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)MnIV(O)]+-M n+ (1-Mn+, M n+ = Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)MnIII(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-Sc3+ ≈ 1-Al3+ > 1-Y3+ > 1-Lu3+ > 1-Zn2+ > 1-Mg2+ > 1-Ca2+, 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-Ca2+ > 1-Mg2+ > 1-Zn2+ > 1-Lu3+> 1-Y3+> 1-Al3+ ≈ 1-Sc3+; 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.