Theoretical Study of the Interconversion of O₂-Binding Dicopper Complexes

Abstract

The structures and interconversion pathway between the [Cu2(μ-η2:η2-O2)]2+ and [Cu2(μ-O)2]2+ isomers of model systems with three ammonia ligands per copper center are investigated using both density functional theory with a B3LYP functional (B3LYP-DFT) and multiconfigurational perturbation theory (CASSCF/CASPT2). Both methods lead to thoroughly different results for the relative energy of both isomers. The CASPT2 results reveal an intrinsic stabilization of the [Cu2(μ-O)2]2+ isomer, thus indicating that the presence of a [Cu2(μ-η2:η2-O2)]2+ core in the respiratory proteins must be brought back to the presence of bulky capping ligands and/or to external effects caused by solvents and counterions. Both isomers are found to be diamagnetic. For the [Cu2(μ-η2:η2-O2)]2+ isomer an antiferromagnetic coupling constant, −2J, of 4209 cm-1 is calculated at the CASPT2 level. On the other hand, in the [Cu2(μ-O)2]2+ isomer, the lowest 3Bu state is calculated at 9316 cm-1, but is found to correspond instead to an oxygen π* → σ* excitation.