Synthesis of cobalt(III) complexes with novel open chain oxime ligands and metal–ligand coordination in aqueous solution

Cobalt(III) complexes with new open chain oxime ligands: N,N′-bis(2-hydroxyiminopropionyl)-1,2-aminoethane (H2pen) and N,N′-bis(2-hydroxyiminopropionyl)-1,3-diaminopropane (H2pap) have been investigated. Single crystals of Co(papH−1)(Im2)·CH3OH (1) and Co(papH−1)(MEA)2·1.5H2O (2) (where Im = imidazole, MEA = monoethanolamine) suitable for X-ray crystallography were grown by slow evaporation of methanol/water solutions at room temperature. The molecular structures have been determined using single-crystal X-ray diffraction methods. The potentiometric and spectrophotometric results in aqueous solution reveal that both of the open chain ligands show a very high efficacy in the coordination of Co(II) ions. As it has been indicated, differences between the two oxime ligands in complexing ability may be attributed to the longer –CH2– chain in H2pap and by that a better fit of the relatively large Co(II) ion to the accessible binding site. One of the complex species confirmed under inert atmosphere, namely of type Co(LH−1)− (where L = pap or pen), has been shown as the “active” form, capable of dioxygen uptake followed by irreversible oxidation to Co(III).

Graphical abstractTwo open chain oxime ligands (H2pap, N,N′-bis(2-hydroxyiminopropionyl)-1,3-diaminopropane and H2pen, N,N′-bis(2-hydroxyiminopropionyl)-1,2-aminoethane) reveal high efficacy in coordination of Co(II) ions followed by autooxidation to Co(III) under O2. Differences in complexing ability are attributed to the longer –CH2– chain in PAP and better fit of the relatively large Co(II) ion to the accessible binding site.Figure optionsDownload full-size imageDownload as PowerPoint slide