19-Tungstodiarsenate(III) Functionalized by Organoantimony(III

Article pubs.acs.org/IC

19-Tungstodiarsenate(III) Functionalized by Organoantimony(III) Groups: Tuning the Structure−Bioactivity Relationship Peng Yang,† Zhengguo Lin,† Gabriela Alfaro-Espinoza,† Matthias S. Ullrich,† Ciprian I. Raţ,‡ Cristian Silvestru,‡ and Ulrich Kortz*,† †

Department of Life Sciences and Chemistry, Jacobs University, P.O. Box 750 561, 28725 Bremen, Germany Department of Chemistry, Supramolecular Organic and Organometallic Chemistry Centre (SOOMCC), Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Arany Janos Str. 11, RO-400028 Cluj-Napoca, Romania

‡

S Supporting Information *

ABSTRACT: A family of three discrete organoantimony(III)-functionalized heteropolyanions[Na{2-(Me2HN+CH2)C6H4SbIII}AsIII2W19O67(H2O)]10− (1), [{2-(Me2HN+CH2)C6H4SbIII}2AsIII2W19O67(H2O)]8− (2), and [{2-(Me2HN+CH2)C 6 H 4 Sb III }{WO 2 (H 2 O)}{WO(H 2 O)} 2 (B-β-As III W 8 O 30 )(B-α-As III W 9 O 33 ) 2 ] 14− (3)have been prepared by one-pot reactions of the 19-tungstodiarsenate(III) precursor [AsIII2W19O67(H2O)]14− with 2-(Me2NCH2)C6H4SbCl2. The three novel polyanions crystallized as the hydrated mixed-alkali salts Cs3KNa6[Na{2(Me2HN+CH2)C6H4SbIII}AsIII2W19O67(H2O)]·43H2O (CsKNa-1), Rb2.5K5.5[{2(Me2HN+CH2)C6H4SbIII}2AsIII2W19O67(H2O)]·18H2O·Me2NCH2C6H5 (RbK-2), and Rb 2.5 K 11.5 [{2-(Me 2 HN + CH 2 )C 6 H 4 Sb III}{WO 2 (H 2 O)}{WO(H 2 O)} 2 (B-βAsIIIW8O30)(B-α-AsIIIW9O33)2]·52H2O (RbK-3), respectively. The number of incorporated {2-(Me2HN+CH2)C6H4SbIII} units could be tuned by careful control of the experimental parameters. Polyanions 1 and 2 possess a dimeric sandwich-type topology, whereas 3 features a trimeric, wheel-shaped structure, representing the largest organoantimony-containing polyanion. All three compounds were fully characterized in the solid state via single-crystal Xray diffraction (XRD), infrared (IR) spectroscopy, and thermogravimetric analysis, and their aqueous solution stability was validated by ultraviolet−visible light (UV-vis) and multinuclear (1H, 13C, and 183W) nuclear magnetic resonance (NMR) spectroscopy. Effective inhibition against six different types of bacteria was observed for 1 and 2, and we could extract a structure−bioactivity relationship for these polyanions.

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chemically active [B-α-AsIIIW9O33]9− subunits linked by a WO6 hinge and possesses various connection modes with metal ion guests; (ii) its adaptive solution behavior allows for flexible isomerization and/or dissociation−association processes by careful control of reaction parameters;8 and (iii) the dimeric and dilacunary nature of {As2W19} is attractive for the assembly of asymmetric or chiral clusters.7c Keeping in mind the desired application, selection of a suitable POM precursor is important, but also the attached organic groups play a crucial role for the properties of the resulting organometallic polyanions. For example, a large number of organotin(IV)-containing POMs are known, including various organotin(IV) guests.2e−i,4a−c,e,5−7 The subclass of phenyltin(IV)-containing heteropolyanions mostly comprises monomeric or dimeric architectures, with the steric effect of the aromatic ring probably preventing further aggregation.5g,h,6a,d,e,7b,g,k On the other hand, using a sterically less demanding electrophile, dimethyltin(IV), has resulted in much larger oligomeric POM assemblies,7a,c,f,i such as the spectacular ball-shaped W108 cluster

