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 co ppers provide the four electrons necessary to fully reduce dioxygen, g enerating two mu(3)-oxo bridges. This complex is best described as a l ocalized, 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 the se geometric, electronic, and magnetic properties of the trinuclear cl uster and provide insight into their origin. Since the trinuclear copp er complex has a 3+ charge, the Cu3O2 core is one electron too oxidize d 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, ra ther than on an O ligand (which would then be an oxyl) due to the stro ng stabilization of the oxo valence orbitals which derives from bridgi ng to the Cu(II) centers. The communication between the coppers is wea k, 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 e lectronic coupling between the Cu(III) and the Cu(II) ions. In the ide alized D-3h high symmetry Limit which would be the favored geometry in the case of complete electronic delocalization, the triplet ground st ate is orbitally degenerate and subject to a large Jahn-Teller distort ion [E' x e'] toward the observed C-2v structure. This combination of a large Jahn-Teller distortion and weak electronic coupling leads to l ocalization of the Cu(III) on one metal center.