Redox chemistry of copper acetate binuclear complexes in acetic acid-methanol mixture as solvent

Abstract

The redox chemistry of the photochemically generated bioinorganic model (CH 3COO) 2Cu II Cu I(OOCCH 3) 2 and the parent species (CH 3COO) 2Cu IICu II(OOCCH 3) 2 (complex I) are reported. The purpose is to achieve a more complete physicochemical characterization of the acetate binuclear copper complex and consequently a closer evaluation of its behaviour as a model of type III Cu sites in cuproproteins. Complex I may be sequentially reduced (broad not resolved peaks around −0.2 and −0.4 V) and reoxidized (peaks at 0.16 and 0.36 V), but in competition with a thermal disproportionation to Cu II and Cu 0. Controlled potential electrolysis (CPE) at −0.4 V measures quantitatively the overall reduction process including disproportionation and involves four electrons per mole of initial complex I. Structural changes seem to play a role in the oxidation-reduction processes as suggested by ip/√v vs v studies (Nicholson and Shain's criteria), and they make the processes irreversible. The mixed-valence Cu II-Cu I species could not be produced by CPE at any voltage before the second reduction peak; however, its electrochemical generation by pulses at −0.4 V render direct experimental evidence that the reduction process is sequential. The voltammograms of the Cu II-Cu I species generated photochemically are more complicated: the reduction and oxidation peaks are centered at −0.2 and 0.0 V, respectively. The species Cu II-Cu II originated from the oxidation of this mixed valence complex, called as complex II, shows similarities and differences with complex I. The results are discussed in relation to the structure stability and to the copper interactions in the binuclear species considered as models for the copper protein centers.