Cobalt(II) Dioxygen Carriers Based on Dinucleating Ligands. Part 2: Kinetics of Dioxygen Binding

We consider here the kinetics of the reaction of formation of dioxygenated Co(II) complexes with the binucleating ligands 1,23-bis(methylamino)-3,6,9,12,15,18,21-heptaazatricosane (Me2Octaen), 1,4,7,10,13,16,19,22,25-nonaazacycloheptacosane ([27]aneN9), 1,4,7,10,13,16,19,22,25,28-decaazacyclotriacontane ([30]aneN10), and 1,4,7,10,13,16,19,22,25,28,31-undecaazacyclotritriacontane ([33]aneN11). The most important result is that the Co(II) complexes with these polyamines in aqueous solution and at room temperature are very flexible in binding dioxygen: by Changing the value of the ligand to cobalt concentration ratio (R =CL/O/CCo/O) and the pH of the solution, it is possible to modulate the dioxygen uptake obtaining two-to-one or one-to-one, Co/O2 superoxo complexes. As to the kinetics of the formation of these complexes, we find that the rate-determining step is the transformation of a cobalt-ligand precursor complex, inert towards dioxygen, to yield an active form, whereas the reaction of dioxygen uptake is fast in all the investigated systems