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

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 O₂, 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 (Me₈Fc) to a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)Cr^III(O₂)]⁺ (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 Me₈Fc to 1 does not occur. The second-order rate constants (k_et) of ET from Me₈Fc 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, where k_MCET is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two Mⁿ⁺ ions are bound to the one-electron reduced species of 1. Such a binding of two Mⁿ⁺ 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([Mⁿ⁺]). The log k_MCET values increased linearly with the increasing Lewis acidity of Mⁿ⁺ (ΔE), which was determined from the g values of O₂ ^•--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.