INTRODUCTION Polyoxometalates (POMs) represent a well-recognized class of molecular-scale metallic oxides with an unmatched structural variety and manifold of fascinating properties (e.g., catalysis, magnetism, electro/photochromism, medicine).1 Covalent attachment of organic or organometallic moieties to POM matrices allows for modulating various properties on the molecular level, such as lipophilicity, solubility, stability, and bioactivity. Such POM derivatives integrate the functionality of both, the attached organic component(s) and the inorganic metal-oxo framework, allowing for specialized applications otherwise hard to achieve, such as designing potent biological inhibitors (e.g., antibacterial, antiviral, antitumor).1h,2 Preformed lacunary heteropolytungstates can serve as welldefined, multidentate, inorganic O-donor ligands, allowing for strong binding to d- and f-block metal ions. By using this mechanistically straightforward approach, organometallic fragments such as organoruthenium(II),3 organogermanium(IV),2k,l,4 and organotin(IV)2e−i,4a−c,e,5−7 have been successfully grafted on POMs as well. Within the rich reservoir of lacunary precursors, the easily available, divacant, lone-pair-containing 19-tungstodiarsenate(III), [AsIII2W19O67(H2O)]14−, is particularly interesting for the following reasons: (i) it comprises two © 2015 American Chemical Society

Received: September 23, 2015 Published: December 14, 2015 251

DOI: 10.1021/acs.inorgchem.5b02189 Inorg. Chem. 2016, 55, 251−258

Article

Inorganic Chemistry [{Me2SnIV(H2O)}24{Me2SnIV}12(A-XW9O34)12]36− (X = PV, AsV), which is the largest organometallic-POM known to date.7d In sharp contrast to the well-documented organotin(IV)POM family, the study of polyanions functionalized by organoantimony(III/V) groups is still in its infancy. Only a few polyoxostibonates9 and organoantimony(V)-containing POMs have been communicated so far.2a,10 In 2009, our group pioneered the class of structurally characterized organoantimony(III/V)-containing POMs, with the discovery of the solution-stable phenylantimony(V)-incorporated tungstophosphate, [{PhSbV(OH)}3(A-α-PVW9O34)2]9−.11a Later, we reported the first three examples of organoantimony(III)containing heteropolytungstates, which are also solution-stable and exhibit interesting antibacterial activity.11b More recently, we prepared a new family of (PhSb III )-containing tungstoarsenates(III), which are solution-stable at physiological pH and show a highly interesting structure-dependent bioactivity.11c In order to reveal the influence of the functional groups on the organoantimony(III) species, with respect to the biomedical activity and selectivity of the resulting POMs, we decided to investigate the reactivity of the Me2NCH2derivatized phenylantimony(III) species {2-(Me2NCH2)C6H4SbIII} with the POM precursor [AsIII2W19O67(H2O)]14−, including biological studies focusing on antibacterial activity.

