Synthesis, characterization, and antimicrobial study of mixed ionic complexes of Cobalt(ii) and Zinc(ii) dipicolinates with n,n-donor ligands

Abstract

Within the framework of this study, two mixed ionic complex salts of cobalt(II) and zinc(II) dipicolinates with 1,10-phenanthroline and 4-(dimethyl)aminopyridine as N,N-donor ligands were synthesized in aqueous solutions. The two salts [Co(DMAP)6]2+[Zn(dipic)2]2-·XH2O (compound A) and [Co(phen)3]2+[Zn(dipic)2]2-·XH2O (compound B) which appeared as crystals of red and orange colouration respectively were characterized using various physico-chemical analytical techniques such as melting point determination, solubility tests, conductimetry, powdered X-ray diffraction, IR, UVVisible and photoluminescence spectroscopies. Their ability to inhibit microbial growth was also assessed on some bacterial and fungal strains. The molar conductivities of compounds A (121.2 S.cm2.mol-1) and B (118.5 S.cm2.mol-1) showed that each of the synthesized compounds is made up of two ions in aqueous solution. Both compounds did not melt up to a temperature of 330 ºC, which is the threshold temperature of the instrument used. This further confirmed the strong nature of the forces holding the constituent particles. The solubility test results suggests that both compounds are ionic with A soluble in both polar protic and polar aprotic solvents while B on its part was only soluble in polar aprotic solvents. The IR spectra revealed that each of the synthesized materials contained water molecules of crystallization as indicated by broad bands at (3400 cm-1) for compounds A and (3173 cm-1) for compounds B. Shifts in vibration bands in both compounds A and B with respect to the ligands used could be attributed to complexation while vibration bands that fall in the lower region (900-400 cm-1) corresponding to metal to oxygen or nitrogen bonds in the synthesized compounds further confirm the formation of new bonds. The UV-Visible and photoluminescence spectra of both compounds indicated the presence of d-d electronic transitions within the d7 orbitals of Co2+ which are responsible for the colouration of the synthesized compounds, which equally supposes an octahedral geometry in the coordination spheres of the central metal ions. The antimicrobial activities of the target materials A and B were evaluated against five bacterial strains (Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Acinetobacter baumannii and Shigella flexineri) and two fungal strains (Candida tropicalis and Candida albicans) by microdilution in liquid medium. Compounds A and B both showed antimicrobial activities on some bacterial strains and no activity against any of the fungal strains. Compound A has a good antibacterial activity against Staphylococcus aureus while compound B shows a weak activity against Escherichia coli.