Synthesis and characterization of cobalt(II), nickel(II), copper(II) and zinc(II) complexes with Schiff base derived from 4-amino-3-mercapto-6-methyl-5-oxo-1,2,4-triazine

A few (1:1) and (1:2) metal complexes of cobalt(II), nickel(II), copper(II) and zinc(II) have been isolated with ligand derived from the condensation of 4-amino-3-mercapto-6-methyl-5-oxo-1,2,4-triazine with 2-acetylpyridine (L(1)) and characterized by elemental analysis, conductivity measurements, infrared, electronic, (1)H NMR spectral data, magnetic and thermogravimetric analyses. Due to insolubility in water and most of the common organic solvents and infusibility at higher temperatures, all the complexes are thought to be polymeric in nature. A square-planar geometry was suggested for copper(II) and octahedral proposed for cobalt(II), nickel(II) and zinc(II). Some of the chemically synthesized compounds have been screened in vitro against the three Gram-positive (Staphylococcus aureus, Staphylococcus epidermidis and Bacillus subtilis) and two Gram-negative (Salmonella typhi and Escherichia coli) organisms. It is observed that the coordination of metal ion has pronounced effect on the microbial activities of the ligand. The metal complexes have higher antimicrobial effect than the free ligands.     

Synthesis, characterization of copper(II), cobalt(II), nickel(II), zinc(II) and cadmium(II) complexes of [7-hydroxy-4-methyl-8-coumarinyl]glycine and a comparitive study of their microbial activities

A new Mannich base, [7-hydroxy-4-methyl-8-coumarinyl]glycine [MCGH(2)], has been prepared by the condensation of 7-hydroxy-4-methyl-coumarin with glycine and formaldehyde. Its conformational changes on complexation with transition metal ions [copper(II), cobalt(II), nickel(II), zinc(II) and cadmium(II)] have been studied on the basis of elemental analysis, conductivity measurements, spectral (infrared, (1)H NMR, electronic, EPR), magnetic and thermogravimetric studies. The infrared spectral studies of all the complexes reveal that the ligand has coordinated through -NH and two hydroxyl groups. The conductance data of the complexes suggest them to be non-electrolytes. Fluorescence property of the ligand and its metal complexes was studied and concluded that the ligand MCGH(2) can be a potential candidate for the determination of zinc(II) and cadmium(II) at room temperature by fluorimetric method. The antimicrobial activity of all the compounds was studied against Gram negative (Escherichia coli) and Gram positive (Bacillus cirroflagellosus) bacteria and fungi, Aspergillus niger and Candida albicans. It was observed that the coordination of metal ion has a pronounced effect on the microbial activities of the ligand. All the metal complexes have shown higher antimicrobial effect than the free ligand.     

Synthesis, spectral characterization, in vitro antibacterial, antifungal and cytotoxic activities of Co(II), Ni(II) and Cu(II) complexes with 1,2,4-triazole Schiff bases

A series of metal complexes of cobalt(II), nickel(II) and copper(II) have been synthesized with newly synthesized biologically active 1,2,4-triazole Schiff bases derived from the condensation of 3-substituted-4-amino-5-mercapto-1,2,4-triazole and 8-formyl-7-hydroxy-4-methylcoumarin, which have been characterized by elemental analyses, spectroscopic measurements (IR, UV-vis, fluorescence, ESR), magnetic measurements and thermal studies. Electrochemical study of the complexes is also reported. All the complexes are soluble to limited extent in common organic solvents but soluble to larger extent in DMF and DMSO and are non-electrolytes in DMF and DMSO. All these Schiff bases and their complexes have also been screened for their antibacterial (Escherichia coli, Staphylococcus aureus, Streptococcus pyogenes, Pseudomonas aeruginosa and Salmonella typhi) and antifungal activities (Aspergillus niger, Aspergillus flavus and Cladosporium) by MIC method. The brine shrimp bioassay was also carried out to study their in vitro cytotoxic properties.     

Novel platinum(II) and palladium(II) complexes of thiosemicarbazones derived from 5-substitutedthiophene-2-carboxaldehydes and their antiviral and cytotoxic activities

A series of thiosemicarbazones and their platinum(II) and palladium(II) complexes have been synthesized. The chemical structures of ligands and their complexes were characterized by UV–Vis, IR, ¹H NMR, ¹³C NMR, MS spectra, elemental analysis and TGA. The antiviral and cytotoxic activities of all compounds have been tested. Results of broad antiviral evaluation showed that none of the compounds evaluated endowed with anti-DNA or -RNA virus activity at subtoxic concentrations except for the palladium complex 1b. This compound exhibited slightly selective inhibition against cytomegalovirus. The platinum complex 4a exhibited the best cytostatic activities against human cervix carcinoma. Ligands 2, 4 and 5 showed cytostatic potential. The palladium complexes were in general less cytostatic than the corresponding platinum complexes or unliganded congeners.     