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in 12 mL of water, and then 2-(Me2NCH2)C6H4SbCl2 (0.032 g, 0.100 mmol) in 3 mL of ethanol was added dropwise with vigorous stirring. The resulting solution was stirred for 20 min and then the pH was decreased to 3.0 by addition of 1 M HCl. The solution was stirred for another 20 min and then filtered, and a few drops of 0.5 M RbCl were added. The final solution was cooled to 4−5 °C. Light yellow, needleshaped crystals were obtained after 3 days, which were filtered off and air-dried. Yield: 0.074 g (26% based on W). Elemental analysis (%): Calcd: Rb 3.48, K 3.50, Sb 3.97, As 2.44, W 56.94, C 5.28, N 0.68, H 1.27; Found: Rb 3.79, K 3.26, Sb 3.62, As 2.51, W 56.95, C 3.26, N 0.81, H 1.77. IR (2% KBr pellet, ν/cm−1): 1622 (m), 1457 (w), 1433 (w), 1410 (w), 1206 (w), 951 (m), 863 (m), 724 (s), 459 (w). Synthesis of Rb2.5K11.5[{2-(Me2HN+CH2)C6H4SbIII}{WO2(H2O)}{WO(H2O)}2(B-β-AsIIIW8O30)(B-α-AsIIIW9O33)2]·52H2O (RbK-3). K14[AsIII2W19O67(H2O)]·23H2O (0.396 g, 0.070 mmol) was dissolved in 12 mL of water, and then 2-(Me2NCH2)C6H4SbCl2 (0.016 g, 0.050 mmol) in 3 mL of ethanol was added dropwise with vigorous stirring. The resulting solution was stirred for 20 min and then the pH was decreased to 3.0 by addition of 1 M HCl. The solution was stirred for another 20 min and then filtered, and a few drops of 0.5 M RbCl were added. The final solution was cooled to 4−5 °C. Light yellow, blockshaped crystals were obtained after 3 weeks, which were filtered off and air-dried. Yield: 0.127 g (30% based on W). Elemental analysis (%): Calcd: Rb 2.35, K 4.96, Sb 1.34, As 2.48, W 58.80, C 1.19, N 0.15, H 1.36; Found: Rb 2.24, K 5.10, Sb 1.33, As 2.60, W 58.80, C 1.26, N 0.12, H 0.69. IR (2% KBr pellet, ν/cm−1): 1624 (m), 1458 (w), 1382 (w), 955 (m), 899 (s), 853 (w), 777 (s), 752 (s), 692 (m), 485 (w), 458 (w). X-ray Crystallography. Single crystals of CsKNa-1, RbK-2, and RbK-3 were mounted in a Hampton cryoloop with light oil to prevent water loss. Data collections were performed at 100 K on a Bruker Kappa X8 APEX II CCD single-crystal diffractometer equipped with a sealed Mo tube and a graphite monochromator (λ = 0.71073 Å). The SHELX software package (Bruker) was used to solve and refine the structures.13 An empirical absorption correction was applied using the SADABS program.14 The structures were solved by direct methods and refined by the full-matrix least-squares method (∑w(|F0|2 − | Fc|2)2) with anisotropic thermal parameters for all heavy atoms included in the model. The hydrogen atoms of the organic groups were introduced in geometrically calculated positions. The H atoms of the crystal waters were not located. It was not possible to locate all counter cations via X-ray diffraction (XRD), probably because of severe crystallographic disorder, which is a common problem in POM crystallography. Therefore, the exact number of counter cations and crystal waters in the compounds was determined by elemental analysis, and the resulting formula units were further used throughout the paper and in the CIF file for overall consistency. In the Supporting Information, the crystal data and structure refinement for the three compounds is summarized in Table S1 in the Supporting Information, and selected bond lengths and angles are listed in Tables S2 and S3 in the Supporting Information. Cambridge Crystallographic Data files CCDC-1416788 (CsKNa-1), CCDC-1416789 (RbK-2), and CCDC1416790 (RbK-3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/ data_request/cif. Antibacterial Activity: Determination of Minimal Inhibitory Concentrations (MIC) for Bacterial Cells. The MIC studies were carried out following our earlier work.11b The Gram-positive bacteria Paenibacillus sp., Bacillus subtilis, and Clavibacter michiganensis, as well as the Gram-negative bacteria Vibrio sp. Gal12, Pseudomonas putida DSM 291, and Escherichia coli DH5α were used in the assay. The concentration of polyanions 1 and 2 in deionized water used in the MIC assay was 10 mg/mL (see Table S4 in the Supporting Information).

EXPERIMENTAL SECTION

Materials and Physical Measurements. The precursors K14[AsIII2W19O67(H2O)]·23H2O8a and 2-(Me2NCH2)C6H4SbCl212 were synthesized according to published procedures and the identity of the products was confirmed by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. All other reagents were purchased from commercial sources and used without further purification. The IR spectra were recorded on KBr disks using a Nicolet-Avatar 370 spectrometer between 400 cm−1 and 4000 cm−1. Elemental analyses were performed by CNRS, Service Central d′Analyze, Solaize, France. Thermogravimetric analyses (TGA) were carried out on a TA Instruments SDT Q600 thermobalance with a 100 mL min−1 flow of nitrogen, with the temperature being ramped from 20 °C up to 800 °C at a rate of 5 °C min−1. The ultraviolet−visible light (UV-vis) spectra were recorded in 1 cm quartz cuvettes on a Varian Cary 100 Bio UVvis spectrophotometer in the range of 200−800 nm. The NMR spectra were recorded on a 400 MHz instrument (JEOL, Model ECX) at room temperature, using 5 mm tubes for 1H and 13C NMR, and 10 mm tubes for 183W NMR, with respective resonance frequencies of 399.78 MHz (for 1H NMR), 100.71 MHz (for 13C NMR), and 16.69 MHz (for 183W NMR). The chemical shifts are reported with respect to the references Si(CH3)4 (1H and 13C) and 1 M aqueous Na2WO4 (183W). Synthesis of Cs 3 KNa 6 [Na{2-(Me 2 HN + CH 2 )C 6 H 4 Sb I I I }AsIII2W19O67(H2O)]·43H2O (CsKNa-1). K14[AsIII2W19O67(H2O)]· 23H2O (0.264 g, 0.046 mmol) was dissolved in 13 mL of 1 M NaOAc/AcOH buffer (pH 5.0), and then 2-(Me2NCH2)C6H4SbCl2 (0.016 g, 0.050 mmol) in 3 mL of ethanol was added dropwise with vigorous stirring. The resulting solution was stirred for 30 min and filtered, and a few drops of 0.25 M CsCl were added. The final solution was cooled to 4−5 °C. Light yellow, platelike crystals were obtained after 1 week, which were filtered off and air-dried. Yield: 0.128 g (44% based on W). Elemental analysis (%): Calcd: Cs 6.27, K 0.61, Na 2.53, Sb 1.91, As 2.35, W 54.89, C 1.70, N 0.22, H 1.60; Found: Cs 5.89, K 0.44, Na 2.68, Sb 1.92, As 2.40, W 54.98, C 1.76, N 0.19, H 0.82. IR (2% KBr pellet, ν/cm−1): 1619 (m), 1461 (w), 1384 (w), 1207 (w), 946 (s), 890 (s), 792 (s), 743 (s), 669 (m), 480 (m). Synthesis of Rb2.5K5.5[{2-(Me2HN+CH2)C6H 4Sb III} 2As III2 W 19O 67 (H2 O)]·18H2 O·Me 2NCH2 C6H 5 (RbK-2). K14[AsIII2W19O67(H2O)]·23H2O (0.264 g, 0.046 mmol) was dissolved

