Interpenetrated Uranyl–Organic Frameworks with bor and pts

Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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Interpenetrated Uranyl−Organic Frameworks with bor and pts Topology: Structure, Spectroscopy, and Computation Chao Liu,†,‡,⊥ Fang-Yuan Chen,§,⊥ Hong-Rui Tian,† Jing Ai,† Weiting Yang,† 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, 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, P. R. China ∥ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: Two novel three-dimensional interpenetrated uranyl−organic frameworks, (NH4)4[(UO2)4(L1)3]·6H2O (1) and [(UO2)2(H2O)2L2]·2H2O (2), where L1 = tetrakis(3-carboxyphenyl)silicon and L2 = tetrakis(4-carboxyphenyl)silicon, were synthesized by a combination of two isomeric tetrahedral silicon-centered ligands with 3-connected triangular [(UO2)(COO)3]− and 4-connected dinuclear [(UO2)2(COO)4] units, respectively. Structural analyses indicate that 1 possesses a 2-fold interpenetrating anion bor network, while 2 exhibits a 3-fold interpenetrated 4,4-connected neutral network with pts topology. Both compounds were characterized by thermogravimetric analysis and IR, UV−vis, and photoluminescence spectroscopy. A relativistic density functional theory (DFT) investigation on 10 model compounds of 1 and 2 shows good agreement of the structural parameters, stretching vibrational frequencies, and absorption with experimental results; the time-dependent DFT calculations unravel that low-energy absorption bands originate from ligand-to-uranium charge transfer.

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INTRODUCTION Over the past decade, uranium coordination chemistry is continuously of interest because of their rich structural diversities and fascinating physical and chemical properties, especially their tremendous importance in the nuclear fuel cycle.1−7 However, in contrast to the burgeoning of metal− organic frameworks (MOFs) based on main-group elements, transition metals, and lanthanides,8 uranyl−organic frameworks (UOFs) still remain less developed. Uranium possesses various oxidation states from 2+ to 6+.9 Among them, UVI is the most prevalent in nature and features a special form of a linear uranyl dication (UO22+) with two kinetically inactive oxo atoms seated in the axis. Thus, ligands tend to coordinate to the uranyl center in the equatorial plane, for instance, in the manner of involving four to six donor atoms to form square, pentagonal, and hexagonal-bipyramidal geometries.10−12 Because of characteristic coordination environments of [OUO]2+, uranyl crystalline materials favor the formation of low-dimensional structures, whereas three-dimensional (3D) uranyl frameworks are relatively rare.13−18 Nonetheless, 3D uranyl compounds usually exhibit thermal stability superior to that of lowdimensional analogues and promising properties such as porous adsorption,19 photoelectronic effects,20 nonlinearoptical properties,21 and ionizing radiation detection.22 © XXXX American Chemical Society

We have been interested in the preparation of silicon-based connecting units for the construction of 3D UOFs with fascinating topological structures. Compared to their carbon analogues, the increased bond-angle flexibility, decreased conformational rigidity, and longer bond lengths to silicon have been shown to contribute to the formation of coordination polymers with novel topological and structural forms.23 To date, there are only a handful of rigid tetrahedral carboxylate connectors known in the literature for the construction of MOFs, but related studies are rare because of their difficulty and low yield often in preparations. This contrasts markedly with the very large number of MOF studies involving linear/bent organic dicarboxylates and trigonalplanar/pyramidal organic tricarboxylates.24 Despite this, tetrahedral connectors remain of high interest because their inherent 3D nature is expected to assist in the formation of novel 3D-networked MOFs. In this study, we chose two isomeric Td-symmetrical 4-connected tetrahedral silicon-centered ligands, tetrakis(3-carboxyphenyl)silicon and tetrakis(4carboxyphenyl)silicon, as the construction agents. It should be Received: September 4, 2017

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DOI: 10.1021/acs.inorgchem.7b02274 Inorg. Chem. XXXX, XXX, XXX−XXX

