A comparison of the thermodynamics of O[bond]O cleavage for dicopper complexes in enzymes and synthetic systems.

The preferred state, the peroxide Cu(2)(II,II) or the bis-mu-oxo Cu(2)(III,III) states, for oxygen-bridged copper dimers with nitrogen donors is reinvestigated. Experiments have indicated that for the enzymatic complexes with histidine ligands the peroxide state should be favored, at least for hemocyanin, while for the synthetic complexes with strained ligands the bis-mu-xo state should be intrinsically favored. The present B3LYP study essentially agrees with these results. The quite different results obtained in CASPT2 and some previous B3LYP studies for these systems are investigated and discussed. The conclusion, drawn in an earlier study, that the Cu(2)(III,III) state is an unlikely intermediate in the enzyme mechanisms of tyrosinase and catechol oxidase, still remains.