Spectral and redox studies on mixed ligand complexes of cobalt(III) phenanthroline/bipyridyl and benzoylhydrazones, their DNA binding and antimicrobial activity

Cobalt(III) complexes of the type [Co(N-N)2L](ClO4)2.H2O [where L=anionic form of para-substituted benzaldehyde-benzoylhydrazone (BHBX-); X=H, Me, OMe, OH, Cl or NO2; N-N=2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen)] have been synthesized and characterized through UV-Vis, IR, NMR and electrochemical studies. The IR spectral frequencies support the mode of coordination of BHBX to the metal through the imino nitrogen and enolic oxygen atoms. The electronic absorption spectra exhibit metal to ligand charge transfer (MLCT) transition around 450 nm together with intraligand (IL) bands that are comparable to that of [Co(phen/bpy)3]3+. In acetonitrile solution these complexes show two well defined redox couples corresponding to Co(III/II) and Co(II/I) processes. Binding of these complexes with herring sperm DNA have been investigated by spectroscopic and voltammetric methods. The lower binding constant values of these complexes with respect to the [Co(phen/bpy)3]3+ are ascribed to the polar interaction of the substituted benzoylhydrazone moiety with the sugar-phosphate backbone of the DNA. The UV spectrum shows reasonable hypochromism with slight (2-4 nm) red shift, while the cyclic voltammogram shows decrease in current intensity along with a very small shift in the formal potential of the Co(III/II) redox couple. These experimental results indicate that phen mixed ligand complexes bind to DNA through an intercalative mode more effectively than their bpy counterparts. These complexes are also found to have good antimicrobial activity.