Structural Tuning and Sensitization of Uranyl Phosphonates by

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Structural Tuning and Sensitization of Uranyl Phosphonates by Incorporation of Countercations into the Framework Si-Fu Tang*,† and Xiaomin Hou‡ †

College of Chemistry and Pharmaceutical Sciences and ‡Shandong Province Key Laboratory of Applied Mycology, College of Life Science, Qingdao Agricultural University, Changcheng Road 700, Chengyang District, Qingdao 266109, China

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S Supporting Information *

ABSTRACT: By the introduction of terephthalic acid, tetramethylammonium chloride, and Zn2+ ions, three new uranyl triphosphonates with varying crystal structures, namely, [H3O][(UO2)2(HL)]·H2O (1), [NMe4][(UO2)2(HL)(H2O)]·H2O (2), and [(UO2)3Zn(H2L)2(H2O)2]·3H2O (3), where H6L = benzene-1,3,5-triyltris(methylene) triphosphonic acid, have been successfully synthesized and characterized by means of powder and single crystal XRD, IR, EA, TGA, UV−vis, and luminescence. These three compounds all possess threedimensional framework structures with hydrium, tetramethylammonium, and Zn2+ as the respective countercations. The uranium(VI) center luminescence of compounds 1 and 2 is completely quenched. However, the incorporation of Zn2+ into the matrix of uranyl phosphonate in the case of compound 3 results in the emerging of the typical intense vibronic emissions of U(VI), demonstrating that zinc phosphonate can behave as sensitizer of uranyl phosphonates. The quenching and sensitization mechanism were also discussed.

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be finely tuned by adding quaternary ammonium hydroxides to the reaction system. The introduced quaternary ammonium cations could serve as counterions and structure directing agents. Usually, three-dimensional anionic framework structures could be obtained in the case of small quaternary ammonium cations (such as tetramethylammonium), whereas two-dimensional layers could be obtained instead in the presence of larger cations (such as tetrabutylammonium). The success of this strategy proves to be an effective approach to tune the structure of uranyl phosphonates and deserves more attention to get a better understanding. The photophysical properties of uranyl complexes are very interesting and can find many potential applications, such as chemical sensors,4,5 luminescent thermometer,23 and so on. However, not all uranyl compounds are fluorescent and the underlying mechanism is still not well understood.24 Selfquenching via nonradiative decay pathways could happen in some uranyl complexes. In particular, it has been reported that the luminescence of uranyl complexes can be quenched by the introduction of transition metals (such as Cu2+, Fe2+, Co2+, and Ni2+) due to energy transfer to the d−d excited state, followed by nonradiative decay.25−28 In the field of lanthanide complexes, the antenna effect and d−f sensitization are usually adopted to enhance emission intensity. For example, lanthanide Schiff base compounds, d10 metals (Zn2+, Cd2+, Ag+) are usually added to form heterometallic complexes

INTRODUCTION Uranium-organic coordination polymers or frameworks have been extensively researched1−3 because of their close relation with waste nuclear processing and interesting properties, including luminescent sensing,4,5 photocatalysis,6 optics,7−9 adsorption,10,11 and so on. Among the uranium-containing complexes, uranyl phosphonates are of particular interest. In recent years, uranium phosphonates have seen tremendous expansion of research efforts to construct esthetic architectures 12 and exploit new applications, such as proton conductivity,13 ion exchange,14,15 gas storage/separation,16 magnetism,17 and so on. Due to the linear feature of the uranyl cation, further coordination is constrained to the equatorial plane, which greatly limits the structural diversity of uranyl complexes. Comparing with the carboxylic acid group, the phosphonic acid group has one more oxygen atom and thus possesses more coordination modes, especially when considering it can adopt diverse protonation degrees. This character can offset the limitation of linear uranyl cation and has proven to be a valuable approach to enrich the structural diversity of uranyl complexes. Some effective strategies have been developed to tune the structure of uranyl phosphonates, including the alteration of synthesis parameters (pH, M/L molar ratio, and temperature), incorporation of organic structure directing (OSD) agents18/alkali19/transition metal ions,20 introduction of an ancillary ligand,21 and decoration of the phosphonate ligand with additional functionalities.12 In our recent work,22 we reported that the structure of uranyl triphosphonates could © XXXX American Chemical Society

