Oxygen activation and deactivation in the presence of simple metal-chelates

Molecular oxygen is a paramagnetic molecule having a triplet ground state. In the presence of some metal-complexes, e.g. of cobalt, copper and iron this ground state is abolished, and oxygen is in an activated state. First of all the addition of molecular oxygen to a metal complex could involve a formal oxidation of the metal center and a concomitant reduction of the bound oxygen. The question is, whether this state of oxygen is the reactive species in oxygenation reactions. With some chemical and physicochemical experiments we would like to show evidence for the singlet character of such a bound oxygen in some cases. The reaction is the oxygenation of 1.5-dihydroxy-naphthalene (1) to give juglone (2) in the presence of Co-salen in high yield. This reaction is affected by specific singlet oxygen quenchers, such as DABCO, sodium azide and Ni(II)-chelates. By measuring the ultraweak chemiluminescence, these findings could be supported.