Structural Variation within Heterometallic Uranyl Hybrids Based on

Article pubs.acs.org/crystal

Structural Variation within Heterometallic Uranyl Hybrids Based on Flexible Alkyldiphosphonate Ligands Weiting Yang,†,∥ Fei-Yan Yi,†,∥ Tao Tian,*,†,‡ Wan-Guo Tian,†,§ and Zhong-Ming Sun*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China ‡ Key Laboratory of Groundwater Resources and Environment, Ministry of Education, College of Environment and Resources, Jilin University, Changchun, Jilin 130021, China § School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, China S Supporting Information *

ABSTRACT: Five novel zinc uranyl diphosphonates have been hydrothermally synthesized by using a series of flexible diphosphonate ligands, including ethane1,2-diyldiphosphonic acid (H4EDP), propane-1,3-diyldiphosphonic acid (H4PDP), and butane-1,4-diyldiphosphonic acid (H4 BDP). Compound Zn(H 2tib)(UO2)2(EDP)(HEDP)(H2EDP)0.5·3H2O (EDP-ZnU1, tib = 1,3,5-tri(1H-imidazol-1-yl)benzene) comprises dimeric U2O12 unit condensed by two UO7 pentagonal bipyramids, which are further connected by Zn-centered polyhedra and EDP ligands resulting in a 3-dimensional framework. Compound [Zn(bipy)(H2O)](UO2)(PDP) (PDP-ZnU1, bipy = 2,2′-bipyridine) also features U2O12 dimers and Zn-centered polyhedra, but a layered arrangement is formed. Different from that in PDP-ZnU1, the uranium exists in the form of UO6 tetragonal bipyramid and is surrounded by four PDP ligands to generate the layered structure of Zn(bipy)(UO2)(PDP) (PDP-ZnU2). ZnO2N2 tetrahedra are connected on both sides of the layers. Both Zn 2 (phen) 4 (UO 2 ) 3 (BDP)(HBDP) 2 ·4H 2 O (BDP-ZnU1, phen = 1,10-phenanthroline) and Zn2(bipy)2(UO2)3(HBDP)2(H2BDP)2 (BDP-ZnU2) contain U2O12 dimers and UO6 tetragonal bipyramids. In BDP-ZnU1, uranyl centers are bridged by BDP to form a 2-dimensional structure, on which Zn(phen)2 are decorated. Whereas in BDPZnU2, uranyl phosphonate layers are connected by bridging ZnO3N2 to produce framework structure. All of these compounds have been investigated by IR and photoluminescent spectroscopy. Their characteristic green light emissions have been attributed to transition properties of uranyl dications.

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INTRODUCTION Great efforts have been devoted to the coordination chemistry and material science of metal phosphonates due to their fantastic structural diversities and potential applications, such as electro-optical, ion-exchange, catalysis, and so on.1−4 The organic ligands are crucial for the construction of metal phosphonates, and various ligands have been designed and synthesized to isolate metal phosphonate materials, mainly including derivatives of alkylphosphonic acids, arylphosphonic acids, and N,N′-piperazinebis(methylenephosphonic acid). The inorganic metal centers involve most transition metals and lanthanide elements. In contrast, actinide phosphonates have been less investigated. Notwithstanding, still various uranium phosphonate complexes possessing potential applications in ion-exchange,5 proton conductivity,6 chiral materials,7 and biomaterials8,9 have been developed. As the inert nature of the two “yl” oxo atoms of the linear UO22+ species, which is the general existing form of uranium,10−17 1-dimensional (1D)18−26 and 2-dimensional (2-D) uranyl coordination polymers26−29 are favored. Thus a challenge in richening structural diversities is how to deviate from the propensity of uranium(VI) complexes to form low-dimensional architectures. In an effort to construct novel uranyl inorganic−organic hybrids, © 2014 American Chemical Society