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RESULTS AND DISCUSSION Synthesis and Structure. The mono-{2-(Me2HN+CH2)C6H4SbIII}-substituted, sandwich-type [Na{2-(Me2HN+CH2)252


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Inorganic Chemistry C6H4SbIII}AsIII2W19O67(H2O)]10− (1) is prepared via reaction of the POM precursor salt K14[AsIII2W19O67(H2O)]·23H2O with an equimolar quantity of 2-(Me2NCH2)C6H4SbCl2 in 1 M NaOAc/AcOH buffer at pH 5.0. Polyanion 1 consists of two trivacant [B-α-AsIIIW9O33]9− Keggin units, which sandwich a central belt comprising a {2-(Me2HN+CH2)C6H4SbIII} group, one {trans-WO(H2O)} moiety, and a Na+ ion, leading to an overall dimeric structure with idealized Cs symmetry (see Figure 1a). Alternatively, 1 can be viewed as a clamshell-like

trapped in the two lacunary sites of [AsIII2W19O67(H2O)]14−, leading to an overall structure with idealized C2v symmetry. The POM precursor enforces a distorted square basal plane for each Sb atom with the apical site occupied by a terminal aryl group pointing outward (see Figure 2a). The bond lengths of Sb−O

Figure 2. (a) Combined polyhedral/ball-and-stick representation of [{2-(Me2HN+CH2)C6H4SbIII}2AsIII2W19O67(H2O)]8− (2); (b) top view of the isosceles triangle in the central belt of 2. Color code: WO6, cyan octahedra; W in the central belt, black; As, yellow; Sb, orange; O, red; N, blue; and C, gray balls. Hydrogen atoms are omitted for the sake of clarity.

Figure 1. (a) Combined polyhedral/ball-and-stick representation of [Na{2-(Me2HN+CH2)C6H4SbIII}AsIII2W19O67(H2O)]10− (1); (b) top view of the scalene triangle in the central belt of 1. Color code: WO6, cyan octahedra; W in the central belt, black; As, yellow; Sb, orange; Na, pink; O, red; N, blue; and C, gray balls. Hydrogen atoms are omitted for the sake of clarity.

(2.121(10)−2.447(10) Å) and Sb−C (2.158(14)−2.186(14) Å) are within the usual ranges.11b,c,16 The two SbIII centers and the central W atom almost form an ideal isosceles triangle, with side lengths of 4.4 and 4.5 Å, respectively, and an Sb···Sb distance of 4.0 Å (see Figure 2b). In analogy with polyanion 1, the two dangling functions (Me2HN+CH2) of 2 are protonated and oriented away from each other in the solid state to avoid steric constraints. Interestingly, we discovered a free, nonprotonated Me2NCH2C6H5 molecule, which must result from cleavage of the Sb−C bond, because of the more-acidic medium used for the synthesis of 2 (pH 3), as compared to the synthesis of 1 (pH 5). This free Me2NCH2C6H5 molecule can be identified unambiguously in both the solid state and in solution by single-crystal XRD and 13C NMR spectroscopy, respectively (see Figures S1 and S14b in the Supporting Information). Cleavage of an Sb−C bond has been seen previously in POM chemistry during the synthesis of [{PhSbV(OH)}3(A-αPV W9O34) 2]9− by reaction of Ph2SbCl3 with Na9[A-αPVW9O34]·7H2O under acidic hydrothermal conditions.11a We also discovered that polyanion 2 is formed in best yield at pH 3, whereas the yield decreases with increasing pH, and at pH >7, only the monomeric, flowerpot-shaped [{2-(Me2NCH2)C6H4SbIII}3(B-α-AsIIIW9O33)]3− is formed.11b In addition to polyanions 1 and 2, we also prepared the trimeric cyclic polyanion [{2-(Me2 HN +CH2 )C6H 4SbIII}{WO2(H2O)}{WO(H2O)}2(B-β-AsIIIW8O30)(B-αAsIIIW9O33)2]14− (3). Polyanion 3 is composed of two [B-αAsIIIW9O33]9− and one [B-β-AsIIIW8O30]9− subunits fused by one mer-{(trans-WO(H2O))OSb} and two {trans-WO(H2O)} bridges in a triangular fashion, which is further capped by a {2(Me2HN+CH2)C6H4SbIII} moiety, leading to an unprecedented, wheel-shaped topology with idealized C1 symmetry (see Figure 3a). The two W atoms in 3 are 6-coordinated, bridging the [B-β-AsIIIW8O30]9− to the two [B-α-AsIIIW9O33]9− units via four μ2-oxo ligands, with an external aqua- and an internal oxo-ligand in trans orientation (see Figure 3b). It is

