Synthesis, Ligational and Biological Properties of Cobalt(II), Copper(II), Nickel(II) and Zinc(II) Complexes With Pyrazinedicarboxaimide Derived Furanyl, Thienyl and Pyrrolyl Compounds

Preparation, ligational and biological properties of some pyrazinedicarboxaimide derived furanyl, thienyl and pyrrolyl compounds with Co(ll), Cu(ll), Ni(ll) and Zn(ll) metals are described. Magnetic moments, electronic, infrared, nuclear magnetic resonance spectra and elemental analysis data indicate that co-ordination of the ligands with the metal ions take place through the pyrazine ring nitrogen, azomethine nitrogen and heteroatom of heterocyclic ring system. The compounds are all novel and are proposed to possess an octahedral geometry for Co(ll) and Ni(ll), and a distorted octahedral geometry for Cu(ll) and Zn(ll) complexes. The comparative biological properties of the title ligands and their metal chelates against different bacterial species are also described.

Pyrrol-2-aldehyde (1.9 g, 0.02 mol)in absolute ethanol (35 mL) was added to a solution of pyrazine-2,3-dicarboxamide (0.01 mol) in n-butanol (30 mL). Then 5-10 drops of concentrated sulfuric acid were added in it and mixture refluxed for 4 h. This reaction mixture, on cooling gave a solid product which was filtered, washed with n-butanol (2x15 mL) and dried. The solid thus obtained was crystallized in hot aqueous n-butanol (50 %) to give L 3 (1.58 g) (59 %). To a hot n-butanol solution (30 mL) of the ligand (0.01 mol) was added an ethanolic solution (25 mL) of the respective metal(ll) chloride salt (0.01 mol). The mixture was refluxed for 2 h. The resulting mxture was cooled, filtered and reduced to half of its volume (20 mL). The concentrated solution so obtained was left overnight at room temperature which resulted in the formation of a solid product. The product thus formed was filtered, washed with n-butanol (2x10 mE), then with ethanol (2x10 mL) followed by ether (2x20 mE) and dried. Crystallization in hot aqueous n-butanol (50 %) gave 1 (0.67 %), 2 (65 %), 3 (58 %), 4 (67 %), 5 (65 %), 6 (69 %), 7 (55 %), 8 (60 %), 9 (65 %), 10 (58 %), 11 (68 %)and 12 (65 %).  The ligand/complex (30 (g)in DMF (0.01mL) was applied on a paper disc, [prepared from blotting paper (3 mm diameter)] with the help of a micropipette. The discs were left in an incubator for 48 h at 37o 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 Chemical Company, 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 120o C and then poured into previously washed and sterilized Petri dishes and stored at 40o 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 use for the application of bacterial strains.

Application of Discs.
A sterilized forceps was used for the application of paper disc on the already inoculated agar plates. When the discs were applied, they were incubated at 37o C for 24 h. The zone of inhibition was then measured (in diameter) around the disc.

RESULTS AND DISCUSSION
The ligands were prepared by adopting the same procedure as reported earlier by US14,15. The structural determination of these ligands was done with the help of their IR, 1H-NMR, 13C-NMR and microanalytical data (Table & 2). The tentative assignment of some of the important R bands of ligand is recorded in Table 1. The IR spectra of the free ligands show characteristic absorption bands at 1670, 1625, 1575 and 1515 cm-1. These bands are assigned to v(C=O), v(C=N) and v(C=C) pyrazine ring stretches, respectively. The disappearance of the band at 3260 cm-1 due to v(NH) and appearance of a new band at 1625 cm-1 due to azomethine linkage confirmed the formation of ligands L1, L2 and L3. The 1H-NMR and 13C-NMR spectra ( Table 2) also display signals assignable to the azomethine and other expected ring protons and carbons assigned for the proposed structures of ligands. Carbon-13 was assigned by mainly comparing the values with the reported16 values. Also, the microanalytical data (Table  1)  All the complexes (1-12) are air and moisture stable solids. They are soluble in DMF, DMSO and water and insoluble in other solvents. The conductivity measurement of these complexes (22-28 ohm-l, cm2. mol-1)in DMF shows that they are all non-electrolyte17,18

Infrared Spectra
The bonding of the ligand to the metal ion was investigated by mainly comparing the infrared spectra of the free ligands with the spectra of their metal complexes (Table 3). The infrared spectra show: Nickel(II) and Zinc(II) Complexes a A positive shift of bands at 1515 and 613 cm-1 due to skeletal modes of the pyrazine ring, indicating19,20 co-ordination through pyrazine ring nitrogen. b Insignificant shift of v(C=O) at 1670 cm-1 indicated that it is not co-ordinated to the metal atom. c The spectra of all complexes indicated that a band at 1625 cm-1 due to azomethine v(C=N) linkage was shifted towards lower frequency by 5-10 cm1, respectively, indicating that the ligands are involved in co-ordination to the metal atom via the azomethine nitrogen.
d The new bands appearing in the spectra of metal complexes and not observed in the spectra of the ligands within 442-455 cm-1, 375-386 cm-1 and 510-525 cm-1 assigned to M-O, M-S and M-N modes, respectively, indicated that the heteroatoms X are also involved in co-ordination.
These results however, indicated a structure of the type shown in Figure 2  The room temperature magnetic susceptibility measurements ( Electronic Spectra Electronic spectra of the metal complexes are recorded in Table 2. The spectra of the cobalt chelates show three bands observed at 8715-9540 cm-1, 17355-18125 cm-1 and 29115-31235 cm -1 which may be assigned to 4T ---> 4T2g (F), 4Tlg 4A2g (F) and 4Tlg --4Tlg (P) transitions, respectively, and are suggestive24,25 of octahedral geometry around the cobalt ion. Three bands observed at 9325-10132 cm-1, 15820-17225 cm-1 and 26375-27650 cm-1 in the spectra of the nickel(ll) chelates are due to spin-allowed transitions 3A2(F 3T2g (F), 3A2g --3Alg (F) and 3A2g ---> 3Tlg (P), transitions, respectively, in an octahedrl environment26,27. The copper(ll) chelates show bands around the region 23155-23418 cmand 29450-30675 cm-1.The lower energy band may be assigned to the transition 2Eg 2T2g as 10 Dq due to a distorted octahedral environment27 and the band in the region 29450-30675 cm-1 can be attributed to ligand metal charge transfer. The absorption spectra of Zn(ll) complexes similarly, show a band at 13125-13514 cm-1 due to d-d transitions in its distorted octahedral environment28.
Based on the above evidences, it is proposed that cobalt(ll) and nickel(ll) complexes have an octahedral geometry ( Figure 2) whereas Cu(ll) and Zn(ll) complexes have a distorted octahedral geometry in which the ligands behave as tridentate and accommodate themselves in such a way that a stable chelate ring is formed around the metal atom thus attains a stable configuration.

Antibacterial Studies
The title ligands in comparison to their metal complexes were screened against bacterial species Escherichia coil, Pseudomonas aeruginosa, Staphylococcus aureus and Klebsiella pneumonae in order to determine their antibacterial properties. The antibacterial activity was tested at a concentration 30(g/0/01 mL in DMF using paper disc diffusion method.
The results of these studies reported in Table 5 showed that ligands and all their metal complexes are biologically active against one or more bacterial species and the metal complexes have been shown to be more antibacterial than the simple uncomplexed parent ligands.