Synthesis, characterization and antimicrobial activity of Fe(II), Zn(II), Cd(II) and Hg(II) complexes with 2,6-bis(benzimidazol-2-yl) pyridine ligand

2,6-Bis(benzimidazol-2-yl)pyridine (L) ligand and complexes [M(L)Cl(2)] and [Fe(L)(2)](ClO(4))(2) (M=Zn, Cd, Hg) have been synthesized. The geometries of the [M(L)Cl(2)] complexes were derived from theoretical calculation in DGauss/DFT level (DZVP basis set) on CACHE. The central M(II) ion is penta-coordinated and surrounded by N(3)Cl(2) environment, adopting a distorted trigonal bipyramidal geometry. The ligand is tridentate, via three nitrogen atoms to metal centre and two chloride ions lie on each side of the distorted benzimidazole ring. In the [Fe(L)(2)](ClO(4))(2) complex, the central Fe(II) ion is surrounded by two (3N) units, adopting a octahedral geometry. The elemental analysis, molecular conductivity, FT-Raman, FT-IR (mid-, far-IR), (1)H, and (13)C NMR were reported. The antimicrobial activities of the free ligand, its hydrochloride salt, and the complexes were evaluated using the disk diffusion method in dimethyl sulfoxide (DMSO) as well as the minimal inhibitory concentration (MIC) dilution method, against 10 bacteria and the results compared with that for gentamycin. Antifungal activities were reported for Candida albicans, Kluyveromyces fragilis, Rhodotorula rubra, Debaryomyces hansenii, Hanseniaspora guilliermondii, and the results were referenced against nystatin, ketaconazole, and clotrimazole antifungal agents. In most cases, the compounds tested showed broad-spectrum (Gram positive and Gram negative bacteria) activities that were either more effective than or as potent as the references. The binding of two most biologically effective compounds of zinc and mercury to calf thymus DNA has also been investigated by absorption spectra.     

Synthesis and antitumor studies on novel Co(II), Ni(II) and Cu(II) metal complexes of bis(3-acetylcoumarin)thiocarbohydrazone

The synthesis, structure, physico-chemical investigation and biological studies of some metal complexes of thiocarbohydrazone ligands are described. The ligand is obtained by condensation of N,N'-thiocarbohydrazide with 3-acetylcoumarin. The metal complexes of Co(II), Ni(II) and Cu(II) with bis(3-acetylcoumarin)thiocarbohydrazone were synthesized and isolated as solid products and characterized by analytical means as well as by spectral techniques such as FT-IR, (1)H NMR and EPR and UV spectrometry. The ligand acts as bidentate, through NO or NN, neutral in coordinating the M(II) ions. The bonding sites are the azomethine nitrogen, lactone carbonyl oxygen and respective anion counterparts. The metal complexes exhibit either octahedral or distorted octahedral structures. The complexes are found to be soluble in dimethylformamide and dimethylsulphoxide. Molar conductance values in dimethylsulphoxide indicate the non-electrolytic nature of the complexes. The compounds tested in present study have shown promising cytotoxic activity when screened using the in vitro method and at the same time were shown to have good activity when tested using the Ehrlich Ascites Carcinoma model. Preliminary antimicrobial screening shows the promising results against both bacterial and fungal strains.     

synthesis, characterization and in vitro antiamoebic activity of 5-nitrothiophene-2-carboxaldehyde thiosemicarbazones and their Palladium (II) and Ruthenium (II) complexes

