Silver Ion-Mediated Heterometallic Three-Fold Interpenetrating Uranyl

Oct 22, 2015 - Recent advances in structural studies of heterometallic uranyl-containing coordination polymers and polynuclear closed species. Pierre ...
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Silver Ion-Mediated Heterometallic Three-Fold Interpenetrating Uranyl−Organic Framework Lei Mei,†,∥ Qun-yan Wu,†,∥ Shu-wen An,† Zeng-qiang Gao,§ Zhi-fang Chai,†,‡ and Wei-qun Shi*,† †

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ School of Radiological and Interdisciplinary Sciences and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China §

S Supporting Information *

ABSTRACT: A unique case of a uranyl-silver heterometallic 3-fold interpenetrating network (U-Ag-2,6-DCPCA) from a multifunctionalized organic ligand, 2,6-dichloroisonicotinic acid, in the presence of uranyl and silver ions is reported. It is the first report of a heterometallic uranyl−organic interpenetrating network or framework. Notably, a (4,4)-connected uranyl building unit in U-Ag-2,6-DCPCA, which is available through combined influences of structural halogenation and silver ion additive on uranyl coordination, plays a vital role in the formation of a 3-fold interpenetrating network. Halogen substitution effectively changes structural features and coordination behaviors of isonicotinate ligand and contributes to the control of uranyl coordination. Meanwhile, it exerts influence on the stabilization of 3-fold interpenetrating networks by halogen−halogen interactions. Theoretical calculation suggests that the silver ion should mainly serve as an inductive factor of uranyl species through strong Ag−N binding affinity, directly leading to the formation of a (4,4)-connected uranyl building unit and finally a heterometallic 3-fold interpenetrating network. Related experimental results, especially an interesting postsynthetic metalation, afford further evidence of this induction effect.

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INTRODUCTION Actinide materials have aroused intense interest from chemists for their close relation with nuclear fuel cycles as well as potential applications in many fields.1−6 The research of actinide chemistry, especially actinide coordination chemistry and their fission products in solution and solid can afford sophisticated knowledge of 5f elements and be helpful to propose feasible strategies for nuclear waste management. As one the most important actinides, the intense research focused on uranium presents lots of hybrid materials with rich topologies and intriguing physicochemical properties, largely obtained by the combination of the linear triatomic uranyl (UO22+) cation together with organic ligand moieties.7,8 Uranyl coordination chemistry as well as its speciation is of great significance in the nuclear industry in terms of nuclear waste separation and disposal. Though many factors, such as pH, temperature, concentration, etc., are involved in controlling uranyl coordination and speciation, uranyl coordiantion is largely dominated by chemical structure and coordination behavior of organic ligands.8,9 Different organic molecules, including carboxylates,9−15 phosphonates,16−21 etc., have been used as ligands to fabricate lots of uranyl-bearing compounds with a variety of uranyl coordination patterns. Generally, when the synthesis © XXXX American Chemical Society

reactions are conducted using a similar experimental procedure, different organic ligands with remarkable distinction originated from varied functional groups or backbones can lead to various uranyl coordination patterns.9 Besides, subtle variation of ligand structures through structural modifications can also induce obvious changes of uranyl coordination under similar conditions. For example, benzoic acid analogues or terephthalic acid15,22,23 is prone to form a monomeric uranyl moiety in eight-coordinated hexagonal bipyramid geometry by coordination of three sets of η2-carboxylate groups on the equatorial plane (Figure 1a). When introducing halogen atoms at the 4position of the phenyl group in benzoic acid, this type of halogen-bearing derivative affords seven-coordinated uranyl monomers, which are further bridged by carboxylate groups to form a uranyl dimer24 (Figure 1b). On the other hand, isonicotinic acid with molecular variation through replacing the phenyl group by the pyridinyl group forms another type of seven-coordinated edge-sharing uranyl dimer, leading to an infinite one-dimensional (1D) chain via bridging of carboxylate groups25 (Figure 1c). For all of the cases above, besides the differences in electronic structures (such as the induction-effect Received: August 27, 2015

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Figure 1. Different changes of uranyl coordination via subtle variation of ligand structures: (a) terephthalic acid; (b) 4-chlorobenzoic acid; (c) isonicotinic acid; (d) 2,6-dichloroisonicotinic acid.

Scheme 1. Participation of Heterometallic Elements into Uranyl-Organic Compounds Achieves a Significant Effect on Uranyl Coordination

uranyl moiety similar to that for halogenated benzoic acid (Figure 1d). Although hydrogen bonding of pyridinyl nitrogen atoms is also found in the final complexes in uranyl compound of halogenated isonicotinic acid, it has no obvious effect on the uranyl coordination. This interesting result suggests the powerful ability of halogenation to affect and control the uranyl coordination environment by halogen bonding. Moreover, incorporation of halogen bonds in uranyl-organic hybrid materials will enrich the library of uranyl-organic compounds based on weak interaction, provide insight into the role of halogen bonds on the formation of uranyl compound, and even promote a deeper understanding of the bonding nature of halogen bonds.24,27,28

