Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations

Formal reduction potentials of highly oxidizing and short-lived
radical cations of substituted biphenyls generated by pulse radiolysis
in 1,2-dichloroethane (DCE) were measured using a redox equilibrium
ladder method. The effect of halide ion–radical interactions
on reduction potentials of biphenyls was examined by utilizing the
ability of DCE to release Cl– in the vicinity of
the radical cation. The Hammett correlation of measured potentials
across a range of over 700 mV shows saturation at high Hammett sigma
values. This effect has been explained by both ion-pairing and hemicolligation
interactions between biphenyl radical cations and Cl– and appears to modulate reduction potentials by as much as 400 mV.
This finding offers a convenient way to manipulate the energetics
of electron transfer involving organic redox species.