Construction of Uranyl Organic Hybrids by Phosphonate and in Situ

Article pubs.acs.org/IC

Construction of Uranyl Organic Hybrids by Phosphonate and in Situ Generated Carboxyphosphonate Ligands Chao Liu,†,‡ Weiting Yang,† Ning Qu,§ Lei-Jiao Li,† Qing-Jiang Pan,*,§ 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, P. R. China § Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The hydrothermal reaction of uranyl ions with (5-methyl-1,3phenylene)diphosphonic acid (H4MPDP) in the presence of additives such as nitric acid, N-bearing species, and heterometal ions yielded five new uranyl organic hybrids: (H3O)[(UO2)5(H2O)4(H3DPB)2(H2DPB)(HDPB)]·2H2O (1), (Hphen)(phen)[(UO2)3(H2DPB)(HDPB)] (2), (H2dipy)[(UO2)3(MPDP)2] (3), Zn(bipy)(UO2)(MPDP) (4), and Co(bipy)(UO2)(MPDP)·H2O (5) (H5DPB = 3,5-diphosphonobenzoic acid; phen = 1,10-phenanthroline; dipy = 4,4′-bipyridine; bipy = 2,2′-bipyridine). Single-crystal X-ray diffraction (XRD) demonstrates that 1 and 2 are 3D frameworks constructed of uranyl centers and carboxyphosphonate DPB ligands; the latter were formed via the in situ oxidation of H4MPDP. In the homometallic uranyl diphosphonate 3, less common UO6 square bipyramids connected by MPDP ligands were incorporated to form the 2D assembly. A further introduction of heterometal ions produced two heterobimetallic uranyl phosphonates 4 and 5. Both of them show layered structures, formed by UO6 square bipyramids linked by MPDP ligands with heterometal-centered polyhedra decorated on the sides of the layers. It is found that the pH and heterometal ions have significant effects on the structures of the complexes. In addition to the syntheses and XRD characterization, the spectroscopic properties of these uranyl complexes were also addressed. To complement the experimental results, density functional theory calculations were carried out on several model complexes that feature a homo- or heterobimetallic molecular skeleton. Geometrical/electronic structures, IR spectra, and electronic absorptions were discussed.

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INTRODUCTION

layers to frameworks, building on a diphosphonic acid with hydroxyl and methyl groups.31,32 The architectures of uranium phosphonates can be further modified by incorporating heterometal ions and adding Nbearing species. The latter are served as counterions, templates, and coligands coordinating to metal centers (like transitionmetal ions, which are regarded as heterometal ions relative to uranium). A number of imidazole and pyridine derivatives have been utilized to isolate uranyl phosphonate complexes.33−37 It is found that the N-bearing additives and phosphonate ligands have a synergetic effect on the dimensions of the resulting structures, particularly for those heterometallic uranyl compounds.33−36 In this study, a methyl-functionalized aryldiphosphonate ligand, (5-methyl-1,3-phenylene)diphosphonic acid (H4MPDP), is adopted based on the following considerations. First, the methyl group will affect the coordination between the phosphonate moieties and uranium centers, thus leading to

Interest in the rational design, synthesis, and understanding of uranium coordination polymers has generated plenty of complexes with a wide range of structural diversities1−10 and various potential applications in photochemistry, nonlinear optics, ionic conductivity, ion exchange, intercalation chemistry, etc.11−17 It is of great importance to investigate the nature and structure of complexes formed between uranium ions and phosphonate ligands. This will rationalize the strong affinity of a phosphonate (−PO3) group to the uranium centers in separation processes.18,19 Among the various phosphonate ligands studied, those attached with other functional groups, such as hydroxyl, carboxylate, and methyl, are of particular interest because of their steric and electronic effects and the differential coordination tendency of the functions for metal centers, thus leading to unique structures.20−25 For example, Albrecht-Schmitt et al. reported a number of homo- and heterometallic uranyl complexes constructed with phosphonoacetate.26−30 Recently, we synthesized a series of uranyl complexes that display different assemblies from chains to © XXXX American Chemical Society