[AsIII2W19O67(H2O)]14− unit that has taken up one {2(Me2HN+CH2)C6H4SbIII} group and a Na+ cation. Bond valence sum (BVS) calculations15 indicated that the central, hinge-like W atom has a terminal oxo-group (W19−O19T = 1.706(8) Å) pointing inside the central cavity and, trans to it, a terminal water molecule (W19−O1W = 2.242(8) Å), as also observed for the [AsIII2W19O67(H2O)]14− precursor.8a The Sb atom adopts a distorted, square-pyramidal coordination sphere with four μ2-oxo ligands positioned in the equatorial plane (Sb−O = 2.124(8)−2.361(8) Å) and the terminal organic group occupying the apical position (Sb−C = 2.206(11) Å). Notably, rather than forming an intramolecular N → Sb interaction, the Me2NCH2 pendant arm is twisted away from the metal center, with its N atom being protonated under the acidic environment (pH 5.0) leading to an ammonium group (Me2HN+CH2), which has been seen previously.12 The Na ion in 1 is tightly bound to five oxo bridges (Na···O = 2.321(10)− 2.385(9) Å) without any terminal ligand. Moreover, because of the fact that three different elements (Sb, W, Na) occupy the three addenda positions in the central belt, the distances among them are different, leading to an irregular triangular arrangement (see Figure 1b). We discovered that NaOAc/AcOH buffer is crucial for the formation of polyanion 1, because, in the absence of a sufficient concentration of Na ions, only polyanion 2 could be obtained. Reaction of 2-(Me2NCH2)C6H4SbCl2 and K14[AsIII2W19O67(H2O)]·23H2O in a molar ratio of 2:1 in an aqueous pH 3.0 medium, resulted in [{2-(Me2HN+CH2)C6H4SbIII}2AsIII2W19O67(H2O)]8− (2). Polyanion 2 also possesses a sandwich-type structure, but it incorporates two {2-(Me2HN+CH2)C6H4SbIII} groups, rather than just one in 1. In 2, two such protonated organoantimony(III) moieties are 253


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Inorganic Chemistry

Figure 3. (a) Combined polyhedral/ball-and-stick representation of [{2-(Me2HN+CH2)C6H4SbIII}{WO2(H2O)}{WO(H2O)}2(B-β-AsIIIW8O30)(Bα-AsIIIW9O33)2]14− (3); (b) the tungstate framework of 3. Color code: WO6, cyan octahedra; {WO2(H2O)} and {WO(H2O)} linkers, violet octahedra; {B-β-AsIIIW8O30} unit, light green polyhedra; {B-α-AsIIIW9O33} unit, light blue polyhedra; W, black; As, yellow; Sb, orange; O, red; N, blue; and C, gray balls. Hydrogen atoms are omitted for the sake of clarity.