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μL), and deionized water (1.5 mL) was loaded into a 20 mL Teflonlined stainless steel autoclave. The autoclave was sealed and heated at 180 °C for 3 days and then cooled to room temperature. The solution pH was 3.2 before the reaction and 2.7 at the end. Yellow blocklike crystals were isolated after being washed with deionized water and allowed to air-dry at room temperature (yield 32 mg, 58% based on uranium) Anal. Calcd for C28H24O16SiU2: C, 30.01; H, 2.16. Found: C, 30.08; H, 2.18. X-ray Crystal Structure Determination. Suitable single crystals for title compounds were selected for single-crystal XRD analyses. Crystallographic data were collected at 296 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. Structures were solved using direct methods (SHELXT and Olex2) and then refined using SHELXL-2014 and Olex2 to convergence.29 All hydrogen atoms were placed by geometrical considerations with isotropic displacement parameters equal to 1.2 times those of the parent atoms 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 Table S1. Crystallographic data are given as CCDC 1578616 and 1570425.

reasonable to construct highly symmetrical 3,4- or 4,4connected UOFs with novel topological structures. Herein, two novel 3D interpenetrated uranyl−organic coordination polymers, (NH4)4[(UO2)4(L1)3]·6H2O (1) and [(UO2)2(H2O)2L2]·2H2O (2), have been successfully obtained via the utilization of uranyl ions and Td-symmetrical tetrahedral ligands (Scheme 1). Note that, although interpenetrated UOFs Scheme 1. Rigid Polycarboxylate Ligands with a Tetrahedral Silicon-Centered Linker

have been studied in several cases,25,26 interpenetrated UOFs with high connectivity, such as 3,4- or 4,4-connected UOFs, are still scarce.27 Structural analyses indicate that 1 features 2-fold parallel interpenetration with 3,4-connected bor network topology, while 2 possesses a 4,4-connected network with pts topology and comprises three sets of identical singlet networks. Their syntheses, structures, spectroscopies, and computational simulations have been described in detail.

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Table 1. Crystallographic Data and Structure Refinement Parameters for 1 and 2 compound empirical formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z F(000) ρcalcd (g cm−3) μ(Mo Kα) (mm−1) R1/wR2 [I > 2σ(I)]a R1/wR2 (all data)

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. Zinc uranyl acetate (99.8%) and NH3·H2O (25%) were obtained from Sinpharm Chemical Reagent Co. Ltd. Tetrakis(3-carboxyphenyl) silicon and tetrakis(4-carboxyphenyl)silicon were synthesized according to a previously reported procedure.28 Powder X-ray diffraction (XRD) data were collected on a D8 Focus (Bruker) diffractometer at 40 kV and 30 mA with monochromated Cu Kα radiation (λ = 1.5405 Å) with a scan speed of 5°/min and a step size of 0.02° in 2θ. Elemental analyses of carbon, hydrogen, and nitrogen were conducted on a PerkinElmer 2400 elemental analyzer. IR spectra were collected using a Nicolet 6700 Fourier transform infrared spectrometer with a Diamond ATR objective. The photoluminescence (PL) spectra were recorded with a F-7000 luminescence spectrometer equipped with a xenon lamp of 450 W as an excitation light source. Solid-state UV−vis absorption measurement was performed using a Hitachi U-4100 spectrophotometer. The N2 absorption measurement was performed on ASAP 2020 and Autosorb MP-1 apparatuses. The residual solvent of the as-synthesized 1 was exchanged sequentially with fresh anhydrous ethanol. The resulting exchanged sample was evacuated (10−3 Torr) successively at room temperature and 120 °C. (NH4)4[(UO2)4(L1)3]·6H2O (1). A mixture of Zn(UO2)2(OAc)6·7H2O (50 mg, 0.05 mmol), tetrakis(3-carboxyphenyl)silicon (26 mg, 0.05 mmol), NH3·H2O (25%, 40 μL), and deionized water (1.5 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. The solution pH was 3.7 before the reaction and 3.0 at the end. A minor yellow blocklike product was isolated (yield 8 mg, 12% based on uranium) with a major yellow powder. Anal. Calcd for C84H66N4O33Si3U4: C, 37.42; N, 2.07; H, 2.46. Found: C, 38.03; N, 2.47; H, 2.56. Synthesis of [(UO2)2(H2O)2L2]·2H2O (2). A mixture of Zn(UO2)2(OAc)6·7H2O (50 mg, 0.05 mmol), tetrakis(4-carboxyphenyl)silicon (26 mg, 0.05 mmol), tetraethylammonium hydroxide (25%, 30

