A Ligand-Bridged Heterotetranuclear (Fe2Cu2) Redox System with Fc/Fc+ and Radical Ion Intermediates

The redox pair [(μ-abcp)­{Cu­(dppf)}2]2+/+ (abcp = 2,2′-azobis­(5-chloropyrimidine) and dppf =1,1′-bis­(diphenylphosphino)­ferrocene) has been structurally characterized to reveal the lengthening of the NN and shortening of the CNazo bonds on reduction, each by about 0.04 Å. These and other charge forms, [(μ-abcp)­{Cu­(dppf)}2]n+ (n = 0, 3+, 4+), have been investigated spectroelectrochemically (UV–vis–near-IR, EPR) to reveal an abcp-based second reduction and a stepwise ferrocene-centered oxidation of the 2+ precursor. In contrast to the small but detectable comproportionation constant of Kc = 17 for the Fc/Fc+ mixed-valence (3+) charge state, the monocationic radical complex exhibits a very large Kc value of 1016.