Incorporating Guest Molecules into Honeycomb Structures

Four uranium(VI)-polycarboxylates frameworks with honeycomb (6,3) nets have been assembled from solvothermal systems, which contain different guest ...
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Incorporating Guest Molecules into Honeycomb Structures Constructed from Uranium(VI)-Polycarboxylates: Structural Diversities and Photocatalytic Activities for the Degradation of Organic Dye Hao-Hong Li, Xian-Hua Zeng, Hong-Yan Wu, Xiang Jie, Shou-Tian Zheng, and Zhi-Rong Chen* College of Chemistry, Fuzhou University, Fuzhou, Fujian 350116, China S Supporting Information *

ABSTRACT: Four uranium(VI)-polycarboxylates frameworks with honeycomb (6,3) nets have been assembled from solvothermal systems, which contain different guest molecules (transition metal complexes and methyl viologen). These compounds exhibit photocatalytic activities for the degradation of rhodamine B. Interestingly, the methyl viologen-containing one possesses the highest efficiency, which can be attributed to the electron acceptor nature of methyl viologen for the stabilization of active peroxide anions.

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corrosion.24,25 Therefore, the construction of stable, efficient, inexpensive, and band gap tunable photocatalysts is still ongoing. To our interest, UOFs with photocatalytical activity for oxidation of organic pollutants have emerged,3,4,18,19 which inspired us to synthesize water-insoluble UOFs with enchanced photocatalytic performance. Host−guest type UOFs might have better photocatalytic efficiencies because the guest molecules can stabilize the degradation intermediates or photocatalytic species and as a result promote the photocatalytic efficiencies.19 Uranium(VI)-polycarboxylates frameworks with honeycomb (6,3) net are potential candidates for host−guest type UOFs with photocatalytic activity, which have been known, but only a limited number of compounds have been reported.15 Transition metal/phenanthroine complexes and viologen have been widely used in biology probes, electrochemistry, and photochemistry because of their versatile electronic behavior.26−29 We want to introduce transition metal complexes and viologen into UOFs to improve their photocatalytic degradation performance. And in this communication, we report the structures and photocatalytic properties of four

ctinide-bearing hybrid materials, especially those constructed from hexavalent uranium, have continued generating great interest because of their structural diversities and functional applications. 1,2 The hexavalent uranium generally existed as linear UO22+ uranyl can coordinate to organic ligands including polycarboxylates, phosphates, germanates, arsenates, and molybdates to result in uranyl−organic extended structures (1-D chains, 2-D layers, or 3-D frameworks).3−9 Among the versatile bridging ligands, polycarboxylates such as thiophene-2,5-dicarboxylate (TDC),10,11 benzene-1,4-dicarboxylate (BDC),12,13 and naphthalene-1,4-dicarboxylate (NDC)14 are the most commonly used ligands in the construction of uranyl−organic frameworks (UOFs). UOFs can exhibit many interesting physical or chemical properties, including photoluminescence,15−17 photocatalysis,3,4,18,19 and photoelectric conversion,20−22 among which the visible-lightdriven photocatalysis behavior is still in its infancy. Photocatalysis used for the green ecological elimination of harmful pollutants seems significant. So far, because of the narrow adsorption bands, most of the commercial solid photocatalysts (TiO2, ZnO) are UV light-driven, which limits their practical applications.23 In addition, some visible light-driven narrowband gap semiconductors such as CdS and CdSe are photochemically unstable owing to their sensitivity to photo© 2014 American Chemical Society

Received: August 22, 2014 Revised: December 2, 2014 Published: December 5, 2014 10

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Figure 1. Schematic representation of the layer stacking (a); single honeycomb layers constructed from TDC (b: compound 1, 4), TDC/OX (c: compound 2), and BDC (d: compound 3).

