Synthesis, Structure, and Antibacterial Activity of a Thallium(III

Oct 5, 2016 - Synopsis. The discrete 16-tungsto-2-silicate [Tl2{B-β-SiW8O30(OH)}2]12− represents the first structurally characterized thallium-cont...
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Synthesis, Structure, and Antibacterial Activity of a Thallium(III)Containing Polyoxometalate, [Tl2{B‑β-SiW8O30(OH)}2]12− Wassim W. Ayass,† Tamás Fodor,‡ Zhengguo Lin,† Rachelle M. Smith,† Xiaolin Xing,† Khaled Abdallah,† Imre Tóth,*,‡ László Zékány,‡ Magda Pascual-Borràs,§ Antonio Rodríguez-Fortea,§ Josep M. Poblet,§ Linyuan Fan,⊥ Jie Cao,⊥ Bineta Keita,∥,▽ Matthias S. Ullrich,† and Ulrich Kortz*,† †

Department of Life Sciences and Chemistry, Jacobs University, P.O. Box 750561, 28725 Bremen, Germany Department of Inorganic and Analytical Chemistry, University of Debrecen, Egyetem tér 1, 4032 Debrecen, Hungary § Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Marcel·lí Domingo 1, 43007 Tarragona, Spain ⊥ Key Laboratory of Cluster Science, Ministry of Education of China; Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, 100081 Beijing, P. R. China ∥ Laboratoire de Chimie-Physique, UMR 8000 CNRS, Université Paris-Sud, F-91405 Orsay, France ‡

S Supporting Information *

NMR spectrometry showing 205Tl−203Tl spin−spin coupling is an extraordinarily powerful tool for studying the structure and dynamics of compounds with more than one Tl center.11,12 Herein, we report on the synthesis and solution characterization of the novel Tl3+-containing 16-tungsto-2-silicate [Tl2{Bβ-SiW8O30(OH)}2]12− (1), which represents the first structurally characterized discrete thallium-containing metal oxide. In polyanion 1, two octahedrally coordinated Tl3+ ions are sandwiched between two lacunary {B-β-SiW8} Keggin-type fragments (Figure 1).

ABSTRACT: We have synthesized and structurally characterized the first discrete thallium-containing polyoxometalate, [Tl2{B-β-SiW8O30(OH)}2]12− (1). Polyanion 1 was characterized in the solid-state and shown to be solution-stable by 203/205Tl NMR, electrospray ionization mass spectrometry, and electrochemical studies. The antibacterial activity of 1 was also investigated.

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olyoxometalates (POMs) are a well-known class of discrete, anionic metal−oxo clusters comprising early transition metals in high oxidation states.1 The structural and compositional variety is unmatched, and, hence, such compounds are of interest in many different areas such as catalysis, medicine, magnetism, and materials science.2 Vacant (lacunary) heteropolytungstates can be considered as inorganic ligands that coordinate to all kinds of oxophilic electrophiles such as d or f block metal ions or main-group elements.2 Thallium-containing compounds are widely used in the electrical, medical, and even glass manufacturing industries.3 However, little work has been done on thallium-containing POMs. Thallium(I) salts of conventional POMs such as the Keggin ion, paratungstate, and metatungstate have been prepared.4 Thallium-containing metal oxides with extended structures5 or Zintl phases containing thallium have also been reported.6 In 1953, Magneli7 first described the structure of hexagonal tungsten bronze AxWO3, and later Shivahare4 (1964) and Bierstedt et al.7 (1965) isolated Tl2W4O13 and Tl0.3WO3, respectively. In 1980, Tourné’s group reported a TlIII-containing Keggin ion based on elemental analysis.8 Considering that no structurally characterized discrete thallium-containing POM had been reported to date, we decided to investigate the reactivity of Tl3+ ions with lacunary heteropolytungstates. The aqueous solution chemistry of Tl3+ is dominated by a high tendency for hydrolysis, and the formation of thallium(III)hydroxo complexes starts already at pH 0.5. In contrast to lighter congeners of group 13, Tl3+ does not form polynuclear hydroxo complexes,9 and compounds with more than one Tl atom are known mainly in organothallium chemistry.10 Usually, 203/205Tl © XXXX American Chemical Society