Synthesis of new Palladium(II) and Ruthenium(II) complexes of the type, [Pd(L)Cl(2)] and [Ru(eta(4)-C(8)H(12))(L)Cl(2)] [where, L = thiosemicarbazones derived from 5-nitrothiophene-2-carboxaldehyde and cycloalkylaminothiocarbonyl hydrazines] have been isolated by the reaction of [Pd(DMSO)(2)Cl(2)] and [Ru(eta(4)-C(8)H(12))(CH(3)CN)(2)Cl(2)] with 5-nitrothiophene-2-carboxaldehyde thiosemicarbazones. The spectral data revealed that the thiosemicarbazones act as bidentate ligands, making use of thionic sulphur and the azomethine nitrogen atom for coordination to the central metal ion. Microdilution method was used for the assessment of antiamoebic activity of all the compounds against HK-9 strain of Entamoeba histolytica. Among all the thiosemicarbazones, 5-NT-4-BPTSCN (3) showed significant antiamoebic activity (IC(50) - 2.56 microM). Enhancement of antiamoebic activity resulted by introducing palladium and ruthenium metals in the thiosemicarbazone moiety. All the Pd(II) and Ru(II) complexes of 5-nitrothiophene-2-carboxaldehyde thiosemicarbazones were found more active then their respective ligands. The complexes 1a-4a, 1b and 3b showed antiamoebic activity.     

Copper(II) complexes with substituted thiosemicarbazones of thiophene-2-carboxaldehyde: synthesis, characterization and antiamoebic activity against E. histolytica

In an effort to develop potent antiamoebic agents, a series of thiosemicarbazone (TSC) ligands 1-5 derived from thiophene-2-carboxaldehyde and N4-substituted thiosemicarbazides has been prepared and characterized using various spectroscopic techniques. Treatment of the ligands with cupric chloride produced the copper(II) complexes [Cu(TSC)2Cl2] 1a-5a where ligand bind through thionic sulfur and the azomethine nitrogen. The possible geometries of the complexes were assigned on the basis of magnetic moment, electronic and thermal patterns as well as infrared spectral studies. The thiosemicarbazones and their copper complexes were tested for their in vitro antiamoebic activity against HK-9 strain of Entamoeba histolytica and showed significant growth inhibition. The results revealed that these complexes are effective chemicals in inhibiting amoebal growth, with compound 5 (having -N(CH3)(C6H5) substituent at N4) and complexes 1a-5a being more effective than the commercial antiamoebic drug, metronidazole, based on IC50 values. These data also indicated that the compounds 3a and 5a are most effective among the complexes studied (IC50=0.26 microM of 3a and IC50=0.21 microM of 5a versus IC50=1.81 microM of metronidazole).     

Synthesis and characterization of dinuclear macrocyclic cobalt(II), copper(II) and zinc(II) complexes derived from 2,2,2('),2(')-S,S[bis(bis-N,N-2-thiobenzimidazolyloxalato-1,2-ethane)]: DNA binding and cleavage studies

New homodinuclear macrocyclic complexes of cobalt(II), copper(II) and zinc(II) were isolated from the newly synthesized ligand 2,2,2',2'-S,S[bis(bis-N,N-2-thiobenzimidazolyloxalato-1,2-ethane)]. The structures of the complexes were elucidated by elemental analysis, molar conductance measurements, IR, 1H NMR, 13C NMR, electronic and ESI-MS spectroscopic techniques. In complex 1, Co(II) ions possess a tetrahedral coordination environment composed of O2S2 donor atoms while its Cu(II) and Zn(II) counterparts 2 and 3, respectively, reveal a six coordinate octahedral structure, defined by the O2S2 donors from the macrocyclic ring and two chloride ions. Molar conductance and spectroscopic data also support the proposed geometry of the complexes. DNA binding properties of complexes 1-3 were investigated using electronic absorption spectroscopy, fluorescence spectroscopy, viscosity measurements and cyclic voltammetry. The absorption spectra of complexes 2 and 3 with calf thymus DNA showed hypochromism, while complex 1 showed hyperchromism attributed to a partial intercalation and electrostatic binding modes, respectively. The intrinsic binding constant K(b) of complexes 1-3 were determined as 16.6 x 10(4) M(-1), 4.25 x 10(4) M(-1) and 3.0 x 10(4) M(-1), respectively. The decrease in the relative specific viscosity of calf thymus DNA with increasing concentration of the complexes authenticates the partial intercalation binding mode. Gel electrophoresis of complex 2 with plasmid DNA demonstrated that complex exhibits excellent "artificial" nuclease activity.     

