Anaerobic and aerobic complexation of Co(II) by a polyamine ditopic ligand containing the phenolic moiety

The basicity behavior and coordination properties of the ligand 2,6-bis-{[bis-(2-aminoethyl)-amino]methyl}-phenol (L) in aqueous solution toward Co(II), are reported and discussed together with those of the species formed with Ni(II) and Cu(II) ions. Moreover, binuclear cobalt complexes of L were investigated as possible binders for molecular dioxygen. The studies were carried out using potentiometric measurements (298.1 K, I=0.15 mol dm−3), UV–Vis absorption spectra, and 1H and 13C NMR spectra in aqueous solution. L behaves as a pentaprotic base in the range of the potentiometric measurements; the UV and NMR experiments revealed that in L, HL+, H2L2+, and H3L3+ species there is the simultaneous presence of positive and one negative charge on the ligand. L forms mono and binuclear complexes with Co(II) ion. The binuclear species occur with the complete deprotonation of L; the oxygen of the phenolate moiety bridges the two metal ions forcing them to stay close to each other, preorganizing the complexes to bind an additional molecule. This is the case of the binuclear cobalt species ([Co2(H−1L)(OH)j](3−j)+ (j=0–2)), which cooperate to bind one molecule of dioxygen, giving rise to mono-oxygenated species. The stoichiometry of such species ([Co2(H−1L)(OH)jO2](3−j)+ (j=0–2)), and other experimental data are in agreement with the formation of a μ-peroxocomplexes where the oxygen molecule bridges the two metal atoms. The experiments showed that the oxygenation reactions are reversible.