Figure 1. Polyhedral representation of 1. Color legend: WO6, dark red; octahedra, SiO4, dark gray; tetrahedra, Tl, light blue; O, light red. The most basic types of O atoms are labeled as OA, OB, and OC. The WA and WB atoms have different 203/205Tl−183W spin−spin coupling constants.

Polyanion 1 was synthesized as follows: the dilacunary POM precursor K8[γ-SiW10O36] was first prepared according to a published procedure.13 A 20 mL solution of this POM precursor (400 mg, 0.135 mmol) in double deionized water was added to solid thallium nitrate, Tl(NO3)3·3H2O (0.060 g, 0.135 mmol) under vigorous stirring for 30 min at room temperature. Caution! Received: August 9, 2016

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DOI: 10.1021/acs.inorgchem.6b01921 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Thallium and its compounds are toxic and should be handled with care. Once the solution was added, a temporary formation of brown Tl2O3 was observed. The dark-brown color changed to light brown after a few seconds under vigorous stirring. After 30 min, the excess of Tl2O3 was finely filtered, resulting in a clear, colorless solution. Around 2 mL of 1 M NH4Cl was added to the solution, resulting in a final pH of around 4.2. Approximately 2 weeks later, white colorless crystals of (NH4)5K7[Tl2{B-βSiW8O30(OH)}2]·19H2O (NH4K-1; yield, 60 mg, 17%) suitable for X-ray analysis were formed upon slow evaporation at room temperature in a 50 mL beaker. The product was characterized in the solid state by IR spectroscopy (KBr pellet, cm−1): 982 (w), 943 (s), 852 (vs), 800 (m), 723 (vs), 523 (w) (Figure S1). Elem anal. Calcd (found): N, 1.37 (1.40); K, 5.4 (5.5); W, 57.60 (57.76); Tl, 8.00 (7.97); Si, 1.10 (1.09); H, 1.18 (1.17). The number of crystal waters was confirmed by thermogravimetric analysis (TGA; Figure S2). Single-crystal X-ray diffraction (XRD) analysis revealed that polyanion 1 crystallizes in a monoclinic crystal system with space group P21/m. The architecture of 1 presents a polyanion with idealized C2h symmetry, which consists of two Tl3+ centers and two {B-β-SiW8O31} POM units. The {B-β-SiW8O31} units were formed from the [γ-SiW10O36]8− precursor by rotational isomerization and loss of tungsten. The first POM comprising a {B-β-SiW8O31} unit was reported in 2005.14 The {M2(B-βSiW8O31)2} structure type has been seen before, with M being 3d metal ions.15 The two equivalent Tl3+ centers in polyanion 1 are both six-coordinated, and each Tl ion is coordinated to two {B-βSiW8O31} lacunary POM fragments via four terminal O atoms of the two complete tungsten−oxo triads and two terminal O atoms of two {SiO4} hetero groups. The Tl−O bond length ranges from 2.156(1) to 2.238(1) Å, and the distance between both Tl atoms is 3.338(1) Å. Bond-valence-sum (BVS) calculations16 (Table S2) confirm that the oxidation state of the two Tl centers is 3+. On the basis of elemental analysis, polyanion 1 is diprotonated, and BVS and density functional theory (DFT; vide infra) suggest that they are located on the two μ3-O atoms (labeled OC in Figure 1 and O2S1 and O2S2 in the CIF file). Preliminary DFT calculations were performed in order to evaluate the most probable protonation sites and proton distribution on 1 in aqueous solution. The structure was optimized initially for the nonprotonated anion [Tl2{B-βSiW8O31)}2]14−, and the most characteristic X-ray bond distances were rather well reproduced: Tl−O(Si) 2.28 Å, W− O(Si) 2.25−2.43 Å, W−O(Tl) ca. 1.81 Å, and Tl···Tl 3.37 Å. Three types of O atoms in 1 were found to be the most nucleophilic according to molecular electrostatic potential distributions (see the SI), i.e., the triply bridging OA and OC sites and the doubly bridging OB sites (Figure 1). The most stable diprotonated form of 1 has two protons on two of the four equivalent OB sites. When the protons are placed on the OC and OA sites, the energy increase is only 2.1 and 3.9 kcal·mol−1, respectively. Therefore, it is expected that the protons will likely move among these three positions in solution. In the solid state, the most accessible basic sites (OA and OB) are blocked by direct interactions with countercations, making OC the most favorable site to be protonated, as suggested by BVS. NH4K-1 was also studied by 205Tl and 203Tl NMR in water (Figure 2). Both isotopes have a spin I of 1/2 and natural abundances of 70.5% and 29.5%, respectively. At first sight, both spectra appear as pseudotriplets, attributed to the spin−spin coupling between two sterically identical Tl atoms. The central peaks are assigned to polyanions with homonuclear 205Tl−205Tl

