Tuning Electron-Transfer Reactivity of a Chromium(III)-Superoxo Complex Enabled by Calcium Ion and Other Redox-Inactive Metal Ions

摘要

Calcium ion plays an indispensable role for water oxidation by oxygen-evolving complex (OEC) composed of a manganese—oxo cluster (Mn_4CaO_5) in Photosystem II. In this context, the effects of Ca~(2+) ion and other redox-inactive metal ions on the redox reactivity of high-valent metal—oxo and metal—peroxo complexes have been studied extensively. Among metal—oxygen intermediates involved in interconversion between H_2O and O_2, however, the effects of Ca~(2+) ion and other redox-inactive metal ions (M~(n+)) on the redox reactivity of metal—superoxo complexes have yet to be reported. Herein, we report that electron transfer (ET) from octamethylferrocene (Me_8Fc) to a mononuclear nonheme Cr(III)—superoxo complex, [(Cl)(TMC)Cr~(III)(O_2)]+ (1), occurs in the presence of redox-inactive metal ions (M~(n+) = Ca~(2+), Mg~(2+), Y~(3+), Al~(3+), and Sc~(3+)); in the absence of the redox-inactive metal ions, ET from Me_8Fc to 1 does not occur. The second-order rate constants (K_(et)) of ET from Me_8Fc to 1 in the presence of a redox-inactive metal ion increased with increasing concentration of M~(n+) ([M~(n+)]), exhibiting a second-order dependence on [M~(n+)]: k_(et) = k_(MCET)[M~(n+)]~2, where k_(MCET) is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two M~(n+) ions are bound to the one-electron reduced species of 1. Such a binding of two M~(n+) ions associated with the ET reduction of 1 resulted in a 92 mV positive shift of the one-electron reduction potential of 1 (E_(red)) with increasing log([M~(n+)]). The log k_(MCET) values increased linearly with the increasing Lewis acidity of M~(n+) (△E), which was determined from the g values of O_2~(·-)—M~(n+) complexes. The driving force dependence of log k_(et) of MCET from ferrocene derivatives to 1 in the presence of M~(n+) has been well-evaluated in light of the Marcus theory of electron transfer.