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

Experimental evidence for a dismutation equilibrium in the mixed-valence (M V) [Mn+ -M(n+1)+] system type has been elusive and its existence can be est ablished only when the oxidation-reduction processes involved are reversibl e. Previous research in the field of binuclear Cu(II) Cu(II), Cu(I)-Cu(I) a nd the related MV Cu(II)-Cu(I) complexes allowed us to obtain electrochemic al evidence for the disproportionation equilibrium in some of these systems . In this communication we report and discuss experiments with [(RCOO)(2)Cu (II)-Cu(I)(OOCR)(2)](-) (R = CH3, Ph) type MV complexes that give direct no n-electrochemical experimental evidence for the presence in solution of the disproportion equilibrium: 2[(R-COO)(2)Cu(II)-Cu(I)(OOC-R)(2)](-) double left right arrow (R-COO)(4)Cu -2(II) + [(RCOO)(4)Cu(I)(2)](2-) It was possible to isolate the different components of the disproportionati on equilibrium by varying temperature and solvent conditions. To our knowle dge, this is the first non-electrochemical experimental evidence of this eq uilibrium for copper MV complexes in solution. Furthermore, as is discussed in the text, these results may be the basis for giving an alternative poin t of view to that given in some studies already reported in the literature, which relate to solvent and temperature effects on the intensity and energ y of the intervalence transfer bands and also to changes in the EPR spectra of MV species. (C) 2000 Elsevier Science B.V. All rights reserved.