Received: November 19, 2016

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DOI: 10.1021/acs.inorgchem.6b02765 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for the Title Complexes compound empirical formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z, ρcalcd (Mg m−3) μ(Mo Kα) (mm−1) F(000) R(int) GOF R1/wR2 [I > 2σ(I)]a R1/wR2 (all data) a

1 C28H26O49P8U5 2584.40 triclinic P1̅ 14.3893(7) 14.7147(7) 15.3534(8) 98.3060(10) 93.8760(10) 96.2960(10) 3185.8(3) 2, 2.694 12.979 2332 0.0334 1.062 0.0494/0.1323 0.0644/0.1454

2 C38H33N4O24P4U3 1767.65 monoclinic P21/n 14.8008(9) 12.2543(7) 30.2243(19) 90 90.9910(10) 90 5481.1(6) 4, 2.134 9.039 3268 0.0738 1.074 0.0605/0.1580 0.0826/0.1745

3 C24H22N2O19P4U3 1480.41 monoclinic P21/c 11.9899(6) 17.4465(9) 17.2385(9) 90 96.6360(10) 90 3581.8(3) 4, 2.745 13.790 2672 0.0675 1.012 0.0525/0.1281 0.0759/0.1425

4 C17H14N2O8P2UZn 739.64 monoclinic P21/n 14.2844(7) 9.7642(5) 14.5701(7) 90 93.7250(10) 90 2027.88(17) 4, 2.423 9.366 1384 0.0345 1.040 0.0228/0.0522 0.0274/0.0540

5 C17H16N2O9P2UCo 751.22 monoclinic P21/c 10.689(2) 10.786(2) 18.730(4) 90 94.020(4) 90 2154.0(7) 4, 2.317 8.481 1412 0.0478 1.011 0.0339/0.0761 0.0466/0.0816

R1 = ∑[ΔF/∑(Fo)]; wR2 = {∑[w(Fo2 − Fc2)]}/∑[w(Fo2)2]1/2, where w = 1/σ2(Fo2). temperature. Yellow blocks of 1 were isolated. Yield: 39 mg (74% based on uranium). (Hphen)(phen)[(UO2)3(H2DPB)(HDPB)] (2). The synthetic procedure was similar to that of 1 except the addition of phen (18 mg, 0.1 mmol; pH = 1.28). Minor yellow rods of 2 along with an unidentified powder were obtained. (H2dipy)[(UO2)3(MPDP)2] (3). UO2(NO3)2·6H2O (50 mg, 0.1 mmol), H4MPDP (25 mg, 0.1 mmol), dipy (16 mg, 0.1 mmol), nitric acid (20 μL, 65%), and deionized water (1.0 mL) were loaded into a 20 mL Teflon-lined stainless steel autoclave (pH = 2.03) and heated at 180 °C for 3 days. The resulting product was washed with distilled water and ethanol and allowed to air-dry at room temperature. Yellow plates of 3 were isolated. Yield: 35 mg (71% based on uranium). Zn(bipy)(UO2)(MPDP) (4). Zn(UO2)2(OAc)6·7H2O (50 mg, 0.05 mmol), H4MPDP (25 mg, 0.1 mmol), bipy (16 mg, 0.1 mmol), nitric acid (20 μL, 65%), and deionized water (1.0 mL) were loaded into a 20 mL Teflon-lined stainless steel autoclave (pH = 2.55) and heated at 180 °C for 3 days. The resulting product was washed with distilled water and ethanol and allowed to air-dry at room temperature. Yellow plates of 4 were isolated. Yield: 48 mg (65% based on uranium). Co(bipy)(UO2)(MPDP)·H2O (5). UO2(NO3)2·6H2O (50 mg, 0.1 mmol), H4MPDP (25 mg, 0.1 mmol), Co(NO3)2·6H2O (29 mg, 0.1 mmol), bipy (16 mg, 0.1 mmol), nitric acid (20 μL, 65%), and deionized water (1.0 mL) were loaded into a 20 mL Teflon-lined stainless steel autoclave (pH = 2.33) and heated at 180 °C for 3 days. The resulting product was washed with distilled water and ethanol and allowed to air-dry at room temperature. Red blocks of 5 were isolated. Yield: 51 mg (68% based on uranium). X-ray Crystal Structure Determination. Suitable single crystals for the title complexes were selected for single-crystal XRD analyses. Crystallographic data were collected at 150 K on a Bruker Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined on F2 by full-matrix least squares using SHELXTL.38 Nonhydrogen atoms were refined with anisotropic displacement parameters during the final cycles. All hydrogen atoms were placed by geometrical considerations and added to the structure factor calculation. A summary of the crystallographic data for the title complexes is listed in Table 1. Selected bond distances and angles are given in Tables S1−S5.

