Ab initio study of the O2 binding in dicopper complexes

The structures and stabilities of [ Cu-2(mu-eta(2):mu(2)peroxo)] (2+) ( A) and [ Cu-2(mu- oxo)](2+) ( B) complexes with three NH3 ligands per copper are investigated using DFT and high-level ab initio methods. These are model systems for active centers in enzymes like hemocyanine and tyrosinase. Previous studies have shown that at the DFT/ B3LYP level the peroxo form A is more stable than the mu- oxo form B, while the opposite was found using CASPT2 ( Flock M, Pierloot K ( 1999) J Phys Chem 103: 95). At the two computational levels, the energy difference of the isomers differed by more than 30 kcal/ mol. In this work this problem is reinvestigated using a localized orbital description and multireference configuration interaction ( MRCI) methods. It is found that CASPT2 strongly over- corrects the correlation effect and MRCI predicts structure A to be energetically lower than B, in qualitative agreement with B3LYP and experiment. However, B3LYP seems to stabilize the biradicalic structure A too much, and this effect depends approximately linearly on the amount of exact exchange in the B3LYP density functional. Reducing the amount of exact exchange to 10 - 15% yields good agreement between MRCI and B3LYP.