Experimental evidence of the disproportionation equilibrium in copper mixed-valence complexes

Experimental evidence for a dismutation equilibrium in the mixed-valence (MV) [Mn+ -M(n+1)+] system type has been elusive and its existence can be established only when the oxidation-reduction processes involved are reversible. Previous research in the field of binuclear Cu(II)-Cu(II), Cu(I)-Cu(I) and the related MV Cu(II)-Cu(I) complexes allowed us to obtain electrochemical evidence for the disproportionation equilibrium in some of these systems. In this communication we report discuss experiments with [(RCOO)2Cu(II)-Cu(I)(OOCR)2]- (R = CH3, Ph) type MV complexes that give direct non-electrochemical experimental evidence for the presence in solution of the disproportion equilibrium: 2[(R-COO)2Cu(II)-Cu(I)(OOC-R)2]-⇔ (R-COO)4Cu2(II) + [(RCOO)4Cu(I)2]2- It was possible to isolate the different components of the disproportionation equilibrium by varying temperature and solvent conditions. To our knowledge, this is the first non-electrochemical experimental evidence of this equilibrium for copper MV complexes in solution. Furthermore, as is discussed in the text, these results may be the basis for giving an alternative point of view to that given in some studies already reported in the literature, which relate to solvent and temperature effects on the intensity and energy of the intervalence transfer bands and also to changes in the EPR spectra of MV species