Received: October 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b02904 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Parameters of Compounds 1−3a,b formula fw space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd/g cm−3 abs coeff/mm−1 F(000) θ range completeness/% reflns collected independent reflns/Rint GOF on F2 final R indices [I > 2σ(I)]: R1, wR2 R indices (all data): R1, wR2

1

2

3

C9H15O15P3U2 932.18 P1̅ 9.9256(13) 10.6108(13) 12.3642(15) 110.333(4) 91.174(4) 110.090(4) 1132.2(2) 2 2.734 14.561 836 3.052−25.000 98.4 19989 3928/0.0714 1.120 0.0723, 0.1651 0.1021, 0.1791

C13H26NO15P3U2 1005.32 Pbca 9.9357(6) 20.5884(11) 23.5290(13) 90 90 90 4813.1(5) 8 2.775 13.713 3680 2.978−24.999 96.7 60625 4206/0.0937 1.062 0.0337, 0.0639 0.0511, 0.0692

C18H32O29P6U3Zn 1677.71 P1̅ 12.2265(5) 13.8550(5) 14.9627(6) 73.4580(10) 71.5370(10) 78.0260(10) 2285.43(16) 2 2.438 11.411 1536 2.918−24.999 96.4 41452 7981/0.0663 1.015 0.0462, 0.1041 0.0686, 0.1143

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

a

Single-Crystal Structure Determination. The diffraction intensity data sets of compounds 1−3 were collected on a Bruker SMART APEX II CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å) at room temperature. SAINT was used for integration of intensity of reflections and scaling.31 Absorption corrections were carried out with the program SADABS.32 Crystal structures were solved by direct methods using SHELXS.33 Subsequent difference Fourier analyses and least-squares refinement with SHELXL-201334 allowed for the location of the atom positions. All non-hydrogen atoms except O2w in compound 1, O2w in compound 2, and O5w in compound 3 were refined with anisotropic thermal parameters. The hydrogen atoms on the water molecules of 1−3 were located from the difference Fourier map. All hydrogen atoms were refined using a riding model. The crystal of compound 1 was twin. The twin refinement was performed on the data with SHELXL-2013 using TWIN (−1 0 0, 0 −1 0, 0.395 0.937 1) and BASF commands. The BASF factor in the final refinement gave a value of 0.01107. The tetramethylammonium cation in compound 2 was disordered and refined over two positions with site occupancy ratio of 78/22. Some water molecules in the channels of compound 3 were disordered and could not be modeled; their electron density peaks were removed by the SQUEEZE routine implemented in PLATON.35 The crystallographic details are summarized in Table 1. The data have been deposited in the Cambridge Crystallographic Data Centre (CCDC), deposition numbers CCDC 1872052−1872054 for compounds 1−3. Synthesis of Compound 1. H6L (0.0360 g, 0.1 mmol), terephthalic acid (0.0332, 0.2 mmol), (UO2)(OAc)2·2H2O (0.0848 g, 0.2 mmol), 10 mL of DI water, and two drops of HF (40%) were mixed and stirred in a Teflon-lined autoclave. Afterward, it was sealed and heated at 160 °C for 4 days and allowed to cool to room temperature in a time period of 12 h. Yellow block crystals could be separated from colorless block crystals with satisfying yield (0.0578 g, 62% based on metal source). Elemental analysis (%) calcd for C9H15O15P3U2 (932.18): C 11.60, H 1.62%; found: C 11.69, H 1.78%. IR(KBr, cm−1): 3440.7 (b, s), 3213.1 (sh, m), 1604.6 (m), 1510.5 (w), 1457.9 (m), 1402.5 (m), 1384.7 (w), 1243.4 (m), 1131.1 (s), 1014.4 (s), 983.0 (s), 968.1 (s), 920.8 (s), 895.3 (s), 825.4 (m), 759.3 (w), 712.1 (w), 691.8 (m), 645.1 (w), 576.1 (w), 552.0 (m), 538.5 (m), 498.0 (m), 472.5 (w), 452.2 (w). Synthesis of Compound 2. H6L (0.0720 g, 0.2 mmol), tetramethylammonium chloride (0.0219 g, 0.2 mmol), (UO2)-