unique structures different from that constructed by 1,3phenylenediphosphonic acid. Second, the methyl group of the aromatic ring has the potential to be oxidized in situ to show a carboxyl function.36 This will produce the ligand 3,5diphosphonobenzoic acid (H5DPB), featuring two functions for the possible binding of metal centers. Further modulation of synthesis variables by adding N-bearing species and secondary metal ions influences the product formation. Herein, five uranyl phosphonates, (H3O)[(UO 2)5(H2O) 4(H3DPB)2(H2DPB)(HDPB)]·2H 2 O (1), (Hphen)(phen)[(UO 2 ) 3 (H 2 DPB)(HDPB)] (2), (H2dipy)[(UO2)3(MPDP)2] (3), Zn(bipy)(UO2)(MPDP) (4), and Co(bipy)(UO2)(MPDP)·H2O (5) (phen = 1,10-phenanthroline; dipy = 4,4′-bipyridine; bipy = 2,2′-bipyridine), were isolated under hydrothermal conditions, with the former two formed via an in situ ligand reaction. Their syntheses, structures, IR, and photoluminescence as well as a density functional theory (DFT) study are described and discussed in detail.

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EXPERIMENTAL SECTION

Caution! Standard procedures for handling radioactive material should be followed, although the uranyl compounds used in the laboratory contained depleted uranium. Materials, Syntheses, and Characterization. All chemicals were purchased commercially and used without further purification. IR spectra were collected from single crystals using a Nicolet 6700 Fourier transform infrared spectrometer. The spectra were collected with a diamond attenuated-total-reflectance objective. Powder X-ray diffraction (PXRD) patterns were performed on a D8 Focus (Bruker) diffractometer with Cu Kα radiation field emission (λ = 1.5405 Å, continuous, 40 kV, 40 mA, and increment = 0.02°). The photoluminescence spectra were recorded with a F-7000 luminescence spectrometer equipped with a 450 W xenon lamp as an excitation light source. The photomultiplier tube voltage was 400 V, the scan speed was 1200 nm min−1, and both the excitation and emission slit widths were 5.0 nm. (H 3 O)[(UO 2 ) 5 (H 2 O) 4 (H 3 DPB) 2 (H 2 DPB)(HDPB)]·2H 2 O (1). UO2(NO3)2·6H2O (50 mg, 0.1 mmol), H4MPDP (25 mg, 0.1 mmol), nitric acid (200 μL, 65%), and deionized water (1.0 mL) were loaded into a 20 mL Teflon-lined stainless steel autoclave (pH = 1.02) and heated at 180 °C for 3 days. The resulting product was washed with distilled water and ethanol and allowed to air-dry at room

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RESULT AND DISCUSSION Syntheses. In situ ligand reaction contains a process wherein the organic species undergoes certain reactions to

B


Article

Inorganic Chemistry

bipyramidal environments defined by two oxo atoms in the axis and five oxygen atoms in the plane. The short UO doublebond distances range from 1.741(7) to 1.774(7) Å with O UO bond angles of 177.8(6)−179.7(5)°. For U1 and U2, five equatorially coordinated oxygen atoms are from four phosphonate groups of four DPB ligands, while four phosphonate oxygen atoms from four ligands and a coordinated water molecule form the pentagonal planes of U3 and U4. U5 is coordinated by three phosphonate oxygen atoms from three ligands and two aqua ligands. The equatorial U−O bond lengths locate in the range of 2.264(7)−2.558(7) Å. As shown in Figure 2, these uranyl centers are bridged exclusively through

