Synthesis, Characterization, Physicochemical Studies and Antimicrobial Evaluation of Mixed Ligand Complexes Involving Co(II) with 2,2′-Dipyridylamine and Dicarboxylic Acids

In the current research, the complex formation equilibria of 2,2′-dipyridylamine (DPA) with the metal ions Cu(II), Ni(II), Co(II), Mn(II), and Zn(II) are investigated potentiometrically. The relation between the stability constants of the formed complexes and the properties of the central metal ions was investigated. In addition, the solvent effect on the protonation of DPA and Co-DPA complex formation was also investigated and discussed. The formation of the ternary complexes Co(DPA)L (L = some selected dicarboxylic acids) was studied in aqueous solutions at 25 °C and 0.1 mol dm−3 ionic strength. Stability constants and stoichiometry are reported for the complexes formed in solution. The results show that ternary complexes are formed by a simultaneous reaction mechanism. The speciation of all the complexes was resolved. The effect of chelate ring size of the dicarboxylic acid complexes on their stability constants was also examined. The concentration distribution of the complexes in solution was evaluated. The solid complexes [Co(DPA)L] (where L is oxalic, malonic, succinic or 1,1-cyclobutanedicarboxylic (CBDC) acid) have been synthesized and fully characterized with the help of elemental analyses, infrared spectra, magnetic and conductance measurements. Spectroscopic studies and magnetic measurements (Meff) suggest a tetrahedral geometry for Co(II)-complexes. The measured molar conductance values in DMSO indicate that the complexes are nonelectrolytic in nature. The isolated solid complexes have also been screened for their pharmacological activities against some selected bacteria and fungi. The activity ata show that the complexes have a significant activity against Escherichia coli (gram negative) and Staphylococcus aureus (gram positive), as well as an antifungal activity against Aspergillus flavus and Candida albicans.