Semirigid Tripodal Ligand Based Uranyl Coordination Polymer

Apr 4, 2018 - Synopsis. A series of novel uranyl coordination polymer isomers based on a semirigid tripodal ligand were synthesized. They share a simi...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Semirigid Tripodal Ligand Based Uranyl Coordination Polymer Isomers Featuring 2D Honeycomb Nets Xiao-Lin Zhang,†,‡,∥ Kong-Qiu Hu,‡,∥ Lei Mei,‡ Yu-Bao Zhao,*,† Yi-Tong Wang,§ Zhi-Fang Chai,‡ and Wei-Qun Shi*,‡ †

School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China § China International Engineering Consulting Corporation, Beijing 100089, China ‡

S Supporting Information *

ABSTRACT: We report five novel uranyl coordination polymers, [(CH3)2NH2]UO2(BTPCA) (1), [(CH 3)2NH2]UO2(BTPCA) (2), [(CH3) 2NH 2]2 [UO2 (BTPCA)][UO2(BTPCA)]·(H2O)5.5 (3), [(CH3)2NH2]2(UO2)2(BTPCA)2·(H2O) 3 (4), and [(CH3)2NH2]UO2(BTPCA) (5), by the utilization of semirigid ligand 1,1′,1″-(benzene1,3,5-triyl)tripiperidine-4-carboxylic acid (H3BTPCA) and uranyl nitrate through solvothermal reactions. Single-crystal X-ray diffraction analysis reveals that the five compounds share a similar structure composition and local coordination mode to the exclusion of disordered water or DMF molecules. Each UO2(COO)3− motif is connected to six neighboring units through three BTPCA3− ligands, generating an infinite uranyl honeycomb (6, 3) net. The structures of all the five compounds consist of 2D honeycomb nets of various degrees of distortions, which are induced by the flexibility of piperidine rings. The dimethylamine cations and solvent molecules fill in the space between layers. Therefore, these five compounds are isomers in a broad sense. Notably, both compounds 3 and 4 possess 2-fold interpenetrated structures. For compound 5, the distance between the neighboring 2D honeycomb nets is 7.253 Å. This is the largest distance between the 2D honeycomb nets in uranyl-based coordination polymers, to the best of our knowledge. In addition, compounds 1, 2, and 4 are also characterized by infrared spectroscopy (IR), thermogravimetric analysis (TG), powder X-ray diffraction (PXRD), and luminescence properties.

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INTRODUCTION Over the past 20 years, the study of coordination polymers (CPs) has remained an active research field. An increasing number of chemists take part in the research of CPs, not only due to the versatile and disciplined architectures1−3 but also because of their potential applications in various scientific fields, involving magnetism,4 nonlinear optics,5 biomaterials,6 catalysis,7,8 luminescence sensors,9 ion exchange,10 gas storage and separation,11 and so on. Under such circumstances, a great quantity of coordination polymers have been synthesized and reported in the literature.12,13 Most of the assembled coordination polymers involve dblock transition metals, main group metals, or 4f lanthanide metals as structure units. As a latecomer, the synthesis and study of actinide-based coordination polymers have also caught increasing attention, benefiting from the flourish of nuclear chemistry in recent years.14−17 As the key element in the nuclear fuel cycle,18−21 uranium has been widely used in the synthesis of actinide-based CPs in light of its unique fission behavior (pure alpha emitting nature and long half-life) as well as its amazing structural and physicochemical properties.22−32 The triatomic species uranyl (UO22+) is the most stable species of uranium under the ambient environment. The two axial oxygen atoms force the additional ligands to coordinate in the © XXXX American Chemical Society

equatorial plane, often generating tetragonal, pentagonal, and hexagonal bipyramidal coordination geometries.6,31−41 It is worth mentioning that the three carboxylate group coordinated unit [UO2(RCOO)3]− can be regarded as the regular triangular secondary building unit (SBU), which can be used to construct specific topological structures containing a three-node unit.14,42,43 In our previous work, we reported two novel (3, 4)-connected UOFs (featuring ctn-type topology or bor-type topology respectively) by combining the uranyl carboxylate SBUs with tetrahedral Td symmetrical H4MTB (tetrakis(4carboxyphenyl)methane) organic linkers.34 Farha and coworkers obtained a water-stable UOF based on 4,4′,4″,4‴(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid (H4TBAPy) with pseudo 4-fold symmetry and featuring tbo-type topology.43 Besides the poly(carboxylic acid) ligands, the most commonly used ligands are di- or tricarboxylic acid ligands. It is expected that the triangular [UO2(RCOO)3]− SBUs can be bridged by the rigid linear or triangular linkers to form 2D honeycomb nets.44−48 However, when triangular SBUs are connected with the rigid carboxylate ligands, the resulting compounds lack the structural Received: January 22, 2018

