Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn ^IV (O)] ⁺ -M ⁿ⁺ (1-M ⁿ⁺ , M ⁿ⁺ = Ca ²⁺ , Mg ²⁺ , Zn ²⁺ , Lu ³⁺ , Y ³⁺ , Al ³⁺ , and Sc ³⁺ ) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn ^III (OH)] ⁺ , with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M ⁿ⁺ ). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M ⁿ⁺ on the reactivity of 1-M ⁿ⁺ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M ⁿ⁺ , such as 1-Sc ³⁺ ≈ 1-Al ³⁺ > 1-Y ³⁺ > 1-Lu ³⁺ > 1-Zn ²⁺ > 1-Mg ²⁺ > 1-Ca ²⁺ , follows the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M ⁿ⁺ , the higher the reactivity of 1-M ⁿ⁺ becomes. In sharp contrast, the reactivity of 1-M ⁿ⁺ in the HAT reaction was reversed, giving the reactivity order 1-Ca ²⁺ > 1-Mg ²⁺ > 1-Zn ²⁺ > 1-Lu ³⁺ > 1-Y ³⁺ > 1-Al ³⁺ ≈ 1-Sc ³⁺ that is, the higher is Lewis acidity of M ⁿ⁺ , the lower the reactivity of 1-M ⁿ⁺ in the HAT reaction. The latter result implies that the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M ⁿ⁺ cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.