where the uranyl (UO22+) oxygen atoms locate at the apexes (Figure 1). The U−Oaxial bond lengths range among 1.741(4)− 1.765(4)Å (Table s1−4), exhibiting typical double uranyl bond characters.9,15,32,33 The Oaxial−U−Oaxial angles in the uranyl cations are in the range of 177.6(2)−178.88(18)°, which are consistent with the linear nature of the UO22+ cation (Table s1−4).15,32,33 In the equatorial plane, uranium centers are coordinated by six carboxylate oxygen atoms from three chelating polycarboxylates ligands. So the coordination modes of all carboxyl groups are bidentate chelated with averaged C− O distances of about 1.270 Å, which are generally in agreement with those coordinated carboyxly groups.9,15,32,33 The U−O equatorial bond distances are more averaged (2.444(4)− 2.541(3) Å) compared with those U−O distances without the guest molecules.9,33 The O−U−O bond angles in the equatorial planes are in the range of 51.71(15)−70.39(12)° (Table s1−4), deviating from the ideal 60° for a regular hexagonal arrangement. Adjacent UO8 hexagonal-bipyramids are bridged by dicarboxylate ligands to give a 2-D honeycomb layer (Figure 1). In this layer, the building units are (UO2)6(L)6 (L = TDC, BDC, and OX) hexagons, but these hexagons are not regular and differ greatly from those bridged by 1,4naphthalenedicarboxylate (NDC).15 These hexagonal open motifs exhibit a large cavity/channel of about 15.66 to 21.1 Å. In the (UO2)6(TDC)6 (for 1 and 4, Figure s1a) and (UO2)6(TDC)4(OX)2 (for 2, Figure s1b) hexagons, TDC ligands adopt two configurations of endo- and exo- with S atom pointing toward or outward the hexagon center (Supporting Information, Figure s1). In addition, the layers of four compounds adopt versatile configurations; for 2, 3, the layers are generally flat, but for in 1 and 4, they are not flat with bend

honeycomb-like uranium(VI)-polycarboxylates frameworks containing transition metal/phenanthroine complexes and methyl viologen, namely, {[Fe(phen) 3][(UO2) 2(TDC)3(TDC)]·6H2O}n (1), {[Ni(phen)3][(UO2)2(TDC)2(OX)]}n (2), {[Cu(phen)2Cl]2[(UO2)2(BDC)3]·5H2O}n (3), and {(MV)[(UO2)2(TDC)3]}n (4) (TDC = thiophene-2,5-dicarboxylate, OX = oxalic acid, BDC = benzene-1,4-dicarboxylate, MV = methyl viologen, i.e., N,N′-diethyl-4,4′-bipyridinium). Methyl viologen iodide MV·2I was synthesized according to the literature.30,31 Four compounds were obtained from the reaction of UO2(OAc)2·2H2O or UO2SO4·3H2O, polycarboxylates, and guest molecules in water via the hydrothermal method (for details, see Supporting Information). The products remain stable in water and air. All compounds were characterized by single-crystal X-ray structural analysis, IR, and elemental analysis (for details and crystallographic data, see Supporting Information). IR absorption bands observed among 800−950 cm−1 can be assigned to the asymmetric stretching vibration of OUO. The structures of four compounds contain two-dimensional honeycomb layers and guest molecules (transition metal/ phenanthroine complexes and viologen), among which hydrogen bonds are observed for structural stabilization. In particular, 2 exhibits the first example of a honeycomb layer constructed from mixed ligands of TDC and OX. The honeycomb layers exhibit (6,3) net topologies (Figure 1), in which UO2 cations are bridged by polycarboxylates ligands. Transition metal/ phenanthroine complexes and viologen cation are captured in the cavities of honeycombs. These (6,3) nets are structural relative to that of (NH4)UO2(BDC)1.5·2.5H2O, in which only NH4+ is encupsuled.15 All the uranium centers of 1−4 are in slightly distorted UO8 hexagonal-bipyramidal environments, 11

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photocatalytic activities were further driven by visible light excitation. We select N-containing dye, rhodamine B (RhB), as a model pollutant for degradation experiments.37 In the photodegradation of RhB, 40 mg catalyst powders were suspended in 80 mL of RhB solution (concentration: 10 ppm), so the reactions are heterogeneous. The wavelength and absorption intensity changes of RhB under the irradiation of xenon-lamp without and with the presence of catalysts are revealed in Figure s6. As illustrated by Figure s6, the adsorption peaks changes little without the presence of catalyst, but with the presence of catalysts, the adsorption spectra of RhB decrease to different extents with the lengthening of the irradiation time, suggesting that the degeneration reactions on RhB have occurred. In particular, the blue shifts from about 556 to 493 nm might be attributed to the deethylation of RhB,40 which suggests that the chromophores reflecting the characteristic color of RhB have broken down, and the degradations of dye proceed in the presence of 1−4 particles. Figure 3 shows the rates of RhB