1 C84H76N4O38Si3U4 2785.87 trigonal R3̅c 26.8972(10) 26.8972(10) 64.009(6) 90 90 120 40104(5) 12 15528 1.370 4.920 0.0376/0.1020 0.0545/0.1068

2 C28H24O16SiU2 1120.62 monoclinic C2/c 24.871(4) 6.4533(10) 22.710(4) 90 119.612(3) 90 3168.9(9) 4 2072 2.349 10.321 0.0263/0.0588 0.0351/0.0620

R1 = ∑(ΔF/∑Fo); wR2 = {∑[w(Fo2 − Fc2)]}/∑[w(Fo2)2]1/2, where w = 1/σ2(Fo2).

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RESULTS AND DISCUSSION Syntheses. Both UOFs were prepared using Zn(UO2)2(OAc)6·7H2O as the uranium source. Replacement of zinc uranyl acetate with uranyl nitrate in the procedure used to synthesize those compounds affords an unidentified powder instead. Note that, although the zinc(2+) ion was not coordinated to the networks, the slow release of uranyl ions by zinc uranyl acetate is crucial for the formation of those compounds. The addition of aqueous ammonia is essential for the synthesis of 1 because it acts as a mineralizing agent in the reaction process, and the protonated ammonium ion acts as charge compensator in the structure of 1. In addition, synthesis of the two compounds strongly depends on the reaction temperature, which required at least 160 °C or higher. Structure of Compound 1. This compound crystallizes in the trigonal R3̅c space group, which is uncommon for uranyl compounds. The asymmetric unit consists of two uranium sites, B

DOI: 10.1021/acs.inorgchem.7b02274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry one L1 ligand, three NH4+ sites, and two water molecule sites (Figure 1). U1 resides in the inversion axis with one-third

Figure 2. (a) Schematic representation of 3- and 4-connected nodes in 1. (b) Simplified network connectivity showing the bor topology for 1. (c) Octahedral cage and packing mode in 1.

the octahedron, while the uranyl cations are located in the center of the faces (Figure 2b). Each octahedral cage is then linked with six neighboring ones by sharing common vertex to form a 3D framework (Figure 2c). This leads to a highly open single network of 1, which contains multidimensional equivalent channels with an aperture of 11.3 Å (Figure 3a). Therefore, to keep the stability of the whole framework, a 2fold-interpenetrating network is adopted, which forms a honeycomb-like structure (Figure 3b). Ammonium ions are located in the void space of the framework and balance the charge. Topological analyses indicate that 1 features a 3,4connected network with bor network topology (with the Schläfli symbol [{62.84}3{63}4]; Figure 4). This type of topology is common in transition-metal systems but still rare in uranyl−organic materials. Despite its interpenetrated structure, the free volume of 1 is still impressive because of the existence of octahedral cages. PLATON calculations indicate that the potential free volume in the framework is 19173.4 Å per unit cell, which is 46.4% of the crystal volume. The N2 sorption test of the desolvated 1 has been performed. A N2 uptake of 92.36 cm3 (STP) g−1 and a Brunauer−Emmett− Teller surface area of 198.83 m2 g−1 were observed (Figure S9). After the sorption test, the structural integrity of 1 was retained, which was confirmed by a powder XRD experiment (Figure S6). Structure of Compound 2. Single-crystal XRD studies on 2 reveal a 3D framework. It crystallizes in the monoclinic space group C2/c and contains one uranium atom, half of a L2 ligand, and one lattice water molecule in the asymmetric unit (Figure 1b). The uranium center is seven-coordinated by two axial −yl oxo atoms and five oxygen atoms in the equatorial plane,

Figure 1. ORTEP representations of the asymmetric units of 1 (a) and 2 (b). Thermal ellipsoids are drawn at the 30% probability level.

