The Impact of Huge Structural Changes on Electron Transfer and Measurement of Redox Potentials: Reduction of ortho-12-Carborane

A massive structural change accompanies electron capture by the 1,2-dicarba-closo-dodecaborane cage molecule (1). Bimolecular electron transfer (ET) by pulse radiolysis found a reduction potential of E⁰ = −1.92 V vs Fc^+/0 for 1 and rate constants that slowed greatly for ET to or from 1 when the redox partner had a potential near this E⁰. Similarly, two electrochemical techniques could detect no current at potentials near E⁰, finding instead peaks or polarographic waves near −3.1 V, which is 1.2 V more negative than E⁰. Voltammetry could determine rate constants, but only near −3.1 V. DigiSim simulations can describe the irreversible voltammograms but require electrochemical rate constants near 1 × 10^–10 cm/s at E⁰, a factor of 10^–10 relative to molecules undergoing facile ET. This factor of 10^–10 compared to ∼10^–5 for bimolecular ET presents a puzzle. This puzzle can be understood as a manifestation of one of the “Frumkin Effects” in which only part of the applied voltage is available to drive ET at the electrode.