Figure 2. Experimentally (black) and theoretically simulated (red) 144.26 MHz 205Tl (top) and 142.86 MHz 203Tl (bottom) NMR spectra of compound 1 (∼6 mM in a 0.04 M acetic acid/sodium acetate buffer; pH 4.1). Coupling constants shown: (a) 2J(205Tl−183WA) = ca. 475 Hz; (b) 2J(205Tl−183WB) = ca. 354 Hz; (c) 2J(203Tl−183WA) = ca. 470 Hz; (d) 2 203 J( Tl−183WB) = ca. 350 Hz; (e) 2J(205Tl−203Tl) = 2J(203Tl−205Tl) = 2670 Hz (same value in both spectra).

or 203Tl−203Tl coupling, whereas the satellite peaks belong to heteronuclear 205Tl−203Tl coupled polyanions, respectively. The peak intensities agree very well with the expected ones based on the isotope ratios, 29.5/2:70.5:29.5/2 (205Tl NMR) and 70.5/ 2:29.5:70.5/2 (203Tl NMR); see the SI for details. This finding is in full agreement with the solid-state structure having two Tl atoms in identical positions, and it proves unequivocally that the dimeric structure of 1 is preserved in solution. The chemical shift difference between the satellite peaks represents the coupling constant 2J(205Tl−203Tl) = 2670 Hz. The spectra indicate further fine structure of the recorded peaks caused by spin−spin coupling with 183W atoms (14.3%). The two different values for 2J(203Tl−183W) are ca. 470 and 350 Hz, respectively, and the corresponding values for 2 205 J( Tl−183W) are ca. 1% larger (see the Figure 2 caption for details). These values can be rationalized by coupling to two structurally inequivalent types of W atoms, being two bonds away from the Tl centers. These couplings may actually be averaged effects of two types of 183W centers (WA and WB) with similar chemical environments. The Tl···W distances are 3.64 and 3.65 Å for WA and 3.72 and 3.73 Å for WB, respectively. However, the relatively broad spectral lines prohibit such a detailed investigation. The 1H couplings do not play a role because of fast exchange, and the effects of 29Si (I = 1/2, 4.7%) and 17O (I = 5 /2, 0.037%) are also not detectable. We have also simulated the Tl NMR spectra considering the symmetry of 1, the relative positions of the Tl and W atoms, the natural abundance of the NMR-active nuclei, and the spin−spin coupling constants measured experimentally (Figure 2). The experimentally obtained NMR spectra are a superposition of several spectra with different combinations of NMR-active nuclei. In our simulated model, only the Tl−O−Tl and Tl− O−183W couplings were considered, leading to 54 different spin systems. The easily detectable shape difference between the B