wherein the d-block of the matrix functions as sensitizers of the lanthanide centers.29 Similarly, it could be possible to sensitize or enhance the luminescence of uranyl phosphonates by incorporation of d10 metals. However, it is still not reported in the field of uranyl phosphonates. Keeping these in mind, it is necessary to carry out a work, focusing on the structural tuning and sensitization of uranyl phosphonates. In this work, we present that the structure of uranyl triphosphonates can be effectively tuned by countercations by adding terephthalic acid and tetramethylammonium chloride, whereas the luminescence could be sensitized by incorporation of Zn2+. Three new uranyl phosphonates, namely, [H3O][(UO2)2(HL)]·H2O (1), [NMe4][(UO2)2(HL)(H2O)]·H2O (2), and [(UO 2)3Zn(H2L) 2(H2O)2]· 3H2O (3), where H6L = benzene-1,3,5-triyltris(methylene)triphosphonic acid, have been successfully synthesized and thoroughly characterized. Here, we report on their syntheses, crystal structures, luminescent properties, and energy transfer mechanisms.

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EXPERIMENTAL SECTION

Materials and Instruments. The synthesis of benzene-1,3,5triyltris(methylene)triphosphonic acid (H6L) has been reported in our previous work.30 Elemental analyses were performed on a Vario EL III elemental analyzer. Powder X-ray diffraction patterns (see Figures S1−S3 in the Supporting Information) were obtained on a Bruker D8 Advance diffractometer using CuKα radiation. IR spectra (see Figure S4 in the Supporting Information) were recorded in the range of 4000−400 cm−1 on a Thermo Fisher Nicolet iS-10 FTIR Spectrometer with KBr pellets. Thermogravimetric analyses (TGA) were carried out on a NETZSCH STA 449C unit at a heating rate of 10 °C/min under an air atmosphere. Fluorescent analyses of compounds 1−3 were performed on a HITACHI F-4600 FL spectrophotometer. The UV−vis spectra of ligand H6L and compounds 1−3 were recorded on a HITACHI U-3900 spectrophotometer. The pH values of the reaction systems were determined using a Sartorius pH meter (PB-10). Caution! Standard precaution should be taken during the handling of uranium-containing materials. B

DOI: 10.1021/acs.inorgchem.8b02904 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (OAc)2·2H2O (0.0848 g, 0.2 mmol), 10 mL of DI water, and two drops of HF (40%) were mixed and stirred in a Teflon-lined autoclave. Afterward, it was sealed and heated at 160 °C for 4 days and allowed to cool to room temperature in a time period of 12 h. Yellow prism crystals could be obtained with satisfying yield (0.0788 g, 80% based on metal source). Elemental analysis (%) calcd for C13H22NO14P3U2 (985.28): C 15.53, H 2.61, N 1.39%; found: C 15.59, H 2.71, N 1.46%. IR(KBr, cm−1): 3442.6 (b, s), 1635.9 (m), 1606.5 (m), 1484.5 (m), 1458.9 (w), 1411.7 (w), 1397.7 (w), 1384.7 (w), 1243.9 (m), 1227.9 (m), 1156.1 (s), 1109.4 (s), 1056.8 (s), 1004.3 (s), 969.5 (s), 940.1 (s), 908.3 (s), 894.3 (sh, m), 813.8 (w), 760.3 (w), 710.6 (w), 694.2 (w), 556.4 (m), 523.1 (m), 498.5 (w), 451.7 (m). Synthesis of Compound 3. H6L (0.0720 g, 0.2 mmol), urea (0.0120, 0.2 mmol), uranyl zinc acetate (0.0848 g, 0.1484 mmol), 10 mL of DI water, and two drops of HF (40%) were mixed and stirred in a Teflon-lined autoclave. Afterward, it was sealed and heated at 80 °C for 4 days and allowed to cool to room temperature in a time period of 12 h. Yellow block crystals could be obtained with satisfying yield (0.0481 g, 58% based on metal source). Elemental analysis (%) calcd for C18H32O29P6U3Zn (1677.71): C 12.89, H 1.92; found: C 12.81, H 2.07%. IR(KBr, cm−1): 3449.6 (b, s), 1635.4 (m), 1602.1 (m), 1511.9 (m), 1456.5 (m), 1416.9 (m), 1384.7 (sh, w), 1237.1 (m), 1139.3 (w), 1060.7 (s), 1023.1 (s), 987.4 (sh, s), 920.4 (m), 891.4 (m), 824.4 (w), 759.8 (w), 715.9 (m), 642.7 (w), 573.2 (w), 513.5 (m).

