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

Abstract

Calcium ion plays an indispensable role for water oxidation by oxygen-evolving complex (OEC) composed of a manganese-oxo cluster (Mn4CaO5) in Photosystem II. In this context, the effects of Ca2+ 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 H2O and O-2, however, the effects of Ca2+ ion and other redox-inactive metal ions (Mn+) on the redox reactivity of metal-superoxo complexes have yet to be reported. Herein, we report that electron transfer (ET) from octamethylferrocene (Me(8)Fc) to a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)Cr-III(O-2)]+ (1), occurs in the presence of redox-inactive metal ions (Mn+ = Ca2+, Mg2+, Y3+, Al3+, and Sc3+); in the absence of the redox-inactive metal ions, ET from Me(8)Fc to 1 does not occur. The secondorder rate constants (k(et)) of ET from Me(8)Fc to 1 in the presence of a redox-inactive metal ion increased with increasing concentration of Mn+ ([Mn+]), exhibiting a second-order dependence on [Mn+]: k(et) = k(MCET)[Mn+2](2), where k(MCET) is the fourthorder rate constant of metal ion-coupled electron transfer (MCET). This means that two Mn+ ions are bound to the oneelectron reduced species of 1. Such a binding of two Mn+ ons associated with the ET reduction of! resulted in a 92 mV positive shift of the one-electron reduction potential of 1 (E-red) with increasing log( [Mn+]). The log kmcET values increased linearly with the increasing Lewis acidity of Mn+ (Delta E), which was determined from the g values of O-2(center dot-)-Mn+ complexes. The driving force dependence of log k(et) of MCET from ferrocene derivatives to 1 in the presence of Mn+ has been well-evaluated in light of the Marcus theory of electron transfer.