Study and Potential Biological Relevance of the aqueous Chemistry of Cobalt(II) in the Presence of Citric Acid

Cobalt as a trace metal ion is essential to humans. It is well established that Cobalt participates in many biological processes. Thus its interactions with low molecular weight physiological ligands are of special biological interest. The species are highly significant in providing soluble bioavailable forms of several metal ions, including cobalt, to biochemical processes. In an effort to comprehend the chemistry of Co(II) in biologically relevant media, citric acid, a low molecular weight physiological ligand, and cobalt(II) were investigated in-depth, in view of their interaction in aqueous solutions. Speciation studies, carried out in aqueous solutions, pointed to several chemical species emerging from the cobalt(II)-citrate system, under varying stoichiometries and in a wide pH range. The proposed species include CoAH, CoA-, CoA24-, Co2A2H-24- or CoAH-12-. Moreover, synthetic efforts targeting soluble species of Co(II), from the same cobalt(II)-citrate system in aqueous media, led to the isolation of the first dinuclear complex anion [Co2(C6H5O7)2(H2O)4]2- at pH ~5, in the form of its potassium (1) and sodium salts (2). Complexes 1 and 2 were characterized analytically, and spectroscopically (FT-IR, UV/Visible). X-ray crystal structure studies on 1 and 2 revealed the presence of two Co(II) ions, octahedrally coordinated by two citrate ligands. The octahedral environment around each Co(II) ion was complemented by another singly bonded citrate ligand belonging to the adjacent Co(II) unit, and two water molecules. Magnetic susceptibility study on 2 ? the presence of a very weak interaction between the ? Co(II) ions, essentially rendering them independent within the dimer. Further EPR studies in combination with the ? for solid and solution data will provide valuable information on the ? of the complex anion of 1 and 2 in the solid state and in solution