A Ligand-Bridged Heterotetranuclear (Fe<sub>2</sub>Cu<sub>2</sub>) Redox System with Fc/Fc<sup>+</sup> and Radical Ion
Intermediates
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Abstract
The
redox pair [(μ-abcp){Cu(dppf)}<sub>2</sub>]<sup>2+/+</sup> (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 CN<sub>azo</sub> bonds on reduction, each
by about 0.04 Å. These and other charge forms, [(μ-abcp){Cu(dppf)}<sub>2</sub>]<sup><i>n</i>+</sup> (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 <i>K</i><sub>c</sub> = 17
for the Fc/Fc<sup>+</sup> mixed-valence (3+) charge state, the monocationic
radical complex exhibits a very large <i>K</i><sub>c</sub> value of 10<sup>16</sup>