especially 3-dimentional (3-D) assemblies, the modulation and modification of the organic ligands is a very effective strategy. So far, most reported uranyl phosphonates are based on rigid skeleton phosphonate ligands.7,30−36 More recently, our group utilized a series of flexible alkyldiphosphonic acids to synthesize uranyl complexes, and a family of uranyl diphosphonates with 2-D and 3-D structures have been obtained, in which the network of EDP-U4 features large nanochannels.37,38 Common features for these ligands are their flexible backbones and versatile coordination modes, which are of benefit for construction of new architectures. Another approach to increase the occurrence of 3-D assembly is binding additional metal centers with different coordination preferences.39−49 Notable examples are a group of heterometallic uranyl phosphonates, [M2(UO2)6(PO3CH2CO2)3O3(OH)(H2O)2]· 16H2O (M = Mn(II), Co(II), Cd(II)) with a cubic openframework structure.50 Our interest in exploration of the Zn system has being continued due to the successful isolation and characterization of a family of zinc uranyl complexes with uncommon properties, such as heterometallic UO−Zn Received: December 11, 2013 Published: February 7, 2014 1366

dx.doi.org/10.1021/cg401850v | Cryst. Growth Des. 2014, 14, 1366−1374

Crystal Growth & Design

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cation−cation interactions.38,51 In addition, we also found Nligands play a key role in formation of uranyl phosphonate compounds. They can serve as coligands, space fillings, or charge compensators. In this work, a group of alkyldiphosphonic acids, ethane-1,2diyldiphosphonic acid (H4EDP), propane-1,3-diyldiphosphonic acid (H 4 PDP), and butane-1,4-diyldiphosphonic acid (H4BDP), combined with some N-ligands are used as construction agents to isolate zinc uranyl phosphonate compounds. Five bimetallic phosphonates, namely, Zn(H2tib)(UO2)2(EDP)(HEDP)(H2EDP)0.5·3H2O (EDP-ZnU1), [Zn(bipy)(H2O)](UO2)(PDP) (PDP-ZnU1), Zn(bipy)(UO2)(PDP) (PDP-ZnU2), Zn(phen)2(UO2)1.5(BDP)0.5(HBDP)· H2O (BDP-ZnU1), and Zn(bipy)(UO2)1.5(HBDP)(H2BDP) (BDP-ZnU2) (tib = 1,3,5-tri(1H-imidazol-1-yl)benzene, bipy = 2,2′-bipyridine, and phen = 1,10-phenanthroline) have been obtained. Their syntheses, structures, and infrared spectroscopy, as well as photoluminescence, have been investigated.

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crystals of all the uranyl phosphonates using a Nicolet 6700 FT-IR spectrometer. The spectra were collected with a diamond ATR objective. The fluorescence spectra were performed on a Horiba Jobin Yvon Fluorolog-3 fluorescence spectrophotometer, equipped with a 450 W Xe-lamp as the excitation source and a monochromator iHR320 equipped with a liquid-nitrogen-cooled R5509-72 PMT as detector. Powder X-ray diffraction patterns were performed on a D8 Focus (Bruker) diffractometer with Cu Kα radiation field-emission (λ = 0.15405 nm, continuous, 40 kV, 40 mA, increment = 0.02°). Synthesis of EDP-ZnU1. A mixture of Zn(UO2)(OAc)4·7H2O (40 mg, 0.04 mmol), H4EDP (20 mg, 0.1 mmol), tib (20 mg, 0.07 mmol), and deionized water (1.0 mL) was loaded into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 160 °C for 2 days, and then cooled to room temperature. Yellow plat-like crystals were isolated, yield 11 mg (39% based on uranium), initial pH 3.5, final pH 2.0. Anal. Calcd (wt %) for C20H32N6O22P5U2Zn: C, 17.10; H, 2.30; N, 5.98. Found: C, 17.95; H, 3.45; N, 5.64. Phase purity was confirmed by powder X-ray diffraction pattern (Supporting Information (SI) Figure S1). Synthesis of PDP-ZnU1. A mixture of Zn(UO2)(OAc)4·7H2O (40 mg, 0.04 mmol), H4PDP (20 mg, 0.1 mmol), bipy (20 mg, 0.13 mmol), CsCl (10 mg, 0.06 mmol), and deionized water (3.0 mL) was loaded into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 160 °C for 2 days, and then cooled to room temperature, initial pH 2.0, final pH 1.5. Yellow rod-like crystals were isolated with minor unidentified powder phase. Synthesis of PDP-ZnU2. A mixture of Zn(U.O2)(OAc)4·7H2O (40 mg, 0.04 mmol), H4PDP (20 mg, 0.1 mmol), bipy (20 mg, 0.13 mmol), and deionized water (1.0 mL) was loaded into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 160 °C for 2 days, and then cooled to room temperature, initial pH 2.0, final pH 2.0. Yellow block-like crystals were isolated with unidentified powder phase. Synthesis of BDP-ZnU1. A mixture of Zn(UO2)(OAc)4·7H2O (40 mg, 0.04 mmol), H4BDP (20 mg, 0.1 mmol), phen (20 mg, 0.11 mmol), and deionized water (3.0 mL) was loaded into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 180 °C for 3 days, and then cooled to room temperature. Yellow plat-like crystals were isolated, yield 13 mg (41% based on uranium) initial pH 2.0, final pH 1.5. Anal. Calcd (wt %) for C60H66N8O28P6U3Zn2: C, 30.31; H, 2.80; N, 4.71. Found: C, 30.95; H,

