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).