create a new ligand with modified functional groups. Furthermore, the generated ligand is incorporated in crystalline coordination polymeric products. In our recent work, a methylfunctionalized phosphonate ligand, (2,5-dimethyl-1,4phenylene)diphosphonic acid, was in situ oxidized to a carboxyphosphonate ligand, 2,5-diphosphonoterephthalic acid, under hydrothermal conditions and resulted in two uranyl carboxyphosphonate complexes.36 Similarly, in this study, two uranyl complexes of 3,5-diphosphonobenzoic acid were obtained through in situ ligand reaction under strong acidic conditions (pH < 1.5) with nitric acid as the oxidant (Scheme 1). Besides, the in situ oxidation of MPDP facilitates the Scheme 1. In Situ DPB Formation from MPDP under Hydrothermal Conditions

formation of single-crystal products suitable for structure determination, while the direct assembly of the uranyl cation and DPB ligand only generated crystalline powder products. Reducing the acidity of the reaction (pH > 2.0) while adding N-bearing species and heterometal ions produced three homoand heterometallic uranyl diphosphonate complexes, 3−5, which all comprise layered structures. Upon a decrease in the pH of the mixtures, no crystalline product but amorphous powder was formed. These results demonstrated that the pH is a key factor for the formation of either carboxyphosphonate or phosphonate complexes. Meanwhile, the additives of N-bearing materials and secondary metal ions also play important roles in the structures of the resultant products. Structural Descriptions. This compound crystallizes in the triclinic space group P1̅. The asymmetric unit of 1 contains five unique uranium cations and four unique DPB ligands (Figure 1). All of the uranium cations are in the common pentagonal-

Figure 2. Illustration of the 3D network of 1 along the c axis.

the phosphonate moieties, while the carboxylate groups of the DPB ligands are free. The connection between the UO7 pentagonal bipyramids and the phosphonate ligands forms the 3D anionic framework structure of 1. In order to satisfy the charge balance, one solvent water molecule was protonated. 2 adopts a 3D framework structure that incorporates common UO7 pentagonal bipyramids and less common UO6 square bipyramids (Figure 3). U1 and U3 are found in the 7fold coordination environment. Five-coordinated oxygen atoms around U1 are provided by three phosphonate groups and one

Figure 1. Depiction of the local coordination environment in 1. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: A, 2 − x, −y, 1 − z; B, x, 1 + y, z; C, 1 − x, −y, 2 − z; D, 1 − x, −y, 1 − z; E, 1 − x, 1 − y, 1 − z.

Figure 3. Depiction of the local coordination environment in 2. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: A, −0.5 − x, −0.5 + y, 0.5 − z; B, x, −1 + y, z; C, 0.5 − x, −0.5 + y, 1.5 − z; D, 0.5 − x, 0.5 + y, 1.5 − z; E, −x, 2 − y, 1 − z. C


Article

Inorganic Chemistry

A 2D structure is present for 3. Its symmetric unit consists of four distinct uranium sites, two MPDP ligands, and one dipy molecule. As shown in Figure 5, all of the uranium cations are

chelating carboxylate moiety. Three of the equatorially coordinated oxygen atoms of U3 are from three phosphonate groups and the other two are aqua ligands, while U2 is 6-foldcoordinated by two axial “yl” oxygen atoms and four equatorial oxygen atoms from four phosphonate moieties of four ligands. The uranyl units have average UO bond distances of 1.770(2) Å for U1, 1.733(2) Å for U2, and 1.771(9) Å for U3. The OUO bond angles are 177.5(5)° for U1, 179.1(5)° for U2, and 179.0(4)° for U3. The equatorial U−O bonds to the phosphonate oxygen atoms are in the distances of 2.28(8)− 2.347(8) Å, whereas the U−O bonds to the carboxylate group and the coordinated water molecule are slightly longer at 2.451(9)−2.467(8) and 2.52(2)−2.59(2) Å, respectively. The linkage of UO7 and UO6 by the DPB ligands creates the whole framework structure with 1D channels along the b axis (Figure 4a). Half of the carboxylate groups in this structure are chelated

Figure 5. Depiction of the local coordination environment in 3. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: A, 1 − x, 1 − y, 1 − z; B, 1 − x, −y, 1 − z; C, x,−1 + y, z; D, x, 0.5 + y, z; E, 1 − x, 0.5 + y, 1.5 − z; F, x, 0.5 − y, 0.5 + z.