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DOI: 10.1021/acs.inorgchem.8b00168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

IR measurements were performed on an infrared spectrometer with the machine model of Bruker Tensor 27 in the range 400−4000 cm−1. PXRD measurements were performed by a diffractometer (Bruker D8 Advance, Cu Kα radiation, λ = 1.5406 Å) from 5° to 50° with a step size of 0.02°. TGA data were obtained on a TA Q500 analyzer under an air atmosphere. The measured temperature is between 298 and 1073 K, and the heating rate is 5 K/min. Synthesis. All of the uranyl coordination polymers in this work were solvothermally synthesized under autogenous pressure using 10 mL Teflon-lined Parr-type autoclaves. [(CH3)2NH2]UO2(BTPCA) (1). UO2(NO3)2·6H2O (0.50 M, 60 μL), H 3 BTPCA (9.26 mg, 0.02 mmol), HNO 3 (4 M, 60 μL), dimethylformamide (DMF, 2 mL), and 1-octyl-3-methylimidazolium chloride (4.61 mg, 0.02 mmol) were placed into a 10 mL autoclave. The autoclave was sealed, heated in an oven to 150 °C for 72 h, and then cooled to ambient temperature. The yellow crystals of compound 1 were obtained. Yield: 3.6 mg. [(CH3)2NH2]UO2(BTPCA) (2). UO2(NO3)2·6H2O (0.50 M, 60 μL), H 3 BTPCA (9.26 mg, 0.02 mmol), HNO 3 (4 M, 60 μL), dimethylformamide (DMF, 0.5 mL), and ultrapure water (1.5 mL) were placed into a 10 mL autoclave. The autoclave was sealed, heated in an oven to 100 °C for 48 h, and then cooled to ambient temperature. The yellow crystals of compound 2 were obtained. Yield: 3 mg. [(CH3)2NH2]2[UO2(BTPCA)][UO2(BTPCA)]·(H2O)5.5 (3). UO2(NO3)2· 6H2O (0.50 M, 60 μL), H3BTPCA (9.26 mg, 0.02 mmol), HNO3 (4 M, 40 μL), and dimethylformamide (DMF, 2 mL) were loaded into a 10 mL autoclave. The autoclave was sealed, heated in an oven to 180 °C for 72 h, and then cooled to ambient temperature. The yellow crystals of compound 3 were obtained. Yield: 3.3 mg. [(CH3)2NH2]2(UO2)2(BTPCA)2·(H2O)3 (4). UO2(NO3)2·6H2O (0.50 M, 60 μL), H3BTPCA (9.26 mg, 0.02 mmol), HNO3 (4 M, 100 μL), dimethylformamide (DMF, 1.5 mL), and ultrapure water (0.5 mL) were loaded into a 10 mL autoclave. The autoclave was sealed, heated in an oven to 150 °C for 72 h, and then cooled to ambient temperature. The yellow crystals of compound 4 were obtained. Yield: 4 mg. [(CH3)2NH2]UO2(BTPCA) (5). UO2(NO3)2·6H2O (0.50 M, 60 μL), H 3 BTPCA (9.26 mg, 0.02 mmol), HNO 3 (4 M, 20 μL), dimethylformamide (DMF, 0.5 mL), and ultrapure water (1.5 mL) were loaded into a 10 mL autoclave. The autoclave was sealed, heated in an oven to 100 °C for 48 h, and then cooled to ambient temperature. The pale yellow crystals of compound 5 were obtained as well as a large number of impurities.

diversity. A reasonable strategy is introducing a semirigid ligand such as 1,1′,1″-(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid (H3BTPCA, Scheme 1). This semirigid tricarboxylic acid Scheme 1. Semirigid Tripodal Ligand H3BTPCA Used in This Work

ligand was first used to synthesize a zinc ion based MOF by Sun’s group.49 It was then diffusely used to coordinate with a lanthanide metal or transition metal to obtain MOFs.50−53 Nevertheless, this ligand has never been used to construct actinide-based coordination polymers, to the best of our knowledge. In this work, we report five uranyl-based coordination polymers with H3BTPCA as the sole ligand. It is interesting that the structure compositions and coordination modes of these five compounds are similar to the exclusion of the disordered solvent molecules. All of them consist of 2D honeycomb nets of various degrees of distortions, which are induced by the flexibility of piperidine rings in H3BTPCA ligands. These five compounds are isomers with a brief formula [(CH3)2NH2]UO2(BTPCA) from a broad perspective.

