NICKEL(II) AND COBALT(II) COMPLEXES OF POTENTIALLY QUINQUEDENTATE MACROBICYCLIC LIGANDS - REVERSIBLE BINDING OF DIOXYGEN TO A COBALT(II) COMPLEX

Two new macrobicyclic ligands which are potentially quinquedentate have been prepared and characterised. Variable-temperature NMR studies show that the nickel(II) complex of one of these ligands exists as a mixture of two conformers at low temperature which take part in two independent exchange processes as the temperature is raised. In the first, one conformer undergoes a form of self exchange and in the second, the two conformers interconvert. The ESR spectrum of the cobalt(II) complex of one of the new ligands indicates that it exists as a mixture of four- and five-co-ordinate species, with the five-co-ordinate material arising from intermolecular binding of the ligand secondary amine group to the cobalt(II) ion. This complex binds dioxygen reversibly and the O2 adduct has reasonable stability with respect to autoxidation.