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

摘要

The reactions of cobalt(II) chloride with racemic malic acid (H(3)mal = C4H6O5) result in the isolation of monomeric, dimeric and tetrameric cobalt malato complexes: (NH4)(2)[Co(R-Hmal)(S-Hmal)]. 2H(2)O (1)[CO2(R-Hmal)(S-HmaI)(H2O)(4)], center dot 2nH(2)O (2), K-4[Co-4(OH)(2)(R-mal)(2)(S-Mal)(2)(H2O)(4)] center dot 10H(2)O (3) and trans[Co(R-H(2)Mal)(S-H(2)mal)(H2O)(2)]- 2H(2)O (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 Coll ion is octahedrally coordinated by two tridentate malates via their alpha-hydroxy, alpha-carboxy and beta-carboxy groups. The malate ligands in 2 coordinate with the cobalt ion via their alpha-hydroxy and alpha-carboxy groups, while the beta-carboxy group acts as a bridging ligand for the other two cobalt ions, forming a novel dimeric unit [Co-2(R-Hmal)(S-Hmal)(H2O)(4)], which further connects into a layered structure through links from the oxygen atoms of the beta-carboxy groups. Complex 3 is a tetranuclear mixed-valence species. Both of the Co-II ions exist in trans-[Co(R-mal)(S-mal)(H2O)(2)] units, which are linked by a Co-2(III)(OH)(2) unit with bridging alpha-alkoxy and beta-carboxy groups. Compound 4 is the main product of reaction between cobalt chloride and excess malate under weakly acidic conditions.