Aluminum-Substituted Keggin Germanotungstate [HAl(H2O)GeW11O39]4-: Synthesis, Characterization, and Antibacterial Activity.

We report on the new monosubstituted aluminum Keggin-type germanotungstate (C4H12N)4[HAlGeW11O39(H2O)]·11H2O ([Al(H2O)GeW11]4-), which has been synthesized at room temperature via rearrangement of the dilacunary [γ-GeW10O36]8- polyoxometalate precursor. [Al(H2O)GeW11]4- has been characterized thoroughly both in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis as well as in solution by cyclic voltammetry (CV) 183W, 27Al NMR and UV-vis spectroscopy. A study on the antibacterial properties of [Al(H2O)GeW11]4- and the known aluminum(III)-centered Keggin polyoxotungstates (Al-POTs) α-Na5[AlW12O40] (α-[AlW12O40]5-) and Na6[Al(AlOH2)W11O39] ([Al(AlOH2)W11O39]6-) revealed enhanced activity for all three Al-POTs against the Gram-negative bacterium Moraxella catarrhalis (minimum inhibitory concentration (MIC) up to 4 μg mL-1) and the Gram-positive Enterococcus faecalis (MIC up to 128 μg mL-1) compared to the inactive Al(NO3)3 salt (MIC > 256 μg mL-1). CV indicates the redox activity of the Al-POTs as a dominating factor for the observed antibacterial activity with increased tendency to reduction, resulting in increased antibacterial activity of the POT.

Polyoxometalates (POMs)[1] represent a broad class of anionic inorganic clusters with versatile structural topologies resulting in a variety of chemical and physical properties that can be modulated by molecular design. These features make them attractive materials in a wide range of fields like catalysis,[2] electrochemistry,[3] magnetochemistry,[4] and biological chemistry[5] including protein crystallography.[6] Among the variety of POMs reported to date, Keggin-type POMs represent the largest family. The incorporation of metal ions into the vacant site(s) of lacunary POMs derived from their plenary counterparts upon controlled hydrolysis of the framework is one of the most powerful synthetic approaches used to encapsulate well-defined metal sites with heteroatoms ranging from 3d transition metals over lanthanides to main-group III elements.[1] Regarding the number of main-group-III-containing POMs reported to date, only a few examples deal with the preparation and full structural characterization of aluminum(III)-substituted POMs (Al-POMs; Table S1) due to hydrolysis of the aluminum species in an aqueous environment.[7] Most of the reported Al-POMs have been studied toward their properties as Lewis catalysts,[8] where the incorporation of a high-sensitivity NMR-active 27Al nucleus into the metal–oxo framework allows for detailed speciation and stability studies of the catalytic species. In contrast to catalytic studies, the biological applications of Al-POMs have not been investigated yet.[1] Among the investigated biological applications, the antibacterial properties of POMs have been a subject of growing interest[5] considering the continuing emergence of antibacterial resistance worldwide due to excessive or improper use of antibiotics. Because of their high negative charge, strong acidity, and geometry, POMs offer alternative modes of antimicrobial action by exhibiting synergy with some conventional antibiotics or direct antibacterial activity against both Gram-negative and Gram-positive bacteria.[5,9] Only a few all-inorganic POMs have been shown to exhibit antibacterial activity (Table S2) on their own accord[5] despite their higher water solubility, which would make them more attractive for application in biological systems compared to their hybridized counterparts. Herein, we report the synthesis and thorough characterization of a new monosubstituted Keggin-type germanotungstate, (C4H12N)4[HAlIIIGeW11O39(H2O)]·11H2O ([Al(HO)GeW]), which was subjected to an antibacterial study.