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EXPERIMENTAL SECTION

Materials and Methods. Caution! Because of the radioactive and chemically toxic nature of uranyl nitrate hexahydrate, UO2(NO3)2· 6H2O, suitable precautions for safety and protection must be taken. All the reagents and solvents used were purchased and used without further purification except for the H3BTPCA ligand, which was synthesized by following a literature report.49

Table 1. Crystal Parameters and Structure Refinement Results for Compounds 1−5 Structure Fw T/K space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g/cm3) μ (mm−1) Rint R1a[I > 2σ(I)] wR2b(all data) GOF on F2 a

1

2

3

4

5

C21H27O8N6U 729.51 170(2) Pc 14.850(3) 15.1481(11) 9.081(2) 90 105.51(2) 90 1968.3(7) 2 1.231 11.926 0.0464 0.0326 0.0736 1.031

C21H27O8N6U 729.51 170(2) P1̅ 9.0539(6) 11.6661(7) 14.8682(9) 100.760(2) 100.772(2) 99.577(2) 1482.31(16) 2 1.634 5.523 0.0555 0.0243 0.0675 1.041

C46H60O21.5N14U2 1629.14 170(2) P21/n 16.8743(6) 15.6576(8) 26.3284(10) 90 91.021(2) 90 6955.1(5) 4 1.556 13.638 0.0802 0.0565 0.1337 1.047

C46H64O19N14U2 1593.17 299(2) P1̅ 14.643(14) 14.836(12) 17.114(11) 67.435(17) 77.25(2) 61.39(3) 3011(4) 2 1.757 5.451 0.0756 0.0475 0.1093 1.054

C21H27O8N6U 729.51 170(2) P3 16.9057(3) 16.9057(3) 10.6735(3) 90 90 120 2641.82(12) 1 0.441 4.442 0.0450 0.0373 0.0924 1.051

R1 = ∑(Fo − Fc)/∑Fo. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. B

DOI: 10.1021/acs.inorgchem.8b00168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry X-ray Crystal Structure Determination. Single crystal X-ray data of compounds 1−5 were collected on a Bruker APEXII X-ray diffractometer equipped with a CMOS PHOTON 100 detector with a Mo Kα X-ray source (Kα = 0.71073 Å) or a Cu Kα X-ray source (Kα = 1.54178 Å). The indexing, integrating, and scaling of diffraction data were performed using DENZO and SCALEPACK from the HKL program suite (Otwinowski and Minor, 1997).54 The direct methods were used to solve the crystal structures, which is refined with fullmatrix least-squares on F2 using SHELXL-2014. All non-hydrogen atoms were refined anisotropically. For compound 3, the SQUEEZE routine of PLATON was used to remove the diffraction contribution from disordered solvents. 55 For compounds 1, 2, and 5, [(CH3)2NH2]+ cations can be found on the electron density map but are highly disordered. Therefore, the SQUEEZE routine of PLATON was used to remove the diffraction contribution from [(CH3)2NH2]+ cations and disordered solvents. For compound 5, due to the serious disorder present, the BTPCA3− ligand splits into two parts with a ratio of almost 1:1. Most hydrogen atoms cannot been added by the order “HADD” or “HFIX” due to the structure disorder. Details of crystal parameters and structure refinement for compounds 1−5 are summarized in Table 1. Important bond lengths and angles of compounds 1−5 are listed in Table 2.

