Density functional theory (DFT) calculations have been carried out to elucidate the mechanism of self-oxidation of a Cu(II) complex octaaza dinucleating macrocyclic ligand. The reaction is bimolecular and spontaneous, in which amine groups of one macrocycle are oxidized and the Cu^II centers of a second macrocylic complex are reduced. No additional oxidation or external base agents are required. DFT calculations predict the reaction to proceed via a two-step mechanism, in which the first step is proton transfer between two reactant complexes. This is followed by a second transfer step in which an electron and proton are transferred together between the two complexes. Concurrent with this external transfer there is also an internal electron transfer in which the ligand reduces the metal center to give the imine product bound to Cu^I. The complexity of this final step differs from the generally accepted mechanisms for transition metal catalyzed amine to imine oxidation in which protons and electrons are transferred individually.