apparent that the [AsIII2W19O68(H2O)]14− precursor underwent partial decomposition and rearrangement during the reaction, which most likely is driven by the small ratio (1:1.5) of organoantimony electrophile to the {As2W19} precursor combined by the absence of Na ions, preventing formation of 2 and 1, respectively. The Sb atom in 3 is tetra-coordinated, featuring an unusual seesaw configuration, by three μ2-oxo bridges to the [B-α-AsIIIW9O33]9−, [B-β-AsIIIW8O30]9−, and mer-{(trans-WO(H2O))OSb} units, respectively, with the fourth position occupied by the organic ligand (Me2HN+CH2)C6H4. The bond distances of Sb−O fall in the range of 1.990(14)− 2.214(14) Å, and the O−Sb−O angles are in the range of 82.9(5)°−167.3(5)°. It is worth noting that polyanion 3 represents the largest cluster yet found in organoantimony− POM chemistry. Initially, we discovered the formation of 3 in a reaction solution of polyanion 2, which was left standing for ∼2−3 weeks, resulting in two types of crystals, RbK-2 and RbK-3, the latter as a minor byproduct (see Figure S2 in the Supporting Information). After performing single-crystal XRD on RbK-3, we obtained the structure of this novel polyanion and then optimized the synthetic procedure accordingly, with the optimum being a stoichiometric ratio of 1:1.5 for 2(Me2NCH2)C6H4SbCl2 and K14[AsIII2W19O67(H2O)]·23H2O, respectively. We note that the solid-state packing of RbK-3 involves hydrogen bonds between the (Me2HN+CH2)C6H4 functionalities of neighboring polyanions, which seems to favor the crystallization process (see Figure S3 in the Supporting Information). Hence, it is perhaps not completely surprising that our attempts to isolate the phenylantimony(III) analogue of 3 were unsuccessful. We discovered that acidic reaction conditions resulted in improved yields for 2, most likely because protonation of the organic pendant arm eliminates the possibility of Lewis acid− base interaction between N and Sb, and, hence, no steric hindrance occurs. At the same time, very acidic conditions can lead to cleavage of the hydrolytically metastable Sb−C bond. All Sb atoms in 1−3 are in the +3 oxidation state and no additional O atoms are protonated, as shown by BVS calculations. The negative charge of the polyanions is balanced by alkali-metal counter cations in the solid state, as shown by elemental analysis. IR Spectroscopy. The Fourier transform infrared (FT-IR) spectra of CsKNa-1, RbK-2, and RbK-3 are presented in

Figures S4−S6 in the Supporting Information. The absorption bands appearing in the region of 1461−1206 cm−1 can be assigned to the C−C stretching vibrations of the phenyl rings and the bending vibrations of −NH2 and −CH2 groups from the pendent arms, whereas the signals at 752−669 cm−1 are attributed to aromatic C−H out-of-plane vibrations.17 The intense peaks at 890 cm−1 (CsKNa-1), 863 cm−1 (RbK-2), 899 and 853 cm−1 (RbK-3) correspond to the respective As−O antisymmetric vibrations. The other signals below 1000 cm−1 can be assigned to terminal WO as well as bridging W−O− W stretching modes.18 The broad bands located at 1619 cm−1 for CsKNa-1, 1622 cm−1 for RbK-2, and 1624 cm−1 for RbK-3 originate from the bending modes of the crystal water molecules.19 Thermogravimetric Analysis. Thermogravimetric analyses (TGA) were performed on crystalline samples under nitrogen flow, and two weight-loss steps were observed for all three samples (see Figures S7−S9 in the Supporting Information). Taking RbK-2 as an example, the first weightloss step of 4.9% represents a dehydration process up to ∼258 °C, resulting in the loss of 16 crystal water molecules (calc. 4.7%). The number of crystal waters determined by TGA is slightly lower than that obtained by elemental analysis (18 molecules), which probably reflects slightly different handling of the samples before performing the measurements. Further weight loss during the second step covering the temperature range from 310 °C to 663 °C is largely due to the loss of organic moieties and structural reorganization of the polyanion. UV-vis Spectroscopy. The stability of polyanions 1−3 in aqueous medium was assessed by UV-vis spectroscopy. Two absorption bands at 200 nm and 250−254 nm were detected, which can be attributed to the pπ−dπ charge-transfer transitions of the Ot → W band for the former, and pπ−dπ charge-transfer transitions of the Ob,c → W bands and π−π* charge-transfer of the aromatic ring for the latter.20 Meanwhile, the spectra of 1 and 2 were monitored as a function of pH, from pH 3 to pH 7, by adding very diluted HCl or KOH solutions, respectively, to adjust the pH in the acidic or basic direction. In both cases, the position and intensity of the bands were essentially unchanged, suggesting that no major structural changes occurred in the pH range of 3−7 over a period of at least 5 h (Figures S10 and S11 in the Supporting Information). In contrast, polyanion 3 underwent a rapid decrease of the 254


