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

Abstract

The reactions of cobalt(II) chloride with racemic malic acid (H 3mal = C 4H 6O 5) result in the isolation of monomeric, dimeric and tetrameric cobalt malato complexes: (NH 4) 2[Co( R-Hmal)( S-Hmal)] · 2H 2O ( 1), [Co 2( R-Hmal)( S-Hmal)(H 2O) 4] n · 2 nH 2O ( 2), K 4[Co 4(OH) 2( R-mal) 2( S-mal) 2(H 2O) 4] · 10H 2O ( 3) and trans-[Co( R-H 2mal)( S-H 2mal)(H 2O) 2] · 2H 2O ( 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 Co II 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 [Co 2( R-Hmal)( S-Hmal)(H 2O) 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 Co II ions exist in trans-[Co( R-mal)( S-mal)(H 2O) 2] units, which are linked by a Co III 2(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.