Synthesis and characterization of Anderson-Evans type polyoxometalates, antibacterial properties

In the present work, the new aluminium-substituted polyoxometalates of the Anderson-Evans type have been prepared and structurally defined by the reaction of aluminium (III) chloride hexahydrate and sodium tungstate dihydrate/sodium molybdate dihydrate in an aqueous basic medium. Elemental analysis, FT-IR, TGA, 1H NMR, and 31P NMR analysis revealed that these polyoxometalates had the following formula: [Ph4P]3[Al(OH)6Mo6O18]·4H2O 1, [Ph4P]3[Al(OH)6W6O18]·4H2O 2, [C7H10N]3[Al(OH)6Mo6O18]·4H2O 3, [C7H10N]3[Al(OH)6W6O18]·4H2O 4. The compounds 1 and 2 show promising antibacterial activity against gram-positive Staphylcoccus aureus ATCC 25923 and gram-negative Escherichia coli ATCC 25922 bacteria.


Introduction
Polyoxometalates (POMs) are metal oxide polyanionic clusters.A large number of POMs have been reported since the first POM, (NH 4 ) 3 [PMo 12 O 40 ], was reported by Berzelius in 1826.These numerous POMs compositions and structural architectures are generally divided into two fundamental classes such as isopolyanions [M x O y ] m− and heteropolyanions [X z M x O y ] n− .Among these heteropolyanions, the Keggin, Anderson-Evans and Wells-Dawson types are the most widely studied in the literature [1][2][3][4][5][6][7].The chemistry of POMs is an extensive and growing area of modern coordination chemistry.Much attention has been focused on engineering materials, photocatalysis, nanotechnology, catalysis, electrochemistry, biochemistry, and medicine .
Anderson-Evans type POMs are an important subfamily among the polyoxometalates because they are composed of a single metal atom supported by a polytungstate or polymolybdate.There are two types of Anderson-Evans POM: The general formula of the A-type is [X n+ M 6 O 24 ] (12-n)-with the heteroatom in its highest oxidation state (e.g., Sb 5+ , Te 6+ , I 7+ ), the B-type has the general formula [X n+ (OH) 6 M 6 O 18 ] (6-n)-where the heteroatom has lower oxidation states (e.g., Co 2+ , Ni 2+ , Al 3+ , Fe 3+ ) [5].In the B-type hydroxy groups can be replaced by the organic ligands to obtain a new polyoxometalate, thus providing different modifications [32][33][34][35].The binding tendency of metal ions is utilized to obtain Anderson-Evans type heteropolyanions.Due to the high binding tendency of aluminium metal, [Al(OH) 6 Mo 6 O 18 ] 3-is used to obtain new materials.However, although there are many Anderson-Evans type POMs, studies on aluminium-containing Anderson-Evans type structures are very limited in the literature.In this paper, it is aimed to provide a new perspective to the studies in the literature with the synthesis and antimicrobial applications of new aluminium-containing Anderson-Evans type compounds.When we examine the studies on this subject in the literature, we come across the following studies: In this study, four aluminum substituted Anderson-Evans type POMs were synthesized and fully characterized by spectroscopic methods.The antibacterial activity of these compounds was investigated.

2.Experimental section 2.1. General methods
All chemicals used in this study were all of analytical grade purchased from Sigma-Aldrich, or Merck and used without purification. 1H and 31 P NMR spectra were recorded on a AVANCE III 400 MHz NaNoBay FT-NMR spectrometer operating at 400 MHz ( 1 H), ( 31 P) in DMSO-d 6 .FT-IR spectrum was obtained from a sample powder palletized with KBr on Perkin Elmer LR 64912 C spectrometer over the range 400-4000 cm -1 .C, H and N elemental analyses were performed on a LECO-932 CHNS element analyzer.Thermogravimetric analysis (TGA) was carried out on a Hitachi Exstar TG/DTA 7300 thermal analyzer in flowing N 2 between 25 and 800 °C at a heating rate of 10 °C/min.

Synthesis of compounds
Two solutions were prepared separately.Solution 1: Na 2 MO 4 •2H 2 O (1.30 mmol) (M=Mo, W) was dissolved in water (10 mL) under stirring.Solution 2: AlCl 3 •6H 2 O (0.52 mmol) was dissolved in water (10 mL) under stirring.Solution 2 was added to solution 1.The resulting mixture was kept at 60 o C and acidified with glacial acetic acid.In 1-2 h at this temperature, the mixture was cooled to room temperature.After o-toluidine hydrochloride (excess) was dissolved in 2 mL water; Ph 4 PBr (excess) was dissolved in chloroform and precipitate was formed by dropwise addition.The solid was filtered, washed with H 2 O and finally dried under a vacuum at 60 ºC.

Antibacterial tests
Antibacterial tests of all compounds (1-4) were carried out by disk diffusion method with gram-positive and gram-negative bacteria.Staphylcoccus aureus ATCC 25923 was used as gram-positive bacteria, whereas Escherichia coli ATCC 25922 was used as gram-negative bacteria.Bacteria stored at -20 °C in media containing 16% glycerol were inoculated on Mueller Hinton Agar (MHA) by streaking method and incubated at 37 °C for 16-24 h.After incubation, bacterial colonies of which purity was checked were selected and inoculated into sterile Mueller Hinton Broth media using sterile loops.Density was adjusted as Mc Farland 0.5 using DEN-1B Densitometer device.Sterile Mueller Hinton Broth medium was used as a control in the Mc Farland measurement.(McFarland 0.5: corresponds to 1-2.10 8 bacteria.)Bacteria were inoculated into different petri dishes for each bacterium, covering the entire surface with sterile swab sticks.Fifteen minutes were waited.After 20 microliter samples were impregnated on the blank discs and the blank discs were placed in the petri dish, they were incubated for 16-24 h at 37 °C after waiting for another 15 min.(Sterile empty discs treated with DMSO served as a negative control whereas discs impregnated Erythromycin was used as a positive control.)After incubation, if there is a zone formed by the sample, it was observed and measured.Zone formation of the antibiotic used as a positive control was confirmed.Experiments were repeated three times.

TGA
In order to characterize the thermal stability of compounds 1-4, their thermal behavior was investigated by TGA.In this investigation, heating rates were suitably controlled at 10 °C min -1 under N 2 atmosphere and the weight loss was measured from ambient temperature up to 800 °C.The thermal behavior of all metal compounds is generally similar.Firstly, the thermograms of 1-4 (Figures S11-S14) show weight loss which starts at room temperature with a dehydration corresponding to the loss of four crystal water molecules.Table 1 shows the other degradation products for compounds 1-4 and the TGA results obtained are in agreement with the calculated value.

Antibacterial analysis
The antibacterial activity of the new Anderson-Evans type POMs (1-4) was studied against one gram-positive bacteria (Staphylcoccus aureus ATCC 25923) and one gram-negative bacteria Escherichia coli ATCC 25922.The results of the  antibacterial activities are presented in Table 2 and Figures 2-5.Compound 1-2 have inhibitory action against both microorganisms.But compound 3-4 have no antibacterial activity.This difference in compounds is thought to be due to the counter ion.It has been reported in the literature that POM compounds containing PPh 4 + cation show antimicrobial activity [9].Compounds 1 and 2 exhibited potent antibacterial activity for Staphylcoccus aureus and Escherichia coli as the standard drugs Erythromycin 36 mm and 23.5 mm, respectively.

Table 1 .
Results for the TGA analysis.
a Inhibition zone diameter in millimeters, CZ: contact zone.