Cobalt(II) dioxygen carriers based on dinucleating ligands. Part 1. Thermodynamics of dioxygen binding in aqueous solution

The equilibria involving the uptake of dioxygen (O-2) to the dicobalt(II) c omplexes of the macrocyclic ligands [27]aneN(9) (1,4,7,10,13,16,19,22,25-no naazacycloheptacosane) and [30]aneN(10) (1,4,7,10,13,16,19,22,25,28-decaaza cyclotriacontane), and of the acyclic polyamine Me(2)octaen (1,25-dimethyl- 1,4,7,10,13,16,19,22,25-nonaazapentacosane) have been studied by potentiome tric measurements in 0.15 mol dm(-3) NaClO4 aqueous solution at 298.1 +/- 0 .1 K, and the relevant equilibrium constants have been reported. All three ligands form stable oxygenated complexes of the type [Co2L(O-2)](4+) and [C o2L(O-2)OH](3+), in which the O-2 molecule is bound in the end-on mode to a single metal ion. In the case of the macrocyclic complexes the coordinatio n of O-2 and OH- reveals a cooperativity effect, i.e. a significant stabili ty increase is observed when one of these species coordinates to a dinuclea r complex containing the other one, with respect to the same reactions invo lving (Co2L)(4+). This behaviour suggests the interaction via hydrogen bond ing between the two species (O-2 and OH-) coordinated to different cobalt(I I) ions. (C) 2000 Elsevier Science B.V. All rights reserved.