EXPERIMENTAL SECTION

Caution! Standard procedures for handling radioactive material should be followed, although the uranyl compounds used in the lab contained depleted uranium. Materials, Syntheses, and Characterization. All chemicals were purchased commercially and used without further purification. Diphosphonic acid and tib were synthesized according to a previously reported procedure.52,53 The organic ligands are listed in Scheme 1.

Scheme 1. Schematic Representation of the Ligands

Elemental analyses of C, H, and N were conducted on a Perkin−Elmer 2400 elemental analyzer. Infrared spectra were collected from single

Table 1. Crystal Data and Structure Refinement for Title Complexes empirical formula Fw crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z T/K λ (Mo Kα)/Å F(000) ρcalcd (Mg/m3) μ (Mo Kα)/mm−1 R1/wR2 (I > 2σ(I))a R1/wR2 (all data) a

EDP-ZnU1

PDP-ZnU1

PDP-ZnU2

BDP-ZnU1

BDP-ZnU2

C20H32N6O22P5U2Zn 1404.80 monoclinic P21/n 9.536(2) 28.337(7) 13.672(4) 90 93.694(5) 90 3687.1(16) 4 293(2) 0.71073 2636 2.531 9.720 0.0630/0.1247 0.1140/0.1440

C13H16N2O9P2UZn 709.63 monoclinic P21/c 10.7002(5) 18.6131(9) 9.4676(5) 90 98.2260(10) 90 1866.20(16) 4 293(2) 0.71073 1320 2.518 10.175 0.0319/0.0655 0.0417/0.0689

C13H14N2O8P2UZn 691.60 monoclinic P21/c 8.635(2) 10.233(2) 20.868(5) 90 100.692(3) 90 1811.8(7) 4 293(2) 0.71073 1288 2.536 10.473 0.0254/0.0564 0.0337/0.0595

C60H66N8O28P6U3Zn2 2377.86 triclinic P1̅ 12.2596(9) 12.6030(10) 13.0430(10) 99.2250(10) 105.5800(10) 105.0420(10) 1816.5(2) 1 293(2) 0.71073 1132 2.174 7.535 0.0396/0.0767 0.0590/0.0853

C36H54N4O30P8U3Zn2 2115.42 triclinic P1̅ 9.5793(5) 11.5325(6) 14.0533(8) 84.9710(10) 72.1400(10) 78.8080(10) 1448.94(13) 1 293(2) 0.71073 994 2.424 9.483 0.0343/0.0692 0.0451/0.0771

R1 = Σ(ΔF/Σ(Fo)); wR2 = (Σ[w(Fo − Fc2)])/Σ[w(Fo2)2]1/2, w = 1/σ2(Fo2). 1367



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and one N atom of tib. The first EDP group serves as a pentadentate ligand linking four uranyl cations and one zinc atom, in which P(1) and P(2) phosphonate groups adopt tridentate and bidentate coordination modes, respectively. The second EDP ligand is similar but with O(16) protonated based on the analyses of P−O distances (P(4)−O(16): 1.564(11) Å). The third EDP as a didentate ligand bridges two Zn atoms, in which the PO3 group takes unidentate coordination mode, leaving one terminal oxygen (P(5)−O(19): 1.496(11) Å) and one OH group (P(5)−O(18): 1.559(11) Å). The connection of U2O12 dimer, ZnO3N tetrahedra, and pentadentate phosphonate groups produces a layered assembly, which is further connected by bidentate EDP to result in a 3-D framework with elliptic channels in [001] and [100] directions (Figure 2 and Figure 3). The protonated tib molecules occupy the void space of EDP-ZnU1 through coordinating to Zn atoms (Figure 3).