Table 2. Important Bond Lengths (Å) and Angles (deg) Compound 1 U(1)− O(8) U(1)− O(1) U(1)− O(2)

1.760(11) 2.459(7) 2.458(9)

U(1)− O(7) U(1)− O(4) U(1)− O(6)

1.788(9)

U(1)−O(3)

2.450(7)

2.466(7)

U(1)−O(5)

2.433(8)

2.498(8)

O(8)− U(1)− O(7)

178.5(4)

Compound 2 U(1)− O(2) U(1)− O(5) U(1)− O(8)

1.771(3) 2.460(3) 2.466(3)

U(1)− O(1) U(1)− O(6) U(1)− O(3)

1.772(3)

U(1)−O(4)

2.449(3)

2.460(3)

U(1)−O(7)

2.468(3)

2.493(3)

O(2)− U(1)− O(1)

179.17(13)

Compound 3 U(1)− O(2) U(1)− O(6) U(1)− O(8) U(2)− O(4) U(2)− O(11) U(2)− O(12)

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RESULTS AND DISCUSSION Synthesis. Compounds 1−5 were synthesized under solvothermal conditions at different temperatures. We also tried to synthesize a uranyl compound based on the H3BTPCA ligand under hydrothermal conditions. However, we did not obtain any crystal except for amorphous precipitates, which may be attributed to the very low solubility of the H3BTPCA ligand in the water. Therefore, organic solvent DMF was introduced into the reaction system due to the good solubility of the H3BTPCA ligand in DMF. On the other hand, during the reaction process, the DMF will decompose and dimethylamine will be produced, which facilitates the deprotonation of the H3BTPCA ligand. In addition, the protonated dimethylamine cations balance the negative charges of main U-BTPCA frameworks. The acidity of the reaction system is also very important besides the solvents. Without the additional acid, the most frequent outcome of the reaction was the formation of yellow amorphous precipitates or the microcrystals which are not suitable for X-ray diffraction analysis. This phenomenon may be induced by the excessive decomposition of DMF and a large amount of dimethylamine decrease the acidity of the solution, which leads to the formation of uranyl oxide. Therefore, an appropriate amount of nitric acid was added to the reaction system. The additional acid can also inhibit uranyl hydrolysis and induce BTPCA3− ligands to coordinate with unhydrolyzed uranyl ions, which can be verified by the experiment results. The five obtained compounds are isomers and share a similar structure composition and local coordination mode to the exclusion of disordered solvent molecules. Each BTPCA3− ligand is connected to three unhydrolyzed uranyl ions, and each uranyl ion is connected to three BTPCA3− ligands. Crystal Structures. [(CH3)2NH2]UO2(BTPCA) (1). As shown in Table 1, compound 1 crystallizes in a monoclinic space group Pc. The asymmetric unit consists of an uranyl ion and a BTPCA3− ligand as depicted in Figure 1a. The uranium atom adopts hexagonal-bipyramidal coordination geometry and is chelated by three carboxylate groups with the U−O bond distances ranging from 2.433(8) to 2.459(7) Å in the equatorial plane. And two “yl” oxygens are 1.760(11) and 1.788(9) Å away from uranium atom with the O7UO8 angle 178.5(4)°. All these bond distances and angles are in good

U(1)− O(2) U(1)− O(4) U(1)− O(5) U(2)− O(12) U(2)− O(9) U(2)− O(10)

U(1)− O(1) U(1)− O(3) U(1)− O(4)

1.737(8) 2.469(9) 2.472(10) 1.783(13) 2.470(10) 2.511(9)

1.730(8) 2.470(7) 2.473(8) 1.736(8) 2.468(7) 2.526(7)

1.691(8) 2.438(15) 2.472(16)

U(1)− 1.753(8) O(1) U(1)− 2.468(9) O(7) U(1)− 2.526(9) O(10) 1.783(13) U(2)− O(14) U(2)− 2.477(10) O(16) O(2)− 178.7(5) U(1)− O(1) Compound 4 U(1)− 1.747(8) O(1) U(1)− 2.467(8) O(16) U(1)− 2.485(7) O(15) U(2)− 2.430(8) O(8) U(2)− 2.466(7) O(13) 179.6(4) O(2)− U(1)− O(1) Compound 5 U(1)− O(2) U(1)− O(3) U(1)− O(4)

1.730(9) 2.438(15) 2.472(15)

U(1)− O(5) U(1)− O(9) U(2)− O(3) U(2)− O(13) U(2)− O(15) O(3)− U(2)− O(4)

2.448(10) 2.488(9) 1.759(12) 2.450(10) 2.479(9) 179.5(5)

