Dicopper complexes, mixed-valence

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

The mechanism of the catalytic reaction proved indeed to be very different from that found for [Cu2([22]py4pz)( r-0H)](C104)3 H20. Thus, in the first step of the reaction, a stoichiometric oxidation of catechol by the dicopper(II) complex takes place however, only one electron is transferred in this stoichiometric reaction, resulting in the formation of a semiquinone radical and a mixed-valence Cu"Cu species. Interestingly, the dicopper(II) complex was found to be essentially dinuclear in solution nevertheless, only one of the two copper(II) ions was found to participate in the redox process, whereas the second one played a purely structural... 

Mixed-valence dicopper complexes have been of some interest as models for the spectroscopically informative half-met hemocyanins 156, 157). These complexes are described as Class I if the unpaired electron is localized on one copper ion (i.e., [Cu(I)Cu(II)]) and Class III if it is fully delocalized (i.e., [Cu(1.5)Cu(1.5)]) intermediate situations... 

Houser RP, Young VG Jr, Tolman WB. A Thiolate-bridged, fully delocahzed mixed-valence dicopper(I,II) complex that models the Cua biological electron-transfer site. J. Am. Chem. Soc. 1996 118 2101-2102. 

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