the OUO bond angle is almost linear (177.9(9)°) and the U−Our are 1.78(2) and 1.815(18) Å. Different from U1, it has an octahedral coordination geometry. The U−Oeq bond distances at the equatorial plane are found in the range of 2.268(16)−2.327(16) Å, which are comparable to those of other uranyl phosphonates.36 The calculated bond-valence sums of U1 and U2 are 5.92 and 6.00, respectively, that are in agreement with U(VI).37 The penta-deprotonated triphosphonate ligand displays an up-up-down configuration. Two of the three phosphonate groups are fully deprotonated and bridge three uranyl units. The third one is monoprotonated and exhibits a chelate-bridging coordination mode, binding two uranyl units. The triphosphonate ligand is octadentate, and its coordination mode can be denoted as μ8:η1:η1:η1:η0:η1:η2:η1:η1:η1 (see Scheme 1a). The uranyl cations are bridged by Scheme 1. Coordination Modes in Compounds 1−3

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RESULTS AND DISCUSSION Structural Description of Compound 1. Compound 1 crystallizes in triclinic P1̅ space group with two molecules in each unit cell. In each asymmetric unit, there are one pentadeprotonated triphosphonate ligand (HL)5−, two crystallographic independent uranyl cations, one hydronium ion, and one lattice water molecule (see Figure 1), corresponding to a

these triphosphonate ligands to form a three-dimensional framework structure with one-dimensional channels of 5.474(11) × 6.995(3) Å2 running along the b-axis (see Figure 2). In the channels, protonated water molecules are accommodated, balancing the charge of the framework. The potential solvent accessible void of the framework is estimated to be 24.3% of per unit cell volume. O−H···O interactions are

Figure 1. Depiction of the coordination environments of the triphosphonate ligand and uranyl cations in 1 (thermal ellipsoids are given at 30% probability). A: 1 − x, −y, −1 − z; B: −1 + x, −1 + y, z; C: 1 − x, 1 − y, −1 − z; D: x, y, 1 + z; E: 1 − x, 1 − y, −z; F: x, y, −1 + z; G: 1 + x, 1 + y, z.

formula of [H3O][(UO2)2(HL)]·H2O. For U1, the OUO bond angle is 176.9(10)°, and the UOur (ur: uranyl) bond distances are 1.762(16) and 1.796(19) Å. The uranyl cation is pentadentate coordinated at the equatorial positions with the U−Oeq (eq: equatorial) distances found in the range of 2.281(18)−2.527(18) Å, forming a normal pentagonal bipyramid coordination geometry. For U2, in the uranyl unit,

Figure 2. Three-dimensional packing structure of compound 1 viewing along b-axis. The −CPO3 tetrahedrons and UOx (x = 6, 7) polyhedrons are shaded in green and yellow, respectively. C

DOI: 10.1021/acs.inorgchem.8b02904 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry observed between the water molecules and phosphonate oxygen atoms (see Table S1). Structural Description of Compound 2. Compound 2 crystallizes in orthorhombic Pbca space group with eight molecules in each unit cell. The asymmetric unit is composed of one tetramethylammonium cation, two crystallographic independent uranyl cations, one penta-deprotonated triphosphonate ligand (HL)5−, one aqua ligand, and one lattice water molecule (see Figure 3), indicating a formula of

Figure 4. Three-dimensional packing structure of compound 2 viewing along a-axis. The −CPO3 tetrahedrons and UOx (x = 6, 7) polyhedrons are shaded in green and yellow, respectively.

charge of the framework and form plenty of C−H···O and O− H···O interactions (see Table S1). It should be noted that the crystal structure of compound 1 is different from that of [NMe4][UO2(H3L)][H2O]22 which is also synthesized from H6L and uranyl acetate. In [NMe4][UO2(H3L)][H2O], the triphosphonate ligand is triply deprotonated and adopts a different coordination mode which can be denoted as μ5:η0:η0:η1:η1:η1:η0:η1:η0:η1. Structural Description of Compound 3. Compound 3 crystallizes in triclinic P1̅ space group with two molecules in each unit cell. Its asymmetric unit is composed of three crystallographic independent uranyl cations, one zinc(II) ion, two tetra-deprotonated triphosphonate ligands (H2L)4−, two aqua ligands, and three lattice water molecules (see Figure 5),