6-fold-coordinated by two axial oxo atoms and four equatorial oxygen atoms from four MPDP ligands, forming less common square bipyramids. The UO distances in the axis range from 1.759(8) to 1.796(9) Å with OUO bond angles of 178.7(4)−180.0(2)°. The U−O bond lengths in the equatorial plane are in the range of 2.259(9)−2.312(7) Å, which are typical for the 6-fold uranium polyhedron (average 2.28 Å in the literature).38 An illustration shown in Figure 6a displays the connection between the centers and the MPDP ligands, which leads to a layered assembly with methyl moieties as the branches. Such layers stack along the a axis with protonated dipy cations in the interlayer space (Figure 6b).

Figure 4. (a) View of 2 projected along the b axis showing the channels filled with protonated phen cations. (b) View of π···π stacking interactions among the phen molecules and that between the phen and DPB ligands.

to U1 centers, and the other half are dangling into the channels. The measured opening size of the channel is 0.67 × 1.32 nm (O···O distance, including the van der Waals diameter). Protonated phen cations reside in the channels for space filling and charge-compensate. The templates are stacked together along the c axis and held together by π···π interactions with intercentroid distances of 3.59−3.82 Å (Figure 4b). Such interactions also exist between the phen and benzoate group of the DPB ligand (intercentroid distances: 3.59 and 3.73 Å).

Figure 6. (a) Representation of the uranyl phosphonate layer in 3 parallel to (011). (b) View of the layer packing with protonated bipy cations in the interlayer space. D


Article

Inorganic Chemistry 4 is a heterometallic uranyl phosphonate complex that incorporates UO6 and ZnO2N2 polyhedra. As shown in Figure 7, the uranium has a square-bipyramidal surrounding, in which

Figure 7. Depiction of the local coordination environment in 4. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: A, 1.5 − x, 0.5 + y, 0.5 − z; B, 1 − x, 1 − y, −z; C, 0.5 + x, 0.5 − y, 0.5 + z.

the equatorial plane is formed by four phosphonate oxygen atoms from four MPDP ligands. In the axis, the OUO bond angle is 177.2(1)° with UO bond distances of 1.772(3) and 1.786(3) Å, while the U−O bonds to the phosphonate groups in the equatorial plane are arranged from 2.276(3) to 2.301(3) Å. The zinc ion is coordinated by two oxygen atoms from two phosphonate groups [Zn−O: 1.884(3) and 1.893(3) Å] and two nitrogen atoms from one bipy [Zn−N: 2.025(4) and 2.081(4) Å], thus forming a distorted tetrahedron. The UO6 square bipyramids are bridged by the MPDP ligands to form the layered assembly, with ZnO2N2 tetrahedra coordinating on both of its sides (Figure 8a). The layered assemblies interact with each other via week π···π interaction between the bipy molecules (4.0 Å) to form a 3D supramolecular structure (Figure 8b). 5 also represents as a 2D heterometallic uranyl phosphonate architecture, which consists of UO6 and CoO3N2 polyhedra linked by the MPDP ligands (Figure 9). The square plane around the uranyl centers is defined by four phosphonate oxygen atoms from four ligands with U−O bond lengths in the range of 2.244(4)− 2.269(4) Å. In the axis, the two oxo atoms are seated with UO bond distances of 1.767(5) and 1.791(5) Å. The OUO bond angle is 176.5(2)°. The Co2+ ion is coordinated by two nitrogen atoms from one bipy, two phosphonate oxygen atoms from two ligands, and one uranyl oxygen atom. The Co−O bond length to the uranyl cation is 2.200(5) Å, slightly longer than that to the phosphonate moieties [1.934(5) and 1.944(4) Å]. It is noted that a rare U OCo interaction occurred in this structure, although the uranyl oxygen atoms are commonly inert. 27,39,40 The connection of the uranyl units through the ligands leads to a layered structure (Figure 10a). Cobalt centers are coordinated on the sides of the layer via bonding interaction with the uranyl units and the ligands. As depicted in Figure 10b, the heterometallic uranyl phosphonate layers stack along the a axis and interact with each other via strong π···π interactions between the bipy molecules (3.42 Å), thereby leading to a 3D supramolecular structure. Some interesting comparisons can be observed from the coordination environments around the uranium centers and the