dependent acidity and the steric hindrance of a bulky group), another factor, that is weak interaction, shows a close relation with the change of uranyl sphere: the formation of a carboxylate-bridged uranyl dimer from halogenated benzoic acid can be partly attributed to the halogen−halogen interaction of adjacent uranyl moieties, while the infinite 1D chain is related to the hydrogen bonding capacity of pyridinyl nitrogen atoms. We are interested in uranyl coordination behevior of halogenated isonicotinic acid devatives, a case of ligand combining pyridine group and halogenation. The initial results reveal that the stable symmetrical 2,6-dichloroisonicotinic acid (H-2,6-DCPCA) gives a cocrystal26 (namely, as U2,6-DCPCA here) with isolated carboxylate-bridged uranyl dimers halogen-bonded with each other, which is in fact a B

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Ag+ ion, only U-2,6-DBPCA was obtained, without Ag+ involved in the final product. X-ray Single Crystal Structural Determination. X-ray diffraction data of U-Ag-2,6-DCPCA were all collected on a Agilent SuperNova X-ray CCD diffractometer with a Mo Kα X-ray source (λ = 0.71073 Å) at room temperature. Standard Agilent Crysalis software was used for the determination of the unit cells and data collection control. The crystal structures were solved by means of direct methods and refined with full-matrix least-squares on SHELXL-97. Data collection of the compound Cu-2,6-DCPCA was performed with synchrotron radiation facility at BSRF (beamline 3W1A of Beijing Synchrotron Radiation Facility, λ = 0.71073 Å) using a MAR CCD detector. The crystal was mounted in nylon loops and cooled in a cold nitrogen-gas stream at 100 K. Data were indexed, integrated, and scaled using DENZO and SCALEPACK from the HKL program suite. The crystal structures were solved by means of direct methods and refined with full-matrix least-squares on SHELXL-97. The crystal data of U-Ag-2,6-DCPCA and Cu-2,6-DCPCA are given in Table 1.

We are further exploring more intriguing structures based on halogenated ligands. A good method for assembling new types of uranyl-organic compounds is to introduce other heterometallic elements, such as 3d or 4f metal ions, into the system of uranyl and isonicotinate derivatives.25,29−33 The introduction of heterometallic centers in uranyl-organic compounds will afford not only their various novel topologies but also potential properties and functions.34−36 Moreover, the isonicotinate ligands, which only take up carboxylate groups to coordinate with uranyl ions, can afford uncoordinated N-donors of pyridinyl groups to the heterometallic ions for binding. Herein, we report a unique case of uranyl-silver heterometallic 3-fold interpenetrating network (namely, as U-Ag-2,6-DCPCA) from 2,6-dichloroisonicotinic acid. Interestingly, during the fabrication process of this 3-fold interpenetrating network, the additional silver (Ag+) ion not only serves as linking nodes through nitrogen donors of isonicotinate but also contributes to the alteration of uranyl coordination of 2,6-dichloroisonicotinate (2,6-DCPCA) (Scheme 1). It is notable that, although several uranyl-organic interpenetrating networks15,23,37−40 (including two types of interpenetrating modes, i.e. paralleled or inclined) have been reported, actinide-based interpenetrating structures are still rare. Our findings here represent the first case of a heterometallic uranyl-organic interpenetrating network or framework. Structural features for this heterometallic 3fold interpenetrating network as well as its possible formation pathway have been presented and discussed using the results from experimental and computational investigations.

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Table 1. Crystal Data and Structure Refinement for U-Ag2,6-DCPCA U-Ag-2,6-DCPCA formula formula weight crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K Dc, g/cm3 μ (Mo Kα) (mm−1) theta(Min, Max) [deg] F(000) R1, wR2 [I ≥ 2σ(I)] R1, wR2 (all data) residue dens. [e/Å3]

EXPERIMENTAL SECTION

General Methods. Caution! Suitable measures for precautions and protection should be taken, and all operations should follow the criteria while handling such substances although natural uranium was used in the experiment. All chemical reagents were purchased from commercial sources and used as received. Thermogravimetric analysis (TGA) was performed on a TA Q500 analyzer over the temperature range of 20−800 °C in air atmosphere with a heating rate of 5 °C/min. Powder X-ray diffraction (PXRD) measurements were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range 5−80° (step size: 0.02°). Solid-state fluorescence spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets in the range of 4000−400 cm−1 on at Bruker Tensor 27 spectrometer. Synthesis. The compound U-Ag-2,6-DCPCA was synthesized hydrothermally under autogenous pressure by using a Teflon-lined stainless-steel bomb from a mixture of uranyl nitrate hexahydrate, silver nitrate, and 2,6-dichloroisonicotinic acid using a method similar to that of U-2,6-DCPCA: 200 μL of UO2(NO3)2·6H2O (0.5M, 0.1 mmol) and 100 μL of AgNO3 (1 M, 0.1 mmol) solution were added to a suspension of 2,6-dichloroisonicotinic acid precursor (0.2 mmol) in water (1 mL) in a stainless-steel bomb. After treating with ammonium hydroxide (10 μL), the mixture was sealed, kept at 150 °C for 48 h, and cooled to room temperature to give yellow-green crystals, which were then filtered off, washed with water, and dried. Other attempts, using other metal ions (Cu2+, Zn2+, Co2+, and Ni2+) to replace Ag+ ion or using 2,6-dibromoisonicotinic acid to replace 2,6dichloroisonicotinic acid, were conducted under similar conditions described above. When using other metal ions (Cu2+, Zn2+, Co2+, and Ni2+) to replace Ag+ ion, only the hydrothermal system of uranyl nitrate hexahydrate, copper nitrate, and 2,6-dichloroisonicotinic acid affords a new copper-2,6-dichloroisonicotinate compound (Cu-2,6DCPCA), while all the other hydrothermal systems give U-2,6DCPCA without any heterometallic elements of starting materials (Cu2+, Zn2+, Co2+, or Ni2+) involved. When using 2,6-dibromoisonicotinic acid to replace 2,6-dichloroisonicotinic acid in the presence of

