Binuclear Cobalt(II) Complexes of Macrocyclic and Macrobicyclic Ligands as Oxygen Carriers

Abstract

The macrocyclic and macrobicyclic ligands, O-BISDIEN, and O-BISTREN, form thermo-dynamically stable hydro×o-bridged binuclear cobalt(ll) complexes. Both of these complexes react with dioxygen to form kinetlcally stable peroxo-bridged complexes that do not undergo (metal-centered) degradation at appreciable rates. The loss of dioxygen from the µ-peroxo-µ-hydroxo-dicobalt(lll) O-BISDIEN complex is estimated to have the first order rate of about 2 × 10-8 sec-1. The ligand is not attacked in this reaction but is converted to the binuclear hydroxo-bridged cobalt(lll) complex. The rate of degradation of the dioxygen complex formed from O-BISTREN is kinetically even more stable, with no measurable rate of degradation in water solution from ~0 °C to 75 °C. Even if the degradation were to take place (a reaction which is very slow for the O-BISDIEN complex and even slower for that formed from O-BISTREN) the ligands are not attacked during the degradation reactions, and the active Co(ll) complexes can be regenerated by electrolytic reduction of the corresponding Co(lll) complexes. In any case, the very slow rates of degradation of and the lack of ligand attack, make these binuclear cobalt(II) complexes the most effective and efficient oxygen carriers ever reported, and are highly recommended for the separation of dioxygen from air.