Figure 8. (a) Representation of the heterometallic uranyl phosphonate layer in 4. Coordinated bipy molecules were deleted for clarity. (b) View of the layer packing with coordinated bipy molecules in the interlayer space.

Figure 9. (a) Depiction of the local coordination environment in 5. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: A, 1 − x, 2 − y, −z; B, 1 − x, 0.5 + y, 0.5 − z; C, x, 1 + y, z.

coordination modes and protonation states of the organic ligands. The uranium centers are found in common 7-fold coordination and show an exclusive affinity for the phosphonate moiety. This is consistent with the hard/soft acid/base theory. On the other hand, the phosphonate groups in this structure display one bidentate mode and two kinds of tridentate modes E


Article

Inorganic Chemistry

modes. Both 1 and 2 are synthesized via an in situ oxidation reaction in the presence of nitric acid. The initial pH value is low. It is not surprising that all free carboxylic groups and uncoordinated phosphonate moieties are protonated. The slight asymmetries of the C−O bonds are indicative of the presence of a C−OH group. The large P−O bond distances reveal the presence of protonated phosphonate groups.41 For 3−5, constructed by the MPDP ligand, only less common UO6 square bipyramids and one hexadentate coordination mode for the ligand occur (F in Scheme 2). Nevertheless, the different connections between the uranyl units and ligands as well as the incorporation of organic species and heterometal ions lead to the distinct structures for 3−5. IR Spectroscopy. Complexes 1−5 were characterized by IR spectra (Figure S5). The strong peaks centered around 920− 900 cm−1 are ascribed to the asymmetric stretching modes ν3 of the uranyl dication, while those centered in the range of 840− 820 cm−1 are arising from the symmetric stretching vibrations ν1. The corresponding theoretical assignment will be shown below. The bands in the range from 1060 to 970 cm−1 and in the low-frequency region from 770 to 600 cm−1 are dominated by the O−P−O bending and P−C stretching vibrations. The peaks in the area 1750−1260 cm−1 are attributed to the vibrations of the aromatic rings and carboxylate groups. The C−H stretching vibrations of the aromatic ring are displayed at 3100 cm−1. Luminescent Property. Uranyl complexes can emit green light centered near 520 nm when excited with UV light. This phenomenon has been documented for decades, and the photoluminescent spectra of uranyl complexes often consist of several peaks vibronically coupled to the symmetric and asymmetric vibrational modes of the uranyl cation. The photoluminescent properties of partial compounds were studied, and the spectrum of solid uranyl nitrate hexahydrate also has been characterized for reference. As shown in Figure 11, the emission spectra of 1, 3, and 4 display typically vibronically coupled charge-transfer features of uranyl complexes with 4−6 clearly resolved vibronic transitions in the range of 460−620 nm. These emission peaks correspond to the

Figure 10. (a) Representation of the heterometallic uranyl phosphonate layer in 5 parallel to (011). Coordinated bipy molecules were deleted for clarity. (b) View of the layer packing along the a axis with coordinated bipy molecules in the interlayer space.

for the binding of uranium units. Then the DPB ligands finally serve as tetradentate, pentedentate, and hexadentate linkers in 1 (Scheme 2A−C). For 2, two coordination spheres, UO6 and Scheme 2. Summary of the Coordination Modes of the Ligands in the Title Compounds

UO7, are observed for the uranium centers. In addition to the phosphonate groups, the carboxylate moieties also coordinate to the uranium units. Besides, free carboxylic groups still exist in the structure. For the DPB ligands in the structure, two coordination fashions (D and E in Scheme 2) are displayed with the phosphonate groups in the bidentate and tridentate