Synthesis, characterization and antiamoebic activity of new indole-3-carboxaldehyde thiosemicarbazones and their Pd(II) complexes

In continuation of our research on thiosemicarbazones and their metal complexes as antiamoebic agents, a new series of indole-3-carboxaldehyde thiosemicarbazones (TSC) 1-7 were prepared by condensing indole-3-carboxaldehyde with cycloalkylaminothiocarbonyl hydrazines. Their palladium(II) complexes of the [Pd(TSC)Cl2] type, were synthesized upon coordination with [Pd(DMSO)2Cl2]. The chemical structures of all the compounds were established by elemental analyses, electronic, IR, (1)H NMR and (13)C NMR spectral data. The structure of the complexes was further established by thermogravimetric analysis and FAB MS. Spectroscopic data revealed that thiosemicarbazones act as bidentate ligands, making use of thione sulphur and azomethine nitrogen atom for coordination to the Pd(II) ion. Among all the compounds evaluated for antiamoebic activity using HM1:IMSS strain of Entamoeba histolytica, all palladium complexes were found to be more active than their respective ligands. Moreover, ligand 5 and complexes 1a-3a, 5a and 7a showed antiamoebic activity, at lower IC(50) doses when compared to the reference drug metronidazole with IC(50)=1.81 microM.     

Synthetic, structural and biochemical studies of polynuclear platinum(II) complexes with heterocyclic ligands

"Non-classical" di- and trinuclear Pt(II) complexes with polydentate nitrogen ligands; ionic [(PtCl(2))(2)(tptz)(2)(mu-PtClNCPh)]Cl (1) [tptz =2,4,6-tris(2-pyridyl)-1,3,5-triazine], [(PtCl(2))(2)(bptz)(2)(mu-Pt)]Cl(2) (2) [bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine] and neutral [(PtCl(2))(2)(tptz)(2)(mu-PtCl(2))](H(2)O)(4) (3), [(PtCl(2))(2)(mu-tppz)](CHCl(3)) (4) [tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine] complexes, have been prepared and structurally characterized. The neutral tptz and tppz complexes present three and two separate PtCl(2) moieties, respectively, in a cis position, presumably acting in a bifunctional mode towards DNA; the cationic tptz and bptz complexes contain monofunctional and bifunctional bridging Pt(II) moieties, respectively, (other Pt(II) moieties in the complexes are bifunctional). All complexes were tested for their biological activity. Both tptz complexes, neutral and ionic, show a potent cytotoxic activity and reduced cell viability in a concentration-dependent manner that was evaluated in a panel of different cancer cell lines: human HT29 colon-rectal carcinoma, HepG2 hepatoma, MDA-MB-231 breast cancer and MG63 osteosarcoma cells; their activity was higher than cisplatin, IC50 values have been calculated for the active compounds and flow cytometric analysis for the tptz complexes performed. Therefore, these new platinum drugs warrant further investigation into their antitumor activity against different types of tumors.     

Synthesis and anti-inflammatory effects of some bis(2-benzimidazolyl)thioethers and their copper(II) chelates,orally administered to rats

A series of bis(2-benzimidazolyl)thioethers and their copper(II) chelates has been prepared and tested. All the ligands and copper(II) complexes synthesized were orally assayed in the rat for anti-inflammatory activity in both acute (carrageenan edema) and chronic (adjuvant arthritis) models of inflammation. Some compounds were also examined for gastric-irritancy potential. Among tested molecules, the copper complex with the unsubstituted lead structure (NSN) showed the best pharmacological activities, which seem to be dependent upon the presence and bioavailability of some intact complex in the gastrointestinal tract after oral administration. The compound did not exhibit any capacity to elicit significant lesion development of the gastric mucosa of stress-sensitized rats.Structure—activity relationships of the copper complexes are discussed.     

Synthesis and characterization of new Pt(II) and Pd(II) complexes with 2-quinolinecarboxaldehyde selenosemicarbazone: cytotoxic activity evaluation of Cd(II), Zn(II), Ni(II), Pt(II) and Pd(II) complexes with heteroaromatic selenosemicarbazones

New complexes of Pt(II) and Pd(II) with 2-quinolinecarboxaldehyde selenosemicarbazone were synthesized and characterized by elemental analysis, NMR and IR spectroscopy and molar conductivity measurements. The assumed geometry of Pt(II) and Pd(II) complexes was square planar where the ligand was tridentately coordinated via the quinoline and imine nitrogen atoms and the selenium atom. The cytotoxic activity of the new Pt(II) and Pd(II) compounds, as well as of some previously synthesized Cd(II), Zn(II) and Ni(II) complexes with the same or analogous ligand, was tested against a panel of three human cancer cell lines: human cervix carcinoma cells (HeLa), human melanoma cells (FemX) and breast cancer cells (MDA-361). All investigated compounds, except Pt(II) complex, possess a strong dose-dependent cytotoxic activity of the same order of magnitude as cisplatin (CDDP). The investigation of potential of these compounds to induce HeLa cell cycle perturbations was also evaluated.     

