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 C2v 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 o...