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Inorganic Chemistry absorption bands in question during the first 10 min, indicating major structural changes (Figure S12 in the Supporting Information). Multinuclear NMR Spectroscopy. To further examine the solution behavior of polyanions 1−3, multinuclear (1H, 13C, and 183W) NMR spectroscopy was performed on solid samples redissolved in H2O/D2O. It should be noted that, for CsKNa-1 and RbK-2, the solutions were carefully adjusted to pH 7 by adding diluted KOH solution. The 1H and 13C NMR spectra showed the expected signals of the organic groups (see Figures S13a and S13b and S14a and S14b in the Supporting Information). The 183W NMR spectrum of CsKNa-1 dissolved in H2O/ D2O resulted in a spectrum with 10 signals (see Figure 4 and

Figure 5. 183W NMR spectrum of RbK-2 recorded in H2O/D2O at room temperature.

at physiological pH, which is a necessary requirement for their potential usefulness in biological studies. Antibacterial Activity: Determination of Minimal Inhibitory Concentrations (MIC) for Bacterial Cells. Antimony potassium tartrate and stibophen are well-known as antiprotozoal and antihelmintic agents.21 Consequently, the antitumor and antimicrobial activities of antimony-containing complexes have been investigated in detail.22 Herein, six different bacterial species were utilized to evaluate the bioactivity of freshly prepared CsKNa-1 and RbK-2. As illustrated in Table 1, the K14[AsIII2W19O67(H2O)] POM precursor was also tested as a reference, but did not impact bacterial growth. In contrast, both {2-(Me2 HN+ CH2 )C 6H4SbIII}-substituted polyanions 1 and 2 successfully inhibited the growth of the selected Gram-positive and Gram-negative bacterial strains. It becomes apparent that solubilizing organoantimony(III) species in aqueous media by incorporation into suitable POMs not only helps with respect to fine-tuning of hydrophobic/hydrophilic properties, but also with mutual stability. So far, the detailed antimicrobial mechanism of organoantimony-containing polyanions is still elusive. It is presumed that the organoantimony(III)-containing POMs might interfere with peptidoglycan production inside the cell wall and thus ultimately causing death of the bacteria.22n Moreover, based on the same sandwich-type POM skeleton of 1 and 2, the inhibitory bacterial effect is proportional to the number of incorporated {2-(Me2HN+CH2)C6H4SbIII} groups, which substantiates a previously proposed structure−activity relationship.11c It is of prime interest that the phenylantimony(III) analogues [(PhSbIII){Na(H2O)}AsIII2W19O67(H2O)]11− (PhSb-1) and [(PhSbIII)2AsIII2W19O67(H2O)]10− (PhSb-2) exhibit a significantly higher antibacterial activity than 1 and 2, suggesting that only a slight variation of the organic group attached to Sb (such as a functional group in ortho position) has a remarkable impact on the bioactivity of the resulting polyanion. This observation is important, because it allows for fine-tuning the biological activity of organoantimony-containing POMs by systematic and subtle variation of the organic groups.

Figure 4. 183W NMR spectrum of CsKNa-1 recorded in H2O/D2O at room temperature.

S15 in the Supporting Information). Polyanion 1 has point group symmetry of Cs and, hence, 19 W atoms can be classified into 9 magnetically inequivalent types with two W atoms for each (δ = −87.2, −95.1, −102.9, −103.3, −105.9, −138.1, −141.1, −157.8, −174.3 ppm), plus the unique, central hinge W atom (δ = −222.1 ppm). Therefore, a total of 10 signals with intensity ratios of 2:2:2:2:2:2:2:2:2:1 are expected, which is exactly what we observed, and, hence, the solid-state structure of 1 is maintained in aqueous solution. The 183W NMR spectrum of RbK-2 dissolved in H2O/D2O resulted in a spectrum comprising 6 signals (δ = −96.8, −97.9, −100.1, −136.8, −158.3, −159.3 ppm) with relative intensities 4:2:4:4:4:1, which is in complete agreement with the C2v symmetry of polyanion 2 in the solid state (see Figure 5). This result also suggests that the two aryl groups can rotate freely in solution. The low stability of polyanion 3 in solution, already suggested by UV-vis data (vide supra), was confirmed by 1H and 13C NMR (see Figures S13c and S14c) measurements as well as 183W NMR (see Figure S16 in the Supporting Information) measurements, which all indicate structural transformation of this polyanion in aqueous solution. In short, a thorough characterization of CsKNa-1 and RbK-2 in solution by UV-vis and multinuclear (1H, 13C, and 183W) NMR spectroscopy confirmed their long-term solution stability 255