U(1)−O(3)

2.436(8)

U(1)−O(6)

2.473(8)

U(2)− O(11) U(2)− O(14) U(2)−O(7)

1.729(8) 2.445(7) 2.475(7)

O(11)− U(2)− O(12)

178.3(4)

U(1)− O(3) U(1)− O(4) O(1)− U(1)− O(2)

2.438(15) 2.472(15) 180.0

agreement with those in the literature.56−58 As shown in Figure 1b, each uranyl-centered motif is connected to six neighboring units through three BTPCA3− ligands, generating an infinite uranyl honeycomb (6, 3) net (Figure 1c). The adjacent U···U distances range is from 15.288(1) to 16.559(2) Å in the honeycomb net (Figure S1a). In light of the short distance (0.36 Å) between adjacent 2D nets, the neighboring 2D nets adopt the ABAB stacking mode (Figure S2a) to extend to 3D structure (Figure 1d). [(CH 3) 2NH 2]UO 2(BTPCA) (2). As shown in Table 1, compound 2 crystallizes in a triclinic space group P1.̅ The asymmetric unit of compound 2 also contains a crystallographically independent UO22+ and a BTPCA3− anion, as illustrated in Figure 2a. The equatorial plane of U1 is coordinated with six oxygen atoms from three BTPCA3− C

DOI: 10.1021/acs.inorgchem.8b00168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Asymmetric unit structure of compound 1. (b) Coordination sphere of the uranyl center in compound 1. (c) 2D honeycomb net of compound 1. (d) Neighboring 2D honeycomb nets adopt the ABAB stacking mode to extend to a 3D structure. Color scheme: U, yellow; C, gray; O, red; N, blue. H atoms were omitted for clarity.

Figure 2. (a) Asymmetric unit structure of compound 2. (b) Coordination sphere of the uranyl center in compound 2. (c) 2D honeycomb net of compound 2. (d) Neighboring supermolecular networks adopt the ABAB stacking mode to extend to a 3D structure. Color scheme: U, yellow; C, gray; O, red; N, blue. H atoms were omitted for clarity.

ligands. At the apical positions, the U1 is coordinated with two uranyl oxo groups. Similar to compound 1, each [UO2(COO)3]− unit is connected to six neighboring units through three BTPCA3− ligands, generating an infinite and wave-shaped uranyl honeycomb (6, 3) net (Figure 2b, c) with adjacent U···U distances ranging from 15.809(1) to 16.992(1) Å (Figure S1b). However, the 3D structures of the two compounds are quite different. As shown in Figure S3, the shortest hexagonal loop of the 2D net is quite twisty due to the flexibility of the BTPCA3− ligand. This twisty structure results in an interesting phenomenon in which part of the framework of the 2D net stretches into the hexagonal hole of the

neighboring 2D net (Figure S3b). Therefore, the adjacent two equivalent honeycomb nets construct a noninterpenetrated supermolecular network (Figures S2b, S3c). Then the neighboring supermolecular networks adopt the ABAB stacking mode to extend to a 3D structure (Figure 2d). [(CH3)2NH2]2[UO2(BTPCA)][UO2(BTPCA)]·(H2O)5.5 (3). The result of X-ray crystallographic analysis shows that compound 3 crystallizes in a monoclinic space group P21/n (Table 1). The asymmetric unit consists of two crystallographically independent UO22+ cations (U1 and U2) and two crystallographically independent BTPCA3− ligands (L1 and L2) (Figure 3a). As shown in Figure 3b and 3c, the coordination mode of D

DOI: 10.1021/acs.inorgchem.8b00168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Asymmetric unit structure of compound 3. (b) Coordination sphere of the uranyl center in compound 3. (c) 2D honeycomb net of compound 3. (d) Neighboring 2D honeycomb networks interpenetrate each other, forming the final 2-fold interpenetrated network, and neighboring interpenetrated networks adopt the ABAB stacking mode to extend to a 3D structure. Color scheme: U, yellow; C, gray; O, red; N, blue. H atoms were omitted for clarity.