DOI: 10.1021/acs.inorgchem.6b01921 Inorg. Chem. XXXX, XXX, XXX−XXX

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usually needs either very strong soft ligands such as bromide or cyanide ions or strongly chelating organic ligands such as ethylenediaminetetraacetate.18,21 The two {B-β-SiW8O31} lacunary POM units with five terminal oxo groups each appear to be excellent polydentate ligands for Tl3+ ions. The Si−O sites behave as μ3-O atoms, and the other O-atom donors bind each Tl center inside four six-membered chelate rings. The structure of 1, where the Tl centers are “closely packed”, is highly stable toward dissociation into monomers, or the loss of Tl3+ in the form of insoluble Tl2O3. We also investigated the antibacterial activity of 1 at physiological pH (Table S7). The minimum inhibitory concentration (MIC) of 1 is compared to that of Tl(NO3)3 and Tl(OOCCH3) (Table S7), with the latter being a known inorganic antibiotic against Gram-negative bacteria.22 Interestingly, both Tl salts show the same activity. In a phosphate buffer (0.5 M, pH 7), the Tl3+ salt is not soluble, in contrast to polyanion 1. The latter shows a high antibacterial activity against Gram-positive bacteria, especially Bacillus aquimaris and Bacillus subtilis, exhibiting a 32-fold-increased sensitivity toward 1 compared to the plain Tl+ and Tl3+ salts. A potential application of 1 could be as a component of painting materials in low concentration. In conclusion, we have synthesized and structurally characterized the first discrete thallium-containing polyanion 1 by a multitude of solution and solid-state analytical techniques, as well as DFT studies. It was shown by 205Tl and 203Tl NMR and ESIMS that 1 stays intact in solution. We have also prepared other Tl-containing POMs, which will be reported in due course.

central and satellite peaks can be attributed to the fact that the 183 W atoms are coupled to homonuclearly coupled Tl atoms (A2 spin system), resulting in magnetic nonequivalence of the chemically equivalent two 205Tl or 203Tl atoms, i.e., creating an AA′X spin system. In such a spin system, the Tl−W splitting is just half of that observed for the satellite signals (see the SI). The theoretical spectra shown in Figure 2 have been simulated as the superposition of 60 Hz wide Lorentzians with the calculated positions and relative integrated intensities of peaks. The very good similarities strongly support the theory that polyanion 1 remains structurally intact in solution. The chemical shift value of ca. 2206 ppm is consistent with a 3+ oxidation state for the Tl atoms being six-coordinated to O atoms in an octahedral geometry in water.17,18 Comparing the 2J(205Tl−203Tl) = 2670 Hz coupling constant to literature values is difficult because published constants are mostly based on organothallium compounds in nonaqueous solvents. The 2J(205Tl−203Tl) = 2560 Hz coupling constant for (TlOEt)4 is very similar to ours,19 but values ranging from 1920 to 19835 Hz for thallium−metal carbonyls have also been reported.12 The solution behavior of 1 was also studied by electrospray ionization mass spectrometry (ESI-MS).20 The major peaks observed in the spectrum of NH4K-1 dissolved in water show a series of envelopes related to various species based on 1 with different numbers of associated K atoms and protons (Figure 3).

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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.6b01921. Details on synthesis, XRD, 205/203Tl NMR, BVS, IR, UV−vis, electrochemistry, TGA, DFT, and antibacterial studies (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

Figure 3. Negative-ion mass spectrum of NH4K-1 showing the different fragmented species and their assignments.