Figure 11. Emission spectra of 1, 3, 4, and UO2(NO3)2·6H2O (λex = 420 nm) showing green-light emission from uranyl dications. F


Article

Inorganic Chemistry electronic and vibronic transitions S11−S00 and S10−S0v (ν = 0− 4). The difference in these spectra may be due to the coordination surrounding of uranium, UO 6 and UO 7 polyhedral environments, and the influence of the organic ligands and heterometal ions. DFT Calculations. To complement experimental results, four model complexes were optimized using relativistic DFT.42 The important structural feature, a homo- and heterobimetallic uranyl-M (M = U, Zn, and Co) skeleton, is taken into account. See the optimized structures in Figure 12. Notably, the

Å) distance and long (1.90 Å) UO distance were calculated for 4a. The lengthening of the latter is caused by the interaction of its -yl oxo with a zinc ion, being at 2.08 Å. Obviously, the calculated Zn−Oyl distance is greatly underestimated relative to the experimental value of 4 (3.03 Å). This is attributed to the approximation of the theoretical model. The simplicity of the ligand in calculations greatly reduces steric repulsion, which consequently prefers strong Oyl → Zn coordination to lower energy of the system. The structural feature of the experimental 5 has been reflected by those of the theoretical 5a and 5a′, albeit with some differences of the geometry parameters related to the UOCo moiety. 5a and 5a′ correspond to the doublet and quartet electronic spin states, respectively, because cobalt(II) has a 3d7 electronic configuration and uranium(VI) is supposed to be closed-shell. The experimental Co−Oyl distance of 2.20 Å is comparable to the computed distances of 1.91 and 2.05 Å. This strong interaction results in the adjacent UO bond being significantly lengthened by 0.08 Å relative to the other one in the calculation, but not that large of an elongation (only 0.02 Å) is found in the experiment. Additionally, energetic calculations show that 5a (doublet state) is 12.6 kJ/ mol more stable than 5a′ (quartet state). Thus, the former will be discussed below. As shown in Figure S8, two types of UO stretching vibrational frequencies were calculated for 3a and 3b. The absorption bands at 722−795 and 846−891 cm−1 are attributed to the symmetric and asymmetric UO stretching modes, respectively. This agrees with the experimentally obtained 820− 840 and 900−920 cm−1 vibrations indicated earlier in this work. Notably, only one asymmetric UO stretching vibrational frequency was obtained for 4a (844 cm−1) and 5a (773 cm−1). Careful inspection of all vibrational modes did not find the symmetric ones. We conjecture that perturbation of the TM ion is responsible for this. Reduction of the UO frequencies from 4a to 5a demonstrates that the interaction strength follows the order of Zn−Oyl < Co−Oyl. This also confirms theoretically computed and experimentally determined TM− Oyl distances (Table S8). The electronic structures of 3a, 3b, 4a, and 5a were calculated. As expected, these uranyl complexes possess lowlying 5f(U)-character unoccupied orbitals (Figures 13 and S9− S11). The π*(bipy) virtual orbital inserts between them for the heterobimetallic uranyl-TM complexes. Mixed with phosphorus and oxygen groups, the aryls contribute to most of occupied orbitals. The σ(UO) bonding orbitals are found in the lowenergy region, for instance, HOMO−13 (H-13), H-14, H-16, and H-20 of 3a (Figure S9). As seen in Figure 13, one can note that the 3d(Co)-dominant character forms the low-lying occupied orbitals (α-spin HOMO ∼ H-4) of 5a, corresponding to dz2, dxz, dyz, and dxy, respectively. We also find σ(UO)-type orbitals like H-20−H-22 but do not observe the Oyl−Co bonding one, even upon careful inspection of the electronic structures of 5a. Time-dependent DFT (TD-DFT) calculations yielded the electronic absorption spectra of the above four model complexes in Figure S12. The adsorption peaks related to σ(UO) are found in the high-energy (i.e., shortwavelength) region, which corresponds to experimentally resolved vibronic emission bands around 520 nm. Both of them show the same origin from UO bonding.