occupancy and has crystallographic disorder, and was split into two components (68:32 atomic site occupancy ratios; Figure S5). Its coordination environment is defined by two “yl” atoms and six equatorial oxygen atoms from the carboxylate groups of three L1 ligands, thus creating a hexagonal bipyramid. Comparatively, U2 is in a general position and also adopts a hexagonal-bipyramidal geometry. The axial UO bond lengths of the U2 atom are measured in the range of 1.736(4)− 1.753(3) Å. which are in good agreement with the typical bond lengths reported for uranium-containing materials.28 Within the equatorial plane, the U−O bond lengths of the U2 atom range from 2.418(10) to 2.477(12) Å, which are typical for an 8-foldcoordinated uranium(VI) cation. The silicon-based ligand binds four uranyl centers, with the four carboxylate groups adopting chelating modes. The central silicon atom of the ligand shows a tetrahedral geometry with an average bond angle of 109.5(5)°. As described above, the carboxylate group adopts a chelating coordination mode in 1. The ligand can be considered as a 4connected node to link four uranium(VI) centers, and each uranyl center is coordinated by three different ligands. The minimal closed loop of 1 consists of three uranyl units and three ligands (Figure S1). As shown in Figure 2, the connections between six silicon-centered tetrahedral ligands and four uranium(VI) centers result in an octahedron-shaped building unit. The silicon-centered tetrahedra occupy vertices of C

DOI: 10.1021/acs.inorgchem.7b02274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Simplified 2-fold-interpenetrating bor network of 1. The equivalent 2-fold-interpenetrated networks are depicted in red and blue.

Figure 3. (a) Singlet network of 1 with large channels. Hydrogen atoms are omitted for clarity. (b) Honeycomblike structure of 1 with a 2-fold-interpenetrating network.

thereby forming a common pentagonal bipyramid. Axially, the OUO angle is 176.95(18)°, and the UO lengths are 1.745(4) and 1.756(4) Å, respectively. Equatorially, four oxygen atoms are from the carboxylate groups of three different ligands, and the leaving one is donated by a water molecule. The oxygen atoms on the equatorial sphere are arranged from 2.289(4) to 2.472(4) Å. Two pentagonal coordination spheres are bridged and chelated by four carboxylates, forming a dinuclear [(UO2)2(COO)4] secondary building unit (SBU). Different from its isomer L1 ligand in 1, which only adopts chelating modes for the carboxylate groups, the carboxylate moieties of the L2 ligand in 2 bind uranium centers in two different coordination modes, a chelating mode binding one uranium atom and a bidentate mode bridging two uranium atoms. Similar to L1, the central silicon atom of L2 is also in a tetrahedral environment defined by four carbon atoms with an average bond angle of 109.4(7)°. The connection between the dinuclear SBUs and tetrahedral carboxylate ligands leads to a 3D framework structure of 2 (Figure 5a), in which the dinuclear SBU is linked with four

Figure 5. (a) Singlet network of 2 with large channels along the c axis. Hydrogen atoms are omitted for clarity. (b) 3-fold-interpenetrating network of 2 viewed along the c axis. The benzene rings are simplified as rods, and all of the hydrogen atoms are omitted for clarity.

different ligands and the organic block binds four different SBUs. As a result, both the inorganic and organic building units serve as 4-connecting nodes (Figure 6a). Then a 4,4-connected network with pts topology is simplified (Schläfli symbol of {42.84}; Figures 6b). The minimal closed loop of 2 consists of two dinuclear uranium units and two tetrahedral ligands. In D