C36H16Cl12 N6O18U2Ag2 1937.75 monoclinic P21/c 30.8827(12) 8.0912(3) 21.0669(8) 90 107.723(1) 90 5014.3(3) 4 296 2.567 7.928 0.7, 25.1 3600 0.0324, 0.0570 0.0490, 0.0617 −0.88, 0.56

CCDC-1017271 (U-Ag-2,6-DCPCA) and CCDC-1419715 (Cu-2,6DCPCA) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Quantum Chemical Calculation. In density functional theory (DFT) calculations, hybrid functional, B3LYP41 with Gaussian 09 program42 was used for structural optimization. The small-core quasirelativistic effective core potentials (RECP), which replace 60 core electrons for uranium atom, have been adopted here in combination with the corresponding basis set with a segmented contraction scheme.43−45 For light atoms (H, C, N, and O), the polarized allelectron 6-31+G(d) basis set was used for geometry optimizations and energy calculations. It can provide reliable results for uranyl complexes with the level of theory according to the previous studies.46−50 Mayer bond order and electron density of the bond critical point as well as the corresponding Laplacian were carried out using Amsterdam density functional (ADF 2013) package.51,52 BP86 method53 and the Slater type orbital (STO) basis set with the quality of triple-ζ plus polarization (TZP) basis set were used,54 without frozen core. The scalar relativistic (SR) effects were taken into account using the zeroorder regular approximation (ZORA) approach.55 C

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Figure 2. (a) The asymmetric unit and four-connected structural extension of U-Ag-2,6-DCPCA; (b) slight departures of η2-carboxylate oxygen donors from the uranyl equatorial plane; (c) all the 2,6-DCPCA moieties bending out of the uranyl equatorial plane in U-Ag-2,6-DCPCA, unlike the good coplanarity found in U-2,6-DCPCA.26

Table 2. Selected Experimental and Calculated Bond Lengths (Å), the Corresponding Mayer Bond Orders (MBO), Electron Density [ρ(r), au] and its Laplacian [∇2ρ(r), au] for the Two Possible Intermediates of Uranyl Dimers, U-6L and U-4L-2H2O U-6L

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U1O1 U1O2 U2O3 U2O4 U1−O5 U1−O6 U1−O7 U1−O8 U1−O9 U2−O10 U2−O11 U2−O12 U2−O13 U2−O14

U-4L-2H2O

expt.

calc.

MBO

ρ(r)

∇ ρ(r)

expt.

calc.

MBO

ρ(r)

∇2ρ(r)

1.767 1.757 1.749 1.752 2.285 2.488 2.518 2.341 2.342 2.336 2.338 2.483 2.521 2.292

1.769 1.764 1.764 1.769 2.416 2.497 2.493 2.383 2.377 2.380 2.379 2.490 2.493 2.415

2.035 2.038 2.035 2.040 0.362 0.409 0.396 0.354 0.355 0.359 0.359 0.411 0.400 0.361

0.299 0.302 0.302 0.298 0.062 0.057 0.057 0.063 0.062 0.064 0.063 0.058 0.057 0.062

0.427 0.427 0.427 0.427 0.230 0.180 0.184 0.254 0.265 0.255 0.261 0.183 0.183 0.230

1.754 1.751 1.751 1.754 2.427 2.469 2.441 2.338 2.316 2.338 2.316 2.469 2.441 2.427

1.762 1.766 1.766 1.762 2.582 2.496 2.455 2.358 2.309 2.358 2.309 2.496 2.455 2.582

2.056 2.060 2.060 2.056 0.115 0.413 0.425 0.396 0.403 0.396 0.402 0.414 0.425 0.115

0.304 0.301 0.301 0.304 0.043 0.057 0.063 0.067 0.072 0.067 0.072 0.057 0.063 0.043

0.427 0.427 0.427 0.427 0.159 0.182 0.194 0.267 0.315 0.267 0.315 0.182 0.194 0.159