2.96; N, 4.48. Phase purity was confirmed by powder X-ray diffraction pattern (SI Figure S2). Synthesis of BDP-ZnU2. A mixture of Zn(UO2)(OAc)4·7H2O (40 mg, 0.04 mmol), H4BDP (40 mg, 0.2 mmol), bipy (20 mg, 0.13 mmol), and deionized water (1.0 mL) was loaded into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 160 °C for 2 days, and then cooled to room temperature. Yellow rod-like crystals were isolated, yield 17 mg (61% based on uranium) initial pH 1.6, final pH 1.0. Anal. Calcd (wt %) for C36H54N4O30P8U3Zn2: C, 20.44; H, 2.57; N, 2.65. Found: C, 20.73; H, 2.76; N, 2.48. Phase purity was confirmed by powder X-ray diffraction pattern (SI Figure S3). X-ray Crystal Structure Determination. Suitable single crystals for title compounds were selected for single-crystal X-ray diffraction analyses. Crystallographic data were collected at 293 K on a Bruker Apex II CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Data processing was accomplished with the SAINT program.54 The structures were solved by direct methods and refined on F2 by full-matrix least-squares using SHELXTL-97.55 Nonhydrogen atoms were refined with anisotropic displacement parameters during the final cycles. All hydrogen atoms were placed by geometrical considerations and were added to the structure factor calculation. A summary of the crystallographic data for these title complexes is listed in Table 1. Selected bond distances and angles are given in SI Tables S1−S5.

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RESULTS AND DISCUSSION Structure of EDP-ZnU1. Compound EDP-ZnU1 crystallizes in a monoclinic space group P21/n. The asymmetric unit of EDP-ZnU1 consists of two crystallographically unique uranyl dications, one zinc atom, two and a half ethylenediphosphonate ligands, and one tib ligand (Figure 1). Figure 2. Polyhedral representation of the 3-D framework with elliptic channels in EDP-ZnU1. The tib is deleted for clarity.

Structure of PDP-ZnU1. This compound crystallizes in space group P21/c, and contains one crystallographically distinct uranium atom, one zinc atom, one PDP ligand, and

Figure 1. ORTEP representation of the asymmetric unit in EDPZnU1. Thermal ellipsoids are drawn at the 50% probability level. Symmetry code A: 0.5 + x, 0.5 − y, −0.5 + z; B: −0.5 + x, 0.5 − y, −0.5 + z, C: 1 + x, y, z.

Both of the UO22+ cations are equatorially coordinated by five bridging-O atoms from four phosphonate groups, forming two pentagonal bipyramids, which are condensed to a dimer through edge sharing. The UO distances in the axis are in the range of 1.756(10) to 1.785(9) Å, and the bond angles of O(1)U(1)O(2) and O(3)U(2)O(4) are 177.9(4)° and 179.1(5)°, respectively. The calculated bond-valence sums for U(1) and U(2) are 6.09 and 6.12, respectively, which are consistent with the formal valence of U(VI).56,57 The Zn atom is in distorted tetrahedral environment defined by three bridging-O atoms from three unique phosphonate ligands

Figure 3. Framework structure of EDP-ZnU1 with large channels filled by coordinated tib molecules. 1368



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one bipy ligand in its asymmetric unit (Figure 4). The U atom is seven coordinated by two symmetrical “yl” oxo atoms (U

Figure 6. Structural view of PDP-ZnU1 showing the arrangement of the layers.

Structure of PDP-ZnU2. PDP-ZnU2 also crystallizes in space group P21/c, and comprises a layered structure. Different from PDP-ZnU1, the U atom is shown in square plane bipyramidal geometry in PDP-ZnU2 (Figure 7). The calculated

Figure 4. ORTEP representation of the asymmetric unit in PDPZnU1. Thermal ellipsoids are drawn at the 50% probability level. Symmetry code A: x, 1.5 − y, 0.5 + z; B: 2 − x, 0.5 + y, 0.5 − z; C: 2 − x, 2 − y, −z.

