Nitrogen hybridization controls peroxo-oxo equilibrium in ethylenediamine bound binuclear [Cu2O2] complexes

Abstract

The reaction of O2 with binuclear Cu(I) enzymatic active sites as well as synthetic complexes result in the formation of [Cu2O2]2 + core which can exist either in a η2η2-peroxo dicopper (II) species or bis(μ-oxo) dicopper (III) species. These species are proposed to exhibit different reactivity towards organic substrates. The factors that control the equilibrium between these two isoelectronic electromers have been investigated for decades. Using DFT calculations on [Cu2O2]2 + core supported by ethelenediammine ligands, where the substitutions on the nitrogen are systematically varied, the electronic structure contributions to the relative stabilities of these species is investigated. The calculations show that the CuN bond length varies anomalously with the electron donor ability of the ethylenediamine derivatives reproducing experimental trends. The ground state wavefunctions show that the relative contribution of the 2p and 2s orbitals in the nitrogen-based donor orbitals vary depending on the substituents. Bulky substituents use donor orbitals having distinctly more N2p contribution than the unsubstituted or mildly substituted ammines which use donor orbitals with more N2s contribution. This results in anomalous variation of the CuN distances and relative peroxo-oxo energies with ligand donor strength.