Electronic and Geometric Structure of a Trinuclear Mixed-Valence Copper(II,II,III) Cluster

The four-electron reduction of dioxygen to water by trinuclear copper clusters is of great biological significance. Recently we reported the crystal structure of a trinuclear model complex in which the three coppers provide the four electrons necessary to fully reduce dioxygen, generating two μ₃-oxo bridges. This complex is best described as a localized, mixed-valence Cu(II,II,III) system which has C_2v effective symmetry. The magnetic properties of this trinuclear cluster have been investigated by MCD and SQUID magnetic susceptibility. The two Cu(II) ions are found to be ferromagnetically coupled with a triplet/singlet splitting of 14 cm^-1. Density functional calculations reproduce these geometric, electronic, and magnetic properties of the trinuclear cluster and provide insight into their origin. Since the trinuclear copper complex has a 3+ charge, the Cu₃O₂ core is one electron too oxidized to permit each atom to be in a preferred oxidation state (2+ for Cu and 2− for O). The extra hole in this highly oxidized Cu₃O₂ cluster is found to be localized on one Cu, which is therefore a Cu(III) ion, rather than on an O ligand (which would then be an oxyl) due to the strong stabilization of the oxo valence orbitals which derives from bridging to the Cu(II) centers. The communication between the coppers is weak, as it involves superexchange through the oxo bridges which provide nearly orthogonal orbital pathways between the copper ions. This leads to a ferromagnetic interaction between the two Cu(II) ions and weak electronic coupling between the Cu(III) and the Cu(II) ions. In the idealized D_3h high symmetry limit which would be the favored geometry in the case of complete electronic delocalization, the triplet ground state is orbitally degenerate and subject to a large Jahn−Teller distortion [E‘ ⊗ e‘] toward the observed C_2v structure. This combination of a large Jahn−Teller distortion and weak electronic coupling leads to localization of the Cu(III) on one metal center.