Figure 4. (a) Asymmetric unit structure of compound 4. (b) Coordination sphere of the uranyl center in compound 4. (c) 2D honeycomb net of compound 4. (d) Neighboring equivalent 2D honeycomb networks interpenetrate each other, forming the final 2-fold interpenetrated network, and neighboring interpenetrated networks adopt the ABAB stacking mode to extend to 3D structure. Color scheme: U, yellow; C, gray; O, red; N, blue. H atoms were omitted for clarity.

compound 3 is similar to compounds 1 and 2. Each uranyl cation is connected with three BTPCA3− ligands and each BTPCA3− ligand bridges three [UO2(COO)3]− SBUs, which is the typical feature of honeycomb net. As mentioned above,

both compounds 1 and 2 contain only one crystallographically independent 2D honeycomb net. However, compound 3 consists of two kinds of crystallographically independent 2D honeycomb nets. One net is composed of U1 and L1, and the E

DOI: 10.1021/acs.inorgchem.8b00168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Asymmetric unit structure of compound 5. (b) Coordination sphere of the uranyl center in compound 5. (c) 2D honeycomb net of compound 5. (d) Neighboring 2D honeycomb nets adopt the AAAA stacking mode to extend to a 3D structure. Color scheme: U, yellow; C, gray; O, red; N, blue. H atoms were omitted for clarity.

Figure 6. (a, b) Two kinds of conformations of BTPCA3− ligand (cis, cis, trans and cis, cis, cis). (c, d) The carboxyl groups of the BTPCA3− ligand may occupy two kinds of positions: the equatorial bond and the axial bond. Color scheme: C, gray; O, red; N, blue; H, white.

crystallizes in a triclinic space group P1̅ (Table 1). The asymmetric unit consists of two crystallographically independent UO22+ cations (U1 and U2) and two crystallographically independent BTPCA3− ligands (L1 and L2) (Figure 4a). The two uranyl cations adopt similar hexagonal-bipyramidal coordination geometry. As illustrated in Figure S5a and S5b, one U1 atom, two U2 atoms, one L2 ligand, and two L1 ligands construct an approximate planate hexagonal loop with the adjacent U···U distances ranging from 15.418(1) to 17.086(1) Å. On the other hand, one U2 atom, two U1 atoms, one L1 ligand, and two L2 ligands construct a quite twisty hexagonal loop with the adjacent U···U distances ranging from 14.684(1)

other one is composed of U2 and L2 (Figure S4a, S4b). In the preceding 2D honeycomb net, the adjacent U1···U1 distances ranges from 15.520(4) to 16.848(5) Å. In the latter, the adjacent U2···U2 distances range from 15.515(4) to 16.848(5) Å. Although the two honeycomb nets are crystallographically independent, their size and configuration are similar. Therefore, they interpenetrate each other to form a 2-fold interpenetrated network (Figures S2c, S4c). Furthermore, neighboring interpenetrated networks adopt the ABAB stacking mode to extend to a 3D structure (Figure 3d). [(CH3)2NH2]2(UO2)2(BTPCA)2·(H2O)3 (4). The result of X-ray crystallographic analysis shows that compound 4 also F

DOI: 10.1021/acs.inorgchem.8b00168 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Thickness of the honeycomb nets in compounds 1−5: (a) compound 1; (b) compound 2; (c-1, 2) two kinds of honeycomb nets of compound 3; (d) compound 4; (e) compound 5.

