A Trinuclear Intermediate in the Copper-Mediated Reduction of O$_2$: Four Electrons from Three Coppers

The reaction of metal complexes with dioxygen (O$_2$) generally proceeds in 1:1, 2:1, or 4:1 (metal:O$_2$) stoichiometry. A discrete, structurally characterized 3:1 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O$_2$ bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O$_2$ at the trinuclear active sites of multicopper oxidases is discussed.