DOI: 10.1021/acs.inorgchem.7b02274 Inorg. Chem. XXXX, XXX, XXX−XXX

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attributed to the different synthesis conditions for those two compounds. Compound 1 was synthesized in the presence of aqueous ammonia and contains an anionic framework that is charge-balanced by ammonium cations. In contrast, compound 2 was constructed using tetraethylammonium hydroxide as the organic template and features a neutral network. The UO22+ ions in 1 are in the hexagonal-bipyramidal coordination geometry, which can be simplified as D3h-symmetrical triangular SBUs and are useful in constructing UOFs with layered topological structures, such as the honeycomb 6,3-connected network, by linking the linear or triangular connectors. However, when a tetrahedral polycarboxylate linker replaces those linear or triangular linkers, layered networks do not occur in 1. As expected, a novel 3D 2-fold-interpenetrating framework with bor topology was formed through continuous symmetric connections of the D3h-symmetrical uranyl unit and the Tdsymmetrical tetrahedral ligand. In contrast, compound 2 was built from the UO22+ ion in pentagonal-bipyramidal geometry, and two pentagonal bipyramids are connected together to form a dinuclear [(UO2)2(COO)4] unit. Interestingly, this coordination style was also observed in the 3D triple-interlocked polythreading uranyl compound [(UO2)3(H2O)4L2]·6H2O [L = 4,4′,4′′-(phenylsilanetriyl)tribenzoic acid], synthesized by our group using a tripodal silicon-centered linker.25f However, despite these similarities, the connection between the dinuclear units and tetrahedral carboxylate ligands finally leads to a 3fold-interpenetrating pts framework of 2. The two assynthesized compounds demonstrate that the tetrahedral polycarboxylate ligands are a reasonable choice for the construction of high-dimensional uranyl materials with interesting topologies. Thermal Stability Measurement. Thermogravimetric analysis (TGA) was conducted to characterize the thermal stability of two compounds (Figure S8). For 1, an initial weight loss from the structure (∼9.0%) began just after 45 °C and was complete by 155 °C, consistent with the loss of solvent water and ammonium. The framework then began to become unstable for removal of the organic ligand and started to decompose at 350 °C. For 2, the TGA curve clearly shows three-step weight losses. The initial weight loss of 3.1% in the temperature range of 35−120 °C corresponds to 2 lattice water molecules per formula unit (calcd 3.21%). The second weight loss of 3.4% started at 170 °C and completed at 230 °C, which is ascribed to the 2 coordinated water molecules (calcd 3.21%). Then the framework is stable until 420 °C; after that, the structure starts to collapse. IR Spectra. Within the IR spectrum, the stretching vibrations of the O−H groups from the coordinated water and lattice water were detected around 3570 and 3450 cm−1 for 2 (Figure S11). The sharp peak around 2979 cm−1 in 1 is attributed to the phenyl C−H stretching mode (Figure S10). The bands at 1536, 1520, 1500, and 1470 cm−1 in 1 and 1678, 1537, and 1490 cm−1 in 2 are assigned to benzene skeleton vibrational modes. The peaks from the carboxylate ligand (νas and νs of −COO−) were displayed around 1587 and 1420 cm−1 in 1 and 1580, 1404, 1380, and 1360 cm−1 in 2. The bands located at 1047 cm−1 in 1 and 1096 cm−1 in 2 are dominated by the Si−C characteristic. The asymmetric and symmetric stretching peaks of ν(UO) were observed around 930 and 880 cm−1 in 1 and 933 and 860 cm−1 in 2.29 UV−Vis Spectroscopy. The diffuse-reflectance UV−vis spectra of 1 and 2 were also investigated. As shown in Figure S12, the spectra of both compounds are dominated by

Figure 6. (a) Schematic representation of 4-connected nodes in 2. (b) Simplified single network of 2 viewed along the c axis.

addition, a big loop also was observed, which is constituted by four dinuclear SBUs and four ligands (Figure S3). It is interesting that one 3D network penetrates through the other two from the big loop but not through the small loop at all (Figure S4). The single network of 2 possesses a large void space with the largest channel dimensions of approximately 8.8 × 8.8 Å along the c axis. To avoid the formation of such an open-framework structure, it forms a 3-fold-interpenetrating network, which is shown in Figures 5b and 7. Calculation by PLATON gives a free void volume ratio of 3.8%, which suggests that the void volume has been eliminated by the interpenetrating molecules. Compounds 1 and 2 both display fascinating 3D networks with different topologies resulting from the different coordination modes of the tetrahedral ligands. The distinct coordination environments of the UO22+ ion in the two compounds can be

Figure 7. Simplified 3-fold-interpenetrating PtS network of 2. The equivalent 3-fold-interpenetrated networks are depicted in green, blue, and red. E

