Bendola Publishing Journal of Transition Metal Complexes Vol. 3 (2020), Article ID 246098, 11 pages doi:10.32371/jtmc/246098 Bendola Research Article Synthesis, Characterization, Physicochemical Studies and Antimicrobial Evaluation of Mixed Ligand Complexes Involving Co(II) with 2,2 ′ -Dipyridylamine and Dicarboxylic Acids Faten M. Ali Zainy, 1 Aisha M. Turkustani, 1 and Azza A. Shoukry 2 1 Chemistry Department, Faculty of Science, University of Jeddah, P.O. Box 80327, Jeddah 21589, Saudi Arabia 2 Department of Chemistry, Faculty of Science, Cairo University, 12613 Giza, Egypt Address correspondence to Azza A. Shoukry, azzashoukry@hotmail.com Received 8 April 2020; Revised 2 May 2020; Accepted 6 May 2020 Copyright © 2020 Faten M. Ali Zainy et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract 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 (M eff ) 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 data 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. Keywords Co(II); 2,2 ′ -dipyridylamine (DPA); dicarboxylic acids; potentiometry; stability constant; effect of solvent; biological activity 1. Introduction Metals have an esteemed place within medical biochemistry, although until recently this was restricted only to organic drugs. Recently, more research has been directed to the area of inorganic chemistry, which led to the developments in the treatment of cancer, diabetes, and ulcers as well as in the development of neurological, cardiovascular, and anti-inflammatory drugs. Metal coordination complexes have been widely studied for their antimicrobial [1,2] and Figure 1: The chemical structure of DPA. anticancer properties [3], particularly those containing the late first row transition metals (i.e., cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn)) which are biologically relevant as they are associated with various biomolecules related to essential physiological activities [4]. Many drugs possess modified pharmacological and toxicological properties when administered in the form of metallic complexes. It was observed that the biological activity of these drugs increases on complexation with metal ion [5, 6]. The factors affecting the stability of metal complexes, such as electrostatic, hydrophobic or steric interaction between the ligands, have been extensively studied based on the idea that these factors could play an important role in enzyme-metal ion-substrate systems [7]. Binary and ternary chelations occur commonly in bio- logical fluids, as millions of potential ligands like amino acids, peptides or their derivatives or analogues, and hetero- cyclic N-bases are likely to compete for biologically impor- tant transition metal ions such as Cu(II), Ni(II), and Zn(II) found in vivo. These chelations, especially complexes that contain the two different types of bioligands (i.e., heteroaro- matic nitrogen-base 2,2 ′ -dipyridylamine (DPA) and dicar- boxylic acids), may be considered as models for substrate- metal ion-enzyme interactions and other metal ion mediated biochemical interactions. From the biological point of view, Co(II) is one of the most important trace elements in the world of animals