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U-Ag-2,6-DCPCA is very similar to that of U-2,6-DCPCA,26 the participation of ligands in uranyl coordination is different from them. Besides two sets of 2,6-dichloroisoniconate (2,6DCPCA) ligands, water molecules also take part in the uranyl coordination for U-2,6-DCPCA, whereas another type of η1mode 2,6-DCPCA replaces the water molecules in U-Ag-2,6DCPCA. IR spectrum of U-Ag-2,6-DCPCA (Figure S1) reveals an attenuation of signal at ∼3200 cm−1 corresponding to crystal water and a slight shift of [OUO] bond vibration (from 935 to 942 cm−1), which is consistent with the structural change between U-2,6-DCPCA and U-Ag-2,6-DCPCA. The significant change of coordination pattern induces a slight departure of η2-carboxylate oxygen donors (O6 and O7 around U1, O12 and O13 around U2) from the uranyl equatorial plane (Figure 2b). As shown in Table 2, O6 gives departure angles of 93.88(17) and 85.19(17) from uranyl center (O1U1O2),

RESULTS AND DISCUSSION

Structure Description. The uranyl-silver heterometallic compound, U-Ag-2,6-DCPCA, was obtained from the hydrothermal reaction of uranyl nitrate and 2,6-dichloroisonicotinic acid in the presence of silver nitrate at 150 °C for 2 days as yellowish-green block crystals. Crystallographic analysis (Table 1) shows that U-Ag-2,6-DCPCA crystallizes in the monoclinic space group P21/c, containing two 7-fold coordinated mononuclear uranyl units bridged by two μ2-(O, O)carboxylate groups in its asymmetric unit (Figure 2a). The coordination sphere of each uranyl center is further completed by a η2-mode and a η1-mode carboxylate group and forms pentagonal bipyramid geometry with equatorial U−O distances ranging from 2.285(4) to 2.521(4) Å (Table S1). It is interesting to find that, though the dimeric uranyl pattern in D

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Figure 3. Heterometallic interpenetrating network found in U-Ag-2,6-DCPCA. (a, b) Single-stranded 2D (4,4)-connected network with a cavity of 31.65 Å × 24.27 Å; (c, d) 3-fold 2D interpenetrating network; (e, f) 2D 3-fold interpenetrating network viewed from the b axis.

Figure 4. Halogen−halogen interactions found between adjacent layers (a) with Cl−Cl distances of ∼3.05, 3.56, and 3.68 Å (b) based on a multifunctionalized linker, 2,6-dichloroisoniconate (c) for the 3-fold interpenetrating network of U-Ag-2,6-DCPCA. Three layers are drawn in different colors (layer A, green color; layer B, blue color; layer C, pink color).

while those for O7 are 84.64(16) and 92.90(17). O12 and O13 atoms show a similar trend (O12:87.03(16) and 85.41(17), O13:91.43(16) and 94.02(18). This special distortion in geometry here seems to be closely related with the crowded coordination environment of uranyl center in U-Ag-2,6DCPCA. For the same sake, we find that six 2,6dichloroisoniticate ligands of uranyl exhibit no good coplanarity but a bend out of one plane (Figure 2c).

In U-Ag-2,6-DCPCA, the uranyl building unit coordinated entirely with six carboxylate groups enables it to further interact with Ag+ ions from four different directions (Figure 3a) and finally form a two-dimensional (2D) (4,4)-connected network with a cavity of 31.65 Å × 24.27 Å (Figure 3b). Structural analysis indicates that this special heterometallic network can interpenetrate each other so as to fill the huge cavity space of the 2D network and afford a 3-fold interpenetrating structure (Figure 3c,d). In fact, following the pioneering work on uranylE

