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

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.