Co(II) and Co(III) hydroxamate systems: A solution equilibrium study

The interaction of Co(II) with monohydroxamates (Aha, Bha and MeAha), dihydroxamates (2,5-DIHA, 2,5-H,H-DIHA, 2,4-DIHA, 2,3-DIHA, 2,2-DIHA, 3,4-DIHA and 3,3-DIHA) and trihydroxamates (DFB and DFC) was investigated in aqueous solution. According to the pH-metric results mono-, bis- and tris-complexes are formed with monohydroxamic acids, but if the metal to ligand ratio is less than 1:3, mixed hydroxo complexes also exist at high pH. With dihydroxamates, the stability trend of the mono-chelated complexes (Co(HDIHA)) follows the trend of the basicity of the ligands, but significant effect of the length and structure of the connecting chain on the stability of the bis-chelated complexes was found. Interestingly, the best fit of the pH-potentiometric results was obtained without the tris-chelated species (Co2(DIHA)3), unlike the case of 2,5-H,H-DIHA, and this was also supported by both the ESI-MS and UV–Vis results. Different protonated and non-protonated complexes (depending on the pH) are formed with the two trihydroxamate-based siderophores. The higher stability of the complexes formed with DFC is originated from the conjugation of the double bonds (situating one in β-position to each hydroxamate function) and the hydroxamate functions in this siderophore. The redox stability of the Co(II)–siderophore complexes was also investigated. Following the oxidation of the central metal ion to Co(III), cyclic voltammetric investigation of the complexes was made. Both the DFB and DFC containing complexes showed irreversible reduction of the Co(III) complexes. Based on the approximate stability constants determined by the cyclic voltammetric results, extremely high stability tris-chelated complexes can be formed in the Co(III)–DFB and especially in the Co(III)–DFC systems.