DOI: 10.1021/acs.inorgchem.5b01988 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry organic interpenetrating networks by Cahill et al.,39 Jacobson et al.,23 Sun et al.,38 and Chai et al.37 reported similar interpenetrating networks in parallel interpenetration type. Recently, Wang et al.15 and Thuery et al.40 both reported a rare case of inclined interpenetration (also named as polycatenation) from aromatic tricarboxylic acids. However, multimetallic uranyl-organic interpenetrating structures are still lacking due to the difficulty of assembling orderly different metal centers in one backbone. The 3-fold interpenetrating U-Ag-2,6-DCPCA obtained here represents the first heterometallic interpenetrating network or framework among all the uranyl-containing materials. Interestingly, halogen−halogen interactions can be found between adjacent interpenetrating networks (Figure 4a). The Cl−Cl distance of ∼3.05 Å between Cl(5) and Cl(10′) (Figure 4b) is shorter than the sum of van der Waals radii (ca. 3.6 Å), suggesting strong halogen−halogen interaction. The approximately equal C−Cl···Cl angles (θ1 and θ2) are both close to 180°, indicating it could be of type-I.56,57 Another two halogen−halogen interactions (Cl(5)−Cl(6′), 3.68 Å, Cl(9)− Cl(10′), 3.56 Å) of type-II have been also found but are much weaker than the former one (Figure 4b). The simultaneous participation of chlorine-based halogen−halogen interactions as well as coordination bonds from carboxylate group and nitrogen atom enable the 2,6-dichloroisoniconate to serve as a multifunctionalized linker in the 3-fold interpenetrating network (Figure 4c). It is notable that the chlorine-based halogen−halogen interactions should be an important characteristic of heterometallic uranyl-silver interpenetrating network in close relationship with halogen-substitution of the bifunctional isoniconate ligand. Besides the typical halogen−halogen interaction between adjacent layers of a set of uranyl-silver heterometallic 3-fold interpenetrating network, there are also hydrogen bonds between different sets of 2D networks via O(1W) and H(20) along the c axis. This type of hydrogen bond contributes to the crystal packing of 2D networks in crystal lattice, though it is very weak. Characterization of U-Ag-2,6-DCPCA by Powder X-ray Diffraction (PXRD), Thermogravimetric Analysis (TGA), and Fluorescence Spectroscopy. The PXRD pattern of yellow-green crystals obtained from the hydrothermal reaction of uranyl nitrate hexahydrate, silver nitrate, and 2,6-dichloroisonicotinic acid is in good accordance with the simulated spectra from single crystal data of U-Ag-2,6-DCPCA (Figure S3), indicating its high purity as only one phase. Thermogravimetric results (Figure S4) do not display obvious weight loss until the temperature goes up to ∼300 °C, demonstrating relatively high thermal stability of the 2D 3-fold interpenetrating network. Fluorescence spectrum shows a typcial set of “five-finger” peaks of uranyl species, but a slight red shift of these peaks can be found compared to those of U-2,6DCPCA and uranyl nitrate hexahydrate, which might be attributed to their different uranyl coordination modes (Figure 5). Combined Influences of Structural Halogenation and Silver Ion Additive on Uranyl Coordination. The initial motivation for the incorporation of additional 3d or 4f metal ions into the system of uranyl and isonicotinate derivatives is to use these ions as cross-linking nodes by binding to the uncoordinated N-donors of pyridinyl groups. However, it is interesting to find out that the Ag+ ion not only serves as crosslinking node but also affects the coordination mode of uranyl center and makes it undergo a structural change from a uranyl

Figure 5. Fluorescence spectrum of U-Ag-2,6-DCPCA using those of U-2,6-DCPCA and uranyl nitrate as a control.

dimer with four isonicotinate ligands in U-2,6-DCPCA to another dimer with a total of six isonicotinate ligands in U-Ag2,6-DCPCA. Hence, the addition of Ag+ ion here in this hydrothermal system induces a fascinating 3-fold interpenetrating heterometallic uranyl-organic network. Obviously, a (4,4)-connected uranyl building unit, which is available in U-Ag-2,6-DCPCA through the interesting induction effect of Ag+ ion, plays a vital role in the formation of the 3-fold interpenetrating network. As a basic skeleton of interpenetrating network, the formation of this special uranyl motif largely depends on the coordination behevior of organic ligand. Hence, besides Ag+ ion as an additive, the structural factor of organic ligands may also affect the formation of U-Ag2,6-DCPCA. For example, in consideration of the structural feature of halogenation for the 2,6-dichloroisonicotinate ligand, the effect of halogen atom was first taken into account. We compared both the uranyl compound and uranyl-silver compound from the 2,6-dichloroisonicotinate ligand system to those from the isonicotinate system (Figure 6). Unlike the structural change of uranyl coordination found in the halogenated isonicotinate system, the Ag+ ion just replaces the proton of pyridinium and coordinates to the deprotonated pyridine, without any effect on the uranyl unit in the isonicotinate system.25 This disparity might be related to their different coordination modes with uranyl: isonicotinate ligand gives 1D infinite chain with uranyl centers fully coordinated by hydroxyl and carboxylate groups (UICP25), while halogenated isonicotinate ligand gives isolated carboxylate-bridged dimers with part of uranyl coordination sites occupied by weakly bonded water, which affords the possibility of replacement by other ligands. Moreover, as demonstrated above, halogen-based halogen bonds play a role in the stabilization of 3-fold interpenetrating networks. Therefore, halogen substitution changes the coordination trend of the isonicotinate ligand and exerts influence on the alteration of uranyl coordination by the Ag+ ion. In terms of the discussion, it is the combined action of structural halogenation and silver ion additive that tunes the uranyl coordination modes and leads to a 3-fold interpenetrating heterometallic uranyl-organic network. Discussion on the Possible Formation Pathway of UAg-2,6-DCPCA. As demonstrated above, the alteration of uranyl coordination should be attributed to a synergistic effect of structural halogenation and Ag+ ion additive. Another important question is how the impact of Ag+ ion occurs. F

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Figure 6. Uranyl compound and uranyl-silver compound display similar uranyl units for the isonicotinate system,25 while those for the 2,6dichloroisonicotinate ligand system26 are different, due to a significant structural change resulting from ligand replacement.