The synthesis of [Al(HO)GeW] starts with the preparation of the literature-known dilacunary germanotungstate building block K8[γ-GeW10O36]·12H2O (GeW).[10] The addition of 2 equiv of Al(NO3)3·9H2O to a stirred reaction mixture of 1 equiv of GeW (pH 7.2) resulted in a significant decrease of the pH value from 7.2 to 3.0. Readjustment of the pH value to pH 3.8 via the dropwise addition of a K2CO3 solution (2 M) led to the in situ rearrangement of GeW to the monolacunary [β2-GeW11O39]8– (GeW) POM species, which incorporates an aluminum(III) metal center and crystallizes as the pure tetramethylammonium (TMA) salt (CCDC 1936850) in a 20% yield based on tungsten (procedure 1). The in situ formation of the GeW unit can be explained by the pH decrease upon the addition of Al(NO3)3 to the reaction mixture (procedure 1, Scheme ).[10] Using the GeW unit as a precursor, the yield of the desired [Al(HO)GeW] was optimized (procedure 2). To avoid the undesired isomerization of the GeW building block, Al(NO3)3 was initially dissolved in water, giving an acidic solution of pH 2.1. The portionwise addition of GeW increases the pH to 3.8, which was kept in the range between 3.8 and 4.0 via the subsequent addition of a K2CO3 solution (2 M) over a time period of 15 min to ensure integrity of the GeW unit and further formation of the desired [Al(HO)GeW] anion. The analytically pure [Al(HO)GeW] was precipitated as the TMA salt, thereby giving increased yields of 80% based on tungsten compared to synthetic procedure 1 starting from GeW (Figure S2).

Scheme 1

Schematic Representation Showing the Syntheses of [Al(HO)GeW]

[Al(HO)GeW] can be obtained as single crystals suitable for single-crystal X-ray crystallography via rearrangement of the dilacunary [GeW] precursor (procedure 1) in a yield of 20% based on tungsten. The portionwise addition of the monolacunary [GeW] to a solution of Al(NO3)3 leads to the isolation of [Al(HO)GeW] in higher yields of 80% based on tungsten (procedure 2). The pH was kept at 3.8–4.0 in both procedures via the constant addition of K2CO3 solution (2 M). Black and red spheres represent the germanium(IV) and oxygen ions, respectively. Gray transparent octahedra for aluminum(III) and magenta polyhedra for {WO6}.

Single-crystal X-ray diffraction (SXRD) measurements were performed on single crystals obtained from procedure 1, revealing that [Al(HO)GeW] crystallizes in the monoclinic space group P21/c (Tables S3–S5). The crystal structure of [Al(HO)GeW] represents a monosubstituted β2-Keggin-type polyanion with idealized C1 symmetry. Occupation of the vacant site with aluminum(III) results in the monosubstituted Keggin-type architecture. The aluminum(III) metal center exhibits a distorted octahedral coordination environment with one terminal H2O ligand and Al–O bond lengths ranging from 1.823 to 2.037 Å and shows a disorder with tungsten in a 90:10 aluminum/tungsten ratio. Powder XRD measurements were performed on [Al(HO)GeW] and compared to the corresponding simulated spectrum, thereby showing the homogeneity of the bulk sample (Figure S4). Besides XRD, [Al(HO)GeW] was characterized in the solid state by attenuated-total-reflectance IR spectroscopy (Figure S1), showing the terminal W=O and bridging W–O–W vibrations typical for the Keggin-type polyoxotungstate framework. The bands at 1630 and 2960 cm–1 are attributed to the vibration and deformation bands of TMA methyl groups. The number of water molecules in (C4H12N)4[HAlIIIGeW11O39(H2O)]·11H2O was determined using thermogravimetric analysis (TGA). The two weight-loss regions (Figure S3) are attributed to losses of 11 water and 4 TMA molecules, respectively. The UV–vis spectrum of [Al(HO)GeW] is characterized by an absorption maximum at 275 nm attributed to the p(Ob) → d*(W) ligand-to-metal charge-transfer typical for the Keggin-type framework (Figure S5).[11] The 183W NMR spectrum of a freshly prepared aqueous solution (pH 6.8) of [Al(HO)GeW] (18.4 mM) reveals the domination of [Al(HO)GeW] in solution, which possesses C1 symmetry and gives rise to 11 183W NMR signals (all tungsten atoms are chemically unique; Figure ).[12] The additional signal at −84.3 ppm (6.5% based on integration values) can be assigned to α-[GeW12O40]4–, which is in accordance with the shift at −81.9 ppm previously described in the literature.[13]27Al NMR measurements were performed on [Al(HO)GeW] dissolved in D2O (pH 6.8), revealing one broad peak at 10.2 ppm, which can be attributed to the octahedrally coordinated aluminum(III) present in [Al(HO)GeW] (Figure S6).