DOI: 10.1021/acs.inorgchem.7b02274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry vibronically coupled charge-transfer bands in the region of 375−500 nm due to the uranyl unit. Eight well-resolved peaks at 385, 398, 409, 421, 433, 456, 471, and 488 nm were observed for 1, while there are four peaks at 420, 460, 475, and 492 nm for 2. The broad peaks in the region of 250−380 nm in the spectra of 1 and 2 may arise from the ligand-to-metal charge transfer (LMCT). The peak centered at 238 nm for 2 could involve the participation of more ligands. Luminescent Spectroscopy. Because of the symmetric and asymmetric vibrational modes of the nearly linear UO22+ cation, uranyl materials generally exhibit the charge-transferbased emission of green light between 520 and 530 nm. Thus, the PL properties of 1 and 2 were studied, in conjunction with the benchmark compound UO2(NO3)2·6H2O. As seen in Figure 8, 1 displays prototypical “five-finger” peaks of uranyl

Figure 8. Emission spectra of 1, 2, and UO2(NO3)2·6H2O.

Figure 9. DFT-optimized structures for various model complexes.

materials [478(w), 494(s), 515(s), 538(m), and 563(w) nm]. The characteristic emissions generally correspond to the electronic and vibronic transitions S11−S00 and S10−S0v (v = 0−4).30 Notably, characteristic peaks of 1 resemble those of UO2(NO3)2·6H2O, which shows well-resolved sharp emission peaks at 470(w), 488(s), 510(s), 533(m), and 558(w) nm but only displays about a 10 nm red shift in wavelength. In contrast, only three emission peaks, locating at 459(w), 472(w), and 520(s) nm, are resolved for 2. Apparently, many of the finestructure emissions are lost between 480 and 600 nm. The difference in the spectra of 1 and 2 may originate from the influence of the coordination environments defined by the carboxylate ligands.31−33 Relativistic DFT Computation. To simulate compounds 1 and 2, 10 molecular model compounds as shown in Figure 9 were examined using a relativistic DFT.34 See the computational details in the Supporting Information. Moieties around the L1 ligand of 1 and L2 of 2 were addressed in the model compounds, which would reasonably reflect the structural features of real compounds. Overall agreement has been obtained between the calculated and experimental data, within 0.05 Å for distances and 2° for angles (Table S3). The UO distances for model compounds 1_1U, 1_2U, 1_4U, and 1_4U-Ph were calculated to be 1.81 Å, which is longer than experimental value of 1.77 Å for 1. The calculated U−Oc bond lengths (2.50−2.51 Å) are slightly longer than the corresponding experimental ones (2.42−2.48 Å). A comparison finds that the UO bond lengths (1.80 Å) of the molecular model

compounds of 2 are close to the experimental values (1.75− 1.76 Å). The calculated U−Oc (2.40 Å, mean value) and U−Ow (2.58 Å) distances fall within the range of experimental values 2.29−2.30 and 2.42−2.47 Å. Oc and Ow correspond to the oxygen atoms in carboxylic acid and water, respectively. One can note the slight overestimation of the calculated results over the experimental data, which is related to the nature of the generalized gradient approximation functional. The uranyl ion remains linear, reflected by the calculated OUO angles within 177° and 180° (exptl = 180° for 1 and 177° for 2). In Figures 10 and S13−S17, we theoretically simulated IR vibrational spectra of four model compounds of 1 as well as 2_U1−U2 and 2_U1−U2 of 2 on the basis of frequency calculations. One can see that only a slight difference is found among the calculated results; what is more, they are comparable to corresponding experimental ones (Figures S10 and S11). This indicates that the choice of theoretical models has almost no effect on the IR spectra. Regarding 2_U1−U2 (Figure 10), its absorption peaks at 838 and 926 cm−1 are attributed to the symmetric and asymmetric UO stretching vibrational modes, respectively.35 This agrees with the experimentally obtained 860 and 933 cm−1 stretches for 2. In addition to the UO vibrations, the O−H stretching vibrations of water molecules that equatorially coordinate to uranyl ions display bands ranging from 3569 to 3734 cm−1, which are comparable to 3570 and 3450 cm−1 of 2 in the experiment. Characteristic peaks calculated at 3097−3142 and 2938 cm−1 are assigned to the phenyl and alkyl C−H stretching vibrational modes, F