bond positions (e, e, e). However, for compound 2, the two carboxyl groups of H3BTPCA ligand occupy the equatorial bond positions and the other one occupies the axial bond position (Figure S7 (a, e, e)). Although compound 3 contains two kinds of H3BTPCA ligands, both of them adopt the same conformations with the H3BTPCA ligand of compound 2 (Figure S8). In compound 4, one H3BTPCA ligand adopts the (e, e, e) conformation, while the other H3BTPCA ligand adopts the (a, e, e) conformation. By comparing the 2D structures of compounds 1−5, it is easy to find that the presence of axial bond increases the structural diversity of the 2D honeycomb nets. Isomerism and Interpenetrate Structure. As mentioned above, the structure compositions and coordination mode of compounds 1−5 are similar if excluding the disordered solvent molecules. Therefore, these five compounds are isomers from a broad perspective. Both compounds 3 and 4 possess 2-fold interpenetrated structures. Interpenetration is one of the most important features of coordination polymers. It is also ubiquitous in compounds with 2D honeycomb structures. Herein, we compare the 2D structures of compounds 1−5 to identify why compounds 3 and 4 possess interpenetrated structures while other ones do not. As shown in Figure 7a, the minimal repeating unit of the 2D honeycomb net contains only one hexagonal loop and the thickness of the honeycomb net is 3.52 Å, similar to the length of the uranyl (3.45 Å). Therefore, it is difficult for the 2D honeycomb net to interpenetrate each other due to the large steric hindrance. On the other hand, the minimal repeating unit of the 2D honeycomb net in compound 4 contains two hexagonal loops and the thickness of the honeycomb net is 6.31 Å, which is much larger than the length of the uranyl (3.48 Å). This stair-shaped and crooked structure allows the neighboring equivalent honeycomb nets to interpenetrate each other, forming a 2-fold interpenetrated network. Based on the same principle, compound 3 possesses an interpenetrated structure while compound 2 or 5 do not. Chemical Analysis (PXRD, IR, TGA, and Luminescence Properties). The PXRD patterns of compounds 1−5 were recorded to confirm their phase purity. As shown in Figure S11a, S11b, and S11d, the PXRD patterns of compounds 1, 2, and 4 are consistent with their simulated results, respectively. For compound 3, some diffraction peaks belonging to the simulated pattern of compound 4 are observed in the experiment result, which indicates that compounds 3 and 4 are concomitant. For compound 5, due to a large number of impurities, many diffraction peaks that do not belong to simulated pattern are observed in the experiment result.

to 17.086(1) Å. Each hexagonal unit connects with two equivalent hexagonal units and four crystallographically independent hexagon units via ligand-sharing mode, to generate the expected stair-shaped 2D honeycomb net (Figures 4c and S5d). This special stair-shaped structure allows the neighboring equivalent honeycomb nets to interpenetrate each other, forming a 2-fold interpenetrated network (Figure S2d). Furthermore, neighboring interpenetrated networks adopt the ABAB stacking mode to extend to a 3D structure (Figure 4d). [(CH3)2NH2]UO2(BTPCA) (5). The result of X-ray crystallographic analysis shows that compound 5 crystallizes in a trigonal space group P3 (Table 1). The asymmetric unit of compound 5 contains one-third of the crystallographically independent UO22+ cation and one-third of the BTPCA3− anion, as depicted in Figure 5a. In addition, each uranyl cation is connected with three BTPCA3− ligands and each BTPCA3− ligand bridges three uranyl cations, which can be further extended into a 2D honeycomb net (Figure 5b, c). As mentioned above, all the hexagonal loops of compounds 1−4 present a certain degree of distortion. However, in compound 5, the minimum honeycomb unit is a regular hexagonal with an adjacent U···U distance of 16.906(0) Å (Figure S1e). In addition, the distance between the neighboring 2D honeycomb nets is 7.253 Å, which is quite larger than the thickness of the honeycomb net (3.421 Å). Therefore, the neighboring honeycomb nets of compound 5 adopt the AAAA stacking mode to extend to a 3D structure (Figures S2e and 5d). The space between the layers is filled with a large amount of disordered water molecules and disordered dimethylamine cations. Conformation of the H3BTPCA Ligand. As described above, although compounds 1−5 share similar coordination modes and structural units, their 3D stacking modes and structures are completely different due to the variable conformation of BTPCA3− ligands. As shown in Figure 6a and 6b, the tripodal ligand H3BTPCA may possess two kinds of conformations (cis, cis, trans and cis, cis, cis) due to the flexibility of three piperidine rings. On the other hand, because of the chair conformation of the piperidine ring, the carboxyl groups of the H3BTPCA ligand may occupy two kinds of positions: the equatorial bond and the axial bond (Figure 6c, 6d). Although a series of compounds involving the H3BTPCA ligand have been reported, all the carboxyl groups adopt the equatorial bond mode in these compounds (Table S1). This could be understood in light of the lower energy and higher structure stability of the equatorial bond. Indeed, as shown in Table S1 and Figures S6 and S10, all the carboxyl groups of the H3BTPCA ligands in compounds 1 and 5 occupy the equatorial G

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compounds is relatively low and the 2D honeycomb nets are relatively close to the plane. In compounds 2, 3, and 4, some of the carboxyl groups in the BTPCA3− ligands locate in the axial bond site of piperidine rings, leading to the significant distortion of the ligands and 2D honeycomb nets. Both compounds 3 and 4 possess 2-fold interpenetrated structures due to the suitable steric configuration. Considering all the BTPCA3− ligands in compounds 1−5 adopt the (cis, cis, trans) conformation, we will try to synthesize uranyl-based compounds involving the H3BTPCA ligand with the (cis, cis, cis) conformation in further work.