angles of 114.893(2)) and 146.38(11)° respectively, as shown in Figure s2. Adjacent layers are stacked together with the guest cations, and versatile weak interactions among them can be found. Inter- and intramolecular hydrogen bonds can be seen in four compounds, which stabilize the structures (Table s5−8, Figure s3). Besides, in 1, π···π stacking interactions among guest [Fe(phen)3]2+ cations and [(UO2)2(Tda)3]n2n− layer can be detected (Figure s3) with centroidal distances of 3.7426 and 3.6514 Å (Table s9). But the π···π stacking interactions are absent in 2−4. The photoluminescent measurements on all solid state compounds at room temperature were recorded (Figure s5), but fluorescence could not be detected in 1. The fluorescence spectra of 2 and 3 consist of the typical well-defined bands in the range of 450−600 nm, corresponding to the transitions from the first excited electronic level to the symmetric and antisymmetric vibration levels of the uranyl ions.34,35 But the fluorescence spectrum of 4 exhibits a broad signal in the range of 500−600 nm without resolution. The resolution difference may be due to overlap contributions of the electronic transitions stemming from the different uranyl centers, which has been previously observed in a series of uranyl phthalates.1,36 We here want to find new high-efficient and easily recycled photocatalytic materials containing photochemical active uranyl (UO22+) units for solving pollution problems.3,18,19,37 In order to disclose the photoresponse regions, the solid state diffusereflectance UV/vis measurements on the as-synthesized compounds were conducted (Figure 2). Below 400 nm, all

Figure 3. Concentration change of RhB irradiated under a xenon lamp as a function of irradiation time with or without the presence of 1−4, Ct and C0 stand for the RhB concentrations after and before irradiation.

degradation (measured as RhB concentration versus irradiation time) in an aqueous solution in the presence of 1−4. For photocatalyst 4, the full degradation time for RhB is about 80 min; in contrast, for photocatalyst 1−3, after the irradiation time of 180 min, the highest degradation rate is 62.2%. It is obvious that 4 exhibits the highest degeneration efficiency among four catalysts, and it is also much higher than those of previous works, such as [Ag(bipy)(UO2 )(bdc)1.5 ] and [Ag2(phen)2UO2(btec)].3,4,18,19 But why 4 has the fast degradation kinetics is still difficult to explain at the current stage. The degeneration mechanism about this system has been explained as the presence of uranyl species in solution, which can be photoexcited and effectively activated the dye molecules.3,41,42 According to this mechanism, in the final step, the HOMO of *UO22+ will be reoccupied by a electron of α-H from substrate, but its excited electron is still in the LUMO and further captured by electronegative species such as O2 in solution. Furthermore, O2 will transform into highly active peroxide anions, which are responsible for the oxidation and total degradation of RhB.43 It is universally accepted that viologens are good electron acceptors with excellent redox behavior, so in this situation, MV2+ can stabilize these highly active peroxide anions and contribute to high degradation

Figure 2. UV−Vis absorption spectra of 1−4.

compounds exhibit two peaks at about 250 and 300 nm. The peak at about 250 nm can be attributed to the π−π* charge transfer of phenanthroine and viologen, but the strong absorptions at 297 nm (for 1), 303 nm (for 2), 313 nm (for 3), and 321 nm (for 4) are the typical absorptions rooting from the electronic transition within the UO double bonds, which have been proven to be the active centers for the photocatalytic performance.4,38 These peaks are approximate to the visible region. And the absorptions in the region from 420 to 480 nm are attributed to the ligand to metal charge transfer (LMCT) between the O atoms of polycarboxylates and empty orbitals of the U(VI) centers.39 Because the absorption rendered by U O double bonds are close to the visible region, their 12

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performance.44 In order to verify the reusability of four compounds as photocatalysts, the samples were recovered from reaction systems with the filtration method. Among the 40 mg catalysts, the recovered amounts of four samples are about 37 mg (about 92%). Because of the unavoidable solubility, a small amount of loss can be observed. The PXRD patterns of four recovered samples are nearly identical with those of the asprepared samples (Figure s6 in the Supporting Information), indicating that four catalysts can be reused. In conclusion, we have reported here four new uranium(VI)polycarboxylates frameworks with honeycomb (6,3) nets, which contain different guest molecules. These compounds exhibit photocatalytic activities for the degradation of rhodamine B. And the methyl viologen-containing one possesses the highest activity for the degradation of rhodamine B as a model pollutant, which is capable of photocatalyzing the degradation of the stable organic dye RhB upon application of a visible light irradiation.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthetic procedures, photocatalytic testing method, tables of bond distances and angles, hydrogen bond details, figures giving additional views and PXRD patterns of four compounds. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC-909137, 909132, 861960, and 940564 contain the supplementary crystallographic data for this paper.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support of this research by National Natural Science Foundation of China (NOS: 21271043). REFERENCES

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