Synthesis and antiamoebic activity of new 1-N-substituted thiocarbamoyl-3,5-diphenyl-2-pyrazoline derivatives and their Pd(II) complexes

Some 1-N-substituted thiocarbamoyl-3,5-diphenyl-2-pyrazoline derivatives (L), 1-8 were synthesized by a base-catalyzed Claisen-Schmidt condensation of benzaldehyde with acetophenone followed by cyclization with various N-4 substituted thiosemicarbazides. The palladium(II) complexes [PdLCl2], 1a-8a of these ligands were obtained by reacting them with [Pd(DMSO)2Cl2]. All compounds have been characterized by means of elemental analyses, electronic, IR, 1H NMR and mass spectroscopic data, while the complexes have additionally been characterized by thermogravimetric patterns. The in vitro antiamoebic activity was evaluated against the HM1:IMSS strain of Entamoeba histolytica and the results were compared with the standard drug, metronidazole. The preliminary test results showed that the complexes had better antiamoebic activity than their respective ligands. Moreover, the complexes showed better inhibition of the test organism. The results suggest that the ligands 4, 7 and the complexes 2a-4a, 6a-8a were found with IC50 lower than that of the standard drug metronidazole and thus are better inhibitor of growth of E. histolytica.     

Novel bidentate complexes of Cu(II) derived from 5-nitrofuran-2-carboxaldehyde thiosemicarbazones with antiamoebic activity against E. histolytica

The novel analogues of 5-nitrofuran-2-carboxaldehyde thiosemicarbazones 1-10 were synthesized and their copper(II) complexes 1a-10a were obtained by means of coordination with cupric chloride. All these compounds have been characterized by elemental analysis, IR, electronic spectra and thermogravimetric patterns while ligands have also been characterized by 1H NMR spectral studies. These copper complexes are bidentate and possess octahedral geometry around Cu(II) ion. Their antiamoebic activities were carried out to ascertain their effectiveness in comparison to their corresponding thiosemicarbazones. A number of these complexes possess noteworthy potencies towards HK-9 strain of Entamoeba histolytica in vitro. The complexes 2a-7a, 9a and 10a showed less IC50 value than metronidazole, the drug of choice for amoebiasis. Moreover, complexes 2a and 9a have shown the most promising antiamoebic activities (IC50 = 0.38 microM of 2a and IC50 = 0.34 microM of 9a versus IC50 = 1.81 microM of metronidazole). These results indicate that the metallated thiosemicarbazone may be lead molecule to inhibit growth of E. histolytica.     

Solvatochromism, DNA binding, antitumor activity and molecular modeling study of mixed-ligand copper(II) complexes containing the bulky ligand: bis[N-(p-tolyl)imino]acenaphthene

Four mixed-ligand copper(II) complexes of the nitrogen ligand bis[N-(p-tolyl)imino]acenaphthene 1 (p-Tol-BIAN). These complexes, namely [Cu(p-Tol-BIAN)2](ClO4)2 2, [Cu(p-Tol-BIAN)(acac)](ClO4) 3, [Cu(p-Tol-BIAN)Cl2] 4 and [Cu(p-Tol-BIAN)(AcOH)(2)](ClO4)2 5, were prepared and characterized. Solvatochromism of the novel copper complexes in various solvents has been studied. Molecular mechanics (MM+) and molecular dynamic simulations have been performed to learn more about the solvatochromic behaviour and the DNA binding affinity of these complexes.     

Cytotoxic activity and chemical reactivity of cis-platinum(II) and trans-palladium(II) complexes with diethyl (pyridinylmethyl)phosphates

A series of square-planar platinum(II) and palladium(II) complexes of the formula cis-[PtCl2L2] and trans-[PdCl2L2] [L stands for diethyl (pyridin-2-ylmethyl)phosphate (2-pmOpe) or diethyl (pyridin-3-ylmethyl)phosphate (3-pmOpe) or diethyl (pyridin-4-ylmethyl)phosphate (4-pmOpe)] have been synthesized and tested in vitro for their cytotoxicity against mouse leukemia L1210 cells. The results indicated that the cis-platinum complexes showed superior activity than trans-palladium complexes, but lower in comparison to cisplatin. The chemical reactivity of the tested complexes has been determined in an in vitro NBP test. The platinum complexes exhibited very high chemical reactivity in NBP test, higher than cisplatin. The results showed no correlation between cytotoxicity and chemical reactivity for platinum complexes. Two platinum(II) complexes {cis-[PtCl2(2-pmOpe)2], cis-[PtCl2(3-pmOpe)2]} have been synthesized and characterized by IR, 1H NMR, 31P NMR, and elemental analysis.     

