to K. C. Wong Magna Fund in Ningbo University.

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