Synthesis, Characterization, and Antibacterial Activity of Ni-Substituted Krebs-type Sandwich-Tungstobismuthates Functionalized with Amino Acids

Four new Ni-substituted Krebs-type sandwich-tungstobismuthates, K4Ni2[{Ni(β-ala)(H2O)2}2{Ni(H2O)}2{Ni(H2O)(η2-β-ala)}2(B-β-BiW9O33)2]·49H2O {(β-ala)4(Ni3)2(BiW9)2}, K3.5Na6.5[{Ni(η3-L-asp)}2(WO2)2(B-β-BiW9O33)2]·36H2O·L-asp {(L-asp)2(NiW)2(BiW9)2}, K4Na6[{Ni(gly)(H2O)2}2(WO2)2(B-β-BiW9O33)2]·86H2O {(gly)2(NiW)2(BiW9)2}, and K2Na8[{Ni(η2-serinol) (H2O)}2{Ni(H2O)2}2(B-β-BiW9O33)2]·42H2O {(serinol)2Ni4(BiW9)2} have been synthesized by one-pot solution methods. All compounds have been characterized in the solid state by single-crystal X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), elemental and thermogravimetric analyses, and infrared spectroscopy (IR), as well as by UV–vis spectroscopy in solution. The antibacterial activity of all compounds was studied against four bacterial strains by the determination of the minimum inhibitory concentration (MIC). The results showed that only {(β-ala)4(Ni3)2(BiW9)2} demonstrates antibacterial activity (MIC is in the range from 8 to 256 μg/mL) compared to three other Ni-Krebs sandwiches.


General information
All reagents were obtained commercially from Sigma Aldrich (Austria), AlfaAesar and Merck with a high-purity grade and were used as purchased without further purification. Unfunctionalized 3}2(WO2)2(B-β-BiW9O33)2]·36H2O {(NiW)2(BiW9)2} has been synthesized according to the published procedure. 1 Elemental analysis (EA): Elemental analysis was performed with an iCAP 6500 series inductively coupled plasma-optical emission spectrometry (ICP-OES) spectrometer (Thermo Scientific, USA). The ICPOES was equipped with a standard sample introduction system consisting of a concentric nebulizer and a cyclonic spray chamber. Transportation of sample solutions was performed by the peristaltic pump of the iCAP 6500 coupled to an ASX-520 auto sampler (Cetac, USA). Per element two sensitive and non-interfered emission lines were used, the first line for measurement and the second line for quality control. Elemental microanalysis of C/H/N/O contents was performed by Mikroanalytisches Laboratorium (University of Vienna, Faculty of Chemistry). An EA 3000 (Eurovector) was used for C/H/N/S-analysis. O-determination was performed by high-temperature digestion using the HT 1500 (Hekatech, Germany) pyrolysis system in combination with the EA 3000 system.

Attenuated total reflection Fourier-transform Infrared Spectroscopy (ATR-IR):
All ATR-IR spectra were recorded on a Bruker Vertex70 IR Spectrometer equipped with a single-reflection diamond-ATR unit. Frequencies are given in cm -1 , intensities denoted as w = weak, m = medium, s = strong.
Thermogravimetric analysis (TGA): was performed on a Mettler SDTA851e Thermogravimetric Analyzer under air and N2 with a heating rate of 5 °C min -1 in the region 25-700 °C. These results were then used to calculate the number of crystal waters present in each compound.
Single crystal X-ray diffraction (SXRD): X-ray intensity data were measured on a Bruker APEXII diffractometer equipped with a CCD (charge-coupled device) area detector, Incoatec Microfocus Source IS (30 W, multilayer mirror, Mo-K), and an Oxford Cryosystem (Cryostream 800 Plus LT) device. The following software was used: the Bruker Apex3 suite 2 the SHELX programme suite for structure solution (SHELXT 3 ), structure refinement (SHELXL 3 ) and OLEX2 4 as graphical user-interface.
Powder X-ray diffraction (PXRD): PXRD was performed on a Bruker D8 ADVANCE diffractometer, Cu Kα radiation, λ = 1.54056 Å, LYNXEYE silicon strip detector and a SolX energy dispersive detector with a variable slit aperture of 12 mm, 8° ≤ 2θ ≤ 50°. The simulated pattern was obtained from the SXRD data collected on single-crystal of each compound using Mercury programme 5 .
UV/Vis spectroscopy: UV/Vis spectra were collected on a Shimadzu UV 1800 spectrophotometer.
Minimum inhibitory concentrations (MICs) were determined by the broth microdilution method according to the guidelines of the Clinical Laboratory Standards Institute 6 . Stock solutions of compounds were prepared in 20 mg/mL or 10 mg/mL concentration in sterile H2O. Double dilutions of tested compounds in 96-well microtiter plates were prepared in 1024 -2µg/ml concentration range for high molecular weight complexes. E.coli and S. aureus were grown on Mueller-Hinton agar plates (by Becton Dickinson, USA) and E. faecalis and M.catarrhalis where grown on Mueller-Hinton agar with 5% defibrinated sheep blood. Inocula were prepared by direct colony suspension method and plates were inoculated with bacteria in a final concentration of 5x10^5 CFU/mL. Results were determined by visual inspection after 20-22h incubation at 37°C in ambient air. Testing was performed by the standard broth microdilution method with azithromycin as the reference antibiotic 7 to assess test validity. Scheme S1. Schematic representation of three distinct synthesis approaches. A) The synthesis procedure 1 yielded {(β-ala)4(Ni3)2(BiW9)2}. B) The synthesis procedure 2 yielded {(Lasp)2(NiW)2(BiW9)2}. C) The synthesis procedure 3 yielded {(gly)2(NiW)2(BiW9)2} and {(serinol)2Ni4(BiW9)2}. Color code: red polyhedra, {WO6}; gray spheres, bismuth; blue spheres, tungsten; green spheres, nickel; red spheres, oxygen; black spheres, carbon; turquoise sphere, nitrogen.

Unit cell dimensions [Å] and [°]
13.1186 (16) Figure S13. Vis spectrum of [3 mM] {(L-asp)2(NiW)2(BiW9)2} in H2O at 25 °C for 18 hours (Vis absorption was recorded every hour). The growth of a shoulder peak and corresponding rise in absorbance could be attributed to the release and partial hydration of the nickel ion from Krebs-type anion, which is fairly similar to the visible spectrum of nickel nitrate aqueous solution ( Figure S14).   The absorption increase could be attributed to the dissociation of L-aspartic acid from Krebstype anion or a change in its chelation mode towards nickel ion, which changes the nickel ion coordination environment and absorption.