dichloroisonicotinate ligand prior to uranyl coordination seems not feasible. On the basis of the discussion above, a second possible pathway is proposed as that Ag+ ion serves as an inductive factor of uranyl species so as to alter uranyl coordination. According to the molecular structures of uranyl building units in U-2,6-DCPCA and U-Ag-2,6-DCPCA, two possible intermediates of uranyl dimers (U-4L-2H2O and U-6L, L = 2,6-DCPCA) were offered. Both of the structures of U-4L2H2O and U-6L were optimized at the B3LYP/6-31+G(d) level of theory (Figure 8), and the analysis of bonding feature including Mayer bond order, the electron density at bond critical point (BCP), the corresponding Laplacian, and the canonical valence molecular orbitals (MOs) was also performed. The selected experimental (data from crystal structure) and calculated (data from theoretical optimization) U−O bond lengths are listed in Table 2. It can be clearly seen that the calculated U−O bond length are in good accordance with the molecular segments of experimental structures in crystals except two U1−O5 and U2−O14 bonds with the largest deviation of 0.15 Å. This deviation should be attributed to possible weak interactions of coordinated water or carboxylate group corresponding to O5 and O14 in crystal, which were not considered in the theoretical case. Moreover, it is interesting that the intermediate U-6L optimized retains a molecular structure with a noncoplanar geometry, although nitrogenbinding Ag+ ions are not taken into account, indicating the crowned coordination sphere of uranyl after the replacement of water by another two 2,6-dichloroisoniticate ligands to be the origin of noncoplanarity for the uranyl dimer in U-Ag-2,6DCPCA. These geometrical results indicate that the method we used here is favorable for studying this molecular system. In terms of the U−O bonds for U-4L-2H2O and U-6L, the axial

Considering the replacement of water molecules by dichloroisonicotinate in U-Ag-2,6-DCPCA, we first speculate that the impact exerted by the Ag+ ion is achieved by the enhancement of uranyl coordination capacity of the dichloroisonicotinate ligand over water molecules after binding to the additional Ag+ via nitrogen donors. Therefore, the coordination capacity of 2,6-DCPCA (L) before and after binding to the Ag+ ion was assessed by DFT calculation, where two possible binding modes between the Ag+ ion and dichloroisonicotinate ligand were taken into account. Figure 7 shows the natural charge on

Figure 7. Natural charges on the oxygen atoms of water molecule, isolated ligand, L-Ag-L, and L-Ag(H2O)-L (L = 2,6-DCPCA).

the oxygen atoms of water molecule, isolated ligand, L-Ag-L and L-Ag(H2O)-L acquired from the natural bonding analysis (NBO). Although the QO value of the isolated ligand water is expectedly smaller than that of water molecule, the natural charge seems not to be improved after coordination with Ag+ ions, as seen from QO values the of L-Ag-L and L-Ag(H2O)-L. It means that the impact of Ag+ ion through binding to G

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Figure 8. Proposed model of two possible intermediates of uranyl dimers, U-4L-2H2O and U-6L (L = 2,6-DCPCA), according to the molecular structures of uranyl building units in U-2,6-DCPCA and U-Ag-2,6-DCPCA.

Figure 9. Diagrams of canonical valence MOs of U-4L-2H2O (a) and U-6L (b) (L = 2,6-DCPCA). The numbers denote the sequence of the MOs.

which suggests weaker binding affinity of water than that of 2,6DCPCA in this geometry when coordinated with the uranyl ion. The significant difference between U-4L-2H2O and U-6L may be related closely with different origins of the coordination oxygen atoms: the oxygen atoms come from two water molecules and four 2,6-dichloroisoniticate ligands for U-4L2H2O, while the oxygen atoms come from a whole of six 2,6dichloroisoniticate ligands for U-6L. The result indicates that

U−O bonds for the two structures are similar (about 1.76−1.77 Å), whereas the equatorial ones are more or less different. Most of the U−O bond lengths of U-4L-2H2O are slightly shorter (0.02−0.07 Å) than U-6L (for the same position on the uranyl complexes). Two exceptions are also found: U1−O5 and U2− O14 bond distances in U-4L-2H2O are about 0.16 Å longer than those of U-6L. The same phenomenon can be seen in the crystal structures of U-2,6-DCPCA and U-Ag-2,6-DCPCA, H

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Figure 10. A proposed formation pathway concerned with an Ag+-mediated equilibrium: in the absence of Ag+, 2,6-DCPCA intends to form energetically favorable U-4L-2H2O and finally obtains compound U-2,6-DCPCA, while the Ag+ ion present together with uranyl ion will be more inclined to coordinate with U-6L with more binding sites and promote the reaction equilibrium to U-6L.