Figure 3. Depiction of the coordination environments of the triphosphonate ligand and uranyl cations in 2 (thermal ellipsoids are given at 30% probability). A: −0.5 + x, 0.5 − y, 1 − z; B: −1 + x, y, z; C: 2 − x, −y, 1 − z; D: 2.5 − x, −y, 0.5 + z; E: −0.5 + x, y, 1.5 − z; F: 0.5 + x, 0.5 − y, 1 − z; G: 1 + x, y, z; H: 2.5 − x, −y, −0.5 + z; I: 0.5 + x, y, 1.5 − z.

[NMe4][(UO2)2(HL)(H2O)]·H2O. Similar to those in compound 1, one uranyl center is heptadentate, whereas another one is hexadentate, showing typical pentagonal bipyramid and octahedral coordination geometry, respectively. Around U1, the OUO angle is 177.3(3)°, the UOur bond lengths are 1.759(6) and 1.761(6) Å. The equatorial coordination positions are filled with one aqua ligand and four phosphonate oxygen atoms from four triphosphonate ligands. The U−Oeq bond lengths are in the range of 2.300(6)−2.532(6) Å, which are comparable to those of other uranyl phosphonates.36 In the case of U2, the OUO angle is 177.1(3)°, and the UOur bond lengths are 1.770(6) and 1.771(6) Å. At the equatorial plane, the uranyl cation is coordinated by four phosphonate oxygen atoms from four triphosphonae ligands. The U−Oeq bond lengths fall in the range of 2.292(5)−2.343(6) Å. The calculated bond-valence sums of U1 and U2 are 6.11 and 6.09, respectively, which are consistent with U(VI).37 The monoprotonated triphosponate ligand is also octadentate and possesses a similar coordination mode (μ8:η1:η1:η1:η0:η1:η2:η1:η1:η1; see Scheme 1b) as that in compound 1, which means it binds eight uranyl cations with its eight phosphonate oxygen atoms. The UO6 and UO7 polyhedrons are bridged by the CPO3 tetrahedrons into corrugated layers in the ac-plane by sharing common vertexes (see Figure S5 in the Supporting Information). These layers are further connected by the benzene rings into three-dimensional framework structure with one-dimensional rhombus channels running along the a-axis (see Figure 4). The channels are filled with tetramethylammonium caions and lattice water molecules which balance the

Figure 5. Depiction of the coordination environments of the triphosphonate ligand and uranyl cations in 3 (thermal ellipsoids are given at 30% probability). A: 3 − x, 1 − y, 1 − z; B: 2 − x, 2 − y, 1 − z; C: −1 + x, 1 + y, −1 + z; D: 2 − x, 2 − y, −z; E: −1 + x, y, z; F: 1 − x, 2 − y, 1 − z; G: 4 − x, 1 − y, 1 − z; H: 1 + x, y, z; I: 1 + x, −1 + y, 1 + z.

indicating a formula of [(UO2)3Zn(H2L)2(H2O)2]·3H2O. In the three uranyl units, the OUO angles are 177.9(4), 178.3(4), and 177.4(4)°, the UOur bond lengths are 1.771(8) and 1.780(8) Å, 1.763(9) and 1.779(9) Å, 1.773(9) and 1.783(9) Å (see Table 2), respectively. The U−Oeq bond lengths fall in the ranges of 2.293(7)−2.535(7) Å, 2.314(8)−2.600(8) Å, and 2.285(10)−2.550(10) Å, respectively. The bond-valence sums of U1, U2, and U3 are D

DOI: 10.1021/acs.inorgchem.8b02904 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Bonds (Å) in Compounds 1−3a compound 1 U(1)−O(10) U(1)−O(11) U(1)−O(3)#1 U(1)−O(9) U(1)−O(6)#2 U(1)−O(5)#3 U(1)−O(6)#3

