Electron-transfer reactions of copper(I) and copper(III) complexes

Article abstract of DOI:10.1021/ic50204a025

The reactions of bis(2,9-dimethyl-1,10-phenanthroline)copper(I), Cu¹(dmp)₂⁺, and of bis(2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonate)cuprate(I), Cu^I(dpmp)₂^3-, with copper(III)-oligopeptide complexes are quite rapid with rate constants varying from 2 × 10⁴ M^-1 s^-1 to 2.5 × 10⁸ M^-1 s^-1, indicating that outer-sphere electron transfer occurs. The cross exchange rate constants, k₁₂, for the reactions Cu^III(peptide) + Cu^IL₂→^k12 Cu^II(peptide) + Cu^IIL₂, where L is dmp or dpmp^2-, are determined for a series of copper(III) peptides, where the E°₁₂ values vary from -0.11 to +0.32 V. The values of k₁₂ for the reduction of copper(III) by Cu^I(dmp)₂⁺ exhibit a correlation with E°₁₂ consistent with the Marcus theory. This is not the case for the reductions with Cu^I(dpmp)₂^3-, which have unusual behavior in two regards. First, the observed first-order rate constants reach limiting values as the concentration of excess reductant increases, but limiting values are not observed if Cu(III) is used in excess. Second, the k₁₂ rate constants (obtained under conditions where the rate depends on both the Cu(III) and the Cu(I) concentrations) approach a limiting value of 10⁶ M^-1 s^-1 and are independent of E°₁₂. Neither anomalous behavior is observed for the Cu_I(dmp)₂⁺ reactions with Cu(III) complexes.