Ferrocene-Derived Pyrazinoyl and Nicotinoyl Schiff-Bases: Their Synthesis, Characterization and Biological Properties

A novel class of acetylferrocene-derived Schiff-bases such as 2-pyrazinoyl-1-(2-ferroceneylmethylene)- hydrazide (HL1) and 2-nicotinoyl-1-(2-ferrocenylmethylene)hydrazide (HL2) have been synthesized and characterized by their IR, 1H NMR, 13C NMR and microanalytical date. The biological effect induced due to the coupling of ferrocene molecule with the aroylhydrazines e.g., pyrazinoylhydrazine and nicotinoylhydrazine has been studied against bacterial species such as Escherichia coli, Pseudomonas aeruginosa , Staphylococcus aureus and Klebsiella pneumonae.


INTRODUCTION
Many studies have highlighted TM an extensive use of ferrocene and ferrocene-containing molecules in substituent and supramolecular chemistry, while the application of ferrocene compounds in medicinal chemistry has not been investigated at a large scale although few reports 5'6 have indicated that the replacement of aromatic group by the ferrocenyl moiety in penicillin and cephalosporines improve their antibiotic activity. Aroylhydrazines have been shownT'8 to possess modest in vitro bacteriostatic properties against microorganisms such as Mycobacterium tuberculosis, Mycobacterium smegmatis, Candida albicans and Aspergillus niger. Preliminary studies have also shown 9'1 that such hydrazine-derived compounds are potent inhibitors of DNA synthesis in a variety of cultured human and rodent cells and their metal(II) complexes produce significant inhibition of tumor growth when given to mice bearing a transplanted fibrosarcoma 1. Although the bioactive forms and their cytotoxic activity is equal or greater than that of many chelators previously known to possess such properties. Moreover these compounds are relatively non-toxic to mice and show some selectivity in their effects. Because of these promising results we have previously synthesized many such novel aroylhydrazines or hydrazine-derived compounds and their various transition metal(II) chelates, and have tested and reported '5 their biological activity. Considering 16 18 that interesting redox-active properties due to FeI-Fe already exist in ferrocene molecules, we thought it now, worthwhile to combine both the chemistry of ferrocene and already known82 biologically active aroylhydrazines such as pyrazinoylhydrazine and nicotinoylhydrazineand and explore their biological properties induced by coupling with ferrocenyl group. For this purpose we have synthesized and characterized some ferrocene-derived Schiff-bases (HLt and HLz) (Fig. 1) and wish to report their biological properties in this paper, which may provide a useful information and may serve as a novel potential area of research which has been ignored before. CH3  The Schiff-base (30 g) in DMF (0.0 lmL) was applied on a paper disc [prepared from blotting paper (3 mm diameter)] with the help of a micropipette. These discs were left in an incubator for 48 h at 37 C and then applied on the bacteria grown agar plates. Preparation of Agar Plates. Minimal agar was used for the growth of specific bacterial species. For the preparation of agar plates for Escherichia coli, MacConkey agar (50 g), obtained from Merck, was suspended in freshly distilled water (1 L). It was allowed to soak for 15 minutes and then boiled on a water bath until the agar was completely dissolved. The mixture was autoclaved for 15 minutes at 120 C and then poured into previously washed and sterilized Petri dishes and stored at 40 C for inoculation.

Procedure of Inoculation.
Inoculation was done with the help of a platinum wire loop which was made red hot in a flame, cooled and then used for the application of bacterial strains. Application of Discs.
A sterilized forceps was used for the application of paper discs on the already inoculated agar plates. When the discs were applied, they were incubated at 37 C for 24 h. The diameter of the zone of inhibition was then measured.

RESULTS AND DISCUSSION
The Schiff-bases HL and HL2 (Fig 1) were prepared by a simple condensation reaction. Both of them are soluble in polar solvents such as methanol, ethanol and DMF, but are insoluble in weakly polar or non-Zahid H. Chohan

and M. Praveen
Metal-Based Drugs polar solvents. The structural determination of these Schiff-bases was done with the help of their IR, H NMR, 3C NMR and microanalytical data. The IR spectra of the HL and HL2 showed well-defined v(C=O) a,nd v(N-H) modes and no stretching due to the presence of v(OH) frequency in the region at 3365-3420 cm was found indicative of their probable keto form (Fig. 1A) than the enol form (Fig. 1B). The important IR frequencies of HLI showed some characteristic bands at 3215, 1715, 1620 and 955 cm"1. These were assigned 9'2 to v(N-H) v(C=O), v(C=N) and v(N-N) stretches respectively. Similarly, the IR spectra of HL2 showed characteristic absorption bands at 3215, 1725, 1615, 1550 and 955 cm " assigned2r to v (N-H), v(C=O), v(C=N), v(C=C) and v(N-N) stretches respectively. The disappearance of the band at 3160 cm " in both the Schiff-bases due to v(NH2) and appearance of a new band at 1615-1620 cm " due to Schiff-base azomethine linkage 22 confirmed the formation of HL and HL2. The IH NMR and 13C NMR spectra also displayed signals assignable to all other carbons and hydrogens expected in their region respectively23. Also, the absence of henolic protons in the IH NMR spectra confirmed the keto form configuration of these Schiff-bases. The H and 3C signals of feerocene moiety were assigned by comparing their shifts with the experimental evidences24'25. Furthermore, the microanalytical data of C, H and N confirmed their proposed structures (Fig. 1A).

Antibacterial Properties
Antibacterial properties of HLI and HL2 in comparison to the simple acetylferrocene (L3) were studied against bacterial species Escherichia coil Pseudomonas aeruginosa, Staphylococcus aureus and Klebsiella pneumonae. These were tested at a concentration of 30 lag/0.01 mL in DMF solution using a paper disc diffusion method devised and reported 26'27 earlier by us. The results of these studies reproduced in Table  indicated that acetylferrocene and both the Schiff-bases (HLI and HL2) showed variable activity against one or more bacterial strains. In comparison to acetylferrocene (L3), the Schiff-base derivatives (HL and HL2) were found to be more biologically active. These studies however, provided a useful information about the biological activity of ferrocene-containing compounds and the knowledge that this activity/potency could become more pronounced when more potent compounds are coupled with ferrocene molecule and thus introduce a new potential class of biologically active compounds.  [10][11][12][13][14] (45-64 %); +++, [14][15][16][17][18] (64-82 %); ++++, [18][19][20][21][22] (82-100 %). Percent inhibition values are relative to inhibition zone (22 mm) of the most active compound with 100 % inhibition.