Figure 12. DFT-optimized structures of uranyl model complexes.

experimental complex 3 (Figure 5) is modeled by 3a, which only contains a large-ring framework constituted by two uranyl moieties and equatorial MPDPs. One additional complex 3b is designed in theory, where the phosphonato ligands of one uranyl are replaced with bipy. It can be regarded as a “transition” structure from 3 to 4/5 because the latter complexes possess the bipy equatorial ligand. Computational details are shown in the Supporting Information. When comparing 3a (calcd) and 3 (expt) in Table S6, a good overall agreement was found. The UO bond lengths were calculated to be 1.82−1.86 Å, which are slightly longer than the experimental values of 1.76−1.80 Å. The difference mainly originates from the GGA functional overestimating the bond lengths.43 The calculated U−Oeq bond lengths (2.20−2.38 Å) fall well within the range of the experimental values (2.26−2.31 Å). The uranyl dication remains linear with a OUO angle of 175°, close to the experimental value of 179°. The calculated UO bond orders range from 2.12 to 2.37, suggesting partial triple-bonding character, as is normally found for uranyl(VI) complexes. The Oeq → U coordination is best described as a weak single bond because of their bond orders between 0.61 and 0.91. For heterobimetallic uranyl-TM (transition metal) complexes, a comparison finds close values between the calculated and experimental data for most geometry parameters. The difference falls within 0.09 Å for the distances and 2° for the angles. However, bond lengths related to the UOTM linkage show a large discrepancy. For example, one short (1.82

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CONCLUSIONS In summary, we have synthesized a series of uranyl phosphonates and carboxyphosphonates based on MPDP and G


Article

Inorganic Chemistry

moieties together participate in the connection with UO7 polyhedra and phosphonate groups, creating 1D channels filled by protonated phen cations. Luminescent studies reveal that 1, 3, and 4 exhibit typical green-light emission originating from the uranyl centers. The absorption spectra from TD-DFT calculations agree with the above experimental assignment related to the UO bond. Moreover, detailed electronicstructure information was provided. In brief, the current study demonstrates that novel uranyl coordination polymer architectures will be achievable via modulation of the ligands with functional groups, the additives, as well as the synthetic conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02765. Selected bond lengths and angles, PXRD patterns, IR spectra, computational details, optimized geometry parameters, and Cartesian coordinates of model complexes, orbital diagrams, and simulated IR and electronic absorption spectra (PDF) X-ray crystallographic CIF files for 1−5 (CCDC 1469920−1469924) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhong-Ming Sun: 0000-0003-2894-6327 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for support of this work by the National Natural Science Foundation of China (Grants 21571171, U1407101, 21301168, 21273063, and 51402286).

Figure 13. Diagrams of α-spin orbitals for 5a from the TD-PBE/TZP/ ZORA/COSMO calculation.

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its in situ generated DPB ligands under hydrothermal conditions. Three uranyl phosphonate complexes of 3−5 are 2D structures formed by less common uranyl square bipyramids and MPDP ligands. Among them, 3 is a homometallic uranyl complex with protonated dipy cations as the templates, while 4 and 5 are heterometallic uranyl complexes with Zn(bipy) and Co(bipy) coordinated by phosphonate groups, respectively. It is noted that, in 5, a rare UOylCo connection is observed. This Co−Oyl bonding interaction has been well reproduced by DFT calculations of heterobimetallic 5a and 5a′. It also results in a relatively low UOyl stretching vibrational frequency. Although most calculated geometry parameters are close to the experimental values, the Zn−Oyl and adjacent UOyl distances were greatly under- and overestimated in theory, respectively. This is attributed to the approximation of the theoretical model, which reduces the steric repulsion and favors Oyl → Zn coordination to stabilize the whole system. For the other two uranyl carboxyphosphonate complexes 1 and 2, both feature 3D framework structures. 1 displays common UO7 pentagonal bipyramids linked by phosphonate groups. In 2, 6-foldcoordinated uranium centers and in situ generated carboxylate

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Construction of Uranyl Organic Hybrids by Phosphonate and in Situ

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