1.762(16) U(2)−O(12) 1.796(19) U(2)−O(13) 2.281(18) U(2)−O(8) 2.283(17) U(2)−O(7)#4 2.399(15) U(2)−O(2)#5 2.480(18) U(2)−O(1)#1 2.527(18) compound 2

1.78(2) 1.815(18) 2.268(16) 2.277(15) 2.327(16) 2.316(15)

U(1)−O(10) U(1)−O(11) U(1)−O(7) U(1)−O(6)#1 U(1)−O(1)#2 U(1)−O(2)#3 U(1)−O(1W)

1.759(6) U(2)−O(12) 1.761(6) U(2)−O(13) 2.300(6) U(2)−O(9)#4 2.322(6) U(2)−O(8) 2.349(5) U(2)−O(4)#1 2.412(5) U(2)−O(5)#5 2.532(6) compound 3

1.771(6) 1.770(6) 2.292(5) 2.294(5) 2.296(6) 2.343(6)

U(1)−O(20) U(1)−O(19) U(1)−O(8) U(1)−O(5)#1 U(1)−O(17)#2 U(1)−O(18) U(1)−O(17) U(2)−O(22) U(2)−O(21) U(2)−O(15) U(2)−O(12)#3 U(2)−O(1)#2 U(2)−O(2)#4

1.771(8) 1.780(8) 2.293(7) 2.312(7) 2.406(7) 2.419(8) 2.535(7) 1.763(9) 1.779(9) 2.314(8) 2.330(8) 2.380(8) 2.396(8)

2.600(9) 1.773(9) 1.783(9) 2.285(10) 2.320(8) 2.340(8) 2.345(8) 2.550(10) 1.748(12) 1.897(8) 1.930(9) 2.076(19) 2.327(6)

U(2)−O(1)#4 U(3)−O(24) U(3)−O(23) U(3)−O(13)#5 U(3)−O(7)#2 U(3)−O(3)#6 U(3)−O(16) U(3)−O(2W) Zn(1)−O(1W) Zn(1)−O(4) Zn(1)−O(4)#7 Zn(1)−O(11)#8 Zn(1)−Zn(1)#7

Figure 6. Three-dimensional packing structure of compound 3 viewing along b-axis. The −CPO3 tetrahedrons, ZnO4 tetrahedrons, and UO7 pentagonal bipyramids are shaded in green, cyan, and yellow, respectively.

These inorganic layers are further linked by the organic moieties into a three-dimensional framework structure. Structural Tuning of Uranyl Phosphonates. In our previous work, tetraalkylammonium hydroxide was added into the reaction system, which resulted in the formation of a series of uranyl triphosphonates with diverse crystals structures.22 These uranyl triphosphonates usually possess an anionic framework structure or intercalation structure with guest tetraalkylammonium cations accommodated in the channels or between the layers, balancing the charge and behaving as structural directing agents. To continue this work, a few of the carboxylic ligands (terephthalic acid and oxylic acid) and tetraalkylammonium chlorides (NMe4Cl, NEt4Cl, NPr4Cl, NBu4Cl, and NMe3C16Cl) were examined. Compound 1 was obtained in the presence of terephthalic acid. It was found that terephthalic acid was not involved in the framework and its crystal structure was different from other uranyl triphosphonates.22 Protonated water molecules can be found in the channels of compound 1, suggesting hydronium cations can be formed at low pH conditions and function as countercations. Compound 2 was obtained in the presence of tetramethylammonium chloride at the same reaction conditions used for compound 1. Interestingly, the crystal structure of compound 2 is totally different from that of [NMe4][UO2(H3L)][H2O] in our previous work,22 although they both contain tetrametylammonium as countercations. The displacement of uranyl acetate by uranyl zinc acetate leads to the formation of compound 3, in which Zn2+ ions are successfully incorporated into the framework. The pH values of the reaction systems were determined to be 1.86, 1.96, and 2.05 for compounds 1− 3, respectively. These results clearly demonstrate that pH value and the incorporation of countercations play important roles during the speciation of uranyl phosphonates and the crystal structure of uranyl phosphonates can be effectively tuned by the incorporation of countercations. Thermal Stabilities. Thermal gravimetric analysis (TGA) measurements were carried out in the temperature range of 30−800 °C to investigate the thermal stabilities of compounds 1−3 (see Figure 7). Compound 1 displayed three main weight losses in the examined temperature range. The first one of