Synthesis, X-ray structure and cytotoxic effect of nickel(II) complexes with pyrazole ligands

Here we present the synthesis of the new Ni(II) complexes with chelating ligands 1-benzothiazol-2-yl-3,5-dimethyl-1H-pyrazole (a), 5-(2-hydroxyphenyl)-3-methyl-1-(2-pyridylo)-1H-pyrazole-4-carboxylic acid methyl ester (b) and 1-benzothiazol-2-yl-5-(2-hydroxyphenyl)-3-methyl-1H-pyrazole-4-carboxylic acid methyl ester (c). These ligands a–c create solid complexes with Ni(II). The crystal and molecular structures of two complexes were determined by X-ray diffraction method. Thermal stability of two complexes with ligand c by TG/DTG and DSC methods were also shown. Cytotoxic activity of all the complexes against three tumour cell lines and to normal endothelial cells (HUVEC) was also estimated. Complexes with ligand c exhibited relatively high cytotoxic activity towards HL-60 and NALM-6 leukaemia cells and WM-115 melanoma cells. Cytotoxic effectiveness of one of these complexes against melanoma WM-115 cells was two times higher than that of cisplatin. The protonation constant log K = 9.63 of ligand b corresponding to the phenol 2-hydroxy group has been determined in 10% (v/v) DMSO/water solution (25 °C). The coordination modes (formation of two monomeric species: NiL and NiL₂) in the complexes with Ni(II) are discussed for b on the basis of the potentiometric and UV/Vis data.     

Ruthenium (II) nitrofurylsemicarbazone complexes: new DNA binding agents

Complexes of the type [Ru(II)Cl(2)(DMSO)(2)L], where L are 5-nitrofurylsemicarbazone derivatives, were prepared in an effort to combine the potential anti-tumor activity of the metal and the free ligands. The new complexes are excellent DNA binding agents for calf thymus DNA. So, their in vitro anti-tumor activity was tested in cellular models and the complexes were found to be non-cytotoxic on the tumor cell lines assayed, neither in aerobic conditions nor in the bio-reductive assay performed. Redox behavior, lipophilicity and stability were studied in order to explain the lack of cellular cytotoxic effects. The complexes resulted 10-100 times more hydrophilic than the parent ligands thus the bio-activity of these compounds would be compromised by their inadequate lipophilic properties.     

Synthesis, characterization and antibacterial activity of cobalt(III) complexes with pyridine-amide ligands

The ligands 2-(N-(X-pyridyl)carbamoyl)pyridine (X=2, 3 or 4 for HL1-HL3, respectively) and 2,6-bis(N-(Y-pyridyl)carbamoyl)pyridine (Y=2, 3 or 4 for H2L4-H2L6, respectively) in their mono- and di-deprotonated forms have been used to synthesize kinetically stable cobalt(III) compounds [Co(L1-3)3] (1-3) and Na[Co(L4-6)2] (4-6), respectively. The Co(III) ion is in octahedral environment and is surrounded by three bidentate ligands in complexes 1-3 and two tridentate ligands in complexes 4-6. Ligands coordinate the cobalt center via amidic-N and pyridine-N centers forming a 5-membered chelate ring. Complexes 1-6 have thoroughly been characterized by the various spectroscopic analyses (1H NMR, 13C NMR, UV-vis, IR, mass), elemental analysis, and conductivity measurement. All complexes have been assayed for in vitro antimicrobial activity against clinically isolated resistant strains of Pseudomonas, Proteus, Escherichia coli and standard strains of Pseudomonas aeruginosa (MTCC 1688), Shigella flexneri (MTCC 1457), Klebsiella planticola (MTCC 2272). All cobalt compounds show mild to moderate activity. However, complexes [Co(L1)3] (1) and Na[Co(L4)2] (4) were found to have potent activity against standard and pathogenic resistant bacteria used in the study. Their MIC ranged from 2.7 to 187 microg/ml. In vitro toxicity tests demonstrated that all complexes were less cytotoxic than that of gentamycin on HEK cell lines and the results reveal that these complexes can act as potent antimicrobial agents.