Figure 1

183W NMR spectrum of [Al(HO)GeW] in D2O at pH 6.8. Signal assignment was according to refs (12) and (13). [Al(HO)GeW] was dissolved in water to obtain a 60 mg mL–1 solution (18.4 mM). The total recording time was 60 h, and the chemical shifts were measured relative to an external 1 M Na2WO4 standard.

Considering the solution stability of [Al(HO)GeW] under physiological conditions (pH 6.8), antibacterial studies against Moraxella catarrhalis, a Gram-negative human mucosal pathogen that causes middle ear infections in children,[14] and Enterococcus faecalis, a Gram-positive bacterium responsible for life-threatening sepsis and urinary tract and meningitis infections,[15] were performed. For comparability of the antibacterial activity of [Al(HO)GeW], the literature-known aluminum-substituted polyoxotungstates (Al-POTs) α-[AlWO] and α-[Al(AlOH)WO] synthesized and characterized by Weinstock et al.[12] were prepared, and their purity and stability at physiological pH were proven by 27Al NMR spectroscopy (Figures S7 and S8). 27Al NMR measurements were performed on solutions of the Al-POTs incubated in the Müller–Hinton–Bouillon (MHB) medium[16] (pH 7.4) at 37 °C overnight to recreate the conditions used for the antibacterial tests. The 27Al NMR spectra of [Al(AlOH)WO], α-[AlWO], and [Al(HO)GeW] in a 50% D2O/MHB medium solution remain unchanged (Figures S9−S11). All three Al-POTs reveal enhanced antibacterial activity in the order α-[Al(AlOH)WO] < [Al(HO)GeW] < α-[AlWO] compared to the inactive Al(NO3)3 (Table S7). In contrast to the expected trend that would suggest increased antibacterial activity with an increasing negative charge of the corresponding polyanion,[17]α-[AlWO] exhibits the overall highest activity with minimum inhibitory values of MIC = 4 μg mL–1 against M. catarrhalis (Table S7) and MIC = 128 μg mL–1 against E. faecalis. For a better understanding of the observed trend, cyclic voltammetry (CV) was performed on 2 mM solutions of [Al(HO)GeW], α-[AlWO], and [Al(AlOH)WO] in a MHB medium, revealing three reversible redox waves attributed to tungsten(VI)/tungsten(V) transitions (Table S6 and Figures S12–S14) for all representatives, with a clear shift of the peak cathodic currents to more positive potentials in the order α-[Al(AlOH)WO] < [Al(HO)GeW] < α-[AlWO], which is in accordance with the observed activity trend of the Al-POTs (Figure ). On the basis of the CV measurements, the activity trend can be explained by the different redox activity of the POTs with increased tendency to reduction, resulting in an increased antibacterial activity of the POT. Considering the charge densities q/M (charge q divided by the number of tungsten atoms M) of the polyanions (0.36 for [Al(HO)GeW], 0.42 for α-[AlWO], and 0.54 for [Al(AlOH)WO]), recent findings show that POMs, as superchaotropes, with intermediate charge density (q/M = 0.4) interact considerably strongly with various surfaces of different or mixed polarities[18] such as the bacterial cytoskeleton of the Gram-positive E. faecalis, rendering the intact α-[AlWO] (q/M = 0.42) and [Al(HO)GeW] (q/M = 0.36) to exhibit the highest activity (MIC = 128 μg/mL) out of the series.

Figure 2

Superimposed cyclic voltammograms of α-[Al(AlOH)WO], [Al(HO)GeW], and α-[AlWO] in a MHB medium at pH 6.8. Working electrode, glassy carbon (d = 3 mm); reference electrode, Ag/AgCl; scan rate, 50 mV s–1; concentration of POMs, 2 mM.

In conclusion, the new monosubstituted Keggin-based aluminogermanotungstate [Al(HO)GeW] was synthesized and thoroughly characterized in the solid state and solution. A study on the antibacterial activity of [Al(HO)GeW], α-[AlWO], and α-[Al(AlOH)WO] revealed the redox activity of the polyanions to be a crucial factor determining the overall antibacterial activity. These findings highlight the importance of lacunary POMs as all-inorganic ligands, which can be used to tune the redox behavior of otherwise antibacterially inactive metal centers such as Al.