9b denotes the interactions between the uranyl ions and the ligand for the crystal U-6L. On the basis of the optimization results of U-4L-2H2O and U-6L, we make a further comparison between the stability of these two possible intermediates. They were proposed to be formed through two thermodynamic reactions following two equations:

the binding abilities of the urany ions with the ligand are sensitive to the coordination environment. The Mayer bond order, the electron density at bond critical point (BCP), and the corresponding Laplacian were analyzed to explore the bonding nature of U-4L-2H2O and U-6L (Table 2). It is important to point out that the Mayer bond order for the UO bond is about 2.00, remaining of double bond character. The values of Mayer bond order for U−OL are scope with 0.35−0.43, which reveal that the U−OL bonds have ionic character. As to the U−Ow bond in the structure U-4L-2H2O, the values of Mayer bond decrease to 0.115, which suggests that the affinity of uranyl ion between the 2,6-DCPCA ligand is stronger than that of water molecule. These results are in accordance with the corresponding bond length analysis. The bonding natures of the U−O bonds are also revealed by the topological analysis of the electron density. As we known, the bonding interactions can be characterized and classified according to the properties of the electron density (ρ). In general, the electron density at BCP ρ(r) > 0.20 au describe a covalent bond, while ρ(r) < 0.10 au is for an ionic bond. The electron density of UO are over about 0.30, indicating that the UO bonds are covalent bonds. As for the other bonds connected with uranium atom, their electron densities are lower 0.072, which reveals that the interactions between uranyl ions and ligand as well as water molecule are predominant ionic bond character. In order to get more insights about the interaction between uranyl ion and organic ligand, the canonical valence molecular orbitals (MOs) relevant to the uranyl ion and ligands are provided in Figure 9. It is worth mentioning all the MOs shown here possess σ characters and are mainly contributed by the p atomic orbital of the oxygen atom of the ligand/water and the f and d orbitals of uranium atom. It is clearly seen for the crystal U-4L-2H2O in Figure 9a that the MO(269) domains the interaction between the uranyl ions and the water molecules, and the others represent the electron density overlap between the uranyl ions and the ligand. In addition, the MOs in Figure

2[UO2 (H 2O)5 ]2 + + 4L = U‐4L‐2H 2O + 8H 2O + 4H+ a

ΔG = −342.16 kcal/mol

(1)

2[UO2 (H 2O)5 ]2 + + 6L = U‐6L + 10H 2O + 4H+ a

ΔG = −284.53 kcal/mol

(2)

U‐4L‐2H 2O + 2L = U‐6L + 2H 2O ΔG = 57.63 kcal/mol

(3)

a

The Gibbs free energy of H+ ion uses the experimental value of −263.982 kcal/mol.58 The change of the Gibbs free energies (ΔG) reveals that U4L-2H2O is more stable than U-6L. It suggests that the intermediate U-4L-2H2O is favorable to form in the absence of Ag+ ions, which excellently explains that only U-2,6-DCPCA can be obtained without the Ag+ ions. On the other hand, we notice that U-6L exhibits more binding sites of Ag+ ions (due to inertness of bridged carboxylate groups, U-4L-2H2O has only two binding sites, but U-6L has four). So when simultaneously adding Ag + ions into uranyl and H-2,6-DCPCA, the coordination of Ag+ and U-6L will be preferred and only UAg-2,6-DCPCA compound can be accessible. It means that the Ag−N binding energy between U-6L and Ag+ ions maybe make up the disadvantage of ΔG in the formation of U-6L. Energy decomposition analysis using ADF program affords the Ag−N binding energy (200.27 kcal/mol for -N-Ag-N-, 178.22 kcal/ mol for -N-Ag(H2O)-N-), both of which are largely over the energy difference (57.63 kcal/mol) between U-4L-2H2O and I

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Inorganic Chemistry U-6L. These results suggest that the Ag−N bond is the driving force for the formation of the U-6L intermediate and the U-Ag2,6-DCPCA compound in the solution contained Ag+ ions. According to the calculation Results and Discussion above, a proposed formation pathway concerned with an Ag+-mediated equilibrium was given as followed (Figure 10): in the absence of Ag+, 2,6-DCPCA intends to react with uranyl to form energetically favorable U-4L-2H2O and finally obtains compound U-2,6-DCPCA; when Ag+ ion is present together with uranyl ion, it will be more inclined to coordinate with U-6L with more binding sites and promote the reaction equilibrium to U-6L, subsequently forming the 3-fold interpenetrating heterometallic uranyl-organic network, U-Ag-2,6-DCPCA. Verification of the Proposed Formation Pathway and Postsynthetic Transformation. According to the above formation pathway, we attempt to use 2,6-dibromoisonicotinic acid to replace 2,6-dichloroisonicotinic acid or other metal ions (Cu2+, Zn2+, Co2+, and Ni2+) to replace the Ag+ ion. All these hydrothermal systems were conducted under the condition similar to U-Ag-2,6-DCPCA. First, when the dichloro ligand was changed for the corresponding dibromo ligand, only uranyl-isonicotinate (U-2,6-DBPCA, 2,6-DBPCA = 2,6-dibromoisonicotinate) was finally obtained, even for the case of the Ag+ ion. It might be related to larger steric hindrance of bromo atoms (atomic radius: Br, 115 pm; Cl, 100 pm) around the nitrogen atom in the isonicotinate ligand, preventing Ag+ coordination to nitrogen donors. Then, we attempt to employ other similar metal ions such as Cu2+, Zn2+, Co2+, and Ni2+ to replace Ag+ in the same hydrothermal system. Surprisingly, only copper-isonicotinate (Supporting Information, Tables S2 and S3 and Figure S5) or uranyl-isonicotinate (U-2,6-DCPCA) was obtained. Considering the relatively smaller ionic radii59 of these metal ions (Cu2+: 87 pm, Zn2+: 88 pm Co2+: 89 pm; Ni2+: 83 pm) than that of the Ag+ ion (129 pm), single ions of them should get access to the nitrogen atom of the isonicotinate ligand more easily. However, when these transition metal ions coordinate with two 2,6-dichloroisonicotinate ligands bearing two bulky chlorine atoms around nitrogen coordination sites, metal ions with smaller sizes always lead to significant “head to head” repulsion forces. This type of significant repulsion force will also make access to nitrogen donors of 2,6-DCPCA more difficult. Hence, the failure to prepare a similar heterometallic uranyl compound from other metal ions (Cu2+, Zn2+, Co2+, and Ni2+) with smaller sizes is probably due to bulky chlorine atoms around nitrogen donors, which leads to their poor access to the nitrogen donors of 2,6-DCPCA. In all, these results above are inconsistent with the formation pathway proposed above, indicating the important role of coordination bonds between metal ions (such as Ag+) and icotinate ligands in promoting the equilibrium reaction to the direction of final heterometallic uranyl compounds. Both the experimental and calculated results suggest the addition of Ag+ ion into the hydrothermal system of uranyl and 2,6-dichloroisonicotinic acid plays an important role in tuning the uranyl coordination sphere through its inductive effect. We are wondering if this induction effect is still working when adding the Ag+ ion into the hydrothermal system with established U-2,6-DCPCA. Interestingly, it was found that, in the presence of the Ag+ ion, U-2,6-DCPCA can transform into a different product, which gives a powder X-ray diffraction pattern identical to U-Ag-2,6-DCPCA (Figure 11). Crystallographic analysis reveals it to be indeed U-Ag-2,6-DCPCA. A previous work dealing with postsynthetic rearrangement/