O: 1.768(5) and 1.773(5) Å) and five bridging-O atoms in equatorial plane (U−O: 2.270(4) − 2.540(4) Å), which are from four PDP ligands. Two UO7 pentagonal bipyramids condense to a U2O12 dimer. The calculated bond-valence sum for the uranium atom indicates 6.09. The Zn atom is five coordinated by two bridging-O atoms from two PDP groups, two N atoms from one bipy and an aqua ligand, resulting into a distorted square pyramid. The PO3 groups in the PDP ligand both adopt tridentate coordination modes: one bridges two uranyl dimer and one zinc atom via corner sharing; the other joins one uranyl dimer (edging sharing) and one zinc atom (corner sharing). The U2O12 dimers are linked by PDP ligands through edge and corner sharing, generating a layer on which the ZnO3N2 polyhedra are decorated (Figure 5). Such layers are stacked forming the whole structure of PDP-ZnU1 (Figure 6). Figure 7. ORTEP representation of the asymmetric unit in PDPZnU2. Thermal ellipsoids are drawn at the 50% probability level. Symmetry code A: 1 − x, −0.5 + y, 0.5 − z; B: −x, 0.5 + y, 0.5 − z; C: 1 − x, 0.5 + y, 0.5 − z.

bond-valence sum indicates 6.02 for the uranium atom. Besides, the Zn atom is four coordinated by two bridging-O and two N atoms resulting into a tetrahedron. The PDP ligand as a hexadentate linker bridges four uranyl centers and two zinc atoms. The connection of UO6 tetragonal bipyramids, ZnO2N2 tetrahedra, and PDP ligands leads to a layered arrangement (Figure 8). Coordinated bipy molecules hold together between the interlayers via weak π···π interactions with intercentroid distances of 3.84 Å (Figure 9). Structure of BDP-ZnU1. This compound crystallizes in a triclinic space group P1̅. Its asymmetric unit contains two crystallographically distinct uranium sites, one zinc atom, two BDP sites, and two phen molecules (Figure 10). U(1) is in pentagonal bipyramidal environment defined by two “yl” oxo

Figure 5. Polyhedral representation of the layer formed by UO7 pentagonal bipyramidal dimer, ZnO3N2 polyhedra, and PDP ligands in PDP-ZnU1. 1369



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Figure 10. ORTEP representation of the asymmetric unit in BDPZnU1. Thermal ellipsoids are drawn at the 50% probability level. Symmetry code A: x, 1 + y, z; B: −x, −y, 1 − z; C: −x, −1 − y, 1 − z; D: −1 − x, −1 − y, 1 − z. Figure 8. Layer formed by UO6 tetrabipyramids and ZnO2N2 tetrahedra, as well as EDP ligands, in PDP-ZnU2.

uranyl cations and one zinc atom. The U2O12 dimer, UO6 tetragonal bipyramids, and BDP ligands are connected into a layered assembly, on which the Zn(phen)2 are decorated (Figure 11). Such layers are packed via the π···π interactions (3.59 Å) between the phen molecules (Figure 12).

Figure 11. Polyhedral representation of the layer formed by UO7 pentagonal bipyramidal dimer, UO6 tetragonal bipyramids, ZnON4 polyhedra, and BDP ligands in BDP-ZnU1.

Structure of BDP-ZnU2. Compound BDP-ZnU2 crystallizes in the triclinic space group P1̅ and features a 3-D framework structure. There are two crystallographically distinct uranyl sites, one zinc atom, two BDP ligands, and one bipy in its asymmetric unit (Figure 13). Similar to BDP-ZnU1, the two unique uranium atoms are in square plane bipyramidal and pentagonal bipyramidal geometry, respectively, and a U2O12 dimer is formed by two UO7 polyhedra via edge sharing. The valences of uranium atoms are 6.02 for U(1) and 6.07 for U(2) on the basis of calculated bond-valence sum. The zinc atom is in a distorted square pyramidal environment, which is defined by three bridging-O atoms from three BDP groups, and two N atoms of one bipy. Two distinct BDP ligands possess two types of coordination modes: the one containing P(1) and P(2) groups both with bidentate mode chelates three uranyl centers and one zinc atom; the other one involves P(3) and P(4) groups with tridentate and bidentate coordination modes, respectively, and bridges three uranyl cations and two zinc atoms. Based on the analyses of their P−O distances, O(6), O(9), and O(13) are protonated (P(1)−O(6): 1.556(5) Å;

Figure 9. Arrangement of the layered structure in PDP-ZnU2.

