A violet mixed-valence copper-mercaptoethylamine complex generated electrochemically or by reversible interaction with oxygen

Abstract

The synthesis, characterization and the reversible interaction with oxygen of a Cu(I)-mercaptoethylamine complex (I) in aqueous solution and room temperature is reported. As a result of the interaction with oxygen a violet species II is formed which has a similar absorption spectrum (λmax = 500 nm) to a violet species III obtained electrochemically from a solution of the Cu-mercarptoethylamine complex in a nitrogen atmosphere. In the absence of dissolved oxygen both violet species are stable but in presence of air they decompose into a greenish-white precipitate which does not contain the mercaptoethylamine ligand in its structure. A mechanism for the complex I-oxygen interaction is proposed which involves a superoxo mixed-valence species as intermediate. The electronic assignation of the 500 nm absorption band is also discussed. The remarkable special characteristics, i.e. (i) the simplicity of the ligand containing a soft donor atom; (ii) the fact that the reversible uptake of oxygen occurs at room temperature and (iii) the spectroscopical similarity of species II and III described above, allow new insights into the interaction of metal complexes with oxygen. These results suggest that a mixed-valence species may be associated with the O2 binding or oxygen activation processes in copper proteins. Also, considering the results recently published by M. Saraste et al., the relevance of the mercaptide bridged mixed-valence species II and III as models for the CuA site in cytochrome oxidase is also discussed