Single- and double-sided functionalized hybrid organic-inorganic Anderson polyoxomolybdates with Ga(III) and Fe(III) positioned as central heteroatoms have been synthesized in a mild, two-step synthesis in an aqueous medium. Compounds 1-4 were isolated as hydrated salts, [TBA]3[GaMo6O18(OH)3{(OCH2)3CCH2OH}]×12 H2O (1) (TBA = tetrabutylammonium), Na3[FeMo6O18{(OCH2)3CCH2OH}2]×11 H2O (2), [TMA]2[GaMo6O18(OH)3{(OCH2)3CNH3}]×7 H2O (3) (TMA = tetramethylammonium), and Na[TMA]2[FeMo6O18(OH)3{(OCH2)3CNH3}](OH)×6 H2O (4). All the compounds were characterized based on single-crystal X-ray diffraction (SXRD), FTIR, UV/Vis, thermogravimetric, ESI-MS, NMR, and elemental analyses. Compound 1 was also crystallized with two smaller organic cations, giving [TMA]3[GaMo6O18(OH)3{(OCH2)3CCH2OH}]×n H2O (5) and [GDM]3[GaMo6O18(OH)3{(OCH2)3CCH2OH}]×n H2O (6) (GDM = guanidinium) and were characterized based on UV/Vis, NMR, FTIR, and elemental analyses. The use of these compounds as additives in macromolecular crystallography was investigated by examining their hydrolytic stability by using ESI-MS in a pH range of 4 to 9. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that BSA remains intact in a solution containing up to 100 equivalents of 1 or 4 over more than four days at 20 °C. Zeta potential measurements demonstrate that 1-4 induce charge inversions on the positively charged surface of BSA (1 mg mL(-1)) with concentrations starting as low as 1.29 mM for compounds 1 and 2, which have the highest negative surface charge.

The antibacterial activity of 29 different polyoxometalates (POMs) against Moraxella catarrhalis was investigated by determination of the minimum inhibitory concentration (MIC). The Preyssler type polyoxotungstate (POT) [NaP5W30O110]14− demonstrates the highest activity against M. catarrhalis (MIC = 1 μg/ml) among all tested POMs. Moreover, we show that the Dawson type based anions, [P2W18O62]6−, [(P2O7)Mo18O54]4−, [As2Mo18O62]6−, [H3P2W15V3O62]6−, and [AsW18O60]7− are selective on M. catarrhalis (MIC range of 2-8 μg/ml). Among the six tested Keggin type based POTs ([PW12O40]3−, [H2PCoW11O40]5−, [H2CoTiW11O40]6−, [SiW10O36]8−, [SbW9O33]9−, [AsW9O33]9−), only the mono-substituted [H2CoTiW11O40]6− showed MIC value comparable to those of the Dawson type group. Polyoxovanadates (POVs) and Anderson type POMs were inactive against M. catarrhalis within the tested concentration range (1-256 μg/ml). Four Dawson type POMs [P2W18O62]6−, [(P2O7)Mo18O54]4−, [As2Mo18O62]6−, [H3P2W15V3O62]6− and the Preyssler POT [NaP5W30O110]14− showed promising antibacterial activity against M. catarrhalis (MICs < 8 μg/ml) and were therefore tested against three additional bacteria, namely S. aureus, E. faecalis, and E. coli. The most potent antibacterial agent was [NaP5W30O110]14−, exhibiting the lowest MIC values of 16 μg/ml against S. aureus and 8 μg/ml against E. faecalis. The three most active compounds ([NaP5W30O110]14−, [P2W18O62]6−, and [H3P2W15V3O62]6−) show bacteriostatic effects in killing kinetics study against M. catarrhalis. We demonstrate, that POM activity is mainly depending on composition, shape, and size, but in the case of medium-size POTs (charge is more than −12 and number of addenda atoms is not being higher than 22) its activity correlates with the total net charge.

Polyoxometalates (POMs) are transition metal complexes that exhibit a broad diversity of structures and properties rendering them promising for biological purposes.