atoms (UO: 1.761(5) and 1.777(5) Å) and five equatorial bridging-O atoms (U−O: 2.280(5) to 2.510(5) Å). As in EDPZnU1 and PDP-ZnU2, such UO7 polyhedrons share edges with each other to produce a dimer. Differently, U(2) locates at inversion center, and is six-coordinated by four equatorial oxygen atoms from four BDP ligands, leaving two oxo atoms in the axis (UO: 1.779(5) Å). Bond-valence sums indicate 6.11 for U(1) and 5.97 for U(2). The zinc atom is five coordinated by four N atoms of two phen and one oxygen atom from one BDP, forming a trigonal bipyramid. The PO3 groups in one distinct BDP assume bidentate coordination modes, which bridge two uranyl centers, leaving one OH group (P(1)−O(2): 1.544(6) Å) and one terminal oxygen (P(2)−O(6): 1.510(6) Å), respectively. The phosphonate groups in the other distinct BDP ligand adopt tridentate coordination modes, and bind two 1370



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Figure 14. Polyhedral representation of the 3-D framework with channels in BDP-ZnU2.

Figure 12. Structural view of BDP-ZnU1 showing the arrangement of the layers.

Figure 15. Structural view of the channels in BDP-ZnU2 filled by coordinated bipy molecules.

that not only are diverse architectures represented, but also varied coordination modes are displayed by the flexible diphosphonate ligands (Scheme 2). For EDP, the PO3 groups adopt unidentate, bidentate, and tridentate coordination manners connecting U or Zn atoms, and the ligands show as bidentate, pentadentate, and hexadentate linkers. For PDP, only bidentate and tridentate coordination way is observed for the PO 3 groups. The PDP ligands serve as tetradentate, pentadentate, and hexadentate linkers taking four coordination modes. Whereas in the BDP ligand, six coordination ways with tetradentate, pentadentate, and hexadentate linker for the phosphonate ligands are observed. These results reveal that BDP possesses more coordination modes than do EDP and PDP. However, these phosphonate ligands connect up to one Zn atom. Another structural feature is the U2O12 dimer condensed by two UO7 pentagonal bipyramids and/or UO6 tetragonal bipyramids as the appearance of U(VI) in these compounds. In EDP-ZnU1 and PDP-ZnU1, a same chain formed by uranyl dimers and PO3C tetrahedra can be isolated if discarding the Zn atoms (Figure 16). Such chains are connected by C2H4 and C3H6 alkyl chain to generate layered uranyl phosphonate partial structures of EDP-ZnU1 and PDP-ZnU1, respectively. Differ-

Figure 13. ORTEP representation of the asymmetric unit in BDPZnU2. Thermal ellipsoids are drawn at the 50% probability level. Symmetry code A: −x, −y, 1 − z; B: 1 − x, 1 − y, −z; C: x, −1 + y, z; D: −x, 1 − y, 1 − z; E: x, y, −1 + z.

P(2)−O(9): 1.572(5) Å; P(4)−O(13): 1.570(5) Å). The synergetic connection of U2O12 dimers, UO6, and ZnO3N2 polyhedra by BDP ligands results in a 3-D framework structure with channels along the [100] direction (Figure 14). Coordinated bipy molecules protrude into the channels and almost eliminate the void space of BDP-ZnU2 (Figure 15). Structure Discussion. From view of the coordination chemistry, the PO3 group in phosphonate ligands can adopt unidentate, bidentate, and tridentate coordination modes. In reported uranyl ethylenediphosphonates, only bidentate and tetradentate diphosphonate ligands are assumed.14 With the addition of Zn atom and the increasing of the flexibility of the diphosphonate ligands, more versatile coordination manners would be achieved. We summarize the known zinc uranyl alkyldiphosphonates and combine this work. It is demonstrated 1371



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Scheme 2. Summary of Coordination Modes of Flexible Alkyldiphosphonate Ligands in Zinc Uranyl Phosphonates

Figure 16. Uranyl phosphonate partial structures of synthesized compounds; the chains formed by U- and P-centered polyhedra are highlighted. Color code: green, uranium; purple, P; gray, C.

ent from that of PDP-ZnU1, only individual UO6 tetragonal bipyramids are linked by PDP ligands to produce the uranyl phosphonate layer of PDP-ZnU2. When the ligand is further

lengthened, both uranyl dimer and UO6 groups serve as the basic units in BDP-ZnU1 and BDP-ZnU2. The same chains as in EDP-ZnU1 and PDP-ZnU1 can be separated from layered 1372