*E-mail: [email protected]. Tel.: +36 52 512900. *E-mail: [email protected]. Notes

For example, the highest-intensity peak centered at m/z 886.34 can be assigned to the 5− charged dehydrated species [K2HTl2(SiW8O29)(SiW8O30)]5−, which indicates that complete dehydration has taken place during the ionization process. The other main peaks and assignments are shown in the inset of Figure 3. These observations were further supported by UV−vis spectroscopy and electrochemistry. UV−vis spectra of 1 dissolved in phosphate media (pH 4.3, 3.3, and 7.2) and water show that it is stable for at least 24 h (Figures S5−S8). A complementary cross-check of this stability was obtained in phosphate media (pH 4.3 and pH 3.3) by cyclic voltammetry and controlled potential electrolysis focusing on the thallium redox processes (Figures S9 and S10). Tl NMR, ESI-MS, UV−vis, and electrochemistry suggest that polyanion 1 maintains its integrity in aqueous solution. Nevertheless, this solution stability is very interesting. In order to avoid hydrolysis of “soft” Tl3+ ions in aqueous solution, one

The authors declare no competing financial interest. ▽ Retired.

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ACKNOWLEDGMENTS U.K. acknowledges support by the German Science Foundation (Grant DFG-KO 2288/20-1), Jacobs University, and CMST COST Action CM1203 (PoCheMoN). I.T. and T.F. grateful to the Hungarian Scientific Research Fund (K-109029), Dr. M. Braun (ATOMKI, Debrecen) for elemental analysis, and Dr. A. Bodor (ELTE, Budapest) for recording the 203/205Tl NMR spectra at 5.87 T. We also acknowledge L. Yassine for assistance in the design of the TOC graphic. J.M.P. thanks MINECO (Grant CTQ2014-52774-P) and Generalitat de Catalunya (Grants 2014SGR199 and XRQTC) for funding support and ICREA foundation for an ICREA ACADEMIA award. Figure 1 was generated using Diamond, version 3.2, software (Copyright 1999 Crystal Impact GbR). C

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(14) Bassil, B. S.; Nellutla, S.; Kortz, U.; Stowe, A. C.; van Tol, J.; Dalal, N. S.; Keita, B.; Nadjo, L. The Satellite-Shaped Co-15 Polyoxotungstate, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}]5‑. Inorg. Chem. 2005, 44, 2659−2665. (15) Winter, R. S.; Long, D. L.; Cronin, L. Synthesis and Characterization of a Series of [M2(β-SiW8O31)2]n− Clusters and Mechanistic Insight into the Reorganization of {β-SiW8O31} into {αSiW9O34}. Inorg. Chem. 2015, 54, 4151−4155. (16) Brown, I. D.; Altermatt, D. Bond-valence Parameters Obtained From a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (17) Hinton, J. F. Thallium NMR Spectroscopy. Bull. Magn. Reson. 1992, 23, 90−108. (18) Tóth, I.; Györi, B. Thallium: Inorganic Chemistry. In Encyclopedia of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: New York, 2005; Vol. 10, pp 6699−6711. (19) Schneider, W. G.; Buckingham, A. D. Mercury, Thallium and Lead Resonances. Discuss. Faraday Soc. 1962, 34, 147−155. (20) Izarova, N. V.; Vankova, N.; Banerjee, A.; Jameson, G. B.; Heine, T.; Schinle, F.; Hampe, O.; Kortz, U. A Noble-Metalate Bowl: The Polyoxo-6-vanado(V)-7-palladate(II) [Pd7V6O24(OH)2]6−. Angew. Chem., Int. Ed. 2010, 49, 7807−7811. (21) Tóth, I.; Brücher, E.; Zékány, L.; Veksin, V. Equilibrium Studies on the AlIII-, GaIII-, InIII- and TIIII-Ethylenediaminetetraacetate-Halide and -Sulphide Systems. Polyhedron 1989, 8, 2057−2064. (22) Kohguchi, K.; Ede, K. I.; Sagara, Y.; Nakamura, M. Effect of Thallium Acetate on the Growth of Bacteria. Kurume Med. J. 1969, 16, 163−168.

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DOI: 10.1021/acs.inorgchem.6b01921 Inorg. Chem. XXXX, XXX, XXX−XXX