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Figure 10. Theoretically calculated IR spectra of the model compound 2_U1−U2 (top and bottom). Note that the region between the wavelengths of 400 and 1400 cm−1 is magnified in the bottom, where the UO stretches (red numbers) are marked.

respectively. The carbonyl stretch related to the −COO− group was computed at 1780 cm−1, while the experimental measurement only finds two very weak absorption peaks around 1770 and 1829 cm−1. In Figure 11, we present diagrams of partial orbitals of 2_U1−U2 from the time-dependent DFT (TD-DFT) calculation. Notably, the energy levels of these frontier molecular orbitals involved in the absorption transition with an oscillator strength (100*f) larger than 0.16 are described. It is shown that low-lying unoccupied orbitals are primarily of U 5f character. The lowest unoccupied molecular orbital (LUMO) is contributed by U fδ character around the U2 structural unit. Above it, the fδ(U1) forms the L+2 (short for LUMO+2) orbital. In the relatively high-energy region, the fϕ of uraniums contributes to the L+4 and L+9 orbitals. Addtionally, both the L+3 and L+11 orbitals have fδ character, differing in the distribution of electron density around the U1 or U2 structural parts. High-lying occupied orbitals are mainly π(Ph) character, as shown in Figures 11 and S18. We did not observe the silicon-character orbitals in this energetic region (HOMO to H−9). The calculated electronic absorption spectra of 2_U1−U2 are simulated in Figure 12, compared with experimental solidstate ones. Six characteristic absorption bands were experimentally measured, for instance, peaks at 492, 475, 460, 420, 340−290, and 238 nm. The calculated results show similar general peaks but occur in relatively low-energy regions. The

Figure 11. Energy levels and diagrams of partial frontier molecular orbitals of 2_U1−U2, which are involved in the relatively more intense absorption transitions from the TD-DFT calculation.

association of Table S4 and Figures 11 and 12 attributes these absorptions to the π(Ph) → fδ(U) charge-transfer character. Similarly, general absorption patterns (Figure S19) were calculated for other model compounds including 2_2U1, 2_2U2, and 2_2U1−2U2. In addition, we also calculated model compounds of 1 for their absorption spectra. Unfortunately, large-molecule ones, 1_4U and 1_4U-Ph, failed in their self-consistent-field calculations. As seen in Figure S20, the results of 1_1U and 1_2U would describe the partial characteristics of the experimental absorption bands of 1 (Figure S12). These TD-DFT calculations also reveal that the low-energy absorption bands of 1 arise from LMCT character.

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CONCLUSION In conclusion, we have designed and synthesized two novel high-connected 3D interpenetrated UOFs by using two isomer tetrahedral Td-symmetrical silicon-centered linkers. Because of diverse coordination from ligand to uranium center, these two compounds possess different topological structures. Compound 1 features 2-fold interpenetration with bor topology. Compound 2 exhibits a 4,4-connected 3-fold-interpenetrated pts framework. A total of 10 model compounds were used to G

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*E-mail: [email protected]. Website: http://zhongmingsun. weebly.com/. ORCID

Qing-Jiang Pan: 0000-0003-2763-6976 Zhong-Ming Sun: 0000-0003-2894-6327 Author Contributions ⊥

These authors contributed equally.

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 21722106, 21571171, and U1407101), Jilin Province Youth Foundation (Grants 20130522132JH and 20130522123JH), and SRF for ROCS (State Education Ministry).

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Figure 12. Simulated absorption spectra of the model compound 2_U1−U2 under the TD-DFT calculations (bottom), compared with the solid-state one of 2 (top).

simulate real compounds 1 and 2, which show good agreement of their geometry parameters and vibrational stretches between computation and experiment. The experimentally measured absorption bands were assigned as ligand-to-uranium-center charge transfer.

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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.7b02274. Selected bond lengths and angles, optimized geometry parameters, calculated absorptions, structures and diagrams, XRD patterns, TGA curves, UV−vis, IR, and luminescent spectra, computational details, and Cartesian coordinates (PDF) Accession Codes

CCDC 1570425 and 1578616 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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