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ZnU2. This is typical for compounds containing UO22+ dications, which usually consist of several emission peaks. The two luminescent spectra differ only in the peak resolution, and five emission peaks are resolved at 487, 502, 524, 547, and 572 nm. These emission peaks correspond to the electronic and vibronic transitions S11−S00 and S10−S0v (v = 0−4). However, EDP-ZnU1 displays no characterized uranyl emission. As far as we know, not all the uranyl species possess luminescent property, and the mechanism is often hard to explain.58,59

uranyl phosphonate structure of BDP-ZnU2. In comparison, the 2-D uranyl phosphonate partial structure of BDP-ZnU1 comprises another chain constructed by single uranyl centers and dimers as well as PO3C polyhdedra. Zn-centered polyhedra are decorated on both sides of these uranyl phosphonate layers in EDP-ZnU1, PDP-ZnU1, PDP-ZnU2, and BDP-ZnU1. In EDP-ZnU1, the ZnO3N tetrahedra are further connected by bidentate EDP ligand to form 3-D framework. Whereas in BDP-ZnU2, the ZnO3N2 polyhedra as linkers connect adjacent uranyl phosphonate layers to generate the 3-D network structure. It is worth noting that the zinc atoms only connect to phosphonate groups, no Zn−O−U connection was observed in all these uranyl diphosphonates. Varied uranyl units, versatile coordination modes of phosphonate ligands, and differential connection ways of Zn-centered polyhedra lead to these rich structures. It is clearly foreseen more novel uranyl phosphonates will be achieved by modulation of synthetic condition. IR Spectroscopy. The IR spectra of title zinc uranyl diphosphonates are displayed in SI Figure S4. The symmetric stretching vibrations v1 are displayed in the range of 822−762 cm−1, while the antisymmetric stretching modes v3 are observed in the area 968−848 cm−1. The bands located about 1100 cm−1 and in the low wavenumber region from 725 to 570 cm−1 are dominated by the O−P−O bending and P−C stretching vibrations. The peaks around 2930, 2860, and 1440 cm−1 are attributed to the CH2 stretching modes. The stretching vibrations of OH are indicated around 3430 cm−1. The stretching vibrations of the aromatic N-heterocyclic coligands are indicated in the bands of 1520−1620 cm−1. Photoluminescent Properties. Because uranyl complexes usually exhibit green light emission, which is always related to the symmetric and antisymmetric vibrational modes of the uranyl cation, the photoluminescent properties of these synthesized compounds were investigated except PDP-ZnU1 and PDP-ZnU2. From the spectra shown in Figure 17, several emission peaks are clearly observed for BDP-ZnU1 and BDP-

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CONCLUSION In summary, we have isolated a series of zinc uranyl phosphonates by modulation of flexible diphosphonic acids as the ligands. EDP-ZnU1 is a 3-D framework structure featured dimeric uranyl unit. PDP-ZnU1 and PDP-ZnU2 are both present as layered arrangements, which contain U2O12 dimers and UO6 tetragonal bipyrimids, respectively. Despite both U2O12 dimers and UO6 tetragonal bipyrimids serving as the building units, BDP-ZnU1 and BDP-ZnU2 comprise layered and framework structures, respectively. Some conclusions can be yielded from these results. First, these flexible diphosphonate ligands exhibit various coordination modes in these compounds, where PO3 groups in EDP with unidentate, bidentate, and tridentate ways, in PDP with only tridentate forms, and in BDP with bidentate and tridentate modes. Second, Ncontaining coligands play a key role in the coordination and connection of Zn atoms, further influencing the final constructions. Last, the addition of zinc atom enriches the structural diversity of uranyl phosphonates. This work demonstrates more new uranyl coordination polymers will be exploited by extending phosphonate ligands, addition of basic templates or coligands, as well as second metals, and modulation of the reaction conditions.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic cif files, selected bond lengths and angles, powder X-ray diffraction patterns, and IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

Authors W.Y. and F.-Y.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support of this work by National Nature Science Foundation of China (21171662, 21201162, 21301168), Jilin Province Youth Foundation (20130522132JH, 20130522123JH, 20130522170JH), and SRF for ROCS (State Education Ministry). T.T. is thankful for support of the China Postdoctoral Science Foundation (CPSF 2013M541324).

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REFERENCES

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Figure 17. Emission spectra of BDP-ZnU1 (λex = 280 nm) and BDPZnU2 (λex = 328 nm). 1373



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