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

• Published in: Journal of the American Chemical Society 2019-01, Vol.141 (3), p.1324-1336
• Main Authors: Sankaralingam, Muniyandi, Lee, Yong-Min, Pineda-Galvan, Yuliana, Karmalkar, Deepika G, Seo, Mi Sook, Jeon, So Hyun, Pushkar, Yulia, Fukuzumi, Shunichi, Nam, Wonwoo
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.8b11492

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.