Correlations of Synthetic, Spectroscopic, Structural, and Speciation Studies in the Biologically Relevant Cobalt(II)−Citrate System: The Tale of the First Aqueous Dinuclear Cobalt(II)−Citrate Complex

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Abstract
Synthetic efforts targeting soluble species of Co(II) with the low molecular mass physiological ligand citric acid led to the isolation of the first dinuclear complex [Co2(C6H5O7)2(H2O)4]2-, at pH ∼5, in the form of its K+ (1) and Na+ (2) salts. Both 1 and 2 were characterized analytically, spectroscopically (FT-IR, UV/visible, EPR), and magnetically. Complex 1 crystallizes in the monoclinic space group P21/n, with a = 10.348(5) Å, b = 11.578(6) Å, c = 12.138(6) Å, β = 112.62(2)°, V = 1342(1) Å3, and Z = 2. Complex 2 crystallizes in the monoclinic space group P21/c, with a = 9.234(4) Å, b = 11.913(4) Å, c = 11.728(6) Å, β = 99.93(2)°, V = 1271(1) Å3, and Z = 2. X-ray crystallography on 1 and 2 reveals the presence of two Co(II) ions, in a dinuclear assembly, octahedrally coordinated by two citrate ligands in a tridentate fashion. The octahedral environment around each Co(II) is complemented by another singly bonded citrate belonging to the adjacent Co(II) unit and two water molecules. Magnetic susceptibility and EPR studies on 1, in the solid state, corroborate the X-ray results, indicating a weak interaction between the two Co(II) ions. Moreover, EPR and UV/visible studies in solution suggest that 1 does not retain its dimeric structure, yielding a mononuclear octahedral Co(II)−citrate species. Detailed speciation studies suggest the presence of a number of species including the mononuclear complex [Co(C6H5O7)]-, optimally present around pH ∼5. In consonance with EPR and UV/visible spectroscopy, [Co(C6H5O7)]- is likely the scaffolding unit on the basis of which the dimer [Co2(C6H5O7)2(H2O)4]2- is isolated from aqueous solutions. Collectively, this comprehensive study offers significant structural insight into the Co(II)−citrate speciation and the elucidation of the role of Co(II) in biological fluids.