Metal ion-promoted intramolecular electron transfer in a ferrocene-naphthoquinone linked dyad. Continuous change in driving force and reorganization energy with metal ion concentration

Thermal intramolecular electron transfer from the ferrocene (Fc) to naphthoquinone (NQ) moiety occurs efficiently by the addition of metal triflates (Mⁿ⁺: Sc(OTf)₃, Y(OTf)₃, Eu(OTf)₃) to an acetonitrile solution of a ferrocene-naphthoquinone (Fc-NQ) linked dyad with a flexible methylene and an amide spacer, although no electron transfer takes place in the absence of Mⁿ⁺. The resulting semiquinone radical anion (NQ^.-) is stabilized by the strong binding of Mⁿ⁺ with one carbonyl oxygen of NQ^.- as well as hydrogen bonding between the amide proton and the other carbonyl oxygen of NQ^.-. The high stability of the Fc⁺-NQ^.-/Mⁿ⁺ complex allows us to determine the driving force of electron transfer by the conventional electrochemical method. The one-electron reduction potential of the NQ moiety of Fc-NQ is shifted to a positive direction with increasing concentration of Mⁿ⁺, obeying the Nernst equation, whereas the one-electron oxidation potential of the Fc moiety remains the same. The driving force dependence of the observed rate constant (k_ET) of Mⁿ⁺-promoted intramolecular electron transfer is well evaluated in light of the Marcus theory of electron transfer. The driving force of electron transfer increases with increasing concentration of Mⁿ⁺ [Mⁿ⁺], whereas the reorganization energy of electron transfer decreases with increasing [Mⁿ⁺] from a large value which results from the strong binding between NQ^.- and Mⁿ⁺.