Therefore, further measurements were conducted on compounds 1, 2, and 4 due to their high purity. As shown in Figure S12, the IR spectra of compounds 1, 2, and 4 are similar. On this account, we will describe in detail the infrared spectrum of compound 1 as a representative. There is a broad and large peak between 3260 and 3660 cm−1 due to the presence of a lattice water molecule. The characteristic absorption peaks of the 1,3,5-triazine ring can be observed at 1666, 1535, and 1304 cm−1, respectively. The C−H stretching vibration absorptions of the piperidine ring can be observed from 2820 to 3025 cm−1. At the same time, there are two sharp peaks appearing near 1375 and 1700 cm−1 by reason for due to the stretching vibration absorption of the carboxyl group. Normally, the presence of the uranyl cation can be confirmed by the absorption peak near 925 cm−1, which is caused by the asymmetric stretching. In compound 1, the corresponding characteristic absorption peak can be observed near 913 cm−1. On the other hand, the absorption peak caused by the symmetric stretching of uranyl cation appears near 803 cm−1. The thermal stability of 1, 2, and 4 was investigated by thermogravimetric measurement in the temperature range 25− 800 °C under constant air flow with a heating rate of 5 °C· min−1 (Figure S13). The weight loss of the three compounds show some plateaus due to the loss of free water molecules, the decomposition of dimethylamine cations, and carboxylate groups in the BTPCA ligand. Although they begin to lose weight at different temperatures (100 °C for compound 1 and 120 °C for compounds 2 and 4), their decomposition processes stopped at the same temperature of 570 °C. With the further temperature increase, all the TG curves maintain the platform; the remaining products should be U3O8. The solid sample of compound 4 loses 66% weight, which is consistent with the theoretical value 65.6%. The luminescent properties of compounds 1, 2, and 4 have been investigated under the same experimental conditions and procedures. As shown in Figure S14, for compounds 1 and 4, no characteristic emission of uranyl cations was observed. For compound 2, five peaks (482, 501, 523, 546, and 570 nm) were observed in the spectra, which are representative fluorescence spectra peaks of uranyl cation. The most intense peak is positioned at 501 nm. Compared to UO2(NO3)2·6H2O, the fluorescence spectra of compound 2 exhibit a slight blue shift of 9 nm. This blue-shift phenomenon can also be observed for previously reported uranyl compounds.27

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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.8b00168. Typical figures including PXRD, IR spectrum, TGA curves (PDF) Accession Codes

CCDC 1817502 and 1828596−1828599 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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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.-Q.S.). *E-mail: [email protected] (Y.-B.Z.). ORCID

Lei Mei: 0000-0002-2926-7265 Wei-Qun Shi: 0000-0001-9929-9732 Author Contributions ∥

X.-L.Z. and K.-Q.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the support of the Science Challenge Project (JCKY2016212A504). We are also thankful for the support of the National Natural Science Foundation of China (Nos. 21577144, 21701178) and the Major Research Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of the Natural Science Foundation of China (91426302, 91326202). We appreciate the help from beamline 3W1A of Beijing Synchrotron Radiation Facility (BSRF) for X-ray single crystal measurements.

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CONCLUSION In conclusion, five novel uranyl-based coordination polymers with semirigid ligand H3BTPCA were synthesized and characterized. Structural analysis indicates that the structure compositions and coordination modes of these five compounds are similar if excluding the disordered solvent molecules. Each uranyl cation is connected with three BTPCA3− ligands and each BTPCA3− ligand bridges three uranyl cations, which further extended into the 2D honeycomb net. All five compounds consist of 2D honeycomb nets with different kinds of stacking modes. Therefore, they can be regarded as isomers with a simple formula [(CH3)2NH2]UO2(BTPCA) from a broad perspective. On the other hand, because of the flexibility of piperidine rings, the 2D honeycomb nets in different compounds exhibit various degrees of distortion. In compounds 1 and 5, all the carboxyl groups in BTPCA3− ligands locate in the equatorial bond site of piperidine rings. Therefore, the degree of distortion of the ligands in these two

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REFERENCES

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