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.