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Symmetry transformations used to generate equivalent atoms: for 1: #1 −x + 1, −y, −z − 1; #2 −x + 1, −y + 1, −z − 1; #3 x − 1, y − 1, z; #4 −x + 1, −y + 1, −z; #5 x, y, z + 1. For 2: #1 −x + 2, −y, −z + 1; #2 x − 1/2, −y + 1/2, −z + 1; #3 x − 1, y, z; #4 x − 1/2, y, −z + 3/2; #5 −x + 5/2, −y, z + 1/2. For 3: #1 −x + 3, −y + 1, −z + 1; #2 −x + 2, −y + 2, −z + 1; #3 −x + 2, −y + 2, −z; #4 x − 1, y + 1, z − 1; #5 −x + 1, −y + 2, −z + 1; #6 x − 1, y, z; #7 −x + 4, −y + 1, −z + 1; #8 x + 1, y, z.

calculated to be 5.94, 5.96, and 6.08, respectively, that are in agreement with U(VI).37 The zinc ion (Zn1) is tetrahedrally coordinated by one aqua ligand and three phosphonate oxygen atoms, forming a typical tetrahedral coordination geometry. The Zn−O bond lengths are in the range of 1.748(12)− 2.076(19) Å, which is comparable to other zinc phosphonates.38 The two tetra-deprotonated triphosphonate ligands adopt different coordination modes. One is octadentate, linking to six uranyl cations and two zinc ions, whereas another one is heptadentate, linking to six uranyl cations and one zinc ion. Their coordination modes can be denoted as μ8:η2:η1:η1:η2:η1:η0:η1:η1:η0, and μ7:η0:η1:η1:η1:η0:η1:η1:η2:η1, respectively (see Scheme 1c,d). These triphosphonate ligands bridge the uranyl units into a dense three-dimensional framework structure (see Figure 6). One-dimensional slits along the b-axis can be observed with the phosphonate hydroxyl groups dangling the slit walls, forming plenty of O− H···O and C−H···O interactions with other phosphonate oxygen atoms (see Table S1). The structure can be viewed as the UO7 pentagonal bipyramids, and ZnO4 tetrahedrons are interconnected by the CPO3 tetrahedrons into 2D-layers in the ac-plane by sharing common vertexes or edges (see Figure S6). E

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5f orbitals and the oxygen 2p orbitals.39 The peaks around 416 nm are originated from the vibrational-coupled electronic transitions within the uranyl unit.40,41 For compound 3, no absorption feature of Zn2+ ion can be observed because the 3d10 electronic configuration has no d−d transition. Luminescence Studies. The emission spectra of ligand H6L and compounds 1−3 were investigated (see Figure 9a). The free ligand H6L displays a broad emission band with a maximum centered at about 429.6 nm, corresponding to the π* → π transition. As we know, uranium(VI)-containing compounds usually emit green light and display characteristic five-band emission peaks arising from the electronic and vibronic transitions of S11−S00 and S10−S0v (ν = 0−4) upon direct excitation of the uranium center. However, compounds 1 and 2 all display ligand centered emission bands with maxima lying at about 469.6 nm; no characteristic emission bands of U(VI) can be observed. This is similar to some uranyl triphosphonates reported in our recent work.22 Different from 1 and 2, compound 3 shows the typical fine peaks at 512.4, 532.2, 554.4, 581.0, and 610 nm, which are slightly blue-shifted in comparison with the benchmark (uranyl nitrate hexahydrate: 487, 509, 532, 558, 586, and 612 nm). Until now, the quenching mechanism of uranium-containing compounds is still not well understood. Usually it is qualitatively ascribed to nonradiative decay processes. To have a better or quantitative understanding of the quenching mechanism, it is necessary to have a closer study. In general, effective energy transfer could occur with two necessary prerequisites: (1) Appropriate energy difference (2100−3200 cm−1) between the energy level of the triplet state ligand and the excited state UO22+. Too small of an energy difference may lead to back energy transfer, causing energy loss via nonradiative pathways. Too large of an energy difference would make it hard to sensitize the luminescence of uranium and will also cause energy loss in the nonradiative intersystem crossing process. (2) Appropriate donor−acceptor distance, herein is the distance between the ligand H6L and uranium center. In the current work, the donor−acceptor distances in compounds 1−3 are appropriate. The energy levels of the triplet state of ligand H6L and excited state UO22+ have been reported to be 27 930 and 21 321 cm−1 (see Figure 9b), respectively.42,43 Obviously, the energy level of the triplet state H6L is higher than that of uranyl cation, but too high (ΔE = 6609 cm−1) to allow effective energy transfer. Therefore, it is believed that the typical uranium(VI) emissions in compounds 1 and 2 are self-quenched via nonradiative processes, especially when disordered ions are accommodated in the channels.44 Upon the incorporation of Zn2+ into the framework, compound 3 shows the typical uranium emission peaks. The emerged characteristic emission signals of uranyl suggest the probable sensitization of UO22+. However, the UV−vis absorption spectra show that the absorption of ligand H6L and uranyl cation occurs in the same region (see Figure 8); direct excitation of U(VI) cannot be precluded. New evidence is needed to confirm whether UO22+ has been sensitized. If the uranium center has been sensitized by the d-block, the luminescence intensity will rely on the efficiency of the energy transfer from the ligand to uranyl center. Therefore, steadystate emission spectra under various excitation wavelengths were recorded (see Figure 9c). It was found that the uranium emission intensity displays a wavelength dependence in the steady-state emission spectra. With the changing of excitation wavelength from 300 to 360 nm, the emission intensity of

