Hydroxo-bridged dinuclear cobalt(II) complexes of OBISDIEN and OBISTREN as oxygen carriers

The hydroxo-bridged dinuclear cobalt(II) complexes of the macrocyclic and macrobicyclic dinucleating ligands OBISDIEN and OBISTREN form thermodynamically stable dioxygen adducts. Although the values of the oxygenation constants of cobalt complexes usually increase with an increase in the number of basic amino groups coordinated to the cobalt centers, these complexes are unusual in that the oxygenation constants of the complex formed from OBISDIEN is about three orders of magnitude higher than the analogous complex formed from OBISTREN. The difference in behavior must be due to differences in steric strain. Since crystal structures are not available, steric effects were estimated by the use of molecular mechanics. By application of the molecular modelling program SYBYL it was determined that the distortions of the cryptand ligand necessary to form the hydroxo-bridged dioxygen complex are much greater and more energetic than those of the macrocyclic ligand. Both ligands were found to form excellent oxygen carriers because of facile oxygenation-deoxygenation reactions of their dinuclear cobalt complexes, and because of almost immeasurably slow metal-centered degradation. Moreover, the ligands were not changed during degradation, so that the oxygen-carrying complexes could be easily regenerated by electrolytic reduction.