pH and molar ratio dependent formation of monomeric, dimeric and tetrameric cobalt malate complexes

The reactions of cobalt(II) chloride with racemic malic acid (H3mal = C4H6O5) result in the isolation of monomeric, dimeric and tetrameric cobalt malato complexes: (NH4)2[Co(R-Hmal)(S-Hmal)] · 2H2O (1), [Co2(R-Hmal)(S-Hmal)(H2O)4]n · 2nH2O (2), K4[Co4(OH)2(R-mal)2(S-mal)2(H2O)4] · 10H2O (3) and trans-[Co(R-H2mal)(S-H2mal)(H2O)2] · 2H2O (4). The formations of the malato complexes are dependent on the pH value, the molar ratio of the solutions, the reaction temperature and the counterions. In the water-soluble compound 1, the CoII ion is octahedrally coordinated by two tridentate malates via their α-hydroxy, α-carboxy and β-carboxy groups. The malate ligands in 2 coordinate with the cobalt ion via their α-hydroxy and α-carboxy groups, while the β-carboxy group acts as a bridging ligand for the other two cobalt ions, forming a novel dimeric unit [Co2(R-Hmal)(S-Hmal)(H2O)4], which further connects into a layered structure through links from the oxygen atoms of the β-carboxy groups. Complex 3 is a tetranuclear mixed-valence species. Both of the CoII ions exist in trans-[Co(R-mal)(S-mal)(H2O)2] units, which are linked by a CoIII2(OH)2 unit with bridging α-alkoxy and β-carboxy groups. Compound 4 is the main product of reaction between cobalt chloride and excess malate under weakly acidic conditions.

The formation of monomeric, dimeric and tetrameric cobalt malate complexes is dependent on the pH value and molar ratio of the reactants, while less relevant to the temperature and counterions. The dimeric species further connects into a two-dimensional coordination polymer, and the tetramer is a mixed-valence cobalt(II/III) complex.Figure optionsDownload as PowerPoint slide