Antimicrobial, insect sterilizing and ovicidal activity of some cobalt(II) and cobalt(III) complexes

Twenty-one mixed-ligand complexes of cobalt(II) and cobalt(III) have been screened for their antimicrobial, insect sterilizing and ovicidal activities. Three of these cobalt(III) complexes exhibit broad antimicrobial spectra, including against human bacterial pathogens, dermatophytes and plant pathogenic fungi, while one exhibits feeble activity against a human pathogenic bacterium. These results have been compared with the activity of the corresponding cobalt(II) complexes, which have been found to be inactive, while the free ligands show reduced activity compared with the cobalt(III) complexes. Change in biological activity induced by a particular complex appears to be dependent on the composition of the first co-ordination sphere. Two of these complexes showed 50% inhibition of the conidial germination of Helminthosporium oryzae and Alternaria triticina. These results indicate their potential for use against human and plant pathogenic microbes. Minimum inhibitory concentrations of the cobalt(III) complexes were determined. Three of these cobalt(III) complexes have been tested for insect sterilizing and ovicidal activities on Dysdercus koenigi F. Positive sterilizing and ovicidal actions of [Co(BSOP)(NH3)2]NO3 (where BSOP-H2 is the Schiff base derived from salicylaldehyde and orthophenylene-diamine) were obtained. Possible mechanism(s) of all these activities are discussed qualitatively.