C-3 Substituted Lawsonemonoximates of Holmium(III): Synthesis, Characterization, and Antimicrobial Activity

A series of five new metal complexes of Ho(III) with C-3 substituted derivatives of lawsonemonexime (2-hydroxy-1,4-naphthalenediene-1-oxime) were synthesized. The compounds were characterized by melting point, elemental analysis, IR spectroscopy and magnetic susceptibility. The antimicrobial activity of the compounds were determined by disk diffusion method and broth micro-dilution techniques using Mueller Hinton medium against the following organisms: S. aureus ATCC 6538P, Klebsiella pneumoniae, NCTC 418, Pseudomonas aeruginosa ATCC 27833, Salmonella typhimurium ATCC 23564, E. coli U 1777, E. coli HB101, Proteus morganii NCIM 2860, Providencia stuartii NCIM 2799 and Acinetobacter baumannii U 24. The chelates of Ho(III) with lawsonemonoxime and Ho(III) with 3-bromolawsonemonoxime showed a variable antimicrobial activity against all organisms tested except Pseudomonas and Klebsiella spp. S. aureus was found more sensitive to all ligands and chelates tested; but the MIC values of chelates were considerably less; thus having more antimicrobial effect.


INTRODUCTION
Antimicrobial or antineoplastic activity of lawsone was studied by Lime [2] showed that lawsone exhibits fungicidal activity. Anderson et al [3] studied phthiocol which is the quinone compound associated with certain bacteria. Some oxime derivatives of lawsone and their chelates showed antimicrobial activity. Mhaske et al [4] have reported the antimicrobial activities of 3-nitrolawsonemonoxime, 3-aminolawsonemonoxime and their bivalent metal chelates where as Dandawate [5] and Gaikwad [6] studied the chelates of some rare earths with lawsonemonoxime derivatives. Joshi et al [7] have reported on rare earth studies where as Shen et al [8] studied lanthanide metal complexes for antimicrobial activities. But the chelates of Ho(III) with lawsonemonoximes (figure 1) and their antimicrobial activities are being reported for the first time in the present studies. All the chemicals and solvents used were of analytical grade. Lawsone (2-hydroxy-l,4naphthalenedione), dichlone (2,3-dichloro-1,4-naphthalenedione) and menadione (2-methyl-1,4naphthalenedione) were purchased from Fluka (Germany). Phthiocol was synthesized from menadione by Fieser's method [9]. The 3-chlorolawsone was prepared from dichlone. All the ligands (figure 1) used (lawsonemonoxime derivatives)were synthesized by treating the solution of a lawsone derivative with solution of hydroxylamine hydrochloride. The entire mixture was heated at 60C, cooled and then neutralized with hydrochloric acid (2M) causing precipitation of the corresponding lawsone-l-oxime derivative. The melting points were recorded after recrystallization of the ligands from methanol. The compounds were tested for solubility in water, methanol, DMF, DMSO and acetonitrile.

Preparation of the Chelates
To a hot solution of 3.0 mmol of ligand (table 1), (0.568 g of lawsonemonoxime, 0.60 g of phthiocolmonoxime, 0.671 g of 3-chlorolawsonemonoxime, 0.804 g of 3-bromolawsonemonoxime and 0.945 g of 3-iodolawsonemonoxime)in 25 mL of ethanol, an aqueous solution of 1.00 mmol of of Holmium(llI) Synthesis, Characterization and Antimierobial Activity metal(Ill)chloride hexahydrate (0.379 g of HoCI3.6H20) was added. The pH of the mixture was adjusted to 5.5 to 6.5 using aqueous ammonia (1:20 v/v). The mixture was refluxed for 3 h and then cooled overnight.
The precipitate was filtered off, washed with water followed by hot methanol and dried in vacuum over fused CaCI2 at ambient temperature. A solubility of the complexes was tested in methanol, water, DMF, DMSO and acetonitrile.
The carbon, hydrogen analysis and residue (Ho203) were determined using a Hosli-Holland C, H Analyzer. The magnetic studies were carried out at room temperature by the Faraday technique using mereury(II) tetrathioeyanatoeobaltate as ealibrant. The infrared spectra were recorded in nujol mulls on a Perkin-Elmer FTIR Speetrophotometer (Model 1600, range 4000-450 em'l). In brief, Mueller Hinton agar (HiMedia Lab, India) was prepared as per instructions of the manufacturer and plates were poured around 56C to set and then placed in the incubator at 37C for overnight to test the sterility of medium. Meanwhile, an overnight nutrient broth culture of each strain was taken and diluted to reach O.D. of 0.5 MaeFarland standard with nutrient broth, of which 50 tL was inoculated on plate and spreader with glass spreader. After h the disks (prepared from Whatman No 42, 6 mm diameter) impregnated with the compounds (prepared in DMSO to water 3" 1) were applied and the plates incubated at   [5]. The (C=O) stretching frequency at 1600-1630 cmfound in the IR spectra of ligands is shifted to a lower frequency region by 20-30 cmafter complexation. This indicates redistribution of electron density in the quinonoid ring i.e. weakening of bond.
The band at 1570-1590 cmin the spectra of ligands is assigned to (C=N) vibrations, it is shifted to lower frequency by 40-70 cmin the complexes (table 3), which indicates a coordination through the oximino nitrogen. A quinone absorption is observed at 1285-1295 cm-l.