Figure 7. TGA curves of compounds 1−3.

about 3.63%, which occurred in the temperature range of 59− 150 °C, could be ascribed to the removal of guest water molecules (calc. 3.87%). With the increasing of temperature, continuous weight losses could be observed, indicating the decomposition of the framework. For compound 2, the first weight loss (3.74%) could be found in the temperature range of 60−150 °C, matching well with the theoretic value (3.66%) of one aqua ligand and one lattice water molecule in each formula. Upon further heating, the framework started to decompose continuously. In the case of compound 3, a weight loss of about 7.54% was observed at 130 °C, indicating that there are seven water molecules in each formula (theoretic: 7.58%). This value is larger than that from the single-crystal structure (5.53%), suggesting that there are some disordered water molecules in the channels and cannot be modeled during structure solution. UV−vis Spectroscopy. The UV−vis absorption spectra of ligand H6L and compounds 1−3 were recorded and are shown in Figure 8. The free ligand H6L exhibits the π → π* transition band with a maximum at about 270 nm. Compounds 1−3 all show two typical absorption bands of uranyl complexes. The former shapeless peaks in the region of 200−286 nm can be ascribed to the ligand to metal charge transfer (LMCT) of the uranyl unit, corresponding to the hybridization of the uranium

Figure 8. Solid state UV−vis absorption spectra of ligand H6L and compounds 1−3. F

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of UO22+. Therefore, the sensitization of the uranium center in compound 3 is evident. Additionally, the emission spectrum of compound 3 also displays one weak band around 469 nm, indicating that the energy transfer is still less efficient.45 Thus far, it can be supposed that energy transfer would take place upon the excitation of the ligand, then intersystem crossing and antenna transfer would occur, and finally transferring to the excited states of the UO22+ cations.

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CONCLUSION By the introduction of terephthalic acid, tetramethylammonium chloride, and Zn2+ ions, three new uranyl triphosphonates have been hydrothermally synthesized and thoroughly characterized. Structural disparity between these three compounds is largely attributed to the difference of pH and the incorporation of countercations which would influence the speciation of uranyl triphosphonates. The fluorescence properties of these compounds were investigated. It was found that the incorporation of Zn2+ can sensitize the emission of uranyl phosphonates.

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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.8b02904. XRD patterns, IR spectra, O···O, C−H···O, and π···π interactions, crystal structures of compounds 1−3 (PDF) Accession Codes

CCDC 1872052−1872054 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 Author

*E-mail: [email protected]. ORCID

Si-Fu Tang: 0000-0002-7151-9876 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the Natural Science Foundation of China (No. 21171173) and the Advanced Talents Foundation of Qingdao Agricultural University.

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Figure 9. (a) Solid state emission spectra of ligand H6L and compounds 1−3 at room temperature. (b) Energy levels of the triplet state of H6L and excited state of UO22+. (c) Solid state emission spectra of compound 3 under different excitation wavelengths.

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