Figure 11. Powder X-ray diffraction for the hydrothermal transformation product of U-2,6-DCPCA in the presence of the Ag+ ion (U-2,6-DCPCA + Ag+, middle), which is different from that of the reagent (U-2,6-DCPCA, top) identical to that of U-Ag-2,6-DCPCA (bottom).

metalation from UICP with isonicotinate ligand was reported by Cahill et al. Similar to the case of UICP and UICP-Ag, the postsynthetic product of U-2,6-DCPCA through the Ag+ ion here is the same to that prepared directly from uranyl and organic ligand, that is, U-Ag-2,6-DCPCA. However, the postsynthetic transformation pathways for UICP and U-2,6DCPCA are remarkably different: the transformation from UICP to UICP-Ag only undergoes a simple deprotonation and Ag+ coordination, whereas the transformation from U-2,6DCPCA to U-Ag-2,6-DCPCA needs a rearrangement of uranyl building unit and a subsequent coordination of Ag+ ion due to structural discrepancy between U-2,6-DCPCA and U-Ag-2,6DCPCA. In regard to the possible formation pathway of this postsynthetic transformation, it should undergo a direct change of uranyl units from U-4L-2H2O to U-6L. Again, it is the Ag+ ion that promotes this energy-unfavorable process (ΔG = 57.63 kcal/mol, see eq 3). The possible transformation process maybe is similar to the Ag+-induced formation pathway mentioned above, only with the starting material (U-2,6DCPCA) undergoing a dissolution process under hydrothermal conditions and a subsequent precipitation along with the Ag+ ion in the form of U-Ag-2,6-DCPCA (Figure S6). From this aspect, this process is more like the situation of the copper ion induced metalation process, which underwent a dramatic rearrangement of uranyl uints.

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CONCLUSIONS We report a unique case of a uranyl-silver heterometallic 3-fold interpenetrating network, U-Ag-2,6-DCPCA, from 2,6-dichloroisonicotinic acid. Our finding here represents the first heterometallic uranyl-organic interpenetrating network or framework. The alteration of uranyl coordination modes to a four-connected uranyl dimer by the combined action of structural halogenation and silver ion additive plays a vital role in constructing this heterometallic uranyl-organic network. Halogen substitution changes the coordination trend of the isonicotinate ligand and exerts influence on the stabilization of 3-fold interpenetrating networks by halogen−halogen interactions. Both the experimental and calculated results suggest the Ag+ ion mainly serves as an inductive factor of uranyl species so as to regulate uranyl coordination. Therefore, our work here enriches the family of actinide materials with J

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fascinating structures and functions, but also affords a good method of tuning structures of actinide compounds by exogenous factors.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01988. Typical figures including powder X-ray diffraction and thermogravimetric analysis for U-Ag-2,6-DCPCA, figure of crystal structure and table of selected bond distances and angles for Cu-2,6-DCPCA are included (PDF) Crystallographic information file (CIF)

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AUTHOR INFORMATION

Corresponding Author

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

L.M. and Q.-y.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of this work by the Major Research Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of the Natural Science Foundation of China (91426302 and 91326202) and the National Natural Science Foundation of China (11405186 and 11275219), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA030104), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We appreciate the help from the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences for providing Gaussian 09 program. We appreciate the help from Prof. Daofeng Sun for X-ray single crystal measurements.

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L

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