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Inorganic Chemistry

Table 1. MIC Determination of Various Polyanions against the Growth of Gram-Positive and Gram-Negative Bacteria MIC Determination (μg/mL) Gram-positive Paenibacillus sp. Bacillus subtilis Clavibacter michiganensis Gram-negative Vibrio sp. Gal12 Pseudomonas putida DSM 291 Escherichia coli DH5α

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1

2

[AsIII2W19O67(H2O)]14−

PhSb-1

PhSb-2

500 250 500

250 250 250

no inhibition no inhibition no inhibition

250 125 250

125 62.5 250

250 500 1000

250 1000 500

no inhibition no inhibition no inhibition

125 125 500

62.5 62.5 250

CONCLUSIONS We have rationally synthesized the three {2-(Me2HN+CH2)C6H4SbIII}-functionalized 19-tungstodiarsenates(III) [Na{2(Me2HN+CH2)C6H4SbIII}AsIII2W19O67(H2O)]10− (1), [{2(Me2HN+CH2)C6H4SbIII}2AsIII2W19O67(H2O)]8− (2), and [{2-(Me2HN+CH2)C6H4SbIII}{WO2(H2O)}{WO(H2O)}2(Bβ-AsIIIW8O30)(B-α-AsIIIW9O33)2]14− (3), following a facile onepot aqueous procedure by careful adjustment of the reagent ratio, ionic strength, counter cations, as well as solvent type and pH. Polyanions 1−3 have been structurally characterized in the solid state by single-crystal XRD, IR spectroscopy, and thermogravimetric analysis, and in solution by UV-vis and multinuclear (1H, 13C, and 183W) NMR spectroscopy. Polyanions 1 and 2 possess a dimeric sandwich-type topology, whereas the trimeric, wheel-shaped 3 is the largest organoantimony-containing POM known to date. Regarding formation mechanism of the three novel polyanions, the dilacunary POM precursor [AsIII2W19O67(H2O)]14− is preserved for 1 and 2 with incorporation of one and two organoantimony(III) electrophiles, respectively. On the other hand, 3 is composed of one {B-β-AsIIIW8O30} and two {B-α-AsIIIW9O33} units linked in a cyclic assembly by one mer-{(trans-WO(H2O))OSb} and two {trans-WO(H2O)} bridges, indicating a unique transformation−decomposition pathway of the dilacunary [AsIII2W19O67(H2O)]14− precursor. Polyanions 1 and 2 are solution-stable at physiological pH, hence allowing for biological studies. Both 1 and 2 were shown to inhibit the growth of a total of six different Gram-positive and Gram-negative bacterial species, along with a tunable bioactivity achieved by controlling the number and type of introduced organoantimony(III) groups. Hence, it is reasonable to believe that the bioactivity of such organoantimony-containing polyanions could be deliberately optimized by virtue of tuning the structure−bioactivity relationship, which drives immense potential for their applications as biomedical agents with improved efficacy and selectivity. Besides being of interest as biological inhibitors (e.g., antibacterial, antiviral, antitumor), such polyanions may also be highly valuable as co-crystallization agents of large biomolecules (see, for example, the 2009 Nobel Prize for ribosome crystallization). An extension of our work in several of the above-mentioned directions is currently in progress.

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Crystallographic data for CsKNa-1 (CIF) Crystallographic data for RbK-2 (CIF) Crystallographic data for RbK-3 (CIF) Tables with the main crystallographic and refinement parameters (Tables S1−S3), as well as polyanion concentrations in the MIC assay (Table S4); representation of polyanion 2 with the free Me2NCH2C6H5 molecule (Figure S1); optical micrographs of crystals of RbK-2 and RbK-3 (Figure S2); hydrogen bonds between neighboring polyanions 3 (Figure S3); FT-IR spectra (Figures S4−S6); TGA thermograms (Figures S7−S9); UV-vis spectra (Figures S10−S12), as well as 1H, 13C, and 183W NMR spectra (Figures S13−S16) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 421 200 3235. Fax: +49 421 200 3229. E-mail: u. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS U.K. thanks the German Research Council (DFG KO-2288/ 20-1), Jacobs University, and CMST COST Action CM1203 (PoCheMoN) for support. P.Y. sincerely acknowledges the China Scholarship Council (CSC) for a doctoral fellowship. C.I.R. and C.S. greatly acknowledge the financial support from the National Research Council (CNCS) of Romania, through Research Project No. PN-II-ID-PCE-2011-3-0933. We thank Annika Moje for performing the elemental carbon analysis. Figures 1−3 were generated using Diamond Version 3.2 software (copyright, Crystal Impact GbR).

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REFERENCES

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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.5b02189. 256


Article

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19-Tungstodiarsenate(III) Functionalized by Organoantimony(III

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